THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA LOS ANGELES PROPETRTV or THE: LIBRARY CH L ' f IM m nf ilNES j L Los Angeles, California ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY BY DIPL. ING. W. RODENHAUSER, E.E. CHIEF ENGINEER, ELECTRIC FURNACE DEPARTMENT, ROCHLING EISEN UNO STAHLWERKE, VOLKLINGEN, GERMANY CHIEF ENGINEER, GESELLSCHAFT FCR ELEKTROSTAHLANLAGEN, M. B. H., BERLIN J. SCHOENAWA, Metallurgist FORMERLY WITH PRAGER EISENINDUSTRIE, KLADNO, AUSTRIA, AND WORKS MANAGER OF ROCHLING EISEN UNO STAHLWERKE AND C. H. VOM BAUR, E.E. FORMERLY CHIEF ENGINEER AMERICAN ELECTRIC FURNACE COMPANY Translated from the Original by the latter and now Completely Rewritten THIRD EDITION, REVISED TOTAL ISSUE, FIVE THOUSAND NEW YORK JOHN WILEY & SONS LONDON: CHAPMAN & HALL, LIMITED 1920 Copyright, 1913, 1917, 1920, by C. H. VOM BAUR PRESS OF THE PUBLISHERS PRINTING COMPANY, NEW TORK, U. 8. A. Library lot* PREFACE FOR THIRD EDITION THE giant strides made in all forms of essential industrial- ism during the Great War did not neglect the electric furnace. Indeed, it fared better than even the optimists did hope. This, taken together with the practical uniformly good steel made in them by experienced men, heat after heat, has placed this industry on a more solid and dignified footing. Added evi- dence of this is the one thousand electric furnaces melting and refining steel which are either built or building. The next ten years will no doubt witness more than double the number of electric furnaces described in this volume than those which are in existence to-day. C. H. VOM BAUR. NEW YORK CITY, December 6th, 1919. PROPEZRTV OF THE LIBRARY CHAMBER OF MINES AND OIL Los Angeled, California 622973 PREFACE TO THE SECOND EDITION IN ENGLISH THE belief expressed in the writer's preface five years ago has been well borne out. To-day there are over 212 electric furnaces in the iron and steel industry in the United States and Canada out of a total of 562. Over half a million tons of electric steel are being made annually the world over, and production only started less than a score of years ago. The average size of furnaces has also steadily increased, together with the number of days they are in operation yearly. Several newcomers have made their appearance, with the usual pain and effort accompanying such creations. Among these it is a pleasure to record due credit to M. S. Vincent, who was so largely responsible for bringing out the American designed Rennerfelt. Cordial thanks are here also acknowledged to many friends for valuable information and data, especially to E. F. Cone for his cheerful help at all times. C. H. VOM BAUR. NEW YORK, September 2ist, 1917. PROPETR-TV OR THE LIBRARY CHAMBER OF MINES AND OIL Los Angeles, California PROPER-TV OF- THE: LIBRARY CHAMBER OF MINES AND OIL LosAngeles, California PREFACE TO THE GERMAN EDITION ELECTRIC furnaces and their use in the manufacture of steel and iron have been described in books by Borchers, Neumann Askenasy, and others. Their treatises have either described so fully the whole subject of electro-metallurgy that only a very small space could be allotted to electric iron and steel, or else, as in Neumann's volume, only a glance is given at the early experiments which were made when these furnaces were first introduced. Hence, there is need for a work thoroughly describing electric' furnaces, which are designed only for the steel and iron industry. For practical reasons the book is divided into two parts, of which the first deals with all questions relative to the construc- tion of these electric furnaces, and the apparatus used, while the other part takes up the practical use of electric furnaces in the steel mill and all its metallurgical reactions. While undertaking this work the authors were conscious of the difficulty of describing each type of furnace entirely from personal observation. This difficulty, however, confronts all who are similarly situated, as these electric furnaces have only recently been introduced into the iron trades and it is practically impossible to know each type from one's own experience. As both practical and theoretical men differ regarding the advantages of these furnaces for steel and iron making, it is not to be expected from this book that any one type of furnace is pictured as being better than any other type. Wherever possi- ble, therefore, results are given which are based on actual ex- perience, although much other material has been used. THE AUTHORS. VOLKLINGEN, SAAR, IQII. vii PREFACE TO THE EDITION IN ENGLISH THE preparation of this work in English was undertaken in the belief that electric furnaces for the iron and steel industry would have their greatest future on the North American Conti- nent. Especially is this true of furnaces making electric steel. Specifications are daily becoming stricter for steel rails, steel castings, and tool steel. Electric steel rails, costing but little more than the ordinary kind, are found to be unbreakable in service, when laid beside open hearth and Bessemer rails. In these latter, scores of breakages have occurred in one season. The future of electric steel rails consequently seems assured. Electric steel castings have also been on the market for the past four years. They are looked upon with favor alike by the foundryman and the customer, not only because the highest class of steel may be made from the cheapest raw material, but also because of the high percentage of good castings and their freedom from blow-holes. The ability to make homogeneous tool steel, free from gases, and at low cost, brought the electric furnace into commercial use over a decade ago. In this field it promises to displace com- pletely the old and small crucible pot which has been in use since the year 1740. With these three principal fields now open to electric furnace products, it cannot be long before all other domains in the use of steel will be invaded. The cost of producing electric steel is lower now than that of the crucible process, or of the small converter process, and even less than that of the open hearth process, as practised with lo-ton furnaces or under. A success can, therefore, be confidently predicted for electric furnaces and their manufacture of iron and steel. A few changes were found necessary, in adapting the German to the edition in English, and some fresh material has been added. X PREFACE The translator gladly takes this opportunity to thank many friends for information and assistance. Dr. G. B. Waterhouse, of Buffalo, kindly gave the benefit of his extended experience in connection with the metallurgy of iron and steel as set forth in Part II of this book. To Mr. A. H. Strong, of New York, special thanks are due for valuable aid rendered in the various chapters on induction furnaces. Mr. Magnus linger, of the transformer and furnace department of the General Electric Company, very kindly read the proofs of many chapters of Part I. No one has had a larger or more successful experience in building trans- formers and furnaces than Mr. Unger. Finally, thanks are due to Dr. D. A. Lyon for much new material added, mainly to the chapters on electric pig-iron furnaces. C. H. VOM BAUR. NEW YORK, September 7th, 1912. PREFACE TO PART I THE realm of Steel and Iron manufacture has in the past ten years had a new world of possibilities opened to it by the intro- duction of the electric furnace. Before finding a commercial foothold among ironmasters, it was in use making ferro alloys. Even earlier than this the electric furnace was manufacturing aluminum and calcium carbide. It has been the aim of the present publication on Electric Steel and Iron Furnaces to produce a book for the practical man; a comprehensive manual of practical information, yet one ex- plaining the electric laws and phenomena involved, and the scientific principles upon which the work rests. The under- standing of these electrical laws is practically necessary, for in electric furnace literature we constantly find assertions con- tradicting the simplest of them. The authors also hope in this manner to render the book of service to the general student of this branch of Electro- Chemical Engineering, and to state especially the principal laws which the construction and operation of electric furnaces entail, without giving long mathematical discussions. Short arithmetical examples nevertheless are given dealing with actual furnace problems. Care has been taken to mention only those things which have some value in the develop- ment of Electric Steel and Iron furnaces, rather than to dwell upon theories of little moment. The furnaces most extensively used, such as those of Stassano, Heroult, Girod, Kjellin, and Rochling-Rodenhauser are described in detail, and compared with an ideal Electric Furnace. This seems to be the best course to pursue, for in this way an unfair criticism of the differ- ent systems can best be avoided. In Chapter XIV, "General Review," some furnace designs are briefly discussed which have obtained only a limited use or xi xii PREFACE which have not yet left the experimental stage, and finally, the electric shaft furnace is described at length. The discussions are accompanied by a large number of cuts and reproductions. The demands of actual practise have always been given the greatest consideration. Accordingly, the latest results obtained from good trials with electrodes in arc furnaces are mentioned, as are others of the same order. This volume should, therefore, be a welcome adviser to the furnace builder, the student, and in fact to anybody who is interested in electric furnaces for the pro- duction of steel and iron. The authors have written in the hope that these pages will aid in the further expansion and success of the electric iron and steel trade. WM. RODENHAUSER. VOLKLINGEN, SAAR, IQII. CONTENTS PART I ELECTRIC FURNACES, THEIR THEORY, CONSTRUCTION, AND CRITICISM CHAPTER I HISTORICAL PAGE Some data relating to the development of electrical engineering, ... i Tests of Davy and Pepys, 3 Suggestions by Wall, 4 by Pichon 4 by William von Siemens, 5 by de Laval, ; 6 by Taussig, 8 The electric furnace of Stassano, 8 of Heroult, 9 of Kjellin 9 Report of the Canadian Commission under Dr. Haanel, 9 The electric furnace of Girod 10 of Rochling-Rodenhauser, 10 of Gronwall, Lindblad & Stalhane 10 CHAPTER II SOME LAWS AND FUNDAMENTAL PRINCIPLES OF ELECTRICITY Ohm's Law, II Resistance of a conductor, 1 1 Units of measurement : Ampere, volt, ohm, 12 Temperature coefficient, 13 Conductors of the second class 15 Series connection, 16 Parallel connection, 17 I. Kirchoff's Law 19 Combination resistances, 20 Arithmetical example, .21 Joule's Law 23 The derivation of heat generated 23 of power, 24 of work, , 25 xiii x j v CONTENTS CHAPTER III EFFECTS OF THE ELECTRIC CURRENT PAGE The action of heat, ' 26 1. Direct resistance heating, 26 Gin, Electric furnace of, 27 Arithmetical example therefore, 28 Current density, permissible in copper conductors, 29 2. Induction heating 32 3. Indirect resistance heating 32 Borchers, Laboratory furnace of, 33 Hera us, Laboratory furnace of 34 Girod, Crucible furnace of 35 Helberger, Crucible furnace of, 35 4. Arc heating 37 Chemical action, 37 Motor effect 39 Action of two magnets upon each other 40 Lines of force of a current-carrying conductor, 41 Direction of lines of force 41 Action between a magnet and electrical conductor, 42 of two electrical conductors upon each other, 43 Lines of force of coils, 44 Pinch effect, 44 CHAPTER IV POWER FACTOR (cos<) AND ALTERNATING CURRENT THEORY IN GENERAL Periodicity, frequency, cycle, 47 Line diagram, 47 Angular velocity 48 Induction, induced currents, 49 Self-induction, current of self-induction, 50 Phase difference, 51 Vector diagram, 51 Coefficient of self-induction 52 Apparent resistance, 54 Power in alternating current circuits, 57 factor, ....... 58 Losses on account of induction phenomena 59 Eddy or Foucoult currents, 59 Hysteresis losses 60 Three phase current, polyphase current, 60 Star or Y connection 61 Delta connection, .....62 CONTENTS xv CHAPTER V GENERAL CONDITIONS FOR THE OPERATION OF ELECTRIC FURNACES PAGE Ad v antages of electric furnaces, 65 Demands made of an ideal electric furnace, . 66 Influence of the kind of current, 68 of the frequency, 70 of changes in the load, 71 Regulating power of the furnace temperature, 72 Electric efficiency, '. .'.... . 72 Furnace and hearth arrangement, . 73 Influence of electric heating . . . . 73 Circulation of the molten metal, . 75 Influence of water cooling, ....'... 75 CHAPTER VI THE ARC FURNACES IN GENERAL The arc, ... . . .... . . . 77 Radiation furnaces, . ...... ..... . . . ' . . . 79 Combined arc and resistance furnaces, . . ....... . . . 79 The electrodes of arc furnaces, 80 Current density in electrodes, 82 Efficiency of electrodes, 84 Burning away of electrodes 89 Electrode consumption, . . . . . . . 89 coverings, . . ... . . ' . . ..-,.',. . . . 97 cooling, . ... . . . ... . . v< . . . 99 regulation, . . . -. . ... . ' . ;. . . . 102 Thury regulator; Seecle regulator .... . ... . . ,:- . 103 CHAPTER VII THE STASSANO FURNACE Stassano shaft furnace, no hearth furnace, .' 112 rotating furnace, ; 112 Comparison with an ideal furnace, 120 Installation costs, 124 Issuing of licenses, 124 CHAPTER VIII THE HEROULT FURNACE Historical, 125 The Furnace, 126 Comparison with an ideal'furnace 139 Installation costs 147 Issuing of licenses 148 xvi CONTENTS CHAPTER IX THE GIROD FURNACE PAGE Historical, 149 The furnace 150 Comparison with an ideal furnace, 156 of electrode cross-section with a Girod and Heroult . . . 159 Installation costs 162 Issuing of licenses, 163 CHAPTER X THE RENNERFELT FURNACE Historical 165 The furnace, 166 Comparison with ideal furnace 169 Installation costs 173 Issuing of licenses 175 CHAPTER XI THE INDUCTION FURNACE IN GENERAL Principle of the transformer, . 176 of the induction furnaces, . 177 Cylinder winding, tube winding, disk winding, 181 Suggestions by de Ferranti, 181 by Colby, * . 185 by Kjellin, , . . . 185 by Frick, 185 Arrangement for lessening the stray fields, 187 Suggestion by Rochling and Rodenhauser . . 188 CHAPTER XII THE KJELLIN FURNACE Historical, 189 The furnace, . 189 Influence of the furnace contents on the power factor, . . . , . 194 Comparison with an ideal furnace, 201 Issuing of licenses, 208 CHAPTER XIII THE ROCHLING-RODENHAUSER FURNACE Its beginning 209 The furnace, .. , ..... .' . 213 Regulating transformers, auto transformers 230 Installation costs, 239 Issuing of licenses, 240 CONTENTS xv ii CHAPTER XIV THE ELECTRIC SHAFT FURNACE PAGE The Stassano electric shaft furnace, . . . '. 241 The Keller electric shaft furnace, 242 The Heroult electric shaft furnace, 243 The test furnaces of Gronwall, Lindblad & Stalhane 244 The Gronwall, Lindblad & Stalhane electric shaft furnace 247 Influence of carbon on the energy taken up, 250 Results of operation ''.",. 251 Installation costs, 255 Issuing of licenses, 256 CHAPTER XV GENERAL REVIEW The Chapelet arc furnace (Giffre, Allevard), . . ....... . 257 The Keller arc furnace . . . . . T . . 258 The Nathusius arc furnace, . . . . . . . . . . . . 260 The Gin induction furnace, . 263 The Schneider-Creusot induction furnace, 263 The Gronwall, Lindblad & Stalhane induction and arc furnace, . . . 264 Greaves-Etchells furnace, ."..'. . . ... . . 265 Snyder furnace, . ... .... . . .. 269 Greene furnace, . ... ^ . . . , . . 227 The Hiorth combination furnace and induction furnace, . ... . 272 Moore furnace, . . . '. .-.'.. . . . 274 Booth-Hall furnace . . . > . . . . . ... . . . 276 Vom Baur furnace, . . . I; .... . . .,. 279 Ludlum furnace, . . . . ; . . . . . . ". . . . . 284 CHAPTER XVI FINAL CONSIDERATIONS Economical - . 287 Statistics, . . . . . . . . . ....... . . . . 291 PART II A. MATERIALS FOR FURNACE CONSTRUCTION AND THE COSTS OF OPERATION MATERIALS FOR FURNACE CONSTRUCTION Their general requirements, 292 "Schamotte" fire-bricks, 294 Acid or silica bricks 295 " Half Schamotte " fire-bricks, 295 Carbon bricks and carbon for ramming in place, 295 xviii CONTENTS PAGE Basic bricks and materials for ramming in place, . ..... . . . 296 Chrome iron ore .....;. 296 Dolomite, .'*...... 296 Dolomite plant 296 Tar, 296 Magnesite and magnesite bricks 297 Mortar ." . . 297 Fluxes for the rammed part of the lining, 298 Form of hearth and durability of lining, 300 THE COSTS OF OPERATION Influence of the kind of furnace on the quality of steel, 300 General operating costs, 301 Charge, 301 Loss in working, 302 Comparison of the heating cost in the open-hearth and electric furnace, 304 Comparison of the heating cost in the crucible and electric furnace, . 306 Statistics concerning electric steel production in principal countries, . 307 The amount of power used and its influence, 308 Comparison of the heating cost in the electric shaft and ordinary blast furnace, 310 Unit price for electric power, 311 Slag-making materials, 312 Labor, 3-12 Costs for lining or furnace maintenance 313 Amortization costs, 314 Cost of electrodes, 315 Auxiliary arrangements, 315 Consumption of tools, 316 Total costs of operation of the electric shaft compared with the ordinary blast furnace, 317 Total costs of operation of the Stassano furnace, 319 Total costs of operation of the Girod furnace, 320 Total costs of operation of the Heroult furnace, 322 Increased cost through desulphurization by means of Ferro-Silicon, . . 323 Total costs of operation for the Rochling-Rodenhauser furnace, . . . 324 B THE ELECTRO-METALLURGY o* IRON AND STEEL Introduction, 326 THE ELECTRIC SMELTING OF IRON ORES WITH THE PRODUCTION OF IRON AND STEEL The smelting of ore in the Stassano furnace, . . . . . . . . 335 The smelting of ore in the Gronwall, Lindblad & Stalhane electric shaft furnace, 338 CONTENTS XIX PAGE The smelting of ore in the Rochling-Rodenhauser induction furnace, . 339 Chemical balance, 341 Smelting results, 343 Criticism of ore smelting in the electrode-hearth furnace, ... . 349 The smelting of ore in the electrode shaft furnace, 351 Ore smelting tests in the special Heroult furnace, 355 Criticism of this nethod of smelting, . . .... . . . 360 Ore smelting in the Gronwall, Lindblad & Stalhane furnace, . . 361 Efficiency of the furnace, . . ... . . -. . . . . 368-372 Criticism of the furnace, . -. . . . 374 Ore smelting in the Noble furnace, . . ....'. . . . 375 Later results in Sweden, . .... ; . 379 Ore smelting in the rebuilt Noble furnace, 383 Metallurgy, . .... . 389 Conclusion, 393 Pig iron from steel turnings, 394 The future of electric steel, . 395 Making pig steel in the electric furnace 396 THE USE OF THE ELECTRIC FURNACE FOR MELTING, FOR THE REFINING OF PIG IRON, AND FOR THE PRODUCTION OF ORDINARY AND SPECIAL QUALITY STEEL The impurities in steel: phosphorus, sulphur, silicon, copper, arsenic, carbon, oxygen, manganese, aluminum, vanadium, titanium, . . 399 The slag-producing materials, ferro-alloys, etc., used in the electric fur- nace: Ferro-manganese, ferro-chrome, ferro-silicon, lime, fluor-spar, iron ore, carbon, . ; . . . . . . . 410 The electric furnace as a melting furnace for iron and steel and iron alloys of every kind, 412 Melting of pig iron, 413 Melting of ferro-manganese, 414 Duplexing with the cupola 416 The electric furnace as a mixer, 422 The electric malleable casting, . . .. . . 420 Pig-iron refining, . . . . 423 Production of special quality steel in the electric furnace, . ... . 427 From previously refined metal with low phosphorus and sulphur, . 428 From previously refined metal with considerable phosphorus and sulphur, . . ,. 432 The metallurgical course of operations of an electric furnace charge, . 437 The special qualities of electric iron and steel, ........ 438 Final considerations, 440 Comparison of heating costs in the open-hearth and electric furnace, . 442 INDEX, 445 xxi INDEX TO ABBREVIATIONS USED IN PART ONE A = Work or Energy. Cos = Power factor. E = Potential per phase. . j e = Potential in volts = Effective value for A. C. e = Maximum value of potential, e' = Instantaneous value of potential. 6L = Potential to overcome the self-induction. e r = Potential to overcome the ohmic resistance. I = Current per phase. i = Current in amperes = Effective value for A. C. i r = Watt component of current. im wattless component of current. k = Heat conductivity. KVA = Kilovolt- amperes. KW = Kilowatt. KW Hr = Kilowatt hours. L = Self-induction. 1 Length of a conductor in metres, m = Angular velocity. N = Flux, p = Power. p' = Instantaneous value of power. Q = Energy in heat units, q = Section in square millimetres, r = Resistance in ohms. a Turns. T = Time of a cycle, t = Time. VA = Volt amperes. A = = Current density per square millimetre. v = Cycles per second. p = Specific resistance per I metre length and I square millimetre section. = Specific conductivity. P pi = mean electrical resistivity per cubic centimetre. pi = Mean electrical resistivity per cubic inch. = x = Specific conductivity per centimetre. Pi T = Temperature gradient. d = Diameter. PART I ELECTRIC FURNACES Their Theory, Construction and Criticisms ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Part One CHAPTER I HISTORICAL REVIEW GREAT interest is today manifested in electric steel and its production. Not only are the different iron and steel works installing electric furnaces or considering their adoption when enlargements become a good investment, but political economists are also carefully following the progress made in the electric- steel industry. The daily press frequently contains accounts of the importance of electric iron and steel. Considering the great and almost universal interest shown today in the new industry, it must seem astonishing that but fifteen years ago hardly a thought was given to the practical utilization of the electric furnace for producing steel. This remarkable growth originat- ing in the laboratory seems to justify us in following the de- velopment of the electric furnace, and in tracing the causes which have made its entrance into the great industries possible. In the first instance we must clearly understand that all electric furnaces are naturally apparatus in which electrical energy is consumed for the purpose of transforming it into heat. Their development on a large scale was therefore not possible until electrical engineering had succeeded in producing sources of current which furnished it economically, continuously and .of sufficient size. At the beginning of the last century thermopiles, or galvanic cells, as they are used today for operating house bells, or telephone 2 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY circuits, were the only sources of electric current at the disposal of the user of electricity, so that we see the electric current of this period confined in its application to the laboratories of the scientist. It was only toward the middle of the igth century that a strong development started which has its foundation in Faraday's discovery of induction. In 1831 Faraday found that each time he brought a strong magnet near to, or moved it away from, a coil of wire, the ends of which were separated a very small distance from each other, a tiny spark appeared at the point of interruption. We say, therefore, that Faraday found that the magnet induces a current in the electric conductor (the coil of wire). This discovery brought a great light into the darkness which until then had covered the practical generation of electricity; for, hardly a year after Faraday's discovery, we find the first magnet-electric machine, which was built by Pixii. This was the first form of a dynamo machine, that is, of a machine which transforms rotary motion into electric energy. In Pixii's ma- chine a coil of wire was arranged in the magnetic field of a strong ordinary horseshoe magnet, in such a way that when the coil was rotated, induced currents were produced, as discovered by Faraday. This first machine was soon followed by improved designs, which, even at this early period of electrical science in the fifties of the last century, were put to use in supplying light in lighthouses on the coasts of France and England. The next step forward was accomplished by H. Wilde in 1866 in Manchester, by the construction of an electric machine, whose magnets were electro-magnets. For these a small machine with ordinary magnets furnished the current. A Wilde generator of this type, which required about 3 h. p. for driving the exciting machine and about 1 5 h. p. for the main dynamo, was able to melt a bar of platinum 6 mm. thick (about y^ inch) and 60 cm. (two feet) long. The above mentioned electric magnetic machines were, however, despite their considerable power, unable to introduce electricity for general uses. An important forward step was still lacking until Werner von Siemens discovered the "dynamo HISTORICAL 3 electric principle," which he laid before the Berlin Academy on January 17, 1867. According to this principle the "residual" magnetism that remains in even the softest iron is sufficient to produce an extremely weak current by which the magnetism can be strengthened more and more. The employment of this discovery in the construction of dynamos now made it possible to use electro-magnets instead of the ordinary permanent magnets heretofore used, and, with this improvement, we have the dynamo as it is today. It is used the world over, following the Siemens principle. But even after Siemens' discovery considerable time elapsed before any great change occurred in the output of large electric generators. It was the invention of the incandescent lamp, first generally known in Europe through the Paris international electric exhibition in 1881, which brought about this develop- ment. Electrical power-houses in ever increasing numbers and sizes now appeared, and today we see them in nearly every city. If we finally call to mind the well-known first great power trans- mission of a current at 30,000 volts pressure over a distance of 170 km. (106 miles), between Lauffen and Frankfurt, Germany, which was shown to the world in 1891 at the time of the Frank- furt Exhibition, we find ourselves in the midst of tremendous advances of electrical science. If we now turn to the history of the development of the electric furnace itself, we find its first traces at the beginning of the i gth century; that is, at the same time in which the sole means of producing electric currents was the thermopile, and at which time no thought of any electrical science existed. The first to consider the practical exploitation of electric energy by converting it into heat, probably was Davy, who, about 1810, during his experiments in the electrolysis of aluminum oxide, ex- cluded any influx of heat from the outside, and produced the heat necessary for the experiment by the electric current itself, which he obtained from an apparatus naturally very inferior, from the view-point of our modern ideas. His apparatus consist- ed of a platinum plate connected with one pole of a thermopile of 1000 plates, the other pole being connected with an iron wire. 4 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The latter projected from the upper side into a layer of clay carried by the platinum plate, which was in connection with the other pole. When the circuit was established the iron wire became white hot. and melted where it was in contact with the clay. A far more perfected arrangement was that of Pepys, who in 1815 welded an iron wire by heating it with an electric current. Pepys' apparatus can be looked upon as the first form of the FIG. i. class of electric furnaces today known as resistance furnaces. A soft iron rod was slotted with a fine saw in the direction of its axis and the slot was filled with diamond dust. The rod was then wound with wire and heated to red heat for 6 minutes by means of an electric battery (Fig. i). An examination of the iron wire showed that the diamond dust had disappeared, and that the iron had changed with the absorption of carbon. In this experiment iron was for the first time treated by the applica- tion of electric heat. It is interesting to find that in 1843 A. Wall made the suggest- ion that pig iron be treated and converted by electrical means. In 1853, IO y ears later, we find in a French patent granted to Pichon, the first electro- thermic furnace. The patent claim is as follows: "economical and application of the electric light to metallurgy and particularly metallurgy of iron." The furnace reproduced by Fig. 2 shows the original design of a furnace indirectly heated by electric arcs. Such furnaces are used even to- day with some changes in the design given us by Stassano. The ore or metal which Pichon tried to melt in his furnace was dropped between electrodes of considerable area through which the electric current passed. It was expected HISTORICAL 5 that the charge would melt under the influence of the tempera- ture of the arc, and collect in the bottom, which in turn was to be heated. Pichon's idea was to build his type of furnace on a large scale, this being clearly indicated by the dimensions of the electrodes which were to be 3 m. (10 ft.) long and to have a cross-section of 60 sq. cm. (9.3 sq. inches). It is interesting to observe that Pichon's suggestion appeared exactly at the time in which the first attempts were being made to illuminate the sea-coasts by means of electric light supplied by permanent magnet-electric dynamos. Electrical science, which thus called this furnace into existence, was, however, unable to further the realization of Pichon's daring plans, so capable of life as later developments show. The magnet-electric machine was by a large margin incapable of furnishing the current necessary for the operation of Pichon's furnace. Many different schemes were tried, in the years immediately following Pichon, to utilize the electric current in the production of iron, but they failed, being in advance of their time. The English patents of William von Siemens, of the years 1878 and 1879, next bring developments in the design of electric furnaces. They contain nearly all the important details of the modern arc furnaces, and for this reason they will be examined somewhat more closely. Siemens used different types of furnaces. The first design consisted of a crucible surrounded by a metallic case, through the bottom of which pro- jected one pole of an electric circuit. That part of the electrode in direct touch with the charge was provided with a point of platinum or other substance capable of resistance of great heat, in order to avoid contaminating the charge. The second electrode, which was connected with the other pole of the electric circuit, entered through the cover of the furnace and was cooled with water or other liquid. Figure 3 shows the arrangement of the furnace. Siemens later changed the design, making the elec- trode, which entered from the top, of carbon, while the lower metallic electrode was cooled with water. A heat-protecting 6 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY FlG. 4. covering was provided for this furnace. The crucible was placed in a larger case of metal and . the space between the two filled with charcoal or other poor conductor of heat, as shown in the design, Fig. 4. Siemens finally used a form of furnace very similar to that of Pichon. Two carbon electrodes were inserted in the sides of a crucible in such a position that they remained above the top of the charge, and the arc formed between them did not come in con- tact with the material to be melted. This furnace is shown in Fig. 5. With it Siemens succeeded in melting 10 kg. (22 Ibs.) of steel per hour. He also reduced iron ore and fused metals of high melting point such as platinum, taking about one-quarter of an hour to liquefy 4 kg. (8.8 Ibs.) of the latter substance. Siemens figured theoretically that the combustion of i kg. of coal under the boilers of a dynamo- electric generating plant would produce i kg. of melted steel. Siemens' furnaces, in regard to their practical construction, attained a high degree of perfection. They were equipped with automatic devices for adjusting the carbons and to keep the arc length always the same. He also utilized the directive qualities of the electro-magnet in order to obtain the best heating effects. All modern constructions of arc furnaces are adapta- tions of this original design, differing simply in size and form and other minor respects. The reason why the Siemens furnace failed of successful introduction on a commercial scale, lies in the fact that current was still too expensive; it cost too much in those days to be of use in melting iron in electric furnaces. With the suggestions of Siemens, the furnace subject seemed for the time exhausted. Aside from a long list of unimportant patents the ensuing time shows no progress until the appearance of the interesting patent of de Laval, of the year 1892. Fig. 6 HISTORICAL shows this furnace. The hearth of a cylindrical furnace is divided into two parts by a bridge cooled with water. At the bottom of each of the two compartments metal or carbon elec- trodes are inserted and connected with a source of alternating FIG. 7. FIG. 8. current. The furnace was to be charged from the top by re- moving the cover and first introducing a quantity of molten magnetic oxide of iron. The succeeding charges were to be of spongy iron. In operating the furnace it was intended to cover the bridge with a layer of oxide or other fusible substance which would act as a resistance. With an iron-refining furnace oxidized iron was to be used. The object sought was to have the spongy iron undergo a refining process in falling through the material forming the resistance; and this was the purpose for which de Laval's furnace was designed. De Laval saw a great future for this furnace and the extent of his hopes can best be realized from the fact that with Nobel, in 1895, he laid plans for a power plant of some 35,000 h. p., to be used in melting iron by electricity. These bright hopes failed of realization, and de Laval's furnace has today an historical interest only, as the first example of a furnace to melt iron by direct resistance. But the plans show that we have arrived at an epoch in the development of electro- chemistry, by aid of which, and that of great water-power it is possible to consider operating electric furnaces on a large scale. 8 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Another type of resistance furnace, the Taussig, appeared in 1893. No longer is a separate liquid resistance required. The charge itself, whether metal or ore, forms the resistance, and in consequence the furnace takes the shape of a horizontal groove rather than a vertical one as Fig. 7 shows. This furnace also remained unused.. It is evident from the above that in the middle of the nineties of the last century there existed a number of designs for electric furnaces, and that the process of heating metal baths by elec- tricity was well understood. But the iron industry had so far refused to take the electric furnace seriously. This is even more astonishing considering the pathfinding and successful employ- ment of the Heroult furnaces of 1887 and 1888 in the aluminum industry, and the use of the electric furnace in the manufacture of calcium carbide in 1894. Both these industries had already attained their full growth when, with the new century at last, the interest of the iron industry in the electric furnace began to awaken. The main reasons for this late beginning of the electro- steel and electro-iron industries may be sought, first, in the high development of the existing process for the manufacture of steel and iron, which seemed to preclude any possibility of cheapening steel; and, second, in the fact that nothing definite was known about the quality of the product of the electric furnace. The first practical constructions of furnaces to melt and refine iron appeared at the same places where the iron industry itself had come into being; that is, places having favorable water-power. Here the electric furnaces could obtain cheap current for experimental purposes. This leads to that point of the development which produced the present furnaces, to be considered more closely in later chapters; at this time, therefore, only the historical facts will be recorded in a general way. In 1898 Stassano took out a patent in different countries claiming: "A method for the practical production of liquid wrought iron of any degree of carbon and of liquid alloys of iron by means of the electric current." Stassano's furnace under- HISTORICAL 9 went many constructive alterations as a result of experiments made to obtain a practical apparatus, but his furnaces even as used today are based on the old principle of heating. The next most important type of furnace used today, the Heroult, appeared in the years 1899 and 1900, and almost at the same time Kjellin with his induction furnace succeeded in pro- ducing an apparatus of practical use in the iron 'industry. All these three furnaces were operated by electricity generated by .means of water-power. The Stassano in upper Italy, the Heroult in Savoy, and the Kjellin in Sweden, and their practical success, first drew the interest of the iron industry. An impor- tant contributing factor also was a report by Dr. Haanel, chief of a commission of experts sent by the Canadian Government to Europe to study the electric furnace. This report first brought together the different existing types of furnaces and considered them in their relation to each other. The plants of Gysinge, Kortfors, La Praz, Turin, and Livet were inspected, and it was found that a Kjellin furnace in Gysinge produced a superior quality of steel from raw materials consisting of charcoal iron and scrap iron. In Kortfors, and also in La Praz, the Heroult steel process was in operation, any desired quality of steel being produced by the method of first melting scrap and then refining it by the use of a large variety of slags. The Stassano furnace in Turin was not in operation at the time of the commission's inspection. In Livet a furnace by Keller, of a construction similar to that of Heroult, was busy melting iron direct from the ore. From the above it is evident that in 1904, the time of the Canadian Commission's tour, the electro-steel industry had at- tained a healthy existence, at least where in proximity to water- power developments. The principal hindrance to the introduc- tion of the electric furnace in the iron industry had now been overcome. In the production of steel the problem was to keep the iron from absorbing the carbon of the electrodes, and both Kjellin and Heroult successfully solved the difficulty although in different ways. Kjellin avoided the use of electrodes entirely, while Heroult brought the electric current into the furnace 10 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY through carbon electrodes, following the method used (for ex- ample) in the aluminum industry, where the electrolysis of the molten mass is desired. He, however, arranged his furnace so that there always remained a layer of slag interposed between the metal and the carbon, thus avoiding contact between the two. Stassano sought to attain his goal in the reduction of iron directly from the ore and only later turned to the scrap-iron method. Heroult and Kjellin were the first to regularly engage in the business of melting scrap iron in the electric furnace. And again it is Heroult to whom credit is due for the develop- ment of the art, to the end that from a charge of ordinary scrap any desired quality of steel may be obtained by refining it. Improvements in the machinery for generating electric currents, especially in the design of gas engines of large capacity, had in the mean time opened the way for other cheap methods of producing electricity, so that the electro-steel industry was no longer limited to water-power locations. In 1905 there appeared the first such industry in Germany, in the works of Richard Lindenberg in Remscheid. The installation consisted in one Heroult furnace. In the same year the Rochling Iron and Steel Works erected a Kjellin furnace and were the first to try the experiment of running an electric furnace in conjunction with a great iron establishment. There remains to be mentioned that during this period the now much used Girod furnace appeared in a small way, and that the year 1906 was marked by the appearance of the Rochling- Rodenhauser furnace, of which the following chapters will speak more fully. With the latter there now existed an induction furnace by means of which (as also with the Heroult furnace) a superior steel of any desired quality could be obtained from a charge of any kind of raw material. In recent years successful efforts have been made to produce iron by means of the electric-shaft furnace of Gronwall, Lindblad & Stalhane and others in California. More modern times have arrived and in the following chapters will be found a discussion of the present day furnaces as used in the electric-steel industry. L. I EB R A t-t Y CHAI Los CHAPTER II SOME FUNDAMENTAL LAWS AND TERMS OF ELECTRICAL ENGINEERING BEFORE turning to a discussion of the different types of electric furnaces used today, it is necessary to have a clear under- standing of some of the fundamental electrical laws and terms, for it is only through a knowledge of this, that an electric furnace can rightly be judged and a correct picture conceived of the occurring phenomena. In order to begin with the most impor- tant foundation for all electrical investigations we have first to deal with Ohm's law. This law says: Drop in potential Current = - p . f Resistance or if i denotes the current, e the drop in volts and r the resistance The resistance r of a conductor is determined by the equation: cl where / denotes the length and q the cross-section of the con- ductor, while c is a constant depending upon the material. It can be shown, for instance, that under the same electrical conditions and measurements, a copper conductor will convey 5X times the current that an iron conductor will. The reason underlying this is that evidently copper conducts electricity better than iron. We therefore speak of the different conduc- tivities of different materials. The above-mentioned phenomena may, however, be explained, since different materials give entirely different resistances, even though the dimensions may be the same. The resistance factor dependent on this material is called the "specific resistance" 11 12 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY and is mathematically indicated by p, so that the formula for the resistance of a conductor is, r = c and may now be written : r = p q p q The different conductivities are consequently the reciprocal values of the specific resistances. In order to be able to apply our first law, the "ohmic law," we must clearly establish the electrical units. The current quantity is measured in Amperes, the potential or pressure in Volts, and the resistance in Ohms. Originally the unit of resistance as established by Siemens consisted of a column of mercury one metre long and of one square millimetre cross-section at a temperature of o centigrade. In place of this resistance unit, we have the ohm to-day, which equals 1.063 Siemens units and corresponds to a mercury column 1.063 m. long, of i sq. mm. cross-section at o C. The generally applied resistance unit, the ohm, was established in such a way that, with a pressure of one volt, and a resistance of one ohm, a current of one ampere resulted. We now know what the dimensions of the mercury column are, and under which conditions it has a resistance of one ohm, and since the resistance of a conductor is, l_ I is in metres r ~ p q W ' re q in square millimetres we may calculate the specific resistance for mercury, which is .. i ohm = p or p = i : 1.063 = -9473 In the above description of the resistance unit the temperature was always given. This was not done without having an object in view, for the specific resistance of a conductor is not only dependent on the material, but also on its temperature. There- fore, equal conductors at different temperatures have different resistances, and consequently with the same voltage they carry different currents. LAWS AND FUNDAMENTAL PRINCIPLES OF ELECTRICITY 13 The conditions governing the alterations of the specific resistance with changing temperatures have been established, by making exact measurements with the different materials. In a similar way the specific resistances were determined. Concerning the changes in temperature, it was shown that the resistance of the metals and their alloys rose with increasing temperatures and in accordance with the following formula: where r = the resistance at o r t = the resistance at t a and /3 are numerical constants, which have to be specially determined for each conductor. For practical purposes when within moderate temperature differences, it will suffice if we use the following formula: r t = r (i + a i) It is well known that the specific resistance of a metal de- pends so much on various substances mixed with it, that accurate figures, as they are known for the absolutely pure metal, have practically only a comparatively small value. It is sufficient therefore to figure practically with the following values: Material p Copper ........................................ %5 Brass, Iron, Platinum, Zinc ...................... ^o German Silver, and similar German Silver alloys: Monel metal ............................... % to % Carbon varies between .......................... 100 and 1,000 The exact values of the specific resistances, and the tempera- ture coefficients, are given for some of the materials which are used in the construction of electric furnaces. It is to be noted that the temperature coefficient for metals is positive, i.e., with rising temperature its resistance increases. In contradistinc- tion to this, carbon and non-metallic conductors are negative, i.e., with rising temperatures the resistance is decreased. 14 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Material P at 15 C a 03 to 05 -J- OOtQ Lead 22 + 0041 Iron Copper .10 tO .12 .018 to .019 + .0045 + .OO37 07 to 08 -J- OOI5 German Silver Nickel Platinum Silver .15 to .36 -15 .12 tO .16 .016 to .018 + . OOO2 to . OOO4 + .0037 + .0024 to .0035 + . 0034 to . 0040 Steel 10 to 25 + 0052 Zinc 06 + .0042 Carbon 100 tO 1000 . 0003 to . 0008 The temperature coefficients in the above table are only exactly accurate within comparatively small temperature in- tervals, in fact, are absolutely accurate only within the limits of o to 30 C. This is why the temperature coefficient only gives an approximate idea of the increase in the specific resist- ance. For instance, that which interests us the most is iron, with its growing temperatures. Unfortunately exact measure- ments of the resistance of iron at high temperatures are extremely difficult to make, and this is why we see so many contradictory statements concerning these. It is evident that the resistance of solid and also fluid iron varies with its chemical composition. To some extent a certain portion of the iron content will include gases and slag particles at the beginning of the run, and this causes it to have a higher resistance than it would have at the end of the heat, yet keeping the composition the same. But all of these influences are practically negligible; for in the many years' experience of the author in operating electric furnaces, there has never been any significant or practically real influence of the refining which could be credited to a change in the re- sistance of the iron. Thus the above-mentioned causes, which could theoretically call forth a change in the resistance, may be neglected in practise. We may now use the formulas and values which are sufficiently correct for practical purposes, and which are obtained by exact measurements for low temperatures. It LAWS AND FUNDAMENTAL PRINCIPLES OF ELECTRICITY 15 has been established that the specific resistance of ordinary basic bessemer iron at temperatures from o to 160 C., would change according to the following formulas: Pt = .13 (i + 5 X io~ 3 / + 3.6 X icrV) If in accordance with this formula we figure the value of p t for 1700, we obtain This result seems somewhat too high, as far as it can be judged with the operating values on hand which were obtained by the author, who has had the best results when figuring with a mean value of p = 1.66 when designing electric furnaces. Gin figured according to Neumann with a resistance of iron of .000175 nm P er cubic centimetre. This would correspond to a specific resistance of p = 1.75. This figure also shows that the result calculated above for the specific resistance of fluid iron is too high at 2.12. In this book p = 1.66 will always be used as the specific resistance of molten iron. Even though this value cannot lay claim to any theoretical accuracy, the calculations will, however, give results which correspond sufficiently with practical operating conditions. It was remarked upon before that carbon, in contradistinc- tion to the metals, has a negative temperature coefficient, so that with increasing temperatures the resistance of the carbon decreases. This phenomenon we have, however, in a much more extraordinary measure, with the so-called "conductors of the second class." Under this heading we mean materials, or bodies, which at ordinary temperatures have practically no conductivity, or at least so small a one as not to be worthy of consideration. With increasing temperatures these conductors of the second class attain steadily bettering conductivities, so that they can eventually be used as conductors directly. We shall have to deal with conductors of the second class in detail, when discussing the various furnace types. It may be well here to speak of the well-known application of a conductor of the second class, in the form of the filament or glower of the 16 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Nernst lamp, which consists of porcelain and magnesia, which substances are non-conductors at ordinary temperatures. The glower of the Nernst lamp must therefore be warmed up first, before it is in any way capable of taking up the lighting current. This pre-heating was at first accomplished with the heat of an ordinary match, whereas it is to-day done electrically. All substances which are used for fire-resisting materials for the lining of electric furnaces, are similar in a way to the filament of a Nernst lamp, i.e., all constructional material for the hearth or roof becomes a more or less good conductor, at the high tem- peratures which are prevalent in electric furnaces, and this is, of course, taken into consideration in their construction, as will be made evident later on. The constructional material used most, for the. lining of electric furnaces, is dolomite or magnesite, aside from the pro- tecting brickwork used as a backing, and aside from the roof material. This material is mixed with tar and pressed into bricks, which are later on used in the furnace hearth, or it is directly tamped into the furnace. The furnace walls produced in this way have a small conductivity in their cold state, so that in such a case they may be regarded as non-conductors. They lose their resistance with increasing temperatures very fast, as is shown in Fig. 9, which shows the results of an investigation, in graphic form, of the resistance measurements of magnesite and tar rods, in relation to the temperature. The curve shows plainly how the specific resistance suddenly falls. This is desig- nated by p in the figure. It also gives a characteristic picture of the conductivity conditions, as they appear with conductors of the second class. We will recur to this matter again in due course. As we have already dealt with the prime laws of all electrical calculations, we will now deal briefly with the possibilities of different connections. This leads us to the series and parallel connections. Both connections will again be met in the detailed description of electric furnaces. As both have their advantages and disadvantages, it seems well that the prime difference be clearly stated. LAWS AND FUNDAMENTAL PRINCIPLES OF ELECTRICITY 17 We will therefore again recur to the analogy between elec- tricity and water. Let us suppose we had a water-power of very high fall but of comparatively very little water, and that several water-wheels were to be driven with it. If the wheels could be operated at any place it would be best, with the small Temperature FIG. 9. amount of water available, that it be used time and again, or sausage-like in series, through the different wheels. That is to say, the water-wheels could be arranged at different elevations of the fall, and thus by dividing the pressure an equal amount of water could be used to drive each wheel. The small amount of water is similar to a small current, the high fall or great differ- ence in pressure is similar to a high voltage. In such a case if the current present can be used in the electric apparatus installed, but only a portion of the prevalent voltage is required, then the apparatus utilizing the current can be so made, that it only absorbs a portion of the voltage. In this case, therefore, the same current and same amperage flows through the various apparatus using it, and we consequently speak of a ''series connection." 18 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY We may, however, also conceive of a case where a water-fall consists of a considerable quantity of water, but has only a small pressure. A well-known case is on the Mississippi River at Keokuk, where eventually 230,000 HP, at 25 cycles, 3-phase current will be generated. Here it would be practically im- possible to use this immense volume of water in one turbine. We are therefore obliged to separate these large volumes of water where each part materially has the same fall, between the intake and the tail-race. We have here then a case where several turbines are arranged next to each other. This case also has its simile in electrical engineering, the best known case being perhaps the ordinary incandescent lamp, where the circuit is also so arranged that only a small part of the main current flows through each lamp, at the same voltage for each. There may be any number of lamps next to each other, or as we say, they receive their current in parallel. Both cases are used in electric-furnace constructions. The Heroult furnace is an example of a series connection, while the Girod furnace employs a parallel connection. These furnaces are described later on, but a few words here concerning their method of connections will not be amiss. Figure 10 shows the schematic wiring diagram of a Heroult furnace, while Fig. n shows the main features of the connection in a Girod furnace. It is evident, that in Fig. 10, the current would flow through first one and then the other conductor, whereas in Fig. n the two points of the circuit are connected together by means of two conductors connected iri parallel. Of course Ohm's law is applicable in both cases. Accordingly with a resistance of r in the conductor and having a voltage e between its terminals, we would have a current of i = amperes. If the voltage e, in Fig. n, is prevalent between the points A and B, Ohm's law will, of course, hold for each parallel connected conductor. Suppose that between the points A and B, we have the voltage e and between these points we also have the resistances r\ and r 2 , the resultant current being ii and i 2 , respectively. LAWS AND FUNDAMENTAL PRINCIPLES OF ELECTRICITY 19 Then in a similar way as with the water-wheels, the main current will be equal to the sum of its parts, i.e., i = ii + i. We there- fore have the first of Kirchhoffs laws: "At each point of division the sum of all the incoming currents equals the sum of all the outgoing currents, or at each point of division the sum of all currents equals zero." With this it is assumed that the incoming currents are regarded as positive and the outgoing as negative currents. r FIG. 10. FIG. ii. From Fig. 1 1 it follows that, ii = and i 2 = or e r\ r z i r\ and e = i z r z . From this it follows that i\ X r\ = i% X r z or we have the proportion i\ : i z :: r z : r\, i.e., currents which flow parallel to each other vary inversely as the resistances of the parallel connected conductors." We will now investigate how large the combined resistance becomes, i.e., the resistance which is there, when both the parallel connected conductors r { and r z are opposed to the current flowing. In order to answer this question, suppose the two parallel connected conductors to be replaced by a single conductor, having the same combined resistance r. This would not change 20 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY the current conditions, and with the voltage e between A and B remaining the same, we would also have the total current * remaining, so that, i = as before. As demonstrated before i = i\ -f 2*2 If we substitute for these current volumes their corresponding voltage and resist- ance equivalents we have: 666 III consequently = 1 or = 1 J r r, r 2 r ri r 2 This gives the size of the combined resistance. = ri x rz r\ + r z In the same way this rule also holds for n parallel connected circuits. Designating the combined resistances again by r, we have, JL , i + i + i + . . -L r ri r z r 3 n As the reciprocal of the resistance is designated as the conduc- tivity, we may define this equation by: " The conductivity of a combination of conductors is equal to the sum of the conductivities of the single conductors. 1 ' If the n conductors are equal to each other, then 11,1,1, i n ri = i 1 1-....-! = or r = r ri TI ri ri n n i.e., "The combined resistance, of n parallel connected resist- ances, equal to each other, is equal to the nth part of any single resistance" In accordance with the above, it is now possible to give an arithmetical example of electric-circuit conditions, as they are often found with electric furnaces. LAWS AND FUNDAMENTAL PRINCIPLES OF ELECTRICITY 21 The circuit is to consist of two carbon blocks or electrodes in touch with a connecting iron block. The carbons and the iron may have equal cross-sections, and may be round of, say, 350 mm. diameter (about 14 inches). The length of each carbon block is to be 1.5 metres (about 60 inches), and the length of the iron block i metre (about 40 inches). In calculating the resistance of each part of the circuit we know that r = p X . In glancing at Fig. 12, we may take the circuit as consisting of two equal parts, which are connected in series in the first place, and in parallel in the second case. The resistance r' ', as shown by the figure, is composed of the resist- ance r c of the carbon block, and the resistance r Fe or half of the iron block. Consequently, r c = PC - = 5o^T = -00779 ohm. (Here the average value for p c is taken from the table on page 14, where the specific resis- tances of carbon are given.) lp e . SO r Pe = p Fe = .11 - 27" =.000000572 ohm. - 27" 35 Fio Here the average value for p Fe was also taken from the table. Even this short example shows how very small the iron resistance is compared with the carbon. For even were the carbon and iron, in the above example, of equal lengths, the iron resis- tance value would have risen to three times the value given ( = ), but even so the carbon would still have a resistance V .5oo/' 2,500 times as great as that of iron, and this is entirely on account of the extraordinary differences in their specific resistances. Turning again to the example, we find that we know the re- sistance of the parts of which the circuit is composed. It is r' = r c + r Fe = .007790572 ohm. 22 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY In case of a series connection the current would have to traverse this resistance twice, so that this resistance would be, r H = 2 r' = 2 X .00779 = - OI 558 ohm. In the second case, i.e., with a parallel connection, we have a total resistance composed of a combined resistance, consisting of two similarly constituted parts, each having a resistance of r' = r c + r Fc = .007790572 ohm. As the combination resistances are alike and in parallel, we have n = 2 for the above case, or r p = = .003895 + ohm. It is evident, therefore, with the same conductor under the same conditions, but in the first case having been in series, and in the other case connected in parallel, that the series connection has four times the resistance of the parallel con- nection. This extraordinarily different resistance of the two connections is, of course, not without its influence on the current and voltage conditions. This is evident from Ohm's law, where i = and the example before us with equal voltages gives us four times the current with the parallel connection, as it does with the series connection. Or as e = i r, and if we wanted equal currents in both cases, we would have to have four times the voltage in the case of the series connections, compared to the parallel arrange- ment of conductors. It may be well to mention here, that the above example only holds for arc furnaces, with series or parallel connected electrodes, when the electrode measurements are alike, as they were assumed to be in the example. What action is there then, when an electric current flows through a conductor? It has always been evident in order that the current may flow through a conductor that a definite voltage was necessary, in order to overcome the resistance. Consequent- ly, when an electric current flows through any conductor a LAWS AND FUNDAMENTAL PRINCIPLES OF ELECTRICITY 23 certain work fe accomplished, which must come to the surface in one form or another. In our case, we find work produced by the current, showing in the conductor again as heat. In what degree this heat is developed, is given us by Joule's law. This was established by the English physicist Joule, and experiment- ally determined by him. It is as follows: " The heat developed by a current flowing through a conductor, is directly proportional to the time, proportional to the resistance and proportional to the square of the current," or Q = C i? rt, where Q = the heat generated / = the time the current is flowing i = strength of the current r = the resistance and C = a constant dependent on the units chosen. If the current i is measured in amperes, the potential e in volts and the time / in seconds, then C = .24, which has been determined by most accurate measurements. Therefore Q = .24 ?r t gram calories or, as according to Ohm's law e = ir, we have Q = .24 eit gram calories. If the heat is to be measured in kilogram calories, it is to be noted that i Kg. calorie = 1000 gram calories, so that the right side of our equation is to be divided by 1000. If the work produced is not to be expressed in kilogram calories, but in metre kilograms (or the equivalent of the foot lb.), we find that i Kg. calorie = 424.7 metre kilograms. If the work delivered be designated by A , we have .24 eit A = 424.7 = .1019 eit m. kg. now. 1019 = i.e., it is equal to the reciprocal of accelera- 9.0! tion, consequently A = metre kilograms. 24 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY We usually denote the work delivered in one second as effect or power, and have chosen p to denote it, hence, eit p = -JT- metre kilograms. With electrical measurements we do not desire to determine the effect in kilogram metres, but in volts X ampers, or in short voltampers, otherwise known as watts, while the work in joules = .24 eit, or as eit is measured in watt-seconds, watt-hours or kilo- watt-hours, all in accordance with the measurement of time, be it in seconds or hours, and whether the power is to be inserted in watts or kilowatts. Thereby i kilowatt = 1000 watts. For the conversion of metric horse-power into watts, the lollowing are determining factors: i HP metric = 75 kg. m.; as the electric power e i . p = ^- m kg. = ei watt i.e., i m. kg. = 9.81 watt, consequently i HP = 75 m. kg. = 75 X 9.81 watt = 736 watts. 1 This gives the relation between the mechanical and electrical units. If we arrange the determined factors in the form of a table, we obtain the following: The heat generated in a conductor by a current is, Q = .24 eit gram calories = .24 i~ rt gram calories where e is to be inserted in volts i is in amperes r is in ohms / is in seconds. The power, or the effect is p = ei watt or volt amperes = i 2 r watt. ^ne British HP = 33,000 ft. Ib. per minute = 746 watts. LAWS AND FUNDAMENTAL PRINCIPLES OF ELECTRICITY 25 Here " i kilowatt = 1000 watts i m. kg. i m. kg. = 9.806 watt = - i HP = 736 watts = 75 75 seconds m. kg. seconds The delivered work is given by the formula, A = eit watt-seconds or joules = r rt watt-seconds or joules Here i watt-second = .24 gram calories = i joule = . 10198 m. kg. i watt-hour = 3600 joules = 864.5 gram calories = 367.1 m. kg. i kilowatt-hour = 1000 watt-hours = 864.5 kilogram calories = 367ii4m. kg. i m. kg. = 2.35 gram calories = 9.806 watt-seconds. CHAPTER III EFFECTS OF THE ELECTRIC CURRENT THE effects of the electric current which interest us most, are its heating effects. Let us therefore consider what possi- bilities electrical engineering offers for the production of heat. i. DIRECT HEATING BY RESISTANCE In the foregoing chapter we have seen that when an electric current flows through a conductor, heat is developed and the quantity produced is Q = .24 eit gram calories = .24 ? rt gram calories. If we for example force an electric current through an iron conductor the temperature of this conductor will increase. The heating will therefore be greater and more rapid with increasing current and increasing resistance of the iron conductor. Possible methods for making this resistance larger become evident from a glance at the formula recently mentioned which reads: I r= ^. An increase of resistance occurs if the cross-section is reduced, or if the length of the iron conductor or the liquid metal bath is increased. The method of heating thus explained may be called direct heating by resistance for the heating is affected solely by the inherent resistance of the metal to be heated. The Taussig furnace, mentioned in Chapter I, is an example of an electric furnace built on this principle. When we consider that the direct heat by resistance is not confined to the metal it is desired to heat and that any such method of heating must also naturally cause heat to appear in every wire used to conduct the electric current, we encounter the first difficulty which operates against a practical utilization 26 EFFECTS OF THE ELECTRIC CURRENT 27 of the direct heat by resistance for the purpose of heating metal baths. An iron bath is a comparatively favorable condition at that, for the resistance being r p the specific resistance p is of considerable importance in creating the higher degree of heat. This point is, of course, carefully considered in electrical engineer- ing and it is for this reason primarily that copper is used for electrical circuits. Copper by virtue of its small specific resistance (p = .018 to .019) is one of the best conductors for the transmis- sion of heavy currents, for it permits the use of relatively small cross-sections without attaining too high a temperature. In contrast thereto the specific resistance of iron, for example (p = .1 to .12), would be the deciding factor -from the stand- point of heat loss, and with equal cross-sections for copper and iron we would have a greater heat in the latter in the proportion of .1 to .018. With equal cross-sections for iron and copper, and equal lengths of the conductors, iron will attain a consid- erably higher temperature. Espe- cially as its heat will be further increased by the fact that the metal in the heat protective covering of the electric furnace will rise to higher temperatures by reason of its posi- tive heat coefficient, which in turn adds to its resistance. Yet the fact FIG. 13. FIG. 14. FIG. 15. still remains that the specific resistance of iron must be placed as being very low. With a system of pure resistance heating and with a resistor of iron, exceedingly high current strengths would be required to produce the temperatures needed in electric fur- naces, and the currents would have to be exceedingly strong even if the cross-section of its iron resistor should be made very great 28 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY and the furnace bath cross-section very small in order to increase the resistance as much as possible. An example will serve to illustrate more clearly the condi- tions of this kind of electric furnace employing direct heat resistance. Suppose we assume that by means of pure resistance heating, a fluid iron bath of one ton is to be supplied with 200 kw. In this case the heat would be generated only by the current over- coming the resistance of the bath. This is the amount of energy with which an electric furnace of i-ton capacity would normally be operated. In order to make the iron bath of as large a resistance as possible, the molten iron is to be contained in a long channel of the smallest practicable cross-section. A furnace of this kind having a long narrow channel has also been patented by Gin. Figs. 13, 14 and 15 show views of this furnace, re- spectively longitudinal cross-section, vertical cross-section and a plan view. We might use this arrangement as an example. The size of cross-section has been taken as 10 cm. (4 inches) high and 5 cm. (2 inches) wide. This has been done in order not to reduce the channel cross-section to such a degree, that the furnace would be rendered inoperative. Or if the cross- section were made much smaller, the cooling surface would become extraordinarily large, and thus cause very large losses. This would be entirely independent of the metallurgical difficul- ties which would ensue if any slagging work were attempted in the narrow channels. The size channel chosen therefore would still be practically workable. The following assumptions are therefore made for this example : Capacity of furnace I ton = (1,000 Kg.) Energy consumption A = 200 kw. Cross-section of bath q = 5,000 sq. m.m. Specific gravity of molten iron, about 7.0 Specific resistance p of molten iron, about 1 .66 From this it. follows that: Volume of iron = - - = 142.8 cu. decimetres. EFFECTS OF THE ELECTRIC CURRENT 29 142 - = 285.6 d.c.m. = 28.5 Length of the iron column L metres = about 94 ft. Hence the resistance would be: r = p X = 1.66 X = .00946 ohm. As the Joule effect A = r r, we have ~A It is evident therefore that a very considerable current would have to be supplied. This also means large copper cables for bringing the current to the furnace, as those of inadequate cross- section would heat up too much. The following table shows the currents usually permitted in wires and cables: AREA CARRYING CAPACITIES Circular Mils. (d) i Mil. = .oor Inch Square Mils. (d*x. 7 854) Rubber Insulations Amperes Other Insulations Amperes 4106. 3225. 12. 16. 6529. 5128. 17- 23- 10381. 8153- 24. 33- 16509. 12960. 33- 46. 26250. 20617. 46. 65- 41742. 32784. 65- 92. 66373. 52130. 90. 131. 83694. 65733. 107. 156. 105538. 82887. 127. I8 5 . I33079. 104520. 150. 220. 167805. I3I790. 177- 262. 2II600. 166190. 210. 312. 250000. 196000. 275- 412. 4OOOOO. 313900. 330. 500. 500000. 392000. 390- 590. 600000. 47IOOO. 450. 680. 700000. 549500. 500. 7 60. 800000. 628000. 550. 840. 1000000. 785400. 6 5 0. IOOO. 30 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The above table is for insulated wires, whereas bare conduct- ors may carry higher current densities. If in the previous example for the i-ton furnace we allow a current density of 1.5 amperes per square millimetre (about 1000 amperes per sq. inch), the secondary conductors leading directly to the furnace would have a cross-section of = 3065 sq. mm. (about 4.75 sq. inches). This would entail .3065 X 10 X 8.9 = 27.28 kg. of copper per meter length (or about 19.5 Ibs. per foot length). Even this value shows that the leads for furnaces of the simple resistance type become extraordinarily expensive, and this is especially so for furnaces of larger capacities. The voltage necessary to force the required current through the iron bath of the i-ton furnace is e = ir = 4598 X .00946 = 43.5 volts. These give us the entire range of electrical conditions, but these must be considered primarily for direct current. When operating electric furnaces for alternating current, there would be certain deviations, which will not be taken into consideration, as direct current gives simpler equations, so that the above calculations are quite sufficient for the case before us. Considering the above circumstances, we may now establish the following: Characteristic marks of electric furnaces having direct resistance heating. a. Concerning the Electric Characteristics As the heating occurs by means of the current passing through, and overcoming the resistance of the iron bath, it is entirely uniform at all places. Furthermore as the heat generated is proportional to the square of the current, the smallest changes in the temperature may be brought about by altering the voltage of the furnace. Such changes would be absolutely uniform throughout the entire bath. In this way by choosing high enough voltages, the EFFECTS OF THE ELECTRIC CURRENT 31 highest temperature may be reached. Besides this the following may be noted : As the iron has a comparatively low specific resistance only at the high temperatures prevalent in liquid iron baths, it follows that direct resistance heating may only be accomplished by applying very heavy currents. For the same reason the voltages required for the operation of these furnaces are comparatively low. The high currents have the disadvantage of demanding expensive cable installations, whereas the low voltage has the advantage of simpler and easier insulation, and the advantage of eliminating all danger which might befall the furnace attend- ants. The high currents can only be decreased by correspondingly increasing the voltage and contracting the bath cross-section, or by increasing the length of the bath. This brings us to: b. The Metallurgical Characteristics Primarily the good points here are the uniform heating, and the easy regulation within narrow limits at any desired high temperature. The disadvantages are : To obtain good electrical conditions it is necessary to use long channels having small cross-section which means large coob'ng surfaces. This is equivalent to high thermal losses which must be covered by expensive electrical energy con- sequently making the power-consumption figures very high. It seems that a regular operation of metallurgical process is precluded, as the working with slag and moreover the changing of slag would almost offer practically unsurmountable obstacles in the line channels. A uniform composition of the furnace contents would hence be unattainable. A lasting durability of the furnace refractories also seems practically unobtainable, considering that the refractory walls between the channels are attacked from both sides by molten iron. This direct resistance furnace, with its channels running to 32 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY and fro, still fails in spite of several electrical advantages. Prac- tical operation has also shown this. For the one furnace of Gin built as here described, was an utter failure. Even so, we see patent applications today of similar ideas, which are to be discarded, as they are bound to be unsuccessful owing to the same inherent weaknesses. These direct resistance furnaces, as just described can there- fore not be considered for practical operation in the iron and steel industry. 2. INDUCTION HEATING If we are able to circumvent the difficulties of leading very heavy currents to the iron bath, we may decrease the resistance of the bath at will if we can only increase the current strength to correspond. This would then allow us to use furnaces with large and wide hearths such as the metallurgist must necessarily demand. The solution of the problem is found in the furnace type known as induction furnaces, which may also be called furnaces with resistance heating. These have the good points of resist- ance heating with the current being caused by induction, without bringing with them the disadvantage, just mentioned above, of the current having to be led to the furnace with immense con- ductors. This is what has enabled these furnaces to attain their great practical importance. On this account induction furnaces are discussed at length in the tenth and following chapters. It suffices to say here, that the induction furnace belongs to that group having a type of direct resistance heating. 3. INDIRECT RESISTANCE HEATING We shall designate all furnaces as resistance furnaces with indirect heating in which the iron itself is not the important resisting element but rather some other conductor of very low conductivity. This conductor, that is the actual heat resisting element, is placed into the circuit and heated so that it can give up the heat generated in it, to the material to be heated. EFFECTS OF THE ELECTRIC CURRENT 33 FIG. 16. As we can now choose for the heat resisting body, a material having a very high specific resistance, the extremely heavy currents are no longer required, which were necessary for the direct resistance heating of molten iron. Consequently cable installation will be less expensive. Such furnaces with indirect heating are often used in labora- tories. We have for instance the type suggested by Borch- ers. (See Fig. 16.) This has carbon blocks or rods of large cross-section which serve as terminals between which a carbon rod of very small cross-section is clamped, which serves as the heating resistance or resistor. The material to be heated is heaped about the small carbon rod. Thus while the current is flowing, the carbon rod heats the desired material indirectly. In practice we find furnaces with similar indirect heating, but they are used mainly for the manufacture of Carborundum. Here we find that a tamped- in mass of powdered coke acts as the heat resisting material. Such designs are not used in the practical manufacture of iron, where the charge to be heated comes in direct con- tact with the heating element, where the latter is usually of carbon. For it is well known that iron absorbs carbon readily until it is finally saturated with it. It is consequently impossi- ble to use carbon as the heating element, not only because the carbon brings impurities to the iron, but primarily because the carbon resistance would be worn away in the shortest time by the iron. In place .of carbon we could suggest the utilization of a conductor of the second class as the heating element. This could be of the same material as the fur- nace lining which surrounds the metal bath; such as dolomite FIG. 17. 34 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY or magnesite held together with 8 to 10 per cent, anhydrous tar as binding material. If we were to place such a heating element or resistor in an iron bath we would have two parallel circuits for the current. In this case a very poor conductor (the heating element) of very high resistance would be in parallel with a very good conductor of very low resistance (the iron). As the currents in parallel circuits are inversely proportional to the resistances practically all of the current would flow through the iron while the con- ductor of the second class would almost be without current. Hence it is established that it is impracticable for any iron process for furnaces having indirect resistance heating to have the heating element in parallel with the iron to be heated. Another possibility of indirect heating could be obtained by utilizing the walls of a vessel, such as a crucible, by heating it with an electric current either directly or indirectly. One of the best known of these furnace types is the Hera'us laboratory furnace, where the heating chamber is composed of a cylindrical tube, into which small crucibles may be placed. The tube of refractory material is wound with a spiral of platinum wire or ribbon, which is placed in the electrical circuit, and thus its heat is transmitted to the furnace chamber. Similar methods, however, have been proposed for several iron processes, one of these being by Girod. Accordingly several crucibles were placed in retorts composed of fire-brick, the bottoms of which were composed of suitable resistances, as shown by Fig. 17. In order not to imperil the retort walls by the heat, various resistances were used for the bottom material. The resistance material itself consisted of carbon and silica. With a furnace of this kind utilizing indirect heating, a tempera- ture of 1400 to 1700 C. was reached. When the cross-section of the heating element was reduced, as shown in the sketch, temperatures as high as 2000 C. were attained. In these furnaces, which Girod used principally for making ferro-alloys, he also melted steel. This necessitated IAAO Kw.- hrs. per ton melted. In the above we have an electric furnace which differs only EFFECTS OF THE ELECTRIC CURRENT 35 from the ordinary crucible by the electrical heating. Even if these have several advantages, the Girod crucible furnace still has the disadvantages of the small size of the common crucible, the difficulty of obtaining a complete uniformity from a greater number of crucibles, the high cost of the crucibles, and. compared to other furnaces, a very high power consumption. All these are reasons why these furnaces have not found a place in the iron industries. This furnace construction had to be mentioned here, in order to give as complete a picture as possible of the various electrical heating possibilities. FIG. 18. FIG. i8n. We still have to mention another indirect heating method, where the walls of the heating chamber are the heating elements themselves, and consequently carry the current. One of these designs is the Helberger furnace. This consists, as Fig. 18 shows, of a crucible, which is placed in circuit by means of carbon contacts, so that the current passes vertically 36 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY through the crucible walls. Helberger uses the ordinary carbon or graphite crucibles. Before using these crucibles they are prepared by a patented process which permits the current passage through the crucible walls only. These furnaces were originally built only for the handling of precious metals. As communicated by the firm of Hugo Helberger, the current conduction from the crucible wall to the metal contents was made more difficult by removing the graphite from the inner surface of the crucible. This graphite removal is accomplished by blowing air into the red-hot crucible. As long as the material is not molten there is no passage of the current. As soon as the contents becomes fluid, it gets into intimate contact with the red-hot lining, which acts as a Nernst filament would, so that some electrical energy also goes through the lining and directly through the bath. This action has no deleterious influence on the charge, as the metal for which these small furnaces are applicable is tapped as soon as it is molten, for a refining of the charge is not necessary or desired. If these furnaces are to be used in steel works for small trial melts, for which they seem excellent, carbon crucibles are used which are nearly always lined with a metal oxide from .4 to 1.2 inches (10 to 30 mm.) thick. These carbon crucibles, so the inventor advises, need only half the voltage of the graphite crucibles, a result of this being that the deviation from the normal working current is not so great, so that in this way it is possible to practically lead the current entirely through the walls of the crucible. The practical design of this furnace is therefore to be regarded as having been well done. The crucible is built together with a regulating transformer, as shown by Fig. i8a. The upper carbon contact covers the heating chamber at the rim only, so that the process going on in the crucible may easily be observed. The crucible is protected against radiation by a fire-brick cylinder. The clamping arrangements holding the carbon contacts are water-cooled. The size of the Helberger furnace is limited on account of the difficulty encountered when manufacturing larger crucibles. Yet, the manufacturers, The Helberger Co., of EFFECTS OF THE ELECTRIC CURRENT 37 Munich, Germany, state that furnaces, up to a capacity of 300 Kg. (660 Ibs.), are being successfully built to-day. 4. ARC HEATING When counting the various possible ways of heating we must not forget to mention the electric arc for this has found the widest application in the iron industry. We will spend much time in the following chapters, therefore, with arc heating and arc furnaces. They are mentioned here only for the sake of completeness. CHEMICAL ACTION Besides the purely thermal action of the electric current the mill man will also be interested in the chemical action which takes place when an electric current is passed through a liquid. The best known example of this is the disassociation of water into its constituents, oxygen and hydrogen. This may be observed by passing a direct or continuous current through water. In so doing the well-known reaction takes place as oxygen is given off at the positive pole and hydrogen at the negative pole. To this belongs also the best known electro metallurgical application of electrolytic action for the smelting of aluminum. According to the method first proposed by Heroult and Hall, both in 1887, the clay is melted by the action of arc heating, and simultaneously the molten mass is separated electrolytically in such a way that the aluminum is freed and collected at the negative pole, whereas at the hanging positive carbon electrode the oxygen is set free, and, together with the carbon of the electrode, escapes as carbonic acid gas. These examples are sufficient to show how chemical action may be brought about by the electric current. In this instance it is to be observed that this action only occurs when direct current flows through the electrolyte to be separated. If on the other hand alternating current is used, where the current direction is constantly changing, then no electrolytic action can take place. For supposing we had an apparatus for the dissociation of water, which was supplied with alternating instead of continuous current. 38 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Then during one moment with the current in one direction, we would obtain oxygen at the electrode, and during the next moment with the reversed current direction we would receive hydrogen. It is evident, therefore, that electrolytic effects do not arise when alternating current is used. From this it also follows that molten iron masses of electric furnaces, through which current passes, are not subject to any chemical action as long as alternating current is used. We could in any event, as in the above example, assume a momentary chemical action, which however would be reversed in the next moment, for even though it appears momentarily, it does not come into play as far as the metallurgical process is concerned. This assumes that the reversal of the chemical effect is not interrupted. When using direct current in iron baths, electrolytic action naturally occurs, by which iron sulphides and iron phosphides may be separated. These suggestions have also found their way into the patent office. Electrolytic actions may however be positively harmful for carrying out metallurgical processes. According to Conrad (see Stahl u. Eisen, 1909, p. 796) we obtain a purer product when using alternating current for the manufacture of ferro silicon, than when using direct current. For when using continuous current the impurities of the charge are reduced, such as calcium, aluminum and other metals, which then find their way into the final product. The designers of the electric furnaces for the iron industry today use alternating current exclusively, because they fear the undesirable influences in the charge due to direct current. It may not be out of place to quote the words here of an ardent supporter of electric furnaces for the iron and steel trades. We quote, therefore, from Prof. Borchers and his address in 1905, a translation of which might be called: "Electrolytic effects were not sought in most reduction and melting tests, whether they endeavored to produce pig iron, or make steel, even though these electrolytic effects are nowhere entirely eliminated. This was particularly so in the arc proc- EFFECTS OF THE ELECTRIC CURRENT 39 esses of earlier periods, where testers positively failed, when endeavoring to produce irons low in carbon. If for instance the iron to be smelted makes one pole of the arc, and carbon blocks the other, then the iron absorbs carbon equally well, whether direct or alternating current be used. We know that an arc between two carbon poles carries carbon vapor across from one pole to the other. For the evaporating point of carbon deter- mines the arc temperature. If one of the poles consists of iron, then the only material remaining for the other pole is carbon, provided direct arc heating is used. In this way the iron will gradually become saturated with carbon even with alternat- ing current. Though we assume that the carbon in the arc wanders only from the anode (positive pole) to the cathode (negative pole), it is evident that the carbon separates itself in its solution in such a way, during a current wave, in going from the carbon pole to the iron, that only a small part of it would return during the current alternation." We perceive, therefore, that we must guard against the harmful absorption of carbon by suitable mean's, when using furnaces operating with carbon electrodes, even when working with alternating current. This is accomplished today by interposing a layer of slag between the arc and the iron, in all furnaces where the arc im- pinges directly on the metal. This slag is then, to be sure, acted upon in a reducing manner by the arc, yet the iron is protected from any union with carbon. In accordance with the foregoing, then, we may definitely establish that no electrolytic action takes place in the great majority of furnaces, which operate exclusively with alternating current. MOTOR EFFECT In the construction and operation of electric furnaces we have to take into account the motor effect of the electric current as well as the thermal and chemical effect. It is just as easy to transform motion into electricity as the reverse, as is very evident from the wide application of the electric motor. v s N A*'//> N|. : 40 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY It exceeds the limits of this book to explain the motor phe- nomenon in detail, still in order to understand and have a correct opinion of the possible and impossible motions of the molten metal in electric furnaces, it appears desirable at least to discuss briefly the reasons for the motion phenomena. It is well known that an ordinary magnet attracts a piece of iron brought into its vicinity, and that motion is caused by means of this magnetism. It is equally well recognized that two magnets, like magnetic poles, repel one another; while unlike poles attract one another. We also speak of lines of force, which surround the space near a magnet, and it is to these lines of force issuing from a magnet that we attribute the / / ^-- ~"~^^^^ \ distant magnetic effect. Sup- 1 1 /^"'"'_ _~_V^^\\\ ! * pose we have two magnets as Fig. 19 shows with their like poles laid next to each other. If we draw the path of the lines of force as shown we may define the repelling action of like poles, by saying : Lines of force having the \\ ^--^ -^--"''X / same direction repel each other; those of opposite direction attract each other. We also obtain motion phenomena, therefore, in accordance with this rule, as a result of two magnets acting on each other. But we will also have these motion phenomena, if a stationary current carrying conductor is brought near an ordinary suspended magnet. In order to do this we may set up an easily movable magnetic needle in its case, and directly above it stretch a wire, which may be connected to a source of electricity. As soon as the current is switched on, the needle will endeavor to set itself at right angles to the wire. The size of the deflection is a direct measure of the current passing through the wire. We find that the deflecting power decreases as the conductor is moved away from the magnet parallel to itself; that the direction of the EFFECTS OF THE ELECTRIC CURRENT 41 needle is reversed when the wire is under instead of above the needle; and that at every position the deflecting power is pro- portional to the current. These phenomena prove that a current carrying wire is surrounded by lines of force throughout its whole length, whose density is greater in the immediate vicinity of the wire, which decreases as the distance from it (the wire) increases. Accord- ingly we may imagine the fields of force of a current carrying wire about as shown by Fig. 20. It is assumed here that the conductor pierces a sheet of paper. On this are drawn the lines of force as they would appear when the current flows. The proof of these lines of force existing concentric to the conductor may easily be had, if a glass plate is used in place of the paper, which is strewn with iron filings. If an electric current is then sent through the wire, which pierces the plate, the iron filings will arrange themselves in direction and density, in accordance with the lines of force. The direction of the lines of force may then be established in compliance with a single rule: // the current carrying conductor is grasped in the right hand so that the out- stretched thumb indicates the direction of the current, then the lines of force will encircle the wire so that they would issue from the ends of the remaining fingers. If we now return to the first test, in which a movable magnet was brought into the magnetic field of an electric conductor, then in diverting the magnet we have a motion phenomena, pursuant to mechanical power, which appears between electric currents and magnets. We can now go a step further and replace the second magnet by a conductor, through which current flows. Even then certain i~o ftWb tfV9^" FIG. 20. 42 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY phenomena will be observable, provided one of the conductors carrying current is movable. For until now, we have found that the motion phenomena are the result of magnetic fields which mutually affect each other. As we have also seen that each conductor carrying current has its own magnetic field, then, in accordance with the foregoing, the appearance of mechanical power is unavoidable, between current carrying conductors lying closely together. Accordingly we may immediately determine the direction of the motion. FlG - 2I - Suppose we have two con- ductors both of which carry current going in the same direction, as shown by Fig. 21 at a and b, here the current would be flowing toward the reader. According to the foregoing rule the direction of the lines of force is quickly determined and is shown by the arrows. We see, FIG. 22. therefore, in the space between the .two conductors that the direction of the lines of force are opposite to each other. As the lines of force of opposite direction attract each other, we may say relative to the current: "Currents of like direction attract each other " and "currents of opposite direction repel each other." EFFECTS OF THE ELECTRIC CURRENT 43 From this it follows that crossed currents and their con- ductors (as shown by Fig. 22) endeavor to arrange themselves parallel to each other, and in such a way that the current in both flows in the same direction. That is, the movable conductor a tries to assume the same direction as the stationary conductor b. The case is also very interesting where one current flows vertically to the other, as shown by Fig. 23. Here the circles or dots represent the points of the arrows, which indicate the direction of the lines of force, while the crosses represent the ends of these arrows. As lines of force in the same direction repel each other, and those of opposite direction attract, then the movable conductor a will endeavor to move in the direction as shown by the arrow. + + o + o o o o o o o > ' 0000 + - + + + + + + -f++ + FIG. 23. In place of the above simple case we may throw some light on the possible motion phenomena in arc furnaces, which may arise due to the electrical conditions which have their electrodes pointed directly against the bath. We may have the effect of two or more currents acting on each other. In this case one of the conductors, namely the molten metal, may be regarded as being movable, within certain limits. For the molten conductor may be mechanically, com- paratively easily influenced, even if only within the limits of the hearth. The conditions are also very similar with induction furnaces, excepting that in the place of the one solid conductor, we have a 44 ELECTRIC PURNACES IN THE IRON AND STEEL INDUSTRY coil of many turns. Figs. 24 and 25 show how the lines of force act: in the former case with the turns wound far apart, and latterly with the turns wound closely together. It follows, therefore, that coils such as these are surrounded by like lines of force as common rod magnets would be, and thereby the laws are known which govern the motion phenomena between active coils and single conductors. Aside from the above explanations it still seems desirable to mention a very special motion in molten conductors, through which current is passing, but which only arises in certain cases. This is the so-called "pinch effect." According to an address by Carl Hering before the Cana- dian meeting of the "American Electrochemical Society," in ^ ^ ^ May, 1909, this pinch effect FlG - 24- occurs when a continuous or alternating current flows through a molten conductor. Then this conductor endeavors to contract itself in the line of its cross-section under the action of electro-magnetic forces. The contracting force is only small, when the current density is low, but grows with in- creasing current density (amperes per square millimetre or square inch), and can, in extreme cases, become so large, that the cross- section at the contracting point may decrease to zero, thereby interrupting the current. The contraction primarily occurs at such places in the hearth which have already been contracted owing to occa- sional irregularities when tamping the lining material in place. The fluid FIG. 25. column of metal conducting current is therefore interrupted at the weakest place in its cross-section, exactly as a rope breaks at its weakest place. In addition to this, there is a depression where the cross-section diminishes, and on this slanting fluid conductor, particles of slag and the like are apt EFFECTS OF THE ELECTRIC CURRENT 45 to follow, which then cause a further increase in the current density, as the conductivity of these impurities is less than that of the molten metal. Furthermore this would be caused at places where the cross-section is already weakened, so that thereby the actions of the pinch effect would be still further increased. This course of things, as pictured above, does not take place in any deleterious or unpleasant fashion with electric furnaces as they are used for the most part in the iron industry today. As the pinch effect only appears with comparatively high current densities, we find that it does not occur at all in arc furnaces. But it causes various motion phenomena with induction furnaces, as we shall presently explain. Motion phenomena which are entirely desirable and advantageous for the working of metal- lurgical processes, may be brought about by artificially narrowing the cross-section of the bath to accomplish the required result. These, by their very nature, would in no way endanger the electrical furnace operation. An explanation of the appear- ance of the pinch effect may be had, if we assume that the FJG 26 fluid mass is composed of many parallel connected conductors, which are all leading like direc- tional currents through them. As currents having the same direction attract each other, the foregoing sentence is applicable here, for in a measure it defines the contracting effect. In electrical furnaces, where the pinch effect causes motion, the liquid seems to be driven along a straight line from the middle and the centre axis toward the ends, so that the fluid mass is lower in the centre than at either end. The weight of the molten metal then causes a flow from the higher lying parts, toward the lower middle section, as shown by Fig. 26. Here the molten conductor is considered to be cut vertically, in the line of the horizontally running current. Without going into further details, it is evident that the motion due to the pinch effect causes an intensive mixing of the charge. This occurs as long as the correct agitation is maintained within the desired 46 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY limits, for the motion can only advantageously effect a rapid chemical reaction, which is needed between the iron bath and the slag. Besides this the quality of the steel can only be bettered by the greatest possible uniformity which is brought about by this circulation. CHAPTER IV POWER FACTOR (COS 0) AND ALTERNATING CURRENT THEORY IN GENERAL IN the previous chapter it was shown that direct current, due to its chemical action, is totally unadapted for electric fur- naces as used in the steel industry, and alternating current is therefore used exclusively to operate electro-steel furnaces. The difference between direct current and alternating current is that in the former the current is always flowing in the same I FIG. 27. FIG. 2-ja. direction, whereas in the latter it changes its direction con- tinually. The required time for one directional change is called the period and is designated by T. Fig. 270 shows a complete wave or cycle. In this figure one cycle, therefore, takes the time, T, which is necessary for the current to swing through a complete wave. Hence, one complete cycle goes from O through to the positive maximum, and from zero to the negative maximum and back to the zero point. For all practical purposes we can assume that the usual alternating current generator gives a sine wave for its electro- motive force. This being the case, it only remains to show how 47 48 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY a sine curve is constructed, and to draw another diagram next to Fig. 270 showing these relations in alternating current circuits. If (as in Fig. 27) we let the radius or radius vector equal the maximum voltage reached in this sine curve, and designate this maximum value by e, the various instantaneous values of the sine curve by e' ', then: e' = e sin a i.e., for every angle a, the ordinates of the sine curve give the corresponding instantaneous potential values as indicated by Fig. 27. In the above equation in place of the angle, however, we can substitute for it the value of the angular velocity and obtain : a = m t (similar to the equation, distance = speed X time), and as the angular velocity w = -77 hence e' = e sin m t where / is the time taken by the radius vector until it has passed through the angle a after leaving the zero or starting point. The whole time corresponding to one cycle is T and the corresponding angle is 2 IT, and by substituting these values in the previous formula, since (a = 2 IT and t = T) 2 TT = m T from which it follows that It is customary to speak of cycles per second or frequency, and as the time of one cycle is equal to T, the frequency u is If we substitute this value in the formula containing m, we get m = 2 TT v. We speak of an alternating current of, say 25 cycles, when this current makes 25 waves each second. The more or less frequently varying direction and strength of the current depending upon the cycles per second, or frequency, POWER FACTOR (COS 0) AND ALTERNATING CURRENT THEORY 49 has a particular bearing on the functions of alternating current circuits. In order to understand these, the so-called induction will next be briefly described. This seems necessary because a clear conception of the induction phenomena is important, in order to understand the induction furnaces which will later be discussed in detail. We obtain an inductive action, for instance, when an electric conductor is moved through a magnetic field so that magnetic lines of force are cut. If we connect the ends of this conductor with a measuring instrument we obtain a deflection, showing the presence of an electric current produced by induction. This current is called induced. We therefore say, " If a conductor is moved in a magnetic field so as to cut magnetic lines of force, an electro-motive force is produced, which will cause a current to flow provided that the conductor has its ends closed so as to form an electric circuit. The electro-motive force and also the current become larger, as more magnetic lines of force are cut in a given time." It is evident that it makes no difference in which way the magnetic field is produced, because it is only necessary for the conductor to cut lines of force. It is, therefore, immaterial whether the conductor is moved through the field of a permanent magnet or through the field of an electro-magnet. It is even sufficient to move it near a wire through which a current is flowing, because this wire is surrounded by lines of force. Until now we have assumed that we have moved the con- ductor in which a current is induced. Instead of that we can move the magnet and hold the conductor; still, as in that case, an electro-motive force is also generated, due to lines of force being cut. We may finally place two conductors side by side, and if we pass a current through one of them, it will generate a magnetic field, the lines of force of which will cut the second conductor. If the current is interrupted, the lines of force dis- appear, only to reappear instantly upon the current being again made. We therefore have a field of constantly changing lines of force and a conductor located in this field. Hence an e.m.f. is induced in the second conductor, exactly as when a magnet 50 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY approaches a conductor from an infinite distance and then recedes again to an infinite distance. The alternating current changes its strength continually, and, as we have seen, it increases twice during each period or cycle from zero to a maximum and consequently decreases from that point again to zero. As a result of this, a conductor carrying an alternating current is surrounded by an alternating magnetic field, which induces e.m.f., or currents, in all conductors within its field. The current-carrying conductor itself thus lies in an alternat- ing field and, from what has been said, it is evident that an e.m.f. will be induced in this conductor by its own field. This action is called self-induction, and the current generated thereby is called the self-induced current. This self -induced current always flows in the opposite direction to the current which produces it. If the primary current, for instance, flows to the right, the induced current will flow in the opposite direction, or to the left, in the same conductor. The self-induced current for this reason does not exist, as the effect is to weaken the primary current. If voltage is applied to a coil, therefore, the current does not immediately reach its maximum value, but does so only after a certain time-interval has elapsed. The highest value is reached after the lines of force are no longer on the increase. We therefore say the current lags behind the voltage. It should be remembered that we obtain the instantaneous values of the voltage as the projections of a rotating radius vector. Therefore, we can likewise get the instantaneous values of the current as projections of a radius vector of a different value. We then obtain the lag of the current behind the voltage, and draw this lag out in the form of a definite angle. This angle is then the measure of the lag. Time difference between current and voltage we call phase displacement, and the angle which the radii vector of the current and voltage make with each other is called the phase angle. The letter has been commonly chosen to designate this angle. We have for instance the vector diagram Fig. 28., which POWER FACTOR (COS 0) AND ALTERNATING CURRENT THEORY 51 pictures the relations as they might be in an alternating current circuit. We have only to imagine the radii vector as rotating about as a centre, to obtain at any time the corresponding values of the current and voltage by drawing the vertical pro- jections of their respective radii vectors. Sometimes this vector diagram is drawn even more simply, see Fig. 29. On what conditions now does this phase displacement depend? We have already seen that this phase displacement is a result of the self-induction. Therefore, the greater the current FIG. 28. FIG. 29. and the faster the changes, the greater is the change in the corresponding magnetic fields, in a given time. The frequency therefore, has a large influence on the phase displacement. Besides this, the self-induction is also dependent on the type of conductor used and its position relative to other conductors. The factor which designates these conditions is called the "co- efficient of self-induction." The mathematical symbol for this is "L." 52 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY We therefore say: The electro-motive force of self-induction is proportional to the coefficient of self-induction, and to the rate of change of current per second, or the frequency. If we compare the relations in an alternating current circuit with those in a direct current circuit, we see, in the latter case, that it takes a definite voltage to force a current i, through the resistance r, and, according to Ohm's Law, we have e = iX r If it is desired to send an equal alternating current, *, through a coil, it also takes a certain voltage, e r = * X r, to overcome the resistance. We have to take into account, though, that with alternating current an electro-motive force due to self-induction is generated, which is always in the opposite direction to the impressed electro- motive force. In order, therefore, to obtain the desired current i, we need not only the voltage, e r = i r, but also an additional pressure e L to overcome the electro-motive force of self-induction. Hence, the total voltage necessary for an alternating current is, e = e r + e L . The alternating current voltage e is composed of two different pressure waves. These waves are displaced by an angle of 90 or % of a period, which can easily be shown by a short mathemati- cal demonstration. The above sentence in italics regarding self-induction, is mathematically expressed as follows: Furthermore, we know that for a sine wave, the formula for an alternating current at any instant is: i' = I sin m t, exactly as the sine wave for the voltage gave e' = e sin m t. POWER FACTOR (COS #) AND ALTERNATING CURRENT THEORY 53 If we substitute another value for i in the equation r di e L = L ; we obtain di d(l sin m i) _^ _ _ _ _ m i cos m i dt dt e' L = (mXiX L) cos m t. We have e r = i r or the instantaneous value e' r = (i r) sin m t. The total voltage is, therefore, e' = e' r -f e' L = (i r) sin m t -f- (i m L) cos m t and as cosmt = sin (m t + 90) B it is evident that the voltage e?=ir) necessary to overcome the coun- ter electro-motive force of self- induction is 90 ahead of the e.m.f. necessary to overcome the ohmic resistance.' From this it follows that these two e.m.f.'s are not to be added arithmetically but geo- metrically. If we draw this as shown in Fig. 30, we have : O A = maximum value of the current = i FIG. 30. The resultant of the two e.m.f.'s is graphically shown as O D, and from it we obtain the total voltage e = e r + e L . From the figure then we have: = i V r + m 2 U It also follows that tan = -y- when is the phase angle 54 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY between the current and the voltage. It is worthy of mention that from the equation e = i vV -f mr L it seems as though the self-induction apparently increases the resistance. Hence, the expression is also called the "apparent resistance" of an alternating current circuit. In order that there shall be no mistake regarding the values which are indicated by measuring instruments in alternat- ing currents, it is well to emphasize here, that so far we have only mentioned the instantaneous and maximum values. As a matter of fact, neither of these values is indicated by the usual alternat- ing current instruments. These values have only been used to more clearly state the relation in a.c. and to make them easier to understand. The instantaneous and maximum values are therefore only of theoretical interest, whereas the a.c. instruments indicate a so-called "effective value." This is obtained from the previous formulas and figures by dividing the maximum values e by \/2. Hence, the effective value of the voltage is e = = and the effective value of the current is 'i l = vl. We can therefore regard the diagrammatic figures as representing the effective values, as these only differ from the maximum values by a constant factor. If we now return to the phase displacement between the current and voltage, we find the question becomes of the greatest interest. What influence has the phase displacement on the power computation ? It was shown in Chapter II. that the power in watts is equal to the product of the current and voltage, that is p = e X i. Unless the so-called power factor, which will be later explained, is unity, this last equation is only applicable to direct current. POWER FACTOR (COS ) AND ALTERNATING CURRENT THEORY 55 Whereas for alternating current the formula becomes, p = e i cos 0. In this equation e is the effective voltage, i the effective current and cos the power factor. In alternating current circuits we call the product e X i the apparent power. It is measured in volt-amperes or kilo-volt- amperes = 1000 volt-amperes. The product e i cos designates the real or effective power and is measured in watts or kilowatts. To verify the equation for the true power really goes beyond the limits of this book. For those, therefore, who are interested in this paragraph, it is added in an abbreviated manner. The equation for the instantaneous energy is p' = e' X '. T The work done in % a period during the time is then r- A =J o 2 e'i'dt and from this we obtain the mean value of the energy. JL L L p = / e'i'dt = -j C 2 e'i'dt 2 J o "a by substituting the values i' = I sin m t and e' = e sin (mt + ) and by completing the integration, we obtain, el p = cos and as e I 7= = e and = = * we get = e * cos <. V2 V2 From this it follows that, providing the voltage and power remain unchanged, the current decreases with an increasing power factor. As the current strength determines the cross- section of the electrical conductor, it naturally interests us to keep the current down, i.e., we. strive to obtain the highest possible power factor. From the above power equation, it follows that, when COS0 = I, 56 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY p = e i and the angle < = o. The other limit is when cos = o or the angle 4> = 90, then the power, p = o. A low power factor, therefore, corresponds with a large phase dis- placement. The meaning of the above may best be enlarged upon by an example: Suppose the electrical circuit contains a coefficient of self- induction L = .002 henry a resistance r = .0125 ohm a frequency v = 50 and therefore m = 2 TT v = 314 voltage e = 150 volts. Then: mL 314 X .002 tan = = = 50.24. r -0125 The angle < corresponding to this value is then 88 50' or nearly 90. Hence cos is nearly zero. The relations are graphically shown in Fig. 31 This shows FIG. 31. that e and e L almost coincide, so that e L practically equal e. The current is then e iso i = 7- = , = 240 amperes. mL .628 Consequently ir = 240 X .0125 = 3 volts = ecos$ and i X e cos = 3 X 240 = 720 watts; With the same current but with cos < = i , we would have ob- tained instead of the above, the power p = 240 X 150 X i = 36000 watts. This example shows us plainly how impossible it is to judge the power in an alternating current circuit by merely reading POWER FACTOR (COS #) AND ALTERNATING CURRENT THEORY 57 the ammeter and the voltmeter, as these two instruments do not in any way indicate what the power factor is. We therefore em- ploy a special instrument to measure the power, a so-called wattmeter, which indicates the watts or kilowatts, directly, where i kilowatt = i kw = 1000 watts. As the example showed that the voltage necessary to overcome the e.m.f. of self-induction (i.e., the vector e L ) is without any influence on the actual power in other words, it delivers no power which can be measured in watts we therefore call this vector the wattless component, and the vector e r = i X r is called the watt component of the voltage. Up to the present we have divided the voltage into two component parts. The one being the watt component e r ir which coincides with the direction of the current, the second being the wattless component e L = i m L which is in quadrature with the former. The power p = e i cos $ = i ( e cos ) where e cos $ = e r ', that is, the power is obtained by multiplying the two unidirec- tional vectors or forces (i and e r ). (See Fig. 30.) Instead of separating the voltage into two components, we could have also separated the current into two forces at right angles to each other. This separation can be done in such a way that one force falls in the direction of the terminal voltage, and being multiplied with this, it gives the resultant power, while the other force is perpendicular to the first one. The equation for the power, p = eicos can be written as p = e (i cos ) = e i r where i r is the watt component of the current and equals i cos . Taking then the values of the example as chosen, we obtain Fig. 32. The directional precedence is given by the curved arrow. Here the total current, i, is shown as lagging behind the voltage by the angle , similarly to the previous example. As the angle $ is approximately 90, then i and i m almost coincide and the wattless component of the current is i = i m = 240 amperes 58 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY whereas the watt component of the current i r approximates zero. It is therefore apparent that the total current i m is only present in order to generate the e.m.f . of self-induction. In other words: It is the wattless component of the current which generates the lines of force. That is why this wattless current is also called the magnetizing current, and this is why it is designated by i m in the accompanying figure. It follows, therefore, from all which has been said of the power factor that: When figuring the size of electrical conductors, the apparent power should always be the determining factor, i.e., the product e X i or current X voltage, in other words, the kilo-volt amperes. On the other hand, the power of the prime FIG. 32. mover only takes into account the actual power, that is the product e i cos 4> or the actual kilowatts. In other words, a poor or low power factor means expensive lines and electrical machinery, whereas it has no influence whatever on the prime mover. It is apparent, therefore, that it is in the interests of an inexpensive installation to have an acceptable power factor. It is to be noted, however, that in the ordinary power houses, the power factor varies between .6 and .8, depending on the sizes of the motors used and at what load these are operating. These values are, therefore, a guide indicating whether or not we have a good power factor. Quite independent of the current lag, we may have induction phenomena which will call forth other and more disagreeable actions than those shown, and as it is the object in designing and POWER FACTOR (COS ) AND ALTERNATING CURRENT THEORY 59 operating electric furnaces to avoid these troubles, we will mention them briefly. We have seen that an alternating current in a conductor will generate another alternating current in any conductor if the sec- ond conductor only lies in the magnetic field of the first conductor. We therefore obtain currents in all conductors which lie in the magnetic field of another conductor, and these currents may cause considerable power losses under certain conditions. It would lead us too far if we were to occupy ourselves deeply with these phenomena. On that account only those possibilities will be mentioned which lead to these power losses in electric furnaces, and the remedies which help to overcome these losses. In the first place there are the induced currents themselves, which may engender considerable losses. As these induced currents are generated in every conductor which is parallel to the main current, they may cause great losses when the conductor carrying the induced current is short-circuited. It is therefore necessary to avoid all designs in which, for example, an iron beam would follow a main conductor, so that it would then be short- circuited on itself. This condition is to be considered only when very heavy currents are present as is altogether the case with electric furnaces. But even here these actions may be avoided by carrying the incoming and outgoing conductors close together. In this way the magnetic fields for instance those made by the two conductors of a single phase circuit are then neutralizing each other, so that we have no action on parallel lying and closed iron parts. There are, however, currents induced in every metal part which is near an alternating current carrying con- ductor. These metallic parts provide splendid conductors for the current through which the current may be short-circuited, so that under certain circumstances a metallic piece of that kind may reach really unlooked-for temperatures. We call these eddy or Foucault currents. They are particularly preva- lent when the metal in question is magnetic, that is, a good con- ductor for the magnetic lines of force. There would be consider- able losses, for instance, in the cooling chambers used in electrode furnaces, to cool the electrodes, if these were made of cast iron 60 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY or cast steel, as both of these materials carry the magnetic flux better than air. We are, therefore, obliged to make these cooling chambers out of copper, red brass, or manganese steel, as these materials are non-magnetic. Another method used to lessen these eddy current losses, is to greatly subdivide the metallic parts in which these eddy currents might appear. Transformer and dynamo armature cores are examples. These cores are built up of sheets as thin as .5 and sometimes only .3 mm. (.02 to .012 inch). Finally we might also have the case where a good magnetic conductor, one of low magnetic reluctance, entirely surrounds an electric conductor. If the magnetic conductor should have a considerable cross-section, then certain power losses arise, due to the constant demagnetizing influence of the alternating current. This loss is known as the hysteresis loss. For this reason, there- fore, we also avoid surrounding electrical conductors with good magnetic conductors in electric furnace construction. It seems well to mention that besides single phase alternating current, polyphase (2 or 3 phase) alternating current is more often used to operate electric furnaces. In order to understand these power circuits, we will add the following: Three phase current is visually distinguishable by having three lines which conduct the current from the source of supply to the apparatus using it. Whereas with single phase current there are only two lines, one line to lead the current to the destination and one return wire. As the name three phase implies, we use three conductors and handle three currents in this power transmission. The vector diagram shows us this the plainest, i.e., the relations be- tween these currents and what the relations are between the different values occurring in three phase power transmission. Fig. 33 shows us three vectors which are separated 1 20 from each other. These vectors indicate the direction of the current as they are actually generated in 3-phase machines and actually consumed in 3-phase apparatus. If we add these currents geometrically, as shown in the figure, we observe that the geomet- POWER FACTOR (COS 0) AND ALTERNATING CURRENT THEORY 61 rical resultant of two current forces always equals the third current. This explains why only three lines are necessary to FIG. 34. conduct a 3-phase current, of which the third conductor may be looked upon as a return wire for the other two. This presupposes of course that the current in each direction or phase is of the same value. 62 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The coils of the generator or those of the power consuming apparatus which are built for 3-phase current, may be connected FIG. 350. FIG. 36. in two different ways with each other. Fig. 34 shows the so- called Star or Y connection, in which the ends of the coils of the generating or receiving apparatus are connected together at the POWER FACTOR (COS ) AND ALTERNATING CURRENT THEORY 63 neutral point A, whereas Fig. 35 shows the so-called Delta con- nection in which the single coils are connected in series, and the connecting points of the coils are led off to the power mains. With the Star or Y connection we may have either the volt- age of one phase or the resultant voltage of two of the phases. The first is the potential between the neutral point A, Fig. 34, and the end of one generator coil, as shown by the connections Aa = Aaz = Aaz = E. The other voltage is the resultant of two of these coils and is across the points a a^, Oz a 3 , a 3 a, and this resultant voltage is designated by e. If in these Star connections the phase voltages should be different, there would however be no difference between the currents flowing in the generator coils and on the line. If / = generator phase current, and i = line current, then I = i, as is evident by consulting Fig. 34. If we, however, view Fig. 35, we instantly perceive that the phase voltage and line voltage are equal to each other or E = e. On the other hand we have different values for the current per phase and the line current. With 3-phase currents for electric furnaces the Y connection is mostly used. What is the relation between these two voltages? If we have the phase voltage , we may obtain the resultant voltage by taking the geometric difference between any 2 phase voltages. If we refer to Fig. 36 we see that, sin 60 = E and therefore the resultant voltage e = 2 E sin 60 = \/3 E = 1.73 E. In the same way it may be shown for A connection that These relations must be known in order to clearly understand the power in 3-phase circuits. We can imagine the 3-phase power being equal to the sum of power of the 3 single phases. We then obtain, p = EI /i cos + Ez h cos + E z h cos and as we assume that the separate phases are balanced or equally 64 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY loaded we may write that p = 3 E I cos <. We saw for the Y connection that e = \/3 E and i = I. If we substitute these values in the power equation, we have e p = 3 = .i cos = v$e i cos $ i.e., we obtain the power in a 3-phase circuit by multiplying the current by the voltage by the power factor and the product by V *' . Finally, it may be said that we have wattmeters which meas- ure the total power, p = \/3 e i cos in 3 phase circuits. It is therefore an easy matter to determine the power factor in a 3- phase circuit, provided other instruments give the values of the current and voltage of the 3 balanced phases. We therefore have In the above p = total power in either a Y or A connection circuit as measured by a wattmeter, i = the current in each line, sometimes only measured by one ammeter, and e = voltage as measured by the usual a.c. voltmeter. CHAPTER V GENERAL CONDITIONS FOR THE OPERATION OF ELECTRIC FURNACES BEFORE we deal with the furnace designs now largely used for steel making, it may be well to discuss a few general questions. An understanding of these is of great importance in order that we may correctly judge an electric furnace. First and foremost the question arises: Why has the steel industry in general an interest in electric furnaces, and what advantages does the electric furnace offer compared to the existing metallurgical apparatus? It is obvious that the advantages will have to be of some moment, if the iron masters are to discard or supplement their hitherto satisfactory methods of procedure. It behooves us then to consider first the proved and peculiar heating effects derived entirely from electricity. We find the following characteristics : 1 . The use of electricity as a heating agent makes an extraor- dinary and quick heat possible, which same is impossible with any system of gas heating. Here it may be noted, that before the introduction of the electric furnace into the steel industry, it was only possible to make refractories stand temperatures of 2000 C., whereas we may now reach any temperature up to 3500 C. in the electric furnace. 2. With the aid of electrical control the heat can be regulated most accurately, so that the charge can be brought to any de- sired temperature and kept there, according to the demands of the process in question. 3. Electricity offers us the cleanest heating agent imaginable, so that we are enabled to avoid all deleterious influence -which other heating agents have; for electric furnaces allow us to oper- ate in any atmosphere, and this prevents reactions taking place 66 66 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY which may be caused by atmospheric elements, gases, or the products of combustion. 4. The characteristics noted, in sections i and 3, allow the steel bath to be refined to any high degree. Sulphur particularly may be entirely eliminated, so- that a high class finished product may be made from impure and cheap raw material. 5. The electric furnace allows us to make crucible quality steel in large quantities (as mentioned in section 4), made from cheap raw material, and yet, at the same time, it turns out a completely homogeneous product. This product has hitherto been possible only in the crucible furnace, where many separate crucibles are used, charged with the purest and most expensive of raw materials. 6. In many cases the product of electric furnaces shows cru- cible quality characteristics, even though the cheapest metal had been charged. This high quality cannot be achieved in any other type of furnace. The reason for this being that the heating agent does not in any way influence the charge and therefore the steel may stay in the electric furnace as long as deemed best, and held at any desirable temperature, mean while allowing the gases to escape. 7. The saving in the additions of ferro alloys is another important consideration, the use of ferromanganese, ferro- silicon, etc., being considerably reduced one-third to one-half less ferromanganese even when added cold. When electrically melted ferromanganese is added to electric steel, the saving is still greater, and of the greatest importance for quantity pro- duction, as the second grade material is considerably reduced. As these are the general principles which make the electric fur- nace valuable to the steel industry, it seems advisable to state the requirements which an ideal electric furnace would demand in order that the above advantages may be best attained. Particu- larly as the numoer of different furnace designs are -numerous. Surely everybody who is confronted with the question of installing an electric furnace, will see first that the installations shall cost the least amount of money, and second that the type used combines the greatest simplicity with the greatest safety during operation. The requirements, therefore, should be as follows: GENERAL CONDITIONS 67 1. The ability to use any prevailing alternating current at any voltage and frequency. 2. The avoidance of any suaaen changes in the load 3. Ease of regulating the incoming current. 4. High electrical efficiency. To which are added the following: 5. A furnace of the tilting variety. 6. Easily surveyed and accessible hearth. 7. The electrical heating or any of its necessary auxiliaries must in no way influence the chemical composition of the steel or the slag. 8. The ability to reach any desired uniform temperature in all parts of the bath, and at the same time avoiding any local under- or over-heating. 9. The furnace should be as versatile in its application as possible. These requirements further stipulate the following: 10. Equally advantageous, rapid, and inexpensive methods of removing all impurities contained in the charge, notably sulphur and phosphorus, and furthermore : 11. The possibility of completely and easily removing any slag in the furnace, and of being quickly and easily able to renew it. 12. Complete uniformity of the material in all parts of the molten metal and consequently a sufficient circulation in the bath. 13. Avoidance of too much agitation in the bath, and there- fore providing an advantageous standing of the metal. 14. The possibility of providing various furnace sizes, which would have to fit prevailing conditions. 15. The highest possible thermal efficiency with all furnace sizes. 1 6. The avoidance of all water cooling. 17. The least possible refractory and initial cost and low- operating cost. 18. The possibility to melt cold scrap economically. 19. And for foundry purposes, the ready adaptability of the furnace to intermittent service. It may be again remarked that the above requirements are 68 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY those which would be expected of an ideal furnace. The furnaces discussed in the following chapters are those in practical use and therefore only partly fulfill the above requirements, some more and some less, so that the exactions made of an ideal furnace only serve as a normal estimate, with which the following various designs are compared. First of all, though, it seems necessary to dwell more in- timately upon the importance of several points. i. Of the furnace operation we required the use of any pre- vailing current. If this requirement were fulfilled it would enable any electric furnace to be connected to an existing central station, no matter if this were a city electric plant or the works' own isolated station. If the electrical power was sufficient in either case, only the connection to the furnace installation and the latter itself would be necessary, so that the expense of a special genera- tor, which would only be ordered for the furnace itself, would be saved. If, on the other hand, the case should present itself where the available power of an existing isolated plant was entirely in de- mand for other purposes, then in this case it would also be advantageous if any available current could be used for the electric furnace, so that the generator installation furnished for the electric furnace could at the same time and in any event be used as a reserve for the remaining generators; or the generators would act as a mutual reserve, as well for the main generator installation as for furnace generators, which would then insure the best service conditions. If the consumer of electric current does not have to take into consideration the conditions existing in a distant central station when connecting to its lines, then such a connection also offers important advantages as it enables the existing central station current to be used. Furthermore the furnace installation in this case can easily be erected in a comparatively small place, besides saving the attendance for one's own power plant, or that required for a rotary transformer. This is entirely independ- ent of the fact that small works are hardly able to generate power GENERAL CONDITIONS 69 as cheaply as it can be sold by large central stations, except- ing when high pressure internal combustion oil-engines are used. Accordingly, it would be desirable, of course, if direct or continuous current could be used for operating electric furnaces, in case a steel mill only possessed a direct current power plant. We, however, saw in the third chapter that on account of the chemical action of direct current, this does not appear suitable for operating electric furnaces, and as direct current can only be changed from a higher to a lower voltage, such as is used for arc furnaces, by means of expensive rotary converters, consisting of driving-motor and gerierator, and if the continuous current were to be used directly from a low voltage plant, the cost of the connecting wires and cables would be extraordinarily .ex- pensive, as the distances are usually considerable; therefore, direct current is practically never used today for any electric furnaces in the iron industry. If, in spite of this, we see the assertion made here and there in advertising mediums, that a furnace may also be operated with direct current, then these assertions are to be approached with the greatest care, for when these are accurately tested, it will always be found that such allegations are misleading. It can accordingly be established that direct current does not come into play at all for operating electric furnaces. These latter may, however, be adjusted to any conditions which are offered by the modern alternating current station. It is well known that at present alternating current stations are built for three phase current, because the electrical conditions are especially favorable. When an electric furnace therefore is to be connected to an existing power plant, we shall no doubt, in the majority of cases, find that it is to be connected to a three phase plant. In this case a three phase furnace shall have a particular advantage which exactly fits into the conditions offered by an existing electric station. A two phase furnace has the same advantage as a three phase furnace, even though the former is to be connected to a three phase circuit, as three phase current may be changed to two phase by means of stationary transformers having the Scott connection. These transformers 70 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY are necessary in such cases to regulate the power fed to the furnace, i.e., these regulating and phase changing transformers would serve the double purpose of simultaneously changing three to two phase current or vice versa, and regulate the current besides. Whereas a single phase furnace under these conditions would necessitate the installation of a rotary transformer, con- sisting of a three phase motor and a single phase generator, which would considerably increase both the initial and the current costs. But even though it would be necessary to install a new alternator to deliver current to the furnace, the three phase (or two phase) furnace has certain advantages. In this case it would be of considerable importance to obtain the least expensive electric plant consistent with economic operation. And it may be of determining importance here as a polyphase alternator costs about 25 to 33 per cent, less than a corresponding single phase alternator, other things being equal. If, on the other hand, single phase current only should be available, then the polyphase furnace would, of course, be more expensive, as the single phase current would then have to be changed to polyphase current by means of a rotary transformer. It appears, therefore, that the utilization of any existing single phase current would be of particular advantage. It seems that being able to use any voltage is of lesser im- portance. For as our requirements have limited us to the use of alternating currents, there no longer remains any noteworthy difficulty in changing or transforming a high central station voltage to a lower furnace voltage. For this change can be made very simply, and almost without loss, by means of stationary transformers, which only entail a comparatively small expense and almost possess an unlimited life. Contrary to the foregoing, we find that it is of great im- portance to be able to use any existing frequency for the electric furnace. Unfortunately, this requirement is not yet completely fulfilled by all of the well-known furnace designs. Among others, the main reason is to be found in the power factor or cos < falling as the frequency rises. (See Chap. 4.) It can, therefore, only be established, (taking into considera- GENERAL CONDITIONS 71 tion that only the practically attainable can be asked,) that an electric furnace should be operated with normal frequencies, meaning thereby 15, 25, 50 and 60 cycles. In order, however, to point out early, of what importance the frequency of an alternator is as regards cost, it may be men- tioned that the costs of a single phase alternator of 25 cycles and a similar one of equal capacity, but of only five cycles, will bear the ratio of i : 2. These figures may perhaps best show, charac- teristically, the influence of abnormally low frequencies. We now come to the second requirement, viz.: the avoidance of all sudden and untoward changes in the load. That such load changes and principally current fluctuations are of the greatest disadvantage to every electrical power plant, needs no explanation. It may only be remarked here that no city light- ing and power plant would allow an electric furnace on its lines, which operated with heavy power fluctuations, without first inter- posing a rotary transformer with suitably heavy fly-wheels or other appurtenances which would be able to absorb these fluctua- tions and thus keep them away from the central station. This same requirement would also have to be met with in every other isolated plant, if any value is placed on its economical operation. With interposed rotary transformers, therefore, the power fluctuations would increase the initial cost. This also holds, provided the furnace is connected to a special generator. For it is evident that the generator must stand the greatest current fluctuations without injury, i.e., the generator must be built for much higher currents than if there were no irregular power surges. In other words, a generator required to operate a furnace, having current fluctuations, could operate a much larger furnace which was free from such fluctuations. To this must be added the fact that the generator's prime mover would run under much more unfavorable conditions, and with a much poorer efficiency, if the current surges are to be overcome, than if it only had to deliver the power uniformly or at a gradually changing rate. The power delivered to an electric furnace, having power fluctua- tions, is similar to that taken by an electrically driven rolling mill or by an electric railway. 72 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY In order to give an arithmetical example, it may be said that normally turbo-generators have a steam consumption of 7.5 kg. per kw.-hr. (16.5 Ibs. per kw.-hr.), whereas turbo-generators for railway service, with their required overload capacity, often have a steam consumption of 8. 25 (18.15) an d more up to 10 kg. per kw.-hr. (22 Ibs.). These figures about give a correct idea of the advantage which an electric furnace has whose operating force is free from fluctuations. This is quite apart from having a less expensive power plant which a smooth running furnace has. Furthermore, a power plant subject to having power fluctuations, is naturally liable to much greater wear than is occasioned by uniformly loaded machines. To the third point, viz.: the ease of 'regulating the electric fur- nace so as to give higher or lower temperatures, nothing more can be added. This is fulfilled as the furnace voltage can be easily changed, by suitable electrical apparatus, so that this require- ment is fulfilled by all furnaces in the same way. Likewise the fourth point leaves nothing to be said regarding the requirement for a furnace with the highest possible efficiency. For, it is self-evident that a poor efficiency would entail a greater power absorption for the same work, and thereby the operating costs might be considerably increased. The remaining requirements refer mainly to metallurgical facts, which are discussed in detail in the second part of this book. That is why they are only given here just sufficiently to enable one to judge the different electric furnace designs. As a comparatively great number of charges are treated in an electric furnace, especially when operating with hot metal, nearly all the furnaces in practical operation to-day are made of the tilting variety. For this allows the teeming to be accom- plished with greater ease, and avoids much trouble caused by the giving away of the tapping hole. Consequently the demand for tilting furnaces today is a general one. In like manner there is recognized the demand for an easily surveyed and accessible hearth. - For, every metallurgical operation will be placed in jeopardy without it. Therefore electric furnaces should have working doors placed at moderate GENERAL CONDITIONS 73 heights above the bath, a little to one side, from which it should be possible to see the entire hearth. This may be required, for instance, in order to exactly determine the condition of the slag, or to be convinced when changing them, that the bath is really free therefrom before endeavoring to make a new slag. This is entirely independent of the fact that side doors are by far the most advantageous and convenient for charging slag. On account of the absence of an easily surveyed hearth, such resist- ance furnaces as described in the third chapter, having channels running to and fro, are absolutely to be discarded. It seems self-evident that we should expect an electric furnace to have its heat, or the necessary appliances required to give it, without influence on the chemical composition of the steel or slag. For it is just by these means that the electric furnace is to prove its superiority over the older gas-heating type. This point is, therefore, to be borne well in mind with every different furnace design. .For suppose we assume that at any time during the metallurgical process, for instance, during the oxida- tion period, the electrical heating should in any way favor the oxidation, then this electrical heat effect would also be present at any other time, i.e., during the reducing period, and the furnace would then consequently be working at a disadvant- age. Thus the harm of these effects is often greater than the good they do, as they are also present when they are not wanted. Every metallurgist will concede that it is justifiable to expect an electric furnace to reach any desired temperature and still avoid any over- or under-heating. That primarily every practi- cally desired temperature must be attainable is evident, when we consider that the electric furnace must enable us to reach the most advantageous temperature for every stage of the metallurgical process. This requirement, therefore, falls to- gether with the one requiring an easy regulation of the incoming energy. With all this, it is of particular importance that the entire furnace contents be heated uniformly, so that over- and under-heating is not to be feared; it is much more likely that there would be an over-heating. The former of these is hardly 74 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY likely to occur in case considerable heat is carried away by the water-cooled appliances in connection with the electrodes. Borchers, in his 1898 address before the "Verein deutscher Eisenhiittenleute, ' ' said : "As a matter of fact we need not fear that we cannot reach almost any temperature by electrical means for this or that purpose, for we shall have to place much greater weight on guarding against wastefulness on account of working with too high temperatures." The ninth point requires the electric furnace to be as versatile in its application as possible, and thereby possess the greatest adaptability in order to work it in conjunction with present or future processes. It goes without saying that it would be par- ticularly advantageous for the electric furnace, if it were possible to make in it the greatest variety of steel, equally well and economically, and of the same good quality. For even though one or the other object of making the steel may primarily be the absolutely determining factor, it is still to be noted that the electric furnace has a far-reaching application even today. However, there are at present still many new fields open to its product. So that even though it does not appear to be absolutely necessary, still by far in the most cases it would appear to be advantageous, provided a qualified electric furnace, or some chosen system, fits into the working program equally well for the reception of a new quality, as the previous material did. The further requirements from the zoth to the i4th are self- evident, if the previous demands made upon the electric furnace are to be fulfilled. As the principal advantage of the electric furnace lies in the fact that it can turn out the highest quality steel from the cheapest raw material, it must consequently be easy to attain the removal of the impurities contained in the charge, provided the electric furnace economically permits what- ever refining there may be to do. First, we shall have to concern ourselves with the entire elimination of the phosphorus and sulphur; while removing the impurities which alloy themselves with the iron, (such as copper, for instance,) is also thus so far impossible in the electric furnace. If all the refining possible is GENERAL CONDITIONS 75 to be carried out, it is absolutely necessary that the slag for re- moving the phosphorus, for instance, can be completely removed from the furnace. For otherwise, when* the metallurgical proc- ess is continued for the removal of other impurities previously taken up by the slag, the phosphorus will again be taken up by the molten metal. The requirement of being able to completely remove slag from the furnace is covered, therefore, by doors enabling us to have an easily surveyed and accessible hearth. It seems just as self-evident that the impurities be removed from all parts of the bath, as it is necessary that all alloys added to it are absorbed equally by all parts of it. Otherwise an un- even material would result. On this account, therefore, a good electric furnace has to have an adequate circulation, which assures the greatest uniformity of material in all parts of the hearth. The desired agitation, however, must not exceed certain limits, as otherwise the advantage of the electric furnace would not be used which allows any slag solutions to be separated from the furnace contents. Finally, in order that the furnace can have a far-reaching application, it is necessary that the furnace be built of such sizes which seem to best fit present or future installations. This is to be kept in mind, for instance, when the furnace is to operate as an adjunct to a converter or an open hearth plant. In such cases, it is, of course, advantageous, if the furnace can receive a whole charge from a converter. It is such reasons as these that make it desirable to build furnaces of the largest capacity. The 1 5th requirement exacted a high thermal efficiency, and no explanation of this is necessary. However a few words may be said regarding the possible influence of using water cooling. First of all, it is evident that energy losses are caused by every cooling means, and water cooling aids this in the strongest degree, thus lowering the efficiency. Water cooling may become partic- ularly harmful when it is used in such manner as to considerably cool those wall parts which encircle the molten metal. For then the danger arises of the fluid iron assuming a certain tough fluidity, at these places, which makes it very hard to obtain a uniform composition of the entire furnace contents. Finally 76 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY the employment of water cooling may easily cause dangerous explosions if the devices used are not very well -protected. If the water cooling is beneath the bath and the molten metal runs into it an explosion will occur, but it is much less dangerous if the water cooling is above the bath, i.e., where the water would run into the melted charge. It only remains to mention the last requirements consisting of the lowest installation costs, likewise the lowest refactory cost, and thereby the lowest operating cost which brings together nearly all the exactions which an ideal furnace has to fulfill. Commercial intermittent operation is also a requirement to which a furnace should be adaptable without badly or completely cracking or checking the bottom, side walls, or roof refractories, even though this demand is more often made, or almost ex- clusively so in foundries compared to steel mills, where in the former it is usual to operate only during the day. It affects the smaller furnaces considerably more than the larger ones, as furnaces of larger than 5 to 6 tons per heat have so far not been adopted in foundries, but are more often from i to 3 tons in size per heat. Lastly, the commercial ability to melt cold scrap is important, and this feature is apparent with all arc furnaces, but with induction furnaces where comparatively much metal must remain in the hearth to facilitate the making of the suc- ceeding charge, cold metal charging only, has been found to be too costly. That is why induction furnaces to-day are used mainly for liquid charges, or for mixed hot and cold metal. Unfortunately, the complete attainment of this ideal has so far not been accomplished by actual practice, as evinced by electric furnace construction. This will be considered in the following chapters, where the constructions, as used, are compared with the stipulated requirements. We will find there, that every furnace design has certain advantages, but also certain disadvan- tages compared with every other electric furnace design. And it is this which makes the choice of a furnace thus far so difficult, for practical experience and the race in the open market have not yet perceptibly proved the superiority of one or another furnace system. CHAPTER VI ARC FURNACES IN GENERAL THE ARC IF the ends of two current carrying wires are brought together so that the current may flow, and if the two ends are then slightly separated, no interruption of the current will take place. But there will appear a small, highly luminous flame between the ends of the wires, which takes the place of the conductor at the point of interruption. With this, then, we have to deal with an entirely different property from that which the electric spark presents. The latter also represents a current transference through the air. But far higher voltages are necessary for the production of a spark than the arc calls for, an example of which we have just given above. In the latter, it is not the air which bridges the current, but the gases emanating from the metal of the wires between which the arc has been struck. The way the arc occurs then is as follows: At the instant when the ends of the two wires are separated, a rise of resistance of such magnitude appears at the point of separation, that, with the current flow, a corresponding and im- portant heating effect takes place. It is under this influence that the metal evaporates at the points of contact. If the separation should be increased, then the distance between the wire ends becomes so filled with metallic gases, that these now take up the current transference at the point of interruption. The metallic gases, however, are much poorer conductors than the metal itself. It follows then that the current in its path, from the end of one wire to the other, has to overcome consider- able resistance. The current flowing through this resistance gap generates such high temperatures, that more metal is gasified at the gap, in this way maintaining the arc. If no provisions have been made for hand or automatic regulation which keeps the distances between the wire ends constant, then the arc will 77 78 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY rupture itself. This will happen as soon as the distance between the rigid wires is so large that the potential provided is no longer great enough to overcome the resistance of the arc. Should the arc be interrupted and it is desired to create it again, then the same ends of the wire must be brought together again, so that the arc may again be struck. It may be noticed, when striking an arc, that the metallic gases of the positive wire end or anode are carried away violently. This keeps the metallic gases together in a comparatively con- tracted area, thus making a definite path for the current. Even though the conducting metallic gas stream heated as a resistance between the electrodes is absolutely necessary in order to maintain the arc, it can be interrupted by thrusting a cold body into the arc stream, although the maintenance of the arc is being upheld by an entirely permissible distance. When the arc is broken a decided cooling off then occurs at the point of interruption. This phenomenon is also to be considered with the operation of arc furnaces. From the above it is evident that every arc furnace furnishes that temperature which is required to gasify the conductors between which the arc is to be made. For the gasification of the conductor ends is the hypothesis upon which the maintenance of an arc rests. The best known arc formation is that which we see in the ordinary arc lamp. Here the arc is usually made between two carbon electrodes. The arcs in electric furnaces are made in a very similar way, for here carbon electrodes are also used to form the arc. As before said, this arc gives a very high temperature, in fact the highest which has so far been reached; for in the carbon we possess the most resistive to gasification conducting material, and this gasifies at about 3500 C. This gives us then the arc temper- ature with which iron and steel baths are heated in arc furnaces. Figs. 37 to 39 show the various possibilities which may be utilized for heating metal baths by the electric arc. In the schematically shown arrangement of Fig. 37 and 370, where the arc is formed directly between two or three carbon electrodes, ARC FURNACES IN GENERAL 79 we have the purest arc heating. The heating of the bath takes place by means of the radiating heat of the arc. The hearth is immediately underneath the arc. These furnaces are generally known today as radiating arc furnaces. The earliest commercial arc furnace using this form of arc heating, Fig. 37, is the Stassano furnace, while Fig. 370 shows the Rennerfelt method of heating. These are the best known arc furnaces using this form of arc heating which will, be discussed in detail in the next chapters. Figs. 38 and 39 show the main idea of two other heating possibilities when using the arc. These methods have the common characteristic of the hanging carbon electrode, which allows the arc to impinge itself directly against the metal. In both of these cases the metal bath is part of the electrical circuit, so that theoretically speaking we no longer have an exclusive FIG. 370. FIG. 37- FIG. 38. arc heating. For the charge, composed of slag and metal, naturally offers a certain resistance to the part of the current, by the overcoming of which heat is generated, no matter what the amount may be. Even though the resistance heating of the metal does not practically enter into the question at all, the designation of calling these furnaces combined arc and resistance urnaces is at least theoretically correct. Nevertheless these two furnaces have radical differences. The one shown by Fig. 38 has the electrodes of all poles or phases above the bath, whereas with the furnace shown by Fig. 39, one pole is above, the other is constructed in a suitable position below the bath. The best known application of the former possibility is the Heroult furnace, whereas the equally well known Girod furnace embodies the second qualification. The question arises, where does the real heating take place, in furnaces as shown by the Figs. 38 and 39 where the arcs impinge directly against the bath? 80 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Although this question is discussed in detail in the chapter on the Girod furnace and an arithmetical example given, still the general answer to this may well be given here, which Borchers gave in 1905 in an address before the "Verein deutscher Eisen- hiittenleute." A translation of this follows: "When the electric current leaves the electrodes, a layer of air, gas or some vapor is formed between the electrode and the slag, which is a heat generating resistance in the circuit. This, therefore, gives us the possibility of arc heating. The bottom surface of the electrode encompasses about 1000 sq. cm. (155 sq. in.), at 3000 amperes. Thus we generate in every .second a heat quantity of (Q = .24 et), or say 30 kilogram calories, in the small space between the electrode and the slag, even though we only assume 40 or 50 volts as the arc voltage. This amounts to over 100,000 calories given off hourly from the foot of the electrode. It follows that the slag layer is the second resistance between the electrode and the metal. The heat thus transformed is dependent on the thickness of the slag layer and on its constantly changing conductivity. If we take for this an additional drop of 10 volts, we add an additional 26,000 calories, which is entirely independent of the small amount of heat appearing in the high conducting iron itself. The main heat therefore manifests itself in the space between the electrode and the slag. The foot of the electrode thereby has the gasifying temperature of carbon. A very considerable portion of the heat, therefore, enters the bath through radiation and through the carbon vapor, having over 3000 temperature, (C.) which is constantly thrown from the electrode onto the slag surface and is for the most part greedily absorbed by the oxygen in the slag." From the foregoing general characteristics of the combined arc and resistance furnaces, as they may be alluded to theoreti- cally, it follows that the heating of the metal bath takes place practically almost exclusively through the arc heating alone, so that the above furnaces are fully entitled to be simply referred to as arc furnaces, which is the case in practice. THE ELECTRODES One of the most important parts of all arc furnaces are the electrodes, at the ends of which the arc is maintained, and which lead the current to the bath. ARC FURNACES IN GENERAL 81 A most resistive to oxidation material is required for electric furnace electrodes in any event, and only carbon meets the requirements for those coming directly in contact with the bath, (if we omit for the moment the electrodes of iron or conductors of the second class,) as of all the metallic conducting materials, carbon alone stands the highest temperatures. It is, of course, to be considered throughout that carbon is very liable to enter into reactions, especially at the temperatures found in electric furnaces, so that the metal bath must be protected by a layer of slag against an undesirable absorption of carbon, as is done for instance in the Heroult, Girod, Rennerfelt and other arc furnaces for steel making. If this is done, carbon offers by far the most desirable material for arc furnace electrodes. These are made in specialty factories, or in case of very large electric furnace installations at their own works. They are made by hydraulic presses, being later on carefully dried and burned. Here one should strive to obtain a complete uni- formity of the mass, and the greatest mechanical solidity. Regarding the electric conductivity, it is to be noted that this varies greatly when using either carbon or the various sorts of amorphous carbon, charcoal, coke or soot. We obtain a higher conductivity, the more the finished electrode approaches the graphitic state, pure graphite electrodes giving the very highest conductivity obtainable. This item is dwelt upon later in detail. Moving parallel with the increase in the electrical conduc- tivity is the heat conductivity, so that when we have these favorable electrical conditions, i.e., when using graphite elec- trodes, we have the smallest Joule or fr losses. To be sure, the largest thermal losses occur at the same time, because graphite electrodes, being good conductors, transmit large heat quantities from the inner furnace to the outside. We then have before us the interesting question concerning the most advantageous composition for the electrodes, i.e., finding out how to gain their best efficiency. This question is of great importance, as the efficiency of the electrodes largely influences the total efficiency of arc furnaces. Before going in to this question, however, we will preface it with a few general remarks. 82 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Next to the heat generated in the electrodes, current density has the greatest influence. That is, the number of amperes per unit of electrode cross-section, which of course accompanies the electrical conductivity. In accordance with a paper read before the "Verein deutscher Eisenhiittenleute," by Professor Borchers, in 1908, an electrode material having the conductivity of arc- lamp carbons, commences to gasify its carbon when the current density is from 10 to 15 amperes per square millimetre (6500 to 9750 amp. per square inch), whereas when the current is from .5 to i.o amperes per sq. millimetre, (325 to 650 per sq. inch), the temperature attained was from 500 to 600 C. Of course these current densities just mentioned do not occur in electrodes for electric furnaces. According to A. Helfen- stein, the electrodes of calcium carbide furnaces reach a red heat with only 9 to 10 amperes per square centimetre (58.0 to 64.5 amps, per sq. inch). The considerably higher temperatures of carbon electrodes as used in practice in arc furnaces is explained by the electrodes not being heated by their ohmic resistance alone (f r loss), as they are heated besides this by the arc temperature at the electrode end. The following table 1 , taken from the book by Wilhelm Borchers, "The Electric Furnace," may show the current densities usually figured with: Electrode Diam. Carbon Cross-Section per Ampere Electrode Cross-Section Load in Amperes per Unit mm. inches sq.mm. sq.in. sq.cm. sq.in. sq.cm. sq.in. 50 1.97 IO 0155 19 63 3-05 10. 65.0 IOO 3-93 12 .0186 78.54 12.12 8.33 53-5 200 7-97 20 .0310 314.16 50.0 5 .00 32-5 300 11.90 30 to 40 .0465 to 706.86 III .2 3-33 to 21.5 to .062 2-5 16.0 400 15 .94 60 to 90 .093 to 1256.64 200.0 1.66 to 10.7 1 .14 I. II 7.2 1 The later (1916) type of carbon electrodes used in the 2o-ton Heroult furnace at Homestead, Pa., was 24 inches in diameter. As they carry from 13,000 to 25,000 amperes per phase, the current density is as low as 26 to 46 amperes per sq. inch (see also page 322), depending on the voltage used. ARC FURNACES IN GENERAL 83 It must be seen from the table that the load per unit of cross- section decreases as the electrode cross-section increases, the essential reason being that the manufacture of electrodes of the best quality becomes more difficult as their cross-section increases. It is also evident, that it is harder to make a completely even mass in a large electrode cross-section, than in a small cross- section. It is likewise much easier to obtain an even annealing for thin electrode rods, than for thick rods. Finally the gasifica- tion of p'art of the binding material of electrodes is much more uniformly and completely accomplished in small cross-sections, than is possible in large cross-sections, in which it is almost im- possible to avoid irregularities. If these facts illustrate the decrease of the permissible current density with increasing electrode cross-sections, and if it appears that the use of too large cross-sections is not advisable, we find that the considerable weight of the carbon electrodes is also forbidding; besides there is irregular solidity with growing cross-sections. Owing to this, it has been found preferable, sometimes, to build up large electrodes of several smaller ones and thus avoid one large electrode block. See Figs. 6oa and 606. Thus we can use in these smaller electrodes the higher per- missible current densities, and attain a smaller total electrode cross-section, which consequently give the much desired lower thermal losses. With all this, we stand anew before the question of what is the best division between the electrical and thermal losses in the electrodes, i.e., how shall their best efficiency be attained? This theme has been extensively discussed in 1909 and 1911 in the Electrochemical and Metallurgical Industry, latterly called Metallurgical and Chemical Engineering. The principle articles are by C. A. Hansen and Carl Hering. Even though these dissertations could not solve the question of the best electrode dimensions completely, still, the results are so important that they are presented here in condensed form. As before mentioned there are two kinds of losses in the electrodes: i. Losses through Joule heat, i.e., those in consequence of the electric current flowing. 84 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY 2. Losses through heat conduction, i.e., those occasioned by the electrode (being a good heat conductor) leading the heat from the inner furnace to the outside. How complicated these conditions become by the cooperation of these two losses is evidenced by one of Hansen's tests, for he obtained the astounding result, when the ohmic resistance and the current density were increased to such an extent that the Joule losses doubled still the total losses remained the same. Of what importance the clearing up of these conditions is, is very evident, when we hear that according to Hansen the losses in a furnace operating with 500 Kw. can easily be 15 per cent, of the total energy input. This would be continually 75 Kw., or with a current cost of $4 cent per Kw.-hour, the electrode losses would cost about 56 cents hourly. When the electrodes are incorrectly dimensioned, it may happen that thermal losses increase to such an extent, that it is no longer possible to keep the whole bath molten. Then only just that part which is directly beneath the arc will stay molten, while the remainder will remain solid, owing to the heat transfer- ence occasioned by the extravagant dimensioning of the electrodes. These examples already show that a saving in the electrode losses may, under certain circumstances, be the deciding factor for the economic working of the electrode furnace, especially if the price of current be high, while in other cases large sums of money could be saved, if we succeeded in approaching as nearly as possible the best theoretical electrode dimensions. In order to become acquainted with the conditions governing the least losses, Hansen made parallel tests with graphite and carbon electrodes which gave the following results. The efficiency of graphite electrodes grows with increasing length and increasing current densities. It is, however, im- possible to force the current density above certain limits, as the electrodes then taken on temperatures that are too high, which might easily destroy the surrounding brickwork. With ordinary carbon electrodes an increasing length causes a decrease in the efficiency, whereas the Joule effect becomes much larger than the thermal losses. ARC FURNACES IN GENERAL 85 The experimentally ascertained conditions of Hansen are, of course, only true between certain limits. It is evident that by continuing to increase the length of graphite electrodes, up to a certain point, a condition would soon result where the losses are a minimum. If this point is exceeded then the Joule effect would increase more rapidly than the heat losses would decrease, and this would result in an increase of the total losses. Similarly the minimum losses would be exceeded if the current density were increased beyond its best value. These reflections led Hering to determine the most favorable electrode dimensions theoretically. Though these determina- tions do not always give the greatest consideration to the con- ditions in actual practice, and the results may only be partly used in practice, still they give such interesting disclosures, re- garding occurring conditions, that they are for this reason worthy of note, and will be given a little later on. Now next it is evident, that under any conditions and inde- pendent of material, an increase in the electrode cross-section in- creases the heat conducting losses, simultaneously, though decreasing the electrical losses. On the other hand a lengthening of the electrode, namely on the inside of the insulating brickwork, causes a decrease of the thermal and an increase of the electrical losses. When both cases are extreme the losses will be infinitely great. It is a fact, however, that the Joule heat as well as the heat carried off through conduction are both generated by electricity. Thus the object is to bring the total losses down to a minimum. For this it is quite necessary to know accurate values of heat conductivity and specific resistance for every electrode material. Furthermore, there should be accurate results on the indepen- dence of these values of the temperature. Fortunately such results are now no longer missing. 1 To this must be added however, that all electrodes are manufactured articles, which are not capable of being prgduced of complete uniformity. The constants of these, (of proved material,) vary somewhat with the area of the electrode, but for practical purposes the varia- 1 See "The Proportioning of Electrodes for Furnaces," A. I. E. E., April, 1910, by Dr. Carl Hering. 80 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY tions of the constants at different sections of the electrodes may be disregarded. Hering, in his computations, assumes what is probably nearly or quite correct in most cases, namely, that the heat gradient through the wall is practically the same as that in the electrode through the wall, and if these two heat gradients are alike, the assumption is correct, that is, he assumes that the electrode is insulated throughout its entire length, so that the heat is only conducted away by the end of the electrode, which is on the out- side of the furnace and there usually cooled with water. He further assumes that the electrode has the exact same cross- section throughout its entire length, and that the change of conductivity with temperature follows a straight line. However, it may be said that the straight line temperature coefficient is not necessary for the correctness of most formulas, provided the correct average value is used, which he calls the "electrode mean value" neither an arithmetic nor geometric mean, but one peculiar to electrodes. There are, therefore, still some assump- tions which are not borne out by the facts, but are, however, necessary in order to make the conditions for theory and practice more distinguishable. With the assumptions as made, Hering shows the conditions as visually shown by Figs. 40 and 41. In Fig. 40, E E represents 7T -- E FIG. 40. -H* FIG. 41. an electrode, which is surrounded about its periphery with a complete heat insulator, so that only the ends remain free, which are for instance kept cool by means of water-cooling. If we now send a comparatively heavy current through the electrode, this will heat the latter strongly at the middle point H, until an equalizing condition occurs. As soon as this is reached, the entire Joule heat will be carried off at the cooled electrode ends, while no heat flow will occur at H. If we now cut the insulated electrode at H , in order to utilize ARC FURNACES IN GENERAL 87 both parts as electrodes for an electric furnace, as it is schemati- cally shown by Fig. 41, there will be no change in the situation, provided the furnace has the same temperature which it formerly had at H, assuming of course that the same current strength as before now flows through the electrodes. Under these con- ditions also, there will be no heat loss from the furnace interior, through the electrodes to the outside of the furnace. The condition given herewith is the ideal one, so that we may have only the minimum electrode losses, with, of course, the previously made assumptions. Provided the assumptions have the limitations as originally laid down, the losses will be equal to Q = Qi + where Qi equals that heat loss, which would be carried from the furnace to the outside by the electrode, if no electric current were flowing, and Qz equals that heat quantity which is solely and alone generated by the current overcoming the electrode resistance = i 2 r. Consequently Q\ = c k r -T-; here c = 4. 18, a constant, which is used for converting gram calories into watts. k = the electrode mean heat conductivity 1 in gram calories per second by i cm. length and i sq. cm. cross-section, with the temperature difference appearing between the hot and the cold electrode ends. T = temperature difference between the hot and cold electrode ends. I = the length of the electrode in centimetres. Furthermore, Qz = i z r = i 2 pi where r denotes the total resistance of the electrodes. Pi = the mean electrical resistivity per cubic centimetre at the occurring temperature difference. / = length in centimetres. q = cross-section in square centimetres. "electrode mean" is that mean value which is the average value tinder electrode conditions as determined by actual research by Hering. See A. I. E. E., 1910. 88 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY We therefore obtain the total losses, which are carried away from the cold end of the electrode, as _ & q i I 2 T I 2 l Pl q' The total energy losses carried off at the cool electrode end, are equal to the sum of the heat losses, which would occur if no current flowed through the electrode, plus half of the heat lost by means of the Joule effect. The losses are a minimum when the pure heat losses are equal to one-half those caused by the Joule effect, i.e., when 4. 18 k T -r = i 2 pi . In this case the total losses are equal I 2 q to the Joule heat losses or i 2 r, and hence no heat would be car- ried from the furnace by conduction. From the equation for the minimum losses, 4. 1 8 k T -J- = i 2 pi , it follows that _i = i rp7 If this result is then substituted for y in the general equation for the total losses, we have: Q = 4. 18 k T r H i 2 pi . In order to attain the minimum losses we have the requirement Qmin = 2.892 V k T pi. The equation for Q min shows that the minimum losses are determined by the material constants k and pi, the temperature differences between the hot and cold electrode ends, and the current strength. It is independent of the absolute dimensions of the electrodes, for of these it is only required to maintain a definite relation between the cross- section and the length in accordance with the equation for -;- If we substitute in the equation for Q min the specific electrical conductivity per cubic centimetre, x = , in place of the specific ARC FURNACES IN GENERAL 89 resistance, we obtain This equation shows that the least losses are fixed for a certain definite temperature on account of the relation between the heat conductivity and the electrical conductivity. In accordance with this, the best material for the electrodes is that which has the lowest ratio of the heat conductivity to the electrical conductivity. From the equation for the minimum losses, it follows that an increase of the temperature difference between the hot and cold electrode ends only influences the losses in proportion to the square root of these differences. If we again consider the equation, r = . 345 i -J 7^- we see, that with a given material, a given current strength, and a given temperature difference, the elec- trode losses would remain the same for entirely different cross- sections, provided the proportion between the cross-section and the length remained unchanged. From this we now learn: // it be desired to save on electrode material when having a minimum of losses, then the electrode is to be made as short as possible. Generally the electrode length is primarily determined by the practical demands of the furnace operation, so that a certain minimum distance of electrode length cannot be exceeded. The length of electrode, therefore, having been determined, the cross-section can be calculated by using the formula -y. It is well to mention here that it can be assumed that all these calculations only retain their full correctness, provided the electrodes are protected by insulation throughout their whole length. It may be repeated though that the heat gradient through the wall is practically the same as that in the electrode through the wall, and, if so, the assumptions are correct. If these two heat gradients differ materially, then an increase of the cross- section, in the same proportion to the length, causes a decided increase in the electrode surface and with it naturallv an in- crease of the heat losses. 90 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY From the derived formulas, it is evident that the current strength influences the size of the losses. Consequently, it would be requisite to have the smallest possible current at high voltages. Unfortunately, this demand cannot be fulfilled, without leaving the total efficiency of the furnace out of consideration, for it is always well to keep in mind that the electrode losses considerably affect the furnace efficiency, but are not the sole factors that carry weight with it. This point is discussed further bn. In place of the current strength in the formula Pi we can insert the current density A = , and obtain This formula produces a combination showing the best con- ditions between cross-section and length together with a current density fit for use. This seems advantageous, because by over- stepping the permissible current density limits, it is very easy to endanger the furnace operation. On page 84 mention has been made of these tests by Hansen. However, this formula also has the disadvantage, that it determines the electrode length arithmetically, which is not fully determinable for practical reasons. And the value of this derived formula, practically only consists in bringing forth a clear idea of the conditions of an ideal case. This, however, does not infer that the above formula is the ideal case and is not considered so by Hering. It should be the ambition of every furnace designer to come as near to this as possible. In order to be able to utilize these rules and references, it is necessary to have useful constants for the different con- ductivities of different electrode materials. Fortunately there is now no lack of these at present. 1 Even though only a few values are given hereafter, it must be observed that they have reference to a certain definite material, which just happened 1 See A. I. E. E., April, 1910. ARC FURNACES IN GENERAL 91 to be used for these determinations, and that products from other factories would give results deviating from these, more or less. Nevertheless, the figures comparing the graphite and carbon electrodes may be regarded as typical, and can conse- quently be used in practise for electrode designs. Hansen gives the following figures for temperature differences up to 3000 Centigrade: Material. Pi k Graphite 1 ............................... 000812 .16 Carbon 2 ................................ 00183 - Ol6 The proportion between the electrical resistance of carbon and graphite is as 2.25 : i, whereas the heat conductivity of graphite is ten times as great as that of carbon. Relative to the current densities Hansen believes it safe to figure with the following values: 3 For graphite, 150 amps, per square inch. This equals 4.3 sq. mm. per amp. or 23.25 amp. per sq. cm. For carbon, 50 amp. per square inch. This equals 13 sq. mm. per amp., or 7.75 amp. per sq. cm. Substituting these values in the ratio -|- in accordance with the equation: for instance for 20000 amp., and 3000 C, we have for graphite, q | .000812 j = -345 X 20000^ . I6X3000 - 9-0 and for carbon, q I .00183 = -345 X 20000 ^^^- = 4*57 i.e., for equal electrode lengths (which would be required for the same furnace) of graphite as well as for carbon, a carbon electrode would have to have - = 4-73 times the cross-section of a graph- ite electrode. 1 Specific resistance ohms per inch cube = .000320. 2 Specific resistance ohms per inch cube = .000721. Compare the values dependent on the cross-section as given on page 82. See also page 81. 92 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY After the relation for ~ is given, we can either assume the required electrode length as given (on account of the practical furnace requirements), and thereafter determine the cross- section, which could then be regulated by permissible current densities. We could, however, also figure from a given current density as a basis, and from the cross-section thus determined, calculate the electrode length, which would then have to have its practical applicability proved. Should we choose the latter method, we may calculate the cross-section based on a certain current density deemed per- missible, and based on Hansen's values, for instance, for the electrode cross-sections of this material. The example cited was for 20,000 amperes. We then have: 2OOOO graphite = = 860 sq. cm. 23-25 , 20000 . N ( = - = 133 sq. inches) on account of the proportion, therefore, of y = 9, we obtain a length of 95 cm., or 37.4 inches. If we assume that this length is satisfactory to the furnace operation, then this same length will, of course, have to be kept for the carbon electrode, and in case a minimum of losses is also desired here, we would have for the carbon electrode cross- section (based on the calculated relation of -y = 42.57) g carbon = 4044 sq. cm. (= 1591 sq. inches). From this, with 20000 amps., we have a current density of 20000 4044 4.9 amp. per sq. cm. , 20000 . , (= - 12.5 amp. per sq. m.), which would show that' according to the values given on page 82, these are sufficiently high, so that an enlargement of the cross- section would recommend itself, and perhaps a simultaneous ARC FURNACES IN GENERAL 93 increase in the electrode length, in order to stay as close as possible to the minimum losses. Besides this it is interesting to become acquainted with the losses as they appear in the given example, either when using graphite or carbon for the electrodes. The equation for the minimum losses was: Q min = 2.89 *V*7^ By substituting the values for graphite, we obtain Q m f n = 36 KW and for carbon Q min = 17 KW., i.e., assuming that the given constants are correct, the losses for graphite would be about twice as large as those for carbon. This condition, however, only holds good, when the electrodes are heat insulated for their entire length as previously mentioned. In accordance with the values heretofore cited on page 82, for the usual current density values for electric arc furnace carbon electrodes, it seems that the figure of 7.75 amps, per square centimetre (50 amps, per square inch), which Hansen gives, is extraordinarily high. It is therefore not advisable to use these figures, which gives much too short electrodes for practical furnace constructions, as the example showed. It is better to use those values given on page 82, which simultaneously take into consideration the influence of the electrode cross- section enlargement. The figures given in the following tables are from tests made by Hansen and published by him. On the one hand for graphite electrodes made by the International Acheson Graphite Co., and on the other for carbon electrodes made by the National Carbon Co.; these may show the influence of the cross-section enlargement on the material constants even a little better. ACHESON GRAPHITE ELECTRODES Diameter or Cross-Section pi = Resistance ohms per cm. cube Diameter or Cross-Section pt = Resistance ohms per in. cube 5.08 cm. diam. 7.62 cm. diam. io.i6xio.i6sq.cm. 15.24x15.24 sq.cm. .00092 to .00093 .OOI03tO .00109 .0009610 .00101 . 00084 to . 00085 2 inches diam. 3 inches diam. 4 in. x 4 in. 6 in. x6in. . 000362 to . 000366 .000406tO .000429 .000378 to .000397 .000331 to .000335 94 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY These measurements were made at a temperature of 25 C. With increasing temperature the resistance of graphite falls as is well known. According to Hansen, this is as follows: At 25 Centigrade 100% 94% 81.5% " 66% " 65% " 68% .- 69% For carbon electrodes Hansen found the following values de- pending on the cross-section: NATIONAL CARBON COMPANY ELECTRODES 400" 8oo c 1200 I600 2000 2200 Size of Cross-Section pi = Resistance in ohms per ccm. cube Size of Cross-Section Pz = Resistance in ohms per inch cube 10.16x10. i6sq.cm. 15.24x15.24 sq.cm. 20.23x20.23 sq.cm. 45.72x45.72 sq.cm. 00457 .00856 .00594 to -OO7I .014 to .0254 4 in. x 4 in. 6 in. x 6 in. 8 in. x 8 in. 18 in. x 18 in. .OOlSo .00337 .00234 to .00279 .00551 to .0100 Referring to the last of these values, it is well to note that this test was made on an electrode delivered 4 years ago, and it is possible that better results have been attained since then, for large electrodes. 1 Hansen also made some investigations with carbon electrodes in order to determine the influence of temperature. He found that, with an increasing temperature, the carbon continually proceeded to graphitize, so that after the electrodes had cooled down, the original figures for the specific resistance no longer held true, but were, instead, much better. The following table shows how the specific resistance of the cold carbon electrode falls, in case the electrode has been pre- viously heated to the temperature shown in the table: 1 In 1916 the National Carbon Co. state that their "Steel furnace elec- trodes have a resistance of about .0025 to .0030 ohms per inch cube." ARC FURNACES IN GENERAL 95 Resistance in the cold condition 100% After heating up to 1200 C 91 .6% " " 1600 C 87.% a o /~> ffrf 2000 C 77- 6 % (( (( (( o /~i x- rrf 2400 C 65.9% " o o /-I /7y 2800 C 5-9% o r< Crf 3SOO C 22.4% Here the last figure approaches that which would be obtained with graphite electrodes under the same conditions. Besides, Hansen gives as an average figure of many tests made with commercial carbon electrodes when heated to 1200 C., a resistance value equal to 60 per cent, of that measured in the cold state. After the electrodes have once been in operation, the uni- formity of the material constants disappear in all parts of the cross-section or the length, owing to the uneven heating of the carbon throughout its entire length. On this account Hansen takes the practical resistance at 1200 C., at only 40 per cent, of its cold figure. As for the rest, we again point to the figures which were used in the arithmetical example on page 76. The remarks regarding the best dimensioning of the electrodes, have a certain practical significance, and that is why they have been discussed here. It is well to be warned, though, that too great stress be not placed on these theoretical opinions. It is to be noted that the derived formulae are only strictly accurate for such cases, where the electrode is protected from heat losses between its hot and cold ends- and that this case never appears in practise. It is further to be observed, that the operation of our arc furnaces necessitates a shortening of the elec- trodes, and consequently considerable electrode lengths appear, which are not taken into consideration in the formula, because they lie outside of the water cooling. Furthermore, the formula are not the only measure for the losses which actually appear in arc furnaces, irrespective of the restrictions just made. Besides the pure radiating losses, there are for instance the contact losses, where the current carrying copper conductor clamps onto the 96 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY electrode. And above all it is a noticeable fact, that on the one hand, the size of the cross-section is of the greatest influence on the efficiency of the furnace, while on the other we see that the electrode length is primarily settled by practical considerations accompanying the furnace operation. We see, therefore, that just this relation between cross- section and length of electrode, which is of striking importance in accordance with the formula for the minimum losses, cannot be freely determined according to the arithmetical values. After all, the benefit of the calculation for the minimum losses lies in the fact that it allows us to ascertain the heat dimensions which lie between the given limits of practical requirements, that we may come as near them as possible. Turning again toward the practical side of the electrode question, we find an interesting work of Hansen's, which deals with the burning away of the electrode or electrode consumption. This question is, of course, of equal importance, as the striv- ing after the least electrical losses, or a high efficiency; for this point is of considerable influence on the operating costs. The consumption of electrodes may occur: 1. In the worst case when the electrode breaks; 2. By the arc formation which causes a gasifying of the carbon and 3. By oxidation. Those under the first heading, which are by far the most unpleasant, seldom or never occur today, as long as the cross- section and lengths used are not too large. Too large cross- sections are always to be avoided, so that if high currents cannot be avoided, it is better to use graphite in place of carbon elec- trodes. Furthermore, it is to be observed that a new electrode must not be placed in the hot furnace in its cold state, as small particles are easily liable to crack off, on account of the great prevailing temperature differences. It is therefore commendable to heat the electrodes slightly before placing them in use. The losses under the second heading are self-evident and unavoidable, so that nothing remains to be said about them. ARC FURNACES IN GENERAL 97 On the contrary a much greater interest manifests itself in the electrode consumption on account of the oxidation. Moissan found that amorphous carbon commences to oxidize at as low a temperature as 375 to 490 C., whereas graphite first begins to oxidize at temperatures of 665 to 690 C. These values though were observed with powdered material and not with solid rods. 1 Finally, Collins, FitzGerald, and Johnson maintain that graphite possesses a greater resistivity against oxidation than carbon does Contrary to this, ' Hansen observed that the losses with graphite electrodes are greater than those with carbon electrodes. In making these tests, graphite rods of the Acheson Graphite Co., and carbon rods of the National Carbon Co. were used. The reason for the higher consumption, when using graphite rods, as given by Hansen, is that at temperatures of 1300 to 1400 C., the graphite particles cracking off are so large, that some of them could be picked up unconsumed. This phenomenon disappeared when the heating occurred in carbonic acid gas, which proves that the cracking off of the electrode particles, when using graphite, leads us back to the oxidizing influence. This investigation shows that it is impossible to accurately determine in advance just what the electrode consumption will be. For this is so dependent on all oxidizing influences, that even the tight or less tight closing of the working doors, or the piercing of the electrodes through the furnace roof, or the working in a more or less reducing atmosphere, may cause considerable changes in the electrode consumption. Oxidizing losses not only affect the electrode consumption, but the power consumption of the furnace as well, i.e., the efficiency. This is evident from the following tests. Hansen operated a small Heroult furnace of 150 kg. (330 Ibs.) capacity, with graphite electrodes of io.i6x 10.16 sq. cm. (16 sq. inches) cross-section and 106. cm. (40 inches) long. 1 These values, however, seem quite reliable since we find data published from electrode manufacturers which give the temperature of oxidation in air at 640 C., and 500 C., respectively, for graphite and carbon. 98 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY With this arrangement he succeeded in melting a 150 kg. charge with 150 kw.-hrs. Later on a similar furnace was operated, but for 300 kg. (660 Ibs.), having the same electrodes. It was established here through various tests, that the power consumption of the larger furnace had the ratio of 1.2 to i compared to the smaller furnace, even though a larger furnace usually has comparatively smaller thermal losses than a smaller furnace. After graphite electrodes of 15.24 x 15.24 cm. sq. (36 sq. inches), and 101.6 cm. (40 inches), long were used, the larger furnace gave a somewhat better power consumption than the smaller one. The tests further showed that- the power consumption rose, as soon as the electrode (the end toward the molten metal) be- came more and more pointed under the oxidizing influences. The difference in power consumption when working with the full cross-section compared to the operation with a pointed one was as much as 30 per cent. This test, as well as others made with various electrode cross- sections in the 300 Kg. (660 Ib.) trial furnace, show that a larger cross-section causes a decrease in the losses. This may be primarily caused by the fact that a larger cross-section permits a more favorable dissemination of energy throughout the whole charge, and furthermore, because the full and larger electrode cross-section acts as an umbrella, which considerably lessens the heat radiation toward the furnace roof. The umbrella action of the electrode also has the additional advantage of keeping the roof from deteriorating too rapidly. This, however, changes as soon as the electrode takes on its pointed form. Hansen established that a more or less strong sharpening to a point of the electrode occurs in all arc furnaces, under the oxidizing influence which takes place during the working period. The trials carried out to protect the electrodes by suitable coverings of car- borundum, water-glass, etc., against the oxidation, have not been successful, for it has not been possible to make the covering durable with the prevalent temperature differences, occurring during the furnace operation. We, therefore, have to figure with a certain burning away of all electrodes, owing to the oxidation. ARC FURNACES IN GENERAL 99 Aside from the three reasons, which have so far been given to determine the electrode consumption, there must still be mentioned the additional loss caused by the stub ends. The length of this stub end depends largely on the distance between the molten metal and the furnace roof. Recently newer methods have been devised which now render it possible to attach the electrode remainders to the new electrodes, thus assuring a most complete use of the electrode material. This is gone into further in the chapter on the Heroult furnace. During the discussion of the electrode conditions, we have often compared the graphite with the carbon electrodes. Is therefore one recommended above the other? To this question this reply may be given: Graphite electrodes mainly have the advantage of greater resistivity, and greater mechanical firmness. This advantage, though, must be purchased at a far higher price, compared to carbon electrodes. Large electrode surfaces tend to save energy, and consequently it is better to work with low current densities. For the graphite electrode loses its im- portance, i. e., its high electrical conductivity, whereas its disadvantage of a high heat conductivity falls heavily in the balance, so that the graphite electrode always has a lesser efficiency than the carbon electrode (see page 94). From all this it is apparent, that one would at first endeavor to utilize carbon electrodes, at least as long as these can still be made of good quality and at the desired cross- sections. It is only with the largest furnaces, where the cross- sections would become so large, that uncertainties would enter the operation, through breakages, for instance, that one would be willing to pocket the disadvantages of the graphite electrode, in order to gain the important advantage of definite and sure operating conditions. THE ELECTRODE COOLING Previously when discussing electrode conditions it was always assumed some water cooling would be arranged at the place where the electrode leaves the furnace roof, by means of which it would be possible, to lower the temperature of the 100 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY electrode as much as 100 or 200 C. It was also shown that the electrode material may occasion considerable losses on account of the oxidation, and this gives us the first reason which forces the application of electrode cooling upon us. If insufficient cooling was provided, so that the electrode, where it issues from the furnace, is not cooled below the tem- perature, where the oxidation begins, then the unavoidable oxidation in the circulating atmosphere would considerably reduce the cross-section. This would be followed by an increase in the electrical resistance, hence a stronger heating up of the cross-section already weakened, and therefore an increasing temperature with increasing consumption, so that in the shortest space of time a change of electrodes would be required. If an intensive water cooling is already unavoidable, this will simultaneously act protectingly on the uniformity of the furnace operation. Thus the contact arrangements which connect the copper conductors to the carbon electrodes are kept from being destroyed. Supposing we assume that the electrode, even outside the furnace, has a comparatively high temperature as well, then there would be such an increase in the heating of the contact pieces, that their hold on the electrodes would be loosened, and with other designs they would burst, so that in both cases the furnace operation would fail, on account of a break in the electrode contacts, quite irrespective of any damage done by the flames shooting through the roof, where the elec- trodes enter. The water-cooling device is also responsible for the long life to-day of the arc furnace contact clamps. At the same time, it fulfils a third and very important purpose. It was shown in Chapter II that all refractory materials used in electric furnace construction are conductors of the second class, and as such obtain higher conductivities with increasing temperatures. This also holds for the brickwork between which the electrodes of arc furnaces lie. It is apparent that these roof bricks become more and more conducting with increasing temperatures, whereas they can be regarded practically as non-conductors with low or even moderate temperatures. In order to avoid a strong oxidation ARC FURNACES IN GENERAL 101 of the electrodes, and to attain the best possible thermal effi- ciency, it is necessary to have the closest fit where the electrodes protrude through the furnace roof. Thus, it is immediately apparent, that when the roof refractories are little resistant, i.e., when their temperature is high, then the small spaces between the electrodes and the surrounding roof bricks are easily bridged over with tiny arcs, which in turn cause currents to flow through the refractory material from one electrode to the other. The current flowing through the brickwork will be higher, as the voltage increases between the different electrodes, as the distance between the electrodes becomes less and the temperature of the brickwork between the electrodes rises. That these currents flowing through the refractory material may be of great importance is shown in an article by Coussergues after seeing a Stassano furnace. In a one-ton furnace, when the arc was interrupted and the voltage was 120, there was still a current of 300 amperes flowing through the brickwork from electrode to electrode. It is to be noted here, that the entrance of the electrodes to the furnace is provided with water-cooling contri- vances. If an attempt were made in such a case as this, to do away with the water-cooling, then the temperature of the brick- work in the neighborhood of the electrodes would rise consider- ably, the resistance between electrode and electrode would thereby further decrease, and still stronger currents would traverse the brickwork. The result would be a considerable increase in the energy consumption, while a strong heating ensues at the wrong place, and at the same time there would be a quick destruction of the very highly heated roof due to the current flowing. In accordance with the foregoing, it is established that the utilization of water cooling with arc furnaces offers important operating advantages, even though there is, of course, a certain heat loss on that account, which is unavoidable up to certain limits. Aside from this there is still, under some circumstances, a small electrical loss, which may appear when currents from the electrode find their way to the cooling chambers, and are thence grounded by the water. 102 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY THE ELECTRODE REGULATION In discussing the arc it was shown that it can only be main- tained, provided a certain distance between the electrode and the bath, or from electrode to electrode in radiating arc furnaces, is not exceeded, as otherwise the arc will be interrupted. It is FIG. 410. therefore necessary to watch the length of the arc. This is easily accomplished with the aid of a voltmeter or an ammeter. The electrodes are then regulated in accordance with readings of the controlling instruments. Even though a manually operated regulation of the electrodes ARC FURNACES IN GENERAL 103 is possible, as, for instance, with the Stassano or Rennerfelt furnace, we find the equipment with automatic regulation, as used in the arc furnaces of Heroult and Girod to-day, has several advantages. In both cases, i.e., either hand or automatic regulation, this is accomplished with the aid of gears, which are often driven by an electric-motor in order to handle them faster and more accurately. In accordance with the indications on the measuring instruments, the motor is started either to the right or left by throwing a double-throw switch, which either raises or lowers the electrode. The General Electric electrode regulator (see Fig. 410), in- vented by Seede, has especial advantages from the standpoint of simplicity, reliability, and low repair costs. There are no moving parts to get out of order, and the few replacements are taken from standard equipment, few in number, and can be quickly replaced. The regulation given by this device is as good as can be had, many hundreds of these regulators giving evi- dence of this service. It is so arranged that besides automatic regulation there is also push-button control of the motors, and independent hand regulation. When automatically operated, the speed is low, whereas when hand regulation is resorted to the speed is high. Both of these, however, are variable. The furnace kilowatt input may also be quickly varied. There are suitable relays to prevent saturating the bath with carbon in case the electrode circuits are opened. Fig. 416 shows the electrical connections. The Thury regulator, invented in 1898, is also used for this automatic regulation. It is made by Ateliers H. Cuenod, A.G., at Chatelaine near Geneva, Switzerland, and in the United States by the Westinghouse Electric and Manufacturing Co., Pittsburg, Pa. The principal part of a Thury regulator is an electro-magnetic scale, which is balanced when the current and voltage condi- tions are normal. When deviations occur in the normal circuit conditions, they throw the lever out of balance. These scales are used then to throw a switch to either one side or the other, 104 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY so that the current for the driving motor enters it either from one or the other side, thus bringing about the corresponding motion of the electrode. The switching mechanism of the old Thury regulator consists of a small constantly running auxiliary motor, which moves a Is not furnished by the General Electric Co., nake connections Iron) automatic regulator panel as shown below FIG. 416. lever back and forth. This lever engages a suitable pawl and ratchet mechanism so arranged that when the electro-magnetic scale is not in balance, it releases one of two pawls which then catches the teeth of a wheel, and causes it to revolve in one direction or another, by the aid of the pendulum motion of the lever, carrying the pawls, at the same time the shaft of ARC FURNACES IN GENERAL 105 this latter wheel carries the switch, which operates the driving motor. The electro-magnetic scales, which bring about the desired regulation, are built for either direct or alternating current. It operates as a volt, ampere, watt or ohm meter and is pro- vided with a regulating resistance, which allows the operating conditions of the furnace to be changed at will. The double- FIG. 42. throw switch which controls the driving motor is either single or double pole. The current is broken between an adjustable copper piece and a block of carbon of generous dimensions so as to equalize the burning away of the contacts or to lengthen the time of contact If several furnaces are to be automatically regulated, only one driving motor is required for all regulators. The apparatus are then mounted on a switchboard, which also carries the con- trol instruments, such as volt and ammeters. IOC ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The regulators are also provided with a manually operated switch, which cuts out the automatic regulation, so that hand regulation may be resorted to. The motor drive for the pulleys is kept, however, which is often very desirable when using hand regulation. There is a train cf gears, of which a pinion and FIG. 43. rack either raises or lowers the electrodes. The electrodes may also be suitably set by the aid of cables or chains. The weight of the electrode and its appurtenances may be partly equalized by a suitable counterweight. The new Type B regulator here shown, Figs. 42 and 43, is ARC FURNACES IN GENERAL 107 an improvement over the old type Thury. In the old model many of the parts subject to wear became out of adjustment, and to obviate this, all parts which, when worn, throw the machine out of adjustment, have been eliminated, thereby making the new regulator better than the older type. Some of the more important changes are as follows: The reciprocating motion is now obtained by means of a set of spiral gears operating a crank pin and block, which latter slides between two guides connected to the rocking lever. This principle of motion brings the movement in a direct vertical line so that wear, if any, is equally divided and the dogs will always engage in the ratchet wheel. The entire movement is enclosed in a gear box and runs in oil, so that wear is reduced to a minimum. The gears have a ratio of about 2^: i and the driving shafts of each regulator are coupled together by means of a flexible coupling. This shaft replaces the countershaft and pulleys of the old type. The carbon contacts have been replaced by copper contact fingers, so arranged as to have a good wiping contact, each contact being provided with blowout coils. The neutralizing arm is now made straight and provided with pivot screws to take up wear. The knife which engages the pawls is so arranged that coarse or fine regulation can be obtained with very little trouble. All parts requiring adjustment have been made easy of access. The whole apparatus is built upon a panel and units can be added as desired. The units are enclosed in a fire-proof steel case and this is mounted on a pipe structure which also carries the rheostats and controllers. The foregoing has briefly discussed the more or less common phenomena and appliances of arc furnaces, and hereafter some of the various designs of arc furnaces will be gone into. The most important things of arc heating may again be briefly stated here. In all arc furnaces the heating of the bath is brought about practically exclusively by the arc itself. There are always tem- peratures of about 3500 C., occasioned by the arc. Even with a moderate heating this temperature cannot be avoided. 108 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Borchers, in his 1908 address before the "Verein deutscher Eisenhiittenleute," said about this: "In arc furnaces there may be many arcs, the arcs may also be brought in more or less great distances from the bath, in order to bring this to a tem- perature of less than 3500 C., but 3500 C. is always generated at some restricted places, and we must operate downwards from this temperature." IT-S.P.Switch TR-Furnace Operating Mech MT-MotorOpera Furnace PT Power Trans former R- Regulator S- Solenoid CR-Auto Device Contacts IG-Main D.P.Switch CG -Fuses RP-Motor Rheostat C- Controller R A- Regulator Rheostat F Electric Furnace I M-D.P.Motor Switch CM -Motor Fuses MR-Motor Driving Auto Reg-. T-Current Transformer A Ammeter FIG. 430. Connections of Thury regulating system. The automatic electrode regulation brings with it decided advantages, all of them tending to reduce the cost of opera- tion and increase the daily tonnage. This is apparent when it is considered that a well-designed machine operates with greater regularity than a man, needing only a little oil, an occasional adjustment, and the replacement of a small part now and then, in order to have it function properly, whereas a man does not always feel the same on Saturday night as ARC FURNACES IN GENERAL 109 on Monday morning. The kw.-hrs. per ton are less, the refractories last longer, the electrode consumption is less, the overhead charge diminishes as the output increases and all of these advantages for only $2,500 more per furnace. Even though the increased output is only 5 or 10% sometimes it is more the advantage of having automatic electrode control is well recognized, and is found on practically all arc furnaces to-day, excepting some of the smaller ones of }4-ton or less capacity ptr heat. REFERENCES " Laws of Electric Losses in Electric Furnaces." Transactions A. E. S. 1909. P. 265. Hering. " Determinations of the Constants of Materials for Electrode Losses," Transactions A. E. S. 1910. P. 151. Hering. " Design of Furnace Electrodes," Electrical World. June 16, 1910. Hering. CHAPTER VII THE STASSANO FURNACE IT was shown in Chapter VI, that among the better known electric furnaces, the Stassano and Rennerfelt furnaces are the only ones which are exclusively operated by arc heating. We may, therefore, also refer to them as radiating arc furnaces. It was Stassano'- 's original ambition to build an electric blast or shaft furnace. His object was primarily to use profitably the rich ore fields of Italy, where native coal is scarce. His first patent, issued in 1898, in England, is based on the following claim: "The utilization of caloric energy of the voltaic arc for primary determining the reduction of oxide of iron and the metals to be combined therewith and afterwards melting the metallic masses reduced, for the purpose of obtaining in a fluid state the product desired, all substantially as set forth." The furnace which Stassano suggested for this trial is shown by Fig. 44 in plan and vertical cross-section. Without describ- ing the first design of this furnace at length, it may be briefly said, that Stassano laid great stress on the point that no air was permitted to enter the furnace. With a furnace of this kind Stassano made his first tests in Rome. With 1800 amperes at 50 volts he succeeded in producing 30 Kg. (66 Ibs.), of metal in one hour. As a result of these trials, a furnace plant for the direct reduction of iron ores was erected at Darfo, in Lombardy, Italy. Despite several changes in the construction of his furnace, Stassano, though keeping his method of heating, was not able to give any permanent life to his electric shaft furnace. When the Canadian Commission made their observation trip in 1904 the installation at Darfo was no longer in existence. In the meantime, Stassano had forsaken the original design of the shaft-like construction, and instead built a hearth furnace 110 THE STASSANO FURNACE 111 with an inclined bottom as shown in Figs. 45 and 46. This furnace in which the ore was charged underneath the arcs, in- FIG. 44. FIG. 45. stead of at the top, as in his shaft-like furnace, was intended for both the reduction of iron ores to pig iron, and the refining of 112 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY pig iron to steel. As the figure shows, the furnace was meant to have three pairs of electrodes, which could all be used at once, or singly, for striking the arc, so that the temperature of the furnace could be regulated. But even with this suggestion for a pure arc furnace Stassano FIG. 47. could not achieve success. He was however able to make a new furnace installation at Turin, Italy, in which he first used a rotating furnace. This furnace was patented in all industrial countries, and dated about the year 1902. As this furnace is in use to some extent today, it will be discussed in detail, showing as it does the first known furnace with purely arc heating. THE STASSANO FURNACE 113 Figs. 47 and 48 show the furnace in vertical and horizontal cross-section. It is very evident from the claim of Stassano's patent that he laid particular stress on the motion of the molten metal in the furnace. As the drawings show (Figs. 47 and 48), the rotary arrange- ment of the furnace necessitates a vertical cylinder. The shell of the furnace is constructed of sheet iron, and is connected at the lower part, near the bottom, with a strong ring-shaped carrier, FIG. 48. which in turn rests on rollers. The motion is usually transmitted by gears driven by an electric motor. At the middle of the furnace bottom, axial to the direction of the furnace, we find the current and water supply. The current is brought in by means of brushes and slip rings, such as are found on any poly- phase motor. The water cooling, which is brought from the fixed to the movable part by suitable means, is needed for two purposes with this furnace. First, it serves as cooling water for the electrodes, and again as the water under pressure for the 114 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY electrode regulation. The figures show that the furnace hearth is covered with a double sort of roof, which is not readily re- movable with this type of furnace. This arrangement allows the heat protecting qualities of the brickwork to be utilized to a great extent, and, as a matter of fact, this method is said to give an extraordinary heat insulation, which can even be bettered by inserting layers of lime or sand. Fig. 47 shows an outlet in the upper part of the melting chamber, allowing a free escape of the gases which are generated during the reaction. This outlet pipe is surrounded with a sand filled covering, into which it dips at its lower end. This pipe does not take part in rotation of the furnace, but is kept in place by suitable means. The gas removing system again betrays the ambition of Stassano to smelt ore, and serves to protect the furnace com- pletely from the entrance of outside air. As a matter of fact, however, this furnace did not give satisfactory results for smelting ores directly. On this account this furnace is today used only for the working up of scrap or for refining hot charges. In such cases, therefore, this gas flue falls away, as in the in- stallation of duplicate Stassano furnaces at the Bonner Maschin- enfabrik, Bonn, Germany. The hearth here has also been given a hexagonal shape, whereas Fig. 48 still shows the round form as used by Stassano. The bottom of the furnace consists of Magnesite brick, 1 as does also the double form of furnace roof. The insulating layers of furnace refractories are partly comprised of tamped in material. The furnace is provided with a door for watching the metallurgical work, for charging the metal, adding the slagging materials and rabbling it off, for taking samples, etc. Besides this the furnace bottom is supplied with a tap, through which the finished material flows. The most essential and most important furnace part is, of course, the arrangement of the electrodes. As the furnace may be built as well for single phase as for three phase, it would have 1 According to Stahl und Risen, page 1066, 1910, the side walls and bottom are said to have lately consisted of tamped in dolomite. THE STASSANO FURNACE 115 two or three electrodes, as the case may be. These pierce the furnace walls as is plainly shown in Figs. 47 and 48 and form an arc or arcs in the middle of the furnace which heat the bath. Stassano laid great stress on the design of bringing the elec- trodes through the furnace. The electrodes enter the furnace by first piercing double walled cylindrical chambers. There is a circulation of water in the space surrounded by both walls, in order to keep the temperature of the outer electrode portion down. There is a regulating cylinder over each cooling cylinder, the former aiding the setting of the electrodes to any desired point. The piston-rod is connected at its outer end by means of a sliding guide rod with the one end of another rod, which carries the electrode itself at the other end, which latter end is in the cooling cylinder. In order to better show this arrange- FIG. 49. ment, the whole design of this electrode regulating apparatus is shown in Fig. 49 on a larger scale. The regulating of the electrodes is accomplished without any automatic regulating apparatus, but is accomplished manu- ally by the aid of the hydraulic cylinder. Any common water pressure of 4 or 5 atmospheres (60 or 75 Ibs.) can be used, so that no special water pumps are needed for the electrode regula- tion. The current carrying parts are naturally easily and well insulated electrically from the furnace shell, as short circuits would otherwise occur through the furnace walls. Stassano did not look with favor upon any automatic regulating appa- ratus for his electrodes, and Osann who studied the operation of the Stassano furnace in detail gave the following reasons in a report in Stahl und Eisen, 1908: "An automatic regulating 116 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY arrangement would be complicated in any event and would not be advisable if for no other reason than this alone because the electrodes are withdrawn while charging; besides this, an elec- trode breaking off now and then is not precluded, and this frag- ment must be removed quickly. This is simply and quickly accomplished by calling to the man who watches the three ammeters, and operates the three corresponding levers which control the hydraulic cylinders for the electrodes. The electrodes can be used up until the remaining stump only protrudes .1 m. (4 inches). Then they are changed, and this change takes only from 3 to 5 minutes, all of which, I have personally assured myself." We now come to the behavior of the furnace during its operation. As already mentioned, the electrodes are withdrawn when the furnace is being charged. When the furnace is about two-thirds charged, the electrodes are brought together, to again form arcs and are then regulated by watching the needles of the ammeters. The charging of the furnace with scrap takes about 15 minutes for a i-ton furnace, and the setting of the electrodes thereafter, takes about two minutes. As soon as the first scrap is melted down, the remainder is charged on top of it, but this time without withdrawing the electrodes, i.e., without any interruption of the current taking place and working with the utmost speed, so as to avoid all radiating losses. The slag- forming materials are charged in the usual way, and the dephos- phorizing slag is likewise removed after the dephosphorizing period is over, similar to the practise with any other electric furnace. In order to easily remove the slag, the furnace is turned far enough so that it may be conveniently removed through the door. This is possible as the furnace axis has a defi- nite angle of about 7 from the vertical, so that the door assumes different positions toward the bath surface during the turning. After this general characterization of the Stassano furnace, we turn to one of its definite examples, viz.: the duplicate furnaces at Bonn of i-ton size, being one of the 1910 Stassano furnace installations. These i-ton furnaces of 2 50- HP are built for three-phase THE STASSANO FURNACE 117 current; no volts is needed to operate them. The current is supplied from a distant central station at an incoming voltage of 5200. This voltage can, of course, not be used directly in the Stassano furnace, and is consequently transformed in a separate transformer, removed from the furnace, and stepped down to the aforesaid no volts. During the normal operating condition, the furnace takes from 1000 to noo amperes at 105 to no volts, and this current is held as steady as possible throughout the en- tire operation. The Stassano furnace having a very good power factor, (as high as .9 to .95 per cent.,) the energy consumption for this i-ton three-phase furnace is 1.73 X noo X no X .95 = 198.86 Kw., or say, 200 Kw. It is necessary to have a man watch the electrical conditions. He regulates the arcing distances of the electrodes, by means of the levers controlling the hydraulic cylinders, and watches the ammeters, one of which is in each phase. The rotating motion of this Stassano furnace in Bonn is transmitted by means of a 5 -HP motor to a tight and loose pulley, connected by a shaft to gears, one of which is a part of the furnace. The electrode diameters of all Stassano furnaces are kept down as much as possi- ble, so that the work is carried on with comparatively high cur- rent densities. In furnaces up to 500 HP, electrode diameters of 80 mm. (3.2 inches) are used. According to an article by Cous- sergues in the Revue de Metallurgie, this diameter is also used in larger furnaces up to 1000 HP. In this case, however, the electrodes are doubled in number. Accordingly, for the 25O-HP furnace at Bonn, for instance, which takes noo amperes with its 80 mm. (3.2 in.) diameter electrodes, whose cross-section is 5024 square mm. (7.78 sq. in.) corresponding to iioo /noo 141 amps.\ = .22 amperes per square millimetre (-^ = per sq in J or 22 amperes per square centimetre. With a 5oo-HP furnace having the same electrode cross- section and about twice the current, the current density would rise to 118 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY 2200 /22oo 282. amps.N = .44 ampere per square millimetre^ - per sq in j or 44 amperes per square centimetre. This, therefore, gives current densities which still substanti- ally exceed those given by Hansen, as mentioned on page 91, even though these values had to be designated as being high enough. We have also then with Stassano furnaces to figure with a substantial heat generation in the electrodes. A short example may show this. New electrodes for a 250-!!? furnace have a length of 1.5 rnetres (59 inches). As the electrodes wear off during the operation, we may figure with an average length of i metre, (39! inches). If we insert besides this the operating value for the resistance of carbon per cubic centimetre, as given by. Hansen and shown on page 76 to be, Pi = .00183 nms P er cubic centimetre, we obtain the resistance of the electrode as being: r = pi where / and q are in centimetres and square centi- metres respectively. Consequently : r = .00183 ~ ~ = .0036 ohm. * 50-24 The drop in voltage in the electrode of a 250-!!? furnace hence is e = ir = noo X .0036 = 3.96 say 4 volts. The energy transformed into heat per electrode is consequently A = ie watts = noo X 4 = 4400 watts, or in all 3 X 4400 = 13200 watts. That is, with a total energy absorption of 200 kw. for the furnace, there is 6.5 per cent, lost through Joule losses (? r) in the electrodes alone. Besides the transformation of electrical energy into heat in the electrodes as just described, several interesting phenomena will be found in the Stassano furnace as shown below. First regarding the length of the arc, with Stassano furnaces with voltages of no up to a maximum of 150 volts, this distance THE STASSANO FURNACE 119 at first is about 10 cm. (4 inches) from electrode to electrode. During the run, however, the arc distance increases up to a length of 30 cm. (about 12 inches). This considerable lengthening of the arc is partly accounted for on the one hand by the high temperature of the furnace atmosphere, and on the other hand through the gasification of the electrode ends caused by the arcs between them. It is to be noticed that the arc sags toward the bath. This phenomenon can only be regarded as favorable to the heatiLg of the metal bath. We, therefore, find with the Stassano furnace, an increasing lengthening of the arc, as the temperature of the furnace atmos- phere increases. The risk must, therefore, be run of having the arc break and making it anew, when charging the furnace with cold material which cuts the arc. On this account, there- fore, particular care should be exercised when charging the furnace, entirely independent of the horizontal arrangement of the electrode rods. If, notwithstanding this care, the arc should still break, then the rise of the furnace temperature is interrupted until the arc is again established. Still there wou'd be no complete interrup- tion of the energy absorption. Thus, according to Coussergues, when visiting the Stassano furnace at Bonn, the arc was inter- rupted, yet 300 amperes per phase at 120 volts were still taken up by the furnace, which is about one-third of the total energy. This energy absorption with an interrupted arc is only then possible, if the refractories are heated to redness. For the energy absorption is dependent upon small arcs establishing themselves between the refractories and the electrode, which carry the current from the electrode to the magnesite bricks, after the latter have become conductors of the second class, due to the high temperature, and may therefore be regarded as heating resistances between the electrodes (see page 17). Finally attention may be drawn to the capability of Stassano furnaces to be heated up electrically, since the charge is com- pletely independent of the arc formation. In this way the furnace is also kept up to temperature during any shut-downs. This is accomplished by heating up for a quarter of an hour with 120 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY the arc, followed by a current interruption for three-quarters of an hour. The above states the specific characteristics of the Stassano furnace. We now come to the comparison of the Stassano furnace with the ideal electric furnace, for which the requirements were laid down in Chapter V. Without entering into a discussion of the purely metallurgical questions which are gone into in detail in Part II of this book, we may say the following: The first requirement stipulated that the electric furnace was to be capable of being operated with any prevailing alternating current at any voltage and periodicity. This requirement is met by the Stassano furnace better than by any other of the well-known arc furnaces, for Stassano furnaces are built for single-phase as well as for three-phase current. At the same time any prevailing periodicity may be used. Opposite this, the necessity of transforming the voltage to that required by the furnace only plays a secondary part, for this transforming takes place in comparatively inexpensive stationary transformers, which hardly call forth any particular vigilance, considering their great operating safety. The second requirement, viz.: the avoidance of sudden power fluctuations, is not fulfilled so well by the Stassano furnace as by other furnaces of the radiating arc type, especially when melting down cold scrap. There are some interruptions during the charging period, as already mentioned, and though the sustaining of the arc is in no way influenced by the melting process, yet, as the arc has no inherent characteristic tending toward stability, but, as experience has shown, has current surges almost con- stantly from electrode to electrode, so that the attendant regu- lating the electrode levers, even with constant attention, makes 20 to 30 regulations per minute. Still Stassano furnaces are more often connected directly to transformers only, although there have been cases where flywheel generators had to be installed to absorb part of the violent power fluctuations. We now come to the third point in which an easy regulation of the current is demanded. This requirement may also be re- garded as being fulfilled, as voltage regulation simultaneously THE STASSANO FURNACE " 121 causes a regulation of the energy supplied to the furnace, which is entirely independent of such energy regulation which is pro- vided by different settings of the electrodes. It was mentioned on page 115 that Stassano avoided every automatic regulation of the electrodes with his furnaces, which would still offer several advantages. These reasons are referred to again at this time. The requirement under 4, viz.: a high electrical efficiency, does not seem to be so completely fulfilled. We have already seen that the high current densities in the electrodes lead to im- portant heat losses, and it does not seem therefore that it is possible to avoid considerable losses. This is even accentuated by the intensive water cooling of the electrodes. There are also heat losses due to the arc not being sufficiently near the bath during the different periods of the melting, the arc also being too far away when making less than a full charge; thus this heat goes to the side walls, but particularly to the roof. That this loss is not inconsiderable is evidenced by the short life of the roof, one week, even with the best operators and refractories other than carborundum. Besides these losses there are the trans- former losses, for changing the voltage to the desired amount, for which about 3 per cent, of the total energy may be allowed. The fifth point, viz.: the tilting arrangement, which Stassano replaced with a turning one, no doubt gives his furnace certain advantages; still, compared to the tilting device, his solution can hardly be regarded as a particularly happy one. The turning or rotating structure requires a really complicated mechanism. As a proof of this it is only necessary to refer to the water supply for the electrode regulation and to the electrode cooling. En- tirely aside from this, it hardly seems advantageous to have a tapping hole, instead of pouring over the lip, when teeming, especially when heats follow each other quickly, as is usually the case when treating hot metal. Even though the requirement of an easily surveyed hearth seems to be completely fulfilled, it is yet to be observed that the almost horizontal arrangement of the electrodes makes the ful- filment of the seventh requirement so much harder. For the breakable electrodes with their comparatively small cross- 122 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY sections are liable to crack off when roughly handled, so that the metallurgical operations in the furnace entail great attention and not a little dexterity. Besides this, the Stassano furnace would have the advantage of influencing the charge the least with its arc heating, in case electrode breakages could be avoided with certainty, as the carbon vapor from the electrodes is not directly against the molten metal. Relative to the avoidance of every under or over heating of the metal, it must be said that the influence of the arc heating as employed by Stassano, i.e., by use of the radiation, is the mildest way in which arc heating can be used at all, as the direct influence of a heating agency of 3500 C., on the metal, is avoided. Without discussing the purely metallurgical demands, the fulfilment or non-fulfilment of which can be readily seen by the construction of the furnace, we find that the requirement of a sufficient but not too strong a circulation of the bath is fulfilled by the rotary arrangement of the furnace. No other mechanical cir- culation appears in the Stassano furnace as it is built today, and it seems, therefore, that if any security is desired for a complete uniformity of the material in its several layers, it is not possible to dispense with the mechanical bath circulation. And these necessary mechanisms must always be designated as being very complicated (for any such metallurgical apparatus as this), no matter how ingeniously the design may have been carried out. Besides the many-sided applications of this furnace, it would seem desirable if they could be built of any possible size. The proof of this is, however, yet to be established. For even though Stassano furnaces of 5-ton size were operated by Stassano him- self, at the plant in his charge in Turin, the plant unfortunately has been shut down. It may, therefore, at present only be regarded as proven that the Stassano furnace of 600 to 1000 Kg. (5/8 to i ton), as it is operated at Bonn for melting up scrap for steel castings, succeeded in giving good results. The furnace does not seem suitable for larger sizes, as the sensitive devices permissible, at any rate, with small furnaces, while easy to watch, are hardly applicable with large furnaces. The high current density, with which 5-ton furnaces are to be operated, THE STASSANO FURNACE 123 also seems unadaptable, while with still larger furnaces where the doubling of the electrode number would be encountered, difficulties could be expected from the simultaneous manual regulation of six electrodes. Larger furnaces would have longer and consequently more breakable electrodes, which would otherwise need much room 124 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY during the furnace's rotation. Finally, the easy working of the furnace becomes difficult with the six horizontal electrodes over- topping the bath. All these reasons make it appear that the Stassano furnace in its present customary form is only useful for small capacities. The requirement of a good thermal efficiency is acceptably fulfilled. For even though the Stassano is the electric furnace where the metal is heated most indirectly, but where the atmos- phere directly above the bath is heated the strongest, and though it is not possible to avoid a considerable heat loss when the furnace door is opened, still it is well to note that the heat in- sulation with the Stassano furnace is extraordinarily well carried out, and that consequently it is possible to attain satisfactory power consumption figures for melting a ton of steel. And these vary between 800 and 1000 Kw. hours per ton of steel for melting cold stock for making steel castings. According to Osann (Stahl und Risen, 1908, p. 660), we find that he begins with a cost of 62 cents for electrodes at the Bonn furnace and $2.75 for refractories per ton of steel, so that we cannot speak here of exactly low refractory costs, which could, however, be considerably reduced by using dolomite bottoms and side walls (Stahl und Eisen, 1910, p. 1060). The installation costs for a i-ton furnace are given by Osann, inclusive of switchboard and foundation, at $8,750. This does not, however, say that the cost of the necessary transformer is included in this price. In Bonn the voltage is stepped down from 5200 to no volts. On the other hand, it may be said that at Bonn they were enabled to connect to an existing central station, so that in case such connection is not possible, the installation cost would be increased by an amount equal to the cost of an isolated plant (250 HP for a i-ton furnace). Fig. 50 shows a Stassano furnace from which the general arrangement is evident. Regarding the sale which these furnaces have had, reference is given to the list in Chapter XV. The giving of licenses for Stassano furnaces is made by the Banner Maschinenfabrik und Eisengieszerei Fr. Mb'nkemoller & Co., Bonn on the Rhine, Germany. CHAPTER VIII THE HEROULT FURNACE HEROULT had already earned great merit in the development of electro-metallurgy, on account of his electric furnace for the production of aluminum. He was the first to discover how to build an arc furnace for refining iron, having vertical electrodes pointing directly at the bath. Before this these furnaces had the objection, that the iron bath greedily absorbed the carbon from the immersed electrodes. On July 4, 1900, Heroult made the suggestion (see German patent No. 139904), that to avoid the absorption of carbon by the metal bath, the slag used to refine the metal should be inserted between the bath and the electrode. According to the patent description the electrodes are to be so far separated from each other and are to dip so little into the slag, that, on the one hand, the resistance between the electrodes within the layer of slag, shall be great enough to force the current from the one electrode through the slag lying directly beneath it to the metal, and from the metal again through the same layer of slag to the other electrode, and that there shall be otherwise no connection between either electrode, and the metal. Further, according to the patent description, the striking of arcs between the electrodes and the metal bath into which the electrodes project, is not precluded, or is it necessary. Regulating the distance between the electrodes and the metal bath, however, is the important part. This must be accomplished in such a way that the slag layer between the electrodes and the metal bath remains hotter and more conductive during the entire refining period, than the layer of slag between the electrodes, because only in this way will the current take the path as prescribed above. 125 120 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY After this general characterization of the Heroult furnace, and before entering into details regarding its construction and operation, we will give a short survey of the development of this furnace. According to the Electrochemical and Metallurgical Industry, 1909, p. 261, Heroult, in his first efforts in building an electric furnace, leaned narrowly toward his type of aluminum furnace. FIG. 51. Five-ton Electric Furnace. In this furnace, as is well known, one pole consists of a hanging carbon electrode, while the other pole was made by the furnace hearth itself. For this purpose the hearth was made of carbon. When it was necessary, however, to obtain a material with the lowest possible carbon content, this style of furnace could no longer be used, as the carbon of the hearth bottom was greedily absorbed by the molten metal. On that account Heroult next made tests with a furnace for the production of low carbon ferro chromium. The bottom of this furnace consisted of chromite bricks in the middle of which a carbon block was inserted which then acted as the bottom THE HEROULT FURNACE 127 electrode. With this method Heroult hoped that a part of the carbon block would be absorbed by the molten metal and that the molten mass would continue to force its way down absorbing carbon as it went, until the exterior radiation of the molten metal would cause it to freeze on the carbon block. Heroult hoped to keep this condition constant, so that there would be an interposition of the frozen metal between the bottom carbon FIG. 52. electrode and the bath, which would at the same time prevent any carbon absorption by the bath. But the tests as carried out did not fulfil his hopes, and so after further trials there was produced the Heroult furnace as we know it today. This has been characteristically shown by the above examples, taken as they are, first of all, from the patent records. Furnaces of this kind were first put in trial in Froges and La Praz, France. The first Heroult furnace in Germany was installed by the firm of Richard Lindenberg of Remscheid in 1905, and put in operation in February, 1906. The first Heroult furnace in the United States was installed by the Halcomb Steel Co., of Syracuse, N. Y. Since then the furnace has come into extensive 128 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY use, thanks to its simple design, and thanks to a thorough knowledge of the metallurgical operations, which have been thoroughly investigated at Remscheid, and elsewhere. Coming now to the furnace itself, we may say the following: Of all the arc furnaces the Heroult furnace resembles most nearly a tilting open hearth furnace. It consists of a steel plate shell of nearly rectangular but sometimes circular form, which has a rounded bottom. Fastened on this are pinions which permit the furnace to roll forward on a rack or, in later designs of the smaller size, the furnace is tilted bodily on supports hinged near the FIG. 53. spout. The furnace is tilted either by means of "a hydraulic cylinder or an electric motor, which latter method permits the control of the pouring very accurately. In the case of both the rolling and the tilting furnaces, it is now possible to pour directly from the furnace into hand ladles. The whole design of the furnace may be seen by consulting the Figs. 51 to 53 inclusive. The lining of the furnace consists of fire-bricks, which are laid directly against the steel plate shell, and on which dolomite is tamped, with the exception that in the United States mag- nesite is generally used instead of dolomite when it can be had- THE HEROULT FURNACE 129 The roof is removable and consists of a steel plate frame lined with fire-brick, the former also having convenient screw eyes so that the entire roof may be readily transported. The hearth may be easily inspected and operated during the charging period as the furnace has from 2 to 4 doors, according to its size, one being over the spout. As the metal bath is needed to conduct the electric current, the current is shut off while charging and naturally while it is empty, which allows the refractories to cool off somewhat. The arched roof of the furnace is pierced by two or three electrodes. Copper cooling chambers are placed at the piercing points, which keep the carbon electrodes outside of the furnace at permissible temperature limits (as discussed in Chapter VI), and simultaneously cool the brickwork at these points. Each electrode hangs from a right-angled support, which is movable in a vertical direction at the furnace. This support, therefore, carries a rack, which is moved by a motor-driven pinion. The use of these small motors in this design permits a mechanical regulation of the electrode positions. In Remscheid these small regulating motors are of the single phase zoo-volt type. These motors operate automatically or by hand, according to whether a higher or a lower position of the electrodes is called for. Nat- urally the electrode clamps are insulated in an improved manner from the furnace casing. Regarding the development of the automatic regulating apparatus of the Thury system, this was described at length in Chapter VI, pages 102-104. The electro-magnetic scales mentioned there are connected as voltmeters to the Heroult furnace in the earlier regulators, as shown by dotted lines of Fig. 54. These have now been changed to ammeters, so that current regulation is obtained and the operation of the furnace is now accomplished with more ease. One regulator is pro- vided for each electrode. With the earlier designs, the voltmeter as shown in Fig. 54 is designated by m in whose place we can imagine the electro-magnetic scales. Two or three scales are provided as each electrode is regulated separately. The scales are influenced by the voltage which lies between the head of the 130 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY electrode and the bath, which receives -the main current. In order to obtain this voltage, an iron rod is imbedded in the furnace bottom, which in turn is connected to the remaining terminals of the magnetic scales. These scales are set so that a difference of 2 volts from the normal will start the regulator FIG. 54. and keep it as near constant as possible. Lately, as the majority of the furnaces installed are fed from transformers from larger power stations, where the voltage is fairly constant, it is the current that requires regulating as is shown by diagram of Fig. 430 of Chapter VI. The design of the furnace is- such that either hot or cold charges may be treated. With cold charges, however, the current fluctuations are still heavy until the whole charge is melted down, the reason being that it is much harder to melt down a stone cold charge in a Heroult arc furnace and maintain the arc than it is to treat hot metal. This is not only because this type of arc furnace always operates better at a higher temper- ature under the influence of which carbon evaporates, but also as the continuity of the current is disturbed with cold charging as the various pieces of scrap make varying and imperfect con- tacts here and there. Furthermore, the appearance associated with the so-called over-regulation causes the electrodes to become unruly when for instance the heavy current fluctuations THE HEROULT FURNACE 131 occur at the arc, rupturing and re-establishing itself. During the melting of a cold charge, continuous fluctuations therefore follow and these continue until the charge has become molten. FIG. 55. During the time of these heavy current fluctuations, that is, while melting the metal, the automatic regulation is sometimes replaced by hand regulation. The series connection of the carbon electrodes has a negative influence on the electrical condi- 800 Kw. 000 3 A.M. 2.A.M. FIG. 56. tions even with a perfectly fluid metal. To illustrate this Fig. 55 shows several current curves as they were recorded by an arc furnace with series connected electrodes. These curves 132 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY were made in 1909. Other curves are shown in Figs. 56 and 57, which show the power fluctuations during the melting and dephosphorizing period and also the finishing period on a 6- gross-ton furnace, operating at about 900 Kw. More lately these furnaces operate with about 1200 Kw. Fig. 57 shows the much steadier power consumption when refining hot metal. At present, with very careful charging of the furnace, it is pos- sible to throw on the automatic regulators simultaneous with throwing on the current. This reduces the current fluctuations somewhat as shown by the difference in the power fluctuations as evidenced by Figs. 55 and 56, the latter of which, also 57, were made in 1915. In order that no misunderstanding may arise regarding the heating method of the Heroult furnace, it is well to especially mention at this time, that Heroult had soon to realize that he must employ an arc to make his furnace operate even though it deviated from the furnace operation of his patent description. THE HEROULT FURNACE 133 In this it was not precluded nor was it necessary that arcs should be struck between the electrode and the bath. Hence to-day the- furnace voltages are chosen so high, that the electrodes are set at about 45 mm. (1.8 in.), above the steel bath. With this setting, it is possible to obviate a carburization of the bath when the slag is interposed, and this is solely caused by the heating action of the arc, (having a length as mentioned above,) heating the metal to the desired temperature. If the electrodes in the Heroult furnace were dipped into the slag, so that no arc exists, then the furnace would be of the pure resistance variety. Should we now calculate the resistance conditions in such a circuit, we shall immediately find, that, under these conditions, practically the whole energy would be changed into heat in the electrodes, without heating the bath materially at all. This is apparent when we compare the resistances of the two carbon or graphite electrodes connected in series, with their comparatively small cross-section and very great length and their high specific resistance, with the resistance of the slag layer and the bath with their very large cross-sections and very short lengths and the very low specific resistances (at least as far as the bath is concerned). These conditions have been clearly recognized by the representatives of the Heroult furnaces. We quote from Prof. Eichhoff of Charlottenburg, the technical adviser of Lichtenberg of Remscheid, his article appearing in Stahl und Eisen, 1909, p. 843, as follows: "It is impossible to heat an arc furnace for steel, by utilizing the heat generated by the resistance of the thin slag layer or the large cross-section of the bath. These resistances only furnish a few per cent, of the heat necessary in the furnace," and again, "Obtaining heat by the rising temperature of the slag with its decreasing resistance, or by utilizing the resistance of the bath, has never been achieved, simply because the slag layer is too thin, and the cross-section of the steel bath too large. Such a view therefore is a fable, which I oppose from the start." This description should suffice to give a perfectly clear picture of the workings of a Heroult furnace, in which then practically the entire heating is obtained from the heat of the arc. 134 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY If we return for the moment to the furnace design, we observe the following: The Heroult furnaces were first built for single- phase currents for from 25 to 33 cycles. To-day they all operate from polyphase, usually 3-phase circuit at 25, 50, and 60 cycles. The Heroult furnace at La Praz operates with 33-cycle current at no volts, single phase. The charge is about 2^/2 tons. At this rate the furnace consumes about 4000 amperes. Two of the i5-ton Heroult furnaces 1 and one 20- ton are operating at the works of the Illinois Steel Co., South Chicago, and another at the plant of the United Steel Co., Canton, Ohio. The hearth of this furnace is circular, over which three electrodes are arranged at the corners of an equilateral triangle. One of these furnaces is operated by 3-phase, 25-cycle current, delta connection, at i GO volts. Under these conditions, the current per phase rises to 12000 amperes. As in other furnaces, the electrodes are auto- matically regulated. The current is taken from a high-tension circuit and stepped down by means of three 750 Kw. transformers in the installation at South Chicago and by means of three 1000 Kw. transformers with the Canton furnace. Accordingly the former 1 5-ton furnace takes 12000 X 100 X 1.73 = 2076 K.V.A. and as the power factor is between .8 and .9, it consumes actually 2076 X .85 = 1760, say 1800 Kw. The difficulty heretofore in building large arc furnaces has lain in the inability of obtaining a large electrode that was durable in service and not having too great electrical or thermal losses. This feature will be alluded to later on. In order to give an idea of the dimensions of the electrodes in Heroult furnaces, it may be well to mention that the electrodes carrying 4000 amperes in the single-phase furnace operating at La Praz have a cross-section of 360 X 360 mm. = 129,600 sq. mm. (14.1 X 14.1 inches = 200 sq. in.), and a length of 1.70 1 In 1912, the Metallurgical and Chemical Engineering reports that a 25- ton Heroult furnace was put in operation at the Gewerkschaft "Deutscher Kaiser," Bruckhausen, Germany. THE H^ROULT FURNACE 135 metres (67 inches). They consequently operate at a current density of 129,600 / 200 ' 40QO =3 2 -4 sq, mm. per amp. \^^^ = .05 sq. in. per amp. 4000 . \ or = 20 amps, per sq. in. 1 If we take into account that as the height of the furnace roof over the bath is 70 cm. (27^ inches), and the clamping length at the top of the electrode is 35 cm. (13^ inches), we find that there is a certain length of usable electrode, which with a total length of 1.75 metres (69 inches), makes the usable portion about 70 cm. (2 7/ / 2 inches). The wwusable portion of the electrode is con- sequently about i metre (39 inches). If we now calculate the electrode voltage losses in accordance with the figures just mentioned, similar to the electrode losses determined for the Stassano furnace, we obtain the following: Assume specific resistance of carbon in operative condition = pi = .00183 ohms per cubic centimetre then as e = i X r, where r = p\ , and / and q are respectively in centimetres and square centimetres, we obtain e = i X PI X = 4000 X .00183 X T = -565 volts. For both electrodes, then the drop is 1.13 volts, because they are connected in series. The result as figured, however, cannot be considered as correct, because the change in the specific resistance with increasing cross-section was not taken into consideration. In the calcula- tions, so far, we kept the probably correct value of .00183 ohm per cubic centimetre, which is in keeping for an electrode of 80 mm. diameter, whereas the electrode of the Heroult furnace in question corresponds to a square having 360 mm. to a side. If this had been taken into consideration, then the value of pi = .014, (when following the values given on pages 93-94), should have been chosen for the electrode condition in its cold state. Should, on the other hand, the values of Hansen be taken, where the 136 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY resistance falls to about 40% in operation, compared to the cold resistance, then the determination should have been figured with PI = .0056 ohm per cubic centimetre. Figuring more correctly then with this value, we obtain, e = 4000 X .0056 X 6 = 1.73 volts per electrode. The drop for both electrodes is consequently 2 X 1.73 = 3.5 volts. This gives a loss three times as high as in the first calculation. This example clearly shows of what importance it is to accurately know the different constants for this material for the different cross-sections. For it is only with these that the determinations of the conditions arising in the electrodes can be figured. Of course it is not to be supposed that this last value gives a final idea of the total losses in the electrodes, because in the calculations just made only the purely electrical losses were judged. This, too, with the rather hazardous assumption that the constant taken for the specific resistance of the carbon elec- trode is correct. Meanwhile, the radiation heat losses have been entirely disregarded. It is undoubted, that the latter raises the total electrode losses considerably, and even though deter- minations regarding radiation heat losses are hardly possible, still it may be said with some certainty, from measurements of other arc furnaces, that the total electrode losses generally, as well as in the Heroult furnace under discussion, will not be below 7 to 10%. These losses do not only appear of this value in the compara- tively small furnaces, such as have just been discussed, i.e., of the 2- to 3-ton size, but especially in the larger sizes. With the size of the furnaces and the increasing cross-sections of the electrodes, the difficulty also grows of obtaining favorable material constants, which is a thing entirely apart from the difficulties to be surmounted of procuring large electrodes of considerable durability. Chapter VI brings out these details. It may be said further regarding the practical operation of the 15 and 25-ton, 3-phast c urnaces at South Chicago, that it has not been found THE H^ROULT FURNACE 137 possible to increase the proportions of the electrodes at will, but lately it has been found possible to obtain an electrode of satisfactory proportions and better material. As has been remarked this 1 5-ton furnace operates with about 12000 amperes per electrode. The conduction of such currents naturally neces- sitates very considerable electrode cross-sections. It was at first tried to produce these electrodes in single large blocks. Ac- cording to the Electrochemical and Metallurgical Engineering, 1909, p. 262, one of these block electrodes had a diameter of 2 ft. (60.9 cm.), by a length of ten ft. (3.048 m.}. The weight of one of these electrodes was about 3200 Ibs. (1451.5 Kg.). The results with these colossal electrodes was hardly satis- factory, as breaks often occurred which disturbed the operation of the charge in a most sensitive way, even though the current density operated with was 28 amperes per square inch, or 4.35 amperes per square centimetre, corresponding to 24 sq. mm. per amp., which is a comparatively high density in spite of the large electrode cross-section. (See Chap. VI, page 82.) On that ac- count they sometimes use the dearer but less troublesome graph- ite electrodes instead of the carbon electrodes. Quoting from the Metallurgical and Chemical Engineering, 1910, p. 179, and following pages, we find that the electrodes as used are made up of Acheson graphite rods, 48 in. long (12 2 cm.), and 8 in. (20.32011.) in diameter. Three such rods are butt-connected to a total length of 144 in. (366 cm.), and three such 144 in. rods are arranged side by side to form a single electrode, consisting (see Fig. 60 a and b) thus of a solid bundle of three rods, each 144 in. (366 cm.) long. The cross-section is therefore 3 X 50.2 = 150.7 sq. in. (3 X 324 = 972 sq. cm.). The consumption of these electrodes is given as averaging 6.6 Ib. (3 kg.), per ton of steel, and this figure is stated to be true for graphite and hot charges. The unavoidable wearing away of the comparatively dear electrodes, naturally causes an increase in the steel conversion costs, which is hardly desired. In the beginning there were additional losses of considerable moment which had to be reck- oned with. These were caused by the unusual lengths of the electrodes in the electrode clamps and the length necessary for the distance between the furnace roof and the slag layer. These 138 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY costs are said to have now been reduced to the irreducible minimum, by using the otherwise worthless stub ends for a new electrode. Figs. 58 and 59 show two possible ways in which the greatest use can be made of the electrodes. Fig. 58 shows the electrode made from shorter pieces with staggered ends held together with graphite screws. This method is also reported to have been used with the 15 -ton furnaces in the United States. Fig. 59 shows a threaded hole in the end of the electrode. On the one hand this scheme enables the conducting clamp to be made of cast copper, as the figure shows, whereas otherwise, should the whole electrode become too short, it can be unfastened at the copper casting, a graphite screw inserted in its place, and a new electrode piece screwed be- tween the too short electrode and the new one. This is also evident from a view at Fig. 59. The latter way of lengthening the electrode is now in general use all over the world. It seems from this that the possible difficulties due to the FIG. 58. FIG. 59. higher resistance at the points of contact are not so great as might be expected from theoretical calculations, especially when the graphite nipple or joint is accurately machined, and when given an extra y& or % turn shortly after being in service. Following this it may be well to relate further details of the operation of the Heroult furnace. If the Heroult furnace is to be heated up after putting in a new lining, or owing to the operation being interrupted by Sunday, it is accomplished by charging the furnace with some coke, which acts as the heating medium and at the same time as the conductor from one electrode to the other, (as long as the heating of the furnace is accomplished electrically). When ready to place the furnace in operation, this coke is raked out and the furnace charged. The current THE HKROULT FURNACE ]39 is then switched on and the automatic regulators thrown in at the same time. In refining molten metal, the automatic elec- trode regulators are thrown in at once. The furnace operation then is decidedly smoother than when melting cold scrap, when hand regulation is sometimes resorted to. As with all other electric furnaces, so also with the Heroult furnace, we find that the power consumption varies greatly with the size of the furnace, with the kind of charge used, and the desired quality of the finished material. A graphic picture of the change of the current consumption varying with the size of the furnace is given by Fig. 60. This data is given by Eichhoff. Here one set of curves represents the conditions for cold and one for hot charges. In the upper set of three curves the lowest one indicates conditions when only one slag is used, the middle curve when two slags are used, and the highest curve when three slags are used, and similarly for the lower set of three curves. In this way the curves show a rising degree of purity in the metal. The table accompanying Fig. 60 gives the quantities directly. Of particular interest are the operating figures which have been achieved with the 1 5-ton furnace. According to the report in the Metallurgical and Chemical Engineering, of 1910, p. 179, ff., the electric furnace is charged with hot metal from the Bessemer converter. On the average here 1 2 charges are made daily, with an average time of i hr. and 15 minutes to 2 hrs. and 15 minutes, the weight of metal averaging from 12 to 14 tons. The average consump- tion of power for this is 200 KW. per ton of steel produced. If we now pass on to the comparison between the Heroult furnace and the ideal furnace, we come to the first demand of an electric furnace, that every existing alternating current can be used. The Heroult furnace fulfils this demand only in part. As every arc furnace needs a certain voltage, the Heroult furnace also demands a specific potential, so that in nearly every instance a stationary transformer becomes a necessity, in which the high pressure of the distant central station is stepped down to the desired 100 to no volts at the furnace. The use of one of these transformers is almost unavoidable with any arc furnace. Other- wise, the Heroult furnace can use any existing alternating 140 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY THE HEROULT FURNACE 141 current of any commercial frequency. In fact, in several recent installations a power factor of 94 or more has been observed and this with 6o-cycle current. No difficulty has been found in the construction of the furnace with three electrodes, in furnaces from the largest to the smallest size. Up to the present time the avoidance of sudden power fluctuations has not been attained when operating Heroult furnaces. At present the Heroult furnace, with its electrodes in series, is credited with having almost the heaviest power fluctu- ations of the better known arc furnaces. However, with the improvement in the automatic regulation and the handling of the furnace, these fluctuations have been reduced very markedly and this without the introduction of reactance coils of any kind. (See Figs. 55, 56, 57.) This, as we have seen, is particularly so when melting down cold stock. The conditions are more favorable as soon as the charge is completely melted, or when only treating hot charges. On the bath of these hearths three electrodes appear which hinder the metallurgical operations somewhat, and lead to greater breakages of the electrodes, than two electrodes alone would, of which it is reported that breakages are rare during operating. In addition the arrangement of three electrodes requires the furnace roof to be pierced three times, which seems so much more dubious, as the flat arched roof is subject to the high temperatures of the arcs, and also to the water cooling around the three electrodes, so that inside of the com- paratively small space of the furnace roof, we have several large differences of temperature arising, which naturally tend to weaken and destroy the roof. Finally it must also be mentioned that three electrodes radiate more heat due to their larger surface than two electrodes do, having the same total cross- section. Furnaces which are operated with three phase current con- sequently only require the installation of stationary transformers, as the central station is usually large enough nowadays to stand the prevailing power fluctuations of arc furnaces of this type. Easy regulation of the power is present in the Heroult furnace, the same as it is in every other electric furnace. 142 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY In judging the electrical efficiency of the furnace, the losses in the transformer are first to be taken into consideration, and then the losses in the carbon or graphite electrodes. In case any rotary transformers have to be used, the considerable losses appearing here have to be added. Their use is rare. In order to give a probable conception of the electrical losses SflHHBfil^^HBL. FlG. 6oa. Heroult 3-phase furnace of 15 tons capacity. Teeming a charge. the efficiency of the transformer may be taken as about 96 to 97%; the electrode losses at about 10%, of which at least 3 to 5% are purely heat losses, and in case rotating transformers have to be used the efficiency of these machines may be taken at about 85%- The further requirements of a tilting furnace, and an easily surveyed and accessible hearth are fully met. It has already been mentioned in which way Heroult knew THE HEROULT FURNACE 143 144 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY how to avoid the undesired reducing action of the electrodes impinging directly on the metal. It is to be noted, however, that this reducing action cannot be altogether avoided, due to the electrodes throwing their carbon vapor stream against the layer of slag, even though the slag layer protects the bath from this action. The prolonged carbonizing action of the arc furnace makes more difficult the oxidizing processes; for instance, during the removal of the phosphorous it cannot be without its influence on the time of treating the charge and the power consumption. When removing the slag it is well to consider that the carbonizing action of the arc remains the same, even though the heating is not interrupted during this period. If we now take up the requirement of the motion of the charge, we find that from reasoning alone, from the standpoint of purely thermal action, it is not present. For, as the arc operates only on the surface of the bath, the hottest parts of the bath are to be looked for here. On account of the electric current, on the other hand, a certain motion of the bath takes place, as this current flows through the electrodes, and a part of the bath which is, to a certain extent, a moving conductor (as the motor action of the electric currents acts as discussed in Chapter III). For, in accordance with the conditions there given, the bath, or the part which is a movable conductor, is pushed to one side, so that the material beneath the electrode is under a certain magnetic pressure, which causes a certain motion in the bath of the Heroult furnace. With all this, it is not correct to assume that the motion caused in the bath of the Heroult furnaces reaches the bottom of the bath. The application of the furnace has a wide scope, due to the fact that it works as well on cold scrap, but with heavier power fluctuations, as on hot metal. It must, however, not be left unsaid that at present the Heroult furnace is the only one that has been built for 25 tons and actually holds 28 tons, which proves the adaptability of this for this size. Up to a certain point naturally the heat losses become proportionately smaller. However, it is to be feared that, for instance, with very large arc furnaces with three electrodes piercing the flat arched roof, THE HEROULT FURNACE 145 that this would cause a frequent roof renewal. This difficulty is being reduced to a very great extent, as in present practice according to size of furnace, whether cold or hot charging, silica roofs with the former method during 1916 lasted respectively 30, 47, and 56 heats in a two-ton furnace, whereas with the larger furnaces and cold charging 200 to 300 heats have been reached before complete renewal is made. In these cases the bottom, as is more usual, was basic. The renewals are more numerous with 3 holes in the roof, than when only pierced with or or two electrodes, other things being equal, owing to the large difference of temperature between the various parts of the covering. This disadvantageous trait remains even though it is con- sidered that the vertical electrodes act with a sort of umbrella action, and in so doing at least keep the most intense heat away from the roof. It is to be noted here that, for instance, the roof of the 1 5-ton furnace at the Illinois Steel Co., has to be changed about every three weeks. According to the Metal- lurgical and Chemical Engineering, a roof such as this costs about $60 with silica brick costing $27 per thousand. The difficulties of very large diameter, 24-26 inches, electrodes have not yet been overcome, as may be judged from the report appearing in the Metallurgical and Chemical Engineering, for 1910, p. i79ff., where the electrode temperature, just where it issues from the furnace was measured and gave 1050 C. It is evident that these electrode temperatures cause a greater con- sumption of the electrodes, so that this may also be looked upon as part of the cause for the consumption of 6.6 Ib. (3 Kg.) of graphite electrodes per ton of steel when charging hot metal. It is also to be noted that it must be possible to change the slag in an electric furnace, as is now done in the open hearth furnaces. The removal of this slag, however, becomes more difficult with the increase in the size of the furnaces, because the slag must be entirely removed. A mere running off of the slag is not sufficient, but a thorough rabbling off is necessary. In taking these conditions into consideration the Electrochemical and Metallurgical Industry, of 1909, p. 262, says in referring to the attainable size of the Heroult furnace: 146 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY "As to the maximum size of furnace which it is now possible to construct, it is the intention to build them up to 30 tons. Very much will depend, however, on the work which has to be accomplished, that is to say, whether one or two slags would be used. In case of one slag, Mr. Turnbull is sure that a 3O-ton furnace is possible, but should two slags be used, owing to the difficulties which might be encountered in raking off the first slag it may be found that a 1 5-ton capacity is nearing the limit. It could certainly be worked quicker than one of a 3O-ton capa- city." Attention is again called here to the influence of the furnace size on the thermal efficiency of Heroult furnaces, and this point is dwelt upon more in detail. Prof. Eichhoff says the following in Stahl und Eisen, 1908, p. 844: "I cannot think of a small furnace that has an efficiency of more than 50%. If the furnaces become larger and larger, then the actual useful absorption of the heat may rise to 70%, for the reason that the furnace surface does not increase in the same ratio as the furnace contents do. As the furnaces become larger the losses gradually decrease, going from 50 to 40, and from 30 to 25%. I can tell you from my own practical experience, that comparing a 3-ton furnace to a i. 5-ton furnace, the effective current increase was only 10%. Hence, the current consump- tion per ton of steel decreases materially. Owing to this fact we are compelled to build larger furnaces, and there is no reason why this cannot be done." Since then there has been built the furnace of 25 tons. For this size the above deductions are correct, however, with the limitations that the furnace efficiency cannot be further increased by further increasing the size of the furnace unit. The efficiency of furnaces of increasing sizes with two electrodes follows the curve of a parabola. However, where three elec- trodes are used, the efficiency will naturally decrease, due to the higher thermal losses, which latter gradually reach the practicable attainable minimum, with the increasing size of furnaces. As Heroult furnaces, however, are built today, these losses will not be less than 25%. It is difficult to calculate definite Heroult furnace installation costs, as these will in all cases be determined largely by local THE HEROULT FURNACE 147 conditions. It is necessary to install a low tension regulating transformer near the furnace, which lowers the higher potential of the central power plant. As the Heroult furnace therefore may be connected to any existing power line, it is not necessary to take into consideration the cost of building a power plant and, in the United States, the cost of installing a 6-ton, three-phase Heroult furnace, with all equipment from the connection with the high tension power line, is about $45,000. In the United States complete units of one and two tons capacity are being sold, ready to set up, for $18,000 and $24,000, in all cases with the usual royalty per ton of metal poured. In all the above calculations, the cost of buildings, etc., are omitted. In closing, the advantages which Heroult himself gave of his furnace, over other arc furnaces, are here set down, especially those opposed to the Girod furnace, which latter is described in the following chapter. The advantages mentioned are taken from the Electrochemical and Metallurgical Industry, for 1909, p. 261: "First The total absence of electrical parts in the furnace proper, it being nothing else but a modified open hearth with the heat introduced above the metal by the electric current in place of gas. This in itself is an important factor as it does away with the bottom pole, considered by Heroult to be always the cause of much trouble in electric furnace work, and allows of any patching necessary to the bottom or side, without inter- fering with the work of the furnace. "Second The heat being introduced by means of two electrodes working in series, the current passing through the bath from one electrode to another and vice versa, necessitates carrying only one-half the current that would be the case should the current flow from one electrode through the bath and then through the bottom of the furnace, if the power is the same in both cases. Thus, all the conductors are reduced to one-half the section required in the other case and the electrodes can perform more efficient work owing to the lesser density of current to be carried." The above-mentioned advantages of the Heroult furnace should be compared to the advantages of the Girod furnace, 148 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY mentioned at the end of the following diaper. Furthermore, the opinion in the first paragraph may be supported. It is correct, of course, that certain advantages accrue by lessening the cross-section of the current carrying conductors. He, how- ever, avoids mentioning that these advantages are only attainable by raising the voltage. The opinion of Heroult that the series connection of the electrodes gives more useful work, is not sub- stantiated in any way. We shall see later on that the total electrode cross-section of the Heroult furnace is not greater than with the Girod furnace, disregarding entirely how incom- prehensible it is that Girod does not also operate with the same current density and the same low current densities as Heroult does. It still remains to be proved that operation with low current densities is an advantage, irrespective of the size of the furnace. One of the 1 5-ton furnaces at South Chicago has been operated with 1 2-inch graphitized carbon electrodes, thus in- creasing the current density four times, compared to some of the earlier methods, when 24-inch round carbon electrodes were used. Lately again, however, as large as 26-inch diameter carbon electrodes have been used in the 20-ton furnace Relative to the use to which the Heroult furnace has been put, reference may be had to the statistics in the closing chapter. Licenses for Heroult furnaces may be obtained in Germany from the Elektrostahl, G. M. b. H., Remscheid, Hasten, and in the United States from the United States Steel Corporation, New York. CHAPTER IX THE GIROD FURNACE THE Girod furnace, as well as the Heroult furnace, deserves the greatest consideration among arc furnaces. Girod originally made ferro alloys in a resistance furnace, in which the heat flow went through the walls, as described in Chapter III. It was in 1906 and 1907 that he turned quite experimentally to the melting of iron. He built a furnace with a capacity of about i to 1^2 tons of a similar type to that used by Heroult, before the latter went over to his electric furnace with series connected electrodes. Where FIG. 61. Heroult did not succeed in obtaining satisfactory results with his furnace, having one pole in the form of a hanging electrode, and the other pole as a bottom electrode, Girod succeeded. Girod' s success has been so great in bringing this furnace to such a fully developed scientific reality, that it was hard to say at first to which of these two contestants, in the arc furnace field, where the metal bath is used to conduct the current, the victory in so far would finally belong. 149 150 ELECTRIC FTTRNACES IN THE IRON AND STEEL INDUSTRY In outward appearances the Girod furnace greatly resembles the Heroult furnace. The furnace casing is made of steel plate and either of the round or rectangular form. This in turn re- ceives a lining of either dolomite or magnesite, making the bath either round or square shaped, as the case may be. The furnace roof is made of silica brick and is removable. The furnace itself is of the tilting variety. Because of this the first furnace at Ugine, France, was provided with trunnions at the side, which allowed the furnace to tilt in its bearings. In the newer design the furnace casing is furnished with a saddle resting in rollers, FIG. 62. as shown in Figs. 61 and 62. The power for the tilting mechan- ism may be of any kind, but is usually an electric -motor. The Girod furnaces are supplied with two doors, one of which serves mainly for the charging and operating of the furnace, while the other is provided with a teeming spout, for the tapping of the furnace. The most interesting part of the Girod furnace is, of course, the arrangement of the electrodes in which centres the whole principle of the furnace. Where in the Heroult furnace the TinC GIROD FURNACE 151 electrodes are of opposite polarity and arranged above the bath, Girod avoids this by placing one pole above and one beneath the bath. When the current strength increases with larger furnaces, and a duplication of the electrodes becomes neces- sary, then these are connected in parallel. This always per- mits electrodes of the same size to be used, and like poles are therefore either only above or below the molten metal. This arrangement, which naturally only allows the electrode above the bath to be of carbon, from which the current flows to the liquid steel in the form of an arc, allows the other pole lying beneath the bath to be of a special formation. In the Girod furnace this bottom electrode consists of a number of soft iron rods, which are arranged at the edges of the hearth, as seen in the horizontal cross-section of Figs. 61 and 62. In order to avoid these bottom electrodes from melting off too far, the parts pro- truding through the furnace bottom are water cooled. During the operation then a part of these electrodes melts away, after which pasty layers, followed by solid ones, issue toward the bottom of the electrode material, as soon as the cooling on one side is balanced by the heating on the other. The part of the electrode which is melted away is about 5 to 10 cm. (2 to 4 inches) long, whereas the space for the water-cooling at the lower end of the iron block is 150 mm. (6 inches) deep. This water cooling not only provides a nearly unlimited durability to the bottom electrodes, but it also materially aids the life of the bottom refractories. From data given by Borchers, the furnace bottom is said to last 120 to 160 heats when melting cold stock, before repairs are necessary. During this time the bottom wears away to the extent of 100 mm. (4 inches), whereas the walls of the furnace need repairing after only 80 heats. It may also be mentioned here, that Girod endeavored to utilize air cooling in place of water cooling for the bottom elec- trode, but at present water cooling is again generally used. What has been said of the Heroult furnace relative to the hanging carbon electrode also applies here. The adjustable electrodes are held in their supports, which are in turn fastened to the furnace. The regulation is automatic and the Thury 152 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY regulators are used. Another similarity is to be found in the method pursued for cooling the furnace roof, where the electrodes enter the furnace. The operation of the furnace and, with it, the duration of the treatment, is much the same with the Girod furnace as with the Heroult. This applies as long as hot charges are being treated, for when it comes to melting cold charges, the Girod furnace shows undeniable advantages over the Heroult furnace. This is because the vertical path of the current does not permit any short circuits at almost full voltage, when the upper electrode touches the top of the scrap pile. When the electrode is lifted clear of the furnace, the scrap entirely fills its interior, and the short circuits are avoided, as the current path necessarily makes a multitude of small arcs between the various pieces of scrap. This equalizes the heating of the whole furnace content, thus causing the whole charge of scrap to gradually collapse and melt. However, it must not be left unsaid that the above conditions are present only when the scrap is charged into the furnace as the best operating conditions of the furnace demand; that is, the scrap is not to be thrown in arbitrarily. The most advan- tageous condition for melting cold stock is when this is in the smallest of pieces, and the conditions become more disadvan- tageous with the growing number of larger pieces. For these latter offer far too little resistance to the current, if the above method were used by starting with the upper electrode touching the top of the scrap pile. Similarly it is always necessary to spread a layer of the smallest sized scrap on the hearth, so that good contact can be made from the start with the bottom elec- trode, the end of which naturally lies a little low after the furnace has been in operation for a while. In order to make a good contact possible between the bottom electrode and the charge, care must be taken that no slag remains in the indentation over the iron electrode, otherwise this cold slag would act as a conductor of the second class, and in this state act as an insulator. We now come to the electrical conditions of the Girod furnace. Heretofore this furnace has been built mostly in two sizes. The THE GEROD FURNACE 153 smaller size of 2>^ tons capacity shown by Fig. 61 and the larger size of 10 and 12 tons shown by Fig. 62. The smaller furnace takes about $00 Kw. and the larger from 1000 to 1200 Kw. As the current is only interrupted by one arc the resistance of the whole circuit of the Girod furnace is comparatively small. From this it follows that a comparatively low voltage suffices, in order to give the furnace its needed energy. The voltage therefore for the 300 Kw. furnace is from 60 to 65 volts, and with the 1000 to 1200 Kw. furnace it is 70 to 75 volts. The single phase furnace has only two poles, one above and one below the bath, naturally only single phase current can be used. As the first Heroult furnaces were operated almost exclusively from 25 cycle circuits, so the Girod furnaces at first operated exclusively from circuits of this periodicity. The first trial furnace of i% tons tapping weight operated from a 35 cycle circuit, using 40 to 60 volts, 4000 to 6000 amperes and giving a power factor of .65%. The low voltage of the Girod furnace naturally necessitates a comparatively large current, and with it very considerable cross-sections in the conductors between the furnace transformer and the furnace. This is very noticeable when comparing the furnace with a Heroult furnace having an equal charging capacity and the same power input. It is this lower voltage which makes this part of the installation more expensive than would be the case with a furnace having a higher operating voltage. We must, however, take into consideration that the lower voltage also has its advantages. We only mention the fact that it is easier to insulate this voltage from the furnace refrac- tories, and there is less danger for those operating the furnace. The Girod furnace now also built polyphase is no longer youngest among the better known arc furnaces. It has since consequently been established how best to operate the furnace. There is, and justly so, the recurrent opinion that the Girod furnace is radically different from the Heroult, owing to the fact that the bath is connected in the circuit in a different way. We have already alluded to the advantage of the current 154 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY passing through the steel and iron in a vertical direction, when melting cold scrap. We desire, however, to discuss the operation of the furnace when the charge is melted. As we have seen, the current passes through the bath in a horizontal direction, in the Heroult furnace, and in a vertical direction in the Girod furnace. With the Heroult furnace, however, mention is never made of any essential influence of the purely resistance heating, which occurs because the current must overcome the resistance of the bath, yet with the Girod furnace we often find an important heating effect ascribed to it. In order that there shall be no misunderstand'ng, it may be said that the different manner in which the current goes through the bath in both furnaces causes different effects, yet these effects do not cause a greater or less resistance heating, (caused by the current passing through the bath,) but rather a difference in the circula- tion phenomenon. This may be decidedly more advantageous in one case than in another. To which misleading points of view our opinions lead us to suppose that the resistance heating, (even with a molten bath,) is of considerable influence, is shown in a short article on the Girod furnace in Stahl und Eisen, for 1908. Here it is pointed out that the depth of the melted iron of the Girod furnace may easily be increased from 30 cm. (12 inches), to 75 cm. (30 inches), or more. With all this the pure resistance heating is supposed to heat the whole bath evenly throughout its total depth. In spite of this, though, Girod with his 10- to 1 2-ton furnaces only used a depth of bath equal to 30 cm. (12 inches). A large surface bath has much greater radiation losses as a consequence than a bath has, having great depth and a lesser surface. The above example of the 1 2-ton furnace really proves that the resistance heating in a Girod furnace can be entirely ignored, as soon as the furnace content is molten. We can also convince ourselves of this arithmetically. If we take, for example, the 2>^-ton Girod furnace, we find by consulting Fig. 61, that with a depth of bath equal to 240 mm. (9.1 inches), the average bath cross-section is about 1200 X 1200 sq. mm. (48 X 48 sq. inches). If we take the specific resistance THE GIROD FURNACE 155 of the bath at 1.66, as given on page 15, we find the ohmic resist- ance of the bath, = P X = 1.66 X 4 - -8 X .or* ohm. A furnace of this kind takes about 300 Kw. at 60 volts. With a power factor of .8% it gives a current of 300,000 6o~^T = 625 amperes ' The energy, therefore, transformed in the bath is: i 2 X r = 6250" X .28 X io- 6 = 10.94 watts. This amount is only of i% of the 300,000 watts delivered to the furnace, and everybody must admit that any such small amount of energy has absolutely no effect on the heating. If, on the other hand, we figure the current density in the bath, we will see that this comparison also shows the heating of the molten metal to be entirely uninfluenced by the current flowing through this resistance, and that the resistance heating of the carbon electrodes is much more important than the resistance heating in the bath. The example we have before us gives. - = ' 1200 X 1200 6250 - 44 ampereS Per SqUare milhmetre 6 2 co 2.71 amperes per ) , = -i r which allots 210 sq. millimetres to 2304 square inch ) i ampere (about .36 sq. in. per ampere). If we compare this with the current density in the carbon electrode, which con- ducts the same current that flows through the bath, and has a cross-section corresponding to a diameter of 350 mm. or 96211 sq. mm. (13^ inches dia. gives 149 sq. inches) with the 300 Kw. furnace, we observe that we only obtain a cross-section ,06211 / 149 .024 square in. \ per ampere of ~, -- = 15.4 sq. mm. I = 6250 ^6250 per ampere. / With all this it is well to note that the comparison of these absolute values gives a much too favorable picture, because no consideration has been taken of the higher specific resistance of the carbon compared to the iron bath. In accordance with 156 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY data on page 15, we figured the specific resistance of fluid iron as p = 1.66. This resistance refers to a length of i m. (39.37 inches), and i sq. mm. (.0155 sq. inch), cross-section. With electrodes in the operating condition we figured p '= .0056 (see page 130). This value corresponds to a length of i cm. (.4 inch), at a cross-section of i sq. cm. (.155 sq. inches). If we convert this value to one corresponding to a length of i m. (39.37 inches), with a cross-section of i sq. mm. (.0155 sq. inches), we acquire the value for carbon in the operating condition, when p = 56. That is to say, the specific resistance of the carbon is I .OO or say, 35 times larger than that for iron. From this it follows that even with equal lengths and cross-sections, 35 times as much energy is transformed into heat in the carbon as in the iron bath. It is obvious that the carbon has a much smaller cross-section and a much greater length than the metal has, evincing that the consequent heat distribution is much more unfavorable for the iron, when considering only the resistance heating. The true ratio is therefore not apparent by the above partial calculation. We will now consider the comparison of this furnace with the ideal furnace. We first come to the availability of any kind of alternating current and refer again to the former remarks, that now besides single phase current, three phase is also available for this type of furnace, as the use of three phase current no longer comes in conflict with the principle underlying the furnace design. Alternating current is usually generated at a commensurately high voltage, brought to the vicinity of the furnace, and there transformed into a stationary transformer to the wished-for low tension current for the furnace. It is just as difficult to entirely avoid the power fluctuations with a Girod furnace as it is with a Heroult furnace; yet it is to be observed that with the Girod furnace the current fluctuations in actual practise are neither as violent nor do they occur as often as they do in the Heroult furnace. In spite of the current fluctuations being smaller, they are yet important enough in a 2^2-ton Girod furnace, which takes 400 Kw. on an average with a power factor of .80%, to recommend that a 50x3 to 550 Kw. THE GIROD FURNACE 157 machine be employed. This example may properly show why the im'tial cost rises which really becomes noticeable here, all due to these power fluctuations. Finally, we may again mention, that the automatic regulation is accomplished by means of Thury regulators. With Girod furnaces, these regulators are set to keep the current constant, and they in turn give the electrodes their proper setting. . The easy regulation of the incoming energy in the Girod furnace is the same as with all other electric furnaces. The electrical efficiency of the furnace is influenced, first by the probable installation of a rotary transformer, or by the losses of the stationary transformer, (neglecting the losses in the conductors,) and finally by the losses at the furnace, due to the electrodes. For a general calculation we can use the following values: Efficiency of the rotary transformer 85% Efficiency of the stationary transformer 96% to 97% Efficiency of the carbon electrodes including the heat conduction losses 90% All Girod furnaces are made of the tilting variety. The hearth is easily surveyed, and perfectly accessible for all operating conditions. We now come to the circulation of the melted metal and once more to the fact that the peculiar path of the current in a Girod furnace is of added importance. This circulation begins with one or more current centres above the bath, and goes to the bottom electrodes set around the periphery of the furnace. Figs. 63 and 64 show the diagrammatic connections for the current paths in a Girod furnace, Fig. 63 being the plan view and Fig. 64 showing the cross-section at a c. The dots and crosses indicate the lines of force, which follow the arrows according to the laws given in Chapter III. As lines of force of the same direction repel while those of opposite direction attract, and as the molten bath in a certain sense can be regarded as a movable conductor, with the vertical electrodes over and under the bath considered as fixed conductors, we find in the molten steel certain circulation phenomena, as shown by the arrows in Fig. 64. That is to say, 158 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY a definite circulation will appear throughout the entire bath, of such a nature, that a current of metal can be observed going from the walls of the furnace toward the centre, from there to the bottom, and back again to the walls. The strength of this circulation phenomenon depends on one hand on the strength of the current which flows through the bath that is then collected at the electrodes, and, on the other hand, on the depth of the FIG. 63. FIG. 64. bath. For it is evident that the circulation in the bath would instantly cease if the metal currents were in a vertical direction, instead of being in an almost horizontal direction. If the bath has a comparatively great depth, we would approach the vertical direction condition. We ascertained before, that as the heating in a Girod furnace is practically entirely from the arc, a great depth of bath is therefore precluded, so we see now that the advantageous mixing in the bath would cease if this were, say, over 40 cm. deep (about 16 inches), this adequate mixing being present with shallow baths or those of normal depth. With poly- phase furnaces one or two phases are purposely unbalanced so that an appreciable amount of current will flow through the neutral or centre point of a star connection, which goes to the bottom electrodes, thus adding to the circulation. The application of the Girod furnace for the steel industry is one of the widest. It has already been said that very good re- sults are obtained with furnaces of the 1 2-ton size. Here, however, they already use four electrodes of considerable cross-sections. There is then no reason why Girod furnaces cannot be built of the same capacities, as, for instance, the Heroult furnaces. THE GIROD FURNACE 159 even though the Girod furnaces operate with a lower voltage than the Heroult furnaces, and both now operate with three phase current. If the Girod only used single phase current we will assume having 1200 Kw. energy at .80% power factor, to be used by means of three phase current at no volts on the one hand and at single phase current at 70 volts on the other, in the former case for a Heroult and the latter case for a Girod furnace. Then, per phase, we obtain for the three phase Heroult furnace, a current of 1200000 ^h = = 7882 amperes, no X 1.73 X .8 and for the single phase Girod furnace the current: '. 1 2OOOOO tg = - = 21429 amperes. 70 X .8 Now Heroult has to deal with 7882 amperes for each phase, i.e., three electrodes are needed each to carry 7882 amperes, whereas Girod has only once to carry a current of 21429 amperes. Suppose we assume that he too uses three electrodes, connected in parallel of course, then each would carry a current equal to 21429 -f- 3 = 7143 amperes. In other words, it would even suffice Girod to have a lesser total electrode cross-section than Heroult, though the latter has a much higher current in the electrodes at the same current density. Or we may say: "The influence of carrying the current in, one way or another, is of so little importance as regards its effect on the carbon electrodes, and that the electrode relation in both types of furnaces may be regarded as being exactly alike." Therefore, the same reasons governing the maximum size of the Heroult furnace cover the Girod furnace also, so that the attainable size of either furnace is on the same footing. No limitation of the applicability of the Girod furnace any longer arises as the furnace is now also built for polyphase current, which precludes expensive rotary trans- former units for large furnaces, and enables current of an existing three phase central station to be used. If no con- sideration need be taken of an existing power plant, even then the single phase generators for large Girod furnaces will be more 160 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY expensive than three phase machines of the same size for Heroult furnaces. Regarding the uninfluencing e/ect of the electric heating on the chemical composition of the bath the comment given on page 140 is also applicable here. This applies to all arc furnaces which have their electrodes directed directly against the metal to be treated. Especially worthy of mention with the Girod furnace is the influence which the water-cooled bottom electrode exercises, even though this influence is said to be of no consequence. To understand this, consider that the circular motion in the bath also continually renews the coldest material over the bottom electrode, so that in spite of the greater temperature difference between the bath surface and hearth bottom, there remain prac- tically the same conditions as in the Heroult furnace. In coming now to the consummate efficiency of the Girod furnace, it may be again said that, compared to the Heroult furnace, the proportions of the carbon electrodes in both furnaces may be looked upon as being equal to each other. From this it follows that not only are the electrical losses equally great, but the thermal losses also, for these are caused by the hanging carbon electrodes. Also, the water-cooling losses, caused by the devices at the roof of the furnace, where the carbon electrodes pierce it, are by no means unimportant. According to the report of Conssergues these are about 10%; for it was established that the power consumption with the Girod furnace decreased 10% when it was operated without the water cooling. The fact that water cooling apparatus is used today on all Girod furnaces as well as on nearly all other arc furnaces, may be explained by the following reasons, (as discussed on page 100,) first, a tighter fit can be made at the water cooling entrance to the furnace, the electrodes being better protected against oxidation, and, secondly, because the water-cooled boxes allow the furnace roofs to be stiffened,' which latter have their life considerably prolonged. Besides these roof and wall radiation losses of the furnace which are about equal in the Girod and Heroult furnaces, there remain still to the detriment of the Girod furnace the losses of the water- cooled bottom electrode. These are avoided in the Heroult THE GIROD FURNACE 161 furnace. 1 We come to the conclusion, therefore, that the losses due to the cooling of the bottom electrode are according to an address by Trasensters "much less important," than those which are occasioned by the cooling where the roof is pierced for the carbon electrodes. In order to further judge the total efficiency of the Girod furnace, the following notation is taken from a report of the firm, Ohler & Co., of Aarau, in Switzerland. (See Electro- chemical and Metallurgical Industry, 1908, pp. 452 and 453.) Here we first find a description of Girod furnace installation at the above works. The furnace is connected to the power of the municipal power plant, through the medium of a motor- generator set. The 2000 volt, 2 phase current system supplies the Ohler Works' motor of 450 HP, running at 560 R.P.M., and is coupled directly to a single-phase alternator giving 4600 to 5000 amperes at 65 to 75 volts and a frequency of 37.4 periods per second. Twelve heavy copper cables, each 20 mm. in diameter and composed of 12 copper wires twisted together, carry the current 10 metres to the furnace. The voltage drop is 2.5 volts from the machine to furnace, so that this short cable installation alone causes a loss of 3 to 4%. At the end of this report we find this statement. It is calculated that the elec- trical part of the plant has an efficiency of 75 to 80%; i.e., 75 to 80% of the energy of the primary current appears as heat in the furnace. A rather approximate estimate of the calorific efficiency of the furnace itself shows about 50% of the current converted into > useful heat. Naturally the efficiency with the Girod furnaces also rises as the furnace increases in size. Yet, it is to be noted that, when 1 With the same construction of the Girod furnace as the Heroult, other things being equal, the efficiency of the Girod furnace must be just that amount less, which corresponds to the water cooling of the bottom electrode. Accord- ing to Stahl u. Risen, July 20, 191 1, by A. Miiller, in a 3-ton Girod furnace, a calorimetric determination of the heat carried out in the cooling water of these bottom electrodes gave 10.1 kilowatt-hours for the 130 minute run and about l.oi per cent., or 2.9 kilowatt-hours per ton of steel produced. The cooling water used in the top electrode carried out 36.7 Kw. hrs., 3.65% of total energy supplied or 10.5 kilowatt-hours per ton of steel. 162 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY the number of the upper electrodes is increased, the efficiency curve decreases here as well. The reasons underlying this were given in the preceding chapter on the Heroult furnace. The costs of a 2>-ton Girod furnace, including the electrode regulators, the switchboard instruments, the tilting mechanism, its motor and short conductors between the furnace and its transformer or the dynamo room, total about $15,000. A large furnace of 10 to 12^ tons with the same equipment will cost about $30,000. To this must be added the transformers and the royalty charges. The cost of a complete Girod furnace installation, but ex- clusive of the transformer or generator, and consisting of an operating and a reserve furnace each of 2 tons capacity, together with the necessary equipment for pouring the steel, and the accompanying buildings, total, according to Borchers, about $40,000 to $60,000. An installation with a 10- to 12^- ton furnace and a reserve furnace of the same size will cost about $60,000 to $80,000. The power consumption with the Girod furnace is about the same as that given for the Heroult furnace. What differences there may be due to a more or less favorable efficiency can be omitted when making arithmetical calculations, as the power consumption figures depend largely on the efficiency of the furnace, the electric power at the terminals, as well as on the charge and the final product. The composition of the final product produces much greater variations in the power con- sumption, than the differences in the efficiency. -This, of course, does not hinder the furnace with the better efficiency to operate with less power and consequently with lower current costs, provided that an equal start is made with like raw materials, and like final products achieved. The electrode consumption with the Girod furnace may be taken to be the same as with the Heroult furnace, for there is no reason why the electrode consumption should be less with one furnace than with the other, when about the same electrode cross-sections are used in either case. Should there be given, nevertheless, larger or smaller values for the consumption THE GIROD FURNACE 103 FIG. 65. 164 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY figures, in one case or another, the larger wear can, in no case, be based on the principle of the furnace. Consequently if one furnace is to have any advantage over the other, it must depend on its more or less successfully constructed details. In order to give the reader an idea what these furnaces look like, Fig. 65 is shown. This pictures a 5-ton polyphase Girod at a German Steel Works. The three electrodes can be plainly seen. As the preceding chapter on the Heroult furnace was closed with Heroult's own opinion of the advantages of his furnace, so this chapter is closed with the deduction of Borchers, where he proves the superiority of the Girod furnace over the Heroult furnace. The quotation is taken from Stahl und Risen, 1909, page 1947, where Borchers says: "I strictly maintain that today there is no electric furnace for the refining of metal which excels the Girod furnace. I make special reference to the uniformity of the current distribution; the uniformity of the heat generation in the bath; the low voltage between poles, the consequent lesser insulation difficulties; followed by the con- sequent lesser danger to the operatives; on account of these circumstances, it excels in its simplicity of construction as a whole, and in its operation." It is well to compare this with the opinion of Heroult given on page 142. Lastly we may add that, if we consider only the evenness of the current distribution, and the heat generation as above mentioned, these alone should be enough to decide the question. That there is an advantage in the lower voltage goes without saying. To these we might add the further advantages of the smaller current fluctuations, especially when melting down cold stock, while the opinion regarding the greater simplicity and the greater safety during the operation of one furnace over the other, may be left to the reader. Regarding the application of the Girod furnace, reference is had to the statistics in the closing chapter. Licenses for Girod furnaces may be had from the inventor, Paul Girod, Ugine, Savoy, France, or from his American representative, C. W. Leavitt, New York. CHAPTER X THE RENNERFELT FURNACE IN 1878 Siemens experimented with a radiating arc furnace having two horizontal electrodes as shown by Fig. 5. Twenty years later Stassano tried out his first radiating arc furnace, single or polyphase, with electrodes rigidly horizontal or nearly so in a vertical plane. This was the first practical application of Siemens' idea for electric furnaces in the iron and steel in- dustry. Both of these applications allowed the flame to remain Y*E- FIG. 650. above the bath without giving it any particular direction toward the steel to be melted or treated. In 1912 Rennerfelt brought out his directionalized radiating arc as shown by Fig. 650. This shows two horizontal electrodes and one vertical coming together at a point to make the arc, the larger amount of current coming from the top electrode and 165 166 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY FlG. 656. forcing the arc flame violently toward the bath. Rennerfelt was granted patents covering this, the first United States patent, dated October 23, 1913, No. 1076518, etc. In the improved form the side electrodes can be tilted in a vertical plane, as shown in the original patent drawing, and better by Fig. 6$b. 1 All three electrodes are lowered as the charge melts, and when molten the best height is chosen for the flame to be above the slag covered charge. It is notable that the material being treated does not conduct the electricity in any way. Before describing further details of this furnace, we mention briefly an account of its development. In 1912, the- Bultfabriken in Halsta- hammer, Sweden, started the first commercial furnace of this design, since which time over one hundred have been placed in operation, due to a simple form of furnace, easily responding to the metallurgical demands made upon it. Regarding the furnace itself, this resembles a tilting open hearth, much as the Heroult furnace does. Perhaps the main resemblance between all three being the fact that these are the main electric furnaces of the arc type, except the Stassano, hav- ing a solid bottom like an open hearth. It consists today of a cylindrical steel plate shell, with the horizontal electrodes pierc- ing the rounded walls, in all sizes up to three tons holding capacity. The furnace hangs in trunnions and tilts by means of an electric motor. The whole design is shown plainly on Figs. 656 and 650. The bottom and sides are made in much the same way as in other furnaces, and the roof is removable. The hearth, being round and having one or two doors, is readily inspected and easily surveyed. The roof is slightly arched, having a rise of six or nine inches (15 to 22.5 mm.) and is dome- shaped. With this size furnace only a single electrode, prefer- ably of Acheson graphite, pierces the roof. The furnace may be easily charged through the doors, as JSeeA. E. S. Vol. XXXI 1917 " Rennerfelt Electric Furnace Oper- ation," by C. H. Vom Baur. THE RENNERFELT FURNACE 167 the electrodes are withdrawn meanwhile. Where the electrodes pierce the furnace refractories, copper or iron cooling chambers keep the electrodes outside the furnace and brickwork reasonably cool. Copper water-cooled clamps are also necessary to fasten the conductors to the electrodes. The electrodes are fed by means of worm gearing, and are either hand controlled, with push-buttons and electric motors, or with electric motors and regulators automatically. With hand control the arc is usually steady, as shown by the curves of Fig. 65^. That FIG. 650. is why, so far, only four furnaces have been equipped with either push-button or automatic electric control. With hand regulation the side electrodes oftentimes do not have to be touched for several minutes and the top electrode once in ten or fifteen minutes. Either hot or cold charges can be treated, and, owing to the inherent characteristics of the arc, tending toward stability as it does, no heavy fluctuations occur. During the melting period choke coils are sometimes placed in series with the current entering the side electrodes which give ten, twenty or thirty volts reactance. These reduce the power factor with sixty-cycle current from 95 to 80%. With cold charges no more fluctuations occur than when treating hot metal, because the steel, being melted, be it in small or large pieces, does not affect the arc flame in any way, as the flame is made independent of the arc, 168 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY i.e., the continuity of the electricity is made as easily with an empty furnace as with a full one. This feature is sometimes called upon to heat the bottom after the furnace has been stand- ing idle, and is the means of avoiding skulls on the bottom, so annoying to the steel maker. Fig. 6$d shows a photographic re- production of the arc flame. The steel bath is always covered with a suitable slag. With the Rennerfelt furnace the entire heat comes from above, as in an open- hearth furnace. Electrically the furnace design FIG. 65d. is sucn tnat an Y frequency can be used to advantage and still main- tain a power factor of 90%. So far, all of these furnaces operate from polyphase circuits. If three phase is supplied from the power house it is changed by means of the Scott connection to two phase. The prevaih'ng voltages at the arc are 120, no, 100, and 80. A three-ton furnace with 750 KVA in transformers (two of 375 KVA each), at 100 volts, gives 3750 amperes at each side electrode, and 5287 in the top elec- trode. Care must be taken while operating to see that the top electrode is lowered properly, otherwise it will not draw its quota of current. The side electrodes may be controlled by automatic means, without any electrical complications, but to attempt to regulate all three automatically and meet all emer- gencies causes serious complications. A three-ton furnace takes 3750 amperes X 100 volts X 2 = 750 KVA, and with its 90% power factor, .90 X 750 = 675 Kw. Acheson graphite electrodes are used usually, and in this case those of 5^ in. and 6 in. diameter give good operating results, especially when having the new cone joint. This gives 3.5 sq. THE RENNERFELT FURNACE IfiO mm. per ampere (182 amperes per sq. in.) for each side electrode and about 2% greater density for the top electrode. The over- hanging portion of the side electrode is one meter (39^ in.) and the length from the electrode clamp to the arc itself is 1.66 meters (60 ins.). With the specific resistance of graphite in an operative condition taken at .00085 hms per cm. cube, the voltage drop is e = i X PI X j = 3750 X .00085 X ^, where / and q are respectively in centimeters and square centimeters, equals 2.86 volts per side electrode. The voltage drop in the top electrode is about the same, giving for the three electrodes, say, 8.50 volts drop. From this it is evident that to raise the arc voltage from 90 to 120, decreases the electrical electrode losses from 10 to 7%. Acheson graphite, not being quite as strong as amorphous carbon electrodes (ratio 8 to 10), more care must be taken in handling them to avoid breakages at the joints. The power consumption varies not so much with the size of the furnace as formerly, but today more with the amount of electrical heat behind the furnace. In 1912, 175 to 200 Kw. for a i -ton furnace was considered ample. Five years later we find 400 Kw. in transformers behind a furnace of this size. Naturally the time for a heat is materially decreased. With a basic bottom melting cold charges and taking off one slag, heats of good steel are regularly taken off, in less than three hours, when operating continuously with a consumption of 670-720 Kw.hr./ton. With an acid bottom, 600 to 625 Kw.hr./tonMs common practice with an experienced and careful attendant, making a steel suitable for castings. Coming now to the comparison between the Rennerfelt furnace and the ideal furnace, the first requirement, namely, that every existing alternating current can be used, is fulfilled only in part, for each furnace operates best at a given potential, hence a set of stationary transformers are required for stepping down voltages as high as 22,000 directly to, say, no at the furnace. The use of transformers with arc furnaces is almost universal. x Ton of 2,000 Ib. avoirdupois. 170 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Seldom with modern installations is the furnace directly con- nected with its own generator, although this was much in vogue in the early years of commercial arc furnaces. The frequency or phase with this furnace is immaterial, and does not affect its operation. That the roof is pierced only once is of advantage, as two or more electrodes coming through here weaken the arch, even if it is dome-shaped. The electrodes at the sides in a 3-ton furnace, for instance, are 18 ins. over the slag line when in their horizontal position. They are tilted down from 7 to 17 degrees for the operating condition, and thus allow of a free view of the hearth, never being nearer than 3 ins. over the bath and as high as 18 ins. when in the horizontal position. They can also be tilted upward, about 7 degrees being enough, to allow of a larger charge in the beginning, and when the bath is melted this additional space between the side electrodes and the molten metal is appreciated by those rabbling off the slag. This tilting of the side electrodes is accomplished by turning a small hand-wheel. From the first, all Rennerfelt furnaces have been able to avoid sudden power fluctuations, owing to the fact that the arc is made between three points of comparative stability, in com- bination with the fact that the arc is forced down upon the bath by means of the preponderating power of the current in the top electrode. Stassano had three electrodes with three- phase current, but his power fluctuations were most violent, as the current surged continuously from one electrode to another in a most irregular fashion, due no doubt to the absence of any of the electrodes having a greater electromagnetic action in one direction than in another, i.e., the Stassano arc is in perpetual, unstable equilibrium whereas the Rennerfelt is the exact opposite insofar, and yet both are pure radiating arcs. When melting cold material no great power fluctuations occur, as shown by the curve Fig. 65^. Not much better regulation could be obtained, but the labor of one man could be saved by using automatic regulators, such as the Thury regulator. Easy regulation of the electric power is obtained with this as with other arc furnaces. THE RENNERFELT FURNACE 171 The electrical efficiency of this furnace is about the same as with other arc furnaces having three electrodes above the bath. There are the usual electrode losses, about 7%, as already dis- cussed for this furnace, besides the transformer losses, 3%. The demand that the furnace be of the tilting variety is easily met and the hearth, being circular, with one or two doors, is accessible and easily surveyed. Even though the carbon vapor with its reducing action may hinder the dephosphorization with its necessary oxidizing con- dition, yet repeated practice shows there is no difficulty what- ever in reducing phosphorus to its lowest limits, i.e., .002 and under, with as low a power consumption as any other arc furnace, substantially as discussed in the second paragraph of the book. Concerning the motion of the charge, there is only that due to thermal action, the same as in an open hearth, all the heat coming from above. The application of the furnace has a wide scope; besides treating steel, gray iron scrap and melting ferro-manganese, is also largely used in the various copper-melting and nickel trades. Cold stock is as easily melted, as it is to treat hot metal. Miscellaneous steel scrap of any sized pieces which can be charged through a door 457 X 508 mm. (18 X 20 ins.) is rapidly melted without any more fluctuation than if there were only the smallest pieces, because the charge is not in the electrical circuit at any time. The Rennerfelt has so far only been built in sizes of three tons or so, and these furnaces are operating well both here and abroad. The life of the roof, through which only one of the three electrodes comes, varies much with the class of service, the experience of the crew and the quality of the refractories. With continuous operation and a magnesite roof and bottom, melting cold charges, as much as 192 heats from one roof have been obtained. With acid refractories throughout and intermittent service, i.e., twelve hours or so daily, 346 heats have been obtained with a high- powered furnace. A roof lasts many weeks, depending upon the conditions, and with a 3-ton furnace with silica brick costing $50 a thousand equals $38 for material. The cost of 172 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY all refractories with an acid hearth in such cases is about 70 to 80 cents per ton of metal poured. Carborundum (C Si) arch brick, covered by a layer of silica arch, and sometimes by Kiesel- guhr brick, have lasted over 200 heats. With an experienced crew a silica roof is cheaper, and with careless operators the carborundum brick roof. The electrode consumption, when using Acheson graphite, varies considerably with conditions. The amount of air leaking into the furnace at the side electrode cooling boxes wears them away perhaps more than any other thing, hence these apertures are kept tight by means of asbestos washers. The electrodes should not be too large, as the wear also depends upon the surface exposed. High-powered furnaces making quicker heats consequently use less electrodes, other things being equal; 2.5 to 3.5 kg. (5.5 to 7.7 Ibs.) per ton of cold metal charged is com- mon with the best practice. This does not include breakages, which can be avoided by taking proper precautions. Slags can readily be changed and necessitate 80 to 90 Kw.hr. per ton for each additional slag with ordinary basic steels. With tool steels more power is usually consumed. The thermal efficiency does not depend so much on the furnace size as formerly, mainly on account of the power now placed on these furnaces. For instance, The i ton has 300 to 400 Kw. " 2 " " S 00 " 600 " " 3 " " 600 " 900 " Larger furnaces are contemplated, using more than one set of electrodes. The efficiency is greatly increased if the operation is continuous, twenty-four hours daily for the week, instead of shutting down each night, even though for only a few hours. If a furnace is shut down overnight, the first heat usually takes 200 and the second 100 Kw.hr. more per ton than the third or fourth heats, which are usually normal. This can be consider- ably cut down by heating the furnace with an oil or gas burner during the idle period. The maximum efficiency can be regarded as approaching 75% when operating under a combination of most favorable conditions. THE RENNERFELT FURNACE 173 The installation costs of Rennerfelt furnaces vary considerably. A 3 -ton furnace complete in every respect, ready to operate, complete with 900 KVA. in transformers, including a heavily discounted royalty, is $37,250. A i-ton under similar con- ditions costs $18,800. These furnaces are also sold without transformers and also under royalty conditions, both reducing the initial investment. All Rennerfelts operate from polyphase current and from various frequencies from 25 to 60. Hand regulation of the electrodes is usually sufficient and automatic electrode regulation would be an additional cost of about $1,500 per furnace. The costs are exclusive of foundations, installation costs, transformer room, etc. The advantages which Rennerfelt himself gives over other arc furnaces may here be cited : 1. The heat is generated with an arc with the absence of exceedingly hard strains on the power supply. 2. On account of the steady power and because of the large flame widely diffused and violently directed downwards as shown on Fig. 69, the heat is communicated to the charge quicker than' with arcs of the thin pencilled type, made between the tips of the electrode and the bath; besides this the arc distance above the bath is variable, . the flame size and power remaining the same or not as desired, thus increasing or decreasing the mushrooming effect of the flame on the bath or material being melted or treated. 3. The furnace operates with polyphase current, and yet is only pierced once through the roof by an electrode; thus the roof is stronger than if two or more electrodes made holes through the roof necessary. 4. The heat gradient in an electric arc is greater if it takes place in the widely spread out zone rather than in a narrow space, and the radiating arc is a more rapid way of transmitting heat than with the shorter flame arc. 5. There is no water cooling below the bath, which conse- quently avoids danger from explosion from this source. 6. The arc being "free burning," i.e., sustained between the 174 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY rMll -.-*Ci J I--*- ''-UL ' trw* i ~ THE RENNERFELT FURNACE 175 tips of three electrodes, the electric flame can be kept in the furnace when there is no metal on the hearth. These claims should be compared with the advantages put forth by Heroult and Girod, as mentioned in their respective chapters. The first Rennerfelt was built like a barrel lying on its curved surface and the side electrodes coming through the flat ends. The models soon following kept the barrel shape but had a door at one flat end, lying on the curved surface as before, but the side electrodes piercing the curved surface of the cylinder. Next came the square shell and the rectangular brick work, then the rounded brick work in the square shell, and lastly the inevitable model approximating more nearly a sphere than all other models, viz.: the cylindrical shape like an upright barrel, with or without a truncated bottom, and always now with a dome-shaped roof, the side-tilting electrodes piercing the curved sides. Fig. 65 e shows this latest model with the side electrodes shown at the maximum tilting angle downwards. They can be tilted upwards also and consequently pass through the horizontal position, which is now no longer their only position. The use to which the Rennerfelt has been put is shown in the table of statistics. Licenses may be obtained in Sweden from the Aktiebolaget Elektriska Ugnar, Stockholm, for prac- tically all of Europe, and in the United States by American Transmarine Co., Inc., New York. 1 1 See A. E. S. XXIX 1916 " The Rennerfelt Electric Arc Furnace," by C. H. Vom Baur. CHAPTER XI THE INDUCTION FURNACE IN GENERAL IT was demonstrated in Chapter IV that an insulated wire of a coil carrying current generates lines of force, and that these lines or fields of force, are continually alternating, when alternat- ing current flows in the coil. These alternating lines of force constitute the well-known underlying principle for all induction phenomena. It is therefore evident that in an electrical con- ductor which lies in the field of another conductor, a current will be induced, which will be proportional to the number of lines of force cut in unit time. This fact immediately gives us the information, by the aid of which we are enabled to obtain any current strength by induction. We merely have to oversee that the conductor in which we desire to induce the current shall be cut with as many lines of force in unit time, as will give the wished-for current conditions. In order to achieve this we encounter these various possi- bilities: Imagine a certain number of lines of force, raised to twice their strength. Then we should find that a turn of wire, lying in this magnetic field, would have twice the electro-motive force generated in it as in a field of only the original strength. When the magnetic lines are doubled, then, the conductor is cut with twice the number of lines of force in the same time. The same effect is accomplished, however, when the field is kept at its original strength, if two turns are used instead of one, where they are both cut by the same number of lines of force. What we have then in this case is an increasing number of turns, and with it a raise in the voltage in the induced coil; because for the moment we may think of these two turns as being sepa- rated in such a way, so as to give us two separate turns, each 176 THE INDUCTION FURNACE IN GENERAL 177 having the same voltage that one turn has now. Finally the potential in the induced circuit may be increased, by raising the velocity of the current alternations, and this leads us to a change in the frequency. And as the induced voltage is proportional to the velocity of the alternating lines of force, it is evident that, a current of 50 cycles will give twice the induced voltage a current of 25 cycles will give, other things being equal. If we now combine the three methods into a formula, which influence the conditions in an induced circuit, we obtain e=CXvXsXN where e denotes the voltage v denotes the frequency s denotes the number of turns N denotes the number of lines of force and C is a constant. We have so far assumed that our lines of force, generated by the aid of a wire coil, sought their paths through the air. This arrangement is, however, very disadvantageous because the air is a very poor magnetic conductor (being only 1/180 as good as iron). The lines of force in this way seek the shortest path, resulting in the consequences (for instance, with a coil of a great number of turns) that only a part of the turns are cut by the total number of lines of force, whereas for the remaining turns only a part of the total lines are taken into consideration at all. In order to keep the lines of force from spreading, or straying, as it is called, we provide a good magnetic conductor for them, which forces them to take advantageous and prede- termined paths, due to the high magnetic conductivity, which in turn gives a good inductive action. These things give us the so-called transformer. Fig. 66 shows the principal arrangement of a transformer as it is commonly used, as well as for induction furnaces. In the figure, A'i and K 2 denote the transformer cores, and J\ and J* the yokes. The wire coils are wound on these cores. The coil receiving the current from an outside source is called the primary, and the coil delivering the useful current is called the secondary 178 ELECTRIC FURNACES IN THE IR'ON AND STEEL INDUSTRY winding. Both coils are separated from each other by suitable insulation. If these yokes and cores were made from solid pieces of iron, then it would not be possible to avoid the considerate losses due to eddy currents, as set forth in Chapter IV. Therefore, in order to bring these losses down to the smallest percentage, the iron cores and yokes are built up of many thin sheets of iron of FIG. 66. FIG. 67. .3 to .5 mm. (.012 to .02 inches) thick. These sheets are insulated from each other by pasting sheets of paper on one side, about i/ 10 as thick as the sheet iron, and the whole then held together by means of screws. Large core cross-sections are divided into separate divisions, which are kept apart by so-called ventilating ducts, by means of which the already low hysteresis and eddy current losses and their consequent heat generation are nullified. Fig. 67 shows one of these core cross-sections. If the primary coil of a transformer is energized with an alternating current, which must necessarily produce an induced current in the closed secondary circuit, then the iron core will be permeated with magnetic lines of force, which is common to both coils. As the primary and secondary coils, besides this, must have the same frequency, we obtain the equations for the volt- ages in both coils, as follows: = C X v X N X and THE INDUCTION FURNACE IN GENERAL 179 from which it follows that; In this ratio we call the factor the ratio of transformation. s i The equation signifies that: The voltage is proportional to the number of turns. By applying a different number of turns in a transformer, we obtain a means whereby any existing voltage may be changed into any other voltage, and one thus suitable for the operation of electric induction furnaces. In this way transformers are nearly always used in alternating current installations. For this method makes it possible to transmit power over great distances at high voltages and at small currents, thus using only smaller and cheaper conductor cross-sections, from the central station to the point of power consumption. At that place then a transformer is erected, by the aid of which, the high primary voltage is changed to any desired secondary voltage, which may be most advantageous for the particular apparatus. We have already observed that transformers are used in this way for arc furnace installations. Alternating current provides such a convenient way of transforming energy in stationary transformers, and this together with its lack of chemical influence constitute the two factors responsible for the reason that all arc furnaces are operated with alternating current to-day. With the present technical perfection of the transformer this last may be regarded as a sort of interposed apparatus, which produces at a different voltage, almost the same amount of energy which it receives. That is to say, the losses in a trans- former are extraordinarily small. With transformers of more than 50 kw the losses are from 2 to a maximum of 3%. Even though the efficiencies of transformers for electric furnaces will fall slightly on account of the necessary overload capacity, yet we may consider, for the sake of simplicity, that the total primary power is given up in the secondary circuit. 180 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Then the primary power pi = e\ ii, and the secondary power pi = e 2 iv, where pi = p* and consequently e\ i\ = eziz- From this it follows that 4 _ * _ that is: _ 12 Ci Si The current is inversely proportional to the voltage and inversely proportional to the number of turns. The foregoing conclusion is of the greatest importance for it solves the building problems of induction furnaces. Induction furnaces in reality are nothing more nor less than properly designed special transformers. Hence every induction furnace has its iron core and yoke, to carry the lines of force, and a primary winding, wound over one part or another of the iron core. FIG. 68. FIG. 69. On the other hand, the secondary winding is com- posed either entirely or for the most part of the bath itself. This point of view enables us to group electric induction furnaces on the one hand into those furnaces where the secondary winding is composed entirely of the bath, and on the other hand into those where, besides the bath being the secondary winding, there is still another winding, made of copper to aid the heating. We denote the former as simple induction furnaces and the latter as combination furnaces. If we take up the first group of simple induction furnaces, we see that the different methods of construction can be dis- tinguished merely by the way the primary coil is placed, relative to the bath. The Figs. 68 to 72 show a number of the most prevalent suggestions. In the figures the steel bath is denoted by the solid black, (the layer of slag is not shown,) THE INDUCTION FURNACE IN GENERAL ]8l the refractories by inclined hatching and the primary winding by cross hatching. Figs. 68 and. 69 show the primary winding in the form of large radial disks, which are under or over the bath, or as Fig. 68 shows it to be both under and over the metal. On the other hand, Figs. 70 and 72 shows the primary winding in the form of a long cylinder, which is placed inside or outside of the ring-shaped hearth. With this arrangement we speak of a transformer with cylinder or tube winding and those of Figs. 68 and 69 as having a disk winding. In all cases the principle of transforming the energy is the same, and in all cases we shall find the ring form hearth, in whose contents the heating currents are produced by means of induction, quite independent of the place in the magnetic circuit, occupied by the primary winding. It is evident that any of these winding schemes can be combined with every other method, and we may therefore state that there is no combination of windings and no placing of it at some part of the transformer, that has not already been patented as being particularly good. It has been shown that we are enabled to obtain any desired current strength in the secondary circuit, by properly winding the primary. The first one to recognize these conditions and use them in the design of an electric furnace was de Ferranti 1 , 1 In this connection proper credit must also be given to Colby. Many years after the invention was made, the Franklin Institute investigated the early patent applications of both Ferranti and Colby and reported, in 1911, in part, in speaking of the patents, as follows: See British patent to Ferranti, No. 700, Dec. 16, 1887, filed January 15, 1887. U. S. Patent to Colby, No. 428,378, May 20, 1890, filed April 14, 1887. U. S. Patent to Colby, No. 428,379, May 20, 1890, filed Sept. 19, 1887. . . . Between the years 1890 and 1900 no notable application of the process appears to have been made. . . . Colby's furnace is most broadly described in his U. S. Patent 428,379. It appears evident that the applicant was one of the first to devise the elemental features of the induction furnace. . . . It is generally conceded that the basic use of the transformer principle to electric furnaces was independently applied by both Ferranti and Colby, the dates of their patent applications being but a few months apart. The tubular water-cooled conductors, the means of supporting them and the connecting devices constitute essentially the features of novelty in the most recent patent 182 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY who patented his apparatus, as shown schematically by Fig. 68, in 1887. Even though his design was never put to practical use, we see how completely de Ferranti and Colby had at that time mastered the problem of heating by induction currents. If we use the furnace form as shown in Fig. 68, in order to obtain a clear view of induction heating, we observe that the middle core of the transformer carries the primary winding and that the furnace hearth is arranged concentric with this. There of Colby. . . . The forms of induction furnace depicted in the early Colby patents closely resemble those adopted in present-day apparatus and although but a joint pioneer in this field, his original designs are distinctive in anticipating the subsequent state of the art. In consideration of its originality and wide and successful commercial use, the Institute recommends to the Philadelphia Board of City Trusts the award of the John Scott Legacy Premium and Medal * to Edward Allen Colby of Newark, N. J., for his Induction Electric Furnace. Adopted at the Stated Meeting of May 3, 1911. (Signed) WALTER CLARK, President, R. R. OWENS, Secretary. GEO. A. HOADLEY, Chairman of the Committee on Science and the Arts. * Medal shown herewith: FIG. 690. Facsimile of medal awarded to Colby for his induction furnace by the Franklin Institute. THE INDUCTION FURNACE IN GENERAL 183 is no secondary winding of copper such as we usually find with ordinary transformers. Should the ring-shaped hearth be filled with molten iron, as shown in the figure, we may regard this ring of iron as the secondary winding, which is composed of only one single turn. Induced currents will, therefore, appear in this iron ring, the same as they would in every other electrical con- ductor which lies in an alternating current magnetic field. As the iron ring comprises in itself one short-circuited turn or a short-circuit consequently all of the energy of the secondary circuit is transformed into heat, as the secondary current has to overcome the resistance of the iron bath. The heat quantity generated is proportional to r r, that is, it is proportional to the product of the square of the current and the resistance. As the resistance of the iron bath may be regarded as being practically constant for a given charge, it is evident that any desired tem- perature may be obtained 1 by raising the current and, of course, first of all, by a proper choice of the primary turns; for the secondary turns with these furnaces are always equal to unity. Suppose we had an induction furnace, possessing 100 turns in its primary winding, and at a definite voltage of, say 1000, it took 100 amperes, we would obtain a secondary current value of Si 100 2 = i\ X = 100. = loooo amperes. On the other hand, if we had a furnace wound with 120 primary turns, and taking the same 100 amperes as before, but at a correspondingly changed voltage, we would obtain a current of, Si 120 *2 = ti X = ioo. = 12000 amperes, in the bath. $2 I These examples show how the number of turns influences the secondary current, and consequently the attainable tem- perature of the bath. It is, therefore, the part of the furnace designer to so choose his proportions, that he may in any case reach the desired temperatures, for his particular case. 1 See Am. Electro-Chemical Society, Sept., 1912, paper by C. H. Vom Baur on "Electric Induction Furnaces for Steel," giving an instance where the temperature of steel in an induction furnace reached 2550 to 2600 C. 184 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY During the operation it is, of course, precluded that any primary turns of the furnace transformer be changed. Still, during the time of operation, temperature changes are desired, which in turn calls forth changes in the energy absorption of the furnace. But even these changes are easily made. We have only to realize that the load on the furnace transformer is brought about solely by the particular resistance of the iron bath, which we may consider as a constant factor for a definite charge. It is now apparent that we have in Ohm's Law i = , a simple remedy for changing the energy, and thereby the current, by simply altering the voltage for the primary winding. Necessarily the secondary voltage and its current are instantly s z changed as e 2 = e\ . If we now review the above, regarding induction furnaces, we find: 1. The charge in induction furnaces is heated, solely and alone by reason of 'the current overcoming the opposed resistance, and to any practically desired temperature. The induction furnace is therefore only a particularly favorable type of resis- tance furnace, which allows a complete and even heating of the metal, without producing any overheating at any point. 2. By changing the primary voltage at any time during the operation of the furnace, the temperature of the charge may be raised or lowered at will, either quickly or slowly. At the same time the heat in the entire furnace contents is altogether uniformly raised or lowered. If all induction furnaces possess these qualities, what differ- ences are there then between the different arrangement of the windings as far as the molten metal is concerned? (See Figs. 68 to 72.) It is well to state that the differences between Figs. 71 and 72 are purely constructive, as the double magnetic path halves the cross-section of Fig. 72, opposite the simple path with the whole cross-section of Fig. 71, yet does not in any way produce any new electrical effects. Therefore, these two types of Kjellin THE INDUCTION FURNACE IN GENERAL 185 furnaces only differ in their outward appearance, without either one or the other of the iron cores giving any advantages worth mentioning. Substantial differences may, therefore, only be found in the arrangement of the coils, and these follow different directions. It is evident that the suggestion of Colby, made in 1887, (the FIG. 70. FIG. 71. FIG. 72. first to surround the hearth with the winding,) necessitates more copper conductors than the second suggestion of Kjellin, of 1900, where the primary winding is inside of the ring-shaped hearth. The whole arrangement of the Kjellin furnace, by reversing this idea, is simpler than the Colby furnace, not only on paper, but also in reality. The Colby furnace, as well as the de Ferranti furnace, are today only of historical importance, except for their later existing patents. This leaves only the accomplishments of Kjellin and Frick for discussion. If we put aside for the moment the fact that the Frick furnace does not permit such a general view of the hearth, or allow the accessibility thereto, on account of the overhanging disk winding, as we have with the Kjellin furnace with its coil removed from the operating conditions, we find that the chief distinction be- tween these furnaces lies in the different circulation of the bath, caused by the changed position of the coil. We saw in Chapter III that the motor effect of an electric current appears, when two conductors with their magnetic fields mutually affect each other. The different position of the winding cannot, therefore, be with- out its influence on the inclination of the bath surface. As will 186 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY be shown in the following chapter, the Kjellin furnace produces the effect of pressing the molten metal toward the outside, so that it stands higher on the outside wall than on the inside. In the Frick furnace, for the same reason, we find a stronger mag- netic pressure on the current carrying molten metal, where the bath and the coil are nearest to each other, and this causes the metal surface to be more depressed at this point than the re- maining part of the hearth. The Frick furnace, therefore, also has an inclination to its bath surface, so that this stands higher at the outside than at the inside. While this slope in the bath is only 4 34' to 5 5' with an 8-ton Frick furnace, according to the published report of von Neumann of the firm of Freidrich Krupp (see Stahl und Eisen, 1910, p. '1071), we see that with a Kjellin furnace of the same size, that it is 24. These differences naturally cause considerable deviation in the circula- tion phenomenon of the bath, so that these are greater in the Kjellin furnace and to its detriment, than they are in the Frick furnace. Even though there are certain differences between the Frick and the Kjellin furnaces, owing to the different position of the windings, still in the essentials of their operation they are en- tirely alike. As the Kjellin furnace opposite the Frick furnace has found a much more extended use, it will suffice if we describe the Kjellin furnace in the next chapter as a representative one. In this the secondary coil is composed solely and alone by the hearth metal itself. The honor is due Kjellin for producing the first practically useful induction furnace. In addition to the group of induction furnaces just men- tioned, in which the secondary coil of the furnace transformer is composed entirely by the bath, there is yet a second group of induction furnaces, which has another common copper winding, besides the short circuited secondary turn which is the bath. This second group of induction furnaces owes its existence primarily to the fact that the furnaces of the first group have a comparatively poor power factor. The cause of this being that the distance between the primary and secondary windings is so THE INDUCTION FURNACE IN GENERAL 187 great, that a large number of the lines of force take their path through the air, without being able to affect the secondary volt- age. We designate these lines of force as, stray lines or leakage lines, and the phenomena itself is called magnetic leakage, and it is this which operates heavily against the power factor. The greater the distance between the primary and secondary winding, the larger the magnetic leakage will be, and the lower the power factor. The leakage may be lessened by placing conductors in the path of these leakage lines, in which secondary currents are generated by induction. As these currents, which are generated by induced currents, always have the opposite direction of the primary or incoming current, (as was shown in discussing the self-induction phenomenon on page 44,) they will in turn send out stray lines of force in the opposite direction into the original stray field, and in this stray field the conductors lie. We may look upon this effect as one where the stray lines are pushed back, and in this way the power factor is raised by the coils, which lie in the space between the primary winding and the bath. Patents show a large number of suggestions, in which second- ary copper windings are to be employed, in order to gain the above result. But the fact must not be overlooked that on the one hand a poor power factor increases the initial cost but does not increase the energy losses, and on the other hand the current generation in the secondary winding (to decrease the magnetic leakage) can only be accomplished with energy so it is immediately evident that danger lurks nigh, in curing a small evil with a larger one. This error is shown by all the designs, whose sole object it is to lessen the stray fields, by means of the secondary copper winding, in which the heat which is generated in these coils is not put to any use; on the contrary this results in only enlarging the cooling appliances for the windings, in order to protect them from too high a temperature. The idea of applying the above-mentioned stray field reducing arrangements to induction furnaces, can hardly be looked upon as induction furnace improvements, (as we have learned to know the furnace in the first group,) as long as no provision is made 188 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY to profitably use the currents generated in the secondary copper winding. This last requirement is fulfilled by the Roechling-Roden- hauser furnace, and, as a result, these furnaces have already come into quite extensive use, whereas all other suggestions to improve the induction furnace power factors from within the confines of the furnace proper are today only on paper. It therefore seems sufficient within the limits of this book, besides describing the Kjellin furnace, to merely narrate the details of the Roechling-Rodenhauser furnace, not only because these two are the only induction furnaces having found ex- tensive use, but also because a discussion of these furnaces will be sufficient to give the reader a clear idea of the workings of induction furnaces. At the same time it will also enable one to adequately judge the value of any other constructional features. If in closing we again mention the essential thing about an induction furnace, we find that the characteristic mark of them all is the common transformer. In the induction furnace we find the application of the built-in transformer to be of the greatest importance to the heating method. For in this way only is it possible to generate the strongest currents directly in the iron bath without con- duct or losses, so that the molten metal itself may be regarded as the source of heat. In his addresses before the "Verein deutscher Eisenhiitten- leute," Borchers says: "Here in the induction furnace we should truly possess the most perfect of electrical heating. Here the generation of heat goes on solely and alone in the metal to be melted, and in the molten bath: the heat transference from other heat sources to the metal is not first required." Again when comparing resistance furnaces, and the induc- tion furnace may be regarded as a resistance furnace, with arc furnaces, Borchers says: "With both furnaces it is possible to reach a temperature of 3500 C. (6332 F.). There will always be 3500 C. at the arc of an arc furnace, while resistance heating enables any temperature up to this point to be reached." CHAPTER XII THE KJELLIN FURNACE THE first induction furnace which made a name for itself as a result of its achievements was the Kjellin furnace. It was conceived in 1899; thus the first trial furnace was placed in operation on March 18, 1900. The furnace was only intended for a capacity of 80 kg. (176 Ibs.), with an energy consumption of 78 kw. Steel castings could be made with this furnace, only with the extraordinarily high power consumption of over 7000 kw hours per ton of steel. With the second furnace of 180 kg. (about 400 Ibs.), which was ready for operation in November, 1900, this amount was reduced to one- third of the original figure. A third furnace followed having a capacity of 1350 (about 3000 Ibs.) to 1800 kg. (about 4000 Ibs.), which was installed in Gysinge on the Dalelf in Sweden. With this furnace they succeeded in bringing down the power consumption to about 800 kw.-hrs. per ton when making steel from cold scrap, and thus, the Kjellin furnace proved its practical and economical adaptability. On account of the successful operation of these furnaces, the Kjellin patents were acquired by Siemens & Halske A.-G.- for the principal countries of Europe, and under their guidance these furnaces were soon used to a considerable extent. In the construction of the Kjellin furnace, the part giving it its characteristic appearance is the transformer, which comes up through the centre of the ring-formed hearth. The first successfully useful Kjellin furnace was the one having a capacity of 1350 to. 1800 kg. (about 3/4000 lb.). This furnace is shown in its later design in Figs. 73 and 74. The original is of the stationary type. The transformer consists of two vertical cores and two horizontal yokes. These are composed of thin iron 190 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY sheets, paper insulated, of the usual transformer construction, so that the magnetic losses are as low as possible. Whereas the yokes and the unwound core of the transformer have a rec- FIG. 73. tangular cross-section, the core carrying the primary winding is made in the form of a cross (see Figs. 67, 73 and 84). This arrange- ment permits, on the one hand, a saving in the copper winding, i FIG. 74. THE KJELLIN FURNACE 191 as the core is of circular form and easily wound, on the other, it provides for successful cooling of the transformer iron on account of its larger surface and thus favorable cooling conditions are provided. For this Kjellin with his first 1^2 -ton furnace, used four one-inch tubes which were placed, one each in the recesses made by the section of cross form. These tubes carried an air circulation of 40 mm. (1.6 inches) water gauge pressure in this winding space, which was thus kept at permissible temperature. This air cooling was also taken from the normal transformer design and utilized in this special construction of furnace trans- former. Besides this, in order to shield the transformer, and especially the coil from the radiated heat, (from the furnace refractories,) the latter is surrounded with a double walled cylinder of brass of ij^ mm - (-06 inches) thickness. Either cooling water or air is passed through this protective cooling, in order to keep the heat from the winding and the transformer. The temperature of the cooling water coming from the pro- tective cylinder was measured during operation and showed 40 to 50 C. Naturally this protective cylinder could not be a closed circuit, or if so, it would form a short circuited turn, which would become heated or even melted under the influence of the currents which would be induced in it. In order to avoid this the protect- ive shield is built as an open double walled ring, while in the Kjellin furnace it is bridged over with wood insulation. On the outside of these cylinders we find the furnace refractories or the brick work, in which there is a ring-shaped space concentric with the winding, which comprises the furnace hearth. The furnace shell is of sheet iron and encloses both cores of the trans- former. After the protective brick work has been placed in tne furnace, the bottom is rammed in. Then a templet having the shape of the hearth, is lowered into the furnace, so that the hearth walls of suitable material may be tamped in. When this work is finished the templet is raised and the hearth is practically ready. The hearth roof consists of special bricks, or of small refractory arches held in iron frames, so that they may be easily removed. This is necessary as the furnace has 192 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY no doors, and the hearth and the progress of the charge can therefore only be watched by lifting off one or more covers. When the furnace is made of the stationary type, it must neces- sarily have the stationary type spout. Subsequent to the design of the first Kjellin furnace as just described, the following constructive changes were made: In order to allow of a thorough cooling of the transformer, it was divided into a number of smaller divisions, which were separated by means of suitable air spaces. This was only following good transformer practise, and the separate sheets were, of course, paper insulated as usual. The air cooling was changed so that there was a more uniform cooling, not only of the transformer iron, but also of the coil. The water cooling of the protective cylinder was avoided and air cooling substituted, this coming from the same ventilating fan feeding the coils. This simplified the furnace construction considerably, and gave equally safe operating conditions. The furnace was made of the tilting variety which materially bettered the conditions for teeming. It may be of interest to mention that Kjellin furnaces have been constructed as though they were self-heating pouring ladles, with which, for instance, the metal could be taken from the open hearth furnaces, then refined and finally poured from the furnace directly into the ingot moulds. The operation of the furnaces is primarily influenced by the fact that the molten metal serves as the secondary winding. Therefore as long as the metal does not possess a conductivity giving an operating voltage having a sufficient heating current, the heating of the furnace by electrical means is impossible. These conditions are the determining factors for the heating of the furnace. As the hearth is of the ring form, it is not feasible to heat the furnace with coke. Care is therefore taken with Kjellin and all other induction furnaces to heat them up with rings of material later to be melted. For making steel, these rings may be cast, welded, or even screwed together, and laid in the furnace. As soon as the current is turned on, induced currents arise in the iron rings, as they become short-circuited THE KJELLIN FURNACE 193 secondary windings. The iron is soon brought to redness, so that the heat thus produced can be used to warm up the furnace. As soon as the furnace walls are red hot, the furnace is charged with fluid metal, and the heating rings are subsequently melted. When this is accomplished, the furnace soon reaches the proper temperature so that the normal furnace operation may begin. If the furnace is operated with hot charges, as is often the case with Kjellin furnaces operating in conjunction with open hearth furnaces, the furnace is fully emptied after each charge and then- charged again with open hearth metal. It is evident that it is possible to fully empty the furnace after each charge. Then when the fluid metal of the new charge, immediately makes a closed ring again, the heating begins simultaneously, provided the primary circuit is closed. The conditions are different when the furnace is charged with cold material. If, under these conditions, the furnace was completely emptied, and a ring made of a large number of pieces of cold scrap, its resistance would be so great, that the proper heating currents could not exist. In this case, it would be found useless to try to obtain a melt. It might, however, be possible to raise the secondary or bath voltage sufficiently, so that arcs would appear between the many small pieces of scrap. In such an event we would obtain heating methods similar to those employed when melting down cold scrap in the Girod furnace. The raising of the voltage necessary to do this, however, leads to difficulties in the transformer design. For this reason, therefore, when working with cold stock, a sufficient portion of the previous charge is left in the furnace to form a closed circuit. If the furnace is now further charged with scrap, it will be melted down by the heat generated in the metal from the previous charge. In this case the cross-section of the bath grows, and a greater absorption of energy takes place, thus hastening the melting. A very quiet melting together of the charge occurs in this way, without any sudden power fluctua- tions. As there is always a molten remainder in the furnace when using the method of cold charging, it is of advantage to keep this remainder as small as possible; still it must be large 194 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY enough to render certain the closing of the molten secondary circuit. The smaller the section of the lower part of the trough, the easier it is to accomplish this. For this reason it is well to make the channel V -shaped. It was previously mentioned that cold charges are melted down without any current fluctuations taking place. We now come to the electrical conditions existing in the in- duction furnace. If we look a little closer at the current con- ditions of the Gysinge furnace, we find that for its operation there is provided a 300 HP water-wheel driving a direct connected 165 to 170 kw. 15 cycle, single-phase generator of 3000 volts. When the furnace content is 1350 kg. (about 3000 Ib.) the power factor is 80%, and with a content of 1800 kg. (about 4000 Ib.) it is 68%. Even these figures show the dependence of the power factor on the size of the charge with Kjellin furnaces. This is also substantiated by the curve in Fig. 77, which was made from results taken from a Kjellin furnace having a maximum capacity of 8>^ tons. This shows, too, how (with other electrical con- ditions remaining the same), the power factor becomes lower ^^^ . JJ.55 0.7 . FIG. 77. with an increased charge. We can, therefore, establish the fact that: "With the same frequency the power factor falls with an increased charge." In searching for the cause of this, we must go back to the causes affecting the power factor. For this purpose we again reproduce the vector diagram originally shown as Fig. 30 in THE KJELLIN FURNACE 195 Chapter IV. We see that the size of the angle < depends on the resistance of the bath r and again upon the factors m and L, In our examples, in both of which the periodicity remains the same, the factor m, depending upon the latter, also remains unchanged. Therefore, only r and L remain as means for reducing the power , factor. It is evident from the dia- gram that when the resistance r of the bath is reduced the angle < becomes larger and the power factor, or cos , consequently de- creases. If the length of channel remains the same, but the cross- section of the bath changes, the re- sistance will change, because r = p and as the example showed, 17HL FIG. 78. that raising the charge from 1350 kg. (about 3000 Ib.) to 1800 kg. (about 4000 Ib.), that is about 33%; and as the cross- section of the bath increased in like ratio, it becomes ap- parent why it is that the power factor falls with an increasing metal charge in the bath. Beside the resistance of the bath, however, the coefficient of self-induction has a noteworthy influence on the size of the power factor. It was shown in Chapter IV that the coefficient of self-induction depends upon the form and arrangement of the conductors. In order to give the reader an idea of this influence, it may be said that for conductors of ring form having a circular cross-section, the following formula for the coefficient of self-induction holds good: L - .(4 log nat-^- - 8). 10-9. Here D denotes the diameter of the wire coil, and d the diameter of the wire itself. This formula, however, is only strictly correct provided the conductor is not in the vicinity of any good magnetic conductors. 196 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY However, it follows that the coefficient of self-induction primarily depends on the surface surrounded by the ring formed conductor, and that the coefficient of self-induction increases, the larger the surface becomes. Besides this the cross-section of the conductor also influences this factor, and as the formula shows, the co- efficient of self-induction becomes a little better with increasing cross-sections of the conductor. This latter influence, however, is too small to nullify the lowering of the power factor, occasioned by the "lowered bath resistance when the cross-section of the bath is increased. The proof of this is plainly seen by the examples given. From what has just been said relative to the power factor it is apparent what the active causes are, and why Kjellin and similar furnaces had to be built with ever decreasing periodicities, for increasing capacities. We have just seen, that the power factor decreases when the charge is increased, due to the lesser resistance to the bath. As we saw in Chapter IV, the lowering of the power factor necessitates a greater current flow than it would have at a higher power factor, in case the furnace is to receive the same power, at a lower power factor and at the same voltage. Heavier currents, however, demand an increase in the copper cross-section of the primary winding, which in turn in- creases the needed space for winding the coil. To this must be added that with an increased capacity the energy absorbed by the furnace is naturally greater, so that the processes to be followed may not be unnecessarily expensive. This, too, necessitates the use of a larger copper conductor, and consequent- ly further increases the winding space. With the same thickness of the furnace refractories, this can only take place, however, when the diameter of the ring shaped hearth is increased; and, as we saw before, this causes a larger coefficient of self-induction, and with it a further decrease in the power factor. With an increasing charge, therefore, the power factor would drop very fast, and this would very quickly lead to impossible operating conditions with the frequency remaining the same. Fortunately by lowering the frequency, we have a means for meeting the lowering power factor. In order to recognize this, if we refer THE KJELLIN FURNACE 197 to Fig. 78 again, we see that the power factor is determined by the equation as r and L are given by the bath conditions, it is only possible to influence the power factor by altering m, and the power factor will, of course, be larger, and so much better, the smaller the quantity m. This quantity m is determined by the equation m = 2 TT v where v equals the number of cycles per second. By 15 10.3 10 Gysinge VGleiwitz Gurtnellen kludno^v^^^ Volklingen ^ 0.4 1.1.5 2 3 3.8 4 5 6 7 8 8.5 9 10 Tons Capacity FIG. 79. lowering the periodicity, the quantity m is reduced, and hence the value of the power factor is kept within reasonable limits for any particular size of furnace. Kjellin also availed himself of this means, and the curve shown by Fig. 79, which appeared in the Elektrotechnische Zeitschrift, in 1907, in an article by Englehardt, shows under what conditions the lowering of the frequency is desirable, with Kjellin furnaces of increasing 198 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY size actually built and operated. The conditions here described are also, of course, more or less applicable for every other induc- tion furnace having a channel hearth. It is well to mention that the lowering of the periodicity is not always feasible as the normal frequencies are 25, 50 or 60. It is not possible to change from one frequency to another by means of stationary transformers, in the manner employed for voltage transformations. If it is desired, therefore, to connect to an existing power station having a higher frequency than would be favorable for the furnace operation, it will be necessary to employ a rotary transformer. In addition a low power factor necessitates comparatively large iron cross-sections in the generator as well as in the transformer, and consequently greater copper lengths for the windings, making a more expensive in- stallation. In order to give an idea of this, we may say that a generator of 25 cycles only costs half as much as one of equal capacity of five cycles. As electrodes are avoided with Kjellin and other induction furnaces, the regulating apparatus for the carbon electrodes themselves is not needed, so that the furnace does not require any movable parts, except the covers. With the absence of electrodes there are consequently no electrode losses, which leads us to the efficiency of the Kjellin furnace. As the furnace is merely a specially designed transformer, the only losses occurring are the ones usually prevalent in ordinary transformers. These losses are due to the iron losses, which are caused by the continually changing direction of the magnetization, and the copper losses in the windings. As the secondary winding in Kjellin furnaces is solely composed of the metal in the hearth, all the losses which ordinarily appear here, are used instead to advantage, because all electrical losses manifest themselves as heat, and in this case the generation of heat is what is desired. Losses, therefore, can only occur in the iron core and in the primary coils. The purely electrical losses of the induction furnace transformer hardly exceed those of the ordinary trans- former. On the whole, the electrical efficiency of the induction furnace is the best obtainable in any electric furnace. As a THE KJELLIN FURNACE 199 proof of the highest efficiency of induction furnaces, it may be said that, the most frictionless transposition of electric energy imaginable into heat takes place here, as the current generated in the secondary circuit, i.e., the induced currents in the iron bath, are generated at their point of origin and directly changed into heat. Induction furnaces may, therefore, be built for any existing voltage, for to generate the particularly high current in the bath is only dependent on a suitable number of turns in the primary winding. It was pointed out, for example, that the furnace at Gysinge, having a capacity of about 1500 kg. (3300 FIG. 80. lb.), is operated at 3000 volts, its primary coil is arranged with 295 turns, so that we have a secondary voltage corresponding to or about 10 volts. As it is possible to use any existing high potential current on the primary side, it is necessary that this part be shielded against coming into any possible contact with other conducting material. This is accomplished by completely encasing the furnace transformer, i.e., the transformer is built with a protecting shell, so that contact with any dangerous parts during the operation of the furnace is practically impossible. In addition to this the protective covering is connected with a copper conductor to the earth, or grounded so that in case any high potential current should strike the protective shield, it would immediately become harmless and flow to earth. Hence 200 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY all danger to those operating the furnace is eliminated, and the best proof of the absolutely safe operation of the furnace, is the fact that thus far no operatives of induction furnaces have been injured. As the operation of the induction furnace is usually not easily understood for non-electricians, the schematic connections of a Kjellin Induction furnace installation are shown in Fig. 80. In this figure the left half shows the electric installation at the central station, and the other side the actual furnace installation. The heavy lines between the central station and the furnace indicate the main high potential conductors. This high tension current is measured with instruments, by interposing so-called potential and current transformers between them and the main conductors so that the instruments only carry low voltage currents. The thin full lines indicate the necessary wiring for this. If we neglect for the moment the dotted lines, we see that the full lines of the circuit in the central station as well as at the furnace show instruments at either place, consisting of an ammeter A , a wattmeter B, and a voltmeter C, which must be watched during the operation of the furnace. D indicates the current transformers for measuring current, and E the potential transformers for measuring the voltage. In order to protect the instruments, the fuses F are inserted, whereas G represents an automatic release, which cuts out the main current when it is overloaded and thereby protects the generator. The generator itself is shown by H. At the furnace we see M which designates the sectors on which copper brushes rub, (similarly to those usually used on a motor.) in order that the furnace may receive its current and still remain in its tilted position. From the contact rails, the current is carried to the primary winding N, in well insulated conductors, which generate the induced cur- rents in the channel O, whose contents simultaneously act as the secondary winding. If we also mention the switch P, at the furnace, which permits the current to be inter- rupted there, we have referred to all the apparatus of the oper- ating circuit. Of great importance is the regulating apparatus of an electric THE KJELLIN' FURNACE 201 furnace, which permits the furnace to receive much or little energy, and thereby enables the furnace to be operated practical- ly. We saw previously with arc furnaces, that besides this apparatus, automatic regulators were also necessary, in order to smooth out the current fluctuations occasioned by the action of electrodes, and to keep a predetermined and constant amount of energy at the furnace. These regulators are wholly absent with induction furnaces, as sudden power fluctuations with induction furnaces are absolutely precluded. We have, there- fore, only to confine ourselves to the apparatus which is necessary to regulate the incoming energy, and for this it is quite sufficient to alter the primary voltage of the furnace. In order to easily change the voltage during the operation at any time, a handwheel rheostat, or regulator is placed at the central station, as well as at the furnace, by the aid of which the magnetizing or exciting current is varied at the alternator. In Fig. 80, / represents the exciter generator, the heavier dotted lines indicate the main wiring of the excitation circuit, and the lighter dotted line denotes the shunt circuit of the exciter gen- erator or exciter. At the furnace is the small regulator L, by the aid of - which the field of the exciter is regulated with a very light current, whereas the regulating resistance K enables the main current of the exciter to be changed. In this way it is possible to regulate the voltage at the central station as well as at the furnace, and in order to keep both regulating platforms in communication with each other, it is usual to have them connected by means of loud-speaking telephones. After this discussion of typical Kjellin furnace switching methods, which are applicable also to other induction furnaces having special generators, we may turn to the comparison of the Kjellin furnace with the ideal furnace. That the Kjellin furnace requires special generators more than any other furnace discussed in detail, was seen when discussing the influence of the power factor; this is the reason for the abnormal frequencies, which, up till now, have been necessary for all induction furnaces having only the ring-shaped hearth. The operation of a Kjellin furnace with other than its own special generator, is not 202 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY practicable and this increases the installation cost, and affects the obvious advantages of the furnace. Among the special advantages of the Kjellin furnace, as the typical representative of the pure induction furnaces, we may count first of all the absolute avoidance of any sudden or undesirable power fluctuations, which must be classified as unavoidable with arc furnaces having vertical or inclined elec- trodes. That there are no reasons for these sudden power changes with Kjellin furnaces we see when we realize, that with this pure resistance heating, sudden power changes could only Wg't Cont't Prod. FIG. 81. occur, with sudden heavy cross-sectional changes of the bath. This condition, though, is positively eliminated because the cross-section can only vary when charging or when tapping the furnace, and as these operations are always the function of a greater or lesser amount of time, it is evident that cross-sectional changes can only appear gradually during this time, and likewise the resistance changes and changes the current strength. This is proved from the operation of all induction furnaces. There is, however, a decided advantage in avoiding any sudden power changes, for it is evident that the generator required for an induction furnace needs to be just large enough to carry the maximum load required over a period of time; whereas a genera- tor for an arc furnace would have to be more liberally propor- tioned, considering the heavy load fluctuations. The curve THE KJELLIN FURNACE 203 of Fig. 57 shows to what degree these power fluctuations occur, and it is interesting to compare this with the one shown by Fig. 81, which latter shows a Kjellin furnace under various methods of operation. These conditions naturally tend to cheapen the construction of the generator for the induction furnace, so that the greater cost occasioned by the generator of abnormal periodicity is at least compensated for to a certain extent. It was seen when discussing the switching mechanism, that the regulation of the incoming energy of a K jell in furnace is accomplished in the simplest way imaginable. It may, therefore, be well to point out again that the regulation of the energy of an induction furnace is accomplished in the most ideal way. For with an electric furnace, the same temperature is found throughout the whole bath, so that any change of the incoming energy alters its temperature gradually without in any way causing any overheating at any one spot, which is always to be dreaded under the electrode in arc furnaces. It has also been mentioned that the induction furnace un- doubtedly has the best attainable electrical efficiency of any electric furnace, because all electrode losses are avoided, and hence only the transformer losses come into play with induction furnaces, except when a special generator is used, and then only the primary copper losses and iron losses appear in the trans- former parts built into the furnace. Transformer losses are, however, present with nearly every arc furnace, thus a transform- er is almost invariably erected as closely as possible to the furnace. As the Kjellin furnace today is always built of the tilting variety, it fulfills one more demand of the ideal electric steel furnace. On the other hand, the Kjellin furnace cannot -fulfill the demand which provides for an easily surveyed and accessible hearth and herein lies its great weakness as compared with other furnaces; its use is therefore restricted to that comparatively small field, in which the requirement of an easily surveyed and accessible hearth is immaterial. Such occasions may, however, occur where a furnace is intended to be a substitute for the crucible furnace, in which a very pure material is mixed in the 204 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY bath, or in case the charge from other furnaces is merely to stand in the electric. The Kjellin furnace or any other induction furnace with a ring-shaped hearth, is found preferable to be used in this way. The advantage it has over the crucible, is that much larger homogeneous quantities of a desired quality can be obtained, whereas crucibles always have a very limited capacity; it is, difficult therefore, to produce large amounts of a predetermined and regular composition. Furthermore, the induction only needs a very limited number of operating men. FIG. 82. Finally, a considerable amount of money can be saved in the crucibles (from $5 to $8 in regenerative furnaces, but as high as $20 a ton in non-regenerative "pan system " oil burning furnaces). As a substitute for the above, the Kjellin furnace seems admirably suited. For most other classes of work, however, the furnace is unsuitable; because it is practically impossible to thoroughly remove the slag from the ring- shaped channel hearth, and thus avoid affecting the purifying process for the succeeding slag. This fact has been proved in practical work by many and ex- tensive tests. One of the next requirements of an ideal furnace is the adequate circulation of the bath by the aid of which the furnace will produce a thoroughly regular material. On account of the magnetic conditions of the Kjellin furnace the circulation of the hearth metal is almost perfect. The proof of this was first published by Englehardt in The Electrotechnische Zeitschrift, in 1907, and is shown schematically in Fig. 82. Here 0/ denotes THE KJELLIN FURNACE 205 the lines of force generated by the primary coil, which take their path through the transformer iron. s ". These lines of force play a very important part, as the iron and the molten metal offer a much lesser resistance to the lines of force than the air does, so that it may be assumed that a part of the secondary lines of force find their way through the molten metal. Both lines of force, a ' and < s ", therefore, essentially flow in opposite directions, as they are generated by currents having opposite directions. (We saw in Chapter IV that the induced current, i.e., the current gen- erated by induction, is always in the opposite direction to the pri- mary current, which is the case here.) Fig. 82, however, shows that the opposite direction of these two lines of force circuits, makes the direction of current flow the same between the primary winding and the bath. It is a fact that lines of force of the same direction repel each other, hence forces must appear between the primary winding and the bath, which endeavor to repel the molten metal from the primary coil toward the outside. In Fig. 83 this force is denoted by P s . Besides this the force of gravity also operates on the bath. Both forces work at right angles to each other, causing a resultant force. Accordingly the surface of the bath inclines at right angles to this resultant, as is shown in Fig. 83. As a matter of fact the inclination of the bath surface can be more or less plainly seen with all Kjellin furnaces. 1 The flow of the metal is from the outside upper edge towards the inner lower one, which in this way provides an intimate mixing of the metal 1 The inclination of the bath of an 8-ton Kjellin furnace is about 24 (See Stahl und Risen, 1910, p. 1071). 206 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY in the bath. One explanation of this flowing or rolling of the bath may be made, if we assume that the parts lying on the outer upper edge are subject to a greater cooling than the inside lower lying metal, so that the higher lying, cooler and consequently specifically heavier metal portions will tend to sink to the bottom, whereas the hot portions will rise. The circulation described has the advantage of the most thorough mixing of the whole furnace contents without mechani- cal aid. With large Kjellin furnaces operating at low frequencies, however, it has frequently happened that the inner wall of the FIG. 83. refractories is quickly destroyed by the circulation directed against it. It was only after decided efforts on the part of the Poldihiitte at Kladno, Austria, in applying the refractories in a special way that they were able to withstand these attacks, so that the lining, even with an 8-ton Kjellin furnace, now lasts six weeks, (164 heats,) when melting cold stock, 1 and 494 heats with hot charges. 2 In the discussion of the Kjellin furnace circulation, it must be stated that the pinch effect mentioned in Chapter III does not come into play as long as the furnace is used for melting iron because the bath cross-sections in relation to the current density are too large when this furnace is thus used. The pinch effect could only be found if the cross-section should be especially narrowed at particular places. If it has been shown that the Kjellin furnace is only a sub- stitute for the crucible, still it may be said concerning the sizes this furnace has attained, that at present the Kjellin furnace ^See also Metallurgical and Chemical Engineering, February, 1913, details of Kladno lining. 2 In 1919, the General Electric Co., of New York, have also brought out an improved bottom and lining for this type of furnace. THE KJELLIN FURNACE 207 has a capacity of 8 tons of steel. One of these furnaces is operat- ing successfully at the works of Friedr. Krupp, at Essen, Germany. Fig. 84 shows the transformer of one of these 8-ton Kjellin furnaces for only five cycles. It is hardly advisable to build Kjellin furnaces of a larger size than this, first because the fre- FIG. 84. quency would have to be reduced still further, and secondly, because it is to be feared that there would be difficulties with the durability of the refractories. Regarding the total efficiency of the furnace, we append the following: Several reports have been made by Englehardt on the Kjellin furnaces. In Stahl und Risen, 1905, page 205, where he speaks of melting a charge consisting of 1/3 pig iron and 2/3 scrap, he figured with a theoretical power consumption of 489 Kw. hours per ton. If we compare the results obtained with the 1.5-ton Kjellin furnace, where with a mixture as above it took 208 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY 966 Kw. hours to melt a ton of steel over a six-hour melting period, and 800 Kw. hours during a four-hour period, this gives a total efficieijcy of 50% for the six-hour melting time, whereas Kjellin himself mentions 47%, and an efficiency of 60% for the four-hour melt. It is interesting to observe how the shortened melting time raises the attainable efficiency of a furnace. The reason for this is that on one hand, the radiation losses are prolonged for six hours, whereas on the other they only occur for four hours. While melting, therefore, it is advisable to operate with as high an incoming energy as possible, and to hasten the work to the greatest extent. The 60% efficiency with a four-hour charge is to be considered as most favorable, considering the small size of the furnace (itf tons). The attainment of such efficiencies, with Kjellin furnaces having such an unfavorable hearth, as far as radiation losses are con- cerned, is only possible because the electrical losses are at a minimum. In spite, however, of the assumed greater radiation losses of the Kjellin furnace as compared with the arc furnace, we find that the induction furnace always has a higher total efficiency than the arc furnace. This becomes even more appar- ent with larger furnaces. In the same article as above, Engle- hardt gives an efficiency of an 8-ton furnace corresponding to a power consumption of 590 Kw. hrs., when melting cold stock. This gives a total efficiency of about 80%. That this figure is attainable as a matter of fact is best proven by the practical operation, where with this ring-shaped hearth a power consump- tion of only 580 Kw. hrs. per ton of steel was attained by the use of suitable heat insulating covers. Regarding the application of this furnace, we refer to the statistics in the closing chapter. The sale of and giving licenses for Kjellin furnaces is handled by the Gesellschaft fur Elektrostahlanlagen in Berlin; in Eng- land and her colonies, except Canada, by the Grondal-Kjellin Co., London, and in the United States and Canada formerly by the American Electric Furnace Co., New York, and at pres- ent by Naylor & Co., agents for Grondal-Kjellin Co., New York. CHAPTER XIII THE ROCHLING-RODENHAUSER FURNACE ALTHOUGH we saw in the previous chapter that the Kjellin furnace and the induction furnace having a ring-shaped hearth, are inapplicable for many uses, and hence at a great disadvantage with the arc furnace, still the induction furnace has important advantages which must not be overlooked, especially where this furnace in its original form finds its best field; viz.: in the re- placement of the crucible furnace. These advantages include the absence of electrodes, and consequent saving in operating costs and also the avoidance of the risk of accidental impurities from the electrodes contaminating the bath, which latter is especially feared when making tool steel. The electrical effi- ciency attainable is also much higher. The absolutely steady furnace operation is almost ideal, and this steadiness is equally excellent for the central station. Finally, we may regard the uniform heating effect throughout the entire bath of the in- duction furnace together with its strong circulation, an advan- tage over the arc furnace, even though the experience thus far gained concerning the influence of the high temperature of the arc on the quality of the steel is not yet extensive enough to form a conclusive opinion on this point. Realizing the good points of the induction furnace referred to above, it was not long before efforts were made to retain the advantages of induction heating. For the disadvantages of the single ring hearth were clearly recognized. Later on pains were taken to alter the hearth in such a way that it would meet the demands of the metallurgist, and to produce thereby an induction furnace which would be equal to any refining work. It was recognized that if at the same time the operating conditions could be bettered (these as we have seen with the Kjellin furnace 210 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY necessitate the use of machines having unusual periodicities), then the induction furnace would be able to enter into successful competition with the arc furnace in any field. It was these considerations which led the Rochling Eisen und Stahlwerke at Volklingen on the Saar, Germany, to consider the problem of re-designing the induction furnace. This de- cision was reached only after it was clearly recognized that the single-ring channel induction furnace was not practical for refin- ing work. The first German Rochling-Rodenhauser patent was applied for on May 6, 1906, and subsequently granted (No. 199354 *). FIG. 85. This patent covers an induction furnace as shown schematically in Fig. 85. It may be seen that both cores of the transformer are provided with coils, in contradistinction to the Kjellin construc- tion. Both cores are surrounded by an induction channel, which are joined between the cores in the middle, forming a roomy work- ing hearth. This middle section is sometimes heated by means of an auxiliary current supplied by a secondary winding wound next to the primary winding, which has the marked advantage of reducing the stray field, and hence improves the power factor. 1 Corresponding to U. S. patent No. 877739 of Jan. 28, 1908. THE ROCHLING-RODENHAUSER FURNACE 211 The furnace principal shown in the sketch is known as the Roch- ling-Rodenhauser furnace. This furnace was investigated for its usefulness at the Rochling Iron & Steel Works from July to September, 1906, by means of a small test furnace holding 60 kg. (132 lb.), and operated from a 50 cycle circuit. Fig. 86 shows this furnace at one stage of the tests with suspended electrodes composed partly of conductors of the second class, being used as a mode of utilizing the auxiliary current, notwithstanding FIG. 86. that the patent specification mentions conductors of the second class for transferring the current, which is the method exclusively employed today. The tests with the small furnace were later continued with a somewhat larger furnace, holding about 300 kg. (660 lb.). In the course of the development a furnace of about 500 to 750 kg. (noo to 1650 lb.), was ordered by and constructed for the Richer Hiittenverein, which company was desirous also of investigating this form of furnace, knowing of the tests carried out at Volklingen. Until this time, the small furnaces were 212 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY provided with covers which had to be lifted when charging, but the Eicher Hiittenverein furnace was the first one to be supplied with doors, thus simplifying the furnace operation. This arrangement may, at the same time, serve to indicate how, with the progress of developments, the work of refining in the furnace was transferred more and more from the channels to the main hearth, where it is carried on today exclusively, the channels serving only as heat carriers, without in any way accomplishing any metallurgical work. One of the determining factors in the further development of the furnace was due to the erection in the spring of 1907, at Volklingen, of an 8-ton Kjellin furnace which operated at only five cycles. The exhaustive tests carried on with the aid of this furnace, furnished convincing proof that the ring-shaped hearth was unsuitable for extensive refining, which was the goal of the Rochling Iron & Steel Works. On the other hand, these small test furnaces, above mentioned, gave the most favorable results in the refining of steel. Because of this a Rochling-Rodenhauser furnace was built and designed for the electric plant which had been installed to operate the five-cycle Kjellin furnace. This first large furnace had a capacity of about 3 tons and was placed in operation on June 22, 1907. It soon demonstrated the ad- vantages of the new furnace principle for large units. In order, however, to render this furnace system adaptable to all conditions, there was still one further step to take, i.e., to derive means to operate the furnace with polyphase current. For as long as it was not possible to use polyphase current directly in the induction furnace, the advantage of the induction furnace in its being able to be operated with any voltage that is available, would be of minor importance. The reason for this being that as it is only possible to operate the furnace with single phase current, it follows that the installation of a rotary trans- former would be necessary when obtaining power from a three- phase circuit. As early as 1907, therefore, the constructive features of a polyphase furnace were considered, and in February, 1908, the first polyphase Rochling-Rodenhauser furnace was placed in THE ROCHLING-RODENHAUSER FURNACE 213 FIG. 86a. Two Phase R.-R. In- duction Furnace. operation. This was designed for 50 cycles and connected to the 3 phase electric plant of the Rochling Iron & Steel Works. The application of the furnace to polyphase current was patented in all industrial countries. These short remarks show the development of the Rochling- Rodenhauser furnace, which can be obtained to-day not only for single phase current, but also for two and three phase cur- rent, for any convenient voltage and for normal fre- quencies. In its present form the Rochling - Rodenhauser fur- nace consists of a casing of strong sheet iron, which is supported by means of a semi-circular saddle and rack ori rollers, thus allowing the furnace to tilt. The tilting may be accomplished in any way desired, but is usually done by means of an electric motor and suitable gearing. The furnace transformer is built into the shell. The upper yoke of the transformer is arranged to be easily removable, while the lower yoke and the cores are securely fastened by bolts, to the furnace casing, so that the transformer may stay securely in position even though the furnace is tilted 45. If we now turn to the furnace in its single phase form as shown in Figs. 87 to 89, which indicates a 5 -ton furnace operating at 15 cycles, 5000 volts, we find two cores of somewhat long-drawn-out rectangular form. The cores are composed of a number of sections, which in turn are built up of paper-covered sheet iron of .3 mm. (.012 inch) thickness, the sections being separated from each other by the ventilating ducts H. Each core carries a primary winding A, and a secondary winding B. The primary winding is connected directly to the incoming voltage intended for the furnace, in the foregoing case, 5000 volts. The current is led to the windings by means of the usual high tension underground cable and thence 214 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY to brushes, by the aid of which it is carried to a copper slip ring. The current is then led directly to the winding, by stationary conductors carried on high tension insulators. These methods of leading the current to the windings have the advantage that the heating may be continued, for instance, when the slag is being rabbled off, i.e., the furnace continues to receive its heat when in the tilted position. The secondary winding lies next to and FIG. 87. separated from the primary winding by an air space, which is both an insulating protection and a cooling chamber. The secondary winding is composed of heavy copper strips and carries very heavy currents at very low voltages. From this secondary winding, copper connections lead upwards from which the current is led to the poleplates E. These connecting pieces are represented by lines in Fig. 87. The whole winding arrangement is surrounded by two cylinders of copper, brass or monel metal, which are separated from each other by an air space. Similarly there is an air space THE ROCHLING-RODENHAUSER FURNACE 215 between the secondary winding and the inner cylinder. The inner cylinder is closed at the top by means of dust catchers in such a way, however, that the cooling air from the furnace trans- former may escape at the upper end with the least resistance. The method of supplying the cooling air is shown by means of the central air duct in Fig. 88, and the air direction is shown by the arrows. As a matter of fact this represents the method of FIG. 88. applying the air supply for Rochling-Rodenhauser furnaces today, for such a centrally located movable duct underneath the furnace is provided from which the air is led to both cores. The air is so divided that the greater part takes its path upward through the winding space, thereby cooling the coils and the transformer cores, whereas a smaller part passes through the space between the two protective cylinders M , in order to keep the heat radiating from the brickwork, away from the whole transformer construction. Air is alone used for cooling from a blower usually of very 216 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY low pressure. At Volklingen for instance with an 8-ton single phase furnace the blower pressure only corresponds to 40 mm. (1.6 inches) water gauge pressure. These cooling arrangements have given the most complete safety to the furnace during the past 8 years' continuous operation. In order that the unavoidable cooling of the furnace trans* former may not cause too great heating losses, much considera- A I FIG. 89. tion was given to the best possible heat protection for the hearth. On that account the outer protective cylinders are surrounded with a layer of granular material, which acts as a heat protector. On the outside of this follows the real refractory mass of either dolomite and tar or magnesite and tar. In order to obtain a hearth of the desired shape as shown in the figure, a wooden or cast iron templet is lowered into the furnace after the bottom has been rammed in, in a similar manner as with the Kjellin furnace. On the side of this templet, the hearth walls are tamped in, which when the templet has been removed leaves the necessary space for the molten metal. The cross-section THE ROCHLING-RODENHAUSER FURNACE 217 a b of Fig. 87 plainly shows that the hearth really has the shape of an 8. It may be seen that the transformer cores are sur- rounded on the outside by the narrow channels C, while between the cores lies the true hearth and working chamber. Working doors are provided at both ends of the hearth, which makes this easily accessible and hence greatly lessens the necessary attend- ance at the furnace, as the entire roof of the furnace covering both hearth and channels remains stationary throughout the whole working period. The channels themselves are not intended for the metallurgical process, but they are of course necessary to provide the induced heating currents for the hearth. Concerning the hearth refractories, it may be mentioned that following the dolomite and tar outer hearth walls, there is provided a layer of coarse-grained heat insulating material, and that finally between this and the furnace shell is placed a ring of heat protecting brickwork. All parts of the roof covering are easily removable, in order that they may be easily lifted off and quickly replaced, in case a new lining is to be rammed in. It was remarked before that the copper secondary winding B leads to the poleplates E. These plates are imbedded in the lining, as shown in Figs. 87 and 89. They are made of soft cast steel, and have the largest possible surface on the side toward the hearth. Between the poleplates and the bath is the hearth wall, which as we have seen consists of dolomite or magnesite and tar, so that the poleplate is protected against direct contact with the molten metal. 1 Mention has been made in previous chapters that the refrac- tories used are conductors of the second class. That is to say, these materials, which are non-conductors at low temperatures, lose their resistance more and more with increasing temperatures, until finally they become comparatively good conductors at the temperatures which are prevalent in electric furnaces. This property of conductors of the second class is sometimes util- ized in Rochling-Rodenhauser furnaces to carry the current from the secondary winding by means of the poleplates to the molten bath itself. In this manner that portion of the lining 1 See " The Practicability of the Induction Furnace for the Making of Steel for Castings." By C. H. Vom Baur. American Foundrymen's Association. Vol. XX, 1912. 218 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY FIGS. 90 and 91. THE ROCHLING-RODENHAUSER FURNACE 219 over the poleplate may be designated as the mass which transfers or conducts the current. At the beginning of the furnace heating this mass will act as a large resistance in the secondary circuit. This holds true as long as the furnace is heated up with iron rings, exactly as is done with KjelJin furnaces, and the same conditions exist when the furnace is charged with its first hot metal, so that at first we only have simple induction heating. As the furnace is further heated, the temperature also rises in the conductor of FIG. 92. the second class in front of the poleplate, and the resistance consequently drops under correct conditions, the secondary winding soon carries a considerable portion of the total energy of the furnace to the bath. With the 8-ton furnace at Volklingen, this result usually takes place in twelve hours. For the normal operation of the furnace there is therefore a double heating; first, we have the single induction heating in which case the ring formed parts of the hearth are to be designated as secondary circuits, and secondly the heating from 220 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY the current of the copper coil secondary winding, which is carried to the bath via the poleplates and conductive lining. The arrows in Fig. 87 show both current plates of the secondary side, which plainly show that the total current flows in the same direction through the hearth. The conditions are much the same in three phase furnaces. These furnaces receive the three conductors from the three phase circuit on three cores, which are very similar to the ones of the single phase furnace, in that each core carries a primary and a secondary winding. Here also we find the coils surrounded with an inner and outer protecting cylinder, through which the ventilating air flows. In building these furnaces, it was the endeavor, of course, to provide a single roomy hearth, which could be easily surveyed and be easily accessible. Following these maxims the designs shown by Figs. 90 to 92 were evolved. The central hearth is consequently surrounded on three sides by the cores. Toward the outside, (corresponding to the arrange- ment of the single phase furnace,) these are encompassed about by induction channels, which join together and form the central hearth A . The yoke is often bent around in the form of a horse- shoe, by the aid of which the cores are connected at top and bottom. In order to make the hearth easily accessible and visible a door is fitted between each two cores, of which the one opposite the central cores is supplied with the tapping spout. The furnace is therefore emptied in that direction. There is a poleplate having two arms near each door toward the bath, which is pro- tected by the conductive lining from the bath, as in the single phase furnace. The arms of the poleplates are connected with one pole of the secondary copper winding, whereas the free ends of the other pole are connected together to the neutral point N, by means of the copper bar connections there shown. In order that there shall be no misconception about the current connections of a Rochling-Rodenhauser furnace Fig. 93 is given which shows the schematic diagram of a single phase. Similarly Fig. 94 shows the schematic diagram of a three phase furnace. THE ROCHLING-RODENHAUSER FURNACE 221 In Fig. 93 both primary coils of the single phase furnace are shown connected in parallel. This also applies to the secondary coils, which are connected in parallel by the poleplates, between which the current flows through the bath. Fig. 94 shows the primary winding for a three phase furnace and their neutral point A 7 1 . The heavier drawn secondary winding of the three cores has one end of each coil connected to the neutral point N 2, while the free ends are also here connected to the pole- Fro. 93. plates, between which the current-carrying lining and the bath are connected as heat-resisting material. Besides this both figures show the hearth and channel limits. The conduits of a channel and hearth form a short circuited secondary, in which the heating currents are directly induced. The operating method resembles that of the open- hearth furnace, as a roomy working hearth is provided, and hence the conditions are present for successful refining work. If the furnaces are to be heated up, and hot metal is obtainable, this heating is accomplished similarly to the method used with 222 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Kjellin furnaces, i.e., iron rings for starting are laid in the furnace channels before the roof is replaced, which rings serve to bring the lining to a red heat under the action of the induced currents. When this is accomplished, hot metal is taken from any con- venient melting furnace and charged in the electric furnace - and with this charging the starting or heat rings are melted, which FIG. 94- permits a quicker heating up of the furnace, as the cross-section is larger and consequently the energy supplied is greater. This heating permits the furnace to be placed in operation in the shortest time. It has, however, the disadvantage that it neces- sitates the use of molten metal from some other melting furnace. Such apparatus is often available and the molten metal may be obtained from converters, open-hearth furnaces, cupolas, crucible pots or even blast furnaces, so that the disadvantage is seldom felt. It is more difficult, when a source of molten metal is not available, to start the electric furnace, and a melting furnace THE ROCHLING-RODENHAUSER FURNACE 223 would therefore have to be furnished just for this purpose. In order to avoid this disadvantage, trials were made at the works of the Rochling Iron & Steel Co., with the object of starting up induction furnaces without the use of molten metal. This test produced satisfactory results, and the method is patented. In accordance with this method, the starting rings are solidly packed with pieces of scrap, steel turnings, etc., until the heating channels and the hearth are completely filled. After the roof is replaced, the current is switched on, and the heating rings soon become red hot under the action of the current. This assumes that with the furnace voltage remaining the same, the absorption of energy will rise. With a 2-ton furnace it is pos- sible, for instance, in twelve hours, to render the entire furnace contents fluid, and the normal operation may then start. When heating up with hot metal about eight hours would be neces- sary in order to proceed with the normal furnace operation. A normal heat with a Rochling-Rodenhauser fur- nace is much the same as with an arc furnace. The first, after which the slag is FIG. 95. dephosphorizing usually occurs completely rabbled off. so that no deleterious material remains to delay the formation of the new slag for desulphurization. The removal of the slag occurs by rabbling through the doors. Of course it is im- possible to remove the slag from the channels, where they sur- round the cores, as these channels are quite unsuitable for this work. On this account they are permanently closed in by the furnace roof, as is the hearth itself. As the slag cannot be removed from the channels some provision must be made so that the slag is prevented from entering the channels. This is 224 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY now accomplished by placing fire-resisting bricks of magnesite or dolomite across the commencement of the channels in such a way that the iron bath in the hearth is i or 2 cm. (3/8" to %"") higher than the lower edge of the channel bridging bricks. In order to avoid as much as possible the heat losses occasioned by the bridging channel bricks coming into direct contact with the metal bath, a further normal roof of ordinary fire-brick is placed over them, which helps to lessen the heat losses by en- training a stationary layer of air. Fig. 95 shows these channel bridg- ing bricks, together with the refractories surrounding the channel. Mention should be made of the behavior of the Rochling- Rodenhauser furnaces when melting down scrap. So far it has not been possible to avoid the necessity of having in the furnace FIG. 96. a portion of the charge, which, as we have seen with the Kjellin furnace also, is necessary when working up scrap, in order to provide the necessary circuit for the induced current, so that the scrap charged in the furnace may be melted down under the influence of the electric heating currents induced in the remaining portion of the previous charge. When scrap is to be melted down, therefore, and no fluid charge is at hand, the disadvantage consists in not being able to pour the entire charge. A certain percentage, say a quarter or a third of the entire contents, must remain in the furnace. The conditions are of course different when operating partly with a fluid charge, as for instance from THE ROCHLING-RODENHAUSER FURNACE 225 a converter or an open-hearth furnace and partly with scrap. Then the conditions are similar to working only with a hot charge, so that there is no reason for leaving any of the charge in the furnace. With a mixed charge, therefore, the metal is fully teemed after each heat, after which some fluid metal is taken from some other furnace and poured into the electric furnace, which permits of the flow of the induced current. Thereupon the metal to be melted is charged gradually or at once, to such a degree that the cold and hot furnace contents at times reaches the roof. After this no attention is necessary until the entire contents is melted down. This takes place without the slight- est current disturbance, while the current and kilowatt curves rise slowly, as shown by the curves in Figs. 96 and 97, which FIG. 97. were taken from an 8-ton single phase furnace, and from a i. 5-ton three phase furnace, respectively, both at Volklingen. In the latter curve it is to be noticed that one division denotes 45 amperes, also 50 volts, also 30 kw., also .1 for the power factor, and ten minutes. The crosses on the bottom line denote that 100 kg. of scrap were charged. It is also of importance from the standpoint of the practical operation of a furnace system that it is convenient to shut down the furnace for a limited time, for instance over Sunday. During such stops, a Rochling-Rodenhauser furnace is partly charged or even filled to capacity. It is then sealed up, after which the current is switched off and the furnace requires no further atten- tion. When it is desired to start up again, the current is switched on for several hours, and the furnace is thus heated up anew. 226 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY When starting up in this way with the furnace fully charged, the 8-ton furnace at Volklingen after a 2o-hour shut-down is ready for normal operation in about six hours. The method of operation of the electric furnace is exactly the same as with the Kjellin furnace, as far as the induction heating is concerned, which is generated by means of the primary coil, in the ring-shaped portion of the iron bath. This is applied in Rochling-Rodenhauser furnaces, twice on single phase, twice on two phase, see Fig. loia, and three times on three phase furnaces. This heating therefore does not require any further explanation. On the other hand it is different with the secondary circuit, which is composed of the copper winding wound directly over the primary coil. The current then flows through the pole- plates, the current-carrying lining, and the metallic bath. The object of the secondary circuit is to raise the power factor, and to aid the heating of the furnace contents. It was seen that the low power factor of the Kjellin furnace, especially the low power factor of the larger sizes, led to the use of machines having very low frequencies, which materially increased the cost of installation. The reason for this decreasing power factor is found in the low bath resistance, together with the high coefficient of self induction, which was caused by the great distance between the coil and the bath. It was therefore necessary to investigate these causes, if the above-mentioned lowering of the power factor was to be avoided. In order to increase the bath resistance the long rectangular form of core was chosen, in place of the more circular shape used with the Kjellin furnace. Furthermore, by placing the winding on two or three cores, it became possible to materially decrease the inner periphery of the induced part of the bath, as compared to that of the Kjellin furnace. This brought about substantial advantages, so that the power factor with Rochling-Rodenhauser furnaces stays much higher than with Kjellin furnaces having equal capacities and equal frequencies, even during the heating up period, i.e., at a time when the poleplate circuits cannot yet do much work, because the current-carrying lining has too high a resistance. In order to further decrease the leakage as much THE ROCHLING-RODENHAUSER FURNACE 227 as possible, because of the comparatively large distance between the primary coil and the bath still remaining, use was made of electric conductors placed in the path of the stray lines of force. This expedient was mentioned when discussing induction furnaces in general. The conductors of Rochling-Rodenhauser furnaces, which ar = 0.9. The results under normal conditions were the production of about 11.5 metric tons of pig iron per 1000 E.H.P. days. Although good results were obtained with various ores, judging from a metallurgical standpoint, it was seen that an electrode entering the furnace with the charge would not satis- factorily solve the problem. For instance the electrode frequent- 244 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY ly rose higher and higher in the shaft of the furnace, so that the material in the bottom got colder and colder. This was caused by the charge becoming too dense, and not allowing the gases to escape easily enough. In this way the resist- ance between the two poles was lessened, and the voltage remaining the same, the upper electrode rose in the furnace. It also brought about con- siderably higher electrode consumption. To sum up the question, a suc- cessful electric shaft fur- nace was not solved by the experiments made in Canada. In the spring of 1907, experiments with electric pig-iron production were begun in Sweden. Messrrs. Gronwall, Lindblad and Stalhane together formed the "Electrometal" Company, with the aim of building and selling electric furnaces. The tests which will now be considered in detail were carried out by them at FIG. 108. Domnarfvet. According to Yngstrom's careful re- port in the Jern-Kontorets Annaler, No. 9, 1909, a current of 7000 volts at 60 periods was used. With this current a 900 HP motor was driven, directly coupled to a 25 period, three phase generator. From this generator the current was led directly TITE ELECTRIC SHAFT FURNACE 245 to the transformers which were arranged near the furnaces and served them. Here also a switphboard with the necessary measuring instruments was set up. These included a watt- meter, three ammeters, and one voltmeter. Underneath were the hand wheels for regulating the electrodes. Gronwall, Lindblad and Stalhane first made use of the results of the former experiments. They therefore endeavored to completely obviate the use of hanging electrodes, and to keep the current in the hearth of the furnace. Fig. 109 shows the first test furnace, which was built to take single phase cur- rent. Each pole consists of a copper plate carrying a graphite block. These blocks are hollowed and lie outside of the furnace proper. Channels lead from them into the furnace which, when filled with molten iron, serve to con- duct the current to and through the charge. Be- sides these two conduction FIG. 109. channels that are arranged on one side of the furnace, there is a third one, as may be seen in the illustration, and which serves for tapping the furnace. After charging, the furnace is run precisely as an ordinary blast furnace, until a considerable amount of metal has collected in the hearth. This insures good conduction from the carbon electrodes to the interior. The blast is then stopped, the current switched on, and the electric heating begun. The course of the current was arranged as follows: It entered at one pole and passed through the metal lying over it into the metal in the channel at one side of the furnace proper, 246 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY from here through the charge to the metal in the channel at the other side, and so to the outgoing pole. Heating is brought about through an overheating of the liquid contents of the furnace on the one hand, and the resistance offered by the charge on the other. This should furnish heat sufficient to smelt the ore. The hearth was made of quartz. In operation it only lasted a very short time, so that the furnace could not be operated for very long. This was because its wave-like surface offered conditions favorable to attack, and brought about quick destruc- tion at the high temperatures reached. The first necessity was to rebuild the furnace. This was done in such a way that the electrodes led into the furnace from opposite sides, as is shown in Fig. 1 10. At the , same time it was hoped that the use of mag- nesite in the hearth would give better service. This, however, was not the case, for the reason that the magnesite, a fairly good conductor even at ordinary temperatures, became too good a conductor at a high temperature, and the ex- periment had to be stopped. This second test ' showed the impossibility of satisfactorily lead- FIG. no. i n g the strong current necessary for heating a shaft furnace into the charge from the bottom. This style of furnace was therefore rejected. The third test furnace approximates in form the one already proposed by Heroult in his patent of 1902. It is shown in Fig. in. The shaft-like construction is furnished with three electrodes, of which one forms the bottom, while the two others are arranged on opposite sides at a medium height. The direction of the current can be so arranged that it either flows horizontally from one shaft electrode to the other, or else goes out through the bottom electrode. In operation the shaft electrodes were destroyed so rapidly that they were replaced by ordinary water cooled electrodes with continuous feed. With this arrangement considerably better results were obtained, but the walls near the shaft electrodes were so rapidly destroyed, because of the intense heat generated, that this style of furnace was also rejected THE ELECTRIC SHAFT FURNACE 247 as unsatisfactory. It, however, pointed the way to a good solution of the question. If care was taken to keep the intense heat, which is produced where the electrodes and charge come in contact, away from the walls, then more favorable results and a greater furnace life would be obtained. These considera- tions led to surrounding the electrodes directly with the charge, so that the heat, which was formerly lost through the walls, could now be used for heating the charge, and at the same time a much greater durability of the furnace walls was obtained. The 1909 test furnace is shown in Fig. 112, the lower part of which may be considered as the final form of the electric shaft furnace. This is the furnace of Gronwall, Lindblad and Stalhane. It has three electrodes penetrat- ing the roof of this hearth and is in general very similar to the ordinary blast furnace, except that the tuyeres are replaced by electrodes. The results show that this construction in its 1911 and 1912 improved form is the most complete and suitable produced, and is perhaps the only one worthy of serious considera- tion. A detailed description is given below. The smelting part of the 1909 furnace forms a large crucible or hearth f 4%" in diameter, 4' n" high. It is lined with mag- nesite. The shaft of the furnace is arranged above the hearth, and has a height of if with an interior diameter of 4' 3" at the widest part. The shaft is supported by a steel framework resting on six iron columns. This makes it possible to independ- ently repair the hearth. The charge falls from the shaft into the hearth through an opening arranged in the roof. It forms an angle or slope of about 50 to 55. This produces a free space between the charge and the roof and walls of the hearth, on which the greatest importance is to be placed. It serves to cool the electrodes and the walls of the furnace. To help in this FIG. in 248 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY purpose the cool waste gases from the top of the shaft are taken and blown through tuyeres into this cooling space. This method also brings heat back to the furnace, and so gives a better heat efficiency. As Fig. 112 shows, three carbon electrodes penetrate the roof of the hearth. In the 1910 model four electrodes, and in the 1911-1912 model six electrodes, are used. The early electrode _B FIG. 112. consisted of two carbon blocks 13" square, so that the total cross- section is 169 sq. ins. The electrodes are made in Sweden from retort carbon, and permit the use of a current of 25.8 amperes per sq. in. The electrode holders consist of strong steel frames. These THE ELECTRIC SHAFT FURNACE 249 are provided with several wedges by means of which the copper plates that carry the current from the cables to the electrodes are firmly pressed against the latter. The electrodes are operated by hand, and the part projecting from the furnace is provided with an asbestos cover to prevent oxidation. The openings for the electrodes have water-cooled seats, and arrangements are provided to prevent the escape of gas, which is most important. When the furnace is put in operation it is run, at first, exactly like an ordinary blast furnace. The electrical heating is only used later. The furnace now described was run, with slight interruptions, from May 7, 1909, to the end of July. The follow- ing notes, taken from the account of the operations, are of special interest. At the beginning of the electric heating the current goes chiefly through the upper part of the charge, which means that the largest amount of heat is produced immediately under the roof, which is strongly heated and partly destroyed. One reason for this is that the lower part of the charge is colder, and therefore offers greater resistance than the upper part. The conditions were greatly improved as soon as the waste gases were blown in. The temperature of the roof was lowered, and the hottest zone sank lower and lower. The result was a lower- ing in the resistance of this part of the charge, so that the current found a more favorable path, and was concentrated in the lower part of the hearth. When this condition was once reached a five days' interruption of the gas-cooling brought about no change from normal running. During the operation of the furnace no big fluctuations of the current were noticed, and even during tapping the instru- ments remained steady. This leads to the conclusion that the resistance of the charge was very constant. The electrodes required very little attention. They were regulated once a day on the average, and in one case they were not touched for five days. The maximum current amounted to 9000 amperes per phase. With 25 cycles a power factor of 0.8 to 0.9 was obtained, with 60 cycles of about 0.7, and other calculations gave 0.535. Natu- rally with a fixed cross-section of electrodes the amount of energy 250 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY that can be used is dependent upon the permitted potential, which in its turn depends upon the resistance of the charge. The higher this resistance, the higher can the voltage be without the strength of current overstepping the permitted maximum. It is therefore of interest to know how to influence the internal resistance, and this consists in the choice of the proper amounts of ore and fuel in the charge. In the following table are given the strengths of current reached with various burdens, and with fixed voltages. Charge with Potential between two Two Phases in Volts Current Strength per Phase Amperes Power with an Average Cos = 0.85 A = 1.73 ei cos < Coke in excess 34 9,6OO 480 kw. Coke not in excess 36 8,800 465 " Too little charcoal. . . '. Sufficient charcoal Too much charcoal 60 54 48 6,300 7,600 7,6OO 555 " 603 " 536 " Too much coke and charcoal 35 9.2OO 471 " Sufficient coke and charcoal .... 48 7,6OO 536 " The operation of the furnace was very simple and uniform, the metal being tapped about every six hours. When judging the efficiency of the furnace it should be remembered that the following sources of loss are to be considered: 1. Cooling of the electrodes with water. 2. The ohmic resistance of the conductors and contacts. 3. The radiation from the furnace. The total loss amounted to from 230 to 270 kw., the higher value coming at the end of the run. The loss is divided about as follows: The water cooling carries away from 118 to 225 kw., which, with a power of about 500 kw., corresponds to a loss of about 25 to 30%. Overcoming the contact resistance takes about 40 kw., and from no to 180 kw. are lost by radiation. The electrodes lose 5.8 kg. (12.8 Ib.) per metric ton by burning away, the total consumption being 13.8 kg. (30.4 Ib.) per metric ton. From another source (E. F. Ljung- THE ELECTRIC SHAFT FURNACE 251 berg, Metallurgie, November, 1909), the consumption of elec- trodes through burning is 8.8 kg. (19.4 lb.), and through waste ends 13.9 kg. (30.6 lb.), a total of 22.7 kg. (50 lb.) per metric ton. This large difference between the loss by burning and the total consumption is brought about by the electrodes not being completely burnt, and the ends having to be replaced by new ones. There is no loss from stub ends in the later designed electrodes which are screwed together. The maintenance cost of the furnace could not be determined exactly, but the furnace worked satisfactorily for 85 days without a stop. The weakest place is the roof of the hearth, which is exposed to the intense heat generated at the electrodes. Accord- ing to Ljungberg, 891,623 kw. hours were used to produce 280 met- ric tons. This means 0.492 h.p. years or 3184 kw. hours per metric ton of pig iron. This is a high figure and has since been lowered to less than 2000 kw. hours, or 0.31 h.p. years, on long runs.* The following tables give the efficiency obtained during the test with different burdens: Charge No. Carbon Consumption (pure carbon) Amount of Theoretical Energy Neces- sary with the Given Carbon Real Power Consumption Electrical Efficiency Kg. Lb. Consumption kw. hrs. kw. hrs H.P.Year 365 days 3 252 555-5 1,470 3,H4 0.483 47% 4 254 560.0 1,438 2,473 0.383 58 5 284 626.1 1,741 3,245 0.505 54 6 294 648.1 1,870 3,334 0.517 56 The economy of making pig iron in the Gronwall, Lindblad 6* Stalhane furnace is given in the chapter on operating costs, hence the following table by Catani is of interest. It is quoted from Neumann, Stahl und Eisen, 1909, p. 276. This table shows how high the price of cufrent per h.p. year can go, with coke at a fixed price, for the electric shaft furnace to compete favorably with the ordinary blast furnace: * See page 250. 252 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Pig Iron Produced per 24 hrs. per H. P. Coke Price Kg. Lbs. $3.81 IS- 7i $7.6l 6 13.2 4.88 7-30 9-76 1 Price of 3 I 7 .6 6.09 9.14 12. 19 \ Power IO 22. O 7 .6l 11.42 15.23 f Per 12 26.4 8-57 12.85 17.14 J h.p. year For comparison with the foregoing figures of the first tests in 1909, the 1910-1911 tests results are here recorded. The November, i9io-April, 1911, test furnace of 2500 h.p., of Gronwall, Lindblad & Stalhane, is shown in vertical cross- section by Fig. 125. During the run the furnace was operated with four electrodes penetrating the roof, the furnace being operated with two phase current, from a three phase circuit by means of Scott connected transformers. The incoming current is 10,000 volts, three phase, 25 cycles. The secondary volts can be regulated between 50 and 90 volts from the high tension side. The arrangements are such that the different phases can work simultaneously with different voltages. The method has greatly facilitated the working. Regulation is also had by different switching from the low tension side. The newer 3500 h.p. furnaces for Hardanger, Norway, using coke instead of charcoal, have the following dimensions: Diameter of hearth 3 meters = 10 ft. at ring 1.5 " = 5 " " at boshes 2.15 " = 7 " Height of shaft 12.0 " =40" Total height of furnace 13.7 " =45 " These Norway furnaces are somewhat different from the Trollhattan furnace. The volume of the shaft is smaller, but its diameter is greater than the Corresponding shaft of a charcoal furnace. The coke in the charge gives it greater conductivity, so that a lower voltage is used. The ratio of volume of charge per day to shaft volume has been taken at 1.55, and the furnace volume has hence been THE ELECTRIC SHAFT FURNACE 253 made 38 cubic metres (about 500 cu. ft.). The furnace hearth is lined with magnesite. The general contour of the furnace walls and roof over the hearth can best be seen by consulting Fig. 125. The roof is cooled as described under operating costs. The gas that is blown through the tuyeres is cleaned in a water scrubber in the latest designs, as shown in Figs. 126 and 127. The electrodes used during the beginning of 1911 were built up of 4 carbons 2 metres (6^2 ft.) long and 330 x 330 mm. (13" x 13") section arranged to form an electrode 660 x 660 mm. (675 sq. in.) section. 17,000 amps, is the permissible maximum or 25 amps, per sq. in. Toward the end of the year this has been changed to a cylindrical electrode of 600 mm. (23.6 in.) diameter, which is gripped much shorter than formerly (see Fig. 126), thus saving 40 kw. The square electrodes were supplied by both the Plania Works of Ratibor, Germany, and from the Hoganas Works, Sweden. The 600 mm. round elec- trodes have lately been furnished by the former works and by Siemens Bros. & Co., Litchenberg, near Berlin. The upper part of the electrodes is covered with sheet asbestos and thin sheet- iron, and the top surface is covered with a thick layer of ground asbestos and silicate of potash. They also have a water-jacket, beneath which gas was blown to cool the roof (see Fig. 124). This practise was not long continued, as the C0 2 burned holes in the electrodes. W T hen starting the furnace, it is thoroughly dried out with wood and charcoal fires, and heated up electrically by filling the hearth with coke and turning on the current. About 3 weeks is taken to burn through an electrode above the so-called "stock line." During January, 1911, the average voltage on each phase was 62.6 volts, and the average current per phase 14,449 amps. The average reading on the wattmeter was 1535 kw. ; the power factor was consequently .88 + %. The efficiency of the furnace has been greatly increased since the tests were made with the 800 h.p. furnace at Domnarfvet, and is discussed under operating costs. The following table indicates the efficiency obtained during the tests as indicated: 254 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY 6 o JS| || if |f || !_J | Si o 5 1 oo' f 10 oo M Q Si *1 V 3 ' n 10 10 4 CO 'Oj-SiOj'SlO s c ; 1 If * 2 1 111 Jll II U| M 00 "* \O >O oo q cocoo q q- CO ^ CO CO CO ""> O a! il 1MI O\ TJ- N ^r ON cO -st" rT . co t^- 04 O C ^ U u lO t^ >O ^~ C, co 01 vO O) H 5 * CO t^. CO 10 Ol Ol CO CO CO ; Cu 1 i oo q o; oj oo i-I w co t-^. OJ . U I * o r i I $ i 1 * 1-1 00 00 O Tj- xO itf *J * O" 00 >O (NI \O \O II s o O O O) O Q .' O) io TJ- q q q ~ 5 I o io o 0; {^ ^ r>. Q lj| if! Jj J 8 | | 8 || i|3| |4i THE ELECTRIC SHAFT FURNACE 255 Comparing the last two sets of figures with the first four sets, it will be seen what a great improvement has been made during 1911. Comparing the above with the 1909 tests shown on page 251, the improvement deserves the recognition it has received, in that over 60,000 h.p. of these furnaces have since been built or are building. It is interesting to know exactly what the first large 2500 h.p. furnace installation of Gronwall, Lindblad & Stalhane cost at Trollhattan, which has a daily capacity of about 20 tons. The furnace house is of steel construction, and brick and both furnace and electric equipment cost more than a subsequent similar installation would, as this was the first one of this size. The cost was as follows: Excavation, railway connection, water-pipes, scale, etc. $10,727 Buildings: Furnace house 14.735 Charcoal storage-house 6,032 Crusher-house, office, laboratory, shops 3,96 1 Furnace I3,m Electric equipment 13.782 Cable and wires 3,832 Gas-motor, pumps, reservoir 3,222 Crushers 1,011 Transformers 3,433 Motors for crushers, etc i,7 2 4 Laboratory equipment, furniture, etc 10,430 $86,000 In order to give an idea as to the size of the necessary plant, it may be said that an output of 10.65 k g- ( 2 3-45 lbs -J, P 61 " h -P- day corresponds to a power consumption of 1736 kw. hrs. per metric ton. With a daily output of 300 metric tons this would need about 35,000 h.p. at the furnace, or about 38,500 h.p. at the power station, when allowing for a long transmission line. If a plant is built for $50.00 per h.p., it would require a capital of $1,925,000. Allowing 9% for interest and amortization, and 3% for taxes, etc., each h.p. year would cost about $6.00 at the power station, or about $7.50 at the furnace. A complete furnace installation for 300 tons would cost about $500,000 and consist of six furnaces of 7,000 h.p. each, 256 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY one furnace remaining in reserve. This estimate is based on the installation costs already obtained, but each furnace would be larger. It only remains to mention that because of the very favorable results obtained at Domnarfvet, 1909, the Jernkontoret of Stock- holm has acquired the patents of Gronwall, Lindblad & Stalhane. The British owners of these patents are the Electro Metals, London, whose American and Canadian representative is American Transmarine Co., Inc., New York. Iron ore reduction or electric pig-iron furnaces of different makes are built or building, among these notably some in northern Italy. Fifteen electric shaft furnaces of the Gronwall, Lindblad & Stalhane design are now (1919) operating or build- ing for Sweden, aggregating 60,000 Kw. One other is destined for Aosta, Italy, and one of these of 3,000 Kw. for Japan. All of the Swedish furnaces are operating with charcoal, although it is stated, that very successful trials have been made on coke. The latt^r's price, however, is at present too high to consider its use. With Swedish magnetites of 57 to 58% iron, the power consumption per metric ton is about 22,000 Kw.hr. With 50% ore it increases to 2,500 Kw.hr. The charcoal con- sumption is averaging 300 to 330 kg. per ton of pig-iron made and the best grade of carbon electrode averages 6 to 7 kg. per ton of pig-iron made CHAPTER XV GENERAL REVIEW IN addition to the methods of furnace construction previously described there are naturally a tremendous number of proposals for the design of electric furnaces. This is best brought out by the many patents that have been issued both for arc and induction furnaces. Although the literature of such patent papers may be very entertaining, and is indeed very often instructive, yet a consideration of the many proposals does not lie within the scope of this book. Most of them are only proposals and will never be tried out. A smaller number disappear quickly after a trial and leave no trace, while the third and smallest part stand trial in one or another plant. They enable the saving of the license fee for a successful furnace, but most of them cost enormous sums for experiments, and very often complications develop when they are put in operation. Although they are not for the most part of value to many people they yet have the advantage that they help to spread the knowledge concerning the properties of electric furnaces further and further. On the other hand, it is naturally only through a fresh consideration of the new methods of construction that a further perfecting of the old or even new ways can be found for reaching the wished-for goal. On this account, therefore, it is perhaps justifiable to con- sider at least a few of the furnaces differing in construction from those used most frequently today. Another reason is that one or the other of them are sometimes discussed in the technical literature. Under the heading of arc furnaces comes first that of Chapelet, which is in use at the plant at Allevard (Isere). It is shown in Fig. 113. We see that the current flows in an arc to the bath 258 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY from a hanging regulated carbon, similarly to the Girod furnace. From the bath it goes through a horizontal channel to a hanging cast-iron electrode that touches the channel. This constitutes the peculiarity of the furnace. It is not apparent that this arrangement offers any advantage over that of the Girod furnace. In the first place the furnace construction is much more difficult and not so accessible as that of the Girod. Further it is to be feared that the metal in the channel between the outer electrode and the bath will force up the furnace bottom, except that part which is not molten, because of the influence of the water cool- ing used for the iron elec- trode. This will bring about difficulties in maintaining the lining, since repairs to the horizontal channel are scarcely possible. The meth- od of working is exactly the same as that of the Girod furnace, that is to say, that heat is produced exclusively by the arc, the resistance offered to the current by the molten material not being of any noticeable value. The details of construction offer little that is worthy of attention. Water cooling is used at the opening in the furnace roof for the entrance of the carbon electrode, at the outer iron electrode, and also at the carbon electrode connections where the current passes from the copper cables. The cylindrical furnace roof is removable. The openings in the front part of the roof are used as working doors, as shown in the illustration. There are several of these furnaces in Allevard, but, according to Coussergues' report, only one is in operation. The Keller furnace, shown in Fig. 114, has still greater FIG. 113. GENERAL REVIEW 259 similarity to the Girod furnace. The only difference is a special arrangement of the bottom electrodes. While Girod, as we have seen, uses several water-cooled steel electrodes that are dis- tributed over the bottom surface of the hearth, Keller uses a furnace bottom formed of a so-called mixed conductor. As FIG. 114. seen in the illustration this bottom consists of a water-cooled iron plate over the whole surface of which are set a number of evenly distributed iron rods from one inch to 1.18" in diameter, between which magnesite is rammed. This is, in itself, a fairly good conductor. In this way a semi-refractory bottom is formed 260 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY with a conduction between that of iron and magnesite. Accord- ing to Keller's results such a bottom is practically unmel table. It is questionable whether his electrode arrangement offers any advantage over that of Girod. It depends on the durability of the furnace hearth in the two cases concerning which only work under practically the same conditions can give conclusions. The production of heat in the two furnaces is in no way influenced by the bottom electrodes. The uniform composition of the FIG. 115. whole furnace bottom in the case of the Keller furnace will not bring about the profitable circulation of the bath found in the Girod furnace. In this case also the original Girod is to be preferred to the newer Keller furnace, provided that the bottom will last as long in the first case as in the second. Often one finds in the patent papers the endeavor to increase the resistance of the bath by means of a suitable shape of hearth, and so bring about an additional resistance heating. As an example, the Nathusius furnace may be mentioned. Fig. 115 gives a section of this furnace taken from the patent papers. It shows a number of electrodes of changeable polarity arranged GENERAL REVIEW 261 above and below the melted material. In this way the current can be forced to flow through and around the molten bath. According to the description given with the drawing, the current flows first from the upper middle electrode b through the slag covering h and the upper layers of the metal bath to the upper outer electrodes a and c, second from the lower middle electrode e to the lower outer electrodes d and /. In addition, however, the current ought to travel from the outer upper electrodes a and c to the outer lower steel electrodes d and f, so that the bath will be enclosed by heat-producing currents. The whole arrangement, as is immediately apparent, repre- sents a combination of the Heroult and Girod furnaces. In the FIG. 116. first place it is presumed that it is possible to heat the bath by current led in through electrodes which have a much smaller section than that of the bath. This is naturally altogether impossible if the electrodes consist of carbon, as is the case with those arranged over the bath, which has an excessively high resistance in comparison with the fluid metal. In addition it can be shown that it is impossible to bring about much heating by means of the water-cooled electrodes, for their section in proportion to the bath is so small that the higher specific resist- ance of the bath can have no important influence. Fig. 116 shows the practical arrangement of a Nathusius furnace that differs from the drawing in the patent papers because of a simpler and therefore better form of hearth. Here a direct heating of the bath, by means of the bottom electrodes, is not 262 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY FlG. 117. taken into consideration because of the greatly increased section. The arrangement of the water-cooled electrodes in the latest furnaces differs from Fig. 115, and according to Neumann's report in Stahl und Risen, 1910, they have a diameter of 8.66", and are covered with a layer of dolomite 7.87" thick. With the passage of the current this layer gives off heat, and so much as is not carried away through the bottom electrodes enters the bath. For increasing this bottom heat- ing an additional 150 kw. trans- former is used for a 5-ton fur- nace. Currents of a maximum of 6000 to 8000 amperes are used, that enter the bath frpm each carbon electrode, when a three phase no volt current is em- ployed amounting to about 2500 amperes. The direct heating of a metal bath n.8" deep and about 78.74" diameter is altogether impossible with these currents. It is therefore also im- possible with the present arrange- ment of the furnace to use the bottom heating alone, although this is advanced as a special ad- vantage of the furnace in question. After all, the small advantage that the bottom heating may bring about must be looked upon as dearly purchased when it is con- sidered that the Nathusius furnace shows a much more complicated construction than the Heroult or Girod alone, and uses practically the same method of heat- ing. Moreover, it has more electrodes than the simpler older furnaces and therefore has greater heat losses. In addition six conductors are used for the current as compared with three FIG. 118. GENERAL REVIEW 263 for the Heroult furnace. Apart from this the method of con- struction does not appear as good as that of either the Heroult or Girod furnaces. In the sphere of induction furnaces one constant endeavor appears to be the production of greater movement in the bath of metal. Most of the proposals show an ignorance of the principles of the induction furnace, for otherwise the designers would know that in these furnaces a completely satisfactory mixing of the whole molten material is produced by the electric FIG. 119. and magnetic conditions themselves. We can, therefore, leave out of consideration all the proposed furnaces that make use of inclined channels of small section through which the hotter material ought to rise, while the colder should descend. Such an arrangement proposed by Gin is shown in Fig. 117. The Schneider-Creusot induction furnace, of which a section is given in Fig. 118. is worthy of notice. This furnace, however, has not been improved since it was first designed. Like the Gin furnace mentioned above, and which appeared much later, it shows an induction channel with several hearth-like widenings. 264 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY All such constructions of induction furnaces have the disadvan- tage that extremely high temperatures must be produced in the narrow channels if the material in the hearths is to be kept hot enough. This brings about a very energetic attack on the lining at these places, and as a result high maintenance costs and frequent delays in the working of the furnace with the Schneider- Creusot furnace refining is only carried out in the hearth A , and the remaining metal is kept free from slag. The use of the small hearth B is therefore not apparent. The arrangement of the furnace cannot be called simple. With this furnace also great value is laid on the increase of movement in the bath due to the great differences in section, and this appears reasonable. For obtaining this circulation the furnace is built on three columns, two of which allow a rise or fall in the furnace, so that during the operation the heating channels or pipes can be in- clined at a sharp angle. The furnace at the Creusot Works is arranged for a one- ton charge. Other types of induction furnaces endeavor to increase the resistance of the bath, and so bring about an improvement in the power factor. The proposal of Gronwall, which is shown in Fig. 119, may serve as an example. We see here the ordinary channel of the induction furnace, greatly elongated on one side. This arrangement naturally brings about a considerable increase in the resistance of the bath, but it has the disadvantage of causing very great radiation losses. Further, it is impossible, according to metallurgical practise, to maintain the division wall that is necessary between the two parallel parts of the hearth, because no refractory material is known that will resist an intense heating from both sides. Further, it may be men- tioned that such a furnace can only be used for the melting of pure materials, for work with slags cannot be carried out even to the small extent possible in the purely ring-shaped furnaces. This proposal, also, has not yet passed the experimental stage. Roberston, in the November, 1911, issue of the Metallurgical and Chemical Engineering, writes of the Gronwall two phase arc furnace. This furnace is the invention of Gronwall, Lindblad GENERAL REVIEW 2G5 & Stalhane. Having originally worked with various types of induction furnaces without great success they decided to design an arc furnace. This furnace operates with two phase current, having two vertical carbons passing through the roof, each one to a phase. See Fig. i iga. The current arcs from the electrodes to the charge, passing through this and then through the basic lining at the centre of the hearth bottom, to the neutral return which is a carbon block fixed in the bottom of the furnace. The top of this bottom electrode comes level with the brick- work so that it does not project into or in any way weaken the basic lining. The hearth of the furnace therefore is not broken by any projections. The fur- nace has three doors, one at each end and one at the spout. Either hand or FlG II90 automatic regulation is pro- vided for the electrodes. This furnace is of the tilting variety, being mounted in curved rails. Heat regulation is obtained by varying the voltage of a special regulating transformer. The normal working voltage is 55, no and 220 and 10% tap voltages. As each phase of a two phase circuit is connected to one of the vertical electrodes the arcs are independently formed, so that if one arc is broken the other remains. This insures steadier running than if both arcs were in series as in the Heroult furnace. The arrangement of two arcs operating in parallel with a neutral return through the bottom produces a vertical as well as a horizontal circulation in the metal bath, slightly different from that in a Girod furnace. The Greaves-Etchells electric furnace is described by the inventors as being of the arc resistance type, and that it com- bines the advantages of both direct arc and resistance heating. It was first introduced by two engineers and metallurgists, 266 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY ' Greaves and Etchells, of Sheffield, England, in 1915. It was the aim of the inventors to produce a furnace which would fill the following conditions: 1. A system which should be applicable to any polyphase supply system, especially those of high frequency, and which should maintain a steadier phase balance, and higher power factor. 2. The generation of heat in such a manner that it was ap- plied as directly as possible to the metal in the furnace, and did not superheat the slag and various portions of the lining more than was essential to maintain the metallurgical conditions necessary for refining steel. 3. Rapid melting and uniform heating of the charge. 4. Automatic circulation of metal giving uniform quality of steel and avoiding use of stirrers and rabbling. 5. The construction of the furnace, which should be free from obvious mechanical defects. In order to obtain an even temperature through the bath of molten metal in the electric furnace, the inventors believe that heat must be applied below, as well as above, the bath. In order to effect this, the hearth of the furnace is specially con- structed. The hearth lining is never less than 500 millimeters (20 inches) thick, and is constructed mainly of dolomite, magnesite, and other materials, in such a manner that it is believed the electrical resistance is high at the inside of the bath in proximity to the charge and decreases rapidly to a negligent quantity at the outside. (The details of the bottom construction are not divulged.) Two phases of the 3-phase low-tension supply are connected to their respective upper graphite or carbon electrodes while the third phase is connected to the bottom of the hearth. The charge thus lies between the three sources of power. The current flowing through the bath generates some heat immedi- ately below the liquid (see page 155 for similar cases), while electric arcs arranged over the bath maintain the slag and surface at the desired temperature. GENERAL REVIEW 207 We quote again from the inventors, " The effect of this bottom heating is to cause 'convection currents' in the molten metal, which ensure a constant circulation of a uniform product. The outside of the furnace bottom remains cold, little or no heat being lost in this direction." The system of transformer ratios is arranged so as to give a perfect balance when the upper electrodes are in equal adjust- ment. The high - tension electric supply is transformed by means of a delta-star connection (see Fig. IIQ&). The transformer connec- tions are such that the short- circuit current of one elec- trode must traverse two transformers in series and in different phase, which automatically lowers the power factor momentarily, and has a cushion effect; the fact that there is always a permanent resistance in FIG. 119 b. the path of the current through the hearth also limits the effect of the short circuits. The combination of these factors provides another means devised for protecting the electrical system from shock, while allowing a high-power factor to be obtained on normal load, which averages 90 to 94%. Automatic electrode regula- tors maintain the current at predetermined figures. Motor and hand regulation are also provided. Arrangements are pro- vided for varying the voltages across the arcs and the ratio of heat generated above and below can be regulated over a wide range. The doors, of special construction and design, effectively exclude air when closed and prevent loss of heat. The furnaces are constructed in various sizes, from J^ to 30 tons, the largest one in operation now is taking 15 tons of cold metal. 268 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Fig. ngc shows the general design of the furnace of the 3- to 30-ton size. They are mounted on rockers operated by an electric motor at the rear of the furnace. There are teaming and slagging spouts and a charging door. (See Fig. ngc.} The mechanically operated electrodes are carried by separate jib arms to winch control. The whole hearth of the furnace is connected to a third transformer winding; the three transformer windings are arranged in star while the pri- maries are in delta. The bottom electrode of the furnace is also con- nected to ground, so that the star point of the transformer group is above the earth po- tential. As shown by the figure, the 3-ton furnaces and larger are now operated with four electrodes. FIG. ngc. Diagrammatically the electrical equipment is equivalent to two individual parts of a smaller furnace. Each pair of electrodes with their respective transformers can be switched individually, enabling one pair of electrodes to be kept in operation while adjustments are made on the other pair. The total overload of the furnace is limited somewhat by this device, because each half of the furnace trips its particular switch when its own pair of electrodes becomes overloaded. It is claimed that due to "convection currents," already mentioned, there is a very rapid refining because the "mov- ing stream of metal carries away the intense heat of the arcs, bringing all parts of the bath in contact with the refining slags." With basic linings, a 3-ton furnace equipped with 900 KVA transformer capacity is melting and refin- ing 3 tons of steel in 2% hours. Six-ton furnaces are melting and completing heats in less than 4 hours per GENERAL REVIEW 269 charge. With acid-lined bottoms these figures have been improved. It is stated by a company operating this type of furnace and another arc furnace of prominent make, that "the average kw.-hr. consumption per ton of steel produced in the furnace (a 3-ton Greaves-Etchells) is comparable to the performance secured in a 6-ton of the other make." The makers of the Greaves-Etchells electric furnace are the Electric Furnace Construction Co., of Philadelphia, Pa., and T. H. Watson & Co., Ltd., Sheffield, England. An adaptation of the orig- inal (1878) Siemens single- phase bottom electrode type of furnace has reappeared for a brief time in a form known as the Snyder. This furnace has a unique type of plug door, shown in the cut, see Fig. 119^. It is very tight, but has the disadvantage of not being able to rabble off slag without opening the entire door. The furnace uses a graphite electrode 4 diameter for a i-ton using 600 KVA, at 60% power factor about 120 to 150 volts at the arc; the density per square inch is high, but as long as the furnace is kept very tight during operation, the graphite electrode con- sumption is below 6 Ib. per net ton of metal melted and poured from an acid bottom, as the majority of these furnaces are operated. Owing to the very high power employed quick heats are possible with acid refractories throughout, less than 550 kw.-hrs for 2,000 Ib. (910 kg.) being common. With continuous operation 10 heats have been made in 24 hours when merely melting, "killing" and pouring. One furnace operating basic, making ingot steel, now makes 6 heats in 24 hours, but has made 7, also 8, but at the great expense of 270 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY the brick work. Owing to the unpopular single phase with the central station managers, together with the low power factor, FIG. 1 190. 60%, and the difficulty always present of burning out the bot- tom electrode, not to mention the most violent electrical surges, always present, this type of furnace gradu- ally, again, took itself off the market, in the steel industry. Following this single -phase furnace came the polyphase Snyder operating 2- phase 3-wire with a bottom contact. (See Fig. age.) Com- pared to the contents of the furnace the steel water-cooled part of the contact with the bath, which protrudes GENERAL REVIEW 271 through the refractory bottom, is not only above the bottom but external to it. Copper cables going to this bottom contact are therefore no longer under the furnace but one side instead. Another feature of this furnace is a hinged roof which can be swung back quickly for an equally quick charging from a FIG. ii9g. specially designed bucket. This latest design, as can be seen from Fig. iiqf, has but one set of parts for tilting the furnace, and also the roof when it is slid back for charging; see also Fig. upg. An interesting thing which has been tried by a user of this furnace is the preheating of the steel scrap to about an annealing temperature, the metal being charged while it is still in its solid condition. This necessitated 94.6 liters (25 U. S. gallons) of fuel oil and saved 180 kw.-hrs. per metric ton with an acid- 272 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY lined furnace. Even if the combined cost of the fuel oil and handling is the same as electricity saved, the process is eco- nomical, as it increases the output about 40% and cuts the time per heat down to a little over an hour. Besides this, it lessens the electrode consumption, the refractory and repair cost per ton of output. It will be interesting to know later on what the saving is with a similar method with a basic-lined furnace. Such preheating as this was long ago proposed by Professor Joseph W. Richards, but this is the first instance we know of where it has been used in conjunction with an electric furnace; and with the constantly rising price of electricity in certain sections, it is expected that this economical method will come into being more and more, especially considering the small outlay for preheating the scrap to be charged. The Greene arc furnace made its appearance on the Pacific Coast during the Great War. Those built so far have a steel bottom electrode and upper carbon or graphite electrode. In principle the furnace is therefore of the original Siemens type. Some later models operate with 2-phase current, also hav- ing a steel bottom contact and two top electrodes of carbon coming vertically through the roof. The latest models are also built in the shape of a barrel lying on its side; in form, much like an earlier Rennerfelt model. The electrodes are regulated only by hand, so far, although automatic electrode control can be attached. The furnace has been made from y^ to 2^-ton sizes per heat, and is often- times operated with an acid bottom. The statistics of this furnace can be seen on another page. It is handled by the Greene Electric Furnace Co., of Seattle, Washington. Naturally there have been many attempts to combine the various types, such as the induction and arc furnace. Fig. 120 shows one, and is that of Hiorth. (Such proposals originated at the time when the causes for the failure of the channel-shaped induction furnaces for refining purposes were not clearly known, and it was thought that the slag temperature was not high enough. In the meantime the successful operation of the Roch- GENERAL REVIEW 273 FIG. 1 20. ling-Rodenhauser furnaces has shown the incorrectness of this reasoning.) In Fig. 120 we see the channel of an induction fur- nace broken by a division wall, which is bridged by a stirrup- shaped electrode. This electrode should only just touch in the slag, and bring it to a very high tempera- ture. This shows a complete ignor- ance of the probabilities. The un- mistakable result of the proposed method of working would be a com- plete freezing up of the metal in the channel on the opposite sides of the electrodes. It would be im- possible to introduce sufficient cur- rent into the bath through the elec- trode with which to produce heat enough, by overcoming the bath re- sistance, to keep the metal fluid. Even so, Hiorth says that he does not consider this furnace construction to be valueless, still we do not find that he has used this method in the single commercial furnace which he has constructed, which is shown in side elevation by Fig. 121. This is a purely induction type of furnace with the primary winding in flat spools similar to the arrangement already pro- posed by de Ferranti and later again by Frick, excepting that Hiorth coils both legs of the magnet. The coils, according to a paper read by Dr. Joseph W. Richards before the American Electrochemical Society, in 1910, are uninsulated copper coils, hollow, and the lower ones water-cooled. The construction of the furnace is such that it may be tilted independently of the magnet. FIG. 121. 274 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY So far, Richards continues, Hiorth uses his furnace only for melting the purest obtainable Swedish Dannemora pig-iron and Dannemora Walloon iron. Yellowish-white blast-furnace slag was being used as a flux. The contents of the furnace being 5 tons, 3 tons were poured at a time and 2 tons left in to start the next charge. The details of a heat run-off are then given, which are here omitted. We quote further: "Assuming 300 calories necessary to melt i kg. of steel, the thermal efficiency of this melting operation is 55% and the furnace radiation loss calculates out 180 kw. at this temperature. FIG. 121 a. It was stated that it took about 170 kw. to keep the charge melted when the furnace was kept up to heat overnight." The power factor varied from .80 at the beginning of the run to .57% at the end, when the metal in furnace was 5.77 tons and at casting temperature. Current used averaged 395 kw. for 6 hours, or 790 kw.-hrs. per ton of steel. As low as 700 kw.-hrs. has been reached in this 5-ton furnace on cold mate- rials. This furnace operates at i2> cycles, 400 to 500 kw. at 250 volts single phase. The Moore electric furnace made its appearance during 1917. It is a 3 -phase arc furnace having 3 vertical electrodes set in the form of an equilateral triangle, placed symmetrically over a circular hearth, at first glance resembling the Heroult furnace. (See Fig. 1216.) GENERAL REVIEW 275 It differs in its electrical connections somewhat, as these are arranged, according to the inventor, "so as to allow an adjustable amount of power to be carried to the charge through the furnace bottom : an amount of power sufficient to produce a circulation and stirring of the bath, resulting in a more rapid refining of the steel and better mixing of alloys; at the same time, the amount of power transmitted through the bottom is limited in such a FIG. 121 b. manner as not to in any way injure or shorten the life of the bottom." Fig. 1 210 shows the low- tension switch which is closed when it is desired to have the current flowing through the bottom. By means of a multiplicity of series, parallel and delta-star connections a number of voltages can be produced at the arc. This causes a rather elaborate system of high-tension wiring in the transformer room. The bottom switch, letting current flow through the conducting hearth, is not always closed, especially not with acid bottoms, as these are not, as is well known, nearly so good an electrical conductor, when white hot, as are the basic hearth refractories. The bottom, or so- called neutral connection, "permits each electrode to obtain 276 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY contact immediately upon its touching the charge." This is of course after the basic bottom is once thoroughly hot. The furnace is more highly powered than some other furnaces, i.e., 1000 KVA at 40 C. rise in transformers for a 3-ton furnace is an example. It has a wide backward tilting angle, allowing the slag to be removed easily. Special attention has been paid to the ready renewal of the refractories. Records of the 3 -ton acid-bottom furnaces have been established at 700 metric tons per month, and a roof lining for 300 heats, the average kw.-hrs. consumption being 560 kw.-hrs. for a "net ton" (908 kg.), equalling 616 kw.-hrs. per metric ton. Either graphite or carbon electrodes can be used. For making ingot steel the furnace is operated basic. Automatic electrode regulation is furnished on all but the smallest furnaces. It is made in i>^-, 3-, 6-, and 1 2-ton sizes, and smaller ones when desired. Fig. 1216 shows a 3-ton furnace. This furnace is handled by the Pittsburg Furnace Co., Milwaukee, Wisconsin. The Booth-Hall arc furnace also made its appearance during the war just passed. It belongs to those having a conducting refractory hearth, when conditions permit, with a basic bottom, and, when operating with an acid bottom, which only be- comes conducting to elec- tricity with difficulty, re- course is had to an auxiliary electrode, making three in all for a 2 -phase 2 -wire system. The electrical con- nection is as shown by Fig. 1 2 ic. Until the basic bottom becomes conducting, the third or auxiliary electrode, shown by black circle in Fig. 121 d, is always used anywhere up to 45 minutes per heat, depending upon the conditions. Great stress is laid by the inventors on the thorough mixing of the metal, due to the cross current in the bath during the latter part of a basic heat when the hearth becomes conducting. FIG. 121 c. GENERAL REVIEW 277 This "mixing action," so called, is claimed to be "very beneficial in refining work, producing a more uniform steel, and, if alloy steels are to be made, largely prevents segregation." In the light of this claim, it must not be forgotten that by far the greater majority of quality and other electric steels to-day are made with electric furnaces having a solid and not a conducting bottom, and it has yet to be shown that any poor steel is made in electric arc furnaces, just because the bottoms are solid and the currents passing through the bath only go through the upper portion per- haps, instead of diagonally through the molten steel from the arc, and thus through the refractory bottom and out. Fig. 121 e shows a 3-ton Booth-Hall furnace. FIG. 121 e. 278 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Furthermore, it is to be remembered that with an acid-bot- tom furnace of this type, when a bottom is practically non- conducting and consequently no cross currents in the molten bath, good steel is made nevertheless with selected scrap, and the inventors by no means admit that their acid steel is inferior to their basic! When operating acid with the auxiliary electrode, this latter is supposed to be either touching the cold scrap or dip- ping in the slag all the time, so that no arc results from this electrode, and hence the two arcs are not in series. There is no particular technical advantage in this, but it avoids patent interference. One of these furnaces of 1.18 metric tons (2600 Ibs. avoirdu- pois), having 600 KVA in transformers behind it, is making heats on a basic bottom without any particular effort at re- fining, and when melting steel scrap regularly, in two hours is averaging 600 kw. hrs. for .908 metric tons (2000 Ibs.), cor- responding to 660 kw. hrs. per metric ton. This was made over a one-month period, allowing for shut-downs at night. Details of this are given hereunder, and also of the 3-ton basic furnace of this type having 1200 KVA feeding it. OPERATION OF BOOTH-HALL BASIC BOTTOM FURNACE 1.18 Kg. (2600 Lbs.) Capacity 600 KVA February, 1919 Total number of heats : 1 16 Total working [days 27 Average heats per working day 4.3 Total charge (Ibs., 294,640) (kg.) 133,400 Average charge per heat (Ibs., 2540) (kg.) 1,150 Total charge (net tons, 147.3) metric tons 133-4 Average daily output (net tons, 5.45) metric tons 4.95 Total kilowatt hours 88,330 Average kilowatt hours (per net ton, 2000 Ibs. avoirdupois = 600) metric tons 660 Total charging time (minutes) 2,944 Average charging time per heat (minutes) 25 Total melting time (minutes) 13,688 Average melting time per heat (minutes) 118 Total pouring time (minutes) 2,320 Average pouring time per heat (minutes) 20 GENERAL REVIEW 279 OPERATION OF BOOTH-HALL BASIC BOTTOM FURNACE 3-Ton Capacity 1200 KVA ANALYSIS Time of Heat Charge Lbs. Kg. Total Power KWH. Power per Ton KWH. Heat No. C. Mn. Si. P. S. 427 .24 56 30 Under .05 Hrs. Min. -55 6000 2722 1640 Net 547 Metric 60 3 428 .20 .70 -30 Under .05 -45 6000 2722 1500 500 550 432 .29 .60 35 Under .05 2-00 6000 2722 1700 567 625 433 31 .67 .28 Under .05 -50 6000 2722 1460 487 537 434 30 .60 .22 Under .05 -50 6000 2722 I52O 507 558 437 .24 -56 32 Under .05 2-05 6000 2722 I6OO 533 588 438 .22 .61 .28 Under .05 -35 5000 2268 1340 536 592 439 23 54 29 Under .05 -55 6000 2722 1450 483 533 440 .27 -59 30 Under .05 -20 4000 1815 II2O 56o 617 An electric steel furnace of new design, which has recently been put in operation, has certain features which distinguish it from some of the more familiar types. The furnace is known as the Vom Baur electric furnace. A 6 ; ton unit of this new furnace was started in February, 1919, since which time others have followed both here and abroad. It seems that very soon after electric furnaces in the iron and steel industry were placed in commercial operation in the United States, some thirteen years ago, it became evident that one of the limits of its possibilities depended on the life of the re- fractories. All kinds and types of electric furnaces have to take care of this item much more than any open-hearth furnace. The experience of many electric furnace operators and designers has been of this nature. The simplest refractories of any arc furnace are those of the single phase, single electrode type, with a bottom electrode, when this has a round hearth and the carbon electrode at its center. It is evident that the heat at the inner contour of the side walls and of the slag line of this furnace is the same at any point thereof. Consequently, when the side walls burn away, they do so equally. That is to say, there are no hot spots on any portion of the side walls or slag line, due to the heat of the arc, and manifestly no portions of the refractories need repairs, when properly laid, prior to some other portion. As the single-phase furnace of any but the smallest 280 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY sizes is not popular with central station managers, it is seldom seen to-day in the newer installations, polyphase furnaces hav- ing almost exclusively taken their place. At first, on account of the many advantages of the polyphase furnace, compared to the single-phase, the refractory feature as above- mentioned did not receive the attention it deserved until the appearance of the Vom Baur electric furnace, which has, with its three electrodes, 3-phase 3-wire, or 2-phase 3-wire, the same good heat distribution as the simple single-phase furnace. FIG. 121 f. This good heat distribution in the Vom Baur furnace is ac- complished by the combination of placing three electrodes in a straight line. The contour of the inner refractories can easily be ascertained so as to give a curve to the refractories which is correct, so that the heat at the slag line, at the side walls, and at the hearth banks is the same, thus assuring among others the equal burning away of all the refractories involved. Only very small allowances from the theoretical have to be made in order to accomplish this purpose. The shape given to these furnaces is shown by the illustra- tion, Fig. 1 2 1/. This shows a well-defined minor axis drawn through the middle electrode, and this shape is the natural GENERAL REVIEW 281 result of the radiated heat reaching points on the minor axis from the three electrodes. With 2-phase theoretically the central electrode would have 41% more heat than either of the end electrodes, but it .is not possible to reach this condition practically and maintain the arcs at their best setting. The current at the central electrode is therefore diminished by sev- eral per cent. Even so, the heat from the central arc is more on an average than from either of the end arcs and, consequently, in order to keep the refractories at their proper distances, the metal bath has a larger surface than the furnace would have if each of the electrodes ever gave off an equal amount of heat. It is well here to quote Ryan (American Foundrymen's Associa- tion, October, 1918), where he says: "The ideal furnace is one which exposes the largest possible surface of the metal to the action of the slag without being subject to freezing conditions at any part of the furnace." With basic operation the advantage is that the metal presents a greater slag area for the same tonnage. As the rate of refining depends not only on the chemical com- position of the slag, the heat at the point of contact between the metal and the slag, but also on the amount of surface en- gaged between the slag and the metal, it follows that the rate of refining is quicker. There is very little poking to be done in this furnace when melting down cold scrap, as there are no hot or cold spots on the side walls or near the doors. The quick melting down with this furnace, as with all others, depends mostly on the amount of transformer capacity which the furnace has. However, as this furnace has an even heat distribution at the side wall refrac- tories, it is more efficient than the other furnaces which lack this feature. Special attention has been given to the doors, which have water-cooled frames, and which fit very snugly, allowing for all expansion, and yet can be tightened after they are closed by giving a few turns to two handles. Structurally the furnace is strong, having a rounded and cone-shaped bottom connected with the upper shell, which is a series of curves. This sets on two rocker arms and the whole is tilted by means of connecting rods from suitably geared 282 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY mechanism in such a way that when the furnace has reached its maximum tilting position and the operating motor should con- tinue to run for any reason, the furnace goes back to its normal position. The tilting mechanism also allows the furnace to tilt slightly backwards some 6 and 7 degrees so that the slag can be taken off at this door instead of from the spout. Both doors are used for charging. See Fig. 12 ig. The center of gravity of the furnace is low, as the heavy part of the standards is no higher than is needed for the full travel of the arm holding the electrodes, and as these have an upward tilt, the upright standards are still further shortened. For carrying the copper cables a much lighter structure is used, all of which can be seen from the illustrations. Either carbon or graphite electrodes can be used and' changed quickly; either hand or automatic electrode control is furnished. The oVal shape and turtle-back construction of the roof makes a strong construction, standard brick being used throughout for the roof construction, excepting a few special electrode bricks. The electrical connections are exceedingly simple, being merely three sets of cables from the transformers to the elec- trodes, and no complication on the high-tension side. Elec- trically the furnace conditions are satisfactory to the central station managers, the power factor being around 90%, and the phases not being distorted more than with other arc furnaces. After continued operation it has been observed that the furnace does not bulge at all, this being another indication of the even heat distribution already mentioned. The furnace is equally well adapted for melting down miscel- laneous steel scrap for castings on either its solid acid or basic bottom, selected scrap for quality and tool steels, or for melting down cast-iron borings, or the like, for use with malle- able iron, or for melting ferro-manganese. The furnace is made in sizes from >^ ton up to 30, and for any electrical condition. Regarding installations of this furnace, reference is had to the statistics on another page. The patents for France, Belgium, Italy, Switzerland, and Spain have been disposed of to Le Flaive 6 s Co., St. Etienne, France. For the GENERAL REVIEW 283 284 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY United States, Canada, and elsewhere, this furnace is handled by C. H. Vom Baur, New York. 1 The Ludlum electric furnace has recently made its ap- pearance. It has a solid refractory bottom, uses 3-phase cur- rent with 3 electrodes in one line with a charging door at either end of its major axis, and the spout at one door. It uses graphite FIG. 121 h. electrodes only. Its efficiency is increased due to the particularly low roof (which, however, has already been raised by means of a steel filler piece, see Fig. 121 h). The roof refractories consequently do not last as long as with the arc furnaces of a similar type, having a corresponding higher roof. The furnace is hexagonal in shape as seen from the picture, and gradually slopes to an oval at the hearth. The manufacturers mention the following details in design of this furnace as follows: The design and shape of the furnace permit the supplying 1 In Scandinavia and Finland this furnace is handled by Henning Broms, Ferdsgatan 6, Stockholm. GENERAL REVIEW 285 of energy to the furnace at a high rate within the critical limits of the refractories, resulting in quick heats and a low power con- sumption per ton of metal, besides giving a thorough refining by reason of the larger area of contact between slag and the steel compared to a round furnace of equal capacity. The arrangement of the electrodes, which are connected to 3 -phase system, results in producing an equal amount of heat at each electrode; the greater amount of heat in proportion to the mass of the bath at the ends compensates for the greater radiation at the doors, and results in an even temperature at the side walls, makes sintering of the lining easy, reduces the danger of the breakage of electrodes and as the maximum space for operating rabbles, test spoons, etc., is available without coming in contact with the electrodes. The entire bath is accessible from the doors, and lining can be inspected and easily repaired, thus simplifying the care of the bottom and increasing its life. The hearth meets the roof with hardly more intervening vertical side walls than shown in the picture. Standard shaped brick are used in the roof and hearth. The arrangement of the electrodes and shape of the bottom result in the formation of a little pool which extends to all three electrodes, at the time they have cut through the charge. This metal protects the bottom refractories from the arcs and allows melting from the bottom up without endangering the lining. The furnace may be provided with either a basic or an acid lining. This furnace is handled by the Ludlum Electric Furnace Corp., New York. Other proposals consist usually of combinations that in most cases would bring about great difficulties in operation, and which offer no advantages over the original furnaces. Here belong those which take an ordinary metallurgical furnace, such as an open hearth, and operate it at certain times by a stoppage of the gas, and the use of carbon electrodes. Also a combination of converter and electric furnace, for instance, a small converter with an arc furnace built in. With these combinations the conditions of operation have not been considered carefully enough. For instance, an open-hearth furnace in comparison 286 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY with an electric furnace has such a high roof, and large working surface of bath, that the heat losses when using carbon electrodes, even if only for the desulphurizing period, would bring about much too high costs. In these cases it is therefore much better to transfer the charge from the open-hearth furnace, or the Bessemer, to a special electric furnace by means of a casting ladle, and to stand the unavoidable heat losses. In this way cheaper and better results will be obtained than with any of the pro- posals mentioned above, and although really seriously tried out up to the present has not been successful. As a conclusion to this review, which is believed to embrace the most valuable proposals in the different spheres, it may be established that, until the invention of further types of con- struction, we have only to deal with those described in detail in the special chapters. These furnaces still show many weaknesses in comparison with the ideal furnace, yet they show that in those with the greatest simplicity the ideal has been closely approached. CHAPTER XVI FINAL CONSIDERATIONS THE purely technical side of the application of electric fur- naces to the iron and steel industry has been considered in the foregoing chapters, so that now something may be said with regard to the economical questions of electric heating.* We have seen already in Chapter I that the development of electric furnaces is closely connected with that of electro-tech- nology. This is still the case when the question as to whether the installation of an electric furnace under certain conditions will be an economic success or not is under discussion. Then, indeed, the cost of the electric current, which is the heating agent of the electric furnace, is of real influence for the success of an electric steel plant. It must be taken into consideration that electricity, in by far the most cases, is much more expensive than the ordinary methods of heating, nevertheless this disad- vantage is more than equalized by other advantages. In this connection we may quote from Borchers' address before the Verein Deutscher Eisenhuttenleute, in 1905. "If we reckon the kilogram of carbon in coke at a high price, say about 0.7I4C., then 1000 kg. calories will cost o.o88c. Very cheap electric power, namely at $9.52 per h.p. year, gives 1000 kg. calories from o.i67c. to o.2i4c. according to the number of work- ing days. This disadvantage of electric heat production is bal- anced by this condition; that the material to be heated, which in this case is the charge itself, accomplishes partly or altogether the transformation of the electric energy into heat. In a certain way it forms, of itself, the source of heat, while in all combustion furnaces the heat goes first to a mixture of gases, and from this to the material to be smelted." * For a more detailed discussion see Part II under "Costs of Operation." 287 288 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY In the above example the price of power taken is very cheap, for with $9.52 per h.p. year, and assuming 300 working days in the year, the kw. hour only costs o.iySc. Such a low figure is only to be reached with the use of very suitable water-powers, while it is unattainable by using blast-furnace gas, provided that the blast-furnace gas is reckoned at a cost corresponding to its heating value. If this is done then it will usually happen that, even with the use of large gas-engines, the kw. hour cannot be furnished lower than 0.357^ to 0.714^ Still more unfavorable are the results if steam is used, although here, also, progressive engineering has brought about a constant cheapening in the price of current. For instance, in well-conducted central stations, with the use of large steam turbines, it has been found possible to produce the kw. hour at about 0.714^, when the coal does not cost more than o.4ic. per kw. hour. This is, of course, provided that the demand for power is very uniform, and free from variation, for otherwise the price per kw. hour is increased considerably. In this connection von Rizzo, in the Electro- technische Zeitschriit, p. 596, 1910, gives a figure of i-3ic., the power being produced by steam, and being used for operating a railroad with a very variable load. The prices given have reference, almost always, to large central stations, such as large iron and steel plants, city stations, etc. With smaller producing plants the price of current naturally rises considerably. It is therefore recommended that small plants should almost always be connected to some large central station for their electric furnace power, if the opportunity is there. Such stations today often furnish power for o.942c. to i.428c. per kw. hour, which is a price that cannot be realized in small power stations, except with high pressure internal combustion oil engines. We see then that the source of power used for the production of electricity can affect the price of current, and therefore the production costs of electric steel. Also the way the current is used plays a very important part, and this depends in the first instance on the method of working. The following table shows how this method of working influences the power consumption: FINAL CONSIDERATIONS 289 It requires for the production of: Pig iron, direct from ore 2,000 Kw. Hrs. Steel, direct from ore 3,000 " Steel from cold pig iron 1,500 " Steel from fluid pig iron 1,000-1,200 /' Steel from cold pig iron and cold scrap 900-1,300 " Steel from molten pig iron and cold scrap. . 600-1,000 " Steel from cold scrap 600-900 Refining of molten low carbon steel to make special quality steel (with very complete chemical purification) crucible steel quality 200-300 " Refining of molten low carbon steel to ordinary electric steel (electric rails) 120 " Retaining pig iron molten for foundry purposes (heated mixer) 50 " These values can naturally only serve as rough estimates, because the composition of the charge and the finished material are absolutely necessary for more exact figures. Further, more or less power will be used according to the efficiency of one or the other furnace, so that with the same charge and finished product different power-consumption figures will be given by two furnaces of different types. The wide limits given for working mixtures of pig iron and scrap are necessary because the power consumption is greatly dependent on the percentage of pig iron and of scrap used, more being necessary with an increase of pig iron. Further details on these points are given in the second part of the book. From what has been said it is apparent that the price of current becomes more of a determining factor (for the efficiency or non-efficiency of electric furnace operation), according to how many of the metallurgical processes necessary for changing ore to steel are carried out in the electric furnace. For instance, when fluid metal from a converter or open-hearth furnace is worked the cost for power, with an average unit price (o.476c. to 0.7I4C. per kw. hour), is about 3% of the production cost; but it increases to 12% with the same price per unit, when scrap is worked. Finally we must remember that all of the furnaces in use 290 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY today have certain special advantages. Unfortunately each type of furnace has also certain disadvantages. These disad- vantages are so closely connected with the methods of heating, that they must be allowed for. If electric steel production is entered into today one of the existing furnaces must be chosen, and its advantages and disadvantages purchased together. It is therefore not without interest to see how widely distributed the various types have become up to date. The following statistical tables date up to July, 1919. The tables show that the more important furnace types have already become so wide-spread that they must be considered to have passed the experimental stage. At the same time the electric furnace has shown that it is of considerable economic importance because it has enabled the production of the very best finished steel from low priced material. Until now the purest and therefore the dearest raw materials were necessary for this purpose. The tables clearly show that this great econo- mic advantage of the electric furnace is becoming known more and more. When we realize that the Stassano, Heroult, and Kjellin furnaces were first brought out in 1900, and the Girod and Rochling-Rodenhauser in 1906 and 1907, the wide-spread dis- tribution of these furnaces takes on greater importance. How quickly this distribution increases is also shown by the table, for in addition to 895 furnaces contracted, all but 80 are in operation. It may be concluded by pointing out that the electric furnace is already firmly established in the iron and steel industry, that the present development of electric furnace plants has been very rapid, and that 'an important future is assured. FINAL CONSIDERATIONS 291 O r*5 f5| O ,r i - 8161 'I 'UBf JBUKXIS Tr IT her induction furn J Reports for 1916 |8 S :|S| and ncludes Kjellin. Rfichling-Rodenhauser. o-metals Company in Great Britain. uoponpui* | % n* : . . Hf ana Wor ."S = IjSj "JJis "t3 c II'" Part Two A. THE MATERIALS USED IN FURNACE CONSTRUCTION, AND THE COSTS OF OPERATION THE MATERIALS USED IN FURNACE CONSTRUCTION BY DIPL. ING. W. RODENHAUSER, E.E. IT has already been pointed out several times that a great advantage of the electric furnace over other metallurgical furnaces is that it enables the generation of desirable high temperatures. Generally speaking this possibility is made use of, and work is carried out at higher temperatures than in gas fired furnaces. On this account, therefore, it is immediately apparent that the materials used for furnace construction, especially those parts in contact with the highly heated charge, have to meet particularly high requirements. Mistakes in the choice of these materials can very quickly bring about trouble in the working of the furnace and, under certain conditions, can completely stop the operation. During the discussion of the different furnaces the materials at present used for their construction were mentioned, but it still appears advantageous in the following pages to treat these materials as a whole, and with special regard to the conditions of service. The first requirement to be demanded is resistance to the high temperatures reached in the furnace. This needs, in the first place, a high melting point, which is usually measured by means of Seger cones. These are named after their inventor Seger. They are small three-cornered pyramids made of various mixtures of silicates, and are about 2.36" high. The softening 292 THE MATERIALS USED IN FURNACE CONSTRUCTION 293 point, with increasing temperatures, is carefully observed. The following table gives the comparison between degrees Centigrade and Seger cone numbers. No. Temp. No. Temp. No. Temp. No. Temp. 022 600 073 9 60 9 1,280 29 1,650 021 650 o6a 9 80 10 1,300 30 1,670 020 670 053 I.OOO ii 1,320 31 1,690 OI9 690 o 4 a I.O2O 12 1,350 32 ,710 018 710 03a 1,040 13 1,380 33 ,730 017 730 O2a 1 ,060 14 1,410 34 ,750 016 750 Ola 1, 080 15 1,435 35 ,770 oi 5a 790 la ,100 16 1,460 36 , ,790 0143 815 2a ,120 17 1,480 ; 37 ,825 oi3a 835 3a ,140 18 1,500 38 ,850 oi2a 855 4a ,160 19 1,520 39 ,880 ona 880 5a ,180 20 1,530 40 1,920 . oioa 900 6a ,200 26 1,580 4i 1,960 oga 920 7 ,230 27 1,610 42 2,000 o8a 940 8 ,250 28 1,630 A material is called refractory if its softening point lies about No. 26, and very refractory if the latter is between 30 and 36. Without going further into the properties which influence the refractoriness of the furnace materials, we will learn the requirements they have to meet. The first consideration is that in electric furnaces the materials are exposed not only to high temperatures, but also to chemical influences. Naturally the results of these influences must be felt as little as possible. We can give as the second requirement great resistance to chemical influences. Unfortunately these harmful influences cannot be altogether prevented, and it is necessary to reduce them as much as possible. This is partially brought about by having the greatest possible density and mechanical solidity of the materials. It is immediately evident that these properties are of important influence on the durability of the furnace masonry and lining, when it is realized that porous material offers much more surface for attack by harmful chemical influences than one that is dense. Further great density is 294 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY synonymous with good mechanical strength, so that a dense material will most successfully withstand the mechanical in- fluences due to movements of the electric furnace. The appearance of cracks in the furnace walls lead to the same bad results as the use of porous material, for they allow the harmful chemical influences to penetrate very deep, and offer a large surface for attack. This, therefore, brings about the requirements of the ability to withstand the influence of changes of temperature. These cracks are due to variations in the temperature, which bring about expansion and contraction in the materials used. The requirements for the materials for furnace construction are therefore: 1. Ability to stand high temperatures. 2. Resistance against chemical influences. 3. Great density and mechanical strength. 4. Permanence of form under changes of temperature. The materials to be considered are as follows, each of which will be taken up separately in the light of the above requirements: 1. " Schamotte " or fire-clay bricks. 2. Acid or silica bricks. 3. Half "Schamotte" or half-silica bricks. 4. Carbon bricks and carbon mixtures for ramming into place. 5. Basic bricks and basic material for ramming into place. 6. Zirconia* bricks. 7. Mortar. " Schamotte " fire bricks are made from burnt fire-clay known as Schamotte or Chamotte, to which unburnt clay is added as a binding material. The clay shrinks more or less during the burning. The Schamotte must therefore be burnt as thoroughly as possible. The more Schamotte in proportion to clay is used in the brick mixture, the less is a strong shrinkage to be feared. Moreover, the shrinkage can be partially neutral- ized by adding quartz or quartzite which expand during heating to the mixture. Under all conditions the shrinkage of the "Schamotte" brick is to be most carefully kept in mind, because, for example, *See "The Use of Zirconia as a Refractory Material," by Audley: British Ceramic Society, 1918. THE MATERIALS USED IN FURNACE CONSTRUCTION 295 the use of this material in furnace roofs would be disastrous As opposed to the "Dinas" silica bricks, which will shortly be described, these fire-bricks have the advantage that they are not so sensitive to changes of temperature, and this advantage is more marked the less unburnt clay is used in the mixture. Their chief importance in electric furnace work is for heat insulation purposes, and they are used in this way, for example, in the induction furnaces. For furnace roofs they are only applicable if the temperature attained is not very high, and therefore at the most can be used only for induction furnaces. Acid or silica bricks are greatly used in electric furnaces for roofs. They are very rich in silica, and are made from quartzite with 95 to 99% silica, and ought to have at least 95% when finished. They are known to the trade as English "Dinas" or Lime Dinas bricks. For the Lime Dinas bricks an addition of i to 2% of lime is used as a binding agent, usually in the form of cream of lime. If clay is used instead of lime the quality is not quite so good, the bricks containing 80 to 90% silica, and being known as Clay Dinas or German Dinas bricks. As has been mentioned, these bricks expand considerably with increasing temperature. This is less noticeable in bricks made from certain quartzites, but is unavoidable even with the best materials. Their greatest use is for the roofs of furnaces with high roof temperatures, such as all the arc furnaces have, and because of their expansion with heat a very flat roof can be maintained. Unfortunately these bricks are very sensitive to changes of temperature, and offer only small resistance to the action of slags. They are therefore practically restricted to roof and side-wall construction. The half Schamotte or half silica bricks are between the two kinds of brick just described. They consist of a mixture of quartz and burnt clay, and have properties corresponding to an excess of one or the other constituent. Carbon mixtures for ramming into place are not used in steel-refining furnaces; because of the great affinity between iron and carbon they are quickly destroyed, and undesirable carbon enters the metal. On the other hand carbon mixtures have been often used in electric shaft furnace experiments in 296 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY their double capacity as refractory materials and conductors of the current. In addition to this a silicon-carbide brick, or so-called "carborundum" brick, is used in the arches over the doors, and for the supports for these doors to just above the slag line, with good results. These brick are very expensive, costing usually 15 to 20 times as much as silica brick. Silicon carbide is a product of the electric furnace. When bricks are made of it, the mass is crushed to small particles, mixed with a binder, and the best grades again burned in an electric heating furnace. Zirconia bricks are used in side walls. Their higher initial cost than magnesite is more than offset by their high melting-point resistance to slags, low thermal conductivity, low ex- pansion value, giving increased production, and higher efficiency. Basic bricks and materials, however, are so important that they are used largely for the hearths and walls of electric steel- making furnaces. They include chrome iron ore, or chromite, dolomite, and magnesite. Of these materials chromite is not used in Germany. It has the disadvantage that if it comes in direct contact with the metal it is rapidly destroyed, and influences the bath in an unwished-for and harmful way. Dolomite is finding increasingly large application. It is a limestone with a large percentage of magnesia, CaC0 3 MgC0 3 and is found in large amounts in Thiiringen and Lothringen, and in various parts of the United States. Its greatest use in the iron and steel industry is as a lining for the basic Bessemer converter for which it is prepared in so-called dolomite plants. The method of preparation may be briefly described : The raw dolomite is either broken by hand, or crushed, to pieces about the size of one's fist. These pieces are then burnt in a shaft furnace to a clinker. The amount of coke necessary is from 20% to 30% of that of the raw stone. After burning, the clinker is ground in a suitable mill, the largest pieces being not more than 10 mm. (0.4") diameter. The ground dolomite is then mixed with about 7 to 10% of hot dry tar, on a moderately heated floor. This mixing is carried out either by hand or in a chili mill, or suitable mixing machine, which is so constructed THE MATERIALS USED IN FURNACE CONSTRUCTION 207 that the material can be heated. Careful attention must be paid to the preparation of the tar if the basic dolomite-tar mix- ture is to have the best properties. The crude tar is distilled in special apparatus at 240-280 C., and is freed in this way from the ammonia water and light oils. It must be mentioned that the burnt dolomite, because of its large lime contents, readily absorbs moisture from the air and falls to powder. It should, therefore, be used as soon after its preparation as possible, and is applicable to the making of bricks, or for ramming into place. Magnesite, as well as dolomite, is finding an increasingly large use for those parts of the furnace in direct contact with the charge, and indeed for all those parts exposed to specially high temperatures. It has an advantage over dolomite in that it is more neutral in character. When mined it has a melting point about equal to Seger cone, No. 42. As it shrinks a great deal when heated, it must, before using, be burnt so thoroughly that it is sintered. For this a temperature of about 1700 C. is necessary, and the'Sp. Gr. rises from 3.19 to 3.65. If the magne- site is to be used direct, it is mixed with tar, like the dolomite, and rammed into place. If, however, it is to be made into bricks, no tar is used. The finely powdered magnesite is forced into shape by the use of very high hydraulic pressure, and then again burnt. Apart from its greater neutrality magnesite has the further advantage over dolomite that it is not so sensitive to the action of moisture, and when burnt to sinter can be kept in storage without fear of spoiling. On account of its great density, how- ever, it easily cracks when subjected to changes of temperature, but does not shrink very much with increasing temperature, and because of this is being used sometimes for furnace roofs. When building electric furnaces a suitable mortar must be used. Here again permanence of form under changes of tempera- ture is of the first importance, and the mortar must be carefully considered in this respect. Also the courses should be laid as close as possible, so that even with expansion of the mortar the joints may remain tight, and expose very small surfaces of at- tack to harmful influences. As a general rule it may be said that the mortar should be of the same nature as the rest of the 298 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY construction, using basic mortar with basic materials, and acid with acid. Thus with dolomite bricks either tar alone is used, or tar mixed with dolomite as a binding material, while with magnesite bricks powdered magnesite is used mixed either with tar or a little hydrochloric acid. With other than basic brick- work a certain amount of quartz or white silica sand is usually mixed with the mortar, an excess of clay being avoided, and care again being taken to have tight joints. Reconsidering the materials of construction, we see that none of them possesses completely the qualities that have been men- tioned, so that they must be carefully chosen and used with proper regard to their properties. This leads us to a point that must riot be left without attention. It is that, under the influ- ence of other materials at high temperatures, certain refrac- tories quickly break down. The following rule must be strictly observed : In the presence of basic influences use basic materials, and with acid influences use acid materials. Therefore, in a furnace in which the basic process is to be carried out, basic materials must be used completely or at least in those parts where the temperature is high enough for one material to act on the other. On the other hand, in the Heroult and Girod furnaces the hearth is basic but the roof is of silica bricks. This is only possible when no danger of fusion is to be feared at the junction of the two materials. It should be remembered that quartz and clay, or mixtures of the two, so greatly lower the melting points of dolomite or magnesite as to bring them from Seger cone 17 to n, which would naturally lead to the rapid destruction of any electric furnace if it took place. The extremely harmful occurrences mentioned here must be prevented, either by the use of basic material exclusively, or else the contact between the acid roof and basic hearth must be removed from the influence of harmful temperatures. It may be mentioned here that the lowering of the fusion point of a refractory material is often brought about purposely, however, only to a certain degree. For instance, in making a hearth of magnesite, slag, clay, or similar material is often added as a flux, to bring about a more ready sintering of the THE MATERIALS USED IN FURNACE CONSTRUCTION 299 mass. In this way the hearth is made denser, and offers greater resistance to the influence of the metal and slags. Naturally only the smallest amount of flux necessary must be used in order not to reduce the melting point too much. In the design of electric furnaces the properties of the mate- rials to be employed should be carefully kept in mind. In the first place the side walls ought to be of such a shape that, after a charge or a short run, repairing can be carried out. This is necessary because of the unavoidable action of the slag on the lining. This is completely possible at present only in that fur- nace which is patterned after the open hearth, except for the rectangular shape. All furnaces allow a certain amount of re- lining, with even the vertical or almost vertical walls it is easily possible to keep these longer than 3 to 4 weeks. After this time a new one is necessary, equally with the Heroult, Girod, or Rochling-Rodenhauser. Notwithstanding this, the previous discussions on costs of operation have shown that the lining costs of the Heroult and Rochling-Rodenhauser are about the same. This is because quicker roof destruction in the arc is counterbalanced by the bottom renewal of the induc- tion furnace. It is because of these considerations that the Stassano and Girod furnaces do not have such good lining costs as the Heroult and Rochling-Rodenhauser. In case bricks are not used the new hearth is sometimes made up of hot magnesite or dolomite mixture rammed into place. If this work has been carried out by hand it can be more profitably done by means of compressed air hammers which are sold by all the firms making compressed- air tools. Air at a pressure of about six atmospheres is used. These tools bring about a great saving in labor, and have the advantage that they also give a much denser and more solid hearth than hand ramming. Naturally if such a lining is heated, which corresponds somewhat to the burning of refractory bricks, an expansion takes place. This will certainly give rise to cracks unless a certain freedom of movement is allowed for. This is well provided for by leaving a space between the hearth and the in- sulating cover, which may be filled with loose granulated material. 300 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY In all arc furnaces the roof is made removable, for repairs are not possible during the operation. In this way a new roof can easily and quickly be put into place. As the bricks used for the roofs are sensitive to changes of temperature, it is evi- dent that there will be more danger the more water-cooled openings there are through which to pass electrodes. A very superior basic bottom material, consisting of about 87% CaO and 13% FeO sintered together, has been brought out by the Coplay Cement Co., Coplay, Pa., U. S. A. This lends itself even better than magnesite for patching basic bottoms need- ing no flux to make it set. Bottoms are also burned in with the arc only, layer by layer, without using flux, which last a year. This material can be exposed to the weather without be- coming hydroscopic. It sells at a reasonable price per ton. THE COSTS OF OPERATION The question of the operating costs of electric-steel and pig-iron processes is undoubtedly the most important one after that of the quality of the product. These two questions there- fore will determine which types of furnace will advance in the future, and which will recede. With regard to the quality it is generally acknowledged that all the accepted types of electric furnaces, those considered in detail in the chapters of the first part of the book, will produce steel that will answer all require- ments. However, the possibility is often mentioned of an un- favorable influence on the quality of the steel of the unneces- sarily high temperatures of the arc furnaces. This possibility is brought up again and again, and was recently spoken of by Henry M. Howe, Professor of Metallurgy in Columbia Uni- versity (E. & M. /., Aug., 1909). During the past ten years of experience, 1900-1919, the high heat of the arc has had no deleterious effects on the steel which can be traced at this time. Howe again says, 1918, of all elec- tric steels, "At any rate, it grows very much easier for us to believe and to accept the evidence of experience, that electric steel is better than open-hearth steel, from the fact that the electric furnace reproduces very closely the conditions of the THE MATERIALS USED IN FURNACE CONSTRUCTION 301 crucible, particularly in that it has very much closer control over conditions. You can get in the electric process much closer controls over the conditions than you can get in the open hearth and very much closer than in the Bessemer." Dr. Jos. W. Richards, disagreeing with Howe on the crucible vs. the electric furnace, says, "You cannot get in the crucible such slag as you wish, you can get only an acid slag, and you cannot make a re- fining slag, whereas in the electric basic, with far inferior material, you can make a basic slag produce steel substantially equal in quality to crucible which is made from much better material." There remains, therefore, only the question of the operating costs to determine which type of furnace to adopt and the econ- omy of electric steel production. Calculations of the operating costs of the different types of furnaces have been published many times, so that it would appear very simple to compare them one with the other. This would immediately show which one would allow the cheapest production of steel, and the difficult question of the choice of the most suitable and economical type of furnace would be solved at one blow. Unfortunately this method is altogether incapable of giving a view corresponding to the real conditions. The figures that one usually finds published are often misleading. However, some of them should be con- sidered later, for with proper care they will give much inter- esting information. First we will see which factors are of importance in influ- encing the operating costs. They are briefly: i. The mate- rials charged. 2. The loss during the operation. 3. The con- sumption and price of the current. 4. The fluxes. 5. The labor costs. 6. The costs of linings and repairs. 7. Amortization. 8. Costs of electrodes. 9. Auxiliary appliances. 10. Tools. In regard first of all to the material charged, it can be of metal below the average in quality, and therefore cheaper, for all electric furnaces which are suitable for refining. This is be- cause these furnaces allow a very complete removal of all harmful impurities. The induction furnaces with ring-shaped hearths, such as the Kjellin furnace, do not allow thorough refining to be carried out, as has been pointed out in the previous chapters. 302 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY For these furnaces, therefore, especially pure and correspondingly dearer raw materials must be chosen, similar to those now used in the crucible process. This includes the high-priced Swedish irons, as well as the purest refined metal, usually made from Styrian charcoal pig iron; also fluid metal already refined in the open-hearth furnace, which can be "killed" in the electric furnace, in exactly the same way as in the crucible. Such a furnace is therefore at a decided disadvantage with regard to the metal charged compared with other furnaces, notwithstanding that a considerably lower power consumption is naturally re- quired for the further working up of pure raw material compared with material that must be first refined in an electric furnace of another construction. For these electric refining furnaces, the cost of the charge can be taken as equally high when comparing the operating costs. When comparing the costs of the open-hearth and basic electric furnace it must not be left out of consideration that the latter has the advantage that it allows the use of more impure and therefore cheaper raw materials, at the same time permitting the production of the highest quality steels. It appears unnecessary to give here any figures on the price of scrap, for as already mentioned this price is strongly depend- ent on local conditions. This also applies to fluid charges from either the blast furnace, cupola, open-hearth furnace, or converter. THE LOSS DURING THE OPERATION This means the material lost during the treatment of the molten metal in the electric furnace. Compared with other furnaces it is very small. When using the purely induction furnace with ring-shaped hearth, in which the purest material must be used, the loss is not considered at all; it can be put down as zero. On the other hand, when carrying out refining in the electric furnace, a certain loss is unavoidable. Altogether apart from the slagging of the impurities, small amounts of the liquid metal are torn away when the slag is removed. This loss is therefore the greater the more impurities are present in the THE MATERIALS USED IN FURNACE CONSTRUCTION 303 charge which necessitates a more frequent making and removal of slags. One will scarcely make a mistake in taking this loss, depending on the charge, as about the same in all electric refining furnaces. It should be figured on the average as 4 to 6 per cent, with a solid charge, and 2 to 3 per cent, with a liquid charge.* In this connection it must be remembered that very light thin scrap can be used in the electric furnace such as waste wire, turnings, etc., without the loss being higher than the figures given above. This is because the strongly oxidizing action of the hot gases of the open hearth is not present in the electric furnace. For comparison it may be mentioned that the loss in the open-hearth scrap process ordinarily amounts to 4 to 8 per cent., and is considerably higher if much of the light scrap, mentioned above, is used. If a pig-iron process is carried out in the electric furnace, which can offer an economic advantage under very favorable prices for current, then more iron is reduced from the ore than is the case in the open hearth, so that the yield is easily greater in amount than that charged. In this respect the electric furnace works more cheaply than the open hearth, and further allows the production of a higher quality of steel than can be produced in the open hearth. This is shown very plainly in that steel is often taken from the open hearth to the electric furnace to be refined, or to be alloyed, etc. In view of these advantages, why is not. the electric furnace used more often for melting, in place of the open hearth? The answer is found immediately if we consider the cost of heating, on the one hand in the open-hearth or crucible furnace, on the other hand in the electric furnace. The fuel consumption in the open hearth, working the scrap process and using bituminous coal, amounts to 22% to 32% of the output. If we take the highest value, and remember that the electric furnace in the sizes used up to now can only be compared with small open- * Borchers gives the loss as 10 to II per cent, in a Girod furnace with a cold charge. 304 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY hearth furnaces, we find that 320 kg. (705.5 Ib.) of coal would be used per metric ton of open-hearth steel. In the electric furnace 700 to 800 kw. hours would be necessary to melt the scrap used in the open hearth, and to refine it to the same grade as ordinary open-hearth steel, provided that the furnace was of 5 to 8 tons capacity. If the coal cost $3.57 per ton, then the heating cost alone of the open hearth would be $1.12 per metric ton. In order that the heating cost in the electric furnace should not exceed that of the open hearth, the kw. hour, with the above assumptions, should cost 0.150. If the kw. hour prices are calculated, which are allowable with different prices of coal and coal consumption, so that the heating costs in the electric furnace do not exceed those in the open hearth, then the following table is obtained: Coal Used per Metric Allowable Price per Kw. Hr. in Cents with Coal at the Following Price per Metric Ton (2,204 Lbs. ): If there is Used per Ton of Metric Open- Ton of Hearth $2.85 13-33 $3-80 $4.28 $4-76 $5-23 *5.7i Electric Steel Steel 22 .0833 .097 .III .126 .140 154 . i66c. 24 .090 .107 . 121 .138 .152 .166 .183 26 . IOO .116 .130 147 .164 .180 .197 750 28 .107 .123 .140 159 .178 195 .214 kw.hrs. 30 .114 133 .152 .171 .190 .209 .228 32 .121 .142 .161 .I8 3 .202 .223 .238 22 .078 .090 .104 .116 .130 .142 .1570. 24 .085 .100 .114 .128 .142 157 .171 800 26 .092 .107 . .123 .138 154 .I6 9 -185 kw.hrs. 28 .100 .116 133 .150 .166 .I8 3 .200 30 .107 .123 .142 159 .178 195 .214 32 .114 .133 .152 .171 .190 .209 .228 22 073 .085 .097 .109 .123 135 i47c. 24 .080 .092 .107 . 121 133 H7 .161 850 26 .086 . 102 .116 .130 145 159 173 kw.hrs. 28 .092 .109 .126 .140 157 .171 .188 30 . IOO .116 133 .150 .166 .183 .202 32 .107 .123 .142 .161 .178 .197 .214 This table clearly shows how very much cheaper the heating THE MATERIALS USED IN FURNACE CONSTRUCTION 305 costs are in the open-hearth than in the electric furnace. With the most unfavorable coal consumption (32%), and very high cost of- coal ($5.71 per metric ton), the kw. hour ought not to cost more than o.238c. If we calculate the kw. year as containing 300 working days, then the kw. year ought not to cost more than $17.14, or the e.h.p. year must not cost more than $12.62. It is evident that these prices for power can only be reached with the most favorable conditions, for example through the use of water-power. It is therefore also clear that the electric furnace can only be used for the melting of scrap, and the pro- duction of steel, similar in quality to ordinary open hearth, in such places where the cheapest natural power is ready for use, or else where small amounts of steel are to be made for which the open-hearth process is unsuitable. In all other cases it is almost always preferable to leave only the final work to the electric furnace. In this way at only a small increase in cost an improved quality is reached, compared with open hearth which is greatly in favor of the electric furnace. The smaller crop of electric steel ingots, and when the latter are rolled into sheets for galvanizing, where the so-called "spangle" is larger and better looking, and takes less spelter, compared to open-hearth sheets, are all offsets which must be taken into consideration. It has been mentioned already that the electric furnace can replace the crucible. If we therefore now consider the heating costs of the electric furnace on the one hand (using this method of working), and the crucible on the other, we obtain the follow- ing: The fuel consumption with crucible melting in coke furnaces not using the waste gases amounts to about 150 to 200 per cent, of coke, and with the use of regenerative gas furnaces, 175 to 200 per cent. coal. If we take it, first, that the same pure charge is to be used in the electric furnace as in the crucible, then the melting capacity alone of the electric furnace comes into consideration. For this the power consumption is 600 to 750 kw. hrs. per metric ton of finished steel depending on the size of the furnace used. For instance, the firm of Krupp has brought the power consumption 306 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY down to 617 kw. hrs. per metric ton in their 8-ton, ring-shaped induction furnaces, of the Kjellin and Frick types. Also, calcu- lating on the melting alone, the power consumption in an 8-ton Rochling-Rodenhauser furnace is only 580 kw. hrs. per ton. Taking the figures given above as a basis, the table on the following page clearly shows how high the cost per kw. hr. may be, in order that electric heating may not be dearer than that in the crucible furnaces with the given unit prices for coal and coke. The table shows how the unit price for power can increase very considerably before the heating cost in the electric furnace Coal Used per Metric Allowable Price per kw. hr. in Cents with the Following Coal or Coke Prices (per Metric Ton): If there is Used Mrtric Crucible Ton of Electric Per cent. $2.38 $2.85 $3-33 $3.80 $4.28 $4.76 $5-23 $5.71 Steel kw. hrs. 150 595 .714 .833 952 I.07I 1.190 1.309 1.428 ) 175 695 833 ..971 I. in 1.250 1.388 1.528 1.666 } 600 200 795 952 I. Ill 1.269 1.428 1.588 1-745 1.904- ) 150 547 659 .769 .881 .988 I.IOO 1.209 I.3I9 \ 175 643 .76 9 .897 1.028 1.115 1.281 1.409 1.540 r 650 2OO 733 .881 1.028 1.171 .1.316 1.464 1.614 1.762 ) 150 .512 .62 .714 .816 .919 1. 02 1 1.123 1.224 j 175 595 .714 833 952 1.071 1.190 1.309 1.428 > 700 200 .681 .8l6 952 1.088 1.226 1.362 1.500 1-633 ) 150 .476 571 .666 .762 857 952 1.047 I-I43 \ 175 557 :666 778 .890 1. 000 1. 112 I.22I 1-333 [ 750 200 635 .762 .890 1.016 1-143 1.269 1-397 1.524 > will exceed that of the ordinary crucible furnaces. We had assumed previously that only pure raw materials were used. In this way, however, only a part of the advantage of the electric furnace is utilized since the remarkable properties that it shows as a refining furnace remain unemployed. We should obtain, therefore, a still more favorable idea of the electric furnace if we used less pure and, therefore, cheaper raw material in the charge. Although from 150 to 250 kw. hrs. more would be THE MATERIALS USED IN FURNACE CONSTRUCTION 307 required, depending upon the degree of purification, this increase would scarcely outweigh the savings brought about by the use of a cheaper charge. It must be further considered that much more labor is re- quired to operate the crucible furnaces than an electric furnace, which can replace many crucibles because of its capacity. This latter property brings about a further advantage, namely, a complete uniformity of the whole cast, while the material from different crucibles shows certain variations. It should also be mentioned that the cost of crucibles is higher than that of the upkeep of an electric furnace. Finally, when one considers that the steel from the electric furnace is of fully equal value to Uiat from the crucible, then the displacing of the crucible by the electric furnace appears inevitable. This is shown by the growth that the electric-furnace industry has had even up to now. The following figures in metric tons are taken from the steel production of Austria-Hungary: Year Crucible Steel Electric Steel 1907 1908 23.215 19,659 4.333 1909 16,083 9,048 1910 17,586 20,028 1911 17,467 22,867 The following table in metric tons shows the steady progress which electric steel has made in the leading steel producing countries of the world: 1918 1917 1916 1915 1914 United States Germany 870.000$ 221,824 147,925 50,000! I2O,OOOt 11,000 304.542 219,700 120,600 48.000! 39,069 10,664 169,918 190,036 49,256 47,247 43.790 6,648 69.412 1 29,000 22,000 23,895 61 24,009 89.336 "f" ..... Austria, Hungary, Bohemia. Canada France Sweden Totals i,420,749j 742,575* 506,895 244,368 1913 1912 1911 1910 1909 1008 United States Germany Great Britain Austria, Hun., Boh.. Canada France Sweden 30,180 101,755 26,837 18,000 t 18,309 74,177 Vi'.SSO 15.992 29.105 60,654 aV.867 '13,850 52,141 26,200 20,028 13,445 13.762 17,700 9.048 50 6,456 t 55 19.536 4.333 93 3 f 4 Totals 176,772 129,964 126.476* * Includes Sweden. t Data not available. t Estimated. 308 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY We have previously shown that, in regard to heating costs, the electric furnace is more economical in almost all cases than the crucible furnace, but that on the other hand it usually is less economical than the open hearth. This naturally brings it about that as much as possible of the melting and refining should be done in the more cheaply operated open hearth; or, in the case of the refining of basic Bessemer metal, in the con- verter. This leaves only refining and desulphurization for the electric furnace, for both of which purposes it is particularly suitable, because of the easy regulation of the temperature, and the removal of the harmful influences which are unavoidable with any other method of heating. It is, therefore, to be expected that the electric furnace will not only displace crucible plants, but will be introduced more and more in connection with open-hearth and Bessemer plants. The power consumption necessary for the work of refining naturally depends greatly on the final product desired, but it is also dependent upon the degree of purity the material has, when charged into the electric furnace. Furthermore, the size of the furnace, as well as the efficiency of the particular type of furnace chosen, has an influence which must not be neglected. In re- gard to these latter influences, the discussion in the first part of the book must be consulted. The only points remaining to be considered are those of the material charged, and the final product required. It is known that in melting in any furnace a higher effi- ciency is obtained the quicker the melting proceeds, that is, the greater the amount of energy supplied, the higher the effi- ciency. With an important lessening of the time necessary for melting, there is a corresponding lowering in the amount of neat lost by radiation, etc. It is also well known that, after the melting stage is once over, the following refining period cannot be lowered at will by increasing the amount of energy introduced, but that this refining work requires a certain time. As already mentioned several times, the slag must be changed more fre- quently, depending on the impurity of the charge and the required THE MATERIALS USED IN FURNACE CONSTRUCTION 300 purity of the final steel. The curve given in Fig. 60 (see Part I, page 140), which shows the power consumption depending upon the size of the furnace with different slag changes, gives a fitting idea of the influence of the impurities in the charge on the power consumption. The figures given should there- fore be considered as approximate. To give more exact values is apparently only possible with a thoroughly fixed type of furnace, of a fixed size, and with an exactly established charge and final material. For example, basic Bessemer metal with about 0.08% P and 0.08% S, requires an average of 250 kw. hrs. per metric ton for refining, in an 8-ton Ro'chling-Rodenhauser furnace, when the final material required is of crucible steel quality with a definite carbon content. With the production of the highest value alloy steels the power consumption under almost the same conditions increases to 280 and even 300 kw. hrs. per metric ton. On the other hand, when making structural steels it falls to 200 kw. hrs. or less. The power consumption is therefore the smallest when only a limited allowing or degasification must be carried out, and not a thor- ough refining of the metal. It then falls even to 100 kw. hrs. and less per ton. It should be remembered that very impure metal was taken for the charge, basic Bessemer, and if metal was taken from the open hearth for example, with 0.03% P and 0.05% S, then under the same conditions there would be a certain lowering of at least 50 kw. hrs. per metric ton when making high quality steels. The considerations given above serve to show that the power consumption figures given in technical papers should be carefully investigated to see what conditions they refer to, for such figures only lead to grave mistakes in many cases. As the electric pig-iron furnace is beginning to be of im- portance, as shown by the action of the Jernkontoret in Sweden, who have built a furnace for a daily output of 20 tons 1 with an energy consumption of 2,500 to 3,000 h.p., a comparison is given 1 In 1919 Average size is 45 tons. 310 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY below between the ordinary blast and electric shaft furnace. The following table, due to Catani, is taken from Neumann's paper in Stahl und Eisen, 1909, p. 276 ff. It shows what unit prices may be paid for electrical power so that the heat- ing cost in the electric furnace does not exceed that of the ordinary furnace, with the given price of coke and output per h.p. day: Weight of Pig Iron in 24 Hours per H.P. Allowable Price per H.P. Year with the Following Coke Prices r kg. Ib. $3.80 $5.71 $7.61 6 13.2 $4-88 S7-3I $9-76 8 I 7 .6 6.09 9.14 12.19 10 22. 7.61 11.42 15-23 12 26.4 8-57 12.85 17.14 By calculation we obtain the following table, taking the h.p. year as equalling 0.736 kw. year, and the year as containing 365 days: Weight of Pig Iron in 24 Hours per Kw. Allowable Price per Kw. hr. in Cents with the Following Coke Prices: kg. lb. $3.80 *S. 7i 17.61 8.15 18 0.0730. O.I09C. 0.1450. 10.9 24 .090 135 .183 13-6 30 .114 .171 .226 16.3 36 .128 .192 .257 In order to be able to form an opinion from the figures given in the table, it is naturally necessary to know what efficiency is possible today with the electric pig-iron furnace, per kw. day. This naturally depends in the first place on the quality of the ore used. In the following chapters the metallurgical part of this work will be gone into, but the table already shows that with THE MATERIALS USED IN FURNACE CONSTRUCTION 311 coke at a relatively high price, the price for electricity must be very low for the electric-shaft or pig-iron furnace to compete with the ordinary one. In Germany, therefore, the electric- shaft furnace apparently has no future. This is clearly shown in the following table by Neumann (Stahl und Eisen, 1904, p. 143). Here the carbon necessary for the reduction of the various ores used in Germany, and that replaceable by electric power is calculated and given in money value. The price of coke is taken as $3.57 per metric ton, and that of power as o.238c. per kw. hr., or $19.04 per h.p. year. Carbon The Carbon Replaceable by Elec- trical Energy In- creased Iron Ore Pig Iron Neces- Re- Corresponds Costs Cost of Elec- sary for Reduction placeable by Kw. hr. Coke Kw. Hr. Coke Kw. Hrs. Heat- ing Bi'bas brown Bessemer 722.9 Ib. 1197.1 Ib. 1400.5 Ib. $2,579 $2.269 $7.881 $5-590 iron ore iron 327.9 kg. 543. kg. 635.3 kg. Dillen- Foundry 910.5 Ib. 1247.8 Ib. 1460.0 Ib. $2,688 $2.364 $8.190 $5.826 b ur ge r iron 413.0 kg. 566. kg. 662.3kg. Luxem- Basic 509.3 Ib. 1261.0 Ib. 1475.7 Ib. $2,717 $2.390 $8.281 $5.890 burg Loth- Bessemer 231.0 kg. 572. kg. 669.2 kg. ringen Min- ette Swedish Basic 1067.0 Ib. 806.9 Ib. 944.1 Ib. $1,636 $1.528 $4-985 $3-457 Magnet- Bessemer 484.0 kg. 336. kg. 428.2 kg. The next question is: What unit prices for electrical power are obtainable today? This has been treated already in Chapter XVI of the first part of the book, and it is therefore sufficient to give here merely the figures on which rough calculations can be based with the use of water-power o.ii9C. and more per kw. hr. and more. With the use of blast-furnace gas-engines 0.3570. to 0.7140. Steam turbines of great efficiency 0.7140. and more Steam-engines 0.9520. Overland and large city central stations 0.9520. " These figures show the values reached under the most favor- able conditions. Apart from these, the prices naturally depend 312 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY very largely on local conditions, so that for more exact calcula- tions these conditions must be considered. Further, the figures refer to the delivery of power at the generators, so that for exact calculations, the transmission losses, and losses in stationary or rotating transformers must also be considered. In the latter case, for example, these can easily amount to 20%, so that the cost of power at the furnace is 20% higher than at the central station. We have now sufficiently considered the influence of current consumption and cost on the operating costs, and can pass on to the other points. The fluxes necessary for the operation of electric fur- naces depend in the first place on the amount of the impu- rities in the charge, and further on the desired composition of the final material. Also, on the method of carrying out the refining process, or on the furnace construction or method of heating, which under certain conditions may bring about a special method of working. As has been pointed out in previous chapters, lime and roll scale or ore are necessary during the oxidation stage. During the deoxidation stage, more lime, together with some sand or fluor-spar, are used to make it liquid, and some powdered carbon or ferro-silicon as special deoxidation medium. Carbon is used only in the Heroult furnace, all other arc furnaces and also the Rochling-Rodenhauser using ferro- silicon, so that in these latter furnaces a somewhat higher ferro- silicon consumption has to be figured upon than in the Heroult furnace. Further, all furnaces working with carbon electrodes use a slightly greater amount of oxidizing agents during the oxida- tion period when compared with induction furnaces, which is due to the reducing action of the carbon vapor. This must be reckoned with, altogether apart from an increased power consumption. The wages or labor costs which are required for the operating of electric furnaces, calculated per ton of steel, are the smaller the greater the capacity of the furnace and the larger the amount THE MATERIALS USED IX FURNACE CONSTRUCTION 313 of steel produced. In almost all cases the labor necessary to operate a small furnace will be completely satisfactory to operate a larger one. If we consider that the size of the various types of furnaces is the same, then the labor necessary for the purely metallurgical work can be taken as equal in amount. It should be determined whether solid or liquid charges are to be worked, and in the former case the kind and amount of scrap to be charged, as well as the kind of auxiliary machinery to be used. If we also suppose that the number of men necessary to handle molds and work on ladles is the same under all conditions, for the different furnaces, (which appears to be absolutely, correct,) then the same amount of labor would be used with all the furnaces for the purely metallurgical work. We have already noticed, however, in the first part of the book that with the Stassano furnace one man is necessary to watch continuously the electrical recording instruments, and to regulate the electrodes according to their readings. Such a man is necessary with all arc furnaces unless they are provided with automatic regulating arrangements, and even if these are. present a continuous supervision of the electrical conditions is necessary while the scrap is being melted, for example in the Heroult furnace, as has been already pointed out in Chapter VIII. This extra man is unnecessary with induction furnaces, and with proper design of the furnace all the switches and regulation devices can be looked after by the first melter without any great or important waste of time. When working with fluid charges in arc furnaces equipped with automatic regulation no important switching work is neces- sary, and the special expense can be saved. These conditions are not without bearing on the amount paid for labor per ton of steel. The lining and repair costs form a very important part of all operating costs. They include labor and the expense of material. The material costs, in the first place, depend largely on local conditions so that correct unit prices cannot be given. Apart from this the wear and tear on the furnace roof and walls 314 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY depend very largely on the method of heating. For this reason we find, for example, that the roof is strongly attacked in all arc furnaces, as it is exposed to the heat radiated from the arc, while an attack on the roof of induction furnaces can scarcely be noticed. The reason is that in the latter case the heat is pro- duced in the metal bath itself so that the roof is protected by the covering of slag, altogether apart from the fact that at no place is a temperature of 3,500 C. produced, as is sometimes the case near the carbon electrodes. In all electric furnaces there is also a certain wearing away of the dolomite or magnesite hearth by the slag. As long as possible this is taken care of by repairs made between the charges. This is done the more easily if all parts of the hearth can be reached from the doors, and if the material used sticks to the places to be repaired. The cylindrical Heroult, Rennerfelt, Girod, and Stassano furnaces, as well as the Rochling-Roden- hauser, only allow such repairs to a certain extent, so that after a run of a certain number of charges the furnaces must be stopped for repairing the walls, and in the case of the Girod, Gronwall, etc., the bottom also. This brings about a certain loss of time and expense for labor, both of which are the greater, depending on the difficulty of making the walls and roofs. The Stassano shows the most unfavorable conditions in this respect, while the Girod and Rochling-Rodenhauser can be prepared for opera- tion in about the same time. Furnace linings last anywhere between 6 and 600 heats, averaging about 100. In regard to furnace repair costs it is evident that with arc furnaces the price of material for the roof as well as the hearth is of determining influence, while for induction fur-, naces the latter alone is of special importance. In general it may be said that the repair and maintenance costs of the furnaces mostly used, namely, the Heroult, Girod, and Rochling-Rodenhauser, do not exceed those of the open hearth, as soon as heats averaging 3 tons and upwards are worked. In open-hearth furnaces this can be taken as 36 to 6oc. per ton, under normal conditions. The depreciation is naturally higher, the more expensive the THE MATERIALS USED IN FURNACE CONSTRUCTION 315 whole plant having the same capacity. It is therefore important to use the plant as completely as possible, and the induction furnace allows this the most easily, as it works without rapid current variations. As this furnace moreover has undoubtedly the best working efficiency, and can be kept under current continuously, even during charging, without the machinery being in danger, there is a saving in time, and therefore an increase in production for a given size of furnace. With an equal amount of total plant cost the deprecia- tion per ton of steel with the induction furnace must be smaller than with other electric furnaces. In regard to the extent of the cost of plant itself, the first part of the book may be referred to. Electrode Costs. This comes into question only with arc furnaces. The conditions affecting the consumption of electrodes were treated in Chapter VI of the first part of the book. It was also proved in Chapter IX that the Girod and Heroult furnaces should be- considered as working with the same electrode conditions, provided that both furnaces are technically of the same excellence. We can, therefore, without further thought put down the electrode consumption in these two furnaces as equally high. Oh the other hand the Stassano furnace, working under altogether different conditions, will give another electrode consumption. The electrode material will also naturally affect the cost per ton of steel. Carbon electrodes vary in price from $115.71 to $200 per metric ton; graphite electrodes, $253.90 to $400. Graphite electrode consumption varies between 2 and 13.6 kg. (4.4 and 30 lb.), and amorphous carbon, between 13.6 and 27.2 kg. (30 and 60 lb.) per metric or gross ton of steel made, exclusive of breakages. Specific cases are given later on. It is perhaps not without value to consider that the mild steel pole. pieces, such as are used in the Rochling-Rodenhauser furnaces, are not attacked. As is well known they are protected from the high temperatures of the bath by a conductor of the second class, which is composed of the lining itself. Through this arrangement every electrode cost disappears. Certain operating costs proceed from the auxiliary machinery 316 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY necessary with all furnaces. For instance, with the Rochling- Rodenhauser furnace there is the air cooling of the transformers, and with all arc furnaces a certain water consumption for cooling the electrodes, or for the governing of the electrodes as in the Stassano furnace. To this also belong the costs of the power necessary for the tilting or turning of the furnaces, and finally also that necessary for automatic regulation, etc. These costs altogether are, however, only very small. With all electric furnaces they only amount to a very few cents per ton. Finally a certain consumption of working tools, rabbles, rods, etc., should not remain unmentioned, which should cause about the same costs for all furnaces. Also when calculating the costs exactly, the power for lighting, operating the travelling cranes, etc., should be considered, which can be taken as equally high for the different furnaces. Finally, there is a license cost which comes into question, concerning the amount of which only the companies owning the patents can give information. As a conclusion some operating costs may be given for different furnaces. It should be again pointed out that such figures and comparisons are to be used with the greatest care because they are based altogether on local conditions, and also on the kind of metal charged and obtained. In regard to the operating costs of the electric shaft furnace it has been pointed out already that it can only compete with the ordinary blast furnace under the most favorable conditions. These conditions exist, for instance, in some parts of the United States, Canada, Norway, Sweden, Japan, and Switzerland, and the following comparison of costs is for Sweden. It has been made by Prof, von Odelstierna of Stockholm, and is taken from the Electro Chemical and Metallurgical Industry, 1909, p. 420. In the charcoal blast furnace: 0.950 metric tons charcoal at $8.00 per ton $7 . 60 Labor i . oo Repairs and general expenses i . 50 Per metric ton #10. 10 THE MATERIALS USED IN FURNACE CONSTRUCTION 317 In the electric shaft furnace: O. 270 metric tons charcoal $2.16 0.3 electric h.p. years at $12.142 3 . 60 Labor x oo 10 Ib. electrodes at 3c. Ib 30 Repairs and general expenses 1 . 50 Per metric ton $8 . 56 According to these figures the use of the electric furnace gives a gain of about $1.54. It is based on the assumption that the Gronwall, Lindblad & Stalhane furnace, which has shown up the best so far, is taken as the electric shaft furnace. The ore taken for the comparison ought to contain 60% metallic iron, and the charcoal 83% carbon. It is further assumed that both furnaces have the same output from 8000 to 10,000 metric tons per year. The prices for ore and raw limestone* are not taken into consideration, as they depend so largely on local conditions. If we compare the results found here with the previously given table of the cost of current for the electric blast furnace, we find complete agreement. For instance, from the table on page 290, we see that if the cost of heating in the two types of furnace is to be equally great the h.p. year should cost $12.19. This is with a production of (8 kg.), 17.637 Ib. pig iron per h.p. day, and a price of coke of $7.61 per metric ton. The figures of Prof, von Odelstierna are based on power at $12.14 per h.p. year, charcoal at $8.09 per metric ton, and an output of i metric ton per 0.3 h.p. year. This corresponds to about (9 kg.), 19.841 Ib. per h.p. day. If it is assumed that the coke and charcoal contain the same carbon then the estimate of von Odelstierna is calculated with a higher output and with a greater price for carbon, both of which points are favorable to the operating costs of the electric-blast furnace. It should, however, be again pointed out that such favorable * In the Metallurgical and Chemical Engineering, Feb., 1912, p. 71, LEFFLER says that in practise it has been found more economical to use unburned limestone, and that among other things burned limestone causes the burden to hang. 318 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY conditions for the electric-shaft furnace are not often present, so that it is restricted to countries poor in fuel and rich in ore and electricity. In this respect it is, however, encouraging to note that, after the five months' test made at Trollhattan, ending April, 1911, (according to The Iron and Coal Trades Review, of Nov. 10, 1911), the pig iron produced per h.p. year equalled 3.79 metric tons or 22.92 Ib. (10.41 kg.) per h.p. day; this corresponds to an output of i metric ton per .262 h.p. year. These later and better figures are the average of the first week's run after again starting up, and are attributed to the improved gas circulation, under the furnace roof which, according to Robertson, the inventors main- tain that the important point is to make this furnace, last as long as possible, and in order to do this they consider it absolutely necessary to have the roof cool. Richards suggests (A.E.S., April, 1912) that the arch of the furnace hearth be protected by water-cooled plates, as is common with open-hearth practise. This, however, as has already been suggested, may decrease the efficiency too much. Lyon states that attempts were made at the Noble Electric Steel Co. in California to preserve the roof of the crucible hearth by water-cooled plates embedded in the brickwork, but these did not prove especially effective. Leffler writes at this time that they would gladly dispense with the artificial gas circulation if they could. As is elsewhere men- tioned, Leffler says that calcined limestone causes the burden to hang. Yet Noble, with his California furnace, says he only uses calcined limestone, and furthermore uses no artificial gas circulation.* In the last tests made at Trollhattan, the repairs and petty expenses cost about $1.60 per ton of pig iron produced. Part of these operating cost repairs are caused by the roof burning away. If half of the above amount were saved by the durability of the roof being increased, it would make, in a 2500 h.p. furnace, producing 25 tons daily, an annual saving of 350 X 25 X .80 = $7,000, enough to pay almost 9% on the investment. * The reason the Trollhattan furnace has gas circulation and the ' Noble furnace none, is because the former is operated as an arc furnace, but the latter as a resistance furnace. THE MATERIALS USED IN FURNACE CONSTRUCTION 319 The cost of producing one ton of electric pig iron during the 5 months period ending in April, 1911, was estimated from the records and from a personal investigation given on the spot to be as follows: 1.52 tons of ore, 67.1% at $2.68 $4.07 .262 kw. year at $13.40 3-51 85. kg. (187 Ib.) limestone at $i.6l per ton 14 416 kg. (915 Ib.) charcoal at $12. . 4.99 5.27 kg. electrodes consumed * at $67 36 Labor 78 Repairs and petty expenses 1 . 60 Interest and sinking fund, 10% on $24,000 35 Total $15.80 The cost of producing one ton from hematite of 50% iron was $16.04. One of the Norwegian companies on the West coast, now (1912) constructing a plant for the smelting of 60% magne- tite, estimated the cost per ton of iron, with electricity at $5.46 a kw. year, at $11.25, using English coke at $5.63 per ton. For the Stassano furnace detailed cost figures are given by Osann in Stahl und Risen, 1908, No. 19. They apply to the furnace described in Chapter VII for one-ton charges, making steel for castings from cold material. The figures are further based on the following special conditions. The furnace remains unused each night for three hours, and 24 hours on Sunday. During these times it is kept warm by electricity, the current being switched on for one-quarter of an hour, and off for three- quarters of an hour. Under this non-continuous operation the furnace gives 3.5 metric tons per day, or 840 metric tons per year of 240 working days. The furnace takes three men per shift, the average wage being given as $1.19. The lining costs $95.24, exclusive of the labor, when magnesite is used. It must be renewed every three weeks, that is, after a production of about 63 metric tons, and requires 4 to 6 days for the renewal. * The total electrodes used per ton of iron produced was 10.28 kg., the difference being attributable to the stub ends, now no longer prevalent, with the new screw type electrode. 320 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The construction cost of the furnace is given as $8,750. Under these conditions the following calculations are given per metric ton of fluid metal: Depreciation $ o . 992 Cost of the charge : I metric ton scrap at $15.95 $ I 5-952 .02 metric ton mill scale at $4.047. ... .081 .02 " " lime at $2.857 057 .008 " " 12% ferro-silicon at $35-71 285 .004 " " 8o%ferro-manganeseat $52.38 209 .0008 " aluminum at $357.00 .. .285 16.869 Cost of power: For melting 900 kw. hrs. at i.O7ic $9.643 For heating during delays i .071 Cost of furnace, lining, and repairs 2.619 Labor 2 . 047 Electrodes . 595 Cooling water . 095 16.070 Total $33-941 According to a more recent article (Neumann, Stahl und Risen, 1910, p. 1066) it is possible to greatly reduce the cost. of the lining when using dolomite for the hearth. At the same time through the use of a purer charge the power consumption for melting drops to 750 kw. hrs., and because of the correspond- ingly less work with slags the furnace can last 70 to 100 heats. Definite figures for the lowering in costs brought about in this way are not known. Cost calculations for the Heroult, Rennerfelt, Snyder and Girod furnaces, have been published. The following are taken from Stahl und Risen, 1908, p. 1825, and apply to a 2-ton Girod furnace. THE MATERIALS USED IN FURNACE CONSTRUCTION 321 If a cold charge is worked, consisting of scrap, turnings, and pig iron, and completely refined to give steel of high value, the costs per metric ton are as follows: .1 ton lime, .1 ton ore, and additions of various alloys, from $ .71410$ .809 Electrodes 952 - i . 190 Labor 1. 142 Furnace maintenance, tools, etc 2 . 857 1,000 kw. hrs., the cost depending on the price of power If melting, without further refining, is all that is necessary, that is to say a similar method of working to that recently car- ried out with the Stassano furnace at Bonn, then the following figures should be used: Lime, etc $0.238 Electrodes 762 Labor 762 Furnace maintenance, tools, etc 2 . 285 Power consumption, 750 kw. hrs For the refining of a liquid steel charge taken from the con- verter or open hearth, the following figures are given: .04 tons lime, and additions $ . 524 to $ .619 Electrodes .381 Labor .571 Furnace maintenance, etc .952 Power consumption about 300 kw. hrs In these tables depreciation and the loss in operation have not been taken into consideration. The latter is given by Borchers as 10 to 11%, who also says that the consumption of electrodes in the larger furnaces ought to amount to 0.012 to 0.015 metric tons per metric ton of steel with cold charges. This gives an electrode cost of $0.571 to $0.762 per metric ton. In regard to the life of the Girod furnace it is stated that with cold charges the walls last about 80, and the bottom about 120 heats, the roof stands 25 to 30 heats with small furnaces and 20 to 25 with large ones. 322 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The costs with the Heroult furnace will scarcely differ in an important degree from those of the Girod, except for the lesser Heroult bottom costs. For the Heroult furnace the following partial results may be shown taken from Metallurgical and Chemi- cal Engineering, 1910, p. 179. They apply to the 15-ton furnace at So. Chicago. Liquid Bessemer metal, of which the composi- tion is not given, is refined in 12- to 14-ton charges the final material containing 0.03% P. and 0.03% S. Power consumption 200 kw. hrs. per metric ton. The furnace roof of silica brick costs $60. It requires changing each Sunday.* With 12 heats a day, and 13 metric tons per charge, this equals 0.0642. Hearth repairs, about $o . 0642 Lining costs, about 1284 f This does not take into consideration the costs per ton of door bricks, which must be replaced at certain times. The electrode consumption is given as (6.6 Ib.) 3 kg. per metric ton. Graphite electrodes are used, and the cost per ton of steel is about $0.91. Neumann gives the loss with a cold charge as 6%, and 2.5 to 3% with fluid charges. With the same kind of charge, how- * The roof problem has recently been the subject of careful study by FitzGerald (see A. I. E. E., June 25, 1912, transactions). A brick made of silicon carbide has been manufactured which it is believed will have a much longer life in the steel furnace than the silica brick now used. The brick is made by taking powdered or granular silicon carbide, mixing it with a suitable temporary binder, such as a solution of dextrine, molding and then heating in an electric furnace to the temperature at which silicon carbide is formed. Bricks made in this way have been used in the roof of an experimental steel furnace in one of these laboratories and then put to the severest test possible. The bottom of the furnace was purposely raised well above the normal level so as to bring the surface of the slag as close to the roof as possible, the actual distance in some experiments being only 10 in. (25.4 cm.). Then the furnace was.worked at double the normal rate of generation of energy so that the heat- ing of the roof was very intense, so much so that an ordinary silica roof would melt down rapidly and be completely destroyed in a single heat. Even under these very severe conditions the silicon carbide roof stood up perfectly. Ex- periments have also been made in other steel furnaces and these results con- firmed. The most serious objection to a roof of this kind is its heat loss and cost. If it lasts a sufficiently long time it is nevertheless economical. t Dolomite taken at $6.00 per metric ton. J This applies to electrodes of Acheson graphite, costing 27 cents per kg., THE MATERIALS USED IN FURNACE CONSTRUCTION 323 ever, the same loss is to be expected in both the Heroult and Girod furnaces. A certain difference in the operating costs of the Heroult and all other electric furnaces arises from the fact that the former uses carbon for deoxidation instead of ferro-silicon. With nor- mal heats, therefore, and deoxidation with carbon the Heroult furnace has to figure on a consumption of about 4 kg. ferro-silicon per metric ton; in the case of other furnaces, and the Heroult also, if deoxidation with carbon is not followed, on about 7 kg. If it is assumed that the Heroult uses 3 kg. petroleum coke, then the following figures are given for deoxidation and de- sulphurization: Heroult furnace, using 3 kg. petroleum coke at $1.90. . . $0.057 Heroult and all other furnaces using 3 kg. ferro-silicon at 7.260. . 216 So that deoxidation and desulphurization by means of ferro-silicon alone is dearer than that by carbon and ferro-silicon by about 159 This is with 50 per cent ferro-silicon costing about $74 per metric or per gross ton. With this alloy costing considerably more than the figure mentioned above, even though the price of petroleum coke increases correspondingly, and even more so, the demarcational advantage of refining with carbon instead of with ferro-silicon becomes more apparent. From an operating standpoint, however, it is a little easier to achieve results with a more expensive method. As mentioned before, this has the advantage, however, that it does not influence the composition of the bath, and so is often used even in the Heroult furnace. such as are used in this furnace. Up to within a short time it was impossible to construct satisfactory electrodes of carbon for the 15- ton Heroult furnace. These are the 20" round amorphous carbon electrodes. Ever since February, 1913, Howland advises that 12" diameter Acheson Graphite are being used with success in these furnaces with only one breakage in 67 used, and that the cost per ton of molten metal refined is no more than with the larger amorphous carbon electrodes. 324 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Advice is received from Gray (J. H.) that with a 6-ton basic Heroult, operating 24 hrs. daily and 6 days weekly, with elec- tricity at one cent per kw.-hr., the cost of refined steel, one or two slags, should on the average not be over $15 per ton, plus the cost of scrap, in normal times. There remain finally the operating costs of the Rochling- Rodenhauser furnaces. Such figures have been published by Wedding. The following apply to a 5-ton furnace working on fluid charges: Power consumption 230-280 kw.-hrs. per metric ton Additions, about $o. 536 Lining costs, using magnetite 595 Wages 178 Air for cooling transformers 050 A 2-ton furnace using polyphase current gave the following costs, when scrap was worked up for making steel castings: Charge i metric ton scrap $15-952 5% loss 798 .01 metric ton roll scale (22. Ib.) 040 .035 " " lime (77. Ib.) 100 .005 " " fluor-spar (15.7 Ib.) 074 .01 " sand (22. Ib.) 014 .004 " ferro- manganese ( 8.8 Ib.) 209 Loss in ferro-alloys remaining behind 157 $17-344 Power consumption, about 900 kw.-hrs., price varies. Lining and repair costs 636 Labor 793 Air for cooling transformers at i.O7ic. per kw.-hr 079 These figures are given for a 2-ton furnace which, working with cold charges, allows a production of 6 to 8 tons per day. Apart from this it should be mentioned that the lining and repair costs when dolomite is used, and liquid charges, only amount to 0.238 to o.428c. with 3- to 8-ton furnaces. It may be mentioned again that all the cost figures given above are only exactly correct for certain predetermined local THE MATERIALS USED IN FURNACE CONSTRUCTION 325 conditions. Care should, therefore, be taken in using them for comparison. The weight of material used ought to be known, and the kind of charge and the final metal required have a great influence. The consideration of the different factors affecting costs given in the first part of the book appears, therefore, to be very valuable, and this part may once again be referred to. B. THE ELECTRO-METALLURGY OF IRON AND STEEL INTRODUCTION UNTIL the invention of the steam-engine the operation of an iron and steel plant required the presence of a waterfall as the source of power for the hammers and blast. If, at the same time, sufficient ore beds and forests were in the neghborhood all the requirements were filled for the prosperity of the plant. The consumption of iron and steel tools was moderate, the plants could operate economically in a modest way and with small water-powers, for with the absence of railroads, etc.. the products found a paying market in the immediate vicinity, and the bring- ing in of foreign goods was almost impossible. The few specially large German water-powers were not needed, and would not be used because the technical knowledge necessary was not sufficient- ly advanced. Conditions changed as the supply of charcoal began to de- crease and the consumption of iron and steel to increase, for the old plants with their associated water-powers and limited amounts of charcoal could only increase their production to a certain amount. The knowledge that ores could be smelted with coke, and the invention of the steam-engine, made it possible to use commercially the immense stores of energy lying dormant in the earth in the form of coal. Soon the plants deserted their old places near the waterfalls, and changed their locations to the coal-fields, where fuel and therefore power were present for application in unlimited amounts. Then succeeded the remarkable newer growth of the iron and steel industry with its attendant immense production. But the consumption of iron and steel constantly increases, coal begins to decrease in amount and become more expensive, and the industry will soon be forced, as in the time of our fathers, to look for a new and constant source of power. Electric energy THE ELECTRO-METALLURGY OF IRON AND STEEL 327 is the first to come into consideration, since it. is possible to pro- duce .it from coal at a moderate cost. Also the railroads have brought the most remote countries into connection, and the enormous water-powers of foreign lands can' be used as sources of cheap electric power. Is it to be wondered at that many technical men are working at the problem of the building of electric furnaces, or that this task should soon be solved economic- ally, when it is known that electric heating produces a higher furnace efficiency than heating with fuel? So we see efforts being made recently to build plants near the larger water-powers, as in the old days, in order to obtain electric power at the lowest cost, and to produce iron and steel from ore by electricity. Also in the industrial countries the electric furnace is gaining importance from day to day, for it is proving capable of pro- ducing higher quality steels equal to crucible steel, from impure raw material. It is the authors' wish that the production of iron and steel by electricity may receive such an impulse that the statements in this little book will very soon be exceeded by the facts. GLUCK AUF! J. SCHOENAWA. THE ELECTRIC SMELTING OF IRON ORES FOR IRON AND STEEL PRODUCTION The usual commercial process by which pig iron is produced is smelting in a blast furnace with fuel, flux .and a blast of air. In the upper part, or shaft, of this furnace a continuous" series of thermal and chemical reactions take place, which reduce the iron and prepare it for its final smelting in the hearth. These preliminary reactions could, if desired, be carried on in a special shaft into which ore is charged and subjected to the action of the hot furnace gases. In the lower part, or smelting zone, of the furnace the reduced and partially carburized iron is melted; the impurities of the ores and fuel are fluxed with the flux added for this purpose, and thereby converted into a liquid cinder, or slag. Besides these thermal effects, some chemical reactions occur which the temper- ature in the shaft was not sufficiently elevated to effect, such as the reduction of the oxides of silicon, manganese and phosphorus (the reduced elements being then absorbed by the iron), the conversion of iron sulphide in part to calicum sulphide, etc. First let us collect the data upon which to base a study of these reactions. Such data are given below; some of them have not been wholly confirmed experimentally, yet .the estimated values are close enough to afford calculations of practical value: i kw. hr. = 864.5 calories, i kw. hr. = 1.34 h.p. hrs. i h.p. hr. = 0.746 kw. hrs. Spec. ht. of iron = 0.20. Spec. ht. of blast furnace slags = 0.30. Spec. ht. of ore = 0.20. Spec. ht. of CO = 0.243. Spec. ht. of C = 0.20. Spec. ht. of air according to its weight = 0.30. Latent heat of fusion of pig iron = 46 calories. Latent heat of slag = 30 calories. 328 THE ELECTRO-METALLURGY OF IRON AND STEEL 320 PLATE I. Test pieces of seamless drawn electric steel tubes (Rochling). The normal tube, and test pieces made from it in the cold state. 330 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY TABLE I WITH THE REDUCTION OF i KG. OF REDUCED MATERIAL IS NECESSARY IS OBTAINED HEAT REQUIRED C. kg. CO kg. CO kg. COzkg. Gain cals. Loss cals. Balance cals. FeO+C=Fe+CO.. FeO+CO = Fe-|-CO 2 Fe 3 O 4 + 4 C=3Fe + 4 CO 0.2I 4 (0.2 1 4 ) 0.286 =0.500 0.500 0.786 529 1,200 707 1,603 795 1,802 1,078 719 540 2,119 2,394 1,332 1,332 l,6 4 8 i,6 4 8 1,876 1,876 2,209 1,947 1,817 7,829 5,966 - 80 3 - 132 - 941 - 45 1081 - 74 -1131 -1228 -1277 -57io -3572 0.667 Fe 3 O 4 + 4 CO = 3Fe + 4 CO 2 (0.286) 0.321 (0.321) 0.436 0.291 0.218 0.857 0.968 =0.667 =0.75 .509 1.048 I.I78 Fe2O 3 +3C=2 Fe + 3 co 0-75 1.017 0.679 0.509 2.OO 2.258 Fe,O 3 +3CO=2Fe+ 3 CO:i MnO 2 +2C= Mn + 2CO Mn 3 4 + 4 C = 3 Mn + 4 co MnO + C = Mn + CO SiO a +2C = Si+2CO P 2 5 + 5C=2P + SCO To reduce 1000 kg. of iron from magnetite requires 1381 kg. of ore. For simplicity the ore may be considered as pure Fes C>4 without any earthy constituents which have to be slagged off. Reduction with pure carbon then takes place according to the following equation: 232 kg. Fes (X + 48 kg. C = 168 kg. Fe -f- 112 kg. CO. The CO therefore measures 4 X 22.4 = 89.6 cu. metres. For the production of a metric ton, 1000 kg. of pure iron 286 kg. of carbon are necessary and 533 cu. m. of carbon- monoxide are produced. In .the blast furnace much larger amounts of carbon than these theoretical calculations call for are required, because carbon is depended upon not only to reduce the ore, but also to furnish the heat required for the process. According to the THE ELECTRO-METALLURGY OF IRON AND STEEL 331 equation Fe 3 O 4 + 4C = 3 Fe + 4 CO, the process can be carried out without the blast being used if the amount of heat is supplied which the table shows is necessary. The amount of gas produced by the reduction would be only about one-tenth of that produced in the blast furnace for the same weight of iron, for in the latter case the gas is diluted with a large nitrogen content. If magnetite is mixed with carbon in the proportion calculated above, and the mixture heated by electricity to the necessary reduction temperature as well as to the melting temperature of about 1300 C., reduction of the magnetite takes place readily. The following rough calculation gives the theoretical power consumption necessary for the production of i metric ton of iron in a condition fluid enough to be readily tapped, which is necessary in practise. 1381 kgs. ore heated to 1300 C. = 1381 Xo.2 X 1300 = . . . 359,060 cals. 286 kgs. carbon heated to 1300 C.= 286X0.2X1300= 74,360 " looo kgs. iron heated to reducing temperature 1000X941 = 941,000 lOOOkgs. iron melted = 1000X46 = 46,000 " 1, 420,420 cals. 1420420 This corresponds to-^- = 1043 kw. hrs. 004.5 From this it is clear that the process requires much less carbon than the blast furnace if considerable electric energy is supplied. In the same manner a high-carbon iron can be produced if sufficient carbon be supplied not only to reduce the ore, but also to supply that which dissolves in the metal. A rough calculation for an iron with 3% carbon is given below: The carbon required is 286 + 30 = 316 kg., and 1030 kg. of pig iron is produced. 1,381 kg. ore heated to 1300 C 359,o6o cals. 316 kg. carbon heated to 1300 C 82,160 I.ooo kg. iron to the reducing temperature 941,000 1,030 kg. iron melted 47.3^0 1,429,600 cals. 332 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY This corresponds to ^~ = 1653.7 kw. hrs. per 1030 kg. 504.5 iron, which equals 1605.5 kw. hrs. per metric ton. It should be remembered that the figures given for the coke consumption in the blast furnace take in all losses through cooling, radiation, etc., and in this respect the efficiency of the blast furnace is not bad. The power consumption given, on the other hand, is only the theoretical minimum, in operation it will be considerably higher, depending on the type of furnace used and its efficiency, etc. Also the figures for carbon consumption are for chemically pure material, while in operation fuel containing ash has always to be figured on so that the minimum carbon consumption in the form of coke, charcoal or similar material is correspondingly higher. The economical side of the smelting of ores by means of the carbon theoretically necessary for reduction and electrical energy to supply the heat for the thermal reactions requires that the saving in coke in the new process must be greater than the expense of the necessary electrical energy. As a result the process has prospective use only under con- ditions where ore and power are cheap and coke is dear, as in some parts of Canada, Italy, Norway, Sweden, Califor- nia, etc. The use of coke can be completely done away with and the iron separated from the ore electrolytically like aluminum, but the necessary power consumption is so extremely high that this method does not appear economical even for the future. Re- cently proposals have also been made to use iron pyrites as the raw material for smelting iron in the electric furnace. It is to be melted and air-blown through the bath until a consider- able amount of ferrous oxide has been formed; then the blast stopped, and the bath allowed to react according to the equation: FeS + 2 FeO = 3 Fe + S0 2 It is scarcely possible that the process will have a great future. THE ELECTRO-METALLURGY OF IRON AND STEEL 333 The process given above of using just enough fuel to combine with the oxygen of the ore and electric heating of the ore-fuel mixture forms the basis of the many recent attempts to smelt iron ore by electro-thermal methods. It should be emphasized that, in electro-thermal processes, as the words themselves indicate, the electricity serves only as the source of heat which brings the charge to the temperature required for reduction and melting. Electrolytic processes where electricity is used both as a source of heat and as a reducing agent are less often employed because only direct current can be used. In regard to this, we may refer again to Part I. Electrical heating of the charge gives the great advantage that, because of the much lower fuel consumption, the influence of the latter on the charge and melted material can be regulated much better, and the operation can be carried out if desired at higher temperatures than used up to now in the blast furnace. This has a great metallurgical advantage for, as is well known, the "reaction ability" of all material increases considerably with increase in temperature. In general it is to be expected that in the smelting of the ordinary iron ores which contain more or less manganese, sulphur, phosphorus, silica, etc., the same reduction reactions will take place as are already known for the blast furnace process, etc., and that with electric smelting an iron of a certain determined purity and analysis will be obtained by regulating the furnace temperature and the slag. The iron will be very low in sulphur, for experience shows that the slagging off of the sulphur is favored by high temperatures, and with the electric furnace the temperature can be raised to any desired amount. Smelting in the electric furnace can also be carried on in such a way that, according to the amount of reducing material used, an iron can be produced of any desired carbon content, even practically free from carbon. However, it is a question whether it is preferable to produce right away a soft material, or to make a higher carbon product and suitably refine this later by special processes. Con- cerning this, local conditions alone can lead to a decision. In smelting lean iron ores, more electric energy is required, 334 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY because the impurities have to be heated to the full temperature of the charge, and, furthermore, additional flux must be added to slag off these useless impurities,, and the extra slag must also be heated to the temperature of the furnace. All this waste makes the process correspondingly more expensive. Raw spathic ore, brown iron ore, etc., must be calcined when smelted, which also requires electrical energy and correspondingly increases the cost. In conclusion: the electric production of iron, which is indeed an "infant industry," must be accomplished without the loss of an unnecessary kilowatt in order to successfully compete with the old economically working blast furnace. In general, therefore, at present usually high percentage iron ores, preferably magnetite and red hematite, are smelted electrically. If it happens that poorer ores have to be used, then they must be previously carefully prepared and concentrated. During this concentration it is well to remove as completely as possible any pyrites, apatite, etc., which may be present, and thereby help in the production of a highly valuable iron of great purity similar to Swedish or Styrian, which will be suitable for the production of high quality steels. The fuel must also be as low as possible in ash, so that the slag volume is not in- creased too much. The size of the material is of secondary importance for suitable reduction, but very fine materials are not willingly used exclusively because of the difficulty of removing the gases produced in reduction. In the first tests carbon and ore were intimately mixed, pressed together with tar and used in the form of briquettes. This briquetting is unnecessary and can be more readily rejected as it is costly, for in those countries where electric smelting is commercially possible because of dear coke the price of tar is also correspondingly high. Electric smelting of iron ore can be carried on in electric steel-making furnaces. The mixture for reduction will either be charged altogether, or else added little by little, depending on the type of furnace. If a pool of liquid pig iron has formed on the hearth, then the reduction of the ore mixture will progress more quickly, for the carbon of the liquid metal takes an energetic THE ELECTRO-METALLURGY OF IRON AND STEEL 335 part in the reduction. The fluid pig iron will then have to be recarburized to the required amount by the carbon of the charge. In regard to the necessary power consumption, that type of furnace will work most favorably with which the radiation loss is the smallest. The Stassano furnace, to the construction of which the first part of this book is devoted, heats the mixture by radiation, for the arcs are outside of the material to be heated. But, as the arcs radiate heat in all directions, and only that much which radiates downwards is used economically, it is to be expected that the efficiency of this furnace will be proportionately low. On the other hand, the electrode consumption will not be very high for the electrodes are not in contact with the charge, and so will not be attacked by the iron ore. i. Smelting of Ore in the Stassano Furnace. (The charge heated by radiation from the arc.) Neumann and Goldschmidt have published results of the following smelting test (Staid und Eisen, 1904, pp. 687, 885). The analyses of the materials used were: Ore: FezO, 93-02% P 056% MnO 62 CaO+MgO 5 SiO 2 3-79 H 2 O 1.72 S 058 Lime: CaO 51.21% FezOj 50% MgO 3.11 SiO 2 90 A1 2 3 50 C0 2 43-30 Charcoal: Carbon 90.42% Ash 3-88% Water 5.70 Pitch: Carbon 59.20% Hydrocarbons... 40.50% Ash 27 Briquettes were made from a mixture of 1000 kg. ore, 125 kg. lime, 1 60 kg. charcoal, 120 kg. pitch (charcoal and pitch together 336 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY containing 230 kg. carbon). These briquettes constituted the charge. According to Stassano the heat requirements per 100 kg. ore in the charge are calculated as follows, the data below being chosen from his tables. Decomposition of the oxide of iron - - in 552.000 Vaporizing the moisture in the ore and charcoal (1.72 +1.21)637 = 1866.41 Calcining the flux 12.5 X 425 .............. = 5312.5 Heating the CO, to 500 C. 542Q X ' l6 X 5 ^ = 987.09 .044 Heating the CO produced - X .0068 X 500 = 5921.667 Melting the iron produced 65 X 350 .......... = 22775.2 Melting the slag produced 13.89 X 600 ....... = 8334.0 157311.427 Produced from the burning of C to CO 20.9 X 2175 45457.500 Leaving 111853.927 These 111,853.927 calories correspond to 129.386 kw. hr. From 100 kg. ore 65.114 kg. of iron will be reduced, so that the power required per metric ton of iron is 1987.6 kw. hrs. This power requirement is, however, only calculated theoretically, and figures concerning the real power consumption have not been published; however, as shown above, the radiation loss with the Stassano furnace must be considerable. An idea of the amount of this radiation loss is obtained from a further test published by Goldschmidt in which the power consumption is given. In this test 70.25 kg. of the same briquettes used in Test No. i were smelted in a 100 h.p. furnace. The output was 30.8 kg. iron with a power consumption of 97.2 kw. hrs. = 132.2 h.p. hrs. The theoretical power consumption for the charge may be calculated on the basis of the analyses given above as follows: THE ELECTRO-METALLURGY OF IRON AND STEEL 337 For the reduction of the iron contained in the final product were necessary 3 - X 192 = 52730.262 For the reduction of the manganese in the final product were necessary '^~ X 94.6 = 48.719 For melting the metal 30.8 X 350 = 10780.00 For melting the slag 6.3 X 600 = 3780.000 For heating and vaporizing the moisture 1.316X637 = 838.292 For calcining the lime 6.25 X 475 = 2968.750 For superheating the steam to 500, 1,316 X 400 X .48 = 252.672 For superheating the CO 2 to 500 C, 2.714 X 500 X 016 = 493-554 0.44 For superheating the hydrocarbons to 500 2.43 X 500 X .27 = 328.05 For superheating the CO produced (3 X 30727 X3i2 + 28.336 ( 112 55 X 500 X .0068 = 2800.131 Total 75020.330 From the combustion of the C to CO were produced 9.883 X 2175 = 21495.525 Leaving 53524.805 As the whole charge consumed 97.2 kw. hrs. = 84012.072 cals., the heat efficiency was: 5,3524.805 X ico % 84012.072 The power consumed per metric ton of iron reduced from its ore is shown to be 3123 kw. hrs. Unfortunately the analysis 338 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY of the metal produced is not given, nor the length of time of the test. The demonstrated efficiency of 61.33% ^ s n t very difficult from the calculated figure. It must be admitted that not only was this a test melt, but that several of the figures calculated gave accidentally very favorable results. In operation the efficiency would undoubtedly be much smaller, for the careful supervision possible with a small test would be lacking. Because of the great heat radiation in the furnace which principally attacks the roof, the life of the roof must be small, and the economical carrying on of the process depends in the first place on the durability of the furnace. In order to make the roof somewhat more durable, either the whole or at least that part attacked the most must be protected by water cooling. This water cooling, however, apart from its complexity will bring about important heat losses, the amount of which will be gone into further in another place. The power consumption per metric ton of iron is seen to be high as was to be expected. Theoretically it is 1643 kw. hrs. or 2680 with a furnace efficiency of 61.33%, compared with a proved figure of 3123. This increase in practise of 443 kw. hrs. is due to the use of ore which is not theoretically pure, and the consequent melting of the slag produced, the burning of the lime, vaporizing the water, etc., a proof that only ores as pure as possible should be smelted. With the smelting of more im- pure ores the power consumption would naturally be con- siderably higher yet. This high power consumption is due to the great radiation loss of this type of furnace, and can therefore scarcely be lessened. Further disadvantages are that no con- tinuous operation is possible, and only small heats can be pro- duced. From this it is evident that furnace types in which, like the Stassano, the charge is only heated by radiation can not be considered in the economical smelting of ore. 2. Ore Smelting in Electric Hearth Furnaces. (Electrodes introduced into the charge.) Theoretically these furnaces should work well because the charge so nearly surrounds the arc that the heat radiated is THE ELECTRO-METALLURGY OF IRON AND STEEL 339 completely absorbed. In operation, however, such a total absorption is impossible, the charge can only surround the arc to a limited extent, and the temperature is so high that radiation through the charge to the walls of the furnace is unavoidable. With such furnaces the lining and the special roofs, if such are present, are particularly strongly attacked by the "stagnant heat," so that it is impossible to maintain continuous operation. Also the electrode consumption will be high, for the electrodes are in contact with the ore mixture and will be attacked. Many ore-smelting tests have been carried out with different types of furnace in recent years in order to remedy the trouble re- sultant upon the attack on the furnace walls, but with uncertain results. In every case the power consumption has been much more favorable than was expected, so that in this respect the question of electric smelting of ore would have been long since set- tled if a furnace construction had been found more suitable for continuous operation. The most recent tests of this kind have been carried out by Messrs. Gronwall, Lindblad & Stalhane, the latest test furnace being shown by figs. 124, 125 and 126, invented by the same men. One metric ton of white iron was produced in 1909 with 0.25 h.p. years equals 2190 h.p., that is " = 1622 kw. hrs., a result that closely approaches the theoretical minimum, and is to be explained perhaps by the very pure ore smelted. Further tests made with the Gronwall, Lind- blad, Stalhane furnace are given elsewhere in Chapter XIV, under "Operating Costs," and under B, " Electro-metallurgy of Iron." THE SMELTING OF ORE IN THE INDUCTION HEARTH FURNACE, SYSTEM ROCHLING-RODENHAUSER The efficiency of this furnace will not be bad for smelting ore, notwithstanding that the charge is only heated by the heat of the molten bath, because the bath is covered with cold charge and the radiation from the lower part of the furnace can be kept low by means of suitable brickwork, etc. Above everything else, however, because of the cooled upper surface of the bath due to the covering of the charge, and the heat- 340 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY ing from within, the roof will show great durability, which is a very important point, if there is to be continuous operation. Those furnaces which work with electrodes and have a roof are com- pelled to use extensive water cooling, sometimes in order to increase the life of the furnace. Through the avoidance of water cooling, a source of considerable loss of heat is avoided, so that the induction furnace is worthy of serious investigation for the smelting of ore. Many smelting tests have been carried out in the Rochling- Rodenhauser furnace, and some reports of them may be given, for up to the present scarcely any results of ore smelting in the induction furnace have been published. High sulphur magnetite in a very fine state of division was used and high sulphur coke breeze, in order to produce a pig iron with 2.6 to 3.0% carbon, and as low in sulphur as possible. Although it was assumed that a greater part of the sulphur would pass away as gas due to the following reaction: FeS + 2 FeO = 3 Fe + S0 2 , yet, by way of precaution, the theoretical amount of lime neces- sary to combine with the sulphur as sulphide of calcium was added to the charge, together with that necessary for the acid gangue, etc. The amount of slag produced in this way was not needlessly increased, although the CaS produced requires a large amount of slag for solution if it is hoped to produce a sufficiently good desulphurization in this way. Analysis of the ore: Fe 3 4 96-38% = 69.79% Fe ( % FeS 2 1.41 = o.66%Fe^ 7045/0 * Mn 3 O 4 0.25 = o.i8%Mn. Si0 2 0.60 P 2 O, 0.05 = 0.02% P. CaO o.io MgO i. 21 100.00 Oxygen combined with Fe & P = 26.62%. Total sulphur in the ore = 0.75%. THE ELECTRO-METALLURGY OF IRON AND STEEL 341 ANALYSIS OF THE COKE BREEZE Carbon 87 . 48% Sulphur i .068% Ash 10.4% The principal constituents of the coke ash were: SiO 2 40.6 % CaO 5.6% Fe ii. 6 % A1 2 O 3 25.40% Oxygen combined with iron = 5.0%. The carbon-monoxide produced passes away unused. Chemical Balance Sheet. 100 kg. ore (i/io of a metric ton) = 70.45 kg. iron. This requires: (a) For reduction and combination of the 26.62 kg. oxygen, C + = CO i60 + 12 C = 28 CO 26.62 X 12 16 19.97 k g- 26.62 X 28 , ~~ and produce = 46.58 kg. CO (b) 70.45 kg. Fe carburized to 3% require ?^JO = 2tl8k c 97 (c) 0.75 kg. S combined with CaO to form CaS require: S + CaO + C = CaS + CO 32 + 56 + 12 = 72 + 28 0.75 kg. S + 1.28 kg. CaO + 0.28 kg. C = 1.70 kg. CaS + 0.70 kg. CO The total amount of carbon is therefore 19.97 + 2.18 -f 0.28 = 22.43 corresponding to 22 -43 X 100 = ^^ kg coke breeze> 57.45 342 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY As, however, the ash of the coke also requires a small amount of carbon for the reduction of its metallic oxides, the calculations should be made with 25.80 kg. of coke breeze. This 25.80 coke breeze contains 0.258 X 1.068 = 0.28 kg. sulphur which must be slagged off as CaS. 0.28 kg. S + 0.49 kg. CaO + o.io kg. C = 0.63 kg. CaS + 0.24 kg. CO. 25.80 kg. coke breeze contains 10.4 X 116 X 25.8 - - 2- = 0.31 kg. Fe, 10000 and 10.4 X 5 X 25.8 ^ , . . , - " -- = 0.13 kg. O, in the form of oxide of iron. 16 + 12 C = 28 CO or 0.13 O + o.io C = 0.23 CO 100 kg. ore therefore require 22.43 + - 10 + ' 10 22.63 kg. carbon or - = 25.87 kg. (57.03 Ibs.) coke breeze. THEORETICAL AMOUNT OF SLAG PER 100 KG. ORE SMELTED From the ore From the coke ash 0.2587 X 10.4 = 2.69 kg. SiO 2 0.60 kg. SiO 2 i. 09 kg. CaO o.io kg. CaO 0.15 kg. MgO 1.2 1 kg. A1 2 O 3 0.69 kg. 1.91 kg. 2.69 kg. from this must be taken 0.166 X 10.4 X 0.2587 = 0.44 kg. Fe 3 4 , leaving 2.69 0.44 = 2.25 kg. slag. CaS produced 1.70 + 0.63 = 2.33 kg. Lime addition for combining with the sulphur 1.28 + 0.49 = 1.77 Lime addition for slag ................................... 1.41 Total lime addition ............................... 3.18 THE ELECTRO-METALLURGY OF IRON AND STEEL 343 The theoretical total amount of slag is 1.91 + 2.25 -f 2.33 + 3.18 = 9.67. In calculating the amount of carbon necessary for reduction, it must be remembered that before the beginning of the test the furnace was filled with 1000 kg. of refined Basic Bessemer metal, which latter had to be recarburized to the required amount. After this the following mixture was charged: 597.5 kg. ore + 183.5 k g- cok e breeze + 19.0 kg. lime = 800 kg., compared with the theoretical amount which does not consider the recarburization of the refined Bessemer metal: 597.5 kg. ore + 154.6 kg. coke breeze + 19.0 kg. lime. The Bessemer metal had a temperature of 1650 C. The ore was charged as uniformly as possible, and in comparatively large amounts. Care was taken that the bath was always covered with the mixture in order to keep the radiation loss as low as possible; a method of working that, in general, was not hard to maintain. The slag produced during the tests was only removed once, and the exact amount was obtained. As the furnace used for the tests was mounted on a scale, the weight of the Bessemer metal charged and the finished material were also obtained exactly. The smelting of the 800 kg. of charge required 1030 kw. hrs. SMELTING RESULT (a) Output of Slag. 99 kg. slag with 9.12% FeO = 7.09% Fe and 2.60% S. Theoretically the slag should contain: 1. From the charge 9.67 X 5.975 = 57-7 8 k g-- -57-7 8 k S- sla S- 2. Lime for slagging (3.18 1.77) = (1.41 X 5-975) = 8. 4 2k g .slag. 3. Excess of coke breeze = 3- k g- sla g- 4. Slag remaining in furnace from previous heat = - ' 20.8 kg. slag. Weight of slag = 90.00 kg. slag. 344 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The weight of this slag is increased by its iron contents 90 X 100 - - = oo kg. which contains 100 - 9.12 X QQ , 7.02 kg. iron. IOO (b) Output of Iron. looo kg. of Basic Bessemer was charged containing: C. 06% P. 093% S. 073%, 1427 kg. electric pig iron were tapped with 2.64% C, 0.02% Si, -73% P> 0.076% S, of which 1427 1000 = 427 kg. were produced from the mixture. The theoretical amount is (1) From the ore 5.975 X 0.7045 = 420.94 (2) From the coke ash, 1.835 X n. 6 X 10.4 2.20 = kg. iron = - loo 423-14 From this kg. iron = ^ went to the slag. 416.12 This weight when calculated to electric pig iron equals 416.12 0.9736 427 kg. The loss of metal in the slag is therefore-^ = 1.6%. 4.2314 CHEMICAL BALANCE (a) The Carbon. 1,000 kg. Basic Bessemer metal with 0.06% C. carbur- ized to 2.64% C. require (2.64 .06) 1,000 = . . . 25.8 kg. C. 427 kg. electric pig iron contain 427 416.12 = IO.88 " The reduction process requires 5.975 X 19-97 = 119.32 " The formation of CaS requires (0.28 + o.io) 5.975 =.. 2.27 " " The reduction of the iron oxide in the coke ash requires 0.10X5.975= 0.98 "" 159.25 kg. C. As 87.48 kg. C = loo kg. coke breeze, this 159.25 kg. C 182.0 kg. coke breeze. THE ELECTRO-METALLURGY OF IRON AND STEEL 345 (b) The Sulphur. Brought in: 597-5 kg. ore at 0.75% S = 4. 48 kg. S l8 3 5 kg. coke breeze at 1 .068 S = i .96 " 20 . 8 kg. slag held back containing i.o%S = ... 0.21 " 6.65kg. S Taken out: 99 kg. slag at 2.6% S = 2.57kg. S 427 kg. electric pig iron at 0.076% S= 0.32 " 2. 8 9 kg. S Therefore 6.65 - 2.89 = 3.76 kg. S or 56% of the total sulphur was gasified. (c) Phosphorus. 1000 kg. Basic Bessemer metal at 0.093% P = o . 93 kg. P 597-5 kg. ore at 0.02% P = 0.12 " P brought in 1 .05 kg. P 1427 kg. electric pig iron at 0.073 need = 1 .05 kg. P (d) The Furnace Gases. 12 C + i6O = 28 CO Therefore (119.32 + 2.27 + 0.98) = 122.57 kg. C + 163.43 kg. O = 286.0 kg. CO 183.5 182.0 = 1.5 excess coke cor- responds to 1.31 C. This was charged in excess, burned with air gives 3.05 kg. CO The burning takes place with 1.74 kg. O, that bring in 6.54 kg. CO The total gas made is 295. 59 kg. HEAT BALANCE The furnace was held at 1300 C. during the test, and the iron tapped at the same temperature. The mixture for reduction, therefore, had to be first heated to this temperature after charg- ing. 346 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY 1. Heat Expended. 1. 597-5 kg. ore require 597.5X1300X0.20= 155,350 cals. 2. 183.5 kg. graphite, 183.5X1300X0.23 = 48,426 ' 3. 19.0 kg. lime require 19X1300X0.21 = 5,187 " 4. 416.12 kg. iron reduced from Fe s O 4 require 1648 X4I6.I2 = 685,765 " 5. 5,975X0.02 kg. P to be reduced from P2O 5 re- quire 0.12X5966= 716 ' 6. 427 kg. pig iron require for melting 427 X46 = . . 19,642 ' 7. 9020.8=69.2 kg. slag require for melting 69.2 X30 = 2,076 " 8. 2.89kg. S changed into CaS require 2.89X1093= 3,159 " 920,321 cals. 2. Heat Brought in. 1,000 kg. Basic Bessemer metal cooled from 1650 to 1300 bring in 1000X350X0.2 = 70,000 cals. Burning 3.76 kg. S to SO 2 = 8,347 " 183.5 kg. coke breeze = 160.53 kg. C. For carbur- izing the pig iron 26.40+10.88 = 37.28 required. The remainder, 123.25 kg. burned to CO bring in 123.25X2473= 304,797 " 1030 kw. hrs. used in the test 1030X864.5 = 890,435 " Heat brought in = 1,273,579 cals. Therefore the efficiency of the furnace equals 920321 X 100 I2 73579 In determining the energy necessary to produce i metric ten pig iron it must be remembered that the basic Bessemer metal charged at 1650 brings in ~ = 81 kw. hrs., for the finished material was tapped at 1300 C. Therefore 1030 + 81 = mi kw. hrs. were required to produce 427 kg. electric pig iron, which equals for the metric ton = 2602 kw. hrs. 427 The following important points were established by means of the test. (i) The efficiency of the furnace is good, as was to be expect- ed. It may be still further increased if the mixture for reduction were charged by machinery and not by hand, so that the frequent THE ELECTRO-METALLURGY OF IRON AND STEEL 347 opening of the working doors and the unavoidable heat losses could be avoided. (2) The reduction of the ore takes place satisfactorily even with the use of dense fuel, chemically inactive, such as coke breeze with a high content of ash. The amount of reduction material necessary closely approaches the theoretical, due to the reducing atmosphere of the electric furnace. (3) The phosphorus in the charge goes entirely into the iron. (4) The sulphur in the charge is lowered more than half, due to the reactions between the oxides and sulphides of iron, so that lime additions to unite with the sulphur are probably unneces- sary. Test 2. In order to increase the efficiency of the furnace ef- forts were made to lower the radiation from the upper surface of the bath by causing the charge to project still deeper into the iron bath in the hearth. A suitable way appeared to be the smelting of the charge in the form of briquettes. The -briquettes were made of the same mixture as used in Test No. i plus 8% of steel- works tar. The whole was ground in a Chili mill, and pressed in an ordinary dolomite press. The briquettes were burned a little before being used. An interesting point is that these partially burned briquettes showed 0.35% reduced metallic iron. A lowering in the power consumption with the use of these briquettes could not be proved, nor any increased smelting efficiency of the furnace compared with Test No. i. The pig iron produced had a low sulphur content, and the chemical balance showed that a greater part had been gasified as S0 2 . The same amount of ore was smelted as in Test No. i. Test 3. As both tests showed that over half the total sulphur was gasified, and the iron was sufficiently low in sulphur, further tests were made on a mixture of ore and fuel without special lime additions. It was thought that because of the smaller slag volume, the power consumption would be lower, and that at the same time the iron would be sufficiently low in sulphur. After Test 2 had shown the lack of efficiency of briquetting, the ore was used fine as it was taken from concentrating, and the coke breeze of the usual size. The results of the test were good. 348 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY A low sulphur white iron was produced, and somewhat fewer kw. hrs. per metric ton were necessary than with Test i, and the furnace efficiency was somewhat higher; that is, the smelting time was somewhat shorter than with Tests i and 2. Test 4. This test was to show whether an addition of granulated iron to the mixture would shorten the time of melting, and give a saving in the energy consumed. It was really a carrying out of the so-called Lash process, which consists of using a mixture of ore, carbon, and slag-producing material with finely divided pig iron, the charge being kept loose and porous with sawdust. The reduction of the ore is helped by the carbide present in the pig iron. An example of a Lash mixture is as follows : Iron ore 54% Cast-iron t urnings or granulated cast-iron 27 Sawdust 4 Limestone 4 Tar 3 Coke... 8 100% From what has been said before, it is to be expected that ore reduction by the Lash process would give no advantage, for in the induction furnace there is present a permanent bath of metal, and therefore with the ordinary ore mixture the known good reactions in the Lash process must take place anyway. In melting a metric ton of pig iron by the Lash process, the power consumption will be rather bad because the iron enclosed in the charge has to be melted electrically. The reduction mixture was charged in exactly the same way as described by Lash. The result of the test, however, gave neither a shorter melting time nor a lower power consumption per metric ton of pig iron from ore. THE ELECTRO-METALLURGY OF IRON AND STEEL 349 CRITICISM OF IRON ORE SMELTING IN THE ELECTRIC HEARTH FURNACE The smelting of iron ore in the electric hearth furnace, which is so simple experimentally, depends on two important factors before it can be carried out commercially. One of them is the power consumption, the other the durability of the furnace lining, that is, the costs for repairs per metric ton of iron produced. The durability of the lining requires that the highest tem- peratures, such as those of the arc, must be avoided because the drop in temperature is too great for it to be taken up by the charge. This question of smelting ore in the electric hearth furnace is therefore only to be solved by a type of furnace that does not work continuously at the highest temperatures, and with which the excess heat which attacks the lining can be carried off. In this case the lining costs will be very small, but a somewhat higher power consumption must be counted on. From the discussion above the only furnace of this type at present is the induction furnace, and the tests show that on the one hand the furnace lining allows continuous operation, and on the other that the power consumption is within such limits that, under certain conditions, successful competition with the blast furnace is permissible. Such conditions are first, that there are no special requirements in regard to the physical properties of the ore and fuel. Even very fine raw materials can be smelted, but the best are of small grain size. This factor becomes more important from day to day, for conditions continually press towards the mining of poorer grade ores and magnetic concentration, so that high percentage concentrates, small in size, are coming on the market. If these concentrates are to be smelted in the blast furnace, they must be first agglomerated or briquetted, a process that even without a binding agent, that is, using high pressure alone, or say sinter- ing, is an additional expense, for in this case a preroasting cannot be avoided. Also in smelting ore in the electric hearth furnace a small 350 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY sized material can be used for reduction with at least equal success to one of moderate size, which means that small waste fuel of any kind is available that up to now has been valueless. Even these two points are so important that under certain con- ditions they will allow the electric hearth furnace to work more economically than the blast furnace. Also in regard to the purity of the ores, especially the sulphur, the electric hearth furnace has great possibilities because of the considerable volatilization that takes place. High sulphur materials can be smelted, therefore, with acid slags and without the lime additions that are absolutely necessary in the blast furnace. Only so much basic flux need be charged as is necessary to give a liquid slag. The concentration of the ores will therefore not have to be carried so far, especially when ores with an acid and basic gangue are to be used at the same time, for by suitable mixing a self- fluxing charge can be obtained. This allows the conclusion to be drawn, that under certain conditions the poorer iron ores can be smelted in the electric hearth furnace without previous prepara- tion, especially if the gangue forms a flux, so that the iron output of the charge is not lowered by the addition of fluxes. A further important advantage of smelting ore in the electric hearth furnace is that the harder steels can be produced direct. It is not favorable to immediately make a soft steel, for the iron bath is first carburized by the reducing material, so that at the end of the heat ore alone must be added in order to remove this carbon. In the next section of the book it is explained how this process is comparatively expensive. Still steel with about 1.5 to 1.8% carbon can be produced direct, and if high in sulphur can be desulphurized at little cost; while, at the same time, if high in phosphorus it can be dephosphorized without removing the carbon, both by means of processes given in more detail in the next chapter. The carbon consumption in the electric hearth furnace is as good as possible when the carbon is only burned to carbon-mon- oxide. Troubles that are always more or less unavoidable in THE ELECTRO-METALLURGY OF IRON AND STEEL 351 the blast furnace disappear altogether, as also the production of the valueless "transition iron," when the furnace is changed from one kind of iron to another. Add to this the simpler operation, the avoidance of water cooling, the possibility of reg- ulating at will the temperature of the metal tapped, and no electrode consumption, are some of the results. All these are points that, under certain conditions, allow the electric hearth furnace to successfully compete with the shaft furnace for smelt- ing ore. THE SMELTING OF IRON ORES IN THE ELECTRIC SHAFT FURNACE The experiments made in the electric hearth furnace make one desirous of studying more economical methods of smelt- ing. The disadvantages of the electric hearth furnace are briefly: 1. Low melting efficiency of the furnace during operation. 2. Large power consumption per ton of iron produced. 3. Frequently too high a consumption of reducing material. The reason for the low furnace efficiency is that the mixture for reduction is charged cold so that it has to be heated electrically to the necessary temperature. As the radiation loss increases with the smelting time per ton, it follows that a shortening of the smelting time would give a better efficiency, and this requires the charging of heated material. This preheating must naturally be brought about without increased consumption of electric or other energy if possible, and the hot waste reduction gases are available without extra cost. They are most suitably used by charging the mixture high in the furnace so that the gases have to pass through it, giving up their heat. This necessitates arranging a shaft on the hearth furnace. The carbon-monoxide produced in the hearth would not only have a thermal effect but also a chemical one, that is, the ore would be partly reduced, so that the furnace then has only to melt the iron in the mixture of iron and ore. In other words only the remainder of the iron ore has to be reduced, and the furnace is released from some of its work. 352 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY If the reduction gases are used only to preheat the charge, then the following rough calculation gives the advantage obtained: 533 cu. m. (1880 cu. ft.) of gases are produced per metric ton of iron, and the same may be used for preheating the charge and be cooled down to 200 C. There will be obtained therefore: 533 X 1 100 X 0.24 = 140712 cals., corresponding to = 004.5 162.8 kw. hrs. In producing a pig iron with 3% carbon there would be a saving in energy of = 10.14%, which means that the metric ton of pig iron will be smelted with 1605.5 ~~ 162.8 = 1422.7 kw. hrs. On the other hand if the waste gases are used only for pre- liminary reduction of the ore, then the following rough calculations are obtained for the limiting case that the CO is all changed to CO 2 . According to the equation Fe 3 4 + 2 C = Fe 3 + 2 C0 2 the metric ton of iron would only require 143 kg. of carbon for reduction. Also, according to the equations: Fe 3 O 4 + 4C = 3 Fe + 4 CO Fe 3 O 4 + 4 CO = 3 Fe + 4 CO 2 i 94IOQ + 45oo only - = 493000 cals. would be necessary. The total carbon required for the produc- tion of a 3% carbon pig iron will be 143 +30 = 173 kg. for 1030 kg. metal, and the following heat balance is obtained for this most favorable case. 1383 kg. ore heated to 1300 C. = 1381X0.2 Xisoo =359,060 cals. 173 kg. C. heated to 1300 C. = 173X0.2X1300 = 44,980 " 1000 kg. iron heated to reducing temperature .. =493,000 " looo kg. iron heated to melting temperature. . . = 47,880 " 944,920 cals. This corresponds to = 1093 kw. hrs. per 1030 kg. pig 864.5 iron, or 1061 kw. hrs. per metric ton. It is here assumed that THE ELECTRO-METALLURGY OF IRON AND STEEL 353 the CO 2 leaves the furnace at 1300 C., and if the excess heat of the CO 2 were further used to preheat the charge, and the gas allowed to escape at 200 C., then the power required would be lowered as follows: From the equation Fe 3 O 4 + 2 C = Fe 3 + 2 C0 2 168 kg. Fe produce 2 X 22.4 = 44.8 cu. m. CO 2 , or 268 cu. m. per metric ton of iron. If the heat from 1300 to 200 is used for preheating then there is obtained 268 X 0.24 X noo = 70752 cals., corresponding to ^p^ = 81.8 kw. hrs. In this case, therefore, 1061 81.8 = 979.2 kw. hrs. are necessary to pro- duce i metric ton of pig iron. From this it may be seen that the use of the furnace gases for reducing the ore brings about a considerable lowering in the power required, just as well as their use for preheating alone. By utilizing these gases as much as possible, the electric furnace is relieved a great deal and the smelting time is considerably shortened. The idea of using the reduction gases is therefore justified particularly as, at the same time, there is obtained a desirable and much lower consumption of reducing material. As is well known, however, carbon-monoxide can only be used up to a certain limited amount for the reduction of ore because the mixture of CO and CO 2 produced has no more reducing influence when a certain percentage of CO 2 is present. In the electric shaft furnace, therefore, .one has to figure on a waste gas that consists largely of CO, and it is apparent that the carbon necessary for reduction will increase with increasing percentage of CO in the waste gases. In smelting magnetite the carbon necessary per metric ton of pig iron with 3% carbon, when the percentage by volume of C0 2 in the waste gases is known, is calculated by the formula 286 (ICQ - 3) + 3 100 + CO 2 % In this way the following table has been prepared: 354 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Per cent. CO 2 in waste gases IOO Kg. carbon required per metric ton pig iron of 3%C. T-lS 90 14.6 80 154. 70 . . 161 60 17^ 50 184 40 198 v> 20 . 2'W 10 2*2 0. . 277 The principle of an addition of a shaft is naturally possible with any electric hearth furnace that has a fairly large hearth, and is the easiest in the case of the arc furnaces, for these always have a comparatively large hearth. The Stassano furnace forms an exception, for here the charge is heated by radiation alone and only the heat below the arc is used. Also the induction furnace can be built so that it is easy to add a shaft, and further as the depth of bath in the induction furnace can be fixed at any desired amount a shaft about 3 m. (10 ft.) high or over is per- missible, which is completely sufficient because of the small amount of reduction gases produced and their slow passage through the shaft. In principle, reduction with gaseous fuel is always preferable to solid fuel, for the latter only reduces the outer layers of the ore. Because of this the use of a gaseous reducing agent should shorten the time of operation and increase the efficiency of the furnace, for the reasons already given. At first it was feared that, with the use of a shaft, the heat would be immediately carried upward from the metal bath and the operation of the furnace thereby made more difficult. These fears, however, were shown to be groundless because preheating helped the furnace so that the same condition was obtained as before, but in a shorter time. It will be shown that the carrying away of heat from the hearth to the shaft only takes place slowly, and that in arc furnaces the heat must be artificially removed from the lower part of the furnace. THE ELECTRO-METALLURGY OF IRON AND STEEL 355 In addition to the economic advantages of the electric shaft furnace compared with the hearth furnace, the disadvantages should not be overlooked. They are: 1 . No very fine material can be smelted, but only pieces that are not too large, nor on the other hand ore smaller than a hazel- nut. 2. In smelting there is no removal of sulphur, therefore with ores, etc., rich in sulphur there must be added the necessary amount of fluxes to slag off the sulphur. 3. The slag must be tapped as a thin liquid, so that for this reason fluxes also must be added, which decreases the output from the charge. Therefore at present only high percentage ores can be used. 4. Only iron with considerable carbon can be produced, not the high carbon steels, and the subsequent refining of the iron is expensive. 5. The electrodes must be burdened only up to a certain amount per sq. cm. of section, so that with coke alone the voltage must be lowered, and with it the furnace efficiency. The first important experi- ments with an electric shaft furnace were carried out by Heroult. SMELTING TESTS IN THE SPECIAL HEROULT FURNACE These very extensive experi ments were carried out at the request of the Canadian Govern- ment in 1906 at Sault St. Marie, Ontario, in a furnace built by Heroult. As the accompanying illustration shows, Fig.. 123, the FIG. 123 35G ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY furnace differs very much from the Heroult steel furnace, and approaches the Girod in principle. The lower part is formed of carbon material stamped into place and constitutes one electrode, while the other, 1.8 m. (72 inches) long, reaches into the shaft from above and can be raised and lowered. The shaft is 1115 mm. fo'-io") high. It is slightly conical and 'is built of fire-brick. The current was delivered to the furnace at 50 volts pressure. Below are given details of these tests which are of the greatest interest because the electrode and furnace lining stood up for at least several days. Test No. 13. The raw materials had the following compo- sition: Wilbur Magnetite, SiO 2 = 6.20% Fe 2 3 = FeO = ^-56.69% A1 2 0, = 2.56% CaO = 2.00% MgO = 6.84% MnO = 0.258% PA = 0.023% P=O.OI% S 0.05% CO, = 3-609% 100.00% Charcoal, Moisture . = 14.00% Volatile matter = 27.56% Fixed carbon = 55.90% Ash = 2 . 54% S = 0.058% Sand, SiO z =81.71% Fe 2 Oj = 0.09% A1 2 O 3 =14-27% CaO = i. 60% MgO = 1.11% Alkali = 1.22% THE ELECTRO-METALLURGY OF IRON AND STEEL 357 The test lasted 61 hrs., 25 mins. The results were: 9573 23 kgs. ore smelted 2 973-75 " charcoal smelted 540.23 " sand smelted 5832. ' pig iron produced 462.67 " charcoal used per metric ton 1726 kw. hrs. used per metric ton The analyses of the pig iron and slag were: Pig iron: Si =0.04 to 3. 7% s = 0.012 " 0.075% p =0.017 " 0.029% Mo = 0.20 " 0.27% Gr. C = 3-53 " 3-7% Total C = 3.92 " 5-18% Slag: SiO 2 = 39-30% PA = traces MgO = 27.06% FeO = 1.21% A1 2 3 = 18.87% CaO = 15-55% MnO = 0.35% S = 0.32% In regard to the charcoal used the theoretical amount neces- sary was determined as follows: In i metric ton of ore there are: Fe as Fe 2 O 3 = 387-94 kgs. Fe " FeO = *79 21 " Slag-forming constituents = 176.00 From this we may calculate the theoretical carbon required: 387.94 kgs. Fe reduced from Fe2Os by C forming CO uses = 1 24 . 52 kg. C 1 79. 2 1 kgs. FeO reduced by C forming CO use.. = 38. 35 kg. C 162. 87 kg. C 567. 12 kgs. Fe as pig iron with 47% require 26.66 kg. C 189. 53 kg. C This equals ^~^= 334 ' 2 kg> per ^ ^ 358 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The actual amount of charcoal used was 462.67 kg. con- taining 462.67 X 0.559 = 258.64 kg. carbon. It follows from this that 334.2 258.6 = 75.6 kg. of the carbon necessary were either replaced by the volatile constituents of the charcoal, or else the CO produced reduced some of the ore in the shaft of the furnace. It is therefore clear that the thermal and chemical processes taking place in the shaft are of the same nature as those in the ordinary blast furnace, whereby the electric furnace is helped. The small power consumption is very remarkable for it only slightly exceeds the theoretical, when the melting of the slag is taken into consideration. From this it is certain that the heat is mostly used in the interior of the furnace and that because of the heat stagnation near the arc the brickwork will be strongly attacked. The test unfortunately had to be discontinued be- cause of the electrode not working properly. Test No. 14. (Time of test: 64 hrs., 30 mins.) The results were: 4943 . 2 kg. Blairton ore smelted 2936.95 kg. charcoal 338.23 kg. limestone 88.71 kg. sand 5386.71 kg. pig iron produced 1968 kw. hrs. used per metric ton 545 kg. charcoal Analyses. (a) The ore: Si0 2 =6.60% Fe 2 O, =60.74 \ rv_, c o c o/ FeO =i 7 .i8[ Fe - 55 ' 85% A1 2 3 = 1.48% CaO = 2.84% MgO = 5-50% Mn = 0.13% P 2 O 6 = 0.037% P=o.oi6% S = 0.57% C0 2 = 4-923% and loss on ignition. THE ELECTRO-METALLURGY OF IRON AND STEEL 359 (b) Limestone: SiO 2 = i-7i% Fe 2 8 A1 2 3 = 0.81% CaCOs =92. 85% CO = 51. 96% MgCO 3 = 4.40% MgO="2.og% P = 0.004% S = 0.052% (c) Pig iron produced: Si = 3-05 to 5. 15% S = 0.027 " 0.332 P = 0.024 " 0.037 Mn = 0.07 " o.i i Graph, car. = 2.72 " 3.46 Total car. = 3.54 " 4.16 (<0 Slag: Si0 2 = 33 to 37% A1 2 8 = 9 18% CaO = 18 ' 30% MgO = 21 ' 30% MnO = O.OI ' 0.05% FeO = 0.4 ' 0.9% S = 2 '3% Test No. 16. (Time of test: 38 hrs., 20 min.) The results were: 2175.6 kgs. Calabogie ore smelted 1611.7 " charcoal 587.9 " limestone 34.1 " quartz 3246.0 " pig iron produced 497.0 " charcoal used per metric ton 1970 kw. hrs. per metric ton Analyses. (a) Ore: Si0 2 = 6.06% FezO, =58.00 | FeO =24.78 f A1 2 0, = 1.00% CaO = 0.40% MgO = 6.00% PA = 0.046% P S = 0.17% CO, = 3-544% and loss on ignition. = 0.02% 360 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY (b) Charcoal: Moisture = 2 . 20% Volatile matter =20.60% Fixed carbon = 74 . 40% Ash = 2.80% (c) Lime. The same limestone was used as in Test No. 14. No analysis was made of the quartz. (d) Pig iron produced: Si = 1.22 to 2.03% s = O.OO6' 0.008% p = 0.047 0.093% Mn = 0.07 0.12% Graphitic C = 3-87 4-55% Total C = 4.40 5-o6% (e) Slag produced: Si0 2 =30-88% A1 2 3 = 9-67% PA = 0.014% CaO =36.14% MgO =20.82% MnO = 0.14% FeO ' = 0.73% S = 1.23% CRITICISM OF IRON ORE SMELTING IN THE HE'ROULT ELECTRIC SHAFT FURNACE The three tests given above show the following consumption of power for the production of one metric ton of pig iron: Kw. Hrs. Charcoal kg. 1,726 463 1,968 545 1,970 497 Average 1,888 501 This power consumption is good, exactly as in all the former tests, because the charge forms a good heat-insulator. Still, this concentration of heat has proved a disadvantage, for with the great drop in temperature between the arc and the walls of the furnace the limited amount of charge surrounding the arc is not enough to absorb it, and the result is a rapid destruction of the lining and uneconomical operation. The ascending reduction gases THE ELECTRO-METALLURGY OF IRON AND STEEL 301 cannot lead away the excess heat near the arc through the charge to the throat, so that the lower part of the furnace is necessarily quickly destroyed by the "stagnant heat." The intended prereduction of the ore is brought about, although only to a moderate degree, so that the carbon con- sumption is still high. The long electrode hanging in the furnace is shown to be a mistake because it is continuously exposed to mechanical wear, and is also chemically attacked by the sur- rounding ore mixture. Because of this delays in operation may be caused. From all this it follows that the problem of electric ore smelting is not to be solved by this Heroult type of furnace, because electrode consumption, delays in operation, and the lining costs exclude economy. The quality of the metal pro- duced, on the other hand, is good. Phosphorus and manganese are completely reduced, the slag can be kept low in iron, and the production of low sulpkur pig iron of any desired silicon is possible. As a reducing agent lump charcoal and also peat coke can be used. THE SMELTING OF ORE IN THE GRONWALL, LINDBLAD & STALHANE ELECTRIC SHAFT FURNACE Gronwall, Lindblad & Stalhane knew that the amount of reducing gases developed was not sufficient to carry the excess of heat present near the arcs from the lower part of the furnace to the shaft where it could be used economically for preheating the charge. They therefore increase the amount of gas by forcing into the lower part of the furnace, by means of a fan, part of the waste gases drawn from the throat. The amount is regulated so that the excessive heat, which would soon lead to the destruction of the lower part of the furnace, is driven into the shaft. Because of the continuous operating troubles experienced with the long Heroult electrode, Gronwall, Lindblad & Stalhane used three electrodes introduced at the sides of the lower furnace in their early tests. A general view of the furnace is shown in the accompanying 362 KLECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY illustration, Fig. 124. In principle it is similar to an ordinary small blast furnace, the electrodes taking the place of the tuyeres. The extensive smelting tests carried out with all kinds of ores allow a definite opinion to be formed as to the practical efficiency of this type of furnace, and show that there is no more difficulty in 'making iron with 4 per cent, carbon in the electric furnace than in the ordinary blast furnace.* Theor- etically, the direct production of the harder steels is also pos- sible, but experience has shown that such steels do not have the required temperature, and must be tapped while thickly fluid, which leads to troubles in operation. In regard to construction the furnace has shown that the expected advantages are obtained. First with reference to avoiding the stagnant heat in the lower part of the furnace which would lead to rapid wearing away. In the first test furnaces, which were built either with none or a very small shaft, the reducing gases escaped at 70 C, but with the new construction the gases at the throat have a temperature of 200 C. to 250 C. (see Fig. 129), and the radiation loss of the shaft is also equalized by these hot gases. From this it follows that the lower part of the furnace will stand up better during operation, but the efficiency of the furnace will not be so great, that is the power consumption necessary per ton of pig iron will be higher. Second, in regard to preheating and preliminary reduction of the ore, while smelting the ore in the electric furnace, having no shaft, only pure carbon-monoxide is produced, the waste gases in the electric shaft furnace give the following analysis: In 1909 In ign a Charge of Red Hematite Fe 2 O 3 Charge of Hematite CO 2 =45% CO 2 CO H CH 4 N O CO =40 27.2 57.5 14.8 o.o 0.5 o.o H 2 =15 b Charge of Magnetite, Fe 3 O 4 Charge of Magnetite, Mch. 16 &* jo C0 2 =30% C0 2 CO H CH 4 N 12.6 71.9 . 13.0 1.7 0.8 19.2 59.7 17.6 2.5 i.o * See American Electro-Chemical Society, p. 400, 1911. Robertson. THE ELECTRO-METALLURGY OF IRON AND STEEL 3f>3 According to the researches of Bauer and Glaessner, the reduction of iron ore by carbon-monoxide begins at about 650 C., and is most active at about 700 C. On the other hand, according to the tests made at Trolhattan in 1911, and given by Robertson, the reduction of magnetite m FIG. 124. by carbon-monoxide takes place at as low a temperature as 300 C. As the above table shows, this furnace gas contains about 72% of that gas, so that reduction of the charge by the gas rich in CO probably takes place throughout the whole of the lower half of the shaft, since the temperatures from the official report of the Jernkontoret on the working of the Trol- hattan furnace for the month of January, 1911, gives a tempera- 364 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY ture at the foot of the shaft (Point No. i, in Fig. 125) at 534 C., and at point No. 4 at 351 C. The readings at point No. 5 were discontinued, but from the other figures given it would appear that the temperature at this point is not below 300 C. As the gases in the electric shaft furnace leave the throat at 200 to 250 C., the chosen height of shaft of 5 metres (16' 4.8") FIG. 125. The Gronwall, Lindblad and Stalhane furnace, design of 1911. is more than sufficient. With the use of the Lange bell alone this height can be lowered, and with the use of the Parry cone at the same time, the effective height can be considerably decreased. The high hydrogen content of the gas comes partly from the hydrogen contained in the reducing agents, but partly from the moisture in the charge, which is decomposed. Hydrogen does not have a special reducing action in the presence of carbon- monoxide, which explains the given high hydrogen content. THE ELECTRO-METALLURGY OF IRON AND STEEL 365 This is shown by recent experiments in the production of pure hydrogen in large amounts which consist of strongly heating iron ore in a muffle furnace, and treating it with water-gas. The ore is reduced, yet almost the whole of the hydrogen passes from the furnace unoxidized, and is used for heating the furnace. The reduced iron is then employed to produce pure hydrogen, by passing steam over it. That an active prereduction takes place in the electric blast furnace is proved by the gas analyses, and the saving brought about in this way should be considered in calculating the amount of the reducing agents to be charged. This saving is based on the ratio of CO 2 to CO in the waste gases, which, for example, in the case of magnetite may be 40 C0 2 : 60 CO. The gas contains 100 carbon to 140 oxygen, the latter coming from the magnetite Fe 3 O 4 , the amount being ^- = 35. This 4 is to say that the reduction process is based on the formula 35 Fe 8 4 + TOO C. According to this 35 X 3 X 56 parts of iron and (iooXi2)+3 parts of carbon should be charged for the production of a pig iron with 3% carbon. If the amount of CO 2 in the gases falls below 30%, then there is an excess of raw material over the carbon present for reduction, because more ore enters the lower part of the furnace, and some additional material rich in carbon must be charged. On the other hand, if the charge contains too much carbon, then the lower part of the furnace becomes filled up with it, and some additional lower carbon material must be charged. In regard to the slag, a singulo-silicate is the best, with the formula SiO 2 2 CaO, and the proper amount of lime to produce this must be added to the charge. As with the ordinary blast furnace so also here it is not profitable to run too basic a slag, as the power consumption increases more than it should.* In this case the slag also very often contains calcium carbide formed by the influence of the arc. The power consumption per ton * The analyses of slag, according to Leffler, which follow show that these have generally been kept more silicious than desirable for the basic lining of the hearth. This, however, has been done for the purpose of obtaining 366 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY of pig iron naturally depends on the amount of slag that has to be melted, because it must be tapped in a fluid condition. Technical knowledge in 1909 was such that only high per- centage ores with 65 to 68% iron could be successfully smelted, which were as low as possible in sulphur, and which in no case gave more than 10% of slag. Further progress was made in the 1910 and 1911 tests such that ores running as low as 53.25% iron and containing .055 sul- phur (Nordmarken coarse washed ore) were successfully smelted. Ores with high sulphur should therefore be roasted before smelting in order to reduce the amount of lime necessary to be added to the charge. This roasting is comparatively easy with ores with an acid gangue, an average result with Swedish magne- tite showing: Before roasting 0.7 % sulphur After 24 hours 0.3 % ' 48 ' o.i % With a large electric shaft furnace plant the waste gases can be used for heating the roasting furnaces. Certain magnetites swell during roasting, do not break up, but change into red hematite. This change probably only makes somewhat lighter the consumption of reducing material and electric energy, for in the ordinary blast furnace 100 parts of magnetite need 100 parts of coke, while the same amount of red hematite takes 90 parts of coke. Fortunately, definite figures on this point have been obtained for electric furnace work, and are as follows : These are taken from the 1910 and 1911 Trolhattan tests. In those singled out for comparison ores of about the same iron content (65%) were chosen. The first test lasted 2096 con- results as closely comparable as possible with the treatment of the same ores by the ordinary blast furnace process. ANALYSES OF SLAG Si0 2 A1 2 O 3 Ti0 2 FeO MnO CaO MgO CaS Pj0 5 Total 41.60 6.85 2.72 1.49 I. 4 8 28.91 16.70 .063 .00 99.813 46.82 5.06 6.89 0.23 33-27 7-97 .023 .041 100.2 THE ELECTRO-METALLURGY OF IRON AND STEEL 367 secutive hours and used 1,760,884 kg. (about 1,760 tons) of natural magnetite ore. The charcoal used per ton of iron equalled 415.7 kg. (914 lb.), containing 70.5% C. The second test lasted 193 hours and used 223,626 kg. (about 223 tons) of magnetite ore of which about 87% was roasted. The charcoal used per ton of iron here equalled 376.3 kg. (829 lb.), containing 73.5% C. The slightly higher carbon in the charcoal content of the latter case is perhaps offset by only 87% of the ore, in this case having been roasted, thus making the comparison with raw and all roasted ore better, and about as it would be if in the one case all the ore had been roasted and the charcoal in each case con- tained the same carbon content. The reduced amount of char- coal used for the roasted ore is about the same as with ordinary blast furnace practise, viz. 10%. Ores with a basic gangue give great trouble in roasting, for the sulphur forms gypsum, and the intended reduction in sulphur is prevented, therefore such ores high in sulphur should not be used in the electric shaft furnace. Fairly rigid requirements are also necessary in the physical properties of the ore to be smelted. The most suitable size is that of a large walnut, and only a little pure ore should be present. Lump ores have, therefore, to be crushed and none can be used which give a considerable percentage of fines on crushing. This is sometimes a great disadvantage because of the brittle character of many magnetites, etc. The reducing agent also ought to be about the size of one's fist, as much as possible, and fine material can only be used with difficulty and in small amount. Formerly, i.e., in the earlier tests only charcoal could be used, or a mixture of coke and charcoal. Since then, however, a 3000 to 3500 HP furnace of the Gronwall, Lindblad & Stalhane type has been completed and is in operation at Har- danger, Norway, where English Durham coke, carrying about .6% sulphur, is being used. This is according to Richards, A.E.C., Society, 1911, p. 417, and from private advices from D. A. Lyon. In regard to the practical operation in 1909, small charges had to be used corresponding to the small size of the furnace, 368 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY that is to say, charges containing about 100 kg. (220 Ibs.) of ore. In the larger furnace of 1911 the average charge over a 6 months' period was 425 kg. of ore (937 Ibs). One-half of the ore should be thrown around the outside, and the rest with the reducing material and lime in the centre. If the charge were placed only in the centre, the 1909 furnace would easily hang, ordinarily due to the separation of carbon. Eighty charges containing 8 metric tons of ore were smelted in 24 hours, which with a 65% ore gives a total output of 5.3 to 5.4 metric tons of metal, obtained at intervals of 6 hours. Because of the small amount of slag, it was allowed to remain in the furnace, and was tapped together with the metal. The results confirm those already obtained with the Heroult furnace, namely, that the smelting process is the same as that of the ordinary blast furnace, so that from a corresponding ore any desired pig iron can be obtained by running a suitable slag, and regulating the furnace temperature. With a high tem- perature the iron contains more carbon, and if at the same time a basic slag is run the manganese of the ore is completely reduced, and a low sulphur iron is obtained because of the complete re- moval of the sulphur in the slag. The silicon content when running a basic slag and high temperature decreases, and under these conditions a part of the phosphorus can remain unreduced in the slag. On the other hand if the slag is acid the manganese is partly slagged off, and with high temperatures a high silicon iron is obtained. Just the same conditions obtain here, there- fore, as in the ordinary blast furnace. In operation it is always desirable to produce an iron as low in carbon as possible, which is the most favorable for foundry purposes, and also for subse- quent-refining into steel. In regard to power consumption, in the tests ending in 1909, 280,307 kilograms of iron were produced in 1903.5 hours, during 5.9% of which no work was done due to troubles with the ma- chinery. For each metric ton, 3181 kw. hrs. were used with an electrode consumption of 30 kg. or 66 Ib. = (0.015%), an d an electrode loss of 8kg. or 17.6 Ib. = (0.004%). The production from the ore was 63.5% and from the charge 60.02%. THE ELECTRO-METALLURGY OF IRON AND STEEL 369 The reducing agent weighed 354. kg. (779. lb.), and con- sisted of 41-7% coke and 58.3% charcoal, and a total of 35.41% was necessary, which corresponds to a consumption of 28% pure carbon. From this data the efficiency of the electric shaft FlG. 126. The Gronwall, Lindblad & Stalhane furnace. Note lower position of electrode clamp. Latest design furnace can be calculated. The pig iron may be taken as con- taining i% silicon and 3% carbon, which leaves 96% iron, the ore as magnetite, and the waste gases as containing 30% CO 2 and 70% CO. 370 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY ioo kg. of the waste gases contain 30 + 70 = 100 kg. carbon, and (2 X 30) + 70 = 130 kg. oxygen. In the reduction of magnetite 130/4 kg. Fe 3 4 must be present to supply the oxygen, FIG. 127. The Gronwall, Lindblad & Stalhane furnace. Gas circulation of 1911. and 130/2 = 65 parts of silica to supply the silica. Reduction takes place according to the following formulas : 1. kg. Fe 3 O 4 -I- ioo kg. C = 30 CO 2 + 70 CO 4 or 13 kg. Fe 3 O 4 + 40 kg. C =12 CO 2 + 28 CO 2. 65 kg. Si0 2 + ico kg. C = 30% CO 2 + 70% C. This gives (12 X 12) -(- (28 X 12) = 480 kg. carbon, which ac- cording to the analysis of the gas gives (i2X44) + (28X28) = 1312 X ioo 1312 kg. gas, that is to say, i kg. carbon gives 4 kg. gas. (i Ib. carbon gives 2.73 Ib. gas.) THE ELECTRO-METALLURGY OF IRON AND STEEL 371 13 kg. of magnetite require 40 kg. carbon for reduction, so that for the smelting of 13 X 3 X 56 = 2184 kg. iron 40 X 12 = 480 kg. carbon are necessary, or for 960 kg. iron 9 X 4oQ _ 210.99 kg. 2l8 4 65 X 28 X 364 kg. silicon reduced from silica require 100 X 12 = 240 kg. carbon or 10 kg. silicon require 6.59 kg. carbon. For carburizing the iron, 30 additional kg. of carbon are necessary, so that the total requirement of carbon necessary for the production of i metric ton of pig iron amounts to 210.99 + 6 -59 + 3- = 2 47-5 8 k g- (545- 8 lb -)- From this there is formed (210.99 + 6.59) X 41.15 = 594.72 kg. waste gases (1311.1 lb.). With an output from the charge of 60%, 960 kg. iron require . 960 X 100 . 960 X 100 a charge of = 1600 kg., with -- -- = 1325.71 DO 72 X 4^ kg. FesO^ This will produce 1600 1325.71 = 274.29 kg. slag from which -- ^-- = 21.43 kg. silica are reduced and enter the iron, leaving 274.29 21.43 = 252.86 kg. (557.4 lb.). Heat requirements. The combustion of i kg. carbon pro- ducing the waste gas analysis given above creates (0.3 X 8080) 4 (0.7 X 2470) = 4153 cals. Reduction of 960 kg. iron from Fe 3 O 4 = 1648 = ........................... 1,582,080 cals. Reduction of 10 kg. Si from SiOg 10X7829= 78,290 Smelting and overheating of 1000 kgs. pig iron = 1000X280= ..................... 280,000 Smelting and overheating of 252.86 kgs, slag 252.86X595= ........................ 150,452 ' Heating of 594.72 kgs. CO 2 and CO to 200 C. 594.72X200X0.245 = .................. 29,145 2,119,967 cals. Heat Supplied: Combustion of 217.53 kgs. carbon = 217. 53 X 4153 = 903,610 cals. Leaving to be supplied by the electric current 1 ,2 1 6,357 Total 2,119,967 cals. 372 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The theoretical amount of power necessary for 1000 kg. of pig 1.216,^7 iron is ' - = 1408 kw. hrs. 864.5 As 3181 kw. hrs. are required daily per metric ton, the efficiency is g = 44%. This low efficiency obtained in the 1909 tests is for a small test furnace run with constant supervision. The remaining 56% is lost by radiation and cooling. In this respect measurements have shown that the water cooling of the three electrodes carried away about 1 20 kw. hrs. per hour. This gives a total loss of 1903.5 X 120 for the entire smelting test, which equals 228,420 kw. hrs. or 22 g' = 815.7 kw. hrs. per metric ton, which corresponds to ^ = 25.6% of the electric energy supplied. Through radiation alone 56 1 25.6 = 30.4% of the energy is lost. It should now be considered whether and by how much the efficiency can be increased with a larger plant. Water cooling will still have to be used, and in this respect the efficiency can scarcely be increased. Apart from this the high water con- sumption, amounting to about % gallon per second (2 liters) is a disagreeable addition. On the other hand the radiation loss would be smaller because the cubic contents increase faster than the radiating surface of the furnace. Most important, however, is the fact that the smelt- ing time per ton of iron will be lowered, and therefore the radia- tion per ton of metal will be considerably smaller with the increase in smelting efficiency. Graphite electrodes will increase the smelting efficiency for they are better conductors than those of carbon, and although FIG. 128. Modified gas circu- lation of 1912. Gronwall, Lind- blad & Stalhane furnace. THE ELECTRO-METALLURCA' OF IRON AND STEEL 373 X they have a higher thermal loss (Chapter VI, Part I), yet this is more than equalized by the increased efficiency. At the furnace at Falun carbon electrodes were used, for there is no plant in Sweden making graphite electrodes. This dependence on electrode plants is necessarily very disadvantageous for all countries not having them. Ex- periments should be made to in- crease the life of the electrodes by mechanical means as much as pos- sible, or the electrode consumption is proportionally high. It will not be much lower with a large furnace, as the electrodes are attacked be- cause of their contact with the ore. Finally the consumption of reducing material is very much higher than it should be theoreti- cally, which of course is also not desirable. Below are given some details of recent test runs in larger furnaces, and it may be seen how these theo- retical considerations have worked out in practise. With regard to the run from Nov. 15, 1910, to April 9, 1911, in the newer Gronwall, Lindblad & Stalhane or Swedish Ludvika Elek- trometal type furnace, 1882.496 kg. (about 1882 tons) of iron were produced in 3501.9 hours, during about 4.4% or 153.7 hours of which no work was done due to troubles with the apparatus. For each metric ton, 2391 kw. hrs. were used with an electrode consumption of 10.28 FIG. 129. Temperatures in the Gronwall, Lindblad & Stalhane electric pig-iron fur- nace. 374 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY kg. (22.6 Ibs.) gross, and 5.27 kg. (n.6 Ibs.) net, per ton of iron produced. The per cent, of iron in the ore was 61.54% and the per cent, iron in the charge 57.00%. The reducing agent weighed 418 kg. (920 Ibs.) per ton of iron produced and consisted of charcoal only, having a carbon content of 80.14%. The pig iron may again be taken as containing i% silicon and 3% carbon, which leaves 96% iron. The average of the gases produced consisted of 23% CO 2 , 60% CO, 10% H, 2% CH 4 , and 5% N. From this data the efficiency of the electric pig-iron furnace may again be calculated as before, and in this case the efficiency is considerably higher, being about 59%. After the furnace at Trollhattan was shut down from June to September, 1911, in order to make such changes as the operation of the furnace had demonstrated would be beneficial and such repairs as were necessary, the furnace was again put into com- mission. During the run from Sept. 3 to Sept. 30, 537.9 tons of pig iron were produced. For each metric ton of pig iron 1 749 kw. hrs. were used with an electrode consumption of less than 10 kg. (22 Ibs.) gross and 5 kg. (n Ibs.) net. The iron in the ore was 67.65% and the iron in the ore and lime was 65.02. The reducing agent weighed only 339.9 kg. now (748 Ibs.), consisting of charcoal. With the same carbon content as before, 72%, this equals 245 kg. or 24.5% pure carbon. From this data the efficiency of the furnace can again be calculated and figured out to 80.5%. This corresponds to an output of over 5 tons of pig iron per kilowatt a year. The above efficiency corresponds favorably with the Swedish charcoal blast furnace of 82% and with 70% the usual coke blast furnace. As a conclusion it may be said that the Gronwall, Lindblad & Stalhane electric shaft furnace is probably the first electric furnace in which ore has been smelted in some degree commer- cially. The weak point has been the furnace roof, which, in the 1909 furnace, either showed such small durability and therefore made continuous operation impossible, or else had to be cooled so strongly that the efficiency of the furnace suffered consider- ably. Further the close limits allowable in the chemical and physical composition of the charge, and the large electrode THE ELECTRO-METALLURGY OF IRON AND STEEL 375 consumption show that the furnace can be employed only under especially favorable operating conditions. The principle is first- rate, especially with regard to making the roof of the lower part of the furnace more durable, as far as possible without the use of water cooling, and so increasing the furnace efficiency. In this place the Lyon experiments conducted at Heroult, California, with the Noble furnace, should be mentioned. The following details are taken from a paper by Otto Frick, in Metallurgical and Chemical Engineering, December, 1911, on "The Electric Reduction of Iron Ores." The Noble furnace is of the same type as that at Trollhattan, and like it in all essential points. This, however, is not the result of mutual understanding or communication. An illustra- tion is given of the furnace at Heroult by Fig. 130. This furnace has now (191 2) 1 been rebuilt seven times. It has three single-phase transformers, each of 750 kilo volt amperes, con- nected to a three-phase system of 2200 volts and 60 cycles. The low tension current is supplied to six graphite electrodes. These electrodes enter into the charge as far as possible, and in this respect the practise differs from that at Trollhattan, where a space is left between the electrodes and the charge. The pressure of the charge on the electrodes is very nearly equal to their breaking strength, so that the additional force arising from a sudden descent of the charge easily causes their breaking at the conical screw joint. This strain can be reduced approxi- mately 30% by lowering the inclination from 35 to 20, and much improvement can be made in the joint. No accurate figures are at hand as to the power consumption, but it has been stated by the manager of the plant that it has averaged 1940 kw. hrs. per ton. With regard to gas circulation it has been found unnecessary to use any in the Noble furnace, where the electrodes penetrate the charge far enough to prevent arcing, so long as they remain unbroken. The question of the smelting of ore in the electric shaft furnace can only be considered solved when the following requirements are met: 1 By 1917 the furnace had been again entirely rebuilt. 376 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY 1. Water cooling lowered as much as possible. 2. The electrode consumption lowered, and the electrodes done away with as much as possible. 3. The radiation loss lowered by the smelting efficiency being raised as much as possible. 4. The waste gases composed of only pure carbon-dioxide, FlG. 130. The Lyon furnace in California. of suitable temperature, in order to lower the amounts of electri- city and reducing material necessary. Requirements i and 2 are very closely connected. So long as electrodes are used, cooling of the electrode heads cannot be THE ELECTRO-METALLURGY OF IRON AND STEEL 377 avoided, which brings about great heat loss, and necessitates a complicated furnace construction. Further great durability of the furnace lining is only possible if the high initial temperature of the arc is avoided, and the most suitable moderate tempera- ture used. This requirement is only met by the induction furnace, for on the one hand electrodes are not used at all; and, on the other hand, as the experiments with the hearth induction furnace have shown the furnace lining is hardly attacked at all when smelting ore. An induction furnace with a wide hearth and a shaft built above is the one to claim the greatest interest for smelting ore, and so much the more that it can be operated at the highest temperatures if required. Any height of shaft can be chosen, so that the waste gases can be efficiently used for prereduction and preheating of the charge. The radiation loss decreases with a larger furnace for the induction as for other furnaces. In regard to the requirement that only pure carbon-dioxide, at a suitable temperature, should be given off, as waste gas, it is well known that carbon-monoxide loses the ability to reduce ore when a certain percentage of carbon-dioxide has been formed. It is therefore theoretically impossible to have a product of pure carbon-dioxide when charging ore and fuel. The complete utilization of the waste gases is therefore only possible if they are burned afterwards, and used as much as possible for preheating the ore. This preheating favors smelting only in that reduction is made more easy by an increase in the degree of oxidation, and also because the sulphur contents are lowered so that a low sul- phur iron can be obtained without the addition of more flux to the charge. With finely divided ores the roasting also brings about a certain amount of agglomeration so that under these con- ditions fine ores, concentrates, etc., can be smelted in the electric shaft furnace. The heating and agglomeration of fine ores, if sufficient fluxing material is present, can be carried out in a revolving cylindrical furnace, the ore being charged wet just as it comes from magnetic separation for instance. Such agglomerat- ing plants are already in satisfactory operation. 378 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY The use of coal dust firing which is recommended for the heating of these furnaces is unsuitable, as it gives rise to high fuel costs, and the ash of the coal makes the ore higher in slag- producing material so that it is more unsuitable for electric fur- nace work. It is, therefore, necessary to operate with waste gases, and an addition of producer gas should only be made when the difference in price between fine and lump ore is sufficiently great to bear the increased cost in fuel needed for the production of the producer gas. Such an ore, however, greatly preheated, cannot be charged directly with the reducing material, as it is immediately reduced, forming carbon-monoxide, and so reduces the furnace efficiency. The greatly heated ore must be charged alone, and the reducing material introduced in the hearth of the furnace at the deepest zone of the shaft. The physical condition of this re- ducing material is not important, if solid, the most suitable size is fine grained. Very small fuels and even valueless waste can be used with complete success. Also fluid- reducing materials such as tar, petroleum, and oil residues of all kinds can be used. This is of special interest to those countries which at present must import coke or charcoal, because these liquid fuels due to their high heating value and low ash contents are brought in at much more favorable freight rates. Finally gaseous reducing agents of all kinds can be used, such as producer gas. The carbon-dioxide should be as low as possible, and if fuels with much moisture are used, such as turf, brown coal, etc., the gas should be cooled as thoroughly as possible to remove the moisture The troublesome precipitation of tar experienced in the cooling of producer gas is no disadvantage to the electric furnace, as opposed to other furnaces, for the tar can be collected, dried in centrifugal machines and used in the furnace as a reducing agent. The carbon-monoxide or the solid liquid or gaseous materials used easily reduce the highly heated charge, and give warm waste gases rich in carbon-monoxide, which can serve to preheat more ore charges if air is added to combine with the combustible constituents. THE ELECTRO-METALLURGY OF IRON AND STEEL 379 In this way it is possible to considerably reduce the consump- tion of reducing material, and to come very near the theoretical minimum; which, in the case of magnetite and the production of a pig iron with 3% carbon, is 143 + 30 = 173 kg. of carbon (381.4 Ibs.) per metric ton. The best figures reached so far as already mentioned are 245 kg. of pure carbon when making a pig iron with 3.64% C. After nearly a year of further experience (215 days) in operat- ing the furnace at Trollhattan, Leffler and Nystrom contributed a supplementary report of 98 pages, to the meeting of the Jern- kontoret at Stockholm, on May 31, 1912. It is not possible to do this report justice here by any abstract of it, still it may be instructive to mention some of the improvements recently made. The new gas circulation system was altered to better dry the gas returned to the furnace. Fig. 128 shows the latest design and is but little different from its predecessor shown by Fig. 127. The cooler acts on the condenser system and requires 100 liters of water per minute to reduce the temperature of the gas so that its moisture content is reduced from 4 grams per cubic meter to .5 gram. Both high and low grade ores were used in this run, so that the furnace output dropped to about 15 tons daily from its normal capacity of 20 tons. This run again demonstrated that economical operations need a rich ore. Fig. 129, which is reproduced from the July, 1912, Metallurgi- cal and Chemical Engineering, shows the temperature and reaction in the furnace shaft. This abstract goes on to say: The temperatures of iron and slag issuing from the furnace varied as follows: Iron 1230 to 1420 C. Slag 1290 to 1460 C. A large table gives the temperatures taken at 8 points in the shaft, just inside the wall and in the middle; also the percent- ages of CO 2 in the gases at these different points. There are various not very important irregularities in the figures, but the general average shows temperatures up to 985 n the middle at the lower part of the shaft and 585 half way 380 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY up; while near the wall it is 420 to 565 at the lowest hole and down to 15 at the highest. The measured percentage of C0 2 shows that reduction takes place ordinarily only one-quarter way up the shaft at the sides and a little over one-half way up in the centre. The extent of the zone of reduction by CO is clearly shown in Fig. 129, in which also some temperatures are indicated. Cooling Water. The contacts and jackets through which the electrodes worked were water cooled. The heat carried away thus varied from 172 to 288 kw., or 10.47 to I 9-3 (average 14.50) per cent, of the power used. Thermal Balance. The heat balance per 1000 kg. of pig iron is worked out for the four weeks, Sept. 3 to Oct. i, 1911, in which the average power used (high tension side) was 1407 kw., and the power consumption 1749 kw. hours per ton of pig iron; the ores worked were the rich Tuolluvaara ores. The heat balance is, per kg. of iron: Combustion C to CC>2 567 calories Combustion C to CO 381 Electric energy 1 504 2452 calories Consumed in reductions 1620 calories Decomposition of limestone 35 Evaporation of water 24 Sensible heat in throat gases 26 Sensible heat in slag 75 Sensible heat in pig iron 300 Cooling water 195 Lost in transformers 43 Lost in conductors 44 Radiation and conduction 90 2452 calories The authors then make some interesting calculations, the results of which are, in brief, as follows: The gas kept in cir- culation was 2.28 times the gas normally produced and escaping. Assuming this gas to enter the furnace at 22 and to enter the shaft at 1000, it carried into the shaft as sensible heat 343,118 THE ELECTRO-METALLURGY OF IRON AND STEEL 381 calories per ton of iron, or 22.9 per cent, of all the heat electrically generated in the crucible. Since it carried with it 22.5 kg. of water vapor and 174 kg. of CO 2 , both of which are decomposed by the glowing carbon, the net heat absorbed in these decom- positions is 160,283 calories, or 10.7 per cent, of the electric energy used. The gas circulation therefore transferred physically and chemically 33.6 per cent. = 1/3 of the electrical energy used from the crucible into the shaft of the furnace. Some later operating data are published by Orten-Boving, in the Canadian Engineer, of May, 1914, of these furnaces when making electric pig iron of three qualities as follows : Si % Mn % P-% S-% For open hearth treat- ment For Lancashire treat- .40- .60 2O 30 .30- .50 2O 30 .oii-.oi8 01 i 018 .015 .015 .020 For Bessemer treatment I . OO-I . 40 2.50-3.00 .015-. 019 .005 Experience has shown that a much more constant product is obtained from the electric than from the old blast furnaces, one of the reasons for this being the large receiver in the lower part of the furnace which acts as a regulator of the quality. The reason for the high Si and Mn in the Bessemer pig is that the temperature of the electro-Bessemer pig is lower than the ordinary Bessemer pig from blast furnaces. The experience obtained points to the following results: It is cheaper to make spiegel than gray pig because: i. More current can be put through the furnace. 2. The current consumption is lower. 3. Thus the production is higher. 4. The electrode consump- tion is lower. 5. The repair costs are lower. The quality of the pig is not influenced by the percentage of iron contents of the ore. The electrode consumption has been comparatively high, being influenced by the following: i. High power consumption. 2. Too lively gas circulation and too large percentage of CO 2 in the gas. (The carbon con- 382 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY sumption is correspondingly lower.) 3. Too large carbon electrodes for the load. Recently the gas from the furnaces has been used as fuel under the open hearths, and this is valued at 50 to 75 cents per ton of electric pig produced. The influence of electric pig in finished steel shows that the change tends to make better steel. Some of the larger furnaces are designed for 8,000 h.p. It was found that when using coke instead of charcoal it is better to operate on burnt lime, to keep the power consumption down, the latter rising when too much CO 2 is produced. Eighteen of these furnaces are now in operation or building, all in Scandinavia. It has been found that the operation of the electric reduction furnace is much simpler than that of a blast furnace. Less labor is required and no' more skill is necessary than with a blast furnace. The total initial cost per ton of output is also lower. For a plant of three furnaces of 3,000 h.p. capacity each, the following staff and labor would be required : One chief engineer, one assistant, two chemists, three foremen, two electricians, ten men in each of three shifts. Below is given the results of a fur- ther continuous run of one furnace belonging to Stromsnas Jern- werks A. B. from October i, 1912, to September i, 1913, 3,000 h.p. Number of charges 26,549 Weight of ore used, tons . (metric) 1 1,338 Weight of limestone, tons 907 Weight of charcoal, tons 2,700 Produced pig iron, tons 7,258.3 Weight of charcoal used per ton of pig iron, Ib 830 Total number of hours when running normal, hr. . . . 7,957 Total power consumed, kw.-hr 1 5,291 Total power consumed per ton iron, kw.-hr 2,107 Weight of pig iron produced per I h.p. yr. tons 3 .05 Weight of pig iron produced per I kw.-yr., tons 4 .14 Total consumption of electrodes, tons 28.42 Consumption of electrodes per ton of pig iron pro- duced, Ib ... 8.7 Details of part of this run are as follows: Continuous Run of the Trolhattan Furnace for Three-Month Periods from October i, 1912, to June 30, 1913. (This furnace THE ELECTRO-METALLURGY OF IRON AND STEEL 383 was run by the Swedish Association of Iron Masters with a view to establishing the practical success of the system as well as to give the various members an opportunity of trying their various kinds of ore. . Thus, in the table below different kinds of ore were used during the period indicated.) The ore from Kiruna and Tuollavara is of the highest quality obtainable in Sweden. It will be seen that the output of the furnace as well as the consumption of electrodes depends largely on the quality of the ore used. Period Oct. i, 1912 to Dec. 31, 1912 Jan. i, 1913 to March 31, 1913 April I, 1913 to June 30, 1913 Number of charges 6,193 7,107 7,28l Kiruna A ore ' .tons 1,047 223.3 799.8 Tuollavara ore tons 973-4 123-3 762.6 Klacka-Lerberg ore tons 885.6 1,453 1,426.5 Persberg ore tons 8.82 47-97 148.4 Total ore tons 2,914.8 3,047-6 3,137-4 Limestone tons 169.94 252.8 273-5 Charcoal . .tons 699 719 738 Pig iron produced tons 1,905.86 1,933-32 2,000.14 Charcoal per ton pig iron . . . . Ib. 825 835 830 Actual working time hr 2,158-5 2,113-7 2,147 Consumed power, kw.-hr. .. units 3,957,565 4,095,588 4,216,544 Consumed power per ton . . units 2,076 2,118 2,108 Produced pig iron per kw.- year tons 4.22 4.14 4-15 Produced pig iron per h.p.- year tons 3.10 3-04 3.05 Consumption of electrodes, total tons 5-307 8.670 7.896 Consumption of electrodes per ton Ib (2.78kg.)6.2 (4.5kg.) 10. o (4 kg.) 8.8 Each of the above furnaces now has six round electrodes. The Noble Electric Steel Company at Heroult, California, through their manager, Crawford, 1 publish results of their tests and operation, while still making foundry pig iron. During 1916-17 they changed to making 80% ferro-manganese which i Metallurgical and Chemical Engineering, page 383, July I9I3- 384 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY they are doing with 4,700 kw.-hr. to the net ton of 2,000 lb., or 5,170 kw.-hr . per metric ton. The plant consists of one 2,ooo-kw. and one 3,ooo-kw. iron furnace, substantially as per Fig. 131, and the usual auxiliary apparatus. To make this installation an economic success caused many misgivings. "With approximately the same costs for power, charcoal, and stock, local conditions caused them to abandon the type of furnace operating commercially in Sweden. "If a certain type of furnace is found to be best suited to make a grade of iron for which the demand is limited, either a market must be found for this product, and that market edu- cated to accept it, or else the design of the furnace, or method of operation must be altered so as to make a grade of 'iron for which there is a market already established. In Sweden, con- ditions permitted them to adopt the former and simpler course, while at Heroult they had to resort to the latter. "The principal users of pig iron on the Pacific slope and far Western States are custom foundries. Specialty foundries, such as those making stoves, bath tubs, pipe, etc., are still relatively few, as are also open~hearth steel furnaces, so that to operate electric furnaces successfully on the Pacific Coast, grades of iron must be produced which meet the demands of general foundry purposes. "There is on the coast an abundance of scrap cast-iron, and foundries making what are spoken of in the East as heavy castings are few in number; hence the popular demand is for a soft high-silicon iron which is a good scrap carrier and can be easily machined when in light castings. "Thus it is apparent their problem became one not merely of making pig iron successfully but of making iron with a silicon content of from 2 to 3 per cent, economically. When a blast furnace works "off " it ordinarily means only a slight concession in price to get rid of the low-grade iron, while with them if iron runs under i per cent, silicon it means a large concession in price to dispose of it. As electric furnaces like blast furnaces have the faculty of misbehaving at times, conditions imposed on them the necessity of having a furnace which would respond THE ELECTRO-METALLURGY OF IRON AND STEEL 385 readily to alterations in the furnace burden and still be of large enough capacity to be efficient. "The following analysis represents the magnetite ore used: Si0 2 3.43% A1 2 O 3 0.81% CaO 0.70% Mg 0.32% Fe 67.86% Mn 0.28% P 0116% CuO Trace S 021% Fe 3 4 79.63% FeA. 14-56% 99-73% "This magnetite lies between quartz and limestone, the analysis of the latter is as follows: A1 2 O 3 ............................................ 0.61% MgO ............................................ 1.12% CaO ............................................ 53.80% FeO ............................................. 20% C0 2 (by diff.) .................................... 43-25% 100.00% "The first furnace closely resembled the Swedish, although independently designed, and was operated intermittently. "But from the style of its construction, it is apparent that it could not be made to respond readily to changes in the burden, and in order to make consistently high-grade foundry iron this is an essential. In a blast furnace, too great an excess of coke in the burden can be taken care of by increasing the quantity of air blown in, but in an electric furnace this is, of course, not feasible, as the oxygen would attack the electrodes. The excess of carbon must be taken care of by an excess of oxygen put into the furnace through increasing the ore in the burden. "The necessary excess one might think could be calculated to a nicety, but because of unknown factors practically the excess must be added gradually until the controls on the slag from the iron and the general working of the furnace show the desired 38G ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY result to have been accomplished. This sometimes takes several days. "If the excess of carbon has been allowed to proceed too long, the furnace will, of course, 'freeze up.' The slag will give up part of its lime content to form calcium carbide, part of its alumina to form aluminium carbide, part of its silica to form silicon carbides, and part to form ferro- silicon, and part of the carbon remaining turns to beautiful sparkling flakes of graphite. Remarkable examples of molecular replacement of the carbon in the charcoal by silicon carbide has also been noted, making beautiful specimens of petrified charcoal. Once a furnace presented these phenomena, though there is no excuse for it, as the decrease in the amount of stock going into the furnace and the daily controls on the slag and metal should give ample warning. The matter of too little carbon gives less trouble, and, if the furnace is producing low silicon and carbon, iron should give none at all. "The question may be fairly put: Why cannot the necessary carbon in the burden be calculated within sufficiently close limits to prohibit any possibility of trouble? Theoretically, of course, it can be. By daily analyses of the furnace gases taken at regular distances as they ascend from the crucible up the stack the ratio in which the carbon is actually being oxidized to CO and CO 2 can be approximately determined, and this, together with the controls on the slag and metal, will keep the carbon within safe limits if the furnace is running with a low carbon burden; that is, making low-silicon iron "When, however, the furnace is running on a high carbon burden, calculated to make a 3 per cent, silicon iron, it is ap- parent that the carbon must be carefully regulated. Insuffi- cient carbon not only lowers the grade of the iron, but intro- duces difficulty by throwing an excess of SiO2 into the slag. Too much carbon lowers the efficiency of the furnace besides leading to the difficulties mentioned above. Practically, it is always necessary to carry an excess of carbon over theoretical calculations to take care of the atmos- pheric oxygen and moisture which is occluded in the charcoal THE ELECTRO-METALLURGY OF IRON AND STEEL 387 and the atmospheric air which is inevitably drawn into the furnace after it has been in use in spite of every practical pre- caution to prevent it. It is evident, of course, that these are to a considerable extent unknown factors; hence the charcoal must be continually varied, as- the control analysis indicates that it is too high or too low. "So, while the shaft type of furnace gave promise of economic success, it operated on the low-silicon, low-carbon, white iron (which our Swedish friends have dignified by the name of pig steel), -it was shut down for making foundry iron." The present long and narrow type furnace was designed by Frickey and others. It is 5.04 meters (16' 6") long, 2.36 i FIG. 131. Sections of No. 7 Furnace. meters (7' 9") wide, and 3.10 meters (10' 2") high. It has four electrodes delta connected and five charging shafts 5.5 meters (18') high, as shown by Fig. 132. This type of furnace seems best adapted to this class of service. The 3 ,ooo-kw. furnace is 8.54 meters (28') long and 3.05 meters (10') wide. The stacks are widened toward the bottom, as in Fig. 131, to prevent the burden from hanging. Between the stacks the top of the furnace is arched, and through the center of these arches the electrodes penetrate vertically into the charge. Acheson graph- ite electrodes, 305 millimeters (12") diameter, are used, using the tapered M. & F. joint. 388 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY So far, no arrangements have been made here to utilize gases, for the best way to utilize these had to be studied, rather as an economic than a metallurgical problem; i.e., whether the saving in charcoal effected by circulating the gases back through the furnace and taking advantage of the reducing action of the CO is greater than the saving in fuel by burning them under lime kilns, charcoal retorts, or elsewhere. Observations led to the adoption of the latter course. Considering the claims made for the cooling effect of the FIG. 132. circulated gases on the furnace roof, this is hardly deemed worthy of practical consideration on this resistance type furnace. If the charge is descending regularly it will protect the roof and if it is not, the slight lowering of temperature, caused by the cool gases, will not prevent the heat radiated from the electrode from melting the roof. Three 750 KVA. transformers furnish three phase delta current between 40 and 80 volts, using 15 steps. The electrode holders are water-cooled cylindrical stuffing boxes made of 98% copper. The electrodes are suspended in these from above and the annular space between the electrode and the holder is packed with a specially prepared graphite THE ELECTRO-METALLURGY OF IRON AND STEEL 389 capable of being compressed to a density equal to that of the electrode itself. This packing material offers no more resistance to the passage of the current than the electrode, and because of its unctuous nature permits the electrode to be raised and lowered without breaking electrical contact. From an electrical standpoint this type of furnace has worked very smoothly. The instruments show but little varia- tion, when things are normal, except that the power factors (which average respectively 90, 85 and 70 per cent.) improve after the furnace is tapped and gradually fall off again as the molten iron accumulates at the bottom. METALLURGY "The stock is charged into the five charging stacks previously mentioned on the basis of 5oo-lb. units of iron ore. This small charging unit, while it entails extra labor on the feed floor, has the advantage of mixing the ore, charcoal, and flux as inti- mately as if the charge were bedded, and homogeneity of charge is very essential. "With this type of electric furnace at least the ore is reduced to a very much greater extent by actual contact with the carbon than by the action of the CO in the stack gases. Some examples of gas analyses will bear this out." Gas Analyses CO 2 O CO CH 4 H 8.6 0.2 57.2 16.0 0.8 9.2 1.4 56.2 12.2 1.2 7-8 0.15 57.1 6.0 0.20 67.9 8.1 o.io 64.8 4.2 0.25 60.7 5.0 o . 40 66 . 6 From these analyses it will be noted that the ratio of CO to CO 2 is very high, but by utilizing the calorific value of the gas, the high charcoal consumption will be balanced to a very ap- preciable extent. 390 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Experiments have been made to some extent with the pos- sibility of utilizing coke or gas carbon instead of charcoal. However, the 72-hour metallurgical coke, as tried, offers two objections: First, its electrical conductivity is so good that much of the current passes between the electrodes in the upper part of the furnace. The smelting zone is thereby raised and the furnace runs hot on top with attendant melting of the arches and cold at the bottom. Second, this coke, because of its density and high crush- ing strain does not break down like charcoal as the burden descends; hence, less surfaces of carbon are exposed to be oxidized by the ore and there is a less intimate mixture of the two. Reduction of the ore takes place more slowly, the silicon in the iron is lowered, the power consumption per ton increases, and thus the efficiency of the furnace is re- duced. However, by adopting certain precautions in crushing the stock and feeding same into the furnace operated on a mixture of 60% coke and 40% charcoal with a very fair degree of furnace efficiency and the grade of the iron was kept up to No. 2 foundry. The possibilities of operating electric iron furnaces with coke instead of charcoal seem to offer a very interesting and necessary field for investigation. At present successful opera- tion of electric iron furnaces depends among other things on an abundant and fairly cheap supply of charcoal. This gen- erally limits their field of activity to well-timbered regions which are usually isolated and where freight rates are high. Many of our coals which make a very poor metallurgical coke for blast furnace use on account of their low crushing strain might be found to make a satisfactory fuel for electric furnace use. The fact that the ore is of so high a grade renders the metal- lurgical problem somewhat different from that usually en- countered. The ore as it comes from the quarry will often run for weeks at a time as low as 2^2% SiO2, and from 67 to 68% Fe; so that to make pig iron which will run from 2 to 3% Si, THE ELECTRO-METALLURGY OF IRON AND STEEL 391 it is necessary to augment the silica in the ore by the addition of barren quartz. The requisite amount of lime or limestone is added so as to give theoretically a slag running about 47% SiO 2 . The following slag analysis will indicate the extent of the silica variation: SiO 2 A1 2 O 3 FeO CaO MgO 54.00 29.70 1.30 12.98 1.17 50.20 28.60 2.40 16.03 2.36 46.13 27.20 0.65 23.10 3.31 Ordinarily slags containing more than 50% Si0 2 are too viscous to run well, but the slags are fluid up to 54% SiO 2 . This may be due to the high alumina ratio. As only from 125 to 140 Ibs. of slag per ton of pig is made it is found more eco- nomical to permit a small percentage of the iron to escape in the slag than to attempt to reduce it. The depth of the green color in the slag also serves as a rough indication for the furnace-men as to whether the carbon ratio of the burden is becoming too high or too low. In calculating the charcoal for the burden it is assumed that all the carbon burns to CO, as it is necessary anyway (for reasons previously given) to carry an excess of charcoal in order to make high silicon iron. Thus, to make 2.75 silicon iron the theoretical quantity of charcoal (containing 85% fixed carbon) necessary is 35% of the pig, whereas there is actually used about 40%. Inasmuch as any necessary change in the burden is distributed over five stacks the furnace responds very rapidly and practically permits of the silicon in the iron to be controlled within a limit of 0.5%. The iron is tapped three times a day into sand pig-beds. A sample is taken from each bed from a pig 250 millimeters (10 inches) long cast for the purpose. The system of grading is somewhat like that used in grading the Southern iron, ex- cept that it is graded entirely by silicon content as the sulphur and phosphorus run uniformly under 0.04% when operating on charcoal alone. The pig is sold on a guar- anteed silicon content with 0.25% limits. The grade card is as follows: 392 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Silicon Per Cent. No. I Silvery 4.5 5.0 No. 2 4.0 4.5 No. i Soft 3.5 4.00 No. 2 " 3.0 3.5 No. I Foundry High 2 . 75 3 . oo No. I " Low 2.50 2.75 No. 2 " High 2.25 2.50 No. 2 " Low 2.00 2.25 No. 3 " High 1.75 2.00 No. 3 " Low 1.50 1.75 No. 4 High 1 . 25 i . 50 No. 4 " Low i. oo 1.25 Some tests were made to determine the relative variation of the silicon and carbon as the burden carbon content increases or 5.5 3t ^ 5.0 33 a4.5 ^ \ 32 ItO 31 " 3.6 _-- f' . N d .~ **! /" 30 3.0 ^Ui 28 -2.0 _^ -x / I / s \ 27 * 0.0 V \ _/ .' 23* < i 108 110 GRAPHIC ILLUSTRATION JttetalTap 98 98 100 102 104 106 Percentage of Carbon in Iron. Percentage of Silicon in Iron. _._ Percentage of Carbon to Ore. : Showing the reeponie of the Carbon and Silicon in the Iron to variations of the Charcoal ratio in the burden.. Starting with a cold furnace. 118 120 122 121 126 FlG. 133. Variation of Silicon and Carbon. decreases. Some of these results are shown by Fig. 133. Each number represents a metal tap made at regular 8-hour intervals. While the silicon responds readily to the carbon ratio, the latter shows but little variation. The slag analyses were not noted at this time. The fracture of this iron is much finer and more uni- form than that of charcoal blast furnace iron of the same grade. A very noticeable characteristic is the homogeneity of the fracture and the almost entire absence of segregations and "hard spots." It is also distinguished by its toughness, so that it has to be cast with very deep notches. THE ELECTRO-METALLURGY OF IRON AND STEEL 393 The softer irons have already won a reputation for themselves as softeners and scrap carriers. Some analyses taken at random of car-load lots of high, medium, and low-grade iron will illustrate how constant all the other metalloids are except the silicon. Analysis on a 2oo-ton lot shipped to a foundry for making steel castings and sold on a guarantee of from 2.75 to 3% silicon and a maximum of 0.04 % of sulphur and 0.04% of phosphorus. Silicon 2 88% Combined carbon 0.09% Graphite carbon 3 38% Sulphur 0.028% Phosphorus 0.031% Analysis on a ico-ton lot shipped for general foundry pur- poses silicon guaranteed from 2.25 to 2.50%: Silicon 2.42% Combined carbon 0.27% Graphite carbon 2 . 94% Sulphur 0.036% Phosphorus 0.023% Analysis on a ico-ton lot shipped to stove works to be mixed with high phosphorus iron for stove castings and sold on guarantee of from 1.75 to 2.00% silicon and 0.04% phosphorus. CONCLUSION The efficiency of this type of furnace increases slightly more than the direct ratio to the increase in load and it is of interest to note that as in the case of the electric steel furnaces, the faster the furnace is operated, the cooler the walls and roof are, and the smoother it operates. Crawford offers the explanation that the faster smelting takes place the faster the cool charge can descend to protect the arches and walls. The long and n'arrow type furnace is not equal technically in its efficiency to the shaft type carrying the same load, though the power con- sumption has been as low as 2,200 kw.-hr. per ton when carrying 3,oookw., but this type offers the possibility of building several furnace units on to each other, like copper blast furnaces. This decreases the radiation and electrical losses. A bank of three or four units arranged in this way will admit of as simple 394 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY metallurgical control as a single unit and will have an efficiency equal to a shaft type of furnace carrying the same load. Further, it can be arranged so that part of the furnace can be frozen up and repaired while the remainder is operating, and for this reason its yearly output should exceed the shaft type. From the nature of its construction it is capable of being made more nearly fool-proof metallurgically, mechanically, and electrically than the shaft type, and, further, the electrode con- sumption is lower, due to the fact that the electrodes penetrate vertically into the charge. While it is hardly agreed with the prophecies made by some that electric furnaces for producing pig iron will eventually be com- petitors of blast furnaces even in the regions where economic con- ditions make the latter possible, Crawford feels that where electric power can be obtained cheaply and where coke and freight rates are high and for making superior grades of iron, electric reduction furnaces will enable many large bodies of iron ore to be worked which would otherwise remain idle and that the electric iron fur- nace, both of the shaft type and of the long and narrow type, each in the field best adapted for it, will make steady progress. The work of Turnbull 1 in making low phosphorous pig iron deserves considerable attention, even though its manufacture has been curtailed since the close of the Great War. This is made in a stationary three-electrode three-phase furnace, solid bottom, 1 200 kw., much resembling the Heroult. Wlien using shell turnings for raw material, about 500 kw. hrs. were needed per metric ton and only 9.5 kg. of amorphous carbon electrode (2 1 Ibs.) . Ferro silicon was replaced by another product, which reduces the cost and helps the carbon content. Twenty- five consecutive heats had the following average analysis: Carbon Silicon Manganese Sulphur Phosphorus 3.19 1.16 .70 .014 .020 This grade of pig iron was also made from a mixture of 50% metallic turnings and 50% iron ore, necessitating about 1 Robert Turnbull, A.E.S., Oct., 1917, and Oct., 1918, "Electric Pig Iron After the War." THE ELECTRO-METALLURGY OF IRON AND STEEL 395 1400 kw. hrs. per metric ton. This furnace was of the batch process and not of the continuous type. With 1200 kw. it could produce 30 tons per day with a batch process furnace using all scrap, but 20 tons using half scrap and half ore with the continuous type on account of the higher load factor. With the latter process no ferro silicon or fluorspar would be required, and the return in metallic contents charged would be 100% as against 90% with turnings alone; besides a saving in furnace repairs, this would probably more than compensate for the extra power, electrodes, and carbon necessary for the reduction of the ore. A mixture of this kind permits the use of iron ore concentrates, the charge being kept porous enough by the presence of the turnings, and as these concentrates can be obtained very low in phosphorus, they could be used to ad- vantage in the production of low phosphorous iron and permit the use of turnings with higher phosphorous contents. As the operation would be conducted in a reducing atmosphere, the resulting carbon in the iron would be perfectly comparable to the blast furnace product. THE FUTURE OF ELECTRIC STEEL This is well discussed by Matthews, 1 and in comparing it with the crucible process, especially in reference to the possibility of replacing crucible steel, he stated, "The point is that each proc- ess has its peculiar field, and while some crucible tonnage may be diverted to electric steels; yet it is more likely that electric steel will find its market in the most exacting requirements of steel for structural and tensile purposes, such as have come about almost simultaneously with the processes themselves, namely, for automobile and airplane parts. Crucible steels for these pur- poses are scarcely commercial because of the difficulties at- tendant upon making very low or medium carbon alloy steels by that method, and the tonnage demanded far beyond what could be met by crucible steels. However, experience has 1 Dr. John A. Matthews, A.E.S., Oct., 1918; also A.I. and S.I., May, 1916, "The Electric Furnaces in Steel Manufacture." .300 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY shown that the electric furnace was apparently invented to meet a new demand rather than to replace an old process." Robin- son 1 presents a paper on the "Triplexing Process of Producing Electric Steel at South Chicago." This represents the largest tonnage installation of electric steel in the world (3-25 ton and 2-15 ton). He said: "The luxury of to-day is the necessity of to-morrow. Safety cannot be measured by price, and public opinion will more and more insistently call for the highest ex- cellence in the automobile and airplane and other forms of fabricated material." During the years that the Halcomb Steel Co. was alone in the electric furnace business in the United States, it was very hard to convince users of its superior quality, and still harder to get makers of open-hearth steels to admit it. Robinson's experience confirms what others have been contending for many years. MAKING PIG STEEL IN THE ELECTRIC FURNACE Some experiments have been made on a very small scale by Keeney. 1 Whether pig steel (carbon less than 2.2%) ever becomes a product on a large scale depends almost entirely on whether or not it can compete with the other processes. At Degerfors, Sweden, it was found that pig steel was more suitable for mak- ing steel in the open hearth than ordinary pig iron, requiring less time for refining. Normal pig iron made in the electric furnace was found to be less suited to the production of open hearth steel than normal blast furnace pig iron. This shows the advantage of producing pig steel rather than pig iron in the electric furnace, when steel is to be the final product. His summary is as follows : i. In the electric-furnace production of pig steel from ore, carbon in the product can be kept below 2.2%, and regulated to an extent by the amount of carbon charged, without result- ing in excessive loss of iron in the slag or in the produc- tion of a pig steel very high in impurities, if a fair grade of ore is used. *T. W. Robinson, May, 1918, A.I. and S.I. 2 A. I. M. E., Feb., 1014. THE ELECTRO-METALLURGY OF IRON AND STEEL 397 2. It is not difficult to slag the greater part of the silicon, phosphorus, and sulphur of the charge, if the furnace is hot and the slag fluid, but conditions are less favorable to the slagging of sulphur than of other impurities in the operation of an electric furnace for pig-steel production, which is, of course, contrary to experience in the manufacture of pig iron. 3. The loss of iron in the slag should not be excessive unless the pig steel produced is of very low carbon content. 4. From the results with the Domnarfvet, Trollhattan, and Heroult furnaces, there does not appear to be great difficulty at- tending the production of pig steel in an electric shaft furnace, and, in fact, experience has shown that there is less diffi- culty in the operation of the electric furnace on pig steel than on pig iron. 5. At any place where there is a market demand for steel and pig iron can be made in the electric furnace at a profit, the steel ultimately produced would be cheaper, if made by the electric reduction of iron ore to pig steel, followed by refining in another furnace if necessary, than if the product of the electric- reduction furnace was pig iron to be subsequently converted to steel in another furnace. Among other tests made are those by Humbert and Hethey. 1 These tests were made in a 6-ton Heroult steel furnace. The results of their tests are given in the table on page 374. The electrode consumption was between 32 and 36.3 kg. per ton of steel made. The wear of the furnace lining was about the same as when melting scrap. When all the ore is reduced the slag should be taken off and the charge treated as with an ordinary pig and scrap charge. Their conclusions and economic advantages are as follows: The authors have come to the conclusion that with the aid of the modern electric furnace, and given satisfactory conditions, the economic manufacture of steel direct from ore is a practical possibility. The material produced will be superior to that manufactured by present methods, and will have properties ^r. Iron & Steel Ins., May, 1914- ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY SERIES A B c Hard Steel Kg., etc. Rail Steel Kg., etc. Soft Steel Kg. Tool Steel Kg., etc. Soft Steel Kg. 6^OO S8SO s6so 43OO CGOO Scrap used 2OO 2OO I2OO 200 Coke I2OO IOOO 93 75O 1050 Lime 95 IOIO 1035 95O 250 Steel made . 3600 3630 358o 2970 3269 Scrap made 120 85 10 215 90 Loss in per cent 6-5 4.16 4.66 18.0 2-54 Electricity consumed per ton in kw.-hr 2459 2862 2843 2604 2709 Time, hours 16 18.35 18.45 H-iS 14.40 Carbon 1-39 .56 .23 .89 .27 Phos OSS O2 T. OIS OI4. 031 Analysis s 0^8 027 O7O O^7 os8 Si 13 .15 .11 .10 05 Mn .26 .28 47 .27 3i 80% FeMn used 12 25 7-5 17-5 40% FeSi used 20 ii 10% FeSi used 5 20 Fluorspar. 20 of the greatest importance and value to the steel user. A special type of furnace will probably be developed, although the standard Heroult furnace is satisfactory for occasional charges. The charge should lie deep in the furnace, to permit violent ebullition of the bath without overflowing. Anthracite electrodes will probably be found most satisfactory owing to their freedom from breakages. The best use for this process will be found in coun- tries that possess readily available sources of water power together with deposits of pure rich ores. Charcoal, coke or anthracite coal can be used as fuel. In countries where cheap power, pro- duced from waste heat such as blast-furnace gases, coke-oven gases, etc., can be obtained, its most useful sphere will probably be found in making high grade steels for springs, drills and similar tools, shafts, and all grades requiring exceptional toughness. The econpmic advantages of this process are: i. One opera- tion instead of several. 2. One furnace instead of a large and THE ELECTRO-METALLURGY OF IRON AND STEEL 399 complicated plant. 3. Simplicity, cleanliness and ease of control of electricity as a source of fuel and power. 4. Ability to use refractory and richer ores, such as titaniferous magnetites, etc. 5. Freedom of the steel from impurities. 6. Speed of manu- facture. 7. General cost depends largely on cost of electric power, but will be cheaper than the electrical production of steel from pig iron. 8. Less labor needed. 9. Metallurgical sim- plicity of the process. 10. Efficient control of quality of steel to be obtained, both from an analytical and physical point of view. The steel made in these tests was of excellent quality. THE USE OF THE ELECTRIC FURNACE FOR MELTING, FOR RE- FINING PIG IRON, AND FOR THE PRODUCTION OF ORDINARY AND SPECIAL QUALITY STEEL Pig iron, that is the iron and carbon alloy, produced in the elec- tric or ordinary blast furnace or in any way, contains other con- stituents, such as silicon, manganese, sulphifr, phosphorus, copper, arsenic, etc., which come from the charge. Some of these ele- ments, such as copper and arsenic, are easily reduced from the ore and enter the metal, and cannot be removed economically by any metallurgical operation. The other elements, such as silicon, man- ganese, sulphur, and phosphorus, can be partly eliminated in the blast furnace and slagged off, and they can also be separated more or less from the finished metal by later metallurgical operations. If the amount of one of these constituents is to be lowered in order to make the metal more suitable for any special purpose it is spoken of as a refining of the metal. Therefore, a lowering in the carbon percentage of the metal is also to be considered as a refining. The refining process can be of various kinds, reducing and oxidizing, or consist of simple reactions such as: FeS + Mn = MnS + Fe. If an electric furnace is to be suitable for refining, then all processes, whether oxidizing or reducing, must be practicable; above all, it must allow the carrying out of all metallurgical operations, such as are now used in the open hearth, converter, etc. The electric furnace, and this must be specially pointed 400 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY out, should not be different from an ordinary furnace, except that the heating is electro-thermal. The electric furnace, as such, except for the lining, should have no influence on the chemical composition of the bath of metal. Arc furnaces do not correspond altogether to these requirements, for an influence of the electrodes on the bath cannot be avoided even with careful operation. Electric heating of the furnace has the great advantage that the influence of the hot gases on the charge, which is present in the furnaces used now, is excluded, so that work can be carried out with an oxidizing, neutral, or reducing atmos- phere at will. Even the maintenance of a neutral or reduc- ing atmosphere is not only very difficult with the present furnaces but really impossible, except with crucible and muffle furnaces. The induction furnace completely meets these requirements, for in it a reducing or oxidizing atmosphere can be obtained as desired. With the arc furnace on the other hand, reduction processes take place very well, but oxidation processes only slowly, due to the reducing action of the electrodes, and with an increased use of oxidizing material there is more electrode consumption. Otto Thalner gives expression to this in his address before the " Oberschlesischen Bergwerksverein deutscher Chemiker," 1909, when he says: "The arc furnace is indeed a good reduction furnace, but a bad refining furnace." An electric furnace, however, to answer all requirements should allow reduc- tion and oxidation processes to be carried out equally well, and this should be pointed out before anything is said about the metallurgy of iron and steel, the influence of impurities, or the refining of the metal. Phosphorus. This exists in the iron in the form of phosphide of iron which dissolves in the metal bath without difficulty up to 1.7% Phos., forming mixed crystals. Phosphorus segregates in both pig iron and steel, for the phosphide has the comparatively low melting point of 910 C. For instance, in gray foundry iron the well known separated bean-shaped pieces are sometimes found, which give the following analyses: THE ELECTRO-METALLURGY OF IRON AND STEEL 401 I I 2 3 Bean-shaped pieces 1.30% P 1.30% P 1. 00% P Solid piece near the beans. . I 0.60% P 0.55% P 0.50% P Analytical proof of its segregation in steel is given in the next section under Sulphur. If a section is cut from a steel high in phosphorus, polished and etched with a solution of copper-ammonium-chloride, by Professor Heyn's method, the places rich in phosphorus will be colored dark, and one is in a position to determine the segregation in the material. As segregated material has considerably lower physical properties than normal material, a low phosphorus should be specified if a high quality is desired, so that if ordinary high phosphorus material is to be used for making high quality steel, it must be dephosphorized. In order to do this an Ameri- can has proposed to destroy the phosphide by the addition of another element according to the equation: Iron phosphide + metal = iron + metal phosphide. Naturally a metal must be chosen that, in the form of phos- phide, does not alloy with the iron but goes into the slag. Such reactions are theoretically possible, and have been carried out practically to a small extent. Even the silica holding desul- phurizing slags of the electric furnace show a certain content of phosphides, 'which can be easily recognized by the garlic-like smell when the slags are moistened with water, but this method of dephosphorizing has not, so far, become of practical import- ance. The removal of phosphorus is only possible with certainty, at present, when the phosphorus is oxidized to phosphoric acid, combined with lime, and removed as slag. The phosphorus is oxidized at low temperatures before the carbon, but at higher temperatures only after the removal of the carbon from the bath. One can therefore dephosphorize high carbon charges without having to previously completely remove the carbon. In this case the temperature of the bath should be kept low, an 402 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY easily fusible basic slag charged rich in oxide of iron, and at the end of the dephosphorization immediately tapped. To a certain degree this method of dephosphorizing requires considerable care and experience for complete success, although the melting of high carbon heats is economical. Dephosphorization is more certain with low carbon and very high temperature, and at the same time strongly oxidizing and basic slags. Further, the oxidation of the phosphorus can be brought about as well by the oxygen in the ores as by that of the air. The maintaining of a basic slag naturally requires that the work be done on a basic hearth. Sulphur. Sulphur can exist in steel as MnS, as well as FeS. The latter can alloy with liquid iron while the former does not alloy with the liquid metal, and is therefore only present in the form of included material. If the bath of metal is allowed to stand long enougty, then the MnS will rise to the surface because of its lower specific gravity, and can be drawn off. This is not possible with the remaining FeS which remains alloyed with the liquid metal. For this reason the slags which separate from the metal, for example from basic Bessemer iron, in casting ladles, or mixers contain a high percentage of sulphur and also manganese, present for the most part as MnS, while the amount of iron is not so great. This is shown by the following average analysis of ladle slag: Iron 6% Manganese 42% Sulphur 10% If these slags come lower in sulphur, then oxidation of the sulphide of manganese by the air or by included oxide has taken place. A high manganese, however, is always a characteristic of these slags so that, after a preliminary roasting, they can be used in the blast furnace as an ore of manganese. Sulphur is harmful to pig iron, steel, and wrought iron, the reason probably being the low freezing point of FeS, whereby during the cooling of the bath of metal it segregates to the centre, and also brings about the red short character of high sulphur material. THE ELECTRO-METALLURGY OF IRON AND STEEL 403 It is therefore necessary to desulphurize the iron as much as possible before it is made into steel, a process that is carried out by the addition of ferro-manganese to the liquid bath, if there is not enough manganese already present. The sulphide of iron is then, decomposed according to the equation: FeS + Mn = MnS + Fe and if sufficient time is given the MnS rises to the surface of the bath into the slag and can be removed. The process only takes place smoothly if a considerable excess of manganese is used. Even in this case, however, no total desulphurization is possible. The sulphur can only be lowered to a certain degree, about 0.05%, which is still considerably too high for- special quality steel. In liquid pig iron or steel, rich in manganese, that has stood long enough before pouring, the sulphur is to be thought of as being present exclusively in the form of FeS. If a microscopic section is taken from high sulphur material, polished and etched as described under "phosphorus," and the dark segregation places examined, then a considerably higher sulphur content is found than in the ground mass, but only the same manganese. If the segregation were a question of the separation of MnS then, with an increasing sulphur content, there would also be noticed an increase in manganese, which is not the case. Below are given some analyses: First material: Mn% S% P% Pure ground mass 0.48 0.067 0.050 Segregate 0.48 O.182 o.ioo Second material: 1. Very black segregate. 0.30 0.097 0.155 2. Gray segregate 0.30 0.055 -79 3. Pure material 0.30 0.040 0.047 The elongated sulphide inclusions often seen under the microscope that are usually assumed to be MnS are perhaps nothing more than inclusions of sulphur holding slag, which during the rolling of the hot ingot were not yet solidified in its 404 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY interior and therefore were rolled out. Also the general appear- ance of red short material during forging and rolling inclines one strongly to the opinion that the Fe FeS alloy separates between the crystals but not inside the crystals themselves. As already mentioned liquid steel produced in the ordinary way and therefore fairly high in sulphur must be further de- sulphurized for the production of good quality steel. For this purpose the electric furnace is suitable. The following two processes are those mostly used for desulphurizing in the electric furnace, and both take place most energetically at high tempera- tures. They also both require the use of a neutral or reducing atmosphere in the furnace and the melting of a strongly basic slag. In order to make these basic slags easily fusible additions of fluor-spar, and quartz in the form of sand, are made. (1) The use of the chemical reaction FeS + CaO + C = Fe + CaS + CO. For carrying out this reaction, therefore, the help of carbon is necessary, and the process can be operated very satisfactorily in the arc furnace, due to the favorable influence of the electrodes. Carbon must be added to the bath, and for this reason the process is used only when it is a question of the production of high carbon steels. In melting very soft steels, one must either take into account a certain carbonization of the bath and later remove the carbon, or else be satisfied with a less complete desulphurization. Even if the carbon is only thrown on the slag covering from time to time, a certain absorption of carbon by the bath cannot be avoided. (2) The use of the chemical reaction between silicon and sulphur whereby SiS is produced which escapes as gas. FeS + Si = Fe + SiS. If at the same time a lime carrying slag is formed on the metal bath a further desulphurization takes place according to the equation : 2 FeS + 2 CaO + Si = 2 Fe + 2 CaS + SiO 2 . The CaS is removed as slag. It is interesting to know that THE ELECTRO-METALLURGY OF IRON AND STEEL 405 both reactions take place almost quantitatively so that scarcely more than the theoretical amount of ferro-silicon must be added to the bath, and if desired a low silicon steel can be produced. The process is often used in the induction furnace and has the advantage that it can be used equally well for high and very low carbon heats. The reaction gives a very fluid slag because of the increase in the amount of silica. Moreover, fluor-spar is also an equally good desulphurizing agent when ferro-silicon is used, according to the equation : 2 FeS + 2 CaF 2 + Si = Fe + 2 CaS + SiF 4 . Further, in regard to desulphurization by means of silicon in the electric furnace a great many theoretical reactions have been suggested, a small selection from which is given below. (a) With the use of burned lime. 1. 2 CaO + SiS = CaS + SiO 2 + Ca -, but there would result Ca + Fe S = CaS + Fe. 2. 2 CaO + 2 SiS = 2 CaS + Si0 2 + Si -, but there would result Si + FeS = SiS + e. 3. 2 CaO + FeS + SiS = 2 CaS + SiO 2 + Fe. The SiS in all these equations is thought of as being pro- duced by Si + FeS = SiS + Fe. Also they all represent the same reaction, namely: 2 CaO + 2 FeS + Si = 2 CaS + SiO 2 + 2 Fe. The slag will be made thinly liquid by the silica produced and, in this reaction, i sulphur requires K silicon. (6) With the use of fluor-spar, 1. 2 "CaF 2 + SiS + FeS = 2 CaS + SiF 4 + Fe. The SiS is produced by the equation FeS + Si = Fe+SiS. 2. 2 CaF 2 + 2 SiS = 2 CaS + SiF 4 + Si. The Si would decompose more FeS. 406 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY Both reactions therefore mean the same, namely: 2 CaF 2 + 2 FeS + Si = 2 CaS + SiF 4 + 2 Fe. The slag will become more basic, that is thicker, and by this reaction also i sulphur requires ]/z silicon. Resume of the equations. (a) 2 CaO + 2 FeS + Si = 2 CaS + Si0 2 + 2 Fe. (b) 2 CaF 2 + 2 FeS + Si = 2 CaS + SiF 4 + 2 Fe. The reactions are the same except that in one the oxide, in the other the fluoride, is the reagent; and by both processes the same amount of silicon is necessary. Desulphurization by the alternate reaction between FeO and FeS and the formation of S0 2 cannot be carried out with fluid metal to complete success, and for this reason it is only suitable for such cases where complete desulphurizaion is not necessary. Silicon. The good influence of a certain silicon content in gray pig iron and gray iron castings is well known. To a certain degree too high silicon in the pig iron is a disadvantage for gray iron castings, particularly for the larger ones, as it brings about a coarsely crystalline structure, and therefore makes weaker castings. On the other hand the silicon in iron or steel can easily be raised by the addition of ferro-silicon to the molten bath. Silicon can be removed from molten iron and steel by oxida- tion, as well by means of ore as by the oxygen of the air, a process that naturally takes place more easily on a basic than on an acid hearth. The silicon burns before the carbon if the temperature is low, at higher temperatures it is only removed completely when the carbon is already partly oxidized, while at high tem- peratures the silica in the slag is again reduced by the carbon in the bath. Copper and Arsenic. Neither of these elements can be removed economically at present from the bath, so that if a certain copper and arsenic content is required in the finished material, an appropriate mixture must be charged. Carbon. The carbon can be removed from the bath by the THE ELECTRO-METALLURGY OF IRON AND STEEL 407 oxygen of the ore or by that of the air, with the formation of carbon-monoxide. If the refining is carried out by means of ore, then iron is reduced, a process that requires heat. It is probable that the metal may dissolve a certain amount of carbon- monoxide, for iron heated in a stream of nitrogen shows a melting point of 1506 C., but when heated in a stream of carbon-mon- oxide only 1406 C., a phenomenon that is explained by the assumption that carbon-monoxide alloys, at least partially, with iron. On the other hand low carbon steel baths easily take up carbon whether the latter is added in a solid, liquid, or gaseous condition, either in the elementary form or as carbon containing alloys such as ferro-manganese, etc. Oxygen. Oxygen may occur in steel combined with other elements, for example, as CaO, SiO 2 , MnO, A1 2 O 3 , etc. These oxides are only mechanically mixed, not alloyed with the steel, and they are usually classed as "inclusions." Such in- clusions are undesirable in high quality steels for they loosen the structure, and so lower the physical properties. In addition to this, however, steel can contain oxygen in the form of ferrous oxide, and such a constituent is especially to be feared for it alloys with the metal, and. like sulphur, brings about red short- ness. It is possible to remove this ferrous oxide from the metal by chemical means, reducing it by other elements according to the equation: FeO + X = XO + Fe. Elements that can serve as reducing agents for ferrous oxide must answer the following requirements: (1) They should not bring about any development of gases in the reduction, for then the metal does not cast quietly, and opportunity is given for the formation of gas inclusions. There- fore reduction by means of carbon or carbides, electrodes, etc., is bad, because the formation of carbon-monoxide is the result. (2) They must have a high volatilization temperature. Therefore the alkali metals are bad to use, for they escape from the bath for the most part as gas without bringing about reduc- 408 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY tion. This is altogether apart from their strong attack on the brickwork. (3) They must easily reduce the ferrous oxide, and for this it is necessary that the metal should dissolve in the bath. In this way only is a complete contact and reaction possible. (4) They must easily slag off, and separate from the metal. Manganese, silicon, aluminum, etc., are generally used as reducing agents, and, recently, for producing high quality steels, certain alloys of silicon with calcium, magnesium, manganese, and aluminum. At the same time vanadium and titanium should be mentioned, for their influence ought to be, in the first place, very strongly reducing on the last traces of ferrous oxide. For ordinary purposes ferro-manganese is mostly used for deoxidation. Its reaction with ferrous oxide, however, only takes place very slowly, and if the deoxidation is to be moderately satisfactory a considerable excess of manganese must be added to the bath. For this reason only high manganese material can be produced which is not applicable as high quality steel for different purposes. The slow influence of the ferro-manganese is caused by the alloy having to become dissolved before it can alloy with the metal. Solid manganese must first melt in the bath before it can carry out its deoxidizing action. Because of this the deoxidation process would be accelerated if liquid ferro-manga- nese were added, and by using this method the amount necessary can be considerably reduced, as the loss of manganese in the shorter time is smaller. In the electric furnace, where the ferro-manganese works in a neutral atmosphere, the minimum amount can natu- rally be used for deoxidation, because the alloy has opportunity to react on the bath for a long time without danger of being burnt by hot gases. Also the necessary excess of manganese in the bath can be lowered, as the manganese can work on the bath without trouble. A disadvantage of this method of de- oxidizing by means of ferro-manganese is that, with the neces- sarily large amount of alloy used, the carbon which is unavoid- ably present in the blast furnace alloy also takes a part in the re- action. It follows that the bath should be allowed to stand for a long time after the ferro-manganese addition in order to allow THE ELECTRO-METALLURGY OF IRON AND STEEL 4Q9 the gas to escape. This gas removal is, however, only complete if the bath has been given some opportunity to take up silicon. Unfortunately, there is no clear explanation for the influence of the silicon. It either reduces the carbon-monoxide dissolved in the bath, or else it makes the metal able to unite with the gases, especially the carbon-monoxide. The latter view is the more probable, for it has been mentioned that iron has a very low melting point when exposed to heat in an atmosphere of carbon-monoxide, which is easily explained by the theory of the existence of an iron-carbon-monoxide alloy. Silicon is moreover a very strongly deoxidizing material, and scarcely more has to be used than the amount theoretically necessary. The silica easily goes into the slag, and there is no production of gas, as the small amount of ferro-silicon used adds practically no carbon. Heats deoxidized by means of silicon can be cast quietly and easily for the reasons just mentioned. Aluminum is also an effective reducing agent, but there is the disadvantage that alumina is produced which, on account of its high melting point, does not slag off completely and some remains as a fine net-work in the metal, lowering the physical properties of the latter. In the production of high quality material the use of aluminum is therefore not to be recommended, above everything no aluminum should be used while pouring into the moulds, for then much less heat is present for melting the alumina than in the furnace. Recently alloys of vanadium and titanium have been recom- mended, the latter produced by the Goldschmidt reaction. They are very effective, but at present their high price limits their use. It may be, however, that the price of vanadium will be lowered when the alloy can be produced in the electric furnace, but due to the formation of carbides special attention must be paid to the making of a low carbon vanadium alloy. Deoxidation requires that during the whole process there should be a purely neutral or reducing atmosphere. Formerly this condition was only obtained in the graphite crucible, silicon being reduced from the crucible walls by carbon, and forming the reducing agent. For this reason crucible steels made from 410 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY a pure charge were, up to the present time, the best obtainable, although low silicon material was only produced with very great difficulty because of the silicon reduced from the acid crucible walls. The melting of low silicon crucible steels had to be done, therefore, in costly alumina crucibles. FLUXES, FERRO ALLOYS, ETC., USED IN THE ELECTRIC FURNACE (1) Ferro-manganese. For reasons of economy the ordinary blast furnace product is used with the average analysis Manganese 80 . 00% Silicon 1 . 20% Phosphorus 0.25% Carbon 6.00% The rather high phosphorus content can be neglected, for only a small percentage of ferro-manganese is used so that the phos- phorus of the charge is practically not increased. Occasionally, pure manganese, which is naturally very expensive, is used for special purposes. (2) Ferro-chromium. Here also the cheap high carbon mate- rial is usually good enough, with the analysis Chromium 64% Carbon 8-9% especially when the alloy is added liquid. The carbon does not produce any gas, for the alloy is only added after the bath is deoxidized. Of course the carbon of the alloy must be consid- ered in figuring the carbon of the steel. The more expensive low carbon alloys are, however, used in many cases. (3) Ferro-silicon. The best is the ordinary 50 per cent, elec- tric furnace grade. High silicon blast furnace pig irons can indeed be used, especially with such a market as the present, but the carbon of this grade of material is higher than desirable. In making high silicon steels, such as dynamo plates, etc., the 90 per cent, alloy is used. (4) Lime. This should, of course, be as free from sulphur and phosphorus as possible. In burning the lime it should be par- THE ELECTRO-METALLURGY OF IRON AND STEEL 4H ticularly remembered that with the use of high sulphfir fuel, such as is generally employed, the lime takes up considerable sulphur, so that with large pieces of lime the sulphur is highest at the outside and decreases towards the centre. Analyses I II Outer shell o.so%S 0.48% S Middle part 0.21% S 0.20% S Core o.os%S 0.06% S One is therefore bound to consider the use of raw limestone, especially for the formation of the refining slag. As lime free from sulphur is needed, the stone could be burned in a shaft or rotating furnace by means of waste gases, so far as they are available. Moreover, tests with the ring furnace have shown that the lime in a chamber does not show the same increase in sulphur at all parts of the chamber. An example is given below. The raw limestone used was very uniform and had 0.05% S. Tests taken from the material after being burnt showed the following results: Average test from the wall of the chamber o.n%S " " in front of the fire o. 16% S " " somewhat further from the fire o. 16% S " " at the door 0.09% S As lime burnt in the ring furnace is mostly used for other purposes, one is in the position to take the low sulphur part and use it specially. If burnt lime is bought it is welPto consider the percentage of moisture and carbon-dioxide contained. On the other hand a small proportion of carbon-dioxide is not a great disadvantage to the process, for the slag must finally show a certain amount of carbon-dioxide, at least for desulphur- izing and deoxidizing. Also certain percentages of magnesia in the lime are a disadvantage as it makes the slag less fusible. (5) Fluor-Spar This should be as low as possible in sulphur and phosphorus, and is suitably paid for according to its contents of fluorine. Also contained magnesia is a disadvantage. (6) Iron Ore for the Carbon Refining Process. Any ore, even brown iron ore, can be used but high percentage ore is recom- 412 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY mended, so that the slag volume and the heat lost in the slag are not too great. Naturally, it is also better to use ore not too high in sulphur, especially if metal has to be worked that is high in carbon, sulphur, and phosphorus. Phosphorus in the ore, on the other hand, is not harmful so that minette ore can be used, for the bath cannot reduce phosphoric acid from the ore. (7) Carbon. A material should be chosen that is low in ash, sulphur, and volatile matter. Graphite, anthracite, petroleum, coke, etc., can be used according to one's wish and the market price. Below are given several analyses of these materials: Vol. matter Ash Sul. Petroleum coke 3-5% o,7% 1-2% Retort carbon 0.6 ' 1.8 1.2 Flake graphite 1.3 6.9 O.5 They are best used in moderate sized pieces. If finely divided material must be used it is best to weigh it out into bags, or -else briquette it. THE ELECTRIC FURNACE AS A MELTING FURNACE FOR IRON AND STEEL, AND IRON ALLOYS OF EVERY KIND The advantages of melting in the electric furnace are chiefly brought about by the possibility of maintaining purely neutral or reducing atmospheres, which means that the hot materials do not attack the furnace walls as in the cupola, air furnace, etc. As is well known the melting of pig iron, etc., in the cupola is attended with a considerable absorption of sulphur, which sensibly affects the final quality. For instance: C% Si% Mn% S% Material before melting 3.50 2.90 1.20 0.035 After melting once 3. 40 2.70 i.io 0.055 After melting twice 3.30 2.50 0.80 0.073 Foundries making low sulphur material such as high quality castings, malleable iron castings, etc., must therefore melt either in the open hearth or air furnace, and notwithstanding this most expensive operation the undesirable action of the furnace gases THE ELECTRO-METALLURGY OF IRON AND STEEL 413 on the charge is not prevented, as is shown by the following analyses: A . Melting of Pig Iron: C% Si% Mn% S% P% (1) Before melting 3.30 1.55 1.67 0.053 0.36 Finished material 3.25 0.66 0.76 0.083 0.37 (2) Before melting 3.18 0.59 1.79 0.075 0.27 Finished material 3.16 0.22 1.22 0.093 0.27 (3) Before melting 3.06 0.72 1.98 0.069 0.23 Finished material 3.02 0.28 0.28 0.090 0.23 The melting in all these cases took place in an air furnace, using bituminous coal with about 0.7% sulphur. B. Steel Castings Melted in a $-Ton Open Hearth Furnace. C s (1) Before melting 0.050 Finished material o . 30 o . 060 (2) Before melting o . 037 Finished material 0.25 o. 050 (3) Before melting o . 048 Finished material 0.45 0.062 Naturally those plants suffer which have to use, anyhow, high sulphur pig iron and fuel; on the other hand, with melting in the electric furnace there is no oxidation of the iron nor of the valuable constituents silicon, manganese, etc., such as is shown in the above analyses. As is well known in the melting of an ordinary foundry iron, a loss of at least 10% of the silicon is calculated, with higher silicon irons still more, and on this account less scrap can be melted than the silicon of the cold foundry iron would allow. In electric furnace melting there is no loss of iron, nor metal loss in the slag, for no melting slag is necessary. Electric melting is particularly important in the production of hard castings. The high manganese pig iron used suffers a high loss of manganese in melting in the ordinary furnace, which is entirely absent with electric melting, so that the amount of the expensive high manganese iron necessary can be considerably lowered. An important point that recommends the electric 414 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY melting of iron for foundry purposes is that in the electric furnace the temperature can be governed as desired. The percentage of bad castings therefore ought to be somewhat reduced, for the casting of cold iron, which may happen with the cupola even with the most careful supervision of the operation, is excluded. Also in regard to the quality of the castings, electric melting should bring about considerable improvement, apart from the avoidance of an increase in sulphur, especially for pipe and thin- walled castings, for which high phosphorous brittle material has now to be used in order to fill out the moulds. As it is possible to increase the temperature of a low phosphorous iron in the electric furnace to such an extent that the same fluidity is pro- duced as with a high phosphorous iron, one can, therefore, under these conditions, produce thin-walled castings from low phos- phorous iron which is not brittle, without getting porous or blowhole castings. The electric furnace is also very suitable for melting ferro- manganese, and all the ferro alloys, which are so much used in steel plants and also recently in foundries. Every metallurgist knows that for quickly completing heats of steel in the furnace or in the ladle, considerably more ferro-manganese must be used if it is added cold than if added liquid. In spite of this, up to now, he has been forced to be satisfied with the use of solid pre- heated ferro-manganese, because the metallurgical furnaces avail- able for melting this easily oxidizable material are not practical as the loss increases immeasurably. The electric furnace is here particularly applicable, for with a reducing atmosphere an oxida- tion of the manganese is excluded. Harden 1 reports saving 44.3% ferro-manganese when poured in its molten state from an elec- tric into steel, instead of using cold lumps; total melting and holding cost about $12 per ton. The finished material is also superior, being free from "ghost lines" and hard lenses. The introduction of the electric furnace into foundries, steel works, etc., will be further favored by the fact that during the melt- ing an excellent mixing of the charge will take place. Until now 'John Harden, 1917, "Fifth General Meeting of Chemists," Stockholm, Sweden; see also Iron Age, April n, 1918, for digest. THE ELECTRO-METALLURGY OF IRON AND STEEL 415 in cupola melting one is compelled to take the metal more or less as it comes, even when making a special material, because, though one may know the composition of the material charged, it is difficult to figure on the loss during melting, and therefore on the final composition. With the electric furnace, on the other hand, where there is no oxidation, one can calculate exactly beforehand the composition of the final fluid metal, apart from the fact that an absolutely uniform material, free from impuri- ties, will be produced. Bad heats, because of low or high sili- con, will be excluded because one can add to the bath the right amount of ferro-silicon on the one hand, or low silicon pig on the other. Attempts have often been made previously to increase the silicon in a low silicon iron by the addition of ferro-silicon to the casting ladle, a process that is only partially successful, for, to absorb the silicon, it must be first melted, which requires a very hot bath of metal and also a certain amount of time. Both conditions are fully met in the electric furnace, but not in the casting ladle Also cast-iron scrap, turnings, etc., can be melted without the scrap being for the most part burned and slagged off as in the cupola. Indeed, this great loss, when melting fine material such as turnings, etc., in the cupola, has forced those plants which have considerable amounts of such scrap to briquette it before melting. The considerable cost of this process is always lower than the saving due to the decreased loss. Also, a low carbon material, similar to cold-blast iron, can be produced without difficulty by the melting in of wrought- iron scrap. The melting of pig iron in the electric furnace can, at the same time, be combined with a refining of sulphur or silicon. In regard to the sulphur its removal is easy if a lime slag is pro- duced. As" the silicon content of the pig iron is almost always high enough, desulphurization readily takes place according to the equations given before, if there is temperature enough. The sulphur enters the slag, which is drawn off. Such desulphurized iron is particularly suitable for malleable iron 416 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY castings, so that even for this high-grade material a cheap high sulphur iron or scrap can be used. The lowering of the silicon will naturally be brought about by the addition of a correspondingly low silicon iron. DUPLEXING WITH THE CUPOLA Elliott 1 mentions his two years' experience making not only gray iron but also malleable iron for castings. Editorially, the Iron Age, 1 in discussing the results, says, "The changes in the metal are striking, when it is considered that the iron is kept under electric conditions less than half an hour. Besides the greater fluidity and a certain amount of refining, espe- cially as to sulphur, the density of the metal has been in- creased, the structure changed and the strength augmented 75 to 90%." Moldenke, in discussing this, pointed out that this process affords a solution of the sulphur problem in cast-iron scrap. In discussing this in detail, Elliott goes on to say that the electric furnace has now been making a super grade of gray iron castings due to the insistent demand for physical prop- erties of a higher order than are commonly characteristic of such iron. This experience covers mostly gray iron for steam-valve bodies, except as otherwise noted. However, similar is the case of cylinders for locomotives using superheated steam, and those for internal combustion cylinders. The two prime qualities most generally wanted in high-grade iron are strength and solidity. The best strength gray iron shows is under compression, and its worst is under impact and vibration; between comes transverse strains and tension. Where unusual strength of iron is required it is usually sub- jected to either impact, vibration, or tension, or where cast iron is at its greatest disadvantage. Under solidity may be included a number of related items such as density, closeness of grain, and freedom from subcutaneous imperfections, such as 'See A. E. S., April, 1919; also Iron Age, April 10, 1919. THE ELECTRO-METALLURGY OF IRON AND STEEL 417 slag inclusions, graphite segregation, blow-holes and shrink- holes. Strong irons without exception require high pouring tempera- tures, because they have high melting points and minimum fluidity. The irons most easily melted, and consequently the most fluid ones, have a high percentage of phosphorus in their make up. Medium and high phosphorous irons comprise by far the greatest part of the melt of the gray iron foundries. These are the popular casting irons, and it is well known that phosphorus has done as much or even more than any other element to popularize gray iron for castings. Still, phosphorus has the grave fault of unfavorably affecting the strength of iron, a thing it accomplishes by forming in the iron mass a net- work of structurally free phosphide, which is quite brittle. Although easily fluid, high phosphorous irons are brittle and generally lacking in strength. Conversely, tenacious strong irons are necessarily low in phosphorus. The fact that strength and fluidity do not go hand-in-hand brings to light the greatest defect of the cupola, namely, its thermal limitations. The melting operation in any furnace is divided into preheating, melting, and superheating. For the first two the cupola is preeminent, but for superheating the cupola has not the same excellence. All ordinary grades of gray iron, those with medium and high percentages of phosphorus, cannot be melted more easily or more economically than in the cupola. For melting efficiency this furnace takes precedence over the reverberatory or air fur- nace and the regenerative type open-hearth (Siemens-Martin) furnace. From the standpoint of process alone, the cupola is blame- less, but from the high ground of product, it shows seri- ous faults. The two worst of these are its weakness as a superheater for molten iron, and its incapacity as a re- finer. The cupola is not a refining furnace, and sometimes the hottest possible iron is not hot enough for the best casting results. With extreme solidity exacted and for really alluring possi- 418 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY bilities and extraordinary eastings, the resulting basic electric furnace is the one to duplex with the cupola. The question arises why is the basic hearth preferred? If superheating only is desired, it is entirely probable that the acid furnace should be given preference; but if an important amount of refining is advantageous the basic furnace should be used. It is true that the acid electric furnace exerts a con- siderably refining influence by virtue of the reducing condition so readily maintained in it. On the other hand, the refining tendency of the basic furnace is so much more pronounced and so readily responding that the iron charge is refined at the same time it is being superheated. By the time the charge has reached its pouring temperature it has also reached. a highly refined condition. Among other results, the sulphur has been reduced very materially, provided the proper basic and reducing slag has been maintained. Sulphur in cast iron has so long been considered a neces- sary evil that its presence not only is commonly con- doned, but occasionally is credited with certain benefits. No doubt there are a few of these last, for instance, where it aids in producing chill. But in general there is good reason to believe that iron is much better for its absence. That it induces unsoundness, when in excess, is general knowledge; and sluggishness of metal is a closely allied evil that travels in the train of sulphur and works unfavorably to solid castings. A medium or high sulphur content is inevitable in the prod- uct of the cupola furnace, which in the basic electric is prac- tically eliminated as a matter of course. The basic electric fur- nace removes most of the sulphur while the metal is being superheated. This reaction gives the electric certain obvious compensating economies in the choice of raw materials used in the cupola phase of the process. Iron coming from the cupola with 0.100% sulphur, after undergoing 25 minutes' treatment in the basic electric, generally contains about 0.03% sulphur. Heats have been made reducing sulphur from 0.099 to 0.022% and from 0.088 to 0.018%. This possibility of so low a sulphur THE ELECTRO-METALLURGY OF IRON AND STEEL 419 in gray iron castings opens up a new field for metallurgical investigations. Carbon regulation is possible to a most useful extent in the electric furnace, total as well as combined and graphitic. By placing steel scrap in the furnace before adding the molten iron from the cupola, total carbon can with accuracy be reduced to any desired amount. Uniformness among different heats, homogeneity in the indi- vidual heat, close carbon regulation, and unlimited temperature are a quartet of benefits of the electric furnace not found in any other furnace. This enables any compositon and exceptional quality to be made, many of which are so often claimed and so rarely reached by the cupola alone, and the cupola product of which goes under many hybrid names. Also correctly balancing of the combined and uncombined carbons is a matter of no great intricacy in the electric, by varying the silicon content. In short, the capriciousness of the cupola, with reference to carbon control, is replaced in the electric by substantial certainties. The well-known advantages of making manganese and other similar additions with these mixtures are here also, and the manganese losses are nil. The one outstanding advantage from the standpoint of composition is the absolute control of the mix- ture, making duplication of results more a matter of correct calculation, and less the effect of happy accident. A concrete example of results actually obtained in every-day running practice is as follows: A rather ordinary mixture of pig iron and foundry scrap was melted as usual in the cupola, transferred to a basic electric, and there superheated and refined under a lime slag. The untreated iron was of a composition regularly giving a standard " arbitration " test of bars that broke under an average transverse load of 1,350 kg. (2,950 Ibs.) with a deflection of 25 mm. (.10 inch). After treatment in the electric furnace, the iron gave the same kind of bars breaking at slightly over 2,000 kg. (4,400 Ibs.) and at a deflection of 2.9 mm. (.115 inch). The specific gravity was increased from 7.10 to 7.25. 420 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY About 25 minutes was the time of electric furnace treatment, and the current consumption was 114 kw. hrs. per metric ton (2,204 Iks.). Broadly speaking, duplexing with the cupola is feasible; first, for castings having an exact unusual tenacity, solidity, and other physical properties; secondly, there are those castings, dif- ficult to run on account of having thin sections and relatively large sizes; thirdly, those of high quality whose extreme foundry cost is but a small part of the total cost of the finished article. For the moment, leaving the cupola, attention is again called to the advantage of using the electric in connection with the direct metal from the blast furnace, the electric furnace filling the triple role of mixer, superheater, and refiner. Such process would make high-class gray iron castings, almost direct from the blast furnace, at a cost very little higher if any, and perhaps lower, than those of the ordinary cupola. THE ELECTRIC MALLEABLE CASTING White iron for malleable castings may be prepared in the electric furnace in two, or possibly more, different ways econom- ically. In foundries where the malleable department is only a small annex requiring but a few tons of castings each day, the best method is to melt down a cold charge of suitable composi- tion in the electric to refine and finally superheat before pouring. A great many hundreds of tons of excellent malleable castings have been made this way by Elliott. In foundries specializing in malleable and consequently running a large daily tonnage, it would be more economical and satisfactory to follow the duplexing process that has been described for electric gray iron. With a basic electric the refining can be made ex- tremely effective, removing with great ease most of the sulphur, which enables the use of higher sulphur raw materials than is customary, and brings about thereby an economy by no means trifling. In addition to these two processes there is the Kranz triplex THE ELECTRIC METALLURGY OF IRON AND STEEL 421 process. It includes the use of the cupola, the side blow con- verter for lowering to a certain part with the cupola melt, car- bon, silicon, and manganese, and the electric for refining the mixed cupola and converter metal. Aside from the possible widening of the field of raw mate- rials so as to admit cheaper grades of iron and scrap, the elec- tric furnace processes for malleable iron have even greater bene- fits than are shown in the product itself. Chief among these is molten iron, hot enough not to freeze either in the ladle before being poured or in the molds after pouring but before the cast- ings are perfectly "run." The raw-material white iron, before being cast into the molds for malleable castings, has a temperature and time range of workable fluidity that is decidedly narrower than has gray iron. Gray iron may drop nearly twice the number of degrees in tem- perature before freezing as may white iron; consequently, the time available for handling white iron in the effective fluid state is only about half that available for gray iron. The practical result of this undesirable characteristic of white iron is that when fluid such iron cannot be held in the ladle for long, and the mortality among small malleable castings as the result of slug- gish metal is often appalling. There being no limit 'to the pos- sible temperature in the electric, the foundrymen operating such a furnace have the pleasing experience of seeing molten white iron that is capable of running even the smallest and most intri- cate castings with losses through dull metal reduced to the practical irreducible minimum. Besides this, there is the ad- vantage of refinement, including desulphurization and deoxida- tion, giving castings of exceptionally high quality. Also, there is the well-grounded idea that the use of the electric furnace may be made to reduce the time necessary for malleableizing, partly through the extreme reduction of the sulphur and partly for other reasons. As a conclusion it may be taken that the electric furnace has found a place in foundries, etc., for melting pig iron, ferro alloys, etc. ; particularly in the production of high quality mate- rial. The advantages are an absence of loss by melting and in 422 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY the slag, the production of the same metal as calculated theo- retically, the use of cheap high sulphur iron, and the melt- ing of more cast-iron scrap. In general, no furnace will be preferable for these purposes, for the bath is heated uniformly enough and there is no electrode action on the metal due to the slag which would cause a loss of manganese because of vaporization. The profitable use of the induction furnace in the melting of fine scrap must be particularly mentioned, for there is always a bath of metal in the furnace. On charging the cold scrap it immediately falls into this bath and is therefore protected from oxidation. The power consumption per metric ton of steel scrap melted in this type of furnace is about 580 kw. hrs., an amount that makes electric melting appear quite economical. THE ELECTRIC FURNACE AS A MIXER Most large steel works that have several blast furnaces, as well as foundries taking metal direct from the blast furnaces, already have mixer plants either to regulate the production, to get a better mixture of the different casts, or to obtain as thorough prerefining as possible. This means a separation of the sulphur brought about by a part of the sulphur slagging off as a sulphide of manganese, if there is sufficient manganese in the iron. The size of the mixers varies a great deal from 25 to 1,000 tons and more capacity. The small mixers are preferably used for iron foundries, such as pipe foundries, that take direct metal, but the prevention of cooling with the small mixers is naturally not very good so that sometimes heating is necessary. If refining is desired in the mixer, then heating by means of fuel is not so profitable and there is opportunity for the electrically heated mixer, which will be similar to an ordinary electric furnace of very large size. Furnaces of more than 30 metric tons capacity have, how- ever, not yet been built, so that in this respect there has been no experience. The requirement for an electric mixer is that the metal should be held at the right temperature. The THE ELECTRIC METALLURGY OF IRON AND STEEL 423 question should be solved by an induction furnace, for here the temperature can be kept at any desired degree, and there is also certainty of an absolute uniformity of the whole metal because of the movement of the bath. In the mixer there would also be a thorough desulphurization of the metal, so that the product would undoubtedly meet the most rigid re- quirements of quality. The electric arc furnace would also do well here. On the other hand, if the mixer metal is to be used for steel- making, and then subjected to subsequent refining processes, heating with ordinary fuels would still in most cases be the more economical. Further, here again the known calculations give weight on one side or the other, namely, which is the more expensive under the conditions present, heating with electricity or direct heating with fuel? THE REFINING OF PIG IRON The refining of pig iron can be carried out very well in the electric furnace, and just as well by the oxygen of ores as by that of the air. In general, the induction furnace here would also come into consideration, for the refining process can be carried out in the arc furnace without great electrode loss and use of consid- erable refining material. The iron could be melted direct in the electric furnace, or the liquid metal could be charged from the blast fur- nace, mixer, cupola, open hearth or special furnaces, after the liquid metal had been previously refined, that is, desul- phurized, etc. Refining with ore in the electric furnace is, however, expen- sive because the reduction of the ore takes place slowly, exactly as in the Talbot, Bertrand-Thiel, and other processes, so that the current consumption caused by radiation is too great. In order to accelerate this reduction, and so save electric energy, one could consider charging the ore highly heated, if peat ^ or some other fuel not suitable for steel-works furnace operation is readily accessible. Concerning the thermal advantages brought about 424 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY by the use of highly heated ore, the following approximate rough calculations give some information. The pig iron contains 3% carbon, and will be refined by pure magnetite heated to 800 C. According to the equation: 232 kg. Fe 3 O 4 + 48 kg. C = 168 kg. Fe + 112 kg. CO, the 30 kg. carbon that are in a metric ton of iron require - 45 = 145 kg. ore. This amount of ore heated to 800 C. holds 145 X 0.2 X 800 = 23200 cals., which equals - = 26.8 864.5 kw. hrs., an amount that helps the electric furnace considerably, so that the use of preheated ore is worth consideration, if the cost of preheating is not too high. There are also proposals to carry out air-blast refining, similar to the Bessemer, in the electric furnace. With arc furnaces the electrodes would have to be drawn up high during the blowing, so that during this operation no heat would be supplied, and the bath would chill, if there were not sufficient silicon and phos- phorus present to balance the heat lost, and bring the metal to the casting temperature of soft steel. In this case the bath must be alternately electrically heated, then blown for a short time, but this gives so many operating troubles that the intended saving due to time saved with blowing is not realized. Iron low i silicon and phosphorus, that cannot be handled by either the acid or basic Bessemer, may be refined with a blast of air in the induction furnace, for here the bath can be heated during the blow. The following rough calculations give some information on the probable results with a lo-ton furnace and a pig iron with 3% carbon, and a temperature of 1300 C. 10 tons iron contain 300 kg. carbon, which would require (12 C + 160 = 28 CO) - = 400 kg. O, 100 kg. air con- tain 23 kg. O, so that 400 kg. O correspond to 1740 kg. air. The temperature of the bath must be raised from 1300 to 1650 C., that is 35oC. The blast may leave the bath at an average temperature of THE ELECTRO-METALLURGY OF IRON AND STEEL '425* 1500 C., although this value is probably too small, for the carbon in the bath heated to 1300 C. will burn at a high tem- perature, and it appears doubtful whether the very hot gas produced will give its heat to the bath completely enough to escape at only 1500 C. There is therefore the following amount of electric energy conducted to the bath. 10000 kg. iron heated 350 C. 10000 X 0.2 X 350 = 700,000 1740 kg. air heat to 1500 C. 1740 X 0.3 X 1500 = 783,000 1,483,000 Brought in: 300 kg. C burnt to CO 741,900 Leaving 741,000 This corresponds to -g^ =857 kw. hrs. Therefore an electric induction furnace of 10 tons capacity will operate with about 800 to 900 kw. hrs. If the efficiency of the furnace is taken as 60%, then 480 to 540 kw. will be sufficient to heat the bath. This shows that the carbon can be thoroughly removed, in -f to , that is iK~i^ hours. The time interval 400 540 under these conditions compared with that of the other air-blast refining processes is very considerable. Air-blast refining must therefore be carried out extremely slowly, or, with frequent interruptions. The proposals to refine in this way have so far found no practical application. From all this it follows that, if there is a mixer available it is a good thing in all cases to bring about as complete refining as possible in the mixer, and so relieve the electric furnace. Better economic results are obtained in refining pig iron if large amounts of mild steel scrap are available to melt with it, so that only a hard steel-like product remains to be treated. The problem of pig-iron refining in the electric furnace now approaches solution, because, for example, for railroad material there is an inclination to use harder qualities of steel than are 426 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY used now. If, in a case like this, the pig iron to be used is sufficiently low in phosphorus so that no refining is necessary, to give a steel low enough to meet specifications, then carbon alone has to be removed. In such a case the electric furnace could work economically in many places. Also with specifica- tions calling for very low phosphorus in the steel the bath high in carbon and phosphorus can be dephosphorized without re- moving the carbon, as mentioned above, by keeping the tem- perature low and forming an easily fusible basic slag, containing oxide of iron, which is drawn off when the phosphorus is low enough in the steel. The refining of pig iron in the electric furnace is not advan- tageous if a low carbon, absolutely phosphorus free, material has to be made from a high phosphorus pig iron. In order to remove the phosphorus the carbon must first be completely taken away, and the bath even overrefined to a certain extent. In general for this purpose the electric furnace cannot compete economically with the open hearth. Further, in this case the cost of fuel and of current have to be weighed against each other. If the pig iron to be refined is high in silicon, as well as phosphorus, two electric furnaces can be used, one with an acid lining for removing silicon and carbon, the other with a basic lining to remove the phosphorus. Still such an iron can be worked in the basic furnace in which case a sufficient amount of lime must be added to prevent the lining from being attacked. Fi- nally, the deoxidation, etc., can be carried out in a third furnace with an acid lining, or in crucibles, and many combinations of the crucible, open hearth furnace, mixer, converter, etc., are possible, the suitability of which must be decided for each separate case. The output when refining with ore is extraordinarily high, because there is no loss and because of reduction from the ore, so that it is over 100%. THE ELECTRO-METALLURGY OF IRON AND STEEL 127 THE PRODUCTION OF SPECIAL QUALITY STEEL IN THE ELECTRIC FURNACE High quality steel production aims at the melting of the softest to the hardest qualities as desired in both alloy and plain steels. Steels that will meet the most rigid requirements in regard to low sulphur and phosphorus on the one hand, and on the other hand be as free from oxygen as possible and in this respect be equal to crucible steel. The best material up to the present has been made by the crucible process, special care being paid to the kind of raw material charged. The use of the purest materials is a first requirement for the crucible process, for naturally no removal of sulphur and phosphorus is possible to any considerable extent. Indeed this dependence on certain kinds of iron, which meet the guarantee of absolute purity, altogether apart from the cost, has finally been the reason for the introduction of the electric furnace, as it made the material forming the charge independent of a fixed source of supply. The general strike in Sweden during 1911 has opened the eyes of the leaders in this industry to the disadvantages that may come when one is forced to use a certain material alone. In comparison with this the electric furnace offers the great advantage that it is not dependent on any certain special raw material, for the most impure materials can be refined so that they become even better than the purest Swedish charcoal iron in regard to purity from phosphorus and sulphur. At the same time deoxidation takes place just as completely as in the crucible because a purely reducing atmosphere is maintained, the steel can be held as long as desired, and the temperature can be regula- ted with more certainty than in furnaces heated with fuel. Sim- ilar conditions are not offered by any other metallurgical furnace, for in them the action of the flame on the bath cannot be entirely prevented. From all this it is seen that electric steel is at least of equal value to crucible steel, for it can be produced practically free from phosphorus, sulphur, and non-metallic inclusions. There- 428 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY fore the use of the electric furnace gives the advantage, that in it ordinary low carbon steel can be improved and made equal to the very best qualities of crucible steel, for from the low carbon steel: (1) Phosphorus and sulphur are completely removed. (2) It is totally deoxidized. (3) It is freed from slags and inclusions. (4) It is accomplished at a lesser cost per ton. PRODUCTION OF SPECIAL QUALITY STEEL IN THE ELECTRIC FUR- NACE FROM PREVIOUSLY REFINED METAL WITH LOW PHOSPHORUS AND SULPHUR It is a side issue in what way the steel is prerefmed, whether in the converter, in the basic or acid open hearth, or in any other way. Also the material can either be charged liquid or cold, but in the latter case, the electric furnace will also be used for melting. If the electric furnace is worked in combination with an ordinary steel plant from which it obtains its charge, then it is most suitable to pour a part of the steel works charge in- to the electric furnace before the deoxidizing additions are made. On the other hand, if larger heats are made in the steel works than the electric furnace is able to take, or for other reasons, then the bath of steel to which the additions have already been made can be partially poured into the electric furnace. Naturally it is preferable that the electric furnace be able to take the whole heat with the restriction that the building of very large electric furnaces is at present troublesome. In regard to this the first part of the book should be consulted. In the case we are considering, the aim of the electric furnace ' is to improve the steel and to produce a material of equal value to open hearth, or crucible steel, in particular: (1) To recarburize the bath to the required hardness. (2) To deoxidize the bath. (3) To bring about the removal of ga and slag inclusions. (4) To alloy the bath as desired. Such a process can be profitable, for example, if there is a Bessemer plant operating, and it is desired to produce from the THE ELECTRO-METALLURGY OF IRON AND STEEL 429 Bessemer steel material equal to high grade open hearth, which will be required for various purposes such as boiler plate, etc. Such material finished by the electric furnace is extremely suitable for particular purposes, so much the more that it rolls and forges well and shows considerably increased physical properties. There is also less second-grade material. If material is charged into the electric furnace to which no deoxidizing additions have been made, then it has to be first completely deoxidized, and the production of steel of a satis- factory quality requires, in the first place, that this deoxidation be carried out very carefully. As shown in a previous chapter it can be done in many ways, with ferro-manganese, ferro-silicon, etc. Which of these materials should be used depends on the quality of steel that has to be produced, particularly whether it is to be a low or high manganese. If a product is to be made as low in manganese as possible then it is best to carry out the deoxidation with ferro-silicon. Immediately after pouring the charge into the electric furnace the first addition of ferro-silicon should be made, preferably in pieces about the size of one's fist, and at the same time the bath should be covered with an easily fusible slag to exclude the air. The kind of slag is governed by the furnace lining: with a basic hearth a neutral or basic slag is charged; with a neutral or acid hearth, on the other hand, one of greater acidity. A suit- able mixture of lime and sand with more or less fluor-spar may be used to form the slag, all of proper size. In regard to the amount of slag it should be mentioned that the bath has only to be completely covered. Furnaces of greater capacity that work with a deep bath use, therefore, a lower percentage of slag than those of less capacity which expose more surface per ton. The first addition of ferro-silicon is given, before the bath is covered with slag in order to save time in the first place, and secondly to prevent the light ferro-silicon from being en- closed in the slag, which is quite thick at first. The slag first turns black due to the absorption of oxide of iron from the bath, for a condition of equilibrium is formed between the oxide dis- 430 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY solved in the bath and the slag. For complete deoxidation it is therefore absolutely necessary that the slag contain no oxide, and so it must always be snow white. This is produced by sprinkling a suitable reducing agent on the slag when it shows a dark color, and maintaining a neutral or reducing atmosphere. In the induction furnace fine ferro-silicon, about pea size, is used in this way, being added from time to time in small amounts. In the arc furnace this ferro-silicon can be produced from the slag if carbon is added. Which of the two methods is the cheaper we will not investigate. If the slag keeps snow white then the melter takes pouring tests and convinces himself of the condition of the steel, and if it does not yet pour quietly, adds more pieces of ferro-silicon to the bath until a further test gives a good result. A completely deoxidized steel, melted with a white slag, must pour without trouble. Alloys such as nickel, manganese, chromium, etc., can now be added in the theoretical amounts, for no slagging of these additions can take place under the white slag covering. No preheating is necessary, and also the cheaper ferro alloys, high in carbon, can be used in the induction furnace for, due to the movement of the metal bath in this furnace, it is impossible for carbides to remain undissolved. In making a steel with low to average manganese content it is the best to give first an addition of ferro-silicon, after which the bath is covered with slag. The final deoxidation can now be made with ferro-manganese, spiegel, etc. After this the slag will first darken, due to the manganese reacting with the oxide of iron forming MnO, which enters the slag. As mentioned above the first requirement for complete deoxidation is that the slag be snow white. The black slag produced must, therefore, be reduced in a suitable manner and made white. In the induction furnace ferro-silicon again serves as a suitable reducing agent and carbon in the arc furnace, both of which are sprinkled on the slag. The reduced manganese goes again into the slag. It therefore passes through a cycle and really only serves as a bearer of the oxygen contained in the metal, so that the deoxidation can be carried out with very small THE ELECTRO-METALLURGY OF IRON AND STEEL 431 amounts of manganese, and a final material with a moderate percentage of manganese can be produced. Instead of ferro-manganese, manganese ore can be used, and in the induction furnace this ore is selectively reduced by ferro-silicon rather than with carbon, but in the arc furnace on the other hand the one would be reduced by carbon under the influence of the arc. It is not necessary to consider here whether it is the cheaper to use ferro-manganese melted in the blast furnace or to reduce manganese in the electric furnace from manganese ore. The remainder of the process of melting is the same as that described under the production of steel free from manganese. If the material has to be harder, that is higher in carbon than the material charged, then after the first addition of ferro- silicon before the slag is made, the necessary amount of carbon is added to the bath. A small excess is given, depending on the size and the physical condition of the carbonizing material used, for a part is burned as it is charged into the furnace. Then comes the slag formation, and it is well to take a test and make a quick color carbon determination to see whether the metal is of the right hardness. The further process is the same as that used for the production of low manganese steel. The tempera- ture is held at such a degree that the small impurities caused by the reduction can separate readily and is gradually increased to the proper casting temperature. It must be remembered that the slag formation requires a certain amount of heat as also the solution of carbon, if any is added, and so cools the bath. At the beginning of the debxidation it is well to give an addition of ferro-silicon even when melting high carbon material. If carbon is added to the bath before the ferro-silicon then there is a vigorous action, and a considerable loss of carbon cannot be avoided. At the most, therefore, one can only add a part of the carbon before the ferro-silicon, and after the deoxidaticn the rest must be added in a special operation to give the hardness required. Also by the addition of carbon before the ferro- silicon scarcely any ferro-silicon will be saved, and on the other 432 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY hand it only increases the time of the operation and the work in the furnace. If the ordinary forging and pouring tests are favorable then the casting of the heat is proceeded with. The slag also is poured into the casting ladle in order to protect the metal from the influence of the air in the first place, and secondly to prevent the slag sticking to the hearth of the empty furnace, and attack- ing the lining. Due to the total deoxidation the electric steel casts so quietly that the addition of aluminum to the stream during pouring is absolutely unnecessary, so that in this respect the quality of the metal does not suffer. Casting is carried out in just the same way as in plants making high quality steels. As a conclusion it may be mentioned that during this finishing process considerable desulphurization is brought about by the silicon present, even when it is not intended, so that in this respect the after treatment of low carbon steel in the electric furnace means a raiher considerable improvement in quality. If an addition of ferro-manganese for deoxidation has already been made in the converter, open hearth, etc., then after pouring into the electric furnace an easily fusible basic slag alone has to be made and kept constantly white, that is free from oxide. This is brought about as mentioned above by ferro-silicon in the induction furnace or just as well by carbon in the arc furnace, where silicon is reduced from the slag by the influence of the arc. This assumes that the steel poured into the electric furnace already contains the necessary percentage of silicon, but if this is not the case then before the formation of the slag the corre- sponding addition of ferro-silicon is given. Naturally in this process also there is a lowering of the sulphur of the charge, even if such is not intended. PRODUCTION OF SPECIAL QUALITY STEEL IN THE ELECTRIC FURNACE FROM PREVIOUSLY REFINED METAL WITH CON- SIDERABLE PHOSPHORUS AND SULPHUR Here also it does not matter in what way the steel is pre- refined, or whether it is charged hot or cold so that the electric furnace has to be also Used as a melting furnace. The aim of THE ELECTRO-METALLURGY OF IRON AND STEEL 433 the after treatment in the electric furnace is to raise the quality either to that of open hearth or the best crucible steel. The metallurgical process in the electric furnace must therefore im- prove the steel in regard to the following points: (1) Eliminate the phosphorus. (2) Deoxidize and desulphurize. (3) Remove the gas and slag inclusions. (4) Recarburize or alloy according to requirements. In regard to the removal of phosphorus and sulphur both elements cannot be removed from the bath in one operation, for the removal of the phosphorus takes place by an oxidizing or refining process, and that of the sulphur, on the other hand, by a reducing process: FeS + CaO + C = Fe + CaS + C O. These operations must, therefore, be carried out one after the other, and it is similar in principle whether the bath is de- sulphurized first and then dephosphorized, or, on the other hand, dephosphorized first and then desulphurized. Which of the two ways is the most suitable depends on the composition of the charge that is put into the electric furnace and on the kind of steel to be produced. If the metal as charged has had no additions so that it is not yet deoxidized, then it is best to dephosphorize first of all as the necessary conditions are present. The total removal of the phosphorus requires an overoxidation of the bath, so that the metal gives a seamy, that is a red short, forging test. When using a charge that is still oxidized therefore only a basic slag has to be charged to favor the soaking of the bath with oxygen, and at the same time give conditions so that the phosphoric acid formed is immediately combined with lime. The require- ments in regard to freedom from sulphur on the part of the slag- making materials are not particularly high, for the bath has to be desulphurized afterwards anyhow, but it is well to use materials as low in sulphur as possible, so as not to raise the sulphur in the bath. Also the phosphoius percentage of the ore is without influence, for an oxidized bath cannot reduce phosphorus, and 434 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY is therefore unable to take phosphorus from the ore or lime- stone. If it is desired, on the other hand, to desulphurize first then the bath must be first completely deoxidized, and after the re- moval of sulphur the bath must be oxidized again to remove the phosphorus. The whole manipulation of the deoxidizing, there- fore, gives no lasting result, and at the same time only the purest ore and limestone can be used to form the refining slag so as to prevent absorption of sulphur by the bath. On the other hand if the charge has already had an addition of f erro-manganese in the converter, open hearth, etc. ; and if a very soft steel has to be made with the lowest possible phos- phorus and sulphur and also practically free from silicon, then it is best to desulphurize first. In this way one can work with a small amount of silicon, and the low silicon remaining in the bath is removed during dephosphorizing. The heat is then finished with the addition of ferro-manganese, and is cast just as in the open hearth process, keeping the slag back. Here also it is necessary to use ore and lime free from sulphur to prevent the bath again taking up sulphur during the subsequent opera- tions. The quality produced is, however, only equal to that of good open hearth not crucible steel. If sulphur-free burnt lime is not available then it is well to use raw limestone if the sulphur is low enough. Apart from this and some other special cases the charge, prerefined and then poured into the electric furnace, will always be dephosphorized first whether deoxidizing additions have been already given in the preliminary furnace or not. This is done because: (i) With desulphurizing first the silicon used is again removed during the dephosphorizing. Therefore the amount of ferro-silicon used is unnecessarily in- creased; (2) the total deoxidation, which is necessary for de- sulphurization, would be made of no use by the subsequent oxidation, (3) the ore and limestone used for the refining slag have to be very free from sulphur. Immediately after pouring the charge into the electric furnace a refining slag should be formed with ore and lime. The size of these materials should not be too great as otherwise THE ELECTRO-METALLURGY OF IRON AND STEEL 435 they only fuse together with difficulty, that is, the formation of the slag takes too long and the time of the heat is increased. On the other hand there is no limit to the fineness of the material, so that, for example, unbriquetted concentrates can be used. The burnt lime is best broken up just before charging, for it quickly takes up moisture and carbon-dioxide from the air. The amount of slag necessary is proportionally small, espe- cially if dephosphorization has already taken place to some extent, for instance, to 0.1%. The bath need not be well covered by the slag, although dephosphorization naturally takes place more quickly if the slag covering is not too small. On the other hand it is well not to unnecessarily increase the amount of slag so as to avoid loss of heat. In the induction furnace, work hi a high manganese charge, that is one already deoxidized in the first furnace, it is well to make a slag with i% ore and 2% lime of the weight of the charge. If the forging test shows that the bath has the right percentage of phosphorus, then the slag is drawn oil", and the last traces removed by means of fresh lime thrown over the bath. This thickens the remainder of the slag so that it can be easily removed. If a high phosphorus charge is to be worked, then it is well not to charge the whole amount of slag necessary at one time for, as mentioned before, it is not recommended to work with too large a slag volume in the electric furnace. In this case it is better to charge the ordinary small amount of slag, remove it when completely used up, and then form a new slag of the same weight. This should be repeated as required. Such a case can happen in practise if the electric furnace charge is taken from a heated prerefming furnace, such as a Wellman-Talbot furnace, in which by exceeding the capacity a material is produced that is high in phosphorus. If the electric furnace is to be used continuously for the refining of such high phosphorus charges, then it is well to figure on this in the construction of the furnace. A shallow bath but a large surface should be used in order to give the refining slag a large attacking surface, and to shorten the time of the process. On the other hand the surface of the bath should not be too great 436 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY for, in this way, the radiation loss increases and the efficiency of the furnace drops. These are important points for the de- signers of the furnace, namely, to make the hearth the right size and shape to properly meet the conditions, and in this respect experience obtained with open hearth furnaces will be valuable. If a high phosphorus charge is worked, with several refining slags, then the first slag is very low in iron but high in lime and phosphoric acid. These slags are of some value in agriculture as low phosphate slags, so that they need not be thrown away. The succeeding slags, however, are rich in oxide of iron and low in phosphorus, and can be used as a first slag for subsequent heats and so be used more completely. If the fluxes are used to exhaustion then the consumption of ore and lime, in reference to the finished material, is considerably reduced. The bath can now be deoxidized, carburized, and desul- phurized. The removal of the oxygen and sulphur takes place together, the first by means of ferro-silicon, carbon, or ferro- manganese depending on the kind of material to be melted; the latter by the addition of ferro-silicon and in the arc furnace by silicon or calcium reduced from the slag by carbon under the influence of the arc. First an addition of ferro-silicon is made that can be a little more in amount than is necessary for deoxidation alone because of the desulphurization also taking place. In general the opera- tion is exactly the same as described in the previous section, namely: "The production of electric furnace material from previously refined metal with low phosphorus and sulphur." It may be mentioned that tungsten has similar desulphuriz- ing properties to silicon, so that tungsten heats can also be made extremely low in sulphur. A peculiar phenomenon must be mentioned which can happen under certain conditions with non-expert handling of the electric furnace. As already said a well deoxidized charge pours quietly, and only pipes a little on solidifying, depending on the temperature and the silicon. On the other hand, if the heat is made too hot, if the bath is not completely covered with slag, if air can enter the furnace, or if the deoxidation slag is THE ELECTRO-METALLURGY OF IRON AND STEEL 437 not kept sufficiently free from metal, then the steel casts very badly, even though it may contain several tenths per cent, of silicon, the forging tests will show the properties of an oxidized material. The causes for this phenomenon are not at present very clear, but it appears probable that at high temperatures a part of the silicon occurs dissolved in the metal as a suboxide, probably with the formula SiO. Because of the similarity between silicon and carbon the possibility of an alloy of iron and silicon-suboxide can be thought of, for, as mentioned before, the existence of an alloy of iron with carbon-monoxide is probable. THE METALLURGICAL COURSE OF AN ELECTRIC FURNACE CHARGE The course of the metallurgical reactions in the Heroult and the Rb'chling-Rodenhauser furnace is given in the two accompanying diagrams (page 409). The curves for the Her- oult furnace were pubb'shed by Thallner in No. 5, 1909, of Kohle und Erz, and are taken from a heat in a 3-ton furnace; while the diagram of the Rochling-Rodenhauser furnace is taken from an ordinary heat made at Volklingen in an 8-ton single- phase furnace built up to take 5 to 6 tons. From a comparison of the two diagrams it is seen first that the time of heat in the Heroult was twenty minutes longer than in the Rochling-Rodenhauser furnace, notwithstanding that the former was only worked with a 3-ton heat, while the latter had 5 tons. Also, the material made in the Rochling-Rodenhauser is at least just as pure as that produced in the Heroult, notwithstand- ing that a much more impure charge was worked in the former. The oxidation period is distinguished, in the Rochling-Roden- hauser furnace, especially at the beginning, by an extraordinarily quick removal of the phosphorus and manganese from the steel. For instance, in the first twenty minutes the phosphorus drops from 0.06 to 0.025, that is 0.035%, while in the Heroult furnace it is only lowered from 0.03 to 0.02, that is 0.01%, in the same time. Also the manganese drops from 0.49 to 0.12, that is 0.37% in 438 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY the first hour in the Rochling-Rodenhauser furnace, while in the Heroult furnace in the same time it is only lowered from 0.21 to is 0.1%. From this it follows that the Rochling-Rodenhauser furnace must be considered as a good oxidizing furnace. The lowering of the phosphorus content of the slag, which is to be noticed during the oxidation period with both furnaces, is due to the slag being diluted from time to time by the addition of roll scale. The removal of sulphur during the oxidation period is rela- tively unimportant with both furnaces, and only after the recarburization, or after the formation of the final slag, does the real desulphurization begin. During this period the sulphur is lowered in the Heroult furnace from 0.07 to 0.012%, while in the Rochling-Rodenhauser a desulphurization from 0.065 to traces is brought about. That the ability of the slag in the Rochling-Rodenhauser furnace to absorb sulphur is at least as great as that in the Heroult furnace is seen from the sulphur content of the slag, which is 1.25% in the first case, and only about 0.06% in the latter as shown by the curves. The amounts of slag-making constituents used in both cases are shown in the diagrams, so that all the details of the refining operation are given that are of interest. THE SPECIAL QUALITIES OF ELECTRIC IRON AND STEEL As already shown any material from the mildest to the hardest quality can be made in the electric furnace. Electric furnace material is distinguished by its freedom from gas and slag inclusions, and can easily be produced with very low man- ganese and completely free from phosphorus and sulphur, and as soft and forgeable as the Swedish qualities. This low carbon electric steel can easily be alloyed, for instance with silicon, for making material for dynamo plates, etc., the production of which in the open hearth furnace is troublesome because of the necessary low casting temperature. The use of the electric furnace for this purpose means, therefore, considerably easier operation. Also the softest material can be flattened out very thin without showing red shortness, and can be used for punching THE ELECTRO-METALLURGY OF IRON AND STIiKL 439 lU-ducinff period Refining curves for the Rochling-Rodenhauser Refining curves for the Heroult furnace, furnace. 440 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY and deep drawn work, deep stamping, etc., for everything where value is put on good malleability; also for the production of chains, the making of tools, etc., and further in those cases where especially soft open hearth qualities from Sweden must be used, such as seamless tubes, horseshoe nails, etc. On account of its purity this low carbon electric steel is much less inclined towards segregation than ordinary low carbon steel, and on this account should be particularly used where the highest require- ments of absolute certainty against brittleness are necessary. In the electric furnace construction steels of any degree of hardness can be produced, of any desired physical properties and chemical analysis, also alloyed with chromium and nickel where it is a question of meeting the highest specifications. These steels at present must be made in the crucible. With the large heats 1 possible in the electric furnace a cer- tainty of absolutely uniform composition is guaranteed for the finished steel, such as is suitable for the production of large forgings low in manganese. The electric furnace material can be easily hardened, and on account of its homogeneity and freedom from slag is an excellent material in such cases where the surface must be dense* and highly polished and show no cracks, such as running taps, etc. In general Plates 3 and 4 show what high requirements are in every way satisfied by elec- tric steel. FINAL CONSIDERATIONS For the smelting of ore the electric hearth as well as the shaft furnace is to be considered, and in each particular case it must be carefully decided which type of furnace has the advan- tage. If very finely divided ores, high in sulphur, are to be worked up into steel by means of small sized reducing material, then the induction hearth furnace should be chosen because of the possi- bility of producing steel direct from such raw materials of any quality desired. Also because when changes have often to be 1 See Osborne Amer. Electro-Chemical Society, 1911, Vol. XIX. * For quality of steel, see also Vom Baur, American Foundrymen's Associa- tion, May, 1911, page 247. THE ELECTRO-METALLURGY OF IRON AND STEEL 441 made in the kind of metal produced, the making of valueless transition products is avoided. On the other hand, if coarse low sulphur lump ore and fuel are available, then the induction shaft furnace should be chosen, especially if the same quality of metal is always to be made, and the reducing material is high in price. The electro- thermal smelting of iron ores can naturally only be considered economically when the saving of coke, etc., compared with the ordinary blast furnace operation is greater than the ex- pense of the necessary electric power, so that it is dependent on the local prices of coke, etc., on the one hand, and electric energy on the other. Electric ore smelting will, however, be favored when one considers that considerably less capital is necessary for the plant than for the building of an ordinary blast furnace plant with the same output. Also the depreciation, etc., per metric ton of iron produced, are considerably lower than with the ordinary blast furnace. It must be further remembered that the quality of electric pig iron is higher than charcoal pig iron, and therefore it should command a higher selling price than the best charcoal iron. For steel making an iron can be readily made low in silicon, which only needs removal of carbon to make steel and forgeable metal. If this refining is carried out in the electric furnace, then it has to compete with the open hearth furnace. Recently Engelhardt at the meeting of the "Verein deutscher Ingenieure," in Berlin, made an interesting comparison between the open hearth furnace on the one hand, and different types of electric hearth furnaces on the other, namely the Heroult, the Girod, and the Induction furnaces. For medium furnace sizes with these three types the produc- tion per h.p. day, with a cold charge, is taken as 20 kg. Cer- tainly this treats the induction furnace somewhat unfavorably, for it has about 10% greater efficiency. The given power con- sumption corresponds to 880 kw. hrs. per metric ton of steel. The electrode consumption, according to the most recent publica- tions, amounts to 28 kg. per metric ton in the Heroult furnace and 17 kg. in the Girod, while with the induction furnace, of 442 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY course, there is not any. The consumption per metric ton of steel is therefore: With the Heroult furnace 880 kw. hrs. + 28 kg electrode. With the Girod furnace 880 kw. hrs. + 17 kg. electrode. With the Induction furnace 880 kw. hrs. + o kg. electrode. In the following table the results of calculations are given to show how much the kw. hr. ought to cost in order that the electric furnace may compete economically with the open hearth furnace using a certain amount of coal per ton at a certain price. The electrodes are taken at 26 marks per 100 kg. ($61.90 per metric ton): OPEN HEARTH FURNACE MAXIMUM COST PER KW. HR. IN CENTS WITH THE Coal Price Cost of coal consumption of coal per per metric ton Girod Heroult Induction by weight metric ton output 25% $3-57 $0.89 minus minus 0.1000 4.76 I.I9 0.0143 " 0.1333 5-95 I. 4 8 o . 0476 " o. 1690 7-14 I. 7 8 0.0833 0.0048 0.2023 30% 3-57 1.07 0.0024 minus o. 1214 4.76 1.42 0.0428 " 0.1618 5-95 I. 7 8 0.0833 0.0048 0.2023 7-14 2.14 0.1238 0.0452 0.2428 35% 3-57 1-25 0.0214 minus 0.1404 4.76 1.66 0.0690 " 0.1880 5-95 2.08 0.1166 0.0381 0.2356 7-14 2.50 0.1642 0.0857 0,2832 40% 3-57 1.42 0.0428 minus 0.1618 4.76 1.90 0.0952 0.0190 0.2142 5-95 2.38 0.1500 0.0714 0.2690 7.14 2.85 0.2047 0.1261 0.3237 It must, however, be remembered that even when producing an ordinary open hearth quality of steel in the electric furnace, material is produced with improved physical properties so that even with a continued regular output there should be a small increased price. If this increase in price is only 5%, and the price per metric ton of open hearth quality be taken as 140 marks THE ELECTRO-METALLURGY OF IRON AND STEEL 443 PLATE II Single Tube test piece from Plate I. 444 ELECTRIC FURNACES IN THE IRON AND STEEL INDUSTRY ($33.33), then there is a surplus per ton of 7 marks ($1.66), which with a power consumption of 880 kw. hrs. per ton makes almost 0.2 cents that the price of power may be increased over the value given in the table. It may be further mentioned that the electrode consumption figures taken by Engelhardt appear somewhat too high, in view of the most recent figures given in Part I of this book, which vary from 10 to 15 kg. per metric ton of solid charge. Therefore the table (on page 304) may be referred to where the electrode consumption, however, is not considered at all; and where the heating costs alone are compared, on the one hand with the use of fuel and on the other with electricity. This table, therefore, gives results similar to those in the table on the preceding page for induction furnaces, with which it agrees exactly. The result of all this is that the electric furnace will not only play an important role in the future, but that it is already a factor which each iron and steel plant must now carefully con- sider. INDEX Abbreviations used, xviii Action of the electric current, 26-44 Additions, 311, 386 Advantage, chief, of electric furnace, 74 Advantages of the electric furnace and steel, 65-66, 74, 302, 408, 413, 419,428 Alternating current, comparison single and polyphase generators, 70 current theory, 47-63 polyphase current in general, 60 Aluminum, 411, 432 Amperes, unit of measurement, 12 Angular velocity (m = 2 T: v), 48 Applicability of the electric furnace (general), 73-74 of the Girod furnace, 158 of the Heroult furnace, 144 of the Kjellin furnace, 204 of the R. & R. furnace, 235 ^ of the Stassano furnace, 122 Arc furnaces in general, 77-79 heating, 37 important points concerning, 109 lengths with the Heroult furnace, 134 with the Stassano furnace, 119 temperature of, 78, 105 the electric, 77-79 Arrangement of an electric pig-iron furnace, 247, 364 of a Girod furnace, 150 of a Heroult furnace, 129-134 of a Kjellin furnace, 190 of a Rochling-Rodenhauser furnace, 209-215. of a Stassano furnace, 114 principle of induction furnaces, 180 tilting furnaces in general, 72 the Girod furnace, 157 the Heroult furnace, 143 the Kjellin furnace, 203 the Rochling-Rodenhauser furnace, 231 the Stassano furnace, 121 Arsenic, 406 Auto-regulating transformer, 230 Auxiliary apparatus, 315, 316 Basic bottom and lining material, 296 445 446 INDEX Basic bricks, 296 Basic laws of electricity and magnetism, 1 1-25 Blast furnace, total operating cost, comparison, 317 Booth-Hall furnace, 276 Borchers' laboratory furnace, 33 Bottom electrodes, influence of the Chapelet furnace, 257 of the Girod furnace, 161 of the Keller furnace, 242 Bricks, basic, 296 carbon, 172 carborundum or silicon carbide, 295, 322 Dinas, 295 dolomite, 296 half Schamotte, 295 half silica, 295 magnesite, 296 silica or acid, 295 Canadian Commission, report of Haanel and, 9 Carbon, 412, 418 bricks, 172, 295, 302, 322 electrodes, their efficiency, 84 their influence with the Girod furnace, 159 with the Heroult furnace, 140 with the Stassano furnace, 117-123 heat conductivity and specific resistance, 91 mixtures for lining, 295 necessary per ton of pig-iron for desulphurizing, 354 Castings, electric steel, 416 Cast iron, low, phosphorus, for thin-walled castings, 414 melted in electric furnace advantage, 414 Chapelet 's ore furnace, 257-258 Charge, course of operations of electric furnace, 437 nature of, 301 Chemical action of the electric current in electric furnace, 37-38 balance, for ore smelting, 344 Chrome iron ore, 297 Clay used as a binder, 294 Colby furnace, 185 medal for, in consideration of originality of his furnace, 182 Cold charge, melting in the Girod furnace, 164 in Heroult furnace, 132-141 in Kjellin furnace, 192-200, 208 in Rochling-Rodenhauser furnace, 232, 235, 223-236, 239 in Stassano furnace, 116 Combined arc and resistance furnaces, 79, 260 resistances, 20 arithmetical examples, 21, 22 INDEX 447 Comparison of costs, crucible and electric, 306 open hearth and electric, 304, 442 Compressed air hammers, 299 Conductivity, 12 Conductor, resistance of a, 12 Conductors, action of two on each other, 43 between magnet and electric, 40-41 of the second class, 15-17 Construction of Girod furnace, 149 of Heroult furnace, 130 of Kjellin furnace, 189 of R. & R. furnace, 213 of Stassano furnace, 1 1 1 Cooling of the electrodes, 99-102 Copper, 406 Cos