LIBRARY OF THE UNIVERSITY OF CALIFORNIA. Class ELECTRO-MAGNETIC ORE SEPARATION BY C. GODFREY GUNTHER WITH ILLUSTRATIONS 1909 HILL PUBLISHING' COMPANY 505 PEARL STREET, NEW YORK 6 BOUVERIE STREET, LONDON, E. C. The Engineering and Mining Journal Power American Machinist /t'C COPYRIGHT, 1909, BY THE HILL PUBLISHING COMPANY ENTERED AT STATIONERS' HALL, LONDON, ENGLAND All rights reserved Hill Publishing Company, New York, U. S. A. PREFACE THIS book has been prepared to gather into convenient form the published information on the magnetic separation of ores. The compilation has been supplemented by data from the writer's obser- vations and an extensive correspondence with mill managers and manufacturers. It has been attempted to include only that which is of present commercial importance. The writer wishes to express his thanks to the many who have aided him in the collection of data for this work, and especially to Messrs. W. R. Ingalls, W. L. 'Austin, Erminio Ferraris, C. Q. Payne, Adriano Contreras, S. Norton, James Hebbard, and Ben- jamin Hodge, and also to the Humboldt Engineering Works Co., Elektro-Magnetische Gesellschaft, Marchegger Maschinenfabrik, United Iron Works Co., and the Dings Electromagnetic Sepa- rator Co. EL PASO, TEXAS, September, 1908. in INTRODUCTION THE magnetic properties of certain minerals have long been recognized, and their concentration through magnetism can lay no claim to novelty. A patent was awarded in England on a process for separating iron minerals by means of a magnet in 1792, and in this country a separator having a conveyor belt for present- ing ore beneath electro-magnets excited by cells was employed in separating magnetite from apatite in New York State in 1852. The earlier attempts at magnetic separation naturally were directed toward the separation of the most strongly magnetic sub- stances. The first separators were employed in separating iron from brass filings and turnings, metallic iron from furnace prod- ucts, and magnetite, the most strongly magnetic of minerals, from gangue. The next step in the process was in roasting, or calcining, certain iron minerals which might by such means be transformed into strongly magnetic compounds and separated from their ad- mixtures. The steady development of improved apparatus and more intense fields has constantly broadened the field of magnetic separation until minerals previously considered nonmagnetic are separated commercially. Beginning with the crude machines which employed perma- nent magnets to attract the magnetic particles and brushes to detach the material so collected, a great variety of separators has been devised and patented, and many of them have been placed in commercial operation. The magnetic separator has been developed, in most instances, for the exploitation of individual ore deposits, and the different types and modifications so produced might well form subject matter for a book. In the United States alone over three hundred patents have been granted. In view of the above facts the broad practice of magnetic sepa- ration is incapable of monopoly and its application is not de- termined by any one machine. The process suitable to the treat- ment of the ore under consideration having been carefully chosen, VI INTRODUCTION it will be found that any one of several machines will perform the functions of the actual separation. In its own field, which will be hereinafter outlined, magnetic separation is a useful adjunct to the specific-gravity processes, but it is in no sense a competitor with these processes except in the concentration of magnetite iron ores, and in this application is a succsss backed up by many years of profitable operation. CONTENTS CHAPTER PAGE INTRODUCTION v I. MAGNETISM APPLIED TO ORE DRESSING 3 II. PRINCIPLES OF MAGNETIC SEPARATION AND PREPARATION OF THE ORE FOR TREATMENT 9 III. SEPARATORS FOR STRONGLY MAGNETIC MINERALS . . 22 IV. SEPARATORS FOR FEEBLY MAGNETIC MINERALS , . . . 61 V. THE CONCENTRATION OF MAGNETITE ORES . . . . . 79 VI. THE SEPARATION OF PYRITE AND BLENDE . . . . .115 VII. THE SEPARATION OF SIDERITE FROM BLENDE , 138 VIII. SEPARATION OF MISCELLANEOUS ORES AND MINERALS 153 ELECTRO-MAGNETIC ORE SEPARATION OF THE UNIVERSITY OF MAGNETISM APPLIED TO ORE DRESSING ALL substances solid, liquid, and gaseous are either attracted or repelled by a magnet, though in most cases this influence is too feeble to be apparent except with delicately adjusted apparatus. The atmosphere has a definite magnetic attractability and the magnetic behavior of solids may be said to be controlled by the magnetic qualities of the surrounding medium; if a substance is more permeable to magnetism than air, it is attracted; if less permeable, it is repelled. The permeability of air (air being the most common medium) is taken as 1, and the permeabilities of all other substances are referred to it as unity. The permeabilities of substances more strongly attracted than air are therefore repre- sented by values greater than 1, and are called paramagnetics ; substances less permeable than air are represented by values less than 1, and are called diamagnetics. The permeability of the diamagnetics is so nearly unity that the phenomenon of magnetic repulsion is riot a familiar one. The lines of force of a magnetic circuit pass along the path of least resistance; in other words, they pass through the most per- meable substance available. Paramagnetic particles, introduced into a magnetic field, tend to aline themselves in the direction of the lines of force in precisely the same manner that a compass needle alines itself with the magnetic meridian. Paramagnetics concentrate the lines of force, while diamagnetics cause the lines of force to go around them. The passage of lines of force through particles induces magnetic polarity in them, and they gather in tufts or chains, North pole to South pole, and are all held by the energizing magnet. The force with which these particles are attracted is a function of their permeability, the intensity of the field, and the time they are subjected to its influence. The paramagnetic minerals have been divided for convenience 3 - 4 ELECTRO-MAGNETIC ORE SEPARATION into two classes : those which are attracted and held by a common permanent magnet, called ferromagnetic minerals., and those not so attracted, referred to as feebly magnetic minerals. The ferro- magnetic minerals are magnetite, pyrrhotite, ilmenite, chromite, and franklinite in typical specimens. Various investigators have made attempts to determine the specific magnetic permeabilities of minerals, but without uniform- ly reliable results. This is due to two causes : the variable mag- netic permeability of the same mineral from different localities (and even of different specimens from the same locality) and the unreliability of the available methods for determining the per- meability of minerals. The rod method, employed in testing the permeability of iron, is not applicable to minerals, as rods of sufficient length of pure minerals are not available. The methods employed are based on a comparison of the permeabilities of crushed and sized minerals with crushed and sized cast iron, or filings. The figures so obtained are affected by the size and shape of the particles tested, the amount tested, whether the charge is packed tight or loose, etc., which makes the results of comparative value only. In view of the above facts the permeabilities of various min- erals, as determined, do not form a reliable guide in the consid- eration of ores, and a preliminary test of the ore in question must be made to determine the applicability of magnetic separation, unless the ore be magnetite or one capable of transformation into magnetic oxide. While chemically pure minerals possess magnetic permeability independently of any iron they may carry, in practice it is almost always the effect of a trace or more of iron, either chemically combined or present as an impurity, that is utilized for separation. In minerals which combined iron renders separable the magnetic permeability varies more or less regularly with the amount of iron combined; but minerals which depend for their separation upon the presence of iron as an impurity are subject to wide vari- ations in permeability, and are therefore more unreliable subjects for magnetic separation. The paramagnetic metals are iron, nickel, cobalt, manganese, chromium, cerium, titanium, palladium, platinum, and osmium. Oxygen is paramagnetic (liquid air is attracted and held by a magnet) and sulphur is diamagnetic; the oxides are therefore MAGNETISM APPLIED TO ORE DRESSING 5 more likely to be magnetic than the sulphides of the same metals, and in like manner the oxides are usually more strongly magnetic than the carbonates. That the chemical composition of a sub- stance does not determine its magnetic properties, however, is strikingly shown in the mineral pyrite (FeS,, 46.7 per cent, iron) which is too feebly magnetic to be separated by the most intense field yet produced ; and also in the bromide of copper, a compound of two diamagnetic elements, which is paramagnetic. The occur- rence of strongly magnetic galena at Gem, Idaho, is another striking instance of the variable magnetic behavior of minerals. The crystalline form of a compound has an effect on its mag- netic properties, as has also water of crystallization. The tem- perature at which separation takes place also exercises an influence : Langguth (" Elektromagnetische Aufbereitung," p. 5) separated readily a zinc blende, warm, which was with difficulty effected when cold. Much has been written concerning the magnetic properties of various salts and alloys, in the investigation of which peculiar manifestations of magnetism have been observed. While throwing light, perhaps, on the magnetic behavior of matter, these results are hardly of importance in the practical subject of magnetic separation. (For the theories regarding the magnetic properties of matter the reader is referred to the writings of Poisson, Cou- lomb, Ampere, Becquerel, Weber, Burgman, Kohlrausch, Plucker, Tyndall, Faraday, Delcasse, Dolter, Wiedman and others.) THE FIELD OF MAGNETIC SEPARATION The applications of magnetism to ore dressing fall naturally under two heads : the concentration of magnetic minerals from their gangues, and the separation of two or more minerals of similar specific gravity in the products of a preliminary water concentration. Magnetic concentration has been applied principally to the treatment of magnetic iron ores, eliminating the gangue, and at the same time effecting a partial separation of phosphorus and sulphur minerals which are frequent and objectionable contaminations. The concentration of these magnetite ores is the oldest, and to-day one of the most important applications of magnetism to ore dressing. A plant for the concentration of siderite from 6 ELECTRO-MAGNETIC ORE SEPARATION gangue has been in operation in France for a number of years, and another is now being constructed in Hungary. Magnetic con- centration has also been applied to the treatment of ores carrying chalcopyrite. This mineral has a tendency to slime, when crushed, which gives rise to an important loss in subsequent wet concentra- tion; but after roasting it is readily saved by magnetic attrac- tion, even if in a fine state of division. There are other minor applications of magnetic concentration such as leucite from lava, manganese ores, garnetiferous schists, etc. In magnetic separation, as distinct from concentration, the ap- plications are more numerous and complex. There occur in nature many combinations of minerals whose specific gravities are too sim- ilar to permit of their separation by any of the usual concentrating devices. In such combinations where one of the minerals is mag- netic, or may be rendered magnetic by the application of heat, magnetism offers an efficient, and often the only, method of separation. For reasons connected with the subsequent reduction of zinc ores the presence of iron is highly objectionable, and ores which carry more than a small percentage of iron are severely penalized. This, together with the similarity of the specific gravities of the iron and zinc minerals often found together, gives rise to one of the most important applications of magnetic separation. Zinc blende frequently occurs with pyrite, marcasite or siderite, all minerals of specific gravities too similar to permit a separation by specific-gravity methods. Pyrite and marcasite are not capable of separation in their raw state, but become magnetic on roasting; siderite is separable by magnetic fields of high intensity, and may also be transformed into a strongly magnetic compound by calcina- tion. Oxidized zinc minerals also occur in important ore bodies with limonite, and here again the difference in the specific grav- ities of the minerals is too slight to permit a separation by milling methods. Limonite is slightly magnetic and may be removed in its raw state by fields of high intensity, and may also be calcined to the strongly magnetic oxide of iron and removed as such. Zinc blende carrying sufficient combined iron to be magnetic occurs in many localities in Colorado and elsewhere in conjunction with pyrite, from which it may be separated by magnetism without preliminary treatment. At Broken Hill, N". S. W., immense ore bodies carry blende to- MAGNETISM APPLIED TO ORE DRESSING 7 gether with rhodonite and garnet, minerals of similar specific gravity. The middling products from water concentration of ores; carrying these minerals are separated by magnetism. The peculiar ore bodies at Franklin Furnace, N. J., are treated exclusively by magnetic separation. Magnetic separation has found application in the treatment of monazite sands, in the separation of tin-tungsten concentrate, for the removal of magnetic contaminations from corundum, in heavy sulphide concentrates, in the separation of chalcopyrite-blende-sid- erite concentrates, and in other cases. The principal applications of magnetism to ore dressing as represented by successful installations have been stated above, but there are many other separations which are entirely practicable but not at present in commercial use. The low prices and high standards of iron ores obtaining in the United States do not per- mit of the exploitation of ore fields which in another country would be of great value. As our purer ores are exhausted, and prices rise, there will be a steady increase in application of mag- netism to the concentration of iron ores, not alone in the treatment of natural magnetite, but also for the lean hematites and limonites which cannot now be worked at a profit. MAGNETIC SEPARATION AS A PROCESS Where applicable, this process possesses all the advantages held, by other separation processes, and, in addition, is independent of gravity. A prerequisite of success in any separation process is the existence of the minerals to be separated as free particles, and in this magnetic separation constitutes no exception. Furthermore, all separating devices work better on sized material than on a mixture of coarse and fine particles. While sizing is necessary in many specific-gravity methods, it is desirable, but not impera- tive, in magnetic separation. The preparation of the ore for treatment by crushing and sizing represents, in any case, a large proportion of the total cost of the process, whether the final sepa- ration be made by jigs and tables or by magnetic separators. The magnetic separator has been developed into an efficient machine which is economical of power, both for operation and excitation of the magnets, not liable to break down or get out of adjustment, is easily operated by anyone with the intelligence neces- 8 ELECTRO-MAGNETIC ORE SEPARATION sary to operate any of the usual concentrating machines, and is not a source of large expense bills for repairs and renewals. To sum the matter up, the only difference between specific- gravity and magnetic-separation processes is that one utilizes dif- ferences in the specific gravities of the minerals to be separated, and the other utilizes the differences in their magnetic permea- bilities. Where, however, the ore must be roasted or dried before separation, this item must be charged against the magnetic treat- ment of which it is a prerequisite. II PRINCIPLES OF MAGNETIC SEPARATION AND PREPA- RATION OF THE ORE FOR TREATMENT To separate successfully a mixture of magnetic and nonmag- netic particles a separator must fulfill the following requirements : It must make a proper presentation to the magnetic field of the mixture to be separated; it must bring about the attraction of the magnetic particles by a uniform field of suitable intensity; it must remove the magnetic particles so attracted from the field and cause their discharge from the separator. PRESENTATION or THE ORE MIXTURE TO THE MAGNETIC FIELD A proper presentation of the mixture to be separated to the magnetic field is of primary importance. The ore must enter the field in such a manner that the individual particles will be free to be attracted according to their permeabilities. The ore must, there- fore, be fed in a thin, even layer or sheet in order that the mag- netic particles may not be hindered in their attraction toward the separating pole by intervening nonmagnetic particles. Theoretic- ally, this layer should be but one particle deep, and in the separa- tion of very feebly magnetic minerals this is carried out in practice. In the separation of magnetite, either natural or artifi- cial, and the ferromagnetic minerals, a deeper feed is permissible, and consequently a greater capacity for the separator. When the feed is more than one particle deep the upward rush of magnetic particles toward the pole is apt to entrain nonmagnetic particles and carry them into the magnetic product. This loss is not a seri- ous one with fields of suitable intensity; that is, with fields just sufficiently strong to attract the magnetic particles. In many sepa- rators provision is made for the removal of entrained particles from the magnetic concentrate by a blast of air or a jet of water while it is still under the influence of the field, or by the turning over 9 10 ELECTRO-MAGNETIC ORE SEPARATION of the magnetic concentrate by causing it to pass from one pole to another of opposite polarity, which operation causes the mag- netic particles to reverse their individual positions as they pass from one pole to the opposite sign. The above considerations apply more particularly to the pres- entation of the ore mixture by conveyor belts, shaking plates, drums, and rolls. When the ore is presented to the magnets as a thin sheet falling past the poles, or when the separation is carried out under water, the feed being introduced in suspension in a stream of water, entrainment is a less serious difficulty. It is also essential to good work that the feed be constant in amount and presented at a uniform distance from the separating pole, that all parts of it may be acted upon equally by the field, the intensity of which varies with the distance from the separating pole. As the intensity of the field is greatest, and the attraction consequently strongest, at the poles, decreasing directly with dis- tance from them, it follows that the ore mixture should be intro- duced into the field as near the separating pole as is practicable. The speed at which the ore is presented to the magnets, or the time the ore is under the influence of the field, is also a factor of prime importance. A definite length of time is necessary for the induction of magnetism, the time required for induction, and consequent attraction, varying inversely with the permeability of the mineral treated. That the speed of passage of the ore through the magnetic field must be regulated according to the per- meability of the mineral separated is well illustrated by the fol- lowing experiment. 1 A Mechernich separator was fitted with a thin conveyor belt passing between the poles of the magnets and so arranged that its speed might be varied at will. A mixture of minerals crushed to pass a 0.75 millimeter aperture was fed upon the belt and passed through the field at different speeds, the intensity of the field re- maining constant. With the belt traveling 100 meters per minute only magnetite was removed by the magnet ; at 70 meters rhodonite was partially removed, but ferruginous blende was quite unaffected ; at 50 meters the rhodonite was completely removed but the blende still remained unaffected ; at 40 meters per minute the blende was partially removed, and at 30 meters completely removed, the in- tensity of the field remaining constant throughout the test. 1 " Elektromagnetische Aufbereitung," E. Langguth, p. 16. PRINCIPLES OF MAGNETIC SEPARATION 11 Separators whose feed is presented to the magnets as a thin sheet falling in front of the separating poles are limited in their application to minerals of high permeability by the speed of the passage of the ore through the field. ATTRACTION OF THE MAGNETIC PARTICLES Magnetic attraction in performing a separation is opposed by some other force, usually gravity, the magnetic particles being lifted from the mixture under separation, or prevented from fall- ing when fed, for instance, upon a revolving drum or cylinder. Gravity is often supplemented by some other agency, as centrif- ugal force, a blast of air or a stream of water acting against the magnetic attraction. The opposing forces of magnetic attraction and gravity, or centrifugal force, may be delicately adjusted, and separations effected between minerals having but slight differences in permeability. The intensity of a magnetic field should be adjusted to the permeabilities of the minerals it is to be called upon to separate, and the field should be uniform throughout its separating zone in order that all portions of the ore fed may be equally acted upon. The air gap between the poles should be as narrow as is permitted by the conveying device and the ore sheet passing between them. The intensity of the field is determined by the ampere-turns of the exciting coils, the cross section, the length and the material form- ing the magnetic circuit, the distance between the poles and the shape of the pole pieces. The intensity of the field is controlled in practice by the current allowed to flow through the exciting coils and the distance between the poles, which in most separators is adjustable. In magnetic separators, for minerals of feeble permeability especially, it is desirable to produce a dense field, or concentration of the lines of force along the separating zone. This may be ob- tained by beveling the pole pieces, by the device of two parallel magnetized cylinders, by a series of sharp projections on the sepa- rating pole or armature placed between the poles, by a laminated construction of pole pieces, or by an armature made up of alternate disks of magnetic and nonmagnetic material. The reason for this concentration is that the lines of force, in their passage across the gap of the separating field between the poles, seek to travel as 12 ELECTRO-MAGNETIC ORE SEPARATION far as possible through the iron of the pole pieces or armature, as offering less resistance to their passage than air, resulting in a concentration of these lines of force where the air gap is shortest. In separators employing but one separating field it is usual in order that no magnetic particle may escape attraction to introduce the feed at the strongest part of the field. Where more than one separating field is employed the ore should be passed through fields of gradually increasing strength. The effect of this is to remove minerals of different permeabilities as separate products, the most strongly magnetic by the first and weakest field, and the most feebly magnetic by the last and strongest field, and to prevent entrainment by avoiding the rush of strongly magnetic particles in a field of greater intensity than is necessary for their attraction. If separators having only one separating field are employed it is usually necessary to operate two or more machines tandem, with fields of progressively increasing intensity. REMOVAL OF THE ATTRACTED PARTICLES FROM THE MAGNETS The removal of the attracted particles from the magnets may be accomplished in several ways, depending upon the form and kind of magnet employed. With separators which draw the magnetic particles against the magnet itself, these particles must be removed either by force or by interrupting the attraction by breaking the current on the exciting coils.' With the old permanent-magnet separators, and with separators employing revolving magnets which do not change their polarity during revolution, scrapers or brushes must be re- sorted to in order to effect the removal of the attracted particles. With wet separators a jet of water may be employed. With electro- magnets the exciting current may be automatically interrupted and the attracted particles allowed to fall; this is only possible with certain constructions and has not been in general use. With separators which employ secondarily induced magnets to effect the separation these may be caused to pass beyond the field of the primaries and the attracted particles so dropped. A con- struction which has found extensive application employs a rotat- ing cylinder, or armature, revolving between the primary poles to effect the separation. Here any point on the cylinder changes its polarity during revolution, and, at a position 90 degrees from the PRINCIPLES OF MAGNETIC SEPARATION 13 separating zone, passes from one sign to the opposite, where the attracted particles are dropped. The cylinder may retain suffi- cient residual magnetism to hold strongly magnetic particles, even at the neutral point, in which case brushes or scrapers may be necessary to overcome the feeble attraction due to this cause. In another construction advantage is taken of a property of the lines of force emanating from a magnet pole to concentrate upon points of magnetic material in other words, employing secondarily induced magnetic points to remove the particles at- tracted by the primary magnet. Here the secondary magnet points are caused to pass out of the influence of the primary magnet, and, upon losing their magnetism, drop the attracted particles. With separators which act by deflecting the magnetic particles from a falling sheet of ore adjustable diaphragms are used to divide the particles according to their degree of deflection from the verticle: any particles which may have become attached to the magnet poles may be dropped by breaking the current for an instant. In separators which employ but one separating zdne, the mag- net, and means of removing the particles attracted by the same, should be so arranged that at least three different products are ob- tained a concentrate, a middling and a tailing. This may be accomplished by gradually decreasing the strength of the field at the discharge and employing adjustable diaphragms to separate the products, the most weakly magnetic falling first and the most strongly magnetic last. NECESSITY OF MAKING A MIDDLING PRODUCT In the crushed ore submitted to any process for separation or concentration there is always a certain proportion of composite particles containing both the valuable mineral and waste, and this may not be avoided, even by excessively fine crushing, which is usually undesirable on account of the quantity of dust or slime produced. These particles are too rich to be allowed to go into the tailing, and too lean to be included in the concentrate ; in any scheme of treatment, therefore, provision should be made for the recovery of such particles as a middling product. With magnetic separators it is usually advisable to carry on the first magnet en- countered by the ore the lowest current which will separate the 14 ELECTRO-MAGNETIC ORE SEPARATION pure magnetic particles, and a sufficient current on the last magnet to remove all the particles carrying a portion of the magnetic min- eral. The result of this is a clean magnetic concentrate from the first magnet and a clean nonmagnetic product, with a middling product, for the retreatment of which provision should be made. Where separators are used which do not yield a middling product two machines should be operated tandem, the first delivering the magnetic product and the second a middling product and non- magnetic discharge. The retreatment of middlings should, of course, be preceded by crushing, and where the ore is roasted for magnetism, a reroast may also be necessary. CLEANING MAGNETITE CONCENTRATE In the separation of strongly magnetic minerals, especially on separators of large capacity, some provision should be made for cleaning the magnetic concentrate from entrained particles of waste. Such cleaning is accomplished in some separators by sub- jecting the concentrate to the repeated action of magnets of al- ternate polarity, the magnetic particles forming loops between the poles, which loops are broken and remade in passing from one pole to the next, and the nonmagnetic particles allowed to fall. In some other constructions a blast of air or jet of water is directed against the concentrate while held by the magnets and the en- trained particles blown or washed out. Repeated treatment of the magnetic product as exemplified by the Edison deviation separator accomplishes the same result. TREATMENT OF FINE MATERIAL In crushing ore a variable amount of dust or slime is produced which may not be separated advantageously in conjunction with the coarser sizes. No especial difficulty is met in the separation of strongly magnetic minerals in a state of fine division either wet or dry; several wet separators are designed to treat ore which has been reduced to slime. The separation of feebly magnetic minerals in a state of fine division is a more difficult problem, as the capacity of the separator is cut down by the thinness of the ore layer which may be treated. In dry-crushing plants the several crushing and separating PRINCIPLES OF MAGNETIC SEPARATION 15 machines are usually housed in, and the flying dust removed from within the casings by exhaust fans and settled in a dust chamber. Dust is a source of danger to the workmen employed about the machines, and is a hindrance to the separation as well. Electric machinery should be installed in a separate building, or dust-tight room, as magnetic dust collecting on magnetized bear- ings, etc., and on motors and dynamos is troublesome. Nonmagnetic dust has a tendency to adhere to magnetic con- centrate, which may be a source of loss, notably in the separation of magnetite and apatite. The dust may be removed by an air blast, or if the trouble be aggravated, resort may be had to wet separation. If the separator is capable of fine adjustment and the ore is accurately sized, fine material may be separated readily, the capac- ity of the separator becoming less the lower the permeability of the magnetic mineral and the finer the material treated. FEEDING DEVICES A usual origin of separator feed is some form of roller or reciprocating feeding device placed beneath a feed hopper. Such feeder should be absolutely automatic, and so connected with the separator mechanism that, should the separator stop, the feeder will stop also. The feeder should spread a thin, even layer of ore upon the conveyor belt, shaking plate, drum or cylinder employed to transport the ore to the separating zones, and the rate of feed should be capable of regulation. The feeder should be so con- structed that if a large piece of ore or other material should find its way past the screening apparatus the feeder will not stop, or necessitate stoppage, for cleaning out. It is usual to place a screen at the feeder, either above or below it, which will eliminate from the feed any oversize particles. In separators which employ con- veyor belts to present the ore to the magnet the feeder should spread a uniform layer across the width of the belt, a couple of brushes being set at the edges of the belt to turn back toward the center any particles which might be shaken off and lost. If a feeder works poorly and does not distribute a uniform ore layer, a piece of canvas so fastened that its lower end will drag on the conveyor belt will be found useful to distribute the feed properly. 16 ELECTRO-MAGNETIC ORE SEPARATION ADJUSTMENTS A magnetic separator should be capable of easy adjustment to suit different ores. A rheostat should be provided to regulate the current on each magnet, and in separators in which the ore is introduced between the poles, the distance between the poles should be capable of adjustment. The amount of feed, the speed at which the ore is presented to the magnets, and the distance of the ore sheet from the separating poles should be capable of regulation, as well as the positions of diaphragms for dividing the separated products. REQUIREMENTS A MAGNETIC SEPARATOR SHOULD FULFILL Besides the ordinary requirements for any steadily operating machine such as automatic operation, economy of power, dura- bility and simplicity of construction, and visibility of working parts a magnetic separator should be provided with a thin, even, regular feed that will present the ore at proper speed as close as may be to the separating poles of the magnets, which should have a concentrated and homogeneous field. The separator should make at least three products; magnetic concentrate, middling, ancl nonmagnetic tailing; should embody some provision for the clean- ing of the magnetic concentrate from entrained nonmagnetic par- ticles, if of high permeability, and should be capable of complete and accurate adjustment. CAPACITY The capacity of a magnetic separator is controlled by the kind of ore treated, by the percentage of magnetic product removed, and by the size to which the ore has been crushed. The effect of the size of the particles treated upon the separator capacity is well illustrated by the results obtained at Ems, Germany, in the re- moval of raw siderite from blende, where the average capacity of a Humboldt-Wetherill separator is 12 metric tons per 10 hours on material between \ and 4 millimeters, but only 3.5 metric tons per 10 hours on the fines passing a ^-millimeter aperture. In general, the more strongly magnetic the mineral removed the greater the capacity of the separator. The Ball-Norton belt sepa- PRINCIPLES OF MAGNETIC SEPARATION 17 .-rator, operating on magnetite ore crushed through 6 mesh, has a capacity of about 20 tons per 10 hours. The capacity of the Dings or the Cleveland-Knowles separator may be taken as 1 ton per hour on roasted pyrite-blende concentrate of average grade. The above figures are taken from representative plants and are generalizations only; the capacities of the several separators are given, when it is possible to do so, in the descriptions of mills in the following chapters. COST OF MAGNETIC SEPARATION The cost of magnetic separation consists of the cost of pre- paring the ore for treatment plus a few cents per ton for super- vision, excitation, and repairs. When the ore must be roasted the cost of this should, of course, be charged against the separation of which it is a prerequisite; the cost of roasting pyrite or siderite to the magnetic oxide should not, under average conditions, ex- ceed 50 cents per ton in a well-equipped plant operated at capacity. Wherever it has been possible to do so, the cost of treatment has been given in the descriptions of mills in the following chapters. 4 TESTING Where it is intended to employ magnetic separation a prelim- inary test of the ore is even more important than with other proc- esses, on .account of the .difference in the magnetic behavior of the same mineral from different localities. Most manufacturers of magnetic separators maintain testing establishments, and will make small scale tests without charge except for any assaying that may be desired. Such tests, if yielding satisfactory results, should be followed by a large scale test under working conditions and personal supervision. While it is impossible to standardize schemes of testing to suit all ores, the following points should be covered: (1) An accurate sample should be used, sufficient in amount to partake of the nature of a mill run; in other words, to be indicative of the results which may be expected from commer- cial operation. (2) Determination should be made of the size to which the ore should be crushed to yield the best results. (3) If there is a choice between direct separation of the raw ore and separation after roasting for magnetism, both methods should be 18 ELECTRO-MAGNETIC ORE SEPARATION tried and results compared ; which might, perhaps, end in a decision to employ a combination of the two methods. (4) Separation of the ore with different amperages on the magnets, different belt or drum speeds, etc., should be made to determine the adjustments necessary to attain the greatest efficiency and capacity. (5) De- termination should be made of the amounts and grades of all products separately, from which data any desired combination of results may be computed. (6) Accounting for all the values in the feed and determination of the sources of loss. PREPARATION OF THE ORE FOR TREATMENT The proper preparation of the ore for separation is as important as the selection of the best method of treatment. No machine should be called upon to treat material for whose separation it was not intended. The cost of crushing, sizing, and, where neces- sary, the cost of roasting the ore in preparation for magnetic treatment is many times as great as that of the actual separation, and the original outlay required for equipment is usually in the same proportion. These subjects are, with the exception of roast- ing for magnetism, fully treated in all the standard works on ore dressing, but a discussion of some of their features must be con- sidered before entering upon the subject of the methods of treat- ment of the different ores amenable to magnetic separation. The subject of roasting for magnetism is taken up in the chapters de- scribing the treatment of the ores to which it is applied. CRUSHING The object of crushing is to free the individual minerals in the ore, the ideal result aimed at being the production of a mixture of particles, each of which is composed of one mineral and nothing else. Such a result is never attained in practice, there remaining always, even after the finest comminution, mixed particles consist- ing of two or more distinct minerals. Ores vary widely in the average size of the particles or crystals of their component minerals, and while one may liberate the bulk of its valuable constituent when crushed to 8 mesh, another ore, of precisely the same mineralogical composition, may require to be crushed to 30 mesh, or even finer. As illustration, at Herrang PRINCIPLES OF MAGNETIC SEPARATION 19 the ore liberates the magnetite when crushed to 8 millimeters, while at Pitkaranta the ore is slimed before separation, which preparation yields but 44 per cent, of the magnetite as particles free from waste. It is apparent therefore that the fineness to which an ore should be crushed should be determined carefully in each individual case. The best way to arrive at the size to which any particular ore should be reduced is to crush and test a sufficiently large sample of it to be indicative of the results which may be expected from treatment on a commercial scale. The ore should be crushed to a size determined by inspection, or by actual measurement, of the particles of the valuable mineral and objectionable impurities to be eliminated, and should be sized, separated, and the several prod- ucts from each size assayed separately. The coarsest size from which a considerable proportion of clean concentrate may be sepa- rated and a clean tailing refused, will be, in general, the size to which the preliminary crushing should be carried, all further crushing being carried out upon the middling product. Graded crushing is employed to minimize the amount of un- dersized particles produced in reducing the ore to pass a given aperture, the ore being broken in two or more stages, and the particles already fine enough screened out after passing each crushing machine, and not subjected to further comminution. The simplex method of crushing the ore in one operation to pass a given aperture is employed where the production of a large amount of undersized material is not counted a source of loss. In mills where the ore is to be separated dry the question arises whether the ore shall be crushed wet and then dried, or dried be- fore crushing. In the latter method, which is extensively employed, the machines should be housed in to prevent the escape of dust into the atmosphere, and an exhaust fan should be provided, with connections to the various machines and a settling chamber, or bag house, where the dust may be collected. The dust from crushing ore is extremely injurious to the workmen exposed to it, and every precaution must be taken to prevent its escape into the atmosphere of the mill. The workmen are required to wear respirators at many plants where there is danger from so-called lead poisoning, and also are shifted from one position to another in order to re- duce the danger to any one man. 20 ELECTRO-MAGNETIC ORE SEPARATION SIZING While, theoretically, the size of a particle of magnetic material should have no effect upon the attraction of a magnet for it, results from practice indicate that small particles are more easily influenced than large particles. It is difficult, for instance, to separate magnetic minerals from each other when the more weakly magnetic mineral is present as the smaller particles. Where there is a wide difference between the magnetic permeabilities of two minerals in a mixture, sizing is usually unnecessary. The more closely the permeabilities of the minerals in a mixture approach each other the closer must the sizing be carried out, and ,the greater the number of sizes treated separately. A further reason for the close sizing of such mixtures is -that with an ore layer of evenly sized particles closer adjustment may be made in the dis- tance of a magnet from the ore stream. In most instances, with the exception of the separation of two or more minerals of similar permeability, a reasonably close sizing before separation gives the best results. Sizing of the finer particles produced by crushing is easily effected by water classification, but as the ores separated wet usually carry magnetite or artificial magnetite as their mag- netic constituent such classification is rarely necessary. An anal- ogous method of classification has been extensively adopted in Europe, where a current of air is employed as a classifying medium in the place of water. SPECIFIC-GRAVITY CONCENTRATION The concentration of ores by specific-gravity methods is a usual preliminary to magnetic separation. With mixtures which require roasting or calcination to render them magnetic, the advantages of such preliminary treatment are too obvious to require mention. DRYING, COOLING, ETC. In dry magnetic separation it is essential that the ore be quite dry, as any appreciable moisture causes the particles to adhere to one another and precludes good work. Ore, as it comes from the mine, can rarely be separated without drying, still less the products PRINCIPLES OF MAGNETIC SEPARATION 21 of wet concentration, even after drying in the air. It is usual to dry the ore after the coarse crushing and before the fine classi- fication, as moist ore clogs the screens. Abroad, a scheme for drying the ore during classification has been successfully carried out: trommels are fitted with jackets through which waste steam is passed, and the ore dried as it passes through. Care must be taken in drying ores not to subject them to suffi- cient heat to render magnetic any minerals which are not in- tended to go into the magnetic product; in the case, for example, of some magnetic-blende ores carrying pyrite, a quite low heat is sufficient to form a film of magnetic oxide on the pyrite and render it sufficiently magnetic to be attracted by the intense fields em- ployed for the separation of magnetic blende. Among the usual forms of drying furnaces are: (a) The re- volving cylinder with inclined axis, through which gases from a combustion chamber are passed; (b) modifications of the shaft furnace; (c) troughs heated from without by steam or by hot gases flowing through them, and fitted with a chain or other conveyor to transport the ore. The several types of furnace will be taken up in connection with the plants with which they are employed. Provision should be made for cooling roasted ore, and in some cases ore from drying furnaces, before allowing it to come into contact with belts, etc., as even a low heat, if sufficiently prolonged, will cause a deterioration in the materials of which they are made. Expedients resorted to for cooling ore comprise transportation by conveyors in which the ore is stirred and exposed to the air until it is cool, cooling floors upon which the ore is spread, contact with cooled surfaces, water-jacketed revolving cylinders and others. With separators which employ high-intensity fields it is advis- able, and often imperative, to pass the ore through a field of low intensity to remove any strongly magnetic particles it may contain before feeding it to the separator proper. Strongly magnetic par- ticles introduced into a field of high intensity will be so strongly attracted as to tear belts, and, if in sufficient amount, to bridge across between the poles and stop the separator. Ill SEPARATORS FOR STRONGLY MAGNETIC MINERALS MANY classifications based on the method of treatment, on differences in construction, etc., have been suggested to include the different types of magnetic separators ; separators with station- ary magnets, and those whose magnets revolve ; separators in which the ore is attracted directly against the magnet, and those which interpose a nonmagnetic belt or drum between the magnet and the particles attracted ; separators which lift the magnetic particles from the mixture, and those which deflect the magnetic particles from a falling sheet of ore, and various others. The classification of most value is that based upon the types of material the dif- ferent separators are suited to treat. For this reason the only classification attempted here is to distinguish between the separa- tors designed to remove ferromagnetic minerals and those designed to treat such feebly magnetic minerals as raw siderite, limonite, etc. A number of separators have been designed to treat a finely divided feed only, and others for use as cobbing machines. The sizes of feed to which the several machines are suited will appear in the descriptions of the individual separators. Descriptions have been published of a large number of sepa- rators whose principal claim to interest is an historical one; the only machines here described are those which are at present of commercial importance. THE BALL-NORTON BELT SEPARATOR This machine employs the principle of a series of magnets of alternate polarity to effect a thorough turning over of the ore while in the influence of the magnetic field, thus permitting en- trained particles of waste to fall from the concentrate. The ore is fed from a hopper by a feed roll upon a horizontal belt which serves to present it to the magnets from beneath. The 22 SEPARATORS FOR STRONGLY MAGNETIC MINERALS 23 magnetic particles are lifted from this feed belt by the magnets and held against a take-off belt running in the same direction and interposed between the ore stream and the magnet poles. The take-off belt is run at a greater speed than the feed belt in order to carry the ore past the magnets in a thinner layer. The belts are made of rubber-covered canvas, and means are provided to Prf Hopper FIG. 1. BALL-NORTON BELT SEPARATOR. alter the speed of belt-travel to suit different ores. As the mag- netic particles are held against the take-off belt, and by its motion carried past the poles alternately opposite in sign, the loops of magnetic particles are broken and reformed as they pass from one pole to the next, permitting entrained particles to fall from the concentrate into a tailing compartment, into which the non- magnetic material remaining on the feed belt also falls. The 24 ELECTRO-MAGNETIC ORE SEPARATION magnetic concentrate is carried past a partition and is dropped from the last magnet into a separate compartment. The series of magnets is made up of 12 poles, those of opposite sign being adjacent, all controlled, in the type machine, by one rheostat. By dividing the poles into two series by suitable con- nections, and employing an additional rheostat, two sections of the field of different intensity may be obtained. The capacity of this machine is from 20 to 35 tons per hour of magnetite ore crushed to pass a J-in. aperture. THE "MONARCH," OR BALL-NORTON DOUBLE-DRUM SEPARATOR This machine embodies the same principle of magnet construc- tion as the Ball-Norton belt separator. It consists of two revolv- FIG. 2. THE BALL-NORTON DOUBLE-DRUM SEPARATOR. A, Feed hopper; B, tailing compartment; C, middling compartment; D, concentrate chute; E, magnets of which the adjacent poles are of opposite sign; F, rougher drum; G, cleaner drum. ing drums with nonmagnetic surfaces placed parallel and close together, within each of which is fixed a composite electro-magnet made up of adjacent poles of opposite sign. The ore is fed at the top of what may be termed the rougher drum, in passing around which the nonmagnetic particles are SEPARATORS FOR STRONGLY MAGNETIC MINERALS 25 thoroughly eliminated, falling into a hopper below. The mag- netic particles are held against the drum by the magnets within, and while passing the poles of opposite sign the loops of magnetic particles are broken and reformed, freeing the nonmagnetic par- ticles, which are removed by a combination of gravity, centrifugal force, and the effect of a blast of air impinging upon the surface of the drum in a direction opposite to its rotation. At a point just below the horizontal diameter of this drum the ore passes beyond the influence of the magnets and is thrown, by centrifugal force, against the face of the adjacent cleaner drum where it is caught and held by the magnets. The cleaner drum revolves at a greater speed than the first drum encountered by the ore and is furnished with weaker magnets; particles of inferior permeability, which were held by the rougher drum, are here thrown off into a middling hopper; the concentrate is carried farther and thrown into a chute after passing beyond the influence of the last magnet pole. The rougher drum makes 40 revolutions per minute and the cleaner drum 50; the magnets in the rougher drum take 10.5 amperes and those in. the cleaner drum 13 amperes. The capacity of this separator, with drums 24 ins. in diam- eter by 24 ins. face, is from 15 to 20 tons per hour of magnetite ore, crushed to pass 16 or 20 mesh. The power required is from to | H. P. for operation, and from 1 to 1.5 E. H. P. for ex- citation. THE DELLVIK-GRONDAL SEPARATOR This type of separator was designed for the treatment of fine material. It consists of a composite electro-magnet of cylindrical form which revolves about a vertical axis. This cylinder, of cast iron, carries a series of six exciting coils, wound in circular grooves cut around its circumference. These coils are separated from each other 60 mm., and are so wound as to give fields of progressively increasing strength from top to bottom opposite the iron spaces between the coils, which form the separating surfaces. The ore, in suspension in water, is fed from a launder against the topmost magnetic ring. This launder, which is curved to cover about 90 degrees of the magnetic cylinder, is supplemented by four other similar launders below it, which serve tp catch and return against the drum any material thrown off by its revolution. 26 ELECTRO-MAGNETIC ORE SEPARATION The magnetic particles stick to the rings between the coils, those not held by the first ring being caught and held by one of the lower rings, each of which has a field of greater strength than the ring next above it. Nonmagnetic particles are washed from the concentrate by a stream of water which plays against the cylin- der. By the revolution of the cylinder the magnetic particles adhering to it are carried opposite a wooden cylinder, carrying secondary magnets, which is mounted parallel to the magnetic cylinder, and which revolves in the opposite direction. This wooden cylinder is studded with a number of iron pegs so placed FIG. 3. THE DELLVIK-GRONDAL SEPARATOR. A, Feed launder; B, splash board; C C, circular launders serving to present the ore in suspension in a stream of water against the magnetized cylinder; D, the separating cylinder; E, exciting coils; F, wooden take-off cylinder; GG, secondarily induced take-off magnets; H, tailing launder; /, concentrate launder. as to come opposite the magnetic rings of the separating cylinder. These pegs, distant 5 mm. from the magnetic rings, concentrate the lines of force from these rings upon their points, giving rise to local fields of greater intensity than the primaries, and so cause the magnetic particles to leap across the gap and attach themselves to the pegs. By the revolution of the wooden cylinder these pegs are carried beyond the influence of the primaries, lose their secondarily induced magnetism, and drop their burden of magnetic particles, which removal is aided by a stream of water. The capacity of this machine is from 30 to 45 metric tons per 24 hours of magnetite ore, crushed to pass a 1-mm. aperture. The magnets require 6 amperes at 31 volts. The separating cylin- der makes 25 R.P.M. and the take-off cylinder 225 R.P.M. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 27 THE GRONDAL TYPE II SEPARATOR This separator consists of two iron disks fastened, 60 mm. apart, to a vertical standard, the space between the disks being occupied by the exciting coils. The disks and coils are station- ary. This circular magnet is covered with a brass ring, around FIG. 4. THE GRONDAL TYPE II SEPARATOR. A, Feed hopper; B, feed launders; CC, soft iron disks; D, exciting coils; E, tailing dis- charge; F, concentrate discharge. the periphery of which a series of iron strips are mounted; and which are magnetized from the disks as long as they are adja- cent to them. The distance between the disks and the brass ring is so varied that the iron strips are magnetized during one half of the revolution only. The ore is slimed and fed, in suspension 28 ELECTRO-MAGNETIC ORE SEPARATION in water, against the brass ring through launders similar to those employed in the Dellvik-Grondal separator. The magnetic par- ticles stick to the iron strips during half the revolution, are thoroughly washed with a jet of water, and, on passing beyond the influence of the magnetic disks, are washed off the strips by a jet of water. The iron strips are coated at the top with a layer of lead and antimony. This layer is thickest at the top of the strip, gradually shading off until at the bottom of each strip the ore comes into direct contact with the iron; this is done to give a field of steadily increasing strength on each strip in the direction of passage of the ore. THE GRONDAL TYPE III SEPARATOR This separator consists of a fixed electro-magnet with hatchet- shaped pole pieces enclosed in brass drums which revolve at 80 revolutions per minute. The surfaces of the drums are fitted with strips of iron which form secondary poles, and against which the magnetic particles are attracted. The ore is introduced into a tank beneath the revolving drum, which is suspended just above the level of the water; the sharp edges of the pole pieces give rise to a concentration of the lines of force which serves to lift the particles of pure magnetite out of the water and against the drum, where they stick to the secondary magnets and are carried by the revolution of the drum out of the field and discharged into a launder. The particles forming the middling product are not lifted from the water, but are sufficiently attracted to separate them from the waste and are discharged through an overflow at the side of the tank. The nonmagnetic particles fall to the bot- tom of the tank and are discharged through pipes. Generally two drums are combined in a twin machine which requires 2 H.P. for operation, and 3.5 amperes at 110 volts for excitation of the mag- nets. The capacity of this machine is 50 tons in 24 hours. THE GRONDAL TYPE IV SEPARATOR This type of separator was designed to deliver magnetite con- centrate as dry as possible from a wet separation. It consists of a brass disk revolving at 1450 R.P.M. beneath an electro-magnet whose pole pieces taper to an edge at their lower extremities. The SEPARATORS FOR STRONGLY MAGNETIC MINERALS 29 30 ELECTRO-MAGNETIC ORE SEPARATION slimed ore is delivered by a launder into a tank beneath the brass disk, and the magnetic particles are drawn up against the disk, from which they are thrown off by centrifugal force in a nearly dry state. About 1 H.P. is required for operation, and 3.5 amperes at 110 volts for excitation of the magnet. THE GRONDAL TYPE V SEPARATOR This machine consists of a brass drum which revolves on a horizontal axis and encloses a series of magnets of alternate polarity of the Ball-Norton type. The difference between the working of this machine and that of the Ball-Norton consists in FIG. 6. THE GRONDAL TYPE V SEPARATOR. 1, Feed; 2, wash water; 3, concentrate; 4, middling; 5, tailing; 6, magnets. feeding the finely crushed ore in the former case, in a stream of water into a tank beneath the separating drum, from which it is raised by the magnets against the drum. This machine re- quires 1 H.P. for operation and 4 to 5 amperes at 110 volts for excitation. It is said to have treated 100 tons of crude ore in 24 hours. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 31 FIG. 7. THE GRONDAL SLIME SEPARATOR. A, Magnets; B, coils; C, settling tanks. 32 ELECTRO-MAGNETIC ORE SEPARATION THE GRONDAL SLIME SEPARATOR This is a stationary electro-magnet with two beveled-edge pole pieces which are suspended above V-shaped settling tanks. The slime, in suspension in water, is introduced at one side of the tank in a shallow stream which flows beneath the pole pieces to a similar discharge at the opposite side. The current on the mag- net, which is suspended close to the water level, but not dipping into the water, is regulated so as to be just too weak to lift mag- netic particles out of the water. The magnetic particles form FIG. 8. THE WETHERILL TYPE F SEPARATOR. A, Magnet poles; B, coils; C, tailing; D, middling; E, concentrate; F, rotating arma- ture; Z, feeding device. bunches in the water beneath the pole pieces and fall to the bot- tom of the tank, from which they are discharged through a pipe. This apparatus is frequently employed for dewatering the pulp from ball mills, in which case a stream of clear water is intro- duced into the tank at the bottom; the sand falls to the bottom and is discharged through a pipe along with the bunches of mag- netic slime collected beneath the magnets. By regulation of the SEPARATORS FOR STRONGLY MAGNETIC MINERALS 33 velocity of the stream of pulp and the amount of clear water added, the size of particles carried over the waste discharge may be adjusted to suit the ore under treatment. THE WETHERILL TYPE F SEPARATOR This machine comprises a separating armature, built up of alternate disks of magnetic and nonmagnetic material. Upon revo- lution between the primary magnets secondary poles are set up at FIG. 4 N FIG. 9. DETAILS OF MAGNET ROLLER, WETHERILL TYPE F SEPARATOR. the edges of the magnetic plates, or disks, of the armature, focus- ing the lines of force from the primaries and causing magnetic particles to stick to the armature until carried beyond the influ- ence of the primary poles. The waste drops off the armature into a receptacle, while the magnetic particles are held until the neutral point is reached, where the magnetism of the disks changes from plus to minus, when they fall into a receptacle. The change in magnetism is gradual, so that by means of suitable partitions, sev- 34 ELECTRO-MAGNETIC ORE SEPARATION eral products may be made on the same separator, the strongly magnetic being the last to fall from the armature. The machine is built in one size only, with 30-in. poles, but the magnets are wound for various strengths of current. The capacity of the machine is large: the makers claim that 400 tons are put through these machines at Mineville, N. Y., in 24 hours. THE FROEDING SEPARATOR This separator consists of a round table of brass 3 mm. thick, and 1.45 meters in diameter, which slopes from center to circum- ference. Beneath this separating surface, which revolves, there is a system of 12 stationary magnets, arranged radially to cover -J FIG. 10. THE FR CEDING SEPARATOR. A, Magnets; B, revolving table; C, tailing discharge; D, concentrate discharge; E, wash water pipes. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 35 of the surface of the table ; beneath the sector, representing $- of the area, there is a gap without magnetic attraction. The magnets are of alternate polarity and have their corners beveled to concentrate the lines of force at the periphery, and are spaced 50 mm. apart. Above the table is a series of movable perforated pipes, which deliver a spray of wash water on the ore under separation. The ore is delivered in a stream of water at the center of the table, and spreads out in a layer of decreasing thickness toward the periphery. The magnetic particles are held against the surface of the table and carried by its revolution to the sector where there is no magnet and here washed off. The nonmagnetic par- ticles are washed off the table by the wash water from the pipes. The alternate polarity of the magnets causes the magnetic par- ticles to turn over in passing from one magnet to another, the entrained waste liberated during this process being washed off by the sprays from the pipes, which are hung 40 mm. above the table. The two products are caught in separate launders at the periphery of the table. Magnetic particles are prevented from being washed off the table by the concentration of the magnetic field due to the beveling of the magnets mentioned before. The capacity of the machine is 2 metric tons per hour, at 10 R.P.M. ; 150 liters of wash water are used per minute; | H.P. is sufficient to operate the moving parts, while the magnets require 8 amperes at 100 volts for excitation. THE ERICKSSON SEPARATOR The construction of this machine is best understood from the accompanying figures. The magnets A and the coils C revolve about the shaft B. The magnet wheels are divided into 21 spokes, the spokes on each side being opposite one another. Be- tween the two halves of the magnet is an annular slot, extending completely around the circle; the walls of this slot are thin sheets of nonmagnetic metal, and this space is filled with water to the height of the axle. The ore is fed by a stream of water at E ; the magnetic particles form bridges in the fields between the opposite spokes, and are carried around by the revolution of the magnets. At K a launder is introduced into the slot, receiving the bridges of magnetic particles, which are washed out of the machine through this launder 'by a strong jet of water. The magnetic 36 ELECTRO-MAGNETIC ORE SEPARATION material is washed, and waste particles removed, by sprays of water playing on the bridges across the slot between the time it is lifted above the water level and the time of its encountering the discharge launder. The nonmagnetic particles fall to the bottom of the tank and are discharged at H. A float, J ' , is con- nected with the discharge opening, H, by a rod; when the water rises above the proper level, because of the introduction of the Stop Cook FIG. 11. THE ERICKSSON SEPARATOR. A, Revolving magnet spokes; B, axle; C, coils; D, separation chamber; E, feed launder; F, slime discharge; G-K, concentrate discharge; H, tailing discharge; J, automatic dis- charge to maintain constant water level. feed, the discharge gate at H is opened and the surplus water, along with the waste, flows from the machine. The capacity of this separator is about 2 metric tons per hour; the magnets take 20 amperes at 110 volts. Nonmagnetic slimes which do not settle readily are drawn off from time to time through the pipe F. THE FORSGREN SEPARATOR This separator comprises five independent separating zones which may be employed, if desired, on different ores and with different strengths of field. This machine consists of two con- centric brass rings mounted with soft-iron secondary poles at- tached to a spider which, by revolution about a vertical axis, causes the rings to pass between the poles of five fixed electro- magnets spaced 72 degrees apart. The ore is fed in the annular SEPARATORS FOR STRONGLY MAGNETIC MINERALS 37 space between the brass rings at points opposite the primary magnets; the magnetic particles in the ore attach themselves to the secondary magnets, while the nonmagnetic particles fall past them into a tailing chute. As the rotation of the brass rings carries the secondarily induced magnets past the fixed primaries they lose their magnetism and the attracted particles fall, first the feebly magnetic particles, which drop into a middling chute, and finally the strongly magnetic particles which drop into a con- centrate chute. From J to 3 H.P. is required for operation, and from 3 to 3.5 amperes for the excitation of each primary magnet. The capacity \ FIG. 12. THE FORSGREN SEPARATOR. A, The annular space in which the separation takes place; B, tailing discharge; C, mid- dling discharge; D, concentrate discharge; E, coils; F, fixed primary magnets. of this machine varies with the size of the material treated: operating on magnetite ore crushed to 1.2 mm. it handles 1| metric tons per separating zone per hour; arranged for cobbing, it handles 2.5 metric tons per separating zone per hour for sizes up to If ins. The brass rings rotate at a speed of from 5 to 10 R.P.M. 38 ELECTRO-MAGNETIC ORE SEPARATION THE EDISON SEPARATOR This machine consists of a series of bar magnets in front of which the ore is allowed to fall in a thin sheet. The magnetic particles are attracted sufficiently to alter their trajectory but /" \ i\ I Upper Magnet ~ He... \ [ Middle Magnet ] j ! I \ Headi / ll! \\ I Lower Magnet I /] IV '/ !,\\ X/i: ' I \ Heads Final Tailings Xailinga Tailings Tailings |i [ 4th Magnet [ y { jX/v4\ // s/ Nt X^ Tailings ilinga FIG. 13. PRELIMINARY MAGNETS, FIG. 14. CLEANING MAGNETS, EDISON SEPARATOR. EDISON SEPARATOR. not enough to draw them against the magnets. The falling sheet thus divided is caught in separate chutes or hoppers. The current on the magnets and the distance from the falling ore sheet to the face of the magnet are capable of adjustment to suit different ores. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 39 A single magnet may be employed to effect the separation, or a number of units in series. In the mill at Edison, N. J., two systems were employed, the first to produce a clean tailing product and a second for the cleaning of the concentrate from the first magnets. The preliminary magnets are arranged as shown in Fig. 13. The ore is fed past each end of the magnets, the magnetic product passing from the machine from each magnet, while the nonmagnetic particles are successively re-treated. This arrange- ment produces a clean tailing with very little loss in magnetic material. The magnets are 12 ins. long, 4 ins. thick and have a separating face 4 ft. 6 ins. wide. The cores are of cast iron (as the magnets are never saturated) and are wound with No. 4 copper wire. The three magnets are wired in series, and each has a different winding, the upper with the fewest and the lower with the greatest number of turns, giving separating fields of con- stantly increasing strength in the direction of travel of the ore. The magnets are excited by 15 amperes at 80 volts. The capacity of the series is 16 tons per hour of ore crushed to pass 0.06 in. A second series of magnets is used to re-treat the magnetic product from the above-described machine after drying and re- crushing. The arrangement is the same, but the magnets are 8 ins. long, and are wound with No. 6 wire: the capacity is 2.25 tons per hour on material crushed to pass 0.02 in.; tailings from the last magnet are waste. These magnets take 10 amperes at 120 volts. The cleaning magnets are arranged in a series of five units, and treat the concentrate from the preliminary magnets after the removal of dust. With this machine the object is the production of a clean magnetic product, and the magnets are arranged as shown above to repeatedly re-treat the magnetite, the tailing being discharged after passing each unit. The magnets are 4 ins. long, 2 ins. thick and have a separating face 4 ft. 6 ins. wide. They all have the same winding of No. 6 wire, are connected in series- and take 17 amperes at 100 volts. The tailing from the upper magnet in this series is run to waste, while the tailing from the four lower magnets is regarded as middling and sent back for re- treatment. The capacity of this machine is about 0.9 ton per hour. 40 ELECTRO-MAGNETIC ORE SEPARATION THE EDISON BELT SEPARATOR This machine consists of a belt 7 ft. wide which travels over two pulleys revolving about horizontal axes in the same vertical plane. Behind the side of the belt which travels upward are placed several electro-magnets staggered across the belt, adjacent magnets being of opposite polarity. The ore is fed against the belt opposite the lowest magnet, the magnetic material adheres to the belt and is carried upward and across it as a result of the arrangement of the poles of the magnets; the nonmagnetic particles fall from the belt. The material fed is in a fine state of division and forms tufts on the surface of the belt which turn over and over in their passage across and up the belt, liberating any particles of entrained waste. The upper magnet extends FIG. 15. THE EDISON BELT SEPARATOR. further toward the edge of the belt than the lower magnets, and the magnetic particles are dropped from it into a series of small buckets riveted to the edge of the belt, and so discharged from the machine. This separator is designed for the removal of non- magnetic particles from a finely divided feed. BALL-NORTON SINGLE-DRUM COBBING SEPARATOR This machine is used for cobbing ores which are not neces- sarily dry; the ore fed is coarse (1J ins.) and the separator puts through a large tonnage with the idea of making a clean concen- UNIVtKbll Y ] OF ' SEPARATORS FOR STRONGLY MAGNETIC MINERALS 41 trate of the pure magnetite pieces, while the tailing is re-treated on other separators after crushing. The separator consists of a drum with nonmagnetic surface which revolves about a composite magnet in the form of a sector of a circle. The attraction is exerted by 16 electro-magnets attached to a spider and mounted Concentrates FIG. 16. THE BALL-NORTON SINGLE-DRUM SEPARATOR. on the shaft of the drum. The magnets are stationary and cover a little more than 180 degrees of the circumference of the drum. They are of alternate polarity, which causes the ore to turn over as it is carried past each of the 16 poles by the revolution of the drum. This turning over permits the nonmagnetic particles to drop off the drum into the tailing hopper. The ore is fed near the top of the drum, and the strongly magnetic pieces are carried past the tailing hopper and thrown off by centrifugal force as they pass beyond the influence of the last magnet, falling into a con- centrate chute. The amperage is regulated so as to pick out the pure pieces of mineral only, allowing composite pieces of ore and waste to go into the tailing to be separated after crushing. THE WENSTROM SEPARATOR This machine consists of a drum made up of alternately mag- netic and nonmagnetic bars, which revolves about a horizontal axis and encloses a stationary magnet. The stationary magnet is cyl- indrical in form and is placed eccentrically within the revolving 42 ELECTRO-MAGNETIC ORE SEPARATION drum; it carries four circular projections, or ridges, between which are wound the exciting coils, so connected that adjacent pro- jections have opposite polarity. The surface of the drum is made up of soft iron bars with nonmagnetic spaces between them usually filled with strips of wood. The bars have projections from the inner surface of the drum which engage the projections from the magnet, making them practically prolongations of the poles of FIG. 17. THE WENSTROM SEPARATOR. A t Fixed electro-magnet; B, separating surface made up of alternate strips of iron and wood; C, projections of magnet which engage the iron strips on the surface of the drum; D, exciting coils; E, revolving drum carrying the magnetic strips; F, feeding chute. the magnet. The projections on alternate bars engage alternately the north and south poles of the stationary magnet, giving adja- cent bars opposite polarity. The projections from the magnet are cut away on one side of a vertical diameter of the drum. The ore is fed at the top of the drum and is carried forward by its revolution; the magnetic pieces are held by the magnetic bars until the vertical diameter is passed, when they fall into a hopper upon the bars becoming demagnetized. The waste falls into a hopper in front on the drum. This machine is designed to treat lump ores which need not necessarily be dry. It is made in two sizes: the larger size is capable of separating 4-in. lumps, is 27 ins. in diameter and 24 ins. across the face, takes 15 amperes at 110 SEPARATORS FOR STRONGLY MAGNETIC MINERALS 43 volts and has a capacity of from 5 to 7 tons per hour. A smaller size has a capacity of 3 tons per hour on ore 1.5 ins. maximum size. THE NEW WENSTROM COBBING SEPARATOR This is a modification of the machine above described. The distance between the ribs making up the surface of the drum of this separator is varied to suit the size of the ore to be treated. For the finer sizes, from -J to 1J ins., the drum is covered with a sheath of German silver. For treating coarse ores the drum is made in diameters from 2 ft. 10 ins. to 3 ft. 4 ins.; the length FIG. 18. THE NEW WENSTROM COBBING SEPARATOR. A, Feeding device; B, magnet core; C, coils; D, projections of magnet which engage iron strips on the drum; E, revolving surface of drum; F, tailing discharge; G, concentrate discharge. of the drum face is 2 ft. Eecently some of these machines have been built with twice this width and divided into two sections, one side for coarse and the other for fine material. The drums make from 16 to 20 revolutions per minute; the electro-magnet requires from 15 to 20 amperes at 110 volts for excitation. The capacity of this separator varies from 5 to 10 tons of crude ore per hour. THE GRONDAL COBBING SEPARATOR resembles the Wenstrpm machine, the drum being made up of ribs alternately iron and brass. The former are in. wide and the latter -$5- in. wide. The drum is operated at a speed of 30 revo- lutions per minute. 44 ELECTRO-MAGNETIC ORE SEPARATION THE PINGS SEPARATOR This separator consists of an inclined shaking conveyor which serves to carry the material to be separated beneath two wheels, each studded with secondarily induced magnets and revolving about vertical axes. The ore is fed from a hopper at the head of the inclined conveyor, and is transported by the shaking move- ment through four zones of separation, due to the magnet wheels. The first magnet encountered by the ore carries the less current and separates the strongly magnetic particles only; the second magnet carries a greater current and separates a middling product ; the nonmagnetic tailing passes off the end of ,the shaking con- veyor. The conveyor is a tray made up of a sheet of ^ in. steel cov- ered with asbestos and mounted upon hangers. A shaking move- ment is imparted to the conveyor by an eccentric, the movement being upward at the feed end and also in the direction of the travel of the ore. The usual speed is 440 strokes per minute. While passing over this conveyor the ore is kept constantly in agitation, thus lessening the chance of entrainment. The con- veyor is 18 ins. wide and 7 ft. long, and may be raised or lowered by means of hand wheels on the hangers, thereby altering its dis- tance from the magnets. By raising one end only, a different and gradually increasing distance from the plate to the magnet wheels may be obtained at each of the four zones of separation. This separator is also built with a conveyor belt in the place of the shaking conveyor. The primary magnets are fixed, and consist of two steel cores, which carry the windings and connect the pole pieces. These pole pieces are made in the form of circular arcs to correspond with the secondary magnets revolving below. The secondary magnets are made of laminated steel and are disposed around the periphery of a bronze carrying wheel 30 ins. in diameter ; they project as cylin- drical knobs about 1 in. below the carrier, and their upper ends are U-shaped to engage closely, but not to touch, the pole pieces of the primary magnets. The magnetic circuit is completed through the steel plate beneath the asbestos covering of the con- veyor. As the individual secondarily-induced magnets are carried by the revolution of the carrying wheel beyond the fields of the SEPARATORS FOR STRONGLY MAGNETIC MINERALS 45 primaries, they lose their magnetism and allow the attracted particles to drop off. These magnets reverse their polarity be- fore entering the field of the opposite pole of the primary, caus- ing a thorough discharge of their burden of magnetic particles. Troughs are provided to carry away the magnetic particles dropped, and may be so arranged as to deliver four distinct products, if it is desired. FIG. 19. THE DINGS BELT TYPE SEPARATOR. In operation, a variety of adjustments may be made, to suit different ores, by altering the amperage on the primary magnets, by changing the distance from the conveyor to the secondary magnets, and by altering the inclination of the conveyor. The ca- pacity of the machine may be taken at 1 ton per hour of properly roasted blende-pyrite concentrate. About one mechanical horse power is required for operation, and from J to 2 electrical horse power for excitation. 46 ELECTRO-MAGNETIC ORE SEPARATION THE HUMBOLDT-WETHERILL TANDEM SEPARATOR, TYPE VII In this a broad conveyor feed belt transports the ore to be separated beneath highly magnetized rollers. These rollers, which revolve in the same direction as the travel of the belt beneath them, pick up the magnetic particles from the ore stream and deposit them on cross belts which remove them to one side. At the end of each cross belt is another magnet which acts upon the magnetic particles as they are thrown off the cross belt, diverting them into suitable receptacles, according to their permeabilities, giving a double separation of the magnetic particles. These sep- arators may be operated at high speed and are said to have a large capacity on strongly magnetic ore or artificial magnetite. THE CLEVELAND-KNOWLES SEPARATOR This machine comprises a conveyor belt which serves to trans- port the material to be separated beneath two cylindrical electro- magnets which revolve about vertical axes at a height of approxi- mately 1 in. above the belt. The first magnet encountered by the ore, usually called the rougher magnet, is the weaker of the two and attracts the more strongly magnetic particles of the ore only; the second, or cleaner, magnet carries a higher amperage on a greater number of turns, and removes such magnetic particles as were not attracted by the first magnet, making a middling product; the nonmagnetic particles pass off the end of the belt. This machine is made in two sizes, with 12-in. and 21-in. belts respectively; a description of the 21-in. belt machine will serve for both. The belt of seamless rubber on a heavy canvas base is carried on two 18-in. pulleys and driven from a line shaft through the pulley at the feed end; provision for taking up stretch in the belt is made by capstan bolts working against the sliding bearing of the pulley at the discharge end. The belt is kept level beneath the magnets by three liner pulleys which are capable of adjust- ment to permit the regulation of the distance between the magnets and the surface of the ore stream. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 47 48 ELECTRO-MAGNETIC ORE SEPARATION The magnets are cylinders 26.5 ins. in diameter, the rougher of cast iron and the cleaner of cast steel and are set to overhang the belt at one side. An annular space -J in. in width is turned out of the bottom of the magnets If ins. from the periphery and is filled with spelter; the magnetic circuit is from the outside shell across the spelter gap to the inner core of the magnet about which the coils are wound. The magnetic particles are attracted and form a bridge across the spelter ring, and, by the revolution of the magnets, are carried to one side where they are scraped off by a brass scraper. The normal speed of the conveyor belt when treating artificial magnetite is 100 feet per minute, and the speed of the magnets is 40 R.P.M. At this speed the operator is capable of treat- ing 1 ton per hour of properly roasted blende-pyrite concentrate of average grade and crushed to pass 4 mesh. The capacity of the 12-in. machine is about one half that amount. The amperage em- ployed varies with the ore and the quality of the roast from J to 2 amperes on the rougher magnet and from 3.5 to 10 amperes on the cleaner magnet. THE STERN-TYPE WET SEPARATOR This separator is built to separate wet concentrates and finely divided material. It is said not to require a preliminary classi- fication of the feed, and to work well on very finely divided ore. This machine consists of a number of electro-magnets mounted on a spider which revolves in a tank partly filled with water. The ends of the revolving magnets are connected by the shaft with the walls of the tank, which form the opposite poles; the separation is accomplished in this space, between the ends of the moving mag- nets and the cylindrical wall of the tank. The ore is fed into the machine at one side, the moving magnets pick up the magnetic particles and carry them above the water level, where they are washed off into a launder by a strong jet of water: the non- magnetic particles are drawn off through the bottom of the tank. The movement of the magnets through the water stirs up the ore thoroughly and permits a thorough separation. The machine operates on a 0.5 H.P. and requires 10 amperes for excitation of the magnets. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 49 I 1* KZl 50 ELECTRO-MAGNETIC ORE SEPARATION THE PRIMOSIGH WET SEPARATOR In general principle this machine resembles the Primosigh sep- arator for dry ores described in the following chapter. The mate- rial to be separated, in a fine state of division, is fed in suspension in a stream of water into the grooves at the top of the magnet FIG. 22. THE STERN-TYPE WET SEPARATOR. SIDE ELEVATION. A, Revolving magnets; B, tailing discharge; C, concentrate discharge. cylinder, which is suspended above a spitzkasten so as to be im- mersed in water during a part of its revolution. The nonmagnetic particles drop away from the pole pieces as soon as they reach the water, while the magnetic particles are carried above the surface of the water and removed by a series of secondarily induced magnet points as in the dry separator. This machine is adapted to the treatment of fine material. Upon a feed ranging from 0.25 mm. down to dust the capacity for a machine with four separating grooves is 0.4 metric ton per hour. Twelve amperes at 80 volts are required for excitation, and J H.P. for revolution. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 51 THE LEUSCHNER TABLE SEPARATOR This separator is designed to treat slime. It consists of a round table with flat surface which rotates above a series of fixed electro-magnets. The magnetic particles are held against the sur- face of the table by the magnets beneath, while the nonmagnetic FIG. 23. THE HUMBOLDT WET SEPARATOR. slime is washed off by jets of water. Beneath one sector of the table there is no magnet and here the magnetic particles are washed off into a separate launder. THE HUMBOLDT SINGLE-ROLLER SEPARATOR: FOR WET SEPARATION This is similar to the above-described separator for the treat- ment of dry ores. The drum revolves partly in water, and the ma- terial to be separated is fed against it, near the lower vertical 52 ELECTRO-MAGNETIC ORE SEPARATION diameter. The nonmagnetic particles sink to the bottom while the magnetic particles are carried farther by the revolution of the drum and washed off by a stream of water. A stream of wash water is directed against the magnetic particles while held against the drum, to remove nonmagnetic dust and entrained particles. The drum is protected by a water-tight mantle of sheet copper. The capacity of these machines varies, with the kind of ore and the size treated, from 500 to 4000 pounds per hour. THE HERBELE WET SEPARATOR In this separator a series of electro-magnets is enclosed in a water-tight casing. An endless belt travels around pulleys at top and bottom of the case containing the magnets, the belt mov- FIG. 24. THE HERBELE WET SEPARATOR. ing downward close to the casting on the side of the ore feed. This apparatus is set vertically in a tank filled with water to a point above the top of the magnets. The ore, best below 30 mesh, as the machine is intended to treat fine material, is fed at the SEPARATORS FOR STRONGLY MAGNETIC MINERALS 53 top of the belt in a stream of water; the nonmagnetic particles fall and are carried straight down by the flow of water, while the magnetic particles, held against the belt by the magnets, are car- ried around the lower pulley and dropped into a separate hop- per. The construction is best understood from the above figure where A is the point at which the ore is fed, in suspension in water; B, the belt which conveys the magnetic particles past the magnets; c-c', the pulleys about which the belt runs; E, the con- centrate hopper ; F, the concentrate discharge ; G, the tailing hop- per; H, the tailing discharge. The actual separation of the non- magnetic particles v from the magnetic takes place at the end of the shield shown close to and opposite the lowest magnet. The feed and discharge of both concentrate and tailing are continuous. The belt is 2 ft. 6 ins. wide. The capacity of ihe machine reaches 35 tons per 24 hours. THE ODLING SEPARATOR In this machine a conveyor belt serves to carry the ore beneath an electro-magnet whose poles extend across, and just above, the conveyor belt. A cross belt running beneath the poles carries the magnetic particles attracted against it to one side, where they are discharged into a chute. The nonmagnetic particles are dis- charged off the end of the conveyor belt. THE HUMBOLDT SINGLE-ROLLER SEPARATOR: FOR DRY SEPARATION This machine consists of a drum whose face is made up of alternately magnetic and nonmagnetic bars, revolving about fixed internal electro-magnets. The primary magnets are placed to cover a part of the lower diameter of the drum; the secondary magnets, carried on the face of the drum, become magnetized by induction while passing the primaries, and pick up the magnetic particles from the stream of ore which is fed beneath the drum. The magnetic particles drop off the drum as the secondary mag- nets become demagnetized on passing out of the field of the primaries. The whole machine is covered with a dust-tight hood. The capacity varies, with the kind of ore and the size to which it is reduced, from 700 to 3000 pounds per hour. 54 ELECTRO-MAGNETIC ORE SEPARATION THE FERRARIS CROSS-BELT SEPARATOR This machine comprises a series of six inverted horseshoe mag- nets placed in line, with a single take-off belt running immediately beneath the poles of all the magnets, and six feed belts running FIG. 25. THE HUMBOLDT DRY SEPARATOR. below the take-off belt and at right angles to it, each feed belt supplying a magnet. The poles of the horseshoe magnets are bent in toward each other, giving a concentrated field at right angles to the feed belts. The magnets are fitted with an iron projection extending a few inches beyond the ends of the poles in the direc- tion of travel of the take-off belt, permitting the magnetic particles to be carried to one side and dropped past the feed belts into sep- arate hoppers. Each magnet is fed with a different size of ore SEPARATORS FOR STRONGLY MAGNETIC MINERALS 55 56 ELECTRO-MAGNETIC ORE SEPARATION except two magnets, which both treat ore passing through a 1 mm. screen, as this size preponderates. Mounted on the separa- tor frame are shaking screens, which deliver .sized products into separate hoppers, which in turn deliver on to the feed belts. The feed belts are 12 ins. wide and travel 1.5 ft. per second. The height between these belts and the magnets is capable of adjust- ment through the small guide rollers shown just below the mag- nets. The distance through which the magnetic particles are lifted varies from 30 to 40 mm. Each magnet requires 2 amperes at 50 volts. The capacity of the apparatus is slightly over 1 metric ton per hour. THE FERRARIS DRUM SEPARATOR This machine comprises a shaking conveyor which feeds the material to be separated from a hopper upon a conveyor belt, which in turn presents it to a magnetic drum, fitted with a belt serving to remove the particles attracted. The magnetic drum is FIG. 27. THE FERRARIS DRUM SEPARATOR. composed of a series of composite pole pieces which dovetail into xme another in a manner best understood from an inspection of the accompanying illustrations. The poles are insulated by a filling of zinc, the whole forming a smooth surface. The exciting coils are placed within the drum, connections with the dynamo being made through disks which dip into cups containing mercury. This ma- chine, fitted with a belt 16 ins. wide, treats about 500 kgm. of calcined limonite-calamine ore per hour. The magnets require 1.5 amperes at 110 volts for excitation. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 57 THE VIAL SEPARATOR The deviation of magnetic particles from a falling sheet of finely divided ore is the principle upon which this separator oper- ates. The attraction is exerted by six horseshoe magnets separated by bronze rings. These magnets, which are arranged horizontally, are enclosed in a brass cylinder which revolves at from 8 to 10 R.P.M. The ore is fed in a thin sheet at a distance of from 5 to 25 mm. from the brass cylinder. The magnetic particles are drawn toward the magnets but are prevented from adhering to them by the brass cylinder; the magnetic and nonmagnetic products are divided by an adjustable diaphragm and fall into separate hoppers. The capacity of the machine is 500 kgm. per hour. The entire apparatus is enclosed in a sheet-iron housing to pre- vent air currents, which would interfere with the separation. The machine treats material passing a screen with 1.5 mm. holes. THE HEBERLE DRY SEPARATOR This machine consists of a brass drum revolving about a series of fixed electro-magnets. The ore is fed against the drum at a horizontal diameter. The nonmagnetic particles fall past FIG. 28. THE HEBERLE DRY SEPARATOR. the drum into a hopper, while the magnetic particles are held against the surface of the drum by the magnets and are carried', by its revolution over the top of the drum to fall into a separate hopper. The drum makes 36 R.P.M. . 6 to 7 amperes at 65 volts are required for excitation, and J H.P. for revolution. 58 ELECTRO-MAGNETIC ORE SEPARATION THE HUMBOLDT RING SEPARATOR This consists of an annular magnet suspended in a horizontal plane within a circular casing. The ore is guided to the magnet by a conical shield, and, passing between the magnet and the cas- ing, the magnetic particles are drawn inward, while the nonmag- netic particles fall past the magnet unaffected. The two products are gathered in two concentric inverted cones, the inner receiving the magnetic portion and delivering it from the separator by means of a spout through the lower, or outer, cone. The sep- arator contains no moving parts. In lieu of an air gap between poles the separation is effected in a zone of dispersion caused by FIG. 29. THE HUMBOLDT RING SEPARATOR. K t Feed cone; M, annular magnet; 7, shield to prevent magnetic particles from adhering to magnet; R, the space in which the separation takes place. a narrowing of the enclosing casing, which induces a magnetic resistance. The operation of the separator is best understood by inspection of the figure given above. The magnetic ring has a diameter of 40 cm., or a separating periphery of about 1.25 meters. The separator is said to have a capacity of 1 metric ton per hour. 1 1 "La Separation Electromagnetique et Electrostatique," D. Korda, p. 38. SEPARATORS FOR STRONGLY MAGNETIC MINERALS 59 THE KNOWLES MAGNETIC SEPARATOR This separator consists of a stationary primary magnet be- tween the poles of which a belt, which is studded with small sec- ondary magnets, is caused to travel. The construction is made clear in the accompanying illustration. The ore is fed from a Side Elevation FIG. 30. THE KNOWLES SEPARATOR. End Vie\ A, Guide roller; B B, poles of the primary magnet; C, driving pulley; F, feed hopper; E, feed plate; I, shaking conveyor; Z, belt studded with secondary magnets; S-P, magnetic oscillator for vibrating feed and conveyor plates; T T, discharge guide plates. hopper upon a reciprocating feed plate, which in turn delivers it upon a reciprocating conveyor plate; this conveyor plate brings the ore close to the belt carrying the secondary magnets ; the plate and belt gradually approach each other, causing the ore particles to move in a magnetic field of constantly increasing strength. The magnetic particles are picked up by the secondary magnets and held until carried past the primary magnet, when they are gradually dropped off in inverse order to their magnetic per- meabilities ,by the gradually decreasing strength of the secondary magnets. The nonmagnetic material falls from the end of the conveyor plate into a separate hopper. The upper pole of the primary magnet is beveled, coming to an edge at its lower end, thus giving a concentrated field at this point: the lower pole is rounded, and being movable, an adjustment of the concentration of the magnetic field is obtainable. The secondary magnets are 60 ELECTRO-MAGNETIC ORE SEPARATION soft-steel rivets, with serrated washers on the lower side of the belt, there are about 200 of these rivets per square foot of belt, Cfe Side Front FIG. 31. DETAIL OF PRIMARY MAGNET, KNOWLES SEPARATOR. copper plated to prevent rusting. The speed of belt travel is 250 ft. per minute. The machine is designed for sizes from 6 to 36-in. belt width, having capacities from 7 to 46 tons per 24 hours. IV SEPARATORS FOR FEEBLY MAGNETIC MINERALS WETHERILL-ROWAND SEPARATOR OR WETHERILL TYPE "E". SEPARATOR THE machine consists essentially of a belt which conveys the ore between the poles of a series of magnets, so arranged that the belt traverses the air gap between opposite poles; the above fig- ure illustrates the principle of this separator. The lower pole of the magnet is flat, the upper pole beveled. This arrangement causes an intense concentration of the lines of force along the lower edge of the upper magnet, and the direction of attraction FIG. 32. SKETCH SHOWING PRINCIPLE OF WETHERILL-ROWAND SEP- ARATOR. A, Feed hopper; B-B' , conveyor belt; C, coils; D, magnet poles ; E, cross belts ; F, nonmagnetic discharge. of a magnetic particle presented to the magnet by the conveyor belt directly above the lower pole, is upward to the beveled edge of the upper pole. A cross belt traveling beneath the upper pole prevents the magnetic particles from sticking to it, and carries them to one side, out of the field. In order to free the mag- 61 62 ELECTRO-MAGNETIC ORE SEPARATION netic particles at the discharge the upper pole is furnished with a tapering iron projection in the direction of travel of the cross belts ; this causes a gradual reduction in the strength of the magnetic field and permits the magnetic particles to be removed from the field and drop away from the cross belt into a hopper. Each magnet in this construction has two separating zones ; the machine is built with one, two, and three magnets having respectively two, four, and six separating zones, the same conveyor belt serv- ing all of them. The conveyor belt is 18 ins. wide and the cross belts 2 ins. wide. The capacity of the separator depends upon FIG. 33. WETHERILL-ROWAND SEPARATOR. A, Feed hopper; B, feed roller; C, conveyor belt; D, cross belts; E, nonmagnetic discharge. the depth of ore feed on the conveyor belt (which must be very thin to prevent entrainment), and upon the speed of the con- veyor belt, which must be slow enough to give the feebly mag- netic particles time to be influenced and picked up by the mag- nets. The speed of the cross belts is adjusted to take care of the magnetic material picked up by the magnets. The ore is fed onto the conveyor belt from a hopper by means of a feed roller turning inside a cylindrical casing. It is well to introduce a coarse screen at this point (or before) to remove large pieces of ore, nails, or other foreign matter which may have passed through a leak in the sizing trommels, as any large piece of magnetic SEPARATORS FOR FEEBLY MAGNETIC MINERALS 63 material will tear the delicate and rapidly moving cross belts by pressing against them under the attractive force of the magnets. The windings on the magnets are such that the second magnet encountered by the ore is stronger than the first and the third stronger than the second, permitting the recovery of products of different degrees of magnetic per- meability. The ore stream,, in passing the mag- nets, has a tendency to gather in ridges, simi- lar to beach sands under the action of waves. These ridges may be smoothed out and the ore layer prepared for the next magnet by means of a strip of heavy canvas placed so as to trail on the conveyor belt. Each magnet is provided with a rheostat to control the exciting currents. The machine is capable of delicate adjustment suitable to variations in the ore fed. The capacity of the E3 machine, having 6 poles, varies from J to 4 tons per hour, depending on the ore treated. THE HUMBOLDT-WETHERILL CROSS-BELT SEPARATOR This is practically the same as the Wetherill-Rowand separator as built in the United States, differing only in details as to the feeding device, construction of frame, etc. FIG. 34. CROSS SECTION OF MAGNET POLES, WETHERILL- ROWAND SEPA- RATOR. FIG. 35. HUMBOLDT-WETHERILL CROSS-BELT SEPARATOR. 64 ELECTRO-MAGNETIC ORE SEPARATION THE MECHERNICH SEPARATOR In this machine the separation is accomplished in a field be- tween two cylindrical poles, of which the upper revolves in the direction of the feed introduced between them. The arrange- ment of the poles is shown in cross section in the figure above. In the earlier type-both poles were cylindrical and both revolved; in the later machines the upper is the separating member and the lower pole is stationary and covered with a nonmagnetic shell which, revolving in the direction of the ore feed, serves to dis- charge the nonmagnetic particles falling upon it. The feed is introduced against the upper pole as shown above; the feed plate is arranged to deliver ore automatically by means of a bumping device ; the ore is pressed against the pole by a weak spring beneath the feed plate, insuring a close contact between the ore stream and the pole. The lines of force are concentrated along a plane FIG. 36. CROSS-SECTIONS OF MAGNETS, MECHERNICH SEPARATOR. A, Feed; B, separating pole; C, stationary pole ; D, feed plate; E, magnetic concen- trate; F, middling; G, nonmagnetic discharge. passing through the axes of both poles; in other words, along the line where they are nearest together. The field gradually decreases in strength as this position is -left, and the magnetic particles drop off the upper pole in reverse order to their per- meability, as by its revolution they are carried out of the con- centrated field toward the neutral point, 90 degrees away. The nonmagnetic particles fall from the upper pole immediately upon leaving the feed plate. By a suitable arrangement of plates be- neath the upper pole several middling products may be made, as well as the magnetic and nonmagnetic products. Any magnetic SEPARATORS FOR FEEBLY MAGNETIC MINERALS 65 material that may still adhere .to the pole is removed near the neutral point by means of a revolving brush of steel wires. The exciting coils are wound on the cylinders themselves. The pear- shaped lower pole produces a greater concentration of the lines of force than the circular section. The principal function of the FIG. 37. MECHERNICH SEPARATOR. A, Feed hoppers; B, separating pole; C, stationary pole; D, feed plate; E, magnetic concentrate; F, middling; G, nonmagnetic discharge. lower pole is as a return for the magnetic flux. The machines are usually tmilt double, with two sets of poles. The whole is en- closed in a sheet-zinc housing to prevent the escape of dust. The machine develops a field of high intensity, and the feed being brought into close contact with the separating cylinder (which is bare) /it is capable of separating feebly magnetic min- erals. The mechanical and magnetic efficiencies are high. The machine is built in two sizes, classified according to the length of the separating poles, respectively, 60 and 80 cm. THE MOTOR-TYPE SEPARATOR This separator consists essentially of an armature revolving between two fixed magnet poles. The ore is fed against the sep- arating roller by means of a feed plate as shown in the above figure; the nonmagnetic particles fall away from the roller, while 66 ELECTRO-MAGNETIC ORE SEPARATION FIG. 38. DETAIL OF MAGNET POLES AND ARMATURE, MOTOR-TYPE SEP- ARATOR. A, Feed plate; B, separating armature; C-C", stationary poles; D, magnetic concen- trate; E, middling; F, nonmagnetic discharge. FIG. 39. MOTOR-TYPE SEPARATOR, SEPARATORS FOR FEEBLY MAGNETIC MINERALS 67 the magnetic particles are carried farther, dropping off in order inverse to their permeabilities. The roller, or armature, is pro- tected by a copper shell which revolves with it. The peculiarity of this separator is that the separating roller, or armature, is set in motion by the current supplied to the magnets, independent of any other source of power. The length of the separating roller is 800 mm.; the power consumption is from 60 to 100 watts. The only wearing parts of this machine are the separating surface and the automatic feed plate. The machine is completely closed in to prevent the escape of dust into the atmosphere of the separating room. THE PRIMOSIGH SEPARATOR FOR DRY ORES This machine consists of an iron core or shaft, 5, upon which six sets of exciting coils, A, are mounted. These coils are pro- tected by the brass rings, 0, which are the separating surfaces. The pole pieces, (7, are connected with the core, 5, by iron disks, and it is in the groove between these pole pieces that the sep- aration takes place. The ore to be separated is fed from hoppers through chutes, E, into the grooves between the pole pieces at the top of the rotating magnets, there being six individual separating zones, each equipped with its own feeding device and take-off brush. The speed of the rotating magnet cylinder is so regulated in conjunction with the current passing through the coils that the nonmagnetic particles are thrown out of the grooves as soon as they acquire the peripheral speed of the rotating cylinder; the weakly magnetic particles are carried a little farther, when centrif- ugal force, assisted by gravity, causes them to fall into suitable receptacles placed beneath the magnet cylinder; the strongly mag- netic particles adhering to the poles are removed by a series of secondary poles consisting of soft-iron points mounted upon brass disks, L, the whole revolving in the direction of the magnet cylin- der. The separator requires one half horse power for revolution, and from 14 to 15 amperes at 80 volts for excitation. The capac- ity of this separator is about 1 metric ton per hour. 68 ELECTRO-MAGNETIC ORE SEPARATION THE ULRICH SEPARATOR A pair of electro-magnets, between the wedged-shaped poles of which a separating armature is revolved, is the essential feature FIG. I. Longitudinal Section FIG. 2. Cross Section L ' ur FIG. 3. Top View FIG. 40. PRIMOSIGH SEPARATOR. A , Coils; B, core; C C, pole pieces; D, separating gap; E, feed; L, disk carrying secondary take-off magnets; O, brass rings covering coils; F, magnetic concentrate; GG, middling products; H, nonmagnetic discharge. of this machine. This armature is a hollow brass cylinder car- rying alternate rings of iron and brass J in. wide and held close together. This cylinder, or armature, is three ft. long and makes SEPARATORS FOR FEEBLY MAGNETIC MINERALS 69 50 R.P.M. about a horizontal axis. Each machine carries four separating cylinders, two above (on the same axis) and two below, the upper magnets carrying a weaker current than the lower. The ore is fed from a hopper by a distributing arrangement having a feed plate, with zigzag channels cast in it, upon the top of the upper cylinders. Here the strongly magnetic particles are held by the concentrated fields at the edges of the iron rings, and de- flected into a receptacle; the material passing unaffected over the first cylinder, falls upon the top of the lower cylinder which is revolving in a stronger field, and the more weakly magnetic particles are removed from the nonmagnetic portion of the feed, which is here discharged from the separator. The capacity of these machines is 25 tons per 24 hours; they require 1.5 E.H.P. and 1.5 M.H.P. for excitation and revolution, respectively. THE PAYNE MAGNETIC SEPARATOR This separator consists of two drums which revolve toward each other in the direction of the passage of the ore, which is fed be- tween them. The upper drum is the separating member; it con- FIG. 41. DETAIL OF SEPARATING SHELL, UPPER DRUM, PAYNE SEPARATOR. tains a stationary electro-magnet, placed to give a strong field between the two drums along the line where they are closest to- gether. The revolving shell is furnished with longitudinal strips of soft steel with a toothed cross section as shown in the above figure. The magnetic lines of force are concentrated along the ridges of these teeth, and give rise to a field of sufficient strength to separate weakly magnetic minerals. The lower drum is also encased with a revolving shell, and serves as a return for the lines 70 ELECTRO-MAGNETIC ORE SEPARATION of force. The ore is fed from a hopper by means of a feed roller upon the shell of the lower drum, which, by its revolution, presents it to the separating drum ; the nonmagnetic particles are discharged by the lower drum, while the magnetic particles are picked up and held on the ridges of the upper drum until carried past the influ- ence of the magnet, when they drop off into a hopper. THE INTERNATIONAL SEPARATOR This machine comprises a cylindrical armature, made up of thin laminated disks of annealed wrought iron, which revolves about a horizontal axis between the poles of an inverted horseshoe magnet. The disks of the armature have saw-tooth edges, the teeth being staggered on adjoining disks, the surface of the arma- ture presenting a great number of sharp points. The pole pieces of the magnet are recessed, and only sufficient space is left between FIG. 42. DETAIL OF SEPARATING ARMATURE, INTERNATIONAL SEPAR- ATOR. them and the armature for the passage of a thin layer of the ore to be separated. The lines of force from the magnet are concentrated upon the points of the armature, giving a strong field capable of attracting weakly magnetic minerals. The magnetic attraction is strongest at a point on the horizontal diameter of the armature, and steadily decreases from this point around to the vertical diameter. The ore is fed at the top of the armature, and, upon being carried SEPARATORS FOR FEEBLY MAGNETIC MINERALS 71 into the field by the rotation of the armature, the magnetic particles adhere to the points of the saw teeth, the nonmagnetic particles sliding off into a hopper. The magnetic particles are carried around underneath the armature and drop off in an order FIG. 43. INTERNATIONAL SEPARATOR. A, Feed hopper; B, separating armature; C, magnet poles; D, magnetic concentrate; E, middling; F, nonmagnetic discharge. inverse to their degrees of permeability. At the vertical diam- eter, where the magnetism of the armature changes polarity, even strongly magnetic particles are thrown off by centrifugal force. By a suitable arrangement of dividing planes a middling product may be made as well as concentrate and tailing. The hoppers receiving the products of separation are adjustable, and may be moved according to the products it is wished to obtain. The posi- tion of each hopper is shown by indicators, and when they are set at the desired points, may be clamped in place by set screws. The field magnet of this separator weighs 9000 Ibs., the whole machine 72 ELECTRO-MAGNETIC ORE SEPARATION 10,000 Ibs. One horse power is used for excitation of the magnet, and one horse power for mechanical operation. The capacity of the separator is from 2 to 4 tons per hour. THE UBALDI SEPARATOR This machine consists of an iron core, in the shape of a ring, carrying exciting coils and having two gaps in which are placed separating armatures. The upper pole pieces are recessed to ad- mit the armatures and the lower pole pieces are tapered to con- centrate the fields at the separating zones. The armatures are FIG. 44. THE UBALDI SEPARATOR. 1 and 2, Separating armatures; 3, rounded pole piece; 4, beveled pole piece; 5, chute for magnetic particles; 6, chute for nonmagnetic particles; 7, middling chute; 8, yoke to control relative strength of fields; 10, coils; 11, core; 12 and 13, pole pieces. fitted with helical ridges which serve to concentrate further the lines of force, and also, by revolution, to transport the attracted particles to one side, where they are dropped into chutes. The second separating zone encountered by the ore is stronger than the first, permitting the machine to deliver a middling product. The lower pole piece of the first separating zone is rounded, caus- ing a partial concentration of the lines of force, while the lower pole piece of the second separating zone is beveled, giving rise to a more complete concentration of the lines of force; the pole SEPARATORS FOR FEEBLY MAGNETIC MINERALS 73 pieces and the separating faces of the armatures are 80 cm. in length. These machines require 1 H.P. for operation and have a capacity of 1200 kgm. per hour on leucite-bearing lava. THE HUMBOLDT-WETHERILL TYPE VI SEPARATOR The construction of this machine, which operates on the principle of deviation of falling particles, is best understood from the above illustrations. One pole of the magnet is tapered down FIG. 45. SHOWING CONCENTRATION OF MAGNETIC FIELD. to a fine edge, the other is split and the two halves carried around the coils to almost meet close to the opposite pole, giving an in- tense field (280 mm. long). The ore is fed from a hopper by means of a roller upon a conveyor belt which passes around a small pul- E F G FIG. 46. HUMBOLDT-WETHERILL TYPE VI SEPARATOR. A, Feed hopper; B, feed belt; C, coils; DD, magnet poles; #, magnetic concentrate; F, middling; G, nonmagnetic discharge. ley close to the field of separation. A second belt travels about the magnet close to the poles, preventing magnetic particles from ad- hering to them. The ore, as discharged over the end of the con- veyor belt, passes into the magnetic field; the nonmagnetic 74 ELECTRO-MAGNETIC ORE SEPARATION particles fall straight down, while the magnetic particles are deflected according to their magnetic permeability into different trajectories and are caught in suitable receptacles. This machine, removing raw siderite from a feed ranging from J to 4 mm. in size of grain, fed separately, puts through 0.6 metric ton per hour; it requires from 8 to 9 amperes at 90 volts. THE WETHEKILL PARALLEL SEPARATOR This consists of a flat conveyor belt, 12 ins. wide and 15 ft. 4 ins. long, between the centers of the pulleys. This belt runs horizontally, at a speed of 100 ft. per minute, and the ore is fed on it in an even layer about J in. thick. At a distance of f in. FIG. 47. WETHERILL PARALLEL SEPARATOR. above the top of the belt is a second belt, parallel to the former and running in the same direction. This second belt is 16 ins. wide, extending 2 ins. beyond the lower belt on each side; it runs at a speed of 125 ft. per minute. Above the upper belt are two magnets with flattened poles, placed close together with the line of their adjacent edges slanting 40 degrees with the edges of two moving belts. The magnetic particles are lifted from the lower belt against the upper, and travel with it and across it, as a result of the diagonal placing of the magnets, and on reaching the edge of the upper belt and passing beyond the magnets, they fall past the narrower lower belt into hoppers. The magnets are wound to carry 6 to 8 amperes at 52 volts. The machine is built to treat material passing a 16-in. screen. The capacity is about 30 tons per 24 hours. SEPARATORS FOR FEEBLY MAGNETIC MINERALS 75 THE WETHERILL HORIZONTAL SEPARATOR This machine is built double, with two magnets and four pole pieces, giving two separating zones. The pole pieces are beveled and rounded at the ends to a in. radius; the belts pass FIG. 48. WETHERILL HORIZONTAL SEPARATOR. A, Coils; B, yokes; C, pole pieces; D, canvas feed belts; E, hoppers; F, feeders; G, chutes; H, guide plates for discharge; L, tailing hopper; M, concentrate (magnetic particles) hopper. 76 ELECTRO-MAGNETIC ORE SEPARATION around a pulley at one end and around the beveled pole piece at the other, being in direct contact with it. The poles are brought up close together in pairs, the beveled edges parallel. The con- struction is made clear in the accompanying plates. The ore is fed upon the belts from the hoppers in a sheet from J in. to -f? in. thick, and is carried around the beveled pole pieces, at which place the nonmagnetic particles fall into a hopper, and the magnetic particles are carried a little farther and fall into other receptacles. The pole pieces are 10.75 ins. wide and the bevel is at an angle of 27 degrees; the opposite pole pieces are 0.92 in. apart normally. This arrangement gives an intense field, and the ore is presented to it at its point of greatest intensity. The adjustments are made between the speed of belt travel, the current on the magnets, and the distance apart of the poie pieces. This machine was used to treat franklinite ore passing 0.058 in. apertures and retained on a screen with 0.01 in. apertures: it treated from 1.5 to 3 tons per hour, three machines in series. The first two machines took 6 to 8 amperes and the third 22 amperes at 52 volts. Two adjustable guide plates below and a little to the side of each sep- arating gap are used to divert the magnetic and nonmagnetic particles into their respective hoppers. THE WETHERILL INCLINED SEPARATOR In this machine the ore is fed upon a conveyor belt which pre- sents it to a magnetic field between two pole pieces similar to those used in the horizontal separator. The pole pieces in this machine, however, are set at an angle of 27 degrees from the horizontal, the plane of the upper pole piece being 1.2 ins. above that of the lower. The construction is made clear by the accompanying illus- trations. The ore is brought by the conveyor belt as close as pos- sible to the gap between the pole pieces ; the magnetic particles are here lifted off the conveyor belt against belts running around the ends of the pole pieces; the belt of the lower pole is the dis- charge belt for the concentrate, which is carried along and dropped into a hopper; the magnetic particles drawn against the upper pole are carried up on a 54-degree incline until past the influence of the separating zone, when they fall back, and by their momen- tum are carried past the gap to join the concentrates on the SEPARATORS FOR FEEBLY MAGNETIC MINERALS 77 lower belt. The magnets are wound to carry from 6 to 8 amperes at 52 volts. The adjustments are made between the distance of FIG. 49. WETHERILL INCLINED SEPARATOR. A, The coils; B, the pole pieces; D, discharge belts; E, pulleys; F, feed hoppers; G, feeders; //.conveyor belt; J, adjustable pulley to bring conveyor belt close to separating gap; L, concentrate hopper; M , tailing hopper; X, beveled edges of pole pieces. the conveyor belt from the gap, in the width of the gap between the poles, and in the current on the magnets. These machines, used three in series in the separation of franklinite ores passing a 78 ELECTRO-MAGNETIC ORE SEPARATION screen with 0.01 in. holes, have a capacity of 3.5 tons per hour. This separator is built double : if a stronger field is desired a yoke is substituted for one set of the pole pieces, leaving but one air gap in the magnetic series. The magnets of this separator are wound the same as those of the horizontal type. 1 1 "Richards Ore Dressing," p. 808. THE CONCENTRATION OF MAGNETITE ORES MAGNETITE is the most strongly magnetic of all minerals, and it is therefore natural that the earliest application of magnetism to ore dressing was for its concentration from gangue. Magnetite ores occur in large bodies in almost all countries, and on account of the high iron tenor of the pure mineral, and the ease with which it is concentrated, its treatment forms one of the most important fields of magnetic separation. Magnetite (composition Fe 3 4 ) has a specific gravity ranging from 5.0 to 5.1, and is sufficiently heavy to permit of its con- centration from gangue by specific-gravity methods, which have had an extensive application. The object of the separation, how- ever, is twofold: the concentration of the mineral in the raw ore to a product of sufficient richness for the blast furnace, and the elimination of phosphorous and sulphur, elements which fre- quently occur with magnetite in nature and which enter into combination with the iron in the furnaces with the production of an inferior metal. The specific gravities of the minerals carrying these objectionable impurities do not permit their complete sep- aration from the magnetite by water concentration. The high magnetic permeability of magnetite, which is 65 per cent, of that of tempered steel, is much greater than the permeabilities of these minerals and permits a separation to be made in magnetic fields of low intensity. The results from the several separators must not be judged on the basis of the percentage of iron in the tailing product, as this figure is controlled largely by factors other than the efficiency of the separator. Iron ores, to be commercially profitable, must carry a high percentage of iron, the low limit being, apparently, between 20 and 25 per cent, iron present as magnetite. This results in a low ratio of concentration and a comparatively small quantity of tailing, and a large percentage of iron in the tailing may represent but a small loss when compared with the total iron 79 80 ELECTRO-MAGNETIC ORE SEPARATION in the ore. The coarseness of the crystallization of the individual minerals, the presence of iron in nonmagnetic form, such as hematite, pyrite, ferruginous silicates, etc., must also be taken into account, while the grade of the concentrate aimed at is also an important factor in determining the efficiency of the separation in terms of the percentage of the iron in the original ore recov- ered as concentrate. The American practise tends toward the production of the coarsest size concentrate consistent with a clean separation and reasonable recovery, employing separators which treat the ore dry. In Sweden it is customary to grind the ore to 1 mm., or even finer, and separate on machines which treat the ore wet, resorting to briquetting to transform the concentrate into a product suitable for the blast furnace. These differences in practise are largely due to the coarser crystallization of the American ores, the Swed- ish ores being more often made up of minerals in a fine state of division. Magnetic cobbing has been successfully applied in both countries, and produces excellent results with ores which carry magnetite in large pieces, and in which apatite and pyrite do not interfere. In Sweden, lump ore from 4 to 5 ins. in size has been cobbed on the Wenstrom separator with the production of a good concentrate, and the separation of lumps 1.5 to 2 ins. in size is regularly carried on in America on the Ball-Norton single-drum separator, and in Sweden on the Wenstrom and Grondal cobbing machines. In the dry concentration of magnetite ores the fine dust formed by crushing is often a source of loss, but is not so counted when some of the newer wet separators are used; in Sweden it is not unusual for over 40 per cent, of the ore fed to the separators to be fine enough to pass a -|-mm. opening. The following table is representative of the best practise in the magnetic concentration of magnetite ores in the United States and Sweden. THE ELIMINATION OF IMPURITIES The objectionable elements occurring with magnetite which are wholely or partially eliminated by magnetic concentration, are, in the order of their importance, phosphorus as apatite, sulphur as pyrite, etc., and titanium as menaccanite or ilmenite Apatite (calcium phosphate, sp. gr. 3.18 to 3.25) is usually THE CONCENTRATION OF MAGNETITE ORES 81 M i J- O .S 'S !* +J ^ rH t^ to CO O (N 00 * 3 C5 CO O CO g C5 CO i s CO 00 (M d gj n o - .s *** nbe O .e H-* .s H-* a 1 a a a HH . . . 0) . ! * ! 1 : to i . w 03 C S3 3 3 1 63 3 63* 3 -o CO w -8 6 ig rH d . o o O ^ O O 13 'g 3 a 13 Cf-i tz & ^4 ^ Jxj *xj ^o t^ *"O 13 13 "e8 O ^ s o 1 1 OQ 09 CQ S 09 05 O s o o O '| 1-5 -9 1 s ^ ^ | 3 *> '> d 1 ^ 1 i cto f . o' r ^ * 1 I 1 3 i 1 rl I 1 W 02 1 ~ cc i I 3 82 ELECTRO-MAGNETIC ORE SEPARATION feebly magnetic, though not sufficiently so to be picked up by magnetic fields of low intensity; a red variety, found at Mineville, N". Y., is sufficiently magnetic to be sometimes drawn into the heads by the Ball-Norton separator. This mineral is a common accessory in magnetite ores; it is quite brittle, and, on being crushed, forms a fine powder which has a tendency to stick to the magnetite grains and so find its way into the concentrate. This tendency is less marked when the concentration is carried out in water, and may be quite thoroughly overcome by the use of a spray of wash water while the magnetite is held by the mag- nets. In dry concentration the use of a blast of air directed against the minerals held by the magnets is .beneficial, or the employment of a separator which turns the concentrate over and over as it is passed from pole to pole of opposite sign. Apatite, when present in quantity in the ore, may form a val- uable by-product, as it may be worked up into soluble form and sold as fertilizer. At Mineville, N. Y., the Old Bed ores carry from 1.35 to 2.25 per cent, phosphorus, and the tailing products find a market for their phosphorus content. Two grades of tail- ing are made : the first called first grade apatite, carries 3.55 per cent, iron and 12.71 per cent, phosphorus, equivalent to 63.55 per cent, bone phosphorus. The second grade apatite carries 8.06 per cent, phosphorus and 12.14 per cent, iron, or an equivalent of 40.30 per cent, bone phosphate. At Svarto, near Lulea, Sweden, the ore carries up to 3 per cent, phosphorus as apatite, averaging 1 per cent., and the tailing product from the separators carries 13.7 per cent, phosphorus. This tailing product is concentrated by jigging, and after fine grinding, is treated chemically for the re- moval of remaining magnetite, calcined with soda ash and sold as fertilizer containing 30 per cent, phosphoric acid in soluble form. Concentrates, to be acceptable at furnaces which turn out the best grades of iron, should not carry more than .01 per cent, phos- phorus; ores which are below this limit command a premium. As the apatite is present principally in the waste particles, the higher the grade of concentrate produced the lower will the percentage of phosphorus be, and tests should be made on the ore under con- sideration to determine the economical limit of concentration and elimination of impurities, where the advantage from these ceases to offset the increased loss of iron in the tailing due to the in- creasing ratio of concentration. THE CONCENTRATION OF MAGNETITE ORES 83 Pyrite (FeS 2 , sp. gr. 4.8 to 5.2) is a common accessory min- eral in magnetite ores. It is nonmagnetic and is not influenced by the most intense magnetic fields; it is easily eliminated in the tailing product when not in an excessively fine state of division. Pyrrhotite (Fe 7 S 8 , sp. gr. 4.5 to 4.65) is, on the other hand, usually ferromagnetic, and is drawn into the magnetite concen- trate. It is not so strongly magnetic as magnetite, and sometimes a partial elimination is accomplished; but, generally speaking, it may not be removed from magnetite by magnetic separation. In the case of some complex ores carrying pyrrhotite, blende in a fine state of division, etc., the sulphur is eliminated by roasting. Mag- netite does not lose its magnetism except when exposed to a red heat for a protracted period, and such roasting may be carried out either before or after separation. Roasting for the removal of sulphur is practised on some concentrates produced in Sweden; the heat employed in briquetting fine concentrate accomplishes at the same time an elimination of the sulphur. Another objectionable element occurring with magnetite is titanium in the form of menaccanite (sp. gr. 4.5 to 5.0, composi- tion the same as hematite but with varying proportions of iron replaced by titanium). This mineral is magnetic, but not to so great a degree as magnetite ; a separation of magnetite and menac- canite may be accomplished, but only at the expense of a serious loss of iron in the tailing product. Titanium is an objectionable constituent in iron ores on account of its tendency to form accre- tions in the blast furnace. Results of tests made to eliminate me- naccanite from magnetite will be found in the following table of beach sands, in which the minerals occur as free particles, forming the raw material for separation : SEPARATION OF BEACH SANDS' LOCALITY Per cent, Fe Per cent. Ti Cumberland, R. I Crude sand. 32 4 6 25 Concentrate. . . . 63 3 2 36 Tailing 11 7 8 76 Crude sand 58 25 8 46 Concentrate 68.45 2 13 Tailing 33.3 11 16 Long Island, N. Y Crude sand 48.49 6 78 Concentrate 69 77 trace Tailing . 36 22 11 4 1 Axel Sahlin, E. & M. J., vol. liii, p. 664. 84 ELECTRO-MAGNETIC ORE SEPARATION MAGNETIC SANDS Many attempts have been made to exploit beds of magnetite sands concentrated by waves and streams along ocean beaches and banks of rivers. Such deposits are abundant at Moisie, on the St. Lawrence, and in smaller developments in the United States at Block Island, on Long Island, along the Great Lakes and on the Pacific Coast; abroad, deposits in Brazil and New Zealand have attracted attention. The writer is not informed of any present commercial operation on such deposits; magnetic impurities in the sands (menaccanite, etc.) and the unreliability of the deposits due to their mode of formation have probably been the chief causes of failure. BRIQUETTING With ores which require fine comminution for the liberation of the magnetite the concentrate produced is usually briquetted, as fine concentrate is not acceptable at the furnaces. While the mill at Edison, N. J., was in operation the ore was crushed to pass ^-in. X i in. openings, and the concentrate briquetted. In Swe- den the briquetting of concentrate is usual. In Sweden the plants installed by The Grondal Kjellin Co. have been very successful. The fine concentrate is pressed into bri- quettes without the use of binding material, the moisture in the concentrate being regulated to obtain briquettes sufficiently firm to be removed from the press and loaded onto the cars used in the furnace. These cars are made of a frame covered with fire-brick and have a tongue cast in the frame at the front end and a groove at the rear end, and along the sides are fitted with a flange which dips into a groove filled with sand in the furnace, a string of these cars thus forming an air-tight platform. The furnace is in the form of a tunnel, with track running down the center, and in the middle has a combustion chamber gas-fired. The air needed for combustion is admitted beneath the gas-tight platform at the feed end of the furnace, and, passing the discharge end, returns above the platforms of the cars with their loads of briquettes, enters the combustion chamber, whence the products of combustion continue above the platform to an outlet near the feed end of the furnace. The cool air circulating beneath the platform keeps the wheels and THE CONCENTRATION OF MAGNETITE ORES 85 framework of the cars cool, becomes heated as it at the same time cools the burned briquettes, and enters the combustion chamber hot; the hot gases in turn heat the briquettes and are themselves cooled before they are liberated from the furnace. Owing to this application of the regenerative principle the thermal efficiency of the furnace is good, the gases escaping at a temperature of less than 100 C. and the consumption of coal averaging 7 per cent, of the weight of briquettes burnt, the principal loss in heat is the FIG. 50. GRONDAL BRIQUETTING PLANT, SWEDEN. evaporation of the water in the briquettes. The temperature in the combustion chamber reaches 1,300 or 1,400 C., and at this heat the particles agglutinate sufficiently to make a firm, hard briquette which will stand rough usage. The time consumed by the operation varies with the ore treated and the degree of desul- phurization required; any sulphur in the concentrate is readily eliminated. Briquettes may be made at a lower temperature through the use of various binding materials: at Pitkaranta, Finland, 3 to 5 'per cent, lime is added to the concentrate which is then briquetted, and, after being allowed to set for two weeks, heated to 800 C. ; at Edi- son, N. J., briquettes were made with a resinous binder. Where no 86 ELECTRO-MAGNETIC ORE SEPARATION binder is used the only requirements are a proper proportion of coarse and fine particles to avoid excessively large interstitial spaces, and a sufficiently high heat to sinter the magnetite particles. It has been estimated (P. McN. Bennie) that the cost of briquetting under conditions obtaining in the Eastern United States would be 45 cents per ton. At Mineville, New York, 1 there are extensive magnetic concen- tration works built by Messrs. Witherbee, Sherman & Co. for the treatment of ores from their mines. The ores are of two classes: the New Bed and the Harmony ores carry from 40 to 69 per cent, iron as magnetite and are low in phosphorus; the Old Bed ores are high in phosphorus, carrying from 1.35 to 2.25 per cent. The apatite varies in color and in the size of crystals; that with a deep red color develops magnetic qualities of sufficient strength to carry some free crystals into the concentrate; it also adheres to the crystals of magnetite in a more marked degree than the green or yellow varieties. The yellow crystals break away freely from the magnetic material. When the magnetite is in large pieces in the crude ore, or in large crystals, it is readily handled by cobbing; when the ore is massive, or when the magnetite and apatite crystals are small and intimately associated, finer crush- ing is necessary for the same degree of concentration. The ore from the Harmony Mines is cobbed on a Ball-Norton single-drum separator, and magnetite recovered in large pieces, the waste going for finer crushing and further magnetic treatment to Mill No. 1. The cobbing plant is near the " B " shaft of the Harmony Mines, the skips dumping into a chute which feeds a 30- X 18-in. Blake crusher weighing 29 tons. The crusher is driven from a jack shaft which is belted to a General Electric induction motor of 100 H.P. operating at 440 volts. The ore is crushed to 1^ ins. and is conveyed from the crusher by a 20-in. Robins belt conveyor to a bin over a Ball-Norton single-drum separator. After passing through the separator the cobbed material and tailing fall on separate 20-in. belt conveyors and are transported up an incline to storage bins. These two conveyors are operated by a rope drive. The cobbed product and the tailing storage bins are placed over and alongside, respectively, two tracks upon which standard-gauge hopper-bottom cars run, connecting with mill, railroad and wharves. The cobbed product is called " Harmony cobbed " ; it is a coarse 1 J. H. Cranberry, E. & M. J., vol. Ixxxi, p. 1082. THE CONCENTRATION OF MAGNETITE ORES 87 magnetite with little gangue, and carries about 61 per cent, iron; it is used to mix with lower-grade ores at the furnaces, where it is desirable on account of its coarseness and uniform grade. The tailing carries sufficient magnetite to be crushed and concentrated in Mill No. 1. Mill No. 1 treats crude ores from the " A " shaft of the Har- mony Mine and the tailing from the cobbing plant. The ore is weighed and dumped into a storage bin which feeds a 30- X 18-in. Blake crusher working at 250 R.P.M. After passing through the FIG. 51. MILL NO. 1, MINEVILLE, NEW YORK. crusher the ore is screened to }-in., the fines going directly to a dryer, while the oversize is passed through a size H Gates crusher, after which it also goes to the dryers. The dryer is built of 4- X 6- X 12-in. furnace-brick. The ma- terial slides over cast-iron tees 5 ins. wide on top and with a shal- low stem arranged in horizontal rows, six in a row, with the rows 6 ins. apart, vertically. The bars, in vertically adjacent rows, are staggered. Six rows parallel to and underneath each other are followed by six similar rows at right angles to the first; this ar- rangement obtains from the top to the bottom of the stack. The dryer is made with a bridge wall and an outside furnace. The gases from the furnace divide at the bridge wall, part passing up the chimney and part into the shaft. There are two openings from the shaft into the chimney, which serve to permit the gases 88 ELECTRO-MAGNETIC ORE SEPARATION to pass from one to the other, which tends to raise the capacity of the dryer by reason of the eddying effect set up. From the dryer the material is fed to a Ball-Norton single- drum separator. The concentrate from this machine goes to a shipping bin and the tailing through a set of Anaconda rolls, 40 X 15 ins., with Latrobe steel shells, operating at 50 R.P.M. Thence the ore is elevated and passed over a f -in. tower screen from which it is fed to two Ball-Norton belt-type separators which make concentrate, a shipping product carried to bins on a Eobins belt conveyor, and tailing which passes to two other separators of the same type but operating with a stronger current. These cleaning separators remove the iron to the economical limit, and the tailing here produced is conveyed to a waste dump. The iron product of the cleaning separators is crushed in Eeliance rolls 36 X 14 ins. fitted with Latrobe steel shells and operating at 100 R.P.M. The final cleaning is effected on two other separators of the same type, the magnetite product is carried to shipping bins by a 20-in. belt conveyor, and the tailing to the dump upon an 11-in. belt conveyor, which handles all the tailing from this -mill. The power supply for this mill comprises four Crocker- Wheeler 50 H.P. direct-cur- rent motors, operating at 220 volts, and a 75 H.P. General Elec- tric motor also employed. Mill No. 1 has a capacity of 800 tons of crude Old Bed ore per day, or of 600 tons of Harmony or New Bed ore ; both figures are for 10 hours. Of the feed 77 per cent, is recovered as con- centrate. A table of average results follows: MATERIAL Iron Per cent. Phosphorus Per cent. Lean Harmony ore 50 26 292 Harmony concentrate 64 10 133 Harmony tailing . 13 97 877 Mill No. 2 treats the Old Bed ore, which is high in phosphorus. The treatment here is similar in many points to that in Mill No. 1, and the points of difference only will be described. The power is furnished by three 60 H.P. General Electric motors, form K, operating on 440 volts. A 10 H.P. motor of the same type is used to drive the conveyors to the shipping bins. The mill THE CONCENTRATION OF MAGNETITE ORES 89 is divided into the crushing, the separating, and the re-treating plants, each of which divisions is independent as to power sup- ply; each motor is arranged to control the machinery and con- FLOW SHEET FOR MILL NO. 1 Ore from mine and cobbing plant storage bin 30 in. X 18 in. crusher | in. screen through oversize Gates H crusher I dryer Ball-Norton single-drum separator [coarse concentrate] tailing 40 in. X 15 in. rolls elevator f in. screen | Ball-Norton belt separator Ball-Norton belt separator [concentrate] tailing tailing [concentrate] Ball-Norton belt separator Ball-Norton belt separator [tailing] [middling] middling [tailing] 36 in. X 14 in. rolls Ball-Norton belt separator Ball-Norton belt separator [tailing] [concentrate] [concentrate] [tailing] veyors without reference to the others. Between each two divisions bins are installed having storage capacity for a two-hours' run. The Wetherill Type F separator is working on the same ma- terial as the Ball-Norton belt separators. The Wetherill Type E separators treat the tailing crushed to 10 and 16 mesh, from the 90 ELECTRO-MAGNETIC ORE SEPARATION main battery of separators and make three products. The first belt removes any magnetite liberated by the secondary crushing, which is re-treated on a Ball-Norton belt separator, which makes a ship- ping concentrate and tailing. The second, third and fourth belts THE CONCENTRATION OF MAGNETITE ORES 91 make a hornblende product, which also carries the magnetic apatite mentioned as sometimes being found in these ores. The nonmag- netic discharge from these separators is called first grade apatite, consisting of apatite with pure white silica. The magnetite product from Mill No. 2 averages 65 per cent, iron and higher. The plant is arranged to re-treat this concentrate and produce a magnetite carrying in excess of 71 per cent, iron, which is sometimes made to supply the demand for the manufacture of the so-called " mag- netite " electric lamps. The mill has a capacity of 800 tons of Old Bed ore in 10 hours. A table showing the average analyses of the crude ore and products of this mill for a year's run, together with the approximate amounts of the several products, follows : MATERIAL Amount daily, tons Iron Per cent. Phosphorus Per cent. Bone Phos- phate, perct. Crude ore Old Bed 800 59.59 1.74 Old Bed Concentrate First-grade apatite. . . . . 680 60 67.34 3.55 0.675 12.71 63.55 Second-grade apatite .... 60 12.14 8.06 40.30 The other elements in the Old Bed concentrate are, silica, 2.2 per cent. ; manganese, 0.08 per cent. ; alumina, 0.90 per cent. ; lime, 3.14 per cent.; magnesia, 0.31 per cent.; sulphur, trace. The first-grade apatite is the material passing off unaffected by the magnets of the Type E Wetherill separators ; the second-grade apa- tite is the discharge from the last three belts of the same sep- arators. At Lyon Mountain 1 or Chateaugay Mines, New York, the ores carry from 25 to 40 per cent, iron, though richer bodies are occasionally found which run from 50 to 55 per cent, iron; the average iron content of the ores treated may be given as 35 per cent. The ore consists of magnetite with orthoclase, quartz, and pyroxene; accessory minerals are titanite, zircone and apatite, all present in small amounts. The magnetite is distributed through the mass, and also occurs in aggregates and stringers. The mill flow sheet follows: The concentrate bins are of 600-ton capacity; there are two tailing bins; one for fine and one for coarse material, each of 1 D. H. Newland and N. V. Hansell, Eng. and Min. Journ., vol. Ixxxii, p. 916. 92 through ELECTRO-MAGNETIC ORE SEPARATION FLOW SHEET FOR MILL NO. 2 Crude ore, scales and bin 30-in. X 18-in. crusher f-in. screen oversize 36 in. X 6-in. two-jaw crusher, 225 r.p.m. 6-mesh screen through oversize 36-in. X 14-in. rolls, 100 r.p.m. 30-mesh I dryer tower screens, 288 sq. ft. screening surface 1 6-mesh 10-mesh 6-mesh oversize Ball-Norton Ball-Norton Wetherill Type Ball-Norton 36-in. X 14-in. beU separator belt separator F separator belt separator rolls 100 /\ /\ /\ /\ r.p.m. tailing [concent.] tailing [concent.] tailing [concent.] tailing [concent.] Ball-Norton belt separator /\ tailing [concent.] 32 X 10-in. high-speed rolls, 300 r.p.m. tower screen 1 6-mesh 10-mesh Wetherill Type E separator Wetherill Type E separator [No. 1 apatite] [No. 2 apatite] magnetite [No. 1 apatite] [No. 2 apatite] magnetite I _. I Ball-Norton belt separator /\ [concent.] [apatite No. 2] THE CONCENTRATION OF MAGNETITE ORES FLOW SHEET FOR LYON MOUNTAIN MILL. 700-ton ore bin 2-inch grizzly through oversize 24-in. X 30-in. crusher 1^-in. grizzly through oversize Gates' gyratory crusher I conveyor f-in. grizzly through oversize 40-in. X 30-in. rolls elevator dryer elevator 450-ton bin, two sections 2 trommels in. to f-in. through 350-ton bin, four sections 4 Ball-Norton double-drum separators [concent.] tailing middling 2 sets 40 X 15-in. rolls 2 Ball-Norton double-drum separators [concent.] tailing middling [concent.] tailing Ball-Norton double-drum separator 2 sets 22 X 16-in. rolls 2 Ball-Norton double-drum separators middling returned to 22 X 16-in. rolls [concent.] [tailing] middling returned to 22 X 16-in. rolls 94 ELECTRO-MAGNETIC ORE SEPARATION 200 tons capacity. The dryer is vertical and 40 ft. in height; the ore drops between cross-laid T-bars coming into thorough con- tact with the heated gases. The furnace is situated 10 ft. above FIG. 53. MINE AND TRESTLE AT LYON MOUNTAIN. the bottom of the dryer, and the cold air feeding the furnace passes through the discharge outlet of the dryer, serving to cool the ore and heat the air before entering the fire-box. The tail- FIG. 54. MILL AT LYON MOUNTAIN. ings are screened in a J-in. trommel, and after grinding, used for locomotive sand; the coarse tailings have found a market as rail- road ballast and material for concrete work. Power is furnished THE CONCENTRATION OF MAGNETITE ORES 95 by two 225 H. P. 3-phase induction motors ; the actual running of the mill requires 250 KW. The capacity of the mill is in ex- cess of 50 tons per hour. Sixteen men on each shift operate the mill; of these four attend to the crushers and rolls, three are required on the separators, one man fires the dryer, another is employed as oiler, one works in the motor room, and there is one foreman; the remainder of the shift dump, weigh, load, and sample the ore. Analyses of the crude ore and products follow: Crude Ore, Per cent. Concentrate Per cent. TaUing, Per cent. Total iron 36 50 64.72 9.70 Iron, as magnetite 34 30 64 53 6.00 Phosphorus 019 0.01 0.028 Titanium 089 0.083 0.096 Manganese 256 250 274 The average concentrate is said now to carry 63 per cent, iron and 0.01 per cent, phosphorus; the tailing being reduced to 4 per cent. iron. The Chateaugay ore commands a premium for the manufacture of low phosphorus iron. At Port Orem, New Jersey, the New Jersey Iron Mining Co. is operating a magnetic-concentration plant on magnetite ores. The ore carries magnetite in stringers and grains in a gangue of quartz and some finely disseminated apatite. It is crushed in breakers and rolls to a size varying from 20 mesh to J in., de- pending upon the ore treated. A modification of the Ball-Norton separator is employed. The ore carries about 25 per cent, iron and 1 per cent, phosphorus; the concentrate carries 61 per cent, iron and from 0.045 to 0.3 per cent, phosphorus; the tailing carries from 11 to 17 per cent.. iron. At Hibernia, New Jersey, the Joseph Wharton Mining Co. is operating a magnetic-concentrating plant on magnetite ores which carry from 38 to 40 per cent, iron, 0.04 per cent, phosphorus, and no sulphur. The ore is crushed by Buchanan breakers and rolls to \ in., and is separated upon a Ball-Norton double-drum separator. One hundred tons of ore yield 40 tons of concentrate, 20 tons of middling, and 40 tons of tailing. The middling is recrushed in tight rolls and repassed. The concentrate carries from 63 to 64 96 ELECTRO-MAGNETIC ORE SEPARATION per cent, iron and 0.008 per cent, phosphorus; the middling product carries 40 per cent, iron, and the tailing from 5 to 6 per cent. iron. Dust is withdrawn from the separator by a fan, and after settling in a dust chamber, is sent to the waste dump. At Lebanon, Pennsylvania, the Pennsylvania Steel Co. is operating a plant equipped with Grondal Type V separators. The capacity of the plant is 300 long tons of 60 per cent, iron con- centrate per twelve-hour shift, from a raw ore carrying 40 per cent. iron. At Solsbury, New York, the Solsbury Iron Co. 1 is completing a magnetic-concentration mill equipped with Ball-Norton single- drum and Ball-Norton belt separators, having a capacity of 500 tons in 20 hours. The ore is passed through gyratory crushers, screened, and the oversize on 1.5-in. screens passed over cobbing separators; the undersize, reduced to 30 mesh, is passed through a drying tower and separated on the belt-type separators. It is expected to ship a product carrying 69 per cent, iron from the 30- mesh material and a 60 per cent, coarse- concentrate from the cob- bing separators. At Mount Hope, New Jersey, the Empire Steel & Iron Co. is completing a magnetic cobbing plant equipped with Ball-Norton separators and having a capacity of 600 tons daily. At Benson Mines, New York, the Benson Iron Ore Co. is building a magnetic-separation mill with an estimated capacity of 3000 tons daily. Steam shovels are used to mine the ore, which is crushed in Edison giant rolls and separated on Ball-Norton sep- arators. At Port Henry, New York, the Cheaver Iron Ore Co. is build- ing a magnetic-concentration mill equipped with Ball-Norton sep- arators. At Herrang, Sweden, 2 the Herrangs Grufaktiebolag is operat- ing a magnetic-concentration and briquetting plant of 50,000 metric tons yearly capacity. The ore carries about 40 per cent, iron with 1.2 per cent, sulphur and 0.003 per cent, phosphorus. The gangue consists partly of pyroxene and garnet. The ore is broken to J in. in breakers and ground in Grondal ball mills to 1 mm. This mill consists of a horizontal cylinder built up of longitud- 1 Communicated by F. R. Switzer, Asst. Treas., Utica, N. Y. 2 Communicated by the Grondal-Kjellin Company, Ltd., London, England. THE CONCENTRATION OF MAGNETITE ORES 97 inal steel ribs, with cast-iron end-plates. Through one end of the cylinder the ore is introduced with water over a roller feeder. The crushing is done by chilled cast-iron balls ranging in size from 6 ins. in diameter downward. No screens are required, the degree of fineness to which the ore is ground being regulated by the speed of the water current passing through the cylinder. The wear of the balls is about 2 Ibs. for each ton of ore ground. The FIG. 55. GRONDAL SEPARATORS AT HERRANG. force required for each mill is from 20 to 25 H.P. : the capacity is from 50 to 100 tons per 24 hours, varying .with the hardness of the ore and the fineness to which it is reduced. The pulp from the ball mills is passed through two V-shaped settling boxes from which the sand is drawn off through a pipe at the bottom ; the slime remaining in suspension in the water is sub- jected to magnetic treatment by a pair of Grondal slime magnets. The sand and magnetic slime are treated on Grondal Type III and Type V separators. The concentrate carries from 60 to 65 per cent, iron with 0.17 per cent, sulphur and 0.0025 per cent, phos- phorus. The tailing product carries from 5 to 15 per cent, iron, and the waste slime 9.6 per cent. iron. The powdered concentrate is pressed into briquettes without 98 ELECTRO-MAGNETIC ORE SEPARATION the use of binding material, the moisture in the concentrate being regulated to give a briquette sufficiently firm to bear handling from the press to the car used in the furnaces. The finished bri- quettes carry 63 per cent, iron with 0.003 per cent, sulphur and 0.0025 per cent, phosphorus; they are hard but porous, the per- centage of porosity being 23.9 per cent. Such a plant as is de- scribed above costs in the neighborhood of $50,000 to erect, and requires 20 men, 200 H.P. and 465 gallons of water per minute to FIG. 56. BRIQUETTING PLANT AT HERRANG. operate. 1 It is probable that where a higher scale of wages ob- tains economical operation would demand labor-saving appliances and permit a reduction in the working force. At Edison, New Jersey, 2 there is a large installation for the treatment of magnetite ores, designed by Mr. Thomas A. Edison and erected by the New Jersey and Pennsylvania Concentrating Co. Between the time of the design of this mill and its completion a severe drop was experienced in the iron-ore market, due to the discovery of the Mesabi ore beds; the mill in consequence has never been operated except in an experimental way. The mill 1 Professor Petersson, E. & M. J., May 11, 1907. 2 Richard's "Ore Dressing," p. 1057. THE CONCENTRATION OF MAGNETITE ORES 99 contains so many valuable ideas and is on such a large scale that it merits description. The plant was designed for 4000 tons capac- ity per 24 hours, but has put through 300 tons per hour, which is at the rate of 6000 tons per 20 hours. The ore consists of magne- tite in a gangue of feldspar with a little quartz and apatite. The ore is ruined in open quarries and contains lumps up to 5 tons in weight. It is loaded by steam shovels and dumped on skips holding 6.5 tons each, which are hauled to the -mill on -ears by locomotive. The skips are of the open, flat form used in quarry work and are suspended by two chains and hooks at the front end and by one chain and hook at the rear; they are lifted at the mill by two electric traveling cranes and then, by unhooking the two front hooks, they are dumped to (1). 1. One No. 1 roller feeder, 3 ft. diameter and 6 ft. long. To hopper 6 ft. square and thence to (2). 2. One pair of No. 1 giant rolls, 72 ins. X 72 ins., set 14 ins. apart. To (3). 3. One pair No. 2, or intermediate rolls, 48 ins. X 60 ins., set 7 ins. apart. By No. 1 bucket elevator to (4). 4. One pair No. 3 or first corrugated rolls, 36 ins. X 36 ins., set 3.5 ins. apart. To (5). 5. One pair No. 4 or second corrugated rolls, 36 ins. X 36 ins., set 1.5 ins. apart. To (6). 6. One pair No. 5 or third corrugated rolls, 24 ins. X 20 ins., set J in. apart. By No. 1 belt conveyor and thence by No. 2 bucket elevator to (7). 7. Three No. 1 fixed screens in series, the upper one having 1J X 3-in. slots and the two lower l^-in. X 2J-in. slots. Oversize, bolts, roots, etc., to dump; underside to (8). 8. One No. 1 dryer in the form of a drying kiln with a dis- tributor at the top. By No. 2 belt conveyor, and thence by No. 3 bucket elevator, followed by Edison distributing conveyor to (9). 9. No. 1 stock house, holding 16,000 tons. By No. 4 bucket conveyor to (10). 10. Bin holding 25 tons. By two No. 2 corrugated roller feed- ers to (11). 11. From (10) and (12). Two sets of No. 6 or three high rolls, 36 ins. X 30 ins., set close together, but the feed opens them to about 1 ins. Only one set is run at a time. The crushed ore is carried in succession by two No. 5 belt conveyors, one No. 6 100 ELECTRO-MAGNETIC ORE SEPARATION bucket conveyor, one No. 5 bucket elevator, one No. 7 Edison distributing conveyor, and twenty No. 3 roller feeders to (12). 12. Two hundred and forty No. 2 fixed inclined screens ar- ranged in sixty sets, with four screens in series in each set, having j2g.-in. X i-in. slots. Oversize to (11) ; undersize to (13). 13. Sixty No. 1 Edison magnetic separators. These are 12-in. magnets and are arranged in twenty sets, with three magnets in series in each set. Heads by two No. 8 belt conveyors to (14) ; tailings by No. 9 belt conveyor to (22). 14. One No. 2 dryer in the form of a drying kiln with a dis- tributor at the top. To (15). 15. From (14), (16), and (19). Two sets No. 7 or three-high rolls, 36 ins. X 30 ins., set close together, but the feed opens them to about J in. Only one set is run at a time. The crushed ore is carried in succession by two No. 10 belt conveyors, one No. 11 bucket conveyor, one No. 6 bucket elevator, one No. 12 Edison dis- tributing conveyor, and twenty No. 4 roller feeders to (16). 16. Two hundred and forty fixed inclined screens, No. 3, ar- ranged in sixty sets, with four screens in series in each set, having ^-in. X -in. slots. Oversize to (15) ; undersize to (17). 17. Ninety-six Edison magnetic separators. They are 8-in. magnets and are arranged in thirty-two sets, with three magnets in series in each set. Heads to (18) ; tailings to (22). 18. Eight dusting chambers. Heavy material to (19); light material to (20). 19. Three hundred and twenty No. 3 Edison magnetic separa- tors. They are 4-in. magnets and are arranged in sixty-four sets with five in series in each set. Heads to (21) ; tailings from first or upper magnets to (22) ; tailings from second, third, fourth, and fifth magnets to (15). 20. From (18). One No. 4 Edison magnetic separator for fine material. Heads to (21) ; tailings are sold for paint. 21. From (19) and (20). No. 2 and No. 3 stock houses with a total capacity of 35,000 tons. From these the concentrates pass in succession through the mixers, the briquetting machines and the baking ovens. 22. From (13), (17), and (19), sand house. Tailings are here sized and sold for mortar sand, etc.; on account of proximity to large cities this material is in demand. The labor required for mining, milling, and briquetting is 311 THE CONCENTRATION OF MAGNETITE ORES 101 men per 24 hours, divided into two shifts of 10 hours each, 46 men and boys mining by day and 46 by night ; 24 men by day and 24 by night in the coarse-crushing house to and including (9) ; 32 men by day and 32 by night in the fine-crushing and separating house; and 66 men by day and 41 by night doing general work. Power is furnished by steam. A single Corliss engine of 300 H.P. runs the dynamos for the magnets, for lighting, and for the two electric cranes, which require 50 to 80 H.P. each. A cross- compound engine of 700 H.P. runs the coarse-crushing plant. A triple-expansion vertical engine of 500 H.P. runs the three-high rolls, elevators, conveyors and fans of the fine-crushing and sep- arating plant. The ore contains about 20 per cent, iron and 0.7 per cent, to 0.8 per cent, phosphorus; the heads of No. 1 magnets (13) contain 40 per cent, iron and the tailings 0.8 per cent, iron; the heads from No. 2 magnets (17) contain 60 per cent, iron; the heads from the dusting chambers (18) contain 64 per cent, iron; the heads from the No. 3 magnets (19) contain from 67 to 68 per cent, iron, the mill tailing carries 1.12 per cent. iron. Analysis of the briquettes show 67 to 68 per cent, iron, 2 to 3 per cent, silica, 0.4 to 0.8 per cent, alumina, 0.05 to 0.10 per cent, manganese, a trace each of lime, magnesia and sulphur, 0.028 to 0.033 per cent, phosphorus, 0.75 per cent, resinous binder, and no moisture. One hundred tons of ore yield about 24 tons of concentrate and 76 tons of tailing. The tailing from No. 1 magnets amounts to 55 per cent, of the ore fed to the mill. An especially noticeable feature of the mill is the absence of graded crushing and sizing; this is allowable because fine ore is not considered a source of loss in the magnetic treatment. At Guldsmedshyttan, Sweden, the Guldsmedshytte Aktiebolag is operating a concentrating and briquetting plant of 60,000 tons yearly capacity similar to the Herrang installation above described. Grondal No. V separators are employed. At Svarto, near Lulea, 1 a magnetite ore rich in phosphorus is being separated for the value of the apatite as well as the cleaned iron concentrate. This plant was erected in 1897 by the Norbot- tom Ore Improvement Co. to treat ores from the Gellivara Mines. The ore carries from 0.01 to 3 per cent, phosphorus, averaging 1 J T. Beckert, Zeit. Ver. D. Ing., vol. xli, p. 1307; E. Langguth, "Electro- magnetische Aufbereitung," p. 61; E. & M. J., vol. Ixv, p. 645. 102 ELECTRO-MAGNETIC ORE SEPARATION per cent. ; the average iron content is 58 per cent. The texture of the ore materially aids in the saving of the apatite, as it consists of sharply defined crystals of the different minerals whose cohesion is low. The run of mine ore is subjected to a rough hand picking and then crushed in a Blake crusher and Swensen rolls to pass a 14 mm. screen. The ore is then, dried in a cylindrical dryer 10 meters long by 1.4 meters diameter, inclined at an angle of 5 degrees. The cyl- FIG. 57. MAGNETIC CONCENTRATION MILL AT GULDSMEDSHYTTAN. inder rotates once in 5 seconds and is heated by a stream of hot gases from a fire box at the lower end. The ore is fed to the cylinder by revolving feed plates and at the discharge falls into rolls which reduce it to pass a 1-mm. screen. The separation is accomplished by four Monarch separators, arranged in two independent units, two machines tandem. The first separator of each unit makes a clean magnetite product, a tail- ing rich in phosphorus, and a middling product which is re-treated on the second separator, which makes two products only, tailing rich in phosphorus, and a concentrate. The dust is removed from the Monarch separators by an exhaust fan and treated on a Her- bele wet-type separator. The iron product amounts to 85 per cent. THE CONCENTRATION OF MAGNETITE ORES 103 of the feed and carries 70 per cent, iron, and 0.127 per cent. phosphorus. The tailing from the separators carries 25.5 per cent, iron and 13.7 per cent, phosphorus. The tailing is jigged and the apatite removed as far as pos- sible from the magnetite by water concentration. The apatite product is then treated chemically for the removal of remaining magnetite and ground to an impalpable powder in a ball mill using flint grinding balls. The powdered apatite is mixed with calcined soda ash and heated to a dull-red heat in a two-stage calcining furnace. The product is finely ground, and as shipped contains 30 per cent, phosphoric acid in soluble form; it is used as a fertilizer. The mill flow sheet follows on page 104. FIG. 58. BRIQUETTING FURNACE AT GULDSMEDSHYTTAN. The capacity of the plant is from 2000 to 2500 metric tons per week. The separators take 7 amperes at 100 volts. At Grangesberg, Sweden, a magnetic concentration plant, equipped with Eriksson, Forsgren and Wenstrom separators, is treating ores carrying magnetite and hematite in a quartz gangue. The mill flow sheet follows on page 105. 1 At Dannemora, Sweden, a magnetic cobbing plant constructed 1 Professor Petersson, E. & M. J., vol. Ixxxiii, p. 889. 104 ELECTRO-MAGNETIC ORE SEPARATION FLOW SHEET FOR THE SVARTO MILL Run of mine ore ! hand picking Blake crusher rolls 14 mm. trommel through oversize rolls _| I cylindrical dryer rolls 1 mm. trommel through oversize- I elevator Monarch separator apatite middling [magnetite] Monarch separator apatite [magnetite] I Monarch separator apatite middling [magnetite] Monarch separator apatite [magnetite] jigs and spitzkasten [magnetite waste] apatite ball mill calcining furnace [soluble phosphate] in 1903 is in operation on small ores; the Wenstrom separator is employed. The run of mine ore is subjected to hand picking, a clean magnetite product carrying up to 60 per cent, being thrown out and sent directly to the furnaces. The ore is lifted by ele- vator to the top floor of the mill and dumped into a bin of 1.5 cu. yds. capacity. The mill flow sheet follows on page 106. 1 ' Professor Petersson, E. & M.J., vol. Ixxxiii, p. 890. THE CONCENTRATION OF MAGNETITE ORES 105 FLOW SHEET FOR THE GRANGESBERG MILL Railroad car grizzly with 3-in. openings through Ferraris oscillating screen with 3 to ns. and %-in. holes U to *XK in. Wenstrom cobbing separator nonmagnetic [magnetite] I picking belt j [hematite] [tailing] oversize hand picked through % in. Eriksson separator tailing [magnetite] [hematite] [tailing] Wenstrom cobbing separator [magnetite] nonmagnetic Forsgren separator nonmagnetic [magnetite] 2 jigs [hematite] [tailing] I Ferraris screens with f-in. and f-in. holes f to % in. I 1 jig [hematite] [tailing] | to | in. I 8 jigs [hematite] [tailing] The crude ore carries magnetite, hematite, and pyrites in peg- matite and schistose material. The ore carries about 40 per cent, iron and the concentrate from 60 to 61 per cent. iron. The con- centrate is roasted to remove sulphur. At Flogbeget, 1 near Ludvika, Sweden, magnetic concentration plant built in 1906 and employing the Grondal Type V separator is in operation on magnetic ores. At Klacka, Sweden, the Klacka-Lerbergs Grufvebolag is oper- ating a magnetic concentration plant equipped with Wenstrom cobbing separators for the sizes coarser than f in. and the Grondal Types I and II for the fine sizes. After passing the ball mills 77 per cent, of the pulp passes 0.15 mm. The ore carries from 38 to 39 per cent, iron, and the con- centrate, amounting to 45.9 per cent, of the feed, 58 to 59 per 1 Professor Petersson, E. & M. J., vol. Ixxxiii, p. 889. 106 ELECTRO-MAGNETIC ORE SEPARATION So THE CONCENTRATION OF MAGNETITE ORES 107 FLOW SHEET FOR THE FLOGBEGET MILL Crude ore Blake crusher 19X24 ins. | 130-ton bin feed rolls conveyor belt 2 Grondal ball mills crushing to 0.8 mm. Grondal Type V separator [tailing] middling Grondal Type V separator [tailing] first concentrate tube mill, grinding to . 2 mm. bucket elevator Grondal Type V separator [tailing] - second concentrate Grondal Type V separator [tailing] [final concentrate] water separator briquetting plant cent. iron. The tailing product carries from 12.7 to 14.6 per cent. iron. 1 The plant is operated by 6 men, and requires 20 H.P. and 200 liters of water per minute. The mill produces 20 metric tons of concentrate per day. At Persberg, Sweden? a Grondal Type I separator is treating low-grade magnetite ore carrying from 15 to 20 per cent. iron. The ore is crushed in a ball mill to pass 5 mm. The finished product carries 57 per cent, iron and amounts to 21 per cent, of the feed. The capacity of the plant is 2500 metric tons per annum. Eight men are employed and 55 H.P. are required to operate the plant. The water consumption is 200 liters per minute. The separator is excited by from 5 to 7 amperes at 30 volts. At Romme, Sweden, a lean magnetite ore carrying 22 to 25 per cent, iron is separated by Grondal Type II separators. The ore is crushed in a ball mill to pass 1.5 mm. The finished product car- ries from 60 to 64 per cent, iron and the tailing averages 10.6 per 1 Dr. Weiskopf, "Stahl und Eisen," vol. xxv, p. 532. 108 ELECTRO-MAGNETIC ORE SEPARATION FLOW SHEET FOR THE KLACKA MILL Crude ore Blake crusher to If ins. conical revolving screen, 1 f-ins. and |-in. holes oversize If to f ins. through f in. Wenstrom cobbing separator Wenstrom cobbing separator [magnetite] [tailing] middling [magnetite] [tailing] middling I I 2 Grondal ball mills Ferraris shaking screen, IJ-mm. holes oversize through \ mm. Grondal Type II separator Grondal Type I separator [magnetite] [tailing] [magnetite] middling Grondal Type II separator [magnetite] [tailing] cent. iron. Each separator puts through \ metric ton per hour ; the magnets are excited by 3 amperes at 90 volts. Fourteen men and 60 H.P. are required to operate the plant. The water used amounts to 600 liters per minute. 1 At Strassa, Sweden, Grondal Type I and Type II separators are treating ore carrying 36.8 per cent, iron, 0.014 per cent, phos- phorus, and 0.11 per cent, sulphur. The ore is crushed to pass 1 mm. in ball mills. The finished product carries 61.58 per cent, iron, 0.006 per cent, phosphorus, and 0.045 per cent, sulphur; it amounts to 45.5 per cent, of the raw ore. The tailing carries 12 per cent. iron. The mill has a capacity of from 30 to 40 metric tons daily and employs 17 men. From 30 to 35 H.P. are required to operate the plant, and from 150 to 200 liters of water are used per minute. The separator is excited by 1.7 amperes at 30 volts. 1 A Grondal Type V separator and a briquetting plant has been added to this installation. At Bredsjo, Sweden, a Grondal Type II separator is treating a magnetite ore carrying 45.3 per cent, iron, 0.0083 per cent, phos- phorus, and 0.198 per cent, sulphur. The ore is crushed to pass i Dr. Weiskopf, "Stahl und Eisen," vol. xxv, p. 532. THE CONCENTRATION OF MAGNETITE ORES 109 1.5 mm. The finished product amounts to 48.6 per cent, of the feed and carries 64 per cent, iron, 0.0023 per cent, phosphorus, and 0.082 per cent, sulphur. The tailing carries 7 per cent. iron. 40 H.P. are required to operate the plant, which employs 4 men and has a capacity of 30 metric tons per day. 1 A Grondal Type V separator has recently been added to this plant. The concentrate is briquetted. The present capacity of the plant is 40,000 metric tons per annum. At Norberg, Sweden, an Ericksson separator is used at the Kall- mora Separating Works, treating magnetite ores. At Bagga, Sweden, a Grondal Type I separator is working on an ore carrying magnetite, hematite, amphibole and quartz. It averages from 30 to 40 per cent. iron. The finished product amounts to 63.7 per cent, of the raw ore and carries from 60 to 62 per cent. iron. Ball mills are used for fine grinding. The mag- nets are excited by from 8 to 10 amperes at 35 volts. 1 At Lomberget, Sweden, a magnetic-concentration mill employ- ing the Forsgren separator has been in operation on magnetite ores since 1903. At Bjornberget, Sweden, a magnetic-concentration mill em- ploying the Eriksson separator has been in operation on magne- tite ores' since 1904. At Kungsgrufvan, Sweden, a magnetic-concentration mill em- ploying the Froeding separator has been in operation on magnetite ores since 1905. At Langgrufvan, Sweden, a magnetic-concentration mill em- ploying the Froeding separator has been in operation on magnetite ores since 1905. A Morgardshammer separator has recently been added to this plant. At Vintjarn, Sweden, a magnetic-concentration mill erected in 1906 is in operation on magnetite ores employing the Hallberg separator. At Hjulsjo, Sweden, a magnetic-concentration mill erected in 1906 is in operation on magnetite ores. The Grondal Type V separator is employed. The concentrate is briquetted. At Lulea, Sweden, the Karlsvik Mill, built in 1906, is treat- ing magnetite ores on Grondal Types IV and V separators. The concentrate is briquetted. The crude ore carries 1 per cent, phos- phorus, which is reduced to 0.005 per cent, in the concentrate. * Dr. Weiskopf, "Stahl und Eisen," vol. xxv, p. 532. 110 ELECTRO-MAGNETIC ORE SEPARATION At Uttersberg, Sweden, the Uttersberg Bruks Aktiebolag is operating a magnetic-concentration mill on magnetite ores. The plant was built in 1906 and has a yearly capacity of 12,000 metric tons. The Grondal Type V separator is employed. The con- centrate is briquetted. At 8yd Varanger, Norway? a magnetic-separation plant hav- ing a yearly capacity of 1,200,000 tons of crude ore is being in- stalled. It will contain 56 Grondal ball mills, 200 Grondal N"o. 5 separators, and 20 Grondal briquetting kilns. The ore will be mined by steam shovels. The test runs on this ore give the follow- ing results : Per cent. Iron Per cent. Sulphur Per cent. Phosphorus Crude ore 38.0 0.066 0.030 Ooncentrate 68.3 0.026 0.014 Tailing. 5 5 Briquette 68 006 014 It is expected to produce 600,000 tons of briquettes yearly, which will be shipped to Germany. At Salangen, Norway, a Grondal concentrating and briquetting plant having a yearly capacity of 300,000 tons of ore is being in- stalled. The test runs on this ore give the following results : Per cent. Iron Per cent. Sulphur Per cent. Phosphorus Orude ore 35 7 039 0.23 Concentrate 69 3 019 009 Tailing 4 9 It is expected to produce 100,000 tons of briquettes yearly, which will be shipped to Germany. At Langbau Eisberg, Kantorp, and Striberg, Sweden? the fol- lowing type of mill is used for the separation of magnetite and hematite from waste: 1 Jour. Can. Mining Inst., vol. xi, p. 153. P. McN. Bennie. 2 Dr. Weiskopf, "Stahl und Eisen," vol. xxv, p. 532. THE CONCENTRATION OF MAGNETITE ORES 111 Feed Gates crusher ball mill with 1-mm. screen Grondal separators, Types I and II [magnetite] nonmagnetic jigs [hematite] [waste] At Pitlcaranta, Finland, a plant equipped with Dellvik-Grondal separators has been in operation since 1894, treating a low-grade magnetite ore. The ore carries magnetite in tough serpentine ac- companied by small amounts of blende, pyrite, chalcopyrite, and pyrrhotite. The ore, which is intimately mixed, is crushed with difficulty; the average size of grain is somewhat less than J mm. The ore carries on an average 30 per cent, iron, of which 80 per cent, only is in the form of magnetite, the balance being chemic- ally combined as sulphides and silicates; it carries from 4 to 5 per cent, sulphur. The first mill was built in 1894 and was en- larged to 350 metric tons daily capacity in 1898; it is situated at Ladogasse 3.5 to 7 km. from the mines, with which it is connected by rail. The tracks from the mines deliver ore into bins 10 meters above the sill floor of the mill, from which the crushers are fed direct. There are four rock breakers which handle ore up to 250 mm. size. From the breakers the ore is delivered in egg size to eight Grondal ball mills. The ball mills are cast-iron cylinders lined with armor plate ; there are two sizes employed. Four of the mills are 1.75 meters in diameter by 0.8 meter long, and four are 2 meters diameter by 1 meter long. The cylinders are turned on an inclined axis, the crushing being accomplished by cast-steel balls. The smaller mills are employed on the more easily crushed ores and put through from 8 to 50 tons in 24 hours; the larger mills were designed especially for the hardest ore and treat 30 tons per 24 hours. The linings are renewed once in 15 months, and fresh balls are introduced from time to time. The ore is crushed to pass 1 mm., but a large percentage is much finer; a screen analysis of the discharge of ball mills follows: 112 ELECTRO-MAGNETIC ORE SEPARATION Size in millimeters Per cent, of Total Over 1 mm . . . 1 7 1 toO 5 .... 1 2 5 to 33 3.8 33 to 25 7.5 25 to 16 22 4 16 to 125 19 6 Through 125 43.8 100.0 Tests on the discharge of the mills show but 44 per cent, of the magnetite to exist as free particles, and as a result the con- centrate rarely exceeds 61 per cent, iron; a higher-grade concen- trate could be made, but it would be at the expense of such a loss in the tailing as to eliminate profit on this low-grade ore. The products from the old mill carried from 65 to 71 per cent, iron in the concentrate and 1 to 1 per cent, iron present as magnetite in the tailing; the new mill concentrate carries from 59 to 61 per cent, iron, and the tailing from J to 1 per cent, iron present as mag- netite. The raw ore contains from 0.08 to 1 per cent, phosphorus; the concentrates average 0.042 per cent, phosphorus; the sulphur in the concentrate is 0.6 per cent., mostly as blende, which mineral is intimately associated with the magnetite. The separators take 8 amperes at 35 volts and put through from 25 to 50 tons of ore per day, according to the iron content. The ball mills deliver by gravity to the separators which are 2 meters above the working floor and 5 meters above the highest waste discharge. The fine concentrate is allowed to drain for a few days and is then pressed into briquettes which are sintered into a firm mass by exposure to a heat of 800 C., which also largely eliminates the sulphur. In 1900 425-kgm. of 61 per cent, concentrate were made from one ton of raw ore. One metric ton of 61 per cent, concentrate cost $3.40 during the same period. Power is derived from a waterfall 7 km. from the mill and transmitted by electricity: the ball mills, crushers, and separators take 160 E.H.P. and the elevator, pumps, and railroad respectively THE CONCENTRATION OF MAGNETITE ORES 113 8, 6, and 25 E.H.P. 8 C. 1 In winter the feed water is warmed to 7 or SEPARATION- OF MAGNETITE AS AN IMPURITY Certain ores of zinc and lead, corundum, etc., carry magnetite where this mineral is regarded as an objectionable impurity and from which it is eliminated by magnetic separation. At Santa Olalla, Huelva, Spain, 2 the Sociedad Minas de Cala is operating a magnetic separating plant on magnetite ores carry- ing chalcopyrite, and also experimenting on a mixture carrying the same minerals with hematite and silica. The ore is reduced by jaw crusher to 3 to 5 cm. and delivered by bucket elevator to hopper bins having capacity for 10 hours' run. From these bins the ore is fed to a Smidt ball mill by an Eriks- son automatic feeder, and reduced to pass 1 mm. This pulp is sent by launder to an Eriksson magnetic separator. The results of the separation follow: SEPARATION OF MAGNETITE AND QUARTZ Fe Per Cent. SiO, Per Cent. Feed 39 33 26 86 Concentrate (1 mm.) 55 20 15 06 Concentrate (^ mm ) .... 61 02 9 26 Waste 6 21 54 50 SEPARATION OF CHALCOPYRITE FROM MAGNETITE Fe Per Cent. Cu Per Cent. Feed 61 55 27 Magnetic product 65 47 06 Nonmagnetic product. 50 00 1 18 1 Gustav Grondal, "Oest. Zeit. B.-, H.- und S.-Wesen," vol. xlix, p. 429; Edouard Primosigh, ibid., vol. xlvii, p. 51; "Revista Minera," vol. liii, p. 109. 2 Communicated by Don Mariano Auguatin, Ingeniero de Minas, Santa Olalla, Spain. 114 ELECTRO-MAGNETIC ORE SEPARATION The reseparation of the magnetite concentrates is made to remove the copper with its combined sulphur; what disposal is made of this iron-copper-sulphur product could not be learned. At the Ryttshytans Zinc Mines, Sweden, a Grondal Type I separator 1 is employed to separate magnetite from blende. In Raglan Township, Ontario, the Canada Corundum Co. 2 employs a magnetic separator in cleaning corundum concentrates. The ore carries corundum associated with magnetite and mica in a feldspathic gangue. The ore is crushed with breaker and rolls and concentrated with jigs and tables. The concentrates passing 8 mesh are dried and the magnetite removed by the separator. The output is about three tons of cleaned concentrates per day. 1 Dr. Weiskopf, "Stahl und Eisen," vol. xxv, p. 532. 2 Richards, "Ore Dressing," p. 1078. VI THE SEPARATION OF PYRITE AND BLENDE THE co-occurrence of the sulphides of zinc and iron is fre- quent; blende and pyrite, or marcasite, are found together in im- portant ore bodies which are worked for the value of the contained zinc, and many lead deposits in their lower horizons carry zinc and iron sulphides. Galena may be separated in the wet way from both of the lighter sulphides, but the specific gravities of the lat- ter are too similar to permit their separation from each other by any method depending upon specific gravity. The presence of iron in zinc ores is very undesirable for metal- lurgical reasons connected with the reduction of zinc, and this, fact, together with the similar specific gravities of the sulphides of these metals, gives rise to one of the most important fields of mag- netic separation. The middling products from mills treating galena-blende-py- rite ores frequently carry an important value in gold and silver locked up in the pyrite. The presence of any considerable amount of zinc in such middling renders it unsalable at the lead smelters, or involves the payment of a heavy penalty for each unit of zine above a certain standard, usually 10 per cent., and their iron con- tent renders them unsalable at the zinc smelters. If, however, the pyrite and blende are separated, two valuable products result. Pyrite (FeS 2 , sp. gr. 4.8 to 5.2) is almost nonmagnetic ; in some specimens it has been reported as diamagnetic, and in others as possessing a feeble paramagnetism ; it is not attracted by the fields of the most powerful separators. On roasting it is readily trans- formed into the magnetic sulphide, and on the continuation of the roast, into a strongly magnetic oxide analogous to the mineral magnetite. Pyrite, in some specimens, on being heated at a quite low temperature for a minute or two, develops an iridescent film of magnetic sulphide, which imparts sufficient permeability to cause it to be attracted by fields of low intensity. The varying magnetic 115 116 ELECTRO-MAGNETIC ORE SEPARATION behavior of pyrite may be in some way connected with its several crystalline forms. Marcasite (FeS 2 sp. gr. 4.6 to 4.85) is similar to pyrite in its magnetic qualities. Sphalerite or blende (ZnS, sp. gr. 3.9 to 4.2) varies in per- meability according to the percentage of isomorphic iron and man- ganese sulphides contained by it. The pure sulphide of zinc, as represented in the light-straw colored varieties, is diamagnetic, while the highly ferriferous variety, called marmatite and "black jack/' in which the combined iron may reach 12 or 14 per cent., may be even ferromagnetic. Pure blende, or " rosin jack," carries 67 per cent, zinc, while "marmatite" rarely carries over 51 or 52 per cent. zinc. Blende carrying as low as J per cent, iso- morphous iron becomes appreciably magnetic upon roasting. Blende is being separated as a magnetic product at several mills in Colorado, in Europe and in Australia. The degree of magnetism appears to depend upon the ratio of the two sulphides contained, which varies from 3 parts ZnS to 1 of FeS 2 , to 5 parts ZnS to 1 of FeS 2 , and in any given ore the individual crystals of sphalerite may vary from the nonmagnetic straw-colored blende to strongly magnetic marmatite. A dark color is not necessarily indicative of high iron content. SEPARATION" OF ROASTED PYRITE AND MARCASITE FROM NONMAGNETIC BLENDE As neither pyrite nor marcasite possesses sufficient permeability to be attracted by even the most intense magnetic fields, a pre- liminary roast is necessary before they may be separated from the blende. There are two methods for rendering iron sulphide magnetic: a slight roast with the formation of the magnetic sul- phide, or a more complete roast with the formation of a magnetic oxide of iron. The magnetic compounds of iron formed by roasting the sul- phide are strongly magnetic and are attracted by fields of low intensity, but, as the quality of the separation made depends en- tirely upon the uniform magnetic quality of the material pre- sented to the separators, the roasting is the most important step in the whole process. Almost any separator can make clean products THE SEPARATION OF PYRITE AND BLENDE 117 when fed with properly roasted material, but no separator can do satisfactory work upon a poorly roasted feed. Upon roasting pyrite or marcasite with access of air a portion of the sulphur is driven off as S0 2 and the nonmagnetic FeS 2 (pyrite) is transformed, superficially at least, to Fe 7 S 8 (analogous to pyrrhotite) which is strongly magnetic. This operation is a difficult one to control in most furnaces, however, as it is easy to oxidize some of the iroc, making a product of uneven permeability. If the sulphur is completely driven off by the roast Fe 3 4 re- sults, which is strongly magnetic. Should the roast be carried farther, another atom of oxygen is taken up by the iron and Fe 2 3 results, which is quite feebly magnetic, being analogous to the min- eral hematite. These two oxides of iron pass from one to the other, according as the atmosphere of the furnace is reducing or oxidizing. The artificial magnetite, the black oxide produced by the roast, loses its magnetism more readily than the natural mag- netite, and is converted into the feebly magnetic red oxide. This may in turn be converted back to the black oxide by exposing it to a reducing atmosphere at the end of the roast. Pyrite and marcasite begin to loose their sulphur and change over into the magnetic sulphide, and finally into the oxides, at a temperature of 370 C., and the roast must be conducted between this point and the ignition point of blende, which is about 600 C. Below 400 to 460 degrees the pyrite does not become thoroughly magnetic and the usual temperature employed is just below the ignition point of blende. If this temperature be slightly exceeded the only result is a superficial oxidation of the blende ; a tempera- ture of 620 degrees was attained without harmful results in some experiments carried out by Messrs. Hofman and Norton. 1 Should this heat be maintained, however, a serious loss would result through the oxidation of the fine particles of blende. In plants where the quantity of material treated is sufficient to make it feasible, the ore, or concentrate, should be sized ber fore- roasting. Eoasting and magnetization take place from the surface inward, and in a mass of ore composed of coarse and fine particles the finer sizes will have been overroasted before the lumps have been affected to their centers. If a medium roast is given the mixture the larger lumps will have centers of un- changed pyrite, while the fine particles may have been converted Trans. A.I.M. E., September, 1904. 118 ELECTRO-MAGNETIC ORE SEPARATION into the nonmagnetic sesquioxide, and it is evident that a clean separation of such a product is out of the question. If a quick, light roast is carried out with a view to the formation of a film of magnetic sulphide on the surfaces of the particles, a fairly uni- form product may result from the treatment of unsized material; the same is true when the roast is carried to the complete forma- tion of the magnetic oxide in a reducing atmosphere. With large lump ore it is difficult to tell when the roast has penetrated to the centers of the lumps, and the process requires too much time. With very fine material the interstitial spaces are small, and, the material having a tendency to pack, the reducing gases reach all the particles with difficulty; also, very fine particles of blende may be converted into the oxide of zinc and pass out of the furnace with the gases, and dust chambers must be provided to save as much of this material as possible. While it may not be definitely stated, certain experimenters have given 8 mesh as the best size for blende- pyrite concentrate for good results in roasting. If the roast be conducted with too free an access of air, the particles will be made up of concentric rings of different mag- netic permeability, the surface will consist of a layer of nonmag- netic sesquioxide beneath which will be found a layer of black magnetic oxide, enclosing, perhaps, a core of unchanged sulphide, the division between the two being marked by a layer of the mag- netic sulphide. Unless the roast has been carried too far the mag- netic oxide will impart sufficient permeability to the whole particle to cause it to be taken up by the magnet, unless decrepitation breaks these layers apart, in which case each behaves according to its individual permeability, and nonmagnetic iron finds its way into the blende concentrate. Separation should be carried out upon ore as it comes from the furnace, cooled in such a manner as not to induce decrepitation, and no part of the separator feed should be crushed after roasting. Particles which have been fritted to- gether should be recrushed, but also reroasted before separation. Much of the concentrate roasted ranges in size from T \ to J in., but after roasting, all but a small percentage of the iron in this concentrate will pass a 20-mesh screen. This is due to the breaking up of the particles under the influence of the roast. If a partially roasted particle of iron sulphide is broken, a network of fine black lines of the oxide will be seen, reaching perhaps the center of the particle, the spaces between them being composed THE SEPARATION OF PYRITE AND BLENDE 119 of unchanged sulphide. This seems to indicate that the roast pro- ceeds more rapidly along the boundaries of the crystals (forming an aggregate of the mineral) than through the individual crystals, causing these aggregates to split up. The tendency of pyrite and marcasite to decrepitate at a lower temperature than blende has been employed to separate these minerals, screening following the roast. , There are many types of roasting furnaces on the market which, with proper management, may be made to do efficient work. The principal requirement is that the admission of air shall be under complete control. Several forms of mechanical furnaces are in extensive use, and the old shaft furnaces may be made to do good work ; for the finer sizes, hearth furnaces do good work. The time required for a good roast may be said to vary from -J hour to 2 hours for fine concentrate, up to 3 or 3 hours for coarse material, roasting for the magnetic oxide. The roasted pyrite or marcasite should be dark-brown in color, almost black; a decided reddish tinge indicates overroasting. The magnetic sulphide is black, and requires little time for its formation. If there is tendency toward overroasting, the air inlets should be sealed up, and all entering the furnace should be made to pass through the fire box, where such is used. The addition of a little coke or hard coal may be resorted to at the end of the roast to re- convert to the black oxide any nonmagnetic red oxide which may have been formed. Subjecting the roasting ore to the action of re- ducing gases is successfully employed in Europe for this purpose. It is probable that in many American plants whose output is not sufficient to warrant the installation of the more expensive types of furnace a small hearth would be advantageous for the re-treatment of the middling product from the separators. The roast may be considered satisfactory when the separators make a recovery of from 85 to 90 per cent, of the total zinc in the raw concentrate, and yield a clean blende product carrying 1.5 to 2.5 per cent, iron due to pyrite. The iron content may, after the best work, reach a much higher figure, due to the presence of com- bined iron in the blende. The Joplin and Wisconsin ores usually do not carry to exceed J per cent, combined iron. The Leadville ores offer a more difficult problem, owing to the presence of blende of all degrees of permeability : an extraction of 75 to 85 per cent, in a product carrying 40 to 50 per cent, zinc is good work. Direct 120 ELECTRO-MAGNETIC ORE SEPARATION separation of these ores yields a lower grade product and a less extraction. SEPARATION OF MAGNETIC BLENDE FROM PYRITE Magnetic blende is of frequent occurrence at Leadville, Col- orado, and other parts of the Rocky Mountain region, and is also found in important ore bodies in Europe and Australia. The Col- orado ores of this type usually carry galena, sphalerite, and pyrite with subordinate amounts of pyrrhotite; the galena is usually ar- gentiferous and the pyrite may or may not carry the precious metals. The sulphide of zinc in these ores varies from straw-col- ored blende to marmatite in other words, from a nonmagnetic mineral to one possessing sufficient permeability to be removed magnetically in its raw state. The determination of a process for the treatment of any ore carrying magnetic blende should be done by actual test on suffi- ciently large samples to indicate commercial results. If the blende is wholly or in large part magnetic, then a direct separation on high-intensity separators is in order. Individual particles of marmatite may show ferromagnetism, but the bulk of the mineral in an ore is usually less strongly magnetic, and a field of high intensity is necessary to obtain a satisfactory recovery. The value of the pyrite in gold and silver plays an important part in the determination of the process to be followed; if its value is neg- ligible, then any loss of blende in the nonmagnetic tailing merely results in a decrease in the percentage recovery of the zinc ; but if the pyrite is valuable for contained gold and silver, this nonmag- netic tailing should be kept below the zinc penalty limit set by the lead smelters to which this product is destined. In choosing be- tween direct separation and separation after roasting, the higher value at the smelter of roasted pyrite over the raw sulphide should be considered. At one plant treating marmatite ores, the nonmagnetic blende remaining in the pyrite tailing is removed by electrostatic sep- arators. When there is much of this nonmagnetic blende in an ore, and the value of the pyrite in gold and silver is sufficient, the nonmagnetic tailing may be roasted, and the iron and blende re- covered separately as clean products. With ores which require roasting, a preliminary treatment of the raw ore on magnetic sep- THE SEPARATION OF PYRITE AND BLENDE 121 arators to remove the strongly magnetic marmatite is advisable, as the actual passing of the ore over a separator represents but a very small proportion of the total cost of preparing the ore for mag- netic treatment, and this magnetic blende would otherwise find its way into the roasted iron tailing. At Koleomo, Colorado, the Kimberly-Wilfley Mines Co. is sep- arating pyrite from galena and blende after roasting to the mag- FIG. 59. WILFLEY ROASTING FURNACE. A, Fire box; B, up-cast flue; C, down-cast or roasting flue; D 1 , D 2 , D 3 , D 4 , water-jackets; E, water-jacket screw conveyor; F, dust chamber; G, dust flue; H, stack; 7, feed opening; J, ore feeder. netic sulphide. The ore carries galena, slightly magnetic blende, and pyrite containing gold and silver values. The ore from the mine, after passing through a breaker, is crushed by 16 X 42-in. rolls to f in. and is then sized and gradually reduced to 14 mesh by three sets of 12 X 36-in. rolls. The crushed ore is transported by a conveyor belt to a Wilfley roasting furnace, a cross section of which is shown in the accompanying figure. ; The ore fed at the top of the furnace falls upon a plate set at an angle of 50 degrees, thence through the down-cast flue, striking upon water-cooled plates, and finally into a water-jacketed screw; 122 ELECTRO-MAGNETIC ORE SEPARATION conveyor which discharges it from the furnace. The roasting is carried out by the hot gases from the fire box together with the heat generated by the oxidizing pyrite upon individual particles, which are cooled to a certain extent before coming into contact with other particles, preventing fritting, and gives a uniform roast to the small and large particles alike. The ore is cooled sufficiently before passing out of the furnace to prevent a continuance of the oxida- tion when it comes into contact with the atmosphere. The dust FIG. 60. DINGS SEPARATORS AT KOKOMO, COLORADO. is collected in F and G, and is discharged from the furnace by a screw conveyor. The fuel consumption is low, most of the heat necessary for the roasting being generated by the oxidation of the pyrite, the coal being used to regulate the temperature of the roast. It is said that during steady operation the temperature of the roasting flue does not vary more than 10 degrees, and a variation in the feed does not cause a variation of more than 100 degrees. The temperature is indicated by a pyrometer, the thermal couple being placed in the roasting flue above the water jackets. The ore, as delivered from the furnace, is elevated and passed through a water-jacketed revolving cylindrical cooler, in which the temperature of the ore is reduced to 60 F., and sent to the sepa- rator bins. Ten Dings separators are employed to effect the separation of the roasted ore. The magnetic iron product from these separators falls upon a belt conveyor which delivers to ship- THE SEPARATION OF PYRITE AND BLENDE 123 ping bins, whence it is shipped to the lead smelters. The nonmag- netic product falls upon another belt conveyor delivering to an elevator, is mixed with water and passed to two Richards classi- fiers, making four sizes ; thence it is fed to eight Wilfley roughing tables. The pulp and middlings are sent to eight other Wilfley tables placed on the floor below and directly beneath the roughing tables. The tabling plant is operated in two units of eight tables each : one lower table receives all the zinc middlings, one all the lead middlings, one all the iron middlings not rendered magnetic in the roast, and one all the silica middlings from four of the rough- ing tables above. The overflow from the classifiers, the crosswash from the feed end of the Wilfley tables, and the dust from the furnace go to a Buckingham filter tank, which classifies and de- waters the fines and makes a thick pulp that is fed to a Wilfley table and to a True vanner fitted with egg-shell belt. It is stated that the roasting of the ore so changes the galena and blende that even the finest particles will not float, and renders easy an other- wise difficult separation. 1 The plant has a capacity of 250 tons per day. At Denver, Colorado? the Colorado Zinc Co. is operating a plant of 100 tons daily capacity, equipped with a Wilfley furnace and Dings separators, with Wilfley tables for the separation of the nonmagnetic product. The ore is crushed dry to 16 mesh, passed through the furnace where from 10 to 15 per cent, sulphur is driven off, transforming the pyrite to the magnetic sulphide. Af- ter cooling in a water-jacketed cylindrical cooler, the ore is passed over 4 Dings separators which remove the iron, amounting to from 40 to 50 per cent, of the ore, while the nonmagnetic product is sent to the tables. The furnace is 8 X 8 ft. in section and 30 ft. high, and has a capacity of 100 tons per 24 hours. With ore carrying 25 per cent, iron as pyrite the furnace requires 1 ton of coal per day when operated at capacity. The separators deliver a finished iron product and a silica-zinc-lead middling which is sent, after sizing, to four Wilfley tables; these tables are operated to make finished zinc concentrate, while the lead-zinc middling and the silica-zinc middling are re-treated on three other tables. It is stated that, due to the action of the roast, the galena and blende 1 Communicated by F. W. Gregory, Kokomo, Colorado, and from E. & M. J., vol. Ixxxv, p. 453. J. M. McClave. 'E. & M. J., vol. Ixxxv, p. 453. J. M. McClave. 124 ELECTRO-MAGNETIC ORE SEPARATION do not, in the finest particles, float, and that the wash-water from the tables runs clear, while when the ore is tabled without roasting the wash water contains from 10 to 15 per cent, of lead-zinc slime. At Galena, Illinois, the Joplin Separating Co. is operating a custom plant whose raw material is derived from the adjacent Wisconsin zinc field. Zinc-iron concentrate from mills not equipped with separating plants forms the greater part of the material treated; raw ores carrying zinc, iron, and lead sulphides are also purchased and, after water concentration, magnetically cleaned. This plant is well managed and, although running on all grades and classes of material, probably represents the best practise of the district. The mill includes a roasting furnace of 40 tons capacity per 24 hours, three Cleveland-Knowles 12-in. belt sep- arators, and a complete equipment of jigs, tables, etc., for the concentration of raw ores preliminary to roasting and magnetic separation. The Wisconsin ores carry blende, galena and marcasite in a limestone gangue. The ore is easily crushed and yields the bulk of its component minerals when crushed to 4 mesh, although a finer comminution is carried out on the middling products from the jigs. The blende is of the variety known as " rosin jack," and rarely carries to exceed -J per cent, combined iron: some darker- colored blende is produced in the district, but none that came under the writer's observation was sufficiently magnetic to be affected by the low-intensity magnetic fields employed in the sep- aration of these ores. The purity of the ores is well illustrated by the fact that 60 per cent, concentrate is the standard grade from which prices are figured, and iron in excess of 2 per cent, is pen- alized at the rate of $1 per unit. In this mill the blende-marcasite concentrate is delivered to the feed hopper above the roasting furnace by a 6-in. bucket ele- tator. The feed hopper is 36 ins. square and slopes from two sides to a point. The feeder, of the stirrup type, delivers into a sheet- iron spout which extends well into the neck of the furnace. The furnace is of the revolving-cylinder type, built by the Galena Iron Works, and is 32 ft. long by 5 ft. in diameter. It is built of boiler plate and lined with fire brick. This shell is fitted with two tires which rest upon two sets of rollers, the dis- tance apart of which may be adjusted to give any desired inclina- tion from the horizontal to the axis of the cylinder, so accelerating THE SEPARATION OF PYRITE AND BLENDE 125 or retarding the passage of the ore through it. Revolution is im- parted to the cylinder by gearing. At either end the furnace is nar- rowed by fire-brick walls to 2 ft. 6 ins. for connection with the fire box at the discharge end, and with the dust chamber, which also serves as foundation for the stack, at the feed end. These con- nections are made through cast-iron necks projecting into the openings at each end of the cylinder. The fire box is fitted with a, grate 4X5 ft. in area, which burns about two tons of soft coal in 24 hours. The roasted ore is discharged through the annular opening between the projecting neck of the fire box and the end of the cylinder. A fire-brick wall reaching the horizontal diameter of the fire-box neck causes the hot gases to impinge on the roof of the cylinder and not to strike the hot ore. A 24-in. rotary blower is mounted alongside the furnace and suitably connected to furnish air under pressure beneath the grate. This is useful in raising the temperature of the charge quickly should it fall below normal and an imperfectly roasted product be likely to result. The roasted ore discharged from the furnace falls upon a cast- iron plate, and is conveyed to one side by scrapers mounted upon a traveling chain, and falls into wheelbarrows. The furnace makes two revolutions in three minutes, the ore remaining in it for approximately two and one half hours. The hot ore is wheeled to a cooling floor 26 ft. wide by 60 ft. long; here it is spread a few inches deep and allowed to remain 12- hours. The ore was formerly cooled by means of a spray of water,, but this gave rise to an excessive amount of fines, produced by the sudden cooling, so that it was abandoned in favor of the cooling floor, in spite of the increased cost of handling entailed. The cooled roasted material is raised to the top of the mill by a bucket elevator and fed into a trommel 36 ins. in diameter. This- trommel is fitted with two screens, each delivering its undersize to a separate bin, while the oversize from both is crushed in tight rolls and returned. The first screen has g*g-in. and the second -ft-in. round punched holes. The roasted concentrate passing -^-in. is treated on separator No. 1, which carries } ampere on the first magnet and 6.5 amperes on the second, making a clean iron tailing product and a clean nonmagnetic zinc product. The material between ^ and fVin. is treated on separator No. 2, which carries the same current, re- 126 ELECTRO-MAGNETIC ORE SEPARATION FLOW SHEET Raw concentrate elevator No. 1 hopper by automatic feeder to 5 ft. X 32 ft. cylindrical roasting furnace by wheelbarrow to 26 ft. X 60 ft. cooling floor by wheelbarrow to oversize tight rolls elevator No. 2 trommel, -ft-in. holes and &-in. holes through A-in. through ^/-in. separator No. 2 separator No. 1 I I I I I I 1st magnet 2d magnet nonmagnetic 1st magnet 2d magnet nonmagnetic [tailing] middling [grade A zinc conct.] [tailing] middling [grade B zinc conct.] middling rolls, set tight elevator No. 3 i^-in. trommel through oversize | separator No. 3 1st magnet 2d magnet nonmagnetic [tailing] middling to tables returned to furnace spectively,. on the two magnets. The middling product from both these machines is crushed in tight rolls to ^ in., and fed to sep- arator No. 3, which also makes three products. The tailing from the first magnet is discarded while the middling from the second magnet is sent to the tables in the concentration mill. The non- magnetic product from this separator carries most of its iron as unchanged pyrite, liberated by the recrushing of the middling from separators Nos. 1 and 2, and is, therefore, fed back to the roasting furnace. THE SEPARATION OF'PYRITE AND BLENDE 127 The speeds of the separator belts are adjusted to suit each class of material treated. The magnetic tailing is run to waste through a launder in which a stream of water is kept flowing, and the cleaned blende is delivered to shipping bins through chutes. The finished product is sold under two grades: grade "A" is the product from the separator treating the coarser size and grade " B " from that treating the finer size ; it has been found here that the coarser the size of the concentrate the higher the grade. The average selling assays on 33 car loads gave grade A 60.49 per cent, zinc and 2.11 per cent, iron, and grade B 57.07 per cent, zinc and 2.19 per cent. iron. The total amount shipped was in the proportion of two cars of B to each car of A. The raw concentrate purchased by this mill carries from 11 to 33 per cent. zinc. The tailing produced averages below 5 per cent., which figure is said to be never exceeded. The efficiency of the plant is given as 85 per cent, of the zinc in the raw concentrate. The mill is equipped with a complete series of rheostats to control the currents on all magnets from the central switch- board. At Hazel Green, Wisconsin, the Kennedy Mining Co. employs a Cleveland-Knowles separator to clean roasted blende-marcasite concentrate. The concentration mill treats 100 tons of ore daily for a production of 25 tons of concentrate, carrying from 41 to 42 per cent. zinc. From the mill the concentrate is delivered to the roaster building by a self-dumping skip and drops into the boot of a bucket elevator, which delivers to the furnace feed hop- per. The furnace is of the cylindrical type usual in the Wisconsin district and is 28 ft. long and 5 ft. in diameter. It is set at an inclination of 4 ins. in 28 ft. and makes two revolutions in 3 minutes. The ore remains in the furnace from 3J to 4 hours. The cylinder is lined with 8-in. fire brick between which are set at intervals projections of refractory material in the form of equi- lateral triangles with an altitude of 3 ins., which serve to lift the ore by the revolution of the furnace and allow it to fall through the hot gases from the fire box. About two tons of soft coal are burned in 24 hours. The roasted concentrate falls from the fur- nace into a paddle conveyor, where it is sprayed with water, which is evaporated immediately by the hot material, which it serves to cool. This conveyor delivers to a bucket elevator delivering into 128 ELECTRO-MAGNETIC ORE SEPARATION a trommel above the separator bins. The roasted concentrate is here sized into two products, through j^ in. and between T ^ in. and J in. ; these two sizes are treated at different times by the separator and the oversize is recrushed. The first magnet of the separator, which is a 21-in. Cleveland-Knowles machine, takes 1.5 amperes and the second magnet 3.5 amperes. The separator belt travels at a speed of about 175 ft. per minute. The magnets revolve at 75 R.P.M. at a height of 1 in. above the belt. The separator treats from 20 to 22 tons of roasted concentrate in 24 hours, which indicates a burden of from 1.25 to 1.5 Ibs. of material on the belt at any one time; this quantity is carried as an even layer one particle deep. The first magnet takes out an iron-tailing product which is run to waste in a launder; the second magnet removes a middling product which is fed to the middling rolls in the concen- tration mill a procedure of doubtful economy. The nonmagnetic material remaining on the belt is almost clean blende, with a little limestone and galena which were not eliminated in the con- centration mill, and not being magnetic, these are concentrated by the separation in the same proportion as the blende. The cleaned zinc product amounts to about 16.5 tons in 24 hours and assays 60 per cent, zinc with 2 per cent. iron. The tailing carries about 5 per cent. zinc. At Platteville, Wisconsin, the Enterprise Mining Co. is separat- ing about 15 tons of blende-marcasite concentrate daily. The con- centrate is delivered from the mill to the roaster bin by a skip run- ning on an incline. The Galena cylindrical roasting furnace is employed, the concentrate remaining in the furnace three hours. The roasted concentrate is sized in a trommel into three products : through -J in., between in. and J in., and oversize. The first two are fed to a 21-in. Cleveland-Knowles separator separately, the bin being provided with a partition to keep them apart; the ^-in. oversize is crushed and refed to the furnace. The separator carries 3 amperes on the first magnet and 5 amperes on the sec- ond. The tailing from the first magnet is run to waste in a wet launder and the middling from the second magnet sent to the jigs in the concentrating mill ; a falling off in the average grade of the cleaned zinc product from 61 to 59 per cent, is said to have re- sulted from so treating the middling. The cleaned zinc concen- trate carries from 58 to 62 per cent, zinc and averages about 2 per cent. iron. The raw concentrate fed to the furnace carries from 40 THE SEPARATION OF PYRITE AND BLENDE 129 to 45 per cent. zinc. The tailing from the separator is said to carry 4.5 per cent. zinc. The Empire Mining Co., of Platteville, is operating a plant similarly equipped. From 18 to 20 tons of raw blende-marcasite concentrate is treated daily for a recovery of from 10 to 15 tons of cleaned zinc. The cleaned zinc concentrate carries from 60 to 63 per cent, zinc and from 1 to 3 per cent. iron. Hocking Valley Coal is burned by the furnace, costing $5 per ton delivered; the fur- nace uses from 1500 to 2000 Ibs. in 24 hours. At Mineral Point, Wisconsin, the Mineral Point Zinc Co. op- erates a custom magnetic-separating plant in conjunction with a zinc-reduction works. The raw material is gathered from all parts of the district and consists chiefly of the products of water con- centration ranging in size from J in. downward; crude ore is oc- casionally treated. The concentrate fed to the furnace ranges from 28 to 34 per cent. zinc. The roasting plant is equipped with two cylindrical furnaces of the Galena type, only one of which is at present in use. This is 20 ft. long by 5 ft. in diameter. The lining of this furnace is of 8-in. special-arch fire brick in which are set four rows of 10-in. brick spaced 90 degrees apart; these elongated brick serve to lift the ore by revolution of the furnace and allow it to fall through the hot gases ; the substitution of 14-in. brick in place of the 10-in. is contemplated. This furnace is equipped with a dust chamber 5 ft. 4 in. wide by 14 ft. in length, is divided horizontally by a plate of heavy sheet iron. The gases from the furnace pass into the lower compartment of the dust chamber, along beneath the plate to the farther end, where a 2-ft. space is left between the end of the plate and the wall of the dust chamber, and thence back over their course, but above the plate, to the stack, which is 30 ins. in diameter and 50 ft. high. The dust is removed through three doors, two on the upper level, and one on the lower, each 12 ins. wide by 2 ft. high. The fire-box grate is 4 X 5 ft. in area, and burns from 1.5 to 2 tons of soft coal, costing $3 per ton, daily. The cylinder makes one revolution in 50 seconds, the ore remaining in the furnace from 3 to 3.5 hours. The temperature of the roast is just sufficient to start a slight fritting at the dis- charge neck of the furnace; that this action is incipient is shown by the fact that little trouble is experienced from overroasting or from particles of zinc and iron which have been cemented together. 130 ELECTRO-MAGNETIC ORE SEPARATION The slight deposit which forms at the discharge neck is removed from time to time with a long chisel. Paddle-and-chain conveyors are used to convey the roasted ore from the furnace, and a small stream of water is sprayed upon the hot material to lay the dust and assist in cooling it; the water used is regulated so that it may be completely evaporated by the heat of the ore before it FIG. 61. MILL OF THE TRIPOLI MINING CO., MINERAL POINT, WIS. reaches the separator bins. The furnace treats about twenty tons of raw concentrate in 24 hours. The roasted concentrate is separated on a Cleveland-Knowles 21-in. separator and upon a Dings separator, set up side by side and working on the same material. Either machine is capable of treating the output from one surface. The Cleveland-Knowles carries 3 amperes on the first magnet and 6 amperes on the second : the Dings separator carries 3 amperes on either magnet. The roasted concentrate is sized before separation into two products, between =J and \ in., and through J in. The concentrate averages 59 per cent, zinc and 2.5 per cent, iron; the middling amounts to THE SEPARATION OF PYRITE AND BLENDE 131 ||I i-H S8S . a 8 Separator % L-in. Cleveland-Knowles. L-in. Cleveland-Knowles. 2-in. Cleveland-Knowles. L-in. Cleveland-Knowles. ings L-in. Cleveland-Knowles. L-in. Cleveland-Knowles. L-in. Cleveland-Knowles. L-in. Cleveland-Knowles. L-in. Cleveland-Knowles. I -in. Cleveland-Knowles. L-in. Cleveland-Knowles. L-in. Cleveland-Knowles. Wm. B. Phillips, T. A. I. M. E., October, 1895. 2 H. A. J. Wilkins and H. B. C. Nitze, T. A. I. M. E., February, 1896. SEPARATION OF MISCELLANEOUS ORES AND MINERALS 167 SEPARATION OF THE OXIDES OF MANGANESE Manganite and pyrolusite are sufficiently magnetic to be capa- ble of concentration on separators with high-intensity fields. To be commercially valuable a manganese ore should carry at least 40 per cent, manganese, although, if accompanied by much iron, a lower grade is suitable for spiegel. The following results are re- ported as having been obtained on a Wetherill separator on culls from a waste heap at Cave Springs, Ga., consisting of particles of chert in a matrix of silicious pyrolusite * : Per Cent, of Total Per Cent. Manganese Per Cent. Quartz Feed 100 28.78 43.00 Concentrate 52 40.91 20.85 Tailing 48 15 54 67 20 Results are reported 2 as having been obtained on an Interna- tional separator showing the production of a 41.8 per cent, man- ganese product from an ore carrying 15 per cent, manganese. SEPARATION OF THE FRANKLIN FURNACE ORES At Franklin Furnace, New Jersey, the New Jersey Zinc Co. is operating a magnetic-separation mill of 1400 tons daily capac- ity. Three plants have been erected by this company, a description of which may be found in Richards's " Ore Dressing," pages 1060- 1065 ; the present mill was erected at a cost of about $600,000, or about $1.75 per ton of annual capacity. 3 The ore consists of franklinite, fowlerite, tephroite and garnet, magnetic minerals, with willemite, zincite, quartz, mica, and calcite. The magnetic minerals are removed from the mixture by twenty-two Wetherill- Rowand separators, the garnet being delivered as a separate prod- uct, and run to waste. This magnetic concentrate is treated in zinc-oxide furnaces and the residue, high in manganese, is sent to > H. A. J. Wilkins and H. B. C. Nitze, T.A.I. M. E., February, 1896. 2 Jour. Canadian Mining Institute, F. T. Snyder, March, 1904. '"Report of the Zinc Commission Appointed to Investigate the Zinc Resources of British Columbia," W. R. Ingalls, p. 88. 168 ELECTRO-MAGNETIC ORE SEPARATION spiegel furnaces. The nonmagnetic product of the separators is jigged to separate the willemite and zincite from the quartz, calcite and mica, and sent to the spelter furnaces. The preliminary crushing of the ore is done by Edison giant rolls and is followed by corrugated and smooth rolls. The ore is reduced to pass a No. 10 slot screen; the sizing is done on Edi- son fixed inclined screens. The ore is dried in an Edison drying furnace. This consists of a stack 3 ft square and 24 ft. high, made of cast-iron plates. The interior is fitted with cast-iron slats 6 ins. wide and inclined at 45; these slats dip alternately to the right and to the left, causing the ore to drop from one to the other and exposing each particle to the drying action of a current of hot gases from a combustion chamber outside the building. The re- sults of the magnetic separation follow: Per Cent, of Feed Per Cent. Iron Per Cent, Manganese Per Cent. Zinc Ore 100 00 Franklinite concentrate 67 48 29 47 13 57 22 94 Zincite and willemite concentrate Tailing 23.99 8 53 2 20 5.15 48.96 A i q It is stated that the cost of separation, exclusive of taxes, amortization, and interest is 40c. per ton of ore. SEPARATION OF WOLFRAMITE Wolframite, sp. gr. 7.1 to 7.5, tungstate of iron and manga- nese, is feebly magnetic; specimens from some localities are re- ported to be strongly magnetic. Wolframite frequently accompa- nies cassiterite in tin ores, and on account of their similar specific gravities (cassiterite 6.4 to 7.02) these minerals may not be sep- arated from each other by specific-gravity methods. The method usually followed in treating crude ore or concentrate of this class is to convert any iron, or copper-iron, sulphides which may be present into magnetic compounds by roasting, and separate these from the mixture by low-intensity magnets, then to pass the re- mainder through a stronger field which takes out the wolframite as a magnetic product and leaves a nonmagnetic tailing carrying SEPARATION OF MISCELLANEOUS ORES AND MINERALS 169 the tin. Numerous tests on large quantities of tin-tungsten con- centrates made preliminary to the installation of separation plants have yielded high extractions, and a number of these plants are at present in operation in Europe and elsewhere. Most of these in- stallations treat the product of preliminary water concentration, but at least one of them is operating on raw ore. Difficulty has been experienced in the separation of wolframite from cassiterite through a tendency of magnetic particles to adhere to the cas- siterite, thereby causing it to be drawn into the magnetic product when a strong field is used for separation; this tendency may be overcome by treating the concentrate with sulphuric acid and dry- ing before pasing it to the separator. Arsenopyrite (sp. gr. 5.67 to 6.3) is of frequent occurrence in ores carrying wolframite: upon roasting this mineral the arsenic is driven off and the result- ing magnetic oxide of iron separated from the wolframite by low- intensity magnets. Care must be taken in the roasting of concen- trates carrying cassiterite that the heat does not rise to such a degree as to cause particles of magnetic oxide to become attached to the cassiterite particles, and so cause them to be drawn into the wolframite product. At Ounnislake Clitters* England, tin-tungsten concentrate is being separated on Humboldt-Wetherill separators. The ore from the mine is crushed in breakers and rolls, sized, and the sands con- centrated on tables and the slimes with Luhrig classifiers. The concentrate is roasted in a Bruckner furnace having a capacity of 10 tons daily; this material carries, raw, 12 per cent, sulphur and arsenic, principally the former, and the iron with which these ele- ments are combined is rendered strongly magnetic in the roast. The roasted concentrate, after cooling, is fed to a Humboldt-Weth- erill separator which carries 4 amperes on the first magnet and 12 amperes on the second. The first magnet removes the strongly mag- netic iron compounds and the second the wolframite, while the tin is contained in the nonmagnetic tailing. The tungsten concentrate and the tin product from the separator are both reconcentrated to eliminate waste. The raw ore yields 0.378 per cent, tin and 0.72 per cent, tungsten. The tungsten concentrate carries from 60 to 64 per cent, tungstate of iron, equal to 46 to 49 per cent. W0 3 . The separator treats 6 tons of concentrate in 10 hours. i E. & M. J., vol. Ixxvi, p. 424, Edward Skewes. 170 ELECTRO-MAGNETIC ORE SEPARATION FIG. 70. At Redruih? Cornwall, England, the East Pool & Agar United Mines Co. is operating a magnetic-separation plant on concen- trates containing wolframite, cassiterite, arsenical pyrites, and chalcopyrite. The ore is crushed by stamps, classified, and con- riG. 71. centrated -on Wilfley tables and Frne vanners, which deliver a con- centrate carrying cassiterite, wolframite, and arsenical pyrites, the 1 E. & M. J., vol. Ixxxiii, p. 941, Edward Walker. SEPARATION OF MISCELLANEOUS ORES AND MINERALS 171 gangue minerals and the ehalcopyrite being eliminated by the con- centration. These concentrates are roasted and the arsenic driven off, to be recovered in the flues, while the residue is again passed over Wilfley tables, yielding a product consisting of about three parts oxide of tin to one part wolframite, and carrying about 5 per cent, magnetic oxide of iron. After drying, this product is passed over a Humboldt-Wetherill separator, which delivers three products: magnetic oxide taken out by the first and weaker mag- net, wolframite taken out by the second and stronger magnet, and a nonmagnetic product carrying the tin. SEPARATION OF MONAZITE SANDS Monazite sands are natural accumulations or concentrations which carry thorium and cerium oxides as their valuable constitu- ents, together with garnet, menaccanite, rutile, zircon, and other heavy minerals as contaminations. The sand is usually concen- trated by washing and the concentrate separated on magnetic sepa- rators. Monazite, sp. gr. 4.8 to 5.1, is feebly magnetic, and is removed from mixtures as a magnetic product in South Carolina, Brazil, and elsewhere. The above-mentioned contaminating minerals are all magnetic in different degrees; menaccanite the most strongly magnetic, followed by zircon, and rutile being the most feebly magnetic; garnet varies considerably in its magnetic properties; all are more strongly magnetic than monazite, which mineral is removed from any nonmagnetic constituents of the sand by the last magnet encountered by the ore, which carries the most current. At Elleriboro, South Carolina, C. P. Meiser is operating Wetherill-Rowand separators on monazite sands. Menaccanite is removed by the first magnet, garnet by the second, and the mona- zite separated from nonmagnetic impurities by the third. At Sapucaia, Brazil, J. L. Weiler is treating monazite sands on a Humboldt-Wetherill separator. The sands carry monazite, menaccanite, wolframite, cassiterite, and quartz. The capacity of the installation is 2 metric tons per hour. At Rio de Janeiro, Charles Ban & Co. are treating monazite sands on Humboldt-Wetherill separators, the installation having a capacity of 3 metric tons per hour. 172 ELECTRO-MAGNETIC ORE SEPARATION The following results are reported from an installation of the Mechernich separator in Brazil working on monazite sands car- rying cassiterite, monazite, and menaccanite : Per Cent. Tin Per Cent. ThO 3 Feed 20 59 78 Tin concentrate 68 30 29 Thorium product 3 32 1 45 Menaccanite tailing 14 The tin recovered in. the tin concentrate amounts to 93.61 per cent, of the total tin in the feed, and 93.75 per cent, of the Th0 2 is recovered in the thorium product. SEPARATION OF LEUCITE FROM LAVA In Italy there are solidified lava streams which average 20 per cent, leucite, which is valuable for its potassium content. Leucite is feebly magnetic and usually carries sufficient iron as an impurity to be susceptible to magnetic separation. The Societa Romana del solfati e chimici, of Rome, is operating two plants for the recovery of leucite from lava. The leucite is accompanied by hornblende, augite, and ferruginous material, which are removed magnetically. Humboldt-Wetherill separators are employed and the plants have a capacity of 90 metric tons per day. The leucite concentrate obtained carries 80 per cent, leucite, and is treated chemically for its potassium. This company is op- erating a plant equipped with TJbaldi separators producing a 95 per cent, concentrate from lava containing 25 per cent, leucite. At Civita Castellana, Italy, the Mechernich separator is em- ployed for the separation of leucite. The concentrate obtained is said to contain 95 per cent, leucite. The leucite here is capable of direct separation. CORUNDUM Corundum, sp. gr. 4.0, is feebly magnetic, and may be strongly magnetic, through its included magnetite; emery, a variety of corundum, is of a dark gray color, due to magnetite. Corundum SEPARATION OF MISCELLANEOUS ORES AND MINERALS 173 concentrate is cleaned by magnetic separation from associated magnetite. HORNBLENDE Hornblende, sp. gr. 2.9 to 3.4, is feebly magnetic. It may be separated as a magnetic product by the more intense separators. Occurring in a magnetite ore, this mineral may be responsible for a loss of iron in the tailing, as it carries iron but is not sufficiently magnetic to be removed with the magnetite by the separators usually employed on these ores. CHROMITE Chromite, sp. gr. 4.32 to 4.6, is usually ferromagnetic; its gen- eral formula is the same as for magnetite, with part of the iron replaced by chromium; analysis, iron protoxide, 32 per cent, chromium sesquioxide 68 per cent. DIAMONDS At The De Beers Consolidated Mines, Kimberly, South Africa, the Humboldt-Wetherill separator is employed to remove magnetic minerals from concentrate carrying diamonds. This concentrate carries magnetite, menaccanite, chromite, and pyrrhotite. GALENA At Gem, 1 Idaho, the Frisco Mining Co. is employing three Dings separators (belt type) for removing lead ore from zinc ore and gangue. The Jead is recovered as a magnetic product. If a piece of apparently pure galena be presented to the magnet in the hand it is attracted apparently as strongly as if it were a piece of iron. This phenomenon may be due to included grains of magnetite. SEPARATION OF BROKEN HILL ORES At Broken Hill, New South Wales, argentiferous galena oc- curs with blende in a gangue composed of quartz, rhodonite, and garnet. Upon concentration these ores yield a middling product consisting of blende, rhodonite, and garnet, with a little galena. 1 Communicated by Mr. W. R. Ingalls. FIG. 72. BROKEN HILL, N. S. W. tiff 1 73. BROKEN HILL, N. S. W. SEPARATION OF MISCELLANEOUS ORES AND MINERALS 175 The re-treatment of this product, an immense quantity of which has accumulated from past operations and which is still being added to, is successfully accomplished by magnetic separation. The blende is feebly magnetic, carrying about 7 per cent, iron and 2 per cent, manganese in combination, and is separated as a mag- netic product. Rhodonite, sp. gr. 3.4 to 3.7 (manganese protoxide, 54.1 per cent.; silica, 45.9 per cent), is feebly magnetic. Crane reports a permeability of 1.0176 for a specimen from. Franklin Furnace, N. J. It is separated as a magnetic product at Franklin Furnace and at Broken Hill. Garnet, sp. gr. 3.1 to 4.3, is in most varieties feebly magnetic. The permeability of this mineral depends upon its composition and specimens of the same variety from different localities exhibit widely differing magnetic qualities. It is separated as a magnetic product at Broken Hill and at Franklin Furnace. The blende is more feebly magnetic than the rhodonite and garnet, which minerals are removed together as a tailing product. The rhodonite occasionally carries silver, in which case it is sent to the lead furnaces and smelted with the galena. The nonmag- netic discharge from the separators, consisting of galena and quartz with a little feebly magnetic blende, is concentrated on jigs and tables. SEPARATION OF BROKEN HILL TAILING AT THE PLANTS OF THE SULPHIDE CORPORATION Plant No. I. 1 This plant has a capacity of 150 tons per day on tailing and middling carrying argentiferous galena and blende with rhodonite, garnet, and quartz. Double-pole Mechernich sep- arators are employed for the separation and make three products : garnet and rhodonite tailing, which is sent underground and used for stope filling ; magnetic blende, a finished product ; nonmagnetic residue, consisting of galena and quartz with a little blende, which is further treated on jigs and tables for the production of a lead and a zinc concentrate and final tailing. The tailing, or middling from the wet concentration mills, is loaded into trucks from the dumps, hauled up an incline and de- livered to a belt conveyor which feeds the drying furnace. This 1 "Australian Mining and Metallurgy," Donald Clark, p. 414. FIG. 74. TAILING HEAPS, BROKEN HILL, N. S. W. FIG. 75. CONVEYOR FROM DUMP, PLANT NO. 2, SULPHIDE CORPORATION. SEPARATION OF MISCELLANEOUS ORES AND MINERALS 177 drying furnace consists of a conical shell revolving about a hori- zontal axis ; the material is fed at the apex of the cone, and is car- ried, by the revolution of the shell, slowly toward the discharge end, where it falls into a conveyor and is transported to the trom- mels and screens. The dryer is heated by gases from a fire box located at the feed end, the gases traversing the furnace in the same direction as the ore. The heat employed is just sufficient FIG. 76. DRYING FURNACE, PLANT NO. 2, SULPHIDE CORPORATION. to dissipate the surface moisture, the sand, as it is discharged, being just too warm to hold in the hand. The dried ore is classified into three sizes : through -J mm., be- tween -J mm. and 1 mm., and from 1 mm. to 3 mm., the oversize at 3 mm. being recrushed and returned to the system. These sized products are treated separately on five double-pole Mechernich sep- arators which yield two magnetic products, garnet and rhodonite, and magnetic blende, and a nonmagnetic product consisting of galena and quartz with a little blende. Dust is withdrawn from each machine by exhaust fans and in addition the men are required to wear respirators for protection against the dust. 178 ELECTRO-MAGNETIC ORE SEPARATION TABLE OF RESULTS ' Per Cent. Zinc Per Cent. Lead Feed . . . 24 7 4 01 Rhodonite middling 19.00 1.80 Blende concentrate 45 41 5 . 75 Nonmagnetic 5 13 5 25 Rhodonite middling re-treated gives Per Cent. Zinc Per Cent. Lead Zinc concentrate 38.00 5.2 Rhodonite tailing ... . ... 5 50 Plant No. 2. 2 This plant is equipped with Motortype sepa- rators and has a capacity of 200 tons per day on the tailing product of the wet-concentration mills working on ore from the Central Mine. This material carries galena and blende with quartz and rhodonite, the latter changing to rhodochrosite in the lower levels. It averages, approximately, 7 ozs. silver, 5 per cent, lead, and 19 per cent. zinc. The various stages in the treatment of this material are: (1) Drying the crude tailing. (2) Preliminary sizing and crushing. (3) Final classification. (4) Magnetic separation. (5) Wet treatment of the nonmagnetic product. The tailing is removed from the dumps in the following man- ner: A horizontal belt conveyor is laid along the toe of the dump from which the tailing is to be drawn off ; this conveyor is so con- structed as to be free to turn about a pivot at the delivery end of Communicated by the Electromagnetische Gesellschaft, Frankfort-a.-M., Germany. 2 Communicated by Mr. James Hebbard, manager of the Sulphide Corpora- tion, Broken Hill, N. S. W. FIG. 77. MOTORTYPE SEPARATOR, PLANT NO. 2, SULPHIDE CORPORATION. FIG. 78. SEPARATORS, PLANT NO. 2, SULPHIDE CORPORATION. 180 ELECTRO-MAGNETIC ORE SEPARATION the belt, and the whole frame carrying the 200 ft. of conveyor is free to traverse a complete circle in azimuth but for the obstruc- tion offered by the dump. A hopper is mounted on rails on this conveyor frame and can be placed at any required position along the conveyor. The tailing is shoveled from the dump into this hopper. When the dump has been removed to a distance of 5 ft. back from the belt, along its entire length, the framework is grad- FIG. 79. SEPARATION MILL, SULPHIDE CORPORATION. ually moved in toward the dump, turning about the pivot at the delivery end, and shoveling resumed. If necessary, the feed may be maintained while the belt is being moved. This movable hori- zontal conveyor discharges onto a fixed inclined conveyor, which in its turn delivers to a revolving dryer which drives off the 2 or 3 per cent, moisture contained by the tailing. The dryer is a revolving cylinder 4 ft. in diameter and 35 ft. long, set at an angle of 6J degrees, and having internal longitudinal ribs which serve to elevate and drop the tailing repeatedly through the heated air as it gravitates toward the lower end of the cylin- der. The tailing is fed from a hopper at the upper end of the cylinder; the fire box is placed at the lower end of the cylinder SEPARATION OF MISCELLANEOUS ORES AND MINERALS 181 into which it discharges its gases through a wrought-iron box with a sloping bottom. The dried tailing falls from the cylinder into this box and is discharged from it into a scraper conveyor leading to an elevator which delivers to the trommel. Here the; dried tail- ing is sized on 2|-mm. screens, the oversize being passed through a pair of 30-in. Cornish rolls and returned by the same elevator to the same trommel. The undersize is transported to the separation mill by a bucket elevator which delivers to a nest of four wind' separators. The wind separators remove the finest particles (ap r proximately from 180 mesh to dust) which are sent to one group of separators ; the doarser product of the wind separators is sent to a series of trommels fitted with screens with f-mm. holes. The classification thus yields three sizes, coarse, medium, and fine, respectively, from f mm. to 2-J mm., from f mm. to 180 mesh, and from 180 mesh to dust. These .sized products are delivered by conveyors to storage -bins which supply the separators. The separation is accomplished on Motortype separators. The relatively strongly magnetic rhodonite is separated directly as a tailing product carrying little zinc. The feebly magnetic blende is removed as a second product, and the remaining galena and quartz, with a little blende, form the nonmagnetic discharge from the separators. The machines are arranged on two floors, each floor having a separate group of magnetic separators for the treatment of each of the three sizes produced by the classification. The ma- chines on the upper floor yield rhodonite tailing, zinc concentrate, and a galena-quartz product. The machines on the lower floor Te- treat the galena-quartz product from the primary machines, group for group, and yield a blende concentrate which is mixed with the blende product from the primary separators, and a final nonmag- netic product which is sent to an auxiliary wet-concentration plant. The rhodonite product and the quartz tailing from the wet- concentration plant are sent underground and there used to fill depleted stopes. The several products from the separators are discharged through rubber pipes which deliver to conveyors running beneath the machines; these conveyors deliver their products to the ship- ping bins alongside the railway track. Two 6-ft. fans and several smaller auxiliary fans are kept con- tinually at work withdrawing the dust from various parts of the 182 ELECTRO-MAGNETIC ORE SEPARATION SEPARATION OF MISCELLANEOUS ORES AND MINERALS 183 plant. The dust so collected is driven into a wooden tower, fitted with suitable baffles, where it rises through a shower of water, by which it is settled and carried to tanks from which it is dis- charged periodically. All the machinery in the plant is operated by individual motors which receive their current from a central power station. In 1906 this plant treated 47,326 tons of tailing assaying 6 ozs. silver, 5.2 per cent, lead, 21.8 per cent, zinc, and recovered 17,- 753 tons of zinc concentrate assaying 39.6 per cent, zinc, a sav- ing of 68.1 per cent. SEPARATION AT THE WORKS OF THE AUSTRALIAN METAL COMPANY, BROKEN HILL, N. S. W. 1 The material treated is tailing from water concentration, car- rying galena and blende with rhodonite, garnet, and quartz. First the garnet and rhodonite are removed as a magnetic product, and finally the blende, leaving the galena with the quartz as a nonmag- netic product, from which the galena is finally removed by water concentration. About 130,000 tons had already been treated at the time of writing. The tailing from the piles is trucked to a hopper which delivers an even feed onto a Eobins belt conveyor, elevated and passed into a revolving dryer running on friction rolls. The heat is just suffi- cient to dissipate the moisture, which amounts to from 2 to 3 per cent. From the dryers the tailing passes to a shaking screen, a hood being placed above the screen to draw off any dust. The material which does not pass through the coarse screen (9 mesh), which is only about 2 per cent, of the total, passes to rolls for crushing. The screened material goes to the boot of an elevator and is car- ried to the top of the building. It is then distributed between two double trommels, with screens giving three products 2, 3, and 4 No. 1 being reduced. No. 1 retained on- a sieve having 9 holes per linear inch. No. 2 passes No. 1 but is retained on a 20-mesh screen. No. 3 passes No. 2 but is retained on a 40-mesh screen. No. 4 passes through the 40-mesh screen. 1 "Australian Mining and Metallurgy," Donald Clark, p. 415. 184 ELECTRO-MAGNETIC ORE SEPARATION The last product contains everything below 40 mesh except dust, which is drawn away through steeply sloping pipes connected with a fan, and sent to a dust chamber. The fines are treated by four Ullrich separators and the coarse by four machines of the same type. Each machine contains four separating armatures set