THE LIBRARY OF THE UNIVERSITY OF CALIFORNIA DAVIS CALIKORXIA STATE NIINING BUREAU. J. J. CRA'WFORD, State Mineralogist. BULLETIN NO. 5. San Francisco, October, 1894. THE CYANIDE PKOCESS ITS PRACTICAL APPLICATION AND ECONOMICAL RESULTS. By dr. a. SCHEIDEL, E.M. SACRAMENTO: STATE OFFICE, : : : A. J. JOHNSTON, SUPT. STATE PRINTING. 1894. LIBRARY UNIVERSITY or CALIFORNIA DAVIS San Fkancisco, October 1, 1894. Hon. J. J. Crawford, State Mineralogist, San Francisco, Calif.: Dear Sir: In accordance with your letter of December 8, 1893, I herewith submit my report on the cyanide process. I have endeavored to describe that process in its practical application and economical results. The information it conveys includes my own experience, and is supplemented from articles which appeared in technical periodicals; also from the records of patents granted by the Patent Offices of the United States and Great Britain, and from the Blue Books issued by the Mining Departments of the British Colonies of Australasia. I am largely indebted for special communications received from metallurgists in charge of prominent companies and important works, and from the officers of the government mining departments of the Australian Colo- nies. It has seemed advisable with some " improvements," and generally with the patents, to simply place them on record without any special comment. Respectfully yours, A. SCHEIDEL, Ph.D., E.M. THE CYANIDE PROCESS ITS PRACTICAL APPLICATION AND ECONOMICAL RESULTS. By A. ScHEiDEL, Ph.D., E.M. The "cyanide process" for extracting gold and silver from ores is based on the fact that a diluted solution of potassium cyanide dissolves these metals, forming, respectively, auro-potassic cyanide and argento- potassic cyanide, from many ores, without dissolving to any material extent the other components thereof. The process consists of treating suitable ores, when finely divided, with a weak solution of potassium cyanide, either by allowing the solution to percolate through the ore or by agitating a mixture of the ore and solution. This part of the opera- tion being completed, the solution is separated from the solid material and the gold and silver are precipitated in metallic form. This process for the extraction of gold and silver is comparatively old in its principle, but modern in its technical application. During the last four years it has been introduced into almost every gold field, and upwards of $14,000,000 in gold and silver have been recovered by the process, which demonstrates beyond doubt that it is one of the most important additions to the wet methods of gold and silver metallurgy. The aim of this paper is to present the history of the process and to describe the ores for which it is adapted, together with their preparation and manipulation during treatment. The economical features of the cyanide process are also dwelt on at some length. The text is illustrated by plans and diagrams. The State Mining Bureau of California was among the first in the United States to investigate the merits of the cyanide process, as set forth in a paper by Dr. W. D. Johnston, in the Xth Report of the Bureau. The process has since found extensive application, and other valuable and interesting papers have been published, but an exact account of the methods employed in all parts of the world is still wanting. This writing is undertaken at the request of Hon. J. J. Crawford, State Mineralogist of California. The facts herein recorded are obtained from the practical experience of the writer in New Zealand and the United States, and of others who have been very successful in the application of the cyanide process. I take great pleasure in expressing my sincere appreciation of and gratitude for the assistance my contributors have extended. I desire especially to acknowledge my obligation to Mr. John S. MacArthur, of Glasgow, Scotland, and to Mr. J. M. Buckland, the general manager of the African Gold Recovery Company, Lim., in Johannesburg, South African Republic. In order to allow a comparatively full description of methods and appliances, in the following pages, theoretical matter is limited to the main chemical reactions incidental to the* process, and an explanation of some of the difficulties most frequently met. To facili- tate the consultation of the paper, I prefix the following synopsis: 6 THE CYANIDE PROCESS. I. History of process: Solubility of gold and silver in cyanide as known to Hagen, Bagration, Eisner, Faraday; its technical application by Wright and Elkington; its metallurgical appli- cation by Rae, Simpson, Endlich and Miihlenberger, Louis Janin, Jr., Dixon, MacArthur and Forrest, Molloy, A. Janin and Merrill, W. D. Johnston. II. Scope of process. III. Chemistry of process. IV. Demonstration of the process. Methods of operation: A. The agitation process. B. The percolation process. (o) Percolation of ores. (6) Percolation of tailings. (c ) Percolation of concentrates. C. Cyanide and cyanide solutions. D. Treatment of the gold solutions. Recovery of the gold and silver. (a) Precipitation by zinc. (b) The Molloy process. (c) The Siemens and Halske procesa. (d) The Pielsticker process. (e) The Moldenhauer process. (/) The Johnston process. V. Percentage of extraction. VI. Working costs of process. VII. Cost of cyanide plants. VIII. Machinery and appliances. IX. Laboratory work. X. Danger in working the process. XL Exemplification of the process. The process in various countries: A. Africa. B. Australasia. , (a) New Zealand. (&) Tasmania. (c ) Western Australia. (d) South Australia, (c) Queensland. (/) New South Wales. ig) Victoria. C. United States of America. (a) Utah. (6) Montana. (c) Colorado. (d) Nevada. (e) Arizona. If) New Mexico. \g) South Dakota. ^ {h) California. D. Mexico, Colombia, Straits Settlements, Russia, Borneo. XII. Summary and conclusions. XIII. Patents: Julio IT. Rae. Improved mode of treating auriferous and argentiferous ores. U. S. patent 61,866, dated February 5, 1S67. Thomas C. Clark. Extracting precious metals from ores. U. S. patent 229,586, dated July 6, 1880. Hiram W. Faucett. Process of treating ore. U. S. patent 236,424, dated January 11, 1881. John F. Sanders. Composition for dissolving the coating of gold in ore. U. S. patent 244,080, dated July 12, 1881. Jerome W. Simpson. Process of extracting gold, silver, and copper from their ores. U. S. patent 323,222, dated July 28, 1885. ^ John Stewart MacArthur, Robert Wardrop Forrest, M.D., and William Forrest, M.B. Improvements in obtaining gold and silver from ores and other compounds. English patent 14,174; 1887. THE CYANIDE PROCESS. • XIII. Patents (continued): John Stewart Mac Arthur, Robert Wardrop Forrest, and William Forrest Process of obtaining gold and silver from ores. U. S. patent 403,202, dated May 14, 1889. John Stewart MacArthur. Metallurgical filter. U. S. patent 418,138, dated December 24, 1889. John Stewart MacArthur, Robert Wardrop Forrest, and William Forrest. Process of separating gold and silver from ores. U. S. patent 418,137, dated December 24, 1889. Edward D. Kendall. Composition of matter for the extraction of gold and silver from ores. U. S. patent, dated September 13, 1892. Bernard Charles Molloy. Improvements in precipitating and collecting metals from solutions containing them. English patent 3,024; 1892. John Cunninghame Montgomerie. Improvements in the extraction of gold and silver from ores or compounds containing the same, and in appa- ratus applicable for use in the treatment of such materials by means of solvents. English patent 12,641; 1892. John Stewart MacArthur and Charles James Ellis. Improvements in ex- tracting gold and silver from ores and the like. New Zealand paten t-speciti- cation, June 29, 1893. Carl Moldenhauer. Improvements in recovering gold and other precious metals from their ores. ISIew Zealand patent-specihcation, August 31, 1893. Carl Pielsticker. Improvements in the extraction of gold and silver from ores. New Zealand patent-specification, December 14, 1893. Alexis Janin and Charles W. Merrill. Process of leaching ores with solu- tions of alkaline cyanides. U. S. patent 515,148, dated February 20, 1894. William David Johnston. Method of abstracting gold and silver from their solutions in potassium cyanides. U. S. patent 522,260, dated July 3, 1894. XIV. List of plans, diagrams, and tables: Details of the false bottoms of the percolation vats. (W. R. Feldtmann.) Plant to treat a minimum of 2,000 tons per month. (MacArthur.) 'Table giving sizes and material of percolation vats. (A. Scheidel.) Zinc box. (MacArthur.) Zinc filter. (A. Scheidel.) Porcelain filter. (A. B. Paul.) Table giving extraction results on various ores. (A. Scheidel.) Discharging tailings-vats at the Langlaagte Estate Company's plant. (W. R. Feldtmann.) Square filter vats at the works of the Crown Company, with doors for the discharging trucks. (Irvine.) Variation No. 1 in designs of cyanide plants. (W. R. Feldtmann.) Variation No. 2 in designs of cyanide plants. (W. R. Feldtmann.) Variation No. 3 in designs of cyanide plants. (W. R. Feldtmann.) Side discharge at percolation vats. (W. R. Feldtmann.) Bottom discharge at percolation vats. (Chas. Butters.) Bottom discharge at percolation vats. (W. E. Irvine.) The cyanide works of the Robinson Company. (Chas. Butters.) The cyanide plant of the Crown Company. (John MacConnell.) The Sylvia Company's Cyanide Works, Tararu, Thames, New Zealand. (A. Scheidel.) Melting room for cyanide bullion in the Sylvia Company's Works, Tararu, New Zealand. (A. Scheidel.) The tailings cyanide works at Waihi. (A. James.) The concentrating and cyanide extraction works of the Sylvia Gold and Silver Mining Company. (A. Scheidel.) The cyanide plant of the Sylvia Company (plan and longitudinal section). (A. Scheidel.) Table showing the concentrating and cyanide process in the works of the Sylvia Company. (A. Scheidel.) Bullion furnace. (A. Scheidel.) Wet-mill cyanide plant, Revenue. (F. B. & R. B.Turner.) Dry-mill cyanide plant. Revenue. (F. B. & R. B. Turner.) U tica Company cyanide plant (plan and longitudinal section). (A. Scheidel.) The vacuum filter, Utica cyanide plant. (A. Scheidel.) The bullion filter, Utica cyanide plant. (A. Scheidel.) Agitator. (J. H. Rac.) Metallurgical filter. (J. S. MacArthur.) Apparatus for treatment of ores, etc., by means of solvents. (J. C. Mont- gomerie.) Improved apparatus for the extraction of gold and silver from ores. (C. M. Pielsticker.) Table giving analysis of gold production in the Witwatersrand District for April, 1894. (Witwatersrand Chamber of Mines.) ; THE CYANIDE PROCESS. XV. List of abbreviations of literature: E. & M. J. — Engineering and Mining Journal, New York. M. I. — Mineral Industry. M. S. P. — Mining and Scientific Press. Tr. A. I. M. E. — Transactions of the American Institute of Mining Engineers. J. S. Chem. I.— Journal of .Society of Chemical Industry, England. J. fr. Chem. — Journal fiir practische Chemie. J. Ch. S. — Journal Chemical Society. Tr. Phil. Soc. — Transactions of the Philosophical Society. M. Sc. — Moniteur Scientifique. A. Ch. Ph.— Annales de Chimie et de Physique. Ch. N. — Chemical News. B. A. I. Sc. — Bulletin de I'Acad^mie Imperiale des Sciences de St. Peters- bourg. B. S. Ch. — Bulletin de la Society Chimique de Paris. HISTORY. I. HISTORY. The fact of gold being soluble in cyanide of potassium solution has been known for a considerable time. Hagen is reported to have men- tioned it in 1806. Dr. Wright, of Birmingham, England, used gold- cyanide solution for electroplating in 1840j he made this application in consequence of his studies of Scheele's report on the solubility of gold- cyanide in a cyanide of potassium solution. J. R. & H. Elkington patented Wright's invention; they speak in tbeir patent-specification of a boiling solution of gold or cyanide of gold in prussiate of potash. The first record in scientific literature of experiments in which metallic gold was dissolved in a cyanide of potassium solution consists in Prince Pierre Bagration's paper in the Bulletin de I'Academie Imperiale des Sciences de St. Petersbourg, 1843, t. 11, p. 136. Bagration, who alludes to Elkington's process, preserved cyanide of potassium solution in a dish, gilded on the inside. He noticed that after eight days the whole gold surface had been attacked. He experimented then with finely divided gold under the influence of the galvanic current; the latter he soon recognized as not of any benefit in the dissolving process. He pre- cipitated the gold out of the cyanide solution by means of the electric current on a cathode of copper. Continued experiments proved the advantage of higher temperature during the dissolving process, and taught the precipitation of gold from its still warm solution by means of silver or copper plates, without the electric current. The higher tem- perature had, however, the disadvantage of the silver and copper being strongly attacked by the cyanide solution during the precipitation process. Bagration extended his experiments to solutions of ferroeyanide, which he found to act like cyanide, but in a much less degree. He further studied the solubility of gold in the form of plates, in cyanide, and found it to be dissolved in such form at a considerable rate at a temperature of 30° to 40° C. He noticed the influence of the air on the reaction. Bagration believes that hydrocyanic acid in a state of generation is a gold solvent, and he concludes his paper with the remark that in the future, cyanide of potassium must be enumerated among the solvents of gold. L. Eisner published in J. fr. Chem., 1844, p. 441, his observations on the reactions of "reguline metals" in an aqueous solution of cyanide. He found that gold and silver were dissolved in potassium cyanide without decomposi- tion of water. "The dissolution of the metals is, however, the conse- quence of the action of oxygen, which, absorbed from the air, decomposes part of the cyanide." His reaction has been expressed by others in the following equation: 2 Au + 4 KCy + -f H2O = 2 AuKCya -f 2 KOH (Gold.) (Cyanide (Oxygen.) (Water.) of potassium.) (Auro-potassic (Potassic cyanide.) hydrate.) It is generally called Eisner's equation. Some years after, Faraday made use of the solubility of gold in cyanide solution for reducing the thickness of gold films (Exp. relations of gold and other metals to light, Tr. Phil. Soc, 1857, p. 147). The basis of the most modern process for the extraction of gold was thus provided. It took many years, however, before the enumerated facts* were made use of for the extrac- tion of gold from ores. In 1867, Julio H. Rae took out United States patent No. 61,866, dated February 5th, for an "improved method of JO THE CYANIDE PROCESS. treating auriferous and argentiferous ores " with a current of electricity in connection with suitable liquids — such, for instance, as cyanide of potassium. Rae's process is an agitation process; he proposed to " expose the auriferous or argentiferous rock to the combined action of a current of electricity and of suitable solvents, and to separate the gold or silver from the rocks containing the same by the action or aid of electricity." The principle of Rae's process, as stated by him, distinguishes his method from the modern cyanide process. His method does not appear to have advanced beyond the laboratory stage or to have found exten- sive and successful practical application, and it sank into oblivion. Since then, cyanide of potassium in connection with gold and silver metallurgy has repeatedly been made a patent claim; in many cases, however, the application recommended is in its principle different from the application which characterizes the modern cyanide process. Thomas C. Clark, of Oakland, Cal. (United States patent No. 229,586, July 6, 1880), roasted his ore to a red heat, and placed it in that condition in a cold bath composed of a solution of salt, prussiate of potash, and caustic soda. H. W. Faucett, of St. Louis, Mo. ( United States patent No. 236,424, January 11, 1881), subjects hot crushed ores to the action of disintegrat- ing chemicals, cyanide of sodium among others, in solution under pressure, the pressure being effected by the steam generated by the con- tact of the hot ores with the chemical solution in a closed vessel. This treatment, like that proposed by Clark, was intended as preliminary to amalgamation. John F. Sanders, of Ogden, obtained United States patent No. 244,080, dated July 12, 1881, for "composition for dissolving the coating of gold in ore." This composition is made of cyanide of potassium and glacial phosphoric acid. He stated that by using this mixture he could dissolve " the impure coatings of gold, leaving the gold free and exposed, and permitting it to be amalgamated." It is evident, therefore, that these processes bear no similarity or relation to the modern cyanide process. For a considerable time, cyanide of potassium has been used in the gold fields of California and Australasia for remov- ing film-coating from gold in ores; its application in the pan-amalga- mation process may have been a source of loss of gold. The application of a cyanide of potassium solution for the extraction of gold and silver direct from their ores, which application had been neglected since Rae, was taken up again by Jerome W. Simpson, of Newark, N. J., who obtained United States patent No. 323,222, dated July 28, 1885, for a process of extracting gold, silver, and copper from their ores. Simpson reduced his ore to a powder and agitated it with a solution of certain salts, which combine chemically with the metal in the ore and form therewith a soluble salt. The salt solution was composed of one pound of cyanide of potassium, one ounce of carbonate of ammonia, one half ounce of chloride of sodium, and sixteen quarts of water. This solution is described as particularly adapted to ores containing gold, silver, and copper in the form of sul- phurets. In the absence of silver no chloride of sodium is used; for ores rich in silver a proportionately larger quantity of chloride of sodium is employed. The metals dissolved in the salt solution were precipitated by means of zinc, suspended therein in form of pieces or plates. Simpson was aware that^ cyanide of potassium, in connection with an electric current, had been used for dissolving metal, and also that zinc had been employed as a precipitant. What he claims as new HISTORY. 1 1 is: (1) The process of separating gold and silver from their ores, which consists in subjecting the ore to the action of the solution of cyanide of potassium and carbonate of ammonia, and subsequently precipitating the dissolved metal by means of zinc. (2) The process of separating metals from their ores, to wit, "subjecting the ore to the action of a solution of cyanide of potassium, carbonate of ammonia, and chloride of sodium, and subsequently precipitating the dissolved metals." My own experiments have proved that the addition of sodium chlo- ride is of no benefit for the extraction of silver. The addition of ammonium carbonate is not beneficial to the extraction of either gold or silver, except under certain conditions, when it may be substituted advanta- geously by an alkali or an alkaline earth; in the presence of base metals it is of disadvantage. Simpson's patent description appears to indi- cate that he had not discovered the most important property of dilute cyanide solutions, namely, that of dissolving, without the addi- tion of other chemicals, the noble (in preference to the base) metals. His patent-claim consists eminently in adding to the cyanide solution the chemicals mentioned above. His process, like that of Rae, is an agitation process. The zinc for precipitating the bullion he used in the form of plates or pieces. There is no record in the technical literature in reference to the application of Simpson's process before the issue of the MacArthur and Forrest patents in 1889. After Rae and Simpson, others have made experiments with cyanide solutions for the purpose of gold and silver extraction from ores. F. M. Endlich and N. H. Miihlen- berger are reported to have filed a caveat in 1885, without, however, securing a patent, the former having apparently become doubtful as to the applicability of cyanide as an economical process (E. & M. J., 1891, p. 86). Louis Janin, Jr., made interesting experiments in the same direction in Park City, Utah, of which he published the results in 1888 (E. & M. J., 1888, p. 548). These experiments refer chiefly to silver extraction, but he mentions as well the results on gold ores; his results appear to have been encouraging, and led to his filing a caveat on May 1, 1886, but this was not pushed on to the taking out of a patent. In the southern hemisphere, W. A. Dixon has made experiments with cyanide on Australian ores as early as 1887. He recorded his results, which are at least of historical interest, in a paper read before the Royal Society of New South Wales. Dixon describes therein the experiments made by him at the instigation of the Government Inspector of Mines, who suggested that the extraction of gold from complex minerals was a subject worthy of investigation. Dixon tried on such ores amalgama- tion and a number of solvents. He found "the aurocyanides of the alkaline metals of sufficient stability to render their use possible for the extraction of the gold." He mentions Bagration's and Eisner's publi- cations and alludes to Rae's patent, of which, however, he possessed no particulars. Dixon feared that "the high price of cyanide, its insta- bility when exposed to the air, and its extremely poisonous qualities," would prove such obstacles as to preclude its use for metallurgical pur- poses. He found the reaction between gold and cyanide slow if " the gold was at all dense"; in presence of alkaline oxidizing agents, how- ever, he found the dissolving process suflliciently rapid. Dixon experi- mented also with the ferrocyanide of potassium. His results generally, did not, as far as known, lead to the metallurgical application of cyanide as a gold and silver solvent. 12 THE CYANIDE PROCESS. T have thus described the history of cyanide of potassium .as a gold dissolving agent from the early laboratory experiments up to its metal- lurgical application for ore extraction; this latter, however, did not gain any practical importance until John S. MacArthur and W. Forrest, of Glasgow, Scotland, took out their patents for the use of cyanide as a gold and silver solvent from ores, and gave thereby the cyanide process a start all over the world. Their patents mark an epoch in gold metallurgy. The results of the application of cyanide, as suggested by them, have been very satisfactory; the $14,000,000 of bullion pro- duced by it during the five short years of its working represents what, by the ordinary methods, would have been irrecoverably lost; hence its value and importance from the standpoint of metallurgy and political economy. The experiments of MacArthur and Forrest with gold dissolv- ing reagents occupied some years before their English cyanide patents were applied for (J. S. MacArthur, J. S. Chem. I., March 31, 1890, No. 3, vol. 9). They drew out a list of possible solvents having a stronger affinity for gold than for sulphides, which included the cyanides, and which they found to solve the problem. Their experiments, conducted first on a small scale and with ores of many kinds and of different sources, were so satisfactory that they gradually worked on a larger scale, and their results formed the basis for the introduction of the cyanide process into most gold-producing fields. Their English patent was applied for October 19, 1887. Since then they applied for and obtained patents in many gold-producing countries. Their United States patents are dated as follows: 403,202, May 14, 1889; 418,137, December 24, 1889; 418,138, December 24, 1889. Their invention is described "as having principally for its object the obtaining of gold from ores, but it is also applicable for obtaining silver from ores containing it whether with or without gold, and it comprises an improved process, which, while appli- cable to auriferous and argentiferous ores generally, is advantageously and economically effective with refractory ores, or ores from which gold and silver have not been satisfactorily or profitably obtainable by the amalgamating or other processes hitherto employed; such as ores con- taining sulphides, arsenides, tellurides, and compounds of base metals generally, and ores from which the gold has not been easily or completely separable on account of its existing in the ores in a state of extremely j&ne division." The patentees describe their invention (I am following United States patent 403,202) as consisting in subjecting the ores to the action of a solution containing a small quantity of cyanide, without any other chemically active agent. In dealing with ores containing per ton twenty ounces or less of gold or silver, or gold and silver, they find it most advantageous to use a quantity of cyanide, the cyanogen of which is equal in weight to from one to four parts for every thousand parts of the ore dissolved in a quantity of water of about half the weight of the ore ; they generally use a solution containing two parts of cyanogen for every thousand parts of the ore. In the case of richer ores, while increasing the quantity of cyanide to suit the greater quantity of gold or silver, they also increase the quantity of water so as to keep the solution dilute; in other words, the cyanide solution should contain from two to eight parts, by weight, of cyanogen to one thousand parts of water, and the quantity of the solution used should be determined by the richness of the ore. The patentees state: "By treating the ores with the dilute and simple solution of a cyanide, the gold or silver is, or the gold and silver are, HISTORY. 13 obtained in solution, while ahy base metals in the ores are left undis- solved except to a practically inappreciable extent; -whereas, when the cyanide is used in combination with an electric current, or in conjunc- tion with another chemically active agent, such as carbonate of ammo- nia, or chloride of sodium, or phosphoric acid, or when the solution contains too much cyanide, not only is there a greater expenditure of chemicals in the first instance, but the base metals are dissolved to a large extent along with the gold or silver, and their subsequent separation involves extra expense, which is saved by their process." Later on MacArthur and Forrest obtained patents covering the use of zinc in a fine state of division for the purpose of precipitating gold and silver from cyanide, chloride, bromide, thiosulphide, sulphate, or other similar solutions; they further protected the use of an alkali or alkaline earth for neutralizing ores preparatory to subjecting the same to the action of cyanogen or of a C3'anide. The MacArthur-Forrest patent-claims con- sist, therefore, in three points: (1) The application of diluted solutions of cyanide (not exceeding eight parts of cyanogen to one thousand parts of water); (2) the use of zinc in a fine state of division; (3) the pre- paratory treatment of the ore, which has become partially oxidized by exposure to the weather, with an alkali or alkaline earth, for the purpose of neutralizing the salts of iron or other objectionable ingredients formed by partial oxidation. It is not the purpose of this paper, which is intended to describe the historical development of the cyanide process and its present forms of application, to enter into a judicial discussion of patent-claims and patent-rights. It is the duty of the historian to date the cyanide process as a commercial success from 1890, when it was introduced as "the MacArthur-Forrest process" on the Witwatersrand gold fields, in the South African Republic. Its success as a metallurgical experiment may be dated from the tests made on a large scale with ore from the New Zealand Crown Mine in June and July, 1888. The practicability of the cyanide process once established, others endeavored to introduce improvements in its application, which they protected by letters patent. A patent which once promised to become of practical importance is that of B. C. MoUoy, of Johannesburg, whose "improvement" consists in the abolition of zinc as a precipitant of gold, and in the revivification of the cyanide of potassium in the solution. In this process the ore is treated with cyanide of potassium as usual; the resulting liquors are passed through a "patent Molloy separator," which consists of an amalgama- tor, the mercury of which is constantly being charged electrolytically with potassium. The potassium on coming into contact with the water of the solution decomposes it with the evolution of hydrogen and the formation of the oxide of the alkaline metal. The nascent hydrogen decomposes the solution of the cyanide of gold, and sets the gold free, which is precipitated upon and collected by the mercury; the metal of the alkaline oxide reacts upon the cyanogen compound, and so repro- duces the cyanide of potassium. The original solution, thus regenerated, is then ready for use again. (In reference to further details see under precipitation of gold and silver, p. 38.) Among other cyanide patent-specifications may be mentioned the fol- lowing: John C. Montgomerie, of Scotland, obtained English patent No. 12,641,1892, "for improvements in the extraction of gold and silver from ores and in apparatus applicable for use in the treatment of such 14 THE CYANIDE PROCESS. by means of solvents." His process is the well-known agitation process of finely divided ore with cyanide solution and the addition of an alka- line oxide " for the purpose of economizing the solvent and expediting its action." The patent-specification does not contain any claims which might be termed either an invention or an improvement, either chemic- ally or mechanically. One of the latest additions to the cyanide patent literature is United States patent 515,148, dated February 20, 1894, of Alexis Janin and Charles W. Merrill, for a process of leaching ores with solutions of alkaline cyanides. (For patent-specification, see Appendix.) They claim as new " the art of leaching ores with solution of alkaline cyanides, which consists in first leaching the- ore with such solutions, then adding to the solution an agent which will precipitate the silver present as a sulphide, and then precipitating the gold in the solution with metallic zinc." The practical advantages of this complication will have to be proved. A patent description of interest, although not a " cyanide process " strictly speaking, is that of E. D. Kendall, of Brooklyn, N. Y., dated September 13, 1892, who claims the use of potassium ferrocyanide com- bined with cyanide of potassium, for extracting gold and silver from ores, etc., as his invention. (For patent-specification, see Appendix.) A further addition to the patent literature is the specification of Mac- Arthur and Ellis, who propose to increase the efficiency and economy of the process in cases in which from the nature of the ores treated or other circumstances, soluble sulphides are formed, which retard and objec- tionally affect the action of the cyanide on the precious metals by adding to the ore or the cyanide solution suitable salts or compounds of metals which will form with the sulphur of the soluble sulphides an insoluble or inert sulphide. For this purpose preference is being given to the metallic salts or compounds in the following order: Salts or compounds of lead — such as plumbates. carbonates, acetate or sulphate of lead — sulphate or chloride of manganese, zincates, oxides, or chloride of mercury, ferric hydrate or oxide. The proportion to Idc used is easily to be ascertained by trials of a few samples in each case. (See patent- specification of John Stewart MacArthur and Charles T. Ellis, in Ap- pendix.) C. Moldenhauer proposes to render the cyanide process more expedi- tious and considerably cheaper by, firstly, adding to the cyanide solution an artificial oxidizing agent, by preference ferricyanide of potassium in alkaline solution, and, secondly, in precipitating the extracted precious metal out of its cyanide solution by means of aluminium, or alloys, or amalgam thereof. (See patent-specification in Appendix.) C. M. Pielsticker reverts to the application of the electric current, in conjunction with the cyanide solution. He proposes to continuously circulate the solvent, to continuously precipitate the dissolved precious metals by electrolysis, and continuously regenerate thereby the reagent. (See patent specification in Appendix.) The latest patent in connection with cyanide treatment of ores is that of Dr. W. D. Johnston, "for abstracting gold and silver from their cyanide solutions by means of pulverized carbon" (for further details, see page 40). To make this report as complete as circumstances permit, I append the specifications of the patents which have been mentioned in the body SCOPE OP PROCESS. 15 of this paper, that the mining public may know the exact wording of descriptions and claims. Such is the history of the cyanide process, rapidly sketched by tracing its development through the phases of its evolution and the intricacies of its patent literature. The modern cyanide process consists in the treatment of ores by means of dilute cyanide of potassium solutions, as a rule without the addition of other chemical substances, and in the subsequent precipitation of the gold and silver from the solution by means of zinc in form of shavings. It is commonly known as the Mac- Arthur- Forrest process. I now propose to enter into a description of the process itself. I embody in it the information given me by Mr. John S. MacArthur, of Glasgow, Scotland. II. SCOPE OF PROCESS. The process can be advantageously applied to many gold ores and many silver ores, and is often suitable for ores which are generally con- sidered as rebellious or refractory. The word " ore '' is here meant to include ores, tailings, concentrates, ajid all similar products from ore. The term "refractory" is used to signify any ore which cannot be satis- factorily amalgamated. The refractory character of such ores can be caused by the presence of base metals in combination with sulphur or arsenic, or otherwise by their physical structure, which prevents the gold from coming in contact with the mercury during the amalgamation process. To the latter class belong the ores in which the gold is "coated" with substances which prevent metallic contact ("rusty gold"). (An excellent instance of such coated ore is that found in the Mount Morgan Mine, in Queensland, where the finely divided gold is coated with a film of what has been termed hydrous peroxide of iron, which makes the gold absolutely refractory to amalgamation.) To the same class of refractory ores belong those in which the gold is so finely divided that the film of air surrounding the auriferous particles prevents amalgama- tion even under the most favorable conditions. The base metals which most frequently accompany refractory ores are iron, zinc, lead, copper, and antimony — usually as sulphides, sometimes as arsenides. When ores containing gold, silver, copper, zinc, iron, etc., are treated with solu- tions of cyanide of potassium, these metals are dissolved more or less, forming soluble cyanides. The solvent action on the base metals can be reduced to a minimum by reducing the strength of the solutions, the readily soluble gold and silver being easily dissolved out with only traces of copper, zinc, etc. The action of these weak cyanide solutions on iron, lead, arsenic, etc., is practically nil, and the solvent action on copper or zinc depends much upon the state of chemical combination in which they exist. The cyanide process is adapted to treat most of such refractory ores as are described above. The principal exce])tions are the ores which contain hydrated copper oxides and copper carbonates, and thdse which contain an appreciable quantity of antimony. "When copper com- pounds exist in a state physically hard, the cyanide solution does not readily act on them; but when the copper compounds are soft, porous, and spongy, the action of the cyanide is so decided as to interfere mate- rially with its action on gold" (MacArthur). In reference to copper 16 THE CYANIDE PROCESS. sulphide I found it no impediment to the process; carbonate of copper, however, was so readily attacked by cyanide that its presence proved absolutely prohibitive to the extraction of silver and interfered seri- ously with the extraction of gold. This most refractory ore, that I am speaking of, came from old workings in the Sylvia Mine, Tararu, New Zealand, where part of the ledge containing a large percentage of copper pyrites had been exposed for many years to the influence of moisture and the atmosphere; the resulting carbonate was hard, but notwith- standing this its reaction on cyanide solutions w^as very marked. One and a fourth ounce of such copper ore, finely divided and shaken for less than fifteen minutes with a 2.73 per cent cyanide of potassium solution, reduced the strength of the solution to 0.05 per cent of cyanide. The treatment of the ore in question proved that the affinity of cyanide to gold is at least equal to that of cyanide to copper, and very much greater than to silver, as, notwithstanding the rapid consumption of cyanide by the copper compound, upwards of 70 per cent of the gold assay- value was extracted by cyanide solution of the usual strength, whereas at the same time absolutely no silver had gone into solution. A pre- liminary treatment of such ore by sulphuric acid had a beneficial eftect on the consumption of cyanide and thereby on the extraction of silver. "In the case of antimonial ores, there is little or no interaction between the antimony and the cyanide, consequently the latter is not taken up; but as gold seems to be very firmly held by antimony, and as the compound is very impervious, the cyanide is unable to penetrate the mass, and to dissolve and separate the precious from the base metals. In the case of both copper and antimon}' the cyanide solution will act, but in the case of copper, if there is much present and acted upon, the consumption of cyanide is so great that the operation is not profitable, and in the case of the antimonial ores, though the cyanide will act with fine grinding and long contact, the expense involved often overbalances the value of the gold contents" (MacArthur). The physical state in which obnoxious compounds are found, is of the greatest importance. Hard-surfaced crystals are, even if finely divided, naturally less acted upon by cyanide than soft, spongy masses of the same size. For tech- nical purposes, cyanide treatment of any ore will be called unsuccessful if the large consumption of cyanide precludes a commercial success, although finally a satisfactory extraction in percentage may be achieved. ni. THE CHEMISTRY OF THE PROCESS. The chemical reaction on which the cyanide process of gold extraction rests is that of the formation of the double cyanide of gold and potassium: 2 Au -f 4 KCy + + HgO = 2 AuKCyj -f 2 KOH (Gold.) (Cyanide (Oxygen.) fWater.) (Auro-potassic (Potassic of potassium.) cyanide.) hydrate.) That of silver extraction produces the double cyanide of silver and potassium: 2Ag -I- 4 KCy -f + H2O = 2 AgKCys + 2 KOH /Silver.) (Cyanide of (Oxygen.) (Water.) (Argento-potassic (Potassic potassium.) cyanide) hydrate.) THE CHEMISTRY OF THE PROCESS. 17 Silver in the metallic state is, however, rarely met with in ores which are subjected to cyanide treatment. The part taken by oxygen in these reactions, first noticed by Prince Bagration and later confirmed by Eisner, has of late been disputed, but again confirmed by McLaurin, who published his experiments (J. Ch. S., 1893, May, p. 724) in refer- ence to the question, and came to the following conclusions: (1) That oxygen is necessary for the solution of gold in cyanide of potassium, and that it combines with the potassium of the potassium cyanide in the proportions required by Eisner's equation; (2) That the rate of solu- tion of gold in a solution of potassium cyanide passes through a maxi- mum in passing from dilute to concentrated solution, and this remark- able variation is capable of explanation by the fact that the solubility of oxygen in a cyanide solution decreases with the concentration. The double compounds of cyanide of potassium and gold and silver, respect- ively, have been described in the Annales de Chimie et de Physique, 53, p. 462, 1858, and in Bull, de la Societe Chimique de Paris, 29, 1878, p. 460. Both compounds are easily soluble in water. The cyanide process, as illustrated by the before-mentioned equations, appears very simple indeed. Its adoption in many places has been very rapid, and its success, particularly on the tailings of the Johannesburg mills, has been great. The practical working and technically success- ful carrying out of cyanide treatment of any ore, even under the most favorable circumstances, is beset with complications, which require a ■careful study of all the circumstances connected with the case. All operations offer occasions for loss and opportunities for improvement. The reaction between cyanide and the metals, so simple in theory, is in practice more or less complicated by the reaction of other ore com- pounds on the cyanide and by other causes which it will be useful to investigate. That such reactions take place is put in strong evidence by the amount of cyanide consumed in treating ores, which is always con- siderably larger than the quantity theoretically necessary to dissolve the gold. In accordance with Eisner's equation, 10 parts of cyanide should dissolve 15.12 parts of gold. In the works at Johannesburg, however, in treating free-milling ore, 40 parts of cyanide are required to dissolve one part of gold; that is to say, 40 parts of cyanide are con- sumed for each part of gold obtained. The main reason for this fact must be looked for in secondary reactions, which as yet have only been partly studied. The great loss of cyanide takes place during the extraction process, and particularly during the first part of it, as proved by the rapid diminution in the strength of the solution. The loss of cyanide in the zinc boxes has often been exaggerated (see page 34). A loss of cyanide occurs by absorption in vats and tanks, which is given as high as one pound per ton of ore in Johannesburg (Butters and Clennell). Some loss will always result from the action of carbonic acid gas, which is always present in the atmosphere, and displaces cyanogen from the alkali, setting prussic acid free, which escapes into the air; if caustic alkali is present the freed prussic acid will be neu- tralized. The extent of loss by hydrolysis requires further investiga- tion. The presence of free sulphuric acid or other products of more or less advanced decomposition of pyritic matter will naturally consid- erably interfere with the simple reaction by increasing the consumption of cyanide, and may, under the most unfavorable circumstances, com- pletely prevent successful treatment. " In many cases tailings which 2cp 18 THE CYANIDE PROCESS. have been exposed to the weather contain oxidized compounds, such as sulphate of iron, and similar sulphates of alumina and magnesia, formed by the action of the metallic sulphates on the earthy constitu- ents of the ore." When this is the case it is advisable to give such tail- ings one or more preliminary water-washings, because the cyanide is partly absorbed and partly decomposed by these substances, as seen in the following equations: FeSO^ + 2 KCy = FeCys + K2SO4 FeaSCSO^) + 6 KCy + 3 HgO = FcaOg -f 3(K2S04) -f 6 HCy ' From these equations it will be seen that the ferrous oxide combines with cyanogen, and that the sulphuric acid, forming the second con- stituent of the ferric salt, liberates hydrocyanic acid, which being vola- tile is not available, and moreover constitutes a loss and a danger. The action of the sulphates of alumina and magnesia has not been generally and sufficiently recognized. These salts act practically as if they were sulphuric acid: hydrocyanic acid is liberated and alumina or magnesia, as the case may be, precipitated, as shown in the following equations: Al23(S04) + 6 KCy -f- 3 H2O = AI2O3 -{- 3 (K2SO4) -f 6 HCy MgSO^ + 2 KCy + H2O = MgO -f K2SO4 + 2 HCy The remedy for these troubles is, as before stated, water-washing, in some cases followed by a lime or soda treatment. Reference only has been made to ferrous sulphate as a soluble salt, but it has been found that the basic ferrous salts, which exist to a greater or less extent in "weathered" tailings, are insoluble in water, and yet act detrimentally on cyanide. In any case it is difficult to wash out the last traces of any soluble substance, and it is wise to economize cyanide by an alkaline treatment. "While ferrous salts, soluble or insoluble, exist in the tailings, the lime or soda combines with the acid and deposits the ferrous oxide or hydrate in the tailings. The ferrous oxide would still absorb cyanogen if a cyanide solution were present, but if the air has free access before a cyanide solution is applied, the ferrous oxide is oxidized to ferric oxide, which does not combine directly with cyanogen. It will thus be seen that where salts of iron have to be dealt with it is advisable to make the alkaline treatment preliminary to permit of the necessary oxidation; but where sulphates of alkaline earths only are in question, the requisite lime or alkali may be added along with the cyanide solution. Where soluble iron salts are present to any extent, the washing should be very thorough, and the solution should be run off from the vat through a separate pipe which has no connection with any of the cyanide pipes" (or better, the washing should take place in a special vat; see page 49). "This matter of salts formed by oxidation arises chiefly in the case of tailings, but it may also happen with concentrates and ores, in which case they are treated as tailings" (MacArthur). Butters and Clennell advance the following equations of possible reactions accomjianying the action of cyanide on pyrites. They illus- trate first the influence of oxygen on pyrites: THE CHEMISTRY OF THE PROCESS. 19 FeSa + H2O + 70 = FeS04 + H2SO4 2 FeS04 H- = FezOg, 2 SO3 (Wittstein) 10 FeS04 + 50 = 2 Fe.^0^, 2 SO3 + 3 Fea (804)3 (Berzelius) (Basic sulphate (Ferric sulphate iusoluble.) soluble.) They describe, then, the reaction of cyanide on such products: FeS04 + 2 KCy = FeCyg + K2SO4 FeCy2 + 4KCy = K4FeCye Ultimately giving rise to 3 K4FeCy6 + 6 FeS04 + 30 = Fe203 + 6 K2SO4 + FevCyjg Ferric salts and cyanide give: ^62(804)3 + 6 KCy = Fe2Cye + 3 K2SO4 and, Fe2Cye + 6 H2O = Fe2(0H)e + 6 HCy Fe2(S04)3 + 6KCy+ 6 H2O = Fe2(0H)e + 6 HCy + 3 K2SO4 A mixture of ferrous and ferric sulphates on addition of cyanide will form Prussian blue when the ferric salt is in excess: 18 KCy + 3 FeS04 + 2 Fe2(S04)3 = 9 K2SO4 + Fe4(FeCy6)3 and Turnbull's blue when ferrous salt is in excess: 12 KCy + 3 FeS04 + Fe2(S04)3 = 6 K2SO4 + Fe3(FeCy6)2 The reactions between the various iron and cyanogen compounds are very complicated, and a number of possible reactions have been illus- trated by equations by various writers, the discussion of which here would take up too much space. In cases where such conditions exist, a preliminary washing with water alone, or with solutions of carbonates or hydroxide of sodium or lime, as described, may be not only useful but imperative. A great sur- plus of alkali should be avoided, on account of its action on the zinc in the precipitation boxes. The loss of zinc will be larger the greater the alkalinity of the solution; besides this, it is apt to form a sulphide of sodium or potassium with the sulphur of ores, which interferes with the extraclion of the silver. Further careful scientific researches in refer- ence to secondary reactions, which accompany the cyanide process, will probably lead to technically important results. The chemistry of precii)itating the metals from cyanide solution will be discussed in con- nection with the description of the various methods employed for that purpose. 20 THE CYANIDE PROCESS. IV. DEMONSTRATION OF THE PROCESS — METHODS OF OPERATION. The cyanide process is worked either by agitating the ore with the solution ("the agitation process"), or by allowing the solution to pass through the ore ("the percolation process"). A. The Agitation Process. When cyanide treatment of ores was first attempted, it was done by agitating the material under treatment with cyanide solution; Rae's cyanide process of 1867 and Simpson's process of 1885 were agitation processes. Generally speaking, agitation, as compared with percolation, expedites and in instances increases extraction, but it requires motive power, which is a source of expense. Wherever large quantities of ore are being treated it has been abandoned in favor of the percolation process. It is useful, however, in many instances where the ores are hard and dense, and of a sufficient high value to pay for the necessary motive power and permit a convenient method of filtration; it is applied where the quantities are limited and is mostly used for treating concen- trates, or such ores as make the treatment of limited parcels by them- selves desirable. The importance of the cyanide agitation process has not been so fully recognized as, in some instances, it deserves. It is natural that if percolation gives as cheaply the same results it will be preferred, but sometimes the agitation system has the advantage of giving quicker, higher, and cheaper returns. Some ores, particu- larly ores containing tellurides and sulphide of silver, give better results by agitation than by percolation. The agitation process, in its present form, is not well adapted to handling very large quantities of ore without a considerable outlay of machinery. Technical improvements of the system, which in suitable cases may make the whole process almost a continuous one, may be expected. The chief appliances for the agita- tion process are the agitator and the filter. Although any vat fitted with revolving arms and barrels, similar to those employed in chlori- nation, may be used successfully for agitation, still an agitator which permits a charge and discharge quickly and safely, which has the least wear and tear, does absorb neither gold nor cyanide, and is cheap in its first cost, corresponds best with all requirements. I have been using wooden barrels, wooden vertical agitators, iron pans, and steel cylindri- cal agitators, and have found the latter construction best suited to the purpose and satisfying all the above conditions. (For description of such an agitator, see Utica cyanide plant, page 89.) ^ For the purpose of extraction, the ore and the cyanide solution are agitated for a time, varying in accordance with the character of the ore, generally ranging from six to twelve hours. I have extracted from com- plex ores, in some instances, upwards of 90 per cent of the assay-value in less than two hours, and in other instances I have found it necessary to continue the operation for twenty-four hours. No general rule can be given; each case has to be investigated and the modus operandi to be selected according to circumstances. (See table showing rate of extrac- tion in relation to time of agitation, attached to the description of the Utica plant, page 94.) The strength of the cyanide solution and the DEMONSTRATION OF PROCESS METHODS OF OPERATION. 21 volume required depend entirely on the character of the ore; as a rule, solutions for agitation should be stronger than those for percolation. Here, like in other matters in connection with cyanide treatment, experi- mental investigation has to advise on best conditions. (See chapter on laboratory work, page 44.) In using barrels as agitators, ore and solu- tion will be charged before the barrel is revolved; if vertical vessels are used, the solution will be charged first, then the stirrer Avill be set in motion, and the ore added by degrees. When the extraction is completed, the mass in the agitator is dis- charged, and the cyanide solution, now containing the gold, is separated from the solid material — i. c, the residues — by any method which local conditions and the character of the ore suggest. Apparatus of different principles have been used for this purpose. Filter presses of various constructions, vacuum filters, and centrifugal machines have been employed. Concentrates, coarse and slimy, can be successfully treated by means of my vacuum filter (see description of Sylvia and Utica plants, p. 79 and p. 89), which permits a quick filtration and a perfect and speedy washing of the residues with a minimum of liquid. In some instances I made successful use of centrifugal force for separating the gold solution and washing the residues. For washing, weak cyanide solutions from previous operations are used, and finally a water-wash is given. The residues are then discharged. The gold solutions should, for practical reasons, be kept separate according to their strength in gold and cyanide; they pass through such appliances as are used for precipi- tating the gold and silver, after which the "liquors" are collected in sumps for use on subsequent charges of ore. In well-appointed works no cyanide solution is allowed to run to waste, as the same amount of liquid remains constantly in circulation. The author took out early in 1893 a caveat in New Zealand for a cen- trifugal apparatus, agitator and separator combined, for the treatment of slimy ores by agitation with cyanide, and subsequent separation of the gold solution by centrifugal force in the same apparatus. Experi- ments have of late been made in the Thames School of Mines, New Zealand, with the treatment of slimy ores by the agitation process in an apparatus which is described as follows: The appliances used in the operation consist of a shallow circular vat, a vacuum cylinder, and an air pump. The vat is provided with four revolving arms, to which soft rubber brushes are fixed. The bottom 'of the vat is fitted with a false bottom, constructed of a wooden grating covered with wool packing. The opera- tion is conducted as follows: The leaching solution, made up to the re- quired strength, is first conducted into the vat. The revolving arms are then set in motion, and the dry pulp or slimes introduced. The agitation is continued for six hours, or until the extraction is complete. A stopcock in a pipe connecting the false bottom of the leaching vat and vacuum cyl- inder, is then opened and the air pump started. The effect is immediate. At once the clear solution begins to drain over into the cylinder, the revo- lution of the arms preventing the slimes from settling and choking up the filter cloth. When the slimes have been drained down to a thick paste, the first wash water is added, the pump again started, and the slimes drained as before. The water-washings are carried on in the same way, and when completed a plug or door is opened and the leached slimes are sluiced out. The whole operation of leaching takes from eighteen to twenty-four hours. The technical and economical practi- 22 THE CYANIDE PROCESS. cability of this method of treating slimy ores appears doubtful and will have to be proved. B. Percolation Process. Percolation is the method generally in use. It is being worked in the United States, in the British Colonies of Australasia, and on a very exten- sive scale in the South African gold fields, and therefore merits a full description. Percolation consists in soaking cyanide solution through ore. The character of the material to be treated, whether ores, concen- trates, or tailings, will demand certain modificatioms of the treatment, without interfering with the principle. (a) Percolation of Ores. — It is advisable to dry-crush the ores; the less dust produced the better for percolation. Screens of thirty meshes to the lineal inch will be found satisfactory in most instances; in some cases a coarser screen may do, or a finer one may be required. It is desirable to crush as coarse as possible without interfering with the percentage of extraction. Stamps are much in use for dry-crushing; mortars with double discharge will give more product and less dust. Rolls are to be preferred, on the ground of their giving a product of greater uniformity. The ore is charged, either directly or through hop- pers, into the vats in which the percolation is conducted. These vats, or tanks, may be constructed of wood, brick and cement, concrete, iron, or steel, and vary in size in accordance with local circumstances and requirements. The largest in existence are the circular brick vats at the Langlaagate Estate and Block B Company's works in Johannes- burg; these vats have a capacity of about 400 tons, and are 40 ft. in diameter by 10 ft. deep. A size in common use in Johannesburg is 20 ft. in diameter and about 6^ ft. in depth, inside measurement, of which I give Mr. J. S. Mac Arthur's description: "This vat is made of the best white pine; the staves are 7 ft. 3 in. long, 4 in. wide, and 2^ in. thick, and fitted with a slight taper upwards, so that the diameter at the top is about 4 in. less than it is at the bottom. The bottom is made of the same kind of material, but is at least 3 in. thick. The pieces are fitted together with dowel pins; it is then fitted into a groove cut in the staves about 6 in. above the ground, and the whole vat is bound together by steel hoops with a 6 in. over-lap an4 at least three rivets. No white lead or packing of any kind should be used in making these vats. If the faces of the wood are true no amount of white lead or other packing can make them truer; if they are not true, neither white lead nor any kind of packing will secure tightness. Besides this the cyanide solution being alkaline would quickly combine with and remove the oil of the white lead, if such were used, and make the vat positively worse than if none had been employed. Circular pieces, cut out of the solid wood and not bent by steam or moisture, are fixed by screws on the bottom of the vat, about 1 in. from the staves, all around the circumference. These circular pieces are about 3 in. thick and 3 in. wide, but the length of each is immaterial, provided always that a complete ring is formed. Wooden slats, about 1 in. thick and 3 in. high, are fixed about 6 in. apart all over the bottom; and an iron pipe, generally 2 in. in diameter, is screwed in from the under side near the center point. The 3 in. space from the bottom to the top of the slats is filled in with round and clean pebbles. Over this surface, formed by slats and pebbles alternately, is stretched DEMONSTRATION OF PROCESS — METHODS OF OPERATION. 23 txi: 24 THE CYANIDE PROCESS. a canvas cloth to act as a filter, which is fastened by stretching it over a circle piece and ramming the cloth tight by pressing an inch rope into the space between the circle pieces and the staves. The canvas filter is made by shaping and sewing the canvas into a circle piece rather larger than the area of the vat bottom. In practice the canvas filter is often protected by covering it with old sacks or cocoa matting, which serves to protect the filter proper from the wear and tear caused by the friction of the ore or by the cutting of spades." (Special vat and tank construc- tions will be given aside from this general description in reviewing large and successful plants in different countries.) "The vat thus protected and fitted is charged with ore, and the cyanide solution is run, prefer- ably from the bottom, by a pipe and rises slowly through the crushed ore. It must not be allowed to rush in or rise violently, as by so doing channels will be formed through which the solution will pass without acting on the ore. Such channels are apt to be formed under any cir- cumstances, and should always be guarded against. After the upward percolation, the stopcocks are shut, and opened again after the desired time of contact has passed, so as to allow of a reversal and downward percolation. The cyanide solution now containing gold is carried through the precipitating appliances and from there into the sump, from which it may again be used for percolation. It is not wise to attempt to make the solution very rich in gold, and it is considered better prac- tice to remove the gold frequently, as it is found that a cyanide solution containing gold is not so active as a similar solution without any." In some instances, however, it has been found of advantage to use gold cyanide solution over again, without first passing the same through the precipitation boxes. (See experiments made by the Robinson Com- pany, in Johannesburg, page 65.) According to the richness of the ore and the fineness of the grinding, percolation may be repeated several times, but after the final percolation with the ordinary cyanide solution,, a washing of weak or waste solution should follow, and the whole opera- tion be completed by a water-wash, after which the residues are discharged. The filling and discharging can be done either by hand or by mechanical appliances. The various methods will be described in connection with the process in South Africa. (b) Tailings are treated in substantially the same way as ores, and, the quantities being large and the grade low, the vats of the largest size and the most complete arrangements for saving labor in charging and discharging are necessary for profitable working; generally speaking, the difficulty of discharging the vats is increased by the increase of their diameter and their depth. " Several difficulties arise in the case of tail- ings, which do not usually present themselves with ores. These diffi- culties are chemical and mechanical. The chemical difficulties have been described in the chapter on chemistry; no general rules can be applied to them; each case has to be investigated and steps be taken accordingly. The mechanical difficulties arise from the tailings being derived from the operation of wet-crushing. When tailings are charged in a wet state into the percolating vat, they are apt to remain in lumps, from which the water has to be expelled by the cyanide solution before the latter can effectually do its work. It is obvious that where tailings are already saturated with water, the cyanide solution will have a diffi- culty in penetrating, and this difficulty is increased when the wet tailings DEMONSTRATION OF PROCESS — METHODS OK OPERATION. 25 are held together in masses between which the cyanide finds an easier channel for flowing than by soaking through them. This is merely a form of the channels above referred to. Assuming, however, that these channels are not formed, the tailings in a wet state mass or pack together to such an extent as seriously to retard percolation. Another difficulty which arises from the tailings being wet is that in clayey ores the slimy portion of the mass is apt to gather into a layer by itself, which if formed of real clay, not only impedes but absolutely prevents percolation. In order to overcome this difficulty the simple method of drying and mixing should be adopted. The drying is a mere preliminary to the essential of thorough mixing. Particles of clay, which are not kept apart by sand, will agglomerate and form water-proof strata. It is impossible, unless the whole material is perfectly dry, to get the particles of clay separated from each other and allow the sand particles to intervene. Even when this is done, the tendency of the clay particles to agglomerate must be guarded against and prevented. The principal precaution necessary is that the solution, whether applied from top or bottom, should not flow more quickly than the dry tailings can absorb it. In many respects upward percolation has an advantage, but principally because the flow of solution is against gravitation. In downward percolation, where the flow of solution and gravitation act together, the whole material tends to become compressed into a cement, through w^iich the solution penetrates but slowly, preferring to take the easier course down the sides of the vat, and in fact going around rather than through the tailings. Alternate upward and downward percolation may be found useful in some cases." The percolation vats are charged with tailings to within a few inches from the top, and their surface is leveled. The cyanide solution of, say from 0.2 to 0.8 per cent of strength, is then permitted to penetrate the tailings, till the liquid covers them. The contents of the vat will settle some inches, which shrinkage depends on the depth of the vat and the percentage of moisture in the material. The solution is permitted to remain undisturbed in contact for say twelve hours; after that time it is allowed to drain ott". As the liquid is drawn off, it is replaced by fresh solution. This operation is continued for a longer or shorter period in accordance with the value of the tailings (about six to twelve hours in the works of the Robinson Company at Johannesburg). After this time, which is termed the "strong solution leaching," a weaker solution, containing say from 0.2 to 0.4 per cent of cyanide, is turned on, which filters through the ore for about eight to ten hours. This weak solution, when drawn off, is treated separately (see above). At last, water is run on the tailings for replacing the last weak cyanide solution. The volume of solution in constant use and circulation remains the same. The weak cyanide solution is the liquor which has previously passed through the process by which the gold and silver are precipitated and has from the sumps been pumped back into the vat. The percolation vats, which used to be square, are, in new works, round. The cyanide solution of the described strength has no appre- ciable deleterious effect either on the wood of the vats or on the iron pipes and iron valves of the pumps. Iron or steel vats may be protected by a coating of coal tar and asphalt, or a solution of asphalt in turpen- tine, preferably put on hot, if special reasons make such protection desirable. The quantity of cyanide solution used for the treatment of 26 THE CYANIDE PROCESS. PLA NT TO TREAT A MINIMUM OF 2000 TONS. PER MONTH. SaaZe. of Fe&t, one ton of tailings amounts generally to half a ton of strong solution and half a ton of weak solution and wash. When the percolating process is finally completed, the exhausted tail- ings, or "residues," are discharged in older works by being shoveled out over the side. More modern works have trap-doors at the bottom of their vats for discharge (see diagram, page 59). "Sluicing out" of the residues is being practiced in several localities. The large vats of the Langlaagte Estate Company's works at Johannesburg, which hold 400 tons of tailings each, are discharged by means of running cranes (see diagram, page 57). It must be always borne in mind that the most complete arrangements for saving labor in charging and discharging large quantities of low- grade material are necessary for profitable working. In order to achieve that end, a plant should offer all such facilities which circumstances permit; it should be so arranged that the tailings will not have to be lifted, but can be dumped into the percolation vats. The size of the vats has been constantly increased. New works, like the Roodeport works at Johannesburg, are supplied with vats 40 ft. in diameter. The table appended illustrates the dimensions of the percolation vats in some of the more important cyanide extraction plants: DEMONSTRATION OF PROCESS — METHODS OF OPERATION. 27 •^r* a • t- lO CO lO o°^ •-' CO o o ^ CO 0-- _ QQ OQ en en tn ai tn m tn « oi m tn m "oSV. CUO bO 60 t>D M tc bD 60 60 60 60 60 60 60 z,c o .s _fl C C f^ C C C C ^ a p 3 a cj a> (D 0) •^ S c3 c3 t4 cS 03 OS 03 03 c3 c3 03 o3 03 03 03 o H H O H H H H H H H H H H H H CO t^ o o o lO o o lO c a M (N CO CO lO o t^ , >> 4J -k^ >, >, >. -M ^ o ^ ^ <+-< ■<*< iq -p ^ -W Si ^ -t< ^ a a 05 CD a .3 a"" a"" 00 o a"" s a i >> >> ^ .3 s >J >, .3 .3 >> .3 .3 5 ^ ^ a -u 'S 'S ^ Xi -S 'S ^ ^ '3 ii ^^ a> _^ ^ -IJ 1 -^ ^ -ij ^ ^ ^ S ««H c3 ■4-H e*-H «*-< <+H «« «*H UH **H <*H iH CD (M Q o O 00 CD o o CO CD o (M •H T-H tH c^ ■* ^ ■q< ^ g c3 !U c3 n3 G c3 (H o3 0) )-. c3 13 C t3 C T3 C o n rj s 3 S D .O 3 3 3 3 g ^ a* o* a* o O O o cr' o' u' O o C o o o CQ cc cc tf « a Ph «2 m Uj M Ph CC! Pi Ph Pi [3 •73 TJ 'O t3 i TS T3 TS T? ^ TS ■73 ■o ^ TJ Ti H ^tJ fi> 3^ •AO 1 ; ; 1 w 1 1 1 o TS •V 73 '^ 03 d c c C C ^1^ o ^ eS ^ r5 c ,__ ^ ,„_, ^__, ,_j ^ __, ,_, __, '3 o o ►J "3 "3 N o 03 d a a o 3 O CO 03 > 03 !> C c3 > 03 a o3 03 cj C 03 03 03 > to S 03 C3 03 > m C oi c3 > to C 03 o3 > tn C 03 03 03 > C 03 0) a> 0} V "o II ^ tl (1 u <~, ^ tH u, ?5 >5 ^ ;?; a ^ H H H H H H H H H >» fjj >> ! ; c 03 c 03 ft 'ft a >. >, >. a o a o o 03 ft a 1 a o CJ 3 <0 •a c 05 P, c a f4 c c3 ft a o o 0) !3 r«=i >> a C3 c o3 o >> ft a o o a si 'A a o CO .5 S o O IH u W >> a 03 p, a o c 03 ft a o 2 o 0) 0) O o o n ft a O O "S a> ft a o o o ft to 03 a '60 C ■o3 ft a o c o GO o o o 0] a s & OS 3 [c 03 c 03 "aj "oS _c 2 0) &> '3 "C O o 60 60 c 03 o a o a H ?= o « ?% ^ o o O '^ 2 Pi CO i-i e4 CO 'U' irf CO t-^ 00 05 o :j 2 CO ■<1< 2 28 THE CYANIDE PROCESS. All vats should be some distance above the ground, so that leaking can be easily detected; concrete foundations for the vats are generally adopted. The wooden tank material is an absorbent of both cyanide and gold, particularly when new. It has been found at the Salisbury works, Johannesburg, that pine wood lying thirty-four hours in a 0.3 per cent cyanide solution reduced it to 0.05 per cent, while cement reduced it to 0.24 per cent. Cement tanks have come into use of late, and have proved satisfactory; such tanks and vats may advantageously be built into excavations in solid ground. Many attempts have been made to discharge tailings-pulp direct from the plates, or ore-pulp direct from the mortars into the percolating vats, but their successful treatment by cyanide when so discharged has been prevented by mechanical causes, the reason being that the material packs so densely in the vats that it makes percolation an extremely tedious operation, and in consequence of the presence of slimes the results are unsatisfactory. The advantages of wet-crushing over dry-crushing are, from an economical standpoint, so obvious, however, that experiments will be continued, and ultimately the drawbacks which now adhere to the method may be overcome. Cyanide of potassium solution has been used, in some instances and in an experimental way, in lieu of water in the mortars, when wet-crush- ing was resorted to, but does not appear to be practiced anywhere at present. An innovation in percolation consists in the circulating system, which will be described in detail in connection with the practice of the cyanide process in South Africa (see page 46). (c) Percolation of Concentrates. — "These are treated similarly to ores, but being generally richer require a greater number of percola- tions, and thereby a much longer time. In most cases, their quantity is limited, and the size of the percolation vats varies in accordance with the quantity." I have, in most cases, given agitation the prefer- ence to percolation for treatment of concentrates, on account of its greater cheapness and rapidity; in Africa most companies prefer the latter method. Percolation of concentrates requires about twenty days, the reason for which will be found partly in the coarser character of the gold, partly in its being in the form of amalgam, and mainly in the difficulty the solution has in penetrating between the faces of the sul- phuret crystals. A difficulty sometimes arises in the percolation of concentrates, owing to the crystalline form of iron pyrites and galena. These minerals crystallize in cubes, and when suspended partially or wholly in a fluid tend to range themselves face to face, so that a section of such a mass deposited from a fluid would resemble a brick wall in structure. This difficulty does not arise in the case of sand or minerals which crystallize in other systems. Whenever it occurs, it may be over- come by mixing the cubical sulphurets with coarse sand. C. Cyanide and Cyanide Solutions. The best strength of solutions to use in either percolation or agitation depends entirely upon the nature of the ore, and it is impossible to set any rule. The strength of solutions generally used varies from one eighth to one per cent of cyanide. (In reference to the determination of the correct strength to be used in treating any class of ore, see chapter on laboratory work, page 44.) " For convenience and economy of work, DEMONSTRATION OF PKOCESS — METHODS OF OPERATION. 29 the solutions are generally divided into three classes: No. 1, No. 2, and No. 3, of which No. 1 is the strongest and No. 3 the weakest. Assuming that the material under treatment does not require a preliminary alka- line wash, or that such treatment has already been completed, it is usual to run on a weaker solution, say No. 2, in the first place, and after its percolation to use No. 1, and then No. 3 in the same manner, finishing with a water-wash, the first portion of which is run into and forms part of the No. 3 solution. These different solutions are kept separate after percolation, and when charged with gold are subjected by themselves to the precipitating process. In some works sumps are used as reservoirs, and the solution is pumped direct from them onto the ore; but space permitting, it is considered better practice to have reservoirs for each solu- tion above the percolation vats, from which the flow can be more easily regulated." For the purpose of bringing weak solutions up to a certain standard, it is advisable to use a very strong solution, of which enough is added to bring the weak solution up to the required strength. This method has to a great extent taken the place of the old method of using solid cyanide to bring weak solutions up to high standards. The strength of the cyanide solutions, which it is of great economical impor- tance to determine, is tested according to Liebig's method by means of a one tenth standard solution of nitrate of silver, which is made by dis- solving seventeen grams of pure nitrate of silver in one litre (1,000 cc.) of distilled water. Liebig's method is based on the fact that silver cyanide is soluble in excess of potassium cyanide, with formation of a double cyanide of silver and potassium: KCy + AgN03 = AgCy + KNO3 (Potassic cyanide.) (Argentic nitrate.) (Argentic cyanide.) (Potassic nitrate.) AgCy + KCy = KAgCy^ (Argentic cyanide.) (Potassic cyanide.) (Argentic potassic cyanide.) As soon as the whole of the cyanide has been converted into a soluble silver salt, an additional drop of silver nitrate will produce a permanent precipitate of the insoluble simple cyanide of silver: KAgCya + AgNOs = KNO3 + AgCy (Argentic-potassic cyanide.) (Argentic nitrate.) (Potassic nitrate.) (Argentic cyanide.) A measured portion of the perfectly clear cyanide solution which is to be tested is taken; if necessary some distilled water is added, and the standard silver solution is gradually added from a graduated burette, until a permanent white cloud is formed. As each cubic centimetre of the silver solution is equal to 0.013 grams of potassium cyanide, by multiplying the number of cubic centimetres consumed by 0.013 the amount of cyanide in the solution tested is found in grams, from which the percentage can easily be calculated. A convenient silver solution for the purpose of analyzing cyanide solutions is one of such strength that every cc, added to 10 cc. of the solution which is to be tested, cor- responds with 1 per cent pure cyanide of potassium. "The cyanide solutions are apt to form, by continued exposure to the air, carbonate of ammonia; and as this salt interferes very seriously with the determina- tion of the cyanide, it is well to add a few drops of solution of iodide of potassium, which forms a pale-yellow cloud insoluble in ammonia, 30 THE CYANIDE PROCESS. which indicates completion of the reaction" (MacArthur). The mode of analysis, as described above, calculates the amount of cyanide of potassium in a solution by ascertaining its contents of cyanogen. If the cyanogen is partly combined with sodium instead of potassium, the per- centage of cyanide appears in the analysis higher than it really is; the value of commercial cyanide of potassium should therefore be ascer- tained by determining its contents of sodium, if any, as it is possible, by manufacturing a mixture of cyanide of potassium and cyanide of sodium, to produce cyanide, which according to the ordinary method of estimation, contains apparently more than 100 per cent of potassium cyanide. The analysis of cyanide solutions for gold and silver will be described in the chapter on laboratory work (page 44). The Cyanide usually employed for ore extraction is of two classes, one of which, manufactured in Scotland, contains from 70 to 80 per cent of pure potassic cyanide; the other is manufactured in Germany and con- tains upwards of 98 per cent. The latter grade is preferable, because it contains no carbide of iron, the presence of which in the former not only involves periodical cleaning out of the dissolving tank, but also is liable to precipitate gold should any of it come in contact with gold cyanide solution. The price of the best quality of cyanide of potassium, guar- anteed to contain upwards of 98 per cent cyanide, is at present 50 cents per pound in the United States, delivered at seaports. D. Treatment of the Gold Solutions (Recovery of the Bullion). All methods of treating ores, as described, yield solutions containing more or less gold and more or less cyanide of potassium. The next step is to recover the gold and silver. The chief method for that end consists in their precipitation by means of finely divided zinc; it forms one of the patent-claims of MacArthur and Forrest, and is generally in use. Another method is that of B. C. Molloy, of Johannesburg, who decomposes the cyanide solution of gold by means of an alkali metal, and amalgamates the bullion thus liberated; cyanide of potassium is regenerated by this process. The Molloy process has been in use on a small scale in South Africa, but has apparently gone into disuse again, for the official list of the Chamber of Mines of Johannesburg for March does not return any gold as extracted by that process, as had been the case in former months. Other processes for bullion precipitation are: The Siemens and Halske process of precipitating the noble metals by electrolysis on lead sheets, which method is in use at the Worcester works in Johannesburg; the Pielsticker process of using electrolysis, constantly applied to the circulating solution; the Moldenhauer process of precipi- tation by means of aluminium, and the Johnston process of using char- coal as reducing agent. No information is available on the technical application of the three processes last named. (a) Bullion Precipitation by Zinc. — The solutions from the perco- lation vats, or from the filter appliances if the agitation process is used, are run through boxes which are divided into chambers with double partitions. The first partition does not reach to the bottom of the box, the next one not quite to the top, and so on; they compel the solution to enter each chamber from below, and pass through a perforated bottom,. DEMONSTRATION OF PROCESS — METHODS OF OPERATION. 31 r N ^ Z ^ o ^ O 32 THE CYANIDE PROCESS. on which the finely divided zinc is placed. After passing through the zinc, the solution leaves the chamber at its top; then it descends between the double partition to the space below the perforated bottom of the suc- ceeding chamber, where it undergoes the same treatment, and so forth in all successive ones (see sketches, pp. 31, 33). "This arrangement has been adopted because the gold is precipitated on the zinc in a state of fine division, and would, if deposited on the upper surface, prevent the further flow of the solution; but being deposited on the under surface, the gold precipitate falls ofl" and leaves the passage clear. Each precipi- tating box may contain ten to twelve double chambers, and no matter how rich the solution is at the inflow, it should not contain more than a few grains per ton at the outflow." In some works the zinc boxes are up to 40 ft. long. There is, however, no advantage in going beyond a limited number of chambers, as precipitation of the metals takes place chiefly in the first few compartments of the box. (See description of tJtica plant, p. 89). I give here Mr. J. S. MacArthur's description of his construction of filter boxes, and the mode of working them which is used by the MacArthur-Forrest patentees in South Africa: " The gold precipitate falls through the gauze 'a' (see cut, p. 31) into a chamber which communi- cates with an inclosed launder or gutter. From day to day fresh zinc is added, always, adding it in the last chamber and bringing the partly consumed zinc up a step, so that the first chamber contains zinc half consumed and rich in gold, while the last chamber contains fresh zinc containing no gold. At intervals of about two weeks, there is a clean-up, and the gold is collected by stirring the zinc so as to cause the gold precipitate to fall ofl". When this is done, the stopper ' b ' is raised and the gold precipitates fall through the opening into the launder 'B.' When this has been completed for each chamber, the launder is dis- charged through the opening ' C " The precipitation boxes are usually made of wood, and although I have been well satisfied with such mate- rial (kauri-pine in New Zealand), I substituted it in California by steel, which I found in every respect an excellent material for the purpose. I could not ascertain any increased loss of cyanide by the use of iron as box material; the galvanic action of iron and zinc in contact on the cyanide seems by some writers overrated (see description of Utica plant, p. 89). Of this apparatus, which is based on the same principle, but which in its construction is simpler than the one described, I give the appended diagram. The total length of the apparatus is 9 ft., the size" of the chambers is 9 in. by 9 in. by 14 in. deep; the distance of the par- titions between each chamber is 1 in. The perforated and movable false bottom of each chamber is of steel, which is an advantage over wire sieve bottoms, which easily become clogged b}^ bullion. The real bottom of each chamber has a faucet of 1 in. diameter, which discharges the liquid and the finely divided bullion into a tank below the apparatus, from where, after settling (under addition of some alum, if saving of time is an object), it is transferred to a vacuum filter, as described further on. "The zinc used for precipitation purposes should be the best quality found in commerce, and should not contain arsenic or antimony; a small percentage of lead, however, does no harm, but rather tends to promote rapid action by forming a voltaic couple with the zinc" (MacArthur). The metal is preferably used as shavings, or filiform, as these forms DEMONSTRATION OF PROCESS — METHODS OF OPERATION. 33 give in practice the most surface for the least weight and do not pass readily through a sieve, whereas the gold, which is precipitated as a fine powder, does. Shavings have the advantage of not forming lumps so easily in the precipitation boxes as filiform zinc; the latter has, however, the greater advantage of being cheaper in its preparation, as no remelt- ing of the commercial zinc is required. It is prepared by cutting sheet zinc into disks, a number of which are placed together and turned on a lathe with an ordinary chisel. The zinc linings of the cyanide packing- cases, for which there is no market, may be turned to account in that way. 3cp 34 THE CYANIDE PROCESS. In reference to the cost of preparing the zinc, I may quote the Nigel Gold Mining Company in Johannesburg, where one native, working about eight hours a day, can easily keep the works going, with an output of about two thousand ounces of gold monthly; the consumption of zinc is about twenty pounds daily. As a rule, one cubic foot of zinc shavings in the precipitating box is sufhcient for the precipitation of the gold from two tons of solution per twenty-four hours, or, roughly speak- ing, from the same weight of ore (see the zinc for bullion precipitation in Africa on page 53). Zinc in sheets and granulated zinc have been tried for bullion reduction, but with indifferent results, on account of their limited surface. Zinc amalgam and zinc dust have not answered, for mechanical reasons — zinc dust packing too tightly and zinc amalgam not offering sufficient surface in proportion to its weight. The precipita- tion of the metals in the zinc boxes takes place rapidly; the zinc in the compartment near the influx will be much more quickly charged with bullion than that in those more distant, and the zinc will be consumed in proportion. Zinc on which bullion is already deposited is more active than new zinc; it is therefore advisable to replace the dissolved zinc in the upper by zinc from the lower chambers, and to add the fresh zinc in the last compartment. The generation of hydrogen in the boxes is liable, by polarization, to partly interfere with the bullion precipitation; the zinc in the boxes should be stirred up occasionally to avoid this. The zinc boxes are cleaned up once or twice a month; for that purpose the inflow of the solution is stopped. The zinc shavings are stirred with a rod, which causes the fine bullion to fall ofl" and to pass through the perforations of the false bottoms, and through the faucets at the real bottoms, into the box below, where it settles readily, on the addition of a little alum. A jet of water will further wash the zinc in the chambers. This method of operating takes only a few minutes, and has been used by me in California. The liquid standing above the settled bullion is returned to the zinc box; the bullion itself, unavoidably mixed with fine zinc, is transferred through a fine sieve onto a vacuum filter. If a final cleaning-up is desired, it will be necessary to dissolve the whole of the zinc, impregnated with bullion, in acid; such necessity will, however, rarely arise. The manip- ulation itself if required, offers no difficulties. The precipitated bullion is very finely divided, and provision should be made to prevent its flow- ing away with the liquid out of the precipitation boxes (see page 52). The process of bullion precipitation by zinc is, generally speaking, a satisfactory one, although not free from objections; all operations with and manipulations of the precipitated bullion require care to avoid loss. The action of the zinc on gold solution is theoretically very simple — a simple substitution of gold by zinc according to the equation: 2 KAuCy2 + Zn = K2ZnCy4 -f- 2 Au (Auro-potassic cyanide.) (Zinc.) (Zinc-poiassic cyanide.) CGold.) One pound of zinc should precipitate about six pounds of gold. The actual consumption is, however, considerably larger, and amounts to from 5 oz. to 1 lb. of zinc per ounce of gold recovered. A constant gen- eration of hydrogen gas in the precipitation boxes proves the effect of the potassic hydroxide on the zinc, and probably a decomposition of cyanide of potassium, going on parallel with the decomposition of the DEMONSTRATION OF PROCESS — METHODS OF OPERATION. oO double gold cyanide. A considerable loss of zinc occurs generally in refining the precipitated bullion, which always contains a high percentage of that metal. The double salt of auro-potassic cyanide appears to be one of the most stable of gold salts; its decompo- sition by zinc is, however, practically complete; an excess of cyanide of potassium in the solution does not redissolve precipitated gold in the boxes as long as there is zinc present. The cyanide of potassium formed into a double salt with zinc during the gold-reducing process is not available for dissolving gold in new operations. If a surplus of caustic soda has been used for neutralizing acid salts in the ore without follow- ing washing, the loss of zinc will naturally be increased. A white pre- cipitate is constantly accompanying the reduction of bullion in the zinc boxes, undoubtedly the result of the action of alkali on the zinc and of the zinc-potassium oxide on the double cyanide of zinc and potassium, Avhich is always present in the solution, forming the insoluble cyanide of zinc; ferrocyanide of zinc is also formed in the boxes. Ferrocyanide of zinc is formed in the percolation vats when the double cyanide of zinc and potassium comes in contact with the iron salts in the ore, and, as it is insoluble, to this cause is due the constant removal of zinc from the solution with the residues (Buckland). The gold precipitate on the zinc is, as a rule, brown to black, with sometimes a metallic luster; it is mostly slimy, and when dry it seldom contains more tban 40 or 50 per cent of gold and silver, the remainder being finely divided zinc and its accompanying impurities, such as carbonate of lead. It may also con- tain copper, if that metal is present in solution. (In the instance of treating concentrates containing carbonate of copper from the Sylvia Mine, New Zealand, the gold solution contained a very appreciable quantity of copper; this complicated matters by causing the copper to precipitate with the gold and cover the zinc, thus forming a galvano- plastic coating, which made it necessary to dissolve the whole of the zinc for the purpose of obtaining the bullion, till I found an addition of cyanide to the solution, before it enters the zinc boxes, useful for the prevention of the deposition of the copper.) I always found mercury in the zinc bullion in not inconsiderable quantities when concentrates had been treated by agitation; such mer- cury must have been derived from amalgam, and mercury saved with the pyrites on the concentrators. Gmelin and others describe mercury as absolutely insoluble in cyanide. There is, however, no doubt of the correctness of my observations; the mercury must have been dissolved in the cyanide solution, which entered perfectly clear into the zinc boxes; solubility of gold amalgam in cyanide may offer the explanation. Traces of antimony and arsenic have also been found in the bullion. The pre- cipitation of silver goes on by zinc simultaneously with the gold; it is even more rapid and complete than that of gold. (See my table on the process in the Utica Works, page 91.) Other apparatus than the described boxes have been suggested for bullion precipitation by zinc — for instance, earthenware and porcelain vessels have been recommended. They have the apparent advantnge of cleanliness; tbeir construction, however, makes the cleaning-up of the bullion difficult, the connection between the single cells being compli- cated; they have not become a practical success in works of any extent; they are all based on the principle of the solution penetrating the zinc from below and running off at the top. The precipitates, obtained as 36 THE CYANIDE PROCESS. n a 1 -4-3- described from the zinc boxes, are transferred to a sieve, made of No. 1 punched battery screen or a 40-mesh wire screen, through which they are washed onto a filter in connection with a vacuum chamber, where they are liberated from the adhering cyanide solution and reduced from their very voluminous state into a more compact form. This filtration will always be found slow on account of the extremely slimy character of the bullion. For filtering and washing the bullion slimes, filter presses may be suggested. By the screening process the coarser particles of zinc are separated from the bullion, but the bullion still contains a large percentage of very fine zinc, of which it is advisable to remove as much as possible before melting. Bullion Refining. — The means for this purpose are calcination or roasting and acid treatment. I use for roasting (see description of Sylvia plant, p. 79) a muffle furnace, where the slimes are dried, and then calcined for the promotion of the oxidation of the base metals. The calcining process is generally in use in South Africa, and will be described with the cyanide practice in Johannesburg, page 54. I gener- ally prefer sulphuric acid treatment, with following washing and drying of the bullion. The acid treatment is a comparatively simple operation, and does not require, even for large quantities of bullion, any other apparatus than wooden tubs, the increased temperature produced by the reaction of the acid on the zinc making application of artificial heat superfluous. The separation of the acid solution from, and the Avashing of, the bullion is best done by decantation, and completed on the bullion filter mentioned above. It is advisable to liberate the bullion as much as possible from base metal before melting, which is otherwise connected with loss of gold by evaporation caused by the volatilization of zinc; besides, the zinc fumes are very disagreeable. The presence of oxides of base metals (as obtained by calcination) makes the melting tedious and expensive on account of the detrimental influence of the slag on the melting pots. The presence of a high percentage of base oxides pre- vents the use of graphite crucibles and compels the use of clay pots. Bullion, when treated by acid as described, does not offer any difficulties in melting, if proceeded with in the following manner: DEMONSTRATION OF PROCESS — METHODS OF OPERATION. 37 The bullion, after having been treated with sulphuric acid and washed with water, is dried by suction on the vacuum filter as much as pos- sible, after which it is easily detached from the filter cloth. The mass is then charged into the muffle (see plans of Sylvia cyanide plant, page 77). The heat is kept low to drive off the moisture; it is then increased gradually to dark red; after about one hour's calcination, during which time the oxidation of base metals which escaped removal by acid treatment is going on, the mass presents a gray-brown appear- ance. Attention has to be paid to the draught to prevent loss of the fine precipitates. Bullion resulting from the treatment of concentrates will invariably give off quicksilver vapors; condensation yielded only a small quantity. The calcination process completed, the roasted bullion is carefully transferred from the muffle, by means of a small shovel, into a wrought-iron box for cooling purposes. It now presents itself in the form of lumps, approaching more or less the spherical form, of the size of peas, largely mixed with dust. When sufficiently cooled, it is charged into a pulverizing cylinder of sheet-iron, 3 ft. long and 2 ft. in diameter, which is revolved by means of a pulley. Large pebbles are charged into the apparatus with the bullion to aid pulverization. Borax, with prefer- ence borax glass, and soda ("ammonia-process soda") are added into the barrel, in proportions according to experience, for securing a fusible clear slag of light specific gravity. If the bullion is base on account of a large proportion of zinc oxide, which happens only if acid treat- ment was not properly conducted, a silicious flux, like sand or glass, has to be added. Acid sulphate of soda and fluorspar have been occa- sionally found useful as additional fluxes. During pulverization, a thorough mixture of bullion and fluxes will take place. Moisture in the fluxes should be avoided, as it is a certain source of loss in melting, the escaping water carrying fine bullion out of the pot. Plumbago crucibles are very well adapted for melting bullion, pre- pared as above described; they stand almost as many operations as with battery gold. In melting, some borax is first put into the crucible; the bullion mixture from the pulverizing and mixing cylinder is not added all at once, but as e,ach portion melts and sinks down, fresh quantities of it are put in. The melting goes on speedily and in the most satisfac- tory manner. When the whole quantity is finally charged, the tempera- ture is kept very high for some time, to give the small bullion globules a chance to collect. The contents of the pot are poured into a heaf-ed mold in the usual manner. No chemical losses will be experienced by this method of bullion melting. Bullion obtained by the described manipulation will be found to be at least 950 fine. The bullion pi'o- duced by cyanide works generally varies in fineness according to the attention paid to its refining. Gold purchasers buy bullion on assay, and refiners charge higher rates for baser bullion; it is therefore as a rule cheaper in the end to produce clean bullion. Bullion precipitation by means of zinc is not free from objections; its practice is connectetl with a loss of cyanide and with the introduction into the process of a new compound (the double zinc-potassium cyanide), which is, to say the least, not an advantage; the consumption of precipitating agent (zinc) is far in excess of the amount theoretically required, and no opportunity is offered for a regeneration of the cyanide. The precipitation itself is, however, a very efficient and simple procedure, not requiring either motive power or more than ordinary attention, and the treatment of the 38 THE CYANIDE PROCESS. precipitates cannot be said to offer any obstacles which would charac- terize the process as metallurgically inefficient. In fact, generally speak- ing, the clean-up of the bullion precipitated by zinc, if properly handled on the lines explained above, will hardly be found more troublesome than a mill clean-up. Although the precipitation of the gold by zinc is unquestionably a weak point in the cyanide process, no other method has as yet taken its place to any extent. Had other methods, which require motive power, careful adjustment of costly and delicate machinery, and constant attention preceded it, its discovery would probably have been con- sidered an improvement of importance. (b) The MoUoy Process. — A method for precipitating bullion which has obtained some technical importance has been advanced by B. C. Mol- loy, of Johannesburg, whose process is protected by English letters patent No. 3,024, dated 16th February, 1893. Molloy uses for precipita- tion purposes sodium or potassium amalgam, which is formed electro- lytically from a solution of carbonate in contact with a bath of mercury. The alkali metal combines with the cyanogen of the gold compound, forming an alkali salt of the cyanogen, while the gold is instantly amal- gamated. This auriferous amalgam is then strained and melted as in an amalgamation mill. The process of precipitating and collecting the bullion is carried out in an amalgamating apparatus, the bottom of which is partially covered with mercury. On this mercury rests the solution from which the metals are to be precipitated. The mercury is charged electrolytically with an alkaline metal (by the electrolysis of an alkaline salt used in a porous vessel in contact with a mercury cathode). The alkaline metal, or its amalgam, when coming to the surface of the mercury and in contact with the water of the solution, decomposes the water, the alkaline metal combining with the oxygen of the water to form an alkaline oxide; the hydrogen of the decomposed water is at the same time evolved in a nascent state from the surface of the mercury which is in contact with the solution from which the gold is precipitated, and absorbed by the mercury. The gold is released, from the mercury in the ordinary manner by straining and distillation. Another, " though much less advantageous method," suggested by Molloy, is the mechanical addition to the mercury of potassium or other alkaline metals, or amal- gam of the same. In both cases the original solution of cyanide of potassium is regenerated and ready for use again. The reaction is aa follows: K.COg + elect, current = Ko + CO2 + 0. KAuCya -f K = Au -f 2 KCy. No information could be gained of the actual working results. Other methods of bullion precipitation by means of the electric cur- rent have been suggested. (c) The Siemens and Halske Process for bullion precipitation by electrolysis on lead sheets has become of technical importance. The precipitation plant consists in boxes through which the gold solution passes; these contain the anodes, Avhich are iron plates, and the cathodes, which are lead sheets, stretched between iron wires fixed in a light wooden frame, which is suspended between the iron plates. It is claimed DEMONSTRATION OF PROCESS — METHODS OF OPERATION. 39 that the adoption of this method of bullion precipitation permits the use of very weak cyanide solutions for extracting purposes, followed by a •considerable reduction in the cost of treatment. Mr. A. von Gernet has given the following details as to the practical working results obtained at the Worcester works, in Johannesburg, with the Siemens and Halske process, which has been tried there for four months on a large scale, after long and exhaustive preliminary experi- ments at the works of the Rand Central Ore-Reduction Company: " There are now in use five leaching vats of 2 ft. in diameter with 10 ft. staves, each holding 2,700 cub. ft. One tank is discharged and filled every day. The strong solution used contains from 0.05 to 0.08 per cent cyanide, and the weak washes 0.01 per cent. The actual extraction of fine gold has averaged 70 per cent, while the consumption of cyanide has been ^ lb. per ton of tailings treated. "The precipitation plant consists of four boxes 20x8x4 ft. Copper wires are fixed along the top of the sides of the boxes, and convey the current from the dynamos to the electrodes. The anodes are iron plates 7 ft. long, 3 ft. wide, and ^ in. thick. They stand on wooden strips placed on the bottom of the box, and are kept in vertical position by Avooden strips fixed to its sides. In order to effect circulation in solutions passing through the box, some of the iron sheets rest on the bottom, while others are raised about 1 in. above the level of the solution, thus forming a series of compartments similar to those of a zinc box, the difference being that the solution passes alternately up and down through successive compartments. The sheets are covered with canvas to prevent short circuit. The lead sheets are stretched between two iron wires, fixed in a light wooden frame, which is then suspended between the iron plates. The boxes are kept locked, being opened once a month for the purpose of the ' clean-up,' which is carried out in the following manner: The frames carrying the lead cathodes are taken out one at a time. The lead is removed and replaced by a fresh sheet, and the frames returned to the box, the whole operation taking but a few minutes for each frame. By this means the ordinary working is not interrupted at all, and the cleaning out of the boxes, which is necessary in the zinc process, is only required at very long intervals. The lead, which contains from 2 to 12 per cent of gold, is then melted into bars and cupelled. The consumption of lead is 750 lbs. per month, equal to 3 cents per ton of tailings. The working expenses, including filling and discharging tanks, come to 80 cents per ton, which is divided as follows: Filling and discharging, 20 cents per ton; cyanide, 12^ cents; lime, 2.4 <;ents; iron, 4.4 cents; caustic soda, 10 cents; lead, 2.2 cents; natives' wages and food, 3.8 centos; coal, 9.2 cents; white labor, 9.2 cents; stores and general charges, 6.5 cents; total, 80 cents. At the "Worcester works 100 tons are being treated per day; when working on a large scale it is iinticipated that the expenses will be further reduced." (The Mining Journal of London, October 27, 1894.) The advantages claimed for this process are that electrical precii)ita- tion is independent of the amount of cyanide or caustic soda contained in the solution, therefore in the treatment of tailings very dilute solu- tions can be used; for generating the current necessary in a 3,000-ton plant, 2,400 Watts are required, equal, theoretically, to 3} horse-power, and actually requiring about 5 indicated horse-power. The process has produced 755 ozs. of gold during the month of July of this year. a ,nd 40 THE CYANIDE PROCESS. (d) The Pielsticker Process applies likewise the electric current as the precipitating agent. A description of this process, illustrated hy a diagram, will be found in the patent-specification (see Appendix). No information in reference to practical results could be obtained. The process attained of late a certain notoriety on account of the patent litigation now pending between the owners of the MacArthur-Forrest patent and the Cyanide Gold Recovery Syndicate (Limited) of London^ who control the Pielsticker patent. (e) The Moldenhauer Process of bullion precipitation consists in the application of aluminium, or alloys, or amalgam thereof, in the pres- ence of a free alkali. It is claimed that aluminium separates the gold very quickly from the cyanogen solution without entering into combina- tion -with the cyanogen, but simply reacting with the caustic alkali which is present at the same time, forming therewith an aluminate. The precipitation of gold by aluminium takes place as follows: 6 AuK (CN)2 + 6 KHO + 2 Al -f 3 HoO = 6 Au + 6 KCN + 6 HCN + 6 KHO + AlgOg 6 Au + 6 KCN + 6 HCN + 6 KOH + Al203= 6 Au + 12 KCN + 6 H.O + AlgOg. The whole of the cyanide of potassium which has been combined with the gold is being regenerated, and the consumption of the cyanide is limited to the loss involved by such secondary reactions as act decom- posing. (See "chemistry of the process.") The discoverer of this method of bullion precipitation claims that the quantity of aluminium required for precipitating the same quantity of precious metal, is about four times less than the amount of zinc required to produce the same effect. No results of this process applied on a large scale have as yet been made public. (f ) The Johnston Process of abstracting gold and silver from their solutions of alkaline cyanide (United States patent 522,260, see Appen- dix) consists in the use of pulverized carbon, preferably in the form of charcoal. "The pulverized carbon is placed upon suitable sup- ports so as to form it into filters, through a series of which the cyanide liquid is caused to pass successively, leaving the metal deposited upon the carbon. The gold and silver are then recovered by carefully burning the carbon and smelting the residue with the usual fluxes. By thus employing a series of filters, through which the solution is passed suc- cessively, 95 per cent of the precious metal contained in the solution is recovered. When only one filter is employed, only about one fourth of the gold can be extracted." V. PERCENTAGE OF EXTRACTION. The percentage of extraction depends on the character of the ore. As I mentioned before, the process is suitable for many ores which for chemical and mechanical reasons are refractory. The commercial ques- tion in the selection of a metallurgical process for treatment of a certain jcr cciii/ ui ^„„- ... biic guiu, iii&teuu ui oo pur cent us snown oy assay; ine returns of the second month yielded 80 per cent, instead of 91 per cent; after the third month the actual results came up to the extraction, as per assay — 89 per cent. Similar experiences have been made in the Sylvia Company in New Zealand and elsewhere. It has been recommended to TABLE SHOWING RESULTS OF CYANIDE TREATMENT OF ORES. Material. Operator. Assay per Ton. Value per Ton. Value. Percentage Extracted. Cost of Name of Mioe. Character. Quantity Treated. Gold. silver. Gold. Silver. In Ore Parcel. Extract from Ore Parcel. Gold. Silver. per Ton. Tons. 5 0.5 280 224 30 145 16.9 5 11 9.3 90 164 206 9 5 5 1 0.3 0.5 9.5 10.5 21 37.9 5 1 263.3 43.1 72.7 192 614 0.6 5 7 7.6 0.9 1 1 I 111 75 0.9 138 19 1 116 111 130 166 422 1,015 4.5 5 4.5 4.2 3 6.8 116 14.1 4.1 1.5 13.3 4.5 17.5 4.5 1,600 10.9 12.6 1.6 7.1 7.2 14 5.7 1,000 2.6 1.5 78 5.2 4.6 5 P.McIntyre 07.. dwt. gr. 1 10 16 ... 8 4 1 6 2 1 2 20 1 2 20 1 2 1 1 18 10 3 15 2 14 3 15 8 7 5 7 1 6 23 2 11 10 2 8 4 1 6 12 3 6 23 3 16 4 3 7 3 17 22 2 6 4 2 1 10 2 8 4 2 19 21 1 12 148 15 8 222 7 12 324 8 10 '"3 '6 2.3 16 3 1 6 3 1 7 18 3 10 15 1 16 22 102 4 6 123 4 11 "b "e i2 oz. dwt. gr. 22 16 18 3 15 308 12 20 488 3 6 504 2 18 '2S ie 13 19 14 17 4 16 18 23 2 19 8 1 1 6 47 22 52 9 6 ""6 "6 i7 84,1 90.0 86.3 81.8 91.1 822 90.7 91.3 87.0 87.2 88.0 88.0 85.1 91.2 90.3 89.3 80.3 lOO 96 88 85.5 87.7 83.3 86.0 91.09 90.6 93 93 93 94 90.29 81.18 84.37 80.78 90.20 86.31 97.90 97.20 86.11 86.34 88.70 80.32 77.90 89.10 91 89 91 91.6 91.8 91.8 79.4 82 82 9.S 90 80.2 71.77 """98'"'" 90.3 90 98 84 96 "m"" 97 "rob"" 90 70 94 2 clacK jacK, A AuDtraifa' Slimes P.McIntyre P.McIntyre.. Day Dawn, Block & Windham, Australia... Day Dawn, Block & Windham, Australia... Day Dawn, Block &. Windham, Australia.. Day Dawn, Block & Windham, Australia... Day Dawn, Block & Windham, Australia... General Grant, Australia -.- Golden Gate, Australia 4 5 6 P.McIntyre.... P.McIntyre.. 8 9 10 11 12 13 P.McIntyre P.McIntyre. P.McIntyre Sludge.... Concentrates Mills United, Australia Mills United, Australia P.McIntyre 15 P.McIntyre 1^ Mount Morgan, Australia Ore, ironstone, and kaolin. 19 P.McIntyre New Towers Fxtend Australia P.McIntyre. New Towers Extend., Australia P.McIntyre... •10 New Towers Extend., Australia.. P.McIntyre.. Sludge Ore Ore Ore Alhurnia, New Zealand... Crown.New Zealand 56.78 70.5 79 79 79 94 79.78 77.06 83.70 74,60 16.10 35.16 100 97.10 67 68.7 59.58 60 64.45 28.46 61 46.6 49 49 49.8 49.8 ""98"" 98 83 61.65 "s'g'gs 63 54.47 3 83 $3 37 MacConnell MacConnell Crown' New Zealand Ore . . MacConnell $41,350 00 3 50 Kenilworth, New Zealand Kenilworth, New Zealand MacConnell Ore V Silverton, New Zealand Silverton, New Zealand Sylvia, Tararu, New Zealand MacConnell . 38 39 40 Concentrates, slimes Concentrates, slimes A. Scheidel A. Scheidel.... A. Scheidel A. Scheidel. A. Scheidel. 1^ 42,200 00 Sylvia, Tararu, New Zealand Sylvia, Tararu, New Zealand Sylvia, Tararu, New Zealand 42 43 44 Jigger concentrates Coarse concentrates 4S Waihi, New Zealand Robert Rose. Robert Rose Robert Rose Robert Rose Robert Rose Robert Rose G. & 8. Extr. Co. of Amer.. .1125 00 22 50 22 50 22 60 23 00 23 00 12 00 13 00 13 00 6 00 20 00 10 80 1.945 00 1,705 00 2,775 00 3,010 00 } 32,000 00 42 87 63 83 60 57 43 31 113 74 54'11 2,920 00 1,386 23 63 20 43 06 198 50 22 64 220 60 63 50 52"86' 166 42 66 70 253 68 301 16 487 20 157 63 «,0OO 00 16 03 56 24 4,240 OO 207 41 10 08 133 36 2 25 4li 1 76 47 Ore 1 50 4K Waihi, New Zealand Waihi, New Zealand Waihi, New Zealand Boulder City, U.S. A Boulder City, IT. S. A 1 38 4» Ore 1 38 hO 138 M $0 60 60" 44 26 00 1 04 iei'is' 11 IS 2 46 1 40 3 51 "i'he """42 1 49 48 00 39 00 42 92 63 00 4 00 pi 00 66 65 61 20 44 20 138 00 69 26 "'"'l'i546"49" 79 97 49 23 233 07 25 95 262 60 67 42 6i"25" 183 62 67 50 289 63 340 17 915 60 170 50 2 25 W. 2 60 .W 3 00 M ChicagcU. S. A 1 00 55 G. &S. Extr. Co.of Amer. 3 00 hH El Capitan. U. S. A. . Eureka, Cal„ D. S. A Flor do Marzo, U. S. A Golden CloudLD. S. A. Golden Rose, U, S. A Gregory, U. S. A KingsOre, U. S. A... ELematite Frue concentrates 1 13 57 5S A. Scheidel 3 60 11 05 51) 8 40 28 80 16 00 9 10 15 00 11 20 ""6 '20 13 20 "is '66" 4 00 2 40 27 00 10 40 8 80 38 00 "39'26" 16 40 29 04 2 50 KO 2 86 til 4 00 m. 3 00 t)3 Or5 64 McKeivie, tj. s. A, :::::;".:'.;:::;:::;'.;:::".'.: Mercur, Utah, U. S. A Old Charlie, U. S. A. Siliciousore 73 ""96"" 33.6 84 87 83.4 62 82 2 25 Ore Gill S.Peyton 2 40 ti« 1 50 67 Oro Grande U. .S. A 311. and pyrites Ore with antimony G. & 8. Extr. Co. ol Amer.. G. & 8. Extr. Co. of Amer.. G. & S. Extr. Co. of Amer. G. &S. Extr. Co.of Amer.. 1 21 69 Poorman, U. S. A Poorman, U. S. A.... Poorman, U. S. A Poorman, U. S. A Revenue, U. S. A :"."."."': " Revenue, U. S. A. R. M, Terror, U. S. A. .; ::.\V.:\[:^[[[[ Rosecrance, tJ. 8. A Utica Mine, California, U. S. A '.'.'.' Western Belle, U. 8. A 3 00 500 70 6 83 Stephanite Siliciousore 6 49 2 00 V3 Tailings F. B. &R. B.Turner G. & S. Extr. Co. of Amer. 74 6 78 3 75 "'4 "92" 2 24 4 29 36 63 62 62 "■"232'05 83 88 166 66 88.6 90 93.18 92 88 82.78 75 88 "6"8;6"' 48 61.32 2 50 77 Canvas concentrates Silicions.. A. Scheidel G. & 8. Extr. Co. of Amer.. 3 27 4 40 Wolferine. U.S. A.. Chicago, Old Mexico... 600 3re 2 31 PERCENTAGE OF EXTRACTION. 41 ore has to be considered parallel with the chemical, and that process should be adopted which permits the extraction of the largest percentage of bullion at the lowest cost, and with the least investment of capital. The cyanide process is, for this reason, the best yet discovered for the treatment of the tailings of the South African gold fields, although giving only an average of extraction of about 70 per cent, of which about 60 per cent is recovered (see page 52). No other process gave, at the same expenditure, any results approaching it. The conditions of the Witwatersrand ores are considered particularly favorable for the process, yet the extraction figures are, in most instances, not high. The per- centage of extraction in various mills in Johannesburg will be given in the chapter on the process in Africa, page 60. The ores and tailings in New Zealand, where cyanide treatment of dry-crushed ores is carried on extensively, give better results. The Waihi ores, pure quartz, the gold free, but exceedingly fine, the silver in form of sulphides, no sulphurets of base metals, give an extraction of from 85 to 91 per cent of the gold assay-value, the silver returns varying from 43 to 51 per cent. The ore of the Crown mines, which resembles those of Waihi, but containing occasionally telluride of gold, yields on an average 93 per cent of gold and 79 per cent of silver. Concentrates, if satisfactory at all in cyanide treatment, give as a rule very high figures. A considerable quantity of concentrates from the Sylvia Mine in New Zealand, of a very complex character, being composed chiefly of zinc-blende and copper pyrites, with a large percentage of galena and iron pyrites, were treated by me by cyanide, and gave very satisfactory results under conditions where no other means of treatment were at disposal. The said concentrates are classified by the dressing plant; the fine slimes rich in bullion and galena gave as high an extraction as 95.43 per cent of the gold and 86.69 per cent of the silver; coarse concentrates gave an average of 80.32 per cent of the gold and 50 per cent of the silver. A large parcel of very fine sulphurets (from the canvas plant) from the Utica Mine, California, consisting of pure iron pyrites in finest division, mixed with more or less fine sand and carbonate of lime, proved an excellent material for cyanide treatment; the extraction averaged 93.18 per cent of the gold value, rising in some instances as high as 96.57 per cent. The coarse concentrates from the Frue vanners did not give such good results, if treated direct; their reduction to greater fineness, however, improved results. An appended table shows the results of successful treatment of parcels of ores from various sources. It is to be regretted that no corresponding table, giv- ing a like description of ores treated with unsatisfactory results, can be produced for comparison, which would be useful and instructive. The recovery of the bullion should correspond with the extraction shown by assays; in practice, however, there is often a discrepancy, which rnay be explained by various causes; new vats, particularly those of wood, absorb both gold and cyanide, and considerable differences in the returns will be felt during the first weeks of their use. In the Waihi Company's works in New Zealand, for instance, 116 tons of ore, of an assay-value of about -$25 per ton, returned after the first month only 75 per cent of the gold, instead of 85 per cent as shown by assay; the returns of the second month yielded 80 per cent, instead of 91 per cent; after the third month the actual results came up to the extraction, as per assay — 89 per cent. Similar experiences have been made in the Sylvia Company in New Zealand and elsewhere. It has been recommended to 40 (c as t] a di: Noi proc litig pate who the; enc€ ver) tion whii The and T the limi posi ofb for ' less resr pub solu dix of ( pon liqu the the em I cess recc the The percentage of extraction depends on the character of the ore. As I mentioned before, the process is suitable for many ores which for chemical and mechanical reasons are refractory. The commercial ques- tion in the selection of a metallurgical process for treatment of a certain PERCENTAGE OF EXTKACTION. 41 ore has to be considered parallel with the chemical, and that process should be adopted which permits the extraction of the largest percentage of bullion at the lowest cost, and with the least investment of capital. The cyanide process is, for this reason, the best yet discovered for the treatment of the tailings of the South African gold fields, although giving only an average of extraction of about 70 per cent, of which about 60 per cent is recovered (see page 52). No other process gave, at the same expenditure, any results approaching it. The conditions of the Witwatersrand ores are considered particularly favorable for the process, yet the extraction figures are, in most instances, not high. The per- centage of extraction in various mills in Johannesburg will be given in the chapter on the process in Africa, page 60. The ores and tailings in New Zealand, Avhere cyanide treatment of dry-crushed ores is carried on extensively, give better results. The Waihi ores, pure quartz, the gold free, but exceedingly fine, the silver in form of sulphides, no sulphurets of base metals, give an extraction of from 85 to 91 per cent of the gold assay-value, the silver returns varying from 43 to 51 per cent. The ore of the Crown mines, which resembles those of Waihi, but containing occasionally telluride of gold, yields on an average 93 per cent of gold and 79 per cent of silver. Concentrates, if satisfactory at all in cyanide treatment, give as a rule very high figures. A considerable quantity of concentrates from the Sylvia Mine in New Zealand, of a very complex character, being composed chiefly of zinc-blende and copper pyrites, with a large percentage of galena and iron pyrites, were treated by me by cyanide, and gave very satisfactory results under conditions where no other means of treatment were at disposal. The said concentrates are classified by the dressing plant; the fine slimes rich in bullion and galena gave as high an extraction as 95.43 per cent of the gold and 86.69 per cent of the silver; coarse concentrates gave an average of 80.32 per cent of the gold and 50 per cent of the silver. A large parcel of very fine sulphurets (from the canvas plant) from the Utica Mine, California, consisting of pure iron pyrites in finest division, mixed with more or less fine sand and carbonate of lime, proved an excellent material for cyanide treatment; the extraction averaged 93.18 per cent of the gold value, rising in some instances as high as 96.57 per cent. The coarse concentrates from tlie Frue vanners did not give such good results, if treated direct; their reduction to greater fineness, however, improved results. An appended table shows the results of successful treatment of parcels of ores from various sources. It is to be regretted that no corresponding table, giv- ing a like description of ores treated with unsatisfactory results, can be produced for comparison, which would be useful and instructive. The recovery of the bullion should correspond with the extraction shown by assays; in practice, however, there is often a discrepancy, which rnay be explained by various causes; new vats, particularly those of wood, absorb both gold and cyanide, and considerable differences in the returns will be felt during the first weeks of their use. In the Waihi Company's works in New Zealand, for instance, 116 tons of ore, of an assay-value of about .$25 per ton, returned after the first month only 75 per cent of the gold, instead of 85 per cent as shown by assay;' the returns of the second month yielded 80 per cent, instead of 91 per cent; after the third month the actual results came up to the extraction, as per assay — 89 per cent. Similar experiences have been made in the Sylvia Company in New Zealand and elsewhere. It has been recommended to 42 THE CYANIDE PROCESS. soak the wooden parts of a new cyanide plant with paraffine to prevent absorption; a coat of asphalt dissolved in bi-sulphide of carbon will be found a good preventive for the absorption by wood. The chief sources of chronic losses are to be found in the imperfect separation of the gold solution from the exhausted ore residues, and in the faulty methods of dealing with the bullion after its precipitation by zinc. There is no reason why the actual returns should differ from the returns as estab- lished by assay, provided all mechanical losses are prevented. In refer- ence to the losses in the Johannesburg mills, see chapter on the process in Africa, page 52. VI. WORKING COSTS OF THE PROCESS. As may be deduced from the whole tenor of this paper, the working costs of the cyanide process vary within wide limits and depend on many circumstances. Locality is a prime factor in the costs of working any process, and expenses must be high where operations have to be carried on in an inaccessible situation, or where there is dearth of fuel, water, building material, etc. Apart from the question of locality, the cost depends principally upon three factors: The nature of the ore. The price of labor. The price of cyanide. When an ore contains acid salts and demands an alkali treatment, the price of the alkali must necessarily be added to other costs; and where the ores are slimy, recourse must be had to drying and mixing appliances, which also increase the cost to an extent depending on local ■circumstances. The principal labor involved in the process is the charg- ing and discharging of the vats, or, if agitation is used, the charging of the agitator and the removing of the exhausted material from the filter appliances. The charging and discharging of the percolation vats may, under ordinary circumstances, be contracted for at a rate of about 25 '/>AAA/TS. . V V y ' ^ _ao ^0 overhead storage vats. Having been made up to strength, it is ready to run direct into the leaching vats again. The discharge system indi- cated is the ' bottom discharge.' No. 3 design is a combination of the two previous ones, and is advantageously fitted with a pipe service to enable one, if desired, to run solutions up through the sand in the leaching vats. As shown in the sketch, the plant is designed for side discharge; but of course any system of discharge may be applied to any of the three arrangements of plant. EXEMPLIFICATION OF PROCESS — THE PROCESS IN AFRICA. 57 gji 15 TO y y ^p %° S o 4o 58 THE CYANIDE PROCESS. Buildings. — " The majority of plants now erected have only the zinc boxes inclosed in buildings, and there is little objection to having no weather protection when cemented vats only are used, but with wooden vats, exposure to the sun and weather undoubtedly causes increased leakage. -pv:7i. rELDT/iAAryv-'- S/Z)r2?/SC//yi/)GE 4-r/^jE:/9CO^./l'r/OA^J<^7'S ■SCyt/ f 7 //vcAy^^Trcor- Lahor. — "The men employed in a plant of average capacity (5,000 tons per month) are: One manager, one assayer, two shift men, one mechanic, two native gangers, and the native crew." The Nigel Com- pany employ two white men of twelve-hour shifts in the works, whose duty it is to " make-up," pump in, and drain ofl" solutions, and to attend EXEMPLIFICATION OF PROCESS — THE PROCESS IN AFRICA. 59 Cn.IiUTTEES' BOTTOM DISCHARGE AT PERCOLATTOy VATS. n./j.iRrjyES bottom discuau> T)! o s r^ i§ 8 GO 00 00 lO~ i-H iH X> CO (M CD T« 05 .-1 CD CO CO lO r^ o CO CD O 00 o a> 05 §J CD U5 iH 05 T-l CO O o o 00 ■o< 05 00 CS in a r^ CO o" o" -* Oi' t^ •»« e»3" ■<<*" ^" 05" o CS tH (>; (M » O-l 1-1 in CO 00 r- Oi t- 05 a> o r^ 05 I^ oo 05_ 00 -* (M CO I^ CD -l .H rH iH rH iH 1-1 iH t^ "« >, iH § i o e 22 >i 1— i Oi 00 00 00 00 00 00 05 05 00 t^ o iH" «0 •-0 s 6 O r~ iH >o 1^ ^ ■* CO 05 lO CO f^ e^i •* •<)< CD t- ■^ CD t^ CO CO CO VO CD_ o_ 1-1 lO t- CO CO oo A a co 03 CO •S s O K *"• in ) 1-1 CD >n IM 05 C^J CI 00 1^ C) ^ co" CO_ CO CO c4" U5 ift" C5_ o 05" oo" 05_ CO ■>#" <<> S lO ■^ lO lO lO O U5 O CO CD CO t- 05 CD § J. t 9> '3 O CO CD V CO lO CO e (M CS S 03 ■«o >, ^ GO o «) *"• CD r~ 05 SS CO 05 CO t-- CD CO CI T-t CO ■< 00 t^ 05 CO x> B > o J<5 x> a fC£. . REFEJiESCE A — Pfrcoialort. B~Sumpi C — fi€iervofr D—Disiohttg Tank /or Cyr Launder % t Ht\ L~Atr-pvmp. M-Centn/ugaJ Pmnp N—Petitm Wheel Afotor. O—Frtsh-valer Supply f'tpe* P— Solulkm Pipet, Reservoir to Percolator. ^ nt t m n. I PercoLtttort to Q- Weai-tUptor Ptpes j y^^^^^ chamber R—Pump Pipes S—Atr-pump Pipe. T — TratKways U— Melting Room V — Assay Room W — L^iboratory. X—Batanee Rdom Y—Dweltmg Rmn . LONGtTVDtMAL StCTiOft . EXEMPLIFICATION OF PROCESS THE PROCESS IN AUSTRALASIA. 75 day. It is so arranged that the requisite amount of strong solution may- be run into the reservoir by simply turning a handle. A 4 in. centrif- ugal pump serves for returning the solution from the sump to the reser- voir, and also an 8 in. vacuum pump, which is capable of producing a vacuum of 26 in. of mercury. A line of pipe runs along above each row of percolation vats, with a connection at each tank for the hose and nozzle. One man can empty a vat containing upward of 40 tons in two hours. A tramway connects the tailings-dams with the w^orks, and two sets of lines run over the top of each vat, so that the tailings may be equally distributed without the necessity for handling. The chief cliaracteristic of the plant is its extreme simplicity and the easy access to any portion of it; the absence of any subdivisions or partitions within the main building exposes the whole of the plant constantly to the eye of the operator. " The system employed of running the solutions into parallel launders instead of pipes, enables the solutions from each vat to be separately and readily sampled and any mishap may be at once detected. The usual method of procedure is as follows: Side-tipping trucks are run from the tailings-pit over the top of the vat which is to be charged. The contents of the trucks are tipped onto cross-bearers resting on struts, which serve to break the fall of the tailings, and to divide them equally over the bottom of the vat. Both tramway and bearers are supported entirely independent of the vats, so that no vibrations ma}'- be communicated to the latter. A charge consists of sixty-five truck-loads — about 33 tons, dry weight — and as soon as the vat is full, ' strong ' solution — about 6 tons of 0.7 per cent — is run onto the top from the elevated reservoir. Provision is made for either upward or downward percolation, but the latter is usually adopted. The solution is now permitted to gravitate through the mass of the charge, and to eventually percolate through the false bottoms into the series of launders in which it is conducted to No. 1, No. 2, or No. 3 zinc precipitation box, according to its strength in bullion and cyanide. About twenty-four hours after the ' strong ' solu- tion, about an equal amount of ' weak' solution (0.25 per cent) from the sumps is pumped on and allowed to gravitate. The residues are now washed with about 10 tons of water in two charges, which are rapidl}" drawn off by suction, and which displace the ' weak ' solution and leave the residues free of either dissolved bullion or cyanide. The solutions run from the zinc boxes to the sumps, whence they are pumped to the reservoir or percolation vats, to be used over again for sluicing or weak solutions as required. A clean-up of the gold in the zinc boxes takes place fortnightly." The New Zealand Mining Report of 1893 contains the description of an interesting experiment which was made some time ago in the Waihi works, with an apparatus by which ore was intended to be rapidly extracted by a cyanide solution acting in a jigging motion on the ore in iron cylinders. The cylinders used were, however, too long and narrow, containing as they did some 10 ft. in depth of ore, which the solution had to be forced through. The effect of this was that the solution could not be made to percolate through the whole of the ore, but passed up between the cylinder and the ore, the solution being forced into the cylinder by a pump, at a pressure of 100 lbs. per square inch. This pressure should have been sufficient to force the solution through, but as the pulverized material offered greater resistance than the contact 76 THE CYANIDE PROCESS. between the material and the side of the cylinder, the solution went through the weakest spot, and had little effect on the ore. The process ("the Bohm Process") proved a failure. The unquestionable success of the percolation process with the Crown and Waihi mines has led to its adoption by a number of other com- panies which treat either ores or tailings by the process, as the Te Komata and Waiorongomai mines at the Upper Thames, and two or three companies on the Kuaotunu gold field. The Kuaotunu ore, in which the gold is exceedingly fine, is especially adapted for the treatment, the only difficulty experienced — a mechanical one — is caused by the amount of slimes formed by some of the ores, which interfere with filtration. The plants on that field do not offer any special point of interest; they are of small extent and give satis- factory results. The best one, that of the Tryfluke Company, will, however, be described on account of the attempts made therewith to run the tailings direct from the battery into the percolation vats. The plant consists of four working tanks, each 12 ft. wide, 16 ft. long, and 4 ft. deep, having a filter-bed of 3 in. in thickness, covered with a coarse cloth. The depth of ore in the tanks is about 3 ft. 6 in. and about 6 tons of 0.25 per cent cyanide solution are used per charge. This is what is termed the strong solution. The tap, which allows the filtrate to flow away, is so regulated as to take about 24 hours for that purpose. After flowing through six compartments of a filter box, which are filled with fine zinc-turnings, the solution passes into a sump, 18 ft. long by 14 ft. wide and 4 ft. deep, from where it is pumped into the tank again, thus forming the second solution. This latter is allowed to filter through as fast as possible, and, after going through the boxes filled with zinc, it flows into another sump of the same dimensions as that already men- tioned. The ore is then washed with pure water, after which the material is shoveled out of the tanks and run onto the waste dump. The second solution in the sump, previously referred to, is pumped into a reservoir placed at a higher level than the working tanks. This reservoir is 10 ft. long, 8 ft. wide, and 5 ft. deep. The solution is made up to the required strength before again being used. The company tried to run the tailings directly into the tanks from the battery, but they, like others, found that the amount of slimes in the ore prevented the cyanide solution from filtering, and they are now making arrangements to run the tailings into a large pit, from which they will be lifted into the cyanide tanks. (Extract from N. Z. Gov. Mg. Rep.) Ali works so far referred to are situated in the North Island; on the large gold fields of the South Island the process has not found more than experimental application. Experiments have been made with gold-bear- ing cement from the extensive deposits on the west coast, where almost inexhaustible quantities of conglomerates, containing black magnetic oxide of iron and very small quantities of gold, are found. Such experi- ments were made, for instance, in the Reefton School of Mines, by treating the cement in lumps, but they were not always successful, apparently for mechanical reasons. When the cement is crushed, a very good percent- age is said to be extrd'cted, the gold being fine and well suited for the purpose. Tests with tailings from the Inangahua River gold fields have given good results, and a plant for working a considerable deposit of tailings is nearing completion at Boatman's. The only instance in New Zealand where the agitation system has •XEMPLIFICATIOX OF PROCESS THE PROCESS IX AUSTRALASIA. 77 I 9-^ ^^ si ^ ilk] 111 S ft 78 THE CYANIDE PROCESS. Si i I I I I ^ « > r^ >! 3: **l '0 1— w •^ ^ ?= 1 - 1 cc < o 2 o -1 cc < o h- o o < s i:; 'i 9? v! ^ ^ !o tN**'>'>. EXEMPLIFICATION OF PROCESS THE PROCESS IN CALIFORNIA. 91 The mode of working the plant is this: The cyanide solution is charged into the agitator, the paddles are set in motion by revolving the shaft, the ore is charged b}'- degrees, and the agitation is kept up for the required time, after which the pulp is discharged from the agitator onto the filter. The vacuum pump is then set in motion, and filtration under the influence of atmospheric pressure will at once commence. The solu- tion will soon be sucked through; then washing follows, first with liquor from former operations, which has already passed through the zinc boxes, and finally with clear water. These operations of filtering and washing take about two hours. It is advisable to suck the tailings as dry as possible before each new wash is put on, which permits the com- plete removal of the gold solution with a very small amount of liquid, one half ton of which is sufficient for washing a charge of two tons of ore. If the filtration has been properly managed, no degree of continued washing can improve the results. The -filtered solutions are clear. The first or original solution, together with the first wash, will be run oft' into one of the two solution tanks in front below the vacuum filter; the follow- ing washes run into the other. These tanks are 8 ft. long by 3 ft. wide and 3 ft. deep, made of i in. steel. Each of the tanks is in connection with a zinc precipitation box, 9 ft. long by 21 in. deep and 9 in. wide, divided into ten compartments; there is an interval of 1 in. between each two compartments. The false perforated steel bottoms of the chambers, which can be removed if desired, are 2^ in. above the true bottom of the box (see diagram, p. 31). The bottom of the box has a number of 1 in. iron faucets, one corresponding with the center of each filter compartment; the sides of the box are 4 in. higher than the par- titions within, which insures absolute safety against the liquid running over the sides of the box, if one or the other compartments should become blocked. The gold solution flows into the box through a 1 in. cock, enters the first compartment from below through the perforated false bottom, percolates the zinc shavings placed thereupon, leaves it, and enters the second, and so forth. A steel settling tank. 12 in. deep, 12 in. wide, and 9 ft. 3 in. long, is placed below the precipitating box for receiving the bullion when cleaning up (see general demonstration of process, above). The zinc used consists in turnings of ^o o' i^^- thick, turned from cast zinc cylinders on a lathe; 2 lbs. fill one compartment. The solution passes through the box at the rate of 700 gallons in twenty-four hours. The bullion precipitation of the solution is very efficient as it passes from compartment to compartment, which amounts to passing ten times through a zinc column of 14 in. high by 9 in. square. As shown by the following table of analysis. One Ton of Liquid contains— Gold Silver. Originallv Oz. Dwts. 5 14 16 5 2 1 1 Grs. I 14 4 1 22 15.43 14.96 13..36 12.34 Oz. Dwts. 2 4 5 1 Grs. g After 14 in. of zinc column After 28 in. of zinc column 3 10 After 42 in. of zinc column . . 17 After 56 in. of zinc column .-. 12 After 70 in. of zinc column 11 After 84 in. of zinc column 3 After 98 in. of zinc column After 112 in. of zinc column . After 126 in. of zinc column After 140 in. of zinc column 92 THE CYANIDE PROCESS. 11X5 rof y/^> ■Sf^t'x rofi MrrA/A/^tfe xjmc j-^a c frs/frr- ^'creH CLort/ j'£/)n>VArra F/i.reft eorroM, £yO*ff:0 CJtt/c^ i o/rCfrvD/Af/tL secT/o/if Jl^'A. SCM£/aSi '* r//sBi/ii/oyv FjiTS» % //vc^^ 7roor The solution leaving the zinc box contains only 12.34 grs. of gold, or 0.0045 per cent of its original contents, and only 3 grs. of silver, or 0.0028 per cent of the original silver value. Simultaneously the solu- tions were analyzed for available cyanide, but no decrease in the strength of the solution, which remained constantly at 0.3185 per cent, could be ascertained. "At another period I studied the solubility of zinc in BXEMPLIFICATlON OF PROCESS — THE PROCESS IN CALIFORNIA. 93 cyanide solution, of which I give the following figures: 0.2634 grs. of filiform zinc were submerged in 50 cc. cyanide solution of 0.26 per cent; after seven days of frequent agitation these were reduced to 0.2584 grs., and after fifty-six days to 0.2252 grs., which means that after seven days 1.98 per cent, and after fifty-six days 14.47 per cent of the zinc were dissolved. From this observation it follows that the loss of cyanide in the precipitating boxes, by means of its being taken up by zinc, is sometimes overestimated." The washes pass through a similar precipi- tating box. The liquids, when leaving these boxes, go as liquor No. 1 and liquor No. 2, into tanks of the same size as the solution tanks, from where a pump will deliver them wherever wanted. Liquor No. 1 serves for making up the new solution for the next charge; liquor No. 2 is used for washing purposes on the vacuum filter. No liquor ever leaves the works; the quantity in circulation remains stationary. The bullion obtained from the zinc boxes is passed, as formerly described, through a sieve onto the bullion vacuum, which itself is a miniature reproduc- tion of the vacuum filter (see diagram). It has the following dimen- sions: Length, 2^ ft.; width, 2 ft.; total depth, 1 ft. 6 in. The per- forated filter-bottom is fixed 12 in. above the true bottom. The bullion is very slimy; in fact, it is the more slimy the freer it is from zinc; its filtration and washing take some time. When the mass is tolerably dry, it is put into a wooden tub and treated with diluted sulphuric acid. The heat of the reaction I have always found sufiicient to make the operation a speedy and satisfactory one; the bullion is then permitted to settle, the liquid is siphoned off through the bullion filter, and the solid matter is washed by decantation with water. This washing process is continued until all soluble salts are removed. The bullion is then par- tially dried on the filter, and finally dried in a small muffle furnace; complete drying of the bullion by artificial heat before the acid treatment is not advisable. The thoroughly dry bullion is pulverized and well mixed with soda and borax, and melted in a plumbago crucible as described before. The bullion thus obtained is 946 fine; the slag is clean, it contains the usual few granules of bullion, but, freed from them, does not give any assay results. The bullion could be still further refined, but to no commercial advantage. The steel of the tanks has not as yet shown any effects from cyanide, nor does it exercise any influence on the solutions. All apparatus is composed of plates and sheets riveted together; leakages, if any, can be easily stopped by a varnish made of asphaltum dissolved in bi-sulphide of carbon. As mentioned, this plant has been constructed for the treatment of slime concentrates from the canvas plant; such concentrates contain a vary- ing percentage of carbonate of lime, in some instances as much as 95 per cent, which, however, does not interfere mechanically or otherwise with their satisfactory extraction by cyanide. Such conditions would make chlorination all but impossible, as alluded to before. For agita- tion the material requires an amount of solution equal to 30 per cent of its weight, and six hours of time. The described plant is capable of treat- ing a much larger amount of slimes than are usually produced per day by the canvas plant; its services are therefore only periodically required. The average consumption of cyanide, calculated from a large tonnage of slimes treated, amounted to 4.3 lbs. per ton, costing $2 27; the labor amounts to $1; the total expenses of treatment by cyanide to $3 50 per ton. The average extraction amounts to 93.18 per cent of the gold, and 94 THE CYANIDE PROCESS. 90 per cent of the silver assay-value; as high as 96.57 per cent of the gold has been extracted in some instances. The extraction of the gold during the agitation goes on as shown by this table: Treatment of Slimes by Agitation. Gold per Ton. Extraction— Per Cent. Sample before treatment Sample after 1 hour's agitation Sample after 2 hours' agitation Sample after 3 hours' agitation Sample after 4 hours' agitation Sample after 5 hours' agitation Sample after 6 hours' agitation Sample after 7 hours' agitation Sample after 8 hours' agitation $88 00 13 00 11 00 7 00 7 00 6 00 5 00 5 00 5 00 85.23 87.50 92.05 92.05 93.18 94.31 94.31 94.31 Within the first hour 85.23 per cent of the gold are extracted; during the following five hours the increase of extraction is slow and irregular; after six hours no further extraction takes place. For experimental pur- poses I continued agitation up to twelve hours without improving on the result. The treatment of the slime concentrates by agitation was pre- ferred on account of its quicker, cheaper, and better results, as compared with percolation. All Utica concentrates, as in fact all concentrates I ever had to deal with, contain a small amount of amalgam, part of which is found on the bottom of the agitator; part of it leaves the works with the tailings, and is recovered in Hungarian riffles and on amal- gamated silver plates; part of the mercury goes undoubtedly into the cyanide solution, and is precipitated with the bullion in the zinc boxes. Other sulphurets, such as the Frue vanner concentrates of the Utica, Madison, and Eureka mines, were treated on a more or less extensive scale by the same plant; results were, however, not very satisfactory on account of their coarseness. All sulphurets of the Utica Mine are pure sulphide of iron. The fine canvas-plant concentrates, although less clean, are as a rule richer in gold; their extraction averaged 93.18 per cent, whereas the vanner concentrates gave only 81.38 per cent, which, although reasonably good at the rate of -$4 per ton cost of treatment, cannot compete with a chlorination treatment, which yields 90 per cent of a $50 ore at a cost of $6 50. (The large size of the Utica chlorination works oflers special advantages and permits chlorination at this figure, which is much lower than the cost anywhere else in California.) A large number of tests proved that a high percentage of the gold is contained in the coarser particles of the sulphurets; this will account to some extent for the comparatively low percentage of cyanide extraction. It would lead too far to give here the results of the large number of experiments in reference. The experiments were extended to roasted concentrates after their reduction to uniform size, which gave on a small scale excellent results (98.09 per cent extraction). I am indebted to Colonel Hay- ward, Mr. Charles D. Lane, and Mr. James Cross, of the Utica Mine, for their permission to publish the diagrams and the described results of the cyanide works which I erected for them. The cost of the plant is divided as follows: Grading and foundations - - $200 00 Biiilding 300 00 Shafting, belting, and puttmg into place 135 00 Agitator 260 00 Vacuum filter --- - -- 165 00 EXEMPLIFICATION OF PROCESS — THE PROCESS IN MEXICO, ETC. 95 Three tanks - $160 00 Two zinc boxes 260 0 Endlich, F. M :... 6-11 Errors in the estimate of extraction 53 Exemplification of the process 46 INDEX. 137 P » Page. Faraday 6-9 Faucett, H. W 10-104 Feldtmann, W. R 55-58 Ferreira Company 62-65 Ferricyanide and cyanide, Moldenhauer process 14 Ferrocyanide experiments &-11 Ferrocyanide for extracting gold and silver 9-11 Filling" and discharging of vats 26 Filter 79 Filter presses 21 Filter presses for slime treatment 52 Filtration by centrifugal force 21 Financial success of cyanide process in South Africa 60 Fineness of bullion in Crown Eeef Company.. 55 Nigel Company 55 Rooinson Company 55 Utica Company 43 Fineness of ore _ 22 Fitness of ore for cyanide treatment .; 96 examination for 44 Fisher, H. T 88 Forest, R.W. and W. ...6-12-110-115 G Generation of hydrogen in the zinc boxes. 34 of hydrocyanic acid... 17 Gernet, A. von _ 39 Gmelin 35 Gold and Silver Extraction Company of America, Limited 84 Gold production of South Africa ._ 67 Gold Run Mine 88 Gold solutions, treatment of 30 Golden Reward works 88 Gordon, H. A. ..69-71-73 Greighton Mining and Milling Company, Georgia 88 H Hagen 6-9 Halske. See Siemens & Halske •. 38 Hay ward, A 94 Hauraki gold fields, cyanide plants on the 69 Havilah, Kern County, California 88 Henderson Mountain Mining Company, Montana 85 Henry Nourse Company, JohannesDurg 62 History of process. 6-9 Hydraulic slime separator at Salisbury works 49 1 Iconoclast Mine, Cal... 89 Irvine, W. E 59 J Janin, A 6-14-125 Janin, Louis, Jr 6-11-84 Johannesburg ores -. 46 Johnston, W. D. 5^14-40 E Kendall, E. D 14-117 Kuaotunu gold field 76 L Labor in cyanide works 68 Laboratory work 44 Lane, C. D 90 Langlaagte Estate Company 22-60-63-65 cement tanks 22 Royal Gold Mining Company 63 liebig's method of testing cyanide 29 138 INDEX. Page. Lime treatment, preliminary of ore 18-50 Loss of cyanide by absorption in vats and tanks 17-28 by action of carbonic acid. -. 17 by hydrolysis 17 gold in Johannesburg 52 zinc by galvanic action ..- 32 M MacArthur, J. S 5-12-110-112-115-126 Machinery and appliances . - - 43 McLaurin 17 Mercur Mining and Milling Company, Utah _ 84 Mercury in zinc boxes 35 Merrill, C. W ..6-14-125 Methods of operation ... 20 Meyer and Charlton Company 62-65 Mexico - 95 Mineralogist, California, State 5 Mitchell Creek Gold Mine, New South Wales 81 Moldenhauer, Carl 14-40-127 Molloy, B. C -.6-13-38-118 Molloy process for bullion precipitation ^.. ... 38 Molloy separator ... 13 Montana 85 Montgomery, T. C 13-121 Moratock Mine, N. C... 88 Mortars with double discharge 22 Mt. Morgan Mine 15 Muffle furnace for bullion refining 36 Miihlenberger, N. H _ 6-11 N Nevada --- - 87 New Golden Mountain Gold Mining Company, Victoria. 83 New Mexico 87 New South Wales . 81 New Zealand 69 Nigel Company ...49-53-55-63 Nitre for bullion refining in South Africa. 54 Number of plants in South Africa , . ... 48 O Otis crusher. 71 Output, total, of Rand mines 65-66-87 Oxygen, action of, in cyanide treatment . 9 P Patents 6 Patent royalty in New Zealand . 80 Paul, A. B 36-88 Percentage of extraction 40-52 in South Africa 41 in New Zealand 41-79 in United States of America 41 Percolation of concentrates - .- 28 ores .._- .- 22 tailings. _ --- .-- 24 process - - .... 20-22 Pielsticker, Carl M - - ....14-40-129 Plant in South Denver 86 Precipitation of bullion by aluminium 40 by charcoal 40 by electricity - 38-40 by zinc - - 34 Preliminary experiments .- 44 Primrose Company, Johannesburg 65 Profits of cyanide treatment in South Africa 65 Puzzler Gold Mining and Milling Company. 87 Q Queensland ■ --- — 81 INDEX. 139 B Page. Radoe, W. A 53-55-60 Rae, Julio H ...6-9-101 Rand Central Ore Reduction Company 39-63 Rate of gold extraction at Utica Mine 94 Reactions, secondary 17 Recovery of bullion 30-53 Recovery ot bullion at Nigel Company 53 Reese Mill, Cal 89 Refractory ores, definition ot ._ 15 Residues, discharging of, by running cranes 26 sluicing out of--- 26 Returns per ton of tailings in Johannesburg -. 66 Revenue, jNIontana --. .._ 85 Revivification of cyanide 38 Robinson Company _.. 25-55-60 plant - 63 Rolls for dry-crushing - _- 42 Rose, R. ..- - _- - 71 Rotary distributor for mixing coarse and fine tailings - 49 Russia - - - -.. 94 S Salisbury works, Johannesburg 28-49 Sanders, I. F ___ 106 Scheidel, Dr. A ..- 3^7-33^77-78-89-92 Scope of process 6-15 Screens for dry crushing 22 Selection of site for plant - 43 Shasta Gold Recovery Company 88 Sheba Mine -- - 47 Side discharge of tailings .-- - 48 Siemens and Kalske process - 38 Silver, precipitation by zinc 92 Simpson, Jerome W -.6^10-107 Slimes, deleterious to percolation - 52 Smith, Halford G __ 65 Soda treatment of ore 18 Sodium amalgam for bullion precipitation 38 Solubility of gold and silver in cyanide 9 South America .-. _ _ 95 South Australia -_ 81 South Dakota 87 Standard Mine, California .- _. 95 Stebbins and Porter... _ 88 Straits settlements _, 95 Strength of solution in Crown Reef works 61 Strength of solutions in Johannesburg 50 Strong solution leaching 50 Sulphates of alumina, action of on cyanide 1. 18 Sulphates of magnesia, action of on cyanide * is Sulphides _ " 15 Sulphuric acid, preliminary treatment by ig Summary and conclusions __ qq Sylvia Mine, Tararu, New Zealand _ ...16-35-41-79 T Tailings .. _ 24 channel formation in percolation - 24 mechanical difficulties in percolation "_ 25 shrinkage in vats " 25 Tasmania "" 80 Taylor, James - - ^_ 81 Te Komata Mine - ..11" 76 Tellurides "ll[""l[. 20 Tellurides in Cripple Greek - 1111 87 Time of treatment-- 1.111 25 Time required for percolation of concentrates 1... 111111 28 Total production by cyanide process ll_] 5 Trap doors at vats HH 26 Treatment of gold solutions —11.1111 30 ore and tailings pulp direct from the battery 111111 28 precipitates in Johannesburg H 54 140 INDEX. Page. Tryflnke Company, plant of - 76 Turner, F. B. and R. B. 85 Turner, J. K 86 U United States oi America _._ 83 Utah 84 Utica Mining Company, Cal 7&-89 V Vacuum filter, Scheidel's 21-79-90 at Utica Mine — -. 90 Value of cyanide gold in South Africa 60 Rand tailings 52 Vats of brick and cement 22-28-48 circular , 48 false bottoms of --. - 23 made of iron or steel. 22 made of wood - - - - --- 22 wooden, MacArthur construction 22 size of, in common use « 22 table giving dimensions and materials of ..-•. 27 Victoria -. .- - 83 Virginia Gold Mining Company, South Australia 81 W Waihi Company, New Zealand 69-71 Waiorongomai Mines - 76 Watts, W.L - 88 Webber, G. E., Jr 51-53-55 Western Australia 8(' Witwatersrand gold fields 46 custom works 68 Worcester Works 39 Working costs of process 42 in Utica cyanide works, California --- 93 Wright, Dr 6-9 Z Zinc amalgam as precipitant - ..- 34 as precipitant -- - 30 box, MacArthur's description of .-- .-- 32 boxes, material of --- — 32 box, Scheidel's construction .♦ 32 dust as precipitant - - - 34 ferrocyanide of - 35 filter, A. B. Paul's ..-- 36 filters of earthenware and porcelain 35 final cleaning up of .- 34 for precipitation of bullion 53 loss of^ by alkali 19-54 precipitation of gold and silver, theory of -. 34 preparation of filiform --- 33 preparation of shavings 34 quality of, used for precipitation 32 THIS BOOK IS DUE ON THE LAST DATE STAMPED BELOW JUL AN INITIAL FINE OF 25 CENTS WILL BE ASSESSED FOR FAILURE TO RETURN THIS BOOK ON THE DATE DUE. THE PENALTY WILL INCREASE TO 50 CENTS ON THE FOURTH DAY AND TO $1.00 ON THE SEVENTH DAY OVERDUE. HU ^^1981 JUN 1 9 1981 APR 3 1989 JUNi^9 JAN04199r NOV 28 1389:!t5 RECEIVED M07 2 ^ 1989 PHSYSCIUBRARy APR OLW^l RECaVEDj MftR2« 1991 ■fl Book SIip-15H<-8,'57(,CS107s4)456 16)^P>9? California. Dept. of natural resources. Di- vision of mines. PHYSICAL SCIENCES LIBRARY Call Number: TN21; A3 no, 5 Ttv2-4 C3 vno. ^ UNIVERSITY OF CALlFO«« DAVIS 164892