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Professor of Metallurgy, Colorado School of Mines, Golden, Colo.
The purpose of this paper is not the presentation of anything new or unique in cyanide treatment, but rather to give a brief statement of the present status of the process, together with the steps required to reach the present condition.
The solubility of gold in alkaline cyanide solutions has been known for a century, having first been announced by Hagen in 1806.
Scheele, the Swedish chemist, knew of the solubility of aurous cyanide in potassium cyanide and probably also of the solubility of metallic gold itself.
Prince Bagration in a remarkable investigation, the details of which were published in the Bulletin of the Imperial Academy of Sciences of St. Petersburg, 1842, Vol. II, page 136, brings to notice many important points. of interest in cyanide practice. He anticipated Elsner in showing the importance of aëration; he showed that the electric current was not necessary to effect solution, and that the dissolved gold could be precipitated upon metallic copper or silver surfaces, either with or without the aid of an electric current.
In 1844 Elsner (J. Pr. Chem., 1844, p. 441) announced that "The dissolution of the metal is, however, the consequence of the action of oxygen, which, absorbed from the air, decomposes part of the cyanide."
Michael Faraday made use of the solvent power of potassium cyanide solutions for obtaining thinner films of gold than could be produced mechanically.
The first patent in which the solvent power of potassium cyanide solutions on the precious metals in ores was a factor, was that granted to Julio H. Rea, of Syracuse, New York, numbered 61,866 and dated February 5, 1867. In this paper Rea makes the following claims:
I. The within described process of treating auriferous or argentiferous rock by exposing the same to the combined action of a current of elec
'Read before the Colorado Section of the Western Society of Technical Chemists and Metallurgists, November 12th, 1907.
tricity and of suitable solvents or chemicals (e. g., cyanide of potassium). substantially as herein specified, or any others which will produce the same effect.
2. Separating gold or silver from the rocks containing the same by the action or aid of electricity, substantially as described (i. e., by deposit on a cathode of copper or other suitable metal).
3. Using the agitator (a coil of platinum wire) as an electrode.
It is evident that Rea did not know that potassium cyanide was a solvent of the precious metals independently of the presence of an electric current, and while his device partakes more of the nature of a toy than a metallurgical machine, yet there is the germ of the process as it now stands in Rea's little agitators and proposed practice.
In Letters Patent, No. 229,580, issued July 6, 1880, Thomas C. Clark of Oakland, Cal., is granted the following claim: "The process of disintegrating and desulphurizing ores and freeing the precious metals therein contained; consisting in first roasting the ore to a red heat, and while in that condition placing it in a cold bath composed of a solution of salt, prussiate of potash and caustic soda, or caustic potash, in about the proportion named, substantially as herein described. This solution is made up as follows: Thirty gallons of cold water to which common salt is added to saturation; to this is added a solution containing one pound of potassium cyanide, and also a solution of one pound caustic soda in water."
With the exception of its great strength and the common salt, which was added to aid in the solution of the silver, this solution corresponds to the solution of the present day.
Hiram W. Faucet was granted patent No. 236,424, January 11, 1881, in which sodium cyanide is mentioned as one of many chemicals to be used; and some of these chemicals would react with the cyanide and destroy its efficiency, the process proposed seems to possess little merit.
John F. Sanders, Ogden, Utah. Letters Patent No. 244,080, July 12, 1881, proposed using a solvent of potassium cyanide and glacial phosphoric acid, in the ratio of sixteen to one, to dissolve the coating from the surfaces of particles of gold and thus render it more readily amalgamable.
Jerome W. Simpson, of Newark, N. J., Letters Patent No. 323.222, July 28, 1885, proposed to dissolve gold, silver and copper from ores by use of a solvent consisting of potassium cyanide one pound, ammonium carbonate one ounce, sodium chloride one-half ounce, and water sixteen quarts. After agitation of the fine ore with this solution it was allowed to settle, after which the metals were precipitated on a plate of zinc suspended in the clear solution.
Simpson states in his application: "I am aware that cyanide of potassium when used in connection with an electric current has been used for
dissolving metal, and also that zinc has been employed as a precipitant, and the use of these I do not wish to be understood as claiming broadly."
MacArthur and the Forrests, whose names are so closely associated with the cyanide process, made their original application for a patent in this country October 19, 1887, but because of, delay in filing complete specifications, the patent was not issued until May 14, 1889, and was, numbered 403,202. The following claim is contained in the patent allowed: "The process of separating precious metal from ore containing base metal, which process consists in subjecting the powdered ore to the action of a cyanide solution containing cyanide in the proportion not exceeding eight parts of syanogen to one thousand parts of water."
In this patent emphasis is laid on the fact that cyanides alone are used without such other active agents as were mentioned by former patentees, and that because of this and of the extremely dilute solutions used, a selective action is exercised by the solvent, gold and silver being principally dissolved, while the other metals are left unattacked.
In patent No. 418,138, issued December 24, 1889, to John Stewart MacArthur, precipitation by use of zinc shavings and the popular zinc box, substantially as used at the present time, were both covered. The use of an alkali or an alkaline earth to prevent undue cyanide consumption was covered by patent No. 418,137, issued December 24, 1889, to MacArthur and the two Forrests.
Providing these patentees had made extravagant claims for royalties for the use of the methods covered by the patents issued to them they might have hindered the advance of metallurgy to a very marked extent.
Fortunately, however, they were men of sufficient gauge and possessing enough foresight to see that moderate claims would be far better for all concerned, and because of their position the remarkable progress in the treatment of low-grade ores of gold has been possible.
The MacArthur and Forrest patents have expired in this country, so the question concerning their rights is now a purely ethical one. Personally, I have always felt that notwithstanding their great contribution to the metallurgy of the precious metals, that they were not entitled to patents covering such broad claims as those in the papers issued to them, and it seems to me that a comparison of their patents with those issued carlier shows them to have been anticipated in all essential points.
It remained for these gentlemen, however, to place the cyanide process upon a commercial foundation and it is to them that we are indebted for the immense strides made in the early stages of its development.
In the practical application of the process I think no one will disagree with me when I make the assertion that the percolation method is to be preferred and always to be used when results justify it. This method is so simple, so easily carried out and so cheap that where satisfactory extraction can be obtained by its use it is to be adopted. Unfortunately, however, be
cause of inability to percolate material as received, or of the necessity for finer grinding in order to make a satisfactory saving, agitation, or preferably, a combination of agitation of slimes and percolation of sands after hydraulic classification into these two products, becomes necessary. Of course with the various devices for separating solution and slime; the problem of slime treatment is much simpler than in the earlier days of the process, and so much better results are obtainable after finer grinding that notwithstanding the greater difficulties in handling, the agitation treatment is rapidly extending.
The selection of a suitable machine for fine grinding is all-important, and, to my mind, the pebble mill stands alone as fulfilling the conditions far more perfectly than any other device. Of course I am aware that the grinding pan has its ardent advocates, but it seems to me that tube mills are much to be preferred.
Of the various devices for agitation, each has its supporters; whether air or mechanical agitation shall be used, and if the latter, what form shall be adopted-paddles, propeller blades, screws, or other forms, or whether the pump circulation method, is a problem for the designer of the plant or the manager to wrestle with.
How shall we separate our slime and solution? In the words of the classic Georgian, "Where are we at?"
If we use a submerged filter do we pay a royalty to a syndicate possessing certain letters patent? If so, how much and under what limitations? Are all submerged filters actually subject to the claims of this one company for infringement? I am asking these questions for information. As fully as anyone, I think, I appreciate the great contribution made by George Moore to the treatment of slimes, when he first placed his excellent filter before the public, and I feel that he should have a just reward for what he has done. But how is this reward to be made? Is there a sufficiently new idea in this filter to support the sweeping claims of the company now exploiting it? Is there a new principle involved? And who ever heard pressure filters connected with Moore's name a year ago?
I feel rather strongly on these points, but do not wish to be misunderstood. Where a man has made a distinct contribution to the advancement of any industry he is entitled to a liberal reward, but in my opinion this contribution should be a distinct thing, not an adaptation of something known for a long time to a somewhat different use.
The fault for the present situation with reference to slime filters is to be placed, in my opinion, in the United States Patent Office.
A patent once granted should be protected to the fullest extent by the government; the questions of originality and of infringement should be taken up and thoroughly investigated in the patent office, and the subsequent issue of any patent should be sufficient proof that there was no infringement by it on existing patents. If this were true at the present time