United States Patent Office Patented Mar

3,311,468 United States Patent Office Patented Mar. 28, 1967 2 3,351,468 continuously or batchwise as desired. Other objects and SVRESR RECGVERY PRO CESS advantages will appear as the description proceeds. Charies Davidoff, 1:8 Roliagiiii Road, The attainment of the above objects is made possible Maiaasset, N.Y. 1936 by my method of recovering silver from a solution of No Drawing, Fied Dec. 26, 1963, Ser. No. 333,635 5 alkali metal silver cyanide comprising adding to the solu 6 Caias. (Ci. 75-63) tion at least about 0.25 mole of an alkali metal hydro sulfite compound for each mole of silver in the solution, This invention relates to a method for recovering silver, maintaining the resulting solution, preferably at an ele and more particularly to a method for recovering silver vated temperature above about 150 F., until cessation from a solution of alkali metal silver cyanide or other sil 10 of precipitation of the silver, and removing the precipi wer salt. tated silver from the solution. The above process has It is common knowledge that solutions of silver cyanide been found to constitute a surprisingly simple, economical complex exist in several fields of industry from which the method of expeditiously recovering silver from solutions silver must be recovered for obvious reasons of economy. of alkali metal silver cyanide in up to 100% yields with Thus, in the mining industry, a widely employed method out the need for the previously required acid-proof equip of recovering silver from ore containing the same involves ment. Further, although the said process exhibits its leaching the silver from the ore with an aqueous solu greatest advantages in the treatment of solutions contain tion of alkali metal cyanide, a solution of sodium or ing the highly stable alkali metal silver cyanide complex, potassium silver cyanide being thereby obtained. For it will be understood that the invention also includes use plating with silver, plating solutions are frequently solu 20 of such process for recovering silver from solutions of tions of sodium or potassium silver cyanide, from which the relatively more unstable silver compounds such as solutions, either new, excess or used, the silver must be silver fulminate, nitrate and thiosulfate and the like. recovered. Also, a method of recovering silver from As the alkali metal hydrosulfite compounds useful as silver alloys such as scrap jewelry and the like involves precipitants in the process of this invention there may be treatment with an aqueous solution of alkali metal 25 employed an alkali metal hydrosulfite such as sodium or cyanide. potassium hydrosulfite, or a substance which liberates In contrast to the many well established simple pro same in the silver-containing solution such as the addi cedures for recovering silver from its uncomplexed ionic tion compounds of an alkali metal hydroSulfite with an state such as monovalent silver, no really satisfactory aliphatic aldehyde as exemplified by formaldehyde, acet method exists for recovering silver from its alkali metal 30 aldehyde and the like. Such addition compounds are cyanide complex. According to one presently employed commonly known as, respectively, sodium formaldehyde procedure, the solution of silver cyanide complex is first sulfoxylate, sodium acetaldehyde sulfoxylate and the like treated, as by vacuum, to remove free oxygen which is and the corresponding potassium compounds. needed in conjunction with the alkali metal cyanide to The process of my invention has been found to be high dissolve the silver but which interferes with the subse ly effective in selectively precipitating silver in substan quent precipitation with zinc. The deoxygenated solution tially pure and readily filterable form from alkali metal is then treated with zinc dust, usually for several hours silver cyanide solutions containing such other base metals and preferably overnight, to precipitate the silver and or contaminants also dissolved therein as copper, Zinc, form a silver-zinc slurry. A small amount of lead acetate nickel, lead, and platinum and the like. Such solutions, is usually added to help coagulate the silver-zinc slurry. 40 as described above, are formed in the cyanide treatment The slurry is then filtered, the filter cake is treated with of silver-containing ores and alloys, and in electroplating acid to dissolve out the zinc and lead, and the mixture processes employing the silver cyanide complex. By way filtered again and the silver filter cake washed free of of example, solutions of this type are formed in the porous acid. According to a further improvement on this in cup technique for electrolytically dissolving silver-base volved process, and to purportedly diminish or eliminate 45 metal alloys according to which the alloy is used as anode the need for preliminary deoxygenation of the pregnant in an electrolyte of alkaline cyanide such as sodium or solution, the zinc dust is first treated with alkali bisulfite potassium cyanide while the cathode is shielded by a por and, desirably, alkali metal cyanide but this improvement ous cup to prevent the silver from plating thereon. The obviously fails to eliminate the disadvantages of this proc resulting electrolyte to be treated by the process of this ess inherent in its overlong duration, plural filtrations, 50 invention therefore contains dissolved therein the silver, need for acid-proof equipment, etc. Further, any lead as cyanide complex, and base metals originally present in precipitated with the silver must be completely removed. the alloy. If any slight amounts of such base metals pre Similar considerations apply when this zinc precipitating cipitate with the silver, they may be readily leached out process is employed for treating the so-called acid silver 55 by treating the precipitate with acid, if their presence in plating solutions containing the silver cyanide complex the recovered silver is prohibited. Any gold present in the silver cyanide solution will be precipitated with the buffered with an organic acid such as citric or acetic acid silver by the present process, but can be readily sepa or their ammonium or other alkaline salts to maintain a rated from the silver, for example by leaching the silver pH at or below 7. from the precipitate with nitric acid and then treating It is an object of this invention to provide a process for 60 the resulting silver nitrate solution by the process of this recovering silver from solutions of silver cyanide complex invention or by any other known process. This method which will not be subject to one or more of the above of recovering gold from solutions of alkali metal gold disadvantages. Another object of this invention is the cyanide is disclosed and claimed in my copending applica provision of a new and improved method for recovering tion Ser. No. 333,717 filed on even date herewith entitled, silver from such solutions. Still another object of the in 65 “Gold Recovery Process.' The gold can of course be vention is the provision of such a method which is rela allowed to remain with the silver if desired. tively brief in duration, high in yield, and/or low in cost In carrying out the process of this invention, the alkali of labor, materials and/or equipment. A further object metal hydrosulfite compound, or mixture thereof, is is the provision of such a process which may be operated added to the silver cyanide solution in molar proportions 3,311,468 3. 4 relative to the silver in the solution of at least about Example 2 0.25:1 or more, usually about 0.5-5:1 and optimally The procedure of Example 1 is repeated but using the about 0.5-1: 1. It is always preferable to employ an following materials yielding a solution having a pH of excess of the precipitant, which is relatively inexpensive, 10.8: to ensure recovery of all the silver in the solution. The 5 rate of precipitation varies directly with the temperature, Grams and for practical purposes, the solution containing the Silver cyanide -------------------------------- 4.5 silver cyanide and precipitant should be maintained at KCN --------------------------------------- 4.5 elevated temperatures of about 150 F. up to the boiling Similar results are obtained. point, and preferably about 180-200 F. until cessation Example 3 of precipitation of the silver which may take from about 10 5 minutes to 2 hours and usually about 4 to 4 of an The procedure of Example 1 is repeated but using the hour. The process is substantially independent of the following materials yielding a solution having a pH of 8.7: pH of the solution which may range from about 14 for Grams the highly alkaline solutions down through neutrality to 5 Silver cyanide -------------------------------- 4.5 about 3 in the case of solutions buffered with phosphates KCN --------------------------------------- 4.5 and/or carboxylic acids such as citric, acetic, etc. Simi Citric acid ----------------------------------- 1.0 larly, the process is operative on solutions containing any Further, sodium formaldehyde sulfoxylate is employed amount or concentration of alkali metal silver cyanide, as precipitant instead of sodium hydrosulfite. Similar the minimum concentration being established in any par 20 ticular instance by economic factors; unless dictated by results are obtained. other considerations, silver concentrations so low as to Example 4 result in the value of the recovered silver being below The procedure of Example 1 is repeated but using so the cost of the recovery process would ordinarily not be dium acetaldehyde Sulfoxylate as precipitant, and the employed herein. The maximum operative concentra following materials yielding a solution having a pH tion is of course that of a saturated solution. As will be of 4.6: readily understood, the attainment of optimum results Grams will be dependent on proper adjustment, determinable by Silver cyanide ------------------------------- 4.5 routine experimentation, of the temperature and duration KCN -------------------------------------- 4.5 of treatment, proportions of precipitant to silver, identity 30 Citric acid ---------------------------------- 35.0 of the precipitant, pH of the solution, etc.

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