Art. XLII.—On the_ Regeneration of Cyanide-solutions.

Transactions and Proceedings of the Royal Society of New Zealand, Volume 36, 1903, Page 481

Art. XLII.—On the_ Regeneration of Cyanide-solutions. By Godfrey Doveton, M.A.I.M.E. Communicated by Professor James Park. [Read before the Otaga Institute, 13th October, 1903.] Case 1.—The recovery of the cyanide from the inert compound K2ZnCN4, which is formed during the precipitation of the metals upon zinc-shaving, filiform, zinc,. &c—zinc fume, emulsion, or dust. Thus— Zn + 4. KCN + 2, H2O = ZnK2CN4, + 2 KOH + H2. To the solution containing the double cyanide of zinc a certain amount of a caustic alkali is added. Thus, with caustic potash— ZnK2Cy4, + 4 KHO = Zn(CK2 + 4 KCN + 2 H2O. The requisite amount of caustic alkali is readily found by a simple laboratory experiment. To the solution containing potassium-cyanide and zincate of potash is now added a certain quantity of alkaline sulphide, - slightly less than actually required to throw down all the zinc as sulphide. Thus— Zn(CK)2 + 4 KCN + 2 H2O + Na2S = ZnS + 4 KCN + 2 KHO + 2 NaOH. The requisite amount of alkaline, sulphide is regularly found by simple laboratory experiment. Case 2.—Recovery of total cyanide, where so desired, either, from foul solutions or weak waste solutions. A. Determine amount of available KCN. B. Determine amount of total KCN. C. Determine amount of protective alkali. If much of C is present it will, be found of advantage to partially, neutralise, to prevent high consumption of zinc salt in the following operation. D. To the tank of solution add sufficient zinc salt to precipitate all the cyanides as zinc-cyanide. Thus, with zinc-chloride— 2 KCN + ZnCl2 = ZnCy2 + 2 KCl.

Also— K2ZnCN + ZnCl3 = 2 ZnCy2 + 2 KCl. Also, with the copper compound, if any— ZnCl2 + K2Cu2CN4 = ZnCu2CN4 + 2 KCl. Collect the granular precipitate of ZnCy2—and in case of copper ZnCu2CN4—by means of filter press, or any other means of filtration or decantation. Reject the supernatant liquor, which will be found to be entirely free from either available cyanide, total cyanide, copper, and containing perhaps a doubtful trace of the precious metals. The Au and Ag are almost entirely precipitated on addition of the ZnCl2. And add to it a concentrated solution of caustic alkali, sufficient to dissolve the cyanide precipitate. Thus— ZnCy2 + 4 KHO = Zn(OK)2 + 2 KCN + 2 H2O. Thus, with copper compound— ZnCu2CN4 + 4 KHO = Zn(OK)2 + K2Cu2CN4. To the highly concentrated solution of potassium-cyanide and potassie cupro-cyanide (if copper be present in the original mill solution), also containing zincate of potassium, we now add the requisite quantity of alkaline sulphide for the purpose of removing the latter compound, Zn(OK)2, care being taken to use, as previously directed, a slightly less amount of Na2S than is actually required to remove the whole of the zinc. It is now desired to recover the cyanide now held up by the copper in the solution as C2Cu2CN4. Case 3.—The highly concentrated solution of KCN, almost entirely freed from zincate of potash, but containing a portion of its cyanide combined in the inert compound K2Cu2CN4, is now subjected to the action of a very weak electric current in a precipitation-box similar to that used in the Siemens-Halske process, though from the fact that a highly concentrated solution is to be operated upon the box may be much smaller than ordinarily; in practice, 100 tons of foul or waste mill solution can be readily concentrated into 5 tons. A Current of 2.5–3 volts is found most suitable, with a volume of .0076 amperes per square foot. The negative pole of the dynamo or batteries, as the case may be, is connected with the cathodes, which are very thin copper plates, not necessarily pure. The positive pole of plating dynamo or batteries is connected with the anodes of the box, which are thick zinc plates. The electrical action causes the potassic cupro-cyanide to split up, the copper precipitates readily upon the cathodes, along with any bullion which may be contained in the solu-

tion, and HCN is evolved, which in its turn attacks the zinc plates, or anodes, and combines therewith to produce additional double cyanide of zinc and potassium—ZnK2CN4. Thus, the solution during passage through the box is deprived of its copper, the place of the double cyanide of copper and potassium being taken for an equivalent amount of the double cyanide of zinc and potassium, from which latter salt the cyanide may readily be obtained in the available form of free potassium-cyanide by the method explained previously in Case 1. Should it be desired to ship the cyanogen product to market, it is shipped readily as the simple zinc-cyanide ZaCy2, the precaution being taken to pack it, while damp, an airtight cases. When allowed to dry much of it is converted to carbonate. To make the ZnCy2 available for gold-extraction purposes it is, as previously explained, dissolved in a caustic alkali, the excess of zincate of soda or potash being removed by addition of an alkaline sulphide. N.B.—It is advantageous, to promote perfect precipitation both in the precipitation of the cyanides with zinc salt and in the precipitation of the zinc as ZnS, and in the electrical precipitation of the copper and bullion, to cause active circulation, which is best occasioned by the employment of compressed air. In the electrical precipitation-box the distance between cathode and anode should be ½ in. Beginning with a thick plate of zinc and a very thin sheet of copper, you end with a thin frayed sheet of zinc and a thick plate of copper containing small amounts of bullion.