The extraction of gold and silver from ores with alkaline cyanide solutions is well known. Cyanide solutions are also good solvents for many base metal minerals, particularly most of the copper minerals. Leaver and Woolf, who determined the solubilities of copper minerals in 0.10 pct sodium cyanide solution, showed that malachite, cuprite, chalcocite, and bornite are readily soluble in cyanide; least soluble are chalcopyrite and chrysocolla.
The sample of commercial zinc ore used in this investigation to produce zinc concentrates was ball mill feed ranging from ½ in. to fines. The ore was crushed to —10 mesh and riffled into 600-g charges for zinc flotation. Analysis revealed 4.54 pct Zn and 0.005 pct Cu.
Aero Brand cyanide, technical calcium cyanide analyzing 49.5 pct NaCN equivalent, was used for all cyanide leaching tests. For each test three rougher concentrates floated as described were combined (total weight close to 180 g) and the water content adjusted to 150 ml. The percent solids in the pulp was thus 55. After a weighed amount of cyanide flakes had been added, the pulp was agitated on rolls for a given period, then filtered and washed. Filtrate and washed concentrate were assayed for copper content.
The solvent effects of 0.125, 0.187, and 0.250 pct NaCN equivalent solutions were compared for leaching periods of 15 min. Residue and filtrate were assayed for copper content. The copper content of the filtrate plus wash was calculated back to the original volume of cyanide solution (150 ml).
15 min leaching with 0.25 pct NaCN equivalent solution extracted 74.6 pct of the copper in the zinc concentrate. The leached product assayed 0.013 pct Cu, appreciably below the required specification of 0.020 pct.
Portions of zinc concentrate were leached with 0.25 pct NaCN equivalent solution for various time periods. The amount of cyanide in each solution was equivalent to 4.17 lb NaCN per ton of zinc concentrate treated.
By calculation, the solution contained at least 0.043 pct free NaCN equivalent (17.2 pct of the original cyanide added). This was determined by calculating the amount of cyanide complexed by the copper and zinc as Na3Cu(CN)4 and Na2Zn(CN)4 and subtracting this amount from the total cyanide determined by distillation.
The acid leach liquor, after being stripped of HCN, contained a pale buff precipitate. This was filtered off and the precipitate and filtrate were assayed for copper content.
The acid solution remaining after cyanide regeneration contained a buff precipitate which consisted of CuCN and CuCNS. This precipitate accounted for about 45 pct of the copper leached out of the zinc concentrate. The remaining 55 pct was copper sulfate. In equivalent amounts the copper-bearing acid solution was as effective as copper sulfate for sphalerite activation..