The Chemistry of Cyanide-Metal Complexes in Relation to Hydrometallurgical Processes of Precious Metals

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Abstract
Thermodynamic stability diagrams have been constructed for the cyanide-metal-sulphide/telluride/selenide systems. These were used to study the solution chemistry of the various metal-cyanide complexes in connection with cyanide leaching of gold and silver from complex sulphide, arsenosulphide, telluride and selenide ores. The results show that the formation of the metal-cyanide complexes and their stability relative to the metal oxides and sulphides depend critically on the total concentration of the cyanide and metal ions. At lower concentration ratios of Tc(CN)/Tc(Me), Ag, Cu, Ni and Zn all form insoluble metal cyanide precipitates. At higher concentration ratios, the precipitates are dissolved to form soluble cyano-metal complexes. The formation of gold and silver sulphides does not affect the stability region of aurocyanide and silver-cyanide complexes. Gold and silver telluride and selenide are stable in the presence of cyanide. These results suggest that direct leaching of gold and silver tellurides and selenides with cyanide is probably ineffective. The metal sulphides are classified into two groups according to their stability relative to the cyano-metal complexes. The application of these results to the cyanide leaching of gold and silver is discussed.