Leaching of silver sulfide with ferricyanide–cyanide solution

Significantly lower silver extraction than gold has been commonly observed in traditional gold-silver cyanidation process, especially in heap leaching operations. The different mineralogy of gold and silver in the ores is suspected to be the main reason, e.g., the occurrence of low solubility acanthite (Ag2S) typically results in low overall silver extraction.Leaching of silver sulfide with ferricyanide–cyanide has been investigated with the purpose of increasing the silver extraction. Ferricyanide proved to be an effective oxidant to dissolve silver sulfide in cyanide solution. The main reaction involved under the experimental leaching conditions was determined, indicating the formation of thiocyanate as the sulphur reaction product.The dissolution rate of Ag2S in ferricyanide–cyanide leaching system (0.5 g/L Fe as ferricyanide) in a rotating disc experiment gives the value of 21 μmol m− 2s− 1 (0.78 μm/h), much higher than that in aerated cyanide solution (13 μmol m− 2s− 1, or 0.38 μm/h). The effect of pH, temperature, ferricyanide and cyanide concentration, and rotating speed on the dissolution rate of Ag2S were examined. A diffusion-controlled process was confirmed with an activation energy of 6.7 kJ/mol.Study on the leaching kinetics of several mixed sulfide minerals in ferricyanide–cyanide solution shows that the presence of non-silver sulfide minerals may decrease silver extraction from silver sulfide to different extents. A significant decrease in silver extraction is observed in the presence of chalcopyrite (CuFeS2) and pyrrhotite (Fe7S8). It is believed that chalcopyrite and pyrrhotite consume ferricyanide, making it unavailable for silver sulfide leaching.