<Original Paper>Leaching of Silver Sulfide in Weakly Acidic Thiourea Solutions

Thiourea leaching of silver sulfide was studied in weakly acidic sulfate solutions containing ferric sulfate as a moderate oxidant. In the presence of ferric ions, the sulfide sulfur was converted into elemental sulfur and the silver was extracted as its thiourea complex ion, Ag(NH_2CSNH_2)_3^+. In the leaching without oxidant, silver sulfide was leached according to the H_2S forming reaction, where the elimination of H_2S from the leach liquor, for instance by nitrogen gas flow, significantly favored the leaching. In the both cases, the presence of thiourea was necessary for the extraction of silver from the sulfide. The moderate leaching conditions such as fairly low acid concentration at a level around 0.06mol/l and the operation temperature not higher than 50℃, and a high concentration of thiourea promoted the silver extraction mainly due to the increased stability of the thiourea complex ion of silver. It was also observed that silver sulfide was easily extracted in the oxidative thiourea leaching where sulfide ore consisting mainly of pyrite, arsenopyrite, stibnite and chalcopyrite coexisted