An Electrochemical Study of the Dissolution of Pure Gold and Gold-Silver Alloys in Thiosulfate Leach Solutions

Abstract

A fundamental electrochemical study of gold and gold/silver alloys in thiosulfate solutions showed that although the oxidation is a complex process, there was no selective dissolution of either gold or silver from the alloys during oxidation. The presence of copper and thiourea catalyses the oxidation of the alloys, resulting in good leaching for thiosulfate systems when utilising either of these species. Surprisingly for copper containing solutions, the oxidation rate is largely independent of potential, and is higher for pure gold than for the 4% Ag and 8% Ag alloys. For thiourea containing solutions, the rate of alloy oxidation increases with increases in the silver content of the alloy. In the absence of copper of thiourea, the only alloy which showed appreciable oxidation was 50% Au / 50% Ag, although the rate of oxidation did increase with an increase in silver content from 0 to 50%.