Eco-friendly leaching of gold from a carbonaceous gold concentrate in copper-citrate-thiosulfate solutions

Thiosulfate is considered to be one of the most promising alternative reagents to cyanide due to its non-toxicity and excellent leaching efficiency. However, the use of ammonia as the additive and high reagent consumption have severely limited the development and commercial application of the traditional copper-ammonia-thiosulfate leaching system. In this work, the potential use of an environmentally friendly copper-citrate-thiosulfate leaching system for gold leaching from a refractory carbonaceous gold concentrate has been examined. The experimental results showed that when citrate served as the stabilizer, the copper-citrate-thiosulfate system exhibited similar extraction capability for gold with a significant decrease in thiosulfate consumption compared to the traditional copper-ammonia-thiosulfate system. Thiosulfate consumption increased with Cu2+ and S2O32− concentration, leaching time and temperature, but decreased with citrate concentration and initial pH. The mechanism of the catalysis of gold dissolution in copper-citrate-thiosulfate solutions by the binuclear cupric-citrate complex (Cu2Cit2H−24−) is presented based on the thermodynamic analysis. The stability of the Cu2Cit2H−24− complex decreased with increasing Cu2+ and S2O32− concentration, increased with an increase of citrate concentration. Moreover, the microwave roasting process applied to concentrate facilitated the subsequent gold leaching process probably by exposing encapsulated gold to thiosulfate solutions.