Thermodynamic evaluation of gold oxidation and reduction mechanisms in ammonium thiosulfate solution

The copper-catalyzed gold oxidation and reduction mechanisms in ammonium thiosulfate solution were investigated by Eh-pH diagram based on the optimum reagent combination for a 60% (by weight) -75-μm silicate ore containing 16-g/t Au with the maximum gold extraction of 92% and the minimum ammonium thiosulfate consumption of 23 kg/t for 24-hours of leaching at ambient temperature. The gold recovery by cementation was nearly 100% using Zn and Al powders at metal/Au mass ratios of 30 and was around 95% using Cu powder at a Cu/Au+ mass ratio of 50. To oxidize the gold efficiently with less thiosulfate consumption, the stable region of [Cu(NH3)4]2+ should be as wide as possible in the region where [Cu(S2O3)2]3- is more stable than [Cu(NH3)2]+ in the Eh-pH diagram. To recover gold using Zn, Cu or Al powder efficiently, the solubility of [Cu(S2O3)2]3- should be higher than [Cu(NH3)2]+ on copper precipitation. Zinc powder may recover gold more efficiently than copper powder with a similar trend on the effect of ammonia, copper and thiosulfate. However, the large amount of copper will be co-precipitated along with gold because the stability of zinc ammine and thiosulfate complexes are higher than that of copper’s. Aluminum may reduce gold efficiently at higher thiosulfate concentration and pH. These results were confirmed and compared to the optimum reagent combination for afiner-ground, 100% (by weight) -75-μm, sample. It was concluded that the optimum reagent combination was 0.5-mol/dm3 NH4OH, 0.0001-mol/dm3 CuSO4·5H2O and 0.05-mol/dm3 (NH4)2S2O3, which resulted in 94% gold extraction and an ammonium thiosulfate consumption of 3 kg/t. Gold and copper stabilities agreed with the thermodynamic rulefound on a coarser ground ore (60% by weight -75 μm). The lower reagent combination will reduce gold oxidation power, but significantly minimizes reagent consumption. Erom this pregnant solution, nearly 100% of the gold was successfully recovered by zinc precipitation at a Zn/Au+ mass ratio of 30. Aluminum and copper precipitations resulted in less than 40% gold recoveries at metal/gold mass ratios of 30. The thiosulfate oxidation mechanism is believed to be in agreement with the Belousov-Zhavotinsky reaction.