The role of heavy metal ions in gold dissolution in the ammoniacal thiosulphate system

A study has been conducted to understand how heavy metal ions affect the dissolution behaviour of gold in the ammoniacal thiosulphate system. The effect strongly depended on ion types and concentrations, and reagent concentrations. Zn had a slightly positive effect on gold dissolution at very low concentrations, and would retard it at high concentrations. Cd, Co, Cr and Ni retarded gold dissolution to different extents at all reagent concentrations, and the dissolution kinetics of gold decreased with the increase in the ion concentrations. The free thiosulphate concentration decreased in the presence of heavy metals, especially at higher concentrations. The Eh–pH diagrams of the Metal–NH3–S2O32− systems indicated that the predominant species for Cd, Co, Ni and Zn in the leaching region are Cd(NH3)42+, Co(NH3)2+, Ni(NH3)62+ and Zn(NH3)42+, respectively. On the other hand, Co(OH)2, NiO and ZnO may also be present in the solutions containing high concentrations of Co, Ni and Zn, respectively. These oxy- or hydroxyl-metal species may hinder gold dissolution. The Co3+/Co2+ couple can be used as an alternative to Cu2+/Cu+. At low lead concentrations, the predominant species for Pb in the leaching region is Pb(OH)+ in the Eh–pH diagram of the Pb–Au–NH3–S2O32− system. AuPb2 may also be present in the leaching region. However, PbO is possibly the predominant species at high Pb concentrations. Consequently, Pb accelerated gold dissolution at low concentrations, and retarded it at high concentrations. Pb retarded gold dissolution at low ammonia concentrations because PbO is the predominant species in such cases. The EDS result indicated the presence of Pb species at the surface of the gold plate leached in the 20 mg/L Pb system.