Thermodynamic analysis of ammoniacal thiosulphate leaching of gold catalysed by Co(III)/Co(II) using Eh-pH and speciation diagrams

In this paper, Eh-pH and speciation diagrams for gold leaching in the Co-NH3-S2O32−-H2O system are constructed from thermodynamic calculations. The diagrams show that thermodynamically stable redox couples of Co(NH3)63+/Co(NH3)x2+ (x = 1-6) can be formed and drive the dissolution of gold in ammoniacal thiosulphate solutions. Based on the diagrams, a possible mechanism for the Co(III)/Co(II) electrochemical-catalytic leaching of gold with ammoniacal thiosulphate is proposed. The calculation also reveals the differences in the oxidization potential and structural stability of the Co(NH3)63+ and Cu(NH3)42+, which explains the much reduced thiosulphate consumption in the Co-NH3-S2O32−-H2O system compared to that in the Cu-NH3-S2O32−-H2O system. Results presented in this paper offer a theoretical basis for the optimization of the cobalt-catalysed thiosulphate leaching of gold.