Thiosulphate leaching of gold in the presence of orthophosphate and polyphosphate

The effects of sodium orthophosphate and hexametaphosphate, were investigated in ammoniacal thiosulphate leaching of pure gold and a sulphide gold ore. Hexametaphosphate and orthophosphate readily complexed with copper(II) ions and prevented the substitution of thiosulphate into the copper(II) inner coordination sphere, thereby stabilising thiosulphate against oxidation by copper(II). In pure gold leaching, the kinetics and overall gold dissolution increased with increasing hexametaphosphate and orthophosphate concentrations. In contrast, there was an optimal hexametaphosphate concentration of about 0.8 g/L for the sulphide ore leaching. The gold leaching was significantly retarded at a hexametaphosphate concentration beyond 1.6 g/L. Silver leaching followed a similar trend to gold leaching. Orthophosphate slightly increased the overall extent of gold and silver extractions. Thiosulphate consumption decreased with increasing hexametaphosphate and orthophosphate concentrations in the leaching of the sulphide ore. After 48 h leaching, the thiosulphate consumption decreased from 8.15 kg/t under the standard leach condition to 5.19 kg/t at 1.0 g/L orthophosphate, and to 5.56 kg/t at 1.0 g/L hexametaphosphate. The enhanced gold leaching by hexametaphosphate was attributed to stabilisation of thiosulphate, reduced interactions between gold/thiosulphate and the sulphide minerals, dispersion of the slurry system and improvement of the leach slurry rheology.