Effect of thiosulphate salts on ammoniacal thiosulphate leaching of gold
Gold leaching with calcium, sodium and ammonium thiosulphate salts was investigated in both pure gold and ore leaching systems. The type of thiosulphate salts was found to influence significantly the solution and surface chemistry, and hence the leaching of pure gold and the pyrite concentrate in the ammoniacal thiosulphate solutions. Calcium thiosulphate gave a higher gold extraction rate with negligible consumption of thiosulphate in the leaching of pure gold. Sodium thiosulphate had the least thiosulphate consumption and the lowest gold extraction rate in the leaching of pure gold. Ammonium thiosulphate had a better gold extraction rate than sodium thiosulphate, but had the highest thiosulphate consumption due to lower solution pH in the leaching of pure gold. The difference in the leaching of pure gold by the thiosulphate salts was attributed to the ability of ion-pair formation of thiosulphate ion with the cations. The mineralogy affected substantially the gold leaching of the ores with different thiosulphate salts. The leaching of the sulphide gold ore with a low-sulphide content followed the same trend as the leaching of pure gold, where the dissolution behaviour of gold with different thiosulphate salts governed the gold extraction rate. In contrast, the extraction rate of gold from the pyrite concentrate was dependent not only on the leaching behaviour of gold but also on the interactions of the associated sulphide minerals. Ammonium thiosulphate had a marginally higher gold extraction rate from the pyrite concentrate than calcium thiosulphate, due to the greater extent of leaching of the copper sulphide minerals in the ammonium thiosulphate solution. The thiosulphate consumption was the lowest in the leaching of the pyrite concentrate with calcium thiosulphate. Sodium thiosulphate had the lowest gold extraction rate from the pyrite concentrate despite low thiosulphate consumption. Ammonium thiosulphate is preferred for the leaching of high-sulphide ores and calcium thiosulphate is preferred for the leaching of low-sulphide ores.