The role of oxygen in thiosulphate leaching of gold

The role of oxygen in thiosulphate leaching of gold has been investigated in both pure gold and ore systems. The gold dissolution decreased in the presence of oxygen in the pure gold system. Air or oxygen bubbling caused higher consumption of thiosulphate and leaching passivation. Nitrogen bubbling largely increased gold dissolution and this beneficial effect appeared to be more noticeable at a higher nitrogen flowrate. Nitrogen bubbling stabilised thiosulphate and prevented the passivation of gold. The use of oxygen pre-saturated water had little effect on gold dissolution. The counter current leaching tests showed that both gold passivation and the products due to thiosulphate decomposition affected the gold dissolution. The leaching solutions could be re-used without any impact on gold dissolution under a nitrogen atmosphere.Mineralogy affected the thiosulphate leaching system under different atmospheres. The leaching behaviour of a sulphide ore with only a small amount of sulphides was determined by the leaching behaviour of gold. However, air bubbling gave a higher overall recovery in the leaching of a pyrite concentrate, while nitrogen bubbling only marginally increased the overall gold recovery. Oxygen injection enhanced the dissolution of sulphides, releasing more gold from the sulphide matrices. Nitrogen injection hindered the corrosion process of the sulphides, affecting the overall gold recovery. Nitrogen bubbling decreased the thiosulphate consumption in the leaching of the pyrite concentrate, while air bubbling increased the thiosulphate consumption. It is suggested that high sulphide containing ores should be pre-treated to fully or partially break down the sulphide matrices to liberate gold, followed by gold leaching under a nitrogen atmosphere.