Gold leaching by copper(II) in ammoniacal thiosulphate solutions in the presence of additives. Part I: A review of the effect of hard–soft and Lewis acid-base properties and interactions of ions

Gold leaching in thiosulphate media has attracted renewed interest of many researchers over the last three decades due to fast kinetics in the presence of some oxidants/ligands and its adaptability to gold ores which are unsuitable for direct cyanidation. The acidity (pH) and the concentrations of copper(II), ammonia, thiosulphate and polythionates produced by the reaction between copper(II) and thiosulphate and other added background reagents including chloride, carbonate, sulphite, sulphide, phosphate and cations such as silver(I), lead(II) and counter ions of thiosulphate salts affect the rate per unit area and extent (%) of gold dissolution. The literature data on the beneficial and detrimental effects of background reagents on gold dissolution from rotating discs, suspended foils and particles of gold ores/concentrates are reviewed. The reaction mechanism(s) with applications to leach systems are also reviewed and discussed on the basis of the Lewis acid-base and hard–soft properties and interactions of metal ions and anions. These interactions determine the ability of background reagents to affect the residual copper(II) concentration and EH, surface reaction with gold by catalytic action and/or by preventing the formation or removing the passivating layers on gold.

► Stabilisation of Cu(II) by coordination enhance residual Cu(II) and gold leaching.
► Co-ligands and thiosulphate ion-pairs are beneficial for anodic reaction of gold.
► Removal of degradation/passivation products on the gold surface enhances leaching.
► Ionic hard–soft and Lewis acid-base interactions/properties explain leach results.