Gold Leaching in Thiosulfate Oxygen Solu

Thiosulfate has received much attention as an alternative non-cyanide lixiviant for gold recovery over the last three decades. In particular, a number of studies have shown that an ammoniacal copper(II)-sulfate/sodium thiosulfate system offers fast leaching kinetics but there are difficulties in controlling the complex solution chemistry and there are concerns over the use of ammonia. Recently, thiosulfate leaching of gold in the absence of ammonia has shown to be a useful alternative to cyanide, as evident from the thiosulfate gold processing plant recently commissioned at Barrick’s Goldstrike operation in the USA for treating a pressure oxidised double refractory ore. In this work, the dissolution of gold as gold(I)-thiosulfate in non-ammoniacal thiosulfate solutions has been studied using a rotating electrochemical quartz crystal microbalance (REQCM), rotating gold disc electrode (RDE), gold powder and various gold ores in the presence of sulfide minerals or activated carbon. The electrochemical studies found that the gold oxidation is enhanced by increases in temperature, thiosulfate concentration and the addition of low concentrations of copper(II)-sulfate. Oxygen reduction was more effective on the surface of sulfide minerals than on the gold surface, offering an opportunity for galvanic interaction. The subsequent leaching tests using REQCM showed that the gold leach rate in the thiosulfate-oxygen system without any additives is in the order of 10⁻⁷ mol m⁻² s⁻¹, two orders of magnitude lower than a typical cyanidation rate. However, a temperature of 50 °C, high oxygen concentration, 2 mM copper(II)-sulfate addition, in conjunction with the galvanic effect of sulfide minerals, dramatically improved the gold leach rate to the same order of magnitude of a typical cyanidation rate of order 10⁻⁵ mol m⁻² s⁻¹. This study showed that the thiosulfate-oxygen system in the presence of activated carbon could be a promising alternative to cyanidation for treating some sulfidic gold ores.