Improvement of thiosulfate stability in gold leaching

Ammonium thiosulfate is a noncyanide lixiviant for gold leaching. However, the major problem with this leaching method has been the high consumption of reagents. To leach a mild-refractory copper-bearing gold ore, the authors used a reagent combination of 0.3M (NH4)2S2O3, 0.03M CuSO4 and 3M NH3 at pH 10.2 for 24 hours. In the testing, thiosulfate consumption was 30 kg/t of ore. Dissolved oxygen and copper species were found to be responsible for the rapid decomposition of thiosulfate. The thiosulfate consumption could be reduced to around 17 kg/t by the addition of an equivalent amount of a chelating agent, by reducing the copper sulfate addition to 0.001M or by replacing CuSO4 byNiSO4. An increase in pulp density from 28.6% to 61.5% also reduced the thiosulfate consumption to around 17 kg/t. Thiosulfate consumption couldbefurther reduced to 10 kg/t by regulating the dissolved oxygen concentration.