Thiosulfate leaching kinetics of gold in the presence of copper and ammonia

In recent years a significant amount of research has focused on thiosulfate as an alternative lixiviant for gold. However few fundamental studies have been undertaken due to the complexity of the solution chemistry. This study investigates the effect of various parameters on the gold leaching kinetics and the undesirable homogeneous copper(II) reduction by thiosulfate. The leach kinetics are measured at essentially constant solution conditions using a rotating electrochemical quartz crystal microbalance, which measures mass changes in real time and hence leach rates within minutes. The gold leaching kinetics were found to be chemically controlled and the rate was found to decrease with time as the copper(H) concentration decreased. This can be attributed to passivation of the gold surface by the products of the copper(II)-thiosulfate reaction. Optimum leach conditions derived from this study are 0.4M ammonia, 0.1M thiosulfate, pH > 11.4, and temperature at 30°C.