Fundamental aspects of the gold thiosulfate leaching process

Many studies have been conducted over the past 3 decades on the leaching of gold in solutions containing thiosulfate, ammonia and copper. However, most of these studies have been carried out using ore samples, with very few fundamental studies of this system being attempted. The majority of the published studies have failed to acknowledge the undesirable side reaction between copper(II) and thiosulfate, let alone account for its effect. The fact that the solution chemistry is constantly changing makes it very difficult to perform a fundamental study of gold leaching. With the advent of the relating electrochemical quartz crystal microbalance (REQCM), such fundamental studies are possible as this instrument measures mass in real time. This has allowed leach kinetics to be measured in a matter of minutes and hence at essentially constant solution conditions. The present paper highlights that a sufficient concentration of copper(II) and ammonia are required in order to achieve appreciable gold leach rates. However, the use of air (oxygen) to maintain the copper(II) concentration catalyses the undesirable copper(II) reaction with thiosulfate. The presence of ammonium ions is shown to have an effect on this undesirable reaction as well as on the gold leaching reaction. Preliminary electrochemical studies into gold electrowinning using the REQCM show that gold(I) thiosulfate cannot be readily reduced to metallic gold in the potential region at which cementation usually occurs. However, it is well known that gold cementation onto copper occurs rapidly. The current study addresses this discrepancy by studying the effect that copper and silver have on the reduction of gold thiosulfate. It has been found that the presence of both silver and copper enhance gold deposition, particularly at low overpotentials. While this is beneficial in cementation, the presence of copper and silver in solution has been found to cause passivation of the cathode surface during electrowinning at higher overpotentials.