Kinetic aspects of gold and silver leaching in ammonia-thiosulfate solutions

The leaching of gold and silver in thiosulfate solutions containing copper and ammonia was studied using a rotating electrochemical quartz crystal microbalance (REQCM). The REQCM allows the measurement of mass changes in situ, and thus, the leaching rate can be readily obtained for a wide range of solution concentrations. It was found that silver leaching occurs quite rapidly, with the rate being limited by cupric tetrammine diffusion. However, the leaching of gold was found to be more complex, with the reaction being hindered by the presence of a transient film. At steady state, it will be shown that the reaction is dependent on the relative concentrations of thiosulfate, ammonia and copper, and under most experimental conditions, the reaction is chemically controlled. (C) 2001 Elsevier Science B.V. All rights reserved.