Kinetic limitations of gold leaching in ferric chloride media Part III: Comparison of kinetics in batch leaching and electrochemical studies

Extensive research efforts have been devoted to developing a technology capable of replacing cyanidation for gold extraction. Sponsored by Barrick Gold Corporation, we investigated the kinetic limitations of using ferric chloride for gold leaching under conditions relevant to heap leaching. The batch reactor studies (Part I) found that gold leaching plateaued in all leaching tests performed at 35 degrees C. In the subsequent electrochemical studies (Part II), it was found that the formation and adsorption of AuClads during leaching passivated the surface hindering gold dissolution. In Part III of this study, we compared the leaching kinetics obtained by the batch reactor and the electrochemical studies under the same conditions. This analysis helps understand the similarities and differences in the leaching kinetics and mechanisms obtained with the two techniques. A kinetic equation with surface passivation was derived that relates the batch leaching of gold to the concentrations of total ferric and total chloride. This model supported that the accumulation of adsorbed species, AuClads, led to the plateauing of the leaching curves. The reaction order with respect to the total ferric concentration approximated to the theoretical value of 0.5 for both techniques. However, the reaction order with respect to the total chloride concentration could vary between one and two, with one indicating the electrochemical step being rate-limiting and two indicating the solubilization step restricting the leaching process.