Kinetic limitations of gold leaching in ferric chloride media Part II: Potentiodynamic polarization and electrochemical impedance spectroscopy

Abstract

The gold industry is in search of an alternative to cyanidation due to its inefficiency in treating challenging ores and increasing public scrutiny of its effects on human health and the environment. Chloride has been used in the past for gold extraction and has been identified as a promising candidate to replace cyanide. The Part I of this research found that gold extraction plateaued under all leaching conditions relevant to heap leaching. The hypothesis was that the formation and accumulation of an intermediate, AuClads, on the gold surface passivated the surface as leaching progressed. The Part II of this research tested this hypothesis using potentiodynamic polarization and electrochemical impedance spectroscopy. The potentiodynamic polarization results show that the kinetics of gold dissolution was restricted by the cathodic reduction of ferric species when the ferric concentration was below 0.3 M; the anodic oxidation of gold was the rate-limiting step when the free chloride activity was low. The gold dissolution was faster in the presence of the monovalent salts. The electrochemical impedance spectroscopy analysis revealed that the low chloride activity allowed the accumulation of the adsorbed species on the gold electrode surface, thereby restricting the charge transfer process and hindering gold dissolution.