An electrochemical study of gold cementation with zinc powder at low cyanide concentration in alkaline solutions

Abstract

The half-reactions involved in gold cementation on zinc powder from low concentration alkaline cyanide solutions were studied in a steady state regime, employing electrodes of glassy carbon and graphite paste with zinc powder. The effects of pH, cyanide and initial gold concentrations were investigated using various electrochemical techniques. The results were used to determine the controlling step of the cementation at low cyanide concentration and to interpret the influence of the variables on this process. Mixed potentials and associated currents were determined from the Evans' diagrams constructed using sampled current–potentials curves from chronoamperometric results for half redox reactions. These values do not adequately describe the global cementation reaction because gold reduction in low cyanide concentration solutions is greatly influenced by a strong contribution from adsorptive processes. This behaviour is different from that found by other authors for concentrated gold and cyanide solutions, where the process is controlled by the complex ion diffusion (Au(CN)2−). The strong component of adsorption found in the present work does not permit the determination, using the Evans' diagrams, of the cementation velocity. Direct monitoring of the mixed potential was proposed, employing an electrode made of zinc powder in carbon paste submerged in a gold cyanide solution. The results of these experiments at low cyanide concentrations show that zinc oxidation is controlled by the formation of different passivating layers, the nature of which depended on the pH of the solution, and that the gold reduction reaction is strongly influenced by adsorption phenomena.