Surface processes in silver and gold cyanidation

Rate processes for silver and gold cyanidation are controlled by a series o f coupled electrochemical reactions. These include anodic metal dissolution and oxygen reduction. This study demonstrates basic differences between si lver and gold dissolution both in reference to the metal and the accompanyi ng oxygen discharge. The kinetics of oxygen reduction are compared for silv er and gold. At a fixed voltage, the cathodic current for gold is less than that of silver except at large negative potentials where the kinetics are oxygen diffusion-limited. A mathematical model is presented for oxygen redu ction based upon two paths: one path with peroxide formation followed by al ternate catalytic decomposition and desorption, and a second path with cont inued peroxide reduction to hydroxyl ions. The model permits separate calcu lations of the first and second waves of oxygen reduction. Based upon mixed potential models, silver dissolution is shown to be controlled by both the first and second waves of oxygen reduction with the number of electrons ap proaching four for high cyanide concentrations. Gold dissolution is control led by surface reactions with mixed potentials associated mainly with the f irst wave of oxygen control and limited to two-electron transfer. The model permits an evaluation of the number of electrons which may be transferred for a given potential and shows this number is voltage-dependent. Silver di ssolution is shown to include both diffusion or mixed diffusion plus charge transfer kinetics, influenced by the degree of agitation and both oxygen a nd cyanide concentration-dependent. Gold dissolution kinetics are controlle d by crystal-dissolution overpotentials and are independent of agitation, M odels of oxygen discharge and metal dissolution are used to explain observe d characteristics and kinetics of silver and gold leaching. (C) 2000 Elsevi er Science B.V. All rights reserved.