Gold dissolution and activation in cyanide solution: kinetics and mechanism

The rate of gold dissolution was measured in clear solution using a gold rotating disk electrode. The data, obtained at various cyanide concentrations, are in agreement with literature observations. The dissolution rates were independent of electrode rotation speed for air-saturated solutions and cyanide concentrations above 5 mol m(-3) and were well below external mass transfer-limited rates for cyanide and oxygen. An activator molecule, NMI, was shown to increase the rate of dissolution and served to illustrate the importance of surface reactions in the dissolution mechanism. With increasing activator concentration, the dissolution rate first increased to a maximum value and then decreased. The optimum activator concentration at the maximum dissolution rate was found to be a function of the cyanide level. The results suggest competitive adsorption between the activator and cyanide species. A dissolution mechanism in which the active site contains two (or more) gold atoms is proposed. Dissolution kinetics are controlled by reactions on the crystalline gold surface in regions independent of potential and identified by measured potentials when both cyanide and oxygen are present. The data are explained by crystalline surface mass transfer away from the active surface site followed by charge transfer and dissolution. Model predictions are in reasonable agreement with observations. The proposed mechanism is based solely on kinetics measurements and calls attention to the need for further mechanistic research. (C) 2000 Elsevier Science B.V. All rights reserved.