Gold dissolution and activation in cyanide solution: kinetics and mechanism

The rate of gold dissolution was measured in clear solution using a gold ro tating 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 cy anide concentrations above 5 mol m(-3) and were well below external mass tr ansfer-limited rates for cyanide and oxygen. An activator molecule, NMI, wa s shown to increase the rate of dissolution and served to illustrate the im portance 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 maxim um dissolution rate was found to be a function of the cyanide level. The re sults suggest competitive adsorption between the activator and cyanide spec ies. A dissolution mechanism in which the active site contains two (or more) gol d atoms is proposed. Dissolution kinetics are controlled by reactions on th e crystalline gold surface in regions independent of potential and identifi ed by measured potentials when both cyanide and oxygen are present. The dat a are explained by crystalline surface mass transfer away from the active s urface site followed by charge transfer and dissolution. Model predictions are in reasonable agreement with observations. The proposed mechanism is ba sed solely on kinetics measurements and calls attention to the need for fur ther mechanistic research. (C) 2000 Elsevier Science B.V. All rights reserv ed.