The cyanidation of silver metal: Review of kinetics and reaction mechanism

Published rate data are analysed for the chemical and electrochemical dissolution of silver metal from rotating discs in aerated/oxygenated cyanide solutions at ≈25 °C, pH 11 and different partial pressures of oxygen. The current status of the reaction mechanism is also reviewed. Speciation analysis of 0.01 mM silver(I) in 1–100 mM cyanide solutions shows that Ag(CN)2− is the predominant complex (50%) at cyanide concentrations < 20 mM. However, at higher cyanide concentrations, Ag(CN)32− (up to 60%) and Ag(CN)43− (up to 10%) can be formed. Thus, it is important to consider a silver(I) : cyanide ion ratio of 2 or 3 in the Levich equation to calculate the diffusion coefficient of cyanide ion. Likewise, it is important to consider a silver(I) : oxygen ratio of 1 : 0.5 to calculate the diffusion coefficient of oxygen. This indicates the reduction of oxygen to hydrogen peroxide in the surface reaction. Analysis of exchange current density data for silver oxidation as a function of cyanide concentration shows the involvement of between 1 and 2 cyanide ions in the surface reaction. The limiting rate of silver dissolution at high cyanide concentrations (2.5 × 10− 5 mol m− 2 s− 1 at ≈21 kPa oxygen pressure) represents the maximum surface coverage by cyanide. This value is in close agreement with the rate constant of the surface reaction 4 × 10− 5 mol m− 2 s− 1 based on the pure kinetic current of the mixed “charge transfer plus diffusion” model proposed by Li and Wadsworth [Li, J., Wadsworth, M.E., 1993. Electrochemical study of silver dissolution in cyanide solutions. J. Electrochem. Soc. 140, 1921–1927].