Kinetics and mechanisms of gold dissolution by ferric chloride leaching

Gold dissolution was investigated in ferric chloride solution, being one alternative cyanide-free leaching media of increasing interest. The effect of process variables ([Fe3+] = 0.02-1.0 M, [Cl−] = 2-5 M, pH = 0-1.0, T = 25-95 °C) on reaction mechanism and kinetics were studied electrochemically using rotating disk electrode with ωcyc = 100-2500 RPM and Tafel method. The highest gold dissolution rate (7.3 · 10−4 mol m−2 s−1) was achieved at 95 °C with [Fe3+] = 0.5 M, [Cl−] = 4 M, pH =1.0 and ωcyc = 2500 RPM. Increase in gold dissolution rate was observed with increase in temperature, ferric ion concentration and chloride concentration, but gold dissolution rate did not have a clear dependency on pH. Redox potential was found to vary between 636 and 741 mV vs. SCE during experiments. According to the calculated equilibrium and measured open circuit potentials, gold was suggested to dissolve as aurous ion Au+ and form AuCl2−, rather than auric ion Au3+ and form AuCl4−. Further, it is suggested that AuCl2− does not oxidize to AuCl4− under the investigated conditions. Levich plot and the calculated activation energies suggested that gold dissolution was limited by mass and electron transfer. According to a mechanistic kinetic model developed in the current work, intrinsic surface reaction mainly controls gold dissolution, especially at higher rotational speeds (>1000 RPM). Uncertainties in the model parameters of the mechanistic kinetic model were studied with Markov chain Monte Carlo methods.