The characteristics of gold open circuit potential in ferric chloride leaching

Gold dissolution was investigated in ferric chloride media, promoted as one of the most promising alternatives for cyanide-free gold leaching. The characteristics of gold open circuit potential (OCP) and its relation to gold dissolution rate was investigated. Furthermore, the reaction mechanism of gold dissolution in ferric chloride leaching was discussed. The effect of solution variables, [Fe³⁺] = 0.02-1.0 M, [Cl^-] = 2-5 M, T = 25-95 °C and pH = 0-1.0, on OCP and dissolution rate were studied electrochemically using rotating disk electrode (RDE) and linear polarization resistance (LPR) method with ω_cyc = 1000 RPM. The investigated gold OCPs in ferric chloride solutions were found to vary between 597 and 688 mV vs. saturated calomel electrode (SCE) and gold dissolution rates between 1.2·10^-5-5.0·10^-4 mol s^-1 m^-2. Increase in temperature (25-55 °C) and ferric concentration (0.02-1.0 M) changed OCP into more positive values, while increase in chloride concentration (2-5 M) decreased it. The logarithm of gold dissolution rate was shown to increase linearly with increasing OCP at the temperature range 25-95 °C, ferric concentration range 0.02-0.5 M and chloride concentration range 2-4 M. A notable increase in the gold dissolution reaction rate was evident at T > 55 °C.