Electrochemical studies of gold dissolution in cupric and ferric chloride solutions

Currently, gold is produced predominantly by the cyanide leaching process. However, there is a demand for alternative methods to replace cyanidation, mainly due to environmental concerns. Chloride leaching has been suggested as one of the most promising cyanide-free processes due to its high leaching rates, high metal solubility, and the possibility to dissolve refractory minerals without pre-treatment. The phenomena of gold dissolution in cupric and ferric chloride leaching are not fully understood, and there is still no commercialized gold chloride leaching process operating on industrial scale. The three main objectives of the thesis were as follows: i) to investigate the electrochemical behavior of gold dissolution in cupric and ferric chloride solutions, ii) to determine the dependency between the redox potential, open circuit potential (OCP), and gold dissolution rate in order to build a tool to estimate gold dissolution in cupric and ferric chloride solutions, and iii) to verify the gold dissolution rate determined by electrochemical experiments using an industrial raw material in cupric chloride solution. The electrochemical test methods included linear polarization resistance, polarization (Tafel method), and cyclic voltammetry measurements. Additionally, quartz crystal microbalance measurements were performed to define the dissolution rate. It is suggested that gold dissolves through direct oxidation by metallic chloride complexes to aurous gold, i.e., AuCl2–, in the investigated cupric (T = 28–95 °C, [Cu2+] = 0.02–1.0 M, [Cl–] = 1.0–5.0 M, pH = 0.0–2.0, and ω = 100–2500 RPM), and ferric (T = 25–95 °C, [Fe3+] = 0.02–1.0 M, [Cl–] = 2.0–5.0 M, pH = 0.0–1.0, and ω = 100–2500 RPM) chloride solutions. It is posited that AuCl2– species do not oxidize to AuCl4– species; however, the formation of AuCl4– through a disproportionation reaction is possible. An increase in temperature, oxidant, and chloride concentration, as well as rotational speed (i.e., mass transfer) promoted the gold dissolution rate, but the pH did not show any clear effect on cupric and ferric chloride leaching in the investigated conditions. Ferric chloride leaching of gold resulted in higher redox potentials, OCPs, and dissolution rates than cupric chloride leaching. It was found that the redox potential could be used to estimate the gold dissolution rate, since it had a positive linear correlation with the logarithm of the gold dissolution rate. Additionally, the redox potential could be defined from the values of the process variables. However, it was found that the OCP could not be used as a single parameter to estimate the gold dissolution rate, as the correlation between the OCP and the logarithm of the gold dissolution rate was insignificant. Batch leaching experiments with a gold-containing tailing verified the results of the electrochemical experiments in cupric chloride solutions. The effect of process variables on the gold dissolution rate was clearer in the electrochemical tests; however, the same trends of parameters and their impacts were also visible in the batch leaching tests.