Features affecting the cupric chloride leaching of gold

The current study investigated gold dissolution in cupric chloride solution, which is one of the most promising alternatives to replace cyanide, although not yet in operation on industrial scale. In this paper, the gold dissolution reaction rate and mechanism were studied by varying the process variables of temperature (28-95 degrees C), cupric concentration (0.02-1.0 M), chloride concentration (1-5 M), rotational speed (100-2500 RPM), and pH (0.5-2.0). The parameters affecting either the anodic or cathodic reaction, or both, were identified for the first time to the best of the authors' knowledge in this wide range and with these parameter intervals. Tafel and linear polarization methods as well as cyclic voltammetry were used for pure gold with both rotating disk and stationary gold electrodes. An increase in the gold dissolution rate was observed with an increase in temperature, chloride concentration, and rotational speed. Additionally, an increase in cupric concentration ([Cu2+] = 0.02-0.75 M) promoted the gold dissolution rate, whereas the gold dissolution rate decreased with [Cu2+] from 0.75 to 1.0 M. The conditions for maximizing the gold dissolution rate in cupric chloride solution were concluded to be T > 55 degrees C, [Cu2+] = 0.5-0/5 M, [Cl-] = 5 M, and pH = 1.0 and the highest gold dissolution rate (2.9.10(-4) mol M-2 s(-1)) was achieved at 95 degrees C with [Cu2+] = 0.5 M, [Cl-] = 5 M, pH = 1.0, and omega(cyc), = 2500 RPM. The pH was shown not to affect the gold dissolution rate at all, but only to affect the solubility of the oxidant. It was suggested that gold dissolved as aurous species in the conditions of this study, although the increase in chloride concentration promoted the dissolution of gold as both, auric and aurous, species. The reaction mechanism was interpreted using mixed potential theory. An increase in temperature was shown to promote only the cathodic reduction of cupric ion to cuprous at lower temperatures (28-55 degrees C); however, both the anodic gold dissolution reaction and cathodic cupric reduction reaction were enhanced at higher temperatures (65-95 degrees C). The cathodic reaction was also enhanced with an increase in cupric concentration (0.02-0.5 M), whereas the anodic reaction was promoted when the cupric concentration was increased from 0.5 to 0.75 M. When the cupric concentration was increased from 0.75 to 1.0 M, the cathodic reaction rate decreased. However, the reason for the decrease in the cathodic reaction rate was not clear. An increase in chloride concentration enhanced the cathodic reaction in the investigated range (1-5 M), whereas an increase in rotational speed (i.e., improved mass transfer) increased the anodic gold dissolution rate, specifically at low rotational speeds.