Selective electrochemical recovery of gold and silver from cyanide aqueous effluents using titanium and vitreous carbon cathodes

In this paper, a different approach for the electrochemical recovery of gold-rich alloys is presented. A filter-press-type electrochemical flow reactor was used and low adherent metal layers were obtained on highly polished vitreous carbon (VC) and titanium (Ti) flat cathodes. On VC, the metallic coating peeled off, being recovered as small flakes outside the reactor. On Ti, the metallic coating could be easily scraped off with a plastic spatula. Gold, silver and copper ions were recovered from real cyanide effluents in conditions such that reduction reactions were mass transfer controlled. Cyclic voltammetry was used for the identification of the potential range in which reduction of Au(I) was the main process in order to improve selectivity. Subsequently, controlled potential electrolysis was carried out at potentials from -1.1 to -1.6 V vs. the saturated calomel electrode (SCE) on VC and from -1.0 to -1.2 V vs. SCE on Ti cathode. Lower potentials favored greater selectivity and alloys containing 76% and 84% of gold were obtained on Ti and VC, respectively. Concentration decay profiles, as a function of potential and electrolysis time, presented pseudo-first-order kinetic and mass transfer coefficients for Au(I) in the order of magnitude of 10(-5) m s(-1) on both titanium and VC, in the region of potential in which electro-recovery presented greater selectivity for gold. Cathodic efficiencies were higher on the Ti cathode in which 23% was observed for a gold-rich alloy recovered at -1.0 V, and 15% when only gold is considered. (C) 2004 Elsevier B.V. All rights reserved.