Electrodeposition of a Silver-Gold Alloy (DORÉ) from Thiourea Solutions in the Presence of Other Metallic Ion Impurities

With the increasing complexity of gold and silver containing minerals, the thiourea system has been considered a possible alternative for precious metal leaching. In the present work, direct electrodeposition is proposed as a viable method to recover gold and silver values from thiourea leaching solutions. With this purpose in mind, thermodynamic and electrochemical analyses of this complex system were undertaken. The thermodynamic reduction potentials of copper and lead ions were shown to be very close to those of gold and silver, favoring the formation of codeposits of these metals.In order to complement the thermodynamic analysis, kinetic studies were performed using potentiostatic electrodeposition with anodic stripping. A vitreous carbon electrode was shown to be sufficiently inert over the entire potential range studied to allow its use for the investigation of the metal deposits formed. Regarding these deposits, it was demonstrated that the presence of a small amount of gold (10 ppm), in solutions containing silver, favorably affects the energy and kinetics of the gold-silver alloy (doré) deposition process. In this same manner, it was also shown that the presence of lead(II) (10 ppm), zinc(II) (10 ppm) or iron(II) (100 ppm) favors the alloy deposition by shifting the initial reduction potential to less negative values while maintaining the same deposition rate. On the other hand, the addition of 200 ppm of copper (II) provokes a slight decrease in the reduction rate, although the potential at which the deposition begins is practically the same as that observed for solutions of only gold and silver.