Mechanism of selective gold extraction from multi-metal chloride solutions by electrodeposition-redox replacement

The electrodeposition-redox replacement (EDRR) process is a promising method for a selective extraction of minor metals from complex mixtures. When it is performed in a benign medium (such as sodium chloride solution or deep eutectic solvent), the EDRR method provides a non-toxic alternative for a conventional cyanide-based process. The detailed reaction mechanisms of the EDRR in Cu–Au systems, as well as the effect of the reaction medium are elucidated in this article. Electrogravimetric studies show that the EDRR process comprises three distinct stages: (1) deposition of Cu at a constant applied potential; (2) dissolution of deposited Cu at open circuit conditions in reaction with dissolved species in solution; (3) reduction of Au to elemental form in reaction with various Cu species. It is discovered that the recovery of Au takes place surprisingly via both the redox replacement between Cu and Au at the surface and the homogeneous Au reduction by Cu(I) species in solution. Both of these reaction pathways are facilitated by open circuit conditions (redox replacement step) between electrodeposition cycles and the utilization of other sacrificial elements in the solution is crucial. The use of aqueous chloride solution is advantageous over 1:2 ChCl:EG for the increased Au recovery (94.4%) and the purity of the product (93.7%), although it consumes slightly more electricity. Therefore, the EDRR enables energy and resource efficient selective extraction of Au from multi-metal industrial solutions even when it is present at low concentrations.