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.

The recovery of precious metals (Ag and Au) by electrodeposition-redox replacement (EDRR) method was studied in three different solutions: sulfate based Zn-Fe-Ag solution, chloride based Cu-Fe-Au solution and industrial multi-metal chloride solution. The common factor to these solutions is that the base (Zn or Cu) metal is present in g/L scale while the precious metal is present in ppm or lower level. The results showed that the presence of impurity Fe (in g/L scale) decreases the recovery efficiency of precious metals but at the same time, it may lead to selective dissolution of the base metal, resulting in the higher purity of the precious metal deposit. Moreover, a successful deposition of Au on carbon nanotube (CNT) films from industrial multi-metal solution was demonstrated.

It is becoming ever more clear that the natural resources – especially metals – are gradually being depleted from the Earth’s crust. Therefore, secondary sources such as industrial residues, waste ...