Towards continuous selective electrowinning of gold from waste printed circuit boards in acidic chloride medium: a parametric study

Fast technological progress in the electronics industry leads to an ever-growing generation of electronic waste or e-waste. Proper recycling of e-waste is, therefore, of great importance, not only for ecological reasons but also to valorize its material content. With gold being the most valuable metal in electronic waste, it is the main driver for recycling, even though it is only present in minor concentrations compared to base metals like copper and iron. This study presents a continuous flow module for potential-controlled electrowinning of gold out of AuCl₄⁻ complexes from a multi-metal solution containing nickel, copper, iron, and gold ions, as a representative solution of a typical Printed Circuit Board leachate after a pre-washing and gold leaching step. Concentrations as low as 0.1 mM of AuCl₄⁻ were selectively reduced to metallic gold on a glassy carbon plate in a continuous flow cell for 4 h with a maximum recovery of up to 86%. Both electrochemical and engineering parameters were investigated, such as the impact of the flow rate, potential, presence of competing reactions from other ions in the solution, and the addition of turbulence promoters in an attempt to optimize the electrowinning. An increase in recovery was observed by raising the flow rate, whereas the potential showed an optimum at + 0.4 V vs Ag/AgCl, with lower potentials leading to more side reactions and lower current efficiencies. Addition of turbulence promoters mainly played a role in the surface utilization and crystal growth, leading to similar overall recoveries but with a more homogeneous deposit and lower variation. With this technique, it is demonstrated that selective gold electrowinning from small concentrations in the presence of base metals, such as copper, nickel, or iron, is feasible in an efficient manner.