A Study on a Two-Step Leaching Process of Electronic Waste for the Recovery of Precious Metals

Abstract

The decrease of natural ore on earth over time has attracted much attention to seek for alternatives to overcome shortage of the limited non-renewable natural resources. Hydrometallurgical methods have become a widespread method for the separation and recovery of metals from electronic waste in support of a sustainable society and a circular economy. However, the heavy consumption of solvents, discharge of toxic wastewater and corrosion during the process have highlighted the need for an environmental-friendly leaching process. Recently, volatile fatty acids (VFAs) produced from organic waste anaerobic digestion have gained attention as a viable leaching solvent because of their biodegradability, reduced toxicity, and low corrosiveness. In this study, a two-step leaching of light-emitting diodes (LEDs) material was studied by sequential application of both volatile fatty acid and thiourea solution respectively. A copper foil was subjected as preliminary investigation of copper (Cu) leaching by using inorganic ligands (sodium nitrate), conventional organic ligands (citric acid), and volatile fatty acids (acetic acid, propionic acid, butyric acid and valeric acid) in the presence of hydrogen peroxide as an oxidizing agent. It was found that the acetic acid from volatile fatty acids displayed highest Cu leaching efficiency at pH 4. The acetic acid was selected as the best solvent for subsequent studies of Cu leaching from LEDs. Furthermore, the kinetic study was performed using shrinking core model and showed that product layer diffusion is the rate-controlling step for Cu leaching from LEDs. Finally, the residues from base-metal leaching were subsequently dissolved into thiourea solution. This process offers two main advantages which secures a secondary supply of critical metals and a more environmentally friendly solvent for hydrometallurgical metal recovery using VFA from by organic waste.