Platinum Group Metal Recycling from Spent Automotive Catalysts Using Reusable Hydrophobic Deep Eutectic Solvent

Abstract

This report describes a green recycling process for spent automotive catalysts (SACs), which contain trace amounts of platinum group metals (PGMs), as well as Al and Mg. Traditional hydrometallurgical recycling involves the leaching of SACs with concentrated inorganic acids, which involves the dissolution of unwanted metals and produces large volumes of wastewater. Herein, we propose a non-aqueous direct leaching process using hydrophobic deep eutectic solvents (DESs). A hydrophobic DES comprising trihexyl(tetradecyl)phosphonium chloride and decanoic acid was a particularly efficient and selective leaching medium for PGMs. This hydrophobic DES, combined with an organic acid and an oxidant additive, enabled efficient leaching of PGMs from a SAC, while minimizing the leaching of unwanted metals, such as Al and Mg. Under optimized conditions (24 h, 80 °C, stirring at 400 rpm), the Pt, Pd, and Rh leaching efficiencies reached 90%, 100%, and 89%, respectively, while the leaching of other metals was limited to less than 5% each. Individual PGMs were isolated from the hydrophobic DES using simple procedures, i.e., through contact with various aqueous solutions. The recovery rate of each metal to the aqueous phase was approximately 80% for Rh and Pt and 90% for Pd. Furthermore, the hydrophobic DES could be regenerated and reused for further SAC leaching. The leaching rate of PGMs with regenerated DES achieved values equivalent to those obtained with DES before regeneration. The system proposed herein provides valuable insights for the development of sustainable PGM recycling processes from SACs.