A Comparative Evaluation of Leaching Reagents of Platinum Group Metals from Spent Catalytic Converters Using Microwave Heating

Platinum group metals (PGMs) are increasingly recycled from spent automotive catalysts due to their scarcity in nature. Many studies were developed using different leaching reagents to achieve greener benefits and reduce environmental pollution. This article reports for the first time a direct comparison of leaching reagents on the efficiency of PGMs using microwave (MW) assisted heating. The leaching system employed HCl with/without the addition of complexing (citric acid) or oxidizing agents (HNO₃, H₂O₂, NaClO, NaClO₃). It was shown that the 11.65 M HCl could leach all elements (94 % Pt, 82 % Pd and 74 % Rh) at 210 °C. The addition of oxidant(s) decreased the PGMs leaching efficiencies due to the formation of a protective oxide layer through surface passivation reaction. In some cases, the addition of complexing or oxidizing (H₂O₂) agents and a high leaching temperature (>180 °C) employed in the MW leaching could enhance PGMs leachabilities, particularly for Rh (>90 %). A detailed temperature study of the MW leaching demonstrated that HCl could obtain excellent recoveries (>90 %) for Pt and Pd at temperatures ≥150 °C. Therefore, this research illustrates that partially replacing the HCl with citric acid is a promising approach to achieve a greener and more efficient leaching process of PGMs.