Combined Microwave Assisted Roasting and Leaching to Recover Platinum Group Metals from Spent Automotive Catalysts

A microwave assisted two-step platinum group metal (PGM) extraction process for spent automotive ceramic catalysts was developed. The first step consists of a microwave (MW) sulfation roasting process of spent catalyst in the presence of NaHSO₄∙H₂O or KHSO₄ and NaClO₃, for which the roasting parameters (i.e. MW power, roasting time, ClO₃^-:HSO₄^- molar ratio, salt:catalyst weight ratio) were studied and optimized. During roasting a solid salt mixture and spent catalyst (weight ratio salt:spent catalyst = 5, molar ratio NaClO₃:NaHSO₄·H₂O = 0.05) was volumetric heated (750 W, 30 min) through absorption of MWs without the use of any other heat source. Whereas for the subsequent step, a short (30 min, 105 °C) MW acidic leaching process in 1 M HCl solution at a liquid to salt ratio of 10 was evaluated to give the best PGM leachabilities (i.e. Pd 96 ±1%, Pt 85 ±5% and Rh >96%). An in depth experimental study of the roasting process showed that sulfation roasting attacks both the cordierite (Mg₂Al₄Si₅O₁₈) honeycomb material and rare earth oxides of the wash coat, to form sulfate salts. During roasting metallic Pd oxidized. Mass loss during roasting was ascribed to evaporation of NaHSO₄∙H₂O hydration water and subsequent decomposition of NaHSO₄.