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.