The dissolution of platinum-group metals (PGMs: Pt, Pd, and Rh) from spent automotive catalysts using aqua regia or HCl/Cl2 is significantly inhibited by the low solubility of ZrO2, CeO2, and PGM oxides (PtO2, PdO, and Rh2O3), as well as by the formation of solid solutions between these oxides and the coating layer. In this study, a NH4HSO4 molten salt treatment was investigated as a pretreatment to improve the recovery of PGMs. The molten salt reduced the PtO2 and PdO to metallic Pt and Pd, which are soluble in aqua regia, and decomposed the coating layer containing ZrO2, CeO2, and Rh2O3, thereby releasing the PGMs as ultrafine particles. These results demonstrate that the NH4HSO4 molten salt treatment enhances the recovery of PGMs from spent automotive catalysts.
Murai et al. (Thu,) studied this question.