Efficient HCl leaching of platinum group metals from waste three-way catalysts: A study on kinetics and mechanisms

Waste three-way catalysts (TWCs) have attracted much attention due to the presence of platinum group metals (PGMs) and hazardous substances such as heavy metals and organic matter. The extraction of PGMs from waste TWCs using hydrochloric acid (HCl) has been extensively researched. However, the addition of oxidizing agents like H₂O₂ and aqua regia is necessary to facilitate PGMs dissolution, which poses significant environmental and operational hazards. Hence, developing a green PGMs recovery process without oxidants is imperative. Previously, we investigated the process of Li₂CO₃ calcination pretreatment to enhance the leaching of PGMs from waste TWCs by HCl, focusing on the process and mechanism of Li₂CO₃ calcination pretreatment. In this study, we focused on the leaching process of HCl after pretreatment. Our investigation includes a detailed examination of leaching kinetics and mechanisms. The optimal leaching conditions were: leaching temperature of 150 °C, leaching time of 2 h, HCl concentration of 12 M, and liquid-solid ratio of 10 mL/g. The experiments resulted in maximum leaching rates of about 96%, 97%, and 97% for Pt, Pd, and Rh, respectively. However, given the presence of heavy metals, attention needs to be paid to the harmless treatment of waste acids and leaching residues. The Pt and Pd leaching process is controlled by a mixture of interfacial chemical reactions and internal diffusion, and dominated by internal diffusion, while the leaching process of Rh is controlled by interfacial chemical reactions. Li⁺ in Li₂PtO₃, Li₂PdO₂, and Li₂RhO₃ preferentially leached and underwent ion-exchange reactions with H⁺, promoting the dissolution of Pt, Pd, and Rh in HCl.