Recovery of Pt, Pd, and Rh from spent automotive catalysts through combined chloride leaching and ion exchange: A review

Three platinum group metals (PGMs), platinum (Pt), palladium (Pd), and rhodium (Rh), are key components in automotive catalytic convertors, playing a pivotal role in controlling harmful emissions. The recycling and recovery of Pt, Pd, and Rh from spent automotive catalysts (SACs) have gained increasing attention as essential measures to mitigate resource depletion, supply risks, and environmental impacts. Due to the growing demand for automobiles and increasingly stricter environmental regulations, a substantial amount of spent automotive catalysts is generated annually, leading to increased interest in their efficient recycling and recovery of the PGMs they contain. Hydrometallurgical processes, particularly chloride leaching and ion exchange, have emerged as promising methods for efficient PGM extraction and separation from these discarded catalysts. This review includes a critical examination of recent advances and innovations in both chloride leaching and ion exchange methods, highlighting their effectiveness in terms of Pt, Pd, and Rh recyclability and recovery efficiency from spent catalysts. The study offers valuable insights into the efficacy of their recycling from SACs through various processes. The importance of investigating the solution chemistry of PGMs in chloride media is highlighted and the leaching of SACs has been explored using various chloride media, including AlCl3, NaCl, CaCl2, MgCl, and NH4Cl, alongside a range of inorganic and organic leaching agents such as HCl, H2SO4, HNO3, acetic acid, citric acid, and oxidizing agents like H2O2, NaClO, NaClO3, Fe3+, Cl-2, and Cu2+. This work is critically reviewed, examining the influence of key parameters investigated on the leaching efficiency of PGMs, such as HCl, Cl-, and oxidizing concentrations, temperature, solid-to-liquid ratio (S/L), particle size, and leaching time. Furthermore, it evaluates the effectiveness of pretreatment techniques such as calcination, salt roasting, and pre-reduction methods involving high temperatures, hydrogen gas flow, formic acid, hydrazine hydrate, and Zn-vapor treatments. The review then turns to the efficacy of the ion exchange method, utilizing a diverse range of anion exchange resins for the selective adsorption of PGMs as well as various elution reagents for the selective desorption of PGMs from loaded resins, aiming to recover them selectively from chloride leach solutions. Therefore, this study seeks to contribute to the development of strategies for recycling and reusing PGMs from SACs, with a view to reducing the industry's dependence on primary raw materials and promoting principles of the circular economy.