Securing a Scarce Metal: Hydrometallurgical Strategies for Rhodium Recovery

Rhodium is a scarce platinum-group metal of considerable economic and technological importance, primarily owing to its use in automotive emission-control catalysts and chemical processes. Its low natural abundance, uneven distribution in primary deposits, and dependence on complex production and refining routes make its supply particularly sensitive to changes in demand and market conditions. In this context, both the efficient processing of primary resources and the recovery of rhodium from secondary materials are important for improving resource efficiency and diversifying supply. This review examines the occurrence and concentration of rhodium in natural resources, its primary production, and the hydrometallurgical approaches applied to rhodium-bearing secondary materials, with particular emphasis on spent automotive catalysts. The composition of these materials and its implications for processing are discussed alongside the chemical reactions involved in rhodium dissolution, the influence of solution chemistry on rhodium speciation, and the behaviour of accompanying platinum-group and non-noble metals throughout the process. Particular attention is given to leaching and separation techniques like precipitation of chemical compounds or metal, solvent extraction, ion exchange, adsorption and membrane-based separation. Proposed process flowsheets are presented and compared to illustrate potential technological approaches to rhodium recovery. The review identifies key challenges in integrating individual separation methods into complete recovery processes and highlights the need to validate proposed flowsheets experimentally using real feed materials, with particular emphasis on achieving high overall rhodium yields and product purity while ensuring economic and environmental viability.