A Review on the Recent Developments in the Dissolution of Rhodium and Iridium Metals from Spent Catalysts.

Rhodium and iridium, both belonging to the platinum group metals, are critical metals due to their scarcity and broad industrial applications, particularly in automotive catalysts. With increasing global demand for PGMs and the depletion of primary resources, recycling spent catalysts has become essential for sustainable resource management and reducing environmental impact. This review focuses on recent advancements in the dissolution of Rh and Ir from spent catalysts, highlighting various hydrometallurgical leaching methods. Aqua regia, while commonly used, shows limited efficiency for both Rh and Ir due to their chemical inertness. More advanced techniques such as HCl-based systems with oxidizing agents like NaClO or NaClO3, microwave-assisted leaching, and dry aqua regia have demonstrated improved dissolution efficiency. There is some difference in the dissolution behavior between Rh and Ir metal by the reported leaching systems. Owing to the stronger chemical inertness of iridium metal, its dissolution requires different approaches from the dissolution of Rh. Microwave-assisted leaching offers superior Rh recovery rates, while Ir dissolution benefits from dry aqua regia, which reduces energy consumption by generating chlorine in situ at lower temperatures. Additionally, alloy fragmentation has proven effective for Ir dissolution by increasing surface area and enhancing chemical reactivity. The environmental and operational advantages of dry aqua regia are highlighted, presenting them as viable options for industrial recycling. This review would contribute to the development of sustainable processes for the recovery of these critical metals from secondary resources. [ABSTRACT FROM AUTHOR]