Hydrometallurgical Recovery of Rare Earth Elements from Metallurgical Slags (2020–2026): A Critical Review

Abstract : Metallurgical slags generated from ironmaking, steelmaking, ferroalloy production, and molten salt electrolysis are increasingly recognized as secondary resources for critical raw materials, particularly rare earth elements (REEs) such as scandium, yttrium, and light REEs, which are incorporated into complex silicate, aluminate, and fluoride phases formed at high temperatures. This review critically evaluates hydrometallurgical routes for REE recovery across a wide range of slag systems, including blast furnace, basic oxygen furnace, electric arc furnace, bauxite residue–derived, FCC catalyst, and molten salt electrolytic slags, covering direct leaching approaches (acidic, alkaline, and ammoniacal), hybrid roast–leach processes (sulfation, chlorination, and alkali roasting), and downstream separation techniques such as selective precipitation and solvent extraction. Particular emphasis is placed on the role of slag mineralogy, phase assemblage, and glassy matrices in controlling leaching kinetics, selectivity, and impurity co-dissolution, with silicate-rich slags identified as the most challenging systems due to their polymerized structure, which limits reagent accessibility and often requires thermal pretreatment to achieve recoveries above 80–90%, typically at high reagent consumption (>50–300 kg/t). Comparative evaluation reveals that reported performance is frequently dominated by recovery metrics, while key parameters such as selectivity, reagent intensity, and process integration remain underreported, such that high extraction efficiencies do not necessarily translate into industrial feasibility. The main limitations across existing approaches include silica gel formation, extensive co-dissolution of matrix elements, and the generation of secondary residues, all of which negatively impact process stability and economic viability; moreover, most reported systems remain constrained by poor selectivity, high reagent intensity, and lack of continuous pilot-scale validation, limiting their industrial transferability. Future progress, therefore, depends on shifting from isolated process optimization to integrated, mineralogy-driven process design, supported by reduced reagent consumption, simplified separation flowsheets, and validation under industrially relevant conditions, positioning metallurgical slags as strategic secondary resources capable of supporting diversified and resilient REE supply chains within circular economy systems.