Secondary resources and advanced separation technologies for critical metal recovery: progress and challenges

Critical metals (CMs) are indispensable to advanced manufacturing, underpinning catalytic, metallurgical, electrical, magnetic, and luminescent functions. This review provides a comprehensive assessment of CM characterization in secondary resources and evaluates recent advances in recovery technologies. Its objectives are to: (i) analyse CM distribution across diverse secondary resources, (ii) summarize progress in state-of-the-art recovery methods, (iii) examine major challenges to effective recovery, and (iv) identify research gaps and future directions. The results show that metallurgical waste, spent catalysts, spent batteries, and electroplating waste contain the highest CM concentrations, up to 10 – 60 % Co and Ni, and 5–30 % rare earth elements such as Nd, Dy, and Pr. Among recovery technologies, green solvents show the strongest potential, achieving up to 100 % of Ni, Co, and Li recovery under optimized conditions. Flotation and nanotechnology perform strongly for Ni, Co, and Li (80 – 100 %), whereas membranes show inconsistent and lower median recoveries, indicating limited efficiency. Current development is increasingly focused on selectivity, sustainability, and scalability to address complex waste matrices. Significant potential exists for the advancement of hybrid systems designed to optimize process efficiency, including bioleaching, solvent extraction, membranes, and adsorbents with 99 % recovery efficiency. Integrating Artificial Intelligence and Machine Learning improves the extraction of rare earth elements to 98.6 %. Furthermore, extraction techniques can be integrated with electrochemical systems to significantly reduce the generation of liquid waste. Integrating CM recovery into circular economy strategies and industrial symbiosis is strongly recommended to maximize efficiency and sustainability.