Abstract:Critical metals are fundamental to the global energy transition and strategic emerging industries. Clay-type deposits, representing significant sources of rare earth elements (REEs), lithium, nickel, and cobalt, have garnered widespread attention due to their extensive distribution and potential for in-situ extraction. This review systematically examines three representative clay-type resources: ion-adsorption type REE deposits, lithium-rich clays, and lateritic nickel ores. It aims to synthesize the mineralization processes, occurrence mechanisms of the critical metals, and the corresponding separation and extraction technologies. The primary objective is to elucidate the intrinsic link between metal occurrence states and extraction efficiency, analyze common challenges in conventional processes, explore advanced intensification mechanisms, and ultimately provide a theoretical and technical foundation for the clean, efficient, and low-carbon utilization of these resources. The analysis is conducted by critically reviewing and synthesizing a broad body of scientific literature and technological reports. The methodological approach involves a systematic comparison of different extraction processes (e.g., (NH?)?SO? leaching, electric field-assisted extraction, high-pressure acid leaching, roasting-acid leaching) for the three clay-type deposits. The review delves into the micro-scale mechanisms governing extraction, focusing on interfacial complexation, the modulation of confined diffusion within clay structures, and the coupling of reaction-separation processes. Furthermore, it assesses the role of advanced characterization techniques (e.g., XAS, NMR, μ-XRF) and emerging digital twin technologies in understanding these mechanisms and optimizing processes. The occurrence state of critical metals (including ion-exchangeable, structurally bound, and adsorbed forms within clay minerals like kaolinite, halloysite, and montmorillonite) fundamentally dictates the selection and design of extraction strategies. For instance, REEs in ion-adsorption deposits are primarily recovered via ion exchange, whereas structural lithium and nickel often require aggressive methods like acid decomposition or roasting. Conventional processes, however, face persistent challenges such as poor selectivity, co-dissolution of impurities (e.g., Al, Fe, Si), and significant environmental burdens. Recent advances are addressing these limitations. Enhanced separation is achieved by: 1) Employing composite lixiviants (e.g., organic acid-inorganic salt mixtures) and bioleaching to strengthen interfacial complexation and improve selectivity. 2) Utilizing physical conditioning (e.g., granulation) and external fields (e.g., electric, ultrasonic, microwave) to overcome kinetic limitations imposed by confined diffusion within clay nano-/micro-pores. 3) Developing coupled process intensification strategies, such as integrating in-situ leaching with electrokinetic collection for REEs, and combining roasting with magnetic separation or pressure leaching for lateritic nickel ores, which significantly improve metal recovery, reduce reagent consumption, and minimize environmental impact. The integration of multi-scale in-situ characterization with process data and machine learning is emerging as a powerful pathway for building predictive models that can intelligently optimize extraction processes from mineral occurrence to separation behavior. The extraction and separation of critical metals from clay-type minerals are inherently controlled by their specific occurrence states. Future technological progress hinges on a deeper, multi-scale understanding of the underlying micro-mechanisms. Key research directions should include: 1) Applying advanced in-situ characterization techniques to unravel metal binding mechanisms and migration pathways at the atomic/molecular level. 2) Developing novel, highly selective, and reusable green lixiviants and functional materials. 3) Gaining a fundamental understanding of the coupling mechanisms of external fields (e.g., electric, microwave) with chemical reactions to create low-energy, short-flow processes. 4) Promoting the systematic integration of multi-source data to construct intelligent predictive models and digital twins for process optimization. Concurrently, the industry must prioritize a green and low-carbon transition across the entire value chain, focusing on the harmless treatment and valorization of tailings and wastewater, and establishing circular economy models that integrate “green mining-precise separation-waste utilization”. This holistic approach is essential for securing a sustainable supply of critical metals and supporting global carbon neutrality goals.

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