A review of the occurrence and extraction of key coal-associated metals: gallium, germanium, lithium, and uranium

Coal and its associated resources serve as crucial non-traditional sources for recovering strategic rare metals, such as gallium (Ga), germanium (Ge), lithium (Li), and uranium (U). This paper provides an in-depth analysis of the occurrence mechanisms and extraction pathways of these four elements within coal-measure products. Utilizing sequential chemical extraction, micro-area in-situ characterization, and coupled thermal analysis, the study reveals that Ga is primarily hosted within the aluminosilicate lattice through isomorphous substitution of Al. In contrast, Ge exhibits strong organophilic affinity, existing as chelated states and demonstrating high volatility during thermal processing. Li is highly dispersed within clay minerals and prone to encapsulation by high-temperature glassy phases. The occurrence state of U evolves with coalification, undergoing a directional migration from organic complexation to inorganic mineral phases. In light of these complex occurrence characteristics, the core of the extraction process lies in matrix restructuring and element release. Pretreatment technologies, such as high-temperature decarbonization, complex salt roasting, and alkali roasting, can effectively disrupt stable mineral matrices or organic functional groups. During the separation stage, by employing gradient leaching, green organic acid dissolution, and highly selective resin adsorption, the optimal extraction efficiencies achieved for gallium, germanium, lithium, and uranium reached 98.00 %, 94.64 %, 99.64 %, and 95.80 %, respectively, demonstrating the feasibility of efficient multi-metal recovery and purification. Future research should focus on developing low-temperature activation additives, elucidating microscopic migration mechanisms, and establishing a green, clean, closed-loop recycling system across the entire industry chain.