Research progress on the recovery of strategic metals lithium and gallium from coal-based solid wastes: from mineral deconstruction to resource utilization

With the rapid expansion of the new energy vehicle and semiconductor industries, the demand for lithium and gallium resources has increased significantly. Coal and its solid wastes, particularly coal gangue (CG), coal fly ash (CFA), and coal gasification slag (CGS), are abundant and rich in lithium and gallium, thereby representing a potential source for these critical elements. This paper presents detection methods for identifying lithium-rich and gallium-rich phases in coal and its solid wastes, describes the lithium and gallium content in these materials, and analyzes their respective occurrence states. Currently, the technologies for recovering lithium and gallium from CG, CFA, and CGS primarily involve pretreatment, leaching, and extraction/separation processes. Pretreatment aims to release lithium and gallium by destroying the stable structure of their mineral phases through activation techniques such as thermal, mechanical, or chemical methods. Leaching technologies use acids, bases, water, or microbial metabolites (organic acids) to extract valuable elements from CG, CFA, and CGS. The extraction/separation of lithium and gallium from leachates mainly utilizes techniques such as solvent extraction, adsorption, precipitation, and ion imprinting. Furthermore, this paper evaluates the advantages and limitations of existing recovery methods through case studies and discusses prospects for developing synergistic extraction techniques targeting multiple elements from coal-based wastes. Finally, the presence of lithium and gallium in low-rank coal suggests the potential for future recovery of these elements from such sources.