Rare earth elements (REEs) are crucial in modern technologies and materials, such as magnets, batteries, catalysts, and defense systems. Due to limited mining operations and concentrated global supply chains, alternative REE sources are urgently needed. Coal-based materials offer a promising secondary resource, as REEs are distributed across the coal value chain from raw coal to combustion byproducts. Low-rank unburned coal, while containing lower REE concentrations than fly ash, is produced in large quantities and contains fewer metal impurities. These characteristics make it a valuable yet underutilized feedstock for REE recovery. In this study, we developed a supercritical fluid–assisted extraction and multistage stripping process with tributyl phosphate–nitric acid (TBP–HNO₃ ) to enable direct recovery of REEs from the solid matrix of unburned coal. Supercritical carbon dioxide (scCO₂ ), nitrogen (scN₂ ), and air (scAir) were used to evaluate their effects on REE extraction selectivity and efficiency. All supercritical fluids enhanced REE selectivity, with scCO₂ exhibiting the highest selectivity. With scCO₂ , REEs were concentrated up to 18 mg L^-1in the first stripping stage, with nearly 100% purity achieved by the fifth stripping stage. The enhanced selectivity is attributed to the scCO₂’s ability to dilute TBP, which weakens weakly bound cation complexes such as Ca 2+ and Mg 2+ , while preserving strong coordination between TBP and REE–nitratecomplexes. This study demonstrates that scCO₂ substantially enhances the REE recovery from low-grade materials, highlighting low-rank coal as a sustainable resource for REE extraction that supports energy technologies and reduces dependence on primary mining.

Zixuan Xie

Washington University in St Louis

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