Recent global geopolitical tensions have exacerbated the scarcity of rare earth elements (REEs), which are critical across many industries. REE-rich coal fly ash (CFA), a coal combustion residual, has been proposed as a potential source, but current convention recovery methods remain inefficient. They are energy and material intensive and generally leach elements indiscriminately, requiring additional downstream processing. A green, efficient method of recovering REEs from CFA would not only provide an attractive solution to REE scarcity, but also to CFA management. CFA contains many toxins present at trace levels, including arsenic, cadmium, chromium, lead, mercury, selenium, uranium, and thorium. Recent catastrophic spills have provoked concerns about potential environmental contamination, which have prompted governmental regulation and ash pond closures.  Here we demonstrate a new valorization process based on the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf<sub>2</sub>N]) for selective extraction of REEs from different CFAs. Efficient extraction relies on [Hbet][Tf<sub>2</sub>N]’s thermomorphic behavior with water. REEs were preferentially extracted over bulk elements from CFAs into the IL phase then recovered in a subsequent stripping step, regenerating the IL. Several optimizations were explored, including alkaline pretreatment, which improved REE leaching efficiency from the more recalcitrant Class-F CFAs, and adding supplementary betaine, which improved REEs and bulk elements’ partitioning between the IL and aqueous phases. Weathered CFA showed slightly higher REE leaching efficiency than unweathered CFA, and Class-C CFA demonstrated higher leaching efficiency but less selective partitioning than Class-F CFAs. Significantly, this method consistently exhibits a particularly high extraction efficiency for scandium.  This work unlocks a new strategy for CFA refinement for REE recovery and is poised to address to problems at once: steadily increasing global demand for REEs, and growing pressure on utility companies to address massive stockpiles of CFA. Furthermore, because the IL is regenerative, this method is less chemically-consumptive than existing strategies. To the authors’ best knowledge, this study represents the first work to demonstrate direct application of an ionic liquid to CFA for efficient recovery of REEs.

Preferential recovery of rare earth elements from coal fly ash using a recyclable ionic liquid

ACS Spring Meeting 2021

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