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Optimization of Iron Removal in the Recovery of Rare-Earth Elements from Coal Fly Ash Using a Recyclable Ionic Liquid.

Rare-earth elements (REEs) are essential for modern technologies, and the United States currently lacks a secure domestic supply. Coal combustion residuals, specifically coal fly ash (CFA), can be a potential source. Our previous work demonstrated that REEs could be preferentially extracted from CFA using the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]), and the process yielded a mildly acidic REE-rich solution with coextracted Fe and regenerated IL. In this study, we investigated three strategies to limit Fe coextraction: magnetic separation, complexing salts, and ascorbic acid (AA) reduction. Magnetic separation of CFA was ineffective in significantly lowering the Fe content in the IL phase. When NaCl was used instead of NaNO3 during extraction, chloride complexation lowered iron distribution to the IL phase over the aqueous phase (DFe) by five folds, from ∼75 to ∼14, while REE leaching (LREEs) and recovery (RREEs) both increased. Using AA for iron reduction lowered the overall amount of Fe extracted and further decreased DFe to ∼0.16, effectively shifting Fe preference from the IL phase to the aqueous phase. Combining the strategies of NaCl, AA, and supplemental betaine addition, leaching and extraction of REEs from CFA by [Hbet][Tf2N] were achieved in higher efficiency for REE recovery with minimized Fe concentration.