Rare earth elements (REEs), including lanthanides, scandium and yttrium, are essential for modern technologies and the United States currently lacks a secure domestic source and supply chain. Coal combustion residuals, specifically coal fly ash (CFA), has been investigated as a potential source. Previous work done by our group has demonstrated that REEs can be preferentially extracted from CFA using a recyclable ionic liquid (IL), betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]). The novel IL extraction process applies IL to alkaline-pretreated CFA, yielding a mildly acidic REE-rich solution contaminated with Fe and regenerated IL that can be reused in extraction. The objective of the current work is to improve the process to limit Fe coextraction into the IL phase, and three strategies were investigated: magnetic separation, complexing salts, and reduction using ascorbic acid. While magnetic separation was unsuccessful due to the dispersion of Fe throughout the CFA matrix, the other strategies were successful. Using NaCl instead of NaNO3 promoted Fe-Cl complexation and decreased Fe’s preference for the IL phase by more than five folds. Using ascorbic acid decreased this preference even further, to where Fe showed a strong preference the AQ phase over the IL phase. Combining these two with other strategies identified in our previous work with CFA-[Hbet][Tf2N] leaching, namely alkaline pretreatment and adding supplemental betaine, generated an REE-rich acidic solution with very low concentrations of Fe.

Minimizing iron co-extraction in recovery of rare earth elements from coal fly ash using a recyclable ionic liquid

ACS Spring 2022

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.