Rare earth elements (REEs) are critical to a wide range of high-tech applications and renewable energy production, but their supply is geopolitically constrained and their extraction practices are often environmentally unsustainable. Coal fly ash (CFA), previously considered a waste and environmental threat, has recently gained attention as a promising low-grade REE source. Developing a less energy-demanding, more environmentally-friendly, and highly selective recovery process for recovering REEs is essential to a more resilient future. Here we present a proof-of-concept for a novel REEs extraction process where supercritical fluids enhance the ability of tributyl phosphate (TBP) to selectively extract REEs directly from solid CFA matrices. For the first time, we showed that supercritical nitrogen and supercritical air could work like supercritical carbon dioxide as green solvents for REE selective extraction. We achieved up to 79% extraction efficiency for REEs during the SCF extraction and significantly removed the impurities with less reactivity with the extractant, such as Mg and Al. Then, by harnessing different water affinities of metal ions, we applied multistage stripping to further separate the REEs from Ca and Fe impurities. In this study, we have demonstrated that the process extractant is reusable, minimizing the associated organic chemical demand and organic waste generation. By selectively extracting REEs at high concentrations from CBR wastes, our newly developed process can significantly support the sustainable supply of REE from waste and underutilized resources.

Supercritical fluid extraction with multistage stripping for selective recovery of rare earth elements from coal fly ashes

ACS Spring 2022

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