Sustainable extraction of rare earth elements from coal fly ash leachates using a recyclable ionic liquid

The growing demand for rare earth elements (REE) has prompted interest in their recovery from alternative sources such as coal fly ash (CFA). This study explores the use of the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf ₂ N], for selective extraction of REE from leachates derived from a Class C CFA. While previous studies have demonstrated the effectiveness of using [Hbet][Tf ₂ N] to extract REE from different types of CFA in direct ash-IL systems, this study extends the approach to four CFA leachates prepared with HCl, HNO ₃ , H ₂ SO ₄ , and citrate, respectively. The leachates were subjected to IL-based extraction at varying pH levels and in the presence of additives such as ascorbic acid and betaine. Among the systems tested, [Hbet][Tf ₂ N] can recover REE from the HCl and citrate leachates at average efficiencies of 51% and 47%, respectively, comparable to the recovery efficiency of 49% in ash–IL extraction. Additionally, the co-extraction of bulk elements was significantly reduced in the leachate-IL systems. Adjusting leachate pH to ~ 11 was found to be critical for optimal REE extraction. The addition of ascorbic acid effectively suppressed iron co-extraction without compromising REE recovery. Recycling experiments demonstrated that [Hbet][Tf₂N] retains its performance over five cycles with manageable losses. These results reveal the promise of [Hbet][Tf ₂ N] for effectively recovering REE from leachates of solid wastes, highlighting its applicability as a sustainable strategy for other aqueous REE feedstocks.