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

The growing demand for rare earth elements (REEs) has prompted interest in their recovery from alternative sources such as coal fly ash (CFA). This study explores the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide, [Hbet][Tf2N], for selective extraction of REEs from leachates of a Class C CFA. While previous studies have demonstrated the effectiveness of using [Hbet][Tf2N] to extract REEs from different types of CFA in direct ash-IL systems, this study investigates four CFA leachates prepared using HCl, HNO3, H2SO4, and citrate. Extraction experiments were conducted across varying pH levels and with additives such as ascorbic acid and betaine. Among the systems tested, [Hbet][Tf2N] achieved REE recoveries of 51% and 47% from the HCl and citrate leachates, respectively, comparable to 49% REE recovery in ash-IL extraction. Co-extraction of bulk elements was significantly reduced in the leachate-IL systems. Optimal REE extraction occurred near pH 11, and 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][Tf2N] for effectively recovering REEs from leachates of solid wastes, highlighting its applicability as a sustainable strategy for other aqueous REE feedstocks.