Ionic Liquid Recovery of Rare-Earth Elements from Coal fly Ash: Process Efficiency and Sustainability Evaluations
Laura Stoy, Jiale Xu, Yamini Kulkarni, Ching‐Hua Huang
Abstract
Previous work found that valuable rare-earth elements (REEs: lanthanides, yttrium, and scandium) can be recovered from coal fly ash (CFA), a waste product from coal combustion, using the ionic liquid (IL) betainium bis(trifluoromethylsulfonyl)imide ([Hbet][Tf2N]). Based on unique thermomorphic properties, this IL/water mixture preferentially extracts REEs from CFA solids and separates REEs from bulk elements (Si, Al, Ca, and Fe). To better understand the behaviors of various metals in CFAs, this study investigated 18 additional elements (29 total) and confirmed good selectivity of this new process, yielding low concentrations (<26 wt %) of bulk elements, little (<1.6 mg/kg) trace elements, and co-extracted thorium in the IL phase, while uranium did not leach at all. The study also identified several operational optimizations, including pH (no impact observed for pH 2–7), temperature (optimal at 75–85 °C of the studied 45–85 °C range), and duration of leaching (optimal at 3 h of the 0.5–12 h range). The process is compared with several other methods of CFA solid extraction and leachate separation for REE recovery to place the new method in context with existing literature. Finally, several process sustainability improvements are recommended, including the use of microwave heating, water and IL recovery strategies, and beneficial uses of the residual solids.