Rare earth elements (REEs), a group of 15 lanthanide metals along with scandium and yttrium, are classified as critical metals in several economies. Coal fly ash, an abundantly available legacy waste, is a secondary source for several REEs. However, the sustainable and selective extraction of REEs is a bottleneck in current recovery processes. In this study, three low – grade brown coal fly ash samples were examined to extract REEs, by employing benign organo-carboxylic acids (acetic, oxalic and citric acid) and a deep eutectic solvent. The study examined the effects of the reagent, reaction temperature, acid concentration, solid: liquid (S/L) ratio and time. Results from screening these parameters showed that the type of carboxylic acid, temperature, and concentration are the dominant process parameters affecting REE recovery. Subsequent process optimization achieved high REE recovery (54 to 100%) with the following leaching conditions: 1.31 mol L −1 of citric acid at 90 °C, with a S/L of 1:50 g/mL and time of 4 h. Lab scale results were further tested through a scale of operations study (scaling factor: 250× ), yielding comparable results and supporting a scientifically robust, sustainable, and scalable process design for REE recovery from secondary resources.
Thomas et al. (2026) studied this question.