Concentration, leachability, and optimization of leaching of rare earth elements (REEs) from the coal and its boiler-derived and gasifier-derived ashes

Rare earth elements (REEs) in coal and coal-derived materials are finely disseminated within stable mineral matrices, limiting their beneficiation and extraction. This study investigated the concentration, leachability, and extractive behavior of REEs in feed coal and its corresponding gasification ash, bottom ash, and fly ash. Grinding to −106 µm and −25 µm was performed to evaluate REE liberation, while sequential chemical extraction procedure (SCEP) assessed REE speciation and mobility. Direct acid leaching experiments were conducted using 0.5–2 M HCl, liquid-to-solid (L/S) ratios of 4–10 mL/g, temperatures of 60–150 °C, and varying leaching durations. Response surface methodology was applied to optimize the major leaching parameters. Grinding to −25 µm enriched total REEs (TREEs) by 0.36–16.25%, with the highest enrichment observed in the fly ash. SCEP showed that most REEs (79–93% TREEs) occurred in residual aluminosilicate fractions, whereas Sc exhibited comparatively higher mobility. Under common optimal leaching conditions (1 M HCl, 5.33 mL/g, 72°C, and 90 minutes), TREE recoveries ranged from ~29 to 58%, with leachates containing ~5018–18,743 ppm of total Al, Fe, Ca, Mn, and Ti impurities. The results demonstrate that direct mild acid leaching alone is insufficient for selective and efficient large-scale REE recovery from the investigated coal and coal-derived materials.