Effective Recovery of Rare Earth Elements from Coal Ash Wastes by a Modified Sequential Extraction Procedure

Abstract

In this study, a simple and ecofriendly sequential extraction procedure was employed to recover rare earth elements (REEs) from coal ash effectively and its leaching efficiency from coal ash was compared with those of conventional procedures (Tessier and European Community Bureau of Reference (BCR)). Citric acid, which is a weak and environmentally benign acid, substituted acetate used in the existing extraction procedures, proving highly effective for extracting REEs from coal ash. Microwave-assisted thermal treatment further enhanced the REE leaching efficiency from coal ash, resulting in an overall recovery of 69.63%, which is 3.76 and 2.54 times higher than those achieved by the Tessier and BCR procedures, respectively. The results suggest that the majority of REEs was strongly associated with CaO, Fe₂O₃, and Al₂O₃, but not with SiO₂. The modified proposed sequential extraction procedure in this study can be used for the development of simple and effective recovery methods for REEs from coal ash wastes.