Extraction and separation of rare earth elements from coal and coal fly ash: A review on fundamental understanding and on-going engineering advancements

Rare earth elements (REEs), a group of 15 lanthanides, have garnered widespread interest due to their application in batteries, lightweight alloys and medical equipment, amongst several other applications. Despite their growing demands, there are restricted sustainable engineering practices that could build a steady supply of these metals, with mineral and metal mining being the major pathway for REE production. This predicament, in turn, provides a window of opportunity for REEs to be mined from secondary sources such as coal fly ash, e-wastes & mine tailings. Mining from secondary sources, also called urban mining, offers alternative REE production pathways, reduces supply monopolization from limited mining locations around the world. Coal fly ash, a residual by-product of the coal combustion process, promises to be an important source of different REEs associated with minerals in the parent coal. The utilization of such waste stream(s) from legacy coal-fired power plants, embodies the underlying principles of waste utilization. Understanding fly ash properties through characterization techniques such as SEM-EDX, XRD, synchrotron-based µ-XRF and µ-XANES sheds light onto various minerals, with which the REEs, are often associated in coal fly ash, and subsequently play a crucial role in developing sustainable leaching and separation methodology. This review focuses on the latest and most significant results published in REE extraction from coal and it's fly ash through hydrometallurgical pathways such as direct acid leaching, solvo-metallurgical pathways utilizing deep eutectic solvents and ionic liquids, and the subsequent metal recovery processes via solvent separation process, electrochemical separation, and membrane separation technique.