Leaching of Rare Earth Elements from Coal Fly Ash via Ammonium Chloride Roasting Coupled with Microwave-Assisted Citric Acid–Ammonium Fluoride

Rare earth elements (REEs) are strategically important in the energy, defense, and other sectors. As the global energy transition accelerates, recovering REEs from value-bearing solid wastes has become an important route for building future strategic reserves. Coal-based solid wastes are regarded as promising secondary REE resources; however, conventional recovery technologies remain limited in terms of selective enrichment, environmental performance, and economic viability. This study developed an integrated metallurgical process combining ammonium chloride activation roasting with microwave-assisted leaching in a citric acid-ammonium fluoride system. The results showed that, at a citric acid concentration of 1 mol L −1 , an ammonium fluoride dosage of 9 %, a liquid-to-solid ratio of 50:3 mL g −1 , a microwave temperature of 80 °C, a microwave power of 600 W, and a microwave irradiation time of 120 min, the REE leaching efficiency from coal fly ash reached 80.45 %. During roasting activation, HCl gas generated by the decomposition of ammonium chloride disrupted the aluminosilicate phases in the coal fly ash and exposed encapsulated REE-bearing sites. During the subsequent leaching stage, microwave irradiation intensified molecular motion in the solution and solid phases and enhanced mass transfer at the solid–liquid interface. H + promoted the dissolution and dissociation of REE-bearing phases, whereas F - interacted with the aluminosilicate structure, weakened and disrupted its framework, and formed soluble fluoride complexes. Citric acid simultaneously complexed REE ions, thereby facilitating REE leaching. Kinetic analysis indicated that the microwave-assisted citric acid-ammonium fluoride leaching process was jointly controlled by interfacial reaction and product-layer diffusion. This study developed a metallurgical process integrating ammonium chloride activation roasting with microwave-assisted leaching in a citric acid-ammonium fluoride system.