Study on the leaching behavior differences of rare earth elements from coal gangue through calcination-acid leaching

•Bastnaesite and monazite were found in the coal gangue from Jungar Coalfield.•Calcination makes rare earth carriers into more soluble fractions.•78.8% of REY and 88.5% of Al were extracted at optimal conditions.•REY leaching is controlled by interfacial transfer and diffusion through solid layer.•The higher extraction rate of LREY than HREY is affected by monazite in coal gangue. Critical metals in coal-based byproducts have been gradually regarded as one of the most important alternative resources due to the market parameters, strategic perspective, and advanced technology. In this study, the mechanism of calcination-acid leaching for rare earth elements (REY) recovery from coal gangue of Jungar coalfield was proposed based on the results of sample characteristics, extraction law, kinetic analysis, and typical mineral leaching. The micro-particles of bastnaesite and monazite were found and embedded in the kaolinite via micro-observation with SEM-EDS. Kaolinite in coal gangue calcined at 600 ℃ was transformed into highly positively reactive metakaolinite, enhancing the dissolution of aluminum (Al) and exposing the rare earth minerals encapsulated within the particles. Meanwhile, bastnaesite was thermally decomposed into rare earth oxides and rare earth fluorides. Rare earth oxides are more easily dissolved by acid, and the fluoride ions and the aluminum ions form a stable complex ion [AlF6]3-, which promotes the dissolution of rare earth fluoride. In addition, a high REY and Al recovery (78.8% of REY, and 88.5% of Al) came true from calcined coal gangue at optimal conditions. The leaching kinetics showed that REY leaching followed the hybrid control model of interfacial transfer and diffusion through solid layer, while Al leaching is controlled by chemical reaction. The leaching test of monazite shows that the leaching efficiency of LREY is higher than that of HREY indicating the leaching difference of LREY (82.3%) and HREY (61.3%) in coal gangue is mainly influenced by monazite. This may be related to the crystal structure and the radius of trivalent rare earth ions. This study provides a useful reference for the extraction of REY from coal gangue.