Recovery of rare earth elements through coal electrolysis, acid leaching, and organic solvent extraction–a comparative study

This study presents the first investigation of coal electrolysis for recovering rare earth elements from low-ash sub-bituminous coal. Compared to conventional acid leaching under equivalent conditions, electrolysis significantly enhanced recovery rates—increasing light rare earth elements by 12.5–33.3% and heavy rare earth elements by 28.6–86.7%. While X-ray diffraction analysis showed both methods similarly affected mineral composition through partial dissolution of clay minerals, electrolysis more effectively extracted rare earth elements associated with organic structures, particularly those bound to aliphatic carboxyl and hydroxyl side chains. Fourier transform infrared spectroscopy confirmed greater reduction in carbon–oxygen bond vibrational modes and degradation of carboxyl functional groups during electrolysis. Thermogravimetric analysis revealed a positive correlation between dry ash-free fixed carbon content and rare earth element concentration, suggesting additional rare earth element association with aromatic carbon structures. Supplementary organic solvent extraction using ethanol and toluene achieved approximately 10% extraction efficiency, with praseodymium showing strong organic-phase affinity while gadolinium associated primarily with inorganic matrices. Notably, organic solvent extraction increased rare earth element recovery after acid leaching but not following electrolysis, indicating electrolysis had already liberated extractable rare earth elements.