Activation Characteristics and Aluminum Recovery from Coal Gangue: Structure–Phase–Reactivity Relationships Revealed by Thermal Analysis, Kinetics, and Acid Leaching

Coal gangue is a typical coal-based solid waste and its efficient utilization is of great importance for waste minimization and resource recovery. In this study, the thermal behavior, kinetic characteristics, and aluminum leaching performance of four representative coal gangue samples (XJ-1, SX-1, NX-1, and IM-1) were systematically investigated. TG-DTG analysis revealed that the decomposition process can be divided into five stages, with major mass losses occurring during organic pyrolysis and kaolinite dehydroxylation. The apparent activation energies calculated from the three isoconversional methods (FWO, KAS, and Starink) were highly consistent, with correlation coefficients (<i>R</i> ²) mostly above 0.95. XJ-1 and SX-1 exhibited higher <i>E</i> _a values that increased with conversion, indicating a "difficult-to-activate" type, whereas NX-1 and IM-1 showed significant decreases in <i>E</i> _a within the main decomposition region, corresponding to an "easy-to-activate" type. Acid leaching experiments further confirmed that NX-1 and IM-1 achieved the highest aluminum leaching efficiencies (>59%) after calcination at 750-800 °C, while XJ-1 and SX-1 remained below 50% even under optimum conditions. All samples exhibited reduced leachability at 850 °C due to recrystallization. Complementary structural and phase analyses supported these findings. Overall, this study establishes a clear structure-phase-reactivity relationship and classifies coal gangue into easy-to-activate and difficult-to-activate types, providing theoretical guidance for its classified utilization and process optimization of acid leaching.