Characteristics, Mineral Synergy Mechanisms, and Spontaneous Combustion Risk of Low-Temperature Oxidation Under Coexisting Conditions of Coal and Gangue

During coal mining, coal and associated gangue are often stored together in surface waste dumps, and low-temperature oxidation of the coexisting piles poses a higher spontaneous combustion risk than pure gangue piles; however, the underlying synergistic mechanisms remain unclear. In this study, coal and gangue from the Wuhai mining area were mixed at coal-to-gangue mass ratios of 10:0, 7:3, 5:5, 3:7, and 0:10 and subjected to programmed-temperature oxidation combined with gas chromatography, and were further characterized by FTIR, XPS, and XRD. The results show that coal, rich in aliphatic −C−H, aromatic −C=C−, and oxygen-containing groups, is the primary carbon reactant, whereas gangue, composed mainly of quartz, kaolinite, and pyrite, acts as a mineral matrix. As the proportion of gangue increases, the crossing point temperature, critical temperature, and cracking temperature generally rise, whereas the absolute O2 consumption and the absolute release amounts of CO and C2H4 decrease; these trends primarily reflect the physical dilution effect. In contrast, the crossover temperatures measured in the mixtures were systematically lower than the mass-weighted theoretical values, while the experimentally determined O2 consumption and the release amounts of CO and C2H4 exceeded the theoretical values, thereby confirming the synergistic promoting effect of low-temperature oxidation reactions. And the apparent activation energy increased from 59.81 kJ/mol for pure coal to 65.31 kJ/mol for pure gangue, exhibiting “dilution-dominated, catalytically modulated” behavior. Mechanistically, gangue promotes coal oxidation via a ternary coupling: exothermic pyrite oxidation supplies an additional local heat source, quartz and kaolinite form a heat-conduction network, and the Fe3+/Fe2+ valence cycle catalyzes radical chain reactions. Therefore, under the experimental conditions of this study, the coexistence of coal and gangue can promote low-temperature oxidation reactions, thereby increasing the spontaneous combustion risk in gangue accumulation zones; provides a scientific basis for the precise prevention and control of spontaneous combustion in coal–gangue coexisting piles.