Preparation of layered double hydroxides from coal gangue in a deep eutectic solvent system and their application in flame retardancy of coal gangue

Coal gangue, a major solid waste from coal mining, is prone to spontaneous combustion during long-term storage, releasing toxic gases and creating serious environmental and safety concerns. In this work, coal gangue was converted into layered double hydroxides (C-LDH) using a deep eutectic solvent, and the resulting nanosheets were incorporated with gangue for flame-retardant applications. Thermal analysis and CO release experiments showed that the addition of C-LDH effectively delayed combustion and suppressed toxic gas emissions, with optimal loadings significantly improving both ignition resistance and heat-release performance. Structural characterization indicated that the inhibition mechanism proceeds in two stages. At intermediate temperatures, the release of interlayer water and carbonate provides gas-phase dilution and heat absorption, while at higher temperatures the LDH-derived oxides generate compact barriers and catalyze CO oxidation, thereby stabilizing the system. Notably, C-LDH demonstrated regeneration capability, as samples subjected to moderate combustion and subsequent hydration partially recovered their layered structure and continued to provide multi-stage inhibition throughout the combustion process, while further reducing heat release. These results demonstrate a sustainable strategy in which coal gangue is valorized into recyclable flame-retardant additives, delivering both environmental benefits and practical potential for mitigating spontaneous combustion hazards.