Low-temperature pre-carbonization modulation of the surface properties and floatability of low-rank coal

Low-rank coal (LRC) exhibits significant potential as a precursor for coal-based carbon materials due to its high chemical reactivity. However, achieving high-value utilization depends on the ultra-clean preparation of LRC. In this study, a combined process of low-temperature pre-carbonization and flotation is proposed to obtain ultra-clean coal. The evolution of surface chemical composition, morphology, pore structure, and wettability at different pre-carbonization temperatures was investigated, and the preparation mechanism was further explored. The results demonstrate that pre-carbonization effectively reduces oxygen-containing functional groups and enhances surface hydrophobicity. A critical transition occurs at 250 °C, where decarboxylation reduces the O = C-O content to 16.43% and increases the contact angle from 112.4º to 123.5º. Flotation tests on the 250 °C pre-carbonized sample yielded ultra-clean coal with an ash content of 0.81% and a yield of 40.36%. Calculations based on E-DLVO theory confirm that the enhanced flotation performance results from strengthened hydrophobic attraction and weakened electrostatic repulsion between coal particles and bubbles. This study elucidates the mechanism by which low-temperature pre-carbonization promotes flotation efficiency, offering a novel strategy for yielding ultra-clean coal with high application value.