New Insights into the Mechanism of Gangue Contamination in Coking Coal Flotation: Roles of Water Entrainment and Slime Coating

Fine gangue contamination limits the flotation upgrading of coking coal middlings, yet the relative roles of water entrainment and slime coating remain unclear for different gangue minerals. This study investigated the contamination behaviors of quartz, kaolinite, and calcite through flotation tests, bubble-particle attachment measurements, zeta potential analysis, DLVO calculations, SEM-EDS observations, foam drainage and settling tests. Results showed that the addition of gangue minerals reduced combustible recovery, and the combustible recovery decreased from 91.68% for low-ash coal to 86.87%, 84.77%, and 82.55% for the coal–quartz, coal–kaolinite, and coal–calcite systems, respectively. Quartz exhibited limited coal–gangue association and a stronger tendency toward water entrainment, whereas calcite showed more pronounced surface attachment, consistent with slime-coating contamination. Kaolinite displayed intermediate behavior, with particle association that could not be fully explained by classical DLVO calculations. These findings demonstrated that gangue contamination depended on the combined effects of interfacial interactions, particle association, and water-mediated transport. The integrated approach provides a basis for identifying mineral-specific contamination tendencies and developing targeted strategies to improve flotation selectivity.