Enhanced flotation of coal gasification fine slag: mechanistic insights into functional group-regulated interfacial behavior of polar–nonpolar compound collectors

Carbon–ash separation of coal gasification fine slag (CGFS) is essential for its resource utilization. However, oxygen-containing groups and a developed porous structure of residual carbon limit the selectivity of conventional nonpolar collectors such as kerosene (KER). In this study, polar–nonpolar compound collectors were constructed by introducing esters, ketones, aldehydes, and ethers into kerosene. The effects of functional group type and blending ratio on CGFS flotation were systematically investigated, and the interfacial regulation mechanisms were elucidated through interfacial characterization and flotation kinetics. The results showed that appropriate polar modification significantly enhanced carbon–ash separation, with esters, ketones, and aldehydes exhibiting stronger synergistic effects, whereas ethers showed limited enhancement. Comprehensive evaluation revealed that the KER–10% ethyl laurate (EL), KER–5% ethyl benzoate (EB), KER–10% 2,5-hexanedione (2,5-HD), and KER–10% undecanal (UDA) systems exhibited relatively favorable performance. KER–10% EL reduced the concentrate ash content to 55.99%, while KER–10% 2,5-HD increased the tailings ash content to 94.38%. The introduced polar functional groups reduced the surface tension, improved reagent dispersion, regulated interfacial charge distribution, and enhanced selective adsorption onto residual carbon. Among them, KER–5% EB exhibited the highest flotation rate constant (0.0148 s −1 ), selectivity index (1.5709), and wettability modification capability, achieving a relatively balanced flotation performance. Furthermore, KER–10% EL was more effective for concentrate upgrading, while KER–10% 2,5-HD favored deep decarbonization of tailings. These differentiated functions provide guidance for the targeted design of compound collectors for residual carbon recovery and tailings resource utilization.