Interface Mechanism of Promoting Low-Rank Coal Flotation by Characteristic Groups in Hydrophilic Moieties of Cationic–Anionic Surfactants

Flotation is an interfacial process involving gas, liquid, and solid phases, where polar ionic promoters significantly influence both gas–liquid and solid–liquid interfaces during low-rank coal (LRC) flotation. This study examines how the structures of hydrophilic groups in cation–anion mixed promoters affect the interfacial flotation performance of LRC pulp using flotation tests, surface tension tests, wetting heat tests, and molecular dynamics simulations. Results indicate that cation–anion mixed promoters enhance the LRC floatability to varying degrees. When the cationic hydrophilic head contains a benzyl group and the anionic head contains an ethoxy group, both the floatability and selectivity improve. These mixed promoters exhibit superior surface activity compared to single ionic solutions, particularly with ethoxy-containing anions, which demonstrate an increased density and viscoelasticity at the gas–liquid interface. The combination of a benzyl cation and an ethoxy anion results in dense adsorption at the solid–liquid interface, maximizing wettability differences between organic matter and mineral surfaces. This is attributed to hydrogen bonds and π–π interactions between the promoter and the coal surface, enhancing adsorption selectivity. Hydrophobic chains shield polar sites on the LRC surface, promoting water molecule diffusion and providing sites for nonpolar oil molecule adsorption, thereby improving LRC flotation performance.