Interface Mechanism\nof Promoting Low-Rank Coal Flotation\nby Characteristic Groups in Hydrophilic Moieties of Cationic–Anionic\nSurfactants

Flotation is an interfacial process involving gas, liquid,\nand\nsolid phases, where polar ionic promoters significantly influence\nboth gas–liquid and solid–liquid interfaces during low-rank\ncoal (LRC) flotation. This study examines how the structures of hydrophilic\ngroups in cation–anion mixed promoters affect the interfacial\nflotation performance of LRC pulp using flotation tests, surface tension\ntests, wetting heat tests, and molecular dynamics simulations. Results\nindicate that cation–anion mixed promoters enhance the LRC\nfloatability to varying degrees. When the cationic hydrophilic head\ncontains a benzyl group and the anionic head contains an ethoxy group,\nboth the floatability and selectivity improve. These mixed promoters\nexhibit superior surface activity compared to single ionic solutions,\nparticularly with ethoxy-containing anions, which demonstrate an increased\ndensity and viscoelasticity at the gas–liquid interface. The\ncombination of a benzyl cation and an ethoxy anion results in dense\nadsorption at the solid–liquid interface, maximizing wettability\ndifferences between organic matter and mineral surfaces. This is attributed\nto hydrogen bonds and π–π interactions between\nthe promoter and the coal surface, enhancing adsorption selectivity.\nHydrophobic chains shield polar sites on the LRC surface, promoting\nwater molecule diffusion and providing sites for nonpolar oil molecule\nadsorption, thereby improving LRC flotation performance.