Interfacial interaction mechanism between alkali metal ions and cationic/anionic surfactants and insight into low-rank coal flotation

In the low-rank coal flotation process, the modification and adhesion behavior of particles can be reflected by the interaction of the phase interface. Therefore, the interfacial mechanism of alkali metal ions (K⁺, Na⁺) in gas-surfactant solution (DTAC, SDS) and the coal-liquid interfaces were investigated by dynamic surface tension and contact angle measurements. The results showed that the salt ions can affect the adsorption kinetic behavior of surfactants at the gas–liquid interface as well as the wetting performance of the solution on coal. Besides, through the interfacial energy and wetting work could judge the tendency of the reactivity and flotability of a solution. In this case, the adsorption experiments, SEM, XPS, probe method and hydrophilicity test were further carried out to verify the surface information of the modified coal. The results illustrated that the DTAC molecule absorbed on coal is realized by the N⁺–OH⁻ ionic bond, which is firstly proved in the coals modification; while the SDS molecule can be absorbed by O–HO hydrogen bond, which were also proved by the surface free energy analysis. Moreover, the flotability performance of modified coal on the residue solution showed the DTAC could increase the hydrophobicity of the coal, while the SDS-modified coal should be paid attention to the occurrence of hydrophobic tail adsorption.