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The poor flotation performance of low-rank coal is due to the large number of oxygen-containing functional groups on the coal surface. To improve the hydrophobicity of low-rank coal surface, dodecyl trimethyl ammonium bromide (DTAB) and sodium dodecyl benzene sulfonate (SDBS) were selected as cationic and anionic surfactants, and their promotion mechanisms were theoretically investigated using a combination of quantum chemical density functional theory (DFT) and molecular dynamics simulation (MD). The results of DFT calculations showed that the absolute value of the energy difference between the HOMO orbital of DTAB and the LUMO orbital of low-rank coal was the smallest compared with that of SDBS and water molecules, and the adsorption distance and interaction energy between low-rank coal and DTAB at adsorption equilibrium were the smallest, indicating that DTAB was able to replace the water molecules on the surface of low-rank coal. The MD simulation results showed that DTAB can adsorb directionally on the surface of low-rank coal, effectively covering the hydrophilic sites and inhibiting the adsorption of water molecules. The adsorption orientation and electrostatic repulsion of SDBS lead to a smaller degree of weakening of the interaction energy between coal and water. In the coal/DTAB/water system, the coal-water interaction was the smallest and the number of hydrogen bonds formed was the lowest. DTAB can promote the migration of water molecules, improve the hydrophobicity of low-rank coal surface and increase the flotation efficiency.

Effect of ionic surfactants on flotation of low-rank coal: a DFT calculation and MD simulation study