Selective adsorption of different functional groups on kaolinite and coal surfaces: an MD and DFT study

The selective flotation of kaolinite from high-ash fine coal slime offers a promising pathway to overcome the challenge of clay–coal separation. This study employed density functional theory (DFT) calculations, molecular dynamics (MD) simulations, and flotation experiments to compare the selective adsorption of different functional groups on kaolinite (001), (00-1), and coal surfaces, with the aim of identifying effective reagents for kaolinite flotation. DFT and MD results show that octylphosphonic acid exhibits the strongest interaction, lowest diffusion coefficient, smallest frontier orbital energy gap, and lowest adsorption energy on kaolinite (00-1), indicating selective recognition of the Al-OH surface. Octylamine adsorbs strongly on both kaolinite (001) and coal but weakly on kaolinite (00-1), whereas octanoic acid is intermediate. Flotation tests confirm these trends: octylphosphonic acid gives the highest kaolinite recovery (60.93%) and concentrate ash (78.37%) at 150 g/t while strongly suppressing combustible recovery, making it the most promising reagent for kaolinite flotation. Octylamine gives the highest combustible recovery (72.98%) at 450 g/t, whereas its lowest kaolinite recovery (25.93%) is observed at 600 g/t, indicating that it preferentially floats coal and is unsuitable for kaolinite flotation. Octanoic acid is intermediate. Overall, the phosphonic acid group is highly selective for the kaolinite Al-OH surface, the carboxyl group is moderate, and the amino group is unfavorable for kaolinite flotation. This study provides a fundamental theoretical basis for the design of selective flocculants in the coal-kaolinite separation system.