Research on the Hydrophilicity of Non-Coal Kaolinite and Coal Kaolinite from the Viewpoint of Experiments and DFT Simulations

Abstract

Coal is often coated by coal kaolinite in flotation, resulting in an increase in concentrate ash. The natural hydrophilicity of minerals is the key factor to determining its flotation behavior. The results of studies on the contact angle of non-coal kaolinite and coal kaolinite samples found that the contact angle of coal kaolinite was bigger than that of non-coal kaolinite and the hydrophilicity of the latter was stronger. To investigate the mechanism of the hydrophilic difference between non-coal kaolinite and coal kaolinite, the adsorption of a single water molecule on non-coal kaolinite and coal kaolinite (100) and (00 1 ¯ ) surfaces was calculated with the first principle method of the density functional theory (DFT). The calculation results showed that hydrogen bonds were formed between the hydrogen atom and the oxygen atom of the surface and the hydrogen atom and the oxygen atom of the water molecule after the water molecule was adsorbed on the kaolinite (100) and (00 1 ¯ ) surface. The adsorption process of water molecules on the kaolinite surface was physical adsorption with Van der Waals force existing between them. Regardless of whether the kaolinite (001) surface or the kaolinite (00 1 ¯ ) surface was doped with a carbon atom, the adsorption of a single water molecule was weakened, with a weaker hydrogen bond formed. The calculated results explained the difference of hydrophilicity between non-coal kaolinite and coal kaolinite samples from the molecular and atomic viewpoint.