A fundamental investigation was carried out on coal-coal, kaolinite-kaolinite, and coal-kaolinite interactions using the atomic force microscopy colloidal probe technique at different pH and calcium concentrations to explore the effect of kaolinite on fine coal flotation in the present study. The results showed that the kaolinite particles seem to be always detrimental to the flotation process. In deionized water, a repulsive force was always detected in the kaolinite-kaolinite interaction while kaolinite-coating phenomenon was not observed on a coal surface under the neutral pH. However, the repulsive force made it more difficult for the ultra-fine kaolinite particles to settle in the tailing thickener and the unsettled particles returned to the flotation flowsheet, which decreased the flotation efficiency because of the water entrainment. With the addition of Ca2+, the repulsive electrostatic force between kaolinite and kaolinite decreased. The attractive van der Waals force could dominate the kaolinite-kaolinite interactions, enhancing the sedimentation classification performance of fine kaolinite in tailing thickener. However, the interaction forces between coal and kaolinite changed from being weakly repulsive to strongly attractive and increased monotonously, resulting in the occurrence of the kaolinite-coating phenomenon during flotation. (C) 2016 Elsevier B.V. All rights reserved.