The effect of kaolinite and montmorillonite on fine coal flotation in deionized water and calcium solution were studied through flotation experiments on a mixed coal-kaolinite/montmorillonite system in the present study, respectively. Fuerstenau upgrading curve was used to evaluate the whole flotation selectivity and the interaction force between kaolinite/montmorillonite and coal was measured using atomic force microscopy (AFM) colloidal probe technique. It was found that kaolinite and montmorillonite caused different issues in coal flotation process. In deionized water, the results show that a lower flotation recovery and worse selectivity were obtained for coal-montmorillonite system compared with that of coal-kaolinite. AFM results show that slightly jump-into-contact phenomenon was observed in coalmontmorillonite interaction while a repulsive force was always detected in coal-kaolinite interaction. The heterocoagulation between coal and montmorillonite was responsible for the low flotation recovery and bad selectivity in deionized water for coal-montmorillonite system. In presence of 0.1 mol/L calcium ion, the flotation recovery increased considerably while the selectivity of both systems was deteriorated. The jump-into-contact and high adhesion force were always found in coal-kaolinite/montmorillonite AFM force curves, indicating attractive force dominated the interaction between kaolinite/montmorillonite and coal. The electrostatic repulsive force between coal and clay particle or air bubble is fully suppressed by Ca2+, aggravating the kaolinite/montmorillonite coating and also leading to an increase of flotation recovery. (C) 2017 Elsevier Ltd. All rights reserved.