Coal slime water contains a large amount of clay minerals such as kaolinite, which brings great difficulties for flocculation and sedimentation. In order to clarify the influence mechanism of these mineral properties on the coagulation process, the interaction between the particles of coal or kaolinite were calculated by the extended DLVO theory, and the coagulation process of 20 g/L coal and kaolinite suspension with a CaCl2 dosage of 4.5 mmol/L was monitored by a focused beam reflectometer at the stirring speeds of 60, 100 and 150 r/min. The results showed that the electrostatic interaction played a dominant role in the range of 2?200 nm for particle surface distance. The kaolinite particles had larger electronegativity, so it was more difficult to approach to and collide to each other during the coagulation process. The higher rotation speed increased the momentum of particles and was beneficial to increase the collision frequency and shorten the time required to complete the coagulation. Under the experimental conditions, the coagulation times of coal and kaolinite were shortened from 74 and 123 s to 47 and 89 s, respectively. The hydrophobic force played a leading role in the range of particle spacing less than 2 nm, which determined the adhesion efficiency of the particles. With strong hydrophobic surface, coal particles were easier to adhere to each other after collision, and can resist higher fluid shearing action. As the result, coal particles can agglomerate to form large agglomerates of 100 μm from its initial size of 19.32 μm, while kaolinite was difficult to obtain agglomerates larger than 30 μm due to their hydrophilicity.