The influence of concentration and disperse composition on the process of flocculation of polydisperse coal slime is examined. It is established that the rate of deposition of the formed aggregates of floccules at constant consumption of flocculant is reduced with an increase in the concentration of solid phase in slime exceeding 30 g/dm 3 . This is explained by the fact that with an increase in the concentration of finely dispersed solid particles, the interphase surface increases, which leads to the non-uniform distribution of polymer at the surface of the particles and decreases the effectiveness of its adsorption. With an increase in the content of solid phase with coarseness exceeding 40 µm, the effectiveness of flocculation (settling velocity and strength of floccules) grows. We proposed a criterion of effectiveness of the process of formation of durable aggregates, which is the residual speed after mechanical action. This indicator characterizes the size of floccules and their strength. Dependences obtained in the course of experimental studies make it possible to intensify the process of forming durable floccules at the minimum consumption of flocculant, as well as to control the process of flocculation by regulating the concentration and disperse composition of polydisperse suspensions.