The motion of coal particles and the breakaway effect in an industrial dense medium cyclone of body diameter 1000 mm are studied by a computational fluid dynamics model. In the model, mixture multiphase model is employed to describe the flow of the dense medium (comprising finely ground magnetite contaminated with non-magnetic material in water) and the air core, where the turbulence is described by the well-established Reynolds Stress Model. The stochastic Lagrangian Particle Tracking method is used to simulate the flow of coal particles. It is shown that for coarse coal particles with size larger than 1 mm, the separation is mainly determined by the difference between the inward pressure gradient force and the outward centrifugal force, and the effect of drag force can be ignored. For finer particles with size less than 1 mm, however, the effect of stochastic drag force becomes significant. As a result, fine particles have an apparent fluctuating velocity and a relatively uniform distribution in the cyclone; and the drag force is so dominant that particles flow with fluid closely and the pressure gradient force is relatively so small to be able to effectively separate particles by density in the radial direction. This results in a significantly deteriorated separation efficiency of fine particles and the breakaway effect.