Computational investigation of the mechanisms of the “breakaway” effect in a dense medium cyclone

• Both particle size and density are critical parameters in a dense medium cyclone.
• The flow of coarse particles is dominated by the inward pressure gradient force.
• The flow of fine coal particles is dominated by the drag force.
• The different roles of different forces result in the “breakaway” effect.

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.

A numerical study of the particle flow and breakaway effect in a DMC is carried out using a CFD-LPT approach. The figure shows the simulated results of the partition coefficient as a function of density (a) and size (b). When particle size is less than 1 mm, the partition curves become flat significantly. It is interesting that the separation of particles less than 0.14 mm is irresponsible to particle density. The roles of different forces on particles of different sizes and densities are analyzed and the results are found to be useful in understanding the mechanisms about the breakaway effect and the particle separation behaviour in a DMC.Figure optionsDownload as PowerPoint slide