Systematic study of effect of particle size distribution in a dense medium cyclone by Johnson's S_B function

Dense medium cyclone (DMC) is a high-tonnage device that is widely used to upgrade run-of-mine coal in the 0.5-50 mm size range. It is known that the performance of a DMC depends on the coal particle size distribution but quantitative relationships have not been established yet. In this work, the effect of the particle size distribution in a DMC is studied in detail using the combined Computational Fluid Dynamics (CFD) and Discrete Element Method (DEM). In particular, Johnson's S_B function, which is capable of representing a wide range of size distributions, is employed to describe the particle size distribution of coal. The function has two parameters, i.e., particle median size d_0.5 and distribution parameter σ_j, for a given size range. It is found that for a constant σ_j, the operational head and medium differential decrease dramatically, and the separation efficiency deteriorates rapidly when d_0.5 increases from 6 to 40 mm. For a constant d_0.5, the DMC performance is sensitive to σ_j, particularly when d_0.5 and σ_j are small. Both the medium differential and split decrease at first and then almost remain constant as σ_j increases from 0.4 to 1.0, and the separation performance follows the similar trend as well. The simulation results are also analysed in terms of medium and particle flow patterns, particle-fluid, particle-particle and particle-wall interaction forces to elucidate the underlying mechanism. For example, the decrease of pressure gradient force and viscous drag force represents the loss of swirling energy and then contributes to the drop of operational pressure and worse separation efficiency. The results should be useful to better design and control DMC operations, particularly for various coal types with different particle size distributions.