CFD and CFD-DEM Modelling of enhanced gravity separators to improve understanding and performance

Abstract

Computational Fluid Dynamics (CFD) and Computational Fluid Dynamics coupled with the Discrete Element Method (CFD-DEM) investigations of a FLSmidth Knelson concentrator model KC-MD3 and a Gravity Mining ltd. Laboratory sized Multi-Gravity Separator (Micro- MGS) have been undertaken to understand the fluid dynamics, physics of the separation processes and explore the possibility of improving the efficiency of the equipment. CFD simulation results for both equipment were validated by experimental data with good agreement and limited CFD-DEM analysis was also conducted for the Knelson using a coupling of Ansys Inc. Fluent and DEM Solutions ltd. EDEM software. Fine grids were carefully concentrated around the water-air interface of both Knelson and Micro-MGS to obtain excellent resolution of the dynamics of the interface region. For the Knelson concentrator, single particle sedimentation was simulated and also investigated experimentally for silica, magnetite and tungsten for two positions of the downcomer. Also, multi-particle sedimentations were investigated using CFD-DEM. The results suggests that the released position of the particles could affect the amount of unwanted light particles of silica that could be captured in the grooves of the Knelson Concentrator. The water-air interface was observed to be very dynamic and varies spatially and with time. Vertical interfacial structure was observed in the interface as a sign of instability. The instability disappears with increase in thickness of the flowing film. The multi-particle sedimentation in the Knelson concentrator shows that the particles of silica and magnetite follow a helical streamline path from bottom to the top of the cone, but the tungsten particles form a dispersed cloud within the fluid. CFD modelling of the Micro-MGS was completed. To optimise computation resources and simulation time, the Micro-MGS was scaled down by 20% by applying hydrodynamic scaling laws. This was required to meet a limitation of 500,000 grids of the educational licence of the Ansys Inc. Fluent software. The Micro-MGS simulation shows a potential fluid resonance when the rotating frequency is equal to shaking frequency of about 4 Hz. Experimental results of scheelite particle separation at this resonance frequency shows a significant change in recovery compared to non-resonant frequencies. However, the results are not sufficient to determine whether this difference is as a result of the resonance or due to other changes in fluid and particle bed behaviour resulting from changes to shake frequency. Specific further experimentation is suggested to better assess the impact of a resonant effect