Model investigation of a dry vibrated fluidized sinkhole system for separating coarse particles based on density

A simulation model of a dry, vibrated, fluidized medium (VFM) with sinkhole arrangement was developed and used to investigate the separation of relatively dense, coarse particles. The objective of the new model was to emulate a most unusual experimental result involving the sinkhole arrangement, separation densities much higher than the bulk density of the fluidized medium. The VFM was simulated using spherical sand particles $225\mu m$ in diameter, and density of 2500 kg/m³, while spherical coarse particles 2 to 4 mm in diameter, with density ranging from 2100 to 8400 kg/m³, were used as the density tracers. Coupled computational fluid dynamics (CFD)/discrete element method (DEM) was used to simulate interactions for up to 10 s duration. Remarkably, the model reproduced separation densities much higher than the bulk density of the suspension. Different combinations of frequency and amplitude of vibration, air flow velocity, and volume of VFM were used. The separation density was found to scale directly with the amplitude, and scale with the frequency to the 0.33 power. Vibration intensity correlated poorly with separation density. The critical condition governing the tendency of a particle to float or sink was examined in terms of the volume fraction and the density of the bed profile in the vicinity of the sinkhole. A pronounced reduction in the bed density is evident near the base of the VFM for particles that sink. The average solid volume fraction is observed to vary from 0.58 during initial settling, decreasing as the particles sink. Interestingly, re-circulation of the VFM is needed for a tracer particle to sink. Re-circulation is dependent on, and increases with, airflow as the bed expands up to a maximum beyond which it again reduces.