Novel jamming mechanism for dry separation of particles by density

Abstract

Previous work on a novel separation device called the Sink-Hole Fluidizer has been extended to investigate the effect of increasing the Sink-Hole diameter from 13 mm up to 60 mm. This device consists of a pair of mesh screens with 1 mm square apertures mounted above the surface of a vibrated fluidized bed. The upper mesh contains a large central hole referred to as a Sink-Hole. The fluidized granular media, nominally 0.2 mm sand particles, expanded up through the large Sink-Hole, spread radially and deposited across the upper mesh, and then sifted back into the lower fluidized zone. The effective density of the medium at the mouth of the Sink-Hole was determined by examining the probability of large tracer particles of a given density floating over or sinking through the Sink-Hole. It was seen previously for a 13 mm diameter Sink-Hole that the separation density exceeded the bulk density of the fluidized medium, and often also even the skeletal density of the granular media itself. Similar results were achieved here, even for a significant increase in the Sink-Hole diameter up to 60 mm. Moreover, the separation density was largely independent of the size of the tracer particles covering the range 2.8 to 8.0 mm, a sign of genuine density-based separation. The underlying mechanism was attributed to a self-organised, tenuous, jamming condition that develops at the mouth of the Sink-Hole. As the diameter of the Sink-Hole increased, the separation density decreased, while the speed at which the sorting of the test particles occurred increased significantly. A 60 mm diameter Sink-Hole achieved a steady state separation within approximately 10 s, and a remarkably sharp density-based separation.