Density segregation of granular material in a rotating cylindrical tumbler

Many mining operations use large quantities of water to separate valuable minerals from less valuable gangue. This dependence on liquid separation has an environmental impact in terms of energy and water use and also implies a cap on production due to the availability of water. To address these problems, the CSIRO has developed the CSIRO Rotational Classifier, which - by using the phenomena of rotational segregation - can quickly separate dry granular material in terms of size and/or density without the use of any liquids. The purpose of this paper is to obtain a deeper understanding of how rotational segregation can separate particles of different densities in a rotating cylinder, free from any interstitial fluids. This was accomplished by analyzing a cross section at the 20% fill level in a 50% full classifier, which contained a 50-50 ratio of glass and lead beads. The granular bed was sampled at different time intervals over a 60 second period with a classifier rotation rate of 2 rpm. These experiments resulted in a high segregation level of 0.9 in 20 seconds and 0.95 by 60 seconds (where a level of 1 implies full segregation). The results then underwent image analysis and were subsequently compared to results from a discrete element method (DEM) model where similar segregation ratios, albeit at longer timescales, were obtained. This study gave a further insight into the segregation process particularly in terms of axial formation of the segregated core which may one day be used in the separation of minerals