Gravity Separation of Ultrafine Coal using Centrifugal Forces

A new separator suitable for recovering ultrafine feed, less than 0.2 mm, by gravity separation was investigated. The separator consists of a fluidized bed mounted within a high speed centrifuge, utilizing fluidization water to suspend the particles against the high G forces. Further, the fluidized bed housing incorporates closely spaced planar channels, inclined at 20^o to the direction of the G Forces, similar to a Reflux Classifier which operates at 1G. The work from this study shows the capacity advantage achieved from the high G Forces and that achieved from the parallel inclined channels actually multiply, delivering remarkably high capacity. For example, in one case the superficial flow velocity through the channels was more than 3000 times the settling velocity of a particle at the separation size, even though the number of Gs was only 73. The plates comprising the system occupy 50% of the flow area, so the actual capacity advantage is half of this result. Nevertheless, this new system has the potential to change the way ultrafine particles are processed in the coal industry. In principle the high capacity device could ultimately compete directly with flotation, and offer new solutions to processing coal tailings, where conventional flotation has failed. This project succeeded in establishing a clear path for developing a full-scale commercial device.

This report outlines the design of the separator, and the experimental work conducted. A significant number of experiments were conducted using silica flour in order to quantify the separation size achieved from semi-batch elutriation experiments. This work helped to insure the system performed in a manner consistent with our theoretical model.  The work also helped to prove the finding that the G forces combine with the inclined channel segregation, delivering a powerful synergy. A series of fine coal elutriation experiments showed that a strong inertial lift force delivers enhanced separation from fine dense silica. Fine coal separations conducted on specific Float-Sink fractions showed excellent separation performance, with Ep values typically 0.07 to 0.09 for particles much finer than 0.20 mm, even at low solids concentrations. There was also remarkably little variation in the separation density with particle size down to 0.038 mm. In another experiment, a flotation feed with a +0.038 mm feed ash of 8.5% was separated into a product of 4.1% ash and reject ash of 13.5%. This worked demonstrated the powerful selectivity available at high G forces. A similar performance was achieved under continuous steady state conditions.