The dry beneficiation of duff coal in a dense medium fluidised bed
Langner, Daniël Johannes
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Traditionally wet beneficiation is the predominant method in order to beneficiate coal due to its sharp separation efficiency; however large quantities of water are required in order to process the coal. Water scarcity, especially in arid coal rich countries, is therefore encouraging the development of dry coal beneficiation technologies. The use of a dense medium fluidised bed (DMFB) has received a great deal of consideration in recent years, specifically in China. The use of a DMFB on a large scale was mostly concentrated on the beneficiation of coarser (+6 mm) coal fractions. Although the technology proved to be successful for these coarser fractions, little work has gone into the beneficiation of the smaller (-6 mm) coal fractions. The purpose of this paper was therefore to determine whether the DMFB can successfully remove impurities from the duff (small) coal particles (-5.6 +0.5 mm). A seam 4 run-of-mine coal, from Witbank, was used during the project which had an initial ash yield percentage of 22.95 % and a CV of 24.16 MJ/kg. The fluidised bed was operated with and without vibration, and the influence of different dense media was tested. Magnetite, sand and a fine discard coal were used as fluidising medium. It was found that the addition of magnetite was able to segregate coal particles according to density, but did not improve the destoning capabilities of the fluidised bed. This was mainly attributed to the very fine nature of the magnetite, which caused back mixing, plug-flow and turbulent behaviour within the bed. Furthermore the separation of magnetite from the coal was troublesome. From all the different dense media, sand was found to give the best separation efficiencies. The use of sand as fluidising medium created a more stable bed, which aided in the density separation. The results therefore indicated that sand is a lucrative alternate to magnetite. A fine discard coal, which was used as a fluidising medium, gave moderate results. However the destoning capabilities of the fluidised bed, with the use of this fine discard medium, were in most cases better than when magnetite was used. Hence, the addition of a fluidising medium did not improve the separation efficiencies of the fluidised bed, and the fluidisation of coal without any medium gave the best results. The addition of vibration did not have a significant impact on the destoning capabilities of the fluidised bed, when no medium was added. However, when a medium was used in order to beneficiate the coal, the addition of vibration improved the sharpness of separation. It was concluded that the use of a fluidised bed is a viable process in order to remove high ash value material from a typical South Africa coal with a size range between 0.5 and 5.6 mm.
- Engineering