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coarse coal continued Figure 6: Preparation of fine feed for round two testing by screening the first round clean product at ¾ inch. Figure 7: Circuitry used in the second round of fine coal testing of the DriJet sorter. priate for the upgrading of finer particles. The feed for these experiments was prepared by screening the clean coal product from the first round of testing at ¾ inch (see Figure 6). The plus ¾ inch material was collected and set aside, while the minus ¾ inch was then passed through two additional stages of sorting using the new set of operating conditions. The resultant test data is summarized in Figure 7. Figures 8 and 9 provide plots of the recovery-ash and recovery-rejection data obtained from the testing of the finer coal feed. As expected, the separation of both of the finest size frac- Figure 8: Size-by-size combustible recovery and clean coal ash obtained while the sorter was configured for fine coal cleaning. 50 www.coalage.com tions (½ x ¼ inch and ¼ inch x 4 mesh) improved dramatically by reconfiguring the sorter electronic setting to conditions more suitable for treating finer solids. After the first stage of cleaning, the feed ash content for the ½ x ¼ inch fraction was reduced from 27.5% down to 16.6%. A second stage of re-cleaning further reduced the ash down to 12.9%. As expected, the ¼ inch x 4 mesh size did not respond as well, achieving clean coal ash values of 29.5% and 21.6%, respectively, after two stages of cleaning a feed stream containing 32.5% ash. Nevertheless, this level of performance was still considered to be good given that the sorter technology was primarily designed for upgrading plus ¼ inch solids. Discussion The pilot-scale test program provided some important information regarding the operational characteristics of the dry sorter technology for coal cleaning applications. For example, the data indicate that the technology performs best when the unit has been configured to treat a specific narrow particle size fraction. In fact, the data suggest that high levels of separation performance may be realized by prescreening the feed coal into different size classes then treating each size using a sort optimized for that particular particle size class. This upfront preprocessing step is not considered to be a serious issue; however, since coal sizing is a normal occurrence in all coal processing operations. Also, this type of size-by-size circuitry would allow each sorter to be optimized for a given size class so that maximum throughput capacity could be attained for the lowest overall investment in capital equipment. Another interesting observation obtained from the test data is that the performance begins to deteriorate significantly below a critical particle size. This finding supports the manufacturer's recommendations that only particles coarser than about ¼ inch are best suited for upgrading using the current configuration of the coal sorter technology. From an engineering perspective, the particle size constraint is not surprising considering the requirement that a single layer of particles needs to be presented to the X-ray scanner. The limitation imposed by particle presentation makes it possible to estimate the theoretical maximum production that can be attained using the new sorter technology. The effective spatial volume (Q) moving through the scanner can be calculated using: Figure 9: Size-by-size combustible recovery and ash rejection obtained while the sorter was configured for fine coal cleaning. where W is the width of the scanner belt, Dp is the particle diameter (bed height), V is the belt velocity and β is the particle packing efficiency. For spherical monosized particles placed back-to-back along the conveyor, β cannot exceed a value of π/6 (i.e., ratio of sphere-to-cube volume). From these expressions, the maximum mass flow rate (M) that can be passed as a single layer of particles through the separator is given by: June 2013