Coal Age

JUN 2016

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June 2016 www.coalage.com 39 fine coal recovery continued This is problematic because: • Plant feed changes, continuously; • Mining conditions and fines loading get typically worse, rarely better; • Plant circuitry or its operating param- eters are often modified to cope with changing situations and markets; and • Fines generation within plant circuitry frequently occurs, and is particularly onerous with friable, high-value met- allurgical coals. In most plants, screen drain material is a high-quality, clean coal stream that is recirculated back to the screen bowl feed. However, this material often does not report directly to the screen bowl feed. It reported somewhere upstream of the screen bowl feed and is thus exposed to additional degradation in each pump and additional process losses in each interme- diate circuit. Since the screen drain stream is typically double the amount shown on the flow sheet and finer than the original screen bowl feed, this recirculating stream contributes significant additional fines loading to a potentially heavily loaded or overloaded fine coal circuit. Plant Bottleneck With many coal preparation plants, the fine circuits are the process capacity bot- tleneck. The recirculating load and circuit overload issues tend to increase separating densities, consume available flotation car- rying capacity, and reduce both yield and recovery efficiency. A heavy recirculating load is a major issue. After assessing the situation at site, we help answer the fol- lowing questions: • How much of a recirculating load do you have in the screen drain? • If the fines circuit is the plant bottle- neck, how can this be improved? • Does removing the screen drain, a fine clean coal recirculating load, free up the fine circuit for additional capacity, addi- tional recovery and/or reduced moisture? • How much of the recirculating load is effectively recovered to product? • Does the screen drain tonnage increase with increased screen bowl life/wear? Ultimately, what benefits for the en- tire operation can be achieved simply by processing the screen drain in a more effective and controllable way? Apart from higher coal recovery, benefits may include additional plant capacity, lower reagent consumption, cleaner water, and increased circuit capacity/availability in other circuits of the plant. On-site Test Results Nearly all test results to date indicate that only about one-half of the total screen bowl drain tonnage going into recircula- tion is effectively recovered to product. Just because the material is fully recircu- lated does not mean it is recovered in full. Indeed, most coal preparation engineers readily accept that 40-70% of the minus 325 Mesh material in screen bowl feed streams is lost during processing even when the screen drain is being recirculated. However, in the author's experience, processing highly fria- ble and easily fractured coals, much of the minus 200 Mesh material in the screen bowl feed is frequently lost despite (or perhaps, because) recirculation has been employed. In fact, the combination of additional degra- dation, circuit overloading, and related pro- cess losses often causes 1/2 or more of the total amount of screen drain material to be lost in the process. Field tests show that, when the screen drain material is processed through the SCI unit, the recovery of this material is between 93%-99% for all tests to date. With the screen drain tonnage removed from recirculation, the tonnage handled and the moisture produced by the existing screen bowl both decrease. In addition, the product yield from the screen bowl typically increases. These results can be easily explained since the SCI unit is removing a recircu- lating load that is not only finer than the "fresh feed" to the circuit but has not been further attrited during additional recir- culation. The combined use of the screen bowl and the Sub325 centrifuge more ef- fectively reduce the overall cake moisture and increase the recovery of fine clean coal from the fine circuits. The remaining fine coal circuit is less loaded and more efficient. Additionally, the SCI bowl effluent is generally >90% minus 10 micron and typically higher in fine silicate and clay content. The SCI unit therefore upgrades the product quality of the ultra-fines recovered. Case Study One: Steam Coal At one steam coal plant, the flow sheet showed a screen bowl dry feed tonnage of 103 tons per hour (tph) feeding two screen bowls. The bowl effluent was 3 tph and the screen drain was 4 tph, according to the flow sheet. On-site testing and measure- ment of the screen drain material showed more than 16 tph of screen drain tonnage on a dry basis. This represents a fourfold increase to the flow sheet estimate. The re- sulting SCI product was 21 tph (16 dry tph plus 23% moisture). As shown in Table 1, the measured performance for processing the screen drain with the SCI unit was more than 96% solids recovery using percent solids to balance the streams. This test removed 16 dry tph of clean coal from the plant re- circulating load. Additionally, the effluent from the SCI unit at this location was 1.5% solids with a dry ash of 73%. Ash balance yield indicates a weight recovery of 95.5%, in good agreement with above solids bal- ance. It must be noted that coal recovery, combustible matter recovery, on an ash basis, is 98.5% because the SCI effluent is very high in ash and contains mainly clay, not valuable coal. The clean coal tonnage and yield were measured with and without the SCI unit running. During the test, the SCI tonnage went on the ground in a separate pile. The tonnage on the clean coal belt went down slightly, 0.3%, during the SCI test. However, the total plant production gain determined by belt scales with the addition of the SCI tonnage was a net 1.6% yield increase while the SCI Sub325 unit was processing screen drain material. At a similar facility the plant yield increase was projected to be 1.1% of the ROM feed. In both cases, the net effect on product quality was minimal. Table 1 — Sub325 Mass Balance for Steam Coal Case Study

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