1607 Figure 8- Existing process flowsheet of Dahibari Washery A designed process flowsheet for washery grade, W-VI coal has been developed through washability data of Dahibari OC mine (50 – 0 mm) by using LIMN: Flowsheet processor for optimum recovery of clean coal. In the proposed flowsheet, screening of -50 mm coal is done at 10 mm and deshaling of 50+10 mm coal is done to in Jig to remove the shale before feeding to washery; re-crushing of Jig floats to 10 mm and mixing of natural (-)10 mm; Classify the crushed coal into coarse, fine and ultra-fine fractions as -10+1.0 mm, -1.0+0.25 mm and (-)0.25 mm sizes; beneficiation of 10.0+1.0 mm coal fraction in Dense Media Cyclone (DMC) in two stage due to high NGM value for sharp separation; in first stage beneficiation is done at lower gravity to obtain floats (clean coal: 18% ash) & sink; rewashing of sink in second stage at higher gravity to obtain middlings (ash < 40%) and rejects; beneficiation of -1.0 +0.25 mm in Fine Dense Media Cyclone/ Reflux Classifier is utilised to produce fine clean coal & middlings; beneficiation of -0.25 mm in Column flotation cells to produce clean coal & rejects. The top size of feed is selected as 10 mm size for improving the liberation and washability as finer crushing helps in distribution of NGM to higher gravity section. The variation of NGM with cut density for coal of top size as 13 mm (plot-1) and 10 mm (plot-2) is shown in Figure 9. It can be seen that NGM decreases from 48% to 30% (for 18.0% ash), shows that liberation characteristics has improved on crushing to 10 mm size instead of 13 mm. Figure 9- Near Gravity Material (NGM) vs Specific Gravity curve Dense Media Cyclone is chosen for the treatment of intermediate size fraction, 1+0.25 mm because in Reflux Classifier, the loss of yield to tailings is more due to high NGM at cut density of separation. So, for sharp separation and maximising yield of clean coal, fine DMC is selected inspite of complexity in operating small diameter DMC. The input washability data (50 – 0 mm) feed in the LIMN model is simulated as per the process flowsheet selected. The principal of incremental ash is used modelling of the process flowsheet as well as controller for Reflux Classifier & Column flotation has been applied.
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