1602 Figure 3- Process flowsheet of New Bhojudih Washery 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 -10.0 +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 float (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 Reflux Classifier is obtain fine clean coal & middlings; beneficiation of -0.25 mm in Column flotation cell to obtain 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 4. It can be seen that NGM decreases from 49% to 38% (for 18.0% ash), shows that liberation characteristics has improved on crushing to 10 mm size instead of 13 mm. Figure 4- Near Gravity Material (NGM) vs Specific Gravity curve In DMC, centrifugal force is critical to obtaining higher separation performance and is also key to setting the lower particle size limit for efficient separation. For coal particles that are fine, the settling of the particles must be controlled by the equilibrium of centrifugal force and viscous force. Nevertheless, when the particle size comes close to 2 mm, the viscous force begins to dominant over the centrifugal force. For particle diameters less than 1 mm, the viscous force becomes predominant, DMC performance decreases and density cut-point (offset) increases significantly [9]. Therefore, the lower particle size is chosen as 1 mm in coarse coal separation by DMC. Among Intermediate Size Beneficiation, fine dense media cyclone is no doubt the best equipment (very sharp separation, Ep: 0.05-0.08) till date, but due to complexity in operation – precise control of
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