364 The input washability data (50 – 0 mm) feed in LIMN model is simulated as per the process flowsheet selected. The principal of incremental ash is used in modelling of the process flowsheet as well as controller for Reflux Classifier & Column flotation has been applied. Mechanical flotation cells are most widely used machine for the flotation of fine coals (-0.5 mm). However, recent studies confirmed that flotation is less efficient for coal particle > 0.25 mm. Mechanical flotation cells are unable to recover low ash coal and with significant yield losses due to inherent design limitations like excessive turbulence, feed short circuiting, and less control on bubble size. To improve recovery of low ash clean coal, flotation column technology is increasingly becoming popular because it possesses attributes that include high residence time, wash water system, quiescent zone, and bubble generator system. The use of wash water minimizes the quantity of hydrophilic particle entrained in the rising froth which reports to the overhead product stream. The externally mounted spargers facilitate bubble particle contacting in a high-shear environment increases attachment rate. The controlled bubble size can be generated by varying the air flow, slurry flow rate and reagent dose. The superior efficiency of column flotation makes it possible to reduce process time, minimize the number of cleaner stages and volume in circulating load in the flotation circuit. Thus, lesser foot print is required, low energy consumption, saving in collector requirement compared to Mechanical flotation cells. Therefore, in this process flowsheet Column Flotation has been used for -0.25 mm instead of Conventional Mechanical Flotation. 2.2 The Dahibari Washery (1.6 Mtpa), BCCL is NLW coal washery, located at Dahibari, P. O- Kumardhubi, Dist- Dhanbad (Jharkhand). The washery is designed to wash washery grade, WVI coal (avg. ash-46%) from Seam Brindabanpur & Kalimati, Dahibari OC Mine of Chanch Victoria Area, Jharia Coalfield of BCCL. The screen analysis of raw coal (50 mm) feed to the washery is described in Table 3 and float & sink data of 50 mm is described in table 4. It is three-product washery producing clean coal (18% ash), middlings (<40% ash) and rejects (> 60% ash). The washery is under operation and O&M is done BOM Operator (M/s ACB) and brief description of process flow of the washery is mentioned below and is also presented in Figure 8: 1. Screening of as received raw coal 50 mm at 13 mm to obtain 50+13 mm & 13 mm size fraction 2. Deshaling of 50 + 13 mm in Jig to obtain float & sink (shale/ rejects) 3. Crushing of float (50 + 13 mm) to 13 mm and mixing with the natural (-) 13 mm fraction 4. Desliming of (-) 13 mm coal at 0.5 mm to remove fines (- 0.5 mm) 5. Washing of 13+0.5 mm in Heavy Media Cyclone to produce clean coal (i.e. o/f of HM cyclone) & sink (i.e. middlings) 6. Washing of (-) 0.5 mm fine in Water Only Cyclone to produce u/f fraction, which is fed to Spiral for futher treatment and o/f fraction to froth flotation 7. Clean & middlings fraction obtained from Spiral is recirculated W/O Cyclone and discards obtained is mixed with rejects. 8. Overflow of W/O Cyclone is treated in froth flotation to produce clean coal & rejects. Table 3 - Screen-cum-ash analysis Size (mm) Wt % Ash %
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