Coal washeries for beneficiation of high-ash coal:

Coal washeries for beneficiation of high-ash coal:

Coal seam in some of the countries has drift origin, resulted in intimate mixing of mineral matter with coal, giving rise to more ash content. Coal washeries are to reduce ash content in coal. For coking coal and non-coking / energy coal quality improvement, coal washeries are used. Coal having drift origin, coal-ash distribution in the coal matrix is so interwoven that coal is essentially required to crush to smaller sizes for better liberation of coal and ash particles.

Coal washing is a process of separation mainly based on difference in specific gravity of coal and associated impurities like shale, sand and stones etc so that we get relatively pure marketable coal without changing the physical properties of the coal. Coal beneficiation largely depends on gravity difference of coal and ash particles after passing through sizing. In general, technologies adopted for coal washeries are: (a) Jigs; (b) Heavy Media Baths; (c) Heavy Media Cyclones; (d) Froth Floatation; (e) Water-only Cyclones.

The major equipments used in a coal washery are: (a) De-shaling jig; (b) washing jig; (c) heavy media bath for coarse coal; (d) heavy media cyclone for smaller size coal; (e) froth floatation for treating fines below 0.5mm; (f) water-only-cyclone / hydrocyclone; (g) cyclone washer. Magnetite having specific gravity in the range of 4.8 to 5.4 is used in washery for washing process which is ground to –200 mesh and mixed in water to maintain the desired gravity in heavy media bath or hydrocyclone for separation of coal with its impurities. The froth flotation plant uses light diesel oil and pine oil, as conditioning and frothing agents. The other main supporting equipments used in Washeries are : Rotary Breaker; Feeder Breaker; Gyratory Crusher, Single/double roll Crushers, Impact crushers, Hammer mill ,Ball Mill etc.; Tipplers; Vibro/Reciprocating feeders; Vibrating Screens; Various kinds of pumps; Vacuum pumps; Compressors; Blowers; Conveyor Beltings and gear boxes; Bucket elevator, scrapper conveyor, flight conveyors etc.; Magnetic Separator; Thickeners; Filters; Gravity Controllers; Automatic Ash Analysers.

The following advantages for using washed coal in thermal power stations:

1. Reduction in emissions of particulate matter

2. Owing to reduced ash content in coal, reduction in size of coal handling plant at power station end; reduction in size of ash disposal unit; smaller ash ponds

3. Reduction in ash will result in less wear and tear of ball mills, induced draught and forced draught fans; less leakage of boiler tubes and less consumption of fuel oil for flame stabilization

4. Owing to 2 and 3 above, the capital cost of the power station will come down for the same design capacity

5. Railways will carry thermal coal with less ash resulting in increased freight carrying capacity for the railways

6. Reduction in freight charges to the power station

7. Better control in thermal power station operations and the control settings would not need to be changed frequently to take care of fluctuations in the heat value in coal feed

8. Increased plant availability and resultant higher Plant Load Factor (PLF)

Continuous research and development efforts, including trial and adoption of latest equipment/ technology are required, in order to get improvement upon existing system of coal washery. Moreover, the environmental issues are also to be looked upon.