Pollution associated with coal washeries for beneficiation of 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 / thermal 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.
Pollution and its control: During various operations in coal washeries, a lot of particulate matters and gaseous pollutants are generated causing a serious air pollution problem in the area. Besides, coal washeries release very large amount of solid and liquid waste causing serious environmental problems. The washeries reduce the ash content of coal to 17.5% or less. This process consumes clear water in the range of 0.2 to 0.25 m3/tonne of raw coal input. The washeries are operated in the closed water circuit system but still about 12-18% of raw water is discharged as effluent. Though, effluent is treated in settling tank but sometimes overloading results into failure of closed circuit system. The rejects (15 to 25% of input) are dumped near available land without caring for stability of dump, as a result dumps near the river bank cause erosion of coal particles during rainy season and accumulation of fine coal particles on the bottom of river / lake bed.
In general, typical coal washery effluents characteristics are:
1. Physical appearance - Blackish brown to deep black in colour
2. pH – 7.1 to 7.8
3. Suspended solids – 800 to 4000 mg/L
4. Particle size – 0 to 150, 150 to 200, 200 to 300 micron
5. Dissolved solids – 300 to 1500 mg/L
6. Oil & grease – 15 to 200 mg/L
Though many of the washeries have adequate settling tanks but disposal of solid waste required to be accelerated, specially in developing countries to check with the pollution. Proper the reject dump management is required to minimise soil erosion.
Another very useful process of generating electricity with the help of washery rejects by using eco-friendly Fludised Bed Combustion (FBC) technology. By adopting technology which uses washery rejects, the disposal of fines and rejects can be utilized properly and economically; as coal of drift origin require to grind coal before beneficiation and thereby generation of fines are more.