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The intrinsic quality of Indian coal, along with the dominant practice of open cast mining, has meant that run of the mine (ROM) Indian coal contains a huge share of ash and other minerals.  ROM coal typically has high ash content from 30-50 percent and low calorific value (2,500-5,000 kcal/kg).  In general, high ash content creates problems for coal users that include erosion, difficulty in pulverisation, poor emissivity and flame temperature, low radiative transfer, and generation of excessive amounts of fly-ash containing large amounts of un-burnt carbon.  In addition, the transport of ROM coal across long distances is wasteful, as it carries large quantities of ash-forming minerals that results in infructuous use of rail and port capacity.  The transport of high ash coal across long distances also contributes to emission of carbon dioxide (CO2) and other greenhouse gases (GHG) from the mode of transport (rail and road).

Thus, before coal can be sold to consumers, it must be cleaned. Earlier only coking coal was being washed because steel making needs coking coal of ash of 17 to 18%. The raw coal ash in Indian coking coal varies from 25 to 30%. Now-a-days, Ministry of Environment & Forests has put restriction on the use of high ash coal in power sectors, which necessitated priority to wash non-coking coal also.

The raw coal extracted out from the coal mine is processed through different techniques to achieve the desired qualities. These result in higher economic value of run of-mine (ROM) coal. The environmental impacts of burning of coal is also to be kept in mind. The mineral matters of the coal is to be reduced during its processing such that, the emissions of sulphur dioxide (SO2), carbon dioxide (CO2) and particulate matters are minimized during burning. The technology of coal washing or coal preparation is applied to produce specific desirable coal products from the run-of-mine coal without the change of the physical identity.

The choice of process equipment involved in coal washing depends on factors such as the type of coal being treated; the market requirement and the economics. Continuous research and development efforts, including trial and adoption of latest equipment/ technology are being done Coal Washing Technologies under trial in India.

• Increased generation efficiency, mainly due to the reduction in energy loss as inert material passes through the combustion process

• Increased plant availability

· Reduced investment costs

• Reduced operation and maintenance (O&M) costs due to less wear and reduced costs for fuel and ash handling

• Energy conservation in the transportation sector and lower transportation costs

• Less impurities and improved coal quality

• Reduced load on the air pollution control system; and

• Reduction in the amount of solid waste that has to be disposed off

Coal washing is accomplished by one of two major processes, by density separation or by froth flotation. Both processes depend on the fact that the particles of which a coal sample are made have different densities. When water, for example, is added to the sample, particles sink to various depths depending on their densities. The various components of the sample can thus be separated from each other.

In some cases, a liquid other than water may be used to achieve this separation. In a heavy medium bath, for example, a mineral such as magnetite or feldspar in finely divided form may be mixed with water, forming a liquid medium whose density is significantly greater than that of pure water.

A number of devices and systems have been developed for extracting the various components of coal once they have been separated with a water or heavy medium treatment. One of the oldest of these devices is the jig. In a jig, the column of water is maintained in a constant up-and-down movement by means of a flow of air. Clean coal particles are carried to the top of the jig by this motion, while heavier refuse particles sink to the bottom.

Another method of extraction, the cyclone, consists of a tank in which the working fluid (water or a heavy medium) is kept in a constant circular motion. The tank is constructed so that lighter clean coal particles are thrown out of one side, while heavier refuse particles are ejected through the bottom.

Shaking tables are another extraction method. As the table shakes back and forth, particles are separated by size, producing clean coal at one end and waste products at the other.

In cylindrical separators, a coal mixture is fed into a spinning column of air that throws the heavier waste particles outward. They coat the inner wall of the cylinder and fall to the bottom, where they are drawn off. The clean coal particles remain in the centre of the air column and are drawn off at the top of the cylinder.

Froth flotation is the most widely used method for cleaning fines. Froth flotation cells utilize the difference in surface characteristics of coal and refuse to clean ultra-fine coal. The coal-water mixture is conditioned with chemical reagents so that air bubbles will adhere only to the coal and float it to the top, while the refuse particles sink. Air is bubble dup through the slurry in the cell and clean coal is collected in the froth that forms the top. This type of cleaning is very complex and expensive and is principally for metallurgical coals. One of the commonest steps to improve the performance of a flotation unit is to separate the pyrite at an earlier stage using cyclones, spirals or tables. Separation of clean coal from waste materials can be achieved in froth flotation by varying factors, such as pH of the solution, time of treatment, particle size and shape, rate of aeration, solution density, and bubble size.

Washed coal is usually dewatered to some extent as excess moisture lowers the deliverable heat content in the coal and increases the weight of coal. Dewatering equipments includes less costly vibrating screens, filters, or centrifuges to the more costly heated rotary kilns or dryer units.