Coal preparation serves several purposes. One important purpose is to increase the heating value of the coal by mechanical removal of impurities. This is often required in order to find a market for the product. Run-of-mine coal from a modern mine may incorporate as much as 60 percent reject materials.
Coal Preparation plant generally use gravity process equipment to separate the refuse from the product (coal). Coal has a specific gravity between 1.35 and 1.5, while the refuse rock has a Specific Gravity of 2.1 to 2.3. Heavy Media is the most popular method of cleaning coarse sizes, jig plants are probably the second most common method used for coarse coal. Heavy media cyclones are being used more often for fines size fractions. Flotation is generally used to clean the -28 mesh size fraction, although spirals and heavy media cyclones have shown success in cleaning down to 100 mesh coal feed. Spirals are generally used for middling sizes ( 10 mesh to 60 mesh).
Coal Processing Description
As it leaves the mine, coal varies widely in size, ash content, moisture content, and sulfur content. These are the characteristics that can be controlled by preparation. Sizes range upward to that of foreign materials, such as a chunk of rock that has fallen from the mine roof or a metal tie;large pieces of coal from a very hard seam are sometimes included. Ash content ranges from three to sixty percent at different mines. Most of the ash is introduced for the roof or bottom of the mine or from partings (small seams of slate) in the coal seam. This ash, called extraneous ash, is heavier than 1.80 specific gravity. The remaining ash is inherent in the coal crushing plant. The density of coal increases with the amount of ash present. The moisture content of the coal is also of two types. The surface moisture, that which was introduced after the coal was broken loose from the seam, is the easier to remove. This moisture is introduced by exposure to air, wet mining conditions, rainfall (in stockpiles), and water sprays. The remaining moisture, called “bed”, “cellular”, or “inherent” moisture, can be removed only by coking or combustion. This moisture was included during formation of the coal.
Air pollution control often requires partial removal of pyrites with the ash to reduce the sulfur content of the coal. Ash content often must be controlled to conform to a prescribed quality stipulated in contractual agreements. Because of firing characteristics, it is often as important to retain the ash content at a given level as it is to reduce it.
Freight savings are substantial when impurities are removed prior to loading. Finally, the rejected impurities are more easily disposed of at the mine site remote from cities than at the burning site, which is usually in a populated area.