Coal Mining Process
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. 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.
Foreign materials are introduced into the coal during the mining process, the most common being roof bolts, ties, car wheels, timber, shot wires, and cutting bits.
Sulfur in coal occurs as sulfates, organic sulfur, and pyrites (sulfides of iron). The sulfates usually are present in small quantities and are not considered a problem. Organic sulfur is bound molecularly into the coal and is not removable by typical coal preparation processes. Pyrites generally are present in the form of modules or may be more intimately mixed with the coal. Coal preparation plants remove only a portion of the pyritic sulfur; therefore the degree of sulfur reduction depends on the percentage of pyrites in the coal, the degree to which this is intimately mixed with the coal, and the extent of coal preparation.
All of the materials described above are combined with the coal to form the ROM feed. Coal, as referred to above, denotes the portion of the feed that is desired for utilization.