07 January 2010

Methods Of Mining

Underground Mining

Underground modes of access include drift, slope, and shaft mining, and actual mining methods include longwall and room and pillar mining. Drift mines enter horizontally into the side of a hill and mine the coal within the hill. Slope mines usually begin in a valley bottom, and a tunnel slopes down to the coal to be mined. Shaft mines are the deepest mines; a vertical shaft with an elevator is made from the surface down to the coal. In western Kentucky, one shaft mine reaches 1,200 feet below the surface.

In room and pillar mining, the most common type of underground coal mining, coal seams are mined by a "continuous miner" that cuts a network of "rooms" into the seam. As the rooms are cut, the continuous miner simultaneously loads the coal onto a shuttle or ram car where it will eventually be placed on a conveyor belt that will move it to the surface. "Pillars" composed of coal are left behind to support the roof of the mine. Each "room" alternates with a "pillar" of greater width for support. Using this mining method normally results in a reduction in recovery of as much as 60 percent because of coal being left in the ground as pillars. As mining continues, roof bolts are placed in the ceiling to avoid ceiling collapse. Under special circumstances, pillars may sometimes be removed or "pulled" toward the end of mining in a process called "retreat mining." Removing support during retreat mining can lead to roof falls, so the pillars are removed in the opposite direction from which the mine advanced: hence the term "retreat mining."

Longwall mining is another type of underground mining. Mechanized shearers are used to cut and remove the coal at the face of the mine. After the coal is removed, it drops onto a chain conveyor, which moves it to a second conveyor that will ultimately take the coal to the surface. Temporary hydraulic-powered roof supports hold up the roof as the extraction process proceeds. This method of mining has proven to be more efficient than room and pillar mining, with a recovery rate of nearly 75 percent, but the equipment is more expensive than conventional room and pillar equipment, and cannot be used in all geological circumstances. As mining continues, roof bolts are placed in the ceiling to avoid ceiling collapse. In longwall mining, only the main tunnels are bolted. Most of the longwall panel is allowed to collapse behind the shields (which hold the roof as coal is excavated).

Surface Mining

Surface-mining methods include area, contour, mountaintop removal, and auger mining. Area mines are surface mines that remove shallow coal over a broad area where the land is fairly flat. Huge dragline shovels commonly remove rocks overlying the coal (called overburden). After the coal has been removed, the rock is placed back into the pit. Contour mines are surface mines that mine coal in steep, hilly, or mountainous terrain. A wedge of overburden is removed along the coal outcrop on the side of a hill, forming a bench at the level of the coal. After the coal is removed, the overburden is placed back on the bench to return the hill to its natural slope. Mountaintop removal mines are special area mines used where several thick coal seams occur near the top of a mountain. Large quantities of overburden are removed from the top of the mountains, and this material is used to fill in valleys next to the mine. Augur mines are operated on surface-mine benches (before they are covered up); the coal in the side of the hill that can't be reached by contour mining is drilled (or augured) out. Drift, contour, mountaintop removal, and augur mining are more common in the Eastern Kentucky Coal Field, and area, slope, and shaft mining are more common in the Western Kentucky Coal Field.


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