VI.

Coal Mining

Coal mining is the removal of coal from the ground. The mining method employed to extract the coal depends on the following criteria: (1) seam thickness, (2) the overburden thickness, (3) the ease of removal of the overburden, (4) the ease with which a shaft can be sunk to reach the coal seam, (5) the amount of coal extracted relative to the amount that cannot be removed, and (6) the market demand for the coal.

The two types of mining methods are surface mining and underground mining. In surface mining the layers of rock or soil overlying a coal seam are first removed after which the coal is extracted from the exposed seam. In underground mining, a shaft is dug to reach the coal seam. Currently, underground mining accounts for approximately 60 percent of the world recovery of coal.

A.

Surface Mining

Surface mining is used to reach coal reserves that are too shallow to be reached by other mining methods. Types of surface mining include open-pit mining, drift mining, slope mining, contour mining, and auger mining.

A.1.

Open-pit Mining

In open-pit mining, or strip mining, earth-moving equipment is used to remove the rocky overburden and then huge mechanical shovels scoop coal up from the underlying deposit. The modern coal industry has developed some of the largest industrial equipment ever made, including shovels capable of holding 290 metric tons of coal.

To reach the coal, bulldozers clear the vegetation and soil. Depending on the hardness and depth of the exposed sedimentary rocks, these rocky layers may be shattered with explosives. To do this, workers drill blast holes into the overlying sedimentary rock, fill these holes with explosives, and then blast the overburden to fracture the rock. Once the broken rock is removed, coal is shoveled from the underlying deposit into giant earth-moving trucks for transport.

A.2.

Drift Mining

Drift mining is used when a horizontal seam of coal emerges at the surface on the side of a hill or mountain, and the opening into the mine can be made directly into the coal seam. This type of mining is generally the easiest and most economical type because excavation through rock is not necessary. If coal is available in this manner, it is likely to be mined.

A.3.

Slope Mining

Slope mining occurs when an inclined opening is used to tap the coal seam (or seams). A slope mine may follow the coal seam if the seam is inclined and exposed to the surface, or the slope may be driven through rock strata overlying the coal to reach a seam. Coal transportation from a slope mine can be accomplished by conveyor or by track haulage (using a trolley locomotive if the grade is not severe) or by pulling mine cars up the slope using an electric hoist and steel rope if the grade is steep. The most common practice is to use a belt conveyor.

A.4.

Contour Mining

Contour mining occurs on hilly or mountainous terrain, where workers use excavation equipment to cut into the hillside along its contour to remove the overlying rock and then mine the coal. The depth to which workers must cut into the hillside depends on factors such as hill slope and coal bed thickness.

A.5.

Auger Mining

Auger mining is frequently employed in open-pit mines where the thickness of the overburden is too great for open-pit mining to be cost-effective. Open-pit mining would require the lengthy and costly removal of the overburden, whereas auger mining is more efficient because it cuts through the overburden and removes the coal as it drills. In this technique, the miners drill a series of horizontal holes into the coal bed with a large auger (drill) powered by a diesel or gasoline engine. These augers are typically about 60 m (200 ft) long and 0.6 to 2.1 m (2 to 7 ft) in diameter. As these enormous drills bore into the coal seam, they discharge coal like a wood drill producing wood shavings. Additional auger lengths are added as the cutting head of the auger penetrates farther into the coal. Penetration continues until the cutting head drifts into the top or bottom of the coal seam, into a previous hole, or until the maximum torque (energy required to twist an object) of the auger is reached.

A.6.

Satellite Aids to Surface Mining

In the late 1990s some coal mining enterprises used technologies such as the global positioning system (GPS) to help guide the positioning of mining equipment. Satellites operated by the United States Air Force Space Command and leased to companies for commercial use track the position of mining equipment against a map of a mine’s topography. This map uses colors to distinguish soil that should be excavated, soil that should remain in place, and areas that should be filled in. The equipment driver observes this visual information on a monitor while operating the equipment. Some coal mining enterprises have used GPS to increase mining efficiency up to 30 percent.

B.

Underground Mining

Underground, or deep, mining occurs when coal is extracted from a seam without removal of the overlying strata. Miners build a shaft mine that enters the earth through a vertical opening and descends from the surface to the coal seam. In the mine, the coal is extracted from the seam by various methods, including conventional mining, continuous mining, longwall mining, and room-and-pillar mining.

B.1.

Conventional Mining

Conventional mining, also called cyclic mining, involves a sequence of operations that proceed in the following order: (1) supporting the roof, (2) ventilation, (3) cutting, (4) drilling, (5) blasting, (6) coal removal, and (7) loading. First, miners make the roof above the seam safe and stable by timbering or by roof bolting, processes intended to prevent the roof from collapsing. At the same time, they create ventilation openings so that dangerous gases can escape and fresh air can reach the miners. Then one or more slots—a few centimeters wide and extending for several meters into the coal—are cut along the face of the coal seam, also known as the wall face, by a large, mobile cutting machine. The cut, or slot, provides easy access to the face and facilitates the breaking up of the coal, which is usually blasted from the seam by explosives known as permissible explosives. This type of explosive produces an almost flame-free explosion and markedly reduces the amount of noxious fumes in comparison with conventional explosives. The coal may then be transported by rubber-tired electric vehicles (shuttle cars) or by chain (or belt) conveyor systems.

B.2.

Continuous Mining

Continuous mining involves the use of a single machine known as a continuous miner that breaks the coal mechanically and loads it for transport. This mobile machine has a series of metal-studded rotating drums that gouge coal from the face of the coal seam. One continuous miner can mechanically break apart about 1.8 metric tons of coal per hour. Roof support is then installed, ventilation is advanced, and the coalface is ready for the next cycle. The method used to transport the coal requires the installation of mobile belt conveyors.

B.3.

Longwall Mining

The longwall mining system uses a remote-controlled self-advancing roof in which large blocks of coal are completely extracted in a continuous operation. Hydraulic or self-advancing jacks, known as chocks, support the roof at the immediate face as the coal is removed. As the face advances, the roof is allowed to collapse behind the remote-controlled, roof-building machinery. Miners then remove the fallen coal. Coal recovery is comparable to that attainable with the conventional or continuous mining systems.

B.4.

Room-and-Pillar Mining

Room-and-pillar mining is a means of developing a coalface and, at the same time, retaining supports for the roof. With this technique, rooms are developed from large, parallel tunnels driven into the solid coal, and the intervening pillars of coal are used to support the roof. The percentage of coal recovered from a seam depends on the number and size of protective pillars of coal thought necessary to support the roof safely. Workers may remove some coal pillars just before closing the mine.