III.

Coal Mining

Coal has been mined for more than 1,000 years, and large-scale mining was practiced as early as the 18th century. The first coal mine in America was opened in Virginia, in the Appalachian bituminous field, during the 1750s; the mining of anthracite began in the late 1700s. Extensive mining in the United States commenced about 1820; until 1854 more than half of all the coal that was produced in the U.S. was Pennsylvania anthracite. In 2000, anthracite production was about 4.15 million metric tons, compared to about 970 million metric tons of bituminous coal and lignite.

Two principal systems of coal mining are used: surface, or strip, mining and underground, or deep, mining. Strip mining, which is a form of quarrying, is possible only when the coal seam is near the surface of the ground. In large surface mines, huge power shovels and draglines are used to remove the earth and rock (overburden) from above the seam; modern shovels have bucket capacities of as much as 290 metric tons. Smaller shovels then load the coal directly into trucks. The chief advantage of strip mining over underground mining is the enormous saving of time and labor. The daily output per person in strip mines is many times that in underground mines.

As a supplement to strip mining, or when other mining techniques are not adequate, augers are used to bore horizontally into exposed coal seams. The loosened coal then flows into a conveyor for loading into trucks. A newer development is a boring machine, called a push-button miner, that can tunnel as deep as 300 m (1,000 ft) into the coal seam, dumping the coal into mobile conveyors pulled by the machine.

In underground, or deep, mining, the coal seam is reached through vertical or inclined shafts, or, if the deposit is located in a mountain, through level or nearly level tunnels. The coal deposit is usually marked out in “rooms,” which vary in size according to local conditions. The coal is cut and blasted away, with pillars of coal left to support the roof. In the longwall system of working, a machine with steel teeth is raked along the face, and the broken coal drops onto a conveyor belt. As the machine moves forward, steel supports are advanced to support the roof directly over the working face. The roof behind the coal face is allowed to collapse.

In the conventional method of mining, power cutters have supplanted the traditional tool of the miner, the pick. The miner makes an undercut with these cutters about 15 cm (about 6 in) wide and as much as 2.7 m (9 ft) deep across the face of the coal seam, often close to the floor of the room. Deep holes are then drilled at the top of the face, and charges of safety-approved explosives or cartridges of compressed air are tamped into them. The explosive blast brings down and partially shatters a large chunk of the coal face, which is then loaded by machines into low, electrically propelled shuttle cars that bring the coal to a central loading point. From there it is hauled to the surface by either rail cars or giant conveyor belts. Most of the U.S. underground production of bituminous coal is mined by so-called continuous-mining machines, which eliminate the separate steps of cutting, drilling, blasting, and loading. These huge machines, capable of mining up to 10.8 metric tons of coal per minute, tear coal from the face and load it onto built-in conveyor belts. The belts transport the coal to waiting shuttle cars or mine conveyor belts that carry it to the surface. The coal is then transferred to a preparation plant, where it is screened, washed, sorted into various sizes, and sometimes crushed before shipment.

A recent development, called the shortwall system, combines the continuous-mining machine with the use of longwall steel supports at the face. The continuous miner operates under the protective canopy of the supports and the roof is allowed to cave in as in the longwall system.

Among the chief problems in underground mines are ventilation and roof support. Ventilation is important because of the presence in coal mines of dangerous gases such as methane and carbon dioxide. Large fans and blowers must be used to maintain the circulation of pure air. In order to prevent the spread of coal dust, which can be highly explosive, mine interiors are frequently sprayed with limestone dust, a process known as rock-dusting. To provide support for the roofs of tunnels and work spaces, steel roof bolts that bind together the overlying rock layers are inserted into the mine ceiling.

In the U.S., mining employment generally declined through the late 1960s. In the early 1970s, however, with the Arab oil embargo (see Energy Supply, World: The Energy Crisis), coal reemerged as an important fuel. As production increased, so did employment—to more than 200,000 men and women in the early 1980s. By 1990, the coal mining employment dropped to 147,000 workers and by 2000 to 80,000.