Mining

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Mining

Mining is the process by which ores or related materials are extracted from the Earth . Ore is defined as a rock or mineral, generally metallic, which can be mined, processed, transported, and sold at a profit. Therefore, the classification of an Earth material as ore depends as much on economics and technology as geology . Nonmetallic substances that are commonly mined but not considered to be ores include coal , phosphate, and sand and gravel. The term groundwater mining is sometimes used to describe the withdrawal of groundwater from an aquifer more rapidly than it is recharged by infiltration or underground flow from adjacent areas.

Mining can occur either at Earth's surface in strip mines or open pit mines, or beneath the surface in underground mines. The method used depends on the depth, lateral extent, and economic value of the rock being mined. The deepest underground mine on Earth, which is 2.4 mi (3.8 km) deep, is a South African gold mine. The open-pit Bingham Canyon Mine near Salt Lake City, Utah, is more than 2.5 mi (4 km) wide and more than 0.62 mi (1 km) deep. It is the largest man-made excavation on Earth. Excavation of the pit began in 1906 and has continued into the early twenty-first century, producing primarily copper with smaller amounts of gold, silver, and molybdenum.

History of mining

Many metals occur in their native state or in readily accessible ores. Thus, the extraction and working of metals dates much further back in time than does the mining industry. Some of the earliest known mines were those developed by the Greeks in the sixth century b.c. As were mines for many centuries thereafter, the workers in these mines were slaves and prisoners of war. By the time the Roman Empire reached its peak, it had established mines throughout the European continent , in the British Isles, and in parts of North Africa . The first scientific description of mining operations was the book De Re Metallica by the Saxon physician Georgius Agricola (1494–1555). De Re Metallica, which remained an authoritative reference for nearly 200 years, was translated from Latin to English by mining engineer and former United States president Herbert Hoover (1874–1964) and his wife Lou Henry Hoover (1874–1944).

Surface mining

Surface mining is less expensive and safer than underground mining. About 90% of the rock and mineral resources mined in the United States and more than 60% of the nation's coal is produced by surface mining techniques. Coal mining accounts for about half of all surface mining, extraction of sand, gravel, stone, and clay for another 35%, phosphate rock for about 5%, and all metallic ores, for about 13%.

There are four kinds of surface mines: open-pit mines, strip mines, mountaintop mines, and alluvial (or placer) mines. Open-pit mines consist of deep cone-shaped holes or pits that are excavated in rock that is first loosened by blasting. In order to prevent the sides of the pit from collapsing, open-pit mines must be continually widened as they are deepened. Open-pit mines are used when the ore is of low grade, meaning that the amount of metal per cubic meter or kilogram of rock is small, and disseminated, meaning that the metals are distributed throughout large volumes of rock rather than being concentrated in veins . The size of open-pit mines, which often take decades to excavate, makes it uneconomical to reclaim the pits by filling them with rock.

Strip mines consist of shallow excavations, perhaps 109 yd (100 m) or less, that are used to mine tabular bodies of rock such as coal. Soil and rock above the mined material, known as overburden, are removed and set aside. Because the strip mines consist of shallow excavations, the overburden can be economically replaced, re-contoured to resemble the original topography, and replanted.

Mountaintop mining came into widespread use in Appalachian coal fields following the 1997 passage of the Surface Mining Control and Reclamation Act by the United States Congress. Mountaintop mining is similar to strip mining in that the overburden above a tabular coal deposit is removed. Instead of being stockpiled and used to restore the original topography, however, the overburden is used to fill adjacent valleys. Although mountaintop mining is an inexpensive method of mining coal in mountainous areas, the filling of valleys can have negative environmental impacts. Mountain-top mining in West Virginia was halted by a court order in 2002 on the grounds that the practice violated the Clean Water Act, but changes to the act may again make the method legal.

Alluvial mining is a form of surface mining used to recover heavy minerals such as gold from sand and gravel beds, including stream beds, known as placer deposits. In some cases these deposits can be removed mechanically by agitating the sand and gravel in simple pans. A more sophisticated and efficient way of separating placer minerals from the sand and gravel is a sluice box, a long, shallow box with wooden separators placed along its bottom. As sand and gravel is shaken in the sluice box, lighter sand grains are washed away and heavier metals are left behind. A particularly destructive form of alluvial mining is hydraulic mining, in which pressurized water is used to wash away large amounts of sand and gravel. Dredges are used in other large-scale alluvial mining operations.

Underground mining

Underground mining involves the excavation of tunnels and rooms beneath Earth's surface. Compared to surface mining, underground mining is expensive and dangerous. Therefore, it is used primarily in situations where high-value ores such as gold are concentrated in narrow veins or other unusually rich deposits. Unlike surface mines, underground mines can also be excavated beneath bodies of water. Salt mines more than 328 yd (300 m) deep, for example, extend beneath Lake Erie near Cleveland, Ohio, and Detroit, Michigan.

The vocabulary of underground mining has developed over several centuries. Shafts are vertical passages excavated downward from Earth's surface, whereas raises and winzes are vertical passages excavated upward and downward, respectively, between horizontal workings beneath the surface. An adit is a horizontal passage excavated into a hillside, whereas an incline is a sloping passage excavated inward from a hillside. Horizontal underground passages following the trend of the ore body are known as drifts. An open room beneath the surface is a stope and its roof is known as the back.

Underground mines are excavated using a variety of methods. Room-and-pillar mining is the excavation of large open rooms supported by pillars. Coal and rock salt (halite) are commonly mined using room-and-pillar methods. Longwall mining is a form of underground mining widely used in the coal industry. A coal seam is completely removed using specialized machines, leaving no support and allowing the overlying rock to slowly subside as the seam is mined. Open-stope mining, in contrast, consists of rooms without any supporting pillars. It is employed if the ore body is small or the rock is strong enough to withstand collapse into the stope. Sub-level caving and block caving involve the excavation of vertical chutes and horizontal passages beneath an ore body, which is then allowed to collapse into the openings under its own weight. Gloryhole mining is a term used to describe caving methods that result in the formation of a crater or depression on the surface above the mine.

Certain water-soluble minerals can be removed from the Earth by dissolving them with hot water piped into the ground under pressure . This practice is known as solution mining. The minerals dissolve in the hot water and then are carried to the surface. In the Frasch process, a system of pipes is sunk into a known deposit of sulfur at some depth under ground. Steam forced into one pipe melts the sulfur, which is then extracted in a liquid form through a second pipe.

Resources

books

Agricola, Georgius. De Re Metallica. Translated by Herbert Clark Hoover and Lou Henry Hoover. New York: Dover Publications, 1950.

Brown, Maurice Russell, and Herbert Ralph Rice. Mining Explained in Simple Terms. Toronto: Northern Miner Press, 1968.

Hustrulid, William A., and Richard L. Bullock, eds. Underground Mining Methods: Engineering Fundamentals and International Case Studies. Littleton, Colorado: Society for Mining, Metallurgy, and Exploration, 2001.

Thomas, L. J. An Introduction to Mining: Exploration, Feasibility, Extraction, Rock Mechanics. Sydney: Hicks Smith & Sons, 1973.

other

Charleston Gazette Online. "Mining the Mountains." 2002 [cited January 15, 2003]. <http://wvgazette.com/static/series/mining/>.

Kentucky Department for Surface Mining Reclamation and Enforcement. "Mining Education." November 20, 2002 [cited January 15, 2003]. <http://www.kyenvironment.org/nrepc/dsmre/NRDSMRE/dsmremay13/mining_education.htm>.

United Mine Workers of America. "What Coal Miners Do." [cited January 15, 2003]. <http://www.umwa.org/mining/colminrs.shtml>.

David E. Newton William C. Haneberg

KEY TERMS

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Adit: —A horizontal passage constructed into a hillside to gain access to underground mineral deposits.
Overburden: —Material that must be removed in order to gain access to an ore or coal bed.
Panning: —The process of separating metallic nuggets and flakes from sand and gravel by shaking them in a pan filled with water.
Placer: —An accumulation of metallic flakes or nuggets, typically gold, in sand and gravel deposited by running water.
Raise: —Vertical passages extending upward from a level in a subsurface mine.
Room-and-pillar: —A technique for mining certain kinds of minerals, especially coal, in which vertical shafts of the mineral are left in place to support the roof of the mine.
Shaft: —A vertical tunnel constructed downward from the surface to gain access to underground mineral deposits.
Sluice box: —A device used to separate metals and metallic ores from sand and gravel.
Stope: —An open room produced by the removal of ore and in which mining is occurring.
Subsidence: —A sinking or lowering of the Earth's surface.
Winze: —A vertical passage extending downward from a level in a mine.