Sunday, September 21, 2008

Modern surface mining

Modern surface mining
Trucks loaded with coal at the Cerrejón coal mine in Colombia.When coal seams are near the surface, it may be economical to extract the coal using open cut (also referred to as open cast or open pit) mining methods. Typically, for coal, strip mining is used. Strip mining exposes the coal by the advancement of an open pit or strip. The earth above the coal seam(s) is known as overburden. A strip of overburden next to the previously mined strip is usually drilled. The drill holes are filled with explosives and blasted. The overburden is then removed using large earthmoving equipment such as draglines, shovel and trucks, excavator and trucks, or bucket-wheels and conveyors. This overburden is put into the previously mined (and now empty) strip. When all the overburden is removed, the underlying coal seam will be exposed as a strip known as a 'block'. This 'block' of coal may be drilled and blasted (if hard) or otherwise loaded on to trucks or conveyors for transport to the coal preparation (or wash) plant. Once this strip is empty of coal, the process is repeated with a new strip being created next to it.

Open cast coal mining recovers a greater proportion of the coal deposit than underground methods, as more of the coal seams in the strata may be exploited. Opencast coal mines can cover many square kilometers.

Most open cast mines in the United States extract bituminous coal. In South Wales open casting for steam coal and anthracite is practiced. In Australia and South Africa open cast mining is used for both thermal and metallurgical coals. Surface mining accounts for around 80% of production in Australia, while in the USA it is used for about 67% of production. Globally, about 40% of coal production involves surface mining.

Mountaintop removal is a form of surface mining that takes place at the topmost portion of a mountain, and is a technique that is commonly applied in Appalachia in the United States. Utilized for the past 30 years, mountaintop mining involves removing the highest part of the mountain for the maximum recovery of coal. The process is highly controversial for the drastic changes in topography, the practice of hollow fills, or filling in valleys with mining debris, and for covering streams and disrupting ecosystems.

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