Surface mining
From Wikipedia, the free encyclopedia
Surface mining is a type of mining in which soil and rock overlying the mineral deposit is removed. It is the opposite of underground mining, in which the overlying rock is left in place, and the mineral removed through shafts or tunnels.
Surface mining is used when deposits of commercially useful minerals or rock are found near the surface; that is, where the overburden (surface material covering the valuable deposit) is relatively thin or the material of interest is structurally unsuitable for tunneling (as would usually be the case for sand, cinder, and gravel). Where minerals occur deep below the surface—where the overburden is thick or the mineral occurs as veins in hard rock— underground mining methods are used to extract the valued material. Surface mines are typically enlarged until either the mineral deposit is exhausted, or the the cost of removing larger volumes of overburden makes further mining uneconomic.
In most forms of surface mining, heavy equipment, such as earthmovers, first remove the overburden - the soil and rock above the deposit. Next, huge machines, such as dragline excavators, extract the mineral.
The large impact of surface mining on the topography, vegetation, and water resources has made it highly controversial. Surface mining is subject to state and federal reclaimation requirements, but adequacy of the requirements is a constant source of contention. Unless reclaimed, surface mining can leave behind large areas of infertile waste rock.
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[edit] Types of surface mining
There are four main forms of surface mining, detailed below.
[edit] Strip Mining
Strip mining is the practice of mining a seam of mineral by first removing a long strip of overlying soil and rock (the overburden). It is most commonly used to mine coal or tar sand. Strip mining is only practical when the ore body to be excavated is relatively near the surface. This type of mining uses some of the largest machines on earth, including bucket-wheel excavators which can move as much as 12,000 cubic feet of earth per hour.
There are two forms of strip mining. The first, and more common one is area stripping, which is used on fairly flat terrain, to extract deposits over a large area. As each long strip is excavated, the overburden is placed in the excavation produced by the previous strip.
Contour stripping, usually used in hilly terrain, involves removing the overburden above the mineral seam near the outcrop in hilly terrain, where the mineral outcrop usually follows the contour of the land. Contour stripping is often followed by auger mining into the hillside, to remove more of the mineral. This method commonly leaves behind terraces in mountainsides.
Among others, strip mining is used to extract the oil-impregnated sand in the Athabasca Tar Sands in Alberta. It is also common in coal mining.
[edit] Open-pit mining
- For more details, see Open-pit mining
Open-pit mining refers to a method of extracting rock or minerals from the earth by their removal from an open pit or borrow. Although open-pit mining is sometimes mistakenly referred to as "strip mining", the two methods are different (see above).
[edit] Mountaintop removal
Mountaintop removal (MTR) is a relatively new form of coal mining that involves the mass restructuring of earth in order to reach the coal seam as deep as 1,000 feet below the surface. It is used where a coal seam outcrops all the way around a mountain top. All the rock and soil above the coal seam are removed and the spoil placed in adjacent lows such as hollows or ravivnes. Mountaintop removal replaces previously steep topography with a relatively level surface.
The technique has been used increasingly in recent years in the Appalachian coal fields of West Virginia, Kentucky, and Tennessee in the United States. The profound changes in topography and disturbance of pre-existing ecosystems have made mountaintop removal highly controversial.
[edit] Dredging
Dredging is a method often used to bring up underwater mineral deposits. Although dredging is usually employed to clear or enlarge waterways for boats, it can also recover significant amounts of underwater minerals relatively efficiently and cheaply.
[edit] See also
| Mining techniques | |
|---|---|
| Surface mining: | Open-pit mining, Quarrying, Strip mining, Placer mining, Mountaintop removal |
| Sub-surface mining: | Drift mining, Slope mining, Shaft mining, Hard rock mining, Borehole mining |
