Definitions

Surface mining

Surface mining

Surface mining is a type of mining in which soil and rock overlying the mineral deposit are 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 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.

Types of surface mining

There are five main forms of surface mining, detailed below.

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 meters of earth per hour.

There are two forms of strip mining. The more common method 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 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. Bucket-wheel excavators are widely used for this purpose, however, they are prone to damage and require many millions of dollars to repair.

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).

Mountaintop removal

Mountaintop removal mining (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 soil placed in adjacent lows such as hollows or ravines. 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, Virginia and Tennessee in the United States. The profound changes in topography and disturbance of pre-existing ecosystems have made mountaintop removal highly controversial.

Advocates of mountaintop removal point out that once the areas are reclaimed as mandated by law, the technique provides premium flat land suitable for many uses in a region where flat land is at a premium. They also maintain that the new growth on reclaimed mountaintop mined areas is better able to support populations of game animals.

Critics contend that mountaintop removal is a disastrous practice that benefits a small number of corporations at the expense of local communities and the environment. A U.S. Environmental Protection Agency environmental impact statement finds that streams near valley fills from mountaintop removal contain high levels of minerals in the water and decreased aquatic biodiversity. The statement also estimates that 724 miles of Appalachian streams were buried by valley fills from 1985 to 2001.

In common with other methods of coal mining, processing the coal mined by mountaintop removal generates waste slurry (also called coal sludge), which is usually stored behind a dam on-site. Many coal slurry impoundments in West Virginia exceed 500 million gallons in volume, and some, including the Brushy Fork impoundment in Raleigh County, exceed 7 billion gallons. Such impoundments can be hundreds of feet high and be in close proximity to schools or private residences. The most controversial sludge dam at present sits 400 yards above Marsh Fork Elementary School. The sludge pond is permitted to hold 2.8 billion gallons of toxic sludge, and is 21 times larger than the pond which killed 125 people in the Buffalo Creek Flood.

Kentucky's Martin County Sludge Spill occurred after midnight on October 11, 2000 when a coal sludge impoundment broke through into an underground mine below, propelling 306 million gallons of sludge down two tributaries of the Tug Fork River. The spill polluted hundreds of miles of waterways, contaminated the water supply for over 27,000 residents, and killed all aquatic life in Coldwater Fork and Wolf Creek.

Blasting at a mountaintop removal mine expels coal dust and fly-rock into the air, which can then disturb or settle onto private property nearby. This dust contains sulfur compounds, which corrodes structures and tombstones and is a health hazard.

Although MTR sites are usually reclaimed after mining is complete, reclamation has traditionally focused on stabilizing rock and controlling erosion, but not reforesting the area with trees. Quick-growing, non-native grasses, planted to quickly provide vegetation on a site, compete with tree seedlings, and trees have difficulty establishing root systems in compacted backfill. Consequently, biodiversity suffers in a region of the United States with numerous endemic species. Erosion also increases, which can intensify flooding. In the Eastern United States, the Appalachian Regional Reforestation Initiative works to promote the use of trees in mining reclamation.

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.

Highwall Mining

Highwall mining is another form of surface mining that evolved from auger mining. In highwall mining, the coal seam is penetrated by a continuous miner propelled by a hydraulic Pushbeam Transfer Mechanism (PTM). A typical cycle includes sumping (pushing forward) and shearing (raising or lowering the cutterhead boom to cut the entire height of the coal seam). As the coal recovery cycle continues, the cutterhead is progressively pushed into the coal seam for 20 feet. Then, the Pushbeam Transfer Mechanism (PTM) automatically inserts a 20-foot long rectangular pushbeam into the center section of the machine between the powerhead and the cutterhead. The pushbeams system can penetrate nearly 1000 feet or 305 meters into the coal seam. Some highwall mining systems use augers enclosed inside the pushbeams that prevent the mined coal from being contaminated by rock debris during the conveyance process. Using a video imaging and/or a gamma detector, the operator can see and guide the continuous miner's progress. Highwall mining can produce thousands of tons of clean coal in contour-strip operations with narrow benches, previously mined areas, or trench mine applications.

Recovery is much better than augering, but the mapping of areas that have been developed by a highwall miner are not mapped as rigorously as deep mined areas. Very little spoil is displaced in contrast with mountain top removal, however a lot of capital is required to operate & own a highwall miner.

Mapping of the outcrop as well as data derived from core holes & samples taken during the bench making process are taken into account to best project the panels that the Highwall miner will cut. Obstacles that could be potentially damaged by subsidence & the natural contour of the Highwall are taken into account, and an Surveyor points the Highwall miner in a line mostly perpendicular to the Highwall. Parallel lines represent the panels cut into the mountain (Up to 1000' deep), because changing the Azimuth during the mining process results in missing a portion of the coal seam.

Environmental and health issues

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 reclamation 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 (i.e. it generates 75% of industrial waste in Spain).

In the United States, the Surface Mining Control and Reclamation Act of 1977 mandates reclamation of surface coal mines. Reclamation for non-coal mines, is regulated by state and local laws, which may vary widely.

Human health

Surface mining creates problems for the human health: gases, dust in suspension, noises and vibrations from the machines and explosions, that can create lung or nervous diseases.

References

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