The mining method used to extract a particular mineral deposit depends on the type of deposit and the economics involved with extraction. When the deposit occurs in veins or beds that extend deep into the Earth's crust or when the overburden (overlying soil and rocks) is very thick, underground mining methods are required. When the deposit is near the surface, surface mining methods such as open pit or quarrying may be most efficient.
|Surface mining methods are commonly used for mineral resources that are close to the surface, low in quality, and large in quantity. Even though the mineral resource is near the surface, however, the overburden must be removed before the mineral resource can be extracted. Bulldozers and power shovels are used to remove the overburden and to mine the mineral resource. Some mineral resources that are mined using surface methods include coal, gold, iron, gypsum, and copper. Special surface mining methods used for specific conditions include open pit mining and strip mining, which involve the same basic methods and equipment. While quarrying and placer mining are also surface mining methods, they use significantly different methods and equipment as discussed below.|
Quarrying is a mining method in which large blocks of rock are mined intact or the rock itself is the intended final product. Rocks such as granite, limestone, marble, or sandstone are often mined this way. The large blocks of rock are mined by cutting them out of the deposit with saws. Then these blocks are lifted out with cranes, loaded onto trucks, and shipped to a location where they are further cut to smaller blocks, slabs, or tiles, or, in the case of limestone, ground to a powder. The blocks of rock are used mainly for making buildings, bridges, or other large structures. Some, like granite or marble, may also be polished and used for floors or decorative building facades or monuments.
Placer mining is a method used to recover precious metals such as gold or platinum and other metals such as tin from sand or gravel at the surface in or near stream beds. The sand and gravel are mixed with water and swirled and shaken so that the valuable heavy metals will sink, whereas the relatively light sand will stay on top and be washed away. Gold panning is a type of placer mining. (For a detailed account of historic gold mining practices, see "A Golden Opportunity for Science" in the February 1993 issue of Science and Children.)
|Underground mining is a method used if the mineral resource is of higher quality and is buried deep within the Earth. Engineers design the best way to mine a particular ore body. Since no two ore bodies are alike, different mines use different underground methods to extract the ore. Some of these techniques are called block caving, room and pillar, longwall, or cut and fill. The ore must be drilled, blasted, or excavated, and transported above-ground, moving in various mechanized carriers or conveyor belts to the shaft, where it is hoisted to the surface. Minerals such as coal, potash, lead, nickel, and zinc are often mined by underground methods.|
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