Small scale mining of surface deposits dates back thousands of years. For example, in Roman Britain, the Romans were exploiting all major coalfields (save those of North and South Staffordshire) by the late 2nd century AD.[1] While much of its use remained local, a lively trade developed along the North Sea coast supplying coal to Yorkshire and London.[1] The Industrial Revolution, which began in Britain in the 18th century, and later spread to continental Europe and North America, was based on the availability of coal to power steam engines. International trade expanded exponentially when coal-fed steam engines were built for the railways and steamships. The new mines that grew up in the 19th century depended on men and children to work long hours in often dangerous working conditions.[2] There were many coalfields, but the oldest were in Newcastle and Durham, South Wales, the Central Belt of Scotland and the Midlands, such as those at Coalbrookdale. The oldest continuously worked deep-mine in the United Kingdom is Tower Colliery in South Wales valleys in the heart of the South Wales coalfield. This colliery was developed in 1805, and its miners bought it out at the end of the 20th century, to prevent it from being closed. Tower Colliery was finally closed on 25 January 2008, although production continues at the Aberpergwm drift mine owned by Walter Energy. Coal was mined in America in the early 18th century, and commercial mining started around 1730 in Midlothian, Virginia.[3] Coal-cutting machines were invented in the 1880s. Before this invention, coal was mined from underground with a pick and shovel. By 1912, surface mining was conducted with steam shovels designed for coal mining. Methods of extraction. The most economical method of coal extraction from coal seams depends on the depth and quality of the seams, and the geology and environmental factors. Coal mining processes are differentiated by whether they operate on the surface or underground. Many coals extracted from both surface and underground mines require washing in a coal preparation plant. Technical and economic feasibility are evaluated based on the following: regional geological conditions; overburden characteristics; coal seam continuity, thickness, structure, quality, and depth; strength of materials above and below the seam for roof and floor conditions; topography (especially altitude and slope); climate; land ownership as it affects the availability of land for mining and access; surface drainage patterns; ground water conditions; availability of labor and materials; coal purchaser requirements in terms of tonnage, quality, and destination; and capital investment requirements.[4] Surface mining and deep underground mining are the two basic methods of mining. The choice of mining method depends primarily on depth of burial, density of the overburden and thickness of the coal seam. Seams relatively close to the surface, at depths less than approximately 180 ft (50 m), are usually surface mined. Coal that occurs at depths of 180 to 300 ft (50 to 100 m) are usually deep mined, but in some cases surface mining techniques can be used. For example, some western U.S. coal that occur at depths in excess of 200 ft (60 m) are mined by the open pit methods, due to thickness of the seam 60–90 feet (20–30 m). Coals occurring below 300 ft (100 m) are usually deep mined.[5] However, there are open pit mining operations working on coal seams up to 1000–1500 feet (300–450 m) below ground level, for instance Tagebau Hambach in Germany. 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, open pit, or strip) mining methods. 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. Large Open Cast mines can cover an area of many square kilometers and use very large pieces of equipment. This equipment can include the following: Draglines which operate by removing the overburden, power shovels, large trucks in which transport overburden and coal, bucket wheel excavators, and conveyors. In this mining method, explosives are first used in order to break through the surface, or overburden, of the mining area. The overburden is then removed by draglines or by shovel and truck. Once the coal seam is exposed, it is drilled, fractured and thoroughly mined in strips. The coal is then loaded on to large trucks or conveyors for transport to either the coal preparation plant or directly to where it will be used.[6] Most open cast mines in the United States extract bituminous coal. In Canada (BC), Australia and South Africa open cast mining is used for both thermal and metallurgical coals. In New South Wales open casting for steam coal and anthracite is practised. Surface mining accounts for around 80 percent of production in Australia, while in the US it is used for about 67 percent of production. Globally, about 40 percent of coal production involves surface mining
Posted on: Fri, 08 Aug 2014 18:23:10 +0000