Mineria Subterranea
Páginas: 7 (1588 palabras)
Publicado: 7 de junio de 2012
7
SECTION TWO
COAL MINING
>> Coal is mined by two methods – surface or ‘opencast’ mining and underground or ‘deep’ mining. >>
The choice of mining method is largely determined by the geology of the coal deposit. Underground mining currently accounts for about 60% of world coal production, although in several important coal producing countries surface mining is more common. Surfacemining accounts for around 80% of production in Australia, while in the USA it is used for about 67% of production.
Underground Mining There are two main methods of underground mining: room-and-pillar and longwall mining.
In room-and-pillar mining, coal deposits are mined by cutting a network of ‘rooms’ into the coal seam and leaving behind ‘pillars’ of coal to support the roof of the mine. Thesepillars can be up to 40% of the total coal in the seam – although this coal can sometimes be recovered at a later stage. This can be achieved in what is known as ‘retreat mining’, where coal is mined from the pillars as workers retreat. The roof is then allowed to collapse and the mine is abandoned. Longwall mining involves the full extraction of coal from a section of the seam or ‘face’ usingmechanical shearers. A longwall face requires
careful planning to ensure favourable geology exists throughout the section before development work begins. The coal ‘face’ can vary in length from 100-350m. Selfadvancing, hydraulically-powered supports temporarily hold up the roof while coal is extracted. When coal has been extracted from the area, the roof is allowed to collapse. Over 75% of the coalin the deposit can be extracted from panels of coal that can extend 3km through the coal seam. The main advantage of room–and-pillar mining over longwall mining is that it allows coal production to start much more quickly, using mobile machinery that costs under $5 million (longwall mining machinery can cost $50 million). The choice of mining technique is site specific but always based oneconomic considerations; differences even within a single mine can lead to both methods being used.
Surface Mining Surface mining – also known as opencast or opencut mining – is only economic when the coal seam is near the surface. This method recovers a higher proportion of the coal
8
World Coal Institute
deposit than underground mining as all coal seams are exploited – 90% or more of thecoal can be recovered. Large opencast mines can cover an area of many square kilometres and use very large pieces of equipment, including: draglines, which remove the overburden; power shovels; large trucks, which transport overburden and coal; bucket wheel excavators; and conveyors.
Definition Overburden is the layer of soil and rocks (strata) between the coal seams and the surface.
process,the coal is separated from other impurities by being floated in a tank containing a liquid of specific gravity, usually a suspension of finely ground magnetite. As the coal is lighter, it floats and can be separated off, while heavier rock and other impurities sink and are removed as waste. The smaller size fractions are treated in a number of ways, usually based on differences in mass, such as incentrifuges. A centrifuge is a machine which turns a container around very quickly, causing solids and liquids inside it to separate. Alternative methods use the different surface properties of coal and waste. In ‘froth flotation’, coal particles are removed in a froth produced by blowing air into a water bath containing chemical reagents. The bubbles attract the coal but not the waste and areskimmed off to recover the coal fines. Recent technological developments have helped increase the recovery of ultra fine coal material.
The overburden of soil and rock is first broken up by explosives; it is then removed by draglines or by shovel and truck. Once the coal seam is exposed, it is drilled, fractured and systematically mined in strips. The coal is then loaded on to large trucks or...
SECTION TWO
COAL MINING
>> Coal is mined by two methods – surface or ‘opencast’ mining and underground or ‘deep’ mining. >>
The choice of mining method is largely determined by the geology of the coal deposit. Underground mining currently accounts for about 60% of world coal production, although in several important coal producing countries surface mining is more common. Surfacemining accounts for around 80% of production in Australia, while in the USA it is used for about 67% of production.
Underground Mining There are two main methods of underground mining: room-and-pillar and longwall mining.
In room-and-pillar mining, coal deposits are mined by cutting a network of ‘rooms’ into the coal seam and leaving behind ‘pillars’ of coal to support the roof of the mine. Thesepillars can be up to 40% of the total coal in the seam – although this coal can sometimes be recovered at a later stage. This can be achieved in what is known as ‘retreat mining’, where coal is mined from the pillars as workers retreat. The roof is then allowed to collapse and the mine is abandoned. Longwall mining involves the full extraction of coal from a section of the seam or ‘face’ usingmechanical shearers. A longwall face requires
careful planning to ensure favourable geology exists throughout the section before development work begins. The coal ‘face’ can vary in length from 100-350m. Selfadvancing, hydraulically-powered supports temporarily hold up the roof while coal is extracted. When coal has been extracted from the area, the roof is allowed to collapse. Over 75% of the coalin the deposit can be extracted from panels of coal that can extend 3km through the coal seam. The main advantage of room–and-pillar mining over longwall mining is that it allows coal production to start much more quickly, using mobile machinery that costs under $5 million (longwall mining machinery can cost $50 million). The choice of mining technique is site specific but always based oneconomic considerations; differences even within a single mine can lead to both methods being used.
Surface Mining Surface mining – also known as opencast or opencut mining – is only economic when the coal seam is near the surface. This method recovers a higher proportion of the coal
8
World Coal Institute
deposit than underground mining as all coal seams are exploited – 90% or more of thecoal can be recovered. Large opencast mines can cover an area of many square kilometres and use very large pieces of equipment, including: draglines, which remove the overburden; power shovels; large trucks, which transport overburden and coal; bucket wheel excavators; and conveyors.
Definition Overburden is the layer of soil and rocks (strata) between the coal seams and the surface.
process,the coal is separated from other impurities by being floated in a tank containing a liquid of specific gravity, usually a suspension of finely ground magnetite. As the coal is lighter, it floats and can be separated off, while heavier rock and other impurities sink and are removed as waste. The smaller size fractions are treated in a number of ways, usually based on differences in mass, such as incentrifuges. A centrifuge is a machine which turns a container around very quickly, causing solids and liquids inside it to separate. Alternative methods use the different surface properties of coal and waste. In ‘froth flotation’, coal particles are removed in a froth produced by blowing air into a water bath containing chemical reagents. The bubbles attract the coal but not the waste and areskimmed off to recover the coal fines. Recent technological developments have helped increase the recovery of ultra fine coal material.
The overburden of soil and rock is first broken up by explosives; it is then removed by draglines or by shovel and truck. Once the coal seam is exposed, it is drilled, fractured and systematically mined in strips. The coal is then loaded on to large trucks or...
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