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Surface coal mining in Wyoming in the United States.
A coal mine in Bihar, India.

The goal of coal mining is to economically remove coal from the ground. Coal is valued for its energy content, and since the 1880s is widely used to generate electricity. Steel and cement industries use coal as a fuel for extraction of iron from iron ore and for cement production. In the United States, United Kingdom, and South Africa, a coal mine and its structures are a "colliery". In Australia, "colliery" generally refers to an underground coal mine.

Contents

History

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 Aberpergwym drift mine nearby. Coal was mined in America in the early 1700s, and commercial mining occurred around 1730 in Midlothian, Virginia.[1] Coal-cutting machines were invented in the 1880s. Before the 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: regional geologic 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.

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 feet (55 m), are usually surface mined. Coals that occur at depths of 180 to 300 feet (91 m) are usually deep mined but, in some cases, surface mining techniques can be used. For example, some western U.S. coals that occur at depths in excess of 200 feet (61 m) are mined by open pit methods, due to thickness of the seam (60-90 feet). Coals occurring below 300 feet (91 m) are usually deep mined.[2]

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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 use in order to break through the surface of the mining area. The coal 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 direct to where it will be used[3].

Most open cast mines in the United States extract bituminous coal. In Australia and South Africa open cast mining is used for both thermal and metallurgical coals. In South Wales open casting for steam coal and anthracite is practiced. Surface mining accounts for around 80% of production in Australia, while in the USA it is used for about 67% of production. Globally, about 40% of coal production involves surface mining.[4]

Area mining

Strip mining exposes the coal by removing the overburden (the earth above the coal seam(s)) in long cuts or strips. The spoil from the first strip is deposited in an area outside the planned mining area. Spoil from subsequent cuts is deposited as fill in the previous cut after coal has been removed. Usually, the process is to drill the strip of overburden next to the previously mined strip. The drill holes are filled with explosives and blasted. The overburden is then removed using large earthmoving equipment such as draglines, shovel and trucks, excavator and trucks, or bucket-wheels and conveyors. This overburden is put into the previously mined (and now empty) strip. When all the overburden is removed, the underlying coal seam will be exposed (a 'block' of coal). This block of coal may be drilled and blasted (if hard) or otherwise loaded onto trucks or conveyors for transport to the coal preparation (or wash) plant. Once this strip is empty of coal, the process is repeated with a new strip being created next to it. This method is most suitable for areas with flat terrain.

Equipment to be used depends on geologic conditions. For example, to remove overburden that is loose or unconsolidated, a bucket wheel excavator might be the most productive. The life of some area mines may be more than 50 years. [5]

Contour mining

The contour mining method consists of removing overburden from the seam in a pattern following the contours along a ridge or around a hillside. This method is most commonly used in areas with rolling to steep terrain. It was once common to deposit the spoil on the downslope side of the bench thus created, but this method of spoil disposal consumed much additional land and created severe landslide and erosion problems. To alleviate these problems, a variety of methods were devised to use freshly cut overburden to refill mined-out areas. These haul-back or lateral movement methods generally consist of an initial cut with the spoil deposited downslope or at some other site and spoil from the second cut refilling the first. A ridge of undisturbed natural material 15 to 20 feet (6.1 m) wide is often intentionally left at the outer edge of the mined area. This barrier adds stability to the reclaimed slope by preventing spoil from slumping or sliding downhill.[6]

The limitations on contour strip mining are both economic and technical. When the operation reaches a predetermined stripping ratio (tons of overburden/tons of coal), it is not profitable to continue. Depending on the equipment available, it may not be technically feasible to exceed a certain height of highwall. At this point, it is possible to produce more coal with the augering method in which spiral drills bore tunnels into a highwall laterally from the bench to extract coal without removing the overburden.[2]

Mountaintop removal mining

Mountaintop coal mining is a surface mining practice involving removal of mountaintops to expose coal seams, and disposing of associated mining overburden in adjacent "valley fills." Valley fills occur in steep terrain where there are limited disposal alternatives. Mountaintop removal combines area and contour strip mining methods. In areas with rolling or steep terrain with a coal seam occurring near the top of a ridge or hill, the entire top is removed in a series of parallel cuts. Overburden is deposited in nearby valleys and hollows. This method usually leaves ridge and hill tops as flattened plateaus.[2] The process is highly controversial for the drastic changes in topography, the practice of creating head-of-hollow-fills, or filling in valleys with mining debris, and for covering streams and disrupting ecosystems.[7][8]

Spoil is placed at the head of a narrow, steep-sided valley or hollow. In preparation for filling this area, vegetation and soil are removed and a rock drain constructed down the middle of the area to be filled, where a natural drainage course previously existed. When the fill is completed, this underdrain will form a continuous water runoff system from the upper end of the valley to the lower end of the fill. Typical head-of-hollow fills are graded and terraced to create permanently stable slopes.[9]

Room and pillar mining

Room and pillar mining consists of coal deposits that are mined by cutting a network of rooms into the coal seam. Pillars of coal are left behind in order to keep up the roof. The pillars can make up to forty percent of the total coal in the seam. Though, this can be extracted at a later stage[10]

Underground mining

Coal wash plant in Clay County, Kentucky.

Most coal seams are too deep underground for opencast mining and require underground mining, which method currently accounts for about 60% of world coal production.[11] In deep mining, the room and pillar or bord and pillar method progresses along the seam, while pillars and timber are left standing to support the mine roof. Once room and pillar mines have been developed to a stopping point (limited by geology, ventilation, or economics), a supplementary version of room and pillar mining, termed second mining or retreat mining, is commonly started. Miners remove the coal in the pillars, thereby recovering as much coal from the coal seam as possible. A work area involved in pillar extraction is called a pillar section. Modern pillar sections use remote-controlled equipment, including large hydraulic mobile roof-supports, which can prevent cave-ins until the miners and their equipment have left a work area. The mobile roof supports are similar to a large dining-room table, but with hydraulic jacks for legs. After the large pillars of coal have been mined away, the mobile roof support's legs shorten and it is withdrawn to a safe area. The mine roof typically collapses once the mobile roof supports leave an area.

There are five principal methods of underground mining:

  • Longwall mining accounts for about 50% of underground production. The longwall shearer has a face of 1,000 feet (300 m) or more. It is a sophisticated machine with a rotating drum that moves mechanically back and forth across a wide coal seam. The loosened coal falls on to a pan line that takes the coal to the conveyor belt for removal from the work area. Longwall systems have their own hydraulic roof supports which advance with the machine as mining progresses. As the longwall mining equipment moves forward, overlying rock that is no longer supported by coal is allowed to fall behind the operation in a controlled manner. The supports make possible high levels of production and safety. Sensors detect how much coal remains in the seam while robotic controls enhance efficiency. Longwall systems allow a 60-to-100% coal recovery rate when surrounding geology allows their use. Once the coal is removed, usually 75 percent of the section, the roof is allowed to collapse in a safe manner.[12]
  • Continuous mining utilizes a machine with a large rotating steel drum equipped with tungsten carbide teeth that scrape coal from the seam. Operating in a “room and pillar” (also known as “bord and pillar”) system—where the mine is divided into a series of 20-to-30 foot “rooms” or work areas cut into the coalbed—it can mine as much as five tons of coal a minute, more than a non-mechanised miner of the 1920s would produce in an entire day. Continuous miners account for about 45% of underground coal production. Conveyors transport the removed coal from the seam. Remote-controlled continuous miners are used to work in a variety of difficult seams and conditions, and robotic versions controlled by computers are becoming increasingly common.
  • Blast mining or conventional mining, is an older practice that uses explosives such as dynamite to break up the coal seam, after which the coal is gathered and loaded on to shuttle cars or conveyors for removal to a central loading area. This process consists of a series of operations that begins with “cutting” the coalbed so it will break easily when blasted with explosives. This type of mining accounts for less than 5% of total underground production in the U.S. today.
  • Shortwall mining, a method currently accounting for less than 1% of deep coal production, involves the use of a continuous mining machine with movable roof supports, similar to longwall. The continuous miner shears coal panels 150-200 feet wide and more than a half-mile long, having regard to factors such as geological strata.
  • Retreat mining is a method in which the pillars or coal ribs used to hold up the mine roof are extracted; allowing the mine roof to collapse as the mining works back towards the entrance. This is one of the most dangerous forms of mining owing to imperfect predictability of when the ceiling will collapse and possibly crush or trap workers in the mine.

Production

Coal is mined commercially in over 50 countries. Over 7,036 Mt/yr of hard coal is currently produced, a substantial increase over the past 25 years.[13] In 2006, the world production of brown coal and lignite was slightly over 1,000 Mt, with Germany the world’s largest brown coal producer at 194.4 Mt, and China second at 100.6 Mt.[14]

Coal production has grown fastest in Asia, while Europe has declined. The top coal mining nations (figures in brackets are 2007 estimate of total coal production in millions of short tons)[13] are:

Most coal production is used in the country of origin, with around 16% of hard coal production being exported.

Global coal production is expected to reach 7,000 Mt/yr in 2030, with China accounting for most of this increase. Steam coal production is projected to reach around 5,200 Mt/yr; coking coal 620 Mt/yr; and brown coal 1,200 Mt/yr.[15]

Coal reserves are available in almost every country worldwide, with recoverable reserves in around 70 countries. At current production levels, proven coal reserves are estimated to last 147 years.[16] However, production levels are by no means level, and are in fact increasing and some estimates are that peak coal could arrive in many countries such as China and America by around 2030.

Modern mining

Technological advancements have made coal mining today more productive than it has ever been. To keep up with technology and to extract coal as efficiently as possible modern mining personnel must be highly skilled and well trained in the use of complex, state-of-the-art instruments and equipment. Future coal miners have to be highly educated and many jobs require four-year college degrees. Computer knowledge has also become greatly valued within the industry as most of the machines and safety monitors are computerized.

In the United States, the increase in technology has significantly decreased the mining workforce from 335,000 coal miners working at 7,200 mines fifty years ago to 104,824 miners working in fewer than 2,000 mines today. As some might see this as a sign that coal is a declining industry its advances has reported an 83% increase of production from 1970 to 2004[citation needed].

Safety

Dangers to miners

The Farmington coal mine disaster kills 78. 1968, WV, USA

Historically, coal mining has been a very dangerous activity and the list of historical coal mining disasters is a long one. Open cut hazards are principally mine wall failures and vehicle collisions; underground mining hazards include suffocation, gas poisoning, roof collapse and gas explosions. Firedamp explosions can trigger the much more dangerous coal dust explosions, which can engulf an entire pit. Most of these risks can be greatly reduced in modern mines, and multiple fatality incidents are now rare in some parts of the developed world.[17]

However, in lesser developed countries and some developing countries, many miners continue to die annually, either through direct accidents in coal mines or through adverse health consequences from working under poor conditions. China, in particular, has the highest number of coal mining related deaths in the world, with official statistic 6,027 deaths in 2004.[18] To compare, 28 deaths were reported in the U.S. in the same year.[19] Coal production in China is twice that in the U.S.,[20] while the number of coal miners is around 50 times that of the USA, making deaths in coal mines in China 4 times as common per worker (108 times as common per unit output) as in the USA.

When compared to industrial countries such as China, the fatality rate is low in the U.S. However, in 2006, fatal work injuries among miners in the U.S. doubled from the previous year, totaling 47.[21] These figures can in part be attributed to the Sago Mine disaster. The recent mine accident in Utah's Crandall Canyon Mine, where nine miners were killed and six entombed, speaks to the increase in occupational risks faced by U.S. miners.[22]

Chronic lung diseases, such as pneumoconiosis (black lung) were once common in miners, leading to reduced life expectancy. In some mining countries black lung is still common, with 4000 new cases of black lung every year in the USA (4% of workers annually) and 10 000 new cases every year in China (0.2% of workers).[23] Rates may be higher than reported in some regions.

Build-ups of a hazardous gas are known as damps, possibly from the German word "Dampf" which means steam or vapor:

Safer times in modern mining

Improvements in mining methods (e.g. longwall mining), hazardous gas monitoring (such as safety-lamps or more modern electronic gas monitors), gas drainage, electrical equipment, and ventilation have reduced many of the risks of rock falls, explosions, and unhealthy air quality. Statistical analyses performed by the U.S. Department of Labor’s Mine Safety and Health Administration (MSHA) show that between 1990 and 2004, the industry cut the rate of injuries by more than half and fatalities by two-thirds. However, according to the Bureau of Labor Statistics, mining remains the second most dangerous occupation in America.[24] New braces called Atlas Cribs contain a mix of hardwoods and a main lateral element that make these braces stronger than other braces used in the past. The new cribbing system takes up 41 percent less area than existing ones and may be up to 50 percent more efficient in terms of airflow.[25]

Environmental impacts

Coal mining can result in a number of adverse effects on the environment. Surface mining of coal completely eliminates existing vegetation, destroys the genetic soil profile, displaces or destroys wildlife and habitat, degrades air quality, alters current land uses, and to some extent permanently changes the general topography of the area mined, [26] This often results in a scarred landscape with no scenic value. Rehabilitation or reclamation mitigates some of these concerns and is required by Federal Law, specifically the Surface Mining Control and Reclamation Act of 1977.

Mine tailing dumps produce acid mine drainage which can seep into waterways and aquifers, with consequences on ecological and human health. If underground mine tunnels collapse, this can cause subsidence of land surfaces. During actual mining operations,methane, a known greenhouse gas, may be released into the air. And by the movement, storage, and redistribution of soil, the community of microorganisms and nutrient cycling processes can be disrupted.

Coal mining by country

Australia

Coal is mined in every state of Australia as well as the Northern Territory. It is mostly used to generate electricity, and 75% of annual coal production is exported, mostly to eastern Asia. Coal provides about 85% of Australia's electricity production.[27] In 2007, 428 million short tons of coal was mined in Australia.[13]

China

The People's Republic of China is by far the largest producer of coal in the world, producing over 2.8 billion tons of coal in 2007, or approximately 39.8 percent of all coal produced in the world during that year.[13] For comparison, the second largest producer, the United States, produced more than 1.1 billion tons in 2007. An estimated 5 million people work in China's coal-mining industry. As many as 20,000 miners die in accidents each year.[28]

Most Chinese mines are deep underground and do not produce the surface disruption typical of strip mines. Although there is some evidence of reclamation of mined land for use as parks, China does not require extensive reclamation and is creating significant acreages of abandoned mined land which is unsuitable for agriculture or other human uses, and inhospitable to indigenous wildlife. Chinese underground mines often experience severe surface subsidence (6-12 meters), negatively impacting farmland because it no longer drains well. China uses some subsidence areas for aquaculture ponds but has more than they need for that purpose. Reclamation of subsided ground is a significant problem in China.

Because most Chinese coal is for domestic consumption and is burned with little or no air pollution control equipment, it contributes greatly to visible smoke and severe air pollution in industrial areas using coal for fuel. Air pollution control equipment is being installed on some plants, but there are unconfirmed reports it is only turned on when inspectors visit. China's carbon dioxide emissions may increase 30% in 2008 due to increased coal combustion.[citation needed]

Colombia

Opencast coal mine at Cerrejón

Some of the world's largest coal reserves are located in South America, and an opencast mine at Cerrejón in Colombia is one of the world's largest open pit mines. Output of the mine in 2004 was 24.9 million tons (compared to total global hard coal production of 4,600 million tons). Cerrejón contributed about half of Colombia's coal exports of 52 million tons that year, with Colombia ranked sixth among major coal exporting nations. The company planned to expand production to 32 million tons by 2008.

The company has its own 150 km standard-gauge railroad, connecting the mine to its coal-loading terminal at Puerto Bolívar on the Caribbean coast. There are two 120-car unit trains, each carrying 12,000 tons of coal per trip. The round-trip time for each train, including loading and unloading, is about 12 hours. The coal facilities at the port are capable of loading 4,800 tons per hour on to vessels of up to 175,000 tons of dead weight. The mine, railroad and port operate 24 hours per day. Cerrejón directly employs 4,600 workers, with a further 3,800 employed by contractors. The reserves at Cerrejón are low-sulfur, low-ash, bituminous coal. The coal is mostly used for electric power generation, with some also used in steel manufacture. The surface mineable reserves for the current contract are 330 million tons. However, total proven reserves to a depth of 300 metres are 3,000 million tons.

South Africa

South Africa is one of the ten largest coal producing[29][30] and the fourth largest coal exporting[31] country in the world.

United States

The American share of world coal production remained steady at about 20% from 1980 to 2005, at about 1 billion short tons per year.

In a conference with the West Virginia Coal Association former President George W. Bush said that there is no more reliable source of electricity than coal[32] and put coal at center of US energy independence.[33]

Ukraine

More than 90% of Ukraine’s coal production comes from the Donets Basin.[34] The country's coal industry employs about 500,000 people.[35]

Other coal business

See also

References

  1. ^ Historical Overview Of The Midlothian Coal Mining Company Tract, Chesterfield County, Virginia, Martha W. McCartney, December, 1989
  2. ^ a b c Christman, R.C., J. Haslbeck, B. Sedlik, W. Murray, and W. Wilson. 1980. Activities, effects and impacts of the coal fuel cycle for a 1,000-MWe electric power generating plant. Washington, DC: U.S. Nuclear Regulatory Commission.
  3. ^ "Coal Mining." World Coal. World Coal Institute. 10 Mar. 2009 <http://www.worldcoal.org/pages/content/index.asp?PageID=92>.
  4. ^ World Coal Institute - Coal Mining
  5. ^ U.S. Department of the Interior, Office of Surface Mining Reclamation and Enforcement (1987). Surface coal mining reclamation: 10 years of progress, 1977-1987. Washington, D.C.: U.S. Government Printing Office.
  6. ^ U.S. Department of the Interior, Office of Surface Mining Reclamation and Enforcement (1987). Surface coal mining reclamation: 10 years of progress, 1977-1987. Washington, D.C.: U.S. Government Printing Office.
  7. ^ Mountaintop mining
  8. ^ U.S. Environmental Protection Agency, Region 3. 2005. Mountaintop mining/valley fills in Appalachia: Final programmatic environmental impact statement. Washington, DC: U.S. Government Printing Office. [1]
  9. ^ U.S. Department of the Interior, Office of Surface Mining Reclamation and Enforcement (1987). Surface coal mining reclamation: 10 years of progress, 1977-1987. Washington, D.C.: U.S. Government Printing Office.
  10. ^ "Coal Mining." World Coal. World Coal Institute. 10 Mar. 2009 <http://www.worldcoal.org/pages/content/index.asp?PageID=92>.
  11. ^ World Coal Institute - Coal Mining
  12. ^ "Coal Mining." World Coal. World Coal Institute. 10 Mar. 2009 <http://www.worldcoal.org/pages/content/index.asp?PageID=92>.
  13. ^ a b c d U.S. Energy Information Administration. 2008. World Coal Production, Most Recent Estimates 1980-2007 (October 2008). http://www.eia.doe.gov/emeu/international/coalproduction.html [accessed 11-2-08].
  14. ^ U.S. Energy Information Administration. 2008. World Coal Production, Primary and Secondary, 2006 (October 2008). http://www.eia.doe.gov/emeu/international/coalproduction.html [accessed 11-2-08].
  15. ^ World Coal Institute - Coal Production
  16. ^ World Coal Institute - Coal Facts 2007
  17. ^ Respiratory Protection in Coal Mines
  18. ^ Deconstructing deadly details from China's coal mine safety statistics | CLB
  19. ^ Mine Safety and Health Administration (MSHA) - Statistics - Coal Mining Fatalities by State - Calendar Year
  20. ^ World Coal Institute - Coal Production
  21. ^ Bureau of Labor Statistics. (2006). Census of Fatal Occupational Injuries Summary, 2006 Washington D.C.: U.S. Department of Labor. Available: http://www.bls.gov/news.release/cfoi.nr0.htm
  22. ^ Associated Press. (2007, September 4). Panel to Explore Deadly Mine Accident. New York Times: http://www.nytimes.com/aponline/us/AP-Utah-Mine-Collapse.html
  23. ^ fossil fuel disasters
  24. ^ cfch0005.pdf
  25. ^ Southern Illinois University Carbondale. "New Technology Will Enhance Coal Mine Safety." ScienceDaily 22 July 2008. 10 March 2009 <http://www.sciencedaily.com/releases/2008/07/080717221913.htm>.
  26. ^ U.S. Department of the Interior. 1979. Permanent Regulatory Program Implementing Section 501(b) of the Surface Mining Control and Reclamation Act of 1977: Environmental Impact Statement. Washington, D.C.: U.S. Department of the Interior.
  27. ^ "The Importance of Coal in the Modern World - Australia". Gladstone Centre for Clean Coal. http://www.gc3.cqu.edu.au/modern-world/index.php#australia. Retrieved 2007-03-17. 
  28. ^ Where The Coal Is Stained With Blood, TIME, March 2, 2007
  29. ^ Schmidt, Stephan. "Coal deposits of South Africa - the future of coal mining in South Africa". Institute for Geology, Technische Universität Bergakademie Freiberg. http://www.geo.tu-freiberg.de/oberseminar/os07_08/stephan_Schmidt.pdf. Retrieved 14 January 2010. 
  30. ^ "Coal Mining". World Coal Institute. http://www.worldcoal.org/coal/coal-mining/. Retrieved 14 January 2010. 
  31. ^ "Coal". Department of Minerals and Energy (South Africa). http://www.dme.gov.za/energy/coal.stm. Retrieved 14 January 2010. 
  32. ^ http://www.chronwatch-america.com/articles/3329/1/President-Bush-Speaks-About-Coal-and-Gas/Page1.html
  33. ^ http://www.platts.com/Coal/highlights/2008/coalp_co_080108.xml
  34. ^ Ukraine - Mining: Coal Mining
  35. ^ Ukraine - Coal, eia.doe.gov

Further reading

  • Daniel Burns. The modern practice of coal mining (1907)
  • Chirons, Nicholas P. Coal Age Handbook of Coal Surface Mining (ISBN 0-07-011458-7)
  • Hamilton, Michael S. Mining Environmental Policy: Comparing Indonesia and the USA (Burlington, VT: Ashgate, 2005). (ISBN 0-7546-4493-6).
  • Hayes, Geoffrey. Coal Mining (2004), 32 pp
  • Hughes. Herbert W, A Text-Book of Mining: For the use of colliery managers and others (London, many editions 1892-1917), the standard British textbook for its era.
  • National Energy Information Center, Greenhouse Gases, Climate Change, Energy, http://www.eia.doe.gov/oiaf/1605/ggccebro/chapter1.html, retrieved 2007-10-16 
  • Charles V. Nielsen and George F. Richardson. 1982 Keystone Coal Industry Manual (1982)
  • Saleem H. Ali. Minding our Minerals, 2006. [2]
  • A.K. Srivastava. Coal Mining Industry in India (1998) (ISBN 81-7100-076-2)
  • The Department of Trade and Industry, The Coal Authority, http://www.coal.gov.uk/resources/cleanercoaltechnologies/CoalMineandbedmethane.cfm, retrieved 2007-10-16 
  • James Tonge. The principles and practice of coal mining (1906)

External links


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