Editing Coal mining (section)

===Underground mining===
{{Main|Underground soft-rock mining}}
[[File:Coal Washer.JPG|thumb|A coal wash plant in [[Clay County, Kentucky]]]]
[[File:Continuous Miner.jpg|thumb|A remote Joy HM21 continuous miner used underground]]
Most coal seams are too deep underground for opencast mining and require underground mining, a method that currently accounts for about 60 percent of world coal production.<ref name="CoalMining2009"/> In deep mining, the [[room and pillar|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.<ref>{{Citation |title=Room and pillar mining |date=2024-04-20 |work=Wikipedia |url=https://en.wikipedia.org/w/index.php?title=Room_and_pillar_mining&oldid=1219912988 |access-date=2024-04-23 |language=en}}</ref>

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 After the large pillars of coal have been mined away, the mobile roof support's legs shorten and it is withdrawn to a safe area.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.{{Citation needed|date=January 2021}}

There are six principal methods of underground mining:
* [[Longwall mining]] accounts for about 50 percent of underground production. The longwall shearer has a face of {{convert|1000|ft|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 onto an armored chain conveyor or 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 percent 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.<ref name="CoalMining2009" />
* [[Continuous mining]] utilizes a continuous miner machine with a large rotating steel drum equipped with tungsten carbide picks 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 (5–10 m) "rooms" or work areas cut into the coalbed—it can mine as much as 14 tons of coal a minute, more than a non-mechanised mine of the 1920s would produce in an entire day. Continuous miners account for about 45 percent of underground coal production. [[Conveyor system|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. Continuous mining is a misnomer, as room and pillar coal mining is very cyclical. In the US, one can generally cut up to around {{convert|20|ft|m|abbr=off|sp=us|0}}. This may be increased with [[Mine Safety and Health Administration|MSHA]] permission.  In South Africa, the limit may be as high as {{Convert|12|m|ft}}. After the cutting limit is reached, the continuous miner assembly is removed and the roof is supported by the use of a roof bolter, after which the face has to be serviced before it can be advanced again. During servicing, the "continuous" miner moves to another face. Some continuous miners can bolt and rock dust the face, two major components of servicing, while cutting coal, while a trained crew may be able to advance ventilation, to truly earn the "continuous" label. However, very few mines are able to achieve it. Most continuous mining machines in use in the US lack the ability to bolt and dust. This may partly be because the incorporation of bolting makes the machines wider, and therefore, less maneuverable.<ref>{{Cite web |title=Rock bolt {{!}} Reinforcement, Anchoring & Stability {{!}} Britannica |url=https://www.britannica.com/technology/rock-bolt |access-date=2024-04-23 |website=www.britannica.com |language=en}}</ref>
* [[Room and pillar|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, however, where there was space to leave the head and floor coal there is evidence from recent open cast excavations that 18th-century operators used a variety of room and pillar techniques to remove 92 percent of the ''in situ'' coal. However, this can be extracted at a later stage (''see'' [[retreat mining]]).<ref name="CoalMining2009">{{cite web|title=Coal Mining. World Coal|work= World Coal Institute |date=10 March 2009 |url=http://www.worldcoal.org/pages/content/index.asp?PageID=92>|archive-url=https://web.archive.org/web/20090428202846/http://www.worldcoal.org/pages/content/index.asp?PageID=92|archive-date=28 April 2009}}</ref>
* [[Rock blasting|Blast mining]] or conventional mining, is an older practice that uses [[explosive]]s such as [[dynamite]] to break up the coal seam, after which the coal is gathered and loaded onto 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 percent of total underground production in the US today.{{citation needed|date=October 2014}}
* [[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 roof will collapse and possibly crush or trap workers in the mine.{{citation needed|date=October 2014}}