Longwall mining
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Longwall mining is form of underground coal mining where a long wall (about 250-400 m long typically) of coal is mined in a single slice (typically 1-2 m thick). The longwall "panel" (the block of coal that is being mined) is typically 3-4 km long and 250-400 m wide. The gate road along one side of the block is called the maingate, the road on the other side is called the tailgate. The end of the block that includes the longwall equipment is called the face. The other end of the block is usually one of the main travel roads of the mine. Fresh air travels up the main gate, across the face, and then down the tail gate. Once past the face the air is no longer fresh air, but return air. Return air is extracted by ventilation fans mounted on the surface.
The longwall equipment consists of a number of hydraulic jacks, called chocks, roof supports or shields, which are placed in a long line up to 400 m in length in order to support the roof. An individual chock can extend to a maximum cutting height of up to 5 m. The coal is cut by a rotating drum with bits called a shearer that moves along the length of the face in front of the chocks, disintegrating the coal. The coal is then removed from the coal face by a moving chain or AFC (articulated face conveyor) (also called armoured face conveyor) to the main gate. Here it is loaded onto a conveyor belt and transported to the surface, usually via a network of conveyor belts. At the main gate the coal is usually reduced in size in a crusher, and loaded onto the first conveyor belt by the beam stage loader (BSL).
As the shearer removes the coal, the chocks move forward into the newly created cavity. As mining progresses and the entire longwall progresses through the seam, the cavity behind the longwall, known as the gob, increases. This gob collapses under the weight of the overlying strata. This collapsing can lower surface height considerably, causing serious problems like changing the course of rivers and severely damage building foundations. The mining company is required by law to compensate landowners who are harmed, but it is still a contentious issue[citation needed].
An advantage of longwall mining is that over 80 percent of the coal is recovered, compared with about 50 percent for the more traditional room and pillar method.[1] In addition, subsidence is largely immediate, allowing for better planning and more accountability by the mining company.[2]
[edit] External links
- University of Wollongong (Australia). Educational resource on longwall mining.
[edit] Notes
