|Longwall Underground Mining|
The other major form of underground coal extraction is longwall mining. In longwall mines, equipment used in Room and Pillar mining is used to develop two or three parallel entries in a coal seam. The longwall mine entries or headings are usually separated by several hundred feet of solid coal. At a certain distance advancing into the coal seam (depending on mine-specific geology), the headings are connected by driving a crosscuts, or in longwall mines, cross headings to connect the entries to form a block of solid coal called a panel. Specialized equipment, known as longwall machines, that are usually custom-built for specific mines then begin extracting the coal along the face of the panel. These machines include a shear or plow device mounted on a track that moves back and forth across the panel cutting the coal with each pass of the machine, referred to as the cutting head. As the cutting head extracts the panel of coal, the longwall machine advances further into the coal seam extracting more of the seam as it advances through successive passes across the panel, allowing for recovery of up to 85 percent of a given coal seam and reserve. The cutting head is attached to a special type of armored conveyor that gathers the coal as it is cut by the head and moves it to a loading point where it is transferred to the main haulage system for movement to the surface and the coal preparation plant. Both the cutting head and the special conveyor are protected by a special form of hydraulic roof support devices known as shields. The shields support the roof immediately above the longwall machine and are designed to advance as the cutting head moves deeper into the coal reserve keeping the roof support parallel to coal face. Figure A-3 is a photograph of a longwall shear and Figure A-4 provides an illustration of a Longwall mining system.
Longwall Mining Machine with Shear Cutting head
Longwall Mining Illustration