Introduction - Coal Waste Impoundments: Risks, Responses, and Alternatives - page 4

ADVANCED COAL CLEANING
Increased mechanization in the underground coal mining industry has
decreased selectivity and increased the volume of refuse. Equipment, such as
continuous miners or longwall shearers, often takes roof and floor rock in
addition to in-seam partings ( Sidebar 1.1 ). Equipment currently used to mine
and transport coal produces more fine coal particles than did earlier equipment.
Rotating cutter heads, feeder-breakers, and transfer points in
SIDEBAR 1.1 UNDERGROUND COAL MINING METHODS IN
THE UNITED STATES
About 39 percent of coal produced in the United States comes from
underground mines (Chircop, 1999). Underground coal is mined by the
following methods: conventional, longwall, continuous, and shortwall.
Longwall and continuous mining are used effectively in combination
(Chircop, 1999).
Conventional mining processes include drilling and blasting the coal.
This method, one of the oldest mining methods, can be effective in certain
geologic areas. Today, it is used for only 5 percent of underground coal
mining (Chircop, 1999).
More than 45 percent of underground coal is mined by longwall
mining (Chircop, 1999). This method is gaining in popularity because it
can improve coal recovery to 80 percent, and it enhances miners' safety
(Peterson et al., 2001). Production rates depend on the width of the block,
the thickness of the coal seam, and the technology used to transport the
raw coal out of the mine. Rotating drums, steel plows, or mounted
shearers traverse back and forth across the block width and excavate
blocks 600 to 1,200 feet wide and 5,000 to 7,500 feet long (Chircop, 1999;
Peterson et al., 2001). Longwall length capabilities have been steadily
increasing: some Western operations now achieve lengths of over 10,000
feet (e.g., Twentymile in Colorado and SUFCO in Utah). The miners and
mining equipment are protected by moving hydraulic roof supports called
shields (Chircop, 1999). After an area has been mined, the roof collapses.
Continuous mining is a mechanized method utilizing mechanical
cutting machinery. Although longwall mining has moved to the forefront
(Chircop, 1999; NRC, 2001), continuous mining is still important in coal
production. Continuous mining equipment is used to develop the areas for
longwall mining. The continuous mining method uses a room-and-pillar
system, whereby mined-out “rooms” are supported by coal “pillars.” An
operator, who maintains visual contact with the machine, can control this
machinery remotely, thereby increasing miners' safety.
Shortwall mining refers to mining with a continuous mining machine,
moving roof supports, and excavating blocks 150 to 200 feet wide and
more than one half mile long (Chircop, 1999). This method is currently not
used to any appreciable extent.