Geology Reference
In-Depth Information
Methods of Coal Mining
T
he principal methods of conventional and continuous methods of mining are discussed in the following
paragraphs.
Room-and-Pillar Coal Mining
The room-and-pillar method of mining has changed little throughout the years since its development. The mining
method can apply to drift, slope, or vertical shaft mines. In room-and-pillar mining, coal seams are mined by
developing a network of
“
rooms
”
in the coal seam. Between the
“
rooms
”
are
“
pillars
”
(walls)ofcoalleftinplace
to support the roof of the mine. The
left behind can account for as much as 40% of the total coal in the
seam, although much of this can be recovered during later stages of retreat mining (World Coal Institute, 2005).
In early conventional mining, the
“
pillars
”
were excavated by using hand tools (picks and shovels). Then hand
drills and explosives were used to loosen the coal and the coal was then hauled from the mine. Today,
mechanized equipment undercuts the coal, drilling machines make the drill holes for explosives, and scoops
load the blasted coal into shuttle cars or conveyor systems to bring the coal to the surface. In continuous mining,
the
“
rooms
”
are the same, but differ in the type of mechanization used to mine the coal. Modern
advances in technology have led to the development of the continuous miner, a machine that cuts into the full
thickness of the coal seam. The self propelled continuous mining machine rips the coal from the working face, in
a preplanned pattern of room-and-pillars, with an array of hardened steel cutting teeth mounted on rotating
wheels. The excavated coal passes through the continuous mining machine onto a conveyor system or into
shuttle cars that transports the coal from the mine (Schmidt, 1979, p. 168). The continuous mining machine has
increased the tonnage of coal mined per man shift enormously (Lindbergh and Provorse, 1980, p. 68). Room-
and-pillar mining accounts for about 49% of all the underground coal mined in the United States today
(American Coal Council, 2008). A significant disadvantage of the room-and-pillar method is that it can leave
50
“
rooms
”
and
“
pillars
”
-
60% of the coal underground in the supporting
“
pillars
”
(World Coal Institute, 2005), but much of this coal
can later be recovered by retreat mining (Figure 2.2.1).
Longwall Coal Mining
The longwall-mining method is a continuous mining system that was developed for mining coal deposits uniform
in thickness and slope or where the overburden pressures may crush support pillars or for improving productivity.
This method relies on the complete extraction of the coal in a designated area referred to as a panel (of solid coal).
As the panel is mined, complete subsidence or caving of the overlying rock strata occurs into the mined-out area
behind the working mine face.
Longwall mining can be conducted as advance longwall mining or retreat longwall mining. In either method
of longwall mining, the production of coal takes place along a face (wall of coal) that is blocked-out between
the two parallel panel entries. The panel in the area designated to be mined can be 305m (~1000 ft) wide by
3050m (~10 000 ft) or more deep (Energy Information Administration, 2008). Steel arches and hardwood
packs provide the parallel panel entries with roof support. Coal is removed from the face which is
perpendicular to the parallel entries. Mining is accomplished by a shearer or cutting tool forced against
the face of the coal seam hydraulically and pulled laterally across the width of the panel. The shearer
consists of rotating drums studded with cutting bits that are raised or lowered depending on seam thickness.
The shearer operator moves with the machine as it advances along the face. The broken coal is deposited on
a conveyor system running parallel to the face which carries the coal to the parallel entries and out of the
mine.
The roof of the mine is supported by hydraulically powered, self-advancing roof supports attached to the mining
machine. As the mining machine advances into the solid coal panel, the roof supports also advance keeping
overburden from collapsing into the exposed face of the coal. At the same time, the roof left unsupported is allowed
to break and cave immediately behind the support line with each face advance. This permits most of the weight of
the overburden to rest on the broken, caved spoil material as mining advances. The longwall-mining machine,
conveying system, roof support, and guidance systems are advanced hydraulically after each pass of the coal
shearer.
