Source: http://www.uky.edu/KGS/coal/coal_mining.htm
Underground Mining
Underground modes of
access include drift, slope, and shaft mining, and actual mining methods
include longwall and room and pillar mining. Drift mines enter horizontally
into the side of a hill and mine the coal within the hill. Slope mines usually
begin in a valley bottom, and a tunnel slopes down to the coal to be mined.
Shaft mines are the deepest mines; a vertical shaft with an elevator is made
from the surface down to the coal
In room and pillar
mining, the most common type of underground coal mining, coal seams are mined
by a "continuous miner"
that cuts a network of "rooms" into the seam. As the rooms are cut,
the continuous miner simultaneously loads the coal onto a shuttle or ram car
where it will eventually be placed on a conveyor belt that will move it to the
surface. "Pillars" composed of coal are left behind to support the
roof of the mine. Each "room" alternates with a "pillar" of
greater width for support. Using this mining method normally results in a
reduction in recovery of as much as 60 percent because of coal being left in
the ground as pillars. As mining continues, roof bolts are placed
in the ceiling to avoid ceiling collapse. Under special circumstances, pillars
may sometimes be removed or "pulled" toward the end of mining in a
process called "retreat mining." Removing support during retreat
mining can lead to roof falls, so the pillars are removed in the opposite
direction from which the mine advanced: hence the term "retreat
mining."
Longwall mining is
another type of underground mining. Mechanized shearers are used to cut and
remove the coal at the face of the mine. After the coal is removed, it drops
onto a chain conveyor, which moves it to a second conveyor that will ultimately
take the coal to the surface. Temporary hydraulic-powered roof supports hold up
the roof as the extraction process proceeds. This method of mining has proven
to be more efficient than room and pillar mining, with a recovery rate of
nearly 75 percent, but the equipment is more expensive than conventional room
and pillar equipment, and cannot be used in all geological circumstances. As
mining continues, roof bolts are placed in the ceiling to
avoid ceiling collapse. In longwall mining, only the main tunnels are bolted.
Most of the longwall panel is allowed to collapse behind the shields (which
hold the roof as coal is excavated).
Surface or Opencast Mining
Surface-mining methods
include area, contour, mountaintop removal, and auger mining. Area mines are
surface mines that remove shallow coal over a broad area where the land is
fairly flat. Huge dragline shovels commonly
remove rocks overlying the coal (called overburden). After the coal has been
removed, the rock is placed back into the pit. Contour mines are
surface mines that mine coal in steep, hilly, or mountainous terrain. A wedge
of overburden is removed along the coal outcrop on the side of a hill, forming
a bench at the level of the coal. After the coal is removed, the overburden is
placed back on the bench to return the hill to its natural slope. Mountaintop
removal mines are special area mines used where several thick coal seams occur
near the top of a mountain. Large quantities of overburden are removed from the
top of the mountains, and this material is used to fill in valleys next to the
mine. Augur mines are operated on surface-mine benches (before they are covered
up); the coal in the side of the hill that can't be reached by contour mining
is drilled (or augured) out
Variables Influencing
Mining Methods
Source: http://www.desrizal.com/reading-186-Variables-Influencing-Mining-Methods.html
When
selecting a mining method, it
is important to maintain flexibility within the method, in case something goes
wrong. Any mining layout must establish the following facilities. Creation of
the above facilities is dependent on the following variables:
Physical
Properties of the mined rocks and ore. The stress/strength relationships of rocks and
ore determine the horizontal and vertical stope dimensions which will stand
without support. This in turn establishes the appropriateness of one of the
three basic stoping methods ...
- Naturally supported
- Artificially supported
- Unsupported (caving)
Orebody
Dimensions - The strike length, width (footwall to hanging wall)
and height of the orebody determine the maximum spans that are possible, which
in some cases will exceed the permissible spans. For greater orebody widths,
longitudinal stopes will give way to transverse stopes. Longitudinal stopes
constitute mining along strike and transverse stopes are perpendicular to
strike. Stope and pillar dimensions must be established.
Orebody
Orientation - the dip of the orebody determines the part gravity
will play in the mining operation. In this course, the following orebody
classifications are used ...
- Flat Dip: 0-20o
- Medium Dip: 20-50o
- Steep Dip: 50-90o
Ore
Grade - The grade of the orebody influences considerations
of selective or mass production methods and thus the need and method for pillar
recovery.
Structural
Features - Major faults, dykes, and rock mass jointing
influence the location and size of stopes and pillars.
Depth
of Operation - This is a factor in so far as stress increases with
depth ... in addition, hoisting heights, and hence costs, also increase.
Proximity
of Other Orebodies - Orebody proximity determines the method of mining
one orebody or lens with respect to an adjacent ore block. It also determines
extraction sequences, location of access, pillar sizes, etc., and whether
mining progresses from hanging wall to footwall or footwall to hanging wall.
Ground
Surface Effects ... Considerations with respect to restriction of
subsidence, e.g. where there are lakes, towns, etc. above; the requirements of
land reclamation and waste and tailings disposal; and the control of effluents
in surface water discharge may all impact on the choice of mining method. The
climate and topography must also be considered.
Labour
and Supplies - Cheap, locally available, labour does not
necessarily mean low costs. Instead, mining methods must generally be modified
for different parts of the world in accordance with the competency of available
labour. The presence of skilled labour but in short supply influences the
choice of methods and equipment. Local availability of supporting materials
such as timber, fill, tailings, cement, etc. also influence mining practice.
Capital
Available ... Costs in mining are of two types, capital and
operating. If the capital for certain desirable expenditures is not available,
operating costs are likely to be higher. For example, the open stoping method
in some cases ties up less capital than shrinkage stoping. Some methods at a
higher operating cost may require less development expenditure. In the overall
picture retreating methods may be more economical than advancing methods. In
each case a capital outlay might be recoverable through lower operating costs
when sufficient tonnage is in sight. The judicious outlay of capital is always
desirable. The difficulty arises in defining judicious in terms of developed
and prospective ore at various stages in the life of a mine.
Physical
Location - The location and jurisdiction of the orebody
(political, provincial, federal, foreign) must be taken into account as well as
ownership (park, reservation, private, common property) or boundary with
another mining operation.
