Surface Coal Mining:
Surface coal mining generally involves the following sequence of
unit operations:
(1) clearing the land of trees and vegetation,
(2) removing and storing the top layers of the unconsolidated soil
(topsoil),
(3) drilling the hard strata over the coal seam,
(4) fragmenting or blasting the hard strata with explosives,
(5) removing the blasted material, exposing the coal seam, and
cleaning the top of the coal seam,
(6) fragmenting the coal seam, as required, by drilling and
blasting,
(7) loading the loose coal onto haulage conveyances,
(8) transporting the coal from the mine to the plant, and
(9) reclaiming lands affected by the mining activity.
Methods adopted in Surface Mining:
When the coal seams occur
near the surface, it may be
economical to extract the coal using
open cut
(also referred to as open cast, open pit, mountaintop removal
or strip) mining methods. Open cast coal
mining recovers a greater proportion of the coal deposit than underground methods. Large open cast
mines can cover an area of many square kilometers and use very
large pieces of equipment.
Surface mining techniques can be broadly classified into the
following types:
(1) contour strip mining,
(2) area strip mining,
(3) open-pit mining, and
(4) auger mining.
Contour strip mining:
Contour strip mining is a method of surface mining or in which bulldozers, power
shovels, or
stripping wheels are used to remove large chunks of earth in
terraced strips in order to extract coal
from a seam or series of seams on a hill or mountain.
The contour mining method consists of removing overburden from
the seam in a pattern following the
contours along a ridge or around the hillside.
The process of contour
mining begins with constructing roads to access the coal seam elevation and
the top or the mountain. A bench is excavated in the mountain at
the coal seam elevation, allowing for
room for the mining equipment and facilities.
The limitations of this method of mining are both economical and
technical .When the burden
undertaken exceeds the coal obtained, the result would be a
financial loss. The amount of waste
accumulated after this method of mining is comparatively very
high to open pit mining.
Area strip mining :
Strip mining exposes coal by removing earth above each coal
seam. This earth is referred to as
overburden and is removed in long strips. The overburden from
the first strip is deposited in an area
outside the planned mining area and referred to as out-of-pit
dumping. Overburden from subsequent
strips are deposited in the void left from mining the coal and
overburden from the previous strip. This
is referred to as in-pit dumping.
"Strip mining" is the practice of mining a seam of mineral, by first removing a
longstrip of
overlying soil and rock (the overburden). It is most commonly used
to mine coal and lignite
(brown coal).
Area strip mining, applied where the terrain is flat, commences
with a trench or “box cut” made
through the overburden to expose a portion of the coal seam.
This trench is extended to the limits of the property in the
strike direction.
After coal removal, a second cut is made parallel to the first
one, and the overburden material from
this cut is placed in the void of the first cut. The process is
repeated in successive parallel cuts until
the stripping ratio indicates that continued surface mining is
uneconomical.
Open-pit mining:
This type of mining involves the extraction of rock or minerals
from the earth by forming an open pit.
This process differs from the other method which requires
digging into the earth. This method of
mining is carried out where useful minerals or rocks are found
near the surface.
The open-pit method
is generally practiced where thick coal seams are overlain by
thick or thin overburden; it is also used
for mining steeply pitching coal seams.
In open-pit mining of the coal seam, several benches are
established in both the overburden strata and
the coal seam. In the beginning stages of mining, considerable
volumes of overburden materials must
be accumulated in large dump areas outside the mine
Auger mining:
Auger mining is a relatively low cost method of coal mining and is
practical in areas where the
overburden (material covering the coal seam) is too thick to be
removed economically or where the
coal seam is too thin for underground mining.
Auger mining is a surface mining technique used to recover additional coal from a seam located
behind a highwall produced either by stripping or open-pit mining.
Holes are horizontally drilled at regular intervals to depths of
as much as 1,000 feet. As the cutting
head of the auger bites into the coalface, the cut coal is carried
out by the screw portion of the bit.
Once the hole is mined to its required depth, the auger machine is
moved a few feet and another hole
is drilled.
Auger mining can create pits up to 500 feet into the highwall depending on the conditions of the site
and the type of auger used. This mining method is generally
inexpensive; however, coal recovery
rates are low. Auger mining
is usually associated with contour strip mining.
Coal recovery rates approach 60 percent with this method. The cutting heads of some augers are as
high as 2.5 metres. As each
stem works its way into the coal seam, additional auger stems are added,
so that hole depths of more than 60 to 100 metres are not
uncommon. Problems of subsidence, water
pollution, and potential fires are associated with augering.
Highwall mining is an adaptation of auger mining.
Instead of an auger hole, an entry into the coal seam is made by a
continuous miner, remotely
operated from a cabin at the surface.
The cut coal is transported by conveyors behind the miner to the
outside. Using a television camera,
the operator can see and control the miner’s progress.
The entry can be advanced 300 to 400 metres into the coal seam,
after which the miner is retreated to
the surface and repositioned to drive an entry adjacent to the
previous one.
Advantages over augering include higher productivity, greater
safety, and lower cost.
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