Page 152 - Synthetic Fuels Handbook

138 CHAPTER FIVE

In the current context, coal mining depends on the following criteria: (a) seam thickness,
(b) the overburden thickness, (c) the ease of removal of the overburden (surface mining),
(d) the ease with which a shaft can be sunk to reach the coal seam (underground mining), (e) the
amount of coal extracted relative to the amount that cannot be removed, and (f) the market
demand for the coal.
There are two predominant types of mining methods that are employed for coal recov-
ery. The first group consists of surface mining methods, in which the strata (overburden)
overlying the coal seam are first removed after which the coal is extracted from the exposed
seam (Fig. 5.8). Underground mining currently accounts for recovery of approximately
60 percent of the world recovery of coal.

Underground Original Dragline removing Surface mining
mining methods land surface mountain top methods
Dozer along
Rock spoil contour bench

Coal beds
Auger mining
Dragline in
Drift mine pit
Coat elevator Miner’s elevator Slope mine Coal beds
Rock spoil

Shaft mine S.G.
FIGURE 5.8 Coal mining. (Reproduced with permission. Copyright 2000. Kentucky Geological
Survey, University of Kentucky).

5.3.1 Surface Mining
Surface mining is the application of coal-removal methods to reserves that are too shallow
to be developed by other mining methods.
The characteristic that distinguishes open-pit mining is the thickness of the coal seam
insofar as it is virtually impossible to backfill the immediate mined out area with the origi-
nal overburden when extremely thick seams of coal are involved. Thus, the coal is removed
either by taking the entire seam down to the seam basement (i.e., floor of the mine) or by
benching (the staged mining of the coal seam).
Frequent use is made of a drift mine in which a horizontal seam of coal outcrops to the
surface in the side of a hill or mountain, and the opening into the mine can be made directly
into the coal seam. This type of mine is generally the easiest and most economic to open
because excavation through rock is not necessary.
Another surface mine is a slope mine in which an inclined opening is used to trap the
coal seam (or seams). A slope mine may follow the coal seam if the seam is inclined and
outcrops, or the slope may be driven through rock strata overlying the coal to reach a seam
that is below drainage. Coal transportation from a slope mine can be by conveyor or by
track haulage (using a trolley locomotive if the grade is not severe) or by pulling mine cars
up the slope using an electric hoist and steel rope if the grade is steep. The most common
practice is to use a belt conveyor where grades do not exceed 18°.