Coal is mined by either underground or
surface methods. In underground mining the coal beds are made
accessible through shaft, drift, or slope entries (vertical,
horizontal, or inclined, respectively), depending on location of the
bed relative to the terrain.
The most widely used methods of coal
mining in the United States are termed continuous and conventional
mining. The former makes use of continuous miners which break the
coal from the face and load it onto conveyors, shuttle cars, or
railcars in one operation.
Continuous miners are of ripping,
boring, or milling types or hybrid combinations of these. In
conventional mining the coal is usually broken from the face by means
of blasting agents or by pressurized air or carbon dioxide devices.
In preparation for breaking, the coal
may be cut horizontally or vertically by cutting machines and holes
drilled for charging explosives. The broken coal is then picked up by
loaders and discharged to conveyors or cars.
A method that is increasing in use is
termed long-wall mining. It employs shearing or plowing machines to
break coal from more extensive faces. Eighty long-wall mines are now
in operation.
Pillars to support the roof are not
needed because the roof is caved under controlled conditions behind
the working face. About half the coal presently mined underground is
cut by machine and nearly all the mined coal is loaded mechanically.
An important requirement in all mining
systems is roof support. When the roof rock consists of strong
sandstone or limestone, relatively uncommon, little or no support may
be required over large areas.
Most mine roofs consist of shales and
must be reinforced. Permanent supports may consist of arches,
crossbars and legs, or single posts made of steel or wood.
Screw or hydraulic jacks, with or
without crossbars, often serve as temporary supports. Long roof
bolts, driven into the roof and anchored in sound strata above, are
used widely for support, permitting freedom of movement for machines.
Drilling and insertion of bolts is done
by continuous miners or separate drilling machines. Ventilation is
another necessary factor in underground mining to provide a proper
atmosphere for personnel and to dilute or remove dangerous
concentrations of methane and coal dust.
The ventilation system must be
well-designed so that adequate air is supplied across the working
faces without stirring up more dust. When coal occurs near the
surface, strip or open-pit mining is often more economical than
underground mining.
This is especially true in states west
of the Mississippi River where coal seams are many feet thick and
relatively low in sulfur. The proportion of coal production from
surface mining has been increasing rapidly and now amounts to over 60
percent.
In preparation for surface mining, core
drilling is conducted to survey the underlying coal seams, usually
with dry-type rotary drills. The overburden must then be removed. It
is first loosened by ripping or drilling and blasting.
Ripping can be accomplished by
bulldozers or scrapers. Overburden and coal are then removed by
shovels, draglines, bulldozers, or wheel excavators. The first two
may have bucket capacities of 200 yd3 (153 m3).
Draglines are most useful for very
thick cover or long dumping ranges. Hauling of stripped coal is
usually done by trucks or tractor-trailers with capacities up to 240
short tons [218 metric tons (t)]. Reclamation of stripped coal land
is becoming increasingly necessary.
This involves returning the land to
near its original contour, replanting with ground cover or trees, and
sometimes providing water basins and arable land.
5 comments:
You’ve provided great information in your blog. Many thanks for sharing the information in your blog.
precision hole drilling
I like the writing style. It is just perfect. The blog post is very connective and attractive. All the information I found in post is very useful. Thanks a lot.
quartz substrate
You writing is always fabulous. This is third time I’ve read your blog and I find the information very useful.
quartz machining
It looks that you have put tremendous effort finding the facts. I really appreciate your research work. Hope to read more articles on your blog site. Keep it up dear.
custom optical coatings
Not only your contents but also the way you present the issue is awesome. Particularly this one is articulating the sense of the discussion. Your word power is awesome. Keep it up.
ceramic machining services
Post a Comment