The information on this page was prepared by:
| Joseph Ruhl | jruhl@usc.edu | Computer Engineering and Computer Science |
| TianLi Woon | twoon@usc.edu | Electrical Engineering |
Coal: Mining
The coal mining industry is now recognized as one of the safest, with a lower rate of nonfatal injuries and illnesses per 100 employees than the agriculture, construction or retail trades. Mining accounted for 2.1 percent of U.S. fatal injuries in 2001. Of all fuel and non-fuel commodities mined, the quantity of coal currently produced ranks third behind crushed stone, sand & gravel. The largest coal producing state is Wyoming, with 369 million tons of production in 2001. In the past five years, the U.S. has produced more than one billion tons of coal each year.
http://www.teachcoal.org/aboutcoal/articles/coalreserves.html
TYPES OF COAL
Anthracite: The “hard coal”, formed from bituminous coal when great pressures developed in folded rock strata during the creation of mountain ranges; has the highest energy content of all coals and is used for space heating and generating electricity. Anthracite averages 25 million Btu per ton.
Anthracite
Bituminous: The “soft coal” formed when greater pressure was applied to subbituminous coal; most commonly used for electric power generation in the U.S.; has a higher heating value than either lignite or subbituminous, but less than that of anthracite. Bituminous coal averages 24 million Btu per ton.
Bituminous
Subbituminous: Formed from lignite when it came under higher pressure; a combustible mineral formed from the remains of trees, ferns and other plants that existed and died during the time of the dinosaurs. A dull black coal with a higher heating value than lignite that is used primarily for generating electricity and for space heating. Subbituminous coal averages 18 million Btu per ton.
Subbituminous
Lignite: Increased pressures and heat from overlying strata caused buried peat to dry and harden into lignite, a brownish-black coal with generally high moisture and ash content and lower heating value. However, it is an important form of energy for generating electricity, particularly in the American Southwest. Lignite averages 14 million Btu per ton.
Lignite
METHODS OF COAL MINING
1) Room and pillar method
More than two-thirds of the coal produced underground is extracted by continuous mining machines in the room-and-pillar method. The continuous mining machine contains tungsten bits on a revolving cylinder. The continuous miner breaks the coal from the face and then conveys it to a waiting shuttle car which transports it to the conveyor belt to be moved to the surface. No blasting is needed. After advancing a specified distance, the continuous miner is backed out and roof bolts are put in place. The process is repeated until the coal seam is mined.
2) LONG WALL LINING
Another method, called longwall mining, accounts for about 20 percent of production. This method involves pulling a cutting machine across a 400 to 600 foot long face (longwall) of the coal seam. This machine has a revolving cylinder with tungsten bits that shear off the coal. The coal falls into a conveyor system which carries it out of the mine. The roof is supported by large steel supports, attached to the longwall machine. As the machine moves forward, the roof supports are advanced. The roof behind the supports is allowed to fall. Nearly 80 percent of the coal can be removed using this method. The remaining 11 percent of underground production is produced by conventional mining which uses explosives to break up the coal for removal.
3) SURFACE MINING
Most surface mines follow the same basic steps to produce coal. First, bulldozers clear and level the mining area. The topsoil is removed and stored for later use in the reclamation process. Many small holes are drilled through the overburden (dirt and rock above the coal seam) to the coal seam. Each is loaded with explosives which are discharged, shattering the rock in the overburden. Giant power shovels or draglines clear away the overburden until the coal is exposed. Smaller shovels then scoop up the coal and load it onto trucks, which carry the coal to the preparation plant. Once the coal is removed, the land is returned to the desired contour and the topsoil is replaced. Native vegetation and/or trees are planted.
CRUSHING COAL
1) JAW CRUSHER
It is one of the main types of primary crushers in a mine or ore processing plant. Rocks or ore is crushed by the �V� shaped jaw until it is small enough to exit through the gap at the bottom.
http://66.113.204.26/mining/jaw_crusher.htm
2) CONE CRUSHER
This is another major type of crushers in a mine or ore processing plant. The crushing action is caused by the closing of the gap between the mantle line (movable) mounted on the central vertical spindle and the concave liners (fixed) mounted on the main frame of the crusher. The gap is opened and closed by an eccentric on the bottom of the spindle that causes the central vertical spindle to gyrate. The vertical spindle is free to rotate around its own axis.
http://66.113.204.26/mining/cone_crusher.htm
3) BALL MILL and ROD MILL
A Ball Mill grinds material by rotating a cylinder with steel grinding balls, causing the balls to fall back into the cylinder and onto the material to be grounded. A rod mill is very similar, except it uses a long rod to grind the material. 
http://66.113.204.26/mining/ballmill.htm
4) HAMMER MILL
A hammer mill crushes material that is friable, by impacting it against a rotating hammer (typically traveling between 750 RPM and 1800 RPM). Then the material is forced against a rugged solid plate called a “breaker plate” which further degrades the particle size. Finally, the material is forced over a discharge grate by the hammers, where crushed finer particles drop through the discharge grate and larger particles travel around for another crushing cycle, until they fall through the discharge grid. This is not a popular method of grinding ore due to the high maintenance costs from the rapid abrasion rate of the rotating hammers.
http://66.113.204.26/mining/hammil.htm
5) ROLL CRUSHER
The particles are drawn into the gap between the rolls by their rotating motion and a friction angle formed between the rolls and the particle, called the nip angle. The two rolls force the particle between their rotating surface into the ever smaller gap area, and it fractures from the compressive forces presented by the rotating rolls.
http://66.113.204.26/mining/rollcrush.htm
COAL PREPARATION PLANTS
At this prep plant, all feed coal (ROM) is crushed to -2″ before entering the plant circuits. The 2″ x 10 mesh coal is screened and goes to the primary dense medium cyclone circuit. The 2″ x 10 mesh coking coal is recovered from the refuse here. The middlings (-10 mesh x 60 mesh) are processed in the Secondary Dense Medium Cyclone Circuit to recover the coal here, this produces a higher ash coal and is used as steam coal. The coking coal product conveyor has an intermediate screen that enables material to be diverted in varying proportions to steam coal if necessary to enhance the coking properties.
The -10 mesh coal is deslimed at 100 mesh and then sized at 10 mesh x 100 mesh is processed in the spiral circuit to produce coking coal. The -100 mesh slimes and -60 mesh coal is then processed in the flotation circuit to produce coking coal.
The cleaned coal is dried in centrifuges and a belt filter press. Coarse rejects are crushed and then recombined with the fine tailings for disposal to the refuse fill. All ROM coal production is beneficiated in the coal preparation plant. Approximately 2,000,000 tonnes of cleaned coal is produced annually, with the plant processing about 3,000,000 tons of raw coal annually.
http://66.113.204.26/mining/coal/coalprep.htm
A Coal preparation plant, nestled among the mountains. http://66.113.204.26/mining/coal/coalprep.htm
Another photo of a coal prep plant.
http://66.113.204.26/mining/coal/coalprep.htm
This graphic shows the general process of a coal plant. First, the mined coal is loaded into a stockpile, with a reclaim tunnel beneath it. Then, the coal is transported to a raw coal silo, usually 10,000 ton capacity, for feed to the plant at a constant rate. In this instance the first stage is a crushing/screening plant, with heavy media processing (for coarse coal sizes – 2″ x 10 mesh), spirals for the middling sizes (10 mesh x 60 mesh), flotation for the -60 mesh fine coal feed. The cleaned coal is then transported to the next plant, which is a series of thermal dryers for reducing the moisture content and thereby raising the BTU value of the coal. The final dried coal product is then transported to the clean coal silo’s for loading onto a train for transport to the end user. In this case, a steel mill and a steam power generation plant.