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How is coal mined?

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    How is coal mined?




    Coal Mine






    How is coal mined?

    As was the case 50 years ago, most coal is produced from two major types of mines-- underground and surface. But the methods for recovering coal from the earth have undergone drastic changes in the past 25 years, as a consequence of technological advances.
    Fifty years ago when most coal mining was done manually, underground mines accounted for 96 percent of the coal produced each year. Today, almost 60 percent is produced from surface mines. Most underground mines in the United States are located east of the Mississippi River, although there are some in the West, particularly in Utah and Colorado.



    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. 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.


    Half of the minable surface coal in the United States is located in the West, but significant amounts are also present in Appalachia and Midwestern states. Surface mining is used when the coal seam is located relatively close to the surface, making underground mining impractical. Before a company can surface mine, it must gather information about the site regarding growing conditions, climate, soil composition, vegetation, wildlife, etc. With this information, the company then applies to the state or federal government for a permit to mine. The company must post a bond for each acre of land it mines to assure that it will be properly reclaimed.
    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. Coal companies operating surface mines must comply with strict requirements and regulations of the Federal Surface Mining Control and Reclamation Act. A crucial part of the surface mining process is restoring a mined site to acceptable ecological conditions, which means it must be made as productive as it was prior to mining. There are farms, parks, wilderness and recreation areas on what was once surface mines.
    The major stigma associated with the coal industry today is the abandoned or "orphan" mines of the early coal mining years. These orphan mines are systematically being reclaimed under the Surface Mining Act taxes coal producers at the rate of 35 cents a ton for surface mined coal, 10 cents a ton for lignite mined coal, and 15 cents a ton for underground mined coal. The tax is paid to the government and is used to reclaim the orphaned mines.
    Provided by National Energy Foundation.


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    Uncovering Coal is the Biggest Job
    Overburden is removed by four different methods:
    1.
    Blasting utilizing explosives to cast the overburden
    2.
    Dozers moving overburden short distances prior to drag line operations
    3.
    The dragline removing the bulk of the overburden
    4.
    Shovels stripping and loading overburden and coal into the haul trucks Blasting loosens materials, casts overburden
    Both overburden and coal can be very difficult to excavate. Therefore, blasting is necessary in order to loosen the overburden and coal before it is moved by the heavy "yellow-iron" equipment. A specially designed powder truck transports, mixes and loads explosives into each drill hole. A technique called "cast blasting" is used to help remove the overburden. During cast blasting, the drill holes and explosive charges are designed so that a portion of the overburden is cast laterally by the force of the explosion into the adjacent mined-out pit. This technique, when utilized, reduces by up to 25% of the amount of overburden that the dragline must handle. Cast basting is a significant cost savings when compared to utilizing trucks and shovels. There is less wear and tear on equipment, less consumption of diesel fuel, and fewer employees are required.


    Dozers move overburden short distances
    Many other pieces of support equipment are needed to conduct day-to-day activities. UCM uses dozers including a Komatsu 475A-5 dozer and several Caterpillar dozers including the D11R-CD, Carry Dozer. They are some of the largest tracked dozers manufactured in the world. Two road graders operate during day and night shifts to keep the haul roads smooth for the big trucks. Two water trucks provide dust control on the roads. Additionally, a high-pressure nozzle mounted on the water trucks serves as ready reaction equipment for fighting fires.
    Dragline does the "dirt" work
    The most efficient machines for moving large volumes of dirt are draglines. In 1977, UCM made a major investment by purchasing a 1300W Bucyrus-Erie Walking Dragline. The dragline arrived in component parts on 26 railcars and 40 trucks during December 1977. It took 11 months to assemble the 2,100-ton machine. Named "Ace-in-the-Hole" by Healy school children, the dragline is the largest land mobile machine in Alaska. The acquisition of "Ace-in-the-Hole" made it possible to double coal production and initiate UCM's Korean export contract in 1985. With its 325-foot boom, the dragline has a reach of 270 feet. The bucket weighs 32 tons and will hold 33 cubic yards of material. In one 24-hour period, the dragline can move 24,000 yards of dirt, leaving a strip of uncovered coal 145-feet (or more) wide. Stability for this heavy machine is achieved by the large steel plate (the tub) which rests on the ground during drag-line operations. Wheels or tracks would be impractical due to its extreme weight. The dragline moves by "walking." Shoes on both sides of the machine lift the base partially off the ground and drag it backward. Each step takes about 40 seconds and moves the dragline a distance of approximately 7 feet. While it takes only one person to operate the controls, there are two operators on duty at all times. One operates the dragline from the cab while the other does routine maintenance. For safety reasons, the operators switch positions every hour. Since the dragline was commissioned in 1978, it has operated for more than 100,000 hours of digging. It moves approximately 6 to 7 million cubic yards of overburden every year.

    The dragline is powered solely by electricity. A Load Stabilizer System consisting of a 40-ton flywheel and motor/generator serves as a buffer between the dragline and the local electrical grid. The flywheel stores energy to be used whenever the dragline's electrical demand increases too quickly for the local grid. This process enables the dragline to operate without causing power fluctuations for other electrical customers.
    The dragline has several buckets, which are used on a rotational basis. UCM utilizes several different configurations of bucket including 33-, 35-, and a high-production 42 cubic-yard bucket. Each bucket is used on the dragline for approximately 4 to 6 months. After that period, the bucket must be serviced repairing critical surfaces of the bucket vulnerable to the abrasive excavated materials. The bucket is repaired by welding wear plates on critical surfaces vulnerable to the abrasive materials that are excavated.

    Shovels and backhoes strip overburden and load coal While the dragline does the majority of the excavation work at the mine, additional excavators are needed. Two track-mounted O&K excavators, RH120C backhoe and an RH170 shovel, are used to load coal and to strip overburden in areas difficult for the dragline to maneuver and trucks with rock and gravel for construction projects. The backhoe has a 16 cubic-yard bucket and is powered by twin 567 horsepower diesel engines. A large hydraulic shovel, the O&K RH-170, was to UCM's fleet in August 1997. Its shovel has a 26 cubic-yard bucket and can load the 150-ton trucks in four passes.
    Trucks and other equipment support operations A fleet of six trucks is used to haul coal and overburden (dirt, topsoil, gravel and rocks). During 1995, the Caterpillar 785 haul truck was added to the UCM fleet. The truck has a capacity of 150 tons, more than 50% greater than the previous 95-ton Dresser HaulPaks. Local modifications are made to the 150 ton truck after delivery from the manufacturer. UCM customizes the truck bed by adding an additional 16 inches in height of the bed. The additional height allows a larger volume of coal (which is lighter than overburden) to be hauled.

    Many other pieces of support equipment are needed to conduct day-to-day activities. UCM utilizes dozers including a Komatsu 475A-5 and several Caterpillar dozers including the D11R-CD for routine daily earthmoving requirements. These dozers are some of the largest tracked dozers manufactured in the world. Two road graders operate during day and night shifts to keep the haul roads smooth for the larger trucks. Two water trucks provide dust control on the roads. Additionally, a high-pressure nozzle mounted on the water trucks serves as ready reaction fire fighting equipment, should it be needed.

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    MINING COAL - METHODS - History

    Historically, underground methods were used to mine coal. When a coal seam was to be mined a tunnel was driven to access the coal. Branching off this main tunnel were smaller tunnels that ran in the coal seam and allowed it to be mined. As time and mining progressed each mine consisted of many miles of these workings.
    The earliest mines were usually worked largely by hand; the miners’ using picks and shovels to dig and load the coal into one ton coal cars. "Pit ponies" pulled trains of coal cars on rails to surface, though in some, the miners pushed the coal cars themselves.
    These techniques were inefficient and by the 1900s, mechanization was becoming common in coal mining although in some mines, to reduce chances of sparks and explosions, hand mining continued for many years.
    MINING COAL - METHODS - Pillar and Stall


    In pillar and stall workings, coal was removed from large areas underground, but much of the coal was left behind. These were the pillars that supported the roof of the mine and normally prevented the mine from caving in as the miners cut deeper into the coal seam. Sometimes these pillars were removed after the main parts of the seam had been extracted.

    Tunnels and workings also required timbers or "pitprops" which supported the rocks above and prevented smaller rocks from falling onto the miners.


    MINING COAL - METHODS - Long Wall




    Long wall mining worked in a different way. Tunnels were dug into the coal seam, usually supported by pit props and pillars of rock and then miners worked along face of the coal seam removing coal as they worked.

    Timbering was carried out along the areas being mined but the area in behind from which the coal had been mined was allowed to collapse.

    Long wall mining was usually done on thin coal seams up to about 5 feet (1.5 metres) in thickness.


    MINING COAL - METHODS - Underground Railways


    Coal was moved from the workings to the surface by underground railways. When mining started here in 1898, small spark free compressed air powered locomotives or "dinkys" were used to haul trains of one ton coal cars to surface.
    Steam locomotives could not be used underground because of the danger of their fires starting an explosion. Electric locomotives were seldom used in the early mines and only in areas that were completely free of explosive gases.






    As time passed and technology advanced the railways were replaced by electric powered loaders and conveyor belts.





    MINING COAL - METHODS - The Tipple


    In the early underground coalmines, the most obvious structure at the mine site or "pit head" was the tipple. These structures processed the coal into different products and were were usually built on a railway siding so that the coal could be loaded into railway hopper cars for shipment.

    The picture shows the Michel Tipple in 1909.














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