Surface mining is used for about 40% of coal production in the world. When coal deposits are near the surface, it may be inexpensive to extract the coal using surface mining methods. The different types of surface mining are modern surface mining, strip or area mining, contour mining, and mountaintop removal mining.
The most usual surface mining method is strip or area mining. This method is most apt for areas with flat terrain. Strip mining exposes the coal by removing the earth above the coal seam in long cuts or strips. When all the earth above the coal seam is removed, the underlying coal seam will be exposed. The exposed coal block may be drilled and blasted. Once this strip is empty of coal, the strip mining process is repeated with a new strip being created next to it.
The contour mining method is the surface mining method which is most commonly used in areas with rolling to steep terrain. This method involves removing the earth above the coal seam in a pattern following the contours along a ridge or around a hillside. This method may cause severe landslide and erosion problems. To solve these problems, a variety of methods were devised to use freshly cut overburden to refill mined-out areas. There are limitations on contour strip mining. When the operation reaches a predetermined stripping ratio, it is not profitable to continue.
Mountaintop removal coal mining is a surface mining method which involves removal of mountaintops to expose coal seams, and disposing of associated mining overburden in nearby valleys and hollows fills. Mountaintop removal mining method combines area and contour strip mining methods.
Modern Open cast surface mining methods recovers a greater proportion of the coal deposit than underground methods.
Deep underground mining is needed when coal seams are found too deep underground. The main underground mining methods are Longwall mining, Continuous mining, Blast mining, Shortwall mining and Retreat mining.
Longwall mining covers for about 50% of underground production. It uses a sophisticated machine with a rotating drum that moves mechanically back and forth across a wide coal seam. Longwall mining helps for high levels of production with high safety. Sensors used in this process of mining helps in detecting the amount of coal remaining in the seam while robotic controls helps in enhancing the efficiency of the process.
Continuous mining is used for about 45% of underground coal production. Continuous mining uses a machine with a large rotating steel drum equipped with
tungsten carbide teeth to scrape coal from the seam.
Blast mining accounts for less than 5% of total underground production in U.S. Blast mining is an older practice that uses explosives such as dynamite to break up the coal seam. The broken coal is then gathered and loaded on to shuttle cars or conveyors and carried to a central loading area.
Shortwall mining method is used only for less than 1% of deep coal production. This method also utilizes a continuous mining machine with moveable roof supports, similar to longwall mining method.
Retreat mining method uses pillars or coal ribs to hold up the mine roof. This method is the one of the most dangerous mining method as it is impossible to predict when the ceiling or roof will collapse and crush or trap the mine workers.
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