Surface Mining

SURFACE MINING METHODS

Surface mining operations typically  involve some sort of strip mining during the lifespan of a given mine. Strip mining, in general is characterized by the removal of overburden to expose the mineral for extraction. Surface mining is typically employed in situations where the overburden is relatively thin, or where underground mining would not be economically feasible. The main types of surface mining nclude:

·       area mining,

·       contour mining,

·       mountaintop mining, and

·       auger/highwall mining.

Area mining is typically conducted on flat or gently rolling terrain, and is used to extract coal over a large area. An area mine starts with an initial cut to expose the coal seam to be removed. The seam may then be removed, creating an open pit. Spoils and overburden are placed in a valley fill or some other disposal site. Materials that may be prone to leaching acid- or toxic-forming materials may be segregated from the rest of the overburden so that they may be isolated from exposure to oxygen and water. As the operation progresses, spoil from new cuts is used to backfill pits left from previous cuts .

Contour mining takes place in mountainous or rolling hill areas where it is uneconomical or infeasible to remove all of the overburden from a particular coal seam, and mining is limited to the side of a mountain or to the end of a ridge line. Typically mining operations progress along the outcrop of a coal seam, removing overburden inward toward the mountaintop or ridge core until the highwall limit of that coal seam is reached. The highwall limit is determined by its stripping ratio. This results in mine cuts that wrap around mountaintops or ridge lines parallel to the contour of the land in a sinuous pattern dictated by topography .

Mountaintop mining is an extreme version of area strip mining. In a mountaintop mining operation the over burden is removed to expose the coal seam. The rocky material is then deposited in hollows and valleys adjacent to the mine, creating a valley fill. After coal extraction is completed, the area is reclaimed as a flat space. This method of mining results in alterations of the topography and drainage of the area. Mountaintop mining is sometimes used to re-mine areas previously mined by underground techniques. In some cases, coal from several seams may be extracted to maximize the profitability of mining

the area .

Auger mining and highwall mining are predominately secondary extraction techniques, used after mining with one of the other methods of surface mining. When the stripping ratio becomes too high to justify further excavation, the final boundary formed by the mine is called the highwall. Rather than abandoning or covering the mine, and leaving valuable minerals behind, augers and continuous highwall miners are used to recover a portion of the coal remaining in the highwall. A traditional auger uses open thread steel drill sections behind a cutting bit. The auger is positioned adjacent to the coal seam and breaks up the coal as it slowly rotates through the seam. As it does so, chunks of coal are drawn out through the open drill thread. Augers can penetrate a coal seam to a depth of approximately 130m (about 400ft), recovering between 30-40% of the coal .