|Surface mining is used when coal seams are close to the earth's surface. Under this procedure, topsoil and rock above the seam, called the overburden, are removed and placed to one side, exposing the coal. It is then removed, loaded onto trucks and hauled to a preparation plant or site.
A preparation plant is usually located on a mine site, but one preparation plant can serve a number of mines. There, the coal is crushed, sized and washed to prepare it for use by a particular customer. Coal washing is the process of separating coals of various sizes, densities and shapes by allowing them to settle in a fluid.
Afterward, the overburden is regraded to the desired contour and then restored to a productive use through the planting of trees, the growing of crops or some other method. This is called land reclamation.
Specialized machinery such as power shovels and draglines are used in surface mining. This equipment is so large that it might function in the same area for years of even decades. Even the trucks used at these mines are too massive for public roads. Hauling 150 tons of coal or more at a time, the trucks use private roadways.
The power shovels work from the floor of a pit, moving slowly on steel treads along the coal they have exposed. Their dippers take huge bites of the earth along one wall of the pit. The machines then pivot and drop their load on the other side in a series of ridges.
In contrast, the draglines work from the surface of the edge of a pit, above the coal, dropping giant, open-end buckets from the ends of their long booms and, then, fill their buckets by dragging them back towards the machines by lines or cables. That is how they got the name draglines.
The big shovels and draglines do not remove the coal, though. They uncover the seams so coal can be picked up by smaller shovels or front-end loaders.
While surface mining has proven to be fast and efficient, with a high percent of coal recovery, the correction of its aftereffects long has been a concern of both government and the coal industry.