Surface and Underground: Coal Mining
Coal can be extracted from the earth by two means: surface mining and underground mining. The characteristics of the coal deposit in question dictate the technique that must be used. If a seam falls near the surface, opencast or open-pit mining procedures are employed. Explosives are first employed to break through the "overburden", meaning the upper layers of rock and soil. The coal is mined by drilling the seam in strips, and the extracted material is transported via conveyor belt.
If the seam is too deep beneath the surface, the company will turn to the pair of underground mining techniques. The first of these, longwall mining, makes use of a highly specialized machine to extract the coal from one whole section of the seam by loosening it from the rock and allowing it to fall on the conveyor belt which will move it to the surface. Using the second technique, room and pillar mining, the coal seam is cut into a series of rooms, making sure to shape roof-supporting pillars to avoid collapse. On a related note, once the seam is exhausted, the process known as retreat mining involves the extraction of coal from those pillars. As you might imagine, this process must be undertaken with extreme care for the safety of both workers and equipment.
Gold: From Placer Mining to Sodium Cyanide
In placer mining, a stream bed is essentially sifted for materials such as precious metals and gemstones. Early modern placer mining operations used open-pit and hydraulic mining and sluicing techniques. Panning, a more primitive form of this type of mining, was used in the various gold rushes in the 19th century in the United States. Today, cyanide extraction is the method of choice for many gold mining operations, as the technology has taken many strides forward in the past several years.
More on Surface Mining
Surface mining techniques have been widely applied to the acquisition of many types of minerals and other resources. Included in this list are the aforementioned open-pit mining, mountaintop removal mining and strip mining. With strip mining, the overburden is stripped away by massive machines such as the bucket-wheel excavator. An open pit, also called a borrow pit, is used to extract minerals and rock from the earth in the open air. Mountaintop removal mining (MTR) is accomplished with explosives that simply blast away the overburden. This waste is displaced by fleets of large trucks to nearby, convenient locations such as valleys.
Highwall mining involves the penetration of a coal seam by a continuous miner, an advanced piece of machinery that sumps and shears the seam. This mining technique is another example of how technological advancements in the industry enhance efficiency and protect miners’ lives. Highwall mining evolved from the process of augering, though the former recovers far more coal than the latter.