school energy&environment

Coal extraction

The set of operations leading to the identification of coalfields and to their assessment is called mining prospecting. As with oil, an analysis of airplane pictures is conducted to identify geologically interesting areas, then soil samples are studied to obtain more detailed information. After having identified the coalfield and its position and shape, the mine building site is started. If the coal is found a at depth not exceeding 30 m, it is extracted in open pit mines, where the field is made accessible after the elimination of the top layers of rocks and soil by means of explosive charges. If the coal is deeper than 30m, the field is accessed by digging underground mines including at least two galleries to let miners and machines reach the coal. In open pit mines the coal is extracted after having removed the rocks above. In the case of underground mines, the extraction is carried out in two ways: the method of “abandoned pillars” and that of “long faces”. The first method consists in extracting coal while leaving “pillars” to support the roof of the mine. The second method envisages the use of a series of supporting structures called “scaffoldings” which can be easily removed and support the roof in the mining area. While the extraction goes on, the scaffoldings are removed and the roof collapses. The two methods differ in terms of their impact on the ground. The removal of coal, if the mine is not supported, leads to a more or less gradual sinking of the ground above it. In environmentally valuable areas, the method of abandoned pillars is preferred. In other areas the preference goes to the long faces, which, thanks to a more intensive exploitation of the mine, lead to a coal output, which is 4/5 times greater. After being extracted, coal is processed in order to make it suitable for commercial needs. In particular, it is milled and riddled to obtain the size required on the market and is washed to remove impurities.