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Depending on the depth of the coal layers (seams), extraction is achieved by means of open-cast (down to a depth 500m) or underground mines. |
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| The Forzando Mines (Witbank - South Africa). |
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An opencast operation has the appearance of an enormous hole, organised a bit like a stadium with terraces, along which machines dig into the seams. But what a stadium! The biggest opencast mines are several kilometres in length and hundreds of meters in depth. In underground mines, the coal is extracted by means of vertical or inclined shafts. At each level where coal is present, these shafts are linked together by a vast network of galleries (10 to 20 m² in cross-section). These networks can be made up of several dozens of kilometres of galleries. The coal is extracted by enormous machines (the coalcutters). It is transported to the surface where it is separated from the sands and clays by flotation (the coal floats and the other mineral materials, known as “steriles”, sink to the bottom).
Extraction by opencast mining is more productive and less expensive than underground mining. As a result it is more profitable. Working conditions are also a significantly less dangerous. Unfortunately, opencast mining is less environmentally friendly; the countryside is disfigured and the surface activity tends to pollute the atmosphere within the locality.
The majority of coalmines in the world are underground. |
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| Coal mining in the Forzando mines (Witbank - South Africa). |
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Methods of extraction of coal:
- Opencast mining: the soil layers located above the first coal layer (the overburden) are removed. Then extraction can begin. When the hole is sufficiently large, digging continues as far as the subsequent coal layer, where the coal is extracted in the same way, simultaneously with continued extraction of the first layer, and so on. Each of the coal layers is called a “discovery”. The mine gradually becomes a giant amphitheatre, the terraces of which are made up of the layers of coal being extracted. It is more a terracing activity than mining. Giant excavators dig out the coal. Their buckets can contain up to 300 tons of rock. The production from an opencast or strip mine starts 2 to 5 years after the initial work. The technique is less expensive, more profitable and less dangerous than extraction by underground mining. Nevertheless it is little used in Europe, where the coal is generally too deeply buried.
- Underground mines: to reach the coal-bearing rocks, vertical shafts are dug, into which lifts and other systems for links with the surface are installed. At the levels of the coal-bearing seams (layers),
digging takes place horizontally, following each seam for as long as possible. Several techniques are used:
In each seam exploited, regularly spaced large pillars of coal are left in place to support the roof: this is called the “room and pillar method”;
Or two parallel tunnels are dug and a machine (a giant rasp or undercutter) goes backwards and forwards between these tunnels, extracting coal at each step: it is the “long coal face” or “long wall” method, which allows recovery of a little more coal than the preceding method. As the coalface moves forward, the roof is allowed to collapse behind (miners talk about “thundering”). The main disadvantage is that the effects of these roof collapses are sometimes felt on the surface, and the buildings and roadways located above the mine suffer the consequences of subsidence: they crack and sometimes even disappear into a hole! To solve this problem, the deliberate collapsing of the roof can be replaced by a filling procedure: the «sterile» rocks replace the extracted coal. But that is more expensive;
In mountainous regions, the galleries can be dug horizontally, directly into the side of the hill, whereas, on the plains, infrastructures are necessary to bring the coal-bearing rocks to the surface.
Pumping and drainage equipment is essential although if the mines are in the mountains the evacuation of water usually happens naturally.
Conveyors or trains of large wagons transport the extracted coal to the vertical shafts. Finally the coal is taken up to the surface by means of a bucket conveyor driven by very powerful electric motors. |
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