Extracting Coal in Underground or Open Pit Mines10/12/2010
Depending on the depth of the layers, or seams, coal is extracted in open pit or underground mines. Whatever the technique used, extracting this precious ore from the bowels of the earth is a huge undertaking.
To reach the coal, if the deposit is not too deep, wells are drilled sideways so trucks can circulate. If it is very deep down, vertical wells are drilled and a system of elevators and connections to the surface are installed. The well is then drilled horizontally to follow each coal layer or seam as far as possible.
There are several techniques available.
• The room-and-pillar method involves leaving large pillars spaced at regular intervals in each seam to support the ceiling. At each level where coal is present, the wells are linked to a network of huge galleries (10-20m2 sections) that can extend over dozens of kilometers.
• The longwall method involves using a machine called a cutter that looks like a huge plow. This machine slowly pushes and cuts through the coal seam. It recovers the ore as it goes along and lets the ceiling collapse behind it - this is called caving. One disadvantage is that these collapses can sometimes be felt at the surface and buildings or roads located above the mine suffer the consequences; cracking and sometimes even falling into the hole! These problems can be solved with backfilling, where sterile rocks replace the extracted coal, but this is very expensive.
Conveyor belts or train carriages (mine cars) transport the harvested coal to vertical extraction wells. Elevators- driven by powerful electric motors- bring it back up to the surface.
It is then cleaned- sand and mud is removed by flotation, i.e. it is immersed in a liquid made up of water and tiny magnetite particles. The different elements separate naturally- the coal floats and the other ores, called tails, sink to the bottom.
In mountainous regions, galleries can be drilled horizontally into the side of the hill, so infrastructure for bringing the ore back up to the surface is not needed.
Open Pit MiningAn open pit mine is a huge hole that looks a bit like a sports stadium, with terraces along which earth-moving vehicles drill into seams. But what a stadium! The largest open pit mines are several kilometers long and hundreds of meters deep.
First of all, the layer of earth above the first coal seam has to be removed - this can be up to 200m down. All the earth around the base of the seam is removed and then the coal starts to be extracted. When the hole is large enough, the miners dig down to the next coal layer. The coal is extracted as before while extraction of the first layer continues and so on.
Each coal layer is called an excavation. The mine starts to look like a gigantic amphitheatre, where the terraces are made up of coal layers being extracted.
Huge excavators harvest the coal (the biggest of these is 240 meters long, longer than 2 football stadiums, and 96 meters high, higher than a 38 story building!). The buckets on these excavators can contain up to 300 tons of rock.