Mine (resource extraction)
A mine (noun) is a place where natural resources are extracted from the ground. To mine (verb) is to engage in processes of extraction from those places, and mining refers to the actual processes of extraction. Mine and mining are also used as adjectives in a large number of instances (E.g., mine shaft, mine portal, mine worker, mining equipment, mining technology).
Mining is a very ancient human activity. Archeological expeditions show mining predates the invention of writing.
Materials extracted from the ground in mines have included:
- metallic nuggets, and metallic ore;
- fossil fuels, like coal, tar sands, and oil-shale;
- salt, and other chemicals, like potash;
- gems, like diamonds;
- construction material, like rock, sand and gravel;
- fossil water.
Various techniques are used to extract valuable materials from the Earth.
Open pit mining
When the valuable material is at or relatively near the surface it is either removed from the ground, or the over-burden is removed first.
Open-pit mines used for extracting rocks, sand or gravel are often called "quarries", or "borrow pits". Although quarries are open pit mines, their designation is used when the material being extracted does not require additional processing to become final product. Thus limestone or marble is quarried, gold is mined.
Historically, mine owners made no effort to fill in the pits left behind. Because they are often rocky, and contain no topsoil, it is rare for the vegetation to grow back in the pits, or on the piles of overburden. There has been a significant change in this practice in the last century and although the open pits are not refilled there is a significant amount of remediation that is done to improve the land surrounding the pit.
Exposure to the weather can leach toxic material into the local groundwater, from the exposed surface of the pit, and on the surface of the piles of overburden.
Another large group of mining techniques requires that the tunnels be used to access the area of mineralization. Typically these deposits occur at a depth that makes them impossible to mine as open pits.
Evolving technology has enable miners to go more deeply underground. Cave-ins, and the build-up of toxic or explosive gases and aerosols has remained a constant problem. The development of steam power allowed miners to pump out ground-water to get at minerals below the natural water-table.
"Placer mining" is the name given to extracting valuable material that has already been removed from the ground, and gone through some natural sorting, through erosion caused by the natural flow of water. The sandbanks lining the courses of streams, downstream from outcroppings of metals like gold and copper often contain minute nuggets of relatively pure metal. The sand-sized grains of metal are more dense than ordinary sand, so the grains are concentrated in layers, when the river currents build up the sandbank.
It can be easier to extract valuable material from these sandbanks than it is to extract the material through digging in the ground. Placer miners can use the waterpower of the flowing stream. And they can use the stream's water for processing the sand.
Panning for gold is a form of placer mining with a low environmental impact. But in larger scale placer mining the miners build sluices, divert the course of the stream, and dig through the sandbank placing it in the water flowing in the sluice, for sorting. A side effect of this kind of placer mining can be a significant build up of silt in the downstream water, making the stream uninhabitable for native species that need clear water.
The techniques through which metal ores are processed typically leave toxic detritus, called "tailings". The toxic silt left over from processing oil sands are also called "tailings". Some material is extracted by mixing toxic chemicals, like cyanides, mercury or arsenic with the ore; this may produce toxic water runoff.
Historically mine owners made no effort, or made ineffective efforts, to prevent toxic runoff of effluents from processing the material. In recent decades mine owners in some jurisdictions have been forced build tailing ponds to contain the toxic effluent from processing. There have been high-profile breaches of the containment dikes of tailing ponds.