Open-pit mining or quarrying
Open-pit mining is the most common mining technique, when it comes to new mines. Through this technique the soil is completely excavated, and the the seperation of the minerals and the remaining soil happens completly above ground.
The first step in this extraction process is the mining of the ore. First the vegetation will be removed and the ground will be dug down to the ground layers, which contain the valuable ores. This results in the disrupting of valuable ecosystems, and the fertile upper layer of soil (formed by an age-long sequence of natural processes and vital micro-organisms) is removed and destroyed.
The excavation technique consists of systematically inflate of the bottom, which results in the detachment of the bottom material. The ground layers are sequentially dug so that a cup-shaped hule (a pit) is produced, hich has a staircase structure on the walls. These ensure that the transport of the ground material is still possible. The depth of the craters will vary, depending on the minerals contained in the ground, and the price of these minerals. How deeply they dug, the more expensive it gets. So it's possbile that a part of the minerals remain unexplored. The craters can be deeper than a kilometer
(eg. Yanacocha mine in Peru). To retrieve the remaining minerals, without making the pit deeper, this technique is sometimes combined with underground mining methods.
The excavated soil material consists of a mineral-free and mineral part. The mineral-free soil material is discharged into heaps of wat rock, and the mineral content of soil material is transported to crushers.
Here begins the second phase, the pulverisation process. Here, the ores are crushed, split and scraped into smaller particles. This requires a huge amount of energy, which makes this step the most expensive one in the extraction process. The crushed ores are sedimented in large heaps, which can reach a height of more than 100m.
To seperate the mineral atoms (eg. gold atoms, copper atoms, ...) from the residual fraction, these heaps are sprayed with a chemical solution, causing acidic drainage. This method is called 'heap leaching', and can use - among others - cyanide in the case of gold, or sulfuric acid for copper or nickel (sulfuric acid heap leaching). The mineral atoms bind with these chemicals and form complexes that are captured. Afterwards, the mineral will be extracted from this complex. This technique requires a huge amount of water (per kilogram mined gold, they use one million liters of water) and chemicals. Many of these chemicals end up in the environment through accidental spills or accidents. Although it's technologically possible to limit the amount of cyanide in waste watr to < 0,0001%, there are still large scale mining projects where there's 10% of cyanide in waste water, by poor recovery and/or degradation. The cyanide decomposes fast in the nature when it's in contact with oxygen, but under certain forms it can cover distances of 60 kilometers in rivers. Swimming in water with a concentration of a milligram per liter cyanide is lethal to mammals, for the human being is 0,3mg per decilter a lethal dose. Consequently, during the seperation process they must take sufficient precautions, and used water and the treated rocks must be treated in the appropriate manner. This doesn't always happen, that's proved on January 30, 2000 in Romania. Through a crack dam was 50-100 tons of cyanide released into the Lapus, a river of the Tisza Basin. As a result of this accident, more than 120 fish died in Hungary. When cyanide end up in the environment, the heavy metals bound to the cyanide will be the biggest problem on a long-term basis. Cyanide breaks down relatively quickly, but heavy metals remain forever in the environment.
The use of cyanide:
In the gold mining industry the must taka an additional and significant risk into account. Current technology uses cyanide to seperate the gold from the worthless rocks. Cyanide is a highly toxic substance, the lethal dose for humans is 50-200 mg.
This technique is used when there are low-grade ores in the soil. The majority of the reclaimed ground material is therefore waste. The more valuable the mineral, the more waste. The current mining of gold is more than 99,9% of the mined material waste. The San Martin mine in Honduras for example, has an average yield of one gram of gold per ton of rock eight (0,000000001%).
Once the mine is depleted, the open-pit site is backfilled with waste rock. Very often the tailings contain cyanide residues. These are simply covered with clay and new soil, supposedly to prevent seepage. It allows the spread of toxic substances, only with a slight delay.
Through promotional videos the mining companies let you know that they restore the open-pit mine. With technologies such as phytoremediation, mining sites will be covered with plants, making it seem as if the earth recovers quickly. To be more effective, such an approach should be included in the 'business plan'. The upper fertile layer of the soil (which is removed at the beginning) could then be saved, to use again later in the rehabilitation.
Studies spreken van een hersteltijd die langer kan zijn dan duizenden jaren. De verontreiniging blijft dus gewoon bestaan of via doorsijpeling zelfs doorgaan. De uitgeputte mijnbouwsites blijven dan ook lang nadat de mijnen zijn uitgeput met hekken omringd om te voorkomen dat dieren eten van de gecontamineerde planten. Local plants and microorganisms should be collected and cultivated for subsequent re-planting on the site. Local communities could be involved to restore the local ecosystem and maintaining it sustainably. However, in reality phytoremediation is rarely so thorough, and is it just a quick solution afterwards. Mining sites aren't often restored, and left highly contaminated.
Strip mining and quarrying
These are two similar techniques in open-pit mining, but these are generally less used. Only for coal and ornamental rocks are these techniques relevant.
Sublevel caving is a technique that is used when surface mining is no longer economically feasible and the ore sediment is steep and deep. The ores are extracted from various intermediate levels from drilling tunnels with explosives. The rock and ore mass is discharged from below.
Example: Goldex mine in Quebec, Canada
Block caving is an underground mining technique that is sometimes used under open-pit mines. It's a technique in which there are several funnel-shaped holes drilled under a solid block to be mined ore. Due to the force of gravity, the upper ores are pulverised and flow to the drain tunnels. Then they collect the ore and drain it to the surface. The process can cause a visible subsidence above ground.
Example: North Park copper and gold mine in New South Wales, Australia
Room and pillar mining
"Room and pillar mining" of "bord and pillar mining" is a mining engineering suitable for horizontal ore sediments. Ore is a mined in a horizontal space where pillars remain untouched, they ensure the stability of the area. There are also several large pillars that create different spaces, therefore they are independent of each other in case of collapse.
Example: Plutonic gold mine in Australia
Blasthole stoping is a technique used when there are regulary steep contoured ore sediments and when the rock is very strong. Sublevel stoping and vertical retreat (VCR) mining are two variations on this technique.
Example Blasthole stoping: Fruta Del Norte gold and silver mine in Ecuador.
This technique is used for narrow steep ore sediments. Ore is broken up from below towards the top in horizontal layers. They let most part of the crushed ore on site, until the entire layer is broken. Then the ore is carried off from te bottom up.
Example: the Sleeping Giant gold mine in Quebec, Canada.
Cut and fill stoping
Cut and fill stoping is a technique that can be applied in many situations. Ore is mined in horizontal or slightly inclined layers, and then the open space is re-filled with useless waste-rock or tailings sans. This layer is sometimes still refilled, stabilised with concrete.
Example: Crixas gold mine in Goias, Brazil.