Open-pit mining
From Wikipedia, the free encyclopedia
- This is an article about a specific type of surface mining. For a more general treatment, see that article.
Open-pit mining, or opencast mining, refers to a method of extracting rock or minerals from the earth by their removal from an open pit or borrow.
The term is used to differentiate this form of mining from extractive methods that require tunneling into the earth. Open-pit mines are used when deposits of commercially useful minerals or rock are found near the surface; that is, where the overburden (surface material covering the valuable deposit) is relatively thin or the material of interest is structurally unsuitable for tunneling (as would be the case for sand, cinder, and gravel). For minerals that occur deep below the surface—where the overburden is thick or the mineral occurs as veins in hard rock— underground mining methods are used to extract the valued material.
Open-pit mines that produce building materials are commonly referred to as quarries. People in some English-speaking countries are unlikely to make a distinction between an open-pit mine and other types of open-cast mines, such as quarries, borrows, placers, and strip mines.
Open-pit mines are typically enlarged head until the mineral resource is exhausted. When this occurs, the exhausted mines are sometimes converted to landfills for disposal of solid wastes. However, some form of water control is usually required to keep the mine pit from becoming a lake.
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[edit] Extraction
Open Cut mines are dug on benches, which describe horizontal levels of the mine. These benches are usually on 3 metre or 6 metre levels, depending on the size of the machinery being used. Quarries rarely use benches, as the majority of quarries are dug using (relatively) small-scale machinery.
Most walls of the pit are generally dug on an angle less than vertical, to prevent and minimise damage and danger from rock falls. This depends on how weathered the rocks are, and the type of rock, and also how many structural weaknesses occur within the rocks, such as a fault, shears, joints or foliations.
The walls are stepped. The inclined section of the wall is known as the batter, and the flat part of the step is known as the bench or berm. The steps in the walls help prevent rock falls continuing down the entire face of the wall. In some instances additional ground support is required and rock bolts, cable bolts and shotcrete are used. De-watering bores may be used to relieve water pressure by drilling horizontally into the wall, which is often enough to cause failures in the wall by itself.
A haul road is situated at the side of the pit, forming a ramp up which trucks can drive, carrying ore and waste rock.
Waste rock is piled up at surface near the edge of the open cut. This is known as the waste dump. The waste dump is also tiered and stepped, to minimise erosion.
Ore which has been processed is known as tailings, and is generally a slurry. This is pumped to a tailings dam or settling pond, where the water evaporates. Tailings dams can often be toxic due to the presence of unextracted sulfide minerals, some forms of toxic minerals in the gangue, and often cyanide which is used to treat gold ore via the cyanide leach process.
[edit] Rehabilitation
After mining finishes, the mine area must undergo rehabilitation. Waste dumps are contoured to flatten them out, to further stabilise them. If the ore contains sulfides it is usually covered with a layer of clay to prevent access of rain and oxygen from the air, which can oxidise the sulfides to produce sulfuric acid. This is then generally covered with soil, and vegetation is planted to help consolidate the material. Eventually this layer will erode, but it is generally hoped that the rate of leaching or acid will be slowed by the cover such that the environment can handle the load of acid and associated heavy metals. There are no long term studies on the success of these covers due to the relatively short time in which large scale open pit mining has existed. It may take hundreds to thousands of years for some waste dumps to become "acid neutral" and stop leaching to the environment. The dumps are usually fenced off to prevent livestock denuding them of vegetation. The open pit is then surrounded with a fence, to prevent access, and it generally eventually fills up with ground water. In arid areas it may not fill due to the deep groundwater levels.
[edit] Typical open cut grades
Gold is generally extracted in open cut mines at 1 to 2 ppm (grams per ton) but in certain cases, 0.75ppm gold is economic. This was achieved by bulk heap leaching at Alkane Minerals Ltd. Peak Hill mine in western New South Wales, near Dubbo.
Nickel, generally as laterite, is extracted via open cut down to 0.2%. Copper is extracted at grades as low as 0.15% to 0.2%, generally in massive open cut mines in Chile, where the size of the resources and favorable metallurgy allows economies of scale.
Materials typically extracted from open-pit mines include:
- Clay
- Coal
- Coquina
- Gravel and stone (stone refers to bedrock, while gravel is unconsolidated material, as found in glacial or fluvial deposits)
- Granite
- Gritstone
- Gypsum
- Limestone
- Marble
- Metal ores, such as copper, iron, gold, and molybdenum
- Oil shale
- Sand
- Sandstone
- Slate
[edit] Open-pit mines
This list includes only those large open-pit mines for which an article exists in Wikipedia.
- Adams Mine – controversial abandoned mine in Kirkland Lake, Ontario.
- Berkeley Pit - former copper mine in Butte, Montana, United States; now a toxic lake.
- Chuquicamata – Chilean copper mine; the largest open-pit copper mine in the world.
- Colomac Mine – gold mine in Northwest Territories, Canada.
- Cerrejón – coal mine in Guajira Department, Colombia.
- Diavik Diamond Mine – diamond mine in Northwest Territories, Canada.
- Ekati Diamond Mine – diamond mine in Northwest Territories, Canada.
- El Chino Mine – copper mine in Grant County, New Mexico, United States
- Hull-Rust-Mahoning Mine – largest open pit iron mine in the world near Hibbing, Minnesota.
- Kennecott Copper Mine – largest excavation in the world; copper mine in Salt Lake County, Utah, United States;
- Lavender Pit – copper mine in Cochise County, Arizona, United States.
- Mirny Mine – diamond mine in Mirny, Eastern Siberia
- Pascua Lama – gold and silver mine in Atacama, Chile (in project)
- Penrhyn Quarry – slate quarry in Wales.
- Pine Point Mine – lead and zinc mine in Northwest Territories, Canada.
- Super Pit – gold mine near Kalgoorlie, Western Australia.
- The Big Hole, a diamond mine in Kimberley, South Africa that reaches a depth of more than 1,000 m (3,300 ft).
- Udachnaya pipe – diamond mine in Yakutia, Russia.
[edit] See also
- List of minerals
- List of stone
- Definition of "ore"
- Ore genesis
- Mineral resource classification
- Bagger 288, an enormous piece of pit-mining equipment
| Mining techniques | |
|---|---|
| Surface mining: | Open-pit mining, Quarrying, Strip mining, Placer mining, Mountaintop removal |
| Sub-surface mining: | Drift mining, Slope mining, Shaft mining, Hard rock mining, Borehole mining |
