Open-pit mining

Open-pit, open-cast or open cut mining is a surface mining technique of extracting rock or minerals from the earth by their removal from an open pit or borrow.

Twincreeksblast
Rock blasting at the large open-pit Twin Creeks gold mine in Nevada, United States. Note the size of the excavators for scale (foreground, left), and that the bottom of the mine is not visible.
Ende Gelände 2017 CHB 23 (cropped)
The giant bucket-wheel excavator in the German Rhineland coal mines are among the world's biggest land vehicles.

Description

This form of mining differs from extractive methods that require tunnelling into the earth, such as long wall mining. Open-pit mines are used when deposits of commercially useful ore or rocks 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 tunnelling (as would be the case for sand, cinder, and gravel).Minerals that have been found underground but are difficult to retrieve due to hard rock, can be reached using a form of underground mining[1].

Open-pit mines that produce building materials and dimension stone are commonly referred to as "quarries."

Open-pit mines are typically enlarged until either the mineral resource is exhausted, or an increasing ratio of overburden to ore makes further mining uneconomic. 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, if the mine is situated in a climate of considerable precipitation or if any layers of the pit forming the mine border productive aquifers.

Open-pit mining is to be considered one of the most dangerous sectors in the industrial world. Open-pit mining also causes significant effects to the ecological land. This type of mining causes changes to vegetation, soil, and bedrock, which ultimately contribute to changes in surface hydrology, groundwater levels, and flow paths[2]

Extraction

Sunrise Dam open pit
Note the angled and stepped sides of the Sunrise Dam Gold Mine, Australia.
Heavy machinery extracting lignite from Garzweiler surface mine in Germany during 2008.

Open-cast mines are dug on benches, which describe vertical levels of the hole. These benches are usually on four-to-sixty-metre (13 to 197 ft) intervals, depending on the size of the machinery that is being used. In contrast, many quarries do not use benches, as they are usually shallow.[3]

Most walls of the pit are generally mined on an angle less than vertical, to prevent and minimize damage and danger from rock falls. This depends on how weathered the rocks are (eroded rocks), and the type of rock, and also how many structural weaknesses occur within the rocks, such as a faults, 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.[4]

A haul road is usually situated at the side of the pit, forming a ramp up which trucks can drive, carrying ore and waste rock.[5]

Waste

Waste rock is hauled to a waste dump. Waste dumps can be piled at the surface of the active pit, or in previously mined pits.

Leftover waste from processing the ore is called tailings, and is generally in the form of a slurry. This is pumped to a tailings dam or settling pond, where the water is reused or evaporated. Tailings dams can 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. If proper environmental protections are not in place, this toxicity can harm the surrounding environment.[6]

Untopping

A form of open-cast quarrying may be carried out as 'untopping'. This is done where a previous underground mine is becoming uneconomic or worked-out, but still leaves valuable rock in place, often as a result of pillar and stall working. Untopping removes the overburden from above this, opens up the mine from above, and then allows the previously 'trapped' minerals to be won.

Untopping was a feature of Welsh slate workings in the 1930s and 2000s, where Martyn Williams-Ellis, manager at Llechwedd found that earlier Victorian workings could be kept profitable with the newly mechanised techniques for bulk excavation to extract their pillars, and more recently across a number of worked-out mines.[7]

Rehabilitation

New Antelope Rochelle Coal Mine
Opencut coal mine loadout station and reclaimed land at the North Antelope Rochelle coal mine in Wyoming, United States.

After mining finishes, the mine area may undergo land 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, a phenomenon known as acid mine drainage.[8] 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.[9] 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 deep groundwater levels.[10] Instead of returning the land to its former natural state, it may also be reused, converting it into recreational parks or even residential/mixed communities.[11]

An open-pit sulfur mine at Tarnobrzeg, Poland undergoing land rehabilitation
An open-pit sulfur mine at Tarnobrzeg, Poland undergoing land rehabilitation

Typical open cut grades

Gold is generally extracted in open-pit mines at 1 to 5 ppm (parts per million) but in certain cases, 0.75 ppm gold is economical. This was achieved by bulk heap leaching at the Peak Hill mine in western New South Wales, near Dubbo, Australia.[12]

Nickel, generally as laterite, is extracted via open-pit down to 0.2%. Copper can be extracted at grades as low as 0.11% to 0.2%.

Materials typically extracted from open-pit mines include:

See also

References

  1. ^ Read "Evolutionary and Revolutionary Technologies for Mining" at NAP.edu.
  2. ^ Chen, Jianping; Li, Ke; Chang, Kuo-Jen; Sofia, Giulia; Tarolli, Paolo (2015-10-01). "Open-pit mining geomorphic feature characterisation". International Journal of Applied Earth Observation and Geoinformation. 42: 76–86. doi:10.1016/j.jag.2015.05.001. ISSN 0303-2434.
  3. ^ Mark A. Prelas; Galina Popovici; Louis K. Bigelow (23 September 1997). Handbook of Industrial Diamonds and Diamond Films. CRC Press. pp. 496–. ISBN 978-0-8247-9994-6.
  4. ^ "Open Pit Surface Mine" Mine Engineer Community (2000) accessed 19 December 2011
  5. ^ G.E. Blight; A.B. Fourie; G.R. Wardle (1 January 1999). Geotechnics for Developing Africa: Proceedings of the 12th regional conference for Africa on soil mechanics and geotechnical engineering, Durban, South Africa, 25–27 October 1999. CRC Press. pp. 49–. ISBN 978-90-5809-082-9.
  6. ^ "Mining Waste" European Commission Environment (17 November 2011) accessed 19 December 2011
  7. ^ "Quarry Proprietors - The Welsh and English Entrepreneurs". Slatesite.
  8. ^ Walter Geller; Helmut Klapper; Wim Salomons (6 December 2012). Acidic Mining Lakes: Acid Mine Drainage, Limnology and Reclamation. Springer Science & Business Media. ISBN 978-3-642-71954-7.
  9. ^ Vasudevan Rajaram; Subijoy Dutta; Krishna Parameswaran (30 June 2005). Sustainable Mining Practices: A Global Perspective. CRC Press. pp. 207–. ISBN 978-1-4398-3423-7.
  10. ^ "MINE REHABILITATION" Department of Mines and Petroleum (October 2006) accessed 19 December 2011
  11. ^ "Mines2Cities" The Mines2Cities Project (April 2016) accessed 19 April 2016
  12. ^ "Peak Hill Gold Mine" Major metallic mines, deposits & projects (2010) accessed 19 December 2011

External links

Amber Coast

The Amber Coast is the name given to a coastal strip of the Baltic Sea in the northwest of Kaliningrad (Russia, Kaliningrad Oblast, Sambia Peninsula, formerly northern East Prussia in Germany). In this area amber (Baltic amber) has been excavated since the mid-19th century and up to today in open-pit mining. Two deposits – Palmnikenskoe and Primorskoe, containing 80% of world amber reserves, were found near Yantarny on the Western coast of the Sambia Peninsula in 1948-1951’s.

Berbești Coal Mine

Berbești Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Berbești, Vâlcea County. The legal entity managing the Berbești mine is the National Company of Lignite Oltenia which was set up in 1997.The exploitation has four open pits Seciuri, Olteț, Berbești-Vest, Panga that produced 2.5 million tonnes of lignite in 2008. The mine has around 1,800 workers and is endowed with 13 bucket-wheel excavators, seven spreaders, one mixed machine and five deposits spreader. The total proven recoverable reserves of the mine amount to 67 million tonnes of lignite.

Bustuchin Coal Mine

Bustuchin Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Bustuchin, Gorj County with estimated coal reserves of 17.7 million tonnes. The legal entity managing the Bustuchin mine is the CE Craiova which was set up in 2003.

Ceptura Coal Mine

Ceptura Coal Mine is an open-pit mining exploitation it is considered one of the largest in the country of Romania located in Ceptura, Prahova County with estimated coal reserves of 18 million tonnes. The legal entity managing the Câmpulung mine is the Ploieşti National Coal Company which was set up in 1957.

Comănești Coal Mine

Comănești Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Comănești, Bacău County with estimated coal reserves of 8.4 million tonnes. The legal entity managing the Comănești mine is the Ploieşti National Coal Company which was set up in 1957.

Câmpulung Coal Mine

Câmpulung Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Câmpulung, Argeş County with estimated coal reserves of 16.5 million tonnes. The legal entity managing the Câmpulung mine is the Ploieşti National Coal Company which was set up in 1957.

Căpeni Coal Mine

Căpeni Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Baraolt, Covasna County with estimated coal reserves of 8.4 million tonnes. The legal entity managing the Căpeni mine is the Ploieşti National Coal Company which was set up in 1957.

Filipești Coal Mine

Filipești Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Filipeștii de Pădure, Prahova County with estimated coal reserves of 8.4 million tonnes. The legal entity managing the Filipești mine is the Ploieşti National Coal Company which was set up in 1957.

Husnicioara Coal Mine

Husnicioara Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Husnicioara, Mehedinţi County. The legal entity managing the Husnicioara mine is the National Company of Lignite Oltenia which was set up in 1997.The exploitation has two open pits Husnicioara - Vest, Zegujani that produced 3.1 million tonnes of lignite in 2008. The mine has around 700 workers and is endowed with five bucket-wheel excavators, three spreaders, one mixed machine and four deposits spreader. The total proven recoverable reserves of the mine amount to 67 million tonnes of lignite.

Jilț Coal Mine

Jilț Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Mătăsari, Gorj County. The legal entity managing the Jilț mine is the National Company of Lignite Oltenia which was set up in 1997.The exploitation has two open pits Jilț Sud, Jilț Nord that produced 7 million tonnes of lignite in 2008. The mine has around 2,400 workers and is endowed with 15 bucket-wheel excavators, seven spreaders, two mixed machines and one deposit spreader. The total proven recoverable reserves of the mine amount to 285.8 million tonnes of lignite.

Motru Coal Mine

Motru Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Motru, Gorj County. The legal entity managing the Motru mine is the National Company of Lignite Oltenia which was set up in 1997.The exploitation has two open pits Lupoaia, Roșiuța that produced 6.6 million tonnes of lignite in 2008. The mine has around 2,300 workers and is endowed with 13 bucket-wheel excavators, seven spreaders, two mixed machines and two deposits spreader. The total proven recoverable reserves of the mine amount to 108 million tonnes of lignite.

Pralea Coal Mine

Pralea Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Căiuți, Bacău County with estimated coal reserves of 5.5 million tonnes. The legal entity managing the Pralea mine is the Ploieşti National Coal Company which was set up in 1957.

Prigoria Coal Mine

Prigoria Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Prigoria, Gorj County. The legal entity managing the Prigoria mine is the CEN Craiova which was set up in 2004.

Rovinari Coal Mine

Rovinari Coal Mine is an open-pit mining exploitation, the largest in Romania, located in Rovinari, Gorj County. The legal entity managing the Rovinari mine is the National Company of Lignite Oltenia which was set up in 1997.The exploitation has four open pits Tismana I, Tismana II, Gârla - Rovinari Est and Pinoasa that produced 8 million tonnes of lignite in 2008. The mine has around 3,000 workers and is endowed with 23 bucket-wheel excavators, 14 spreaders, three mixed machines and one deposits spreader. The total proven recoverable reserves of the mine amount to 610 million tonnes of lignite.

Roșia – Peșteana Coal Mine

Roșia – Peșteana Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Rovinari, Gorj County. The legal entity managing the Roșia - Peșteana mine is the National Company of Lignite Oltenia which was set up in 1997.The exploitation has three open pits Roșia, Peșteana Nord, Peșteana Sud-Urdar that produced 7.2 million tonnes of lignite in 2008. The mine has around 2,900 workers and is endowed with 19 bucket-wheel excavators, 12 spreaders, one mixed machine and three deposits spreader. The total proven recoverable reserves of the mine amount to 112 million tonnes of lignite.

Surface mining

Surface mining, including strip mining, open-pit mining and mountaintop removal mining, is a broad category of mining in which soil and rock overlying the mineral deposit (the overburden) are removed, in contrast to underground mining, in which the overlying rock is left in place, and the mineral is removed through shafts or tunnels.

Surface mining began in the mid-sixteenth century and is practiced throughout the world, although the majority of surface coal mining occurs in North America. It gained popularity throughout the 20th century, and surface mines now produce most of the coal mined in the United States.In most forms of surface mining, heavy equipment, such as earthmovers, first remove the overburden. Next, huge machines, such as dragline excavators or Bucket wheel excavators, extract the mineral.

Sărmășag Coal Mine

Sărmășag Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Sărmășag, Sălaj County with estimated coal reserves of 10.6 million tonnes. The legal entity managing the Sărmășag mine is the Ploieşti National Coal Company which was set up in 1957.

Voivozi Coal Mine

Voivozi Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Popeşti, Bihor County with estimated coal reserves of 3.9 million tonnes. The legal entity managing the Voivozi mine is the Ploieşti National Coal Company which was set up in 1957.

Șotânga Coal Mine

Șotânga Coal Mine is an open-pit mining exploitation, one of the largest in Romania located in Șotânga, Dâmboviţa County with estimated coal reserves of 5.01 million tonnes. The legal entity managing the Șotânga mine is the Ploieşti National Coal Company which was set up in 1957.

Mining techniques
Surface
Sub-surface

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