Open-pit mining

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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. Twincreeksblast.jpg
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.
The giant bucket-wheel excavators in the German Rhineland coal mines are among the world's biggest land vehicles. Ende Gelande 2017 CHB 23 (cropped).jpg
The giant bucket-wheel excavators in the German Rhineland coal mines are among the world's biggest land vehicles.

Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, [1] is a surface mining technique that extracts rock or minerals from the earth.

Contents

Open-pit mines are used when deposits of commercially useful ore or rocks are found near the surface where the overburden is relatively thin. In contrast, deeper mineral deposits can be reached using underground mining. [2]

Kittila Gold Mine in Kittila, Finland is the largest primary gold producer in Europe. Kittila mine.jpg
Kittilä Gold Mine in Kittilä, Finland is the largest primary gold producer in Europe.

Open-pit mining is considered one of the most dangerous sectors in the industrial world.[ not verified in body ] It causes significant effects to miners' health, as well as damage to the ecological land and water. Open-pit mining causes changes to vegetation, soil, and bedrock, which ultimately contributes to changes in surface hydrology, groundwater levels, and flow paths. [5] Additionally, open-pit produces harmful pollutants depending on the type of mineral being mined, and the type of mining process being used.

Extraction

Note the angled and stepped sides of the Sunrise Dam Gold Mine, Australia. Sunrise Dam Gold Mine open pit 11.jpg
Note the angled and stepped sides of the Sunrise Dam Gold Mine, Australia.
A bucket-wheel excavator extracting lignite from Garzweiler surface mine in Germany during 2008

Miners typically drill a series of test holes to locate an underground ore body. From the extracted samples, they can determine the extent of the ore. This helps them determine the likely location of the veins or benches of ore and its commercial value. [6] Open-pit mines that produce building materials and dimension stone are commonly referred to as quarries.

Open-cast mines are dug on benches, which describe vertical levels of the hole. The interval of the benches depends on the deposit being mined, the mineral being mined, and the size of the machinery that is being used. Generally, large mine benches are 12 to 15 metres thick. [7] In contrast, many quarries do not use benches, as they are usually shallow. [7] Mining can be conducted on more than one bench at a time, and access to different benches is done with a system of ramps. The width of each bench is determined by the size of the equipment being used, generally 20–40 metres wide. [6] Downward ramps are created to allow mining on a new level to begin. This new level will become progressively wider to form the new pit bottom. [6]

Most walls of the pit are generally mined on an angle less than vertical. Waste rock is stripped when the pit becomes deeper, therefore this angle is a safety precaution to prevent and minimize damage and danger from rock falls. However, this depends on how weathered and eroded the rocks are, and the type of rocks involved. It also depends on the amount of 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. [8]

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. [9]

After closure

Open-pit mines are typically worked until either the mineral resource is exhausted, or an increasing ratio of overburden to ore makes further mining uneconomic.[ citation needed ] After open-pit mines are closed, they are sometimes converted to landfills for disposal of solid waste. Some form of water control is usually required to keep the mine pit from becoming a lake. Several former open-pit mines have been deliberately converted into artificial lakes, forming areas such as the Lusatian Lake District, the Central German Lake District or the Upper Palatinate Lake District. A particular concern in the formation of these lakes is acid mine drainage.

Impacts

Waste

Open-pit mines create a significant amount of waste. Almost one million tons of ore and waste rock can move from the largest mines per day, and a couple thousand tons moved from small mines per day. [6] There are generally four main operations in a mine that contribute to this load: drilling, blasting, loading, and hauling.

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. [10]

Pollutants

Open-pit mining involves the process of disrupting the ground, which leads to the creation of air pollutants. The main source of air pollutants comes from the transportation of minerals, but there are various other factors including drilling, blasting and the loading and unloading of overburden. [11] These type of pollutants cause significant damage to public health and safety in addition to damaging the air quality. The inhalation of these pollutants can cause issues to the lungs and ultimately increase mortality. [11] Furthermore, the pollutants affect flora and fauna in the areas surrounding open-pit mines.

Open-pit gold mining is one of the highest potential mining threats on the environment as it affects the air and water chemistry. The exposed dust may be toxic or radioactive, making it a health concern for the workers and the surrounding communities. [12]

Deforestation

Open-pit nickel mining has led to environmental degradation and pollution in developing countries such as the Philippines and Indonesia. [13] [14] In 2024, nickel mining and processing was one of the main causes of deforestation in Indonesia. [15] [16] Open-pit cobalt mining has led to deforestation and habitat destruction in the Democratic Republic of Congo. [17]

Hydrology in mining

Open-pit mines operating in an area with heavy groundwater features may eventually face hydrology-related problems. This includes heaving and bursting of the mine floor due to excessive uplift pressure. A groundwater control system must be installed to fix problems caused by hydrology. The formation of an appropriate open-pit slope design, changes throughout the life of a mine. It is based mainly on an ever-increasing understanding of the rock mass conditions, including groundwater and associated pressures that may be acting within the slopes. The reduction of groundwater related to pore pressures is a crucial aspect of determining whether or not a geotechnical engineering design for open-pit slopes is attainable. Groundwater control systems, which include dewatering and depressurization wells, may also have a large impact on local groundwater. Because of this, an optimization-based version of the control system is required to ensure that local and regional hydro-geological impacts are within acceptable ranges. [18]

Open Pit depressurization is the process of removing tensions or pressure from different areas of a mine. Depressurization helps to make open-pit mines more stable and secure. By using an integrated mine slope depressurization program the likelihood that mine plans can be achieved, and at an acceptable level of risk increase drastically. [18] [19] Depressurization allows considerable expansions of a mine, and can extend the life of the mine by 10 to 15 years. One technique used in depressurization is annealing. Annealing is the slow heating and cooling of a metal, alloy or glass. This slow heating and cooling relieves the internal stress of surrounding areas. Annealing will increase a material's workability and durability, which overall increases open-pit mine safety. [20] When groundwater pressures cause problems in open-pit mines, horizontal drains are used to aid in accelerating the slope depressurization process. Which helps to prevent large scale slope failure in the mine. Horizontal drains are used to lower pore pressure by reducing groundwater head, which enhances slope stability. [18]

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 room and pillar mining. 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. [21]

Rehabilitation

A loadout station and reclaimed land at the North Antelope Rochelle open-pit coal mine, in Wyoming, United States New Antelope Rochelle Coal Mine.png
A loadout station and reclaimed land at the North Antelope Rochelle open-pit coal mine, in Wyoming, United States

After mining ends at a location, the mine area may undergo land rehabilitation. Waste dumps are contoured to flatten them out, to further stabilize them. If the ore contains sulfides it is usually covered with a layer of clay to prevent ingress of rain and oxygen from the air, which can oxidize the sulfides to produce sulfuric acid, a phenomenon known as acid mine drainage. [22] 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. [23] 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. [24] In Germany, the world's largest producer of lignite (virtually all of which these days is mined open-pit), the former mines are usually converted to artificial lakes. To mitigate the problem of acid mine drainage mentioned above, flooding is often done with the water of nearby rivers instead of using groundwater alone. In some cases, calcium oxide or other basic chemicals have to be added to the water to neutralize the pH-value.

Tarnobrzeg, zalew Machowski.jpg
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. [25]

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%.

Largest open-pit mines

Open-pit mining is a common method to extract minerals and samples from the Earth. Due to being cost-effective, this method is very popular and is used all over the world. Listed are the world's ten largest open-pit mines in 2015. [26]

See also

Related Research Articles

<span class="mw-page-title-main">Mining</span> Extraction of valuable minerals or other geological materials from the Earth

Mining is the extraction of valuable geological materials and minerals from the surface of the Earth. Mining is required to obtain most materials that cannot be grown through agricultural processes, or feasibly created artificially in a laboratory or factory. Ores recovered by mining include metals, coal, oil shale, gemstones, limestone, chalk, dimension stone, rock salt, potash, gravel, and clay. The ore must be a rock or mineral that contains valuable constituent, can be extracted or mined and sold for profit. Mining in a wider sense includes extraction of any non-renewable resource such as petroleum, natural gas, or even water.

<span class="mw-page-title-main">Quarry</span> A place from which a geological material has been excavated from the ground

A quarry is a type of open-pit mine in which dimension stone, rock, construction aggregate, riprap, sand, gravel, or slate is excavated from the ground. The operation of quarries is regulated in some jurisdictions to manage their safety risks and reduce their environmental impact.

<span class="mw-page-title-main">Placer mining</span> Technique of mining stream bed deposits for minerals

Placer mining is the mining of stream bed deposits for minerals. This may be done by open-pit mining or by various surface excavating equipment or tunneling equipment.

Gold cyanidation is a hydrometallurgical technique for extracting gold from low-grade ore by converting the gold to a water-soluble coordination complex. It is the most commonly used leaching process for gold extraction. Cyanidation is also widely used in the extraction of silver, usually after froth flotation.

<span class="mw-page-title-main">Tailings</span> Materials left over from the separation of valuable minerals from ore

In mining, tailings or tails are the materials left over after the process of separating the valuable fraction from the uneconomic fraction (gangue) of an ore. Tailings are different from overburden, which is the waste rock or other material that overlies an ore or mineral body and is displaced during mining without being processed.

<span class="mw-page-title-main">Mining engineering</span> Engineering discipline

Mining in the engineering discipline is the extraction of minerals from the ground. Mining engineering is associated with many other disciplines, such as mineral processing, exploration, excavation, geology, metallurgy, geotechnical engineering and surveying. A mining engineer may manage any phase of mining operations, from exploration and discovery of the mineral resources, through feasibility study, mine design, development of plans, production and operations to mine closure.

<span class="mw-page-title-main">Acid mine drainage</span> Outflow of acidic water from metal or coal mines

Acid mine drainage, acid and metalliferous drainage (AMD), or acid rock drainage (ARD) is the outflow of acidic water from metal mines and coal mines.

<span class="mw-page-title-main">Industrial wastewater treatment</span> Processes used for treating wastewater that is produced by industries as an undesirable by-product

Industrial wastewater treatment describes the processes used for treating wastewater that is produced by industries as an undesirable by-product. After treatment, the treated industrial wastewater may be reused or released to a sanitary sewer or to a surface water in the environment. Some industrial facilities generate wastewater that can be treated in sewage treatment plants. Most industrial processes, such as petroleum refineries, chemical and petrochemical plants have their own specialized facilities to treat their wastewaters so that the pollutant concentrations in the treated wastewater comply with the regulations regarding disposal of wastewaters into sewers or into rivers, lakes or oceans. This applies to industries that generate wastewater with high concentrations of organic matter, toxic pollutants or nutrients such as ammonia. Some industries install a pre-treatment system to remove some pollutants, and then discharge the partially treated wastewater to the municipal sewer system.

<span class="mw-page-title-main">Gangue</span> Commercially worthless material that surrounds a wanted mineral in ore

In mining, gangue is the commercially worthless material that surrounds, or is closely mixed with, a wanted mineral in an ore deposit. It is thus distinct from overburden, which is the waste rock or materials overlying an ore or mineral body that are displaced during mining without being processed, and from tailings, which is rock already stripped of valuable minerals.

<span class="mw-page-title-main">Surface mining</span> Type of mining in which the soil/rock above mineral deposits is removed

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 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.

<span class="mw-page-title-main">Heap leaching</span> Industrial mining process used to extract precious metals from ore

Heap leaching is an industrial mining process used to extract precious metals, copper, uranium, and other compounds from ore using a series of chemical reactions that absorb specific minerals and re-separate them after their division from other earth materials. Similar to in situ mining, heap leach mining differs in that it places ore on a liner, then adds the chemicals via drip systems to the ore, whereas in situ mining lacks these liners and pulls pregnant solution up to obtain the minerals. Heap leaching is widely used in modern large-scale mining operations as it produces the desired concentrates at a lower cost compared to conventional processing methods such as flotation, agitation, and vat leaching.

<span class="mw-page-title-main">Bingham Canyon Mine</span> Worlds largest open-pit copper mine, located in Utah, United States

The Bingham Canyon Mine, more commonly known as Kennecott Copper Mine among locals, is an open-pit mining operation extracting a large porphyry copper deposit southwest of Salt Lake City, Utah, in the Oquirrh Mountains. The mine is the largest human-made excavation, and deepest open-pit mine in the world, which is considered to have produced more copper than any other mine in history – more than 19,000,000 short tons. The mine is owned by Rio Tinto Group, a British-Australian multinational corporation. The copper operations at Bingham Canyon Mine are managed through Kennecott Utah Copper Corporation which operates the mine, a concentrator plant, a smelter, and a refinery. The mine has been in production since 1906, and has resulted in the creation of a pit over 0.75 miles (1,210 m) deep, 2.5 miles (4 km) wide, and covering 1,900 acres. It was designated a National Historic Landmark in 1966 under the name Bingham Canyon Open Pit Copper Mine. The mine experienced a massive landslide in April 2013 and a smaller slide in September 2013.

Lateritic nickel ore deposits are surficial, weathered rinds formed on ultramafic rocks. They account for 73% of the continental world nickel resources and will be in the future the dominant source for the mining of nickel.

<span class="mw-page-title-main">In situ leach</span> Solution mining process for recovering certain dissolved metals

In-situ leaching (ISL), also called in-situ recovery (ISR) or solution mining, is a mining process used to recover minerals such as copper and uranium through boreholes drilled into a deposit, in situ. In-situ leach works by artificially dissolving minerals occurring naturally in the solid state.

<span class="mw-page-title-main">Outline of mining</span> Overview of and topical guide to mining

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<span class="mw-page-title-main">Environmental impact of mining</span> Environmental problems from uncontrolled mining

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Mining in the United States has been active since the beginning of colonial times, but became a major industry in the 19th century with a number of new mineral discoveries causing a series of mining rushes. In 2015, the value of coal, metals, and industrial minerals mined in the United States was US$109.6 billion. 158,000 workers were directly employed by the mining industry.

<span class="mw-page-title-main">Pinge</span>

A Pinge or Binge ("binger") is the name given in German-speaking Europe to a wedge-, ditch- or funnel-shaped depression in the terrain caused by mining activity. This depression or sink-hole is frequently caused by the collapse of old underground mine workings that are close to the Earth's surface. Unlike natural landforms, a Pinge is a direct result of human activity. The term has no direct equivalent in English, but may be translated as "mining sink-hole", "mine slump" or, in some cases, as "glory hole".

<span class="mw-page-title-main">Environmental impact of iron ore mining</span>

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<span class="mw-page-title-main">Ridgeway Mine</span> Open-pit mine in South Carolina, U.S.

The Ridgeway mine was a gold and silver open-pit mine near Ridgeway, South Carolina. In its eleven years of operation between 1988 and 1999, it produced 1,500,000 ounces of gold and 900,000 ounces of silver. The mine's two ore bodies are part of the gold-rich Carolina Slate Belt rock package that runs through the upstate Piedmont foothills. The operator of the mine, the Ridgeway Mining Company, is a subdivision of Kennecott Minerals. Kennecott is a subdivision of Rio Tinto which is one of the world's largest mining companies.

References

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  17. ↑ "How 'modern-day slavery' in the Congo powers the rechargeable battery economy". NPR . 1 February 2023.
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