Strip mining (see coal mining), open-pit (or open-cut) mining, and quarrying are the most common mining methods that start from the earth's surface and maintain exposure to the surface throughout the extraction period. The excavation usually has stepped, or benched, side slopes and can reach depths as low as 1,500 ft (460 m). In strip mining, the soft overburden, or waste soil, overlying the ore or coal is easily removed. In open-pit mining the barren rock material over the ore body normally requires drilling and blasting to break it up for removal. A typical mining cycle consists of drilling holes into the rock in a pattern, loading the holes with explosives, or blasting agents, and blasting the rock in order to break it into a size suitable for loading and hauling to the mill, concentrator, or treatment plant. There the metals or other desired substances are extracted from the rocks (see metallurgy).
Under certain circumstances surface mining can become prohibitively expensive and underground mining may be considered. A major factor in the decision to operate by underground mining rather than surface mining is the strip ratio, or the number of units of waste material in a surface mine that must be removed in order to extract one unit of ore. Once this ratio becomes large, surface mining is no longer attractive. The objective of underground mining is to extract the ore below the surface of the earth safely, economically, and with as little waste as possible. The entry from the surface to an underground mine may be through an adit, or horizontal tunnel, a shaft (see shaft sinking), or vertical tunnel, or a declined shaft. A typical underground mine has a number of roughly horizontal levels at various depths below the surface, and these spread out from the access to the surface. Ore is mined in stopes, or rooms. Material left in place to support the ceiling is called a pillar and can sometimes be recovered afterward. A vertical internal connection between two levels of a mine is called a winze if it was made by driving downward and a raise if it was made by driving upward.
A modern underground mine is a highly mechanized operation requiring little work with pick and shovel. Rubber-tired vehicles, rail haulage, and multiple drill units are commonplace. In order to protect miners and their equipment much attention is paid to mine safety. Mine ventilation provides fresh air underground and at the same time removes noxious gases as well as dangerous dusts that might cause lung disease, e.g., silicosis. Roof support is accomplished with timber, concrete, or steel supports or, most commonly, with roof bolts, which are long steel rods used to bind the exposed roof surface to the rock behind it.
Although surface and underground mining are the most common techniques, there are a number of other mining methods. In solution mining the valuable mineral is brought into a liquid solution by some chemical or bacteria. The resultant liquid is pumped to the surface, where the mineral or metal is taken out of solution by precipitation or by ion exchange (e.g., the Frasch process). In glory-hole mining a steep-sided, funnel-shaped surface excavation is connected to tunnels below it. Rocks blasted off the sides of the excavation fall into the tunnels, from which they are then removed. Gopher mining is an old-fashioned method still used in very small mines. Narrow, small holes are driven in order to extract the ore (e.g., gold) as cheaply as possible. In placer mining no excavation is involved; instead, gravel, sand, or talus (rock debris) is removed from deposits by hand, hydraulic nozzles, or dredging. The ore is separated from the waste by panning or sluicing.
Associated with mining are many environmental concerns. Large-scale excavation is often necessary to extract a small amount of ore. Ore extraction disrupts the topsoil and can displace local animals and plants, and sometimes native human populations. Runoff can contaminate nearby water sources with pollutants such as the mercury and sodium cyanide used in gold mining. Waste materials and smelters can cause sulfurous dust clouds that result in acid rain. Abandoned strip mines have often been used as unregulated landfills for hazardous wastes. Several pieces of legislation in the United States, the Surface Mining Control and Reclamation Act (1977) and the Comprehensive Environmental Response, Compensation, and Liability Act, or Superfund Act (1986), address these issues, but enforcement has been difficult.
Another act that affects mining in the United States is the 1872 Mining Act. This now controversial act, which was originally designed to encourage settlement of the West, allows mining companies to purchase land for $2.50 per acre. In the late 20th cent., despite many efforts at reform, the law and the $2.50 per acre price still stood, despite the fact that the ore contained in the land could be worth billions of dollars.
See R. Peele and J. A. Church, ed., Mining Engineer's Handbook (3d ed.; 2 vol., 1941); R. S. Lewis and G. B. Clark, Elements of Mining (3d ed. 1964); E. Pfleider, ed., Surface Mining (1968); G. C. Amstutz, Glossary of Mining Geology (1971); C. Gregory, A Concise History of Mining (1981); M. K. Tolba (United Nations Environment Programme), Saving Our Planet (1991); A. Warhurst, Environmental Degradation from Mining and Mineral Processing in Developing Countries (1994).
Technique for the surface mining of coal by removing the soil and rock overburden above a seam and extracting the exposed mineral. The method is used to best advantage where the coal seam is thin and not deeply buried. (Thicker and deeper seams would be extracted by open-pit or underground mining.) Strip mining is most economical where flat terrain and horizontal seams permit a large area to be stripped. Where deposits occur in rolling or mountainous terrain, a contour method is used that creates a shelf with a slope on one side and an almost vertical wall on the other. A variety of equipment is used, including dozers, scrapers, hydraulic shovels, draglines, and bucket-wheel excavators. Concern over the environmental effects of strip mining have resulted in numerous requirements for the reclamation of excavated land.
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Oldest method of recovering gold from alluvial deposits. It takes advantage of gold's high density, which causes it to sink more rapidly from moving water than the lighter siliceous materials with which it is found. Panning, used by 19th-century miners, employed a pan in which a few handfuls of the gold-bearing soil or gravel and a large amount of water were placed; by swirling the pan's contents, the miner washed the siliceous material over the side, leaving the gold and heavy materials behind. Dredging is the most important placer-mining method today. Used worldwide is the bucket-ladder dredge, with its continuous chain of buckets rotating around a rigid adjustable frame called the ladder. In sluicing, a slightly sloping wooden trough called a box sluice, or a ditch cut in hard gravel or rock called a ground sluice, is used as a channel along which gold-bearing gravel is carried by a stream of water. Riffles placed transversely along the bottom of the sluice cause the water to eddy into small basins, retarding the current so that gold may settle.
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Excavation of materials from the Earth's crust, including those of organic origin, such as coal and petroleum. Modern mining is costly and complicated. First, a mineral vein that can likely produce enough of the desired substance to justify the cost of extraction must be located. Then the size of the vein or deposit is determined, and mining engineers decide the best way to mine it. Most of the world's yearly mineral production is extracted by surface mining, which includes open-pit mining, strip mining, and quarrying. For ore bodies that lie a considerable distance below the surface, underground mining must be considered. In both techniques, excavating and extracting mineral substances involve costly combinations of drilling, blasting, hoisting, and hauling, as well as measures for health and safety and reduction of environmental impact.
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Extraction of coal deposits from the Earth's surface and from underground. Because coal was the basic energy source that fueled the Industrial Revolution, the resulting industrial growth supported the large-scale exploitation of coal deposits. In the late 20th century, open pit mines replaced underground mines as the principal source of coal in the industrial nations. The mining of coal from surface and underground deposits today is a highly productive, mechanized operation.
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Mining is the extraction of valuable minerals or other geological materials from the earth, usually (but not always) from an ore body, vein or (coal) seam. Materials recovered by mining include bauxite, coal, copper, gold, silver, diamonds, iron, precious metals, lead, limestone, magnesite, nickel, phosphate, oil shale, rock salt, tin, uranium and molybdenum. Any material that cannot be grown from agricultural processes, or created artificially in a laboratory or factory, is usually mined. Mining in a wider sense comprises extraction of any non-renewable resource (e.g., petroleum, natural gas, or even water).
Since the beginning of civilization people have used stone, ceramics and, later, metals found on or close to the Earth's surface. These were used to manufacture early tools and weapons. For example, high quality flint found in northern France and southern England were used to set fire and break rock. Flint mines have been found in chalk areas where seams of the stone were followed underground by shafts and galleries. The mines at Grimes Graves are especially famous, and like most other flint mines, are Neolithic in origin (ca 4000 BC-ca 3000 BC). Other hard rocks mined or collected for axes included the greenstone of the Langdale axe industry based in the English Lake District.
The oldest known mine on archaeological record is the "Lion Cave" in Swaziland. At this site, which by radiocarbon dating proves the mine to be about 43,000 years old, paleolithic humans mined mineral hematite, which contained iron and was ground to produce the red pigment ochre. Mines of a similar age in Hungary and are believed to be sites where Neanderthals may have mined flint for weapons and tools.
Ancient Egyptians mined malachite at Maadi. At first, Egyptians used the bright green malachite stones for ornamentations and pottery. Later, between 2,613 and 2,494 BC, large building projects required expeditions abroad to the area of Wadi Maghara in order "to secure minerals and other resources not available in Egypt itself. Quarries for turqoise and copper were also found at "Wadi Hamamat, Tura, Aswan and various other Nubian sites on the Sinai Peninsula and at Timna. Mining in Egypt occurred in the earliest dynasties, and the gold mines of Nubia were among the largest and most extensive of any in Ancient Egypt, and are described by the Greek author Diodorus Siculus. He mentions that fire-setting was one method used to break down the hard rock holding the gold. One of the complexes is shown in one of earliest known maps. They crushed the ore and ground it to a fine powder before washing the powder for the gold dust.
Mining in Europe has a very long pedigree, examples including the silver mines of Laurium, which helped support the Greek city state of Athens. However, it is the Romans who developed large scale mining methods, especially the use of large volumes of water brought to the minehead by numerous aqueducts. The water was used for a variety of purposes, and included using it to remove overburden and rock debris, as well as washing comminuted or crushed ores, and driving simple machinery. Spain was one of the most important mining regions, but all regions of the Roman Empire were exploited. They used reverse overshot water-wheels for dewatering their deep mines such as those at Rio Tinto. The Celts, for example, who were native to Britain, had mined minerals for centuries, but when the Romans came, the scale of the operations changed dramatically. The Romans needed what Britain possessed, especially gold, silver, tin and lead. Mining in the Medieval period is best known through the work De Re Metallica (1556) of Georg Agricola, who described many different mining methods then used in German or Saxon mines. Use of water power in the form of water mills was extensive, and were employed in crushing ore, raising ore from shafts and ventilating galleries by giant bellows.
They followed the ore veins underground once opencast mining was no longer feasible. At Dolaucothi they stoped out the veins, and drove adits through barren rock to drain the stopes. Alternatively, the veins may have been trenched, keeping the upper part open to the air. At deeper levels, stoping will have been necessary. The same adits were also used to ventilate the workings, especially important when fire-setting was used. At other parts of the site, they penetrated the water table and dewatered the mines using several kinds of machine, especially reverse overshot water-wheels. They were used extensively in the copper mines at Rio Tinto in Spain, where one sequence comprised 16 such wheels arranged in pairs, and lifting water about . They were worked as treadmills with miners standing on the top slats. Many examples of such devices have been found in old Roman mines and some examples are now preserved in the British Museum and the National Museum of Wales.
Lead and silver were widely exploited during the Roman period, in Wales, the Mendips and Pennines. A lead-silver mine at Charterhouse, Somerset for example, was guarded by a small fort and was next to the mine workings and settlement for the miners. Lead pigs have been found at several localities in Britain, and mining was so successful, that the Spanish complained about over-production depressing the market. The silver was valuable bullion, while the lead found numerous applications, especially in roofing and plumbing.
There were many iron mines in Roman Britain. The index to the Ordnance Survey Map of Roman Britain lists 33 iron mines: 67% of these are in the Weald and 15% in the Forest of Dean. The majority of mine workers were slaves. The work conditions were poor, and up to 12% of miners died every year.
In the early colonial history of the Americas, "native gold and silver was quickly expropriated and sent back to Spain in fleets of gold- and silver-laden galleons. Turquoise dated at 700 A.D. was mined in pre-Columbian America; in the Cerillos Mining District in New Mexico, estimates are that "about 15,000 tons of rock had been removed from Mt Chalchihuitl using stone tools before 1700.
Mining in the United States became prevalent in the 19th century. As with the California Gold Rush in the mid 1800s, mining for minerals and precious metals alongside ranching was very important in the Westward Expansion to the Pacific coast. With the exploration of the West, mining camps were established and "expressed a distinctive spirit, an enduring legacy to the new nation;" Gold Rushers would experience the same problems as the Land Rushers of the transient West that preceded them. Aided by railroads, many traveled West for work opportunities in mining. Western cities such as Denver and Sacramento originated as mining towns.
According to the NAICS classification system, there are ten industries that are classified as belonging to the Mining division. Over 2008, the three riskiest industries within the Mining division are expected to be: Oil and Gas Extraction, Coal Mining and Other Metal Ore Mining. These three industries combined account for around 2.3% of America's gross domestic product.
Mining techniques can be divided into two basic excavation types:
The science of extractive metallurgy is a specialized area in the science of metallurgy that studies the extraction of valuable metals and minerals from their ores, especially through chemical or mechanical means. Mineral processing (or mineral dressing) is a specialized area in the science of metallurgy that studies the mechanical means of crushing, grinding, and washing that enable the separation (extractive metallurgy) of valuable metals or minerals from their gangue (waste material).
Modern mining companies in some countries are required to follow environmental and rehabilitation codes, ensuring the area mined is returned to close to its original state. In some countries with pristine environments, such as large parts of Australia, this is impossible despite the best intentions. Some mining methods have devastating environmental and public health effects.
Mining can have adverse effects on surrounding surface and ground water if protection measures are not exercised. The result can be unnaturally high concentrations of some chemical elements, notably arsenic and sulfuric acid, over a significantly large area of surface or subsurface. Coal mining releases approximately twenty toxic chemicals, of which 85% is said to be managed on site. Combined with the effects of water and the new 'channels' created for water to travel through, collect in, and contact with these chemicals, a situation is created in which massive contamination can occur. In well-regulated mines, hydrologists and geologists take careful measures to mitigate any type of water contamination that could be caused by mines. In modern American mining, operations must, under federal and state law, meet standards for protecting surface and ground waters from contamination, including acid mine drainage (AMD). To mitigate these problems water is continuously monitored at coal mines. The five principal technologies used to control water flow at mine sites are: diversion systems, containment ponds, groundwater pumping systems, subsurface drainage systems, and subsurface barriers. In the case of AMD, contaminated water is generally pumped to a treatment facility that neutralizes the contaminants.
Some examples of environmental problems associated with mining operations are:
Although such issues have been associated with some mining operations in the past, modern mining practices have improved significantly and are subject to close environmental scrutiny. Modern mining practises aim to lessen environmental impacts from mining, and the ultimate aim is to return the local environment to as close to pristine as is possible. In many cases, the most significant environmental impact longer-term is visual, with pits and mine dumps prominent landscape features.
To ensure completion of reclamation (restoring mine land) most governments and regulatory authorities around the world require that mining companies post a bond to be held in escrow until productivity of reclaimed land has been convincingly demonstrated. Since 1978 the mining industry has reclaimed more than 2 million acres (8,000 km²) of land in the United States alone. This reclaimed land has renewed vegetation and wildlife in previous mining lands and can even be used for farming and ranching.
For further reading on reclamation of former mining sites, please see Restoration ecology.
While exploration and mining can sometimes be conducted by individual entrepreneurs or small business, most modern-day mines are large enterprises requiring large amounts of capital to establish. Consequently, the mining sector of the industry is dominated by large, often multinational, mostly publicly-listed companies. See Mining companies for a list. However, what is referred to as the 'mining industry' is actually two sectors, one specializing in exploration for new resources, the other specializing in mining those resources. The exploration sector is typically made up of individuals and small mineral resource companies dependent on public investment. The mining sector is typically large and multi-national companies sustained by mineral production from their mining operations.
Miners today do more than just dig in the Earth's subsurface. There are many different jobs, direct and indirect, in the mining industry, ranging from engineers and lab technicians to geologists and environmental specialists. Beyond employment directly linked to mine-site activity, the modern mining industry also employs many other professionals, including accountants, lawyers, sales representatives, public relations specialists, not to mention thousands of men and women involved who manufacture the machines and equipment necessary to mine minerals.
Employment in the mining industry offers highly competitive wages and benefits, especially in rural or remote areas. According to the Bureau of Labor Statistics (BLS), wages for coal miners are 30% higher than the wage earned by the average American. Employees possessing at least a bachelor's degree in mining or geological engineering can earn a median pay of over $80,000 annually.
The US Bureau of Labor Statistics estimates that 675,000 are employed in the natural resources and mining sector. Estimated employment by selected specific commodity (including mine, mill, smelter, and quarry workers) listed below is from US Geological Survey Mineral Commodity Surveys:
The mining industry has an experienced but aging workforce with a mean average age of 50 years and median of 46 years. Indeed, while the industry will require new employees to meet future demand, the largest dilemma currently facing mine operators is finding employees to fill vacancies left by a generation of miners, mine engineers, senior managers, technical experts and others who are set to retire between 2005 and 2015. However, the industry is struggling to meet that demand due to current low enrollment levels in mining education programs at American colleges and universities.
Mining is regulated under a comprehensive federal safety law (Federal Mine Safety and Health Act) that is administered by the Department of Labor's Mine Safety and Health Administration (MSHA). Currently under federal law, and enforced by MSHA each U.S. miner must have an approved worker training program in health and safety issues, including at least 40 hours of basic safety training for new underground miners with no experience; 24 hours for new miners at surface mines with no experience; plus eight hours of annual refresher training for all miners.
Safety has long been a controversial issue in the mining business especially with sub-surface mining. While mining today is substantially safer than it was in the previous decades, mining accidents are often very high profile, such as the Quecreek Mine Rescue saving 9 trapped Pennsylvania coal miners in 2002.
Mining ventilation is often seen to be a safety concern for many miners and their families. Poor ventilation of the mines causes exposure to harmful gases, heat and dust inside sub-surface mines. These can cause harmful physiological effects, even death. The concentration of methane and other airborne contaminants underground can generally be controlled by dilution (ventilation), capture before entering the host air stream (methane drainage), or isolation (seals and stoppings).
Methane gas is a common source of ignition for explosions in coal mines and can propagate into the more violent coal dust explosions. Explosions can be prevented or mitigated by eliminating ignition sources, minimizing methane concentrations and coal dust accumulations, generalized rock dusting, and by using passive and active barriers to suppress propagating explosions. High temperatures and humidity may result in heat-related illnesses, including heat stroke which can be fatal. Dusts can cause lung problems, including silicosis, asbestosis and pneumoconiosis (also known as miners lung or black lung disease).
A ventilation system is set up to course a stream of air through the working areas of the mine. The air circulation necessary for the effective ventilation of a mine is generated by one or more large mine fans, usually located above ground. In the United States, the main fans at coal mines are required to be above ground. Air flows in one direction only, making circuits through the mine such that each main work area receives a supply of fresh air.
Mining is regulated under the federal Mine Safety and Health Act by MSHA, which employs nearly one safety inspector for every four coal mines. Underground coal mines are thoroughly inspected at least four times annually by MSHA inspectors. In addition, miners can report violations, and request additional inspections. Miners with such concerns for their work safety cannot be penalized with any threat to the loss of employment.
Immediately reportable accidents and injuries are:
Additionally, the Mine Safety and Health Act authorizes the National Institute for Occupational Safety and Health (NIOSH), part of the Centers for Disease Control and Prevention under the U.S. Department of Health and Human Services to develop recommendations for mine health standards for the Mine Safety and Health Administration; administer a medical surveillance program for miners, including chest X-rays to detect pneumoconiosos (black lung disease) in coal miners; conduct on-site investigations in mines; and test and certify personal protective equipment and hazard-measurement instruments.
Statistical analyses performed by the U.S. Department of Labor's Mine Safety and Health Administration (MSHA) show that between 1990 and 2004, the industry cut the rate of injuries (a measure comparing the rate of incidents to overall number of employees or hours worked) by more than half and fatalities by two-thirds following three prior decades of steady improvement.
Underground mining, like continuous mining, tends to be more technologically sophisticated because of the dangers and expense of subsurface tunneling.
Trams are used to transport miners, minerals and waste.
Mining machinery manufacturers include Atlas Copco, Joy Mining Machinery, Bucyrus International, Caterpillar, Komatsu, Volvo, Hitachi, Terex, Dresser Industries, Kawasaki, Eimco Elecon India Limited, and Liebherr.
Placer mining is a type of surface mining, usually for gold, tin, and other metals, and gemstones. Ore, typically unconsolidated gravels (alluvium), is fed into machinery that may consist of a hopper, shaking screen or trommel which frees the minerals from the gravels. The target minerals are then concentrated using sluices or jigs.
The number of abandoned mines in the United States remains an unknown, ranging "from the National Park Service's tally of 2,500 on its lands, to the Mineral Policy Center's assessment of 560,000 abandoned mines on public and privately owned lands." Many of these abandoned mines are associated with abandoned neighboring towns often referred to as ghost towns. Experts strongly warn against entering or exploring old or abandoned mines.
In the U.S., the estimation is that approximately 25% of the abandoned mine lands (AML) sites pose physical safety hazards. Old mines are often dangerous and can contain deadly gases. Since weather may have eroded the earth/rock surrounding it, the entrance to an old mine in particular can be very dangerous. Old mine workings, caves, etc. are commonly hazardous simply due to the lack of oxygen in the air (a condition in mines known as blackdamp). Blackdamp is a deadly killer which provides no warning to any individual or group of individuals entering such an environment.
Every year, dozens are injured or killed in recreational accidents on mine property. It is only fair to note, however, that the majority of the deaths are unrelated to mine exploration. Drownings in open quarries and ATV accidents on abandoned mine properties are the main cause of accidental death. The U.S. Department of Labor notes that since 1999, "more than 200 people have died in recreational accidents at surface and underground active and abandoned operations across the country." Due to these circumstances, MSHA (Mine Safety and Health Administration) launched the "Stay Out – Stay Alive" campaign, which is a national public awareness campaign aimed at warning and educating children and adults about the dangers of exploring and playing on active and abandoned mine sites.
In the U.S., the Abandoned Mine Land Initiative, launched by the Western Governor's Association and the National Mining Association is also an effort focusing on reporting the number of high-priority AML sites. The initiative identifies, measures and reports on the progress of current reclamation cleanup programs on an annual basis. In the Americas region, the United Nations Environment Programme (UNEP) and the Chilean Copper Commission (COHILCO) co-hosted a workshop to address the problem of abandoned or "orphaned" mines. Including a representative from the UN, ten countries were represented from North, Central and South America with an eleventh participant being Japan.