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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.
The extraction of minerals from ore can be done two ways: placer mining, which uses water and gravity to concentrate the valuable minerals, or hard rock mining, which pulverizes the rock containing the ore and then relies on chemical reactions to concentrate the sought-after material. In the latter, the extraction of minerals from ore requires comminution, i.e., grinding the ore into fine particles to facilitate extraction of the target element(s). Because of this comminution, tailings consist of a slurry of fine particles, ranging from the size of a grain of sand to a few micrometres. [1] Mine tailings are usually produced from the mill in slurry form, which is a mixture of fine mineral particles and water. [2]
Tailings are likely to be dangerous sources of toxic chemicals such as heavy metals, sulfides and radioactive content. These chemicals are especially dangerous when stored in water in ponds behind tailings dams. These ponds are also vulnerable to major breaches or leaks from the dams, causing environmental disasters, such as the Mount Polley disaster in British Columbia. Because of these and other environmental concerns such as groundwater leakage, toxic emissions and bird death, tailing piles and ponds have received more scrutiny, especially in developed countries, but the first UN-level standard for tailing management was only established 2020. [3]
There are a wide range of methods for recovering economic value, containing or otherwise mitigating the impacts of tailings. However, internationally, these practices are poor, sometimes violating human rights.
Tailings are also called mine dumps, culm dumps, slimes, refuse, leach residue, slickens, or terra-cone (terrikon).[ citation needed ]
The effluent from the tailings from the mining of sulfidic minerals has been described as "the largest environmental liability of the mining industry". [4] These tailings contain large amounts of pyrite (FeS2) and Iron(II) sulfide (FeS), which are rejected from the sought-after ores of copper and nickel, as well as coal. Although harmless underground, these minerals are reactive toward air in the presence of microorganisms, which if not properly managed lead to acid mine drainage.
Between 100 million and 280 million tons of phosphogypsum waste are estimated to be produced annually as a consequence of the processing of phosphate rock for the production of phosphate fertilizers. [5] In addition to being useless and abundant, phosphogypsum is radioactive due to the presence of naturally occurring uranium and thorium, and their daughter isotopes. Depending on the price achievable on the uranium market, extraction of the uranium content may be economically lucrative even absent other incentives, such as reducing the harm the radioactive heavy metals do to the environment.
Bauxite tailings is a waste product generated in the industrial production of aluminium. Making provision for the approximately 70 million tonnes (150 billion pounds) that is produced annually is one of the most significant problems for the aluminium mining industry. [6]
Red mud, now more frequently termed bauxite residue, is an industrial waste generated during the processing of bauxite into alumina using the Bayer process. It is composed of various oxide compounds, including the iron oxides which give its red colour. Over 97% of the alumina produced globally is through the Bayer process; for every tonne (2,200 lb) of alumina produced, approximately 1 to 1.5 tonnes (2,200 to 3,300 lb) of red mud are also produced; the global average is 1.23. Annual production of alumina in 2023 was over 142 million tonnes (310 billion pounds) resulting in the generation of approximately 170 million tonnes (370 billion pounds) of red mud. [7]
Due to this high level of production and the material's high alkalinity, if not stored properly, it can pose a significant environmental hazard. As a result, significant effort is being invested in finding better methods for safe storage and dealing with it such as waste valorization in order to create useful materials for cement and concrete. [8]
Less commonly, this material is also known as bauxite tailings, red sludge, or alumina refinery residues. Increasingly, the name processed bauxite is being adopted, especially when used in cement applications.Coal refuse (also described as coal waste, rock, slag, coal tailings, waste material, rock bank, culm, boney, or gob [9] ) is the material left over from coal mining, usually as tailings piles or spoil tips. For every tonne of hard coal generated by mining, 400 kg (880 lb) of waste material remains, which includes some lost coal that is partially economically recoverable. [10] Coal refuse is distinct from the byproducts of burning coal, such as fly ash.
Piles of coal refuse can have significant negative environmental consequences, including the leaching of iron, manganese, and aluminum residues into waterways and acid mine drainage. [11] The runoff can create both surface and groundwater contamination. [12] The piles also create a fire hazard, with the potential to spontaneously ignite. Because most coal refuse harbors toxic components, it is not easily reclaimed by replanting with plants like beach grasses. [13] [14]
Gob has about four times as much toxic mercury and more sulfur than typical coal. [9] Culm is the term for waste anthracite coal. [9]Early mining operations often did not take adequate steps to make tailings areas environmentally safe after closure. [15] [16] Modern mines, particularly those in jurisdictions with well-developed mining regulations and those operated by responsible mining companies, often include the rehabilitation and proper closure of tailings areas in their costs and activities. For example, the Province of Quebec, Canada, requires not only the submission of a closure plan before the start of mining activity, but also the deposit of a financial guarantee equal to 100% of the estimated rehabilitation costs. [17] Tailings dams are often the most significant environmental liability for a mining project. [18]
Mine tailings may have economic value in carbon sequestration due to the large exposed surface area of the minerals. [19]
The fraction of tailings to ore can range from 90 to 98% for some copper ores to 20–50% of the other (less valuable) minerals. [20] The rejected minerals and rocks liberated through mining and processing have the potential to damage the environment by releasing toxic metals (arsenic and mercury being two major culprits), by acid drainage (usually by microbial action on sulfide ores), or by damaging aquatic wildlife that rely on clear water (vs suspensions). [21]
Tailings ponds can also be a source of acid drainage, leading to the need for permanent monitoring and treatment of water passing through the tailings dam; the cost of mine cleanup has typically been 10 times that of mining industry estimates when acid drainage was involved. [22]
The greatest danger of tailings ponds is dam failure, with the most publicized failure in the U.S. being the failure of a coal slurry dam in the West Virginia Buffalo Creek Flood of 1972, which killed 125 people; other collapses include the Ok Tedi environmental disaster in New Guinea, which destroyed the fishery of the Ok Tedi River. On average, worldwide, there is one big accident involving a tailings dam each year. [22]
Other disasters caused by tailings dam failures are, the 2000 Baia Mare cyanide spill and the Ajka alumina plant accident. In 2015, the iron ore tailings dam failure at the Germano mine complex in Minas Gerais, Brazil, was the country's biggest environmental disaster. The dam breach caused the death of 19 people due to flooding of tailings slime downstream and affected some 400 km of the Doce river system with toxic effluence and out into the Atlantic Ocean.
Tailings deposits tend to be located in rural areas or near marginalized communities, such as indigenous communities. The Global Industry Standard on Tailings Management recommends that "a human rights due diligence process is required to identify and address those that are most at risk from a tailings facility or its potential failure." [23]
Historically, tailings were disposed of in the most convenient manner, such as in downstream running water or down drains. Because of concerns about these sediments in the water and other issues, tailings ponds came into use. The sustainability challenge in the management of tailings and waste rock is to dispose of material, such that it is inert or, if not, stable and contained, to minimise water and energy inputs and the surface footprint of wastes and to move toward finding alternate uses. [21]
Bounded by impoundments (an impoundment is a dam), these dams typically use "local materials" including the tailings themselves, and may be considered embankment dams. [1] Traditionally, the only option for tailings storage was to contain the tailings slurry with locally available earthen materials. [24] This slurry is a dilute stream of the tailings solids within water that was sent to the tailings storage area. The modern tailings designer has a range of tailings products to choose from depending upon how much water is removed from the slurry prior to discharge. It is increasingly common for tailings storage facilities to require special barriers like Bituminous Geomembranes (BGMs) to contain liquid tailings slurries and prevent impact to the surrounding environment. [25] The removal of water not only can create a better storage system in some cases (e.g. dry stacking, see below) but can also assist in water recovery which is a major issue as many mines are in arid regions. In a 1994 description of tailings impoundments, however, the U.S. EPA stated that dewatering methods may be prohibitively expensive except in special circumstances. [1] Subaqueous storage of tailings has also been used. [1]
Tailing ponds are areas of refused mining tailings where the waterborne refuse material is pumped into a pond to allow the sedimentation (meaning separation) of solids from the water. The pond is generally impounded with a dam, and known as tailings impoundments or tailings dams. [1] It was estimated in 2000 that there were about 3,500 active tailings impoundments in the world. [18] The ponded water is of some benefit as it minimizes fine tailings from being transported by wind into populated areas where the toxic chemicals could be potentially hazardous to human health; however, it is also harmful to the environment. Tailing ponds are often somewhat dangerous because they attract wildlife such as waterfowl or caribou as they appear to be a natural pond, but they can be highly toxic and harmful to the health of these animals. Tailings ponds are used to store the waste made from separating minerals from rocks, or the slurry produced from tar sands mining. Tailings are sometimes mixed with other materials such as bentonite to form a thicker slurry that slows the release of impacted water to the environment.
There are many different subsets of this method, including valley impoundments, ring dikes, in-pit impoundments, and specially dug pits. [1] The most common is the valley pond, which takes advantage of the natural topographical depression in the ground. [1] Large earthen dams may be constructed and then filled with the tailings. Exhausted open pit mines may be refilled with tailings. In all instances, due consideration must be made to contamination of the underlying water table, amongst other issues. Dewatering is an important part of pond storage, as the tailings are added to the storage facility the water is removed – usually by draining into decant tower structures. The water removed can thus be reused in the processing cycle. Once a storage facility is filled and completed, the surface can be covered with topsoil and revegetation commenced. However, unless a non-permeable capping method is used, water that infiltrates into the storage facility will have to be continually pumped out into the future.
Paste tailings is a modification to the conventional methods of disposal of tailings (pond storage). Conventional tailings slurries are composed of a low percent of solids and relatively high water content (normally ranging from 20% to 60% solids for most hard rock mining) and when deposited into the tailings pond the solids and liquids separate. In paste tailings the percent of solids in the tailings slurry is increased through the use of paste thickeners to produce a product where the minimal separation of water and solids occurs and the material is deposited into a storage area as a paste (with a consistency somewhat like toothpaste). Paste tailings has the advantage that more water is recycled in the processing plant and therefore the process is more water efficient than conventional tailings and there is a lower potential for seepage. However the cost of the thickening is generally higher than for conventional tailings and the pumping costs for the paste are also normally higher than for conventional tailings as positive displacement pumps are normally required to transport the tailings from the processing plant to the storage area. Paste tailings are used in several locations around the world including Sunrise Dam in Western Australia and Bulyanhulu Gold Mine in Tanzania. [26]
Tailings do not have to be stored in ponds or sent as slurries into oceans, rivers or streams. There is a growing use of the practice of dewatering tailings using vacuum or pressure filters so the tailings can then be stacked. [27] This saves water which potentially reduces the impacts on the environment in terms of a reduction in the potential seepage rates, space used, leaves the tailings in a dense and stable arrangement and eliminates the long-term liability that ponds leave after mining is finished. However although there are potential merits to dry stacked tailings these systems are often cost prohibitive due to increased capital cost to purchase and install the filter systems and the increase in operating costs (generally associated electricity consumption and consumables such as filter cloth) of such systems.[ citation needed ]
While disposal into exhausted open pits is generally a straightforward operation, disposal into underground voids is more complex. A common modern approach is to mix a certain quantity of tailings with waste aggregate and cement, creating a product that can be used to backfill underground voids and stopes. A common term for this is high density paste fill (HDPF). HDPF is a more expensive method of tailings disposal than pond storage, however it has many other benefits as it can significantly increase the stability of underground excavations by providing a means for ground stress to be transmitted across voids – rather than having to pass around them – which can cause mining induced seismic events like that suffered previously at the Beaconsfield Mine Disaster.
Usually called riverine tailings disposal (RTD). In most environments, not a particularly environmentally sound practice, it has seen significant utilisation in the past, leading to such spectacular environmental damage as done by the Mount Lyell Mining and Railway Company in Tasmania to the King River, or the poisoning from the Panguna mine on Bougainville Island, which led to large-scale civil unrest on the island, and the eventual permanent closing of the mine. [22]
As of 2005, only three mines operated by international companies continued to use river disposal: The Ok Tedi mine, the Grasberg mine [22] and the Porgera mine, all on New Guinea. This method is used in these cases due to seismic activity and landslide dangers which make other disposal methods impractical and dangerous.
Commonly referred to as STD (Submarine Tailings Disposal) or DSTD (Deep Sea Tailings Disposal). Tailings can be conveyed using a pipeline then discharged so as to eventually descend into the depths. Practically, it is not an ideal method, as the close proximity to off-shelf depths is rare. When STD is used, the depth of discharge is often comparatively shallow, and extensive damage to the seafloor can result due to covering by the tailings product. [28] If the density and temperature of the tailings product is not controlled, it may travel long distances, or even float to the surface.
This method is used by the gold mine on Lihir Island; its waste disposal has been viewed by environmentalists[ who? ] as highly damaging, while the owners claim that it is not harmful. [22]
Phytostabilisation is a form of phytoremediation that uses hyperaccumulator plants for long-term stabilisation and containment of tailings, by sequestering pollutants in soil near the roots. The plant's presence can reduce wind erosion, or the plant's roots can prevent water erosion, immobilise metals by adsorption or accumulation, and provide a zone around the roots where the metals can precipitate and stabilise. Pollutants become less bioavailable and livestock, wildlife, and human exposure is reduced. This approach can be especially useful in dry environments, which are subject to wind and water dispersion. [29]
Considerable effort and research continues to be made into discovering and refining better methods of tailings disposal. Research at the Porgera Gold Mine is focusing on developing a method of combining tailings products with coarse waste rock and waste muds to create a product that can be stored on the surface in generic-looking waste dumps or stockpiles. This would allow the current use of riverine disposal to cease. Considerable work remains to be done. However, co-disposal has been successfully implemented by several designers including AMEC at, for example, the Elkview Mine in British Columbia.
During extraction of the oil from oil sand, tailings consisting of water, silt, clays and other solvents are also created. This solid will become mature fine tailings by gravity. Foght et al (1985) estimated that there are 103 anaerobic heterotrophs and 104 sulfate-reducing prokaryotes per milliliter in the tailings pond, based on conventional most probable number methods. Foght set up an experiment with two tailings ponds and an analysis of the archaea, bacteria, and the gas released from tailings ponds showed that those were methanogens. As the depth increased, the moles of CH4 released actually decreased. [30]
Siddique (2006, 2007) states that methanogens in the tailings pond live and reproduce by anaerobic degradation, which will lower the molecular weight from naphtha to aliphatic, aromatic hydrocarbons, carbon dioxide and methane. Those archaea and bacteria can degrade the naphtha, which was considered as waste during the procedure of refining oil. Both of those degraded products are useful. Aliphatic, aromatic hydrocarbons and methane can be used as fuel in the humans' daily lives. In other words, these methanogens improve the coefficient of utilization. Moreover, these methanogens change the structure of the tailings pond and help the pore water efflux to be reused for processing oil sands. Because the archaea and bacteria metabolize and release bubbles within the tailings, the pore water can go through the soil easily. Since they accelerate the densification of mature fine tailings, the tailings ponds are enabled to settle the solids more quickly so that the tailings can be reclaimed earlier. Moreover, the water released from the tailings can be used in the procedure of refining oil. Reducing the demand of water can also protect the environment from drought. [31]
As mining techniques and the price of minerals improve, it is not unusual for tailings to be reprocessed using new methods, or more thoroughly with old methods, to recover additional minerals. Extensive tailings dumps of Kalgoorlie / Boulder in Western Australia were re-processed profitably in the 1990s by KalTails Mining. [32]
A machine called the PET4K Processing Plant has been used in a variety of countries for the past 20 years to remediate contaminated tailings. [33]
The UN and business communities developed an international standard for tailings management in 2020 after the critical failure of the Brumadinho dam disaster. [3] The program was convened by United Nations Environment Programme (UNEP), International Council on Mining and Metals (ICMM) and the Principles for Responsible Investment. [3]
Bauxite is a sedimentary rock with a relatively high aluminium content. It is the world's main source of aluminium and gallium. Bauxite consists mostly of the aluminium minerals gibbsite, boehmite and diaspore, mixed with the two iron oxides goethite and haematite, the aluminium clay mineral kaolinite and small amounts of anatase and ilmenite . Bauxite appears dull in luster and is reddish-brown, white, or tan.
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.
A slurry pipeline is a specially engineered pipeline used to move ores, such as coal or iron, or mining waste, called tailings, over long distances. A mixture of the ore concentrate and water, called slurry, is pumped to its destination and the water is filtered out. Due to the abrasive properties of slurry, the pipelines can be lined with high-density polyethylene (HDPE), or manufactured completely from HDPE Pipe, although this requires a very thick pipe wall. Slurry pipelines are used as an alternative to railroad transportation when mines are located in remote, inaccessible areas.
Open-pit mining, also known as open-cast or open-cut mining and in larger contexts mega-mining, is a surface mining technique that extracts rock or minerals from the earth.
The Bayer process is the principal industrial means of refining bauxite to produce alumina (aluminium oxide) and was developed by Carl Josef Bayer. Bauxite, the most important ore of aluminium, contains only 30–60% aluminium oxide (Al2O3), the rest being a mixture of silica, various iron oxides, and titanium dioxide. The aluminium oxide must be further purified before it can be refined into aluminium.
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.
Froth flotation is a process for selectively separating hydrophobic materials from hydrophilic. This is used in mineral processing, paper recycling and waste-water treatment industries. Historically this was first used in the mining industry, where it was one of the great enabling technologies of the 20th century. It has been described as "the single most important operation used for the recovery and upgrading of sulfide ores". The development of froth flotation has improved the recovery of valuable minerals, such as copper- and lead-bearing minerals. Along with mechanized mining, it has allowed the economic recovery of valuable metals from much lower-grade ore than previously possible.
Mineral processing is the process of separating commercially valuable minerals from their ores in the field of extractive metallurgy. Depending on the processes used in each instance, it is often referred to as ore dressing or ore milling.
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.
A coal preparation plant is a facility that washes coal of soil and rock, crushes it into graded sized chunks (sorting), stockpiles grades preparing it for transport to market, and more often than not, also loads coal into rail cars, barges, or ships.
A tailings dam is typically an earth-fill embankment dam used to store byproducts of mining operations after separating the ore from the gangue. Tailings can be liquid, solid, or a slurry of fine particles, and are usually highly toxic and potentially radioactive. Solid tailings are often used as part of the structure itself.
The Climax mine, located in Climax, Colorado, United States, is a major molybdenum mine in Lake and Summit counties, Colorado. Shipments from the mine began in 1915. At its highest output, the Climax mine was the largest molybdenum mine in the world, and for many years it supplied three quarters of the world's supply of molybdenum.
The Batu Hijau mine is an open pit copper-gold mine operated by PT. Amman Mineral Nusa Tenggara. The mine is the second largest copper-gold mine in Indonesia behind the Grasberg mine of PT. Freeport Indonesia. The mine is located 1,530 kilometres (950 mi) east of the Indonesian capital Jakarta on Sumbawa, an island in West Nusa Tenggara Province, more precisely in the southern part of West Sumbawa Regency. The mine is the result of a ten-year exploration and construction program based on a 1999 discovery of the porphyry copper deposit. Production began in 2000.
Environmental impact of mining can occur at local, regional, and global scales through direct and indirect mining practices. Mining can cause erosion, sinkholes, loss of biodiversity, or the contamination of soil, groundwater, and surface water by chemicals emitted from mining processes. These processes also affect the atmosphere through carbon emissions which contributes to climate change.
An ash pond, also called a coal ash basin or surface impoundment, is an engineered structure used at coal-fired power stations for the disposal of two types of coal combustion products: bottom ash and fly ash. The pond is used as a landfill to prevent the release of ash into the atmosphere. Although the use of ash ponds in combination with air pollution controls decreases the amount of airborne pollutants, the structures pose serious health risks for the surrounding environment.
Rampura Agucha is a zinc and lead mine located on a massive sulfide deposit in the Bhilwara district of Rajasthan, India. Rampura Agucha is located 220 km (140 mi) from Jaipur. It is north of Bhilwara, and northwest of Shahpura. Rampura Agucha is 10 km (6.2 mi) southeast of Gulabpura on NH 79. The mine is owned by Hindustan Zinc Limited (HZL), and has the world's largest deposits of zinc and lead.
Mount Polley mine is a Canadian gold and copper mine located in British Columbia near the towns of Williams Lake and Likely. It consists of two open-pit sites with an underground mining component and is owned and operated by the Mount Polley Mining Corporation, a wholly owned subsidiary of Imperial Metals. In 2013, the mine produced an output of 38,501,165 pounds (17,463,835 kg) of copper, 45,823 ounces of gold, and 123,999 of silver. The mill commenced operations in 1997 and was closed and placed on care and maintenance in 2019. The company owns 20,113 hectares (201.13 km2) of property near Quesnel Lake and Polley Lake where it has mining leases and operations on 2,007 hectares (20.07 km2) and mineral claims on 18,106 hectares (181.06 km2). Mineral concentrate is delivered by truck to the Port of Vancouver.
Red mud, now more frequently termed bauxite residue, is an industrial waste generated during the processing of bauxite into alumina using the Bayer process. It is composed of various oxide compounds, including the iron oxides which give its red colour. Over 97% of the alumina produced globally is through the Bayer process; for every tonne (2,200 lb) of alumina produced, approximately 1 to 1.5 tonnes of red mud are also produced; the global average is 1.23. Annual production of alumina in 2023 was over 142 million tonnes resulting in the generation of approximately 170 million tonnes of red mud.
The environmental impact of iron ore mining in all its phases from excavation to beneficiation to transportation and beyond may have detrimental effects on air quality, water quality, biological species, and nearby communities. This is predominantly a result of large-scale iron ore tailings that are released into the environment which are harmful to both animals and humans.
The structural failure of tailings dams and the ensuing release of toxic metals in the environment is a great concern. The standard of public reporting on tailings dam incidents is poor. A large number remain completely unreported, or lack basic facts when reported. There is no comprehensive database for historic failures. According to mining engineer David M Chambers of the Center for Science in Public Participation, 10,000 years is "a conservative estimate" of how long most tailings dams will need to maintain structural integrity.
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