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.
Tailings dams rank among the largest engineered structures on earth. The Syncrude Mildred Lake Tailings Dyke in Alberta, Canada, is an embankment dam about 18 kilometres (11 mi) long and from 40 to 88 metres (131 to 289 ft) high. The dam and the artificial lake within it are constructed and maintained as part of ongoing operations by Syncrude in extracting oil from the Athabasca oil sands; it is the largest dam structure on earth by volume, and as of 2001 it was believed to be the largest earth structure in the world by volume of fill. [1]
There are key differences between tailings dams and the more familiar hydroelectric dams. Tailings dams are designed for permanent containment, meaning they are intended to "remain there forever". [2] Copper, gold, uranium and other mining operations produce varied kinds of waste, much of it toxic, which pose varied challenges for long-term containment. [3]
An estimated 3,500 active tailings impoundments stand around the world, although there is no complete inventory, and the total number is disputed. In an average year, it would be expected that between two and five "major" tailings dam failures would occur, along with 35 "minor" failures. [4] Assuming the 3,500 figure is correct, this failure rate is "more than two orders of magnitude higher than the failure rate of conventional water retention dams". [5] A 2020 assessment of responsible mining practices by the Responsible Mining Foundation, found that companies have made little or no progress in improving the documentation and safety practices of these ponds. [6]
Unlike water retention dams, the height of a tailings dam is typically increased (raised) throughout the life of the particular mine. Typically, a base or starter dam is constructed, and as it fills with a mixture of tailings and water, it is raised. Material used to raise the dam can include the tailings (if their properties are suitable), earthfill, or rockfill. [7] It is increasingly common for barrier systems such as geomembranes to be incorporated into tailings dams. Impermeable barriers can prevent or reduce seepage thereby increasing the geotechnical and environmental stability of the dam. [8]
There are three types of dam raises, the upstream, downstream and centerline, named according to the relative position of the new crest of the dam to the previous. The specific design used is dependent upon topography, geology, climate, the type of tailings, and cost. An upstream tailings dam consists of trapezoidal embankments being constructed on top but toe to crest of another, moving the crest further upstream. This creates a relatively flat downstream side and a jagged upstream side which is supported by tailings slurry in the impoundment. The downstream design refers to the successive raising of the embankment that positions the fill and crest further downstream. A centerlined dam has sequential embankment dams constructed directly on top of another while fill is placed on the downstream side for support and slurry supports the upstream side. [9] [10]
The largest three tailings dams are:
Rank | Name [11] | Country | Year completed | Structure height [m] | Structure volume [12] [106 m3] | Reservoir volume [109 m3] | Installed capacity [MW] | Type |
---|---|---|---|---|---|---|---|---|
1 | Syncrude Tailings Dam Mildred MLSB [13] | Canada | 1995 | 88 | 540 [14] /720 | 0.35 | NA | TE |
2 | Syncrude Tailings Dam Mildred SWSS [15] | Canada | 2010 | 40–50 | 119 [14] | 0.25 [14] | NA | TE |
3 | ASARCO Mission Mine Tailings Dam | United States | 1973 | 30 [16] | 40.1 | 0 [17] | NA | ER |
Type: TE – Earth; ER – Rock-fill; PG – Concrete gravity; CFRD – Concrete face rock fill
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An embankment dam or caldera is a large artificial dam. It is typically created by the placement and compaction of a complex semi-plastic mound of various compositions of soil or rock. It has a semi-pervious waterproof natural covering for its surface and a dense, impervious core. This makes the dam impervious to surface or seepage erosion. Such a dam is composed of fragmented independent material particles. The friction and interaction of particles binds the particles together into a stable mass rather than by the use of a cementing substance.
The Syncrude Tailings Dam, impounding the Mildred Lake Settling Basin (MLSB), is an embankment dam that is, by volume of construction material, the largest earth structure in the world in 2001. It is located 40 km (25 mi) north of Fort McMurray, Alberta, Canada, at the northern end of the Mildred Lake lease area owned by Syncrude Canada Ltd. The dam and the tailings reservoir within it are constructed and maintained as part of ongoing operations by Syncrude in extracting oil from the Athabasca oil sands. Other tailings dams constructed and operated in the same area by Syncrude include the Southwest Sand Storage (SWSS), which is the third largest dam in the world by volume of construction material after the Tarbela Dam.
The Merriespruit tailings dam disaster occurred on the night of 22 February 1994 when a tailings dam failed and flooded the suburb of Merriespruit, Virginia, Free State, South Africa. Seventeen people were killed as a result.
The Church Rock uranium mill spill occurred in the U.S. state of New Mexico on July 16, 1979, when United Nuclear Corporation's tailings disposal pond at its uranium mill in Church Rock breached its dam. The accident remains the largest release of radioactive material in U.S. history, having released more radioactivity than the Three Mile Island accident four months earlier.
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.
RAVEN (Respecting Aboriginal Values and Environmental Needs) is a charitable organization that provides financial resources to assist Aboriginal nations within Canada in lawfully forcing industrial development to be reconciled with their traditional ways of life, and in a manner that addresses climate change and other ecological sustainability challenges.
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The Fujinuma Dam, was an earth-fill embankment dam in Sukagawa City, Fukushima Prefecture, Japan. It was established on the Ebana River, a tributary of the Abukuma River, 16 km (10 mi) west of the city office of Sukagawa City. Construction on the dam commenced in 1937 and it was completed in 1949 after construction was halted due to World War II. The dam's primary purpose was irrigation. It failed on 11 March 2011 after the 2011 Tōhoku earthquake.
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Baogang Tailings Dam, also known as the Baotou Tailings Dam or Weikuang Dam, is a tailings dam in Inner Mongolia, China, on the outer ring of the city of Baotou, about 20 kilometres from the city centre. The dam is filled with tailings and waste slurry from nearby rare earth mineral refinery plants. Accounts of the tailings dam appeared in western media outlets after a visit in 2015 by British writers Tim Maughan, Liam Young and Kate Davies from Unknown Fields, a "nomadic design studio" from London. Footage posted on YouTube by Maughan appears to show him collecting samples from the floor of the dam. Maughan's account contrasts with the Chinese media's own reporting of the rare earth industry in the area. In 2016, Chinese authorities identified contamination of farmlands surrounding the dam.
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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