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Tailings dam

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Syncrude Tailings Dam, Fort McMurray, Alberta

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]

Structure

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Bituminous geomembrane installation on the base and walls of a tailings storage facility.

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]

Wheal Jane Tailings Dam, West Cornwall, England

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]

Largest

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See also: List of largest dams in the world

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

Concerns

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This section is an excerpt from Tailings dam failure.[edit]
Brumadinho dam disaster in 2019
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.[18] 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.[19]

See also

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References

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  1. ^ Morgenstern, Norbert R. (19–20 September 2001). "Geotechnics and Mine Waste Management – Update" (PDF). Swedish Mining Association, Natur Vards Verket, European Commission. Retrieved 27 April 2014.
  2. ^ "Tailings Dams: Where Mining Waste is Stored Forever". FRONTLINE. Retrieved 28 January 2019.
  3. ^ Culbert, Lori (24 November 2001). "Story of a shattered life: A single childhood incident pushed Dawn Crey into a downward spiral – Vancouver Sun". Retrieved 28 January 2019.
  4. ^ Martin, T.E.; Davies, M.P. "Trends in the stewardship of tailings dams" (PDF). www.infomine.com. Archived from the original (PDF) on 21 November 2011. Retrieved 30 July 2020.
  5. ^ Azam, Shahad; Li, Qiren (December 2010). "Tailings Dam Failures: A Review of the Last One Hundred Years" (PDF). www.infomine.com. Archived from the original (PDF) on 26 November 2013. Retrieved 30 July 2020.
  6. ^ Antistatique. "Thematic Results | RMI Report 2020". 2020.responsibleminingindex.org. Retrieved 16 April 2021.
  7. ^ Blight, Geoffrey E. (1998). "Construction of Tailings Dams". Case studies on tailings management. Paris, France: International Council on Metals and the Environment. pp. 9–10. ISBN 1-895720-29-X. Retrieved 10 August 2011.
  8. ^ McLeod, Harvey; Bjelkevik, Annika (2021). TAILINGS DAM DESIGN Technology Update (PDF) (Report). ICOLD Committee on Tailings Dams. p. 91.
  9. ^ "Properties of Tailings Dams" (PDF). NBK Institute of Mining Engineering. Archived from the original (PDF) on 1 October 2011. Retrieved 10 August 2011.http://mining.ubc.ca/files/2013/03/Dirk-van-Zyl.pdf
  10. ^ Raj K. Singhal, ed. (2000). Environmental issues and management of waste in energy and mineral production: Proceedings of the Sixth International Conference on Environmental Issues and Management of Waste in Energy and Mineral Production: SWEMP 2000; Calgary, Alberta, Canada, May 30 – June 2, 2000. Rotterdam: Balkema. pp. 257–260. ISBN 90-5809-085-X. Retrieved 9 November 2015.
  11. ^ Talk:List of largest dams in the world#Phantom Dams
  12. ^ Talk:List of largest dams in the world#Structure Volume
  13. ^ D. Nicol (1994) "The Syncrude Mildred Lake Tailings Dyke Redesign", 18th Int. Congr. Large Dams.
  14. ^ a b c Estimate based on height, dimensions from Google Earth and, where available, cross section. Accuracy ±15%
  15. ^ "Microsoft Word – Baseline Report on Fluid Deposits revE" (PDF). Retrieved 16 February 2011.
  16. ^ Estimate based on structure volume and dimensions from Google Earth
  17. ^ Zero reservoir size because full of tailings
  18. ^ Azam, Shahad; Li, Qiren (December 2010). "Tailings Dam Failures: A Review of the Last One Hundred Years" (PDF). www.infomine.com. Archived from the original (PDF) on 26 November 2013. Retrieved 30 July 2020.
  19. ^ David M Chambers, "Long Term Risk of Releasing Potentially Acid Producing Waste Due to Tailings Dam Failure". Center for Science in Public Participation. Page 3 of 12. CSP2.org
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