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    How are waste materials managed at mine sites?

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    The type, amount, and properties of mine waste produced at different mines vary depending on the resource being mined, process technology used, and geology at the mine site. While many mine wastes are benign, mining companies manage their waste in order to deal with the large volumes of waste produced and to prevent the release of contaminates into the environment. Waste management plans are developed as part of the mine approval process in Canada, and consist of waste storage area selection and design, strategies to address problematic waste, and long-term stabilization of waste as part of mine closure.

    Origins of waste at mine sites

    Like the majority of human activities, mining operations produce waste materials. “Waste” is a general term for material which currently has little or no economic value. [1] The soil and rock which is removed to gain access to buried ore, and the material (water, solids, and gases) left behind after the ore has been processed to remove the valuable commodities, are considered to be waste materials. However, the difference in mineral content between ore and waste rock can change depending on market conditions and available extraction technology, and there are a number of cases where material that was once considered waste has become a resource for modern mining operations. [2]

    Types of mine waste

    There are different types of mine waste materials which vary in their physical and chemical composition, their potential for environmental contamination, and how they are managed at mine sites. Types of mine waste include: [2]

    • Overburden: Overburden includes the soil and rock that is removed to gain access to the ore deposits at open pit mines. It is usually piled on the surface at mine sites where it will not impede further expansion of the mining operation – moving large volumes of material is expensive. Overburden generally has a low potential for environmental contamination, and is often used at mine sites for landscape contouring and revegetation during mine closure.
    • Waste rock: Waste rock is material that contains minerals in concentrations considered too low to be extracted at a profit. Waste rock is often stored in heaps or dumps on the mine site, but may be stored underwater with tailings if it contains a lot of sulphide minerals and has a high potential for acid rock drainage formation. Waste rock dumps are generally covered with soil and revegetated following mine closure, although there are cases of waste rock being re-mined due to an increase in mineral market prices or improvements in extraction technology.
    • Tailings: Tailings are finely ground rock and mineral waste products of mineral processing operations. Tailings can also contain leftover processing chemicals, and are usually deposited in the form of a water-based slurry into tailings ponds (sedimentation lagoons enclosed by dams built to capture and store the tailings), although offshore tailings disposal has been successful in some cases. Tailings dams are discussed in further detail below.
    • Slags: Slags are non-metallic by-products from metal smelting, and were historically considered to be waste. Slags are largely environmentally benign, and are being used increasingly as aggregate in concrete and road construction.
    • Mine water: Mine water is produced in a number of ways at mine sites, and can vary in its quality and potential for environmental contamination. Water at mine sites is frequently monitored and various water management strategies have been developed to reduce the amount of mine water produced, and treat the water before it is discharged to the environment.
    • Water treatment sludge: Sludge is produced at active water treatment plants used at some mine sites, and consists of the solids that had been removed from the water as well as any chemicals that had been added to improve the efficiency of the process. Although ways of recycling the sludge are being explored, the majority of sludge has little economic value and is handled as waste. Disposal of water treatment residues in underground mine workings is the least expensive option where it is permitted and environmentally safe. In extreme cases where the sludge is rich in cadmium or arsenic, it may be classified as hazardous waste and require special handling and disposal. [3]
    • Gaseous wastes: Gaseous wastes include particulate matter (dust) and sulphur oxides (SOx).  The majority of emissions to the atmosphere are produced during high-temperature chemical processing such as smelting, and vary in their composition and potential for environmental contamination. Environmental control technologies such as gravity collectors, cyclones, and electrostatic precipitators are capable of removing up to 99.7% of dust and fumes, and wet scrubbers typically remove 80-95% of sulphur oxide emissions. [4, 5] In Canada, the atmospheric sulphur dioxide emissions from metal smelters have decreased by 37% between 2003 and 2010. [6]

    Environmental impacts of mine waste

    The environmental impact of mine waste depends on its type and composition, which vary considerably with the commodity being mined, type of ore, and technologies used to process the ore. [2] For instance, where waste rock and tailings contain significant quantities of sulphide minerals and are exposed to air and water, acid rock drainage (ARD) can occur. [7] As a result, every mine requires its own waste characterization, prediction, monitoring, control, and treatment. [8] Many mine wastes are environmentally benign, and can be used for landform reconstruction, vegetation covers, and road and dam construction. [8]

    The major environmental impacts from waste disposal at mine sites can be divided into two categories: the loss of productive land following its conversion to a waste storage area, and the introduction of sediment, acidity, and other contaminates into surrounding surface and groundwater from water running over exposed problematic or chemically reactive wastes. [2, 8]

    Management of mine wastes

    Mine wastes require careful management to ensure the long-term stability of storage and disposal facilities, and to prevent and minimize air, water, and soil contamination. [2] The inappropriate or unsafe management of wastes at mining operations continues to generate opposition from local communities, the general public, and non-government organizations, and has contributed to the negative public perception of the mining industry. [2] Technological advances and changes in regulations have resulted in significant changes in waste management practices over the last 10 to 20 years, and mine wastes at modern mines are generally better managed than they have been in the past. [9, 10] Waste management plans are frequently developed before a mine is constructed, and the reclamation of waste rock dumps and tailings ponds are increasingly incorporated into the designs of new mines. [1, 9] In addition, in many parts of the world authorities require a proper waste management plan before they will issue a mining permit. [7]

    Mine waste management practices and storage facilities used at different mines are based on common design principles, but are optimized by mine engineers depending on specific site conditions. [9] These designs take into account the potential for extreme events, such as earthquakes and floods. [11] Guidelines on waste management and mine closure have been developed at international, national, and regional levels, and provide an advisory framework for best practices in mine waste management. [9]

    The usual approach to managing wastes is to contain and collect them at the point of production, treat the wastes to make them environmentally safe if necessary, and dispose of them to the land, water, or air. [2] The waste management method used at a particular mine depends mainly on an evaluation of cost, environmental performance, and risk of failure. [12] Successful management of tailings and waste rock is based on selecting appropriate waste storage locations, and proper material characterization, including the accurate prediction of long-term chemical behaviour. [12] Solid mine waste (overburden, waste rock, solidified tailings, slag, dust) can be used as backfill in underground or open pit workings, stored in piles on site or underwater to prevent ARD from occurring in the case of problematic wastes, used in construction of roads and dams at the mine, or recycled. [1] Water can be recycled and reused for dust suppression and mineral processing, or treated and discharged into the environment. [10]

    Tailings management

    Because tailings are composed of fine particles (sand, silt, and clay-sized material), and often have a high water content, they have been particularly troublesome to manage. In the past, tailings were deposited directly into rivers or wetlands, which would introduce sediment and contaminates into those water bodies and in many cases adversely affect aquatic life. [1] Tailings are currently used as backfill in underground mines, stored in open pits, dried and stacked, or pumped into tailings ponds on site. [1]

    Although there have been a number of incidents where the dams securing tailings ponds have been breached, mining engineers have been learning from the enquiries into tailing dam failures, and have improved tailings dam design and execution. A compilation of worldwide tailings dam failure statistics between 1909 and 1999 shows an improving trend as mining companies have learned from past mistakes and as regulators have imposed more stringent regulations and conducted more inspections. [7] In the 1970s, there were 44 tailings dam failures, in the 1980s, 27 failures, and in the 1990s, only 7 failures. [7] Modern tailings dam design is very technical, and takes a number of site-specific factors into consideration, such as rainfall and flooding predictions, earthquake response, seepage control, tailings discharge method and rate, and changes over the lifetime of the dams. [7] Non-critical structures are typically designed to withstand a 1-in-100-year flood, while more critical structures are designed for a 1-in-1000-year event or above. [13]

    In response to concerns over tailings dam failures and water contamination, some mines are opting to produce thickened tailings, which are pressed or have chemicals added to remove excess water. [2] Thickened tailings can be mixed with cement and used in construction or as backfill in underground mines. [1] Although producing thickened tailings is often more expensive than storing the tailings in a pond, the use of thickened tailings is increasing, especially in arid areas where water availability is an issue. [2]

    Turning mine wastes into a resource

    The large volumes of waste produced at mining operations are expensive to manage, and are frequently cited as an obstacle in the environmental sustainability of mining. The mining industry plays a leading role in waste management, and is one of few industries that recycles its own waste. [8] Uses of mine waste include: [10 p. 406]

    • Waste rock: Can be reprocessed to extract minerals and metals, used as backfill, landscaping material, aggregate in road construction, or feedstock for cement and concrete
    • Manganese tailings: Manganese tailings have been used in agro-forestry, buildings and construction materials, coatings, resin, glass, and glazes
    • Clay-rich tailings: Clay-rich tailings have been used for making bricks, floor tiles, and cement
    • Slag: Slag is often used for road construction, and in concrete and cement
    • Red mud: Bauxite red mud is solid alkaline waste produced in aluminium refineries. Red mud  has been used as a soil amender, in waste water treatment, and as a raw material for glass, ceramics, and bricks
    • Mine water: Mine water is used for dust suppression and mineral processing, industrial and agricultural uses, as a coolant, and as a source of drinking water
    • Water treatment sludge: Sludge from ARD treatment, which is high in iron, has been sold commercially for use in pigments
    • Sulphur oxide emissions: Many smelters have installed acid plants to convert sulphur dioxide to sulphuric acid, a useful industrial chemical [4]

    Management of waste following mine closure

    Despite the recycling and reuse of many wastes at mine sites, the majority of waste produced is still placed into storage facilities, and the reclamation and long-term management of these facilities has become an important part of modern mine development and mine closure. [10] Regulators may require any waste storage structures to remain stable for a minimum of 100 to 200 years, which means they must withstand extreme events such as floods and earthquakes. [11] Mine closure activities often involve containing and covering tailings to prevent their escape into the environment; minimizing the amount of water seeping from the tailings into surface or groundwater; covering waste rock piles and exposed materials with topsoil and planting vegetation to prevent erosion; and designing the final land formation to minimize erosion and post-closure maintenance. [2] Plans for mine closure and site cleanup are required as part of the mine permitting process in Canada and these plans are updated after additional study [19].  It is also common in Canada for government agencies to issue a new permit on shutdown to cover mine closure. [19]

     

    Case Study: Myra Falls Mine, Campbell River, Vancouver Island, B.C.

    Myra Falls Mine is a zinc, copper, lead, gold, and silver mine located at the south end of Butte Lake in the Strathcona-Westmin Provincial Park, 80 kilometers southwest of Campbell River on Vancouver Island, British Columbia. [14] Myra Falls Mine is owned by the Belgian mining company Nystar after it acquired Breakwater Resources, the mine’s previous owners, in April 2011. [15] The mine began as an open pit in 1966, but now active mining is only taking place underground over more than 240 kilometers of tunnels and shafts. [14]

    Ore is blasted and crushed underground before being hoisted to the mill on the surface. [14] Water is added and the ore is ground up to sand-sized particles; froth floatation cells are then used to separate the metal concentrates from the resulting slurry, and the tailings are piped to a tailings pond behind a man-made dam. [14] About half the tailings (the coarser fraction) settle out of the slurry, and are removed, mixed with cement, and piped back down to the mine as a paste which is used to back-fill the mine workings. [14] The underground mine does not have a waste rock dump because so little waste rock is mined and brought to the surface. [14]

    Much of the water coming from or flowing over the mine site becomes contaminated with dissolved metals including aluminum, zinc, copper, and iron as a result of acid rock drainage (ARD). [16] In the late 1970s, it was discovered that the concentration of zinc in Butte Lake was increasing as a result of inflows of mine water. [16] A number of water control and treatment facilities were installed in the 1980s, which were very successful in reducing levels of contamination coming from the mine. [16]  All the water from the mine, mill, and tailings pond is collected in six polishing ponds, and lime is added to raise the pH and remove the metals. [14] The resulting sludge is pumped to the tailings pond for storage. [14] Most of the water used in the mill is pumped back and re-used, and the water is tested daily to make sure it meets regulatory requirements. [14]

    Myra Falls is the only mine in B.C. located within a Provincial Park. [14] The Strathcona-Westmin Provincial Park is a Class B park, which means certain activities (commercial logging, hydroelectric development, or mining) may be permitted, where they would not be in a Class A park. [17, 18] An area of 3,328 hectares was separated out of the Strathcona Class A Park in 1965 and given special designation because of value of the minerals found in the area and the decision by the provincial government to allow exploration and mining. [14] While the mine controls 36 square kilometers (3,600 hectares) it only has surface footprint of 1.7 square kilometers (170 hectares). [14] The objective of the Myra Falls mine closure plan is to reclaim the land to a Class A park. [14]

    The current reclamation plan calls for the waste rock from the original pit mine to be used to backfill the open pit, and a cover system will prevent further ARD formation at the ponds and old waste dumps. [14] Trials of different materials that will form the cover system are being conducted, and the best material so far has been made with a mixture of mine tailings and sludge from the polishing ponds. [14] Revegetation and landscaping were undertaken around the mine site following the completion of construction activities in 1985, disused roads and facilities have been decommissioned and removed, and plots were set up to identify the types of plants which would be best suited for revegetation of the mine site. [16] So far CA$ 2 million has been spent on reclamation, and another CA$ 10 million dollars has been set aside for future work. [14] The total reclamation cost is expected to be CA$ 21.1 million. [14]



    Show References

    References

    1 Hudson-Edwards, K.A., H.E. Jamieson, and B.G. Lottermoser. Mine Waste: Past, Present, Future. 2011 [cited 2012 August 31]; Available from: http://facstaff.uww.edu/bhattacj/mine_waste_overview.pdf.
    2 Rankin, W.J., Minerals, metals and sustainability : meeting future material needs. 2011, Collingwood, Vic.: CSIRO Pub.
    3 Younger, P.L., S.A. Banwart, and R.S. Hedin, Mine Water: Hydrology, Pollution, Remediation. 2002, Dordrecht, The Netherlands: Kluwer Academic Publishers.
    4 Vallero, D.A., Fundamentals of Air Pollution. 2007, Amsterdam: Elsevier.
    5 U.S. Environmental Protection Agency. Module 6: Air Pollutants and Control Techniques -- Sulfur Oxides -- Control Techniques. Basic Concepts in Environmental Sciences 2010 [cited 2012 September 06]; Available from: http://www.epa.gov/apti/bces/module6/sulfur/control/control.htm.
    6 Canada, Environment Canada. National Pollutant Release Inventory. 2012 [cited 2012 May 24]; Available from: http://www.ec.gc.ca/inrp-npri/default.asp?lang=En&n=4A577BB9-1.
    7 Rajaram, R. and R.E. Melchers, Chapter 6: Waste Management, in Sustainable Mining Practices -- A Global Perspective, V. Rajaram and S. Dutta, Editors. 2005, A. A. Balkema Publishers, a member of Taylor & Francis Group: Leiden, The Netherlands. p. 193-230.
    8 Lottermoser, B., Mine Wastes: Characterization, Treatment and Environmental Impacts, 2012, Springer: New York. p. 400.
    9 Van Zyl, D., et al. Mining for the Future. 2002 [cited 2012 August 14]; Available from: http://www.mining.ubc.ca/mlc/presentations_pub/Pub_LVW/68_mftf-mainreport.pdf.
    10 Lottermoser, B.G., Recycling, Reuse and Rehabilitation of Mine Wastes. Elements, 2011. 7: p. 405-410.
    11 Caldwell, J. Mine Closure: The Basics of Success. [Online course] 2011 Version: 11 May 2011 [cited 2012 September 10]; Available from: http://www.edumine.com/xutility/html/menu.asp?category=xcourse&course=Xclosure.
    12 European Commission. Reference Document on Best Available Techniques for Management of Tailings and Waste-Rock in Mining Activities. 2009 [cited 2012 August 30]; Available from: http://eippcb.jrc.ec.europa.eu/reference/BREF/mmr_adopted_0109.pdf.
    13 Robertson, A. and S. Shaw. Mine Closure. InfoMine E-book 2002 [cited 2012 July 23]; Available from: http://www.infomine.com/publications/docs/E-Book%2002%20Mine%20Closure.pdf.
    14 Minerals Resources Education Program of BC. Myra Falls. mineralsed 2009 [cited 2012 September 06]; Available from: http://www.mineralsed.ca/s/MineProfile.asp?ReportID=534373.
    15 Nyrstar. Myra Falls Mine. 2012 [cited 2012 September 06]; Available from: http://www.nyrstar.com/operations/Documents/NYR1288%20MYRA%20FALLS%20230911.pdf.
    16 Dyk, R.P.v. Environmental Control and Reclamation at Westmin, Myra Falls Operations. Proceedings of the 11th Annual British Columbia Mine Reclamation Symposium 1987 [cited 2012 September 07]; Available from: https://circle.ubc.ca/bitstream/handle/2429/14206/1987%20-%20van%20Dyk%20-%20Environmental%20Control%20and%20Reclamation.pdf?sequence=1.
    17 B.C. Parks. Strathcona-Westmin Provincial Park. 2012 [cited 2012 September 07]; Available from: http://www.env.gov.bc.ca/bcparks/explore/parkpgs/strath_wm/.
    18 B.C. Parks. Summary of the Park and Protected Areas System. 2012 [cited 2012 September 07]; Available from: http://www.env.gov.bc.ca/bcparks/aboutBCParks/prk_desig.html#ClassB.
    19 Canada, Natural Resources Canada. 4. Mine Closure, Mining Sequence: Mining Information for Aboriginal Communities, 2011 [cited 2012 July 13]; Available from: http://www.nrcan.gc.ca/sites/www.nrcan.gc.ca.minerals-metals/files/pdf/mms-smm/abor-auto/pdf/kit-clo-eng.pdf.

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