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Topic 4: Mine wastes




                                     From a series of 5 lectures on
             Metals, minerals, mining and (some of) its problems
                               prepared for London Mining Network
                                                                 by
                                                      Mark Muller
                                      mmuller.earthsci@gmail.com
                                                     24 April 2009
Outline of Topic 4:

• Types of mine waste: mine waters, tailings, sulphidic wastes
• Rock dumps
• Focus on tailings dams
           Tailings dam construction methods
           Water balance in tailings dams
           Tailings dam failure, with case studies
• Thickened paste disposal
• In-pit disposal
• Riverine tailings disposal
           Case study on riverine tailings disposal
• Submarine tailings disposal
           Case study on submarine tailings disposal
• Focus on radioactive wastes of uranium ores
           Radioactive minerals, radioactive decay products and health risks
           Release of radioactive minerals into the environment by oxidation
           Impact of release of radioactive minerals
Mineral extraction: from mining to metal
Mining




                                           Mineral
                                           concentrate




         METAL EXTRACTION




                                           Metal




                                    Figure from Spitz and Trudinger, 2009.
Mines wastes:

Mine wastes are problematic because they contain hazardous substances that
   can be (or are) released into the environment around the mine – heavy
   metals, metalloids, radioactive elements, acids, process chemicals –
   and therefore require treatment, secure disposal, and monitoring.

Wastes are not only produced during mining, but also at mineral
  processing plants and smelter sites and include effluents, sludges,
  leached ore residues, slags, furnace dusts, filter cakes and smelting
  residues.

Mine wastes may be in the form of: solid waste, water waste, or gaseous
   waste.

Environmental contamination and pollution as a result of improper mining,
   smelting and waste disposal practices has occurred, and still occur,
   around the world (Lottermoser, 2007).
Mine wastes:

Open-pit mining      Produces waste rock:
Underground mining   either barren host rock (referred to as “spoils” in coal
                     mining),
                     or “ore” that is too low-grade, overburden soils and sands.


Mineral processing   Produces processed solid wastes that includes tailings and
Hydrometallurgy      sludges with different physical and chemical properties.

                     Tailings can be used as mining back-fill, but are generally
                     contained on surface.

                     Also produces mill-water and other processing waste-water
                     also produced, as well as atmospheric emissions.
Sulphidic mine wastes:

Sulphide wastes are the biggest problem on mines because of
   potential for generating acid mine waters. Pyrite is the major
   concern.

Sulphide minerals occur abundantly in many types of deposits
    - Metallic ore (Cu, Pb, Zn, Au, Ni, U, Fe)
    - Phosphate ores
    - Coal seams
    - Oil shales
    - Mineral sands

Sulphide minerals may be exposed (just about) everywhere in mines
    - Tailings dams
    - Waste rock dumps and coal spoil (overburden) heaps
    - Heap leach piles
    - Run-of-mine and low-grade ore stockpiles
    - Waste repository embankments
    - Open-pit floors and faces
    - Underground workings
    - Haulroads and road cuts
Acid mine waters:

“Acid mine drainage” (AMD) refers to a particular process whereby low pH
    mine water is formed from the oxidation of sulphide minerals. It
    provides one of the most significant hydrological impacts of mining.
    AMD is particularly prevalent in both metallic mineral and coal mines.



Some authors refer to “Acid rock drainage” (ARD), “acid sulphate waters”
  (ASW); and also “acidic ground water” (AG) when referring to impacted
  ground-water specifically.
Waste-rock disposal – rock dumps:

“Waste-rock” is rock emerging from the mine that will not be processed
   further. It is either “ore” that is below the cut-off grade, or is simply
   the barren host-rock to the mineral deposit.

Rock dumps contain an wide variety of different rocks and minerals that
   is site specific, depending on the nature of the ore deposit and the
   host-rock. If sulphide minerals are present in any of the rocks, there
   is the potential for acid mine drainage.

Generally rock dumps are not sealed at their base, and the risk of
  acid water incursion into the surface drainage system or subsurface
  aquifers is very high.

Rock dumps are also highly porous to water flow, and therefore
   increases significantly the risk of AMD production.
Top-down storage: waste
                                       rock is dumped over an
                                                                                Rock dumps
                                       advancing face.




                                                                                Bottom-up storage:
                                                                                waste rock is
                                                                                dumped in a series
                                                                                of piles, and later
                                                                                spread out and
                                                                                flattened, to be
                                                                                covered by the next
                                                                                layer of dumping.




Trucks (the size of houses) dump 200-ton loads of waste rock from an open pit mine in
Nevada. A composite storage approach is used here: top-down dumping is following after
an earlier phase of bottom-up dumping.
http://science.nationalgeographic.com/science/enlarge/dumping-waste-rock.html
Waste-rock disposal – rock dumps:

Typically a “plume” of contaminated water (either acidic or not) and
   precipitated waste products is developed below and around a rock
   dump.

                                       Figure from Lottermoser, 2007, reproduced from Jurjovec et al., 2002.




              DUMP
                                                 SURFACE




                                       Potential for lateral migration
                                       of contaminated or acidic
                                       water within subsurface
                                       aquifers




  Schematic cross-section of a sulphide waste dump showing a plume of acid water seeping
  into the ground. Also shown is how various subsurface minerals (at this particular site) help
  to buffer, or neutralise, the acid. The initial highly acidic pH value of 1, directly below the
  dump, is buffered back to a neutral pH value of 7 at some depth below the dump.
Tailings disposal:

Tailings are (generally) stored in engineered structures or impoundments,
    called “tailings storage facilities” or “tailings dams”. It is estimated
    that there are at least 3,500 tailings dams worldwide (Davies and
    Martin, 2000).

Tailings dams should be constructed to:
- Contain waste materials indefinitely, and provide long term stability
    against erosion and mass movement.
- Achieve negligible seepage of tailings liquids into ground and surface
    waters to prevent contamination of these waters.
- Prevent failure of dam structures.

The overriding issue with tailings dams is getting the liquid out of
   them, safely, both during mining and afterwards.
Tailings disposal:

In an alternative disposal approach (that is often highly criticised), no
    impoundment is used at all, and tailings are pumped directly into rivers
    (riverine tailings disposal), lakes (lacustrine disposal) or into the
    ocean and onto the seafloor at some water (submarine tailings
    disposal – STD).
Tailings composition:

Tailings consist of a liquid and solid component: generally about 20 – 40
    weight percent solids (Robertson, 1994). The composition of both is
    highly site-specific, depending on the ore and gangue minerals and
    the nature of the water (fresh or saline) and processing chemicals used.


Tailings waters may be alkaline (cyanide used in processing), acidic
    (sulphuric acid used in processing) or saline (saline water used in
    processing). They are a complex cocktail of residues of the processing
    chemicals. The waters are highly chemically reactive.

                                                GRAIN SIZES OF SOLIDS

Tailings solids. Solids are very fine
    grained.




                                                      Figure from Lottermoser, 2007.
Tailings disposal methods


Different disposal methods
are used at different mines,
sometimes in combination,
depending on local
circumstances and
constraints.

Factors may include:
Composition of tailings
Climate
Local land use
Local topography
Costs
Environmental impacts
Safety concerns




TSF = “Tailings storage facility”
        (i.e., tailings dam)


                                    Figure from Spitz and Trudinger, 2009.
Tailings disposal on surface – tailings dam styles or configurations


Topographic conditions around the
mine generally dictate the
configuration of the tailings dams.

Additional storage capacity can be
obtained by filling depressions or
valleys in the topography.


3 configurations of tailings dams used

- Paddock (or ring-dyke): 4 dam walls

 needed

- Hill-side: 3 dam walls needed

- Cross-valley: 1 or 2 dam walls
  needed.




                                                          Figure from Spitz and Trudinger, 2009.
Tailings dams – construction:

Tailings dams hold up to several hundred million cubic meters of water
saturated tailings – they can be very, very large structures.

The fundamental constructed elements of a tailings dam are:

- Dam walls (dykes) to contain the tailings. These are normally constructed

 using waste rock and material available at the dam site. The maximum
 wall height is reported currently to be about 100 m.

- Impermeable liners at the base of the dam to prevent leakage of fluids.
  Linings may consist of geomembranes (polyethylene or PVC), or clay
  layers, or a combination of the two.

- Drainage ditches around the periphery of the tailings dam to collect
  seepage.

- Under-drains to facilitate drainage and consolidation of the tailings in the
  dam. (Not all tailings dams have under-drains installed). Without under-
  drains, tailings dams can only dry-out by evaporation and seepage, which
  generally takes a long time (years after mining has ceased).
Tailings dams – construction

                                                                             Tailings dam at Chatree
                                                                             Gold Mine (Thailand)
                                                                             shortly after
                                                                             commissioning, showing
                                                                             under-drains installed
                                                                             in a herring-bone
                                                                             pattern. Under-drains
                                                                             significantly improve
                                                                             water drainage from the
                                                                             tailings dam, thereby
                                                                             reducing water saturation
                                                                             of tailings sediments and
                                                                             improving geotechnical
                                                                             strength and safety of the
                                                                             dam.

                                    Figure from Spitz and Trudinger, 2009.


Best practice tailings dam construction will consist of:
(i) drains beneath the dam walls,
(ii) double liners under the dam, with a leak detection system between layers,
(iii) under-drains at the base of the tailings and a liquid recovery system.
Tailings dams – construction

                                         Mature, but active,
                                         tailings dams located
                                         south of Johannesburg,
                                         South Africa. These
                                         dams are receiving the
                                         final tailings products of
                                         the reprocessing of
                                         numerous old mine-
                                         dumps spread around
                                         Johannesburg. The
                                         mines were closed in
                                         the 1960s.




http://www.panoramio.com/photo/2399572
Tailings dams – construction

Dam walls are built up successively, from a                      Solid tailings become segregated in
“starter dyke”, during the mine lifetime. Three                  the tailings dam, based on their
methods of successive build-up are commonly                      grain-size and distance from the
used.                                                            discharge point.
                    Surface

UPSTREAM
METHOD
                    Liner




DOWNSTREAM
METHOD




                                                                 Fine-grained                Coarse-grained
CENTRELINE                                                       sediments settle            sediments settle
METHOD                                                           further from the            closest to the
                                                                 discharge point,            discharge point,
                                                                 and are                     and are
                                                                 significantly less          significantly more
In the “upstream” method, note how much thinner the dams         permeable                   permeable – they
walls are, and how much less construction material is            (porous).                   drain more easily.
used. Also note that new embankment material overlies
                                                                 These sediments             These sediments
earlier tailings deposits, which may not have adequate
                                                                 have lower shear            have higher shear
strength to support the weight of the embankment,                strength.                   strength.
especially if water saturation levels in the tailings suddenly
increase, or in the face of earthquake-induced tailings
liquefaction.
                                                                                      Figures from Lottermoser, 2007.
Tailings dams – water balance

Tailings dams remain wet during their entire operational life, and only start drying out
     after decommissioning.

Contamination-plumes below tailings dams are normally much reduced compared to
   rock-dumps, due to the low porosity of tailings materials and the low permeability
   of the liner at the base of the tailings dams.


 Water extracted for re-use                                          High potential for sulphide oxidation and
 from decant pond              Precipitation of salts at             acid development in area immediately
                               edge of decant pool                   above saturated zone




                                                    Beach    UNSATURATED
 Hill-side
                                                                    ZONE

                                            SATURATED ZONE                                               Drainage
                                                                                                         ditch
                              Liner

 Water exchange below the                                             Dam-wall may be saturated at its
 tailings dam depends on                                              base, particularly if the decant
 permeability of the liner                                            pond is too close to it – saturation
                                                                      weakens the strength of the wall


                                                                           Figure modified from Spitz and Trudinger, 2009.
Tailings dams – failure:

More than 50% of tailings dams worldwide are built using the upstream
   method, although it is well recognised that this construction method
   produces a structure which is highly susceptible to erosion and failure
   (Lottermoser, 2007) – less construction material is used, and the dam
   walls are thinner. Statistically, every 20th upstream tailings dam that
   is built, fails (a 5% failure rate), and there have been about 100
   documented significant upstream tailings dam failures (Davies and
   Martin, 2000).

Lottermoser (2007) catalogues 26 tailings dam failures that have
    occurred within the last twenty years, and 13 within the last 10
    years.

There are at least 138 known significant tailings dam failures to date. (
   http://www.wise-uranium.org/mdaf.html; Spitz and Trudinger, 2009;
   UNEP, 2001)

Most failures, whatever the construction method, have occurred in humid,
   temperate regions. There have been very few failures in semi-arid and
   arid regions.
Tailings dams – failures 1909 to 2000, per decade




                                                                                    Contemporary
                                                                                    failure rate of
                                                                                    tailings dams is
                                                                                    much higher than
                                                                                    water supply dams.

                                                                                    Average failure
                                                                                    rate for 1998 to
                                                                                    2008 was 1.3
                                                                                    failures per year.


Low numbers of failures recorded in early
years due to: (i) lower numbers of tailings
dams and (ii) less complete records of
failure from these years.


                                         Figure from Spitz and Trudinger, 2009 (Based on data from UNEP, 2001).
Tailings dams and rock dumps - selected list of major failures

 Date               Location             Incident                               Release                Impact
                                         Tailings dam failure during wall                              17 people missing. Cyanide release to
 2006 April 30      Miliang, China       raise                                  ?                      local river
                                                                                         3
                                                                                950 000 m coal waste   Contamination of 120 km of rivers and
 2000 October 11    Inez, USA            Tailings dam failure                   slurry                 streams. Fish kills

                    Grasberg, Irian Jaya Waste rock dump failure after    Unknown quantity heavy 4 people killed. Contamination of
 2000 May 4         (West Papua)         heavy rain                       metal bearing wastes   streams
                                                                                                 2,616 ha farmland and river basins
                                                                                                 flooded with tailings. 40 km of stream
                                                                                         3
                    Los Frailes,        Collapse of dam due to foundation 4.5 million m of acid, contaminated with acid, metals and
 1998 April 25      Aznalcóllar, Spain  failure                           pyrite rich tailings   metalloids
                                                                                         3
                                                                          4.2 million m cyanide  80 km of local river declared
 1995 August 19     Omai, Guyana        Tailings dam failure              bearing tailings       environmental disaster zone
                    Merriespruit, South                                                          17 people killed. Extensive damage to
                                                                                       3
 1994 February 22   Africa              Dam wall breach after heavy rain 600 000 m               town
                    Olympic Dam, South Leakage of uranium tailings dam
                                                                                       3
 1994 February 14   Australia           into acquifer                     5 million m            ?
                    Ok Tedi, Papua New Collapse of waste rock dump and 170 Mt waste rock and 4
 1989 August 22     Guinea              tailings dam                      Mt tailings            Flow into local river

                                         Failure of fluorite tailings dam due
                                                                                            3
 1985 July 19       Stava, Italy         to inadequate decant construction      200 000 m              269 people killed. Two villages buried
                                         Embankment failure of platinum
                  Bafokeng, Impala,      tailings dam due to excessive                                 15 people killed. Tailings flow 45 km
                                                                                              3
 1974 November 11 South Africa           seepage                                3 million m            downstream
                                         Failure of coal refuse dam after                              150 people killed. 1,500 homes
                                                                                            3
 1972 February 26   Buffalo Creek, USA   heavy rain                             500 000 m              destroyed
                                         Tailings move into underground
 1970 September 25 Mufulira, Zambia      workings                               1 Mt                   89 miners killed
                   Aberfan, Great        Liquefaction of coal refuse dam
 1966 October 21   Britain               after heavy rain                       ?                      144 people killed
                                         Liquefaction of 2 tailings dams                               250 people killed. Tailings traveled 12
 1965 March 28      El Cobre, Chile      during earthquake                      2 Mt                   km downstream, destroyed El Cobre

List selectively extracted from Lottermoser, 2007, with further information added from http://www.wise-uranium.org/mdaf.html
Tailings dam failure – Stava, Italy, 19 July 1985
When a tailings dam breach occurs, some or all of the tailings migrate out of the impoundment and
flow downstream. Obstructions in the path of the flow are either swamped or carried downstream.
A disastrous dam failure and flow of tailings occurred in 1985 at Prestavel mine in Stava, Italy.
The dam breached as a result of heavy rains which caused overtopping. The flow travelled down
the valley through the town of Stava, killing 268 and destroying 62 buildings and 8 bridges.




    Stava before the breach.               Stava covered by tailings as they
    www.wise-uranium.org/mdafst.html       travel through the valley.
                                           www.wise-uranium.org/mdafst.html       From TAILSAFE, 2004.
Tailings dam failure – Los Frailes, Aznalcóllar, Spain, 25 April 1998
A tailings dam failed at Los Frailes mine in Aznalcóllar, Spain in 1998. The failure is thought to
have occurred as a result of the marl foundations of the dam being eroded by the acid seepage
from the tailings that passed through the embankment walls. The weakness in the foundations
combined with the minimal length of beach (i.e., ponded water was encroaching the embankment)
caused high stress in the foundations, thus resulting in the failure of the embankment material.
In total, 4.6 million cubic meters of toxic tailings and effluent poured into the Río Agrio and
Río Guadiamar Rivers. Note: marl is a clayey limestone and it dissolves in acid.


                                                                            Aerial photo of breached
                                                                            embankment.
                                                                            www.tailings.info




                                                                                    From TAILSAFE, 2004.
Tailings dam failure – Mufulira, Zambia, 25 September 1970
On the 25th September 1970 an underground breach of No. 3 tailings dam occurred at the Mufulira
Mine in Zambia. As the night shift crew were on duty, the tailings dam above them collapsed
causing nearly 1 million tons of tailings to fill the mine workings, killing 89 miners. A sinkhole
opened on the surface allowing surface water to continue to pour into the workings.
Two years prior to the disaster, sink holes opened up within the No.3 tailings pond due to roof
collapse underground, and a surface depression developed in the impoundment. There were also
two cases of minor mud ingress into the mine a few months before the main failure. Management
were reluctant to accept and investigate the potential impact of future sink holes. Finally, a sink
hole opened connecting the underground workings and the tailings in the impoundment.




Aerial photo of the sinkhole in No. 3 dam.        Sinkhole in No. 3 dam and processing plant.

                                                               From www.tailings.info and TAILSAFE, 2004.
Tailings dams – failure – causes:

Poor choice of site, poor dam design, poor dam construction, or poor
   management


Liquefaction of tailings and dam: Liquefaction describes the change in
   behaviour, from “solid” to liquid, of a liquid-saturated sedimentary unit in
   response to increased pore-fluid pressures (pores are the spaces
   between particles) – the solid particles literally loose contact with each
   other and the unit loses its physical cohesiveness. High pore-fluid
   pressures are induced by ground motions resulting from earthquakes
   (e.g., Veta de Agua, Chile, 3 March 1985), mine blasting, or nearby
   motion and vibrations of heavy equipment.

Rapid increase in dam wall height: If an upstream dam is raised and the
   dam filled too quickly, very high internal pore pressures are produced in
   the tailings and dam walls, decreasing the dam stability and leading to
   dam failure (e.g., Tyrone, USA, 13 October 1980).
Tailings dams – failure – causes:

Foundation failure: If the base below the dam is too weak to support the
   weight of the dam, movement along a failure plane will occur (e.g., Los
   Frailes, Spain, 25 April 1998).

Excessive water levels: Dam failure can occur if the top of the saturated
   zone in the tailings dam rises too high. Flood inflow, high rainfall, rapid
   melting of snow, and improper water management may cause
   excessive water levels. If “over-topping” of the embankment occurs,
   breaching, erosion, and complete failure of the dam walls are possible
   (e.g., Baia Mare, Romania, 30 January 2000). It is important to keep
   decant pond as small as possible and as far as possible from the
   containing embankments.

Excessive seepage: Seepage within or beneath the dam causes erosion
   along the seepage flow path. Excessive seepage may result in failure
   of the embankment (e.g., Zlevoto, Yugoslavia, 1 March 1976).
Tailings dams – failure – consequences:

Release of huge volumes of tailings, that may enter underground
   workings, towns and villages or spill into waterways and travel
   downstream, polluting streams for considerable distances and covering
   large surface areas with thick, metal-rich mud, and causing significant
   environmental damage to impacted ecosystems.

Significant loss of life.
Thickened discharge and paste disposal

    Thickened tailings
                                             AIR-PHOTO   PLAN VIEW
    are discharged
    from central “riser”
    and a series of
    outer risers to
    create a set of
    cone shaped
    impoundments.

    The “risers” are
    moved up
    incrementally as
    the layers of
    tailings material
    build up.

    Figure is greatly
    vertically
    exaggerated:
    the slope of                         1 – 3°
    “beaches” is
    only 1 to 3°




Figure from Spitz and Trudinger, 2009.
Thickened discharge disposal – advantages and disadvantages:

See e.g., Williams and Seddon, 1999; Brzezinski, 2001.

Advantages over conventional tailings disposal are that:

(i) The disposal site covers a much smaller surface area,
(ii) tailings are not segregated into coarse and fine components, which
      improves the geotechnical properties of the pile,
(iii) water consumption is significantly reduced,
(iv) process chemicals are recovered with the water, rather than left with the
      tailings,
(v) contaminated water drainage into the subsurface and surface water
      systems is reduced,
(vi) The resulting cone shaped deposit provides an attractive landform (say
      the miners!), more amenable to rehabilitation.

Disadvantages of the method include:

(i) The operations are subject to dust generation,
(ii) failure due to liquefaction is not ruled out entirely during the period
     required to dry the paste (McMahon et al., 1996).
In-pit waste disposal:
Tailings may be pumped into mined-out open pits (as well underground mine workings)
     for final disposal.

Backfilling an open-pit eliminates the formation of an open-pit lake.

Any backfill material placed below the water table will form part of the subsurface
   acquifer. The extent to which the water level inside the open pit equilibrates with
   the regional water table will depend on whether or not the open pit is lined with clay
   or other impermeable layer.

Water-waste reactions may lead to the mobilisation of contaminants into ground waters.


 Backfilled open-pit showing
 return of the water table to
 pre-mining levels.

 Sulphidic tailings with high
 acid generating potential are                                              Water
 placed at a depth below the                                                saturated
 final level of the water table
 (to limit oxygen supply to the
 sulphides and hence
 minimise the risk of acid
 water development).
                                                                Figure from Lottermoser, 2007.
Riverine tailings disposal:

Riverine tailings disposal is currently used in more than a few modern
   mining projects, e.g., the copper mines at:
    - Grasberg-Ertsberg, Indonesia
    - Porgera, Papua New Guinea
    - Ok Tedi, Papua New Guinea
    - Bougainville (closed), Papua New Guinea

Riverine disposal is “preferred” in these areas because earthquakes,
   land-slides and very-high rainfall makes the construction of tailings
   dams geotechnically “impossible”.

Miners argue that high natural sediment loads in rivers, generated by the
   high rainfall, is able to dilute the mine tailings discharges. (Nonsense –
   tailings volumes are huge compared to the natural sediment load).

Tailings can be neutralised before disposal into the river systems (but they
   are not always).

Historically riverine tailings disposal from mines was commonly practiced.
Riverine tailings disposal – impacts:

The solids and liquids of tailings are transported down rivers for
   considerable distances: tens to hundreds to thousands of
   kilometers.

Sulphide minerals in discharged tailings generally oxidise in oxygenated
   river waters, creating the potential for acidification of waters.

Problems include:
    - Significantly increased sedimentation and turbidity in the river
       system, and associated flooding of lowlands.
    - Contamination of the stream and floodplain sediments with metals,
       and associated impact on aquatic ecosystems.
    - Diebacks of rainforests and mangrove swamps.
Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea:

Ok Tedi open-pit mine is located at 1,600 m elevation in the Star Mountains,
   in a high rainfall, mudslide and earthquake prone region.

The mine produces a copper-gold-silver concentrate for export, accounting
   for a significant proportion (about 16%) of PNG’s total annual export
   income (Enright, 1994; Murray et al., 2000).

In 1976, the state of Papua New Guinea authorized BHP, Australia’s biggest
    mining corporation, to prepare a development plan for the mine. Four
    years later, the government committed to a partnership in Ok Tedi Mining
    Limited with a 20 percent shareholding. The other shareholders were
    BHP (the major shareholder), Amoco Minerals, and a consortium of
    German companies (World Resources Institute report
    http://archive.wri.org/page.cfm?id=1860&z=?, and references therein)

Mine construction was authorised in August 1981, with production scheduled
   to begin May 1984. The Environmental Impact Assessment was only
   completed in June 1982, a year after construction started, at which
   time the decision not to mine was no longer an option (Townsend and
   Townsend, 2004).
Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea:

A tailings dam was constructed initially, but was swept away by a
    landslide just before production started in 1984. At that time, the
    PNG government controversially granted permission to the mine’s main
    shareholder and operator (BHP) to utilise riverine tailings disposal.
    Riverine disposal is thus allowed under, and is in compliance with,
    PNG laws and regulations. (Which does not necessarily make it
   environmentally or socially desirable though).

Since 1986, tailings have been discharged, and waste rock dumps
   have been left to erode, into the headwaters of the Ok Tedi and Fly
   river systems, which subsequently drain, via the Strickland River and
   estuary, into the Gulf of Papua, over a total distance of over 1,000 km
   (Hettler et al., 1997).
Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea

The volume of tailings generated and deposited into the Ok Tedi and Fly rivers is enormous.
The discharge rate amounts to about 160,000 tons of waste per day. About 1,400 million
tons of waste is estimated to have been released into the tropical river system during the
period 1984 – 2007.


                    Ok Tedi gold and copper mine (Papua New Guinea)




                                  5 June 1990                                   26 May 2004

  Image source: “One Planet, Many People: Atlas of our Changing Environment”, UNEP, 2005.
Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea:

Impacts on the environment include:

-   Increased river turbidity. The small grain size (<100 μm diameter) and
    large quantity of waste has increased the sediment load to the middle
    Fly River by 5 – 10 times the normal load, impacting on aquatic life.

-   Increased sedimentation. The wastes are deposited everywhere along
    the river, all the way down to the Gulf of Papua, but particularly on the
    floodplains of the middle and lower Fly River. Large areas of tropical
    lowland rainforests and mangroves have also been covered with a thin
    veneer of waste.

-   Metal contamination of sediments. Deposited sediments are
    enriched in copper and gold, and contamination moves into the river
    waters themselves, with high potential toxicity to fish populations and
    communities living along the rivers.
Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea:

Social impacts:

By 1989, river communities were struggling to produce enough food,
   and a social impact study in 1991 showed that environmental
   degradation was causing severe hardships to peoples living
   downstream from the mine.
“This chronic build-up of waste has had a devastating effect on the 50,000 people who live in the
     120 villages along the two rivers and depend on them for subsistence fishing and other river-
     based resources. Before the mine, taro and bananas were commonly grown in village
     gardens and riverside sago palms often provided the mainstay of local diets. But since the
     early 1990s, the build-up of sediment in the rivers and subsequent flooding of forests have
     dramatically altered the local environment. Fish stocks have fallen by 70–90 percent,
     animals have migrated, and about 1,300 square kilometers of vegetation have died or
     become blighted, forcing villagers to hunt and fish over larger distances (BHP report 1999:
     9; Higgins 2002: 2). Copper concentrations in the water are about 30 times background
     levels, though the river still meets World Health Organization drinking water standards (BHP
     report 1999: 8–9)”. (World Resources Institute report: http://archive.wri.org/page.cfm?
     id=1860&z=?)

A 2001 study showed that even if mining were to stop [then], the sheer
   volume of tailings already in the river, and continued erosion from the
   waste rock dumps adjacent to the mine, would see the problems grow
   worse over the next forty years.
Riverine tailings disposal – case study – Ok Tedi, Papua New
   Guinea:

“High-value” out of court compensation settlements have been made by
    BHP in favour of local communities affected by the mine (MAC report
    http://www.minesandcommunities.org/article.php?a=622 and World
    Resources Institute report http://archive.wri.org/page.cfm?id=1860&z=?).

In August 1999, BHP announced that it regarded the mine as being
    incompatible with its environmental values.

In February 2002, BHP withdrew from the mine. Their 52 percent
    equity share was transferred to an offshore trust, set up on behalf of
    the Papua New Guinea people. The PNG government gave BHP
    Billiton legal indemnity from responsibility for future mine-related
    damage to the Ok Tedi ecosystem (although the legality of this deal
    may still be challenged in the country’s courts).

The mine is still currently operating, and although a limited dredging
   operation has been introduced, mine waste disposal into local rivers
   continues. Operations are scheduled to end in 2010.
Submarine tailings disposal

Coagulants and flocculants
used to bind particles together
to form a thicker mixture to
prevent wide dissemination of                     The euphotic layer is defined as
the tailings-plume underwater                     the depth reached by only 1% of
                                                  photosynthetically active light

                                  (High density
                                  polyethylene)




                                                                                     Greater
                                                                                     than 50 m
                                                                                     water
                                                                                     depth



                                                                  Seafloor


De-aeration and mixing with
seawater to increase density                       Plume of lighter      Final resting place
of slurry                                          tailings material     of tailings on the
                                                                         sea-floor



                                                              Figure from Spitz and Trudinger, 2009.
Submarine tailings disposal (STD):

STD is used in coastal settings where the earthquakes, land-slides and
  very-high rainfall (as for riverine disposal) make construction of tailings
  impoundments geotechnically unfeasible.

The aims of STD are:
    - to place the tailings into a deep marine environment which
       has minimal oxygen concentrations – thereby avoiding sulphide
       oxidation and acid generation.
    - to prevent tailings from entering the shallow, biologically
       productive, oxygenated zone.

Tailings are discharged at water depths of greater than 50 m, create a
    plume of material in the vicinity of the discharge point, and
    subsequently settle on the sea-floor.

STD has a very damaging impact on seafloor ecosystems. There is
  high potential for metal uptake by fish and bottom dwelling organisms.
Submarine tailings disposal – case study – Black Angel, Greenland


    Open adits in the
    footwall below the
    massive sulphide
    orebody.



            Cable-car
            entrances
            to mine




The “angel” is a
contorted pelite bed
(metamorphosed
mudstone), and not the
orebody itself.

                         View of the 700 m cliff face that overlooks Affarlikassaa Fjord at
                         Black Angel Mine.




                                                                    Figure from Lottermoser, 2007.
Submarine tailings disposal – case study – Black Angel, Greenland:

Black Angel lead-zinc underground mine is located on the west coat of
   Greenland, about 500 km north of the Arctic Circle. The orebody was
   mined between 1973 and 1991, with a total production of 11 million tons
   of ore, consisting of sulphide minerals sphalerite and galena (and
   pyrite) (Asmund et al., 1994).

The mine is located at the top of a 700 m cliff face above the junction of the
   4-km-long Affarlikassaa Fjord and the 8-km-long Qaumarujuk Fjord.

Waste rock was allowed to accumulate at the base of the cliff in a 0.4
  million ton rock-dump at the shoreline of the Affarlikassaa Fjord.

Mined ore was transported by cable-car across the fjord to an industrial
   area for processing using conventional selective flotation.

Tailings were discharged directly into Affarlikassaa Fjord. The total
    amount of tailings discharged was about 8 million tons, containing
    elevated arsenic, cadmium, copper, lead, and zinc values (Poling and
    Ellis, 1995).
Submarine tailings disposal – case study – Black Angel, Greenland:

While tests prior to mining indicated elevated metal concentrations in
   seaweed and mussels due to the natural exposure of the orebodies to
   weathering and erosion, problems relating to the tailings disposal
   quickly emerged.

Within a year of starting STD, distinctly elevated lead and zinc values
   were found in waters and biota of the entire fjord system.
   Extensive investigation at this stage indicated that (Poling and Ellis,
   1995):

(i) The assumption that all the metals in the tailings would be present
    only in insoluble sulphide minerals was incorrect – the tailings in fact
    contained minerals that could be dissolved in sea-water.

(ii) The assumption that the discharged tailings would be permanently
     protected [from oxidation] by stagnant bottom waters in the fjord was
     incorrect – the disposal site in fact did not have a permanently layered
     water column, and complete mixing of the fjord waters [including
     the oxygenated upper layers] took place during winter.
Submarine tailings disposal – case study – Black Angel, Greenland:

Changes made subsequently to the processing and discharge methods:
(i) Minerals processing changes reduced the lead content in the tailings
     from 0.4% Pb in 1973 to 0.18% Pb in 1989.
(ii) Increasing the density of the tailings, by addition of seawater and
     coagulation and flocculation chemicals, helped reduce the extent of
     dispersion of metals away from the submarine deposition site (Asmund
     et al., 1994).

These changes reduced, but did not eliminate, the elevated metal levels.
   The tailings discharge resulted in the metal enrichment of water,
   suspended particulate matter, sediment and biota in the Affarlikassaa
   and Qaumarujuk fjords up to 70 km away from the tailings outfall
   (Loring and Asmund, 1989; Elberling et al., 2002).

While analyses of seals and fish species largely revealed no metal
   contamination during mining, deep sea prawns and capelins, as well
   as the livers of certain fish species and sea-birds contained lead
   concentrations above the safe consumption limit (Asmund et al.,
   1994).
Submarine tailings disposal – case study – Black Angel, Greenland:

Since mine closure in 1991, metal concentrations declined in fjord waters,
   as well as in animal and plant life (Asmund et al., 1994), but dispersion
   and release of metals from the tailings still continues (Elberling et al.,
   2002).

In hindsight:
    “detailed mineralogic, leaching, and oceanographic studies, which
    are now conventional at proposed new mines, would have
    produced more detailed information on which to base the decision
    whether submarine tailings disposal (STD) was appropriate at this
    particular site” (Poling and Ellis, 1995).
Black Angel Mine, Greenland:

The case is not yet closed.....
Press Release 28/05/2008:
“A&R [Angus & Ross] (AIM: AGU.L), a zinc/lead mining company focused on re-opening the
    Black Angel Mine in Western Greenland, is pleased to announce that its wholly owned
    subsidiary, Black Angel Mining A/S, has been awarded a 30 year licence to mine zinc, lead
    and silver ore from the Black Angel Mine.” (http://www.angusandross.com/AR-NEW/news/PR-28-05-
    08-Mining-license.htm)


  “Pillar mining will require strategically placed backfill.
  The pillar mining plan with the use of backfill [shown
  right] has been developed by Golders of Vancouver”
  “Phase One is expected to last for 4 years, during
  which time 1.3 million tonnes [of ore] is expected to
  be mined.”
  http://www.angusandross.com/AR-NEW/pages/proj-black-angel-
  phase1.htm


 A&R are currently refurbishing the mine for “Phase One” which will
 “concentrate on the development of infrastructure and extraction of the
 pillars from the old mine” and also the “production of 'dry concentrate'
 in the mine” (http://www.angusandross.com/AR-NEW/pages/proj-black-angel-phase1.htm)
Black Angel Mine, Greenland:

Production of “dry concentrate”.....
“Processing of the mined ore was to take place in a mill in Europe according to the Bankable
    Feasibility Study (BFS). This possibility still exists, but the fall in metal prices since the
    completion of the BFS makes it less attractive than before. In this context our technical
    team is working on a solution to produce concentrate on site. The nature of the ore makes
    is suitable for 'dry concentration' e.g. by gravity concentration or optical ore sorting. Such
    concentrate could be shipped directly to a smelter thus significantly reducing shipping
    costs. (http://www.angusandross.com/AR-NEW/pages/proj-black-angel-phase1.htm)

Press release 19/02/2009:
“US specialist Wardrop Engineering, a Tetra Tech Company ("Wardrop"), and Canadian based
     SGS Minerals Services UK Limited ("SGS"), have been selected as the main nominated
     contractors for the development of the Black Angel Mine mineral processing and waste
     handling plant..... to be installed inside the Black Angel Mine”.
“This will consist of a primary and secondary crushing circuit, pre-concentration by optical ore
     sorting, with milling and fine grinding feeding a conventional froth flotation plant. Premium
     grade Zinc (59-61% Zn) and Lead concentrates (69-71% Pb) will be produced. These will
     be shipped to the logistics hub at Maarmorilik for bonded product storage as part of the
     recently announced off-take agreement with Swiss metal trader MRI Trading AG”.
(http://www.angusandross.com/AR-NEW/news/PR-19-02-09-tech-app-of-contractors.htm )

And of the fate of the large volumes of tailings that will be produced
  by milling and “conventional” froth flotation.... Not a word.
Worldwide uranium mining and waste production:

There are probably more than 500 million tons of uranium tailings
   located around the world (Waggitt, 1994). Uranium mine tailings are
   defined as “low-level” radioactive wastes, and their long term
   containment is a great environmental concern.


     World’s ten largest uranium mines in 1997.
     (From Hockley et al., 2000, using data from Uranium Institute).
Radioactive wastes of uranium ores:

The mineral processing of hard-rock uranium ores proceeds along the same
   route as typically used for sulphide or gold bearing ores. Either
   sulphuric acid or ammonium carbonate (alkali) leaches are used to
   dissolve the uranium-bearing oxide minerals from the mined ore rocks.

The “pregnant” uranium-bearing leach solution is subsequently chemically
   processed to extract the uranium and produce yellowcake.

Vat leaching. The ore processing may include crushing and grinding of ore
   rock followed by vat leaching – which will generate waste waters (both
   mill-water and process-water) and large volumes of tailings.

Heap leaching. Alternatively, low-grade uranium ore may be processed in
   leach heaps, generating waste that consists largely of process-waters,
   with little or no tailings.

Waste rock dumps, old leach heaps and tailings dams are all potential
  areas where dissolved uranium can be mobilised into surface and
  subsurface water systems.
Radioactive wastes of uranium ores:

While uranium oxide minerals form the basis of uranium ores (primarily
   uraninite, UO2), sulphide minerals are also ubiquitous in uranium
   orebodies. Particularly where pyrite and marcasite (FeS2) are present
   and exposed by mining, acid mine drainage may develop in
   workings and mine wastes. More detail on AMD follows in Topic 5.

Thorium occurs together with uranium in uranium ore deposits.

The mining of placer and mineral sand deposits for gold, diamond,
   sapphire, ruby, titanium (in ilmenite and rutile) and tin (in cassiterite)
   also accumulates gangue minerals that contain radioactive
   uranium and thorium (e.g., the minerals monazite, xenotime, zircon,
   tantalite, columbite). If accumulations of such gangue-mineral wastes
   are allowed to weather and break down, both uranium and thorium
   may enter surface and subsurface waters.

Phosphate mining for both fertilisers and Rare Earth Elements (contained
   in the mineral monazite) may also generate uranium-bearing waste
   products.
Uranium radioactive decay series



Uranium-238 (92 protons, 146                                                          Series starts with
                                                                                      radioactive isotope
neutrons) accounts for 99.28%
of the Earth’s uranium.


   Critical U-238 decay products:
   Radium-226
   Radon-222 (gas)
                                                                                      Series ends with
                                                                                      stable lead isotope

Uranium-235 (92 protons, 143
neutrons) accounts for 0.71% of
the Earth’s uranium. Its decay
products are therefore
negligible.
                                                                                            Low abundances and
                                                                                            very short half-lives
                                                                                            with respect to radium
                                                                                            (Ra) and radon gas (Rn)
Thorium-232 (90 protons, 142                                                                isotopes generated by
neutrons) is the most abundant                                                              uranium-238 decay –
radioactive thorium isotope.                                                                therefore negligible
                                                                                            with respect to the
                                                                                            impact of U-238.




                                    Table from Lottermoser, 2007, and references therein.
Impacts of uranium and thorium radioactive decay:

All three types of radiation (alpha, beta and gamma) from all parent and
     daughter radionuclides are extremely damaging to living organisms:
     (i) living cells and tissue are directly damaged, and (ii) water molecules
     in the organisms are damaged, releasing free radicals and chemicals
     that are toxic.

Alpha particles are not deeply penetrating, and when external to the body,
   are stopped by the outer layer of skin. They are particularly damaging
   to internal organs when ingested or inhaled.

Radium-226 (Ra-226). Is particularly of concern for several reasons:
(i) With a half life of 1,622 years it persists in uranium mine wastes.
(ii) Compared to uranium and thorium, Ra-226 is more easily liberated from
      minerals in uranium orebodies during natural weathering and mineral
      processing. It is also more soluble in water and therefore more mobile
      in the environment.
(iii) Ra-226 behaves biologically similarly to calcium (Ca) and forms
      compounds that can be taken up by humans, plants and animals.
(iv) Ra-226 has a high radiotoxicity and accumulates in bones.
(v) It decays to a further problematic radioactive element – radon-222 gas.
Impacts of uranium and thorium radioactive decay:

Radon-222 (Rn-222). Radon is a colourless, tasteless and odourless
   gas, that is the most abundant isotope of radon. Although Rn-222
   has a short half-life (3.8 days) and decays quickly, it occurs in
   abundance and is constantly replenished due to the abundance of
   its very long-lived “parent” U-238.

Rn-222 is of concern for several reasons:
(i) It is constantly replenished by U-238.
(ii) It is soluble in water and therefore mobile within the environment.
(iii) When Rn-222 is inhaled by humans its decay products are solid and
      become lodged in the lungs, and are themselves highly radiotoxic –
      polonium-218, lead-214, and bismuth-214 – emitting α, β and γ
      radiation and inducing lung cancer. Radioactive lead-210, near the
      end of the decay series, has a half-life of 22.5 years, so will remain
      resident in lungs for most of a person’s lifetime, emitting β radiation
      and generating further radioactive “progeny”.
Radioactive wastes of uranium ores:

While the hydrometallurgical processing of uranium ore is very selective and
   efficient in extracting uranium, not all of the uranium is extracted, and
   tailings will always contain small amounts of uranium. Moreover,
   most of the undesired (from an extractive point of view) and undesirable
   (environmentally) daughter radionuclides from the U-238 decay
   series end up in the tailings.

As a result of the selective extraction, only 15% of the initial radioactivity of
   the orebody is transferred to the uranium yellowcake concentrate, while
   75% of the radioactivity remains with the tailings (Landa, 1999;
   OECD, 1999; Abdelouas et al., 1999).

Unlike acids which can (in principle) be neutralised, and free cyanide and
    cyanide complexes which can (in principle) be destroyed or will degrade
    naturally with time, radioactivity and radioactive elements cannot be
    destroyed. All one can hope to achieve in dealing with radioactive
    mining wastes is to immobilise the radioactive minerals, prevent
    dissolution of uranium and thorium from them, and isolate them from the
    environment safely and permanently (which is not easily achieved).
Oxidation and dissolution of uranium wastes
                  Atmospheric                                         Rock dumps at Sherwood
                  oxygen (O2)               Water (H2O)               Uranium Mine,
                                                                      Washington State, USA,
                                                                      before reclamation. The
                                                                      mine operated from 1976
                                Uraninite (UO2) +                     to 1985. Subsequent
                                                                      reclamation work
                                sulphuric acid (H2SO4)
                                                                      completed in June 2000.
                                                                      Photo August 1985.



                                                  Uranyl sulphate
                                                  (UO2SO4)
                                                  dissolved in
                                                  water
                                                                      http://ecorestoration.montana
                                                                      .edu/mineland/histories/miner
                                                                      als/sherwood/default.htm#


       2   UO2     +   2   H2SO4        +    O2           2   UO2SO4        +    2   H2O
      Uraninite        Sulphuric acid       Oxygen        Uranyl sulphate            Water
       (solid)          (dissolved)         (gas)           (dissolved)              (liquid)

Note: the sulphuric acid is generated by oxidation of coexisting sulphide minerals (acid mine drainage).
Oxidation and dissolution of uranium wastes:

Uraninite (UO2) in uranium ores can be broken down by the process of
   oxidation when exposed at the surface in rock dumps or tailings dams.

The resulting oxidised uranium compounds are highly soluble in water, highly
   mobile and easily dispersed in surface or subsurface drainage
   systems for significant distances away from the mine site.

Uranium oxidation-dissolution can occur in both acidic and alkaline waters,
   given the presence of an oxidising agent (atmospheric oxygen) to trigger
   the process.

Acid conditions particularly favour the dissolution of uranium. As sulphide
   minerals are also ubiquitous in uranium orebodies, acid conditions are
   very commonly generated through sulphide oxidation (see Topic 5).

Oxidised uranium mineral forms that are found dissolved in water, or
   precipitated as salts adjacent to surface water, are highly toxic and
   include uranyl sulphate UO2SO4 (yellowcake!) and uranium sulphate
   U(SO4)2 (under acidic conditions) and uranyl carbonate complexes
   UO2(CO3)n (under alkaline conditions).

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Topic 4: Mine wastes

  • 1. Topic 4: Mine wastes From a series of 5 lectures on Metals, minerals, mining and (some of) its problems prepared for London Mining Network by Mark Muller mmuller.earthsci@gmail.com 24 April 2009
  • 2. Outline of Topic 4: • Types of mine waste: mine waters, tailings, sulphidic wastes • Rock dumps • Focus on tailings dams Tailings dam construction methods Water balance in tailings dams Tailings dam failure, with case studies • Thickened paste disposal • In-pit disposal • Riverine tailings disposal Case study on riverine tailings disposal • Submarine tailings disposal Case study on submarine tailings disposal • Focus on radioactive wastes of uranium ores Radioactive minerals, radioactive decay products and health risks Release of radioactive minerals into the environment by oxidation Impact of release of radioactive minerals
  • 3. Mineral extraction: from mining to metal Mining Mineral concentrate METAL EXTRACTION Metal Figure from Spitz and Trudinger, 2009.
  • 4. Mines wastes: Mine wastes are problematic because they contain hazardous substances that can be (or are) released into the environment around the mine – heavy metals, metalloids, radioactive elements, acids, process chemicals – and therefore require treatment, secure disposal, and monitoring. Wastes are not only produced during mining, but also at mineral processing plants and smelter sites and include effluents, sludges, leached ore residues, slags, furnace dusts, filter cakes and smelting residues. Mine wastes may be in the form of: solid waste, water waste, or gaseous waste. Environmental contamination and pollution as a result of improper mining, smelting and waste disposal practices has occurred, and still occur, around the world (Lottermoser, 2007).
  • 5. Mine wastes: Open-pit mining Produces waste rock: Underground mining either barren host rock (referred to as “spoils” in coal mining), or “ore” that is too low-grade, overburden soils and sands. Mineral processing Produces processed solid wastes that includes tailings and Hydrometallurgy sludges with different physical and chemical properties. Tailings can be used as mining back-fill, but are generally contained on surface. Also produces mill-water and other processing waste-water also produced, as well as atmospheric emissions.
  • 6. Sulphidic mine wastes: Sulphide wastes are the biggest problem on mines because of potential for generating acid mine waters. Pyrite is the major concern. Sulphide minerals occur abundantly in many types of deposits - Metallic ore (Cu, Pb, Zn, Au, Ni, U, Fe) - Phosphate ores - Coal seams - Oil shales - Mineral sands Sulphide minerals may be exposed (just about) everywhere in mines - Tailings dams - Waste rock dumps and coal spoil (overburden) heaps - Heap leach piles - Run-of-mine and low-grade ore stockpiles - Waste repository embankments - Open-pit floors and faces - Underground workings - Haulroads and road cuts
  • 7. Acid mine waters: “Acid mine drainage” (AMD) refers to a particular process whereby low pH mine water is formed from the oxidation of sulphide minerals. It provides one of the most significant hydrological impacts of mining. AMD is particularly prevalent in both metallic mineral and coal mines. Some authors refer to “Acid rock drainage” (ARD), “acid sulphate waters” (ASW); and also “acidic ground water” (AG) when referring to impacted ground-water specifically.
  • 8. Waste-rock disposal – rock dumps: “Waste-rock” is rock emerging from the mine that will not be processed further. It is either “ore” that is below the cut-off grade, or is simply the barren host-rock to the mineral deposit. Rock dumps contain an wide variety of different rocks and minerals that is site specific, depending on the nature of the ore deposit and the host-rock. If sulphide minerals are present in any of the rocks, there is the potential for acid mine drainage. Generally rock dumps are not sealed at their base, and the risk of acid water incursion into the surface drainage system or subsurface aquifers is very high. Rock dumps are also highly porous to water flow, and therefore increases significantly the risk of AMD production.
  • 9. Top-down storage: waste rock is dumped over an Rock dumps advancing face. Bottom-up storage: waste rock is dumped in a series of piles, and later spread out and flattened, to be covered by the next layer of dumping. Trucks (the size of houses) dump 200-ton loads of waste rock from an open pit mine in Nevada. A composite storage approach is used here: top-down dumping is following after an earlier phase of bottom-up dumping. http://science.nationalgeographic.com/science/enlarge/dumping-waste-rock.html
  • 10. Waste-rock disposal – rock dumps: Typically a “plume” of contaminated water (either acidic or not) and precipitated waste products is developed below and around a rock dump. Figure from Lottermoser, 2007, reproduced from Jurjovec et al., 2002. DUMP SURFACE Potential for lateral migration of contaminated or acidic water within subsurface aquifers Schematic cross-section of a sulphide waste dump showing a plume of acid water seeping into the ground. Also shown is how various subsurface minerals (at this particular site) help to buffer, or neutralise, the acid. The initial highly acidic pH value of 1, directly below the dump, is buffered back to a neutral pH value of 7 at some depth below the dump.
  • 11. Tailings disposal: Tailings are (generally) stored in engineered structures or impoundments, called “tailings storage facilities” or “tailings dams”. It is estimated that there are at least 3,500 tailings dams worldwide (Davies and Martin, 2000). Tailings dams should be constructed to: - Contain waste materials indefinitely, and provide long term stability against erosion and mass movement. - Achieve negligible seepage of tailings liquids into ground and surface waters to prevent contamination of these waters. - Prevent failure of dam structures. The overriding issue with tailings dams is getting the liquid out of them, safely, both during mining and afterwards.
  • 12. Tailings disposal: In an alternative disposal approach (that is often highly criticised), no impoundment is used at all, and tailings are pumped directly into rivers (riverine tailings disposal), lakes (lacustrine disposal) or into the ocean and onto the seafloor at some water (submarine tailings disposal – STD).
  • 13. Tailings composition: Tailings consist of a liquid and solid component: generally about 20 – 40 weight percent solids (Robertson, 1994). The composition of both is highly site-specific, depending on the ore and gangue minerals and the nature of the water (fresh or saline) and processing chemicals used. Tailings waters may be alkaline (cyanide used in processing), acidic (sulphuric acid used in processing) or saline (saline water used in processing). They are a complex cocktail of residues of the processing chemicals. The waters are highly chemically reactive. GRAIN SIZES OF SOLIDS Tailings solids. Solids are very fine grained. Figure from Lottermoser, 2007.
  • 14. Tailings disposal methods Different disposal methods are used at different mines, sometimes in combination, depending on local circumstances and constraints. Factors may include: Composition of tailings Climate Local land use Local topography Costs Environmental impacts Safety concerns TSF = “Tailings storage facility” (i.e., tailings dam) Figure from Spitz and Trudinger, 2009.
  • 15. Tailings disposal on surface – tailings dam styles or configurations Topographic conditions around the mine generally dictate the configuration of the tailings dams. Additional storage capacity can be obtained by filling depressions or valleys in the topography. 3 configurations of tailings dams used - Paddock (or ring-dyke): 4 dam walls needed - Hill-side: 3 dam walls needed - Cross-valley: 1 or 2 dam walls needed. Figure from Spitz and Trudinger, 2009.
  • 16. Tailings dams – construction: Tailings dams hold up to several hundred million cubic meters of water saturated tailings – they can be very, very large structures. The fundamental constructed elements of a tailings dam are: - Dam walls (dykes) to contain the tailings. These are normally constructed using waste rock and material available at the dam site. The maximum wall height is reported currently to be about 100 m. - Impermeable liners at the base of the dam to prevent leakage of fluids. Linings may consist of geomembranes (polyethylene or PVC), or clay layers, or a combination of the two. - Drainage ditches around the periphery of the tailings dam to collect seepage. - Under-drains to facilitate drainage and consolidation of the tailings in the dam. (Not all tailings dams have under-drains installed). Without under- drains, tailings dams can only dry-out by evaporation and seepage, which generally takes a long time (years after mining has ceased).
  • 17. Tailings dams – construction Tailings dam at Chatree Gold Mine (Thailand) shortly after commissioning, showing under-drains installed in a herring-bone pattern. Under-drains significantly improve water drainage from the tailings dam, thereby reducing water saturation of tailings sediments and improving geotechnical strength and safety of the dam. Figure from Spitz and Trudinger, 2009. Best practice tailings dam construction will consist of: (i) drains beneath the dam walls, (ii) double liners under the dam, with a leak detection system between layers, (iii) under-drains at the base of the tailings and a liquid recovery system.
  • 18. Tailings dams – construction Mature, but active, tailings dams located south of Johannesburg, South Africa. These dams are receiving the final tailings products of the reprocessing of numerous old mine- dumps spread around Johannesburg. The mines were closed in the 1960s. http://www.panoramio.com/photo/2399572
  • 19. Tailings dams – construction Dam walls are built up successively, from a Solid tailings become segregated in “starter dyke”, during the mine lifetime. Three the tailings dam, based on their methods of successive build-up are commonly grain-size and distance from the used. discharge point. Surface UPSTREAM METHOD Liner DOWNSTREAM METHOD Fine-grained Coarse-grained CENTRELINE sediments settle sediments settle METHOD further from the closest to the discharge point, discharge point, and are and are significantly less significantly more In the “upstream” method, note how much thinner the dams permeable permeable – they walls are, and how much less construction material is (porous). drain more easily. used. Also note that new embankment material overlies These sediments These sediments earlier tailings deposits, which may not have adequate have lower shear have higher shear strength to support the weight of the embankment, strength. strength. especially if water saturation levels in the tailings suddenly increase, or in the face of earthquake-induced tailings liquefaction. Figures from Lottermoser, 2007.
  • 20. Tailings dams – water balance Tailings dams remain wet during their entire operational life, and only start drying out after decommissioning. Contamination-plumes below tailings dams are normally much reduced compared to rock-dumps, due to the low porosity of tailings materials and the low permeability of the liner at the base of the tailings dams. Water extracted for re-use High potential for sulphide oxidation and from decant pond Precipitation of salts at acid development in area immediately edge of decant pool above saturated zone Beach UNSATURATED Hill-side ZONE SATURATED ZONE Drainage ditch Liner Water exchange below the Dam-wall may be saturated at its tailings dam depends on base, particularly if the decant permeability of the liner pond is too close to it – saturation weakens the strength of the wall Figure modified from Spitz and Trudinger, 2009.
  • 21. Tailings dams – failure: More than 50% of tailings dams worldwide are built using the upstream method, although it is well recognised that this construction method produces a structure which is highly susceptible to erosion and failure (Lottermoser, 2007) – less construction material is used, and the dam walls are thinner. Statistically, every 20th upstream tailings dam that is built, fails (a 5% failure rate), and there have been about 100 documented significant upstream tailings dam failures (Davies and Martin, 2000). Lottermoser (2007) catalogues 26 tailings dam failures that have occurred within the last twenty years, and 13 within the last 10 years. There are at least 138 known significant tailings dam failures to date. ( http://www.wise-uranium.org/mdaf.html; Spitz and Trudinger, 2009; UNEP, 2001) Most failures, whatever the construction method, have occurred in humid, temperate regions. There have been very few failures in semi-arid and arid regions.
  • 22. Tailings dams – failures 1909 to 2000, per decade Contemporary failure rate of tailings dams is much higher than water supply dams. Average failure rate for 1998 to 2008 was 1.3 failures per year. Low numbers of failures recorded in early years due to: (i) lower numbers of tailings dams and (ii) less complete records of failure from these years. Figure from Spitz and Trudinger, 2009 (Based on data from UNEP, 2001).
  • 23. Tailings dams and rock dumps - selected list of major failures Date Location Incident Release Impact Tailings dam failure during wall 17 people missing. Cyanide release to 2006 April 30 Miliang, China raise ? local river 3 950 000 m coal waste Contamination of 120 km of rivers and 2000 October 11 Inez, USA Tailings dam failure slurry streams. Fish kills Grasberg, Irian Jaya Waste rock dump failure after Unknown quantity heavy 4 people killed. Contamination of 2000 May 4 (West Papua) heavy rain metal bearing wastes streams 2,616 ha farmland and river basins flooded with tailings. 40 km of stream 3 Los Frailes, Collapse of dam due to foundation 4.5 million m of acid, contaminated with acid, metals and 1998 April 25 Aznalcóllar, Spain failure pyrite rich tailings metalloids 3 4.2 million m cyanide 80 km of local river declared 1995 August 19 Omai, Guyana Tailings dam failure bearing tailings environmental disaster zone Merriespruit, South 17 people killed. Extensive damage to 3 1994 February 22 Africa Dam wall breach after heavy rain 600 000 m town Olympic Dam, South Leakage of uranium tailings dam 3 1994 February 14 Australia into acquifer 5 million m ? Ok Tedi, Papua New Collapse of waste rock dump and 170 Mt waste rock and 4 1989 August 22 Guinea tailings dam Mt tailings Flow into local river Failure of fluorite tailings dam due 3 1985 July 19 Stava, Italy to inadequate decant construction 200 000 m 269 people killed. Two villages buried Embankment failure of platinum Bafokeng, Impala, tailings dam due to excessive 15 people killed. Tailings flow 45 km 3 1974 November 11 South Africa seepage 3 million m downstream Failure of coal refuse dam after 150 people killed. 1,500 homes 3 1972 February 26 Buffalo Creek, USA heavy rain 500 000 m destroyed Tailings move into underground 1970 September 25 Mufulira, Zambia workings 1 Mt 89 miners killed Aberfan, Great Liquefaction of coal refuse dam 1966 October 21 Britain after heavy rain ? 144 people killed Liquefaction of 2 tailings dams 250 people killed. Tailings traveled 12 1965 March 28 El Cobre, Chile during earthquake 2 Mt km downstream, destroyed El Cobre List selectively extracted from Lottermoser, 2007, with further information added from http://www.wise-uranium.org/mdaf.html
  • 24. Tailings dam failure – Stava, Italy, 19 July 1985 When a tailings dam breach occurs, some or all of the tailings migrate out of the impoundment and flow downstream. Obstructions in the path of the flow are either swamped or carried downstream. A disastrous dam failure and flow of tailings occurred in 1985 at Prestavel mine in Stava, Italy. The dam breached as a result of heavy rains which caused overtopping. The flow travelled down the valley through the town of Stava, killing 268 and destroying 62 buildings and 8 bridges. Stava before the breach. Stava covered by tailings as they www.wise-uranium.org/mdafst.html travel through the valley. www.wise-uranium.org/mdafst.html From TAILSAFE, 2004.
  • 25. Tailings dam failure – Los Frailes, Aznalcóllar, Spain, 25 April 1998 A tailings dam failed at Los Frailes mine in Aznalcóllar, Spain in 1998. The failure is thought to have occurred as a result of the marl foundations of the dam being eroded by the acid seepage from the tailings that passed through the embankment walls. The weakness in the foundations combined with the minimal length of beach (i.e., ponded water was encroaching the embankment) caused high stress in the foundations, thus resulting in the failure of the embankment material. In total, 4.6 million cubic meters of toxic tailings and effluent poured into the Río Agrio and Río Guadiamar Rivers. Note: marl is a clayey limestone and it dissolves in acid. Aerial photo of breached embankment. www.tailings.info From TAILSAFE, 2004.
  • 26. Tailings dam failure – Mufulira, Zambia, 25 September 1970 On the 25th September 1970 an underground breach of No. 3 tailings dam occurred at the Mufulira Mine in Zambia. As the night shift crew were on duty, the tailings dam above them collapsed causing nearly 1 million tons of tailings to fill the mine workings, killing 89 miners. A sinkhole opened on the surface allowing surface water to continue to pour into the workings. Two years prior to the disaster, sink holes opened up within the No.3 tailings pond due to roof collapse underground, and a surface depression developed in the impoundment. There were also two cases of minor mud ingress into the mine a few months before the main failure. Management were reluctant to accept and investigate the potential impact of future sink holes. Finally, a sink hole opened connecting the underground workings and the tailings in the impoundment. Aerial photo of the sinkhole in No. 3 dam. Sinkhole in No. 3 dam and processing plant. From www.tailings.info and TAILSAFE, 2004.
  • 27. Tailings dams – failure – causes: Poor choice of site, poor dam design, poor dam construction, or poor management Liquefaction of tailings and dam: Liquefaction describes the change in behaviour, from “solid” to liquid, of a liquid-saturated sedimentary unit in response to increased pore-fluid pressures (pores are the spaces between particles) – the solid particles literally loose contact with each other and the unit loses its physical cohesiveness. High pore-fluid pressures are induced by ground motions resulting from earthquakes (e.g., Veta de Agua, Chile, 3 March 1985), mine blasting, or nearby motion and vibrations of heavy equipment. Rapid increase in dam wall height: If an upstream dam is raised and the dam filled too quickly, very high internal pore pressures are produced in the tailings and dam walls, decreasing the dam stability and leading to dam failure (e.g., Tyrone, USA, 13 October 1980).
  • 28. Tailings dams – failure – causes: Foundation failure: If the base below the dam is too weak to support the weight of the dam, movement along a failure plane will occur (e.g., Los Frailes, Spain, 25 April 1998). Excessive water levels: Dam failure can occur if the top of the saturated zone in the tailings dam rises too high. Flood inflow, high rainfall, rapid melting of snow, and improper water management may cause excessive water levels. If “over-topping” of the embankment occurs, breaching, erosion, and complete failure of the dam walls are possible (e.g., Baia Mare, Romania, 30 January 2000). It is important to keep decant pond as small as possible and as far as possible from the containing embankments. Excessive seepage: Seepage within or beneath the dam causes erosion along the seepage flow path. Excessive seepage may result in failure of the embankment (e.g., Zlevoto, Yugoslavia, 1 March 1976).
  • 29. Tailings dams – failure – consequences: Release of huge volumes of tailings, that may enter underground workings, towns and villages or spill into waterways and travel downstream, polluting streams for considerable distances and covering large surface areas with thick, metal-rich mud, and causing significant environmental damage to impacted ecosystems. Significant loss of life.
  • 30. Thickened discharge and paste disposal Thickened tailings AIR-PHOTO PLAN VIEW are discharged from central “riser” and a series of outer risers to create a set of cone shaped impoundments. The “risers” are moved up incrementally as the layers of tailings material build up. Figure is greatly vertically exaggerated: the slope of 1 – 3° “beaches” is only 1 to 3° Figure from Spitz and Trudinger, 2009.
  • 31. Thickened discharge disposal – advantages and disadvantages: See e.g., Williams and Seddon, 1999; Brzezinski, 2001. Advantages over conventional tailings disposal are that: (i) The disposal site covers a much smaller surface area, (ii) tailings are not segregated into coarse and fine components, which improves the geotechnical properties of the pile, (iii) water consumption is significantly reduced, (iv) process chemicals are recovered with the water, rather than left with the tailings, (v) contaminated water drainage into the subsurface and surface water systems is reduced, (vi) The resulting cone shaped deposit provides an attractive landform (say the miners!), more amenable to rehabilitation. Disadvantages of the method include: (i) The operations are subject to dust generation, (ii) failure due to liquefaction is not ruled out entirely during the period required to dry the paste (McMahon et al., 1996).
  • 32. In-pit waste disposal: Tailings may be pumped into mined-out open pits (as well underground mine workings) for final disposal. Backfilling an open-pit eliminates the formation of an open-pit lake. Any backfill material placed below the water table will form part of the subsurface acquifer. The extent to which the water level inside the open pit equilibrates with the regional water table will depend on whether or not the open pit is lined with clay or other impermeable layer. Water-waste reactions may lead to the mobilisation of contaminants into ground waters. Backfilled open-pit showing return of the water table to pre-mining levels. Sulphidic tailings with high acid generating potential are Water placed at a depth below the saturated final level of the water table (to limit oxygen supply to the sulphides and hence minimise the risk of acid water development). Figure from Lottermoser, 2007.
  • 33. Riverine tailings disposal: Riverine tailings disposal is currently used in more than a few modern mining projects, e.g., the copper mines at: - Grasberg-Ertsberg, Indonesia - Porgera, Papua New Guinea - Ok Tedi, Papua New Guinea - Bougainville (closed), Papua New Guinea Riverine disposal is “preferred” in these areas because earthquakes, land-slides and very-high rainfall makes the construction of tailings dams geotechnically “impossible”. Miners argue that high natural sediment loads in rivers, generated by the high rainfall, is able to dilute the mine tailings discharges. (Nonsense – tailings volumes are huge compared to the natural sediment load). Tailings can be neutralised before disposal into the river systems (but they are not always). Historically riverine tailings disposal from mines was commonly practiced.
  • 34. Riverine tailings disposal – impacts: The solids and liquids of tailings are transported down rivers for considerable distances: tens to hundreds to thousands of kilometers. Sulphide minerals in discharged tailings generally oxidise in oxygenated river waters, creating the potential for acidification of waters. Problems include: - Significantly increased sedimentation and turbidity in the river system, and associated flooding of lowlands. - Contamination of the stream and floodplain sediments with metals, and associated impact on aquatic ecosystems. - Diebacks of rainforests and mangrove swamps.
  • 35. Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea: Ok Tedi open-pit mine is located at 1,600 m elevation in the Star Mountains, in a high rainfall, mudslide and earthquake prone region. The mine produces a copper-gold-silver concentrate for export, accounting for a significant proportion (about 16%) of PNG’s total annual export income (Enright, 1994; Murray et al., 2000). In 1976, the state of Papua New Guinea authorized BHP, Australia’s biggest mining corporation, to prepare a development plan for the mine. Four years later, the government committed to a partnership in Ok Tedi Mining Limited with a 20 percent shareholding. The other shareholders were BHP (the major shareholder), Amoco Minerals, and a consortium of German companies (World Resources Institute report http://archive.wri.org/page.cfm?id=1860&z=?, and references therein) Mine construction was authorised in August 1981, with production scheduled to begin May 1984. The Environmental Impact Assessment was only completed in June 1982, a year after construction started, at which time the decision not to mine was no longer an option (Townsend and Townsend, 2004).
  • 36. Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea: A tailings dam was constructed initially, but was swept away by a landslide just before production started in 1984. At that time, the PNG government controversially granted permission to the mine’s main shareholder and operator (BHP) to utilise riverine tailings disposal. Riverine disposal is thus allowed under, and is in compliance with, PNG laws and regulations. (Which does not necessarily make it environmentally or socially desirable though). Since 1986, tailings have been discharged, and waste rock dumps have been left to erode, into the headwaters of the Ok Tedi and Fly river systems, which subsequently drain, via the Strickland River and estuary, into the Gulf of Papua, over a total distance of over 1,000 km (Hettler et al., 1997).
  • 37. Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea The volume of tailings generated and deposited into the Ok Tedi and Fly rivers is enormous. The discharge rate amounts to about 160,000 tons of waste per day. About 1,400 million tons of waste is estimated to have been released into the tropical river system during the period 1984 – 2007. Ok Tedi gold and copper mine (Papua New Guinea) 5 June 1990 26 May 2004 Image source: “One Planet, Many People: Atlas of our Changing Environment”, UNEP, 2005.
  • 38. Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea: Impacts on the environment include: - Increased river turbidity. The small grain size (<100 μm diameter) and large quantity of waste has increased the sediment load to the middle Fly River by 5 – 10 times the normal load, impacting on aquatic life. - Increased sedimentation. The wastes are deposited everywhere along the river, all the way down to the Gulf of Papua, but particularly on the floodplains of the middle and lower Fly River. Large areas of tropical lowland rainforests and mangroves have also been covered with a thin veneer of waste. - Metal contamination of sediments. Deposited sediments are enriched in copper and gold, and contamination moves into the river waters themselves, with high potential toxicity to fish populations and communities living along the rivers.
  • 39. Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea: Social impacts: By 1989, river communities were struggling to produce enough food, and a social impact study in 1991 showed that environmental degradation was causing severe hardships to peoples living downstream from the mine. “This chronic build-up of waste has had a devastating effect on the 50,000 people who live in the 120 villages along the two rivers and depend on them for subsistence fishing and other river- based resources. Before the mine, taro and bananas were commonly grown in village gardens and riverside sago palms often provided the mainstay of local diets. But since the early 1990s, the build-up of sediment in the rivers and subsequent flooding of forests have dramatically altered the local environment. Fish stocks have fallen by 70–90 percent, animals have migrated, and about 1,300 square kilometers of vegetation have died or become blighted, forcing villagers to hunt and fish over larger distances (BHP report 1999: 9; Higgins 2002: 2). Copper concentrations in the water are about 30 times background levels, though the river still meets World Health Organization drinking water standards (BHP report 1999: 8–9)”. (World Resources Institute report: http://archive.wri.org/page.cfm? id=1860&z=?) A 2001 study showed that even if mining were to stop [then], the sheer volume of tailings already in the river, and continued erosion from the waste rock dumps adjacent to the mine, would see the problems grow worse over the next forty years.
  • 40. Riverine tailings disposal – case study – Ok Tedi, Papua New Guinea: “High-value” out of court compensation settlements have been made by BHP in favour of local communities affected by the mine (MAC report http://www.minesandcommunities.org/article.php?a=622 and World Resources Institute report http://archive.wri.org/page.cfm?id=1860&z=?). In August 1999, BHP announced that it regarded the mine as being incompatible with its environmental values. In February 2002, BHP withdrew from the mine. Their 52 percent equity share was transferred to an offshore trust, set up on behalf of the Papua New Guinea people. The PNG government gave BHP Billiton legal indemnity from responsibility for future mine-related damage to the Ok Tedi ecosystem (although the legality of this deal may still be challenged in the country’s courts). The mine is still currently operating, and although a limited dredging operation has been introduced, mine waste disposal into local rivers continues. Operations are scheduled to end in 2010.
  • 41. Submarine tailings disposal Coagulants and flocculants used to bind particles together to form a thicker mixture to prevent wide dissemination of The euphotic layer is defined as the tailings-plume underwater the depth reached by only 1% of photosynthetically active light (High density polyethylene) Greater than 50 m water depth Seafloor De-aeration and mixing with seawater to increase density Plume of lighter Final resting place of slurry tailings material of tailings on the sea-floor Figure from Spitz and Trudinger, 2009.
  • 42. Submarine tailings disposal (STD): STD is used in coastal settings where the earthquakes, land-slides and very-high rainfall (as for riverine disposal) make construction of tailings impoundments geotechnically unfeasible. The aims of STD are: - to place the tailings into a deep marine environment which has minimal oxygen concentrations – thereby avoiding sulphide oxidation and acid generation. - to prevent tailings from entering the shallow, biologically productive, oxygenated zone. Tailings are discharged at water depths of greater than 50 m, create a plume of material in the vicinity of the discharge point, and subsequently settle on the sea-floor. STD has a very damaging impact on seafloor ecosystems. There is high potential for metal uptake by fish and bottom dwelling organisms.
  • 43. Submarine tailings disposal – case study – Black Angel, Greenland Open adits in the footwall below the massive sulphide orebody. Cable-car entrances to mine The “angel” is a contorted pelite bed (metamorphosed mudstone), and not the orebody itself. View of the 700 m cliff face that overlooks Affarlikassaa Fjord at Black Angel Mine. Figure from Lottermoser, 2007.
  • 44. Submarine tailings disposal – case study – Black Angel, Greenland: Black Angel lead-zinc underground mine is located on the west coat of Greenland, about 500 km north of the Arctic Circle. The orebody was mined between 1973 and 1991, with a total production of 11 million tons of ore, consisting of sulphide minerals sphalerite and galena (and pyrite) (Asmund et al., 1994). The mine is located at the top of a 700 m cliff face above the junction of the 4-km-long Affarlikassaa Fjord and the 8-km-long Qaumarujuk Fjord. Waste rock was allowed to accumulate at the base of the cliff in a 0.4 million ton rock-dump at the shoreline of the Affarlikassaa Fjord. Mined ore was transported by cable-car across the fjord to an industrial area for processing using conventional selective flotation. Tailings were discharged directly into Affarlikassaa Fjord. The total amount of tailings discharged was about 8 million tons, containing elevated arsenic, cadmium, copper, lead, and zinc values (Poling and Ellis, 1995).
  • 45. Submarine tailings disposal – case study – Black Angel, Greenland: While tests prior to mining indicated elevated metal concentrations in seaweed and mussels due to the natural exposure of the orebodies to weathering and erosion, problems relating to the tailings disposal quickly emerged. Within a year of starting STD, distinctly elevated lead and zinc values were found in waters and biota of the entire fjord system. Extensive investigation at this stage indicated that (Poling and Ellis, 1995): (i) The assumption that all the metals in the tailings would be present only in insoluble sulphide minerals was incorrect – the tailings in fact contained minerals that could be dissolved in sea-water. (ii) The assumption that the discharged tailings would be permanently protected [from oxidation] by stagnant bottom waters in the fjord was incorrect – the disposal site in fact did not have a permanently layered water column, and complete mixing of the fjord waters [including the oxygenated upper layers] took place during winter.
  • 46. Submarine tailings disposal – case study – Black Angel, Greenland: Changes made subsequently to the processing and discharge methods: (i) Minerals processing changes reduced the lead content in the tailings from 0.4% Pb in 1973 to 0.18% Pb in 1989. (ii) Increasing the density of the tailings, by addition of seawater and coagulation and flocculation chemicals, helped reduce the extent of dispersion of metals away from the submarine deposition site (Asmund et al., 1994). These changes reduced, but did not eliminate, the elevated metal levels. The tailings discharge resulted in the metal enrichment of water, suspended particulate matter, sediment and biota in the Affarlikassaa and Qaumarujuk fjords up to 70 km away from the tailings outfall (Loring and Asmund, 1989; Elberling et al., 2002). While analyses of seals and fish species largely revealed no metal contamination during mining, deep sea prawns and capelins, as well as the livers of certain fish species and sea-birds contained lead concentrations above the safe consumption limit (Asmund et al., 1994).
  • 47. Submarine tailings disposal – case study – Black Angel, Greenland: Since mine closure in 1991, metal concentrations declined in fjord waters, as well as in animal and plant life (Asmund et al., 1994), but dispersion and release of metals from the tailings still continues (Elberling et al., 2002). In hindsight: “detailed mineralogic, leaching, and oceanographic studies, which are now conventional at proposed new mines, would have produced more detailed information on which to base the decision whether submarine tailings disposal (STD) was appropriate at this particular site” (Poling and Ellis, 1995).
  • 48. Black Angel Mine, Greenland: The case is not yet closed..... Press Release 28/05/2008: “A&R [Angus & Ross] (AIM: AGU.L), a zinc/lead mining company focused on re-opening the Black Angel Mine in Western Greenland, is pleased to announce that its wholly owned subsidiary, Black Angel Mining A/S, has been awarded a 30 year licence to mine zinc, lead and silver ore from the Black Angel Mine.” (http://www.angusandross.com/AR-NEW/news/PR-28-05- 08-Mining-license.htm) “Pillar mining will require strategically placed backfill. The pillar mining plan with the use of backfill [shown right] has been developed by Golders of Vancouver” “Phase One is expected to last for 4 years, during which time 1.3 million tonnes [of ore] is expected to be mined.” http://www.angusandross.com/AR-NEW/pages/proj-black-angel- phase1.htm A&R are currently refurbishing the mine for “Phase One” which will “concentrate on the development of infrastructure and extraction of the pillars from the old mine” and also the “production of 'dry concentrate' in the mine” (http://www.angusandross.com/AR-NEW/pages/proj-black-angel-phase1.htm)
  • 49. Black Angel Mine, Greenland: Production of “dry concentrate”..... “Processing of the mined ore was to take place in a mill in Europe according to the Bankable Feasibility Study (BFS). This possibility still exists, but the fall in metal prices since the completion of the BFS makes it less attractive than before. In this context our technical team is working on a solution to produce concentrate on site. The nature of the ore makes is suitable for 'dry concentration' e.g. by gravity concentration or optical ore sorting. Such concentrate could be shipped directly to a smelter thus significantly reducing shipping costs. (http://www.angusandross.com/AR-NEW/pages/proj-black-angel-phase1.htm) Press release 19/02/2009: “US specialist Wardrop Engineering, a Tetra Tech Company ("Wardrop"), and Canadian based SGS Minerals Services UK Limited ("SGS"), have been selected as the main nominated contractors for the development of the Black Angel Mine mineral processing and waste handling plant..... to be installed inside the Black Angel Mine”. “This will consist of a primary and secondary crushing circuit, pre-concentration by optical ore sorting, with milling and fine grinding feeding a conventional froth flotation plant. Premium grade Zinc (59-61% Zn) and Lead concentrates (69-71% Pb) will be produced. These will be shipped to the logistics hub at Maarmorilik for bonded product storage as part of the recently announced off-take agreement with Swiss metal trader MRI Trading AG”. (http://www.angusandross.com/AR-NEW/news/PR-19-02-09-tech-app-of-contractors.htm ) And of the fate of the large volumes of tailings that will be produced by milling and “conventional” froth flotation.... Not a word.
  • 50. Worldwide uranium mining and waste production: There are probably more than 500 million tons of uranium tailings located around the world (Waggitt, 1994). Uranium mine tailings are defined as “low-level” radioactive wastes, and their long term containment is a great environmental concern. World’s ten largest uranium mines in 1997. (From Hockley et al., 2000, using data from Uranium Institute).
  • 51. Radioactive wastes of uranium ores: The mineral processing of hard-rock uranium ores proceeds along the same route as typically used for sulphide or gold bearing ores. Either sulphuric acid or ammonium carbonate (alkali) leaches are used to dissolve the uranium-bearing oxide minerals from the mined ore rocks. The “pregnant” uranium-bearing leach solution is subsequently chemically processed to extract the uranium and produce yellowcake. Vat leaching. The ore processing may include crushing and grinding of ore rock followed by vat leaching – which will generate waste waters (both mill-water and process-water) and large volumes of tailings. Heap leaching. Alternatively, low-grade uranium ore may be processed in leach heaps, generating waste that consists largely of process-waters, with little or no tailings. Waste rock dumps, old leach heaps and tailings dams are all potential areas where dissolved uranium can be mobilised into surface and subsurface water systems.
  • 52. Radioactive wastes of uranium ores: While uranium oxide minerals form the basis of uranium ores (primarily uraninite, UO2), sulphide minerals are also ubiquitous in uranium orebodies. Particularly where pyrite and marcasite (FeS2) are present and exposed by mining, acid mine drainage may develop in workings and mine wastes. More detail on AMD follows in Topic 5. Thorium occurs together with uranium in uranium ore deposits. The mining of placer and mineral sand deposits for gold, diamond, sapphire, ruby, titanium (in ilmenite and rutile) and tin (in cassiterite) also accumulates gangue minerals that contain radioactive uranium and thorium (e.g., the minerals monazite, xenotime, zircon, tantalite, columbite). If accumulations of such gangue-mineral wastes are allowed to weather and break down, both uranium and thorium may enter surface and subsurface waters. Phosphate mining for both fertilisers and Rare Earth Elements (contained in the mineral monazite) may also generate uranium-bearing waste products.
  • 53. Uranium radioactive decay series Uranium-238 (92 protons, 146 Series starts with radioactive isotope neutrons) accounts for 99.28% of the Earth’s uranium. Critical U-238 decay products: Radium-226 Radon-222 (gas) Series ends with stable lead isotope Uranium-235 (92 protons, 143 neutrons) accounts for 0.71% of the Earth’s uranium. Its decay products are therefore negligible. Low abundances and very short half-lives with respect to radium (Ra) and radon gas (Rn) Thorium-232 (90 protons, 142 isotopes generated by neutrons) is the most abundant uranium-238 decay – radioactive thorium isotope. therefore negligible with respect to the impact of U-238. Table from Lottermoser, 2007, and references therein.
  • 54. Impacts of uranium and thorium radioactive decay: All three types of radiation (alpha, beta and gamma) from all parent and daughter radionuclides are extremely damaging to living organisms: (i) living cells and tissue are directly damaged, and (ii) water molecules in the organisms are damaged, releasing free radicals and chemicals that are toxic. Alpha particles are not deeply penetrating, and when external to the body, are stopped by the outer layer of skin. They are particularly damaging to internal organs when ingested or inhaled. Radium-226 (Ra-226). Is particularly of concern for several reasons: (i) With a half life of 1,622 years it persists in uranium mine wastes. (ii) Compared to uranium and thorium, Ra-226 is more easily liberated from minerals in uranium orebodies during natural weathering and mineral processing. It is also more soluble in water and therefore more mobile in the environment. (iii) Ra-226 behaves biologically similarly to calcium (Ca) and forms compounds that can be taken up by humans, plants and animals. (iv) Ra-226 has a high radiotoxicity and accumulates in bones. (v) It decays to a further problematic radioactive element – radon-222 gas.
  • 55. Impacts of uranium and thorium radioactive decay: Radon-222 (Rn-222). Radon is a colourless, tasteless and odourless gas, that is the most abundant isotope of radon. Although Rn-222 has a short half-life (3.8 days) and decays quickly, it occurs in abundance and is constantly replenished due to the abundance of its very long-lived “parent” U-238. Rn-222 is of concern for several reasons: (i) It is constantly replenished by U-238. (ii) It is soluble in water and therefore mobile within the environment. (iii) When Rn-222 is inhaled by humans its decay products are solid and become lodged in the lungs, and are themselves highly radiotoxic – polonium-218, lead-214, and bismuth-214 – emitting α, β and γ radiation and inducing lung cancer. Radioactive lead-210, near the end of the decay series, has a half-life of 22.5 years, so will remain resident in lungs for most of a person’s lifetime, emitting β radiation and generating further radioactive “progeny”.
  • 56. Radioactive wastes of uranium ores: While the hydrometallurgical processing of uranium ore is very selective and efficient in extracting uranium, not all of the uranium is extracted, and tailings will always contain small amounts of uranium. Moreover, most of the undesired (from an extractive point of view) and undesirable (environmentally) daughter radionuclides from the U-238 decay series end up in the tailings. As a result of the selective extraction, only 15% of the initial radioactivity of the orebody is transferred to the uranium yellowcake concentrate, while 75% of the radioactivity remains with the tailings (Landa, 1999; OECD, 1999; Abdelouas et al., 1999). Unlike acids which can (in principle) be neutralised, and free cyanide and cyanide complexes which can (in principle) be destroyed or will degrade naturally with time, radioactivity and radioactive elements cannot be destroyed. All one can hope to achieve in dealing with radioactive mining wastes is to immobilise the radioactive minerals, prevent dissolution of uranium and thorium from them, and isolate them from the environment safely and permanently (which is not easily achieved).
  • 57. Oxidation and dissolution of uranium wastes Atmospheric Rock dumps at Sherwood oxygen (O2) Water (H2O) Uranium Mine, Washington State, USA, before reclamation. The mine operated from 1976 Uraninite (UO2) + to 1985. Subsequent reclamation work sulphuric acid (H2SO4) completed in June 2000. Photo August 1985. Uranyl sulphate (UO2SO4) dissolved in water http://ecorestoration.montana .edu/mineland/histories/miner als/sherwood/default.htm# 2 UO2 + 2 H2SO4 + O2 2 UO2SO4 + 2 H2O Uraninite Sulphuric acid Oxygen Uranyl sulphate Water (solid) (dissolved) (gas) (dissolved) (liquid) Note: the sulphuric acid is generated by oxidation of coexisting sulphide minerals (acid mine drainage).
  • 58. Oxidation and dissolution of uranium wastes: Uraninite (UO2) in uranium ores can be broken down by the process of oxidation when exposed at the surface in rock dumps or tailings dams. The resulting oxidised uranium compounds are highly soluble in water, highly mobile and easily dispersed in surface or subsurface drainage systems for significant distances away from the mine site. Uranium oxidation-dissolution can occur in both acidic and alkaline waters, given the presence of an oxidising agent (atmospheric oxygen) to trigger the process. Acid conditions particularly favour the dissolution of uranium. As sulphide minerals are also ubiquitous in uranium orebodies, acid conditions are very commonly generated through sulphide oxidation (see Topic 5). Oxidised uranium mineral forms that are found dissolved in water, or precipitated as salts adjacent to surface water, are highly toxic and include uranyl sulphate UO2SO4 (yellowcake!) and uranium sulphate U(SO4)2 (under acidic conditions) and uranyl carbonate complexes UO2(CO3)n (under alkaline conditions).