Topic 4: Mine wastes

Topic 4: Mine wastes

From a series of 5 lectures onMetals, minerals, mining and (some of) its problems

prepared for London Mining Networkby

Mark Muller [email protected]

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 methodsWater balance in tailings damsTailings 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 risksRelease of radioactive minerals into the environment by oxidationImpact of release of radioactive minerals

Figure from Spitz and Trudinger, 2009.

Mineral extraction: from mining to metal

Mining

Mineralconcentrate

Metal

METAL EXTRACTION

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 Underground mining

Mineral processingHydrometallurgy

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

Produces processed solid wastes that includes tailings and 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.

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

Rock dumpsTop-down storage: waste rock is dumped over an 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.

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.

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.

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

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

SURFACEDUMP

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.

Figure from Lottermoser, 2007.

Tailings solids. Solids are very fine grained.

GRAIN SIZES OF SOLIDS

Tailings disposal methods


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

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

Factors may include:Composition of tailingsClimateLocal land useLocal topographyCostsEnvironmental impactsSafety concerns

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.

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.


(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.

Best practice tailings dam construction will consist of:


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

http://www.panoramio.com/photo/2399572


Figures from Lottermoser, 2007.

Dam walls are built up successively, from a “starter dyke”, during the mine lifetime. Three methods of successive build-up are commonly used.

In the “upstream” method, note how much thinner the dams walls are, and how much less construction material is used. Also note that new embankment material overlies earlier tailings deposits, which may not have adequate strength to support the weight of the embankment, especially if water saturation levels in the tailings suddenly increase, or in the face of earthquake-induced tailings liquefaction.

UPSTREAM METHOD

DOWNSTREAM METHOD

CENTRELINE METHOD

Surface

Liner

Solid tailings become segregated in the tailings dam, based on their grain-size and distance from the discharge point.

Fine-grained sediments settle further from the discharge point, and are significantly less permeable (porous).

These sediments have lower shear strength.

Coarse-grained sediments settle closest to the discharge point, and are significantly more permeable – they drain more easily. These sediments have higher shear strength.

Tailings dams – water balance

Figure modified from Spitz and Trudinger, 2009.

Drainage ditch

Liner

Hill-side

SATURATED ZONE

UNSATURATED ZONE

High potential for sulphide oxidation and acid development in area immediately above saturated zone

Dam-wall may be saturated at its base, particularly if the decant pond is too close to it – saturation weakens the strength of the wall

Water extracted for re-use from decant pond

Water exchange below the tailings dam depends on permeability of the liner

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.

Precipitation of salts at edge of decant pool

Beach

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.

http://www.wise-uranium.org/mdaf.html

Tailings dams – failures 1909 to 2000, per decade

Figure from Spitz and Trudinger, 2009 (Based on data from UNEP, 2001).

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.

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.

Tailings dams and rock dumps - selected list of major failures

List selectively extracted from Lottermoser, 2007, with further information added from http://www.wise-uranium.org/mdaf.html

Date Location Incident Release Impact

2006 April 30 Miliang, ChinaTailings dam failure during wall raise ?

17 people missing. Cyanide release to local river

2000 October 11 Inez, USA Tailings dam failure950 000 m3 coal waste slurry

Contamination of 120 km of rivers and streams. Fish kills

2000 May 4Grasberg, Irian Jaya (West Papua)

Waste rock dump failure after heavy rain

Unknown quantity heavy metal bearing wastes

4 people killed. Contamination of streams

1998 April 25Los Frailes, Aznalcóllar, Spain

Collapse of dam due to foundation failure

4.5 million m3 of acid, pyrite rich tailings

2,616 ha farmland and river basins flooded with tailings. 40 km of stream contaminated with acid, metals and metalloids

1995 August 19 Omai, Guyana Tailings dam failure4.2 million m3 cyanide bearing tailings

80 km of local river declared environmental disaster zone

1994 February 22Merriespruit, South Africa Dam wall breach after heavy rain 600 000 m3

17 people killed. Extensive damage to town

1994 February 14Olympic Dam, South Australia

Leakage of uranium tailings dam into acquifer 5 million m3 ?

1989 August 22Ok Tedi, Papua New Guinea

Collapse of waste rock dump and tailings dam

170 Mt waste rock and 4 Mt tailings Flow into local river

1985 July 19 Stava, Italy Failure of fluorite tailings dam due to inadequate decant construction 200 000 m3 269 people killed. Two villages buried

1974 November 11Bafokeng, Impala, South Africa

Embankment failure of platinum tailings dam due to excessive seepage 3 million m3

15 people killed. Tailings flow 45 km downstream

1972 February 26 Buffalo Creek, USA Failure of coal refuse dam after heavy rain 500 000 m3

150 people killed. 1,500 homes destroyed

1970 September 25 Mufulira, ZambiaTailings move into underground workings 1 Mt 89 miners killed

1966 October 21Aberfan, Great Britain

Liquefaction of coal refuse dam after heavy rain ? 144 people killed

1965 March 28 El Cobre, ChileLiquefaction of 2 tailings dams during earthquake 2 Mt

250 people killed. Tailings traveled 12 km downstream, destroyed El Cobre

http://www.wise-uranium.org/mdaf.html

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 covered by tailings as they travel through the valley. www.wise-uranium.org/mdafst.html

Tailings dam failure – Stava, Italy, 19 July 1985

Stava before the breach.www.wise-uranium.org/mdafst.html

From TAILSAFE, 2004.

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.

Tailings dam failure – Los Frailes, Aznalcóllar, Spain, 25 April 1998

Aerial photo of breached embankment. www.tailings.info

From TAILSAFE, 2004.

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.

Tailings dam failure – Mufulira, Zambia, 25 September 1970

From www.tailings.info and TAILSAFE, 2004.

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

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

PLAN VIEWAIR-PHOTOThickened tailings 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 “beaches” is only 1 to 3

1 – 3

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 placed at a depth below the final level of the water table (to limit oxygen supply to the sulphides and hence minimise the risk of acid water development).


Water saturated

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).


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

Image source: “One Planet, Many People: Atlas of our Changing Environment”, UNEP, 2005.

Ok Tedi gold and copper mine (Papua New Guinea)

5 June 1990 26 May 2004

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.


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.


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.


“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


Greater than 50 m water depth

De-aeration and mixing with seawater to increase density of slurry

Coagulants and flocculants used to bind particles together to form a thicker mixture to prevent wide dissemination of the tailings-plume underwater

Plume of lighter tailings material

Final resting place of tailings on the sea-floor

Seafloor

The euphotic layer is defined as the depth reached by only 1% of photosynthetically active light

(High density polyethylene)

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


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

Cable-car entrances to mine

Open adits in the footwall below the massive sulphide orebody.

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

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).


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.


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).


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)

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)

“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

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.


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

Table from Lottermoser, 2007, and references therein.

Series ends with stable lead isotope

Series starts with radioactive isotope

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

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) isotopes generated by uranium-238 decay – therefore negligible with respect to the impact of U-238.

Critical U-238 decay products:Radium-226Radon-222 (gas)

Thorium-232 (90 protons, 142 neutrons) is the most abundant radioactive thorium isotope.

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”.


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

Water (H2O)Atmospheric oxygen (O2)

Uraninite (UO2) + sulphuric acid (H2SO4)

Uranyl sulphate (UO2SO4) dissolved in water

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

http://ecorestoration.montana.edu/mineland/histories/minerals/sherwood/default.htm#

Rock dumps at Sherwood Uranium Mine, Washington State, USA, before reclamation. The mine operated from 1976 to 1985. Subsequent reclamation work completed in June 2000. Photo August 1985.

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).

Topic 4: Mine wastes

Education

Transcript of Topic 4: Mine wastes