Title: Martin Fahey
1Engineering with Tailings
Some Environmental Aspects of Tailings Management
Keynote Lecture4th International Congress on
Environmental Geotechnics Rio de Janeiro,
Brazil, 13 August 2002
Department of Civil Resource EngineeringThe
University of Western Australia
2Acknowledgements
- Co-Authors
- Dr Tim Newson Department of Civil Engineering,
University of Dundee, UK(formerly Research
Fellow, The University of Western Australia) - Dr Yoshimasa Fujiyasu Department of Civil and
Environmental Engineering, University of South
Carolina (formerly PhD student, The University
of Western Australia - Some people who provided slides or other
information - Hugh Jones (Golder Associates)
- David Cooling (Alcoa)
- Ian Piggott (Collie Coal)
- John Carras (CSIRO)
3There are many arts and sciences of which a
miner should not be ignorant De Re
MetallicaGeorgius Agricola (1556) (Translated
by Herbert Hoover, mining engineer, later
President of the United States, 1929-1933)
4Vale of Avoca, Ireland Idyllic Beauty Spot
Avonmore Avonbeg meet to form the Avoca
River The Meeting of the Waters There is not
in this wide world a valley so sweetAs that vale
in whose bosom the bright waters meet Thomas
Moore, 1779 - 1852
5Reality Acid Drainage since 18th Century
- Mining for copper at Avoca since 2nd century
- Revived in about 1720
- A thriving salmon fishery downstream of the mine
was wiped out shortly afterwards by acid drainage - This stretch of the Avoca River has been dead
ever since, and is one of the most polluted in
Ireland
6Tailings Storage Facilities (TSF)
- The design life of a TSF is, effectively,
perpetuity. A TSF could be considered to have two
phases to its life a depositional phase with
active human involvement, followed by an erosion
free, environmentally benign, stage with no
further human intervention, forever - Guidelines on the Safe Design and Operating
Standards for Tailings Storages. Department of
Minerals and Energy, Western Australia
7Requirements for TSFs
- The stability of the tailings and the retaining
structure must be guaranteed in the long-term
forever. - In the long-term, there must be no escape of
tailings to the environment through wind erosion
or water erosion. - Any seepage or water release that occurs from the
tailings must not contain any deleterious
products (acid, cyanide, heavy metals, etc). - The TSF must be rehabilitated to whatever level
is required this will vary with location and
local regulation.
8Requirements for TSFs
- The stability of the tailings and the retaining
structure must be guaranteed in the long-term
forever. - In the long-term, there must be no escape of
tailings to the environment through wind erosion
or water erosion. - Any seepage or water release that occurs from the
tailings must not contain any deleterious
products (acid, cyanide, heavy metals, etc). - The TSF must be rehabilitated to whatever level
is required this will vary with location and
local regulation.
9Stability is easily guaranteed
Then why so many failures?
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1222 February 1994, the Merriespruit (South Africa)
tailings dam failed with catastrophic results 17
people were killed and many houses were
devastated as a result of the extensive mudflow
13Golden Cross Mine, New Zealand
14Overtopping - prime cause of failure
50 m high tailings storage filled to the brim
(Hopes Hill, Western Australia, 1994)
freeboard 0 A good example of worst practice
15Baia Mare, Romania Overtopping
30 January 2000 almost 100,000 m3 tailings water
with high concentration of cyanide was spilled
into the Zazar and Lápos water courses that
belong to the catchment area of river Szames,
which flows into the river Tisza.
16Baia Mare Cyanide concentrations
Cyanide quickly breaks down on exposure to
atmosphere, and hence does not persist in
tailings
17Fish kills, Baia Mare disaster, Hungary
- Cyanide quickly breaks down on exposure to
atmosphere, and hence does not persist in
tailings (hmmmph!) - In this case, environmental conditions
(temperature, ice on water etc) contributed to
persistence, resulting in fish kills many km
downstream
18Los Frailes, Aznacollar, Southern Spain
19Requirements for TSF
- The stability of the tailings and the retaining
structure must be guaranteed in the long-term
forever. - In the long-term, there must be no escape of
tailings to the environment through wind erosion
or water erosion. - Any seepage or water release that occurs from the
tailings must not contain any deleterious
products (acid, cyanide, heavy metals, etc). - The TSF must be rehabilitated to whatever level
is required this will vary with location and
local regulation.
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26Requirements for TSF
- The stability of the tailings and the retaining
structure must be guaranteed in the long-term
forever. - In the long-term, there must be no escape of
tailings to the environment through wind erosion
or water erosion. - Any seepage or water release that occurs from the
tailings must not contain any deleterious
products (acid, cyanide, heavy metals, etc). - The TSF must be rehabilitated to whatever level
is required this will vary with location and
local regulation.
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28nor for next 400 years!
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31Below-ground leakage of pollutants
TSF
Ground surface
Pervious soil
Groundwater
flow
Impervious Aquitard
32Requirements for TSF
- The stability of the tailings and the retaining
structure must be guaranteed in the long-term
forever. - In the long-term, there must be no escape of
tailings to the environment through wind erosion
or water erosion. - Any seepage or water release that occurs from the
tailings must not contain any deleterious
products (acid, cyanide, heavy metals, etc). - The TSF must be rehabilitated to whatever level
is required this will vary with location and
local regulation.
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34Engineering with Tailings
- TSF engineers are building structures that have
design life of hundreds (thousands?) of years - Two approaches
- Engineering the environment to store the tailings
as received from the mill - accept what you are given, and make the most of
it - Engineering the tailings to produce the most
favourable outcome - try to modify/improve what you are given
Hugh Jones (Golder Associates) coined the term
designer tailings to indicate the requirement
to engineer the tailings to suit the long-term
performance of the TSF
35Some issues
- Design for stability
- the upstream method of construction ???
- Acid drainage prevention and remediation
- Modification of tailings by thickening
- Reduction of tailings hazard by changing
chemistry - Dust suppression by tailings modification
36Upstream Construction Method
37Upstream Construction
38Upstream Method of Construction
- According to the WISE (World Information Service
on Energy) websitethe upstream technique is no
longer regarded an acceptable option for tailings
dam construction in Chile, where several
liquefaction-type failures have occurred with
tailings dams located in steep mountain valleys
in areas of high seismic activity (Supreme
Decree 86 Regulation on the Construction and
Operation of Tailings Dams, 1970). - Question Is upstream method worth the cost??
- very low initial cost, but potentially very high
long-term cost
39Discussion on Stability Issues
- Stability in seismically-active areas discussed
by Professor Ramon Verdugo - Static liquefaction of tailings dealt with by
Professor Andy Fourie
40Some issues
- Design for stability
- the upstream method of construction ???
- Acid drainage prevention and remediation
- Modification of tailings by thickening
- Reduction of tailings hazard by changing
chemistry - Dust suppression by tailings modification
41Acid Drainage from Tailings
- Production of Sulphuric Acid when sulphur-bearing
minerals (e.g. pyrite) are exposed to oxygen and
water - Pyrite is often associated with coal and
metal-ore deposits - Oxidation of pyrite is complex, involving
chemical, biological and electrochemical
reactions, and hence varies with environmental
conditions - Acid drainage frequently contains Fe, Al, SO4 and
heavy metals such as Cu, Pb, Hg, Cd, etc - Of a total of about 7 billion tonnes of
metal-mine and industrial minerals tailings in
Canada, it is estimated that 1.9 billion tonnes
is acid generating
- Parker, G. Robertson, A. (1999). Acid Drainage.
Australian Minerals Energy Environment
Foundation, Occasional Paper No. 11
42Metal Precipitates, Mt Lyall, Tasmania
43Engineering to Prevent Acid Generation
(After Marszalek, 1996)
44Sub-aqueous Disposal to prevent AMD
Extensive work carried out under the MEND program
in Canada on subaqueous tailngs disposal to
prevent AMD
45Remediation of Acidic Waters from AMD
www.wvu.edu/agexten/landrec/passtrt/passtrt. htm
46Anoxic Limestone Drain (ALD)
Problems Consumption of the limestone.
Precipitation of metals/sludge on limestone
results in armouring of the limestone,
preventing further reactions (anoxic conditions
meant to prevent this)
47Wetlands for AMD remediation
- Wetlands (natural or man-made) useful for final
polishing of effluent from anoxic limestone
drain (or other) neutralisation system
May have a number of stages
48Reactive Wall (U. of Waterloo)
Benner et al (1999) For a reactive wall they
installed at the Nickel Rim mine site near
Sudbury, Ontario, the reactive component
consisted of 20 municipal compost, 20 leaf
compost and 10 wood chips, mixed with 50 pea
gravel
49Reactive Wall (U. of Waterloo)
50Muja Open Cut Coal Mine, Collie, WA
Perth
Muja Open Cut Coal Mine Part of the Collie
Basin Only coal-mining area in WA. Owned by
Griffin Energy Supplies coal directly to Muja
Power Station
Kwinana
Mandurah
200 km
WA
200 km
Indian Ocean
Waroona
Muja
Bunbury
Collie
51Muja Coal, WA Chicken Creek wetlands
overflow weir
weir
area 4 lake
passive organic treatment
picnicarea
52Muja Coal Chicken Creek wetlands
53Fluidised limestone bed (FLB) system
- pH too low (3.0) for wetlands
- Fluidised Limestone Bed (FLB) reactor used first
- Acidic water (pH 3.0) pumped into base of
container - Limestone fluidised by upward flow
- tumblingand abrasion of grains together
prevents buildup of armouring - Reasonable quality limestone required
- not so friable as to disintegrate in fluidisation
process
f 1 m
Out
Fine limestone gravel
3 m
Low-pH water
54FLB at Muja Part of two-stage process
Settling pond - after 1st stage, heavy metals
precipitated
55Performance of FLB at Muja
- 45 litre/s (4000 m3/day)
- Stage 1 pH increases from 2.0 to 4.5
- Settling pond - for precipitation of some heavy
metals - Stage 2 pH increases from 4.5 to 5.8
- Discharge to wetlands for final polishing
- Running cost about 100 per week
- Limestone 10 per tonne
- Main cost is for transport of the limestone to
the site - (Note performance results by personal
communication - not yet confirmed)
56Some issues
- Design for stability
- the upstream method of construction ???
- Acid drainage prevention and remediation
- Modification of tailings by thickening
- Reduction of tailings hazard by changing
chemistry - Dust suppression by tailings modification
57Engineering the Tailings Thickening
- Many of the problems associated with tailings
storages revolve around the high water volumes
pumped out with the tailings - Thickening involves removing some of this water
prior to deposition in the TSF - slight dewatering - thickened tailings
- greater dewatering - paste
- extensive dewatering - tailings cake
- Properly engineered, thickening can produce
significant benefits - up front costs more than compensated for by lower
long-term costs
58Some Benefits of Thickening
- Enhanced water recovery (less water available for
evaporation and seepage loss in the TSF) - water
may be costly in arid areas. - If the tailings are thickened sufficiently,
segregation may not occur on the tailings beach
this is important for the central thickened
discharge method of tailings disposal. - The higher deposited solids content means that
consolidation and evaporation outcomes are
enhanced. - Dusting may be less of a problem (if there is
sufficient clay content to act as a binder for
the silt sized and fine sand sized fractions, and
segregation does not occur). - Having less water on the TSF will often have
benefits for stability of the walls, and also for
reduced base seepage. - Thickening is usually required for tailings to be
used for underground fill.
59Engineering Required
- Thickener design evolving rapidly
- mechanical design, flocculants
- now a specialist industry
- Pumping characteristics must be understood
- the rheology of the thickened tailings must be
fully understood - does material show shear thinning or shear
thickening behaviour? - Transport method critical (centrifugal pumps,
positive displacement pumps, conveyor belt,
trucking?) - The flow behaviour on tailings beach (or
underground) must be undersood (rheology)
60Alcoas WA Operating Locations
Perth
Booragoon
Mandurah
Dwellingup
Indian Ocean
Waroona
Alcoas WA Operations supply around 15 of the
world market. Very high volumes of red mud
tailings - very high pH - potential major
environmental problem
Collie
61Schematic of Alcoas Dry Stacking Process
FLOCCULENT
CYCLONES
SUN
CLOUD
EVAPORATION
DECANT
SPRINKLERS
RUNOFF
DRY DISPOSAL AREA
UNDER DRAINAGE
62Kwinana Residue Storage Areas
63Alcoas Pinjarra Refinery Thickener
64Measuring Red Mud Slurry Properties
Vane Shear Testing
Slump Testing (slumps at different solids)
65Rheology of Red Mud
Initial
Sheared
- Red mud behaviour is well described by the
Herschel-Buckley model ???y?k?n - n and k do not vary significantly in the
range of concentrations Alcoa normally deal with - The yield stress is the parameter most indicative
of the shear history dependence.
Shear Stress ? D?P/4L (Pa)
Initial
Sheared
50Solids
47Solids
Shear Rate ? 8V/D (sec-1)
66Rheology of Red Mud
47 Superthickener Underflow Effect of mixing
time
100
Shear Stress ? D?P/4L (Pa)
initial state
180 min
1500 min
4080 min
8040 min
12660 min
15450 min
10
1
10
100
1000
Shear Rate ? 8V/D (sec-1)
67Final strength gain from evaporation
- Remaining alumina forms surface crust due to
drying - reduces drying efficiency dramatically
68Mud Farming Ploughing to enhance drying
69Use of an Amphirol to aid ploughing
70Alcoas Perceived Benefits of Dry Stacking
- The height of the deposit can be increased
economically - Less land is required
- Significantly reduced risks to the ground water
- Safety hazards are significantly reduced
- Deposit can be readily accessed to rehabilitate
- Overall reduction in costs
71Some issues
- Design for stability
- the upstream method of construction ???
- Acid drainage prevention and remediation
- Modification of tailings by thickening
- Reduction of tailings hazard by changing
chemistry - Dust suppression by tailings modification
72Alcoa Carbonation of Red Mud
- High pH (gt13) of red mud is major part of the
long-term hazard (caustic soda NaOH used in
refining process) - Carbonation involves the addition of CO2 to
thickened residue slurry - NaOH is converted to carbonate
- alumina is precipitated as dawsonite
- CO2 reaction with the solids (conversion of
residual lime to calcite and conversion of TCA to
calcite and gibbsite) - The resulting liquor associated with the residue
- reduction in pH
- reduction total dissolved solids
- reduction in trace metal concentrations (As, V,
Ga, Se)
73Carbonation prototype plant at Kwinana
CO2 storage and vaporiser units
CO2 mixing tanks
74Trial drying beds, Kwinana refinery
Untreated
Carbonated
75Drainage water quality monitoring
76Drying bed monitoring Shear Strength
Strength Target (20 kPa)
77Comparison of drying rates
78Overall Benefits of Carbonation
- Reduced risk to clay and synthetic seals
- reduced potential for ground water contamination
- Improved quality of run-off and drainage water
- reduced pH, Al and trace metal concentrations in
leachate - Reduced Drying Area Costs
- reduction in
- drying area
- potential for dust
- reliance on mechanical ploughing equipment
- Avoid future classification as a Hazardous Waste
- Basel convention has adopted hazardous criterion
of pH 11 - EU considering imposing classification and taxing
system - Greenhouse benefit
- use of residue as a sink for CO2
79Neutralisation Combine waste streams
- Many mine wastes can generate acid (previous
discussion) - Alumina refining produces high-pH residue (red
mud - What about combining the two waste streams to
neutralise the two? - The Bauxsol technology (a proprietary process
owned by Virotec International Ltd) - understood to involve using processed red mud
to neutralise acid drainage or acidic tailings
waters (http//www.virotec.com/)
80Producing Bauxsol
According to Virotech, the processing required to
produce Bauxsol involves pH neutralisation of
the red mud without reducing its acid
neutralising capacity. After this is done,
reaction conditions are created to form brucite,
calcite, aragonite. The new carbonate,
hydroxide and hydroxycarbonate minerals that are
formed preserve the acid neutralising capacity of
the solids. Much of the sodium originally present
in the red mud is released during this treatment
and stabilisation process
81Applying Bauxsol to tailings water
82Spraying Bauxsol, Mt Carrington Toe Dam
83Results, Mt Carrington
84Results, Mt Carrington
85Challenges
- Modify chemistry of tailings to reduce or
eliminate the hazards due to toxicity - pH modification prior to disposal (for acidic or
basic tailings) - combining tailings streams to provide in-built
buffering capacity against acid generation - cyanide removal ?
- Think laterally to consider if waste streams from
different industries can be combined to
reduce/eliminate the hazards associated with one
or both
86Some issues
- Design for stability
- the upstream method of construction ???
- Acid drainage prevention and remediation
- Modification of tailings by thickening
- Reduction of tailings hazard by changing
chemistry - Dust suppression by tailings modification
87Dust from Tailings Dams
88Dust generation from tailings
- Tailings have often very narrow grading - often
fine sand and silt sizes - very susceptible to
dusting once they are dry - Tailings dust can contain harmful materials
- heavy metals, arsenic, radioactivity, asbestos.
- Dust blowing from tailings dams is a very
powerful means of generating community concern,
anger, opposition. - Short term as well as long term issue.
89Dust suppression
- During operations
- watering (e.g. Alcoas sprinkler systems)
- keep submerged (not always possible or desirable)
- Permanent
- capping with topsoil, re-vegetation
- expensive
- in arid or semi-arid climates, difficult to
maintain vegetation cover in the long term - may need capillary breaks to isolate cover from
tailings - spraying with synthetic membrane of some sort
- permanence questionable
- Engineering the tailings prior to deposition
90Sprinklers for dust suppression at Alcoa
91Surface re-vegetation (KCGM, Kalgoorile)
Trials using different thicknesses of waste rock
cover as capillary barrier over high salt
content tailings Surface rehabilitation of this
type is very expensive!
92Tailings modification for dust suppression
- Tailings with some clay content (say gt 10)
appear less susceptible to dusting - e.g. in WA, in many open-pit gold-mining
operations, the upper levels are often oxide ore,
with significant clay content in the tailings - schedule mining, or stockpile some oxide ore, so
that the last layer of tailings (1 m or so?) has
clay content - thickening may be required to prevent segregation
of this layer, otherwise only central area of the
TSF will contain a clayey cap. - Tailings with high salt content appear to be
resistant to dusting - in WA, high-salinity groundwater used for gold
processing - salt causes many problems - but prevents dust
93Salt crusting efficient dust suppression
94Micro Wind Tunnel (MWT)
- MWT
- Developed by CSIRO Australia
- Tests the resistance of tailings surfaces to
dusting
95Results of MWT Testing
- Results expressed as Dust Yield Potential (DYP)
- Freshwater tailings surface
- DYP 32.9 g/m2/sec
- Hypersaline tailings surface (4 months to 2 years
after completion of deposition) - DYP 0 to 1.2 g/m2/sec
96Additives to prevent dusting ?
- Addition of saline water to final tailings layer
not suggested as general solution to dusting - causes problems of reduced evaporation, toxic to
plants etc - But this evidence suggests that additives to
final layer might be a viable solution to dusting
problem - Challenge to find suitable cheap non-toxic
additive(s) to produce durable non-dusting final
layer
97Requirements for Engineering
- All of the engineering with tailings examples
presented require detailed characterisation of
the tailings materials (preferably well before
mining starts in earnest) - mineralogy, grading, rheology, chemistry,
acid-generating potential (acid buffering
potential) - Past practice has often neglected much of this
characterisation work - Even the most basic tailings storage scheme
(pouring slurried tailings into impoundment)
requires detailed geotechnical testing,
understanding, and modelling of basic processes
98Modelling Tailings Deposition
- 2nd half of paper deals with some of the detailed
aspects of tailings deposition - sedimentation and beaching in sub-aerial
deposition - consolidation under self-weight
- effect of base drainage
- effect of evaporation
- factors affecting evaporation (salinity salt
crusting) - numerical modelling of filling / consolidation /
evaporation behaviour - Illustrates the attention to detail required in
even this limited aspect - Same attention to detail is required in all other
areas
99Conclusions
- There is a history of long-term problems from
tailings storages, often lasting long after
mining activity has ceased. - Tailings storages worldwide continue to
experience full scale failures, escape of
material by wind (dust) and water erosion, acid
drainage, cyanide escape, etc. - Tailings management must be given at least as
much attention as any other part of the mining
process right from the start of the project.
100Conclusions (Cont.)
Quote from the report of the tribunal set up to
investigate the causes of the Aberfan disaster in
South Wales in 1966, where children attending the
local school were killed by failure of a coal
mine waste tip
As we shall hereafter seek to make clear, our
strong and unanimous view is that the Aberfan
disaster could and should have been prevented.
But the Report that follows tells not of
wickedness, but of ignorance, ineptitude and a
failure in communication.
- Ignorance on the part of those charged at all
levels with the siting, control and daily
management of tips
- bungling ineptitude on the part of those who had
the duty of supervising and directing them, and
- failure on the part of those having knowledge of
the factors that affect tip safety to communicate
that knowledge and see that it was applied.
101Conclusions (Cont.)
- There have been many advances since 1967 in
knowledge of how to manage tailings. - Much of this knowledge is readily available (much
of the material for this paper came from the
internet). - many useful guides, codes of best practice, case
studies, are available - Much still to be done to improve our ability to
engineer tailings to achieve benign long-term
outcomes. - Improvement is not an optional luxury - the
future of the mining industry requires it.
102There are many arts and sciences of which a
miner should not be ignorant De Re
MetallicaGeorgius Agricola (1556)