The Management & Treatment of Acid and
Metalliferous Drainage
Dr Jeff Taylor
Introduction: AMD Sources
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Acid and Metalliferous Drainage or AMD is the most long lived and damaging water
quality issue facing the mining industry worldwide.
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AMD impacts on profitability, reputation, environment, future projects.
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AMD derived from waste rock, tailings, pit wallrock, underground voids, heap leach
pads, ore stockpiles, concentrate stockpiles, slag, etc.
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Commonly assumed that tailings is the highest AMD risk material, due to:
– High sulfide content.
– Fine particle size.
– Large tonnages.
– Unsaturated deposits.
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HOWEVER, Waste rock is primary acidity producer on essentially all mine sites (normally
60-80 wt.% of acid and metal loads).
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AMD can appear as acid and metalliferous, neutral and metalliferous or simply saline.
AMD long lived post closure – mining started 3000 BC
Waste Rock AMD Sources
Waste Rock AMD Sources
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Angle of repose pathways for air and water.
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Exacerbates air-entry and AMD generation in sulfidic waste rock.
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Facilitates AMD discharge from sulfidic waste rock piles.
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Conventional approach is to install a cover system at closure to
block air and water pathways.
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Thin cover systems (alone) cannot be expected to effectively limit
oxygen entry and acidic and metalliferous discharge.
Management measures: AMD from waste rock piles
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Geochemical characterisation (static and kinetic testwork).
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Waste rock classification (PAF, NAF, AC, NMD, SD).
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Waste segregation / selective handling.
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Carbonate blending.
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Waste encapsulation in air-entry control cells.
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Oxygen consuming layers / cells.
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Subaqueous disposal / water covers.
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Avoid end-dumping: BULC dump construction (see below).
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Alkalinity generating cover systems.
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Waste rock + tailings co-disposal ± saturation.
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Net percolation control – “store and release covers”.
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Passive treatment or active treatment – only if there are no other options.
Management measures: BULC waste rock dump construction
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More emphasis needed on strategic waste rock pile construction, with
less reliance on thin soil cover systems to manage AMD risks postclosure.
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“Base-Up Layered and Compacted” (BULC) waste rock dump aims to:
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Avoid or minimise AMD generation.
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Supersede the need for AMD treatment.
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Facilitate successful closure and relinquishment across the mining sector.
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“Base-Up” – avoids preferential flow pathways.
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“Layered” thin-lift configuration – also avoids preferential flow pathways
and facilitates compaction.
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“Compacted” – lowers air and water entry, AMD generation and water
discharge.
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Various aspects of the BULC method demonstrated at sites in Australia,
NZ and SE Asia.
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In line with Federal Government Leading Practice Handbook “Preventing
AMD”.
Management Measures: BULC waste rock dump construction
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Avoids preferential flow pathways.
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Lowers air entry.
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Lowers AMD generation.
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Lowers water entry / discharge.
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Enhances carbonate neutralisation reactions.
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Facilitates silicate neutralisation reactions.
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Facilitates compaction which improves geotechnical stability.
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Multiple, stringent air entry control layers can be
incorporated.
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Water addition (for compaction) can further lower air entry.
Challenges – AMD Management – Dump Construction
Iron Mountain – California, USA
(20–40 tonnes H2SO4/day)
Berkeley Pit – Montana, USA
(30–50 tonnes H2SO4/day)
Bingham Canyon – Utah, USA
(~20 tonnes H2SO4/day)
Mt Lyell, Australia
(80 tonnes H2SO4/day)
Mt Morgan, Australia
(20 tonnes H2SO4/day)
Brukunga, Australia
(2 tonnes H2SO4/day)
AMD Treatment Options
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Passive treatment:
– Multiple systems involving organic carbon ± limestone.
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Active (chemical) treatment (in-situ and fixed):
– Fe, Al, Mn, Cu, Pb, Zn, Cd, Co, Ni, Cr, Hg, As, Sb, Se, Ag, Mo, Tl, Be, U.
– Hydrated lime neutralisation.
– High density sludge.
– Low density sludge.
– Sulfide precipitation.
– Aeration / oxidation.
– Chemical reduction.
– Adsorption / flocculation – filtration.
In Situ Treatment
In Situ Treatment
– In Situ Treatment
DayOpportunities
1, April 4
Day 2, April 5
1400
DayOpportunities
118, July 31 – In Situ Treatment
1200
Hydrated lime, tonnes
1000
800
600
400
200
0
0
20
40
60
80
100
Days of treatment
120
In-situ
treatment Options
AMD Treatment
Conclusions
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AMD can generate enduring legacies – none good!!
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Relinquishment of mining leases is very difficult when AMD persists.
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Focus on appropriate mine planning and operational controls can dramatically
retard or prevent AMD.
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Waste rock materials generate the majority of water pollution from minesites (ie.
acid, metals, salinity). Much more than tailings.
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New waste rock construction methods provide substantial reductions in acidity
discharges from waste rock. Oxygen control is the key.
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Passive treatment systems can assist with acidity loads <150 kg H2SO4/day.
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For larger acidity loads, there are a variety of in-situ treatment methods can
lower the cost of pollution control during operations and post closure.
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Fixed plant treatment systems can address essentially AMD treatment issues.
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Treatment in perpetuity needs to be avoided at all costs.