Acid Mine Drainage
11.11.2015
H-ESD : Environmental and Sustainable
Development
Michael Staudt, GTK
Table of contents
Acid Mine Drainage
• Excercise
• Steps of the excercise
• Equations
Managing Sulphidic
Mine Wastes and
Acid Drainage
Acid Drainage
Caused by the oxidation of sulphide
minerals, especially iron sulphides,
associated with mining
Oxidation produces sulphate ion
which when dissolved in water
forms sulphuric acid
Acid Drainage
Some effects:
Acid drainage affects water
quality downstream
Rehabilitation becomes more
difficult
Metal ions are released
Acid Drainage
Acid drainage is one of the most significant
environmental issues facing the mining
industry.
Canadian liability estimated as C$ 2-5
billion
Australian liability estimated as A$
60M/year
in the USA 20,000 km of streams and
rivers adversely affected
Longevity of the Problem
• Acid drainage may not develop immediately
• Acid drainage can continue for tens to
thousands of years
Rio Tinto region, Spain; for more than
2000 years
 Many examples more than 50 years
with little reduction in rate of acidic
drainage
What is Acid Drainage?
• Oxidation of sulphidic minerals, especially in connection
with mining
– Exposure to air and water
– Increase in surface area
– Reactive minerals
• Pyrite (iron sulphide) most common sulphide
mineral associated with mines
• Other iron and other metal sulphides
• Drainage of acid away from its source
FeS2 + 3.75 O2 + 3.5 H2O = Fe(OH)3 + 2 SO42- + 4 H+
(Iron sulphide + Oxygen + Water = Ferric Hydroxide + Aqueous sulphuric acid)
Factors Influencing Acid Drainage
•
•
•
•
•
•
•
Water (required for oxidation and transport)
Oxygen availability
Physical characteristics of the material
Temperature, pH
Ferric (Fe+3)/ferrous (Fe+2) ion equilibrium
Microbiological activity
Presence of neutralising minerals
– Carbonates are most effective
– Silicates & aluminosilicates may contribute
• Chemistry of receiving waters
Impacts of Acid Drainage
• Potential for reuse of water on mine is
limited
– corrosion problems for equipment
• Toxic effects to aquatic ecosystems
– acidity and dissolved metals
• Toxic effects on downstream vegetation
• Adverse impacts on ground water
• Limits uses of downstream water
– Irrigation, stock watering, recreation,
fishing
• Causes difficulties in revegetation and
stabilising mine wastes
Best Practice Approach
• During feasibility stages:
– Characterise acid
generating potential of
materials
– Characterise mobility
of potential
contaminants such as
heavy metals
– Estimate the potential
for oxidation products
to migrate to the
environment
– Estimate effects on
host environment
Identifying and Predicting Acid Drainage
• When characterising rock types at site important
characteristics include:
– Geological description
– Mineralogy of both ore and waste
– Fracturing
• Sampling and analysis:
– Acid-base accounting
– Simulated oxidation, usually with hydrogen
peroxide
– pH and conductivity tests of paste or slurry
– Total and soluble metal analysis
– Geochemical Kinetic Tests
• Humidity cells
• Column Leach Tests
Acid Drainage Control Strategies
• Control requires:
– Data on physical and chemical properties of
materials
– Risk assessment
– Strategies to minimise oxidation
• Control strategies
– Containment and isolation
– Treatment of acid drainage
Soil Covers
• Materials
– Imported materials e.g. clay, soil
– Low-sulphide waste rock, if compactable
– Geotextile fabrics
– Covers may require zones
• Base (main sealing) layer - high water retention, low
permeability
• Middle layer - water reservoir (may have higher permeability)
• Surface layer (barrier zone) - erosion protection and/or
substrate for plant growth
Isolation
Revegetated and c ontoured c over material
(surface c apping and water storage m edium )
Top non-sulphidic waste layer
Free
dumped
non-sulphidic
waste
Sulphid ic waste
Basal layer
Original ground surfac e
15
BEST PRACTICE ENVIRONMENTAL
MANAGEMENT IN MINING
Free
dumped
non-sulphid ic
waste
Water Covers
• Most readily used in high
rainfall, low evaporation
areas
• Creation of a permanent
lake or swamp
• Use of an existing lake or
the sea
• Flooding of underground
tunnels and pits
Blending
• Mixing of acid and
non-acid forming
waste rock
• Incorporation of
alkaline materials
•Lime
•Fly ash
•Kiln dust
Bacterial Inhibition
Bacteria can catalyse sulphide oxidation
Applying bactericides can slow the process
Effect may be short-term only
Some success claimed in USA coal industry
Used in establishing a vegetation cover before
acid production starts
Treatment Systems
• Collection of acid drainage followed by neutralisation
– Passive Anoxic Limestone Drains (PALID)
• Drainage passed through a channel of coarse limestone
gravel in the absence of oxygen
– Successive Alkalinity Producing Systems (SAPS)
• Variation on PALID
– Wetland treatment systems
• Newer treatments, moving from experimental to operational
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Bioreactors
KAD (kaolin amorphous derivative)
Bauxite derivatives
‘Green rust’ precipitation
Passive Treatment Systems
Cross section through an anoxic limestone drain
19
BEST PRACTICE ENVIRONMENTAL
MANAGEMENT IN MINING
Treatment Systems
Conceptual design of a
wetland system for treating
Acid Mine Drainage
20
BEST PRACTICE ENVIRONMENTAL
MANAGEMENT IN MINING
Monitoring
An essential component of sulphidic waste management
• Classification of materials
• Point source monitoring
• Monitoring surface water and ground water in both up- and
down-stream gradients
• Monitoring of effectiveness of control measures
Monitoring
Rock materials:
Waters:
•Static and kinetic
geochemical tests
•Water flux through
stockpiles
•Physical stability:
cracking, erosion
•pH, conductivity, SO4-2
•Other major ions (Ca+2, Mg+2,
Al+3, Na+, K+)
•Alkalinity
•Metals/metalloids (Fe, Al, As,
Cd, Cu, Zn, Mn, Pb)
•Toxicity to organisms