The Basics of
Acid Mine Drainage
By
Andy Robertson and Shannon Shaw
Disclaimer
• These slides have been selected from a set used as the basis of a
series of lectures on Acid Mine Drainage presented in 2006 at
the University of British Columbia, Vancouver, BC.
• No attempt is made here to provide linking text or other verbal
explanations.
• If you know about Acid Mine Drainage, these slides may be of
interest or fill in a gap or two—going back to basics never hurts
the expert.
• If you know nothing of Acid Mine Drainage, these slide may be
incomprehensible, but on the other hand they may be an easy
way to ease into a tough topic—good luck.
Overview of ARD
Metal Sulphide + Water + Oxygen => Acid + Metal
[M]S + H2O + O2 => H2SO4 + [M(OH)x]
(not stoichiometrically balanced)
Acid + Alkali => “Salt” + Carbon Dioxide
H2SO4 + CaCO3 => CaSO4 + CO2
• Environmental Impact from:
• Acidity
• Metals in solution (in acid or alkaline environments)
• Salinity
• Sludge precipitates
Bacterial Catalization of Oxidation
Temperature Effects on Oxidation
Buffering of ARD during Oxidation of
a Mineral Assemblage
Buffering of ARD during Oxidation of
a Mineral Assemblage
Buffering of Mineral A
(e.g. calcite, dolomite)
Buffering of Mineral B
(e.g. ankerite, siderite)
pH
Buffering of Mineral C
(e.g. Al(OH)3)
Buffering of Mineral D
(e.g. feldspars)
Time
Mechanisms Controlling ARD in
Tailings
Precipitation
Surface Discharge
Tailings
Oxidation Zone
Neutralization Zone
Dam
Process Water
Seepage
Mechanisms Controlling ARD in
Waste Rock
Precipitation
Oxygen Diffusion
Sulfide Waste Rock
Advective Air
Transport
Infiltration
Surface Runoff
Basal
Drainage
Seepage
Collection
Ditch
Mechanisms Controlling ARD in Open
Pits
Precipitation
Surface Water Runoff
Pre-Mining
Groundwater
Table
Infiltration
ARD Seepage
Post-Mining
Groundwater
Table
Residual Sulphide
Rock Debris
Residual
Sulphides
ARD Seepage
Ore Body
ARD Seepage
Groundwater Flow
Through Rockmass
Mechanisms Controlling ARD in
Underground Workings
Precipitation
Backfill Alternatives
Open Pit
Water
Flow
A
GloryHole
B
Sulphide
Exposure
Infiltration
Mine
Workings
Open Stope
Rockfill
Residual
Sulphide Exposures
(see inset backfill
alternatives)
ARD
Mine
Workings
ARD
Ore Body
C
Tailings
(uncemented)
D
Tailings
(cemented)
Pre-Mining
Groundwater
Table
Post-Mining
Groundwater
Table
Sulphide Minerals
Pyrite (FeS2)
Marcasite (FeS2 )
Galena (PbS)
Arsenopyrite (FeAsS)
Pyrrhotite (Fe(1-x)Sx)
Chalcopyrite (CuFeS2)
Sphalerite (ZnS)
Bornite (Cu5FeS4)
Alkali Minerals
• Types
– Carbonates
• Calcite (CaCO3)
• Dolomite (Ca,Mg(CO3)2)
– Hydroxides
• Fe(OH)3
• Al(OH)3
– Silicates
– Clays
Development of ARD
• Water chemistry depends on:
– Rate and extent of oxidation
– Rate and extent of metal release
– Quantity of material
– Contained metals
– Site hydrology and
climate
– Accumulation of
oxidation products
– pH/solubility controls,
flowpath reactions
– Control technology
Site Characterization
• Design
• Field investigation & Sampling
• Lab testing
New Mines vs. Existing Mines
• New Mines
• ARD probably not evident
• Objective is to determine ARD potential
• Fresh samples used for testing and prediction
• Long term behavior based on kinetic testing, modeling and
prediction
• Existing and Abandoned Mines
• ARD may be evident/mature
• Field reconnaissance used to define ARD
• Historic data (time trends) extremely useful
• Limited laboratory testing required
• Field instrumentation and monitoring possible
• Background altered, requires simulation
Design
• Review existing data, e.g:
– Geology & mine plan
– Drill core logs
– Water quality monitoring results
– Assays on ore/waste rock and tailings
– Waste type volumes
– Waste placement history
 Develop reconnaissance & sampling plan
Field Investigations
• Objectives
– Detect early signs of ARD
– Determine potential for ARD
– Assess factors that control ARD
– Evaluate control measures
– Determine environmental impact
– Assess compliance with regulatory standards
Field Investigations
• What to bring:
– Eyes that know what to look for
– pH and conductivity meters
– Acid bottle, hydrogen peroxide,
sulfate kit
– Geological pick, hand lens,
sampling bags, camera, GPS unit
– Site map, history, data
2.2
Field Investigations
• Things to look for:
– Visible pyrite or other sulfides (oxidation) & calcite
– Red, orange, yellow, white, blue staining (precipitates, water)
–
–
–
–
Dead vegetation or bare ground
Melting snow or steaming vents on waste
Dead fish or other biota
Low pH in seeps, groundwater, decants & streams
Field Investigations
• Things to log in the field:
–
–
–
–
–
–
–
–
–
Paste pH
Paste conductivity
‘Colour’
Lithology
Sulfide content
Secondary mineralogy
Degree of ‘fizz’
Moisture content
Grain size
Field Investigations
• General Methodology
– Visual observation of site
– Paste pH and water quality data
– Field extraction testing
– Classify types of wastes
– Solids sampling (for lab testing)
Field Investigations
• Geochemistry:
– Low paste pH of mine wastes
– High conductivity of waste paste
– Contaminants in leach extraction tests
– Static (ABA) tests
• Products from Reconnaissance:
– Physical disturbance and drainage map
– Waste deposit map and characterization
– Exposed rock map and characterization
– Paste pH and conductivity survey
– Observations and sampling map
– ARD site assessment report
TDS vs pH
Field Paste pH vs. Field Paste TDS
Field Paste TDS
2200
2000
Dike samples
1800
Leach Pad Samples
1600
Pit Samples
1400
Waste Rock Samples
1200
1000
800
600
400
200
0
0.0
2.0
4.0
6.0
8.0
Field Paste pH
10.0
12.0
14.0
Sample Selection (New Mines)
• Step 1: On geological sections:
– Define rock types
– Define sulfide and alkali mineral distribution
– Preliminary rock units classification
• Step 2: Sample each rock unit class allowing for:
– Area distribution of class
– Variability of rock
• Step 3: Perform static lab tests and use results to refine rock unit
classification
• Step 4: Sample each new rock class and repeat Step 3 until satisfied.
• Step 5: Sample each rock class for appropriate kinetic testing and use
results to refine rock classification
• Step 6: Repeat Step 5 until satisfied with classifications and
characterization.
Sampling (existing mines)
• Steps:
– Define geology, mineralization,
waste ‘types’ etc.
– Define objectives (i.e. sampling
for reveg, cover, water quality
evaluations etc. may have
different focus)
– Consider mine plan and waste
placement history
– Identify sources of samples
– Initial sampling and testing
program
– Further sampling if necessary
Sampling
(Existing Mines)
• A Becker hammer-type drill rig
can be used in order to minimize
sample crushing and the
geochemical disturbance of the
samples
• Samples typically collected at
specified intervals (e.g. every 10
ft) & paste pH and EC measured,
• A sub-set of samples can then be
selected using observations and
field measurements as a guide for
more detailed laboratory testing
Test Methods
• Static ARD Tests
– balance between potentially acid generating and consuming
– tool for waste management
– includes geological/mineralogical
characterization
– individual samples
• Short-term Leaching Tests
– readily soluble component
• Kinetic Tests
– oxidation and metal leaching rates
– water chemistry prediction
Geochemical Static Tests
• Objective:
Potentially Acid Generating Minerals
vs
Acid Neutralizing Minerals
• Cautions for ARD assessment:
– pH of alkalinity (NP) determination
– Assumes instant availability of NP
– Assumes all sulphur/sulphide
minerals reactive
– Ignores reaction rates (kinetics)
– Extrapolation to field
Geochemical Static Tests
• Procedures
• Paste pH and conductivity on the ‘as received’ fines
• Acid-Base Accounting Tests
• Net Acid Generation (NAG) - also an accelerated kinetic test
• B.C. Research Initial Test
• Lapakko Neutralization Potential Test
• H2O2 Oxidation (modified for siderite correction)
• Net Carbonate Value (NCV) for ABA Tests
• Leach extraction analyses
• Forward acid titration tests
• Multi-element ICP analyses
Detailed procedures can be found on: www.enviromine.com and in
prediction course on www.edumine.com
Geochemical Static Tests
Definitions:
AP
= acid potential
= % S x 31.25
NP
= neutralization potential
NNP
= net neutralization potential
= NP - AP
NP:AP ratio
= NP/AP
All expressed as:
kg CaCO3 equivalent/tonne, or
CaCO3 eq./1000 tonnes
Example:
S=2%
AP = 62.5 kgCaCO3/t
NP = 90 kgCaCO3/t
NNP = 27.5 kgCaCO3/t
NP/AP = 1.4:1
Note: units and acronyms used are different in Australiasia, local references should be
sought for correct usage, terminology, guidelines etc.
Interpretation
20
NP (kg CaCO3/t equiv)
Non-acid
generating
1:1 ratio
3:1 ratio
15
Start with
‘guidelines” or
general criteria
for classification,
then develop
site- specific
criteria
Uncertain acid
generating potential
Potentially acid
generating
10
5
12
Potentially acid
generating
10
0
5
AP (kg CaCO3/t equiv)
Typically criteria are
based on a ‘set’ of
tests, not just one
type of test e.g. ABA
& NAG results
10
Paste pH
0
15
8
Uncertain acid
generating potential
Non-acid
generating
20
6
4
2
0
-50
-30
-10
10
30
Net Neutralisation Potential (NP-AP) (kg CaCO3/t equiv)
50
NAG Test
• Developed in Australia as an alternative and/or compliment to ABA test,
• Developed as a “one-off” test that can assess the net acid generation potential
–both acid generation and acid neutralization – in one test.
• NAG test varies among users, typically:
– Adding 250 mL of 15% H2O2 at room temp to 2.5 g of sample
pulverized to pass 200 mesh.
– React for 12 h then boiled until visible reaction ceases (or Cu catalyst
added) or initial reaction period is extended to 24 h
– Measure pH of the reacted solution (NAGpH)
– Titrate reacted solution with NaOH to a specified pH end-point (pH 4.5
and/or pH 7) to determine the NAG value of the sample.
Interpretation
• There are numbers of modifications to the test for different scenarios,
including:
– Sequential addition NAG test (multiple additions of H2O2)
– Kinetic NAG test (track pH, temperature and EC during test)
– Modifications to account for organic matter effects (analyze for organic
acids and sulphuric acid in reacted solution, extended boiling step).
– Modifications to leach carbonates prior to NAG test (i.e. measure of
acidity not net acidity).
• NAG results are generally interpreted as such:
– If the final NAGpH is > 4.5, sample said to be non-acid forming
– If the final NAGpH is < 4.5, the sample is said to be
potentially acid forming
– The NAG value then provides a quantitative assessment of potential acid
formation in units of kg CaCO3/t equivalent (or kg H2SO4/t equivalent)
Applications of the NAG test
• In conjunction with ABA tests etc to reduce the risk of misclassification
• As an operational scale management tool (e.g. for segregation
of different material types)
• For identifying material for prioritization (e.g. AML ranking)
• As an indicator test that can be run on greater number of
samples than if using other methods due to the fact it is quick,
simple and inexpensive
• Used very widely in Australasia
Some potential pitfalls
• Organic matter, Cu, Pb and MnO2 can catalyze decomposition
of H2O2. Samples high in these parameters can have
unpredictable results (O’Shay et al., 1990)
• Samples with a lot of Zn can be buffered between pH of ~ 4 to 5
by the formation of Zn(OH)2 (Jennings et al., 1999)
• NAG test can underestimate potential acidity if samples have
(Amira, 2002):
– Sulphide content > ~1%
– High carbonate content
– High organic content
• Not as ‘conservative’ as ABA testing
Example – Ok Tedi
11
10
9
8
7
6
NAGpH
Unce rtain
NAF
5
4
Unce rtain
PAF
3
2
1
-300
-200
-100
Dredge Site
0
NAPP kgH2SO4/t
Floodplain
100
River Sediment
[Rumble et al. 2003 ICARD proceedings]
200
300
Example – Ok Tedi
• Single addition NAG test showed the dredged material was
NAF – but river bars showed elevated SO4 and metals and
slightly depressed pH
• Sequential NAG test consistently showed a drop in the NAGpH
of the material below 4.5 after additional H2O2 additions
• perhaps due to
presence of Cu or
higher S content
[Pile et al. 2003 ICARD proceedings]
Short-term Extraction Tests
• Objective
• Determine readily soluble load
• Determine acid soluble load
• Procedure
• Uncrushed sample including fines
• Agitate in deionised water or mild acid
• Filter and analyse filtrate
Sample Wt.
Vol.
pH
Cond.
[SO4]
%
[Cu]
mg/L
SO4
mg/L
% Cu
(g)
(mL)
1
100
200
5.5
68
300
10
2
5
2
100
200
2.5
150
848
95
14
80
* Always account for dilution in concentration assessments
Kinetic Testing
• Objectives
– Validation of static test results and boundaries
– Determination of leaching behaviour
– Simulation of site conditions
– Evaluation of extent of oxidation
– Evaluation of stored products
– Prediction of drainage water quality
– Produces raw data for modeling
– Investigate factors controlling ARD
– Selection of control measures
Kinetic Testing
Humidity Cells
• Objective
– Predict lag to, and rate of, acid generation
– Semi-qualitative water quality prediction*
• Advantages
– Widely used in North America in the past
– Simple to operate
– Appropriate for fine samples, disseminated
mineralization
• Disadvantages
– Crushed sample - does not address surface area,
mineralogy
– Not representative of waste rock
– High flushing rate, saturation, pH & solute modification
* Always account for dilution in concentration assessments
Columns
• Objective
– Evaluate kinetics of oxidation & leaching for waste rock
– Data to predict drainage water quality
• Advantages
– Representative of rock pile size distribution
– Development of local pH environments
– Evaluate storage/flushing
– Evaluate control options
– Estimate production rates
• Disadvantages
– Size of sample required
– Interpretation of data
– Edge effects
– High flush rates
– Laboratory conditions of temp and oxygen availability
Kinetic Testing Data
Field Test Plots
• Objective
– Evaluate leach kinetics & drainage water quality in field
conditions
• Advantages
– Representative of site conditions
– Calibration of water quality prediction
– Test control options on a realistic scale
– Already exist?
• Disadvantages
– Limited control of test conditions
– Time required
– Expensive for new installations
– Maintenance and damage
– Interpretation of results
Field Test Plots
Field Test Plots
Field Test Plots
Field Barrel Tests
ARD Model
Sulphide Oxidation
Acid Neutralization
Mineral Dissolution
ARD Model
(pore-water)
Secondary Mineral
Precipitation
‘Scale-up’ to
Field Conditions
Metal Attenuation
Ion Exchange
Dynamic Systems
Sulfate Generation Over Time
Water Quality
Examples:
• Highly Acid Generating Rock Seepage
– pH <2.5, SO4 > 4000 mg/L
– High Cu (>5 mg/L), Zn (>3 mg/L), Fe (10’s mg/L), Al (>10’s mg/L)
• Moderately Acid Generating Rock Seepage
– pH 3.5-5.5, SO4 2000-4000 mg/L
– Moderate Cu (0.5-5 mg/L), Zn (0.3-3 mg/L), Fe (0.3-10mg/L), Al (0.110mg/L)
• Neutral pH/Metal Leaching Rock Seepage
– pH 5.5-7.5, SO4 ~ 2000 mg/L
– Moderate Zn (>0.3 mg/L), +/- As, Cd, Ni
– Low Cu (<0.5 mg/L), Fe (<0.3 mg/L), Al (<0.1 mg/L)
• Buffered/Low Metal Leaching Rock Seepage
– pH 7-8, SO4 <2000 mg/L
– Negligible Cu, Zn, Fe, Al etc
•
Note: in arid climates evapo-concentration can drastically change any of these water
types, salinity can become an issue in particular for revegetation purposes
Chemical-Physical Interactions
• The time dependant change in geotechnical characteristics
of a rock results from:
– Physical Weathering - e.g. sheeting due to unloading;
thermal expansion and contraction, abrasion, salt and
ice crystal growth; slaking due to clay mineral
expansion and contraction during wetting and drying
– Chemical Weathering - e.g. oxidation; hydrolysis;
dissolution; diffusion; precipitation
• These weathering processes may result in an increase or a
decrease in rock strength, and an increase or decrease in
permeability. Most commonly a decrease in shear strength
and permeability occur.
Pre-mining Alteration
• The natural geothermal processes that are associated with
sulphide ore genesis alter alumino-silicate minerals in the
rock mass.
• Sericite-clay and chlorite-epidote altered zones
surrounding such ore bodies often exhibit reduced strength
properties and an increased propensity to slake when
exposed to air and water.
• Additional alteration occurs as a consequence of exposure
of the mineral deposits to air and water and the resulting
oxidation of pyrite and further hydrolysis of the aluminosilicates.
Mineral Alteration
• Under non-acidic conditions, primary minerals like feldspars
weather to form clay and amorphous hydroxide minerals, such as
kaolinite and gibbsite
• Under acidic and sulphate-rich conditions, produced by pyrite
oxidation, alumino-silicates weather far more rapidly. Aluminum
is highly soluble under these conditions.
• Acid leaching is concentrated on weak zones such as fractures in
rock particles and mineral cleavages causing a breakdown of the
rock fabric.
• When this occurs over natural sulphide bodies it results in the
production of gossan or oxide zones, often with high percentages
of clays, including smectite clays.
Consequence of Mining Pyritic Rock
• Mining of altered and acid-generating sulphide containing
waste rock increases, by several orders of magnitude, the
surface area of rock surface exposed to air and water
resulting in hugely increased rates of slaking (physical
weathering) as well as geochemical weathering.
• Hydrolysis, fragmentation and breakdown of the rock
fabric, results in an increase in the percentage of fines,
including clays.
• This in turn results in changes in both the permeability and
shear strength of the mine rock
Oxidation Products Mass Balance
• 1% by weight of sulfide sulfur can produce:
• 3.2% by weight of sulfuric acid and this can hydrolyze
• 4.3% by weight of Feldspar to jarosite and clay.
• The sulfur in rock containing 5% by weight sulfide sulfur can
hydrolyze up to 430 lbs/ton of mine rock.
Surface Enrichment
Reacted
Zone
Partially
Reacted
Sulfides
Unreacted
Sulfides
Secondary
Alteration
at High T
Sulfide ore
fragment showing
reaction zone,
shrinking unreacted
core and expanding
rim (reacted zone).
After Bartlett, 1998.
Air
O2
Oxidation
Products
Trickle
Leaching
Film
A
A’
Reacted Zone
Ore fragment after
extensive chemical
weathering along
fissures due to
internally generated
acid from pyrite
oxidation After
Bartlett, 1998.
Unreacted Core
Weathering along
fractures and fissures
Diffuse Reaction Zone
“The rock leaching kinetics are complicated by changing microporosity, pH,
solution concentrations of several species, and chemical weathering and
disintegration of the rocks by the generated sulfuric acid.”
Additional Observations From Dump
Leaching
• The average rock particle size, and permeability to both
percolating leach solutions and airflow, tends to decrease
with extended leaching time.
• This is a major factor preventing adequate aeration and
continued economic leaching as the mine dumps age.
• Basic igneous host rocks are generally less resistant to acid
weathering and disintegration than more siliceous rocks
• Ores that contain clay, or minerals that weather to clay,
rapidly lose permeability
H2 O O2
O2
H2 O O2
O2
Oxidized
Zone
H2SO4
Oxidized Zone
& Dissolved Zone
Oxidation &
Dissolution Fronts
Oxidation Front
H2SO4
Dissolved
Zone
Dissolution Front
Reduced Zone,
Not Dissolved
High elevations in humid regions
Reduced Zone,
Not Dissolved
Valley bottoms, Cut slopes, Cavern
walls, Ground under house floor
Flux of oxygen and water carrying sulfuric acid
After: Chigira and Oyama, Engineering Geology (1999).
0
Surface oxidized zone
10
20
30
40
Oxidized zone
Dissolved zone
Dissolution transition zone
Fresh Rock
50
After: Chigira and Oyama, Engineering Geology (1999).
Geological engineering aspects of the weathering of sedimentary rocks
Observations From Natural Slopes
• In addition to the general mechanical properties, a remarkable
strength loss at the dissolution front, and the increase of
smectite at the oxidation front of mudstone, could lead to the
generation of landslides. Indeed, landslides with sliding
surfaces along or beneath the oxidation front are quite common
in mudstone areas. ----- these rocks weather very rapidly if the
environment is artificially changed.
After: Chigira and Oyama, Engineering Geology (1999).
Natural oxidation and weathering scars
Natural oxidation and weathering scar slopes
Debris flows from natural oxidation and weathering scar slopes
Successive debris flows from natural oxidation and weathered slopes
Example of ARD conditions in a Waste Rock Pile
)S m( dnoC ets aP
)%( tn etnoC erutsioM
51
01
5
0
000,01
Hp ets aP
000,1
01
8
6
4
2
0
02
02
02
04
04
04
06
06
06
08
08
001
001
001
021
021
021
041
041
041
)tf( htp eD
08
)tf( htp eD
0
)tf( htp eD
0