the basics of acid mine drainage by andy robertson and shannon shaw
TRANSCRIPT
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The Basics ofAcid Mine Drainage
ByAndy Robertson and Shannon Shaw
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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.
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Overview of ARD
Metal Sulphide + Water + Oxygen => Acid + Metal[M]S + H2O + O2 => H2SO4 + [M(OH)x]
(not stoichiometrically balanced)
Acid + Alkali => “Salt” + Carbon DioxideH2SO4 + CaCO3 => CaSO4 + CO2
• Environmental Impact from:• Acidity• Metals in solution (in acid or alkaline environments)• Salinity• Sludge precipitates
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Bacterial Catalization of Oxidation
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Temperature Effects on Oxidation
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Buffering of ARD during Oxidation of a Mineral Assemblage
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Buffering of ARD during Oxidation of a Mineral Assemblage
pH
Time
Buffering of Mineral A (e.g. calcite, dolomite)
Buffering of Mineral B (e.g. ankerite, siderite)
Buffering of Mineral C (e.g. Al(OH)3)
Buffering of Mineral D (e.g. feldspars)
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Mechanisms Controlling ARD in Tailings
Precip ita tion
Tailings
Dam
O xidation Zone
Seepage
Surface D ischarge
Neutra lization Zone
Process Water
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Mechanisms Controlling ARD in Waste Rock
Precip ita tion
Seepage Collection
D itch
Surface Runoff
In filtra tionBasal
Drainage
Sulfide Waste Rock
Advective Air Transport
Oxygen Diffusion
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Mechanisms Controlling ARD in Open Pits
In filtra tion
P recip ita tion
S urface W ater Runoff
G roundw ater F low Through R ockm ass
Pre-M in ingG roundwater
Table
Post-M iningG roundwater
Table ARD Seepage
ARD Seepage
ARD Seepage
ResidualSulphides
Residual SulphideRock Debris
O re B ody
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Mechanisms Controlling ARD in Underground Workings
Post-M ining G roundwater
Table
ResidualSulphide Exposures
(see inset backfilla lternatives)
In filtra tion
AR D
Precip ita tion
G lory-Hole
O pen P it
M ineW orkings
M ineW orkings
AR D
Tailings(cem ented)
O re B ody
Pre-M in ing G roundwater
Table
Backfill A lternatives
Tailings(uncem ented)
C D
Rockfill
B
O pen S tope
A
SulphideExposure
WaterF low
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Sulphide MineralsPyrite (FeS2) Pyrrhotite (Fe(1-x)Sx)Marcasite (FeS2 ) Chalcopyrite (CuFeS2)Galena (PbS) Sphalerite (ZnS)Arsenopyrite (FeAsS) Bornite (Cu5FeS4)
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Alkali Minerals• Types
– Carbonates • Calcite (CaCO3)• Dolomite (Ca,Mg(CO3)2)
– Hydroxides• Fe(OH)3
• Al(OH)3
– Silicates – Clays
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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
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Site Characterization
• Design• Field investigation & Sampling• Lab testing
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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
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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
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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
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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
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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
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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
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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)
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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
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Field Paste pH vs. Field Paste TDS
0
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0
Field Paste pH
Fie
ld P
ast
e T
DS
Dike samples
Leach Pad Samples
Pit Samples
Waste Rock Samples
TDS vs pH
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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.
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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
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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
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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
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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
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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
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Geochemical Static TestsDefinitions:AP = acid potential
= % S x 31.25NP = neutralization potentialNNP = net neutralization potential = NP - APNP:AP ratio = NP/APAll expressed as: kg CaCO3 equivalent/tonne, or CaCO3 eq./1000 tonnes
Example:S = 2 %AP = 62.5 kgCaCO3/tNP = 90 kgCaCO3/tNNP = 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.
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Interpretation Start with ‘guidelines” or general criteria for classification, then develop site- specific criteria
Typically criteria are based on a ‘set’ of tests, not just one type of test e.g. ABA & NAG results
0
5
10
15
20
0 5 10 15 20
AP (kg CaCO3/t equiv)
NP
(kg
CaC
O3/
t equ
iv)
1:1 ratio3:1 ratioNon-acid
generating
Potentially acid generating
Uncertain acid generating potential
0
2
4
6
8
10
12
-50 -30 -10 10 30 50Net Neutralisation Potential (NP-AP) (kg CaCO3/t equiv)
Pas
te p
HNon-acid
generatingPotentially acid
generatingUncertain acid
generating potential
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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.
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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)
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Applications of the NAG test
• In conjunction with ABA tests etc to reduce the risk of mis-classification
• 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
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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
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1
2
3
4
5
6
7
8
9
10
11
NAGpH
-300 -200 -100 0 100 200 300NAPP kgH2SO4/t
River SedimentFloodplainDredge Site
UncertainNAF
Uncertain PAF
[Rumble et al. 2003 ICARD proceedings]
Example – Ok Tedi
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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
[Pile et al. 2003 ICARD proceedings]
• perhaps due to presence of Cu or higher S content
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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
* Always account for dilution in concentration assessments
Sample Wt.
(g)
Vol.
(mL)
pH Cond. [SO4]
mg/L
%
SO4
[Cu]
mg/L
% Cu
1
2
100
100
200
200
5.5
2.5
68
150
300
848
10
95
2
14
5
80
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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
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Kinetic Testing
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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
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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
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Kinetic Testing Data
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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
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Field Test Plots
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Field Test Plots
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Field Test Plots
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Field Barrel Tests
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ARD Model
ARD Model(pore-water)
Acid Neutralization
Sulphide Oxidation
Secondary Mineral Precipitation
Mineral Dissolution
Ion Exchange
Metal Attenuation
‘Scale-up’ to Field Conditions
Dynamic Systems
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Sulfate Generation Over Time
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Water QualityExamples: • 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 (0.1-10mg/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
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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.
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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 alumino-silicates.
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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.
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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
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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.
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Plagioclase crystal with sericite alteration
Chloritized biotite
Sericitized plagioclase
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Sulfide ore fragment showing reaction zone, shrinking unreacted core and expanding rim (reacted zone). After Bartlett, 1998.
TrickleLeaching
Surface Enrichment
ReactedZone
Partially Reacted Sulfides
Air
O2UnreactedSulfides
Secondary Alteration at High T
Cu2+
Oxidation Products
Film
Cu2+Oxidant
A’A
Co p
per
Co n
tent
Oxi
dant
Con
cent
rat io
n
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Ore fragment after extensive chemical weathering along fissures due to internally generated acid from pyrite oxidation After Bartlett, 1998.
Unreacted Core
Reacted Zone
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.”
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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
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Flux of oxygen and water carrying sulfuric acid
H2O O2 H2O O2
O2
H2SO4
H2SO4
O2
High elevations in humid regions Valley bottoms, Cut slopes, Cavern walls, Ground under house floor
OxidizedZone
Oxidation Front
DissolvedZone
Dissolution Front
Reduced Zone, Not Dissolved
Oxidized Zone& Dissolved Zone
Oxidation & Dissolution Fronts
Reduced Zone, Not Dissolved
After: Chigira and Oyama, Engineering Geology (1999).
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0
10
20
30
40
50
Dep
th (
m)
Hai
zum
e F
m. (
Ms)
Kak
inok
idai
Fm
. (S
and
y m
s)
Toy
onik
awa
Fm
. (S
s &
cgl
)
Yam
aya
Fm
. (S
s)
Surface oxidized zone
Oxidized zone
Dissolved zone
Dissolution transition zone
Fresh Rock
After: Chigira and Oyama, Engineering Geology (1999).
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Geological engineering aspects of the weathering of sedimentary rocks
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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).
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Natural oxidation and weathering scars
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Natural oxidation and weathering scar slopes
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Debris flows from natural oxidation and weathering scar slopes
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Successive debris flows from natural oxidation and weathered slopes
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0
20
40
60
80
100
120
140
0 5 10 15
Moisture Content (%)
De
pth
(ft
)
0
20
40
60
80
100
120
140
1,000 10,000
Paste Cond (S)
De
pth
(ft
)
0
20
40
60
80
100
120
140
2 4 6 8 10
Paste pH
De
pth
(ft
)
Example of ARD conditions in a Waste Rock Pile