The Basics ofAcid Mine Drainage

ByAndy 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 DioxideH2SO4 + 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

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)

Mechanisms Controlling ARD in Tailings

Precip ita tion

Tailings

Dam

O xidation Zone

Seepage

Surface D ischarge

Neutra lization Zone

Process Water

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

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

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

Sulphide MineralsPyrite (FeS2) Pyrrhotite (Fe(1-x)Sx)Marcasite (FeS2 ) Chalcopyrite (CuFeS2)Galena (PbS) Sphalerite (ZnS)Arsenopyrite (FeAsS) 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

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

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 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.

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

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 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

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

1

2

3

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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

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

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

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

ARD Model(pore-water)

Acid Neutralization

Sulphide Oxidation

Secondary Mineral Precipitation

Mineral Dissolution

Ion Exchange

Metal Attenuation

‘Scale-up’ to Field Conditions

Dynamic Systems

Sulfate Generation Over Time

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

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 alumino-silicates.

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.

Plagioclase crystal with sericite alteration

Chloritized biotite

Sericitized plagioclase

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

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.”

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

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).

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).

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

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140

0 5 10 15

Moisture Content (%)

De

pth

(ft

)

0

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120

140

1,000 10,000

Paste Cond (S)

De

pth

(ft

)

0

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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