The Control & Management 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.

ARD Prevention & Control Measures

• Primary, secondary and tertiary controls• Oxygen control• Groundwater control• Surface water control• Covers• Collection and treatment

Control Technologies

• Prevention– Control designed and implemented before the event of ARD– No acid product storage

• Abatement and Mitigation– Control implemented after the fact– Acid product storage

• Approaches to Control– Primary - control of acid generation– Secondary - control of migration of contaminants– Tertiary - collection and treatment

WASTE TYPE

ACID GENERATION CONTROL

YES

N0

YES

NOARD MIGRATION CONTROL

YES

NO

COLLECTION AND TREATMENT

WASTE ROCKDUMPS/STOCKPILES

TAILINGS HEAP-LEACHPILES

UNDERGROUNDWORKINGS

OPEN PITS

IS WATER COVER FEASIBLE?DESIGN & IMPLEMENT

EVALUATE OTHER METHODS

• SEGREGATION & BLENDING• CONDITIONING• BASE ADDITIVES

• BACTERICIDES• COVERS & SEALS

IS SUFFICIENT CONTROLACHIEVED? DESIGN & IMPLEMENT

EVALUATE AVAILABLE METHODS

• COVERS & SEALS • DIVERT SURFACE WATER • INTERCEPT GROUND WATER

DESIGN & IMPLEMENT

DESIGN COLLECTION & TREATMENT SYSTEM(S)

• PASSIVE SYSTEMS• ACTIVE SYSTEMS

IS SUFFICIENT CONTROLACHIEVED?

PRIMARY

SECONDARY

TERTIARY

ARD Control Technology Selection

Segregation & Blending• Segregation:

– Feasibility of sulphide removal• Sometimes applicable to tailings which can be floated• Not applicable to waste rock

– Feasibility of separation by rock unit classification• Depends on variability and selective mining capability• Requires:

a) Long range planning for designing of waste dumps and coarse scheduling

b) Short range planning to schedule haulage to correct destinations by time period

c) Accurate, reliable in-field sampling, testing and prediction (blast hole sampling and modeling)

d) Very strict effective operations control

Segregation & Blending

• Blending methods:– Layering– Coarse blending by scheduling– Fine blending by truck loads and dozer pushing– Alkali addition

BlendingD

IST

RIB

UT

ION

DUE TO SLAKING CHARACTERISTICS

100:1

NP:AP RATIO0.01:1 0.1:1 0.3:1 1:1 3:1 10:1

ACID GENERATINGPOTENTIAL FROMSULFIDES

o:1o

DUE TO DURABILITY% AVAILABILITY - LOW PERCENTAGE

% AVAILABILITY - LARGE PERCENTAGEACID CONSUMING

LIMESTONE

Blending

NP:AP RATIO

0.01:1 0.3:10.1:1 1:1 100:110:13:1 oo:1

APPROXIMATELY 35%ACID GENERATING

POOR BLEND

DIS

TR

IBU

TIO

N

Blending

3:11:1 100:1

NP:AP RATIO

0.3:10.1:10.01:1

DIS

TR

IBU

TIO

N

APPROXIMATELY 20%ACID GENERATING

INTERMEDIATE BLEND

o:1o10:1

Blending

DIS

TR

IBU

TIO

N

NP:AP RATIO

0.01:1 0.1:1 1:10.3:1 100:110:13:1 oo:1

ACID GENERATINGAPPROXIMATELY 8%

THOROUGH BLEND

Oxygen Control

• Process by which oxygen enters reactive waste deposits:– Diffusion– Convection

(thermal, wind pressure)– Barometric Pumping

• P1V1 = P2V2

-2

-4

-6

1

10

10

10

DD M illington

and Shearer

Currie

Direct Proportionality

0.0 0.40.2

0a

1.00.80.6

M oisture content (vol.w ater/vol.void)

= 0.5 /T 2 = 0.7

Van Brakeland Heertjes

Diffusion Coefficient as a Function of Saturation

Oxygen Effectiveness of a Single Layer ‘Dry’ Sandy Till Cover

0.5

0.60.0

0.8 1.0

1 Sept1 July

1 May

1 NovFine-texturedlayer

(i.e. =0)

SATURATIO N (VO L.WATER/VO L.VOID)

SATURATIO N (VO L.WATER/VO L.VOID)

Drying of the fine-grained layer caused by capillary waterflow upw ards during the dry period. The fine-grained layeris represented by the silt (Ks=5x10 m /s)-8

Z (

m)

Z (

m)

Ks= 5x10 m /s

1 May

-8

0.5

0.60.0

0.8 1.0

0.50.5

0.00.0

0.60.40.2 1.00.8

Fine-texturedlayer

1.0

1.5

1 Aug

1 Nov

1 S

ept

1 J

un

e

1 Ju

ly

1 O

ct

Sandy moraine

O Effectiveness of ‘moist’ covers2Oxygen Effectiveness of a Layered ‘Moist’ Cover

Drying of the fine-grained layer caused by capillary waterflow upwards during the dry period. The fine-grained layer is represented by the silt (Ks=5X10^-8 m/s)

-7

2.0

O Effectiveness of ‘moist’ covers2

6.04.0 10.08.0 12.0

SANDY TILL

CLAY TILL

10

10

10

10

10

10

D N O S A J M A M F J J

Seasonal variation of the m ass transfer coefficient for oxygendiffusion through 1 m m oraine layers during norm al years.

Sandy moraine above a course-grained layer. Acapillary barrier is form ed at the bottom of the moraine layerSandy moraine directly above the waste sand.

Clayey moraine directly above the waste sand.

-10

-11

-9

-8

-6

Oxygen Effectiveness of Various ‘Moist’ Covers

Hydraulic Balance Using a Permeable Surround

Hydraulic balance using a permeable surroundExamples: Rabbit Lake Pit; Key Lake Pit

Hydraulic Cage

Hydraulic Cage

Surface Water Control

• Avoid stream channels and valleys• Install diversion ditches and berms• Install collection ditches• Separate clean from contaminated runoff• Install covers to minimize infiltration• Provide erosion protection

Soil Covers

• Types of Covers:

• Simple– Permeability depends on grain size– Compaction– Oxygen diffusion depends on moisture content

• Compound

• Complex– Variable– Multi-layered

waste

low density

high density

waste

moisture

Grey Eagle Tailings Cover

Tertiary Control

• Active Treatment– Collection of drainage– Chemical treatment– Require continuous operation

• Passive Treatment– Limestone trenches– Wetlands– Sulphate reduction– Intended to function without maintenance

Collection, Storage, Treatment & Sludge Disposal

• Both collection and treatment are transient functions but must by ready to function at all times

• Storage and sludge disposal facilities requires ‘dams’ with:– Long term stability

• Resistance to extreme events (floods, earthquakes, tornadoes and terrorist or vandalism acts)

• Resist the perpetual degradation forces of erosion, sedimentation, weathering, frost action, biotic and root penetration and anthropogenic activity

– Containment to prevent leakage and discharges– Isolation of sludges to prevent re-dissolution and migration

Collection• Objectives:

– Collect all seepage and drainage– Minimize volume to treatment process– Provide surge control

• Achieved by:– Ditching to collect surface flows– Groundwater flows - ditches, wells (drawdown), cutoff

walls• Difficulties:

– Identification of all sources– Seasonal variations, peak flows, holding capacity– Maintenance and operational requirements– Control of hydraulic and chemical loading

Collection• Objectives:

– Collect all seepage and drainage– Minimize volume to treatment process– Provide surge control

• Achieved by:– Ditching to collect surface flows– Groundwater flows - ditches, wells (drawdown), cutoff

walls• Difficulties:

– Identification of all sources– Seasonal variations, peak flows, holding capacity– Maintenance and operational requirements– Control of hydraulic and chemical loading

Water Treatment

• Objective is to remove from solution:– Acidity

• by neutralization– Heavy metals

• by hydrolysis and precipitation• co-precipitation

– Metal such as As, Sb• by complexation and precipitation as arsenate,

antimonate• co-precipitation

– Deleterious substances eg. suspended solids• settling, flocculation, precipitation, HDS

ground limestone gypsum

gypsumslaked lime

Chemical Treatment• Neutralization Process Chemistry

H2SO4 + CaCO3 + H2O CaSO4.2H2O + CO2

H2SO4 + Ca(OH)2 CaSO4.2H20

• Also use NaCO3 and NaOH

• Produces– Gypsum and metal hydroxide sludge.– Gypsum saturated (~ 3,000 ppm) water = high TDS– Very low density (5 to 30% solids depending on

process)

Chemical Treatment

• High Density Sludge Process– Process

• recycle treatment sludge (thickener underflow)• up to 50% recycle• premix lime and recycled sludge• then combine with influent ARD

– Advantages• reduced lime consumption• high density/lower volume sludge• larger precipitate particles “seeds”• increased removal of suspended solids• more efficient dissolved metal removal

Chemical Treatment

• Considerations:– Metal removal limited by solubility– Optimum pH for hydroxide precipitation– Acceptable final effluent pH– Complex Chemistry

• interactions with other constituents• complexing agents, coprecipitation• surface adsorption• mixed hydroxides

– Ferric iron can also act as flocculant/adsorbent– Sludge density and disposal

• Cannot design plant from theoretical concepts alone.

Sludge Disposal

• Concern– Long term chemical stability

• Issues– Changes in solution chemistry - pH– Leach testing - EPA 1312, SWEP test?– Special waste classification– Disposal to limit flushing – Include with tailings

• Research and more experience in sludge stability required.

Passive Treatment

• Wetland: – Soil is at least periodically saturated or covered with water– Peat bogs, cattail marshes, swamps.– Effluent directed to natural or constructed wetland with

emergent vegetation– Ability to treat depends on:

• water flow distribution • residence time• seasonal, climate

– Low strength feeds, polishing process

Wetlands• Advantages

– Adaptability to acid drainage and elevated metals – Low capital costs of natural wetland systems – Low operational costs for constructed wetland (?)– Provide wildlife habitat and flood control

• Disadvantages – Capital costs of earth moving requirements– Land area requirement – Treatment during winter is reduced– Impacts on wildlife are still unknown– Heavy metal loads in vegetation– Polishing process

Passive Treatment

• Sulphate Reduction– Part of wetland, at depth– Anaerobic bacterial treatment – Establish anaerobic conditions on solid medium, – Bacterial reduction of SO4

2- to H2S– Precipitation of metal sulphides– Convert excess to elemental sulphur– Possible treatment in a flooded open pit after closure

Land Application

• The LAD relies on the cation exchange in the soils and plant uptake of constituents.

• Solutions are irrigated over the surface to enhance evaporation and minimize surface water discharge.

• Can have issues related to increasing concentrations of Se, SO4 and other constituents in the water as a result of on-going oxidation

• Must evaluate the agronomic limits for various parameters

Biotreatment Processes

• Example: Landusky• An integrated, staged process system using biological

denitrification, biological selenium removal and biological cyanide oxidation

• Biotreatment technology utilizes a mixture of reduction and oxidizing bacteria that have been demonstrated to perform at site temperatures of ~6oC

• Other processes such as that of BioteQ• Bacterial reduction of sulphate and metal extraction as

sulphides• Utilizes sulphur and nutrients for bacterial growth

Monitoring and Maintenance

• Long term monitoring should be the minimum required to:1. Detect and define changes which require reaction and

reclamation2. Demonstrate performance where changes from required

performance standards are expected or suspected.• All monitoring results should be subject to pre-defined analysis

with defined alert and decision making levels and criteria. Any monitoring for which there are not defined decision criteria and response should be questioned.

• Site inspections and reconnaissance is a cost effective, efficient and effective monitoring methodology if done systematically with pre-established reference points (monuments, stations, photographs and survey records)

Monitoring and Maintenance• Two types of monitoring:

– Monitoring to establish performance or initial transient effects, i.e.:

• Seasonal trends (e.g. depth of frost penetration)• Vegetation establishment• Dissipation of contaminant plume

– Monitoring for expected or suspected change in compliance, i.e.:

• Water quality discharged from a treatment plant• Erosion of a tailings dam spillway• Financial performance of a trust fund

The former should be discontinued once performance is established, the latter must be sustained as long as a

change, suspected change or compliance requirements persist

Maintenance• Some sites can be returned to a self sustaining condition that,

after a demonstration period of monitoring, will require no further interaction by man

• Many sites require ongoing monitoring and maintenance to ensure that performance standards are maintained.

• Typical maintenance items include:– Diversion and spillway structure cleaning out and repair– Erosion gully repair– Fence repair and access control– Prevention of root and rodent penetration of covers– Maintenance of contaminated water collection and

management systems (passive care)– Operation and maintenance of water treatment plant and

sludge disposal systems (active care)

Requirements for Containment and Reclamation

• Chemical stability– Contaminants must not leach and move

• Physical stability– Solids must not move

• Land use and aesthetics– Must be useful and look good

Physical Stabilization

• Dumps– Erosion protection– Prevent water mounding– Cut off airflow pathways– Diversions– Resloping– Toe berms– Relocating

• Diversions– Control erosion– Remove sediment and debris– Control overtopping

Physical Stabilization

• Tailings dam– Spillway maintenance– Drainage and dewatering– Plug decants– Erosion protection– Covers– Dam stabilization including berms– Maintain internal drainage

• Covers– Revegetation– Erosion control– Drainage channels– Control disruption

Physical Stabilization

• Open pits– Backfilling– Slope crest laybacks– Fencing or berming and ditch– Flooding with or without neutralization

• Underground mines– Controlled flooding with or without neutralization– Hydraulic plugs– Shaft caps and access plugs– Subsidence stabilization– Glory hole fencing or filling

Land Use

• Reclamation, in terms of land use, means measures taken so that the use or conditions of the land or lands is:– Restored to its former use or condition, or– Made suitable for an acceptable alternative use

• This can be accomplished via:– Land form engineering– Revegetation– Land use planning– Land use management

Long Term Monitoring and Maintenance

• Maintenance and monitoring must be provided by a long term custodian

• Funding for such activity must be derived either from income from sustainable land use on the site or from an ‘endowment’ or ‘trust fund’

• There must be ‘something in it’ for the long term custodian to accept the responsibility of long term maintenance and monitoring