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The Control & Management 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.

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The control management of acid mine drainage l.jpg

The Control & Management ofAcid Mine Drainage

By

Andy Robertson and Shannon Shaw


Disclaimer l.jpg
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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ARD Prevention & Control Measures

  • Primary, secondary and tertiary controls

  • Oxygen control

  • Groundwater control

  • Surface water control

  • Covers

  • Collection and treatment


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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


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ARD Control Technology Selection

WASTE TYPE

WASTE ROCK

DUMPS/STOCKPILES

HEAP-LEACH

PILES

UNDERGROUND

WORKINGS

TAILINGS

OPEN PITS

PRIMARY

ACID GENERATION CONTROL

YES

IS WATER COVER FEASIBLE?

DESIGN & IMPLEMENT

N0

  • SEGREGATION & BLENDING

EVALUATE OTHER METHODS

  • CONDITIONING

  • BASE ADDITIVES

  • BACTERICIDES

  • COVERS & SEALS

IS SUFFICIENT CONTROL

ACHIEVED?

YES

DESIGN & IMPLEMENT

SECONDARY

NO

ARD MIGRATION CONTROL

  • COVERS & SEALS

  • DIVERT SURFACE WATER

EVALUATE AVAILABLE METHODS

  • INTERCEPT GROUND WATER

IS SUFFICIENT CONTROL

ACHIEVED?

YES

DESIGN & IMPLEMENT

NO

TERTIARY

COLLECTION AND TREATMENT

  • PASSIVE SYSTEMS

DESIGN COLLECTION &

TREATMENT SYSTEM(S)

  • ACTIVE SYSTEMS


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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:

        • Long range planning for designing of waste dumps and coarse scheduling

        • Short range planning to schedule haulage to correct destinations by time period

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

        • Very strict effective operations control


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Segregation & Blending

  • Blending methods:

    • Layering

    • Coarse blending by scheduling

    • Fine blending by truck loads and dozer pushing

    • Alkali addition






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Oxygen Control

  • Process by which oxygen enters reactive waste deposits:

    • Diffusion

    • Convection

      (thermal, wind pressure)

    • Barometric Pumping

      • P1V1 = P2V2



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


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Oxygen 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)



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Hydraulic Balance Using a Permeable Surround

Examples: Rabbit Lake Pit; Key Lake Pit



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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


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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

moisture

waste



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Tertiary Control

  • Active Treatment

    • Collection of drainage

    • Chemical treatment

    • Require continuous operation

  • Passive Treatment

    • Limestone trenches

    • Wetlands

    • Sulphate reduction

    • Intended to function without maintenance


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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


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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


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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


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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


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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)

ground limestone

gypsum

gypsum

slaked lime


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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


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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.


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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.


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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


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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


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Passive Treatment

  • Sulphate Reduction

    • Part of wetland, at depth

    • Anaerobic bacterial treatment

    • Establish anaerobic conditions on solid medium,

    • Bacterial reduction of SO42- to H2S

    • Precipitation of metal sulphides

    • Convert excess to elemental sulphur

    • Possible treatment in a flooded open pit after closure


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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


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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


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Monitoring and Maintenance

  • Long term monitoring should be the minimum required to:

    1. Detect and define changes which require reaction and reclamation

    2. 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)


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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


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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)


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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


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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


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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


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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


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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


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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


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