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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

The Control & Management ofAcid Mine Drainage

By

Andy Robertson and Shannon Shaw

disclaimer
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
ARD Prevention & Control Measures
  • Primary, secondary and tertiary controls
  • Oxygen control
  • Groundwater control
  • Surface water control
  • Covers
  • Collection and treatment
control technologies
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
ard control technology selection
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
segregation blending
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
segregation blending7
Segregation & Blending
  • Blending methods:
    • Layering
    • Coarse blending by scheduling
    • Fine blending by truck loads and dozer pushing
    • Alkali addition
oxygen control
Oxygen Control
  • Process by which oxygen enters reactive waste deposits:
    • Diffusion
    • Convection

(thermal, wind pressure)

    • Barometric Pumping
      • P1V1 = P2V2
oxygen effectiveness of a layered moist cover
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)

hydraulic balance using a permeable surround
Hydraulic Balance Using a Permeable Surround

Examples: Rabbit Lake Pit; Key Lake Pit

surface water control
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
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

tertiary control
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
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
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
collection25
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
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
chemical treatment
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

chemical treatment28
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 treatment29
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
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
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
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 treatment33
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
land application
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
slide35
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
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)
monitoring and maintenance37
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
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
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
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 stabilization41
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 stabilization42
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
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
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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