Tailings Facilities An Introduction from an Insurance Perspective

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Tailings Facilities - An Introduction from an
InsurancePerspective

• Basic Planning and Design Considerations

2
Typical Mine Site Layout
3
Procedures in Tailings Production

• Crushing
• Grinding
• Leaching Concentration
  Heating
• Dewatering
• Tailings Slurry Disposal

4
Tailings Grain Size Curves
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Tailings Impoundments - Operating Components

• Tailings Delivery - pumps, pipelines, and bridges
• Tailings Impoundment - retention dams, tailings,
  tailings pond, diversion ditches, and seepage
  control
• Pond Water Return - pump barge, decant conduits,

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Tailings Impoundment Life Cycle

• Planning
• Construction
• Operation
• Closure

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Key Elements in Tailings Impoundment Design

• Siting Studies
• Retention Dam Design
• Water Management
• Tailings Management
• Contamination - Cyanide and Acid Rock Drainage

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Factors Influencing Impoundment Siting

• Location and elevation relative to the mill
• Topography
• Hydrology and catchment area
• Geology
• Groundwater

• Length of tailings and return-water pipelines
  Capital and operating costs for the pumps
• Embankment layout Embankment fill requirements
  Diversion feasibility
• Long-term water accumulation Flood-handling
  requirements
• Availability of natural borrow type and volume
  Seepage losses Foundation stability
• Rate and direction of seepage movement
  Contamination potential

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Catchment Diversion Planning
Runoff diverted by dyke
Runoff diverted by conduit beneath
tailings(rare) usually have diversion ditches
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A Cross-Valley Downstream Dam

• 200 ft. high downstream dam in a semi-tropical
  area
• Decant conduit pond discharge through right
  abutment
• Dam raised by added slices of fill on downstream
  slope

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A Cross-Valley Tailings Dam

• Tailings is discharged to storage area upstream
  of the dam.
• Clearwater pond is retained by the dam
• Dam and decant are raised as the tailings level
  rises

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Basic Tailings Dam Section Types
A1
UPSTREAM
A2, A22A1
CENTRELINE
A3, A33A1
DOWNSTREAM
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Upstream Tailings Dams
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Poderosa Tailings Dam Peru, 1998
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INCO R4 - Modified Upstream
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Centreline Construction
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Brenda Centreline Tailings Dam
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L-L Dam - Centreline Construction
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Downstream Tailings Dam
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Downstream Tailings Dam
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Downstream Construction
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Tailings Dam Design Criteria

• Impoundment must be stable under all static and
  transient loads must identify failure modes
• Design criteria from CDA FOS of 1.5 for long
  term FOS of 1.3 for short term
• Impoundment must be stable for seismic loading
  and must pass design flood

• Transient load design magnitudes are selected
  from consequence-based criteria such as CDA
• High consequences mean design to MCE and PMF
• Surface and groundwater quality must meet
  regional and global water quality standards

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CLASSIFICATION BASIS
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EARTHQUAKE BASED ON CLASSIFICATION
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INFLOW DESIGN FLOOD BASED ON CLASSIFICATION
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REVIEW FREQUENCY BASED ON CLASSIFICATION
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Typical Dam Zonation - Earthfill
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Internal Seepage Control
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Water Balance

• Water accumulates in tailings impoundment from
  tailings transport and runoff
• Return water to mill for processing circuit
• Design impoundment water handling so that
  impoundment is not overtopped
• Want a closed system with no loss to environment
  without treatment

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Schematic Water Balance
gt
_
Inflows
Outflows
- Precipitation - Surface Water Runoff - Tailings
Process Water
- Evaporation - Reclaim Water To Mill
SPILLWAY DISCHARGE
Seepage Losses
Pond Storage
Storage In Tailings Voids
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Tailings Management

• The rate of rise of a tailings impoundment
  depends on mill production and design of tailings
  distribution systems within the pond
• Must be able to predict storage requirements with
  time so that retention dam construction can stay
  ahead of rising tailings surface

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Tailings Volume Balance
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Tailings Deposition Planning
Construct digital terrain model
Run terrain filling model by spigoting from
different locations
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Tailings Discharge Techniques
Spigotting
Open Discharge
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Single Discharge Point
Tailings discharged into middle of pond forming
beach at slopes less than 1
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Water Quality

• Chemical factors affecting water quality are pH,
  salinity, and toxicity
• Mill effluents that are products of the leaching
  process most likely to cause contamination. Acid
  leaching causes low pH increasing heavy metal
  solubility
• Tailings and waste rock with pyrite may result in
  low pH due to production of sulphuric acid by
  oxidation. This is ARD (Acid Rock Drainage).
  Low pH increases heavy metal solubility which are
  toxic at low concentrations.
• Cyanide used in gold extraction is highly toxic
  but is unstable and rapidly degrades with time

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Closure Design

• All tailings impoundments need to be designed
  from the outset so that they can be closed
  efficiently Designing for Closure
• Closure design is driven by water management
  considerations - passing floods and water quality
  
• Low pH water containing heavy metals persist in
  perpetuity must treat with lime before release
  or prevent ARD by dry and/or wet covers
• Cyanide is not a closure issue because of its
  instability

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Tailings Design Optimization

• Unlike water retention dams, tailings dams are
  constructed and operated simultaneously
• Can optimize performance and reduce cost by
  monitoring
• Monitoring programs key to improvements but
  usually neglected
• Technology exists to construct safe tailings
  impoundments (same failure rate as water
  retaining dams)

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Trends in Tailings Impoundment Design

• Tailings dams are becoming some of the highest in
  the world Los Leones in Chile is over 200 m
  high Antamina is scheduled to be 230 m high
• Closure and abandonment of mine sites in North
  America is expensive
• The ARD problem is not yet satisfactorily solved
  so that mine sites can be abandoned with
  confidence
• Mining companies are acutely aware of tailings
  and environmental problems