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Environmental Issues in Tailings Management
Aims of this Module( courtesy of Best Practice Environmental Management in Mining Training Kit)
To develop an understanding of:
Objectives of tailings storage
Suitable sites
Design of tailings facilities
Operating and monitoring a tailings
facility
Closure and reclamation of the facility
Objectives of Tailings Storage
To store tailings so that the:
Impoundment structure remains stable
Impact on local residents is minimised
Environmental impacts are minimised
Storage facility can be rehabilitated when
closed
Life Cycle of Tailings Containments
Design ConstructOperate
and
monitor
Maintai
n and
modify
Decommissio
n and closeAftercare
Common Environmental Problems
Dam safety and stability
Water pollution
Surface and ground water
Air pollution (dust)
Visual impact
Inadequate reclamation and restoration
Tailings Characterisation
Physical properties
Determine resistance to erosion; possible options for reuse such as construction
Chemical properties
Determine potential for drainage waters to damage plant and animal life
Mineral content and oxidation state
Determines specific gravity, which affects
segregation and consolidation of particles
Stability
A stable embankment for the tailings dam is
essential.
Important factors are:
Height and slope of the embankment
Characteristics of the foundation and
embankment materials
• Geology and composition
• Strength
• Degree of compaction
Seismicity
Earthquakes are a serious threat to tailings
dams
Loose, saturated sands in a dam are
likely to liquefy during an earthquake
Liquefaction causes a sudden
additional shear force on the dam
wall that may cause it to fail
Analyses for tailings dams must consider
this extra shear force
Ground and Surface Water Pollution
Seepage from tailings can pollute ground
water.
Pollution of streams and rivers is caused by:
Spills (usually from breaks in pipelines)
Seepage through embankments;
Overflow during extreme wet conditions
Dust Generation
Many tailings are fine-grained. When the
tailings dry out, wind erosion can cause a dust
nuisance.
Maintaining a wet surface will prevent
this; however, this may conflict with other
management strategies for promoting
consolidation of tailings.
Factors Affecting Site Suitability
• Regulatory requirements
• Meteorology
• Topography and mapping
• Surface water
• Ground water
• Geotechnical and geochemical characteristics of soils
• Properties of tailings
– Physical
– Chemical
Major Factors Determining Site Suitability
• Each country has its own legal requirements.
– These may cover:
• Water quality
• Cultural and heritage significance
• Specific design requirements
• Dust emissions and noise pollution
• Zoning of the tailings storage area
• Water balance is a key issue.
– Necessary data:
• Rainfall (annual and monthly)
• Evaporation rates
• Humidity, temperature, solar radiation
• Wind strength and direction
• Past or infrequent storm events such as typhoons or floods
Topographical Survey and Mapping
• Information should include– Surface contours at 1 m intervals
– Drainage patterns
– Boundaries of the site
– Roads and services
– Houses and other structures
– Cultural and heritage sites
– Current land use
Geotechnical and Geochemical Measurements
• Characteristics of local soils
• Condition of the soil forming the foundation of the containment
• Availability of construction materials
• Structure of any rock formations
• Seismic risk data
• Possible chemical reactions of soils with tailings
Surface and Ground Water
• Flow data for natural water courses
• Flood plain and flood records
• Water quality
• Environmental flows
• Hydrogeology
• Preferred flow paths
• Zone of ground water discharge
• Water quality
• Upstream and downstream water use
Tailings Properties
• Mineral and chemical composition of solids
– Heavy metals, radio-nuclides, specific gravity
• Physical characteristics
– Settling and consolidation behaviour, permeability
• Strength characteristics
Detailed Site Investigations
Soil profile
Water table
In situ materials
Construction materials
Rehabilitation materials
Design Considerations
Types of storage facility
Dumps, impoundment dams, existing
mine pits
Liner systems
Construction techniques
Closure systems
Types of Storage Facility
Storage dumps
Avoids large water impoundment
Includes downstream catchment dams to catch
runoff and drainage water
Example: Thickened central discharge
Tailings thickened in a large settler or by cyclone
separation
Discharged onto a sloped bed
Evaporation speeds consolidation
Results in a large conical dump
Cobar & Canada – CTD tailings disposal
Types of Storage Facility
• Storage dams
– Constructed impoundments for retaining slurry and reclaiming water
• Paddock or “turkey nest dams”
• Cross valley impoundments with a single dam wall
• Impoundments constructed on a hill slope
• Storage in disused workings
– Worked-out open-cut or underground workings for containment
• Advantage: low cost; good technique for mineral sands mining.
• Problems: Low evaporation rate; poor consolidation; reclamation of the surface is difficult. May contaminate ground water.
Liner Systems
Liners reduce seepage of leachate through the
base of the storage area.
All liners leak; rate of leakage depends
on:
Size of hydraulic head above the liner
Length of time the hydraulic head is
applied to the liner
Hydraulic conductivity of the liner
(leakiness)
Level of care during installation and
Construction Techniques
Four main construction approaches
Conventional water dams (walls
completed then dam filled)
Staged construction using tailings (+/-
rock or soil)
Upstream
Downstream
Centreline
Upstream Construction
Starter dam is built from rock fill or mine waste
Lifts are constructed upstream from the initial dam
Lifts can be constructed from dried compacted
tailings in the beaches or from waste rock
Tailings discharge must be controlled to make sure
the coarser sandy tailings are deposited near the
dam wall
And alternative strategy uses hydraulic methods to
separate sand from slimes and raise the dam wall
Upstream construction techniques (A mechanical and
B hydraulic)
A
B
Upstream Construction --Summary
• Advantages
– Requires least material for initial starter dam.
– Low cost
• Disadvantages
– Limited rate of height increase (< 5m/y)
– Tailings discharge must be carefully controlled
• Disadvantages (con’t)
– Not well suited to large runoff inflows or water storage
– May be subject to liquefaction in earthquakes
– Dam capacity is reduced with each raise
– Not suitable for significant water storage
Downstream Construction
• Starter dam of waste rock fill
– Lifts are constructed downstream from the initial dam using sand or waste rock
– Compatible with any type of tailings
– Can be used for water storage
– Good earthquake resistance
– Requires greatest amount of dam fill
– Dam fill volumes increase for each successive lift
– Often most expensive method
Downstream construction techniques (A mechanical
and B hydraulic)
A
B
Centreline Construction
Crest of the wall remains central on the starter
dam with progressive raises.
Construction materials may be waste rock or
coarse tailings.
Embankment must be partly permeable to control
water pressure.
Tailings placement is important to keep ponded
water away from the dam.
Land, River or Marine Tailings
Disposal
Deposition methods
Land-based impoundment
Underground disposal
Thickened discharge
Managed submarine disposal
Riverine disposal
Tailings Deposition
Tailings pumped to dams and discharged through single
or multiple spigots
Subaqueous deposition
Central thickened discharge
Dry filter cake
Physical or chemical additives for stabilisation
Operating and Monitoring
Observations of the storage facility
Instrumentation for monitoring stability of
impoundment structures
Piezometers (instruments for measuring
pressure head) are used to measure:
the phreatic surface
excess pore pressures
Water Management
• Minimising the amount of water escaping from the tailings dam will reduce pollution
– Strategies include:
• Recirculating process water
• Reducing seepage
• Water cycles
– Natural water cycle
• Rainfall, evaporation, infiltration, runoff
• Mining water cycle
• Deposition, reclaim, seepage
– Water balance must consider both cycles and extreme events: floods, drought
Water Cycle in a Tailings Dam
Operating and Monitoring
Ground water monitoring
Factors to consider for monitoring plan
Composition of tailings stored
Design of the storage facility
Potential for seepage to migrate
ground water flow
Possible interaction of leachate with soils
Probable pathways for leachate migration
Operating and Monitoring
General surveillance
Annual inspection and review by original
designer or experienced engineer
Monitoring
Surface water
Ground water
Biological
Closure Systems
Considerations
Leaching to ground or surface water
Surface drainage and erosion
Stable surface cover to prevent wind
erosion
Post-closure maintenance
Covers for Closure
Covers provide protection from erosion.
Factors to consider include:
Physical and chemical characteristics of tailings
Amount of rainfall
Type of containment
Type of liner
Wet climates will require covers that reduce infiltration.
Possible cover materials: gravel, vegetation, topsoil, fill and
geotextile in various combinations
Summary: Tailings Containment
• Problems
– Contamination of surface and ground water
– Dam safety and stability
– Visual impact
– Dust
– Reclamation and restoration of the area
• Best practice strategies
– Ensure site suitability using detailed site investigations
– Choose dam design, liner system and construction technique to suit site and type of waste
– Plan closure strategy
– Monitor operation