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