A Dewatered Tailings Practice –Trends and Observations

Michael Davies1, John Lupo4, Todd Martin1, Ed McRoberts1, Marcelo Musse2, and David Ritchie3

AMEC Earth & EnvironmentalVancouver1, Santiago2, Toronto3, Denver4

18 October 2010Presented by: Dr. Michael Davies, Vice-President AMEC

Black Eyes Keep Coming…..

Hungary tailings breach must be a forewarning for AlbertaSierra Club Atlantic – October 7, 2010

Villagers 'lose everything' in tailings tideJanos Gal, Agence France-Presse · Thursday, Oct. 7, 2010

Black Eyes Keep Coming…..

Hungary tailings breach must be a forewarning for AlbertaSierra Club Atlantic – October 7, 2010

Villagers 'lose everything' in tailings tideJanos Gal, Agence France-Presse · Thursday, Oct. 7, 2010

Girl missing after dam bursts in Mozambique(AFP) – October 11, 2010

Dewatered Tailings

Traditional Slurry

Thickened/Paste

Filtered

WaterContent

(decreasing water losses)

Dewatered Tailings – Classification

Tailings Management Basics

Conventional tailings impoundments• Lots of water!

Dewatered Tailings• Can range from high slump to low slump to a “solid”• At maximum dewatering, can form stable, compacted stack with

no need for dam construction• Useful in recovery of process water constituents• Useful for projects with cold climates, arid conditions, high

seismicity, sensitive water balance, etc. • Increasingly useful for “image” concerns• Applicability at larger scale mining developing relatively quickly

Cycloning or Classification

• Hydrocyclones use more pressure than spigots to separate coarse from fine particles

• Coarse material is diverted to the underflow and finer material to the overflow

• Single or double-staged cycloning (in series) can be used• Key criteria for a satisfactory use of cyclones to build tailings

dams:• Split of the material to the flow• Quality of the underflow, particularly the d10 size• Underflow must be permeable, and the material must act as a

suitable filter for slurry • Density of the underflow material

• too wet, a wet, sloppy heap• too dry, a steep, unstable pile

Cycloning or Classification

Cycloned Tailings mixed with Coarse Coal Refuse

Co-Disposal

Thickened and Paste Tailings

Dewatered but still a slurry

Thickened First thickened piles were far from panaceas Taking what happens inefficiently in tailings impoundment and doing it in

the process circuitPaste A nomenclature challenge – pumping boundary seems artificial Underground paste backfill where much of sound development research

occurredThickened/Paste To obtain benefits, must remain non-segregating Far better to operate based upon rheological properties than upon solids

content (or terminology)

Thickened Tailings Examples - Australia

Thickened Tailings at its best

Sunrise Dam Gold Mine in Western Australia.

Thickened Tailings on a Large Scale

Note Channels

Paste Tailings

• There is a “grey” (and artificial) area between thickened/paste tailings nomenclature

• Maintaining this name discrimination may not serve any useful purpose

• Paste tailings are often defined as tailings that have been significantly dewatered to a point where they: require positive displacement versus centrifugal pumping do not have a critical flow velocity when pumped do not segregate as they deposit produce minimal (if any) bleed water when discharged

• In spite of above, a major challenge can be flow velocity in pipe ($)

• One thing proven by case histories – the laboratory scale never matches the operating scale

Underground Paste Backfill

Surface Paste

Myra Falls, British ColumbiaRelatively low tonnage, very wet environment

Note Channels

Bulyanhulu, Tanzania Relatively Low Tonnage, Arid Environment

Surface Paste

Note Channels

Surface geochemical processes between beach lifts – not case that there is zero oxidation with paste/thickened tailings

Surface Paste

Filtered Tailings

• Filtering concentrate is a commonplace activity at mines

• Paste plants often include tailings filtration• In past 10 to 15 years, advances in filter technology

has seen wider range of stacked tailings (most often metal mines,

particularly mid-sized gold properties) increased tonnage potential decreased CAPEX/OPEX on per tonne basis

Five years of operating experience

Filtered Tailings Dry Stack Pogo Mine, Alaska

Filtered Tailings Dry Stack Greens Creek Mine, Alaska

Raglan – pressure filtered to wopt

La Coipa – vacuum filtered to 2% over wopt

Filtered Tailings - Climatic ExtremesRaglan, PQ and La Coipa, Chile

World’s Largest Tailings StackLa Coipa Mine, Chile

Nominally 20,000 tpd since 1990

Dewatered Tailings – No Tailings Ponds?

• A “Dry Landscape” once a mine decides to start dewatering their tailings is indeed seductive but seldom is it accurate

• During Operations, only case where “no pond” can be stated with confidence is for filtered tailings

• One culprit in the “no pond” myth is the surprising large portion of the recent thickened/paste literature implying the elimination of tailings ponds

A tailings pond allows:• Storing the pore fluids expressed through bleed and consolidation• Saturation which limits oxidation - experience to date with

thickened/paste tailings demonstrates oxidation of these materials willoccur

• A facility to store and provide water during drier periods.• Many others including clarification, geochemical polishing, water

management buffering, geothermal factors, etc.

No Water Management Ponds?

Thickened Tailings Facility

Arid Thickened/Paste Tailings

Central Thickened Discharge Facility, Australia

• For tailings intended to be self supporting in a large cone the specification of the design angle is highly critical

• An unfortunate number have been laid out with design slopes that were far steeper than those ultimately achieved in the real operating conditions

• A common theme of many such slope designs (i.e., for thickened tailings or paste tailings) is the reliance on small scale flume or “fish-tank tests”

• While universally appealing given their common use, these tests are in fact fundamentally wrong

• Non-segregating tailings flow on beaches does not occur via simple sheet flow observed in very low discharge rate experiments

Thickened and Paste Tailings Beach Slopes

Thickened and Paste Tailings Beach Slopes

2

tan tan V zh×××

=π

θθδδ

d C sin u

⋅=γ

θ

•“δz/δV” is function of the cone slope and the current height

•if the slope angle is fixed, only way to get sheet flow to all parts of the cone is to continually adjust either Q or Cu

Thickened and Paste Tailings Beach Slopes

• Experience and theory indicate that it is invalid to assume deposition slopes and processes that are:

• independent of slope length and/or deposit height• derived on the basis of flume tests that due to scale inevitably

exclude many of the key processes, including post-depositional dewatering

• There can be plenty of useful information gained from bench scale tests with dewatered tailings but providing an indication of what the operating slopes will be is not one of them

Current Global Trends - Chile

• High number of large tonnage operations• Water conservation potential at many of these mines• Dewatered tailings are being considered on multiple projects • Examples of actual dewatered tailings projects currently in

operation include:– La Coipa (filtered)– Mantos Blancos (filtered) – El Peñón (filtered)– El Indio Mine (currently closed, filtered)

• Projects that will soon be in operation include the Las CenizasProject (small scale, paste) and the Esperanza Project which will be Chile’s first large scale paste tailings project

• Chilean regulators have already included different types of dewatered tailings in governance literature

Current Global Trends - Canada

• Slow, but steady, increasing use of dewatered tailings or small to medium sized mines

• Slow adoption in oil sands due to economic/technical challenges at large scale

• Lower tonnage in the northern regions of Canada (and Alaska), have more readily embraced thickened/paste and filtered tailings and some of the best global examples of these technologies exist in this region

• Some of the least successful thickened tailings facilities also exist in Canada – however, these projects have provided some valuable lessons

Current Global Trends - Australia

• Filtered tailings have not been embraced quite as quickly as in other regions

• The region is by far the leader in thickened tailings applications

• Pioneering work by M.P.A. Williams and his colleagues has resulted in some excellent project examples

• Potential filtered and paste tailings in the project “pipeline”

• Several projects in Papua New Guinea and Eastern Indonesia (region) evaluating dewatered tailings

Current Global Trends – Middle East

• A significant proportion of newer mines using thickened tailings – usually at appreciable throughputs

• Several large scale thickened tailings projects, for example, in Iran

• Scarce water resources continues to drive consideration of dewatered tailings for most new mines in region

Current Global Trends - Throughputs

• Besides regional trends, another important trend is the size of the facilities using dewatering technologies

• Current operations using thickened/paste tailings are mostly under 30,000 tpd

• Current filtered operations are generally under 10,000 tpd• Exceptions in each case with operations of greater throughputs

currently existing• Authors aware of proposed mines in advanced project stages

where:• thickened tailings in excess of 100,000 tpd• filtered tailings in the range 70,000 tpd

• Significant increase in potential throughput coming from a combination of more precedents, increased economies of scale, advances with dewatering equipment and increased “drivers” for dewatered tailings

Current Global Trends

Publication/Branding Dichotomy

Dewatered Tailings – The Future

The collective experience of the authors indicates: • Dewatered tailings have become a viable alternative for

mining projects of essentially any tonnage • The main limitation being in ensuring the right technology

is selected based upon the nature of the tailings• Misconceptions about from an alarming amount of

misinformation available in current literature • With increased use of these methods, more case records

will be developed and better information will flow to operators, designers, regulators and educators and improved decisions will result

Dewatered Tailings Classification

Dewatering Facilities1

Conventional Slurry Thickened Paste CakeNone (cyclones possible) Coagulants/Flocculants

Cyclones/ThickenersOn-line chemical modifications

Deep bed/cone thickeners

Additional chemicals

Filters (vacuum or pressure)

Typical Slurry Density2 30-40% solids 45-65% solids 65-70% solids 80-85% solids (unsaturated)

Typical conveyance Centrifugal pumps Positive displacement pumps

Viscosity reducers

Trucks or conveyers

Bleed Water Fines to pond with significant water to manage

Considerable water to manage

Little to no bleed water to manage

Consolidation only –can be negligible

Deposit Character Segregating Possibly segregating Non-segregating Soil-like (non-segregating)

Physical strength model Fluid rheology Fluid rheology Transition to soil mechanics

Soil mechanics

Beach Slopes Flat with concavity Lesser concavity. Slightly steeper than conventional slurry. Beaches form through channelized flow

Approaching sheet flow but still has channelization. Beaching can be inhibited by strength

N/A

Post DepositionalSettlement (Consolidation)

Creep may still effect non-linear finite strain predictions Approaching traditional soil mechanics models

Traditional soil mechanics models

Containment of Tailings Retention structures for all subaerial deposition

Can be self-supporting on very low angle slopes. Modest retention structures still required

Can be self-supporting on modest slopes. Can potentially be developed with minimal retention structures

Self-supporting at high angle slopes without need for retention structures

1Centrifuges, not listed, can develop tailings product intermediate to paste-cake2Typical densities are for mineral tailings from metal mining. Variations in behavior with other mined materials can be expected.