williams 2014

TAILINGS AND MINE WASTE 2014

APPLYING SOIL MECHANICS PRINCIPLES TO TAILINGS DEWATERING,

DENSIFICATION AND STRENGTHENING

David Williams

TAILINGS AND MINE WASTE 2014

Conference Sponsors

AMEC Earth & Environmental Knight Pisold and Co.

Ausenco MWH

BASF Chemical MineBridge Software, Inc.

CETCO Paterson & Cooke

ConeTec Robertson GeoConsultants, Inc.

DOWL HKM SRK Consulting, Inc.

Engineering Analytics, Inc. Tetra Tech, Inc.

Gannett Fleming URS

Golder Associates, Inc. Community Sponsor

CDM Smith

Overview Water recovery from tailings is most efficiently

achieved in-plant, but this must be balanced against efficient management of tailings disposal and cost

Densification and strengthening of tailings is best achieved by depositing them in thin layers and allowing time for consolidation and desiccation

This can be assisted by amphirolling to drain surface water down tailings beach and increase surface area exposed to desiccation, and by subsequent dozing to compact tailings

TAILINGS AND MINE WASTE 2014 3

Constraints under which TSFs Must Operate

Climatic and topographic setting of TSF Processing plant tailings production rates Manage supernatant tailings water Meet discharge water quality Maximise tailings settled dry density Rehabilitate TSF on closure to ensure stability,

minimise environmental impacts, and achieve some post-closure land use / function


Good Tailings Management

Spigotting in thin lifts

Maintaining a small decant pond


Good Tailings Management

To achieve optimal water recovery and tailings dry density

Which will minimise required TSF wall raises Possibly allow upstream wall raising on

tailings, potentially using tailings to construct raise

Will facilitate closure


Operational Tailings Water Balance

Rainfall Evaporation fromponded water and

wet tailings Tailings input

Seepagerecovery

Original groundwater tableGroundwater mounding

Wall seepageFoundation seepage

DecantStored wet tailingsPhreatic surface



TW + RR + WW = RW + EW + SE + SF + SW TW = tailings input water RR = TSF catchment rainfall and runoff WW = net waste water RW = water recycled to plant EW = entrained water SE = surface evaporation SF = seepage into foundation SW = seepage through wall



Best-known water volumes are initial % Solids, rainfall, and evaporation from ponded water

Water volumes that can be determined include entrained water, runoff, input and storage of waste water, and evaporation from wet, desiccating and dry tailings

Water volumes that are least well-known are seepage into TSF foundation and through wall


Operational Tailings Water Balance An Australian Example

Average annual rainfall = 845 mm (range 308 to 1,542 mm, since 1994)

Highest daily rainfall recorded = 272 mm (and 776 mm over 3 days)

Water discharged into TSF ~5 times rainfall Pumping capacity ~20 mm/day, of which

return water accounts for ~15 mm/day Leaving ~5 mm/day in reserve for rainfall


Tailings Continuum (Davies, 2004)

Complex water managementInefficient water recovery

Containment requiredSeepage likely

Rehabilitation difficult

Likely low OpEx and CapEx,but high rehabilitation cost

Simple water managementEfficient water recovery

Process chamical recoveryMinimal containment required

Negligible seepage lossesProgressive rehab. possible

Stable tailings mass

High OpEx and CapEx,but low rehabilitation cost

Tailings slurry(typically segregating)

Thickened tailings(dewatered, ideally non-segregating)

Paste tailings(Dewatered, ideally non-bleeding)

"Wet" filter cake(near-saturated)

"Dry" filter cake(85 to 70% saturated)

Pumpable

Non-pumpable

CONTINUUM

Slurry-like: No particle/particle

interaction Saturated

No effective stress

Soil-like: Particle/particle

interaction Effective stresses

and suction Shear strength


Consistency of Tailings

High density slurry

Filtered (dry cake) Centrifuged (wet cake)

Low slump paste High slump paste


Transporting Filtered Tailings

CRICOS Provider No 00025B

By truck

By conveyor and stacking

13 TAILINGS AND MINE WASTE 2014 13

In-Plant Recovery of Tailings Water THICKENING NOTES

Conventional and high rate

~25% Solids for coal tailings and red mud 40-50% Solids for metalliferous tailings Transportable by centrifugal pumping Beaching at ~1%

High compression thickening

Higher % Solids Just pumpable by centrifugal pumps Beaching at up to 5%

Paste thickening

Raises % Solids to between 45% (red mud) and 75% Solids (metalliferous)

Requiring transport by expensive diaphragm or positive displacement pumps

Consistency of toothpaste Requiring considerable management

Centrifuging and filtration

Solid-like Potentially transportable by conveyor or truck


Water Recovery from TSF

Generally limited to recovery of supernatant water, plus seepage through wall

Other tailings water is lost to: Entrainment within tailings Evaporation from decant pond and wet tailings Seepage into foundation

In order to maximise recovery of supernatant water: Direct supernatant water to decant pond Minimise size of decant pond and rapid recovery Maintain decant pumps and water return pipelines


Post-Closure Tailings Water Balance and Quality


Post-Closure Tailings Water Balance and Quality

RR = SO + EW + SE + SF + SW RR = TSF catchment rainfall and runoff SO = spillway overflows EW = entrained water SE = surface evaporation SF = seepage into foundation SW = seepage through wall


Downstream vs. Upstream TSF Wall Raising

Typically, TSF walls initially constructed as starter dams using borrow material

Wall raising can be: Downstream using borrow material Upstream partially on desiccated tailings using

borrow material and/or tailings, if suitable

2 m downstream wall raise

2 m high upstream raise


Upstream Wall Raising using Tailings

Harvesting tailings

Completed raise Compacting tailings

Moisture-conditioning


Excessive Wall Settlement due to Borrow Pit being Infilled with Slimes

Failure

Resulting kink


Amphibious Excavator, Amphirol and D6 Swamp Dozer

Parked Amphirol

D6 Swamp Dozer

Amphibious Excavator


Principles of Farming Tailings by Amphirol

In good weather in a dry climate, farming can be achieved in a month-long cycle

Amphirols exert 3 to 5 kPa bearing pressure (c.f. ~35 kPa for a D6 Swamp Dozer)

Some surface drying and strengthening is required to allow safe and efficient amphirol operation

Too heavy a bearing pressure and/or too soft a tailings surface leads to bogging


Principles of Farming Tailings by Amphirol

An amphirol should: Essentially float on lightly desiccated surface Create trenches down beach to facilitate surface

drainage Maximise surface area exposed to evaporation

and strengthening Expose un-desiccated tailings on further farming

An amphirol should not over-shear tailings by repeated farming, ~4 passes is optimal


Amphirols Dont Float!


Subsequent Farming by D6 Swamp Dozer

A D6 Swamp Dozer (bearing pressure of ~35 kPa) can be used once tailings have gained sufficient shear strength and bearing capacity to safely support it

A dozer may be used after amphirolling or simply after tailings has desiccated naturally on exposure

Dozing improves already desiccated tailings by compaction, leading to a further increase in dry density and shear strength TAILINGS AND MINE WASTE 2014 25

Schematic Slurried Tailings Shear Strength Profiles with Depth

Self-weight + Amphirol +2 m Fill + Desiccation

Desiccation and fill are most effective for consolidation and strengthening


Safe Fill Height Placed with a D6 Swamp Dozer

Conventional bearing capacity analysis gives:

H = Nc.sv /F.fill - He ~ 0.143 sv - 1 H = safe fill height (m) Nc = bearing capacity factor (~5.14 for a strip) sv = appropriate (vane) shear strength (kPa) F = appropriate factor safety (perhaps ~2) fill = unit weight of fill (~18 kN/m3) He = equivalent height represented by D6 Swamp Dozer (~1 m)


Strength Gain due to Fill Placement

Peak shear strength of loaded tailings will increase as it drains, according to:

= .tan , up to 18 H.tan 30o or 10 H = Increase in peak shear strength = Increase in effective stress due to fill loading, up to fill height H x Unit weight of fill (~18 kN/m3) = Drained friction angle of tailings ~30o


Capping Wet and Dry Tailings

Hydraulic capping of soft, wet tailings

Thin lifts and desiccation to facilitate capping


Capping Tailings

Bow-wave failure

Pushing shallow fill on a broad front Surcharging edge

Bearing capacity failure


Covers over Reactive Tailings

Rainfall runoffEvapotranspiration

from vegetated surface

Rainfall

Nominal 0.5 m compacted seal, if required

Minor net percolation Construction platform/capillary break, if required

Limited oxygendiffusion and

net percolation PAF or saline tailings

Seepagealong ~1% slope Infiltration and storage

Nominal 1 m of growth medium

Evaporationfrom ponded water

Evapotranspirationfrom vegetated surface

Rainfall

Infiltration and storage

Nominal 0.5 m compacted seal, if required

Minor net percolation Construction platform/capillary break, if required

Limited oxygendiffusion and

net percolation PAF or saline tailings

Seepagealong ~1% slope

Nominal 1 to 2 m ofloose, rocky soil mulch

Rainfall-shedding Wet climates Store and release Dry climates


Conclusions

Good tailings deposition and tailings water management can achieve optimal water recovery and maximise tailings dry density, which will: Minimise tailings storage volume required and minimise

wall raises Possibly allow upstream wall raising on tailings, potentially

using tailings to construct raise Facilitate closure

Soil mechanics principles underlie dewatering, densification and strengthening of tailings


Slide Number 1Slide Number 2OverviewConstraints under which TSFs Must OperateGood Tailings ManagementGood Tailings ManagementOperational Tailings Water BalanceOperational Tailings Water BalanceOperational Tailings Water BalanceOperational Tailings Water Balance An Australian ExampleTailings Continuum(Davies, 2004)Consistency of TailingsTransporting Filtered TailingsIn-Plant Recovery of Tailings WaterWater Recovery from TSFPost-Closure Tailings Water Balance and QualityPost-Closure Tailings Water Balance and QualityDownstream vs. Upstream TSF Wall RaisingUpstream Wall Raising using TailingsExcessive Wall Settlement due to Borrow Pit being Infilled with SlimesAmphibious Excavator, Amphirol and D6 Swamp DozerPrinciples of Farming Tailings by AmphirolPrinciples of Farming Tailings by AmphirolAmphirols Dont Float!Subsequent Farming by D6 Swamp DozerSchematic Slurried Tailings Shear Strength Profiles with DepthSafe Fill Height Placed with a D6 Swamp DozerStrength Gain due to Fill PlacementCapping Wet and Dry TailingsCapping TailingsCovers over Reactive TailingsConclusions

williams 2014

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