P. Simms, S. Sivathayalan, F. Daliri

Carleton University

Desiccation in dewatering and strength development of high density hard rock tailings

Stress history of high density tailings stack layers : Effect of degree of desiccation

Settling o r s/w consolidation

Stress history of thickened tailings stack layers:

4

2

1

Geotehcnical Stability?Cost?Acid Rock Drainage?

Geotechnical Stability?

Set

tling

S

ettlin

g

ICOLD and UNEP

(2001) Drying to

Shrinkage Lim

it?

3

Effect of salts? Cracks?

Consolidation

Thickness of fresh-layerRheology

Site topography

Conceptual model of factors influencing “drying time”

Within a range, initial solids concentration does not affect initial degree of settling (column test data)

5

How we predict the rate of drying?

Hypothesis: We can use standard unsaturated flow codes, that are used in soil cover design, to predict rate of drying Models such as SoilCover, SVFlux, Hydrus…Complicating factors with tailings: salts, cracking, significant volume change

Matric suctiondata from a two-layer test

Initial period of hindered settling

Placement of Second layer

So we have found that this approach has a certain skill,what happens when we extrapolate to deep layers of tailings?

Hypothetical analyses performed to analyze what happens when a fresh layer is deposited over several (1m to 10 m) of previously desiccated tailings

We used a range of parameters (Water retention curves, saturated hydraulic conductivity) typical to many hard rock tailings (air-entry value from 50-500 kPa, saturated hydraulic conductivity between 10 -6 and 10-8 m/s)

Potential evaporation rate varied between 2 and 10 mm/day.

Drainage and no-flow bottom boundary conditions applied

Influence of fresh layer thickness on drying rate

PE = 5 mm /day

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

0 5 10 15 20 25 30 35

Mid

-laye

r gra

vim

etric

wat

e con

tent

Time after deposition (Days)

Drying of 0.5 m layer over relatively dry tailings

Drying of 0.2 m layer over relatively dry tailings

Uniform drying for 0.5 m layer

Uniform drying for 0.2 m layerThese and similar predictions published in 2010 Paste and thickened tailings conference

Multilayer Deposition in a Drying Box

Volumetric Water Content Sensors

Tensiometers

Senix Distance Sensors

Scale

Desiccation Process in the Drying Box

Day 1 Day 2

Day 3 Day 4

Day 5 Day 6

Evolution of Cracks

Drying Time

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2

4

6

8

10

12

0 2 4 6 8 10 12 14 16

AE (m

m/d

ay)

Time (Days)

Layer 1 predicted

Layer 1 Measured

Layer 5 Predicted

Layer 5 Measured

Modelling of drying box

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.00 2.00 4.00 6.00 8.00 10.00 12.00 14.00

Mid

-laye

r gra

vim

etric

wat

er co

nten

t

Time after deposition (Days)

Layer 5 data

Specific prediction for drying box

Modelling of drying box

Modelling of drying box

Generic predictions compared to field and drying box results

16

Predictions from Paste 2010 paper

Drying box – only 1 m of tailings

Monotonic Results of Desiccated/Rewetted Samples

0 2 4 6 8 10 120

5

10

15

20

25 Wd=30%, OCRd=1Wd=28%, OCRd=1Wd=23%, OCRd=1Wd=19%, OCRd=1.7Wd=17%, OCRd=2.6Wd=12%, OCRd=4.1Wd=4%, OCRd=8

Shear Strain %

She

ar S

tress

(kP

a)

Settling &No desiccation

Vane Shear Results

The influence of stress history on shear strength

0 2 4 6 8 10 120

5

10

15

20

25 Wd=30%, OCRd=1Wd=28%, OCRd=1Wd=23%, OCRd=1Wd=19%, OCRd=1.7Wd=17%, OCRd=2.6Wd=12%, OCRd=4.1Wd=4%, OCRd=8

Shear Strain %

Shea

r Stre

ss (k

Pa)

4

1

3

2 41

2

Desiccation H

istory

Set

tling

3

If only desiccation to 25 % instead of 20% (SL) required, significant reduction in needed drying time

20

Important Outcomes:

Time to reach end of SW consolidation, for lifts up to 1 m and as low as 65% solids concentration, seems to be within 48 hours. Only small influence of initial solids concentration on final void ratio post-consolidation and post-drying

Samples experiencing settling without desiccation exhibited strain softening response. A relatively small degree of desiccation changes the response to strain hardening.

Though desiccation to SL and beyond increases the shear strength, it requires substantially longer drying time.

The degree of desiccation should be determined based on required site specific shear strength.

AcknowledgementsFormer and current graduate students who have contributed to this work: Bereket Fisseha, Julio Henriquez, Rachel Bryan, Adedeji Dunmola, Shabnam Mizani, Adrian Manlagnit, Parviz Heidarian, Mary-Ellen Gleeson, and Farzad Daliri

Collaborator Siva Sivathayalan (co-supervisor of Farzad Daliri)

Thank you Golder Associates, Musselwhite Mine, and Barrick Gold

0

1

2

3

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5

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8

9

10

0 10 20 30 40 50 60 70

AE (m

m/d

ay)

Time (Days)

First Layer SecondLayer ThirdLayer

FourthLayer

First Resaturation

SecondResaturationFifthLayer

Actual evaporation in drying box

Preliminary Results

5 Days 8 Days 11 Days