Laurentian University School of Engineering

EVALUATION OF FROZEN BACKFILL EVALUATION OF FROZEN BACKFILL

FOR OPEN STOPE MINING FOR OPEN STOPE MINING

IN PERMAFROST CONDITIONSIN PERMAFROST CONDITIONS

Daniel L. CluffDaniel L. Cluff

James Gallagher, Ali Jalbout, James Gallagher, Ali Jalbout,

Vassilios Kazakidis, Graham Swan.Vassilios Kazakidis, Graham Swan.

CIM 2008CIM 2008

Video of Large form

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

PROBLEM STATEMENTPROBLEM STATEMENT

To investigate the requirements of frozen

consolidated fill for blasthole/panel mining

at mines in permafrost.

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

PROJECT OBJECTIVEPROJECT OBJECTIVE

Provide critical insight to the design and

process parameters controlling the use of

rockfill, tailings, ice and water for open

stope mining at Raglan Mine.

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

CHALLENGESCHALLENGES

• Strength requirement• Availability and mechanical properties of inert material• The water as the binder • The distribution system

General design parameters for fill material

Additional design parameters of fill material in permafrost zone• The impact of subzero temperatures on design variables• The thermal properties of the mixed materials. • The distribution, mixing, storage and placement of material• The time effects for fill once placed in a stope

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

TESTING FACILITY AT NORCAT MINETESTING FACILITY AT NORCAT MINE

SampleLVDT Top Plate

JacksBottom Plate

Sample

Jacks

Freezer

Data acquisition

Video of testing facility

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Compressor

Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

TESTS USING ROCKFILLTESTS USING ROCKFILL

• Through trial and error, a new method of mixing and dumping was devised, whereby the rock and snow were mixed prior to being dumped into the forms.

• The snowmaking machine was abandoned in favor of fresh snow which was readily available on site.

• A screen was used to filter out rocks larger than 8”.

• A smaller loader bucket about 1.5 ft3 was used to allow for better control of measurement and better mixing.

Video of tailings and rockfill mix method

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

TESTS USING ROCKFILL AND TAILINGSTESTS USING ROCKFILL AND TAILINGS

• Tailings were mixed with development waste rock at NORCAT mine.

• The mixture was then dumped into the small forms measuring 2’ in diameter and 4’ in height.

• The samples were then compressed inside these forms to simulate the compression effect of a 35m column of fill (stope height).

• After freezing at -6°C, UCS testing of tailings indicated a strength consistently above 1.0 MPa.

Results met Raglan Mine’s strength requirements of 1 MPa

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

Low Strength ResultsLow Strength Results

• Mixtures without added tailings had strengths well below 1.0 MPa.• These mixtures consisted of cold rock and moisture (sprayed water

and ice crystals).• Mixing was done either by loader (excavator) where the two materials

were mixed prior to dumping into forms, or by hand, whereby snow was added after dumping each bucket of rock.

Numberof tests

% Dry Rock

% DryTailings

% Crystal ice Added

% WaterAdded

(sprinkled)

%Moisturein Rock

Total % Moisture content in mix

Strength (MPa)

3 87 - 10 0 3 13 0

4 82 - 5 10 3 18 0.05-0.17

2 81 - 6 (wet snow) 10 3 19 0.24-0.4

Temperature was -6o at the time of the UCS testing

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

Tests with Sufficient StrengthTests with Sufficient Strength

Numberof tests

% Dry Rock

% DryTailings

% Crystal ice Added

% WaterAdded (sprinkled)

% Moisture in Tailings

%Moisturein Rock

Total % Moisture content in mix

Strength (MPa)

2 60 26.7 0 6 17 3 13.3 1.2-1.4

2 57 25 5 (wet snow) 6 17 3 18 1.1

1 53.5 23 5 (wet snow) 12 17 3 23.5 1.6

2 0 77 0 6 17 0 23 > 1.8

1 0 80 0 3 17 0 20 > 1.8

(temperature was -6°C for all UCS tests)

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

Role of Constituents in Frozen BackfillRole of Constituents in Frozen Backfill• Water

• Is added in the liquid state – upon freezing becomes the binder.• Releases latent heat of fusion.• Has a high specific heat capacity. cw = 4.187 J/g

• Ice• Is contained in rock and tailings or added – subzero.• Provides an offset to the latent heat released by the water.• Has medium specific heat capacity. ci ≈ 2.0 J/g

• Rock• Waste rock is economically disposed of in backfill.• Has low specific heat capacity. cR ≈ 0.8 to 1.0 J/g • Provides heat sink to absorb excess heat from water.• Is a solid aggregate with rough surfaces suitable for adherence.

• Tailings • Properties similar to rock, heat capacity, heat sink, economic filler.• Fills voids between rock increasing the strength and density.• Can be used to deliver the desired water/moisture content to the backfill.

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

THE MIX DESIGNS USED IN THIS PHASETHE MIX DESIGNS USED IN THIS PHASE

0%

10%

20%

30%

40%

50%

60%

70%

80%

90%

100%

Bac

kfil

l co

mp

on

ent

per

cen

tag

e

0 0.11 0.32 1.1 1.3 1.6 >1.8Uniaxial compressive strength (MPa)

Frozen backfill -6 oC mix design versus strength

Rock % Tailings % Added Water % Voidage Pre-existing ice Added ice %

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

LARGE SCALE TESTLARGE SCALE TEST• The large form, which was 2 m in diameter and 6.1

m high, was used to examine large scale effect and determine the stiffness of the material.

• Mixing was done by the loader. Before dumping, all constituents were dropped into the form together to aid the mixing process.

• Jacks were placed inside the form (one 2 ft, and another 8 ft, from the bottom) along with temperature and displacement sensors to record the necessary data to conduct stiffness tests.

• The form was then removed, and the pillar was left to stand on its own. Even after 3 warm days, the rockfill column fell down only with the help of a scooptram.

Constituents% (by

weight)Initial T

(˚C)Final T

(˚C)

Rock ~85 -5 to -10

-6Ice Crystals ~5 0

Water Added 8~10 +5

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

LARGE SCALE ROCKFILL PILLARLARGE SCALE ROCKFILL PILLAR

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

THERMODYNAMIC MODELLING OF THERMODYNAMIC MODELLING OF FROZEN BACKFILL FROZEN BACKFILL

• An model to calculate the resting temperature the backfill mix reaches shortly after being placed, based on the initial temperatures and heat capacities of the constituents was developed to study mix design scenarios.

• The heat diffusion equation in cylindrical coordinates was analytically solved and fitted to cooling curve data obtained from the cylindrical samples to determine the thermal conductivity of the mix design.

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

THERMAL EQUILIBRIUMTHERMAL EQUILIBRIUM

Using the specific heat capacities, densities, initial temperatures, mass and latent heat in water as inputs the thermal equilibrium model of the backfill calculates the latent heat remaining that will delay the onset of freezing of the backfill into a solid mass.

For constituents at an initial temperature of -20oC and Water at 5oC

the percent energy distribution for each constituent and latent heat

0%10%20%30%40%50%60%70%80%90%100%

0% 2% 5% 7.50% 10%Percent water added to the backfill

Per

cen

t co

ntr

ibu

tio

n e

ner

gy

bal

ance

Rock Tailings Ice crystals Liquid water Latent Heat

The effect of initial constituent temperatures on the final

temperature of the backfill for 5% and 10% water

at 5oC with the addition of 0%, 5%, and 20% ice

-20

-15

-10

-5

0

5

-40 -35 -30 -25 -20 -15 -10 -5 0 5

Initial temperature of rock ice and tailings oC

Fin

al t

emp

erat

ure

of

ba

ckf

ill

o C

5% water 0% ice

5% water 5% ice

5% water 20% ice

10% water 0% ice

10% water 10% ice

10% water 20% ice

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

THERMAL PROPERTIES THERMAL PROPERTIES OF FROZEN BACKFILLOF FROZEN BACKFILL

The thermal conductivity was determined for three distinct backfill samples by fitting the analytical solution to the measured cooling curves obtained

Thermodynamic properties of selected frozen backfill mix designs

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0.62.2 to2.5

1.73 to

6.23

0.15to 4.0

0.15to 2.5

1.73 to

3.463.633.3

ThermalConductivity

W/(mK)

1000916 to 922

1500to

2500

1600 to 2100

1300to

1500

2200 to

2500225021001420

DensityKg/m3

WaterIceFoundation materials

Sand dry to

saturated

Clay Dry to

saturated

Concrete mass

Tailings, ice,

water

Rock, tailings,

Icewater

Rock, ice

water

Typical valuesExperimental values

Properties

Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

CONCLUSIONSCONCLUSIONS• The addition of only ice crystals and water to rockfill does not produce sufficient

strength with the mixing process that was followed.

• Initial results indicate that rockfill in combination with moisturized tailings (above

25% dry weight) can provide sufficient strength for open stoping operations.

• The backfill set time is highly sensitive to the initial temperatures of the

constituents.

• The amount of liquid water used as binder should be minimized as it introduces

latent heat to the mix, which will delay the set time.

• Frozen backfill is an environmentally friendly solution for open stoping in

permafrost.

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Laurentian University School of Engineering

Phase 1: Experimental WorkPhase 1: Experimental Work

FUTURE WORKFUTURE WORK

• Process and composition design for application to Raglan Mine.

• Determination of representative material properties of frozen

tailings and rockfill mixes for a range of temperatures and

compositions for the purposes of modelling.

• Analytical and numerical modelling of heat-flows and stope wall

stability to simulate the long-term in-situ conditions.

• A site trial for process finalization and long term monitoring of

temperature and stability.

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