Frozen Backfill for Underground Mine Applications

Frozen Backfill for Underground Mine Applications Daniel Cluff1 & Vassilios Kazakidis2 1Camborne

School of Mines University of Exeter, UK

2Laurentian

Laurentian University School of Engineering

University, Sudbury, Canada

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Need & Challenge The need for an environmentally‐friendly mixture/process for creating cement‐free frozen backfill in underground mines in cold regions. The challenge is to create a mixture Which: 1) Has sufficient strength to meet the requirements of the particular application; 2) Minimizes the setting requirements, imposed by a mine schedule, by taking advantage of sub‐zero temperatures; 3) Is technically and economically feasible for introduction to mining environments by utilizing existing material and resources.


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Raglan Mine

The benefits of using a frozen backfill in lieu of the standard backfill options are immense from an operational, environmental, and cost perspective. In a frozen backfill delivery system attention must be given to specific details and controls relating to the delivery technology for the constituents. In addition to having an environmentally friendly alternative, where the fill blends seamlessly into the permafrost, the cost of using a 3% cemented rockfill in a 3,000 tonne/day operation can exceed $10 million per year, just for the cost of the cement alone. The transportation and plant costs are significant additional costs in a mine in cold regions.


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Initial Concept Waste rock, tailings with ice dropped into stope

Snow machine Stope overcut

Backfilled stope


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Design Variables The composition and grading of the dry fill material i.e. waste rock and tailings; The pre-existing moisture content of the waste rock and tailings; The amount of ice crystals added; The amount of liquid water added; The form of the ice crystals added; The mixing process;

The initial temperatures of the rock, tailings, ice crystals and water; The ambient temperature and thermal conductivity of the permafrost and mix; The volumes and temperatures of airflow near the frozen rockfill stope.


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Work Plan • Initial testing & analysis - Testing at NORCAT facilities

- Heat diffusion analysis • Lab testing & mixing process analysis

Completed

• Data analysis • Thermodynamic modelling • Numerical modelling strength - thermal properties • Site testing on a large scale


Funding ??

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Testing & analysis

Freezer

Sample Jacks

Compressor


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Testing Program at NORCAT Strength ! Required a scoop To knock it down

The use of a 6.1 m high by 1.65 m diameter steel form was needed to perform stiffness measurements and determine the thermal behavior on a larger scale than is available in laboratory testing.


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Testing Program at NORCAT A walk‐in freezer and a press were installed underground at the NORCAT research mine. Testing included measuring the load, displacement and temperature profile over time.


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The mixture containing rockfill and tailings exceeded the strength of 1 MPa UCS strength required by the mine. The strength of samples containing rockfill, tailings, water and ice crystals was similar to the samples containing no added ice crystals, which shows the potential for adjusting the moisture content of the tailings to provide the desired ice fraction. Samples consisting only of tailings and water had the highest strength exceeding 1.8 MPa demonstrating a high ductility: even when it was laterally deformed during the UCS testing it did not buckle. Sample size 24” by 48”


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Effect on the UCS strength of adding tailings to the mix. The initial work emulated a cemented Rockfill, which was shown experimentally to have insufficient strength. The addition of tailings to the mix in an amount approximately equal to the voidage in the rockfill was successful in developing sufficient strength.


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Mix Design Composition vs. UCS Backfill component percentage

Frozen backfill -6 oC mix design versus strength

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

0.11

0.32

1.1

1.3

1.6

>1.8

Uniaxial compressive strength (MPa)

Rockfill %

Tailings %


Added Water %

Voidage

Pre-existing ice

Added ice %

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Laboratory set up at Laurentian University The hopper has separate insulated compartments for rock and tailings with drop doors. The material is lowered to about -20 oC in the adjacent freezer, weighed and loaded into their respective hopper compartments.

The spray jets are located under internal baffles used to mix the material as it falls. Water flow rates and pressures are measured as the material falls to determine the percentage in the mix.

The temperatures of the five thermocouples are transmitted wirelessly to a data logger.


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Lab testing & mixing process Result of a Frozen Backfill pour test in the Laurentian University Laboratory


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Lab testing and analysis


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For constituents at an initial temperature of -20°C and water at 5°C. The percent energy distribution for each constituent and latent heat in the mix. The latent heat contained in the mix becomes dominant at water percentages greater than 5%.

Methods of mitigating this include ensuring the tailings and rock are below the threshold temperatures or chilling the water to remove latent heat.


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A typical cooling curve for a mix. Five thermocouples were located at strategic points the geometric center, axial midpoints and outer edges.


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Constituent Specific heat Measured Corrected Temperature Latent Percentage capacity J/gK Mass Kg Mass Kg Initial oC Heat J/g in mix Rock 0.8 60 57.72 -20 54.87% Tailings 0.9 40 36 -20 34.22% Ice 1.90 0 6.28 -20 5.97% Water 4.186 5.2 5.2 1 333.15 4.94% Moisture content Rock 3.80% Tailings 10.00% Total ice 10.91% Total Mass 105.2 Kg Predicted Temperature 272.7 K -0.49 oC •The above table contains values needed for the prediction of the set-temperature. •A simple heat balance is calculated based on the mass and thermal properties of the constituents. •If the set-temperature is above 0 oC then a delay in the development of strength can occur. •In some modeled scenarios this could be as long as two years and will impact on the stability of the host rock. •This can be avoided by using the appropriate mix design which includes the appropriate ratio of solid materials to water and strict adherence to the temperature guidelines. •The temperature guidelines are a function of the thermal properties of the solids and the amount of water added.


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U(n,k)(ρ, φ, z, t) = -Σ nΣ kA(n, k)Uie-(τ/σµ)λ(n,k)t Jo((ρ/a)ζn)sin(kπz/L) Property(↓) Mix design(→)

Rock ice water

Rock tailings ice water

Tailings ice water

1420

2100

2250

Thermal conductivity W/m K

3.3

3

3.6

Apparent specific heat J/goC

2.58

3.2

3.89

9 x 10-7

4.5 x 10-7

4.1 x 10-7

47

97

110

Density Kg/m3

Diffusivity m2/s Time to reach -6oC hrs

Theory and data for the cooling curve of a cylindrical sample of rock, tailings, ice and water backfill held at -6oC

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Temperature at the geometric center of the sample

0 -1 -2 -3 -4 -5 -6

Theory

Data

-7 0

50000

100000

150000

200000

250000

Time in seconds


300000

350000

400000

Where: •Jo is a Bessel’s function. •ζn is the nth zero of the Jo. •a is the radius and ρ is the distance along the radius. •L is the height and z is the distance along the height. •n and k are indices 1,2,3,…∞. •τ is the thermal conductivity. •µ is the density. •σ is the specific heat capacity. •К = τ /σµ is the diffusivity of the Backfill. •A(n, k) is the initial temperature Kelvin

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Proposed Concept 1

2 Cold rockfill &tailings 3 4

sprinkler Cold water

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6 Mixture of waste/water


Stope Overcut Bonding strength builds as the water freezes on the particle surfaces of moisturized waste

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Conclusion 1. A stable frozen mass with sufficient strength for particular application to underground mines is possible. 2. The establishment of equilibrium: a) time dependent b) controlled by the site-specific conditions c) properties of the inert material.

3. The mechanism for mixing of water and frozen solids in a material handling distribution system during the placement in an open stope needs to be tailored to the individual mine.


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Acknowledgements Laurentian University Xstrata NSERC NORCAT


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Thank you Any Questions?


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Acknowledgements • Laurentian University • Xstrata • NSERC • NORCAT


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