Laurentian University School of Engineering Laurentian University School of Engineering Laurentian University School of Engineering 1 EVALUATION OF FROZEN BACKFILL FOR OPEN STOPE MINING IN PERMAFROST CONDITIONS Daniel L. Cluff James Gallagher, Ali Jalbout, Vassilios Kazakidis, Graham Swan. CIM 2008 Video of Large form 1

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work PROBLEM STATEMENT To investigate the requirements of frozen consolidated fill for blasthole/panel mining at mines in permafrost. 2

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work PROJECT 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. 3

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work CHALLENGES 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 4

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work TESTING FACILITY AT NORCAT MINE Sample LVDT Top Plate Jacks Bottom Plate Sample Jacks Freezer Data acquisition Video of testing facility 5 Compressor

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work TESTS 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 ft 3 was used to allow for better control of measurement and better mixing. Video of tailings and rockfill mix method 6

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work TESTS 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 7

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work Low 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. Number of tests % Dry Rock % Dry Tailings % Crystal ice Added % Water Added (sprinkled ) %Moisture in Rock Total % Moisture content in mix Strength (MPa) (wet snow) Temperature was -6 o at the time of the UCS testing 8

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work Tests with Sufficient Strength Number of tests % Dry Rock % Dry Tailings % Crystal ice Added % Water Added (sprinkled ) % Moisture in Tailings %Moisture in Rock Total % Moisture content in mix Strength (MPa) (wet snow) (wet snow) > > 1.8 (temperature was -6°C for all UCS tests) 9

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work Role 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. c w = 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. c i ≈ 2.0 J/g Rock Waste rock is economically disposed of in backfill. Has low specific heat capacity. c R ≈ 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. 10

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work THE MIX DESIGNS USED IN THIS PHASE 11

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work LARGE 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 Ice Crystals~50 Water Added8~

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work LARGE SCALE ROCKFILL PILLAR 13

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work THERMODYNAMIC MODELLING OF 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. 14

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work THERMAL EQUILIBRIUM THERMAL 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 -20 o C and Water at 5 o C 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 Percent contribution energy balance RockTailingsIce crystalsLiquid waterLatent Heat 15

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work THERMAL PROPERTIES OF 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 to to to to to Thermal Conductivity W/(mK) to to to to to Density Kg/m 3 WaterIce Foundation materials Sand dry to saturated Clay Dry to saturated Concrete mass Tailings, ice, water Rock, tailings, Ice water Rock, ice water Typical valuesExperimental values Properties

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work CONCLUSIONS 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. 17

Laurentian University School of Engineering Laurentian University School of Engineering Phase 1: Experimental Work FUTURE 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. 18