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Artificial Ground Freezing Proceedings_Theme_02_Proceedings IMWA 2011 22/08/2011 12:43 AM Page 113 Aachen, Germany “Mine Water – Managing the Challenges” IMWA 2011 Artificial Ground Freezing: An Environmental Best Practice at Cameco’s Uranium Mining Operations in Northern Saskatchewan, Canada Greg Newman¹, Lori Newman¹, Denise Chapman², Travis Harbicht² ¹Newmans Geotechnique Inc., 311 Adaskin Cove, Saskatoon SK, S7N 4P3, Canada ²Cameco Corporation, 2121 11th St. West, Saskatoon SK, S7M 1J3, Canada Abstract Cameco Corporation (Cameco) is becoming a leader in the application of artificial ground freezing for controlling water and ground stabilization at its existing and in-development high-grade uranium mines in Canada. Artificial ground freezing (AGF) has been used in the mining industry over the past 125 years for support of shaft sinking, tunneling, and foundation excavation. The AGF process involves chilling a brine so- lution to between -25 °C and -35 °C in a large, conventional refrigeration plant. The brine is then circulated through the ground within steel pipes in a linear or grid pattern to remove heat from the ground and freeze water within the soil/rock pore spaces to improve soil strength and decrease the mobility of liquid water. Tra- ditional AGF projects are usually short-term in nature, which has limited the relevance of the long-term effi- ciency and environmental benefits of the process. Freezing projects at the northern Saskatchewan uranium mine sites typically will be active for 10 years or longer, providing an opportunity to consider the development of a best-practice philosophy for all phases of the project, from design through to operations and decommis- sioning of freeze systems. Key Words artificial ground freezing, water control, shaft sinking, tunnelling Introduction turns to ice. When a cold heat sink (i.e. cold brine Cameco strives to continuously improve environ- pipe) is introduced in the ground, stored thermal mental performance in all aspects of operations. energy flows from the ground toward the heat Main areas of focus have been waste reduction, sink and the temperature of the ground drops. improved air quality, reduced energy consump- The warmer brine then flows back to the refriger- tion, and decreased water intake and treated water ation plant where it is re-cooled and circulated release volumes. Ground freezing has been recog- back through the system. The rate of ground cool- nized as a critical component of Cameco’s mining ing is dependent on several things: the thermal success: creating a barrier to groundwater flow, conductivity or ability of the ground to transport strengthening weak ground mass, and providing heat, the temperature of the ground and the brine, control of radon gas release. This paper will focus the ability of the ground to release heat (e.g. heat on the environmental management benefits that capacity), the magnitude of the heat gradient es- can be gained through the implementation of tablished between the warm ground and the intro- ground-freezing technology at uranium mining duced cold heat sink, and finally the amount of operations in northern Saskatchewan. Most no- water stored in the ground (Newman 1997). The tably, the use of long-term ground-freezing tech- amount of water is important because when nology provides ongoing groundwater inflow water changes phase from liquid to solid, it re- management thereby substantially reducing leases latent heat which must also be removed treatment and release volumes. In addition, oper- from the system through the cold heat sink. ating procedures and practices to reduce energy consumption has been developed and has ex- plored the potential for heat-recovery systems as part of freeze-plant operations which can be ap- plied to other operational areas, decreasing en- ergy demands. Other innovative uses for freezing are being investigated in support of future shaft sinking which could also further enhance the en- vironmental benefits of this technology. How Ground Freezing Works Artificial ground freezing (AGF) is a process by which stored thermal energy (e.g. heat) is re- moved from soil or rock to such an extent that the water held in the interstitial pores in the ground Figure 1 Typical underground ore freezing con- cools to below the phase change temperature and cept. Rüde, Freund # Wolkersdorfer (Editors) 113 Proceedings_Theme_02_Proceedings IMWA 2011 22/08/2011 12:43 AM Page 114 IMWA 2011 “Mine Water – Managing the Challenges” Aachen, Germany A typical ground freezing system is shown in surface at an unconformity between highly frac- Figure 1 (Newman 2000). In this image a series of tured silicified sandstone above and relative dry freeze pipes have been installed in a linear pattern granitic basement rock below. The fractured sand- to optimize the rate of cooling and create a frozen stone contains water in a fracture network con- barrier wall. A freeze pipe is a two-part system that nected to ground surface and as such the water consists of an outer steel casing with a sealed bot- pressure at the depth of the ore zones is as high as tom cap and an inner high-density polyethylene 6000 kPa and has unlimited flow potential. In the tube. Cold brine, typically calcium chloride (CaCl₂) vicinity of the orebodies themselves, the ground chilled to between -25 °C and -35 °C, is introduced is often de-silicified with potentially flowing sand inside the polyethylene tube where it flows to the and unconfined squeezing clay. To complicate end of the freeze pipe before returning to the top matters, the pore-water in the ore zone vicinity of the hole in the annular gap between the tube contains high concentrations of radon gas which and the outer steel casing. It is during the return comes out of solution as soon as the pore water is flow to the top of the freeze pipe that the cold depressurized (e.g. enters the mine workings brine draws heat from the surrounding ground. below) (Werniak 1999). A typical cross-section of Ground freezing requires front-end design and the geology and associated mining-related haz- analysis to determine the rate of freezing, the op- ards is given in Figure 3. This particular image is timum freeze-pipe configuration, and the size of representative of the conditions at the high-grade refrigeration or freeze plant necessary to accom- McArthur River mine located in northern plish the project goals. Finite element analysis Saskatchewan, Canada. (FEA) is used to develop a design and to size the During early design phases at McArthur River freeze plant so it can be procured and so that there was discussion about the potential for using power consumption rates can be known ahead of a combination of cement grouting and pumping time. A typical ground-freezing heat-load curve is to stabilize the ore zone and control the pore shown in Figure 2. In this figure the units of heat water. Cement grouting will work in circum- extraction are kilowatts per meter (kW/m) which stances where the ground or rock formations are is an energy value per unit time per unit length of susceptible to grout penetration. This would be freeze pipe. It is quite typical to have many thou- the case in the fractured sandstone, but the highly sands of meters of installed freeze pipes surround- altered ground directly adjacent to the ore zones ing a mining area and so it is not uncommon to proved to be too variable and the risks associated need several megawatts of electrical power avail- with trying to grout the zone were deemed too able to drive the system. Obviously any opportu- high. Likewise, allowing inflow and having suffi- nity to reduce this energy requirement, or to cient pumping capacity to remove the water car- recover the waste heat energy, is a good practice ries very high uncertainty. In addition, allowing to adopt. the high radon-bearing water to flow intentionally into the mine poses an unnecessary risk to mine Why Ground Freezing is Needed in Uranium workers. Finally, if very high inflow volumes were Mines allowed, large amounts of water would have to be The high-grade ore deposits at Cameco’s pumped to surface and treated prior to release to McArthur River and Cigar Lake uranium mines the environment – a load which could exceed are located between 450 and 600 m below ground 3000 m³/hr. For these reasons grouting and Figure 2 Typical power consumption curve per Figure 3 Cross-section of ore and original freeze meter of installed freeze pipe. wall concept at McArthur River Mine. 114 Rüde, Freund # Wolkersdorfer (Editors) Proceedings_Theme_02_Proceedings IMWA 2011 22/08/2011 12:43 AM Page 115 Aachen, Germany “Mine Water – Managing the Challenges” IMWA 2011 pumping water were deemed to be a non-viable The subsequent delay to mine development, as primary control strategy and ground freezing was well as the water pumping and treating required considered not only the most feasible operational to remediate the shaft, has proven costly. Cameco solution, but also is beneficial in that it reduces a has applied the knowledge gained from this expe- significant volume of water which would require rience. In the future, mine shafts will be sunk management. using a unique combination of freeze-wall barrier Several implications of the challenging ground with thawed central core, instead of a grout-cur- conditions are evident in Figure 3 as they relate to tain approach. More details of this best-practice ground freezing. First of all, given the high-pres- approach to using freezing in support of shaft sure water and the poor ground conditions in the sinking are provided in the following section. vicinity of the ore, it is not possible to mine the ore without depressurizing the water and stabiliz- Best Practices for Implementing AGF ing the ground, both achieved by ground freezing.
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