Étude du comportement thermique d'une halde à stériles en milieu nordique

A major challenge for the mining industry is the environmental reclamation of waste rock piles (WRPs) potentially generating acid rock drainage (ARD). ARD occurs especially when the sulfide minerals present in the mine waste rock are exposed to water and atmospheric oxygen. The resulting oxidation reaction causes the release of acid, iron, and sulfate. The acid leachate thus formed is then likely to percolate into the WRP, to become rich in various contaminants, and then to flow into the natural environment, leading to long-term ecosystem degradation over large areas. The formation of ARD can be controlled by acting on one or more agents according to various techniques. Temperature control with insulating covers is a unique technic to northern regions. This method aims to incorporate mine waste rocks into permafrost. At low temperatures, the kinetics of geochemical and biochemical reactions are almost zero. As a result, oxidation reactions from reactive mineral waste rocks are slowed down, reducing the production of contaminants. Also, the freezing controls the movement of fluids in the pile, which limits the migration of contaminants. Meadowbank is a gold mine located in the Kivalliq region of Nunavut (65 ° N). At Meadowbank, an insulation cover is in place for the reclamation of the WRP. The special feature of the Meadowbank cover is that it consists of non-reactive waste rock (NPAG). In order to evaluate the effectiveness of the cover at keeping potentially acid-generating waste rocks (PAG) below the freezing point, 14 thermistors were installed on the waste rocks pile between 2013 and 2015. The objective of this study is to evaluate the thermal behavior of an insulating cover system built on the Meadowbank WRP. The specific objectives of the project are: (1) characterize the cover materials and the reactive waste rock; (2) sample and analyze in situ temperature measurements; (3) develop a numerical model calibrated according to material properties and field conditions; and (4) identify the main heat transfer mechanisms in the Meadowbank WRP from in situ measurements and numerical analysis results. To achieve the objectives, characterization work on hydro geological, mineralogical and thermal properties was undertaken, for both types of materials, NPAG and PAG. The results show that the materials have typical hydro geological properties of waste rock (saturated hydraulic conductivity of 1.3 x 10-1 cm/s for NPAG and 1.1 x 10-1 cm/s for PAG) with air entry value of less than 5 cm. The hydro geological properties are also very similar between the two types of materials, except that the NPAG (CU = 284 and a D10 = 0.04 mm) contains a greater proportion of fine particles than the PAG (CU = 13 and a D10 = 1.01 mm). NPAG is mainly composed of talc (31%) and chlorites (26%), while the two main minerals identified for PAG are quartz (44%) and chlorites (14%). The characterization of the thermal properties of the solids was evaluated at 5.27 W/m K for the PAG and 4.80 W/m K for the NPAG. Further study of the thermal properties of solids for NPAG demonstrated the importance of considering the oriented component of thermal conductivity for certain minerals, such as talc, when the needle probe method was used. Moreover, according to the tests carried out on different particle sizes, no scale effect of the thermal properties is expected for the two materials. The evaluation of in situ temperature profiles demonstrated that the pile had not yet reached thermal equilibrium. At the interface between the cover and the reactive waste rock, the mean global temperature was -6.2 ° C. The depths of the active layer were evaluated between 1.9 m and 7.9 m, with an average of 3.9 m. In 2016 and 2017, three thermistor chains revealed lower thaw depths than the NPAG/PAG interface. However, a decrease in depths was observed between years of the study. Temperature profiles also showed latent heat indices in the first eight metres of the WRP for four stations. Numerical modeling has been used to better understand the processes of heat transfer in the pile. The one-dimensional numerical model in phase change conduction has been calibrated according to material properties and field conditions. Calibration of the model demonstrated that the properties of the materials had little influence on the model. Meteorological parameters during calibration revealed no trend in slope orientation, nor significant differences in maximum temperatures near the NPAG/PAG interface with net radiation variation. In general, the calibrated models had cooler temperatures than the temperatures measured for all year, but the trend was roughly the same as measured temperatures. Numerical modeling in 2D showed the presence of edge effects, but the effect was pronounced in cold temperatures and had little influence on warm temperatures near the surface, so on the thaw depth. The temperature profiles of 2D modeling compared to 1D modeling was very similar and showed that 1D temperature analysis can be considered adequate to predict the overall thermal behavior of a waste rock pile such as Meadowbank. Finally, research work allowed to better understand heat transfer in the Meadowbank waste rock pile. As a result of this work, it is recommended to continue the data collection with a particular attention to the thermistor chains where the thaw was observed under the cover. It would also be interesting to evaluate the critical temperature in order to make the connection between the temperatures and the acid generation potential of the waste rock. Finally, it would be relevant to measure water levels and study ice formation near the base of the active layer to see the effect on water movement and heat transfer.