Évaluation de la performance de recouvrements miniers pour contrôler le drainage minier acide en climat arctique

In arctic regions where permafrost is present, the mechanisms leading to the generation of acid mine drainage (AMD) by mine tailings are slown down by the low temperatures. The occurrence of frozen conditions also contributes to limiting the transport of contaminated water into the receiving environment. Even though such conditions are observed, AMD-generation can still be observed in arctic conditions and mine site reclamation must be achieved. Many types of covers are available to control AMD-generation from tailings storage facilities located in the Arctic, including the insulation cover. The insulation cover seeks to integrate reactive mine tailings into the natural permafrost and control the tailings temperature to limit the generation of AMD. In order to be effective, an insulation cover must permanently maintain the tailings below a target temperature at which the oxidation of sulfide minerals is negligible. However, this approach can be vulnerable to changes in climatic conditions as its long-term performance is strictly based on controlling the temperature of tailings. For this reason, its performance must be demonstrated based on anticipated future changes in climatic conditions. In this context, a more robust alternative could be the use of insulation covers with capillary barrier effects (ICCBEs) because they control both the tailings temperature and the vertical migration of oxygen. ICCBEs use the temperature lowering properties of an insulation cover to reduce the period of time at which the temperature of the tailings are greater than the target temperature and the contrast in the hydrogeological properties of the cover layers to limit flux of oxygen available to the oxidation reactions during the periods when the temperature at the tailings-cover interface are greater than the target temperature. Even if this alternative approach is appealing, many aspects related to its performance must be validated before being implemented at full scale, including the effect of time on the properties of cover materials and their capacity to be effective under field conditions. This study aims at evaluating the short- and long-term performance of several covers constructed at Meadowbank mine for controlling sulfide oxidation. The tested covers include two insulation covers (2- and 4-m-thick) made of non-potentially acid-generating (NPAG) waste rocks and a 2 m-thick ICCBE constructed with a moisture-retaining layer made of fine compacted NPAG waste rocks and protective and capillary break layers made of raw non-compacted NPAG waste rocks. More specifically, this research project seeks to determine the main thermo-hydro-geotechnical properties of the cover materials and tailings, assess the performance of several cover configurations based on laboratory characterizations and field experimental cells, assess the durability of the NPAG waste rock unit to freeze-thaw and wet-dry cycles at the laboratory, analyze the long-term behaviour of insulation covers in the long-term using numerical modelling, and develop a generalized design framework for insulation covers. Laboratory characterizations showed that the loose and compacted waste rocks have the hydrogeological properties required to be used as capillary break and moisture-retaining layers, respectively. A difference in saturated hydraulic conductivity of four orders of magnitude was observed between the two materials (1 × 10-1 vs 1 × 10-5 cm/s), and residual suction conditions (125 cm of water) are reached in the loose waste rocks before the air entry value (250 – 300 cm of water) of the compacted waste rocks is reached. Oxygen diffusion tests performed on compacted waste rocks indicated a effective oxygen diffusion coefficient – saturation behaviour similar to typical granular materials. Oxygen consumption tests performed at the laboratory on tailings at different temperatures highlighted that the reaction rate of Meadowbank mine tailings becomes negligible at 0 °C. Accordingly, the target temperature of Meadowbank tailings was established at this temperature. Thermal characterizations indicated thermal conductivity of the solid particles of 4.80 and 5.80 W/m K for the waste rocks and tailings, respectively. A predictive thermal conductivity model was calibrated to best represent the data obtained in the laboratory. No significant scale and structural effects on the thermal conductivity were observed for the waste rocks. Therefore, the calibrated models can be extended to field conditions with confidence. The thermal and hydrogeological behaviour of the two insulation cover field experimental cells was monitored for 4.5 years. The tailings and the lower 0.50 m of cover materials were saturated. These saturation conditions have an impact on the cover material's thermal conductivity and on the distribution of the internal temperatures. The monitoring of internal temperatures indicated that conduction is the main heat transfer mechanism controlling the thermal behaviour of the studied insulation covers. The 2-m insulation cover showed temperatures at the tailings-cover interface greater than 0 °C for 94 to 124 days every year, whereas under the 4-m cover, the temperatures at the tailings-cover interface stayed below 0 °C. The comparison of the oxygen fluxes measured for the uncovered versus covered tailings indicated that the oxygen fluxes were reduced by 70 to 90 % by the insulation covers. The thermal and hydrogeological behaviour of the insulation cover with capillary barrier effects field experimental cell constructed at Meadowbank mine was monitored for 3.5 years. Results show thaw depths periodically reaching the tailings and temperatures at the tailings-cover interface greater than 0 °C for 39 to 57 days every year. During the period of time at which the temperature at the tailings-cover interface was greater than 0 °C, the average degree of saturation of the moisture-retaining layer was 84 %. In these saturation conditions, total yearly diffusive oxygen flux passing through the moisture-retaining layer inferior to 2 mol/m2/yr were calculated, thus confirming the efficacy of the cover to act as an efficient oxygen barrier. Considering the potential for future changes in climate, ICCBEs represent an attractive alternative to insulation covers, especially when the tailings are highly reactive. The laboratory tests performed to assess the durability of the waste rock unit with respect to freeze-thaw and wet-dry cycles indicated acceptable mass losses. The variations in terms of the material's physical and mechanical properties after 20 and 80 freeze-thaw and wet-dry cycles were within the acceptable limits for materials of high durability. The high durability of the waste rock unit is owed to its high density, low water absorption index, and low apparent porosity. Based on the results, no problem related to the physical integrity of cover materials or water quality is to be expected from the weathering of the waste rock unit. Ultimately, Meadowbank mine's waste rock unit is an excellent and durable construction material for mine closure purposes. Considering all the results obtained from laboratory characterizations and the monitoring of the field experimental cells, a numerical model was developed to assess the long-term behaviour of insulation cover configurations similar to those tested at Meadowbank. Considering an increase in air temperature of 6.0 °C over 100 years and post-operations ground thermal conditions, the models showed that the temperature of the tailings under the 2 and 4-m insulation covers decrease over the first 25 – 35 years due to the process of freezing. Then, the temperature at the tailings-cover interface gradually increase due to the impact of the increase in air temperature over time. The temperatures of the tailings under the 2-m cover become constantly below 0 °C after 4 years. Then the temperature at the tailings-cover interface becomes greater than 0 °C at year 82 and for 120 days at year 100. Under the 4-m cover, the temperature of the tailings becomes constantly below 0 °C after 9 years. The temperature at the tailings-cover interface is maintained below 0 °C year-round after the freezing period. A parametric demonstrated that the cover thickness, thickness of the tailings layer, variation in the increase in air temperature and moisture conditions are the main factors affecting the thermal behaviour and performance to control sulfide oxidation of the cover configurations. Ultimately, a generalized design framework comprised of eight steps was proposed. This design approach provides a framework useful for the industry to design insulation covers in the Arctic.