Techniques de controle du drainage minier acide basees sur les effets capillaires

Acid mine drainage (AMD) ensuing from the oxidation of sulphide minerals contained in mining wastes remains one of the critical challenge for the mining industry. This oxidation reaction can cause acidification and heavy metal release in surface and ground water. To inhibit the acid formation, one must exert control on the constitutive elements (sulphide, water or oxygen) of oxidation reactions. In a humid environment, such as the one found in Quebec, studies have shown that preventing oxygen from reaching the reactive material constitutes a practical approach for controlling the production of AMD. A cover with low gas permeability or a technique that limits its transport can be use to this effect. Capillary effects that occur in porous media can be applied toward gas migration control by keeping a soil close to saturation, thus reducing oxygen diffusion through it. This research project evaluates two practical means using capillary effects in porous materials (soils and tailings) to control the production of AMD. It is divided in two main parts, each focusing on a control method.;The first part of the project involves the concept known as "elevated water table" as a management and reclamation method for acid generating tailings. This concept is based on the fact that tailings in the capillary zone are maintained in a state close to full saturation, which inhibits oxygen diffusion and controls AMD generation. The study addresses the practical aspects of the elevated water table concept: the oxidation rates that can be expected (efficiency) and the water table level's influence on these. Results from column tests performed on three different types of tailings at different water table levels show how the oxidation depends on the degree of saturation associated to capillary rise.;A complementary numerical parametric study, based in part on the tests performed in the lab, was conducted to evaluate the impact of various factors on the water content profiles in the tailings, including the evaporation rate, thickness and properties of the protection layer, water level, and hydraulic properties of the sulphide tailings. The oxygen fluxes associated with the different conditions are also evaluated and analysed. Results show how these factors may influence the water content profiles in the tailings and the oxygen fluxes causing acidification of the leachate. They were related to the efficiency of the concept for preventing the generation of AMD. Criteria to be used in a preliminary approach are also proposed here.;The second main component of the research project deals with the hydrogeological behaviour and the efficiency of covers with capillary barrier effects (CCBE), based on an actual large-scale cover system and on a series of in situ tests and numerical calculations. The CCBE at the core of this investigation has been constructed in 1999 at the Lorraine mine site, Quebec, Canada. It has been instrumented and monitored using TDR probes (for water content measurements), Watermarks (for suction measures) and observation wells to help assess its hydrogeological behavior and its performance in limiting oxygen diffusion. Modified oxygen consumption tests were also done to evaluate the oxygen flux through the cover.;Water content and suction data, measured from 1999 to 2003, show that the capillary barrier effects are effective where needed on the site ( i.e., where the water table is below the tailings surface). Oxygen fluxes estimated from monthly water content measurements and those inferred from the modified oxygen consumption tests fully meet the design objectives. A two dimensional numerical model of the Lorraine mine site was also built to evaluate the capacity of a mathematical model to represent realistically the in situ conditions on a large scale site. The unsaturated flow calculation results are compared with observations made in situ . The model is based on laboratory data and conditions gathered on site. It represents a north - south transect across the old tailings pond and the cover. Numerical results obtained for three years show a good agreement between measured and calculated values. The results obtained from the different methods used in this research to evaluate the CCBE performance concur and confirm the efficiency of this cover in limiting oxygen flux to the acid generating tailings.;The study has demonstrated further the CCBE's ability to control the oxygen flux related to acid mine drainage (AMD). Nonetheless, it has also been shown that detrimental factors such as freeze-thaw cycles can affect their long term performance. In this regard, laboratory tests were performed to assess the effects of freeze-thaw on the hydraulic conductivity, water retention curve, and pore size distribution. Results for the tested soil samples show variations in their hydraulic properties due to the presence of induced fissures.;The presence of fissures induces variations in the internal structure of the samples, which are confirmed by results from mercury intrusion porosimetry tests. This variation in internal structure can influence the soil's ability to perform as expected. After freeze-thaw cycles, the flow of water and diffusion of oxygen could be concentrated in these new cracks. Depending on the connectivity of the fissures, the additional flux may be significant and can affect the long term performance of the covers. In situ testing was also done on a CCBE, showing evidence of fissuring after several winters. For this case however, the changes induced in the water retention layer (micro) structure were not sufficient to influence significantly the CCBE performance.*.;*This dissertation is a compound document (contains both a paper copy and a CD as part of the dissertation). The CD requires the following system requirements: Micrsoft Office.