Performance de recouvrements isolants partiels pour controler l'oxydation de residus miniers sulfureux.

摘要

An important challenge for the mining industry is the reclamation of tailings storage area, especially when tailings can generate acid mine drainage (called AMD). AMD is caused by the reaction of sulfide minerals in tailings exposed to air and water. Integrating tailings within the permafrost by the use of an insulation cover is a reclaiming method designed and used especially for Nordic conditions.;The main objective of this project is to evaluate the performance of partial insulation covers (0.4 m and 1.2 m thick) built on the Raglan Mine tailings storage facility. The method used to assess the performance of the partial insulation covers includes oxygen consumption tests (OC tests) for measuring tailings reactivity under the partial insulation covers. Moreover, temperature, volumetric water content and suction measurements at various depths in the cover material and in sulfide tailings allow to study the effect of the partial covers' hydro-thermal behavior on tailings reactivity for this particularly site.;Cover and tailings materials were characterized for their mineralogical, physical, hydrogeological and thermal properties. Results show that the tailings hydrogeological properties are typical for that kind of material (saturated hydraulic conductivity around 1x10-5 cm/s and air entry value around 20-30 kPa). Tailings have a good potential for acid generation with their high sulfide content (10 to 20% wt) and low neutralization potential. Tailings reactivity at different degrees of saturation was evaluated in laboratory, and oxygen consumption test variability was assessed in situ in the Raglan Mine tailings storage facility. Cover material (0-20 mm) has a saturated hydraulic conductivity near 1x10-1 cm/s and an air entry value around 1 kPa; its geochemical behavior is not considered problematic. In situ characteristic soil freezing curves for tailings and cover material agree well with water retention curves obtain in laboratory for suction greater than 1x103 cm. In situ water retention curves agree well with the laboratory water retention curve for cover material but underestimate water retention for tailings.;Oxygen fluxes between 0 moles·m-2·an -1 and 544 moles·m-2·an-1 were measured for non-covered tailings and between 4 moles·m -2·an-1 and 525 moles·m-2·an -1 for tailings with 0.4 m and 1.2 m partial insulation covers. Results of the study show that tailings hydrogeological conditions in the tailings storage facility are favorable for their oxidation (40%< Sr < 60%). Effect of temperature on tailings reactivity had previously been observed and this effect is well described with the Arrhenius law for the Raglan Mine case. Tailings reactivity is considerably decreased for temperature between -2°C and -6°C; this temperature interval can be considered as an objective for controlling mineral oxidation of Raglan Mine tailings. Tailings reactivity will vary according to the emplacement in tailings stack and the period of the year.;Partial insulation covers (0.4 m and 1,2 m) are unsaturated for most of the time (Sr < 40%) and maximal tailings temperature (5 cm under surface) is 7°C, 1.4°C, and 25.2°C for tailings covered with 0.4 m, 1.2 m, and non-covered tailings. The partial insulation covers performance for the control of tailings oxidation was assessed with the temperature profile and oxygen consumption test methods. Partial insulation covers do not affect the oxidation period length, but reduce the period where tailings are highly reactive. Performance of 0.4 m and 1.2 m partial insulation covers is 52 +/- 18% and 65 +/- 25% respectively, based on the mean annual oxygen flux with and without cover. The study has shown that the efficiency of partial insulation covers to control tailings oxidation varies according to the period of the year. They have a beneficial effect most of the year, except for the period between from the end of September 2011 and mid-December 2011, and from the end of September 2011 to the end of October 2011 for the 1.2 m and 0.4 m respectively.;The results of this study are interesting for the management of the progressive reclamation plan, and for the understanding of the efficiency of the proposed final insulation cover for the Raglan Mine tailings storage facility. For the performance evaluation of the final cover scenario, a laboratory study of the tailings reactivity in controlled conditions should be undertaken to identify the temperature at which tailings oxidation is stopped. Furthermore, the performance of reclamation scenario is evaluated on the effluent water quality; the next challenge will be to relate the oxygen flux with the water quality effluent. A thermo-hydro-geochemical modelling would allow the understanding of the mid-long term behavior of Raglan Mine tailings stack and to include the climate change scenarios in the cover performance evaluation.