Evaluation du comportement geochimique de steriles potentiellement generateurs de drainage neutre contamine a l'aide de cellules experimentales in situ.

摘要

One of the main environmental problems associated to mining is the surface storage of solid mine waste such as mine tailings and waste rocks. When exposed to the ambient climatic conditions, these wastes can react and generate contaminants in the drainage water. Because of their proprieties (heterogeneous waste with wide-spread grain-size distribution, highly variable hydro-geotechnical proprieties, high porosity, high angularity, etc.) waste rocks are often considered more problematic than tailings in an environmental point of view.;Nickel contaminated CND can be observed at Tio mine site where the concentrations at the final effluent occasionally exceed environmental regulation criteria. The deposit exploited at Tio mine (owned by QIT- Fer et Titane inc.) is the largest massive ilmenite deposit in the world. It is located in the Cote-Nord region of the province of Quebec in Canada. This deposit has been exploited since the 1950's, and has generated a large quantity of waste rock. Previous studies performed on the CND generating potential at Tio mine and elsewhere have highlighted many weaknesses in the existing tools used to predict water quality from waste rock piles. Results showed that laboratory tests are often not very representative of the reality (significant differences between the lab concentrations and those observed on the field).;To fully understand the geochemical behaviour of Tio mine waste rocks, six different waste rocks were selected (waste rocks with different ore grade, freshly blasted or oxidized waste rocks) and submitted to intermediate scale (25 to 30m3) field kinetic tests. The construction of 7 experimental cells was done in the fall 2006. It is worth mentioning that at the Tio mine site, the major part of the waste rocks are exposed to natural surface conditions while a certain portion of the waste rocks has been deposited underwater in lakes. To better understand the impact of the deposition conditions of the waste piles on the generation of CND, two types of cells were built: submerged and exposed. More specifically, six cells are exposed to natural surface conditions, while the seventh cell is submerged. All cells allow for the collection of percolation water (or surface water for the submerged cell) and the calculation of flow rates. For the submerged cell, a piezometer also allows sampling of the water within the waste rocks.;Results analysis from the percolating waters in the experimental cells during the years 2006 to 2008 allows to categorize the different waste rocks studied as non acid-generating in the short and long term. However, results show that the waste rocks have a CND potential for nickel that would come from, according to our characterisation work, pyrite oxidization (nickel is closely related to pyrite). During the tests, concentrations in nickel were measured in leachates of all cells (but below the Directive 019 criterion). However, for the cells containing altered, high hemo-ilmenite grade waste rocks, leachates have punctual nickel concentrations close to the Directive 019. Concentrations of up to 0.946 mg/L were observed while the Directive 019 criterion is 1 mg/L for a punctual sample. Results also showed that nickel concentrations in the cells' leachates are higher for the altered waste rock than for the freshly blasted material. Sorption tests have shown that fresh waste rocks have a higher nickel retention potential than the oxidized waste rocks, which mostly explains the difference in nickel concentrations in the leachate from the cells with fresh and oxidized waste rocks.;As mentioned previously, the different minerals present in the waste rocks are susceptible to react with the surrounding water and air, and therefore affect the quality of the percolating waters in the waste piles. The chemical reactions can lead to an important acidification of the drainage waters; the term acid mine drainage (AMD) is used to describe this phenomenon. Another type of problematic drainage (called contaminated neutral drainage CND), not known until recently and therefore less studied, can also be caused by the reactivity of minerals containing elements soluble at near-neutral pH.;The study showed that a change in the deposition conditions (from natural conditions to submerged conditions) destabilizes the retention links and causes a release of elements (for a relatively short period of time) in the interstitial water of the submerged cell. This phenomenon has been observed during the 2006-2007 years. However, in 2008, the elemental concentrations from samples taken in the sampling well were reduced to values similar to those of the naturally exposed cells.;Finally, the leaching rates obtained in experimental cells were compared with those obtained at the Puyjalon pile. This comparison showed that the experimental cells can represent relatively well the conditions found in the waste rock pile. Even if the experimental cells cannot simulate the pile in its totality, the leaching rates obtained at both scales were in the same range. Finally, it is important to mention that even if this project is based on a particular case (Tio mine), the predictive approach proposed can be transferable to other mines with CND problems from waste rocks.