Importance de la minéralogie et des conditions climatiques et opérationnelles sur les risques de dépôt des résidus miniers filtrés

« RÉSUMÉ : Les résidus miniers sont déposés de manière conventionnelle sous forme de pulpe à une teneur en eau massique élevée, dans des parcs ceinturés par des digues. Toutefois, les structures de confinement présentent un risque élevé de rupture, pouvant entraîner des conséquences environnementales majeures, telles que la contamination des eaux et des sols environnants, notamment en raison de la libération de métaux lourds et de produits toxiques liée à la nature physique et chimique des résidus. Une alternative pour réduire ces risques géotechniques est la filtration, visant à réduire la teneur en eau des résidus lors du dépôt. Les propriétés géotechniques sont alors améliorées mais cet état désaturé favorise l’oxydation des résidus réactifs exposés, augmentant les risques de génération de drainage minier acide (DMA). Cependant, les réactions d’oxydation à l’origine du DMA peuvent prendre un certain temps avant de générer un lixiviat avec un pH acide. À ce jour, le nombre d’études concernant la gestion des résidus filtrés réactifs demeure limité, en particulier en lien avec les stratégies visant à prévenir la génération de DMA. Mieux comprendre leur comportement géochimique dès leur dépôt est nécessaire afin d’orienter la conception et leur gestion. L’objectif de ce projet de recherche est de comprendre les mécanismes qui permettent d’assurer la stabilité géochimique des parcs à résidus filtrés dès le début des opérations minières et jusqu’à la restauration finale. Pour ce faire, les travaux vont porter sur l’optimisation de leur conception sur la base de la composition minéralogique des résidus, des paramètres de dépôt et des conditions climatiques d’exposition.»

Abstract

« ABSTRACT : Tailings are conventionally disposed of as a pulp with an elevated water content, in impoundments surrounded by containment structures. However, containment structures present a high risk of failure, which can lead to major environmental consequences, such as contamination of the surrounding water and soil, particularly due to the release of heavy metals and toxic products linked to the physical and chemical nature of the tailings. An alternative to reduce these geotechnical risks is filtration, which aims to lower the water content of tailings during deposition. Geotechnical properties are thereby improved; however, this unsaturated state promotes the oxidation of exposed reactive tailings, increasing the risk of acid mine drainage (AMD) generation. Nevertheless, the oxidation reactions responsible for AMD may take some time before producing a leachate with an acidic pH. To date, the number of studies addressing the management of reactive filtered tailings remains limited, particularly with regard to strategies aimed at preventing the generation of AMD. A better understanding of their geochemical behavior from the time of deposition is essential to guide design and management practices. The objective of this research project is to understand the mechanisms that ensure the geochemical stability of filtered tailings storage facilities from the beginning of mining operations through reclamation. To achieve this, the study focused on optimizing their design on the basis of on the tailings’ mineral content, deposition parameters, and the climatic exposure conditions. The first part of this project examined the time required for exposed tailings to generate an acidic pH below 6, a duration referred to as the tailings’ critical time. The determination of this critical time was based on laboratory kinetic tests as well as on numerical simulations. Initially, five cells containing LaRonde tailings, saturated at 79%, 83%, and 94%, were used to evaluate the effect of the degree of saturation on the critical time. Additionally, two cells containing LZ5 tailings, both subjected to a degree of saturation of 84%, were used to compare the effect of the mineral content on the critical time. Then, the effect of saturation variation was assessed using two cells in which the degree of saturation decreased each month in increments of ±10% (ranging from 100% to 70%). Finally, based on the results obtained during this initial series of tests, four cells were used to investigate the effect of temporal fluctuations between a saturated state (100%) and an unsaturated state (<90%). In a second phase, extrapolations performed via 1D numerical simulations using the MIN3P software, and calibrated with laboratory results, provided additional data by simultaneously varying both the degree of saturation and the mineral content of the tailings. Laboratory results showed that maintaining a degree of saturation higher than 90% delayed the onset of the critical time. Furthermore, an acidic pH was observed between 72 and 93 days in cells subjected to a degree of saturation below 90%. Numerical extrapolations showed the tailings’ neutralization potential (NP) of the carbonates present in the tailings was the most determining parameter for prolonging the critical time. However, not all carbonates have the same effect on the critical time: ankerite alone was unable to maintain the pH value higher than 6, compared to calcite. Finally, a simple model to calculate the critical time was proposed.»