Laboratory and field study of oxygen flux and hydrogeological behaviour of monolayer covers made of low-sulfide tailings combined with an elevated water table placed over acid-generating mine tailings

Nicolas J. Rey, Isabelle Demers, Bruno Bussière, Mamert Mbonimpa, Sylvain Lortie

Dans les comptes rendus d’articles de la conférence: GeoOttawa 2017: 70th Canadian Geotechnical Conference; 12th joint with IAH-CNC

Session: Mining - Reclamation and Covers I

ABSTRACT: An experimental protocol including laboratory and field tests was conducted in order to evaluate and optimize the reclamation concept of Doyon-Westwood pond #2 and 3: the monolayer cover combined with elevated water table. The efficiency of this method based on the capacity to maintain sulfidic tailings at high water saturation, in order to control their oxidation and consequent acid production, depends on the proper parametrization of the level of the water table, cover thickness and grain size distribution. In this paper are presented the results of volumetric water contents and suctions, monitored at different positions inside four laboratory columns submitted to wetting-drainage cycles, and one field cell exposed to natural atmospheric conditions, in which was deposited Doyon sulfidic tailings surmounted by a cover made of low-sulfide tailings. Oxygen fluxes estimated with oxygen diffusion-consumption tests, and the measurement of oxygen gradient inside the cover, are also discussed. Un protocole expérimental incluant essais de laboratoire et de terrain a été mené pour évaluer et optimiser le concept de restauration des parcs #2 et 3 de la mine Doyon-Westwood : le recouvrement monocouche avec nappe phréatique surélevée. L™efficacité de cette méthode basée sur la maintenance à haute saturation en eau des résidus sulfureux afin de contrôler leur oxydation et la génération consécutive d™acide, dépend du paramétrage adéquat du niveau de nappe, de l™épaisseur et de la granulométrie du recouvrement. Dans cet article sont présentés le suivi des résultats de teneur en eau et de succion, en divers points de quatre colonnes de laboratoire soumises à des cycles de lixiviation, ainsi qu™une cellule de terrain exposée aux conditions atmosphériques naturelles, dans lesquelles ont été déposé des résidus sulfureux Doyon surmontés d™un recouvrement fait de résidus peu sulfureux. Des flux d™oxygène estimés avec des essais de diffusion-consommation ainsi que par mesure des gradients sont aussi discutés. INTRODUCTION 1 The monolayer cover combined with Elevated Water Table (EWT) is a fairly new restoration concept designed to prevent the formation of acid mine drainage (AMD) from sulfidic tailings stored in retention ponds, where the water table level can easily be raised and controlled. It was implemented at full scale in only a few places and showed mitigated results, related mostly to design or conception flaws (Dobchuk et al. 2013; Ethier 2013). A few parametric laboratory studies however proved its efficiency when correctly set up (Demers et al. 2009a; Ouangrawa et al. 2009; Pabst et al. 2011). Due to the relatively scarce literature on the subject, and real cases of application, a certain degree of skepticism of the industry regarding this approach persists. Nevertheless, the robustness of the setting as well as its long term reliability, compared to other technologies such as water covers, encourages further investigation. Moreover the approach is particularly well suited for integrated mine waste management (Demers et al. 2009a). Indeed, progresses in the field of tailings desulphurization (Benzaazoua et al. 2000) have allowed considering the use of low-sulfide tailings to constitute the cover, which brings numerous advantages. Indeed, environmental footprint is attenuated, volumes for tailings accumulation are saved, transportation costs are reduced (drastically if the cover is deposited hydraulically), and finally the residual sulfides remaining in the cover material can temporarily limit oxygen fluxes by consuming a portion of the oxygen going through the cover (Demers et al. 2009b; Mbonimpa et al. 2003; Mbonimpa et al. 2011). Following that principle, IAMGOLD Corporation operating the Westwood gold mine in Abitibi, Quebec, has opted for the integration, at the gold production stage of operations, of a desulphurization process designed to produce low-sulfide tailings. This material will later be used for the construction of a monolayer cover designed to reclaim the nearby Doyon-Westwood retention ponds #2 and 3, also managed by the company. The sulfide concentrate by-product resulting from the treatment will be incorporated into the backfill material of Westwood underground stopes. The cover may be deposited hydraulically using a pipeline. Nevertheless it is important to note that this method can lead to horizontal and vertical heterogeneities in the properties of the cover.

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