Comparison of the precipitation of calcium carbonates to the dissolution of uranium in uranium mine tailings

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The natural resource sector is an important area of Canada's economy. Of the many natural resources that are mined, uranium mining is centred in northern Saskatchewan. The uranium ore bodies contain many mineral phases beside uranium oxide that need to be disposed of in a responsible fashion. Solid mining waste can be placed in a tailings management facility, which is designed to contain the waste elements and limit interaction with the external environment. To understand how a tailings facility will behave throughout the lifetime of the mining operations that feed the tailings management facility (TMF), and after the site has been decommissioned, it is important to understand the chemistry of the tailings and how they change with time. The JEB tailings management facility located at the McClean Lake operation contains many metals from the separation of uranium oxide, including iron (Fe), arsenic (As), molybdenum (Mo), and uranium (U), among other elements. By comparing the concentration of dissolved species in pore water samples and analysis of the solids in the tailings, coupled with modelling of the behaviour of the tailings, it has been proposed that uranium oxide solids could become soluble in the tailings if they react with carbonate ions to produce soluble uranyl carbonate complexes in the pore water. However, carbonate ions could also react with calcium sulphate (gypsum) and produce calcium carbonate, which would precipitate under the conditions of the tailings facility (near neutral pH). In our most recent collaboration, we have used X-ray microprobe in combination with micro-X-ray absorption near-edge spectroscopy to identify the bulk and trace Ca-bearing species in the TMF, which include gypsum, anhydrite, calcite, aragonite, dolomite, and Ca-bearing clays. With this information in hand, we propose to expand this research to compare how the concentrations of Ca carbonates change with age to the concentrations of solid U minerals, and to relate this to variations in the aqueous bicarbonate and uranyl carbonate complex concentrations in the TMF pore water. This study is necessary to determine if the precipitation of Ca carbonates in the TMF is involved in controlling the aqueous U concentration.