Mineralogical Controls on Aluminum and Magnesium in Uranium Mill Tailings: Key Lake, Saskatchewan, Canada

The mineralogy and evolution of Al and Mg in U mill tailings are poorly understood. Elemental analyses (ICP-MS) of both solid and aqueous phases show that precipitation of large masses of secondary Al and Mg mineral phases occurs throughout the raffinate neutralization process (pH 1–11) at the Key Lake U mill, Saskatchewan, Canada. Data from a suite of analytical methods (ICP-MS, EMPA, laboratory- and synchrotron-based XRD, ATR-IR, Raman, TEM, EDX, ED) and equilibrium thermodynamic modeling showed that nanoparticle-sized, spongy, porous, Mg–Al hydrotalcite is the dominant mineralogical control on Al and Mg in the neutralized raffinate (pH ≥ 6.7). The presence of this secondary Mg–Al hydrotalcite in mineral samples of both fresh and 15-year-old tailings indicates that the Mg–Al hydrotalcite is geochemically stable, even after >16 years in the oxic tailings body. Data shows an association between the Mg–Al hydrotalcite and both As and Ni and point to this Mg–Al hydrotalcite exerting a mineralogical control on the solubility of these contaminants.