CHARACTERIZATION OF THE ALUMINUM AND MAGNESIUM SECONDARY MINERAL PHASES AND THEIR POTENTIAL CONTROL OF CONTAMINANTS IN URANIUM MINE TAILINGS

The Deilmann uranium Tailings Management Facility (DTMF) is located at Cameco Corporation’s Key Lake mill, Saskatchewan, Canada. The tailings discharged from the Key Lake mill into the DTMF are high pH (pH ≈ 10.1), oxic (Eh ≈ 200 mV), and contain elevated concentrations of potential environmental contaminants including As, Ni, Se, and Mo. Extensive studies established secondary amorphous iron hydroxides (ferrihydrite) as the primary control on the solubility of these elements of concern (EOC) and show that, under current environmental conditions in the tailings, the ferrihydrite should remain stable for thousands of years. If, however, the redox environment in the tailings becomes reducing (e.g., via bacterial reduction), the impact on the solubility of the sequestered EOCs is unknown. As such, other potential mineralogical controls on EOCs must be explored. Al and Mg minerals are the dominant secondary phases in the DTMF tailings. These minerals are poorly characterized and may provide additional controls on EOCs. To characterize the Al and Mg mineral phases and their potential control of EOCs in the DTMF, synthetic Key Lake mill raffinates were processed in a pilot scale bulk neutralization process to replicate the secondary mineral phases deposited in the DTMF. The dominant Al and Mg phases observed in the neutralized products were amorphous Al(OH)₃ and hydrotalcite-like compounds. Amorphous Al(OH)₃ precipitated at pH = 4 and sequestered As and Se, and, to a lesser extent, Ni and Mo. Hydrotalcite-like compounds precipitated at pH 9.5. Experiments to characterize the potential EOC controls of the hydrotalcite-like compounds are currently in progress. Understanding the Al and Mg secondary mineral phases will provide a more complete understanding of the long-term fate of EOCs in the DTMF and other uranium tailings management facilities.