Aluminum and Magnesium Mineralogical Controls of Elements of Concern in Bulk Neutralized Uranium Mill Tailings

Abstract

The Deilmann Tailings Management Facility (DTMF) is located at Cameco Corporation’s Key Lake U mill in northern Saskatchewan, Canada. A large portion of the deposited tailings is comprised of secondary mineral phases that precipitate during bulk neutralization of raffinate - an acidic, U-barren waste solution. The tailings discharged from the mill into the DTMF contain elevated concentrations of elements of concern (EOCs) including As, Ni, Se, and Mo that originate from the U ore. Extensive studies established that secondary Fe mineral phases (e.g., ferrihydrite) act as primary controls on the solubility of these EOCs. However, the identification of the presence of large quantities of uncharacterized secondary Al and Mg mineral phases in the tailings suggested that they may provide additional geochemical controls on EOCs. A lab-scale continuous model of the Key Lake bulk neutralization circuit was constructed to study the mineralogy and evolution of the secondary phases which precipitate during raffinate neutralization and are discharged as part of the tailings. Characterization of these phases showed amorphous AlOHSO₄ and Al-substituted ferrihydrite as prominent precipitates in the pH 4.0 slurries, and amorphous Al(OH)₃ and MgAlSO₄-type hydrotalcite in the pH 9.5 slurries. X-ray elemental mapping found As, Se, and Mo associated to amorphous AlOHSO₄, Al(OH)₃, and Al- substituted ferrihydrite, while Ni was associated to hydrotalcite. Extended X-ray absorption fine structure (EXAFS) spectroscopy indicated that As was bonded to multiple phases (amorphous Al(OH)₃, ferrihydrite, and hydrotalcite). Analysis of reference compounds suggest that As is chemically bonded to these phases through bidentate-binuclear linkages. These results show that the EOCs present in raffinate can be sequestered by both Fe and Al/Mg phases. The results of the study also showed that, even in the absence of secondary Fe mineral phases, EOCs can be effectively sequestered from raffinate, thereby demonstrating that multiple EOC controls exist in the final tailings.