ARSENIC SPECIATION OF NEUTRALIZED URANIUM MILL WASTES: PROTECTING SASKATCHEWAN'S GROUNDWATER THROUGH CHARACTERIZATION OF TAILINGS PRECIPITATES

Uranium milling in northern Saskatchewan, Canada produces waste materials (tailings) containing elements of concern (EOCs: e.g. arsenic, molybdenum, selenium, and nickel) that have the potential to adversely impact local groundwaters and surface waters. As part of a laboratory-based pilot study at Cameco’s Key Lake milling operation, three uranium feed blends were created from current and potential ore deposits being considered: McArthur River and Millennium. The ore blends were processed through a pilot mill to yield neutralization precipitates that would ultimately contribute to the mill final tailings. The mineralogical controls on EOCs in these neutralized tailings were assessed to determine the stability of secondary arsenic mineral phases in the resulting oxic and high pH (Mean Eh = 200 mV, pH = 10) tailings environment. Precipitates were collected from the underflows of two thickeners, at successive pH set points in the mill waste neutralization process. These samples were subjected to XRD, EMPA, ICPMS and synchrotron-based X-ray absorption spectroscopy. Comparison of near edge XAS spectra to model compounds showed that arsenic exists predominantly in the +5 oxidation state for all samples tested. This finding was supported by geochemical modeling and pH/Eh diagrams for soluble arsenic species. Principle component analysis and EXAFS curve fitting suggests that arsenic sorbed to ferrihydrite dominates the precipitates, confirming earlier studies. This finding is an important first step to understanding the stability of these arsenic-containing precipitates in the tailings of different uranium ores. As studies have indicated that adsorption of arsenic to ferrihydrite during co-precipitation can incorporate into crystalline forms such as hematite during phase transformation, it is predicted to be stable under current tailings conditions for up to 10,000 years.