The Evolution of Arsenic in the Uranium Mill Process at Mcclean Lake Mine, Saskatchewan, Canada

Abstract

The McClean Lake Uranium Mill in Northern Saskatchewan was designed to accommodate the wide variation in the percentage or arsenic which occurs naturally within uranium ore bodies of the Athabasca Basin without producing adverse environmental effects. After over 15 years of mill operations and the direct study of arsenic through the mill process and in the tailings, a detailed understanding of the geochemical evolution of arsenic has been achieved.Arsenic enters the mill as reduced arsenide minerals such as niccoline (NiAs), rammelsbergite (NiAs₂) and gersdorffite (NiAsS) which are not always fully oxidized during the leaching process. Residual primary arsenic mineralization is, therefore, transferred to the tailings preparation process in an ustable form. It undergoes oxidation to secondary mineral phases controlled by the final circumneutral tailings geochemistry in the Tailings Management Facitly (TMF). The oxidation of arsenides to stable arsenates is a two-step process: there is a rise in As³⁺ values in the tailings pore water and then a subsequent fall as As³⁺ is oxidized to As⁵⁺ and precipitated as the mineral scorodite.Verifying this model of arsenic mineral speciation in the tailings is essential to predicting the long term evolution of arsenic mineralization in the TMF and ensuring that the process is protective of the environment. Most recently, X-ray Absorption Near-Edge Structure (XANES) spectroscopy was used to analyze fifty samples from three boreholes through the TMF. The large sample set in this study provided a detailed picture of the arsenic mineral speciation in the tailings at various ages, enabling identification of the unique arsenic mineral species present. The results of the XANES analysis identified that scorodite was the most abundant arsenic mineral species. Comparisons of the XANES data with arsenic concentrations measured in the tailings solids and pore water indicated the arsenide minerals do oxidize to scorodite over time. Overall, as predicted, scorodite controls the release of arsenic in the tailings, maintaining As⁵⁺ pore water concentrations at < 1 mg/L.