Uranium-bearing phases in a U-mill disposal site in Northern Canada: Products of the interaction between leachate/raffinate and tailings material

The Above Ground Tailings Management Facility (AGTMF) is a licensed tailings receiving facility at the Key Lake mine site, Northern Saskatchewan, Canada. Tailings within the AGTMF were deposited between 1983 and 1995 and are derived from mining and milling of the Gaertner and Deilmann ore-bodies at Key Lake. The tailings are primarily composed of quartz, phyllosilicate and clay gangue minerals, minor amounts of Ni–Co–S-arsenides, iron oxide minerals and gypsum. They typically contain U concentrations on the order of between 50 and 300 ug/g U. A small area of the AGTMF receives occasional mill-process upset solutions with an initial pH-range of 2–6. The solutions contain elevated activities of U-bearing aqueous-species and are rapidly buffered to neutral and sub-alkaline pH values during interaction with surroundings tailings solids having a pH of approximately 10. This buffering and neutralization process results in the precipitation of gypsum and minor amounts of uranyl minerals. Phases of the autunite-group (chernikovite, (H3O)[(UO2)(PO4)](H2O)3 and troegerite, (H3O)[(UO2)(AsO4)] (H2O)3 with minor NH4+) occur predominantly in the gypsum matrix but can also occur as crusts on silicates. The occurrence of troegerite is due to elevated concentrations of As within the deposited tailings solids. Surface alteration on phyllosilicates within tailings affected by the mill process solutions results in elevated concentrations of K at the phyllosilicate–water interface, which are ideal nucleation sites for the formation of single crystals of zippeite, K3(H2O)3[(UO2)4(SO4)2O3(OH)]. Use of NH4-sulfates and ammonia in the extraction and yellow-cake precipitation cycles and subsequent entrainment of (NH4)+ species in the tailings results in the occurrence of (NH4)-bearing phases of the zippeite- and autunite-group. Literature data on dissolution features, dissolution kinetics and mineral solubilities suggests that the minerals of the autunite group control the mobility of U in the contact zone between the mill-process solutions and the tailings.

► Uranyl-phases in an U-tailings site.
► Raffinate–tailings interaction.
► U-bearing gypsum.
► Ammonium-bearing autunite and zippeite.