Aging Of Reduced Arsenic Minerals In Uranium Mill Tailings At The McClean Lake Operation

Abstract

The primary arsenic minerals in the uranium ore processed at the McClean Lake Operation are rammelsbergite, niccolite and gersdorffite. During processing, a large fraction, typically (80 - 95%), of these reduced minerals is oxidized and dissolves primarily as As⁵⁺ in the leachate solution. In the tailings preparation circuit the dissolved As⁵⁺ is precipitated as a poorly crystalline form of the mineral scorodite. Following subaqueous deposition in the tailings management facility, a temporal rise and fall in arsenic pore water concentrations has been observed. This is due to the oxidation of the residual quantities of reduced arsenic minerals initially present in the tailings sediment. The XANES (x-ray absorption near edge spectrometry) technique has shown that As^1- is gradually oxidized to As³⁺ and then finally to As⁵⁺ in the tailings sediment. This oxidation process results in a temporary accumulation of As³⁺ in solution and is the source of the rise and fall in arsenic concentration observed.