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Oxidation of Mine Tailings from Rankin Inlet, Nunavut, at Sub-Zero Temperatures

by J.L. Meldrum, MASc (co-supervised with H. Jamieson)

Acid-generating sulphidic tailings from a Ni-Cu mine were exposed on the shores of Hudson Bay for 30 years before burial in a drained bedrock basin. To determine if encapsulation in permafrost will maintain the tailings, and their saline pore water, in a chemically inert state, a field and experimental program were employed. Three years after burial, several boreholes were drilled to collect tailings and pore water samples, and to initiate long-term thermal monitoring. Columns charged with pyrrhotite-bearing, unsaturated tailings were studied at temperatures between 30°C and –10°C. Oxygen consumption was measured directly to determine the effect of low temperatures on oxidation rate.

Results of the column experiments indicate that significant oxidation of the Rankin Inlet tailings occurs at 30°C but at lower temperatures this rate is substantially reduced. At –10°C, unfrozen water was still present in the columns, but oxygen consumption was below the detection limit. The highest measured oxygen flux correlates with a temperature increase of 1°C, consistent with exothermic sulphide oxidation reactions. Thermal modeling and direct measurements have shown that the freezing of the tailings in Rankin Inlet is progressing. The tailings are expected to be ice-bonded approximately 15 after burial.

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