Effects of multiple freeze–Thaw cycles on oil sand tailings behaviour

Abstract

Continuous deposition of flocculated tailings into a deep containment facility (known as deep deposits) is gradually becoming widely acceptable as an alternative to multiple thin layered deposits, due to its reduced footprint and material handling requirements for tailings disposal. However, capping deep tailings deposits remains challenging due to the extremely slow self-weight consolidation, uncertainty in the prediction of consolidation and low surficial strength. Consequently, the resulting tailings deposits remain very soft and need to be desiccated at the surface to support capping for reclamation. This paper evaluated the effects of natural dewatering processes such as multiple freeze–thaw cycles and evaporative drying on surface crust formation for a deep oil sands tailings deposit. This paper also explored how changes in temperature gradient, initial solids content and initial water chemistry or a change in treatment process can influence the freeze–thaw dewatering process. The results suggested that the tailings samples subjected to a lower temperature gradient resulted in overall higher gravimetric solids contents and subsequent higher shear strengths at the surface after five freeze–thaw cycles. When coupled with evaporative drying, the combined freeze–thaw–drying processes further contributed to surface crust formation (strength at the surface >100 kPa). The electrical conductivity measured at each cycle and after the drying–wetting phase predicted that the upward migration of the solute contributed to the overall volume reduction during the freeze–thaw process due to osmotic and osmotically induced consolidation.