The CT or consolidated tailings process involves chemical amendments to combine the clays and fines in oil sands mature fine tailings or thickened tailings with the coarser sand components to create a nonsegregating tailings (NST) mixture that will rapidly consolidate. Over the years, several amendment chemicals have proven to be useful in controlling the fluid tailings properties so that they may support a sand loading and remain nonsegregating. Suncor has several years of commercial scale operating experience with gypsum as the CT process aid and in the years leading up to the commercialization of the CT process at Suncor, carbon dioxide was also investigated as a CT process aid. With the concerns over carbon dioxide related to the Kyoto protocol, the extent to which carbon dioxide is trapped and chemically sequestered in the CT process has been investigated. The mechanism by which carbon dioxide addition affects the strength of the mature fine tailings or fluid tailings component has been investigated, and the potential for carbon dioxide sequestration has been quantified. Depending upon the availability of gypsum as a CT or NST additive, carbon dioxide could be a useful alternative.
Creation of a suitable CT mixture involves creation of a nonsegregating mixture of sand, clay, and water; rapid initial settling (water release) of the mixture; and ultimate consolidation of the mixture. Extensive studies by Scott et al.1 have demonstrated that there is a wide range of sand-to-fines ratios, solids contents, and gypsum addition levels where these criteria are met. Poor control of calcium levels in the release water and the scaling problems associated with high calcium and bicarbonate concentrations have been major problems associated with gypsum CT. To circumvent these problems, polymeric flocculants and aluminum salts (alum) have been investigated at Syncrude Research2 and CANMET3 as substitutes for gypsum, with alum showing some promise. In these studies, the dewatering rate and segregation index of alum CT were shown to be similar to those for gypsum CT. Calcium ion concentrations in the release water of alum CT are significantly lower than in the release water of gypsum CT. However, there is a decrease in the release water alkalinity (bicarbonate) with alum - CT, which can potentially increase the caustic demand in extraction where bicarbonate alkalinity has been shown to improve the conditioning and flotation process. The bicarbonate ion disperses the clays, improving oil sand conditioning and flotation.
The potential for using CO2 as a substitute for gypsum was evident from high viscosities observed in CO2- saturated MFT4. Under normal circumstances, high MFT viscosity is essential for good CT production. In fact, one would need about 3000 ppm of gypsum in the MFT to match the viscosity produced by CO2-saturated MFT. In the highly buffered CT mixture, pH reduction associated with CO2 addition quickly reverts to the initial pH of the MFT.
At low clay-to-water ratios, (C:W), the nonsegregating property of a CT mixture is controlled by the extent to which clay flocs can support sand grains. Divalent cations achieve this strength through double-layer compression, which promotes coagulation of the clays3.
