Microstructure and macroscopic behaviour of polymer amended oil sands mature fine tailings

After several years following deposition, oil sand fine tailings settle to a solids content of 30 ~ 40% and are subsequently termed mature fine tailings (MFT). Once they have reached this state, MFT do not appreciably dewater, even after several decades. The particle size and dispersed structure of the fines (and possibly residual bitumen), negatively impacts the consolidation behavior of MFT. Several technologies currently being trialed to accelerate dewatering of MFT, such as in-line flocculation, centrifuge, and tank thickening, flocculate the tailings using a polymer. The addition of polymer results in substantial dewatering in the short term (24 hours) after in-line flocculation, and increases dewatering in tank thickening and centrifugation. This is well understood in industry. However, the polymer may also change longer term dewatering behaviour by potentially changing the consolidation characteristics and water–retention characteristics of the tailings. To assess the effect of polymer on all aspects of dewatering behaviour, column experiments simulating deposition of in-line flocculated tailings, were undertaken for different times, different thicknesses, and under different environmental conditions. This allowed for the study to separate initial dewatering from self-weight consolidation, subsequent desiccation, and further consolidation of desiccated tailings when they are buried by fresh deposition. The behaviour was analyzed by tracking fabric changes using mercury intrusion porosimetry and scanning electron microscopy. Differences in dewatering behaviour following initial self-weight consolidation due to differences in polymer dose appear to be minimal. Consolidation behaviour of previously desiccated tailings converges to the same properties by 80 kPa vertical effective stress. Optimizing for dose should therefore be done in terms of self-weight consolidation. It was observed that optimizing for yield stress may yield a higher optimal dose than for dewatering. Key words: consolidation, desiccation, dewatering, hydraulic, microstrcuture, shear strength, suction, tailings, yield stress