Determination of optimum polymer dosage for dewatering of oil sands tailings using torque rheology

Abstract

The slow settlement of fine clay particles in oil sands tailings, the by-product from the process of bitumen extraction from the oil sands ores takes many years, leading to the build-up of an enormous volume of fluid fine tailings in tailings ponds over time, which also gives rise to the biggest challenge in tailings management – poor dewatering rate and long consolidation time. Adding polymers can promote the flocculation of clay particles, and thus, can improve the dewaterability of tailings. However, choosing an ideal polymer dosage for a particular tailings composition or fine-tuning the polymer dosage based on the changes in tailings composition has always been a challenge for the oil sands industry. This study examined the potential of two rheological methods, which are based on torque rheology and measure the rheological parameters (i.e., peak torque and totalized torque) to determine the optimum polymer dosage. When the tailings sample was subjected to a shearing force in a torque rheometer and a polymer solution was injected into that sample simultaneously, the rheometer measured the torque on the impeller. The peak torque and the totalized torque were determined from the torque-time rheogram and their interpretations provided useful information on the strength of the elastic floc network, which was formed after adding the polymer, and also on the degree of flocculation. When the polymer dosage was increased in a stepwise manner and the resulting peak torques and totalized torques were plotted against the corresponding polymer dosages (six polymer dosages in total: 0.2, 0.4, 0.6, 0.8, 1.0, and 1.2 g/kg of dry solids), a local decrease was observed in both plots at the dosage of 0.8 g/kg of dry solids, which also corresponded to the shortest suction time determined from the capillary suction time tests and the highest solids settlement obtained from the settling tests at the same dosage. Hence, the results suggested that the measurement of peak torque and totalized torque could be used as a reliable control parameter for the optimization of polymer dosage for the dewatering of tailings.