Effects of Polymers on short-and long-term dewatering of oil sands tailings

Each year, while extracting bitumen from the oil sands ores, the oil sands industry generates a large volume of tailings. Although the polymers have been found to improve the release of water from the tailings and accelerate the dewatering and settling processes in the short-term, longer-term dewatering processes such as consolidation are crucial for making the tailings deposits safe and strong enough for reclamation. Unamended tailings have been found to demonstrate time-dependent creep and thixotropic behaviour. However, whether such behaviour influences the long-term dewatering and consolidation performance of polymer amended/flocculated tailings deposits is unknown. This research investigated the creep, thixotropy, and structuration in polymer amended tailings from an experimental perspective using different polymer dosages, mixing methods, and boundary conditions. Flocculated tailings developed a thixotropic strength and also a higher preconsolidation pressure and reduced compressibility, which was clearly a structuration effect that impeded the self-weight consolidation in longer-term by limiting the creep compression. Similar experiments performed on the Leda clay showed that the structuration effect observed in flocculated tailings is not atypical of the structuration effect observed in natural clays. The potential of the UV-vis spectrophotometry and torque rheometry techniques for determining the optimum polymer dosage was investigated. The results indicated that the techniques could determine the optimum polymer dosage and help to minimize the operational costs and improve the dewaterability in the short-term. Longer-term column dewatering tests showed that the choice of polymer dosage could influence the dewatering and settlement behaviour of tailings in the long-term. Different polymer dosages resulted in different degrees of structuration with the higher dosages showing a relatively prominent structuration behaviour but also a suboptimal dewaterability. Hence, if the structuration effect is considered, the optimum polymer dosage for the long-term dewatering mechanisms may not be the same as the polymer dosage that produces optimum dewatering performance in the short-term. Based on the experimental findings, an empirical method was proposed for predicting the degree of structuration in a particular tailings-polymer mixture; such a method would allow the operators to minimize the negative effects of structuration through the selection of an appropriate polymer dosage or deposition management plan.