Characterization and Centrifuge Dewatering of Oil Sands Fine Tailings

Abstract

Oil sands mining operations in northern Alberta, Canada generates an ever growing volume of fine tailings that are disposed off hydraulically in the tailings ponds. The caustic extraction process results in dispersed tailings that do not dewater under conventional gravity settling. Centrifugation along with polymer addition has recently demonstrated promising results for these tailings at the pilot-scale level. The complex colloid-water-polymer interactions govern the dewatering behaviour of the tailings during this process. The main objective of this research was to characterize and improve the dewatering properties of oil sands fine tailings using centrifuge technology in conjunction with polymer amendment. MFT was found to be a fine grained material with clay size fraction of 53% and possessing a moderate water adsorption capacity (wl = 55% and wp = 25%). The solids comprised of 55% quartz and 40% clay minerals (kaolinite and illite) and showed a specific surface area of 43 m2/g and a cation exchange capacity of 29 cmol(+)/kg. Likewise, the pore water was dominated by Na+ (776 mg/L) and HCO3

(679

mg/L), related to the extraction process and by Cl- (518 mg/L) and SO4 2- (377 mg/L), related to ore geology. A basic pH (8.15), a high EC (3280 μS/cm), and a high ZP (-46 mV) indicated a dispersed MFT microstructure. Centrifugation improved MFT dewatering through the physical mechanisms of particle segregation, assemblage formation, and flow channeling. For the investigated g-factor of up to 2550 g, the released water increased by 4.7%, the entrapped water decreased by 30% and the sediment solids content increased by 7%. The corresponding decrease in pH was from 8.15 to 7.2, EC from 3295 μS/cm to 2530 μS/cm, and ZP from -40 mV to -28.7 mV. These data confirmed aggregate formation and an effective capture of clay particles mainly in the intermediate suspension zone. Centrifugation along with polymer amendment was found to improve MFT dewatering up to 630 g (because of assemblage formation due to increased particle collisions and polymer adsorption) beyond which dewatering declined (owing to floc breakage due to excess centrifugation and surface saturation). For 10 mg/L polymer, the released water increased by 17%, the entrapped water decreased by 58% and the sediment solids content increased by 13%. The corresponding values at 20 mg/L were found to be 34%, 82%, and 18%, respectively. The physicochemical parameters were found to correlate well with tailings dewatering. The decrease in pH, EC, and ZP up to 630 g is due to water dilution, charge neutralization, and shear plane displacement, beyond which opposite phenomena occurred. At the optimum and 10 mg/L polymer, the pH value decreased to 7.5, EC decreased to 2615 μs/cm, and ZP decreased to -26 mV. The corresponding values at 20 mg/L were found to be 7.5, 2234 μs/cm, and -21 mV, respectively.