Dewatering Behavior of Centrifuged Oil Sand Fine Tailings for Surface Deposition

Date
2013-12
Authors
Owolagba, John Oladele
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Publisher
Faculty of Graduate Studies and Research, University of Regina
Abstract

Tailings footprint reduction efforts have led to the investigation of several treatment technologies (natural, physical, chemical and physicochemical processes) to improve tailings dewatering. Most of these methods develop slurries with solids content ranging between 50% and 60% (Devenny, 2010). However, this solids content is not enough because the material cannot be readily reclaimed as a soil. The required improvement is achieved through subsequent dewatering in the disposal facility by evaporation that, in turn, is governed by the prevalent climate. Centrifuge technology is being adapted as part of long time management of oil sand fine tailings. The main objective of this research was to investigate the dewatering behavior of centrifuged oil sand fine tailings for surface deposition using laboratory investigation program and numerical modeling. A material with 60% solids (e = 1.5) was obtained using a bench-scale centrifuge. The fine grained material (52% clay fraction) with a moderate water adsorption capacity (wl = 40% and wp = 20%) indicated an air entry value of 1000 kPa and a residual suction of 30000 kPa. The correct way of representing the soil water characteristic curve for tailings was found to be the one based on degree of saturation along with simultaneous volume change measurements. The tailings mainly dewatered during normal shrinkage while remaining saturated whereas volume reduction was negligible beyond the shrinkage limit. Likewise, dewatering under an effective stresses of 160 kPa was found to be 67% along with a compression index of 0.36. The saturated hydraulic conductivity measured 3 x 10-10 m/sec that decreased marginally during consolidation and rapidly (10- 10 m/sec to 10-18 m/sec) due to suction. The unsaturated soil properties and the atmospheric parameters were fully coupled using one-dimensional seepage model. Volume decrease due to desiccation was also investigated by correlating laboratory measured shrinkage with model results. The model predicted results were found to largely depend on how the material properties were quantified, site condition and the applied atmospheric forcing parameter. Cyclic variations in the degree of saturation under different climatic scenario decreased over the summer months. The driest condition showed degree of saturation of 50% corresponding to a suction 5000 kPa and solids content of 78%. Whereas the mean condition showed a degree of saturation of 70% corresponding to a suction of 2500 kPa and a solids content of 72%. While wettest condition showed a degree of saturation of 89% corresponding to 700 kPa matric suction and a solids content of 67%. These results were significantly impacted with increasing depth. Most predicted variations narrowed down such that the curves converge at 6 m depth. The top 4 m was found to be the active zone of soilatmosphere interactions therefore, a 4 m annual surface deposition will be most appropriate for this class of material. A minimum settling potential of 8% was observed. The one dimensional nonlinear finite strain consolidation model successfully captures the self-weight dewatering behavior of centrifuged oil sand fine tailings due to homogenous nature. The consolidation process was highly influenced by the initial tailings treatment resulting in high solids content. This initial condition (s = 60%) makes these tailings behave more like a soft soil. Pore water pressure dissipation was so small in one year that no significant compression occurred resulting in relatively small gain in effective stress. Similarly, volume compressibility and initial solids was found to be responsible for the amount of consolidation resulting in negligible reduction in void ratio and relatively small increases in solids content.

Description
A Thesis Submitted to the Faculty of Graduate Studies and Research In Partial Fulfillment of the Requirements for the Degree of Master of Applied Science in Environmental Systems Engineering, University of Regina. xv, 131 p.
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