Field and laboratory measurements of oil sands tailings centrifuge
cake deposits
Louis Kabwe, Ward Wilson and Don Scott
Department of Civil & Environmental Engineering, University of Alberta, Edmonton, AB, Canada
Robert Donahue, Applied Geochemical Engineering Inc., Edmonton, AB, Canada
Nan Wang, Syncrude Canada Ltd, Fort McMurray, AB, Canada
ABSTRACT
This paper discusses the dewatering of fluid fine tailings (FFT) by centrifugation to produce a cake-like
material. Previous testing of FFT by Syncrude in 2007 and 2008 at the Mildred Lake Settling Basin proved the
concept of centrifugation and created deposits for geotechnical and desiccation testing. The in-situ testing conducted
on the two-year-old desiccated layer of centrifuge cake deposit indicated a shear strength and hydraulic conductivity
of approximately 30 kPa and 1x10-8 m/s, respectively. In 2010, Syncrude and Total E&P Canada conducted a large
field pilot plant for centrifugation to produce a cake material that was placed on the old centrifuge cake deposit in
thicker layers using trucks. The objective of the research was to evaluate this method of dewatering the FFT in order
to increase the solids content and shear strength of the material during and following deposition. Evaporation
measurements taken from mini-lysimeters showed that the ratio of actual evaporation to potential evaporation was
reduced by half over a period of two months. These results and the reasons for this reduction are discussed.
The present study focuses mainly on the evaluation of the geotechnical properties of the frozen core samples
collected from the centrifuge second lift deposits during the winter of 2011. Large strain consolidation tests performed
on the FFT thick deposit from Syncrude’s Mildred Lake Settling Basin are presented and discussed. Three important
relationships of the large strain consolidation test (effective stress – void ratio (compressibility), void ratio – hydraulic
conductivity (permeability) and void ratio – shear strength (strength)) were measured directly. The compressibility plot
indicates an overconsolidation stress which has to be exceeded for consolidation to take place. Results show that
these three relationships for the FFT thick lift deposit can be predicted with power law mathematical equations with a
high correlation coefficient. This information can provide an important tool in the evaluation of FFT consolidation
behaviour.