Field and laboratory measurements of centrifuge cake deposits for oil sands reclamation

Louis K. Kabwe, G. Ward Wilson, J. Don Scott, Robert Donahue

In the proceedings of: GeoRegina 2014: 67th Canadian Geotechnical Conference

Session: Mining Geotechnics

ABSTRACT: Previous testing of fluid fine tailings (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 2-year-old desiccated layer of Marquis centrifuge cake deposit indicated a shear strength and hydraulic conductivity of approximately 30 kPa and 1x10-8 m/s, respectively. In 2010 Syncrude conducted a large field pilot plant for centrifugation to produce a cake material that was placed on the old Marquis centrifuge cake deposit in thicker layers using trucks. 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. Three important relationships of the large strain consolidation test were measured directly: 1) effective stress Œ void ratio (compressibility); 2) void ratio Œ hydraulic conductivity (permeability); and 3) void ratio Œ shear strength (strength). The compressibility plot indicates an overconsolidation stress which has to be exceeded for consolidation to take place. Results show that the permeability and strength 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. RÉSUMÉ Les essais anterieurs effectués sur des residus fins du bassin de decantation à Mildred Lake (FFT) par Syncrude en 2007 et 2008 ont validé le concept de centrifugation et ils ont généré des dépôts pour les epreuves geotechniques et de dessiccation. Les essais in-situ effectués sur un dépôt de la couche desséchée de gâteau de centrifugeur Marquis, vieille de 2 ans, ont produit des résultats de la force de cisaillement de 30 kPa et de 1x10-8 m/s de conductivité hydraulique. En 2010 Syncrude avait effectué en pilote la centrifugation et l'épandage par camions des couches plus épaisses des produits de centrifugation entreposées par dessus le dépôt anterieur de gâteau de centrifugeur Marquis. La présente étude est concentrée surtout sur l'évaluation des propriétés geotechniques des échantillons congelés recueillis de la deuxieme couche des dépôts de centrifugeur de l'hiver 2011. Trois rapports importants de l‚épreuve de consolidation ont été mesuré directement: du rapport vide et la tension efficace (la compressibilité), la conductivité hydraulique (la perméabilité), du rapport vide et la force de cisaillement (la force). La courbe de compressibilité indique une tension de surconsolidation qui doit être excédée pour initier la consolidation. Les résultats montrent que la perméabilité et la force de la couche épaisse de dépôt de FFT peuvent être prédits avec la loi de puissance exponentielle des équations mathématiques avec un coefficient de corrélation elevé. Ces informations peuvent fournir un instrument important dans l'évaluation de comportement de consolidation de FFT. 1 INTRODUCTION Oil sands mining operators such as Syncrude Canada Ltd. produce large volumes of tailings. Disposal of these tailings is presently managed in external disposal ponds. The tailings slurry is generally comprised of a warm aqueous suspension of sand, silt, clay, residual bitumen, salts, surfactants and naphtha at a pH typically between 8 and 9. The resulting discharge flow segregates, with the coarser fraction of the tailings slurry settling out rapidly to form long beaches that become progressively finer down slope. The solids contained in the fresh slurry, called thin fine tails (TFT), flowing into the pond settle relatively fast (over several days) creating a 'clean' water cap containing few solids. Settling continues to take place below the water cap, and typically within a few days a solids content of 15% by weight is achieved. Hindered sedimentation continues within the pond, and the TFT within the pond gradually forms fluid fine tailings (FFT) which ranges between 30% and 45% solids by weight depending on depth and time since deposition. Further de-watering and consolidation of the tailings within the pond occurs very slowly because the water chemistry in the tailings is dominated by sodium and chloride, which disperses clay minerals present in the tailings. To improve oil sands tailings facilities' reclamation techniques, alternate methods of managing and reclaiming the FFT are being investigated (Matthews et al. 2011). Previous testing of FFT by Syncrude in 2007 and 2008 proved the concept of centrifugation and created deposits

RÉSUMÉ: and laboratory measurements of centrifuge

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Louis K. Kabwe; G. Ward Wilson; J. Don Scott; Robert Donahue (2014) Field and laboratory measurements of centrifuge cake deposits for oil sands reclamation in GEO2014. Ottawa, Ontario: Canadian Geotechnical Society.

@article{GeoRegina14Paper228,author = Louis K. Kabwe; G. Ward Wilson; J. Don Scott; Robert Donahue,title = Field and laboratory measurements of centrifuge cake deposits for oil sands reclamation,year = 2014}