The oil removal efficiency for the ex situ extraction of bitumen from oil sands, or ex situ washing of oil-contaminated sand and related processes is determined by the balance of forces at the oil/water and solid/fluid interfaces. The objective of this work is to estimate the balance of forces at the interface using dimensionless numbers, and their use in evaluating and engineering ex situ soil washing processes. To this end, bitumen was removed from bitumen-coated sand particles using a twostep process. In the first step, the particles were mixed with a suitable solvent (toluene) used, primarily, to reduce the viscosity of bitumen. The particles were then mixed with water or an aqueous surfactant solution capable of producing low interfacial tensions with the solvent-bitumen mixture. The fraction of oil retained after washing was evaluated as a function of interfacial tension, solvent/bitumen ratio, mixing time, mixing velocity, and particle size. These ex situ washing conditions were normalized using dimensionless film and particle-based Weber and Capillary numbers. The fraction of oil retained by the particles was plotted against these dimensionless numbers to generate capillary curves similar to those used in enhanced oil recovery. These curves reveal the existence of a critical film-based Weber number and a particle-based Capillary number that can be used in the design or evaluation of soil washing processes. The film-based Weber number also explained literature data that associates interfacial tension with the removal of oil from oil-based drill cuttings, as well as field observations on the role that particle size plays on the removal of oil in soil washing operations.

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Bitumen froth is a water-in-bitumen emulsion (∼30 wt % water, 60 wt % bitumen, and 10 wt % of solids) stream obtained during the water-based extraction process of mined oil sands. The separation of water (to 2 wt % or less) and solids (to 0.5 wt % or less) from the froth is necessary to prevent corrosion, catalyst deactivation, and fouling in downstream processes. In naphthenic froth treatment (NFT), aromatic naphtha is added to reduce the density and viscosity of bitumen to aid in this separation, which often requires the addition of demulsifiers and centrifugation. This work looks at simulating the dewatering of froth using a bench-scale mixer and heptol 80/20 (80 vol % heptane; 20 vol % toluene) as a simulated naphtha solvent. Power dissipation during mixing, water contents, image analysis of micrographs, and acoustic spectroscopy were used to examine the dewatering process as a function of time for three froth samples with different compositions. Gravity drainage, in the absence of additives, led to a residual water content, after 2 h, ranging from 1.7 to 3.7 wt % for the three different samples, consistent with the typical residual water reported for these systems. Micrographs of the diluted froth show the eventual disappearance of large water drops and the prevalence of smaller emulsified drops (<10 μm) in the residual water. An examination of this residual water using acoustic spectroscopy showed that up to 0.8 wt % water is in the form of ∼0.3 μm submicron drops that cannot be removed by gravity or centrifugation. A dewatering model using an initial drop size distribution (DSD) of water drops also supports the existence of a substantial amount of submicron drops. A low-shear dewatering test suggests that most of this submicron water was in the froth before simulated froth treatment, formed potentially during bitumen extraction and transportation, prior to solvent dilution and dewatering. Cryo-SEM imaging further supports this hypothesis. Studies of water solubilization in toluene–asphaltene and toluene–naphthenic acid systems suggest that up to 0.5 wt % of this submicron water could be originated from water–asphaltene association. The presence of high solid contents in the froth correlated with high residual water and submicron water contents, pointing to the potential role of solids in the formation of submicron drops.

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