Batch extraction tests show that, for Athabasca oil sands, the water-based conditioning/flotation process can be adjusted from 80 to 50 ° C conditions without substantial changes in optimal process aid addition level or primary oil recovery obtained. When the process temperature is further reduced to 25 ° C however, an order of magnitude reduction in primary oil recovery is obtained, suggesting that one or more key process variables have undergone a substantial change. Our studies with process additives suggest that several key physical properties undergo major changes, including bitumen viscosity, interfacial tension, and interfacial charge. If these are addressed, then comparable optimum primary oil recoveries can be achieved under all of 25, 50, or 80 ° C conditions. This is a significant result in terms of identifying the key mechanism(s) by which good primary froth recovery can be achieved. It is shown that the interfacial property changes, in particular, are consistent with the expected thermodynamic conditions necessary for efficient bitumen separation and flotation.
Oil sands are unconsolidated sandstone deposits containing bitumen, which is chemically similar to conventional crude oil, but has a greater density (a lower API gravity) and a much greater viscosity. Because sediments were brought in to the Athabasca deposit area from different sources and at different times, the oil sands occur as a mixture of sediment types, overlain by varying thicknesses of non-oil bearing formations(1, 2), so that a diverse number of distinct depositions can be discerned(2–5). Accordingly, the oil bearing sands have great variability in their compositions and properties and while in oil sand processing the general principles of mineral flotation apply, oil sand composition and structure, and their variations, have a great impact on the way the flotation must be operated.
The hot water flotation process for oil sands is a separation process in which the objective is to separate bitumen from mineral particles by exploiting the differences in their surface properties. The slurry conditioning process involves many process elements, including ablation, mixing, mass and heat transfer, and chemical reactions leading to the separating of bitumen from the sand and mineral particles. Adopting the water-wet model for Athabasca oil sand, one assumes that a thin aqueous film already separates the bitumen from the sand; this separation needs to be enhanced. Disengagement of bitumen from solids will thus be favoured if their respective surfaces can be made more hydrophilic, since a lowering of surface free energy will accompany the separation. The phase separation is enhanced by the effects of mechanical shear and disjoining pressure.
Although there are many variables, including water addition ratios, mechanical energy input levels, chemical addition levels, temperatures, and residence times, process efficiency is more sensitive to some variables than to others(6, 7). Early studies led to the identification of base (NaOH) addition level as the preferred process variable [see the review in Reference (8)] and it was shown by Sanford(9) that NaOH addition level could be controlled in response to fines level in the feed.
