The Hot Water Extraction Process (HWEP) is presently used in Alberta to extract bitumen from surface mineable oil sands. Tailings remaining after bitumen extraction, are disposed of in sedimentation ponds where the coarse sand and some of the fines settle rapidly to form the dykes surrounding the pond. About one half of the silt and clay and almost all of the unextracted bitumen remain in suspension and are carried out towards the centre of the pond. Eventually, these solids form a sludge having a gel-like structure and containing a significant proportion of the process water (1).
In previous work (2) we have used selective separation techniques to identify several solids fractions, which although occurring in relatively minor amounts in sludge, have the capability of contributing significantly to its structure. These same solids fractions have also been found in the parent oil sands ore and the corresponding fresh tailings. Of special interest is a colloidal solid material showing a tendency for gel formation. Other fractions are associated with various amounts of organic matter and may be separated based on their surface active nature; a particularly useful technique is oil assisted or emulsion flotation in combination with centrifugation. These partiy oil wettable solids are also capable of forming structures through interaction with free bitumen and naphtha, also present in the sludge. This type of structure formation would involve the process of immiscible liquid phase welting (3).
It has been demonstrated (4) that when sludge is treated under certain conditions to remove selectively the more hydrophobic materials, the gel structure can be broken. The bulk of the solids then settle out as a dense sediment leaving a dilute suspension of ultra fine particles comprising only about 20% of the original solids. These results indicate that the concentration of solids necessary for gel formation may be critical and that separation of only a small fraction of solids is required to disrupt or prevent the formation of sludge.
With some minor modification the separation scheme, shown in Figure 1, can be used to fractionate oil sands ore, sludge or tailings. The only change required is that, for sludge or tailings, a certain amount of the connate water must first be removed by centrifugation, followed by replacement with fresh water. After vigorous agitation and recentrifugation the original sample will then form three layers which are readily separable from each other On top is a bitumen layer containing hydrophobic solids (BS) closely associated with significant amounts of strongly bound organic matter These solids probably do not participate in sludge structure but can reduce bitumen separation by increasing its density. A second, aqueous layer, containing a stable dispersion of ultrafine particles (AS) forms between the bitumen and a compacted sediment, containing the bulk of the residual solids (RS). The colloidal, or ultrafines suspension only forms during this particular sequence of treatments.
Further size separation of the ultrafines can be achieved by centrifugation at different speeds.
