FIELD OF THE INVENTION
The present invention relates to a process for treating an aqueous oil sand slurry to enhance the probability of bitumen droplet contact and coalescence.
BACKGROUND OF THE INVENTION
Oil sand, as known in the Fort McMurray region of Alberta, comprises water-wetted sand grains having viscous bitumen flecks trapped between the grains. It lends itself to separating or dispersing the bitumen from the sand grains by slurrying the as-mined oil sand in heated water so that the bitumen flecks disperse into the aqueous phase.
The bitumen in McMurray oil sand has been commercially recovered at applicant's plant for the past 25 years. Initially this was done using the following general scheme (referred to as the “hot water process”):
- dry mining the oil sand at a mine site that was kilometers from an extraction plant;
- conveying the as-mined oil sand on conveyor belts to the extraction plant;
- feeding the oil sand into a rotating tumbler where it was mixed for a prescribed retention time with hot water (80° C.), steam, caustic and naturally entrained air to yield a slurry typically having a temperature of 80° C. During this operation, bitumen flecks were heated and became less viscous. Chunks or lumps of oil sand were ablated or disintegrated. The sand grains and bitumen flecks were dispersed or separated in the water. To some extent bitumen flecks contacted, coalesced into droplets and grew in size. The bitumen droplets contacted air bubbles and coated them or connected with them to become aerated bitumen. The term used to describe this overall process in the tumbler is “conditioning”;
- the slurry produced was then diluted with additional hot water and introduced into a large, open-topped, conical-bottomed, cylindrical vessel (termed a primary separation vessel or “PSV”). The diluted slurry was retained in the PSV under quiescent conditions for a prescribed retention period. During this period, aerated bitumen droplets rose and formed a froth layer. The froth overflowed the top lip of the vessel and was conveyed away in a launder. The sand grains sank and were concentrated in the conical bottom—they left the bottom of the vessel as a wet tailings stream. Middlings, a watery mixture containing solids and relatively non-buoyant bitumen, extended between the froth and sand layers. The term used to describe this step is “spontaneous flotation”;
- the tailings and middlings were withdrawn, combined and sent to a secondary flotation process carried out in a deep cone vessel wherein air was sparged into the vessel to assist with flotation. This vessel is referred to as the TOR vessel. It and the process conducted in it are disclosed in U.S. Pat. No. 4,545,892. The bitumen recovered was recycled to the PSV; and
- the middlings from the deep cone vessel were further processed in air flotation cells to recover contained bitumen. The term used to describe the mechanisms in the TOR and air floatation cells are collectively referred to as “secondary flotation”.
A fairly recent change with respect to this procedure involved:
- supplying heated water at the mine site;
- pre-crushing the dry, as-mined oil sand, for example to −24 inches;
- mixing the pre-crushed oil sand with the heated water and entraining air, at the mine site, to form a pumpable slurry having a temperature, for example in the order of 50° C.; and
- pumping the slurry through a pipeline directly to a PSV to subject it to spontaneous flotation.
This procedure relies on the mechanisms of conditioning being completed satisfactorily as the slurry moves through the pipeline so that, when the slurry is retained in the PSV, a viable proportion of the contained aerated bitumen reports to the froth layer.
It needs to be understood that the composition and processability of oil sand varies, often significantly. As a consequence, we have noted that, from time to time, the pipelined oil sand slurry produces unsatisfactory primary froth yield in the PSV. We believe that one reason for this may be that the residence time in the pipeline may be too short for that particular oil sand and that the bitumen droplets do not coalesce and grow to a size suitable for aeration and flotation.
SUMMARY OF THE INVENTION
With this background in mind, we have devised an apparatus and process for treating an aqueous aerated oil sand slurry to increase the probability that bitumen droplets will contact and coalesce to grow in size. In addition, small non-aerated droplets may coalesce and grow to a size that will readily aerate.
More particularly:
- an assembly is provided comprising a coalescer and a bitumen/sand separator (such as a PSV or a cycloseparator). These units are operatively connected in series to process a stream of aqueous oil sand slurry;
- the coalescer is an elongate closed vessel forming a chamber having a circular cross-section, the vessel further having means, such as a tangential inlet at its inlet end, for introducing slurry feed and inducing it to rotate about the vessel's longitudinal axis as it advances through the chamber, and an outlet at its other end; slurry is injected under pressure into the vessel chamber through the inlet means with the result that bitumen droplets are caused to move inwardly and to concentrate in the central core of the advancing flow, whereby the bitumen droplets contact, coalesce and grow; and
- the so-treated slurry stream is then expelled through the coalescer outlet and is conveyed through a pipeline into the separator, for separation of sand and bitumen.
DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic showing a circuit for processing a stream of aqueous, aerated bitumen slurry; and
FIG. 2 is a partly broken away side view of the coalescer, forming part of the circuit of FIG. 1, with arrows indicating the slurry flow through the vessel inlet, its rotational flow along its length and its exit through the outlet.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The preparation of slurry, pipelining and bitumen/sand separation of an aqueous oil sand slurry is known technology in the oil sand industry. Typically, the as-mined oil sand is pre-crushed to −24 inches using a double roll crusher 1. The crushed oil sand is then mixed with heated water and air and partly conditioned in a mixer 2. Oversize material is rejected by passing the slurry through a screen 3. The aerated slurry is collected in a pump box 4 and is then pumped through a pipeline 5, thereby advancing conditioning, and is introduced into a sand/bitumen separator 6, such as a cycloseparator.
In accordance with the present invention, a coalescer 7 is connected with the pipeline 5, upstream of the separator 6.
The coalescer 7 is an elongate, closed, preferably cylindrical vessel 8 forming an interior chamber 9 of circular cross-section. The vessel 8 has a tangential inlet 10 at one end. The inlet 10 is connected with the pipeline 5. At its opposite end, the vessel 8 has an outlet 11, preferably axial, which is connected with the separator 6.
In use, pipeline slurry is injected under pressure into the chamber 9 through the tangential inlet 10. The slurry rotates as it proceeds to advance longitudinally through the chamber 9. Under the influence of centrifugal force, the lighter bitumen and water migrate inwardly and the heavier sand migrates outwardly. The entire stream, now somewhat stratified, is expelled through the outlet 11 and is introduced back into the pipeline 5 and eventually into the separator 6 for separation of the bitumen and sand.
By centrally concentrating the bitumen droplets in the stream moving through the coalescer 7, a greater opportunity or probability is provided whereby the individual bitumen droplets may contact, coalesce and grow into larger droplets better able to unite with air bubbles to form buoyant bitumen. As a result, the performance of the separator 6 may be enhanced.
Although a preferred embodiment has been shown and described, it will be appreciated by those skilled in the art that various changes and modifications might be made without departing from the scope of the invention, as defined in the appended claims.
Patent Application
20050082205
