Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
1 FIELD OF THE INVENTION -
2 This invention relates to a circuit for mixing oil sand
3 in hot water to produce a slurry suitable for conveyance in a
4 pipeline.
S BACKGROUND OF THE INVENTIQ~
6 The invention has been developed in connection with
7 mixing oil sand in hot water. While not limited to that
8 application, it will now be described in connection therewith.
9 Bitumen, a heavy oil, is currently being extracted on
a commercial basis from oil sand. Presently, two very large
11 scale commercial operations are producing synthetic crude oil
12 from oil sand in the Fort McMurray district of Northern Alberta.
13 At each of these operations, the oil sand is strip-
14 mined and conveyed on conveyor belts, often several kilometers
in length, to an extraction plant. At the extraction plant, the
16 bitumen is separated from the solids and recovered. This is
17 accomplished using a process known as the 'hot water process'.
18 The hot water process involves mixing the oil sand with
19 hot water (95C) and a small amount of caustic in a rotating
horizontal drum (or 'tumbler'). Steam is added to the mixture
21 as it moves through the tumbler, to ensure that its exit
22 temperature is about 80C. In the tumbler, the bitumen is
23 separated from the solids, lumps of the cohesive oil sand are
24 ablated and disintegrated and minute flecks of freed oil coalesce
to form larger globule. In addition air bubbles are entrained
26 in the slurry. Some of the oil flecks contact air bubbles and
27 coat them, whereby the oil (or bitumen) is aerated. The term
28 "conditioning" is used to denote thy sum of the mechanisms
1 occurring in the tumbler. On leaving the tumbler, the slurry is
2 diluted with additional hot water and retained under quiescent
3 conditions for a prolonged period in a thickener-like vessel
4 referred to as a primary separation vessel ("PSV"). In the PSV,
other bitumen lobules attach to and film around bubbles of air
6 entrained in the slurry. Much of the aerated bitumen rises to
7 form froth on the surface of the vessel contents. This froth is
8 recovered. drag stream is withdrawn from the central part of
9 the PSV and this drag stream is processed in a bank of sub-
aerated flotation cells to produce a secondary yield of boatmen froth. The froth streams are combined and further processed to
12 remove entrained water and solids and yield essentially pure
13 bitumen.
14 Now, the belt conveyors extending between the mine and
the extraction plant are characterized by a number of problems.
16 They are expensive to install, operate and maintain. And their
17 use requires that the solids, which have no value, must be
18 conveyed to the extraction plant and then returned by truck to
19 the mine pits for disposal. In addition, the tumblers cannot be
increased in size to permit of improvement of the system. They
21 are presently so large that it would be technically difficult to
22 manufacture them in a larger size and convey them to the plant
23 site. As a result, it is difficult to reduce the heat
24 requirements of the process by lowering the slurry temperature,
because such a step would require increasing the tumbler
26 retention time, which would necessitate larger tumblers.
27 In a co-pending application, applicants teach use of
28 a pipeline to convey an aqueous slurry of the oil sands from the
29 mine site to the extraction plant. The pipeline slurry may be
1 fed directly to the PSV, thereby eliminating the need for the
2 tumbler. The invention in the co-pending application is based
3 on the discovery that the slurry will undergo adequate
4 conditioning in the pipeline over a distance that is
significantly shorter than the length of pipeline needed to get
6 Kit to the extraction plant. In addition, the slurry will not be
7 over-conditioned if it continues to move through the pipeline
8 after conditioning is complete. conditioning is considered to
9 be complete if good bitumen recovery in the form of good quality
froth can be achieved in the downstream PSV.) This pipeline
11 scheme has the further advantage that most of the coarse solids
12 may be removed in a settler positioned part way along the length
13 of the pipeline.
14 So pipe lining of the oil sand in slurry form between
the mine and the PSV is now considered by applicants to be a
16 viable procedure.
17 The present invention is directed toward providing a
18 mixer circuit which satisfactorily blends the oil sand with hot
19 water and entrains air to yield a consistent, dense (e.g. about
60% - 65% by weight solids) aerated slurry, preferably having a
21 relatively low temperature (e.g. 50C), that is amenable to
22 pipeline conveyance.
23 In this connection, it needs to be appreciated that oil
24 sand is tacky, cohesive, erosive material incorporating a
significant content of "oversize". Oversize is a term applied
I to the rocks, oil sand lumps, and clay lumps that occur in oil
27 sand (often up to a size of 20 inches.
28 If one were to feed a stream of oil sand into a tank
29 containing hot water and proceed to withdraw a mixture from the
1 base of the tank with a pump, the oil sand would simply pipe up
2 in the kink, fill it, and plug the pump. So a mixer circuit for
3 this purpose must be capable of suspending the oil sand in the
water with which it is mixed.
It has been mentioned that it is desirable to produce
6 a dense slurry. This need arises from the fact that one wants
7 to minimize the amount of hot water supplied at the mine site
8 for this purpose. Heating water is expensive and there are many
9 reason why these plants need to conserve water to the maximum.
And of course the mixer circuit has to be capable of
11 coping with the oversize material. Equipment having moving
12 parts, such as a tank equipped with paddle mixers, would be
13 inappropriate for use with the erosive sand associated with
14 oversize chunks.
SUMMARY OF THE INVENTION
16 In accordance with the invention, as-mined but
17 preferably processed oil sand is mixed with streams of recycled
18 slurry and fresh hot water in the cylindrical chamber of a
I vertically oriented, open-topped mixer vessel, to produce a
I slurry. The slurry exits the mixing chamber through a centrally
21 positioned bottom outlet and is screened to remove oversize,
22 thereafter entering the chamber of a holding vessel. Part of the
23 slurry moving through the holding vessel is recycled, to provide
24 the previously mentioned recycled slurry stream entering the
I mixer vessel. This is done by pumping it through a pipe loop
~26 that communicates with the mixing chamber through an inlet that
27 feeds the slurry tangentially to the inner surface of the mixer
28 vessel wall.
1 queue recycled slurry is therefore controllable and
2 mechanically given energy by the pump in the recycle loop. Due
3 to its tangential entry into the mixing chamber, the slurry
adopts the form of a rotating vortex, into which the oil sand and
fresh water are added and into which air is entrained. The oil
6 sand is fed into the vortex as a free-flowing stream that moves
7 along a downwardly extending trajectory. The trajectory is
8 directed to cause the stream of oil sand to impinge and enter the
9 vortex adjacent the latter's upper end. The added oil sand and
fresh water mix with the rotating recycled slurry to produce a
11 satisfactorily consistent, dense, aerated slurry leaving the
12 mixer vessel through its bottom outlet. The intensity of the
13 vortex can be varied by adjusting the output of the recycle loop
14 pump.
In a preferred feature, the fresh water stream is
16 injected into the mixing chamber tangentially to the inner
17 surface of the mixer vessel wall. This incrementally increases
18 the energy supplied to the vortex, although the main energy
19 contributor retains the dense, pumped, recycled slurry.
The proportion of the slurry, produced by the mixer
21 vessel, which is recycled is quite large. The rate of
22 recirculation is maintained so as to ensure that the vortex is
23 capable of accepting and suspending the dry oil sand. typically
24 the rate of recirculation is 2 to 3 times the discharged slurry
rate.
26 The mixer circuit is characterized by the following
27 features:
I - the mixer vessel's upright circular bounding
29 surface of relatively small diameter is coupled
1 with a pumped, dense, tangentially-directed
2 recycle stream to create a relatively thick and
3 fast-moving vortex that has been found to be
4 capable of dispersing and suspending the dry oil
sand while only about 35 to 40% by weight fresh
6 water is consumed in creating the slurry;
7 - the recycle loop, having a pump, is used to
8 contribute most of the energy needed to carry out
9 the mixing function;
- the screen is provided between the two vessels to
11 remove the oversize, so that recycle and product
12 pumping can be accomplished; and
13 - the mixer vessel does not incorporate moving parts
14 and can accommodate the passage there through ox
the oversize.
16 DESCRIPTION OF THE DRAWING
17 Figure 1 is a schematic sectional side view of the
18 mixer circuit.
19 DESCRIPTION OF THE PREFERRED EMBODIMENT
The mixer circuit 1 comprises a vertically orientated
21 mixer vessel 2 forming a cylindrical, open-topped mixing chamber
22 3. The mixer vessel 2 has a conical bottom which forms a
23 centrally positioned bottom outlet 4.
24 A vibrating screen 5 is positioned beneath the outlet
4, to retain and reject oversize material 6 unsuitable for
26 subsequent pumping.
1 A holding vessel 7, forming an open-topped chamber 8,
2 is positioned beneath the screen 5, to receive the slurry passing
3 through the latter.
4 A recycle pipe loop 9 connects the holding vessel
chamber 8 with the mixing chamber 3. The loop 9 connects with
6 an inlet port 10 adapted to feed recycled slurry tangentially to
7 the lower end of the inside surface 11 of the mixer vessel wall
8 12.
9 A variable pump 15 is connected into the recycle loop
9, for pumping slurry from the holding vessel chamber 8 into the
11 mixing chamber 3.
12 A conveyor 16 is provided to feed oil sand 17 from a
13 point spaced to one side of the vertical axis of mixer vessel 2.
14 The oil sand forms a frilling stream that follows a downward
and lateral trajectory and penetrates into the slurry vortex 18,
16 which has been formed by pumping slurry through the inlet port
17 10 and into the mixing chamber 3.
18 A line 19, connected with a source (not shown) of hot
19 water, is connected with a port 20 adapted to feed the water
tangentially to the mixer vessel inner surface 11.
21 In practice, the rate at which the oil sand is fed to
22 the mixer vessel 1 tends to be irregular. As a result, the
23 swirling vortex 18 can overflow the rim of the vessel. To cope
24 with this problem, an inwardly projecting flange 21 is provided
around the rim, to serve as an annular dam. If slurry vises
26 about the dam, an overflow conduit 22 is provided to drain it
27 into the holding vessel chamber 8.
1 A line 23 and outlet pump 24 withdraw product slurry
2 from the holding vessel 7, for conveyance to the pipeline (not
3 shown).
4 The operation and performance of the mixer circuit 1
are exemplified by the following test results from a pilot run
6 using the circuit.
7 Example
8 A mixer circuit in accordance with Figure 1 was tested
9 in the field. Roy cylindrical section of the mixer vessel had
a 4 foot diameter and 4 foot height, with a 15 conical section
11 at its base. A 12 inch bottom outlet was provided. A vibrating
12 screen was positioned beneath the outlet, for rejecting plus 1
13 inch material.
14 Oil sand, pre-crushed to -5 inches, was introduced at
90 tons/hour and mixed with fresh hot water (90C), added at the
16 rate of 360 gallons/minute, and recycled slurry. The slurry was
17 recycled at a rate sufficient to maintain the vortex.
18 The product from the holding vessel had a density of
19 about 1.6 (about 60% by weight solids) and temperature of about
50C. The density was consistently maintained within 10% for a
21 period of more Han 2 hours.
