Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
BACKGROUND OF THE INVENTION
This invention relat:es to the hot water process
for extracting bitumen from tar sand. More particularly, it
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1 30 relates to a system for controlling the density or solids
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content of the primary separation vessel (PSV) underflow and
fluidizing it to render it pumpable and non~plugging.
A large proportion of the world's known
hydrocarbon reserves exists in the form of tar sands. One
large deposit of this material is found along the banks of
the Athabasca River in Alberta. The tar sand exists in the
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~ form of water-wet grains of sand, sheathed in a film of bitumen.
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~ In treating the tar sand to recover commercially useful -~
; products, it is first necessary to separate the bitumen from
the water and sand.
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-~ The method commonly employed to extract the -~
i bitumen from the mined tar sand is known as the hot water
'3,' process. In the first step of this process, tar sand, hot water
and steam are fed into a rotating tumbler and mixed therein.
The hot water is supplied at a temperature of about 180F
and in amounts sufficient to supply a slurry containing about
20 - 25~ by weight water. The residence time within the -
tumbler is typically four minutes and the exit temperature of
the slurry is about 180F. While in the tumbler, the tar sand
disintegrates and the bitumen particles are liberated from the
i sand.
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¦ The t~bler product is passed through a screen
to remove lumps and rocks and is then flooded with additional
hot water to further disperse the sand and bitumen particles.
A typical flooded, aqueous, aerated slurry will have a
composition of~7% bitumen, 43% water and 50% solids, and its
temperature will be about 160F - 180F.
The ~looded slurry is then continuously fed ' `1-
into a primary separation vessel. This vessel is conventionally
3Q a cylindrical settler having a conical bottom. In the
vessel, most of the large sand particles (i.e. plus 200 mesh),
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~ fall to the bottom and leave through an outlet as a primary ~ ;
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tailings stream. Most of the bit~men particles, rise to
~` the top of the vessel and form primary bitumen ~roth. This
froth overflows the vessel wall into a launder for removal.
A middlings stream, typically comprising about
~- 5 77~ water/ 21% solids and 2% bitumen, is continuously with-
drawn from the intermediate zone of the primary vessel. The
middlings stream is processed in a secondary recovery flotation
-~ cell to produce secondary froth and a secondary tailings
stream.
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:' 10 SUMMARY OF T~IE INVENTION
It is desirable to operate the primary separation
, vessel so as to keep the underflow or tailings relatively dry.
fi This is done to reduce the loss of bitumen with this stream,
;f since bitumen loss increases as the water content of the
primary tailings stream increases. In practice, the PSV
is operated to try to keep the primary tailings solids content
;f above about 65% by weight, preferably above 7~%. A typical
! primary tailings stream may comprise 30% waterl 69.3% solids
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I and 0.7~ bitumen. The solids are mainly coarse in nature,
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as shown in Table I hereinbelow which provides an illus~rative -
particle siæe analysis.
Due to the high coarse sand content, the primary
tailings stream must be diluted to improve its pumpability
and to reduce the possibility o the solids settling out and
~ 25 plugging the conduit carrying it to the tailings distribution - :
i~ ~ system.
f The secondary tailings stream is a good deal
more dilute than the primary tailings stream and carries a
large proportlon of fine clay and silt solids particles in
it. A typical secondary tailings stream comprises 78.4% by ;~
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weight water, 21.1% solids and 0.5% bitumen. Table 1 shows
an illustrative particle size analysis ~or this stxeam.
TABLE I
Size fraction Primary Secondary
(microns?_ _ tailings tailings -~
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~ 5 0 - 5 2.6 11.
- 5 - 10 .2 10.7
10 ~ 20 1.5 10.4
20 - 3Q .8 ~.5
30 - 44 1.4 4.3
_ 44 6.6 ~1~6
44 - 74 3.2 10.5
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~ 74 - 147 48.~ 46.0
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~ ~ 147 42.1 1.8
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In accordance with one aspect of this invention,
Z the PSV is operated to produce a dry primary tailings stream.
At least part of the secondary tailings stream is combined
therewith between the PSV outlet and the pump to produce a ~ ;
. single pumpable stream. As a result, the fine solids of
the secondary tailings stream help to keep the coarse solids
~i 20 of the primary tailings stream in suspension and process water
is conserved.
It is also desirable to provide a responsive and
safe system for controlling the rate at which the primary
tailings are withdrawn from the PSV. In this connection, a
variable-speed close-coupled pump is direct connected to the
PSV downcomer conduit and pumps the combined tailings through
a line to the tailings distribution system. The speed of
Z~ the pump is controlled to ensure that the solids content of
the PSV tailings is maintained close to the desired level. ~;
In a preferred feature, the speed of the close-
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~` coupled pump ~s controlled responsive to the torque on the
PSV rake shaft or to the density of the PSV tailings stream.
In the case where the PSV is operating on low ~ines tar sand
~eed, the torque-sensing system is used. With high fines tar
sand ~eed, the density~sensing system is used.
Broadly stated, the invention comprises an
improvement in the hot water process for recovering bitumen
~rom tar sand wherein aqueous tar sand slurry is fed to a primary
separation vessel having a sand rake and tailings outlet and
conduit to produce a primary bitumen froth stream, a relatively
- dry primary tailings stream comprising coarse solids, and an
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aqueous middlings stream comprising fine solids and bitumen,
said middlings stream being processed in a secondary recovery
~lotation cell to produce a secondary bitumen froth stream ~ ;
and a relatively Eluid secondary tailings stream comprising
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water containing fine solids. The improvement comprises the ~ ~
steps o~: pumping at least part of the secondary tailings -
stream with a variable speed pump through a conduit which
3 joins the primary separation vessel outlet conduit and combining
said secondary tailings with the primary tailings, said
secondary tailings being supplied at a rate su~ficient to
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I produce a stream o~ combined tailings which is pumpable;
and then pumping the combined tailings stream through a
conduit with a variable-speed close-coupled pump to deliver it ;. h
to a tailings distribution system, the speed o~ said close- -
coupled pump being controllad to maintain the density of the
primary tailings stream close to a pre-determined value.
DESCRIPTION OF T~E DRA~ING
The figure is a schematic ~iew o~ the no~el
~; 30 system.
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~ DESCRIPTION OF PREFERRED EMBPDIMENT
- With re~erence to the Figure, tar sand slurry
is introduced into a primary separation vessel 1 of conven-
`~ tional designO In the vessel,floatàble bitumen in the slurry
` 5 rises to the surface of the vessel contents. Here this bitumen
forms primary froth which is recovered in the launder 2.
A middlings stream is wikhdrawn through an
~; outlet 3 intermediate th~ ends of the vessel by a pump 4
and transferred through -the conduit 5 to a secondary separation
vessel 6. The vessel 6 is a conventional flotation unit,
wherein air is~supplied through an agitation shaft 7 to the
agitator blades 8. The air becomes dispersed in the middlings
;~ in the form of fine bubbles and displaces the bitumen to the
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~;~ surface where it forms secondary froth. This froth is recovered
in the launder 9.
,~ . .; Tailings streams are produced from the bottom
outlets 10, 11 of the primary and secondary vessels 1, 6. ~-
As previously stated, it is desirable to maintain the primary
tailings solids content at about 70~ ~ however this is a
~; 20 stream which is difficult to pump and is subject to rapid ;;~
-~ settling out of solids. Therefore a portion of the secondary
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~ tailings, which are relatively dilute, is pumped by a variable-
i speed pump 12 through a conduit 13 which connects with the
downcomer conduit 14 of the primary separation vessel. The
~ 25 secondary tailings mix with the primary tailings to form a
"'! dilute stream. The combined tailings stream is pumped by
the variable-speed close~coupled pump 15 through the conduit
~,' 16 to a tailings distribution 17 for transfer to a settling
pond.
1~30 A nuclear density gauge 18 establishes a
~ measure of the density of the combined tailings stream passing ~
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through the conduit 16, This continuous reading is trans-
mi-tted to a controller on the secondary tailings pump 12 and
the speed of the pump is varied to maintain the solids content i
of the combined stream below 65% ~y weight, thereby ensuring
pumpability and non-plugging of the conduits.
A second nuclear density guage 19 establishes
a measure of the density of th~ primary tailings stream as
it leaves the outlet 10. This continuous reading can be
transmitted to a controller on the primary tailings pump 15
and the speed of the pump can be varied to maintain the
density of the primary tailings stream close to a pre-determined
value, i.e. that value corresponding to a solids content of
about 70%. This method of monitoring the primary tailings
solids content is particularly useful when high fines tar
sand is being fed to the circuit.
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In the event that low fines tar sand is being
fed to the primary separation vessel, it is preferred to
l establish a measure of the torque on the shaft 20 of the
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l vessel rake 21. A rake torque recorder 22 can be used for
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this purpose and its signals transmitted to the controller
j on the primary tailings pump 15. The speed of the pump 15
can thus be varied responsive to these signals to maintain
the density of the primary tailings stream at the desired level.
The invention is characterized by several ~-
advan~ages. The use of a close-coupled pump and closed conduit
system ensures a responsive and safe operation. The use of
the secondary tailings as a fluidizing agent for the primary
tailings reduces the possibility of the contained solids
settling out downstream. In addition, the water requirements ~ ;
of the process are less than would be the case if fresh water
was used, as has been advocated in the prior art. Finally, the
control systems used in conjunction with the pumps are
particularly responsive.
