Note: Descriptions are shown in the official language in which they were submitted.
82-033
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AN IMPROVED PROCESS TO FLOAT BITUMEN FROM MINERAL SLIMES RESULTING FROMTAR SANDS PROCESSING
Field Of The Invention
The present invention relates to an improved method for the recovery
of bitumen from the waste tailings resulting from the hot water extraction
process of a tar sands operation. More particularly, the invention
involves the benefication of these waste tailings which are mineral
sludges or slimes comprising bitumen and minerals in an aqueous system and
this invention enables the recovery of the bitumen in such waste ma-
terials. The invention comprises the air Elotation of the aqueous sludge
in specific equipment under specific conditions to effectively float the
bitumen away from the aqueous mineral suspension.
Background Of The Invention
Tar sands are also known as oil sands or bituminous sands. The
sand deposits are found in numerous locations throughout the world, e.g.
Canada, United States, Venezuela, Albania, Rumania, Malagasy and U.S.S.R.
The largest deposit, and the only one of present commercial importance is
in the northeast Province of Alberta, Canada.
o
Tar sand is a three-component mixture of bitumen, mineral and
water. Bitumen is the component for the extraction of which tar sands are
mined and processed. The bitumen content is variable, averaging 12 wt.%
of the deposit, but ranging from about 0 to 18 wt.%, and as used herein
bitumen includes hydrocarbons. Water typically runs 3 to 6 wt.% of the
mixture, increasing as bitumen content decreases. The mineral content
constitutes the balance.
The "hot-water" process for primary extraction of bitumen from
tar sands consists of three major process steps (a fourth step, final
extraction, is used to clean up the recovered bitumen for further
processing). In the first step, called conditioning, tar sand is mixed
with water and heated with open steam to form a pulp of 70-85 wt.% solids.
Sodium hydroxide or other reagents are added as required to maintain the
pH in the range oE about 8.0-8.5. In the second step, called separation,
the conditioned pulp is diluted further so that settling can take place.
The bulk of the sand-sized particles (greater than 325 mesh screen)
rapidly settles and is withdrawn as sand tailings. Most of the bitumen
rapidly Eloats (settles upward) to form a coherent mass known as bitumen
Eroth which is recovered by skimming the settling vessel. An aqueous
middlings layer containing some mineral and bitumen is formed between
these layers. A scavenger step may be conducted in the middlings layer
from the primary separation step to recover additional amounts of bitumen
therefrom. This step usually comprises aerating the middlings. The
froths recovered from the primary and scavenger step can be combined,
diluted with naphtha and centrifuged to remove more water and residual
mineral. The naphtha is then distilled off and the bitumen is coked to
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a high quality crude suitable for further processing. Hot water processes
are described in the prior art. Tailings can be collected from the
aforementioned processing steps and generally will contain solids as well
as dissolved chemicals. The tailings are collected in a retention pond
in which additional separation occurs. The tailings can also be con-
sidered as processing water containing solids which are discharged from
the extraction process. The tailings comprise water, both the natural
occurring water and added water, bitumen and mineral. This invention is
directed to a bitumen flotation step in the process for recovering the
bitumen from these tailings, which bitumen would otherwise be wasted and
also would be an environmental hazard.
Discussion Of The Prior Art
Methods for treating sludge formed in a retention pond used to
store tailings from a hot water extraction of bitumen from tar sands are
disclosed in Canadian Pat. Nos. 975,696; 975,697; 975,698; 975,699 and
975,700 all issued Oct. 7, 1975 to H. J. Davitt. The Eirst mentioned
Canadian Patent discloses removing sludge from a pond, placing the sludge
in an air scavenger treating zone wherein the sludge is aerated and
agitated concurrently to form an upper bitumen froth layer and a lower
tailings of water and mineral water. The lower tailings can be discharged
into a retention pond. The upper bitumen froth is sent to a settling zone
wherein two layers are formed, an upper bitumen layer reduced in mineral
matter and water and a lower layer comprised substantially of mineral
matter and water with minor amounts of bitumen. The latter lower layer
is recycled back to the air scavenger treating zone while the upper
bitumen layer is processed further to recover the bitumen. Canadian Pat.
s~
No. 975,697 discloses a process similar to that described in the previous
patent with an additional step in that a portion of the lower layer, which
otherwise would be recycled back to the air scavenyer treating zone, is
returned to the retention pond. Canadian Pat. No. 975,698 discloses
feeding the sludge from a retention pond to an air pressure zone wherein
the sludge is aerated at superatmospheric pressure to aerate bitumen in
the sludge. Canadian Pat. No. 976,699 discloses feeding sludge recovered
from a retention pond to a settling zone and permitting the sludge to form
an upper froth layer and a lower tailings layer. Canadian Pat. No. 975,700
discloses feeding sludge to an air scavenger treating zone wherein the
sludge is aerated and agitated concurrently and resulting froth is
separated in the scavenger treating zone, while the tailings are returned
to the pond. However, none of the previously discussed patents discloses
or suggests the specific method of this invention for treating froth in
order to obtain improved bitumen recovery.
U.S. Pat. No. 3,594,306, E. W. Dobson, ~uly 20, 1971, discloses
upgrading froth from a secondary recovery operation (generally a flo-
tation scavenger zone treating the bi.tumen-rich middlings from a separa-
tion zone) by allowing the froth to settle in a settling zone whereby an
upper layer is formed which is substantially upgraded in bitumen content
compared to the secondary froth. The lower layer formed in the settling
zone can be recycled. Again, nothing in the aforementioned U.S. patent
discloses or suggests the improved method of this invention for treating
froth.
U.S. Pat. No. 4,018,664, F. A. Bain et al, Apr. 19, 1977,
discloses a method for treating sludge from a retention pond associated
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with hot water extraction of bitumen from bitumen sands. The method
involves withdrawing sludge from a pond, diluting and mixing it with
water, and settling to obtain a froth layer, a middle layer containing
less solids than the original sludge, and a lower layer containing
increased solids over the original sludge. Agitation and/or aeration,
for example, aeration sufficient to mildly agitate the sludge, are
disclosed as beneficial and essential to the extent that proper mixing
is achieved. Proper mixing presumably means that the sludge and dilution
water are in such close association that samples taken anywhere in the
mixture all would contain essentially the same amount of water. However,
nothing in the aforementioned patent suggests the specific method of this
invention for treating froth obtained from sludge.
A more recent disclosure of interest is U.S. 4,401,552 which
involves the benefication of froth obtained from tar sands wherein the
froth is diluted with water at ambient temperature and the diluted froth
agitated and aerated vigorously for a short time to obtain an upper layer
containing most of the bitumen, which bitumen layer is skimmed off for
recovery. There is no suggestion in this patent disclosure to the
speciEic parameters used in the bitumen flotation step of this invention
which lead to significantly improved bitumen recovery.
Summary Of The Invention
The present invention embodies an improved method for re-
covering bitumen from a waste sludge containing bitumen, mineral and
water. The sludge is obtained from a retention pond used to store
tailings from water extraction of bitumen from tar sands and is converted
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to a froth from which the bitumen is reeovered. The method of the
invention will generally be employed on a conditioned sludge; e.g. a
sludge conditioned by removing carbon dioxide and methane from the sludge
to effect some viscosity reduction and thus further reduce the viscosity
of the sludge, preferably with an additive. The thus conditioned sludge
is then subjected to air flotation in accord with the invention in one
or more WEMC0 or equivalent flotation cells at a specific speed of the
cell rotor and at a specific volumetric air ratio. The froth thus
obtained is subsequently deaerated, diluted and the bitumen recovered,
preferably by hydrocycloning followed by centrifugation. By controlling
the rotor speed and air flow in accord with the invention, the flotation
of minerals in the tailings sludge is suppressed (i.e., bitumen froth
yields are high).
~eseription Of The Drawing
Fig. 1 is a schematic representation of a hot water tar sands
extraction proeess whieh embodies applieant's improved bitumen recovery
proeess.
Detailed Description Of The Invention
Referring now to Fig. 1 whieh is a flow diagram showing the
overall operation of a bitumen reeovery proeess~ sludge from the tailings
pond is fed through line 1 into the eonditioning tank A which may be a
standard six blade (flat) turbined vessel with four baffles flotation
cells or equivalent devices are also useful as conditioning tanks. These
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tailings and sludges generally contain from about 3 to about 20 per cent
by weight of bitumen and from about 10 to about 55% by weight of minerals.
Conditioning of the sludge in tank A involves a first step of removing
the carbon dioxide and methane present in the sludge and this effectively
reduces viscosity of the sludge. This first step is done, preferably,
by sparging air into the turbine-agitated conditioning tank containing
the sludge through line 12 for about 6 to about 10 minutes. Preferably
for a commercial operation, the air sparging ratio (i.e., the volume of
the conditioning vessel in cubic feet divided by the volume of air per
minute in cubic feet per minute) should be about 1. While it is important
to remove both the carbon dioxide and methane, the initial reduction of
viscosity is due to removal of carbon dioxide, the methane removal having
little effect on visocity. In order to effect conditioning of-the sludge
the agitator Reynold's number should be greater than about 10,000,
preferably 100,000. When most of the carbon dioxide has been removed as
determined by a decrease of viscosity of the sludge to about 800 cps or
below, depending on the particular sludge, the viscosity of the sludge
is further reduced to a value below about 100 cp (60 RPM in a Brookfield
viscometer) and this is done by addition of an appropriate additive to
the tank through line 15. Suitable additives for viscosity reduction are
clay dispersants such as, preferably, sodium silicate, which is added to
the sludge in the tank in an amount of from about 100 to about 2000 ppm
while maintaining agitation. An alternative to sodium silicate, but less
preferred, is the use of water alone as disclosed in U.S. 4,401,552
discussed above. Other viscosity clay dispersants which are viscosity
reducing agents may also be used such as phosphates, hydroxides, and the
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li]ce. Upon completion of the condition step, the sludge will have a pH about 8Ø
The conditioned feed stream (line 2) is then taken to a holding tank
(not shown) or fed directly to a first flotation cell where the process of
this invention is effected. The flotation cell should be a specific type of
flotation cell shown in Fig. 1 as B and designated as a "Rougher" flotation
cell. The specific cell which should be used in this invention is the well
known WEMC0 (a registered trademark of WEMC0 of Sacramento, California, U.S.A.)
type or equivalent flotation cell which is an induced air type of flotation
cell. These cells will preferably be operated in two or more banks of cells,
each bank consisting of four or more cells. For maximum cell efficiency,
which requires minimum rate of flow through any one unit, the units are placed
in parallel lines, each line consisting of two or more banks of the
flotation cells. Furthermore, in accord with this invention, the WEMC0
cells are operated at a lower rotor tip speed than normally used (normal
tip speed is about 1050 to 1100 rpm) and with additional air blown into
the cell. Thus, to achieve the efficiency of high bitumen recovery, tip
speed of the cell rotor in the process of this invention will be from
about 600 to about 750 ft/min and preferably about 675 ft/min and
additional air will be introduced to the cells (line 13) by a blower so
that the volume of air blown in is an amount wherein the ratio of the
volume of the cell to the volume of air per minute (e.g., volumetric air
ratio) is from about 1 to about 4, preferably about 2; e.g.,
volume of cell = 1 to 4. Expressed in terms of cubic feet per
volume of air/min
minute per cell, the flow rate will be maintained between about 5 and
about 20 CFM/Cell (20 CuFt cells), preferably about 10 CFM/Cell. Another
parameter of significance is the flow rate of material through the cells
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when operated in a continuous system. When cells other than WEMCO cells
are used, such as Galigher cells, it is found that a significant amount
of mineral material is floated as well as bitumen and this markedly
reduces the bitumen recovery. However, it will be understood that other
cells may be modified to meet the parameters required for this invention
and thus be made equivalent to the WEMCO cell used in this invention.
The Rougher froth (line 3) floated in the "Rougher" flotation
cells during a residence time of from about 30 to about 90 minutes
comprises from about 4 to about 30% bitumen, about 10 to about 30%
minerals with the balance being water. An optional, but preferred Tail
Skimmer C may be used to receive any unfloated bitumen in the Rougher
tails (line 4) and separate the froth, which is recovered (line 6) from
the tails (line 5) which tails are dispersed to a storage pond.
The separated Rougher froth (line 3) and any Skimmer froth
(line 6) is subsequently processed for recovery of the bitumen. This
separated froth is gravity fed into a Froth Settler D where the mineral
tailings are allowed to drain off (line 8). The froth in Settler D is
diluted with water (line 14) in accord with the process of U.S. 4,401,552
to enhance the bitumen in the froth. This enriched froth (line 7) which
comprises from about 7 to about 30% bitumen, about 15 to about 30%
minerals and the balance being water, is fed to a deaerator F where steam
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sparging (line 16) is used to deaerate the froth. The deaerated froth
is diluted with hot naphtha (line 17) and heated to 1800F to reduce its
viscosity to a value suitable for the hydrocyclones (G) which separate
the diluted bitumen into an overflow stream containing concentrated
hydrocarbon and -20 micron particles of minerals. The underflow stream
(line 19) comprises mainly water and mineral particles over 20 microns
n slze.
The overflow from the hydrocyclones (line 18) is fed to disc
centrifuge H where the hydrocarbon product (line 21) is concentrated to
95~ or more and the mineral is reduced to less than 3.5% by weight. Tails
from the centrifuge (line 20) are disposed of by sending to a storage
pond.
The tails from the Froth Settler D (line 8) are pumped to a
further upgrading system of "Cleaner Cells" E for further bitumen re-
covery. The Cleaner Cells are similar to the Rougher Cells and are
operated in similar banks of cells. The floated bitumen (line 9) is
recycled to the Froth Settler D and the tails (line 10) are sent to
disposal.
Typical preferred operating parameters for a large commercial
plant are as follows:
Conditioning
Flow rate: 7000 USGPM
Average Sodium silicate dosage: 400 ppm
Conditioning time: 6 minutes
Turbine plus air agitation Reynolds number 104-105
Vol of cond vessel (Ft3) = 1
Alr sparglng ratlo =
Volume of air/min (CFM)
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Rougher Flotation Cells
Flow rate: 7000 USGPM
Residence time 68.5 min.
Average Recovery = 84%
Flow rate through 1000 Ft3 WEMCO cells = 1000 USGPM
Volumetric air ratio: Volume of cell = 2
Volume of air/min
Rougher Tails Skimmer
Flow rate: 4,000 USGPM
Residence time: 15 min
Recovery: 10%
Flotation Froth Settler
Flow rate: 5,500 USGPM
Residence time: 45 min
Recovery: 70%
Cleaner Cells For Froth Settler Tails.
Flow rate: 4000 USGPM
Residence time: 30 Min
Average Recovery: 90%
Volumetric air ratio: Volume of cell = 2
Volume of air/min
By following the above-described over-all process a sludge
feed (line 1) containing 4% by weight of bitumen and 29% mineral was
processed with the result that the product bitumen stream (line 21) was
upgraded to contain 95% bitumen and 1.14% mineral.
Table I illustrates the specific improvement of bitumen re-
covery achieved by the invention. As seen in the table the bitumen re-
covery increases substantially when the flow of air is increased over the
normally induced amount of air in the WEMCO cell.
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TABLE I
Effect of Air Flow On Bitumen Recovery
I A WEMCO Cell On A Tar Sands Tailings Sludge ~
Residence Bitumen Recovery (~) *
Time Induced Air Only With Additional
(Min) (40 CFM) Air Flow
18
31
33 42
44 50
Tip speed of agitation = 677 ft/min
* Average values
+ Volumetric air ratio = 2
As can be seen from the above data, the lower agitated tip
speed alone (induced air only) is inadequate for improved performance,
but the combination of lower tip speed and use of increased air
flow in the flotation cell gives significantly higher bitumen recovery.
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