CA1247550A - Process to float bitumen from mineral slimes resulting from tar sands processing - Google Patents

Abstract

ABSTRACT

The present invention is an improved method for recovering 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 to a froth from which the bitumen is recovered. The improved method involves subjecting the sludge to air flotation in a flotation cell at a controlled rotor speed and at a controlled air flow, deaerating the froth thus obtained and recovering the bitumeh.

Description

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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.

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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 ~Z4755~
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 S~

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 ,, ~

~;~47S50 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) 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 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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Claims (2)

1. Claim 1 1. A method for improving the recovery of bitumen from a waste sludge containing bitumen, mineral and water wherein said sludge is subjected to bitumen recovery by air flotation which comprises subjecting the sludge to said air flotation in an induced air type of flotation cell operated at a rotor tip speed of between about 600 and about 750 ft/min and wherein the air flow through said cell is augmented by blowing in air in an amount so that the ratio of the volume of the cell to the volume of air per minute is from about 1 to about 4 to obtain a froth from which bitumen is subsequently recovered.
   Claim 2 The method of Claim 1 wherein the flotation cell is a WEMCO
   cell.
   Claim 3 The method of Claim 2 wherein the volumetric air ratio is about
2. 2.
   Claim 4 A method for improving the recovery of bitumen from a waste sludge containing bitumen, mineral and water wherein said sludge is subjected to bitumen recovery by air flotation which comprises subjecting the sludge to air flotation in a WEMCO type flotation cell operated at a rotor tip speed of between about 600 and about 750 ft/min, augmenting the air flow through said cell by blowing in air in an amount so that the volumetric air ratio is about 2 to obtain a froth; subjecting the bituminous froth to a froth settler wherein the mineral tailings are drained off; diluting the froth from said settler with water to enhance the bitumen in said froth; deaerating the diluted froth; and separating a bitumen product from said froth.