CA1252409A - Process to recover 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 the combined steps of conditioning the sludge to reduce viscosity by (a) removing carbon dioxide and methane from the sludge and (b) further reducing the viscosity of the sludge with an additive, subjecting the thus conditioned sludge to air flotation in a flotation cell, deaerating the froth thus obtained and recovering the bitumen.

Description

~5~

AN IMPROVED PROCESS TO RECOVER BITUMEN FROM MINERAL SLIMES RESULTING EROM
TAR 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 is carried out by a combination of specific steps which condition the aqueous sludge and effect air flotation in specific equipment to 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, Venezuelar 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.
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 of 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 froth which is recovered by skimming the settling vessel. An aqueous middlings layer containing some mineral and bitumen is formed between : -2 ~ ~5,5~

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 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 process for recovering the bitumen from these tailings, which bitumen would otherwise be wasted and also 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 first 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

2~

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. This Canadian patent and the others also disclose that sodium silicate can improve bitumen recovery when used in connection with aeration and agitation.
Canadian Pat. 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 scavenger 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 improved method of this invention for treating froth obtained by agitation and aeration of pond sludge.

~q ~5~

U.S. Pat. No. 3,594,306, E. W. Dobson, July 20, 1971, discloses upgrading froth from a secondary recovery operation (generally a flo--tation scavenger zone treating the bitumen-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 treating froth o~tained by treatment of pond sludge as disclosed herein.
U.S. Pat. No. 3,738,930, V. P. Kaminsky, June 12, 1973, dis-closes forming a froth from a middlings stream from a primary cell. The formed froth is produced in a secondary cell and as it leaves the secondary cell it is treated to a fresh hot water wash which deaerates the secondary formed froth. The combination of the hot water wash and deaerated froth is subjected to intimate contacting in a froth washer cell and within a quiescent settling zone a more concentrated (as to bitumen) froth is formed. The temperature range of the hot water washer is 100-200F. U.S. Pat. No. 3,784,464, V. P. Kaminsky, June 18, 1974, discloses apparatus which can be used in the hot water washing of secondary froth.
U.S. Pat. No. 3,296,117, S. Ross, et al, Jan. 3, 1973, discloses upgrading Eroth from a primary recovery operation (wherein fresh tar sand and water are contacted) by water washing the froth. The washing involves contacting incoming froth countercurrently with in-coming water. The water used contains an additive such as tetrasodium pyrophosphate and the temperature of the water washing zone is maintained in the range of 104 F. to 200 F. The water washed product, an emulsion, is separated from solids (contained in the froth) and contacted with a selective demulsified mixture whereby a water-free oil phase and an oil-free water phase are obtained and separated. U.S. Pat. No. 3,331,765, G. R. Canevari, et al, July 18, 1967, discloses a similar process using a different demulsifier mixture. U.S. Pat. No. 3,330,757, J. A. Bichard, July 11, 1967, also discloses a similar process using a chelating agent with the water wash for the froth produced in the primary tar sandwater mixture step. 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 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. The present invention is a significant improvement over the process of U.S. 4,401,552.

Summary of the Invention 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 bitumen is recovered. The improved method involves the combined steps of conditioning the sludge by (a) removing carbon dioxide and methane from the sludge whicn effects some viscosity reduction and (b) further reducing the viscosity of the sludge, preferably with an additive, subjecting the thus conditioned sludge to air flotation in a series of WEMCO (a registered trademark of WEMC0 of Sacramento, California, U.S.A.) or equivalent flotation cells and the froth thus obtained is deaerated, diluted and the bitumen recovered, preferably by hydrocycloning followed by centrifugation.

Description of the Drawing Fig. 1 is a schematic representation of a hot water tar sands extraction process and how one embodiment of applicants' method could be incorporated herein.
Fig. 2 is a graph showing the importance of time at which a viscosity reducer is added.

Detailed Description of the Invention_ Referring now to Fig. 1 which is a flow diagram showing the overall operation of the process of the invention, sludge from the tailings pond is fed through line 1 into the conditioning 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 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. The process of this invention will recover about 80~
of the bitumen in the sludge. 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 removalhaving 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 agi-tation. 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 like. It is critical to the operation of this invention, however, that the sludge be conditioned first by agitation to remove CO2 and methane and then reduce the viscosity. As can be seen in Fig. 2, addition of sodium silicate before sparging with air does not enable the viscosity to be reduced to the necessary level of below about 100 centipoise (cp). 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 which is a specific type of flotation cell shown in Fig. 1 as B and designated as a "Rougher" flotation cell. The specific cell which must be used in this invention is the well known WEMCO 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, 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 WEMCO
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 - l t 4 . - Expressed ln terms of cublc feet per volume of air/mln minute per cell, the flow rate will be maintained between about 5 and about 20 CFM/Cell (20 CuFt cells), preferably about lO CFM/Cell. 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 lO 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 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 sparging (line 16) is used to deaerate the froth. The deaerated froth is diluted with hot naphtha (line 17) and heated to 180F 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) com-prises mainly water and mineral particles over 20 microns in size.
The overElow 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 Y) 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 Fro-th 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 ~ir sparging ratio = Vol. of cond. vessel ~Ft3) = 1 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 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.

Claims

Claim 1 1. An improved method for recovering bitumen from a waste sludge containing bitumen, mineral and water which comprises a) conditioning said sludge by removing carbon dioxide and methane and thereafter reducing the viscosity of the sludge to a value below about 100 centipoise;
b) subjecting the conditioned sludge to 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 wherein 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;
c) subjecting said froth to a froth settler wherein the mineral tailings are drained off;
d) diluting the froth from said settler with water to enhance the bitumen in said froth;
e) deaerating the diluted froth; and f) separating a bitumen product from said froth.

Claim 2 The process of Claim 1 wherein the flotation cell is a WEMCO
cell.

Claim 3 The method of Claim 1 wherein sodium silicate is added to the sludge after removal of the CO2 and methane.

Claim 4 The method of Claim 3 wherein conditioning is carried out with an agitator Reynold's number greater than about 10,000.

Claim 5 The method of Claims 1, 3 or 4 wherein the air sparging ratio in the conditioning step is about 1.

Claim 6 An improved method for recovering bitumen from a waste sludge containing bitumen, mineral and water which comprises a) conditioning said sludge by removing carbon dioxide and methane and thereafter reducing the viscosity of the sludge to a value below about 100 centipose by the addition of sodium silicate;
(b) subjecting the conditioned 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 and wherein the air flow through said cell is augmented by blowing in air in an amount wherein the volumetric air ratio is about 2 to obtain a froth;
c) subjecting said froth to a froth settler wherein the mineral tailings are drained off;
d) diluting the froth from said settler with water to enhance the bitumen in said froth;

e) deaerating the diluted froth; and f) separating a bitumen produce from said froth.

Claim 7 The method of Claim 6 wherein the air sparging ratio in the conditioning step is about 1.