CA2075721C

CA2075721C - Process for cleaning up wet tailings ponds

Info

Publication number: CA2075721C
Application number: CA002075721A
Authority: CA
Inventors: Thomas P. Clarke
Original assignee: Individual
Current assignee: Individual
Priority date: 1992-05-01
Filing date: 1992-05-01
Publication date: 1996-02-20
Anticipated expiration: 2012-05-01
Also published as: CA2075721A1

Abstract

A process for the clean-up of wet tailings ponds produced in the operation of an oil sands extraction process includes the steps of removing matter from all layers of a pond and vacuum filtering the combined matter to obtain clarified water, bitumen and dry tailings.
With this process, tailings ponds may be completely removed and the water contained therein recycled to the extraction process. The uncovered area may be mined and the dry tailings may be used to reclaim mined-out areas. Residual bitumen recovered from the pond material contributes to a reduction of the clean-up costs. Environmental damage is minimized by complete removal of the pond.

Description

Process For Cleanin~ UP Wet Tailin~s Ponds This invention relates to a process for treating both the liquid and the solid phase of a tailings pond generated in the operation of an oil sands water extraction process, and more particularly to a process for the removal of the whole tailings pond matter with dry tailings, water and bitumen being the major end products.
Various industrial mining processes currently employ large quantities of water for the extraction of minerals or oil from a natural carrier medium such as sand in the case of oil sand extraction processes. Consequently, large amounts of wet tailings are produced.
The wet tailings produced in oil sand extraction processes, usually include the whole oil sand body removed from the mining area plus added process water and chemicals used in the extraction process less the recovered bitumen. Wet tailings consist of process water (about 45%), mineral matter (about 54%) including sand and some fine solids such as silt and clay, as well as some residual bitumen (about 0.8%). The tailings are regarded as commercially not valuable and are discarded into special holding areas, the so-called tailings ponds. The sand portion of the tailings settles out rapidly from the tailings water while the suspended fines, settle only very slowly, if at all. Fines are defined as having a particle size of 44 microns or smaller. Good grade Oil Sands fines content is up to about 10%, poor grade Oil Sands content is up to about 35% and clay sands have a fines content of about 60%. The tailings sludge which remains when the sand has settled out contains these fines as well as the residual bitumen, chemicals and water. Thus, since enormous amounts of water are required for the oil sand extraction process and not all the bitumen is extracted, it would be economical and environmentally responsible to recycle the water and to extract the residual bitumen contained in the tailings ponds.
However, the settling rate of the fines and oil-fines emulsions formed in the tailings ponds is very slow so that it has not been possible to dry-up the tailings. Consequently, gigantic tailings ponds are currently used to provide sufficient retention time for the water to clarify. Additional tailings ponds must be provided for the tailings of present and future oil sands processing.

There are a number of economical and environmental problems associated with the tailings ponds. The ponds cover mineable tar sands deposits and moving their contents would require large capital investments greatly reducing the profitability of the uncovered deposits. The ponds have to be surrounded by dikes which are expensive to construct. Furthermore, the tailings ponds are contaminated with residual bitumen from the extraction process and various extraction yield enhancing chemicals as well as flocculants. The residual bitumen content of the ponds is about 0.8% of the tailings pond matter and is due to extraction inefficiencies of the processes currently used and plant upsets. This residual bitumen may be recovered, thereby reducing the costs of tailings pond clean-up. Finally, environmental authorities have found that the cont~ ~nAnts in the pond are gradually seeping out into the ground water, surrounding lakes and rivers and other adjacent fresh water bodies. The contamination problem is of great concern and requires immediate attention. The most sensible solution would be to remove all existing tailings ponds and to only operate oil sands extraction processes which do not produce wet tailings. One such process, which produces dry tailings, is disclosed in U.S. Patent 4,240,897.
Several processes and methods have been proposed to overcome the problems associated with the tailings ponds.
HEPP (CA 892,548) teaches the recovery of clarified water from the aqueous phase of a tailings pond by flocculating the finely divided mineral in the water and subsequently subjecting the aqueous phase to a vacuum precoat filtration. Tailings ponds water is admixed with a flocculant and a filter aid such as diatomaceous silica and the resulting mixture is passed through a vacuum filtration zone.
Substantially clarified water is recovered from the vacuum filter zone and recycled into the hot water process. The sludge layers which constitute the major portion of the pond volume and contain the residual bitumen remain in the pond. Thus, HEPP does not teach the removal of all tailings pond material but only the recycling of the surface water in the pond.
KUTASINSKI (CA 1,036,524) describes a process of solidifying the fluid sludges in tailings ponds for the recovery of bitumen and clarified water, the latter being recycled to the hot water extraction process. The pond water is admixed with a chemical additive and fed under pressure into the sludge material near the bottom of a tailings pond. This results in a rapid mixing of the chemical additive with the sludge and coagulation of the sludge solids into an inert solid material. At the same time, bitumen contained in the aqueous sludge separates out as froth on the surface of the tailings pond and a major portion of the retained water is separated. The bitumen froth iscollected from the pond's surface and transferred to the hot water extraction process for conversion to synthetic oil. Thus, although Kutasinski teaches the recovery of both clarified water and bitumen from the tailings pond, this is only achieved by adding a further chemical substance to the sludge, which remains in the tailings pond.
YONG (CA 1,124,895) teaches the treatment of tailings pond sludge with a flocculant to decrease the volume of the sludge by increased dewatering. This is achieved by treating the tailings of a hot water extraction process with hydrolyzed starch flocculants. These flocculants increase the strength of the sludge in the tailings pond.
Furthermore, sand is admixed with the tailings in order to increase the self-weight of the sludge. A porous piston effect is achieved for the compressing of and, thus, dewatering of the sludge layer. The sludge layer of a tailings pond which has been treated with a hydrolyzed starch flocculant is capable of supporting a substantial sand surcharge. The permeability of the sludge layer is also improved by treatment with the hydrolyzed starch flocculants, which affords an enhancement of the degree of compaction and dewatering of the sludge.
In a related patent (CA 1,140,281) YONG describes a process which is very similar to the one discussed immediately above, however, instead of adding sand directly to the mixture of tailings and hydrolyzed starch flocculants, a layer of sand is distributed over the sludge layer in the tailings pond. The sand layer functions as a heavy porous piston which compresses and dewaters the sludge layer beneath. A second layer of sludge may be laid over such a deposited sand layer and then the second sludge layer, itself, may be subjected to a surcharge brought about by another sand layer. For relatively deep tailings ponds, a number of such alternate layers of treated sludge and sand may be employed to obtain a very high degree of compaction and dewatering.

Thus, none of the prior art processes teach or even suggest a complete removal of the whole tailings ponds material and, consequently, none of these processes overcome the above mentioned problems associated with existing tailings ponds.
The invention now provides a process for cleaning up tailings ponds from oil sand extraction processes, which overcomes these disadvantages by permitting the removal and treatment of material from all layers of a tailings pond. Clarified water, bitumen and substantially dried solids are obtained. The dried solids may be used for filling in and reclaiming mined out areas. With this process, existing tailings ponds may be completely removed to allow mining of the uncovered areas.
Furthermore, the disclosed process provides for additional bitumen recovery, reuse of the tailings ponds water and minimization of the environmental impact of the pond.
Accordingly, the invention provides a process for cleaning up a pond made from wet tailings produced in the operation of an oil sands water extraction process, comprising the steps of (a) removing material from the pond and producing a filterable stream of material including solid matter, bitumen, water and dispensed solids;
(b) passing the stream of material into a filter unit having a filter medium;
(c) filtering the stream of material through the filter medium to form a filter cake on the filter medium and to recover water;
(d) adding a diluent and passing the diluent through the filter cake and the filter medium to extract residual bitumen present in the stream of material and captured in the filter cake;
(e) adding heated wash water and passing the wash water through the filter cake to recover diluent retained in step d and to extract additional bitumen;
(f) reducing the moisture content of the filter cake; and (g) removing the filter cake from the filter medium for disposal of the filter cake material as dry tailings.
The stream of material removed from the pond preferably includes material from all layers of the pond and the filter unit is preferably a vacuum filter unit providing for a vacuum filtration in step c.

_ 5 _ 2075721 In another embodiment, the process further includes the steps of elutriating the stream to the filter unit and decanting a portion of a substantially solid free liquid layer above the filter medium.
In yet another preferred embodiment of the invention, the tailings pond clean-up operation is integrated with a conventional oil sands water extraction process by mixing the wet tailings produced in the extraction process with the stream of tailings pond material before passing it into the filter unit.
Preferred embodiments of the invention will now be further described by way of example only and with reference to the following drawings, wherein Figure l schematically illustrates a preferred embodiment of a process in accordance with the invention; and Figure 2 schematically illustrates another preferred embodiment of a process in accordance with the invention.
In the process illustrated in Figure 1, material is removed from all layers of a tailings pond 10 and passed through a vacuum filter unit 22 for removal of bitumen and clarified water and for the production of dried tailings which can be used to reclaim a mined out area (not shown). The process can be used to completely remove and clean up the tailings pond 10. Tailings pond 10, unless constantly agitated, includes a surface layer 12 of relatively clarified water and a sludge layer 14, which generally contains water and mineral matter, unrecovered bitumen, as well as chemical substances used in the extraction process.
The material of sludge layer 14 is dredged from pond 10 by appropriate machinery, in this embodiment a dredging platform 16. The clarified water of surface layer 12 is either pumped off and recycled for use in an oil sand water extraction process or mixed with the dredged out sludge in a mixing unit 18 to produce a pumpable solid/liquid mixture.
When several ponds are cleaned-up simultaneously, the clarified surface water of one pond may be admixed with the sludge material dredged from another pond. Conventional pumps used in existing extraction operations for the transport of the wet tailings can be used to sluice the mixture as a stream of tailings ponds material 20 to vacuum filter unit 22 for further processing. To reduce the tailings ponds liquid contents more rapidly, the clarified surface water can be separately pumped to the filter unit 22. The dredging of the sludge layer 14 and the pumping of the water layer 12 may be done simultaneously by separate pumps mounted on the dredging platform 16. The stream of material 20 is transported to filter unit 22 with an intermediate heating by steam injection in unit 21, if required, to provide a higher extraction efficiency in the subsequent steps of the process described in the following. No specific ratio of solids to water is required, since the lower the water content, the smaller the size of the filter required. Vacuum filter unit 22 has a number of radially positioned filter segments (Pyron* filters available from the Bird Company, South Walpole, Mass., U.S.A.; not shown), which are sequentially used for a continuous filtering operation. Each filter segment is passed under the stream of material 20, whereby the solid matter of the stream 20 accumulates on the filter segment to form a filter cake and clarified water and a small fraction of fines are recovered in a first filtrate 28. The coarse sand particles in the solid matter stream settle quickly onto the filter segment and provide a filter bed cake for capturing bitumen and the fine clay particles ("fines") also in the stream. At high clay fines contents in the stream of material 20, the stream of material is elutriated with warm process water to improve the filtration rate and a portion of the relatively solid free liquid layer obtained above the filter medium is decanted and combined with the first filtrate 28. The PYRON filters are composed of separate individual leafs which can be independently tilted at any angle of up to 180 and for any desired time required for the decanting operation. The PYRON filters also allow the selection of any quantity of elutriant (water) and time of elutriation.
After filtration, the dried filter cake is passed under a hot diluent to extract bitumen from the filter cake. The diluent used in this preferred embodiment is heavy naphtha which is added at a preferred weight ratio of heavy naphtha to filter cake of about 0.06 to achieve a bitumen recovery of up to 99%. The heavy naphtha boiling range is 240-250 F Initial Boiling Point (IBP) to about 375 F End Boiling Point (EBP). A second filtrate 32 which includes hot diluent and bitumen is obtained. Subsequently, the filter cake is passed under a hot wash water stream (130-208 F) to recover more bitumen and diluent still trapped in the filter cake and possibly the filter medium. A third filtrate 34 is obtained which includes hot wash water, bitumen and some * Trade-mark dissolved matter. The dried filter cake is removed from the filter unit by rotation of the filter segment through 180 and transported as dry tailings to a tailings dump (not illustrated) in a mined out area. The term "dry tailings" used throughout this specification designates tailings of a water content of about 8 to 10 wt%, which are dry to the touch.
The first filtrate 28 may be recycled to the mixing unit 18. The second filtrate 32, containing the extracted recovered bitumen and naphtha solvent mixture is forwarded to a conventional froth clean up unit 30 currently used in oil sands extraction processes. However, other clean up units which perform essentially the same function may be used. The third filtrate 34, including the hot wash water plus recovered bitumen and solvent naphtha, is forwarded to clean up unit 30 to recover the bitumen and solvent naphtha.
In clean up unit 30, the filtrates 32 and 34 are separated into clean dry bitumen and diluent and tailings 33 including mainly water.
The tailings 33 are recycled to mixing unit 18, or recycled as wash water to filter unit 22 with an intermediate heating by steam injection in a heating unit 31, to recover any residual bitumen and diluent. The tailings may also be used for pulping the oil sands feed of a conventional oil sand extraction process or for sluicing the primary separator cell bottoms of the conventional processes. The clean dry bitumen and diluent is transported to upgrading facilities (not shown) where the diluent is separated for recycling to filter unit 22 and the bitumen is upgraded to synthetic crude oil.
In another process illustrated in Figure 2, the tailings ponds clean-up is integrated with a conventional hot water oil sands extraction process (CHWEP), which uses a hot water extraction unit 40, a separation unit 42 and a froth clean-up unit 44. In this operation, oil sands from a mining area are mixed in extraction unit 40 with hot water and chemicals and subsequently fed to separation unit 42 where the mixture is separated into a froth 13, water and suspended mineral matter, and solid tailings. The froth 13 which includes recoverable bitumen is transported to the froth clean-up unit 44. Here the froth 13 is mixed with a hydrocarbon diluent such as naphtha. Clean dry bitumen and diluent are obtained and transported to upgrading facilities for further processing as described above in relation to Figure 1. In the ~?

process of Figure 2, a stream of pond material 20, which is obtained as described above with reference to Figure 1, is combined with the wet tailings streams produced by the separation unit 42 and the froth clean up unit 44 respectively. The combined streams are then fed into filter unit 22 and processed as described in relation to Figure 1. However, froth clean up unit 44 of the oil sands extraction operation is used in place of separation unit 30 (Figure 1). The streams may be combined in any appropriate ratio as long as the fines content of the combined streams is sufficiently low to prevent blinding of the filter and to allow an effective filtering operation. To aid the filtration rate, the combined streams may be elutriated with hot water and a portion of the relatively solid-free liquid layer may be decanted and combined with the first filtrate 28 and recycled to the hot water extraction process. A
floculant is added to the decant stream to reduce the fines content, if required. Appropriate floculants are mentioned below. Also, if the extraction process already includes a vacuum filtration step, the mixing ratio of the streams will be determined by the total capacity of the filter unit, the total wet tailings output of the extraction process and the time frame available for completion of the tailings ponds clean-up.
Thus, the mixing ratio will be influenced to a large degree by legislation which is to be put in place by governments in relation to the clean-up of tailings ponds. The clarified water obtained is recycled to the hot water extraction unit 40.
In both of the above described processes, a flocculant may be added to the stream of pond material 20 or to the pumped-off pond water to prevent blinding of the filter at high fines contents in the stream of material 20 and to increase the filtering rate thereby reducing the fine clay particles concentration in the various filtrate streams.
Flocculant may also be added to the first filtrate 28 before recycling it to the mixing unit 18 or to a conventional oil sands extraction process. Appropriate flocculants are well known in the art and include, calcium oxide, calcium chloride, aluminum sulfate, calcium sulfate, polyalkylene oxides, the calcium salt of ethylene diamine tetraacetate, guar flow, high molecular weight acrylamide polymers, acrylic or methacrylic acid derivatives, aminoalkyl acrylates, aminoalkyl acrylamides, N-alkyl substituted aminoalkyl esters of acrylic or methacrylic acids, etc.

9 207572i Although naphtha is the diluent preferably used in the disclosed process, other appropriate diluents are well known in the art.
Dredging methods other than the one described above may be used for removal of the tailings pond contents. Hydraulic dredging machinery employed for the claiming of land lost to beach erosion may be used, for example. Instead of pumping off the clarified surface water of a pond and subsequently dredging the remaining sludge, the complete pond may be agitated by heavy pumping equipment mounted on a barge to achieve mixing of the layers of the pond. Air may be pumped down into the bottom sludge layer of the pond to assist the mixing operation. The resulting mixture is then transported to the filter unit 22. The layer of sludge which will invariably remain at the bottom of the pond once the mixed phase is removed may be dredged and processed as discussed above.
Although the present process has been described in relation to a vacuum filter unit, it will be apparent that other means of solid/liquid separation, such as a centrifuge may be used.

Claims

1. Process for cleaning up a pond made from wet tailings produced in the operation of an oil sands water extraction process, comprising the steps of a) removing material from the pond for producing a stream of material including solid matter, water and dispensed solids;
b) passing the stream of material into a filter unit having a filter medium;
c) elutriating the stream of material being passed to the filter unit with water;
d) decanting a portion of a substantially solid free liquid layer above the filter medium;
e) filtering the stream of material through the filter medium to form a filter cake on the filter medium and to recover clarified water;
f) adding a diluent and passing the diluent through the filter cake to extract in a first filtrate residual bitumen captured in the filter cake;
g) adding heated wash water and passing the wash water through the filter cake to recover diluent retained in step f) and to extract additional bitumen in a second filtrate, h) reducing the moisture content of the filter cake; and i) removing the filter cake from the filter medium for disposal of the filter cake material as dry tailings.

2. A process as defined in claim 1, wherein the stream of material is removed from a pond having at least two layers and includes material from all layers of the pond.

3. A process as defined in claim 1 or 2, wherein the filter unit is a vacuum filter unit and the filtering in step e) is achieved by vacuum filtration.

4. A process as defined in claim 3, comprising the further step of extracting bitumen from the second filtrate obtained in step g).

5. A process as defined in claim 3, wherein the diluent is heated before addition in step f).

6. A process as defined in claim 3, comprising the further step of extracting diluent and bitumen from the first filtrate obtained in step f).

7. A process as defined in claim 3, further comprising the step of heating the stream of material removed from the pond in step a) before step b).

8. A process as defined in claim 3, wherein at least a part of the clarified water is recycled for addition in step g).

9. A process as defined in claim 6, wherein the diluent extracted from the first filtrate obtained in step f) is recycled for addition in step f).

10. A process as defined in claim 3, further comprising the step of adding a flocculant to the stream of material for increasing the filtering efficiency of the filter unit.

11. A process as defined in claim 4, 5, 6, 7 or 10, further comprising the step of heating the stream of material obtained in step a) to a temperature corresponding to the temperature of wet tailings produced in a hot water oil sand extraction process.

12. A process as defined in claim 1, 2, 4, 5, 6, 7, 8, 9 or 10, wherein the filter cake is subjected to vacuum for reducing the moisture content of the filter cake.

13. A process as defined in claim 12, further comprising the step of drying the filter medium with hot air after removal of the filter cake.

14. A process as defined in claim 13, wherein the stream of material removed from the pond in step a) has the same composition as a wet tailing stream produced in a hot water oil sand extraction process.

15. A process as defined in claim 14, wherein the stream of material is produced by dredging a sludge layer of the pond and mixing the dredged-out material with additional water in a ratio selected for achieving the desired composition of the stream of material.

16. A process as defined in claim 15, wherein the additional water is at least one of pond water, fresh water and the clarified water obtained in step e).

17. A process for cleaning up a pond including wet tailings produced in the operation of an oil sand extraction process, comprising the steps of a) removing material from the pond for producing a stream of material including solid matter, unrecovered bitumen water and dispensed solids;

b) mixing the stream of material with a stream of wet tailings to obtain a mixed stream, the wet tailings being produced in an oil sand hot water extraction process using a separation unit for generating a bitumen froth and wet tailings and a froth clean up unit for separating the bitumen from the froth;
c) passing the mixed stream into a filter unit having a filter medium;
d) elutriating the mixed stream being passed to the filter unit with water;
e) decanting a portion of a substantially solid free liquid layer above the filter medium;
f) filtering the mixed stream through the filter medium to form a filter cake on the filter medium and to recover clarified water;
g) combining the portion decanted in step e) with the clarified water recovered in step f) for recycling to the separation unit.
h) adding a diluent and passing the diluent through the filter cake to extract in a first filtrate bitumen captured in the filter cake;
i) adding heated wash water and passing the wash water through the filter cake to recover diluent retained in step h) and to extract additional bitumen thereby producing a second filtrate;
k) reducing the moisture content of the filter cake; and l) removing the filter cake from the filter medium for disposal of the filter cake material as dry tailings.

18. A process as defined in claim 17, wherein the stream of material is removed from a pond having at least two layers and includes material from all layers of the pond.

19. A process as defined in claim 17 or 18, wherein the filter unit is a vacuum filter unit and the filtering in step f) is achieved by vacuum filtration.

20. A process as defined in claim 19, wherein the filter unit is a filter unit of a hot water extraction process producing dry tailings.

21. A process as defined in claim 19, comprising the further step of extracting bitumen from the second filtrate obtained in step i).

22. A process as defined in claim 19, wherein the diluent is heated before adding the diluent in step f.

23. A process as defined in claim 19, comprising the further step of passing the filtrate obtained in step h) to the froth clean up unit for the extraction of bitumen and diluent from the filtrate.

24. A process as defined in claim 19, wherein the clarified water is recycled for addition in step i).

25. A process as defined in claim 23, wherein the diluent extracted from the filtrate obtained in step h) is recycled for addition in step h).

26. A process as defined in claim 23, wherein the diluent extracted from the first filtrate obtained in step h) is recycled for use in the hot water extraction process.

27. A process as defined in claim 19, further comprising the step of adding a flocculant to at least one of the stream of material, the stream of wet tailings, the mixed stream and pond water added in the mixing unit.

28. A process as defined in claim 17, 18, 20, 21, 22, 23, 24, 25, 26 or 27, further comprising the step of heating the stream of material before step b) to a temperature corresponding to a temperature of the stream of wet tailings.

29. A process as defined in claim 17, 18, 20, 21, 22, 23, 24, 25, 26 or 27, wherein the filter cake is subjected to vacuum for reducing the moisture content of the filter cake.

30. A process as defined in claim 30, further comprising the step of drying the filter medium with hot air after removal of the filter cake.

31. A process as defined in claim 31, wherein the stream of material is produced by dredging a sludge layer of the pond with at least one of pond water, fresh water and the clarified water obtained in step f).