Note: Descriptions are shown in the official language in which they were submitted.
207572 1
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.
207572 1
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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
207572 1
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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.
2075721
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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
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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
207572 1
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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
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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.
