Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
PROCESS FOR IMPROVING DEWATERING CHARACTERISTICS
OF OIL SANDS TAILINGS BY TREATMENT WITH LIME
FIELD OF THE DISCLOSURE
The present disclosure relates in general to methods and processes for
dewatering
tailings produced by ore-water slurry-based processes for extraction of
bitumen from
surface-mineable oil sands ore. The disclosure relates in particular to
methods and
processes for treating mature fine tailings (MFT) to promote the release of
water
therefrom.
BACKGROUND
The oil sands deposits in the Athabasca region of Alberta, Canada are
estimated to
contain about 30 billion cubic meters (equivalent to about 180 billion
barrels) of heavy
hydrocarbons in the form of bitumen, and constituting the third largest
hydrocarbon
deposit in the world. The typical composition of these oil sands, by weight,
is about 12%
bitumen, 82% to 85% mineral solids, and 3% to 6% water. The fraction of solids
smaller
than 44 microns (i.e., micrometer, or m) in size are called "fines",
consisting largely of
silt and clay particles. Clay particles are commonly defined as particles
smaller than 2
microns in size.
The clay fraction in oil sands ore is a significant factor affecting ore-water
slurry-
based bitumen extraction processes as well as processes for disposal of the
tailings
produced by such extraction processes. The widely-used Clark Hot Water
Extraction
(CHWE) process uses caustic sodium hydroxide (Na OH) as an additive to promote
bitumen recovery efficiency from oil-sands ore-water slurry. The use of NaOH
as an
additive to the slurry increases the pH (a measure of the acidity of
suspensions and
slurries, defined by pH = - log [H+1, where [Tr] is the molar hydrogen ion
concentration)
of the ore-water slurry to about 8.0 to 9.0, causing asphaltic acids of
bitumen to be
activated to produce surfactant species. Surfactant species are water-soluble
and reduce
water surface tension and bitumen-water interfacial tensions, thus promoting
the
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liberation of bitumen from the oil sands ore matrix. At the same time,
however, they
promote dispersion of silt/clay fraction of the ore in the extraction process
slurry.
These surfactant species promote hydrophilic characteristics of bitumen
droplets
surfaces, and suppress kinetics of coalescence and aeration of bitumen
droplets;
therefore, they slow down the formation of bitumen-rich froth in the
extraction process.
As a result, long retention times, and/or large vessels, are needed for the
primary and
secondary recovery processes. This increases the capital and operating costs
of oil sands
plants, one of the disadvantages of the CHWE process.
In commercial oil sands operations, the residual slurry (or "tailings")
remaining
after extraction of bitumen from the ore-water process slurry is typically
hydraulically
transported from the extraction plant to tailings ponds and deposited therein.
This
tailings slurry is typically a mixture of sand particles, dispersed fines
(silt and clay
particles), water, and residual bitumen. The tailings slurry is made up of
about 55%
solids (by weight), and this solids fraction is typically about 82% sand, 17%
fines smaller
than 44 microns, and about 1% residual bitumen (by weight). The remaining 45%
(by
weight) of the tailings slurry is water, which typically contains significant
residual
amounts of various other substances including toxic chemicals.
When the tailings slurry is discharged into the tailings ponds, the coarse
sand
particles tend to segregate quickly and form a "beach"; however, the remaining
"fine
tails" (i.e., a slurry containing about 6% to 10% silt and clay fines in
suspension)
accumulate in the tailings ponds. Fine tails settle quickly to about 15% to
20% by
weight. After a few years, though, the fine tails transform into a gel-like
fluid material
commonly called "mature fine tails" (MFT), containing about 30-32% solids by
weight
(predominantly silt and clay particles), with the remaining constituent being
water
making up roughly 85% of the MFT by volume. This large volume of typically
toxic
water trapped in mvr ponds is a significant environmental concern.
Industry has been striving for decades to find effective ways to reduce the
water
content of MFT in order to decrease tailings pond volumes, to recover water
trapped in
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MFT for treatment and recycling to the bitumen extraction process (thus
reducing the
volume of fresh water import required for the extraction process), and to
facilitate
removal and disposal of MFT solids. However, these efforts have been and
continue to
be frustrated by the inherent stability of MFT and the resultant difficulty of
separating its
main constituent elements ¨ i.e., water and fines.
The stable structure of the MFT is believed to be largely attributable to the
use of
NaOH in the bitumen extraction process, because NaOH in the ore-water slurry
results in
the bonding of sodium ions (Nat) onto the surfaces of clay particles in the
slurry, which
in turn promotes swelling of the clay with water. In addition, and as
previously noted,
NaOH in the ore-water slurry promotes solubility of asphaltic acids present in
bitumen,
which reduce water surface and bitumen-water interfacial tensions (which is
good for
bitumen extraction), but which also promote dispersion of silt-clay particles
in the slurry
(which is not desirable in tailings). It is also suspected that hi-wetted clay
particles
(mostly kaolinite type) of submicron dimensions (i.e., less than 0.2 gm), also
called ultra-
fines, contribute to the formation of the stable gel-like structure of MFT.
In summary, MFT is a toxic by-product of oil sands ore-water slurry-based
bitumen
extraction processes. The existing inventory of MFT is estimated at over a
billion cubic
meters, which both government and the oil industry recognize as a serious
environmental
liability.
For the reasons discussed above, there is a need for improved methods and
processes for dewatering MET to facilitate reduction of MFT inventories, along
with
other benefits arising from dewatering of NIFT.
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BRIEF SUMMARY
The present disclosure teaches methods and processes for improving the
dewatering characteristics of MFT by increasing the yield stress of MFT, and
thus
reducing the plasticity MFT, by treating MFT with lime in the form of
quicklime (i.e.,
calcium oxide, or CaO) and/or hydrated lime (i.e., calcium hydroxide, or
(Ca(OH)2).
Quick lime is produced by thermal decomposition of limestone (CaCO3) to CaO
and
carbon dioxide (CO2) by the chemical reaction: Ca CO3 --> Ca0 + CO2. In
aqueous
environments, quick lime reacts with water (1120) and forms hydrated lime by
the
reaction: Ca0 + H20 Ca(OH)2.
As taught herein, treatment of MFT with lime promotes the release of water
from
MFT and thus reduces the volume of MFT. Treatment of MFT with lime
preconditions
the clay fraction of the MFT by converting sodium-clay of high swelling
characteristics
to calcium-clay of low swelling characteristics, and improves the chemistry of
water
released from MFT by making it more suitable for recycling to the bitumen
extraction
process.
The present disclosure teaches the use of lime as an additive at higher
dosages (and
accordingly resulting in higher slurry pH ranges) than in other oil-sands-
related uses of
lime, such as taught in Canadian Patent No. 2,581,586 (Ozum) and U.S. Patent
No.
7,931,800 (Ozum), directed to suppression of dispersion of silt/clay-sized
particles in
bitumen extraction process slurries; and Ozum's Canadian Patents No. 2,188,064
and No.
2,522,031, directed to disposal of oil sands tailings as non-segregating
tailings (NST). It
has been observed that treatment of MFT with lime in comparatively high
dosages:
= chemically alters clay surfaces such that clay particles in MFT bind
together; and
= reduces the attraction of clay surfaces to water and thus improves the
dewatering
characteristics of the MET.
In a first process in accordance with the present disclosure, MFT may be
treated in
situ with Ca0 at selected dosages, followed by capping with sand, "composite
tailings"
(or CT, an engineered tailings product with non-segregating characteristics),
and/or NST.
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In a second process in accordance with the present disclosure MFT may be
treated with
CaO and then blended with overburden soils for permanent deposition.
In another application of processes in accordance with the present disclosure,
existing MFT tailings ponds may be partially dewatered by first draining any
surficial
free water layer present on the ponds above the MFT layer, then treating the
drained
water with Ca0 and recycling the treated water back to the bitumen extraction
process.
By use of this process, fresh water import to the extraction process would be
reduced,
thus reducing the environmental impact of the extraction process. The
appropriate CaO
dosage for treating the water from the tailings ponds would depend on the
chemistry of
the water. As a general guideline, however, beneficial results would typically
be
expected from treating the oil sands tailing pond water with sufficient Ca0 to
raise the
pH of the treated water to around 8.2 to 9.0 (or even higher if the treated
water is to be
blended with fresh water or other recycled tailings pond water, such that the
pH of the
blended water would be around 8.2 or 9.0).
After the surficial water has been drained to expose the MFT layer, the MFT
can be
treated in situ with CaO at selected dosages and then pipelined or otherwise
conveyed to
a site for blending with overburden soils for subsequent permanent deposition
in selected
locations. In a variant of this process, the MFT could be transported from the
tailings
ponds to another location for treatment with CaO prior to blending with
overburden soils
and permanent deposition of the blended material.
Processes in accordance with the present disclosure are not restricted to use
in
connection with oil sands tailings. By way of non-limiting examples, processes
taught
herein could also be implemented for dewatering of industrial tailings
produced by
mining, agriculture, and pulp-and-paper production processes that produce fine
tailings
material including silt, clay, or wet organic-based particulates with strong
affinity to
water and resistance to settling in aqueous environments.
The appropriate dosage of lime to achieve desired results will be affected by
variable factors including but not limited to the chemistry of the water
fraction of the
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MFT being treated, and the characteristics and concentration of clay particles
in the MFT.
Subject to that qualification, treatment of MFT with CaO at dosages in excess
of 1,000
ppm (parts per million) by weight, can be expected to:
= increase the MFT's yield stress ("yield stress" in this context referes
to a
rheological characteristic of fluids and suspensions which could be expressed
by
Bingham's stress-strain relation);
= reduce the MFT"s plastic limit ("plastic limit" being one of the well-
known
"Atterberg limits" corresponding to critical water contents of fine-grained
soils);
and
= reduce the affinity of clay particles in the MFT for water;
all of which consequences of lime treatment produce beneficial alterations of
MFT
characteristics for the implementation of dewatering processes.
The Atterberg limits for MET include (a) the "liquid limit", defined as the
moisture
content (mass of water/dry mass of MFT) at which MFT stop behaving as a liquid
and
start behaving plastically; and (b) the "plastic limit", defined as the
moisture content at
which the MFT stops behaving plastically. The plasticity index (or simply
"plasticity")
of MFT is the range of moisture contents within which the MFT exhibit plastic
behavior
¨ which is the difference between the liquid limit and the plastic limit.
Plastic limit,
therefore, is expressed by the formula:
Plasticity Index = Liquid Limit - Plastic Limit.
Reduction in the plastic limit of MFT is a strong indication that the MFT's
dewatering
characteristics have been enhanced.
In laboratory tests conducted by the inventor, Atterberg limits of two MFT
samples
with different sodium (Na), bicarbonate (HCO3"), and clay contents were
measured after
treating the MFT with lime (CaO) at dosages ranging between 1,000 mg-CaO/kg-
MFT
(i.e., 1,000 ppm, or 1,000 parts Ca0 per million parts MFT by weight) and
100,000 mg-
CaO/kg-MFT (100,000 ppm). Test results showed that the MFT's plasticity
reduced as
the dosage of Ca0 was increased.
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Curing time also appeared to be a factor in the reduction of MFT plasticity;
the
largest plastic limit reductions were observed after about 28 days of curing
time. As an
example, when two different MFT samples were treated with Ca0 at 7,000 ppm
dosage,
after a curing time of 28 days the liquid limits of the two samples had been
reduced from
80% to 90% to about 65%, and the plastic limits of the two samples had
increased from
about 34 to 42. It was observed that the reduction in liquid limit may have
been a more
significant than the increase in the plastic limit in reducing the plasticity
of MFT treated
with CaO.
The use of CaO as an oil sands slurry additive at elevated dosages as
described in
the present disclosure reduces Atterberg limits plasticity, improves yield
stress, and
enhances the dewatering properties of the MFT. These effects would improve the
strength of blended MFT/overburden deposits in the long term and promote the
formation
of a robust soil deposits, due in part to pozzolanic reactions between the
CaO, clay, and
overburden soils in the blended material. Pozzolanic reactions are reactions
between the
added CaO and the silicic (Si(OH)4) and aluminate (A/(01-1)4-) functional
groups of the
kaolinite clays in the MFT, which takes place at elevated pH, most likely at
pH values
above 11Ø These reactions result in the formation of calcium silicate
(CaH2SiO4.2H20)
and different calcium aluminum hydrates increasing the strength of MFT. These
are
relatively slow reactions, and it may require long retention times (from a few
hours or a
few days to months) for the lime-treated MFT to realize the full potential
strength gain
that can result from these reactions.
Any excess Ca0 in blends of CaO-treated MFT and overburden soils could be
expected to react with components of the overburden soils by the same ion
exchange and
pozzolanic reactions, which would further promote the stability of the blend.
Excess
CaO would simultaneously be converted to calcium carbonate (CaCO3) by reacting
with
the atmospheric carbon dioxide (CO2) by the reaction Ca0 + CO2 ----> CaCO3.
Excess
CaO in the final deposit would also act as a catalyst for the oxidation of
residual bitumen
to humic acids, which from an environmental perspective would be an additional
benefit
from the use of Ca0 as an additive to promote dewatering of MFT.
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DETAILED DESCRIPTION
The previously-mentioned patents by Ozum demonstrated that CaO could be used
as additives for various different several oil sands plant operations, within
a range of
dosages resulting in slurry pH values from 8.0 to 12.0 or higher. The
effectiveness of
CaO as an additive for oil sands plant operations arises from the fact that,
in aqueous
environments it provides divalent calcium ions (Ca2 ) and increases slurry pH
by
providing hydroxyl ions (OH). These characteristics of CaO, providing Ca2+ and
OH-
simultaneously, makes it a useful additive for bitumen extraction and tailings
treatment
operations in which complex reactions occur between CaO and process water,
silt and
clay particles, and bitumen present in the oil sands ore. Depending on the Ca0
dosages
used, and in turn the resultant slurry pH, these reactions promote beneficial
improvement
in certain aspects of the bitumen extraction processes used in oil sands
plants. Addition
of Ca0 to oil sands ore slurry also improves the chemistry of release water,
because of
the chemistry of Ca0 in aqueous slurries of high salinity, specifically Nat,
bicarbonate
(HCO3), and high clay concentrations.
The present disclosure focuses on the treatment of mature fine tails (MFT)
with
high dosages of lime in the form of quick lime (CaO) or hydrated lime
(Ca(OH)2).
Typical effective lime dosages may range from 1,000 mg-CaO/kg-MFT (i.e., 1,000
ppm
by weight) to 10,000 ppm, with lime dosages in a preferred embodiment being
between
3,000 and 10,000 ppm (i.e., from 3 to 10 kg of Ca0 per tonne of MFT). However,
it is
believed that Ca0 doses as high as 50,000 ppm or even higher may produce
beneficial
results, bearing in mind that an appropriately effective lime dosage in
specific cases may
be affected by variable factors such as the chemistry and characteristics of
the constituent
elements of the particular MFT to be treated.
MFT treatment in accordance with the methods disclosed herein appears to work
better if the existing MFT are diluted with water (which may be process water)
as
necessary to produce a slurry having a solids content of about 20% by weight,
particularly for MFT having a comparatively high fraction of solids smaller
than 44
micron (or -325 mesh).
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In the case of in situ treatment of MFT with lime, the resultant yield stress
may
need to be in the order of a few hundred Pascals (Pa) to withstand the weight
of sand or
other material that might be used to cap the treated MFT. Such a threshold
yield stress
could be reached within a few days depending on the selected lime dosage, and
also
depending on HCO3" concentration in the tailings water, which buffers pH
increases
induced by the addition of lime. In addition, the rate of yield strength gain
in lime-treated
MFT may be affected by the Na+ concentration in the process water, as higher
Na+
concentrations could have a suppressing effect on the rate of chemical
reaction between
CaO and clay.
The inventor performed slump tests to measure MFT's strength gain shortly
after
treatment with CaO at dosages of 3,000 ppm and 7,000 ppm (mass basis of MFT).
These
tests were conducted using cylindrical containers having a 9.0 cm diameter.
For the MFT
treated with CaO at 3,000 ppm, the initial and final heights of the samples
were 10.8 cm
and 5.5 cm. For the MFT treated with Ca0 at 7,000 ppm, the initial and final
heights of
the samples were 14.8 cm and 8.5 cm for 7,000 ppm CaO dosage treatment. The pH
of
the MFT immediately after lime treatment was reported as 10.80 (at a
temperature of
23.2 C) for MFT treated with quick lime at 3,000 ppm, and 12.05 (at 24.0 C)
for MFT
treated with quick lime at 7,000 ppm.
As may be appreciated from these test results, the strength gain immediately
after
lime treatment is much larger for a 7,000 ppm dosage than for 3,000 ppm
dosage.
Because of variations in composition of oil sands MFT produced at different
plants, it can
also be expected that the pH increase in MFT resulting from lime addition may
vary
between MFT samples. For a particular MFT sample, the pH of the lime-treated
MFT
was measured as 9.44, 10.80, 11.34, 12.05 and 12.58 for quick lime dosages of
1,000
ppm, 3,000 ppm, 5,000 ppm, 7,000, and 10,000 ppm respectively (with all pH
measurements having been made at about 24 C).
Strength gained in MFT by lime treatment was also quantified by yield stress
measurements. Yield stress is a measure for the Bingham plasticity of a fluid,
where the
fluid remains rigid when the stress imposed on the fluid is smaller than the
yield stress. It
was observed that the yield stress gain of lime-treated MFT is a function of
the lime
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treatment dosage, the length of time elapsed between lime treatment and yield
stress
measurement, as well as the compositional characteristics of the MFT, such as
percent
clay fraction of the solids, clay type, water chemistry, and the temperature
at the time of
treatment and at the time of testing. In the latter regard, all of the above-
noted tests were
carried out at temperatures in the range of 20 C to 24 C.
In all of these tests, significant yield stress gains were observed upon the
treatment
of MFT with lime. Initial yield stress gain was over 100 Pa at 7,000 ppm CaO
dosage for
almost all tests, which corresponds to pH of the treated MFT at about 12Ø It
was
observed that yield stress increases in time, with strength gains in most
cases being over
300 Pa and in some cases as high as 500 Pa.
It was also observed that MFT composition, particularly clay amount, clay
type,
and process water chemistry, especially sodium (Na) and (HCO3-)
concentrations, were
also factors that can influence longer-term yield stress gain. In all cases,
however, the
measured yield stress gain in MFT samples of different origins were
sufficiently high to
represent beneficial improvement of the tested MFT's dewatering
characteristics. The
complex nature of the reaction between lime and clay materials makes it
difficult to
model. However, it was observed that the yield stress achieved by treatment of
MFT
with CaO at more than 3,000 ppm, and more particularly between 5,000 ppm and
10,000
ppm CaO, for MFT with lesser clay contents, would result in sufficient yield
stress gain
to enable dewatering of the MFT in the tailings ponds by in situ treatment
with lime
followed by capping with sand, NST, or CT.
In this patent document, any form of the word "comprise" is to be understood
in
its non-limiting sense to mean that any element or feature following such word
is
included, but elements or features not expressly mentioned are not excluded. A
reference
to an element by the indefinite article "a" does not exclude the possibility
that more than
one such element is present, unless the context clearly requires that there be
one and only
one such element. Wherever used in this document, the terms "typical" and
"typically"
are to be understood in the sense of representative or common usage or
practice, and are
not to be understood as implying invariability or essentiality.
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