Note: Descriptions are shown in the official language in which they were submitted.
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DOCKET NO.: NS-468/469
MIXING SYSTEMS FOR MIXING OIL SANDS TAILINGS AND POLYMER
FIELD OF THE INVENTION
The present invention relates to a process for dewatering oil sands tailings.
In
particular, tailings are treated with a polymeric flocculant such as a water
soluble polymer
having a moderate to high molecular weight and an intrinsic viscosity of at
least 3 dl/g
(measured in 1N NaC1 at 25 C) to form larger structures (flocs) that can be
efficiently separated
from the water when ultimately deposited in a deposition area.
BACKGROUND OF THE INVENTION
Oil sand generally comprises water-wet sand grains held together by a matrix
of viscous
heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or
heavy hydrocarbon
molecules which contain a significant amount of sulfur, nitrogen and oxygen.
The extraction of
bitumen from oil sand using hot water processes yields fine tailings composed
of fine silts,
clays, residual bitumen and water. Mineral fractions with a particle diameter
less than 44
microns are referred to as "fines." These fines are typically clay mineral
suspensions,
predominantly kaolinite and illite.
The fine tailings suspension is typically 85% water and 15% fine particles by
mass.
Dewatering of fine tailings occurs very slowly. When first discharged in
ponds, the very low
density material is referred to as thin fine tailings. After a few years when
the fine tailings have
reached a solids content of about 30-35%, they are referred to as mature fine
tailings (MFT),
which behave as a fluid-like colloidal material. MFT, which has a low solids
to fines ratio
(<0.3), is often referred to as a type of fluid fine tailings (FFT). FFT is
generally defined a
liquid suspension of oil sands fines in water with a solids content greater
than 1% and having
less than an undrained shear strength of 5 kPa. The fact that fluid fine
tailings behave as a fluid
and have very slow consolidation rates significantly limits options to reclaim
tailings ponds. A
challenge facing the industry remains the removal of water from the fluid fine
tailings to
strengthen the deposits so that they can be reclaimed and no longer require
containment.
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Accordingly, there is a need for an improved method to treat fine tailings to
reduce their
water content to create dry stackable tailings and reclaim the land on which
fine tailings are
disposed,
SUMMARY OF THE INVENTION
It has been discovered that proper mixing of a flocculant such as a high
molecular
weight nonionic, anionic, or cationic polymer with oil sands fine tailings
such as FFT is critical
to creating the right floc structure that will dewater the tailings rapidly.
It is contemplated that
the present invention can be used in conjunction with centrifugation of the
flocculated fine
tailings in, for example, decanter centrifuges; thickening of the flocculated
fine tailings in
The two main methods of mixing polymer with oil sand tailings with polymer are
static/in-line mixing and dynamic mixing. Dynamic mixing utilizes a motor
driven mixing
device such as an impeller to cause fluid mixing while static/in-line mixing
uses the energy
contained within the flowing fluid stream to mix the polymer and oil sand
tailings with
polymer. Given that mixing energy is directly coupled with flow rate for a
static mixing
The present invention applies in particular to thixotropic suspensions where
the
viscosity and yield strength of the slurry can be manipulated and optimized to
improve mixing
of polymeric flocculants and other process aids. The current application is
directed to a
process for dewatering oil sands tailings by treating the tailings with
flocculant by providing in-
In one aspect, an end of pipe mixing process for mixing oil sand tailings with
polymer is
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flocculating oil sand tailings and polymer with an end of pipe device. The
first step is to very
quickly disperse the polymer into the oil sand tailings and then allow a flow
conditioning
period for floc growth. Quick dispersion of polymer into oil sand tailings can
be enhanced if a
conventional static mixer is placed directly upstream of the end of pipe
flocculating device to
pre-shear the raw oil sand tailings. Pre-shear is used to reduce the raw oil
sand tailings
viscosity plus induce small scale eddies to aid in the dispersion of the
polymer in the oil sand
tailings.
It is also important that the piping be sized as to ensure turbulent flow of
the raw oil
sand tailings. The end of pipe polymer injection device may consist of polymer
jets that shoot
into the raw oil sand tailings stream to quickly disperse the polymer
throughout the oil sand
tailings. After the polymer is completely dispersed in the oil sand tailings,
a large stilling
chamber or flow conditioning section is required to promote floc growth. The
flow
conditioning section then overflows the flocculated oil sand tailings into a
thin lift or other
accelerated dewatering deposits. In one embodiment, the flocculated oil sand
tailings are
treated by centrifugation.
In one embodiment, the stilling chamber has to be of adequate size as to not
cause
excessive floc breakup and thereby reducing the ability of the flocculated
mixture to release
water prior to dewatering by, e.g., centrifugation.
In another aspect, a mixing system is provided that that will allow for in-
line mixing of
raw oil sand tailings with polymer within the pipeline itself. There are two
important steps
when flocculating oil sand tailings with polymer in an in-line mixing system.
The first step is
to very quickly disperse the polymer into the oil sand tailings and then allow
a flow
conditioning period for floc growth through the pipeline. Quick dispersion of
polymer into oil
sand tailings can be enhanced if a conventional static mixer is placed
upstream of the polymer
injection to pre-shear the raw oil sand tailings. Pre-shear is used to reduce
the raw oil sand
tailings viscosity plus induce small scale eddies to aid in the dispersion of
the polymer in the oil
sand tailings.
It is also important that the piping be sized as to ensure turbulent flow of
the raw oil
sand tailings. The polymer injection/dispersion section may consist of an
injection device that
will quickly disperse the polymer throughout the oil sand tailings. After the
polymer is
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completely dispersed in the oil sand tailings, a flow condition section is
required to promote
floc growth. The conditioning section should not be excessively long otherwise
it will result in
excessive floc breakup, thereby reducing the ability of the flocculated
mixture to release water.
The present invention is particularly useful with, but not limited to, fluid
fine tailings
(FFT) such as MFT. Thus, a process is provided for flocculating and dewatering
oil sands fine
tailings in a pipeline, comprising:
= pumping a tailings feed having a solids content in the range of about 1 0
wt% to about
45 wt% through a pipeline;
= injecting an effective amount of a polymeric flocculant into the tailings
feed to provide
1 0 an initial quick dispersion of the polymeric flocculant into the
tailings feed; and
= providing a subsequent conditioning environment to form flocs and release
water
without overshearing.
In one embodiment, the conditioning environment is a lower energy environment
that
the dispersion environment. In another embodiment, a coagulant such as gypsum
is also added
to the tailings feed. In another embodiment, polymer is injected using an
injector design
selected from (1) a plurality of side ports in a spool section of the
pipeline, (2) a single side port
in a spool section of the pipeline, and (3) a plurality of spargers, each with
a plurality of holes,
in a spool section of the pipeline. In another embodiment, an in-line mixer or
static mixer is
used in conjunction with a polymer injector.
In one embodiment, the oil sands tailings is fluid fine tailings, such as MFT,
which fluid
may be optionally diluted with water to provide the tailings feed having a
solids content in the
range of about 10 wt% to about 45 wt%. In another embodiment, the tailings
feed has a solids
content in the range of about 30 wt% to about 45 wt%.
In one embodiment, the polymeric flocculant is a water soluble polymer having
a
moderate to high molecular weight and an intrinsic viscosity of at least 3
dl/g (measured in IN
NaCl at 25 C). In one embodiment, the polymer dosage is about 1000 g/Tonne of
tailings and
the polymer jet shear rate is about 200
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In one embodiment, the removed flocculated oil sands fine tailings are added
to at least
one centrifuge to dewater the oil sands fine tailings and form a high solids
cake and a low solids
centrate.
In another embodiment, the removed flocculated oil sands fine tailings are
added to a
thickener to dewater the oil sands fine tailings and produce thickened oil
sands fine tailings and
clarified water.
In another embodiment, the removed flocculated oil sands fine tailings are
transported
to at least one deposition cell for dewatering.
In another embodiment, the removed flocculated oil sands fine tailings are
spread as a
thin layer onto a deposition site.
In one embodiment, the process further comprises pre-shearing the oil sands
fine
tailings using at least one in-line or static mixer prior to polymer
injection. Without being
bound to theory, it is believed that, in certain cases, pre-shearing may
increase the maximum
dewaterable solids loading.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings wherein like reference numerals indicate similar
parts
throughout the several views, several aspects of the present invention are
illustrated by way of
example, and not by way of limitation, in detail in the figures, wherein:
FIG. 1 is a schematic of one embodiment of the present invention for mixing
oil sands
fine tailings with polymer.
FIG. 2 is a schematic of another embodiment of the present invention for
mixing oil
sands fine tailings with polymer.
FIGS. 3A to 3F show schematics of a variety of in-line polymer
injectors/mixers useful
in the present invention.
FIG. 4 is a bar graph showing the hydraulic mixing times for a variety of in-
line polymer
injectors/mixers.
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FIGS. 5A to 5C shows three examples of polymer injectors useful in the present
invention.
FIG. 6 shows an example of an embodiment of the present invention where a pre-
mixer
is used to first shear the MFT.
FIG 7 is a CoV plot showing a comparison of Computational Fluid Dynamics (CFD)
simulations with and without pre-shearing.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The detailed description set forth below in connection with the appended
drawings is
intended as a description of various embodiments of the present invention and
is not intended to
represent the only embodiments contemplated by the inventor. The detailed
description
includes specific details for the purpose of providing a comprehensive
understanding of the
present invention. However, it will be apparent to those skilled in the art
that the present
invention may be practiced without these specific details.
The present invention relates generally to a process for treating tailings
derived from oil
sands extraction operations and containing a fines fraction, and dewatering
the tailings to
enable reclamation of tailings disposal areas and to recover water for
recycling. As used herein,
the term "tailings" means tailings derived from oil sands extraction
operations and containing a
fines fraction. The term is meant to include fluid fine tailings (FFT) such as
mature fine
tailings (MFT) from tailings ponds and fine tailings from ongoing extraction
operations (for
example, thickener underflow or froth treatment tailings) which may bypass a
tailings pond.
The tailings are treated with a flocculant to aggregate the solids prior to
dewatering by thin lift,
accelerated dewatering such as rim ditching, centrifugation, etc.
As used herein, the term "flocculant" refers to a reagent which bridges the
neutralized or
coagulated particles into larger agglomerates, resulting in more efficient
settling. Flocculants
useful in the present invention are generally anionic, nonionic, cationic or
amphoteric
polymers, which may be naturally occurring or synthetic, having relatively
high molecular
weights. Preferably, the polymeric flocculants are characterized by molecular
weights ranging
between about 1,000 kD to about 50,000 kD. Suitable natural polymeric
flocculants may be
polysaccharides such as dextran, starch or guar gum. Suitable synthetic
polymeric flocculants
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include, but are not limited to, charged or uncharged polyacrylamides, for
example, a high
molecular weight polyacrylamide-sodium polyacrylate co-polymer.
Other useful polymeric flocculants can be made by the polymerization of
(meth)acryamide, N-vinyl pyrrolidone, N-vinyl formamide, N,N
dimethylacrylamide, N-vinyl
acetamide, N-vinylpyridine, N-vinylimidazole, isopropyl acrylamide and
polyethylene glycol
methacrylate, and one or more anionic monomer(s) such as acrylic acid,
methacrylic acid, 2-
acrylamido-2-methylpropane sulphonic acid (ATBS) and salts thereof, or one or
more cationic
monomer(s) such as dimethylaminoethyl acrylate (ADAME), dimethylaminoethyl
methacrylate
(MADAME), dimethydiallylammonium chloride (DADMAC), acrylamido propyltrimethyl
ammonium chloride (APTAC) and/or methacrylamido propyltrimethyl ammonium
chloride
(MAPTAC).
In one embodiment, the flocculant comprises an aqueous solution of an anionic
polyacrylamide. The anionic polyacrylamide preferably has a relatively high
molecular weight
(about 10,000 kD or higher) and medium charge density (about 20-35%
anionicity), for
example, a high molecular weight polyacrylamide-sodium polyacrylate co-
polymer. The
preferred flocculant may be selected according to the oil sand tailings
composition and process
conditions.
The flocculant is generally supplied from a flocculant make up system for
preparing,
hydrating and dosing of the flocculant. Flocculant make-up systems are well
known in the art,
and typically include a polymer preparation skid, one or more storage tanks,
and a dosing
pump. The dosage of flocculant may be controlled by a metering pump. In one
embodiment,
the dosage of flocculant ranges from about 400 grams to about 1,500 grams per
tonne of solids
in the FFT. In one embodiment, the flocculant is in the form of a 0.4%
solution.
As used herein, "fluid fine tailings" or "FFT" is a liquid suspension of oil
sand fines in
water with a solids content greater than 2%. "Fines" are mineral solids with a
particle size
equal to or less than 441.1. "Mature fine tailings" or "MFT" are FFT with a
low solids to fines
ratio (SFR), i.e., less than about 0.3, and a solids content greater than
about 30%.
As used herein, the term "in-line flow" means a flow contained within a
continuous
fluid transportation line such as a pipe or another fluid transport structure
which preferably has
an enclosed tubular construction.
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. .
FIG. 1 is a flow diagram of one embodiment of the process of the present
invention. In
this embodiment, oil sands fine tailings are mature fine tailings (MFT)
obtained from a settling
basin 110. However, it should be understood that the fine tailings treated
according the process
of the present invention are not necessarily obtained from a settling pond and
may also be
obtained from ongoing oil sands extraction operations.
The tailings stream from bitumen extraction is typically transferred to a
settling basin 10
where the tailings stream separates into an upper water layer, a middle MFT
layer, and a bottom
layer of settled solids. The MFT layer is removed from between the water layer
and solids
layer via a dredge or floating barge having a submersible pump. In one
embodiment, the MFT
has a solids content ranging from about 10 wt% to about 45 wt%. In another
embodiment, the
MFT has a solids content ranging from about 30 wt% to about 45 wt%. In one
embodiment,
the MFT has a solids content ranging from about 37 wt% to about 40 wt%. The
MFT is
preferably undiluted.
The MFT is then pumped through pipeline 112 (in-line flow). In this
embodiment, the
MFT first passes through an in-line static mixer 114 for pre-shearing the MFT.
Suitable static
mixers for use in the present invention for pre-shearing oil sands fine
tailings are known in the
art. A static mixer is a motionless mixer which is inserted into a housing or
pipeline with the
objective of manipulating fluid streams, in this instance, to significantly
accelerate the in-line
reaction of flocculation. Typical designs of static mixers comprise plates,
baffles, helical
elements or geometric grids positioned at precise angles to direct flow and
increase turbulence.
In the embodiment of FIG. 1, the pre-sheared MFT is then introduced into an
end of
pipe chamber 116 comprising a mixing chamber 118 where polymer 122 is injected
as a jet
using a jet mixer as shown in FIG 3B. Other in-line polymer injection and
mixing concepts can
also be used. End of pipe chamber 116 further comprises a stilling chamber 120
where
complete flocculation of the FFT can occur without overshearing of the flocs
taking place. The
flocculated FFT can then be further treated by centrifugation or directly
deposited in thin
sloping layers (thin-lift), subjected to accelerated dewatering (rim ditching)
or deposited into
other tailings deposition cells.
FIG. 2 is a flow diagram of another embodiment of the process of the present
invention.
In this embodiment, fluid fine tailings (FFT) are primarily mature fine
tailings (MFT) obtained
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from a settling basin 210. However, it should be understood that the fine
tailings treated
according the process of the present invention are not necessarily obtained
from a settling pond
and may also be obtained from ongoing oil sands extraction operations,
The MFT is pumped through pipeline 212 into an in-line first static mixer 214
for pre-
shearing the MFT. Suitable static mixers for use in pre-shearing the MFT are
known in the art.
The pre-sheared MFT is then introduced into a polymer injection/dispersion
zone 230 where
polymer 232 is injected using a polymer injector, for example, a 3" Tee 80 on
an 8" pipe 312, as
shown in FIG 5A, or a 1.5" Tee 84 on an 8" pipe 312, as shown in FIG. 5B, Tee
injectors were
investigated due to the simplicity of their design.
In one embodiment, polymer
injection/dispersion zone 230 comprises a venturi, as shown in FIG. 5C. With
reference to FIG.
5C, venturi 86 can be inserted between two pieces of pipe 312' forming
pipeline 312. Polymer
is injected into the venturi 86 via a plurality of polymer lines (not shown)
connected to a
plurality of polymer inlets 88.
It is understood, however, that other polymer injectors/mixers can be used in
the
polymer injection/dispersion zone 230. FIG. 3 shows a number of different
polymer
injectors/mixers that could be used in the present invention. FIG. 3A
illustrates sparger-like
injectors with spatial distribution; FIG 3B shows various nozzle-type
injectors; FIG 3C shows
a number of venturi-type injectors/mixers; FIG. 3D shows polymer injection
coupled with static
mixers, FIG, 3E show wake mixers/injectors and FIG 3F shows vortex generators.
The MFT/polymer mixture continues through the pipeline 212 so that flow
conditioning
can occur, i.e., complete flocculation of the MFT without over-shearing of the
flocs taking
place. The flocculated MFT can then be further treated by centrifugation,
directly deposited in
thin sloping layers (thin-lift), subjected to accelerated dewatering (rim
ditching) or deposited
into other tailings deposition cells.
Thus, in both embodiments described above, it important to have a first step
comprising
quick dispersion of the polymer into the FFT followed by a second step of
providing a
subsequent lower energy region to promote floc growth. A well-dispersed
polymer/MFT
product flowing through the conditioning pipe or stilling chamber develops
increasingly large
flocs, builds rheological strength, and then begins to dewater, either within
the pipe or stilling
chamber.
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In one embodiment, a coagulant is also introduced into the in-line flow of
FFT. As used
herein, the term "coagulant" refers to a reagent which neutralizes repulsive
electrical charges
surrounding particles to destabilize suspended solids and to cause the solids
to agglomerate.
Suitable coagulants include, but are not limited to, gypsum, lime, alum,
polyacrylamide, or any
combination thereof. In one embodiment, the coagulant comprises gypsum or
lime. In one
embodiment, the dosage of the coagulant ranges from about 300 grams to about
1,500 grams
per tonne of solids in the FFT.
Exemplary embodiments of the present invention are described in the following
Examples, which are set forth to aid in the understanding of the invention,
and should not be
construed to limit in any way the scope of the invention as defined in the
claims which follow
thereafter.
Example 1
The measure of uniformity or "mixedness" that is most often used is the
radical
variation coefficient (CoV). A low CoV number indicates better uniformity of
flocculant and
tailings (good mixedness). FIG. 4 is a bar graph which illustrates the
hydraulic mixing time
necessary to achieve a CoV reduction using a variety of polymer injection
devices and 32%
MFT. It was observed that, with the simpler designs such as the quill, tee and
sparger, it took
longer for CoV reduction to occur with MFT having a relatively high solids
content (32%).
However, use of Computational Fluid Dynamics (CFD) suggested that mixing of
32% MFT and
polymer fiocculant could be improved by using a pre-mixer. FIG. 7 is a CoV
plot versus
distance from the injector point (m) showing a comparison of Computational
Fluid Dynamics
simulations of polymer injectors comprising a sparger, a 2" quill in a 12"
pipe and a 3" Tee in
an 8" pipe without and with pre-shearing. It can be seen from FIG. 7 that with
pre-shearing
(pre-mixing) a lower CoV number could be obtained for all injectors, which
indicates better
uniformity of flocculant and tailings (good mixedness) when using 32% MFT.
Example 2
In one test, a plurality of KomaxTM flanged carbon steel static mixers were
placed in an
8" pipeline upstream from the polymer injection site for pre-mixing MFT.
Following the static
mixers, a venturi (4" contraction) having eight (8) port openings was used for
injecting
polymer. Following the venturi was a length of 8" pipe, as shown in FIG. 6.
The flocculated
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MFT was deposited in a ditch and observed for dewatering properties. It was
observed that the
addition of the Komax mixers promoted flocculation and allowed excellent
dewatering results
over a wide range of conditions, including a different number of injectors,
polymer dosages and
MFT solids wt%. A Tee injector was also tested and provided good dewatering
conditions at
discharge. It was further observed that aggressive post mixing after polymer
injection was
actually detrimental to instant dewatering observed at the end of pipe and at
the discharge.
One of the more surprising results when using a pre-mixer for pre-shearing the
MFT
was that much higher solids content MFT could be used. For example, MFT having
a wt%
solids of 31% and the MFT density (kg/m3) of 1.24 still showed good dewatering
properties
with good water runoff in the ditch when using a MFT flow rate (m3/hr) of 250
and a polymer
dosage (kg/Tonne MFT) of 1050.
The scope of the claims should not be limited by the preferred embodiments set
forth in
the examples, but should be given the broadest interpretation consistent with
the description as
a whole.
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