Note: Descriptions are shown in the official language in which they were submitted.
CA 02886983 2015-03-30
NS-534
TAILINGS-POLYMER MIXING OPTIMIZATION BY CONTROLLING THE
DISCHARGE ENVIRONMENT
FIELD OF THE INVENTION
The present invention relates to a process for dewatering tailings such as oil
sands tailings. In particular, the present invention is directed to a process
for optimizing
the dewatering of polymeric flocculant-treated tailings by controlling the
deposition or
discharge conditions of polymeric flocculant-treated tailings in a deposition
area.
1.0 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 sand using hot water processes yields
large
volumes of 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
sometimes
referred to as mature fine tailings (MFT) which behave as a fluid-like
colloidal material.
The more general term used for tailings which behave as fluid-like colloidal
suspension
is fluid fine tailings or FFT. 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
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strengthen the deposits, so that they can be reclaimed and no longer require
containment.
One method used to dewater fluid fine tailings such as MFT is to treat the
tailings
with polymeric flocculants to form large flocs which will release the water
more rapidly.
However, optimizing dewatering of fluid fine tailings such as MFT is achieved
only with
ideal mixing of the tailings and flocculant, and the operating window for
ideal mixing is
often very narrow. The current state of the art attempts to achieve optimum
mixing in a
mixing vessel or in-line mixer located at some distance along a discharge pipe
(see, for
example, CA 2,789,678 and CA 2,678,818). Hence, the tailings and polymer
mixture is
typically optimally mixed at a mixer discharge or at the end of a pipe.
Furthermore, the
nature of the mixing at deposition is much different than in a mixer, in a
pipe, or during
discharge.
A tailings and polymer mixture that is optimally mixed after the mixer stage
or at
the end of pipe has the potential to be over-mixed upon discharge. There is
then a
reliance on a gentle transport and/or deposition of the flocculated tailings
to maintain the
optimum mixed condition after the tailings polymer mixture has been deposited
for
maximum dewatering. Often, this is very difficult to control.
There is a need in the industry for a method of ensuring that optimal mixing,
and
therefore dewatering, is still maintained when the tailings are finally
discharged into the
tailings disposal area.
SUMMARY OF THE INVENTION
The present invention uses the discharge conditions as an integral and
critical
part of the total mixing protocol for mixing tailings with polymeric
flocculant.
It was surprisingly discovered that the discharge conditions could be
manipulated
to produce optimally mixed tailings (e.g., MFT) and flocculant, thereby
maximizing
dewatering of the tailings. The mixing under discharge conditions can be
controlled, for
example, by adjusting the height of a vertical tailings discharge pipe, by
adjusting the
velocity of a tailings-flocculant mixture into a plunge pool, or by use of a
weir box.
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Thus, the overall mixing of the polymer and tailings can be controlled by
optimizing
these discharge parameters.
Depending upon the extent of any upstream mixing, the discharge conditions are
controlled to optimize the mixed condition of the finally deposited flocculant
treated
tailings. This optimization might provide either incremental mixing or the
entirety of the
mixing energy necessary to produce optimally flocculated and dewatering
tailings. The
current state of the art generally involves mixing in a static or dynamic
mixer followed by
pipeline transport or mixing in the pipeline itself. However, additional
mixing may occur
during deposition which can result in over-mixing, i.e., over-shearing of the
formed flocs,
which can result in poor dewatering of the tailings.
It was surprisingly discovered that the discharge conditions could be
manipulated
to produce an optimally mixed tailings (e.g., MFT) and flocculant, thereby
maximizing
dewatering of the tailings. The mixing under discharge conditions can be
controlled, for
example, by adjusting the height of a vertical tailings discharge pipe, by
adjusting the
velocity of a tailings-flocculant mixture into a plunge pool, or by adjusting
the discharge
distance into a weir box. Thus, the mixing of the polymer and tailings is
controlled by
optimizing these discharge parameters.
In one aspect, a process for dewatering tailings is provided, comprising:
= providing a tailings feed having a solids content in the range of about
10 wt% to
about 70 wt 70;
= adding an effective amount of a polymeric flocculant to the tailings feed
and,
optionally, mixing the polymeric flocculant and tailings feed mixture in a
mixer or
a pipeline;
= transporting the tailings feed and polymeric flocculant mixture to a
deposition
area; and
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= optimizing the dewatering rate of the tailings feed and polymeric
flocculant
mixture by controlling the discharging conditions used for discharging the
mixture
to a deposition area.
In one embodiment, the discharging conditions are controlled by adjusting the
height of
a vertical tailings discharge pipe. In another embodiment, the discharging
conditions
are controlled by discharging the tailings feed and polymeric flocculant
mixture into a
plunge pool and adjusting the velocity of discharge into the plunge pool. In
another
embodiment, the flocculated tailings are discharged into an optimally designed
weir box,
for instance, by optimizing end of pipe to weir wall distance or weir box
dimensions.
In one embodiment, the tailings feed and polymeric flocculant mixture is
transported through a pipeline. In one embodiment, the flow of the tailings
feed and
polymeric flocculant mixture through a pipeline is laminar flow.
In one embodiment, a coagulant is also added to the tailings feed either prior
to
or after the addition of the polymeric flocculant.
In one embodiment, the tailings are oil sands tailings, such as fluid fine
tailings,
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 1N NaCI at 25 C).
Additional aspects and advantages of the present invention will be apparent in
view of the description, which follows. It should be understood, however, that
the
detailed description and the specific examples, while indicating preferred
embodiments
of the invention, are given by way of illustration only, since various changes
and
modifications within the scope of the invention will become apparent to those
skilled in
the art from this detailed description.
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BRIEF DESCRIPTION OF THE DRAWINGS
The invention will now be described by way of an exemplary embodiment with
reference to the accompanying simplified, diagrammatic, not-to-scale drawing:
FIG. 1 is a graph showing yield stress (Pa), capillary suction time (sec), and
torque (Nm), versus time, of an MFT sample which has been mixed with a
polymeric
flocculant in a dynamic mixer.
FIG. 2 is a schematic of one mixing protocol of present invention.
FIG. 3 is a photograph of a standpipe useful in discharging a polymeric
flocculant
and tailings mixture into a deposition site in one embodiment of the present
invention.
DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION
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
inventors.
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 practised without
these specific
details.
The present invention relates generally to a process for dewatering tailings
such
as tailings that are derived from oil sands extraction operations. In general,
a tailings
feed is provided having a solids content in the range of about 10 wt% to about
70 wt%
and an effective amount of a polymeric flocculant is added to the tailings
feed and
transported through a pipeline to a deposition area. The tailings and the
polymeric
flocculant are optionally mixed in a mixer such as a dynamic mixer or an in-
line or static
mixer or in a pipeline prior to being subjected to further mixing upon
discharge.
However, it is understood that all of the mixing energy necessary to form
optimally
dewatering tailings can solely occur at discharge.
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Hence, the mixing and therefore dewatering rate of the polymeric flocculant
and
tailings is optimized by controlling the discharge conditions under which the
polymeric
flocculant and tailings mixture is placed in a deposition area. Thus, much, or
perhaps
all, of the mixing energy needed to form optimally dewatering flocs can
actually be
provided during the discharging of the tailings in the deposition area.
As used herein, the term "oil sands tailings" means tailings derived from oil
sands
extraction operations and containing a fines fraction. The term is meant to
include fluid
fine tailings (FFT) from tailings ponds and fine tailings from ongoing
extraction
operations (for example, thickener underflow or froth treatment tailings). The
tailings
1.0 are treated with a flocculant to aggregate the solids and aid in the
consolidation and
dewatering of the tailings.
As used herein, the term "flocculant" refers to a reagent which bridges
particles
into large agglomerates or flocs, resulting in more efficient settling and
dewatering.
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 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).
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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 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.
In one embodiment, a coagulant is also added to the tailings feed. 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.
It is important that the tailings and flocculant are properly mixed in order
to form
large, stable, rapidly dewatering flocs. It was discovered that, the discharge
conditions
can be controlled to add an optimized amount of incremental mixing or to
provide the
entirety of the mixing energy in order to optimize the dewatering rate of the
tailings-
flocculant mixture. The current state of the art, which involves tailings and
flocculant
mixing prior to pipeline transport or mixing in the pipe, is sensitive to
whether the flow
conditions in the pipe are laminar or turbulent, and demands deposition
conditions that
do not create a less than optimum mixture.
FIG. 1 is a graph showing the effects of mixing time in a dynamic mixer on a
mature fine tailings (MFT) sample having a solids content of 35.8 wt% and an
anionic
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=
polyacrylamide polymeric flocculant. In this example, samples of MFT were
taken from
a dynamic mixer at various time periods (in minutes) post flocculant polymer
addition.
The torque, which is a measure of the turning force on the impeller, was
plotted against
time (in minutes) over the entire period of the test. The yield stress (Pa)
and capillary
suction time (CST in seconds) were also measured at various time intervals
after about
3.5 minutes of mixing of polymer and MET. CST is used to determine how quickly
water
can be filtered from a flocculated sample. A high CST may indicate both under-
mixing
of tailings and flocculant or over-mixing of tailings and flocculant.
For optimally flocculated tailings, it is desirable that the CST be low and
that the
yield stress be relatively high, indicating good dewatering and good floc
formation,
respectively. It can be seen in FIG. 1 that there is a conditioning period
(i.e., mixing
period) required for good floc formation. During this conditioning period,
yield stress is
low (indicated by low torque) and CST is high (indicating poor dewatering). It
can be
seen than maximum yield stress is obtained at about 4.5 minutes post
flocculant
addition and, at this time, CST begins to drop, indicating good dewatering
conditions.
However, if mixing continues past about 5.5 minutes post-flocculation, the
yield stress
begins to decline and the CST begins to rise. This indicates over-shearing,
where the
large flocs are broken down resulting in poor dewatering. Thus, the optimal
operation
window is likely between about 4.0 to about 5.1 minutes post flocculant
polymer
addition. Hence, the optimal operating window for mixing is quite narrow and
it occurs
as the yield stress is changing over a wide range.
Thus, FIG. 1 clearly shows that when mixing time increases beyond the optimal
operating window, yield stress of the sample is reduced and the CST increases.
The
yield stress change with mixing time could easily represent a transition from
laminar to
turbulent flow, complicating the ability to control mixing in a pipeline after
a dynamic or
static mixer. Thus, it was discovered that having less initial mixing of
flocculant and
tailings, e.g., upstream of transport, is more desirable, with additional
mixing occurring
at the downstream end of transport, i.e., at the discharge end of the
flocculation
process.
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=
It was discovered that by utilizing pipe discharging mixing conditions to add
mixing downstream, the tailings (e.g., MFT) and polymer mixture can be
optimized for
maximum water release. Rather than attempting to optimize mixing in the
process and
piping, mixing is optimized at the discharge point. FIG. 2 is a schematic
showing one
embodiment of the present invention. MFT 12 is transported through a pipe 16
where
polymeric flocculant is injected therein by means of an injection system such
as a T-
inlet. In this embodiment, the mixing protocol 10 may optionally comprise a
mixer 18,
such as a dynamic mixer, which mixer 18 mixing conditions can be controlled to
ensure
that only preliminary mixing of the MFT 12 and polymeric flocculant 14 is
obtained, i.e.,
where the CST is still high and the yield stress is still low. Thus, the
mixture is still in the
conditioning zone as shown in FIG. 1. The polymeric flocculant and MFT mixture
is
then transported via pipeline 16 to discharge 20, where additional mixing is
provided
upon discharge of the polymeric flocculant/MFT mixture into deposition site
22. In one
embodiment, it may be desirable to maintain laminar flow conditions in the
pipe 16.
In one embodiment, optimized mixing at the discharge can be achieved with a
vertical drop from a standpipe, as shown in Figure 3. In another embodiment,
the
flocculated tailings can be discharged into a plunge pool. In another
embodiment, the
flocculated tailings can be discharged into a weir box. In one embodiment, the
slope of
the deposit that the flocculated material will run down can be controlled to
control
mixing.
Example 1
In this example, the tailings used are mature fine tailings (MFT) which
generally
have a solids content of about 35 wt% and a fines content of about 90 wt%. The
polymeric flocculant used in this example is 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 polymer dosage ranged from about 750-850 g/tonne dry weight of
tailings.
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In this example, the MFT and polymeric flocculant mixture was under-mixed pre-
discharge and then discharged into a pit by means of a standpipe as shown in
FIG. 3.
A capillary suction test (CST) was performed on the tailings at the top of the
standpipe,
immediately prior to discharge. Samples collected at the discharge of the
standpipe
showed very high CST values (i.e., greater than about 700 sec), indicating
under-mixing
and that a very poor degree of dewatering would be expected. However, further
mixing
was added as a result of the mixture being discharged from a considerable
height into
the pit. Additional mixing is defined by the discharge height of the
standpipe; in this
instance, the standpipe was 3 meters in height. Thus, the polymeric
flocculant/MFT
mixture was falling into the deposition site as a velocity of about 7.7
meters/sec. It can
be seen in FIG. 3 that the deposit created by this standpipe discharge is
showing a
significant degree of dewatering, as indicated by the arrow in FIG. 3. A
significant
amount of water is released from the MFT and is collecting at the end of the
pit.
Thus, the mixing created by the standpipe was determined by the vertical
discharge height. The mixing conditions might also be adjusted by modifying
the
discharge velocity. Mixing might further be optimized by control of the
discharge beach
slope, or by a combination of all of these factors. Thus, the use of discharge
conditions
to control mixing and optimizing dewatering rate is an important improvement
relative to
the current state of the art.
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