Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02791544 2012-10-05
DOCKET NO,: NS-449
BITUMEN REMOVAL FROM TAILINGS CENTRIFUGE CENTRATE
FIELD OF THE INVENTION
The present invention relates to a process for removing residual bitumen from
oil sands
tailings, particularly from centrate derived from centrifugation of the
tailings.
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 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."
When oil sand tailings are pumped to the deposition area, the coarse sands
settle quickly
on the beach while the fine tailings run off the beach and flow by gravity to
the tailings ponds.
The low density run-off material is referred to as thin fine tailings. The
thin fine tailings
suspension is typically 85% water and 15% fine particles by weight. Dewatering
of fine
tailings occurs slowly. After a few years when the fine tailings have reached
a solids content of
about 30-35% and are commonly referred to as fluid fine tailings (FFT) which
typically
contains about 2 wt% bitumen.
Attempts to recover bitumen from the FFT have been largely unsuccessful.
Skimming
bitumen from the tailings pond is no longer practiced due to high operating
costs and
difficulties in dealing with the froth produced. FFT may be processed for
dewatering and
reclamation through decanter centrifuges, producing a cake and a centrate
(i.e., liquid stream).
The centrate is usually recycled as dilution water or discharged to the
tailings pond. However,
centrate can have variable bitumen contents ranging from 0.01% to 1.9%. Higher
centrate
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bitumen contents often result in slugs of bitumen discharged from the
centrifuges. These high
bitumen contents in centrate may cause problems for water systems during
recycle, bitumen
accumulations in the centrifuge feed that interfere with flocculation or cause
mats of bitumen to
form on the surface of tailings ponds.
Accordingly, there is a need for an improved method to remove residual bitumen
from
oil sands tailings centrifuge centrate.
SUMMARY OF THE INVENTION
The current application is directed to a process for removing residual bitumen
from oil
sands tailings, particularly from centrate derived from centrifugation of the
tailings. The
present invention is particularly useful with, but not limited to, fluid fine
tailings. It was
surprisingly discovered that by conducting the process of the present
invention, one or more of
the following benefits may be realized:
(1)
the centrate represents an easy stream to remove bitumen due to having a
relatively low solids content;
(2) the
cleaned centrate may be recycled as centrifuge dilution water, without build-
up of bitumen mats or slugs which can accumulate with time or in upset
conditions; and
(3) if returned to the tailings pond, the cleaned centrate results in less
bitumen being
deposited upon the surface of the pond.
(4) the recovered bitumen has production value when returned to the oil
sand
processing plant.
Thus, use of the present invention provides both environmental and economic
incentives
for removing bitumen from the centrate derived from centrifugation of oil
sands tailings.
In one aspect, a process for removing residual bitumen from oil sands tailings
is
provided, comprising:
= optionally diluting the tailings with sufficient water to yield a tailings
feed
having a solids content in the range of about 18 wt% to about 36 wt%;
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= adding one or both of a coagulant and a flocculant to the tailings feed
to form a
centrifuge feed;
=
centrifuging the centrifuge feed to produce a cake and a centrate having a
solids
content of less than about 3 wt%;
= introducing the centrate into a flotation device so that bitumen froth
and cleaned
centrate are formed;
= recycling the cleaned centrate as dilution water or discharging the
cleaned
centrate to a tailings pond; and
= further processing the bitumen froth from the flotation device.
The bitumen froth may be processed either by adding it to the primary
extraction feed, or by
treating it in a new or existing froth treatment plant. In either case, the
flotation bitumen froth
may be first pre-cleaned by gravity removal of a portion of the free water
phase.
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 removing
bitumen
from centrate produced from the centrifugation of oil sands tailings.
FIG. 2 is a microscopic image of centrate froth showing the water continuous
phase at
the top (dark area) and the bitumen continuous phase at the bottom
(fluorescence mode, 450-
490 nm incident light).
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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 removing residual
bitumen from
oil sands tailings, particularly from centrate derived from centrifugation of
the tailings. 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) from tailings
ponds and fine tailings from ongoing extraction operations (for example,
thickener underflow
or froth treatment tailings) which may bypass a tailings pond.
FIG. 1 is a flow diagram of the process of the present invention. In one
embodiment,
the tailings are primarily FFT obtained from tailings ponds. However, it
should be understood
that the fine tailings treated according the process of the present invention
are not necessarily
obtained from a tailings pond, and may also be obtained from ongoing oil sands
extraction
operations.
In a tailings pond, the tailings stream separates into an upper water layer, a
middle FFT
layer, and a bottom layer of settled solids. The FFT layer may be removed from
between the
water layer and solids layer via a dredge or floating barge having a
submersible pump. The
FFT may then be transported to a centrifuge plant for processing.
The FFT 10 is optionally diluted with water within a suitable vessel such as a
tank. The
source of water is preferably tailings water. Seepage of tailings water may
arise from sand dike
construction rather than from the pond, and can be stored in a water tank for
use in processing.
Sufficient water is added to achieve a centrifuge feed having a solids content
in the range of
about 18 wt% to about 36 wt%, preferably greater than about 30 wt%, for the
most economic
usage of the centrifuge equipment. Dilution provides a consistent feed to the
centrifuge to
ensure stable machine operation. Optionally, a coagulant is introduced into
the in-line flow of
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FFT prior to entering a mixer. 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 coagulate. Suitable coagulants include, but are not
limited to, gypsum,
lime, alum, cationic polymers, or any combination thereof. In one embodiment,
the coagulant
comprises gypsum or lime. 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. 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.
The FFT is then pumped from the tank into a mixer. Additional water and a
flocculant
are introduced into the in-line flow of the FFT at a line prior to entering
the mixer. The source
of water is preferably tailings water. As used herein, the term "flocculant"
refers to a reagent
which reacts with the FFT solids to form flocs and through rearrangement
reactions increases
the strength of the flocculated FFT. 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 kDa to about
50,000 kDa.
Suitable natural polymeric flocculants may be polysaccharides such as dextrin,
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 having a medium charge density (about 20-35%
anionicity).
Other useful polymeric flocculants can be made by the polymerization of
(meth)acrylamide, 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). The preferred flocculant may be selected according to the FFT
composition and
process conditions.
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The flocculant may be 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 mixing skid, one or more storage tanks, and a dosing
pump. The dosage
of flocculant is 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 0.2-0.4% solution.
The additional water is provided to disperse the flocculant into the forward
flow of the
FFT for better flocculation. The FFT and diluted flocculant are further
combined within the
mixer. The flocculated FFT is mixed in a manner so as to avoid overshearing
which results in
floc breakage and re-suspension of the fines within the water. Suitable mixers
include, but are
not limited to, simple pipe tee mixers, in-line static mixers, dynamic mixers,
and continuous
stirred-tank reactors (CSTR's). Preferably, the mixer is a tee mixer
positioned before the feed
tube of the centrifuge. Alternatively, the diluted flocculant may bypass the
mixer and be fed
directly through the feed tube of the centrifuge for addition to the FFT.
At a centrifuge plant 12, the flocculated FFT is transferred to a centrifuge
for
dewatering. In one embodiment, the centrifuge is a solid bowl decanter
centrifuge. The cake
14 is collected and transported via a conveyor, pump or transport truck to a
disposal area where
the cake is stacked to maximize dewatering by natural processes.
In one embodiment, the centrate 16 has a solids content of less than about 3
wt%. The
centrate 16 is transferred to a flotation device at a bitumen recovery plant
18 to recover as much
bitumen from the centrate as possible in the form of froth 20. Bitumen
recovery may be
accomplished using various flotation devices including, but not limited to, a
gravity separator, a
mechanical flotation cell, a separator equipped with aeration downpipes, a
flotation column, or
any combination thereof.
In one embodiment, the centrate 16 may be transferred to a gravity separator
to enable
quiescent separation of the bitumen from the centrate 16. A gravity separator
typically includes
a shallow cone end and a rake at the bottom of the cone for further
concentrating the bitumen
froth by releasing any entrapped solids and water. Aeration of the centrate 16
promotes the
attachment of bitumen to air bubbles, creating a lower-density bitumen froth
20 which floats to
the upper portion of the gravity separator. The resulting bitumen froth 20
overflows the weir of
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the vessel into a launder extending around the rim of the gravity separator
for removal for
downstream processing.
In one embodiment, the centrate 16 may be transferred to a mechanical
flotation cell
which typically includes a mixer and diffuser mechanism at the bottom of the
mixing tank to
introduce air and provide mixing action, thereby intensifying the aeration of
the bitumen
droplets.
In one embodiment, the centrate 16 may be transferred to a separator equipped
with
aeration downpipes which combines the centrate 16 with air in a downpipe where
high shear
creates the turbulent conditions required for aeration of the bitumen
droplets. The very high
interfacial surface area and intense mixing result in rapid bitumen attachment
to the air bubbles.
In one embodiment, the centrate 16 may be transferred to a flotation column
which
includes air spargers to introduce air at the bottom of a tall column while
introducing the
centrate above. The countercurrent motion of the centrate 16 flowing down and
the air flowing
up provides mixing action to aerate the bitumen droplets. Counter current
froth washing with
water may also be employed to produce an enhanced froth quality.
As an optional enhancement to any of the bitumen flotation embodiments, the
flotation
overflow may be cleaned by removal of a significant portion of the free water
phase, using a
gravity-based froth cleaner, This is done to reduce the impact of water and
solids from the free
water phase on downstream processing. The froth cleaner is usually a simple
gravity separator,
fed by the flotation overflow stream, and producing a cleaned froth overflow
stream, and a
water phase underflow stream, containing only small amounts of bitumen, to be
returned to
tailings for disposal. In one embodiment, the cleaned froth 20 contains 30-40%
bitumen.
Following use of any one of the above flotation devices, froth cleaners or the
like, the
bitumen froth 20 is withdrawn and transferred to either the primary extraction
feed, or directly
to a froth treatment plant 22. The bitumen which is present in the bitumen
froth 20 comprises
both non-asphaltenic material and asphaltenes. Froth treatment is the process
of eliminating the
aqueous and solid contaminants from the bitumen froth 20 to produce a clean
bitumen product
for downstream upgrading processes. The bitumen froth 20 is diluted with a
hydrocarbon
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solvent to reduce the viscosity and density of the oil phase, thereby
accelerating the settling of
any dispersed phase impurities by gravity or centrifugation.
Either a paraffinic or naphthenic type diluent may be used. Examples of
paraffinic type
diluents include C4 to C8 aliphatic compounds and natural gas condensate,
which typically
contains short-chained aliphatic compounds and may also contain small amounts
of aromatic
compounds.
Examples of naphthenic type diluents include toluene (a light aromatic
compound) and naphtha, which may be comprised of both aromatic and non-
aromatic
compounds. The difference in the bitumen produced by use of either a
paraffinic or naphthenic
type diluent can be attributed largely to the presence of aromatics. Aromatics
have the ability to
hold asphaltenes in solution, whereas paraffinic type diluents cause
asphaltene precipitation.
Recovery of the hydrocarbon solvent from the diluted bitumen component is
typically
conducted in a recovery unit before the bitumen is delivered to a refinery for
further processing.
The remaining bitumen-depleted, cleaned centrate 24 may be either recycled as
dilution
water in the centrifugation process or discharged back to the tailings pond
26.
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
In a first pilot test, centrate produced from the centrifugation of fluid fine
tailings was
collected in open topped rectangular tanks prior to discharge to a settling
basin. The tanks
served as gravity separators, and the froth formed on top of the centrate
within the tanks.
Samples of the froth were collected and analyzed. The froth was found to
contain 57% bitumen
and 9% solids. Without bitumen recovery, discharge of the centrate into the
Mildred Lake
Settling Basin resulted in the formation of a large visible mat of bitumen.
Example 2
In a second pilot test, six samples of froth which foimed in the centrate tank
of an
Andritz A14 centrifuge were taken. The analyses of the froth samples are set
out in Table 1.
The froth samples had considerably poorer froth quality compared to the first
centrifuge pilot.
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The present FFT centrifuge prototype samples had bitumen contents ranging from
8.5 wt% to
28.4 wt%, with solids contents from 11.4 wt% up to 18.9 wt%.
Table 1
Date
09/28/11 09/28/11 09/28/11 09/28/11 09/29/11 09/29/11
Time 10:23 13:40 15:28 16:55 14:57
15:49
Bitumen, wt% 11.7 17.0 15.8 8.5 22.7
28.4
Water, wt% 68.6 66.9 65.0 72.2 60.9
56.3
Solids, wt% 16.8 15.8 18.9 18.3 11.4
12.4
Solids <5.5 m, % 55.1 60.9 62.7 54.6 55.9
56.2
Solids <44 [1.m, % 96.5 97.8 98.3 96.9 96.3
96.4
Solids d50, pm 4.6 3.8 3.6 4.7 4.5
4.5
As shown in FIG 2, the froth samples were mostly water-continuous. The bitumen
continuous phase was found to have 30 vol% of degraded bitumen, with mostly
dendrites and
sheets ranging from 100 pm to 150 p.m. The bitumen continuous phase was also
found to
contain a high amount of dispersed water droplets that were less than 10 p.m
in size. Both
water continuous and bitumen continuous phases were observed, with the
dispersed water drops
in the bitumen continuous phase showing up as dark circles.
Further characterization tests were conducted on the bitumen from the centrate
froth
samples (Table 2). The bitumen was separated from the centrate froth samples
using a
combination of Dean Stark extraction with toluene, followed by removal of the
toluene with the
standard rotary evaporator method. The bitumen asphaltenes and MCR
characterization data
fall within the range of normally expected values.
Table 2
Date 09/28/11 09/28/11 09/28/11 09/28/11 09/29/11
09/29/11
Time 10:23 13:40 15:28 16:55 14:57
15:49
Extraneous Matter, wt% 0.3 0.3 0.2 0.2 0.5
0.5
C5 Asphaltene Content, wt% 15.1 15.3 15.0 13.8 16.2
16.6
Micro-carbon Residue, wt% 12.9 12.8 12.6 12.6 13.4
13.8
Example 3
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The centrate froth samples were also processed using the 1 G and Cold Spin
tests, as
detailed below, to evaluate the simulated froth treatment processability in
inclined plate settler
and centrifuges.
Summary of Cold Spin Test
The cold spin test was performed as follows:
= Collect a froth sample in a 250m1 jar, and add naphtha to achieve a
naphtha to
bitumen ratio (N/B) of 0.7;
= Heat to 80 C, then mix for 10 minutes on a shaker table;
= Centrifuge for 10 minutes at 2000 RPM; and
= Take a 1 gram sample of hydrocarbon layer, and deteimine water content with
a
Karl Fischer Titration.
Summary of a 10 test
The 10 test was performed as follows:
= Collect a froth sample in a 1 litre jar, and add naphtha to achieve a 0.7
N/B;
= Heat to 80 C, then mix for 20 minutes on a shaker table;
= Further mix with a baffled tank/impeller mixer at 700RPM for 10 minutes;
= Place the jar in a 80 C water bath, and allow to settle for 2 hours;
= At 0, I, 3, 5, 7, 10, 15, 20, 30, 60, 90, 120 minute marks, collect 1
gram samples,
and determine water contents with a Karl Fischer Titration.
The 1-G tests of the froth samples showed no froth separation at all, possibly
due to the high
water and emulsion content of the centrate froth samples. In the Cold Spin
tests, it was found
that an average of 1% water remaining in the diluted bitumen, which falls
within the range of
1% to 2% expected for normally separating froths (Table 3). The water content
in the diluted
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bitumen samples ranged from 0.25% to 3%. The tests indicate that the froth
would likely be
difficult to process in the inclined plate settlers, but may be treatable with
a centrifuge-based
froth treatment process with naphtha dilution. This behavior is consistent
with the 30%
degraded bitumen observed in the microscopy evaluations.
Table 3
Date
_ 09/28/11 09/28/11 09/28/11 09/28/11 09/29/11 09/29/11
Time 10:23 13:40 15:28 16:55 14:57
15:49
Cold Spin Test Actual 0.88 0.73 0.65 0.95 0.65
1.13
Naphtha/Bitumen Ratio
Cold Spin Water in Diluted 0.8 0.47 0.56 0.25 3.01
1.1
Bitumen, wt%
Example 4
During the second pilot test, nine 1 m3 totes of FFT centrifuge centrate were
collected
for evaluation in a test loop. The centrate was circulated through a flow loop
through an
aerator, and transferred to an open topped tank where froth was removed. Three
tests were
performed, one with "normal" centrate and two with "off-spec" centrate
containing a higher
amount of solids and bitumen. Samples were collected for analysis, and a
complete material
balance was performed and shown in Table 4. These results indicate that the
froth qualities are
very "lean" at less than 10% bitumen content, potentially indicating the need
for froth cleaning
(i.e. removal of the free water phase) prior to further processing. Recovery
of centrate bitumen
is expected to have its highest production potential during periods of "off-
spec" FFT centrifuge
operation, when the centrate contains more bitumen
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Table 4 Mass balances of centrate bitumen flotation tests
Test# Centrate Name Bitumen Water Solids Bitumen Water Solids
Content Content Content Recovery Recovery Recovery
CC-1 off-spec 'Feed , 0.99 90.55 8.45
100.00% 100,00% 100.00%
Froth 4.32
85.55 10.14 36.60%, 7.95% 10.09%
Free bitumen 13.02 76.81 10,17 0.41% 0,03%
0.04%
Tailings 0.68
91.02 8.30 62.99% 92.02% 89.87%
CC-2 off-spec Feed 1,02
91.44 7.54 100.00% 100.00% 100.00%
Froth 4.94 84.96 10.11 43.82% 8.41%
12.14%
Free bitumen , 7.41 82.86, 9.73 0.12% 0.02%
0.02%
Tailings 0.63
92.09 7.28 56.06% 91,58% 87.84%
CC-3 on-spec Feed 0.06
99,66 0.28 100.00% 100.00% 100.00%.
Froth 7.31 , 80,14, 12,55 45.17% 0,30%
16.70%
Free bitumen 5.16 90.64 4,20 0.47% 0.01%
0.08%
Tailings 0.03
99,73 0.24 54.36% 99.69% 83.21%
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