Note: Descriptions are shown in the official language in which they were submitted.
CA 02864857 2014-09-25
NS-457
BITUMEN RECOVERY FROM OIL SANDS TAILINGS
FIELD OF THE INVENTION
The present invention relates to a process for removing bitumen from oil sands
tailings, and in particular, a process for bitumen recovery from tailings from
storage
ponds.
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 that 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.
Generally, the fine tailings are discharged into a storage pond for settling
and
dewatering. When first discharged in the pond, the very low solids content
material is
referred to as thin fine tailings. After a few years, the tailings separate
into an upper layer
of water, a settled layer of coarse solids and a fluid fine tailings (FFT)
layer between the
upper water layer and the bottom layer of settled coarse solids. The fluid
fine tailings
generally have a solids content of about 10-45 wt% and behave as a fluid-like
colloidal
material.
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A substantial amount of bitumen remains in the tailings stream from oil sand
= extraction. For example, there is approximately 20 MBbl of bitumen per
100 Mm3 of
fluid fine tailings.
The tailings bitumen represents a large loss given the commercial value of
this
potentially useable hydrocarbon. Furthermore, the tailings bitumen interferes
with
tailings operations, including reducing the efficiency of tailings treatments.
In addition,
tailings bitumen may represent an environmental risk by accumulation in the
storage
ponds.
Accordingly, there is a need for a method to recover bitumen from tailings in
the
storage ponds.
SUMMARY OF THE INVENTION
The current application is directed to a method to recover bitumen from
tailings in
the storage ponds, which will be referred to herein as storage pond tailings.
In accordance with a broad aspect of the present invention, there is provided
a
method including:
= combining storage pond tailings with a heated tailings stream to form a
tailings mixture, the storage pond tailings having a temperature and a
solids content and the tailings mixture having a resulting solids content
less than the solids content of the storage pond tailings; and
= treating the tailings mixture to recover bitumen therefrom.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings wherein like reference numerals indicate similar
components and steps 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:
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Figure 1 is a process flow diagram of a typical hot/warm water based oil sand
= extraction practice.
Figure 2 is a schematic process flow diagram of an embodiment of the present
invention for recovering bitumen from oil sands tailings.
Figure 3 is a graph showing the effect of ratios of heated tailings (P6 Tails)
to
storage pond tailings (FFT) on bitumen flotation kinetics.
Figure 4 is a graph showing the effect of ratios of heated tailings to storage
pond
tailings on the cumulative bitumen recoveries and the cumulative solids to
bitumen ratios.
Figure 5 is a graph showing the effect of ratios of heated tailings to storage
pond
tailings on cumulative bitumen recoveries and grades.
Figure 6 is a graph showing the effect of ratios of heated tailings to storage
pond
tailings with respect to a Gaudin Selectivity Index.
Figure 7 is a graph showing bitumen recoveries from treatment of samples of
heated tailings (6Tail) and storage pond tailings (FFT).
Figure 8 is a graph showing bitumen to solids ratios of froth from treatment
of
samples of heated tailings and storage pond tailings.
Figure 9 is a graph showing bitumen to water ratios of froth from treatment of
samples of heated tailings and storage pond tailings.
Figure 10 is a graph showing the effect of feed solids content on bitumen
recovery
and froth weight from treatment of a mixture of heated tailings and storage
pond tailings.
Figure 11 is a graph showing the effect of feed solids content on bitumen
froth
quality expressed as bitumen to solids ratio from treatment of a mixture of
heated tailings
and storage pond tailings.
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Figure 12 is a graph showing the effect of temperature on bitumen recovery and
grade of froth from the mixture of heated tailings and storage pond tailings.
= Figure 13 is a graph showing the effect of feed solids contents and
volumetric
ratio of heated tailings to storage pond tailings on bitumen recovery.
Figure 14 is a graph showing the effect of feed solids contents and volumetric
ratio of heated tailings to storage pond tailings on froth quality.
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 current application is directed to a method for recovering bitumen from
storage pond tailings. In the method, a tailings mixture is formed by
combining storage
pond tailings with a heated tailings stream and the tailings mixture has a
solids content
less than those of the storage pond tailings. The tailings mixture is then
treated to recover
bitumen from the tailings mixture. In so doing, bitumen may be recovered from
the
storage pond. In addition, a treated tailings stream may be generated, which
is more
suitable for further handling than storage pond tailings.
Storage pond tailings generally have a solids content of about 10 to 45 wt%.
The
storage pond tailings may be, for example, fluid fine tailings. Fluid fine
tailings can have
a solids content of about 10 to 45 wt% but generally the solids content is in
the range of
about 30 to 40 wt%.
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Storage pond tailings are obtained from a tailings storage pond, also called a
tailings pond, settling pond, settling basin, etc. These ponds are generally
in an outdoor
setting and, thus, are exposed to normal outdoor conditions. Thus, storage
pond tailings,
including fluid fine tailings, can have a temperature of about 1 to 25 C, but
most often
5 are at about 5 to 20 C.
Storage pond tailings may be obtained by pumping or otherwise drawing tailings
from a storage pond. After a residence time in a storage pond, storage pond
tailings may
separate into an upper water layer, an FFT layer and a bottom layer of
settled, coarse
solids. In one embodiment, the storage pond tailings are predominantly FFT.
The FFT
may be removed from between the water layer and the solids layer, for example,
via a
dredge or floating barge having a submersible pump.
Heated tailings are derived from various stages of oil sand processing. Figure
1 is
a flow diagram of a typical hot/warm water oil sand extraction process showing
the
various tailings streams that are produced. These tailings streams useful in
the present
invention are indicated in Figure 1 with an asterisk (*). As-mined or pre-
crushed oil sand
ore 10 is first mixed with slurry water 12 having a temperature generally
around 50-80 C
(and, optionally, caustic) in a slurry preparation unit 14 such as a tumbler,
mix box, etc.
and the resultant oil sand slurry is transported through a hydrotransport
pipeline 16 with
air injection for conditioning prior to bitumen separation. The conditioned
slurry is then
optionally diluted and subjected to gravity separation in a primary separation
vessel 18,
where three layers are formed: coarse tailings 20, middlings 22 and bitumen
froth 24
(commonly referred to as primary bitumen froth). The middlings 22 are further
treated in
a secondary recovery unit 26, such as flotation cells or the like, where
further bitumen
froth 28 (commonly referred to as secondary bitumen froth) is recovered and a
finer
tailings stream 30 is produced. The secondary bitumen froth 28 may either be
combined
with primary bitumen froth 24 for further treatment or may be recycled back to
the
primary separation vessel 18. The finer tailings stream 30 (i.e., the
secondary
separation/flotation tailings) generally will have a temperature ranging from
about 35 C
to about 50 C. The solids contents and sand to fine ratios (SFR) in the finer
tailings
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stream 30 typically contain 10-40% solids depending upon the amount of
dilution water
= added. The finer tailings steam 30 is a tailings stream useful in the
present invention.
Bitumen froth produced during bitumen separation generally comprises about 60
wt% bitumen, about 30 wt% water and about 10 wt% solids and, thus, needs to be
further
cleaned prior to upgrading. Generally, the bitumen froth is first deaerated in
a deaerator
32, for example, a steam deaerator, and then subjected to froth treatment 34
using a
hydrocarbon solvent such as naphtha or paraffin. Solvent-diluted froth (often
referred to
as dilfroth) is then subjected to at least one stage of gravity- or centrifuge-
based
separation to produce solvent-diluted bitumen 36 (often referred to as
dilbit), having
reduced solids and water. The tailings that are produced during froth_
treatment are
generally referred to as froth treatment tailings 38 and have a solids content
typically
about 20-25% and a temperature of about 80 C. The froth treatment tailings 38
is another
tailings stream useful in the present invention. However, since the froth
treatment
tailings 38 still have a considerable amount of solvent associated with them,
the froth
treatment tailings 38 are further treated in a solvent recovery unit 40 where
the solvent is
stripped with steam and hot froth treatment tailings 42 are produced.
Generally, the hot
froth treatment tailings 42 that are produced after hydrocarbon solvent
removal are the
most useful tailings stream in the present invention.
The coarse tailings 20 can also be further treated. In one embodiment, coarse
tailings 20 can be used to form composite tailings with fluid fine tailings
(HT). In
particular, coarse tailings 20 produced from primary bitumen separation 18 may
be
optionally first screened in a rotating, stationary or vibrating screen 44 to
remove large
lumps and the screened tailings are then subjected to separation in a
plurality of
hydrocyclones 46. The hydrocyclone overflow 48 is another tailings stream
useful in the
present invention. The hydrocyclone overflow 48 typically has a temperature of
about
C to about 50 C. The solids content in the hydrocyclone overflow 48 is
typically in
the range of 2-25% solids, depending on the feed properties and the
hydrocyclone
operation conditions. The hydrocyclone underflow 49 is then mixed in mix box
50 with
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fluid tine tailings 52 from tailings ponds and gypsum 54 to form non-
segregating
composite tailings 56.
In another embodiment, hydrocyclone overflow 48 is further treated in at least
one thickener 57 to produce thickened tailings 58 for disposal and thickener
overflow 59,
which is another tailings stream useful in the present invention.
The heated tailings useful in the present invention are selected to have a
solids
content less than that of the storage pond tailings, such that when combined,
the
combination of heated tailings and storage pond tailings has a dilution
greater than that of
the storage pond tailings. RCW may be added to the combined tailings to
achieve an
optimal flotation feed density if necessary.
In addition, the heated tailings have a temperature greater than the
temperature of
the storage pond tailings to be employed. As such, the combination of heated
tailings and
storage pond tailings has a temperature greater than that of the storage pond
tailings.
To facilitate operations, the heated tailings may be a tailings stream that is
already
heated, rather than heated solely for this purpose. For example, as noted
above, tailings
may be used from a primary bitumen extraction process, which is the process
through
which mined oil sands ore is first treated. In one embodiment, for example,
the heated
tailings may be fine tailings 30, froth treatment tailings 38, hot froth
treatment tailings 42
and/or hydrocyclone overflow 48.
As noted above, the most useful source of heated tailings is the hot froth
treatment
tailings 42. These tailings 42 may contain solids and water, likely some
amount of
residual bitumen and possibly traces of one or more additives such as solvents
from the
froth treatment process. Generally, the hot froth treatment tailings include
solids of less
than 25% and generally less than 20%.
=
Also, the hot froth treatment tailings are generally much wanner than storage
pond tailings, for example, having a temperature of about 80 to I00 C and
generally 85
to 95 C.
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While recovery of tailings bitumen from storage pond tailings has previously
been
so difficult as to be uneconomic, it has been determined that combining the
storage pond
tailings with a heated tailings stream may dilute and heat the resultant
tailings mixture
such that recovery of the tailings bitumen from storage pond tailings may
become
economically viable. The combined tailings mixture has a resulting mixture
temperature
that is greater than the temperature of the storage pond tailings and a
resulting solids
content that is less than the solids content of the storage pond tailings. In
one
embodiment, for example, the tailings mixture includes a temperature of
greater than
20 C and possibly greater than 30 C and a solids content of less than 20% and
possibly
of less than the gel point for the storage pond tailings, which for fluid fine
tailings is less
than about 13%. In general, it was discovered that the more dilute the FFT
feed, the
easier it is to float the bitumen therein.
Tailings mixtures of the present invention can be treated to remove bitumen.
The
treatments may include various processes including, for example, flotation,
froth cleaning
and froth treatment. The recovery of bitumen from the tailings mixture is much
better
than the recovery of bitumen from storage pond tailings alone, without
addition of heated
tailings.
In addition to bitumen, the method generates a treated tailings stream. The
treated
tailings stream at least contains less bitumen and is, therefore, more suited
to further
handling. For example, the treated tailings stream may be more suitable for
disposal than
the storage pond tailings. The treated tailings stream, having some or all of
the bitumen
removed, may be receptive to further treatments.
Because the heated tailings may also contain some bitumen, the present method
provides an efficiency by combining treatments for bitumen recovery for
storage pond
tailings and heated tailings coming out of the primary, first stage oil sand
processes. This
highly synergistic process for bitumen recovery from FFT takes the advantage
of the heat
from the heated tailings and the dilution from the water in the heated
tailings by
combining bitumen incentive from two waste streams in one process. Thus, two
separate
processing facilities would not be required.
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One embodiment of the invention is shown in Figure 2. The illustrated method
is
for removing bitumen from storage pond tailings, here in the form of fluid
fine tailings
(FFT) 60. FFT 60 can have a solids content of about 10 to 45 wt% but generally
the
solids content is in the range of about 30 to 40 wt%. Being from an outdoor
site and
having had a long residence time in a storage pond, FFT may have an ambient
temperature of about 1 to 25 C, but most often are at about 5 to 20 C.
The FFT is combined with a heated tailings stream, here shown as hot froth
treatment tailings 62. Hot froth treatment tailings are obtained from the
treatment of
bitumen froth using any suitable froth treatment process, including without
limitation,
processes using a froth treatment diluent such as naphtha or paraffin, a
gravity settler
such as an inclined plate settler, enhanced gravity separation apparatus such
as a scroll
centrifuge and/or solvent recovery unit. Preferably, hot froth treatment
tailings have been
treated in a diluent/solvent recovery unit to remove a large portion of the
solvent
remaining with the tailings.
Hot froth treatment tailings 62 are generally at a temperature of about 80 to
100 C
and generally include 5 to 20% solids by weight and 1 to 12% bitumen by weight
in
water. Hot froth treatment tailings 62 may be further comprised of an amount
of a froth
treatment diluent which is present as a result of separating the froth
treatment tailings
from the bitumen froth. Where the heated tailings include a froth treatment
diluent, the
froth treatment diluent may be comprised of a naphthenic type diluent and/or a
paraffinic
type diluent.
Preferably, however, the heated tailings contain little or no froth treatment
diluent,
because the froth treatment diluent has been recovered from the hot froth
treatment
tailings in a tailings solvent recovery unit process or a similar process.
Such a process
increases the temperature of the tailings considerably.
In one embodiment, hot froth treatment tailings 62 contain, on average, about
17% solids, 3% bitumen, 80% water and a small amount, for example about 0.2%,
of
naphtha by wt.
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The FFT and hot froth treatment tailings are combined to form a tailings
mixture
64. The tailings mixture is selected to have a temperature greater than the
temperature of
the storage pond tailings and a solids content less than the solids content of
the storage
pond tailings. Thus, the FFT is heated and diluted by forming the tailings
mixture.
5 The FFT and hot froth treatment tailings are combined in various ratios.
Generally, to treat useful volumes of FFT, the ratio is less than 3:1 hot
froth tailings to
FFT. When FFT is combined with hot froth tailings at ratios of less than 1:1,
the high
fines content of FFT tends to adversely affect bitumen recovery and froth
quality. Thus,
ratios of 1.5:1 to 2.5:1 (heated tailings to FFT) are most useful.
10 Tailings mixture 64 may, for example, have a temperature of greater than
20 C,
for example between 30 C and 60 C.
Tailings mixture 64 may, for example, also have a solids content of less than
20%,
for example 5 to 20%wt. In one embodiment, it is useful to bring tailings
mixture to a
dilution less than the gel point of the fluid fine tailings, which is
generally less than 13%
solids by weight.
If desired, tailings mixture 64 may be further diluted and/or heated by
addition 66
of recycle water (RCW) 68a and/or heat such as waste heat 70 to achieve the
desired
conditions of temperature and dilution. Waste heat 70 may include hot water
from
cooling towers, such as upgrading cooling towers, or other waste heat streams.
In one embodiment, where it is difficult to achieve desired dilutions (<13%)
with
just the heated tailings, water such as recycle water 68a may be added to
dilute tailings
mixture 64. However, the lower temperature of recycle water, which is
generally close to
the temperature of storage pond tailings (i.e. 5 to 20 C), may drive down the
temperature
below desirable levels. As such, if recycle water 68a is added to tailings
mixture 64,
generally the recycle water and/or the tailings mixture are heated.
The tailings mixture may be treated 72 to recover bitumen 73 therefrom. During
this treatment process, it may be useful to maintain the appropriate mixture
temperature
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and dilution. In one embodiment, for example, recycle water (RCW) 68b may be
added
74.
Bitumen recovery treatment 72 may include conditioning, solvent extraction,
etc.
to obtain bitumen 73. In the illustrated embodiment, for example, bitumen
recovery
treatment 72 includes flotation 76, froth cleaning 78 and froth treatment 80
to obtain
bitumen 73.
Flotation 76 is an operation in which components of a mixture are separated by
passing a gas through the mixture so that the gas causes one or more
components of the
mixture to float to the top of the mixture and form a froth. Froth flotation
may be
performed using flotation cells or tanks, flotation columns or any other
suitable froth
flotation apparatus, which may or may not include agitators or mixers, and
froth flotation
may include the use of flotation aids, including without limitation,
surfactants and
frothing agents.
For example, flotation 76 may include conditioning the tailings mixture by
aeration, agitation, etc. in order to facilitate separation of the tailings
bitumen from the
solids. Conditioning may include agitating the tailings mixture, with this
kinetic energy
aerating the bitumen and causing it to attach to air bubbles to float as froth
85 and
separate from the solids and water, which may be separated as tailings 82.
Flotation 76
may agitate tailings mixture 64 in any suitable manner, including, without
limitation, by
stirring and/or by mixing including by piping or gas injection. In particular,
subjecting
the tailings mixture to froth flotation may be performed using any suitable
froth flotation
apparatus.
Flotation 76 concentrates the bitumen in the froth 85. The froth may be
collected
as an overflow product. When separated from the underflow of tailings 82,
froth 85 may
be subjected to froth cleaning 78.
Froth cleaning 78 produces dilute tailings 87a, including mostly water and
some
solids, and an improved froth 88 including the bitumen. Froth cleaning 78 may
subject
froth 85 to various processes including any or all of dewatering, gravity
settling, solvent
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extraction, etc. For example, froth cleaning 78 may include the addition of an
amount of
= a hydrocarbon solvent to incoming froth 85 for solvent extraction.
Solvent extraction is an operation in which components of a mixture are
separated
by adding to the mixture a suitable liquid solvent, here a hydrocarbon
solvent, which
dissolves or dilutes one or more components of the mixture, thereby
facilitating
separation of components of the mixture. Solvent extraction apparatus may be
employed
such as including gravity settlers (including without limitation, gravity
settling vessels,
inclined plate separators, and rotary disc contactors) and enhanced gravity
separators
(including without limitation, centrifuges and hydrocyclones).
The hydrocarbon solvent is a substance containing one or more hydrocarbon
compounds and/or substituted hydrocarbon compounds which is suitable for use
for
diluting bitumen.
Generally, the hydrocarbon solvent may include any suitable
naphthenic type diluent or any suitable paraffinic type diluent.
A naphthenic type diluent is a solvent that includes a sufficient amount of
one or
more aromatic compounds so that the solvent exhibits the properties of a
naphthenic type
diluent as recognized in the art, as distinguished from a paraffinic type
diluent. In this
document, a naphthenic type diluent may therefore include solvents such as
naphtha and
toluene.
A paraffinic type diluent is a solvent that includes a sufficient amount of
one or
more relatively short-chain aliphatic compounds (such as, for example, CS to
C8 aliphatic
compounds) so that the solvent exhibits the properties of a paraffinic type
diluent as
recognized in the art, as distinguished from a naphthenic type diluent. In
this document, a
paraffinic type diluent may therefore include solvents such as natural gas
condensate.
If bitumen froth quality is poor, recycle water (RCW) 68b may be added during
froth cleaning to dilute the froth to facilitate cleaning. This is especially
true if froth
cleaning is conducted using a flotation column. If solvent extraction is
employed for
froth cleaning, it may not be necessary to dilute the froth with further
water.
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Froth 88 is then treated 80 to concentrate and recover the bitumen 73.
Treatment
80 may include for example clarification, concentration and dewatering such as
by
solvent recovery, centrifugation, etc.
The process removes bitumen from the storage pond tailings. The bitumen
recovery may be greater than 15% and in some cases recovery may be greater
than 50%.
Tailings from bitumen recovery treatment 72, for example tailings 82 from
flotation 76, may be passed for disposal 84. Dilute tailings 87a from later
processes of
bitumen recovery may be disposed of, along with tailings 82, and/or the dilute
tailings
may be returned 87b for dilution of the tailings mixture 64. Another source of
dilution
water may be the tailings 87c from froth treatment.
Tailings 82, which may contain some tailings 87a, may be disposed of in any of
various ways, such as by returning to a tailings storage pond. However, due to
the
effectiveness of the above-noted process, the tailings contain only a minor
amount of
bitumen and may be suitable for treatment to concentrate solids. For example,
tailings 82
may be subjected to dewatering treatments such as may include flocculation 90
and
thickening or centrifugation 92, to generate thickened tailings or centrifuge
cake 94 and
water 98. In this process, the tailings are treated with flocculant 96 prior
to dewatering
by centrifugation 92 to aggregate the solids and to recover the water.
In one embodiment, for example, flocculant 96 is introduced into the in-line
flow
or directly to a mixer to mix with tailings 82. 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 IcD. Suitable natural polymeric flocculants may
be
polysaccharides such as dextran, starch or guar gum. Suitable synthetic
polymeric
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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)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).
In one embodiment, the flocculant 96 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
82 and
process conditions.
The flocculant 96 is 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 96 is controlled by a metering pump. In
one
embodiment, the dosage of flocculant 96 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.
When the flocculent 96 contacts tailings 82, it starts to react to form flocs
formed
of multiple chain structures and solids. Tailings 82 and flocculant 96 are
combined at
least to some degree within a mixer. Since flocculated material is shear-
sensitive, it must
be mixed in a manner so as to avoid over-shearing. Over-shearing is a
condition in which
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additional energy has been input into the flocculated tailings, resulting in
release and re-
suspension of the fines within the water. Suitable mixers include, but are not
limited to,
T mixers, static mixers, dynamic mixers, and continuous-flow stirred-tank
reactors. In
one embodiment, the mixer is a T mixer positioned before the feed tube of the
centrifuge
5 employed for centrifugation 92. In one embodiment, flocculation may be
achieved by
introducing flocculant directly to tailings 82 in a feed line to the
centrifuge or thickener.
Flocculation 90 produces a suitable feed 100 which can be dewatered and
thickened by centrifugation 92. The feed 100 is transferred to the centrifuge
for
dewatering. In one embodiment, a centrifuge useful in centrifugation is a
solid bowl
10 decanter centrifuge. Solid bowl decanter centrifuges are capable of
dewatering materials
which are too fine for effective dewatering by screen bowl centrifuges.
Extraction of
centrate water 98 occurs in the cylindrical part of the bowl, while dewatering
of solids by
compression of the centrifuge cake 94 takes place in the conical part of the
bowl.
Separation of the water 98 and centrifuge cake 94 using a solid bowl decanter
centrifuge
15 may be optimally achieved using low beach angle, deep pool depths, high
scroll
differential speed, and high bowl speed rpm.
In one embodiment, water 98 has a solids content of less than about 3 wt%. The
centrate water 98 may be collected and either discharged 102 back to the
tailings pond or
diverted 86 into a line for recycling for feed dilution or other processes
such as flocculant
dilution.
In another embodiment, the flocculated material is treated with a thickener,
resulting in thickened tailings (TT) product 94 and the water stream 98. The
TT product
may have a solids content of 40 ¨ 50%, while the water stream may contain less
than 1%
solids.
In one embodiment, the cake 94 has a solids content of at least about 50 wt%.
The cake or TT 94 may be collected and transported via a conveyor, pump or
transport
truck to a disposal area. At the disposal area, the cake or TT 94 is stacked
to maximize
dewatering by natural processes including consolidation, desiccation and
freeze thaw via
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1 to 2 m thick annual lifts to deliver a trafficable surface that can be
reclaimed. In
another embodiment, cake or TT 94 can be placed in deep pits where dewatering
includes
desiccation and freeze thaw, but primarily consolidation. In another
embodiment, cake to
IT is placed at the bottom of End Pit Lakes.
Exemplary embodiments of the present invention are described in the following
Example, which is 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
Tests were conducted to show bitumen recovery from fluid fine tailings.
An FFT sample was obtained from the Syncrude site in Fort McMurray, Alberta,
Canada. The sample contained 33.79% solids and 1.97% bitumen. Kerosene was
added
at 834 g/t as a bitumen collector.
A 2.3 ¨ 2.5m3 sample of the FFT was added to a separator tank and mixed for 5
minutes, followed by aeration for a total of 33 minutes. The temperature was
ambient at
about 22 C.
The bitumen froth was collected by hand and analyzed.
Though kerosene was added to facilitate flotation, the bitumen recovery was
less
than 1%. The froth quality was very low with about 2% bitumen.
The bitumen in undiluted FFT is hard to recover by flotation at ambient
temperatures.
Example 2
Tests were conducted to study bitumen recovery from fluid fine tailings when
combined with froth treatment tailings.
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Four FFT samples were obtained from an HT Centrifuge Field Test Plant at the
Mildred Lake Settling Basin from the Syncrude site in Fort McMurray, Alberta,
Canada.
The compositions of the FFT samples are shown in Table 1. The samples contain
an average bitumen content of 2.34%, solids content of 37.11% and water of
60.74%.
The average ¨44um fines content is 94.16% and the average ¨2um clay content
28.51%.
The consistency of the FFT sample particle size distribution (PSD) was good.
Solids content of the tested FFT probably represented a "high-end" of the
expected levels from a commercial operation, for example dredge-recovered FFT.
A
typical delivered FFT density is expected at about 30-35% solids.
Table 1 Compositions of FFT samples for the lab tests
Bitumen Water -44 um
Name % Solids% -2 lam % %
FFT-1 2.35 61.39 37.04 27.78 93.10
FFT-2 2.37 60.81 36.98 28.81 95.22
FFT-3 2.35 60.38 37.33 28.90 94.19
FFT-4 2.29 60.39 37.34 28.53 94.12
Average 2.34 60.74 37.17 28.51 94.16
Samples of hot froth treatment tailings were obtained from the Syncrude site
in
Fort McMurray, Alberta, Canada. In Syncrude operations, hot froth treatment
tailings,
which are those tailings having been through both froth treatment and solvent
recovery,
are known as Plant 6 tailings (P6 tails). The assays of two samples of Plant 6
tailings are
given in Table 2. The samples contain an average bitumen content of 2.65%,
solids
content of 10.58% and water content of 85.60%. The average -44um fines content
is
83.51% and the average -2um clay content 17.98%. So the particles of Plant 6
tailings
are relatively coarser than FFT.
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Table 2 Compositions of Plant 6 tailings samples for the lab tests
Bitumen Water -44 um
Name % cyo Solids% -2 tim cyo %
P6 2.67 85.57 10.94 18.39 83.26
Tails-1
P6 2.62 85.63 10.22 17.57 83.75
Tails-2
Average 2.65 85.60 10.58 17.98 83.51
It is known that Plant 6 tailings contain heavy minerals such as rutile,
ilmenite,
zircon and pyrite, etc. To investigate the flotation behavior of Plant 6
tailings without the
heavy minerals, an additional Plant 6 tailings sample was de-sanded by
siphoning the
upper fine slurry after homogeneously mixing the Plant 6 tailings and then
statically
settling for 1.5 min. The compositions of the de-sanded fines (DS-Finesl and
DS-
Fines2) and sand materials (DS-Sandl and DS-Sand2) are given in Table 3.
Table 3 Compositions of the de-sanded Plant 6 tailings samples for
the lab tests
Name Bitumen Water Solids% -2 urn % -44 um
cyo 0/0
DS- 2.75 84.93 11.20 18.27 84.24
Fines1
DS- 2.77 85.17 11.25 17.66 83.24
Fines2
Average 2.76 85.05 11.23 17.97 83.74
DS- 1.75 27.45 70.31 4.49 21.40
Sand1
DS- 2.00 26.97 69.73 4.10 18.72
Sand2
Average 1.88 27.21 70.02 4.30 20.06
As shown in Table 3, the bitumen content and the solids content in the de-
sanded
fines fraction are slightly higher compared with those in Table 2, while the -
44im fines
content and the -21am clay content are not changed. In the sand fraction,
however, the
solids content was increased to 70% and the -441..im fines content and the -
21,im clay
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content are significantly reduced to 20.1% and 4.3% respectively. As only a
very small
amount of sand was removed from the raw Plant 6 tailings, the PSDs of the de-
sanded
fines fraction overlapped with those of the raw Plant 6 tailings.
Flotation tests were conducted using the FFT and Plant 6 samples. The test
matrix for the lab flotation tests is shown in Table 4. The main test variable
for the
flotation tests is the ratios of Plant 6 tailings to FFT which determine the
mixed feed
solids content and temperature for both the raw and de-sanded Plant 6
tailings. Table 4
also shows the estimated and measured feed solids contents, the calculated and
measured
temperatures, and the time to reach to a steady temperature (i.e.,
equilibrium) for mixing
the two feed materials. Table 4 shows that the calculated and measured
temperatures are
quite consistent. The time to reach to a steady temperature was usually 30-60
seconds.
The estimated flotation feed solids contents based on the Plant 6 tailings
solids content,
the FFT solids content and their mixing ratios are quite close to the measured
feed solids
contents.
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Table 4 Test matrix for the lab flotation tests
Ratio by Aerati
Temperature Time
Vol. Agitat'n on Feed solids % C (sec) to
Test Plant PL6Tails ml/ Estimat Measu Calcula Measu equilibri
No. 6 tails / FFT rpm min ed red ted red urn
1:2 1500 150 31.3% 30.66 41 41.4 56
P6M-1 Raw
%
1:1 1500 150 27.1% 25.57 52 53.6 45
P6M-2 Raw 0/0
2:1 1500 150 22.7% 22.33 64 64.3 43
P6M-3 Raw oh)
3:1 1500 150 20.3% 70
65.6 55
P6M-4 Raw 21.15
cyo
P6M-8 De- 1:2 1500 150 30.4% 30.17 41 42.3
35
sanded Vo
P6M-9 De- 1:1 1500 150 26.0% 25.71 52 51.6
46
sanded
P6M- De- 2:1 1500 150 20.4% 21.14 64 63.6
43
10 sanded
P6M- De- 3:1 1500 150 18.5% 19.83 70 68.2
51
11 sanded
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Flotation Procedure
A Denver flotation machine and a 1.5-liter cell equipped with a warm water
jacket
were used for the lab flotation tests. Before the flotation tests, the samples
were
homogenized using a mechanical mixer. Predetermined volumes of samples were
taken
out from the FFT and Plant 6 tailings sample pails and put into beakers
respectively. To
heat up the Plant 6 tailings to 90 C, simulating the similar temperature in
operation, and
to minimize the changes in solid-bitumen interaction, the Plant 6 tailings
sample was
warmed up in an oven without any mechanical disturbance. Even under such
conditions,
it was observed that there was a bitumen layer floating at the top of the
warmed slurry. It
was possibly due to the heat convection of the slurry, resulting from the
temperature
changes inside the slurry. The water jacket temperature was set at a value as
calculated
based on the ratio of Plant 6 tailings to FFT. After mixing FFT and Plant 6
tailings at a
certain ratio to reach a steady temperature, aeration started and froth
samples at times of
3, 6, 10 and 20 min were collected into 4-oz glass jars, weighed and held for
analysis.
The flotation tailings after taking the above-noted 4-oz sample were stored in
a 2-liter
beaker for subsequent flocculation and settling tests. For the de-sanded Plant
6 tailings,
the flotation test procedure was the same as above.
Flocculation Procedure
Six flotation tailings samples from the above flotation tests were selected to
perform flocculation and settling tests with graduated cylinders. The test
numbers were
P6M-4, P6M-11 and P6M-12. All settling tests were conducted at room
temperature
(21 C) except for a repeated one (P6M-11) at 50 C for comparison purpose. An
anionic
flocculant was employed, which is an anionic polyacrylamide-sodium
polyacrylate co-
polymer with a high molecular weight (about 10,000 kD or higher) and a medium
charge
density (about 20 to 35% anionicity). The polymer is available as SNF A3338.
The
flocculant was used at a fixed dosage of 800 g/t, which was prepared using
recycle water
(RCW).
The flocculation procedure is summarized as follows.
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= Calculated the solids weight in the tailings sample and the amount of
flocculant solution to be added;
= Poured the tailings sample into a 2-liter beaker;
= Pre-mixed the sample at 600 rpm for 2 min with an impeller (Lightnin
A310 impeller 3.4" diameter (86 mm) x 3/8" bore diameter). The distance
between the
impeller bottom and the beaker bottom was 2.5 cm;
= Kept the mixer running at 600 rpm and simultaneously injected the pre-
determined amount of 0.4% flocculant solution into the tailings sample through
a tube in
3 min and then allowed 0.5 min additional mixing;
= Poured the above flocculated sample into a 2-liter graduated cylinder;
= Recorded the settling heights versus time over a period of 24 hours with
an AITF camera system;
= During the testing, took supernatant samples to measure the solids
contents at 10 min and 24 hrs and took a sediment sample to determine the
solids content
at 24 hrs.
Gaudin Selectivity Index
Recovery and grade of concentrates are most popularly used to evaluate
flotation
performance in mineral processing and oil sand industries. However, sometimes
they are
not enough to evaluate the separation efficiency because they depend on the
feed grades.
Gaudin selectivity index is one of the supplemental terms used to gauge
flotation
selectivity. It was defined as a geometric mean of the relative floatability
and the relative
rejectability for component a against component b (Taggart A., (1954).
Handbook of
Mineral Dressing, Section 19 Sampling and Testing, p.19-195-19-196).
The Gaudin selectivity index is expressed in Equation (1).
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R, 1Y2
Gaudin S I. ¨
R!, Ja
(1)
where Ra is recovery of component a in concentrate, Rb recovery of component b
in concentrate, Ra/Rb relative floatability of a to b; Ja is rejection of
component a in
tailings, Ja=100-Ra in a separation process with two outputs; Jb is rejection
of component
b in tailings, Jb=100-Rb in a separation process with two outputs; Jb/Ja is
relative
rejectability of b to a.
If Gaudin selectivity index is equal to 1, there is no selective separation
between
the two components. Gaudin selectivity index was used to evaluate the
flotation
selectivity between bitumen and solids in this series of lab flotation tests.
Results and Discussion
As mentioned earlier, this series of tests were conducted with both raw and de-
sanded Plant 6 tailings being combined with FFT at different ratios. The test
results are
presented for raw Plant 6 tailings and de-sanded Plant 6 tailings
respectively.
Raw Plant 6 Tailings and FFT Tests: The effect of ratios of Plant 6 tailings
to FFT
on bitumen flotation kinetics is shown in Figure 3, in which the feed solids
content and
temperature are also indicated. As mentioned earlier, the Plant 6 tailings
sample was pre-
heated to 90 C and the FFT sample was kept at ambient temperature of 21 C.
When
changing the volume ratios of Plant 6 tailings to FFT in the flotation feeds
from 1:2 to
3:1, the feed solids contents were accordingly decreased from 30.66% to 21.15%
and the
temperatures were increased from 41.4 C to 65.6 C. As shown in Figure 3, the
ratios of
Plant 6 tailings to FFT had a significant effect on the bitumen flotation
kinetics. The
more dilute the flotation feed, the faster is the bitumen flotation rate.
Figure 4 shows the effect of ratios of Plant 6 tailings to FFT on the
cumulative
bitumen recoveries and the cumulative solids to bitumen ratios (SBR) in the
froth
products. It is clear that the solids to bitumen ratios were reduced with the
enhanced
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ratios of Plant 6 tailings to FFT from 1:2 to 3:1. Data is shown for each of
the tested
flotation residence times, in each data series.
Figure 5 demonstrates the effect of ratios of Plant 6 tailings to FFT on
cumulative
bitumen recoveries and grades. It is evident that more dilution of FFT with
the Plant 6
tailings resulted in higher bitumen recovery and grade.
The Gaudin selectivity index between bitumen and solids is shown in Figure 6.
When the ratio of Plant 6 tailings to FFT was 1:2 which gave a flotation feed
of 30.66%
solids, the selectivity index was close to 1. It means that at such a dilution
ratio, almost
no selective separation between bitumen and solids took place. With the
enhanced ratios
of Plant 6 tailings to FFT from 1:2 to 3:1, the selectivity between bitumen
and solids was
gradually improved. At the 3:1 dilution ratio of Plant 6 tailings to FFT, the
flotation feed
density was 21.15% solids and the bitumen recovery was 25%.
De-sanded Plant 6 Tailings and FFT Tests: This series of tests with the de-
sanded
Plant 6 tailings were performed to investigate the flotation behavior when the
heavy
minerals were removed from the Plant 6 tailings. The de-sanded fines were
mixed with
FFT at different ratios for the flotation tests. The de-sanded Plant 6
tailings sample was
pre-heated to 90 C and the FFT sample was kept at an ambient temperature of 21
C.
When changing the volume ratios of de-sanded Plant 6 tailings to FFT in the
flotation
feeds from 1:2 to 3:1, the feed solids contents were accordingly decreased
from 30.17%
to 19.83% and the temperatures were increased from 42.3 C to 68.2 C. The test
results
were very similar to those noted above for raw Plant 6 tails. In particular,
the more dilute
the flotation feed, the better the results.
Flocculation and Settling Tests: Six flocculation and settling tests on the
lab
flotation tailings samples were performed with SNF A3338 at a fixed dosage of
800 g/t in
order to check the flocculation behavior of the combined Plant 6 tailings and
FFT. The
settling data are given in Table 5. The more dilute the flocculation feed, the
faster was
the settling rate.
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Table 5 Settling data of the selected lab flotation tailings flocculated with
800 g/t
SNF A3338
P6Tails/ Feed 10min 24hr 24hr 24hr
FFT
supernatant supernatant sediment release
Test # water
Solid %
by
by Vol. s% Solids% Solids% Solids% Vol.
P6M4 21.15
3:1 N/A 0.60%
30.13% 35.65%
P6M11 19.83
3:1 0.62% 0.56%
32.29% 36.54%
P6M11 at 50 C 19.83
3:1 0.58% 0.54%
33.58% 42.37%
P6M12 25.90
1:1 N/A 0.44%
29.33% 8.66%
It is interesting to notice in Table 5 that the flocculation at 50 C
significantly
increased the initial settling rate and the water release compared with that
at ambient
temperature of 21 C for the same samples from the combined de-sanded Plant 6
tailings
5 and FFT at 3:1.
Large floc particles were observed, indicating a good flocculation was
achievable
with 800 g/t SNF A3338 for the combined Plant 6 tailings and FFT. The
flocculated
materials may be suitable for centrifugation.
Example 3
10 Test Materials and Set-Up
Six 20-L pails of Plant 6 tailings (6Tails), three pails of FFT and six pails
of
recycled process water (RCW) were obtained from the Syncrude site. The
composition
and particle size distribution of tailings samples were analyzed and the
results are
summarized in Table 6.
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26
Table 6 Properties of Tailings Samples
Sample Bitumen Water Solids 44 um
d50 SFR
ID % % % % Urn
,
6Tail 1/6 2.39 81.34 16.88 63.05
24.76 0.586
6Tail 2/6 2.4 82.87 17.55 71.69
15.78 0.395
6Tail 3/6 2.29 83.47 16.29 80.23
12.04 0.246
6Tail 4/6 2.59 83.79 16.91 72.17
16.06 0.386
6Tail 5/6 2.26 83.11 16.73 75.38
14.73 0.327
6Tail 6/6 , 2.23 83.02 16.66 75.11
14.46 0.331 ,
FFT 1/3 2.71 63.3 35.46 89.19
5.74 0.121
FFT 2/3 2.59 64.5 34.77 88.82
5.85 0.126
FFT 3/3 2.65 63.81 35.70 93.76
5.34 0.067
Although bitumen content in 6Tails and FFT were similar (about 2.5%), there
were much
more fines (-44 mm) present in FFT. In addition, the sand to fines ratio (SFR)
was low
for both samples (< 0.6).
Test Set-Up
Sample homogenization: A specifically designed baffle was fixed in a 20-L
pail.
To ensure that coarse sands at the bottom of the pail could be suspended and
homogenized completely in the pail, a powerful hand drill with a mixer was
used to mix
the slurry. Before each test, a given amount of sample was taken from the well
mixed 20-
L sample pail.
Flotation test: After exploratory tests to evaluate the impact of impeller
selection,
agitation intensity (rpm), aeration rate and cell volume, it was found that a
2-L Denver
flotation cell with 1200 rpm and 0.8-L/min. aeration could give reasonably
stable froth in
the cell. Therefore, a 2-L Denver flotation cell was used for all the
flotation tests.
Test Procedures
The basic steps involved are summarized below:
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= Agitate both 6Tails and FFT samples in their holding 20-liter buckets to
ensure
homogeneous mixing of the slurries.
= Calculate and weigh the 6Tails, FFT and RCW samples based on the total
volume
of 2-L for each test, the ratio of 6Tails to FFT, the target combined feed
solids
content and the original tailings solids contents (Table 6).
= Pre-heat the mixture of the weighted 6Tails, FFT and RCW samples in an
oven to
the target temperature. Pour the mixture samples into the 2-L flotation cell
and
turn on the agitation at 1200 rpm for 2 min. conditioning.
= Set the thermostat of water bath connected with the water jacket to the
target
temperature and maintain it during the subsequent flotation test.
= Turn on airflow meter to control the aeration rate at a given value.
Start timing
immediately when beginning the aeration. Collect bitumen froth into separate
jars
directly at flotation time of 3, 5, 10 and 25 min. respectively.
= After finishing flotation, collect the flotation tailings in a 2-L
beaker, weigh the
flotation tailings, and estimate solids content in the tailings.
Bitumen Flotation Recovery
To evaluate the role of mixing 6Tails with FFT in flotation performance,
baseline
flotation tests using FFT (12.5% solids) and 6Tails (as received) alone at
different
temperatures were carried out first. The results in Figure 7 showed that
bitumen recovery
from 6Tails was faster than those from FFT. Some of the reasons could be
attributed to
smaller bitumen droplets present in FFT, and fewer fines in 6Tails. In
addition, the
residual bitumen from 6Tails was the bitumen floated to the froth, while the
bitumen in
FFT was those unrecoverable during extraction. Increasing operating
temperature from
to 40 C accelerated bitumen recovery for FFT samples. Increasing flotation
25 temperature could reduce slurry and bitumen viscosity, accelerate
bitumen liberation
from solids particles, and increase activation energy for bitumen-bubble
attachment, all
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contributing to accelerated bitumen recovery. However, for 6Tail samples,
increasing
operating temperature from 40 to 90 C (typical temperature of plant 6
tailings) virtually
did not increase bitumen recovery.
It can also be noted from Figure 7 that more than 90% bitumen recovery was
obtained from 6Tails, and about 75% from FFT, which was much higher than the
targeted
bitumen recovery of > 50%. These results demonstrated the importance of
dilution of
FFT for improving bitumen recovery. In Figure 7, the FFT in the flotation feed
was
diluted to 12.5% solids, while in Example 2 the lowest flotation feed was
about 21.15%
solids, which resulted in about 25% bitumen recovery.
Bitumen Froth Quality
Analyzing bitumen froth quality through bitumen to solids ratio (Figure 8) and
bitumen to water ratio (Figure 9) revealed that froth quality from 6Tail and
FFT was poor.
In particular, in commercial oil sands extraction, bitumen to solids and
bitumen to
water ratio in the froth could be in the range of 4 and 2, respectively. As
shown in Figure
8, bitumen to solids ratio was less than 0.5 for FFT, and 0.3 for 6Tails. It
is obvious that
fines present in the tailings could be responsible for the poor froth quality.
Since particle
size from 6Tail was larger than that from FFT (Table 6), the lower bitumen to
solids ratio
from 6Tail in the froth would suggest that the bitumen from 6Tails was less
liberated
from sands than those in FFT. Another reason could be that the heavy minerals
enriched
in the froth are still hydrophobic and float together with bitumen to the
froth, making the
froth quality poorer. This observation would suggest that the lower bitumen
recovery
from FFT than 6Tails (Figure 7) would be mainly attributed to smaller bitumen
droplet
sizes (not the liberation), with lower collision and attachment probability of
the bitumen
droplets to bubbles in FFT than those in 6Tails.
Effect of Feed Solids Content
This set of tests was conducted to determine what feed solids content could
give
optimized bitumen recovery and froth quality.
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In these tests, a starting mixture of ratio 2:1 for 6Tails: FFT was diluted
with
recycle water to arrive at samples with 5%, 7.5%, 10%, 12.5%, 15%, 17.5% and
20%.
For each sample 'flotation was conducted at temperatures 25 C, 40 C and 55 C.
Aeration
was continued for 25 minutes.
As shown in Figure 10, up to 90% bitumen recovery was obtained, and bitumen
recovery did not change too much with increasing solids content in the feed.
However,
the froth weight changed significantly. Increasing feed solids content
increased the froth
product weight. Two straight lines could be drawn, in terms of froth weight
vs. feed
solids content (Figure 10): the first from 5% to 12.5% solids, and the second
from 12.5%
to 20% solids. When the feed solids content was higher than 12.5 wt%, the
increase in
froth weight became even faster, with a steeper slope of the straight line. In
this case, the
entrapment of solids into bitumen-bubble aggregates recovered to the froth
could
contribute to the increased froth weight. Since bitumen recovery virtually did
not
increase, the increased froth weight with increasing feed solids content would
indicate the
froth quality could become worse. Indeed, as shown in Figure 11, bitumen froth
quality
as expressed by bitumen to solids ratio (B/S) in the froth decreased with
increasing feed
solids content. Especially when the feed solids content was higher than 10 -
12.5%, the
ratio reduced more significantly. Therefore, considering operation capacity
and froth
quality, a feed solids content of 12 wt% or less is most useful.
Effect of Temperature
To evaluate the effect of temperature on bitumen recovery from the tailings
mixture, mixtures were prepared with 6Tails/FFT at a ratio of about 2 having
temperatures of 25, 40 and 55 C. In general, it was discovered that there is a
reverse
relationship between recovery and concentrate grade (or froth quality):
increasing
recovery could make froth quality worse. To have a reasonable comparison, the
plot of
recovery-grade curve is normally used in mineral flotation separation to
identify
optimized conditions: the curve on the far right side gives the best
performance. For this
reason, the 25-minute cumulative bitumen recovery was plotted against the 25-
minute
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cumulative bitumen content in the froth for each of the tested feed solids
content from
5% to 20%.
As clearly shown in Figure 12, the tests at 40 C gave the best flotation
performance, i.e., at the same concentrate grade or bitumen content in the
froth, a highest
5
bitumen recovery, on average, was obtained at 40 C. Raising extraction
temperature to
55 C virtually did not improve flotation separation performance, although
increasing
extraction temperature decreases bitumen and slurry viscosity, accelerates
bitumen
liberation from the sands, and enhance bitumen-bubble attachment, with the
potential of
increasing bitumen recovery and bitumen froth quality. The exact reasons for
such
10
extraction behavior remain to be explored. From the test results, it appeared
that using
extraction temperature at about 40 C was sufficient to have maximized
extraction
performance.
Effect of 6Tails to FFT Ratio
This set of tests was aimed at establishing suitable volumetric ratios of
6Tails to
15 FFT
for the treatment of two tailings mixed together. Samples were prepared with
volumetric ratios of 6Tails to FFT of 1.5, 2 and 3. These samples were diluted
to 5%,
7.5%, 10%, 12.5%, 15%, 17.5% and 20%. To minimize the effect of other
operating
variables the extraction temperature was fixed at 40 C. The total flotation
time was 25
minutes.
20
Figure 13 shows the bitumen recovery at different volumetric ratios of 6Tails
to
FFT and solids content in the feed. For the ratio of 6Tail to FFT at 1.5,
bitumen recovery
decreased with increasing solids content in the feed. However, with raising
volumetric
ratio of 6Tail to FFT to 2 and 3, virtually no difference in bitumen recovery
was observed
with the tested solids content in the feed. It is known that increasing solids
content in the
25 feed
increased slurry viscosity, which could retard bitumen-bubble interactions and
bitumen-bubble aggregates rising to the froth. Since the solids in FFT are
much smaller
than those in 6Tails (Table 6), the actual amount of fine solids in the feed
would be much
VVSLegal\053707\00381\10864803v1
CA 02864857 2016-03-04
31
higher for the ratio of 1.5, as compared to 2 and 3, resulting in reduced
bitumen recovery
with increasing total solids content in the feed at the ratio of 1.5.
Another observation from Figure 13 was that bitumen recovery of 6Tails alone
(from Figure 7) was always higher than that for 6Tail-FFT mixture, as
expected. But the
lowest recovery of the mixture at a higher feed solids content of >15% was
even lower
than FFT alone diluted at 12.5% solids. However, for the mixture at 12.5% feed
solids,
the obtained bitumen recovery was 85.23%, which was higher than 79% for FFT
alone
(from Figure 7), thus there is a synergistic benefit from combining the 6Tails
with the
FFT.
Figure 14 shows the effect of mixing FFT with 6Tails at different volumetric
ratio
on bitumen froth quality as expressed by bitumen to solids ratio in the froth
at 40 C.
Mixing FFT with 6Tails dropped bitumen froth quality, due to lower froth
quality of
6Tails than that of FFT (Figure 8). Increasing solids content in the feed
decreased froth
quality, probably resulting from a higher probability of mechanical entrapment
of solids
inside bitumen-bubble aggregates. Increasing the volumetric ratio of 6Tails to
FFT also
dropped froth quality, because of the lower froth quality of 6Tails than that
of FFT. By
considering bitumen recovery and froth quality, it appeared that the
volumetric ratio of
6Tails to FFT of about 2 was most suitable for bitumen removal from the
mixture.
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