Note: Descriptions are shown in the official language in which they were submitted.
I
TREATMENT AND DEWATERING OF OIL SANDS FINE TAILINGS
FIELD
[0001]The technical field generally relates to the treatment of fine tailings
derived
from mining operations, and more particularly relates to the treatment of
thick fine
tailings and froth treatment tailings derived from oil sands mining.
BACKGROUND
[0002]Tailings derived from oil sands mining operations are often placed in
disposal ponds for settling. The settling of fine solids from the water in
tailings
ponds can be a relatively slow process and can form a stratum of thick fine
tailings.
[0003]Certain techniques have been developed for dewatering fine tailings.
Dewatering of fine tailings can include contacting with a flocculant and then
depositing the flocculated material onto a sub-aerial deposition area where
the
deposited material can release water and eventually dry. Other techniques for
treating thick fine tailings include addition of gypsum and sand to produce
consolidated tailings.
[0004]In some scenarios, it can be desirable to pre-treat the fine tailings
stream
prior to dewatering and adding the flocculant. However, there are a number of
challenges related to pre-treating the fine tailings stream and to processing
the
material to facilitate efficient reclamation.
SUMMARY
[0005]In one aspect, a process for treating a froth treatment tailings stream
is
provided. The process includes: subjecting the froth treatment tailings stream
to
flotation, to produce an aqueous underflow including contaminants of concern
(CoCs) and suspended solids; adding an immobilization chemical to the aqueous
underflow in order to chemically immobilize the CoCs; adding a polymer
flocculant
to the aqueous underfiow in order to flocculate the suspended solids, thereby
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producing a flocculated material; and dewatering the flocculated material to
produce: an aqueous component depleted in the CoCs and the suspended solids;
and a solids-enriched component including the chemically immobilized CoCs and
flocculated solids.
[0006]In one implementation, the froth treatment tailings stream can include
untreated froth treatment tailings obtained from an output of a froth
treatment
process. Further, the froth treatment tailings stream can include froth
treatment
mature fines tailings retrieved from a froth treatment tailings pond, a froth
treatment
tailings slurry obtained by excavating a beach of a froth treatment tailings
pond,
and/or a centrifuge cake obtained from centrifuging untreated froth treatment
tailings obtained from an output of a froth treatment process, the centrifuge
cake
being diluted prior to the flotation.
[0007]Subjecting the froth treatment tailings stream to flotation can include
generating gas bubbles that aid in the flotation. Generating the gas bubbles
includes can be performed by at least one of dissolved air flotation,
decomposition
of chemicals, air induction and CO2 addition. Generating the gas bubbles can
also
include contacting an oxidizing agent that react with organic materials. In
some
scenarios, the gas bubbles can include microbubbles.
[0008]In one implementation, the process further includes providing an in-line
flow
of the aqueous underflow. The immobilization chemical and the polymer
flocculant
can be added in-line. The immobilization chemical can be added as an aqueous
immobilization solution into the in-line flow of the aqueous underflow, and
the
process can further include in-line mixing of the aqueous immobilization
solution
and the aqueous underflow.
[0009]The immobilization chemical can be selected from multivalent organic
salts.
[0010]In some scenarios, in-line conditioning of the suspended solids to form
the
flocculated solids in a water release zone can be performed.
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[0011]In one implementation, dewatering the aqueous underflow can include
depositing the flocculated material onto a sub-aerial deposition area, thereby
allowing separation of the aqueous component from the solids-enriched
component. Alternatively, in another implementation, dewatering the aqueous
underflow can include depositing the flocculated material into a pit, thereby
allowing separation of the aqueous component from the solids-enriched
component.
[0012]The process can further include forming a permanent aquatic storage
structure (PASS) for retaining the solids-enriched component and a water cap,
wherein the solids-enriched component: forms a consolidated solids-rich lower
stratum below the water cap; and retains the immobilized CoCs and inhibits
migration of the CoCs into the water cap.
[0013]Adding the immobilization chemical and the polymer flocculant can be
performed in various orders. For example, adding the immobilization chemical
and
the can be performed prior to adding the polymer flocculant. Alternatively,
adding
the polymer flocculant can be performed prior to adding the immobilization
chemical. Alternatively, the immobilization chemical and the polymer
flocculant can
be added simultaneously.
[0014]The dewatering of the flocculated material can be performed prior to
adding
the immobilization chemical, or after adding the immobilization chemical and
the
polymer flocculant.
[0015]In some implementations, the flotation further produces a froth
concentrate
overflow including residual bitumen. The process can further include treating
the
froth concentrate overflow to recover at least one of the residual bitumen,
diluent
used in a froth treatment process, and heavy minerals. Treating the froth
concentrate overflow can also include subjecting the froth concentrate
overflow to
a solvent separation step.
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[0015a] In some implementations, there is provided a process for treating oil
sands fine tailings, comprising: subjecting the oil sands fine tailings to
flotation to
produce a froth concentrate overflow comprising residual bitumen and diluent
and
an aqueous underflow comprising contaminants of concern (CoCs) and
suspended solids; and treating the froth concentrate overflow to recover at
least
one of the residual bitumen and the diluent.
[0015b] In some implementations, there is provided a process for treating
froth
treatment tailings comprising residual bitumen and diluent, comprising:
treating the
froth treatment tailings to produce a diluent-depleted froth treatment
tailings
stream; and subjecting the diluent-depleted froth treatment tailings stream to
dewatering.
Date Recue/Date Received 2021-03-17
4
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]Figure 1 is a process flow diagram of an oil sands mining operation,
including tailings dewatering operations.
[0017]Figure 2 is a process diagram showing an exemplary flotation unit.
(0018] Figures 3a and 3b are flow diagrams of exemplary froth treatment
tailings
dewatering operations.
[0019]Figures 4a to 4e are flow diagrams illustrating optional examples of
fine
tailings dewatering operations.
[0020]Figure 5 is a process flow diagram illustrating the treatment of the
froth
concentrate overflow obtained by flotation.
DETAILED DESCRIPTION
Overview of the process
[0021]Various techniques that are described herein enable the treatment of
fine
tailings streams, such as thick fine tailings (e.g., mature fine tailings) or
froth
treatment tailings (FTT). The fine tailings stream can be subjected to
flotation, prior
to sending the flotation underflow stream to treatment and dewatering
operations.
The flotation can be facilitated by generating gas bubbles (such as
microbubbles)
in various ways, as will be described in detail herein. Dewatering operations
will
also be described in detail herein. In the case of froth treatment tailings, a
froth
concentrate overflow including heavy minerals and organic materials can be
further treated to recover residual bitumen, diluent from the froth treatment
process
and/or heavy minerals.
[0022]It is understood that the "organic materials" found in oil sands include
bitumen and other "insoluble organic materials". It is understood that the
term
"bitumen" as used herein, can include bitumen components such as maltenes
and/or asphaltenes in varying proportions. It is understood that the maltenes
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consist of the fraction of the bitumen which is soluble in n-alkane solvents,
including pentane, hexane and/or heptane. It is also understood that the
asphaltenes consist of the fraction of the bitumen which is soluble in light
aromatic
solvents, such as benzene or toluene, and precipitates in n-alkane solvents.
The
"insoluble organic materials" (also referred to herein as "tightly bound
organic
materials") can for example include humic materials (i.e., humins) which can
form
chemical complexes with some of the heavy minerals. It is understood that the
"insoluble organic materials" consist of the organic materials which are
insoluble
in n-alkane solvents and light aromatic solvents at atmospheric pressure and
at
the boiling temperature of the solvents, and can also be understood as being
non-
bitumen components.
[0023]1t is understood that the "heavy minerals" in the oil sands tailings
refer to a
portion of the solids present in the oil sands tailings, including minerals
such as
zircon, rutile, anatase, ilmenite, pyrite, iron oxides and monazite. These
minerals
generally have oleophilic surfaces with adsorbed insoluble organic material.
The
remainder of the solids of the oil sands tailings generally includes
"hydrophilic
minerals" such as quartz, feldspars and sand.
[0024]It should be understood that the term "fine tailings" (also referred to
in the
art as "fluid tailings") as used herein, may refer to various types of oil
sands tailings,
such as thin fine tailings (i.e., extraction tailings that have formed from
run-off of
sand dump operations), thick fine tailings (i.e., settled tailings from a
tailings pond
¨ an example of which being mature fine tailings), or froth treatment tailings
(or
streams derived from froth treatment tailings such as froth treatment mature
fine
tailings, froth treatment centrifuge cake, etc.).
[0025]Referring to Figure 1, in a bitumen extraction operation, oil sands ore
10 is
mined and crushed in a crushing unit 12 to obtain a crushed ore 13. The
crushed
ore 13 is then mixed with water 14 (e.g., hot water) in a mixing unit 16 (for
example,
a rotary breaker) to form an aqueous slurry 18. The aqueous slurry 18 is
conditioned (for example during transport) to prepare the bitumen for
separation
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from the aqueous slurry 18 by adding additives (for example, caustic soda) to
the
aqueous slurry 18. The aqueous slurry 18 is then transported to a primary
separation vessel 20 for separation into primary bitumen froth 22 and coarse
tailings 24 (also referred to as primary tailings).
[0026]In some scenarios, the primary separation vessel 20 can also produce
middlings 26 which can be sent to a secondary separation vessel 28 to be
separated into secondary bitumen froth 30 and secondary tailings 32. The
secondary bitumen froth 30 can be fed back to the primary separation vessel
20,
as shown in the Figure or, alternatively, can be directly added to the primary
bitumen froth 22.
[0027]The bitumen froth 22 typically includes between about 40 wt% and about
70
wt% bitumen, between about 20 wt% and about 50 wt% water, and between about
wt% and about 15 wt% solid materials. The solid materials in the bitumen froth
22 typically include hydrophilic mineral materials and heavy minerals which
can
include adsorbed insoluble organic material.
[0028]The primary tailings 24 and secondary tailings 32 generally include
between
about 45 wt% and about 55 wt% solid materials, between about 45 wt% and about
55 wt% water, and residual bitumen (typically between about 1 wt% and about 3
wt% bitumen). The solid materials in the primary and secondary tailings 24, 32
are
mainly sand and other fine hydrophilic mineral materials, and can include
residual
heavy minerals. The primary tailings 24 and secondary tailings 32 (that can be
generally referred to or combined as extraction tailings or whole tailings 29)
can
be further treated and dewatered, as will be described further below.
[0029]The bitumen froth 22 is treated in a froth treatment process 34 in which
the
bitumen froth 22 is diluted with a diluent 36 to obtain a diluted bitumen
froth. The
diluent 36 can be either a naphthenic type diluent or a paraffinic type
diluent. The
naphthenic type diluent can for example include toluene, naphtha or other
light
aromatic compounds. The paraffinic type diluent can for example include C4 to
C8
aliphatic compounds and/or natural gas condensate. The diluted bitumen froth
is
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then separated into a bitumen product 38 (which can be further upgraded or
used
as is) and froth treatment tailings 40 including contaminants of concern
(CoCs,
such as residual bitumen, naphthenic acids, various salts etc.) and suspended
solid materials (such as hydrophilic mineral materials, heavy minerals and
insoluble organic materials), water and diluent 36. The froth treatment
tailings 40
can be further treated and dewatered, as will be described further below. The
froth
treatment tailings can also be further treated to recover residual bitumen,
diluent
and/or heavy minerals therefrom, as described in Canadian patent application
No.
2,889,586.
[0030]Still referring to Figure 1, in one implementation, froth treatment
tailings 40
are deposited in a froth treatment tailings pond 42 for settling, to form a
top water
region 43 and a bottom froth treatment mature fine tailings region 44 in the
tailings
pond 42. The froth treatment mature fine tailings 44 are then subjected to
flotation
46 to produce an aqueous underflow stream 47 and a froth concentrate overflow
49. The froth concentrate 49 can be subjected to recovery operations 50 (e.g.,
solvent extraction, filtration, drying) to recover residual bitumen, heavy
minerals
and/or diluent. The aqueous underflow stream 47 includes CoCs and suspended
solids, and can be treated and dewatered 48 in an effort to clean the stream.
For
example, the treatment and dewatering operation 48 can include the injection
of
an immobilization chemical to chemically immobilize the CoCs and/or injection
of
a polymer flocculant to flocculate the suspended solids and form a flocculated
material. Dewatering of the flocculated material can then produce an aqueous
component depleted in the CoCs and the suspended solids, and a solid-enriched
component including the chemically immobilized and flocculated solids. While
Figure 1 depicts separate tailings pond 54 for thin fine tailings 53 and
tailings pond
42 for froth treatment tailings 40, it should be understood that a single
tailings pond
can be used to recover both the thin fine tailings 53 and the froth treatment
tailings
40. Mature fine tailings from that single tailings pond can then be subjected
to a
flotation step followed by treatment and dewatering steps similar to treatment
and
dewatering 48. Alternatively, it is also understood that the fine tailings can
be
deposited in more than two tailings ponds.
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[0031]It should be understood that while the implementation of Figure 1
describes
that the flotation step 46 is performed on froth treatment mature fine
tailings 44,
other froth treatment tailings streams can be subjected to the flotation step.
For
example, the froth treatment tailings 40 directly obtained from froth
treatment
process 34 can be directly subjected to the flotation step 46 without being
decanted
in a tailings pond. Alternatively, the beach of the froth treatment tailings
pond 42
can be excavated and the excavated solids can be diluted to form a froth
treatment
tailings slurry that can be subjected to the flotation step 46. In yet another
alternate
implementation, the froth treatment tailings 40 can be centrifuged to produce
a
centrifuge cake. The centrifuge cake can then be diluted and the resulting
diluted
centrifuge cake can be subjected to the flotation step 46.
[0032]Still referring to Figure 1, the extraction tailings 29 can be subjected
to a
similar flotation step 46', and treatment and dewatering operation 48'. In one
implementation, the extraction tailings are subjected to a sand dump step 52
prior
to obtain thin fine tailings 53 that can be deposited into a tailings pond 54
for
settling. After settling, the thin fine tailings can produce a water top layer
55 and
Mature fine tailings 56. The water top layer 55 can be reused as recycle water
in
the oil sands extraction operation, for example as input water in mixing unit
16, for
forming aqueous slurry 18. The mature fine tailings 56 can be extracted from
the
pond 54, and subjected to flotation 46'. Residual organics 49' can be
recovered as
an overflow stream, and an aqueous underflow stream 47' can be subjected to
treatment and dewatering operations 48'. In some implementations, the fine
tailings that have been settling in a tailings pond for some time (e.g., what
can be
referred to as "legacy tailings") can be subjected to a flotation step,
followed by
treatment and dewatering operations 48, 48'.
[0033]The flocculated material to be treated and dewatered in the treatment
and
dewatering operations 48, 48' can be deposited onto a sub-aerial deposition
area
or into a pit, for allowing the aqueous component and the solids-enriched
component to separate. Overtime, a permanent aquatic storage structure (PASS)
58 can be formed for retaining the solids-enriched component and a water cap
62.
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The solids-enriched component can form a consolidated solids-rich lower
stratum
60 below the water cap, such that the immobilized CoCs are retained by the
solids-
enriched component and migration of the CoCs into the water cap is inhibited.
The
treatment and dewatering operation 48, 48', as well as the formation of the
PASS
will be described in further detail below.
Flotation of fine tailings streams
[0034]Now referring to Figure 2, various types of fine tailings 64 can be
subjected
to flotation 46 to produce an aqueous underflow that can be further treated
and
dewatered using the process described herein. The flotation 46 can be
facilitated
by generating gas bubbles (such as microbubbles). The gas bubbles can be
generated by various methods. For example, the gas bubbles can be generated
by at least one of dissolved air flotation, air induction, decomposition of
chemical
reagents, and gas addition such as CO2 addition. In the implementation shown
at
Figure 2, gas 66 is injected into the flotation unit 46 to generate gas
bubbles 68.
[003511n some implementations, an oxidizing agent 70 can be injected into the
flotation unit 46 to react with at least part of the organic materials and
generate
gas bubbles 68 that aid in the flotation. The gas bubbles 68 can for example
include
CO2 bubbles. In some scenarios, the oxidizing agent 70 can react with organic
materials coated on the heavy minerals (i.e., insoluble organic materials
and/or
bitumen coated on the surface of the heavy minerals) in order to oxidize the
coated
organic materials and generate the gas bubbles 68. The gas bubbles 68 can be
adsorbed on the surface of the heavy minerals, thereby aiding in the
flotation. In
some implementations, enough gas bubbles 68 are generated by the reaction
between the oxidizing agent 70 and the organic coatings so that the flotation
is
mostly induced by the gas bubbles 68. In other implementations, the flotation
is
mostly or completely induced by other techniques (such as dissolved air
flotation,
air induction and/or gas addition, as described above). In some scenarios, the
flotation segregates the diluent 36, bitumen and diluent, insoluble organic
materials
and the heavy minerals into a froth layer which is recovered as froth
concentrate
CA 2983961 2017-10-27
10
72 overflowing from the flotation unit 46. The aqueous underFlow 73 including
water
and hydrophilic mineral materials settles down by gravity and is recovered
from
the flotation unit 46 to be sent to treatment and dewatering operations 74.
Mechanical agitation 76 can also be provided to aid in the flotation.
[00361ln some scenarios, the flotation step results in a sand/water underflow
that
is sufficiently clean to be introduced into a dedicated disposal area (DDA)
that can
form a PASS over time. Compared to a tailings pond (a thin fine tailings pond
or a
froth treatment tailings pond), the sand/water underflow obtained after the
flotation
step is typically cleaner. In order to increase the likelihood of obtaining a
cleaner
sand/water underflow, the flotation step can be configured so as to err on the
side
of having some water and fines in the froth concentrate rather than allowing
more
organics and minerals to migrate in the aqueous underflow.
[0037]As mentioned above, the techniques described herein relate to the
treatment of fine tailings that include constituents of concern (CoCs) and
suspended solids. The fine tailings can be subjected to various treatments
including at least one of flotation, chemical immobilization of the CoCs,
polymer
flocculation of the suspended solids, and dewatering. In some implementations,
the fine tailings can be subjected to treatments including flotation, chemical
immobilization of the CoCs, polymer flocculation of the suspended solids, and
dewatering.
[0038]The froth concentrate overflow produced by the flotation step can be
further
treated to recover the residual bitumen, heavy minerals and/or diluent used in
the
froth treatment process. In some scenarios, the separation step results in an
upgrader-compatible hydrocarbon stream that does not contain a significant
amount of heavy minerals (i.e., trace amounts) and/or water. The heavy
minerals
stream that can be obtained following the separation step can contain some
organics as a result.
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11
Treatment of the flotation overflow
[0039]Referring to Figure 5, the froth concentrate 49 is contacted with an
extraction agent 263 (e.g., a solvent), and fed into separation unit 264 for
separation into a first fraction 266 and a second fraction 268. The first
fraction 266
includes insoluble organic materials and a portion of the heavy minerals. The
second fraction 268 includes bitumen, a portion of the diluent, a portion of
the water
initially present in the froth concentrate and a portion of the extraction
agent 263.
The concentration of asphaltenes in each one of the first and second fractions
depends on the type of extraction agent 263 used. In some implementations, the
separating of the froth concentrate 49 into the first and second fractions
includes
precipitating some of the organic materials by adding the extraction agent
263. The
separating of the froth concentrate can also include filtering the froth
concentrate
49. In such case, adding the extraction agent 263 to the froth concentrate 49
forms
a diluted froth concentrate mixture, solubilizes maltenes and precipitates
insoluble
organic materials. Alternatively, gravity separation, such as centrifugation,
can be
used to separate the first and second fractions. In some scenarios, the
extraction
agent 263 also solubilizes water in addition to solubilizing the diluent and
the
bitumen. In some scenarios, a first portion of the asphaltenes can be
solubilized
while a second portion precipitates, with the amount of solubilized and
precipitated
asphaltenes depending on the nature of the extraction agent 263. The filtering
of
the diluted froth concentrate mixture then enables recovery of the first
fraction 266
as a filter cake (or retentate) and the second fraction 268 as a filtrate.
[0040]In some implementations, the filtering of the diluted froth concentrate
mixture can take place using pressure or vacuum force. The filtration can for
example include drums, horizontally or vertically stacked plates or horizontal
belts.
Alternatively, the filtering of the diluted froth concentrate mixture can be
performed
by cross-flow filtration (also referred to as tangential flow filtration),
wherein the
diluted froth concentrate mixture is injected tangentially across the surface
of a
filter (as opposed to into the filter in conventional filtering processes).
The filtration
can be a batch filtration or a continuous filtration.
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[0041]In some implementations, the filter cake including heavy minerals (to
which
insoluble organic material can be adsorbed onto) and optionally asphaltenes is
produced with a sufficiently high solids content to be trucked or conveyed to
a
desired location. The filter cake can be further treated to recover the
insoluble
organic materials, the precipitated asphaltenes (when present) and the heavy
minerals. The filter cake and/or the froth concentrate can also be used as
feedstock
in mineral processing plants to recover and/or purify the heavy minerals.
[0042]In some implementations, the extraction agent 263 is selected such that
(i)
the water, the diluent and the bitumen are soluble in the extraction agent 263
below
80 C; and (ii) the extraction agent 263 precipitates the insoluble organic
materials
and enables a partitioning of the diluted froth concentrate into the first
fraction 266
including the heavy minerals and the insoluble organic materials, and the
second
fraction 268 as a filtrate including the extraction agent 263, the diluent and
the
maltenes. In some implementations, the extraction agent 263 is added in a
concentration of about 30 wt% to about 60 wt% or about 40 wt% to about 60 wt%
of the froth concentrate 49. In some scenarios, a substantial amount or
virtually all
of the solids present in the froth concentrate are filtered off in the first
separation
unit, and are recovered in the filter cake, such that the filtrate only
includes a
residual or trace amount of solids such that downstream processing of the
filtrate
does not employ solids handling or separation steps. In some implementations,
the solvent includes naphtha or a paraffinic solvent.
[0043]In some implementations, the second fraction 268 is fed into an
extraction
agent recovery unit 270 and separated into a recovered agent stream 272,
recovered water 274 and an agent-depleted bitumen-enriched stream 276
including a portion of the diluent. For example, the second separation unit
270
includes a liquid-liquid separation unit and/or a distillation unit. In some
implementations, the separation of the second fraction 268 is performed in a
closed
pressure vessel. The separation of the second fraction 268 can be performed at
a
temperature above the boiling point of the extraction agent 263 (at the
operating
pressure).
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[004411n some implementations, the solvent-depleted bitumen-enriched stream
276 is supplied to an upgrading unit 278 such as a diluent recovery unit
(DRU).
The DRU can be operated at a temperature similar to the temperature of the
separating of the second fraction 268. The solvent-depleted bitumen-enriched
stream 276 can be separated into diluent 277 which can be recycled for re-use
in
the froth treatment unit 34, and a bitumen-enriched stream 280. The bitumen-
enriched stream 280 can be further upgraded, mixed with other bitumen products
or used as is.
[0045]In some implementations, the recovered solvent stream 272 is recycled
for
re-use as part of the extraction agent 263. In some scenarios, up to 99% of
the
extraction agent 263 added to the froth concentrate 49 can be recovered from
the
second separation unit 270.
[0046]The recovered water 274 can be sent for disposal in a tailings pond, or
can
be directly recycled for re-use as water 14 (e.g., hot or warm water) for
forming the
oil sands slurry 18.
Treatment and dewaterinq of thick fine tailings
[0047]The long-term result of treating and dewatering the tailings can be a
permanent aquatic storage structure (PASS) that includes a water cap suitable
for
supporting aquatic life and recreational activities. Techniques are described
to
facilitate the deposition of treated thick fine tailings at a deposition site
that over
time becomes the PASS. In some implementations, the solids separated from
water during the dewatering of the thick fine tailings do not need to be
relocated,
e.g., from a drying area, as can be the case for other known techniques for
dewatering thick fine tailings. Rather, the solids remain in place and form
the basis
of a sedimentary layer of solids at the bottom of the PASS. Previous
techniques
for treating tailings are known to use polymer flocculation for dewatering a
stream
of thick fine tailings. However, the PASS technique additionally provides for
treating the thick fine tailings to provide chemical immobilization of CoCs
that would
otherwise remain in or transfer into the water, such that the water layer that
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14
inherently forms over the solid, sedimentary layer has CoCs removed allowing
for
the water cap to be of such a quality it can support aquatic life. Although
the size
of a PASS can vary, in some implementations the PASS can contain a volume of
100,000,000 to 300,000,000 cubic metres and can be approximately 100 metres
deep at its greatest depth. With a PASS of this scale, flocculated material
from the
treated thick fine tailings can be directly deposited onto a sub-aerial
deposition
area that is proximate and/or forms part of the PASS footprint. Within a
relatively
short period of time following closure of a mine that is reclamation of the
tailings is
complete. That is, the solids feeding treated thick fine tailings into the
PASS, e.g.,
years, are contained in the base of the PASS and CoCs are immobilized within
the solid layer. The water cap is of a quality to support aquatic life and/or
recreational activities.
[00481For example, in the context of oil sands mature fine tailings (MFT) that
include CoCs such as dissolved metals, metalloids and/or non-metals,
naphthenic
acids and bitumen, the chemical immobilization can include the addition of
compounds enabling the insolubilization of the metals, metalloids and/or non-
metals, as well as naphthenic acids, in addition to chemical bridging of
bitumen
droplets with suspended clays. The MET can also be subjected to polymer
flocculation, which can include the addition of a polymer flocculant solution
followed by pipeline conditioning. The MET that has been subjected to
immobilization and flocculation can then be dewatered. The dewatering can be
performed by supplying the flocculated tailings material to a dewatering
device
and/or a sub-aerial deposition site. While MET derived from oil sands
extraction
operations will be discussed and referred to in herein, it should be noted
that
various other contaminant-containing tailings and slurry streams can be
treated
using techniques described herein. For example, froth treatment tailings or
mature
fine tailings extracted from a froth treatment tailings pond can be treated
using
techniques described herein.
[004911t should be noted that the term "constituents" in the expression
"constituents-of-concern" (CoC) can be considered to include or correspond to
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15
substances that are considered as "contaminants" by certain institutions,
regulatory bodies, or other organizations, which can vary by jurisdiction and
by
evaluation criteria.
[0050]In some implementations, subjecting the thick fine tailings to chemical
immobilization and polymer flocculation facilitates production of a
reclamation-
ready material, which can enable disposing of the material as part of a
permanent
aquatic storage structure (PASS).
[0051 'Tailings are left over material derived from a bitumen extraction
process.
Many different types of tailings can be treated using one or more of the
techniques
described herein. In some implementations, the techniques described herein can
be used for "thick fine tailings", where thick fine tailings mainly include
water and
fines. The fines are small solid particulates having various sizes up to about
44
microns. The thick fine tailings have a solids content with a fines portion
sufficiently
high such that the fines tend to remain in suspension in the water and the
material
has slow consolidation rates. More particularly, the thick fine tailings can
have a
ratio of coarse particles to the fines that is less than or equal to one. The
thick fine
tailings have a fines content sufficiently high such that polymer flocculation
of the
fines and conditioning of the flocculated material can achieve a two-phase
material
where release water can flow through and away from the flocs. For example,
thick
fine tailings can have a solids content between 10 wt% and 45 wt%, and a fines
content of at least 50 wt% on a total solids basis, giving the material a
relatively
low sand or coarse solids content. The thick fine tailings can be retrieved
from a
tailings pond, for example, and can include what is commonly referred to as
"mature fine tailings" (MFT).
[0052]MFT refers to a tailings fluid that typically forms as a layer in a
tailings pond
and contains water and an elevated content of fine solids that display
relatively
slow settling rates. For example, when whole tailings (which include coarse
solid
material, fine solids, and water) or thin fine tailings (which include a
relatively low
content of fine solids and a high water content) are supplied to a tailings
pond, the
CA 2983961 2017-10-27
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tailings separate by gravity into different layers over time. The bottom layer
is
predominantly coarse material, such as sand, and the top layer is
predominantly
water. The middle layer is relatively sand depleted, but still has a fair
amount of
fine solids suspended in the aqueous phase. This middle layer is often
referred to
as MFT. MFT can be formed from various different types of mine tailings that
are
derived from the processing of different types of mined ore. While the
formation of
MFT typically takes a fair amount of time (e.g., between 1 and 3 years under
gravity
settling conditions in the pond) when derived from certain whole tailings
supplied
from an extraction operation, it should be noted that MFT and MFT-like
materials
can be formed more rapidly depending on the composition and post-extraction
processing of the tailings, which can include thickening or other separation
steps
that can remove a certain amount of coarse solids and/or water prior to
supplying
the processed tailings to the tailings pond.
[0053]In one implementation, the thick fine tailings are first subjected to
chemical
immobilization, followed by polymer flocculation, and then dewatering to
produce
a solids-enriched tailings material in which CoCs are immobilized. CoCs can
sometimes be referred to as contaminants in the sense that the presence of
certain
constituents can be undesirable for various reasons at certain concentrations,
within certain matrices, and/or in certain chemical forms.
Chemical immobilization
[00541-Thick fine tailings can include a number of CoCs depending on the
nature
of the mined ore and processing techniques used to extract valuable compounds
from the ore. Thick fine tailings can include dissolved CoCs, dispersed CoCs
that
are immiscible in water, as well as fine suspended solids.
[0055]For example, thick fine tailings derived from oil sands mining can
include
metals (e.g., heavy metals), polyatomic non-metals (e.g., selenium),
metalloids
(e.g., arsenic), surfactants (e.g., naphthenic acids), residual bitumen, as
well as
other CoCs. The CoCs can exist in various forms and as part of various
compounds in the tailings material. In order to reclaim the thick fine
tailings, the
CA 2983961 2017-10-27
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CoCs can be treated so that the eventual landform that includes the treated
tailings
meets regulatory requirements.
[0056]In some implementations, a process for treating thick fine tailings
includes
immobilization of bitumen; removal of toxicity due to surfactants, metals, non-
metals and/or metalloids; and polymer flocculation of the slurry material to
reduce
hydraulic conductivity of the resultant treated fine tailings landform.
[0057]In some implementations, the thick fine tailings can be treated with an
immobilization chemical, which can include multivalent cations (e.g.,
trivalent or
divalent). The multivalent cation can be added as part of an inorganic salt.
The
multivalent salts can be added to the thick fine tailings pre-dissolved in an
aqueous
solution, which can be acidic or neutral for example. Various multivalent
inorganic
salts can be used as immobilization chemicals. For example, aluminum sulphate
(e.g., in acid solution which can be sulfuric acid), aluminum potassium
sulphate,
iron sulphate, or chloride or hydrated calcium sulphate (gypsum) can be used
for
chemical immobilization of certain CoCs. For example, the trivalent cation
Fe3+ can
be added as part of iron (Ill) sulphate Fe2(SO4)3. Addition of ferric sulphate
to the
thick fine tailings can provide certain advantages, such as lower potential
H2S
emissions.
[0058]The multivalent cation added to thick fine tailings can perform various
functions. One function is that the multivalent cation can form a cation
bridge
between negatively charged bitumen droplets and negatively charged clay
particles in the fine tailings. This bitumen droplet bridging can help
immobilize the
bitumen within the solids-enriched material that is formed after dewatering of
the
treated tailings. Chemical bridging of bitumen droplets with clays can
decrease the
potential for gas bubbles to adsorb onto bitumen and migrate out of the solids-
enriched material; or chemical bridging of bitumen droplets with clays can
increase
the density and viscosity of the bitumen droplet and prevent upward migration
in
the deposit through buoyancy effects as the deposit densifies. Thus, the
bitumen
CA 2983961 2017-10-27
18
can remain immobilized within the solid material and thus inhibiting its
migration
into adjacent water regions.
[0059]Another function of the multivalent inorganic salt is to insolubilize
certain
CoCs present in the thick fine tailings. For instance, surfactants, metals,
non-
metals, metalloids and other compounds can be present in soluble form in the
water of the fine tailings material. In thick fine tailings derived from oil
sands,
surfactants such as naphthenic acids are considered CoCs in terms of water
toxicity. In addition, compounds such as selenium and arsenic can also be
present
and subject to certain regulatory requirements. The addition of the
multivalent
inorganic salt enables such dissolved CoCs to be precipitated and to remain
insolubilized so that the CoCs cannot re-solubilize. Insolubilization
decreases the
risk of the CoCs migrating out of the solid material or entering the water
column.
[0060]In some implementations, chemical immobilization is performed with
addition of a coagulant that destabilizes particles in the thick fine tailings
through
double-layer compression and modifies the pore water chemistry. In this sense,
the immobilization chemical can include or be a coagulant for coagulating CoCs
from the thick fine tailings to form coagulated CoCs. The coagulant can
include a
multivalent inorganic salt as described above and can include other various
conventional coagulant species. Chemical immobilization by addition of the
coagulant to the thick fine tailings can be performed before, during or after
flocculation as will be further described in relation to Figures 4a to 4e,
although
pre-addition can be a preferred mode of operation in many cases.
[0061]Certain chemicals referred to herein can be known as coagulants in the
field
of water treatment and can therefore can be referred to as "coagulants" in the
present application. However, it should be noted that such chemicals are used
herein for the purpose of immobilization in PASS techniques rather than mere
coagulation as would be understood in the water treatment industry, for
example.
In this sense, the terms "coagulant" and "immobilization chemical" can be used
interchangeably as long as the coagulant performs the function of
immobilization
CA 2983961 2017-10-27
19
as described in the present application. It should still be noted that certain
immobilization chemicals described herein can or cannot perform the function
of
coagulation. In some implementations, the so-called coagulant is added to the
fine
tailings in quantities superior to what is known in the water treatment
industry for
coagulation, e.g., superior to 350 ppm, which is used for purpose of mere
coagulation rather than immobilization. It is noted that in many cases the
immobilization chemical that is added will in effect cause some or substantial
coagulation. It is also noted that immobilization chemicals that generally do
not
cause coagulation can be used in conjunction with a separate coagulant
chemical
that provides coagulation effects.
Immobilization chemical addition and mixing into thick fine tailings
(0062] When the immobilization chemical is added upstream prior to
flocculation,
certain features of the immobilization chemical injection and the subsequent
mixing
can be provided for enhancing the pre-treatment (e.g., pre-coagulation) prior
to
flocculation. For example, the immobilization chemical injector, subsequent
mixers, as well as pipeline length and diameter leading up to the flocculant
injector
can be designed and provided to ensure a desired immobilization chemical
mixing
and coagulation time to facilitate benefits of pre-coagulation. In some
scenarios,
the immobilization chemical injector can be an in-line addition unit, such as
a T or
Y pipe junction, and at least one static mixer can be provided downstream of
the
immobilization chemical injector. It should nevertheless be noted that the
immobilization chemical injector can take other forms and have alternative
constructions for adding the immobilization chemical. For example, the
immobilization chemical injector can be configured for injecting an
immobilization
chemical solution that includes immobilization chemical species in solution
(e.g.,
in an aqueous acid-containing solution), and can thus be adapted for liquid-
phase
injection of the immobilization chemical solution into an in-line flow of the
thick fine
tailings. Alternatively, certain immobilization chemicals can be added in dry
form
(e.g., powders) and the immobilization chemical addition unit can in such
cases be
CA 2983961 2017-10-27
20
designed for dry addition. The immobilization chemical addition unit can
include an
in-line dynamic mixer (e.g., paddle mixer type) or other types of mixer units.
[0063]In some implementations, immobilization chemical dosage can be
determined based on various factors, including properties of the thick fine
tailings
and the configuration of the immobilization chemical addition unit and
subsequent
mixer devices that can be present. For example, in some implementations, the
immobilization chemical can be added as an immobilization chemical solution by
in-line addition into the in-line flow of the thick fine tailings followed
immediately by
a mixer, such as a static mixer. Immobilization chemical dosage can be
determined
and provided based on the solids content and/or density of the thick fine
tailings
as well as the given mixer design (e.g., number and type of static mixers).
For
example, the mixer effects can be pre-determined in terms of the shear
imparted
to the immobilization chemical-tailings mixture, which can depend on thick
fine
tailings properties and other operating parameters, such as flow rate and
temperature of the fluid.
[00641Immobilization chemical dosage determination can take various forms. For
example, given a particular thick fine tailings density and a given mixer
design, a
range of effective immobilization chemical dosages can be determined along
with
an optimal immobilization chemical dose. Such determinations can be based in
laboratory experiments (e.g., using batch mixers units, such as stirred
vessels)
and/or small scale pilot experiments (e.g., small continuous in-line addition
and
mixing units). In addition, immobilization chemical dispersion targets for
dispersing
the immobilization chemical upon addition into the thick fine tailings can be
determined and used to provide an appropriate pipe length and diameter leading
up to the immobilization chemical injector to ensure turbulent flow of the
tailings at
the immobilization chemical injector. For example, target dispersion shear
rates
can be tested on laboratory and/or small-scale units, and the pipeline leading
to
the immobilization chemical injector as well as the operating conditions
(e.g., flow
rate) for larger scale operations can be determined accordingly. For example,
should a certain Reynolds Number (Re) of the thick fine tailings flow be
targeted
CA 2983961 2017-10-27
21
for immobilization chemical addition, the pipeline diameter and flow rate can
be
provided to ensure a minimum turbulence level based on density and viscosity
of
the thick fine tailings to be treated. Once the system is operational and the
pipeline
diameter is fixed, the minimum turbulence level can be achieved by controlling
certain operating variables, such as flow rate (e.g., regulated by an upstream
pump) and potentially the density and/or viscosity of the thick fine tailings
(e.g.,
regulated by dilution or heating).
[0065]In some scenarios, immobilization chemical dosage and dispersion
requirements can be determined in part or primarily based on thick fine
tailings
density and a given mixer design. It should also be noted that other methods
can
be used to design the system for immobilization chemical addition, dispersion
and
subsequent transportation to the flocculation step. In some implementations,
the
flow regime of the thick fine tailings is turbulent at the immobilization
chemical
addition point and a static mixer is provided just downstream of the
immobilization
chemical addition point to produce a thoroughly mixed coagulating material
(which
can also be referred to as a pre-treated material in general as coagulation
can or
cannot be present), which is then transported via pipeline toward the
flocculation
step.
[0066]Pipeline design, flow rate control and determining properties of the
thick fine
tailings can be used to achieve a first turbulent flow regime at the
immobilization
chemical addition point, while mixer design downstream of the immobilization
chemical addition point can be used to achieve a second turbulent flow regime
at
that point in the process. The first and second turbulent flow regimes can
have
different minimum target thresholds or target ranges.
[0067]It should also be noted that a single immobilization chemical addition
point
or multiple immobilization chemical addition points can be used. Each
immobilization chemical addition point can have a subsequent mixer
arrangement,
and the dosage at each addition point can be determined based on the
properties
of the incoming tailing stream as well as the downstream mixer design.
CA 2983961 2017-10-27
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Pipelining pre-treated thick fine tailings to flocculation
[0068]After addition of the immobilization chemical, a series of kinetics-
limiting
reactions occurs between the immobilization chemical and components of the
thick
fine tailings. In some implementations, these reactions result in pH and
rheology
changes in the coagulating thick fine tailings (which can also be referred to
as the
pre-treated TFT) during pipeline transportation. It should be noted that the
changes
in pH and rheology can further affect the subsequent process steps, in
particular
the flocculation stage. Impacts of the mixing intensity on pH and rheology are
further discussed below and also described in the experimentation section.
[0069]In terms of pH, when the immobilization chemical is a basic compound
that
is added as part of an acid-containing solution (e.g., alum in a sulfuric acid
solution), the pH of the resulting immobilization chemical-tailings mixture
can show
an initial decrease followed by an increase as the mixture buffers back to a
higher
pH. Other tests have shown pH can go down as low as 4.5 or 5 after addition of
an immobilization chemical acidic solution.
[0070]In some implementations, the pipeline that transports the coagulating
material to flocculation can be configured and operated to impart at least a
target
pipe-mixing level to the coagulating material prior to flocculation. For
example, the
pipeline can be provided with sufficient length and diameter to impart pipe-
shear
mixing so that the pH of the material has bounced back to a minimum target
value
or within a target range. The target pH bounce-back value can be, for example,
the
initial pH of the thick fine tailings or a desired pH based on optimal
activity of the
flocculent. In some scenarios, the target pH bounce-back value can be between
7.5 and 8.5. The target pH bounce-back value can also be based on the lowest
pH
that is obtained, e.g., a pH increase of 5%, 10%, 15%, 20%, 25%, 35%, 45%,
55%,
65%, 75%, 85%, 95% or higher based on the lowest pH value that is obtained
from
the initial decrease after immobilization chemical addition.
[0071]In addition, the pipeline transporting the coagulating material can be
configured in terms of mixing intensity and/or total mixing energy imparted to
the
CA 2983961 2017-10-27
23
material. For example, higher mixing intensities can result in a more rapid pH
decrease followed by a more rapid pH increase. Thus, the flow rate and
pipeline
diameter, which can impact mixing intensity, can be considered in addition to
the
pipeline length in order to provide the dimensions and conditions to impart
adequate mixing energy over an adequate time scale to achieve the target pH
bounce back values when the coagulated material reaches the flocculant
injector.
[0072]Furthermore, properties of the thick fine tailings (e.g., clay-to-water
ratio
CWR) can also be measured and used to configure the pipeline transporting the
coagulating material. Lower CWR can at some mixing intensities result in more
rapid pH decrease and bounce-back, notably at the tested 100 RPM mixing
intensity where the pH changes for 0.2 CWR were faster compared to 0.35 CWR.
Thus, CWR or other properties (e.g., density) of the thick fine tailings can
be used
to determine desired pipeline configurations and dimensions to achieve target
pH
bounce back values.
[0073]In some implementations, when the coagulating material is subjected to
pipeline transportation and pipe-shear based mixing certain rheological
changes
can occur. For example, pipeline mixing can be performed for a sufficient time
and
under shear conditions that cause the coagulating material to reach a post gel-
stage state, which can reduce polymer flocculant dosage in the subsequent
step.
More particularly, the pipeline mixing can be conducted to cause the
coagulating
material to increase in yield strength and reach a generally gel-like state
having
gel-like properties, and then the pipeline mixing can be continued so that
this gel-
like material returns to an ungelled state having slurry-like fluid
properties. In this
manner, the pipeline mixing can be conducted to ensure adequate progression of
the coagulation/immobilization reactions between the immobilization chemical
and
components of the thick fine tailings while avoiding the difficulties that
would occur
if the flocculant were mixed with a gelled, high yield strength material. In
this
regard, it should be noted that gel-like materials have higher yield strength
and
would be more difficult to mix with the flocculant. Therefore, adding the
flocculant
to the coagulated slurry after the gel-like material has been "broken" and the
yield
CA 2983961 2017-10-27
24
strength has decreased significantly, can facilitate rapid and thorough mixing
of
the flocculent and reduced flocculent dosage requirements. Imparting
sufficient
pipeline shear energy to the coagulating material can be done to achieve such
a
post gel-stage material prior to flocculation. Shear intensity and duration as
well as
total mixing energy can be assessed in order to provide a pipeline
configuration
and operating conditions (e.g., pipeline diameter and length, flow rates,
etc.) which
can also be based on properties of the material (e.g., density, CWR,
viscosity, yield
strength, etc.).
[0074]In some implementations, the pipeline mixing of the coagulating material
can also be provided to ensure a turbulent flow regime or a target turbulence
level
of the coagulated slurry at the flocculent addition point. The coagulating
material
can thus have different flow regime properties along the pipeline due to its
changing properties. The pipeline diameter and length as well as the flow rate
can
be provided such that the thick fine tailings have turbulent flow regimes at
the
immobilization chemical addition point and at the flocculent addition point
while the
flow regime of the coagulating material at certain points in between these two
addition points can be non-turbulent or laminar. In order to provide such flow
regime properties, a number of factors can be manipulated including flow
rates,
pipe sizes (length and diameters), immobilization chemical mixer type and
operation, immobilization chemical dosage, and incoming thick fine tailings
properties (e.g., viscosity or density, which can be manipulated by pre-
dilution, for
example).
[0075]It should be noted that different flow regimes can be used upon
injection of
the immobilization chemical and/or flocculent depending on the mixing
requirements of the corresponding injected chemical at the initial mixing
state.
Laminar flow regime can be therefore used for initial mixing upon injection of
certain chemicals.
[0076]In an in-line system, it should be noted that timing of the flocculent
injection
is related to the distance between the immobilization chemical and flocculent
CA 2983961 2017-10-27
25
injection points. The distance between those injection points can also be
characterized by the mixing of the pre-treated fine tailings between the
immobilization chemical and flocculent injection points, in terms of intensity
and
time. Thus, mixing time and mixing distance can both be used to assess the
impact
of mixing on the coagulating material and the flocculent addition point. As
mentioned above, the immobilization chemical pipeline mixing and the
flocculent
injection point can be provided such that the flocculent is added once the
coagulated material has left a gel-stage and/or experienced pH bounce back. In
another example, injecting the polymer flocculent downstream of the
immobilization chemical injection point such that pipeline mixing is within a
critical
mixing range can facilitate enhanced flocculation. Critical mixing ranges can
be
determined for open-pipe configurations by using various empirical and/or
computational methods. In addition, in dynamic paddle mixers it has been found
that the optimum polymer flocculent dosage decreases as the critical mixing
constant (K) increases (e.g., (KO of 20 to 12,000). Kc values determined for
batch
or in-line stirred tank impeller vessels may be used to help predict critical
mixing
ranges for in-line open-pipe operations, and Camp Number-based scaling
methods can be used.
[0077]In some implementations, pre-shearing is performed to enhance uniform
shearing within the coagulated tailings before injection of the flocculent. In
addition,
one or more in-line high-shear static mixer(s) (or other in-line shear
devices) can
be used to enhance or ensure mixing of the core of the coagulated tailings
within
the pipe to further reduce the yield stress within the pipe.
[0078]In some implementations, the coagulating material is subjected to
sufficient
mixing (e.g., pipeline shear mixing) to reach a generally stable yield stress
plateau
after descending from a crest in terms of its yield stress properties. In some
scenarios, the mixing is conducted to reach a target yield stress value or
range or
to reach a target yield stress reduction based on the maximum or average crest
value of yield stress (e.g., 30% to 80%, 40% to 70%, or 50% to 60% reduction
of
the maximum or average crest value). For example, with alum dosage of 1800 ppm
CA 2983961 2017-10-27
26
the maximum yield stress is about 25 Pa which decreases to a plateau value of
about 10 Pa to 12 Pa which represents a reduction of 52% to 60% of the
maximum.
[0079]It should be noted that certain polymer flocculants can be sensitive to
pH
and rheology variation. Consequently, both polymer flocculant consumption and
deposit performance can be impacted by the polymer flocculant injection
location
downstream of the immobilization chemical injection location. In some
implementations, timing of the flocculant injection can be enhanced based on
properties including yield strength and/or pH of the pre-treated thick fine
tailings
that is subjected to flocculation. Certain enhancement techniques and details
related thereto will also be discussed in the experimentation section. It
should also
be noted that the pipeline transporting the coagulating material can have
various
arrangements, including a single pipeline composed of a series of pipe
sections or
a pipeline network that includes a splitter leading into multiple parallel
pipelines
that can rejoined into a single pipeline prior to flocculation. Such pipeline
networks
can be configured to increase pipeline shear imparted to the material, and can
also
be controlled and operated to impart different levels of shear to the material
when
desired. It is also noted that the pipeline can include one or more shear
devices
(e.g., static mixer) arranged along its length to impart part of the desired
shear to
the material, and such shear devices and pipeline can be arranged so that the
material can either pass through or bypass the shear devices.
[0080]Thus, various pipeline configurations can be provided in order to
produce a
pre-treated coagulated material that is ready for flocculation. For example,
mixing
intensity, mixing time, pipeline length and diameter, immobilization chemical
dosage, yield stress of the material, and flow rate are relevant
interconnected
factors that can be managed to produce the pre-treated coagulated material
having
target pH, yield stress and flow regime characteristics at the flocculation
point. For
in-line systems that include a simple pipeline from the immobilization
chemical
mixer to the flocculant injector, pipeline length and diameter can be designed
in
view of flow rate and tailings properties (notably density) in order to impart
pipe
shear energy in an intensity and over a time period that enable the target pH,
yield
stress and flow regime characteristics.
CA 2983961 2017-10-27
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[0081]This pipeline can have a single diameter along most or all of its
length, or it
can have different diameters at particular locations along its length to
achieve
desired effects at certain locations. For example, the pipeline can include a
pipe
section proximate the immobilization chemical addition point with a first,
relatively
small diameter to impart higher shear rates (i.e., higher shear intensity) to
cause a
sharp pH reduction and/or a sharp yield stress increase at that upstream
location.
The pipeline can also include a subsequent intermediate pipe section that has
a
second, larger diameter and a pipe length that provide a desired shear energy
and
residence time for the coagulating material. This intermediate pipe section
can be
configured to impart a desired mixing energy and intensity to achieve the
desired
pH and yield stress characteristics, but is not necessarily concerned in a
direct
manner with turbulence or flow regime. Next, the pipeline can include a
downstream pipe section that feeds into the flocculant injector, and this
downstream pipe section can have a third, smaller diameter to ensure
turbulence
as the material contacts the flocculant. This downstream pipe section could be
relatively short in length as it simply has to ramp up the turbulence of the
material
to a desired level prior to flocculant addition and is not necessarily
designed for
imparting a given amount of energy for the pH or yield stress evolution.
Various
other pipeline configurations are also possible for achieving desired pH,
yield
stress and flow regime characteristics. For example, alternatively, pipe
section can
be increased to ensure laminar flow.
Flocculation
[00821A polymer flocculant can be added to the fine tailings in order to
flocculate
suspended solids and facilitate separation of the water from the flocculated
solids.
The polymer flocculant can be selected for the given type of fine tailings to
be
treated and also based on other criteria. In the case of oil sands MFT, the
polymer
flocculant can be a medium charge (e.g., 30%) high molecular weight anionic
polymer. The polymer flocculant can be a polyacrylamide-based polymer, such as
a polyacrylamide-polyacrylate co-polymer. The polymer flocculant can have
CA 2983961 2017-10-27
28
various structural and functional features, such as a branched structure,
shear-
resilience, water-release responsiveness to fast-slow mixing, and so on.
[0083]It should be noted that polymer flocculant is not limited to a medium
charge,
as altering the pH can influence the charge requirements. In some
implementations, the polymer flocculant charge is selected in accordance with
pH.
[0084]In some implementations, the overall flocculation and dewatering
operations
can include various techniques described in Canadian patent application No.
2,701,317; Canadian patent application No. 2,820,259; Canadian patent
application No. 2,820,324; Canadian patent application No. 2,820,660; Canadian
patent application No. 2,820,252; Canadian patent application No. 2,820,267;
Canadian patent application No. 2,772,053; and/or Canadian patent application
No. 2,705,055. Such techniques¨including those related to flocculant
selection;
rapid dispersion; pipeline flocculation and water-release condoning; Camp
Number-based design and operation; injector design and operation; sub-aerial
deposition and handling; pre-shearing; pre-thinning; and pre-screening¨can be
used or adapted for use with techniques described herein related to chemical
immobilization, polymer flocculation and dewatering. It should also be noted
that
various techniques described in such documents can be adapted when included
with techniques described in the present application, such as chemical
immobilization and coagulation as well as post-flocculation handling,
discharging
and management.
[0085]In some implementations, the polymer flocculant is added as part of an
aqueous solution. Alternatively, the polymer flocculant can be added as a
powder,
a dispersion, an emulsion, or an inverse emulsion. Introducing the polymer
flocculant as part of a liquid stream can facilitate rapid dispersion and
mixing of the
flocculant into the thick fine tailings.
[0086]In some implementations, the polymer flocculant can be injected into the
pre-treated thick fine tailings using a polymer flocculant injector. For
example,
static injectors and/or dynamic injectors can be used to perform flocculant
addition.
CA 2983961 2019-06-11
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The injection can be performed in-line, that is, into the pipeline for
example. A
length of the pipeline downstream of the flocculant injection point can be
dedicated
to dispersion of the polymer flocculent into the pre-treated thick fine
tailings,
thereby producing treated thick fine tailings that is ready for conditioning
and
eventual dewatering.
[00871As mentioned further above, the incoming pre-treated thick fine tailings
that
has been subjected to coagulation can arrive at the flocculent injector with
certain
pH, yield stress, and flow regime characteristics that facilitate flocculant
dispersion,
mixing and reaction with suspended solids.
[0088]Immediately after flocculent injection (e.g., via a co-annular injector
where
flocculant inlets are spaced away from the pipe side wall and are distributed
around
an annular ring through which the pre-treated tailings flow), there can be a
dispersion pipe section that receives the flocculating material and imparts
pipe
shear energy to the material. The dispersion pipe length as well as polymer
flocculant dosage can be provided based on various factors, which can include
the
density and/or clay content of the thick fine tailings as well as the
flocculent injector
design. In some scenarios, for a given injector design and density of the
thick fine
tailings, optimum ranges of polymer flocculant dosage and dispersion pipe
length
can be determined, particularly when the target pH, yield stress, and flow
regime
characteristics have been provided. More regarding process modelling will be
discussed in further detail in the experimentation section below.
Pipeline conditioning and transport after flocculation
[008911n some implementations, the process includes pipeline conditioning of
the
treated thick fine tailings after flocculent addition and dispersion. The
pipeline
conditioning can notably be adapted to the type of dewatering, deposition and
disposal that will be conducted (e.g., ex situ dewatering devices, sub-aerial
deposition in thin lifts, or discharging into a pit to form a permanent
aquatic storage
structure (PASS), as will be discussed in greater detail below). For
dewatering by
sub-aerial deposition in thin lifts, the pipeline conditioning can be
conducted to
increase the yield stress of the flocculated material to a crest or maximum
where
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the material presents gel-like characteristics, and then reduce the yield
stress and
effect floc breakdown to form a flocculated material in a water release zone
yet still
having relatively large flocs. For dewatering within a PASS, the pipeline
conditioning can be modified such that the floc breakdown reduces the flocs to
smaller sizes that provide settling time and settled volume characteristics
for
formation of the PASS. The floc size for thin lift dewatering can be provided
to
promote rapid initial water release a separation from the flocculated solids,
while
the floc size for the PASS implementation can be provided to promote both fast
settling time and small settled volumes. For example, the target floc size for
dewatering by sub-aerial deposition in thin lifts can be greater than about
100 pm,
about 150 pm, about 200 pm, or about 250 pm; while the target floc size for
dewatering via the PASS implementation can be between about 50 pm and about
200 pm, between about 50 pm and about 150 pm, or between about 75 pm and
about 125 pm. The target floc size can be treated as an average floc size for
process control and measurement. The floc size for the PASS implementation can
be provided in order to balance competing effects of settling speed and
settled
volumes, which will depend on the starting CWR of the thick fine tailings, in
order
to achieve a CWR of at least 0.65 within one year after discharge into the
PASS
containment structure. The target floc size depends on polymer dosage of the
thick
fine tailings, regardless of the starting CWR. For example, with a starting
CWR of
about 0.1, the target floc size can be provided to achieve above 80% volume
reduction within one year of discharge, whereas with a starting CWR of about
0.4,
the target floc size can be provided to achieve above 32% volume reduction
within
one year of discharge.
[0090]Floc size reduction can be achieved by subjecting the treated thick fine
tailings to pipeline shear sufficient to break down larger flocs to form
smaller flocs
while avoiding over-shearing the material where the flocs would be
substantially
broken down and the material would generally return to its initial slow
settling
characteristics. The pipeline shear can include high shear rates and/or
sufficiently
small pipe diameters in the conditioning section. The conditioning pipeline
can be
configured and implemented based on pre-determined target values for shear
CA 2983961 2017-10-27
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rates and total shear energy to impart to the material, based for example on
empirical and/or modelling information. It should also be noted that the
system can
include monitoring equipment for measuring the approximate floc size (e.g., in-
line,
at-line or off-line) so that the conditioning pipeline can be adapted and/or
regulated
based on the measured floc size to provide the shear necessary to be within a
target floc size range.
[0091]In some implementations, the conditioning pipeline terminates at a
discharge point where the treated thick fine tailings are supplied to the
dewate ring
device or site. In alternative implementations, the conditioning pipeline
feeds into
a conveyance pipeline that transports the treated thick fine tailings to the
discharge
location under reduced shear conditions. The conditioning and conveyance
pipelines can be configured together to provide a target total shear energy to
the
material prior to deposition as well as high initial shear (i.e., in the
conditioning
pipeline) followed by lower shear (i.e., in the conveyance pipeline).
[0092]In some implementations, the total shear energy imparted to the treated
thick fine tailings prior to discharge is sufficiently high to reach the
target floc
breakdown and yet within a range to facilitate water clarity and settling
characteristics within the PASS. For example, it was found that, at optimum
polymer dosage an average shear rate within150 s-1 for 30 minutes could be
imparted after flocculant addition to coagulated thick fine tailings. Based on
this
value, a conditioning and conveyance pipelines can be designed and implemented
to operate within this envelope. More regarding conveyance will be discussed
below.
[0093] Water separation from the flocs within the PASS can include several
physical mechanisms. Settlement can be understood as volume reduction of the
flocculated material, such that settlement is obtained by settling,
consolidation and
other volume reduction mechanisms. For example, during water separation,
settling mechanisms where solid flocs and grains fall downward through the
liquid
phase can evolve into consolidation mechanisms. Modeling settlement within the
CA 2983961 2017-10-27
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PASS can combine various input data including settling data, consolidation
data
and other water-separation data.
Conveyance and discharge of treated thick fine tailings
[0094]As mentioned above, the system can include a conveyance pipeline that is
sized and configured for imparting a reduced or minimum shear to the material
from the conditioning pipeline until discharge. This can be particularly
advantageous when the distance from the flocculent injector to the discharge
point
is substantial or sufficiently great such that simple continuation of the
conditioning
pipeline would impart excess shear and risk over-shearing the material prior
to
discharge. The conveyance pipeline can be provided to have a larger diameter
compared to the conditioning pipeline in order to reduce shear during this
transportation step. Alternatively, the conveyance step can include other
methods
or systems that do not necessarily involve increasing pipe diameter, such as
splitting the flow of treated thick fine tailings coming from a single
conditioning
pipeline into multiple conveyance lines and operating the conveyance lines at
reduced flow rates, thereby reducing shear imparted to the material prior to
discharge.
[0095]Flow rate and pipe diameter can be controlled in tandem in order to
reduce
the shear sufficiently to substantially maintain the floc size during
conveyance (i.e.,
from conditioning to discharge). In some scenarios, the floc size change
during
conveyance is kept within 150 pm while keeping the floc size within 50 pm to
200
pm. Thus, if the initial floc size prior to conveyance is at the maximum
target size
of 200 pm, then the maximum floc size change should be 150 pm such that the
floc size upon discharge is at least 50 pm. If the initial floc size is
smaller than 200
pm, then the maximum floc size change should be kept at a lower level to
ensure
a minimum floc size of 50 pm upon discharge. Alternatively, when the initial
floc
size prior to conveyance is above 200 pm, then the floc size change can be
greater
than 150 pm. In general, the floc size prior to conveyance and after
conveyance
can be targeted and the process conditions (e.g, shear conditions) can be
managed such that the floc size upon discharge is within the desired range.
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[0096] In a first implementation the treated thick fine tailings is discharged
into the
containment structure of the PASS directly after the pipeline conditioning
stage.
The discharge section of the pipeline is in direct fluid communication with
the
conditioning section of the pipeline. In this dewatering scenario, the in-line
injection
of the immobilization chemical (e.g., coagulant) and the flocculant can be
located
on a buttress, upstream of the conditioning pipeline which can be provided
sloping
down from the buttress toward the discharge location. In this scenario, the
chemical injection assets (e.g., immobilization chemical injector and
flocculant
injector) can have to be relocated repeatedly as the level of the PASS rises
with
time, e.g., to maintain the slope of the conditioning section of the pipeline.
The
treated thick fine tailings are then discharged into the pit of the PASS to
allow the
flocs to settle and the water to separate and form an upper layer, thereby
forming
the water cap. Without a conveyance pipeline there can be certain challenges
and
constraints in terms of operation and relocation of the chemical injection
units.
[0097] In a second implementation, the treated thick fine tailings are
conveyed to
the discharge location after the pipeline conditioning stage. The pipeline
geometry
can be adapted to include a conveyance pipe section or arrangement, which is
in
fluid communication with the conditioning pipeline. In addition, the chemical
injection assets can be provided in a central location that would not require
relocation as the level of the PASS rises, as opposed to the first
implementation.
In addition, the conditioning section of the pipeline can also be located off
the
buttress, which can enhance accessibility and operational aspects of that
step. The
conditioning can be performed to condition the flocs and the treated thick
fine
tailings to a state where continuing pipeline shear would not have a
significant or
beneficial impact on the terminal floc sizes or settling behavior of the
discharged
material in the PASS. The flocculated and conditioned thick fine tailings can
then
be sent to the discharge section of the pipeline, via the conveyance section.
The
conveyance section of the pipeline can be located on a sloped ramp or
earthwork
to facilitate distribution to the discharge section. The presence of a
conveyance
section therefore facilitates efficient relocation of system assets over time
(e.g., as
only conveyance and discharge assets can have to be relocated) as well as
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centralization of chemical injection units in more suitable locations for
operation,
maintenance, chemical supply, and so on. The conveyance system facilitates
stable operation of the chemical addition and conditioning steps for reliable
production of treated thick fine tailings with desired characteristics, while
the low-
shear conveyance system provides enhanced adaptability and flexibility for
transporting ready-to-deposit material to a variety of different discharge
points
operating at any given time and different discharge points that can change
location
over time.
[0098]In terms of the conveyance method, in an in situ or ex situ dewatering
case,
conveyance of the flocculated and conditioned thick fine tailings can be
controlled
to maintain the floc size at an optimal value or within an optimal range for
dewatering until deposition into the containment structure of the PASS. For
example, lengths and diameters of the pipes can be chosen in accordance with
various parameters including the distance to the discharge section and the
attrition
resistance of the flocs from the treated fine tailings. In addition, the
conveyance
pipes can be configured, positioned and operated such that no additional
pumping
is required to transport the material to the discharge locations. For example,
the
conveyance pipes can be positioned on a sloped section of the PASS containment
structure having an inclination sufficient for the material to flow under
gravity and
remaining head provided by upstream pumps to the discharge locations.
[0099]In terms of discharge methods, in an in situ dewatering case, the
treated
thick fine tailings can be discharged continuously into the subaerial pit over
a
relatively long period of time (e.g., rise rate of about 20 meters per year)
with the
release water coming to the surface and the solids settling to the bottom. The
discharge points can sometimes be submerged in the water or within the
underlying tailings deposit, but the primary discharge method would include
discharging the material onto the top of the fluid and/or onto a solid earth
surface
proximate to the fluid surface. The discharge should be designed and managed
to avoid over-shearing or destroying the flocs in order to facilitate initial
high water
release and good settling rates. Thus, the discharge points should not be
located
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at a significant height above a solid surface which could lead to a high-
energy
impact causing over-shearing.
[0100]In some implementations, floating pipe sections with discharge ends can
be
used to gain access to underutilized areas of discharge. The floating can be
equipped with floating devices or can be supported by other means.
[0101]In an ex situ dewatering case, where the bulk of the water has been
removed prior to deposition, the discharge method can be modified, such as
distributing the discharge to prevent water pooling and modifying the pipe
sections
and discharge ends to accommodate higher-solids material.
[0102]It should also be understood that similar principles can apply to both
the
conveyance section and the discharge section to maintain the floc size in an
optimal range for the desired water release and settling characteristics. For
example, the conveyance section can be designed to include a plurality of
pipes
for splitting the flow of treated fine tailings coming from the conditioning
section.
Similarly, the discharge section can be designed to include a plurality of
pipes for
splitting the flow of treated fine tailings coming from the conditioning
section or the
conveyance section.
Dewaterinq
[0103]As mentioned above, various dewatering techniques described in several
Canadian patent applications can be used in the context of the techniques
described herein. It should be noted that the overall process can include
several
dewatering steps, which will be discussed in greater detail in relation to
Figure 3a
and 3b, for example. In general, dewatering can be done by a solid-liquid
separator
(SLS) or by sub-aerial deposition/discharge. A combination of SLS and sub-
aerial
dewatering can also be performed.
[0104]Various types of SLS's can be used. For example, belt filters and/or
thickeners can be used to separate a solids-depleted water stream from a
solids-
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enriched tailings material, both of which can be subjected to further
processing
after dewatering.
[0105]In the case of dewatering by sub-aerial deposition, various dewatering
mechanisms can be at work depending on the deposition and post-deposition
handling methods that are used. For instance, thin lift deposition can promote
release water flowing away from the deposited material followed by dewatering
by
freeze-thaw, evaporation, and permeation mechanisms. For deposition that is
performed to promote the formation of a much thicker lower stratum of treated
fine
tailings with an upper water cap, the lower stratum can dewater with
consolidation
as a significant dewatering mechanism. More regarding this will be discussed
in
relation to forming and managing the permanent aquatic storage structure
(PASS)
for the fine tailings and CoCs.
Characteristics of PASS landform
[0106]In some implementations, as mentioned above, a permanent aquatic
storage structure (PASS) can be built via in situ and/or ex situ dewatering of
thick
fine tailings that has been subjected to chemical immobilization and
flocculation. A
summary of some characteristics of the PASS landform is provided below.
[0107]The containment structure of the PASS can be a former mine pit, which
can
include various in-pit structural features such as benches and in-pit dykes.
After
mining is complete, preparation of in-pit structures and landforms (e.g.,
dykes,
dumps, temporary dams, pit walls) can be undertaken. Placement of the treated
fine tailings can then begin. The treated fine tailings can be discharged in
various
ways at different stages of forming the PASS. The treated fine tailings can be
discharged within the pit in accordance with tailings management and
reclamation
considerations. During or after placement of the treated fine tailings,
additional
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37
landforms, surface water inlets and outlets, and operational infrastructure
can be
constructed as part of the overall PASS system.
[0108]The PASS can be seen as a type of end pit lake ¨ but how it is formed
and
its target characteristics are different than a conventional end pit lake. For
example,
the discharged fine tailings are pre-treated before depositing into the
landform that
will become the end pit lake, to enhance dewatering and stability of the
landform.
Conventional end pit lakes are formed by placing tailings into the mine pit
(i.e., the
landform), capping with water, and treating the water within the landform. In
an oil
sands application, a conventional end pit lake directly deposits untreated MFT
into
the landform. In contrast, the PASS is formed from pre-treated material such
that
the MFT is dewatered at deposition and the water released from the MFT is pre-
treated to chemically immobilize CoCs in the solids layer formed at the base
of the
PASS. Thus the PASS has several advantages over conventional end pit lakes,
such as more consistent immobilization characteristics throughout the sediment
layer, accelerated dewatering, and mitigation of long-term risks related to
CoCs in
the tailings.
[010911n a PASS, the CoCs are immobilized prior to deposition in the landform.
Fresh water dilution can be used in the aquatic reclamation process, in
addition to
the chemical immobilization of CoCs in the sedimentary layer. Note that fresh
water
dilution, meaning dilution of the already present pre-treated water cap, is
different
than relying on a fresh water cap to overlay fluid fine tails that were
deposited
untreated into the landform (i.e., as in a conventional end pit lake). The
PASS in a
reclaimed state will have no persistent turbidity, no (or negligible) bitumen
in the
water cap and toxicity and metals below guidelines required to support aquatic
life.
By contrast, a conventional end pit lake uses a fresh water cap and microbial
activity as the aquatic reclamation process, and steps are not taken
specifically to
remove bitumen from water released from the fine tailings. A conventional end
pit
lake will have low persistent turbidity.
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Process implementations
[0110]Referring to Figures 3a to 3b, there are two main process
implementations
particularly in terms of the dewatering of the flocculated tailings material.
Figure 3a
illustrates an in situ process where the dewatering includes depositing the
flocculated tailings material onto a dedicated disposal area and optionally
forming
a permanent aquatic storage structure (PASS), while Figure 3b illustrates an
ex
situ process wherein the dewatering includes supplying the flocculated
tailings
material to solid-liquid separator (SLS). The tailings material injected into
the
flotation unit in these two process implementations can be any suitable fine
tailings
stream, including thick fine tailings (e.g., mature fine tailings) or froth
treatment
tailings.
[0111]The processes illustrated in Figures 3a and 3b have several common
elements. The fine tailings 110 can be retrieved from a tailings pond or from
a
treatment operation and supplied to a flotation unit 46. Gas 66 can be
injected into
the flotation unit 46 to generate gas bubbles 68 that aid in the flotation. An
aqueous
underflow 120 is retrieved from the flotation unit 46 and can be supplied by
pipeline
to various processing units. It should be noted that the fine tailings 110 can
be
subjected to various preliminary treatments before or after addition to the
flotation
unit, and before any further treatment. Such preliminary treatment steps may
include at least one of dilution, coarse debris pre-screening, pre-shearing,
thinning
and/or chemical treatments to alter certain chemical properties of the fine
tailings
stream 110. An immobilization chemical 124 is added to the aqueous underflow
120 to produce a pre-treated tailings stream 126. The pre-treated tailings
stream
126 is then combined with a polymer flocculant 128, which can be added in-line
via a co-annular injector. The polymer flocculant 128 can be added so as to
rapidly
disperse into the tailings, forming a flocculating tailings material 130. The
flocculating tailings material 130 can then be subjected to shear conditioning
in
order to develop a flocculated material 132 suitable for dewatering.
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[0112]In some implementations, as illustrated in Figure 3a, the flocculating
tailings
material 130 is subjected to pipeline conditioning 134, which can be the only
conditioning that causes the flocculated material 132 to attain a state in
which
release water readily separates and flows away from the flocs. Alternatively,
other
shear mechanisms can be provided. The flocculated material 132 can then be
dewatered. Figure 3a illustrates a scenario where the dewatering includes
depositing the flocculated material 132 onto a sub-aerial dedicated disposable
area (DDA) 136, which can be a beach or built using earthwork techniques. Each
DDA 136 can have a deposition region that has a sloped base to facilitate
release
water flowing away from the deposited material and promote such rapid
separation
of the release water from the flocs.
[0113]Still referring to Figure 3a, over time the structure and operation of
the DDAs
136 can be managed such that a PASS 138 is formed. The PASS 138 includes
containment structures 140 for containing the material, a water cap 142, and a
solids-rich stratum 144 below a water cap. During formation of the PASS 138,
the
water cap 142 results from the dewatering of the treated material. The release
water separating from the flocs can be the primary source of water for the
water
cap 142 such that the quality of the water in the water cap is directly
related to the
immobilization of CoCs. It is also possible to add fresh water 137 or another
source
of water into the PASS as it is forming such that the water cap includes water
from
sources other than the pore water of the tailings. The solids-rich stratum
includes
flocculated solids as well as the immobilized CoCs, which can include bitumen-
clay complexes, insolubilized surfactants (e.g., naphthenic acids),
insolubilized
metals (e.g., arsenic and selenium) and thus inhibits migration of the CoCs
into the
water cap or water column. Once the PASS 138 is substantially formed, a fresh
water stream 146 can be added to the PASS and an outlet water stream can be
withdrawn from the PASS, so as to create a flow-through with the water cap 142
in order to maintain the water level and/or gradually reduce certain CoC
levels to
facilitate supporting freshwater plants and/or phytoplankton. In some
implementations, the PASS 138 can be formed by expelling treated tailings
therein
for a period of time (e.g., 20 years) in order to fill the PASS to a desired
level.
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During this formation period, the water cap 142 can be substantially composed
of
tailings pore water that has separated out, as well as precipitation and
optionally
some other water sources that can be used to account for evaporation. Then,
after
the formation period (e.g., 20 years), water flow-through is implemented. The
water
flow-through can include connecting the PASS 138 with existing waterways. The
water flow-through provides certain inlet and outlet flows of water into and
out from
the water cap, and gradually reduces salt levels in the water cap. The water
flow-
through can be provided such that the water cap has a certain salt content
below
a threshold in a predetermined period of time (e.g., below a desired value
within
years after initiating the flow-through), and salt levels can be monitored in
the
water cap, the inlet flow and the outlet flow.
[0114]4 recycle water stream 148 can be withdrawn from the PASS for recycling
purposes. In addition, recycle water 148 is withdrawn from the water cap 142
and
can be supplied to various processing units, e.g., as polymer solution make-up
water 150 and water 152 for use in extraction operations 154.
[01151Referring now to Figure 3b, the flocculated material 132 can be supplied
to
an SLS 156 instead of a DDA for the main dewatering step. The SLS 156 can be
various different types of separators. The SLS 156 produces a water stream 158
and a solids-enriched stream 160. In some implementations, the immobilization
chemical can be added upstream of the SLS 156, as stream 124 for example. In
other implementations, a downstream immobilization chemical stream 162 can be
added into the solids-enriched stream 160, to produce a depositable tailings
material 164 that can be deposited into a DDA 136. It should also be noted
that
the immobilization chemical can be added at both upstream and downstream
points (e.g., streams 24 and 62). In the scenario illustrated in Figure 3b,
the DDA
136 can be managed such that over time a PASS 138 is formed. Due to the
upstream separation of water 158 in the SLS 156, the water cap 142 of the PASS
in the ex situ dewatering scenario can be thinner than that of the in situ
scenario.
Indeed, in the ex situ scenario, a portion of the release water, which can be
the
primary source of water for the water cap 142, is withdrawn from the solid-
liquid
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41
separator as recycle water 158, thereby reducing the water level of the water
cap
142 in comparison to the in situ scenario. Depending on a desired water cap
depth,
water from other sources can be added to the water cap in the ex situ
implementation if there is insufficient water from the remaining tailings pore
water.
[0116]Turning now to Figures 4a to 4e, there are several potential process
implementations for effecting flotation followed by contaminant immobilization
as
well as polymer flocculation of suspended solids present in the fine tailings.
In
general, flotation can effected directly on the fine tailings stream, whereas
chemical immobilization and polymer flocculation can be effected at different
points
in the process and by using different chemical addition approaches.
[0117] Referring to Figure 4a, fine tailings stream 110 is subjected to
flotation 46
to produce a froth concentrate overflow 111 and an aqueous underflow 120. The
aqueous underflow 120 can be combined with the immobilization chemical 124 to
produce the pre-treated tailings 126, which is then combined with the polymer
flocculent 128 so that a flocculated tailings material 132 is produced and
then
subjected to dewatering 166. The dewatering step 166 results in a water stream
168 and a solids-enriched stream 170. It can be noted that the scenario of
Figure
4a is a generalized version of the process similar to that of Figures 3a and
3b
insofar as the immobilization chemical 124 is added to the thick fine tailings
prior
to the flocculent 128.
[0118] Referring to Figure 4b, the immobilization chemical 124 and the
flocculent
128 are added simultaneously into aqueous underflow. The resulting flocculated
tailings material 132 is then supplied to the dewatering step 166. The co-
addition
of the immobilization chemical 124 and flocculent 128 can be done by
introducing
the two additives via a single addition line or injector, or by introducing
the two
additives via separate lines or injectors at a single point of the aqueous
underflow
120 such that the two additives undergo mixing and reaction with the aqueous
underflow at substantially the same time.
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[0119]Referring to Figure 4c, the aqueous underflow 120 can be subjected to
chemical immobilization and polymer flocculation by introducing a single
additive
172 that has both immobilization groups and polymer flocculation groups. For
example, a calcium-based anionic polymer flocculant, including calcium cation
groups and polymer flocculant groups, could be used to enable both chemical
immobilization and polymer flocculation. Polymer flocculants based on
multivalent
cations instead of monovalent cations, such as sodium, can provide the
additional
immobilization functionality. The anionicity, calcium content, molecular
weight,
mixing properties, and other polymer properties can be adapted according to
the
characteristics of the thick fine tailings to obtain desired immobilization
and
flocculation functionalities. Thus, in some implementations, a single additive
that
includes a multivalent cation and an anionic polymer can be used. It should be
noted that such additives could be introduced as part of an aqueous solution
where
the additive is fully dissolved, for example.
(0120] Referring to Figure 4d, the aqueous underflow 120 can first be
subjected to
flocculation to produce a flocculation stream 174 that is then subjected to
chemical
immobilization by addition of a downstream immobilization chemical 176,
thereby
producing a treated tailings stream 178 which can be supplied to the
dewatering
step 166. In such scenarios, shear and mixing imparted to the tailings between
the
flocculant addition and the dewatering can be adapted to provide suitable
shear to
flocculate the tailings, mix the immobilization chemical to enable the desired
insolubilization and immobilization reactions, while avoiding overshearing the
flocs.
[0121]Referring now to Figure 4e, the aqueous underflow 120 can first be
subjected to flocculation to produce a flocculation stream 174 that is then
subjected
to dewatering 166 to produce the water stream 158 and the solids-enriched
stream
160. This scenario is similar to that illustrated in Figure 3b insofar as a
dewatering
step 166 (e.g., using an SLS 156 as in Figure 3b) is performed prior to
addition of
downstream immobilization chemical 162. Thus, the solids-enriched stream 160
can be subjected to downstream immobilization prior to disposal or further
treatment of the resulting solids-rich stream 180 (e.g., further dewatering
such as
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via beaching or deposition into the PASS). In addition, the water stream 158
can
also be subjected to an immobilization treatment by addition of an
immobilization
chemical stream 182 to produce a treated water stream 184 for recycling or
deposition into a holding tank, pond, or as part of the water cap of the PASS.
The
immobilization chemical stream 182 added to the water stream 158 can include
the same or different compounds and can have the same or different
concentration
profile as the immobilization chemical 162 added to the solids-enriched stream
160. In some implementations, the immobilization chemical streams 162 and 182
are prepared or obtained from a common chemical source 186 and can be
formulated differently for their respective applications.
[0122]It should be noted that various other scenarios beyond those illustrated
in
Figures 4a to 4e are possible in order to subject an aqueous underflow from a
flotation step of fine tailings and/or a derivative stream to both chemical
immobilization and polymer flocculation. The process implementation can be
selected depending on various factors, such as the characteristics of the fine
tailings and its CoCs, the properties of the immobilization chemical and
polymer
flocculant in terms of reactivity and mixing with the tailings (e.g.,
dewatering device
or via deposition, weather, deposition variables such as lift thickness and
surface
slopes), make-up water chemistry, pipeline configurations, and deposition or
PASS
capacity.
[0123]Experimentation and calculations regarding chemical immobilization
compounds, flocculation and other process parameters related to treating and
dewatering thick fine tailings can be found in Canadian patent applications
Nos.
2,921,835 and 2,958,873.
(0124] It should be noted that the techniques described herein can be used to
treat
fine tailings derived from oil sands extraction operations as well as various
other
fine tailings or slurries that include CoCs such as surfactants, metal
compounds
and/or hydrocarbons or other compounds immiscible in the water phase of the
slurries. Whether applied to oil sands fine tailings or other types of fine
tailings,
CA 2983961 2017-10-27
44
various implementations described herein enable effective and efficient
conversion
of the fine tailings into a viable aquatic landform and facilitates permanent
storage
of thick fine tailings in a reclaimed landscape. In addition, in some
implementations,
a number of operational and environmental compliance constraints can be dealt
with such as facilitating large scale storage of legacy and newly generated
fine
tailings in a permanent aquatic landform that is ready for reclamation within
a
relatively short timeframe (e.g., 10 years) from the end of mine life, while
enabling
efficient overall tailings management.
CA 2983961 2017-10-27
