Note: Descriptions are shown in the official language in which they were submitted.
CA 02567185 2006-10-31
BITUMEN AND THERMAL RECOVERY FROM OIL SAND TAILINGS
FIELD OF THE INVENTION
The present invention relates generally to a process and a process line for
recovering residual bitumen and heat from oil sand tailings produced during an
oil sands extraction process.
BACKGROUND OF THE INVENTION
Oil sand, such as is mined in the Fort McMurray region of Alberta,
Canada, generally comprises water-wet sand grains held together by a matrix of
viscous bitumen. It lends itself to liberation of the sand grains from the
bitumen,
preferably by slurrying the oil sand in heated water, allowing the bitumen to
move
to the aqueous phase.
For many years, the bitumen in McMurray oil sand has been commercially
recovered using a hot water process well known in the art. Generally, oil sand
is
mixed in a tumbler with hot water having a temperature of approximately 80-
90 C, steam, caustic (e.g., sodium hydroxide) and naturally entrained air to
yield
a slurry having a temperature typically around 80 C. The slurry so produced is
diluted with additional hot water to produce diluted slurry having a
temperature of
about 65 C to about 80 C. The diluted slurry is introduced into a large, open-
topped, conical-bottomed, cylindrical vessel termed a primary separation
vessel
(PSV) where the more buoyant aerated bitumen rises to the surface and forms a
froth layer.
However, while the hot water process assured good bitumen recoveries
for all grades of oil sand, the thermal energy requirement per tonne of oil
sand
processed for the steam production and for heating hot flood water is very
high.
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Recently, in an attempt to reduce the thermal energy requirement for
bitumen extraction from oil sands, a low energy extraction process or the "LEE
process" for bitumen extraction was developed, which is generally described in
Canadian patent No. 2,217,623 and United States Patent No. 6,007,708. The
LEE process generally comprises the following steps:
= dry mining the oil sand;
= mixing the mined oil sand with water in predetermined proportions near
the mine site to produce a slurry containing entrained air and having a
controlled density in the range of about 1.4 to about 1.65 g/cc and
preferably a temperature in the range of about 20 C to about 50 C or
higher;
= pumping the slurry through a pipeline having a plurality of pumps spaced
along its length, preferably adding air to the slurry as it moves through the
pipeline, to condition the slurry (i.e., ablating the larger lumps of oil sand
to
release bitumen and allowing the bitumen flecks to coalesce and attach to
air bubbles);
= diluting the conditioned slurry with flood water and introducing the
diluted
slurry into a primary separation vessel (PSV) to float the aerated bitumen
and separate it from the middlings and tailings (primary tailings). The froth
is maintained at a temperature of at least 35 C in the PSV by use of a
heated water underwash to optimize separation of the bitumen. Primary
tailings, primarily comprising coarse solids, water, and residual bitumen,
which settle to the bottom of the PSV, and secondary tailings, primarily
comprising fines, water, and residual bitumen, which are produced from
the further processing of the PSV middlings in flotation cells to remove
bitumen still remaining in the middlings, are disposed of accordingly.
While the thermal energy requirements of the hot water process are
significantly reduced in the LEE process, nevertheless, thermal energy in the
form of heated flood water is still required for slurry preparation, slurry
dilution
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and for the PSV underwash to ensure the overall PSV slurry temperature of at
least 35 C.
Finding sources of thermal energy for the LEE process, however,
becomes problematic as oil sands mining and extraction operations are being
located at considerable distances away from upgraders such as cokers, which
are an economical source of thermal energy. These satellite oil sands
operations
still require considerable supplemental heat input to achieve the targeted
bitumen
recoveries. Thus, the heat input comes predominantly from natural gas
delivered
through a gas turbine operated with auxiliary burning as well as by utilizing
natural gas fired auxiliary boilers.
Currently, both the heat (thermal energy) and any residual bitumen
present in the primary and secondary tailings are lost in the tailings
deposition
process. In fact, using optimum LEE process conditions still results in only
about
90 to about 94% bitumen recovery depending on the ore blend, pipeline
conditioning and recycle water chemistry. Thus, it would be beneficial, both
from
an energy conservation and an improved bitumen recovery point of view, to
capture the heat and bitumen in tailings.
Thus, there is a need for a process that can be used for both bitumen and
heat recovery from oil sand tailings.
SUMMARY OF THE INVENTION
The present invention relates to a process and a process line for
recovering residual bitumen and heat from oil sand tailings produced during an
oil sands extraction process. The present invention is of particular
importance
when a low energy extraction process such as the LEE process described above
is used for bitumen extraction at sites relatively remote from readily
accessible
sources of heat. However, it is understood that the present invention can be
used with any oil sand extraction process including those that use extraction
temperatures higher than those used in the LEE process.
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As described above, bitumen present in oils sands is extracted from oil
sands by first forming an oil sand slurry with either hot or warm water. Oil
sand
slurry is then conditioned either in a tumbler or more recently by pumping the
slurry through a pipeline. Primary separation of bitumen from solids present
in oil
sand slurry may occur in large capacity gravity settlers called primary
separation
vessels (PSVs), where the slurry is divided into primary bitumen froth,
middlings
(primarily comprised of warm water, fines and bitumen) and coarse tailings
(primarily comprised of coarse solids, warm water, and residual bitumen),
which
are generally referred to as primary tailings.
The bitumen still remaining in the middlings fraction is often recovered in
flotation cells where air is added and further separation of bitumen from
solids
occurs. The tailings that are separated during flotation are commonly referred
to
as secondary tailings and are primarily comprised of fines, warm water and
residual bitumen. The present invention can be used to recover heat and
bitumen from either primary tailings, secondary tailing or, preferably, from
pooled
primary and secondary tailings ("pooled tailings").
It is understood that the present invention can be used on any oil sand
tailings produced as a result of the separation of bitumen from solids present
in
an oil sand slurry. For example, separation means other than a PSV can be
used to separate bitumen from solids, thereby producing oil sand tailings, for
example, cycloseparators as described in CA 2,246,841 or incline plate
settlers
or a combination of cycloseparators and inclined plate settlers.
Thus, in accordance with one aspect of the invention, a process is
provided for recovering heat in the form of cleaned warm water and residual
bitumen from oil sand tailings produced during an oil sands extraction
process,
said oil sand tailings including coarse solids, warm water, fines and bitumen,
comprising:
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= removing at least a portion of the coarse solids from the oil sand
tailings to
produce a coarse solids fraction and a reduced solids tailings fraction
primarily including fines, warm water and bitumen;
= separating at least a portion of the bitumen from the reduced solids
tailings fraction to produce a bitumen fraction and a warm water and fines
fraction; and
= removing at least a portion of the fines from the warm water and fines
fraction to produce cleaned warm water and a concentrated fines fraction.
By "oil sand tailings" is meant any solids fraction obtained after the
separation of bitumen from the solids present in oil sand slurry and includes
primary tailings, secondary tailings and pooled tailings.
By "fines" is meant particles such as fine quartz and other heavy
minerals, colloidal clay or silt generally having any dimension less than
about 44
pm.
By "coarse solids" is meant solids generally having any dimension greater
that about 44 pm.
In general, the concentrated fines fraction includes particles such as fine
quartz and other heavy minerals, colloidal clay or silt generally having a
nominal
average dimension of about 100pm.
Preferably, the cleaned warm water produced by the present invention has
less than 2 wt% total solids, more preferably less than 1 wt% total solids,
and
most preferably less than 0.5 wt% solids, and has a temperature between about
20 C to about 50 C or higher.
In one embodiment, the heat present in the cleaned warm water can be
used for oil sands extraction. More particularly, in one embodiment, the
cleaned
warm water can be used to prepare oil sand slurry. In another embodiment, the
cleaned warm water can be used to dilute oil sand slurry prior to separating
the
bitumen from the solids present in the oil sand slurry, for example, in a PSV.
In
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another embodiment, the cleaned warm water can be added directly to the
Utilities water heating infrastructure for thermal conservation. Thus, the
heat
present in the cleaned warm water is conserved thereby reducing the overall
thermal energy that needs to be supplied from external sources.
In one embodiment, the concentrated fines fraction that is removed from
the warm water and fines fraction is deposited in a tailings disposal site.
In one embodiment, coarse solids are removed from the oil sand tailings
by means of one or more screen. In another embodiment, coarse solids are
removed from the oil sand tailings by means of one or more hydrocyclone. In
yet
another embodiment, coarse solids are removed by means of a combination of
one or more screen for removing the larger coarse solids and one or more
hydrocyclone for removing the smaller coarse solids.
In one embodiment, the bitumen is separated from the reduced solids
tailings fraction by means of one or more flotation device, wherein the
bitumen
floats to the top of the flotation device to produce the bitumen fraction,
leaving
behind the warm water and fines fraction. In one embodiment, the flotation
device is a flotation cell. In another embodiment, other flotation devices
known in
the industry can be used, for example, but not limited to, any mineral
flotation
device such as a Jameson CellTM, a contact flotation cell, a mechanical
flotation
cell, a Tailings Oil Recovery Vessel (TORV) or a flotation column.
In one embodiment, the fines are removed from the warm water and fines
fraction to produce cleaned warm water by feeding the warm water and fines
fraction into one or more thickener having a substantially shallow sloped
bottom
and allowing the fines to settle on the substantially shallow sloped bottom to
form
the concentrated fines fraction. In a preferred embodiment, a processing aid
is
added to the thickener such as a flocculant, a coagulant or a combination of
both
to aid in the settling of the fines. The coagulant is preferably a cationic
coagulant.
Suitable flocculants are well known in the art and include
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polyacrylamide. Suitable coagulants are well known in the art and include
polyamine, gypsum, lime, alum or any combination thereof.
In one embodiment, the coarse solids fraction is mixed with the
concentrated fines fraction and gypsum is added to the mixture to produce
composite tailings.
In one embodiment, the process further comprises cleaning the bitumen
fraction in a froth cleaner to produce a cleaned bitumen overflow and a froth
cleaner underflow. In another embodiment, the process further comprises mixing
the froth cleaner underf low with the oil sand tailings. In yet another
embodiment,
the process further comprises mixing the froth cleaner underflow with the
reduced solids tailings fraction.
In accordance with another aspect of the invention, a process is provided
for recovering bitumen and heat from a conditioned oil sand slurry,
comprising:
= introducing the conditioned oil sand slurry into a primary separation
vessel
to form a top layer of bitumen froth, a middle layer of middlings including
primarily warm water, fines and residual bitumen and a bottom layer of
primary tailings including primarily coarse solids, warm water and residual
bitumen;
= delivering the middlings to one or more primary flotation device to
remove
at least a portion of the residual bitumen from the middlings to produce a
secondary bitumen froth and secondary tailings including primarily fines,
warm water and residual bitumen;
= mixing the secondary tailings with the primary tailings to produce a
pooled
tailings fraction;
= screening out at least a portion of larger coarse solids from the pooled
tailings fraction to produce a screened tailings fraction;
= feeding the screened tailings fraction to one or more hydrocyclone to
remove at least a potion of smaller coarse solids to produce a smaller
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coarse solids fraction and a reduced solids tailings fraction primarily
including fines, warm water and bitumen;
= delivering the reduced solids tailings to one or more secondary flotation
device to separate at least a portion of the bitumen from the reduced
solids tailings fraction to produce a bitumen fraction and a warm water and
fines fraction; and
= feeding the warm water and fines fraction into one or more thickener to
remove at least a portion of the fines from the warm water and fines
fraction to produce cleaned warm water and a concentrated fines fraction.
In one embodiment, the process further comprises delivering the cleaned
warm water back to the primary separation vessel. In another embodiment, the
process further comprises mixing the warm water with conditioned oil sand
slurry
prior to introducing the conditioned oil sand slurry into the primary
separation
vessel.
In one embodiment, the process further comprises introducing the bitumen
fraction into one or more froth cleaner to produce a tertiary bitumen froth
and a
froth cleaner underflow. In a further embodiment, the process further
comprises
mixing the froth cleaner underflow with the pooled tailings fraction prior to
screening.
In one embodiment, the smaller coarse solids fraction is mixed with the
concentrated fines fraction and gypsum is added to the mixture to produce
composite tailings.
In accordance with another aspect of the invention, a process line for
recovering heat in the form of cleaned warm water and recovering residual
bitumen from oil sand tailings produced during an oil sands extraction
process,
said oil sand tailings comprising coarse solids, fines, warm water and
bitumen,
said process line comprising:
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= one or more hydrocyclone for removing at least a portion of the coarse
solids in the oil sand tailings to produce a reduced solids tailings fraction
including primarily fines, warm water and bitumen;
= one or more flotation device for receiving the reduced solids tailings
fraction and separating out the bitumen from the warm water and fines to
produce a bitumen fraction and a warm water and fines fraction; and
= one or more thickener for receiving the warm water and fines fraction and
facilitating the settling of the fines to form cleaned warm water and a
concentrated fines fraction.
In one embodiment, the process line further comprises one or more
screen to remove larger coarse solids prior to introducing the oil sand
tailings into
the hydrocyclones.
In another embodiment, the process line further comprises a froth cleaner
for receiving the bitumen fraction to produce a cleaned bitumen overflow and a
froth cleaner underf low.
In one embodiment, the thickener has a substantially flat bottom. In
another embodiment, the thickener has a rake, such that as the rake moves
through the sludge, it provides channels for the liquid supernatant to move
upward as the solids settle downward.
When particularly used with the LEE process, the present invention results
in an increase in overall bitumen recovery to greater than 95% and a warm
water
recovery commensurate with the anticipated warm water needs but generally
greater than 25%. Furthermore, use of a thickener to facilitate the settling
of
fines, in particular, in the presence of a flocculant and/or a coagulant
results in
more compact tailings that are easier to dispose.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic setting forth the process in accordance with an
embodiment of the invention.
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FIG. 2 is a schematic showing the process line of an embodiment of the
present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is exemplified by the following description. This
embodiment of the present invention is described for recovering heat and
residual bitumen from pooled primary and secondary tailings.
A schematic of an inline or in series process of the present invention is
shown in FIG. 1. Conditioned oil sand slurry, for example, slurry produced by
the
LEE process and conditioned in a pipeline, is fed into primary separation
vessel
(PSV) 10 and allowed to separate under quiescent conditions into a top layer
of
bitumen froth 11, commonly referred to in the art as "primary froth", a middle
layer of middlings 16, including primarily warm water, fines and residual
bitumen,
and a bottom layer of coarse tailings 14, including primarily coarse solids,
warm
water and residual bitumen, which are commonly referred to in the art as
"primary tailings".
Bitumen froth 11 is typically removed from the PSV via launder 12 and
collected for further upgrading by upgrading processes known in the art.
Middlings layer 16 is removed from PSV 10 and at least a portion of the
residual
bitumen still remaining in the middlings fraction may be recovered in a series
of
primary flotation cells (often collectively referred to as the primary
flotation
circuit) where air is added to the cells and the residual bitumen floats to
the top of
the primary flotation cells to form primary flotation cell overflow. FIG. 1
shows a
series of three flotation cells 18, 19, 20 whereby the underflow from the
previous
flotation cell is fed into the flotation cell next in line. For example, as
shown in
FIG. 1, the underflow from flotation cell 18 is fed into flotation cell 19 and
the
underflow from flotation cell 19 is fed into flotation cell 20. Primary
flotation cell
overflow 21 from the last flotation cell 20, which is commonly referred to in
the art
as secondary froth, is removed into pump box 22 and may be recycled back via
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pump 24 to PSV 10. In the alternative, the secondary froth can be further
cleaned by cleaning processes known in the art.
The primary flotation cell underflow 26 from the last in the series of
flotation cells, e.g., flotation cell 20, is commonly referred to in the
industry as
"secondary tailings" and is conventionally transported to sand disposal site.
However, in the present embodiment of the invention, the secondary tailings
are
removed to pump box 28 and then either pumped via pump 30 and returned to
the PSV 10 into the bottom layer of primary tailings 14 or pumped via pump 31
and pooled with primary tailings 14 that have been removed from the PSV via
pump 32. The primary and secondary tailings can be pooled in a pump box or
tailings distributor (not shown), a mixing tank, a pipeline or the like. On
average,
the pooled primary and secondary tailings (hereinafter referred to as "pooled
tailings") will have a temperature in the range of about 30 to about 50 C,
usually
around 35 C.
The residual bitumen and energy (in the form of warm water) contained in
the pooled tailings are recovered as follows. In the present embodiment, the
pooled tailings are first screened using screen 34 to remove the larger coarse
solids present in the pooled tailings in order to protect the hydrocyclones
that are
used to remove smaller coarse solids. Screen 34 reduces the size of the coarse
solids in the pooled tailings from about 5" down to possibly as small as about
1"
in any dimension. Thus, the larger sized solids such as stones, charcoal and
the
like are removed and disposed of accordingly. It is understood that more than
one screen can be used at this step to accommodate larger volumes of pooled
tailings. In another embodiment, the screens can be replaced with one or more
larger cyclones that are specifically designed to remove solids larger than
2".
The screened pooled tailings stream 35 is then fed to one or more
hydrocyclones 36 (e.g., hubs of hydrocyclones) to further remove smaller
coarse
solids (e.g., sand). A hydrocyclone overflow comprising primarily bitumen,
fines
and warm water (generally containing about 80 wt% of the water) and a
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hydrocyclone underflow of coarse solids or tailings (generally containing
about 20
wt % of the water) are produced in hydrocyclones 36.
The hydrocyclone underflow of primarily smaller coarse solids can
generally be disposed of in one of two ways. First, the cyclone underflow is
delivered into compartment 39 of pump box 38 where it can be diluted with cold
water to form a pumpable coarse solids slurry that can be pumped via pump 40
to tailings disposal sites. Alternatively, a coagulant such as gypsum can be
added to the cyclone underflow, along with thickener 60 underflow 70
(discussed
in more detail below) or Mature Fine Tailings ("MFT") produced in previously
existing oil sand tailings disposal sites, present in pump box 38 instead of
cold
water to form "composite tailings" or "CT", so called because a non-segregated
mixture is formed due to the fines being interspersed between the coarse
solids.
This high density mixture can then be pumped to appropriate disposal sites.
The hydrocyclone overflow, which primarily contains up to about 80 wt%
of the water plus fines and bitumen, can be further treated to separate out
the
valuable bitumen and to capture the heat present in the cyclone overflow in
the
form of substantially clean reusable warm water. Hydrocyclone overflow is
added to one or more high energy air and slurry contact cells, such as a
flotation
cell as known in the art. It is understood that other aerated separation means
or
flotation devices known in the industry can be used, for example, but not
limited
to, any mineral flotation device such as a Jameson CeIITM, a contact flotation
cell,
a mechanical flotation cell, a Tailings Oil Recovery Vessel (TORV) or a
flotation
column. It is also understood that more than one secondary flotation cell may
be
used. In FIG. 1, two flotation cells 42 and 43 are shown.
Overflow from hydrocyclone 36 is fed into compartment 41 of pump box
38 and then fed to one or both flotation cells 42 and 43 where a portion of
the
residual bitumen floats to the top to form bitumen fractions 44 and 45,
respectively. Bitumen fractions 44 and 45 can be further cleaned in froth
cleaner
46. In froth cleaner 46, bitumen rises to the top to form froth cleaner
overflow, or
"tertiary bitumen froth", which contains substantially cleaned bitumen.
Tertiary
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bitumen froth 51 can be recycled back to PSV 10 via pump 52 to knock out any
solids still remaining therein. In the alternative, tertiary froth can be
further
upgraded by upgrading processes known in the art.
Froth cleaner 46 is typically a gravity separator having a shallow cone end
and a rake at the bottom of the cone for further concentrating the bitumen
froth
by releasing any entrapped solids and water. The froth cleaner underflow 50
can
be recycled back via pump 54 to either screen 34, hydrocyclone 36 or back to
secondary flotation cells 42 and 43 for further treatment to recover any
further
residual bitumen.
In other embodiments, bitumen fractions 44 and 45 could go directly to
PSV 10 without further cleaning in froth cleaner 46.
The secondary flotation cell underflows 48 and 49, which underflows
contain mostly warm water and about 10 to 15 wt% solids, are collected in pump
box 56 and pumped via pump 58 to one or more thickeners 60. Thickener 60
comprises a substantially shallow sloped bottom 62 and heavy-duty rake drive
mechanism 64 to move the settled tailings or sludge to the centre outlet 66.
Thickener 60 is an efficient method to gravity concentrate a substantial
portion of
fines from the hydrocyclone overflow into thickener underflow. The rake 68, as
it
moves through the sludge, provides channels for the liquid supernatant to move
upward as the solids settle downward. The thickener underflow 70, which
underflow is also referred to herein as concentrated fines fraction or
thickened
tailings ("TT"), is pumped via pump 72 for disposal, or to the hydrocyclone
underflow compartment 39 of pump box 38 for making a non-segregated mixture
of coarse solids and fines (CT).
In one embodiment, a flocculant and/or a coagulant can be added
to the secondary flotation cell underflows 48 and 49 prior to feeding the
underflow to the thickener 60 to improve thickening of the solids. Suitable
flocculants or coagulants include, but are not limited to, polyacrylamide,
polyamine, gypsum, lime, alum or combinations thereof. The thickener overflow
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74 comprises substantially clean, high quality warm water 76 having less than
about 2 wt% total solids, preferably less than 1.0 wt% solids, most preferably
less
than 0.5 wt% total solids. This warm water can thus be reused in the oil sand
extraction process thereby conserving energy and helping meeting the heated
water demands of the LEE process. In particular, warm water 76 can be used in
oil sand slurry preparation. In the alternative, or in addition, warm water 76
can
be used to dilute conditioned oil sand slurry prior to introducing it into the
PSV.
In the further alternative, or in addition, warm water 76 can be used as PSV
underwash. Finally, the warm water, if not of sufficient quality, can be used
in a
heat exchanger for efficient heat transfer to a more suitable process water
stream.
Various tailings fractions produced from the overall process described
above can be further treated and/or disposed of as follows. As previously
mentioned, hydrocyclone tailings/tails or coarse tailings/tails can be treated
in
one of two ways. The coarse tailings can be diluted with cold water and
disposed of in various tailings disposal sites. In one embodiment, the diluted
coarse tailings can be further treated in stacking cyclones for dewatering and
the
dewatered coarse tailings used for deck construction or cell construction, as
is
known in the art. In another embodiment, the diluted coarse tailings are
pumped
directly to tailings disposal sites where the coarse sand settles out on the
beach
and the fines/water flow by gravity to a lower elevation and colleted therein
(referred to as "coarse tails beaching"). In a further embodiment, MFTs that
are
produced in existing tailings disposal sites can be added to the coarse
tailings
and gypsum added thereto to form CT.
The thickener underf low 70, which underflow is also referred to herein as
concentrated fines fraction or thickened tailings (TT), can also be pumped
back
to compartment 39 of pump box 38, mixed with the hydrocyclone tailings and
treated with gypsum to produce Composite Tailings as described above. In the
alternative, the TT can be directly deposited into tailings disposal sites.
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A process line of an embodiment of the present invention is now described
with reference to FIG. 2. In the embodiment shown in FIG. 2, two PSVs 210,
210' are used for separating out primary bitumen froth from conditioned oil
sand
slurry. It is understood that the process line may include more than two PSVs
or
only one PSV. Middlings 216 and 216', respectively, from each PSV are fed into
primary flotation circuits 217 and 217'. Primary flotation circuit 217 is
comprised
of a plurality of flotation cells 218, in series, and primary flotation
circuit 217' is
also comprised of a plurality of flotation cells 218', in series. The
underflow from
one flotation cell is fed to the next in line flotation cell. The flotation
cell
underflow from the last in line flotation cell of each primary flotation
circuit is then
removed into pump box 228, 228', respectively, to form secondary tailings 226,
226'.
Coarse tailings or primary tailings 214, 214' are removed from PSVs 10,
10', respectively, and pumped into tailings distributor 229. Secondary
tailings
226, 226' are also pumped into tailings distributor 229, which along with
primary
tailings 214, 214' form pooled tailings. Alternatively, streams 214, 214' and
226,
226' can be processed separately as non-pooled tailings. Pooled or non-pooled
tailings are pumped via at least one pump 233 into at least one screen 234,
where the larger coarse solids (i.e., greater than 2" in any dimension) are
removed. The screened pooled tailings 235 are then fed into at least one
hydrocyclone 236 for removal of smaller coarse solids thereby primarily
leaving
behind solids having a nominal average dimension of about 100pm in the
hydrocyclone overflow.
The underflow from the hydrocyclone 236 (i.e., comprising the smaller
coarse solids) is collected in at least one pump box 238, where it is either
diluted
with cold water prior to being pumped for disposal or gypsum along with MFT or
TT are added to thicken the underflow to form non-segregating or composite
tailings before disposal. The overflow from hydrocyclone 236 is then fed into
at
least one secondary flotation circuit 239, each secondary flotation circuit
comprising two flotation cells 242, 243. Flotation cell overflow, or bitumen
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fraction 244, may be further treated in froth cleaner 246 as described above.
Flotation cell underflow, comprising primarily warm water and fines are first
contained in at least one pump box 256 and pumped to at least one thickener
260. Thickener underflow, i.e., thickened tailings or concentrated fines
fraction,
is deposited in tailings disposal sites or sent to pump box 238 to form non-
segregating tailings. Thickener overflow, i.e., cleaned warm water, is then
used
in utilities or for PSV feed dilution as previously described.
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