Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02677479 2009-09-16
HEAT AND WATER RECOVERY FROM OIL SANDS WASTE STREAMS
FIELD OF THE INVENTION
[0001] The present invention relates generally to recovery of heat and water
from
waste streams produced during oil sands extraction.
BACKGROUND OF THE INVENTION
[0002] Oil sands are deposits comprised of bitumen, clay, sand, and connate
water,
and make up a significant portion of North America's naturally-occurring
petroleum reserves.
To produce a marketable hydrocarbon product from the oil sands, the bitumen
must be
recovered from the oil sands matrix. Depending on geographic location, bitumen
may be
recovered by surface mining or in-situ thermal methods, such as steam assisted
gravity
drainage (SAGD), cyclic steam stimulation (CSS), vapor extraction process
(VAPEX), liquid
addition to steam for enhancing recovery (LASER) or derivatives thereof.
[0003] Because the bitumen itself is a highly viscous material, separating it
from the
extracted sands poses certain practical difficulties. The common industry
practice for bitumen
recovery (or extraction) from surface mineable oil sands is based on the Clark
Hot Water
Extraction (CHWE) process. This process is a water-based bitumen extraction
process,
where hot water, air, and process aides are added to crushed ore. An oil-rich
froth "floats" or
rises through the mixture as a hydrocarbon phase. The result is an extract
that typically
comprises two parts: a hydrocarbon phase known as a bitumen froth stream, made
up of
bitumen, water and fine solids, and an aqueous phase known as extraction
tailings, made up
of coarse solids, fine solids, water and some unrecovered hydrocarbon. The
bitumen froth
stream typically comprises bitumen (approximately 60% by weight), water
(approximately
30% by weight) and solids (approximately 10% by weight), and must undergo a
froth
treatment process to separate the organic compounds from the water and the
solid
contaminants. Due to the high viscosity of the bitumen froth, the first step
is typically the
introduction of a solvent, usually a hydrocarbon solvent such as a naphtha or
a paraffinic
solvent. This step is known as froth separation (FS), and helps to accelerate
the separation
of solid particles dispersed within the froth by both increasing the density
differential between
the bitumen, water, and solids and lowering bitumen viscosity. Separation is
carried out by
any number of methods, such as centrifugation or simply allowing solids to
settle by gravity.
The result of the froth treatment process is diluted bitumen and a second
tailings stream,
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The result of the froth treatment process is diluted bitumen and a second
tailings stream,
known as froth treatment tailings, made up of water, solids, residual solvent
and residual
hydrocarbon, that requires secondary treatment to recover the residual solvent
and prepare
the tailings for disposal. The first step in this secondary treatment process
is to recover as
much of the residual organic solvent as possible through any number of
processes known
collectively as tailings solvent recovery (TSR). Recovered solvent can then be
reused in the
FS process. Tailings from a TSR unit, known as TSRU tailings, are then
disposed of into
storage (or tailings) ponds. The specific properties of the tailings will vary
depending on the
extraction method and processing step from which they originate, but in all
cases, the tailings
streams are essentially spent water, process aids, residual hydrocarbon and
waste solids left
over once the usable bitumen has been removed.
[0004] While effective, the extraction treatment processes require the use of
significant quantities of water, and heat to raise the temperature of the
water, which
significantly increases the cost associated with recovery of petroleum from
the bitumen-laden
oil sands. Most of the process inputs, including the water and energy used in
the processes
end up in the product streams; over 60% of the enthalpy invested in the
extraction process is
"lost" to the tailings stream, along with approximately three barrels of water
per barrel of
bitumen extracted.
[0005] One known method of recovering the water is to simply store the
tailings
stream in tailings ponds, and allow the solid components to settle and
separate from the
water over time. The heat content of tailings escapes into the atmosphere,
while the tailings
water is retained for future use, with some loss due to evaporation. This
tailings storage
method is ineffective for at least three reasons: firstly, it is not very
efficient, as a significant
amount of time is required for most of the solid materials to settle out of
the tailings by
operation of gravity alone; secondly, it does not allow for the recovery of
any of the large
amount of energy contained within the tailings stream in the form of heat,
which is significant,
as tailings are initially sent to the ponds for storage at temperatures
between 20 and 90 C;
thirdly, the resulting tailings ponds are voluminous, occupying a large
footprint of land that
cannot be used for any other purposes during the settling process.
[0006] Rather than simply storing the tailings in ponds, it is desirable to
recover as
much of the invested water and enthalpy from the tailings stream as possible
to reduce the
overall cost of extracting bitumen from the oil sands, to minimize land
footprint and to
minimize fresh water withdrawal. The energy and water recovered can ideally be
reused in
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the extraction process. This has the advantage of improving the overall energy
efficiency of
the extraction process. It is further desirable to minimize the volume of
tailings that must be
disposed. By removing as much water as possible from tailings, the waste
streams can be
substantially reduced to the sand, clay and other minerals originally
extracted from the oil
sands. In this form, the tailings can be easily disposed of through, for
example, direct mine
refill to reduce or eliminate tailings ponds and minimize land footprint.
[0007] Several attempts to recover heat, water and other reagents from
tailings
streams are known. Exemplary methods are disclosed in U.S. Patent Nos.
4,343,691,
4,561,965 and 4,240,897, all to Minkkinen. These patents are directed to heat
and water
vapor recovery using a humidification/dehumidification cycle. U.S. Patent No.
6,358,403 to
Brown et al. described a vacuum flash process used to recover hydrocarbon
solvents from
heated tailings streams. There has been, however, a lack of success in
effective water and
energy recovery.
[0008] For both economic and environmental reasons, it is desirable to provide
an
alternative method for recovering water and energy from tailings streams, as
well as to
reduce the overall volume of tailings that must be disposed of.
SUMMARY OF THE INVENTION
[0009] It is therefore an object of the present invention to obviate or
mitigate at least
one disadvantage of known systems or methods.
[0010] In one aspect, there is provided methods and systems for treating
tailings
produced during oil sands extraction. A tailings stream is preheated in a heat
exchanger,
thereby reducing the energy required to dry the tailings. The preheated
tailings are then
dried, thereby changing the tailings to dry stackable tailings or thickened
tailings suitable for
mine backfill. Heat and high-quality water are recovered from the drying
operation and re-
used in the preheat operation, or in the oil sands extraction process, thereby
reducing the
overall heat and water requirements.
[0011] In accordance with another aspect, there is provided a method for
treating a
tailings stream produced during extraction of bitumen from oil sands,
comprising: drying the
tailings stream in a dryer using input steam in indirect thermal contact with
the tailings stream
to produce a dried tailings stream, whereby the input steam and evaporated
water from the
tailings evaporated during drying convert to a condensed water effluent; and
recycling steam
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produced from the dryer back into the dryer, wherein the input steam comprises
the recycled
steam and optionally make-up steam.
[0012] In accordance with yet another aspect, there is provided a system for
treating
a tailings stream produced during extraction of bitumen from oil sands,
comprising: a dryer
for receiving the tailings stream and input steam, drying the tailings stream
using heat from
the input steam, and producing a condensed water effluent; and a steam
recycling unit for
recycling steam produced from the dryer back into the dryer, wherein the input
steam
comprises the recycled steam and optionally make-up steam.
[0013] In accordance with still another aspect, there is provided a method for
treating
a tailings stream produced during extraction of bitumen from oil sands,
comprising: a)
introducing the tailings stream and a flue gas into a first heat exchanger to
preheat the
tailings stream, b) introducing the tailings stream into a second heat
exchanger to preheat
the tailing stream, wherein step a) is performed before step b) or step b) is
performed before
step a), to produce a preheated tailings stream, c) drying the preheated
tailings stream in a
dryer using input steam and evaporated water from the preheated tailings
evaporated during
drying produced vapor in indirect thermal contact with the preheated tailings
stream to
produce a dried tailings stream, whereby the input steam and the evaporated
water from the
preheated tailings convert to a condensed water effluent, and d) passing the
condensed
water effluent from the dryer to the second heat exchanger to preheat the
tailings stream.
[0014] In accordance with still another aspect, there is provided a system for
treating
a tailings stream produced during extraction of bitumen from oil sands,
comprising: a first
heat exchanger for receiving the tailings stream and a flue gas, and
preheating the tailings
stream using heat from the flue gas; a second heat exchanger for receiving the
tailings
stream and a condensed water effluent and preheating the tailings stream using
heat from
the condensed water effluent, wherein the first or the second heat exchanged
is placed
upstream of the other; and a dryer for receiving the preheated tailings stream
and input
steam, heating the preheated tailings stream using heat from the input steam
to produce a
dried tailings stream, and producing the condensed water effluent produced in
the second
heat exchanger.
[0015] Other aspects and features of the present invention will become
apparent to
those ordinarily skilled in the art upon review of the following description
of specific
embodiments of the invention in conjunction with the accompanying Figures.
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BRIEF DESCRIPTION OF THE DRAWINGS
[0016] Embodiments of the present invention will now be described, by way of
example only, with reference to the attached Figures, wherein:
Fig. 1 is a flow diagram illustrating an overview of a method of tailings
treatment according to one disclosed embodiment;
Fig. 2 is a schematic of an example of a tailings treatment system in
accordance with one disclosed embodiment; and
Fig. 3 is a schematic of an example of a tailings treatment system using a
flue
gas in accordance with one disclosed embodiment.
DETAILED DESCRIPTION
[0017] Generally, in one embodiment the present invention provides a method
and
system for using drying to recover water and heat energy from tailings streams
in oil sands
extraction, such as mature fine tailings, coarse tailings, TSRU tailings, fine
tailings, and froth
separation tailings. Because of the high energy required by conventional
drying, using it in
the tailings treatment process has not previously been considered practical;
however, by
recovering heat energy from the evaporated water through drying in accordance
with one
aspect of the present invention, drying processes may now form an integral
part of the
treatment process. As will become apparent in view of the following examples
and
embodiments, water and heat recovered during drying of a tailings stream can
be used to
preheat the rest of the tailings stream, thereby reducing the amount of energy
required to be
input for the drying operation. Preheating is optional. While the following
examples are
presented in the context of tailings streams generated by a bitumen extraction
process, one
of ordinary skill in the art will appreciate that embodiments of the present
invention may be
implemented in any oil sands extraction process that produces a stream of
tailings
comprising solid(s) and liquid(s). The system and methods described herein are
equally
applicable to any process using or generating any aqueous slurry or mine
tailings. For the
purposes of this description, the general terms "tailings" and "tailings
stream" will be used to
denote any aqueous slurry, mine tailings or other solid-liquid mixture used or
generated in
mining or industrial operations from which heat and/or water can be recovered.
Further, the
tailings may be provided directly from the extract from the oil sands, from
any secondary (or
subsequent) recovery or any portion of the process that generates water and/or
solids
CA 02677479 2009-09-16
including tailings ponds. The tailings may or may not also comprise solvents
which were
added to the oil sands to assist in the bitumen extraction process.
[0018] A drying operation generally comprises heating the tailings stream,
thereby
providing energy required for the water in the stream to undergo vaporization.
Once the
phase change occurs, water vapor is liberated from the tailings stream and can
be collected
through a variety of methods, including, but not limited to, vacuum or steam
sweeping. In a
drying operation, a dryer must first add sufficient heat, referred to here as
sensible heat, to
raise the temperature of the tailings stream to the boiling point of water.
The dryer must then
add the energy required for the water to undergo the change from its liquid
phase to its
vapour phase. This energy is typically referred to as the heat of
vaporization. At all
practicable pressures, however, the drying process requires the addition of a
substantial
amount of energy. In fact, the major part of heating requirement can be
captured from the
vapour produced during drying. In accordance with one aspect of the present
invention, the
recovered heat can be sent directly to the drying operation. Any recovered
heat can be
supplemented by additional energy added to the drying operation through a heat
transfer
medium such as fuel or steam to reach the energy threshold required for the
drying process.
[0019] Figure 1 shows a general overview of an embodiment of the invention.
Tailings stream 100 from a bitumen separation process undergoes preheat 110.
The
preheat step is optional. A heat exchanger is particularly suitable for the
preheat operation,
as it allows for indirect transfer of heat from one substance to another by
conduction through
a wall or other barrier separating the two substances. In this manner, the two
substances
may exchange heat between one another without ever coming in direct contact
with one
another or being mixed in any way. Because of the nature of tailings streams
in oil sands
extraction, the environment within the heat exchanger may be highly
susceptible to fouling,
or the accumulation of solid material along its inner surfaces. Accordingly,
in one
embodiment of the invention, the heat exchanger is a self-cleaning heat
exchanger of any
self cleaning technology known in the art. Examples include circulating
fluidized bed
exchangers (such as those designed by Klaren BV, Rotterdam, The Netherlands),
turbulence
inducing or scraping devices, or heat exchangers with an on-line cleaning
design (using
circulating balls), etc. However, as one of ordinary skill in the art will
appreciate, other heat
exchangers capable of indirect transfer of heat from either a liquid or
gaseous substance to a
tailings stream may be used.
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[0020] By way of background in respect of circulating fluidized bed exchangers
designed by Klaren, BV, self-cleaning heat exchange technology can be applied
in most
vertically oriented shell and tube exchangers. The fouling prone fluid flows
upward inside the
tubes, is charged with solid particles that are swept upward with the fluid
producing a
scouring action on the walls of the tubes as they travel. A unique
distribution system in the
inlet channel provides a uniform distribution of liquid and particles into all
the tubes. From the
outlet channel, the particles are carried to the separator where they
disengage from the liquid
and are returned through the external downcomer into the control channel and
from there
through the connecting line between control channel and inlet channel into the
inlet channel.
The flow of particles is activated by the control liquid flow, which is a
fraction of the total liquid
flow supplied to the exchanger. By changing the control liquid flow, the
intensity of the
cleaning action can be varied. If desired, the cleaning can also be applied
intermittently.
[0021] Preheated tailings stream 115 flows from preheat operation 110 into the
drying operation 120. Make-up steam 150 provides dryer 120 with a part of the
enthalpy
required by the drying process. As preheated tailings stream 115 is dried, it
is separates into
two distinct portions, namely, water vapor 180 and solid cake 140. The water
content of the
solid cake 140 may be adjusted to the desired end-product composition process
requirements, and could range from a pumpable product to a dry cake. The solid
cake may
comprise insufficient water to flow, which is usually below about 60 mass %
water. The
water content of the solid cake output could also be between 40 and 60 mass %,
or between
0 and 40 mass %. The water vapor 180 is removed from the dryer and commingled
with
make-up steam 150. The addition of stream 180 to make-up steam 150 provides
the rest of
enthalpy requirement for the drying process through condensation in an
indirect fashion.
Following heat transfer, input steam 150 is condensed and removed from drying
operation
120 as condensed water effluent 160. The condensed water effluent 160 is the
result of
water vapor 180 released from the tailings 115 and steam introduced to carry
out the drying.
Condensed water effluent 160 is then used in preheat operation 110. For
example, if the
preheat operation employs a heat exchanger, the enthalpy of the condensed
water effluent
160 is transferred to tailings stream 100 through the indirect heat transfer
between the
compartments of the heat exchanger. This has the effect of preheating the
tailings stream
100, raising its temperature and reducing the energy required to carry out the
heating in
drying operation 120. Following the indirect heat transfer, much of the
enthalpy from
condensed water effluent 160 has been passed to the tailings stream, and
leaves preheat
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operation 110 as high purity water 170. A significant benefit of this approach
in addition to
the drying of the tailings, is capturing the originally contaminated water
present in the tailings
stream as high quality, distilled water. In another embodiment of this
invention, the recovered
water stream is subsequently used in the extraction process or utilized in an
integrated
thermal in-situ and mining and extraction operation to generate steam.
[0022] Figure 2 shows an example of the tailings treatment system in
accordance
with an embodiment of the invention. Tailings stream 200 flows through heat
exchanger 210,
for instance at a temperature of approximately 36 C, where it is preheated by
condensed
water vapor effluent 240, as discussed above. Suitable temperatures include
those below the
boiling point of water. As discussed below, a portion of tailings stream 200
may form a
bypass stream 295. The preheated tailings 220 then enter dryer 230. The
tailings are dried
with heat energy from input steam 237, provided by ejector 235. This steam is
a mixture of
the water evaporated during drying and make up steam added to the system
through ejector
235. The role of the ejector is to produce slight vacuum inside the dryer 230,
remove the
produced water vapor 238, mix the produced water vapor 238 with make-up steam
236, and
to provide the combined steam at a higher pressure and temperature than the
released
vapor from tailings. Alternatively, produced steam 238 can be compressed using
a
compressor, such that minimal or no make-up steam 236 would be required. Dryer
230 is
optionally and preferably in an indirect dryer configuration, whereby input
steam 237 injected
into the dryer is contained within a shell surrounding the dryer cavity, and
does not come into
direct contact with the tailings. The heat required for drying, therefore, is
indirectly transferred
from the steam through the shell to the tailings. Input steam 237 loses heat
to the preheated
tailings, and, as input steam 237 condenses, the tailings will absorb the
latent heat of
condensation of the water vapor. Any indirectly heated dryer may be used (for
example a
rotary drum or paddle), but may optionally comprise design features that mix
the product
and/or scrape the drum walls to minimize thermal resistance. An example of a
commercial
dryer is the K-S biosolids dryer system for biosolids and biological sludge
drying (Komline-
Sanderson, Peapack, NJ, U.S.A). An indirect steam dryer is described in U.S.
Patent No.
5,291,668. Dryer 230 may comprise one or more dryers, in series or parallel,
at least one of
which uses indirect steam heating as discussed herein. Other dryer units could
use electrical
heating for example. The steam used in the dryer could be superheated or at
any
pressure/temperature required.
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[0023] As the preheated tailings 220 are dried, water vapor escapes as
produced
steam 238. This steam is substantially free of minerals, as the salts
originally contained in
the tailings stream remain with the dried solids. Produced steam 238 is piped
back and
mixed with makeup steam 236, such that the enthalpy released during the drying
operation
can be conserved and re-used as the drying operation continues. Hydrocarbons
and
solvents originating in the preheated tailings stream 220 that are volatilized
and subsequently
condensed in the condensed water vapor effluent 240, in addition to the water,
can be
separated from the recovered water and used at any point in the process. Water-
hydrocarbon separations means known in the art may be used (for example oil
removal
filters, membranes, centrifuges, or separators). For instance, a conventional
two-phase
separator in stream 240 or 239 could be used. Decantation could also be used
to remove
hydrocarbon solvents. However, since the condensation of produced steam 238
results in
clean, high quality water, one of ordinary skill in the art will appreciate
that any water
collected therefrom may be put to any variety of uses, including, but not
limited to, other
phases of the oil sands extraction process. Non-limiting examples of uses for
recovered
water include boiler feed water for SAGD operations, utility steam, makeup
water, etc.
[0024] Streams 239 or 240 may be the equivalent of about 48000 m3/d of warm or
hot water. This water may be used in bitumen extraction to significantly
reduce the fresh
water requirements of a conventional bitumen mining and extraction operation,
by about half,
for instance. Alternatively, it may be used in a SAGD operation to supply more
than the
necessary boiler feed water (BFW) in a 20,000 bbl/d to 30,000bbl/d thermal in-
situ operation
that may be adjacent to the mining/extraction operation. The recovery of this
water is an
alternative method to make BFW from process affected water.
[0025] According to one embodiment of the invention, input steam 237 has
temperature and pressure from ejector 235 of 103 C and 113 KPa, respectively.
The input
steam should be at a sufficiently higher temperature than both the inlet
tailings temperature
and boiling point of water at the cavity operating pressure to transfer heat
across the dryer
wall at the desired rate. Optionally and preferably, ejector 235 also creates
a low-pressure
environment, for instance below atmospheric pressure, within the cavity of
dryer 230, such
that the boiling point of water is reduced, leading to a lower temperature of
the dried tailings.
The ejector controls pressure in the dryer cavity and in the dryer shell using
makeup steam
236 as a motive fluid. The pressure in the dryer cavity may be reduced to any
practical limit.
Parameters that set the limit are in part based on make up steam 236 pressure,
ejector
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pressure ratio, mass flow rate ratio between make-up steam 236 and produced
steam 238
This lowers the boiling point of water within the cavity, and the water
contained within the
tailings will consequently undergo a phase change to a gaseous state at a
lower temperature
and pressure. In this manner, the ejector can serve two functions: raise the
pressure of
produced steam stream 238 so that it can be introduced to dryer 230 at a
higher
temperature, and lower the pressure in the cavity of dryer 230 so that
tailings water will
evaporate at a lower temperature.
[0026] As it condenses, input steam 237 leaves dryer 230 as condensed water
effluent 240, at a temperature of approximately 83 C, or higher than the
tailings temperature
in accordance with this example. Condensed water vapor effluent 240 then flows
through a
second compartment of heat exchanger 210 and, through indirect heat transfer,
preheats the
tailings stream 200 currently flowing through the first compartment. During
normal operation
of this embodiment of the invention, sufficient sensible heat is transferred
from condensed
water effluent 240 to the tailings stream 200, such that the temperature of
the tailings
increases from 36 C to 55 C, thereby greatly reducing the energy required by
dryer 230 to
dry the preheated tailings. Following preheat, the condensed water vapor
effluent 240 leaves
heat exchanger 210 as liquid water at a temperature of 60 C. The recovered
water 239 may
then be collected and used in the extraction process or to make boiler feed
water for steam
generation in an integrated thermal in-situ and mining/extraction operation.
Higher
throughput may also be achieved where less than the full amount of steam is
condensed in
the dryer, but a portion of the steam is condensed in the heat exchanger 210.
Optionally, the
preheat heat exchanger may be eliminated, by increasing the length or surface
area of the
dryer such that condensed water vapor effluent 240 leaves the dryer at about
60 C.
[0027] A further aspect of the present invention provides for converting a
tailings
stream to dry, stackable tailings that can be disposed of through, for
example, direct mine
refill, water capping, overburden capping, capping with consolidated tailings,
or sand
layering. An output of the drying operation is solid cake 250, which comprises
dried clay,
sand, residual hydrocarbons and other mineral deposits from the taiiings.
Because solid cake
250 has been separated from much of the water originally contained in tailings
stream 200,
the overall volume of tailings to be disposed is significantly reduced and
thus reduces the
footprint of the tailings pond. Optionally, the solid cake is mixed with
bypass stream 295,
which is some portion of untreated tailings stream 200, for instance 5 to 20%,
or about 10%
in this example. The bypass stream assists in processing a larger volume of
tailings and
CA 02677479 2009-09-16
produces a pumpable material so that expensive trucks or conveyors are not
required. To
make the tailings particularly suitable to backfill, the moisture content
should range from 0 -
40%, preferably between 10 - 20%. This final mixing step is carried out by
mixer 280 and
results in thickened tailings 290, which is composed of a substantial portion
of solids tailing
material, for instance approximately 75% solid tailings material. Thickened
tailings 290 can
then be disposed of in any desired manner, although they are particularly
suitable for "back
fill," or refilling the oil sands mine from which the original unprocessed
bitumen was
extracted.
[0028] According to one embodiment of the invention, solid cake 250 is divided
into
two parts: solid cake remainder 270 and recycled solid stream 260. Solid cake
remainder
270 then undergoes the optional mixing treatment discussed above, followed by
disposal.
Recycled solid stream 260, which in this example is approximately 50% of solid
cake 250, is
piped back and mixed with preheated tailings 220 as the drying operation
continues. This
increases the solid content of the dryer's feed, thereby reducing fouling
within the dryer itself
Approximately 50% of solid cake 250, is piped back and mixed with preheated
tailings 220 to
increase the solid content of the dryer feed to an acceptable level (for
example over 35%). In
this example, the solid content of the dryer's feed was increased to
approximately 37% by
combining with the recycle stream. As the solid content increases, materials
become less
sticky in the dryer, which is an advantage in a high temperature operation.
[0029] Figure 3 shows another embodiment of the present invention, whereby a
tailings stream undergoes two separate preheat operations to further improve
the efficiency
of the treatment process. Elements common between this embodiment and the
exemplary
embodiments previously discussed are marked in Figure 3 with labels matching
those in
previous figures, and function as described above.
[0030] Tailings stream 300 enters a first heat exchanger 301, which functions
according to the same principles described above with reference to heat
exchanger 210. As
discussed above, the heat exchangers are optionally and preferably self-
cleaning heat
exchangers to minimize fouling. First, heat exchanger 301 receives hot flue
gas 305 and
uses it as the heat source to initially preheat tailings stream 300. Flue gas
305 (optionally
and preferably with minimal sulfur content) may be sourced from any other step
in the oil
sands extraction process or an integrated thermal in-situ operation that
produces a gas
capable of flowing through first heat exchanger 301 and providing heat energy
to tailings
stream 300, such as from a boiler. Optionally and preferably, flue gas 305
passes through
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fan 310, which serves to increase the pressure inside first heat exchanger
301, because flue
gas is at atmospheric pressure, and should be increased to account for the
pressure drop
within exchanger 301. Fan 310 may also be a compressor. In one example of the
embodiment shown in Figure 3, tailings 300 enter heat exchanger 301 at a
temperature of
36 C. Flue gas 305 is at a temperature of approximately 150 C, and provides
sufficient heat
to the tailings in first heat exchanger 301 so as to increase the temperature
of the tailings to
60 C. An induced fan could be used to pull the flue gas through the heat
exchanger. This
would be positioned on stream 313 and has the benefit of handling a lower
temperature,
higher density gas.
[0031] Following the first preheat operation, the cooled flue gas and any
condensed
water vapour 306 from first heat exchanger 301 is collected and separated in
separator 311.
This allows the system to recover additional water, as flue gas 305 may
contain significant
water content. The water content of cooled flue gas 306 is collected from
separator 311 as
condensed liquid 312, which, if desired, may be added to water 239 (discussed
above with
reference to Figure 2), thereby becoming part of the total water recovered
from the tailings
treatment process, or become feed for another process. The remaining
components of
cooled flue gas 306 are released from separator 311 as separator flue gas 313.
[0032] The tailings preheated during the first preheat operation leave first
heat
exchanger 301 as initially preheated tailings 302. These tailings may then
undergo a second
preheat operation in heat exchanger 210, which transfers heat to initially
preheated tailings
302 from condensed water vapor effluent 240 as discussed above with reference
to Figure 2.
The remaining elements shown in Figure 3 function in the same manner as the
corresponding elements shown in Figure 2, and the elements in Figure 3 are
consequently
labeled with identical identifiers for ease of reference to the discussion
above. In particular, a
bypass stream 295 is taken from the tailings stream 300 and mixed with solid
cake
remainder 270 by mixer 280 to form thickened tailings output 290. Preheated
tailings 220 are
fed to dryer 230 and are at, according to this example, approximately 90 C.
Condensed
water vapor effluent 240 from dryer 230 is fed into heat exchanger 210 at
approximately
99 C in this example. Produced steam 238 released from the dryer 230 is mixed
with
makeup steam 236 to form input steam 237 which is introduced into dryer 230 by
ejector
235. Solid cake 250 exits the dryer once the drying operation is complete.
Optionally and
preferably, a portion of solid cake 250 is separated and sent as recycled
solid stream 260 to
mix with preheated tailings 220 to help reduce fouling inside the dryer.
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[0033] The embodiments of the invention that include two distinct preheat
steps may
provide even greater efficiency to the overall tailings treatment process, as
enthalpy from flue
gas 305 which might otherwise be lost is instead transferred to the tailings
stream, thereby
further reducing the heat energy required by dryer 230. In addition, water
contained in the
flue gas 305 which might otherwise be lost is instead at least partially
collected by separator
311, and forms part of the total water collected during the treatment process.
However, as
one of ordinary skill in the art will appreciate, flue gas 305 may be sourced
from another
industrial or extraction process and still function in accordance with
embodiments of the
invention.
[0034] In accordance with another embodiment of the invention, valuable heavy
minerals can be recovered from oil sand tailings. Heavy minerals are defined
herein as
minerals having a specific gravity greater than about 2.85, and including,
without being
limited to, such minerals as rutile, ilmenite, leucoxene, siderite, anatase,
pyrite, zircon,
tourmaline, garnet, magnetite, manzite, kyanite, staurolite, mica, and
chlorite. Material
extracted from oil sands often includes deposits of usable heavy minerals such
as titanium
and zirconium. In order to recover usable resources from the material, solid
cake 250 may
also be subjected to a heavy minerals recovery step. Several non-limiting
examples include
gravity, electrostatic, chemical, and magnetic separation techniques, although
another
suitable method for extracting heavy minerals from a solid cake may be used
for this
purpose. This step allows for recovery of additional valuable resources from
treated tailings,
further improving the recovery of valuable resources during oil sands
extraction, thereby
making the entire process more economically viable and reducing the amount of
waste
material that must be disposed of. It should be noted that heavy metal
recovery or another
separation process (such as coarse solid removal using a separation device)
may be
employed at any point in the tailings treatments as additional embodiments of
the present
invention.
[0035] Several other advantages of treating tailings streams through drying in
accordance with the present invention may include, but are not limited to:
recovering
extraction process water as high-quality warm water; achieving a net gain in
sensible heat by
an appropriate heat integration scheme; producing dry, stackable tailings,
thereby reducing
dykes in current tailings treatment and reducing the overall volume of
tailings that must be
disposed of; reducing the footprint of oil sands extraction, and allowing for
the potential
recovery of valuable heavy metal oxides, such as titanium and zirconium
oxides.
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[0036] Example
[0037] With reference to Figures 2 and 3, Table 1 provides flow rates and
temperatures, according to one embodiment, for certain streams identified by
their reference
numbers.
[0038] Table 1. Flow rates and Temperatures
Reference Flow rate Temperature
number (stream) tons per hour ( C)
Figure 2
200 2820 36
220 2540 55
236 100 147
240 2055 83
270 580 95
290 865 75
295 280 36
239 2055 60
Figure 3
220 2540 90
236 12.5 147
239 1950 64
240 1950 99
300 2820 36
302 2540 60
305 1100 150
313 1030 40
239 and 312 2035 63
together
[0039] In the preceding description, for purposes of explanation, numerous
details
are set forth in order to provide a thorough understanding of the embodiments
of the
invention. However, it will be apparent to one skilled in the art that these
specific details are
not required in order to practice the invention.
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[0040] The above-described embodiments of the invention are intended to be
examples only. Alterations, modifications and variations can be effected to
the particular
embodiments by those of skill in the art without departing from the scope of
the invention,
which is defined solely by the claims appended hereto.
