Note: Descriptions are shown in the official language in which they were submitted.
BACKGnOUND OF THI~ INVE,NTION
This invention relates to the hot water process for
treating bituminous sands, such as Athabasca tar -,ands, and,
more particularly, to the treatment of the water and clay-
containing effluent discharged from the process.
Tar sands (which are also known as oil sands and
bituminous sands) are sand deposits which are impregnated
with dense, viscous petroleum. Tar sands are found through-
out the world, often in the same geographical area as con-
ventional petroleum. The largest deposit, and the only one
of present commercial importance, is in the Athabasca area
in the northeast of the Province of Alberta, Canada. This
deposit is believed to contain over 700 billion barrels of
bitumen. For comparison, this is just about equal to the
world-wide reserves of conventional oil, 60% of which is
found in tbe middle east.
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Athabasca tar sand is a three-component mixture of
bitumen, mineral and water Bitumen is the value for the ex-
traction of which tar sands are mined and processed. The
bitumen content is variable, averaging 12 wt.% of the deposit,
but ranging from 0 to 18 wt.%. Water typically runs 3 to 6
wt.% of the mixture, increasing as bitumen content decreases.
The mineral content is relatively constant ranging from 84
to 86 wt.%.
Several basic extraction methods have been known for
many years for separating the bitumen from the sands. In the
so-called "cold water" method, the separation is accomplished
by mixing the sands with a solvent capable of dissolving the
bitumen constituent. The mixture is then introduced into a
large volume of water, water with a surface agent added, or
a solution of a neutral salt in water. The combined mass is
then subjected to a pressure or gravity separation.
The hot water process for primary extraction of bitu-
men from tar sands consists of three major process steps (a
fourth step, final extraction, is used to clean up the re-
covered bitumen for downstream processing.) In the firststep, called conditioning, tar sand is mixed with water and
heated with open steam to form a pulp of 70 to 85 wt.% solids.
Sodium hydroxide or other reagents are added as required to
maintain pH in the range 8.0 - 8.5. In the second step,
called separation, the conditioned pulp is diluted further
so that settling can take place. The bulk of the sand-size
mineral rapidly settles and is withdrawn as sand tailings.
Most of the bitumen rapidly floats (settles upward) to form
a coherent mass known as froth which is recovered by skimming
the settling vessel. A third stream may be withdrawn from
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the settling vessel. This stream, called the middlings drag
stream, may be subjected to a third processing step, scaven-
ging. This step provides incremental recovery o~ suspended
bitumen and can be accomplished by conventional ~roth flo-
tation.
The mineral particle size distribution is particularly
significant to operation of the hot water process and to
sludge accumulation. The terms sand, silt, clay, and fines
are used in this speci~ication as particle size designations
wherein sand is siliceous material which will not pass a 325
mesh screen. Silt will pass 325 mesh, but is larger than 2
microns, and clay is material smaller than two microns in-
cluding some siliceous material of that size.
Conditioning tar sands ior the recovery o~ bitumen
consists of heating the tar sand/water ieed mixture to pro-
cess temperature (180-200F), physical mixing of the pulp
to uniform composition and consistency, and the consumption
(by chemical reaction) oi the caustic or other reagents added.
Under these conditions, bitumen is stripped ~rom the individ-
ual sand grains and mixed into the pulp in the ~orm of dis-
crete droplets oi a particle size on the same order as that
oi the sand grains. The same process conditions, it turns
out, are also ideal ior accomplishing deilocculation of the
clays which occur naturally in the tar sand feed. Deiloccu-
lation, or dispersion, means breaking down the naturally
occurring aggregates of clay particles to produce a slurry
oi individual particles. Thus, during conditioning, a large
~raction o~ the clay particles become well dispersed and
mixed throughout the pulp.
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Those skilled in the art will therefore understand
that the conditioning process, which prepares the resource
(bitumen) for efficient recovery during the following process
steps also prepares the clays to be the most difficult to
deal with in the tailings disposal operations.
The second process step, called separation, is actu-
ally the bitumen recovery step, (the separation having already
occurred during conditioning). The conditioned tar sand pulp
is screened to remove rocks and unconditionable lumps of tar
sands and clay. The reject material, "screen oversize", is
discarded. The screened pulp is further diluted with water
to promote two settling processes: globules of bitumen, es-
sentially mineral-free, settle (float) upward to form a co-
herent mass of froth on the surface of the separation cells;
and, at the same time, mineral particles, particularly the -
sand size mineral, settle down and are removed from the bottom
of the separation cell as tailings. The medium through which
these two settling processes take place is called the middlings.
Middlings consists primarily of water, with suspended fine
material and bitumen particles.
The particle sizes and densities of the sand and of the
bitumen particles are relatively fixed. The parameter which
influences the settling processes most is the viscosity of
the middlings. Characteristically, as the fines content rises
above a certain threshold (which varies according to the com-
position of the fines), viscosity rapidly achieves high values
with theeffect that the settling processes essentially stop.
In this operation condition, the separation cell is said to
be "upset". Little or no oil is recovered, and all streams
exiting the cell have about the same composition as the feed.
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As feed fines content increases, more water must be
used in the process to maintain middlings viscosity within
the operable range.
The third step o~ the hot water process is scavenging.
The feed fines content sets the process water requirement
through the need to control middlings viscosity which, as
noted above, is governed by the clay/water ratio. It is
usually necessary to withdraw a drag stream of middlings to
maintain the separation cell material balance, and this stream
of middlings can be scavenged for recovery of incremental
amounts o~ bitumen. Air flotation is an effective scavenging
method for this middlings stream.
Final extraction or froth clean-up is usually accom-
plished by centrifugation. Froth from primary extraction is
diluted with naptha, and the diluted froth is then subjected
to a two stage centrifugation. This process yields an oil
product of an essentially pure (diluted) bitumen. Water and
mineral removed from the froth constitute an additional tail-
ing stream which must be disposed of.
In the terminology of extractive processing, tailings
is the throwaway material generated in the course of extrac-
ting the valuable material from an ore. In tar sands pro-
cessing, tailings consist of the whole tar sand ore body
plus net additions of process water less only the recovered
bitumen product. Tar sand tailings can be subdivided into
three categories; viz: (1) screen oversize, (2) sand tailings
(the fraction that settles rapidly), and (3) tailings sludge
(the fraction that settles slowly). Screen oversize is typ-
ically collected and handled as a separate stream.
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Tailings disposal is all the operations required to
place the tailings in a final resting place. One obvious
long-range goal of tailings disposal is to replace the tail-
ings in the mined out area in a satisfactory form. Thus, there
are two main operating modes for tailings disposal: (1) dike
building-hydraulic conveying of tailings followed by mechan-
ical compaction of the sand tailings fraction; and (2) over-
boarding-hydraulic transport with no mechanical compaction.
Recently, in view of the high level oi` ecological
consciousness in Canada and the United States, technical in-
terest in tar sands operation has begun to focus on tailings
disposal. The concept of tar sands tailings disposal is
straighti'orward. Visualize mining one cubic foot of tar
sands. This leaves a one cubic foot hole in the ground. The
ore is processed to recover the resource (bitumen) and the
remainder, including both process material and the gangue
constitutes the tailings; tailings that are not valuable and
are to be disposed of. In tar sands processing, the main
process material is water and the gangue is mostly sand with
some silt and clay. Physically, the tailings consists o~ a
solid part (sand tailings) and a more or less fluid part
(sludge). The most satisfactory place to dispose oi these
tailings is, of course, the existing one cubic foot hole in
the ground. It turns out, however, that the sand tailings from
one cubic foot or ore occupy just above one cubic foot. The
amount of sludge is a variable, depending on ore quality and
process conditions, but may run up to 0.3 cubic feet. The
tailings simply will not iit into the hole in the ground.
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The historical literature covering the hot water pro-
cess for the recovery of bitumen from tar sands contains little
in the way of a recognition that a net accumulation of liquid
tailings or sludge would occur. Based on analysis of field
test unit operations which led to the Great Canadian Oil
Sands plant design near Ft. McMurray, Alberta, the existence
of sludge accumulation was predicted. This accumulation came
to be called the "pond water problem". Observations during
start-up and early commercial operations at Ft. McMurray
(1967-1969) were of insufficient precision to confirm the
prediction. Since 1969, commercial operating data have con-
firmed the accumulation in GCOS' tailings disposal area of
a layer of fine material and water (sludge) which settles and
compacts only very slowly, if at all.
At the GCOS plant, for dike building, tailings are
conveyed hydraulically to the disposal area and discharged
onto the top of a sand dike which is constructed to serve as
an impoundment for a pool of liquid contained inside. On the
dike, sand settles rapidly, and a slurry oi fines, water and
minor amounts of bitumen flows into the pond interior. The
settled sand is mechanically compacted to build the dike to
a higher level. The slurry which drains into the pond in-
terior commences stratification in settling over a time scale
; of months to years. As a result of this long-term settling,
two layers form. The top 5 to 10 feet of the pool are a
layer of relatively clear water containing 0 to 5 wt.% solids.
Below this clear water layer is a discontinuity in solids
content. Over a matter of a few feet, solids content in-
creases to 10 - 15 wt.%, and thereafter, solids content increases
regularly toward the pond bottom. In the deepest parts of the
pond, solid contents of over 50 wt.% have been recorded. This
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second layer is called the sludge layer. The solids content
of the sludge layer increases regularly from top to bottom
by a factor of 4-5. The clay-water ratio in this layer in-
creases also, but by a lower factor of 1.5 - 2.5. The clays,
dispersed during processing, apparently have partially re-
flocculated into a very fragile gel network. Through this
gel, fines of larger-than-clay sizes are slowly settling.
Overboarding is the operation in which tailings are
discharged over the top of the sand dike directly into the
liquid pool. A rapid and slow settling process occur but
their distinction is not as sharp as in dike building and
no mechanical compaction is carried out. The sand portion
of the tailings settles rapidly to form a gently sloping
beach extending from the discharge point toward the pond in-
terior. As the sand settles, fines and water drain into the
pool and commence long-term settling.
In summary: (1) tar sands contain clay minerals, ~2)
in the hot water extraction process, most of the clays become ~ !
dispersed in the process streams and traverse the circuit,
exiting in the tailings, (3) the amount of process water in-
put is fixed by the clay content of the feed and the need to
control viscosity of the middlings stream, (4) the amount
of water required for middlings viscosity control represents
a large volume relative to the volume of the ore itself, and
(5) upon disposal, clays settle only very, very slowly; thus,
the process water component of tailings is only partially ~`
available for reuse via recycle. That which cannot be re-
cycled represents a net accumulation of tailings sludge.
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The pond water problem is then: to devise long-term
economically and ecologically acceptable means to eliminate,
minimize, or permanently dispose o~, the accumulation of
liquid tailings or sludge.
Flocculation of the drag stream in order to improve
the settling characteristics thereto has been proposed and
practiced in the prior art. In flocculation, individual par-
ticles (in this case clay particles) are united into rather
loosely bound agglomerates or flocs. The degree of floccu-
lation is controlled by the probability of collisions be-
tween the clay particles and their tendency toward adhesion
after collision. Agitation increases the probability of
collision and adhesion tendency is increased by the addition
of flocculants.
Reagents act as flocculants through one or a combina-
tion of three general mechanisms: (1) neutralization of the
electrical repulsive forces surrounding the small particles
which enables the van der Waals cohesive force to hold the
particles together once they have collided; (2) precipita-
tion of voluminous flocs, such as metal hydroxides, that en-
trap fine particles; and (3) bridging of particles by natural
or synthetic, long-chain, high-molecular-weight polymers.
These polyelectrolytes are beIieved to act by adsorption
(by ester formation or hydrogen bonding) of hydroxyl or amide
groups on solid surfaces, each polymer chain bridging be-
tween more than one solid particle in the suspension.
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Among the various reagents which have been found
useful for flocculating clay are are: aluminum chloride,
polyalkylene oxides, such as polyethylene oxide, compounds
of calcium such as calcium hydroxide, calcium oxide, calcium
chloride, calcium nitrate, calcium acid phosphate, calcium
sulfate, calcium tartrate, calcium citrate, calcium sulfonate,
calcium lactate, the calcium salt of ethylene diamine tetra-
acetate and similar organic sequestering agents. Also useful
are quar flour or a high molecular weight acrylamide polymer
such as polyacrylamide or a copolymer or acrylamide and a
copolymerizable carboxylic acid such as acrylic acid. Ad-
ditional flocculants which have been considered include
the polymers of acrylic or mephacrylic acid derivitives, for
example, acrylic acid, methacrylic acid, the alkali metal
and ammonium salts of acrylic acid or methacrylic acid,
acrylamide methacrylamide, the aminoaklyl acrylates, the ~;
aminoalkyl acrylamides, the aminoaklyl methacrylamides and
the N-alkyl substituted aminoaklyl esters oi' either acrylic
or methacrylic acids.
Those skilled in the art will understand that a sat-
isfactory solution to the tar sands "pond water problem"
must be economically, as well as ecologically acceptable.
Despite the considerable attention which has been paid to
the use of flocculants in the treatment of tailings from the
hot water extraction process, the quantities needed to obtain
ecologically acceptable results have not been economically
acceptable.
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Objects of the Invcntion
It is therefore a broad object of our invention to
provide mcans for improving the effectiveness of flocculating
agents for treating tar sands tailing streams which carry
suspended clay particles.
It is another object of our invention to provide such
means which is economical to cmploy in the treatment of
high volume tar sands tailings streams.
It is a more particular object of our invention to reduce
the particle size of the suspended clay particles in tar
sands tailings streams by grinding in order to render the
sludge more receptive to subsequently applied flocculating
agents.
Brief Summary of *he Invention
Briefly, these and other objects of the invention are
achieved by grinding the sludge prior to its exposure to
a flocculating agent in such a manner that the particle size
is reduced, preferably through planar cleavage perpendicular
to the c-axis of the particle crystals in order to create
mostly negatively charged sites. As a result of the grinding,
the polymer chains attach themselves better onto the clay
colloids and hence reduce the amount of flocculant required
to produce the same flocculation obtained without employing
the grinding step.
Description of the Drawing
The subject matter of the invention is particularly
pointed out and distinctly claimed in the concluding portion
of the specification. The invention, however, both as to the
manner in which the grinding step is carried out and its
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rclationshi~ to the tot~l process, may bcst be undcrstood by
refcrence to the following dcscription taken in connection
with the drawing of which the single figure is a schematic
representation of a hot water extraction process wherein the
invention finds particular use.
Detailed Description of the Invention
- Referring now to the single figure, bituminous tar sands
are fed into the system through a line 1 and pass to a
conditioning drum or muller 18. Water and steam are introduced
to the muller through another line 2. The total water so
introduced in liquid and vapor form is a minor amount based
on the weight of the tar sands processed. The tar sands,
conditioned with water, pass through a line 3 to the feed
sump 19 which serves as a zone for diluting the pulp with
additional water before passage to the separation zone 20.
The pulp tar sands are continuously flushed from the feed
sump 19 through a line 4 into a separator 20. The settling
zone within the se~arator 20 is relatively quiescent so that ~;~
bituminous froth rises to the top and is withdrawn by a
line 5 while the bulk of the sand settles to the bottom as
a tailings layer which is withdrawn through line 6.
A middlings stream is withdrawn through line 7 to be
processed as described below. Another middlings stream,
which is relatively oil-rich compared to the stream withdrawn
through line 7, is withdrawn from the cell via line 8 to a
flotation scavenger zone 21. In this zone, an air flotation ;;
operatlon is conducted to cause the formation of additional `-
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oil roth which passcs from the scavcngcr zone through line
9 in mixture with the primary froth from the separator 20
to a froth settler 22. An oil-lean water stream is removed
from the bottom of the scavenger zone 21 through line 10 to
be further processed as described below. In the settler zone
22, some further oil-lean water is withdrawn from the froth
and removed through line 11 to be mixed with the oil-lean
water from the flotation scavenger zone, the sand tailings
stream from the separation zone and a portion of a lower
middlings withdrawn from the separation zone.
The oil-lean water from the froth settler, the scavenger
zone, and the separator, and the tailings from the settler,
all of which make up an effluent discharge stream, are treated
in the sand separation zone 20 by, for example, a single
gravity settling process. The sand is withdrawn by a line
13 and discarded and a process water stream is withdrawn by
a line 14 to a grinding zone 30.
In the grinding zone 30, the fines-containing process
water is ground by ball-grinding, sand-grinding, or any
other suitable grinding process by which the fines particle
size can be substantially reduced, mostly through planar
cleavage perpendicular to the c-axis of the particle
crystals. The effluent from the grinding zone is conducted,
by line 32, to a flocculation zone 24.
In the flocculation zone 24, a substantial amount of the
clay suspended in the effluent is coagulated, and a slurry
of coagulated clay and process water is withdrawn in line
15 to a centrifuge zone 25. A portion of the effluent
from the flocculation zone 24 may also be passed, via a line
40, to a settling pond 36 wherein the enhanced flocculation
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characteristics permit a more rapid and complete settling
process to take place over a substantial period of time.
In the ccntrifuge zone ~5, coagulated clay is separated
from the process water and discarded via line 16. Water
substantially reduced in clay and sand content compared to
the effluent discharge is recovered from the centrifuge
zone and is recycled by a line 17 to be mixed with fresh water
and charged into the hot water extraction process.
Sludge from the subsurface reg;on of the settling zone
36 is withdrawn through line 40 by a pump 38 and is
transferred by a line 34 to the grinding zone 30 ~or treatment
. .
as hereinabove described. The proportions of the respective
feeds through llne 14 and 34 to the grinding zone 30 depend
upon the needs of a given system at a given time, and those
skilled in the art will appreciate that each source may supply
from zero to 100~ of the charge to the grinding zone 30 at
a given time. Similarly, the proportions of the effluent
from the flocculation zone 24 which are applied to the
centrifuge zone 25 or returned to the settling pond 36 will
20 also depend upon the system needs, and each may vary from ;~
zero to 100% of the total.
In order to quantitatively evaluate the results obtained
with the method of the present in~ention, grinding experimellts
were performed on 4.1 and 11.7 wt.% oil-removed sludge
suspensions. Ball-mill grinding was employed although the
vast quantities which must be processed in a tar sands ~ ~
operation may be as readily, and more economically, handled -~ ~-
by other forms of grinding such as sand-grinding.
Ball-grinding was carried out for twenty-four hours on
the sludge suspension, and the ground sludge was thcn exl~osed
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to various concentrations of a cationic polyacrylamide
flocculant 573 C and then centrifuged for predctermincd
periods. The results were compared to corresponding
unground sludge samples exposed to like flocculant
concentrations. The results of the comparative tests for the
4.1 wt.% sludge samples are given in Table 1.
~A~LE 1 Effect of ball-mill grTndlng versus no-grlnding on flocculant requi~e-
rents for flocculation of the sludge ~initial concent~ation 4.1~
S~RL ulant Conc Treatment tumulative Centrifugation Time at 280 9 mi
~ o 40 1 60 320
.
Sol Id Conccntrat ion, ~ (W~W)
50 ppm Unground 4.1 4.4 4.6 ' 6.1
81 100 " " 4.1 4.5 5.1 18.7
82 200 " " 4.~ 4.6 19.9 24.5
83 400 " " 4.11 7 . 2 20. 5 23 . 6
~ 84 50 ~ Cround 4.116;7 21.9 23.6
; 85 100 " " 4.117.2 21.9 23.1
~6 200 " " . 4.117.2- 21.9 22.7
87 400 " " 4.11~.0 20.2 21.2
It will be observed that the solids concentration obtained
was approximately the same for all prcportions of flocculant
added to the ball-ground sludge. That îs, after ball grinding,
the lowest proportion ~50 ppm) was as effective as thc
highest proportion t400 ppm) of flocculant added to the
unground suspensions as may be understood from a comparison
of the results obtained from samples 84 and 83, respectively.
The test procedure was also conducted on 11.7 wt.%
oil-removed sludge suspensions to determine if corresponding
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effects could be obtained on somcwhat thic~er suspensions.
The results of the 11.7 wt.~ tests are prescnted in
Table 2.
T~ble 2 Effect of ball-mill grlndtng vcrsus no-grtnding on locculant require-
ments for flocculation of the sludge (initial conccntration 11.7~,U/~)
S~RL 573C Floc~ Treatmcnt Cumulative Ccntrifusation Timc at 2809, m;n.Lab. ulant Conc.
, 0 ~0 160 320
Solid Conccntration, ~ (U/U)
128 0 ppmVnground 11.7 11.7 12.0 12.6
-0 129 100 " " 11.7 12.0 12.1 12.8
130 200 " ^' 11-7 ~ 1l-9 12.4 13.3
131 400 " " 11.7 12;2 16.6 25.6
132 o ppmGround , 11.7 16.0 24.3 ~ 2B.3
133 100 " " 11.7 19.5 24.6 -2B.7
134 200 " " 11.7 20.6 26.3 30.~
135 40û " ", 1~-7 2!.0 26.3 30.6
It will be observed from Table 2 that grinding the
sludge suspension before the addition of the flocculant very
significantly reduces the amount of flocculant required
as indicated by comparison of sample #131 and samples
132 and 135 which produced similar results in terms of
solids concentration reached upon centri~ugation. This. it
will be understood that grinding sludge prior to flocculation
results in a very significant decrease in the amount of ;
flocculant needed to obtain meaningful results.
Plocculants, natural or synthetic, usually are of high ;
molecular weight which may range from 500,000 to 6-8 million.
It is generally believed that the higher the molecular
weight, the better the flocculating ability of the polymer.
However, the flocculating ability varies considerably
depending upon the nature of the material which is to be
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flocculated. In order for the flocculant to be effective,
it is necessary forlthe polymer chain to be attached to
several particles and subsequently to engulf or entrap
other particles into a floc. Cationic polymers are
bonded to the negatively-charged sites and anionic on the
positively-charged sites, ~usually edges) on the clay surfaces.
Non-ionic polymers a~tach themselves through hydrogen
bonding and non-specific van der Waals bonds. Grinding
according to the method of this invention apparently increases
the electronic charge through the creation of new surfaces
and also increascs the specific surface area, this enhancing
the reactivity of the ground material. In addition, the
physical hindrances, such as coating of the exchange sites
with bitumen or amorphous materials, are likely to be removed
by the process. This treatment of wet grinding apparently
makes the polymer chains adapt themselves better onto the
clay colloids and hence reduces-the amount of flocculant
required to produce the same results as are achieved without
grinding.
While the principles of the invention have now been made
clear in an illustrative embodiment, there will be immediately
obvious to those skilled in the art many modifications of
structure, arrangement, proportions, the elements, materials,
and components used in the practice of the invention which
are particulalry adapted for specific environments and
operating requirements without departing from those principles.
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