Note: Descriptions are shown in the official language in which they were submitted.
Background of the Invention ^
This invention relates to the hot water process for
treating bituminous sands, such as Athabasca tar sands,
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 asconventional 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 the middle east.
. .
' ' ~
- . ., .. ~
.
Athabasca tar sand is a thrcc-component mixture of
bitumen, mineral and water. Bitumen is the value for the
extraction of which tar sands are mi~ed and proccssed.
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 bitumen
from tar sands consists of three major process steps (a
fourth step, final extraction, is used to clean up the
recovered bitumen for downstream processing.~ In the first
step, called conditioning, tar sand is mix&d 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of the sand-size mineral rapidly settles and is withdrawn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 cohcrent mass known as froth which
is recovered by skimming tlle settling vessel. A third
stream may be withdrawn from the settling vessel. This
stream, called the middlings drag stream, may be subjected
to a third processing step, scavenging. This step provides
incremental recovery of suspended bitumen and can be
accomplished by conventional froth flotation.
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 specification as particle si~e
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 including some siliceous material of that size.
Conditioning tar sands for the recovery of bitumen
consists of heating the tar sand/water feed mixture to process
temperature (180-200F~, physical mixing of the pulp to
uniform composition and consistency, and the consumption
(by chemical reaction) of the caustic or other reagents
added. Under these conditions, bitumen is stripped from the
individual sand grains and mixed into the pulp in the form
of discrete droplets of a partic]e size on the same order
as that of the sand grains. The same process conditions,
it turns out, are also ideal for accomplishing
deflocculation of the clays which occur naturally in the tar
sand feed. Deflocculation, or dispersion, means breaking
down the natural]y occurringaggregates of clay particles
to produce a slurry of individual particles. Thus, during
conditioning, a large fraction of the clay particles become
well dispersed and mixed througllollt the pulp.
s~
Those skillcd in the art will thercforc understand that
the conditioning process, which prcpares the resource
(bitumen) for efficient recovery during the, following process
steps also prepares the clays to be the most difficul~ to
deal with in the tailings disposal operations.
The second process step, called separation, is actually
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,
essentially mineTal-free, settle (float) upward to form a
coherent mass of froth on the surface of the separation cells;
and, at the same time, mineral particles, particularly the
sand size mincral, 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
suspendcd 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 thresllold (which varies according
to the composition of the fines), viscosity rapidly achieves
high values with the effect that the settling processes
essentially stop. In this operating condition, the scparation
cell is said to be "upset". Little or no oil is rccovered,
and all streams exiting the cell have abou~ the same composition
as the feed.
.
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s~
As feed fines content increases, more water must he
used in the process to maintain middlings viscosity within
the operable range.
The third step of 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 of bitumen. Air flotation is an
effective scavenging method for this middlings stream.
Final extraction or froth clean-up is usually accomplished
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 tailing
stream which must be disposed of.
In the terminology of extractive processing, tailings
is the throwaway material generated in the course of extract-
ing the valuable material from an ore. In tar sands
processing, 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 slow]y).
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Screen oversize is typically collected and handled as a
separate stream.
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
tailings in the mined out area in a satisfactory foTm.
Thus, there are two main operating modes for tailings
disposal: (1) dike building-hydraulic conveying of tailings
followed by mechanical compaction of the sand tailings
fract;on; and (2) overboarding-hydraulic transport with no
mechanical compaction.
Recently, in view of the high level of ecological
consciousness in Canada and the ~nited States, technical
interest in tar sands operation has begun to focus on
tailings disposal. The concept of tar sands tailings disposal
is straightforward. ~isualize 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 dis~osed 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 of a solid part (sand tailings) and a more or less
fluid part (sludge). The most satisfactory place to
dispose of these tailings is, of course, the existing one
c~lbic foot hole in the ground. It turns out, however, that
the sand tailings from the one cubic foot of ore occupy
just about one cubic foot. The amount of sludge is a
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s~ -
variable, dcl)cnding on ore quality and process conditions,
but may run up to 0.3 cubic feet. The tailings simply
will not fit into the hole in the ground.
The historical literature coveringthe hot water process
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 (1967-69) were of insufficient precision to
confirm the prediction. Since 1969, commerical operating
data have confirmed 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 of 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 interior 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
o to 5 wt.% solids. Below this clcar l~ater layer is a
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discontinuity in solids content. Over a matter of a few
- feet, solids content increases to 10-15 wt.%, and thereafter,
solids content increases reg~larly toward the pond bottom.
In the deepest parts of the pond, solid contents of over
50 wt.% have been recorded. This second layer is
called the sludge layer. ~he solîds content of the sludge
layer increases regularly from top to bottom by a factor
of 4-5. The clay-water ratio in this layer increases also,
but by a lower factor of 1.5 - 2.5. The clays, dispersed
dur~ng processing, apparently have partially reflocculated
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
interior. 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 input is fixed by the clay content of the feed and
the need to control viscosity of the middlings stream,
~) the amount of water required for middlings viscosity control
represents a largc volume re~ative to the volume of the ore
3~ itself, and (5) upon disposal, clays settle only very very
.
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Y~5~ .
slowly; tl-us, the process water componcnt of tailings is only
partially availablc for reuse via recycle. That which
can't be recyclcd rcpresents a net accumulation of ~ailings
sludge.
The pond ~ater problem is then: to devise long-term
economically and ecologically acceptable means to eliminate,
minimize, or permanently dispose of, the accumulation of
liquid tailings or sludge.
Flocculation of the drag stream in order to improve the
settling characteristics theretohas been proposed and
practiced in the prior art. In flocculation, individual
particles (in this case clay particles) are united into rather
loosely bound agglomerates or flocs. The degree of
flocculation is controlled by the probability of collisions
between the clay particles and their tendency toward adhesio
after collision. Agitation increases the probability of
collision and adhesion tendency is increased by the addition
of flocculants.
Reagents act as flocculants through one OT a combination
of threegeneral mechanisms: (1) neutralization of the
electrical repu~sive forces surrounding the small particles
which enables the vander Waals cohesive force to hold the
particles together once they have collided; (2) precipitation
of voluminous flocs, such as metal hydroxides, that entrap
fine particles; and (3) bridging of particles by natural
or synthetic, long-chain, high-molecular-~eight polymers.
These polyelectrolytes are believed to act by adsorption
(by ester formation or hydrogcn bonding) of hydroxyl or amide
groups on solid surfaces, each polymcr chain bridging
bet~een morc than one solid particle in the suspension.
.
g
~mong the various reagents which have been found
useful for flocculating clay are: aluminum chloride,
polyalkylene oxides, such as polyethylene oxide, compounds
of calcium such as calcium hydroxide, calcium o~ide,
calcium chloride, calcium nitrate, calc;um acid phosphate,
calcium sulfate, calcium tartrate, calcium citrate,
calcium sulfonate, calcium lactate, the calcium salt of
ethylene diamine tetraacetate and similar organic
sequestering agents. Also useful are quar flour or a high
10 molecular weight acrylamide polymer such as polyacrylamide
or a copolymer of acrylamide and a copolymerizable
carboxylic acid such as acrylic acid. Additional flocculants
which have been considered include the polymers of acrylic
or methacrylic acid derivitives, for example3 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 of ei'ther acrylic or methacrylic acids.
Those skilled in the art will understand that a satisfactory
solution to the "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 for tar sands, no flocculant, or flocculant family
known in the prior art has been able to meet these
fundamental criteria.
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Objects of t}lC Invention
_
It is therefore a broad objcct of our invention to
providc an effective flocculating agent for treating tar
sands tailing streams which carry suspended clay particles.
It isanother objcct of our invention to provide such
a flocculating agent which is economical to prepare and
employ in the treatment of tar sands tailing streams.
In another aspect, it is yet another object of our
invention to provide such a flocculant which is safe and
easy to handle and which itself offers no ecologically
undesirable side effects.
It is a still further object of our invention to
provide a flocculant which does not require the prior removal
of oil to be effective in flocculating sludge suspensions
within the tailing stream from a hot water bitumen
extraction process.
Brief SuJn ary of the Invention_ _
~ riefly, these and other objects of the invention are
achieved by employing synthesized flocculants comprising
starches. Starches are polysaccharides containing many
monosaccharides joined together in long chains. Upon
complcte hydrolysis by chemical or enzymatic means, starch
yields monosaccharides. Ilydrolyzed corn and potato
starches are effective as flocculants in destabilizing
dilute as well as thick sludge suspensions. Potato starch
flocculants are generally superior to corn starch flocculants,
and those potato starch flocculants are equally effective
on oil-removed and no-oil removed-sludge suspensions.
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Descri~tion of thc Drawi~
Thc sub~ect matter o the invcntion 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 flocculants are prepared
and the method of employing them, may best be understood
by reference to the following description 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.
_etailed Description of the Invention
Referring now to the single f;gure, 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 separator 20 is relatively
quiescent so that bituminous froth rises to the top and
is withdrawn via 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,
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h~
which is relatively oil-rich comllared to thc stream
withdrawn through line 7, is withdrawn ~rom the cell
via line 8 to a flotation scavenger zone 21. In this
zone, an air flotation operation is conducted to cause
the ~ormation of additional oil froth which passes from the
scavenger 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 Z2, some further
oil-lean water is withdrawn from the froth and removed
through line 11 to be mixed with the oil lean water stream
from the flotation scavenger zone, the sand tailings stream
from the separation zone and a portion o-f the lower middlings
withdrawn from the separation zone. The bitumen from the
settler is removed through line 12 for further treatment.
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 simple gravity setting process. The sand is withdrawn
by a line 13 and discarded, and a process water stream is
withdrawn by a line 14 to the flocculation zone 24.
In the flocculation zone 24, a substantial amount of
clay suspended in the process water is coagulated~ and a
slurry of coagulated clay and process water is withdrawn
in line 15 to a centrifuge zone 25. In the centrifuge zone,
coagulated clay is separatcd from the process water and
discarded via line 16. ~ater substantially rcduced in clay
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and s~lnd con~ent com~ red 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 process.
As previously discussed, a substantial amount of
flocculants have been investigated and none are known to
have been both effective and economical when used in
treating tailings from the hot water process for extracting
bitumen from tar sands. I-lowever, according to the present
invention, it has been found that hydrolyzed starches
synthesized from corn and potato starches can effectively
meet these criteria. The major fraction of starch per se
is water insoluble. To prepzre the hydrolyzed starch,
a 20,000 ppm stock solution was prepared by refluxing
a mixture of the starch and an aqueous so~ution containing
the requisite amount of electrolyte. The hydrolysis was
considered complete when the insoluble starch was converted
into a clear colloidal solution. Henceforth in this
specification, these hydrolyzed starches will be referred
to as starch flocculants. A summary of the prepared starch
f]occulants is given in Table 1 on the following page:
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TA~LE 1. Sumrn~ry of prepared starct~ floccul~nts from corn an~ potato starches
_____ _ ________ __ _
SMRL lype of St~rch Nature ~nd Concentration of Electrolyt~ A~ded
L b. Flocculant
_ _ _ __ .
1 Na starch 0.05 N NaOH
2 Ca starch 0.05 N C~(OH)2
3 AQ starch 0.10 N AQCQ3
4 Na AQ starch 0.0~ N ~aOH ~ 200 ppm AQ
Ca AQ starch 0.05 ~ Ca(OH2) + 200 ppm A~
6 Na AS,P04 starch 0.05 N ~'aOH + 200 ppm AQ ~- 20~ ~pm P04
7 Ca ~QPOIl starch 0.05 N Ca(OH2) i 200 ppm AQ + 200 ppm P04
8 AQP04 s~arch O.l N AQCQ3 + 200 ppm P04
A~ was a~ded using ~Q2(S04)3.18 H20
P04 was a~ded using 1~a3P011.12 ~2
In order to test the effectiveness of the synthesized
starch flocculants, two sludge suspensions containing 5.5
and 17.3 wt.% solids, respect;vely, were employed. In
addition, synthetic polyacrylamide f]occulants were used
for co~nparative purposes. Test criteria used were:
reflitration rates, self-settling and sedimentation upon
centrifugation at a relative centrifugal force of 790g
at the bottom of the tube for 30 minutes. The results of
reflitration tests and preliminary tests on self-
set~ling indicated that the starch flocculants prepared
from potato starch were superior to those prepared from
corn st~rch; therefore, Table 2 presents only the
sedimentation-upon-centrifugation studies done with potato
starch floccu]ants.
~?~
LE 2. Solids conccntration tn cakc and s-lpcrnatant upon scdimcntatiol~ by
ccntrifugat;on using diffcrcnt flocculants.
__ _
'Trcatment _ _ Flocculant Initial Solids Final Solids Conc,, %(\~/W)
No Type Conccntrat;On Conc-, ~(W/~I) Cake Supcrnatant
. _ _ _ _
Polyacrylamide Floccul ants
1 Nonc (untreated sludge) 17.3 1~2.~ 2.4
2 1820~(anionic) 200 ppm 17.3 39 9 1.1
3 573C (cationic) 200 ~ 17~3 37,7 1.7
lû 4 19n6N(non-ionic) 200 " 17.3 42.~ 2.4
Potato Starch Flocculants
Na Starch 200 ppm 17.3 36,6' o.o
6 AQ starch 200 " 17.3 35.8 o,o
7 Na AQ starch 200 " 17.3 37.0 o.o
Ca ~Q starch 200 " 17.3 36.3 0.0
9 Na AQP01~ starch 200 " 17.3 41.7
Ca ~QP04 starch 200 " 17.3 41.9 0.0
Il AQP04 starch 200 " 17.3 42.~ 0.0
.
,
12 None (untreated sludge) 5.5 35.4 0.4
13 Na AQ starch 200 " 5.5 36.o 0.2
14 Ca AQ starch 200 " 5.5 35.6 0.2
_ .
From the data set forth in table 2, it is evi.dent that the
starch flocculants are decided].y superior to the polyacrylamide
flocculants vis-a-vis the quality of the resultant
supernatant. For those in which no flocculants were used
or in which synthetic polyacrylamide flocculants were used,
the supernatant had up to 2.4 wt.% solids in it, whercas
the runs in which the starch flocculants were employed with
a 17.3 wt.% slud~e concentration had no suspended solids
in the supernatant at all. Among the starch flocculants,
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it appcars that those starchcs containing AQP04 were the
best. ~urther, it was found that the starch flocculants
are e~ually effective on no-oil-removed sludge as in treating
oil-removed sludge whereas the polyacrylamide flocculants
were more effective on oil-removed than on no-oil-removed
sludge suspensions.
The fines contained in the sludge suspension associated
with the hot water process for extracting bitumen from tar
sands consists of primary, as well as secondary minerals.
. 10 Primary minerals, which are mostly quartz and some feldspars,
have very low specific surface areas and little of any
kind of charge. In contrast, the secondary minerals, which
are mostly kaolinite and illite with some montmorillonite
and intergrade mixed-layer minerals, have high specific
surface areas and a substantial amount of negative charge.
There is also some positive charge, usually disposed at
the edges of the crystals of solids.
As previously noted, starches are polysaccharides
containing many monosaccharides joined together in long
chains. Upon complete hydrolysis, by chemical or enzymatic
means, starch yields monosaccharides. Starch consists
primarily of two components: amylose and amylopectin. The
amylose fraction makes up from 10 to 20% of the starch
and is water soluble. The other portion, amylopectic,
constitutes 80 to 90% of the starch and is water insoluble.
; The molecular weight of the starches varies from 10,000
to 1,000,000. The mechanism by which starch functions as
a destabilizing agent for sludge appears to be one where
the free hydroxyl groups of the starch attach themselves
onto the surfaces of solid particles, probably through
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hydrogcn bonding. The clay particles with adsorbed
starch polymcrs are then no longer able to attract water
molecules as before, and hence, attract each other and
are flocculated. The presence of electrolyte in the system
enhances the effectiveness of the starch flocculants by
reducing the repulsive forces between the electric double
layers of the solid particles, thereby ma~ing it easier
for the starch polymer to adsord and form a floc. The
presence of phosphate is notably helpful, because the
starch polymers can readily interlink through this radical.
It may be noted that potato starch contains 0.07 to 0.13%
phosphate and has generally been considered as a better
flocculant than those starches containing no phosphate.
Further, starches with branched chains appear to be more
effective than straight chain varieties.
While the principles of the invention have now been
made clear in an illustrative embodiment, there will be
immediately ohvious 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 particularly adapted for
speciflc environments and operating requirements without
departing from those principles.
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