Note: Descriptions are shown in the official language in which they were submitted.
llZ4895
CROSS-REFERENCE TO RELATED APPLICATIONS
This specification includes material in common with:
Canadian patent application Serial No. 338,892, entitled "Method
of Dewatering the Sludge Layer of an Industrial Tailings Pond"
by Raymond N. Yong; Canadian patent application Serial No. 338,919,
entitled "Dike Building Material Comprising Sand and Treated
Sludge" by Raymond N. Yong; and Canadian patent application
Serial No. 338,921, entitled "Method for Surcharging with Sand
the Sludge Layer of a Tar Sands Tailings Pond" by John O. L.
Roberts. All these related applications are assigned to the same
assignee as this application and were filed on even date herewith.
1124895
BACKGROUND OF THE INVENTION
Tar sands (which are also known as oil sands and bitu-
minous sands) are sand deposits which are impregnated with
dense, viscous, petroleum. Tar sands are found throughout
the world, often in the same geographical areas as conventional
petroleum. The largest deposit, and the only one of present
commercial importance, is in the Athabasca region in the north-
east of the province of Alberta, Canada. This deposit is be-
lieved to contain perhaps 700 billion-one t;illion barrels of
bitumen. For comparison, 700 billion barrels is just about
equal to the world-wide reserves of conventional oil, 60% of'
which is found in the Middle East. While much of the Athabasca
deposit is not economically recoverable on a commercial scale
with current technology, nonetheless, a substantial portion is
situated at, or very near, the surface where it may fairly
readily be mined and processed into synthetic crude oil, and
this procedure is being carried out commercially on a very
large scale by Great Canadian Oil Sands (now Suncor Inc. - Oil
Sands Division) and Syncrude near Fort McMurray, Alberta.
Athabasca tar sands is a three-component mixture of bitu-
men, mineral and water. Bitumen is the valuable cornponent for
the extraction of which tar sands are mined and processed. The
bitumen content is variable, averaging 12wt~ of the deposit,
but ranging from zero to 18wt%. Water typically runs 3 to 6wt%
of the mixture, and generally increases as the bitumen content
decreases. The mineral content is relatively constant, ranging
from 84 to 86wt%.
While several basic extraction methods to separate the
bitumen from the sand have been known for many years, the "hot
3Q water" process is the only one of present commerci~l significance
1~24895
and is employed by both GCOS and Syncrude. The hot water pro-
cess ~or achieving primary extraction of bitumen ~rom ~ar sand
consists of three major process steps (a fourth step, ~inal ex-
traction, is used to clean up the recovered bitumen from down-
stream processing). In the first step, called conditioning, tar
sand is mixed with water and heated with open steam to form a
pulp of 70 to 85wt% solids. Sodium hydroxide or other reagents
are added as required to maintain pH in the range of 8.0-8.5.
In the second step, called separation, the conditioned pulp is
diluted ~urther so that settling can take place. The bulk oi
the sand-size mineral rapidly settles and is withdrawn as sand
tailings. Most of the bitumen rapidly floats (settles upwardly)
to form a coherent mass known as froth which is recovered by
skimming the settling vessel. A third stream, called the mid-
dlings drag stream, may be withdrawn from the settling vessel
and subjected to a third processing step, scavaging, to provide
incremental recovery of suspended bitumen.
The mineral particle size and type distribution is par-
ticularly significant to the operation of hot water process and
to sludge accumulation. The terms 7'sand", "silt", "clay" and
"fines" are used in the specifica~ion as a simplified approxima-
tion of mineral particle size wherein sand is siliceous material
which will not pass 325 mesh screen, silt will pass 325 mesh,
but is larger than 2 microns and clay is material smaller than
2 microns, including some siliceous material of that size. Fines
includes both silt and clay, but excludes sand. It should be
again noted that ~hese ~esignatio~s are simpli~ied approximations.
For an elegant and in-depth discussion o~ particle size and type
in tar sands sludges, reference may be taken to the article en-
titled 'IMineral Particle Interaction Control of Tar Sand SludgeStability" by Yong and Sethi which appears in The Journal o~
Canadian Petroleum l~echnology, Volume 17, Number 4 (Octo~er-December
1~7~).
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As previously indicated, conditioning tar sands for the
recovery of bitumen consists-of heating the tar sands/water
feed mixture to pracess 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 discreet droplets of a 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 fines,
particularly the clays, which occur naturally in the tar sand
feed. Deflocculation, or dispersion, means breaking down the
naturally occurin~ aggregates 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 throughout the pulp.
Those skilled in the art will therefore understand that
the conditioning process, which prepares the ~itumen resource
for efficient recovery during the succeeding process steps, also
prep~res the clays to be the most difficult to deal with in the
tailings disposal operation.
The second p~ocess step, called separation, is actually
the bitumen recovery step since separation occurs during the
conditioning step. The conditioned tar sand pulp is first
screened to remove rocks and unconditionable lumps of tar sands
and clay, and the reject material, "screen oversize", is discarded.
The screened pulp is then further diluted with water to promote
two settling processes: globules of bitumen, essential.ly mineral-
free, float upwardly to form a coherent mass of froth on the
surface of the separation cells; and, at the same time, mineral
particles, particularly the sand-sized mineral, settle downwardly
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~124895
and are removed from the bottom of the separation cell as tail-
ings. The medium through which these two settling processes
take place is called the middlings. The 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, and viscosity is directly related to fines content.
Characteristically, as the fines content rises above a certain
threshold, which varies according to the composition o~ the
fines, middlings viscosity rapidly reaches high values with the
effect that the settling processes essentially stop. In this
operating 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. Thus, as feed
fines content increases, more water must be 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 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 re-
co~ery of incremental amounts of bitumen. Air flotation is an
effective scavenging method for this middlings stream.
Final extraction or froth clean-up is typically accomp-
lished by centrifugation. Froth from primary extraction is
diluted with naphtha, and the diluted froth is then subjected
to a two-stage centrifugation. This process yields an essentially
pure diluted bitumen oil product. Water and mineral removed from
l~Z4895
the froth during this step constitutes an additional tailings
stream which must be disposed of.
In the terminology oi' extractive processing, tailings is
the throw-away material generated in the course of extracting
the valuable material i'rom an ore. In tar sands processing,
tailings consists of the whole tar sand ore body plus net ad-
ditions of process water less only the recovered bitumen productO
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-
ly). Screen oversize is typically collected and handled as a
separate stream.
Recently, in view of the high level of ecological con-
sciousness in Canada, United States, and elsewhere, technical
interests in tar sands operation, as well as other diverse ore
handling operations, has begun to focus on tailings disposal.
The concept of tar sands tailings disposal is straightforward.
If one cubic ~oot of tar sands is mined, a one cubic foot hole
is left in the ground. The ore is processed to recover the bitu-
men fraction, and the remainder, including both process materialand the gangue, constitutes the 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, t~e tailings (other than over-
size3 consist o~ a solid part (sand tailings) and a more or less
fluid part (sludge). The most satisfactory place to dispose of
these tail~ngs is, of course, in the e~isting one cubic foot hole
in the ground. It turns out, however, that the sand tailings
alone ~rom the one cubic i'oot of ore occupy just about one cubic
3Q foot. The amount of sludge is variable, depending on ore quality
and process conditions, but averages about 0.3 cu~ic ~eet. The
tailings simply will not fit back into the hole in the ground.
~12489S
The historical literature covering the hot water process
for the recovery of bitumen from tar sands contains little in
the way of a recognition that a net accumulation of sludge would
occur. Based on analysis of field test unit operations which led
to the Great Canadian Oil Sands plant design near Fort 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
Fort McMurray (1967-1969) were of insufficient precision to con-
firm the prediction. Since 1969, commercial operating data haveconfirmed the accumulation in GCOS' tailings disposal area o~ a
sludge layer of fines material and water which settles and com-
pacts only very slowly, if at all, after a few years. For a
number of reasons, this sludge layer, in common with similar
sludge layers observed in tailings ponds associated with mining
and extracting processes of many kinds, is particularly important
and difficult to deal with.
At the GCOS plant, for dike building, tailings are con-
veyed hydraulically to the-disposal area and discharged onto the
top of a sand dike which is constructed to serve as an impound-
ment for the pool of fluid contained inside. On the dike, the
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 strengthen the dike as it is
built to a higher level. The slurry which flows into the pond's
interior commences stratification in settling over a time scale
of months to years.
O~e~boarding is the operation in which tailings are dis-
charged over the top of the sand dike directly into the liquid
pool. Rapid and slow settling processes occur, but their dis-
tinction is not as sharp as in dike building, and no mechanical
llZ4895
compaction is carried out. The sand portion of the tailings
settles rapidly to form a gently sloping beach extending from
the discharge position towards the pond interior. As the sand
settles, fines and water commence long-term settling in the pond.
The exceedingly complex behavior and characteristics of
tailings ponds have only recently come to be understood beyond
the simplistic categorization of various zones such as clarified
water, transition, and sludge/slime. Since a tailings pond em-
ployed in conjunction with the hot water process for processing
tar sands is fairly typical, the following characteristics of
the layers or zones in such a tailings pond is a good general
example.
Tailings from the hot water process containing a dilute
suspension of fine materials in water, together with sand, are
discharged to the tailings pond. The formation of sludge by
settling of these tailings is attributable primarily to the
presence of dispersed clay minerals. Many of the factors which
determine the rate at which the clay minerals settle and the
characteristics of the sludge formed are set within the tailings
discharge. These include intitial clay concentration (clay/water
ratio), relative proportions of various clay mineral species,
particle size, condition of clay surfaces and pore water chemis-
try. Experience and laboratory analysis indicate that all these
factors vary significantly from time to time depending on the
composition of the tar sands feed and the process conditions.
Typically, tailings are discharged over the beach (either
directly or from dike construction) where most of the sand settles.
The run-off flows continuously into a ~luid pool or pond from
which water is simultaneously withdrawn as recycle to the tar
3~ sands extraction process. Here, additional important determin-
ants of settlirg behavior are imposed. These include rate of
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inflow and out~low in relation to sur~ace area and clarified
water volume, pond depth, and degree of agitation of pond con-
tents, either through inflows and outflows or via thermal or
by wind effects. While initial temperature is inherent in the
tailings streams, temperatures in the pond are obviously deter-
mined by numerous other factors as well.
Experience and laboratory analyses indicate that when a
partly settled sludge remains undisturbed for between several
months and about two years in a deep pond, it separates into
two distinct layers, a virtually clear water layer on top and
a sludge layer beneath. The density of the sludge layer increases
gradually with depth due mainly to the presence of more sand and
silt particles. These settle either not at all or very slowly
because of the signi~icant yield strength of stagnant sludge.
The clay/water ratio increases only slightly with depth in the
upper part of the pond and scarcely at all in the lower part.
Aiter one or two years, little further change in sludge volume
occurs. Consolidation at the bottom of the pond is so slow that
detection of consolidated material is difficult. Sludge formed
in this manner is virtually unchanging over periods of years or
decades and for practical purposes may be regarded as terminal
sludge.
An active pond involving continuous inflow and outflow is
more complex. Experience and laboratory tests indicate that,
following discharge to the pond, clay particles undergo an aging
process varying in length from a few days to many weeks. Prior
$o completion of the aging process, the clay particles do not
begin to settle. However, once $hey commence to do so, the pro-
cess proceeds quite rapidly according to the principles of Stokes
Law until a claytwater ratio o~ about 0.13/1 is reached at which
other factors evidently predominate over Stokes Law. In the upper-
l~Z4895
most part of a well managed pond, these effects result in a moreor less clear water layer at the top underlaid by a layer o~
relatively dilute sludge more or less sharply differentiated
from it. This may be termed the sedimentation zone; its volume
is determined by the rate of clay inflow and the average aging
time required. If the water layer is permitted to become too
small in relation to the clay inflow, water out~low and aging
tîme, the upper part of the pond becomes overloaded, the clear
water layer virtually disappears and the sedimentation zone be-
comes much larger since clay is then recycled through the process.GCOS operated under such conditions or on the edge of them through
much of the early years.
Sludge in the lower part of a deep active pond which has
been in operation ~or some years is similar to that from an inac-
tive pond; i.e., it may be regarded as terminal sludge. The space
below the sedimentation zone and above the terminal sludge may be
regarded as a transition zone lacking clear boundaries at top and
bottom. It is characterized by a gradual increase in clay/water
ratio with depth and owes its existence to the long time needed
to attain the terminal sludge condition. Its thickness is pri-
marily a function of the average clay inflow rate in relation to
volume.
In summary, an active pond normally has a well-defined
clear water layer at the top which can, however, disappear i~
overloading occurs. Beneath this is sludge which increases in
density with depth. There are generally no clearly defined boun-
daries within this sludge except on occasion a layer of separated
bitumen near the interface between water and sludge. I~owever,
the sludge may be considered as consisting of three zones each
3~ involving successively larger orders of magnitude of time scale
for measurable dewatering to occur, and each characterized by the
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1124895
predominancs of differing dewatering parameters. These three
zones may be termed respectively a sedimentation zo~e, a trans-
ition zone and a terminal ~ludge zone.
Thus, (1) tar sands contain clay mineral, (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, (4) the amount of water required for mid-
dlings viscosity control represents a large volume relative tothe volume of the ore itself, and (5) upon disposal, clays settle
only very, very slowly; thus, the water component of tailings is
only partially available for reuse via recycle. That which can-
not be recycled represents a net accumulation of tailings sludge.
The pond ~ater problem, therefore, is to devise long-term,
economically and ecologically acceptable means to eliminate,
minimize, or permanently dispose of the accumulation of sludge.
Experience has demonstrated that the problem requîres a multi-
faceted approach toward its solution, and the present invention
is directed at achieving one aspect of the solution: a more
thoroughly dewatered sludge layer which, as a consequential re-
sult, obtains a greater quantity of clarified water for recircu-
lation into the process if necessary in the particular system.
Flocculation of the tailings stream in order to improve
the settling characteristics of an industrial process tailings
pond has been proposed and practiced in the prior art. In flocc~-
lation9 individual particles are united into rather loosely-bound
ag~lomerates or flocs. The degree of flocculation is controlled
by the probability of collision bet~een the particles and their
tende~cy toward adhesion after collision. Agitation increases
the pr~bability of collision, and adhesion tendency is increased
by the addition of a flocc~lant.
llZ4895
Reagents act as flocculants through one or a combination
of three general mechanisms: tl~ neutralization of the electrical
repulsive forces surrounding the small particles which enables
the van de Waals cohesive force to nold 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 weight polymers. These polyelectrolytes are believed
to act by absorption (by ester formation or hydrogen bonding) of
hydroxyl or amide groups on solid surfaces, each polymer chain
bridging between ~lore than one solid particle in the suspension.
A remarkable number of flocculants have been employed in
the prior art to obtain precipitation of particles in tailings
ponds of various industrial processes as well as in sewage treat-
ment facilities. However, a distinct step forward in the art has
been achieved by the use of hydrolyzed corn and potato starch
flocculants as described in co-pending Canadian application
S.N. 275,214, filed March 31, 1977 and entitled "Destabiliz~tion
of Sludge with Hydrolyzed Starch Flocculants" and by the use of
wheat starch flocculants as set forth in co-pending Canadian ap-
plication S.N. 308,619, filed August 2, 1978, and also entitled
"Destabilization of Sludge with Hydrolyzed Starch Flocculants".
These specific hydrolyzed starch flocculants, particularly taking
into account the economics of carrying out flocculation on a
large scale, enjoy high performance characteristics for their
ability to bring about rapid precipitation to a substantially
terminal settled condition. This characteristic is especially
valuable for use in those processes, such as the hot water process
for obtaining bitumen from tar sands, in which there is a critical
need to recycle clarified water from the tailings pond back into
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llZ4895
the process. However, experience has indicated that the simple
use of these hydrolyzed starch flocculants, or for that matter
any other known flocculant, results in very little, if any, im-
provement on the ultimate degree of dewatering of the sludge
layer. That is, the terminal status of the sludge layer is just
about the same as would be obtained over a much longer period of
time by natural settling processes, and this terminal condition
is unsatisfactory in that it includes too much water, is too vol-
uminous, and is too unstable.
Nonetheless, it is not accurate to say that all character-
istics of ~ sludge layer obtained as a result of flocculants by
the aforementioned hydrolyzed starch flocculants is the same as
that achieved naturally or by the use of other flocculants~ In
point of fact, certain very desirable characteristics to the
sludge layer are obtained from the use of the hydrolyzed starch
flocculants which are not achieved by natural settling or by the
use of any other flocculant presently known, and it is on the
appreciation and use of these characteristics that the present
invention is based. More particularly, it has been found that
~0 the permeability and shear strength characteristics of the sludge
layer are both very much increased; as a result, previously im-
possible dewatering techniques may be employed to compact and
stabilize the sludge layer and to extract additional amounts of
clarified water therefrom.
~t has been proposed in the past, as another approach to
alleviating pond water problems, to store the fines in the inter-
stices between the sand grains in the material employed for dike
building. Such a process is disclosed in Canadian patent
1,063,956, issued October 9, 1979, and entitled "Method of
Sludge Disposal Related to the Hot Water Extraction of Tar Sands"
and corresponding U.S. patent 4,008,146, issued February 15, 1977.
1124895
The experience with the procedure described in that reference
is ihat the height to which the dike can be built is somewhat
limited; however, it has now been discovered that if the sludge
mixed with the sand to prepare the dike building material has
been treated with the aforementioned hydrolyzed starch floccu-
lants, the strength of the resultant material is notably in-
creased such that the dike can be built higher, thereby not
only permitting a deeper tailings pond, but also storing more
sludge in the interstices between the sand grains comprising
the dike.
SUMMARY OF THE INVENTION
._
It is a broad object of this invention to minimize the
vo'lume of sludge stored in an industrial process tailings pond.
In another aspect, it is an object of this invention to
provide means for mixing sand with the sludge layer of an in-
dustrial process tailings pond to increase its sel~-weight
thereby achieving a porous piston ef~ect for compressing, and
thus dewatering, the sludge layer.
In a more specific aspect, it is an object of this in-
vention to provide means for controlling the sludge layer of
an industrial process tailings pond by mixing sand with sludge
which has been treated with hydrolyzed starch flocculant to
increase its strength such that the mixture, by self-weight,
achieves a porous piston effect for compressing, and thus de-
~-atering, the sludge layer.
D~SCRIPTION OF THE DRAWTNGS
. _
The subject ma-tter of the invention is particularly pointed
Ollt and distinctly claimed in the conclucling portion of the
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llZ4895
specification. The invention, however, both as to organization
and method of operation, may best be understood by reference to
the following description taken i~ conjunction with the accom-
panying drawing of which:
Figure 1 is a somewhat simplified block diagram of a hot
water process for covering bituminous tar sands into bituminous
froth for subsequent upgrading to synthetic crude oil;
Figure 2 is a partial cross-sectional view which illus-
trates, conceptually and simplistically, the distribution of
water and sludge in a tailings pond associated with the apparatus
illustrated in Figure 1;
Figure 3 is a view similar to Figure 2 and sho~s the re-
sults of prior art attempts to surcharge the sludge layer of a
tailings pond with sand;
Figure 4 illustrates the effect of surcharging the sludge
layer of the tailings pond with sand after the sludge layer has
been treated with specific hydrolyzed starch flocculants;
Figure 5 illustrates the effect obtained by alternating
layers of surcharging sand with sludge previously treated with
specific hydrolyzed starch flocculants;
Figure 6 illustrates the e~fect of internal surcharging
obtained by mixing sand with sludge which has been or is simul~
taneously treated with specific hydrolyzed starch flocculants;
Figure 7 illustrates the effect of employing a combination
of internal and external surcharging techniques with sludge which
has been treated with specific hydrolyzed starch flocculants;
Figure 8 illustrates a general approach for increasing
the amount of fines stored in the interstices between adjacent
sand grains in a dike;
Figure 9 illustrates an exemplary specific method for adding
hydrolyzed starch flocculant to the tailings of a tar sands hot
water process;
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1124895
Figure 10 illustrates a method ~or the addition o~
hydrolyzed starch ~locculants to sludge, accompanied by sand
inclusion, ~ound in the tailings system of a tar sands hot water
process;
Figure 11 illustrates a combination of the technique~
illustrated in Figures 9 and 10 by which a more rapid recovery
of clarified water may be obtained; and
Figures 12a, 12b, 12c, and 12d illustrate a sequence oi
operations by which external sand surcharge to the sludge la~er
of a tailings pond located in a cold environment can be obtained.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to Figure 1, bituminous tar sands are ~ed
into the system through a line 1 and passed to a conditioning
drum or muller 18. ~ater and steam are introduced into the
muller through another line 2. The total water so introduced
in li~uid and vapor form is a minor amount based on the weight
of the tar sands processed. The tar sands, heated and conditioned
with steam and water, pass through a line 3 to a screen 29. The
purpose of the screen 29 is to remove ~rom the pulp any debris
such as rock or oversized lumps of clay as indicated generally
at 30. The oversize material is discarded at a suitable site.
The conditioned pulp passes through a line 31 to a ~eed sump 19
which serves as a zone for diluting the pulp with additional
water before it enters a separation zone 20.
The diluted pulp is continuously flushed from the feed
sump 19 through a line 4 into the separation zone 20. The settling
zone within the separator 20 is relatively ~uiescent so that ~itu-
minous ~roth rises to the top and is withdrawn through a line 5
while the bu]k of the sand component settles to the bottom as a
tailings layer which is withdrawn through line ~. It will be
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1124895
understood, of course, that the tailings streams can be trans-
ferred individually, with or without downstream treatment, as
indicated by the alternate lines 23, 24 and optional treatment
processes 70, 80.
A relatively bitumen-rich middlings stream is withdrawn
through line 8 to maintain the middlings layer between the froth
and the sand layer at a functional viscosity. This middlings
material is transferred to a flotation scavenger zone 21 where
an air flotation operation is conducted to bring about the for-
mation of additional bituminous froth which passes from thescavenger zone 21 through line 9, in conjunction with the pri-
mary froth from the separation zone 20 passing through line 5,
to a froth settler zone 22. A bitumen-lean water stream is
removed from the bottom of the scavenger zone 21 through line 10.
In the froth settler zone 22, some further bitumen-lean water
is withdrawn ~rom the froth and removed through line 11 to be
mixed with the bitumen-lean water stream from the flotation scav-
enger æone and the sand tailings stream from the separation zone
20. The bitumen from the settler 22 is removed through line 12
for further treatment, typically final extraction.
Bitumen-lean water from the froth settler 22, the scavenger
zone 21, and the separation zone 20, all of which make up an ef-
fluent discharge stream carried by line 7, are discharged into
a tailings pond 15 which has a clarified water layer 26 and a
sludge layer 27. The sand included in the tailings stream quickly
settles in the region 14, and the fines-containing water flows
into the body of the pond 1~ where settling takes place. Water
from the clarified water layer 26 may be withdrawn by a pump 28
for recycle through a line 17 to be mixed with fresh makeup water
and charged into the hot water p~ocess.
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1124895
Referring now to Figure 2, the sludge layer 27 of the
tailings pond 15 is overlayed with a clarified water layer 26.
(As previously noted, this is a considerable simplification,
but is adequate and appropriate for an understanding of the
present invention ) The sand bottom 23 of the pond defines
the lower limit of the sludge layer 27 which, as previously
discussed, increases a mineral-to-water ratio from top to bottom.
The characteristics of the sludge layer 27 so formed is unaccep-
tably and insufficiently dewatered and compacted to minimize
the pond volume required to contain the sludge and to obtain
a stable sludge structure.
It has been proposed in the past to "surcharge" a sludge
layer with a layer of sand whereby the sand acts as a permeable
piston to compress the sludge and force water out of it. All
attempts to carry out this surcharging concept have met with con-
plete failure or have been performed under conditions which yield
only marginal, if any, benefits under very limited conditions.
Seej by way of example, U.S. Patent 4,036,752, issued July l9,
1977, and entitled "Dewatering Clay Slurries."
What has been observed in practice, when such techniques
have been attempted in large, relatively deep tailings ponds, is
illustrated in ~igure 3. As a layer of sand 2~ is broadcast over
the sludge layer 27, the sand layer is observed to tilt and dump
through the sludge layer as shown generally in the region 32.
The sludge layer is simply incapable of supporting a useful sur-
charge of sandO Thus, in the prior art, sand surcharging has been
theoretically interesting, but totally impractical as a process
for dewatering and compacting sludge, and this has been the case
whether the sludge was allowed to settle naturally or the settling
process was accelerated by the use of flocculants.
llZ4895
However, it has been determined that the use of the
specific hydrolyzed starch flocculants described in the above-
referenced Canadian patent applications produces a sludge layer
with remarkably enhanced shear strength and permeability charac-
teristics, and an appreciation of this fact resulted in recon-
sideration of the heretofore substantially impractical and dis-
carded sand surcharge concept. Throughout the remainder of this
specification the term "hydrolyzed starch flocculant" means one
of the specific starch flocculants disclosed in th~ above-referenced
Canadian applications or a chemical or fully-functional equivalent
comprising, for example, hydrolyzed starch with polyelectrolytes
and a low dielectric constant fluid rendered in aqueous form.
As shown in Figure 4, a sludge layer 33 which has been
treated with a hydrolyzed starch flocculant is capable of sup-
porting a substantial sand surcharge which operates as a porous
piston to compact and dewater the sludge layer. In addition,
the observed improved permeability of the sludge layer 33 resul-
ting from treatment with a hydrolyzed starch flocculant affords
an enhancement to the degree of compaction of dewatering which can
be achieved. Furthermore, as shown in Figure 5, sludge layer 33,
treated with a hydrolyzed starch flocculant, is sufficiently
strong that a second layer of sludge 35 may be layed over the
sand layer 34 and the second sludge layer, itself, may be sub-
jected to a surcharge brought about by another sand layer 36.
For relatively deep tailings ponds, a number of such alternate
layers of treated sludge and sand may be employed to obtain a
very high degree of compaction and dewatering.
It has also been proposed in the prior art to mix sludge,
which has been flocculant-treated with sand to o~tain a ma~erial
which, in effect, i5 "internally surcharged." One may refer, by
way o~ example to U.S. Patent 3,680,693 issued August 1, 1972,
895
and entitled "Process for the Treatment of Slime and Waste
Solids." While this technique has been promising, the amount
of sand which can be added to the sludge has been limited by
the strength of the sludge and, as previously noted, no pre-
viously known floccuIant affords the strength and permeability
enhancement to the sludge layer observed to result from use
of the hydrolyzed starch flocculants previously identified.
It has now been found that sand mixed with sludge treated with
one of these hydrolyzed starch flocculants results in a material
which, indeed, exhibits important internal surcharge character-
istics resulting in a compacted sand/sludge layer 37 as illus-
treated in Figure 6. Furthermore, as shown in Figure 7, a
combination of internal and external sand surcharging techniques
may be employed in which the mixed sand/treated-sludge layer 37
is itself overlayed with a sand layer 38. In addition, of course,
the multilayering technique illustrated in Figure 5 is equally
applicable~
It has been obser~ed at the Suncor-Oil Sands Division plant
that on the order of 35~ of the fines (and a larger portion of
the clay component) is discharged into the tailings pond; the
remainder is stored in the interstices between adjacent sand grains
or is discarded as lumps which are part of the oversize. It has
been proposed in the past to increase the quantity of silt, and
particularly the quantity of clay, stored in the interstices be-
tween adjacent sand grains in the material employed to build a
pond-impounding dike. By way of example, one may refer to pre-
viously referenced U.S~ ~atent 4,008,146, issued February 15, 1977,
and entitled "Method of Sludge Disposal Related to the Hot Water
Extraction o~ T~r Sands~' and also to the corresponding Canadian
patent 1,063,956, i~sued Octoker 9, 1379. As disclosed in
that re~erence, sand and sludge are admi~ed in a pre-
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~124895
scribed fashion, and the resultant material is discharged at
the dike site to effect dike building. This is an important
concept, but its use in practice has been somewhat limited be-
cause the stability of the resulting dike structure is insuf-
ficient to permit building the dike to a height which represents
storage of meaningful additional quantities of fin~s.
It has now been determined that, if sand is admixed with
sludge which has been treated with a hydrolyzed starch flocculant,
an important increase in the strength of the resulting material,
when employed for dike building, is observed such that the resul-
ting structure is much more stable. Thus, substantially higher
dikes can be built, and significantly large quantities of silt
and, particularly, clay can be stored in the interstices between
adjacent sand grains in the material.
An exemplary procedure for storing silt and clay particles
in the interstices between adjacent grains of sand in a sand dike
is illustrated in ~igure 8. A tailings pond 41 is enclosed by
dike walls 42 and contains a clarified water layer 43 and a sludge
layer 44. Sludge is withdrawn from the pond 41 via sludge with-
drawal means 51 and is transferred to a line 47 by a pump 46
which is supported by flotation means ~5 on the surface of the
pond 41. The sludge material is transferred from the line 47 into
a line 50 where it is combined with, by way of example, tailings
material from the hot water extraction process for recovering
bitumen from tar sands, This waste water stream from the extrac-
tion process is primarily water and sand, but inclues minor amounts
of silt, clay, and bitumen. Thus ~ the combined streams which are
transferred from line 50 into a settling zone 52 contain a sub-
stantial amount of sand.
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~lZ4895
In the settling zone 52, an upper layer 53 and a lower
layer 54 are formed. The upper layer is withdrawn through line
55 and is transferred into a line 56, where it is combined with
beach run-off water transferred from zone 61 via line 57, and
added to the retention pond 41.
The lower layer 54 in settling zone 52 is withdrawn through
a line 58 and is transferred to an inclined sand pile 59 situated
adjacent a dike 60. The lower layer 54 of the settling zone 52
typically comprises on the order of 2% bitumen, 39% sand, 9% silt,
4% clay, and 46% water. This mixture is dispersed over the sand
pile to form additional sand layers whereby a part of the clay,
silt, and water in the stream is retained in the interstices of
the sand layers. The remainder of the aqueous stream percolates
down the inclined sand pile zone and settles into the retention
zone 61. A pump 62 in the retention zone 61 withdraws the aqueous
portion of that pond and transfers it into the line 57 where, as
previously noted, it is combined with the stream from the upper
layer of zone 52 in the line 56.
Thus, a part of the sludge from the tailings pond 41 is
removed and dispersed with the sand of the wastewater stream over
the pond dike wall to carry out dike building. Substantially in-
creased quantities of the sludge withdrawn from the pond are stored
in the interstices of the sand pile zone 59 thereby providing a
means for reducin~ the solids content and, more importantly, clay
content of the tailings pond 41. It may be noted that tailings
pond 41 and retention zone 61 can be unitary wherein the sand
pile 59 is located on the dike walls 42 of the tailings pond 41.
In that manner, only one pond is necessary to conduct the whole
process, and there is no need to transfer clarified water ~-om
the zone 61 to ~he zone 41.
llZ48~5
If, as previously discussed, the sludge layer 44 in the
retentlon pond 41 has been treated with the hydrolyzed starch
flocculant, the strength of the resultant sand/sludge mixture
discharged onto the sand pile 59 to increase the height of the
dike will be very much stronger, thereby permitting the dike to
be built to a substantially greater height without compromising
its integrity.
An exemplary system for adding hydrolyzed starch floccu-
lant to the tailings from the separation zone 20 discharged
through the line 6 and alternative line 23 (Figure 1) is illus-
trated in Figure 9. Tailings from the separation cell are trans-
ferred, via line 23, to a sand separation zone 71 in which the
sand component rapidly settles to the bottom for discharge as
wet sand through a line 72 to a tailings sump 73. Tailings water
is withdrawn from the sand separation zone 71 at a higher point
via line 74 into which the hydrolyzed starch flocculant is intro-
duced through a line 75. The flocculated tailings water is then
discharged into a thickening pond 76 which functions as a holding
zone during the several days residence period required for the
Z~ flocculant to settle the fines (principally clay) well below the
sur~ace. Optionally, the hydrolyzed starch flocculant may be
broadcast on the surface of the thickening pond as indicated in
the region 77, or a combination of flocculant dosing techniques
may be applied to the tailings water. Virtually clear water
may be withdrawn from the upper layer of the thickening pond 76
~ia line 78 for recycle into the hot water process.
Thickened tailings water is drawn from the lower regions
of the thickening pond 76 and is transferred, via line 79, to
the tailings sump 73. The content of the tailings sump 73, which
will be a sand and flocculated thickened tailings water mixture,
is ~ithdrawn via line 81 and transferred to a sand pond 82. In
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llZ489~
the sand pond 82, further settling takes place and, because of
the use of the hydrolyzed starch flocculant, an effect takes
place corresponding to that illustrated in Figure 6; i.e., a
higher degree of dewatering and compaction results than would
be obtained if another type of flocculant were used. As a re-
sult, a clarified water layer 160 is also present on the surface
of the sand pond 82, and this clarified water layer may be with-
drawn by pump 83 for transfer via line 84 to a primary tailings
pond 85.
Tailings from downstream incremental bitumen recovery pro-
cesses, which ess~ntially comprise fines-laden water, may also be
conducted via line 24 for discharge into the primary tailings pond
8~. Hydrolyzed starch flocculant may also be added to this tail-
ings stream as indicated at 87 in order to maintain the floccu-
lant dosage in the primary tailings pond 85 at an optimum level.
Clarified water is withdrawn by pump 88 from the upper level of
the primary tailings pond 85 for recycle via line 89 to the hot
water process.
Figure 10 illustrates an examplary system for accomplish-
ing addition of hydrolyzed starchflocculant to sludge accompanied
by sand inclusion to obtain the effect illustrated in Figure 6
and discussed above. Tailings from the separation cell are con-
veyed via line 23 to a sand separation zone 90 wherein the sand
component rapidly settles to the bottom for discharge through
line 91 to a tailings sump 92. Fines-containing tailings water
is withdrawn from an upper region of the sand separation zone 90
through line 93 for discharge into a primary tailings pond 94.
The primary tailings pond 94 also receives, via line 24, the tail-
ings from the downstream processes ~or extracting incremental
amounts of bitumen. As indicated at 95, hydrGlyzed starch floccu-
lant may be added to this stream to maintain the flocculant dosage
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1124895
in the primary tailings pond at a desired level. Clarified
water is withdrawn, by pump 96, for recycle via line 97 b~ck
into the hot water process.
Sludge is withdrawn from the sludge layer of primary
tailings pond 94 by a pump 98 and is transferred via line 99
to an auxiliary pond 100 which functions essentially as a sludge
holding area. Sludge is withdrawn from the auxiliary pond 100
by a pump 101 and is transferred via line 102 to the tailings
sump 92. It will be understood that, if the sludge withdrawal
rates from the primary tailings pond 94 is commensurate with the
capacity of the tailings sump 92, the transfer of sludge to the
auxiliary pond 100 need not necessarily be carried out. As a
practical matter, such nice adjustments cannot always be achieved,
and it is therefore often desirable to provide the auxiliary pond
100.
Hydrolyzed starch flocculant is added to the wet sand/sludge
mixture by injecting it into the sludge stream from the auxiliary
pond 100 (as indicated at 103), by adding the flocculant to the
tailings sump 92 (as indicated at 104), and/or by adding the
flocculant to the mixture discharged from the tailings sump 92
through line 105 for discharge into a third pond 106. In the
third pond 106, a high degree of dewatering and compaction of
the sand/hydrolyzed starch flocculated sludge mixture, generally
as depicted in ~igure 6, takes place. As a result, clarified
water from a layer 161 may be withdrawn, by pump 107 from the
upper layer of the pond 106 and transferred via line 108 to the
primary tailings pond 94 from which it is available as recycle
water to the hot water process.
It may be noted that the system sludge has a bitumen con-
tent which may be sufficient for economic recovery as the priceof crude oil continues to increase. For that reason, provision
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l~Z4~395
may be made to bypass section 102a of line 102 by a circuit
which includes line 109, optional tertiary bitumen recovery
process 100, and line 111.
Fi~ure 11 illustrates a system which combines the tech-
niques illustrated in Figures 9 and 10 in order to obtæin a
higher rate o~ recovery of recycle water and, particularly,
to minimize the containment volume required to hold the sludge.
Such a higher water recovery rate may be dictated by the fresh
water requirements of the entire hot water process system or,
in a given installation, may only be necessary during periods
when relatively poor (i.e., high in clay content~ tar sands feed
is being worked. The containment volume problem is critical at
sites of limited area and is, for e~ample, more important at
the Suncor-Oil Sands Division lease site than the fresh water
aspect.
Tailings from the separation cell are transferred, via
line 23, to a sand separation zone 140 in which the sand compon-
ent rapidly settles to the bottom for discharge as wet sand throug~
a line 141 to a tailings sump 142. Tailings water is withdrawn
from the sand separation zone at a higher point via line 143
into which hydrolyzed starch flocculant is introduced through
a line 151. The flocculated tailings water is then discharged
into a thickening pond 152 which functions as a holding zone dur-
ing the residence period (on the order of up to one day~ required
for the flocculant to settle the fines (principally clay) well
below the surface. Optionally, the hydrolyzed starch flocculant
may be broadcast on the surface of the thickening pond as indica-
ted in the region 157, or a combination of flocculant dosing
techniques may be administered to the tailings water. Virtually
clear recycle water may be withdrawn from the upper level of the
thickening pond 152 via line 157 ~or rec~cle into the hot water
- -26-
895
process. Thickened tailings water is withdrawn from the lower
region of the thickening pond 152 and is transferred, via line
153, to the tailings sump 142.
Because clay particles undergo an aging process varying
in length from a few days to many weeks before they begin to
settle, an individual practical installation may require the
addition of a holding pond 170 which receives the tailings
water via a line 171. Aged tailings water is withdrawn through
line 172 and transferred to the thickening pond 152.
A first tailings pond 144 receives, via line 24, the tail-
ings from downstream processes for extractin~ incremental amounts
of bitumen. As indicated at 145, ~ydrolyzed starch flocculant
may be added to this stream to maintain the flocculant dosage in
the first tailings pond at a desired level. Clarified water is
withdrawn, by pump 146, for recycle, via line 147, back into the
hot water process along with the recycle water obtained from the
thickening pond 152.
Sludge is withdrawn from the sludge layer of the first
tailings pond 144 by pump 148 and is transferred via line 149 to
a second tailings pond 150 which functions essentially as a sludge
holding area. Sludge is withdrawn from the lower region of the
second tailings pond 150 by a pump 131 and is transferred via
line 132 to the tailings sump 142. It will be understood that
if the sludge withdrawal rate from the first tailings pond 144
is commensurate with the capacity of the tailings sump 142, the
trans~er of sludge to the second tailings pond 150 need not neces-
sarily be carried out.
~ydrolyzed starch floccùlant is added to the wet sand/sludge
mixture by injecting it into the sludge stream from the second
tailings pond 150 as indicated at 133, by adding the flocculant
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11i~4895
to the tailings sump 142 as indicated at 134, and/or by adding
the flocculant to the sand/sludge mixture discharged from the
tailings sump 142 through line 135 into a third tailings pond
136, as generally indicated at 139. In the third tailings pond
136 a high degree of dewatering and compaction of the sand/hydro-
lyzed starch flocculated sludge mixture, in the manner depicted
in Figure 6, is obtained. As a result, clarified water may be
withdrawn, by pump 137, from the upper layer 162 of the third
tailings pond 136 for transfer via line 138 to the first tailings
pond 144 from which it is available as recycle water to the hot
water process.
As previously noted, the sludge has a significant bitumen
content. Hence, optional tertiary bitumen recovery may be sought
in the bypass loop comprising line 154, process 155, and line 156,
disposed around the line section 132a between the pump 131 and
the tailings sump 142.
It may be noted, with respect to the discussions relevant
to Figures 8, ~, 10, and 11, that, in many instances, the plur-
ality of ponds illustrated ~or clarity in explaining the processes
may often be, in practice, a single pond. In that instance, cer-
tain of the process steps, such as pumping clarified water and/or
sludge between the ponds, takes place naturally so that no special
provisions need be made for carrying out these steps.
It will be appreciated by those skilled in the art, of
course, that the systems illustrated in Figures 9, 10, and 11
are merely exemplary of approaches toward practical installation
which will vary with the process material, type of process, cli-
mate, and according to many other factors. The approaches in-
volved are basically to employ the thickening pond, sludge recycled
3~ from the field, or a combination of both. The ways in which
these approaches can be applied together or separately are quite
numerous. Merely by way of example, (1) either one or both
sludges may be added to the tailings before sand separation; (2)
either one or both sludges may be added to the tailings after
sand separation (such as into a tailings sump); (3) extra stages
involving repeated sand separation and remixing with fresh sludge
may be added with recycle of surplus sludge back to the thicken-
ing pond or out to the field; or (~) a settling vessel or cyclone
may be used for sand separation or the displacement technique
disclosed within previously referenced U.S. Patent 4,088,146,
may be used.
Figures 12a, 12b, 12c, and 12d illustrate sequential steps
in a process by which an external sand surcharge achieving the
result illustrated in Figures 4, 5, and 7 can be obtained in re-
gions (such as northwest Alberta~ having harsh winters. Consider~
as shown in Figure 12a, a first summer period in which a first
auxiliary pond 110 contains sludge received, by way of example,
from a primary tailings pond, not.shown in Figures 12a, 12b,
12c, or 12d. The sludge is withdrawn by pump 111, for transfer~
via line 112 to a second auxiliary pond 113. Hydrolyzed starch
flocculant may be added, as indicated at 114, if the sludge has
not previously been treated with the starch ~locculant or if the
dosage needs to be renewed or increased. The sludge transfer
from pond llO to pond 113 is carried out throughout the summer~
Subsequently, as illustrated in Figure 12b, during the first
winter, sludge from the primary tailings pond is transferred into
the first auxiliary pond 110 via line 115. Because of the harshly
cold environment at the site of the Atha~asca tar sands, a thick
ice layer 116 forms on top the sludge 117, After the ice ha~s be-
come sufficiently thick to bear the weight of heavy machinery,
layer 118 o~ sand is spread on top of the ice layer 1l6
~29-
llZ4895
Upon spring thaw, the ice layer 116 melts to permit the
sand layer 118 to settle on top the hydrolyzed starch flocculant
treated sludge layer 117 to be supported thereby and to function
as a porous piston to effect further dewatering and compaction
of the sludge layer 117. During the second summer, Figure 12c,
sludge is again withdrawn from the first auxiliary pond 110 by
the pump 111 and is transferred via line 112 to the second aux-
iliary pond 113 for deposit as another sludge layer 119 over the
sand layer 118. Hydrolyzed starch flocculant is added as in-
dicated at 114 if the transferred sludge has not been previously
treated to the desired dosage.
During a second winter, Figure 12d, sludge from a primary
tailings pond is again received into the first auxiliary pond
110 via line 115. In the second auxiliary pond, a new ice layer
120 forms on top the second sludge layer 11~, a~d when the ice
layer 120 reaches sufficient thickness, a second layer of sand
121 is spread over it such that, upon spring thaw, the sand layer
121 settles atop the sludge layer 119 to obtain additional ex-
ternal surcharging of the entire system belbw it.
The foregoing yearly cycle may be repeated until the cap-
acity of the second auxiliary pond is reached whereupon another
auxiliary pond can begin to receive sludge from the first aux-
iliary pond 110.
It will be readily apparent that many diverse techniques
may be employed to emplace a sand surcharge over a sludge layer
in a tailings pond. For example, the sand may simply be broad-
cast over the pond surface as illustrated in previously referenced
U.S. Patent 4,036,752, or any other wor~able technique may be used
to obtain the effect illustrated in Yigure 4, etc., so long as
3~ the sludge layer is first treated with hydrolyzed starch floccu-
lant to improve its shear strength and permeability characteristics.
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1~24~95
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 struc-
ture, arrangement, proportions, the elements, materials, and
components used in the practice of the invention which are par-
ticularly adapted for specific environments and operation re-
quirements without departing from those principles.
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