Note: Descriptions are shown in the official language in which they were submitted.
MP/3-22321 /CGC 2094 ~ 02392246 2oo2-os-2s
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OIL SANDS SEPARATION PROCESS
This invention relates to oil recovery processes; which comprises a main
separation stage and
a waste sedimentation stage. The objective is to treat waste sediment from a
tar sand
processing facility in a novel and efficient manner.
Tar sands (which are also known as oil sands or bituminous sands) are sand
deposits, which
are impregnated with dense, viscous, petroleum. Tar sands are found throughout
the world,
often in the sarr~e geographical areas as conventional petroleum. The largest
deposit, and
the only one of present commercial importance, is in the Athabasca region in
the northeast
of the province of Alberta, Canada. This deposit is believed to contain
perhaps 700 billion to
one trillion barrels of bitumen. A substantial portion of the deposits are
situated at, or very
near, the surface where they may be readily mined and processed into synthetic
crude oil.
This procedure is being carried out commercially on a very large scale near
Fort McMurray,
Alberta.
Athabasca tar sands is a three-component mixture of bitumen, mineral and
water. bitumen is
the valuable component for which the extraction of tar sands are mined and
processed. The
bitumen content varies, averaging about 12wt~/o of the deposit, but ranging
from zero to
18wt%. Water typically runs from 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
knovvn for many yearn the "hot water" process is the one of the most
significant. The
present invention is not limited to a particular separation process and the
following
description is for backing: The hot water process for achieving primary
extraction of
bitumen from tar sand consists of three major process steps (a fourth step,
final extraction, is
used to clean up the recovered bitumen from downstream processing). (n the
first step,
called conditioning, tar sand is mixed with water and heated with open steam
to farm a pulp
of 70 to 85wtMo 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 further so that settling can take place: The bulk of the sand-
size mineral
CA 02392246 2002-06-28
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rapidly settles and is withdrawn as sand al ings. Most of the bitumen rapidly
floafis (settles
upwardly) to form a coherent mass known as froth which is recovered by
skimming the
settling vessel (sometimes called the primary separation vessel or separation
cell). A third
stream, called the middlings drag stream; may be withdrawn from the settling
vessel and
subjected to a third processing step, scavenging, to provide incremental
recovery of
suspended bitumen.
As previously indicated; conditioning tar sands for the recovery of bitumen
consists of
heating the tar sands/water feed mixture o process temperature {180-200 F),
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 deflocculafiion of the fines,
particularly the clays,
which occur naturally in the tar sand feed. Deflocculation; or dispersion,
means breaking
down the naturally occurring 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 bitumen resource for efficient recovery during the succeeding
process steps,
also prepares the clays to be the most difficult to deal with in the tailings
disposal operation.
The second process 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 wo settling processes: globules of bitumen, essentially mineral-
free, float
upwardly to forma coherent mass of froth on the surface of the separation
cells; arid, at the
same time, mineral particles, particularly the sand-sized mineral, settle
downwardly and are
removed from the bottom of the separation cell as tailings. The medium through
which
these two settling processes take place is railed the middlings: The middiings
consists
primarily of water with suspended fine material and bitumen particles.
CA 02392246 2002-06-28
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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 of 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 ail 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 recovery or incremental amounts of bitumen. Air flotation is
an effective
scavenging method for this middlings stream.
Final extraction or froth clean up is typically accomplished 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 the' froth during his step constitutes an
additional tailings
stream, which must be disposed of.
In the terminology of extractive processing; tailings is the throw-away
material generated in
the course of extracting the valuable material from an ore: In,tar sands
processing; tailings
consists 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) coarse or sand tailings (the fraction that settles
rapidly), and (3) fine
or tailings sludge (the fraction that ettles slowly). Screen oversize is
typicaNy collected and
handled as a separate stream.
Recently, in view of the high level of ecological consciousness in Canada,
United States, and
elsewhere, technical interests in tar sands operation, as well another diverse
ore handling
CA 02392246 2002-06-28
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operations, has begun to focus on tailings disposal: The concept of tar sands
tailings
disposal is straightforward. If one cubic foot of tar sands is mined, a one
cubic foot hole is
left in the ground. The ore is processed to recover the bitumen fraction, and
the remainder,
including both process material and 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, the tailings
(other than
oversize) consist of a solid part (sandvtailings) and a more or lessftuid part
(sludge). The
most satisfactory place to dispose of these tailings is; of course, in the
existing one cubic toot
hole in the ground. It turns out, however; that the sand tailings alone from
the one cubic
foot of ore occupy just about one cubic foot. The amount of sludge is
variabEe, depending
on ore quality and process conditions, but average about 0.3 cubic feet. The
tailings simply
will not fit back into the hole in the ground.
Commercial operating data confirms that a sludge layer is accumulating in the
tailings
disposal area which settles and compacts 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. 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 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 mecha~icaliy compacted o 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:
Overboarding is the operation in which tailags are discharged over the top of
the sand dike
directly infio the liquid pool. Rapid and slow settling processes 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 position owards the pond interior. As the sand settles, fines and
water commence
long-term settling in the pond.
CA 02392246 2002-06-28
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
initial clay
concentration (clay/vvater ratio), relative proportions of various clay
mineral species, particle
size, condition of clay surfaces and pore water chemistry. 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-offflows continuously into a fluid
pool or pond
from which water is simultaneously withdrawn as recycle to the tar sands
extraction process.
Here; additional important determinants of settling behavior are imposed.
These include
rate of inflow and outflow in relation to surface area and clarified water
volume, pond depth,
and degree of agitation of pond contents, 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 determined by numerous other factors as well.
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 mare sand and silt particles. These settle
either not at
all or very slowly because of the significant 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. After one or two years; iittlefurther change in sludge volume
occurs.
Consolidation at the bottom of the fond 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.
Following
discharge to the pond, clay particles undergo an aging process varying in
length from a few
days to many weeks. Prior to completion of the aging process, the clay
particles do not
CA 02392246 2002-06-28
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begin to settle. However, once they commence to do so, the process proceeds
quite rapidly
according to the principles of Stokes Law until a clay/water ratio of about
0.13/i is reached
afi which other factors evidently predominate over Stokes law. In the upper
most part of a
well managed pond; these effects result in a more or less clear water layer at
the top
underlaid by a layer of relatively dilute sludge mare or less sharply
differentiated from it. This
layer or diluted sludge may be termed the edimentation 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 outflow and aging time,
the upper
part of the pond becomes overloaded; the clear water layer virtually
disappears and the
sedimentation zone becomes much larger since clay is then recycled through the
process.
Sludge in the lower part of a deep activepond which has been in operation for
some years is
similar to that from an inactive pond; i.e., it may be regarded as terminal
sludge: The space
belovv 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 primarily a function of
the average clay
inflow rate in relation to volume.
In summary, an active pond normally has a vveil-defined clear v eater layer at
the top which
can, however, disappear if overloading occurs. Beneath this is sludge which
increases in
density with depth. There are generally'no clearly defined boundaries within
this sludge
except on occasian a layer of separated bitumen near the interface between
water and
sludge. However, the sludge may be considered as consisting of three zones
each involving
successively larger orders ofimagnitude of ime scale for measurable dewatering
to occur,
and each characterized by the predominance of differing dewatering parameters.
These
three zones may be termed respectively a sedimentation zone, a transition zone
and a
terminal sludge zone.
A continuing challenge remains the development of a long-term economically and
ecologically acceptable means to eliminate, minimize, or permanently dispose
of the
accumulation of sludge. The problem requires a multi-faceted approach tovvard
its solution,
and the present invention is directed at achieving one aspect of the solution:
a more
CA 02392246 2002-06-28
thoroughly and efficiently dewatered sludge layer which, as a consequence;
eliminates the
requirement for an active point to function both as a solids disposal and a
water clarification
site, and thus allows for semi-solid or paste to be disposed of in the primary
disposal site.
Flocculation of the tailings stream in order to improve fihe settling
characteristics of an
industrial process tailings pond has been proposed and practiced in the prior
art. In
flocculation, individual particles are united into rather loosely-bound
agglomerates or flocs.
The degree ofiffocculation is controlled by the probability of collision
befiween the particles
and their tendency toward adhesion after collision. Agitation increases the
probability of
coilision, and adhesion tendency is increased by the addition of a flocculant.
Reagents act as flocculants through one or a combination of three general
mechanisms: (1 )
neutralization of the electrical repulsive forces surrounding the small
particles which enables
the van de Waals cohesive force to hold the particles together once they have
toliided; (2)
precipitation of voluminous flocs; such as metal hydroxides, that entrap fine
particles; and
(3) bridging of particles by natural of 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 more than one solid particle in the suspension.
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 treatment
facilities. However; a
distinct step forward in the art has been achieved by the use of hydrolyzed
corn and potato
starch floccuiants. These specific hydrolyzed starch flocculants (which are
characterized by
the aqueous hydrolysis of the starch in the presence of one or more metal
salts), 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 or
sands, in
which there is a critical need to recycle clarified water back into the
process. However,
experience has indicated that the simple use of these hydrolyzed starch
flocculants, or for
that matter any other known fioccuiant results in very little, if any,
improvement on the
ultimate degree of dewatering of the sludge layer. That is, the terminal
status of the sludge
~ 02392246 2002-06-28
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 voluminous, and is too unstable.
Nonetheless, it is not accurate to say,that all characteristics of a sludge
layer obtained as a
result of flocculation 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 circumstances
that the
present invention is based. More particularly, it has been found that the
permeability and
shear strength characteristics of the sludge layer are both very much
increased; as a result,
previously impossible dewatering techniques may be employed to compact and
stabilize the
sludge layer and to extract additional amounts of clarified water therefrom.
It has been proposed in the past, as another approach to alleviating pond
water problems, to
store the fines in the interstices between the sand grains in the material
employed for dike
building. Such a process is disclosed in Canadian Pat. No. 1,063,956, entitled
"Method of
Sludge Disposal Related to the Hot Water Extraction of Tar Sands" and ixsued
Oct. 9, 1979;
and corresponding U.S. Pat. No. 4,408;146, +ssued Feb: 15; 1977. The
experience with
the procedure described in that reference is that the height to vvhich 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 flocculants, the strength of the resultant material is
notably increased 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 gains comprising
the dike.
There have been numerous proposals in he literature to try to accelerate the
sedimentation
by flocculating the clay waste, and there have been proposals to improve the
structure ofi the
substantially solid clay sediment by adding sands or other materials to the
clay waste:
Examples of disclosures of such mineral recovery processes utilizing
flocculants are U.S. Pat.
Nos: 3,418,237, 3;622,087, 3,707,523; 4,194,969, 4;224,149, 4,251;363,
4,265,770,
CA 02392246 2002-06-28
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4,342,653, 4,555,346, 4;690,752, 5,688,404, 6;077,441 and 6,039,189 which are
each
incorporated herein by reference.
Despite the numerous proposals to use flocculants, in practice it has been
found that their
use frequently is not Lost effective. Even vvhen flocculant is used to promote
sedimentation
and the provision of a supernatant which can be recycled, the quality of the
supernatant
fiends to be rather poor because the supernatant tends to be contaminated with
unflocculated clay particles.
In particular, the main separation stage often includes the use of treatment
chemicals such as
flocculation agents or, especially; flotation agents and the efficiency of
their use is decreased
(and thus the required dosages are increased) when the water which is used in
the flotation
or other procedures during the separation stage contains suspended clay
particles.
In order to minimize contamination and lack of clarity of the supernatant, it
vvould be
desirable to conduct the sedimentation under conditions which provide a
considerable
depth of sedimenting waste, so as to allow for a deep layer of supernatant
above the
sediment; thereby permitting supernatant to be drawn off ata height which is
as far above
the lower sedimented material as is passi6le. Unfortunately it is difficult to
provide for this in
lagoons as they end normally to be relatively shallow. In particular, the
problem becomes
more acute as the lagoons become filled, over the years; with an increasing
depth of
substantially solid clay sediment.
A further problem arises from the fact that it is necessary to make optimum
use of lagoon
areas because of the undesirability of creating new lagoons. Accordingly there
is an
increasing tendency to need to continue using lagoons until it is impossible
to deposit any
more solid clay sediment in them, and so there is an increasing tendency to
want to use
lagoons which are substantially full and are too shallow for useful
sedimentation. There is an
increasing need to utilize lagoon areas more efficiently.
It is known to utilize sedimentation columns, for instance tubular metal
tanks, which are
constructed above ground level. Provided such a column has sufficient height
it will
eventually allow for the formation of a useful depth of supernatant.
Unfortunately the
CA 02392246 2002-06-28
volume of aqueous clay wastes vvhich are generated in oil bearing ands
recovery processes
can be so large that it l impracticable even to contemplate the provision of
column
separating flanks of this type.
It is also well known to, extend the-life of a lagoon by digging the solid
clay sediment out of
it, but this is labor intensive and does not provide any direct solution to
the need to conduct
the recovery process efficiently and to give a good quality supernatant.
The object of the invention is to provide a oil bearing sands recovery process
by which it is
possible to obtain supernatant for flotation or other separation-steps of
improved quality, by
which it is possible to utilize sedimentation lagoons more efficiently, and by
which it is
possible to recover additional hydrocarbon values normally lost to the
sedimentation
process.
An oil bearing sands recovery process according to the invention comprises a
main
separation stage in which oil bearing sand is slurried with water and
separated into an
enriched fraction and a dilute aqueous clay waste; and a waste sedimentation
stage in which
the dilute aqueous clay waste is sedimented in one or more settling lagoons to
provide a
substantially solid clay sediment and supernatant and the supernatant is
recycled to the main
separation stage, and the waste edimentation stage comprises feeding the
dilute aqueous
clay waste into a well which has been formed in the grounds flocculating the
dilute aqueous
waste in the well by mixing polymeric flocculant into the waste, sedimenting
the flocculated
waste in the well to provide a pumpable hickened clay sediment and a
supernatant,
recycling the supernatant from the well back to the main separation stage,
pumping the
thickened clay sediment from beneath the supernatant in the well to one or
more final
lagoons and allowing the thickened clay sediment to undergo further
sedimentation to
provide a substantially solid clay sediment in the one or more final lagoons.
In general, the invention is applicable to any process in which the separation
of oil bearing
sands values from the raw mined rock or other material comprises slurring with
water and
thereby producing large volumes of a diluted aqueous clay waste which is then
subjected to
sedimentation in lagoons. Generally the clay waste has the characteristic of a
very slowly
consolidating clay mineral such as kaolonite, itlite; chlorite; smectite,
montmorillonite,
CA 02392246 2002-06-28
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attapulgite, or other typical naturally occurring clay minerals. The aqueous
clay waste may
also contain fine particles of other minerals such as sand; quartz or other
minerals indigenous
to the oil bearing sands source.
The preferred oil bearing sands recovery process to which the invention is
applied is the
recovery of oil or bitumen values-from oil bearing sands ; for instance as is
practiced in the oil
bearing sands recovery processes in lNestern Canada. Other mineral recovery
processes to
which the invention may be applied include any of those where the natural
settling rate of
the sediment is sufficiently slow that lagoon settlement is appropriate and
where the settling
can be promoted by a flocculating agent: Another particularly preferred
example is bauxite
mining and refining systems.
When the process is oil bearing sands recovery process, the main separation
stage can
involve any of the conventional separation procedures in Such processes. For
instance it may
involve cycloning the slurry and may involve subjecting the slurry to
flotation. Frequently
the material may be recycled one or more times through one or more separation
procedures. The advantage of the present invention is the formation of recycle
stream with
reduced suspended solids. A relatively dirty recycle stream can affect the
viscosity of the
primary separation veSSel and cause excessive losses in the underflow and
excessive solids in
the recovered oil.
The dilute aqueous clay waste is a slurry consisting primarily of waste clay
particles in water.
However the clay slurry or waste may contain some useful course mineral values
or residual
bitumen values. The waste may be primary clay separation waste, typically
having a 6 - 20%
solids content and containing some useful mineral or bitumen value, or a
secondary clay
separation waste typically having a-louver solids content and less coarse
material or bitumen
in it. In particular, the' clay slurry or waste will often consist of or
contain a secondary clay
waste, namely a waste obtained from a flotation or other separation process.
In some
processes, the primary and secondary clay wastes are treated separately while
in other
processes the primary and secondary, clay wastes are mixed together.
In general, the dilute aqueous clay waste generally contains not more than 20%
and uSUally
not more than 10% total clay solids by weight, but usually contains at least
0.1 % and usually
CA 02392246 2002-06-28
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at least 0.596 by weigha total clay solids. The solids generally consist
wholly or mainly of clay
fines but can include some coarser waste or some coarser mineral values, such
that the
coarser material can be sedimented from the clay while the fines remain in
suspension. The
clay fines usually constitute at least 2096 and usually at least 109~o by
weight of tha dry
matter of the waste. The clay fines, which rnay constitute the majority of the
dry matter, will
have physical and chemical characteristics typical of clay waste and, in
particular, these
characteristics are such that lagoon settlement and evaporation is usually the
only practical
way for converting the fines into a substantially solid sediment.
If the dilute aqueous clay waste contains coarse mineral values or other
coarse settleable
material, these materials may be sedimented from the waste as it flows through
a ditch
towards the well (for instance as described in U.S. Pat. No: 5,688,404) or
these values may
be sedimented in a lagoon prior to the well treatment of the pre$ent
invention. Thus the
waste containing the mineral values may be directed into an entry area of a
settling lagoon,
and the resulting reduction in flow velocity which occurs as the waste enters
the lagoon
causes sedimentation of the mineral values primarily in the entry area. The
mineral values
can then be recovered from the base of the entry area, or if appropriate from
the base of the
entire lagoon by excavation.
The dilute aqueous clay waste, optionally after preliminary sedimentation of
coarse materials,
then flows into a well that has been forrrted in the ground. The well can be
located within or
in the vicinity of a primary lagoon, existing mine cuts; a channel, an
emergency spillway or
secondary containment area or virgin territory. In a preferred embodiment, the
well is
located in any convenient location, such as a primary lagoon, more
particularly at the base
or waste entrance of a primary lagoon. The welt rnay be made by excavation of,
for
example, a square, rectangle; circle or oval area in the ground to a
sufficient depth. if
desired the well can be lined in order to prevent erosion of the walls, but
this is usually
unnecessary.
As a result of utilizing,a well, instead of a settlement column, it is
possible to generate a very
large volume and deep settlement zone,very cost'effectively. A supernatant
layer and a
thickened sediment foyer aye foFmed within-the well from the dilute aqueous
waste layer fed
into the well. The supernatant can be removed; pumped or otherwise drawn from
the top
CA 02392246 2002-06-28
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of the well provided this removal does not disturb the thickened;sediment
layer at t'be
bottom of the well: Generally the supernatant is taken from the well by
overflowing the well
during substantially continuous fieed into the well. The removal of the
supernatant can be in
any convenient manner, for instance;through a channel and piped back to the
separation
stage, in which event the well can be dug in any suitable location.
As described above; a 'preferred ernbodiment'provides that the-well is dug in
an area that
facilitates easy introduction of clay waste and easy return of clarified water
and may be at the
base' of a primary lagoon and the supernatant overflows from the top of the
well and may
flow across the exposed base of the primary lagoon. Generally the
p~imarylagoon would
have been used already for collecting solid clay sediment from the mineral
recovery process
and so the supernatant flows over the substantially solid clay sediment in the
primary
lagoon:
The flow of the supernatant from the welt over the sediment before recycling
of the
supernatant to the main separation stage has the effect of palishing the
supernatant and
thereby increasing the clarity and :reducing the suspended salids of the
supernatant which is
returned to the flotation or other main separation step.
Generally, in the preferred embodiment, the well is formed in a lagoon-that is
already
substantially filled with substantially solid clay sediment. Thus, by
practicing the invention, a
lagoon that has already been substantially filledwith,sediment can be given a
new and very
important purpose by excavating a treatment well and then relying on the
existing sediment
in the lagoon to provide a polishing of the supernatant. The rate of increase
of sediment in
the lagoon as a result of this polishing process is extremely slow and so the
lagoon can be
given an a'Irnost indefinite extension in its useful life.
The phrase "substantially full" means that the lagoon is too shallow to be
useful 'for
separation of clear supernatant from sediment, for instance as a result of the
horizontal
component of the flow velocity exceeding the vertical component of the
settling rate.
Suitable dimensions for the well in the inventive process comprise a depth of
from about 6
to about 6Q or deeper as convenient, preferably about 25 to about 60, feet and
an upper
CA 02392246 2002-06-28
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surface area (generally approximating a square or round area) giving a flow
rate of 0.01 to 1,
preferably 0;1 to 0.5 U:S. gallons per minute per square foot: The required
surface area
depends on the flow rate of the clay waste.
The size of a conventional primary lagoon is from about SO toabout 2000;
generally 250 to
1000, acres. The distance of travel o~ the supernatant over the substantially
solid clay
sediment in the primary lagoon is generally at least 300 feet and usually at
(east 700 feet,
e.g., 1000 to SOOO feet. The flow: conditions of the supernatant as it travels
from the well
overflow to the primary lagoon exit are preferably such as to cause minimum
disturbance of
clay which is sedimenting down onto the substantially solid clay sediment in
the base of the
lagoon: Preferably the depth of the supernatant above the ettliug clay and the
solid clay
sediment is such that there is a layer of at least about b inches; generally
at least about 1 foot
and preferably at least about 3 feet deep which appears, s the naked eye, to
be clear. The
speed of flow of the supernatant is generally in the range of about 0.0000U1
to 0.1 gallons
per minute per square foot, so as to promote the opportunity of sedimentation
and to
minimize the risk of disturbing the sedimenting clay.
The dilute aqueous clay waste is flocculated in the well by mixing a polymeric
fiocculant infio
the waste in such a way as to promote optimum flocculation with minimum
polymer usage.
The polymeric flocculant can be added in solid form but more usually is added
as a
preformed solution in conventional maryner, typically having a polymer
concentration of
about 0.1-2R~o by weight. The polymeric flocculant can be added to the waste
after the
waste has entered the well but may be added to the waste before the-waste
enters the well
or may be added to the recycled supernatant prior to mixing in the well. The
addition point
can be just prior to the entry to the well or it can be at a substantially
earlier position, for
instance as described in U.S. Pat. No. 5,688,404:
Generally the polymeric flocculant is added to the waste as it flows through a
mixing device
which discharges into the well. The mixing device can be a duct or other
suitable device,
such as a tank, or small well formed in the grown, through which the waste
flows with
sufficient turbulence o promote good mixing of the flocculant into the waste.
The
turbulence may be generated solely by the rate of flow through the duct or by
baffles or
other turbulence indacers by the injection of water within the duct. If
desired, mechanical
CA 02392246 2002-06-28
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rotors or other mechanical mixing apparatus can be provided to achieve
suitable mixing of
the polymeric flocculant into the waste; sufficient to give substantially
uniform flocculation.
The polymeric flocculant can be any water soluble polymeric flocculant which
is capable of
promoting flocculation and therefore separation of the aqueous waste into a
supernatant
and a thickened clay sediment. The polymer is generally a water soluble
polymer formed
from one oc more ethylenically unsaturated monomers. The monomers may be non-
ionic,
anionic or cationic. Similarly, the polymer may be non-ionic, anionic or
cationic, or it may
be amphoteric.
Suitable anionic monomers include ethylenically unsaturated: carboxylic or
sulphonic
monomers such as aryclic acid, methacrylic acid and 2-acrylamido-2-methyl
propanesulfonit
acid (AMPS) (a US trademark of the Lubrizol Corporation). Acrylamide is a
suitable non-ionic
monomer. Suitable cationic monomers are dialkylaminoalkyl (meth) -acrylates
and
acrylamides, usually as their quaternary ammonium or acid addition salts, or
diallyl dimethyl
ammonium chloride.
Preferred anionic polymers are copolymers of 3-7090 by weight generally 10-
SO~Yo by weight
anionic monomers such as acrylic acid (usually as sodium .acrylate) and/or
AMPS with other
monomers generally acrylamide. Particularly preferred anionic copolymers are
Percol 3:36,
Percol 727, Percol 358 al( from Ciba Specialty CHemicals, Water Treatments
Inc. Suitable
cationic polymers are formed of l -50aYo by weight, generally 2-15~a by weight
cationic
monomer such as dimethyl aminoethyl-acrytate or -rnethacrylate acid additions
or
quaternary salts together with other monomers; generally acrylamide.
Particularly preferred
cationic copolymers are Percol 455, Percot 352; also from Ciba Specialty
Chemicals, Water
Treatments Inc.
The molecular weight of the polymer is generally such that the polymer has an
intrinsic
viscosity ("IV") (measured using a suspended level viscometer, 1 N sodium
chloride buffered
to pH 7 at 20 DEG C) of at least 4 dI/g and usually at least 8 dl/g. When the
polymer is
anionic, the IV is typically 10-30 d1/g and when it is cationic the IV is
typically 8-15 dl/g:
CA 02392246 2002-06-28
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The polymer can made by gel polymerization, reverse phase bead polymerization
ar reverse
phase emulsion polymerization or by any other suitable technique in known
manner.
The effective dosage of the polymer is selected in conventional manner for
sedimentation
applications and is usually 0.01 to 1, preferably about 0:0125 to about 0.75
pounds polymer
per ton solids in the waste which is being flocculated.
The selection of the polymer and dosage amount can be conducted by
conventional
selection procedures so as to obtain 'the optimum combination of clarity and
depth of
supernatant and the rate of settling on the one hand and purnpable thickened
clay ediment
on the other.
The theoretical residence time of the dilute aqueous clay waste in the well is
usually from S
minutes to S hours; preferably 1 O minutes to 3 hours, e.g.; 45 to 120
minutes.
Promotion of the flocculation process can be achieved by mixing dilution water
with the
flocculant solution into the dilute aqueous clay waste because: the waste
entering the well
often has a solids content above the value which gives opfimurn settling:
rate. The optimum
amount of dilution water can-be determined by routine esfiing. An added
benefit of this
process, when operated at the optimum dilution rate, such that polymer usage
is optimized,
is that a film of oil is observed floating on the surface of the supernatant.
This oil fitm;is
substantial enough that in time it vvill accumulate to an amount which can be
recovered by
any convenient manner, thus adding to the total bitumen recovery, which is the
object of
the present invention.
The thickened clay sediment is removed from the welt at a position
significantly below the
supernatant and/or at a time such that removal does not undesirably impair the
quality of
the supernatant. The removal may be continuous or discontinuous. The solids -
content of
sediment will generally increase towards the bottom of the well and, in order
to minimize
the risk of the well gradually filling,. up with sediment; it is therefore
desirable to remove the
thickened clay sediment from as close to the bottom of the well as possible.
CA 02392246 2002-06-28
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The sediment that is removed from the well generally has a solids content at
least 2 or 3
times and often up to 10 times the solids content of the original dilute
aqueous clay waste
stream which is being flocculated. Often the solids content of the thickened
sediment is
from about 10 to about 30% dry weight solids. Measured by taking a sample of
the
thickened sediment of known weight and evaporating the liquid component o~
moisture at a
known temperature (typically 105 degrees Celsius) in a standi~rd laboratory
drying. The
solids content should preferably be as high as is practicable but must not be
so high that the
sediment is not conveniently pumpable.
Removal can preferably be accomplished by pumping, such as by using a fixed
pump
positioned on the ground near the side of the well and connectwi by a pipe to
near the base
of the welt for drawing off thickened.sediment from the well, or a floating
pump which floats
on the supernatant arid has a pipe extending dovvn to near the base of the
well.
The thickened sediment is removed from the weH. to one or more final lagoons
where it is
spread over the lagoon and allowed to sediment and evaporate to form the
desired final
substantially solid clay sediment. Because the thickened sediment removed from
the well
has a much higher solids content than conventional clay waste, the amount of
sedimentation and evaporation which is required to provide the final solid
sediment is much
less than in conventional processes, and there may be no incentive to try to
recycle any
supernatant (because of the large amount of supernatant which has been
recycled from the
well). Accordingly the final one or more lagoons do not have to have as deep a
seftling
volume as is normally considered necessary. As a result, the thickened
sediment can be
pumped into lagoons which are partially or almost entirely full of sediment.
The invention thus the advantage that it can simultaneously give good recovery
of
supernatant (often of very high clarity) while using lagoons which would
normally be
considered to be too full and to shalioyr for many purposes.
The following are examples of the invention.
Exa a 1: A oil bearing sands recovery process is conducted by slurring: oil
bearing sands
with water and separating the desired bitumen in the primary separation
vessel, from which
CA 02392246 2002-06-28
. ~~
generally bitumen, coarse sand tailings and fine clay tailings are removed.
These fractions
are subjected to further separations t;including flotation) to give oil
bearing fractions and
aqueous clay waste having a solids content which varies between about 0.29~o
solids and
2096 solids and which contains a residual amount of bitumen values.
