Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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METHOD FOR REDUCING THE SLUDGE
CONTENT OF A TAILINGS POND
BACKGROUND OF THE INVENTION
This invention relates to the hot water process
for treating bituminous sands, such as Athabasca tar
sands, and more particularly, to means by which the
sludge layer within a tailings pond is diminished,
thereby achieving a consequent increase in the clarified
water layer.
Tar sands (which are also ~nown as oil sands and
bituminous 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 area in the northeast of the Province
of Alberta, Canada. This deposit is believed to contain
perhaps 700 billion-l trillion 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.
Athabasca tar sand is a three-component mixture of
bitumen, mineral and water. Bitumen is the value for the
extraction of which tar sands are mined and processed.
The bitumen content is variable, averaging 12 wt.% of the
deposit, but ranging from 0 to 18 wt.%. Water typically
runs 3 to 6 wt.% of the mixture, increasing as bitumen
content decreases. The mineral content is relativ~ly
constant ranging from 84 to 86 wt.%.
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l~hile several basic extraction methods have been
known for many years for separating the bitumen from the
sands, the "hot water process" is the only one of
present commercial significance. The hot water process
for primary extraction of bitumen from tar sands consists
of three major processsteps (a fourth step, final
extraction, is used to clean up the recovered bitumen
for downstream processing.) In the first step, called
conditioning, tar sand is mixed with water and heated with
open steam to form a pulp of 70 to 85 wt. % solids.
Sodium hydroxide or other reagents are added as required
to maintain pH in the range 8.0 - 8.5. In the second
step, called separation, the conditioned pulp is diluted
further so that settling can take place. The bulk of the
sand-size mineral rapidly settles and is withdrawn as
sand tailings. Most of the bitumen rapidly floats ~settles
upward) to form a coherent mass known as froth which
is recovered by skimming the settling vessel. A third
stream may be withdrawn :Erom the settling vessel. This
stream, called the middlings drag stream, may be subjected
to a third processing step, scavenging. This step provides
incremental recovery of suspended bitumen and can be
accomplished by conventional froth flotation.
The mineral particle size distribution is particularly
significant to operation of the hot water process and to
sludge accumulation. The terms sand, silt, clay, and
fines are used in this specification as particle size
designations wherein sand is siliceous material which will
not pass a 325 mesh screen. Silt will pass 325 mesh, but
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is larger than 2 microns, and clay is material smaller
than two microns including some siliceous material of
that size.
Conditioning tar sands for the recovery of bitumen
consists of heating the tar sand/water feed mixture to
process temperature ~180-200F), physical mixing of the
pulp to uniform composition and consistency, and the
consumption ~by chemical reaction) of the caustic of
other reagents added. Under these conditions, bitumen is
stripped from the individual sand grains and mixed into
the pulp in the form of discrete droplets of a particle
size on the same order as that of the sand grains. The
same process conditions, it turns out, are also ideal for
accomplishing deflocculation of the clays which occur
naturally in the tar sand feed. Deflocculation, or
dispersion, means breaking down the 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 resources
(bitumen) for efficient recovery during the following
process steps also prepares the clays to be the most
difficult to deal with in the tailings disposal operations.
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The second process step, called separation, is
actually the bitumen recovery step, ~the separation having
already occurred during conditioning). The conditioned
tar sand pul~ is screened to remove rocks and
unconditionable lumps of tar sands and clay. The reject
material, "screen oversize", is discarded. The screened
pulp is further diluted with water to promote two
settling processes: globules of bitumen, essentially
mineral-free, settle (float) upward to form a coherent
mass of froth on the surface of the separation cells;
and, at the same time, mineral particles, particularly
the sand size mineral, settle down and are removed from
the bottom of the separation cell as tailings. The medium
through which these two settling processes take place is
called the middlings. 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 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 all
streams exiting the cell have about the same composition
as the :Eeed.
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As ~eed ~ines 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 middliings viscosity which, as
noted above, is governed by the c~Lay/water ratio. It is
usually necessary to withdraw a drag stream of middlings
to maintain the separation cell material balance, and this
stream of middlings can be scavenged for recovery of in-
cremental amounts of bitumen. Air flotation is an effec-
tive scavenging method for this middlings stream.
Final extraction or froth clean-up is typically ac-
complished by centrifugation. Proth from primary extrac-
tion is diluted with naptha, and the diluted froth is then
subjected to a two stage centrifugation. This process
yields an oil product of an essentailly pure (diluted)
bitumen. Water and mineral removed from the froth, during
this step constitute an additional tailing stream which
must be disposed of.
In the terminology of extractive processing, tailings
is the throwaway material generated in the course of ex-
tracting the valuable material from an ore. In tar sands
processing, tailings consist of the whole tar sand ore
body plus net additions of process water less only the
recovered bitumen product. Tar sand tailings can be `
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subdivided into three categories; viz: ~1) screen oversize,
~2) sand tailings (the fraction that settles rapidly),
and ~3) tailings sludge ~the fraction that settles slowly).
Screen oversize is typically col]ected and handled as a
separate stream.
Recently, in view o-F the high level of ecological
consciousness in Canada and the llnited States, technical
interest in tar sands operation has begun to focus on
tailings disposal. The concept of tar sands tailings
disposal is straightforward. Visualize mining one cubic
foot of tar sands. This leaves a one cubic foot hole in
the ground. The ore is processed to recover the resource
~bitumen) and the remainder, including both process
material and the gangue, constitutes the tailings; tailings
that are not valuable and are to be disposed o-f. 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) consists
of a solid part ~sand tailings) and a more or less fluid
part ~sludge). The most satisfactory place to dispose
of these tailings is, of course, in the existing one cubic
foot hole in the ground. It turns out, however, that the
sand tailings from the one cubic foot of ore occupy just
about one cubic foot. The amount of sludge is a variable,
depending on ore quality and process conditions, but may
run up to 0.3 cubic feet. The tailings simply will not
fit into the hole in the ground.
The historical literature 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 liquid tailings or sludge would occur. Based on
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analysis of field test unit operations which led to the
Great Canadian Oil Sands plant design near Ft. McMurray,
Alberta, the e~istence of sludge accumulation was
predicted. This accumulation came to be called the
"pond water problem". Observations during start-up
and early commercial operations at Ft. McMurray ~1967-1969)
were of insufEicient precision to confirm the predlction.
Since 1969, commercial operating data haveconfirmed the
accumulation of GCOS~s tailings disposal area of a layer
of fine material and water (sludge) which settles and
compacts only very slowly, if at all, after a few years.
At the GCOS plant, for dike building, tailin~s are
conveyed hydraulically to the disposal area and discharged onto
the top of a sand dike which is constructed to serve
as an impoundment for a pool of liquid contained inside.
On the dike, 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
build the dike to a higher level. The slurry which drains
into the pond interior commences stratification in
settling over a time scale of months to years. As a result
of this long-term settling, two layers form. The top 5
to 10 feet of the pool are a layer of relatively clear
water containing 0 to 5 wt. % solids. Below this clarified
water layer is a discontinuity in solids content. Over
a matter of a few feet, solids content increases to 10-15
wt. %, and thereafter, solids content increases regularly
toward the pond bottom. In the deepest parts of the pond,
sclid contents of over 50 wt. % have been recorded. This
second layer is called the sludge layer. The solids
content of the sludge layer increases regularly from
top to bottom by a factor of 4-5. The clay-water
ratio in this layer increases also, but by a lower
factor of 1.5-2.5. The clays, dispersed during
processing, apparently have partially re-flocculated
into a very fragile gel network. Through this gel, fines
of larger-than-clay sizes are slowly settling.
Overboarding is the operation in which tailings are
discharged over the top of the sand dike directly into
the liquid pool. A rapid and slow settling process occur
but their distinction is not as sharp as in dike building
and no mechanical compaction is carried out. The sand
portion of the tailings settles rapidly to -Eorm a gently
sloping beach extending from the discharge point toward
the pond interior. As the sand settles, fines and water
drain into the pool and commence long-term settling.
In summary: (1) tar sands contain clay minerals,
~2) in the hot water extraction process, most of the clays
become dispersed in the process streams and traverse the
circuit, exiting in the tailings, (3) the amount of process
water input is fixed by the clay content of the feed and
the need to control viscosity of the middlings stream,
~4) the amount of water required for middlings viscosity
control represents a large volume relative to the volume
of the ore itself, and (5) upon disposal, clays settle
only very very slowly; thus, the process water component
of tailings is only partially available for reuse -
via recycle. That which can't be recycled represents a net
accumulation oE tailings sludge.
The pond water problem is then: to devise long-
term economically and ecologically acceptable means
to eliminate, minimize, or permanently dispose o-f,
the accumulation of liquid tailings or sludge.
It is therefore a broad object of this invention to
control the size of the sludge layer in a tailings pond.
In another aspect, it is an object of this
invention to provide means for withdrawing the sludge
from the sludge layer and discarding the clay constituents
as agglomerated masses.
In a more specific aspect, it is an object of this
invention to provide, in the hot water process for
processing bituminous tar sands into synthetic crude oil,
means for controlling the sludge layer of an associated
tailings pond by withdrawing sludge therefrom and
introducing sludge into the conditioning drum where it
agglomerates into clay masses of sufficient size for discard
with the oversize.
The manner in which these and other objects of the
invention are achieved is particularly pointed out and
distinctly claimed in the concluding portion of the
specification. The invention, however, both as to
organization and method of operation, may best be
understood by reference to the following description taken
in conjunction with the drawing which is a somewhat
simplified block diagram of a hot water process for
converting bituminous tar sands into synthetic crude oil.
DETAILED DESCRIPTION OF THE INVENTION
Referring now to the drawing, bituminous tar sands
are fed into the system through a line 1 and pass -to a
conditioning drum or muller 18. Water and steam are
introduced to the muller through another line 2. The
total water so introduced in liquid and vapor form is
a minor amount based on the weight of the tar sands processed.
The tar sands, heated and conditioned with steam and
water, pass through a line 3 to a screen 29. The
purpose of the screen 17 is to remove from the pulp any
debris such as rocks or over-sized lumps of tar sand as
indicated generally at 30. This oversize material is
discarded at a suitable site. The screened pulp passes
through a line 31 to the feed sump 19 which serves as a
zone for diluting the pulp with additional water before
passage to the separation zone 20.
The pulp tar sands are continuously flushed :Erom the
feed sump l9 through a line ~ into a separator 20. The
settling zone within the separator 20 is relatively
quiescent so that bituminous froth rises to the top and
is withdrawn via line 5 while the bulk of the sand
settles to the bottom as a tailings layer which is
ithdrawn through line 6.
A relatively bitumen rich middlings stream is withdrawn
through line 8 and transferred to a flotation scavenger
zone 21. In this scavenger zone, an air flotation
operation is conducted to cause the formation of additional
bituminous :Eroth which passes from the scavenger zone
through line 9 in mixture with the primary froth from the
separation zone 20 to a froth settler zone 22. A
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bitumen-lean water stream is removed from the bottom of
the scavenger zone 21 through line 10 to be further
processed as described below. In the froth settler zone
22, some further bitumen-lean water is withdrawn from the
froth and removed through line 11 to be mixed with the
bitumen-lean water stream from the flotation scavenger
zone and the sand tailings stream from the separation zone.
The bitumen from the settler 22 is removed through line
12 for further treatment, typically upgrading to synthetic
crude oil.
Bitumen-lean water from the froth settler 22, the
scavenger zone 21, and the separation zone 20, all of which
make up an effluent discharge stream carried by line 7,
are discharged into a settling pond 15 having 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 15 where settling takes place. As previously
noted, in an actual settling pond, the demarcation
between a clarified water layer 26 and a sludge layer
27 is ill-defined and variable, and the characteristics
of the sludge layer 27 change from top to bottom. Water
from the clarified water layer 26 may be withdrawn by
a pump 28 for recycle by a line 17 to be mixed with fresh
water and charged into the hot water process.
According to the present invention, sludge from the
layer 27 is withdrawn by a pump 23 and is transferred
through a line 2~ to the conditioning drum 18. Those
skilled in the art will recognize that the introduction
of clay-laden sludge into the conditioning drum 1~ is
totally contrary to the prior art teachings. However,
it is believed that clay lumps which already have their
pores filled, or partly filled with water are less
and less easily dispersed in a clay slurry as the solids
content of the slurry increases. As a result, rather
than dispersing, the clay lumps tend to "grow" such
that a large proportion of them are discarded as oversize
when subjected to the screen 29. Thus, the sludge layer
27 is diminished with a consequent increase in volume
of the clarified water layer 26 of the tailings pond.
While the principles of the invention have now been
made clear in an illustrative embodiment, there will be
immediately obvious to those skilled in the art many
modifications of structure, arrangements, proportions, the
elements, materials and components, used in the practice
of the invention which are particularly adapted for
specific environments and operating requirements without
departing from those principles.
