Note: Descriptions are shown in the official language in which they were submitted.
~ -0Gl (GPM-685) ~275~6~
IMPROVED RECOVERY OF HEAVY BITUMEN IN TAR SANDS PROCESSING PROCESS
BACKGROU~D OF THE INVENTION
This invention relates to the hot water process for trea~ing
bituminous sands, such as Athabasca tar sands, and more particularly
to the treatment of those tar sands which when treated in the usual
manner do not lend themselves to efficient recovery of bitumen.
Tar sands (which are also known as oil sands and bituminous
sands) are sand deposits which are impregnated with dense, viscous
petroleum. Tar sands are found throughout the world, often in the
same geographical areas as conventional pe~roleum. ~he 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 - 1 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 ranglng from 0 to 18 wt.%.
Water typically runs 3 to 6 wt.% of the mixture, increasing as bitumen
content decreases. The mineral content is relatively constant ranging
from 84 to 86 wt.~.
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Several basic extraction methods have been known for many
years for separating the bitumen from the sands. In the so-called
"cold water" method, the separation is accomplished by mixing the
sands with a solvent capable to dissolving the bitumen constituent.
The mixture is then introduced into a large volume of water, water
with a surface agent added, or a solution of a neutral salt in
water. The combined mass is then subjected to a pressure or gravity
separation.
The hot water process for primary extraction of bitumen
from tar sands consists of three major process steps (a fourth step,
final extraction, is used to ~lean 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 foxm 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.S. In the second
step, called separation, the conditioned pulp is diluted further so
that separation can take place wherein the bulk of the sand-si~e
mineral rapidly settles and is withdrawn as sand tailings. Most of
the bitumen rapidly floats (settles upward) to form a coherent mass
known as froth which is recovered by skimming the settling vessel.
A third stream may be withdrawn from 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. These steps are discussed in detail in the literature
of tar sands processlng.
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The conditioning step of tar sands for the recovery of
bitumen from tar sands consists of heating the tar sand/water feed
mixture to process temperature (180 - 200F.), physical mixing of
the pulp to unifor~ composition and consistency, and the consumption
(by chemical reaction) of caustic to bring the pH of the system to
8.0 to 8.5. Under these conditions, bitumen is stripped ~rom the
individual sand grains and mixed into the pulp in the form of discrete
droplets of a particle size on the same order of that of the sand
grains. The same process conditions, it turns out, are also ideal
for accomplishins 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. Such a procedure is very effective for efficient
recovery of bitumen with about 90~ of the tar sands treatea. How-
ever, about 10~ of the tar sands mined do not lend themselves tv
efficient recovery by such technique. It has been observed that
when these abnormal tar sands are treated in the usual manner, the
bitumen which would be expected to float in the separation step
actually sinks to the bottom and is carried away with the mineral
fines as waste. It is an ob~ect of this invention to provide a
process for efficien~ bitumen recovery from such heavy tar sands.
BRIEF DESCRIPTION OF THE INVENTION
In accord with this invention it has been found that the
abnormal tar sands which contain transition metals, particularly
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il , do not lend themselves to the usual conditioning process, but
these abnormal tar sands can be efficiently handled for high bitumen
recovery by weakening the strong interaction between the bitumen and
minerals caused by the metals, which are concentrated in the clays
contained in the tar sands. These metals bind the bitumen to the
clays through polar interactions (i.e. by Van der Waal type bonding
as determined by infra-red spectra) and thus cause the bitumen to be
carried down with the clays and minerals. This bond weakening is
accomplished by any one of several techniques such as oxidation or,
preferably, adjusting the pH of the material to be conditioned to a
pH of just below 7.5; e.g. about 7.0 to about 7.4. Subsequent to
the pH adjustment, the tar sands material is treated with sodium
silicate solution to reduce its viscosity and it is the combination
of these steps that results in a high recovery of bitumen from the
abnormal tar sands.
DISCUSSION OF PRIOR ART
In the recovery of the bitumen from tar sands a large
volume of tailings comprised of hydrocarbon sand and water are sent
to waste and, because they cannot be discharged to rivers and
streams because of environmental problems, are stored in large
ponds. Much effort has been made to treat these tailings for re-
covery of water and bitumen with numerous techniques being developed.
Of interest here is the technique disclosed in Canadian Patent
1,023,677 which teaches the use of carbon dioxide to reduce the
pH of tailings in sedimentation reservoirs to a value of about 4.0
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o about 6.0 whereby the chemical bonding between bitumen and mineral
particles are broken and the hydrocarbons rise to the surface so
that the water may be recycled or put into a natural water course.
Although in the present invention the Van der Waal forces between
bitumen and minerals are weakened, the use of CO2 or acid alone is
inadequate to permit the successful processing of the bitumen for
recovery as the mass becomes too viscous for handling. Thus, the
prior art technique of pH reduction to about 4 to about 6 with
carbon dioxide as applied to tar sands tailings is not germane to
the process of this invention which relates to those peculiar tar
sands containing transition metals.
DETAILED DISCUSSION OF THE INVENTION
As indicated above, the conditioning step used in the hot
water process for bitumen recovery from Athabasca tar sands using
caustic soda to adjust the pH to about 8.5 generally is very effective
for about 90% of the tar sands processed. The remaining 10%, however,
do not satisfactorily give up their bitumen with such treatment and
the bitumen recovery of these abnormal tar sands has been observed
to be as little as 25% of the expected amount.
In accord with this invention, it has been established
that these abnormal tar sands contain unexpectedly high amounts of
transition metals, particularly iron, concentrated largely in the
clay content of the tar sands. It has been found that it is when
the iron content exceeds abou~ 1.0% that the adverse effect vn bitumen
recovery is observed. Iron and the other transitional metals in
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reduced form are present in the tar sands together with anions such
as carboxylates, naphthalatesl sulfonates and the like, which effect
rather strong bonding between the bitumen and clay. Since the clay
is of high density, it carries the bitumen down with it as it sinks
to the bottom of the separation vessel and thus bitumen recovery is
lowered significantly.
To overcome this phenonemon, the process of this invention
weakens this bonding either by a specific oxidation step or by ad-
justing the pH of the system to a value of below 7.5 (preferably
about 7.0 to about 7.45) whereby the polar anions are converted to
non-polar free acid groups. However, this pH adjustment alone is
insufficient to effect satisfactory commercial separation of the
bitumen because the pH adjustment results in a significant increase
in viscosity of dispersed clays which adversely affects bitumen
separation. Accordingly, it is necessary to reduce the viscosity of
the system and this is preferably done by the addition of a viscosity
reducinq agent such as an aqueous solution of an alkali metal silicate
(e.g., sodium silicate) or an alkali metal phosphate (e.g., sodium
hexametaphosphate). Upon carrying out the combined steps of pH
adiustment and viscosity réduction it is observed that bitumen
flotation is signiicantly enhanced and very high bitumen recovery
is achieved.
~ he pH adjustment of the tar sands material is readily
achieved by addition of an acid to the aqueous suspension of tar
sands until the system has a pH of just below 7.5. Preferred acids
will be hydrochloric, sulfuric, carbonic (or C02 gas~ phosphoric
and the like~ All that is required is that the acid be added with
thorough agitation until the desired p~ is reached.
The viscosity adjustment step is also easily carried out
and is done so that the adjusted pH to below 7.5 is not affected.
Preferably, an aqueous solution of sodium silicate (about 40~ solids)
is added to the aqueous suspension of tar sands so as to provide
about 20-50 parts of silicate per million parts of tar sands, but
the correct amount is easily ascertained by using viscosity measure-
ment as a control. The desixed viscosity for the treated tar sands
will be about 1-5 centipoise and the appropriate amount of silicate
will be used to achieve this value~
As indicated, the interaction between the bitumen and
minerals in abnormal tar sands may be weakened by oxidation, and
such oxidation may be carried out in numerous ways. Thus, for example,
stockpiling of the tar sands in air and dynamiting the ore body will
be an effective oxidation means. Also, introduction of oxidizing
agent~ into the aqueous suspension of tar sands is effective.
However, the pH adjustment method discussed above has the advantage
of also flocculating the clays present and is the preferred method.
Subsequent to the pH and viscosity adjustment by the method
of the invention, the tar sands suspension is handled in the usual
manner, the bitumen which floats to the top being separated off, a
middlings layer taken for further usual handling and the clay minerals
being removed from the bottom of the separator.
The following examples will illustrate th~ method of the
invention.
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E~amele 1
A laboratory batch extraction unit is used to approximate
the hot water extraction process of tar sands. In this unit the tar
sand sample and water are slurried in a stainless steel pot of the
batch extraction unit and the mixture is neutralized and reduced in
viscosity. A predetermined amount of flood water is introduced and
the floated primary froth retrieved. Further mixing and aeration
allows the recovery of secondary froth. Samples of tar sand feed
and the recovered froths are submitted for bitumen/water/solids
analysis, thus allowing bitumen recoveries to be calculatecl on an
input basis. This apparatus and technique is described by E. C.
Sanford and F. A~ Seyer, American Chemical Society Division of Fuel
Chemistry, Pregenist 23 (4)~ 54-62 (1978) and by the same authors in
CIM Bulletin 72 (8031, 164-169 (1979).
The procedure which is used for this example employs one
sample which corresponds to the usual extraction procedure, where
caustic is added to the conditioning unit to a pH of about 8.0, a
second caustic treated sample where the pH is reduced with hydrochloric
acid (or CO2) to a pH just below 7.5 (about 7.45) and a third sample
where sod1um silicate is also added in an amount of 50 ppm to an
acid treated sample (pH about 7.45]. The bitumen recoveries in each
of these samples using a tar sands containing less than 1% iron and
with a tar sands containing more than 1% iron is shown in Table I.
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TABLE I
PERCENT RECOVERY OF BITUMEN
FROM TAR SANDS CONTAINING IRON
Iron Content Iron Content
Less than 1~ More Than l~
TREATMENT MEAN Standard MEAN Standard
Deviation Deviation
__ _ = _
CA~STIC 91.S ]4.7 76.0 23~4
TO pH 8.0
NEUTRALIZATION83.8 18.8 ~,1.2 28.6
WITH HCl
TO pH 7.45
NEUTRALIZATION + 96.4 5.25 85.6 14.0
SODIUM SILICATE
_
As can be seen from the table, the bitumen recovery is
significantly improved only by adjusting the pH to 7.45 and lowering
the viscosity by use of sodium silicate. Also evident from the table
is the greater consistency of high recovery which results in a
higher level of confidence for the process.
Example 2
Samples of abnormal tar sands which sink to the bottom of
the separation vessel during hot water extraction and give poor
bitumen recovery are allowed to age in air for about two weeks (e.g.,
under oxidizing conditions)~ At the end of the aging period the
samples are subjected to extraction and are found to give high recovery
of bitumen.
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Example 3
When normal tar sands which give good bitumen recovery are
soaked in a sodium dithionite solution to subject them to a reducing
environment, they become abnormal in the extraction process and sink
to the bottom o the extraction vessel, giving poor recovery of bitumen.
This process can be reversed either by oxidizimg with a hydrogen
peroxide solution or by ollowing the procedurle of Example 1 with
mineral acid and silicate addition.
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