Note: Descriptions are shown in the official language in which they were submitted.
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PROCESSES FOR TREATING A TAILINGS STREAM
FIELD OF THE ART
[0001] The present disclosure relates generally to processes for treatment of
a
tailings stream.
BACKGROUND
[0002] Bituminous sands, or oil sands, are a type of petroleum deposit. The
sands
contain naturally occurring mixtures of sand, clay, water, and a dense and
extremely viscous
form of petroleum technically referred to as bitumen (or colloquially "tar"
due to its similar
appearance, odor, and color). Oil sands are found in large amounts in many
countries
throughout the world, and most abundantly in Canada and Venezuela. Oil sand
deposits in
northern Alberta in Canada (Athabasca oil sands) contain approximately 1.6
trillion barrels of
bitumen, and production from oil sands mining operations is presently
approximately one
million barrels of bitumen per day.
[0003] Oil sands reserves have only recently been considered to be part of the
world's oil reserves, as higher oil prices and new technology enable them to
be profitably
extracted and upgraded to usable products. They are often referred to as
unconventional oil or
crude bitumen, in order to distinguish the bitumen extracted from oil sands
from the free-
flowing hydrocarbon mixtures known as crude oil traditionally produced from
oil wells.
[0004] Conventional crude oil is normally extracted from the ground by
drilling oil
wells into a petroleum reservoir, and allowing oil to flow into them under
natural reservoir
pressures. Although artificial lift and techniques such as water flooding and
gas injection are
usually required to maintain production as reservoir pressure drops towards
the end of a
field's life. Because extra-heavy oil and bitumen flow very slowly, if at all,
towards
producing wells under normal reservoir conditions, the sands may be extracted
by strip
mining or the oil made to flow into wells by in situ techniques which reduce
the viscosity,
such as by injecting steam, solvents, and/or hot air into the sands. These
processes can use
more water and require larger amounts of energy than conventional oil
extraction, although
many conventional oil fields also require large amounts of water and energy to
achieve good
rates of production.
[0005] Water-based oil sand extraction processes include ore preparation,
extraction and tailings treatment stages wherein a large volume of solids-
laden aqueous
tailings is produced. One such extraction process is called the hot water
process. In the hot
water process the displacement of bitumen from the sands is achieved by
wetting the surface
of the sand grains with an aqueous solution containing a caustic wetting
agent, such as
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sodium hydroxide, sodium carbonate or sodium silicate. The resulting strong
surface
hydration forces operative at the surface of the sand particles give rise to
the displacement of
the bitumen by the aqueous phase. The name of the process comes from the fact
that the
system is operated at temperatures near the boiling point of water. Once the
bitumen has been
displaced and the sand grains are free, the phases can be separated by froth
flotation based on
the natural hydrophobicity exhibited by the free bituminous droplets at
moderate pH values
(Hot water extraction of bitumen from Utah tar sands, Sepulveda et al. S. B.
Radding, ed.,
Symposium on Oil Shale, Tar Sand, and Related Material - Production and
Utilization of
Synfuels: Preprints of Papers Presented at San Francisco, California, August
29 ¨
September 3,1976; vol. 21, no. 6, pp. 110-122 (1976)).
[0006] The recovered bitumen froth generally consists of 60% bitumen, 30%
water
and 10% solids by weight. The recovered bitumen froth needs to be cleaned to
reject the
contained solids and water to meet the requirement of downstream upgrading
processes.
Depending on the bitumen content in the ore, between 90 and 100% of the
bitumen can be
recovered using modern hot water extraction techniques.
[0007] Hydrophilic and biwetted ultrafine solids, mainly clays and other
charged
silicates and metal oxides, tend to form stable colloids in water and exhibit
a very slow
settling and dewatering behavior, resulting in tailing ponds that can take
several years to
manage. The slow settling of fine (<44 pm) and ultrafine clays (<1 pm) and the
large demand
of water during oil sand extraction process have promoted research and
development of new
technologies during the last 20 years to modify the water release and to
improve settling
characteristics of tailings streams. These include process additives such as
variations in pH,
salinity and addition of chemical substances. Currently, two technologies
commonly used in
the oil sands industry are the consolidated tailings (CT) process and the
paste technology.
Gypsum is used in the CT technology as a coagulant while polyelectrolytes,
generally
polyacrylamides of high density, are used as flocculants in the paste
technology. Flocculants,
or flocculating agents, are chemicals that promote flocculation by causing
colloids and other
suspended particles in liquids to aggregate, forming a floc. Flocculants are
used in water
treatment processes to improve the sedimentation or filterability of small
particles.
[0008] Various inorganic and/or organic flocculants are currently in use in
tailings
treatments. The adequate dosage of gypsum and/or flocculants during the
tailings disposition
improves the oil sands process efficiency because these substances act as
modifiers of the
interaction forces responsible for holding particles together. Consequently,
the addition of
these chemicals can enhance the settling rate of tailings for consolidation
and land
reclamation and promote the recovery of water and its recirculation in the oil
sands process.
Recently some silicates and silica microgel have been proposed for treating
tailings and
separation of ultrafine solids. Silica could cause some problems, however,
because it could be
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precipitated by the presence of excess calcium and magnesium. The resulting
silicate scale could foul pipe surfaces and
other surfaces.
100091 When tailings are treated with gypsum (calcium sulfate) to flocculate
the particles there is a problem
that excess calcium can be present in the water recycled from the tailings,
and this calcium can make it very hard to extract
bitumen from oil sands.
100101 US Patent No. 5,804,077 discloses a method for treating whole aqueous
tailings produced by a water-
based extraction process to recover bitumen from oil sand, said tailing
containing suspended coarse sand and clay fines,
comprising desanding the whole tailings by settling out substantially all of
the sand to yield desanded tailings; adding
about 100 to 200 ppm of calcium sulfate to the desanded tailings; settling the
mixture to produce clarified water and
sludge; and recycling the clarified water to the plant as process water.
100111 The Canadian government has recently required all companies exploiting
the Canadian Oil Sands to
have plans to treat their aqueous dispersions of tailings so that the tailings
will be trafficable within ten years.
BRIEF SUMMARY
100121 A process is provided for treating a tailings stream which comprises
water and solids such as sand and
clay the process comprising the steps of: (i) adding a cement to the tailings
stream; and (ii) separating at least a portion of
the cement and solids from the tailings stream.
10012a1 In another aspect it is provided a process for treating a tailings
stream which comprises water and solids,
the process comprising the steps of:
(i) adding a cement to the tailings stream; and
(ii) separating at least a portion of the cement and the solids from the
tailings stream; wherein the cement
comprises one or more hydraulic cements.
BRIEF DESCRIPTION OF THE FIGURES
[00131 Figure 1 shows a particle size distribution as a function of time for
tailings streams that were untreated,
treated with cement, and treated with cement and a polymer flocculant, in
accordance with one or more of the examples.
100141 Figure 2 shows particle size distribution as a function of time for
tailings streams that were untreated,
treated with cement, and treated with cement and a polymer flocculant, in
accordance with one or more of the examples.
DETAILED DESCRIPTION
100151 Processes for treating tailings streams are provided, wherein the
tailings are treated with cement to
produce coagulated solids. After the treatment the coagulated solids are
usually left for gravity settling or are mechanically
separated.
100161 It has been surprisingly discovered that tailings streams, in
particular oil sands tailings streams, can be
treated with cement to remove fine or ultrafine solids from
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tailings streams. In an exemplary embodiment, the process can be used for
consolidation and
dewatering and consolidation of tailings streams.
[0017] Tailings
[0018] The expressions "tailings", "tailings stream", "process oil sand
tailings", or
"in-process tailings" as used herein refer to tailings that are directly
generated as bitumen is
extracted from oil sands. Process tailings contain coarse and fine particles.
The solid particles
contain a majority of coarse particles. The coarse particles are essentially
comprised of
silicates (e.g. sand) and clays. Fine tailings are generated after the process
tailings are sent to
a settling pond. Eventually, the coarse tailings settle fast to the bottom of
the pond, leaving a
weak gel of fine tailings (average diameter <44 pm) suspended in the water.
Fine tailings
tend to be almost entirely composed of clays. While fine tailings primarily
consist of particles
that are smaller than 44 p,m in diameter, the majority of the solids in the
process tailings have
diameters between 44 and 1000 tin) and above. Ultrafine solids (<1 pm) may
also be present
in the tailings stream. The tailings can be one or more of any of the tailings
streams produced
in a process to extract bitumen from an oil sands ore. The tailings are one or
more of the
coarse tailings, fine tailings, and froth treatment tailings. In exemplary
embodiments, the
tailings may comprise paraffinic or naphthenic tailings, for example
paraffinic froth tailings.
The tailings may be combined into a single tailings stream for dewatering or
each tailings
stream may be dewatered individually. Depending on the composition of the
tailings stream,
the additives may change, concentrations of additives may change, and the
sequence of
adding the additives may change. Such changes may be determined from
experience with
different tailings streams compositions.
[0019] In one embodiment, the tailings stream is produced from an oil sands
ore
and comprises water and solids, for example sand and fines. In one embodiment,
the tailings
stream comprises at least one of the coarse tailings, fine tailings, ultrafine
tailings or froth
treatment tailings.
100201 Cement
[0021] In exemplary embodiments, the cement process aid may be any of a
variety
of cements and pozzalanic materials. In one embodiment, the cement contains
one or more
hydraulic cements. Exemplary hydraulic cements include Portland cement,
Portland-based
cement, pozzolana cement, gypsum cement, high alumina cement, slag cement,
silica cement,
kiln dust or mixtures thereof. Exemplary Portland cements may be those
classified as class A,
C, H and G cements according to American Petroleum Institute (API)
specification for
materials and testing for well cements. They can also be classified by ASTM
C150 or EN 197
in classes of I, II, III, IV and V. In one embodiment, the cement is a
hydraulic cement that
comprises calcium, aluminum, silicon, oxygen and/or sulfur which may set and
harden by
reaction with water. In one embodiment, the cement is an alkaline cement. In a
particular
embodiment, the cement comprises a mixture of two or more hydraulic cements..
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[0022] In one embodiment, the cement comprises one or more types of Portland
cement. Portland cement is the most common type of cementitious material used
around the
world. Tt consists mainly of calcium silicates and aluminates and some iron-
containing
phases. When mixed with water, Portland cement undergoes various hydration
reactions
resulting in raised pH as well as generation of new species including calcium
silicate hydrates
(CSHs). CSH may bind strongly to other mineral grains, resulting in
aggregation and a
settling process.
[0023] Portland cement (also referred to as Ordinary Portland Cement or OPC)
is a
basic ingredient of concrete, mortar, stucco and most non-specialty grout.
Portland cement is
a mixture that results from the calcination of natural materials such as
limestone, clay, sand
and/or shale. In particular, Portland cement comprises a mixture of calcium
silicates,
including Ca3Si05 and Ca2SiO4, which result from the calcination of limestone
(CaCO3) and
silica (Si02). This mixture is known as cement clinker. In order to achieve
the desired setting
qualities in the finished product, calcium sulfate (about 2-8%, most typically
about 5%),
usually in the form of gypsum or anhydrite, is added to the clinker and the
mixture is finely
ground to form the finished cement powder. For example, a typical bulk
chemical
composition of Portland cement is about 61 to about 67 wt% calcium oxide
(CaO), about 12
to about 23 wt% silicon oxide (Si02), about 2.5 to about 6 wt% aluminum oxide
(A1203),
about 0 about 6 wt% ferric oxide (Fe203) and about 1.5 about 4.5 wt% sulfate.
The properties
of Portland Cement can be characterized by the mineralogical composition of
the clinker.
Major clinker phases present in Portland cements include: Alite (3CaO.Si02),
Belite
(2CaO.Si02), Aluminate (3Cao.A1203) and Ferrite (4Ca0. A1203.Fe203).
[0024] In an exemplary embodiment, the cement is a fine powder mixture which
contains more than 90% Portland cement clinker, calcium sulfate and up to 5%
minor
constituents (see European Standard EN197.1).
During the preparation of the cement, a grinding process may be controlled to
obtain a
powder with a broad particle size range, in which typically 15% by mass
consists of particles
below 5 pm diameter, and 5% of particles above 45 pm. The measure of particle
fineness
usually used is the "specific surface area", which is the total particle
surface area of a unit
mass of cement. The rate of initial reaction (up to 24 hours) of the cement on
addition of
water is directly proportional to the specific surface area. Typical values
are 320-380 m2.kg-1
for general purpose cements, and 450-650 m2.kg I for "rapid hardening"
cements.
[0025] In an exemplary embodiment, supplementary cementitious materials, such
as fly ash, silica fume or natural pozzolans may be used together with the
cement. As used
herein, a pozzolan is a material which, when combined with calcium hydroxide,
exhibits
cementitious properties.
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[0026] Tailing Treatment Process
[0027] It is an objective of the exemplary processes described herein to
separate the
solids, while enabling recovery of as much of the water as possible.
Surprisingly, by using the
exemplary processes, a faster settling rate and a more complete separation of
the solids from
the water has been achieved, improving process efficiency relative to
conventional processes
for treating tailings streams.
[0028] In an exemplary embodiment, a process for treating a tailings stream
which
comprises sand, clay fines and water may comprise the steps of: (i) adding a
cement to the
tailings stream; and (ii) separating the solids from the flocculated stream.
In an exemplary
embodiment, the process may further comprise adding a flocculant, for example
a polymer
flocculant. In exemplary embodiments, the flocculant may be added before,
concurrently
with, or after the cement is added to the tailings stream.
100291 According to the embodiments, the separation step may be accomplished
by
any means known to those skilled in the art, including but not limited to
centrifuges,
hydrocyclones, decantation, filtration, thickeners or another mechanical
separation method.
[0030] The addition step of the embodiments results in the production of
coagulated solids. In one embodiment, the process provides efficient
dewatering of the
tailings and no other chemicals are necessary as cement alone is a
sufficiently potent
coagulant. In other embodiments, the process may further comprise adding a
coagulant or a
flocculant, or a combination thereof, for example a polymer flocculant. The
step of adding a
cement to the tailings stream may be simultaneous or sequential with the step
of adding one
or more polymer flocculants to the tailings stream. In certain embodiments,
the process
comprises adding sequentially the cement, then adding the one or more polymer
flocculants
to the tailings stream. In other embodiments, the process comprises adding
simultaneously
the cement and the one or more polymer flocculants to the tailings stream.
[0031] In exemplary embodiments, the tailings stream or solids comprises sand.
100321 In exemplary embodiments, the process comprises a desanding step. In an
exemplary embodiment, the cement, and optionally the flocculant, may be added
to the
tailings stream before desanding or after desanding. Desanding is a process
wherein the
tailings are settled for a period of time to form desanded tailings as the
supernatant.
Desanding may also be done for example by hydrocyclone.
[0033] In exemplary embodiments, one or more polymer flocculants having a
molecular weight of greater than about 100,000 Daltons or greater than about
1,000,000
Daltons may be added to the tailing stream during or after the cement is added
to the tailings
stream. In exemplary embodiments, the one or more polymer flocculants is a
high molecular
weight non-ionic polyacrylamide flocculant or a medium molecular weight high
charged
polyacryl ate flocculant.
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[0034] In certain embodiments, the clays in the supernatant, which may be
present
as a very dilute suspension, can be removed if desired or necessary by any
means known in
the art, or by the addition of more of the cement.
[0035] In certain embodiments, the process may optionally comprise adding one
or
more cationic coagulants or cationic flocculants to the tailings stream. The
one or more
cationic coagulants or flocculants may be added to the tailings stream before
or at the same
time as the cement, or added to the flocculated stream after the separation
step. In an
exemplary embodiment, a cationic coagulant or flocculant may be added to the
supernatant.
In exemplary embodiments, the cationic flocculant or coagulant is a
poly(dially1 dimethyl
ammonium chloride) compound; an epi-polyamine compound; a polymer that
contains one or
more quaternized ammonium groups, such as acryloyloxyethyltrimethylammonium
chloride,
methacryloyloxyethyltrimethylammonium chloride,
methacrylamidopropyltrimethylammonium chloride,
acrylamidopropyltrimethylammonium
chloride; or a mixture thereof. In exemplary embodiments, one or more
inorganic coagulants
may be added to the tailings stream. An inorganic coagulant may, for example,
reduce,
neutralize or invert electrical repulsions between particles. Exemplary
inorganic coagulants
include inorganic salts such as aluminum sulfate, ferric chloride, lime,
calcium chloride,
magnesium chloride, or various commercially available iron or aluminum salts
coagulants.
[0036] In the exemplary embodiments, the process may provide enhanced
flocculation of the solid materials in the tailings, better separation of the
of solids from water,
an increased rate of separation of the solids from the water, and/or may
expand the range of
operating conditions which can be tolerated while still achieving the desired
level of
separation of solids from water within a desired period of time.
[0037] The exemplary processes described herein may provide flocculant bed
with
higher densities, leading to settled beds that can dewater faster and build
yield strength faster
than comparable treatments without cement. In a particular embodiment, the
exemplary
processes accelerate dewatering of the tailings.
[0038] In exemplary embodiments, the processes may be used to dewater the
tailings so as to provide trafficable solids, or solids which possess a yield
stress of greater
than about 5000 Pa after one year, or a yield stress of greater than about
10000 Pa within five
years. In certain embodiments, the cement is added to the tailings stream to
accelerate
dewatering or to produce tailings that achieve 2500 Pa yield stress after
centrifugation.
[0039] In the exemplary embodiments, the dewatered solids may be handled or
processed in any manner as necessary or desired. In one embodiment, the
dewatered solids
should be handled in compliance with governmental regulations. In some
embodiments, the
resultant solids may be disposed of, sent to a tailings pond for additional
settling, or when
solids are a concentrated source of minerals, the solids may be used a raw
materials or feed to
produce compounds for commercial products. In the exemplary embodiments, the
separated
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water may be handled or processed in any manner as necessary or desired. In
one
embodiment, the separated water may be recycled to the process ("recycled
water"). For
example, the recycled water may be added to the crushed oil sands ore for
bitumen
extraction. Recycled water may also be added to the process at any point where
water is
added.
[0040] In the exemplary embodiments, the processes may be carried out at broad
pH conditions, such as a pH of about 6 to about 12, or about 8.5 to about
10.5. In certain
embodiments of the process, it is not necessary to adjust the pH.
[0041] In the exemplary embodiments, the processes may be carried out at
temperature of about 0 C to about 100 C, or about ambient temperature to about
90 C, or
about 20 C to about 90 C.
[0042] In one
embodiment, the processes produce at least about 20 %, at least
about 25 %, at least about 30 %, at least about 35 %, at least about 40 %, at
least about 45 %,
or at least about 50 %, by weight, of bed solids.
[0043] In one embodiment, the processes produce less than about 2 wt%, less
than
about 1.5 wt%, less than about 1 wt%, less than about 0.5 wt%, or less than
about 0.3 wt%
solids in the supernatant.
[0044] In the exemplary embodiments, the dosage of the cement can be any
dosage
that will achieve a necessary or desired result. In one embodiment, the dosage
of cement
added to the tailings stream is in the range of about 10 to about 10000 grams
cement per ton
of suspended dry solids (g/t), about 100 to about 5000 g/t, about 100 to about
2000 g/t, about
50 to about 1700 g/t, about 100 to about 1600 g/t, about 500 to about 1150
g/t, or about 500
to about 1000 g/t. In one embodiment, the dosage of cement is about 300 g/t,
about 350 g/t,
about 400 g/t, about 450 g/t, about 500 g/t, about 550 g/t, about 600 g/t,
about 650 g/t, about
700 g/t, about 750 g/t, about 800 g/t, about 850 g/t, about 900 g/t, about 950
g/t, about 1000
g/t, about 1050 g/t, about 1100 g/t, about 1150 g/t, about 1200 g/t, about
1250 g/t, about 1300
g/t, about 1350 g/t, about 1400 g/t, about 1450 g/t, about 1500 g/t, about
1550 g/t, or about
1600 g/t. In a particular embodiment, the dosage of cement is the dosage
effective to
coagulate fine tailings.
[0045] In one embodiment, the dosage of the cement added to the tailings
stream is
in the range of about 100 ppm to about 2000 ppm, about 200 ppm to about 2000
ppm, about
300 ppm to about 2000 ppm, about 400 ppm to about 1500 ppm, or about 400 ppm
to about
1000 ppm.
[0046] As used herein, the terms "polymer," "polymers," "polymeric," and
similar
terms are used in their ordinary sense as understood by one skilled in the
art, and thus may be
used herein to refer to or describe a large molecule (or group of such
molecules) that contains
recurring units. Polymers may be formed in various ways, including by
polymerizing
monomers and/or by chemically modifying one or more recurring units of a
precursor
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polymer. A polymer may be a "homopolymcr" comprising substantially identical
recurring
units formed by, e.g., polymerizing a particular monomer. A polymer may also
be a
"copolymer" comprising two or more different recurring units formed by, e.g.,
copolymerizing two or more different monomers, and/or by chemically modifying
one or
more recurring units of a precursor polymer. The term "terpolymer" may be used
herein to
refer to polymers containing three or more different recurring units.
[0047] In exemplary embodiments, the polymer flocculants may be anionic or
nonionic. Any anionic or nonionic polymer flocculants having a molecular
weight of greater
than about 100,000 Daltons, that are known in the art may be used in the
processes described
herein. Nonlimiting examples of exemplary polymer flocculants include, for
example,
flocculant-grade homopolymers, copolymers, and terpolymers prepared from
monomers such
as (meth)acrylic acid, (meth)acrylamide, 2-acrylamido-2-methylpropane sulfonic
acid, and
ethylene oxide. In one embodiment, the polymer flocculant is an anionic
polymers. In one
embodiment, the polymer flocculant is a nonionic polymers. In one embodiment,
the polymer
flocculant is a mixture of anionic polymers and nonionic polymers.
[0048] In the exemplary embodiments, the dosage of the one or more polymer
flocculant can be any dosage that will achieve a necessary or desired result.
In one
embodiment, the dosage of the one or more polymer flocculant is about 25 g/dry
T to about
1000 g/dry T, about 50 g/dry T to about 500 g/dry T, about 50 g/dry T to about
400 g/dry T,
about 50 g/dry T to about 300 g/dry T, about 50 g/dry T to about 100 g/dry T,
or about 50
g/dry T to about 200 g/dry T. In one embodiment, the dosage of the one or more
polymer
flocculant is less than about 500 g/dry T, less than about 400 g/dry T, less
than about 300
g/dry T, less than about 200 g/dry T, or less than about 100 g/dry T.
[0049] In one embodiment, the dosage of the one or more polymer flocculant
added
to the tailings stream is in the range of about 100 ppm to about 2000 ppm,
about 200 ppm to
about 2000 ppm, about 300 ppm to about 2000 ppm, about 400 ppm to about 1500
ppm, or
about 400 ppm to about 1000 ppm.
EXAMPLES
[0050] Example 1.
[0051] In this example, consolidation of oil sands tailings by using Portland
cements is examined by mixing tailings containing 22.17% solids with Portland
cements. A
40 ml sample of tailings was centrifuged at G-force 670 for 10 min. Residual
fine solids in
supernatant were measured gravimetrically. As given in Table 1, it has been
found that
centrifuging the tailings resulted in 1.36% solids in supernatant and 69.62%
solids in settled
bed. Centrifugation of tailings treated with 500 ppm of cements resulted in
0.36% or 0.23%
residual fine solids in supernatant and 57.53% or 53.36% solids in settled
beds, providing
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evidence of a reduction in suspended fine solids in supernatant. More
significant reduction of
suspended fine solids can be obtained using cement treatments at dosage of
1000 ppm or
higher. Some of the variation in solid content of settled beds when tailings
are treated with
cement can be attributed to differences in solids packing from dense close
packing to more
open network and immobilization of fine particles which in turn can result in
faster
dewatering.
[0052] Table 1. Consolidation of oil sands tailings by addition of Portland
Cements.
Experiment Cem A Dosage Cem W Solids in Solids in
Number (PPm) Dosage Supernatant Settled Bed
(PPm) (wt%) (wt%)
1 None None 1.36 69.62
2 500 None 0.36 57.53
3 1000 None 0.09 47.98
4 2000 None 0.00 46.79
4000 None 0.00 51.35
6 None 500 0.23 53.36
7 None 1000 0.07 48.70
8 None 2000 0.00 50.10
9 None 4000 0.00 47.54
Cem A: Grey Portland cement Type I/IT (Lehigh Cement, Alberta, Canada)
Cem W: White Portland cement Type I (Federal White Cement, Ontario, Canada)
[0053] Example 2. Effect of Cement and Polyacrylamide Flocculant Treatment on
Aggregation of Fine Tailings.
100541 In this example, aggregation of fine tailings was investigated using an
inline
particle size measurement by dynamic light scattering. Particle size
distributions (PSD) were
determined with a Beckman Coulter LS230, which measures the angular dependence
of
scattered light (mainly in the forward direction). A fine fraction of oil sand
tailing solids was
initially separated from coarser fraction by gravitation over a period of 4
hours.
Subsequently, a 1.0 mL sample was dispersed in the impeller driven flow loop
of the analyzer
containing tap water (approximately 700 mL) without additional chemical
dispersants.
Particle size distributions were computed as equivalent-sphere size
distributions based on
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Mic scattering and Fraunhofer diffraction formalisms applied to the scattering
data.
Measurements were taken during each test at 30, 60 and 120 seconds after
addition of the
solids to the diluent under continuous agitation at a single impeller speed.
This procedure was
used to assess whether a steady-state size distribution had been achieved. In
most cases,
particle size distributions reached steady state after 60 sec of mixing and
were used for
comparisons. Then 0.1 g of Portland cement was added to the suspended fines
slurry and
PSD was monitored over the time intervals of 5 minutes. Then 0.5 mL of polymer
flocculant
solution (0.4% in water of Superfloc4) N-300 or Superflocg A-190K, both
available from
Kemira Oyj) was added to the above diluted slurry. Results are presented in
Figures 1 and 2.
These figures show the particle size distribution as a function of time upon
addition of
cement and a high molecular weight non-ionic polyacrylamide flocculant (Figure
1) or a
medium molecular weight high charged polyacrylate flocculant (Figure 2) to a
sample of fine
tailings.
[0055] As represented in PSD graphs, fine solids agglomerate upon addition of
cement and larger aggregates are formed. In this regard, cement material show
similar
behavior as inorganic coagulants for agglomeration of fine solid particles. In
order to further
enhance agglomeration of fine solids, a flocculant may be added. As shown in
Figures 1 and
2, addition of a polymer flocculant to the fine tailings treated by cement,
resulted in complete
elimination of ultrafine solids (less than 1 micron) and formation of larger
aggregates as large
as 200 ¨ 600 microns within 60 seconds.
[0056] Example 3. Rheological Investigations.
[0057] Some insight on the microstructure of networks of flocculated particles
may
be acquired from their rheological behavior. Rheological investigations were
conducted at 22
C on an Anton-Paar MCR 300 rheometer equipped with a six-bladed Anton Paar ST
22-6V-
16/106 spindle. Measurements are accomplished by inserting the spindle into a
flocculated
sample until the tips of the vanes were covered, and then logarithmically
increasing the shear
stress from 0.1 Pa until the sample began to flow. The yield stress value can
be determined
by calculating the point at which a log-log plot of shear stress versus
viscosity deviated from
linearity. Samples of oil sand tailings (22.17% solids in process water) were
treated with
cement and a high molecular weight non-ionic polyacrylamide flocculant
(Superflock N-300,
available from Kemira Oyj). The fluids were centrifuged at G-force 670 for 5
min. The
supernatant is decanted and settled beds were used for rheological
measurement. Residual
fine solids in supernatant were measured gravimetrically. Rheological
measurement on
settled beds showed an increase in shear stress for cement treated tailings
compared to other
treatments as given in Table 3 and suspended fine solids in supernatant are
significantly
reduced and eliminated.
[0058] Table 3. Analytical data for settled beds from treated tailings.
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Experiment Cement Dosage Flocculant Dosage Yield Stress Solids in
Number (PPm) (PPnl) (Pa) Supernatant
(wt %)
1 None None 217 1.82
2 None 500 1299 0.74
3 500 500 1609 Less than 0.01
4 1000 500 2573 Less than 0.01
[0059] Example 4. Capillary Suction Time Tests.
[0060] The Capillary Suction Time (CST) test has been used since the 1970's as
a
practical, yet empirical method for characterizing dewatering and the state of
colloidal
materials in wastewater treatment facilities. The main use for the CST is to
determine
filterability of the flocculated solids after the addition of coagulant and/or
flocculant aids. The
dewatering properties of materials excised from settled beds of treated
tailings were measured
using a capillary suction time method as described in Standard Method APHA
2710 G
(APHA 1999). CST measurements were carried out using a CST instrument from
Venture
Innovations Inc. (California, USA). The CST is the time interval it takes an
aqueous solution
to traverse between two radial positions in a filter paper (Whatman No. 40, 9
cm) under the
influence of capillary suction. A low CST value implies good sludge
dewatering; i.e. the
water from the paste releases quickly with little impediment. Each measurement
was
conducted at least in triplicate and the average presented here. The time for
distilled water
blanks (10.8 + 0.7 sec) was subtracted from sample times to improve
comparisons and to
account for variation in characteristics of the filter paper used. Data given
in Table 4,
indicates that cement and polymer flocculant treatments resulted in generally
more expanded
beds with better dewatering rates (lower CST values) than did untreated or
treatment with
polymer flocculants only.
[0061] Table 4. Dewatering characteristics of tailings with chemical
treatments.
Experiment Cement Dosage Flocculant Dosage CST
Number (PPnl) m) (sec)
1 None None 218.4
2 None 500* 114.0
3 1000 500* 60.6
4 None 500** 84.0
1000 500** 24.1
*High molecular weight non-ionic polyacrylamide flocculant (Superflock N-300,
Kemira)
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** Medium molecular weight high charged polyacrylatc flocculant (SuperflocgA-
190K,
Kemira)
[0062] In the preceding specification, various exemplary embodiments have been
described. It will, however, be evident that various modifications and changes
may be made
thereto, and additional embodiments may be implemented, without departing from
the
broader scope of the exemplary embodiments as set forth in the claims that
follow. The
specification and drawings are accordingly to be regarded in an illustrative
rather than
restrictive sense.
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