Note: Descriptions are shown in the official language in which they were submitted.
CA 02792786 2012-10-16
NS-465, 466, 467
OIL SANDS FLUID FINE TAILINGS DEWATERING USING RIM DITCHING
FIELD OF THE INVENTION
The present invention relates to a process for dewatering oil sands tailings
using rim
ditching by modifying tailings properties with additives, and removing water
and rainfall by
drainage, ditching and decant structures to yield a trafficable deposit for
reclamation.
BACKGROUND OF THE INVENTION
1 0 Oil sand generally comprises water-wet sand grains held together by a
matrix of viscous
heavy oil or bitumen. Bitumen is a complex and viscous mixture of large or
heavy hydrocarbon
molecules which contain a significant amount of sulfur, nitrogen and oxygen.
The extraction of
bitumen from sand using hot water processes yields large volumes of tailings
composed of fine
silts, clays and residual bitumen which have to be contained in a tailings
pond. Mineral fractions
with a particle diameter less than 44 microns are referred to as "fines."
These fines are typically
quartz and clay mineral suspensions, predominantly kaolinite and illite.
The fine tailings suspension is typically 85% water and 15% fine particles by
volume.
Dewatering of fine tailings occurs very slowly. When first discharged in the
pond, the very low
density material is referred to as thin fine tailings. After a few years when
the fine tailings have
reached a solids content of about 30-35%, they are sometimes referred to as
mature fine tailings
(MFT). Hereinafter, the more general term of fluid fine tailings (FFT) which
encompasses the
spectrum of tailings from discharge to final settled state. The fluid fine
tailings behave as a fluid
colloidal-like material. The fact that fluid fine tailings behave as a fluid
and have very slow
consolidation rates limits options to reclaim tailings ponds. A challenge
facing the industry
remains the removal of water from the fluid fine tailings to increase the
solids content well
beyond 35% and strengthen the deposits to the point that they can be reclaimed
and no longer
require containment.
wsLegah053707\00008\8268987,1
CA 02792786 2012-10-16
. .
Accordingly, there is a need for an improved method of dewatering tailings.
SUMMARY OF THE INVENTION
The current application is directed to a process for dewatering oil sands
tailings using rim
ditching by modifying tailings properties with additives, and removing water
and rainfall by
drainage, ditching and decant structures to yield a trafficable deposit for
reclamation. The
present invention is particularly useful with, but not limited to, fluid fine
tailings. It was
surprisingly discovered that by conducting the process of the present
invention, one or more of
the following benefits may be realized:
(1) Modifying the tailings properties by use of additives enhances strength
development, drainage and evaporation in the resulting deposit. In particular,
lime alone or a
mixture of lime and gypsum combined with the tailings increases cracking of
the deposit thereby
enhancing evaporation, and reduces the total amount of dissolved solids
content in the release
water. Addition of a polymeric floeculant (typically a high molecular weight,
medium charge
density anionic polyacrylamide) to the tailings improves initial and
subsequent water drainage to
a greater extent than lime and gypsum. Other additives which manipulate the
initial strength and
dewatering of the fluid fine tailings such as alum, other flocculant
formulations, or combinations
of inorganic and organic mineral suspension modifiers would also be very
effective in this
application.
(2) Initial release water is removed rapidly from the deposit by pumping
from sumps,
or from the toe of the deposit, depending upon the deposit design
(3) Removing subsequent release water and rainfall from the deposit is
managed
efficiently using drainage, ditching and decant structures. The rapid removal
and control of
release water and rainfall or runoff accelerates consolidation and dewatering.
Thus, use of the present invention yields a tailings deposit which becomes
trafficable
soon after its disposal in a rim ditch, and enables reclamation of tailings
disposal areas.
In one aspect, a process for dewatering tailings is provided, comprising:
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= mixing the tailings with a sufficient amount of an additive or a mixture
thereof;
= depositing the resulting mixture into a containment area;
= rapidly removing the initially released water from the deposit to one or
more
sumps, and allowing the deposit to reach a sufficient strength; and
= removing remaining deposit water and rainfall through one or more of a
network
of ditches, a decant tower, or a plurality of dike drainage structures to
yield a
rapidly consolidating deposit for reclamation.
BRIEF DESCRIPTION OF THE DRAWINGS
Referring to the drawings wherein like reference numerals indicate similar
parts
throughout the several views, several aspects of the present invention are
illustrated by way of
example, and not by way of limitation, in detail in the figures, wherein:
FIG. 1 is a diagram showing the approximate FFT test dimensions and piezometer
locations.
FIG. 2 is a photograph of untreated FFT after ten months showing little
consolidation and
shallow cracks.
FIG. 3 is a photograph of lime and gypsum treated FFT showing extensive
consolidation
and deep cracks.
FIG. 4 is a graph showing the weight loss of a standard 20 L pail of water
over time, with
the rate corresponding to approximately 2 cm of water per week.
FIG. 5 is a graph showing pressure drop over time for an untreated FFT sample.
FIG. 6 is a graph showing pressure drop over time for a FFT sample treated
with 0.25%
lime and 0.25% gypsum.
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FIG. 7 is a graph showing pressure readings over time for the chemically
amended FFT
test.
FIG. 8 is a graph showing pressure readings over time for the control,
untreated FFT.
FIG. 9 is a graph showing the later dewatering rate for the treated FFT
sample.
FIG. 10 is a photograph showing the experimental set up for the pan drying
tests at the
start of the experiment (left) and after thirty days (right).
FIG. 11 is a graph showing the drying rates for the samples of FIG. 10.
FIG. 12 is a graph showing the solids fraction reached after eleven days of
drying for a
variety of polymers and lime/gypsum additives.
FIG. 13 is a graph showing the drying rate for the 45 kg tests for different
polymer types.
FIG. 14 is a graph showing the reduced evaporation rate over time with gypsum
addition
to FFT.
FIG. 15 is a graph showing the drying rate over time of FFT with varying lime
additions.
FIG. 16 is a photograph of the FFT with varying lime additions of FIG. 15.
FIG. 17 is a graph showing the evaporative drying rate over time of FFT with
various
lime and gypsum combinations. The inset is a photograph of the samples.
FIG. 18 is a photograph showing the dewatering behavior for 0.25% lime, 1000
g/tonne
A3338 polymer, and untreated FFT samples (top to bottom).
FIG. 19 is a photograph showing the initial pour for a 4 m3 dewatering test
with a high
molecular weight, medium charge density anionic polyacrylamide flocculant at
1360 g/tonne
solids (initial solids content of 25%) (top panel); and rim ditch water
release showing a close up
of cracking, view after ditching, and water collecting in the rim ditch
(bottom panel, left to right).
FIG. 20 is a graph showing the solids fraction increase over time for the 4 m3
rim ditch
test.
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DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The detailed description set forth below in connection with the appended
drawings is
intended as a description of various embodiments of the present invention and
is not intended to
represent the only embodiments contemplated by the inventor. The detailed
description includes
specific details for the purpose of providing a comprehensive understanding of
the present
invention. However, it will be apparent to those skilled in the art that the
present invention may
be practiced without these specific details.
The present invention relates generally to a process of improving the
dewatering of
tailings in rim ditching applications. Rim ditching is a common method of
accelerating the
dewatering of tailings, whereby the degree of saturation is controlled by
preventing standing
water from accumulating on the tailings deposit. The pressure of the material
above helps to
squeeze water out of the deposit. When enough strength is reached, a
continuous ditch is created
around the edge of the deposit to allow for accumulation of the water pushed
from the pore
spaces. With extensive deposit cracking and the construction of a ditch to
collect water and
guide it to a collection sump, tailings dewatering can be enhanced. The more
rapidly strength
develops in the tailings deposit, the more quickly and deeply the rim ditch
can be constructed.
However, vagaries of the weather (i.e., control and removal of rainfall) make
rim ditching
challenging to manage.
The process of the invention includes modifying the proper-ties of the oil
sands tailings by
use of additives to enhance deposit cracking and strength development,
drainage and evaporation
in the resulting deposit; and removing water and rainfall from the deposit
efficiently by use of
particular drainage, ditching and decant structures which accelerate
consolidation and
dewatering. The process thus forms a tailings deposit which is trafficable
soon after its disposal
in the rim ditch deposit containment area.
As used herein, the term "tailings" means tailings derived from oil sands
extraction
operations and containing a fines fraction. The term is meant to include fluid
fine tailings (FFT)
from tailings ponds and fine tailings from ongoing extraction operations (for
example, flotation
tailings, thickener underflow or froth treatment tailings) which may or may
not bypass a tailings
pond. In one embodiment, the tailings are primarily FFT obtained from tailings
ponds given the
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=
significant quantities of such material to reclaim. However, it should be
understood that the fine
tailings treated according the process of the present invention are not
necessarily obtained from a
tailings pond, and may also be obtained from ongoing oil sands extraction
operations.
In the process of the invention, a sufficient amount of an additive or a
mixture of
additives is first added to the tailings. In one embodiment, the additive is
introduced into an in-
line flow of the FFT. As used herein, the term "in-line flow" means a flow
contained within a
continuous fluid transportation line such as a pipe or another fluid transport
structure which
preferably has an enclosed tubular construction. In one embodiment, the
additive is combined
with the FFT in a mixer. Suitable additive introduction can include, but are
not limited to,
dynamic mixers, T mixers, static mixers, and continuous-flow stirred-tank
reactors. Preferably,
the mixer is a dynamic mixer in order to better optimize the additive/FFT
interaction. A typical
dynamic mixer is powered by an electric motor and contains one or more mixing
elements which
perform a rotary motion about the axis of the flow path. The speed and
configuration of the
mixing elements can be easily modified to fine-tune the mixing process for
products which are
susceptible to variations in raw material. Mixing is conducted for a
sufficient duration in order
to allow the tailings and additive to combine properly and to ensure the
efficiency of the
additive.
The preferred additive or mixture of additives may be selected according to
the tailings
composition and process conditions. However, optimum additives have been
identified for the
effective dewatering of tailings and production of amenable recycle water.
Suitable additives
include, but are not limited to, flocculants, coagulants, an additive
comprising at least one
multivalent cation, and other reagents that modify the rheology and cracking
behaviour of the
tailings deposit.
As used herein, the term "flocculant" refers to a reagent which bridges the
neutralized or
coagulated particles into larger agglomerates, resulting in more efficient
settling. Flocculants
useful in the present invention are generally anionic, nonionic, cationic or
amphoteric polymers,
which may be naturally occurring or synthetic, having relatively high
molecular weights.
Preferably, the polymeric flocculants are characterized by molecular weights
ranging between
about 1,000 kD to about 50,000 kD.
Suitable natural polymeric flocculants may be
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polysaccharides such as dextran, starch or guar gum. Suitable synthetic
polymeric flocculants
include, but are not limited to, charged or uncharged polyacrylamides, for
example, a high
molecular weight polyacrylamide-sodium polyacrylate co-polymer.
Other useful polymeric flocculants can be made by the polymerization of
(meth)acryamide, N-vinyl pyrrolidone, N-vinyl formamide, N,N
dimethylacrylamide, N-vinyl
acetamide, N-vinylpyridine, N-vinylimidazole, isopropyl acrylamide and
polyethylene glycol
methacrylate, and one or more anionic monomer(s) such as acrylic acid,
methacrylic acid, 2-
acrylamido-2-methylpropane sulphonic acid (ATBS) and salts thereof, or one or
more cationic
monomer(s) such as dimethylaminoethyl acrylate (ADAME), dimethylaminoethyl
methacrylate
(MADAME), dimethydiallylammonium chloride (DADMAC), acrylamido propyltrimethyl
ammonium chloride (APTAC) and/or methacrylamido propyltrimethyl ammonium
chloride
(MAPTAC).
In one embodiment, the flocculant comprises an aqueous solution of an anionic
polyacrylamide. The anionic polyacrylamide preferably has a relatively high
molecular weight
(about 10,000 kD or higher) and medium charge density (about 20-35%
anionicity), for example,
a high molecular weight polyacrylamide-sodium polyacrylate co-polymer.
It will be appreciated by those skilled in the art that various modifications
(e.g., branched
or straight chain modifications, charge density, molecular weight, dosage) to
the flocculant may
be contemplated.
As used herein, the term "coagulant" refers to a reagent which neutralizes
repulsive
electrical charges surrounding particles to destabilize suspended solids and
to cause the solids to
agglomerate. Suitable coagulants useful in the present invention include, but
are not limited to,
lime (calcium oxide), slaked lime (calcium hydroxide), gypsum (calcium sulfate
dehydrate),
polyamine, alum, or any combination thereof.
Lime has the advantage of not generating deleterious compounds. In one
embodiment,
the additive comprises lime in a concentration ranging from about 0.10% to
about 0.25%. In one
embodiment, the additive comprises a mixture of lime and gypsum. In one
embodiment, the
mixture comprises lime in a concentration ranging from about 0.10% to about
0.25%, and
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gypsum in a concentration ranging from about 0.10% to about 0.25%. In one
embodiment, the
concentrations of lime and gypsum are the same. In one embodiment, the mixture
comprises
0.125% lime and 0.125% gypsum. In one embodiment, the mixture comprises 0.185%
lime and
0.185% gypsum. In one embodiment, the mixture comprises 0.25% lime and 0.25%
gypsum.
The optimum combinations will vary depending upon the initial water chemistry
of the fluid fine
tailings and any desire to control the resulting release water chemistry.
As used herein, the term "multivalent" means an element having more than one
valence.
Valence is defined as the number of valence bonds formed by a given atom.
Suitable multivalent
inorganic additives may comprise divalent or trivalent cations. Divalent
cations increase the
adhesion of bitumen to clay particles and the coagulation of clay particles,
and include, but are
not limited to, calcium (Ca2+), magnesium (Mg2+), iron (Fe2+), and barium
(Ba2+). Trivalent
cations include, but are not limited to, aluminium (A13'), iron (Fe3+).
Various reagents may be
added to raise or lower the pH of the FFT, while also improving dewatering,
consolidation, and
deposit crack formation. Such reagents include, but are not limited to,
sulphuric acid, carbon
dioxide, phosphoric acid, sodium phosphate, sodium carbonate, hydrochloric
acid, calcium
oxide, calcium hydroxide, potassium hydroxide, sodium silicate, Portland
cement, and others.
Preferably, an alkaline pH ensures that the release water will ultimately have
a basic pH
amenable for recycling to the extraction process which is typically conducted
under conditions of
alkaline pH.
As demonstrated in the Example, addition of either lime alone or a mixture of
lime and
gypsum to the FFT increases cracking of the deposit thereby enhancing
evaporation, and reduces
the total amount of dissolved solids (TDS) content in the release water. As
used herein, the term
"TDS" means the total amount of mobile charged ions including minerals, salts
or metals
dissolved in a given volume of water. TDS is used as a common parameter for
assessing water
quality. High concentrations of TDS in release water are considered
detrimental to bitumen
recovery through disruption of extraction chemistry, and scaling, corrosion
and fouling of
equipment. However, lime addition precipitates calcium carbonate from the
release water. In
one embodiment, a mixture of lime and gypsum is preferred due to having a
synergistic effect.
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Addition of a polymeric flocculant to the FFT improves drainage to a greater
extent than
lime and gypsum, although cracking is not as extensive as with lime and
gypsum. In one
embodiment, the dosage of flocculant ranges from about 400 grams to about
1,500 grams per
tonne of solids in the FFT. Accordingly, the tailings may be treated with an
additive or mixture
of additives either prior to treatment with a flocculant, or following
treatment with a flocculant.
Adding the additive prior to treatment with a flocculant minimizes the amount
of required
flocculant which is relatively expensive. Adding the additive after flocculant
treatment may
improve deposit performance and release water quality.
The treated tailings may be deposited into a retaining impoundment which may
be
constructed in a mined-out pit or in a specifically constructed disposal area.
The impoundment is
of a sufficient size to retain the treated tailings. Tailings can be deposited
using various subaerial
techniques, the choice of which affects how the tailings deposit will
initially form and settle
within the impoundment. Preferably, the treated tailings are deposited at a
controlled rate to
optimize the release of water. When the evaporation rate from the tailings
deposit is sufficient, a
crust forms on top of the deposit. The additive treatment causes the formation
of cracks in the
crust and throughout the interior of the deposit, thereby increasing the
surface area for
evaporation and providing a network of cracks or channels through which the
water may drain
and be recovered. The released water is rapidly removed using one or more
pumps and/or by
directing it via ditches to sumps or other collection points where it can be
removed. Any suitable
water removal pumps as are known in the art may be used.
Removal of any remaining deposit water and subsequent rainfall is managed by a
network of ditches which channel the water and rainfall from the deposit to
one or more sumps.
As used herein, the term "sump" means a pit designed to receive water. The
sump is of a
sufficient size to retain water. The sump is capable of simultaneously
decanting rainwater and
accommodating subsidence as the deposit dewaters. The deposit and ditching
network are
monitored and maintained regularly to ensure an absence of standing water
which may inhibit
deposit dewatering.
The ditches may be formed once water release from the deposit appears to have
decreased and the deposit has sufficient strength to maintain a depression or
low point in the
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treated tailings material. It will be recognized by those skilled in the art
that this would typically
occur as the deposit approaches the liquid limit. The term "liquid limit"
means the moisture
content at which a material begins to behave as a liquid. However, depending
upon the amount
of additive and the nature of the crust formed on the deposit, it may be
possible to reach liquid
limit behavior at lower solids content. Without being bound by any theory, the
purpose of the
ditch network is to control rainfall; however, as the deposit increases in
strength, the material
from the ditches provides a stress which increases consolidation.
Preferably, the pumps and sumps are positioned remote from the point at which
the
treated tailings are deposited in order to minimize flat or dead areas where
water may collect and
inhibit the dewatering process.
In one embodiment, removal of any remaining deposit water and rainfall is
controlled by
a decant tower positioned at the lowest point of the deposit. The decant tower
is an intake
structure comprising a vertical or inclined hollow tower which allows standing
water to be
pumped out of the tower or drain by gravity via a conduit or pipe into a sump.
In one
embodiment, the decant tower comprises a cascade decant tower. The cascade
decant tower
allows water to flow over a variable weir into the sump. The height of the
weir is adjustable to
accommodate the reduction in the elevation of the deposit as a result of
dewatering and
consolidation. In one embodiment, multiple dike drainage structures may be
stacked to create a
sump of a desired depth in accordance with the rim ditch deposit geometry.
The collected water and rainfall may be recycled to the extraction process so
that the
amount of makeup water required is minimized. Once the tailings deposit has
eventually dried
and appears to have a suitable density to allow load-bearing, the deposit may
be used as a
trafficable surface for reclamation. Any common tailings management approach
would be used
to accelerate the reclamation process, including the addition of wick drains
to the deposit,
loading the deposit with a surcharge of sand or coke, or other common methods
or combination
of methods.
Exemplary embodiments of the present invention are described in the following
Example,
which is set forth to aid in the understanding of the invention, and should
not be construed to
limit in any way the scope of the invention as defined in the claims which
follow thereafter.
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=
Example 1
The effects of the addition of lime and gypsum to fluid fine tailings in a rim
ditch process
were assessed, with particular focus upon evaporative drying and comparing the
drying rate with
and without the addition of inorganic reagents. 100 kg test cells and two 14
m3 or 17,000 kg test
cells were used. The two large cells were instrumented to measure pore
pressure as a function of
depth in the cell and selected samples were evaluated for water chemistry
changes. The pore
pressure dissipation with evaporation was monitored for over one year for the
large cells and
evaporation rate for about eight weeks for the smaller 100 kg cells.
Lime and gypsum were added to FFT samples and mixed for more than thirty
minutes
before being transferred into the 14 m3 test cells. The smaller 100 kg tests
were thoroughly
mixed using a large hand held mixer. Lime and gypsum additives were added on a
weight of
slurry basis. Where possible, water samples were collected to detelinine the
water chemistry.
After drying, re-wetting experiments were conducted to determine re-wetting
and run off water
chemistries. Standard methods were used to evaluate sample composition, water
chemistry,
particle size distribution, and clay content.
i. Large (14 m3) bin tests
Two separate, large volume FFT samples were characterized (Table 1), Although
the
two samples represented FFT collected several weeks apart, the solids,
bitumen, and clay
contents were similar. Table 1 includes the milli-equivalents of methylene
blue adsorbed by the
solids as determined by Dean-Stark and slurry methylene blue index (MBI)
methods. In both
cases, the results were about the same, indicating that the FFT samples
exhibited similar clay
contents. The clay content was confirmed with the sedigraph analysis,
indicating that
approximately 53% of the solids are clay sized in both of the samples.
1 1
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Table 1
Sample Bitumen Water Mineral % MBI Slurry % Passing %
Passing
( /0) (%) (%) Passing meq/100g MBI 44 um 2
45 um meq/100 g (sedigraph
(sedigraph
sieve method)
method)
FFT 1.7 63.2 34.9 97.7 10,1 8.2 98.6
53.5
untreated
FFT 0.8 60.0 33.6 95.1 9.8 8.0 98.1
53.2
treated
Figure 1 shows the approximate locations of four (4) piezometers, 1.38m,
0.95m, 0.52m
and 0.09m (from top to bottom), installed in the deposit to measure pressures
as a function of
depth. Each bin was filled to the 190 cm level at the start of the experiment,
which was also the
height of a collapsible tube designed to drain the surface water away as the
FFT consolidated.
Figures 2 and 3 show the untreated and treated (0.25% lime, 0.25% gypsum by
weight)
samples, respectively, near the end of the experiment. A fan was used at each
bin to mimic wind
conditions.
The treated FFT clearly performed better since no water was collected by
drainage for the
untreated FFT. Ditching for the untreated FFT was abandoned due to lack of
strength gain.
Strength gain allows for the maintenance of a ditch. A significant amount of
water was collected
for the treated FFT. The water chemistry results are set out in Table 2. Due
to a passive water
collection system, samples were not provided for water chemistry analysis for
days or weeks
after collection. Consequently, carbon dioxide from the air may have reduced
the pH. The
release water contained essentially no solids. The increase in total dissolved
solids over time
may be due to evaporation of water from the deposit sample. The lack of any
release water from
the untreated FFT test was attributed to the fact that evaporation was faster
than drainage.
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Table 2
Sample Ca K Mg Na Cl SO4 HCO3 CO3 pH Ion
Total
ID
balance dissolved
solids
Pail 1 329 29 4 1313 712 2446 64 13 8.66 1.03
4911
Day 1
Pail 2 328 29 5 1328 705 2423 60 15 8.71 1.05
4893
Day 1
Pail 3 351 34 4 1336 708 2431 63 14 8.68 1.07
4942
Day 1
Day 10 330 5 1571 905 3034 74 8 8.51 1.08
Day 15 488 39 5 1721 1018 3426 89 0 7.85 0.99
6785
Day 17 477 39 11 1687 952 3178 89 0 7.97 1.05
6435
Day 21 501 41 6 1789 1090 3594 92 0 7.83 0.97
7113
The initial water release from the treated FFT was about 542 kg (over eight
days), after
which water drainage stopped due to problems with the retractable drain pipe
in the deposit. The
542 kg of water represented about 7.5 cm depth given the 7 m2 surface area of
the test bin.
Figure 4 shows the pan evaporation rate as determined from the loss of weight
of a water sample
over time. The rate of water loss was 0.17 kg per day or 1.2 kg per week. With
the area of the
water pail at approximately 610 cm2, the evaporation rate was about 2 cm per
week. The rate of
pressure change in the piezometers should reflect the sum of the water lost
due to evaporation
plus the water which was drained as the sample consolidated, but these changes
are essentially in
the noise of the piezometer readings when corrected for atmospheric pressure.
7 cm of water
represented approximately 0.7 kPa with atmospheric pressure being in the range
of 92 0.2.
Figures 5 and 6 show the sensor pressure in kPa for untreated and treated FFT
(0.25%
lime, 0.25% gypsum), respectively. Figure 6 shows that there was a steady drop
in corrected
pressure (0.2 kPa per week) corresponding to a loss of water of about 2 cm per
week in the
treated FFT. In Figures 5 and 6, the highest pressure readings are for the
deepest piezometers.
When the piezometer is no longer under water, the pressure reading approaches
zero. The
untreated FFT showed a rate of pressure drop of approximately half of the rate
(Figure 5)
compared to the treated FFT, indicating a significant improvement in
dewatering and
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. ,
consolidation compared to the untreated FFT. Figure 5 also indicated problems
with the
piezometers in the untreated sample after about 25 weeks (note the increased
scatter in the
readings), but this has no impact on the conclusions regarding the significant
difference in
evaporation rate for the treated and untreated FFT. The evaporation rate from
a slurry sample is
seldom equal to the rate from a liquid water surface, but for the test bins,
the fan would improve
the evaporation rate. In addition, the extensive and deep cracks in the
treated FFT would
increase the effective evaporative surface area for the FFT slurry sample.
ii. Small Scale Evaporation Rate Experiments
Small scale tests were conducted to assess the relationship between chemical
addition and
dewatering via evaporation. The data showed that there is likely an optimum
lime/gypsum
dosage which optimizes cracking and evaporation without leading to reduction
in evaporation
due to salt accumulation. Three sets of small scale tests were conducted to
establish a reasonably
scaled index test to evaluate dewatering. These were lab scale (1-2 kg) and
two lab pilot scales
at nominally 45 kg or 150 kg.
a) Lab Scale Dewatering Tests (1-2 kg)
Initial small scale testing of various polymer and inorganic ion mixtures was
conducted
using a 5% slope and a paper towel base to prevent the mixture from sliding to
the bottom of the
pan. Two polymers were tested, both high molecular weight, medium charge
density, anionic
polyacrylamides. Polymer A was a predominantly branched polymer, while polymer
B was
predominantly straight chained. The inorganic ion mixture tested was
lime/gypsum. The
polymer addition was 500 g/tonne on a slurry basis, or about 1400 g/tonne of
dry solids. The
lime and gypsum was added as 0.25% of each on a slurry basis. The results are
shown in Figures
10 and 11. The tests were performed in duplicate. With reference to Figure 10,
illustrated from
top to bottom, the top two trays are untreated FFT, the next two trays are FFT
treated with
polymer A, the next two trays are FFT treated with polymer B, and the bottom
two trays are FFT
treated with gypsum and lime.
As shown in Figure 10, the polymer samples had sufficient strength to sit on
the upper
part of the drying tray, and little or no cracking was evident as the samples
underwent significant
shrinkage as they dried. The lime/gypsum sample filled the tray more uniformly
and underwent
14
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significant cracking. Since cracking increases evaporation by increasing
available surface area,
these tests were repeated with larger 2 kg samples spread across the tray in
order to try to create
a uniform sample cracking opportunity. The A and B polymers released a
significant amount of
water at the start of the experiment as evidenced by the saturated paper
towel. The lime and
gypsum sample did not develop an initial strength and coverage of the pan was
more uniform
than that for the FFT and polymer mixes. Figure 11 shows that the drying rates
for all samples
were similar where 1.2 kg of sample was set up for drying.
Figure 12 shows the increase in solids fraction after eleven days of drying
for a variety of
polymers and lime and gypsum samples. The data confirm the above experiments,
with the
exception that when polymer was introduced under low shear conditions,
significantly greater
cracking was observed, along with measurably greater drying rates. The drying
process is
complex, with surface area, shrinkage, and salt accumulation all playing a
role in the drying rate.
In addition, polymer mixing is important. Drainage of water is also a factor,
as observed by the
difference in the amount of water adsorbed into the paper backing of the 1.2
kg tests. Since these
small scale tests did not allow for quantification of the amount of drainage
water relative to the
evaporated water, larger tests were run to collect and quantify drainage and
evaporated water.
b) Lab Pilot Dewatering Tests (45 kg)
Large plastic containers containing about 45 kg of treated FFT were evaluated
for drying
rate with three anionic polyacrylamides. Polymer C is a very high molecular
weight medium
charge density polyacrylamide, and Polymer D is a very high molecular weight
low charge
density polymer. Figure 13 and Table 3 summarize the results over 3 weeks.
w S Le ga1\053707\00008\8268987v1
CA 02792786 2012-10-16
Table 3
Sample 0 hours 168 hours 336 hours 504 hours
(1 week) (2 weeks) (3 weeks)
FFT 1 0.96 0.90 0.85
700 ppm polymer A 1 0.92 0.70 0.48
1000 ppm polymer A 1 0.87 0.70 0.41
1300 ppm polymer A 1 0.83 0.74 0.36
700 ppm Polymer C 1 0.93 0.87 0.74
1000 ppm Polymer C 1 0.93 0.80 0.58
1300 ppm Polymer C 1 0.90 0.80 0.54
700 ppm Polymer D 1 0.97 0.86 0.72
1000 ppm Polymer D 1 0.96 0.73 0.60
1300 ppm Polymer D 1 0.94 0.75 0.54
This test scale was not large enough to allow for removal of surface or
drainage water,
even though for some test samples, significant amounts of water were released.
Dewatering was
defined only by the pan evaporation rate until all of the water had evaporated
from the surface
and shrinkage and cracking mechanisms began to play a role. As a result of
this limitation, it is
difficult to gauge accurately the dewatering performance that would occur in
the field where a
deposition slope would allow for water drainage and exposure of the slurry
surface for
evaporative dewatering.
c) Lab Pilot Dewatering Tests (150 kg)
Boxes were constructed and lined with plastic. Fans were used to ensure a
consistent and
rapid drying rate. As shown in Figure 14, there was a reduction in evaporation
rate with gypsum
addition (0.1% gypsum by weight, 0.15% gypsum by weight, 0.2% gypsum by
weight, 0.25%
gypsum by weight, and 0.3% gypsum by weight) relative to the FFT alone, This
behaviour is
typical of soils with elevated salt content since the salt precipitation tends
to seal the pore throats
and soil channels through which the water evaporates. Further, it can be seen
from Figure 15
that when lime was added (0.25% gypsum by weight/0.25% lime by weight, 0.1%
lime by
weight, 0.15% lime by weight, 0.2% lime by weight, and 0.25% lime by weight),
there was an
increase in evaporative drying via two mechanisms. The first is the increased
cracking which
increases the surface area and therefore the evaporation rate (see Figure 16).
The second is a
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potential reduction in the total dissolved salts due to a cold lime softening
effect. The elevated
pH due to the lime addition precipitates calcium carbonate (due to the calcium
and bicarbonate
concentrations) from the FFT pore water. These salts are then no longer
available to concentrate
at the sample surface as drying takes place. Figures 15 and 16 also show the
synergistic effect of
lime and gypsum addition, where the best performance is seen with the
combination of reagents.
Figure 17 shows both the salt shut off of evaporation, and the optimization
that occurs
with cracking of the sample surface (i.e., the greater the cracking, the
greater the evaporation
rate, just as was observed in the smaller scale tests). The high gypsum
content tailings dewatered
more slowly than the control (untreated FFT) due to salt accumulation at the
surface, hindering
evaporation. In the lime gypsum combinations, the inset shows the increase in
cracking with
lime and gypsum in combination, and the figure shows the concomitant increase
in dewatering
rate.
In order to assess the cracking phenomenon, 150 kg tests were conducted with
shallow
pans to maximize the area of the deposit, and thus the cracking behaviour. As
shown in Figure
18, there was significantly greater cracking for the lime treated sample
compared to either the
polymer treated or the control FFT samples. The upper sample is the
lime/gypsum treated FFT,
followed by the polymer treatment, and then the control or untreated FFT.
However, this
experimental set up did not permit weighing of the samples as drying occurred,
and the shallow
aspect ratio did not allow for proper quantification or qualitative assessment
of the cracking
depth. Yet it can be seen that there are significant differences in drying
rate. The lime treated
FFT dried much faster than either the polymer treated or untreated FFT samples
(Figure 18).
The scale of this experiment enabled collection of run-off water. The water
chemistries
of selected samples are set out in Table 4. Depending upon the additives, the
lime softening
effect was evident by the reduction of the total dissolved salts when lime was
added. Even with
some gypsum addition, the appropriate amount of lime resulted in a net
decrease in the total
dissolved salts. Without being bound by any theory, it may be possible to
improve cracking and
dewatering behaviour, and decrease the effect of evaporative reduction with
salt precipitation.
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Table 4
Pore Waters Ca K Mg Na S Fe CI SO4 HCO3 CO3 pH
IB TDS
FFT 18 18 13
1327 47 0 937 135 1228 251 8.4 1.04 3974
FFT, 8 12 9 885 42 3 941 48 0 441 10.53
0.945 2390
0.25% lime
FFT, 14 13 4 1159 333 0 964 954 0 0 9.39 1.1
3439
0.125%
gypsum,
0.125% lime
d) Lab Pilot Rim Ditch Simulation (4000 kg)
Since it is generally thought that drainage dominates in rim ditch
applications, a large
scale test was conducted (Figure 19). Approximately 4 m3 of 25% solids treated
FFT (Polymer
E, a medium to high charge density, high molecular weight polyacrylamide) at
1360 g/tonne of
solids) was left to dewater over several weeks. An initial rapid water release
was observed.
F--
Figure 19 shows the cracking behavior and settling after about twenty-eight
days, with water still
collecting in the ditch that was created to collect drainage water. The upper
left photograph
shows the initial filling of the bin with the polymer treated FFT, and the
upper right photograph
I -
shows the initial water release. The bottom left and bottom centre photographs
show the
cracking occurring after the initial water was removed and evaporation is
occurring. The bottom
right photograph shows the rim ditch that was created and the deposit water
continuing to collect.
Figure 20 shows the average solids content increase in the bin as drainage
water was removed.
This demonstration of rapid dewatering, coupled with the previous smaller
scale test results,
confirmed that polymers would be suitable for rim ditch applications.
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