Note: Descriptions are shown in the official language in which they were submitted.
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DEPOSITING AND FARMING METHODS FOR DRYING FINE TAILINGS
FIELD OF THE INVENTION
The present invention generally relates to the field of treating oil sand fine
tailings.
BACKGROUND
Oil sand fine tailings have become a technical, operational, environmental,
economic
and public policy issue.
Oil sand tailings are generated from hydrocarbon extraction process operations
that
separate the valuable hydrocarbons from oil sand ore. All commercial
hydrocarbon
extraction processes use variations of the Clark Hot Water Process in which
water is
added to the oil sands to enable the separation of the valuable hydrocarbon
fraction from
the oil sand minerals. The process water also acts as a carrier fluid for the
mineral
fraction. Once the hydrocarbon fraction is recovered, the residual water,
unrecovered
hydrocarbons and minerals are generally referred to as "tailings".
The oil sand industry has adopted a convention with respect to mineral
particle sizing.
Mineral fractions with a particle diameter greater than 44 microns are
referred to as
"sand". Mineral fractions with a particle diameter less than 44 microns are
referred to as
"fines". Mineral fractions with a particle diameter less than 2 microns are
generally
referred to as "clay", but in some instances "clay" may refer to the actual
particle
mineralogy. The relationship between sand and fines in tailings reflects the
variation in
the oil sand ore make-up, the chemistry of the process water and the
extraction process.
Conventionally, tailings are transported to a deposition site generally
referred to as a
"tailings pond" located close to the oil sands mining and extraction
facilities to facilitate
pipeline transportation, discharging and management of the tailings. Due to
the scale of
operations, oil sand tailings ponds cover vast tracts of land and must be
constructed and
managed in accordance with regulations. The management of pond location,
filling, level
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control and reclamation is a complex undertaking given the geographical,
technical,
regulatory and economic constraints of oil sands operations.
Each tailings pond is contained within a dyke structure generally constructed
by placing
the sand fraction of the tailings within cells or on beaches. The process
water,
unrecovered hydrocarbons, together with sand and fine minerals not trapped in
the dyke
structure flow into the tailings pond. Tailings streams initially discharged
into the ponds
may have fairly low densities and solids contents, for instance around 0.5-10
wt%.
In the tailings pond, the process water, unrecovered hydrocarbons and minerals
settle
naturally to form different strata. The upper stratum is primarily water that
may be
recycled as process water to the extraction process. The lower stratum
contains settled
residual hydrocarbon and minerals which are predominately fines. This lower
stratum is
often referred to as "mature fine tailings" (MFT). Mature fine tailings have
very slow
consolidation rates and represent a major challenge to tailings management in
the oil
sands industry.
The composition of mature fine tailings is highly variable. Near the top of
the stratum the
mineral content is about 10 wt% and through time consolidates up to 50 wt% at
the
bottom of the stratum. Overall, mature fine tailings have an average mineral
content of
about 30 wt%. While fines are the dominant particle size fraction in the
mineral content,
the sand content may be 15 wt % of the solids and the clay content may be up
to 75 wt%
of the solids, reflecting the oil sand ore and extraction process. Additional
variation may
result from the residual hydrocarbon which may be dispersed in the mineral or
may
segregate into mat layers of hydrocarbon. The mature fine tailings in a pond
not only has
a wide variation of compositions distributed from top to bottom of the pond
but there may
also be pockets of different compositions at random locations throughout the
pond.
Mature fine tailings behave as a fluid-like colloidal material. The fact that
mature fine
tailings behave as a fluid significantly limits options to reclaim tailings
ponds. In addition,
mature fine tailings do not behave as a Newtonian fluid, which makes
continuous
commercial scale treatments for dewatering the tailings all the more
challenging. Without
dewatering or solidifying the mature fine tailings, tailings ponds have
increasing
economic and environmental implications over time.
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There are some methods that have been proposed for disposing of or reclaiming
oil
sand tailings by attempting to solidify or dewater mature fine tailings. If
mature fine
tailings can be sufficiently dewatered so as to convert the waste product into
a reclaimed
firm terrain, then many of the problems associated with this material can be
curtailed or
completely avoided. As a general guideline target, achieving a solids content
of 75 wt%
for mature fine tailings is considered sufficiently "dried" for reclamation.
Some known methods have attempted to treat oil sand tailings with the addition
of a
chemical to create a modified material that can be deposited. The chemically
modified oil
sand tailings have conventionally been sent subsurface or dumped and stacked
onto a
deposition area according to the area's availability and proximity to the
chemical addition
site and left to dry. The variability of the raw oil sand fine tailings and
the process
operating conditions of chemical addition can lead to variability in the
physical properties
of the resulting modified tailings material that is deposited. Consequently,
known
techniques for treating and then depositing fine tailings have had various
difficulties and
disadvantages.
Management of deposition and drying must deal with significant quantities of
fine tailings
with variable compositions and properties. For instance, bag filters, track-
packing, filter
pressing and other techniques are unsuitable for deposition and post-
deposition
handling of oil sand fine tailings. Known methods for deposition and post-
deposition
management of treated oil sand tailings have a variety of drawbacks including
inefficient
use of land and energy, bottlenecks in the process, uncontrolled dewatering,
mechanical
equipment clogging, difficulties in releasing, draining and recovering water
and inefficient
use of drying mechanisms.
Given the significant inventory and ongoing production of fine tailings at oil
sands
operations, there is a need for techniques and advances in fine tailings
drying.
SUMMARY OF THE INVENTION
The present invention responds to the above need by providing methods for
drying oil
sand fine tailings.
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Accordingly, the invention provides a method for drying oil sand fine
tailings, comprising
chemically treating the fine tailings to produce modified fine tailings
comprising
flocculated fine tailings; providing a deposition cell having a sloped bottom
surface;
depositing the modified fine tailings into the deposition cell such that the
sloped bottom
surface allows the flocculated fine tailings to form a deposit that undergoes
buildup and
channelless advancement over the sloped bottom surface and allows gravity
drainage of
release water away from the deposit; and allowing the deposit to stand within
the
deposition cell and for drying.
The sloped bottom surface of the cell advantageously cooperates with the
flocculated
fine tailings to allow gravity to induce the deposit to gently advance over
the deposition
cell and the release water to flow away from the advancing deposit, so as to
avoid
channeling and allow the flocs to retain capture of the fines, to particularly
improve the
initial dewatering of the deposit which accelerates the overall drying. The
sloped bottom
surface allows improved drainage of both release water contained in the
original fine
tailings and precipitation that may occur since the deposition cells are
located outdoors.
The invention also provides a method for drying oil sand fine tailings,
comprising
providing a deposition cell comprising a head region; a toe region spaced away
from the
head region; and a sloped bottom surface extending from the head region to the
toe
region such that the toe region is at a lower elevation than the head region;
depositing
flocculated fine tailings at the head region of the deposition cell, to form a
deposit that
undergoes buildup and moves down the sloped bottom surface, the deposit
forming a
built-up area and a lower area; and plowing the deposit while wet to spread
the modified
fine tailings from the built-up area toward the lower area, to ensure water
release
conditioning while avoiding over-shearing and maintaining sufficient shear
strength of
the flocculated fine tailings to allow standing.
The plowing of the deposit advantageously cooperates with the flocculated fine
tailings
which have increased yield strength yet can have variable properties upon
deposition, to
ensure sufficient water release conditioning, improve land utilization and
accelerate the
overall drying.
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The invention also provides a method for drying oil sand fine tailings,
comprising
depositing chemically modified fine tailings comprising flocculated fine
tailings into a
deposition cell so as to form a standing deposit; allowing the standing
deposit to partially
dewater and dry to form a dried upper crust; and harrowing the standing
deposit to break
up the dried upper crust, expose wet regions there-beneath and create furrows
in the
standing deposit.
The harrowing of the deposit advantageously cooperates with the flocculated
fine tailings
ones they have formed a dried upper crust, to both enhance evaporative drying
mechanisms and create furrows to enhance drainage of release water permeating
out of
the wet regions of the deposit and of precipitation that may occur since the
deposition
cells are located outdoors, thereby improving the overall drying of the oil
sand fine
tailings.
In some aspects, there is provided a method for drying fine tailings,
comprising:
chemically treating the fine tailings to produce modified fine tailings
comprising
flocculated fine tailings;
providing a deposition cell having a sloped bottom surface;
depositing the modified fine tailings into the deposition cell such that the
sloped
bottom surface allows the flocculated fine tailings to form a deposit that
undergoes buildup and channelless advancement over the sloped bottom surface
and allows gravity drainage of release water away from the deposit; and
allowing the deposit to stand within the deposition cell and for drying.
In some aspects, there is provided a method for drying fine tailings,
comprising:
providing a deposition cell comprising:
a head region;
a toe region spaced away from the head region; and
a sloped bottom surface extending from the head region to the toe region
such that the toe region is at a lower elevation than the head region;
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depositing flocculated fine tailings at the head region of the deposition
cell, to
form a deposit that undergoes buildup and moves down the sloped bottom
surface, the deposit forming a built-up area and a lower area; and
plowing the deposit while wet to spread the modified fine tailings from the
built-up
area toward the lower area, to ensure water release conditioning while
avoiding
over-shearing and maintaining sufficient shear strength of the flocculated
fine
tailings to allow standing.
In some aspects, there is provided a method for drying fine tailings,
comprising:
depositing chemically modified fine tailings comprising flocculated fine
tailings
into a deposition cell so as to form a standing deposit;
allowing the standing deposit to partially dewater and dry to form a dried
upper
crust;
harrowing the standing deposit to break up the dried upper crust, expose wet
regions there-beneath and create furrows in the standing deposit.
In some aspects, there is provided a method for drying a colloidal fluid
having non-
Newtonian fluid behavior, comprising:
chemically treating the colloidal fluid to produce a modified fluid comprising
flocculated colloidal fluid;
providing a deposition cell having a sloped bottom surface;
depositing the modified fine tailings into the deposition cell such that the
sloped
bottom surface allows the flocculated fine tailings to form a deposit that
undergoes buildup and channelless advancement over the sloped bottom surface
and allows gravity drainage of release water away from the deposit; and
allowing the deposit to stand within the deposition cell and for drying.
In some aspects, there is provided a method for drying a colloidal fluid
having non-
Newtonian fluid behavior, comprising:
providing a deposition cell comprising:
a head region;
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a toe region spaced away from the head region; and
a sloped bottom surface extending from the head region to the toe region
such that the toe region is at a lower elevation than the head region;
depositing flocculated colloidal fluid at the head region of the deposition
cell, to
form a deposit that undergoes buildup and moves down the sloped bottom
surface, the deposit forming a built-up area and a lower area; and
plowing the deposit while wet to spread the flocculated colloidal fluid from
the
built-up area toward the lower area, to ensure water release conditioning
while
avoiding over-shearing and maintaining sufficient shear strength of the
flocculated colloidal fluid to allow standing.
In some aspects, there is provided a method for drying a colloidal fluid
having non-
Newtonian fluid behavior, comprising:
depositing chemically modified colloidal fluid comprising flocculated
colloidal fluid
into a deposition cell so as to form a standing deposit;
allowing the standing deposit to partially dewater and dry to form a dried
upper
crust;
harrowing the standing deposit to break up the dried upper crust, expose wet
regions there-beneath and create furrows in the standing deposit.
In some aspects, there is provided a method for drying fine tailings,
comprising:
providing a pair of deposition cells, each deposition cell having:
a head region;
a toe region spaced away from the head region; and
a sloped bottom surface extending from the head region to the toe region
such that the toe region is at a lower elevation than the head region;
the pair of deposition cells having a toe-to-toe arrangement;
depositing flocculated fine tailings at the head region of each deposition
cell to
form a corresponding deposit and allow gravity drainage of release water away
from the deposit toward the corresponding toe region; and
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providing a common drainage ditch in between the toe regions of the pair of
deposition cells for receiving the release water from both of the deposition
cells.
In some aspects, there is provided a method for drying fine tailings,
comprising:
providing a pair of deposition cells, each deposition cell having:
a head region;
a toe region spaced away from the head region by a length; and
a sloped bottom surface extending from the head region to the toe region
such that the toe region is at a lower elevation than the head region;
the pair of deposition cells having a head-to-head arrangement; and
depositing flocculated fine tailings from common distribution piping to form a
corresponding deposit and allow gravity drainage of release water away from
the
deposit toward the corresponding toe region.
In some aspects, there is provided a method for drying fine tailings,
comprising:
providing a radial deposition cell having:
a head region having a deposition point;
a toe region spaced away from the head region; and
a sloped bottom surface extending from the head region to the toe region
such that the toe region is at a lower elevation than the head region, the
sloped bottom surface having at least a partial-conical shape; and
depositing flocculated fine tailings at the deposition point to form a deposit
and
allow gravity drainage of release water away from the deposit toward the toe
region.
In some aspects, there is provided a method for drying fine tailings,
comprising:
providing a deposition cell having:
a head region at which flocculated fine tailings are deposited;
a toe region spaced away from the head region; and
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a sloped bottom surface extending from the head region to the toe region
such that the toe region is at a lower elevation than the head region;
depositing the flocculated fine tailings into the deposition cell to form an
initial lift
and allow gravity drainage of release water away from the initial lift toward
the toe
region and to form a dewatered initial lift having sufficient geotechnical
stability to
support a subsequent lift; and
depositing the subsequent lift over top of the initial lift to allow gravity
drainage of
subsequent release water away from the subsequent lift toward the toe region.
In some aspects, there is provided a method for drying fine tailings,
comprising:
providing a deposition cell having:
a head region;
a toe region spaced away from the head region; and
a sloped bottom surface extending from the head region to the toe region
such that the toe region is at a lower elevation than the head region;
depositing the flocculated fine tailings at the head region of the deposition
cell to
form a deposit and allow gravity drainage of release water to produce a
partially
dewatered deposit;
excavating the partially dewatered deposit to relocate the partially dewatered
deposit to a secondary deposition area; and
depositing additional flocculated fine tailings into the deposition cell to
form an
additional deposit and allow gravity drainage of additional release water away
from the additional deposit toward the toe region.
In some aspects, there is provided a method for drying fine tailings,
comprising:
providing a deposition cell having:
a head region;
a toe region spaced away from the head region; and
a sloped bottom surface extending from the head region to the toe region
such that the toe region is at a lower elevation than the head region;
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adding a flocculant to the fine tailings to produce flocculated fine tailings;
submitting the flocculated fine tailings to solid-liquid separation to produce
water-
depleted flocculated fine tailings; and
depositing the water-depleted flocculated fine tailings into the deposition
cell to
form a deposit and allow gravity drainage of additional release water away
from
the deposit toward the toe region.
In some aspects, there is provided a method for drying fine tailings,
comprising:
adding a flocculant to the fine tailings to produce flocculated fine tailings;
depositing the flocculated fine tailings into a deposition cell to form a
deposit and
allow gravity drainage of additional release water away from the deposit;
wherein the flocculated fine tailings are supplied to solid-liquid separation
equipment prior to depositing.
In some aspects, the flocculated fine tailings are produced in a pipeline and
comprise
the flocculated fine tailings and a portion of the release water within the
pipeline prior to
depositing.
In some aspects, the sloped bottom surface is generally planar with a
generally constant
slope from the head region to the toe region.
In some aspects, the head region is narrower than the toe region.
In some aspects, the bottom surface is generally convex in the lateral
direction.
In some aspects, the flocculated fine tailings are expelled for deposition at
a flow rate
between about 1000 gal/min and about 3000 gal/min.
In some aspects, the plow device is operated to provide a single plowing
sweep.
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In some aspects, the upper crust has a thickness between about 1 inch and
about 4
inches when harrowed.
In some aspects, the harrow device is a disc harrow comprising a plurality of
discs sized
so that the discs penetrate below the dried upper crust and into a wet region
thereunder.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig 1 is a topographical map schematic of various embodiments of deposition
cells that
may be used with the method of the present invention.
Fig 2 is a general representative graph of shear yield stress versus time
showing the
process stages for an embodiment of an MFT flocculation technique.
Fig 3 is a general representative graph of shear yield stress versus time
showing the
process stages for another embodiment of the flocculation technique.
Fig 4 is a graph showing the relationship between shear stress and shear rate
for an
MFT sample, illustrating the non-Newtonian nature of MFT at higher solids
contents.
Fig 5 is a side cross-sectional view of a pipeline reactor for use with
embodiments of the
method of the present invention.
Fig 6 is a partial perspective transparent view of a pipeline reactor for use
with
embodiments of the method of the present invention.
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Fig 7 is a partial perspective transparent view of the pipeline reactor of Fig
6 with cross-
sections representing the relative concentration of flocculent solution and
MFT at two
different distances from the injection location.
Fig 8 is a close-up view of section VIII of Fig 7.
Fig 9 is a close-up view of section IX of Fig 7.
Fig 10 is a side cross-sectional view of a variant of a pipeline reactor for
use with
embodiments of the method of the present invention.
Fig 11 is a side cross-sectional view of another variant of a pipeline reactor
for use with
embodiments of the method of the present invention.
Fig 12 is a side cross-sectional view of another variant of a pipeline reactor
for use with
embodiments of the method of the present invention.
Fig 13 is a partial perspective transparent view of yet another variant of a
pipeline
reactor for use with embodiments of the method of the present invention.
Fig 14 is a graph of shear yield stress versus time comparing different mixing
speeds in
a stirred tank for mature fine tailings treated with flocculent solution.
Fig 15 is a bar graph of water release percentage versus mixing speeds for
mature fine
tailings treated with flocculent solution.
Fig 16 is a graph of yield shear stress versus time in a pipe for different
pipe flow rates
for mature fine tailings treated with flocculent solution.
Fig 17 is a schematic representation of treating mature fine tailings with a
flocculent
solution.
Fig 18 is another schematic representation of treating mature fine tailings
with a
flocculent solution.
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Fig 19 is another schematic representation of treating mature fine tailings
with a
flocculent solution.
Figs 20 and 21 are graphs of percent solids as a function of time for
deposited MFT
showing drying times according to trial experimentation.
Fig 22 is a graph of second moment M versus MFT flow rate for different
mixers.
Fig 23 is a graph of percent solids versus time of a drying deposit according
to an
embodiment of the method.
Fig 24 is a graph of percent solids versus time of a drying deposit according
to another
embodiment of the method.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
The methods described herein were developed to overcome a number of challenges
with respect to managing material produced during oil sand fine tailings
drying
operations. For convenience, the overall drying operation will be referred to
as the "MFT
drying process".
The methods of the present invention are used in conjunction with chemically
altering
raw oil sand fine tailings to produce modified fine tailings. One technique of
producing
the modified fine tailings is through flocculation and subsequent pipeline
handling and
conditioning, which will be described further below in order to contextualise
the
deposition and farming methodologies of the present invention. For now,
briefly put, the
oil sand fine tailings are preferably treated with a flocculent solution by in-
line dispersion
and are then conditioned by inputting sufficient energy to cause the formation
and
rearrangement of flocculated fine tailing solids to increase the yield shear
strength while
enabling water release without over-shearing the flocculated solid structure.
The
modified fine tailings are then ready to be deposited.
=
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According to embodiments of the present invention, material for treatment
consists of oil
sand fine tailings. "Oil sand fine tailings" means tailings derived from oil
sands extraction
operations and containing a fines fraction. They include mature fine tailings
from tailings
ponds and fine tailings from ongoing extraction operations that may bypass a
pond, and
combinations thereof. In the present description, the abbreviation MET will be
generally
used, but it should be understood that the fine tailings treated according the
methods of
the present invention are not necessarily obtained from a tailings pond.
Both deposition of the modified fine tailings and post-deposition management
involve a
number of challenges.. Improper deposition or post-deposition management can
reduce
efficiency or even prevent drying of the deposit. It is inadvisable for
continuous and
large-scale operations to deposit modified tailings unsystematically on
various surface
areas and simply allow it to stand. A methodology geared to oil sand fine
tailings and
accounting for release water quality and quantity, drying rates and land use
efficiency
has thus been developed.
In one embodiment, the method provides a deposition cell allowing effective
drainage of
release water, controlled stacking and advancing of the flocculated fine
tailings within the
deposition cell while avoiding "channelling". Deposited material that has
insufficient
shear strength for a given cell design will entrain flocculated tailings in
the release water
resulting in channelling of flow. Channelling has several negative impacts on
the
deposition and overall drying process. More regarding channelling will be
discussed
further below.
Referring to Fig 1, the deposition cell 100 may have several different
designs. Each cell
100 preferably has a head region 102 at which the modified fine tailings are
deposited
and a toe region 104 spaced away from the head region 102 by a certain length.
The
sloped bottom surface extends from the head region 102 to the toe region 104
such that
the toe region 104 is at a lower elevation than the head region 102.
In one embodiment, the method first comprises chemically treating raw fine
tailings to
produce modified fine tailings comprising flocculated fine tailings. A
deposition cell is
provided comprising a bottom surface having a slope. The chemically modified
fine
tailings are deposited into the cell such that the slope causes drainage of
release water
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away from the flocculated fine tailings and allows the flocculated fine
tailings to stack
upward and advance over the bottom surface while having sufficient shear
strength on
the slope to resist channeling entrainment with the drainage of the release
water.
There are various challenges with respect to managing material produced by the
MFT
flocculation process and maximizing the rate of drying of the material. These
challenges
relate to the interaction between shear strength and viscosity variability of
the modified
MFT material produced by the flocculation process, the deposition thickness,
the
permeability of the modified MFT material, the drainage of released water and
precipitation, and the design of cell slope.
Due to the variability of raw MFT feed quality to the MFT flocculation
facility and process
operating conditions, the physical properties of the modified MFT material
that is
produced can vary. The variability of properties such as shear strength,
viscosity, and
permeability can affect the deposition, dewatering and drying performance. For
a given
deposition cell slope design, shear strength influences the ability of the
flocculated MFT
to stack on the deposit surface. More particularly, for a given quantity of
placed material,
the cell slope design influences whether the material will form a thicker lift
over a smaller
surface area or, conversely, a thinner lift over a larger area. Additional
practical
operating considerations influence the quantity of material and, hence, lift
thickness
placed in a given operating period for a given shear strength material. Since
part of the
MFT drying process relies on evaporation, thicker lifts create a challenge in
maximizing
the overall rate of drying of the MFT.
The ability of flocculated MFT to build up and advance on the cell slope is
also important
to separating flocculated MFT from initial release water.
Modified MFT material with insufficiently high shear strength will entrain
flocculated MFT
in the released water resulting in "channelling" of flow. Channelling has
several
detrimental impacts to the performance of the MFT drying process. First, the
modified
MFT will not effectively distribute across the head (deposition) end of the
cell resulting in
areas of no material being placed. This reduction in land utilization reduces
the overall
technology efficiency and necessitates greater land requirements for a given
production
capacity. Second, channelled material may flow to the toe of the cell where it
will drain or
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be pumped back into the pond, thereby reducing fines capture efficiency within
the cell.
Third, channelled material may entrain or erode previously placed wet
flocculated MFT
or previously placed dried MFT and entrain this additional material to the toe
of the cell
where it will be drained back into the pond, again impacting fines capture
efficiency.
In one embodiment, the deposited flocculated fine tailings undergo buildup and
channelless advancement while allowing drainage of release water. "Channelless
advancement" means that the deposit advances over the bottom surface as a
substantially uniform entity with minimal or no channelling. Unacceptable
channelling
occurs when a part of the flocculated tailings has insufficient shear strength
for a given
cell design and operating conditions, causing a contaminating quantity of
flocculated
tailings to be entrained in release water which forms one or more localized
flow channels
which run through the deposit and down the deposition cell and thereby results
in
contamination of the drainage water:
Conversely, modified MFT material with excessively high shear strength may not
effectively spread across the lower regions of the cell surface before
operability
considerations, such as lift thickness at the head of the cell, force
premature termination
of material placement in the cell or sub-par water release. This similarly
reduces the
effective utilization of available surface area and the dewatering potential.
Drainage of both released water and precipitation from the deposit area is
another
important aspect to efficiency and success of the MFT drying technology.
Modified MFT
material that sits in ponded water will have little to no driving force to
either release
further water or undergo evaporation. Design of the cell slope and formation
of cell
drainage paths are advantageous in managing removal of this water.
For a given cell design, drainage can be further hindered by variability in
the raw MFT
feed and process conditions that result in the formation of heterogeneous
deposits with
uneven surface topography. The uneven surfaces create localized opportunities
for
ponded water to form and negatively impact drying rates. Cell slope design can
improve
the drainage to a certain degree, but the issue can still persist in some
cases.
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Regarding the design of cell slope, as the cell slope increases, high shear
strength and
high stacking angle material will spread more readily leading to a thinner
effective
deposition lift height which can improve drying rate and improve free water
drainage.
However, excessive cell slope lowers the shear strength threshold for inducing
channelling of flocculated material and hence increases erosion of the cell
deposit and
reduces fines capture efficiency. Conversely, reducing the cell slope can
result in the
opposite challenges including increasing the amount of ponded surface water
and
creating thicker lift material at the head end of the cell. Both of these
issues serve to
decrease drying rates of the deposited material.
The method of the present invention is an improvement to the management of
cell
design and post-deposition operations that provides uniform and consistent
results
across the variable range in flocculated MFT properties that are produced by
the MFT
flocculation and drying process, allowing advantageous flexibility in
operation of the MFT
drying operation with minimized negative impact on drying rates.
In an embodiment of the present invention, the method allows channelless
advancement
of the deposited MFT in a fixed cell slope design despite variability in shear
strength of
the material.
The deposition cell design and deposition techniques will now be described in
further
detail.
The deposition cells may have a variety of dimensions. In one embodiment, the
cells are
approximately 200-250 m long with a cell slope between 1% and 7%. Preferably,
the
slope may be between 2% and 5%. The cell slope design, in combination with the
flocculation and deposition conditions, allows stacking of the flocculated MFT
and
segregation of released water from flocculated MFT over the range of typical
properties
of deposited material. The slope is preferably fixed and generally constant
from the head
to the toe of a given cell. However, the bottom surface may have different
slopes in
different directions at different locations of the cell and may be straight or
curved.
As the shear strength of the flocculated MFT increases, so does its stacking
angle. For
a given cell slope and given quantity of placed material, this translates into
increased lift
CA 02684232 2009-10-30
12
thickness at the point of deposition (head) and decreased distance of
spreading down
the length of the cell. Additional considerations for creating a toe region in
the cell for
collection and drainage of release water without impinging on the deposited
MFT factor
into the overall design of the cell length. For cell slopes between 1% and 7%
and a
given targeted lift thickness, it has been found that 200-250 m length cells
are
advantageous for managing the MFT deposit and release water. According to
embodiments of the method, the gentle slopes also enable a cost savings in
building and
grading.
Experiments have also been conducted with shallower slopes and it was found
that this
leads to significant ponded water and drainage issues resulting in decreased
rates of
drying. Experiments have also been conducted using high slopes of about 9%
where
significant channelling of flow and ineffective dispersal of material across
the head end
of the cell (reduced land utilization efficiency) were observed.
There are virtually no restrictions on design of cell width. Rather multiple
deposition
points are used within the cell to maximize dispersal of the flocculated MFT
across a
given cell width. The spacing between, and hence number of, deposition points
within a
given cell is dependent on the specific deposition device and can vary from 1
m to 50 m.
The number of specific deposition points operated at a given time is
determined by
optimal operating conditions (target flows) per the specific deposition device
in use and
the overall flowrate of the MFT drying process. Typically two to three
deposition points
are used.
In one aspect, the depositing is performed via a plurality of outlets
distributed widthwise
across the head region of a single cell. In one optional aspect, the outlets
may be
operated so that the flocculated MFT is expelled from one outlet at a time. In
this way,
the flocculated MFT is expelled from a first outlet to form a first mound of
flocculated fine
tailings at the head region. Expelling from the first outlet is ceased when
the surface
slope of the first mound is sufficient to cause channeling of the modified
fine tailings
deposited thereon, and a second outlet is then operated to expel the
flocculated MFT to
form a second mound of flocculated fine tailings beside the first mound. It
should be
understood that the first and second and subsequent mounds may be directly
adjacent
to each other or remote from each other as desired. It should also be noted
that a set of
CA 02684232 2009-10-30
13
outlets may be operated to expel flocculated MFT simultaneously until multiple
mounds
reach a certain height and surface slope before operating another set of
outlets.
The layout of cells within a given area may take on a variety of
configurations to further
improve the overall efficiency especially of land utilization for MFT drying
within a given
available space. The layout is preferably configured for minimization of
unnecessary
berms, establishing common drainage ditches, establishing and minimizing
common
road access paths, and utilizing already present grading to minimize
earthworks
requirements. The layout may also be configured such that rectangular cells
are
arranged with head ends abutting to allow discharge from common distribution
piping
along a centre berm. According to some embodiments, the layout may also be
configured such that pipeline sections transporting the flocculated MFT impart
sufficient
shear conditioning to reach the water release zone (see Figs 2 and 3). In the
case that
some pipeline sections impart shear to bring the flocculated MFT only to the
flocculation
conditioning zone, the outlet of the corresponding pipeline section may be
provided with
a mechanical shearing mechanism or the deposit at that location may be plowed
according to various post-deposition "farming" techniques that will be further
described
below.
In one embodiment of cell design represented in Fig 1, the cells 100 may be
rectangular
and may be typically about 50m wide by 200-250m long. These cells 100 may be
arranged side by side over a given beach surface area and can either be
arranged along
the pond edge for direct drainage of release water into the pond. Such
arrangements
allow open discharge to a pond 106 via a pond beach area 108. The cells 100
may also
be arranged in rows with the toe end 104 of the cells 100 abutting onto each
other to
share a common drainage ditch 110.
In another embodiment of cell design also represented in Fig 1, the cells 100
may be
"radial" having a central deposition point that discharges in a radial arc.
The arc may be
up to 180 as illustrated, or in some cases up to 360 when the cell is
configured
essentially as a conical hill. The cell slope is best described as cone shaped
in that it
has a 1-7% slope outwards in a linear direction from the deposition point much
like a
cone. The cell surface area may be additionally sloped in a second direction
towards a
CA 02684232 2009-10-30
14
common corner of the conical cell for accumulation and drainage of release
water
through a single collection point.
These radial cells enhance distribution of MFT over the cell surface area by
having a cell
width that increases with length into the cell. Due to single point discharge
mechanisms,
flocculated MFT flow slowly spreads outward as it travels the length of the
cell. This
results in unutilized area at the head of the cell where spreading is minimal.
A radial cell
thus can improve surface area utilization over a rectangular cell.
Average deposit lift thicknesses are targeted to between 20 cm and 50 cm. This
has
been found to be an advantageous balance between optimizing drying rates and
practical implementation. The upper limit is restricted by the strength of the
material and
the need to create a lift thin enough to dry in a reasonable time frame. The
lower limit is
restricted by practical operability and design considerations including
frequency of valve
switches between cells and quantity of piping. Thinner lifts equate to shorter
deposit
spread lengths which, in-turn, translate into more cells per given surface
area and hence
more piping to access each cell and associated reduction in land utilization
due to
increased piping corridors and road access points.
Experiments have also been conducted with lifts as deep as 1 m and it was
found that
this thickness does not dry as advantageously as lifts between 20 cm and 50
cm.
There are also some post-deposition "farming" techniques that may be used to
improve
the MFT drying. The post-deposition techniques enhance both the distribution
of
flocculated MFT over the cell area and the drying rate of the deposited
material.
Preferably, plowing and harrowing are both used at the appropriate times after
the
deposition has been performed as described herein. However, it should be
understood
that plowing or harrowing may be performed following other schemes of MFT
deposition.
In some preferred embodiments, the method implements the use of mechanical
equipment including plow devices and disc harrows to address challenges
relating to
thick deposits and surface water drainage. These techniques may also provide
advantages when the MFT is not optimally flocculated or sheared.
CA 02684232 2009-10-30
The plowing is preferably performed while the deposit is still wet at its
upper surface.
Preferably, the plowing is performed using a plow device that acts to spread
flocculated
MFT, post-deposition, uniformly over the cell area, thereby placing material
in areas
where the flocculated MFT did not spread including regions at the bottom/toe
of the cell
and regions along the width at the head end of the cell, and moving material
placed in a
thick lift at the head end of the cell towards the toe end where the lift
thickness is much
thinner. The plowing has the additional advantageous effect of allowing water
release
conditioning, when necessary in the given deposit, while avoiding over-
shearing and
maintaining sufficient shear strength of the flocculated fine tailings to
allow standing.
Thus, the plowing is preferably preformed using a mechanism that can provide a
controlled amount of shear uniformly across the deposit, rather than a high-
shear local
agitation device.
The plow device may comprise a cross-member extending across the cell and
displacement means for displacing the cross-member toward the toe of the cell
while it
pulls high material toward the toe and allows the material to fill low areas
of the cell. The
plow device may have a concave surface facing the toe for improving the
scooping
action of the deposited material and an appropriate amount and distribution of
shear. For
example, the plow device may be constructed from piping cut lengthwise to form
a semi-
circular piece of half-pipe. This half-pipe is attached to a rig that supports
it at a given
height, such as between 8 and 18 inches above the ground surface, and allows
the
assembly to be attached to a dozer or other displacement mechanism. The half-
pipe is
sunk into the deposit to the proper depth and the dozer drags the half-pipe
plow through
the deposit thereby pulling material from a location where it is thicker to a
location where
less material is present, thereby creating a generally uniform thickness
deposit.
The plowing and spreading of the material can improve drying rates through a
plurality of
actions. First, it improves land utilization. Areas that had no placed MFT are
now being
used thereby increasing surface area for evaporation. Second, in suboptimally
flocculated MFT, it provides opportunity for release water that is trapped
subsurface in
the flocculated MFT to reach the surface and evaporate, thereby improving
drying rates.
Third, in undersheared MFT deposits, the activity of the plow may be
sufficient to provide
extra shear to the deposit and allow the material to reach a dewatering (water
release)
CA 02684232 2009-10-30
=
16
state. Undermixed or undersheared deposits would otherwise only be subject to
evaporation processes if left unplowed.
It has been seen through operation of MFT drying that a majority of the water
released
through the dewatering phase occurs in the first couple of days upon
deposition. After
the first couple of days, dewatering rates taper off and evaporation processes
start to
dominate the rate of drying. In some embodiments, because over-aggressive
plowing
has the potential to overshear flocculated MFT, thereby hindering the
dewatering
process, it has been found that it is advantageous to implement plowing of
cells no
sooner than two days after deposition. This allows for maximized dewatering
rates prior
to implementing plow operations to enhance evaporation processes for such
embodiments. In rare cases, if a very high shear strength material is placed
in a shallow
slope area, a very thick lift may form during operations at the point of
deposition, and
thus earlier plowing may be appropriate. Practical considerations may also
lead to the
use of the plow during deposition to aid in spreading of the material.
According to another farming technique, the harrowing is performed once a
dried crust
has formed at the upper surface of the deposit. Once the dewatering phase of
the MFT
drying technology is complete and the evaporation process has begun, a crust
layer
begins to form on the MFT deposit. The permeability of the flocculated MFT
leads to
both trapped release water as well as deposit moisture below the crust layer
will not
readily migrate to the surface for drainage and evaporation. The deposit has
relatively
low permeability compared to coarse tailings, containing higher contents of
sand or
coarse particles, resulting at a certain point in the process in slower water
migration
rates such that evaporative mechanisms become more dominant. For thicker lift
deposits, trapped release water and deposit moisture below the crust can
result in
lengthy drying times. In ideally dosed and ideally mixed/sheared MFT, the
deposited
material will form cracks that create drainage channels and expose further
surface area
for evaporation. This aids in accelerating the drying rate. However, the ideal
dosing and
mixing range may be narrow with some chemical addition techniques, creating a
practical challenge in achieving ideal dosing and mixing under all operating
conditions.
Given a variety of process fluctuations including MFT density, MFT flowrate,
bitumen
content, and piping distance to the specific cell, either or both the dose and
the mixing
could be substandard resulting in some under- or over-dosed material or some
under- or
CA 02684232 2009-10-30
17
over-sheared flocculated MFT. As a result, crack formation and the benefit it
provides to
MFT drying rates is not always achievable.
To accelerate the rate of drying in non-ideal dosing or mixing scenarios, the
present
invention provides a harrowing method in flocculated MFT deposits. In one
embodiment,
a conventional farm disc harrow device is attached to a dozer and dragged
through the
deposit. Disc harrowing is applied once a dry crust layer has formed and is
utilized to
turn over the surface layer of the deposit and expose the wet material under
the crust
layer. This serves to maximize the overall rate of evaporation of water from
the deposit.
If disc harrowing is executed in a lengthwise direction through the deposit it
has the
added benefit of creating furrows that act as drainage paths to the toe of the
cell helping
in alleviating ponded water from shallow sloped or non-heterogeneous areas of
the
deposit thereby improving drying rates.
The farming techniques have provided numerous observed advantages. They create
a
uniform deposit thickness over the entire cell area, thereby maximizing
surface area
utilization and maximizing drying rates over the entire cell. They facilitate
the drainage of
ponded surface water and the release and drainage of free water trapped below
the
dried surface of the deposit. They allow overturning of dried surface material
to expose
wet material underneath thereby enhancing the rate of evaporation mechanisms.
They
synergistically enable improved timing of substantially gravity-controlled
drainage
dewatering and substantially evaporation-controlled drying of the deposited
MFT.
The present invention also provides a number of options for final handling of
the
deposited flocculated MFT to tailor the MFT drying process to site specific
conditions
and available solids disposal options.
One deposit handling option is referred to as in-situ deposition. Deposited
material is left
in-place in the deposition cell and a subsequent deposit lift is placed over
top using the
above described methodologies. Prior to a subsequent lift, the material in the
drying cell
preferably achieves a moisture content of less than about 25% by weight to
promote
geotechnical stability. Depending on the specific chemical used to modify the
MFT, the
relationship between the geotechnical properties and moisture content may
vary. In-situ
material can be left in place indefinitely.
CA 02684232 2009-10-30
18
Another deposit handling option is excavation. Flocculated MFT has sufficient
shear
strength to be excavated as a solid material as early as four days post
deposition
provided. Depending on specific objectives, this material can be disposed in a
mud
dump or spread on secondary drying areas for mechanical manipulation using
disc
harrowing techniques as described above. This allows quick turnover of the
deposition
cell for re-use thereby minimizing the number of active cells for a given
operation.
Materials can also be excavated once dried to over 75 wt% solids.
In another embodiment, the cycle time period of depositing the modified
tailings is
controlled so that each deposition cell is filled to the desired lift over a
time period not
exceeding about one day. When the deposition occurs over a longer timeframe,
the
initially deposited material begins to release water out of the top of the
material, creating
a 20-40 mm liquid film on which subsequently deposited material flows more
quickly.
Thus, the modified fine tailings are preferably deposited over a timeframe
sufficient to
avoid significant water release film formation on the top of the material
during deposition,
allowing the deposit to dewater and dry as a substantially unitary aggregate.
As mentioned in a previous section, the deposition and farming methods of the
present
invention are used in conjunction with chemically altering raw oil sand fine
tailings to
produce modified fine tailings.
A preferred technique of producing the modified fine tailings through
flocculation and
subsequent pipeline handling and conditioning will now be described. This
preferred
technique will be referred to herein below as the "flocculation process" or
the
"flocculation technique".
Referring to Figs 2 and 3, the general stages of the flocculation process will
be
described. The oil sand fine tailings are treated with a flocculent solution
by in-line
dispersion of the flocculent solution into the fine tailings, then
conditioning the fine
tailings by inputting a sufficient energy to cause the formation and
rearrangement of
flocculated fine tailing solids to increase the yield shear strength while
enabling water
release without over-shearing the flocculated solid structure that can then
form a non-
CA 02684232 2009-10-30
19
flowing deposit. The flocculated fine tailings are then deposited and may be
farmed to
promote the water release and allowed to dry.
Certain terms employed hereinbelow should be read in light of the following
additional
definitions:
"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.
"Flocculent solution comprising a flocculation reagent" means a solution
comprising a
solvent and at least one flocculation reagent. The flocculent solution may
contain a
combination of different flocculation reagents, and may also include
additional
chemicals. The solvent comprises water but may include other compounds as
well, as
desired. Flocculation reagents are compounds that have structures which form a
bridge
between particles, uniting the particles into random, three-dimensional porous
structures
called "flocs". Thus, the flocculation reagents do not include chemicals that
merely act
electrostatically by reducing the repulsive potential of the electrical double
layer within
the colloid. The flocculation reagents have structures for forming floc
arrangements upon
dispersion within the MFT, the flocs being capable of rearranging and
releasing water
when subjected to a specific window of conditioning. The preferred
flocculation reagents
may be selected according to given process conditions and MFT composition.
"Molecular weight" means the average molecular weight determined by
measurement
means known in the art.
"Dispersion", as relates to the flocculent solution being introduced into the
in-line flow of
MFT, means that upon introduction within the MFT the flocculent solution
transitions
from droplets to a dispersed state sufficient to avoid under-reacting or over-
reacting in a
localized part of the MFT which would impede completion of the flocculation in
the
subsequent conditioning stage to reliably enable dewatering and drying.
CA 02684232 2009-10-30
,=
"Flocculation conditioning" is performed in-line and involves the flocculation
reagent
reacting with the MFT solids to form flocs and through rearrangement reactions
increase
the strength of the flocculating MFT.
"Water release conditioning" means that energy is input into the flocculated
MFT so as to
initiate rearrangement and breakdown of the structure to release water from
the
flocculated matrix. The energy input may be performed by in-line shearing or
by other
means. "Release of water" in this context means that water selectively
separates out of
the flocculated MFT matrix while leaving the flocs sufficiently intact for
deposition.
"Over-shearing", which is a stage that defines the limit of the water release
conditioning
stage and is to be avoided, means that additional energy has been input into
the
flocculated MFT resulting in dispersing the structure and resuspending the
fines within
the water. Over-sheared MFT releases and resuspends fines and ultrafines
entrapped
by the flocs back into the water, essentially returning to its original fluid
properties but
containing non-functional reagent.
"Non-flowing fine tailings deposit" means a deposited flocculated MFT that has
not been
over-sheared and has sufficient strength to stand within a cell while drying.
While the
water release from the flocs is triggered by conditioning, the MFT deposit may
have
parts that continue to release water after it has been deposited. The drying
of the MFT
deposit may then occur by gravity drainage, evaporation and permeation. The
removal of
water from the flocculated MFT may also occur before deposition, for instance
when a
stream of release water separates from the flocculated MFT upon expelling for
deposition.
"Yield shear strength" means the shear stress or pressure required to cause
the MFT to
flow.
In the flocculation technique, the oil sand fine tailings are primarily MFT
obtained from
tailings ponds given the significant quantities of such material to reclaim.
The raw MFT
may be pre-treated depending on the downstream processing conditions. For
instance,
oversized materials may be removed from the raw MFT. In addition, specific
components
of the raw MFT may be selectively removed depending on the flocculation
reagent to be
CA 02684232 2009-10-30
21
used. For instance, when a cationic flocculation reagent is used, the raw MFT
may be
treated to reduce the residual bitumen content which could cause flocculent
deactivation.
The raw MFT may also be pre-treated to provide certain solids content or fines
content
of the MFT for treatment or hydraulic properties of the MFT. The fine tailings
may also be
obtained from ongoing oil sand extraction operations. The MFT may be supplied
from a
pipeline or a dedicated pumped supply.
The flocculation technique is preferably conducted in a "pipeline reactor"
followed by
deposition onto a deposition area. The pipeline reactor may have various
configurations,
some of which will be described in detail herein below.
The MFT to be treated is preferably provided as an in-line flow in an upstream
part of the
pipeline reactor. The properties of the MFT and its particular flow
characteristics will
significantly depend on its composition. At low mineral concentrations the
yield stress to
set the MFT fluid in motion is small and hydraulic analysis can approximate
the fluid
behaviour of a Newtonian fluid. However, as mineral concentration increases a
yield
stress must be overcome to initiate flow. These types of fluids are a class of
non-
Newtonian fluids that are generally fitted by models such as Bingham fluid,
Herschel-
Bulkley yield-power law or Casson fluid. The rheological relationship
presented in Fig 4,
illustrating a yield stress response to shear rate for various mineral
concentrations in a
MFT sample, considers MFT as a Bingham fluid. MFT may also be modelled in
viscometric studies as a Herschel-Bulkley fluid or a Casson Fluid.
Empirical data and modelling the rheology of in-line MFT have confirmed that
when a
flocculent solution is added by conventional side injection into a Bingham
fluid MFT,
solution dispersion is very sensitive to flow rate and diameter ratios as well
as fluid
properties.
Particularly when the flocculent solution is formulated to behave as a non-
Newtonian
fluid, the dispersion stage of the flocculation technique may be performed to
cause rapid
mixing between two non-Newtonian fluids. Rapid non-Newtonian mixing may be
achieved by providing a mixing zone which has turbulence eddies which flow
into a
forward-flow region and introducing the flocculent solution such that the
turbulence
CA 02684232 2009-10-30
22
eddies mix it into the forward-flow region. Preferably, the flocculent
solution is introduced
into the turbulence eddies and then mixes into the forward-flow region.
Figs 5 and 6 illustrate a pipeline reactor design that enables such rapid
mixing of non-
Newtonian fluids. The MFT is supplied from an upstream pipeline 10 into a
mixing zone
12. The mixing zone 12 comprises an injection device 14 for injecting the
flocculent
solution. The injection device may also be referred to as a "mixer". The
injection device
14 may comprise an annular plate 16, injectors 18 distributed around the
annular plate
16 and a central orifice 20 defined within the annular plate 16. The MFT
accelerates
through the central orifice 20 and forms a forward-flow region 24 and an
annular eddy
region 22 made up of turbulence eddies. The injectors 18 introduce the
flocculent
solution directly into the eddy region 22 for mixing with the turbulent MFT.
The
recirculation of the MFT eddies back towards the orifice 20 results in mixing
of the
flocculent solution into the MFT forward-flow. The forward-flow region 24
expands as it
continues along the downstream pipe 26. For some mixer embodiments, the
forward-
flow region may be a vena-contra region of a jet stream created by an orifice
or baffle.
The main flow of the MFT thus draws in and mixes with the flocculent solution,
causing
dispersion of the flocculent solution, and flocculation thus commences in a
short distance
of pipe. The injection device 14 illustrated in Figs 5 and 6 may also be
referred to as an
"orifice mixer". For the mixer of Figs 4 and 5, the preferred range of orifice
diameter "d"
to downstream pipe diameter "D" is 0.25 ¨ 0.75.
Figs 7-9 illustrate the performance of an orifice mixer based on computational
fluid
dynamic (CFD) modeling and empirical data obtained from a test installation on
a MFT
pipeline reactor. The MFT flow rate in a 2 inch diameter pipe was 30 LPM and
flocculent
solution was injected at about 3 LPM. The 2 inch long orifice mixer had an
orifice to
downstream pipe diameter ratio d/D = 0.32 with six 0.052 inch diameter
injectors located
on a 1.032 inch diameter pitch circle. Due to the density difference between
the MFT and
flocculent solution, a useful method of characterizing the degree of mixing is
to
determine the second moment M of the concentration C over the pipe cross
section A in
the following equation where C is the mean concentration for the fully mixed
case (thus
directionally M = 0 is desired).
CA 02684232 2009-10-30
23
M = ¨1 ¨,C ¨1)2dA
A A\ c
In Figs 7-9, the dark areas represent MFT that has not mixed with the
flocculent solution
(referred to hereafter as "unmixed MFT"). Just downstream of the mixer, the
unmixed
MFT region is limited to the central core of the pipe and is surrounded by
various
flocculent solution-MFT mixtures indicative of local turbulence in this zone.
As the
flocculent solution is miscible in MFT, the jetting of the flocculent solution
into the
turbulent zone downstream may cause the flocculent solution to first shear the
continuous phase into drops from which diffusion mixing disperses the
flocculent into the
MFT.
The CFD model was based on a Power-law-fluid for the flocculent solution and a
Bingham-fluid for the MFT without reactions. The Bingham-fluid approximation
takes into
account the non-Newtonian nature of the MFT as requiring a yield stress to
initiate flow.
Bingham-fluids are also time-independent, having a shear stress independent of
time or
duration of shear. Preferably, the CFD model is primarily used to determine
and improve
initial mixing between the flocculent solution and the MFT.
The injection device 14 may have a number of other arrangements within the
pipeline
reactor and may include various elements such as baffles (not shown). In one
optional
aspect of the injection device shown in Fig 10, at least some of the injectors
are oriented
at an inward angle such that the flocculent solution mixes via the turbulence
eddies and
also jet toward the core of the MFT flow. In another aspect shown in Fig 11,
the orifice
has a reduced diameter and the injectors may be located closer to the orifice
than the
pipe walls. The injectors of the mixer may also be located at different radial
distances
from the centre of the pipeline. In another aspect, instead of an annular
plate with a
central orifice, the device may comprise baffles or plates having one or
multiple openings
to allow the MFT to flow through the mixing zone while creating turbulence
eddies. In
another aspect shown in Fig 12, the injectors face against the direction of
MFT flow for
counter-current injection. Fig 13 illustrates another design of injection
device that may be
operated in connection with the flocculation process. It should also be noted
that the
injection device may comprise more than one injector provided in series along
the flow
direction of the pipeline. For instance, there may be an upstream injector and
a
CA 02684232 2009-10-30
24
downstream injector having an arrangement and spacing sufficient to cause the
mixing.
In a preferred aspect of the mixing, the mixing system allows the break-up of
the plug
flow behaviour of the Bingham fluid, by means of an orifice or opposing "T"
mixer with
MFT and flocculent solution entering each arm of the Tee and existing down the
trunk.
Density differentials (MFT density depends on concentration ¨ 30 wt%
corresponds to a
specific gravity of ¨ 1.22 and the density of the flocculent solution may be
about 1.00)
together with orientation of the injection nozzles play a role here and are
arranged to
allow the turbulence eddies to mix in and disperse the flocculent solution.
The following table compares the second moment values for the orifice mixer
(Fig 4) and
a quill mixer (Fig 13) at various locations downstream of the injection
location for the
same flows of MFT and flocculent reagent solution.
Downstream Distance
L/D Orifice Mixer (Fig 4) Quill Mixer (Fig 12)
1 11.75 5.75
2 3.17 3.65
3 1.75 2.89
1.10 2.24
0.65 1.39
Near to the injection point of the orifice mixer as shown on Fig 8, there is a
larger region
of unmixed polymer surrounding a strong MFT jet with a "M" value of 11.75.
However,
the mixing with the MFT jet occurs very rapidly so that by 5 diameters
downstream of the
injection point shown as Fig 9 with a second moment M value of 1.10. In
contrast, for the
quill mixer as shown Fig 13, the initial mixing with a second moment M value
of 5.75 only
improves to 2.24 by 5 diameters downstream of the injection point. Mixing by
the orifice
mixer is preferred to the quill mixer.
Preferably, the mixing is sufficient to achieve an M <2 at UD = 5, and still
preferably the
mixing is sufficient to achieve an M < 1.5 at L/D = 5, for the pipeline
reactor. Controlling
the mixing at such preferred levels allows improved dispersion, flocculation
and
dewatering performance.
CA 02684232 2009-10-30
In this flocculation technique, initial mixing of the flocculent solution into
the MFT is
important for the flocculation reactions. Upon its introduction, the
flocculent solution is
initially rapidly mixed with the fine tailings to enhance and ensure the
flocculation
reaction throughout the downstream pipeline. When the flocculent solution
contacts the
MFT, it starts to react to form flocs made up of many chain structures and MFT
minerals.
If the flocculent solution is not sufficiently mixed upon introduction into
the pipe, the
flocculation reaction may only develop in a small region of the in-line flow
of tailings.
Consequently, if the tailings are subsequently mixed downstream of the polymer
injection, mixing will be more difficult since the rheology of the tailings
will have changed.
In addition, the flocs that formed initially in the small region can be
irreversibly broken
down if subsequent mixing imparts too much shear to the flocs. Over-shearing
the flocs
results in resuspending the fines in the water, reforming the colloidal
mixture, and thus
prevents water release and drying. Thus, if adequate mixing does not occur
upon
introduction of the flocculent solution, subsequent mixing becomes problematic
since
one must balance the requirement of higher mixing energy for flocculated
tailings with
the requirement of avoiding floc breakdown from over-shearing.
The initial mixing may be achieved and improved by a number of optional
aspects of the
flocculation process. In one aspect, the injection device is designed and
operated to
provide turbulence eddies that mix and disperse the flocculent solution into
the forward
flow of MFT. In another aspect, the flocculation reagent is chosen to allow
the flocculent
solution to have decreased viscosity allowing for easier dispersion. The
flocculent
solution may also be formulated and dosed into the MFT to facilitate
dispersion into the
MFT. Preferably, the flocculation reagent is chosen and dosed in conjunction
with the
injection conditions of the mixer, such that the flocculent solution contains
sufficient
quantity of reagent needed to react with the MFT and has hydraulic properties
to
facilitate the dispersion via the mixer design. For instance, when a viscous
flocculent
solution displaying plastic or pseudo-plastic non-Newtonian behaviour is used,
the mixer
may be operated at high shear injection conditions to reduce the viscosity
sufficiently to
allow dispersion into the MFT at the given hydraulic mixing conditions. In yet
another
aspect, the flocculation reagent is chosen to form flocs having increased
shear
resistance. Increased shear resistance enables more aggressive, harsh mixing
and
reduces the chance of premature over-shearing of the resulting flocs. The
increased
shear resistance may be achieved by providing the flocculent with certain
charge
CA 02684232 2009-10-30
26
characteristics, chain lengths, functional groups, or inter- or intra-linking
structures. In
another aspect, the flocculation reagent is chosen to comprise functional
groups
facilitating rearrangement and selective water release. In another aspect, the
flocculation
reagent is chosen to form large flocs facilitating rearrangement and partial
breakdown of
the large flocs for water release. In another aspect, the flocculation reagent
may be an
organic polymer flocculent. The polymer flocculent may have a high molecular
weight,
such as above 10,000,000, or a low molecular weight. The high molecular weight
polymers may tend to form more shear resistant flocs yet result in more
viscous
flocculent solutions at the desired dosages. Thus, such flocculent solutions
may be
subjected to higher shear injection to reduce the viscosity and the turbulence
eddies
may be given size and spacing sufficient to disperse the flocculent solution
within the
pipeline mixing zone.
In another aspect, the flocculation reagent may be chosen and dosed in
response to the
clay concentration in the MFT. The flocculation reagent may be anionic,
cationic, non-
ionic, and may have varied molecular weight and structure, depending on the
MFT
composition and the hydraulic parameters.
It should be noted that, contrary to conventional teachings in the field of
MFT
solidification and reclamation, the improvement and predictability of the
drying process
rely more in the process steps than in the specific flocculation reagent
selected. Of
course, some flocculation reagents will be superior to others at commercial
scale,
depending on many factors. However, the flocculation process enables a wide
variety of
flocculation reagents to be used, by proper mixing and conditioning in
accordance with
the process steps. By way of example, the flocculent reagent may be an organic
polymer
flocculent. They may be polyethylene oxides, polyacrylamides, anionic
polymers,
polyelectrolytes, starch, co-polymers that may be polyacrylamide-polyacrylate
based, or
another type of organic polymer flocculents. The organic polymer flocculents
may be
obtained from a flocculent provider and subjected to selection to determine
their
suitability to the specific commercial application.
Initial mixing was further assessed in a conventional stirred mix tank by
varying the initial
speed of the mixer. Fig 14 presents indicative lab test results comparing
rapid mixing
(230 RPM) and slow mixing (100 RPM). The test results with the mixer at the
higher
CA 02684232 2009-10-30
27
initial speed developed flocculated MFT with a higher shear yield strength
significantly
faster than tests with the mixer at a lower speed. For the lower speed, the
time delay
was attributable to dispersing the flocculent solution into the MFT. Moreover,
Fig 15
indicates that the fast initial mixing also resulted in higher initial water
release rates,
which results in reduced drying times.
While the lab scale stirred tank demonstrated benefits from fast mixing, other
results also
demonstrated the effect of over-mixing or over-shearing, which would break
down the
flocculated MFT such that the MFT would not dewater. The lab scale stirred
tank is
essentially a batch back-flow reactor in which the mixer imparts shear firstly
to mix the
materials and secondly to maintain the flocculating particles in suspension
while the
reactions proceed to completion. As the operational parameters can be easily
adjusted,
the stirred tank provides a valuable tool to assess possible flocculation
reagent
performance. Lab scale stirred tank data may be advantageously coupled with
lab
pipeline reactor tests and CFD modelling for selecting particular operating
parameters
and flocculation reagents for embodiments of the continuous in-line process of
the
flocculation process.
The MFT supplied to the pipeline reactor may be instrumented with a continuous
flow
meter, a continuous density meter and means to control the MFT flow by any
standard
instrumentation method. An algorithm from the density meter may compute the
mineral
concentration in MFT and as an input to the flow meter determine the mass flow
of
mineral into the pipeline reactor. Comparing this operating data to
performance data for
the pipeline reactor developed from specific flocculation reagent properties,
specific MFT
properties and the specific pipeline reactor configurations, enables the
adjustment of the
flowrate to improve processing conditions for MFT drying. Operations with the
mixer in a
12 inch pipe line processing 2000 USgpm of MFT at 40% solids dewatered MFT
with a
pipe length of 90 meters.
Referring back to Figs 5 and 6, after introduction of the flocculation reagent
in the mixing
zone 12, the flocculating MFT continues into a conditioning zone 28. The
conditioning
stage of the flocculation process will be generally described as comprising
two main
parts: flocculation conditioning and water release conditioning.
CA 02684232 2009-10-30
28
At this juncture, it is also noted that for Newtonian fluid systems, research
into
flocculated systems has developed some tools and relationships to help predict
and
design processes. For instance, one relationship that has been developed that
applies to
some flocculated systems is a dimensionless number called the "Camp number".
The
Camp number relates power input in terms of mass flow and friction to the
volume and
fluid absolute viscosity. In non-Newtonian systems such as MFT-polymer mixing
both
pipe friction and the absolute viscosity terms used in the Camp number depend
on the
specific flow regime. The initial assessment of the pipeline conditioning data
implies the
energy input may be related to modified Camp number. The modified Camp number
would consider the flocculating agent, the rheology of the flocculated MFT in
addition to
the flow and friction factors.
Flocculation conditioning occurs in-line to cause formation and rearrangement
of flocs
and increases the yield shear stress of the MFT. Referring to Figs 5 and 6,
once the
MFT has gone through the mixing zone 12, it passes directly to the
flocculation
conditioning zone 28 of the pipeline reactor. The flocculation conditioning
zone 28 is
generally a downstream pipe 26 with a specific internal diameter that provides
wall shear
to the MFT. In one aspect of the process, the flocculation conditioning
increases the
yield shear stress to an upper limit. The upper limit may be a single maximum
as shown
in Fig 2 or an undulating plateau with multiple local maximums over time as
shown in Fig
3. The shape of the curve may be considered a primary function of the
flocculent solution
with secondary functions due to dispersion and energy input to the pipeline,
such as via
baffles and the like.
Water release conditioning preferably occurs in-line after the flocculation
conditioning.
Referring to Figs 2 and 3, after reaching the yield shear upper limit,
additional energy
input causes the yield stress to decrease which is accompanied by a release of
water
from the flocculated MFT matrix. Preferably, the water release conditioning
occurs in-line
in a continuous manner following the flocculation conditioning and before
deposition. In
this case, the water release may commence in-line resulting in a stream of
water being
expelled from the outlet of the pipe along with depositing flocculated MFT.
The release
water will quickly flow away from the MFT deposit, especially in a sloped
deposition cell,
while the MFT deposit has sufficient strength to gradually advance and then
stand in the
deposition cell. Here, it is preferred to have no high-shear units such as
pumps in the
CA 02684232 2009-10-30
29
downstream pipe. The hydraulic pressure at the MFT pipeline reactor inlet is
preferably
established so that no additional pumping which may over-shear the flocs would
be
required to overcome both static and differential line head losses prior to
deposition. In
some aspects, the deposited MFT is not disturbed with further shearing after
deposition,
but rather is left to dry after in place. Alternatively, instead of being
performed in-line, the
water release conditioning may occur in a controlled shearing apparatus (not
shown)
comprising baffles, an agitator, a mixer, or a rotary separator, or a
combination thereof.
The water release conditioning may also occur after the flocculated MFT is
deposited, for
instance by a mechanical mechanism in an ordered fashion. In such a case, the
flocculated MFT would be deposited as a gel-like mass at a shear yield
strength allowing
it to stand but tending not to promote water release until additional energy
input is
applied. By conditioning the flocculated MFT back down from a yield stress
upper
threshold, the process avoids the formation of a gel-like water-retaining
deposit, reliably
enabling water release and accelerated drying of the MFT.
Care should also be taken not to expel the MFT from a height that would
accelerate it to
over shear due to the impact on the deposition cell or the previously
deposited MFT.
The flocculation conditioning and the water release conditioning may be
controlled in-line
by varying the flow rate of the MFT. Preferably, the flow rate may be as high
as possible
to increase the yield stress evolution rate of the flocculating MFT, while
avoiding over-
shear based on the hydraulic shear of the pipeline to the deposition area.
Tests were
conducted in a pipeline reactor to determine conditioning response. Fig 16
identifies the
response to varying the pipeline flow rate. A 34 wt% solids MFT was pumped
through a
2 inch diameter pipe at a flow rate of about 26 LPM for the low flow test and
about 100
LPM for the high flow test. A 0.45% flocculent solution was injected at about
2.6 LPM for
the low flow test and at about 10 LPM for the high flow test. At high flows,
the maximum
yield shear stress of the flocculated MFT occurs earlier than at low flows.
This observed
response indicates that the total energy input is an important parameter with
input
energy being hydraulic losses due the fluid interacting with the pipe wall in
this case.
Referring to Figs 5 and 6, the conditioning zone 28 may include baffles,
orifice plates,
inline static mixers or reduced pipe diameter (not shown) particularly in
situations where
layout may constrain the length of the pipeline reactor, subject to limiting
the energy
CA 02684232 2009-10-30
input so the flocculated MFT is not over sheared. If the flocculated MFT is
over sheared,
the flocs additionally break down and the mineral solids revert back to the
original
colloidal MFT fluid which will not dewater.
When the yield stress of the flocculated MFT at release is lower than 200 Pa,
for some
embodiments of the flocculation technique, the strength of the flocculated MFT
may be
inadequate for dewatering or reclamation of the deposited MFT. Thus, the yield
shear
stress of the flocculated MFT should be kept above this threshold. It should
be
understood, however, that other flocculation reagents may enable a flocculated
MFT to
dewater and be reclaimed at a lower yield stress. Thus, although Figs 2 and 3
show that
a yield stress below 200 Pa is in the over-shearing zone, these representative
figures do
not limit the process to this specific value. When an embodiment of the
process used
20% - 30% charge anionic polyacrylamide high molecular weight polymers, the
lower
threshold of the yield shear stress window was about 200 Pa, and the
flocculated MFT
was deposited preferably in the range of about 300 Pa and 500 Pa, depending on
the
mixing and MFT solids content. It should also be noted that the yield shear
stress has
been observed to reach upper limits of about 400 - 800 Pa in the pipeline
reactor. It
should also be noted the yield shear stress of the MET after the initial water
is released
when the MFT is deposited has been observed to exceed 1000 Pa.
In general, the process stage responses for a given flocculation reagent and
MFT are
influenced by flocculent type, flocculent solution hydraulic properties, MFT
properties
including concentration, particle size distribution, mineralogy and rheology,
dosing levels
and energy input.
The flocculation technique provides the advantageous ability to predict and
optimize the
performance of a given flocculent reagent and solution for deposition and
dewatering
MFT. The mixing zone ensures the efficient use of the flocculation reagent and
the
pipeline conditions of length, flow rate and baffles if required provide the
shear =
necessary to maximize water release and avoid over-shearing when the MFT is
discharged from the pipeline reactor.
After the in-line water release conditioning, the flocculated MFT may be
deposited. The
conditioned MFT is suitable for direct deposition into one or more deposition
cells as per
CA 02684232 2009-10-30
31
the method of the present invention, where water is released from the solids,
drained by
gravity and eventually further removed by evaporation to the air and
optionally
permeation into the deposition cell barrier. The deposition cells may be made
of
materials to facilitate draining and permeation. The MFT deposit may be farmed
as per
the procedures described herein, so that the deposit dries so as to reach a
stable
concentration of the MFT solids for reclamation purposes. Rather than direct
deposition
from the pipeline reactor into the cells, solid-liquid separation equipment
may be used
prior to deposition, provided the shear imposed does not over-shear the
flocculated
MFT. The MFT pipeline reactor may be used to treat MFT or other tailings or
colloidal
fluids having non-Newtonian fluid behaviour for deposition and farming. The
application
of cell design, plowing and disc harrowing may be performed on other treated
tailings
streams for dewatering processes including on pastes produced from thickeners,
cyclones, and centrifuges, polymer-treated thin fine tails or scavenger bank
tailings
streams, and the like.
The raw oil sand fine tailings may be MFT continuously provided from a pond or
thin fine
tailings which are provided from ongoing extraction operations and bypass a
pond. In a
preferred aspect of the method, it is MFT that is dredged or pumped from a
barge from a
tailings pond such that the MFT has a solids content over 20 wt%, preferably
within 30 -
40 wt%, and has a fines content of at least 75 wt% on a solids basis,
preferably from 75 -
95 wt% on a solids basis. Such MFT is preferably undiluted, such that no water
is added
either alone or as a carrier for sand. Such solids and fines contents
cooperate with the
MFT flocculation process and further with the MFT deposition and farming
methods to
enable fines capture in the flocs in addition to advantageous pre- and post-
deposition
dewatering and eventual drying of the deposit. Such high fines contents
further enable
the improved flocculation of the MFT and deposition dewatering. Alternatively,
when the
fine tailings are thin fine tailings, the fines content is preferably at least
50 wt% on a
solids basis, which also allows advantageous dewatering and drying.
EXAMPLES
Example 1:
Fig 23 provides details on a set of operational conditions and drying rates
obtained in
exemplary MFT deposition. The conditions are summarised below:
CA 02684232 2009-10-30
32
Cell dimensions m x m 165 x 43
Cell Area m2 6110
Cell slope ok 2
MFT flow rate gpm 2000
Total MFT deposited m3 2800
Polymer flocculent dosage ppm 870
Farming No plowing or harrowing
Example 2:
Fig 24 provides details on another set of operational conditions and the
drying rates
obtained in exemplary MFT deposition. The conditions are summarised below:
Cell dimensions m x m 180 x 36
Cell Area m2 6632
Cell slope 2
MFT flow rate gpm 2000
Total MFT deposited m3 2000
Polymer flocculent dosage ppm 1040
Plowing on 9th day and
Farming
harrowing on 15th day
Example 3:
For the deposition and farming methodology, embodiments of the method of the
present
invention were contrasted to other possible techniques.
The method has the ability to treat variable shear strength material in a
fixed cell slope
design that minimizes the channelling effects for the material produced from
the MFT
drying process. The co-implementation of post-deposition farming techniques,
in
conjunction with the cell slope design, further improves the drying and
further addresses
the challenges relating to thick deposits, and surface water drainage.
Embodiments of the method of the present invention allow improvements in
materials
handling with respect to other possible technologies, such as bag filters,
filter presses,
and track packing operations.
Example 4:
CA 02684232 2009-10-30
33
Trials showed that if the deposit is left unplowed, water drains from the head
(thickest) to
the toe (thinnest) region, flowing the hydraulic gradient dictated by the cell
slope design.
This was confirmed by strength and solids content observations. Fast drying of
shallow
lift sections is often aided by the effect of drainage path short circuit
created during
plowing.
Example 5:
As mentioned in the above description, lab scale stirred tank tests were
conducted to
assess mixing of a flocculent solution into MFT. The lab mixer was run at
initial speeds
of 100 RPM or 230 RPM. The dosage of 30% charge anionic polyacrylamide-
polyacrylate shear resistant co-polymer was about 1000 g per dry ton. Figs 14
and 15
show that the fast initial mixing shortens the yield stress evolution to
enable dewatering
and also increases the water release from the MFT.
Example 6:
As mentioned in the above description, lab scale stirred tank tests were
conducted to
assess mixing of different dosages of flocculent solution into MFT. The lab
mixer was run
at speeds of 100 RPM or 230 RPM for flocculent solutions containing different
doses of
dissolved flocculation reagent. The dosages of flocculent ranging from 800 to
1200 g per
dry tonne of MFT indicated adequate mixing and flocculation for dewatering.
The
flocculation reagent here was a 30% charge anionic polyacrylamide-polyacrylate
shear
resistant co-polymer with a molecular weight over 10,000,000. A dosage range
of 1000 g
per dry tonne 20% was appropriate for various 30% charge polyacrylamides for
MFT
with clay content of 50 to 75 %.
Example 7:
As mentioned in the above description, continuous flow pipeline reactor tests
were
conducted. Results are shown in Fig 16 comparing high and low flow rates. A 34
wt%
solids MFT was pumped through a 2 inch diameter pipe at a flow rate of 26 LPM
for the
low flow test and 100 LPM for the high flow test. A 0.45% organic polymer
flocculent
solution was injected at 2.6 LPM for the low flow test and at 10 LPM for the
high flow
test. The distance from injection to deposition was 753 inches or 376.5 pipe
diameters.
The 2 inch long orifice mixer had an orifice to downstream pipe diameter ratio
d/D = 0.32
CA 02684232 2009-10-30
34
with six 0.052 inch diameter injectors located on a 1.032 inch diameter pitch
circle. For
the high flow test the six injector diameters were increased to 0.100 inch.
Example 8:
As mentioned in the above description, computational fluid dynamic (CFD)
modelling
was conducted. The CFD modeling considered the flocculent solution as a Power-
law-
fluid and the MFT as a Bingham-fluid in the mixing zone and confirmed both the
adequate mixing of the injection device of Figs 4 and 5 and the inadequate
mixing of the
conventional side branch tube as discussed in the Background section under the
same
conditions. The MFT flow rate in a 2 inch diameter pipe was 30 LPM and polymer
solution was injected at 3 LPM. The 2 inch long orifice mixer had an orifice
to
downstream pipe diameter ratio d/D = 0.32 with six 0.052 inch diameter
injectors located
on a 1.032 inch diameter pitch circle. The MFT had a density of 1250 kg/m3 and
a yield
stress of 2 Pa while the polymer solution had a density of 1000 kg/m3, with a
power-law
index n = 0.267 and a consistency index of 2750 kg s2/m.
Furthermore, the visualization shown in Figs 7-9 is only possible by CFD
modelling due
to the opaqueness of actual MFT. For MFT, the CFD model incorporates non-
Newtonian
fluid behaviours into the hydraulic analysis to develop a robust design for a
variety of
possible combinations and permutations between various MFT properties and
flocculation reagent solutions.
Example 9:
As described above, the flocculation technique relies on its process steps
rather than in
the specific flocculation reagent selected. A person skilled in the art may
select a variety
of flocculation reagents that enable in-line dispersion, flocculation, water
release and
non-flowing deposition. One selection guideline method includes taking an MFT
sample
representative of the commercial application and using a fast-slow mixer test
to observe
the water release capability of the flocculent. In the fast-slow mixer test,
the flocculent is
injected into the mixer running at a fast mixing rate and after a delay of 7
seconds the
mixer is switched to slow mixing. Water release may then be assessed. For
instance,
test have been run at 230 RPM (corresponding to a shear rate of 131.5 s-1) for
fast
mixing and 100 RPM (corresponding to a shear rate of 37. s-1) for slow mixing.
A fast-
slow mixer test was conducted on 10%, 20%, 30% and 40% charge anionic
CA 02684232 2009-10-30
polyacrylamide flocculants and the 30% charge anionic polyacrylamides enabled
superior water release. The use of such 30% charge anionic polyacrylamides in
the
pipeline reactor and CFD modeling validated this approach. In addition, the
fast-slow
mixer test was conducted on high and low molecular weight linear anionic
polyacrylamide flocculents and the high molecular weight polyacrylamides
enabled
superior water release. The fast-slow mixer test may be combined with the CFD
model
to test the mixing of the flocculent solution at the density of the desired
formulation. Such
cross-validation of flocculation reagents and solutions helps improve the
flocculation
process and deposition/farming method operating conditions and validate
preferred
flocculation reagents and solutions.
Example 10:
Trials were performed and showed that a flocculation reagent could be injected
into MFT
in-line followed by pipeline conditioning, deposition and drying. Figs 17-19
schematically
illustrate different experimental setups that were used. For Figs 17 and 18,
the
flocculated MFT was deposited onto beaches and for Fig 19 into a deposition
cell.
The MFT was about 36 wt% solids and was pumped from a pond at flow rates
between
300 and 720 gal/min. The flocculent solution was injected in-line at different
locations.
One of the flocculent reagents used was a 30% charge anionic polyacrylamide-
sodium
polyacrylate co-polymer with a molecular weight over 10,000,000. The
flocculated MFT
ws conditioned along a pipeline and then expelled out of spigots arranged in
series.
In order to monitor the progress of the drying, samples were taken and
analyzed for
percent solids. The drying times to achieve 75 wt% solids ranged from 5 to 7.5
days
depending on the sample location. Deposition areas having a slope showed
faster
drying. Figs 20 and 21 show some results at two different sample points of the
drying
times of deposited MFT.
Dosages between 0.6 Kg to 1.1 Kg per dry tonne of MFT provided preferred
drainage
results, and much cleaner effluent water than those outside this range. Trials
revealed
that incorrect dosage may reduce dewatering for a number of reasons. If the
dosage is
too low, some of the MFT goes unflocculated and overall there is a lack of
dewatering
performance. Overdosing flocculent applications may also lead to reduced
dewatering
CA 02684232 2009-10-30
36
due to allowing water to become bound up in semi-gelled masses with the solids
making
it more difficult to provide conditioning sufficient to allow water release
with the given
pipeline dimensions and hydraulic conditions. Both of these situations were
observed
and dosage adjustments were made to compensate. In addition, water quality
depends
on dosage control. Overdosing or inadequate mixing (resulting in localised
overdosing)
resulted in poor water quality with at times over 1 wt% solids. Increased
dosing control,
the preferred dosage range and rapid initial mixing helped resolve water
quality issues
and improve dewatering and drying of the deposited MFT. Other observations
noted that
the deposited MFT dewatered and dried despite significant precipitation, thus
resisting
re-hydration from precipitation.
Reclamation of the MFT deposits was further observed as vegetation from seeds
tossed
=
on the deposition area was later noted to be growing well.
Example 11:
One of the challenges to successful treating of MFT is the process variations
encountered in operations. It may be desired to use a side injection nozzle to
for mixing
liquids into MFT. Using the mixing algorithm developed for the MFT pipeline
reactor
model, Fig 22 compares a typical side injection nozzle to the orifice nozzle
of Fig 5 on a
2 inch pipeline for a range of MFT flows based on:
¨ The MFT is 30 wt% solids and modeled as a Herschel-Bulkley fluid with a
yield
stress of 2 Pa and high shear rate viscosity of 10 mPa s. Density was 1250
kg/m3.
¨ The flocculent solution was modeled as Power Law fluid with n = 0.267 and
consistency index (k) of 2750 kg s2/m. Density was 1000 kg/m3 and the flow
rate
was 1/10 the MFT volume flow rate
¨ The orifice mixer had a 0.32 orifice ratio.
¨ The flow area for injecting the polymer solution was the same for both
mixers.
Fig 22 illustrates that the orifice mixer of Fig 5 provides significantly
preferred mixing
than the conventional side injection nozzle over the range of MFT flows.
