Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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A METHOD FOR PRODUCING A NON-SEGREGATING WASTE STREAM
The present invention relates to a method and
system for producing a non-segregating waste stream.
BACKGROUND
Mining operations suffer from significant waste
disposal difficulties. One such difficulty is differential
settling of the produced waste stream.
In many current open-pit mining operations, for
example, waste streams are disposed of by pipelining the
waste stream slurry to an external tailings confinement
facility, which is essentially a man-made pond enclosed with
a dyke system that contains the waste material. Poor
settling characteristics of fine inorganic solids create an
uppermost solids layer that consists of material that has
limited bearing capacity. There is a desire to reclaim
mined surfaces, but the low bearing capacity of the top
layer of the tailings ponds presents a technical barrier for
achieving this.
One example of where this problem is presented is
in oil sands mining operations, such as those found in
Alberta, Canada. In order to separate valuable bitumen from
a surrounding inorganic matrix, large amounts of water and
process aids are currently added to mined ore. After
separating the bitumen, a large volume of waste slurry
remains, which comprises water, sand, dissolved organic and
inorganic material, and fine, poorly settling, solids. Once
disposed in the tailings confinement, the fine, poorly
settling solids do not settle with the other, coarser solids
creating a low bearing capacity top layer that is unsuitable
for land reclamation.
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It has been recognized that one means of
generating an in-pit deposit of a waste slurry that is
suitable for land reclamation is to generate a waste slurry
that does not show differential settling rates between the
coarser and finer solids. Several attempts have been made
to prevent differential settling. To date, these attempts
have shown little success. In the oil sands industry, these
attempts have focused on increasing the yield stress of the
tailings, but this has failed to generate tailings resistant
to differential settling effects under process conditions.
SUMMARY
According to one broad aspect of the present
invention, there is provided a substantially non-segregating
waste stream.
According to another broad aspect of the present
invention, there is provided a method for producing a
substantially non-segregating waste stream comprising: (a)
determining the apparent viscosity of the waste stream; and
(b) modifying the waste stream to increase the apparent
viscosity to at least a threshold level over which the waste
stream becomes substantially non-segregating.
According to another broad aspect of the present
invention, there is provided a method for recovering water
from a waste disposal site comprising: (a) determining the
apparent viscosity of a waste stream comprising water and
solids; (b) modifying the waste stream to increase the
apparent viscosity to at least a threshold level over which
the waste stream becomes substantially non-segregating; (c)
depositing the modified waste stream at the waste disposal
site; (d) allowing sufficient time to permit settling of the
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solids from the water in the modified waste stream; and (e)
recovering the water.
BRIEF DESCRIPTION OF THE DRAWINGS
Figure 1 is a schematic representation of a tailings pond
exhibiting differential settling.
Figure 2 is a schematic representation of differential
settling.
Figure 3 is a schematic representation of non-differential
settling.
Figure 4 is a plant layout for generating substantially non-
segregating tailings according to one embodiment of the
present invention.
Figure 5 is a chart illustrating the effect of apparent
viscosity on settling.
Figure 6 shows rheograms of a number of different fluids
having different rheological properties.
Figure 7 is the accompanying apparent viscosity profiles for
the fluids of Figure 6.
DETAILED DESCRIPTION
A schematic representation of a mine tailings
stream produced under current operating conditions is shown
in Figure 1. Upon introduction of the mine tailings stream,
a poorly settling fine solids layer (100) forms on a settled
coarse solids layer (110) in a tailings pond (120),
surrounded by a dyke (130). The poorly settling fine solids
layer (100) is a result of differential settling, which may
be better understood from Figures 2 and 3.
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When an homogeneous combined solids slurry is
introduced into a containment unit, a heavier fraction will
settle to the bottom of the containment unit due to
gravitational effects. This is shown in Figure 2. In a
typical oil sands operation, the heavier fraction is rich in
sand, with the upper fraction comprising a mixture of fines
suspended in water. The fine slurry upper layer of the
tailings pond will have insufficient bearing capacity to be
suited for land reclamation purposes.
ASTM provides a definition for "fines" as
particles having a diameter of 74 m or less. In practice,
fines are primarily composed of silica and clay. "Coarse"
solids have particles with a larger diameter than the fines
(e.g., above 75 m). In practice, these particles are
primarily sand.
If differential settling were not to occur, the
homogeneous slurry would settle under its own weight in a
homogeneous fashion. This is shown in Figure 3. In time, a
water layer will form over the settled layer, which water
can be recycled into, for example, the oil sands mining
operation process, reducing the amount of water that must be
drawn from external water sources. The consolidated slurry
forms a soil layer that will have sufficient bearing
capacity to allow for land reclamation.
There have been a number of different approaches
taken in an attempt to obtain non-segregating tailings.
According to one theory, it has been suggested that the
yield strength of the tailings mix be increased in order to
minimize particle settling. On a fundamental level, the
terminal settling velocity of a particle is determined by
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force equilibrium on the particle. Referring to standard
textbooks on fluid mechanics (see, for example, Bird, R.,
W.E. Steward and E.N. Lightfoot, "Transport Phenomena",
1960, John Wiley & Sons), the terminal settling velocity for
a single particle in a Newtonian fluid is given by Formula
1.0:
V - gOPdz
s 1grI
Formula 1.0
where:
g is the gravitational acceleration constant [m/sZ];
Ap is the difference between particle and fluid
density [kg/m3];
d is the particle diameter [m]; and
ri is the fluid viscosity [Pa=s].
In Formula 1.0, the density difference for a
tailings recipe comprising silica particles and water is
typically about 1500 kg/m3. From the functional form of the
equation, it is therefore obvious that the settling velocity
can only become zero if the fluid viscosity is infinitely
high. This suggests that there will always be differential
settling (and therefore segregation) when particles settle
in a Newtonian fluid.
It is possible to generate tailings mixes that
exhibit non-Newtonian behaviour. Unlike with Newtonian
fluids, it is theoretically possible to obtain non-
segregating tailings with a non-Newtonian fluid. In order
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to keep a particle suspended in a non-Newtonian fluid (i.e.,
have zero settling velocity), the fluid must have sufficient
strength to counteract the buoyancy force acting on the
particle. In rheological terms, this means that the fluid
must exhibit yield strength characteristics (see, for
example, Hunter, R.J., "Introduction to modern colloid
science", Oxford University Press, 1993; and Hiemenz, P.C.,
"Principles of colloid and surface chemistry", Marcel
Dekker, 1997). Physically, the fluid's yield strength
required for keeping a particle suspended is given by
Formula 2.0:
tio = a0pgd
Formula 2.0
where:
tio is the yield strength in Pa;
a is an empirically determined constant with a
numerical value between 0.048 and 0.2; and
p, d and g are as defined for Formula 1Ø
Applying numbers typical to oil-sands tailings
reveals that a yield strength of approximately 0.33 Pa
should suffice for generating non-segregating oil sands
tailings. But, laboratory testing has revealed that oil
sands tailings samples with yield strength higher than
0.33 Pa still show differential settling under simulated
process conditions.
Another approach taken in an attempt to obtain a
non-segregating tailings has focused on increasing the fines
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content of the tailings. It has been suggested that if the
tested tailings mix is higher in fines content such that a
weight ratio of Fines/(Fines+Water) is higher than a certain
threshold value, then the tailings should be non-
segregating. (See, for example, Fine Tailings Fundamentals
Consortium, 1995. "Vol. III, Volume reduction of CHWE fine
tailings utilizing non-segregating tailings", in "Advances
in oil sands tailings research", Alberta Department of
Energy, Oil Sands Research Division.)
The present inventors have determined that while
the presence of a yield strength of 0.33 Pa may be a factor
in generating non-segregating oil sands tailings, this alone
is insufficient. It is believed that varying the yield
strength alone is insufficient, because this property
describes the slurry under stagnant conditions, and not
under process conditions. Increasing the
Fines/(Fines+Water) ratio has also not resulted in a
substantially non-segregating tailings. The present
inventors have determined that modifying the apparent
viscosity of a tailings product will reduce the undesired
effects of differential settling and assist in producing
non-segregating tailings.
Any fluid (Newtonian or non-Newtonian) will show
resistance against deformation, e.g. shear. The amount of
stress that has to be applied at a certain deformation (or
shear) rate depends on the rheological properties of the
fluid. For Newtonian fluids, the ratio between shear stress
and shear rate is constant (e.g., doubling the shear rate
doubles the amount of stress required). This constant ratio
is referred to as the fluid viscosity. For non-Newtonian
fluids, the ratio between shear stress and shear rate is
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generally not constant, but is dependent on the shear rate.
For non-Newtonian fluids, an analogous property to
viscosity, i.e. the apparent viscosity, can be defined as
the ratio between shear stress and shear (i.e., deformation)
rate. Apparent viscosity can be measured in a number of
ways, as will be known to the person skilled in the art.
It has been observed that the total amount of
segregation to be expected is a function of the deformation
rate to which the tailings recipe is exposed and the time
the tailings mix is exposed to this deformation rate. As
described above, the deformation rate is directly related to
the apparent viscosity of the mixture.
Tailings slurries are exposed to deformation under
process conditions (e.g., during mixing, pipeline transport,
in-pit placement). To offset this, the apparent viscosity
of the slurry is made sufficiently high during processing so
that a substantially non-segregating tailings is obtained.
In practice, this means determining a threshold apparent
viscosity above which the tailings become substantially non-
segregating. The threshold apparent viscosity can be
determined experimentally by observing at what apparent
viscosity value the tailings mix becomes substantially non-
segregating.
"Substantially non-segregating" as used throughout
this application is intended to encompass waste streams that
show limited differential settling between coarse and fine
particles, which will facilitate generating a sufficient
bearing capacity in situ to meet a desired purpose,
including, without limitation, land reclamation activities.
It will be readily appreciated that when the waste stream is
first introduced into a containment area, it is not
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immediately suited for land reclamation purposes. A
sufficient consolidation time will have to pass before the
material reaches a desired strength.
There are a number of variables that affect the
apparent viscosity, including, without limitation, the
composition of the tailings mix, the degree of dewatering of
the tailings slurry, additives that can modify solution
chemistry, and flow devices used during processing.
Increasing the apparent viscosity to a threshold level can
therefore be achieved in a number of ways. For example,
modifications can be made to mechanical and chemical
variables during processing, as well as mechanical factors
that may act on the slurry during deposition into a tailings
pond. The modifications include, without limitation:
increasing the degree of dewatering of the waste slurry; a
regime of conditioning treatments in the process line-up;
and limiting the deformation rate on the slurry during the
final stages of deposition.
Apparent viscosity is raised when the water
content of a tailings slurry is lowered. There are a number
of different dewatering processes that can be used to lower
water content, which will be known to the person skilled in
the art. Common dewatering options for fine solids include,
without limitation, mechanical thickeners and centrifuges.
For coarse tailings, hydrocylones and vibrating screen
technology are commonly adopted.
Chemical modification during processing may also
be used to increase apparent viscosity. For instance, an
apparent viscosity enhancing chemical agent could be added.
Apparent viscosity is raised in oil sands tailings slurries,
for example, when the electric double layer around clay
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material in the slurry is contracted. To achieve this
divalent or trivalent cations (e.g., Ca2+, A13+) may be added
to the tailings stream. Gypsum (CaSO4), for example, can be
used as a source of Ca2+. Other means of chemical
modification include, modifying the pH of the tailings
stream and/or adding active clays (e.g. Bentonite) to the
tailings product. Other suitable chemical modifications
will be apparent to the skilled person, or can be determined
through routine testing. The appropriate amount of
additives can be determined through routine testing.
Mechanical parameters at deposition into a
tailings pond may also affect apparent viscosity. For
instance, apparent viscosity is adversely affected if the
deformation rate applied to the tailings product is too
high. For example, a common way of introducing tailings
material in the deposition cell is through 'beaching'. In
this method, tailings run down the containment wall into the
containment. During this process, the deformation to which
the material is exposed results in the coarse fraction
segregating almost immediately. When the tailings product
is introduced in a more controlled way (e.g., by means of
feed-wells, diffusers), deformation can be limited and
segregation inhibited.
Any of these modifications can be made alone or in
combination to minimize deformation conditions in order to
meet a defined threshold apparent viscosity suitable for the
intended purpose.
A plant layout for generating a substantially non-
segregating waste stream is exemplified in Figure 4. This
plant layout is described by reference to a tailings
obtained from a typical oil sands mining operation.
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In Figure 4, a waste stream (400) from an oil
sands processing facility is fed into a size classification
unit operation (405). The waste stream (400) may be a
combination of one or more waste streams, and may comprise
one or more of, without limitation, bitumen extraction
tailings, Tailings Solvent Recovery Unit (TSRU) tailings,
and Mature Fine Tailings (MFT) from a conventional external
tailings facility. The size classification unit
operation (405) separates the stream into a fines rich
stream (410) and a coarse rich stream (420). The size
classification unit operation (405) typically consists of
(but is not limited to) an appropriate combination of
hydrocyclones, gravity separators and equivalents thereof.
Stream (410) is fed into a fine tailings
conditioning unit operation (415). In conditioning unit
operation (415), one or more tailings process aids (430) may
be added to the fines rich stream (410). Processing aids
may comprise (but are not limited to) one or more anionic
and/or cationic flocculants, as well as dilution water. The
conditioned fine tailings stream (440) is fed to a fine
tailings dewatering unit operation (425). This unit
operation (425) may comprise a combination of mechanical
thickeners and/or centrifuges or equivalent unit operations.
The unit operation (425) achieves a degree of dewatering of
the conditioned fine tailings stream (440) for generating a
fine paste having a sufficient apparent viscosity to produce
a substantially non-segregating tailings product. Unit
operation (425) produces an overflow stream (450) that is
lean in particles and an underflow stream (460) that is rich
in particles. Stream (450) may be fed to water treatment
unit operation (435), which separates the stream into an
internal recycle stream (470) and a product stream (480).
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Unit operation (435) is aimed at removing sufficient ionic
material from the aqueous phase to make the water acceptable
for recycling into the oil sands processing plant as
stream (480). Unit operation (435) will typically comprise
a final solids removal step and a combination of Reverse-
Osmosis/Nanofiltration or equivalent. Stream (450) may be
treated in whole or in part, and unit operation (435) may
comprise a means for bypassing a part of stream (435)
directly to stream (480). Stream (470) is comprised of a
concentrated 'brine' that can be used as dilution water for
unit operation (405).
The coarse rich stream (420) may not be of high
enough solids content for producing a substantially non-
segregating tailings product. Stream (420) may therefore be
further treated in a coarse tailings dewatering unit
operation (445). This unit operation (445) may comprise
vibrating screen technology, for example, or an equivalent
thereof. A water stream (490) from this unit operation may
contain fine material that is recycled into the size
classification unit (405). Unit operation (445) dewaters
the coarse material to sufficiently low water content for
substantially non-segregating tailings production. A
dewatered coarse materials stream (500) is obtained from
unit operation (445). Both the dewatered fines stream (460)
and the dewatered coarse stream (500) are fed at correct
proportions to product mixing unit operation (455). After
the streams are homogenized sufficiently in unit
operation (455), the homogenized material stream
obtained (510) is fed to deposition unit operation (465),
which pumps out and distributes stream (520) in-pit (not
shown). Unit operation (465) may comprise equipment
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equivalent to feed-wells in commercially available
mechanical thickeners.
Example 1
In Figure 5, laboratory data on a combined coarse-
fine tailings mix is presented. Physical characteristics of
the tailings mix are:
Remoulded yield strengthl: 2 Pa.
Unmoulded yield strength2: > 10 Pa.
Fines/(Fines+Water): 38 wt%
Solids content: 75.5 wt%
1"Remoulded yield strength" is the residual yield strength
after thorough homogenisation.
2"Unmoulded yield strength" is the yield strength if the
tailings mix is left undisturbed.
The yield strength of the sample is higher than
0.33 Pa, and should therefore be non-segregating according
to Formula 2Ø Also, the tested tailings mix is higher in
total solids content and a ratio of Fines/(Fines+Water) than
the segregation boundary as defined in "Advances in Oil
Sands Tailings Research" (see above). This also suggests
that the tailings should be non-segregating.
The tailings recipe was tested by monitoring the
degree of sand drop-out over time under simulated process
conditions. For this purpose, laboratory equipment that
monitors sand fraction in a slurry as a function of time was
used. Process conditions are mimicked by applying shear
rates representative of pipeline transport and deposition to
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the sample while measuring the change in sand fraction in
the sample. A decreasing sand fraction in the sample
indicates that the sand has settled past the detection
point, which means that sand has segregated from the slurry
mix. The lines shown in Figure 5 show the amount of sand
present at a certain height in the test cell. For the
condition indicated as "low apparent viscosity" (typically
around 100 mPa s), it can be seen that after around 6000
seconds, the sand content starts decreasing. The solids
content continues to decrease over time. When the same
sample was tested under higher apparent viscosity
conditions, as shown by lines labelled "intermediate
apparent viscosity" and "high apparent viscosity" (typically
around 500 mPa s), both the time at which segregation
commences and the extent to which segregation occurs
decreases. The apparent viscosity was changed by altering
the shear rate applied to the sample.
Prior teachings suggest that this tailings recipe
should be non-segregating, but segregation is observed under
process conditions. However, it is observed in this example
that increasing apparent viscosity reduces the degree of
segregation, and that the onset of segregation can be
delayed.
Example 2
The rheological behaviour of two tailings mixes is
shown in Figure 6. Figure 6 shows the stress-deformation
rate behaviour of a Newtonian fluid (600), an oil sands
tailings slurry having a low yield stress (610) of about
0.4 Pa, and an oil sands tailings slurry having a high yield
stress (620) of greater than 3 Pa. The Newtonian fluid
(600) has a viscosity that is 10 times the viscosity of
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water. The Newtonian fluid (600) was not actually measured,
but is a theoretical construct included for reference
purposes only.
In Figure 7 the data plotted in Figure 6 is
converted into apparent viscosity values. The horizontal
line (700) illustrates an approximate threshold value for
the apparent viscosity over which segregation was observed
to be negligible. The reference Newtonian fluid from Figure
6 is plotted as line (710). The low yield strength fluid is
plotted as line (720) and also fails to exceed the threshold
value for deformation rates over about 10 s-1: for
deformation rates below 10 s-1, the material acts as non-
segregating, while for values over 10 s-1 the material
segregates. Finally, the higher yield strength fluid is
plotted as line (730). The constraint on deformation rate
is alleviated: the material stays non-segregating throughout
the deformation rate regime shown.
In this example, it is observed that a material
with a yield stress of 0.33 Pa will segregate under process
conditions when the apparent viscosity is not carefully
controlled.
Although the foregoing invention has been
described in some detail by way of illustration and example
for purposes of clarity of understanding, it is readily
apparent to those of ordinary skill in the art in light of
the teachings of this invention that certain changes and
modifications may be made thereto without departing from the
spirit or scope of the appended claims. By way of example,
specific reference to tailings from a mining operation as
the waste stream has been made, but the invention is not
intended to be so limited. The method of the invention may
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be extended to any suitable manufacturing waste stream,
including one comprising organic matter.
It must be noted that as used in the specification
and the appended claims, the singular forms of "a", "and"
"the" include plural reference unless the context clearly
indicates otherwise.
Unless defined otherwise all technical and
scientific terms used herein have the same meaning as
commonly understood to one of ordinary skill and the art to
which this invention belongs.
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