Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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SP 1678
METHOD OF DEWATERING OF A TAILINGS STREAM
The present invention relates to a method of
dewatering of a tailings stream, in particular oil sands
tailings.
Oil sands mining operations generate significant
volumes of so-called 'tailings' that pose unique challenges
for water recovery and land reclamation. Currently, the
legacy Mature Fine Tailings (MFT) or continuously produced
Fluid Fine Tailings (FFT) are stored in tailings ponds and
are typically periodically transported without treatment.
Treatment of tailings can lead to reduced volumes of
tailings and to faster mine closure and reclamation of the
disturbed land.
Several treatments have been suggested. A potentially
effective tailings treatment option is flocculation.
Flocculation uses flocculants that help the clay/fines in
the tailings to form large aggregates which settle thereby
leaving the water for reclamation.
An example of a method for monitoring and controlling
the dewatering of oil sands tailings has been described in
US20160100135. US20160100135 discloses an in-line
monitoring method, wherein an image capture device is
positioned in a flow of flocculated oil sands tailing
through a pipeline for acquiring one or more images of the
flocculated oil sands tailing; collecting the one or more
images; and analyzing the one or more images to ensure
production of alleged optimum floc structures.
A problem of the method as described in U520160100135
is that it does not allow for inline real-time measurement
of process sensitivities and disturbances including (but
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not limited to) variation in the clay content in the
tailings feed stream and amount of flocculant injected into
the tailings stream. Oil sand tailings (like other types of
tailings) can have substantial variability in compositions
and in case the amount of.flocculant is not optimized, this
may result in ineffective flocculation or unnecessary loss
of expensive flocculants. In this respect it is noted that
the method as described in US20160100135 determines 'good'
or 'bad' flocculation based on dewatering only, without
considering the yield strength which is important for
reclamation as well. Also, the method as described in
US20160100135 does not account for under- or overshearing
of the tailings.
Another problem of the method as described in
US20160100135 is that it uses variations in the pixel
brightness of the image as an estimate of the 'well
flocculated' material, which can be specific to MFT/FFTs.
This value changes depending on the composition of a
tailings stream (which may change over time and which may
be different for different types of tailings) and which
cannot be monitored dynamically in the described method.
Another issue with the known method is that the
signal as generated according to the method as described
in US20160100135 is insensitive between well-flocculated
and oversheared state of treated tailings. After the
tailings have become well flocculated the signal
saturates; in this manner, the metric cannot
differentiate between 'well mixed' and 'over mixed'
materials.
It is an object of the present invention to solve,
minimize or at least reduce one or more of the above
problems.
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It is a further object of the present invention to
provide an alternative method of monitoring and
controlling the flocculation and/or dewatering of a
tailings stream.
One or more of the above or other objects may be
achieved according to the present invention by providing
a method of dewatering of a tailings stream, in
particular oil sands tailings, the method at least
comprising the steps of:
(a) providing a tailings stream;
(b) measuring at least the solids concentration, the clay
content and the flow rate of the tailings stream provided
in step (a);
(c) providing a flocculant-containing stream;
(d) measuring the concentration of flocculant of the
flocculant-containing stream as provided in step (c);
(e) combining the flocculant-containing stream provided
in step (c) with the tailings stream provided in step (a)
thereby obtaining a flocculant-enriched tailings stream;
(f) subjecting the flocculant-enriched tailings stream
obtained in step (e) to shear and mixing in a shear
device thereby obtaining flocculated tailings;
(g) separating the flocculated tailings thereby obtaining
a dewatered tailings stream and a water-enriched stream;
wherein the amount of flocculant-containing stream
combined in step (e) is based on the clay content of the
tailings stream as measured in step (b) and the
concentration of flocculant of the flocculant-containing
stream as measured in step (d); and
wherein the amount of shear in step (f) is selected
dependent on the solids concentration, the clay content
and the flow rate of the tailings stream as measured in
step (b).
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It has surprisingly been found according to the
present invention that it allows for inline real-time
measurement of process sensitivities and disturbances
including (but not limited to) variation in the clay
content in the tailings feed stream and amount of
flocculant injected into the tailings stream. As the
present invention makes the amount of flocculant added
dependent on the clay content of the tailings stream the
flocculation process can be optimized, whilst overuse of
(and hence loss of expensive) flocculant can be
minimized.
A further advantage of the method according to the
present invention is that the quality of the flocculated
tailings can be maintained at a target flocculation state
with respect to both dewatering and yield strength values
using automated control of the shear in the shear device.
It has been found according to the present invention
that, from a reclamation perspective, both dewatering and
yield strength are important parameters. A low yield
strength of the flocculated tailings may indicate slow or
ineffective reclamation even with good immediate
dewatering.
In step (a), a tailings stream is provided. The
tailings stream is not limited in any way (in terms of
composition, phase, etc.) and can be any tailings stream
as obtained in mining, oil sands operations, etc.
However, preferably, the tailing stream is an oil sands
tailings stream such as MFT. Also, it is preferred that
the tailings stream is continuously flowing.
Preferably, the tailings stream as provided in step
(a) comprises from 55 to 85 wt.% water, preferably at
least 60 wt.%, more preferably at least 65 wt.%, and
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preferably at most 75 wt.%, more preferably at most 70
wt. %.
Further it is preferred that the tailings stream as
provided in step (a) comprises from 15 to 45 wt.% solids,
preferably at least 25 wt.% and preferably at most 35
wt.%. The solids in the tailings stream are typically
mineral solids with a particle diameter of less than 44
micron ("fines"), as greater solids particles may have
bene removed earlier. Further, in case the tailings
stream is an oils sands stream, it usually contains at
least 5 ppm bitumen, typically at least 10 ppm, more
typically at least 15 ppm, even more typically at least
ppm, sometimes even as high as 10 wt.%.
In step (b), at least the solids concentration, the
15 clay content and the flow rate of the tailings stream
provided in step (a) are measured.
The person skilled in the art will readily understand
that various suitable devices and methods are available
for measuring the solids concentration, the clay content
20 and the flow rate.
Typically, the solids concentration is measured using
an in-line densitometer, e.g. a nuclear densitometer,
coriolis meter, etc. If desired, dependent on the solids
concentration measured in step (b), the tailings stream
may optionally be diluted to a predefined value for the
solids concentration, typically between 20-40 wt.%,
preferably above 25 wt.% and preferably below 35 wt.%.
Typically, the clay content is measured using
spectroscopic techniques such as IR, NIR and FT-NIR. If
desired, non-contacting sensors may be used. Preferably,
in step (b) the clay content is measured using Near-
infrared spectroscopy (NIR; 700-2500 nm), preferably
using a non-contacting NIR device.
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Typically, the flow rate is measured using a flow
meter. Suitable flow meters are e.g. magnetic meters,
coriolis meters, pressure drop devices, etc.
In step (c), a flocculant-containing stream is
provided. The person skilled in the art will readily
understand that the flocculant, the amount thereof in the
flocculant-containing stream and optional other
components are not particularly limited. The flocculant
may be selected from a broad variety of components (or
mixtures thereof). Preferably the flocculant comprises a
polymeric flocculant. The flocculants may be charged or
uncharged. Suitable flocculants are polyacrylamides,
polyacrylates, (co)polymers of ethylene oxide, etc.
Generally, the flocculant-containing stream comprises
0.05-5 wt.% flocculant in aqueous solution (dosed at 10-
5000 g/tons of dry solids), preferably 0.3-0.5 wt.%
(dosed at 500-1500 g/tons of dry solids).
In step (d), the concentration of flocculant of the
flocculant-containing stream as provided in step (c) is
measured. Again, the person skilled in the art will
readily understand that there are many suitable devices
or methods for doing so. Also, the person skilled in the
art will readily understand that the concentration can be
measured directly or by means of a proxy (such as
viscosity) that reflects the concentration. For example,
a viscometer or a rheometer can be used. Usually, in step
(d) also the flow rate of the flocculant-containing
stream is determined.
In step (e), the flocculant-containing stream
provided in step (c) is combined with the tailings stream
provided in step (a) thereby obtaining a flocculant-
enriched tailings stream. The person skilled in the art
will understand that this combining can be done upstream
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of or in the shear device. If desired, some additional
flocculant may be added after exiting the shear device.
Also, if desired the tailings stream may be pre-sheared
before flocculant addition and entering the shear device.
According to the present invention, the amount of
flocculant-containing stream combined in step (e) is
based on the clay content of the tailings stream measured
in step (b) and the concentration of flocculant of the
flocculant-containing stream as measured in step (d).
Preferably, the amount of flocculant-containing stream
combined in step (e) is also based on the density and/or
the flow rate of the tailings stream as measured in step
(b).
In step (f), the flocculant-enriched tailings stream
obtained in step (e) is subjected to shear and mixing in
a shear device thereby obtaining flocculated tailings. As
the person skilled in the art is familiar with shear
devices, this is not further described here in detail.
Usually, the shear device is a static mixer, a dynamic
mixer, a series of stirred tanks, etc. Preferably, the
shear device is a dynamic mixer. Examples of shear
devices are:
- in-line dynamic mixers (i.e. mechanical impellers
installed in-pipe) such as conventional pitched blade
turbines, hydrofoil variants such as A310 (commercially
available from Lightnin (Rochester, NY, US)), XE3, HE3 or
Maxflo W variants (commercially available from Lightnin);
- static mixers (i.e. inserts in empty pipes with
geometrics to promote mixing): Kenics (commercially
available from Chemineer (US)), SMV or SMX variants
(commercially available from Sulzer (Winterthur,
Switzerland)), static mixers with STM elements or
variants obtainable from Statiflo (Macclesfield, UK);
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- stirred mixing tanks (i.e. vessels equipped with
impellers and/or agitators): process tanks with SC-3
impellers (commercially available from Lightnin) or other
dynamic mixers designed for tanks.
According to the present invention, the amount of
shear in step (f) is selected dependent on the flow rate,
the density and clay content of the tailings stream as
measured in step (b). Usually, also the geometry of the
shear device is taken into account. The amount of shear
can for example be adjusted by changing the mixing speed
(or the flow rate in case of a static mixer).
Advantageously, this allows for a feed-forward mechanism
in the shear device based on information obtained
upstream of the shear device. Similarly, a feed-back
mechanism may be present based on information obtained
downstream of the shear device as will be discussed
below.
According to a preferred embodiment of the present
invention, the flocculation state of the flocculated
tailings as obtained in step (f) is determined using one
or more of:
- an image capturing device, preferably a PVM (Particle
Vision and Measurement; commercially available from
Mettler Toledo (Columbus, Ohio, US));
- a particle size measurement device, preferably a FBRM
(Focused Beam Reflectance Measurement; commercially
available from Mettler Toledo);
- the solids concentration of the tailings stream as
measured in step (b); and
- the clay content of the tailings stream as measured in
step (b).
In this respect it is noted, that the present
invention introduces the (new) concept of the
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'flocculation state' of flocculated tailings, at least
for monitoring and controlling the dewatering and yield
strength of oil sands tailings. Irrespective of this new
concept being used, as the person skilled in the art is
familiar with the above-mentioned measurement methods and
devices to obtain the flocculation state, these
measurement methods and devices are not further described
here in detail. According to the present invention, the
image capturing device is particularly used for the
purpose of assessing the dewaterability and yield
strength (by analysis of the images along with analysis
of other signals such as obtained from e.g. an FBRM, a
NIR device and a densitometer, to determine the
flocculation state which is an estimate of the dewatering
and yield strength), and the particle size measurement
device for measuring the size of flocs (which is also
used indirectly to help the estimation of the
dewaterability and yield strength. If e.g. an FBRM is
used, the direct measurement is the floc size, often
called 'chord length'. The signals such as obtained from
e.g. a PVM and an FBRM, in addition to the ones obtained
from e.g. a NIR device and densitometer are then used to
determine the flocculation state, which is an estimate of
the dewatering and yield strength).
Preferably, the flocculation state of the flocculated
tailings as obtained in step (f) is determined whilst in
addition using an image capturing device (preferably a
PVM), that is used to measure the initial dewaterability
and yield strength of the tailings stream provided in
step (a). This additional image capturing device may help
to normalize the signal obtained using the image
capturing device for determining the flocculation state
of the flocculated tailings as mentioned above.
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Of course, if desired and as is preferred according
to the present invention, further flocculation state
determination or analysis methods may be used, e.g. on
the basis of image processing (of the flocs), such as
Eigenfunction, Fourier transformation, fractal
dimensional analysis, variation of pixel brightness,
classifiers, wavelet transformation, mean gradient energy
analysis, entropy (randomness) analysis, spectral weight
analysis, etc.
Also, the person skilled in the art will readily
understand how to record and analyse the signals from the
various instruments using a computer.
According to an especially preferred embodiment
according to the present invention, the determined
flocculation state is used to control (and adjust if
desired) one or more of:
- the amount of shear in step (f); and
- the residence time of the flocculant-enriched tailings
stream in the shear device in step (f). This
advantageously allows for a feedback loop to the shear
device. In this respect it is noted, that the present
invention - apart from introducing the concept of the
'flocculation state' of flocculated tailings as mentioned
above - also introduces the concept of using the
flocculation state for controlling the flocculation
process. By measuring the flocculation state of the
flocculated tailings it can be determined whether the
flocculation has been effective and, if not, e.g. the
amount of shear applied in step (f) can be controlled by
increasing or decreasing the amount of shear, if
appropriate.
If desired, the flocculated tailings obtained in step
(f) may be transported in a pipeline to a remote
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separator before it is separated in step (g), although it
is noted in this respect that the separation already
happens in the pipeline (and/or in the deposit areas
where the water is collected). Hence, the use of a
dedicated separation vessel is possible but not essential
to the invention.
In step (g), the flocculated tailings are separated
thereby obtaining a dewatered tailings stream and a
water-enriched stream. Typically, the dewatered tailings
stream comprises from 30 to 75 wt.% solids, preferably
above 45 wt.% (and usually below 65 wt.%).
Hereinafter the present invention will be further
illustrated by the following non-limiting drawings.
Herein shows:
Fig. 1 schematically a flow scheme of the method for
monitoring and controlling the dewatering of an oil sands
tailings stream according to the present invention.
For the purpose of this description, same reference
numbers refer to same or similar components.
The flow scheme of Figure 1, generally referred to
with reference number 1, shows a densitometer 2, a non-
contacting NIR device 3 and a flowmeter 4 (respectively
for measuring the solids concentration, the clay content
and the flow rate of the oil sands tailing stream 10), a
shear device 6 (in the embodiment of Figure 1 in the form
of a dynamic mixer), a viscometer 5 and flow meter 11
(respectively for measuring the flocculant concentration
and the flow rate of the flocculant-containing stream
20), a first PVM 7, a FBRM 8 and a second PVM 9.
During use, the tailings stream is supplied via line
10. After optional diluting with water using water stream
40, at least the solids concentration (in densitometer
2), the clay content (in NIR device 3) and the flow rate
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(in flow meter 4) of the tailings stream 10 are measured.
If desired, optional chemicals (such as coagulants, e.g.
gypsum and alums) may be added as stream 50.
A flocculant-containing stream is supplied via line
20. The concentration (in viscometer 5) of flocculant and
the flow rate (in flow meter 11) of the flocculant-
containing stream 20 are measured.
Then the flocculant-containing stream 20 is combined
with the tailings stream 10 thereby obtaining a
flocculant-enriched tailings stream. In the embodiment of
Figure 1, the streams 10 and 20 are combined in the
dynamic mixer 6. In the dynamic mixer 6, the flocculant-
enriched tailings is subjected to shear and mixing
thereby obtaining flocculated tailings 30. Subsequently
(not shown) the flocculated tailings are separated
thereby obtaining a dewatered tailings stream and a
water-enriched stream.
The amount of flocculant-containing stream 20
combined in the dynamic mixer 6 is based on the clay
content (measured in NIR device 3) of the tailings stream
10 and the concentration (measured in viscometer 5) of
flocculant of the flocculant-containing stream 20.
Further, the amount of shear in the dynamic mixer 6
is selected dependent on the solids concentration
(measured in densitometer 2), the clay content (measured
in NIR device 3), the flow rate (measure in flow meter 4)
of the tailings stream 10 and the geometry of the dynamic
mixer 6. If desired, the amount of shear can be adjusted
by e.g. changing the mixing speed in dynamic mixer 6.
In the embodiment of Fig. 1 a 'flocculation state
observer model' was developed to produce an estimate of
the flocculation state of the flocculated tailings 30,
considering input from the first PVM 7 (on dewaterability
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and yield strength) and optionally normalized to the
signal produced by the second PVM 9 (providing a baseline
image of the tailing stream 10), the SD/mean of the FBRM
signal obtained in FBRM 8 (on the size of flocs), the
clay content (as measured in NIR device 3) and the solids
concentration (as measured in densitometer 2). This
flocculation state was then used as a control parameter
for controlling the amount of shear applied in the
dynamic mixer 6. This control parameter could also be
used to control the residence time of the flocculant-
enriched tailings stream in the dynamic mixer 6.
The person skilled in the art will readily understand
that many modifications may be made without departing
from the scope of the invention. Further, the person
skilled in the art will readily understand that, while
the present invention in some instances may have been
illustrated making reference to a specific combination of
features and measures, many of those features and
measures are functionally independent from other features
and measures given in the respective embodiment(s) such
that they can be equally or similarly applied
independently in other embodiments.
