Note: Descriptions are shown in the official language in which they were submitted.
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Removal of Mature Fine Tailings from Tailings Ponds and
Screening Debris
Field
[0ool] This disclosure relates to removal of mature fine tailings (MFT)
from tailings ponds,
such as those in the oil sands industry.
Background
[0002] Oil sands tailings are a by-product of bitumen extraction from oil
sands. Tailings
ponds are intended to be temporary settlement basins for tailings. However,
tailings ponds
have become one of the main environmental challenges in the oil sands
industry.
[0003] One component of tailings is mature fine tailings (MFT), which is a
mixture of mainly
water and fine clay. MFT may also include other components that may be
hazardous or that
may have detrimental effects on the environment. In a tailings pond, MFT is
typically found
between a top layer of water and the base of the pond, which may include
settled sand.
[0004] Removal of MFT from tailings ponds is key to reclamation of land, to
recovery of
potentially valuable materials within MFT, and for other reasons. MFT can take
a very long time
to settle or solidify on its own.
[0005] Removal of MFT from tailings ponds is conventionally done using
submersible
pumps or dredging. When pumps are used, they are often lowered from barges to
a depth at
which MFT of desired density is located. However, one complication in MFT
removal is the
presence of debris such as tree branches, vegetation, industrial debris, and
the like in the
tailings pond.
[0006] Another method for removing MFT from tailings ponds is described in
Canadian
patent 2,812,202. This method proposes positioning a hollow cylindrical
structure in the tailings
pond. The structure is open at top and bottom and serves to draw MFT of
desired density into
the bottom of the structure for removal by a pump or similar apparatus from
within the
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structure. This patent suggests that debris can be removed from within the
structure, as the
debris would be located at or near the fluid surface.
[0007] The above technologies are not without their drawbacks.
Particularly, the technique
described in the above-mentioned patent brings debris in close proximity to
the pump or other
apparatus charged with removing MFT from the structure. This can cause the
pump or other
apparatus to become clogged or damaged by debris. In addition, a separate
debris handling
process to remove debris from the structure and process it would need to be
implemented.
Summary
[0008] A hollow conduit is positioned within a tailings pond. A bottom
end of the hollow
conduit is open to inflow of mature fine tailings (MFT) and a top end of the
hollow conduit is
above a surface of the tailings pond. A removal apparatus, such as a
submersible pump, is
positioned within the hollow conduit to remove MFT from within the hollow
conduit. A debris
screen is located at the bottom end of the hollow conduit. The debris screen
is sized to allow
inflow of MFT into the hollow conduit and to prevent inflow of debris having a
size exceeding a
maximum opening size of the debris screen. The risk of damage or clogging of
the removal
apparatus is thus reduced or eliminated and the debris remains in place,
thereby removing the
need for a separate debris handling process.
Brief Description of the Drawings
[0009] The drawings illustrate, by way of example only, embodiments of
the present
disclosure.
[0010] FIG. 1 is a partial section side view of a system for removing
mature fine tailings
(MFT) from a tailings pond.
[0011] FIG. 2 is a schematic view of a tailings pond with the system
deployed.
[0012] FIG. 3 is a perspective view of the system showing detail of a
debris screen.
[0013] FIG. 4 is a plan view of the debris screen.
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[0014] FIG. 5 is a plan view of another debris screen.
[0015] FIGs. 6A ¨ 6E are side views of other systems for removing MFT from
a tailings
pond.
[0016] FIG. 7 shows removal of a portion of the debris screen.
Detailed Description
[003.7] FIG. 1 shows a system 10 according to the present invention. The
system 10 is for
removing mature fine tailings (MFT) from a tailings pond to solve or mitigate
at least one of the
problems discussed above.
[0018] The system 10 includes a hollow conduit 12, a removal apparatus 14,
and a debris
screen 16.
[0019] The hollow conduit 12 is configured to be positioned within a
tailings pond. In this
embodiment, the hollow conduit 12 is shaped as an open-ended cylinder. In
other
embodiments, the hollow conduit 12 takes other shapes (e.g., a rectangular
tube). A bottom
end 20 of the hollow conduit 12 is open to inflow 22 of MFT 24 and a top end
26 of the hollow
conduit 12 is above a surface 28 of the tailings pond.
[0020] The removal apparatus 14 is positioned within the hollow conduit 12
and is
configured to remove MFT from within the hollow conduit 12 for ultimate
removal to a facility
for further processing. In this embodiment, the removal apparatus 14 includes
a submersible
pump 30 suspended within the hollow conduit 12 at an adjustable depth by a
line 32 from a
support 34, which may be affixed to the hollow conduit 12 or to a nearby
barge. The
submersible pump 30 can be a submersible vertical turbine pump or similar. The
line 32 is
configured to support the submersible pump 30 and can include conductive wires
for signal
transmission, so as to convey control commands to the pump 30. A discharge
conduit 36 is
further provided to convey discharge of the pump 30 to outside the hollow
conduit 12. In other
embodiments, more than one submersible pump 30 is provided for increased pump-
out rate
and/or for redundancy.
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[0021] The debris screen 16 is fitted to the bottom end 20 of the hollow
conduit 12. The
debris screen 16 is sized to allow inflow 22 of MFT into the hollow conduit 12
and to prevent
inflow of debris 40 having a size exceeding a maximum opening size of the
debris screen 16. In
this embodiment, the maximum opening size of the debris screen 16 is selected
based on a
clogging characteristic of the submersible pump 30. For example, the maximum
opening size of
the debris screen 16 can be selected to be approximately equal to or smaller
than the
throughlet size of the pump 30. The result is that debris that would present a
clogging or
damage risk to the pump is prevented from entering the MFT removal zone 42
within the
hollow conduit 12, so that the pump 30 can be operated reliably and
effectively. Small debris,
which presents tolerable or no risk of clogging or damaging the pump 30, is
permitted to enter
the MFT removal zone 42. The maximum opening size of the debris screen 16 can
be selected
to maximize flow 22 of MFT into the MFT removal zone 42, while minimizing the
risk of clogging
or damaging the pump 30.
[0022] In this embodiment, the system 10 further includes a buoyancy device
50 connected
to the hollow conduit 12 to allow the hollow conduit 12 to float in the
tailings pond. The
buoyancy device 50 can include one or more chambers filled with air or other
material that is
less dense than the contents of the tailings pond by an amount sufficient to
float the weight of
the system 10. The buoyancy device 50 and height of the hollow conduit 12 are
selected such
that the top end 26 of the hollow conduit 12 floats above the surface 28 of
the tailings pond
and the bottom end 20 is located within MFT of target density. The depths and
densities of the
contents of the tailings pond, such as the MFT 24 and any water cap 52, as
well as the weight of
the system 10 should be taken into account when designing the buoyancy device
50.
[0023] The scientific principles by which MFT is drawn into the hollow
conduit 12 are well
understood. Canadian patent 2,812,202 describes such in detail and they will
not be
unnecessarily repeated here. To summarize, when the hollow conduit 12 is
lowered into the
tailings pond, the strata of the tailings pond is enveloped by the hollow
conduit 12. Water 52,
intermediate material of lower-than-target density (e.g., less dense MFT), and
MFT 24 of target
density may be present in the hollow conduit 12 in roughly the same layers as
outside the
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hollow conduit 12 in the undisturbed pond. Water 52 and intermediate material
is then
removed from the hollow conduit 12, using the pump 30 for example, causing the
free surface
54 in the hollow conduit 12 to drop due to the density differential between
material outside
and material inside the hollow conduit 12. Eventually, hydrostatic equilibrium
is reached
between the material (mainly or entirely MFT) inside the hollow conduit 12 and
water 52 and
other material outside the hollow conduit 12. Thereafter, as the pump 30 is
operated and MFT
is removed from inside the hollow conduit 12, a hydrostatic imbalance is
created between
material outside and material inside the hollow conduit 12. The hydrostatic
imbalance tends to
cause material outside the hollow conduit 12 near its bottom end 20 to flow
into the hollow
conduit 12. Hence, MFT 24 of target density outside the hollow conduit 12 is
drawn into the
hollow conduit 12 and to the removal zone 42 for removal from the tailings
pond. A relatively
continuous inflow 22 of MFT can be achieved, while at the same time debris 40
is
advantageously excluded from the removal zone 42 by the debris screen 16.
[0024] In other words, the bottom end 20 of the hollow conduit 12 is
open to MFT, but not
to problematic debris, and the bottom end 20 is positioned to receive MFT of a
desired density.
The MFT of the desired density fills the hollow conduit 12 to a
hydrostatically equilibrating level
for balancing hydrostatic pressures inside the conduit with hydrostatic
pressures outside the
conduit. As MFT is removed from inside the hollow conduit 12, MFT outside the
hollow conduit
12 flows into the hollow conduit as the hydrostatic pressures seek to restore
equilibrium.
[0025] FIG. 2 shows the system 10 in use in a tailings pond 60. A
removal conduit 62
connected to the pump discharge conduit 36 can be floated on the pond 60 to
deliver MFT to a
shore facility 64 for processing and/or removal from the site.
[0026] FIG. 3 shows the debris screen 16 affixed to the bottom end 20
of the hollow
conduit 12.1n this embodiment, the debris screen 16 includes a structural
framework 70 that is
connected, at connection points 72, to the wall at the bottom end 20 of the
hollow conduit 12.
[0027] As shown in FIG. 4, the structural framework 70 of the debris
screen 16 can include
radially arranged structural members 74 (e.g., C channels, angle iron, box
tubing, etc.) and a
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central member 76. The outer end of each structural member 74 is connected to
the hollow
conduit 12 at a connection point 72. The inner end of each structural member
74 is connected
to the central member 76. Disposed between the structural members 74, the
central member
76, and the hollow conduit 12 is one or more pieces of grating 78. The grating
78 defines the
maximum opening size of the debris screen 16.
[0028] Each of the connections between the structural members 74, the
central member
76, the hollow conduit 12, and the pieces of grating 78 may be permanent
(e.g., welded) or
removable (e.g., bolted, clipped in place, etc.).
[0029] FIG. 5 shows a debris screen 80 according to another embodiment. The
debris
screen 80 includes linearly arranged members 82, such as rods or similar,
having ends affixed to
the bottom end 20 of the hollow conduit 12 at connection points 84. Each
member 82 may be
permanently or removably connected to the bottom end 20. The spacing between
the
members 82 defines the maximum opening size of the debris screen 80.
[0030] FIGs. 6A ¨ E show other embodiments of systems for removing MFT from
a tailings
pond according to the present invention. Individual features of these systems
can be used with
the system 10 and vice versa. That is, any combination of features of the
systems discussed
herein that include a debris screen 16 can be used in other embodiments of the
present
invention.
[0031] As shown in FIG. 6A, a system 100 includes as its removal apparatus
a siphon 102,
and the maximum opening size of the debris screen 16 is selected based on a
clogging
characteristic of the siphon.
[0032] As shown in FIG. 6B, a system 110 includes a hollow conduit 112
configured with an
adjustable wall height, so that the depth of the bottom end 20 of the hollow
conduit can be
adjusted to target different densities of MFT. In this embodiment, the
adjustable wall height is
achieved by forming the hollow conduit 112 from two concentric shells. An
inner shell 114 is
slidably positionable within an outer shell 116.
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[0033] As shown in FIG. 6C, a system 120 includes one or more supports 122
connected to
the hollow conduit 12 and extending downward to support the hollow conduit 12
from the
bottom of the tailings pond. The buoyancy device can be omitted. When the
inner and outer
shells 114, 116 (FIG. 6B) are used, the one or more supports 122 can be
affixed to the outer
shell 106.
[0034] As shown in FIG. 6D, a system 130 includes a deflector 132 disposed
at an outside
surface of the hollow conduit 12 to deflect undesirable fluid 134 away from
the bottom end 20
of the hollow conduit 12. Undesirable fluid, such as water or MFT of density
lower than
targeted, may, by operation of the system 130, be drawn down and into the
inflow 22 of what
is desired to be mainly or exclusively MFT of target density. In this
embodiment, the deflector
132 includes a skirt positioned around the circumference of the hollow conduit
12, the skirt
being angled downward and outward so as to deflect downward flowing
undesirable fluid 134
outward and away from the bottom end 20 of the hollow conduit 12.
[0035] As shown in FIG. 6E, a system 140 includes as its removal apparatus
a dredge 142,
and the maximum opening size of the debris screen 16. The dredge 142 can be of
any suitable
type, such as a ladder dredge, clamshell dredge, bucket dredge, backhoe
dredge, and similar.
The dredge 142 can be suspended from the support 34 (FIG. 1) or similar
suitable support.
[0036] FIG. 7 shows removal of a screen portion 150 of the debris screen
16, while the
system 10 is within a tailings pond. The support 34 (FIG. 1) or similar
suitable support can be
used to remove the screen portion 150. In this example, a lifting hook 152 is
used to hook onto
a lifting eye 154, lifting lug, or similar suitable structure provided to each
removable screen
portion 150 of the debris screen 16. The screen portion 150 can thus be
completely removed
from the hollow conduit 12. This allows for ongoing preventive maintenance,
such as cleaning,
or replacement of screen portions due to damage or the collection/growth of
organic matter,
which may plug the openings in the screen portion 150.
[0037] Advantages of the present invention should be apparent from the
above. For
instance, the debris screen keeps debris away from the pump or other removal
apparatus,
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which helps prevent clogging and damage to the pump or other removal
apparatus. In addition,
a separate debris handling process is not needed, as debris is kept outside
the MFT removal
process and remains in the tailings pond. Debris macerators are also not
required. Further, the
debris screen can be easily removed, while the system is deployed on site or
within a tailings
pond, for cleaning using equipment that is often already on site.
[0038] While the foregoing provides certain non-limiting example
embodiments, it should
be understood that combinations, subsets, and variations of the foregoing are
contemplated.
The monopoly sought is defined by the claims.
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