Note: Descriptions are shown in the official language in which they were submitted.
Method of Excavating a Tailings Lagoon
In mining operations there is a need to store tailings, products resulting
from the ore
extraction process. Typically these are retained in tailings pits or tailings
lagoons
behind a tailings dam. Typically the tailings dam can begin as a low level dam
and be
built up to a greater height periodically as mining progresses to accommodate
tailings
as these are produced. Solid tailings are often used as part of the dam
structure.
Over the last 50 years, over 1800 people have been killed as a result of
tailings dam
"ruptures" in different places around the world, the last two events having
been in
Minas Gerais Brazil.
All these tailings dam "ruptures" were the direct result of hydraulic pressure
being
exerted on the manmade dam which more often than not is built out of local
waste
mineral materials that are compacted to form the dam at the lower part of the
dam/
valley.
More often than not, it is the fluidisation of the slurry behind the dam that
results in
the full hydraulic pressure to the depth of the tailings slurry acting on the
dam
especially in flat face rather than curved face dams.
Some of the "ruptures" are caused by flood rain taking the water level in the
dam to
above the design limit but the mine operators have not developed adequate
mechanisms of removing excess rain water from the lower part of the dam. This
excess of rain water that sits against the inner edge of the dam again has the
effect
of fluidising the settled tailings slurry and also fluidising the compacted
dam itself.
Many tailings dam designs (except dams constructed from concrete) often have
horizontal water drain pipes built into the bottom of the dam walls which
whilst
initial filling of the tailings dam with tailings these pipes are kept closed,
but as the
tailings slurry increases in height in the dam, these pipes are used to vent
water that
has seeped to the bottom of the tailings dam wall to the downstream side of
the dam.
This removal of water assists in ensuring the slurry behind the dam for up to
100
meters upstream of the dam dries out and does not become fluidised.
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The most frequent problem is that these water drainage pipes at the bottom of
the
dam wall that vent water from the upstream side of the dam to the other side
of the
dam downstream become blocked or fractured and no longer are able to take the
water away. This happens especially when the dam is not maintained or when
production has stopped for one reason or another at the metal mine that
produces the
by-product tailings. It can also be a problem that the sheer quantity of rain
ends up
raising the water level in the dam and it overspills and fluidises the toe of
the dam.
The end result of blocked dam water drainage pipes is fluidisation of the
settled
slurry behind the dam and even the fluidisation of the dam itself of which
either or
both can easily result in the rupture of the dam and catastrophic dam failure.
The present application relates to a mechanism and process that will mitigate
against
fluidisation of tailings material close to the dam wall on tailings pits that
have been
inactive for a period of time or that are active but are considered at risk of
"Rupture".
The issues with tailings pits that are considered at risk of "Rupture" is that
the risk of
them being inherently unsafe, nobody wants to venture out onto the dam or into
the
tailings lagoon and even if they did, they are uncertain how to remove water
and or
de-fluidise the tailings especially towards the centre of the dam.
The present invention relates to a method of dewatering a tailings lagoon
retained by
a dam comprising: excavating an excavation hole in the tailings lagoon;
allowing
water from surrounding tailings to enter the excavation hole; and pumping
water in
the excavation hole out of the excavation hole and discharging beyond a toe of
the
dam. Thus the tailings surrounding the excavation holes are dewatered and the
possibility of hydrostatic pressure acting on the dam is reduced.
In an embodiment a pontoon floats in water in the excavation hole and supports
equipment used for the step of pumping water.
In an embodiment before excavating the excavation hole, excavating a channel
in the
tailings lagoon from a shore of the tailings lagoon to a location at which the
excavation hole is to be excavated and floating the pontoon from the shore to
the
excavation hole along the channel. This is a safe way to get the equipment
necessary
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for excavating the excavation hole and for providing water to the pontoon down
the
excavated channel for density control of the slurry generated from the
tailings being
excavated for the area near the dam.
In an embodiment excavating involves breaking down solid (i.e settled)
tailings in the
tailings lagoon into a slurry using water and removing the slurry using a
slurry pump
mounted on the pontoon. In an embodiment the excavating involves breaking down
solid tailings in the tailings lagoon into a slurry using water and removing
the slurry
using at least two submersible slurry pumps ( or at least one submersible
slurry pump
and at least one water pump ( submersible, horizontal or other type )) mounted
on
the pontoon. This way of excavating is efficient and avoids needing to dig out
tailings, which can be more dangerous and time consuming, particularly if the
water
content of the tailings is high.
In an embodiment during the excavating the water for breaking down the solid
tailings is supplied by the pontoon preferably from a shore supply or from
excess
surface water on the tailings dam. This speeds up breaking down of tailings
and
therefore excavation.
In an embodiment the water supplied by the pontoon is at least partly water
extracted from the channel, which may be supplied from a shore supply or that
the
channel has collected from surface water, on the pontoon and/or excavation
hole by
the pontoon.
In an embodiment during the excavating water is provided to the pontoon from a
shore of the tailings lagoon. This speeds up excavation if water content in
the tailings
at the site of excavation is low.
In an embodiment water is supplied to the channel and/or excavation hole
during
excavation from beyond the tailings lagoon. This speeds up excavation if water
content in the tailings at the site of excavation is low.
In an embodiment the amount of water contained in the slurry is controlled to
be a
certain minimum amount. This ensures that any pipes through which the slurry
is
transported from the pontoon to a discharge location can be prevented from
being
blocked with slurry which has too low a water content.
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In an embodiment the pontoon is controlled remotely from a shore of the
tailings
lagoon and can be run unmanned for significant periods. This enhances safety.
In an embodiment dewatering is localised to tailings adjacent the dam. This is
efficient as it is the tailings closest to the dam wall which must be
prevented from
fluidizing. If the tailings closest to the dam wall have low water content,
the
presence of such tailings helps hold back tailings further up the lagoon away
from the
dam with a higher water content.
In an embodiment the present invention provides a method comprising excavating
a
channel in a tailings lagoon from a shore of the tailings lagoon and floating
a pontoon
in water in the channel from the shore along the channel, wherein excavating
involves
breaking down solid tailings in the tailings lagoon into a slurry using water
and
removing the slurry using a slurry pump mounted on the pontoon. This method
allows
solidified tailings from a tailings lagoon to be broken down into a slurry for
easy
transport of the broken down tailings. The tailings may then be moved to a
different
position and/or reprocessed to extract certain materials contained in the
tailings
which were not previously removed. For example, new techniques may allow
greater
extraction of a material such as a precious metal contained in the tailings
than was
possible when the tailings was originally produced. Alternatively, new
techniques or
changes in the value of materials may mean that the tailings may be
reprocessed to
remove materials not previously removed from the tailings.
All of the techniques described herein relating to excavating the channel in a
dewatering method apply equally to the above described method of excavating a
channel.
The present invention will be described by way of example only with reference
to the
following drawings in which:
Figure 1 is a schematic illustration of a pontoon used in the present
invention.
Figure 2 is a schematic diagram of the delivery of an un-assembled pontoon.
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Figure 3 is a schematic cross-sectional diagram of digging a trench adjacent
the
tailings lagoon shore.
Figure 4 is a schematic cross-sectional diagram illustrating moving of the
pontoon into
position.
Figure 5 is a schematic cross-sectional diagram illustrating the pontoon
loaded into
the trench.
Figure 6 is a schematic cross-sectional diagram illustrating initiating
excavating the
channel.
Figure 7 is a schematic cross-sectional diagram illustrating excavating the
channel.
Figure 8 is a schematic plan diagram of the tailings lagoon illustrating the
positions of
the channels.
Figure 9 is a schematic plan diagram of the tailings lagoon illustrating the
positions of
the channels and excavation holes.
Figure 10 is a schematic plan diagram of the tailings lagoon illustrating the
de-
watering of the tailings adjacent the excavation holes.
Figure 11 is a schematic cross-sectional diagram illustrating de-watering of
the
tailings adjacent the dam wall and the disposal of water from the excavation
holes.
Below is a description of an embodiment to make dams which retain a tailings
lagoon
at "risk" more inherently safe by ensuring the tailings behind the dam, say 25
or more
meters down and up to the surface and up to 100 meters from the dam wall is
not
subjected to high water content and thus the material in the dam above this
level will
not become fluidised and as such will stop exerting hydraulic force on the dam
wall
and will actually assist the dam wall hold back tailings material higher up
the tailings
lagoon. By de-fluidising the tailings from the surface to 25 or more meters
down and
from the dam to 100 or more meters upstream of the dam, it will also ensure
that the
dam itself does not become fluidised. The figures 25m down and up to the
surface
and up to 100m from the dam wall are examples and the method can be used to de-
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water a to a shallower level or to a deeper level and closer or further from
the dam
wall.
If dewatering to a greater depth using a submersible pump, a pressure
compensator
reservoir may need to be fitted to the pump motor housing.
To carry out this dewatering operation, two submersible slurry pump pontoons
can be
used with one submersible slurry pump and one water submersible pump on each
pontoon. These modular section quick build pontoons can be constructed on the
edge
of the lagoon 100 meters upstream of the dam, one on a first (e.g the left
hand
looking towards the dam from below the dam wall) side of the lagoon and one on
the
other (e.g. right hand) side of the lagoon. These pontoons would make a
channel
filled with water through the tailings for themselves to float along by
pumping water
jetting and pumping slurry towards the upper end of the lagoon (e.g. 750meters
up
the lagoon) such that the pontoons end up on either side of the centre line of
the dam
(e.g. 75meters either side) and a distance (e.g. 100 meters) from the dam.
These pontoons would be operational 95 % or more of the time from the shore
and
would be controlled from a shore based control cabin linked to the pontoons
with e.g.
cables or wirelessly, for example by radio signals or industrial WiFi, thus
minimising
the risk of operators being out in the tailings lagoon when there is a risk of
dam
rupture.
Each pontoon having used it's submersible slurry pump with a water submersible
or
other type of water pump and density control system to ensure the
concentration of
solids did not go above a predetermined level (say above 40 % by volume) would
be
used to create in their location an excavated pool (for example 30 to 50
meters in
diameter) that was tapered as it went down (e.g. to a depth of up 25 meters,
or if
needed 50 meters with a pressure compensating reservoir fitted to the
submersible
pump.
This excavation hole (perhaps conical in shape) would naturally find water
draining
into it from the tailings. The water would convert the settled tailings from
anything
between 70 % solids to 40 % solids into slurry. The slurry can then be pumped
out and
up the lagoon using the submersible slurry pump. If there was not enough water
available, which might happen if the creation of the conical hole was done in
the dry
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season, water would be supplied additionally down the channel the pontoon has
created from the shore.
Once the excavation of the two excavation holes has been completed, then the
pontoon and submersible pumps would be used to pump water that flowed into the
excavation holes along a pipe to the downstream side of the dam. This can be
achieved with flexible pipes that are led over the top of the dam. If this
process is
kept running 24 hours per day then dewatering of the dam in the most critical
areas
would be very much improved especially if the water drain pipes from the
bottom of
the dam were blocked.
Also when there is rain water coming down the lagoon towards the dam it would
naturally migrate to the two excavation holes via the channels excavated from
the
shore such that it could be pumped away by the submersible pumps. This process
is
designed to keep the water table in the tailings lagoon near the dam wall
between 10
and 20 or more meters below where it would naturally be and thus prevent
fluidisation of tailings material along the dam wall and near the centre of
the dam
which is its weakest point and also assist in preventing the fluidisation of
the dam
itself as occurred at Bento Rodrigues/Samarco.
Exactly the same techniques can be used for the purpose of reprocessing or
repositioning solid (i.e. settled) tailings. That is the excavation techniques
described
can be used to break down solid (i.e. settled) tailings in a tailings lagoon
into a slurry
and remove the slurry using a slurry pump mounted on the pontoon. That slurry
may
then be reprocessed or repositioned. This technique can be used whilst carving
a
channel in a tailings lagoon or at any desired location within the tailings
lagoon where
an excavation hole is excavated.
The process and apparatus will now be described with reference to the figures.
Figure 1 shows a pontoon 10 which in an assembled state. The pontoon 10 may be
modular so that components can be more easily transported to site. As
illustrated in
figures 2 - 8, one or more transport devises such as trucks can deliver
components to
the site where they are assembled into the pontoon 10.
The pontoon 10 comprises at least one float 20. In the example of figure 1,
the
pontoon 10 comprises two floats 20. These are delivered by truck 100 and
placed
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next to each other (figure 2). Struts are used to connect the two floats 20
together
and at least part of a gap between the floats 20 is covered with centre panels
25.
Various cabins may be secured to the pontoon 10, including, for example, a
control
cabin 31, a crew cabin 32 and/or a toilet 33. A framework 40 is assembled and
secured to the pontoon 10. Pipework, flow meters are also installed onto the
pontoon
10.
A submersible slurry pump 50, such as that available from Goodwin, is
installed onto
the framework 40. In an embodiment a submersible pump 60 is installed onto the
framework 40. The submersible pump 60 may be a slurry pump or a water pump
mounted in some manner to the pontoon 10. In an embodiment the two pumps 50,60
are attached at opposite ends of the pontoon 10. The submersible pumps 50,60
can
be raised and lowered from the framework 40, so that the distance of
extraction by
the pumps 50,60 from the pontoon 10 can be varied. In another embodiment the
submersible pumps 50 and or 60 are mounted in fixed position relative to the
deck of
the pontoon 10 and so are fixed in the horizontal position.
In the method, a trench 110 is dug into the tailings 80 of the lagoon from the
shore
70. The trench may, for example, measure 15m long (10-20m long), 12m wide (5-
18m
wide) and 3.5m deep (1.5 to 5m deep) (figure 3). This trench 110 is filled
with water,
either by pumping water into it, or by allowing the trench 110 naturally to
fill with
water, either through water running into it from outside the lagoon or by
water from
tailings surrounding the trench flowing into it. The edge of the trench 110 by
the
shore may or may not be lined with steel piles. The pontoon 10 is then moved
into
the trench 110, for example by using steel or wooden rollers and/or a crane
(figure
4).Or the pontoon may be built in the trench before the trench is filled with
water.
The pontoon 10 is connected to the shore so that electricity, control signals
and/or
water can be provided to the pontoon 10, depending upon its needs. For example
the
pontoon 10 may have its own generator installed, or may rely on power being
provided from the shore. Water can be provided to the pontoon 10 by providing
water from the shore 70 into the trench 110 (and into the channel 128 and/or
excavation hole 150 described below) which then flows to the pontoon 10. A
pump
(e.g. pump 60) of the pontoon 10 then collects water from under the pontoon 10
for
use by the pontoon 10. The flow of water may be controlled dependent upon the
density of tailings in the water being consumed by the pontoon 10.
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As illustrated in figure 5, water can be pumped (illustrated by 85) onto the
tailings to
start breakdown of the tailings, if necessary. The solid tailings 80 is broken
down into
slurry which fills the trench 110. The submersible slurry pump 50 then removes
the
slurry, for example further up the lagoon using pipes 120. The submersible
water
pump 60 can remove water from the trench 110 to be used by the pontoon 10 to
breakdown the solid tailings 80. The submersible water pump 60 may be located
higher than the slurry pump 50 as the concentration of tailings at lower depth
will be
lower.
The submersible slurry pump 50 may also have its own source of water, and it
expels
water 89 to help breakdown tailings 80 at the location at which it is
extracting slurry.
For this purpose a pipe 91 provides water to the submersible slurry pump 50
and a
separate pipe 120 removes the slurry, for disposal elsewhere. Material
breakdown
and pumping is thereby accelerated by the submersible slurry pump. A jetting
ring
can be fitted to the slurry pump to jet the water against the solid tailings
and thereby
accelerate breakdown. Jetting can also be achieved from a plurality, e.g four
( one
or more ) jets, for example mounted on each corner of the pontoon 10. These
jets
have a dual purpose _ Hydro propulsion, and preferably directional control,
for the
pontoon 10 and jetting of solid tailings 80 in the lagoon.
The water pump 60 on the pontoon 10 and the piping and measuring and control
system on the pontoon 10 is used for controlling the density of the slurry
being
pumped out by the slurry pump 50 (for example via a density control circuit).
For
example the amount of water in jet 89 and/ or jet 85 is controlled. The
tailings in
the lagoon may have a water content of between 20 % and 70 %. If you try to
pump
slurries with water contents below 40 % by volume there is a high chance of
the slurry
pipe blocking. So control is applied to provide water at a rate which ensures
the
slurry being pumped has a water content above, for example 30% or 35% ,
preferably
above 40%. The provision of water to the pontoon 10 (for example along the
channel
128 or through pipes from the shore), is controlled so that the appropriate
water
content in the excavated slurry is present. In an embodiment there may be one
or
more additional optional booster pumps 250 (see figure 1) on the pontoon 10
before
or after the density control circuit (after the density control circuit as
illustrated). In
an embodiment such booster pumps are horizontal pumps. The booster pump 250 is
used to provide additional pressure head to the slurry being pumped to the
upstream
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end of the lagoon ( away from the dam) this allows the slurry to be pumped
further as
it over comes the friction loss in the slurry pipeline. As can be seen from
figure 1, the
density control circuit can involve a water bleed valve 260. The water bleed
valve
260 (a proportional valve) controls the amount of water (e.g. from the water
pump
60) which is added into the pipe 120 along which the slurry pump 50 pumps
slurry.
The density of the slurry in the pipe 120 can be measured at any location
(either side
of where a flow meter 280 is shown in the discharge pipe 120 downstream of the
booster pump 250) and the water bleed valve 260 can be controlled to adjust
the
slurry water density accordingly. In a preferred embodiment the water content
in the
slurry in measured on shore. In an embodiment a current drawn by the slurry
pump
50 and/or booster pump 250 is used in the control as an indication of the
water
content of the slurry being pumped. That is, the measurement of the
density/water
content of the slurry is made based on a knowledge of the current/power drawn
by
the slurry pump for different densities/water contents of slurry (e.g. from a
predetermined look-up table or a mathematical relationship, both determined
experimentally) and the actual current drawn by the slurry pump. In an
embodiment
a measurement of the flow rate of slurry is used in the control as an
indication of the
water content of the slurry being pumped. An alternative or additional way of
controlling the water density in the slurry is to control the amount of water
provided
in water jets 85 from the pontoon 10 and/or in water jets 89 exiting the
slurry pump
50. An increase in water flow in either of those jets results in an increased
water
content in the slurry pumped by the slurry pump 50. In an embodiment the
density
control circuit may additionally or alternatively raise and lower the slurry
pump 50
relative to the pontoon 10. Raising the slurry pump towards the pontoon 10
results in
the water content in slurry increasing because particles in the slurry tend to
sink,
meaning that the water content is greatest near the surface and reduces with
depth.
One or more of these methods or alternative methods can be used to adjust the
water
content of the slurry being pumped from the pontoon, ensuring that pipes 120
leading
from the pontoon 10 do not become blocked with slurry due to the density of
water in
the slurry being too low.
Figure 6 show the pontoon extending the trench 100 into the solid tailings 80.
This is
achieved by continuing to breakdown the solid tailings at a leading edge of
the
pontoon 10. A water jet 87 at the trailing edge of the pontoon 10 can propel
the
pontoon 10 forward into the lagoon. In an embodiment individually controllable
water jets directed to the sides (for example one at each corner of the
pontoon 10)
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can be used to steer the pontoon 10 so that omnidirectional control is
possible. Thus
the pontoon 10 extends the trench 110 to bore a channel 128 towards a desired
point
in the lagoon and floats on water in the channel 128 (figure 7). The flow of
water
from the submersible pump 60 to breakdown the tailings (85) and to propel the
pontoon (87), as well as the operation of the submersible slurry pump are
controlled
in any way by switching valves which may be on the pontoon 10 or located on
the
shore 70. In a preferred embodiment, the control is remote from a control
station
200 on the shore 70, so that for normal operation no operators need to be on
the
pontoon 10.
Figure 8 illustrates the progress of the pontoons 10 from the left and right
of the
Lagoon towards the chosen location for dewatering along the channels 128
excavated
by the pontoons 10.
In the case that the lagoon has a layer of water on top of the tailings 80, it
may not
be necessary to excavate a trench 110 and channel 128 as described above with
reference to figures 3-7. For example, if the tailing 80 has one meter of more
of
water covering it, the pontoon can be floated out to the chosen location (such
as
illustrated in figure 8) at an appropriate area near the dam 5.
Once the pontoons are in position in the lagoon, for example 100m from the dam
wall
(e.g 200 - 20 m from the dam wall, preferably 150-50m from the dam wall) and,
in
the case to two pontoons 10 being used, each 75m from the lagoon centreline
(e.g.
20-100m from the centreline), the pontoons 10 stop excavating the channel 128.
Thus the excavation holes are positioned in the tailings lagoon such that
dewatering is
localised to tailings adjacent the dam wall 5. The pontoons 10 then start to
excavate
holes 150 in the tailings 80 (figure 9). The excavation holes can be of any
size. A size
of about 30m diameter (7-75m in diameter, preferably 10-50m in diameter) and
to a
depth of 10 to 75m, preferably to a depth of 25-50m is thought suitable. The
excavation holes 150 are excavated in the same way as the channels, using
pumped
water to breakdown the solid tailings (for example pumped from the end of the
submersible slurry pump 50) and by lowering the submersible slurry pump 50 as
the
excavation hole gets deeper.
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The depth of the excavation hole 150 is desirably greater than the depth of
the
channel 128 leading to it. In an embodiment the diameter of the excavation
hole 150
is greater than the width of the channel 128 leading to it.
Once the excavation holes 150 are dug in the tailings 80 and the slurry
pumping stops,
water from the surrounding tailings 80 will drain into the excavation holes
150
(illustrated as 160 in figure 10). If necessary additional water can be pumped
into the
excavation holes via the channels (or via a pipe) to assist pontoon 10
stability. Water
drawn from the solid tailings 80 surrounding the excavation holes 150 can then
safely
be discharged beyond the lagoon, for example pumped by either or both pumps
50,60
via pipes 170 up and over the dam wall Sand beyond the toe of the dam ( figure
11).
In this was tailings 80 in the proximity of the dam wall 5 are de-watered, so
that the
tailings cannot fluidize, meaning that the dam wall 5 only has to resist the
force of
the tailings 80 acting on the dam wall 5 and not hydrostatic pressure. For the
case
where a dam is 600m long and the tailings 50m high, the average force of a
fluidised
tailing load is approximately 12.25MN per meter of wall length. This compares
to
800kN per meter of wall in the case of the tailings having low moisture
content,
namely less than 7% of the water loading.
The slurry that will be discharged due to the creation of the channels 128
and/or
dewatering holes 150 upstream of the dam wall 5 may optionally be captured in
geotextile cloth bags to reduce turbidity (cloudiness) of any effluent water
discharged
downstream of the dam 5 whilst capturing the tailings to prevent further
environmental contamination moving the tailings from their present location.
The dewatering holes 150 created to reduce the hydraulic pressure of the dam
wall 5,
once created, may optionally have installed a high volume submersible pump
capable
of handling dirty water, typically they would be similar to those found at the
entrances to underground mines that prevent underground mines flooding during
rainy
seasons or flash rainfall, being able to cope with high volumes of dirty water
(typically from 0% up to 15% solids) on an ad hoc basis to further reduce the
possibility
of any standing water anywhere on the tailings pit.
Although the invention is described using two pontoons 10, any number of
pontoons
can be used. For example, only one pontoon 10 may be used, or three or more
may be used, depending on the desired speed and extent of dewatering and the
size
of the lagoon.
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