Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02742041 2011-06-02
ENHANCED DEWATERING OF SLURRIES
TECHNICAL FIELD
The present invention concerns a method to enhance the dewatering of settling
ponds,
particularly tailings, slurries, soft soils, dredge spoils and the like.
BACKGROUND
The reference to any prior art in this specification is not, and should not be
taken as an
acknowledgement or any form of suggestion that the prior art forms part of the
common general
knowledge.
Tailings, also called slimes, leach residue, or slickens (hereafter
collectively referred to as
"slurries") are the waste materials left over after the mechanical and
chemical processes are
used to extract the desirable fraction from the non-desired fraction of a
mined ore. Slurries,
generally include ground rock and process effluents that are generated in the
mine processing
plant. The slurries are usually in a slurry form (a mixture of fine particles
ranging from the size
of a grain of sand to a few microns and fluid). Slurries sometimes also
include process
additives to enhance settling and consolidation.
In order to prevent the uncontrolled release of slurries into the environment,
mines or other
processing facilities usually have a disposal facility in the form of a
tailings dam or pond
(hereafter collectively referred to as "settling ponds"). This is a convenient
method of storage
because, as previously mentioned, the slurries are usually in the form of a
slurry when they are
discharged from the concentrator.
The integrity of a settling pond is one of the most important environmental
issues for any mine
during the project's life. In many instances, the slurry represent a
significant environmental
hazard containing, for example, uranium or other toxic heavy metals.
Additionally some
processing method utilise compounds such as copper sulfate, xanthate,
hydrocarbons or
cyanide, which will be present to some degree in the slurry and are hazardous
to the
environment. Several major environmental disasters have been caused by
settling pond
failures. However, damage to the environment can also occur without failure of
a settling pond.
CA 02742041 2011-06-02
2
This kind of damage is much less obvious and may take the form of acid
drainage or dry slurry
dust being blown away from the settling pond site.
Generally the lower the settled density of the slurry the greater the volume
and area that is
required to achieve safe and secure storage. This incurs a high capital cost
to match the
environmental risk. In some cases natural dewatering of slurries by
consolidation and
evaporation takes many months or even years. As these slurries have a low
inherent strength
they cannot be expanded or closed to allow commencement of rehabilitation.
Over the last century the volumes of slurry being generated has grown
dramatically as the
demand for minerals and metals has increased and lower and lower grades of ore
are being
mined. This is not surprising when considering that the volume of slurry
requiring storage can
often exceed the in-situ total volume of the ore being mined and processed. As
such, many
techniques have been employed to try and reduce the area required for
effective slurry disposal
or to increase the slurry load of an existing slurry disposal area. Generally
all such techniques
employed involve trying to improve the consolidation and dewatering of the
slurry through the
addition of flocculating compounds. That is, improvements are made before
disposal operations
occur.
Another technique to increase the capacity of a settling pond is called
"upstream construction"
which involves the construction of new parts of the embankment of a settling
pond partially on
top of existing slurry deposits impounded during a previous stage, thus the
dam crest moves
"upstream". As the technique involves the construction over existing slurry
deposits,
foundation strength of the slurry deposits is critical to ensure the long-term
integrity and
stability of the growing embankment. One way to improve the foundation
strength is through
the effective dewatering of slurry deposits.
For the most part, the dewatering of slurries follows accepted and well
understood processes.
In general, these processes are:
- drainage and self weight consolidation (decantation and underdrainage); and
- evaporation.
The changing dynamics and influences of these processes are such that a
failure to manage one
of these processes will in all likelihood prevent effective dewatering due to
the other processes.
CA 02742041 2011-06-02
3
By managing these processes, a means of improving the dewatering
characteristics of slurries
can be achieved after disposal operations.
Thus, there is a need for an alternative method to enhance the dewatering of a
settling pond.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a method to enhance the
dewatering of a
settling pond which may overcome at least some of the abovementioned
disadvantages, or
provide a useful or commercial choice.
According to an aspect of the present invention, there is provided a method to
enhance the
dewatering of a settling pond, said method including:
(a) identifying one or more parameters of the settling pond, said one or more
parameters selected from a group including one or more of initial slurry
density,
target slurry density, initial depth of slurry, and target depth of slurry;
(b) nominally dividing the settling pond into one or more sub areas;
(c) depositing a slurry into the sub areas to a depth equal to the target
depth of
slurry;
(d) allowing the slurry to consolidate under gravity and release fluid;
(e) further consolidating the slurry in each sub area with mechanical means;
and
(f) repeating step (e) periodically until the target slurry density is
reached.
In another aspect of the present invention, there is provided a method to
enhance the dewatering
of a settling pond, said method including:
(a) identifying one or more parameters of the settling pond, said one or more
parameters selected from a group including one or more of initial slurry
density,
target slurry density, initial depth of slurry, target depth of slurry, and
regional
climatology;
(b) preparing the settling pond by:
(i) nominally dividing the settling pond into one or more sub areas, each sub
area having a slurry discharge point and a drainage collection point; and
(ii) constructing a barrier at least partially around each sub area;
CA 02742041 2011-06-02
4
(c) depositing a slurry into the sub areas to a depth equal to the target
depth of
slurry;
(d) allowing the slurry to consolidate under gravity and release fluid;
(e) further consolidating the slurry in each sub area with mechanical means;
(f) repeating step (e) periodically until the target slurry density is
reached; and
(g) monitoring the one or more parameters of the settling pond and repeating
steps
(c) to (f) as required.
Preferably, the barrier may be constructed to a height of at least 120% the
target depth of slurry.
Preferably, step (b) further comprises the sub step of constructing one or
more guidance barriers
adjacent the slurry discharge point, said guidance barriers being adapted to
guide a discharge of
slurry from the slurry discharge point. Step (b) may also further comprise the
sub step of
placing one or more markers in the sub areas, said markers indicating the
target depth of slurry.
Preferably, the slurry may be allowed to consolidate under gravity and release
fluid in step (d)
for between about 24 hours to about 72 hours.
In a preferred embodiment, the mechanical means of step (e) is adapted to
provide low ground
pressure. More preferably, the mechanical means comprises a vehicle adapted to
plough and
consolidate material over which it traverses.
Preferably, step (e) further comprises the sub steps of-
(i) constructing one or more drainage barriers adjacent the drainage
collection
point, said drainage barrier being adapted to collect run-off slurry and fluid
released from the slurry;
(ii) ploughing the slurry in a path from the drainage collection point to the
slurry
discharge point;
(iii) re-ploughing the slurry along said path from the slurry discharge point
to the
drainage collection point;
(iv) repeating sub steps (i) and (ii) about one vehicle width from said path;
and
(v) repeating sub steps (i) to (iii) across each sub area; and
(vi) draining the fluid collected by the drainage barriers, and
wherein each sub area is ploughed at an even speed.
CA 02742041 2011-06-02
Preferably, step (f) comprises repeating step (e) about 2 to about 5 days
until the target slurry
density is reached.
5 In an embodiment of the present invention, a method is provided that
provides the potential to
increase the final density of deposited slurry enabling more slurry to be
deposited in a settling
pond, thereby reducing the area required for effective slurry disposal. The
method also
provides the potential to increase the foundation strength of existing slurry
deposits thus
providing a better foundation for increasing the capacity of an existing
settling pond through
techniques such as upstream construction. Additional secondary advantages
provided are:
- denser slurry deposits reduce the environmental risk of a settling pond
failure by,
for instance, seismic activity;
dewatered slurry can be used as construction material in, for instance,
upstream
construction; and
- reduced operational areas and a more even drying process reduce the risk of
settling pond dust generation as precipitates formed at the surface are
further
consolidated by mechanical means into the drying slurry, and the mechanical
means creates a surface roughness reducing the potential for dust lift off.
As used herein the term "settling pond" refers to any reservoir open to the
atmosphere and
collect slurry-like material. The term encompasses tailings dams, waterlogged
marshes and the
like.
As used herein the term "slurry" and variations such as "slurries" refer to
tailings, slimes, leach
residues, slickens and other like waste material left over after the
mechanical and chemical
processes mineral extraction processes. The term encompasses any material with
slurry-like
properties.
As used herein the term "drainage collection point" refers to the low point in
a sub area or
operational area designated to collect run-off fluid.
As used herein the term "slurry discharge point" refer to the location in a
sub area or
operational area from which slurry is discharged.
CA 02742041 2011-06-02
6
As used herein the term "top of the sub area" refers to an edge of a sub area
or operational area
adjacent the slurry discharge point.
As used herein the term "bottom of the sub area" refers to an edge of a sub
area or operational
area opposite the top of the sub area, the edge being adjacent the drainage
collection point.
In one embodiment of the present invention, step (a) of the method may involve
the
identification of one or more parameters of the settling pond. The method may
involve the
identification of further parameters such as regional climatology and settling
pond personnel
operation parameters such as shift rosters. Preferably, the method involves:
the identification of
one or more parameters of the slurry including initial slurry density, target
slurry density, initial
depth of slurry, target depth of slurry, specific gravity, vertical
permeability of the saturated
slurry, and variability in slurry initial viscosity; identification of one or
more parameters of the
settling pond including area of operation, angles of repose, locations of
slurry discharge points,
locations of drainage collection points and drainage structures; and
identification of one or
more parameters of the regional climatology including median rainfall, median
evaporation and
the relationship between pan evaporation and lake evaporation.
Upon identifying the key parameters, the method according to a preferred
embodiment of the
present invention may then involve step (b), the preparation of the settling
pond. This may
involve nominally dividing the settling pond into one or more operational sub
areas. The
number of sub areas chosen may be dependent upon the size of the settling
pond. The number
of sub areas chosen may then be used to develop a schedule of operations.
Typically, a settling
pond on average is divided into approximately twenty sub areas.
In a preferred embodiment, each sub area may have an associated slurry
discharge point and
drainage collection point. The identification of the slurry discharge point
and drainage
collection point in each sub area establishes an operational axis (the line
between the slurry
discharge point and the drainage collection point).
In dividing the settling pond into a variable number of sub areas,
consideration may be given to:
the ability to access stranded equipment should the need arise (i.e., the sub
areas should not be
too wide).
CA 02742041 2011-06-02
7
In an embodiment of the present invention, a barrier may be constructed to
extend at least
partially around each sub area. The barrier may be of any suitable size, shape
or configuration
and constructed from any suitable material adapted to separate one sub area
and the contents
thereof from adjacent sub areas. The barriers may be engineered structures.
Preferably,
however, the barrier will be prepared from earth materials or previously
dewatered slurry.
Typically, the barriers will only be constructed for sub areas with variable
slurry characteristics
and may be negated for sub areas with minimal variability in slurry
characteristics. The barriers
may be prepared using any suitable means. Typically, the barrier may be
prepared using
mechanical means. Preferably, the height of the barrier will be approximately
120% of the
planned depth of slurry.
Preferably, markers may be placed in each sub area. The markers may be of any
suitable size,
shape or construction adapted to indicate the target depth of slurry. The
markers may be natural
markers (i.e., rocks or logs left over from cleared land). Preferably, the
markers will be
sacrificial. In a preferred embodiment, markers may be placed every 100 m on
either side of
each sub area.
According to a preferred embodiment of the present invention, step (c) of the
method involves
depositing a slurry into the sub areas to a depth equal to the target depth of
slurry. The slurry
may be deposited into each sub area via the associated slurry discharge point.
Preferably, the
deposition of slurry should cease when the slurry front is approximately 25 m
from the bottom
of the sub area. This is to account for the momentum present in the slurry as
the rate of flow of
slurry slows.
Depending on the density and viscosity of the slurry, one or more guidance
barriers may be
constructed adjacent the slurry discharge point, said guidance barriers being
adapted to guide
the discharge of slurry from the slurry discharge point. The guidance barriers
may be of any
suitable size, shape or configuration and may be constructed from any suitable
materials. The
guidance barriers may be engineered structures. Preferably, the guidance
barriers will be
sacrificial structures. In a preferred embodiment, the guidance barriers may
be formed from
earth materials or previously dewatered slurry materials.
Once a slurry is deposited into the sub areas, the deposited slurry under step
(d) may be allowed
to consolidate under gravity and release fluid. Preferably, the released fluid
may then drain via
CA 02742041 2011-06-02
8
the drainage collection point. The slurry may continue to drain fluid until
the slurry reaches
"field capacity" or a density at which excess fluid will stop being shed by
consolidating forces
at a rate equal to the vertical permeability of the slurry. The "field
capacity" is variable
depending on the properties of the slurry. Typically, a slurry is allowed to
consolidate for
between about 24 to about 72 hours.
Upon allowing the deposited slurry to consolidate, in accordance with step (e)
further
consolidation of the deposited slurry in each sub area may be undertaken with
mechanical
means, preferably adapted to provide low ground pressure. The mechanical means
may be of
any suitable size, shape or construction suitably adapted to further
consolidate the deposited
slurry by ploughing the deposited slurry. Preferably, the mechanical means is
a vehicle. Most
preferably, the mechanical means is a Twin Archimedes Screw Tractor adapted to
consolidate
and dewater materials over which it traverses, commonly referred to as a
MudMasterTM
(Residue Solutions Pty Ltd; http://www.residuesolutions.com.au).
In a preferred embodiment, step (e) involves:
(i) ploughing the slurry in a path from the drainage collection point to the
slurry
discharge point;
(ii) re-ploughing the slurry along the path of sub step (i) from the slurry
discharge
point to the drainage collection point;
(iii) repeating sub steps (i) and (ii) about one vehicle width from said path;
and
(iv) repeating sub steps (i) to (iii) across each sub area.
The initial ploughing of a slurry may commence at any location in a sub area.
However,
preferably, ploughing may commence at the edge of a sub area and continue in
an uninterrupted
straight line to the top of a sub area (i.e., the end at which the slurry
discharge point is located).
Ploughing should preferably occur at an even speed and without stopping until
the vehicle
reaches the top end of the sub area. Stopping of the vehicle prior to reaching
the top end may
result in a situation where the slurry will envelop the vehicle making further
progress difficult.
Additionally, stopping places additional stress on the foundation layers and
can lead to bogging
on restart due to the large force required to commence movement.
The initial plough may liberate a large amount of fluid, which will then
collect at the drainage
collection point at the bottom of the sub area. Depending on the
characteristics of each sub area
CA 02742041 2011-06-02
9
it may be necessary to construct one or more drainage barriers adapted to
collect run-off slurry
and fluid released by the deposited slurry. The drainage barrier may be of any
suitable size,
shape or configuration and may be constructed out of any suitable material.
The drainage
barrier may be an engineered structure. Preferably, the drainage barrier will
be constructed
from earth materials or previously dewatered slurry material.
Fluid that is collected by the one or more drainage barriers may be allowed to
drain. The
collected fluid may be allowed to drain by breaching the one more drainage
barriers.
Preferably, the breached drainage barriers are rebuilt prior to further
ploughing. An additional
advantage of preparing for the one more drainage barriers is that they provide
a means of
stilling released fluid to allow suspended slurry material to settle out
thereby minimising the
impact of suspended slurry on liquid management systems.
Once released fluid has ceased collecting at the one or more drainage barriers
and the collected
fluid has been drained, in accordance with step (f) further ploughing may
commence.
Typically, further consolidation of the slurry in accordance with step (f) may
occur about two to
about five days after the previous plough. As before, the ploughing preferably
should
commence from the bottom of each sub area to the top of each sub area. Further
ploughing
may be undertaken between the previous plough lines thereby ploughing
untouched slurry
deposit areas.
Once the entire sub area has been re-ploughed, additional fluid will be
released and collect at
the one or more drainage barriers. As with after the previous plough, the
collected fluid should
be drained.
Typically and in accordance with step (f), step (e) is repeated until the
target slurry density is
reached. Ideally, when further consolidation results in a smooth slurry
surface and the tracking
of the vehicle leaves indentations of less than 10 mm the slurry may be
considered to be
completely dewatered and the sub area can be prepared for a further slurry
deposit.
Typically, routine measurements are made to identify when the target slurry
density is reached.
Measurements may be made using any suitable means. Preferably measurements may
be made
using a hand-held shear vane shear tester together with routine slurry coring
to develop a
density/shear strength curve. Once sufficient measurements have been made the
shear vane
CA 02742041 2011-06-02
measurements may be used to infer the slurry density. An advantage of taking
routine
measurements are that rapid density determinations may be made thus allowing
the forecasting
of future slurry deposition schedules.
5 In parallel with the consolidation process, fluid may also be removed by
evaporation. The
evaporative drying of slurries tends to follow the classic three stage drying
process. In a slurry
environment these stages are evident as the deposited slurry initially
simulates a free water
surface. In the absence of an external energy source, the potential
evaporation rate determined
by ambient climate conditions is the maximum possible evaporation rate. This
stage will
10 continue at close to the potential evaporation rate until the available
fluid content is decreased
and the rate of evaporation is then controlled by the liquid transfer
properties of the slurry. The
third stage commences when the reduction in fluid content reduces evaporative
loss to the rate
of vapour transfer between fluid droplets held in the interstitial voids in
the slurry. A key
difference when comparing the drying of slurry to the drying of the soil is
that the removal of
fluid by evaporation from the entrained fluid will result in the accumulation
of precipitated
impurities. This tends to create a desiccated layer on the surface of the
slurry further reducing
the evaporation rate.
The limiting issue in evaporative drying is not the potential evaporation rate
but the area over
which it is acting. By routinely ploughing a drying slurry, fresh moist
surfaces are exposed and
the surface area increases significantly as opposed to a flat surface. In
addition, by repeatedly
turning over the slurry, any fluid impurities precipitated at the evaporative
surface are
recombined with the slurry and have limited impact on the drying rate. It is
thus possible to
maintain a very high net evaporative loss even as the net evaporation rate
decreases.
The actual evaporation rate is dependent on the properties of the slurry and
is measured rather
than predicted.
As the consolidation process is repeated, dewatering of the slurry leads to
higher densities and
strengths. Within the slurry this imparts a characteristic where the slurry
progressively changes
from a uniform slurry to a crumbling solid. Initially the passage of the
vehicle results in
minimal impact to the slurry other than a slight indentation along the plough
lines that act as a
drain removing fluid and rainfall run-off. After repeated ploughing the plough
lines will begin
to remain open. This process progressively results in an expansion of the
evaporating surface
CA 02742041 2011-06-02
11
area. At its maximum extent this can increase up to about 57% (not including
the increase in
evaporating surface due to the crumbling of the slurry solids). This, in part,
compensates for
the reduction in the evaporation rate (particularly when the slurry moves to
the third stage of
evaporation) resulting in a near constant rate of evaporative loss and a more
rapid dewatering
process.
In yet another aspect of the present invention, there is provided a method to
improve the
foundation of a portion of a settling pond by enhancing the dewatering of the
portion of the
settling pond, said method including:
(a) identifying one or more parameters of the portion of the settling pond in
need of
improved foundations including target slurry density;
(b) consolidating the portion with one or more mechanical means; and
(c) repeating step (b) until the target slurry density is reached.
In step (a) of the method, one more parameters may be identified using any
suitable means
known to a person skilled in the art. The one or more parameters identified
include the target
slurry density and may additionally include identifying the portion of the
settling pond
requiring foundation improvement including access and drainage restrictions
from the adjacent
wall portions of the portion of the settling pond.
Under step (b), consolidation of the portion by ploughing the portion with the
one or more
mechanical means may commence from any location in the portion and may be of
any
particular path suitably adapted to create a surface drain to direct released
fluid from the
consolidated portion to the drainage collection point. Typically, the one or
more mechanical
means are one or more vehicles adapted to provide low ground pressure.
Preferably, the one or
more mechanical means may commence consolidation of the portion by:
(i) ploughing a path in the portion approximately 50 in in length, or as far
as
possible, in a direction approximately 45 degrees from an edge of the portion;
(ii) re-ploughing said path in the reverse direction;
(iii) repeating sub steps (i) and (ii) about one path width from the initial
path; and
(iv) repeating sub steps (i) to (iii) along the edge.
CA 02742041 2011-06-02
12
By repeating sub steps (i) to (iii), above, a "herringbone-like" surface
drainage pattern is
ploughed into the portion, which may direct the drainage of released fluid
into the centre of the
portion, away from the edge.
A low ground pressure vehicle may be tracked across a long the edge of the
portion to ensure
that unimpeded drainage occurs along the plough paths of the portion adjacent
the edge will
become disturbed due to the routine changing of direction of the one or more
vehicles.
Under step (c), once the entire portion has been ploughed along the edge, the
portion may be
further consolidated by re-ploughing the initial plough path thereby deepening
previously
plough paths and then, if the target density is not reached, splitting the
previous plough paths by
ploughing between the previously ploughed paths.
If necessary, the length of the plough path can be extended to increase the
width of the
foundation footprint, primarily to start foundation development further out
away from the edge.
In a preferred embodiment, slurry density is periodically monitored throughout
steps (a) to (c).
Initially there is practically no vane shear strength measurable. This means
that dewatering
activity will need to be maintained even though there will be no changes in
shear strength
detected. As a result, initial monitoring will concentrate on visual changes
to the slurry surface.
These changes will occur due to the dewatering process and are manifested by
the movement of
the fluid from the slurry. The key changes that may be initially observed
include:
- evidence that plough paths are starting to remain in place and do not
collapse after the
one or more vehicles have passed;
- the location/volume of fluid released;
- weather condition and local impacts;
- the drainage pathways from released fluid and rainfall run-off;
- evidence of carbonation or crystallisation of salts from the released fluid;
- operator feedback on machine effort, ride height and directional tracking to
ascertain
underlying slurry consistency; and
- operation of any surface drainage pumping.
CA 02742041 2011-06-02
13
To accommodate these observations a layered monitoring approach may be
required. This may
include:
- daily photography of the operational area;
- weekly vane shear measurements taken every one hundred metres approximately
midway a long the plough path of the one or more vehicles; and
- fortnightly surveying of the foundation area as access permits.
In yet a further aspect of the present invention, there is provided a method
to enhance the
dewatering of a settling pond, said method including:
(a) identifying one or more parameters of the settling pond, said one or more
parameters selected from a group including one or more of initial slurry
density,
target slurry density, initial depth of slurry, target depth of slurry, and
regional
climatology;
(b) preparing the settling pond by:
(i) nominally dividing the settling pond into one or more sub areas, each sub
area having a slurry discharge point and a drainage collection point;
(ii) constructing a barrier at least partially around each sub area to a
height at
least 120% the target depth of slurry;
(iii) constructing one or more guidance barriers adjacent the slurry discharge
point, said guidance barriers adapted to guide a discharge of slurry from
the slurry discharge point; and
(iv) placement of one or more markers in the sub areas, said markers
indicating the target depth of slurry;
(c) depositing a slurry into the sub areas to a depth equal to the target
depth of
slurry;
(d) allowing the slurry to consolidate under gravity and release fluid for
between
about 24 hours to about 72 hours;
(e) further consolidating the slurry in each sub area with mechanical means
adapted
to provide low ground pressure, wherein said mechanical means is a vehicle
adapted to plough the slurry, and wherein the slurry is further consolidated
by:
(i) constructing one or more drainage barriers adjacent the drainage
collection point, said drainage barrier adapted to collect run-off slurry
and fluid released from the slurry;
CA 02742041 2011-06-02
14
(ii) ploughing the slurry in a path from the drainage collection point to the
slurry discharge point;
(iii) re-ploughing the slurry along said path from the slurry discharge point
to
the drainage collection point;
(iv) repeating sub steps (i) and (ii) about one vehicle width from said path;
and
(v) repeating sub steps (i) to (iii) across each sub area; and
(vi) draining the fluid collected by the drainage barriers, and
wherein each sub area is ploughed at an even speed;
(f) repeating step (e) periodically until the target slurry density is
reached, wherein
periodically is about every 2 to about 5 days; and
(g) monitoring the one or more parameters of the settling pond and repeating
steps
(c) to (f) as required.
BRIEF DESCRIPTION OF THE DRAWINGS
Preferred features, embodiments and variations of the invention may be
discerned from the
following Best Modes Of Carrying Out The Invention which provides sufficient
information for
a skilled addressee to perform the invention. The Best Modes Of Carrying Out
The Invention is
not to be regarded as limiting the scope of the preceding Summary Of The
Invention in an way.
The Best Modes will make reference to the following drawing:
Figure 1 is a diagram of a consolidation pattern of a method of the invention
according to a
preferred embodiment.
BEST MODES OF CARRYING OUT THE INVENTION
Example I - a method to enhance the dewatering of a settling pond.
This example describes a method to enhance the dewatering of a settling pond
according to an
embodiment of the present invention.
Step 1
One or more parameters of a settling pond including the initial slurry
density, target slurry
density, initial depth of slurry, target depth of slurry, location of slurry
discharge points,
CA 02742041 2011-06-02
drainage collection points, angle of repose and regional climatology are
identified.
Step 2
The settling pond is then prepared by dividing the settling pond into one or
more sub areas.
5 Each sub area having a slurry discharge point and drainage collection point.
Depending on the
characteristics of the slurry, constructing a barrier formed from earth
materials or dewatered
slurry (if available) at least partially around each sub area. The barrier
being constructed to a
height at least 120% the target depth of slurry.
10 If needed, depending on the characteristics of the slurry, constructing one
or more guidance
barriers adjacent the slurry discharge point for each sub area from earth
materials or dewatered
slurry (if available) to guide the discharge of slurry into each sub area.
The placement of one or more sacrificial markers in each sub area indicating
the target depth of
15 slurry.
Step 3
Depositing slurry via the slurry discharge point into each sub area to a depth
equal to the target
depth of slurry. Ceasing the deposition of slurry into each sub area when the
front of the
deposited slurry is approximately 25 in from the bottom of the sub area to
account for the
residual momentum of the deposited slurry.
Step 4
Allowing the deposited slurry to consolidate under self-weight and gravity and
release fluid for
typically between about 24 hours to about 72 hours. The deposited slurry will
continue to
release fluid until the deposited slurry reaches "field capacity" or a density
at which fluid will
stop being released by the consolidating forces at a rate equal to the
vertical permeability of the
slurry.
Step 5
Further consolidation of the deposited slurry in each sub area commences once
there is no more
fluid being released by the deposited slurry. Further consolidation is
undertaken with a vehicle
adapted to provide low ground pressure (i.e., mechanical means). The vehicle
being adapted to
plough or turn over the upper layer of the deposited slurry. The vehicle
ideally will be a Twin
CA 02742041 2011-06-02
16
Archimedes Screw Tractor specifically adapted to consolidate and dewater the
materials over
which it traverses, commonly referred to as a MudMasterTM (Residue Solutions
Pty Ltd;
http://www.residuesolutions.com.au).
Prior to commencing to plough each sub area, one or more earthen drainage
barriers should be
constructed at the bottom end of each sub area, the end opposite the slurry
discharge point and
adjacent the drainage collection point. The drainage barriers should be
constructed from earth
materials or dewatered slurry (if available) and be constructed in such a
manner as to collect
fluid released from the consolidated slurry and any run-off rain or slurry.
The deposited slurry should be initially ploughed in a direct path from the
bottom end where the
drainage collection point is located to the top end where the slurry discharge
point is located.
Once the vehicle reaches the top end, the vehicle turns around ploughs a fresh
path toward the
bottom end approximately one vehicle's width (- 4 m) from the initial plough
path. This
process is repeated until the whole sub area has been ploughed.
When ploughing it is important a constant speed is maintained and that the
vehicle only ever
stops at the top end of the sub area. This ensures the vehicle does not bog in
the slurry.
A large amount of fluid, often called "bleed" water, will be released by the
further
consolidation of the deposited slurry. This released fluid is allowed to
collect in the one or
more drainage barriers. Prior to commencing further ploughing this collected
fluid should be
drained by simply breaching the drainage barriers with for instance a swamp
excavator. The
drainage barriers are repaired prior to further ploughing.
Step 6
Step 5 is then repeated along the same plough paths or by splitting the
previously ploughed
paths by ploughing between the previous plough paths. The slurry density is
periodically
measured to monitor when target slurry density is reached. The slurry density
is measured by
using a hand-held shear vane shear tester together with routine slurry coring
to develop a
density/shear strength curve. Once sufficient measurements have been made the
shear vane
measurements can be used to infer the slurry density.
Example 2 - a method to improve the foundation of a portion of a settling pond
CA 02742041 2011-06-02
17
This example describes a method to improve the foundation of a portion of a
settling pond
according to an embodiment of the present invention.
Step I
Identifying one or more parameters of the portion of the settling pond as
outlined in Step I of
Example 1, above, including identifying the portion of the settling pond in
need of improved
foundations and the target slurry density.
Step 2
Consolidating the portion with a vehicle (i.e., mechanical means) as indicated
in Step 5 of
Example 1, above.
Referring to Figure 1, the consolidation of the portion 2 of the settling pond
I comprises the
process of ploughing a path 4 approximately 50 m in length (or as far as
possible) in a direction
approximately 45 degrees from an edge 6 of the portion 2. The vehicle then re-
plough the same
path in the reverse direction (i.e., back towards the edge). This process is
then repeated
approximately one plough width (w; i.e., approximately 4 m) along the edge 6
until the entire
edge 6 of the portion 2 has been ploughed.
As a result of the consolidation process a "herringbone-like" surface drainage
pattern 8 is
formed, which is adapted to drain fluid released from the consolidation of the
slurry along the
plough paths 4 away from the edge 6 of the portion 2.
A low ground pressure swamp excavator can be used to track across the portion
2 adjacent the
edge 6 to ensure that unimpeded drainage occurs along the plough paths 4,
which will have a
propensity to be disturbed by the continual changing in direction of the
vehicle.
CA 02742041 2011-06-02
18
Step 3
Step 2 is then repeated to re-plough the same plough paths thereby deepening
the plough paths.
As with Example 1, the slurry density is periodically measured to monitor when
target slurry
density is reached. Depending on how far away from the edge 6 the portion 2 of
the settling
pond I is located in need of improved foundations, the initial plough paths of
Step I can be
extended further away from the edge 6.
If the target slurry density is not reached, the previously re-ploughed paths
are then split by
ploughing between the previously ploughed paths. Step 3 is repeated until the
target slurry
density is reached.
A skilled addressee will appreciate that many embodiments and variations can
be made without
departing from the ambit of the present invention.
In compliance with the statute, the invention has been described in language
more or less
specific to structural of methodical features. It is to be understood that the
invention is not
limited to specific features shown or described since the means herein
described comprises
preferred forms of putting the invention into effect.
Throughout the specification and claims, unless the context requires
otherwise, the term
"comprise", or variations such as "comprises" or "comprising", will be
understood to apply the
inclusion of the stated integer or groups of integers but not the exclusion of
any other integer or
group of integers.
