Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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TITLE: ELECTROKINETIC PROCESS AND APPARATUS FOR
CONSOLIDATION OF OIL SANDS TAILINGS
FIELD OF THE INVENTION
This invention relates generally to the broad field of pollution
control. More particularly, this invention relates to methods and apparatus
that can be used to mitigate the persistent nature of certain types of
tailings ponds, such as tailings ponds filled with waste products from tar or
oil sand recovery processes. Such mitigation is for, among other things,
the purpose of allowing land reclamation to occur.
BACKGROUND OF THE INVENTION
Oil or tar sands are a source of bitumen, which can be reformed
into a synthetic crude or syncrude. At present a large amount of
hydrocarbon is recovered through surface mining. To obtain syncrude,
the hydrocarbons must be first separated from the sand base in which it is
found. This sand based material includes sands, clays, silts, minerals and
other materials. The most common separation step used on surface
mined tar sands is the hot water separation process which uses hot water
to separate out the hydrocarbons. However, the separation is not perfect
and a water based waste liquid is produced as a by-product which may
include small amounts of hydrocarbons, heavy metals and other waste
materials, but is mostly a stable colloidal mixture of water and clay, and
other materials. This is waste liquid is called Mature Fine Tailings (MFT)
and is collected in onsite reservoirs called tailings ponds.
Oil extraction has been carried out for many years on the vast
reserves of oil that exists in Alberta, Canada. It is estimated that
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750,000,000 m3 of MFT have been produced. Some estimates show that
550 km2 of land has been disturbed by surface mining, yet only 267 ha
(less than 0.5%) has received certification as being reclaimed. Even this
small area was not mined, nor used for associated processing operations,
but was only used for the storage of overburden.
The MFT ponds present three environmental and economic issues:
water management, sterilization of potentially productive ore and delays in
reclamation. Although concentrations vary, MFT can typically comprise
50 to 70% water. This high water content forms, in combination with the
naturally occurring clays, a thixotropic liquid. This liquid is quite stable
and persistent and has been historically collected in large holding ponds.
Very little has been done to treat the MFT that has been created and so it
continues to build up in ever larger holding ponds. As development of the
tar sands accelerates and more and more production is brought on line,
more and more MFT will be produced. What is desired is a way to deal
with the MFT that has been and will be generated to permit land
reclamation, a way to release captured water and to provide access to the
productive ore located beneath such ponds.
MFT represent a mixture of clays (illite, montmorillonite and
kaolinite), water and residual bitumen resulting from the processing of oil
sands. In some cases MFT may also be undergoing intrinsic
biodegradation. The biodegradation process creates a frothy mixture,
further compounding the difficulty in consolidating this material. These
clays, most particularly, sodium montmorillonite found in MFT are
expansive; i.e., volumetric changes of as much as 30% can occur
between wetting and drying. It is estimated that between 40 and 200
years are required for these clays to sufficiently consolidate to allow for
reclamation of tailings ponds. Such delays will result in unacceptably
large volumes of MFT, and protracted periods of time before reclamation
can take place unless a way to effect disposal and reclamation is found.
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It is known that the application of an electrical field to a dielectric
material results in certain electro-kinetic phenomena, including electro-
osmosis, the movement of water from an anode to a cathode;
electrophoresis, the movement of ions in the water to oppositely charged
electrodes and electrostriction, a result of the application of an electrical
field that results in mechanical work which deforms the dielectric material.
Electro-osmosis has been used to dewater solid or consolidated clay soils
for construction projects to improve bearing capacity. Electrophoresis has
been used in many industries, such as the pharmaceutical industry and
ceramics industry to produce high grade separations. Electrostriction has
been used on a small scale to create high density ceramics. In a
electrical resistance heating treatment at Fargo, ND (Smith et al., 2006)a,
where the applied electric field ranged between 0.46 to 0.8 volt/cm an
electro osmotic phenomenon was observed with AC current. Examples of
applications of electrical fields in various circumstances can be found in
the following prior patents.
United States Patent No. 3,962,069
United States Patent No. 4,107,026
United States Patent No. 4,110,189
United States Patent No. 4,170,529
United States Patent No. 4,282,103
United States Patent No. 4,501,648
United States Patent No. 4,960,524
United States Patent No. 5,171,409
United States Patent No. 6,596,142
a Smith, G.J., J. von Hatten, and C. Thomas (2006) Monitoring Soil
Consolidation during Electrical
Resistivity Heating. Proceedings of the Fifth International Conference on
Remediation of Chlorinated and
Recalcitrant Compounds, May 22-25, 2006, Monterey, CA,
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The application of electrical current to treat oil sands tailings has
also been tried, as shown in U.S. Patent 4,501,648. However, this
teaches a small device with a tracked moving immersed electrode onto
which is deposited clay solids. The electrode is moved out of contact with
the liquid and then the solids are scraped off the electrode. A chemical
pre-treatment step is required to achieve the desired deposition rate on
the immersed electrode. While interesting, this invention is too small to
be practical for MFT treatment and requires a chemical pre-treatment step
which adds to the cost. What is desired is a better way to deal with vast
volumes of MFT that will need to be treated.
SUMMARY OF THE INVENTION
According to the present invention, the consolidation of solids
present in MFT occurs in under the application of an electrical field. The
application of an electrical field causes contemporaneously or pana
contemporaneously one or more of the following: (a) dispersion of
particles in a flocculation step with an accompanying release of water, (b)
release of pore water and pore water pressure during the residual
consolidation of the solids, or (c) electrostriction, whereby the flocked
material is compressed under the application of electromotive forces.
MFT, in its original state being a thixotropic liquid cannot support a
load, and given that the liquid is stored in large ponds, there is virtually
no
ability to release pore water pressure by conventional means, such as
compressive loading. Therefore, the present invention provides for a
reduction of the moisture content of the solids such that it is no longer a
thixotropic liquid, preferably by the application of an electrical field to
induce flocculation, releasing pore water and pore water pressure and
then to compress the MFT to express further pore water from the solids to
increase the density to increase the lithostatic loading. An aspect of the
present invention is to provide a mechanism for relief of pore pressure to
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accelerate the consolidation of the solids.
The present invention provides the placing of equipment to allow
the generation of an electrical field (AC, DC, or EM-induced) having a
voltage gradient that can be varied resulting in both electrokinetic floccing
of the MFT and a further consolidation of the flocculated or weakly
consolidated solids. The amount of consolidation provided can be varied
by either the duration of application and/or the magnitude of the voltage
gradient to achieve a desired bearing capacity for the MFT. An
appropriate magnetic force can also be applied to accomplish the same
goals and is comprehended by the present invention although the
electrical field is most preferred.
According to an aspect of the present invention, the electrical field
neutralizes the electrostatic charges on the clay platelets, releasing water
from the MFT pores during an initial flocculation step. Over time the
flocculated solids will settle into a weakly consolidated mass. The
electrical field also creates electro-osmotic flow to the cathodes, where
water can then be pumped away to a location where it can be optionally
treated and recycled. This can also assist further consolidation.
Alternately, or in combination, the use of sand drains, wick drains, or the
like facilitates the release of water from within the weakly consolidated
MFT deposits, relieving pore pressures, further enhancing consolidation.
An electrostrictive force can be applied in varying degrees to achieve the
desired bearing capacity in desired zones of the MFT deposits or, to
simply achieve a consolidation level sufficient to permit effective use of
sand drains, wicks and the like to complete the consolidation process.
Consolidation in active tailings ponds may still be desirable even if
certified reclamation is not desired, because for instance, greater storage
capacity can be achieved.
Therefore, there is provided, according to an embodiment of the
present invention, a method of compacting solids in situ in an oil sands
extraction tailings pond, the method comprising the steps of:
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a) Placing at least two electrodes into the tailings pond in a
predetermined spacing;
b) Connecting the electrodes to a source of electrical power
having a variable voltage, wherein an electrical field of sufficient
strength is established between said at least two electrodes to
induce flocculation of particles in said oil sands tailings and to
simultaneously release water; and
C) Compacting said flocculation solids and removing further
water released from said compacting solids to create a load
bearing material.
In another embodiment of the present invention, there is provided,
an electrode for use in a method of compacting solids in an oil sands
extraction tailings pond, the electrode comprising:
a) A connector to electrically connect said electrode to a
source of power;
b) An electrically conductive body having a size and shape to
permit said body to be inserted into said tailings pond and to
extend below and above said tailings;
C) A means to electrically isolate a portion of said electrode
which extends above said tailings pond.
In another embodiment of the present invention, there is provided,
a method of treating a layer of a tailings pond comprising the steps of:
Providing a cable electrode which can be submerged to a
desired depth;
Position the electrode within the tailings pond at the depth of
the layer to be treated;
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Positioning at least one other electrode at the same depth at
a location remove from the first electrode; and
Connecting the electrodes to a source of power to
encourage flocculation to occur at the depth that the electrodes are
submerged within the tailings pond.
In another embodiment of the present invention, there is provided
an apparatus for use in a method of compacting solids in an oil sands
extraction tailings pond, the apparatus comprising:
a. A network of uniformly-spaced electrodes;
b. A connector to electrically connect said plurality of
electrodes to an AC power source; and
c. A means to support the network of electrodes in said tailings
pond.
In another embodiment of the present invention, there is provided a
method of treating oil sands tailings having at least dispersed fine solids,
water and residual hydrocarbons, said method comprising:
a. Placing a network of electrodes into a volume of oil sands
tailings in a treatment area;
b. Connecting the network of electrodes to a controlled source
of electrical power;
c. Applying an electrical field to said treatment area through
said electrodes by means of a controlled application of power
from said source of electrical power; and
d. Using said electrical field to consolidate said fine solids
within said treatment area.
In a further embodiment of the present invention, the electrical field
applied during the electro-kinetic treatment can be varied at different
depths. For example, by applying the electrical field to the deepest
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depths of the MFT deposits causes the clay particles to flocculate there
first. Afterwards, the conductive zone of the electrodes which creates the
electric filed can be raised to higher elevations to encourage lithostatic
consolidation at a different depth. Alternatively, for especially thick MFT
deposits, the operator may wish to induce flocculation in the deeper
deposits of MFT, and then compact a shallow zone in an amount
sufficient to achieve a 5 kPa bearing capacity. This area could then be re-
covered with overburden to enhance the consolidation of the compacted
treated depths through the use of sand drains or wicks or the like, while
re-vegetation can occur on the replaced overburden.
In a still further aspect of the present invention the flocculation step
and the subsequent consolidation step both involve the release of water
from the thixotropic liquid. If this free water is removed from the tailings
pond for further processing and clean-up, that frees up space in the pond
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for additional MFT to be added. As a result the present invention provides
for a way to increase the capacity of the tailings pond to accept more
MET, by the separation and removal of water content from the MET.
BRIEF DESCRIPTION OF THE DRAWINGS
Reference will now be made to preferred embodiments of the
invention, by way of example only, with reference to the following figures
in which:
Figure I a is a graph depicting an estimation of pressure at depth
for a sample tailings pond;
Figure lb is a graph depicting an estimate of lithostatic pressures
resulting from an electrostriction treatment according to the present
invention at various depths;
Figure 2 is a depiction of a graph showing a change in pressure
with electrical field variance according to the present invention;
Figure 3 is a layout of electrodes in a three spot treatment pattern
according to the present invention;
Figure 4 is a schematic of a further electrode layout with a neutral
pumping well according to a further aspect of the present invention;
Figure 5 is a tubular electrode connection according to the present
invention;
Figures 5a and 5b are enlarged views of a portion of Figure 5.
Figure 6 is an enlarged view of an alternate connection;
Figure 7 is a schematic of a drain of the type that can be used in
the present invention;
Figure 8 is a schematic of a first embodiment of a combined
cathode well structure;
Figure 8a is a top view of the embodiment shown in Figure 8;
Figure 9 is a schematic of a second embodiment of a combined
cathode well structure; and
Figure 10 is a schematic of a variable depth electrode according to
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a further aspect of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
In this specification the term MFT shall mean the tailings that exist
in tailings ponds that arise from the extraction of hydrocarbons, such as
bitumen, from tar or oil sands. As will be appreciated by those skilled in
the art, the exact composition of MFT will vary, depending upon the
composition of the ore being mined due to local variations in such ore.
However, as used herein the term is intended to include compositions of
material that include water, clays, silts, and residual hydrocarbons and
hydrocarbon by products among other things.
The present invention comprehends the application of an
electromagnetic field and most preferably an electrical field to the MFT.
According to one aspect such an electric field can be used to exert a force
on the solids present in the MFT due to electrostriction. Electrostrictionb is
a property of dielectric materials, and is caused by the presence of
randomly-aligned electrical domains (e.g., clay platelets) within the
material. When an electric field is applied to a dielectric material such as
clay particles, the opposite sides of the domains become differently
charged and attract each other, reducing material thickness in the
direction of the applied field, and simultaneously increasing thickness in
orthogonal directions due to Poisson's ratio'. The resulting strain (ratio of
deformation to the original dimension) is proportional to the square of the
polarization (i.e., the voltage gradient). Reversal of the electric field
(e.g.,
A phenomenon first reported by Reuss in 1807 to the Moscow Academy of Science
c When a material is compressed in one direction, it usually tends to expand
in the other two
directions perpendicular to the direction of compression. This phenomenon is
called the "Poisson
effect". Poisson's ratio v is a measure of the Poisson effect.
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under the application of alternating current) does not reverse the direction
of the deformation. Therefore, the same phenomenon is observed under
a magnetic field, DC or AC currents, and under electro-magnetically-
induced current flow, again, either alternating or direct all of which are
comprehended by the present invention.
The electric force density under an applied electrical field to induce
electrostriction is governed by the square of the electrical gradient. From
Brevik (1982)d, to determine the electric force density te', one can make
use of the Helmholtz variational principle under reversible, isothermal
conditions. From this, fei is defined as:
, 1 1
= , ¨V E-,
p
2 2 _ "P T_
Where:
V refers to the vector in the direction of the application of the field
p = mass density (kg.rn-3);
E = permittivity (s4.A2-m-2.kg-1);
E = electric gradient (volt.m-1); and the system is operating at
constant temperature.
The second term in this equation is the electrostriction term.
According to the present invention the application of a preferred
electrical field to create an electrostrictive force on the material first
results
in flocculation of the clay particles. This releases water that was
otherwise bound to the clay particles to form the persistent gel or
d Brevik, I. (1982). Fluids in electric and magnetic fields: Pressure
variation and stability.
Canadian J. Physics, 60, pp 449-455.
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thixotropic MFT liquid. Once flocculation has occurred, the present
invention provides for further water release and consolidation of the clay
solids as explained in more detail below.
In one aspect of the present invention the further consolidation of
According Melloni, et al., (1998)e, the change in density under an
applied electric field can be determined from:
1 2
15 AP = ¨2 ffneo E
Where:
Ap = the change in density under the applied electrical field (kg.m-
3)
p = the density of clay (kg.m-3)
20 C = the compressibility of clay (1)/0)
Ye = electrostriction coefficient (unit-less)
. -.
E0 = dielectric constant (permittivity; s4.A2 m2 kg1- ) for clay
E = electric field (volts.rn-1)
Melloni, A., M. Frasca, A. Garavaglai, A. Tonini and M. Martinelli (1998).
Direct Measurement
of Electrostriction in Optical Fibers. Optics Letters, Vol. 23, No. 9. p 691-
693.
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The electrostriction coefficient used was 0.902 (Melloni, 1998).
One known dielectric constant for montmorillonite is 4.2 0.8 (Ishida, et
al.,2000f). The permittivity of water is 80.37g. Therefore, for MFT which
comprises 50% to 70% water content, the estimated permittivity for MFT
is expected to range between 43.1 and 58.7 s4.A2. m-2. k _
g MFT are
reported to typically have between 50% and 70% water (by weight) but
this is an estimated range only and the present invention can be applied
to materials having either higher or lower water contents without departing
from the scope of the invention. One value for the specific gravity of
montmorillonite is about 2.35 g/cm3 (Webminerals.com; unknown water
content) or 2,350 kg/m3. Commercial bentonite has between 5% and 8%
water by weight. The difference between soft and stiff clay can be less
than 1%. However, water content is not an accurate measure of
stiffness. Solubles present in the clay, or variations in the electrolyte in
the
water supply can cause floccing or defloccing of clay. Typical malleable
clays will have a water content of between 19.5 and 22.5%.
The desired water content for reclamation soils is in the order of
15% to 18%, based on natural water content (by weight) in clay soils.
Therefore, the desired density is between 2,107 to 2,148 kg/m3, with the
flocced MFT density estimated to range between 2,046.25 to 2086.75
kg/m3. Therefore, Ap is 20.25 to 101.75 kg/m3.
The following relationship equates the applied electrical field to the
electrostriction force in Pa:
(t, \2/
Ap=E2
ue
Ishida, T., M. Tomoyuki, and C. Wang (2000) Dielectric-Relaxation Spectroscopy
of Kaolinite,
Montmorillonite, Allophone and Imogolite under Moist Conditions. Clays and
Clay Minerals, Vol. 48, No, 1,
75-84.
g Weast, Robert, C. (ed; 1975). Handbook of Chemistry and Physics. 56th
Edition, CRC Press,
Cleveland, OH.
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Using the values from above, the electrostrictive force to achieve
the desired reduction in moisture content and the associated degree of
consolidation is estimated to be between 15.1 and 18.7 kPa. From the
above relationship to achieve these forces, the applied electric field is
estimated to range from 0.035 volt/m to 0.039 volt/m (within the linear
range of the equations describing electrostriction). Moisture content is not
always a reliable factor in clay stiffness, but provides a reasonable
determination of the amount of compaction required.
The greater the applied electric field, the greater the applied force,
the shorter the time period to achieve the desired degree of
flocculation/compaction, or the greater the degree of compaction that can
be achieved. It will be now understood by those skilled in the art that the
present invention can be applied in various intensities, depending upon a
balance of cost, timing and degree of compaction required. The design of
the delivery system and equipment for the electrical energy can be based
on the balance required between speed, cost and result required in the
tailings pond being reclaimed. For example, the present invention
provides that a step down transformer may be used to convert line
voltages to distribute power to a network of electrodes fully penetrating
the MFT to induce an electrical field resulting in a force within a
predetermined appropriate range.
Turning now to the figures, Figure la depicts in schematic form the
pressure-depth relationship in a notional tailings pond filled with MFT. In
Figure la the x axis is pressure and the y axis is depth. The line 10 is
hydrostatic pressure, the line 12 is the pressure at 70% water content
MFT and line 14 is the pressure at 50% water content MFT. As can be
seen all of the lines are straight meaning that the pressure varies linearly
with depth (assuming water is a non-compressible fluid at a constant
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temperature and there is negligible densification of the MFT). Figure lb
is a schematic of the pressure distribution with depth after an electro-
kinetic treatment according to the present invention, where there is a 30%
reduction in MFT volume as a result of the electro-kinetic treatment of the
present invention. In figure 1 b, because the clay in the MFT has been
flocced according to the present invention, the MFT is now denser and
there has been a gravity-separation of the water from the flocked particles
within the MFT. In Figure lb the line 16 is the hydrostatic pressure and
the lines 18 and 20 represent the pressures at depth for reduced water
content solids, such as solids having 15% water content in line 18 and
18% water content in line 20. These water contents are expressed as a
percentage of the total weight.
As can now be appreciated the pressure profile of Figure lb results
in greater lithostatic pressure with depth than is shown in la. Therefore
the present invention provides a step-wise advance in consolidating the
solids within the MFT, with these steps providing options as the treatment
progresses. The invention involves a process and apparatus to create
and apply an electrical (or magnetic) field with a voltage gradient that is
maintained over a treatment period, and then providing for release of pore
water to increase the density of the material (Figures la, and 1 b) while
the material consolidates.
In general there are two main aspects to the present invention.
The first part is to place the necessary equipment in position to deliver the
desired electrical field to the MFT. This is explained in more detail below.
The second aspect is to identify what happens to the MFT once the
electrical field is applied in a treatment process according to the present
invention. The first result of the application of the electrical field
according
to the present treatment process is that the MFT will begin to flocculate
and gravity separate. After this has occurred, the operator has the option
to continue with electrostriction (described below) or allow the MFT to
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consolidate assisted by such techniques as sand drain, wick drains, etc.
This may be useful to the operator where the tailings pond is in operation
and he wishes to increase capacity to accept additional tailings. This
feature of drain-assisted consolidation further enhances and takes
advantage of natural consolidation started by the application of an
electrical field.
As noted above, after the flocculation step the further application of
the electrical field allows for further application of an electrostriction
force,
which is converted to mechanical work. The relationship between the
applied voltage gradient and the electromotive force is depicted in the
range between 0.001 to 1 volts/m in Figure 2. Figure 2 shows a
schematic relationship between a change in the applied electrical field
and the pressure. In this graph the change in pressure is plotted along
the y axis and the change in electrical field is plotted on the x axis. As
can be seen from the plot line 22, the greater the electrical field the
greater the pressure. Of course there is a limit of how much electrical
energy can be applied.
Generally, since the higher the voltage gradient, the greater the
electromotive force, and as a result, the shorter the treatment time.
However, there are two negative factors in applying a higher gradient: 1)
the current density around the electrodes increases, resulting in "dry-out"
and loss of electrical contact with the pore water carrying the current; and
2) the greater the gradient, the closer electrode spacing, and increased
apparatus costs. The voltage gradients and number and spacing of
electrodes need to be evaluated on a case-by-case basis to determine the
most economical design compared against the timeframe for treatment.
Having described the action of the present invention on the MFT
the apparatus used to effect such action can now be described. The
preferred embodiment of this invention involves the use of a variable
voltage power supply connected to a network of electrodes. Where the
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power source is an AC source, the electrodes are arranged in a triangular
(Figure 3) or hexagonal pattern (Figure 4). In figure 3 there are three
electrodes denoted with the numbers 1, 2, or 3. These electrodes would
be charged at 120 degrees out of phase with one another, with the phase
charge varying with time. According to the present invention, the spacing
between electrodes and the desire voltage gradient is determined through
the conductivity of the pore water in the thixotropic liquid, the desired
degree of consolidation and time to achieve, the volume and geometry of
the treatment volume, and the capability of the power supply.
Figure 4 shows an embodiment of an apparatus for applying an
electrical field to induce a voltage gradient across the area to be treated,
or subsections of the area to be treated. There are six electrodes shown
as El to E6 respectively in a regular hexagonal pattern. A source of AC
power 40, is shown and connected by electrical conductors 42, 44, 46, 48,
50 and 52 to each electrode in turn. As will be understood by those
skilled in the art, each of the electrodes El through E6 will be charged at
60 degrees out of phase with the adjacent electrode, with the phased
charging varying with time. This results in a maximum electrical field
being generated across the long diagonals of the hexagon (e.g. El to E4),
where the electrodes are 180 degrees out of phase (Note: Electrodes E2
to E5 are also 180 degrees out of phase, as are electrodes E3 to E6, and
so on). The larger electrical field will have the effect of causing the
greatest flocculation, at first, and then electrostriction, later, also across
the longest diagonals. This phased charging is also charged sequentially
with time to ensure even application of the electrical field. Thus the
hexagonal pattern noted provides for a useful pattern for applying the
desired electrical field across a substantial area for an AC power source
40.
The AC power source 40 will be provided with a power controller to
permit the voltages being applied to be controlled. Most preferably it
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provides a six phase for the hexagonal geometry and a three phase time
distributed and interphase synchronization power control for the three
phase geometry. While the present description is with respect to an AC
power source, the present invention comprehends the use of a direct
current, or electro-magnetically induced current using a variable voltage
transformer as well. The voltages applied are to be determined based on
the most economic use of electrodes (number and spacing) and the
capabilities of the power supply, but the hexagonal pattern is believed to
provide good results for illustration of an AC application where the volume
of MFT to be treated has simple geometry approximating a cylinder. The
desired voltage supplied by the transformer is dependent on the spacing
of the electrodes, and the conductivity of the interstitial water in the MFT,
which will vary during the treatment as electrophoresis causes the
movement of ions in the pore water. Therefore, the present invention
provides that the voltage applied may be adjusted throughout the
treatment period to respond to changes in the electrical field resulting
from changes in the electrical properties of the MFT as the treatment
progresses. The present invention contemplates that the transformer will
be kept in a safe locked housing and operatively connected to a portable
computer with remote access communication features, such as for
example through a cellular network communications grid.
This
combination permits remote monitoring and access to operate the system.
According to a further aspect of the present invention, the electrical
field generating equipment will include the capability of monitoring the
electrical conductivity of the pore water, both overall and throughout the
treatment area. Overall, the electrical conductivity will be monitored
through variations in current draw at the transformers. Throughout the
treatment area, small diameter slotted CPVC tubing embedded in the
MFT will allow for periodic conductivity measurements to track and
optimize the application of the electrical field.
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Also shown is a neutral electrode 54 located at the center of the
hexagonal spacing of the electrodes. According to one embodiment of
the invention this electrode can also function as a water recovery device.
In this case a pump 56 is used to draw the water out of the hexagon,
through a conduit 58. This water is the water that is freed from the gel by
the flocculation step outlined above. The reclaimed water can then be
optionally treated and recycled as desired using conventional processes.
According to the present invention, these electrodes El to E6 can
be constructed using steel pipe, steel rods, sheet metal pile, electrically
conductive plates suspended on electrical cable or any other electrically
conductive or electro-magnetic material. The electrodes are placed in
position by driving, drilling, using conventional drilling equipment, or pile
driving equipment.
Figure 5 shows an electrode 58 according to one aspect of the
present invention. The electrode includes an electrical connection wire 60
which connects to an electrode head connection 62. The electrode itself
is in the form of hollow metal tube or pipe 64. Although the power
supplied is very low and thus it might not be required, there is also shown
an optional non-electrically conductive sleeve 66 to protect against
accidental electrical shocks to people or the like. The sleeve 66 can be of
any reasonable length but is preferred to provide enough freeboard above
the level of the tailings pond that the electrodes do not become totally
submerged in the pond. The electro.de is most preferable driven into the
MFT below the pond to ensure that it is stable during the treatment
process with sufficient depth of penetration into the subsurface to be
anchored in material below the 30 percent expected reduction in volume
plus an appropriate factor of safety to maintain a stable installation. This
installation depth is thought to provide adequate results in most cases.
The depth of the bottom of the electrode 64, in some embodiments, may
be driven nominally 3 m into native soil. The non-electrically conductive
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sleeve 66 may, in some embodiments, extend to a projected depth to
which MET will consolidate plus a safety factor of 10 m. In figures 5a and
5b there is shown the details of the electrical head connection which can
take the form of a welded flange 70 with a bolt hole 72 for electrical
5 connection. In these figures the flange 70 is welded to the side of the
pipe 64 and the pipe 64 has closed capped top. In an alternate
embodiment of Figure 6 the welded bolt connection 74 is placed centrally
on a cap 76 which covers the open top of the pipe 64.
The present invention comprehends that it may be desirable to
10 remove supernatant water and or water being electro-osmotically drawn
towards the cathode in certain circumstances. In some cases it may be
desirable to leave the water in place, above the flocculated solids, as a
means to provide access to the treatment area by floating barge or the
like, but in other cases, where it is desired to create more room in the
15 pond for fresh tailings the water may be removed. In addition the
present
invention contemplates the use of a wick or drain to help remove
additional pore water from consolidating solids within the pond. An
example of such a drain 88 is depicted in Figure 7, in which the hollow
skeleton 90 supports a water permeable mesh 92. Essentially this drain
20 provides a leak path for pore water to be expressed through the
consolidation process. A Mebradrain, as sold by Cofra, Kwadrantweg 9,
1042 AG Amsterdam, P.O. Box 20694, 1001 NR Amsterdam, The
Netherlands, would produce adequate results.
In a further embodiment the present invention provides as shown in
25 Figure 8 a dual purpose electrode and well. In this example, of a
cathode,
the cathode tubing 100 includes an upper section 102 and a lower section
104. The lower section is made water permeable, such as by being
formed from a wire wound screen. A submersible pump 106 is located
within the lower section 104 to pump the water collecting at the cathode
30 out of the tubing 100 through a riser pipe 108. As noted the tubing 100
is
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provided with a centralizer 110 to keep the pump located within the middle
of the tubing 100 and would electrically isolate the pump from the wall of
the tubing 100. In figure 8a there is shown a top view of the cathode of
figure 8 in which the top 112 is shown with the riser pipe 108, which is
protected by an insulator 114. Figure 9 shows an alternate embodiment
in which the wire screen has been replaced with a perforated pipe section
116.
The present invention also comprehends being able to selectively
treat sections of the tailings pond as local requirements demand. In the
first instance the tailings ponds tend to be vast in area and to facilitate
the
treatment the present invention contemplates creating smaller treatment
areas by means of sheet piling or the like. This can be used to divide the
area of the pond up into smaller areas or cells to facilitate treatment. The
sheet pile can also be used as an electrode in some cases. The use of
the sheet pile wall is used to hydraulically and hydrologically isolate the
treatment cell from the rest of the pond to also allow the supernatant
water to be removed to the extent desirable prior to or during treatment
within the treatment cell.
In addition to dividing the pond into smaller areas for treatment
through the use of cells, the present invention comprehends treating the
pond at various depths to achieve certain desired results. Figure 10
shows a cable electrode 200 which includes an electric cable 202
connected to a source of power and at the free end is an electrode 204.
The electrode 204 can be an electrically conductive plate, bar, tube, or
other electrically conductive element and can be made of any desired
length depending upon the depth of the zone which is to be treated. Most
preferable the cable electrode is inserted within a driven borehole 206
which when it collapses will provide good electrical contact with the
electrode 204, which is further maintained as the pore water is released
during treatment. The MFT surface is denoted by reference character
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208. As can now be appreciated the electrode 204 can be positioned at
any depth within the tailings pond to permit the flocculation and/or
electrostriction to occur at such depth. The electrical cable 202 may be
connected to a pulley system to raise or lower the cable 202 to a desired
depth. The hollow tube 206 may be a driven borehole to ensure the walls
of the hollow tube 206 do not collapse given the Thixotropic nature of
MFT maintaining electrical contact.
As will be appreciated by those skilled in the art, it may be
desirable to control the generation and/or release of methane during
treatment. One method to do so is to add gypsum to the MFT being
treated.
As will further be appreciated, the present invention provides a low
voltage electrical field in the MFT. Higher voltage ranges may lead to
heating up of the near electrode area, and the drying out of this area.
Heating and drying are not desirable according to the present invention,
especially in light of the presence of residual hydrocarbons. Thus, the
present invention comprehends using a voltage that is small enough,
having regard to the properties of the MFT, to avoid such heating and
drying out of the MFT. As well, the supernatant water is desirable as an
additional safety factor. Most preferably the current is being applied to
the MFT below the surface of any such supernatant water.
Although the foregoing description has been made with respect to
preferred embodiments of the present invention it will be understood by
those skilled in the art that many variations and alterations are possible
without departing from the broad spirit of the claims attached. Some of
these variations have been discussed above and others will be apparent
to those skilled in the art.
