U.S. patent application number 13/440386 was filed with the patent office on 2012-10-11 for electrokinetic process and apparatus for consolidation of oil sands tailings.
Invention is credited to Bruce S. Beattie, Paul Garcia, Doug Kimzey, Thomas M. Kroll, JR., James Micak, Robert C. Parrott, Gregory J. Smith.
Application Number | 20120255872 13/440386 |
Document ID | / |
Family ID | 46965264 |
Filed Date | 2012-10-11 |
United States Patent
Application |
20120255872 |
Kind Code |
A1 |
Smith; Gregory J. ; et
al. |
October 11, 2012 |
Electrokinetic Process And Apparatus For Consolidation Of Oil Sands
Tailings
Abstract
A method is provided of treating liquid tailings using
electro-kinetics by creating a variable voltage between two
electrodes in the tailings. Flocculation and water release from the
tailings is induced by establishing an electrical field between the
two electrodes. The electrodes are connected to an electrical power
source having the variable voltage to create a cathode and an
anode. Compacting the flocculation solids and removing further
water released from the compacting solids allows for the creation
of a compacted material having a desired load bearing capacity.
Inventors: |
Smith; Gregory J.;
(Woodbridge, IL) ; Beattie; Bruce S.; (Vista,
CA) ; Parrott; Robert C.; (Knoxville, TN) ;
Micak; James; (Aurora, CA) ; Garcia; Paul;
(Temecula, CA) ; Kimzey; Doug; (Knoxville, TN)
; Kroll, JR.; Thomas M.; (Louisville, TN) |
Family ID: |
46965264 |
Appl. No.: |
13/440386 |
Filed: |
April 5, 2012 |
Current U.S.
Class: |
205/742 ;
204/279; 204/280; 204/284 |
Current CPC
Class: |
C02F 2103/18 20130101;
C02F 2201/46135 20130101; C02F 11/006 20130101; C02F 11/125
20130101; C02F 2103/10 20130101; C02F 1/463 20130101; C02F 1/52
20130101 |
Class at
Publication: |
205/742 ;
204/280; 204/284; 204/279 |
International
Class: |
C02F 1/463 20060101
C02F001/463; C25B 15/00 20060101 C25B015/00; C25B 11/03 20060101
C25B011/03 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 7, 2011 |
CA |
2,736,675 |
Nov 16, 2011 |
CA |
2,758,872 |
Claims
1. A method of treating liquid tailings using electro-kinetics, the
method comprising the steps of: a. Causing at least two electrodes
to come into contact with the liquid tailings; b. Inducing
flocculation of particles in the tailings and releasing water from
the tailings by establishing an electrical field between said at
least two electrodes, the electrodes being connected to a source of
electrical power having a variable voltage to create at least one
cathode and at least one anode; and c. Compacting said flocculation
solids and removing further water released from said compacting
solids to create a compacted material having a minimum desired load
bearing capacity.
2. The method of claim 1 wherein the tailings are at least one of
oil sands extraction tailings and fly ash tailings.
3. The method of claim 1 wherein said flocculation step further
comprises passing the tailings through a conduit containing the at
least two electrodes.
4. The method of claim 3 in which the conduit is a canal.
5. The method of claim 3 in which the at least two electrodes are
augers.
6. The method of claim 3 in which said compaction step further
comprises placing said flocculated solids into a treatment cell or
tailings pond and applying a second variable voltage to the
flocculated solids to compact the flocculated solids through
electrostriction.
7. The method of claim 6 in which the second variable voltage is
created by a second pair of at least two electrodes which are
placed into the treatment cell or tailings pond.
8. The method of claim 6 in which the second variable voltage
applied during the compaction step is higher than the variable
voltage applied during the flocculation step.
9. The method of claim 8 in which the second variable voltage is
greater than 2 V.sub.DC/m.
10. The method of claim 8 in which the variable voltage during the
flocculation step is between 1-2 V.sub.DC/m.
11. The method of claim 3 in which said compaction step further
comprises placing said flocculated solids into a tailings pond and
allowing the flocculated solids to naturally consolidate.
12. The method of claim 1 wherein said compaction step includes
using electrostriction to compact the flocculated solids.
13. The method of claim 1 wherein said compaction step includes
using gravity loading to further compact said flocculated
solids.
14. The method of claim 12 further including the step of inserting
a drain or wick into said flocculated solids to permit pore water
to be expressed from said compacting solids.
15. The method as claimed in claim 13 further including the step of
removing water from tailings as said solids are compacted.
16. The method as claimed in claim 15 wherein said water is pumped
out of said tailings.
17. The method as claimed in claim 16 wherein said electrode
includes an associated pump, electrically isolated from said
electrode, to remove said water.
18. The method of claim 17 wherein the pump is located within a
hollow cathode.
19. The method of claim 1 in which the tailings are located in situ
at a tailings pond, the method further including the step of
partitioning said tailings pond to create at least one cell, and
wherein said step of causing the at least two electrodes to come
into contact with the tailings comprises placing said electrodes
within said cell.
20. The method of claim 19 further including the step of
partitioning said tailings pond into a plurality of cells.
21. The method of claim 20 wherein said cells are formed by sheet
metal pilings.
22. The method of claim 21 wherein said sheet metal pilings are
electrically connected to said source of power and thereby become
one of said electrodes.
23. The method of claim 1 further including the step of sampling
said tailings to determine one or more electrical properties, and
using said measured electrical properties to control the output
from the source of power.
24. The method of claim 1 further including the step of measuring
the electrical properties of said tailings over time and adjusting
said variable voltage across said electrodes in response to changes
detected in said measured electrical properties.
25. The method of claim 24 wherein the at least two electrodes are
used to induce a variable voltage during the compaction step to
cause electrostriction of the flocculated fluids and the variable
voltage applied during the electrostriction step is higher than the
variable voltage applied during the flocculation step.
26. The method of claim 23 wherein said electrical properties vary
as said solids compaction process progresses, and said voltage is
varied as said compaction progresses.
27. The method of claim 1 wherein said source of power is at least
one transformer.
28. The method of claim 27 wherein said at least one transformer is
operatively connected to a computer to permit the power from said
transformer to be controlled.
29. The method of claim 28 wherein said controller includes a
remote access controller.
30. The method of claim 1 wherein said predetermined load bearing
capacity of said compacted solids is about 5 kPa or more.
31. The method of claim 1 wherein the flocculation of the solids
within the tailings is induced by one or more of an AC, DC or
EM-induced electrical field.
32. The method of claim 31 in which the flocculation of the solids
within the tailings is induced by alternating current and in which
the anode and cathode are operating 180.degree. out of phase with
each other.
33. The method of claim 1 wherein said electrical field gradient
can have any range, but the preferred embodiment ranges from about
0.3 volt per centimeter to about 4 volt per centimeter.
34. The method of claim 22 wherein said electrical field gradient
is a substantially uniform field between said electrodes.
35. 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; and c. A means to
electrically isolate a portion of said electrode which extends
above said tailings pond.
36. The electrode of claim 35 wherein said body is hollow and
includes openings to permit water to pass into said electrode.
37. The electrode of claim 36 wherein said openings are screened to
prevent solids from passing into said hollow electrode.
38. The electrode of claim 37 further including a pump located with
said electrode to remove said water from within said hollow
body.
39. The electrode of claim 38 wherein said pump is electrically
isolated from said electrode.
40. A conveyor for use in a method of flocculating tailings, the
conveyor comprising: a. A conduit defining a tailings reservoir
through which tailings may pass, the conduit having an intake
opening and a discharge opening; b. First and second electrodes
connected to a power source, the first and second electrodes at
least partially placed within the tailings reservoir; c. A water
extraction outlet within the tailings reservoir for removing
released water; and d. A means to cause the tailings to pass
through the tailings reservoir from the intake opening to the
discharge opening.
41. The conveyor of claim 40 wherein the first and second
electrodes are a pair of augers.
42. The conveyor of claim 41 wherein the means to cause the
tailings to pass through the tailings reservoir are the pair of
augers.
43. The conveyor of claim 40 wherein the means to cause the
tailings to pass through the tailings reservoir is a pump in fluid
connection with the tailings reservoir.
44. The conveyor of claim 40 in which the first and second
electrodes each have hollow core to allow released water to travel
from the anode to the cathode.
45. 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; positioning the electrode within the tailings pond
at the depth of the layer to be treated; positioning at least one
other electrode at the same depth at a location remote from the
first electrode; connecting the electrodes to a source of power and
encouraging flocculation to occur at the depth that the electrodes
are submerged within the tailings pond.
Description
FIELD OF THE INVENTION
[0001] 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 allows land reclamation to occur.
BACKGROUND OF THE INVENTION
[0002] 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 first 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 hydrocarbon, heavy metals, and other waste
materials. The oil producers currently deal with what they call
Fresh Fine Tailings (FFT) and Mature Fine Tailings (MFT); the
distinction between the two being that MFT are derived from FFT
after allowing sand to settle out over a period of typically 3
years. MFT are mostly a stable colloidal mixture of water and clay,
and other materials, and is collected in onsite reservoirs called
tailings ponds.
[0003] Oil extraction has been carried out for many years on the
vast reserves of oil that exists in Alberta, Canada. It is
estimated that 750,000,000 m.sup.3 of MFT have been produced. Some
estimates show that 550 km.sup.2 of land has been disturbed by
surface mining yet less than 1% of this area has been certified as
reclaimed.
[0004] The FFT and MFT present three environmental and economic
issues: water management, sterilization of potentially productive
ore, and delays in reclamation. Although concentrations vary,
MFT/FFT 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/FFT will be produced. What is
desired is a way to deal with the MFT/FFT that has been and will be
generated to permit land reclamation, release of captured water and
provide access to the productive ore located beneath such
ponds.
[0005] MFT/FFT represents a mixture of clays (illite, and mainly
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. 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 certification can take place unless a way to
effect disposal and reclamation is found. The oil sands producers
are required by a directive of the Energy Resources Conversation
Board to treat their tailings to a bearing capacity of 5 kPa by
2012 and 10 kPa by 2015.
[0006] Applied electrical fields have been used to dewater 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 to create high density
ceramics. In electrical resistance heating treatment at Fargo, N.D.
(Smith et al., 2006).sup.a, electrostrictive phenomenon has been
observed in the application of an electric field to already
consolidated clays where the applied electric field ranged between
0.46 to 0.8 volt/cm. Examples of applications of electrical fields
in various circumstances can be found in the following prior
patents. .sup.a Smith, G. J., J. von Hallen, 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, Calif.,
[0007] U.S. Pat. No. 3,962,069
[0008] U.S. Pat. No. 4,107,026
[0009] U.S. Pat. No. 4,110,189
[0010] U.S. Pat. No. 4,170,529
[0011] U.S. Pat. No. 4,282,103
[0012] U.S. Pat. No. 4,501,648
[0013] U.S. Pat. No. 4,960,524
[0014] U.S. Pat. No. 5,171,409
[0015] U.S. Pat. No. 6,596,142
[0016] The application of electrical current to oil sands tailings
has also been tried, as shown in U.S. Pat. No. 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/FFT treatment and requires a chemical
pre-treatment step which adds to the cost.
[0017] Moreover, the application of electrical fields to treat
small-scale clay deposits may not require efficient use of energy.
However, on a large scale, the application of an electrical current
requiring high power consumption or requiring an application of an
electrical current over a long period of time may be prohibitively
expensive or impossible to carry out due to the available
resources. At remote sites, large-scale access to electrical power
may be limited. Small variations in electrical current draws may
have significant impact on costs and power requirements when
dealing with millions of square meters of MFT and FFT. What is
desired is a better way to deal with vast volumes of MFT/FFT that
will need to be treated. There is a need for a practical system for
dealing with tailings efficiently and quickly.
SUMMARY OF THE INVENTION
[0018] According to the present invention, the consolidation of
solids present in MFT/FFT may occur in multiple phases that can be
initiated contemporaneously or sequentially under the application
of an electrical field. These phases can be controlled by varying
the applied voltage gradient to achieve a desired end point. For
example, if water release and natural consolidation is desired,
then one can apply a voltage gradient that promotes electro-osmotic
flow of low pH water from the anode to neutralize the water
sorption capability of the clay solids in the MFT/FFT. Or if one
desires material that meets a desired bearing capacity, the
electro-osmotic flow described above would occur with or be
followed by an increase in the voltage gradient which facilitates
the application of an electrostrictive force as the electrical
resistance increases. Also envisaged is the ability to take the
treated MFT/FFT at various stages of its consolidation, place in a
mould, dry either in air or in a kiln to create building materials.
The phases may occur in distinctly separate steps, for example, at
different locations. Some phases may be replaced by other steps or
processes, or omitted entirely, depending on the particular needs
for each application.
[0019] Further electro-kinetic processes offer a means to release
water from the MFT/FFT through electrolysis, which creates low pH
water at the anode, which combined with electro-osmosis causes the
migration of this low pH water to the cathode, lowering the pore
water pH to the point of zero charge (P.sub.zc) which in turn
releases the pore water bound in the diffuse double layer of the
clay structure.
[0020] These phases include the initial water release under the
influence of an electric field in a flocculation step with an
accompanying release of water, followed or contemporaneously
occurring with the secondary release of pore water during the
electro-osmotic flow of low pH water produced at the anode which
when electro-osmotically transported through the MFT/FFT
neutralizes the diffuse double layer. This results in the release
of ions from the pores which are transported to the anode and
cathode via electrophoresis, electromigration and electro-osmosis.
With the ions released and transported to the anode and cathode,
the electrical resistance of the wet tailings increases, which
allows the application of higher voltage gradients at lower current
draw, or improved energy efficiency to achieve compaction through
electrostriction. As a result, in one embodiment, the
electrokinetic remediation process involves two distinct and
separate steps involving the water release/ion
release/flocculation, followed by compaction through
electrostriction as the electrical resistance increases. The
draining of water from the tailings also increases the electrical
resistance and provides a means of controlling the process.
[0021] MFT/FFT, 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/FFT to express further pore water from the
solids to increase the density further increasing the lithostatic
loading. In one aspect of the present invention a mechanism is
provided for relief of pore pressure to accelerate the
consolidation of the solids for say, the consolidation of thick
deposits.
[0022] The present invention provides the placing of equipment
in-situ in tailings ponds or ex-situ, for example, in designed
treatment cells, to allow induction of an electrical field (AC, DC,
or EM-induced) having a voltage gradient that can be varied
resulting in electrokinetic floccing of the MFT/FFT,
electro-osmotic flow of low pH water, electrophoretic flow of ions,
and an electrostrictive force causing the flocculated or weakly
consolidated solids to further consolidate. An electrostrictive
force can be varied by either the duration of application and/or
the magnitude of the voltage gradient to achieve a desired bearing
capacity of the MFT/FFT. 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.
[0023] According to an aspect of the present invention, the
electrical field and the low pH water neutralizes the electrostatic
charges on the clay platelets, releasing water from the MFT/FFT
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. Under the application of an
electric field, electrophoresis results in the migration of ions to
the anode and cathode, thus increasing the electrical resistance of
the tailings. The water removal and increase in electrical
resistance can also assist further consolidation along with the
electrostriction. The electrostrictive force can be applied in
varying degrees to achieve the desired bearing capacity in desired
zones of the MFT/FFT 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. The latter option
allows for consolidation in active tailings ponds that are not
seeking certified reclamation, but where for instance, greater
storage capacity is desired. In one embodiment, the tailings may be
left to consolidate in tailings ponds or other settling locations
to allow for natural compaction of the tailings over time.
[0024] Therefore, there is provided, according to the present
invention, a method of treating liquid tailings using
electro-kinetics, the method comprising the steps of: [0025] a.
Causing at least two electrodes to come into contact with the
tailings; [0026] b. Inducing flocculation of particles in the
tailings and releasing water from the tailings by establishing an
electrical field between said at least two electrodes, the
electrodes being connected to a source of electrical power having a
variable voltage to create at least one cathode and at least one
anode; and [0027] c. Compacting said flocculation solids and
removing further water released from said compacting solids to
create a compacted material having a minimum desired load bearing
capacity.
[0028] 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 depths of the MFT/FFT deposits
causes the clay particles to flocculate there first. Afterwards,
the conductive zone of the electrodes which creates the electric
field can be raised to higher elevations to encourage weak
consolidation at a different depth. Alternatively, for especially
thick MFT/FFT deposits, the operator may wish to induce
flocculation in the deeper deposits of MFT/FFT, and then
electrostrictively treat a shallow zone in an amount sufficient to
achieve a 5 kPa or greater bearing capacity. This area could then
be re-covered with overburden to enhance the consolidation of the
non-electrostriction treated depths through the use of sand drains
or wicks or the like, while re-vegetation can occur on the replaced
overburden.
[0029] 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 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 MFT/FFT,
by the separation and removal of water content from the
MFT/FFT.
[0030] In a further embodiment of the present invention, controlled
compaction of MFT/FFT occurs at a location having semi-permanent
treatment cells capable of receiving MFT/FFT. Treatment of the
MFT/FFT may occur in a series of batch treatments, for example, on
a continuous year-round basis.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] Reference will now be made to preferred embodiments of the
invention, by way of example only, with reference to the following
figures in which:
[0032] FIG. 1a is a graph depicting an estimation of pressure at
depth for a sample tailings pond;
[0033] FIG. 1b is a graph depicting an estimate of lithostatic
pressures resulting from an electrostriction treatment according to
the present invention at various depths;
[0034] FIG. 2 is a depiction of a graph showing a change in
pressure with electrical field variance according to the present
invention;
[0035] FIG. 3 is a layout of electrodes in a three spot treatment
pattern according to the present invention;
[0036] FIG. 4 is a schematic of a further electrode layout with a
neutral pumping well according to a further aspect of the present
invention;
[0037] FIG. 5 is a tubular electrode connection according to the
present invention;
[0038] FIGS. 5a and 5b are enlarged views of a portion of FIG.
5.
[0039] FIG. 6 is an enlarged view of an alternate connection;
[0040] FIG. 7 is a schematic of a drain of the type that can be
used in the present invention;
[0041] FIG. 8 is a schematic of a first embodiment of a combined
cathode well structure;
[0042] FIG. 9 is a schematic of a second embodiment of a combined
cathode well structure;
[0043] FIG. 10 is a schematic of a variable depth electrode
according to a further aspect of the present invention;
[0044] FIG. 11 is a perspective view of an embodiment of a conveyor
having rotating electrode screws;
[0045] FIG. 12 is a partial perspective view of the conveyor in
FIG. 11 having a removable insulated panel;
[0046] FIG. 13 is a top view of the conveyor in FIG. 11;
[0047] FIG. 14 is a perspective top view of the conveyor in FIG.
11;
[0048] FIG. 15 is a flow diagram of a method of treating liquid
tailings using electro-kinetics; and
[0049] FIG. 16 shows the distribution of bearing capacities and
moisture content for MFT after flocculation and
electrostriction.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0050] In this specification the terms MFT, or MFT/FFT or FFT shall
mean the tailings that exist in tailings ponds that arise from the
extraction of hydrocarbons, such as bitumen, from tar or oil sands,
or fly ash tailings ponds. As will be appreciated by those skilled
in the art, the exact composition of MFT/FFT 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.
[0051] 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.
[0052] The present invention comprehends the application of an
electromagnetic field and most preferably an electrical field to
the MFT. In one embodiment of the present invention, there are two
aspects: the first being the application of an electric field to
neutralize the diffuse double layer that is formed between the clay
particles and the water, further neutralization as a result of the
electrolytic breakdown of water under the application of an
electric field, whereby acidic conditions are produced at the anode
and basic conditions are created at the cathode, whereby the acidic
water migrates from the anode under electro-osmotic flow to reduce
the pH in the wet tailings to the point of zero charge of the
tailings minerals, releasing water and ions from the tailings
minerals, and electrophoretic transport of the released ions from
the tailings mineral surfaces. The second aspect occurs where a
second electric field is applied such that an electric field can be
used to exert a force on the solids present in the MFT/FFT due to
electrostriction. The second electric field to induce
electrostriction may be created by the same source as the first
electric field mentioned above, or may be created by a separate
source. Electrostriction.sup.b occurs where dielectric materials
are subjected to an electric field. 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.sup.c. 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., 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. .sup.b A phenomenon first reported by Reuss in 1807 to
the Moscow Academy of Science.sup.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.
[0053] The electric force density under an applied electrical field
to induce electrostriction is governed by the square of the
electrical gradient. From Brevik (1982).sup.d, to determine the
electric force density f.sup.el, one can make use of the Helmholtz
variational principle under reversible, isothermal conditions. From
this, f.sup.el is defined as: .sup.d Brevik, I. (1982). Fluids in
electric and magnetic fields: Pressure variation and stability.
Canadian J. Physics, 60, pp 449-455.
f.sup.el=-1/2E.sup.2.gradient..epsilon.+1/2.gradient.[E.sup.2.rho.(.diff-
erential..epsilon./.differential..rho.).sub.T]
Where:
[0054] .gradient. refers to the vector in the direction of the
application of the field
[0055] .rho.=mass density (kgm.sup.-3);
[0056] .epsilon.=permittivity
(s.sup.4A.sup.2m.sup.-2kg.sup.-1);
[0057] E=electric gradient (voltm.sup.-1); and the system is
operating at constant temperature.
[0058] The second term in this equation is the electrostriction
term.
[0059] According to the present invention the application of a
preferred electrical field results in flocculation of the clay
particles by said electric field, with pH neutralization of the
sorbtive capability of the clay as a result of the electro-osmotic
flow of low pH water from the anode. This releases water that was
otherwise bound to the clay particles to form the persistent gel or
thixotropic MFT/FFT 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.
[0060] In one aspect of the present invention the further
consolidation of the solids occurs through electrostriction. To
determine the electric field required to achieve a given amount of
consolidation, the change in permittivity relative to the change in
mass density under a defined electric gradient (E, volt/m) is
determined. Therefore, the present invention provides that it is
possible to correlate changes in the permittivity and as a result
density under an applied electrical field to track the progress of
the electrostriction treatment of the MFT/FFT.
[0061] According Melloni, et al., (1998).sup.e, the change in
density under an applied electric field can be determined from:
.sup.e 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.
.DELTA..rho.=1/2].rho.C.gamma..sub.e.epsilon..sub.0|E|.sup.2
Where:
[0062] .DELTA..rho.=the change in density under the applied
electrical field (kgm.sup.-3)
[0063] .rho.=the density of clay (kgm.sup.-3)
[0064] C=the compressibility of clay (%)
[0065] .gamma..sub.e=electrostriction coefficient (unit-less)
[0066] .epsilon..sub.0=dielectric constant (permittivity;
unit-less) for clay
[0067] E=electric field (voltsm.sup.-1)
[0068] The electrostriction coefficient used was 0.902 (Melloni,
1998). One known dielectric constant for kaolinite is 5.3.+-.0.6
(Ishida, et al., 2000.sup.f). The permittivity of water is
80.37.sup.g. Therefore, for MFT/FFT which comprises 50% to 70%
water content, the estimated permittivity for MFT/FFT is expected
to range between 43.1 and 58.7. MFT/FFT 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. In laboratory testing, moisture
contents of the treated MFT ranged from 15.6% to 72.7%, with
associated bearing capacities ranging from 19,700 kPa to 33.9 kPa.
Therefore reduction in moisture content may not be a good indicator
of bearing capacity. .sup.f Ishida, T., M. Tomoyuki, and C. Wang
(2000) Dielectric-Relaxation Spectroscopy of Kaolinite,
Montmorillonite, Allophane and Imogolite under Moist Conditions.
Clays and Clay Minerals, Vol. 48, No. 1, 75-84..sup.g Weast,
Robert, C. (ed; 1975). Handbook of Chemistry and Physics. 56.sup.th
Edition, CRC Press, Cleveland, Ohio.
[0069] Bearing capacities are dependent on compaction effort, which
is governed by the applied force and the duration of the
application of the desired force. Either parameter can be varied to
achieve the desired bearing capacity. This is expected to vary as
to the mineralogy of the MFT/FFT and the pore water chemistry
varies as a result of variations in the ore from one location to
the next.
[0070] The following relationship equates the applied electrical
field to the electrostriction force in kPa:
.DELTA.p=1/2E.sup.2.rho.((.epsilon.-.epsilon..sub.0).sup.2/6.epsilon..su-
b.0)
[0071] From the above relationship to achieve these forces, the
applied electric field is estimated to range from 1 volt/cm to 4
volt/cm (within the linear range of the equations describing
electrostriction). From the above equation, this translates into an
electrostriction pressure of between 1.23 and 19.7 kPa, which can
be varied by the applied gradient or time of treatment to achieve
the desired compaction effort for the desired bearing capacity.
[0072] The greater the applied electric field, the greater the
applied force, the shorter the time period to achieve the desired
degree of compaction, or the greater the degree of compaction that
can be achieved. However, this may also result in the greater the
amount of energy consumed, relating directly to cost. Further,
water balance is important. The higher the applied electric field
the greater the potential for increases in temperature and hence
drying of the MFT/FFT. Drying MFT/FFT results in loss of electrical
circuit and hence the electro kinetic treatment. 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 or ex-situ treatment
cells. 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/FFT to
induce an electrical field resulting in a force within an
appropriate range. Turning now to the figures, FIG. 1a depicts in
schematic form the pressure-depth relationship in a notional
tailings pond filled with MFT or FFT. In FIG. 1a 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 temperature and there is negligible densification of the
MFT). FIG. 1b 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 FIG.
1b, 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 FIG. 1b 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.
[0073] As can now be appreciated the pressure profile of FIG. 1b
results in greater lithostatic pressure with depth than is shown in
1a. Therefore, in one embodiment, 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 can be
varied and is maintained over a treatment period, and then
providing for release of pore water to increase the density of the
material (FIGS. 1a, and 1b) while the material consolidates.
[0074] In one aspect of the present invention, the method of
treating liquid tailings is applied in situ at a tailings pond in
which there are two main steps. 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 water is released through this
flocculation process as a result of the electrical field and
through electro-osmotic flow of low pH water. After this has
occurred, the operator has the option to continue with
electrostriction (described below) or allow the MFT to 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.
[0075] As noted above, after the flocculation step the further
application of the electrical field allows for further application
of electro-osmotic flow of low pH water to lower the clay point of
zero charge to neutralize the sorbtive characteristics and the
application of an electrostriction force, which is converted to
mechanical work. The relationship between the applied voltage
gradient and the electromotive force is linear in the range between
100 to 400 volts/m and depicted in FIG. 2. FIG. 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 and the cost
associated with applying a higher than necessary electrical
field.
[0076] The higher the voltage gradient, the greater the
electromotive force, and as a result, the shorter the treatment
time. However, there are three 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; 2) the greater the
gradient, the closer electrode spacing, and increased apparatus
costs; and 3) The electrical resistance of the MFT and FFT
increases as water is released, making the timing of the
application of higher electrical fields important. 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.
[0077] One apparatus used to effect the action of the present
invention on MFT/FFT is described below. One embodiment of this
invention involves the use of a variable voltage power supply
connected to a network of electrodes. Where the power source is an
AC source, the electrodes are arranged in a triangular (FIG. 3) or
hexagonal pattern (FIG. 4). In FIG. 3 there are three electrodes
denoted with the numbers 1, 2, or 3. These electrodes would be
charged 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.
[0078] FIG. 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 E1 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 E1 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. E1 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 electrical field will be preferably initiated
at less than 200 V/m, increasing as the water and ions are released
resulting in increased electrical resistance, allowing for greater
voltage gradients to be applied more efficiently, across the
longest diagonals to efficiently apply electrostriction. 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.
[0079] The AC power source 40 will be provided with a power
controller to permit the voltages being applied to be varied. Most
preferably it 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) 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);
and, the timing of the water release from the MFT/FFT and the
subsequent increase in electrical resistance. 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/FFT, which will vary during the treatment as electrophoresis
and electro-migration 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/FFT 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.
[0080] 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 and
voltage drops, 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, periodic conductivity measurements through such
means as small diameter slotted CPVC tubing embedded in the MFT/FFT
will permit the operator to track and optimize the application of
the electrical field.
[0081] 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 MFT/FFT by the flocculation step, the
electro-osmotic flow of low pH water, and the electrostrictive
compaction of the MFT/FFT and reduction in pore volume outlined
above. The reclaimed water can then be optionally treated and
recycled as desired using conventional processes.
[0082] According to one aspect of the present invention, these
electrodes E1 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. For in situ treatment, the electrodes
are placed in position by either through driving, drilling, using
conventional drilling equipment, pile driving equipment, or in the
case of treatment cells specifically constructed for this purpose,
placed in accordance with the design placement with the MFT/FFT
pumped into the treatment cell.
[0083] FIG. 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 tube or pipe 64. Also
shown is a 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 or treatment cell
that the electrodes do not become totally submerged in the
pond/cell. In FIGS. 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 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 FIG. 6
the welded bolt connection 74 is placed centrally on a cap 76 which
covers the open top of the pipe 64.
[0084] The present invention comprehends that it is usually
desirable to remove supernatant water and or water being
electro-osmotically drawn towards the cathode. 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. In most cases the removal of water to
increase the electrical resistance of the MFT/FFT facilitates
increasing the voltage gradient to increase the electrostrictive
force as desired. As an option 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 FIG. 7, in which the hollow skeleton 90
supports a water permeable mesh 92. Essentially this drain provides
a leak path for pore water to be expressed through the
consolidation process.
[0085] In a further embodiment the present invention provides as
shown in FIG. 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 out of the tubing 100 through a riser
pipe 108. As noted the tubing 100 is 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 FIG. 8a there is shown a top view of the cathode of FIG. 8
in which the top 112 is shown with the riser pipe 108, which is
protected by an insulator 114. FIG. 9 shows an alternate embodiment
in which the wire screen has been replaced with a perforated pipe
section 116.
[0086] The present invention also comprehends being able to
selectively treat sections of the tailings pond/treatment cell 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, or by providing hydraulic control by
manipulating the electro-osmotic flow to create pressure barriers
around the treatment area. This can be used to divide the area of
the pond up into smaller areas or cells to facilitate treatment.
The sheet pile may 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.
[0087] 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. FIG. 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 hollow tube 206 to which
water can be added to maintain good electrical contact with the
electrode 204, which is further maintained as the pore water is
released during treatment. As can now be appreciated the electrode
204 can be positioned at any depth within the tailings pond to
permit the flocculation, water release and/or electrostriction to
occur at such depth.
[0088] A 100,000 bbl/day production facility produces 50,000 tonnes
per day of FFT, which is equivalent to approximately 33,500 m.sup.3
of FFT per day. The water release/ion release/flocculation step
operates at a lower voltage gradient than the compaction step. One
means of accomplishing this difference voltage gradient in a
quasi-continuous operating mode is envisaged in FIGS. 11-14. A
central canal (not shown) will feed wet tailings to a series of
conveyors 300, 302. The conveyors 300, 302 are composed of two
counter-rotating intermeshed electrode screws or augers, with one
representing an anode and the other a cathode. These
screw/electrodes 300, 302 will have the capability to reverse
polarization, or be operated in an alternating current mode, with
one operating 180.degree. out of phase with the other, or other
desired phasing. The conveyors 300, 302 lie within a conduit 304.
As shown in FIG. 11, the conduit 304 is a canal. At the sides of
the canal, serviceable dewatering screens 306 separate the main
conduit 304 from a pair of troughs 326, 328. Service doors 308 are
connected to the pair of troughs 326, 328. The conveyors 300, 302
are powered by electric motors 310. Removable insulated panels 314
(FIG. 12) may be used to cover the conduit 304. As shown in FIG.
13, an intake 312 lies on the upstream end of the canal 300 and a
discharge 316 lies on the downstream end of the canal. As shown in
FIG. 14, during the operation of the conveyor system, incoming
tailings arrive through the intake 312 into the upstream end 318 of
the conduit 304. As the tailings travel through the canal,
flocculation begins to occur, as generally shown at 320. As the
tailings continue through the canal, water is removed for treatment
at the downstream end 322 of the canal. As shown in FIG. 11, the
troughs 326 and 328 may lie on the downstream end 322 of the
conduit 304 adjacent to the discharge. The troughs 326, 328
function as a water extraction outlet. Denser tailings 324 are
discharged through the discharge 316 to the next treatment step.
Further, the electrodes 300, 302 have a water-filled core, with
screens to allow the low pH water generated at the anode to migrate
to the cathode. A water extraction outlet may be connected directly
to the hollow core of the cathode of the conveyors 300, 302 to
allow for water removal. Any outlet allowing for the removal of
water from the tailings reservoir may be used as a water extraction
outlet. The spacing between the screw blades dictates the residence
time needed to achieve the deflocculation.
[0089] The conveyors 300, 302 may be powered by any suitable means
for causing rotation of the screws. In other embodiments, movement
of the tailings through the conduit 304 may be caused by other
means, such as a conveyor belt or a rotary pump, so long as the
means cause the tailings to pass through the tailings reservoir
from the intake opening to the discharge opening. In other
embodiments, the conduit 304 may be a pipeline to transport
tailings. Separate cathodes may be placed into the canal instead of
the counter-rotating screws. The cathodes may be distinct from the
means for causing the tailings to pass through the tailings
reservoir and may only function as cathodes and, or may, as in the
example in FIGS. 11-14, function as both a cathode and a conveyor.
In other embodiments, more than two cathodes may be used to induce
flocculation of the tailings.
[0090] As shown in FIG. 15, in one embodiment of the present
invention, tailings may be treated using flocculation and
compaction. There is shown a method 400 of treating liquid tailings
using electro-kinetics. At 402, at least two electrodes are caused
to come into contact with the tailings. The electrodes may be
placed into a tailings pond or ex-situ treatment cell or FFT/MFT
may be moved into contact with the electrodes. At 404, flocculation
of particles and releasing of water from the tailings is induced in
the tailings by establishing an electrical field between the two
electrodes. The two electrodes are connected to a source of
electrical power having a variable voltage to create at least one
cathode and at least one anode. Following the flocculation of
particles, the flocculated solids are compacted and further water
is released at 406 to create a compacted material having a minimum
desired load bearing capacity. Compaction and further water release
may be induced through the application of a second variable
voltage; a reapplication of the variable voltage from the
flocculation step, for example, at a higher variable voltage; or
through a non-electrokinetic process such as natural consolidation
or forced compaction, such as through the application of wicks or
sand drains. The compaction and further water release step may be
carried out at a separate location using separate electrodes to
induce the second variable voltage. The separate location may be in
situ in a tailings pond or ex-situ at additional treatment
cells.
[0091] To reduce the size of tailings to a manageable size, MFT may
be pumped into intermediate cells in which flocculation of the
material may be applied prior to electrostriction of the material.
Flocculation and water removal may be applied during transportation
of MFT or FFT from location to location, for example using a
processing cell such as described in FIGS. 11-14. The
transportation of FFT may be challenging and steps would need to be
taken to ensure that material does not settle out and damage any
equipment used to transport the FFT.
[0092] Where fly ash tailings are treated, the flocculation step
may be followed by compaction and further water removal in which
the compaction step does not include the application of
electrostriction. The properties of fly ash tailings may prevent
the effective use of electrostriction to compact the tailings.
Other techniques for compacting tailings such as those discussed
throughout this specification may be used.
[0093] Electrokinetic experiments were performed in laboratory on
MFT material produced from Syncrude's Mildred Lake Mine. In the
embodiment discussed in the testing, the flocculation step and
electrostriction steps were applied one after the other using a
higher variable voltage during the electrostriction step. The
results of this testing is discussed below.
[0094] The electrokinetic test cells used for the experiments in
this study include a reactor which consists of an electrokinetic
cell, two electrode compartments, two electrode reservoirs, a power
supply, a multi-meter, flow control valves, and gas vents. The
reactor was designed to simulate one-dimensional transport of
contaminants under an induced electric potential and was also used
to determine the compaction achieved under higher voltage gradients
(up to 4.3 V.sub.DC/cm) and the volume of water that can be
recovered.
[0095] To perform the 1-D pretesting testing, a Plexiglas tube
measuring approximately 3.8 cm in diameter and 14.2 cm long was
filled with MFT. At the ends of the tube, filter paper discs were
placed between the MFT and the porous stones capping each end. Each
end was then sealed with an integrated end cap equipped with an
electrode providing even distribution of voltage from one end of
the tube to the other. MFT material was placed into the cell with
no headspace. The voltage gradient can be varied up to 4.3
V.sub.DC/cm, which is at the mid-range of the linear range of the
equations for describing electrostriction. Water drainage was
provided at the cathode end of the apparatus.
[0096] The 2-D test cells measure 20 cm high by 20 cm wide, by 5 cm
deep. On either side of the test cells are located the anode and
cathode in water-filled reservoirs. In the 2-D cell, approximately
0.5 cm of sand was place for drainage during the treatment. Across
the top of the sand layer and over the plastic screens separating
the electrode reservoirs from the sample, a geotextile material was
placed and a silicone sealant applied where the fabric met the
walls of the test cell to maintain separation of the materials. The
MFT was filled to a level of approximately 18 cm from the base of
the cell. The remainder of the cell was filled with tap water to a
height of approximately 18 cm from the base of the cell to mimic
conditions in a tailings pond and ensure current flow throughout
the MFT. This left approximately 1 cm of freeboard in the cell. The
MFT was placed in the cell, placed in layers to minimize void
spaces while filling. The MFT was measured at approximately 18 cm
thick, with the electrode reservoirs filled with tap water to
approximately the same level as the MFT. The voltage gradient could
be applied up to 2.36 V.sub.DC/cm (236 V.sub.DC/m) with the
available equipment and for the most part, the maximum voltage
gradient was used.
[0097] Samples were obtained from each of the three pails to be
tested for the following characteristics:
[0098] Moisture content per ASTM D2216;
[0099] Unconfined Compression Testing per ASTM 2166;
[0100] Testing of produced water for total Dissolved solids, pH,
major ions (Ca, Mg, Na, K, CO.sub.3, Cl, SO.sub.4 and
HCO.sub.3),
[0101] Vane shear testing (ASTM 2573-08) as a screening tool to
evaluate whether the MFT had achieved the desired strength;
and,
[0102] Scanning electron microscopy to determine structure and the
changes that result with treatment.
[0103] This testing was performed at the completion of the
electro-kinetic compaction treatment.
[0104] Initial baseline measurements of applied voltage, voltage
drop across a water filled and then a MFT filled cell and amperage
were made. The electrode compartments, consisting of graphite
electrodes, were connected to the compartments which were then
filled with potable water (pH=7.7, redox potential=150 mV,
electrical conductivity=280 mS/cm). The initial water elevations in
both the reservoirs were kept the same in order to prevent a
hydraulic gradient from forming across the cell that would be
opposite the electrical field and affect the electro-osmotic
direction of flow. Initial baseline voltage drop measurements were
made to compare against the previous data to calculate changes in
permittivity and as a result, the applied compactive pressure. The
voltage, voltage drops, and current draw through the MFT sample as
well as pH, redox potential, and electrical conductivity (EC) of
the aqueous solutions in the water drained from the cathode
reservoir were monitored during the testing. At the end of testing,
aqueous solutions from both the cathode and anode compartment and
reservoirs from several tests were combined for chemical
analysis.
Dewatering of the MFT occurs as a result of a number of mechanisms:
[0105] Neutralization of the diffuse double layer; electrolytic
decomposition of water and the electro-osmotic flow of reduced pH
water from the anode to the cathode results in the neutralization
of the point of zero charge of the soil minerals; [0106] The
compaction of the MFT itself through electrostriction, effectively
squeezing water from the material; and [0107] Potentially, gas
generation displacing water from the pores. Monitoring consisted of
the following: [0108] Voltage and amperage being supplied to the
test cell; [0109] Water drained during the application of the
electrical field; [0110] Voltage drop across the test cell (2-D
only); [0111] Other observations such as physical condition of the
sample; and [0112] The test cells were opened on a periodic basis
to observe changes and on some occasions to measure bearing
capacity using a Humbolt H-4212MH Pocket Shear Vane Tester.
[0113] The application of an electrical field generally resulted in
the migration of water to the cathode within 2 hours of the
initiation of the electrical field. Associated with the release of
water, the MFT is observed to shrink, and vapor bubbles begin to
form. Within 48 hours of the initiation of the electrical field,
water is released ranging from 24 to 34% of the sample volume as
summarized below.
[0114] A 1-D test set-up was used to determine if there may be an
optimal voltage gradient for electrostrictive treatment. Three
tests were planned, but after conducting the second test, it was
apparent that the ideal voltage gradient may be outside the range
allowed by the equipment. Two voltage tests were conducted: 1) 2.92
V.sub.DC/cm and 2) 3.2 V.sub.DC/cm. An additional test at
approximately 2.5 V.sub.DC/cm was planned, but cancelled when the
trend in Table 1 was observed.
TABLE-US-00001 TABLE 1 Comparison of 1-D testing Showing Power
Consumption and Water Production for Two Different Voltage
Gradients. Cumulative Cumulative % Elapsed Volt- Amper- Power Water
Cumulative Time age age Input Production Water Pro- (hrs) (Volts)
(Amps) (KW-hr/m.sup.3) (ml) duction 3.2 Volts.sub.DC/cm 0:00:00
60.1 0.04 0.00 0:05:00 60.3 0.03 0.05 1:00:00 60.2 0.01 0.58
2:35:00 60.2 0.01 1.10 21:50:00 60.2 0.01 4.27 29 19 22:55:00 60.2
0.01 4.45 31 20 44:15:00 60.2 0 7.95 53 34 2.92 Volts.sub.DC/cm
0:00:00 56.8 0.05 0:40:00 56.8 0.06 0.52 2:20:00 56.8 0.06 2.07 29
19 3:15:00 56.8 0.05 2.92 33 22 21:25:00 56.8 0.01 17.01 36 24
22:40:00 56.8 0 17.20 23:00:00 56.8 0.01 17.20 24:25:00 56.8 0.02
17.42
While this represents a small data set, the indication is that
operating at higher voltage gradients results in lower overall
power consumption, and may result in greater water release.
[0115] With the application of the electrical field, effervescence
was almost immediately observed at both the anode and the cathode.
As discussed in relation to electrolysis and zero point charge,
this would represent electrolytic breakdown of the water resulting
in the production of oxygen gas at the anode and hydrogen gas at
the cathode. It should also be noted that a faint chlorine gas
odour was observed on several occasions when opening the treatment
cell to inspect the sample for progress. From the publicly
available information, there are chloride salts present in the MFT,
and electrolysis would result in the conversion of chloride to
chlorine gas as shown below.
2Na++2e-.fwdarw.2Na (sodium metal at the (-)cathode).
2Cl--2e-.fwdarw.Cl2 (chlorine gas at the (+)anode).
[0116] These trace amounts are not believed to be harmful or be of
concern at full scale treatment production.
[0117] It was observed that within 15 minutes of the application of
the electrical field that the MFT at the anode reservoir had begun
to floc (i.e., the MFT had formed tuft-like particles,
approximately 1-2 mm long) with the flocked material falling into
the anode reservoir. It is believed that this flocking may be the
result of one or a combination of two mechanisms: 1) the electrical
field neutralizes the surface charge (or zeta potential) on the
mineral surfaces, releasing the water; or 2) the generation of
reduced pH at the anode through electrolysis, which migrates to the
cathode, in turn also reduces the zeta potential to the point of
zero charge (P.sub.zc, estimated pH=3.5 to 3.6 for kaolinite; and 4
to 9.6 for illite).
[0118] At elapsed time 161:25 the following was observed. At the
two electrode ends of the cell, the MFT still clings to the walls
of the cell. The material at either end of the test cell is MFT
that is squeezed against the Plexiglas.TM. wall is a result of the
electrostriction force. However in the mid-section, the MFT shows
signs of shrinkage, and the MFT has pulled away from the cell
walls. There are significant differences in water levels at the two
electrodes. This is a manifestation of the electro-osmotic pressure
induced by the electric field, from which, the permeability of the
treated MFT can be estimated. Using this, it may be possible to
engineer the electro-osmotic hydraulic pressure to provide
hydraulic containment for in situ treatment in the tailings pond.
This has the potential of significantly lowering costs through the
elimination of much of the treatment infrastructure associated with
ex-situ treatment.
[0119] There was a vertical to sub-vertical orientation to the
layering of the MFT reflecting the electrostrictive compression
perpendicular to this direction, due to compactive forces from the
anode to the cathode. This results in greater compaction
perpendicular to the direction of this force.
[0120] The test cell had plainly evident shrinkage cracks that
result from the electro-kinetic compaction (EKC) treatment, as well
as the mineral precipitate that forms on the surface.
[0121] The electrical resistance and the resulting power draw over
time for 2-D EKC application for test run MFT5 was calculated. As
expected, resistance and power measurements were mirror images of
each other. In general, after an initial drop in electrical
resistance, there is a generally increasing resistance trend as the
MFT is compacted. Since V=IR and the voltage gradient is held
essentially constant, as the resistance increases, the current draw
decreases.
[0122] However, the current draw decreases as water is drained from
the test cell. Electrostriction is dependent on the voltage
gradient. Therefore according to the present invention maintaining
water drainage while still maintaining the electrical circuit
provides a means of minimizing the power consumption, and hence
costs.
[0123] In this experiment, the highest power draw occurred during
the first 20 hours of EKC treatment. During this period, the pH at
the anode is decreasing and the low pH water is being
electro-osmotically drawn through the MFT to the cathode. This
results in the release of water from the MFT. It is generally
recognized that electro-osmotic flow occurs at voltage gradients
ranging from 1 to 2 V.sub.DC/cm. Electrostriction occurs at voltage
gradients of 2 V.sub.DC/cm and higher. From the perspective of
compacting the MFT, there is little benefit in doing this where the
water content is high. Water is considered an incompressible fluid
and compaction theory informs us that it is better performed at
more optimal moisture contents.
[0124] Compaction effort is a term used to denote a specific
compaction operation. A specific compaction operation may specify
the number of passes that a sheepsfoot roller moves over a section
of fill. For this invention the compaction effort is defined as the
applied compaction pressure multiplied by duration.
Electrostrictive force is proportional to the square of the voltage
gradient as noted above. Therefore, according to the present
invention, it is desirable to maximize the voltage gradient and to
apply the highest compactive force practical, to increase the
bearing capacity while also reducing the moisture content. Unlike
conventional compaction, the present invention does not seek to
compact at the optimum moisture content. At a voltage gradient of
2.34 V/cm, the change in pressure is 4.69 kPa. At 4 V.sub.DC/cm,
the change in pressure is 19.7 kPa or almost 3 times higher than at
a gradient of 2.34 V.sub.DC/cm. Further, when the higher voltage
gradient is applied at the latter stages of treatment, the power
consumption is less. If compaction effort is defined as applied
force times duration (kPa-hrs) the present invention provides a
means of maximizing the compaction effort, while minimizing the
time to achieve the desired bearing capacity. This operation can be
performed in a manner where a higher compaction effort occurs at
conditions of lower current draw (and hence lower power
consumption).
[0125] The overall power consumption is reduced by operating at a
lower voltage gradient initially and then increasing the voltage
gradient as the electrical resistance increases. As well, when
considering the compaction effort for the variable voltage
application from the application of 1,330.6 kPa-hrs is to be
compared to the 1,082 kPa-hrs at a constant voltage gradient of
2.34 VDC/cm. The experimented results indicate that 2.34 VDC/cm
provides more than adequate compaction, so a reduced compaction
effort to 1,082 kPa-hrs would result in a further reduction in
energy consumption of 19.9%. The savings in energy arise by
operating at higher voltage gradients (higher electrostrictive
pressure) and higher electrical resistances. According to the
present invention a continuous feedback loop on the applied current
can be used to provide real-time control on the power input. In
this way power consumption can be optimized in real time as the MFT
is being treated using compaction effort as a guide.
[0126] Electro-osmotic velocities were calculated at 1.5
V.sub.DC/cm and 2.34 V.sub.DC/cm to determine whether a lower
voltage gradient would impact on treatment time and costs. It was
determined that the electro-osmotic flow velocity at 2.34
V.sub.DC/cm was 19% faster (or a 6 hour difference in pH wave
travel time) than at a voltage gradient of 1.5 V.sub.DC/cm.
Therefore, a slower travel time in the electro-osmotic water
release portion of the treatment is expected.
[0127] The 2-D test cell has septum ports spaced 5 cm apart that
allow for the insertion of metal probes to measure voltage drops
within the MFT as it is being treated. These voltage drops can be
used to determine permittivity and the electrostrictive pressure.
With this, real-time pressure measurements can be determined. This
is important in the application of EKC, since electrostrictive (ES)
forces can be both positive and negative, and monitoring requires
that the forces result in compaction of the MFT. When the pressure
becomes negative, the polarity can be reversed to increase the
pressure.
[0128] Only a limited amount of EKC occurs during the first 24 hrs,
yet the most significant power consumption occurs; greater
compaction occurs during the period from 48 to approximately 96
hrs. Reversing the polarity also results in increasing the
compaction pressure. As a result, the present invention comprehends
that EKC be operated in a mode focusing on electro-osmosis to
release water for approximately the first 24 hours, increasing the
voltage gradient thereafter.
[0129] Unconfined compression testing was performed by obtaining
samples from the 1 and 2-D test cells. For the 1-D cells, the
sample was pushed from the apparatus and trimmed to form a right
angle cylinder. For the 2-D cells, samples were prepared using two
methods: 1) a tube was inserted into the MFT in the test cell
apparatus, which allowed for some testing in different
orientations; and, 2) the EKC treated MFT was removed from the test
cell and re-molded into right-angle cylinders for testing.
[0130] The tests were performed not only to demonstrate that the
required bearing capacity can be achieved, but also to evaluate
variations in treatment. For example:
[0131] Variations in horizontal and vertical bearing
capacities;
[0132] Changes in applied gradient; and
[0133] Variations in moisture content.
[0134] The application of the electrostrictive force is from anode
to cathode. As a result, the unconfined compression test shows
greater bearing capacity in the horizontal direction versus the
vertical. This was seen in the failure planes in the unconfined
compression testing. An observed main diagonal failure plane
represented the failure occurring that is being measured as the
bearing capacity. Observed subvertical failure planes were the
result of the electrostrictive compactive force being applied
horizontally across the sample.
[0135] Results from samples obtained from one test run (MFT2),
where horizontal and vertical bearing capacities are compared are
summarized below.
TABLE-US-00002 TABLE 2 Summary of comparing horizontal to vertical
bearing capacities Identifier Bearing Capacity (kPa) Moisture
Content (%) MFT 2 Horizontal 19,669.79 15.6 MFT 2 Vertical 10.53
58.2 MFT 2 Horizontal 987.05 43.99
[0136] Samples were tested after undergoing EKC treatment at 2.34
V.sub.DC/cm, followed by treatment at 4.3 V.sub.DC/cm. This was
done by taking the MFT treated in the 2-D cell and then re-molding
the treated material into the 1-D cell. This was done to see if
there was a loss in bearing capacity on remolding and whether
additional compaction could be achieved. This test was to try to
determine, if at full scale the removal of the treated MFT from the
treatment cells would result in loss of bearing capacity and also
whether material when remolded as fill could be effectively
compacted. Presented below is a comparison of the bearing
capacities from 2.34 V.sub.DC/cm and 4.3 V.sub.DC/cm
treatments.
TABLE-US-00003 TABLE 3 Comparison of Bearing Capacities achieved by
Differing Voltage Gradients Identifier Bearing Capacity (kPa)
Moisture Content (%) MFT4 2.3 V.sub.DC/cm 33.923 72.7 MFT4 4.3
V.sub.DC/cm 66.089 45.91
[0137] Vane shear testing, as noted above was used as both a
screening tool to add to the database on bearing capacities. Vane
shear testing as a screening tool, involved opening up the test
cell during treatment and obtaining the data. At the completion of
treatment, the vane shear measurements were made both in the test
cell and as with the unconfined compression tests, then remolded
material in a bowl.
TABLE-US-00004 TABLE 4 Summary of Vane Shear Bearing Capacity Tests
Sample ID Bearing Capacity (kPa) MFT2 Anode 19.6 MFT2 Mid 137.3
MFT2 Cathode 88.3 MFT3 Anode 39.2 MFT3 Cathode 49.0 MFT4.sub.10 -1
25.5 MFT4.sub.10 -2 19.6 MFT4.sub.10 -3 21.6 MFT4.sub.10 -4 25.5
MFT4.sub.10 -5 31.4 MFT5-1 88.3 MFT5-2 58.8 MFT5-3 49.0
[0138] At the completion of each unconfined compression test, the
samples were weighed and placed in a muffle furnace for drying and
then re-weighed to determine moisture content. This data is
presented graphically in FIG. 16, and shows that moisture contents
on the samples that achieved 20 kPa or better, ranged between 30
and 50%. Also there appears to be a general trend of increasing
bearing capacity with decreasing moisture content. Therefore,
according to the present invention, it is believed that significant
effort to reduce moisture content is not required to achieve the
required bearing capacities of 5 to 10 kPa.
[0139] The specific gravity testing was complicated by the porous
nature of the dried treated MFT. The tests were run two ways: 1)
the dried treated MFT was ground using a mortar and pestle and
inserted in the pycnometer for weighing; and 2) small pieces of the
dried treated MFT were inserted into the pycnometer for weighing.
In both cases the results were lower than anticipated. It is
believed that even in its ground up state, there remains
significant air filled pore spaces that cannot be fully water
filled to obtained accurate determinations. The data from this
testing is summarized below in Table 5.
TABLE-US-00005 TABLE 5 Specific Gravity Values Empty Pycnom-
Pycnom- Pycnom- Pycnom- eter + eter + eter + Specific Specific eter
Dry Soil Dry Soil + Water Gravity Gravity Mass Mass Water Mass Mass
UIC (Lambe) 37.6 47.6 122.5 119.6 1.408 1.406 32 42 119.6 117.6
1.25 1.248
Water recovered during the EKC process was analyzed for the
following parameters:
TABLE-US-00006 Barium Copper Nickel Calcium Magnesium Manganese
Potassium Sodium Boron Zinc Chloride Sulfate Carbonate
Bicarbonate
[0140] Another parameter to monitor is effect of naphthenic acids.
Naphthenic acids are natural constituents of petroleum, formed
through the oxidation of naphthenes, representing as much as 4% of
raw petroleum by weight, and represents an important component of
the waste generated during petroleum processing. In the Athabasca
oil sands, naphthenic acids become dissolved and concentrated in
tailings water as a result of the hot-water process used to extract
bitumen from mined oil sands. A consequence of the hot water
extraction process is that the alkalinity (pH=8) promotes
solubilization of naphthenic acids (pKa.about.5), thereby
concentrating them as mixtures of sodium salts in aqueous tailings
(sodium naphthenate). The actual amounts of naphthenic acids in the
tailings ponds are typically between 80 and 110 mg/l.
[0141] The present invention contemplates monitoring leaching of
naphthenic acid in the vicinity of the cathode and evaluating if
compounds form, such as sodium naphthenic. Adequate treatment of
any such compounds is desirable.
[0142] Summarized below are the major ion data in both mg/l and
meq/l.
TABLE-US-00007 TABLE 6 Major Ion Data Evaluation Calcium
(Ca.sup.+2) Sodium (Na.sup.+) Mg/l Meq/l % Mg/l Meq/l % 11.5 0.574
0.58 2040 88.74 89.25 Total Potassium (K.sup.+) Magnesium
(Mg.sup.2+) Cations Mg/l Meq/l % Mg/l Meq/l % Meq/l 259 6.62 6.66
42.5 3.50 3.52 99.43 Carbonate (CO.sub.3.sup.2-) Bicarbonate
(HCO.sub.3.sup.-) Mg/l Meq/l % Mg/l Meq/l % 4160 138.53 92.1 680
11.15 7.41 Total Sulphate (SO.sub.4.sup.2-) Choride (Cl.sup.-)
Anions Mg/l Meq/l % Mg/l Meq/l % Meq/l 12 0.25 0.17 17 0.48 0.32
150.4 Charge/balance error = -20.4; pH = 12.2; TDS = 7222.
The charge balance error shows that there are more anions than
cations. This is to be expected given that water is recovered from
the cathode reservoir where the water and anions are drawn to the
cathode reservoir as a result of the attractive forces. Given the
faint chlorine odour, it is expected that chloride is electrolyzed
to chlorine gas. It is also believed that the water chemistry is
also out of balance because of the fate of sodium chloride. As
noted above, sodium is expected to be consumed in the reaction of
naphthalenic acid to sodium naphthenate:
C.sub.9H.sub.17COOH+Na.fwdarw.C.sub.10H.sub.17NaO.sub.2
[0143] This testing has shown that electro-kinetic compaction
treatment according to the present invention is effective for
treating liquid tailings. The treatment described in the
experiments applied a combination of a number of mechanisms:
eletro-osmosis, electro-migration and electrostriction that when
combined and appropriately sequenced can cost-effectively treat
tailings. These experiments indicate that the treatment of MFT and
FFT using this process will meet the requirements and goals of oil
sands producers. The testing consistently achieved 100 kPa bearing
pressures, well above the requirements imposed on the oil sands
producers of treating their tailings to a bearing capacity of 5 kPa
by 2012 and 10 kPa by 2015. Because the electrical resistance
varies throughout the treatments tested, the voltage gradients and
as a result the electro-osmotic flow and compative forces can be
varied to take place when the electrical resistance is at its
greatest and hence the power draw is minimized to produce higher
voltage gradients. The present invention also provides that
multi-stage application of electro-kinetics processes, such as the
application of separate flocculation and electrostriction steps may
be employed to achieve the same benefits. Due to the volume of
material being treated, a few pennies savings per cubic meter can
result in significant overall cost savings or improved operating
margins.
[0144] In one embodiment, MFT/FFT are subjected to the application
a flocculation step prior to the application of an electrostriction
step through the application of a single electric field. The
applied voltage gradient is increased over time. During the
flocculation step the application of the electric field is applied
in a preferred voltage gradient range between 100 V/m and 200 V/m.
Preferably, the voltage gradients increased slowly over time. For
example, the voltage gradient might begin at 100 V/m and will
increase as flocculation occurs until it eventually reaches 400 V/m
at the conclusion of the electrostriction step. As the process
continues, the voltage gradient will preferably be increased until
it reaches a value around 400 V/m. To achieve even results, it is
preferable that increases to the voltage gradient are done slowly.
As the voltage gradient increases, the application of the electric
field will first cause flocculation and will eventually cause
electrostriction of the material. Electrostriction will generally
occur at voltage gradients above 200 V/m. An initial voltage
gradient of 50 V/m or lower is possible, but lower voltage
gradients would mean that the process as a whole would take longer.
Increasing the voltage above 400 V/m during the electrostriction
step is also possible, but a voltage gradient higher than 400 V/m
may result in inefficiencies because the end product would achieve
a level of compactification higher than that is required by current
Alberta regulations. The distance between the electrodes will have
an impact on the time it takes for flocculation and
electrostriction to occur. The farther the distance between the
electrodes the longer the process will take. The exact voltage
gradient to be applied at any time can be determined by a feedback
loop which is dependent on observations of the properties of the
materials being flocculated and compacted. Generally, the voltage
gradient will increase over time, but those increases may not be
linear.
[0145] 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.
* * * * *