CAPACITATIVE ELECTROKINETIC DEWATERING OF SUSPENSIONS

Abstract

Capacitive electrokinetic densification, decontamination and dewatering of suspensions and soils can be performed while controlling and/or preventing chemical and pH changes in the densified material and extracted water. High electrical capacitance electrodes or Electric Double Layer Capacitor (EDLC) electrodes are used which can operate without redox reactions occurring on their surfaces until their developed voltage reaches the standard electrode potential of the electrode. Water-retaining, flexible covers for the EDLC electrodes have drainage and filtering capabilities and are made of a fabric which allows the passage of ions, water and electricity therethrough and facilitate continuous electrical contact between the EDLC electrode and the surrounding suspension.

Claims

1. A capacitive electrokinetic process for densifying solids and recovering water from colloidal suspensions without changes in chemical composition and pH of the densified material and the extracted water, the process comprising the steps of: a) providing an insulated container, the insulated container including at least one outlet drain line for removing water therefrom; b) providing an electric current power supply for generating an electric field through the suspension, wherein the power supply includes a positive pole and a negative pole and is capable of polarity reversal; c) providing at least one pair of high capacitance electric double-layer capacitor (EDLC) electrodes connected to opposite poles of the power supply, wherein each electrode of the at least one pair of EDLC electrodes has a specific capacitance of at least 1.0 farad per gram and is spaced apart from the other electrode within the insulated container; d) providing each EDLC electrode with an electrode cover for placement over the EDLC electrode, wherein each electrode cover is made of a fabric which allows the passage of ions, water and electricity therethrough and facilitates electrical contact between the EDLC electrode and the surrounding suspension; e) providing each electrode cover with a drain conduit, wherein each drain conduit is positioned below its corresponding EDLC electrode and is hydraulically connected to its corresponding electrode cover and to the at least one outlet drain line; f) providing a suspension to be treated in the insulated container; g) applying a potential difference between the EDLC electrodes via the power supply to generate an electric field through the suspension, wherein electric current flow through the suspension is established and maintained by the EDLC electrodes without the occurrence of redox reactions at their surfaces so long as the potential difference between the EDLC electrodes and the ions in the surrounding suspension ions is below the standard electrode potential of the ions in the surrounding suspension; h) detecting the initiation of redox reactions at the EDLC electrode surfaces; i) upon the detection of redox reactions at the EDLC electrode surfaces, reversing the electric field direction through the suspension by reversing the polarity of the potential difference applied between the EDLC electrodes via the power supply; j) removing water from the insulated container through the at least one outlet drain to achieve water extraction from the suspension and densification of solids within the suspension; k) removing the treated suspension from the insulated container; and l) repeating steps (g) through (k).

2. The process of claim 1, wherein the step of (h) detecting the initiation of redox reactions at the EDLC electrode surfaces is performed by detecting pH changes in the water recovered at the at least one outlet drain line.

3. The process of claim 1, wherein the step of (h) detecting the initiation of redox reactions at the EDLC electrode surfaces is performed by detecting voltage changes between the EDLC electrode surfaces and the suspension.

4. The process of claim 3, wherein detecting voltage changes between electrodes and various points in the suspension is used as a guide to affect removal of the densified material from the dewatering cell.

5. The process of claim 1, wherein the specific capacitance of the high capacitance EDLC electrodes is more than 1 farad per gram and less than 5 farads per gram.

6. The process of claim 1, wherein the specific capacitance of the high capacitance EDLC electrodes is more than 5 farads per gram and less than 50 farads per gram.

7. The process of claim 1, wherein the specific capacitance of the high capacitance EDLC electrodes is more than 50 farads per gram and less than 400 farads per gram.

8. The process of claim 1, wherein the specific capacitance of the high capacitance EDLC electrodes is more than 400 farads per gram.

9. The process of claim 1, wherein the suspension to be treated is a colloidal suspension containing charged particles and electro-active materials.

10. The process of claim 1, wherein the suspension to be treated is selected from the group consisting of oil sands tailings, clay-water suspensions such as Mature Fine Tailings (MFT), dispersions or suspensions of inorganic particles that are a by-product of mining, manufacturing or other industrial processes, food and food processing waste suspensions, biological wastes, and biomass sludges.

11. An apparatus for capacitive electrophoretic densification of solids and capacitive electro-osmotic removal of fluids from a colloidal suspension, the apparatus comprising: a) an insulated container for receiving and containing a suspension to be treated, the insulated container including at least one outlet drain line for removing water therefrom; b) an electric current power supply for generating an electric field through the suspension, wherein the power supply includes a positive pole and a negative pole and is capable of polarity reversal; c) at least one pair of high capacitance electric double-layer capacitor (EDLC) electrodes connected to opposite poles of the power supply, wherein each electrode of the at least one pair of EDLC electrodes has a specific capacitance of at least 1.0 farad per gram and is spaced apart from the other electrode within the insulated container; d) at least one pair of electrode covers placed over the at least one pair of EDLC electrodes, wherein each electrode cover is made of a fabric which allows the passage of ions, water and electricity therethrough and facilitates electrical contact between the EDLC electrode and the surrounding suspension; and e) at least one pair of drain conduits positioned below the at least one pair of EDLC electrodes, wherein each drain conduit is hydraulically connected one of the at least one pair of electrode covers and to the at least one outlet drain line;

12. The apparatus of claim 11, wherein the suspension to be treated is a colloidal suspension containing charged particles and electro-active materials.

13. The apparatus of claim 11, wherein the suspension to be treated is selected from the group consisting of oil sands tailings, clay-water suspensions such as Mature Fine Tailings (MFT), dispersions or suspensions of inorganic particles that are a by-product of mining, manufacturing or other industrial processes, food and food processing waste suspensions, biological wastes, and biomass sludges.

14. The apparatus of claim 11, wherein the specific capacitance of the high capacitance EDLC electrodes is more than 1 farad per gram and less than 5 farads per gram.

15. The apparatus of claim 11, wherein the specific capacitance of the high capacitance EDLC electrodes is more than 5 farads per gram and less than 50 farads per gram.

16. The apparatus of claim 11, wherein the specific capacitance of the high capacitance EDLC electrodes is more than 50 farads per gram and less than 400 farads per gram.

17. The apparatus of claim 11, wherein the specific capacitance of the high capacitance EDLC electrodes is more than 400 farads per gram.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0048] FIG. 1 is a schematic representation of the electric potential pattern (voltage drop) within a prior art electrochemical cell;

[0049] FIG. 2 is a schematic representation of one embodiment of an apparatus of the present invention;

[0050] FIG. 3 is a schematic representation of a multicompartmental embodiment that includes a number of interceptor drain positioned between the electrodes;

[0051] FIG. 4 is a cross section view of the multicompartmental embodiment presented in FIG. 3;

[0052] FIG. 5 is an illustration of a preferred embodiment of an electrode assembly for a capacitive electrokinetic dewatering apparatus, according to the present invention

[0053] FIG. 6 illustrates a capacitive electrokinetic dewatering apparatus according to the invention

[0054] FIG. 7 presents a graphical output of voltage developed vs. time for first cycle of a constant current dewatering test.

DETAILED DESCRIPTION OF THE INVENTION

[0055] All terms used herein, relating to physical properties not specifically defined herein, are assumed to be used in their engineering sense and usability for the intended purpose. For example, when a pipe is referred to as a pipe drain, it is assumed that it has the required engineering specifications with respect to perforations and structural integrity and internal and external diameter to function as a pipe drain without excessive hydraulic resistance as a properly designed pipe drain would.

[0056] As defined herein, the terms active electrode and counter electrode can also mean anode and cathode, depending on their charge.

[0057] The terms ion or ions refer to an atom or molecule with a net electric charge due to the loss or gain of one or more electron. In electrolytes or in entrained water within a suspension, ions are hydrated ions which means that they are covered by a shell of water molecules. The amount of charge of an ion depends on the number of electrons lost or gained. For any ion missing or gaining one electron, the net charge is equals to that of an electron equal to 1.60217662?10.sup.?19 Coulombs.

[0058] The term insulated container refers to a container made of non-conductive material such as plastics or my metal covered by insulating paint, or a container internally lined with such non-conductive materials such as plastics.

[0059] The terms suspension or slurry refer in general to suspensions of electro-active materials in a fluid such as aqueous suspensions of fine mineral solids, particulate/liquid dispersions or suspensions, or low-density mixtures of suspended loads of surface-charged particles in a fluid, and in particular to clay-water suspensions such as Mature Fine Tailings (MFT), dispersions or suspensions of inorganic particles that are a by-product of mining, manufacturing or other industrial processes, food and food processing waste suspensions, biological wastes, and biomass sludges or oil sands tailings sludge, irrespective of the existence of other organic or inorganic constituents in the mix.

[0060] Electrode covers as referred to herein are preferably made of fabrics having the combined capabilities of being a water retaining (wettable) or hydrophilic, flexible single or multilayered fabrics with drainage and filtering capability and the capability to pass electric currents (ions) when wet and gravitationally drained. The draining capability of these fabrics also allow for passage of electro-osmotically generated water flow to pipe drains attached to them when used as electrode covers while also allowing for deposition of densified material thereon.

[0061] The capability of a fabric to function as an electrode cover is a result of the fabric having sufficient hydraulic conductivity to pass water across the thickness or through it and along its length or width without appreciable excess pressure buildup within its pores/passages. The capability of the fabric to function as a filter is the result of the fabric having appropriate pore sizes to prevent or effectively reduce the passage of suspension particles, so that the particles do not effectively plug the pores. The capability of the fabric to pass ions and water through its pores/internal passages when wet but gravitationally drained is the result of the fabric having the ability to form continuous ion flow paths through the fabric when wet while possessing such fine texture as to mobilize capillary forces to retain water on its inner porosity even when gravitationally drained.

[0062] Procedures for evaluating the drainage, the filtering and ion conduction capabilities (electric conductivity) of such fabrics are well known by those of skill in the art of geotechnical engineering and electrochemical engineering. Non-limiting examples of water retaining fabrics that can function as electrode covers and which also have the capability to pass ions under the influence of electric fields when wet, even when gravitationally drained, include the commercially available ShamWow!? cloth that has been found to be quite suitable for such uses. Other non-limiting examples of suitable electrode cover and drain fabrics can include non-woven fabrics such as stiffened felt or premium felt (sold under the trademark Creatology?) or various grades of non-woven clothing insulation liner (sold under the trademark Pellon?). Other fabrics may also be useful so long as their water retention, filtering, drainage and ion conductance properties match or qualify them for the intended use with respect to the suspension being treated.

[0063] When the ions in a conductive solution or suspension come in contact with a charged electrode, two differing outcomes are possible. If the potential difference between the ions absorbed to the surface of a charged electrode and the electrode body is more than the minimum potential difference needed to initiate electrode redox reactions, then such reactions take place and electrons are exchanged between the ions and the electrode. This is the concept of Standard Electrode Potential well known by people even with basic knowledge of electrochemistry as also referenced for the splitting of water molecules under Equation 1 above. However, if this potential difference is less than the minimum potential difference needed for initiation of redox reactions, that is, if this potential difference is less than the standard electrode potential of the ions absorbed to the electrode, then the ions remain capacitively absorbed to the electrode and nothing further happens. As a numerical example and assuming standard conditions (25 degree Celsius, 1.0 mole per liter concentration at 1.0 atmosphere pressure), if the potential difference between the negatively charged chlorine ions (Cl.sup.?) and a positively charged electrode is less than 1.36 volts (the standard electrode potential for oxidation reaction of chlorine ions), there will be no charge exchange between the electrode and these chlorine ions and these ions will just sit on the electrode without charge exchange. The Standard Electrode Potentials for electrochemical oxidation and reduction of ions are listed on Standard Electrode Potential tables in most references on electrochemistry, as know by persons with ordinary skills in the art of electrochemistry. These tables indicate that if the potential of an electrode is 1.36 volts higher than chlorine ions absorbed to it, the extra electron on the chlorine ion (Cl.sup.?) would leave the ion and enter the positively charged electrode, thus oxidizing the chlorine ion and transforming it into chlorine element (gas). With a view to Equation 2, this means that if the amount of charge Q capacitively absorbed to a given electrode with a capacitance C is less than the amount calculated at a voltage V of 1.36 volts, the voltage between the two charged plates of this internal capacitor (one plate being the absorbed hydrated ions separated by their hydration shell from the second plate that is the charged electrode body) is less than the minimum required for charge exchange, then nothing happens and the ions just sit on the electrode. However, if this internal capacitor is filled up to 1.36 volts potential difference between the capacitor plates, then the electrons would leave the chlorine ion and enter the electrode. This voltage of 1.36 volts is the minimum potential difference needed for this oxidation reaction. For appreciable rates of reaction at such electrodes, an over potential will have to be applied. The necessity for a minimum voltage development and filling of the capacitance of the electrode to such minimum voltages before electrode reactions occur means that if the capacitance of the electrode is high, large amounts of ions could be absorbed to it without any electrode reactions. As a numerical example, if an electrode has a mass of 60 grams and a specific capacitance (defined as capacitance per unit mass) of 5 Farads per gram, it would have a capacitance of 300 Farads. This means that as long as the total amount of chlorine ions capacitively absorbed to this electrode are less than 408 coulombs (see Equation 2, Q=C*V or Q=300 farads*1.36 volts=408 coulombs), there would be no electrode reactions at this electrode and no chlorine gas will be generated as the potential difference between these ions and the electrode will be less than 1.36 volts.

[0064] In this case if we assume the electric current I between two oppositely charged high capacitance EDLC electrodes, each with a capacitance of 300 farads placed in a common salt (NaCl) electrolyte is 2 amps and the resistance R of thus constructed circuit (power supply, electrode and the electrolyte) is 10 ohms, then based on the ohm's law, (V=R*I) the voltage supplied by the power source will be (10 ohms*2 amps) 20 volts. In this case the time t in seconds required to fill the capacitance of the positively charged electrode absorbing the chlorine ions will approximately be (t=Q/I) or 408 coulombs/2 amps equal to 204 seconds or 3.4 minutes. This also means that after about 3.4 minutes, electrode reactions could also begin on this high capacitance electrode. In comparison, if the same electrodes were metallic with typical capacitances in the range of a few milli-farads, then their capacitance would fill up to 1.36 in a few milli-seconds (almost instantaneously) and the electrode reactions would start almost immediately. These facts show that by the use of high capacitance EDLC electrodes, rather large electric currents can be sustained through electrolytes without the occurrence of electrode reactions. Further, it is noted if two such electrodes are used within an electrochemical cell, one being positively charged and the other negatively charged, based on Equation 4, the equivalent capacitance of the two EDLC electrodes each with a capacitance of 300 Farads will be 150 Farads. Therefore, when considering both electrodes, the sum of potential differences between both electrodes and the ions absorbed to each of them should be considered.

[0065] In the above example, once the capacitance of the 300 Farad EDLC electrode is reached, that is, after 204 (two hundred and four) seconds under a current of 2 amps, electrode reactions will start. The amount of chlorine gas produced would then follow Faraday's law of electrolysis: The mass of the substance deposited or liberated at any electrode is directly proportional to the quantity of electricity or charge passed, which is one mole of products per 96,485.3 coulombs. This means that the amount of chlorine produced could be controlled by the amount of charge passed through the electrode after its capacitance is filled to 1.36 volts.

[0066] As is well-known in the art of super capacitors, Electric Double Layer Capacitors (EDLC) are also known by such names as super capacitors or ultra capacitors, and use two oppositely charged high-capacitance electrodes. When used as electrodes in electrochemical processes, as described in U.S. Pat. No. 8,715,477, U.S. Pat. No. 9,309,133, and U.S. Pat. No. 9,315,398, each entitled Apparatus and process for separation and selective recomposition of ions by the present inventor, the individual ion absorption capacitance of each electrode is utilized when used in conjunction with other EDLC electrodes or electrodes that undergo redox reactions.

[0067] To qualify as a high-capacitance electrode as referred to herein, such as an EDLC electrode, the specific capacitance (defined as capacitance per unit mass of the electrode) is understood to be at least 1.0 Farad per gram, preferably higher than 5.0 Farads per gram, and ideally higher than 50 Farads per gram. Depending on the surface area, material of construction and the electrolytes used, especially the electrolyte concentration and ion types, specific capacitances of about 400 Farads per gram have been reported in the literature for such ultra capacitors. In practical and industrial applications of this invention, EDLC electrodes can typically have a height of 100 cm, a width of 200 cm, and a thickness of 2 cm, resulting in a mass of 24 kg to 30 kg and a capacitances of several hundred thousand Farads.

[0068] The present invention provides an improved process and apparatus for dewatering of particulate/liquid dispersions or suspensions, including oil sands tailings, using electrophoresis and electro-osmosis processes wherein the chemical composition and pH of the densified material and the extracted water are controlled and could be even unchanged. This invention also teaches specific electrode arrangements for such suspensions, and specific filtering and draining materials and fabrics for use as electrode covers that provide for improved means for the removal of densified material from the vicinity of the electrodes.

[0069] FIG. 5 illustrates a preferred embodiment of an electrode assembly 30 for a capacitive electrokinetic dewatering apparatus, according to the present invention. As shown, a planar sheet electrode 31 is in the form of a high capacitance electric double layer capacitor (EDLC) electrode sheet and is covered by an electrode cover fabric 32. The cover 32 is typically stretched over the electrode 31 in a coextensive manner, creating intimate contact with the electrode and facilitating electrical contact between the EDLC electrode and the surrounding suspension. The electrode cover 32 also allows for effective filtration and drainage of any water entering it to a drain pipe 33. The drain pipe 33 is typically fluidly connected to the electrode cover 32 at the bottom of the electrode and inside the cover and passes through an opening (not shown) in the electrode cover which is in fluid communication with the drain pipe 33. The drain pipe 33 can drain the fluid entering the electrode cover and then pass the fluid through a side wall of the dewatering cell, where the fluid can then be delivered to an outlet drain pipe for further conveyance and containment of the recovered fluid.

[0070] FIG. 5 also illustrates a plastic blade 34 extending across the electrode assembly. This blade is supported by two rods 35 that are used to move it up and down to scrape the densified material off the surface of the electrode cover 32. The mechanical system attached to the rods 35 is not shown, but the rods are powered by a mechanical system which causes the blade to move along the surface of the electrode cover 32 when scraping downward, but when moving back up the blade 34 is positioned away from the electrode cover so as to not make contact with it. Although the blade 34 and rods 35 are shown in FIG. 5 only on one side of the electrode, it is to be understood that there are two of them, one on each side of the electrode assembly.

[0071] FIG. 6 illustrates a capacitive electrokinetic dewatering apparatus 40 according to the invention, which includes an insulated container 41 in the shape of a rectangular box. The container 41 has an open top and is equipped with electrode assemblies 42 and 43, each of which are essentially identical to the electrode assembly 30 in FIG. 5. In FIG. 6, connecting cables 45 electrically connect electrode assemblies 42 to one pole 44 of a reversible direct current (DC) power supply 45, and connecting cables 46 electrically connect electrode assemblies 43 to the other pole 47 of the reversible DC power supply 45. Electrode assemblies 42 and 43 have a depth less than the depth of the insulated container 41. In FIG. 6, pipe drains 48 pass through the side wall of the container 41 and can be connected to a collection drain pipe (not shown) for further conveyance and containment of the recovered fluid. The dewatering cell's insulated container 41 is filled to the top via supply lines (not shown) with a suspension to be treated 49. The scraping blades 34 and support rods 35 of FIG. 5 are not shown in FIG. 6 to avoid crowding the picture, but are assumed to be present. For proper operation and control of the process the power supply 45 should be fully instrumented and equipped with recordable voltmeters and ammeters.

[0072] FIG. 6 shows eight (8) electrode assemblies, four of which are labeled 42 and four of which are labeled 43; however, depending on the cell size, the desired production rate, and the specifications of the suspension, the number of electrode assemblies for a given dewatering application can range from a minimum of two (2) electrode assemblies to any larger number of assemblies. The electrode assemblies 42 and 43 can span the entire width of the cell, forming separated compartments between each electrode assembly, or they can be positioned such that gaps exist between the assemblies and the side walls of the insulated container 41. This positioning of the electrode assemblies 42 and 43 within the container 41 will determine whether the suspension 49 can be delivered into the insulated container 41 via each individual compartment, or on mass to the entire container 41 from a single delivery point, allowing the suspension to flow from one compartment to the other.

[0073] Once the suspension to be treated is delivered to the cell, the electrodes can be energized by the DC power supply 45, delivering electric current to each and every electrode assembly 42 and 43 and establishing current flow within the cell. The polarity of the electrodes in the electrode assemblies 42 and 43 are reversible, and the capacitive electrodes within each assembly can function either as anodes or as cathodes, depending on the polarity of the power source 45 they are connected to.

[0074] With establishment of the electric field between the capacitive electrodes, both electro-osmotic and electrophoretic processes will be initiated due to the electric field generated through the suspension. Water will flow towards the cathodes and the solids from within the suspension will move towards the anodes. As electro-osmotically generated water enters the cathodes' electrode covers, the water is drained off by the electrode covers towards the drain pipes 48 at the bottom of each assembly and right below the electrodes, and from there the water can be conveyed to the outside of the cell. At the same time, the solid materials from the suspension move towards the anodes, and the densified materials adhere to and deposit on the anodes' electrode covers. This densified material is also subject to electro-osmotic process, and water is further removed from the solidifying material at the anodes, due to the water migrating towards the cathodes. Thus, the suspension in the vicinity of the anodes typically forms a lower water content, densified material which is deposited on the anode electrode cover. Some densification of the suspension on the cathode covers due to water movement may also occur. The densified material on the anode cover has a lower water content than the suspension materials located near the cathodes, and the electrical resistivity of this densified material is higher than the remainder of the suspension. As a result, there will be higher voltage drop gradients near the anodes as compared to the rest of the suspension mass between each pair of the electrode assemblies.

[0075] In practical use of such preferred embodiment the capacitance of the individual electrodes and that of the electrode system as well as the chemical composition of the water within the suspension will be known. The known electrode system capacitance and electric current measurements provided by the power supply instrumentation, would allow the operator to determine the total charge Q in units of coulombs moved on to the electrodes at any time t from the start of a charging cycle (Q=I*t), where I is the measured current in amps and t is time in seconds. Then based on Equation 2 the voltage mobilized between the electrodes and the adjacent suspension could be calculated. Comparison of these calculated voltages with the standard electrode potential of ion species present in the suspension water then determines the allowable mobilized potential on the EDLC electrodes based on the standard electrode potentials.

[0076] As an example, with a view to FIG. 6, if the dewatering cell 41 is assumed to have internal dimensions of about 4 meters length and 2 meters width and a height of about 2 meters, we could have four sets of 1 meter deep and 2 meters wide and 3 cm thick electrodes in it. The spacing for these electrodes would then be about 50 centimeters and will result in seven compartments between the electrodes. Each of these electrodes would have a dry mass of about 45 Kilograms and based on a specific capacitance of 10 farads per gram will have capacitance of 450,000 Farads. This translates to a total capacitance of 1,575,000 Farads for the entire electrode systems. If we limit the mobilized voltage for this capacitor system to 1.2 volts which is slightly less than the required potential difference for splitting water to produce hydrogen and oxygen gasses (1.23 volts), the total charge allowed on the electrodes will be 1,890,000 coulombs. With an applied voltage of about 160 volts and a current of 80 amps between the electrodes the required time for the capacitance of these electrodes to fill to 1.2 volts for the first time will be 3375 seconds which is 56.25 minutes. For steady state charging and discharging cycles this time will be almost doubled. During this time the densified material may need to be scraped off several times before the polarity of voltage applied to the two sets of electrodes is reversed. Determination of the occurrence of redox reactions at the electrodes can be aided by a voltage measurement system which can detect resistance buildup within different areas of the suspension within the insulated container, which can also indicate if sufficient densification has occurred. Alternatively pH probes could be installed on drain lines to help detect the occurrence of redox reaction signified by a rise in pH of the extracted water. This information can then be used by the operators as a means of determining the proper time to reverse the polarity of the power source, and/or to activate the scraping mechanism of each electrode assembly.

[0077] Once the polarity is reversed, the EDLC electrodes that were previously acting as cathodes become anodes, and vice versa. Thus, upon reversal of polarity, water will now reverse its direction within the suspension and move towards the new cathodes, and the colloidal solid materials within the suspension will move towards the new anodes, where densified materials will now deposit on their electrode covers.

[0078] As an example, with a view to FIG. 6, if the dewatering cell 41 is assumed to have internal dimensions of about 4 meters length and 2 meters width and a height of about 2 meters, we could have four sets of EDLC electrodes each with a height of one meter, a width of 2 meters, and a thickness of 2 cm, and if the EDLC electrodes are made of an aerogel composite material with a dry density of about 0.75 grams per cubic centimeter and a specific capacitance of about 10 Farads per gram, then each will have a capacitance of 300,000 Farads. With these parameters, and assuming that the dominant salt in the suspension water is common salt (NaCl) and in order to avoid redox reactions at such electrodes, the voltage developed between the capacitive cathode electrodes and the ions absorbed to them should be less than standard electrode potential of 0.829 volts, which is the voltage for the water splitting electrode reaction based on Equation 1. For this electrode functioning as a cathode and receiving current of 75 amps from each side (total 150 amps), it would take approximately 1658 seconds, that is, more than 27 minutes before sufficient charge of 248,700 coulombs (300,000 farads*0.829 volts=248,700 coulombs, see Equation 2) accumulates on each high capacitance EDLC cathode electrode plate to cause redox reactions to begin to occur. Experimental observations show that this is more than enough time to reduce the water content of typical MFT to equivalent solids contents of more than 70%. At this stage the same amount of charge will be stored on the anode counter electrodes. Determination of the occurrence of redox reactions at the electrodes can be aided by a voltage measurement system which can detect resistance buildup within different areas of the suspension within the insulated container, which can also indicate if sufficient densification has occurred. Alternatively, pH probes could be installed on drain lines to help detect the occurrence of redox reaction signified by a rise in pH of the extracted water. This information can then be used by the operators as a means of determining the proper time to reverse the polarity of the power source, and/or to activate the scraping mechanism of each electrode assembly.

[0079] If at this stage the polarity of the potentials applied to the electrodes is reversed, there would be approximately 248,700 coulombs of charge stored on each electrode. This amount of charge would have to be discharge and then the equivalent amount will have to be recharged before the voltage buildup in the electrodes that were previously functioning as anodes and then were transformed to cathodes by polarity reversal to begin causing electrode reactions according to Equation 1 above (water splitting). This will almost double the time duration for each of the electrodes to function without electrode reaction and will constitute the steady state mode of operation for this dewatering cell.

[0080] If however the polarity is not reversed, the electrodes functioning as cathodes will now have their potential with respect to the ions adjacent to them exceed the standard electrode potential and water splitting would occur. This will result in introduction of hydroxide ions at the cathode electrodes that would change the pH of water entering to their location through the electrode cover. These newly generated hydroxide ions will also begin to move towards the anodes through the suspension. At this stage the potential difference between the cathode electrode and the ions and water adjacent to it will include an over potential and will be somewhat higher than the standard electrode potential of water splitting reaction of Equation 1. At the counter anodes however, the potential buildup will continue. If this process is allowed to continue by about 1062 seconds more there will be an additional 159,300 coulombs (1062 seconds*150 amps=159,300 coulombs) of charge moved on to each anode. Combined with 248,700 coulombs previously charged to these electrodes, the total charge on these high capacitance EDLC electrodes will be (248,700+159,300) 408,000 coulombs. This will raise the potential of these anode electrodes with respect to chlorine ions assumed to be present in the water adjacent to them to 1.36 volts (408,000 coulombs/300,000 Farad) which is the standard electrode potential for oxidation of chlorine ions, resulting in production of chlorine gas. Continued operation of the system will then result in production of chlorine gas.

[0081] Based on the above, with reversing the potentials applied redox reactions could be totally avoided. Alternatively, they could be also allowed, to the extend desired by allowing certain degree of their occurrence based on the degree of change desired in the chemical composition and pH of the densified material and the extracted water before the polarity of the potentials applied between the two sets of electrodes are reversed.

[0082] Test Results

[0083] The test setup used is similar to that shown on FIG. 6, wherein the container 41 was a plastic container with a width of 10.8 cm, a length of 20 cm and a depth of 20 cm. Four (4) high capacitance EDLC electrodes, each with a planar dimension of about 6 cm by 6 cm and a thickness of about 1.1 to 1.2 cm were used. Each electrode was also covered by a 0.5 mm thick non-woven clothing insulation liner marketed under Pellon? brand as electrode cover fabric. This fabric was cut to such a size that allowed the complete covering of the electrodes with edges of the fabric glued together to create full encapsulation of the electrodes. These formed electrode assemblies 42 and 43 of FIG. 6 for this test.

[0084] Each EDLC electrode was also equipped with a nut and bolt assembly connecting the electrode to a wire. The bolted connections and the exposed wire connected to each of them were covered by a water-resistant glue (Marine glue by GOOP) to prevent interaction of these metallic wires with the suspension. The drain conduits 48 were clear flexible plastic tubes with an outside diameter of about 5 mm, and with an inside diameter of about 3 mm. Being made of clear plastic, these drainage tubes allowed direct observation of water flow in them. Each drain conduit penetrated the electrode cover by about 3 cm and was positioned below the electrode body and right next to it. This allowed any water passing through the electrode cover to be easily drained. The points of entry of each drain conduit through the electrode cover fabrics were also sealed with the same water-resistant glue.

[0085] The four electrode assemblies were placed along the 10.8 cm width of the plastic container, such that one electrode was practically positioned right next to each side wall and the other two were positioned at equal distances between them. This resulted in electrode spacings of about 1.6 to 1.8 cm. The drain conduits extended to the outside of the plastic container and exited it through the side wall at mid-height of the container on the 10.8 cm by 20 cm deep side. The drain tubes had a length of about 25 centimeters beyond the edge of the container. This allowed the tips of these tubes to be raised to allow the drained water to rise in them or could be lowered to drain the water entering them off. To make sure that the spacing between the electrodes were maintained, each of the four drain lines were passed through a 5 mm in diameter hole drilled on a 1.5 mm thick plastic strip positioned very close to the electrodes. The same was done for the wires. This way the separations between the electrodes were maintained.

[0086] In this test the connection wires from every other electrode were joined together. These wires were then connected to opposite poles of Gamry Reference 3000 potentiostat, as the power supply similar to what is shown on FIG. 6. When the pole for the reference electrode in a potentiostat is connected to one of the active or the counter electrode poles, as for this test, it functions as a DC power supply. Gamry Reference 3000 potentiostat is a high precision device capable of applying constant voltages while measuring the generated current and applying constant currents while measuring the generated voltages. The current and the voltages are displayed on a computer screen and are also numerically and graphically saved. The tests conducted were constant current tests. As such, the capacitance and voltage buildup were analyzed using the following basic relationship for charging capacitors:

V=Q/C+RI(Equation 5)

[0087] Based on the above and at time zero, that is at the beginning of the test, with stored charge Q on the capacitor being zero, the voltage observed, denoted as V, is the product of the current I, that for constant current test is a constant, and the total electric resistance of the circuit R, that is V.sub.i=RI. Assuming this product is constant (as a good approximation), any voltage buildup can be viewed as resulting from charging of the capacitor, with the capacitance C of the EDLC electrode being charged to total charge Q equal to the product of the current I and the time t from the start of charging, i.e. Q=I*t.

[0088] The capacitance of the electrodes system of the two pairs of electrodes used in this test were initially measured when the cell was filled with water from the top of a tailing pond. Allowing the voltage to build up to 1.2 volts above V.sub.i, the resulting capacitance was in the order of 96 Farads. Using this capacitance and allowing for the difference between the applied voltage V and initial voltage V.sub.i to be limited to 1.0 volts (less than electrolysis voltage of water 1.23 volts), it was determined that for a current of 0.8 amps, the time allowed should be about 120 seconds.

[0089] The cell was then filled with MFT from a Suncor tailings pond and the setting for the potentiostat was selected such as it would apply repeated cycles of +0.8 amps and ?0.8 amps for 120 seconds. This insured that the actual potentials mobilized at the electrodes would never be above 1.0 volt even when higher voltages automatically applied by the potentiostat increased to maintain the specified constant current. The graphical output for the first cycle when operating the MFT filled cell is presented on FIG. 7. It shows V.sub.i to be 4.92 volts. The applied voltage to maintain a current of 0.8 amps then increased to 6.2 volts and then dropped to ?3.5 volts when the polarity reversed. In the remaining 120 seconds this voltage further dropped to ?5.8 volts. These variations in the applied voltage to maintain the applied constant currents are then the response of the circuit as the capacitive electrodes are charged and the material densifies.

[0090] When filling the cell with MFT it was made sure that all four electrodes were fully submerged. At the same time a moisture content sample was taken from the MFT that upon drying in a 105 degree Celsius oven by the next day showed that it had a moisture content of 146.6%, corresponding to a solids content of 40.5%. The setup was then allowed to sit idle for about an hour to allow for full saturation of the electrodes and electrode covers. During this time it was observed that a small amount of rather turbid water filled the drain lines. The pH of this water was measured by a pH paper (Fisher scientific, Fisher brand pH paper 0.0 to 14.0). It indicated a pH of about 9.

[0091] The test was then started by powering up the potentiostat while the drain lines were lowered to allow the outward flow of any water entering them. The drain lines from each pair of the electrodes connected to the same pole of the potentiostat were directed to a separate container for collection. The test was carried out by application of 15 cycles of +0.8 amps and (negative) ?0.8 amps, each for 120 seconds. It was observed that within about 20 seconds from the start of each charging cycle, water would flow out of the drain lines connected to the pair of electrodes functioning as the cathodes, with more water coming out of the middle electrode that was positioned between two anodes than the side electrode which was adjacent to one anode. This matched the expectation as the current to and from the middle electrodes are double of the same for the side electrodes.

[0092] With advancement of the test it was also observed that the flow out of the drain lines connected to the cathode electrodes would stop and even reverse when these electrodes switched roles, upon polarity reversal and became anodes. The pH of water coming out was systematically assessed using the same pH paper as before throughout the test at maximum 2 minutes intervals. All measurements throughout the test indicated the same reading of pH=9. At the end of the first 15 cycles of charging and discharging each pair of electrodes 42 milliliters of water had exited the cell. At this point the pH of this sample was measured by a pHTestr 10 waterproof pH meter after its full calibration. The result was a pH of 9.1. The electrical conductivity of this extracted water was measured by a PINPOINT Salinity Monitor conductivity meter which registered a conductivity of 7.3 mS/cm. The sequence of 15 cycles of charging and discharging was continued for two more times with the same pattern of observations, with pH of extracted water remaining in the 9 range as evaluated by pH paper and between 9.1 and 9.2 when measured by the pH meter.

[0093] After the first and the third sequence of 15 cycles of charging and discharging, the electrodes were raised and samples of the densified material on the surface of the electrode covers were extracted for moisture content measurements. The results were the first and the third sequences were 75.0% and 82.4% corresponding to solids contents of 57% and 54% respectively. The total volume of water collected at the end of the test was measured to be 76 milliliters.

[0094] At the end of this phase of the experiment the voltage applied by the potentiostat was raised to constant value of 10 volts at which time the current increased to about 1.94 amps and then began to decrease. Within about one minute after the start of this phase of the test, the pH of the water exiting the cathode drain line increased to 12 range as measured by pH paper. Measurement the pH for the sample collected yielded a pH of 11.8. This clearly shows that once the capacitance of the electrodes reached the point of initiating redox reactions, such reactions were initiated as verified by the observed pH increase of the water coming out of the cathodes.

[0095] The observation that water did flow to and then out of the cathode electrodes used and that the solids from the suspension collected and densified on the surfaces of the electrode covers clearly confirm that electro-osmosis and electrophoresis processes could be caused by capacitively generated electric fields. They further confirm that by the use of high capacitance EDLC electrodes, redox reactions at electrodes could be avoided in capacitive electrokinetic dewatering of suspensions, resulting in maintaining the PH and chemical composition of the extracted water and the densified material constant. They further demonstrate that once the capacitance of such EDLC electrodes are filled, resulting in the development of higher than standard electrode potential differences between the electrode body and the ions absorbed to them, redox reactions are initiated, changing the pH and chemical composition of the water and the densified material.

[0096] The present invention provides a capacitive dewatering apparatus capable of reduced spacing between the capacitive electrodes, which can reduce the voltage level required to achieve a sufficient level of electric field intensity while increasing the rate of densification and dewatering. It also provides a capacitive dewatering process and apparatus in which the composition and pH of the densified material and the extracted water can be controlled or maintained constant/unchanged. Likewise, it provides a capacitive electrokinetic process that minimizes the amount of redox reactions occurring at the electrodes while providing reliable electric contact between the capacitive electrodes and the colloidal suspension and allowing for simultaneous drainage of recovered fluid from the location of the capacitive electrodes. The inventive dewatering process and apparatus disclosed herein increases the exposed surface area of each electrode, in comparison to the volume of the suspension being treated, in comparison to prior art in situ operations, and thus can provide a plant-based capacitive dewatering operation which can produce a high-volume, low-cost treatment for large colloidal suspensions in general, and oil sands tailings in particular.

[0097] While the present invention has been illustrated by the description of embodiments and examples thereof, it is not intended to restrict or in any way limit the scope of the appended claims to such detail. Additional advantages and modifications will be readily apparent to those skilled in the art. Accordingly, departures may be made from such details without departing from the scope of the invention.