Method of performing electrochemical ion exchange

Abstract: An electrode for use in electrochemical deionisation comprises a current feeder disposed asymmetrically in an ion exchange material bonded into a coherent structure i.e. more of the ion exchange material adheres to one side of the current feeder than to the other side. The ion exchange material may be bonded by being provided in intimate admixture with a binder. Such an electrode offers manufacturing ease and the ability to be scaled up in multi-modular form.

Description: This invention relates to a method of performing electrochemical ion exchange and to electrodes for use in electrochemical ion exchange. The electrochemical removal of ions from aqueous solutions onto ion-exchange materials, which may be referred to as electrochemical ion exchange or electrochemical deionization, is known for example from UK patents GB 1 247 732 and GB 2 150 598. It involves establishing an electrochemical cell comprising the aqueous solution as electrolyte, a working electrode and a counter electrode, where at least the working electrode incorporates an ion exchange material such as a resin, and applying a D.C. voltage between the electrodes. For example, to remove cations from the solution, the working electrode incorporates a cation responsive ion exchange material and is made the cathode. A localized change of pH is thereby produced at the electrode due to generation of OH⁻ ions which in turn produce active sites thereon for absorption of cations from the aqueous solution. Subsequent reversal of the voltage enables the absorbed ions to be eluted, so the working electrode can be readily regenerated. The working electrode may comprise a current feeder support carrying an intimate mixture of an ion exchange material, and a binder. The mixture may also include an electrically conducting material. Examples of suitable materials are carbon (e.g. in the form of carbon felt) or platinised titanium (e.g. in the form of an expanded metal mesh) as the current feeder; an ion exchange resin as the ion exchange material; graphite as the electrically conducting material; and an adhesive elastomer as the binder. A particular problem in the use of such a working electrode is to maintain adhesion between the current feeder and the mixture, as in use, gas bubbles may form between them and reduce electrode performance. A way of ameliorating the above problem is provided in GB-B-2 150 598 (corresponding to US-A-4 548 695) where an electrode, comprising a current feeder support carrying an intimate mixture of an ion exchange material and a binder, has an inert, electrolyte- permeable outer envelope adhering to the mixture for maintaining the mixture in contact with the current feeder during use of the electrode in electrochemical deionization. The above-described electrode may, however, need a restrainer to force gases produced at the interface of the current feeder and the ion exchange material to be expelled from the electrode at its edges and not cause distortion of the ion exchange material. Further problems with the above-described electrode are that its production may require more than one stage, that it may be difficult to make flat electrode surfaces, and that scale-up may be difficult. The present invention ameliorates one or more of the above problems by providing a method of performing electrochemical ion exchange using an electrode in asymmetric form. Thus, the invention provides a method of performing electrochemical ion exchange wherein at least one electrode comprises a current feeder in intimate contact with a mixture of particulate ion exchange material and a binder, the binder bonding the current feeder and the mixture into a coherent structure, characterised in that at every part of the current feeder the thickness of the mixture on one surface of the feeder is much less than on the other surface of the feeder so as to enhance diffusion of electrogenerated gas from the current feeder out of the electrode. If desired, the electrode may also have an inert, electrolyte-permeable outer envelope adhering to one or both external surfaces of the said coherent structure. Preferably the ion exchange material is arranged to provide a uniformly thin layer on one surface of the feeder, and a uniformly thicker layer on the other surface. The ion exchange material may be arranged on only one surface of the current feeder, the opposite surface of the current feeder being devoid of ion exchange material. The electrode may be supported in a support module shaped to receive the electrode so that fluid flowing past or through the module contacts an exposed surface of the electrode. It is also preferred that a plurality of such modules can be assembled together to form a multi-cell electrochemical ion exchange apparatus having one or more pathways for fluid to be deionised defined therein and communicating with the electrodes. The electrode of the invention may be made by coating the current feeder with an intimate mixture of the ion exchange material and a solution of the binder in a suitable solvent to give a "wet" electrode; applying the envelope thereto, if required, so that the binder solution enters (or passes through) the pores of the envelope; and drying to give the final electrode. The binder may be an adhesive elastomer. The electrode can be made in a single step moulding process in which the mixture is added to a mould in which the current feeder is first positioned. Once the solvent has evaporated, the electrode can be removed from the mould. Preferably the electrode (and the mould) is rectangular, while the support module defines a channel to accommodate the electrode, the channel being of the same width as the electrode but of greater length, and defining inlet and outlet ports for the liquid at each end of the channel. In use of the electrode, gases are released adjacent the rear of the electrode thus ensuring that the electrode does not expand and obviating the need for a constraint. The outer envelope, if present, has to be electrolyte-permeable in order to allow electrolyte to pass therethrough to permit satisfactory contact between an electrolyte and the ion exchange material in use of the electrode in an electrochemical cell. For example, the envelope may be in the form of a cloth having open pores of size of the order of several micrometres (e.g. less than 20 micrometres). By "inert" is meant that the material of the envelope does not react with any of the reagents or other materials which may be present in use of the electrode in an electrochemical cell and, in particular, does not take part in any electrochemical reaction therein. A specific example of a material for the outer envelope is a non-woven polyamide cloth having, for example a maximum pore size of 18 micrometres. The presence of such a cloth at the rear of the current feeder can aid removal of an electrode from a mould during manufacture. The current feeder may, for example be an amorphous carbon felt or a graphite felt or a metal mesh of, for example stainless steel, platinized titanium, nickel or platinum. Metal current feeders are preferred because of their greater stability during repeated cycling. The ion exchange material may, for example be an ion exchange material such as known in the art, for example AMBERLITE (Registered Trade Mark) IRC50 and 84 (ex Rohm and Haas) and zirconium phosphate (ex Magnesium Elektron). It is preferably, but not essentially, present in finely divided form, for example in a particle size of the order of a hundred micrometres. The intimate mixture may also include an electrically conducting material such as graphite in finely divided form. Subject to appropriate choice of materials and operating conditions, the electrode of the invention may be used as a working electrode in an electrochemical cell for electrochemical deionization of cations or anions or both, for example in water softening or removal of heavy metal cations. As already indicated, the electrode of the invention may be made in a form suitable for scale-up. The invention will now be further described, by way of example only, and with reference to the accompanying drawings wherein: Fig 1 is a schematic sectional diagram of an electrode according to the invention; and Fig 2 is a schematic sectional diagram of a multi-modular electrochemical deionisation apparatus embodying more than one electrode of the invention. Referring to Figure 1, in making an electrode a current feeder 1 is placed in a rectangular recess in a mould 3, and is then covered with a wet mixture of binder, solvent and ion exchange material 2. Preferably the mould 3 is covered by mould-release paper, and to further aid removal of the electrode an electrolyte-permeable cloth is placed in the mould 3 under the current feeder 1. The exposed surface 4 of the ion exchange material 2 carries another electrolyte permeable cloth (not shown). The wet mixture permeates the current feeder 1, but when the solvent evaporates the ion exchange material 2 is asymmetrically distributed, with the bulk of the material 2 on the top surface of the feeder 1 and very little on the rear surface. Typically there might be less than 0.5 mm on the rear surface, possibly between about 0.1 and 0.4 mm and preferably less than 0.2 mm, although the thickness may vary from place to place since the current feeder 1 may not be flat, whereas there might be about 5 mm on the front (top) surface. The preferred thickness on the front surface of the feeder 1 varies with different ion exchange materials 2, as this thickness determines both the theoretical ion exchange capacity of the electrode, and its electrical resistance in operation. When the electrode is dry it is removed from the mould 3. The edges of the current feeder 1 may then be insulated with insulating tape to ensure short-circuits cannot occur. Referring now to Figure 2, the electrode so made is then clipped between a polyethene sheet (not shown) and a counter electrode 5 and located in a flow channel 7 for a liquid to be treated. In the apparatus of Figure 2 two flow channels 7 are depicted, defined by recesses in opposed faces of polymethylmethacrylate sheets 8, 9. The sheets 8, 9 are sealed together by rubber gaskets 6, and clamped tight with bolts (not shown). The recesses in the sheets 8, 9 are of the same width as the electrodes, but about 10 cm longer, so there is an inlet region and an outlet region at the ends of the flow channels 7. One electrode 1, 2 along with its associated counter-electrode 5 is located in each recess, so there are two such electrode assemblies 1, 2, 5 in each flow channel 7. It will be appreciated that since the apparatus is of modular construction, any desired number of flow channels 7 may be created by providing sufficient sheets 9, gaskets 6, and electrode assemblies 1, 2, 5. For operation of the apparatus of Figure 2, electrical connections (not shown) are provided for the electrodes. Also provided is an inlet arrangement (not shown) for causing electrolyte to flow through the flow channels 7 (perpendicular to the plane of Figure 2). In operation, a voltage is applied to the electrodes 1, 5 and electrolyte is passed through the flow channels 7 for ions to be removed therefrom - for example, cations where the ion exchange material 2 is a cation absorber, and deionisation where the material 2 is a cation absorber at a cathode and an anion absorber at an anode. Gas created at the current feeder 1 can diffuse through the thin layer of material 2 at the rear of the electrode, to escape. EXAMPLE (with reference to Figs 1 and 2) A platinised-titanium based electrode (100 cm x 20 cm) was prepared by placing a platinised titanium mesh as current feeder 1 in a PERSPEX (Registered Trade Mark) polymethylmethacrylate mould 3 and adding thereto a mixture comprising zirconium phosphate ion exchange material 2 (840 g; particle size about 20 micrometres; ex Magnesium Elektron) mixed into a solution of a butadiene/styrene copolymer elastomer (148 g) dissolved in 1,1,1 trichloroethane (811 ml). A non-woven polyamide cloth 4 (maximum pore size 18 micrometres, thickness 0.3 mm; sold commercially as VILEDON FT 2118) was applied to the wet electrode surface. The electrode was then dried to give an electrochemical ion exchange electrode in which the current feeder was asymmetrically disposed within the ion-exchange material as shown in Fig 1. In use this electrode is sandwiched between an expanded mesh platinised titanium counter electrode and a sheet of polyethene, and fits closely in the recess in a sheet 8 or 9. Electrolyte undergoing treatment has easy access to the front surface 4 of the electrode, whereas access to the rear surface is considerably inhibited by the close fit of the electrode in the recess.

Claims: A method of performing electrochemical ion exchange wherein at least one electrode comprises a current feeder (1) in intimate contact with a mixture (2) of a particulate ion exchange material and a binder, the binder bonding the current feeder (1) and the mixture (2) into a coherent structure, characterised in that at every part of the current feeder (1) the thickness of the mixture (2) on one surface of the feeder (1) is much less than that on the other surface of the feeder (1), so as to enhance diffusion of electrogenerated gas from the current feeder out of the electrode. A method as claimed in Claim 1 wherein the thickness of mixture (2) is uniform over any one surface of the current feeder (1). A method as claimed in Claim 1 or Claim 2 wherein at every part of the current feeder one surface of the feeder (1) is devoid of the mixture. A method as claimed in Claim 1 or Claim 2 wherein the thickness of the mixture (2), on the surface on which it is less, is less than 0.5 mm. A method as claimed in Claim 4 wherein the said thickness is less than 0.2 mm. A method as claimed in any of the preceding claims also including a sheet of an inert electrolyte-permeable material adhering to a surface (4) of the said coherent structure.

Inventors: BRIDGER NEVILL JOHN, TURNER ANDREW DEREK, BRIDGER, NEVILL JOHN, TURNER, ANDREW DEREK
Applicants: ATOMIC ENERGY AUTHORITY UK, UNITED KINGDOM ATOMIC ENERGY AUTHORITY
Application year: 1991
Main classification: B01J47
IPC codes: C02F1, C25B11, B01J47, C25C7