GB2178759A - Electrochemical ion exchange - Google Patents

Abstract

Ion exchange material having ions absorbed thereon is regenerated by incorporating it into a compartment of an electrochemical cell defined by a cell separator permeable to said ions and including a working electrode. The ion exchange material and the working electrode are not bonded to one another but are in sufficiently close proximity for the pH of the electrolyte adjacent the material to be controlled by the polarity of the working electrode and the current passing. Thus, at an appropriate pH during operation of the cell, the ions are eluted into the electrolyte. Ion exchange material thereby regenerated may be removed from the cell, used to extract ions from an aqueous solution and subsequently returned to the electrochemical cell for regeneration.

Description

SPECIFICATION Electrochemical ion exchange This invention relates to the regeneration of an ion exchange material having absorbed ions and the use of such regenerated material in the removal of ions from an aqueous solution.

The electrochemical removal of ionsfrom aqueous solutions, sometimes referredto as electrochemical deionization, is known. See, for example UK Patent Specification No 1 247732 and its corresponding US Patent Specification No 3533929. Basically, it involves establishing an electrochemical cell comprising the aqueous solution as electrolyte, a working electrode and a counter electrode, where at leastthe working electrode incorporates an ion exchange material such as a resin, and applying a D.C. potential to the working electrode. To remove cations from the solution, a cathode potential is applied totheworking electrode, being a cation responsive electrode.This produces a local wised change of pH thereat due to generation of OH ions which in turn produce active sites onthe electrode for absorption of cations from the aqueous solution. Some authors may term this 'adsorption'.

Regeneration of the working electrode is effected by reversal of its polarity which causes the absorbed cations to be displaced into an aqueous medium The cell may also be operated in analogous manner, where the working electrode is anion responsive, to remove anionsfrom an aqueous solution or, where the working electrode is cation responsive and the counter electrode anion responsive, to remove both anions and cationstherefrom.

In the above-mentioned UK patent specification, the ion exchange material is described as being in intimate mixture with electrically conductive material maintained, for example by use of a binder. However, the ion exchange material will not operate effectively in the'passive' mode, absorbing ions from bulk solution, because the binder slows transportto the ion exchange material. Passage of current is essential for absorption of ions. Also, removal of plateable metal ions from aqueous solution in the above way is complicated bythe metal plating out on the electrical ly conductive material ofthe working electrode when operated as a cathode and when the ion exchange material is a cation-exchanging material.

The invention is concerned with ameliorating the above-mentioned problems and provides a method for regenerating an ion exchange material having ions absorbed thereon comprising the steps of (i) establishing an electrochemical cell comprising an aqueous electrolyte, a working electrode, a second electrode, and a celt separator permeable to the ions and defining a compartment of the cell includingthe working electrode, the ion exchange material being provided in the compartment whereby it is not bonded to the working electrode but is in sufficiently close proximity thereto forthe pH of the electrolyte adiacentthe ion exchange material to becontrolled by the polarity of the working electrode and the current passing;;and- (ii) operating the cell in a regeneration mode to displace the absorbed ions into the electrolyte. The ions displaced in step (ii) may be carried from the electrolyte by diffusion through the separator or by flow of solution.

The ion exchangematerial may be remotefrom the working electrode or in touching contacttherewith, provided it is mechanically separate therefrom.

An ion exchange material so-generated, not being bonded to the working electrode, may be contacted with an aqueous solution containing ions for removal thereby to absorb the ionswithoutthe need for electricity or electrical connection. Such absorption may be carried out in situ inthe compartment or after removal of the ion exchange material as described hereinafter. When the ion exchange material is charged with ions, it may be returned to the cell, if required, and step (ii) carried out to regenerate the material.The aqueous solution containing ions for removal may be in the form of a flowing stream such as an effluent stream and, if desired, a number of different batches of regenerated ion exchange material from say a single cell may be used to treat different streams without each requiring a power supply.

The ion exchange material may be a cation exchange material, when the working electrode is operated as an anode in step (ii), or it may be an anion exchange material arranged astwo ion-exchange half cells in combination in step (i}. If a weakly dissociated resin is used in an adversely alkaline or acid environment, external addition of acidor base will be necessary to effect absorption.

The cell separator, which may, if desired, be selectively permeable to said ions, preferably contains fine (i.e. < 10 m) pores for preventing bulk solution flow, connectedfrom the front to the back of the separator and constituting a high proportion of its area. It may be fabricated of a material such as porcelain, asbestos felt, or sintered plastics. Its thickness is preferablygreaterthan 1 mm and its performace may be predicted in conjunction with the ion exchange material to be used, the aqueous solution to be treated and the current density. The separator may be in theform of a container carrying the ion exchange material in the electrochemical cell wheretheworking electrode is situated in that partof the electrolyte within the container.Such an arrangement permits ready removal ofthe ion exchange material from the cell. Asingle electrode such as platinisedtitanium, which is expensive but very useful, may then be used with a plurality of combinations of ion exchange material and separator. If desired, a porous wall, which is thin relative to the separator, may be provided between the ion exchange material andthe working electrode. This is to facilitate good solution contact with the electrode and removal of electrode and/orion exchange material. If desired the separator and the wall may be in two separable partswhich, in use, completely enclose the electrode.

This allows the regenerated ion exchange material to be removed from the compartment after step (ii) to be used to absorb ions andwithout disturbing the working electrode. Alternatively, the regenerated ion exchange material may remain in position after step (ii) but be isolatedfrom the electrochemical cell byan impermeable removable barrier such as a wall two enable itto be usedto absorb ions. Such a barrier may help to increase breakthrough capacity by promoting plugflowthroughtheionexchange material when the latter is in the form of a bed.

The role ofthe separator is to facilitate a pH change in the electrolyte adjacent the ion exchange material in step (ii} thereby to displace absorbed ions. The eluted ions for removal may be transported away from the ion exchange compartment by diffusion orelectromigration through the separator, or by solution flow through or around the separntor. Where plateable cations are eluted in an anode compartment, they may be removed by electrodeposition in the corresponding cathode compartment.

The ion exchange material maybe a material known in the art such as an ion exchange resin and may be used in the form of beads or as a powder. A chelating resin is particularly effective for passive absorption of divalent non-ferrous metals from weakly acidic solutions. A weak acid resin can be used to absorb non-ferrous metals from neutral solutions and can be regenerated at high current efficiency. A high current efficiency will be achieved when the metal concentration near the ion exchange material is in equilibrium with a low hydrogen ion concentration.

Several ways of carrying outthe invention will now be described as follows. Reference will be made to the accompanying drawings, wherein Figure lisa schematic view of an ion-exchange material/electrode combination; and Figures 2,3 and 4are plots of Cd2+ concentration againsttime obtained in the Examples described below.

Referring to the drawing, a cation exchange resin 1 having metal ions, such as those of Cu or Cd, absorbed thereon is situated in a thick ( > 1 mm) separator2 in the form of a bag fabricated of a membrane. A gas-evolving anode 3 is positioned with in the thick separator 2 and a thin porous wall 4 also in the form of a bag is positioned between the resin 7 and the anode 3. It should be noted that wall 4 need not necessarily be present. In operation, the assembly shown in the drawing is positioned in anelectrolytetogetherwith a cathode to constitute an electrochemical cell. The cell is operated by passing a current to generate hydrogen ions in the region of the electrolyte adjacent the anode 3 and the resin 1.These displace the metal ions on the resin 1, which metal ions pass th rough the separator 2 for removal if required.

When the majority of the metal ions have been removed from the resin 1, the latter 1 is removed from the electrolyte and contacted with an aqueous solution containing metal ions for removal thereby to absorb said metal ions. If desired the resin 1 may be returned to the cell and regenerated by operation thereof as described above.

EXAMPLES 1 to 3 A known quantity of beads of a chelating ion exchange resin (IRC-71 81) in the sodium form was washed with distilled wateirand placed in a beaker together with the calculateduaiitltyofCd2+ (aq) required for the desired percentage loading on the resin. The solution was stirred for approximately an hourtoensure equilibrium. The resin beadswere removed, washed and placed inside a separator alongside a platinised titanium anodeto give an anode assembly as shown in Figure 1.

This assembly was removed to a main cell where, in orderto monitorthe rate of metal regeneration into bulk solution of the main cell, a nickel wire protected by a cation exchange membrane was used as a cathode which made it impossible for the metal to plate out The cell electrolyte was 0.1 M NaN03.

A current ofthe orderof0.2- 0.5 A at current densities of 5 - 20 mAcm -2 on the anode was passed.

The electrochemically generated protons adjacent the anode andthe resin displaced the cadmium ions bound to the resin. These cadmium ionsthen began to pass across the separator into the bulk solution which was stirred. This process continued until the equilibrium of H+/Cd2+ on the resin was achieved, and the bulk cadmium concentration no longer increased.

The bulk solution was analysed by a metal ionselective electrode and glass pH electrode placed in a small sampling vessel remote from the cell. Small quantities of bulk solution were fed to this vessel by a peristaltic pump, operated by a computer-tripped relay,analysed and returned to the cell bya second pump. This gave rise to a plot of Cd2+ concentration against time of passing current. Aliquots were also analysed by atomic absorption spectrometrywith good agreement of results.

The results are summarised in the graphs of Figures 2,3 and 4,the process parametersforwhich are given in the table below.

Figur Separator Separator Current Bulk Resin Material Thickness Density Volume (ml) Weight (g) Cd2+ (mm) (mm) (mAcm-2) Loading (%) 2 'Vyon F' 3.2 5.7 800 8, 43 (porous sintered polythene) 3 jViledon' 0.3 27 400 2, 100(32% (porous total polyamide : regeneration) cloth) 4 'Celloton' 3.0 13 400 2, 100 (65% (a ceramic) total regeneration} EXAMPLE 4 Wet beads of a weak acid ion exchange resin (lRC-84) in the sodium form (300mI) charged with Ni2+ was placed in an anode box (116 cm2 on each face) in an eFectrolytic cell. The electrolyte solution had a pH of 4.end contained 3400 ppm Ni2+, andthe cell was operated using a constant currentof 500 mA, i.e. 2 mAcm2. After 22 hours, the resin loading had decreased from 70% to 27% with a current efficiency of 96%. The final concentration of N i2+ measured in the anode compartment was 4300 ppm, i.e. 0.07 M, and the pH was 3.6

Claims (8)

1. A method for regenerating an ion exchange material having ionsadsorbedthereon comprising the step of (1) establishing an electrochemical cell comprising an aqueous electrolyte, aworking electrode, a second electrode, and a cell separator permeable to the ions and defining a compartment of the cell including the working electrode, the ion exchange material being provided in the compartmentwhereby it is not bonded to the working electrode but is in sufficiently close proximity thereto forthe pH of the electrolyte adjacent the ion exchange material to be controlled bythe polarity of theworking electrode andthe current passing; and (ii) operating the cell in a regeneration modeto displacetheadsorbed ions into the electrolyte.

2. A method as claimed in claim X wherein the regenerated ion exchange material is removed from the cell and is contacted with an aqueous solution containing ions for removal thereby to adsorb the ions.

3. A method as claimed in claim 2wherein the resulting ion exchange material is regenerated as claimed in claimed.

4. Amethodasclaimed incaimlorclaim wherein the aqueous solution containting ions for removal is in the form of a flowing stream.

5. A method as claimed in anyofthepreceding claims wherein the cell separator is in the form of a container carrying the ion exchange material and the working electrode is situated in that part of the electrolyte within the container.

6. A method as claimed in anyofthe preceding claims wherein a porouswall is provided between the ion exchange material and the working electrode.

7. A method as claimed in claim 6 wherein the porous wall and the separator constitute a separable envelope completely enclosing the ion exchange material.

8. A method for regenerating anion exchange material having ions adsorbed thereon substantially as described herein with reference to the examples.