GB2403166A - Electrodeionisation process - Google Patents

Abstract

Ions can be removed from a fluid by an electrodialysis or packed-bed electrodialysis process, carried out between two electrodes (16, 18). To prevent ions that may have a detrimental effect from contacting the electrodes, an ionic barrier (25, 45) is arranged between the electrode and an adjacent channel, the ionic barrier comprising two spaced-apart membranes (C) selectively impermeable to the detrimental ions, and a liquid-permeable bed of ion exchange material (26, 46) between the two membranes. Water is passed through this bed (26, 46), to flush out any contaminants which may diffuse or leak out of the channel through the first membrane (C). The process is carried out by passing the fluid through at least one flow channel (12) packed with anion exchange resin beads (14), defined between membranes (A,B) of different ion-selective characteristics, anions migrating from the flow channel(s) (12) into transfer channel(s) (20).

Description

24031 66 Electrodeionisation Process The present invention relates to a

plant and process for performing electrodeionisation, so as to remove ions from a fluid such as a damp gas or an aqueous or non- aqueous liquid. The ions are preferably recovered in a concentrated form.

A range of different techniques are known for removing ions from aqueous liquids. For example, the process of electrodialysis enables ions to be removed on a continuous basis, the liquid being passed between anion and cation selective membranes while being subjected to an electric field, but operates efficiently only with high concentrations of ions in the liquid. It is therefore unsatisfactory for use with weak acids. Ion exchange is another process used to remove ions, and can successfully treat liquids in which the concentration of ions is low, but it cannot operate continuously, as the ion exchange material will eventually become saturated with absorbed ions and need to be regenerated.

Electrodeionisation has been known for over 40 years, and is sometimes referred to as filled bed electrodialysis; it refers to a process in which the liquid to be treated is passed between ion selective membranes while being subjected to an electric field, the flow channel between the membranes being filled with an ion exchange material.

For example US 2 923 674 (Kressman/Permutit Co.) describes a process said to be particularly applicable to the demineralisation of water, in which the water is passed successively through two compartments each being defined between a cation-selective membrane and an anion- selective membrane, the first compartment being filled with a cation exchange material (to remove cations) and the second with an anion exchange material (to remove anions). A practical device consists of a stack of 2 membranes which are alternately anion-selective and cation-selective, so defining several such parallel flow channels for the water to be treated, alternating with channels in which the ions become concentrated. US 4 969 983 (Parsi/Ionics Inc.) describes a system for removing weakly ionised substances such as silica from water; the flow path contains anionic ionexchange material between an anion-selective membrane and a bipolar membrane, dissociation of water providing hydroxyl ions at the surface of the bipolar membrane and so in the vicinity of the anion resin.

Particular problems arise when treating materials which would react in a detrimental fashion at either an anode or a cathode. For example in the recovery of hydrogen fluoride, the recovered acid is itself corrosive, and the anode must therefore be protected from the corrosive effects. In this same context, in alkaline conditions there could be electrochemical formation of OF at the anode, which is a highly toxic and oxidising gas.

Similarly, where chloride ions are involved, these can generate chlorine gas at an anode in acidic conditions, which again is destructive to both ion exchange membranes and resins, and to electrodes. Where organic compounds are being recovered, these may be oxidised at an anode.

And where nitrates are in solution, these may be reduced to harmful gases NOX at a cathode. In any such situation it would be desirable to prevent contact between the ions that may cause such problems, and the electrodes; such ions are referred to herein as detrimental ions. A way of preventing such contact is described in pending patent application GB 02 30240.4.

According to the present invention there is provided a process for removing ions from a fluid, the process comprising a passing the fluid stream through a deionization plant comprising at least one flow channel for the fluid stream, the flow channel being defined between membranes having different ion-selective characteristics, and there being a transfer channel at at least one side of each flow channel into which ions are transferred from the fluid stream, and the plant also comprising electrodes arranged to subject each flow channel to an electric field transverse to the flow direction so as to urge ions from the flow channel into the transfer channel, wherein there is provided an ionic barrier between at least one of the electrodes and an adjacent channel, the ionic barrier comprising two spaced-apart membranes selectively impermeable to detrimental ions, and a liquid-permeable bed of ion exchange material between the two membranes, and causing a flushing liquid to flow through the bed of ion exchange material in the ionic barrier.

The deionization process may be that referred to as electrodialysis, or that referred to as packed-bed electrodialysis (or electrodeionisation). In the latter case each flow channel is also filled with a fluidpermeable bed of ion exchange material.

The present invention also provides a plant for performing this process.

The ion exchange material in the ionic barrier may be in the form of beads, or a foam or felt structure having the requisite ion exchange properties. It ensures that the ionic barrier does not significantly increase the electrical resistance between the electrodes.

Preferably the ion exchange material in the ionic barrier is an exchanger of ions of opposite polarity to the detrimental ions. The separation between the two membranes forming the ionic barrier may be for example in the range 2 mm to 4 mm.

Where the channel next to the ionic barrier is a transfer channel, this may contain ions at a significantly higher concentration than those in the flow channel. Indeed, flow rates of the fluid to be treated through the flow channel, and of the liquid through the transfer channel, may be such that the concentration in the transfer channel during operation is at least ten times that in the fluid to be treated, and may be as high as 10,000 times. If there is a single ion-selective membrane separating liquids of markedly different concentrations there will tend to be diffusion of any neutral species (e.g. a weak acid such as HF in its un ionised form), and there will be some leakage of ions through the membrane due to imperfections in the membrane (e.g. of anions through a cation or bipolar membrane).

Consequently, if a single membrane is used to prevent detrimental ions from reaching an electrode, some passage of these ions towards the electrode is inevitable, and this is driven by the concentration gradient across the membrane. By using the ionic barrier, the flushing liquid removes these detrimental ions, and maintains a low concentration in the space between the membranes, so that migration of detrimental ions to the electrode becomes negligible. The flushing liquid may be deionised water; and after its passage through the ionic barrier it may be added to the process fluid.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Flqure 1 shows a diagrammatic cross-sectional view of a plant for removing and recycling hydrofluoric acid; - 5 - Figure 2 shows a diagrammatic cross-sectional view of a modification of the plant of figure 1; and Figure 3 shows a diagrammatic cross-sectional view of another modification to the plant of figure 1.

Referring to figure 1 a diagrammatic cross-sectional view is shown of a plant 10 for removing hydrofluoric and nitric acids from an aqueous stream 11. The plant 10 includes several parallel flow channels 12 (only four are shown) for the solution of the acids in water, each flow channel 12 being defined between a bipolar membrane B and an anion-permeable membrane A (the anionic side of the bipolar membrane B being marked by an asterisk), and containing a packed bed 14 of anion-exchange resin beads.

The flow channels 12 are arranged side-by-side between a lead dioxide/lead anode 16 and a graphite cathode 18, the anion-permeable membrane A of each flow channel 12 being at the side nearer to the anode 16 in each case, with concentrate channels 20 between the flow channels 12.

The cathode 18 is immersed in a sulphuric acid electrolyte 19, and the anode 16 is immersed in a sulphuric acid electrolyte 22. The anolyte 22 is separated by an ionic barrier 25 (described below) from the adjacent concentrate channel 20.

It will be appreciated that the drawing is diagrammatic; in particular the electrodes 16 and 18 which form the ends of the stack may in practice be exposed to liquid only on one face, there being resilient seals around the periphery of the exposed parts of the electrodes. Furthermore a recirculating system (not shown) is preferably also provided to circulate the sulphuric acid between the anolyte 22 and the catholyte 19.

In operation, the solution 11 of hydrofluoric acid and dilute nitric acid in water is passed in parallel through all the flow channels 12, preferably flowing in a vertically upwards direction, and an electric voltage is applied between the electrodes 16 and 18 so that a current flows. Water is supplied to the concentrate channels 20. The anionexchange resin bed 14 is initially in its basic state, that is to say saturated with hydroxyl ions. In each flow channel 12, fluoride and nitrate anions are trapped by the ion exchange resin, and are then eluted by hydroxyl ions generated at the adjacent surface of the bipolar membrane B; these hydroxyl ions ensure that the solution is not acid. The trapped anions then migrate under the influence of the electric field through the resin bed 14 (undergoing repeated ion exchange steps) and the anion-permeable membrane A, into the adjacent concentrate channel 20. At the side of each bipolar membrane B adjacent to a concentrate channel 20 hydrogen ions are generated. An aqueous solution of hydrofluoric acid and nitric acid emerges from each concentrate channel 20, while the aqueous solution containing a considerable reduced concentration of anions flows out of all the flow channels 12. Oxygen gas and hydrogen gas are generated by electrolysis at the anode 16 and cathode 18 respectively, and these gases are vented to the atmosphere.

For example a solution 11 containing 25 mM HF and 1 mM nitric acid has been treated at a current density of A/m2, the solution flowing at a speed of 0.1 cm/s through a bed 14 of thickness 2 mm. It was found that 99% of the anions were recovered in a single pass into a concentrate stream of about 0.73 M, the system operating at a current efficiency about 70%. The treated stream had thereby been reduced to a concentration less than l - 7 - 0.05 mM, and the volume reduction factor (comparing feed to concentrate) was greater than 30.

The ionic barrier 25 consists of two cation permeable membranes C between which is a bed 26 of cation-exchange beads. A flow of deionised or softened water 27 is passed through the bed 26, and the resulting output flow is added to the acidic mixture 11 to be treated. As mentioned above, oxygen gas is generated by electrolysis at the anode 16, and hydrogen ions are therefore also generated. Hydrogen ions from the anolyte 22 pass through the ionic barrier 25 and hence into the concentrate channel 20. If there is any molecular diffusion of HF molecules from the concentrate channel 20, or any leakage of fluoride ions, through the adjacent cationpermeable membrane C, the resulting concentration in the bed 26 is minimised by the flow of flushing water 27; any anionic contamination will not bind to the cation-exchange bed 26, but will remain in the bulk liquid, and therefore be flushed out. Migration of HF molecules or fluoride ions through the other cation- permeable membrane C is negligible, because of the very small concentration gradient. For example, the plant may be of working area 400 cm2, treating a stream 11 of over 1200 litres/hour in say 40 flow channels 12, and the rinsing stream 27 may be less than 1 litre/hour.

It will be appreciated that the plant 10 may be modified in various ways and in particular one or both of the cation membranes C of the ionic barrier 25 may be replaced by bipolar membranes. However, a disadvantage of having a bipolar membrane adjacent to the anolyte 22 is that the anionic surface of the bipolar membrane is less stable when adjacent to an oxidising environment.

Alternatively only the membrane adjacent to the concentrate channel 20 might be replaced by a bipolar membrane; hydroxyl ions generated within the bipolar membrane by water splitting would then combine with hydrogen ions migrating away from the anode 16 as discussed above.

Referring now to figure 2 a modification is shown to the plant of figure 1. The modification only affects the anode, and the other features are as in figure 1, and are referred to by the same reference numerals. In the plant 30 of figure 2, a porous carbon anode 32 is separated from the adjacent concentrate channel 20 by the ionic barrier 25 consisting of two cation- selective membranes C spaced apart and with a bed 26 of cation exchange beads between them. The anode 32 may for example be of graphite cloth or fibrous graphite felt, and is not provided with a separate electrolyte. The porous carbon anode 32 acts as a gas diffusion electrode, and its rear surface may be impregnated with a chemically-resistant non-wettable polymer such as polytetrafluoroethylene (PTFE) so that water cannot pass through it although it is gas permeable. The front surface of the anode 32 may be impregnated with an oxidation resistant cationic exchange material, and bonded to the cation-selective membrane C. Deionised water 27 is supplied to the bed 26 to flush out any contaminants that enter the bed. Some water diffuses through the membrane C, so that the porous carbon anode 32 is damp. The rear surface of the anode 32 is vented to the atmosphere so that oxygen formed from electrolysis of water can can diffuse out of the anode 32; the hydrogen ions generated by electrolysis pass through the ionic barrier 25 into the adjacent concentrate channel 20.

In practice a gas diffusion electrode would also be provided as the cathode, for example consisting of a graphite felt whose rear surface is impregnated with PTFE, and whose front surface may be impregnated with - 9 cationic exchange material, and is held in contact with or bonded to the cationic surface of the bipolar membrane B. An ionic barrier is not essential when dealing with this particular solution.

The plants 10 and 30 ensure there is no contact between the concentrate and the anode, which not only prevents corrosion of the anode but also prevents electro-oxidation of materials in the concentrate. Such plants would therefore be appropriate when dealing with oxidisable organic acids such as formic acid or amino- acids.

Substantially the same ionic barrier can also be used to protect a cathode. For example, referring to figure 3 a modification is shown to the plant of figure 1. The modification only affects the cathode, and the other features are as in figure 1, and are referred to by the same reference numerals. In the plant 40 of figure 3, an ionic barrier 45 is arranged between the feed channel 12 and the catholyte 19, the barrier 45 comprising a cation permeable membrane C spaced apart from the bipolar membrane B (on the side nearer to the cathode 18), and with a packed bed 46 of cation exchange material between them. A rinsing stream 47 of deionised water is passed through this bed 46, and then combined with the stream 11 to be treated.

In use, diffusion of ionic fluoride or nitrate from the feed channel 12 towards the cathode 18 is of course inhibited by the applied electric field. Furthermore, diffusion of un-ionised HF is inhibited by the locally alkaline environment at the anionic surface of the bipolar membrane B. which encourages ionisation. But any diffusion or leakage that occurs, as a result of the concentration difference, from the feed stream 12 through the bipolar membrane B, does not reach the catholyte 19.

Any such leakage of neutral molecules or anions will only reach the packed bed 46 and will then be swept away by the rinsing stream 47.

Such an ionic barrier 45 may also be provided along with a gas diffusion electrode as cathode; in this case the front face of the gas diffusion electrode would be in contact with the cation membrane C of the ionic barrier 45.

In the above cases the detrimental ions are anions.

In some cases it may be necessary to prevent cations from reaching an electrode, for example in electrodialysis or electrodeionisation systems for recovery and concentration of metal ions in solution where the cation may precipitate on contact with the electrolyte or may electrodeposit on an electrode (e.g. copper depositing as dendrites on a cathode, or lead depositing as PbO2 at an anode). In this case an ionic barrier would consist of two anion-permeable membranes (or an anion-permeable membrane adjacent to the anionic surface of a bipolar membrane) with the gap between the membranes filled with a bed of anion-exchange material. A rinsing solution might be a dilute acid in this case.

Claims (6)

1. Claims 1. A process for removing ions from a fluid, the process comprising

   a passing the fluid stream through a - deionization plant comprising at least one flow channel for the fluid stream, the flow channel being defined between membranes having different ion-selective characteristics, and there being a transfer channel at at - least one side of each flow channel into which ions are transferred from the fluid stream, and the plant also comprising electrodes arranged to subject each flow channel to an electric field transverse to the flow direction so as to urge ions from the flow channel into the transfer channel, wherein there is provided an ionic barrier between at least one of the electrodes and an adjacent channel, the ionic barrier comprising two spaced-apart membranes selectively impermeable to detrimental ions, and a liquid-permeable bed of ion exchange material between the two membranes, and causing a flushing liquid to flow through the bed of ion exchange material in the ionic barrier.
2. 2. A process as claimed in claim 1 wherein each flow channel is also filled with a fluid-permeable bed of ion exchange material.
3. 3. A plant for removing ions from a fluid, the plant comprising a deionization plant comprising at least one flow channel for the fluid stream, the flow channel being defined between membranes having different ion-selective characteristics, and there being a transfer channel at at least one side of each flow channel into which ions are transferred from the fluid stream, and the plant also comprising electrodes arranged to subject each flow channel to an electric field transverse to the flow direction so as to urge ions from the flow channel into the transfer channel, wherein there is provided an ionic barrier between at least one of the electrodes and an adjacent channel, the ionic barrier comprising two spaced-apart membranes selectively impermeable to detrimental ions, and a liquid-permeable bed of ion exchange material between the two membranes, and means to cause a flushing liquid to flow through the bed of ion exchange material in the ionic barrier.
4. 4. A plant as claimed in claim 3 wherein the ion exchange material in the ionic barrier is in the form of beads.
5. 5. A plant as claimed in claim 3 or claim 4 wherein the separation between the two membranes forming the ionic barrier is in the range 2 mm to 4 mm.
6. 6. A process for removing ions from a fluid.

   substantially as hereinbefore described with reference to] figure 1 or figure 2 or figure 3 of the accompanying drawings.