GB2383275A

GB2383275A - Ion exchange column regeneration by electrodialysis

Info

Publication number: GB2383275A
Application number: GB0130893A
Authority: GB
Inventors: Plc Accentus; Christopher Peter Jones; Stuart Anton Legg; Andrew Derek Turner
Original assignee: Accentus Medical PLC
Current assignee: Accentus Medical PLC
Priority date: 2001-12-22
Filing date: 2001-12-22
Publication date: 2003-06-25
Also published as: GB0130893D0

Abstract

The regeneration of a saturated ion exchange column (10) involves eluting adsorbed material from the column with a regenerant liquid, so generating a solution of the adsorbed material; and then rinsing the column with a rinsing liquid, so generating used rinsing liquid. The process thus generates two waste streams: a small volume, concentrated stream (22), and a larger volume but dilute stream (20). The used rinsing liquid (20) is therefore stored separately from the solution of the adsorbed material; and is treated by electrodialysis (26) to generate both clean rinsing liquid and a concentrate. The total volume of waste liquid is hence considerably reduced.

Description

Ion Exchange Column Regeneration The present invention relates to a method of regenerating an ion exchange column, and to a plant for performing this method.

Ion exchange columns are widely used for removing contaminants, particularly ionic contaminants, from liquid streams. For example in electroplating, the objects being plated are immersed in an electroplating solution; and after electroplating, any remaining solution must be removed from the objects. This may be done in one or more stages with a cleaning liquid such as water. The cleaning or rinsing liquid becomes contaminated with electroplating solution carried over by the objects, and eventually must either be discarded or, more preferably, processed to remove the contamination.

This may be carried out using ion exchange columns. The ion exchange columns eventually become saturated with trapped contaminants, and must be regenerated; with industrial-scale columns the regeneration process itself generates a significant volume of waste water, and it would be desirable to minimize this volume.

According to the present invention there is provided a method of regenerating an ion exchange column, the method including at least the steps of: (a) eluting adsorbed material from the column with a regenerant liquid, so generating a solution of the adsorbed material; and (b) then rinsing the column with a rinsing liquid, so generating used rinsing liquid; wherein the method also includes:

(c) storing the used rinsing liquid separately from the solution of the adsorbed material; and (d) processing the used rinsing liquid by electrodialysis to generate both ionically-depleted rinsing liquid and a concentrate.

This considerably reduces the volume of liquid waste to be disposed of. The ionically-depleted rinsing liquid can be re-used for rinsing the column (in step (b)).

The regenerating method may also include flushing (or back-flushing) of the column prior to the elution step; the liquid used in this flushing step may be combined with the used rinsing liquid and also subjected to electrodialysis. The column may also be washed, generating a concentrated waste, after eluting but prior to the rinsing step.

With electrodialysis the concentrated solution is typically no more than 30 times more concentrated than the liquid being treated. If a larger ratio of concentrations is required this may be achieved by subjecting the concentrate to a further electrodialysis treatment., The concentrate generated by a first electrodialysis stack may be provided as feed to a second electrodialysis stack.

The invention also provides a plant for performing such a method.

The invention will now be further and more particularly described, by way of example only, and with reference to the accompanying drawings in which: Figure 1 shows a flow diagram of an ion exchange

column regeneration system including an electrodialysis unit; and Figure 2 shows a diagrammatic cross-sectional view of the electrodialysis unit of the plant of figure 1.

Referring to figure 1, two ion exchange columns 10 and lOa are shown through which a process liquid is passed in sequence to remove contaminants. For example the ion exchange column 10 might contain ion exchange beads of a strong acid, to remove cations from the process liquid; and the column lOa might contain ion exchange beads of a strong base, to remove anions from the process liquid. There might also be a column (not shown) containing an organic scavenger. When the adsorbative capacity of the ion exchange columns 10 and lOa have been fully utilized, then they must be regenerated. For this purpose the base of each column is provided with inlets 12 and 14 to supply water and an appropriate regenerant liquid respectively, each inlet being provided also with a respective valve. The top of the column is provided with outlets 16 and 18 (and respective valves), the outlets leading to a rinse water storage tank 20 and a concentrate storage tank 22 respectively. The system is shown for column 10; for the other column lOa the system is substantially identical (apart from the nature of the regenerant liquid), and the outlets from column lOa lead to the same tanks 20 and 22.

After closing off flow of the process fluid through the columns 10 and lOa, each column is back-flushed with water to remove any process fluid; this back flushed water is stored in the rinse water storage tank 20. The columns 10 and lOa are then regenerated by passing an appropriate regenerant liquid: for example if the column 10 was trapping cations, then an appropriate regenerant

would be aqueous hydrochloric acid, while if the column lOa was trapping anions then an appropriate regenerant would be sodium hydroxide solution. The regenerant elutes substantially all the adsorbed material, producing a concentrated waste stream which is stored in the concentrate storage tank 22. If there is a column containing an organic scavenger too, then it could be regenerated in a similar way using concentrated brine as the regenerant. Each column 10 and lOa is then washed with water to remove the remaining regenerant and any remaining material that has been eluted; this produces a fairly concentrated aqueous waste stream that is also stored in the concentrate tank 22. Finally, each column 10 and lOa is rinsed with a large volume of clean water to remove the remaining ions; this produces a large volume of dilute waste that is stored in the rinse water storage tank 20. By way of example, with each column 10

3 and lOa of volume 1. 2 m, as a result of one such regeneration process the concentrate tank 20 might contain 1.1 m3 at a concentration of 1.95 M, while the rinse water storage tank 22 might contain 8.2 m3 at a concentration of 0.09 M. If there is also an organic scavenger tank of the same volume to be regenerated, then the volumes might for example be 1.6 m3 in the concentrate tank 20, and 11.5 m3 in the rinse water storage tank 22.

The rinse water storage tank 20 is connected via a pump 24 so as to circulate the rinse water 25 to an electrodialysis unit 26 that extracts ions from the water, generating a more dilute stream 27 that is returned to the rinse water tank 20, and a concentrate stream 28 that is supplied to the concentrate tank 22.

Referring now to figure 2 a diagrammatic crosssectional view is shown of the electrodialysis unit 26.

The unit 26 includes several parallel flow channels 32 (only four are shown) for the liquid from the rinse water tank 20, alternating with several concentrate channels 34 (only three are shown), adjacent channels 32 and 34 being separated by ion-selective membranes. The flow channels 32 and the concentrate channels 34 form a stack, and are arranged between an anode 36 and a cathode 38 immersed in suitable electrolytes. Each flow channel 32 is defined between a cation-permeable membrane C (on the side nearest the cathode 38) and an anion-permeable membrane A (on the side nearest the anode 36), apart from the membranes at the two ends of the stack (adjacent to the anode chamber, and adjacent to the cathode chamber, respectively) which are bipolar membranes B. The anionic side of each bipolar membrane B is indicated by an asterisk.

Such a bipolar membrane B is permeable to neither anions nor cations. It may be considered as comprising a layer of anion-selective material on one side *, and a layer of cation-selective material on the other side. In the presence of an electric field, if the potential difference across the membrane B is at least 0.84 v, then water is split into hydrogen and hydroxyl ions within the membrane B, the hydrogen ions emerging from the cationic side and the hydroxyl ions emerging from the anionic side *. The effect of the bipolar membranes B is to protect the electrodes 36 and 38 from metals that might electrodeposit (and consequently create dendrites that might cause membrane failure), and from anions that might be oxidised at the anode to create oxidising agents such as chlorine (which could damage the membranes), or that might be reduced at the cathode to create toxic off-gases (e. g. NOX formed by a reduction of nitrate).

The electrodialysis unit 26 might contain say one hundred flow channels 32 alternating with concentrate channels 34. The dilute rinse water flows through the flow channels 32, the cations moving through the cationpermeable membrane C and the anions moving through the anion-permeable membrane A into the concentrate channels 34 on either side. The electrodes 36 and 38 may be of platinised titanium, and the electrolyte in each case may be sulphuric acid. The voltage applied across the stack should be less than 1.3 V per flow channel 32. It is desirable to ensure that the outermost channels 32 for the dilute rinse water are adjacent to the electrode chambers at the ends of the stack, as shown, as this reduces the risk of precipitation should the end membrane be damaged. The contents of the rinse water tank 20 might be reduced from 8.1 m3 at a concentration of 0.09 M to 7.5 m3 at a concentration of 0.01 M, so increasing the

3 volume of concentrate waste by 0. 6 zu It should be understood the dilute waste in the rinse water storage tank 20 may be processed by a single passage through the electrodialysis unit 26; in this case it is desirable to provide a spacer in the flow channels 32 so that the rinse water 25 follows a serpentine path.

Alternatively the liquid may be recirculated several times through the unit 26 to progressively decrease the remaining ionic concentration.

It will thus be appreciated that the process of the invention significantly decreases the total amount of waste water generated in a regeneration process, while slightly increasing the volume of concentrated waste. At the same time it significantly reduces the consumption of water.

The invention is applicable to a wide range of ion exchange column systems, including those that trap just one type of ion (say cations), and to columns that are regenerated with acids, alkalis or salt solutions.

Where the rinse water to be treated is neutral or slightly acidic, it is desirable to use an acid electrolyte adjacent to the anode 36 and cathode 38 (as described above), as this reduces the risk of any precipitation occurring within the end membranes.

Similarly, where the rinse water to be treated is neutral or slightly alkaline it would be appropriate to use an alkaline electrolyte for the electrodes 36 and 38, for example sodium hydroxide solution, to minimize risk of precipitation occurring within the membrane; the electrodes 36 and 38 may be of nickel in this case. The use of an alkaline electrolyte is particularly appropriate where the rinse water contains cyanide ions, in order to avoid the risk of HCN release in the event of failure of a membrane. Indeed, where cyanide ions are trapped by the column lOa it is preferable either to keep both the concentrated waste stream and the rinse water waste stream completely separate from those generated by elution of the cation column 10, and to process the rinse water containing cyanide ions separately; or alternatively to add alkaline material such as caustic soda to the concentrate and/or rinse water from the cation column 10 so as to ensure it is alkaline before it is mixed with the corresponding waste streams from the anion column lOa. In another alternative, the liquid stream containing cyanide ions is subjected to an oxidation treatment, to remove any cyanide ions present, before being mixed with the waste stream originating from the cation column 10.

If a still greater reduction in volume of waste

liquid is required, this may be achieved by storing the concentrate stream 27 from the electrodialysis unit 26 in an intermediate storage tank (not shown), and then circulating the liquid from the intermediate storage tank through a second electrodialysis unit (not shown), so as to generate an even more concentrated waste stream to add to the concentrate in the tank 22.

It will be appreciated that under some circumstances the stream formed in the concentrate channels 34 may be sufficiently concentrated that some salts may precipitate; it may be beneficial to monitor the pH of these streams, and to add acid or base accordingly. If surface fouling develops on the membrane surfaces within the unit 26, this may be reduced by a temporary reversal of the polarity of the electric field across the stack, and reversal of the flow directions through the channels 32. It will be appreciated that this method may not be applied for stacks with bipolar membrane protection of the electrodes due to the possibility of delamination of the anion and cation permeable layers. In this case, monovalent cation membranes or a combination of monovalent cation and anion membranes may be used instead, as shown in fig. 2. The flow channels 32 may contain porous beds of ion exchange material to trap ions while allowing them to be transferred into the adjacent concentrate channels 34. There might, for example, be one or more beds of anion-selective ion exchange beads and one or more beds of cation-selective ion exchange beads arranged in succession or as a mixed bed along the flow channel 32. Alternatively, separate anion and cation absorber filled stacks may be used in parallel.

This can improve the extent to which the rinse water becomes ion-depleted on a single pass through the unit 26. In any event the resulting ion-depleted water may then be sufficiently clean to be transferred directly

back to the storage tank for use as rinsing water the next time that the columns 10 are regenerated.

To obtain a greater difference of ionic concentration between the ion-depleted stream 27 and the concentrate stream 28 that is returned to the concentrate tank 22, it may be beneficial to provide two electrodialysis units 26. In this case the concentrate stream generated by the first electrodialysis unit is recirculated through the flow channels of the second electrodialysis unit (so ions are removed from it) before being returned to the concentrate channels of the first electrodialysis unit. Consequently the concentrate stream generated by the second electrodialysis unit is of markedly greater concentration than can be achieved with a single electrodialysis unit 26. The two electrodialysis units may be separate units, or alternatively share an electrode in the electrolyte between two stacks. The power supply will supply current separately to the two electrodes at the extremities of the separate stacks and the common central electrode.

Alternatively they may be arranged side-by-side as a single stack between a single pair of electrodes, the two parts of the resulting stack being separated by a pair of bipolar membranes between which is an electrolyte channel.' As described in relation to figure 2, the electrodes 36 and 38 may be protected by bipolar membranes B. With some waste streams this may not be essential. For example if the concern is to prevent metal deposition onto the cathode 38, it may be sufficient to protect the cathode 38 by a monovalent cation-selective membrane, as monovalent cations such as sodium will not deposit; divalent ions such as copper are prevented from reaching the cathode 38 by the membrane. If the electrolyte

adjacent to the cathode is alkaline (such as sodium hydroxide solution), such a monovalent cation-selective membrane would prevent the passage of any polyvalent cations that might precipitate as hydroxide in or adjacent to the membrane. A cation-selective membrane may also be used to protect the anode 36. If no problem anions are present, it may be adequate to separate the anodic electrolyte chamber from the adjacent flow channel 32 by an anion-selective membrane.

Claims

Claims 1. A method of regenerating an ion exchange column, the method including at least the steps of: (a) eluting adsorbed material from the column with a regenerant liquid, so generating a solution of the adsorbed material; and (b) then rinsing the column with a rinsing liquid, so generating used rinsing liquid; wherein the method also includes: (c) storing the used rinsing liquid separately from the solution of the adsorbed material; and (d) processing the used rinsing liquid by electrodialysis to generate both clean rinsing liquid and a concentrate.
2. A method as claimed in claim 1 also comprising flushing the column prior to the elution step, the liquid used in this flushing step being combined with the used rinsing liquid and also subjected to electrodialysis.
3. A method as claimed in claimed 1 or claim 2 also comprising washing the column, generating a concentrated waste, after eluting but prior to the rinsing step.
4. A method of regenerating an ion exchange column substantially as hereinbefore described with reference to, and as shown in, the accompanying drawings.
5. An ion exchange column including regeneration plant for performing a method as claimed in any one of the

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preceding claims.

15588 MdH P. T. Mansfield Chartered Patent Agent Agent for the Applicants