The commonly-used techniques for de-salinating sea water and the like are many and varied. They include distillation, reverse osmosis, and ultra filtration, and all have disadvantages. De-salination by ion exchange has also been attempted before, but has been found to be impractical, and thus has not been applied commercially.

However, the present invention relates to a modification of such a process-using a combination of ion exchange and electrodialysis techniques-which, it is believed, will overcome the previous impracticalities, and make a commercially viable method.

An ion exchange resin is a resin that in its original state carries, at ion-active sites, loosely- bound ionic groups that in certain circumstances can be exchanged for other ionic groups that bind preferentially to the resin. Accordingly, passing a solution containing these other ions over the resin causes them to be bound to the resin in place of, and thus in exchange for, the ions originally carried by the resin. In this way the ions in solution are removed and replaced by the resin's original ionic groups.

This process is reversible; by passing over the resin a strong solution of the resin's original ions these displace the exchanged ions, so regenerating the resin into its original form.

In sea water the predominant ionic material is common salt, sodium chloride-in its dissolved, and ionic, form this is a collection of sodium ions (the cation Na+, which in electrolysis is attracted towards the negatively-charged cathode) and chloride ions (the anion C1-, which in electrolysis is attracted towards the positively-charged anode). This can be purified by being passed through two ion exchange resins one of which is a cationic exchange resin, and in its basic, original, form is rich in hydrogen cations (H+), and the other of which is an anionic exchange resin, in its basic, original, form is rich in hydroxyl anions (OH-).

During the passage through the hydrogen-ion-rich cationic exchange resin the sea water's sodium ions exchange places with the hydrogen ions at the active sites, and become bonded to the resin. And during passage through the hydroxyl-rich anionic exchange resin the sea water's chloride ions exchange places with the hydroxyl ions at the active sites, and become bonded to the resin. This process of exchange continues until all the sites are used up; at this stage the resin is described as exhausted, and cannot be used any more until it has first been regenerated-the cationic resin by the subsequent passing therethrough of a strong solution of hydrochloric acid (HC1, or, in water, a source of hydrogen cations), the anionic resin by passing therethrough a strong solution of sodium hydroxide (NaOH, or, in water, a source of hydroxyl anions).

Anionic and cationic resins may be used separately -and sequentially-or in a mixed bed format.

This process of ion exchange has, as a well known example, the use of water softeners (e. g. PERMUTIT equipment in domestic and industrial situations) and the use of domestic water purifiers for removing heavy metals (lead and copper cations) and anionic nitrates from water before drinking it.

In an ion exchange resin the resin itself is usually an organic resin such as one of those made by co-polymerising styrene and divinylbenzene. The degree of crosslinking, and hence the internal pore structure, physical strength and swelling characteristics, are related to the proportion of divinylbenzene. The physical form is usually small (circa 2mm) spheres. In normal use the spheres are packed into open ended tubes, and the liquid to be purified is forced under pressure through the rein in the tube, exiting in a purified form and leaving the exchanged ions bound to the resin.

Eventually the resin's active sites are used up, and at that point (easily detected by sensors looking at the output) the resin has to be regenerated. For this purpose it is normal to drive the regenerating concentrate through the resin bed in the opposite direction, as this both flushes out any particulate material that may have collected and also loosens the resin from its packed-down state. Even in situations with lesser ionic loads than sea water the norm is large, even very large, resin beds that require frequent regeneration (as, of course, is the need for two such systems to be run in parallel so that treatment may continue in one whilst regeneration proceeds in the "off-line"other).

Because of their necessarily large size conventional ion exchange systems inevitably consume concomitantly large quantities of power to overcome internal pressure. They also produce significant quantities of concentrated effluent whilst at the same time using large amounts of costly chemicals. Moreover, a conventional ion exchange system can be shown to be flawed for de-salination purposes, in that the resin itself has a finite exchange capacity which is totally inadequate for this purpose-i. e. there are far too many ions present in sea water (the concentration of salt is far too high), and these in effect'swamp'the exchange sites almost instantaneously.

The present invention seeks to avoid or at least mitigate such problems by a new approach to the design and operation of the resin units in an application at present not suited to ion exchange. More particularly the invention suggests that a commercially-viable ion exchange sea water de-salination method could be achieved by utilising a regeneration process which is effected electrolytically and continuously whilst the resin unit is still purifying the salt water, in which electrolytic process the released sodium, chloride and other ions are equally continuously separated by employing an ion permeable membrane through which the displaced ions can pass out of the solution being treated.

Electrolytic regeneration of ion exchange resins is not in itself new, nor is the use of semi-permeable membranes to separate out the released ions. However, until now there has never been any suggestion that these two techniques could be combined, as is now described, to form a complete'salt'removal system capable of viably handling the high ionic load present in sea water. The concept of this type of regeneration, and its application in a sea water context, will be understood from the following comments.

If an electric potential is applied to a solution of positive and negative ions (respectively cations and anions) by dipping a positively-charged electrode, or anode, in at one side, and a negatively-charged electrode, or cathode, in at the other, so that one side of the solution is positive with respect to the other, the resultant electric field/potential gradient across the solution causes the ions to be attracted to, and migrate through the solution towards, the side (and electrode) with the opposite sign. Thus, unlike charges attracting each other, the negative anions move towards the positive anode, while the positive cations move towards the negative cathode. Perhaps surprisingly, the active site ions in an ion exchange resin can be made to behave in the same way; place the resin in an electric field and the ions at the active sites are attracted to, and actually move across the surface towards, the relevant electrode. Indeed, it is not unusual for the conductivity-the willingness of the electrons and ionic components thereof to move under an applied field -of the resin to be as high if not higher than the conductivity of a dilute solution of those ions, and if a suitable field is applied across an ion exchange resin surrounded by a solution of the same ions then most of the ion transfer takes place through the resin rather than through the surrounding solution.

It is this concept of ion transfer across an ion exchange resin in a potential field that is one of the features underlying the present invention, for it is this that permits the continuous regeneration of a partially-used ion exchange resin even while the resin is still carrying out its ion exchange function to purify a liquid. Thus, as the solution passes across the resin, its contaminant ions being exchanged thereby for the resin's own ions and hence being trapped on the resin, so the applied field causes those same trapped ions to migrate across the resin to the relevant electrode, where they leave the resin entirely. By suitably choosing the solution flow rate and the applied electric potential, so it can be arranged that the net effect is that contaminating ions are first trapped on the resin, removing them from solution, and then moved over the surface of the resin to the relevant electrode to be removed from the system altogether; the main output is thus pure water.

Of course, to complete the conversion of the solution to pure water the contaminating ions therein that migrate to the relevant electrode must then be removed from the system altogether, and it is here that there is applied the second feature underlying the present invention, that of the use of an ion-permeable membrane.

An ion-permeable membrane can be effected in a number of ways; one is a charged sheet of semi-permeable plastic (polyethylene, say), while another is an ion exchange resin in the physical form of a semi-permeable membrane. Although the normal physical form for an ion exchange resin is that of small spheres, it is also possible to fashion the resin into thin sheets or membranes, and indeed so to choose the resin base that it is semi-permeable-that is, so that certain ions or molecules will pass therethrough while others do not.

For the purposes of the invention, then, there is produced and used a membrane which is permeable to the contaminating ions alone when they are being moved therethrough by the applied electric field but which is not permeable to the other major components of the solution (and in particular not to the water). If, then, such a membrane is used, within the solution being treated, to isolate the field-producing electrodes from a main body of particulate ion exchange resin supplied with liquid effluent, contaminant ions which migrate across the main body to the relevant electrode will pass irreversibly through the membrane (to what is effectively a collection and disposal vessel on the other side) and thus permanently out of the effluent, so that in due course the solution will, as required, be quite free of those ions.

This, then, is what the invention proposes-a system for purifying aqueous salt solutions (such as sea water) by removing therefrom undesirable ions (the sodium chloride), this system involving the use of a sequence of two cells of ion exchange resin each bounded by ion-permeable membranes, one cation cell, one anion cell, together with means for applying an electric field across each thus-bounded body such that contaminant ions removed from the solution and trapped by the ion exchange resin body are caused to migrate across that body to and through the membrane boundary layer, so exiting the body and thus being removed from the solution, leaving behind pure water to be extracted from the container. The first cell is conveniently the cation cell containing cation ionic exchange resin, and is bounded by cation-permeable membranes, while the second cell is the anion cell containing anionic ion exchange resin, and is bounded by anion-permeable membranes.

In one aspect, therefore, this invention provides de- salination apparatus for the treatment of an aqueous salt solution (such as sea water) to purify it by the removal therefrom of the sodium and chloride ions dissolved therein, which apparatus comprises: a sequence of two sea-solution-feedable ion exchange resin cells, one being a cationic cell and the other being an anionic cell, each cell being an individual physical entity separate from the other; bounding the body of resin in each cell, and operatively relevant to that cell only, an ion-permeable membrane permeable to, and only to, the ions to be removed by that cell; and means for applying an electric field across each thus-bounded body and its membranes, which field is individual to that cell, and not shared by the other cell; whereby, in operation, the salt solution may be fed into the sequence of ion exchange cells, and the ions removed from solution and trapped by the resin in each cell are caused to migrate across the cell's resin body to and through the membrane boundary layer, and so are caused to exit the body and thus be removed from the solution, leaving behind a solution depleted in those ions.

In a second aspect, the invention provides a method for the de-salination of salt water, which method comprises: feeding the salt water to two bodies of ion exchange resin of opposite ionic type each in an ion-permeable membrane permeable to the ions to be removed; and applying an electric field across the thus-bounded body and the membrane, such that ions which are first removed and trapped by the body are then caused to migrate across that body to and through the membrane boundary layer, and so are caused to exit the body and thus be removed from the water, leaving behind relatively pure water to be removed therefrom.

In slightly different words, this second aspect of the invention may be defined as a de-salination method utilising the apparatus of the invention, which method is for the treatment of an aqueous salt solution (such as sea water) to purify it by the removal therefrom of the sodium and chloride ions dissolved therein, in which method: the salt solution is sequentially freed of its cations and anions by ion exchange in a sequence of two cationic and anionic ion exchange resin cells, the ion exchange resin body within each cell being bounded by an ion-permeable membrane permeable to the ions to be removed by that body; and each cell is regenerated by electrodialysis, a suitable electric field being applied for this purpose across each membrane-bounded body and its membranes; so that, in operation, the salt solution is fed into the sequence of ion exchange cells, and the ions removed from solution and trapped by the resin in each cell are caused to migrate across the cell's resin body to and through the membrane boundary layer, and so are caused to exit the body and thus be removed from the solution, leaving behind a solution depleted in those ions.

As noted above, the first cell in the sequence of two cells is conveniently the cation cell containing cationic ion exchange resin and being bounded by cation-permeable membranes, and removes the sodium cations, while the second cell is the anionic cell containing anionic ion exchange resin, and bounded by anion-permeable membranes, and removes the chloride anions.

The invention in both its method and its apparatus guise concerns the purification of salt water-that is to say, water such as sea water-that is relatively clear and free from suspended solids. It is not intended to use on, nor is it suitable for use with, waters that contain significant amounts of solid materials, such as those"muddy"waters found in estuaries. However, the apparatus is entirely suitable for such feedstock once the solid material has removed therefrom in some preliminary stage.

The system of the invention employs two bodies of ion exchange resin to which is sequentially fed the salt solution being treated. Each ion exchange resin is chosen not only to be appropriate for the ions to be removed but also to have suitable physical characteristics (a cationic exchange resin-for removing the sodium cation-will have ion active sites carrying exchangeable hydrogen ions, whilst an anionic exchange resin-for removing the chloride anion-will carry hydroxyl ions). Moreover, as noted above the resin itself is conveniently one of those organic resins made by co-polymerising styrene and divinylbenzene, and is preferably employed in the physical form of small spheres. Typical actual anionic and cationic exchange resins of this general sort and suitable for use in the invention are those sold under the names AMBERJET by Rohm & Haas and DAION by Mitsubishi.

In the invention's apparatus the appropriate ion exchange resin is a body within a cell-that is to say, it is simply"shaped"so that the salt solution can conveniently be fed to, into and through the resin in the most efficient way. A typical such cell may be of square shape but with less depth, being similar to a quarter slice of a cube, with the membranes enclosing the resin on the opposed major faces (as shown in the Figure in the accompanying Drawings, discussed further hereinafter).

The size of the ion exchange resin cell used in the invention may of course be whatever is required to enable a suitable input flow and pure water output.

Compared with the large vessels normally associated with ion exchange systems the cell of the invention is quite tiny and yet still has the required effect.

The containment walls of the cell may be made of any inert non-conductive rigid material, such as PVC or polypropylene.

Each of the ion-permeable membranes is conveniently made from a sheet of the appropriate ion exchange resin bonded onto a thin (2mm thick) permeable support sheet (conveniently of GORTEX), so that the cation-permeable membrane, for instance, is a sheet of cationic ion exchange resin. Suitable such membranes are sold under the names SELEMION by ASAHI glass & NEOSEPTA by TOKYAMA Soda.

The membrane may of course be provided with some additional suitable physical support structure, such as a non-conductive, inert, plastic mesh either built-in or attached to one (or both) faces of the membrane.

In the invention an electric field is applied across each ion exchange resin body and the membrane, so that in operation contaminant ions removed from the effluent and trapped by the body are caused to migrate across the body to and through the membrane boundary layer. The field-conveniently from about 10 to 30 volts/cm, which might result in a current flow of from 0.1 to 3 amps-is applied across the body/membrane- that is to say, generally normal both to the direction of flow of the effluent and to the plane of the membrane -and this is most conveniently arranged by placing outside the body/membrane combination a pair of plate-like electrodes lying generally parallel to but spaced from the membrane, each of these electrodes matching (being much the same size and shape as) the membrane. More specifically, the membrane-bounded ion exchange resin body is placed in what is effectively an electrochemical cell through which a very gentle flow of the same feedstock is passed, so that the salt ions to be removed migrate across the body to and through the membrane boundary layer and then into the cell liquid, where they are retained separate from the remaining feedstock. In this way the salt ions are removed from the solution, leaving behind purer water, and the cell electrolyte becomes a gradually more and more concentrated (aqueous) solution of the removed ions, as it passes through the cell and to waste.

The electrodes themselves may be of any suitable material not itself electrochemically soluble in the electrolyte or attacked by any released atoms. A typical such material is graphite, but gold or platinum have been found more effective through experience.

In such an electrochemical cell arrangement the electrodes may simply be immersed directly in the cell electrolyte. It is best, however, to prevent the salt ions building up in the electrolyte from contacting the electrodes, and perhaps modifying their effect in some detrimental manner, and this effect is minimised by the flow of feedstock keeping the concentration of sodium or chloride ions removed from the feedstock at reasonable levels.

When carrying out the method of the invention- when operating the apparatus of the invention-the salt water to be de-salinated is fed into the first (cationic) ion exchange resin cell, and the sodium cations removed therefrom and trapped by the resin body are caused by the applied electric field to migrate across that body to and through the membrane boundary layer, and so are caused to exit the body and thus be removed from the feedstock, leaving behind a sodium-free solution. This solution then passes to the similar second (anionic) exchange cell, where the chloride anions are removed in a corresponding manner to leave pure, salt-free water.

Feeding the effluent is per se a conventional process, needing no comment here-except perhaps to note that the rate of salt water flow (and thus the resultant rate of water extraction) should naturally be such that all the contaminating ions can be removed therefrom, first trapped by the ion exchange resin cells and then migrating thereacross to pass through the ion-permeable membranes, well within the time taken for the salt water to flow from the input to the output face of the bodies. This will, of course, depend upon the concentration of the contaminating ions. Absolute input/output flow rates will then depend upon the area of the input/output faces, but attainable unit area input/output flow rates will depend to a greater or lesser extent up in the length of the ion exchange resin body (from input to output face), upon the conductivity of both the ion exchange resin cells and the membranes, and upon the applied field (the greater each is, the faster the ions are removed, and the greater a flow rate can be sustained). By way of indication, for sea water a pair of cells each having membrane faces of lOcm2 and with a typical resin bed size of 200 millilitres will de-salinate continuously at a rate of 25 litres per hour.

The apparatus of the invention is remarkably compact, and by connecting large numbers of the cell pairs in parallel as great a treatment rate as is required can be achieved.

The apparatus enables salt water/sea water to be purified on a commercially-viable scale by removal of the contaminating"salt"ions through a process of ion exchange and electrodialysis.

An embodiment of the invention is now described, though by way of illustration only, with reference to the accompanying Drawings in which: the Figure shows diagrammatically a cross-section through the ion exchange part of an apparatus of the invention.

The de-salination apparatus shown in the Figure is basically a sequence of two ion exchange cells (B, H) in each of which there is a body of ion exchange resin within a chamber (C) bounded by ion-permeable membranes (D) and bathed in the electric field provided by pairs of electrodes (E and F, G, H in one, and E and F, I, J in the other), and served by various pipelines (A, A2, A3) along which salt water is fed into, though and out of the cells, as well as other pipelines (Y, Yz, Y3) along which electrolyte is fed into, through and out of the cells.

The salt (sea) water is fed via a number of valves and detectors (flow sensor & conductivity sensor: not shown) through the input pipe A of the first, cationic exchange resin cell B. The internal chamber C in this cell is filled with cationic exchange resin bounded by cation-permeable membranes D (represented by wavy lines). Also within this inner, membrane-defined cell are the two electrodes G and H. In this cationic cell G is the negative electrode whilst H is linked to earth.

The inner, membrane-defined cell lies within the body of the device; the volume (X) around this cell acts as the concentrating outer chamber, and has disposed therein two more electrodes E and F, positive on one side, negative on the other (the source of the potential is not shown).

The outer concentrating cell X is fed similarly with salt water through at the inlet pipe (A), valve (V) and pipeline (Y2), but at a rate some 1/20th of the inner cell feed. This flow which becomes more concentrated as it passes through exits the outer cell at pipe (Y) which is connected to waste.

Sodium-free"solution"leaves the inner cell at the outlet A2, and then passes to the second (anionic resin) ion exchange unit H, which is identically constructed to the first unit but has anionic ion exchange resin both in the inner cell"bed"and bonded into the permeable membrane sides. This anionic cell has inner cell electrodes I and J connected to positive and earth respectively, and its outer section is also slowly fed sea water via pipe Y2.

The now-purified water leaves the unit at pipe A3, where its condition is monitored by sensors for pH and conductivity.

Details of the operation of the system are as follows.

Ion exchange resins are polymers with a fixed ionic charge that can react (exchange) with free ions of opposite charge (counterions). Because of this ability to exchange counterions, ion exchange resins are electrically conductive; under the influence of any electric potential, counterions can transfer across the polymer, resulting in both mass and associated electron transfer. The actual electrical conductivity varies with the mobility and affinity of the counterions with which the resins are in contact, but in general ions are several orders of magnitude more mobile in ion exchangers than they are in freshwater.

In a system consisting of an ionic solution in contact with ion exchange resins, ionic transfer driven by electrical potential will occur almost exclusively through the ion exchanger, and not through the aqueous liquid. The ease of ion transfer, relating to the affinity and mobility of the particular ions in the ion exchange resin, will determine the electrical resistivity of the system. Within limits the ratio of the applied potential to the electrical resistivity of the system will determine the extent of ionic transfer.

The transfer of the ions through the seem results in a transfer of electrons that can be related to electrical current passage, in that one chemical equivalent of salt transferred causes the transfer of 96,494 coulombs, or one faraday, of electric charge.

Ion exchange membranes are made of ion exchange resins manufactured in sheet form. As such, they are semi-permeable to ions: membranes of a particular fixed charge are permeable to counterions and impermeable to co-ions. Ion exchange membranes are also impermeable to water, and can therefore act as a barrier to bulk liquid flow while allowing the transfer of counterions from feedstock under the influence of an electrical potential.

The transfer and semi-permeability properties of ion exchange resins and membranes can be used to de-ionise salt water.

In the apparatus shown in the Figure, the ion exchange membrane sheets D are used as barriers to bulk water flow; they define distinct compartments through which liquid streams containing ions can flow tangentially and relatively independently of one another. The compartments are represented by the spacing between membranes, and in the instance illustrated the salt ions in solution are represented by Na+ and C1-.

In this particular configuration, a DC electrical potential, applied by an external power supply (not shown), will cause the transfer of ions to occur by the mechanisms described above.

In the inner dilution cell compartment C (the ion removal cell), the space between the two membranes is filled with ion exchange resin, denoted by the circles labelled according to their counterion specificity (anion and cation). The transfer of ions is represented schematically by the arrows. Ions entering the ion removal cell compartment C react with the ion exchange resin based on their affinity, concentration, and mobility. The ions then transfer through the resin in the direction of the potential gradient. Ions simultaneously transfer across the membranes, maintaining charge neutrality in all cells. Because of the semi-permeable properties of the membranes and the directionality of the electrical potential gradient, ions in the solution will become depleted in the ion removal cell C and will become concentrated in the other chamber X adjacent the concentrating cell).

As can be appreciated, the use of ion exchange resin in the diluting compartment C is one key to the process of the invention. One reason for this is that without the ionic conductivity of the resins, ion transfer will not occur at a practical rate for this application.

A second reason for the need for resin in the diluting compartment C is more subtle but also important: there is an electrochemical relationship between the electrical potential applied and the equilibrium concentration of hydrogen and hydroxyl ions in water. At localised areas of high potential gradients, significant amounts of these ions are produced. Now, without the incorporation of ion exchange resin, this ion production leads to the disadvantages of localised pH fluctuations, and to inefficient use of electrical current.

In use, the resins become electrochemically regenerated to their original ionic forms to some extent. Under these conditions, the resin acts as a continuously-regenerated ion exchange column, exchanging hydrogen ions and hydroxyl ions in stoichiometric amounts with the salts in the solution. This method accounts for the ability of the system of the invention efficiently to de-ionise salt waters to the 100 to 200 microsiemen/cm region of electrical conductivity.

The resin-enhanced conductivity of the de-ionising stream also causes rapid transfer of ions in the compartments, resulting in low membrane area requirements and low resin bed depths. This allows for equipment designs that are compact with relatively low hydraulic pressure losses.