WO2014206381A1

WO2014206381A1 - The asymmetric ion-exchange membrane and use thereof

Info

Publication number: WO2014206381A1
Application number: PCT/CZ2014/000071
Authority: WO
Inventors: Jan KRIVCIK; David NEDELA; Jaroslav HADRAVA; David TVRZNIK
Original assignee: Membrain S.R.O.
Priority date: 2013-06-27
Filing date: 2014-06-25
Publication date: 2014-12-31
Also published as: CZ2013504A3; CZ307270B6

Abstract

The asymmetric ion-exchange membrane consists of a minimum of two layers where at least one of these layers is the active layer (1) of a common cation-exchange or anion-exchange membrane alternatively reinforced with at least one layer of textile reinforcement. Inert layer (2) of polymer that is porous is connected firmly, principally through co-extrusion or through an adhesive inter-layer, to this active layer (1), and the matrix (2a) of this inert layer (2) provides at least partial blocking of the surface of the active layer (1) while the actual material of this inert layer has advantageously low permeability for the low molecular neutral components of the solution. The invention further pertains to the asymmetrical membrane application method in the electromembrane separation process of the electrodialysis (ED) or electrodeionization (EDI) type.

Description

THE ASYMMETRIC ION-EXCHANGE MEMBRANE AND USE THEREOF

Technical Sphere

The invention pertains to the asymmetric ion-exchange membrane formed at least by two layers while at least one of these layers is active layer of a common cation-exchange or anion- exchange membrane. The invention further pertains to the asymmetrical ion-exchange membrane application method in the electromembrane separation process of the electrodialysis (ED) or electrodeionization (EDI) type.

State of the Art

Electrodialysis (ED) is the electromembrane separation process used to separate the electrolytes from solutions. The equipment for technical realization of the ED process is called electrodialyser. Electrodialyser is a device reminiscent of a frame filter-press with its concept This device is composed of end plates fitted with electrodes on their inner side among which is assembled a so called membrane bundle formed with cation exchange membranes (CM) and anion exchange membranes (AM) regularly alternating from one electrode to the other, and these membranes are staggered with spacers. The arrangement of CM and AM in the membrane bundle provides for almost selective transport of cations or anions through relevant membranes, i.e. electrolyte separation from one hydraulic flow and their transfer into the other hydraulic flow. The spacer is used to delimitate the area for fluid flow between the membranes and to assure its adequate turbulization. Fluid turbulization is ensured by network or partition system. The network or partition fibres force the fluid to change the direction of flow and, in doing so, to assure cross mixing of the fluid layers that results in the ED process intensification. This is important namely where diluted solutions are desalted. The flow chambers delimitated on the AM anode side and on the CM cathode side are so called diluate chambers. The fluid flowing through the diluate chambers is referred to as diluate. The flow chambers delimitated on the CM anode side and on the AM cathode side are so called concentrate chambers. The fluid flowing through the concentrate chambers is referred to as concentrate. The diluate and concentrate spacers are mostly of the same structure, however, with regard to the input and output system they are inserted into the apparatus mutually reversed in horizontal or vertical direction. So called electrode chambers through which electrode solution is flowing are separated hydraulically from both the main currents. Its aim is to remove the gases generated on electrodes under the electrode reactions (mainly 0 , H₂) as well as characteristic shift of anolyte's and catolyte's pH to acid or basic area.

Structural element created from anode to cathode gradually by CM, concentrate spacer, AM and diluate spacer, being referred to as a membrane pair, recurs regularly in the membrane bundle of the electrodialyser. By increasing the number of membrane pairs the electrodialyser's process capacity is increased. The maximum number of membrane pairs of the electrodialyser is limited by the production or technical capabilities of individual components as well as the whole ED module or by the parameters of the direct voltage source. The biggest large-capacity industrial electrodialysers are composed of up to 1000 membrane pairs at present.

When introducing a direct voltage that is the driving force of ED process, electric power passes through the electrodialyser to the electrode terminals while electrolytes are removed from diluate to be transferred by membranes to concentrate. Anode is usually of ATA type (activated titanium anode), i.e. it is made of a titanium activated by the layer of Pt, Ru0₂ etc., cathode is mostly made of stainless steel. In certain cases the electrodes are of Ti/Pt type. This allows for electrode polarity reversion. In such case we speak of the ED process with electrode polarity reversion (EDR). When electrode polarity changes the flow chambers' function is reversed in reference to the membrane bundle symmetry, i.e. the originally diluate chambers are transformed into concentrate chambers, and vice versa. In application, the polarity reversion is usually carried out once in 15 to 60 minutes and it increases resistance of membranes to scaling (formation of the deposits of inorganic nature) and fouling (contamination of the membrane surface by organic substances, microorganisms, and so on).

Electrode solution is usually formed by separated hydraulic current. The aqueous solution of electro-chemically inert electrolyte (Na₂S0₄, NaN(¾) is used as the electrode solution. The following electrode reaction takes place on the anode under the current load

H₂0→2H⁺ + ^0₂ + 2e^~ (1) while on the cathode

2H₂O + 2e^~→ 20H^~ + H₂ . (2) pH of the solution flowing through the anode chamber (anolyte) is therefore slightly shifted to the acid area while pH of the solution flowing through the cathode chamber (catholyte) is slightly shifted to the basic area. Electrode solution is fed into the anode and cathode chamber from the common tank and after having passed the electrodialyser both electrode currents flow back to this tank again and this eliminates pH changes. pH of the electrode solution is usually maintained in the acid area by dosing of acid and this eliminates the risk of so called scaling (precipitation of insoluble inorganic substances) on cathode. The electrode solution tank must be ventilated to prevent accumulation of H₂ generated on cathode and the explosion hazard is eliminated. An exception is the continuous single-pass ED technologies that process water with very low electrolyte concentration (up to about 3 g/L) where the electrode solution branches directly from the processed water and, after having passed the electrode chambers, it is conveyed either to the waste or it is used to dilute concentrate in the concentrate circuit.

The largest applications of ED and EDR exist in the area of partial demineralization of the brackish, fluvial or even the municipal/industrial waste waters aimed to prepare drinking or service water (e.g. for irrigation, cooling circuits, etc.). There is also a series of ED applications for demineralization of crud, fruit juices, wine stabilization, cleaning of organic substances or regeneration of galvanic baths (Zn, Ni).

The disadvantage of ED as a result of the concentration polarization is limited speed of electrolyte separation from diluate and, as a consequence, the need for a large membrane area to reach a deep demineralization. Further disadvantage is incapability of ED to separate the neutral components from diluate without the need to adjust pH. As a result of changed pH, these components are capable of being dissociated into ions, e.g. C0₂, H4S1O4, H₃BO3, NH₃.

However, conditions can be considerably improved under these circumstances by filling the diluate chambers with the ion-exchange mixbed, that is a perfect mixture of strong acid cation exchanger and strong basic anion exchanger in a form of spherical beads. This hybrid process that combines ED with ion-exchangers is called electrodeionization (EDI). The technical realization of the process is a module (EDI). The ion-exchange filling of diluate chambers has a specific function in EDI the assertion of which is conditional upon a low conductivity of the inlet water. The cation and anion exchanger beads expand the effective membrane area into the diluate chamber area through their mutual contacts with further ion-exchange beads of the identical type or with relevant membrane and this results in a tight approximation of both ion-exchange surfaces, i.e. the cation and anion exchanger surface. If a solution with low conductivity flows through the diluate chambers, the solid phase thus creates a low-resistant environment for the preference passing of electric current and, at the same time, the transfer of mass increases.

Large gradients of electrical potential are formed on the oriented bipolar interfaces in connection with depletion of electrolyte from water and this leads to water splitting into the FT" and OH^" ions. These ionts regenerare relevant ion-exchangees and ion-exchange membranes so the diluate chambers can simultaneously function as the continuously regenerated ion-exchange mixbed. This is connected with the capability to transfer the low dissociated components (e.g. C0₂, H4S1O4, H₃BO3, NH₃, etc.) into the ionic forms and to remove them from water.

Electrolyte separation takes place in EDI process in the manner below. Firstly, the ion is transferred through diffusion and migration from the liquid phase (diluate) into the solid phase (cation or anion exchanger depending on the ion type), the ion consequently migrates through the solid phase to appropriate ion-exchange membrane and, finally, it is transferred by this membrane into the adjacent concentrate chamber. In case of the components that are dissociated only after the change of pH, capture takes place simultaneously with dissociation, e.g. C0₂ into HC0₃ ^~, or C0₃ ^2~.

Since having been marketed the EDI technology had witnessed long development and a number of variants of this process has been developed that cover application of the mixbed in thin (2 - 3 mm) as well as thick (8 - 12 mm) diluate chambers, application of laminated or completely separated mixbed of cation and anion exchanger in the thick (9 mm) diluate chambers, application of inert net spacers or ion-exchange fillings (cation exchanger or mixbed) in the concentrate and electrode chambers. The plate and spiral coiled EDI modules have been developed in terms of design. The development in EDI area continues even today while efforts are focused mainly on the configuration of ion-exchange mixbed in the diluate and concentrate chambers and in connection to the diluate chamber thickness with the aim to improve performance and demineralization capability of the module, improve rejection of the selected components (H4S1O4, ¾B0₃), influence distribution of the current density in the module, increase tolerance to the inlet water hardness or limit scaling of CaC0₃, etc.

Today, EDI is a generally accepted technology that is applied in industry as an alternative to the ion-exchange in mixbed columns for the preparation of demineralized or ultraclean water. Given the process sensitivity and limitation, the processed fluid is a water of a quality of the permeate from reversing osmosis (RO) or better (e.g. demineralized water).

Despite its long development, EDI process still has a series of imperfections that are not dealt with sufficiently today. As in all of the membrane separation processes, the main problem with EDI technology is scaling, namely of CaC0₃ or Mg(OH)₂ in the concentrate chambers or on cathode. In order to prevent scaling, the hardness linked with alkalinity of the input water to EDI is limited. Scaling means necessity for frequent shutdowns of technology operation for chemical cleaning with acids.

A considerable trouble, especially in EDI, is also the reverse diffusion of electrolytes and neutral components through the ion-exchange membranes from concentrate into diluate. The other stated trouble limits considerably the EDI product quality for certain compositions of the water to be processed, or specific operation conditions, namely the high water gain. Causes of the last stated phenomenon result from the physico-chemical properties of ion-exchange membranes and the essence of EDI process or from general electromembrane separation processes as ED or EDI. What results from the Donnan equihbrium is the fact that the only moving ions in the ion- exchange material (ion exchanger, membrane), when in contact with highly diluted electrolyte solution, are almost exclusively so called counter-ions, the ions with opposite charge than the fixed functional group. However, the concentration of both counter-ions and co-ions (ions with the same charge as that of the fixed functional group) in the ion-exchange materials grows and selectivity of the ion-exchange material drops with growing electrolyte concentration in the fluid at the interface with the ion-exchange material. The same equilibrium indicates that the concentration of neutral components in the ion-exchange materials is the same (providing we match the activities with concentrations) as in the solution at the interface with the ion-exchange material.

In the EDI process for the preparation of highly clean water, there is a virtually clean water on the diluate side and usually a solution with the conductivity in the order of 10¹ to 10² 8/αη on the concentrate side. In similar way as in ED, the electrolyte concentration shift against relevant flow phase and its growth in the case of concentrate take place at the interface of ion-exchange membrane with diluate and concentrate as a result of existence of the diffusion layer. In the combination with the statement in the previous paragraph there is always a non-zero concentration of co-ions at least on the concentrate side so the concentration gradient of co-ions and neutral components that is a driving force of their reverse diffusion is necessarily generated across the membrane. However the term of "back diffusion" must be understood as a transport of components through the migration mechanism and a diffusion of concentrate into diluate.

The maximum separation rate of electrolyte from diluate in ED is limited by so called concentration polarization and it depends on the electrolyte nature, ion-exchange membrane selectivity and hydrodynamic conditions. In case of EDI it can also be given by the rate of ion exchange between the liquid and solid phase (phenomenon controlled by diffusion), the rate of migration through the solid phase (ion exchangers), etc. The resultant transport of component from the diluate into concentrate chamber is given by the difference in (densities) of currents of this component between both types of ion-exchange membranes. It can be shown that the total separation rate of electrolyte from diluate resulting from reverse diffusion drops at a constant current density and with growing electrolyte concentration in the concentrate. In certain cases, better result can be obtained in this respect by applying higher current density, i.e. at the cost of increased consumption electric power; however, such electrolyte separation rate as that with the absence of reverse diffusion is not attained even then in number of cases. Thus, the reverse diffusion can easily limit the obtainable quality of demineralized product, especially when highly diluted solutions are processed.

The serious problem in EDI is presented namely by the low-molecular substances that are partly dissociated even in the area of pH that is close to the neutral pH, namely in C0₂ and N¾. Depending on the inlet water composition, these substances may exist in the diluate flow phase and in the corresponding concentration, that is, in the flow concentrate phase, or they can be generated locally under specific conditions of EDI process, i.e. in the alkaline environment in anion exchanger in diluate chamber, in the anion-exchange membrane and diffusion layer at its interface with concentrate or in the acid environment in cation exchanger in diluate chamber, in cation-exchange membrane and diffusion layer at its interface with concentrate. This is associated with water splitting at the bipolar interfaces and with the transport of ions of IT" and OH^" solid phase. The ions of HCCV in concentrate at the interface with the cation-exchange membrane are converted into CC½ as a result of low pH while the ions of NH in concentrate at the interface with the anion-exchange membrane are converted into NH₃ as a result of high pH. Local concentrations of the specified components in concentrate at the interface with relevant membrane can be additionally increased directly by the precipitation of HCCV and NH ions from the solid into liquid phase if the concentrate chamber's filling contains anion exchanger or cation exchanger that form a continuous zone across the concentrate chamber, and by their consequent conversion into CO2 or N¾. Thus, the concentration of the neutral components above, namely C0₂ in practice, in concentrate at the interface with relevant (cation-exchange) membrane can easily reach the order of tens of mg 1 while it is virtually zero on the diluate side (it can be shown that the water with a conductivity of 0.1 μ5/οιη can contain a max. of 0.005 mg 1 of free CO2). C0₂, just like the low-molecular neutral component, is capable of fast diffusion through the cation-exchange membrane from concentrate into diluate to contaminate EDI product. We mention mainly the trouble with C0₂ here because alkalinity (HCO₃ ^" and C0₂) is usually the most significant component of TEA RO permeate as typical inlet water into EDI.

The issue of the reverse diffusion of C0₂ and similar substances is not still resolved in satisfactory manner in the current technical practice in the EDI area. One of possible solutions seems to be the application of ion-exchange membranes of higher thickness. This reduces the concentration gradient that is the driving force of diffusion. However, such solution is not economically profitable since ion-exchange membranes are usually very expensive and this limits its profitability. More suitable alternative is the solution according to the Patent Application USA 2013/0092540. Here, the creators placed a cation-exchange monolayer with finer beads size at the surface of the cation- exchange membrane's concentrate side. In principle, it is cheaper alternative to the solution where the cation-exchange membrane of about twice the thickness is used. As shown above, this solution limits considerably the reverse diffusion of C0₂ but it fails to solve the problem completely.

Such module for EDI that is capable within the tolerance limits for the composition of processed water and operation conditions (max. water gain) of efficiently eliminating the effect of back diffusion of the low-molecular components of C0₂, H4S1O₄, H₃B0₃, NH₃ type or electrolytes through the ion-exchange membranes from concentrate into diluate, especially in the cases when the concentrate chambers are filled with ion exchangers, is therefore still missing in the industrial practice. The Principle of the Invention

The asymmetric ion-exchange membrane according to this invention contributes significantly to removing of the issues of back diffusion above. This asymmetrical membrane consists of a minimum of two layers where at least one of these layers is the active layer of a common cation- exchange or anion-exchange membrane alternatively reinforced with at least one layer of textile reinforcement. The principle of the invention consists in the fact that the inert layer of polymer that is porous is connected firmly, principally through co-extrusion or through an adhesive inter-layer, to this active layer, and the matrix of this inert layer provides at least partial blocking of the surface of the active layer while the actual material of this inert layer has advantageously low permeability for the low molecular neutral components of the solution and it is ideally totally impermeable for these components.

The inert layer has advantageously a thickness of 1 to 100 μπα, mainly 1 to 10 μπι. The material of this inert polymer layer is advantageously polyolefin, namely polyethylene or polypropylene, polyester, polystyrene, polymethylmethacrylate, polyvinylchloride (PVC), polyamide, polyvinylidendifluoride (PVDF), (copolymer based on the natural or synthetic rubber or thermoplastic elastomer, polytetrafluorethylene (Teflon), etc.

Principally, the inert substance can be represented by any polymer that is mechanically, chemically and thermally stable under the process conditions and simultaneously impermeable or that has very limited permeability for the neutral components the back diffusion of which should be prevented. Thickness of the inert layer can be optional, however, it is better in terms of practice (total membrane thickness, voltage increase) that this thickness is low, preferably (as stated above) in the order of 1 to 10¹ μιη.

Pores in the inert layer are filled with fluid and they form continuous zone across this layer. This ensures electrical conductivity of this layer and thereby also the asymmetrical ion-exchange membranes according to this invention. Uniform distribution of the pore size and density in the inert layer is preferred. Uneven distribution of the pore density, geometry or size can be used where the separation rate of the selected components must be monitored.

The active layer of the asymmetrical membrane according to the invention can be formed by a common, commercially available or special homogeneous or heterogeneous cation-exchange or anion- exchange membrane. The homogeneous types include the membranes based on the styrene copolymer with divinylbenzene, based on polyacrylate or perfluorinated polymers. Heterogeneous types are represented by the thermoplastic mixtures of milled ion exchangers with the matrix based on styrene copolymer with divinylbenzene or polyacrylate with a polymeric binder as polyethylene (PE), polypropylene (PP) or polyvinylidendifluoride (PVDF). The essence of application method of the asymmetrical ion-exchange membrane depending on the invention in the electromembrane separation process of the electrodialysis (ED) or electrodeionization (EDI) type consists in the fact that the asymmetrical ion-exchange membrane is oriented in such a manner that its inert layer is directed to the concentrate or electrode chambers while the active layer is directed to the diluate chambers and this results in reduced undesirable back diffusion of ions and neutral components.

The asymmetrical ion-exchange membrane can be used in combination with a minimum of one more standard or asymmetrical ion-exchange membrane.

The application principle of asymmetrical ion-exchange membrane consists in limiting the kinetics of back diffusion of ions and neutral components by blocking a part of the surface of active membrane layer on the concentrate side with the inert layer matrix while the surface of active membrane layer on the diluate side is not limited in any manner. It can be shown that this method is capable of increasing current density that corresponds to the back diffusion of ions in the unblocked parts of ion-exchange membrane, however, this possible increase fails to compensate for the decrease of the ion-exchange membrane's active area by the blocked part so this results in reduced ion current by the back diffusion. In case of the neutral components the current density corresponding to the back diffusion is given by the concentration gradient of these components across the ion-exchange membrane. Blocking of partial surface of the active membrane layer on the concentrate side by the inert layer's matrix therefore results in repeated reduction of the neutral component's current by the back diffusion.

However, if the part of the surface of the ion-exchange membrane's active layer is blocked on its concentrate side by the inert layer's matrix, certain increase of electrical resistance may take place in the entire system because the ion path in the asymmetrical membrane's active and inert layer and in the diffusion layer at the interface with membrane and concentrate or in the ion-exchange filling of the concentrate chambers is elongated. At the same time, certain voltage increase in the active part of the asymmetrical ion-exchange membrane and in the diffusion layer at the interface of this membrane and concentrate does not pose any technical problem because the voltage loss ratio in these parts of EDI module is very low in general.

However, it is necessary to realize that excessive blocking of the surface of membrane's active layer by the inert layer's matrix on the part near the concentrate chamber will result in practical elimination of back diffusion of the undesirable components from concentrate into diluate but it simultaneously increase the ohmic loss of the system voltage above the tolerable limit. On the contrary, an insufficient blocking of the surface of membrane's active layer by the inert layer's matrix will increase the ohmic loss in the system in negligible manner but it will not limit substantially the back diffusion of undesirable components from concentrate into diluate. According to this invention, the application field of the asymmetrical ion-exchange membrane falls into demineralization applications of the electromembrane separation processes of ED and EDI type where, on one hand, the diluate is present with very low concentration either of electrolytes, preferentially 10^"2 mol/1 or lower, or of undesirable neutral components and, on the other hand, the concentrate exists where the concentration of electrolyte or undesirable neutral components is higher at least by one order or, preferentially, by two to four orders than in the diluate.

Overview of Figures in the drawings

The attached drawings are used to clear the Invention Principle in detail. The meanings of the drawings follow

Fig. la - structure of the asymmetrical ion-exchange membrane,

Fig. lb - structure of the asymmetrical membrane with the inert layer's section depicted in detail,

Fig. 2 - hydraulic diagram of the electrodialyser,

Fig. 3 - principle diagram of the electrodialysis (ED) process,

Fig. 4 - principle diagram of the electrodeionization (EDI) process,

Fig. 5 - application example of the asymmetrical ion-exchange membranes in EDI module.

Examples of Invention Implementation

Example 1

Asymmetrical ion-exchange membrane (see Fig, la, lb) is formed by two layers. The First layer is the active layer 1_ of the cation-exchange or anion-exchange membrane of homogeneous type with a thickness of 50 to 1000 μπι, based on the styrene copolymer with divinylbenzene, based on the polyacrylate or based on the perfluorinated polymers. Polymeric inert layer 2 based on polyethylene having 1 to 100 μπι in thickness that is porous is fixed to this active layer I - by way of co-extrusion or through the adhesive inter-layer - see the representation of pores 2b on Fig. lb - while matrix 2a of this inert layer 2 simultaneously provides at least partial blocking of the active layer's surface. Permeability of the asymmetrical ion-exchange membrane for the neutral component of C<¾, NH₃ etc. type is lower than permeability of the active layer L Example 2

Asymmetrical ion-exchange membrane (see Fig. 1) is formed by two layers. The first layer is the active layer I of the cation-exchange or anion-exchange membrane of heterogeneous type having 50 to 1000 μιη in thickness, based on the thermoplastic mixtures of milled ion exchangers with the matrix based on styrene copolymer with divinylbenzene or polyacrylate with polymeric binder as polyethylene (PE), polypropylene (PP) or polyvinylidendifluoride (PVDF). Polymeric inert layer 2 based on polyethylene having 1 to 100 μπι in thickness that is typical of its porous nature so it does not restrain the transport of ions by diffusion and migration is fixed to this active layer I by way of co- extrusion or through the adhesive inter-layer. Permeability of the asymmetrical ion-exchange membrane for the neutral component of CO2, NH₃ etc. type is lower than permeability of the active layer L

Example 3

Asymmetrical ion-exchange membrane (see Fig. 1) is formed by two layers. The first layer is the active layer I of the cation-exchange or anion-exchange membrane of homogeneous type identical to Example 1. Polymeric inert layer 2 based on polypropylene having 1 to 100 μπι in thickness that is typical of its porous nature so it does not restrain the transport of ions by diffusion and migration is fixed to this active layer 1 by way of co-extrusion or through the adhesive inter-layer. Permeability of the asymmetrical ion-exchange membrane for the neutral component of C0₂, N¾ etc. type is lower than permeability of the active layer I.

Example 4

Asymmetrical ion-exchange membrane (see Fig. 1) is formed by two layers. The first layer is the active layer1 of the cation-exchange or anion-exchange membrane of heterogeneous type identical to Example 2. Polymeric inert layer 2 based on polypropylene having 1 to 100 μιη in thickness that is typical of its porous nature so it does not restrain the transport of ions by diffusion and migration is fixed to this active layer1 by way of co-extrusion or through the adhesive inter-layer. Permeability of the asymmetrical ion-exchange membrane for the neutral component of C0₂, N¾ etc. type is lower than permeability of the active layer1.

Example 5 Asymmetrical ion-exchange membrane has active layer i of the cation-exchange or anion- exchange membrane and polymeric inert layer 2 in the version as in any of Examples 1 to 4. However, the active layer I of the cation-exchange or anion-exchange membrane is extra reinforced with a minimum of one layer of textile reinforcement.

Example 6

When using the asymmetrical ion-exchange membrane according to any of Examples 1 to 5 in the electromembrane separation process of the electrodialysis (ED) or electrodeionization (EDI) type the asymmetrical ion-exchange membrane is oriented in such a manner that its inert layer 2 is directed to the concentrate C or electrode E chambers while the active layer 1 is directed to the diluate chambers D (see the representation on Fig. 1). This will restrain the undesirable back diffusion of ions and neutral components.

The hydraulic diagram of a typical equipment for electrodialysis (ED) - the plate electrodialyser - is depicted on Fig. 2. Electrodialyser is a device reminiscent of a frame filter-press with its concept. This device is composed of end plates fitted with electrodes on their inner side among which is assembled a so called membrane bundle formed with cation-exchange membranes CM and anion-exchange membranes AM regularly alternating from one electrode to the other, and these membranes are staggered with spacers. Fig. 2 also outlines the passage of electrode solution E, concentrate C and diluate D through the equipment.

The electrodialysis (ED) process principle is apparent from Fig. 3. Even Fig. 3 depicts the alternating cation-exchange membranes CM and anion-exchange membranes AM. the path of concentrate C and diluate D and the direction of electric current I. The big horizontal arrows symbolize the required transport of ions while the dashed horizontal arrows symbolize the undesirable transport of ions. The undesirable transport of ions takes place as a result of non-selectivity of the ion- exchange membranes and reverse diffusion.

In processing the highly diluted solutions with conductivity in the order of 10² μ8/αη or lower, the conditions in the equipment can be further improved substantially in terms of the matter transfer intensification by filling the diluate chambers with the ion-exchange mixbed, i.e. a perfect mixture of strong acid cation exchanger and strong basic anion exchanger in a form of spherical beads as shown on Fig. 4. Inlet of O permeate ΡΛ and outlet of EDI product P2 are also outlined schematically on Fig. 4. Example 7

Any combination of the standard ion-exchange membranes with asymmetrical membranes, preferably the combination of the asymmetrical cation-exchange membrane with the standard or asymmetrical anion-exchange membrane can be used in ED or EDI module. The inert layer of asymmetrical ion-exchange membrane is always directed to the concentrate or electrode chambers while the active layer is always directed to the diluate chambers. The preferred filling of the diluate and concentrate chambers according to this Invention is a mixbed.

The diagram on Fig. 5 depicts combination of the standard anion-exchange membranes AM in combination with the asymmetrical cation-exchange membranes CM in EDI module. What is further outlined here is also the passage of the processed solution F, concentrate C and the outlet of product P from the equipment.

Claims

P A T E N T C L A I M S

1. The asymmetric ion-exchange membrane formed by at least two layers where a minimum of one of the stated layers is active layer ( 1) of a common cation-exchange or anion-exchange membrane, alternatively reinforced with at least one layer of textile reinforcement, typical of the fact that the inert layer (2) of polymer that is porous is connected firmly, principally through co-extrusion or through an adhesive inter-layer, to the active layer (1), and the matrix (2a) of this inert layer (2) provides at least partial blocking of the surface of the active layer (1) while the actual material of this inert layer (2) has advantageously low permeability for the low molecular neutral components of the solution.

2. The asymmetric ion-exchange membrane according to Claim 1, typical of the fact that the inert layer (2) has a thickness of 1 to 100 μπι, predominantly 1 to 10 μιη.

3. The asymmetric ion-exchange membrane according to Claim 1 , typical of the fact that the material of the inert layer (2) is polyolefm, namely polyethylene or polypropylene, polyester, polystyrene, polymethylmethacrylate, polyvinylchloride (PVC), polyamide, polyvinylidendifiuoride (PVDF), (co)polymer based on the natural or synthetic rubber or thermoplastic elastomer, polytetrafiuorethylene (Teflon).

4. The asymmetric ion-exchange membrane according to Claim 1, typical of the fact that the pores in the inert layer (2) create a continuous zone across this layer and the pore size and density (2b) is distributed uniformly.

5. The asymmetric ion-exchange membrane according to Claim 1, typical of the fact that the inert layer (2) has not uniformly distributed pores (2b) for the control of the component separation rate.

6. The application method of the asymmetric ion-exchange membrane according to Claim 1 in the electromembrane separation process of the electrodialysis (ED) or electrodeionization (EDI) type, typical of the fact that the asymmetrical ion-exchange membrane is oriented in such a manner that its inert layer (2) is directed to the concentrate (C) or electrode (E) chambers while the active layer (1) is directed to the diluate chambers (D) and this results in reduced undesirable back diffusion of ions and neutral components.

7. The method according to Claim 6 typical of the fact that the asymmetric ion-exchange membrane will be used in combination with a minimum of one more standard or asymmetrical ion-exchange membrane.