IL29761A - Ion-permeable membrane and its use in electrodialysis - Google Patents

Description

, 29761/2 Ion-permeable membrane and its use in electrodialyois TOKUYAMA SODA KABUSHIKI KAISHA Cs28182 - 2 - 29761/2 This invention relates to a method of treating an electrolyte solution by electrodialysis, and to an ion-permeable membrane for use in that method. , It is known to form a plurality of compartments in an electrodialysis cell by alternately disposing therein ion-selective permeable membranes, one set of which has the property of passing anions but not cations whil'e the other set passes cations but not anions, and to pass a direct current through an aqueous salt solution in the compartments to separate the aqueous solution into concentrated salt water and demineralized water. However, in the use of this method to recover sodium chloride as a concentrated solution from sea-water, it is difficult to obtain a product of satisfactory purity since seawater contains many different ions such for example as Ca , Mg , S0^"~ and CO^ .. Furthermore the process is inefficient in that the power consumed per unit of the sodium chloride recovered is high.

In view of the above difficulties, ion permeable membranes have bee rpposed for the selective dialysis of specifio ions, especially membranes which selectivel pass ions of smaller valence from among those having charges of the same sign. The selectivity of these known ion-selective permeable membranes is as yet not sufficient, and furthermore these membranes are not fully satisfactory from the commercial standpoint. The electric resistances of these membranes are generally high, their durability is poor, and there may a be/ eutrality disturbance during the electrodialysis resulting from the electrolysis of water at the surface of - 3 - . 29761/2 the membrane. In addition the manufacturing operations are complex, and the production cost is high.

For example, the Journal of Applied Chemistry, vol. 6, page 511 (1956) discloses an ion-selective permeable membrane of a structure in which an anion exchange membrane and a cation exchange membrane are laminated* This membrane is difficult to make because ion exchange membran^es of the -two different species must be laminated, and moreover the resulting membrane has an increased thickness. Because of its increased thickness and also because it is a laminate of ion exchange membranes of different species, it has the drawbacks of exceedingly high electrical resistance and susceptibility to concentration polarization.

It is an object of this invention to provide a permselective membrane which may be used to separate inorganio ions whose valence is small, especially monovalent inorganic ions, from a solution containing Inorganic ions of the same sign but of differing electrioal charge, but which has substantially the same electrical resistance as the membranes currently in use and i which ions of charge of opposite sign to those to which the membrane is selective are substantially not transmitted.

In one aspect the invention provides a method of treating by electrodialysis an electrolyte solution containing inorganic ions of the same sign but different valence whereby the ions of different valance are selectively electrodialysed, which comprises passing a direct current through the electrolyte - 4 - 29761/2 while it is contained in two or more compartments each of which is separated from an adjoining compartment by a membrane which is permeable only to ions of the same sign as the ions to be separated and is selectively permeable with respect to such ions, wherein each membrane is formed from an insoluble infusible synthetic organic polymer having dissociable ionic groups of sign opposite to that of the said ions of diffeieit valence bonded thereto, at least one of the" surfaces of each membrane having adsorbed thereon an ion which is substantially incapable of passin through the membrane and has a molecular weight of at least 100 and is derived from an electrolyte having an ionic dissociation constant of at least 0.001, each membrane as a whole preferentially transmitting the ions of the lower valence.

A preferred ion permeable membrane for use in the above process comprises an ion exchange membrane consisting of an insoluble infusible synthetic organic high polymer having a dissociable ionic group bonded thereto, at least one of the surfaces of the membrane having adsorbed thereon at least 0*1 milligram per square decimetre of an ion of molecular weight above 100 derived from a water soluble polymeric electrolyte having an ionic dissociation constant of at least 0.001 the said ion being substantially incapable of passing through the membrane.

It has been found that the pem selectlve membranes of the invention retain their selectivity after one month's continuous use.

The ion exchange Membrane used may be of a conventional - 5 - 29761/2 Ion exchange resin, and preferably contains 0.3 milli-equi-valent per gram of dissociable ionic groups having a dissociation constant of at least 0.001.

Suitable cation exchange membranes contain active acidic functional groups such as -SO^H or -COOH groups bonded to the polymer. Suitable anion exchange membranes may contain active groups containing nitrogen such as quaternary ammonium groups, amino group, guanidyl groups, and dicyandiamidine groups bonded to the polymer matrix.

The ionic substance with which the ion exchange membrane is treated will be referred to hereinafter as the electrolytic treating agent. It must have a molecular weight of above 100 and must be substantially incapable of passing through the membrane. Advantageously it is a polymeric substance.

It is preferred, but not essential, that the electrolytic treating agent should contain active groups with charges of the same sign as the ions to which the resulting membrane is selective. Thus an electrolytic treating agent containing a cation of molecular weight at least 100 may be used with a cation exchange membrane, whereas an electrolytic treating agent containing an anion of molecular weight at least 100 is suitable for treatment of an anion exchange membrane. If the molecular weight of the ion which becomes absorbed in the membrane is less than 100, it easily permeates through the membrane during use.

Electrolytes capable of forming anions having a - 5a - 29761/2 molecular weight above 100 (hereinafter referred to as anionic electrolytic treating agents), include: 1. Compounds containing either a sulphonic acid group or a sulphonate group such as; (a) Aromatic compounds such as benzene and naphthalene containing one or more sulphonic acid groups or sulphonate groups and which may optionally be further substituted, for example by alkyl and nitro groups.

Examples t benzene sulfonic acid and the alkali metal salts thereof, naphthalene sulfonic aoid and.the alkali metal salts thereof, lauryl benzene sulfonic acid and the alkali metal salts thereof. b. Water-soluble polymers having a plurality of sulfonic acid groups or sulfonate groups.

Examples t polystyrene sulfonic aoid) polyvinyl sulfonic acid)and the alkali metal salts thereof. 2. Compound^ containing a sulfate group or a sulfate salt group in their molecule^ and having a molecular weight of above 100 in their dissociated state. a. Sulfuric acid esters of alcohols.

Examples ι lauryl and oleyL sulfates and the alkali metal salts thereof. b. Water-soluble polymers having a sulfate group or sulfate salt group.

Examples t sulfuric acid esters of polyvinyl aloohol.

» Compounds containing a carboxyl group in their molecule^ and having a molecular weight of above 100 in their dissociated state, a. Aliphatic or aromatic compounds which are water-soluble and contain at leas one carboxyl group or a group of a salt thereof. ic Examples : laur$, oleic, stearic and benzoic acids and the alkali metal salts thereof, b. Water-soluble polymers having a plurality of carboxyl groups or groups of the salts thereof.

Examples : polyacrylic and polymethacrylic acids and the alkali metal salts thereof. 4· Compounds containing a phosphoric acid group in their mole-culejf and having a molecular weight of above 100 in their dissociated state.

Examples ι sodium tripolyphosphate, other polyphosphates, alkyl phosphoric acid esters and salts thereof, phosphoric acid esters of cellulose or polyvinyl alcohol.

. Compounds containing a phenolic hydroxyl group in their molecule^ and having a molecular weight of above 100 in their dissociated state.

Examples 1 lauryl phenol 6. Compounds containing a boric acid or arsenic acid group in their molecule and having a molecular weight of above 100 in their dissociated state.

Examples 1 (H0)2-B ^~~^>, , (HO) As(0H)2.

On the other hand, the eleotrolytes capable of forming cations and having a molecular weight of above 100, as used in this invention (hereinafter referred to as cation electrolytic treating agents) include : 7. Compounds containing either a primary, secondary or tertiary amino group in their mpleoule. and having a molecular weight of above 100 in the dissociated state. a. Aliphatic, aromatio and heterocyclic compounds having at least one amino group, and particularly the cationic surfactants.

Examples » long-chain amines having alkyl groups of 12 to 1Θ carbon atoms such as lauryl amine, triethanol- amine monostearate, stearamide- ethyl diethyl- amine, 2-heptadecinyl hydroxyethyl imidazole. b. Water-soluble polymers having a plurality of either amino or imino groups in their molecules.

Examples : polyvinyl imidazole, polyethylene imine, polyvinyl pyridine. 8. Quaternary ammonium salts having a molecular weight of above 100 in their dissociated state.

Examples s lauryl trimethyl ammonium chloride, cetyl pyridinium chloride, stearamide methyl pyridinium chloride, polyvinylpyridinium chloride. 9. Quaternary phosphonium salts having a molecular weight of above 100 in their dissociated state.

Example : . Tertiary sulfonium salts having a molecular weight of above 100 in their dissociated state.

Example : 11. The charged tetra- or hexacoordinated complexes of transition metals having a molecular weight of above 100 in their dissociated state. - 10 - 29761/2 The electrolytic treating agent is readily applied to the ion exchange membrane substrate by dipping the membrane in a solution of the electrolytic treating agent of concentration preferably at least 0.1 ppm by weight, arid particularly at least 0.5 ppm. The conditions under which tie dipping is carried out will vary considerably depending upon the species of the ion exchange membrane used, and the species of the electrolytic treating agent and the concentration thereof in the solution, but the dipping time should be experimentally determined so that the required amount of the electrolytic treating agent is applied.

Alternatively the electrolytic treating agent can be applied to the membrane electropho e ically by introducing the membrane between the cathode and anode in an electrolytic cell containing an aqueous solution of the electrolytic treating agent and passing a direct current between the electrodes. Again the concentration of the aqueous solution of the electrolytic treating agent used must not b less than 0.1 ppm.

Furthermore, electrolytic treating agent may be applied to the ion exchange membrane by a coating operation with a brush or the' like.

The ion exchange membrane can have both of its surfaces treated with the electrolytic treating agent in which case an anionic electrolytic treating agent may be applied to one surface while a cationic electrolytic treating agent is applied to the other surface. Where both surfaces have been treated in this manner, a still better selectivity 11 - 29761/2 may be obtainable. The ion absorbed oh the membrane and and derived from the electrolytic treating agent is of such size that.it does not pass through the minute holes of the ion exchange membrane substrate so that it forms a very thin layer on the ion exchange membrane substrate. it is believed that the macro-ions resulting from the dissociation of the electrolytic treating agen are pulled towa ds the interior of the membrane by the electric fi&eld and hence are not readily dissociated f om the surface of the membrane. It is also possible that when the macro-^ion component has a charge opposite to that of the ionic group of the ion exchange membrane, it is bonded electrostatically to the, membrane surface.

The ion-selective permeable membrane of this invention usually has an electric resistance which is practically the same as that of the untreated ion exchange membrane substrate.

The present cationic ion-selective permeable membrane especially exhibits higher permeability towards monovalent ions such as the hydrogen ion and alkali metal ions than towards di^- or trivalent ions suc as alkaline earth metal ions, aluminium ions and the divalent ions of group VIII metals or ions of still higher valence. Similarly, the anionic membrane exhibits higher permeability towards monovalent anions such as hydroxyl and halogen ions than towards dl- and trivalent ions such as sulphate, carbonate, borate and phosphate ions or ions of still higher valence. The - 12 - 29761/2 Φ1 membrane can however exhibit selective permeability towards divalent cations in the case of an electrolytic solution containing divalent inorganic cations and trivalent inorganic cations. For the present purpose the dimeric ions [H 2] 2+ and Cu2]2+ may be regarded as divalent ions.

The membrane can be used for electrodialysis in place of conventional ion-selective permeable membranes. In a solution containing at least two ions which have charges of the same sign but differ in their valence, the ions of the lower valence can be separated using the ion-selective permeable membrane of this invention. The solution to be separated is placed in.a cell comprising at least two compartments separated by at least one perm^selective membrane, and a direct current is passed through the cell. The membrane will only allow passage of ions carrying charge of a given sign, and among these ions, those of lower charge will be transmitted selectively, while ions of higher valence are generally substantially not transmitted.

An electrodialysis cell of known type, wherein concentrating compartments and diluting compartments are provided alternately by alternately disposing the anion- and cation-seleotive permeable membranes may be used for the present purpose to concentrate a specific ion in one set of compartments and eliminate it from the other set of compartments.

In order to obtain the maximum membrane life, it is preferred that in use the membrane should be positioned so that the surface which vas treated with the electrolytic treating agent is at the anode side iri those oases where - 13 - 29761/2 the macro-ion in the treating agent is cationic, whereas it should be positioned so that its treated surface is at the cathode ■ side in those cases where the macro ion in the treating agent is anionic. the present membrane can be formed In situ when carrying out electrodialysis. An electrolytic solution is introduced into an electrodialysis cell made up by disposing conventional cation and anion exchange membranes alternately and passing a direct current therethrough. The electrolytic treating agent may be added to the electrolytic solution prior to electrodialysist or continuously or intermittently during the electrodialysis, and in each case the ion exchange membranes are converted in situ to the permT^selective membranes of this invention.

In a multi-compartment plant, it is advantageous to add the anionic and/or cationic electrolytic treating agent to the electrolytic solutio to be introduced into the diluting compartment, since the membrane of this invention is rapidly and eficiently formed .

The selectivity may decline after a period of pro- · longed use, but can be restored in situ by adding the electrolytic treating agent to the electrolytic solution during the electrodialysis operation. The membrane can also be regenerated either by dipping it into a solution of the exectrolytic treating agent or by application of the treating agent solution with a brush.

The invention is illustrated in the following Examples in which parts and percentages are by weight unless was passed between silver chlo-rido electrodes. The intramembrane cation transport numbers were computed from the changes that occur in the amount of ions in each compartment before and after passage of the o electric current, and then the relative transport number was obtained by substitution of these values in equation (l). The measurement was made at a temperature of 25°C, and both compartments were vigorously stirred.

In a separate test 5e.paa»*-aljt- a multicompartment type of electrodialysis apparatus constituted by disposing a plurality of pairs of anion- and cation-selective permeable membranes was used, into each of which compartments was introduced seawater o'f the following composition at the rate of 6 cm/sec. A current was then passed through the apparatus via a pair each / of electrodes provided at "bo-bh end* thereof at a current density of 2 amp/dm2 of the membrane area. The effective area of the membrane was 1 dm2 and the temperature of the seawater was 30°C.

Composition 1 CI SO4 Ca Mg K Na Equivalent/liter 0.53 0.05 0.02 0.11 0.01 0.44 The solutions concentrated by electrodialysis gegaowtioly in the alternate compartments were analyzed after their compositions re-ached equilibrium, and the relative transport number was obtained using the approximate equation (2). l?Mi = (CM/CMi) cone. solution/(CM2/CM].) seawater — (2) Example 1 (A) One part of finely divided powder of .polyvinyl chloride, O.9O part of styrene, 0.10 part of 50 divinyl benzene, 0.3 part of dioctyl phthalate and 0.01 part of benzoyl peroxide were homogeneously ( Japanese Industrial Standard ) mixed, and the resulting mixture was applied to a 1.6/mesh polyethylene net.- The so treated net was covered with cellophane on both surfaces, and polymerised by heating for 3 hours at a temperature of 110°C. The obtained film was sulphonated for 24 hours with a 98 sulphuric acid of 50°C 4be¾?e*y to give a cationic exchange membrane having a sulphonio acid group as an exchange group. With tka- -uee thi s cationic exchange membrane , an electrolytic solution of a mixture of 0.2 N NaCl and 0.2 N CaCl2 was subjected to electrodialysis. The cationic transport number, direct electric resistance, and relative transport number (P.. ) , ^a as measured by the two- compartment electrodialysis method, were 0.98, 7 Λ cm^ , and 2.5, respectively, ij test was repeated but (B) jthe above-mentioned electrodialysis, /poly-2-vinylpyridine hydrochloride with a molecular weight of 30 ,000 was added the to -sen-electrolytic solution in the anodic compartment to a conoentra-tion of 20 p. p.m. It was found that the cationic transport number, direct current resistance and relative transport number ( /P. c-as ) are 0.98, % Ω. cm2 and 0.4 . The presence of about 1 mg/dm2 0f poly-2-vinyl-pyridine hydrochloride on the surface of the said membrane was observed.

Subsequently, the electrolytic solution containing poly-2-vinylpyridine hydrochloride was discharged from the dialysis vessel .

The same electrolytic solution as above-mentioned containing no poly-2- electrodialysis vinylpyridine hydrochloride was added thereto , and the /aa e- -O!Kperiittettt repeated eeiKfcetL-otrtr. The results were the same. The of poly-2-vinylpyridine hydrochloride on the surface of this membrane was observed. hydrochloride (C) Instead of the polyvinyl pyridin^, lauryl pyridinium chloride was added to the anodic compartment to a concentration of 300 p. p.m. It was found that the cation transport number, direct current Q resistance and relative transport number (PJJ- ) are 0.95 » H Λ cm^ ·' and 1.2 respectively.

Example 2 A polymeric latex consisting of 30 parts of styrene and 70 fiber parts of "butadiene was applied to a glass fabric. A membraneews-Hsafc--e*e»*e-obtained after drying was sulphonated for hours with a 95 sulphuric acid of 30°C. to -*ke-a?e¾S give a cationic exchange membrane having a sulphonic acid group as an exchange group. With the-twe-of this cationic membrane, an electrolytic solution of a mixture of 0,4 N KC1 and 0.1 N MgCl2 was subjected to electrodialysis. The cation transport number, direct current resistance and relative transport number (P^>) f as measured by the two-compartment electrodialysis method, were 0*99 » 6 -Q. cm2, and 0.9 » respectively.

In the above-mentioned dialysis, a polymeric substance obtained by treating poly-2-vinylpyridine with methyl iodide to tfceapefey- convert about - - of the pyridyl groupsto 4. methyl pyridinium group was added to the electrolytic solution on both sides of the membrane to a concentration of Ρ·Ρ·πι. As a result, the cation transport number, direct current resistance and relative transport number (P^S) were 0.98, 7 SI cm?, and 0.3 » respectively.

The presence of about 0.4 mg/dm2 of poly-2-vinylpyridine hydrochloride on the surface of the said membrane was observed.

When instead of the methyl iodide-treated poly-2-vinyl-pyridine, polyacrylic aoid with a molecular weight of 3»000 was added to both sides of the membrane to a concentration of 300 p.p.m., the cation transport number, direct current resistance and relative transport number (P^S) were 0.99 » 7 ilcm2 and 0.Θ, respectively.

Subsequently, a polymeric substance obtained by treating poly-2-vinylpyridine with methyl iodide to thereby- converia»g about ^ of the pyridyl group/ into A methyl pyridinium groupswas added to the Table I It was observed that 0,6 nig/dm2 of polyethylene imine was deposited on the surface of the membrane so treated. It is clear from this Example that selective permeation of Na ions is markedly improved by treating a cationic exchange membrane with polyethylene imine, and. that the ionic exchange membrane of this invention has a prolonged selectivity.

Example 4 The cationic exchange membrane of Example 1 (A) was immersed for 4 hours at room temperature in a 2?o aqueous solution of polyethylene imine with a molecular weight of 30,000, withdrawn and thoroughly washed with water. The deposition of 5 mg/dm20f polyethylene imine on the sur- Using face of the membrane was observed. i k-tke-¾&o--o£ this membrane, tu»-eie>- cfaOly i-c--¾olubio.rgi a mixture of 0.2 N NaCl and 0.2 N CaCl2 was subjected to electrodialysis. The cation transport number, direct current resistance, and relative transport number (p£a), as measured by the two-compartment electrodialysis method, were 0.98, 11 fi cm2 and 0.5» respectively.

Example 5 ττ .

Using (A) Μ-*-*-%-*θ-"«.ββ·-οί the cationic exchange membrane of Example 1, a mixture of 0.2 N HCl and 0.2 N FeCl3 was subjected to a two-compartment electrodial sis. The relative transport number .(I}) . -¾¾--,fe-¾e--ei-e¾tiOdi¾l^ais uf hu ga±d"(ft)-, polyethylene imine with a molecular weight of 6,000 was added to the anodic compartment to a concentration of 500 p.p.m. frs-trrMsul l, }ihe relative transport number (Pg6) "as 0.05.

Example 6 of (A) A polyvinyl chloride film with--j-" hTcte-rea¾T¾-f- 0.15 mm was immersed for Θ hours at 25°C. in a solution consisting of 0 parts of styrene, 10 parts of a solution containing 50$ divinylbenzene and 0$ ethylbenzihe 20 parts of dioctyl phthalate, 25 parts of petroleum ether and 2 parts of benzoyl peroxide, and withdrawn. The treated film was covered on the surface with cellophane, and heated for 5 hours at 100°C. tfcereby to give a membraneous polymeric substance. This membraneous polymeric substance was chloromethylated for 8 hours at °C. with a solution consisting of 25 parts of chloromethyl methyl ether, 75 parts of carbon tetrachloride and parts of anhydrous stannic chloride, washed thoroughly with methanol, and aminated with a aqueous solution of trimethyl amine, whereby an anionic exchange mem- the ion brane having a trimethylbenzyl ammonium group as an- exchange group was obtained.

Using. ¥i¾h-tho-¾oo-»&» this anionic exchange membrane, -a¾. olootro ■ ■iyfcLo--8oIu.tic-n~o£. a mixture of 0.25 N NaCl and 0.25 N Na2S04 was subjected to a two-compartment electrodialysis. The anion transport number, direct current resistance and relative transport number (PQ^) as measured were 0 , 99 » 7 .Λ cm2, and 0.17 , respectively.

The method was modified i ·' . ·. in that (B) i»"*fee"-s«i electrodialysis'/of (A) a low condensation product of naphthalenesulphonic acid and formalin (trade mark Demor N) was added to the cathodic compartment to a concentration of 1 , 000 p.p.m. As a result, the anion transport number, direct current resistance and relative transport number were 0.98 , 7 /1 cm2, and 0.07 , respectively. When the said treating agent was added to a concentration of 1 , 000 p.p.m., the anion transport number, direct current resistance and relative transport number were found to be 0.98 , 9 A cm2 and 0.04 » respectively.

The deposition of 4 mg/dm2 of the low condensation product of naphthalenesulphonic acid and formalin on the surface of the said membrane was observed.

Example 7 When, in Example 6 (A), there was added, ae-ackfcitive-, poly (sodium styrenesulphonate) obtained by sulphonating polystyrene with a molecular weight of 10 , 000 with i. 9&fo sulphuric acid for 24 hours at 0 to a ' 90 C. and neutralising it with caustic soda, such that—b e concentration of N . of poly(BOdium styrenesulphonate)-aejs-be= 0. 5 by weight, it was found that the anionic transport number, direct current resistance and relative transport number were 0.99 » 7 fl om2 and 0.13 » respectively.

The deposition of 4 mg/dm2 of(polystyrene sulphonate)on the ' surface of the said membrane was observed.

Example 8 A paste consisting of 100 parts of finely divided powder of polyvinyl chloride, 160 parts of 4-vinylpyridine, 10 parts of styrene, mixture parts of a seitufci©*- containing 50 divinylbenzene and 50$ ethyl-benzene, 25 parts of dioctyl phthalate and 3 parts of benzoyl peroxide was applied to a fabric of polyvinyl chloride. The ·.» treated fabric was covered on tooth surfaces with cellophane and heated for 5 hours at 90°C. to t oreby give a membraneous polymeric substance.

This membraneous polymeric substance was quanternarised by treating it for 20 hours at 25°C. with a solution consisting of 50 parts of methanol and 50 parts of methyl iodide whereby an anionic exchange membrane having an N methyl-pyridinium group as an exchange group was obtained.

Under the same conditions as employed in Example 6, a two-compartment electrodialysis was carried out. The anion transport number direct current resistance and relative transport number were 0.99» 3jft.cm2 and 0.16, respectively.

When in the above electrodialysis, poly( sodium acrylate) was added to the cathodic and anodic compartments to a concentration of 100 p.p.m., the anion transport number, direct current resistance, and relative transport number were 0,99» 3-^cm2, and 0.13, respectively.

The deposition of 4 mg/dm2 of the polyacrylate on the surface of said membrane was observed.

Example 9 A IhRn d two-compartment electrodialysis was carried out under using the same conditions as described in Example 6 -wiii-—tfee-use-ef-the same /and membrane astie-ee. in Example 8, each of the additives indicated in Table 2 below was added to the cathodic compartment to the concentration T measured indicated in the same Table. Αβ-β-5?©β«14τ he anion transport number a.ve direct current resistance and relative transport number^ shown in the same- Table weje-e-b-toiaeet-.

Table 2 Example 10 Jfflaea.-a t'wo-compartment electrodialysis was carried out under the same conditions as described in Example 6 with the use of the same membrane as used in Example diamine hydrochloride was added to the anodic and cathodic compartments to a concentration of 50 p.p.m. As the result, the anion transport number, direct current resistance and relative ■ transport number were 0.99» 3 lcm2 and 0.10, respectively.

The deposition of 1 mgdm2 of meta-phenylene diamine hydrochloride on the surface of the said membrane was observed.

Example 11 A Wheit-a two-compartment electrodialysis was carried out under the same conditions as described in Example 6 with the use of the mem- /and brane as used in Example 8¾ each of the additives indicated in Table 3 below was added to the anodic compartment to the concentration indicated this in fche-eaae table. As a result, the anion transport number, direct current resistance and relative transport number shown in the same Table were obtained.

Table 3 Example 12 The same membrane as used in Example 8 was immersed for 3 hours in a $ aqueous solution of a low condensation product of formalin and naphthalenesulphonic acid (Demor N) , withdrawn, and thoroughly washed with water, and then a two-compartment electrodialysis was carried out in the same manner as in Example 6. As the result, the anion transport number, direct current resistance, and relative transport number were 0.98-, J Icm2 and 0,08, respectively.

The deposition of 7 mg/dm2 of a low condensation product of the naphthalenesulphonic acid and formalin on the surface of the said membrane was observed.

Example 13 The same membrane as used in Example 8 was immersed for 1 hour at room temperature in a $ aqueous solution of meta-phenylene- diamine hydrochloride, withdrawn, and wiped on the surface with & filter paper. Thereafter, a two-compartment electrodialysis was carried out according to the method described in Example 6. The anion transport number, direct current resistance and relative transport number obtained were 0.99» 3- cm2, and 0.09, respectively.

The deposition of 4 mg/dm2 of meta-phenylene diamine hydrochloride on the surface of the said membrane was observed.

Example 14 To one side of the same membrane as that of Example 8 was applied three times a 20/o aqueous solution of meta-phenylene diamine hydrochloride, and a 5$ aqueous solution of a low condensation product of formalin and naphthalenesulphonic acid (tradename being. Demor N) was applied three times on the opposite side. After drying for two hours at room temperature, a two-compartment electrodialysis was carried out in the same manner as in Example 6 with the side on which meta-phenylene diamine hydrochloride had been applied being used as the anodic side. The anion transport number, direct current resistance and relative transport number were 0.97» -fl cni2 and 0.06, respectively.

The deposition of 4 mg/dm2 of meta-phenylene diamine hydro-chloride on one surface of the membrane and 1 mg/dm2 of the low condensation product of naphthalenesulphonic acid and formalin on the other was observed.

Example 15 When a two-compartment electrodialysis was carried out in the same manner as described in Example 6 with the use of the same membrane as used in Example 6 , polyethylene imine with a molecular weight of 30,000 was added to the anodic compartment to a concentration of 00 p.p.m. and sodium p-phenolsulphonate was also added to the cathodio compartment to a concentration of 100 p.p.m. The anion trans-port' number, direct current resistance and relative transport number were 0.98, Θ /L cm , and 0.05» respectively.

The deposition of 5 mg/dm2 of polyethylene imine on one surface of the membrane and 1 mg/dm2 of p-phenolsulphonate on the other was observed.

Example 16 A paste composed of 100 parts of finely divided powder of polyvinyl chloride, 220 parts of styrene, 20 parts of a solution containing 50 divinylbenzene and 5 ethylbenz^Ln^, 20 parts of dioctyl phthalate and 2 parts of benzoyl peroxide was applied to a fabric of polyvinyl chloride, and both sides of the so treated fabric were covered with cellophane, followed by heating for 4 hours at 120°C. The resulting membraneous polymeric substance was sulphonated for 12 hours at 60°C. with a 8$ sulphuric acid. An experiment on the concentrating 0 of sea brine was carried out with the use of an apparatus provided with the so obtained cationic exchange membrane and the anionic exchange membrane of Example 8, First, the sea brine was directly subjected to dialysis for 3 days, and then the sea brine, to which was added a low condensation 5 product of formaline'/and naphthalenesulphonic acid (Demor N), was subjected to dialysis for another three days. Subsequently, in the same apparatus, sea brine without the said additive was sub ected to electro- dialysis. The obtained results are shown in Table 4 below.

Table 4 Example 17 The cationic exchange membrane of Example 1 was immersed for 2 hours in excess of a 200 . .m. aueous solution of ol eth lene imine with a molecular weight of 6,000, and the anionic exchange membrane of Example 8 was immersed for 5 hours in a V o aqueous solution of a low condensation product of formalin and naphthalenesul phonic acid. With the use of the so treated exchange membranes, an experiment on the concentrating of sea brine was carried out. As the result, the relative transport number was 0.4 for 0.5 for (P£*) » and 0.06 for (PjjJ ) .

A continued experiment for four successive days did not bring about any ohange in relative transport number.

Example 18 With the use of the membrane of Example 1 (A) and on addition of 4 x 10"' mole/l, of hexamine cobalt complex aalt CCo(Mj) gDClj , electrodialysis was carried out in the same manner as in Example 1 (B) . The obtained results are shown in Table below.

Table The deposition of 8 mg/dm2 of CCo(NH3) 3 on the surfaoe of the said membrane was observed.

Claims (22)

1. An ion-permeable membrane comprising an ion-exchange membrane consisting of an insoluble infusible synthetic organic high polymer having a dissociable ionic group bonded, thereto, at least one of the surfaces of the membrane having adsorbed thereon a least 0.1 millferam per square decimetre of an ion of molecular weight above 100 derived from a water soluble polymeric electrolyte having an ionic dissociation constant of at least 0.001, the said ion being substantially incapable of passing through the membrane.

2. A membrane according to Claim 1, wherein the water soluble polymeric electrolyte contains pendant sulphonic acid or sulphonate groups, sulphate or sulphate salt groups, carboxyl groups or carboxyl salt groups, amino, imino or quaternary ammonium groups.

3. A membrane according to Claim 1, wherein the water soluble polymeric electrolyte is a phosphoric acid ester of cellulose or polyvinyl alcohol.

4. · A membrane according- to any of Claims 1 to 3, wherein at least 0.5 milligrams of the ion are adsorbed per square decimetre of the membrane.

5. A membrane according to any of Claims 1 to 4, wherein the opposite surface of the membrane also has adsorbed thereon an ion which is incapable of passing through the membrane and has a molecular weight of at least 100, which ion may be the same as or different from the ion adsorbed onto the first surface. - 29 - 29761/2

6. A membrane according to any one of Claims 1 to 5t wherein the high molecular weight ions adsorbed onto the two opposite surfaces of the membrane are of opposite sigh,

7. A membrane according to any one of Claims 1 to 6, wherein tue synthetic organic polymer contains at least 0*3 milli equivalent per gram of dissociable ionic groups havong a dissociation constant of at least 0.001.

8. A membrane according to Claim 1, substantially as described herein with reference to the Examples.

9. A method of treating by electrodial sis an electrolyt solution containing inorganic ions of the same sign but different valence whereby the ions of different valence are selectively electrodialysed, which comprises passing a direct current through the electrolyte, hile it is contained in two or-more compartments each of which is separated from an adjoining compartment by a membrane according to any of Claims 1 to 8 which is permeable only to ions of the same sign as the ions to be separated and is selectively permeable with respect to suctions, each membrane as a whole preferentially transmitting the ions of the lower valence.

10. A method according to Claim 9» wherein the ion adsorbed on the membrane is also present in the electrolyte solution.

11. •11· A method according to Claim 9 or 10, wherein the electrolyte solution contains at least 1 part per million by weight of the said adsorbed ion. - 30 - 29761/2

12. A method according to any preceding claim, wherein the membrane is an snLon-exchange membrane and the adsorbed ion is derived from a water soluble compound containing sulphonic acid or sulphonated groups.

13. 15. A method according to Claim 12, wherein the ion of lowest valence which is preferentially transmitted through the membrane is a hydroxy! or halide ion.

14. A method according to Claim 13» wherein the ion is a chloride ion.

15. A method according to any one of Claims 9 to lj, wherein the membrane is a cation-exchange membrane and the adsorbed ion is derived from a water soluble amino compound.

16. A method according to Claim 15» wherein the amino compound is polyethylene imine.

17. A method according to any one of Claims 9 to 11, wherein the membrane is a cation-exchange membrane and the adsorbed ion is derived from a quaternary ammonium salt.

18. A aBbhod according to any one of Claims 15 to 17, wherein the ion of lowest valence which is preferentially transmitted through the membrane is a hydrogen ion or an alkali metal ion.

19. · A method according to Claim 18, wherein the ion is a sodium ion.

20. A method according to any preceding claim, wherein t adsorbed ion is derived from a polymeric substance. - 31 - 29761/2

21. A method according to any preceding, wherein the electrolyte solution is sea-water.

22. A method according to Claim 1, substantially as described herein with reference to the Examples. For the Applicants RTNERS PC/rb