This is a divisional of application Ser. No. 743,294, filed Nov. 18, 1976, now U.S. Pat. No. 4,111,780. 
    
    
     BACKGROUND OF THE INVENTION 
     This invention relates to the process for electrolytic treatment of aqueous alkali halide solution using of ion exchange membranes, and more specifically, it relates to the above kind of process by use of a electrolytic bath assembly comprising an anodic chamber, an intermediate chamber and a cathodic chamber arranged in series each chamber being separated by an ion exchange membrane from its adjoining chamber. 
     The process for the electrolysis of aqueous alkali halide solution in a bath assembly fitted with selectively operating ion exchange membranes has hitherto been known. More recently, with remarkable development of fluorine containing resin-made, ion exchange membranes, this process has been practised on the industrial scale. 
     It has been experienced, however, that the current efficiency is appreciably reduced with increase of alkali concentration in the cathodic chamber, when an ion exchange membrane of the normally procurable kind and type is used. This is especially true, even with use of rather superior membranes providing as high as 80% or higher current efficiency at 20% or lower alkali concentration in the cathodic chamber, this current efficiency frequently becoming 50% or less when the alkali concentration increases to 30% or higher. It has already been proposed to provide an intermediate chamber between the anodic and cathodic chambers in order to avoid the above-mentioned disadvantage resulting from the increased alkali concentration in the cathodic chamber, so as to separate these chambers one after another by the provision of respective ion exchange membranes for the purpose of keeping the difference in the alkali hydroxide concentrations between the neighboring chambers as little as possible, as would necessary for the avoidance of current efficiency reduction. The number of such intermediate chambers may not be limited to only one, thus, it may be increased as occasion desires. 
     With increased number of such intermediate chambers for obtaining as little as possible difference in the concentrations in these chambers for improving the current efficiency, the number of the ion exchange membranes separating these chambers one from another would correspondingly, be increased, with increased interelectrode distances, thereby requiring a correspondingly increased voltage to be impressed. 
     It is, therefore, highly desirous to minimize the number of the intermediate chambers as much as possible it being of still greater advantage to operate the process with a three chamber mode of the electrolytic bath assembly, the number of the intermediate chambers being thus limited to only one, thereby to obtain an operating current efficiency as high as possible. 
     We have extensively investigated, in order to realize a high efficiency process for obtaining high concentration aqueous caustic alkali solution with high current efficiency, the use of the three chamber mode of the electrolytic bath assembly comprising a cathodic, an intermediate, and an anodic chamber arranged in series and fitted with ion exchange membranes acting as the separating walls between the chambers. 
     It may be stressed that the membrane acting as the separating wall between the anodic and the intermediate chamber must be highly durable to gaseous chlorine and caustic alkali present respectively in the anodic and the intermediate chamber. A most preferable one serving for this purpose is a fluorine-containing resin-made anodic ion exchange membrane. As results with the conventional two-chamber system, the current efficiency will appreciably decrease with increase of the caustic alkali concentration in the intermediate chamber which is comparable to the cathodic chamber in the known conventional system, and thus, the alkali concentration in the intermediate one should preferably be lower than 20%. 
     As the cationic permselective membrane acting as the separating wall between the intermediate chamber and the cathodic one may be made of any suitable substance if it is durable to caustic alkali. But, in practice, it must have a low electric resistance in aqueous caustic alkali solution and a high selective penetration performance to Na + . Further, this membrane must have a moderate water penetration performance, since if an excess quantity of water should transfer from the intermediate chamber through the membrane to the cathodic chamber, the alkali concentration prevailing in the latter will be disadvantageously reduced. 
     As, conventionally, when all the quantity of caustic alkali formed in the intermediate chamber is transferred to the cathodic chamber of the three bath chamber system, the alkali concentration in the intermediate chamber must become disadvantageously high, when high concentration caustic alkali is to be attained at the cathodic chamber. In such case as above, the current efficiency will become highly inferior and under extreme conditions, the electrolytic apparatus does not operate properly, the operational troubles being caused by the too much high alkali concentration near the condensation state. 
     We propose, therefore, to separately take out the caustic alkali electrolytically formed in the intermediate chamber, or to transfer part or the whole thereof to the cathodic chamber so as to produce high concentration caustic alkali at the latter chamber, as an alternate measure. 
     According to our experiments, it has been found that in order to provide high concentration caustic alkali by high current and power efficiency with use of the three chamber type bath assembly with ion exchange membranes, the anodic ion exchange one acting as the separating wall between the intermediate chamber and the cathodic one, must have lower penetration performance to water as well as hydroxide radicals, and an especially small electric resistance value in the caustic alkali solution. For this purpose, we have found that those having phenol radicals acting as the ion exchange one are highly suitable. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     The objects, features and advantages of the present invention will become more apparent from the following detailed disclosure of preferred embodiments and numerical examples to be disclosed mostly with reference with to the accompanying drawing, representing schematically the three chamber type electrolytic bath assembly. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In the drawing, the bath assembly comprises an anodic chamber 1, an intermediate chamber 2 and a cathodic chamber 3 arranged one after another into a unit and separated from each other by cationic permselective membranes &#34;A&#34; and &#34;B&#34;, respectively, as shown. Numeral 4 represents an anode fixedly mounted within the anodic chamber or bath. Numeral 5 is a cathode fixedly mounted in the cathodic chamber or bath. 
     Anodic chamber 1 is provided with an inlet 6 through which saturated common salt brine is being fed continuously. The chamber 1 is further provided an outlet 7 through which diluted brine is being continuously taken out. 
     Intermediate chamber 2 is preferably fitted with an inlet 8 through which water or diluted caustic soda solution is continuously fed. The caustic soda solution formed in the intermediate chamber is taken out through an outlet, not shown, and in the form of a diluted caustic soda solution. Or alternatively, part or whole of the product is transferred through a communication pipe 9 to the cathodic chamber 3, so as to elevate the concentration of caustic soda. The thus concentrated caustic soda is being taken out continuously through an outlet 10. 
     In this case, when the membrane &#34;A&#34; is the same and the caustic soda concentration and the current passage rate at the intermediate chamber are unchanged, it may naturally be expected that the quantities or rates of anodic ions and cathodic ions should be unchanged during continued operation of the electrolytic plant. Thus, as an example, the quantity or rate of Na +  -ions passing from the anodic chamber to the intermediate chamber will be unchanged. 
     Part of Na +  -ions thus conveyed to the intermediate chamber will react with OH -  -ions conveyed from the cathodic chamber to the intermediate one, to form caustic soda, while the remaining Na +  -ions will be conveyed through the membrane &#34;B&#34; to the cathodic chamber where they will form caustic soda. 
     The caustic soda formed at the intermediate chamber is taken out in the form of a dilute caustic soda, or alternatively, it will be conveyed therefrom through communication pipe 9 to the cathodic chamber, as above mentioned. It should be thus noted that all the Na +  -ions passed through the first membrane &#34;A&#34; will form caustic soda after all. 
     Further, it must be noted that the electric current efficiency of the electrolytic plant will vary in function of the passing conditions of Na +  -ions through the first membrane &#34;A&#34;. Thus, this efficiency depends upon the alkali concentration prevailing in the intermediate chamber and kept in contact with the first membrane &#34;A&#34;, and being not affected by the alkali concentration in the cathodic chamber and/or the conditions of the second membrane &#34;B&#34;. For this reason, excessively high alkali concentration at the intermediate chamber should be avoided. 
     In order to produce high concentration caustic soda with increased electric current efficiency, cationic permselective high alkali concentration at the intermediate chamber should be avoided, and thus, in turn, the difference in the alkali concentration between the intermediate and cathodic chambers must be kept as high as possible. 
     The alkali concentration at the cathodic chamber 3 depends upon the quantities or rates of water and Na +  -ions transferred to the cathodic chamber, while the quantity or rate of Na +  -ions depends upon the electric current (rate) and the current efficiency. These water and Na +  -ions consist of those which have been conveyed from the intermediate chamber 2 through second membrane &#34;B&#34; to cathodic chamber 3 and those which have been conveyed through communication pipe 9 and in the form of an aqueous caustic soda solution, the both being thus fed from the intermediate chamber. 
     The caustic alkali prevailing in the intermediate chamber is formed by the reaction of OH -  -ions transferred from the cathodic chamber thereto, being however, reduced by those which will further be conveyed towards the anodic chamber, with part of those Na +  -ions transferring from the anodic to the cathodic chamber. The concentration will depend upon the water quantity fed to the intermediate chamber, this water quantity or rate being the total sum of the water content supplied from the anodic chamber through the first membrane &#34;A&#34;, and that supplied from outside through the inlet 8 as occasion may demand. However, this concentration will be increased so far by the water quantity or rate which is conveyed from the intermediate chamber through the second membrane &#34;B&#34; towards the cathodic chamber. 
     In other words, since water is transferred from the intermediate chamber through the second membrane &#34;B&#34;  to the cathodic chamber, the alkali concentration in the intermediate one is reduced so far, while the concentration in the cathodic one is increased correspondingly. Still in other words when expressed conversely, the alkali concentration in the cathodic chamber will be reduced so far, with increase of the quantity of OH -  -ions conveyed from the cathodic chamber through the second membrane &#34;B&#34; towards the intermediate chamber, while the alkali concentration in the last mentioned chamber will increase correspondingly. With one and the same nature and structure of the anodic ion exchange membrane, the water penetration quantity will increase, and indeed, with increase of the electric current passage rate, as well as of the penetratingly transferring quantity of Na +  -ions. On the other hand, the penetrating quantity or rate of OH -  -ions will generally increase, with increase of penetratingly transferred quantity or rate of Na +  -ions. These quantities or rates will naturally vary, depending upon the nature and structure of the membrane. 
     According to our experimental observation, we have found that in the process for obtaining 30% or higher concentration of caustic alkali with 80% or higher electric current efficiency, it is highly recommended to use an a membrane &#34;B&#34;, acting as the separating wall between the intermediate and cathodic chambers, having a water-penetrating rate of 3 g/A.h or less and a OH -  -ions-penetrating rate of 0.4 g/A.h or less, while keeping the alkali concentration (by weight percent) in the intermediate chamber at 10-20% and the alkali concentration in the cathodic chamber at 30-50%. 
     With use of such second membrane &#34;B&#34; having a higher water penetration quantity or rate than 3 g/A.h, and when keeping the caustic alkali concentration at 10-20%, substantial part of the water content, larger than the half fed to the intermediate chamber will penetrate through the membrane towards the cathodic chamber and thus, difficulties will be met in maintaining the cathodic alkali concentration at 30% or higher. On the other hand, with use of such second membrane, having a higher OH -  -ions-penetrating power than 0.4 g/A.h, too much quantity of caustic alkali will be formed at the intermediate chamber and, thus, the alkali concentration prevailing therein will become higher than 20%. 
     As above mentioned, the anodic ion exchange membrane separating the intermediate and cathodic chambers must have a water penetration power lower than 3 g/A.h. For this purpose, the membrane must have lesser hydrophilic radicals as ion exchangeable one. 
     As for these radicals, and as examples, carboxylic radical; phenolic radical or substituted phenol-, mercapto- and phosphoric radical may be used. As most preferable type cationic permselective membrane, however, it may have a pendant type phenolic radical or its derivative, acting as the main ion exchange radical. Such a membrane as above has such favorable performances of lesser water and OH -  -ion permeability and a lesser electric resistance in alkaline range. 
     As an example of such membrane material, a graft copolymer comprising a main chain structure of polyolefine having side chains of hydroxy styrene as main component wherein these side chains are grafted, preferably to a crosslinked structure. Under occasion, the graft copolymer may be sulfonated to a certain degree. 
     As the characterizing features of such phenol radical bearing-membrane material, it may be said that it represents lesser swelling capability to water and lesser electric resistance in aqueous alkali solution. As an example, even if it shows a resistance of 144 ohm-square centimeter in a 26%-NaCl aqueous solution, the value may decrease to only 12 ohm-square centimeter when operated in a 20%-NaOH aqueous solution. Such characteristic feature will be highly advantageous, since the membrane is always used in caustic alkali solution as in the case of the present invention. 
     The membrane provided between the anodic and intermediate chambers must be durable to chemicals, especially to chlorine and acid in the anodic chamber; to caustic alkali prevailing in the intermediate chamber. Further, such membrane must represent a higher current efficiency than 80%, more preferably than 85%, with the alkali concentration of 10-20% at the intermediate chamber. 
     As the preferred membrane material suitable for this purpose, a fluorine-containing resin may be used. As an example, a polymer or copolymer of at least one of the following monomers may be used: Ethylene tetrafluoride; propylene hexafluoride; ethylene trifluoride-chloride; ethyl trifluoride; vinylidene fluoride; α,β,β-styrene trifluoride and the like. The above fluorine-containing monomer or monomers may be copolymerized with an olefine or olefines such as ethylene and propylene. These polymers or copolymers may have introduced therein ion-exchangeable radicals such as a sulfonic acid radical; a sulfuric acid radical, a carboxylic acid radical, and/or a phenolic radical, of which the most preferred is a sulfonic acid radical. The most preferable way for by introduction of these radicals is the polymerization. A further example of the fluorine-containing resin, such as a copolymer comprising a cyclic structure of structural units (A) and (B), and having pendant type sulfonic acid radicals follows: ##STR1## where, 
     R stands for --O--CR 4  R 5  --CR 6  R 7 ) m , 
     R 1  -R 7  stand for fluorine or perfluoroalkyl radicals of C 1  -C 10  ; 
     Y stands for a perfluoroalkyl radical of C 1  -C 10  ; 
     m=0, 1, 2 or 3; n=0 or 1; p=0 or 1; 
     X stands for fluorine, chlorine, hydrogen or trifluoromethyl; 
     X 1  =X or CF 3  (CF 2 ) q  ; q=0 or an integer of 1-5. 
     A commercially procurable copolymer, as above the anti-chlorine and fluorine-containing resin-made, ion exchange membrane manufactured and sold by E. I. Du Pont under the registered trademark: &#34;NAFION&#34;. 
     The ion-exchangeable radicals may be introduced into the fluorine-containing resin by chemical reaction therewith. As an example, sulfonic acid radicals may be introduced into a polymer comprising vinylidene fluoride units, and by chemical reaction therewith. As a further example, sulfonic acid radicals may be introduced into the benzene rings of a polymer of fluorostyrene by a chemical reaction. Styrene monomers, acrylic acid monomers or the like are graft-polymerized to respective polymers which are then subjected to a sulfonation, hydrolysis or the like treatment for the introduction of the ion-exchangeable radicals. Sulfonation products of perfluorostyrene are especially highly useful to provide superior anti-chemical membranes such as the said membrane &#34;NAFION&#34;. 
     The aforementioned fluorine-containing resin-made cationic-permselective membranes may have introduced therein inorganic ion exchangers including Zr; Bi; Ti; Ce, Sb; Sn and/or the like so as to increase the fixed cathodic ion concentration in the body of the membrane. Such improved membranes, when used, can improve the electric current efficiency by 5% or more, in comparison with those per se used in their original or otherwise state. For introducing the inorganic exchanger on or into the inside of the cationic permselective membrane, the latter can be treated with a solution including, by way of example, phosphoric acid, tungstic acid, molybdic acid, acid radical(s) thereof, their hydroxide and/or oxide by the way of coacting, dipping or impregnation, and then, it is brought into contact with a solution containing zirconium ions, or vice versa. As an alternative measure, when the occasion may desire, a gel product of zirconium salt, including said acid radical, hydroxide radical or the like, is prepared for coating or impregnating the ion exchange membrane substrate per se. 
     By the use of the inorganic ion exchanger-containing membrane of the fluorine-containing resin as above mentioned, the range of caustic alkali concentration to be kept at the intermediate chamber for attaining 80% or higher electric current efficiency can be substantially broadened. As an example, with use of the NAFION 315-membrane which does not include the inorganic ion exchanger of the above kind, the current efficiency will amount to 20% of so. By adopting the aforementioned improvement on the membrane, the caustic alkali concentration may be increased to 28% at superior current efficiency and with an output of high concentration caustic alkali as desired. 
     In the following, several numerical examples are given for a still better understanding of the present invention. 
     EXAMPLE 1 
     A three chamber type electrolytic bath assembly, containing an anodic, an intermediate and a cathodic chamber arranged one after another and separated with respective two ion-exchange membranes from each other, was used. 
     As the first membrane &#34;A&#34; separating the anodic and intermediate chambers, NAFION-315-membrane (to be called membrane No. 1 hereinafter), a composite one including NAFION EW-1100; NAFION EW-1500 and a mesh sheet of ethylene tetrafluoride resin, being laminated one after another, was used. 
     On the other hand, as the second membrane &#34;B&#34;, separating the intermediate and cathodic chambers, a commercially procurable ion exchange membrane (to be called membrane No. 2 hereinafter), bearing phenolic radicals or its derivative and manufactured and sold by Maruzen Sekiyu K. K., of Tokyo, was used. 
     A hydrolitic treatment of aqueous NaCl brine was then carried out in the following way: 
     The anodic chamber was fed with aqueous saturated NaCl-solution and the intermediate chamber was supplied with water. The total quantity of aqueous NaOH-solution as produced at the intermediate chamber in the progress of the process was conveyed from the intermediate chamber to the cathodic one in an overflowing way, while high concentration NaOH product solution was taken out from the cathodic chamber. 
     The results are shown in the following Table 1. 
     The water penetration rate and OH -  -penetration rate of the cationic-permselective membrane &#34;B&#34; were measured after removal of communication pipe 9 (shown and described previously). In this case, anodic chamber was fed with aqueous NaCl-solution of a constant concentration and intermediate and cathodic chambers were fed with aqueous NaOH-solutions of respectively constant concentrations. These conditions were so set that output of aqueous NaOH-solutions from the intermediate and cathodic chambers represented respective concentrations of about 19% and 39%. Then, the respective aqueous contents of the supplied NaOH-solution to the cathodic chamber and the discharged one therefrom, for a predetermined time duration, were measured, together with the corresponding aqueous consumption through the hydrolysis, as well as the penetrated aqueous quantities through both membranes, as determined by the respective electric current consumption rates thereat. 
     Further, the quantity of NaOH formed in the intermediate chamber was calculated, by measuring the quantities and concentrations of the supplied NaOH to and the discharged NaOH from the chamber, respectively. The quantity of OH -   consumed in the formation of NaOH was also calculated. Further, the quantity of OH -  transferred to the anodic chamber was calculated based upon the current efficiencies calculated from the quantities of NaOH in the intermediate and cathodic chambers. The thus determined sum of both OH -  -quantities was adopted as the OH -  -quantity transferred from the cathodic chamber to the intermediate one through the second membrane &#34;B&#34;. 
     
                       Table 1______________________________________anode             titanium/ruthenium oxidecathode           steel wire netbrine             26%-aqueous NaCl-             solutionbath temperature  80° C.current density   20 A/dm.sup.2bath voltage      4.0 VNaOH-concentration,in intermediate chamber             19%NaOH-concentration incathodic chamber  39%current efficiency             82%flow rate of NaOH, aq.through communication pipe             1.34 g/A . h.water penetration ratethrough membrane No. 2             1.49 g/A . h.OH.sup.- -penetration ratethrough membrane No. 2             0.22 g/A . h.______________________________________ 
    
     EXAMPLE 2 
     NAFION-315 membrane was dipped in a 10%-solution of ZrOCl 2 .2H 2  O in HCl of 1 N, for two hours and then taken out. The membrane surfaces were wiped with a filter paper. Then, the membrane was dipped in a 85%-H 3  PO 4  -solution for an hour, washed with fresh water and dried in a dryer at 160° C. for an hour. The first membrane &#34;A&#34;, to be called membrane No. 3 hereinafter, was used as the separating wall between the anodic and intermediate chambers, while the second membrane &#34;B&#34;, which was same as the membrane used in Example 1, was used as the separating wall between the cathodic and intermediate chambers. Other electrolytic conditions were same as in Example 1 for the production of NaOH from NaCl. The results are shown in Table 2. 
     Membrane-penetration rates of water and OH -  were determined in the same way as adopted in the foregoing Example 1. 
     
                       Table 2______________________________________anode             titanium/ruthenium oxidecathode           steel wire netbrine             26%-aqueous NaCl-             solutionbath temperature  80° C.current density   20 A/dm.sup.2bath voltage      4.2 VNaOH-concentrationin intermediate chamber             25%NaOH-concentration incathodic chamber  44%current efficiency             85%flow rate of NaOH. aq.through communication pipe             1.65 g/A . h.water penetration ratethrough membrane No. 2             1.04 g/A . h.OH.sup.- -penetration ratethrough membrane No. 2             0.27 g/A . h.______________________________________ 
    
     EXAMPLE 3 
     As the first membrane &#34;A&#34;, a laminated composite membrane NAFION-390-membrane (called membrane No. 4), comprising NAFION EW-1100; NAFION EW-1500 and a mesh sheet of ethylene tetrafluoride resin, was used, and as the second membrane &#34;B&#34; separating between the intermediate and cathodic chambers, same membrane No. 2 as used in the foregoing Example 1, was used. Other electrolytic conditions were same as disclosed therein for the treatment of NaCl. The results are shown in Table 3. Penetrated water and OH -  -quantities were determined as before. 
     
                       Table 3______________________________________anode             titanium/ruthenium oxidecathode           steel wire netbrine             26%-aqueous NaCl-             solutionbath temperature  80° C.current density   20 A/dm.sup.2bath voltage      3.8 VNaOH-concentration, inintermediate chamber             12%NaOH-concentration incathodic chamber  31%current efficiency             87%flow rate of NaOH, aq.through communication pipe             2.17 g/A . h.water penetration ratethrough membrane No. 2             1.65 g/A . h.OH.sup.- -penetration ratethrough membrane No. 2             0.19 g/A . h.______________________________________ 
    
     EXAMPLE 4 
     As the membrane &#34;A&#34;, NAFION-390 membrane (called membrane No. 4) was used, while, as the membrane &#34;B&#34;, membrane No. 2 used in the foregoing Example 1, was used. Aqueous NaCl-solution was then subjected to electrolysis while water is being supplied to the intermediate chamber. Rather dilute NaOH-solution and rather thicker NaOH-solution were taken out concurrently and respectively from the intermediate and the cathodic chamber. The results are shown in Table 4. 
     
                       Table 4______________________________________anode             titanium/ruthenium oxidecathode           steel wire netbrine             26%-aqueous NaCl-             solutionbath temperature  80° C.current density   20 A/dm.sup.2bath voltage      3.8 VNaOH-concentration, inintermediate chamber             10%NaOH-concentration incathodic chamber  45%current efficiency             90%taken-out NaOH-quantity fromcathode chamber   0.806 g/A . h.taken-out NaOH-quantity fromintermediate chamber             0.537 g/A . h.water penetration ratethrough membrane No. 4             2.11 g/A . h.water penetration ratethrough membrane No. 2             1.66 g/A . h.OH.sup.- -penetration ratethrough membrane No. 2             0.29 g/A . h.______________________________________ 
    
     COMPARATIVE EXAMPLE 
     NAFION-315-membranes were used as the first and second membranes &#34;A&#34; and &#34;B&#34;, concurrently. Other electrolytic conditions were same as those employed in the foregoing Example 1. 
     To the cathodic chamber, aqueous 30%-NaOH-solution was fed, while, to the intermediate chamber, aqueous 10%-NaOH-solution was fed. The water supply to the intermediate chamber and current passage conditions were so adjusted that the alkali concentration in the cathode chamber amounted to 30% and the electric current efficiency amounted to higher than 80%. The initial voltage was 5.0 V. After lapse of an hour from the start of the electrolysis, the impressed voltage rose gradually. It was found that gases accumulated in the communicating pipe 9 connecting between the intermediate and cathodic chambers. With further rise-up of the voltage, the temperature at the intermediate chamber rose suddenly and appreciably until the bath liquid began to boil. Further continuation of the electrolysis became thus impossible under these operational conditions. 
     When the water- and OH -  -penetration rates at the NAFION-315-membrane were determined in the same way as before, they amounted to 3.53 g/A.h and 0.21 g/A.h, respectively. 
     From these results, it can be concluded that as the second membrane &#34;B&#34;, the phenolic radical-bearing in the present invention can be better used for the purpose thereof.
This invention relates to an apparatus for the electrolytic treatment of alkali halide solution in a three chamber type electrolytic bath assembly comprising an anodic chamber, an intermediate chamber and a cathodic chamber arranged one after another in series. Each chamber is from its neighboring chamber by means of a cationic permselective membrane. The apparatus is characterized, according to this invention, in that the first one of the membranes separating the intermediate and anodic chambers is made of a fluorine-containing resin, while the second membrane separating the intermediate and cathodic chambers includes, as its main ion exchange radial, a pendant type phenolic radical or derivative thereof. The inventive process, which utilizes the above-described apparatus is characterized in that it utilizes a caustic alkali concentration in the intermediate chamber which ranges from about 10-20 wt.% while the output caustic alkali developed at and delivered from the cathodic chamber amounts to a concentration ranging from about 30 to 50 wt.%.