Process for electrolysis of aqueous alkali metal halide solution

Abstract

An aqueous alkali metal halide solution is electrolyzed in an electrolytic cell using a cation exchange membrane which is a fluorocarbon polymer containing sulfonic acid groups and at least one cation exchange group which is less acidic than the sulfonic acid group with higher proportion of the latter in surface stratum on the cathode side of the membrane than in the entire membrane, while controling proton concentration in the anolyte at not higher than critical proton concentration at which no substantial amount of protons in the anolyte penetrate into the membrane.This invention relates to a process for electrolysis of an aqueous alkali metal halide solution using a cation exchange membrane comprising a fluorocarbon polymer containing sulfonic acid groups and at least one cation exchange group with weaker acidity than the sulfonic acid group.It has been known to the art to utilize for electrolysis of aqueous alkali halide solutions, a cation exchange membrane of a perfluorocarbon polymer containing pendant sulfonic acid groups obtained by saponification of a membrane prepared from a copolymer of tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octene sulfonyl fluoride. This known perfluorocarbon type cation exchange membrane containing onyl sulfonic acid groups, however, has the disadvantage that the membrane tends to permit penetration therethrough of hydroxyl ions back migrating from the cathode compartment because of the high hydrophilicity of the sulfonic acid groups. As a result, the current efficiency during electrolysis is low. This is a special problem when the electrolysis is used for the production of aqueous solution of caustic soda at concentrations of more than 20 percent. In this reaction, the current efficiency is so low that the process is economically disadvantageous compared with electrolysis of aqueous solutions of sodium chloride by conventional mercury or diaphragm processes.The disadvantage of such low current efficiency can be alleviated by lowering the exchange capacity of the sulfonic acid group to less than 0.7 milliequivalent per gram of the H form dry resin. Such lowering results in decrease in water content in the membrane, whereby the concentration of fixed ions in the membrane is relatively higher than in a membrane of higher exchange capacity so that the loss of current efficiency during electrolysis is slightly improved. For example, when a caustic soda solution with a concentration of 20% is to be recovered during electrolysis of sodium chloride, the current efficiency can be improved to about 80%. Improvement of current efficiency by decrease of ion exchange capacity of the membrane, however, results in a serious decrease in the electroconductivity of the membrane, so that the method is at an economic disadvantage. Moreover, it is difficult to produce a commercial cation exchange membrane of perfluorosulfonic acid type which can be improved in current efficiency to approximately 90% by increase of membrane resistance.In order to overcome the drawbacks mentioned above, it has been suggested in U.S. Pat. No. 3,784,399, German Pat. OLS No. 2437395, U.S. Pat. No. 3,969,285 and Japanese Patent Applications No. 84111/1975 and No. 84112/1975 (U.S. Patent Application Ser. No. 701,515) to use cation exchange membranes comprising fluorocarbon polymers containing cation exchange groups with weaker acidity than sulfonic acid on one side of the cation exchange membrane of sulfonic acid type for electrolysis of an aqueous alkali metal halide solution. When electrolysis is carried out by use of such a cation exchange membrane, a high current efficiency can be attained by decrease in water content on the side on which the exchange groups with weaker acidity are present. The thickness of the stratum wherein said groups are present can be as thin as 100 A or more, which is extremely thin as compared with the entire thickness of the membrane, whereby electric resistance can be made very low so as to achieve a lower cell voltage.Even in electrolysis by use of such a cation exchange membrane as mentioned above, however, current efficiency is found to be lowered, and there is an increase of voltage when the proton concentration in the anolyte surpasses a certain critical value. In some cases, cleavage or peel-off may occur on a part of the membrane. Said critical value is dependent on various factors such as the temperature, current density, current efficiency, the anolyte concentration, the thickness of the stratum on the cathode side on which the exchange groups with weaker acidity are present, the thickness of desalted layer, etc. It has now been found that these difficulties usually occur at high temperature, current density and current efficiency, which are just the conditions most favorable for commercial practice of the process from the standpoint of decreased fixed charges and proportional costs.The object of the present invention is to provide a process for electrolysis of an aqueous alkali metal halide solution free from the above problems.The above object is found to be accomplished by use of a specifically selected cation exchange membrane and by carrying out electrolysis while controlling proton concentration in the anolyte at a value which is not higher than a critical value at which no substantial number of protons will penetrate into the membrane. As a result, electrolysis at a high current efficiency with lower voltage can be effected stably for a long period of time.The present invention provides a process for electrolysis of an aqueous alkali metal halide solution in an electrolytic cell in which anode and cathode are separated by a cation exchange membrane to divide said cell into anode and cathode chambers, said cation exchange membrane being made of a fluorocarbon polymer containing cation exchange groups consisting of sulfonic acid groups and at least one weaker cation exchange group with weaker acidity than the sulfonic acid group, the cation exchange groups in the surface stratum on the cathode side of the membrane being richer in said weaker cation exchange groups than in the entire membrane, proton concentration in anolyte being maintained at not higher than the critical proton concentration.The term "critical proton concentration" herein used in the specification and claims refers to the critical value of the concentration of protons in the anolyte at which hydroxyl ions migrating through the cation exchange membrane are neutralized on the interfacial liquid film on the anode side of the membrane so that no substantial amount of protons in the anolyte may penetrate into the membrane, and is generally determined from the equations as set forth below.The cation exchange membrane used in the present invention comprises a copolymer having the repeating units (I) and (II) as shown below:--CF.sub.2 --CXX'-- (I) ##STR1## wherein X represents F, Cl, H or --CF.sub.3 ; X', F, Cl, H or --CF.sub.3 ; R pendant group containing cation exchange groups. Said copolymer may further contain other units derived from copolymerizable monomers. The membrane contains cation exchange groups as mentioned above in the amount of 900 to 2,000, preferably 1,000 to 1,600, in terms of equivalent weight (grams of dry resin containing one equivalent of ion exchange groups). The ratio of the weaker cation exchange groups based on the total cation exchange groups through the entire membrane is up to 40 mol %, the mol percent being based upon the weight of the whole membrane, preferably up to 20 mol %. The thickness of the membrane to be used in commercial application is from 50 to 500 microns, preferably from 100 to 250 microns.According to the presently preferred method, the membrane of the invention is prepared by subjecting the membrane containing sulfonic acid groups as cation exchange groups to chemical treatment, especially on the surface thereof, to convert a part of sulfonic acid groups into the weaker cation exchange groups. Details of the methods for production of the membrane used in the present invention, i.e., the membrane containing sulfonic groups as cation exchange groups as mentioned above, are described in, for example, U.S. Pat. Nos. 3,282,875 and 3,718,627 and GB 1,184,321. The subject matter in these patents are incorporated herein by reference.The cation exchange membrane used in the present invention may have a homogeneous equivalent weight throughout the membrane or alternatively be a composite film consisting of two or more layers with different equivalent weights. In the latter case, the composite consists preferably of two layers with a difference in equivalent weight of 150 or more. The layer with larger equivalent weight is present on the cathode side of the membrane with a thickness of up to 1/2 of the entire thickness of the composite.The cation exchange groups with lower acidity than sulfonic acid group may include carboxylic acid groups, phosphoric acid groups, phosphite groups, sulfonamide groups, N-mono-substituted sulfonamide groups, alcoholic or phenolic hydroxyl groups, thiol groups and sulfinic acid groups. Among them, carboxylic acid groups and phosphoric acid groups are preferable from the standpoint of their characteristics and stabilities. In particular, carboxylic acid groups are most preferred.One example of the cation exchange membranes preferably used in the present invention is the cation exchange membrane comprising a fluorocarbon polymer containing on one side pendant groups R having terminal groups --OCF.sub.2 COOM (wherein M is hydrogen, metal or ammonium ion) with the remainder of terminal groups being the groups --OCF.sub.2 CF.sub.2 SO.sub.3 M (M is the same as defined above). This membrane can be produced by treatment with a reducing agent of a membrane containing sulfonyl derivatives such as sulfonyl halide groups, as disclosed by Japanese Patent Applications No. 84111/1975 and 84112/1975, the subject matter of which is herein incorporated be reference.The proportion of the exchange groups with weaker acidity than sulfonic acid group in surface stratum on the cathode side of the membrane based on the total exchange groups in said stratum may ordinarily fall within the range from 10 to 100 mol %, preferably from 20 to 100 mol %, most preferably from 40 to 100 mol %. The thickness of the stratum, in which the exchange groups with weaker acidity than sulfonic acid group are present, will normally be from 100 A to 20 microns.The membranes used in the present invention are desirably reinforced with backings made of nets of polytetrafluoroethylene fibers or porous films of polytetrafluoroethylene, etc. to increase the methanical strength thereof.The weaker cation exchange groups may be present throughout the entire membrane together with sulfonic acid groups, provided that their proportion increases relative to sulfonic acid groups in the direction towards the surface on the cathode side of the membrane. Further, the weaker exchange groups may also be present on the anode side of the membrane in lower proportion than in the surface stratum on the cathode side of the membrane. Practically, however, to practice commercial electrolysis while using less electric power, it is sufficient and advantageous to have the weaker cation exchange groups present substantially in a surface stratum with a thickness of 100 A to 20 microns only on the cathode side of the membrane, with the cation exchange groups in residual portion, especially on the anode side of the membrane, being mostly sulfonic acid groups.According to the present invention, it is also required to maintain proton concentration in the anolyte at not higher than the critical proton concentration as mentioned above, thereby to maintain a high current efficiency and prevent increase in voltage, and also to prevent the stratum with higher proportion of the weaker cation exchange groups on the cathode side of the membrane from being peeled away from the membrane. By this method, the membrane life can be prolonged to a great extent to make possible stable running periods of long duration.

Description

Claims

1. A process for electrolysis of a sodium chloride solution in an electrolytic cell in which anode and cathode are separated by a cation exchange membrane to divide said cell into anode and cathode chambers, said cation exchange membrane comprising at least one fluorocarbon polymer which contains sulfonic acid cation exchange groups and at least one cation exchange group with weaker acidity than sulfonic acid group, the cation exchange groups in surface stratum on the cathode side of the membrane being richer in said weaker cation exchange group than the balance of the membrane, the proton concentration in the anolyte being maintained at a value up to the critical proton concentration C.sub.H.spsb.+.sup.o as defined by the following formula:

C.sub.H.spsb.+.sup.o = (id/FD.sub.H.spsb.+) (l - y - t.sub.H.spsb.+)

wherein i represents current density (A/dm.sup.2), F the Faraday constant: 96,500 coulomb/eq., d the thickness of the desalted layer (cm), y the transport number of sodium ions through the membrane, D.sub.H.spsb.+ the diffusion coefficient of protons in the anolyte (cm.sup.2 .multidot.sec.sup.-1), and t.sub.H.spsb.+ the transport number of the protons in the anolyte of the formula: ##EQU2## wherein D.sub.Na.spsb.+ and D.sub.Cl.spsb.- represent diffusion coefficients of sodium ions and chlorine ions in the anolyte, respectively, and C.sub.Na.spsb.+ concentration (N) of sodium ions in the anolyte, respectively.

2. A process as in claim 1, wherein the weaker cation exchange group is at least one member selected from the group consisting of carboxylic acid and phosphoric acid groups.

3. A process as in claim 2, wherein the weaker cation exchange group is a carboxylic acid group.

4. A process as in claim 3, wherein the carboxylic acid is represented by the formula --OCF.sub.2 COOM and the sulfonic acid group is represented by the formula --OCF.sub.2 CF.sub.2 SO.sub.3 M wherein M is a hydrogen, metal or ammonium ion.

5. A process as in claim 1, wherein the cation exchange groups in the surface stratum on the cathode side of the membrane consists substantially of the weaker cation exchange groups and those other than in said stratum consist substantially of sulfonic acid groups.

6. A process as in claim 5, wherein the thickness of the surface stratum is from 100 A to 20 microns.

7. A process as in claim 1, wherein the cation exchange membrane is a composite consisting of two layers having different equivalent weights with a difference of at least 150, the layer with larger equivalent weight being on the cathode side of the membrane and having a thickness up to 1/2 of the entire thickness of the composite.