CA1165277A - Process for electrolysis of sodium chloride - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Electrolysis of an aqueous sodium chloride solution is conducted by passing an electric current through said solution in an electrolytic cell separated into an anode chamber and a cathode chamber by a novel cation exchange membrane made of ? perfluoro-carbon polymer and having carboxylic acid groups as the ion exchange groups, said membrane having an ion exchange capacity of car-boxylic acid groups of from 0.5 to 4.0 milli-equivalents per gram of dry resin.

Description

:1~6~77 This invention reIates to a diaphragm electrolysis of sodium chloride which comprises using as diaphragm a specific cation-exchange membrane.

Heretofore, there has been known a process for electrolyzing sodium chloride by use of an electrolytic cell divided into cathode and anode chambers, using as the diaphragm a cation-exchange membrane having sulfonic acid groups which has been obtained by hydrolyzing a membrane composed of a copolymer of perfluorocarbon sulfonyl fluoride with tetrafluoroethylene.
Further, Japanese Patent Application laid open No. 37395/1973 (filed by PPG Industries Incorporated and published June 1, 1973), teaches a membrane prepared from a copolymer of the strUcture ~~-CmF2m)M (CF2 , ~ . However, no membrane is known to be satisfactory in industrial application. For example, when using cation-exchange membranes having sulfonic acid groups, no sufficiently high current efficiency can be attained no matter how the exchange capacity of sulfonic acid groups is controlled.
The reason therefor is that SO3Na groups as the exchange groups in the membrane have significantly dissociated, and therefore even if the exchange capacity is made small to make the membrane dense in structure, OH ions which are high in mobility cannot be prevented from diffusing in reverse from the cathode chamber to the anode chamber, and thus the current efficiency is lowered.
In the case of a conventional perfluorosulfonic acid type cation-exchange membrane having such an ordinary exchange capa-city of 0.5 to 1.5 milli-equivalent per gram of dry resin, it is usual that the current efficiency decreases to 60 to 75% in general, if the concentration of sodium hydroxide in the cathode ,~

`~ ~16~;Z7'7 chamber becomes 10 to 15%. Accordingly, the above-mentioned membrane cannot be used for industrial scale eIectrolysis of sodium chIoride. Further, if the concentration of sodium hy-droxide in the cathode chamber bec`omes higher, the current efficiency becomes smaller than the above-menti;oned value, and thus the use of said membrane necessarily brings about indus-trial disadvantages.
.
The present invention provides a diaphragm electrolysis process, wherein there is used a cation-exchange membrane, comprising (a) - 10 a fluorocarbon polymer having carboxylic acid groups or (b~ a composition containing a polymer having carboxylic acid groups and a fluorocarbon polymer as the diaphragm. The process of the present invention has the advantage that the reverse diffusion of OH ions from the cathode chamber to the anode chamber is successfully inhibited. Accordingly, not only the operation can be effected with a high current efficiency and at a high current density even when sodium hydroxide in the cathode chamber is high in concentration, but also high purity sodium hydroxide at a high concentration can be obtained in the cathode chamber, while high purity chlorine containing almost no oxygen can be produced in the anode chamber. For example, even when the so-dium hydroxide concentration in the cathode chamber is more than 20~, or i8 about 30%, the operation can be effected with such a high current efficiency as 90% or more. Furthermore, the resul-ting sodium hydroxide i8 10 or more times higher in purity thanthat obtai.ned by the conventional process.

It has also been found that when the cation-exchange membrane used in the present invention contains sulfonic acid groups in addition to carboxylic acid groups, the result is best in the above-mentioned current efficiency, current density and sodium hydroxide purity.

According to one embodiment of the present invention, the cation-exchange membrane used contains a polymer having carbox-ylic acid groups and a perfluorocarbon polymer. Alternatively, according to another embodiment, thé carboxylic acid groups are chemically bonded to the perfluorocarbon polymer, Thus, the perfluorocarbon polymer in the cation-exchange membrane may be any of a perfluorocarbon polymer having no ion exchange group or a perfluorocarbon polymer having carboxylic acid groups. The carboxylic acid groups in the membrane inhibit~ the reverse dif-fusion of OH ions, and the perfluorocarbon polymer in the mem-brane prevents the membrane from being chemically corroded by chlorine generated in the eIectrolytic cell.

In one particular aspect the present invention provides a process for the electrolysis of an aquebus sodium chIoride solution which comprises passing an eIectric current throu~h said solution in an electrolytic cell separated into an anode chamber and a cathode chamber ~y a cation exchange membrane made of ~ perfluorocarbon polymer~having carboxylic acid groups, said membrane having an ion exchange capacity of carboxylic acid groups of from 0.5 to 4.0 milli-equivalents per gram of dry resin.

In another particular aspect the present invention provides a cation exchange membrane suitable for use in the electrolysis of an aqueous sodium chloride solution made of perfluorocarbon poly-merlhaving carboxylic acid group8, said membrane having an ion exchange capacity of carboxylic acid groups of from 0.5 to 4.0 milli-equivalents per gram of dry resin.

The cation-exchange membrane used in the electrolysis according to the present invention i5 resistant to solvent and heat under electrolysis condition8. If possible, therefore, the polymers contained in the membrane have desirably been crosslinked. How-ever, when the membrane contains a polymer which is highly re-sistant to solvent and heat due to intermolecular attraction of the polymers despite of the presence of hydrophilic ion exchange groups, the constituent polymers may be linear polymers and are not always required to be cro8slinked.

As mentioned above, the carboxylic acid groups may be bonded chemically to the fluorocarbon polymer. Alternatively, the polymer having carboxylic acid groups may be combined physically together with the fluorocarbon polymer. In the latter case, the ~ 116~77 p~ymer having carboxylic acid groups may be dispersed uniformly throughout the fluorocarbon matrix or it may be present in la-yers on the fluorocarbon polymer. Such heterogeneous cation-exchange resin may be prepared by coating or impregnating a membrane composed of a fluorocarbon polymer with a carboxylic acid group-containing monomer solution, followed by polymeriza-tion.

Generally, in preparing a cation-exchange membrane by physical combination of a polyer having carboxylic acid groupSwith a fluorocarbon polymer, it is advantageous to let the polymer having carboxylic acid groups be present on the surface of the membrane.

When both sulfonic acid and carboxylic acid groups are present ; together in the cation-exchange membrane used in the parent 15 invention, Canadian Patent Application No. 220,976, the membrane comes to have a high electric conductivity, with the result that the cost of power decreases so as to provide an industrial advantage. The industrial advantage can be obtained as long as the ratio of carboxylic acid groups to sulfonic acid groups is 20 in the range from 1/100 to 100/1.

The carboxylic acid groups contained in the membrane of the present invention may be either in the form of a free acid or in the form of metal salts.

The cation-exchange membranes of the present invention compri-sing (a) a fluorocarbon polymer having carboxylic acid groups are as follows:

1. A membrane made from a polymer of perfluorocarbon vinyl ether of the general formula:
CF2=CF-O-(CF2) -X
30 (wherein n is an inte~er of 2 to 12, preferably 2 to 4; and X
is one of the groups of the formulas -CN, -COF, -COOH, -COOR, -COOM and -CONR2R3, where R is an alkyl group having 1 to 10, preferably 1 to 3, carbon atoms; R2 and R3 are individually hydrogen or one of the groups represented by R; and M is sodium, . .

potassium or cesium), tetrafluoroethylene and/or CF2=CF-~-Rf (wherein Rf is a perfluorinated alkyl group having 1 to 3 car-bon atoms) or its hydrolysis product;

2. A polymermembrane made from a polymer of a perfluoroacrylic acid represented by the general formula, CF2=CFCOZ
(wherein Z is fluorine or an alkoxy, amino or hydroxy group) or a perfluoroacrylic fluoride, tetrafluoroethylene and CF2=CF-O-Rf or its hydrolysis product: and On the other hand, the cation-exchange membrane of the present invention, comprising (b) a polymer having a carboxylic acid group and a fluorocarbon polymer are as follows:

3. A membrane which is obtained by impregnating or coating a fluorocarbon polymer membrane, e.g. a homo- or copolymer of lS such monomer as tetrafluoroethylene, hexafluoropropylene or per-fluoromethyl perfluorovinyl ether, with CF2=CF-O-(CF2)n-X, followed by polymerization, or its hydrolysis product;

4. A membrane which is obtained by impregnating or coating a fluorocarbon polymer membrane having no ion exchange group with a vinyl compound having COOR group, followed by polymerization, or its hydrolysis product; and Among the polymers mentioned in the above, the copolymer compri-sing CF2=CF-ORf and CF2=CF-O-(CF2)n-X or CF2=CF-COZ and the co-polymer compri~ing CF2=CF-ORf, CF2=CF2 and CF2=CF-O-(CF2)n-X or CF2=CF-COZ are advantageously used because they can be formed into membranes with extreme ease.

When the monomers are impregnated into or coated on the polymer in the preparation of the above membranes, the polymerization may be effected in the presence of a crosslinking agent or a solvent, if desired.

Typical examples of the fluorinated perfluorocarbon vinyl ether of the general formula:

., , 7 ,~

' C~2=CF-O-(CF2)n~X
are methyl perfluoro-6-oxa-7-oct~noate, methyl perfluoro-5-oxa-6-hept~noate, perfluoro-6-oxa-7-octenoyl fluoride and perfluoro-6-oxa-7-octene nitrile.
.
Typical examples of the vinyl ether of the general formula:
CF2CFORf are perfluoromethyl perfluorovinyl ethers.

Typical examples of the perfluorocarbon polymer having no COOR
group are homopolymers of tetrafluoroethylene,hexafluoropropene, vinylidene fluoride, perfluoromethyl perfluorovinyl ether, chlo-rotrifluoroethylene, 1,1,3,3,3-pentafluoropropene and 1,2,3,3,3-pentafluoropropene, alternating copolymers of these monomers and ; copolymers of these monomers with ethylene.

As crosslinking agents, there may be used fluorinated diolefins of the general formula:
CF2=CF-O-(CF2CF2-0)nCF=CF2, in addition to such diolefin compounds as, for example, divinyl-benzene and butadiene.

As is clear from the above explanation, the process for prepa-29 ring the cation-exchange membrane used in the present invention includes various proces8e8 depending on the monomer8 employed, polymers used a8 sub8trates, and cation-exchange membranes to be obtained.

Generally, the polymerization of a fluorocarbon monomer contai-ning or not containing carboxylic acid groups is mostly effected according to emulsion or suspension polymerization in the pre-sence of a radical polymerization catalyst, and the resulting polymer is moulded into a membrane according to an ordinary moulding procedure such as melt fabrication or the like. If a cation-exchange membrane is to be prepared directly by polymeri-zing a monomer component containing a fluorinated diolefin com-pound monomer, the monomer may be subjected to casting polymeri-zation to prepare a membrane at the time of polymerization.
When a fluorocarbon polymer having ion-exchange groups is - 116~:~77 .

impregnated or coated with acrylic acid or the like monomer having carboxylic groups, and, if necessary with a crosslinking agent and then polymerization is effected, the polymerization may be conducted in the presence of a radical polymerization catalyst or by the action of radiation.

Generally, the cation-e~change membrane used in the present invention has an exchange capacity, in terms of carboxylic acid groups, of 0.1 to 10 milli-equivalent,~ preferably 0.5 to 4.0 milli-equivalent~ per gram of dry resin.

The cation-~xchange membrane used in the present invention may sometimes by reinforced in mechanical strength by incorporating into the membrane a net of fibres of other fluorocarbon polymers.
For industrial purposes, the use of a cation-exchange membrane, which has been lined with Teflon* fibres, is preferable, in 15 general. The thickness of the membrane is 0.01 to 1.5 mm, pre-ferably 0.05 to 1.5 mm, and may be suitably selected according to the specific conductivity and current efficiency of the mem-brane so that the membrane is successfully applicable to the electrolysis of sodium chloride.
,, The cation-exchange membrane u6ed in the present invention con-tains 5 to 50% of water (sodium type). The electrolytic cell employed has been divided by the cation-exchange membrane into a cathode chamber and an anode chamber. Electrolysis is perfor-med by charging an aqueous sodium chloride solution to the anode chamber and water or a dilute sodium hydroxide solution to the cathode chamber, which is recycled to control the concentration of sodium hydroxide at the outlet of the cathode chamber. The concentration of the sodium chloride solution charged to the anode chamber is desirably high, preferably near saturation.

The electrolysis may be effected at a temperature of 0 to 150C, and heat generated due to the electrolysis is removed by cooling a part of the anolyte or catholyte.
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~ Trade Mark ~ ' , .

- llG5;~77 At the time of electrolysis, the cathode and anode chambers are desirably kept under same pressure, so that the cation-exchange membrane can always be maintained vertically. In order to pre-vent the membrane from contacting either electrode spacers which also facilitate discharge of gas can be interposed between the electrodes and the membrane. In the cathode and anode chambers, hydrogen and chlorine are generated, respecti~ely. The separa-tion of the gases from the liquids is desirably conducted by providing a free space at the upper portion of each chamber of the electrolytic cell. With this construction the gases and liquids may be discharged separately, though discharging them together may be effected from the cathode or anode chambers.
When separation of gas from liquid is effected in the uppèr free space within the electrolytic cell, the recycle of the electro-lyte in each chamber can advantageously be promoted by the as-cending action of the formed gases, in general. Particularly, an electrolytic cell, constructed so that the formed gases pass to and ascend the back side of each electrode thus occupying no space between the electrode and the membrane surface, is advan-tageous in that potential depression and power consumption areminimized.

The perpass electrolysis ratio o sodium chloride Gharged to the anode chamber is 3 *o 50%. This varies depending on the current density and the manner of heat removal, but is desirably high, in general.

The liquid in each chamber is desirably stirred by means of the gases generated in the cathode and anode chambers, in addition to the flow of externally supplied fluids. For this purpose also, it is desirable that an electrode having many vacant spaces such as a metal mesh electrode is used so that the liquid in each chamber can be moved, circulated and stirred with as-cending flow of the ga~es.

As the cathode, the use of an iron electrode which has been plated with nickel or a nickel compound is preferable, in gene-ral, from the standpoint of overpotential. As the anode, theuse of a metal mesh or rod rleatroc~which hao been aoated with '' ;
',' ;, ' 116~ 7 an oxide of a noble metal such as ruthenium or the like is pre-ferable, in general, for the present invention. Use of these types of electrodes makes it possible to minimize the space between membranes and electrodes so that power consumption and S potential depression are minimized. By the use of the membranes the movement of OH ions is inhibited and the cathode and anode chambers can be distinctly separated from each other. According-ly, when metal electrodes of dimensional precision are used in combination with the cation-exchange membrane, the space between each electrode and the membrane can be made extremely small, e.g. about l to 3mm, so that eIectrolysis can be effected at a high current density while minimizing the potential depression and while maintaining the power consumption at a low level.
This is a characteristic that cannot be seen in the conventional diaphragm process.

Since the present cation-exchange membrane is resistant to chlo-rine generated in the anode chamber, not only can the operation be effected stably over a long period of time, but also no reverse diffusion of OH ions occurs due to the presence of car-boxylic acid groups in the membrane. Consequently, the pH ofthe anode chamber can be easily maintained at a neutral to slightly acidic pH, and thus the content of oxygen in the chlo-rine generated in the anode chamber can be maintained at such an extremely low degree as less than 500 p.p.m.

When the cation-exchange membrane of the present invention is used, the current efficiency is far higher than in the case of a cation-exchange membrane prepared from a polymer of perfluoro-carbon sulfonic acid, and conseguently with the present membrane, for the production in the cathode chamber of sodium hydroxide with a concentration of more than 20%, the operation can be ef-fected with a current efficiency of at least 80%, and about 90 ; to 98% under optimum conditions. Since the current efficiency ; is high and the amount of consumed power is small, the present invention can economically be conducted even at a current den-sity of 20 to 70 A/dm . The reason why such high current efficiency can be attained is ascribable to the fact that the reversal diffusion of OH ions is inhibited by the cation-exchange ,: ~

116~
:

membrane of the present invention. That is, it is considered that when a cation-exchange membrane contains carbo~ylic acid groups, a part of the carboxylic acid groups are affected by the low pH of the anode chamber to exist as an H-form resin at

5 portions where the layer of the membrane is extremely thin, thereby making the resin higher in density to inhibit effec-tively the reversal diffusion of OH ions. Such effect cannot be obtained by use of a cation-exchange membrane having only sulfonic acid groups which have a high dissociation constant.
10 The above explanation outlines the mechanism of the present invention, but is not intended to be limitative.

The aqueous sodium chloride solution charged to the anode cham-ber is purified, like in the conventional sodium chloride elec-trolysis process. That is, the aqueous sodium chloride solu-15 tion, which is to be recycled and returned, is then subjected to the steps of dechlorination, dissolution and saturation of sodium chloride, precipitation and separation of magnesium, po-f tassium, iron, etc., and neutralization, in the same manner as in the conventional process. It is sometimes desirable to fur-20 ther purify the feed sodium chloride solution with a granular ion exchange resin, particularly a chelate resin, to reduce the calcium content thereof to a permissible limit, preferably to less than 1 p.p.m.

The following examples illustrate some modes of practise of the 25 present invention, but the invention is not limited to the examples.

Example 1 A polymer pr~epared by the copolymerization of methyl perfluoro-

6-oxa-7-oct~noate, perfluoromethyl perfluorovinyl ether and 30 tetrafluoroethylene was subjected to compression moulding to form a membrane of 0.12 mm in thickness.

This membrane was hydrolyzed to obtain a carboxylic acid type cation-exchange resin membrane having an exchange capacity of .,, ,~ .
2.1 milli-equivalent~/gram dry resin.

1161r~:~t77 This cation-exchange membrane, which had an effective area of 100 dm2, was used to divide an electrolytic cell into a cathode chamber and an anode chamber. 50 Units of such electrolytic cell were arranged in series so that the respective adjacent electrodes were brought to a bipolar syste~ and thus an assem-bly of 50 electrolytic cells was prepared.

Using the thus prepared electrolytic cell assembly, electroly-sis was conducted in such a manner that 305 g/l of an aqueous sodium chloride solution was charged to each cell through the inlet of the anode chamber, and an aqueous sodium hydroxide solution was recycled while being controlled at a concentration of 38% by adding water to the outlet of the cathode chamber.
The electrolysis was carried out while applying in series a current of 5,000 amperes to the chambers.

In the above-mentioned electrolysis, the current efficiency of the sodium hydroxide recovered from the outlet of the cathode chamber was 91.6%.

Comparative Example 1 A copolymer of perfluoro[2~2-fluorosulfonylethoxy)-propyluinyl ether] with tetrafluoroethylene was moulded into a membrane of 0.12 mm in thickness, which was then hydrolyzed to prepare a cation-exchange membrane containing 0.90 milli-equivalent/gram dry resin of sulfonic acid groups.

U~ing 50 sheets of this cation-exchange membrane which had an effective area of 100 dm2, electrolysis was conducted in the same manner and by use of the same apparatus as in Example 1, while flowing in series a current of 5,000 amperes to the 50 units of electrolytic cell. As the result, the current effi-ciency at the time of producing sodium hydroxide of 35.1% con-cen'ration was 55.7%, and the amount of NaCl in NaOH was 2,000 p.p.m. Further, the specific electric conductivity of said membrane was 11.3 mmho/cm as measured in a 0.lN aqueous NaOH
solution at 25C.

116~77 The specific electric conductivity of the membrane was measured in the following manner:

The membrane was completely brought into -S03Na form and then equilibrated by dipping at normal temperature for 10 hours in a O.lN aqueous NaOH solution which is supplied continuously.
Subsequently, the membrane was measured in electric resistivity in the solution by applying an alternating current of 1,000 cycles, while maintaining the solution at 25C., and the speci-fic electric conductivity was calculated from the thickness and the effective area of the membrane.

Comparative Example 2 The same copolymer as in Comparative Example 1 was moulded into a membrane of 0.12 mm in thickness, which was then hydrolyzed to prepared a cation-exchange membrane containing 0.65 milli-equivalent/gram dry resin of sulfonic acid groups.

Using this membrane, electrolysis was conducted in the same manner as in Comparative Example 1. As the result, the current eficiency at the time of producing sodium hydroxide of 35.1%
concentration was 73%. The specific electric conductivity of said membrane was 4.5 mmho/cm as measured in a O.lN aqueous NaOH solution at 25C.
"
Example 2 A copolymer of tetrafluoroethylene with perfluoromethyl per-; 1uorovinyl ether was moulded into a membrane of 0.1 mm in thickness. The membrane was impregnated with methyl perfluoro-5-oxa-6-heptenoate, polymerized and then hydrolyzed to prepare a cation-exchange membrane having an exchange capacity, in terms of carboxylic acid groups, of 2.31 milli-equivalent/gram ; dry resin.

Using this cation-exchange resin, electrolysis was conducted in the same manner as in Example 1. As the result, the current efficiency at the time of producing sodium hydroxide o~ 27%

165~

concentration was 98.2%.

Example 3 A ternary copolymer comprising methyl perfluoroacrylate, tetra-fluoroethylene and perfluoropropyl perfluorovinyl ether was moulded into a membrane of 0.12 mm in thickness, which was then hydrolyzed to prepare a cation-exchange resin membrane contai-ning 1.15 milli-equivalent/gram dry resin of carboxylic acid groups.

Using 50 sheets of this cation-exchange membrane which had an efective area of lO0 dm , electrolysis was conducted in the same manner and by use of the same apparatus as in Example 1, while flowing in series a current of 5,000 amperes to the 50 units of electrolytic cell. As the result, the current effici-ency at the time of producing sodium hydroxide of 31.7% concen-tration was 97.2~.

Example 4 A copolymer of CF2=CF-O(CF2)4COONa with tetrafluoroethylene was moulded into a membrane of 0.12 mm in thickness to prepare a cation-exchange membrane containing 1.33 milli-equivalent/gram dry resin of carboxylic acid groups. Using this cation-exchange membrane, electrolysis was conducted in the same man-ner as in Example 1. As the result, the current efficiency at the time of producing sodium hydroxide of 35.8% concentration was 92.9%.

Example 5 A ternary copolymer comprising perfluoromethyl perfluorovinyl ether, tetrafluoroethylene and perfluoro-5-oxa-6-heptenoyl fluoride was moulded into a membrane of 0.12 mm in thickness, which was then hydrolyzed to prepare a cation-exahange membrane containing 1.36 milli-equivalent/gram dry resin of carboxylic acid groups.

S~7 Using this cation-exchange mcmbrane, electrolysis was conduc-ted in the same manner as in Example 1. As the result, the current efficiency at the time of producing sodium hydroxide of 35.5% concentration was 93.3%, and the specific electric conductivity of the membrane was 7.2 mmho/cm.

Example 6 A copolymer of perfluoroacrylic acid with tetrafluoroethylene was moulded into a membrane of 0.12 mm in thickness. This mem-brane contained 1.88 milli-equivalent/gram dry resin of carbox-ylic acid groups.

Using this membrane, electrolysis was conducted in the same manner as in Example 1. As the result, the current efficiency at the time of producing sodium hydroxide of 32.5~ concentra-tion Wab 93.6%.

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Claims (39)

1. THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
   PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
   
   l. A process for the electrolysis of sodium chloride which comprises passing an electric current through an aqueous sodium chloride solution having a calcium content of less than 1 ppm in an electrolytic cell separated into an anode chamber and a cathode chamber by a cation exchange membrane made of perfluorocarbon polymer and having carboxylic acid groups as the ion exchange groups, said membrane having an ion exchange capacity of carboxylic acid groups of from 0.5 to 4.0 milli-equivalents per gram of dry resin, wherein a perfluorocarbon polymer containing carboxylic acid groups is present at least on the surface of the membrane facing the cathode chamber of the cell.
2. 2. The process of Claim 1, wherein said carboxylic acid groups are contained predominantly on said one surface of the membrane and wherein the membrane is disposed in said electrolytic cell such that said surface having the car-boxylic acid groups faces the cathode chamber of the cell.
3. 3. The process of Claim 1 or 2, said membrane having an ion exchange capacity of carboxylic acid groups of from 0.5 to 2.1 milli-equivalents per gram of dry resin.
4. 4. The process of Claim 1 or 2, wherein the electrolysis is conducted at a temperature of 0° to 150°C while charging an aqueous sodium chloride solution into the anode chamber and adding water or an aqueous dilute sodium hydroxide solution into the cathode chamber to adjust the concentra-tion of sodium hydroxide to more than 20%.
5. 5. The process of Claim 1 or 2, wherein the aqueous sodium chloride solution charged to the anode compartment is puri-fied by ion exchange.
6. 6. The process of Claim 1 or 2, wherein the aqueous sodium chloride solution charged to the anode compartment is -1?-purified by ion exchange using a chelate resin.
7. 7. The process of Claim 1 or 2, wherein the membrane in its sodium salt form has a water content of from 5% to 50%
   by weight.
8. 8. A cation exchange membrane suitable for use in the electrolysis of an aqueous sodium chloride solution made of perfluorocarbon polymer and having carboxylic acid groups as the ion exchange groups, said membrane having an ion exchange capacity of carboxylic acid groups of from 0.5 to 4.0 milli-equivalents per gram of dry resin.
9. 9. A cation exchange membrane according to Claim 8, com-prising a perfluorocarbon copolymer having pendant carboxy-lic acid groups as ion exchange groups, said membrane hav-ing an ion exchange capacity of from 0.5 to 2.1 milli-equivalents per gram of dry resin.
10. 10. A cation exchange membrane in accordance with Claim 8 or 9, which has a water content in the sodium salt form of from 5 to 50% by weight.
11. 11. A cation exchange membrane in accordance with Claim 8 or 9 wherein a perfluorocarbon polymer containing car-boxylic acid groups is present on at least one surface of the membrane.
12. 12. A cation exchange membrane in accordance with Claim 8 or 9, wherein a perfluorocarbon polymer containing carboxy-lic acid groups is present on one surface only of the membrane.
13. 13. A cation exchange membrane in accordance with Claim 8, wherein the ion exchange capacity of carboxylic acid groups is in the range of 0.5 to 2.31 milli-equivalents per gram of dry resin.
14. 14. A cation exchange membrane in accordance with Claim 8 or 9, which includes a fibre reinforcement.
15. 15. A cation exchange membrane in accordance with Claim 8 or 9, wherein said perfluorocarbon polymer comprises a perfluorocarbon copolymer of at least one of tetrafluoro-ethylene and CF2=CF-0-Rf wherein Rf is a perfluorinated alkyl group containing 1 to 3 carbon atoms with perfluoro-carbon vinyl monomers containing carboyxlic acid groups or functional groups which can be converted to carboxylic acid groups.
16. 16. A cation exchange membrane in accordance with Claim 8 or 9, wherein said perfluorocarbon polymer comprises a perfluorocarbon copolymer of a perfluorocarbon vinyl mono-mer having the general formula:
    CF2=CF-O- (CF2)n-X
    wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOM or CONR2R3, where R is an alkyl group containing 1 to 10 carbon atoms, R2 and R3 are individually hydrogen or an alkyl group containing 1 to 10 carbon atoms, and M
    is sodium, potassium or cesium, with at least one of tetra-fluoroethylene and CF2=CF-O-Rf wherein Rf is a perfluori-nated alkyl group containing 1 to 3 carbon atoms, which copolymer has been hydrolyzed, if necessary, to form said acid groups.
17. 17. A cation exchange membrane in accordance with Claim 8 or 9, wherein said perfluorocarbon polymer comprises a perfluorocarbon copolymer of a perfluoroacrylic acid having the formula:
    CF2=CF-COZ
    wherein Z is fluorine, an alkoxy group containing 1 to 10 carbon atoms, amino or hydroxy, with at least one of tetra-fluoroethylene and CF2=CF-O-Rf wherein Rf is a perfluori-nated alkyl group containing 1 to 3 carbon atoms, which copolymer has been hydrolyzed, if necessary to form said acid groups.
18. 18. A cation exchange membrane in accordance with Claim 8 or 9, which comprises a combination of a perfluorocarbon polymer having carboxylic acid groups with a perfluorocarbon polymer having no ion exchange group.
19. 19. A cation exchange membrane in accordance with Claim 8, wherein said cation exchange membrane has been prepared by impregnating or coating a membrane comprising a perfluorocarbon homo- or copolymer of at least one monomer selected from tetrafluoroethylene, hexafluoropropene and perfluoromethyl perfluorovinyl ether, with a perfluorocarbon vinyl compound having carboxylic acid groups or derivatives thereof, polymerizing said impregnated or coated vinyl compound, and hydrolyzing, if necessary, to form said acid groups.
20. 20. A cation exchange membrane in accordance with Claim 8, wherein said perfluorocarbon vinyl compound is at least one member selected from the group consisting of compounds having the formula:
    CF2=CF-O-(CF2)n-X
    wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOM or CONR2R3, where R is an alkyl group containing 1 to 10 carbon atoms, R2 and R3 are individually hydrogen or an alkyl group containing 1 to 10 carbon atoms, and M is sodium, potassium or cesium, and compounds having the formula:
    CF2=CF-COZ
    wherein Z is fluorine or an alkoxy group containing 1 to 10 carbon atoms.
21. 21. A cation exchange membrane in accordance with Claim 8, wherein said cation exchange membrane has been prepared by impregnating or coating a membrane comprising a perfluorocarbon homo- or copolymer of at least one monomer selected from tetrafluoroethylene, hexafluoropropene and perfluoromethyl perfluorovinyl ether, with a solution of a perfluorocarbon polymer having carboxylic acid groups or derivatives thereof, and hydrolyzing, if necessary, to form said acid groups.
22. 22. A cation exchange membrane in accordance with Claim 8 or 9, wherein the thickness of the membrane is 0.05 to 1.5 mm.
23. 23. An electrolytic cell containing an anode chamber and a cathode chamber separated by a cation exchange membrane which is suitable for the production of aqueous sodium hydroxide in the cathode chamber from an aqueous solution of sodium chloride which is fed into the anode chamber, said membrane being made of perfluorocarbon polymer and having carboxylic acid groups as the ion exchange groups, said membrane having an ion exchange capacity of carboxylic acid groups of from 0.5 to 4.0 milli-equivalents per gram of dry resin.
24. 24. An electrolytic cell in accordance with Claim 23, wherein said membrane comprises a perfluorocarbon copoly-mer having pendant carboxylic acid groups as ion exchange groups, said membrane having an ion exchange capacity of from 0.5 to 2.1 milli-equivalents per gram of dry resin.
25. 25. An electrolytic cell in accordance with Claim 24, wherein said membrane consists essentially of a physical combination of the perfluorocarbon copolymer having pendant carboxylic acid groups with another perfluorocarbon polymer having pendant carboyxlic acid groups.
26. 26. An electrolytic cell in accordance with Claim 23 or 24, wherein the membrane in the sodium salt form has a water content of from 5 to 50% by weight.
27. 27. An electrolytic cell in accordance with Claim 23 or 24, wherein a perfluorocarbon polymer containing carboxylic acid groups is present on at least one surface of the mem-brane.
28. 28. An electrolytic cell in accordance with Claim 23 or 24, wherein a perfluorocarbon polymer containing carboxylic acid groups is present on one surface only of the membrane.
29. 29. An electrolytic cell in accordance with Claim 23 or 24, wherein the ion exchange capacity of carboxylic acid groups is in the range of 0.5 to 2.31 milli-equivalents per gram of dry resin.
30. 30. An electrolytic cell in accordance with Claim 23 or 24, wherein the cation exchange membrane is fibre rein-forced.
31. 31. An electrolytic cell in accordance with Claim 23 or 24, wherein said perfluorocarbon polymer comprises a per-fluorocarbon copolymer of at least one of tetrafluoroethy-lene and CF2=CF-O-Rf wherein Rf is a perfluorinated alkyl group containing 1 to 3 carbon atoms with perfluorocarbon vinyl monomers containing carboxylic acid groups or func-tional groups which can be converted to carboxylic acid groups.
32. 32. An electrolytic cell in accordance with Claim 23 or 24, wherein said perfluorocarbon polymer comprises a per-fluorocarbon copolymer of a perfluorocarbon vinyl monomer having the general formula:
    CF2=CF-O-(CF2)n-X
    wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOM or CONR2R3, wherein R is an alkyl group contain-ing 1 to 10 carbon atoms, R2 and R3 are individually hydro-gen or an alkyl group containing 1 to 10 carbon atoms, and M is sodium, potassium or cesium, with at least one of tetrafluoroethylene and CF2=CF-O-Rf, wherein Rf is a per-fluorinated alkyl group containing 1 to 3 carbon atoms, which copolymer has been hydrolyzed, if necegsary, to form said acid groups.
33. 33. An electrolytic cell in accordance with Claim 23 or 24, wherein said perfluorocarbon polymer comprises a per-fluorocarbon copolymer of a perfluoroacrylic acid having the general formula:
    CF2=CF-COZ
    
    wherein Z is fluorine, an alkoxy group containing 1 to 10 carbon atoms, amino or hydroxy with at least one of tetrafluoroethylene and CF2=CF-O-Rf wherein Rf is a perfluorinated alkyl group containing 1 to 3 carbon atoms, which copolymer has been hydrolyzed, if necessary, to form said acid groups.
34. 34. An electrolytic cell in accordance with Claim 23, wherein the membrane comprises a combination of a perfluorocarbon polymer having carboxylic acid groups with a perfluorocarbon polymer having no ion exchange group.
35. 35. An electrolytic cell in accordance with Claim 23, wherein said cation exchange membrane has been prepared by impregnating or coating a memhrane comprising a perfluorocarbon homo- or copolymer of at least one monomer selected from tetrafluoroethylene, hexafluoropropene and perfluoromethyl perfluorovinyl ether, with a perfluorocarbon vinyl compound having carboxylic acid groups or derivatives thereof, polymerizing said impregnated or coated vinyl compound, and hydrolyzing, if necessary, to form said acid groups.
36. 36. An electrolytic cell in accordance with Claim 23, wherein said perfluorocarbon vinyl compound is at least one member selected from the group consisting of compounds having the formula:
    CF2=CF-O-(CF2)n-X
    wherein n is an integer of 2 to 12, and X is CN, COF, COOH, COOR, COOM or CONR2R3, where R is an alkyl group containing 1 to 10 carbon atoms, R2 and R3 are individually hydrogen or an alkyl group containing 1 to 10 carbon atoms, and M is sodium, potassium or cesium, and compounds having the formula:
    CF2=CF-COZ
    wherein Z is fluorine or an alkoxy group containing 1 to 10 carbon atoms.
37. 37. An electrolytic cell in accordance with Claim 23, wherein said cation exchange membrane has been prepared by impregnating or coating a membrane comprising a perfluorocarbon homo- or copolymer of at least one monomer selected from tetrafluoroethylene, hexafluoropropene and perfluoromethyl perfluorovinyl ether, with a solution of a perfluorocarbon polymer having carboxylic acid groups or derivatives thereof, and hydrolyzing, if necessary, to form said acid groups.
38. 38. An electrolytic cell in accordance with Claim 23 or 24, wherein said carboxylic acid groups are contained predominantly on one surface of the membrane and wherein the membrane is disposed in said electrolytic cell such that said surface having the carboxylic acid groups faces the cathode side of the cell.
39. 39. An electrolytic cell in accordance with Claim 23 or 24, wherein the thickness of the membrane is 0.05 to 1.5 mm.