Electrolysis of aqueous solution of alkali metal chloride

Claim 2

An electrolysis of an aqueous solution of an alkali metal chloride to produce an alkali metal hydroxide having a concentration of 30 wt.% or more and chlorine which comprises using a modified cation exchange membrane of a fluorinated polymer having essentially only carboxylic acid type ion exchange groups which has an ion exchange capacity of 1.0 to 2.2 meq/g. dry polymer and wherein one surface layer has a lower ion exchange capacity than that of the major portion of the membrane and is from 0.9 to 1.6 meq/g. dry polymer.

Claim 3

An electrolysis according to claim 1 wherein the ion exchange capacity of the modified surface layer of the modified cation exchange membrane of a fluorinated polymer is in a range of 1.0 to 1.5 meq/g. dry polymer which is in a range of from 45 to 95% based on the ion exchange capacity of the non-modified parts.

Claim 4

An electrolysis according to claim 1 or 2 wherein the modified surface layer having lower ion exchange capacity in the modified cation exchange membrane of a fluorinated polymer is in a thickness of at least 0.1μ from the surface.

Claim 5

An electrolysis according to claim 1, or 2 wherein the ion exchange capacity of the surface layer is decreased by contacting with an aqueous solution of an alkali metal hydroxide in a concentration of higher than 40 wt.% at higher than 80° C.

Claim 6

An electrolysis according to claim 4 wherein at least peripheral parts of two sheets of the cation exchange membrane are bonded to form a bonded unit and the bonded unit is dipped into an aqueous solution of an alkali metal hydroxide to decrease the ion exchange capacity.

Claim 7

An electrolysis according to claim 5 wherein the ion exchange groups of the cation exchange membrane are carboxylic acid ester groups.

Claim 8

An electrolysis according to claim 4 wherein the surface layer of the cation exchange membrane of a fluorinated polymer is contacted with an aqueous solution of an alkali metal hydroxide in an electrolytic cell.

Claim 9

An electrolysis according to claim 1, or 2 wherein the modified surface layer of the modified cation exchange membrane of a fluorinated polymer is faced to a cathode side in an electrolytic cell.

Claim 10

An electrolysis according to claim 1, or 2 wherein the cation exchange membrane of a fluorinated polymer which has an ion exchange capacity of carboxylic acid group type ion exchange groups of 0.9 to 2.2 meq/g. dry polymer is made of a copolymer having units (a) and units (b): ##EQU2## wherein X represents fluorine, chlorine, hydrogen or --CF 3 ; X' represents X or CF 3 (CF 2 ) m ; m represents 1 to 5; and Y represents ##STR2## p, q and r respectively represent 1 to 10; Z and R f respectively represent --F or a C 1 -C 10 perfluoroalkyl group; A represents a functional group which can be converted to --COOM in a hydrolysis or a neutralization such as --CN, --COF, --COOR 1 and --CONR 2 R 3 ; R 1 represents a C 1 -C 10 alkyl group; M represents hydrogen or an alkali metal atom and R 2 , and R 3 respectively represents hydrogen or a C 1 -C 10 alkyl group.

Claim 11

An electrolysis according to claim 1 wherein the concentration of the resulting alkali metal hydroxide is in a range of 35 to 40 wt.%.