Electrolytic cell for the electrolysis of an alkali metal chloride and process of using said cell

An electrolytic cell for the electrolysis of an alkali metal chloride, wherein an ion-exchange membrane provided at least on one side thereof with a gas and liquid permeable non-electrocatalytic porous layer, is disposed between an anode and a cathode so that the porous layer is in contact with the facing electrode, said ion-exchange membrane being provided on its porous layer surface with grooves which form continuous void spaces and secure passages for the electrolyte at the interface between the electrode and the ion-exchange membrane.

FIELD OF THE INVENTION 
The present invention relates to an electrolytic cell for the electrolysis 
of an alkali metal chloride. More particularly, it relates to an 
electrolytic cell for the electrolysis of an alkali metal chloride, in 
which an ion-exchange membrane is disposed substantially vertically and 
which is capable of producing chlorine gas containing oxygen gas of a low 
oxygen concentration at the anode at a low cell voltage. 
DESCRIPTION OF THE PRIOR ART 
As a process for producing an alkali metal hydroxide and chlorine by the 
electrolysis of an aqueous solution of an alkali metal chloride, a 
diaphragm method has been used in place of a conventional mercury method. 
Further, in order to efficiently obtain an alkali metal hydroxide having a 
high purity in a high concentration, it has been proposed and put into 
practical application to employ an ion-exchange membrane process. 
On the other hand, from the standpoint of energy saving, it is desired to 
reduce the cell voltage in an ion-exchange membrane process as much as 
possible. For this purpose, various means have been proposed. However, 
this object has not yet adequately been attained for a reason such that 
the electrolytic cell tends to have a complicated structure. 
It has been proposed that the above object can adequately be attained by 
using an electrolytic cell wherein a cation exchange membrane has an 
electrocatalytically inactive gas and liquid permeable porous layer on at 
least one surface thereof, i.e. at least the anode or cathode side of the 
ion exchange membrane. The inventions based on this discovery have been 
made the subject matters of earlier U.K. Pat. No. 29751 or European Patent 
Publication No. 29751. 
The effect for reducing the electrolytic voltage attinable by the use of a 
cation exchange membrane having such a porous layer on its surface, varies 
depending upon the kind, the porosity and the thickness of the material 
constituting the porous layer. However, even when the porous layer is made 
of a non-conductive material as mentioned hereinafter, substantially the 
same voltage reducing effect is obtainable. 
It has also been proposed that when an ion-exchange membrane having a gas 
and liquid permeable porous layer on the surface, is used, the minimum 
cell voltage is attainable if the porous layer is in contact with the 
electrode. However, it has been found that with this electrolytic cell, 
the oxygen concentration in the chlorine gas generated at the anode can 
not necessarily be reduced. 
The cause for such undesirable phenomenon is not entirely clear, but it is 
conceivable that no adequate passage for the electrolyte is secured and 
proton can not readily be supplied to the interface between the ion 
exchange membrane and the anode, and consequently a liquid having a high 
pH will be brought in contact with the anode, whereby the oxygen 
concentration tends to be high. In some cases, such a phenomenon can not 
be neglected for electrolytic cells for industrial purposes. 
The present inventors have continued the study with an aim to suppress such 
a phenomenon, and have found that the above object can adequately be 
attained in a practical manner by providing grooves on the porous layer 
side of the ion exchange membrane to form continuous void spaces and to 
secure passages for the electrolyte at the interface between the electrode 
and the ion exchange membrane having the gas and liquid permeable porous 
layer. 
SUMMARY OF THE INVENTION 
The present invention provides an electrolytic cell for the electrolysis of 
an alkali metal chloride, wherein an ion-exchange membrane provided at 
least on one side thereof with a gas and liquid permeable 
non-electrocatalytic porous layer, is disposed between an anode and a 
cathode so that the porous layer is in contact with the facing electrode, 
said ion-exchange membrane being provided on its porous layer surface with 
grooves which form continuous void spaces and secure passages for the 
electrolyte at the interface between the electrode and the ion-exchange 
membrane. 
Now, the present invention will be described in detail with reference to 
the preferred embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
With respect to the grooves to be provided on the porous layer surface of 
the ion-exchange membrane, the object of the present invention can be 
attained so long as they will provide continuous void spaces and secure 
the passages for the electrolyte at the interface between the ion-exchange 
membrane and the electrode as mentioned above. However, the degree of 
attainment of the purpose of the invention varies depending upon the 
shape, the direction and the number of such grooves. 
In the present invention, the grooves to be provided on the porous layer 
surface of the ion-exchange membrane may preferably have a square, 
circular, triangular or elliptic cross section as illustrated in FIGS. 
1-(i) to 1-(iv). Their width (a) on the porous layer surface is preferably 
from 0.1 to 10 mm, more preferably from 0.5 to 5 mm, and the depth (b) is 
preferably at least 0.03 mm, more preferably from 0.05 mm to a half of the 
thickness of the membrane. The pitch (c) of the grooves may vary depending 
upon the width (a) of the grooves, but is preferably from 0.1 to 20 mm, 
more preferably from 0.5 to 10 mm. The pitch (c) is preferably in 
proportion to the width (a). Namely, it is preferred that the greater the 
width (a), the greater the pitch (c). Further, the length (d) of the 
grooves is preferably at least 5 mm, more preferably at least 10 mm, as 
illustrated in FIG. 2. 
The grooves on the porous layer surface are preferably inclined at an angle 
of upto 60.degree. preferably upto 45.degree. relative to the vertical 
direction or most preferably directed vertically. However, the grooves may 
be inclined at an angle beyond 60.degree., although the effect of the 
present invention will be substantially reduced. In some cases, the 
grooves may be provided in a horizontal direction. The arrangement of the 
grooves on the porous layer surface is preferably determined to have a 
certain geometric pattern as shown in FIG. 2. However, the grooves may 
entirely or partially be randomly arranged. 
Further, the grooves of the porous layer surface may be provided so that a 
plurality of differently directed grooves are provided to cross one 
another, as shown in FIG. 2-(iii) and 2-(iv). In any case, it is important 
that the continuous void spaces are formed and electrolyte passages are 
provided at the interface between the ion-exchange membrane and the 
electrode. Accordingly, by virtue of the above-mentioned grooves on the 
porous layer surface, the void spaces are preferably inclined at an angle 
of upto 60.degree. relative to the vertical direction or most preferably 
directed vertically. Likewise, the length of the void spaces is preferably 
at least 5 mm, more preferably at least 10 mm. Further, it should be 
understood that the present invention is not restricted to the strict 
sense of the term "grooves" on the surface of the ion-exchange membrane, 
and extends to cover, e.g. a case where the porous layer surface are 
partially protruded to provide linear protrusions, whereby the object of 
the present invention is likewise attained. 
Various methods may be employed for the formation of the grooves on the 
porous layer surface of the ion-exchange membrane. It is preferred to 
employ a method wherein the porous layer surface of the ion-exchange 
membrane is roll-pressed by means of a grooved roll having predetermined 
grooves on its surface, or a flat plate pressing method wherein a grooved 
flat plate having grooves of a predetermined shape on its surface is used. 
Further, the porous layer may be provided on the ion-exchange membrane 
surface so that the predetermined grooves are preliminarily formed on the 
porous layer itself. 
The depth of the grooves is not necessarily required to have a 
predetermined relation with the thickness of the porous layer formed on 
the ion-exchange membrane surface. However, the thickness of the grooves 
is preferably greater than the thickness of the porous layer. Namely, the 
depth of the grooves is preferably from 5 to 50 times, more preferably 
from 10 to 30 times, the thickness of the porous layer. 
The ion-exchange membrane having on its surface a gas and liquid permeable 
porous layer to be used in the present invention, may be formed by bonding 
particles on the membrane surface. The amount of the particles deposited 
to form the porous layer may vary depending upon the nature and size of 
the particles. However, it is preferably from 0.001 to 100 mg, preferably 
from 0.005 to 50 mg per cm.sup.2 of the membrane surface, according to the 
study of the present inventors. If the amount is too small, no desired 
effect of the present invention can be obtained, and if the amount is too 
large, the electric resistance of the membrane increases, such being 
undesirable. 
The particles to form the gas and liquid permeable porous layer on the 
surface of the cation exchange membrane may be made of electro-conductive 
or nonconductive inorganic or organic material so long as they do not 
function as an electrode during an electrolysis. However, they are 
preferably made of a material which is resistant to corrosion in the 
electrolytic solution. As typical examples, there may be mentioned a metal 
or a metal oxide, hydroxide, carbide or nitride or a mixture thereof, 
carbon or an organic polymer. 
As preferred specific materials for the porous layer on the anode side, 
there may be used a single substance of Group IV-A of the Periodic Table 
(preferably, silicon, germanium, tin or lead), Group IV-B (preferably, 
titanium, zirconium or hafnium), Group V-B (preferably, niobium or 
tantalum), an iron group metal (iron, cobalt or nickel), chromium, 
manganese or boron, or its alloy, oxide, hydroxide, nitride or carbide, or 
polytetrafluoroethylene, or ethylene-tetrafluoroethylene copolymer. 
On the other hand, for the porous layer on the cathode side, there may 
advantageously be used, in addition to the materials useful for the 
formation of the porous layer on the anode side, silver or its alloy, 
stainless steel, carbon (activated carbon or graphite), or silicon carbide 
(.alpha.-type or .beta.-type), as well as a polyamide resin, a polysulfone 
resin, a polyphenyleneoxide resin, a polyphenylenesulfide resin, a 
polypropylene resin or a polyimide resin. 
For the formation of the porous layer, the above-mentioned particles are 
used preferably in a form of powder having a particle size of from 0.01 to 
300 .mu.m, especially from 0.1 to 100 .mu.m. If necessary, there may be 
incorporated a binder of e.g. a fluorocarbon polymer such as 
polytetrafluoroethylene or polyhexafluoroethylene, or a 
viscosity-increasing agent, for instance, a cellulose material such as 
carboxymethyl cellulose, methyl cellulose or hydroxyethyl cellulose, or a 
water soluble substance such as polyethylene glycol, polyvinyl alcohol, 
polyvinyl pyrrolidone, sodium polyacrylate, polymethylvinyl ether, casein 
or polyacrylamide. The binder or the viscosity-controlling agent is used 
in an amount of preferably from 0 to 50% by weight, especially from 0.5 to 
30% by weight. 
Further, if necessary, there may further be added a suitable surfactant 
such as a long chained hydrocarbon or a fluorohydrocarbon, or graphite or 
other electroconductive fillers to facilitate the bonding of the particles 
to the membrane surface. 
To bond the particles or particle groups (mass) to the surface of the 
ion-exchange membrane, a binder and a viscosity-increasing agent which are 
used as the case requires, are adequately mixed in a suitable solvent such 
as an alcohol, a ketone, an ether or a hydrocarbon to obtain a paste, 
which is then applied to the membrane surface by transfer or screen 
printing. Alternatively, it is possible to deposit the particles or 
particle groups on the membrane surface by forming a syrup or slurry of a 
mixture of the particles instead of the paste of the mixture, and spraying 
or hot pressing the syrup or slurry onto the membrane surface. 
The porous layer-forming particles or particle groups are then preferably 
pressed under heating by means of a press or rolls preferably at a 
temperature of from 80.degree. to 220.degree. C. under pressure of 1 to 
150 kg/cm.sup.2. It is preferred that they are partially embedded in the 
membrane surface. 
The porous layer thus formed by the particles or particle groups bonded to 
the membrane surface preferably has a porosity of at least 10%, especially 
at least 30%, and a thickness of from 0.01 to 200 .mu.m, especially from 
0.1 to 50 .mu.m. The thickness of the porous layer is preferably thinner 
than the thickness of the ion-exchange membrane. 
The porous layer may be formed on the membrane surface in a form of a 
densed layer where a great amount of the particles are bonded to the 
membrane surface or in a form of a single layer wherein the particles or 
particle groups are bonded to the membrane surface independently without 
being partially in contact with one another. In the latter case, it is 
possible to substantially reduce the amount of the particles to form the 
porous layer, and in certain cases, the formation of the porous layer can 
be simplified. 
In the present invention, the ion-exchange membrane on which the porous 
layer is to be formed, is preferably made of a fluorine-containing polymer 
having cation exchange groups such as carboxylic acid groups, sulfonic 
acid groups, phosphoric acid groups or phenolic hydroxyl groups. Such a 
membrane is prererably made of a copolymer of a vinyl monomer such as 
tetrafluoroethylene or chlorotrifluoroethylene with a fluorovinyl monomer 
containing ion exchange groups such as sulfonic acid groups, carboxylic 
acid group or phosphoric acid groups. 
It is particularly preferred to employ a polymer having the following 
repeating untis (i) and (ii): 
##STR1## 
where X is F, Cl, H or --CF.sub.3, X' is X or CF.sub.3 (CF.sub.2).sub.m 
where m is from 1 to 5, and Y is selected from the following groups: 
##STR2## 
where each of x, y and z is from 0 to 10, and each of Z and R.sub.f is 
selected from the group consisting of --F or a perfluoroalkyl group having 
from 1 to 10 carbon atoms. Further, A is --SO.sub.3 M or --COOM, or a 
group which can be converted to such groups by hydrolysis, such as 
--SO.sub.2 F, --CN, --COF or --COOR, where M is a hydrogen atom or an 
alkali metal, and R is an alkyl group having from 1 to 10 carbon atoms. 
The cation exchange membrane used in the present invention, preferably has 
an ion exchange capacity of from 0.5 to 4.0 meq/g dry resin, more 
preferably from 0.8 to 2.0 meq/g dry resin. In order to obtain such an ion 
exchange capacity, the ion-exchange membrane made of a copolymer having 
the above-mentioned polymerization units (i) and (ii), preferably contain 
from 1 to 40 mol %, more preferably from 3 to 25 mol %, of the 
polymerization unit (ii). 
The cation exchange membrane used in the present invention, may not 
necessarily be formed from one type of a polymer and may not necessarily 
have only one type of ion exchange groups. For example, there may be used 
a laminated membrane composed of two types of polymer sheets so that the 
cathode side has a smaller ion exchange capacity, or an ion-exchange 
membrane having weakly acidic exchange groups such as carboxylic acid 
groups on the cathode side and strongly acidic exchange groups such as 
sulfonic acid groups on the anode side. 
These ion-exchange membranes may be prepared by various conventional 
methods. Further, these ion-exchange membranes may preferably be reinfoced 
by a woven fabric such as cloth or a net, or a non-woven fabric, made of a 
fluorine-containing polymer such as polytetrafluoroethylene, or by a metal 
mesh or perforated sheet. The thickness of the ion-exchange membrane of 
the present invention is preferably from 50 to 1000 .mu.m, more preferably 
from 100 to 500 .mu.m. 
When the porous layer is to be formed on the anode side or a cathode side, 
or on both sides of the ion-exchange membrane, as mentioned above, the ion 
exchange groups of the membrane should take a suitable form not to lead to 
decomposition thereof. For instance, in the case of carboxylic acid 
groups, they should preferably take a form of an acid or an ester, and in 
the case of sulfonic acid groups, they should preferably take a form of 
--SO.sub.2 F. 
When the above-mentioned grooves are to be provided on the ion-exchange 
membrane having on its surface a gas and liquid permeable porous layer, 
the operation is preferably conducted in the same manner as in the 
above-mentioned formation of the porous layer on the ion-exchange 
membrane, i.e. in the case where the ion exchange groups of the membrane 
are carboxylic acid groups, the ion exchange groups should preferably take 
a form of an acid or an ester, and in the case of the sulfonic acid 
groups, they should preferably take a form of --SO.sub.2 F. The operation 
is preferably conducted by roll pressing or flat plate pressing, 
preferably at a pressing temperature of from 60.degree. to 280.degree. C. 
under a roll pressing pressure of from 0.1 to 100 kg/cm or a flat plate 
pressing pressure of from 0.1 to 100 kg/cm.sup.2. The formation of the 
porous layer and the formation of the grooves may be conducted 
simultaneously, as mentioned above. 
Any type of electrodes may be applied to the membrane of the present 
invention. For instance, there may be employed perforated electrodes such 
as foraminous plates, nets or expanded metals. As the porous electrode, 
there may be mentioned an expanded metal having openings with a long 
diameter of from 1.0 to 10 mm and short diameter of from 0.5 to 10 mm, the 
wire diameter of from 0.1 to 1.3 mm and an opening rate from 30 to 90%, or 
a punched metal having openings of a circular, elliptic or diamond shape 
and an opening rate of from 30 to 90%. Further, a plate-like electrode may 
also be used. The effectiveness of the present invention is remarkable 
particularly when electrodes having a smaller opening rate are used. 
Further, in the present invention, a plurality of electrodes having 
different opening rates may be employed. 
The anode may usually be made of a platinum group metal or its 
electro-condutive oxides or electro-condutive reduced oxides. On the other 
hand, the cathode may be made of a platinum group metal, its 
electro-conductive oxides or an iron group metal. As the platinum group 
metal, there may be mentioned platinum, rhodium, ruthenium, paradium and 
iridium. As the iron group metal, there may be mentioned iron, cobalt, 
nickel, Raney nickel, stabilized Raney nickel, stainless steel, an alkali 
etching stainless steel (U.S. Pat. No. 4,255,247), Raney nickel-plated 
cathode (U.S. Pat. Nos. 4,170,536 and 4,116,804) and Rodan nickel-plated 
cathode (U.S. Pat. Nos. 4,190,514 and 4,190,516). 
In the case where perforated electrodes are used, the electrodes may be 
made the above-mentioned materials for the anode or cathode. However, when 
a platinum group metal or its electro-conductive oxides are used, it is 
preferred to coat these substances on the surface of an expanded metal 
made of a valve metal such as titanium or tantalum. 
When the electrodes are to be disposed in the present invention, at least 
anode or cathode, preferably both are arranged to be in contact with the 
gas and liquid permeable porous layer having the grooves on the surface. 
On the other hand, in the case of an ion-exchange membrane having a gas 
and liquid permeable porous layer having no grooves on the surface, or an 
ion-exchange membrane having no porous layer on the surface, may be 
arranged in contact with the electrode or it may be arranged with a space 
from the electrode. The contact between the electrode and membrane should 
preferably be made under a moderate pressure, for instance, the electrode 
is pressed against the porous layer under a pressure of e.g. from 0 to 20 
kg/cm.sup.2, rather than strongly pressing the electrode and membrane to 
one another. 
In the present invention, in the case where only one of the anode side and 
the cathode side of the ion-exchange membrane is provided with the porous 
layer, the electrode disposed to face with the side of the ion-exchange 
membrane on which no porous layer is provided, may be disposed in contact 
with or out of contact with the ion-exchange membrane. 
The electrolytic cell of the present invention may be a monopolar type or 
bipolar type so long as it has the above-mentioned construction. With 
respect to the material constituting the electrolytic cell, for instance, 
in the case of the anode compartment for the electrolysis of an aqueous 
alkali metal chloride solution, a material resistant to an aqueous alkali 
metal chloride solution and chlorine, such as a valve metal like titanium, 
may be used, and in the case of the cathode, iron, stainless stell or 
nickel resistant to an alkali hydroxide and hydrogen, may be used. 
In the present invention, the electrolysis of an aqueous alkali metal 
chloride solution may be conducted under conventional conditions. For 
instance, the electrolysis is conducted preferably at a temperature of 
from 80.degree. to 120.degree. C. at a current density of from 10 to 100 
A/dm.sup.2 while supplying preferably a 2.5-5.0 N alkali metal chloride 
aqueous solution to the anode compartment and water or diluted alkali 
metal hydroxide to the cathode compartment. In such a case, it is 
preferred to minimize the presence of heavy metal ions such as calcium or 
magnesium in the aqueous alkali metal chloride solution, since such heavy 
metal ions bring about a deterioration of the ion-exchange membrane. 
Further, in order to prevent as far as possible the generation of oxygen 
at the anode, an acid such as hydrochloric acid may be added to the 
aqueous alkali metal chloride solution to adjust the pH value of the 
solution to preferably less than 3. 
Now, the present invention will be described in further detail with 
reference to Examples. However, it should be understood that the present 
invention is by no means restricted by these specific Examples. 
EXAMPLE 1 
Tetrafluoroethylene and CF.sub.2 =CFO(CF.sub.2).sub.3 COOCH.sub.3 were 
copolymerized in a trichlorotrifluoroethane solvent in the presence of 
azobisisobutyronitrile as a catalyst to obtain a copolymer having an ion 
exchange capacity of 1.25 meq/g dry resin, and a copolymer having an ion 
exchange capacity of 1.80 meq/g dry resin. 
The film having an ion exchange capacity of 1.25 meq/g and a thickness of 
30 .mu.m and the film having an ion exchange capacity of 1.80 meq/g and a 
thickness of 180 .mu.m were subjected to compression molding at a 
temperature of 220.degree. C. under pressure of 25 kg/cm.sup.2 for 5 
minutes to obtain a laminated membrane. 
On the other hand, a mixture comprising 10 parts by weight of zirconium 
oxide powder having a particle size of 5 .mu.m, 0.4 part by weight of 
methylcellulose (a 2% aqueous solution having a viscosity of 1500), 19 
parts by weight of water, 2 parts by weight of cyclohexanol and 1 part by 
weight of cyclohexanone, was kneaded to obtain a paste. The paste was 
screen-printed on the anode side surface of the above cation exchange 
membrane having an ion exchange capacity of 1.80 meq/g, by means of a 
printing plate comprising a tetron screen having 200 mesh and a thickness 
of 75 .mu.m and a screen mask having a thickness of 30 .mu.m provided 
therebeneath and a squeezee made of polyurethane. The layer deposited on 
the membrane surface was dried in air. 
Then, on the other surface of the membrane having the porous layer thus 
formed on the anode side, .alpha.-silicon carbide particles having an 
average particle size of 5 .mu.m were likewise deposited. 
Thereafter, the particle layers on the respective sides of the membrane 
were press-bonded to the respective sides of the ion-exchange membrane at 
a temperature of 140.degree. C. under pressure of 30 kg/cm.sup.2, whereby 
an ion-exchange membrane having a porous layer of 1.0 mg/cm.sup.2 of 
zirconium oxide particles and a thickness of 10 .mu.m on the anode side of 
the membrane and a porous layer of 0.7 mg/cm.sup.2 of silicon carbide 
particles and a thickness of 10 .mu.m on the cathode side of the membrane, 
was obtained. 
The ion-exchange membrane thus having porous layers on both sides, was 
roll-pressed at a temperature of 140.degree. C. under pressure of 20 
kg/cm.sup.2 with a grooved roll, to form a porous layer surface having, at 
the anode side, vertically directed continuous grooves (square cross 
section) having a width of 1.2 mm, a depth of 0.15 mm and a pitch of 1.5 
mm. The membrane thickness was 200 .mu.m at the grooved portions and 350 
.mu.m at non- grooved portions. 
Such an ion-exchange membrane was immersed in an aqueou solution containing 
25% by weight of sodium hydroxide at 90.degree. C. for 16 hours for the 
hydrolysis of the ion exchange groups. On the anode side of the membrane 
thus obtained, an anode prepared by coating a solid solution of RuO.sub.2, 
iridium oxide and titanium oxide on a titanium expanded metal (short 
opening diamer 4 mm, long opening diameter 8 mm) and having a low chlorine 
overvoltage, was pressed to be in contact with the ion-exchange membrane. 
Likewise, to the cathode side of the membrane, a cathode obtained by 
subjecting a punched metal made of SUS 304 (short opening diameter 4 mm, 
long opening diameter 8 mm) to etching treatment in an aqueous solution 
containing 52% by weight of sodium hydroxide at 150.degree. C. for 52 
hours, and having a low hydrogen overvoltage, was pressed to be in contact 
with the ion-exchange membrane. Then, electrolysis was conducted at 
90.degree. C. at a current density of 30 A/dm.sup.2, while supplying an 
aqueous solution of 5 N sodium chloride adjusted to pH2 by an addition of 
hydrochloric acid, to the anode compartment and water to the cathode 
compartment, and maintaining the sodium chloride concentration in the 
anode compartment at a level of 3.5 N and the sodium hydroxide 
concentration of the cathode compartment to a level of 35% by weight. 
As the results, the current efficiency was 95%, the cell voltage was 2.8 V, 
and the oxygen concentration in the chlorine gas obtained at the anode, 
was 0.3%. 
COMATIVE EXAMPLE 1 
The electrolysis was conducted in the same manner as in Example 1 by means 
of the same electrolytic cell and the same ion-exchange membrane except 
that the ion-exchange membrane was not roll-pressed by the grooved rolls. 
As the results, the current efficiency was 95% and the cell voltage was 
2.8 V, but the oxygen concentration in the chlorine gas obtained in the 
anode compartment was 0.6%. 
EXAMPLE 2 
The same cation exchange membrane as used in Example 1 was used except that 
grooves (square cross section) was formed on the anode side porous layer 
surface composed of zirconium oxide particles by roll-pressing so as to 
bring the angle of the grooves to 30.degree. relative to the vertical 
direction. 
The grooves had a width of 2 mm, a depth of 0.1 mm, a length of 20 mm and a 
pitch of 2.5 mm. The thickness of the membrane was 300 m at the 
non-grooved portions. By using this membrane, the electrolysis was 
conducted in the same manner as in Example 1, whereby the current 
efficiency was 95%, the cell voltage was 2.8 V, and the oxygen 
concentration in the chlorine gas obtained in the anode compartment was 
0.3%. 
COMATIVE EXAMPLE 2 
A membrane was prepared in the same manner as in Example 2 except that no 
porous layer on both sides was deposited. By using this membrane, the 
electrolysis was conducted in the same manner as in Example 1, whereby the 
current efficiency was 95%, but the cell voltage was 3.5 V. The oxygen 
concentration in the chlorine gas obtained in the anode compartment was 
0.5%. 
EXAMPLE 3 
Tetrafluoroethylene and CF.sub.2 =CFO(CF.sub.2).sub.3 COOCH.sub.3 were 
emulsion-polymerized in the presence of ammonium persulfate as a catalyst, 
whereby a polymer having an ion exchange capacity of 1.45 meq/g dry resin 
was obtained. 
To this polymer, 2.7% by weight of polytetrafluoroethylene fine powder was 
mixed, kneaded and then formed by an extruder into a film having a 
thickness of 280 .mu.m. 
Porous layers were deposited in the same manner as in Example 1. A layer on 
one side was composed of zirconium oxide particles, and the layer on the 
other side was composed of silicon carbide particles. To the zirconium 
oxide layer side, flat plate pressing by means of a patterned plate was 
applied to form grooves (triangular cross section). The grooves had a 
width on the surface of 0.5 mm, a depth of 50 .mu.m, a length of 5 mm and 
a pitch of 1.5 mm, and the grooves were directed vertically. 
By using this membrane, the electrolysis was conducted in the same manner 
as in Example 1, whereby the current efficiency was 93%, and the cell 
voltage was 2.9 V. The oxygen concentration in the chlorine gas obtained 
in the anode compartment was 0.4%. 
EXAMPLE 4 
A polytetrafluoroethylene cloth was press-bonded to the 1.8 meq/g side of 
the laminated membrane obtained in Example 1, to obtain a cloth-reinforced 
membrane. Then, porous layers were deposited thereto in the same manner as 
in Example 1. 
To the 1.8 meq/g side of this membrane, roll pressing was applied by means 
of a grooved roll to form grooves. The grooves had a width on the surface 
of 1.5 mm, a depth of 30 .mu.m, a length of 10 mm and a pitch of 2 mm. The 
grooves having square cross sections were directed vertically. By using 
this membrane, the electrolysis was conducted in the same manner as in 
Example 1, whereby the current efficiency was 95%, and the cell voltage 
was 2.8 V. The oxygen concentration in the chlorine gas obtained at the 
anode compartment was 0.3%. 
EXAMPLE 5 
Tetrafluoroethylene and CF.sub.2 =CFOCF.sub.2 CF(CF.sub.3)OCF.sub.2 
CF.sub.2 COOCH.sub.3 were copolymerized in a trichlorotrifluoroethane 
solvent in the presence of azobisisobutyronirile as a catalyst to obtain a 
copolymer having an ion exchange capacity of 0.90 meq/g dry weight. 
On the other hand, tetrafluoroethylene and CF.sub.2 =CFOCF.sub.2 
CF(CF.sub.3)OCF.sub.2 CF.sub.2 SO.sub.2 F were likewise copolymerized to 
obtain a copolymer having an ion exchange capacity of 0.91 meq/g dry 
resin. 
The above carboxylic acid polymer and sulfonic acid polymer were 
co-extruded by means of a co-extruder to obtain a film having a thickness 
of 250 .mu.m. The thickness of the carboxylic acid layer was 50 .mu.m, and 
the thickness of the sulfonic acid layer was 200 .mu.m. 
As the porous layers, in the same manner as in Example 1, silicon carbide 
was deposited on the carboxylic acid layer side, and titanium oxide was 
deposited on the sulfonic acid layer side. To the sulfonic acid layer 
side, roll pressing was applied to form the same grooves as in Example 1. 
This membrane was subjected to hydrolysis, and the electrolysis was 
conducted in the same manner as in Example 1 with the sulfonic acid layer 
side being the anode side, whereby the current efficiency was 96% and the 
cell voltage was 2.9 V. The oxygen concentration in the chlorine gas 
obtained in the anode compartment was 0.3%. 
COMATIVE EXAMPLE 3 
The electrolysis was conducted in the same manner as in Example 5 by means 
of the same electrolytic cell and the same ion-exchange membrane except 
that no roll pressing by the grooved roll was applied, whereby the current 
efficiency was 96% and the cell voltage was 2.9 V, but the oxygen 
concentration in the chlorine gas obtained in the anode compartment was 
0.6%.