Note: Descriptions are shown in the official language in which they were submitted.
: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
--2--
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
5Z77
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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
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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,
. .
--5--
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 ,~
--6--
' 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
.
--7--
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.
,, ~
~ Trade Mark
~ ' ,
.
- llG5;~77
--8--
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~
:
--10--
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
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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~
-13-
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
-14-
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%.
,:
., ,
