CA1187839A - Method for electrolyzing alkali metal halide - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

An improved method for electrolyzing alkali metal halide.
Electrolysis is carried out by supplying an aqueous solution of a halide of alkali metal into an anode chamber partitioned by a cation-exchange membrane and water into a cathode chamber to obtain halogen from the anode chamber and hydrogen and alkali hydroxide from the cathode chamber. The cation-exchange membrane is a fluorocarbon polymer membrane which is prepared by arranging one side thereof to have a cation exchange group concentration lower by 10 to 30% within a depth range of 1 to 100µ than that of the remainder of the membrane. The anode chamber is prepared with this side of the membrane which has the lower cation exchange group concentration arranged to face the anode chamber.
By this arrangement, a high purity alkali hydroxide can be manu-factured at a high rate of decomposition of halide of alkali metal and at a high current efficiency.

Description

1 This invention relates to an electrolyzing method ~ employing a novel cation-exchange membrane. The electrolysis 3 is carried out by supplying an aqueous solution of an alkali 4 metal halide to an anode chamber and water to a cathode chamber to obtain halogen from the anode chamber and hydrogen 6 and alkali hydroxide from the cathode chamber. The method of 7 the invention yields a high purity alkali hydroxide at a high 8 rate of decomposition of alkali metal halide and a high 9 current efficiency by employing a cation-exchange membrane.
The membrane is prepared by arranging one side of a 11 cation-exchange group containing fluorocarbon polymer 12 membrane to have a lower concentration of the cation-exchange 13 group than that of the other side of the ~roup within a range 14 of depth from 1 to 100 ~ and by arranging this side of the lS membxane which has the lower exchange group concentration to 16 face the anode chamber, which has an alkali metal halide 17 supplied thereto.
18 Conventional diaphragms designed for use in electrolysis 19 of alkali metal halides include cation-exchange membranes having a sulfonic acid group as the exchange group with a 21 fluorine-containing resin employed as substratum. A typical 22 example of cation-exchange membranes of this class is a 23 sulfonic acid type membrane made of a perfluorocarbon 24 polymer, marketed by Du Pont Co. However, although the membrane is durable its cation transport rate in an 26 electrolytic solution has not been satisfactory.

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3~3 1 ~ence, various methods for improveme~t in this respect

2 have been and continue to be under investigation. The

3 following are examples of such methods:

4 1) Methods in which the concentration of the exchange group on one side of a membrane facing the cathode chamber is 6 arranged to be lower than that of the other side facing the 7 anode chamber.
8 2) Methods in which the exchange group on the side 9 facing the cathode chamber is arranged to be more weakly acidic than that of the other side facing the anode chamber.
11 3) Methods in which weakly acidic exchange groups are 12 employed.
13 It is weLl known that the cost of production generally 14 not only varies with power consumption but also greatly varies with the rate of decomposition of the alkali metal 16 halide employed and the concentration of the alkali hydroxide 17 produced. Further, a manufacturing method that enables 18 production of an alkali hydroxide at a low production cost 19 would be hardly acceptabl for indus trial purposes if the purity of the alkali hydroxide produced were low. In order 21 to provide an efflciently operating industrial process, the 22 balance between the cost of production and the quality of the 23 product must be thoroughly considered in developing an 24 improved membrane or diaphragm.
With the above-mentioned cation-exchange membranes of 26 the prior art employed to obtain high purity a]kali hydroxide 27 by enhancing the rate of decomposition of the alkali metal .~

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halide, however, it has been ~ound that the alkali hydroxide thu~ obtained i3 mi~ed with alkali metal halide and, Purthermore, the current efficiency of the process is low.
An object of the present invention is to provide n method of electrolyzin~
~lkali metal halide which is a si~nificant improvement over such prior art methods as those described above.
Thus, in accordance with the invention, there is provided a fluorocarbon polymer membrane for electrolyzin~ an alkali metal halide comprising a cation-exchan~e ~roup dist~ibuted through said membrane and whose concentration beneath one surface of said membrane is, to a depth of from 1 to 100 u, from 10 to 30 % lower than the concentration of the exchan~e group beneath the opposite surface of the membrane throu~h the remainder of said membrane, the surface having the lower concentration of the e~chan~e group being adapted to face an anode chamber acco~modating said alkali metal halide.
Without bein~ bound by theory, the inventors believe that the une~pectedly hi~h perfomance and excellent results provided by the no~el method disclosed may be explained as follows.
Swellin~ of the me~brane surface on the side of the anode ch~mber increases as the rate of decomposition of the alk~li metal halide in the anode chamber increases. This causes the aqueous solution of the alkali metal halide to enter the membrane, which results in increasin~ water content ~ - 3 -3~

in the membrane. The increased water content lowers the con-centration of fixed ion, resulting in decreased current effi-ciency, movement of the alkali metal halide within the membrane towards the cathode chamber and eventual lowering of the purity of the alkali hydroxide produced.
When the side of the membrane facing the cathode chamber only swells to a very small degree, ~he difference in swelling between the two sides of the membrane eventually becomes suffi-ciently great to break the membrane. In view of this, the degree of swelling of the side of the membrane facing the anode chamber must be controlled. Fur~her, with increasing rate of decomposi tion of the alkali metal halide in the anode chamber, the con-centration of the alkali metal halide tends to decrease. If the concentration of the alkali hydroxide produced increases, the swelling of the side of the membrane facing the anode chamber becomes greater than the swelling of the other side of the membrane facing the cathode chamber and this tendency further contributes to the above-mentioned undesirable results.
Preferably, the exchange membrane should be selected from fluorocarbon polymer membranes having a sulfonic acid group, a carboxylic acid group or a sulfonamide group in the side chain thereof. For example, a polymer expressed by the following generic formula and formed into a film may be used:

2 t CF2 ~ CF2 ~~ C~2 ~ CF ~--4 ~ 2 S O
6 ~

CF-R

9 ~n Im 11 (CF2tp 12 wherein:
13 R: -CF3, -CF2-0-CF3 n: 0 or 1-5 m: 0 3r 1 14 0: 0 or 1 P: 1-6 ~: -S02F, ~S02Cl9 -COORl (Rl: 1 to 5 alk~l group~), C~, CO~
17 The film, if so desired, may be hydrolyzed before use.
18 Further, a pol~mer which is polymerized by adding a 19 third or fourth component to the above stated two-compor.ent system may be employed as the ion-exchange membrane. Such a 21 polymer, for example, may be selected from the following 22 groups A and B and is formed into a film and is then 23 subjected to a hydrolyzing process before use:
Group A:
t CF2 CF2~nL- ~CF2- l~ t-ml i l~3 CF2-CF -C~2-~F2-s2 ( 2) tCF2 CF~t- 2 ~CF2-CF ~ 2 ~r ~2 ( 3 ~ ~ CF2-CF2 ~ 3 ( CF2-CF ~ 3 o CF2-CF2-~a2F

( 4 ~ t CF2 2~ CF 2-CF ~ 4 O

Gl~ CF2 0 (;5~3 CF2-C~2-0-CE'3

(5) ~C~2-~2) 5 (CF2 IF ~ 5 Gr3up :B:
( 1 ) t CF2 a~2 )~e F2~
O
lF2 O-CF 2-COOaH~

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'7~

(2) -tCF2-CF2~p ~CF2-CF ~q lF2 CF2 -~COOCH~

( 3 ~ tCF 2-CE' 2 ) p (- C~ 2 7E' ~tq CF2 (~2 ~2 C~2 COOCH3 ( 4 ) t CF 2-~ 2~ CF 2- CF ~q ( 5 ) tCF2-GF2t~CF2-CF tq o e~F2 IF CF2 aF2 COOCH3

(6) ~F2-CF2~)p (cF2-lF~q COOCH~5

(7) ~CF2 C . 2~CF2-7~ CF2-O
CF3 C~ 2-C~2-C~ 2-COF

~8~ ?2-~ F2t~ 1 3~ 2 ~ r2 O O
I ~? C~2-CF2-a3?2~ooc~ 3 3'7~

(g) -~CF2-C~2~p-9- ~C~2 1 ~ -C~2-C~-r O O
CF3 C~2 ~4 cooaH;5 ~10) ~CF;~ 2)p ~C~2-1~3q ~C~2-~F ~;

O O
CF~ CF2-C~2-COF
(12) tC~2-~F2)p~ 2-~F) tCF f~ t aF~;
) t ~2 C~2)pl~ C~2~C~~)q~ CF2-CF~-O O

F3C~CF
0~ 2-~:F 2-CO~
1 It is also possible to use a membrane which i5 obtained ; 2 from some of the copolymers o Group A shown above and has 3 one side thereof suitably modified with monoamine or di- or 4 poly-amine; a membrane which is obtained from some of the copolymers of Group A and has a arboxlyic acid group 6 introduced therein; or a membrane which is prepared by ; 7 laminating together films made from some oE the polymers of ; 8 the groups A and s. Each of such polymer membranes is 9 preferably prepared by selecting the equivalent weight of resin containing 1 equivalent of the exchange group to be 500 11 to 2800 (hereinafter will be expressed as EW = 500 - 2800).

3~3 g 1 To make the exchange group concentration of one side of 2 the membrane facing the anode chamber less than that of the 3 other side thereof facing the cathode chamber, the following 4 procedures may be employed:
a. the exchange group on one side of the membrane 6 facing the anode chamber may first be converted into a 7 readily decomposable gxoup such as a sulfonyl chloride group

8 or a carboxylic acid salt and is then removed;

9 b. films having different concentrations of the exchange group may be laminated together;
11 c. the side of the membrane facing the anode chamber 12 may be prepared by impregnating it with a monomer which has 13 no gxoups available as exchange groups (or, the monomer 14 together with a cross linking agent~and then the monomer may be polymerized~.
1~ However, it is to be understood that such methods have 17 been mentioned above by way of example only and the present 18 invention is of course nok limited to these methods.
19 The cation-exchange membrane as normally used has a thickness between 0.05 mm and 1.5 mm, taking the specific 21 conductivity of the membrane and current efficiency into 22 consideration.
23 In practising the present invention, the electrolytic 24 cell is equipped with at least an anode, a cathode and an external mea~s for allowing a current to flow to the 26 cation-exchange membrane. The electrolytic cell is divided 3~
-- ].o 1 into an anode chamber and a cathode chamber and the current 2 then f low5 between the anode and cathode. With the apparatus 3 arranged in this manner, electrolysis is carried out when an 4 aqueous solution of an alkali metal halide is supplied to the anode chamber. If necessary, water is supplied to the 6 cathode chamber to adju~t the concentration of the alkali 7 hdyroxide which is to be removed from the cathode chamber.
8 The electrolysis is carried out at temperature between room 9 temperature and 100C and preferably within a range from 50 to 95C. The electrolyzing operation is carried out at a 11 current density of 5 to 50 A/dm . An operation at a 12 current density value exceeding 50 A/dm2 is not always 13 advantageous because the cell voltage then significantly 14 rises.
The aqueous solution of the alkali metal halide is 16 purified before use, as in conventional methods for 17 electrolyzing alkali metal halides. It is particularly 18 desirable that magnesium and calcium are thoroughly removed 19 from the aqueous solution. The concentration of the aqueous solution of the alkali metal halide to be supplied is 21 preferably in a state close to saturation and normally from 22 250 ~/1 to 350 g/l.
23 The cathode is made from iron, stainless steel or a 24 material prepared by plating iron with nickel or a nickel compound. The anode is prepared by coating a titanium net 26 with an oxide of a noble metal such as platinum or ruthenium 27 oxide. By employing metal electrodes with a high dimensional ~.'`

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1 stability, these electrodes can be closely spaced to minimize 2 potential drop and reduce power consumption. A ~uitable 3 spacer may be arran~ed to prevent the electrode from coming 4 into contact with the membrane.
The above and further objects, featuxes and advantages 6 of the present invention will become apparent from the 7 following detailed description of preferred embodiments.
8 However, it is to be understood that these examples are 9 intended to illustrate the invention only and are not to be construed as limiting upon the scope of the invention.

12 One side of a film (EW = 1150, film ~hickness 7 mils) 13 made from a copolymer obtained from copolymerization of the 14 groups CF2 = CF2 and CF2 = CF - 0 - CF2 - IF - 9 -16 was brought into contact with ethylene diamine. The surface 17 thereof was thoroughly washed and then was dried. The 18 cross-section of the film was subjected to a coloring test 19 which revealed that a reaction took place down to a depth of 1.1 mil. Then, after Teflon (trade mark) fiber was 21 introduced into the film khus prepared, the film was heat 22 treated at a temperature between 180 and 200C. The film 23 was then further subjected to hydrolysis to form a 24 cation-exchange membrane. The cation-exchange membrane thus obtained was further treated with hydrochloric acid to 26 ~cidify the exchange group.

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1 Following upon the foregoing, two sheets of the 2 copolymer film were joined together with the film-side which 3 had been treated with ekhylene diamine disposed inside and 4 were fixed within an acrylic frame to-m~ke~ only the sulfonic acid layer o~ the laminated sheet reactive. The composite 6 membrane thus obtained was then subjected to a mixture of 7 equal parts by weight of phosphorus oxychloride and 8 phosphorus pentachloride at 120C for four hours. After 9 the reaction, the membrane was washed with carbon tetrachloride at 80C and dried. The dried membrane was 11 heated at 200C for 2 min. under pressure of 50 kg/cm2.
12 Through these processes, the concentration of the exchange 13 group to a depth of 10 microns of the membrane decreased by 14 12~.
An electrolytic cell having an effective area of 30 x 30 16 cm~ was formed using the cation exchange membrane prepared 17 as described abo~e. The membrane was employed as a 18 partitioning diaphragm separating the anode chamber from the 19 cathode chamber~ with the ethylene diamine reaction layer on the membrane arranged to face the cathode chamber. Satuxated 21 brine was supplied to the anode chamber to provide a 22 concentration of 200 g/l while water was supplied to the 23 cathode chamber to adjust the caustic soda concentration a~
24 the exit of the cathode chamber to 28% hy weigh~. Undex this condition, electrolysis was carried out at a current density 26 value of 30 A/dm2 and a temperature of 80C. Under ~:, 7~

1 stable operating conditions, current efficiency, voltage and 2 the common salt concentration in the aqueous solution of 3 caustic soda were as shown in Table 1 below:
4 Table 1 Common salt contained in 6 Current the aqueous solution of 7 efEiciencyVoltage caustic soda 9 94% 3.7V 13 ppm 11 An electrolytic cell was prepared in the same manner as 12 in Example 1 with the exception that the cation-exchange 13 membrane which was used as the partitioning diaphragm 14 consisted of only the membrane treated with ethylene diamine~
Electrolysis was then carried out under the same conditions 16 as in Example 1 to obtain results as shown in Table 2 below:
17 Table 2 18 Common salt contained in 19 Current the aqueous solution of efficiencyoltage caustic soda _ _ _ 22 87% 3.5V 92 ppm 24 A film (EW = 1100, film thickness 10 mils) made from a copolymer obtained from copolymerization of the groups 26 CF2 = CF2 and CF2 ~ ~ CF2 ~ fF - - CF2 - CF2 S2F

28 was subjected to a hydrolysis process with a mixture of a 10%
29 aqueous solution of equal parts by w~ight of sodium hydroxide ,~

8'7~3~7 1 and methanol. Then, the exchange group was converted to a 2 sulfonic acid type with nitric acid.
3 The membrane thus obtained was subjected to treatment by 4 a solution of phosphorus oxychloride and phosphorus pentachloride at 120C for 50 hours to convert the sulfonic 6 acid into a sulfonyl chloride group. Two sheets of the 7 membrane laminated together and fixed within an acrylic 8 frameO Then only one side o~ the membrane was subjected to 9 reaction with hydriodic acid at 80C for 20 hours, the membrane heated at 200C for 2 min. under pressure of 50 11 kg/cm ~ In the membrane thus processed, there was produced 12 a carboxylic acid group to a depth of 15 microns on the above 13 stated side while, on the other side, the exchange group 14 decreased by 14% to a depth of 11 microns. Then, the membrane was further hydrolyzed with a 10~ aqueous solution 16 of equal parts by weight of sodium hydroxide and methanol.
17 With a cation-exchange membrane obtained in this manner, 18 an electrolytic cell having an effective area of 30 x 30 19 cm was formed using the cation-exchange membrane as partitioning diaphragm separating the anode chamber and the 21 cathode chamber from each other, with the carboxylic acid 22 layer of the membrane arranged to face the cathode chamber.
23 Saturated brine was supplied to the anode chamber to adjust 24 the exit concentration to 180 g/l while water was supplied to the cathode chamber to adjust the caustic soda concentration 26 in the cathode chamber to 30% by weight. Under these 7~

1 conditions, electrolysis was carried out at a current density 2 of 30 A/dm2 and at a temperature of 80C~ Current 3 efficiency, voltage and the concentration of common salt in 4 the aqueous solution of caustic soda measured after 30 days were as shown in Table 3 below:
6 Table 3 7 Common salt contained in 8 Current the aqueous solution of 9 efficiency Voltage ~ tic ~d~
11 92~ 3~9V 12 ppm 13 A further sample of the film material obtained in 14 Example 2 was subjected to hydrolysis, except that it was prepared without the heating process which was carried out in 16 the case of ~xample 2 at 200C under the pressure of 50 17 kg/cm2. Using the membrane thus obtained, electrolysis was 18 carried out in the same manner as .in Example 2 to obtain 19 results as shown in Table 4 below:
Table 4 21 Common salt contained in 22 Current the a~ueous solution of 23 efficiency Voltage caustic soda 88% 3.8V 79 ppm 27 A film (EW = 850, film thickness 6 mils) which was 2a obtained by copolymerization of the groups 29 CF2 = CF2 and CF2 = CF - - CF2 - CIF - - CF2 - CF2 -~C~3 :~8'7~3~

l was subjected to hydrolysis.
2 The membrane which was obtained in this manner was set 3 in a reaction tank arranged to allow only one surface of the 4 membrane to be subjected to a reaction process. The membrane was treated with 60 wt% of potassium hydroxide to have 15% oE
6 the exchange group thereof removed to a depth of 15 microns 7 of the membrane.
8 Then~ electrolysis was carried out in exactly the same 9 manner as in Example 2 with the exception that the concentration of the caustic soda in thP cathode chamber was ll controlled to be 37~ by weight. The results w~re as shown in 12 Table 5.
13 Table 5 ~ , = = = =
14 Common salt contained in Current the aqueous solution of 16 efficiency Voltaqe caustic soda lB 92% 3.9V 13 ppm 19 COMPARISON EXAMPLE 3:

Electrolysis was carried out in exactly the same manner 21 as in Example 3 with the exception that the exchange group of 22 the membrane employed in this case was not removed. The 23 results obtained were as shown in Table 6 below:

24 Table 6 Common salt contained in 26 Current the aqueous solution of 27 efficiency Voltage caustic soda 29 B8% 3 ~ BV 80 ppm EXAMPLE 4:

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1A film ~EW = 1050, film thickness 3 mils) which was 2 obtained from copolymerization of the groups 3CF~ - CF2 and CE'2 = CF - 0 - CF2 - fF - 0 - CF2~CE~ -COOCH3 ~ CF3 and another film (EW = 1250, film thickness 3 mils) which was 6 obtained from copolymerization of the groups 7CF2 = CF2 and CF2 = CF - 0 - CF2- CIF - 0 - CF2 - CF2-SO2F
~ C~3 9 were joined together through a thermal pressure joining process carried out at a temperature below that at wh;ch the 11 two copolymers would decompose. The membrane which was thus 12 obtained was hydrolyzed with a 10~ aqueous solution of equal 13 parts by weight of sodium hydroxide and methanol.
14Then, with the membrane arranged to have the carboxylic acid group thereof facing the cathode chamber, electrolysis 16 was carried out under the same conditions as in Example 3 to l? obtain results as shown in Table 7 below:
18 Table 7 19Common salt contained in Current the aqueous solution of 21 efficiency Voltaqe caustic soda 23 97% 3.7V 15 ppm 24 COMPARISON EXAMPLE 4:
25A film (EW = 1100~ film thickness 3 mils) which was 26 obtained by copol~merization of the groups 27CF2 = CF2 and CF2 = CF - - CF2~ fF - ~ CF2-CF2 - COOCH3 '7~3~`~

1 and another ilm (EW = 1100, ~ilm thickness 3 mils) which was 2 obtained by copolymerization of the groups 3 CF~ = CF2 and C~2 = CF - 0 - CF2- fF - Q - CF2 - CF2-SO2F

were joined together in the same manner as in E~ample 4.
6 ~ydrolysis was also carried out in the same manner as in 7 Example 4.
8 Electrolysis was carried out under the same conditions 9 as in Example 4 to obtain results which were as shown in Table 8 below:
11 Table 8 12 Common salt contained in 13 Current the a~ueous solution of 14 efficiency Voltaqe caustic soda 16 92% 3.6V 90 ppm 17 EXAMPLE 5:
18 A film (E~ = 950, film thickness 6 mils) obtained by 19 copolymerization of the groups CF2 = CF2 and CF2 = CF - 0 - CF2 - ~F - 0 - CF2 - CF2-COQCH3 22 was hydrolyzed with a 10% aqueous solution of equal parts by 23 weight of sodium hydroxide and methanol.
24 A laminated cation-exchange membrane was formed by hot-pressing together two sheets of the cation-exchange 26 membrane thus obtained. The sealed cation-exchange membrane 27 was placed in an autoclave. Then, ethylene tetrafluoride and 28 a~obisisobutyronitrile which was used as initiator were put `~;;~"`'`

~lB~t9 1 into the autoclave. Only one side of the cation-exchange 2 membrane was arranged to be impregnated with the ethylene 3 tetrafluoride for polymerization. As a result, the exchange 4 capacity of the membrane decreased by 20~ to a depth of 2 mils.
6 The membrane was positioned ~ith the layer thereof 7 having a higher exchange capacity arranged to face the 8 cathode ~chamber and an electrolyzing operation was carried 9 out in exactly the same conditions as in Example 4 to obtain results as shown in Table 9 below.
11 Table 9 12 Common salt contained in 13 Current the aqueous solution of 14 efficiency Voltage caustic soda 16 96% 4.0V 15 ppm 17 COMPARISON EXAMPLE 5.
18 A membrane which was obtained in the same manner as in 19 Example 5 was used without application of the ethy]ene tetrafluoride treatment. An electrolyzing operation was 21 carried out in exactly the same manner as in Example 5 to 22 obtain results as shown in Table 10 below:
23 Table 10 24 Common salt contained in Current the aqueous solution oE
26 efficiency Voltag~ caustic soda 28 91% 3.9V 92 ppm

Claims (6)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE PROPERTY OR PRIVILEGE
IS CLAIMED ARE DEFINED AS FOLLOWS:

1. A fluorocarbon polymer membrane for electrolyzing an alkali metal halide comprising a cation-exchange group distributed through said membrane and whose concentration beneath one surface of said membrane is, to a depth of from 1 to 100 u, from 10 to 30 % lower than the concentration of the exchange group beneath the opposite surface of the membrane through the remainder of said membrane, the surface having the lower concentration of the exchange group being adapted to face an anode chamber accommodating said alkali metal halide.

2. A membrane as defined in claim 1, the total thickness of said membrane being from 0.05 to 1.5 mm.

3. A membrane as defined in claim 1 or 2, wherein said cation-exchange group is a sulfonic acid group.

4. A membrane as defined in claim 1 or 2, wherein said cation-exchange group is a carboxylic acid group.

5. A membrane as defined in claim 1 or 2, wherein said cation-exchange group is a member of the group consisting of a sulfonic acid group and a carboxylic acid group.

6. A membrane as defined in claim 1 or 2, wherein said cation-exchange group is a member of the group consisting of a sulfonic acid group and a sulfonamide group.