KR810001353B1 - Process for electrolysis - Google Patents

Abstract

Sodium chloride soln. is electrolyzed in an electrolytic cell using a cation exchange membrane which is a fluorocarbon polymer containing sulfonic acid groups and at least one cation exchange group which is less acidic than the sulfonic acid group with higher proportion of the latter in surface stratum on the cathode slide of the membrane than in the anode membrane, while controlling proton concn. in the anolyte at not higher than critical proton conc. at which no substantial amount of protons in the anolyte penetrate into the membrane.

Description

Electrolysis Method

1 is a schematic diagram showing the position of neutralization.

2 and 3 are diagrams showing values in the case of FIG.

The present invention relates to a method for electrolysis of an aqueous metal halide solution using a cation exchange membrane composed of a fluorocarbon polymer having a sulfonic acid group and one or more cation exchange groups having weaker acidity than the sulfonic acid group.

Perfluorocarbons having sulfonic acid groups in the side chains by saponification of the network of tetrafluoroethylene and perfluoro-3,6-dioxa-4-methyl-7-octensulfonyl fluoride A method of electrolyzing an aqueous alkali metal halide solution using a cation exchange membrane of a polymer is known in the art.

However, this conventionally known perfluorocarbon cation exchange membrane composed of only sulfonic acid groups has a drawback of low current efficiency during electrolysis because the hydrophilicity of sulfonic acid groups is high so that it is easy to permeate the hydroxy ions from the cathode chamber. Particularly, when the electrolysis is performed to prepare a high concentration caustic soda solution of 20% or more, the current efficiency is extremely low, which is economically disadvantageous compared to the salt electrolysis by the conventional mercury method or the diaphragm method. In order to improve the drawback of low current efficiency, lowering the exchange capacity of sulfonic acid group to below 0.7mg equivalent per 1g of H-type dry resin, for example, decreases the water content in the membrane and the fixed ion concentration in the membrane is higher than that of the membrane with high exchange capacity. Since it is relatively high, the current efficiency during electrolysis is slightly prevented. For example, in the case of obtaining sodium hydroxide with a concentration of 20% by electrolysis of salt, the current efficiency is improved to about 80%.

However, improving the current efficiency by reducing the exchange capacity of the membrane significantly increases the electrical resistance of the membrane and makes it impossible to carry out electrolytic economics.In addition, when the membrane resistance is increased, the current efficiency is improved to around 90%. It was extremely difficult to produce a sulfonic acid type cation exchange membrane.

In order to compensate for the above drawbacks, for example, sulfonic acid cations as described in Japanese Patent Laid-Open Nos. 73-44360, 75-66488, 75-92339, Japanese Patent Application Nos. 75-84111 and 75-84112. It has been proposed that a cation exchange membrane made of a fluorocarbon polymer containing an exchange group having a weaker acidity than a sulfonic acid type on one side of the exchange membrane was used for electrolysis of an aqueous alkali metal halide solution.

According to the electrolytic method using such a cation exchange membrane, the water content decreases on the side where an exchanger with weak acidity exists in the membrane, and high current efficiency is obtained. In addition, the thickness of the exchanger layer is usually 100 A ° or more, and is very thin with respect to the thickness of the entire film, so that the electrical resistance is low, and a low precursor voltage is realized.

However, even in the electrolytic method using such a cation exchange membrane, if the hydrogen ion concentration in the anolyte solution exceeds a certain threshold, the current efficiency is lowered, the voltage is increased, and in some cases, a crack or peeling may occur in a part of the membrane. The threshold depends on several factors, such as temperature, current density, current efficiency, anolyte concentration, thickness of the negative electrode thin layer with weak acidity, thickness of desalination layer, and the like.

High temperatures, high current densities and high current efficiencies are good conditions for the industrial implementation of the process in terms of reduced fixed and operating costs, but under these conditions various problems arise.

This is a serious obstacle to the use of cation exchange membranes industrially. This obstacle is a problem which occurs in the conventional cation exchange membrane method and electrolysis. Therefore, the prior art has not considered this obstacle at all.

The present inventors have reached the present invention as a result of intensive studies on the electrolytic method using such a cation exchange membrane.

That is, the present invention is a cation exchange membrane made of a fluorocarbon polymer having one or more types of sulfonic acid groups or cation exchange membranes having a lower acidity than sulfonic acid groups (this cation exchange membrane has a ratio with respect to the total number of exchange groups of sulfonic acid groups on one side. When electrolytic solution of an alkali metal halide solution is used in the surface layer (A surface), using an electrolytic cell divided into two chambers by a cation exchange membrane characterized by being larger than the other surface layer (B surface).

(1) A metal electrode with dimensional stability is provided as an anode on the side of A surface, and a cathode is installed on the side of B surface.

(2) Holding electrolysis temperature at 60-130 degreeC.

(3) Holding the electrolytic current density at 10-80 A / dm ² .

(4) Adjust the concentration of the anolyte solution so that the hydrogen ions are neutralized at the liquid film interface on the A surface side or on the more anode side.

(5) Holding concentration of anolyte with 1-5 rule.

(6) Holding concentration of catholyte in 1-20 rule.

It is an electrolytic method.

Examples of the cation exchange group having a lower acidity than the sulfonic acid group in the present invention include a carboxylic acid group, a phosphoric acid group, a phosphoric acid group, an alcoholic or phenolic OH group, an SH group, a sulfinic acid group, a sulfonamide group, and an N-monosubstituted sulfonamide group. .

Among these, a good exchange group in terms of performance and stability is a carboxylic acid group and a phosphoric acid group, and a particularly good one is a carboxylic acid group.

또는 -CmH₂m-(m은 1-6의 정수)}]를 함유하고, 잔여의 부분이 -SO₃M(M은 상기의 정의와 같고)를 가지고 타면에 -OCF₂COOM(M은 상기의 정의와 같다)를 함유하고 잔여부분이 -OCF₂CF₂SO₃M(M은 상기의 정의와 같다)를 가진 플로로 카본중합체로 이루어진 양이온 교환막이다.The following are mentioned as a suitable example of the cation exchange membrane used in this invention. That is, one side is -SO ₂ NMR ₁ [M is H, a metal or ammonium ion, R ₁ is -CnHzn + 1 (n is an integer of 0-6), a phenyl group or R ₂ MNO ₂ S- {M is Is the same as R ₂

Or -CmH ₂ m- (m is an integer from 1-6)}] and the remaining portion has -SO ₃ M (M is as defined above) and -OCF ₂ COOM (M is Cation exchange membrane consisting of a fluorocarbon polymer having a residue of -OCF ₂ CF ₂ SO ₃ M (M is as defined above).

On the B side, the ratio of the exchanger whose acidity is weaker than the sulfonic acid group to the total number of exchange groups is usually in the range of 10-100%, preferably 20-100%, particularly preferably 40-100%, and more acidic than the sulfonic acid group. The thickness at which the weak exchanger is present is more than 100 A °. The cation exchange membrane in the present invention may be composed of two or more layers having different exchange capacities or different exchange capacities.

In this case, the B side may be placed in a layer having a low exchange capacity. In addition, the membrane used in the present invention is preferably made of polytetrafluoroethylene fibers and reinforced with a porous film made of polytetrafluoroethylene or the like to increase the mechanical strength.

Moreover, as a method of making an exchange group whose acidity is weaker than a sulfonic acid group, there exists a method to mix | blend and exist a sulfonic acid group uniformly in the whole membrane. There is also a method in which not only the B side but also the A side has an exchanger having a lower acidity than the sulfonic acid group. In order to industrially perform stable electrolysis with less power, it is sufficient to have a low acidity exchanger on the B side as a thin layer of 100 A or more.

According to the method of the present invention, by adjusting the concentration of hydrogen ions in the anolyte solution to neutralize the hydrogen ions passing through the membrane at the A surface side and the more anode side, high current efficiency is maintained and voltage rise is prevented. This peeling can be prevented and stable operation can be performed in the long term.

The present invention will be described in detail again by taking electrolysis of the salt as an example.

When the hydroxy ions passing through the membrane are neutralized by the hydrogen ions in the anolyte solution, three cases of a, b and c are generally considered as shown in FIG. In this figure, a shows a case where neutralization occurs in the membrane, b shows a case where neutralization occurs in the liquid membrane interface, and c shows a case where neutralization occurs outside the membrane. Therefore, the symbol in this figure is as follows. Here, the sign relation of FIG. 1 according to Donnan's membrane equilibrium is as follows.

Ci; concentration in the anolyte solution

; i이온의 액막계면에 있어서의 농도

; Concentration in the liquid film interface of i ion

; i이온의 막내농도

; Intracellular Concentration of Ions

Any of the above (a), (b) and (c) is uniformly dependent on many factors such as temperature current density, current efficiency, anolyte concentration, hydrogen ion concentration in the anolyte solution, and thickness of the desalting layer. In general, the case (a) is likely to occur due to a decrease in the anolyte concentration, a high hydrogen ion concentration in the anolyte solution, or a thinner desalination layer. The influence of temperature and current density is very complex. This is because the diffusion coefficient of the ions changes with temperature, and the change in the thickness of the desalination layer is also affected. The case of (a) is likely to occur when parameters such as temperature and current density are high.

In the above description, the hydrogen ion concentration in the anolyte is controlled by various factors such as temperature, current density, current efficiency, anolyte concentration, thickness of the negative electrode thin layer in which an exchanger with weaker acidity than sulfonic acid group exists, and thickness of the desalting layer. In the case of exceeding the critical hydrogen ion concentration, the neutralization position is deflected on the cathode side in the case of (a), and eventually a considerable amount of hydrogen ions reach the B plane. As a result, a part of the relatively weak acidic group contained in the B surface is converted to H-type, so that it is not dissociated, thereby lowering the current efficiency and increasing the voltage. In contrast to the swelling degree of the portion converted to form B, unlike the swelling of other portions that cause cracking in the film, it is considered that peeling of the B surface occurs when the hydrated sodium ions move violently.

According to the method of the present invention, hydrochloric acid is added to the anolyte solution at the inlet of the anode chamber so that the concentration of hydrogen ions in the cathode chamber is not exceeded. As a result, the hydroxyl ions passing through the membrane are neutralized in the vicinity of the interface liquid film on the A surface side or the interface liquid film on the A surface side, whereby a considerable amount of hydrogen ions can be prevented from contacting the surface layer on the B surface side.

As a result, it is possible to maintain a high current density without peeling the surface layer of the B surface layer of the film and to avoid an increase in voltage, thereby making it possible to perform stable operation in the long term.

The range of hydrogen ion concentration in the anolyte used in the present invention is determined in consideration of many factors such as current density and non-uniformity of hydrogen ion concentration in the anolyte solution together with the above various conditions. 10-80 A / dm ² ; Under the conditions of 1-5 regulation of anolyte concentration and 1-20 regulation of catholyte concentration, the range of 1.0 × 10 ^-1 or less is preferably 0.8 × or less, and the patent is 0.5 × 10 ^-1 or less. Used as The lower limit of the concentration of hydrogen ions in the anolyte solution is determined for reasons such as protecting the metal electrode, especially the noble metal-coated anode, and suppressing the oxygen concentration in the chlorine generated in the anode chamber below a certain value. ^That's about ^-5 .

From these viewpoints, it is preferable that the hydrogen ion concentration in the anolyte is high, and therefore, knowing its upper limit and operating in the vicinity thereof is advantageous and industrially advantageous.

Although it will be described again below to help understand that the upper limit of the concentration of hydrogen ions in the anolyte solution is in the above range, it is difficult and difficult to handle the system in which the mass transfer and the current exist as is well known to those skilled in the art. It is an approximate description and the technical scope of the present invention is not limited to the following description.

In the case of (a), considering the resin of hydrogen ions in the liquid film interface, the formula (1) is established.

+)막내에 침투하는 수소이온 : (i/F)(1-y)Hydrogen ions carried as a current in the anolyte: (i / F) t _H + Hydrogen ions carried as diffusion in the desalination layer: (D _H + / d) (C _H +

Hydrogen ions penetrating into the membrane: (i / F) (1-y)

+)…(1)Thus, (i / F) (1-y) = (i / F) t _H + + (D _H + / d) (C _H +

+)… (One)

Where i is the current density: (A / cm ² )

F is a paradigm constant: 96500 (Kurom / eg)

d is the thickness of the desalting layer: (cm)

y is the yield of sodium ions in the film

D _H + is the diffusion coefficient of hydrogen ions in the liquid: (cm ² · sec ^-1 )

t _H + is the yield of hydrogen ions in the liquid and is approximately represented by the formula (2).

_Wherein D _{Na +} ; D _C1- ) is the diffusion coefficient (cm ² · Sec ⁻¹ ) of sodium and chlorine ions in the liquid, respectively.

However, when neutralization occurs at the liquid film interface (b)

+=0이므로, (1)식으로부터 (3)식이 얻어진다.

Since it is + = 0, (3) is obtained from (1).

C _{H +} (b) = (id / FD _{H +} ) (1-yt _{H +} )... (3)

Therefore, the hydrogen ion concentration C _{H +} (case b) in the anolyte solution obtained in the case (b) is obtained by analyzing the equation (2) (3).

In formula (3), when i becomes large, gas generation becomes violent, and as a result, d becomes small, so that C _{H +} (case b) becomes rather small, for example, when the temperature is 90 ° C. and the current density is 50 A / dm ² . The result of calculating C _{H +} (case b) as a parameter for the value of d is shown in Figs. 2 and 3 for two cases where the concentration of the anolyte solution is 2.0 regulation and 4.0 regulation. As the value of the diffusion constant of ions in the liquid, the following resins are conventionally used to obtain the equivalent conductivity in infinite dilution of each ion.

^{DH +: 19 × 10 -5 (} cm 2 · sec -1)

^{DNa +: 4.1 × 10 -5 (} cm 2 · sec -1)

^{DC1 -: 6.0 × 10 -5 (} cm 2 · sec -1)

In the experiments in which the limit current density was obtained, the thickness of the desalted layer was in the range of approximately 0.5-4 × 10 ⁻² cm under ordinary practical electrolytic conditions. Therefore, CH ⁺ (case b) is determined by the combination of the current efficiency and the concentration of the anolyte. When read at 3 degrees, it is as follows.

In the method of the present invention, the range of hydrogen ion concentration in the anolyte solution that is practically used is generally 1.0 × 10 ⁻¹ or less, preferably 0.8 × 10 ⁻¹ or less, particularly preferably 0.5 × 10 ^−1. Hereinafter, although the upper limit is included in the range of the value of the said table | group of group.

That is, according to the method of this invention, the value of CH + determined from the thickness of the desalting layer in temperature, salt concentration, current density, current efficiency, and electrolytic conditions at that time is selected as an upper limit of hydrogen ion concentration. Therefore, the method according to the present invention neutralizes the hydroxy ions passing through the membrane on the A-side side and the membrane on the anode side, and effectively prevents a large amount of hydrogen ions from reaching the B-side, thereby stably obtaining high current efficiency. And mechanisms for preventing voltage rise and peeling are understood.

Hereinafter, the present invention will be described in detail as follows.

Example 1

Tetrafluoroethylene and perfluoro [3,6-dioxa-4-methyl-7-octensulfonyl fluoride] were used as ammonium persulfate ammonium salts, and ammonium salt of perfluoro octenic acid as emulsifier. And tetrafluoro ethylene are emulsion polymerized at a pressure of 4 atmospheres. As a result of measuring the exchange capacity of the obtained polymer by water washing and saponification after titration, it was 0.83 mg equivalent / g per dry resin.

The copolymer was heat-molded with a 0.3 mm thick film, and then ignited at 60 ° C. for 16 hours in 2.5% caustic soda / 50% methanol, and then H-formed at 90 ° C-16 hours in 2% hydrochloric acid, followed by phosphorus pentachloride. In a 1/1 mixed solution of phosphorus oxychloride, the mixture is heated to reflux for 120 ° C to 40 hours to form a sulfonyl chloride. After completion of the reaction 4 by refluxing in carbon tetrachloride four hours and washed strong absorption of 1420cm ^-1 of fluoride vacuum As a result of measuring a spectrum the outer surface area after drying at 40 ℃ flow sulfonyl group appears at 1060cm ^-1 absorption delivery of one sulfonic acid group It was absorbed and was not stained at all by staining with the crystal bioret.

This film is put together in a frame made of acrylic resin, and is fastened by using polytetrafluoroethylene packing. The mold is immersed in 57% aqueous hydrochloric acid solution and only one side of the membrane is allowed to react at 80 ° C to 30 hours. After a -30 after 60 minutes washing in water ℃ measuring surface area outside the spectrum of the surface treatment film sulfonyl fluoride croissant characteristic absorption band 1420cm absorption of the carboxylic acid to exchange ^-1 1780cm ^-1 disappeared, and instead of the absorption appears in Cleveland STAHL Staining with biolette was a layer of about 25μ on one side of the membrane.

The membrane is saponified at 60 ° C.-16 hours in a 2.5-degree caustic soda / 50-methanol aqueous solution and once again taking the surface infrared spectrum of the treated surface, the absorption of the carboxylic acid exchanger shifts to 1690 cm ⁻¹ . The non-conductivity of this membrane was 10.0 × 10 ^-3 mho / cm as measured in 0.1 N caustic soda solution after oxidation treatment at 90 ° C.-16 h in 2.5 N sodium hydroxide / 12.5% sodium hypochlorite solution.

The measurement of the membrane's non-electroconductivity was carried out through the 1000 cycles of alternating current while maintaining the membrane at 25 ° C in a 0.1-degree caustic soda solution after equilibrating the membrane completely with Na salt, changing the liquid in a 0.1-degree caustic soda solution at room temperature for 10 hours. Measure the resistance.

The Na-type cation exchange membrane was immersed in a 90 ° C.-16 hour aqueous solution of 5.0-degree caustic soda and incorporated into an electrolytic cell having a metal anode with a dimension stability and an iron plate cathode toward the cathode side.

The salt concentration on the anode side is 4 rules, the hydrogen ion concentration is 0.03 regulation, and the alkali concentration on the cathode side is 8 regulations, and it is energized for 300 hours at a temperature of 90 ° C. and a current density of 50 A / dm ² . The current efficiency obtained from the hydrochloric acid addition is 91% and the voltage is stable at 3.7V.

The membrane was taken out after the end of energization, and the surface and the cross section were observed under a microscope, but no abnormality was recognized. Moreover, the thickness of the desalination layer at this time was d = 1x10 <^-2> cm when the current density was changed and measured.

Example 2

In Example (1), electricity was supplied for 200 hours under the same conditions as in Example (1) except that the hydrogen ion concentration in the anolyte solution was set to 0.02.

The current efficiency was 90-91%, the voltage was stable at 3.7V, and no abnormality of the film was observed after energizing.

Reference Example 1

In Example (1), energization was carried out for 300 hours under the same conditions as in Example (1) except that the hydrogen ion concentration in the anolyte was adjusted to 0.10.

The current efficiency was unstable with 82-87% and the voltage was 4.0-4.2V.

When the membrane was taken out after the energization and the surface and the cross-section were observed, it was recognized that the thin layer portions on the cathode surface were peeled everywhere.

Example 3

The sulfonyl fluoride-type membrane obtained in Example (1) was electrolytically joined, and ammonia gas flowed to one side to react for 10 hours at room temperature.

The membrane is saponified at 90 ° C.-16 hours in 2.5 N caustic caustic / 30% DMSO and introduced into the precursor and energized for 300 hours under the same conditions as in Example (1).

The current efficiency was 86%, the voltage was 3.2V and stable. After the energization, the membrane was observed in detail under a microscope, but no abnormality was recognized.

Reference Example 2

The membrane obtained in Example (3) was energized for 300 hours under the same conditions as in Reference Example (1).

The current efficiency was 81-82% and the voltage was 3.5-3.6V.

When the surface and the cross section of the membrane were observed under a microscope after the energization, it was recognized that the surface had peeling everywhere.

Example 4

The sulfonyl fluoride type membrane obtained in Example (1) was reacted with n-butylamine at room temperature for 1 hour and then washed with water.

Under the same conditions as in Example (1) except that the film was saponified at 90 ° C.-16 hours in ca. 30% caustic ca./30% DMSO, and incorporated into the precursor to give a hydrogen ion concentration of 0.010 in 30 A / dm ² anolyte. Turn on for 200 hours

The current efficiency was 90% and the voltage was 3.2V. After energization, the membrane was taken out and observed under a microscope, but no abnormality was recognized.

Reference Example 3

Except for setting the membrane obtained in Example (4) to 0.40 hydrogen ion concentration in the anolyte solution, it was energized for 200 hours under the same conditions as in Example (4), and the B surface was significantly peeled off when the membrane was observed.

Example 5

By the same polymerization method as in Example (1), a copolymer of an exchange capacity of 0.74 mg equivalent / g dry resin (hereinafter abbreviated as polymer 1) and 0.91 mg / equivalent dry resin (hereinafter abbreviated as polymer 2) was obtained.

Sulfonyl fluoride type polymers 2 and 2 are molded into 2 mm and 4 mm films, and both films are heat-molded to prepare a composite membrane. Gauge weave woven fabric of 25 vertically and horizontally per multi-filament of 400 filament multi-filament total yarn 200 denier under the side of Polymer 2 of the composite membrane. It is placed on a 0.15 mm polytetrafluoroethylene fabric and heated to 270 ° C. under vacuum to reinforce and intervene in the interior of Polymer 2.

The membrane was changed to sulfonyl chloride type in the same manner as in Example (1), and 57% of the aqueous hydroiodic acid solution was reacted with 80 ° C. for 20 hours, and then saponified and oxidized. A cation exchange membrane having a carboxylic acid layer of mm is obtained.

The carboxylic acid layer of this membrane was brought to the cathode side at a temperature of 90 ° C., a current density of 50 A / dm ² , a salt concentration of 3% in the anolyte solution, a hydrogen ion concentration of 0.01% and an alkali concentration of 8% for 300 hours. When energized, the current efficiency was 95% and the voltage was 3.8V.

Moreover, the thickness of the desalination layer at this time was d = 0.9 * 10 <^-2> cm when measured by the method similar to Example (1).

After energization, the membrane was taken out and observed, but no abnormality was recognized.

Reference Example 4

The current efficiency was 87% and the voltage was 4.3V when the membrane obtained in Example (5) was used and energized for 300 hours under the same conditions as in Example (5) except that the hydrogen ion concentration in the anolyte solution was set to 0.10.

When the cross section of the membrane was observed under a microscope after the end of energization, the carboxylic acid layer on the cathode side was peeled off.

Example 6

A membrane having a phosphate group on the surface layer of 0.2 mm and a residue having a sulfonic acid group is used.

The side having the phosphate group was directed to the cathode, and when it was energized for 200 hours under the same conditions as in Example (1), the current efficiency was 90% and the voltage was stable at 3.7V.

After the energization, the membrane was taken out and observed, but no abnormality was recognized.

At this time, the thickness of the desalination layer was d-1.0 × 10 ⁻² cm when measured in the same manner as in Example (1).

In Fig. 3, the value of CH ⁺ corresponding to d = 1.0 × 10 ⁻² cm and 90% of current efficiency is 0.025, and 0.013 meets the requirements of the present invention.

Reference Example 5

In Example (6), electricity was supplied for 200 hours under the same conditions as in Example (1) except that the hydrogen ion concentration was defined as 0.05. The current efficiency was 83% and the voltage was 4.1V. When the membrane was taken out and observed after energization, the surface layer portion having a phosphate group was partially peeled off.

Example 7

In Example (6), when the membrane containing a carboxylic acid group other than a phosphoric acid group was used, the same result as Example (6) was obtained.

Claims (1)

One having one or more cation exchange groups which are less acidic than sulfonic acid groups and sulfonic acid groups, and the proportion of sulfonic acid groups relative to the total cation exchanger is higher at the surface layer portion at the anode side of the membrane than at the surface layer portion at the cathode side of the membrane. In the electrolysis of NaCl solution in the electrolytic cell in which the electrolytic cell is divided into the anode chamber and the cathode chamber by the cation exchange membrane composed of the polymer, the anode is neutralized at the anode side rather than the liquid membrane interface on the anode side or the liquid membrane interface. A method of electrolyzing an aqueous NaCl solution, characterized by adjusting the concentration of hydrogen ions in the liquid.

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