JPS6256184B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a roughened cation exchange membrane suitable for electrolysis of aqueous alkali chloride solutions.

A method of producing alkali hydroxide by electrolyzing an aqueous alkali chloride solution in an electrolytic cell divided into an anode chamber and a cathode chamber by a cation exchange membrane (ion exchange membrane method)
In recent years, many attempts have been made to save energy, and in particular, methods of reducing electrolysis power by lowering the electrolysis voltage as much as possible are attracting attention. Various methods have been proposed in the past, such as considering the material, composition, and shape of the anode and cathode, or specifying the composition of the cation exchange membrane and the type of ion exchange group used. ,
Although all of them have certain effects, they are not necessarily fully satisfactory industrially.

On the other hand, in recent years, the anode and cathode have been moved as close as possible,
The mainstream method is to minimize the electrolytic voltage portion due to the resistance of the electrolytic solution and bubbles existing between the two electrodes. As an ideal method, a method called SPE electrolysis has been proposed, which attempts to minimize the resistance between the electrodes by integrating the cation exchange membrane and the anode and cathode, but there are still many problems that still need to be solved. industrialization is difficult.

Therefore, instead of integrating the membrane and electrode, it has been proposed to bring the electrode and membrane as close as possible or to treat the membrane surface in order to bring them into close contact for electrolysis. For example, methods for roughening the membrane surface (JP-A-55-110786, JP-A-56-116891, JP-A-57-
70285), a method of forming a porous layer made of metal oxide on the surface (Japanese Patent Application Laid-Open No. 108888/1983), etc.

Surface-treated membranes prepared by any of the above methods can prevent a large voltage increase due to electrolytically generated bubbles that normally occur when the electrode and membrane are brought close to each other.

However, according to studies conducted by the present inventors, the roughened film using the blasting method often damages the film because particles collide with the film at high speed during blasting, forming unevenness. This method results in a decrease in efficiency and is not yet a perfected method. Furthermore, even if the surface is roughened by removing particles such as aluminum, zinc, tin, or nickel after being heated and compressed, the voltage reduction value will be small if you try to maintain high current efficiency. However, this is still not a satisfactory method.

Furthermore, embossing, which is a common surface processing method for plastic film, cannot form desired irregularities, and it is often difficult to completely prevent the voltage increase that occurs when the electrode and the film are brought close to each other. in this way,
For example, it is difficult to simply apply surface roughening, which is a long-known method of making a film surface hydrophilic, as seen in British Patent No. 851021, to cation exchange membranes.

On the other hand, when a porous layer made of metal oxide is formed on the surface, there always remains the problem that the attached porous layer peels off over time.

The present inventors continued their research on a method of manufacturing a membrane that does not have these disadvantages and has the electrolytic voltage as low as possible, and found that specific powder particles were selected and applied to the surface of the base cation exchange membrane. After heating and pressing the particle layer, we have discovered a method that can be economically scaled up without causing a voltage increase even when the electrode and membrane are brought close together and maintaining high current efficiency by removing the particle layer. . By this method, it became possible to perform surface treatment on cation exchange membranes with good reproducibility, but while continuing research for further improvements, they discovered a method that could reduce the voltage even further and completed the present invention.

That is, the present invention provides a roughened cation exchange membrane obtained by heating and pressing silica powder onto the surface of a base cation exchange membrane used in aqueous alkali chloride solution electrolysis, and then eluting the silica powder with an aqueous caustic alkali solution. This is a method for producing a roughened cation exchange membrane characterized by boiling the membrane in a 1 to 10% by weight aqueous caustic solution, which only prevents a large increase in voltage when the electrode and membrane are brought close together. Instead, the present invention provides a surface-treated cation exchange membrane that effectively reduces electrolysis voltage and produces high-quality caustic alkali.

For the base cation exchange membrane used in the present invention, a perfluorocarbon polymer having excellent heat resistance, chemical resistance, mechanical strength, etc. is usually used.

The perfluorocarbon polymer has a cation exchange group and/or a group that can become a cation exchange group, and these groups include a sulfonic acid group (-SO ₃ M, where M is a hydrogen atom or a metal atom),
−SO ₂ F, − which is a precursor of sulfonic acid group
SO ₂ Cl, carboxylic acid group (-COOM, where M is a hydrogen atom or metal atom), -COF, -COOR (R has 1 to 1 carbon atoms), which is a precursor of a carboxylic acid group
5) and -CN. Furthermore, examples of the polymer include polymers represented by the following general formula.

[However, R' = -CF ₃ , -CF ₂ -O-CF ₃ n = 0 or 1 to 5 m = 0 or 1 o = 0 or 1 p = 1 to 6 X = -SO ₃ M (M is hydrogen atom or metal atom), -SO ₂ F, -SO ₂ Cl -COOM (M is a hydrogen atom or metal atom), -COOR ₁ (R ₁ = alkyl group of 1 to 5), -CN, -COF], A polymer obtained by adding a third or fourth component to the above two-component system can also be used.

Specifically, the following can be shown, for example.

(Group A) (Group B) In these polymerizations, the ion exchange capacity
It is preferable to adjust the amount to 0.5 meq/g dry resin to 1.5 meq/g dry resin.

In the present invention, these polymers molded into a membrane can of course be used alone, but
A polymer having a mixture of a sulfonic acid group or a group that can be converted into this group and a carboxylic acid group or a group that can be converted into this group, preferably a polymer having a sulfonic acid group or a group that can be converted into this group, and a carboxylic acid group or a group that can be converted into this group. It is also possible to use a polymer having a layer on each side of the polymer having a group that can be converted into a group.

Such a film-like material is composed of a polymer having a sulfonic acid group or a group that can be converted into this group (for example, a group (A) polymer), and a polymer having a carboxylic acid group or a group that can be converted to this group (for example, (B) group polymer) can be obtained by forming each into a film shape and then gluing them together, or a film of a polymer having only a sulfonic acid group or a group that can be converted into such a group. It can also be obtained by chemically treating only one side of the compound and converting these groups into carboxylic acid groups or groups that can be converted into carboxylic acid groups.

Furthermore, by chemically treating only one side of the polymer film having only carboxylic acid groups or groups that can be converted into such groups, these groups can be converted into sulfonic acid groups or groups that can be converted into such groups. You can get it even if you twist it. Also, the thickness of the membrane used is 50μ
~500μ is generally used, and an appropriate thickness is selected by considering the specific conductivity and current efficiency of the film.

When roughening the surface of a cation exchange membrane, it is most important to select the type of powder particles and the paper that is the carrier for the powder. As a powder particle, the average particle size is 0.01
~20μ, preferably 0.1-10μ silica is used. The silica powder is once formed as a particle layer on the carrier. If silica powder is directly formed on the base cation exchange membrane by means such as coating, the cation exchange membrane will wrinkle or the particle layer will crack, making it impossible to obtain the desired surface roughening. As a carrier,
It is preferable to use paper or art paper (trade name). With other papers, it is difficult to obtain a uniform silica particle layer, and if the particle layer is dried and handled, the particles may fall off or the particle layer may crack, which is undesirable.

In addition, when using powder particles other than silica, such as aluminum, zinc, nickel, tin, etc., even if paper or art paper is used, if the particle layer is dried and handled, the particles may fall off or the particles may fall off. It is not suitable as a surface roughening material because the layer cracks.

As described above, in the case of a combination of silica and paper or silica and art paper, an optimal surface roughening particle layer can be formed.

The thickness of the particle layer formed on the carrier is 5μ to 250μ
μ is preferred. If it is less than 5μ, it is difficult to uniformly roughen the surface of the cation exchange membrane, and
If it is more than 250μ, cracks will occur in the silica particle layer, which is not preferable. Within this range, a sufficient surface treatment effect can be obtained even if the depth of the unevenness varies.

If the particle layer formed on the carrier contains water, it may generate water vapor and break the particle layer during heating and pressure bonding, so it is necessary to dry it beforehand.

The silica powder particle layer formed on paper or art paper is heated and pressed onto the base cation exchange membrane to form such a particle layer on the surface of the base cation exchange membrane. The pressure bonding method may be either a press method or a roll method, which is appropriately selected depending on the membrane form of the base cation exchange membrane. The compression conditions are appropriately selected depending on the form of the cation exchange group, but the temperature is 100%.
~200° C. and a pressure of 5 to 100 Kg/cm ² are preferred.

The silica particle layer formed on the surface of the cation exchange membrane is dissolved and removed in a caustic aqueous solution with a concentration of 1 to 30% by weight at a temperature of 20 to 90°C.

By further treating the cation exchange membrane whose surface has been roughened by the above method in the next step, a high-performance surface-treated ion exchange membrane with a reduced absolute value of voltage can be obtained.

This step involves boiling the roughened cation exchange membrane under normal pressure in a dilute caustic aqueous solution having a concentration of 1 to 10% by weight. Processing time is 0.5 ~
10 hours is recommended. If boiling in water instead of a dilute caustic aqueous solution causes the roughened cation exchange membrane to swell too much, resulting in a high water content, the current efficiency, especially immediately after the start of electrolysis, may decrease and damage the anode. , Also, the current efficiency does not recover easily.

If the concentration of the dilute caustic aqueous solution exceeds 10%, there will be no treatment effect and no voltage reduction effect can be expected. Further, if the treatment temperature is lowered to, for example, 80 to 90° C. under normal pressure, the voltage reduction effect cannot be obtained, which is not preferable. Furthermore, it is preferable to take out the boiled membrane and dry it in air under mild conditions at a temperature of 10 to 50°C. If such a drying step is performed, high current efficiency can be obtained immediately after the start of electrolysis.

The roughening applied to the surface of the cation exchange membrane through the above treatment means that the depth or height from the membrane surface is on average 0.1 to 20 μ, and the average roughening is 10 3 to 10 ³ per cm ² of the membrane surface.
It consists of 10 ^{to 15} minute irregularities, and its cross-sectional shape is
It has an irregular circular shape.

These surface irregularities can be approximately measured using a surface profile measuring device (roughness meter), but in order to accurately judge the extent of the effect, there is a method to determine the depth or height and density from surface and cross-sectional photographs taken using an electron microscope. It is better to adopt it.

The surface roughening of the present invention may be applied to only one side of the membrane, or may be applied to both sides. When applying to both sides, it is preferable to heat and press the silica powder layer on both sides at the same time. When roughening only one side,
It is used by arranging it so that the roughened surface faces the cathode side.

The surface-treated cation exchange membrane obtained as described above is used as a diaphragm for dividing an anode chamber and a cathode chamber in an electrolysis process of an aqueous alkali chloride solution. In this case, the cathode used may be one that can withstand the operating environment, has sufficient catalytic action for the reaction, and has a structure that does not hinder the escape of the produced gas, and may be any commonly used cathode. Bye. Examples include, but are not limited to, materials such as iron, mild steel, nickel, and stainless steel, and porous materials such as wire mesh, expanded metal, lattice, vertical rail, and punched metal. isn't it.

As for the anode, a normal anode that can withstand the usage environment and has sufficient catalytic activity for the desired reaction is used. A porous anode coated with a platinum group metal such as platinum, palladium, ruthenium, or iridium, an oxide of a platinum group metal, or a mixture of an oxide of a platinum group metal and an oxide of a valve metal is used.
During electrolysis, these electrodes may be in contact with the membrane surface or may be apart.

The method of the present invention will be explained below using specific examples. Note that the present invention is in no way limited to these specific examples.

Example 1 CF ₂ = CF ₂ and were copolymerized in 1,1,2-trichloro-1,2,2-trifluoroethane using perfluoropropionyl peroxide as an initiator to obtain a polymer (exchange capacity as sulfonic acid group was 0.91meq/g).
dry resin). This is called Polymer A.

Similarly, CF ₂ = CF ₂ (exchange capacity as carboxylic acid group was 1.1 meq/g). This will be referred to as B polymer.

Next, polymer A is formed into a film with a thickness of 100 μm and polymer B is formed with a thickness of 75 μm, and then these two films are stacked and thermocompressed to form a single film, which is used as a base cation exchange membrane. .

Fine silica powder with an average particle size of about 5 μm was kneaded with water to form a paste of 15% by weight, and then applied onto art paper to obtain a silica particle layer with a thickness of about 50 μm. The art paper carrying the silica particle layer was applied to both sides of the base cation exchange membrane and heated at 160°C and 20Kg/cm ² .
It was crimped. Thereafter, in a 5% by weight aqueous sodium hydroxide solution at a temperature of 80° C., the silica powder pressed onto the surface of the cation exchange membrane was dissolved and removed, and at the same time hydrolysis was carried out.

Next, the cation exchange membrane was boiled in a 2% by weight aqueous sodium hydroxide solution for 3 hours, washed with water, and dried in air at 25°C for 2 days.

The obtained roughened cation exchange membrane was immersed in a 2% by weight aqueous sodium hydroxide solution overnight, and the cation exchange membrane was assembled into an electrolytic cell with the B polymer side facing the cathode.
Using expanded titanium metal coated with ruthenium oxide as the anode and expanded metal made of iron as the cathode, the distance between the anode and cathode was set to 1 mm, and the extraction level of the caustic soda aqueous solution in the cathode chamber was made 20 cm higher than the level in the anode chamber. Electrolysis was performed with the membrane in contact with the anode.

Saturated salt solution is supplied to the anode chamber and water is supplied to the cathode chamber to maintain the caustic soda concentration in the cathode chamber at 33% by weight, and the temperature is 90%.
When electrolyzed at a temperature of 40 A/dm ² at a current density of 40 A/dm 2 , the voltage was 3.25 V and the current efficiency was 96.5%.

Comparative Example 1 Zinc powder with an average particle size of 7 μm was suspended in water to a concentration of 2% by weight, and a layer of zinc powder particles was formed on paper by the filtration method. was heated and crimped.

Next, the zinc powder was dissolved and removed in a 20% by weight aqueous sodium hydroxide solution at 80°C, and was hydrolyzed by treatment in a 10% by weight aqueous sodium hydroxide solution at 90°C for 24 hours.

Electrolysis was carried out under exactly the same conditions as in Example 1, and a current density of 40 A/dm ² , a voltage of 3.30 V, and a current efficiency of 94.0% were obtained.

Comparative Example 2 The roughened cation exchange membrane was treated in exactly the same manner as in Example 1, except that it was boiled in water for 3 hours instead of the 2% by weight aqueous caustic soda solution, and electrolysis was carried out under the same conditions as in Example 1. , voltage at current density 40A/ ^dm2
We obtained results of 3.24V and current efficiency of 93.0.

Comparative Example 3 The roughened cation exchange membrane was treated in exactly the same manner as in Example 1, except that the roughened cation exchange membrane was boiled for 3 hours in a 20% by weight aqueous sodium hydroxide solution instead of a 2% by weight aqueous sodium hydroxide solution. Electrolysis was performed under the following conditions, and results were obtained at a current density of 40 A/dm ² , a voltage of 3.34 V, and a current efficiency of 96.5.

Example 2 0.3% by weight of silica fine powder with an average particle size of 3μ or less
A layer of silica particles with a thickness of approximately 70 μm was formed on paper by a filtration method. The paper carrying the silica particle layer was applied to both sides of the same base cation exchange membrane as in Example 1, and heated and pressed at 160° C. and 100 kg/cm ² . Thereafter, the silica powder pressed onto the surface of the cation exchange membrane was dissolved and removed in a 5% by weight aqueous sodium hydroxide solution at a temperature of 80°C.

Next, the cation exchange membrane was boiled in a 5% by weight aqueous solution of caustic soda for 3 hours, and then boiled in air at 25% by weight.
It was dried for 2 days at ℃. The obtained roughened cation exchange membrane was immersed in a 2% by weight aqueous sodium hydroxide solution overnight, and then electrolyzed under the same conditions as in Example 1.
Current density 40A/ ^dm2 , voltage 3.21V, current efficiency
I got a result of 96.0.

Example 3 A SO 3 H/SO ₃ H/C cation exchange membrane having a carboxylic acid layer with a thickness of approximately 40μ was prepared by chemically treating one side of a Nafion 117 cation exchange membrane commercially available from DuPont, USA.
A cation exchange membrane with a COOH two-layer structure was obtained. The film was subjected to surface roughening treatment in exactly the same manner as in Example 2,
Electrolysis was carried out under the same conditions as in Example 2, and the current density was
At 40 A/dm ² , a voltage of 3.28 V and a current efficiency of 97.0 were obtained.

Example 4 Example 1 except that the drying step after boiling treatment was removed
The surface was roughened in exactly the same manner as in Example 1, and electrolyzed under the same conditions as in Example 1. Initially, both voltage and current efficiency were low, and it took about a week for performance to recover. The electrolysis performance for 7 days was 3.24V voltage, 96% current efficiency, and 6
This performance was maintained over a long period of several months.

Comparative Example 4 In Example 1, both sides of the cation exchange membrane were not treated in any way, and electrolysis was carried out in exactly the same manner as in Example 1, with a current density of 40 A/dm ² , a voltage of 3.65 V, and a current efficiency of 3.65 V.
I got a result of 96.0.

Claims (1)

[Claims] 1. A silica powder layer formed by mixing silica powder and water to form a suspension or paste mixture, supporting the mixture on art paper or paper, and drying the silica powder layer is coated with a cation exchange group. and/or a roughened surface obtained by heating and press-bonding the surface of a perfluorocarbon polymer membrane having a group that can become a cation exchange group, and eluting the silica powder formed on the membrane surface with a caustic aqueous solution. A method for producing a roughened cation exchange membrane, which comprises boiling the ion exchange membrane in a 1 to 10% by weight aqueous caustic solution. 2. The method according to claim 1, which comprises washing with water and drying after the boiling treatment.