JPH05125579A - Method for restarting electrolytic cell - Google Patents

Abstract

PURPOSE:To restart an electrolytic cell after temporarily stopped so that stabilized electrolytic performance is exhibited even after restaring at the time of directly producing a >=42wt.% alkali hydroxide by ion-exchange membrane electrolysis. CONSTITUTION:A double-layer cation-exchange membrane of a cation-exchange layer of a fluorine-contg. polymer having a carboxyl group and a hydrophilic porous body on the cathode side is used, and a >=42wt.% alkali hydroxide is produced in the cathode chamber of the electrolytic cell. When the cell is restarted, the alkali hydroxide in the cathode chamber is kept at <=25wt.%, the temp. is controlled to 20-80 deg.C, and a current is applied.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a temporary electrolytic cell for directly producing 42% by weight or more of high concentration alkali hydroxide in a cathode chamber by electrolysis by ion exchange membrane method electrolysis using a fluorinated cation exchange membrane. It is related to the method of restarting when it is stopped temporarily.

[0002]

2. Description of the Related Art So-called ion exchange membrane method alkaline electrolysis in which a fluorinated cation exchange membrane is used as a diaphragm to electrolyze an aqueous solution of alkali chloride to produce alkali hydroxide and chlorine, Since it can be manufactured with lower energy consumption than the conventional methods up to the above, it is becoming internationally popular in recent years.

In such an ion exchange membrane method alkaline electrolysis, a fluorine-containing ion exchange membrane having a sulfonic acid group as an ion exchange group was used in the early days, but it was difficult to increase the current efficiency. It was Therefore, in recent years, it has been changed to a cation exchange membrane having a carboxylic acid group as an ion exchange group, and as a result, the concentration of alkali hydroxide to be produced is 30 to
At 35% by weight, the current efficiency reaches 93 to 97%, which is almost industrially completed.

On the other hand, in a commercial ion-exchange membrane electrolysis process for obtaining alkali hydroxide, in addition to controlling the alkali hydroxide concentration (catholyte concentration) and temperature during steady operation to appropriate conditions, the electrolysis is stopped. The operation method during non-steady state such as restarting is an important factor for obtaining stable electrolysis performance for a long time.

Regarding the operation method at the time of restarting, generally,
It is preferable to re-energize under the condition as close as possible to the condition at the time of steady operation where high electrolysis performance can be obtained.
In the case of obtaining 0 to 35% by weight of alkali hydroxide, electricity is supplied again under conditions of a catholyte concentration of 25 to 35% by weight and a temperature of 50 to 80 ° C., and then reused. Further, within this condition range, when the concentration of the catholyte is low, the temperature at the time of restarting is lowered to prevent the performance from being deteriorated due to the swelling of the film. In addition, when the concentration of the catholyte is high, the temperature is raised to reduce the electric shock received by the film during re-energization.

On the other hand, in recent years, a fluorine-containing cation exchange membrane having a hydrophilic porous layer provided as a diffusion layer on the cathode side surface has been used, and alkali hydroxide having a concentration of more than 42% by weight can be obtained with a current efficiency of 93% or more. An electrolysis method of directly producing by electrolysis has been disclosed (JP-A-63-310985, JP-A-1-
242794).

However, although these electrolysis methods have shown preferable operating conditions in steady operation, none of them have been shown for unsteady operation such as restarting.

According to the research conducted by the present inventor, the above concentration of 42
In the electrolytic process with a weight percentage of 30% or more, the conventional method is 30.
It has been surprisingly found that the electrolytic performance is significantly reduced when the re-running is performed under conditions close to the steady-state operating conditions used in the ~ 35 wt% electrolysis process. That is, after the electrolysis is stopped, a condition close to a steady operation condition (for example, catholyte alkali hydroxide concentration 40 to 50% by weight, temperature 75 to 9)
It was found that re-energization at 0 ° C. significantly reduced the current efficiency and, in some cases, increased the electrolysis voltage.

The cause of this phenomenon is not clear yet,
It can be considered as follows. That is, in an ion-exchange membrane electrolyzer for producing an alkali hydroxide having a concentration of 42% by weight or more, when electrolysis is stopped, electroosmotic water flowing from the anode to the cathode stops, so that 42% by weight is left on the cathode side of the ion-exchange membrane. It will come into direct contact with excess alkali hydroxide, resulting in extreme dehydration of the film. For this reason, the alkali metal ions in the film are in a very difficult state to move. It is presumed that when current is applied again in such a state, the mobility of the hydroxide ions is relatively increased, so that the current efficiency is lowered.
Further, it is presumed that the reason why the electrolysis voltage generated in a certain case is increased is that the membrane contracts due to the dehydration.

[0010]

DISCLOSURE OF THE INVENTION The present invention is based on an ion exchange membrane method electrolysis using a fluorine-containing cation exchange membrane to produce an alkali hydroxide having a concentration of 42% by weight or more, particularly 45% by weight to 55% by weight. An object of the present invention is to provide a novel restarting method for temporarily exhibiting stable and high electrolytic performance even after restarting when the electrolytic cell manufactured by electrolysis is temporarily stopped and restarted.

[0011]

The present invention thus provides an electrolytic cell for producing a high-concentration alkali hydroxide of 42% by weight or more in the cathode chamber by the ion exchange membrane method electrolysis using a fluorine-containing cation exchange membrane. It is characterized in that when it is restarted after being temporarily stopped, the cathode chamber alkali hydroxide concentration is maintained at 25% by weight or less and re-energization is performed.

In the present invention, the fluorinated cation exchange membrane is a multi-layered cation composed of a cation exchange layer made of a fluoropolymer having at least a carboxylic acid group and a hydrophilic porous layer disposed on the cathode side thereof. Preference is given to using exchange membranes. The porous layer on the cathode side is effective for acquiring over 42% by weight of alkali hydroxide with high current efficiency for a long period of time.

A fluorine-containing carboxylic acid polymer film or a fluorine-containing sulfonic acid polymer film having a smaller specific resistance than the above-mentioned fluorine-containing polymer having a carboxylic acid group in order to reduce the film resistance and impart a large film strength, A cation exchange membrane having a multilayer structure in which a film made of a mixture thereof is laminated on the anode side can also be used, and these cation exchange membranes are woven fabrics made of a fluorine-containing polymer having corrosion resistance such as polytetrafluoroethylene or the like. It can be reinforced with non-woven fabric.

In the present invention, the fluorinated carboxylic acid polymer and the fluorinated sulfonic acid polymer constituting the cation exchange membrane are preferably copolymers having the following polymer units (a) and (b): Become.

(A)-(CF ₂ -CXX ')-, (b)
_{- {CF 2 -CX (Y-} A)} -

Here, X and X'are -F, -Cl, -H or -CF ₃ , and A is -SO ₃ M or -CO ₂ M (M.
Represents a group which is converted into these groups by hydrogen, an alkali metal or hydrolysis), and Y is selected from the following, where Z and Z ′ are —F or a group having 1 to 10 carbon atoms. It is a perfluoroalkyl group, and x, y, and z are 1 to 10
Represents the integer.

-(CF ₂ ) _x- , -O- (CF ₂ ) _{x
-, - (O-CF 2} CFZ) x -, - (O-CFZ-CF 2) x -O- (CFZ ') y -

Further, in addition to the polymer units of (a) and (b), the following polymer units may be contained.

-{CF ₂ -CF (O-Z)}-,-{CF ₂ -CF (O-CF ₂ -CFZ ') _x -O-
Z}-

The above polymer is formed (a) /
The composition ratio (molar ratio) of (b) is preferably a fluoropolymer, preferably 0.5 to 4.0 meq / g dry resin, and particularly 0.7.
Selected to form an ion exchange capacity of ˜2.0 meq / g dry resin.

The above-mentioned fluorine-containing polymer is preferably a perfluorocarbon polymer, and preferable examples thereof include CF ₂ ═CF ₂ and CF ₂ ═CFOCF ₂ CF (CF
₃ ) Copolymer with OCF ₂ CF ₂ SO ₂ F, CF ₂ ═C
F ₂ and CF ₂ = CFO (CF ₂₎ a copolymer of _{2 ~ 5 SO 2 F, CF} 2 = CF 2 and _{CF 2 = CFO (CF 2)} 1
~ ₅ COOCH ₃ copolymer, CF ₂ = CF ₂ and CF
₂ = CFO (CF ₂ ) ₂ to ₅ CO ₂ CH ₃ copolymer, and further CF ₂ ═CF ₂ and CF ₂ ═CFOCF ₂ C
A copolymer with F (CF ₃ ) OCF ₂ CF ₂ COOCH ₃ is exemplified.

On the other hand, the hydrophilic porous layer disposed on the cathode side of the cation exchange layer is preferably formed from a fluorine-containing polymer having inorganic particles and a hydrophilic group. In particular, it is preferable that the hydrophilicity is —SO ₃ M (M is the same as above) from the viewpoint of stability in an alkali hydroxide of more than 42% by weight.

The porous layer composed of the inorganic particles and the fluoropolymer having a hydrophilic group can be produced by various methods. For example, a method of casting a film from a mixture in which inorganic particles or fibrils, which are preferably alkali-resistant, are dispersed in a solution of a fluoropolymer having a hydrophilic group, inorganic particles are mixed with a fluoropolymer having a hydrophilic group, and kneaded. After that, a method of heating and molding to form a thin film, followed by stretching to form a porous film, a porous layer of inorganic particles is formed using an appropriate water-soluble binder such as methylcellulose, and a hydrophilic layer is formed between the two porous layers. A method of sandwiching between films of a fluorine-containing polymer having a group, heating, and pressure bonding may be mentioned.

The inorganic particles are preferably oxides, nitrides, carbides, hydroxides or oxides of elements of the IIa, IIIb, IVb, Vb, IIIa and IVa groups of the long period type periodic table and after the third period. It is selected from boron carbide alone or a mixture thereof, and specifically preferable examples thereof include oxides of titanium, zirconium, niobium, hafnium, tantalum, indium, tin, etc., nitrides, carbides, hydroxides, and silicon. It is selected from oxides and carbides alone or as a mixture.

The multilayer structure of the cation exchange layer and the porous layer can be formed by heating and press-bonding the porous layer and the cation exchange layer obtained above. Further, when the porous layer is formed by a casting method, a mixed solution of inorganic particles and a fluoropolymer having a hydrophilic group can be directly applied to the cation exchange layer to form a multilayer by drying, Furthermore, in order to enhance the adhesive force between the porous layer and the cation exchange layer, a heat compression treatment may be performed.

The above-mentioned multi-layered membrane of the cation exchange layer and the porous layer can be used as it is, but preferably, at least one surface of the cation exchange membrane, particularly preferably, the surface of the anode side is exposed to chlorine gas. By performing the processing for
The long-term stability of current efficiency can be further improved.

As a method for treating the surface of the cation exchange membrane for releasing gas, a method of forming fine irregularities on the membrane surface (Japanese Patent Publication No. 60-26495) or a liquid containing iron, zirconia or the like in the electrolytic cell is used. A method for supplying hydrophilic inorganic particles to the surface of the membrane (JP-A-56-152980) and a method for providing a porous layer containing particles having gas and liquid permeable electrode activity, but not (JP-A-56-75583). And JP-A-57-391
85) and the like. The gas release layer on the surface of the cation exchange membrane can further improve the voltage under electrolysis in addition to the effect of improving the long-term stability of the current effect.

In the present invention, as the electrolysis conditions for producing an alkali hydroxide having a concentration of 42% by weight or more by using the above cation exchange membrane, known conditions as described in JP-A No. 54-112398 mentioned above are used. Can be adopted. For example, the anode chamber is preferably supplied with an aqueous solution of 2.5-5.0 normal (N) alkali chloride, and the cathode chamber is supplied with water or diluted alkali hydroxide or without these, preferably 50 It is electrolyzed at ˜120 ° C. and 5˜100 A / dm ² . In such a case, calcium and magnesium in alkali chloride, impurities heavy metal ions such as iodide ions,
Since it causes deterioration of the ion exchange membrane, it is preferable to make it as small as possible.

The present invention provides an operating method in the case where the electrolysis for producing the high-concentration alkali hydroxide is temporarily stopped for various reasons and then restarted. As a result of repeated studies on the re-use method that produces stable performance, the catholyte solution was re-energized under conditions where the alkali hydroxide concentration was greatly reduced from that during steady operation, and then the catholyte solution was brought to the desired steady operation concentration. It was found that high electrolytic performance can be maintained even after re-use by increasing the alkali hydroxide concentration.

The concentration of the alkali hydroxide in the catholyte upon re-energization is 2
It is preferably 5% by weight or less, particularly 10 to 2
0% by weight is preferred. If it is less than 10% by weight, blister may occur in the film, which is not preferable.

When re-energized, the temperature of the catholyte alkali hydroxide is 2
The temperature is preferably 0 to 80 ° C, and more preferably 30 to 70 ° C. At 80 ° C. or higher, irreversible swelling occurs in the membrane due to the fact that the catholyte concentration is significantly lower than the high concentration in steady operation, and the current efficiency is reduced. Cooling equipment is required to reduce the temperature to 20 ° C. or lower, which is not practical.

After re-energizing at the above-mentioned alkali hydroxide concentration and temperature in the cathode chamber, the concentration of the catholyte can be increased to a desired concentration by adjusting the amount of diluted alkali or water added to the cathode chamber. During this time, the temperature also rises due to the increase in electrolysis voltage accompanying the increase in concentration. In the present invention, when the concentration of the catholyte is 30% by weight, 70 ° C.
It has been found that it is preferable to go through the process of keeping the temperature above because high current efficiency can be obtained after re-use.

[0033]

In the present invention, the reason why stable electrolysis performance is obtained by re-electrolyzing under the condition that the concentration of the catholyte is 25% by weight or less is not always clear. However, according to the present invention, the dehydration state on the cathode side of the membrane is It is considered that, since the water content is restored to an appropriate state by energization at a low concentration of the catholyte alkali hydroxide, the alkali metal ions are also easily moved.

[0034]

The present invention will be further described in the following examples, but the present invention is not limited to these examples. In the electrolysis in Examples and Comparative Examples, an electrolytic cell equipped with a temperature control device having an effective energization area of 0.25 dm ² was used, and titanium was punched metal (minor diameter 4 mm, major diameter 8 mm) and oxidized with ruthenium oxide. The one coated with a solid solution of iridium and titanium oxide is used, and the cathode is made of SUS304 punched metal (short diameter 4 mm,
Raney nickel containing ruthenium (long diameter 8 mm) (ruthenium 5%, nickel 50%, aluminum 45%) was electrodeposited.

The electrolytic cell is arranged so that the anode, the cation exchange membrane, and the cathode are in contact with each other. While supplying 5N aqueous sodium chloride solution to the anode chamber and water to the cathode chamber, the electrolytic chamber is operated in a steady operation. Sodium chloride concentration of 3.2-3.6N
Further, the sodium hydroxide concentration in the cathode chamber was adjusted to 48 to 52% by weight, and electrolysis was performed at 90 ° C. and a current density of 30 A / dm ² .

In each of the examples and comparative examples, after 20 days of initial steady operation, electrolysis was stopped, the catholyte was replaced with an aqueous sodium hydroxide solution having the concentration shown in Table 1, and the temperature of the electrolytic cell was changed in each example. Adjust to the set temperature, 30A / dm ²
After re-energizing at the current density of, the steady operation was performed for 20 days.

The fluorine-containing cation exchange membrane used in Examples and Comparative Examples was formed by the following method. CF ₂ = CF ₂ / C
The ion exchange capacities of the F ₂ = CFOCF ₂ CF ₂ CF ₂ CO ₂ CH ₃ copolymer are 1.25 and 1.4, respectively.
4, Resins A, B, C of 1.80 meq / g dry resin
And CF ₂ = CF ₂ / CF ₂ = CFOCF ₂ CF (CF
₃ ) A resin D having an ion exchange capacity of 1.10 meq / g dry resin composed of an OCF ₂ CF ₂ SO ₂ F copolymer was synthesized. Also, a resin E was obtained by blending the resin C and the resin D in a weight ratio of 1: 1.

Next, a resin A having a thickness of 20 μm, a resin B having a thickness of 100 μm, a resin D having a thickness of 30 μm, and a resin E having a thickness of 10 μm.
Of the film E of FIG.
A cation exchange layer was obtained by performing thermocompression bonding in the order of B, E, and D.

Next, 9.5% by weight of the acid type polymer of Resin D
15. Add ZrO ₂ having an average particle size of 5 μm to the ethanol solution.
A mixed liquid having 8% by weight dispersed therein is prepared, and the mixed liquid is applied to the film A side of the cation exchange layer and dried to form a hydrophilic porous layer having a thickness of 60 μm,
A multi-layer cation exchange membrane consisting of a cation exchange layer and a porous layer was obtained.

Then, a mixed solution prepared by dispersing 20% by weight of SiC having an average particle size of 3 μm in a 25% by weight ethanol solution of an acid-type polymer of resin E was prepared, and the mixed solution was mixed into the above-mentioned multilayer cation exchange membrane. The both sides were sprayed so as to have a solid content of 1.5 mg per cm ² , and a gas-releasing coating was attached.

This film was treated with 25 wt% NaOH aqueous solution, 70
It was hydrolyzed at 16 ° C. for 16 hours and used for electrolysis. The results of the above electrolysis are shown in Table 1.

[0042]

[Table 1]

When re-energization is performed under the conditions of the present invention,
Although stable electrolysis performance is exhibited even after restarting, when re-energization was performed under the conditions outside the present invention, a current efficiency decrease of 3 to 6% occurred before and after restarting.

[0044]

42% by weight by ion exchange membrane electrolysis
When the electrolytic cell for directly producing the above-mentioned high-concentration alkali hydroxide is temporarily stopped and then restarted, stable high electrolytic performance can be obtained by a simple method even after restarting.

Claims (7)

[Claims] 1. When the electrolytic cell for producing a high-concentration alkali hydroxide of 42% by weight or more in the cathode chamber by the ion exchange method electrolysis using a fluorine-containing cation exchange membrane is temporarily stopped and then re-used. A method for reactivating an electrolytic cell, which is characterized in that the alkali hydroxide concentration in the cathode chamber is maintained at 25% by weight or less and then re-energization is performed. 2. The reactivating method according to claim 1, wherein re-energization is performed at an alkali hydroxide concentration in the cathode chamber of 10 to 20% by weight. 3. The temperature of the alkali hydroxide in the cathode chamber is 20 to 80.
The method for restarting according to claim 1 or 2, wherein re-energization is performed at ℃. 4. The concentration of alkali hydroxide in the cathode chamber after re-energization is 4
4. The method of reactivating according to claim 1, 2 or 3, wherein the catholyte temperature at a cathode chamber alkali hydroxide concentration of 30% by weight is 70 ° C. or more before a steady operation at 2% by weight or more is reached. 5. The fluorinated cation exchange membrane is a multi-layer membrane comprising a cation exchange layer made of a fluorinated polymer having a carboxylic acid group and a hydrophilic porous layer on the cathode side thereof. 2, 3, or 4 recurrence method. 6. The method of reactivating according to claim 5, wherein the hydrophilic porous layer comprises inorganic particles and a fluoropolymer having a hydrophilic group. 7. The method for re-initiating according to claim 5, wherein the fluoropolymer having a hydrophilic group is a fluoropolymer having a sulfonic acid group.