JPH0617278A - Method for restarting electrolytic cell - Google Patents

Abstract

PURPOSE:To develop stable high-electrolysis performance after restarting by supplying water or dilute aq. alkali hydroxide soln. to a cathode chamber and holding the soln., then reenergizing the soln. while increasing the concn. thereof at the time of restarting the once stopped electrolytic cell. CONSTITUTION:An aq. alkali chloride soln. is electrolyzed by an ion exchange membrane method electrolysis in the electrolytic cell having a fluorine-contained cation exchange membrane, the alkali hydroxide of a high concn. of >=42wt.% is produced in the cathode chamber. Water or the dilute aq. alkali hydroxide soln. is first supplied into the cathode chamber and is held for >=30 minutes, preferably 30 minutes to one day at 20 to 70 deg.C, at the time of restarting the electrolytic cell after temporarily stopping the electrolytic cell. The water or aq. solution is thereafter replaced with the aq. alkali hydroxide soln. of the concn. lower than the concn. in a ordinary operation state, more preferably 10 to 40%. The reenergization is executed in this state and the aq. alkali hydroxide soln. of the cathode chamber is thereafter increased up to the concn. of the ordinary operation state.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention temporarily stops an electrolytic cell for producing a high-concentration alkali hydroxide of 42% by weight or more in a cathode chamber by ion exchange membrane electrolysis using a fluorinated cation exchange membrane. This is related to the method of reactivating the case.

[0002]

2. Description of the Related Art So-called ion-exchange membrane alkaline electrolysis, which uses a fluorinated cation exchange membrane as a diaphragm and electrolyzes an aqueous solution of alkali chloride to produce alkali hydroxide and chlorine, has a lower energy consumption than conventional methods. Since it is possible to produce high-purity alkali hydroxide at, it has become popular internationally 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 operating method during non-steady state such as restarting is an important factor for obtaining stable electrolytic performance over a long period of time.

Regarding the operation method at the time of restarting, generally,
It is preferable to re-energize under conditions that are as close as possible to the conditions during 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, among these conditions, when the catholyte concentration is low, the temperature at the time of re-use is lowered to prevent the performance from being deteriorated due to the swelling of the film.
On the other hand, when the concentration of the catholyte is high, the electric shock received by the film during re-energization is alleviated by increasing the temperature.

On the other hand, in recent years, an electrolysis method has been disclosed in which a fluorine-containing cation exchange membrane is used to directly produce an alkali hydroxide having a concentration of 42% by weight or more by electrolysis at a high current efficiency of 90% or more (special feature KAISHO 63-310985, JP-A-1
-242794).

[0007] However, although these electrolysis methods disclose preferable operating conditions in steady state operation, methods for unsteady state operation such as restarting are not shown.

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, the conditions are close to the steady operation conditions (for example, the concentration of the catholyte solution is 40 to 50% by weight, the temperature is 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 producing alkali hydroxide with a concentration of 42 wt% or more, when electrolysis is stopped, electroosmotic water flowing from the anode to the cathode stops, so that the cathode side of the ion-exchange membrane is 42 wt% or more. Will come into direct contact with the high-concentration alkali hydroxide of
Extreme dehydration of the membrane will occur. 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]

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

[0011]

Thus, the present invention provides a temporary electrolytic cell for producing a high concentration of 42% by weight or more of alkali hydroxide in the cathode chamber by ion exchange membrane electrolysis using a fluorine-containing cation exchange membrane. In order to reactivate after being stopped, water or a dilute aqueous solution of alkali hydroxide is supplied to the cathode chamber and held for 30 minutes or more, and then the water or the dilute aqueous solution of alkali hydroxide is hydrated at a concentration lower than that in the steady state. It is characterized in that re-energization is carried out in the state of being replaced with alkali, and then the alkali hydroxide aqueous solution in the cathode chamber is increased to a steady-state concentration.

As the fluorine-containing cation exchange membrane used in the present invention, any fluorine-containing cation exchange membrane capable of producing an alkali hydroxide with high current efficiency can be used. Among them, it has a carboxylic acid group. A multilayer cation exchange membrane comprising a cation exchange membrane layer made of a fluoropolymer and a hydrophilic porous layer disposed on the cathode side thereof is preferable. The cathode-side porous layer is effective for obtaining 42% by weight or more 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 specific resistance smaller than that of 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 multi-layer structure in which a film made of a mixture thereof is laminated on the anode side can also be used. Further, these cation exchange membranes can be reinforced with a woven or non-woven fabric made of a fluorine-containing polymer having corrosion resistance such as polytetrafluoroethylene.

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 (a) and (b) polymerized units: Become.

[0015] _{(b) - (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 hydrogen or an alkali metal), and Y is
Selected from the following, wherein Z and Z ′ are —F or a perfluoroalkyl group having 1 to 10 carbon atoms, x,
y represents an integer of 1 to 10.

[0017] _{- (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. In addition,
Z, Z ′ and x are the same as above.

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

The above polymer is formed (a) /
As for the composition ratio (molar ratio) of (b), the fluoropolymer is preferably 0.5 to 4.0 meq / g dry resin, and more preferably 0.
It is chosen to form an ion exchange capacity of 7-2.0 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
Copolymer of F ₂ and CF ₂ ═CFO (CF ₂ ) _2-5 SO ₂ F, CF ₂ ═CF ₂ and CF ₂ ═CFO (CF ₂ ) _1-5
Copolymer with COOCH ₃ , CF ₂ = CF ₂ and CF ₂ =
CFO (CF ₂ ) _2-5 CO ₂ CH ₃ copolymer, and further, CF ₂ ═CF ₂ and CF ₂ ═CFOCF ₂ CF (CF
₃ ) A copolymer with OCF ₂ CF ₂ COOCH ₃ is exemplified.

The hydrophilic porous layer disposed on the cathode side of the cation exchange membrane layer is preferably formed of inorganic particles and a fluoropolymer having a hydrophilic group. High density, it is preferable, especially 42 stability point of view from hydrophilic groups on a weight% or more of the alkali hydroxide is -SO ₃ M (M is as defined above).

The porous layer composed of the above-mentioned inorganic particles and a fluoropolymer having a hydrophilic group preferably has a porosity of 5 to 95.
%, More preferably 10-85%, but it can be produced in various ways. For example, a solution of the fluoropolymer having a hydrophilic group is preferably cast from a mixture of alkali-resistant inorganic particles or fibrils dispersed in a film, a method of drying, inorganic particles to the fluoropolymer having a hydrophilic group. A method of mixing and kneading, then heating, molding to form a thin film, and stretching to form a porous film, a porous layer of inorganic particles is formed using a water-soluble appropriate binder such as methylcellulose, and two such porous layers are formed. A method in which a film of a fluoropolymer having a hydrophilic group is sandwiched therebetween, followed by heating and pressure bonding may be mentioned.

The inorganic particles preferably have a particle size of 0.
1 to 20 μm, preferred examples of which are titanium,
These are oxides, nitrides, carbides, hydroxides, silicon oxides, and carbides of zirconium, niobium, hafnium, tantalum, indium, tin, etc., alone or in a mixture.

The multilayer structure of the cation exchange membrane layer and the porous layer is
It can be performed by heating and press-bonding the porous layer and the cation exchange membrane 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 membrane layer and dried to form a multilayer. Further, it is also possible to carry out a heat pressing treatment in order to further enhance the adhesive force between the porous layer and the cation exchange membrane layer.

The above-mentioned multilayer film of the cation exchange membrane layer and the porous layer can be used as it is, but preferably chlorine gas is released on at least one surface of the cation exchange membrane, particularly preferably on the anode side surface. By performing the treatment for, the long-term stability of the 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. 60-26595) 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 forming a porous layer containing gas- and liquid-permeable particles having no electrode activity (JP-A-56-75583). And JP-A-57-3
9185) and the like are exemplified. 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 using the above cation exchange membrane, for example, the above-mentioned JP-A-54-1123 is used.
Known conditions such as those described in 98 can be employed. For example, the anode chamber is preferably supplied with an aqueous solution of alkali chloride of 2.5 to 5.0 N (N), and the cathode chamber is supplied with water or diluted alkali hydroxide or without them, preferably 50 ~ 120 ° C, 5-100A / dm ²
Is electrolyzed. In this case, impurity heavy metal ions such as calcium and magnesium and iodine ions in the alkali chloride aqueous solution cause deterioration of the ion exchange membrane, and thus it is preferable to make them 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. Also, the following means are adopted to develop stable performance. That is, after stopping the energization of the electrolytic cell,
By gradually replacing the alkali hydroxide in the cathode chamber with water or a dilute aqueous alkali hydroxide solution, the cathode chamber is filled with water or a dilute alkaline hydroxide solution, and then the catholyte is treated with water or a dilute alkali hydroxide solution in a predetermined amount. It was found that high electrolytic performance can be maintained even after restarting by substituting the alkali hydroxide aqueous solution of the concentration and then re-energizing, and then increasing the cathode chamber alkali hydroxide concentration to the desired steady-state operating concentration. is there.

Stopping of the electrolytic cell may occur for various reasons. However, in the case of planned stoppage, energization is performed with the concentration of alkali hydroxide in the cathode chamber being lowered to 40% by weight or less in advance at least 2 hours before the stop. It is preferable to stop.

The diluted alkaline hydroxide aqueous solution used for holding in the cathode chamber is preferably an alkaline hydroxide aqueous solution of 5% by weight or less. If the concentration exceeds 5% by weight, stable electrolytic performance cannot be obtained after re-use even if it is held in the cathode chamber under the conditions of the present invention.

The holding of water or a dilute aqueous alkali hydroxide solution in the cathode chamber is preferably carried out at a temperature of 20 to 70 ° C. for 30 minutes or longer, preferably 30 minutes to 1 day. Especially 30 ~ 6
Holding at 0 ° C. for 1 to 8 hours is preferable. When the holding temperature and the holding time exceed 70 ° C. and one day, the film swells irreversibly and the current efficiency decreases. If it is less than 20 ° C. and 30 minutes, stable electrolytic performance cannot be obtained after re-use.

The concentration of alkali hydroxide in the cathode chamber during re-energization is
It is lower than the concentration of alkali hydroxide in the cathode chamber during steady operation, preferably 10 to 40% by weight, and particularly 15
-30% by weight is preferred. If it is less than 10% by weight or more than 40% by weight, blister may occur in the film, which is not preferable.

The temperature of the aqueous alkali hydroxide solution in the cathode chamber during re-energization is preferably 20 to 90 ° C, more preferably 40 to 80 ° C. A temperature above 90 ° C is not preferable because it exceeds the temperature at the time of steady operation, and a cooling facility is required to make it below 20 ° C, which is not practical.

[0035]

In the present invention, the reason why stable electrolytic performance is obtained is not always clear, but according to the present invention, the dehydration state on the cathode side of the membrane is determined by supplying water or a dilute aqueous alkali hydroxide solution to the cathode chamber. It is considered that the alkali metal ions are likely to move because they return to an appropriate water-containing state.

[0036]

The present invention will be further described below with reference to 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 a titanium punched metal (having a rhombic opening having a short axis of 4 mm and a long axis of 8 mm) was used as an anode. ) Is coated with a solid solution of ruthenium oxide, iridium oxide, and titanium oxide, and the cathode is made of SUS304 punched metal (having a rhombic opening with a short axis of 4 mm and a long axis of 8 mm).
The electrodeposited was Raney nickel containing ruthenium (ruthenium 5%, nickel 50%, aluminum 45%).

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 chloride of the anode chamber is maintained during steady operation. The sodium concentration was adjusted to 3.2 to 3.6 N and the sodium hydroxide concentration in the cathode chamber was adjusted to 50% 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, electrolysis was stopped after 20 days of initial steady operation. The sodium hydroxide in the cathode chamber was discharged, and instead, water was supplied to fill it, and it was held under each "water holding condition" shown in Table 1. Then, the electricity in the cathode chamber was restarted while the water in the cathode chamber was replaced with an aqueous sodium hydroxide solution having the concentration and temperature shown in "Conditions for re-energization" in Table 1. Then, the concentration of the sodium hydroxide aqueous solution in the cathode chamber was raised to a steady-state concentration over 6 hours, and then 2
This is the one that was subjected to steady operation for 0 days.

The fluorine-containing cation exchange membrane used in the examples and comparative examples was formed by the following method. That is, CF ₂ = CF
₂ / CF ₂ = CFOCF ₂ CF ₂ CF ₂ CO ₂ CH ₃ copolymers each having an ion exchange capacity of 1.25,
Resin A of 1.44, 1.80 meq / g dry resin,
B, C, and CF ₂ = CF ₂ / CF ₂ = CFOCF ₂
A resin D having an ion exchange capacity of 1.10 meq / g dry resin composed of a CF (CF ₃ ) 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 10 μm, and a resin E having a thickness of 30 μm.
Of the film E of FIG.
A cation exchange membrane layer was obtained by heating and press-bonding B, E and D in this order.

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-layered cation exchange membrane consisting of a layer and a porous layer was obtained.

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

This membrane was hydrolyzed with a 25 wt% sodium hydroxide aqueous solution at 70 ° C. for 16 hours and used for electrolysis. The results of the above electrolysis are shown in Table 1.

[0044]

[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 conditions outside the present invention, a current efficiency decrease of 3 to 5% occurred before and after restarting.

[0046]

42% by weight by ion exchange membrane electrolysis
When the electrolytic cell for directly producing the high-concentration alkali hydroxide is temporarily stopped and then restarted, stable and 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 having a concentration of 42% by weight or more in the cathode chamber by the ion exchange membrane method electrolysis using a fluorinated cation exchange membrane is temporarily stopped and then reused. After supplying water or a diluted alkaline hydroxide aqueous solution to the cathode chamber and holding it for 30 minutes or more, re-energization is performed with the water or the diluted alkaline hydroxide aqueous solution replaced with an alkaline hydroxide aqueous solution having a concentration lower than the steady state. A method for reactivating an electrolytic cell, which is performed, and then increases the alkaline hydroxide aqueous solution in the cathode chamber to a steady-state concentration. 2. The method according to claim 1, wherein the temperature of the water or the diluted aqueous solution of alkali hydroxide supplied to the cathode chamber is 20 to 70 ° C. 3. The method of reactivating according to claim 1, wherein the aqueous alkali hydroxide solution having a concentration lower than that in the steady state has a concentration of 10 to 40% by weight. 4. The method for restarting according to claim 1, 2 or 3, wherein re-energization is carried out at a temperature of 20 to 90 ° C. of an alkali hydroxide aqueous solution having a concentration lower than that in a steady state. 5. A cation-exchange membrane layer comprising a fluorinated cation-exchange membrane and a fluorinated polymer having a carboxylic acid group.
The method of reactivating according to claim 1, 2, 3, or 4, which is a multilayer film with a hydrophilic porous layer on the cathode side. 6. The method of reactivating according to claim 5, wherein the hydrophilic porous layer is composed of inorganic particles and a fluoropolymer having a hydrophilic group. 7. The method according to claim 6, wherein the fluoropolymer having a hydrophilic group is a fluoropolymer having a sulfonic acid group.