JP3112039B2 - How to restart the electrolytic cell - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a temporary electrolytic cell in which a high concentration of alkali hydroxide of 42% by weight or more is directly produced by electrolysis in a cathode chamber by ion exchange membrane electrolysis using a fluorine-containing cation exchange membrane. The present invention relates to a method for restarting when the system is temporarily stopped.

[0002]

2. Description of the Related Art In a so-called ion exchange membrane method of electrolysis, in which a fluorine-containing cation exchange membrane is used as a diaphragm and an alkali chloride aqueous solution is electrolyzed to produce alkali hydroxide and chlorine, high-purity alkali hydroxide is used. In recent years, it can be manufactured with lower energy consumption than conventional methods, and thus has been spreading internationally in recent years.

[0003] In Louis-exchange membrane method alkaline electrolyte written early days, although fluorine-containing ion exchange membrane of the sulfonic acid groups and ion exchange groups are used, it is difficult to increase the current efficiency Met. For this reason, in recent years, it has been changed to a cation exchange membrane having a carboxylic acid group as an ion exchange group.
At 5% by weight, the current efficiency reaches up to 93-97%, almost reaching the industrially complete range.

[0004] 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-state operation under appropriate conditions, the electrolysis is stopped. In addition, an operation method in an unsteady state such as restarting is an important factor for obtaining stable electrolysis performance over a long period of time.

[0005] Regarding the operation method at the time of re-use,
It is preferable to re-energize under the conditions as close as possible to the conditions at the time of steady operation in which high electrolysis performance can be obtained.
In the case of obtaining 0 to 35% by weight of alkali hydroxide, energization is performed under the conditions of a catholyte concentration of 25 to 35% by weight and a temperature of 50 to 80 ° C, and reused. Further, even within this condition range , when the concentration of the catholyte is low, a decrease in performance due to swelling of the film is prevented by lowering the temperature at the time of reactivation.
Further, when the concentration of the catholyte is high, the electric shock applied to the film at the time of re-energization is reduced by increasing the temperature or the like.

On the other hand, in recent years, a fluorinated cation exchange membrane having a hydrophilic porous layer provided as a diffusion layer on the surface on the cathode side has been used to convert alkali hydroxide having a concentration of more than 42% by weight with a current efficiency of 93% or more. An electrolysis method for directly manufacturing by electrolysis is disclosed (Japanese Patent Application Laid-Open No. 63-310985,
242794).

[0007] However, although these electrolysis methods show preferable operating conditions in steady-state operation, they do not show any method relating to unsteady operation such as restarting.

According to the study of the present inventor, the above concentration 42
In the electrolysis process of not less than 30% by weight, the conventional 30% is used.
It has been surprisingly found that when re-use is performed under conditions close to the steady-state operating conditions used in the ~ 35 wt% electrolysis process, the electrolysis performance is significantly reduced. In other words, conditions close to steady-state operating conditions after electrolysis is stopped (for example, cathode
Room alkali hydroxide concentration 40-50 wt%, temperature 75-9
It has been found that when re-energization is performed at 0 ° C.), the current efficiency is greatly reduced, and in some cases, the electrolytic voltage is increased.

Although the cause of this phenomenon is not yet clear,
It is considered as follows. That is, in an ion-exchange membrane electrolyzer for producing 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. Excessive alkali hydroxide will result in extreme dehydration of the membrane. For this reason, the alkali metal ions in the film are in a state where they hardly move. It is presumed that when current is supplied again in such a state, the mobility of hydroxyl ions relatively increases, and the current efficiency decreases.
Further, the reason why the electrolytic voltage generated in a certain case increases is presumed to be caused by the contraction of the membrane due to the dehydration.

[0010]

SUMMARY OF THE INVENTION According to the present invention, an alkali hydroxide having a concentration of 42% by weight or more, particularly 45% by weight or more to 55% by weight, is obtained by ion exchange membrane method electrolysis using a fluorine-containing cation exchange membrane. An object of the present invention is to provide a new restarting method for temporarily stopping an electrolytic cell manufactured by electrolysis and restarting the electrolytic bath so as to exhibit stable and high electrolytic performance even after restarting.

[0011]

According to the present invention, there is provided 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 fluorine-containing cation exchange membrane. When the reactor is restarted after being temporarily stopped , the concentration of the alkali hydroxide in the cathode chamber is 25% by weight or less and the temperature is reduced.
Is maintained at 20 to 80 ° C. , and re-energization is performed.

In the present invention, the fluorinated cation exchange membrane is a multilayer cation comprising a cation exchange layer comprising a fluorinated polymer having at least a carboxylic acid group and a hydrophilic porous layer disposed on the cathode side thereof. Preferably, an exchange membrane is used. The porous layer on the cathode side is effective for obtaining alkali hydroxide exceeding 42% by weight with high current efficiency over a long period of time.

[0013] membrane resistance in order to impart only One large film strength reduced, fluoropolymer small fluorinated acid polymer is also specific resistance than substance film and a fluorine-containing sulfonic acid polymer film having the carboxylic acid group or cation exchange membrane of a multilayer structure film consisting of the mixture was layered on the anode side you can use. In addition, these cation exchange membranes can be reinforced with a woven or nonwoven fabric made of a corrosion-resistant fluoropolymer such as polytetrafluoroethylene.

[0014] The fluorine-containing carboxylic acid polymer constituting the cation exchange membrane and a fluorine-containing sulfonic acid polymer in the present invention, preferably hydrolysis of the copolymer having polymerized units of the following (i) and (ii) Consists of decomposition products .

(A)-(CF ₂ -CXX ')-, (b)-{CF ₂ -CXX (YA)}- .

[0016] Here X, X 'are, -F, -Cl, -H or -CF _3, A is -SO ₃ M or -CO ₂ M (M
Is hydrogen, an alkali metal, or hydrolysis, etc. to convert to by Riko these groups based), Y is selected from: where Z, Z 'is -F or a carbon number 1 It is a perfluoroalkyl group of 10 to 10, and x and y represent an integer of 1 to 10.

-(CF ₂ ) _x- , -O- (CF ₂ ) _x -,-(O-CF ₂ CFZ) _x -,-(O-CF ZCF ₂ ) _x -O- (CFZ ') _y − .

Further, in addition to the polymerized units (a) and (b), the polymerized polymer may contain the following polymerized units.

-{CF ₂ -CF (O-Z)}- (where Z is the same as above)

It should be noted that (a) /
The composition ratio (molar ratio) of (b) is the hydrolysis of the fluoropolymer.
Things preferably 0.5 to 4.0 meq / g dry resin,
In particular, it is selected so as to form an ion exchange capacity of 0.7 to 2.0 meq / g dry resin.

[0021] The fluorine-containing polymer is preferably a perfluorocarbon polymer is appropriate and preferred examples thereof are CF ₂ = CF ₂ and CF ₂ = CFOCF ₂ CF (CF
₃ ) Copolymer with OCF ₂ CF ₂ SO ₂ F, CF ₂ ＣC
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
Copolymers of _{~ 5 COOCH 3, CF 2 =} CF 2 and CF
₂ = CFO (CF ₂ ) ₂ to ₅ CO ₂ CH ₃ Copolymer, 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 above-mentioned hydrophilic porous layer disposed on the cathode side of the cation exchange layer is preferably formed of inorganic particles and a fluoropolymer having a hydrophilic group. In particular stability viewpoint from hydrophilic groups in an alkali hydroxide of greater than 42 wt% is preferably (the M as defined above) -SO ₃ M is.

The porous layer comprising the inorganic particles and the fluoropolymer having a hydrophilic group can be produced by various methods. For example, a method of forming a cast film from a mixture in which alkali-resistant inorganic particles or fibrils are preferably dispersed in a solution of a fluoropolymer having a hydrophilic group, mixing and kneading inorganic particles with a fluoropolymer having a hydrophilic group was heated after, and formed form thinned, a method of porous film by being stretched, by using a water-soluble appropriate binder such as methylcellulose to form a porous layer of inorganic particles, between two such porous layers A method in which the film is sandwiched between films of a fluoropolymer having a hydrophilic group, heated, and pressure-bonded, may be used.

As the inorganic particles, preferably, IIa, IIIb, IVb, Vb, and III of the long-periodic periodic table are used.
a, oxides and nitrides of elements of the third and subsequent periods of the IVa group,
Carbides, Ru is selected from a single or a mixture thereof hydroxide or boron carbide. Specific preferred examples include titanium, zirconium, niobium, hafnium, tantalum, indium, oxides such as tin, nitrides, carbides,
It is selected from hydroxides, oxides of silicon and carbides alone or in mixtures.

The multilayering of the cation exchange layer and the porous layer can be carried out by heating and pressing the porous layer and the cation exchange layer obtained as described above. When the porous layer is formed by a casting method, a mixed solution of inorganic particles and a fluoropolymer having a hydrophilic group may be directly applied to the cation exchange layer and dried to form a multilayer. Further, in order to increase the adhesive strength between the porous layer and the cation exchange layer, a heat-press treatment can be performed.

The above-mentioned multilayer film of the cation exchange 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 processing for
The long-term stability of current efficiency can be further improved.

As a method of performing a treatment for releasing gas on the surface of the cation exchange membrane, a method of providing fine irregularities on the membrane surface (Japanese Patent Publication No. 60-49595), a method of using a liquid containing iron, zirconia, or the like in an electrolytic cell. A method of supplying hydrophilic inorganic particles to the surface of a membrane (Japanese Patent Application Laid-Open No. 56-152980) and a method of providing a porous layer containing particles having no gas- and liquid-permeable electrode activity (Japanese Patent Application Laid-Open No. 56-75583). And JP-A-57-391.
85) and the like. The gas release layer on the surface of such a 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-mentioned cation exchange membrane, known conditions as described in the above-mentioned JP-A-54-112398 are used. Can be adopted. For example, an aqueous solution of an alkali chloride of preferably 2.5 to 5.0 N is supplied to the anode compartment, and water or diluted alkali hydroxide is supplied to the cathode compartment or not. It is electrolyzed at １２０120 ° C. and 5１００100 A / dm ² . In such a case, calcium and magnesium in alkali chloride, impurity heavy metal ions such as iodine ions,
It is preferable to reduce the size of the ion exchange membrane as much as possible, since it causes deterioration of the ion exchange membrane.

The present invention provides an operation method in which the electrolysis for producing the high-concentration alkali hydroxide is temporarily stopped for various reasons and then restarted. As a result of repeated investigations on the method of restarting to achieve stable performance, re-energizing was carried out under the condition that the concentration of alkali hydroxide in the cathode chamber was greatly reduced from the concentration at the time of steady operation, and then the cathode chamber was brought to the target steady operation concentration. By increasing the alkali hydroxide concentration, it has been found that high electrolytic performance can be maintained even after re-use .

At the time of re-energization, the concentration of alkali hydroxide in the cathode chamber is 2
It is preferably at most 5% by weight , particularly preferably from 10 to 20% by weight. 1
If the content is less than 0% by weight, blisters may be generated on the film, which is not preferable.

The temperature of the alkali hydroxide in the cathode chamber during re-energization is 2
The temperature is preferably from 0 to 80 ° C, particularly preferably from 30 to 70 ° C. At 80 ° C. or higher, irreversible swelling of the film occurs due to the fact that the alkali concentration in the cathode compartment is much lower than the high concentration in the steady operation, and the current efficiency is reduced. Cooling equipment is required to lower the temperature to 20 ° C. or less, which is not practical.

After re-energizing at the above-mentioned alkali hydroxide concentration and temperature in the cathode compartment , the concentration of the alkali in the cathode compartment is increased to the target concentration by adjusting the amount of diluted alkali or water added to the cathode compartment. However, during this period, the temperature also rises due to the rise in the electrolytic voltage accompanying the rise in the concentration. In the present invention, it has been found that it is preferable to perform a process of increasing the temperature to 70 ° C. or more at a cathode chamber alkali concentration of 30% by weight during the increase in concentration, since high current efficiency can be obtained after re-use. .

[0033]

In the present invention, the reason why stable electrolysis performance can be obtained by re-electrolysis under the condition that the alkali concentration in the cathode chamber is 25% by weight or less is not always clear. However, it is considered that since the state returns to an appropriate water-containing state by energization at a low concentration of alkali hydroxide in the cathode compartment , alkali metal ions are easily moved.

[0034]

EXAMPLES The present invention will be further described below with reference to examples, but the present invention is not limited to these examples. The electrolysis in Examples and Comparative Examples was carried out using an electrolytic cell equipped with a temperature controller with an effective energizing area of 0.25 dm ² , and using titanium perforated metal (short diameter 4 mm, long diameter 8 mm) as ruthenium oxide and ruthenium oxide as the anode. A material coated with a solid solution of iridium and titanium oxide is used, and a SUS304 punched metal (short diameter 4 mm,
What used the electrodeposition of Raney nickel containing ruthenium (5% of ruthenium, 50% of nickel, and 45% of aluminum) to 8 mm of major diameter was used.

In the electrolytic cell, the anode, the cation exchange membrane, and the cathode are arranged so as to be in contact with each other, and a 5N aqueous solution of sodium chloride is supplied to the anode chamber and water is supplied to the cathode chamber. Sodium chloride concentration of 3.2-3.6N
The concentration of sodium hydroxide 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 performing the initial steady operation for 20 days, the 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 a re-energization at a current density of, a steady operation was performed for 20 days.

The fluorine-containing cation exchange membrane used in the examples and comparative examples was formed by the following method. CF ₂ = CF ₂ / C
F ₂ = CFOCF ₂ CF ₂ CF ₂ CO ₂ CH ₃ copolymer
Each ion exchange capacity obtained by hydrolyzing 1.2
5, 1.44, 1.80 meq / g Resins A, B, C of dry resin and CF ₂ = CF ₂ / CF ₂ = CFOCF
₂ CF (CF ₃₎ a OCF ₂ CF ₂ SO ₂ F copolymer additive
1.10 meq / g ion exchange capacity resulting from water splitting
Resin D as a dry resin was synthesized. The above resin C and resin D
Was blended at a weight ratio of 1: 1 to obtain a resin E.

Next, a film A having a thickness of 20 μm from resin A, a film B having a thickness of 100 μm from resin B, a film D having a thickness of 30 μm from resin D, and a film D having a thickness of 30 μm from resin E
A film E is formed by a melt extrusion method.
A cation exchange layer was obtained by heating and pressing in the order of B, E, and D.

Next, 9.5% by weight of the acid type polymer of the resin D
14. ZrO ₂ having an average particle size of 5 μm is added to the ethanol solution.
A mixed solution in which 8% by weight is dispersed is prepared, and the mixed solution is applied to the film A surface side of the cation exchange layer and dried to form a 60 μm-thick porous layer having a hydrophilic group ,
A multilayer cation exchange membrane comprising a cation exchange layer and a porous layer was obtained.

Next, a mixed solution in which 20% by weight of SiC having an average particle size of 3 μm was dispersed in a 25% by weight solution of an acid-type polymer of the resin E in ethanol was prepared, and this mixed solution was applied to the above-mentioned multilayer cation exchange membrane. Both surfaces were sprayed to give a solid content of 1.5 mg / cm ² , and a gas-releasing coating was adhered.

This membrane was treated with a 25% by weight aqueous NaOH solution
Hydrolyzed at ℃ for 16 hours and used for electrolysis. Table 1 shows the results of the electrolysis.

[0042]

[Table 1]

When re-energization is performed under the conditions of the present invention,
Although stable electrolysis performance was exhibited even after re-use, when the re-energization was performed under conditions deviating from the present invention, the current efficiency decreased by 3 to 6% before and after re-use.

[0044]

EFFECT OF THE INVENTION 42% by weight by ion exchange membrane electrolysis
Upon temporarily stopping the electrolytic cell for directly producing the high-concentration alkali hydroxide described above and restarting it, stable and high electrolysis performance can be obtained by a simple method even after restarting.

──────────────────────────────────────────────────続 き Continued on front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C25B 1/00-15/08

Claims (4)

(57) [Claims] 1. An electrolytic cell for producing a high-concentration alkali hydroxide of 42% by weight or more in a cathode chamber by ion exchange electrolysis using a fluorine-containing cation exchange membrane is temporarily stopped and then restarted. in the cathode chamber alkali hydroxide, the concentration of 2
A method for restarting an electrolytic cell, wherein re-energization is performed while maintaining the temperature at 5% by weight or less and the temperature at 20 to 80 ° C. 2. After re-energization, the concentration of the alkali hydroxide in the cathode chamber becomes 4%.
Before the steady operation at 2% by weight or more, the cathode chamber alkali hydroxide concentration when the cathode chamber alkali hydroxide concentration is 30% by weight
2. The method according to claim 1, wherein the temperature of the mixture is 70 ° C. or more. 3. A fluorinated cation exchange membrane, a cation exchange layer made of a fluoropolymer having carboxylic acid groups, a multilayer film of a porous layer having disposed hydrophilicity to the cathode side according Item 3. The method for restarting according to Item 1 or 2 . 4. A porous layer having a hydrophilic property, re-hired method according <br/> to claim 3 comprising a fluorine-containing polymer having inorganic particles and hydrophilic groups.