JPS625998B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing caustic alkali, and particularly to a method for producing caustic alkali by electrolyzing an aqueous alkali chloride solution at low voltage using an ion exchange membrane. Instead of using asbestos as a diaphragm to obtain caustic alkali by electrolyzing an aqueous alkali chloride solution using a diaphragm method, several methods have been proposed that use ion exchange membranes to obtain caustic alkali with higher purity and concentration. ing. On the other hand, energy conservation has been progressing worldwide in recent years, and from this point of view, in this type of technology, it is desired to reduce the electrolysis voltage as much as possible. Various proposals have been made as means for lowering the electrolytic voltage, such as considering the material, composition, shape, etc. of the anode and cathode, or specifying the composition of the ion exchange membrane used and the type of ion exchange group. . Although all of these methods have certain effects, most of them have an upper limit where the concentration of the caustic alkali obtained is not so high.
If this value is exceeded, the electrolytic voltage will suddenly increase, the current efficiency will decrease, or the sustainability and durability of the electrolytic voltage drop phenomenon will be poor. In addition, in the production of caustic alkali using an ion exchange membrane having a sulfonic acid group as an ion exchange group, polyvalent cations such as calcium ions remaining as impurities in the aqueous alkali chloride solution that is the electrolyte are A method has been proposed for obtaining caustic alkali with high current efficiency and low voltage by adding a compound containing an anion such as a phosphate group that can form an insoluble gel. (Refer to Japanese Patent Publication No. 52-38519.) However, as clearly stated in the above publication, this method prevents polyvalent cations contained as impurities in the aqueous alkali chloride solution from penetrating into the ion exchange membrane. By preventing the deterioration of the membrane, such as preventing the mobility of alkali metal ions from being inhibited and preventing the formation of cracks in the membrane,
By maintaining the initial performance of the membrane, a decrease in current efficiency and an increase in electrolytic voltage are prevented, and therefore, it is not an active means that necessarily enables electrolysis at low voltage. Furthermore, recently, on the surface of fluorine-containing cation exchange membrane,
A method has been proposed in which an aqueous alkali chloride solution is electrolyzed by bringing gas and liquid permeable anodes and cathodes into close contact to obtain caustic alkali. (Refer to Japanese Patent Application Laid-open No. 54-112398.) This method eliminates the electrical resistance caused by the electrolyte, which was previously thought to be unavoidable in this type of technology.
This is an excellent method for electrolysis at a lower voltage than conventional methods, as it can minimize the electrical resistance caused by bubbles generated from hydrogen and chlorine gas. The electrode in this method is directly bonded to and embedded in the surface of the ion exchange membrane, and the gas generated by electrolysis at the contact interface between the membrane and the electrode easily leaves the electrode, and the electrolyte permeates through it. It is made permeable to gases and liquids. Such porous electrodes are usually formed into a thin layer by uniformly mixing active particles as an anode or a cathode with a substance that binds them, preferably graphite or other conductive material. However, according to the inventor's study, when the electrode is directly bonded to the ion exchange membrane in this way,
For example, in the case of an anode, since it comes into contact with hydroxide ions that diffuse back from the cathode chamber, it is required to have alkali resistance as well as the conventional chlorine resistance, and a special and expensive material must necessarily be selected. In addition, when the electrode and ion exchange membrane are combined, gas is generated as a result of the electrode reaction, causing phenomena such as swelling in the cation exchange membrane.
It was found that membrane performance deteriorated and there was a risk that stable operation could not be expected for a long period of time. In view of these points, the present inventor conducted various research and examinations with the aim of finding a means to obtain caustic alkali by electrolyzing an aqueous alkali chloride solution at as low a voltage as possible. In a method of introducing an aqueous alkali chloride solution into the anode chamber of an electrolytic cell having an anode chamber and a cathode chamber and electrolyzing it to produce caustic alkali in the cathode chamber, a gas- and liquid-permeable membrane is provided on the side of the cathode chamber of the ion exchange membrane. A porous layer having no electrode activity is formed, a cathode is disposed through the porous layer, and a metal or metal ion capable of forming a hydroxide at pH 1 to 5 is added to the aqueous alkali chloride solution to form an anode. It has been found that the above object can be achieved by forming and allowing a thin layer of metal hydroxide to exist on the surface of the ion exchange membrane facing the chamber and performing electrolysis. According to the present invention, when using an ion exchange membrane having a specific type of ion exchange group and concentration of ion exchange groups, for example, in Japanese Patent Publication No. 52-38519, it is necessary to Among valent cations, certain types of cations are rather actively added from the outside, while anions such as phosphoric acid, which are used only to form compounds with them, are not added from the outside. It has been surprisingly discovered that the electrolysis voltage can be effectively lowered without causing any decrease in current efficiency. Further, according to the present invention, the cathode is disposed through the gas- and liquid-permeable porous layer that does not have electrode activity, so that it does not come into direct contact with the membrane.
Therefore, gas generated during electrolysis is not generated at the contact interface between the ion exchange membrane and the electrode, so that the ion exchange membrane does not bulge due to gas generation. Furthermore, since the anode does not need to be in direct contact with the ion exchange membrane, great alkali resistance is not required, allowing a wide range of choices for the anode material. In the present invention, the porous layer that is permeable to gas and liquid and has no electrode activity formed on the surface of the ion exchange membrane is made of a material that has a higher hydrogen overvoltage than the cathode disposed through the layer, such as a non-conductive material. Adopted. It may be formed from any material as long as it satisfies these properties, but from the viewpoint of corrosion resistance, it is preferably made of periodic law group A, group B, group B, and
Oxides, hydroxides, and nitrides of group B and iron group metals, as well as rare earth elements such as silver, beryllium, aluminum, gallium, indium, yttrium, antimony, bismuth, selenium, manganese, or lanthanum, cerium, and thorium. , carbide may be used alone or in a mixture of two or more types. Among these, titanium, zirconium, niobium, tantalum, vanadium, manganese,
molybdenum, tin, antimony, tungsten,
It is particularly preferable to use bismuth oxide, hydroxide, nitride, or carbide because stable performance can be obtained over a long period of time. In order to form a porous layer on the membrane surface using these materials, powders or granules of the materials are preferably bonded with a suspension of a fluorine-containing polymer such as polytetrafluoroethylene. This is achieved by The content of the fluorine-containing polymer used in this case is usually 1.5 to 50% by weight, particularly preferably 2.0 to 30% by weight. Further, if necessary, in order to make the mixture of the two uniform, an appropriate surfactant or other additives such as a pore-forming agent may be added. The content of the substance having no electrode activity in the porous material is
0.01 to 30 mg/cm ² , particularly 0.1 to 15 mg/cm ² is preferred. The formation of these porous layers on the ion exchange membrane surface is
When the electrode disposed through this is a porous layer containing electrode particles, the procedure is substantially the same. That is, it is prepared in the same manner as the method described in JP-A-54-112398, and is bonded to the membrane surface by heating and pressing, preferably partially embedded.
However, if the porous layer has self-supporting properties, it does not necessarily need to be integrally bonded to the membrane surface, and may simply be in contact with it. The porous layer formed on the membrane surface has an average pore diameter of 0.01~2000μ and a porosity of
It is suitable to have a content of 10 to 99%. When these physical properties deviate from the above range, it is not preferable because mass transfer is inhibited or the effect of attaching the porous layer is reduced. Among these ranges, it is particularly preferable to use an average pore diameter of 0.1 to 1000 μm and a porosity of 20 to 95%, since the intended purpose of the present invention can be stably maintained over a long period of time. The thickness of the porous layer is strictly determined by the material forming it and the above-mentioned physical properties, but it is generally 0.1~
It is appropriate to employ 300μ, preferably 1 to 100μ. If the thickness deviates from the above range, the object of the present invention may not be fully achieved or it may become difficult for gas to escape or for the electrolyte to move through the porous layer, which is not preferable. In the present invention, the cathode disposed through the porous layer is composed of a porous electrode such as a porous plate, a mesh, or an expanded metal, or a gas- and liquid-permeable porous layer having cathodic activity. Any electrode can be used. When using either a porous cathode or a porous cathode, the material is selected to have a low hydrogen overvoltage. For example, platinum group metals, alloys thereof, conductive oxides of these metals and alloys, or iron group metals are used. As platinum group metals,
Examples include platinum, rhodium, ruthenium, palladium, and iridium, and iron group metals include iron,
Examples include cobalt, nickel, Raney nickel, and stabilized Raney nickel. On the other hand, when the cathode is composed of a porous material that is permeable to gas and liquid, for example,
This is carried out in the same manner as in the case of obtaining a porous cathode as described in the above publication. That is, the powder or granules of the cathode-forming material are combined with a binder made of a fluorine-containing polymer such as polytetrafluoroethylene together with a conductive material such as graphite, a pore-forming agent, etc. as necessary, and
Formed in a thin layer. On the other hand, the material and shape of the anode are similar to those used in the electrolysis of aqueous alkali chloride solutions using the conventional ion exchange membrane method, and an appropriate distance from the ion exchange membrane is maintained. Next, the ion exchange membrane used in the present invention is made of a polymer containing a cation exchange group such as a carboxyl group, a sulfonic acid group, a phosphonic acid group, or a phenolic hydroxyl group. Particular preference is given to employing fluoropolymers. As a fluorine-containing polymer containing an ion exchange group,
For example, vinyl monomers such as tetrafluoroethylene and chlorotrifluoroethylene and carboxylic acids,
A copolymer with a perfluorinated vinyl monomer having an ion exchange group such as a sulfonic acid or phosphoric acid group is preferred. Furthermore, a film-like polymer of trifluorostyrene into which an ion exchange group such as a sulfonic acid group is introduced may also be used. Among these, when using polymers having the following structures, high purity caustic alkali can be obtained with relatively high current efficiency and low electrolytic voltage. It is particularly preferred as an exchange membrane. Here, X is F, Cl, H or _-CF3 , X' is X or _CF3 ( _CF2 )m, m is 1 to 5, and Y is selected from the following. (-CF ₂ )- _x -A, -O (-CF ₂ )- _x -A,

【formula】

x, y, z are all 0 to 10, and Z, Rf are -
selected from F or _C1-10 perfluoroalkyl groups. Also, A is -SO ₃ M, -COOM or -SO ₂ F, which can be converted into these groups by hydrolysis.
-CN, -COF or -COOR, M is hydrogen or an alkali metal, and R is a _C1-10 alkyl group. The cation exchange membrane used in the present invention is
It is preferred to have an ion exchange capacity of 0.5 to 4.0 meq/g dry resin, especially 0.8 to 2.0 meq/g dry resin. In order to provide such an ion exchange capacity, in the case of an ion exchange membrane made of a polymer having the above-mentioned M and N structures, the polymerized units of N are preferably 1 to 40 mol%, particularly 3 to 25 mol%. Appropriate. Further, the cation exchange membrane used in the present invention does not necessarily need to be formed from one type of polymer, nor does it need to have only one type of ion exchange group. For example, as an ion exchange group, an ion exchange membrane can be used in which the cathode side has a weakly acidic exchange group such as a carboxylic acid group and the anode side has a strongly acidic exchange group such as a sulfonic acid group. It will be done. These ion exchange membranes are manufactured by various known methods. In addition, these ion exchange membranes can be reinforced, if necessary, with a cloth, a woven fabric such as a net, a nonwoven fabric or a metal mesh, a porous body, etc., preferably made of a fluorine-containing polymer such as polytetrafluoroethylene. . Further, the thickness of the ion exchange membrane is 20 to 500μ, preferably 50 to 400μ. In order to form a porous layer having no electrode activity on the surface of these ion exchange membranes, an appropriate form of ion exchange group, such as a carboxylic acid In the case of radicals, they are bonded in their ester form, and in the case of sulfonic acid groups, in their --SO ₂ F form, by the action of pressure and heat. Next, in the present invention, the alkali chloride aqueous solution supplied to the anode chamber is heated to pH 1 to 5 so that a thin layer containing metal hydroxide is formed on the anode side surface of the ion exchange membrane. A metal or metal ion capable of forming a hydroxide is added. Examples of metals used to form such hydroxides include:
Iron group metals consisting of iron, nickel, and cobalt; consisting of titanium, zirconium, and hafnium.
Group B metals include metals such as aluminum, ruthenium, palladium, beryllium, cerium, tin, niobium, copper, scandium, and yttrium.
These can be used alone or in combination of two or more. In addition, in such metals, the ion exchange group of the ion exchange membrane used is a carboxylic acid group,
Particularly when an ion exchange membrane having the above-mentioned preferred fluorine-containing polymer structure is used, hydroxides or hydroxides that can effectively carry out low voltage electrolysis simply by adding these metals as powder to the anolyte. A thin layer of oxide can be formed on the side of the ion exchange membrane facing the anode chamber. For cation exchange membranes other than those mentioned above, the object of the present invention can be achieved to some extent by adding metal powder to the anode chamber in the same manner as described above, but in addition, for example, phosphates, chlorides, Compounds containing anions, such as sulfates, hydroxides, carbonates, and nitrates, or compounds with other non-hazardous cations, such as sodium phosphate, that react with these metals to form compounds. It may be effective to add it. The amount of metal hydroxide thin layer formed on the ion exchange membrane surface by adding such metals or metal ions to an aqueous alkali chloride solution varies somewhat depending on the type of metal used, but usually cations It is appropriate for the metal to be present in an amount of about 0.005 to 50 mg per 1 cm ² of the exchange membrane. If the amount present on the membrane surface is less than the above range, stable low-voltage electrolysis operation and its sustainability will be insufficient; on the other hand, if it exceeds the above range, it is no longer possible to expect better effects. On the contrary, the electrical resistance may become high, and the intended purpose of the present invention may not be achieved, which is not preferable. Among these ranges, it is particularly preferable to adopt a metal content of 0.01 to 20 mg per cm ² of the cation exchange membrane, since stable electrolysis operation at low voltage can be expected over a long period of time. Next, the means for forming the thin layer on the surface of the cation exchange membrane is to add the metal or compound capable of forming the layer to an aqueous alkali chloride solution, which is an electrolyte solution, which is normally introduced into the anode chamber. This is achieved by carrying out electrolysis of alkali chloride. Further, the metal or compound forming the thin layer is
It is not necessarily necessary to continuously supply this during electrolysis operation, and as long as the above-mentioned amount is maintained, it can be supplied only at the beginning of electrolysis or intermittently. Furthermore, in order for such a thin layer to exist, the pH of the aqueous alkali chloride solution, which is the anolyte, plays a large role. The pH is strictly determined depending on the type of thin layer to be made and the type of aqueous alkali chloride solution, but generally a pH of about 1 to 7 is appropriate. If the pH is lower than the above range, a thin layer will not be formed effectively, and if it is higher than the above range, the adhesion strength of the thin layer to the ion exchange membrane surface will be insufficient, inhibiting the voltage reduction effect. Both are undesirable as there is a risk of being exposed. Of the PH range, it is particularly preferable to use PH 1 to 5 because a thin layer can be effectively formed and electrolysis operation can be stably maintained at low voltage. As other conditions for electrolyzing an aqueous alkali chloride solution in the present invention, known conditions may be employed as appropriate. For example, the anode chamber is supplied with a 2.5-4.5N aqueous alkali chloride solution, and the cathode chamber is supplied with water or diluted alkali hydroxide, preferably
Electrolysis is carried out at a temperature of 80-120° C. and a current density of 10-50 A/dm ² . When the method of the present invention is employed, the electrolytic voltage is reduced by about 0.1 to 0.9 V compared to the commonly practiced method for producing caustic alkali using an ion exchange membrane method. In the present invention, the alkali chloride to be subjected to electrolysis is usually common salt, but other alkali metal chlorides such as potassium chloride and lithium chloride may be used as appropriate. Example 1 73mg of titanium oxide powder with a particle size of 44μ or less was added to 50c.c. of water.
To this was added a suspension of polytetrafluoroethylene (PTFE) (trade name: Teflon 30J, manufactured by Dupont) so that the amount of PTFE was 7.3 mg, and to this was added a nonionic surfactant (ROHM& One drop of TRITON The thin layer had a thickness of 31μ, a porosity of 75%, and contained 5mg/cm ² of titanium oxide. Next, this thin layer has an ion exchange capacity of 1.45 meq/g.
A porous PTFE membrane is placed on one side of an ion exchange membrane made of a copolymer of tetrafluoroethylene and CF ₂ = CFO (CF ₂ ) ₃ COOCH ₃ with a thickness of 250μ, and a porous PTFE membrane is placed on the outside of the ion exchange membrane. The thin layer of titanium oxide is attached to the ion exchange membrane by applying pressure at a temperature of 160℃ and a pressure of 60Kg/ ^cm2 , and then
The PTFE membrane was removed to obtain an ion exchange membrane in which a porous layer of titanium oxide was adhered to one side of the ion exchange membrane. The ion exchange membrane was immersed in a 25% by weight aqueous sodium hydroxide solution at 90°C for 16 hours for hydrolysis. After that, on the titanium oxide side of the ion membrane, there is a nickel expanded metal with a major axis of 5 mm and a minor axis of 2.5 mm, and on the other side there is a coating layer containing ruthenium oxide, iridium oxide, and titanium oxide in a ratio of 3:1:4.
After bringing titanium expanded metal with a diameter of 2.5 mm into pressure contact, the ionic structure was used to assemble an electrolytic cell, with the titanium expanded metal side serving as an anode and the nickel expanded metal side serving as a cathode. Ferric chloride is dissolved and supplied to the anode chamber of the electrolytic cell so that 5N saline solution contains 2 mg/l of iron, and the saline solution in the anode chamber has a concentration of 4N PH.
Electrolysis was carried out at 90° C. while maintaining the concentration of caustic soda in the catholyte at 35% by weight by supplying water to the cathode chamber, and the following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 3.02 40 3.41 Also, when electrolysis was carried out at a current density of 20 A/dm ² for 30 days, the cell voltage was 3.03 V, and the increase in voltage was practically It wasn't recognized. Additionally, the current efficiency for caustic soda generation during this period remained constant at 93%. Analysis of the ion exchange membrane after the above electrolysis showed that
0.140 mg of iron was detected per 1 cm ² of the membrane surface, and iron was a hydroxide. Comparative Example 1 Electrolysis was carried out in exactly the same manner as in Example 1, except that iron-free saline solution was supplied, and the following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 3.23 40 3.69 Example 2 In Example 1, zirconyl chloride was dissolved in 5N saline solution so that it contained 2 mg/l of zirconium, and the solution was supplied to the anode chamber. Electrolysis was carried out under exactly the same conditions as in Example 1, except that the pH of the anolyte was kept at 4, and the following results were obtained. Current density (A/dm ² ) Cell voltage 20 3.05 40 3.43 Furthermore, when electrolysis was carried out at a current density of 20 A/dm ² for 30 days, the cell voltage was 3.06 V, with virtually no increase in voltage observed. Ta. During this period, the current efficiency for caustic soda production remained constant at 94%. Analysis of the ion exchange membrane after the above electrolysis showed that
0.158 mg of zirconium was detected per 1 cm ² of the membrane surface, and the zirconium formed a hydroxide. Example 3 In Example 1, instead of titanium oxide, a porous layer containing niobium pentoxide at a ratio of 2 mg/cm ² was attached to the cathode side of the ion membrane, and the porous layer was further added to the supplied brine.
Example 1 except that no HCl was added, the pH was maintained at 4.5, and electrolysis was performed while continuously adding metal aluminum powder to a concentration of 1 mg/l, and after 14 days, the supply of aluminum was stopped. In exactly the same manner, the following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 3.07 40 3.40 In addition, the current efficiency of caustic soda production when electrolysis is continued at 40 A/dm ² is 93.0%, and after 115 days of operation, the current efficiency for generating caustic soda per 1 cm ² of membrane , 0.183mg of aluminum was detected. Example 4 In Example 1, a porous layer containing iron oxide at a ratio of 1 mg/cm ² was attached to the cathode side of the ion membrane instead of titanium oxide, and the pH was adjusted to 4.5 without adding HCl to the supplied brine. Copper powder at 1mg/l
The following results were obtained in exactly the same manner as in Example 1, except that electrolysis was carried out while continuously adding copper so that the amount of copper was continuously added, and after 14 days, the supply of copper was stopped. Current density (A/dm ² ) Cell voltage (V) 20 3.06 40 3.38 In addition, the current efficiency of caustic soda production when electrolysis is continued at 40 A/dm ² is 93.5%, and after 109 days of operation, the current efficiency for generating caustic soda is 93.5 ^% . 0.245 mg of copper was detected.

Claims (1)

[Claims] 1. In a method of introducing an aqueous alkali chloride solution into the anode chamber of an electrolytic cell having an anode chamber and a cathode chamber separated by an ion exchange membrane and electrolyzing it to produce caustic alkali in the cathode chamber, the cathode of the ion exchange membrane A gas and liquid permeable porous layer having no electrode activity is formed on the side surface, a cathode is disposed through the porous layer, and hydroxide is formed in the aqueous alkali chloride solution at pH 1 to 5. 1. A method for producing caustic alkali, which comprises adding a metal or metal ion that can be used for electrolysis.