JPS6223076B2 - - 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 obtaining caustic alkali by electrolyzing an aqueous alkali chloride solution at low voltage using an ion exchange membrane. In recent years, as a method for obtaining caustic alkali by electrolyzing an aqueous alkali chloride solution, the diaphragm method has become mainstream instead of the mercury method from the viewpoint of pollution prevention. As for the diaphragm method, instead of the method using asbestos as a diaphragm, the ion exchange membrane method is attracting attention for the purpose of obtaining higher purity and higher concentration caustic alkali. 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 methods have been proposed to reduce the electrolytic voltage, such as considering the material, composition, and shape of the anode and cathode, or specifying the composition of the ion exchange membrane and the type of ion exchange group used. . Although all of these methods have certain effects, most of them have an upper limit where the concentration of caustic alkali obtained is not very high, and if this is exceeded, the electrolysis voltage will suddenly increase and the current efficiency will decrease. However, they are not always fully satisfactory industrially, such as causing a drop in electrolytic voltage, or being inferior in the sustainability and durability of the electrolytic voltage drop phenomenon. Recently, a method has been proposed in which a gas- and liquid-permeable anode or cathode is brought into close contact with the surface of a fluorine-containing cation exchange membrane to electrolyze an aqueous alkali chloride solution to obtain caustic alkali. (Refer to Japanese Unexamined Patent Publication No. 112398/1983) 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. In this method, the anode and cathode are 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 electrode easily leaves the electrode, and the electrolyte permeates. It is made permeable to gases and liquids so that it can be used. Such porous electrodes usually consist of a uniform mixture of active particles as an anode or a cathode, and a substance that binds them, preferably graphite or other conductive material.
It consists of a porous body formed into a thin layer. However, according to studies conducted by the present inventors, when using an electrolytic cell in which such an electrode is directly bonded to an ion exchange membrane, for example, the anode in the electrolytic cell comes into contact with hydroxide ions that diffuse back from the cathode chamber. Therefore, in addition to the conventional chlorine resistance, alkali resistance is also required, which necessarily requires special and expensive materials.In addition, since the electrode and ion exchange membrane are directly connected, the electrode reaction It has been found that the gas generated during this process may cause partial bulges in the ion exchange membrane, deteriorating membrane performance, and making stable operation difficult over a long period of time. As a result of various studies and examinations aimed at eliminating such drawbacks and finding a means to obtain caustic alkali with as low a cell voltage as possible, the present inventor discovered that a gas having no electrode activity on the surface of a cation exchange membrane. By using an electrolytic cell in which a liquid-permeable porous layer is formed and an anode or a cathode is arranged through the porous layer,
It was discovered that the above purpose could be achieved, and a patent application has already been filed.
It was proposed as No. 55-13012. Subsequently, the present inventor conducted further studies and found that a gas and liquid permeable porous layer with no electrode activity provided on the surface of the cation exchange membrane can be made of a certain material to absorb ions. It has been discovered that even stronger adhesion to the exchange membrane and a more preferable porous layer can be easily obtained with good reproducibility, and that electrolytic operation can be carried out stably over a long period of time at low voltage. Thus, the present invention provides an electrolytic cell in which an anode and a cathode are partitioned by a cation exchange membrane, wherein at least one of the anode and the cathode is made of a gas- and liquid-permeable molten metal formed on the surface of the cation exchange membrane. This method involves electrolyzing an aqueous alkali chloride solution in an electrolytic cell placed through a porous layer made of an oxide to obtain caustic alkali. According to the invention, the electrode is arranged through a gas- and liquid-permeable porous layer that does not have electrode activity, so that it does not come into direct contact with the membrane, and, for example, in the case of an anode, a high alkali resistance is required. This increases the range of choices for anode materials. Further, since the gas generated during electrolysis is not generated at the contact interface between the membrane and the electrode, the above-mentioned troubles to the membrane do not occur. On the other hand, when the present invention is adopted, the cell voltage is unexpectedly low. For example, it is difficult to electrolyze alkali chloride in an electrolytic cell in which a porous electrode such as a mesh is brought into direct contact with the membrane without using the porous layer. In comparison, it was observed that the cell voltage was considerably lower. In the present invention, a porous body made of molten metal oxide is used as the gas and liquid permeable porous layer formed on the ion exchange membrane surface. As the molten metal oxide, preferably used is a metal oxide melted and solidified in an arc furnace. Such molten metal oxides include Group A of the periodic table such as tin and lead, Group B of the periodic table such as titanium, zircon, and hafnium, Group V of the periodic table such as niobium, tantalum, iron, cobalt, etc. , nickel, aluminum, chromium or manganese oxides are preferably employed. In addition, it is appropriate for the porous layer formed on the membrane surface to have an average pore diameter of 0.01 to 2000μ and a porosity of 10 to 99%;
It is particularly preferable to employ μ and a porosity of 20 to 95%. 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 0.5 to 100μ. If the thickness deviates from the above range, there is a risk that the intended purpose of the present invention may not be fully achieved, or that gas release and electrolyte movement through the porous layer may be difficult, so it is preferable. do not have. As a means of actually forming a porous layer using the molten metal oxide, the metal oxide is used in powder form, and in order to satisfy the above physical properties, the particle size is 0.1 to 50μ. is preferable. Such powders are then combined with a suspension of a fluorine-containing polymer such as polytetrafluoroethylene. The amount of fluorine-containing polymer used at this time is:
Usually 2 to 50% by weight, preferably 5 to 30% by weight is appropriate. Further, if necessary, in order to facilitate and uniformly mix the two, an appropriate surfactant or a conductive filler such as graphite may be added. The content of the metal oxide in the porous layer is usually 0.05 to 50 mg/cm ² , preferably 0.1 to 30 mg/cm 2 .
mg/cm ² is appropriate. 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 active particles, the procedure is substantially the same. That is, it is prepared in the same manner as described in JP-A-54-112398, and is bonded to the membrane surface by the action of pressure and heat, preferably embedded.
However, if the porous layer has self-supporting properties, it does not necessarily need to be integrally embedded in the membrane surface, and may be merely in contact. In the present invention, the electrode disposed through the porous layer is, for example, a porous electrode such as a perforated plate, a net, or an expanded metal, or a gas- and liquid-permeable porous layer having electrode activity. An electrode consisting of can be used. In either case of an electrode having voids or an electrode consisting of a gas- and liquid-permeable porous layer, the material forming the anode or cathode is chosen to have a low chlorine overvoltage or hydrogen overvoltage, respectively. That is, as the anode, for example, a platinum group metal, an alloy thereof, or a conductive oxide thereof is used, and as the cathode, for example, a platinum group metal, an alloy thereof, an iron group metal, or the like is used. Examples of platinum metals include platinum, rhodium, ruthenium, palladium, and iridium, and examples of iron group metals include iron, cobalt, nickel, Raney nickel, and stabilized Raney nickel. When using a porous electrode, the electrode can be composed of the material itself that forms the anode or cathode, but platinum group metals, alloys thereof, and oxides thereof may be used. In this case, it is preferable to use these materials by coating the surface of, for example, expanded metal, which is usually made of a base metal such as titanium or tantalum. On the other hand, when the electrode is formed from a gas- and liquid-permeable porous layer, it can be formed in the same manner as the anode or cathode porous layer described in, for example, JP-A-54-112398. That is, the powder or granular material of the anode or cathode forming material is formed into a thin layer using a binder made of a fluorine-containing polymer such as polytetrafluoroethylene, along with graphite or other conductive material if necessary. These electrode porous layers, both cathode and anode, have substantially the same average pore diameter, porosity, air permeability coefficient, and thickness as the porous layer made from the molten metal oxide. In the present invention, when the anode or cathode is disposed through a porous layer made of molten metal oxide formed on the membrane surface, the electrode is preferably disposed in contact with the porous layer. Effective for lowering voltage. In particular, when the electrode is a porous layer having the above-mentioned electrode activity, it is preferable to bring the electrode into contact with the porous layer made of the metal oxide and to integrally bond the two, preferably by heating and/or pressurizing. Effective for lowering cell voltage. but,
These anodes or cathodes do not necessarily need to be placed in contact with the porous layer, and may be placed at appropriate intervals depending on the case. A porous layer made of molten metal oxide can be provided on either or both the anode side or the cathode side of the ion exchange membrane. In addition, when only either the anode or the cathode is disposed via a porous layer made of molten metal oxide according to the present invention, the opposite electrode, the anode or the cathode, has the above-mentioned porosity. An electrode made of a gas- and liquid-permeable porous material is placed directly on the anode side or the cathode side of the cation exchange membrane.
In this case, these electrodes may be provided in contact with the ion exchange membrane surface, or may be provided at predetermined intervals. However, when using an electrode made of a gas- and liquid-permeable porous material, it is preferable to bring it into contact with the membrane surface, and furthermore, to combine it and embed it. 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.
It is particularly preferable to employ a fluorine-containing polymer. Examples of fluorine-containing polymers containing ion-exchange groups include those that react with vinyl monomers such as tetrafluoroethylene and chlorotrifluoroethylene and ion-exchange groups such as sulfonic acid, carboxylic acid, and phosphoric acid groups, or that can be converted into ion-exchange groups. A copolymer with a perfluorinated vinyl monomer having a functional group is preferred. Also usable are trifluoroethylene membrane polymers into which ion exchange groups such as sulfonic acid groups have been introduced, and styrene divinylbenzene into which sulfonic acid groups have been introduced. Among these, it is preferable to use polymers that form the following polymerized units (a) and (b), respectively. (stomach)(-
CF ₂ −CXX′)−(b)

[Formula] Here, X is F, Cl, H or -CF ₃ , X' is X or CF ₃ (CF ₂ ) _n , m is 1 to 5,
Y is selected from the following: (-CF ₂ -) - _X A, -O (-CF ₂ ) - _X A,

【formula】

x, y, and z are all 1 to 10, and Z.Rf is a group selected from -F or a C ₁ to ₁₀ perfluoroalkyl group. A is −COOM, −SO ₃ M or −CN, −
Indicates a functional group _that can be converted to -COOM, _-SO3M by hydrolysis or neutralization, such as COF, _-SO2F , _-COOR1 , _-CONR2R3 . R ₁ is a C _1-10 alkyl group,
M is an alkali metal or a quaternary ammonium group, and represents R ₂ , R ₃ , H or a C _1-10 alkyl group. In the present invention, the concentration of ion exchange groups in the membrane per gram of dry resin made of these copolymers is
It has been found that when a fluorine-containing cation exchange membrane having a fluorine content of 0.5 to 4.0 milliequivalents is used, the intended purpose of the present invention can be particularly fully achieved. When the concentration of ion exchange groups per dry resin is 0.8 to 2.0 milliequivalents, it is preferable because the object of the present invention can be achieved sufficiently and stably, and in particular, the sustainability and durability of performance can be greatly improved. . A preferred cation exchange membrane used in the present invention is a copolymer of the above-mentioned fluorinated olefin monomer and a polymerizable monomer having the above-mentioned ion exchange group or a functional group that can be converted into the group. The resulting non-crosslinkable copolymer preferably has a molecular weight of approximately 100,000 to 2,000,000, particularly 150,000 to 100,000.
10,000 is preferable. Copolymerization of the fluorinated olefin monomer and a polymerizable monomer having an ion exchange group or a functional group convertible to the group, and further a third monomer can be carried out by any known method. It can be done. That is,
Polymerization can be carried out by catalytic polymerization, thermal polymerization, radiation polymerization, etc., using a solvent such as a halogenated hydrocarbon, if necessary. Furthermore, there is no particular restriction on the means for forming an ion exchange membrane from the obtained copolymer, such as press molding, roll molding, extrusion molding, solution casting method,
Any known means such as dispersion molding or powder molding may be employed as appropriate. The membrane thus obtained has a thickness of 20 to 500 mm.
μ, preferably 50 to 400 μ. Furthermore, if the copolymer is not an ion-exchange group itself but a functional group that can be converted into an ion-exchange group before or after the copolymer film-forming step, preferably after the film-forming process, an appropriate treatment is performed accordingly. , these functional groups are converted into ion exchange groups. For example −CN, −
COF, −COOR ₁ , −SO ₂ F, −CONR ₂ R ₃ (M, R ₁ ~
R ₃ is the same as above), it is converted into an ion exchange group by hydrolysis or neutralization with an acid or alkaline alcohol solution. Furthermore, the cation exchange membrane used in the present invention is
If necessary, during film formation, an olefin polymer such as polyethylene or polypropylene, preferably a fluorine-containing polymer such as polytetrafluoroethylene or a copolymer of ethylene and tetrafluoroethylene, may be mixed and molded. . Further, it is also possible to use a cation exchange membrane reinforced with a reinforcing material such as a metal wire, net, or a synthetic resin net, or provided with dimensional stability. 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, in terms of ion exchange capacity, the cathode side is a laminated film of two types of polymers with smaller ion exchange capacity than the anode side, the cathode side is a weakly acidic exchange group such as a carboxylic acid group, and the anode side is a weakly acidic exchange group such as a sulfonic acid group. Ion exchange membranes with ion exchange membranes may also be used. The porous layer, which is preferably bonded to and formed on the surface of these ion exchange membranes, has an appropriate form of ion exchange group, for example, a carboxylic acid group, so as not to cause decomposition of the ion exchange groups of the ion exchange membrane. In the case of a sulfonic acid group, the bonding is carried out in its ester form or acid form, and in the case of a sulfonic acid group, in its -SO ₂ F form, by the action of pressure and heat. Known conditions are employed as process conditions for electrolyzing an aqueous alkali chloride solution in the present invention. For example, an aqueous alkali chloride solution of 2 to 5N is preferably supplied to the anode chamber, and preferably 80 to
Electrolysis is carried out at 120°C and a current density of 10 to 100 A/ ^dm2 .
In such a case, polymerized metal ions such as calcium and magnesium in the aqueous alkali chloride solution cause deterioration of the ion exchange membrane, so it is preferable to keep the content as low as possible. Further, in order to prevent the generation of oxygen at the anode as much as possible, it is preferable to add an acid such as hydrochloric acid to the aqueous alkali chloride solution. The electrolytic cell in the present invention may be of either a monopolar type or a bipolar type as long as it has the above configuration. In addition, the material constituting the electrolytic cell is a material that is resistant to aqueous alkali chloride solutions, acids, and chlorine, such as titanium, for the anode chamber, and a highly concentrated alkali hydroxide material for the cathode chamber. Iron, stainless steel, nickel, etc., which are resistant to hydrogen, are suitable. Further, in the case of using a gas- and liquid-permeable porous electrode in the present invention, a current collector for supplying power to the porous electrode is arranged on the outside thereof. The current collector usually has a chlorine or hydrogen overpotential equal to or higher than the electrode.
For example, the anode is made of noble metal, valve metal, etc., and the cathode is made of mesh, porous material, expanded metal, etc. of iron, nickel, stainless steel, etc. These current collectors are brought into contact with the porous layer by pressing them against the porous layer or the like. Next, the present invention will be explained by examples. Example 1 73mg of molten titanium oxide powder with a particle size of 44μ or less was added to water.
A suspension of polytetrafluoroethylene (PTFE) (trade name: Teflon 30J, manufactured by Dupont) was added to this so that the amount of PTFE was 7.3 mg.
One drop of a nonionic surfactant (trade name: TRITON The thin layer has a thickness of 30μ and a porosity of 75%,
It contained 5 mg/cm ² of molten titanium oxide. Next, this thin layer has an ion exchange capacity of 1.45meq/
g A porous PTFE membrane was 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 resin thickness of 220μ, so that the porous PTFE membrane was on the outside of the membrane. Laminated to temperature 160
℃ and a pressure of 60 kg/cm ² to adhere a porous thin layer to the above ion exchange membrane.
The PTFE membrane was removed to obtain an ion exchange membrane in which titanium oxide adhered to one surface of the ion membrane. The ion exchange membrane was placed in a 25% by weight caustic soda aqueous solution at 90°C.
The ion exchange membrane was hydrolyzed by immersion for 16 hours. After that, a titanium micro-expanded metal coated with a solid solution of ruthenium oxide and iridium oxide was used as an anode on the side of the ionic titanium oxide film.
An electrolytic cell was assembled by contacting the other side of the ion membrane with a Nickel micro-expanded metal as a cathode under pressure. Then, electrolysis was carried out at 90°C while maintaining the saline solution in the anode chamber of the electrolytic cell at a concentration of 4N, and water was supplied to the cathode chamber to maintain the same concentration of caustic soda in the catholyte as 35% by weight.The following results were obtained. Ta. Current density (A/cm ² ) Cell voltage (V) 20 3.09 40 3.41 When electrolysis was carried out at a current density of 20 A/dm ² , the current efficiency for producing caustic soda was 92%. Example 2 A porous layer was attached to the ionic membrane in exactly the same manner as in Example 1 except that molten zirconium oxide was used instead of molten titanium oxide.
Electrolysis was performed under the following conditions and the following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 3.10 40 3.39 When electrolysis was carried out at a current density of 20 A/dm ² , the current efficiency for producing caustic soda was 93%. Example 3 Electrolysis was carried out in exactly the same manner as in Example 1, except that stabilized Raney nickel was further attached to the cathode side surface of the ion membrane at a rate of 5 mg/cm ² in Example 1, and under exactly the same method conditions. The following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 3.00 40 3.32 When electrolysis was carried out at a current density of 20 A/dm ² , the current efficiency for producing caustic soda was 92.5%. Comparative Example Electrolysis was performed under the same method conditions as in Example 1 without attaching anything to the ion membrane, and the following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 3.11 40 3.53 When electrolysis was carried out at a current density of 20 A/dm ² , the current efficiency for producing caustic soda was 93.5%. Example 4 1000 mg of molten tin oxide powder with a particle size of 25μ or less and 100 mg of polytetrafluoroethylene with a particle size of 1μ or less
mg, water 1.0 cc, and isopropyl alcohol 1.0 cc were mixed and kneaded to obtain a paste. The paste has an ion exchange capacity of 1.45meq/g and a dry resin thickness of 220μ.
Polytetrafluoroethylene with CF ₂ =
A porous layer containing 2 mg/cm ² of molten tin oxide was obtained by screen printing on one side of a cation exchange membrane made of a CFO(CF ₂ ) ₃ COOCH ₃ copolymer. Next, using the same method, ruthenium black was deposited on the other side of the ion exchange membrane at a rate of 1.0 mg/cm ² to obtain a cathode layer. These electrode layers were then pressure-bonded to the ion exchange membrane under the conditions of 150℃ and 20Kg/ ^cm2 , and then
The ion exchange membrane was hydrolyzed by immersion in a 25% by weight aqueous solution of caustic soda at 16 hours. Next, titanium expanded metal and nickel expanded metal coated with a mixture of ruthenium oxide and iridium oxide (3:1) are brought into pressure contact with the porous layer and cathode layer as an anode and a current collector, respectively. , 5N-NaCl aqueous solution was supplied to the anode chamber and water was supplied to the cathode chamber to bring the sodium chloride concentration in the anolyte to 4N and the caustic soda concentration in the catholyte to 35% by weight.
Electrolysis was carried out while maintaining the temperature, and the following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 2.82 40 3.10 60 3.35 In addition, the current efficiency for producing caustic soda when electrolysis was carried out at a current density of 40 A/dm ² was 92%. Example 5 Electrolysis was carried out in exactly the same manner as in Example 4, except that a porous layer containing molten iron oxide at a rate of 1.5 mg/cm ² was attached to the ion exchange membrane instead of molten tin oxide. The following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 2.80 40 3.08 60 3.31 Furthermore, the current efficiency for producing caustic soda when electrolysis was carried out at a current density of 40 A/dm ² was 93%. Example 6 In Example 4, a porous layer containing molten niobium pentoxide at a ratio of 2.0 mg/cm ² was attached to the cathode side of the ion exchange membrane, and nothing was attached to the anode side. Electrolysis was carried out in exactly the same manner as in Example 4, and the following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 3.30 40 3.40 60 3.61 In addition, the current efficiency for producing caustic soda when electrolysis was carried out at a current density of 40 A/dm ² was 93%. Example 7 In Example 4, a porous layer containing molten chromium oxide at a ratio of 2.5 mg/cm ² was placed on the anode side of the ion exchange membrane, and a porous layer containing titanium oxide at a ratio of 1.5 mg/cm ² on the cathode side. Electrolysis was carried out in exactly the same manner as in Example 4, except that the materials were brought into close contact with each other, and the following results were obtained. Current density (A/dm ² ) Cell voltage (V) 20 2.95 40 3.22 60 3.45

Claims (1)

[Scope of Claims] 1. An electrolytic cell in which an anode and a cathode are partitioned by a cation exchange membrane, wherein at least one of the anode or the cathode has a gas- and liquid-permeable membrane formed on the surface of the cation exchange membrane. 1. A method for producing caustic alkali, which comprises electrolyzing an aqueous alkali chloride solution in an electrolytic cell placed through a porous layer made of molten metal oxide. 2 Molten metal oxides are in the synchronous table - Group A, the same -
The method of claim 1, wherein the molten oxide is of group B, iron group, aluminum, chromium or manganese. 3. The method of claim 1, wherein the porous layer made from molten metal oxide has an average pore diameter of 0.01 to 2000 microns and a porosity of 10 to 99%.