JPH0220713B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing alkali hydroxide, particularly to a method for obtaining alkali hydroxide by electrolyzing an aqueous alkali chloride solution at low voltage.

In recent years, as a method for obtaining alkali hydroxide by electrolyzing an aqueous alkali chloride solution, the diaphragm method has become mainstream in place of the mercury method from the viewpoint of pollution prevention.

As for the diaphragm method, instead of using asbestos as a diaphragm, several methods have been proposed in which an ion exchange membrane is used for the purpose of obtaining alkali hydroxide with higher purity and higher concentration.

On the other hand, in recent years, energy conservation has been progressing worldwide, and from this point of view, in this type of technology, it is desired to lower 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 alkali hydroxide obtained is not very high, and if this is exceeded, the electrolysis voltage will suddenly increase and the current efficiency will increase. However, these methods have not always been industrially satisfactory, such as a decrease in electrolytic voltage, or poor sustainability and durability of the electrolytic voltage drop phenomenon.

Recently, an alkali chloride aqueous solution is electrolyzed using an electrolytic cell in which an anode and a cathode made of a gas- and liquid-permeable porous layer are closely attached to the surface of a fluorine-containing cation exchange membrane to obtain alkali hydroxide and chlorine. A method is proposed. (Refer to Japanese Patent Application Laid-open No. 112398/1983)
This method enables electrolysis at a lower voltage than conventional methods because it can reduce as much as possible the electrical resistance caused by the electrolyte and the bubbles based on generated hydrogen and chlorine gas, which were considered to be unavoidable in this type of technology. This is an excellent method for doing so.

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 the electrode easily leaves the electrode, and the electrolyte is It is made gas and liquid permeable so that it can penetrate. Such porous electrodes usually consist of a porous body 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. ing.

However, according to the inventor's study, 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. , alkali resistance is required in addition to the conventional chlorine resistance, and special and expensive materials must necessarily be selected. In addition, the lifespans of electrodes and ion exchange membranes are usually very different, so if they are combined, both will have to be discarded when one of them reaches the end of its lifespan, especially in the case of expensive noble metal anodes. , the economic loss is large.

As a result of continuing research on electrolysis methods that do not have these disadvantages and have the lowest possible cell voltage, a gas and liquid permeable porous layer with no electrode activity has been developed on the surface of the cation exchange membrane. When an alkali chloride aqueous solution is electrolyzed in an electrolytic cell in which an anode or a cathode is formed, alkali hydroxide and chlorine can be obtained at an unexpectedly low voltage, and the above purpose is virtually eliminated. The applicant of this application previously filed an application as Japanese Patent Application No. 152416/1983, but the applicant had previously filed an application regarding the method of arranging the electrodes.
As a result of further investigation, the method for producing alkali hydroxide of the present invention was finally discovered. Thus, the present invention provides a cation exchange membrane having a gas- and liquid-permeable porous layer with no electrode activity on at least one surface, disposed between an anode and a cathode; at least one of them is a flexible porous electrode having greater rigidity than the cation exchange membrane, and when the flexible electrode is deformed, the flexible electrodes face each other through the cation exchange membrane. The gist of this invention is a method for producing alkali hydroxide, which is characterized by supplying alkali chloride to the anode side of an alkali chloride electrolytic cell that is in close contact with the electrode surface, and electrolyzing this.

According to the present invention, the electrode is disposed through the gas- and liquid-permeable porous layer, so that it does not come into direct contact with the membrane. Therefore, the anode is not required to have high alkali resistance, and the conventionally widely used electrodes with only chlorine resistance can be used.
At the same time, the electrodes do not need to be combined with the membrane or the porous layer, so that they cannot be disposed of along with the membrane due to its lifetime.

In addition, because the anode and cathode are placed at a nearly uniform distance between the cation exchange membranes with a porous layer, there is no localized current flow, and the current density remains constant even locally. . Since the distance between the electrodes is very small, approximately the thickness of the cation exchange membrane, it is naturally expected that the electrolytic voltage will be very small.

Furthermore, the cell voltage according to the present invention is unexpectedly low compared to, for example, a method of electrolyzing alkali chloride in an electrolytic cell in which the anode or cathode is brought into direct contact with the cation exchange membrane without passing through the porous layer. The cell voltage drops dramatically. This is because, unlike the method described in JP-A-54-112398, this can also be obtained when the porous layer is formed from a substantially non-conductive particle layer that does not have electrode activity.
I have to say that this was an unexpected effect.

The electrode used in the present invention is a porous metal such as a wire mesh or an expanded metal, or a porous metal coated with an electrode active component, and is generally quite thin, with a thickness of about 0.1 to 3 mm.

On the other hand, its size is approximately equivalent to the size of the electrode chamber, and if it is large, for example, 1 m
In some cases, it can reach 2 m.

Even if the area is smaller than this, thin electrodes as described above can be placed facing each other and close to each other via a cation exchange membrane provided with a porous layer, and the distance between the electrodes can be kept almost constant regardless of the location. It is quite difficult to do.

This is because these electrodes have a small thickness relative to their area and are therefore flexible, so they may bend due to pressure fluctuations in the electrolyte, or they may bend during the manufacturing process. .

The present inventors solved these problems by making at least one of the electrodes more rigid and flexible than a cation exchange membrane provided with a porous layer,
It has been discovered that the purpose can be fully achieved by deforming the flexible electrode toward the cation exchange membrane.

The explanation will be given below based on the drawings.

FIG. 1 is a partial cross-sectional explanatory diagram showing an example of the arrangement of a cation exchange membrane provided with a porous layer and an anode and a cathode when carrying out the method of the present invention. 1 is a cation exchange membrane with a porous layer attached, and 2 is an anode in which an anode active component is supported on, for example, an expanded metal, which is usually not completely flat, so that part is slightly exaggerated and drawn in a curved shape. . 3 is a flexible cathode.

5 is a conductive support in which a conductive wavy body is used. The conductive corrugated body 5 is placed in electrical contact with the cathode and forces it towards the porous layer attached cation exchange membrane.

FIG. 2 shows a case where a conductive network is used as the conductive support. It is arranged so as to maintain electrical contact with the conductive network 6 and the cathode, and presses the cathode toward the porous layer-attached cation exchange membrane.

FIG. 3 shows a case where the conductive support is composed of a composite of a conductive network and a conductive wavy body. The conductive multilayer structure 7 is constructed by laminating a conductive network body 71 and conductive wavy bodies 72 and 73.
1, 72, and 73 are maintained in sufficient electrical contact, and this conductive multilayer structure 7 maintains electrical contact with the cathode and extends toward the porous layer-attached cation exchange membrane. Press the cathode. The conductive composite structure 7 does not necessarily need to be composed of one conductive net-like body and two conductive wavy bodies, and may be a stack of an appropriate number of these bodies.

As explained above, the conductive support supports the flexible electrode in a planar manner. The porous layer-attached cation exchange membrane is pushed by the flexible cathode, which is deformed by force, and is deformed into the shape of the anode. In this case, since the rigidity of the flexible cathode is greater than the rigidity of the cation exchange membrane attached to the porous layer, both eventually deform to match the shape of the anode, as shown in the figure. If the rigidity is reversed, a gap may be formed between the anode and the cation exchange membrane attached to the porous layer, which is not preferable.

The porous layer may be provided on both sides of the cation exchange membrane, or may be provided only on one side.

Further, although FIGS. 1 to 3 show the case where the cathode is flexible, it goes without saying that the anode may also be flexible. Although it is possible to make both the anode and cathode flexible, it is usually better to make only one of them flexible.

According to the experience of the present inventors, when a porous layer is provided on only one side, it seems to be better to provide it on the anode side of the cation exchange membrane.

The reason for this has not been fully elucidated, but in general, anodes often do not have sufficient alkali resistance, and when they come into direct contact with a cation exchange membrane, hydroxyl groups diffuse through the cation exchange membrane. This is thought to be due to the negative effects of ions.

Hereinafter, the present invention will be explained in more detail with respect to a case where porous layers are provided on both sides of a cation exchange membrane and only the cathode is flexible, but it is understood that the present invention does not include only such a case. This is clear from the above explanation.

Now, various methods can be considered as means for pressing the flexible cathode in the direction of the porous layer-attached cation exchange membrane. It is a method in which a flexible cathode is pressed against a conductive support. This conductive support is also connected to a negative power source by other conductive means.

In the present invention, the conductive support is a planar conductive support such as a conductive wavy body or a conductive net-like body capable of supporting a flexible cathode, as described above.

In the case of the electrode used in the present invention, for example, an expanded metal such as titanium or tantalum is coated with a platinum group metal such as ruthenium, iridium, palladium, or platinum, an alloy thereof, or an oxide thereof, or an electrode made of platinum, Appropriately known anodes such as platinum group metals such as iridium and rhodium, alloys thereof, and porous plates and mesh bodies made of oxides thereof are used. Among these anodes, when using an anode coated with expanded metal such as titanium with platinum group metals, their alloys, or oxides of these metals or alloys, electrolysis can be performed at particularly low voltages. Therefore, it is preferable.

In the case of a cathode, for example, a substrate such as iron coated with platinum group metals such as platinum, palladium, and rhodium or alloys thereof, mild steel, nickel, stainless steel, etc. Used in the form of expanded metal, etc. Among these cathodes, it is preferable to use a cathode containing a platinum group metal, an alloy thereof, or nickel as an active ingredient, since electrolysis can be expected particularly at low voltage.

On the other hand, the gas- and liquid-permeable, corrosion-resistant porous layer used in the present invention is inactive as an anode or a cathode, respectively. That is, it is made of a material, for example, a non-conductive material, which has a greater chlorine overvoltage or hydrogen overvoltage than the electrodes disposed through the porous layer. For example, the material is titanium,
Zirconium, niobium, tantalum, vanadium,
Manganese, molybdenum, tin, antimony, tungsten, bismuth, indium, cobalt, nickel, beryllium, aluminum, chromium,
Iron, gallium, germanium, selenium, yttrium, silver, lanthanum, cerium, hafnium,
Oxides and nitrides of lead, thorium, rare earth elements, etc.
Carbides may be used alone or in mixtures, among which iron, titanium, zirconium, niobium,
tantalum, vanadium, manganese, molybdenum,
Oxides, nitrides, and carbides of tin, antimony, tungsten, bismuth, etc. alone or in combination are preferred. On the cathode side, iron, hafnium, titanium,
Zirconium, niobium, tantalum, indium,
Oxides, nitrides, and carbides of tin, manganese, cobalt, nickel, etc. alone or in combination are preferred.

When forming the porous layer of the present invention from these materials, the above-mentioned materials are used in powder or particulate form, and are preferably combined with a suspension of a fluorine-containing polymer such as polytetrafluoroethylene. Ru. At this time, if necessary, a porous layer is formed in which a surfactant is used to uniformly mix the two. These mixtures are suitably formed into a layer and then bonded to the surface of the ion exchange membrane using pressure and heat, preferably embedded.

In addition, the physical properties of these porous layers are almost the same on both the cathode and anode sides, with an average pore diameter of 0.01~
2000μ, porosity 10-99%, air permeability coefficient 1×10 ^-5
~10 mol/cm ² ·min·cmHg is suitable.

If any of these physical properties deviate from the above range, the desired low electrolytic voltage cannot be expected, or
Both are unfavorable because there is a risk that the electrolytic voltage drop phenomenon may become unstable. Among the above physical properties, the average pore diameter is 0.1 to 1000μ, the porosity is 20 to 98%, and the air permeability coefficient is 1×10 ^-4 to 1 mol/cm ²・min・cmHg.
It is preferable to employ this method because stable electrolytic operation can be expected especially at low voltage.

In addition, the thickness of such a porous layer is strictly determined by the material used and its physical properties, but generally
It is appropriate to use 0.1 to 500μ, preferably 1 to 300μ.

If the thickness deviates from the above range, the electrical resistance may become high or gas release may become difficult.
This is not preferable because it makes it difficult to move the electrolyte.

In the present invention, the anode placed through the porous layer is provided in contact with the surface of the porous layer. In this way, it is provided through a porous layer, and the electrode may be either the anode or the cathode, but
When provided on both the anode side and the cathode side of the ion exchange membrane, it is particularly preferable for lowering the electrolytic cell voltage.

In addition, when either the anode or the cathode is provided on the ion exchange membrane via the porous layer of the present invention, the counter electrode should have the same composition and shape as in the case of producing ordinary alkali chloride. Adopted.

In fact, as a means of providing an electrode through the porous layer, for example, the powder forming the porous layer is applied to the ion exchange membrane by screen printing or the like, and then the ion exchange membrane is bonded with heat and pressure. A method of forming a porous layer on the surface of the porous layer and pressing an electrode against the surface of the porous layer is used.

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 phosphoric acid group, or a phenolic hydroxyl group, and such a polymer includes a fluorine-containing polymer. It is particularly preferable to adopt Examples of fluorine-containing polymers containing ion exchange groups include vinyl monomers such as tetrafluoroethylene and chlorotrifluoroethylene, and perfluorinated polymers having reactive groups that can be converted into ion exchange groups such as sulfonic acid, carboxylic acid, and phosphoric acid groups. A copolymer of a vinyl monomer with a perfluorinated vinyl monomer having an ion exchange group such as a sulfonic acid, carboxylic acid, or phosphoric acid group is preferably used.

Also usable are membrane polymers of trifluorostyrene into which ion exchange groups such as sulfonic acid groups have been introduced, and styrene dibylbenzene into which sulfonic acid groups have been introduced.

Among these monomers, it is particularly preferable to use monomers that can form the following polymerized units (a) and (b), since highly pure caustic alkali can be obtained with relatively high current efficiency. .

Here, X is fluorine, chlorine, hydrogen or _-CF3 , X' is X or _CF3 ( _CF2 )m, and m is 1 to
5, and Y is selected from the following:

-P-A, -O-( _CF2 )-m(-P,Q,R)A where P is -( _CF2 )-a(-CXX')-b( _-CF2 )
-c
, Q is (-CF ₂ -O-CXX')-d, and R
is (-CXX'-O-CF ₂ )-e, and (P, Q, R)
represents arranging at least one of P, Q, and R in any order. X and X' are the same as above, n=0-1, a, b, c, d, and e are 0-6. A is -COOH, or -CN, -COF, -
Represents a functional group that can be converted to -COOH by hydrolysis or neutralization, such as COOR, -COOM, _{-CONR2R3 ,} etc. R ₁ is an alkyl group having 1 to 10 carbon atoms, N is an alkali metal or quaternary ammonium group,
R ₂ and R ₃ represent hydrogen or an alkyl group having 1 to 10 carbon atoms.

Preferred representative examples of the above Y include the following, in which A has a structure in which A is bonded to carbon containing fluorine.

x, y, z are all 1 to 10, and Z, Rf are -
It is a group selected from F or a perfluoroalkyl group having 1 to 10 carbon atoms, and A is the same as above.

1 g of dry resin composed of these copolymers
When using a fluorine cation exchange membrane with a carboxylic acid group concentration within the membrane of 0.5 to 2.0 milliequivalents,
For example, even if the concentration of caustic soda is 40% or more,
Its current efficiency reaches over 90%. Then, the concentration of carboxylic acid groups in the film per gram of dry resin is
In the case of 1.12 to 1.7 milliequivalents, it is particularly preferable because it is possible to stably obtain the above-mentioned highly concentrated caustic soda with high current efficiency over a long period of time. And to achieve such ion exchange capacity, the above
In the case of a copolymer consisting of the polymerized units of (a) and (b), preferably the polymerized units of (b) are 1 to 40 mol%, especially 3 to 40% by mole.
A suitable amount is 25 mol%.

A preferred ion exchange membrane used in the present invention is a copolymer of the above-mentioned fluorinated olefin monomer and a polymerizable monomer having a carboxylic acid group or a functional group convertible to a carboxylic acid group. It is composed of the resulting non-crosslinkable copolymer, and its molecular weight is preferably about 100,000 to 2 million, especially about 150,000 to 2 million.
1 million is appropriate. Further, in order to produce such a copolymer, one or more of the above-mentioned monomers may be used, and the resulting film may be modified by further copolymerizing a third monomer. For example, CF ₂ = CFORf
(Rf is a perfluoroalkyl group having 1 to 10 carbon atoms)
By using in combination, flexibility can be imparted to the obtained film, or CF ₂ = CF−CF=CF ₂ , CF ₂ =
By using a divinyl monomer such as CFO(CF ₂ ) ₁ to ₃ or CF=CF ₂ in combination, the resulting copolymer can be crosslinked and mechanical strength can be imparted to the membrane. The copolymerization of the fluorinated olefin monomer and a polymerizable monomer having a carboxylic acid group or a functional group convertible to the carboxylic acid group, as well as a third monomer, can be carried out by any known method. . 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, and suitable known means such as press molding, roll molding, extrusion molding, solution casting, dispersion forming, powder molding, etc. can be adopted.

The film thus obtained has a thickness of 20 to 500μ,
Preferably, it is appropriate to make it 50 to 400μ.

In addition, if the copolymer is not a carboxylic acid group itself but a functional group that can be converted into a carboxylic acid group before or after the copolymer film formation process, preferably after film formation, by appropriate treatment accordingly. , these functional groups are converted to carboxylic acid groups. For example, −CN, −
COF, −COOR ₁ , −COOM, −CONR ₂ R ₃ (M, R ₁
~R ₃ is the same as above), it is converted into a carboxylic acid group by hydrolysis or neutralization with an acid or alkali alcohol solution, and when the functional group is a double bond, it is converted into -COF ₂ . It is converted into a carboxylic acid group by reaction. Furthermore, the ion exchange membrane used in the present invention may optionally contain an olefin polymer such as polyethylene or polypropylene, preferably polytetrafluoroethylene or a copolymer of ethylene and tetrafluoroethylene, etc. during membrane formation. It can also be molded by mixing fluorine polymers, or by using fabrics such as cloth, nets, nonwoven fabrics, or porous films made of these polymers as a support, or by using metal wires, nets, or porous bodies. It is also possible to use it as a support to reinforce the membrane. The alkali chloride used for electrolysis is generally sodium chloride, but other alkali metal chlorides such as potassium chloride and lithium chloride are also available. Next, the present invention will be explained by examples.

Example 1 73 mg of tin oxide powder with a particle size of 44 μm or less was suspended in 50 c.c. of water and mixed with a polytetrafluoroethylene (PTFE) suspension (DuPont, trade name: Teflon).
30J) so that the PTFE content was 7.3 mg, and after adding one drop of a nonionic surfactant (Rohm & Haas, trade name: Triton X-100),
After stirring using an ultrasonic stirrer under ice cooling, the porous
A porous tin oxide thin layer was obtained by suction filtration on a PTFE membrane.

The thin layer had a thickness of 30μ, a porosity of 75%, and contained 5mg/cm ² of tin oxide.

On the other hand, using the same method as above,
A thin layer of 73% was obtained.

Next, each thin layer is
An ion exchange membrane made of a copolymer of tetrafluoroethylene and CF ₂ = CFO (CF ₂ ) ₃ COOCH ₃ with 1.45 meq/g resin and a thickness of 250 μ has a porous structure on both sides.
The PTFE membrane was stacked on the body side of the ion exchange membrane, and the porous thin layer was attached to the ion exchange membrane by applying pressure at a temperature of 160°C and a pressure of 60 kg/ ^cm2 .
Thereafter, the porous PTFE membrane was removed to obtain an ion exchange membrane with porous layers of tin oxide and nickel oxide adhered to each surface.

The ion exchange membrane was immersed in a 25% by weight aqueous sodium hydroxide solution at 90° C. for 16 hours to hydrolyze the ion exchange membrane.

Next, we fabricated an anode made of expanded titanium metal with an opening size of 6 mm x 13 mm and a plate thickness of 1.5 mm, coated with ruthenium oxide.
A nickel expanded metal with a thickness of 0.5 mm and a plate thickness of 0.5 mm was used as a cathode, and was arranged as shown in FIG. 1 in the following manner.

As a conductive support, a nickel plate with a thickness of 0.5 mm was made into a corrugated plate with a width of 15 mm and a pitch of 70 mm, and the peaks of this corrugated plate were welded to a nickel expanded metal cathode.

Next, after pressing the conductive support against the anode side, an electrolytic cell was assembled using a known chamber frame using a hollow pipe or the like.

Then, the saline solution in the anode chamber of the electrolytic layer was maintained at a concentration of 4N, and water was supplied to the cathode chamber to maintain the caustic soda concentration in the catholyte at 35% by weight.
The following results were obtained.

Current density (A/dm ² ) Cell voltage (V) 10 2.73 20 2.94 30 3.15 40 3.31 Example 2 Other than welding 20 mesh nickel wire mesh as a conductive support to the nickel expanded metal cathode at one location per ² cm 2 Electrolysis was carried out in the same manner as in Example 1. The results were as follows.

Current density (A/dm ² ) Cell voltage (V) 10 2.68 20 2.89 30 3.09 40 3.26 Example 3 The same nickel corrugated plate and nickel wire mesh used in Examples 1 and 2 were made into a nickel corrugated plate/nickel wave The plate/nickel wire mesh was laminated and welded to obtain a conductive coated structure. Next, the surface of the introduced conductive multilayer structure on the nickel wire mesh side was welded to the nickel expanded metal cathode at one location per 2 cm ² , and the electrolytic cell was assembled and carried out in the same manner as in Example 1. Electrolysis was carried out in the same manner as in Example 1. The results were as follows.

Current density (A/dm ² ) Cell voltage (V) 10 2.69 20 2.90 30 3.12 40 3.27

[Brief explanation of drawings]

FIG. 1 is a partial cross-sectional explanatory view showing the arrangement of a porous layer-attached cation exchange membrane and an anode and cathode for carrying out the method of the present invention using a conductive corrugated body as a conductive support. FIG. 2 is a partial cross-sectional explanatory diagram showing the arrangement of a porous layer-attached cation exchange membrane and an anode and cathode for carrying out the method of the present invention using a conductive network as a conductive support. Figure 3 shows the arrangement of the porous layer-adhered cation exchange membrane and anode and cathode for carrying out the method of the present invention, in which the conductive support is a multilayer structure in which a conductive network body and a conductive wavy body are laminated. It is a partial cross-sectional explanatory view showing. 1 to 3 show the case where only the cathode is flexible.

Claims (1)

[Claims] 1. A cation exchange membrane having a gas- and liquid-permeable porous layer with no electrode activity provided on at least one surface is disposed between the anode and the cathode, and the anode or at least one of the cathodes is a flexible porous electrode having greater rigidity than the cation exchange membrane, the flexible electrode is supported planarly on a conductive support, and the flexible electrode An alkali chloride electrolyzer, characterized in that the flexible electrode, the cation exchange membrane, and the counter electrode are brought into close contact with each other by deforming the flexible electrode. 2. The method for producing alkali hydroxide according to claim 1, wherein the conductive support is a conductive corrugated body. 3. The method for producing alkali hydroxide according to claim 1, wherein the conductive support is a conductive network. 4. The method for producing alkali hydroxide according to claim 1, wherein the conductive support is a conductive multilayer structure formed by laminating a conductive wavy body and a conductive network body.