JP3420790B2 - Electrolyzer and electrolysis method for alkali chloride electrolysis - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for electrolyzing an aqueous solution of alkali chloride with high efficiency to produce an alkali hydroxide, and more particularly, to low power consumption for industrial large-scale alkali chloride electrolysis. Type electrolytic cell and electrolysis method.

[0002]

2. Description of the Related Art So-called soda electrolysis for electrolytically producing sodium hydroxide and chlorine from an aqueous solution of alkali chloride, particularly saline, is the main component of industrial electrolysis and is the main product, sodium hydroxide. It is a useful electrolysis that can use chlorine as a product, and many improvements have been made regarding its production method and electrolytic cell. The most advanced industrial method at present is the so-called ion exchange membrane method, which has an overvoltage of several tens of mV.
In addition, a low insoluble anode and an activated cathode with an overvoltage of about 100 mV are used, and the electrolysis voltage of the ion-exchange membrane sandwiched between them is improved. It has reached a stage where it is almost impossible to desire further energy saving except the unavoidable ohmic loss of about 3V against V.

On the other hand, the product chlorine gas and alkali hydroxide are used 100%, but hydrogen gas is partially used among the substances represented by the total reaction formula 2NaCl + H ₂ O → 2NaOH + Cl ₂ + H _2. However, it is currently underutilized and it is predicted that it will not be utilized in the future. From the viewpoint of theoretical decomposition voltage, the voltage associated with hydrogen generation is about 0.83 V, and if hydrogen generation is not involved, it is possible to reduce the power consumption corresponding to the voltage. A means developed for this purpose is an oxygen gas depolarizing electrode (gas cathode), and by supplying oxygen gas to the cathode, the cathodic reaction can be carried out by conventional H ₂ O + e → OH ⁻ + 1/2 H ₂ (− 0.83 V) from the _{H 2 O + 2e + 1/2 O} 2 → 2OH - ( be substituted in the reaction power consumption savings that can electrolyte which corresponds to approximately 1.2 V in addition theoretically no hydrogen generated by the 0.40 V) It becomes possible to provide the system.

This gas cathode itself is well known (for example, Japanese Examined Patent Publication No. 2-29757 and Japanese Patent Laid-Open No. 59-25179), and all of them achieve a voltage drop of about 0.8 to 1V. The gas electrode has a hydrophobic porous layer on one surface and a hydrophilic layer carrying an electrocatalyst on the other surface or on the hydrophobic layer, and is mainly manufactured by carrying platinum on a conductive carbon surface. However, although these electrodes show good performance in the initial stage of electrolysis, the catalyst itself comes into direct contact with concentrated alkali hydroxide, and the electrochemical reaction takes place in it, so that the resistance of the catalyst is insufficient and the catalyst activity is short-term. However, it is extremely difficult to produce a large-area gas electrode in which gas and liquid do not leak to each other, and no gas electrode has been put into practical use for industrial-scale electrolysis. Furthermore, when air is used as the gas in this gas electrode, carbon dioxide in the air reacts with alkali hydroxide in the electrolytic chamber and is converted into sodium carbonate to prevent clogging of the membrane and prevent it from becoming unusable. Removal of carbon dioxide in the used gas is a big problem.

[0005] The ion exchange membrane before the gas electrode is provided, H ⁺ (anode) in the electrolyte chamber from the gas electrode and OH - ^(cathode)
Is proposed, and this proposal seems to be advantageous for increasing the size of the gas electrode, but the specific conditions of use are unknown, and it has not yet been put into practical use. As for energy saving by using gas electrodes for alkaline chloride electrolysis, there are some predictions for practical use, but concrete means such as means for realization and longer life of gas electrodes are still unseen. It has not been issued.

[0006]

An object of the present invention is to provide an alkaline chloride electrolytic cell using a gas cathode and an electrolyzing method, and more particularly to an alkaline chloride electrolytic cell capable of maintaining a good electrode performance of the gas cathode for a long time and increasing in size, and the electrolytic method. An object is to provide an alkali chloride electrolysis method using a bath.

[0007]

The alkali chloride electrolytic cell according to the present invention is divided by a cation exchange membrane into an anode chamber containing an anode and a cathode chamber containing a gas cathode. An electrolytic cell for alkaline chloride electrolysis characterized by being divided into a solution chamber in contact with the anode chamber and a gas chamber containing the gas cathode by an ion exchange membrane, and the method of the present invention is an electrolyzer having such a configuration. Alkali chloride aqueous solution to the anode chamber of the tank, electrolyzing the alkaline chloride aqueous solution while supplying the oxygen-containing gas to the gas chamber, to produce an alkali hydroxide in the solution chamber, an alkali chloride electrolysis method Is. The present invention will be described in detail below.

When a normal gas electrode is used for alkaline chloride electrolysis, the gas electrode is brought into direct contact with concentrated alkali hydroxide, and the chemical corrosion of the alkali hydroxide causes the catalyst layer of the gas electrode to be consumed, which results in alkaline chloride electrolysis. It is considered to be one of the causes of shortening the life of the gas electrode. However, the present inventors recognized that the role of the cathode in the alkali chloride electrolysis was to supply hydroxide ions to the cathode chamber, and tried to eliminate other functions as much as possible. That is, the electrolytic cell and the electrolysis method of the present invention divide the electrolytic cell into an anode chamber and a cathode chamber by a cation exchange membrane, and further, a solution chamber in which an aqueous alkali hydroxide solution that forms the cathode chamber by the anion exchange membrane exists. And a gas chamber in which the gas electrode is present, and it is intended to prolong the life of the gas electrode by preventing the gas electrode from coming into direct contact with a concentrated aqueous alkali hydroxide solution due to the presence of the anion exchange membrane. .

In this way, the cation exchange membrane, the anion exchange membrane and the gas cathode are arranged, and an aqueous solution of alkali chloride is placed in the anode chamber, water or a diluted aqueous solution of alkali hydroxide is placed in the solution chamber of the cathode chamber, and oxygen or air is placed in the gas chamber. When oxygen-containing gas is supplied, sodium ions in the anode chamber permeate the cation exchange membrane and reach the solution chamber, and hydroxide ions generated in the gas chamber permeate the anion exchange membrane and reach the solution chamber. , Alkali hydroxide is generated in the solution chamber. In this electrolysis flow, permeation of sodium ions in the alkali hydroxide in the solution chamber of the cathode chamber toward the gas chamber is blocked by the anion exchange membrane, and hydroxide ions generated on the surface of the gas cathode are charged by the charge. It is prevented that it permeates the anion exchange membrane and moves toward the solution chamber and again contacts the cathode, and this migration of the hydroxide ions further reliably prevents the contact of the sodium ions with the gas cathode, and substantially The gas cathode does not come into contact with the alkali hydroxide aqueous solution, so that the life of the gas cathode can be extended.

Further, in comparison with the system in which the cathode chamber is divided into a solution chamber and a gas chamber by the gas cathode itself which is porous, the electrolytic cell and the electrolysis method in the present invention are to separate the chambers from the gas cathode and the gas. Since it uses an anion exchange membrane that is much denser than the cathode and has excellent liquid impermeability, gas-liquid leaks, especially alkali hydroxide aqueous solution in the solution chamber, are reliably prevented from permeating into the gas chamber, and the gas cathode is degraded. In addition, since it is possible to suppress a decrease in current efficiency, it can be used for electrolysis on an industrial scale that requires operation at a high current density. However, as the electrolytic reaction progresses, the contact area between the catalyst and the gas becomes smaller, or the movement of the generated ions is slightly restricted, so that the desired performance is not obtained as in the case of a small electrolytic cell such as a fuel cell. It is not realized, and the overvoltage tends to increase especially at a high current density, and it is desirable to operate at a current density of 5 KA / m ² or less for practical use.

The practical current density of alkaline chloride electrolysis is 3
At ~ 4 KA / m ² , it depends on the performance of the anion exchange membrane, but compared to the case of the conventional hydrogen generating cathode, when electrolysis is performed while supplying oxygen to the cathode chamber, about 0.7 V, air is supplied. When electrolysis is performed while supplying, a decrease in electrolysis voltage of about 0.5 V is observed. Further, the carbon dioxide in the above-mentioned supply gas may cause the clogging of the membrane and is not an exception in the present invention, but the clogging of the membrane is substantially caused by passing the oxygen-containing gas through the lime water before the feeding. Can be prevented.
This is presumed to be because the gas and the liquid do not directly associate with each other in the hydrophilic layer as in the conventional case. The electrolytic cell of the present invention is preferably constructed as a conventional filter press type electrolytic cell.

The anion exchange membrane used in the present invention is about 30%.
It must be resistant to high concentrations of high temperature aqueous alkali hydroxide solution. For that purpose, it is preferable to use a fluororesin-based ion exchange membrane like that used in conventional cation exchange membranes, but a hydrocarbon-based ion exchange membrane can also be used without problems in a few months of operation. ,
It is speculated that this is because the migration of hydroxide ions from the cathode side, which is a characteristic action of the gas cathode, and the accompanying water that protects the anion-exchange membrane surface and that there is no agitation effect on bubbles due to gas generation. To be done. However, in the case of continuous operation for one year or more, the above-mentioned fluororesin type ion exchange membrane should be used. The anion exchange membrane that can be used in the present invention is manufactured by Tokuyama Soda Co., Ltd. under the trade name Neoceptor ACLE
-5P, Tosflex IE-SF34, which is a fluorine-based anion exchange membrane manufactured by Tosoh Corporation, and the like. Further, the anion exchange membrane may be formed from a granular anion exchange resin.

This anion exchange membrane prevents the gas cathode from coming into direct contact with the alkali hydroxide aqueous solution, and
It also prevents the aqueous alkali hydroxide solution in the solution chamber from penetrating the anion exchange membrane and coming into contact with the gas cathode. Therefore, the periphery of the anion exchange membrane must be securely fastened by a chamber frame of the electrolytic cell or the like to prevent liquid leakage. Further, it is desirable to use an SPE type in which both the anion exchange membrane and the gas cathode are brought into close contact with each other in order to minimize the liquid resistance between them, but they may be set apart from each other.

The gas cathode installed on the gas chamber side of the anion exchange membrane has a function of generating hydroxyl ions by the reaction of H ₂ O + 2e + 1/2 O ₂ → 2OH ⁻ , and the generated water The acid ions are transferred to the solution chamber by the electric field through the anion exchange membrane. The gas cathode may or may not be of the SPE type that comes into contact with the anion exchange membrane as described above, but it is arranged in the gas chamber so as not to come into contact with the liquid. Therefore, since no association occurs in the liquid phase, even an electrode composed of two layers, a hydrophobic layer and a hydrophilic layer, like a conventional gas electrode, or an electrode in which a catalyst is embedded in the hydrophobic layer Good. As the gas cathode, for example, a conventional gas electrode prepared by mixing a carbon powder carrying a catalyst such as platinum or silver and a water repellent resin such as polytetrafluoroethylene (hereinafter referred to as PTFE) and processing it into a sheet shape may be used. It is also possible to use carbon cloth or a metal mesh as the core material. As a current collector for supplying power to the gas electrode, for example, a silver-plated nickel porous body (expanded mesh or the like) is used, and this is pressed against the gas cathode to supply power.

As the anode accommodated in the anode chamber separated from the solution chamber by the cation exchange membrane, a catalyst layer mainly composed of a noble metal oxide is carried on a valve metal substrate such as titanium known as a dimensionally stable anode. Electrode (DSE)
Is preferably used. As the cation exchange membrane, it is preferable to use a fluorine-based ion exchange membrane used for ordinary ion exchange membrane method alkali chloride electrolysis, and examples of these ion exchange membranes include Nafion under the trade name of DuPont, USA, Asahi Glass Co., Ltd. The brand name is Flemion, and the brand name of Asahi Kasei Co., Ltd., Aciplex F.

Next, the method of the present invention will be described with reference to the accompanying drawings. FIG. 1 is a schematic vertical sectional view showing an example of an alkali chloride electrolytic cell according to the present invention. The electrolytic cell body 1 is divided into an anode chamber 3 and a cathode chamber 4 by a cation exchange membrane 2, and in the anode chamber 3 a DSE is formed so as to be in close contact with the cation exchange membrane 2.
Etc., the anode 5 is installed. The cathode chamber 4 is further provided with a solution chamber 7 on the side of the anode chamber 3 by means of an anion exchange membrane 6, a gas cathode 8 in close contact with the anion exchange membrane 6, and a mesh-shaped current collector 9 for supplying power to the gas cathode 8. Is divided into a gas chamber 10 in which is stored. A salt water supply port 11 and a salt water and chlorine gas extraction port 12 are installed at the lower and upper portions of the side wall of the anode chamber 3, and a diluted alkaline hydroxide aqueous solution supply port 13 and a concentrated aqueous solution are provided at the lower and upper surfaces of the solution chamber 7, respectively. An alkali hydroxide aqueous solution outlet 14 is installed, and a gas chamber 10
The upper and lower side walls of the
15 and an oxygen-containing gas outlet 16 are installed.

When salt water is supplied to the anode chamber 3 of the electrolytic cell body 1, a dilute alkaline hydroxide aqueous solution is supplied to the solution chamber 7, and an oxygen-containing gas is supplied to the gas chamber 10, both electrodes are energized to generate sodium in the anode chamber. Ions permeate the cation exchange membrane 2 to reach the solution chamber 7, are generated on the surface of the gas cathode 8 and permeate the anion exchange membrane 6 to reach the solution chamber 7 and react with the hydroxide ions to react with alkali hydroxide. To generate. Although it is desirable to circulate the alkaline hydroxide aqueous solution in the solution chamber 7 to gradually increase the concentration, even if the concentrated alkaline hydroxide aqueous solution is present in the solution chamber 7, the solution chamber 7 and the gas chamber 10 are separated from each other. The partitioning anion exchange membrane 6 surely prevents the contact of the gas cathode 8 with the concentrated aqueous alkali hydroxide solution and the leakage of the aqueous alkali hydroxide solution, so that the life of the gas cathode 8 is extended. Further, unlike the case of a normal gas electrode, since the solution chamber and the gas chamber are partitioned by an ion exchange membrane, the alkali hydroxide aqueous solution is reliably prevented from leaking and can be used even in a large electrolytic cell.
The electrolyzer shown can also be applied to industrial scale electrolysis.

[0018]

EXAMPLES Next, examples illustrating the electrolysis of an aqueous alkali chloride solution according to the present invention will be described, but the present invention is not limited thereto.

Example 1 As a cathode, a surface of a thin carbon fiber cloth was coated with a catalyst-free graphite powder, and a graphite powder having platinum supported on the surface by a sputtering method was kneaded with a fluororesin to form a thin film. A weight was placed on the cloth so that the gas cathode was baked for 30 minutes at 250 ° C. The amount of platinum carried was 15 g / m ² .

On the surface of this gas cathode is a fluororesin-based anion exchange membrane, Tosflex (T
osflex) IE-SF34 is adhered, and the nickel-made openings 6 are silver-plated from the opposite side of the anion exchange membrane.
An expanded mesh of × 3.5 mm was pressed as a current collector and assembled into an experimental electrolytic cell with an electrolysis area of 50 × 125.
A (ruthenium-titanium) oxide-based DSE electrode was used for a titanium perforated plate as an anode. As the cation exchange membrane, Nafion 90209 (trade name, manufactured by DuPont, USA) was used to partition the anode chamber and the cathode chamber.

As the anolyte, 200 g / liter of saline was circulated and used, and as the catholyte, pure water was added so that the sodium hydroxide concentration was about 32%, and the solution was circulated about 3 times per minute. . As a cathode gas, oxygen gas generated by water electrolysis was sufficiently humidified through a water layer and then supplied to a gas chamber at a pressure of 30 cm of water column.

When electrolysis was carried out at a temperature of 90 ° C. and a current density of 30 A / dm ² , the cell voltage was 2.4 V, showing a decrease of 0.7 V from 3.1 V in the case of using a normal activation cathode. Electrolysis was continued for 6 months, but no deterioration in performance was observed. Further, it was found that the consumption of platinum was about 1 g / m ² , and the consumption rate was extremely small.

[0022]

Example 2 In the same manner as in Example 1, a kneaded material obtained by kneading a powder of a tertiary ammonium anion exchange resin with a PTFE dispersion was applied to one side of a gas cathode supporting a catalyst, and baked at 120 ° C. Further, a PTFE resin containing fine graphite fluoride powder was baked on the surface. When this was used as a cathode and electrolysis was performed in the same manner as in Example 1, the electrolysis voltage was
The voltage was 2.5 V, which was 100 mV higher than that in Example 1, but a decrease of 600 mV was observed as compared with normal electrolysis. Moreover, no deterioration in performance was observed even after a voltage of 6 months. When air that passed lime water was supplied instead of oxygen gas in this example, the voltage was 2.7 V, but other performances were the same.

[0023]

Comparative Example 1 A voltage was applied under the same conditions as in Example 1 except that the anion exchange membrane was not used and the gas cathode was brought into direct contact with an aqueous sodium hydroxide solution. Sodium hydroxide began to leak to the chamber side, the cell voltage started to rise, and 14 days later, the cell voltage exceeded 3 V, and the electrolysis was stopped. After that, the electrolytic cell was disassembled, and the gas cathode was observed. As a result, its hydrophobicity was completely lost.

[0024]

The present invention is divided into an anode chamber containing an anode and a cathode chamber containing a gas cathode by a cation exchange membrane, and the cathode chamber is in contact with the anode chamber by an anion exchange membrane. And an electrolytic cell for alkaline chloride electrolysis, which is characterized by being partitioned into a gas chamber containing the gas cathode.

In the present invention, since the solution chamber in which the corrosive concentrated alkali hydroxide aqueous solution is present and the gas chamber in which the gas cathode is present are partitioned by the dense anion exchange membrane, the gas cathode having poor resistance is directly Direct contact with corrosive aqueous alkali hydroxide solution is avoided, thus extending the life of the gas cathode. Moreover, since the solution chamber and the gas chamber are reliably partitioned by the anion exchange membrane, the alkali hydroxide aqueous solution is prevented from leaking into the gas chamber, and therefore the gas cathode is indirectly corrosive alkali hydroxide. Contact with an aqueous solution is also prevented, and a longer life can be achieved. And as the electrolytic cell becomes larger, it becomes difficult to prevent the leak of the aqueous alkali hydroxide solution into the gas chamber of the solution chamber, but in the electrolytic cell of the present invention, the leak can be almost completely prevented as described above. The invention enables the electrolytic cell to be upsized and thereby to operate on an industrial scale.

Similarly, in the case of the electrolysis method according to the present invention, the life of the gas cathode can be extended by using the anion exchange membrane and the size of the electrolytic cell can be increased.

[Brief description of drawings]

FIG. 1 is a schematic vertical cross-sectional view showing an example of an alkali chloride electrolytic cell according to the present invention.

[Explanation of symbols]

1 ... Electrolyzer main body 2 ... Cation exchange membrane 3 ...
・ Anode chamber 4 ・ ・ ・ Cathode chamber 5 ・ ・ ・ Anode 6 ・ ・ ・ Anion exchange membrane 7 ・ ・ ・ Solution chamber 8 ・ ・ ・ Gas cathode 9
... Current collector 10 ... Gas chamber

   ─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Shuji Nakamatsu               5568-4 Obiwa, Fujisawa City, Kanagawa Prefecture               Race II 102                (56) References JP-A-56-69384 (JP, A)                 JP-A-56-47578 (JP, A)                 JP 54-132498 (JP, A)

Claims (2)

(57) [Claims] 1. A solution chamber and a gas cathode which are partitioned by a cation exchange membrane into an anode chamber containing an anode and a cathode chamber containing a gas cathode, the cathode chamber being in contact with the anode chamber by an anion exchange membrane. An electrolytic cell for alkaline chloride electrolysis, which is characterized in that it is divided into a gas chamber that accommodates. 2. A gas chamber, which is divided into an anode chamber containing an anode and a cathode chamber containing a gas cathode by a cation exchange membrane, and the cathode chamber is in contact with the anode chamber by an anion exchange membrane. Aqueous alkali chloride solution in the electrolytic chamber for alkaline chloride electrolysis divided into a gas chamber containing the alkaline chloride aqueous solution, while performing the electrolysis of the alkaline chloride aqueous solution while supplying an oxygen-containing gas to the gas chamber, in the solution chamber A method for electrolyzing alkali chloride, which comprises producing alkali hydroxide.