JPH01316480A - Production of potassium hydroxide solution - Google Patents

Abstract

PURPOSE:To produce a high-purity KOH soln. from a KCl soln. with high current efficiency by utilizing a fluorine-series cation exchange membrane made of a perfluorocarbon polymer incorporating sulfonate group as an ion exchange group for a diaphragm and specifying amount of water permeability at a time of electrolysis and performing electrolysis. CONSTITUTION:In case of electrolyzing a KCl soln. to produce a KOH soln., a fluorine- series cation exchange membrane made of a perfluorocarbon polymer incorporating sulfonate group as an ion exchange group is utilized as a diaphragm and electrolysis is performed while maintaining the amount of water permeability at the time of electrolysis at 3-4 mols per 1mol K ion allowed to permeated through this diaphragm. At this time, when the concn. of the KCl soln. in an anodic chamber is regulated to about 0.8-2.5mol/l and current density is regulated to about 20-80A/dm<2> and electrolysis temp. is regulated to about 100 deg.C higher than room temp. and the diaphragm having 0.5-2.5 equivalent/g-dry resin exchange capacity is utilized, the concn. of the KOH soln. in a cathodic chamber can be maintained at about 15-45wt.%. Thereby low voltage and high current efficiency are maintained in a range over a long period and the high-concn. KOH soln. is produced with stabilizes electrolysis behavior.

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a method for producing a potassium hydroxide solution by electrolysis of a potassium chloride solution using a diaphragm. More specifically, the present invention relates to a method of performing electrolysis using a fluorine-based cation exchange membrane made of a perfluorocarbon polymer with sulfonic acid groups as ion exchange groups as a diaphragm.

[Prior Art] Potassium hydroxide has a wide range of applications as one of the basic inorganic chemicals.

The potassium hydroxide solution is produced by electrolyzing a potassium chloride solution, and the mercury method and the asbestos diaphragm method are used industrially.

In recent years, in the electrolysis method of sodium chloride solution, in order to prevent environmental pollution and save energy in the process,
An electrolysis method using a new fluorine-based cation exchange membrane has been developed and is being implemented as an industrial process collectively referred to as ion exchange membrane salt electrolysis method.

Among the electrolysis methods for potassium chloride solutions, the electrolysis method using this fluorine-based cation exchange membrane, that is, the ion exchange membrane electrolysis method, is attracting attention, and several electrolysis methods have already been proposed.

For example, Japanese Patent Publication No. 53-3998 discloses a method of electrolyzing a potassium chloride solution while maintaining the water content of a fluorine-containing cation selectively permeable membrane at 3 to 6% by weight.

It has been known for a long time that the water content of an ion-selective membrane is related to the membrane's permselectivity, and the technology disclosed here is fundamental in the ion-exchange membrane electrolysis method. considered to be a thing. In other words, what governs the permselectivity of an ion exchange membrane is the fixed ion concentration CX of the membrane, as has long been known from the theory of Donnan equilibrium and Donnan potential.
(meq/g-1120).

This fixed ion concentration is determined by the ion exchange membrane ff1cr (■eq/g-dry resin), which is also well known as one of the membrane characteristics, and the water content V (g-H20/g-dry resin).
The relationship is CX-Cr1w with .

In order to increase the permselectivity of the membrane, it is necessary to increase % Cr, and it is natural for ion exchange membrane electrolysis to select a water content V that corresponds to Cr. As described above, the water content is a basic concept, but the problem is that it is extremely difficult to measure the water content during electrolysis operation, which is nearly impossible under industrial electrolysis conditions. Therefore,
Control on an industrial scale using this value is extremely difficult.

Furthermore, in JP-A No. 57-35888, in the electrolysis of a potassium chloride aqueous solution using a fluorocarbon-based cation exchange membrane in which the equivalent weight of sulfonic acid groups is 1000 to 1300, calcium in a potassium chloride salt solution in an anode chamber is disclosed. Total concentration of ions and magnesium ions is 0.05a+g/l
discloses a method for electrolyzing potassium chloride salt water under conditions where the potassium chloride concentration is 150g71 or more.

Cation impurities such as calcium and magnesium in salt water accumulate in the membrane and cause an increase in electrolytic voltage and a decrease in current efficiency, so it is extremely important to control this amount to a very small amount. Electrolytic methods have already been implemented industrially. In addition, the salt water concentration was adjusted to 150 g/l (approximately 2.6
mol/l) or more is also considered to be a matter of course based on the experience of salt electrolysis, and the conventionally known salt water concentration range is used.

Furthermore, JP-A No. 59-67379 describes a cation exchange membrane having an ion exchange capacity of 0.80 to 0.95 eq/g-dry resin, and a membrane thickness of 0.1 to 0.2+u. A perfluorosulfonic acid membrane is used to supply a potassium chloride solution with a calcium and magnesium content of 0,1*g/l or less to the anode chamber, and a potassium hydroxide concentration of 20 to 40 wt% in the cathode chamber. A method for electrolyzing an aqueous potassium hydroxide solution is disclosed, which is characterized by maintaining the potassium hydroxide aqueous solution within the range of . This technology can also be considered natural if we infer from today's experience with salt electrolysis.

In this way, various methods for producing potassium hydroxide solution by performing potassium chloride electrolysis using a fluorine-based cation exchange membrane as a diaphragm have been studied, and several proposals have been made based on experience with salt electrolysis. . However, despite such various proposals, potassium chloride electrolysis using an ion exchange membrane has not been put into practical use to date.

[Problems to be Solved by the Invention] An object of the present invention is to develop a process for producing a potassium hydroxide solution using a fluorine-based cation exchange membrane as a diaphragm, which is industrially satisfactory under high current density conditions. This paper proposes an electrolysis method that maintains low voltage and high current efficiency over a long period of time, and produces a highly concentrated potassium hydroxide solution with stable electrolytic behavior.

[Detailed Description of the Invention] In producing a potassium hydroxide solution by electrolysis of a potassium chloride solution, the present inventors used a fluorine-based cation exchange membrane made of a perfluorocarbon polymer with sulfonic acid groups as ion exchange groups as a diaphragm. 3 per mole of potassium ions permeating the water during electrolysis.
The present invention was completed based on the discovery that a highly concentrated potassium hydroxide solution can be produced with stable electrolytic behavior that is industrially satisfactory as described above by performing electrolysis while maintaining the concentration at ~4 mol.

The amount of water permeated here refers to the water flow from the anode chamber to the cathode chamber during electrolysis.
It is the amount of water that permeates through a fluorine-based cation exchange membrane, and is calculated per unit amount of electrolyzed electricity or 111 ions of permeated ions.
It is expressed as the number of moles of water that permeate per number of moles of water. The amount of permeable water is generally considered to be the sum of the water that accompanies the ions that permeate through the ion exchange membrane using the potential gradient as the driving force, and the water that permeates the ion exchange membrane by diffusion using the driving force as the concentration gradient between the anode and cathode chambers. It will be done.

In the ion-exchange membrane salt electrolysis method, the resulting water permeation amount is 3 to 4.5 moles (3 to 4.5 IIol-1120/Na+mol) per sodium ion passing through the membrane. Therefore, in the ion exchange membrane salt electrolysis method, it is known to maintain the water permeation rate at 3 to 4. However, in the method for producing a potassium hydroxide solution, maintaining the amount of water permeation within the above range is not known at all, and was discovered for the first time by the present inventors as described below.

That is, potassium chloride electrolysis is carried out under the conditions normally carried out in salt electrolysis, for example, the anode chamber salt water concentration is 2.6 to 4101/1, the cathode chamber alkali concentration is 20 to 40 vt%, and the current density is 20 to 4 OA/da2. Even so, it is difficult to maintain the amount of water permeation within the range of the present invention.

Figure 1 shows electrolysis of potassium chloride solution and sodium chloride solution using a fluorine-based cation exchange membrane made of perfluorocarbon polymer with sulfonic acid groups as ion exchange groups (exchange volume ff 10.9 m equivalent/g - dry resin). A comparison of water permeability when electrolysis (salt electrolysis) is performed is shown. Two electrolytic systems, current density 4OA/dm2, electrolysis temperature 90
℃, the alkaline concentration in the cathode chamber was kept constant at 35 νt%, and the salt water concentration was varied to determine all water permeation rates.

From the diagram! ! It can be seen that, under the same electrolytic conditions, the amount of permeable water differs by about 1.7 moles between the electrolysis of potassium chloride solution and the electrolysis of common salt, such as 1%, which shows completely different electrolytic behavior.

The preferred electrolytic conditions normally implemented in salt electrolysis are a salt water concentration of about 3 to 4.2 mol/1, but in potassium chloride electrolysis, in this salt water concentration range, the water inlet is outside the range disclosed in the present invention. Therefore, under industrial conditions for a long period of time,
It cannot show stable electrolytic behavior.

The present inventors believe that this amount of water permeation is an extremely important parameter in potassium chloride electrolysis, and after examining various electrolytic conditions and determining the relationship between electrolytic characteristics and water permeation amount, we found that the amount of water permeation during electrolysis It has been found that it is necessary to perform electrolysis while maintaining the amount of potassium ions per mole of potassium ions permeating the membrane at 3 to 4 moles.

When the amount of permeable water is less than 3 mol, there is a tendency for the electrolytic voltage to increase, and this tendency increases especially when electrolysis is performed at a current density of 4QA/da2 or higher, or when the potassium hydroxide concentration in the cathode chamber is set to 35vtX or higher. do.

This is due to the alkali concentration at the membrane and cathode chamber interface and the +
This is presumed to be because the ion concentration increases, resulting in a decrease in the water content of the membrane, an increase in membrane resistance, and a bond between fixed ions in the membrane and K ions.

Under conditions where the amount of permeable water exceeds 4 moles, the current technology is not practical because it is necessary to operate the salt water concentration under extremely low conditions as described below. When the salt water concentration is extremely reduced, the potassium chloride impurity increases in the potassium hydroxide solution, the oxygen impurity increases in the chlorine product of the anode chamber,
Furthermore, due to the presence of a diffusion layer of salt water formed at the interface between the membrane and the anode chamber, the current density approaches the critical current density region, causing an increase in electrolytic voltage and a decrease in current efficiency.

The amount of permeable water during electrolysis is an amount that can be calculated by determining the mass balance of the electrolytic reaction, and therefore, unlike the water content, it is an amount that can be determined during electrolysis operation. It is an amount that can be controlled by changing it. Controlling the amount of water permeation within the above range can be achieved by variously changing the type of fluorine-based cation exchange membrane used and the electrolytic conditions.

When performing potassium chloride electrolysis under economical electrolytic conditions using a fluorine-based cation exchange membrane made of a perfluorocarbon polymer having sulfonic acid groups as ion exchange groups and maintaining water permeation within the range of the present invention, One preferred embodiment is to vary the saline concentration.

According to studies by the present inventors, when other electrolysis variables are held constant, the amount of water permeation during electrolysis has a linear relationship with the concentration of potassium chloride solution in the anode chamber. That is, by controlling the salt water concentration during electrolysis, the water permeability during electrolysis can be controlled within the above range.

From this point of view, the concentration of potassium chloride solution in the anode chamber is 0.
．． 8mol/1 to 2.5mol/L preferably 1ffl
It is to be maintained at 011 to 2.3 mol/1.

If the potassium chloride concentration exceeds the above range, it will be difficult to maintain the lower limit of water permeation according to the present invention, causing an increase in electrolytic voltage. If the potassium chloride concentration is below the above range, as mentioned above, the potassium chloride impurity in the potassium hydroxide solution will increase, the oxygen impurity in the chlorine product in the anode chamber will increase, and furthermore, the interface between the membrane and the anode chamber will increase. Due to the presence of a diffusion layer of salt water formed in the process, the current density approaches the critical current density region, causing an increase in electrolytic voltage and a decrease in current efficiency.

In the method for producing a potassium hydroxide solution of the present invention, it is necessary to use a fluorine-based cation exchange membrane made of a perfluorocarbon polymer and having a sulfonic acid group as an exchange group as a diaphragm.

In salt electrolysis, the fluorine-based cation exchange membrane is usually a two-layer membrane consisting of a sulfonic acid base membrane and a carboxylic acid base membrane, or two layers of sulfonic acid base membranes or two layers of carboxylic acid base membranes with different exchange capacities. It is common to use a so-called two-layer membrane consisting of a sulfonic acid homogeneous membrane, and if a sulfonic acid homogeneous membrane is used, the current efficiency is low and the amount of impurities in the resulting caustic alkali increases.

However, in potassium chloride electrolysis, even if a fluorine-based cation exchange membrane made of a perfluorocarbon polymer having a sulfonic acid group as an exchange group and having a particularly uniform exchange capacity is used, if the manufacturing method limited in the present invention is used, It is possible to obtain a potassium hydroxide solution with high current efficiency and high purity, and conversely, when electrolysis is carried out for a long period of time using a two-layer membrane such as the one used for salt electrolysis as a fluorine-based cation exchange membrane. , undesirable tendencies such as accumulation of impurities and generation of blisters at the interface of the two-layer film are observed. In particular, this tendency increases when electrolysis is carried out at a current density of 40 A/ds2 or higher, or when the potassium hydroxide concentration in the cathode chamber is set to 35 wt% or higher.

This fact also illustrates that salt electrolysis and potassium chloride electrolysis exhibit completely different electrolytic behavior.

The fluorine-based cation exchange membrane used in the present invention, which is made of a perfluorocarbon polymer and has sulfonic acid groups as ion exchange groups, can have an exchange capacity of 0.5 to 2.5 equivalents/g of dry resin.

If the exchange capacity is less than the above range, the resistance of the membrane is high and the electrolytic voltage increases. If the exchange capacity exceeds the above range, problems such as expansion and collapse of the membrane will occur, which will impede stable electrolytic operation.

In the method for producing a potassium hydroxide solution of the present invention, the concentration of the potassium hydroxide solution in the cathode chamber is between tsvt% and 45vt%.
can be maintained within the range of In particular, one of the characteristics of the method for producing a potassium hydroxide solution of the present invention is that it enables industrially stable electrolytic operation for a long period of time even under high concentration conditions of 85 vt% or more.

The current density during electrolysis can be carried out in the range of 20 to BOA/da2. One of the features of the present invention is that electrolytic operation can be carried out even at a current density of 40 A/da2 or more.

In the method for producing a potassium hydroxide solution of the present invention, a fluorine-based cation exchange membrane made of a perfluorocarbon polymer with sulfonic acid as an ion exchange group is used as the diaphragm, and as long as the amount of water permeation is maintained within the range of the present invention. , other conditions may be appropriately used as known conditions.

For example, electrolysis can be carried out at a temperature ranging from room temperature to 100°C.

The anode is a porous base material such as titanium expanded metal coated with a noble metal oxide such as ruthenium or palladium, and is a so-called DSA type anode that has low chlorine overvoltage and high oxygen overvoltage. be able to.

The cathode can be a porous cathode such as expanded metal made of nickel or nickel alloy, or a low hydrogen overvoltage cathode made of this as a base material and coated with a coating containing nickel and sulfur or a nickel oxide.

[Effects of the Invention] The present invention provides an electrolytic method that maintains low voltage and high current efficiency over an industrially satisfactory long period of time and produces a highly concentrated potassium hydroxide solution with stable electrolytic behavior. It has become possible, and its industrial value is great.

[Example] Examples will be described below, but the present invention is not limited thereto.

Example 11 Comparative Example 1. 2.3 A potassium hydroxide solution was produced under the following conditions.

The electrolytic cell is a 10 cm x 10 cm acrylic electrolytic cell, and a perfluorosulfonic acid membrane with a uniform exchange capacity as a fluorine-based cation exchange membrane (exchange capacity is approximately 0.9 m equivalent/
g-dry resin, film thickness 150μ), and the anode was a DSA type with Ru02-TIO2 film on TI.
A 4-inch expanded metal is used as the cathode, and a half-inch expanded metal made of nickel is used as the cathode.
An electrode plated with nickel containing sulfur, which exhibits a low hydrogen overvoltage, was used. Note that the membrane was brought into close contact with the anode side, and the distance between the anode and the cathode was 20+m. The electrolysis conditions were: current density: 40 A/di2, electrolysis temperature: 90° C., cathode chamber alkaline concentration: 40 wt % potassium hydroxide solution, and anode chamber potassium chloride solution concentration of 1.9 aof/I.

The water permeability under these conditions is 34mol-1120/K”
It was mol.

Electrolysis operation continued for 6 months, but the electrolysis 71i pressure was 3.6
(2), which was almost constant, and the current efficiency was 95% or more.

On the other hand, as Comparative Example 1, electrolytic operation was carried out under the same conditions as in Example 1 except that the potassium chloride solution concentration in the anode chamber was set to 2.50101/1. The amount of water permeation at this time was 2X8rAol-1120/Kll1ol. Under these conditions, the initial electrolysis voltage was almost the same as in Example 1, but when electrolysis was continued for several months, the electrolysis voltage gradually increased and reached 4 V or more after 6 months.

In addition, as Comparative Example 2, electrolytic operation was carried out under the same conditions as in Example 1 except that the potassium chloride solution la degree in the anode chamber was set to 0.8 oIol, and the initial water permeation amount was 4.
．． 2a+ol-1t20/K" a+ol, but after a few days, the electrolytic voltage increased to 4 V or more, which is presumed to have approached the critical current density region.

Furthermore, as Comparative Example 3, two types of perfluorosulfonic acid and perfluorocarboxylic acid were used as a fluorine-based cation exchange membrane.
layer membrane (sulfonic acid layer: exchange capacity is approximately 0.9 m equivalent/g-
Dry resin, film thickness 125μ, carboxylic acid layer: exchange capacity is approximately 0.95°C m/g - dry resin, film thickness 25μ),
The carboxylic acid layer was placed on the cathode chamber side, and the other conditions were as in Example 1.
Electrolysis operation was carried out under the same conditions. The water permeability at this time is 3
．． 2 mol-H20/K”s.

It was l. Under these conditions, the initial electrolytic voltage is 3°6■
However, after continuing electrolysis for several months, the electrolysis voltage gradually increased to 4V or more. When the electrolytic cell was disassembled and the membrane was examined, blisters were observed at the sulfonic acid/carboxylic acid interface.

Example 2 Using a perfluorosulfonic acid membrane (exchange capacity: approximately 0.95°C m/g-dry resin, membrane thickness: approximately 150μ), Example 2
Using the same electrolytic cell, anode, and cathode, a potassium chloride solution was electrolyzed to produce a potassium hydroxide solution.

The electrolysis conditions were: current density: 50A/dm2, electrolysis temperature: 90°C, cathode chamber alkaline concentration: 35vt% potassium hydroxide solution, and anode chamber potassium chloride solution concentration of 1.3.
Electrolysis was performed with aof/I. The water permeability under this condition is
It was 3.7 mol-1t20/K mol.

Although the electrolytic operation was continued for 6 months, the electrolytic voltage remained almost constant at 3.6 V, and the current efficiency was 95% or more.

[Brief explanation of the drawing]

FIG. 1 is an example shown in the main text of the specification, and is a diagram showing a comparison of water permeability in potassium chloride solution electrolysis (1) and salt electrolysis (2).

Claims (1)

[Claims] In producing potassium hydroxide solution by electrolysis of potassium chloride solution, a fluorine-based cation exchange membrane made of perfluorocarbon polymer with sulfonic acid groups as ion exchange groups is used as a diaphragm, and the amount of water permeation during electrolysis is determined by A method for producing a potassium hydroxide solution, characterized by carrying out electrolysis while maintaining 3 to 4 moles per mole of permeated potassium ions.