JP3196382B2 - Method for electrolysis of sodium sulfate solution - 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 producing caustic soda by electrolyzing an aqueous sodium sulfate solution by an ion exchange membrane method. More specifically, the present invention provides a method for producing caustic soda by efficiently and stably electrolyzing an aqueous sodium sulfate solution from soda ash and / or sodium bicarbonate as a raw material.

[0002]

2. Description of the Related Art Hitherto, a method for producing caustic soda and chlorine by electrolyzing a saline solution by an ion exchange membrane method has been widely practiced industrially, and high production efficiency and stable operation have been established.

On the other hand, there is known a method for producing caustic soda by electrolyzing an aqueous solution of sodium sulfate instead of saline.

[0004] In the electrolytic reaction of the aqueous solution of sodium sulfate, caustic soda and sulfuric acid are generated as shown in equation (1), and the generated sulfuric acid is reacted with soda ash and / or sodium bicarbonate as shown in equation (2). The aqueous sodium sulfate solution is again converted to an aqueous solution of sodium sulfate, and the sulfuric acid is recycled if electrolyzed. As a whole, a process for producing caustic soda and carbon dioxide gas from soda ash and / or sodium bicarbonate as shown in equation (3).

(Electrolysis) Na ₂ SO ₄ + 2H ₂ O → 2NaOH + H ₂ SO ₄ (1) Na ₂ CO ₃ + H ₂ SO ₄ → Na ₂ SO ₄ + H ₂ O + CO ₂ (2) Na ₂ CO ₃ + H ₂ O → 2NaOH + CO ₂ ↑ (3) In addition, in the conventional electrolysis method, water molecules are electrolyzed as shown in equation (4), and oxygen gas is generated at the anode and hydrogen gas is generated at the cathode. Therefore, extra power is consumed.

SO ₄ ²⁻ + H ₂ O → H ₂ SO ₄ + 1 / 2O ₂ + 2e ⁻ (4) On the other hand, a hydrogen gas diffusion electrode is used for the anode, and while supplying hydrogen gas to the electrode section, electricity is supplied. When decomposed, the generation of oxygen gas at the anode is stopped, and the power consumption is greatly reduced.

SO ₄ ²⁻ + H ₂ → H ₂ SO ₄ + 2e ⁻ (5) As described above, the Glauber's salt electrolysis method is industrially advantageous in that chlorine is not produced as a by-product and power consumption can be significantly reduced. It has great advantages. However, in the method using a gas diffusion electrode, although the initial electrolysis voltage can be reduced, stable electrolysis efficiency has not yet been achieved over a long period of time.

[0008] In the electrolysis of an aqueous electrolyte solution, it is generally not preferable to electrolyze a dilute aqueous solution because the production efficiency of the electrolysis step is reduced. Furthermore, in the case of electrolysis of an aqueous sodium sulfate solution, the concentration of the aqueous sulfuric acid solution produced by electrolysis of a low-concentration aqueous solution of sodium sulfate becomes diluted. When the aqueous sulfuric acid solution is recycled or a high-concentration aqueous sulfuric acid solution is obtained, it is concentrated. And energy consumption is high, which is not preferable.

Therefore, in the step of obtaining the aqueous solution of sodium sulfate by reacting soda ash and / or sodium bicarbonate with sulfuric acid in the formula (2), it is important to obtain a concentrated aqueous solution of sodium sulfate.

Conventionally, as a method of obtaining an aqueous solution of sodium sulfate, a method of mixing an aqueous solution of soda ash and / or sodium bicarbonate with an aqueous solution of sulfuric acid is known. However, in this method, the solubility of soda ash and / or sodium bicarbonate in water is low. Therefore, it is difficult to obtain a concentrated aqueous sodium sulfate solution of 20% by weight or more. That is, the solubility of soda ash in water is about 32% by weight at 50 ° C. and about 30% by weight at 80 ° C. When a soda ash aqueous solution of about 30% by weight and a sulfuric acid aqueous solution of about 20% by weight are reacted. The concentration of the obtained sodium sulfate solution is about 18% by weight. Further, the concentration of the aqueous sulfuric acid solution obtained by electrolysis thereof becomes about 13% by weight, and concentration is required to recycle the aqueous sulfuric acid solution.

The solubility of sodium bicarbonate in water is 30 ° C.
The concentration is only about 10% by weight in the vicinity, and the concentration of the aqueous sodium sulfate solution obtained by reacting the aqueous solution of sodium bicarbonate with the aqueous solution of sulfuric acid is diluted.

In order to react soda ash and / or sodium bicarbonate with an aqueous solution of about 20% by weight of sulfuric acid to obtain a concentrated aqueous sodium sulfate solution of 20% by weight or more, solid soda ash and / or bicarbonate and an aqueous solution of sulfuric acid are used. It must be reacted directly.

That is, about 20% by weight of solid soda ash
About 25% by weight of aqueous sodium sulfate solution is obtained, and the concentration of sulfuric acid aqueous solution obtained by electrolysis is about 20% by weight, so that it can be recycled almost without concentration. Becomes When solid sodium bicarbonate is reacted with an aqueous solution of about 20% by weight of sulfuric acid, an aqueous solution of about 25% by weight of sodium sulfate is obtained in this case, and the concentration of the aqueous solution of sulfuric acid obtained by electrolysis is also about 20% by weight. Becomes
Similarly, the sulfuric acid aqueous solution can be recycled with almost no concentration.

However, in this method, since an excess of soda ash and / or sodium bicarbonate is contained in the aqueous solution of sodium sulfate, these solids accumulate in the battery case or in the piping, and the soda ash is removed. There was a problem that the sodium sulfate forms a solid solution and scales, causing trouble. Furthermore, when electrolysis is carried out using a hydrogen gas diffusion electrode, there is a problem that CO ₂ gas is generated in the anode chamber and the diffusion of hydrogen gas at the gas diffusion electrode is inhibited, so that the electrolysis efficiency is reduced. .

In the method of neutralizing the reaction by mixing soda ash and / or sodium bicarbonate and sulfuric acid in an equivalent amount, it is difficult to continuously dissolve the resulting solution.
When the H was not stable and contained excessive soda ash and / or sodium bicarbonate, the above-mentioned trouble occurred, and stable operation was difficult.

[0016]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for preparing an aqueous sodium sulfate solution from soda ash and / or sodium bicarbonate and an aqueous sulfuric acid solution, and performing highly efficient and stable electrolysis.

[0017]

Means for Solving the Problems The present inventors have conducted intensive studies in order to solve the above-mentioned problems. As a result, the solid soda ash and / or sodium bicarbonate was brought into contact with a sulfuric acid aqueous solution at a temperature of 30 ° C. or more, and the pH was adjusted to 7 or more. After obtaining an alkaline concentrated sodium sulfate solution, sulfuric acid was added to the aqueous solution to adjust the pH to 6 or less, and the above problem was solved by electrolysis at a temperature of 50 ° C. or more, and the present invention was completed. did.

In order to react solid soda ash and / or sodium bicarbonate with a sulfuric acid aqueous solution, a reaction tower is filled with solid soda ash and / or sodium bicarbonate and the sulfuric acid aqueous solution is allowed to flow through the reaction tower by bubbling or the like. Can be carried out by, for example, a method of bringing the particles into sufficient contact. The reaction is carried out at a temperature of 30 ° C. or higher to completely convert sulfuric acid to sodium sulfate.

As the starting material, soda ash and / or sodium bicarbonate can be used not only synthetic products but also natural products or purified products thereof, and each of them can be used alone or in combination.

The concentration of the aqueous sulfuric acid solution to be brought into contact with solid soda ash and / or sodium bicarbonate is preferably about 10% by weight or more, more preferably about 15% by weight to about A range of 30% by weight is desirable. Further, the sulfuric acid aqueous solution may contain Glauber's salt at all. That is, as the aqueous sulfuric acid solution used in the method of the present invention, it is preferable to use a return liquid from the electrolytic cell after electrolysis.

In the above reaction, sulfuric acid is reacted with soda ash and / or
Alternatively, it is necessary to carry out the reaction under the condition where sodium bicarbonate is present in excess, and to make the obtained sodium sulfate aqueous solution alkaline.
The pH of the obtained sodium sulfate solution needs to be 7 or more, more preferably 8 or more.

After obtaining a concentrated aqueous sodium sulfate solution having a pH of 7 or more in this way, sulfuric acid is added to neutralize excess soda ash and / or sodium bicarbonate, and further, to obtain an acidic solution having a pH of 6 or less. The pH of this solution needs to be 6 or less, and more preferably in the range of 3 to 5.

As a result, most of the carbonate ions dissolved in the aqueous solution are diffused and removed as CO ₂ gas.
Sulfuric acid to be added may be either concentrated sulfuric acid or dilute sulfuric acid aqueous solution, and it is also possible to use a return liquid from the electrolytic cell after electrolysis. In addition, it is desirable to remove solids and the like remaining in the aqueous solution of sodium sulfate by a filter or the like before feeding as an electrolytic solution.

The solubility of sodium sulfate in water reaches a maximum at about 30 ° C. and is about 32% by weight. In the range of 50 ° C to 90 ° C, the content is about 30% by weight. In the method of the present invention, the concentration of sodium sulfate in the electrolytic solution is preferably about 20% by weight or more in order to obtain the effects of the present invention. In the method of the present invention, preferred electrolysis conditions include stability and electrolysis efficiency.
The temperature is preferably 50 ° C. or more, more preferably 60 ° C. to 90 ° C. When the aqueous sulfuric acid solution obtained by the electrolysis is recycled, it is not necessary to convert all the sodium sulfate into sulfuric acid by the electrolytic reaction, and it is preferable that about 2 to 10% by weight of the sodium sulfate remains and recycled.

The production of caustic soda by electrolyzing an aqueous solution of sodium sulfate by the method of the present invention can be carried out by a conventionally known ion exchange membrane method. Examples of the ion exchange membrane used as the membrane include a fluorinated ion exchange membrane represented by Nafion membrane manufactured by DuPont, a hydrocarbon ion exchange membrane, and a modified bipolar membrane.
Further, the method of the present invention has a greater effect in the Glauber's salt electrolysis method using a gas diffusion electrode, and enables stable production of caustic soda at a low power consumption rate. For example, when a hydrogen gas diffusion electrode is used for the anode and the electrolysis is performed while supplying hydrogen gas to the anode, the configuration of the electrolytic cell is as shown in FIG.

That is, the anode chamber 2 and the cathode chamber 3 are partitioned by using the cation exchange membrane 1 as a diaphragm. A hydrogen gas diffusion electrode 4 is provided in the anode chamber 2 and a metal cathode 5 is provided in the cathode chamber 3. Further, a hydrogen chamber 10 for supplying hydrogen gas to the hydrogen gas diffusion electrode is provided. Reference numeral 6 denotes an inlet for an aqueous solution of sodium sulfate, which is a liquid to be electrolyzed. Reference numeral 8 denotes an inlet for the diluted caustic soda aqueous solution into the cathode chamber, and reference numeral 9 denotes an outlet for the produced concentrated caustic soda aqueous solution and hydrogen gas. 1
1 and 12 are an inlet and an outlet of hydrogen gas, respectively. The method of the present invention can be applied not only to the above-exemplified two-chamber electrolytic cell but also to sodium sulfate electrolysis using a three-chamber or four-chamber electrolytic cell.

As the gas diffusion electrode, a sheet made of a carbonaceous conductive porous body as a base material and a metal catalyst component such as platinum for promoting an electrode reaction dispersedly supported thereon is used. The method of the present invention can be applied not only to a hydrogen gas diffusion electrode but also to a method of using an oxygen electrode as a cathode and performing electrolysis while blowing oxygen gas. Further, the distance between the ion exchange membrane and the electrode is not particularly limited, and a method of providing the electrode in contact with the ion exchange membrane, an ion exchange membrane-electrode assembly, or the like can be used.

[0028]

EXAMPLES Hereinafter, the preparation of the aqueous sodium sulfate solution according to the present invention and the method for producing caustic soda by electrolysis thereof will be described with reference to examples, but the present invention is not limited thereto.

Example 1 A reaction tower was filled with solid soda ash powder, and the temperature was set to about 50 ° C.
And a 20% by weight aqueous solution of sulfuric acid was allowed to flow and contacted. The liquid was drained in a state where excess soda ash was present, and the solid content was separated by filtration with a filter. The concentration of the obtained aqueous sodium sulfate solution was 25.0% by weight, and the pH was 8.7.

Then, concentrated sulfuric acid was added to the aqueous sodium sulfate solution to adjust the pH to 5.0.

The acidic sodium sulfate solution obtained as described above was supplied to the electrolytic cell shown in FIG. 1 for electrolysis. The anode consisted of a porous film carrying platinum, and the cathode was made of nickel expanded metal. The membrane used is Nafion N-902 manufactured by DuPont, which is a fluorine-containing ion exchange membrane, and the membrane area is about 30 cm ² .

While supplying hydrogen gas at a rate of 1 liter / minute to the hydrogen gas supply chamber on the anode side, the current density was 30 A / d.
m ² , temperature 80 ° C, cathode chamber caustic soda concentration 30% by weight
Electrolysis was performed.

The initial electrolysis voltage is 2.36 V,
After the lapse of 00 hours, the electrolysis voltage increased only by 0.04 V.

Comparative Example 1 An aqueous solution of sodium sulfate was prepared in the same manner as in Example 1 except that concentrated sulfuric acid was not added to an aqueous solution of sodium sulfate obtained by bringing a 20% by weight aqueous solution of sulfuric acid into contact with solid soda ash. The concentration of the aqueous sodium sulfate solution was 25.0% by weight, and the pH was 8.7. This aqueous sodium sulfate solution was electrolyzed by the same hydrogen gas diffusion electrode method as in Example 1. Electrolysis was performed at a current density of 30 A / dm ² , a temperature of 80 ° C., and a concentration of caustic soda in the cathode chamber of 30% by weight.

As a result, the initial electrolysis voltage was 2.56 V, and after 400 hours, the electrolysis voltage increased by about 0.30 V.

Example 2 A reaction tower was charged with solid sodium bicarbonate powder,
While maintaining the temperature at 0 ° C, a 20% by weight aqueous solution of sulfuric acid was brought into contact.
The product liquid was extracted while excess sodium bicarbonate was present to obtain an aqueous sodium sulfate solution.

The concentration of sodium sulfate was 24.2% by weight, and the pH of the solution was 8.5. Then, concentrated sulfuric acid was added to the aqueous sodium sulfate solution to adjust the pH to 4.5.

The aqueous solution of acidic sodium sulfate obtained as described above was supplied to an electrolytic cell, and electrolysis was performed in the same manner as in Example 1. As a result of electrolysis at a current density of 30 A / dm ² , a temperature of 80 ° C., and a concentration of caustic soda in the cathode chamber of 30% by weight, the initial electrolysis voltage was 2.40 V, and after 2000 hours, the electrolysis voltage increased by 0.04 V. Met.

[0039]

According to the method of the present invention, a concentrated and stable aqueous solution of sodium sulfate is obtained from soda ash and / or sodium bicarbonate and an aqueous solution of sulfuric acid, and a high production efficiency is obtained by electrolyzing the aqueous solution. No troubles such as clogging of battery case and piping due to deposition and scaling
In electrolysis or the like using a gas diffusion electrode, there is no hindrance due to generation of CO ₂ gas in a battery case, and a stable operation is achieved with high electrolysis efficiency and almost no increase in electrolysis voltage.

[Brief description of the drawings]

FIG. 1 is a sectional view of an electrolytic cell used in Examples and Comparative Examples.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 Cation exchange membrane 1 2 Anode chamber 3 Cathode chamber 4 Hydrogen gas diffusion electrode 5 Metal cathode 6 Inlet of sodium sulfate aqueous solution 7 Outlet of generated sulfuric acid aqueous solution 8 Inlet of diluted caustic soda aqueous solution to cathode chamber 9 Generated concentrated caustic soda aqueous solution and hydrogen gas Outlet 10 hydrogen chamber 11 hydrogen gas inlet 12 hydrogen gas outlet

Claims (3)

(57) [Claims] In a method for producing caustic soda by electrolyzing an aqueous solution of sodium sulfate by an ion exchange membrane method, a sulfuric acid aqueous solution is brought into contact with solid soda ash and / or sodium bicarbonate at a temperature of 30 ° C. or more, and a pH of 7 or more. A method for electrolyzing an aqueous solution of sodium sulfate, comprising obtaining sulfuric acid solution of sodium sulfate by adding sulfuric acid to the aqueous solution to make the solution acidic to pH 6 or less, and electrolyzing at a temperature of 50 ° C. or more. 2. The method according to claim 1, wherein the concentration of the aqueous solution of sodium sulfate is 20% by weight or more. 3. The method according to claim 1, wherein a hydrogen gas diffusion electrode is used as an anode.