JPS58515B2 - Ion exchange membrane electrolysis method for saline water - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of electrolyzing common salt containing an iron cyanide compound as a raw material in an electrolytic cell divided into an anode chamber and a cathode chamber by a cation exchange membrane.

Generally, several ppm of iron cyanide compounds such as potassium ferrocyanide, potassium ferricyanide, sodium ferrocyanide, and sodium ferricyanide are often added to solid salt for the purpose of preventing caking of the salt.

When electrolysis is performed using such salt, iron cyanate ions are oxidized by chlorine generated at the anode and become iron ions.

Electrolysis of common salt has conventionally been carried out by two processes: the mercury method and the diaphragm method.

In the case of the mercury method, even if salt containing several ppm of iron cyanide compounds is electrolyzed, the iron ions produced by oxidation in the anode chamber do not form amalgam with mercury and have a negative effect on the electrolysis. There was no.

In addition, in the case of the diaphragm method, even if salt containing several ppm of iron cyanide compound is dissolved and electrolyzed, the amount of iron ions produced by oxidation in the anode chamber is less than 2 ppm, which is one of the factors that promotes the clogging of the diaphragm. The influence of impurities such as calcium and magnesium contained in the saline solution at 20 to 5 ppm was significant, and the influence of iron ions derived from iron cyanate compounds was not a problem.

However, in an electrolytic cell that is divided into an anode chamber and a cathode chamber by a cation exchange membrane, a saline solution containing an iron cyanide compound is supplied to the anode chamber and electrolyzed, with chlorine gas flowing from the anode and hydrogen gas flowing from the cathode. When producing caustic soda in the cathode chamber, the cation exchange membrane is much denser than the asbestos diaphragm in the diaphragm method, so hydroxide precipitates are deposited in the cation exchange membrane and on the membrane surface. As a result, the phenomenon of increasing electrolytic voltage and destroying the membrane is much more significant.

Therefore, when using a cation exchange membrane, impurities that can precipitate as hydroxides, such as calcium, magnesium, iron, etc., are all contained in saline solution at 0.0%. Must be kept below lppm.

For the same reason as above, the present inventors discovered that when a saline solution containing an iron cyanide compound is supplied into the anode chamber, it will be oxidized and become iron ions, increasing the electrolytic voltage, so that the iron cyanide compound in the saline solution will also be reduced to 0.5 ppm. I found out what you need to do below.

It is only necessary to remove the iron cyanide compound in the saline solution until it becomes 0.5 ppm or less.

For this purpose, an anion exchange resin is used to remove or
Various methods can be considered, such as a method of removing it as a precipitate.

Any precipitant may be used as long as it forms a hardly soluble salt with iron ions and acid ions, and examples thereof include ferric chloride, copper chloride, zinc chloride, and the like.

However, as a result of intensive study, the inventors of Mokura found that it is economically more advantageous to oxidize and decompose "iron cyanate compounds" to form iron ions before removing them, rather than removing them in the form of iron cyanate compounds. It was found that the electrolytic voltage does not increase.

Calcium, magnesium, etc. in the saline solution are purified by adding soda carbonate, caustic soda, etc. to precipitate calcium carbonate and magnesium hydroxide.

If this liquid is further purified using a chelate resin tower,
Calcium, magnesium, etc. are all 0. It can be purified to 1 ppm or less.

Before the purification process to remove impurities such as calcium and magnesium as described above, equipment is installed to oxidize and decompose the iron cyanide compound, and if it is converted into iron ions, in the process of purifying impurities such as calcium and magnesium, This is preferable because iron ions are also removed and purified at the same time.

All commonly known oxidizing agents such as chlorine, sodium hypochlorite, hydrogen peroxide, sodium chlorate, potassium chromate, and potassium permanganate can be used as oxidizing agents for oxidative decomposition of iron cyanide compounds. However, it is preferable to use chlorine and/or sodium hypochlorite as the oxidizing agent.

These may be added to a saline solution containing iron cyanide compounds, but when using a cation exchange membrane, the salt concentration decreases in the anode chamber and the anode contains chlorine and hypochlorous acid. Remove some of the liquid and redissolve the salt in it.

Preferably, an anolyte in which chlorine gas is dissolved is used so that 30 to 200 ppm of chlorine gas remains in the saline solution containing iron cyanate ions.

Although it may remain in an amount greater than 200 ppm, it is not preferable because it gives off a strong chlorine odor when the salt is dissolved.

In this way, since the anolyte normally contains several hundred ppm of chlorine, the amount of chlorine must be kept below 200 ppm and 30 ppm.
It is preferable to dechlorinate the chlorine and use it so that pm or more remains.

Next, we will discuss the temperature of the oxidative decomposition tank.

When chlorine and/or sodium hypochlorite is used as an oxidizing agent, at temperatures below 60°C, ferrocyanic acid only becomes ferricyanic acid, and practically sufficient decomposition does not occur.

Therefore, it is necessary to maintain the temperature at 60°C or higher.

If the temperature is 60° C. or higher, the higher the temperature, the more rapidly the iron cyanide compound decomposes into iron ions.

However, a temperature of 150° C. or higher is not preferable because the equipment etc. will be severely corroded.

More preferably, the temperature is in the range of 90°C to 110°C.

Within this range, the residence time required for oxidative decomposition will be within 1 hour.

To produce caustic soda using a cation exchange membrane,
Normally, the electrolysis temperature is about 90°C.

Normally, a highly saturated saline solution is supplied to the anode chamber, and the saline solution is used in the anode chamber until the concentration of the saline solution is reduced by approximately half, and then extracted, and the anolyte is used to dissolve the salt. It is used repeatedly.

When hesodium ions move from the anode chamber through the cation exchange membrane into the cathode chamber in the electrolytic cell, about 90 g of water is usually carried along with each mol of sodium ions.

Correspondingly, the anolyte at about 90° C. extracted from the anode chamber is usually replenished with about 1/3 of room temperature water and then room temperature salt is dissolved therein.

As a result, the temperature of the nearly saturated saline solution after dissolving the iron cyanate ion-containing salt is usually about 60° C. or lower.

Therefore, in order to decompose iron cyanate, it is usually necessary not only to dissolve chlorine gas, but also to heat it to a temperature of 60° C. or higher.

On the other hand, electrolytic cells inherently generate heat.

Therefore, in order to continue electrolysis at a constant temperature, it is necessary to constantly cool it by some method.

For this reason, it is not preferable to directly supply the high-temperature saline solution in which the iron cyanate ions have been decomposed at 60° C. or higher to the electrolytic cell.

Therefore, it is preferable to heat-exchange the saline solution containing iron cyanate ions and the saline solution after the iron cyanate ions have been oxidized and decomposed at 60° C. or higher.

This may reduce the amount of steam required for oxidative decomposition.

The cation exchange membrane may be one with sulfonic acid groups, but when a liquid such as caustic soda is generated in the cathode chamber, hydroxide ions tend to move toward the anode chamber, increasing current efficiency. Therefore, as cation exchange membranes, it is recommended to use membranes with weakly acidic ion exchange groups such as carboxylic acid groups, sulfonamide groups, and phosphoric acid groups, or cation exchange membranes in which sulfonic acid groups and these weak acid groups are laminated. It's sad.

Especially in cation exchange membranes with such weak acid groups, if iron ions are present in the anolyte, iron ions will accumulate on or in the membrane and the voltage will increase easily. Ion removal is important.

Next, a typical flow sheet to which the method of the present invention is applied will be explained with reference to FIG.

The present invention is not limited to the flow sheet shown in FIG. 1. In FIG. 1, 1 is the cathode chamber of the electrolytic cell, 2 is the catholyte tank, and between 1 and 2, an aqueous solution of caustic soda is circulated. are doing.

The caustic soda aqueous solution separated in step 2 is discharged from step 3, and the hydrogen gas is discharged from step 4.

5 is a cation exchange membrane, 6 is an anode chamber of an electrolytic cell, 7 is an anolyte tank, and an anolyte is circulated between 6 and 7.

From the chlorine gas separated in step 7, the salt aqueous solution whose concentration has been reduced is extracted to a dechlorination tower 9.

In step 9, make-up water 10 is added to the fresh salt water in which the amount of dissolved chlorine gas is maintained at 30 to 200 ppm, and the pH is adjusted to a level at which magnesium hydroxide does not precipitate in the salt dissolution tank 12, that is, about 9 or less. Caustic soda is added from 11 or not, and is supplied to 12.

From No. 13, a crystalline salt containing potassium iron cyanate or the like as an anti-caking agent is supplied.

The saturated salt solution coming out from 13 is heated in a heat exchanger 14, and further heated in an oxidation decomposition tank 15 to a temperature of 60°C or higher.
It is further heated by water vapor 16.

After the residence time necessary for oxidative decomposition has elapsed in step 15, it is returned to the heat exchanger 14 again.

After being used as a heat source and cooled here, the reaction tank 17
At step 18, soda carbonate, caustic soda, etc. are added.

If necessary, barium carbonate, sodium sulfite, a precipitation accelerator, etc. are added from step 19.

20 is a thickener.

Here, the oxidized and decomposed iron cyanate ions are discharged as iron hydroxide along with magnesium hydroxide, calcium carbonate, and the like.

22 is a filter, and 23 is a chelate resin tower, in which calcium ions, magnesium ions, iron ions, etc. dissolved in the saline solution are reduced to 0.0, respectively. removed to lppm or less.

The purified nearly saturated saline solution is then replenished into the anolyte tank.

Hydrochloric acid is supplied from 24 to keep the pH in the anolyte tank constant.

The caustic soda concentration in the cathode chamber is adjusted by replenishing or not replenishing water from 25.

Example 1 In the flow sheet shown in FIG. 1, salt water with a concentration of 300 to 310 g/l is supplied from 23 to the anolyte tank 7, and hydrochloric acid is supplied from the line 24, and the circulating fluid between 7 and 6 is supplied. was adjusted to have a salt concentration of 175 g/l and a pH of approximately 2.

Further, the generated caustic soda is also circulated between the cathode circulation tank 2 and the cathode chamber 1, and is extracted from the line 3.

Water is replenished from 25 so that this caustic soda concentration becomes 21 parts.

The temperature of the circulating fluid is controlled at 90°C.

A part of the circulating brine was extracted from 7 to 9, and the dechlorination tower 9 was operated so that the chlorine concentration in the fresh brine at the outlet was 30 to 200 ppm.

Makeup water was added from 10, caustic soda was added from 11, and the pH at 12 was adjusted.

Potassium ferrocyanide is 12p as raw salt 13
Common salt containing pm was used.

In step 12, salt was dissolved to give a salt concentration of 310 g/l.

At that time, the concentration of potassium ferrocyanide was 2.2 ppm.
, the outlet temperature was 60°C.

The temperature of the oxidation decomposition tank 15 is 60°C, 70°C, 90°C, 110°C.
Table 1 shows the concentration of potassium ferricyanide in the brine at the outlet of the oxidation decomposition tank when operating at ℃ and pH=46810.

The chlorine concentration in the salt water after dissolving the common salt was 1100 pp, and the residence time of the salt water in the oxidation decomposition tank was 15 minutes.

These results show that a temperature of 60° C. or higher is required to oxidize and decompose potassium ferrocyanide, and the pH is preferably 5 to 9.

The oxidation decomposition tank was controlled at a temperature of 100°C and pH = 6, and a membrane consisting of two layers, a perfluorosulfonic acid layer and a perfluorocarboxylic acid layer, was used as the cation exchange membrane, and the current density was 40A/dm2 for one month. During continuous operation, the voltage was 3.75V and stable throughout.

On the other hand, when the oxidation decomposition tank was bypassed and the operation was performed without decomposing potassium ferrocyanide, the voltage was 3.75V at the start, but it increased over time, and after 3 days, there were more than 4 layers and the voltage continued to increase. there were.

Example 2 In the flow sheet as shown in Fig. 1, 14151
617181920 Although there is no equipment corresponding to 21, in a flow sheet in which the anolyte is supplied to 7 after passing through a column filled with anion exchange resin after 23, the anolyte tank 7 is supplied with a concentration of 300~ 310 g/l of salt water and hydrochloric acid are supplied from line 24, and the circulating fluid between 7 and 6 is adjusted to have a salt concentration of 175 g/l and a pH≈2.

Further, the generated caustic soda is also circulated between the catholyte tank 2 and the cathode chamber 1 and extracted from the line 3.

Water is replenished from 25 so that the caustic soda concentration becomes 21%.

The temperature of the circulating fluid is controlled at 90°C.

A portion of the circulating brine is extracted from 7 to 9, and chlorine in the outlet fresh brine is removed in a dechlorination tower 9.

Makeup water was added from 10, and caustic soda was added from 11.

The pH at 12 was adjusted to 7.

Potassium ferrocyanide is 12p as raw salt 13
Common salt containing pm was used.

In step 12, salt was dissolved to give a salt concentration of 310 g/l.

At that time, the concentration of potassium ferrocyanide was 2.2 ppm,
The outlet temperature was 60°C.

Furthermore, the concentration of ferrocyanate ion in the saline solution after leaving the anion exchange resin tower is 0. The concentrations of calcium ions, magnesium ions, and iron ions were all below 1 ppm.

When such saline solution was electrolyzed at a current density of 40 A/dm2 using a cation exchange membrane consisting of two layers, a perfluorosulfonic acid layer and a perfluorocarboxylic acid layer, the voltage was 3.75V. It was stable.

Example 3 In the flow sheet shown in Fig. 1, 14°1516
The corresponding equipment was removed and a new reaction tank 26 was installed instead.
A flow sheet apparatus shown in FIG. 2 equipped with a thickener 27 and a thickener 27 was used.

12pp of potassium ferrocyanide as raw salt 13
Common salt containing m was used.

In step 12, the salt was dissolved to a salt concentration of 310 g/l.

Also, at that time, the concentration of potassium ferrocyanide was 2.2 pp.
m, and the outlet temperature was 60°C.

Ferric chloride is added from line 28 to the second line of reaction tank 26.
The iron ion concentration was adjusted to 5 mg/l.

Ferrocyanate ions are precipitated and separated as iron ferrocyanide in the thickener 27, and the concentration of potassium ferrocyanide at the outlet of the thickener 27 is 0.5 ppm.
Met.

Further, in the thickener 20 and the chelate resin tower 23, calcium ions, magnesium ions, and iron ions are 0,
It was removed to below lppm.

Such saline solution and hydrochloric acid are each sent to the anode tank 7 at 23
The circulating fluid supplied from line 24 between 7 and 6 is adjusted to have a salt concentration of 175 g/l and a pH≈2.

Further, the generated caustic soda is also circulated between the catholyte tank 2 and the cathode chamber 1 and extracted from the line 3.

Water is replenished from 25 so that the caustic soda concentration becomes 21 range.

The temperature of the circulating fluid is controlled at 90°C.

A membrane consisting of two layers, a perfluorocarboxylic acid layer and a perfluorocarboxylic acid layer, was used as the cation exchange membrane, and when operating at a current density of 40 A/dm2, the voltage was stable at 3.75 V from beginning to end.

[Brief explanation of drawings]

FIGS. 1 and 2 are typical flow sheets to which the method of the invention is applied. 1: cathode chamber, 2: catholyte tank, 5: cation exchange membrane,
6: Anode chamber, 7: Anolyte tank, 12: Salt dissolution tank, 1
3: Crystal salt, 14: Heat exchanger, 15 Dioxide decomposition tank, 20
: thickener, 22: filter, 23: chelate resin tower.

Claims (1)

[Claims] 1. When electrolyzing salt containing an iron cyanide compound as a raw material in an electrolytic cell divided into an anode chamber and a cathode chamber by a cation exchange membrane, the pH of the salt water is 5 to 5. 9. Chlorine and/or sodium hypochlorite concentration is 30 to 2 in terms of chlorine.
00 ppm and at a temperature of 60 to 150°C, the iron cyanide compound in the saline solution is oxidatively decomposed with chlorine and/or sodium hypochlorite to form iron ions, and then removed. An electrolysis method characterized by supplying saline water with a concentration of 5 ppm or less to an anode chamber. The method according to claim 1, characterized in that the feed liquid to the dioxide decomposition equipment and the discharge liquid are subjected to heat exchange. 3. The method according to claim 1 or 2, wherein at least some of the ion exchange groups in the 3-cation exchange membrane are weakly acidic ion exchange groups.