KR820000613B1 - Process for the electrolysis of sodium chloride - Google Patents

Abstract

The acid concn. of dil. aq. NaCl solns. useful in the prodn. of NaOH by electrolysis was controlled by absorbing HCl(g) into the dil. aq. NaCl feed, the HCl(g) being produced from the Cl(g) and H(g) liberated in the anode and cathode compartments, resp., during electrolysis in the presence of a cation exchange membrane. e.g., an aq. 25.5% NaCl soln. was fed at 71.8 ton/h into a bipolar electrolyzer sepd. into anode and cathode compartments by a sulfonated fluoropolymer cation exchange membrane having carboxyl groups on its cathodic face and electrolysis was effected at c.d. 40 A/dm2.

Description

Salt delivery method

Figure 1 shows the process flow diagram for the salt electrolysis,

2 illustrates one embodiment of concentrating a dilute saline solution.

The present invention supplies a salt solution to the anode chamber in the electrolytic cell divided into a cathode chamber and a cathode chamber by a cation exchange membrane between the anode and the cathode, and electrolyzed while absorbing hydrogen chloride gas into the raw saline solution and / or dilute saline solution. The present invention relates to a novel salt electrolysis method for producing chlorine gas in the anode chamber and aqueous sodium hydroxide solution and hydrogen gas in the cathode chamber, respectively.

When a cation exchange membrane is inserted between the anode and the cathode, it is ideal that a current flows only by moving cations from the anode chamber to the cathode chamber, but in reality, a small amount of anions are also moved from the cathode chamber to the cathode chamber. Particularly, in electrolysis in which a salt solution is supplied to the anode chamber to produce caustic soda in the cathode chamber, the mobility of hydroxyl ions present in the cathode chamber is large. Therefore, when a cation exchange membrane is used, an amount of electricity flows due to hydroxyl ions. Moves from the cathode chamber to the anode chamber. The hydroxyl ions moved to the anode chamber are discharged at the anode to produce oxygen gas, and the chlorine gas is generated by decomposing purely chlorine gas or reacting with the generated chlorine gas. As the chlorate accumulates in the anolyte, the solubility of the salt in the anolyte is lowered.

Therefore, in electrolytic operation using a cation exchange membrane, conventionally, mineral acid such as hydrochloric acid was added to the anode chamber to neutralize hydroxyl ions migrated from the cathode chamber. Therefore, it is advantageous to use a high concentration of hydrochloric acid solution, such as 35 to 36% aqueous hydrochloric acid solution in order to prevent a decrease in the salt concentration in the anolyte solution and to maintain large decomposition of the raw saline solution. Previous technical methods have used commercial by-product hydrochloric acid or synthetic hydrochloric acid. That is, by supplying a solution to be used for the hydrochloric acid synthesis apparatus, 35% hydrochloric acid aqueous solution is synthesized as chlorine gas generated in the anode chamber and hydrogen gas generated as a by-product in the cathode chamber. However, when commercial hydrochloric acid is used, the production cost is considerably increased, and since the apparatus for synthesizing 35% hydrochloric acid solution is a very complicated structure, the construction cost is increased and the accompanying cost of cooling water is also increased.

The present invention provides a salt electrolysis method comprising the following steps (1) (2) and (3) when the saline solution is supplied to the anode chamber in an electrolytic cell divided into an anode chamber and a cathode chamber by a cation exchange membrane between the anode and the cathode. It is.

(1) step of synthesizing hydrogen chloride gas with chlorine gas and hydrogen gas generated in the electrolytic cell

(2) absorbing the hydrogen chloride gas synthesized as described above into a raw saline solution and / or a dilute saline solution to adjust the concentration of hydrochloric acid in the absorption solution from 0.1% to 20% by weight;

(3) Figures 1 and 2 of the process and the accompanying drawings for obtaining the concentration of the dilute saline solution and / or the sodium hydroxide aqueous solution generated in the cathode chamber using the reaction zone and the heat of absorption of the hydrogen chloride gas and the heat generated in the electrolytic cell are shown in FIG. Representative process flow diagrams for carrying out the process method of the invention are shown.

It is a feature of the present invention to neutralize hydroxyl ions that migrate from the cathode chamber to the anode chamber by absorbing hydrogen chloride gas directly into the raw saline solution and / or the dilute saline solution.

The dilute saline solution described in the present invention refers to the saline solution consumed during the salt concentration after electrolytic operation in the anode chamber or the saline solution consumed during the salt concentration circulated between the anode chamber and the anolyte circulation tank.

In the ion exchange membrane process method, it is advantageous to set the concentration of the dilute saline solution in order to increase the yield of the used saline raw material. When using well brine as the raw material salt, it is preferable to set the concentration of the dilute salt solution. In some cases, it is also necessary to concentrate the aqueous solution of caustic soda produced in the cathode chamber above, in which case it occurs in the electrolytic cell as described in Japanese Patent Application Nos. 128,857 1975 and 151,663 / 1977. Concentrating using heat is very advantageous. The present invention combines the above-mentioned concentration method which shows the excellent effect.

In other words, the synthesis and absorption processes of the hydrogen chloride gas are all exothermic reactions. In the synthesis process, 22 kcal of heat of reaction is released per mole, and in the absorption process, 15 to 17 kcal of heat of absorption is released per mole. When preparing an aqueous hydrochloric acid solution of 20% or more, the gas partial pressure of hydrogen chloride in the aqueous hydrochloric acid solution is increased while increasing the temperature so that the gas of hydrogen chloride is absorbed less. Therefore, the solution must be cooled to a higher temperature and kept at a lower temperature. Then, their heat of reaction and heat of absorption are lost by the cooling water. According to the present electrolytic method, these heats are dilute in the anolyte solution and are effectively used for concentrating a dilute saline solution or caustic soda solution.

The hydrogen chloride gas is absorbed into the raw salt solution or the dilute saline solution. In the case of high raw saline solutions with concentrations of 300 g to 310 g per liter, great attention should be paid to whether or not salt precipitates in the absorber. In the case of absorption into a dilute aqueous solution of salt, the concentration is considerably low, from 1 l 100 to 250 g, which is more advantageous because there is no fear of precipitation of salt.

The concentration of hydrochloric acid after absorption should be 0.1% to 20% by weight. When the concentration of hydrochloric acid exceeds 20%, the temperature of the saline solution is lowered because the partial pressure increases after the hydrogen chloride gas is absorbed into the saline solution. Must be used, and coolant must be used. In addition, when the concentration of hydrochloric acid is less than 0.1%, a large amount of dilute saline solution must be circulated.

The temperature of the absorbed saline solution ranges from 30 ° C. to 100 ° C., and the absorbing devices that can be used in the present invention include absorption devices that have been used in the past, such as packed towers, porous tube towers, and wet wall towers. wetted wall towers, spray towers, jet washers, venturi washers and the like. Hydrogen chloride gas can be synthesized from a chlorine gas and hydrogen gas generated in an electrolytic cell by a burner combustion method, an ultraviolet irradiation method and a contact method. In addition, the synthesis and absorption operations can be performed simultaneously by a combustion in liguid method. The cation exchange membrane used in the present invention is not uniquely limited, and it is possible to prevent chlorine gas generated in the anode chamber, to transport large amounts of sodium ions, and to use any cation exchange membrane. In view of the need to prevent chlorine gas, it is preferable to use a fluorocarbon type cation exchange membrane. As the ion exchangers, cation exchange groups which have been used conventionally without any limitation, such as carboxylic acid groups, sulfonic acid groups, phosphoric acid groups, sulfonamide groups, hydroxyl groups and the like can be used. It is preferable to use such a membrane because a membrane having a weakly acidic group such as a carboxylic acid group, a phosphoric acid group or a sulfonamide group at the anode portion of the membrane can transport large amounts of sodium ions.

The temperature of the electrolysis operation ranges from 50 ° C to 120 ° C, preferably between 60 ° C and 100 ° C. If the temperature of the electrolysis operation is too low, the voltage of the electrolyzer increases because the electrical resistance increases. On the contrary, if the temperature of the electrolysis operation is too high, boiling occurs and the voltage of the electrolytic cell is increased. In particular, when the hydrogen chloride gas is absorbed in a dilute aqueous solution of salt, a temperature rise state occurs during the absorption process. Therefore, the electrolytic operation temperature is preferably set between 60 ° C and 90 ° C. If the electrolytic operation temperature is to be maintained at a high temperature, it may be advantageous to absorb the hydrogen chloride gas into the brine concentrate solution which is lowered to a lower temperature after the concentration of the raw saline solution or the dilute saline solution is recovered.

DETAILED DESCRIPTION Referring to the accompanying drawings in detail, it will be appreciated that the present invention will be of great help.

In FIG. 1, 1 is a cation exchange membrane, 2 is an anode chamber, 3 is an anode chamber, 4 is an anolyte circulation tank, 5 is a catholyte circulation tank, 6 is a combustion nozzle, 7 is an absorption tower, and 8 is waste gas cleaning. Point to the tower. A circulation line is formed by lines 9 and 10 between the cathode chamber 3 and the anolyte circulation tank 5, and in some cases, an aqueous solution of caustic soda adjusted to a certain concentration by water added via the line 11 is used to circulate the cathode chamber and the anolyte solution of the electrolyzer. Circulated between tanks 5. The resulting aqueous solution of caustic soda is recovered by line 12 and transferred to a concentrator (not shown) of aqueous solution of caustic soda, and a portion of the anolyte is recovered by line 13 and transferred to a concentrator. After being used as a heat source for heating the aqueous solution, line 14 is transferred to the anolyte circulation tank. The generated hydrogen gas is recovered by line 15 after gas-liquid separation in the anolyte circulation tank 5. It is also possible to connect line 11 to line 9, in which case more caustic soda solution is recovered via line 12.

Lines 16 and 17 are circulation lines between the anode chamber 2 and the cathode chamber circulation tank 4, and the chlorine gas produced after gas-liquid separation in the catholyte circulation tank 4 recovers line 18, and part of the dilute saline solution is line 19 It is recovered by the ship and moved to the salt dissolving tower (not shown). Here, after the salt is dissolved, impurities are removed from the saturated saline solution during the purification process, and then, the line 20 is transferred to the catholyte circulation tank 4 as the raw saline solution.

A portion of the generated chlorine gas is recovered on line 21, and a portion of the generated hydrogen gas is recovered on line 22, and all of these gases are transferred to combustion nozzle 6, where hydrogen chloride gas is synthesized. The synthesized hydrogen chloride gas is absorbed into the absorption tower 7 by the dilute saline solution traveling on line 23. After the hydrogen chloride gas is absorbed, the aqueous salt solution with increased acidity and temperature is recovered to the catholyte circulation tank via line 24 to neutralize hydroxyl ions moving from the cathode chamber to the catholyte circulation tank. In addition, the heat of reaction and absorption of hydrogen chloride recovered into the catholyte are transferred to the catholyte in the electrolytic cell and used to concentrate the aqueous solution of caustic soda. After absorption, the remaining waste gas is transferred to waste gas scrubber tower 7 via line 25, where the remaining hydrogen chloride gas and chlorine gas are absorbed in the raw saline solution, which is transported on line 26, and then cooled down and discharged on line 28. It is also possible to connect line 24 to line 16, in which case the concentration of acid in the dilute saline solution withdrawn via lines 19 and 23 is lowered as appropriate. The combustion nozzle 6 and the absorption tower 7 may be installed on line 16 so that the hydrogen chloride gas is absorbed by the aqueous solution of the raw salt.

FIG. 2 shows one example process in which dilute saline solution is concentrated, the numbers shown in FIG. 2 being the same as the names described in the system, 30 is the multi-stage instantaneous evaporator, and the electrolytic operation process. Is the same as FIG.

The dilute saline solution recovered on line 19 is transferred to a hydrogen chloride gas absorption tower 7 to absorb chlorine gas synthesized by line 21 and hydrogen gas synthesized on line 22, followed by Take 24 and move to multistage evaporator. Since the heat of combustion and the heat of absorption of the hydrogen chloride gas raise the temperature of the dilute saline solution, it can be effectively used when the dilute saline solution is concentrated in a multistage instantaneous evaporator. The diluted aqueous saline solution in the multi-stage evaporator is routed via line 31 to the catholyte circulation tank 4 as the aqueous saline solution to be concentrated.

33 indicates boiler steam, boiler steam is used as a heat source for the multi-stage flash evaporator, and 34 indicates condensate recovery line.

After the removal of calcium, magnesium, iron, etc. during the refining operation process (not shown), the raw saline solution is transferred to the multistage instantaneous evaporator via line 20 and transferred as a cooling source, followed by line 32 to catholyte circulation tank 4 Is moved to. In addition, a portion of the raw saline solution is transferred to waste gas scrubber tower 8 via line 26, where line 25 is cooled to cool off-gas flowing from the hydrogen chloride absorption tower 7, absorbing remaining hydrogen chloride and chlorine gas, and The ride is taken to the catholyte circulation tank 4.

The heat of absorption and heat of absorption of hydrogen chloride gas can be used to install the hydrogen chloride absorption tower 7 on line 31 to raise the temperature of the aqueous solution of the concentrated salt solution cooled in the multi-stage flash evaporator 30.

The advantages of the operation method according to the present invention are described as follows.

(a) In the prior art operation, hydrochloric acid is accompanied by water when the aqueous hydrochloric acid solution is added to the catholyte. However, in the present invention, since the saline solution is not diluted with the addition of water, the salt can be maintained in high yield. .

(b) According to the present invention, the synthesis and absorption of hydrogen chloride gas can be carried out as a means of a fairly simple device.

In the conventional hydrochloric acid synthesis tower, a hydrochloric acid solution having a high concentration of about 35% has been usually used. On the contrary, in the present invention, a hydrochloric acid solution having a low concentration of 10% or less is used because only a hydrochloric acid gas is absorbed in a large amount of saline solution. do. Therefore, since the partial pressure of the hydrogen chloride gas in the absorbed liquid phase is smaller, the cost is lower than that of the conventional hydrochloric acid synthesis tower, and the apparatus can be simplified.

(c) In the conventional hydrochloric acid synthesis tower, the tower was normally operated under excessive hydrogen conditions. However, in the present invention, the tower can be operated in the presence of excess chlorine gas (that is, 1.01 to 5 times the theoretical amount). That is, when excess chlorine gas is absorbed into the saline solution, it is injected into the anode chamber, and thus does not cause any harmful result. Therefore, the yield of hydrogen gas to be used can be increased, and operation control can be easily performed. Moreover, since the temperature of the flame by the chlorine of hydrogen gas falls in the excess chlorine gas, the lifetime of an apparatus can be extended for a long time.

(d) In the present invention, since the temperature of the saline solution absorbing the hydrogen chloride gas is increased by the heat of combustion and the heat of absorption, the chlorate accumulated in the saline solution can be decomposed.

(e) In the present invention, the heat of reaction and heat of absorption generated when synthesizing or absorbing the hydrogen chloride gas can be effectively used.

The present invention is described in detail with reference to the following examples. In the examples, "tone" means metric "tone".

Example 1

According to the process flow diagram shown in FIG. 1, the salt electrolysis operation was performed. As the cation exchange membrane, a membrane composed of a fluorocarbon resin having a sulfonic acid group suspended therein and having only a carboxylic acid group at the negative electrode portion of the membrane was used.

The cation exchange membrane was electrolytically operated with a current density of 40 A / dm ₂ in a bipolar electrolyzer divided into an anode chamber and a cathode chamber. A 25.5 wt% saline solution was injected via line 20 at a rate of 71.8 tonnes / hour and the salt concentration in the circulating anolyte was maintained at 14.4 wt%. Water was injected through line 11 at a rate of 16.1 tons / hour, and 21.6% by weight aqueous solution of caustic soda was recovered on line 12 at a rate of 39.5 tons / hour. The current density for caustic soda production was 93% and the electrolyzer voltage was 3.75 volts.

A dilute saline solution with a pH of 3 and a temperature of 88 ° C is transferred to the hydrogen chloride absorption tower 7 via line 23 at a rate of 162.4 tons / hour, where it absorbs hydrogen chloride gas at a rate of 0.6 tons / hour to give an acid concentration of 0.37. % And temperature are increased to 93 ° C. The resulting saline solution is recovered to the anolyte circulation tank, and the dilute saline solution recovered via lines 19 and 23 by installing a separator plate in the anolyte circulation tank is recovered via lines 20, 24 and 27. Does not mix with

The temperature of the circulating anolyte and the circulating catholyte can be maintained at 88 ° C. and 87.5 ° C. by using a part of the circulating catholyte as a heat source for the caustic soda concentrating device, respectively.

Operation was continued for three months under the above conditions, and the oxygen concentration in the chlorine gas was approximately 0.5%, and there was no increase in the concentration of chlorate in the saline solution. The amount of steam required to concentrate 21.6% aqueous solution of caustic soda to 48% was reduced by 0.15 ton per 1 tonne of pure caustic soda produced as compared to the operation of adjusting the concentration of acid as aqueous solution of hydrochloric acid.

Example 2

Salt electrolysis was performed according to the process flow diagram shown in FIG. As the cation exchange membrane and the electrolytic cell, the same ones as used in Example 1 were used. Perform electrolytic operation under current density of 40 A / dm ² to inject 25.5% by weight of saline solution at 49.0 tons / hour through line 20, and to inject water at 16.1 tons / hour through line 11, 21.6 An aqueous solution of caustic soda by weight was recovered on line 12 at a rate of 39.5 tons / hour. The current density for caustic soda production was 93% and the cell voltage was 3.75 volts.

A dilute salt solution with a salt concentration of 14.3 wt% pH 3 and a temperature of 87 ° C. is transferred via line 19 to the hydrogen chloride absorption tower 7 at a speed of 132.1 tons / hour, where hydrogen chloride gas at a rate of 0.6 tons / hour. Absorption increases the concentration of hydrochloric acid to 0.45% and the temperature to 92.4 ° C. This dilute aqueous salt solution is taken via line 24 to a multistage flash evaporator 30. In this multi-stage flash evaporator 30, water was evaporated at a rate of 17.7 tons / hour, the concentrated salt solution was concentrated to 16.5% by weight of salt, 0.52% by weight of hydrochloric acid, and the temperature was cooled to 84.7 ° C. The concentration of saline is maintained at 14.3% by weight as 115 tons / hour is transferred to the anolyte circulation tank 4 at speed.

The operation was continued for 3 months under the above conditions, and the oxygen concentration in the chlorine gas was approximately 0.5, and there was no increase in the concentration of chlorate in the saline solution. The amount of steam required for concentrating the dilute saline solution decreased by 0.11 ton per tonne of pure caustic soda produced as compared to the operation of adjusting the concentration of acid as an aqueous hydrochloric acid solution.

Claims (1)

In the salt electrolytic method of supplying the saline solution to the anode chamber in the electrolytic cell divided into the anode chamber and the cathode chamber by a cation exchange membrane between the anode and the cathode, hydrogen chloride gas synthesized from the chlorine gas and hydrogen gas generated in the electrolytic cell Then, the synthesized hydrogen chloride gas is absorbed into the raw saline solution and / or the dilute saline solution to maintain the concentration of hydrochloric acid in the absorbing solution at 01.-20% by weight, and then the heat of reaction and absorption heat of the hydrogen chloride gas and the heat generated in the electrolytic cell. A salt solution method characterized by concentrating an aqueous solution of sodium hydroxide produced in a diluted salt solution and / or a cathode chamber.

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