JP3249677B2 - Electrolysis method of alkali metal chloride - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for electrolyzing alkali metal chlorides. In particular, in circulating and using the alkali metal chloride-containing liquid having a reduced concentration discharged from the electrolytic cell in the electrolysis method using the ion exchange membrane, it is necessary to prevent the accumulation of the sulfate of impurities remaining in the liquid. The present invention relates to a method for performing electrolysis.

[0002]

2. Description of the Related Art A method for producing alkali metal hydroxides, chlorine and hydrogen by electrolyzing alkali metal chlorides such as sodium chloride and potassium chloride by an ion exchange membrane method generally uses a raw material alkali metal chloride (raw material). (Salt), a concentrated aqueous solution, usually saturated brine or saturated brine is introduced into the electrolysis tank to perform electrolysis, and dilute brine or light brine with reduced concentration is extracted,
The alkali metal chloride and water are added again to circulate and use as saturated brine. In this case, the sulfate contained as an impurity in the raw salt remains in the salt water discharged from the electrolytic cell and is accumulated by being recycled. Since sulfate causes inconveniences such as damage to the ion exchange membrane and reduction in electrolysis efficiency, it is necessary to prevent its accumulation. Currently, as a method for removing sulfate in salt water, barium chloride, barium carbonate, or another barium salt is added to saturated brine to precipitate and remove insoluble barium sulfate produced. It is expensive and toxic, and the barium sulfate precipitate must be treated as industrial waste, which is problematic in terms of operational management and economics.

Japanese Patent Application Laid-Open No. Sho 60-228691 proposes a method of treating light brine discharged from an electrolytic cell by using an anion exchanger to adsorb and remove sulfates.
Since light brine contains a considerably high concentration of chloride ions, the adsorption amount of sulfate ions is small. Furthermore, the anion exchanger adsorbing sulfate ions is regenerated with concentrated brine,
It is necessary to recover salt water from the recycled waste liquid. In addition, an operation for concentrating and cooling the regenerated waste liquid to remove the sulfate is required, and a very complicated process must be performed.

JP-A-4-55313 discloses an ion exchange resin having a polyamine group.
Japanese Patent Laid-Open Publication No. HEI 7-125 proposes a method of treating salt water using zirconium hydroxide. These methods utilize the difference in the selective adsorption of chloride and sulfate ions,
Because of the high concentration of chloride ions, the amount of sulfate ions adsorbed is small and a large amount of anion exchangers must be used.
Further, it is necessary to use a large amount of caustic soda for regeneration and hydrochloric acid for re-adsorption in order to desorb the adsorbed sulfate ions, and it is also necessary to treat a large amount of waste liquid for regeneration. Therefore, it is expensive, the operation is complicated, and systemization is difficult.

[0005]

As described above, the conventional sulfate removing method requires a large amount of chemicals, and requires cumbersome waste disposal and complicated operations.
The present invention has been studied variously for the purpose of developing a method for effectively removing sulfates which is simple in operation management and economically advantageous, and uses an amphoteric ion exchanger having an ion retardation function to convert alkali metal chlorides. An industrially advantageous electrolysis method of alkali metal chlorides was completed by separating the product and sulfate.

[0006]

According to the present invention, in a dissolution tank, a saturated brine prepared from an alkali metal chloride containing a sulfate as an impurity and dissolved water is supplied to an ion exchange membrane electrolytic cell to perform electrolysis. In the electrolysis method, the light brine extracted from the electrolytic cell and having a reduced concentration of alkali metal chloride is circulated to the dissolving tank, the alkali metal chloride and dissolved water are added, and the brine is again supplied as saturated brine to the electrolytic cell. At least a part of the light brine extracted from the tank has an anion exchange group and a cation exchange group,
The solution passes through a separation column filled with the amphoteric ion exchanger in which the amphoteric ion exchanger forms an internal salt to adsorb the alkali metal chloride on the amphoteric ion exchanger. Through which the effluent containing sulphate is removed outside the circulation system, and the effluent containing mainly alkali metal chloride is circulated to the dissolution tank. It lies in the method of electrolysis of alkali metal chlorides.

Next, the method of the present invention will be described in detail with reference to FIG. FIG. 1 is a flowchart showing an example of a plant for implementing the method of the present invention. In the dissolving tank 1, an alkali metal chloride raw salt (containing sulfate as an impurity) sent from the feed line 2, dissolved water from the branch pipe 3 a of the conduit 3, light brine circulated from the electrolytic tank 7, and A high-concentration saline solution to be supplied to the electrolytic cell 7 is prepared using the fractionated solution from which sulfate has been substantially removed. As will be described in detail later, the above-described fractionation liquid refers to a fractionation liquid containing an alkali metal chloride from which sulfate has been substantially removed by the separation tower 15. That is, after the alkali metal chloride aqueous solution is electrolyzed in the electrolytic cell 7, the alkali metal chloride aqueous solution (light brine) discharged from the electrolytic tank 7 and having a reduced content of the alkali metal chloride is chromatographed by the method of the present invention. This is a liquid which has been treated so as to be substantially free of sulfate and containing alkali metal chlorides according to the graph method, and this fraction is discharged to the dissolution tank 1 via the conduit 4.

It is desirable that the concentration of the alkali metal chloride in the salt water is as high as possible.
(Saturated brine) is prepared. The saturated brine prepared in the dissolving tank 1 is sent to a purification facility 6 through a conduit 5 to remove impurities such as calcium salts, magnesium salts, and strontium salts in the saturated brine derived from the raw salt. As a purification method, for example, sodium carbonate and caustic soda are sequentially added to precipitate as calcium carbonate and magnesium hydroxide. If necessary, a purification method such as chelate resin treatment may be used in combination. Next, the saturated brine is sent to an ion exchange membrane type electrolytic cell 7 and electrolysis is performed according to a conventional method.

The electrolytic cell 7 has a membrane 20 made of an ion exchange membrane.
The caustic alkali generated in the cathode chamber is discharged from the conduit 11, the hydrogen gas is discharged from the conduit 9, and the chlorine gas generated in the anode chamber is discharged from the conduit 8 and recovered respectively. You. The electrolysis consumes about 50% of the alkali metal chloride and about 20% of the water in the saturated brine, and the remaining brine (light brine) is withdrawn from the conduit 12 and circulated to the dissolution tank 1. The light brine extracted from the electrolytic cell 7 usually contains 180 to 2 alkali metal chlorides.
It contains about 00 g / l and about 6 to 12 g / l of sulfate. The method of the present invention comprises the use of such light brine, preferably in a dechlorination tower 1.
After dechlorination in step 3, if necessary, although not shown, once stored in a storage tank, the valve 14a is opened, and at least a part thereof is fed to the separation tower 15 by a predetermined amount.

The separation column 15 is packed with an amphoteric ion exchanger having an anion exchange group and a cation exchange group, both of which form an internal salt. As an amphoteric ion exchanger forming an internal salt, for example,
A resin called a cage type is exemplified. Snake cage type resin is a complex obtained by impregnating and polymerizing styrene or acrylic strong basic ion exchanger with acrylic acid.
It is commercially available under the names of "Rita-deion 11A-8" of Dow Chemical Company and "Diaion SR-1" of Mitsubishi Kasei Corporation. As the amphoteric ion exchanger forming the internal salt, a resin having an ion exchange group represented by the following formula (1) directly bonded to a resin matrix composed of an acrylic or styrene-based crosslinked copolymer is also used. Is done.

[0011]

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(Wherein R ¹ and R ² each represent an alkyl group having 1 to 3 carbon atoms, and m and n each represent a number of 1 to 4) Resin having an ion exchange group represented by the formula (1) For example, according to the method described in Japanese Patent Publication No. 60-45942, a halomethyl group is introduced into a styrene-based cross-linked copolymer, and then N, N-dimethylglycine acid anhydride, acid amide, acid halide And N-substituted-amino acids such as lower alkyl esters, and then hydrolyzing the reaction, and a method analogous thereto.

The amphoteric ion exchanger used in the present invention comprises:
Spherical with a particle size of 100-1200 μm, preferably 150-350 μm
It is. The exchange capacity for forming an internal salt of the amphoteric ion exchanger is 1.0 to 6.0 meq / g resin, preferably 2.0 to 4.5 meq / g resin. The water content of the amphoteric ion exchanger is 20 to 80% by weight, preferably 30 to 60% by weight. The layer height of the amphoteric ion exchanger packed in the separation tower 15 depends on the type of ion exchanger,
Depending on the amount of treated water (capacity of the electrolysis plant), usually 1 to 4
About m is good. The light brine dechlorinated in the dechlorination tower 13 still contains a small amount of chlorate. Therefore, when the light brine is acidified, the chlorate is decomposed to generate free chlorine, and the amphoteric ion exchanger is oxidized and deteriorated. It is important to keep it between 7 and 13.

The amount of light brine supplied to the separation tower 15 is determined in consideration of the amount of sulfate in the raw salt, the concentration of sulfate allowed in the saturated brine supplied to the electrolytic cell 7, and the like. The sulfate in the light brine does not need to be completely removed, and may be maintained at a concentration that does not hinder electrolysis. At least, the accumulation of sulfate in saturated brine may be prevented by removing the amount of sulfate associated with the newly added raw salt. For this purpose, the brine 4 circulated through the electrolytic cell 7
Up to 20% may be treated in the separation tower 15. The amount of one light brine supplied to the separation column 15 is the volume of the packed ion exchanger.
0.1 to 0.5 times the volume, and the flow velocity is 1 space velocity (SV).
~ 5 / hour is good. The water passing temperature is preferably from 40 to 80 ° C. After passing a single amount of fresh brine, valve 14a is closed, valve 14b is opened and water is passed from conduit 3b to separation column 15 according to the chromatographic procedure to first elute and elute sulfate, Elute and elute the alkali metal chloride. As the water, it is preferable to use a solution obtained by filtering dissolved water from the conduit 3 through a filter of about 1 micron or softened water. The amount of water flow at one time is 0.2 to 1.1 times the volume of the charged ion exchanger, and the flow rate of water is 1 to 5 / hour in SV, preferably the same as the flow rate of the above-mentioned light brine. The water passage temperature is 40 to 80 ° C., which is preferably the same as the water passage temperature of the above-mentioned light brine.

The flow direction of the light brine and the dissolved water may be either an upward flow or a downward flow when the packed ion exchanger is packed so as to completely fill the inside of the separation tower 15. When the body is filled with an empty tower portion remaining, the flow is downward, and water is passed so that the packed ion exchanger does not flow. A change in the concentration of components in the liquid flowing out of the separation tower 15 is detected by a detector such as a conductivity meter or a refractometer, and the valves 18 and 1 are transmitted through transmission paths 21 and 22.
9 and the effluent containing mainly sulfates is discharged out of the system via an outflow pipe 17, while the effluent containing mainly alkali metal chlorides is passed through a conduit 4 to the dissolution tank 1. Circulate. The operation of opening the valve 14a again to supply the flow of light brine and then flowing the dissolved water is repeated. In the dissolving tank 1, if necessary, further dissolving water is added to the light brine, which is circulated directly from the conduit 12 without passing through the separation tower 15, and the alkali metal-containing effluent from the separation tower, and the raw salt is newly added. To prepare saturated brine. As described above, according to the method of the present invention, the light brine adsorbed on the amphoteric ion exchanger is easily eluted with water and can be used again for the treatment of light brine without regeneration treatment with acid or alkali.

If the processing amount of the light brine is 4 to 20%, the accumulation of new sulfate due to the addition of the raw material salt can be prevented, and the water used for eluting the amphoteric ion exchanger is supplied to the electrolytic cell 7. Since the amount of water consumed is within the range, the separation tower 15
The total amount of the effluent fraction mainly discharged from the alkali metal chloride can be circulated to the dissolving tank 1, minimizing the amount of water used for electrolysis and the amount of alkali metal chloride leaking out of the system. be able to. In addition, the amount of wastewater is small and economical. According to the method of the present invention, there is no need to use a toxic barium salt or the like, no need for troublesome precipitation treatment, no need for an operation for regenerating the resin, and no operation for recovering the alkali metal chloride from the regenerated solution. Since sulfates are also removed as an aqueous solution, handling is easy and operation management is easy. In particular, when the amphoteric ion exchanger having the ion exchange group of the above formula (1) is used, as is apparent from Example 1 and FIG. 2 described later, the alkali metal chloride eluted from the separation column 15 has no tailing and is extremely low. The sulfate can be separated and recovered efficiently. As is apparent from the above-mentioned production method, the production of the amphoteric ion exchanger is relatively easy as compared with the snake cage type resin. The number of anion exchange groups and cation exchange groups in the resin is the same, they associate with each other to form internal salts, and since there is no excess anion or cation exchange groups, the separation ability is high, and the eluate effluent pH does not change. When the snake cage type resin is used for a long period of time, polyacrylic acid may be eliminated from the strongly basic resin. However, such an amphoteric ion exchanger having the ion exchange group of the above formula (1) may have such a risk. However, it has various advantages such as stable use for a long time.

[Embodiment] The method of the present invention is carried out as described above.
An example of a light brine treatment using an amphoteric ion exchanger will be described. However, the present invention is not limited to the following examples unless it exceeds the gist. The light brine used in the following examples contains 195 g / l of sodium chloride, 7.5 g / l of sodium sulfate, and 2.55 g / l of sodium chlorate. Example 1 As an amphoteric ion exchanger, a resin in which an ion exchange group represented by the following formula (2) was bonded to a benzene nucleus of a styrene-divinylbenzene crosslinked copolymer was used.

[0018]

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The above amphoteric ion exchanger was produced as follows. 5 containing 0.1% by weight of polyvinyl alcohol
1 g of benzoyl peroxide as a polymerization initiator was added to a mixture of 92.4 g of styrene, 3.3 g of ethylvinylbenzene, and 4.3 g of divinylbenzene using 00 ml of demineralized water as a dispersion medium, and the mixture was heated at 80 ° C for 10 hours under a nitrogen atmosphere. Dispersion polymerization was performed. The crosslinked copolymer particles thus obtained are heated at 120 ° C.
After drying for 5 hours with heating, the mixture was heated and swelled with 100 g of tetrachloroethylene, then cooled, 200 g of chloromethylmethyl ether was added, 50 g of anhydrous zinc chloride was added, and the mixture was reacted at 50 ° C. for 5 hours. Then, the mixture was poured into 2.5 l of cold water to decompose excess chloromethyl methyl ether, and washed with filtered water.

[0020] 40 g of the obtained chloromethylated resin particles (chlorine content: 22%) and 100 g of benzene were mixed with a stirrer, a thermometer,
Put into a four-necked flask equipped with a reflux condenser and a dropping funnel, and slowly add 55 g of N, N-dimethylglycine methyl ester from the dropping funnel while maintaining the temperature at 50 ° C or lower. The reaction was further performed at 60 ° C. for 4 hours. After completion of the reaction, the resin was collected by filtration and washed with 100 ml of acetone. Next, this resin was added to a 5N aqueous solution of sodium hydroxide (300 ml) in 80 ml.
The hydrolysis reaction was performed at 8 ° C. for 8 hours. Next, the resin was separated by filtration, washed with water and filtered to obtain a resin having an average particle size of 280 μm. The internal salt formation of this resin was 2.17 meq / g-resin, and the water content was 40%. When measured with sodium chloride and hydrochloric acid, the quaternary ammonium group and the carboxyl group were present in the same ratio, and were present without any excess or shortage. The amount of internal salt was measured by measuring 5 ml of the resin, draining it with a centrifuge, immersing it in 250 ml of 0.2 N hydrochloric acid,
Hydrochloric acid consumed by titration was measured and calculated.

The separation column was composed of a jacketed column having an inner diameter of 25 mm, and the column was filled with 500 ml of the amphoteric ion exchange resin to form a chromatographic separation packed bed having a bed height of 1020 mm. 100 ml of light brine having the above composition was passed downward through the packed bed at a space velocity (SV) of 3 / hour (liquid volume 0.2 times volume / resin volume) at a water temperature of 40 ° C., and then 200 ml (0.4 ml) of soft water. (Double volume / resin volume) under the same conditions as those for the light brine. Next, soft brine and then soft water were repeatedly passed under the same conditions as above. At this time, the relationship between the respective concentrations of sodium chloride and sodium sulfate in the effluent from the separation tower and the amount of the effluent was as shown in FIG. In FIG.
The curves 31a and 31b relate to sodium sulfate, and the curves 32a and 32b relate to sodium chloride. In this way, a salt solution substantially free of sulfate can be easily fractionated.

Example 2 The same separation tower as used in Example 1 was filled with 500 ml of a snake cage type amphoteric ion exchange resin Diaion SR-1 [manufactured by Mitsubishi Chemical Corporation] sufficiently swollen with deionized water. , Height 1020 m
A chromatographic separation packed bed of m was formed. This resin had an average particle size of 224 µm, a total exchange capacity of 3.74 meq / g (including 3.09 meq / g of internal salt), and a water content of 41%. 100 ml of light brine having the same composition as in Example 1 was passed downward through the packed bed at a space velocity (SV) of 3 / hour (flow rate 0.2 times volume / resin volume) at a water temperature of 40 ° C. 400 ml (0.8 times volume / resin volume) was passed under the same conditions as for the light brine. Next, soft brine and then soft water were repeatedly passed under the same conditions as above. At this time, the relationship between the respective concentrations of sodium chloride and sodium sulfate in the effluent from the separation tower and the amount of effluent was as shown in FIG. In FIG. 3, 4
The curves 1a and 41b relate to sodium sulfate,
The curves 42a and 42b relate to sodium chloride. In this way, a salt solution substantially free of sulfate can be easily fractionated.

[0023]

According to the present invention, sulfate contained in light brine discharged from an electrolytic tank of an aqueous solution of an alkali metal chloride is efficiently removed by a simple operation without using a special agent, thereby preparing brine for electrolysis. Can be used repeatedly.

[Brief description of the drawings]

FIG. 1 is a flow chart showing an example of a plant for implementing the method of the present invention.
Sheet.

FIG. 2 is a table showing the concentration of components and the amount of effluent fractionated and discharged when components are separated by a separation tower in Example 1.

FIG. 3 is a chart showing the concentration and outflow amount of components of an effluent fractionated when components are separated by a separation tower in Example 2.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Dissolution tank of raw salt 2 Supply path of alkali metal chloride raw salt 3 Pipe of dissolved water 4 Pipe of fractionated liquid containing alkali metal chloride 5 Pipe of saturated brine 7 Electrolyzer 12 Pipe of light brine 15 Separation tower 16 Detection 17 Outflow pipe for sulfate-containing fraction

──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Takayuki Tashiro 1000 Kamoshita-cho, Midori-ku Yokohama-shi, Kanagawa Prefecture Mitsubishi Chemical Research Institute (72) Inventor Ryuichi Sugimoto 1000 Kamoshita-cho Midori-ku Yokohama-shi, Kanagawa Mitsubishi Chemical (56) References JP-A-60-228691 (JP, A) JP-B-60-45942 (JP, B2) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25B 1 / 00-15/08 C01D 3/16

Claims (8)

(57) [Claims] In a dissolution tank, a saturated brine prepared from an alkali metal chloride containing a sulfate as an impurity and dissolved water is supplied to an ion exchange membrane electrolysis tank for electrolysis.
In the electrolysis method, the light brine extracted from the electrolytic cell and having a reduced concentration of alkali metal chloride is circulated to the dissolving tank, the alkali metal chloride and dissolved water are added, and the brine is again supplied as saturated brine to the electrolytic cell. At least a portion of the light brine extracted from the tank was filled with an amphoteric ion exchanger having an anion exchange group and a cation exchange group, and both ion exchange groups forming an internal salt. After passing through a separation column to adsorb the alkali metal chloride on the amphoteric ion exchanger, dissolved water is passed through the separation column as an eluent, and the effluent fraction mainly containing sulfate is passed through the circulation system. A method for electrolyzing an alkali metal chloride, comprising removing an effluent fraction containing an alkali metal chloride to the dissolution tank and removing the effluent fraction mainly containing the alkali metal chloride. 2. The method according to claim 1, wherein the amphoteric ion exchanger is a snake cage type resin. 3. The method according to claim 1, wherein the amphoteric ion exchanger has an ion exchange group represented by the following formula (1) bonded to a crosslinked copolymer resin matrix. Embedded image (Wherein R ¹ and R ² each represent an alkyl group having 1 to 3 carbon atoms, and m and n each represent a number of 1 to 4) 4. The method according to claim 3, wherein the resin matrix comprises a crosslinked copolymer of styrene and divinylbenzene. 5. The method according to claim 3, wherein in the amphoteric ion exchanger, R ¹ and R ² in the formula (1) are methyl groups and m and n are 1. 6. A light brine 4 extracted from the electrolytic cell.
2. The method according to claim 1, wherein about 20% by volume is passed through the separation column. 7. The method according to claim 1, wherein the light brine is dechlorinated, adjusted to pH 7 to 13, and supplied to a separation column. 8. The amphoteric ion exchanger wherein an ion exchange group represented by the following formula (2) is bonded to a benzene nucleus of a resin matrix composed of a crosslinked copolymer of styrene and divinylbenzene. The method of claim 1. Embedded image