Method for purifying high-salt solutions containing nitrogen.

Abstract

This invention provides a highly efficient purification method for removing fluoride ions from a high-concentration nitrogen compound salt solution containing trace amounts of fluoride ions. [Solution] (1) A high-salt concentration aqueous solution containing a nitrogen element salt and fluoride ions is adjusted to a pH of 9 or less. (2) This solution is then brought into contact with a resin adsorbent on which zirconium ions have been supported after introducing sulfonic acid groups into a styrene-divinylbenzene copolymer. (3) The high-salt concentration solution is recovered by separating the solution. (4) The resin is regenerated by bringing the separated resin adsorbent into contact with an alkaline aqueous solution with a pH of 11 or higher. Through these steps, a high-concentration nitrogen compound salt solution containing trace amounts of fluoride ions is purified.

Description

[[Technical Field]]

[0001] The present invention relates to a purification method for highly efficiently removing fluoride ions from a high-concentration nitrogen compound salt solution containing a trace amount of fluoride ions. [[Background Art]]

[0002] Semiconductors are known as the "rice" of various industries, manufactured worldwide, and plants are being expanded and production scaled up to meet the demand for further production growth. Various chemical solutions are used in semiconductor manufacturing; among used chemical solutions, those that can be reused are recycled, while most are discharged as waste liquid.

[0003] In the semiconductor manufacturing process, a wet etching step is performed to remove unnecessary portions for patterning metals and insulating films on a silicon substrate. Buffered hydrofluoric acid (hereinafter referred to as BHF), which is used to remove insulating films mainly composed of silicon oxide, is one of the cleaning chemical solutions used in this step. BHF is a chemical solution composed of hydrofluoric acid and ammonia. By mixing ammonia, which has a pH buffering effect, with hydrofluoric acid that reacts with silicon oxide, BHF has the characteristic of enabling adjustment of the etching rate to perform fine processing. After used BHF is mixed with wastewater from other processes to form a waste liquid containing fluorine and ammonia, low-concentration waste liquid is treated in the factory, while high-concentration waste liquid is difficult to treat in-house and is therefore discharged externally as industrial waste.

[0004] In addition, as a solution other than BHF, a mixed acid of hydrofluoric acid and nitric acid (hereinafter referred to as hydrofluoric-nitric acid) is sometimes used due to its etching characteristics. After used hydrofluoric-nitric acid is mixed with wastewater from other processes to form a waste liquid containing fluorine and nitric acid, low-concentration waste liquid is treated in the factory, while high-concentration waste liquid is difficult to treat in-house and is therefore discharged externally as industrial waste.

[0005] The fluorine and nitrogen compounds contained in these wastewaters are regulated as hazardous substances under the Water Pollution Control Law. When wastewater containing these substances is discharged into public water bodies or sewers, it is necessary to stabilize it so that its concentration is below the wastewater standard value.

[0006] Wastewater containing fluorine is often treated by coagulation and sedimentation using calcium-based chemicals. The fluorine in the wastewater is immobilized as calcium fluoride and disposed of as sludge in landfills along with other impurities. On the other hand, efforts are also being made to recycle the recovered calcium fluoride. Fluorite, which contains calcium fluoride as its main component, is used as a raw material for hydrofluoric acid production and as a flux in the steelmaking industry. If conditions such as purity, unreacted calcium concentration, impurity concentration, moisture concentration, and particle size can be met, it is possible to recycle calcium fluoride recovered from wastewater as a substitute for fluorite. To achieve this, it is necessary to adjust the recovery conditions according to the properties of the wastewater, such as removing various components contained in the wastewater and performing crystal growth to recover easily atomized calcium fluoride at an appropriate particle size.

[0007] Wastewater containing ammonia, a nitrogen compound, is difficult to fix with sparingly soluble salts and is often decomposed using chemical oxidation, biological treatment, or stripping. Chemical oxidation allows for processing in a short time, but requires a considerable amount of chlorine for ammonia, resulting in high chemical costs, so it is only used for wastewater with low nitrogen concentrations. In the case of biological treatment, aerobic treatment has limited processing capacity, and anaerobic treatment requires a digestion and denitrification process, which takes longer than other treatments. Furthermore, due to toxicity to microorganisms, heavy metals and other components must be removed as much as possible through pretreatment. Stripping allows for processing even at high concentrations in a short time, but it requires separating ammonia from the liquid layer and further decomposition using catalytic combustion equipment, necessitating large-scale treatment facilities. While this wastewater is mainly decomposed as described above, efforts are also being made to recover ammonia from the wastewater. For example, by combining a stripping device with a recovery device, if ammonia can be preferentially separated from heavy metals and other components, it becomes possible to recycle the recovered ammonia water or ammonium salt solution. When recycling ammonia water, its use is limited to internal reuse because it is difficult to store long-term and requires further purification before it can be used as a basic raw material for chemical products. On the other hand, ammonium salt solutions are stable, easy to store long-term, and can be recycled as a nitrogen resource for fertilizers and bacterial nutrients.

[0008] Furthermore, when nitric acid is included as a nitrogen compound, immobilization with sparingly soluble salts is difficult, and the product is often diluted and discharged after removing harmful components, or disposed of by incineration. Products with high concentrations of nitric acid are purified by distillation and recycled. When treating hydrofluoric acid, calcium compounds are often used to remove fluorine, so a high concentration of calcium nitrate is dissolved in the treated liquid, making final disposal difficult. However, if other coexisting substances can be controlled, it can be used as a nitrogen resource for fertilizers, etc.

[0009] As shown in Patent Document 1, the applicant, Kohtoku Cleaner Co., Ltd., has devised a method for recovering nitrogen resources and calcium fluoride from wastewater containing fluorine and ammonia, such as BHF, and has proposed a wastewater recycling method. This method can significantly reduce the fluorine concentration in nitrogen resources and expand their range of use. However, when used as a nitrogen resource for a long period of time, there is a need to further reduce the amount of fluorine in nitrogen resources, such as concerns about fluorine accumulation in soil, etc., when used as a fertilizer, and concerns about fluorine accumulation when incorporated into sludge and used as surplus sludge when used as a nutrient agent in biological treatment.

[0010] When producing nitrogen resources such as ammonium sulfate and calcium nitrate, a crystallization process is often carried out in the final stage. This crystallization process allows for high purity and a reduction in the fluorine content in the liquid. However, as shown in Patent Documents 2 and 3, this crystallization process is very complex and uses a large amount of energy. Furthermore, it is applicable to continuously stable processes. In processes using wastewater as raw material, it has been difficult to utilize due to problems such as the constantly changing composition and the difficulty in controlling the timing of wastewater acceptance.

[0011] On the other hand, Patent Document 4 proposes a method for purifying high-concentration sulfate solutions of 0.8 N or higher by using a sulfate-type anion exchange resin to remove monovalent anions in bulk. While this method is applicable to highly ionic monovalent anions such as chloride ions and bicarbonate ions, it has been found that fluoride ions exhibit specific characteristics, such as being difficult to ionize due to the formation of hydrogen bonds and readily forming coordinate bonds as ligands, and therefore cannot be sufficiently removed by the method described in the document. Furthermore, the removed fluoride ions tend to form strong bonds with the anion exchange resin, and even attempts to regenerate them with sulfuric acid or the like have proven insufficient. Moreover, the method proposed in Patent Document 4 is applicable to sulfates such as ammonium sulfate solutions, and is not applicable to non-sulfate solutions such as calcium nitrate. [Prior art documents] [Patent Documents]

[0012] [Patent Document 1] Japanese Patent Publication No. 2023-110804 [Patent Document 2] Japanese Patent Application Publication No. 55-127186 [Patent Document 3] Special Publication No. 2002-527329 [Patent Document 4] Patent No. 6421021 [Overview of the project] [Problems that the invention aims to solve]

[0013] The present invention has been made in view of the above circumstances, and provides a method for efficiently removing fluoride ions from a high-concentration nitrogen compound salt solution containing trace amounts of fluoride ions. [Means for solving the problem]

[0014] As a result of diligent research to solve the above problems, the present inventors have completed a method for purifying a high-salt solution containing nitrogen elements, which efficiently removes fluoride ions by contacting a resin adsorbent, on which zirconium ions are supported after introducing sulfonic acid groups into a styrene-divinylbenzene copolymer, with a high-concentration nitrogen compound salt solution containing trace amounts of fluoride ions adjusted to a specific pH range, and then separating the solution. Furthermore, the resin adsorbent can be regenerated after use by contacting it with an alkaline solution of a specific pH.

[0015] In other words, the present invention provides (1) a high-salt concentration aqueous solution containing a nitrogen element salt and fluoride ions, adjusted to a pH of 9 or less. (2) After introducing sulfonic acid groups into the styrene-divinylbenzene copolymer, it is brought into contact with a resin adsorbent supporting zirconium ions. (3) The solution is then separated to recover the high-salt aqueous solution. (4) The separated resin adsorbent is regenerated by contacting it with an alkaline aqueous solution with a pH of 11 or higher. This is a method for purifying a high-salt solution containing nitrogen elements.

[0016] According to this method, a high-salt concentration aqueous solution containing a nitrogen-containing salt and fluoride ions is adjusted to a pH of 9 or lower by adding an acid such as sulfuric acid, nitric acid, or hydrochloric acid. While it is preferable to use an acid contained in the nitrogen-containing salt, the type of acid added can be appropriately selected according to the properties of the solution. For example, when obtaining a high-salt concentration aqueous solution by treating wastewater discharged from semiconductors, the treatment is often carried out with an excess of calcium. Adding sulfuric acid may cause precipitation, so the addition of hydrochloric acid or nitric acid is preferable. Furthermore, if the pH of the high-salt concentration aqueous solution is 9 or lower, such as when the treated wastewater solution is obtained as an ammonium salt solution (such as ammonium sulfate) using methods like stripping, it can be used as is without adding acid.

[0017] The pH of a high-salt aqueous solution containing nitrogen elements can be set arbitrarily as long as it is 9 or less. Lower pH speeds up the separation rate of fluoride ions, but it also increases the amount of alkaline agent used to regenerate the resin adsorbent after use. Therefore, it is preferable to set the pH to 3 or higher and less than 8, and more preferably to 4 or higher and less than 7.

[0018] Considering the practical value of high-salt aqueous solutions, a concentration of nitrogen-containing salts of 5% by weight or higher is desirable. Furthermore, when comparing efficiency with other fluoride ion removal methods, a fluoride ion concentration of 0.1 ppm to 1000 ppm is desirable in high-salt aqueous solutions.

[0019] To the above solution, a resin adsorbent, which consists of a styrene-divinylbenzene copolymer into which sulfonic acid groups have been introduced and then supported with zirconium ions, is brought into contact. These resin adsorbents can be prepared by known methods. A method such as reacting a zirconium salt with a styrene-styrene sulfonic acid copolymer crosslinked with divinylbenzene can be appropriately selected.

[0020] Regarding the method for bringing a high-salinity aqueous solution containing elemental nitrogen into contact with the above resin adsorbent, the method can be appropriately selected depending on the treatment amount, the contained components, or the concentration. It is possible to select a method in which each is charged into a reaction tank followed by stirring and mixing, or a method in which an adsorption tower is filled with the resin adsorbent and the high-salinity solution containing elemental nitrogen is passed through the adsorption tower.

[0021] The amount of the resin adsorbent relative to the high-salinity aqueous solution containing elemental nitrogen can be determined according to the fluoride ion content of the high-salinity aqueous solution and the saturated fluoride ion adsorption amount of the resin adsorbent. It is preferable to use the resin adsorbent in an amount of 0.01% by weight or more and less than 20% by weight relative to the high-salinity aqueous solution containing elemental nitrogen to be treated. If the amount is less than 0.01% by weight, the high-salinity solution cannot sufficiently contact the resin adsorbent, making it difficult to sufficiently remove fluoride ions; if the amount is 20% by weight or more, the cost of the resin adsorbent required for the treatment increases.

[0022] The contact time between the high-salinity aqueous solution containing elemental nitrogen and the resin adsorbent is preferably 10 minutes or longer. If the contact time is less than 10 minutes, there is a possibility that fluoride ions in the high-salinity aqueous solution cannot be sufficiently removed. There is no particular upper limit for the contact time, but it needs to be determined in consideration of productivity.

[0023] After bringing the high-salinity aqueous solution containing elemental nitrogen into contact with the resin adsorbent, the high-salinity solution and the resin adsorbent are separated. Existing methods can be used for the separation; for example, filtration using a filter or a method of extracting only the solution portion after standing still can be employed. In the method where the solution is passed through an adsorption tower filled with the resin adsorbent during contact, the structure is configured such that no solid matter is discharged on the outlet side, so the liquid can be recovered as it is.

[0024] Most of the fluoride ions in the liquid separated in this separation step have been removed, so when the liquid is used as a fertilizer in the agricultural field or as a nutrient in microbial treatment, the accumulation of fluorine is extremely low, which allows the liquid to be widely used.

[0025] On the other hand, the resin adsorbent removed in the separation process can be regenerated and reused by contacting it with an alkaline aqueous solution with a pH of 11 or higher. As the alkaline aqueous solution, lithium hydroxide aqueous solution, sodium hydroxide aqueous solution, or potassium hydroxide aqueous solution can be used, which are obtained by dissolving alkali metal hydroxides in water.

[0026] The pH of the alkaline aqueous solution can be 11 or higher, but preferably 12 or higher. If the pH is lower than 11, it becomes difficult to dissolve the fluoride ions adsorbed on the resin adsorbent, and sufficient regeneration becomes impossible. On the other hand, regeneration is faster with a higher pH, but since a high concentration of alkaline aqueous solution adheres to the resin adsorbent after regeneration, washing takes time. Therefore, it is desirable to decide on the pH considering the overall productivity.

[0027] Regarding regeneration, the weight ratio of the resin adsorbent to the alkaline aqueous solution is preferably 1 to less than 100 times the amount of the alkaline aqueous solution relative to the resin adsorbent, and more preferably 4 to less than 20 times. If the weight ratio is less than 1, there is a risk that the surface of the resin adsorbent and the alkaline aqueous solution will not be able to make sufficient contact, resulting in insufficient regeneration. Furthermore, if an alkaline aqueous solution of 100 times or more is used, the equipment required for regeneration will be larger, resulting in increased equipment costs.

[0028] The method of this regeneration process can be appropriately selected depending on the processing volume, etc., but options include adding each material to a reaction vessel and stirring and mixing them, or filling a column with a resin adsorbent and passing an alkaline aqueous solution through it. If an adsorption tower is chosen for the fluoride ion removal process, it is possible to regenerate the tower by passing an alkaline aqueous solution through it. By using multiple adsorption towers and operating them alternately, one tower is used for fluoride ion removal while the other is regenerated after adsorption, efficient operation is possible.

[0029] For regeneration, a contact time of 10 minutes or more between the alkaline aqueous solution and the resin adsorbent is preferable. If the contact time is less than 10 minutes, sufficient regeneration may not occur. There is no specific upper limit to the contact time, but it should be determined considering the productivity of the entire process.

[0030] Furthermore, regeneration is not required after each fluoride ion removal step; if there is sufficient removal capacity, the solution can be reused without regeneration. In addition, the alkaline aqueous solution used for regeneration can be reused multiple times as long as the pH is within a predetermined range and the amount of alkali metal and fluoride ions in the solution does not exceed the solubility of the alkali metal fluoride salt. In other words, this invention not only removes trace amounts of fluoride ions contained in high-salt-concentration aqueous solutions containing nitrogen elements, but also concentrates them in the alkaline aqueous solution, making it possible to reuse fluorine. The fluoride ion-containing alkaline aqueous solution after use can be used in the fluorine recycling process as described above, or it can be stabilized according to existing methods for treating fluorine-containing wastewater. [Effects of the Invention]

[0031] According to the method of the present invention, trace amounts of fluoride ions contained in a high-salt aqueous solution containing nitrogen elements can be removed by contacting it with a specific resin adsorbent at a pH of 9 or lower. This method can be widely used as a fertilizer in agriculture and as a nutrient in microbial treatment. Furthermore, by regenerating the treated resin adsorbent with an alkaline aqueous solution with a pH of 11 or higher, it becomes possible to realize an economical process for purifying high-salt solutions containing nitrogen elements. [Brief explanation of the drawing]

[0032] [Figure 1] This is a flowchart illustrating one embodiment of the method for purifying a high-concentration nitrogen compound salt solution containing trace amounts of fluoride ions according to the present invention. [Modes for carrying out the invention]

[0033] An example of an embodiment for carrying out the present invention will be described below with reference to the drawings. As shown in Figure 1, a high-salt concentration aqueous solution containing a nitrogen element salt whose pH has been adjusted to 9 or less in advance, and a trace amount of fluoride ions, and a resin adsorbent in which a styrene-divinylbenzene copolymer has been introduced with sulfonic acid groups and then zirconium ions are supported are mixed in an adsorption tank.

[0034] Subsequently, by separating the high-salt solution from the resin adsorbent, trace amounts of fluoride ions contained in the high-salt aqueous solution are removed, allowing it to be effectively utilized as a fertilizer or nutrient agent.

[0035] Meanwhile, the separated resin adsorbent is mixed with an alkaline aqueous solution with a pH of 11 or higher in a regeneration tank. The fluoride ions then dissolve into the alkaline aqueous solution, and the resin adsorbent is regenerated. The regenerated resin adsorbent is neutralized and washed as appropriate before being reused.

[0036] The present invention will be specifically described below with reference to examples. However, the present invention is not limited to the following examples. [Examples]

[0037] In the examples, the fluoride ion concentration of the samples was measured by pretreatment with steam distillation and color development using lanthanum alizarin complexone spectrophotometric analysis (La-ALC method) for solutions containing nitrate ions and calcium ions. For other solutions, the concentration was measured using a combined fluoride ion selective electrode. [Examples]

[0038] 50 g of a high-salt aqueous solution (1) containing 40% by weight of ammonium sulfate obtained by treating wastewater discharged from the semiconductor industry and 104 ppm of fluoride ions was added to a 100 ml beaker, and sulfuric acid was added while stirring with a magnetic stirrer until the pH of the solution reached 6. While continuing to stir, 2.0 g of MuromacXSS-530B (hereinafter referred to as resin adsorbent (1)), a resin manufactured by Muromachi Chemical, in which a styrene-divinylbenzene copolymer has sulfonic acid groups introduced and zirconium ions supported on it, was added, and stirring was continued for 300 minutes.

[0039] Subsequently, the liquid was filtered to obtain purified high-salt concentration solution (1)-1 and adsorbent resin adsorbent (1)-1. The fluoride ion concentration of purified high-salt concentration solution (1)-1 was measured to be 2.1 ppm, and an ammonium sulfate solution from which fluoride ions had been removed was obtained.

[0040] Furthermore, 100 g of a pH 13 sodium hydroxide aqueous solution was placed in another 100 ml beaker, and while stirring with a magnetic stirrer, adsorbent resin adsorbent (1)-1 was added. After stirring for 300 minutes, the liquid was filtered. The fluoride ion concentration in the filtrate was measured to be 46 ppm, indicating that fluorine desorption was observed. The filtered resin was then washed with water to obtain regenerated resin adsorbent (1).

[0041] Similarly, with the recycled resin adsorbent (1), 30 g of high-salt aqueous solution (1) was placed in a 100 ml beaker, and sulfuric acid was added while stirring with a magnetic stirrer until the pH of the solution reached 6. Then, while continuing to stir, 1.2 g of recycled resin adsorbent (1) was added, and stirring was continued for 300 minutes. After that, the solution was filtered to obtain purified high-salt solution (1)-2 and post-adsorption resin adsorbent (1)-2. The fluoride ion concentration of the purified high-salt solution (1)-2 was measured to be 2.6 ppm, indicating that an ammonium sulfate solution from which fluoride ions had been removed was obtained. Fluoride ions could also be removed from the recycled resin adsorbent. [Examples]

[0042] 50 g of a high-salt aqueous solution (2) containing 30% by weight of calcium nitrate obtained by treating wastewater discharged from the semiconductor industry and 11 ppm of fluoride ions was added to a 100 ml beaker and stirred with a magnetic stirrer, and the pH of the solution was 7. While continuing to stir, 2.0 g of the same resin adsorbent (1) used in Example 1 was added and stirring was continued for 300 minutes.

[0043] Subsequently, the liquid was filtered to obtain purified high-salt concentration solution (2)-1 and adsorbent resin adsorbent (2)-1. The fluoride ion concentration of purified high-salt concentration solution (2)-1 was measured to be 2.2 ppm, indicating that a calcium nitrate solution from which fluoride ions had been removed was obtained.

[0044] Furthermore, 50 g of pH 14 sodium hydroxide aqueous solution was placed in another 100 ml beaker, and while stirring with a magnetic stirrer, adsorbent resin adsorbent (2)-1 was added. After stirring for 300 minutes, the liquid was filtered. The fluoride ion concentration in the filtrate was measured to be 8.4 ppm, indicating that fluorine desorption was observed. The filtered resin was then washed with water to obtain regenerated resin adsorbent (2).

[0045] Similarly, with the recycled resin adsorbent (2), 30 g of high-salt aqueous solution (2) was placed in a 100 ml beaker and stirred with a magnetic stirrer, resulting in a pH of 7. Then, while continuing to stir, 1.2 g of recycled resin adsorbent (2) was added, and stirring was continued for 300 minutes. Afterward, the solution was filtered to obtain purified high-salt solution (2)-2 and post-adsorption resin adsorbent (2)-2. The fluoride ion concentration of the purified high-salt solution (2)-2 was measured to be 2.4 ppm, indicating that a calcium nitrate solution from which fluoride ions had been removed was obtained. Fluoride ions could also be removed from the recycled resin adsorbent. [Examples]

[0046] An adsorption column was prepared by filling a glass container with an inner diameter of 1.12 cm with 10 g of the same resin adsorbent used in Example 1. The high-salt concentration aqueous solution (1) used in Example 1 was adjusted to pH 6 by adding sulfuric acid, and this solution was passed through the adsorption column at a flow rate of 0.5 g per minute for 200 minutes to recover the purified high-salt concentration solution (3)-1. The fluoride ion concentration of this purified high-salt concentration solution (3)-1 was measured to be 3.0 ppm, indicating that an ammonium sulfate solution from which fluoride ions had been removed was obtained.

[0047] Subsequently, the resin adsorbent was washed with pure water at a flow rate of 10 g per minute for 5 minutes, and then a pH 13 sodium hydroxide aqueous solution was passed through the column at a flow rate of 0.5 g per minute for 200 minutes. The fluoride ion concentration of the recovered sodium hydroxide aqueous solution was measured to be 94 ppm, indicating that fluorine desorption was observed.

[0048] Furthermore, the resin adsorbent was washed by passing pure water through this adsorption column at a flow rate of 10 g per minute for 5 minutes. Then, the high-salt concentration aqueous solution (1) used in Example 1 was adjusted to pH 6 by adding sulfuric acid, and this solution was passed through the adsorption column at a flow rate of 0.5 g per minute for 200 minutes to recover the purified high-salt concentration solution (3)-2. The fluoride ion concentration of this purified high-salt concentration solution (3)-2 was measured to be 3.3 ppm, and an ammonium sulfate solution from which fluoride ions had been removed was obtained. Comparative Example 1

[0049] 50 g of the same high-salt-concentration aqueous solution (1) used in Example 1 was placed in a 100 ml beaker, and sulfuric acid was added while stirring with a magnetic stirrer until the pH of the solution reached 6. Then, while continuing to stir, 2.0 g of MuromacXMA-413B-OH (hereinafter referred to as comparative resin adsorbent (1)), an anion exchange resin manufactured by Muromachi Chemical, in which quaternary ammonium ions have been introduced into a styrene-divinylbenzene copolymer, was added, and stirring was continued for 300 minutes.

[0050] Subsequently, the liquid was filtered to obtain comparative purified high-salt concentration solution (1)-1 and comparative adsorbed resin adsorbent (1)-1. The fluoride ion concentration of comparative purified high-salt concentration solution (1)-1 was measured to be 99.3 ppm, indicating that almost no fluoride ions were removed. Comparative Example 2

[0051] 50 g of the same high-salt aqueous solution (1) used in Example 1 was placed in a 100 ml beaker and stirred with a magnetic stirrer, resulting in a pH of 10. While continuing to stir, 2.0 g of the same resin adsorbent (1) used in Example 1 was added, and stirring was continued for 300 minutes.

[0052] Subsequently, the liquid was filtered to obtain comparative purified high-salt concentration solution (2)-1 and comparative adsorbed resin adsorbent (2)-1. The fluoride ion concentration of comparative purified high-salt concentration solution (2)-1 was measured to be 55.1 ppm, indicating that fluoride ions could not be sufficiently removed. Comparative Example 3

[0053] 50 g of the same high-salt aqueous solution (1) used in Example 1 was placed in a 100 ml beaker, and sulfuric acid was added while stirring with a magnetic stirrer until the pH of the solution reached 6. While continuing to stir, 2.0 g of the same resin adsorbent (1) used in Example 1 was added, and stirring was continued for 300 minutes.

[0054] Subsequently, the liquid was filtered to obtain comparative purified high-salt concentration solution (3)-1 and comparative adsorbed resin adsorbent (3)-1. The fluoride ion concentration of comparative purified high-salt concentration solution (3)-1 was measured to be 2.1 ppm, indicating that an ammonium sulfate solution from which fluoride ions had been removed was obtained.

[0055] Furthermore, 100g of a pH 10 sodium hydroxide aqueous solution was placed in another 100ml beaker, and comparative adsorption resin adsorbent (3)-1 was added while stirring with a magnetic stirrer. After stirring for 300 minutes, the liquid was filtered. The fluoride ion concentration in this filtrate was measured, but no fluoride ions were detected, indicating that the resin adsorbent could not be regenerated.

Claims

1. (1) A high-salt aqueous solution containing a nitrogen element salt and fluoride ions, adjusted to a pH of 9 or less. (2) After introducing sulfonic acid groups into the styrene-divinylbenzene copolymer, it is brought into contact with a resin adsorbent supporting zirconium ions. (3) The solution is then separated to recover the high-salt aqueous solution. (4) The separated resin adsorbent is regenerated by contacting it with an alkaline aqueous solution with a pH of 11 or higher. A method for purifying a high-salt concentration solution containing nitrogen elements, characterized by the following:

2. The method for purifying a high-concentration solution containing a nitrogen element according to claim 1, wherein the nitrogen-containing salt is one or more salts selected from ammonium sulfate, ammonium nitrate, ammonium chloride, sodium nitrate, and calcium nitrate.

3. A method for purifying a high-salt solution containing nitrogen elements according to claim 1 or 2, wherein the concentration of nitrogen-containing salts in the high-salt aqueous solution is 5% by weight or more.

4. A method for purifying a high-salt solution containing nitrogen elements according to any one of claims 1 to 3, wherein the fluoride ion concentration of the high-salt aqueous solution is 0.1 ppm or more and 1000 ppm or less.

5. A method for purifying a high-salt solution containing nitrogen elements according to any one of claims 1 to 4, wherein the contact time between the high-salt aqueous solution and the resin adsorbent is 10 minutes or more.

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