JPH0354934B2 - - Google Patents

Description

[Detailed description of the invention]

<Industrial Application Field> The present invention deals with 2,6-naphthalene, which is a chemical product that is not only important as an intermediate for facial cleansing agents, but also as a raw material for 2,6-naphthalene diol, which is a raw material for heat-resistant polymers. The present invention relates to a method for producing disulfonic acid. <Prior art and its problems> 2,6-naphthalene disulfonic acid can be easily produced by sulfonating naphthalene or β-naphthalene sulfonic acid with sulfuric acid or fuming sulfuric acid. By-products such as naphthalenedisulfonic acid, 1,6-naphthalenedisulfonic acid, 1,7-naphthalene trisulfonic acid, and naphthalene trisulfonic acid are unavoidable, and 2,6-naphthalenedisulfonic acid is purer than the isomer mixture of these isomers. requires an operation to separate the
However, only a limited number of methods are known for separating 2,6-naphthalenedisulfonic acid, and all of these methods are extremely unsatisfactory in terms of separation efficiency and operation. Conventionally, 2,6-naphthalenedisulfonic acid and 2,
7-Naphthalenedisulfonic acid was separated using the method described in the PB report (BIOS Final Report No.
1152), that is, adding an alkali metal salt or an alkaline earth metal salt to a sulfuric acid solution of disulfonic acid,
This was done by generating salts of these isomers and taking advantage of their differences in solubility. However, in this method, the difference in solubility of naphthalene disulfonate is not significant, so the resulting 2,6-
The problem was that the naphthalene disulfonic acid metal salt had a low purity and a low recovery rate. Another drawback was that unreacted sulfuric acid could not be reused because inorganic salts were accumulated in the waste acid. As an improved method, 2,6-naphthalenedisulfonic acid is separated by crystallization at a sulfuric acid concentration of 60 to 70%, where its solubility is the lowest, in JP-A-54-163563, and in JP-A-59-26619. has proposed a separation method in which aniline is added as a separating agent and 2,6-naphthalenedisulfonic acid is precipitated as an aniline salt. However, in the former case, the difference in solubility between isomers is not very large, so
The yield and purity of the obtained 2,6-naphthalenedisulfonic acid are low, and although pure 2,6-naphthalenedisulfonic acid salt can be obtained in the latter case, the solubility of the aniline salt is relatively high, so 2,6-
Loss of the aniline salt of naphthalenedisulfonic acid was unavoidable, resulting in a decrease in the yield of 2,6-naphthalenedisulfonic acid and a drawback that aniline, which is a separation agent, could not be completely recovered. <Object of the Invention> The object of the present invention is to solve the drawbacks of such conventional methods, and by using a separating agent, a solution containing 2,6-naphthalenedisulfonic acid compound can be obtained with higher purity. The present invention makes it possible to efficiently produce 6-naphthalenedisulfonic acid, and also provides a method in which the separation agent used can be almost completely recovered and reused. <Structure of the Invention> As a result of studies on the separation of sulfonic acids using various diamines, the present inventors have found that when paraphenylenediamine and its derivatives are used as separating agents, the separating agent is 2,6-naphthalenedisulfonic acid. It was found that only sulfuric acid solutions produce salts that are almost insoluble.
Unlike aniline salts, 2,6-naphthalenedisulfonic acid phenylenediamine salts have extremely low solubility over a wide range of sulfuric acid concentrations, and are therefore characterized by no leakage into the mother liquor. The obtained 2,6-naphthalenedisulfonic acid diamine salt is suspended in an alcohol solvent having 1 to 5 carbon atoms, and then the amine salt is decomposed by adding an alkali such as sodium hydroxide or potassium hydroxide. It can be converted into an alkali salt of 2,6-naphthalenedisulfonic acid. Alkaline salts of 2,6-naphthalenedisulfonic acid are insoluble in alcohol solvents having 1 to 5 carbon atoms, whereas diamines are soluble, so the separating agent can be easily removed by filtration and washing, resulting in high purity. The 2,6-naphthalenedisulfonic acid metal salt can be isolated. On the other hand, since the diamine used as a separating agent completely migrates into the alcohol mother liquor, the diamine can be isolated from this as a mineral acid salt, or the alcohol solvent having 1 to 5 carbon atoms can be removed by distillation. It can be recovered in free amine form and recycled to the 2,6-naphthalenedisulfonic acid separation step. With the present invention, the following can now be accomplished very easily. High purity 2,6-naphthalenedisulfonic acid can be produced in high yield. Since there is no loss of the diamine used as a separation agent to the mother liquor, it can be almost completely recovered and recycled. According to a first aspect of the invention, in a solution containing 2,6-naphthalenedisulfonic acid, the general formula (R ¹ to ^{R 4} are the same or different, hydrogen, chlorine,
A compound that produces an aromatic diamine represented by (a fluorine, bromine, or iodine atom, or a lower alkyl group) is added as a separating agent, and the precipitated 2,6-naphthalenedisulfonic acid diamine salt is isolated. This is suspended in an alcohol having 1 to 5 carbon atoms, an alkali is added, and the precipitated alkali salt of 2,6-naphthalenedisulfonic acid is filtered off. Provided is a method for producing 2,6-naphthalenedisulfonic acid, which comprises washing with alcohols of numbers 1 to 5 to obtain a filtrate (A) containing a separating agent and an alkali salt of 2,6-naphthalenedisulfonic acid. Ru. According to a second aspect of the invention, in a solution containing 2,6-naphthalenedisulfonic acid, the general formula (R ¹ to ^{R 4} are the same or different, hydrogen, chlorine,
A compound that produces an aromatic diamine represented by (a fluorine, bromine, or iodine atom, or a lower alkyl group) is added as a separating agent, and the precipitated 2,6-naphthalenedisulfonic acid diamine salt is isolated. This is suspended in an alcohol having 1 to 5 carbon atoms, an alkali is added, and the precipitated alkali salt of 2,6-naphthalenedisulfonic acid is filtered off. A filtrate (A) containing a separating agent and an alkali salt of 2,6-naphthalenedisulfonic acid are obtained by washing with several alcohols, and a mineral acid is added to the filtrate (A) to precipitate paraphenylenediamine mineral. A method for producing 2,6-naphthalenedisulfonic acid is provided, which comprises isolating the acid salt and recycling it as a 2,6-naphthalenedisulfonic acid separating agent. According to a third aspect of the invention, in a solution containing 2,6-naphthalenedisulfonic acid, the general formula (R ¹ to ^{R 4} are the same or different, hydrogen, chlorine,
A compound that produces an aromatic diamine represented by (a fluorine, bromine, or iodine atom, or a lower alkyl group) is added as a separating agent, and the precipitated 2,6-naphthalenedisulfonic acid diamine salt is isolated. This is suspended in an alcohol having 1 to 5 carbon atoms, an alkali is added, and the precipitated alkali salt of 2,6-naphthalenedisulfonic acid is filtered off. Wash with several alcohols from 1 to 5 to obtain a filtrate (A) containing a separating agent and an alkali salt of 2,6-naphthalenedisulfonic acid, and remove the alcohol solvent from the filtrate (A) by distillation. 2,6-naphthalenedisulfonic acid, which is characterized by being recycled as a separating agent.
A method for producing naphthalenedisulfonic acid is provided. <Specific Structure of the Invention> The present invention will be explained in more detail below by dividing it into four steps. (First step) A step of obtaining a solution containing 2,6-naphthalenedisulfonic acid. For example, disulfonation in which naphthalene or β-naphthalenesulfonic acid is sulfonated with a sulfonating agent such as sulfuric acid or oleum to obtain a reaction solution containing 2,6-naphthalenedisulfonic acid or 2,7-naphthalenedisulfonic acid as the main components. Process. (Second step) Adding a compound that produces a predetermined amount of aromatic diamine to a solution containing 2,6-naphthalenedisulfonic acid such as the disulfonation reaction solution,
A process of separating naphthalene disulfonic acid as a diamine salt. (Third step) The diamine salt of 2,6-naphthalenedisulfonic acid obtained in the second step is suspended in an alcohol solvent having 1 to 5 carbon atoms, and an alkali is added in the presence of an inert gas. , 2,6-naphthalenedisulfonic acid alkali salt is isolated by decomposing it into an alkali salt of 6-naphthalenedisulfonic acid and a diamine, filtering off the precipitated crystals, and washing the filtered mass with an alcohol having 1 to 5 carbon atoms. (Fourth step) The diamine as a separating agent recovered in the alcohol mother liquor in the above third step is removed by distillation and then directly or by adding mineral acid to the mother liquor to precipitate the mineral acid salt of the amine and filtering. After separation, the recycling process is reused in the second process in the form of mineral salts. The solution containing 2,6-naphthalenedisulfonic acid used in the present invention generally refers to a solution containing 2,6- and 2,7-naphthalenedisulfonic acids obtained by sulfonation of naphthalene and β-naphthalenesulfonic acid as main products. It refers to a mixture of isomers, which may contain β-naphthalenesulfonic acid and naphthalenetrisulfonic acid. In addition, a mixed liquid after separating 2,7-naphthalenedisulfonic acid from the isomer mixture in advance, or
The present invention is also applicable to waste acids etc. that have a low concentration of 2,6-naphthalenedisulfonic acid. Further, the sulfuric acid concentration (sulfuric acid/(sulfuric acid) + water) in the sulfuric acid aqueous solution at this time is preferably adjusted to a range of 80% or less, more preferably ~60% or less. This is because if it exceeds 80%, the solubility of the mother liquor of 2,6-naphthalenedisulfonic acid phenylenediamine salt increases, and the separation rate decreases slightly. The molar ratio of the sulfonating agent used in the first step is
It is preferable to use it in a range of 1 to 3 times the theoretical amount. The reaction temperature may be a standard reaction temperature for the purpose of disulfonation, and is generally in the range of 140 to 190°C. The aromatic diamine used in the second step is a compound represented by the following general formula, and can also be used in the form of a salt such as a hydrochloride or a sulfate. (R ¹ to ^{R 4} are the same or different, hydrogen, chlorine,
The amount of diamine used in the present invention is 0.5 to 1.5 times the mole content of 2,6-naphthalenedisulfonic acid, preferably 0.9 It is best to use ~1.3 times the mole. Diamine may be added as a crystal, or
Alternatively, water or a solution of an alcohol solvent having 1 to 5 carbon atoms may be added dropwise. Among the compounds represented by the general formula, R ¹ to ^{R 4}
is hydrogen because it is easily obtained industrially. Furthermore, amide compounds of aromatic diamines and organic acids, such as monoacetyl compounds, can also be used. In this way, in the present invention, 2,6
- in a solution containing naphthalenedisulfonic acid,
In particular, any compound may be used as long as it easily produces the aromatic diamine represented by the above general formula by hydrolysis in its aqueous solution. The alcohol solvent having 1 to 5 carbon atoms used in the third step includes methanol, ethanol, n-propanol, isopropanol, n-butanol, sec
- Any substance that dissolves diamines such as butanol, tert-butanol, n-pentanol, etc.
It doesn't matter whether it is hydrated or anhydrous. The alkali used in the third step is preferably a hydroxide compound of an alkali metal salt such as sodium hydroxide, a hydroxide compound of an alkaline earth metal such as calcium hydroxide, and ammonia, and the amount of the alkali added is 2, For 2,6-naphthalenedisulfonic acid in 6-naphthalenedisulfonic acid diamine salt
The amount is preferably 2.0 to 3.0 times by mole. If the amount is less than 2.0 moles, the diamine salt will not be completely decomposed and the amine will be mixed into the crystal. It is meaningless to use more than 3.0 moles of alkali since it only increases the amount of alkali used. Further, the alkali may be added as an aqueous solution or a solution of an alcohol solvent having 1 to 5 carbon atoms. The temperature during decomposition of the diamine salt is preferably 20 to 50°C. Further, the stirring time varies depending on the conditions and is difficult to limit, but generally requires 0.1 to 10 hours. By satisfying the above conditions, 2,6-naphthalenedisulfonic acid alkali salt can be easily produced from a cake of 2,6-naphthalenedisulfonic acid diamine salt in a high yield of 95-99% and purity of 95-99%. It is possible to produce with high yield and high purity. In the fourth step, if the diamine obtained is directly recycled after the alcohol solvent has been removed by distillation, the concentrate containing some alcohol may be recycled. Also, in the method of adding mineral acid to the mother liquor to precipitate diamine mineral salts, and recycling them in the form of mineral salts after filtration, it is not necessary to completely dry the mineral salts, and the mineral salts contain alcohol. There is no harm in recycling it.
In any of these methods, the diamine recovery rate is as high as 98% or more, and the yield and quality of the alkali salt of 2,6-naphthalenedisulfonic acid obtained by recycling remain unchanged. Moreover, the alcohol solvent recovered in this step is of course reused in the third step. The mineral acid used is preferably sulfuric acid. <Examples> Example 1 347 parts of 60% β-naphthalenesulfonic acid hydrate was placed in a four-necked flask, and the temperature was raised to 180°C. After dropping 253 parts of concentrated sulfuric acid over 1 hour, 180 parts of concentrated sulfuric acid was added.
The sulfonation reaction was continued for an additional 5 hours at °C. The composition obtained at this time was 0.29 mol of 2,6-naphthalenedisulfonic acid, 0.29 mol of 2,7-naphthalenedisulfonic acid,
0.55 mol, 1,6-naphthalenedisulfonic acid 0.04
mole, 0.05 mole of 1,7-naphthalene disulfonic acid, and 0.06 mole of naphthalene trisulfonic acid. water to this naphthalene disulfonic acid isomer mixture.
500 parts of paraphenylene diamine sulfate were added at 90°C. After stirring at the same temperature for 1 hour, cool to room temperature, filter the precipitated crystals,
Washed with 420 parts of water. Methanol was added to this wet cake to suspend it, and then 56 parts of a 50% aqueous sodium hydroxide solution was added. Stirring was continued for 1 hour, and the 2,6-naphthalenedisulfonic acid phenylenediamine salt was decomposed and converted into 2,6-naphthalenedisulfonic acid sodium salt, which was precipitated. This was filtered, washed with 550 parts of methanol, and the 2,6-
Crystals of naphthalene disulfonic acid sodium salt were obtained. Example 2 On the other hand, 38 parts of concentrated sulfuric acid was added to a methanol solution in which paraphenylenediamine was dissolved, and the precipitated paraphenylenediamine sulfate was collected by filtration. Add 347 parts of 60% β-naphthalene sulfonic acid hydrate to a four-necked flask and raise the temperature to 180°C. After dropping 253 parts of concentrated sulfuric acid over 1 hour, 180 parts of concentrated sulfuric acid was added.
The sulfonation reaction was continued for an additional 5 hours at °C. 500 parts of water was added to this naphthalenedisulfonic acid isomer mixture, and at 90°C, the paraphenylenediamine sulfate wet cake recovered earlier and 1 part of paraphenylenediamine sulfate were added thereto. Hereinafter, the same operations as in Example 1 were performed. Examples 3 to 5 The same operation as in Example 2 was repeated three more times. The results of Examples 1 to 5 are shown in Table 1. Note that the numerical value of added paraphenylenediamine sulfate shows parts by weight. In addition, in this recycling, the recovery rate of the separating agent paraphenylenediamine was 98.2 mol%.

[Table] Total amount of naphthalenesulfonic acid
2) Yield example from β〓 naphthalene sulfonic acid 6 Put 128 parts of naphthalene into a four-necked flask,
The temperature was raised to ℃. After 120 parts of concentrated sulfuric acid was added dropwise over 30 minutes, the sulfonation reaction was continued at the same temperature for 3 hours. Thereafter, the temperature was further raised to 180°C, 230 parts of concentrated sulfuric acid was added dropwise over 1 hour, and the sulfonation reaction was continued at the same temperature for 6 hours. At this time, the obtained composition was 0.25 mol of 2,6-naphthalenedisulfonic acid,
2,7-naphthalenedisulfonic acid 0.51 mol, 1,
0.05 mol of 6-naphthalenedisulfonic acid, 1,7-
The amounts were 0.06 mol of naphthalene disulfonic acid and 0.02 mol of naphthalene trisulfonic acid. Add 100 parts of water to this naphthalene disulfonic acid isomer mixture,
A solution of 27 parts of paraphenylenediamine dissolved in 200 parts of water was added dropwise over 30 minutes at 90°C. After stirring at the same temperature for 1 hour, the mixture was cooled to room temperature, and the precipitated crystals were filtered and washed with water. After adding methanol to this wet cake and suspending it, 70 parts of a 50% aqueous potassium hydroxide solution was added. Stirring was continued for 1 hour to decompose the phenylene diamine salt of 2,6-naphthalenedisulfonic acid and phenylene disulfonic acid, converting it to potassium salt of 2,6-naphthalenedisulfonic acid, and precipitating it. This was filtered and washed with 450 parts of methanol to obtain crystals of 2,6-naphthalenedisulfonic acid potassium salt. Example 7 On the other hand, 33 parts of concentrated sulfuric acid was added to a methanol solution in which paraphenylenediamine was dissolved, and the precipitated paraphenylenediamine sulfate was collected by filtration. Put 128 parts of naphthalene in a four-necked flask and add 160 parts
The temperature was raised to ℃. After 120 parts of concentrated sulfuric acid was added dropwise over 30 minutes, the sulfonation reaction was continued at the same temperature for 3 hours. Furthermore, the temperature was raised to 180°C, 230 parts of concentrated sulfuric acid was added dropwise over 1 hour, and the sulfonation reaction was continued at the same temperature for 6 hours. 300 parts of water was added to this naphthalenedisulfonic acid isomer mixture, and the paraphenylenediamine sulfate and 1 part of paraphenylenediamine sulfate recovered earlier were added at 90°C. below,
The same operation as in Example 6 was performed. Examples 8-9 The same operation as in Example 7 was repeated three more times. The results of Examples 6 to 10 are shown in Table 2. Note that the values for added paraphenylene diamine and paraphenylene diamine sulfate are in parts by weight. In this recycling, the recovery rate of paraphenylenediamine was 98.0 mol%.

[Table] Example 11 347 parts of 60% β-naphthalenedisulfonic acid hydrate was placed in a four-necked flask, and the temperature was raised to 170°C. After dropping 253 parts of concentrated sulfuric acid over 1 hour, the sulfonation reaction was continued at the same temperature for 8 hours. At this time, the obtained composition was 0.28 mol of 2,6-naphthalenedisulfonic acid, 0.28 mol of 2,7-naphthalenedisulfonic acid
0.54 mol, 1,6-naphthalenedisulfonic acid 0.05
mole, 0.07 mole of 1,7-naphthalene disulfonic acid, and 0.02 mole of naphthalene trisulfonic acid. 400 parts of water was added to this naphthalenedisulfonic acid isomer mixture, and 40 parts of paraphenylenediamine hydrochloride was added at 80°C. After stirring at the same temperature for 1 hour, it was cooled to room temperature, and the precipitated crystals were filtered off.
Washed with water. Methanol was added to this wet cake to suspend it, and then 56 parts of a 50% aqueous sodium hydroxide solution was added. Continue stirring for 1 hour to decompose the 2,6-naphthalenedisulfonic acid phenylenediamine salt,
It was converted into 2,6-naphthalenedisulfonic acid sodium salt and precipitated. Filter this and methanol
Washing with 500 parts gave crystals of 2,6-naphthalenedisulfonic acid sodium salt. Example 12 On the other hand, a methanol solution in which paraphenylenediamine was dissolved was concentrated under reduced pressure using an evaporator to remove methanol. 347 parts of 60% β-naphthalenesulfonic acid hydrate was placed in a four-necked flask, and the temperature was raised to 170°C. After dropping 253 parts of concentrated sulfuric acid over 1 hour, the sulfonation reaction was continued at the same temperature for 8 hours. 400 parts of water was added to this naphthalenedisulfonic acid isomer mixture, and the above paraphenylenediamine concentrate and 1 part of paraphenylenediamine hydrochloride were added. Hereinafter, the same operations as in Example 11 were performed. Examples 13-15 The same operation as in Example 12 was repeated three more times. The results of Examples 11 to 15 are shown in Table 3. Note that the numerical value of added paraphenylenediamine hydrochloride indicates parts by weight. In addition, in this recycling, the recovery rate of the separating agent paraphenylenediamine was 97.8 mol%.

[Table] Total amount of naphthalenesulfonic acid
2) Yield comparison example from β-naphthalenesulfonic acid 347 parts of 60% β-naphthalenesulfonic acid hydrate was placed in a four-necked flask, and the temperature was raised to 180°C. After dropping 263 parts of concentrated sulfuric acid over 1 hour, the mixture was heated to 180°C.
The sulfonation reaction was continued for 5 hours. At this time, the obtained composition was 2,6-naphthalenedisulfonic acid 0.28
mol, 2,7-naphthalenedisulfonic acid 0.58 mol, 1,6-naphthalenedisulfonic acid 0.04 mol,
The amounts were 0.05 mol of 1,7-naphthalenedisulfonic acid and 0.03 mol of naphthalene trisulfonic acid. 500 parts of water was added to this naphthalenedisulfonic acid isomer mixture, and 119 parts of aniline was added at 60°C. After stirring at the same temperature for 3 hours, the mixture was cooled to room temperature, and the precipitated crystals were filtered and washed with water. Water was added to this wet cake to suspend it, and then 45 parts of 50% sodium hydroxide was added. Stirring was continued for 1 hour to decompose the 2,6-naphthalenedisulfonic acid aniline salt, converting it to 2,6-naphthalenedisulfonic acid sodium salt, and precipitating it. This was filtered to obtain crystals of 2,6-naphthalenedisulfonic acid sodium salt. The yield from β-naphthalenedisulfonic acid was 17.2%, and the purity was 97.1%. Furthermore, the amount of aniline recovered in the mother liquor during filtration of sodium 2,6-naphthalenedisulfonate was 47.1% based on the added aniline. <Effects of the Invention> As is clear from the above description, according to the method of the present invention, 2,6-naphthalenedisulfonic acid can be extracted from a solution containing 2,6-naphthalenedisulfonic acid, even from a solution containing various isomers. 6-Naphthalenedisulfonic acid can be separated almost completely. Furthermore, the diamine used in the separation of 2,6-naphthalenedisulfonic acid can be almost completely recovered and reused in the above separation cycle, which also has an adverse effect on the separation of 2,6-naphthalenedisulfonic acid. The diamine can be fully utilized without any adverse effects.

Claims (1)

[Scope of Claims] 1 In a solution containing 2,6-naphthalenedisulfonic acid, in this solution, the general formula (R ¹ to ^{R 4} are the same or different, hydrogen, chlorine,
A compound that produces an aromatic diamine represented by (a fluorine, bromine, or iodine atom, or a lower alkyl group) is added as a separating agent, and the precipitated 2,6-naphthalenedisulfonic acid diamine salt is isolated. This is suspended in an alcohol having 1 to 5 carbon atoms, an alkali is added, and the precipitated alkali salt of 2,6-naphthalenedisulfonic acid is filtered off. A method for producing 2,6-naphthalenedisulfonic acid, which comprises washing with 1 to 5 alcohols to obtain a filtrate (A) containing a separating agent and an alkali salt of 2,6-naphthalenedisulfonic acid. 2 Into a solution containing 2,6-naphthalenedisulfonic acid, in this solution, the general formula (R ¹ to ^{R 4} are the same or different, hydrogen, chlorine,
A compound that produces an aromatic diamine represented by (a fluorine, bromine, or iodine atom, or a lower alkyl group) is added as a separating agent, and the precipitated 2,6-naphthalenedisulfonic acid diamine salt is isolated. This is suspended in an alcohol having 1 to 5 carbon atoms, an alkali is added, and the precipitated alkali salt of 2,6-naphthalenedisulfonic acid is filtered off. A filtrate (A) containing a separating agent and an alkali salt of 2,6-naphthalenedisulfonic acid are obtained by washing with several alcohols, and a mineral acid is added to the filtrate (A) to precipitate paraphenylenediamine mineral. A method for producing 2,6-naphthalenedisulfonic acid, which comprises isolating the acid salt and recycling it as a 2,6-naphthalenedisulfonic acid separating agent. 3 Into a solution containing 2,6-naphthalenedisulfonic acid, in this solution, the general formula (R ¹ to ^{R 4} are the same or different, hydrogen, chlorine,
A compound that produces an aromatic diamine represented by (a fluorine, bromine, or iodine atom, or a lower alkyl group) is added as a separating agent, and the precipitated 2,6-naphthalenedisulfonic acid diamine salt is isolated. This is suspended in an alcohol having 1 to 5 carbon atoms, an alkali is added, and the precipitated alkali salt of 2,6-naphthalenedisulfonic acid is filtered off. Wash with several alcohols from 1 to 5 to obtain a filtrate (A) containing a separating agent and an alkali salt of 2,6-naphthalenedisulfonic acid, and remove the alcohol solvent from the filtrate (A) by distillation. 2,6-naphthalenedisulfonic acid, which is characterized by being recycled as a separating agent.
Method for producing naphthalene disulfonic acid.