JPH0940604A - Purification of citric acid - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain citric acid almost without consuming auxiliary raw material, almost without forming a waste material, with an easy operation, having a wide range of variation in the operating conditions and capable of highly purifying the compound easily by purifying through the disproportionation process of the citric acid hydrogen salt using an organic solvent. SOLUTION: Citric acid is purified by performing (B) a disproportionation process of a citric acid hydrogen salt by using (A) an organic solvent. Further, acetone, etc., are used as the component A, and calcium monohydrogen citrate, calcium dihydrogen citrate, etc., are preferably subjected to the disproportionation treatment at 50 deg.C for 1hr, and as the component B, a mixture obtained from a precipitate obtained by mixing a crude citric acid obtained by adding glucose to citric acid monohydrate, with an aqueous suspension of a citric acid calcium salt at 0 deg.C, is preferably used.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a method for purifying citric acid.

[0002]

2. Description of the Related Art There is a method of purifying citric acid via calcium citrate. In this method, a salt of calcium citrate is precipitated from crude citric acid and calcium hydroxide to remove impurities, and the salt is decomposed with sulfuric acid to obtain purified citric acid.

There is also a method via calcium monohydrogen citrate. This is a method in which calcium hydrogen citrate is produced from crude citric acid and calcium citrate to remove impurities, and monohydrogen salt is decomposed with sulfuric acid to obtain purified citric acid.

As another method via calcium monohydrogen citrate, JP-A-6-48979 discloses a method for purifying citric acid. This is to react crude citric acid with calcium citrate in the aqueous phase to give calcium monohydrogen citrate, and then disproportionate the calcium monohydrogen citrate in an aqueous solution to liberate citric acid.

[0005]

The prior art methods have some problems to be solved when applied to industrial production.

The method via calcium citrate is as follows:
(1) A large amount of calcium hydroxide and sulfuric acid are required as auxiliary materials, (2) not only a large amount of calcium sulfate waste is generated from these auxiliary materials, but also (3) calcium citrate is generated and decomposed. There is a problem that batch processing is required and a long time is required. The method of decomposing calcium monohydrogen citrate with sulfuric acid is more effective in reducing the amount of auxiliary raw materials used and the amount of waste generated than the method of using calcium citrate, but the problem of the method of using calcium citrate remains. ing.

On the other hand, the method described in JP-A-6-48979 is an excellent method in principle because salt wastes derived from the auxiliary raw materials are difficult to generate. The problems are (1) the free citric acid is an aqueous solution but calcium salts are present, and (2) the second treatment can reduce the calcium salt contamination, but the operation becomes complicated. As a result, the processing time becomes long, (3)
It is effective only when passing through calcium monohydrogen citrate, and (4) it is considered that a large amount of energy is required for concentrating the aqueous solution.

Calcium contained in citric acid has a surprisingly large effect on the purity of citric acid. The molecular weight of citric acid is 192 for 40 atomic weight of calcium
It is. Considering that most of the impurity calcium exists in the form of calcium dihydrogen citrate, citric acid 2
The amount of calcium hydrogen corresponds to about 10 times the amount of calcium. Therefore, the purity of true citric acid directly obtained by a single treatment by the method described in JP-A-6-48979 is extremely low. In particular, when the reaction time was shortened, citric acid containing 8 to 9% calcium was obtained from calcium monohydrogen citrate (reference example of the present invention). Although the (apparent) purity of citric acid is 91 to 92%, it is a problem that should be improved because it is almost equivalent to calcium dihydrogen citrate in terms of composition.

Therefore, the object of the present invention is to (1) hardly consume auxiliary raw materials, (2) hardly generate wastes, (3) easy operation, and (4) a wide variable range of operating conditions. (5) A method for purifying citric acid, which has advantages such as easy purification and high purification.

[0010]

The present inventor has made earnest studies on a method for purifying citric acid which solves the drawbacks of the prior art. As a result, they have found that hydrogen citrate is effectively disproportionated in an organic solvent, and completed the present invention.

That is, the present invention can be achieved by a method for purifying citric acid which is characterized by passing through a disproportionation step of hydrogen citrate using an organic solvent.

Therefore, the characteristic part of the present invention lies in the step of disproportionating the hydrogen citrate salt to produce citric acid, and in the method of the present invention, substances other than impurities can be easily recovered and reused in principle. Therefore, the advantages (1) to (5) described above can be achieved.

"Disproportionation" as used herein
The term is intended to indicate a chemical change in which a chemical species existing as an organic polybasic acid hydrogen salt exchanges hydrogen ions and metal ions with each other to form an organic polybasic acid and an organic polybasic acid salt. . It is self-evident that citric acid is one of the organic polybasic acids.

[0014]

BEST MODE FOR CARRYING OUT THE INVENTION According to the disproportionation of the present invention, free citric acid can be obtained from hydrogen citrate. It involves a novel step of producing free citric acid by disproportionating hydrogen citrate in an organic solvent. In the present specification, hydrogen citrate means citric acid 1
A hydrogen salt, a dihydrogen citric acid salt, or a mixture thereof.

This disproportionation method is characterized by treating solid hydrogen citrate with an organic solvent. The liquid phase observed under the disproportionation treatment condition is only the organic solvent phase microscopically, which is different from the ordinary extraction method utilizing the partition between the two liquid phases.

This method is also different from the method of recovering and purifying an organic acid from a mixture of an organic acid and an organic acid salt. That is, in such a mixture, the organic acid is always present stoichiometrically regardless of the existing state such as solid state or solution state. Recovery and purification of organic acids from such mixtures causes physical changes. This method effectively changes the chemical reaction of the organic acid hydrogen salt and is essentially different from the usual separation method.

The preferred embodiments of the present invention will be described below.
This will be described in detail with reference to the drawings.

First, a step of reacting a base or a citrate with citric acid to form a hydrogen citrate (a step of preparing a hydrogen citrate), and a hydrogen citrate obtained in the step of producing a hydrogen citrate. Is disproportionated in an organic solvent to obtain citric acid and a citrate salt (hydrogen citrate disproportionation step). FIG. 1 shows an embodiment of this purification method. It is a schematic process drawing which simply represented the flow of a process.

(I) Step of Preparing Hydrogen Citric Acid Salt The operation itself of this step (I) is well known and is not particularly novel, but it will be briefly described. From FIG. 1, the desired hydrogen citrate salt (citric acid monohydrogen salt, dihydrogen citric acid salt or a mixture thereof) is (1) a crude citric acid aqueous solution as a raw material, which is reacted with an appropriate amount of citrate. (2) It is obtained by a method such as neutralizing a crude aqueous citric acid solution as a raw material with an appropriate amount of a metal oxide, hydroxide, carbonate or the like.

The crude citric acid used as a raw material may be prepared by any method such as a fermentation method, an enzymatic method, an extraction method and a synthetic method. Impurities in the crude citric acid as a raw material are removed by applying operations such as washing, crystallization and recrystallization to the hydrogen citrate obtained in this preparation step.

From the viewpoint of industrial production, it is considered that hydrogen citrate containing calcium as a cation is particularly easy to use. In fact, in the preceding citric acid purification method,
A calcium salt such as calcium citrate or calcium monohydrogen citrate is used as an intermediate substance.

By the way, it is possible to form calcium dihydrogen citrate from citric acid and calcium. In the present invention, of course, if calcium dihydrogen citrate can be used in the similar method of the prior invention, there should be many industrial advantages in terms of reaction rate and waste. Nevertheless, calcium dihydrogen citrate was not utilized because it was difficult to handle. That is, not only is calcium dihydrogen citrate highly water-soluble and difficult to recrystallize from an aqueous solution, but the aqueous solution is easily decomposed by heating or dilution. The present inventor also studied the production of calcium dihydrogen citrate.

The aqueous solution of calcium dihydrogen citrate can be prepared by reacting calcium citrate, calcium hydroxide, calcium oxide or calcium carbonate with crude citric acid in water. Calcium dihydrogen citrate can be crystallized by rapidly adding an organic solvent to this aqueous solution. Citric acid 2 prepared here
If the aqueous calcium hydrogen solution is diluted, it easily decomposes, so the concentration is preferably 10% or more, and more preferably 20% or more. The organic solvent used for crystallization of calcium dihydrogen citrate is highly water-soluble, and any solvent that does not chemically react with citric acid or hydrogen citrate can be used. Examples thereof include alcohols such as methanol and ethanol, ethers such as dioxane and DME, ketones such as acetone, and nitriles such as acetonitrile. The amount of organic solvent used is
0.5 for the amount of calcium dihydrogen citrate solution
It is practically 2 to 5 times, but more preferably 1 to 3 times. Since calcium dihydrogen citrate is easily decomposed,
The process of crystallizing and separating calcium dihydrogen citrate needs to be performed at the lowest possible temperature and as quickly as possible. Although the separated calcium dihydrogen citrate is washed in the examples, the washing operation can be omitted and the yield can be increased depending on the nature and amount of impurities. Calcium dihydrogen citrate may decompose if the crystallization / separation operation is not performed properly, but even calcium dihydrogen citrate containing such a decomposition product is sufficient for the disproportionation step described below. Can be used.

In the method for preparing calcium dihydrogen citrate, the organic solvent remains in the crystallization mother liquor. The organic solvent can be recovered as an organic solvent containing some water by a known method such as distillation. Since the organic solvent used in this preparation method does not have a problem even if it contains a little water, the recovered organic solvent can be reused.

(II) Disproportionation Step of Hydrogen Citrate This step (II) is the core of the present invention. As shown in FIG. 1, citric acid is produced by treating the hydrogen citrate obtained in the above step (I) in an organic solvent (Formula 1). Further, hydrogen citrate prepared by another method can also be used in this step (II).

Citric acid monohydrogen salt → citric acid citric acid dihydrogen salt → citric acid (Formula 1) As a reaction related to the above Formula 1, it is widely known that an organic acid is produced by an equilibrium reaction in an aqueous solution of an organic acid hydrogen salt. It is known. However, when the aqueous solution of organic acid hydrogen salt is concentrated, the organic acid hydrogen salt is formed again, and it is usually impossible to obtain a free organic acid. Therefore, it has been believed that it is difficult to disproportionate an aqueous solution of an organic acid hydrogen salt to prepare a free organic acid. With a lucky exception,
(1) There is a description (Hirokawa Shoten, Kagaku Daijiten) that the L-calcium hydrogen malate aqueous solution is disproportionated at 60 ° C. (it was difficult to reproduce), (2) water of calcium monohydrogen citrate. Preparation of free citric acid by disproportionation in the phase (Japanese Patent Application Laid-Open No. 6-48979), (3) In relation to these, the present inventor has shown that when an aqueous solution of DL-calcium hydrogen malate is heated, it is disproportionated. It can only be seen that it has been admitted.

The disproportionation step (II) is characterized in that hydrogen citrate is treated in an organic solvent. Therefore, the present disproportionation method enables a general-purpose and effective disproportionation of an organic acid hydrogen salt, which has hitherto been difficult, and produces an organic acid in a state where it can be easily separated. In this respect, it is qualitatively different from the formation of the organic acid by the equilibrium reaction in the aqueous solution.

The disproportionation method of the present invention is applicable to various hydrogen citrate salts, that is, various 1 hydrogen metal citrate and 2 hydrogen hydrogen citrate salts. Since the obtained citric acid is an organic solvent solution, it can be easily concentrated and has a low salt impurity content.
There are more advantages than the method of the '79 publication. The disproportionation step (II), which is the core of the invention, will be described in detail below.

[Organic solvent] In the disproportionation step (II) of the present invention, an organic solvent is used. Any organic solvent can be used as long as it dissolves citric acid at the temperature of the disproportionation treatment and does not pose a problem of alteration during the disproportionation treatment. However, considering the actual operation, an organic solvent capable of dissolving 5% by weight or more of citric acid under the temperature conditions of the disproportionation treatment is preferable. The boiling point of the organic solvent does not directly limit the range of use of the organic solvent, but it is preferable that vaporization / condensation is easily caused from the viewpoint of easy recovery and reuse. From this point of view, the boiling point of the organic solvent is preferably 160 ° C or lower at 1 atm, and more preferably 120 ° C or lower at 1 atm. Specific examples of the even more preferable organic solvent include acetone, ketone solvents such as 2-butanone (MEK), tetrahydrofuran (THF),
Ether solvent such as 1,2-dimethoxyethane (DME), dioxane, diglyme (diethylene glycol dimethyl ether), alcohol solvent such as methanol, ethanol, butanol, pentanol, nitriles such as acetonitrile (AN), ethyl acetate, etc. Examples thereof include ester solvents and halogen solvents such as dichloromethane.

These organic solvents can also be used as a mixture. It is also possible to add small amounts of substances to promote disproportionation. In particular, it is effective to contain a small amount of water or to add water to the extent that an aqueous phase does not appear under disproportionation conditions. In this state, solid hydrogen citrate salts (hydrogen salt and normal salt) are suspended in the organic solvent.

[Disproportionation Temperature and Time] Disproportionation Step (II)
Temperature and time influence the disproportionation reaction in. Disproportionation tends to proceed at higher temperatures. If the disproportionation temperature is set high, the time can be set short. Since the rate of disproportionation also depends on the particle size of the hydrogen citrate salt used, in the examples described below, the disproportionation treatment condition of 50 ° C. for 1 hour is often used.
However, the disproportionation treatment conditions are not limited to the temperature and time exemplified in the examples. The disproportionation treatment conditions are such that the organic solvent used, hydrogen ions in the hydrogen citrate salt, metal ions in the hydrogen citrate salt, and the state of the hydrogen citrate salt do not cause difficulties in the disproportionation operation and the subsequent separation operation. Can be set freely.

[Hydrogen citrate] The present disproportionation method can be applied to various hydrogen citrate. It is also applicable to a hydrogen salt of citric acid prepared by a method other than the preparation step (I). Various cations can be used as hydrogen citrate used in the disproportionation process. For example, it is possible to use alkali metal ions such as ammonium ions, lithium ions, sodium ions, and potassium ions, magnesium ions, calcium ions, strontium ions, alkaline earth metal ions such as barium ions, and various metal ions such as zinc. is there. Practically, use of calcium ion, strontium ion, barium ion and zinc ion is preferable, and calcium ion is particularly preferable.

The hydrogen citrate salt used in the disproportionation step (II) is not limited to the anhydrous salt. That is, the disproportionation method can be applied even if the hydrogen citrate contains water of crystallization, water of adsorption, or water of adhesion.

[Particle size of hydrogen citrate and stirring during disproportionation operation] Under the conditions in which this disproportionation method can be effectively applied, particles of hydrogen citrate are suspended in an organic solvent.
If the size of the particles is small, disproportionation proceeds rapidly, which is advantageous. However, since it can be seen that the particles disintegrate with the progress of the disproportionation reaction, the disproportionation process can be performed even for large particles by prolonging the treatment time and stirring strongly.

[Mixing ratio of hydrogen citrate and organic solvent]
When the mixing ratio of hydrogen citrate and organic solvent is expressed based on the number of moles of hydrogen ion in hydrogen citrate, it is practical that the organic solvent is 0.1 to 50 ml per 1 mmol of hydrogen ion. . Preferably 0.5 to 10 ml of an organic solvent for 1 mmol of hydrogen ion of hydrogen citrate.
It is.

[Treatment after Disproportionation Operation] This disproportionation step (I
In I), after the disproportionation reaction, a solid state citrate and a solution of citric acid in an organic solvent are separated. For the separation of these two (solid-liquid separation), a commonly used separation method such as a filtration method, a centrifugal separation method, or a decantation can be applied. Since the disproportionation rate changes when the temperature is changed and the mixture is left to stand after the disproportionation treatment, it is preferable to quickly separate the citrate salt and the citric acid solution in the organic solvent.

The separated organic solvent solution of citric acid is subjected to a treatment of distilling or evaporating the organic solvent to separate and recover the organic solvent and citric acid. This separation and recovery method is a general method. The organic solvent recovered here can be reused by circulating it in the disproportionation step (II) as shown in FIG. In this disproportionation step (II), high-purity citric acid can be easily recovered in a short time.

An important impurity in the citric acid obtained by the purification method of the present invention is a metal derived from hydrogen citrate. When calcium is used as the metal, the amount of calcium varies depending on the organic solvent used, but 0.03%
~ 0.07% or less. That is, when the purification method of the present invention is used, the purity of citric acid finally obtained is 99.
It can be easily purified to 93 to 99.97% or more.

Further, as shown in FIG. 1, in the disproportionation step (II), the citrates are recovered in a solid state. The recovered citrates can be recycled and used as a raw material for producing hydrogen citrate in the hydrogen citrate preparation step (I). By this operation, the metal oxide, hydroxide, carbonate or base component derived from the citrate used for the preparation of hydrogen citrate can be recovered and reused.

Further, in this disproportionation step (II),
As an embodiment different from that shown in FIG. 1, the recovered citrates can be disproportionated again to release citric acid from the remaining hydrogen citrate. If necessary, it is possible to finally disproportionate all the hydrogen citrate by repeating the disproportionation treatment on the recovered citrates.

As described above, in the purification method according to the present invention, the base component and the organic solvent used as auxiliary materials can be almost completely reused. As a result, it is possible to purify citric acid with little consumption of these auxiliary raw materials, and thus with almost no generation of waste materials derived from the auxiliary raw materials.

[0042]

EXAMPLES The present invention will be described in more detail below with reference to examples. In the present invention, especially salts containing citric acid and calcium are frequently used. Regarding the names and chemical formulas of those salts, citric acid
group) is abbreviated as cit, calcium citrate is Ca ₃ cit ₂ , calcium monohydrogen citrate is CaHcit, and calcium dihydrogen citrate is CaHit.
(H ₂ cit) ₂ . The number of moles described in the following examples and comparative examples are according to these chemical formulas. Further, here, the “disproportionation rate” is expressed by Equation 2 below.

[0043]

[Equation 1]

It is a value represented by

The amount of hydrogen ions in Formula 2 can be measured by using hydrogen citrate or citric acid as an aqueous solution and performing neutralization titration with sodium hydroxide. In order to reduce the titration error, the hydrogen ion concentration (pH) is measured with a pH meter, and the hydrogen ion amount is determined with the time point at which the pH change per added sodium hydroxide becomes maximum as the neutralization point. In this method, the pH at the neutralization point is higher than the acid dissociation constant (pKa ₃ ) of Formula 3 shown below by about 2 to 3 and the titration error is 0.2% or less.

[0046]

Embedded image

(1) Synthesis of pure calcium monohydrogen citrate using crude citric acid as raw material A mixture of citric acid monohydrate (21.0 g, 0.1 mol) and glucose (6.4 g) was used. This crude citric acid was dissolved in 50 ml of water, and calcium citrate 49.8 g
It was mixed with an aqueous suspension (containing 150 ml of water) of 0.1 mol (that is, containing 0.3 mol of Ca and 0.2 mol of citrate) at 0 ° C. The obtained precipitate is separated and cold water 100m
It was washed 3 times with 1 each. Dried under vacuum, 61.9 g
A solid of calcium dihydrogen citrate (0.269 mol, yield 90% of theory) was obtained. The glucose content in calcium monohydrogen citrate was less than 0.02%, and it could be used in the step of producing citric acid.

(2-a) Disproportionation of calcium monohydrogen citrate-Part 1-To 691 mg (3.00 mmol) of calcium monohydrogen citrate obtained above, 3 ml of an organic solvent was added and stirred at 50 ° C. for 1 hour. did. The organic phase is separated by filtration and the residue is the organic solvent 1
Washed twice with each ml. The organic phase and the washing solution were combined and concentrated to obtain citric acid. The organic solvents used and the results are summarized in Table 1.

[0049]

[Table 1]

(2-b) Disproportionation of calcium monohydrogen citrate-Part 2-To the 691 mg (3.00 mmol) of calcium monohydrogen citrate obtained above, 5 ml of DME was added and stirred at 50 ° C. for 1 hour. The organic phase is separated by filtration and the residue is DME 1 m.
It was washed twice with 1 l each. The organic phase and the washing solution were combined and concentrated, and citric acid 70.1 mg (0.365 mmol, reaction rate 3
7%). The case where the reaction temperature was 80 ° C. was also examined, but 67.9 mg (0.354 mmol, disproportionation rate 35%) of citric acid was obtained. The calcium content of these citric acids was 0.03% or less.

(2-c) Disproportionation of calcium monohydrogen citrate-Part 3-To the 460 mg (2.00 mmol) of calcium monohydrogen citrate obtained above, 4 ml of diglyme was added and stirred at 50 ° C. for 1 hour. . The organic phase was separated by filtration and the residue was washed twice with 1 ml of diglyme. 80.4 mg (0.419 mmol, disproportionation rate 63%) of citric acid in the organic phase and washing solution
Met. The case where the reaction temperature was 120 ° C was also examined, but 33.5 mg of citric acid (0.174 mmol,
A disproportionation rate of 26%) was obtained.

(2-d) Disproportionation of calcium monohydrogen citrate-Part 4-Further similarly, to 691 mg (3.00 mmol) of calcium monohydrogen citrate was added 5 ml of methanol, and the temperature was 50 ° C.
For 1 hour. The organic phase was separated by filtration and the residue was washed twice with 1 ml each of methanol. The organic phase and the washing solution were combined and concentrated to obtain 107.7 mg (0.561 mmol, disproportionation rate 56%) of citric acid. The calcium content of this citric acid was 0.07% or less.

Example 2 (1) Synthesis of calcium dihydrogen citrate using crude citric acid as a raw material 8.4 g (40 mmol) of citric acid monohydrate and 2.6 g of glucose (corresponding to a glucose content of 24%) Was used. This crude citric acid was dissolved in 30 ml of water, and 4.98 g of calcium citrate (10 mmol, ie, containing 30 mmol of Ca and 20 mmol of citrate) in a water suspension (containing 10 ml of water) and room temperature (25 ° C).
And mixed. Calcium citrate dissolved and a clear solution was obtained. The solution was cooled to 0 ° C. and the acetone at 10 ° C. 10
0 ml was mixed rapidly. The white precipitate formed was separated and hydrated acetone (acetone: water = 2.5: 1) 20 m
It was washed with 1 and dried. The obtained calcium dihydrogen citrate was 9.6 g (the yield based on the theoretical value was 76%), and the glucose content was 2%, which was sufficiently usable for disproportionation.

(2-a) Disproportionation of calcium dihydrogen citrate-Part 1-To 323 mg of the calcium dihydrogen citrate sample obtained above (317 mg, containing 0.75 mmol of calcium dihydrogen citrate). 3 ml of an organic solvent was added, and the mixture was stirred at 50 ° C for 1 hour. The organic phase is separated by filtration and the residue is 1 m of organic solvent.
It was washed twice with 1 l each. The organic phase and the washing solution were combined and concentrated to obtain citric acid. The organic solvents used and the results are summarized in Table 2.

[0055]

[Table 2]

(2-b) Disproportionation of calcium dihydrogen citrate-part 2-To 317 mg (0.75 mmol) of calcium dihydrogen citrate obtained above, 3 ml of DME was added and stirred at 80 ° C. for 1 hour. The organic phase is separated by filtration and the residue is DME 1 m.
It was washed twice with 1 l each. The organic phase and the washing solution were combined and concentrated to give 146 mg of citric acid (0.76 mmol, disproportionate ratio of 76).
%) Was obtained. The calcium content of this citric acid is 0.03
% Or less.

Comparative Example 1 This comparative example was carried out according to the method described in JP-A-6-489798 or a method similar thereto.

(1) Synthesis of pure calcium dihydrogen citrate using crude citric acid as raw material In the same manner as in Example 1, calcium dihydrogen citrate was synthesized.

(2) Disproportionation of calcium monohydrogen citrate To 691 mg (3.00 mmol) of calcium monohydrogen citrate was added 3 ml of water, and the mixture was stirred at 50 ° C. for 1 hour. The aqueous phase was separated by filtration and the residue was washed twice with 1 ml of water each time.
Analysis of the aqueous phase and the washings 11.3 mg of calcium
(0.282 mmol) and 0.578 mmol of citric acid radical (111 mg as citric acid) were detected. The case where the amount of water used for the reaction was 5 ml was also examined, but after the treatment, calcium 14.9 mg (0.372 mg) was obtained as an aqueous solution.
And 0.860 mmol of citric acid radical (165 mg as citric acid) were detected. These results showed that calcium was present in the obtained citric acid in an amount of 8 to 9% by weight. Judging from the molar ratio of citric acid and calcium, it seemed more appropriate to think that an aqueous solution of calcium dihydrogen citrate was obtained when calcium monohydrogen citrate was treated in water for 1 hour.

Comparative Example 2 This comparative example was also carried out according to the method described in JP-A-6-489798 or a method similar thereto.

(1) Synthesis of pure calcium dihydrogen citrate using crude citric acid as raw material In the same manner as in Example 2, calcium dihydrogen citrate was synthesized.

(2) Disproportionation of calcium dihydrogen citrate To 317 mg (0.75 mmol) of calcium dihydrogen citrate was added 3 ml of water, and the mixture was stirred at 50 ° C. for 1 hour. The aqueous phase was separated by filtration and the residue was washed twice with 1 ml of water each time.
Analysis of the aqueous phase and the washings revealed that calcium was 14.3 mg
(0.359 mmol) and 1.104 mmol of citric acid radical (212 mg as citric acid) were detected. This result showed that 6% by weight of calcium was present in the obtained citric acid.

As a result of analyzing the calcium content in citric acid obtained by the disproportionation in each Example and Comparative Example, the calcium content was low, and the superiority of the present invention over the prior art was confirmed. Furthermore, regarding the solubility of calcium citrate, at 50 ° C, it is approximately 0.2 g / dl water (50 m
Although it was suggested that the solubility of calcium is higher in the comparative example, the solubility of gCa / dl) is higher, but this is due to the effect of coexisting citric acid, and substantially calcium dihydrogen citrate. It is thought that it is because it has become. Regarding the solubility of calcium citrate in water-containing acetone or water-containing DME, it was 2 at 50 ° C.
It was confirmed that the amount was not more than the mg / dl solution solvent (0.5 mgCa / dl).

[0064]

INDUSTRIAL APPLICABILITY In the method for purifying citric acid according to the present invention,
By going through the disproportionation process of hydrogen citrate,
(1) hardly consumes auxiliary raw materials, (2) hardly produces waste, (3) easy operation, (4) wide variable range of operating conditions, (5) easy purification. Due to its advantages, citric acid can be purified easily and efficiently without the problems of auxiliary material consumption and waste production.

[Brief description of drawings]

FIG. 1 is a process drawing showing one embodiment of a purification method according to the present invention.

Claims (1)

[Claims] 1. A method for purifying citric acid, which comprises passing through a disproportionation step of hydrogen citrate using an organic solvent.