JPH03328B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to the purification of wet phosphoric acid by an organic solvent extraction method, and more particularly to a method for suppressing the formation of compounds that accumulate in the solvent phase.

A method is known in which phosphoric acid obtained by decomposing phosphate rock with sulfuric acid, that is, wet phosphoric acid, is purified by extraction with organic solvents such as various alcohols, ketones, ethers, and phosphate esters. The above-mentioned wet phosphoric acid contains various metal impurities derived from the raw material phosphate rock, as well as anionic impurities such as sulfate radicals derived from sulfuric acid. Although the above extraction method relatively removes metal impurities, anionic impurities are not removed and remain in the purified phosphoric acid obtained.

Regarding the removal of various anionic impurities, sulfate radicals are particularly difficult to remove, and when producing phosphoric acid solutions for industrial, pharmaceutical, and food use, sulfate radicals must be virtually completely removed. . For example, according to the Japanese Industrial Standards (JISK1449-1975), sulfate (SO ₄ ) must be removed to 0.003% or less.

As a method for removing this sulfate group, there is a method of adding a barium compound to the purified phosphoric acid obtained to separate out the sparingly soluble barium sulfate (Japanese Patent Publication No. 1983-
16089).

However, the barium sulfate crystals obtained in phosphoric acid are fine and separated from the viscous phosphoric acid solution, making the operation complicated.

As an improved method, there is a method in which the sulfate radicals are substantially completely removed by an extraction method (Japanese Patent Application Laid-open No. 100208-1983).
In this method, phosphoric acid is extracted from wet phosphoric acid using an organic solvent, and then the solvent phase is washed with an alkali metal compound or an ammonia compound to reduce the concentration of sulfate groups in the product by SO ₄ /P _{2 O5} weight ratio
It can be less than 50ppm.

However, as a result of extensive research into extraction methods by the present inventors, it has been found that, depending on the conditions of conventional extraction methods, compounds that cannot be removed by the above methods often form and accumulate in the solvent phase. In other words, even if a barium compound is added to phosphoric acid and the sulfate radicals have been virtually completely removed, the sulfate radicals will increase if the product is stored for a long time or heat treated again, and the solvent from which phosphoric acid was extracted may It has been found that even if the phase is washed with an alkali metal compound, sulfate radicals accumulate in the solvent that has been used for a long period of time. Sulfate groups exhibiting such properties are hereinafter referred to as "accumulative sulfate compounds."

Although the cause of the formation of this accumulative sulfuric acid compound is not clear, the present inventors speculate that it is a complex combination of the sulfuric acid added for the purpose of improving yield, the solvent, and the organic matter in the wet phosphoric acid contained in the solvent. As a result of repeated experiments over a long period of time, the following findings were obtained.

(1) Accumulative sulfuric acid compounds accumulate in the solvent phase over a long period of time and contaminate the resulting purified phosphoric acid, so the amount produced is extremely small.

(2) Accumulative sulfuric compounds are produced only when alcohols or solvents containing alcohols are used as solvents.

(3) The higher the concentration of sulfuric acid in the solvent phase and the higher the temperature of the solvent phase, the more accumulative sulfuric compounds are produced and the amount of accumulated sulfuric acid compounds increases.

(4) When organic substances in wet phosphoric acid accumulate in the solvent phase and the coloring of the solvent phase increases, accumulative sulfuric acid compounds tend to accumulate in the solvent phase.

(5) Accumulative sulfate compounds have very high boiling points and can be removed by distillation of the solvent phase, as can organic substances that accumulate in the solvent phase.

Based on the above knowledge, the present inventors conducted further studies and arrived at the present invention.

The present invention aims to suppress the formation of the above-mentioned accumulative sulfuric acid compounds, and includes an extraction step of extracting wet phosphoric acid with a solvent;
A washing step in which the solvent phase obtained in the extraction step is brought into contact with purified phosphoric acid to remove impurities in the solvent phase, and the phosphoric acid content of the solvent phase obtained in the washing step is extracted with water to obtain purified phosphoric acid. When purifying wet phosphoric acid using an alcohol or a solvent containing alcohol in the presence of sulfuric acid by a countercurrent multistage extraction method with a back extraction process as the main process, the sulfuric acid concentration in the solvent phase at any stage of the extraction process. The temperature of the solvent phase, which has a sulfuric acid concentration of 20 to 50 g/less than 50 g, is reduced to 40° C. by extracting the aqueous solution phase from the intermediate stage, adding sulfuric acid to it, cooling it to 40° C. or less, and returning it to the extraction process.
The gist is to maintain the temperature below °C and to distill and reuse a portion of the circulating solvent phase.

As used herein, "sulfuric acid concentration" means the concentration of sulfate radicals.

The present invention will be explained in detail below.

When extracting phosphoric acid by bringing wet phosphoric acid into contact with an organic solvent, the yield is limited to 70 to 80% because the impurities present in the wet phosphoric acid are combined with the phosphoric acid. In order to obtain a higher yield, e.g. 90-98%, it is necessary to add sulfuric acid to form free phosphoric acid and extract it into the solvent phase. You need to choose the largest one. That is, as a solvent, n-
An essential condition is that the solvent be an alcohol such as butanol or isobutanol, or a mixed solvent of these alcohols and an organic solvent such as a ketone or ether. In addition, in order to efficiently extract phosphoric acid,
The countercurrent multi-stage method is industrially most advantageous, and in order to minimize the amount of sulfuric acid mentioned above, it is essential to form a state with a high sulfuric acid concentration mainly at the stage where sulfuric acid is added. At that time, the sulfuric acid concentration in the solvent phase varies depending on the composition of wet phosphoric acid and operating conditions such as the number of stages and flow rate ratio, but the concentration of sulfuric acid in the stage to which sulfuric acid is added is 20 to 60 g/
become. The sulfuric acid added is usually 96-98% concentrated sulfuric acid, and the temperature at the point where it is added rises to 45-60°C due to the heat of dilution and reaction. When the extraction method is carried out under such operating conditions, accumulative sulfuric acid compounds accumulate in the solvent phase over a long period of time, and are eventually detected in large amounts in the purified phosphoric acid obtained.

In the present invention, in order to achieve a high yield of 90% or more and to suppress the formation of accumulative sulfuric compounds, it is important to improve the sulfuric acid addition method, and in particular, to improve the sulfuric acid concentration and temperature in the solvent phase. Control is an essential condition. Since most of the generation of accumulative sulfuric compounds occurs in the solvent phase, the concentration of sulfuric acid in the solvent phase should be set at 50 g/min.
If it exceeds 50g/
Either the addition of sulfuric acid must be divided into several locations, or the amount of sulfuric acid added must be reduced at some cost to the yield.
Furthermore, the higher the sulfuric acid concentration in the solvent phase and the higher the temperature, the more rapidly the amount of accumulative sulfuric compounds produced increases, so it is necessary to cool the portions of the solvent phase where the sulfuric acid concentration is high. The amount of accumulative sulfuric acid compounds produced is extremely affected by temperature; when the sulfuric acid concentration in the solvent phase is the same, the amount produced at 20°C is
1.7 times at 30℃, 3.0 times at 40℃, 5.7 times at 50℃,
At 60℃, it increases 7.8 times. In the present invention, the aqueous phase is extracted from the intermediate stage, sulfuric acid is added to it, and the temperature is increased to 40°C.
Accumulative acid compounds are removed by cooling the solvent phase preferably to below 30°C and returning it to the extraction step, by maintaining the temperature of the solvent phase with a sulfuric acid concentration of 20 to 50 g/ below 40°C, preferably below 30°C. The generation of can be suppressed.

As for the method of cooling the solvent phase, industrially the simplest method is to add sulfuric acid to the aqueous solution phase, cool it to 40° C. or lower using a heat exchanger, and then bring it into contact with the solvent phase. However, if scaling of gypsum or the like occurs in a heat exchanger or the like due to the composition of wet phosphoric acid, sulfuric acid can be diluted in advance to 50% or less and supplied after cooling. Also, sulfuric acid concentration 20~
It is better to keep the residence time of the 50 g/solvent phase as short as possible, but in extraction operations, the residence time of the solvent phase is often determined by the separation time between the solvent phase and the aqueous solution phase. In the present invention, the residence time of the solvent phase with a sulfuric acid concentration of 20 to 50 g is within 1 hour, preferably 30 to 50 g.
The residence time of 50g/solvent phase is preferably within 30 minutes.

Even under the above conditions, if the process is carried out for a long period of time, a small amount of accumulative sulfuric acid compounds will accumulate in the solvent phase, so it is necessary to remove a portion of the circulating solvent by distillation. Distilling a portion of the solvent simultaneously removes colored organic substances derived from crude phosphoric acid, which also has the effect of suppressing the formation of accumulative sulfuric acid compounds. The distilled solvent is used again for extraction of phosphoric acid. The amount of solvent to be distilled is between 0.2 and 5% of the amount of solvent circulating in the flow rate.

Next, an example of the method of the present invention will be explained in detail based on FIG.

In Figure 3, P ₂ O ₅ 35-54% wet phosphoric acid 1
and the cleaning waste liquid 3 discharged from the extractor D,
Feed extractor A ₁ . In order to extract the phosphoric acid content in this liquid mixture, the solvent 2 is supplied from the other side and brought into countercurrent contact to obtain an extraction phase 6 containing phosphoric acid. At this time, in order to improve the yield, the aqueous phase is extracted from the end stage of extractor A ₁ , mixed with 98% concentrated sulfuric acid 4 in sulfuric acid addition tank B, cooled to below 40°C in heat exchanger C, and then again Return to extractor A ₂ . Impurities in the wet phosphoric acid are discharged from the raffinate 5.

Since a small amount of impurities in the crude phosphoric acid are co-extracted in the extraction phase 6 obtained in the extractor _A1 , it is brought into countercurrent contact with the reflux 7 (part of the purified phosphoric acid 11) in the extractor D. At this time, caustic soda 8 is added to the intermediate stage of the extractor D in order to effectively remove impurities such as sulfate radicals in the crude phosphoric acid.

The purified extraction phase 9 obtained in the extractor D is transferred to the extractor E.
, and brought into countercurrent contact with pure water 10 to obtain purified phosphoric acid 11. A part of the purified phosphoric acid 11 is used for cleaning.

The solvent discharged in extractor E includes extractors A ₁ ,
Since a small amount of the accumulative sulfuric acid compound produced in A ₂ is present, a portion of the organic solvent (contaminated solvent 12) is supplied to distillation column F to obtain regenerated solvent 13 for reuse.
Further, the distillation waste liquid 14 from the distillation column is discarded.

The present invention will be further explained below with reference to Examples, but the present invention is not limited to these Examples.

Percentages relating to concentration are by weight.

In addition, as shown in Figure 4, "accumulative sulfuric acid compounds" are completely decomposed when concentrated to a P ₂ O ₅ concentration of 72.4%.
Since it can be detected as a sulfate radical, in the Examples and Comparative Examples, the P ₂ O ₅ concentration was concentrated to 72.4% (pressure: -660
mmHg...1) in the figure, -400mmHg...2) in the figure) Increase in sulfate root concentration was investigated.

Example 1 In Fig. 3, 100 parts by weight of wet phosphoric acid (specific gravity 1550) with the composition shown below and cleaning waste liquid (specific gravity 1360) 81
After mixing the weight parts and removing the generated precipitate, it was supplied to an 8-stage extractor _A1 .

P ₂ O ₅ 45.1% SO ₄ 3.34 Fe 0.76 Al 0.23 Mg 0.57 On the other hand, 425 parts by weight of n-butanol (specific gravity 0.840) was supplied to the two-stage extractor A ₂ , and phosphoric acid was extracted from wet phosphoric acid and washing waste liquid. I did this. At that time, 12.0 parts by weight of 98% (H ₂ SO ₄ ) concentrated sulfuric acid (specific gravity 1.820) was added to the aqueous phase discharged from extraction stage A ₁ in sulfuric acid addition tank B, and the mixture was cooled to 30°C using heat exchanger C. After that, the extractor A ₂ was supplied again, and 110 parts by weight of the raffinate solution (specific gravity 1.14) flowed out from the extractor A ₂ , and its sulfuric acid concentration was 11.8%. Extractor A ₁ and
The sulfuric acid concentration and temperature of the solvent phase in A ₂ are shown in Figure 1A.
And the results shown in FIG. 1B were obtained.

The phosphoric acid-containing extraction phase obtained in extractor A ₁₈
It was brought into countercurrent contact with 146 parts by weight (part of purified phosphoric acid) of reflux (specific gravity 1.205) in stage extractor D. At that time, 6.7 parts by weight of 48% caustic soda (specific gravity 1.500) was added to the 14th stage from the solvent phase supply stage to improve the removal effect of sulfate radicals, etc.

The purified extraction phase obtained in extractor D was supplied to a 12-stage extractor E, and contacted with 147 parts by weight of pure water from the other side to obtain 180 parts by weight of purified phosphoric acid (specific gravity 1.205).
Additionally, 10 parts by weight of the circulating contaminated solvent was supplied to the distillation column F, and the regenerated solvent (specific gravity 0.840) was again supplied to the extractor _A1 . As a result, the yield was 97.4%, and the P ₂ O ₅ concentration of the purified phosphoric acid obtained was 24.4%.

As a result of concentrating this purified phosphoric acid to a P ₂ O ₅ concentration of 58.0%, concentrated phosphoric acid having the following composition was obtained.

SO ₂ 20ppm Fe 1ppm or less Al 1ppm or less Mg 1ppm or less As a result of further concentrating this concentrated phosphoric acid to a P ₂ O ₅ concentration of 72.4%, the SO ₄ concentration is 30 ppm, and SO ₄ is only 4 ppm (P ₂ O ₅ concentration 58.0 %). Further, the residence time of the solvent phase with a sulfuric acid concentration of 20 to 50 g/in extractors _A1 and _A2 was 30 minutes.

Example 2 Under the same conditions as Example 1, the amount of concentrated sulfuric acid added was 11.2
As a result of changing the addition amount of caustic soda to 5.7 parts by weight, purified phosphoric acid with a yield of 98.4% and a P ₂ O ₅ concentration of 24.6% was obtained. As a result of concentrating this purified phosphoric acid to a P ₂ O ₅ concentration of 58.0%, concentrated phosphoric acid having the following composition was obtained.

SO ₄ 180ppm Fe 1ppm or less Al 1ppm or less Mg 1ppm or less Add barium carbonate to this concentrated phosphoric acid, remove the barium sulfate produced, and reduce the SO ₄ concentration to 20ppm.
Even if the P ₂ O ₅ concentration is further concentrated to 72.4%, SO ₄
The concentration was 30 ppm.

Comparative Example 1 Direct extraction of concentrated sulfuric acid under the same conditions as Example 1
As a result of adding A ₂ to A 2 , the temperature of the solvent phase becomes as shown in Figure 1 B, and the purified phosphoric acid obtained has a P ₂ O ₅ concentration of 58.0%.
Condensed phosphoric acid with the following composition was obtained.

SO ₄ 30ppm Fe 1ppm or less Al 1ppm or less Mg 1ppm or less As a result of further concentrating this concentrated acid to a P ₂ O ₅ concentration of 72.4%, the SO ₄ concentration was 75ppm, and SO ₄ was 30ppm.
(based on P ₂ O ₅ concentration of 58.0%).

Comparative Example 2 As a result of changing the amount of concentrated sulfuric acid added to 17.2 parts by weight under the same conditions as in Example 1, the sulfuric acid concentration in the solvent phase at that time obtained the results shown in FIG. 1A. Moreover, the yield was 99.1%, and as a result of concentrating the obtained purified phosphoric acid to a P ₂ O ₅ concentration of 58.0%, concentrated phosphoric acid having the following composition was obtained.

SO ₄ 1630ppm Fe 1ppm or less Al 1ppm or less Mg 1ppm or less Add barium carbonate to this concentrated phosphoric acid, remove the barium sulfate produced, and reduce the SO ₄ concentration to 20ppm.
As a result of further concentrating to a P ₂ O ₅ concentration of 72.4%,
SO ₄ concentration is 45 ppm, SO ₄ is 16 ppm (P ₂ O ₅ concentration
This was an increase of 58.0% (standard).

Comparative Example 3 As a result of stopping the supply of contaminated solvent to the distillation column under the same conditions as in Example 1, the SO ₄ concentration of purified phosphoric acid concentrated to 72.4% P ₂ O ₅ was 70 ppm, and SO ₄ was 20 ppm.
(based on P ₂ O ₅ concentration of 58.0%).

Example 3 The solvent was changed to n-butanol under the same conditions as Example 1.
As a result of changing the amount of caustic soda from 425 parts by weight to 540 parts by weight of isobutanol and 5.4 parts by weight of caustic soda, the yield was 97.0%, and the resulting purified phosphoric acid was concentrated to a P ₂ O ₅ concentration of 58.0%. , concentrated phosphoric acid with the following composition was obtained.

SO ₄ 20ppm Fe 1ppm or less Al 1ppm or less Mg 1ppm or less As a result of further concentrating this concentrated phosphoric acid to a P ₂ O ₅ concentration of 72.4%, the SO ₄ concentration is 27 ppm, and the SO ₄ is only 2 ppm (P ₂ O ₅ concentration 58.0 %).

Example 4 Wet phosphoric acid with the composition shown below (specific gravity 1.680)
100 parts by weight and 100 parts by weight of the washing waste liquid (specific gravity 1.465) were mixed and supplied to the 7-stage extractor _A1 .

P ₂ O ₅ 52.0% SO ₄ 3.5% Fe 0.46% Al 0.09% Mg 1.00% On the other hand, mixed solvent (methyl isobutyl ketone/n
-Butanol = 4 (volume basis) (specific gravity 0.810)) 355
The weight part was supplied to a three-stage extractor _A2 , and phosphoric acid was extracted from the wet phosphoric acid and the washing waste liquid. At that time, 14.2 parts by weight of 98% concentrated sulfuric acid was added to the aqueous phase discharged from extraction stage A ₁ in sulfuric acid addition tank B, and after cooling to 30°C by heat exchanger C, it was again supplied to extractor A ₂ . The raffinate from extractor A ₂ (specific gravity 1.31)
65 parts by weight of sulfuric acid flowed out, and the sulfuric acid concentration was 28%. The sulfuric acid concentration and temperature of the solvent phase in extractors _A1 and _A2 at that time gave the results shown in FIGS. 2A and 2B.

The phosphoric acid-containing extraction phase obtained in extractor A ₁₅
It was brought into countercurrent contact with 105 parts by weight (part of purified phosphoric acid) of reflux (specific gravity 1.277) in stage extractor D. At that time, 8.7 parts by weight of 45% caustic soda was added to the 10th stage from the solvent phase supply stage to improve the removal effect of sulfate radicals, etc.

The purified extraction phase obtained in extractor D was supplied to a five-stage extractor, and brought into contact with 70 parts by weight of pure water from the other side.
182 parts by weight of purified phosphoric acid (specific gravity 1.277) was obtained.
Additionally, 10 parts by weight of the circulating contaminated solvent was supplied to the distillation column F, and the regenerated solvent was again supplied to the extractor _A1 . As a result, the yield was 98.5%, and the P ₂ O ₅ concentration of the purified phosphoric acid obtained was 37.0%.

As a result of concentrating this purified phosphoric acid to a P ₂ O ₅ concentration of 58%, concentrated phosphoric acid having the following composition was obtained.

SO ₄ 17ppm Fe 1ppm or less Al 1ppm or less Mg 1ppm or less As a result of further concentrating this concentrated phosphoric acid to a P ₂ O ₅ concentration of 72.4%, the SO ₄ concentration is 32 ppm, and SO ₄ is only 6 ppm (P ₂ O ₅ concentration 58.0 %). Also, the sulfuric acid concentration in extractors A ₁ and A ₂ is 20
The residence time of the ~50 g/solvent phase was 45 minutes.

Comparative Example 4 As a result of directly adding concentrated sulfuric acid to A ₂ under the same conditions as in Example 4, the temperature of the solvent phase became as shown in Figure 2 B, and the purified phosphoric acid was added to the P ₂ O ₅ concentration of 58.0%. Condensed phosphoric acid with the following composition was obtained.

SO ₄ 32ppm Fe 1ppm or less Al 1ppm or less Mg 1ppm or less As a result of concentrating this concentrated phosphoric acid to a P ₂ O ₅ concentration of 72.4%, the SO ₄ concentration was 102 ppm, and SO ₄ was 50 ppm.
( _P2O5 concentration _58.0 % standard) increased.

Reference Example In Comparative Example 4, when the solvent was changed to only methyl isobutyl ketone, the yield was 57.0%, but even if purified phosphoric acid was concentrated to a P ₂ O ₅ concentration of 72.4%,
There was no increase in _SO4 .

[Brief explanation of the drawing]

FIG. 1A, B and FIG. 2 A, B show the sulfuric acid concentration and temperature of the solvent phase of Examples and Comparative Examples. FIG. 3 shows an embodiment of the method of the present invention. FIG. 4 is a diagram showing the relationship between P ₂ O ₅ concentration and SO ₄ concentration.

Claims (1)

[Claims] 1. An extraction step in which wet phosphoric acid is extracted with a solvent, a washing step in which the solvent phase obtained in the extraction step is brought into contact with purified phosphoric acid to remove impurities in the solvent phase, and a washing step in which impurities in the solvent phase are removed. Using an alcohol or a solvent containing alcohol in the presence of sulfuric acid, the phosphoric acid content of the obtained solvent phase is extracted with water to obtain purified phosphoric acid. When refining wet phosphoric acid, the sulfuric acid concentration in the solvent phase at any stage of the extraction process is 50 g/or less,
The aqueous solution phase is extracted from the intermediate stage, sulfuric acid is added to it, cooled to 40°C or less, and returned to the extraction process, thereby maintaining the temperature of the solvent phase with a sulfuric acid concentration of 20 to 50 g/40°C or less and circulating it. A method for purifying phosphoric acid characterized by distilling and reusing a portion of the solvent phase. 2. The method for purifying phosphoric acid according to claim 1, wherein the alcohol is n-butanol or isobutanol. 3. The method for purifying phosphoric acid according to claim 1, wherein the alcohol or the solvent containing alcohol is n-butanol or a mixed solvent of isobutanol and methyl isobutyl ketone. 4. The method for purifying phosphoric acid according to claim 1, 2 or 3, wherein the residence time of the solvent phase at a location where the sulfuric acid concentration is 20 to 50 g/min is 1 hour or less. 5. The method for purifying phosphoric acid according to claim 4, wherein the residence time of the solvent phase at a location where the sulfuric acid concentration is 30 to 50 g/min is 30 minutes or less. 6 Of the circulating solvent phase, the flow rate is 0.2 to 5.
A method for purifying phosphoric acid according to any one of claims 1 to 5, wherein % of phosphoric acid is distilled.