DE1904316A1 - Process for the production of highly concentrated phosphoric acid - Google Patents

Description

PATENTANWÄLTE vJUHÖJb DIPL.-ING. H. LEINWEBER dipl-ing. H. ZIMMERMANN

P. 1003 - ? 8 MUnchen2, Rotental7, 2nd Aut ₀ .

τβΐ.-addr. Ulnpat Manch *

τβΐ (οη

^to 29, Jan. 1969

Under sign Lw / XIIl / C

NIPPON KOKAN KABUSHIKI KAISHA, Tokyo, Japan Process for the production of highly concentrated phosphoric acid

The invention relates to a method of making highly concentrated phosphoric acid in the wet process, where phosphate rock is decomposed with sulfuric acid or a mixture of sulfuric acid and phosphoric acid and, with a high yield, highly concentrated phosphoric acid with a content of 40 to 50? ί PpOc as well as high purity calcium sulfate is obtained.

- The known wet process for the production of phosphoric acid consist in the dihydrate calcium sulfate process, the Monohydrate calcium sulfate process, the hemihydrate calcium sulfate process and the non-hydrated calcium sulfate process, corresponding to the reaction between the Phosphate rock and the sulfuric acid formed calcium sulfate. According to the dihydrate calcium sulfate process, the Phosphate rock decomposes under such reaction conditions that a slurry of calcium sulfate dihydrate and Forms phosphoric acid, from which the phosphoric acid is filtered off

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will. If you are highly focused that way Want to produce phosphoric acid, concentration levels are required. This procedure is that oldest and is largely carried out, but it is not yet satisfactory in terms of the yield of phosphoric acid and the quality of that formed as a by-product Calcium sulfate.

The Monshydrat-Caleiumsulfat method consists in the improvement of dihydrate calcium sulfate FSR W driving with respect to the yield of phosphoric acid and the quality of the calcite oil formed. According to this process, the phosphate gas is decomposed under such conditions that the calcium sulfate is formed in the form of hemihydrate; the resulting slurry of calcium sulfate monohydrate and phosphoric acid is then cooled to a temperature at which the calcium sulfate dihydrate is stable and the calcium sulfate monohydrate is converted into the bihydrate. The phosphoric acid is then filtered off from the slurry. ^{Similar to the calcium sulfate dihydrate process, this process also produces} only one phosphoric acid with a content of less than about 30 * P2 ° 5, so that the phosphoric acid must also be further concentrated to achieve higher concentrations.

To achieve a higher concentration of phosphoric acid ! the calcium sulfate hemihydrate method was developed, at which the phosphate rock under such conditions

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is decomposed to form a slurry of calcium sulfate hemihydrate and phosphoric acid, from which then the phosphoric acid is filtered off. According to this method, a phosphoric acid with a larger Concentration (40 to 50jO than can be achieved with the two methods described above; However, the calcium sulfate hemihydrate obtained in this process contains a larger amount of phosphoric acid, so that this process is not suitable for use on an industrial scale.

In the non-hydrated calcium sulfate process the phosphate rock is decomposed under such reaction conditions that a slurry of anhydrous calcium sulfate and phosphoric acid is formed, from which then the phosphoric acid is filtered off.

Although an even greater concentration of phosphoric acid can be achieved with this process than with the calcium sulfate hemihydrate process, there is the factual part, that the calcium sulfate is obtained in anhydrous form and has no practical value in this form.

Regarding the hydration of calcium sulfate hemihydrate in the calcium sulfate dihydrate, the form must also be of _a I pfkristalle considered. As such, powdered natural calcium sulfate or calcium sulfate formed as a by-product was used. However, the rate of crystallization when using such seed crystals is slow, so that a longer period of time is required for complete hydration.

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Furthermore, it was impossible to obtain coarsely crystallized calcium sulfate.

The invention provides a new process for producing highly concentrated phosphoric acid in large quantities Yield without the need for a concentration step.

The invention is based on a wet process for producing highly concentrated phosphoric acid which also calcium sulfate τοη excellent quality is obtained.

Another advantage of the invention is based on the production of highly concentrated phosphoric acid at the same time greatly reduced response time.

The invention is characterized by the use of separately prepared calcium sulfate seed crystals. The seed crystals are produced by the decomposition of a calcium oxide compound with sulfuric acid or a Mixture τοη sulfuric acid and phosphoric acid among those Conditions that stable CtleiUHSulfat-Dihrdrat in the form τοη twin crystal parts arises.

According to the invention, for example, calcined tea and pulTtriaitrtte Phoephitgeetein with sulfuric acid or a mixture τοη sulfuric acid and phosphoric acid decomposes, forming a slurry of phosphoric acid and calcium sulfate hemihydrate. From the resulting Slurry τοη phosphoric acid and calcium sulfate hemihydrate, the phosphoric acid is separated off by filtration. The remaining calcium sulfate hemihydrate is treated with sulfuric acid, a slurry τοη completely hydrated c

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Calcium sulfate dihydrate and dilute phosphoric acid and a relatively small amount of the seed crystals described above are added, so that the calcium sulfate hemihydrate is hydrated to calcium sulfate dihydrate and a mixture of calcium sulfate dihydrate and phosphoric acid is obtained.

The invention will now be explained further on the basis of the following description and the drawing.

In the process scheme shown in the drawing, pulverized phosphate rock is made from a Source 1, phosphoric acid and recycled slurry are placed in a premixer 3 and thoroughly mixed with each other, whereby the majority of the phosphate rock dissolves in the form of monoealcium phosphete. the The resulting slurry is then transferred to a decomposition tank

4 brought. Then sulfuric acid is made from a source 2 introduced into the tanks 4, forming a slurry of calcium sulfate hemihydrate and phosphoric acid .. The Slurry is then put into a first filter device

5 introduced, in which the phosphoric acid is separated from the calcium sulfate hemihydrate; the severed Phosphoric acid is transferred to a closed tank 6 and the calcium sulfate hemihydrate to a hydration tank 8. The phosphoric acid in the closed tank Tank 6 is discharged as product and part of it is transferred to the premixing device 3 as recycled phosphoric acid. Dt · in dtm Kvdntieieruugttink 8 gtM »- »Eltt CtlciuMSulfat-Htlbhydrtt contains tint large · Mtng« Phosphoric acid. In dtn Hydrttieitrunfittnk 8 vtrttn tint

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Phosphoric acid slurry, consisting of dilute phosphoric acid and calcium sulfate dihydrate, introduced; a small amount of sulfuric acid is taken from a source 2 and from a source to complete hydration Source 7 a small amount of a slurry of added fine twin crystals of calcium sulfate dihydrate obtained by decomposing a calcium oxide compound with sulfuric acid or a mixture of sulfuric acid and phosphoric acid; the hydration is carried out under conditions in which the calcium sulfate dihydrate is stable. The finely divided Calcium sulfate dihydrate acts as seed crystals. The calcium sulfate hemihydrate is placed in the hydration tank converted into calcium sulfate dihydrate and the undecomposed elements still present in the calcium sulfate hemihydrate from the previous decomposition stage of the phosphate rock Components containing phosphoric acid are completely replaced. After hydration is complete, the Slurry transferred to a second filter device 9 and subjected to double countercurrent washing; this produces phosphoric acid in great yield and calcium sulfate dihydrate in the form of large crystals Bit of a low content of phosphoric acid is obtained. The obtained from this filtration process and stored in tank 1Qa the first filtrate collected is transferred to the premixing device 3; the second collected in tank 10b The filtrate is transferred to the hydration tank 8.

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Essential factors for the process according to the invention are the regulation of the temperature and concentration at which the phosphate rock is decomposed by the mixture of sulfuric acid and phosphoric acid. The concentration of phosphoric acid should be less than 50 * PpOc in order to prevent the formation of anhydrous calcium sulfate; preferably the FpOc concentration should be less than 45 * in order that the calcium sulfate hemihydrate slurry can be easily filtered off. At higher concentrations of phosphoric acid, the viscosity of the slurry increases, which means that the filtration speed decreases concomitantly.

It has been found that the best results can be achieved with a sulfuric acid concentration between 0.5 and 3.0 *. In this sulfuric acid concentration range, the percentage extraction of phosphoric acid is 96 to 97 *; outside these concentration ranges, the amount extracted is lower. Regarding the filtration speed of the calcium sulfate hemihydrate slurry, it should be noted that the calcium sulfate hemihydrate crystals formed are small at sulfuric acid concentrations below 0.5, so that the filtration speed is low. On the other hand, if the sulfuric acid concentration is over 0.3 * », the crystal size of the calcium sulfate hemihydrate crystals is 50 to 80 microns, so that the slurry can be filtered at a high temperature. In this way the Filtriergeechwindigkeit higher al · in the known calcium sulfate dihydrate process or monohydrate process. From this

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BATH ORIGINAL

it was found that the most appropriate concentration range for sulfuric acid is between 0.5 and 3.0. the The reaction temperature is selected depending on the acid concentration, so that it is necessary to adjust the rate to be kept in a zone suitable for the formation of calcium sulphate hemihydrate. Since, however, according to the invention High concentration phosphoric acid should be produced and not a cooling device for the slurry is used, the reaction temperature is usually kept near the boiling point.

During hydration, it is important to control the concentration and temperature of the acids, as well as the introduction of the seed crystals. The concentration of phosphoric acid varies depending on the amount of phosphoric acid remaining in the calcium sulfate hemihydrate after filtration; but if you have a P _? 0, - if you want to achieve a concentration of 40 to 50 $, the PpO ^ content is 20 to 30 # if one takes into account the amount of water present. However, the acid concentration must be controlled so that the total concentration of phosphoric acid and sulfuric acid is less than $ 35. This upper limit of the acid concentration arises from the fact that the hydration time of the calcium sulfate hemihydrate increases rapidly if this limit is exceeded. The sulfuric acid concentration affects the hydration and the recovery percentage of remaining in the calcium sulfate hemihydrate dtn phosphoric acid. The results of the investigations carried out in this connection are listed in the following tables:

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Table I: Dependence of the hydration time on the Sulfuric acid concentration

time required ^{for complete hydration}

0.9 23.9 4th Hours. 10 Min. * λ1 » seed crystals
give Trains- 2.4 23.9 1 Hours. 40. Min. M. 4.2 23.5 1 Hours. 20th Min. Il 5.8 23.7 1 Hours. 15th Min. dd 1.6 23.0 45 Min. Y * $ 1 seed crystals
give Trains-

3.2 23.5 30 min. (The amount of

\ run guided hydration

/ Q οχ χ or M-5 «sized slurry ⁴ ' ^{8 23} ' ³ 25 min.; _{concerns ] M beZ0} g _e ^

_f g

on the semi-hydrated slurry.

* The seed crystals used consisted of fine

Dihydrate twin crystals produced by decomposing a J.

Calcium oxide compound with sulfuric acid or a mixture I

'from sulfuric acid and phosphoric acid under such conditions'

was obtained in which calcium sulfate dihydrate is stable. j

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Table II: Dependence of the percentage recovery of

remaining phosphoric acid from the sulfur ■ Acid concentration

H ₂ SO ₄ , Jf P ₂ O ₅ , * 0.9 23.9 2.4 23.9 4.2 23.5 4.8 23.3 5.8 23.7 8.3 23.3

Percentage recovery of the remaining phosphoric acid, ί>

From the preceding tables it can be seen that the sulfuric acid concentration must be above 2 # in order to shorten the time required for hydration to complete. On the other hand, the sulfuric acid concentration should be about W> in order to enable a percentage recovery of over 70 $ of the remaining phosphoric acid. The most appropriate sulfuric acid concentration is therefore, taking the above facts into account, between 4 and 10 °. Although the temperature varies depending on the acid concentration, it has been found that satisfactory crystal formation occurs when the temperature is between 60 and 70 ^° C. Instead of circulating the calcium sulfate dihydrate as in the known methods, fine twin crystals obtained by decomposing a calcium oxide compound with sulfuric acid or a mixture of sulfuric acid and phosphoric acid are used as seed crystals under conditions in which

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the calcium sulfate dihydrate is stable. The use of these separately prepared seed crystals leads to a reduction in hydration time and stable formation of large calcium sulfate dihydrate crystals. In particular, if the calcium sulfate dihydrate formed in the main process operation in the circuit as in the _; known processes, the calcium sulfate crystals gradually deteriorate into monocrystals, so that no formation of large crystals takes place and thus the hydration time is extended. On the other hand, if separately prepared seed crystals are gradually added to the calcium sulfate dihydrate prepared in the main process, deterioration of the seed crystals can be effectively prevented even if the calcium salfate dihydrate is repeatedly recycled because the added seed crystals function as active crystal nuclei. Although the amount of the seed crystals to be added varies depending on the hydration conditions, an amount ranges from 0.05 to 1 # of seed ^; crystals, based on the calcium sulfate produced in the main process.

That by decomposing calcined phosphate rock; calcium sulfate hemihydrate obtained from Florida, USA was j hydrated in aqueous phosphoric acid solution and the j thus obtained slurry was used repeatedly to make j so experience with repeated use of calcium! to collect sulfate dihydrate. The influence of the inflicted Seed on the hydration time was considerable, as can be seen from the following Table III:

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Table III: Influence of the addition of seed crystals on hydration time

^ Frequency of repeated use of the slurry of the formed calcium sulfate dihydrate;

! Amount of the added inoculum crystals

0.3 *

1 H. 40min. 1 ^m 30 min. 1 ^st

2 hours. 30 min.

1 H. 20 min.

.1 H.

2 hours 50 minutes 1 hour 30 minutes 1 hour 20 minutes

Hydration conditions concentration

P ₂ O ₅ H ₂ SO ₄

temperature

65

) Amount of recycle slurry

corresponds to the amount of calcium sulfate hemihydrate slurry

j As can be seen from Table III readily effected ^1, the addition of seed crystals, a great reduction in the

Hydration time. If these crystals are not added ; den, the hydration time increases during the process with repeated use of the calcium sulfate dihydrate formed ! . . . . - 13 -

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to; however, this can be achieved by adding seed crystals Increase in the crystallization time can be avoided.

The invention will now be explained further with the aid of the following examples:

504 g of water and 163 g of 75% sulfuric acid were added

to 353 g of phosphoric acid (ΡηΟ, -29.5 *, H ₀ SO ₄ 5.2 #) was added.

The mixture was stirred thoroughly and warmed to 50.degree. While stirring this mixture of phosphoric acid and sulfuric acid was gradually added calcined and powdered phosphate rock (PpOp; 36.856) from Florida, wherein the reaction temperature is kept between 40 and 70 C and so the reaction was carried out. After adding the phosphate rock, stirring was continued for 30 minutes, during which a slurry of the following composition was obtained:

Concentration of acids {^ 2 ^ 5 14,13 * j

IH ₂ SO ₄ 5.6 * j

'I.

Calcium sulfate 15.8 * j

The calcium sulfate formed consisted of calcium sulfate dihydrate in the form of twin crystals with a crystal length of 20 to 30 microns.

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\ $> Calcium sulfate in the slurry was added as seed crystals to the calcium sulfate hemihydrate obtained by decomposing powdered Florida phosphate rock with sulfuric acid and phosphoric acid. The hydration of the calcium sulfate hemihydrate at a temperature of 65 ⁰ C in a mixed acid of 23 # PpOc and 5.8 # HpSO. was completed within one hour and 15 minutes, giving twin crystals with substantially uniform dimensions of 30 to 40 microns 150 to 250 microns.

100 grams of calcined and powdered Florida phosphate rock was added to 848 grams of water while the mixture was thoroughly stirred into a suspension. With continued stirring and heating to 7O ⁰ C were 40.bis sulfuric acid was added and reacted 180 g of 75 #. After the sulfuric acid had been introduced, stirring was continued for a further 30 minutes, whereby a slurry of the following composition was obtained:

Concentration of acids Calcium sulphate content

P ₂ O ₅ ■ 3, 7 * H ₂ SO ₄ 5, 3 * 14, 6 *

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The calcium sulfate obtained lay as a dihydrate in the form of twin crystals with a length of up to 30 microns.

1 # of the calcium sulfate in this slurry was used as seed crystals for calcium sulfate hemihydrate, which was obtained by decomposing calcined and powdered phosphate rock from Florida with a mixture of sulfuric acid and phosphoric acid. This calcium sulfate hemihydrate was hydrated at a temperature of 6O ⁰ C in a mixture of 23.0 # P ₂ O ₅ and 6.2 # H ₂ SO _4. Hydration was complete after one hour and twin crystals with essentially uniform dimensions of 30 to 40 microns by 150 to 250 microns were obtained.

Example 3

1 kg of phosphoric acid produced by the process according to the invention (P ₂ O ₅ 40.9 *, H ₃ SO ₄ 2.096), 1.2 kg Recovered phosphoric acid (P ₃ O ₅ $ 20.8, H ₃ SO ₄ 5.2 #) from the hydration step and 4 kg of a slurry in which the decomposition of the phosphate rock had been completed became 1 kg of calcined and powdered phosphate rock from Florida (P ₂ Oc 36.8 ^) and the mixture was allowed to react with stirring for 30 minutes. The temperature was increased ^{to 108 ° C. here.} Then 0.91 kg of 98% sulfuric acid were added and the mixture was used to decompose the phosphate rock into calcium sulfate hemihydrate and phosphoric acid

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continuously stirred for 1 hour to obtain 8 kg of the reacted slurry. The percent extraction of phosphoric acid at this stage of the process was 97t1 * »4 kg of the slurry was used to recycle through the reaction tank and the remaining 4 kg were filtered, with 1.96 kg of phosphoric acid (P ₃ O ₅ 40.3 *, H ₂ SO ₄ 1.8 *) as the product and 2.04 kg of calcium sulfate hemihydrate (calcium sulfate hemihydrate 1.42 kg; phosphoric acid 0.62 kg).

2.7 g of the washing liquid obtained when washing out the calcium sulphate dihydrate (P ₂ O ₅ 14.8 *), 0.06 kg 98% sulfuric acid, 0.01 kg seed crystal suspension (calcium sulphate content 15 *) consisting of the calcium sulphate described above dihydrate, and 4.81 kg fully hydrated Caleiumsulfat dihydrate Aufsehlimmung were hemihydrate calcium sulphate added to the filtered and the mixture was maintained at a tempera door of 65 ⁰ C and stirred while 9.62 kg calcium sulfate dihydrate slurry was obtained. 4.81 kg of this slurry was recycled to the hydration stage and the remaining 4.81 kg was filtered off. The calcium sulfate was washed twice in countercurrent with 1.39 kg of warm water, 1.2 kg of the first filtrate (P ₂ O ₅ 20.8 *), 2.7 / μg of the second filtrate (P ₂ O ₅ 14.8 * ) and calcium sulfate dihydrate containing 2.3 kg of volatility was obtained. The calcium sulfate thus obtained was in the form of columnar crystals with dimensions of 200 to 300 microns 30 to 50 microns. The remaining phosphoric acid components consisted of 0.28 *

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1904316 40.3 * - 17 - 1.8 * PpO and 0.04 * water soluble P ₂ O ₅ . The quality of the ■
97.1 * Calcium sulfate was outstanding. 1.5 hours Phosphoric acid concentration P ₂ O ₅ 20.8 * (Product) H ₂ SO ₄ 5.2 * Percentage extraction of
Phosphoric acid during the
decomposition 15 minutes. Decomposition time 98.7 * Concentration during hydration
ization P ₂ O ₅ H ₂ SO ₄ Hydration time Yield of phosphoric acid

Beisj) iel__4

1.16 kg of phosphoric acid produced by the process according to the invention (P ₂ O ₅ 45.0 *, H ₂ SO ₄ 2.0 *), 1.10 kg of phosphoric acid produced from rock containing phosphoric acid (P ₂ O ₅ 25.4 *, H ₂ SO ₄ 5.0 *) ^ which was obtained in the hydration step, as well as 4 kg of slurry * in which the decomposition reaction of the phosphate rock was complete, were added to 1 kg of calcined and pulverized phosphate rock (P ₂ Oc 36.8 *) and the mixture became 30 minutes with stirring reacted, the temperature was raised to 11O ⁰ C. Then 0.91 kg of 98% sulfuric acid was added and the mixture was continuously stirred for 1 hour to remove the phosphate rock

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to decompose into calcium sulfate hemihydrate and phosphoric acid. As a result, 8.0 kg of the reacted slurry was obtained. The percentage extraction of the phosphoric acid content in this process stage was 95.5 #. 4 kg of this slurry was used to return to the reaction tank and the remaining 4 kg were filtered, with 1.96 kg of phosphoric acid (P ₂ O ₅ 45 #) as the product and 2.04 kg of calcium sulfate hemihydrate (consisting of 1.42 kg Calcium sulfate hemihydrate and 0.62 kg of phosphoric acid) were obtained.

Then 2.7 kg of washing liquid (P ₃ O ₅ 19 #), 0.06 kg of 98 # Lge sulfuric acid and 0.01 kg of the above-described slurry of seed crystals (calcium sulfate content 15%) were added to the calcium sulfate hemihydrate #) was added, which consisted of a fully hydrated calcium sulfate dihydrate slurry. The hydration was carried out at 65 ⁰ C under Eühren, wherein 9.62 kg of calcium sulfate dihydrate slurry was obtained. 4.81 kg of this slurry was recycled to the hydration stage; the remaining 4.81 kg were filtered off and the remaining residue was washed 2 with 1.29 kg of warm water in countercurrent. 1.19 kg of the first filtrate (P ₂ O ₅ 25.1 #), 2.7 kg of the second filtrate (P ₂ O ₅ 18.7? O and 2.3 kg of moisture-containing calcium sulfate dihydrate were obtained.

As in Example 3, the quality of the calcium sulfate obtained according to this example was excellent and the content of remaining phosphoric acid components was 0.3 # PoOp- and 0.04 # in total

^ά °

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water soluble P2O5

Phosphoric acid concentration PpOc 45,

(Product) H ₂ SO ₄ 0.6 #

Percentage extraction of Phosphoric acid during the Decomposition level 95.5 # j Decomposition time 1.5 hours i Concentration during the hydra- j

tization P ₃ O ₅ 25.1? ί!

₅ H ₉ SO, 5.8Ji

C.

Hydration time 35 min.

Yield of phosphoric acid 98

The superiority of the invention over the known processes can be seen from the following table IV:

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Table IV: Comparisons between the inventive Methods and Various Known Methods

Calcium sulfate dihydrate process

Calcium sulfate Monohydrate

procedure

CaIeium-

sulfate-

Half-hy-

drat-

procedure Anhydrous calcium sulfate process

Disorientation

according to

procedure

Phosphoric acid concentration (Product)

less) less than 'than 30 *

40-50 * 40.50 *

Response time (all in all required Dwell time)

Cooling device

Quality of the calcium sulfate formed as a by-product

Yield of P ₂ O ₅

8 to 10 pcs

8-10 hours

^trfor derl.

bad good

1 -1.5 hours

not

require

derl.

bad 2 - 2 _f 5h

not research derl.

i'm bad

not

erfop-

derl.

Well

small amount

great

small amount small amount

great

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The advantages of the invention can be summarized as follows:

1. Highly concentrated phosphoric acid with a content of 40 to 50 * P ₂ Oc can be produced directly without having to use a concentration process.

2. The phosphoric acid yield is very high and is over 98 *.

3. The calcium sulfate formed as a by-product contains less than 0 _f 3 * of the total phosphoric acid content remaining in the residue and the calcium sulfate crystals formed are large and uniform.

4. Compared to the known method, the reaction time (residence time of the slurry) is increased 1/3 to 1/5 reduced. The physical dimensions of the required device are greatly reduced.

5. The filtration efficiency is high. The calcium sulfate hemihydrate crystals formed have a Length from 50 to 80 microns. The calcium sulfate can filtered at high temperature without the need for complete removal of the liquid. The filtration and washing of the calcium sulfate dihydrate can be because of the large crystal dimensions and low Concentration can be easily performed.

6. There are no cooling devices for them Slurry required

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Claims (1)

Patent claims: 1. Wet process for producing highly concentrated phosphoric acid by decomposing phosphate rock with sulfuric acid or a mixture of sulfuric acid and phosphoric acid, characterized in that one phosphate rock with sulfuric acid or a mixture of sulfuric acid and phosphoric acid among such Reaction conditions implemented so that the bridged calcium sulfate is obtained in the form of hemihydrate, which decomposed Products filtered and the calcium sulfate hemihydrate separated from the phosphoric acid, the separated calcium sulfate hemihydrate with calcium sulfate dihydrate, which by Decomposition of a calcium oxide compound with sulfuric acid or a mixture of sulfuric acid and phosphoric acid was obtained, added at a temperature at which calcium sulfate dihydrate forms directly, and the calcium sulfate hemihydrate is hydrated in a mixture with phosphoric acid and sulfuric acid. 2e method for producing highly concentrated Phosphoric acid, characterized in that phosphate rock is decomposed with sulfuric acid or a mixture of phosphoric acid and sulfuric acid to form a slurry of phosphoric acid and calcium sulfate hemihydrate, the slurry is filtered and the phosphoric acid is separated from the calcium sulfate hemihydrate, the separated calcium sulfate hemihydrate with sulfuric acid, a slurry of fully hydrated calcium sulfate dihydrate and dilute phosphoric acid, as well as a relatively -23- 909836 / 131b small amount of seed crystals clay calcium sulfate dihydrate twin crystals or a mixture of sulfuric acid and phosphoric acid, and the calcium sulfate hemihydrate in this way to form a mixture Calcium sulfate dihydrate and phosphoric acid hydrated, and the hydrated calcium sulfate and phosphoric acid clay separate. 3. The method according to claim 2, characterized in that the seed crystals are produced by pulverized phosphate rock is reacted with phosphoric acid, water and sulfuric acid and the seed crystals consist of calcium sulfate dihydrate twin crystals. ι ■ 909836/1316 Blank page