NL7909057A - Wet-process phosphoric acid prodn. - with calcium sulphate recovery as hemi:hydrate via di:hydrate - Google Patents

Abstract

Prodn. of H3PO4 and CaSO4.1/2H2O is carried out by (a) reacting a phosphate ore with H2SO4 under conditions such that the CaSO4 pptes as hemihydrate; (b) separating the hemihydrate and washing it under conditions such that the hemihydrate is not converted to dihydrate, while cpds. which migh inhibit the conversion of hemihydrate to dihydrate are removed by decompsn. and/or dissolution, (c) removing free moisture from the hemihydrate, (d) converting the hemihydrate to dihydrate by direct cooling, and (e) converting the dihydrate to hemihydrate in known manner. The process permits simultaneous prodn. of conc. aq. H3PO4 (e.g. with a P2O5 content of 40-54 wt.%) and high-purity CaSO4,1/2H2O, which is a more useful by-prod. than CaSO4.2H2O.

Description

• * 3144

Applicant: Unie van Kunstmestfabrieken BV in Utrecht Inventor: Dr. ir. Arnoud WALLER in Rotterdam METHOD FOR THE PREPARATION OF PHOSPHORIC ACID AND CALCIUM SUL-

FAATHEMIHYDRATE

The invention relates to a process for the preparation of phosphoric acid and calcium sulfate hemihydrate.

In the preparation of phosphoric acid along the so-called.

Wet road by reacting a natural phosphate or a mixture of natural phosphates with sulfuric acid or a mixture of sulfuric acid and phosphoric acid, the calcium compounds present in the phosphate are converted into calcium sulfate which precipitates in the form of crystals and can be removed from the reaction mixture. be separated. Depending on the method used, a more or less concentrated phosphoric acid is obtained and the calcium sulfate becomes available as calcium sulfate dihydrate or hemihydrate. Both crystal forms contain impurities in the grid 15 in the form of HPO4 ions which must be removed to limit the P205 losses.

The increasing production of phosphoric acid from natural phosphates involves a correspondingly higher production of calcium sulfate. A problem here is that this calcium sulphate can only be processed to a limited extent to other products and that the remainder must be disposed of as waste product. The possibilities for the latter are increasingly limited as a result of increasingly strict environmental requirements, so that new applications and larger sales opportunities must be sought. In this respect, calcium sulfate in the hemi-hydrate form offers more possibilities, for example for processing into building elements, than in the dihydrate form. For such application, however, it is necessary that the hemihydrate contain as little amount of HPO4 ions and other impurities as possible.

It is known that this can only be achieved by recrystallization of the hemihydrate in, for example, dihydrate or vice versa. The original crystals dissolve and the HPO4 ions are released. When recrystallized, they remain in the crystallization medium. This medium is recycled so that the P2O5 thus recovered eventually ends up in the product acid 40 '.

A method in which during the digestion and conversion of natural phosphate the calcium sulphate is obtained as hemihydrate, which is subsequently recrystallized in dihydrate, is described, for example, in NL-IS 6612150. dihydrate for which the applications are limited is obtained. In addition, special measures must be taken in this method to prevent; that during filtration and washing, the hemihydrate changes into a rigid dihydrate cake by rapid cooling, and furthermore the P2O5 which hereby returns in the crystal lattice can no longer be recovered.

NL-IS 6609093 describes a method in which the digestion of phosphate with sulfuric acid is carried out under such conditions of temperature and concentration that the calcium sulfate precipitates in the dihydrate form and this dihydrate is recrystallized into hemihydrate. However, the product acid obtained by this process has a relatively low P2 O5 content, so that a separate concentration step is necessary for certain applications.

The invention now provides a process by the use of which both a concentrated phosphoric acid and a highly filterable hemihydrate containing very few HPO4 ions can be prepared. For this purpose the calcium sulphate is precipitated as hemihydrate in the digestion of the natural phosphate, this hemihydrate is recrystallized in a simple manner into dihydrate and the dihydrate thus obtained is converted into hemihydrate in a manner known per se.

The invention therefore relates to a process for the preparation of phosphoric acid and calcium sulphate heraihydrate in which crystallization of calcium sulphate dihydrate obtained by reaction of natural phosphate, with sulfuric acid and optionally phosphoric acid in an environment with suitable P2O5-30 and H2 SO4 concentrations at suitable temperature is recrystallized. calcium sulfate hemihydrate, which is characterized in that the reaction of the natural phosphate with sulfuric acid is carried out in such a way that the calcium sulfate precipitates as hemihydrate, separates and washes the hemihydrate from the phosphoric acid and then removes the free moisture therefrom, the composition and the amount of the washing liquid and temperature are such that substantially no recrystallization of the hemihydrate takes place, while compounds which would delay the conversion of hemihydrate to dihydrate are removed by decomposition and / or solution, then the hemihydrate by direct cooling recrystallizes into dihydrate and recrystallizes this dihydrate into hemihydrate in a manner known per se.

45 A weakly acidic environment is necessary for the recrystallization of hemihydrate into dihydrate.

According to the invention this can be obtained in a simple manner by choosing the conditions in the digestion of the phosphate with sulfuric acid such that the calcium-50 sulphate precipitates in the form of hemihydrate florets or stars. Such crystals are highly filterable, but poorly washable. Thus, when erasing and removing free moisture, a small amount of acid remains, which under suitable conditions is sufficient to cause the hemihydrate roses or stars to spontaneously transition into dihydrate roses or 7909057 • 3 stars during cooling. It is not necessary to mix the hemihydrate with a weakly acidic solution and to pass the mixture into a separate crystallizer. The crystallization proceeds without the presence of a continuous liquid phase. The intended rose or star shape is achieved by, upon digestion of the phosphate with sulfuric acid, the concentration of Ca ++ and SO4 ions in the reaction mixture at at least eight times the saturation concentration and the temperature above 70 ° C, preferably between 90 and 120 ° C to keep 10.

The hemihydrate, which is free from free moisture, is cooled by means of air, but other gases can also be used. Furthermore, the cooling can be effected by suction of adhering moisture. Most advantageously, the air is passed through the crystal mass, which is then preferably spread loosely into a layer with a thickness of at most 5 cm. By preventing this layer from being crushed or bruised and by washing, by measures known per se, avoiding crystallization of dihydrate gypsum, it is possible to ensure that the crystals retain their rose or star shape as much as possible and that the layer does not close. A very simple method is obtained by feeding the reaction mixture as such or the wet hemihydrate separated therefrom to a belt filter 25 on which it is successively passed through a washing zone, a liquid removal zone and a cooling zone.

Another advantage of crystallization and recrystallization of rose- or star-shaped crystals is that the recrystallization proceeds considerably faster than if short bars are formed in the reactor.

The attached figure schematically shows an embodiment of the method according to the invention.

Raw phosphate ore is supplied via 1 to dissolving tank 2. Sufficient phosphoric acid from one or more further stages of the process is passed through line 3 herein.

The amount of phosphoric acid required for dissolving and digesting the phosphate 40 can also be fed partly to dissolving tank 2 and partly to reactor 12, but in the embodiment described here such an amount of phosphoric acid is introduced into dissolving tank 2 that, apart from an insoluble rest, all phosphate dissolves and 45 decomposes all carbonates present in the phosphate rock. In this case it is not necessary to grind the phosphate first.

Another advantage is that CO2 is no longer present at the location where the fluorine gases divert, which can lead to annoying mist formation. For this purpose, the 50 CO2 released during the dissolution is removed via 4. The insoluble residue settles and is fed via line 5 to settling tank 16. or discharged. The residual solution, which mainly consists of monocalcium phosphate and phosphoric acid, is transferred via 6 to precipitator 55 7 which is further supplied via 8 barium salts and sulfides, for example BaCl2 and FeS, and via 9 a sulfuric acid-containing 7909057 4 stream which is obtained in a further at the stage of the process. Radium and strontium present in the pure phosphate, like barium and calcium, are precipitated in the form of sulphates in precipitator 7, the cadmium as cadmium sulphide. Up to 10% by weight of the Ca ++ ions is removed in this way in view of the desired Ca + ion concentration in the hemihydrate formation. The resulting mixture flows to a battery of discontinuously operating filters 10 in which the solid is separated so that a clear filtrate is obtained.

This filtrate, which thus contains little or no impurities, is led via 11 to reactor 12, to which also via 13 and 14 a mixture of phosphoric acid and sulfuric acid in which inoculum for the formation of hemi-hydrate crystals is present or S1O2 in soluble form is supplied. . If the crude phosphate contains enough Si to remove the fluorine, the supplementation of S1O2 can of course be omitted. The phosphoric acid-sulfuric acid mixture here comes from the same process step as the sulfuric acid-containing stream fed to precipitator 7. However, it is also possible to work with a mixture from another process step or with only sulfuric acid. Care must then be taken to ensure that sufficient inoculum is supplied to the reactor.

The S1O2 is added to remove the fluorine contained in the raw phosphate. It makes a difference here whether the raw phosphate has a relatively low or a relatively high Al2C> 3 content. In the first case, for example at 0.5-0.6 wt.% AI2O3 in the raw phosphate, the fluorine is preferably removed quantitatively as gaseous S1P4 by adding an excess of S1O2. If the crude phosphate contains a relatively large amount of Al2Q3, for instance 1-5 wt.%, It is preferable to choose another way to remove the fluorine, namely by adjusting the amounts of Ca, 35 Al, SO4, F, rare earths and / or Si, so that through the formation of chukhrovite, it is prevented that all Al enters the phosphoric acid product and that the fluorine is further removed. The adjustment is effected by supplying the required elements via reactor 14 to 14. Both 40 methods of fluorine removal are known per se.

In reactor 12, the temperature and the concentration of Ca ++ and SO4 ions are adjusted and the supersaturation of dissolved calcium-45 sulphate is maintained such that rose or star-shaped hemihydrate crystals are formed from the calcium sulphate formed by reaction of the sulfuric acid with the monocalcium phosphate. on the particles of the inoculum fed through 13. Good results are hereby obtained if the concentration of Ca ++ and SO4 ions is at least eight times the saturation concentration and the temperature is kept at a value between 90 and 120 °. The temperature should generally not be below 70 ° C.

The hemihydrate crystals obtained were contaminated by HPO4 ions which were incorporated in the crystal lattice instead of SO4 ions. In addition, the fluorine contained in the filtrate fed to reactor 12 via 11 passes through gaseous and liquid HF, gaseous S1F4 and compounds of F with Al +++ r ions.

The reaction mixture formed in reactor 12 flows via 15 to settling tank 16. This is a closed vessel in which the gaseous components present in the supplied mixture and water vapor are separated and the mixture is thickened. The gaseous components and the water vapor are removed via 17. This does not contain air or CO2 which serves as an inert carrier, so that these gases do not have to be separated from the air first, for example by washing, but can be condensed quantitatively. The contaminating hemihydrate crystals present settle, so that they do not reach the top discharge 18. The solids concentration in the crystal effluent is controlled, for example, at 20-40% by weight, so that the drained slurry can be treated on a filter without forming an impermeable filter bed.

Other separating devices can also be used instead of a filter.

Settling tank 16 has a third function, besides the separation of gaseous constituents and the thickening of the mixture, namely the discharge of impurities.

The sedimentation tank forms and / or grows out of precipitates which are formed from the phosphate only after passing the filters 10 through accumulation of impurities or addition of S1O2 and a sufficient amount of H2SO4. If the raw phosphate contains a relatively large amount of AI2O3, as already described, the fluorine present is converted into chukhrovite. With sufficient residence time of the mixture in the settling tank, the chukhro viet crystals grow to such a size that they collect on the bottom of the settling tank and can be discharged via the bottom drain and conduit 55. The hemi-35 hydrate crystals settle more slowly and can therefore be discharged at a point higher than the bottom discharge and brought to filter 20 via 19. However, some of the impurities, including the fine chukhrovite particles, do not precipitate but settle more slowly than the 40 hemihydrate florets or form a gel-like substance that collects at the top of the settling tank and can be discharged through top drain 18. If crude phosphate with a low content of AI2O3 is used, a separate discharge of a chukhrovite precipitate via 55 is unnecessary and the discharge can only take place via top discharge 18. In a separator 53, for example a hydrocyclone, a centrifuge, a flocculation device, the majority of these impurities are separated, so that a liquid with few impurities remains. This 50 is fed via 32 to backwash section 31 of filter 20 and used here as washing liquid. The separated impurities go via 54 to solution tank 2 and from there via precipitator 7 to filters 10 where they are removed from the process.

55 In view of the maximum acceptable content of certain impurities in the hemihydrate to be disposed of as end product, it is often not permissible to use the 7909057 r.

6 quantitatively leave any impurities present in settling tank 16 from the settling tank 16 in the process flow. An excess of impurities can be combined with stream 54 via 55, 21, 22 and 3 and precipitator 7 can be fed to filters 10 via dissolving tanks 2 and 5. Another possibility is to treat streams 54 and 22 and / or stream 3 in separate filters.

Filter 20 is preferably designed as a band filter. In view of the further processing of the hemi-10 hydrate crystal mass, a certain minimum permeability of the crystal layer to be formed thereon is desirable. It is therefore advantageous to maintain a relatively small layer thickness of no more than 5 cm and preferably 2-4 cm on the filter. A first portion of the filtrate is cloudy and contains some of the precipitates described above. This is removed via 21. From the second filtrate fraction obtained from the second section of the filter and discharged from it via 23, a fixed amount is discharged as product acid via 24, the remaining part is passed with the first filtrate via 22 and 3 to dissolving tank 2 to dissolve raw phosphate.

The P2 O5 concentration of the product acid is adjusted by controlling the amount of wash water supplied through 50 to wash section of hemihydrate filter 45 and the amounts of water vapor removed through 17 from settling tank 16 and through 29 from recrystallization section 28 of filter 20. With the method described here it is possible to achieve phosphoric acid with a P 2 O 5 concentration of 40-54% by weight. The H2 SO4 concentration of the product acid is adjusted to 0.5-2 wt% by controlling the amount of sulfuric acid supplied to the process via 40.

The contaminated hemihydrate crystals, from which most of the phosphoric acid has been removed, are then washed in washing section 25 with washing liquid supplied via 26, which comes, for example, from washing section 49 of hemihydrate filter 45. It is ensured that the filter cake is washed during washing. and vacuum cleaning does not cool down to such an extent that recrystallization of hemihydrate in 40 dihydrate occurs, since there is a risk of the filter tape closing. The minimum temperature to be observed here is 80 ° C. The filtrate from this wash is discharged through 27 to disperser 30. Washing section 25 is shown in the simplicity for simplicity, but in practice 45 may consist of more steps in which washing is carried out according to the counter-flow principle. If the F and Al present in the raw phosphate are removed as chukhrovite, a prewash with sulfuric acid is necessary to remove the chukhrovite still present in the crystal mass by decomposition 50. If this is not the case, however, a prewash with sulfuric acid is nevertheless desirable, as this leads to very short recrystallization times in the cooling section 28 to be discussed below. The filtrate from this prewash is then returned directly to reactor 12. After 55 prewash, the filter bed should be prevented from closing, for example by thoroughly vacuuming.

7909057 7

After the washing, the filter cake is sucked dry. As a result of the rose or star shape of the hemihydrate crystals, a small amount of weakly acidic liquid remains behind between the needles. In the cooling section 28 following the washing section, in which powerful suction is effected, cooling takes place by evaporation of water still present below the transition point from hemihydrate to dihydrate. The extracted air loaded with water vapor is discharged via 29. The weakly acidic liquid present forms an environment in which the contaminated hemihydrate rapidly transitions into dihydrate in situ. During this transition which takes place via the liquid phase, the HPO4 ions from the hemihydrate do go into solution, but they can again be wholly or partly incorporated in the lattice of the dihydrate crystals.

After the cooling phase, the filter cake is blown off the filter belt by blow-off and / or with the aid of a scraper and is sent to disperser 30. The filter tape is stripped of adhesive plaster in a separate backwashing section 31 by vigorous rinsing with liquid supplied via 32. The used washing liquid is partly passed via 27 to disperser 30. The remaining part goes via 33, 22 and 3 to solution tank 2.

This recirculation of the remaining part of the washing liquid makes it possible to select the amount of liquid used for backwashing independently of the amount of phosphoric acid to be fed via disperser 30 into crystallizer 37 in order to maintain the optimum phosphoric acid-sulfuric acid ratio therein.

It may be advantageous for the recrystallization section 28 of filter 20 to use a different filter material as in the previous sections. In view of this, filter 20 can then be replaced by two separate filters connected by a transport device which leaves the structure of the filter cake unchanged.

In disperser 30, the dihydrate cake is dispersed in dilute acid from wash section 25 of filter 20. The resulting crystal slurry is then passed via 40 and 36 to hemihydrate crystallizer 37 with addition of clean hemihydrate seed material from 39 at the bottom of the crystallizer and optionally is supplied from dryer 51 via 38. In crystallizer 37, which may, for example, be of the screw type or 45 as the riser type schematic here, the average residence time of the crystal mass is set at about one hour to prevent decomposition of the hemihydrate crystals. Furthermore, by controlling the amount of liquid supplied via 27 to disperser 30 and the amount of sulfuric acid supplied via 40 and 41 with 98 wt.% H 2 SO 4 in the crystallizer, an environment is maintained with a P 2 O 5 concentration of 20-30 wt. % P2O5 and 10-20 wt% H2SO4 and a temperature of 75-85 ° C.

The desired particle size of the formed hemi-55 hydrate crystals can be maintained by controlling the supply of seedings via 39 and optionally 38. Via line 42 7909057 8, a germ-containing liquid is passed from the crystallizer to separator 43, for example a centrifuge, which is further supplied via 44, the first filtrate of hemi-hydrate filter 45 and an amount of fresh 98% sulfuric acid are supplied via 56. The hemihydrate crystals present in the supplied streams are partially separated here. The hemihydrate thus separated is passed to solution tank 2 via 46, 22 and 3. The remaining part is fed via 47, 34 and 13 to reactor 12 as seed material and further via 9 to precipitator 7 for sulfuric acid supplementation.

The hemihydrate product leaves the crystallizer in the form of a slurry and reaches a separating device 45, such as, for example, a band filter, via line 48. The hemihydrate formed in crystallizer 37 has the habit of short, thick bars and is therefore both filterable and washable. The first filtrate which is separated on the filter, as already mentioned, is led to separator 43. Washing 20 takes place in washing section 49 in the same manner as on filter 20. For this purpose, clean water is used, which is supplied at 50. Also in this washing and the subsequent dry suction it is prevented that hemihydrate is converted into dihydrate by excessive cooling. The liquid extracted in washing section 49 is fed via 20 to washing section 25 of filter 20 to once again serve as washing liquid.

The dry-sucked filter cake can be further dried in a drying device 51. If desired, a small fraction of the dried hemihydrate crystals is recycled via 38 and 36 as seed material to crystallizer 37. The remainder is discharged as final product via 52. However, it is also possible to lead the product discharged from filter 45 as such to a further processing, for example an installation for the production of building elements or an installation for the preparation of calciurasulfate anhydrite.

The final product contains virtually no HPO4 ions and has a very low content of other impurities.

40 Example

In an apparatus as shown in the figure, but without precipitator 7 and filters 10, 1000 kg of Taiba phosphate per hour, containing 51.0 wt% CaO, 38.4 wt% P2O5 and a small amount of inert material, are added and 45 9200 kg of a solution containing 47.0 wt.% P2O5 and 2.0 wt.% H2SO4 via 3 fed to dissolving tank 2. The phosphate • dissolves, with the exception of 13.2 kg inert, and the resulting 23 .0 kg of CO2 are removed via 4. The inert is withdrawn from the process discontinuously via 5 together with the small amounts of 50 hemihydrate crystals and chukhrovite supplied via 3.

The remaining mixture is charged to reactor 12 along with 2978.0 kg of solution containing 22.6 wt.% P2O5 and 32.1 wt.% H 2 SO 4 (calculated as 98% acid) about 0.5. 1% by weight of inoculum based on the amount of hemihydrate formed 7909057 9 and has a temperature of 114 eC. The temperature in the reactor is adjusted to 110 ° C. Under these conditions, the desired rose-shaped hemihydrate crystals form from the monocalcium phosphate and the sulfuric acid. The crystal slurry formed in reactor 12 is then passed into settling tank 16, where the crystals continue to grow at a temperature of 110 ° C and a nominal residence time of 3 hours. 190.8 kg of water vapor, 15.7 kg of HF and 30.9 kg of S1F4 are removed via 17.

From the bottom of the settling tank, 4654.1 kg of crystal slurry are discharged to filter 20. On this, first a cloudy liquid and then clear phosphoric acid are separated from the crystal mass. The first, 120 kg, is returned to settling tank 16. Of the latter, which contained 47.0 wt.% P2 ° 5 and 2.0 wt.% H 2 SO 4, are removed from the process as product acid via 24 781.1 kg. The remaining part goes to solution tank 2. In the washing section, the filter cake, which has a temperature of 110 ° C, is washed with 577.1 kg of water with a small amount of 20 P2 ° 5 from filter 45. 564 kg of liquid are suctioned containing 40.0 wt% P2O5 and 1.8 wt% H2SO4. The temperature of this liquid is 99.5 ° C. The filter cake, which still contains 583.0 kg of liquid with 5.1 wt.% P2O5 and 0.2 wt.% H2SO4 and has a temperature of 99.6 ° C, is then cooled by passing through ambient air. Due to the accompanying drop in temperature to 60 ° C, the 1345 kg of hemi-hydrate crystals are converted into 1596 kg of dihydrate crystals.

The cooling air exhausts 119.9 kg of water vapor via 29. The filter cake, which further contains 29.7 kg of P2O5, 1/35 kg of H2SO4, 67.4 kg of water and small amounts of impurities, such as FeP04, AIPO4, HF and H2S1F5, is mixed in disperser 30 with the 564 kg of liquid from the washing section of the filter and 994.1 kg of liquid from 35 backwashing section 31 of the filter.

The slurry of dihydrate crystals, 3393.9 kg, obtained in the disperser, is fed to crystallizer 37 as well as 30% of the H 2 SO 4 (98%) supplied at 6 in an amount of 943.9 kg. A temperature of 75.7 ° C is used in the 40 crystallizer

maintained, while the nominal residence time is approximately 1 hour. At 48, 3400 kg of crystal slurry containing 1345.5 kg of hemihydrate crystals, 18.2% by weight of phosphoric acid calculated as P2O5, 7.7% by weight of H2SO4 and 846 kg of water are discharged.

45 This slurry is treated on filter 45 and an amount of liquid of 2040, 3 kg with 30.3 wt.% Phosphoric acid calculated as P2O5, 12.7 wt.% H2SO4 and 846 kg of water is filtered off. This liquid is fed to reactor 12 with 660.7 kg, H2SO4 (98%) and 1.3 kg of hemihydrate seed crystals containing 50 liquid via 34 and 13. The crystal mass remaining on the filter is washed with 917.1 kg of water at 80 ° C which is supplied at 4. 1699.4 kg of hemihydrate crystals, which still contain 340.0 kg of water, are removed from the filter.

After drying, a hemihydrate product with 0.18 wt.% P2O5 and 0.14 wt.% F is obtained.

7909057

Claims (9)

Process for the preparation of phosphoric acid and calcium sulfate hemihydrate, wherein calcium sulfate dihydrate obtained by reaction of natural phosphate with sulfuric acid and optionally phosphoric acid is recrystallized at an appropriate temperature in an environment with suitable concentrations of 5-205 and H2SO4 into calcium sulfate hemihydrate, characterized in that the reaction of the natural phosphate with sulfuric acid is carried out such that the calcium sulfate precipitates as hemihydrate, the hemi-hydrate is separated from the phosphoric acid and washed and then the free moisture is removed therefrom, the composition and the amount of the washing liquid and the temperature being removed. such that substantially no recrystallization of the hemihydrate takes place here, while compounds which would delay the conversion of hemihydrate to dihydrate are removed by decomposition and / or solution, then recrystallizes the hemihydrate by direct cooling to form dihydrate and this dihydrate follow In a manner known per se, it recrystallizes to hemihydrate. 2. Process according to claim 1, characterized in that the reaction of the natural phosphate with sulfuric acid is carried out in such a way that a reaction mixture with a concentration of dissolved calcium sulfate of at least 5% by weight is obtained. 3. Process according to claim 1 or 2, characterized in that the washed hemihydrate, which has been freed from adherent moisture, is recrystallized into dihydrate by air passage. 4. Process according to claims 1-3, characterized in that the wet hemihydrate separated from the reaction mixture in the form of a layer with a thickness of at most 5 cm is successively passed through a washing zone, a liquid removal zone and a zone in which 35. cooling air is passed through the layer. Method according to claim 4, characterized in that a band filter is used for the separation of hemihydrate and phosphoric acid, washing, liquid removal and cooling. 6. Process according to claims 1-5, characterized in that a reaction mixture is started from which is substantially free of fluorine, strontium, radium and cadmium compounds. 7. Calcium sulfate hemihydrate obtained using the method according to claims 1-6. 8. Phosphoric acid obtained using the method according to claims 1-6. 9. Process for the preparation of phosphoric acid and calcium sulfate hemihydrate as described and explained with reference to the drawing. 7909057 IT. ^ ^ .J2 Λ "" é YY a 13 # "11 1214. 17 &" -54 -H_r ^ v I Ψ r Λ "53 n 16-—" —32 a 19 — I r— ^ r.? 5__ 1 / gp8, aa 1 / -. I _§3 25,, 33 ^^ - 31 37 fL ^ O yv 'p - * ^ I 1 ^ 6 ^ -]> 124 * f ^ ^ 2 _ [| T42. "ίϊΐΚ-" Ί u "40 48- - 39-4 50, 38— ^ IV ^ 44 45 ^ Q ^ r ^ 1—11 ~ f ^? 51, 7909057 3144 U.K.F