IL26394A - Process for the preparation of phosphoric acid and gypsum from phosphate rock - Google Patents

Description

SPECIFICATION ■Process for the preparation of phosphoric acid and gypsum from phosphate rock 0K3D 1 S I (we) ALBATROS SUPERPOSPAATFAERIBKEll N.V., a Butch limited liability company, of Maliebaan 31, Utrecht, the Ketherlande do hereby declare the nature of this invention and in what manner the same is to be performed, to be particularly described and ascertained in and by the following statement :- The invention relates to a continuous process for the preparation of phosphoric acid and gypsum from phosphate rock "by acidulation with sulphuric acid or a mixture of sulphuric acid and phosphoric acid.

It is known that phosphate rock on acidulation with sulphuric acid or a mixture of sulphuric acid and phosphoric acid yields phosphoric acid and calcium sulphate. The calcium sulphate can appear in different crystal forms according to the reaction conditions. Thus, at high temperatures, usually between 80° and 90°C, and at a high concentration (e.g. above 30%) of phosphoric acid the hemihydrated form CaSO^ . " appears. At lower temperatures, of the order of 70-75°C , and at lower concentrations (e.g. 20 to 25%) of phosphoric acid, the dihydrated form CaSO^^HgO, called gypsum, appears.

If acidulation conditions favouring the formation of gypsum are employed, it is found that the gypsum crystals include a considerable amount of CaHPO^.c-ILjO which is iso-morphous with gypsum, crystallizing in the same lattice.

These conditions therefore yield a gypsum of unduly high phosphate content thereby lowering the yield of phosphoric acid. These losses may be reduced by increasing the sulphate ion concentration in the reaction mixture, but this leads to a high sulphate contamination of the phosphoric acid obtained and to gypsum showing poor filtration characteristics.

It has therefore been proposed to conduct the acidulation of phosphate rock under such conditions that the calcium sulphate appears initially in the hemihydrated form, and thereafter to convert the hemihydrate to the dihydrate by lowering the temperature and the phosphoric acid concentration. In this way easily filterable gypsum crystals are obtained phate. Furthermore the reduction of the phosphoric acid concentration leads to a rather diluted phosphoric acid solution, A separate step for the removal of water therefore becomes necessary if concentrated phosphoric acid is to be produced.

It has therefore further been proposed to separate the initially formed hemihydrate from the reaction mixture on a filter and to wash it with a diluted phosphoric acid solution, thereafter removing it from the filter and converting it to gypsum by recrystallization from dilute phosphoric acid.

However, premature conversion of the hemihydrate to gypsum frequently occurs during and/or after the washing on the filter and/or during its transport to the recrystallization vessel, thus causing clogging of the transport conduits. In addition, this accidental crystallization takes place under conditions unfavourable to proper dihydrate crystal growth. Moreover, the gypsum obtained is still unduly contaminated with phosphate and shows poor filtration characteristics.

These disadvantages have been eliminated for a considerable part by another process of the art comprising reacting phosphate rock with a first mixture of concentrated phosphoric acid and sulphuric acid to form a slurry of calcium sulphate hemihydrate crystals in a concentrated phosphoric acid solution, separating said hemihydrate crystals from said phosphoric acid solution and withdrawing the phosphoric acid solution from the process, admixing said hemihydrate crystals with a second mixture of dilute phosphoric acid and sulphuric acid, redissolving said hemihydrate crystals in said mixture and precipitating therein gypsum crystals, which are separated from the mixture of phosphoric acid and sulphuric acid.

By using this process it is possible to control the thus improving its quality. Moreover, when aiming at phosphoric acid with a high concentration, a low sulphate ion concentration in the reaction mixture improves the filtration characteristics of the hemihydrate precipitate. Thus, according to that process it is possible to produce a good quality phosphoric acid in a rather high concentration, the content of phosphate in the gypsum produced being lower than obtained with the normally used dihydrate process as previously described.

However, in carrying out that process the phosphoric acid concentration in the recrystallization step is high, i.e. 20-30%, due to the phosphoric acid remaining in the calcium sulphate hemihydrate mass which has been separated from the phosphoric acid solution. This high phosphoric acid level means that a long time is needed for the recrystallization step and that there is still much phosphate included in the gypsum, even though the sulphuric acid concentration in the recrystallization step is maintained at the rather high level of 5-20% in order to counteract inclusion of phosphate in the gypsum.

There are therefore a number of problems to be solved before a satisfactorily economical process can be put into operation. It is an object of the present invention to provide satisfactory solutions to these problems, that is to say, to produce a phosphoric acid of high an^ low sul~ phuric acid content, a gypsum with good filtration characteristics and low phosphate content and to yield these products in less time than hitherto and in a more economical manner.

According to the present invention the above problems are solved by a continuous process for the preparation of sulphuric acid or a mixture of phosphoric acid and sulphuric acid to form a slurry of calcium sulphate hemihydrate crystals in a concentrated phosphoric acid solution, separating the hemihydrate crystals from said phosphoric acid solution and withdrawing said phosphoric acid solution and converting the calcium sulphate hemihydrate crystals into calcium sulphate dihydrate (gypsum) crystals, which process is characterized by washing the calcium sulphate hemihydrate crystals after the separation of the concentrated phosphoric acid solution with an aqueous washing liquid in order to re- t move at least part of the phosphoric acid regained in and adhering to the mass of the hemihydrate crystals, the composition and quantity of said washing liquid and the temperature being selected so as to prevent completely or substantially the transformation of the hemihydrate into another crystal form and to make the liquid remaining in and adhering to the mass of hemihydrate crystals after washing a suitable component for the recrystallization solution to which said mass is transferred, and by recrystallizing the washed calcium sulphate hemihydrate crystals to form gypsum crystals in an aqueous solution containing from 0 to 20% by weight of phosphoric acid, calculated as ϊ^^' anc^ ^rom 2 to 25 by weight of ELpSO^, and separating the gypsum crystals obtained from the recrystallization solution.

The acidulation of the phosphate rock with sulphuric acid to form phosphoric acid and calcium sulphate can be carried out in any possible way provided the conditions are such that calcium sulphate hemihydrate is formed. The phosphate rock and the sulphuric acid are e.g. mixed at temperatures between 75 and 105°C. One or more phosphoric acid solutions obtained in a further stage of the process may be amounts of phosphate rock, sulphuric acid, phosphoric acid and water to be employed are of course not only determined by the formation of the hemihydrate but also by the desired concentration of the resulting phosphoric acid.

The temperature of the acidulation reaction may be controlled in several ways. If, for instance, strong sulphuric acid (90-100%) is used, this strong sulphuric acid may be mixed with recycled phosphoric acid solution whereby heat is generated causing a temperature rise. When operating under reduced pressure water may be evaporated by the generated heat. Thus both the sulphuric acid concentration and the temperature are controlled simultaneously. It is also possible to control the temperature by evaporating water by blowing air through the mixture of sulphuric acid and phosphoric acid solution.

The separation of the concentrated phosphoric acid solution and the calcium sulphate hemihydrate formed by the acidulation is generally carried out on a filter, although any other suitable separative method, such as centrifugation, may also be employed. As a filter in most cases a continuous belt filter is used. Often the first phosphoric acid filtrate obtained still contains some contamina ion. Preferably, it is recycled to the acidulation reactor. The remainder of the filtrate is the phosphoric acid solution having the desired concentration, which is withdrawn from the process.

After the separation of the phosphoric acid solution the calcium sulphate hemihydrate is washed to remove at least part of the phosphoric acid still adhering to the hemihydrate. The composition and quantity of the washing liquid and the temperature are predominantly determined by the requirement that the calcium sulphate hemihydrate crystals should not crystals and that the composition and quantity of the liquid remaining in and adhering to the cake of hemihydrate crystals after washing should be such that the composition of the re-crystallization solution to which the said cake is transferred, can be properly controlled.

Gypsum crystals could be easily formed, if the hemihydrate was simply filtered off and then washed with water or dilute sulphuric acid. A considerable amount of phosphate, which is still present in the cake of hemihydrate crystals in several forms, will then be entrapped in this gypsum. As contrary to the hemihydrate this untimely formed gypsum is not redissolved in the subsequent recrystallization step, the entrapped phosphate will not be liberated and is definitely lost. Moreover, seed crystals of gypsum of unde-sired form result which hinder the proper growth of pure gypsum crystals in the subsequent recrystallization. This formation of gypsum crystals might already take place on the filter to such an extent that the cake particles agglomerate and thus include another portion of phosphate.

Using water or dilute sulphuric acid as a washing liquid moreover has the disadvantage that the liquor obtained by washing the hemihydrate is so dilute that it is substantially unsuitable to be recycled to the acidulation reactor, so that the phosphoric acid and sulphuric acid contained therein are lost.

To avoid the conversion of the hemihydrate into gypsum on the filter a suitable washing liquid should have a low water vapour pressure. As a washing liquid for instance a non-dilute aqueous sulphuric acid solution may be used. For example, in the continuous process of the invention the sulphuric acid contraining mother liquor obtained by filtering of the hemihydrate may be used as a washing liquid. The concentration of sulphuric acid in this mother liquor may be adjusted b admixing fresh strong sulphuric acid. By a proper selection of the amount and the temperature of the admixed strong sulphuric acid and by the use of its dilution heat it is possible to evaporate a predetermined amount of water from the mixed phases thus obtaining the desired composition and temperature of the washing liquid. By these measures gypsum crystals, which may be present in the said mother liquor, are transformed into calcium sulphate hemihydrate crystals. The desired concentration of sulphuric acid in the washing liquid and the amount thereof can be easily determined by preliminary experiments.

By using the washing liquids in accordance with the invention it has become possible to obtain a liquor after washing the hemihydrate, which may conveniently be recycled to the acidulation reactor, thus avoiding losses of phosphoric acid and sulphuric acid.

The conditions under which calcium sulphate hemihydrate is converted into calcium sulphate dihydrate or another crystal form of calcium sulphate are known to the expert in the art and need not be illustrated further.

The composition of the recrystallization solution, to v/hich the hemihydrate crystals are transferred after washing, can be properly controlled by the composition and quantity of the washing liquid for the hemihydrate crystals , as referred to in the above. As will be illustrated hereinafter, there is a preferred range for the phosphoric acid and sulphuric acid content in the recrystallization solution. As the hemihydrate crystal cake contains entrapped and adhering phosphate, phosphoric acid and sulphuric acid, which are the composition and quantity of the washing^ liquid will influence the percentages of these ingredients which will come into this solution. As after the recrystallization the mother liquor obtained by filtering off the gypsum crystals formed is generally recycled either to the acidulation reactor or the recrystallization solution or as part of the said washing liquid, it is clear that by controlling the amount and composition of the liquid transferred with the hemihydrate crystals to the recrystallization solution, an important contribution to a favourable balance of all substances used all over the plant can be obtained. The composition of the washing liquid for the hemihydrate crystals on the filter is of course also dependent on the way and extent of acidulation, which determines the composition of the phosphoric acid phase adhering to the hemihydrate crystals and on the temperature during the washing on the filter.

To avoid that phosphate is included in the gypsum during the recrystallization of the hemihydrate into gypsum the recrystallization temperature should be preferably as high as possible and the phosphate ion concentration as low as possible as will be illustrated hereinbelow. On the other hand it has been found that a certain minimum amount of phosphate ion in the recrystallization solution improves the filterability and washability of the resulting gypsum. If the acidulation of the phosphate rock would result in the formation of a calcium sulphate hemihydrate filtercake which has entrapped such a low amount of phosphoric acid and phosphate that after washing the filter cake with the washing liquid according to the invention this minimum amount of phosphate ion is not present, the washing of the filter cake should be stopped before all adhering phosphoric acid is adhering acid to come into the recrystallization solution.

The following experiments have been carried out to determine the preferred compositions of the recrystallization solution.

Experiment 1 350 cm of each of the aqueous solutions A, B, C and D, the composition of which is found in Table I are maintained at various fixed temperatures. Into each of these 350 cm are added 50 gm of CaSO^^H^O crystals containing various small percentages of total phosphate (calculated as serving as seed crystals. To each of the resulting slurries are added 100 gm of CaSO^.i-H^O containing various percentages of ^2^ * -^ach °^ "^ e mixtures is then left to recrystallize for two hours at their predetermined temperatures and the as well as the content of the water of crystallization of the recrystallized sulphate are then determined. The results are shown in Table I.

Table I Notes: percentages of ?2 5 a (^ sulphate in recrystallization solutions % P % B0 = A B C D determined after two hours.

Experiment 2 This experiment is performed in the same manner as Experiment 1 but with the temperature maintained at 70°C and with other percentages of and sulphate ion in the re- crystallization solution. In this Experiment the percentage of ^2^5 was maintained substantially constant with a varying percentage of sulphate ion or was varied with substantially constant sulphate content. The results are shown in Table II. In the last column the percentage of ΐηΘ gypsum is given after a recrystallization time of 2 hours.

Table II recrystallization recrystallization % of ^2^5 -*-n solution solution gypsum product % P2O5 % so = 4.9 0.1 0.79 .0 5.1 0.24 4.9 10.0 0.14 .0 15.0 0.13 4.9 19*6 0.09 0 2.0 0,32 4.9 2.2 0.46 .7 2.8 0.5 14.5 2.8 0,43 19.5 . 3.0 0.46 0 4.8 0. 9 4.9 5-2 O.3O 9.6 5-5 0.33 14.5 5.6 O.3O 9.3 6.0 Ο.27 These experiments show that indeed the phosphoric acid content in the recrystallization solution should be kept as low as possible to avoid unduly high contamination of the resulting gypsum with phosphate. If under any circumstances the phosphoric acid concentration is rather high, a more elevated sulphuric acid concentration may be helpful for obtaining a low in the gypsum. The total amount of acid, however, should not be too high in order to avoid unduly long recrystallization times. As furthermore the sulphuric acid concentration is essential for obtaining gypsum with a low ^2^5 content, the only way of keeping the acid level low is to minimize the amount of ^2^5' whic enters the recrystallization step.

In general the recrystallization solution used is an aqueous solution containing from 0 to 9.0% by weight of ?2<-)5 and from 2 to 25% by weight of H2S0 . Preferably, the amount of is from 5 "to 15 » The recrystallization temperature is preferably kept below 90°C and may be gradually decreased as the recrystallization proceeds e.g. by maintaining a temperature gradient in the recrystallization step.

As is known from crystallization practice the proper e form and quantity of seed crystals may have a beneficial influence on crystallization. This influence may be obliterated by impurities present. In the process of the invention, however, impurities, originating from the phosphate rock have been removed to a great extent by the filtration and washing of the hemihydrate crystals. Thus the recrystallization solution is substantially free from such impurities, which makes it possible to utilize the benificial action of proper seed crystals to full advantage. This also makes gypsum during the washing of the hemihydrate crystals as previously described is so important. Proper gypsum seed crystals may e.g. be prepared by separate recrystallizatxon of part of the hemihydrate. Alternatively, gypsum crystals obtained by an earlier recrystallizatxon in the process may be recirculated and added as seed crystals.

After the recrystallizatxon has been completed the gypsum crystals are separated from the mother liquor, e.g. by means of filtration. Preferably, the mother liquor is recycled to an earlier stage of the process, e.g. to form part of the washing liquid for the hemihydrate crystals , and/or to the recrystallizatxon solution. In general, the gypsum crystals are washed with water, preferably counter-currently, i.e. by recycling the last filtrate to wash another portion of gypsum. The resulting washing liquor is preferably recycled to the recrystallizatxon solution and the gypsum crystals are withdrawn.

The process of the invention enables a very economic use of sulphuric acid. The sulphuric acid is in fact used to replace the calcium in the phosphate starting material by hydrogen and to form calcium sulphate. Thus the main consumption takes place in the acidulation reaction, to which the predominant part of the sulphuric acid supply is fed. However, in many cases also sulphuric acid is used for the washing of the hemihydrate crystals. It is not consumed, however, in the washing operation and must be recovered to avoid losses. The recovering is carried out by recycling the washing liquor to the acidulation reactor as described in the above, where the sulphuric acid in the washing liquor is consumed in the acidulation reaction. The sulphuric acid adhering to the hemihydrate after washing reacts during the phosphate rock entrapped in the hemihydrate and released therefrom "by the dissolution of the hemihydrate in the recrystallization solution. If desired, additional fresh strong sulphuric acid may "be added to said solution in order to obtain or to maintain the sulphuric acid concentration which is desirable for optimum gypsum yield during the re- crystallization. If too much sulphuric acid would have entered the recrystallization solution, the sulphuric acid concentration thereof can be reduced to the desired level by introducing an increased part of the mother liquor obtained by filtering off the gypsum crystals into the washing liquid for the hemihydrate wash, whence it is returned to the acidulation vessel. The sulphuric acid leaves the process for the greater part in the form of gypsum and for a as free acid in the phosphoric acid product.

Water is introduced at the tail end of the process as wash water for the gypsum crystal cake. Part of this water is lost in this cake and part is evaporated in various steps of the process, e.g. when mixing strong sulphuric acid with recycled phosphoric acid solution before the sulphuric acid is used for the acidulation reaction and v/hen using strong sulphuric acid as part of the washing liquid for the hemihydrate crystals. Thus it is possible to control the temperature in the various steps of the process by properly balancing the amount of water and the amount and concentration of sulphuric acid to be used in these steps. In an industrial process it is convenient to use as a sulphuric acid for the process concentrated sulphuric acid. This makes it possible to evaporate much water in the various steps.

As water is added to the process only as wash water for the gypsum this means moreover that the gypsum can be washed out water can be recycled to earlier steps in the process without loading those steps too much with water. Finally, water may also evaporate during the acidulation reaction and during recrystallization using the heat of reaction and dilution.

According to the process of the invention hot sulphuric acid may also be used, y/hich is not possible when phosphate rock is acidulated with the immediate formation of gypsum. By using hot sulphuric acid an additional amount of water may be evaporated.

According to the invention it is possible to produce phosphoric acid solutions containing more than 30% by weight of ^2^5' e>S* containing 0 by weight or even more, the sulphuric acid content being low e.g. less than about 1.5% by weight and often less than 1% by weight.

The gypsum crystals prepared according to the process of the invention show excellent filtration characteristics and have a phosphate content of less than 0,3% by weight, calculated as °f gypsum prepared according to the known processes. In the gypsum the content of fluorine originating from the phosphate rock is generally below 0.2% by weight.

Another advantage of the process of the invention is that it only takes 2 to 3 hours before a certain portion of phosphate rock is converted into phosphoric acid and gypsum. According to the known processes this reaction period is to 12 hours.

An embodiment of the process of the invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a flow-scheme of a plant for the production Figure 2 is a modified flow scheme.

According to the flow scheme of figure 1 phosphate rock and sulphuric acid are fed into the acidulation reactor 1 through conduits 2 and 3 respectively. Through conduits 4 and 5 phosphoric acid phases from further steps of the process are recycled to reactor .

If it is advantageous in the process to adjust the temperature in the acidulation reactor 1 by cooling, preferably by evaporation cooling, e.g. by a flash cooling system or by blowing air in the liquid, it is convenient to effect this cooling where fresh sulphuric acid and the recycled phosphoric acid phases are mixed. This is illustrated in figure 2, where the phosphoric acid phases carried through conduits 4 and 5 are wholly or partly mixed with the fresh sulphuric acid and passed through a suitable cooler A before being introduced into reactor 1.

If desired, the phosphate rock may be wetted by part of these phosphoric acid phases before being introduced into reactor 1. Vigorous mixing during this wetting step is desirable .

The acidulation in reactor 1 is carried out under such conditions that phosphoric acid and calcium sulphate hemi-hydrate are formed. The effluent from reactor 1 which con-tains the phosphoric acid and calcium sulphate hemihydrate, is passed through conduit 6 to a separating apparatus 7? e.g. a filter, such as a continuous belt filter. Such filters are known to produce the first part of the filtrate as not wholly clean phosphoric acid filtrate due to entrainment of insoluble material through the filter cloth prior to cake formation thereon. This so-called "cloudy" filtrate fraction is recycled to reactor 1 through conduit 5· As a second is obtained the concentration of v/hich can be controlled by-control of the acidulation reaction. This phosphoric acid product is withdrawn from the filter through conduit 8. Then the calcium sulphate hemihydrate crystals, from which the greater part of the phosphoric acid phase has now been removed, are washed with a liquid phase partly originating from the recrystallization step in the process as described in the above, which liquid phase is added through conduit 9· The composition and temperature of this washing liquid may be adjusted by adding through conduit 14 fresh concentrated sulphuric acid to the liquid from the recrystallization step and cooling the mixture by passing it through an evaporating cooler B as shown in figure 2, before using the mixture as washing liquid on filter 7· Part of the washing liquid is withdrawn from the filter through conduit 4 and recycled to the acidulation reactor , if desired after removal of part of its water content in a flasher (A, see fig.2). The filter cake of calcium sulphate hemihydrate with adhering liquid is then transferred from filter 7 through conduit 10 to recrystallization vessel 11. In this vessel the calcium sulphate hemihydrate is re-crystallized to form calcium sulphate dihydrate (gypsum) crystals. The suitable water and sulphuric acid content in this vessel may be maintained on the desired level by recirculation of part of the liquid obtained by filtering the resulting gypsum crystals through conduits 12 and 13 and by adding fresh sulphuric acid through conduits 3, 1 and 15, if necessary. After the recrystallization has been completed, the mass containing the gypsum crystals is withdrawn from the vessel 11 through conduit 16 and fed to filter.17· Part of stream 16 may be recycled to vessel 11 to en of the filtrate is withdrawn through conduit 12. This filtrate may be recycled, if desired, to recrystallization vessel 11 through conduit 12 and/or to filter 7 through conduits 12 and 9. The phase recycled to filter 7 may be mixed with additional sulphuric acid supplied through conduits 3 and 14. The gypsum crystals are washed on the filter 17 with water added through conduit 18 and then withdrawn from the filter at 19 · The washing filtrate containing a small amount of sulphuric acid is recycled to the recrystallization vessel through conduit 13 » It may be of advantage to carry out an extra wash on filter 17 , e.g. by using the last filtrate obtained from filter 17 , which is recycled through conduit 20 as indicated in figure 2.

A continuous process which may be carried out in the apparatuses shown in fig.1 and fig.2 will now be illustrated in more detail in the following Examples without restricting the invention to the embodiments described therein. The amounts referred to in the Examples are the amounts passed through the apparatus in one hour.

Example 1 Using the apparatus shown in figure 2 1000 kg of Kourigha-phosphate rock containing 51 , 2 CaO and 33,6% ^2^5 and 467 kg of I^SO^ as 98% sulphuric acid are added continuously through conduits 2 and 3 respectively to reactor 1 , the temperature of which is maintained at about 90°C.

Through conduit 5 2300 kg of a solution containing 40% phosphoric acid, calculated as ^^ ^ > an(^ ¾S0^ are recycled, through conduit 4 1095 kg of a solution containing 3, 3% phosphoric acid, calculated as being added to reactor . The water content in the acidulation reactor is adjusted by evaporation of 166 kg reactor 1 , from which another portion of 4-1 kg of H20 evaporates, and transported to filter 7· Phosphoric acid having a P20^ content of 4-0% and a H^SO^ content of 1% is withdrawn from filter 7 through conduit 8 in an amount of 821 kg. 1180 kg of recycle liquid containing 15.0% H2S0^ and 2,19% phosphoric acid, calculated as P20 ' fed- through conduits 12 and 9 are mixed with 4-4-8 kg of HgSO^ as a 98% sulphuric acid supplied through conduit 14-, Using the mixing heat 200 kg of HgO are evaporated in B from the mixture before this is used as washing liquid through conduit 9 on filter 7· After having been washed the filter cake of the calcium sulphate hemihydrate crystals is withdrawn from the filter 7 through conduit 10 to vessel 11. It contains 1252 kg of CaS0 . H20 , 583 kg of water, 26 kg of V≥ ^ and 221 kg of H2S0^.

Through conduit 12 976 kg of a solution containing .0% ' are recycled to vessel 11, through conduit 1 3 1273 kg of a solution containing 0,4-9% phosphoric acid, calculated as P^O^, and 3.4·% H2S0^ being added. The recrystallization temperature was 70°C. Fresh sulphuric acid was not supplied. The suspension of the gypsum crystals, formed in vessel 11, from which an additional portion of 125 kg of H^O evaporates, was filtered on filter 7 and washed with 1273 kg of water. From filter 17 1564 kg of gypsum containing 0, 15% P20^ on dry basis (drying at 60°C) and 20„1% crystal water were withdrawn. The ?20^ figure is very low and completely acceptable considering the fact that the phosphoric acid produced contains 4-0% of PoOc * Example 2 Using the same continuous procedure as in example 1 hut with the use of the apparatus shown in fig. a 7% phosphoric acid is produced as follows. 1000 kg of phosphate rock containing 5 . % CaO and 53.6% P20^ and 733 kg of H2S0 as 98% sulphuric acid are fed to reactor 1 through conduits 2 and 3 respectively. 6100 kg of a solution of % Ρ≥0^ and 1,2% HgSO^ are recycled through conduit 5 and 5 4- kg of a solution of 3,5% E^O^ and 25.3% HgSO^ are recycled through conduit 4.

Through conduit 8 708 kg of a phosphoric acid of 47% P2°^ and 1,2% H2S0^ άηϋ withdrawn from filter 7. The washing liquid added to the filter through conduit 9 consists of a mixture of 182 kg of H2S0^ supplied through conduits 3 and 14 and 1435 kg of a solution of 4% P^ and 16% ^SO^ recycled from filter 17 through conduit 12 to conduit 9· Prom filter 7 a filter cake containing 1247 kg of calcium sulphate hemihydrate is withdrawn through conduit 10 into vessel 1 . This filter cake further contains 22 kg of P20^ and 278 kg of H2S0 . Through conduit 12 1435 kg of a solution of 4% P20^ and 16% H2S0 and through conduit 1 3 850 kg of a solution of 2% Γ^Ο^ and 8% H2S0^ are recycled to recrystalliz-ation vessel 1 , the temperature of which is maintained at about 70°C From vessel 11 the gypsum suspension is fed to filter 17 and washed with 890 kg of water through conduit 18. From filter 17 560 kg of gypsum containing 0,22% Ρ≥0^ on dry basis (drying at 60°C) and 20,2% crystal-water are withdrawn. The P20^ figure is very low considering the fact that the phosphoric acid produced contains 47% of P20^.

Example 3 Using the procedure and the phosphate rock described in Example 2 the following process is carried out. 1000 kg of phosphate rock and 600 kg of HgSO^ as 98% sulphuric acid are fed through conduits 2 and 3 respectively to reactor 1 the temperature of which is maintained at about 90°G. 921 kg of a solution of 7 PgO^ and 1,2% H^jSO^ are recycled through conduit 5 and 525 kg of a solution of 3"l% ^2^ and are recycled through conduit 4. From reactor 1 78 kg of water evaporate.

Through conduit 8 67-6 kg of a phosphoric acid of 7 ^2^ and 1.2% ccaiJ. withdrawn from filter 7· The washing liquid added to the filter through conduit 9 consists of a mixture of 281 kg of iL-,S0^ as 98% sulphuric acid, supplied through conduits 5 and 14, and 1259 kg of a solution of 9,2% P20^ and 6.5% E^SO^ recycled from filter 7 through conduit 12 to conduit 9· From filter 7 a filter cake of 2236 kg is withdrawn through conduit 10 into vessel 11. This filter cake consists of 134-2 kg solid substance, 588 kg R^O, 213 kg H2S0 and 93 kg H^PO^. Through conduit 2 2780 kg of a solution of 9,2% ^2^5 and 5% H^SO^ and through conduit 13 1832 kg of a solution of 5 „5% P2°5 a d 3,9% HgSO^ are recycled to recrystallization vessel 11 , the temperature of which is maintained at about 70°C. Prom vessel 11 70 kg of H2O evaporate. A gypsum suspension is fed from vessel 11 to filter 17 and washed with 1832 kg of HgO through conduit 18. From filter 17 750 kg of a filter cake containing 1650 kg of solid substance are withdrawn. The gypsum obtained contains 0.19% "by weight of PpOr on dry basis (drying at 60°C).

Example 4 Using the procedure and the phosphate rock of Example 2 the following process is carried out. 1000 kg of phosphate rock and 416 kg of R"2S0^ as 98% sulphuric acid are fed through conduits 2 and 3 respectively about 90°C. 574- kg of a solution of 4-0% P20^ and 1% H^SO^ are recycled through conduit 5 a d 1870 kg of a solution of 21% ^2^5 an<^ ¾*^ are τβ070^-β^- through conduit 4. From reactor 1 163 kg of E^O evaporate. Through conduit 8 796 kg of a phosphoric acid of 4-0% P20^ and 1% HgSO^ α¾& withdravm from filter 7· The washing liquid added to the filter through conduit 9 consists of a mixture of 466 kg iLpSO^ as a 8% sulphuric acid, supplied through conduits 3 and 14- , and 144-0 kg of a solution of 3 ,5% P2°5 and 21 .4% H2S0 recycled from filter 17 to conduit 9. From filter 7 a filter cake of 2284 kg is withdrawn through conduit 10 into vessel 11 . This filter cake contains 1370 kg of solid substances, 502 kg of H20 , 378 kg of H2S0 and 34 kg of H^PO^. Through conduit 12 25 8 kg of a solution of 3.5% P20^ and 21 .4% HgSO^ and through conduit 1 3 1960 kg of a solution of 12% Η230^ and ,95%?20^ are recycled to re-crystallization vessel 11 , the temperature of which is maintained at about 70°C. From this recrystallizationr' vessel 95 kg of H20 evaporate. A gypsum suspension is fed from vessel 11 to filter 17 and washed with 1960 kg of water through conduit 18 . From filter 7 a filter cake of 2750 kg containing 1650 kg of solid substances is withdrawn. The gypsum contains 0 #14% by weight of ?20^ on dry basis (drying at 60°C ) .

Claims (3)

1. HAVING HOW particularly described and ascertained th$ nature of our said invention and in what manner the same is to be performed, we declare that what we claim is 1. A continuous process for the preparation of phosphoric acid and gypsum by acidulating phosphate rock with sulphuric acid or a mixture of phosphoric acid and sulphuric acid to form a slurry of calcium sulphate hemihydrate crystals in a concentrated phosphoric acid solution, separating the hemihydrate crystals from said phosphoric acid solution and withdrawing said phosphoric acid solution and converting the calcium sulphate hemihydrate crystals into calcium sulphate dihydrate (gypsum) crystals, c h a ra c t e r i z e d b y washing the calcium sulphate hemihydrate crystals after the separation of the concentrated phosphoric acid solution with an aqueous washing liquid in order to remove at least part of the phosphoric t ^ jj acid regained in and adhering to the mass of the hemihydrate crystals, the composition and quantity of said washing liquid and the temperature being selected so as to prevent completely or substantially the transformation of the hemihydrate into ai/other crystal form and to make the liquid remaining in and adhering to the mass of hemihydrate crystals after washing a suitable component for the recrystallization solution to which said mass is transferred, and by re- crystallizing the washed calcium sulphate hemihydrate crystals to- form gypsum crystals in an aqueous solution containing from 0 to 20% by weight of phosphoric acid, calculated as and separating e gypsum crys a s o a ne rom t e re- crystallization solution.
2. 2. A process according to claim 1, in which the aqueous washing liquid has a water vapour pressure which prevents transformation of the hemihydrate into ai/other 3o A process according to claims 1 and 2, in which as the washing liquid part of the mother liquor is used, which has been obtained by separating the gypsum from the recrystallization solution. . A process according to claims 2 and 3, in which the water vapour pressure of the washing liquid is adjusted by the addition of concentrated sulfuric acid. 5= process according to anyone of the claims 1 to , in which the liquor obtained by washing the hemihydrate is recycled into the acidulation reactor. 6. A process according to anyone of claims 1 to 5, in which the aqueous recrystallization solution contains from 5 to 10% by weight of phosphoric acid, calculated as P20^, and from 5 to 15% y weight of H2S0^. 7. A process according to anyone of the claims 1 to 6, in which gypsum crystals, which have been prepared previously, are added to the recrystallization solution as seed crystals . 8. A process according to claim 7? in which the added gypsum crystals are prepared by separate recrystallization of part of the hemihydrate. 9. A process according to anyone of the claims 1 to 8, in which the formation of the gypsum crystals in the recrystallization solution is promoted by recirculating to the recrystallization solution part of the gypsum crystals obtained by an earlier recrystallization. 10. A process according to anyone of the claims 1 to 9, in which the recrystallization temperature is kept below 90°C. 11. A process according to anyone of the claims 1 to 10, in which the recrystallization temperature is gradually 12. A process according to anyone of the claims 1 to 11, in which the gypsum crystals formed are washed counter-currently with water and the resulting washing liquid is recycled into the recrystallization solution.
3. 3. A process according to anyone of the claims 1 to 12, in which the sulphuric acid needed for the reaction with the phosphate rock, is partly introduced into the acidulation reactor and partly into the washing liquid for the hemihydrate crystals, the balance, if necessary, being introduced into the recrystallization solution. 14-. A process according to claim 13, in which hot concentrated sulphuric acid is used. 15· A process according to anyone of the claims 1 to 4-, in which the temperature in the acidulation reaction and/or during the washing of the calcium sulphate hemihydrate crystals is controlled by mixing fresh concentrated sulphuric acid and an aqueous solution obtained at a subsequent step of the process and recycled and by evaporating water. 16. A continuous process for the preparation of phosphoric acid and gypsum substantially as hereinbefore described ■with special reference to the Examples given. 17· Phosphoric acid, obtained by the process according to anyone of the claims 1 to 16, containing more than 30% by weight of a (3- less "than 1.5% "by weight of sulphuric acid. 18. Gypsum obtained by the process according to anyone of the claims 1 to 16, having a phosphate content below 0^3 by weight, calculated as P20^, and a fluorine-content below 0,2% by weight.