IL33681A - Process and apparatus for the preparation of crystalline precipitates by reaction of solutions - Google Patents

Description

33681/2 mo«an *fv nann Process and apparatus or the preparation of crystalline precipitates by reaction of solutions VER IGDE JOJliSTMESTPABRIE ElJ MEKOG-ALBATROS W.V.

C: 31905 The invention has the object to provide a process in which liquids are mixed so as to form a single liquid phase, whereby a crystalline precipitate is produced from ions originally dissolved in at least one of the feed liquids, and in which the precipitate is classified into coarser and finer crystal fractions and possibly wasted.

The invention is particularly useful in the conversion of crude rock phosphate into phosphoric acid and a calcium sulphate hydrate but it is not confined to this use.

According to the invention, two liquids which are miscible with each other and of which at least one contains precipitatable ions, are separately fed to a rotating disc contactor (as herein defined, HDC for short), mixed in the and latter to form a single liquid phase/at the same time to produce a crystalline precipitate;, an upward flow of liquid is maintained in the EDO and made to contain the finer portions of the precipitate; mother liquor is discharged from the top end of the RDCj and the coarser fractions of the precipitate are allowed to settle in the RDC and discharged from the bottom part of the latter.

An HDC, which is know in principle, essentially consists of an upright cylindrical vessel whose inner ©pace is divided into compartments by fixed annular baffles and which comprises a stirring device including horizontal vanes or discs, referred to as rotor discs, mounted on a vertical shaft which extends in the axis of the vessel without contact with the baffles. Usually one rotor disc is located in every compartment at about half the height thereof. For constructional reasons it is preferred that the diameter of the The invention, however, also provides new HDC arrangements particularly suitable for carrying out the process according to the invention.

The British patent specification Ho. 784,520 describes a method and an apparatus for the separation and purification of crystals. The apparatus comprises an upright elongated column divided into three principal sections, namely, a scraped-surface chiller, a filter section and a crystal purification column. A shaft is axially disposed in the upright elongated column and carries a plurality of scraper blades within the chiller and filter section, and radial stirring rods or blades within the purification column from whose wall a number of fixed rods or blades extend inwardly. These fixed rods are located between the stirring rods and prevent the crystal mass from being entrained in rotation by the stirring rods. In the method described in that specification a liquid mixture is chilled to form crystals and the latter move downwards and are heated to melting at the bottom of the apparatus. A part of the pure liquid formed by the melting crystals is drawn off at the bottom and the remainder ascends in the apparatus in counter-current to the descending crystals. As described in the British specification aforesaid this method does not serve for the formation of crystals by the interaction of ions, nor does the apparatus there described comprise a system of fixed annular baffles and rotor discs, which is typical for the RDC.

The attached drawings illustrate, in diagrammatic axial sections, several RDC arrangements suitable for use in the process according to the invention.

Figure 1 shows an RDC in whioh a crystalline precipitate is to be produced. The RBG comprises an upright cylindrical vessel with conical bottom, fixed annular baffles and a stirring device. The latter comprises horizontal discs 3 ("rotor discs") fixed to a rotatable shaft disposed in the 3¾is of the vessel. The feed liquids, which are to be mixed in the REG, are introduced through orifices 4 in the wall of the vessel- It may be advantageous to divide the supply of the liquids over several orifices provided at various heights of the vessel* The liquids mix within the vessel and the single liquid phase thus formed rises in the vessel. The velocity of the ascending current can be controlled by regulation of the feed of the liquids, whereby the time of dwell of the mixture in the RDQ can be adapted to the speed of formation and growth of the crystals*]: The small nascent crystals are entrained upwards by the ascending current* As they move upwards they grow and follow the ascending current less n¾ less, until they finally sink in counter-current to the ascending liquid.and settle on the bottom of the vessel from which the precipitate can be removed through an orifice The feed liquids introduced into the RDC are usually aqueous solutions of electrolytes. By one embodiment of the invention, the feed solutions contain different ions capable of reacting with each other when the liquids are mixed. By another embodiment of the invention, only one of the feed liquids is a solution of an electrolyte while another feed liquid is different from the solvent of that solution but miscibl solubility of the solute which is thereby precipitated at least partly.

The mother liquor leaves the RDC at the top through an overflow 4a. It may be used wholly or in part for dissolving further portions of the electrolyte, or one of the electrolytes used as starting material, and the solution thus made up is re-introduced Into the RDC at a lower level.

For the separation of the iner crystal fractions carried to the top of the RDC by the ascending liquid, the liquid discharged at the top of the RDC may be passed into a chamber in which the liquid rises more slowly than in the RDC and vrtiere the liquid is not , or almost no , stirred.

This chamber may be disposed on top of the upright RDC vessel and have a larger diameter than the latter. The velocity of the ascending current in that chamber can also be decreased, instead of or in addition to making the chamber wider than the RDC vessel, by removing part of the mother liquor at the top of the RDC through a side line and feeding only the remainder of the liquor from the RDC into the chamber. It is advantageous to make up the whole or part of the liquid removed through the side line with more electrolyte starting material and to recycle the solution into the RDC. The finer crystals remaining in the liquid from the preceding cycle can act as seed crystals in the RDC.

According to Figure 2, a chamber 6 having neither fixed annular baffles nor rotor discs is disposed on top of the RDC vessel 1* From the lower part of chamber 6 a part of the mother liquor, in which fine crystals are still present, have the same diameter the ascending current is slower in chamber 6 than in vessel 1 owing to the removal of some liquid through line 7· Mother liquor which is free or nearly free from crystals* is removed at the top of chamber 6, e.g. through an overflow channel 8 with discharge line 9· Φηβ liquid removed through line 7 can be made up in a vessel 10 with further portions of starting material and the solution is recycled to vessel 1 through line 11 with the aid of a pump 12. Instead of, or in addition to, the liquid discharged through lias 7» the liquid discharged through line 9 may thus be made up. Fresh feed solution is introduced into the vessel 1 through lines 13 for reaction with the recycle feed.

If the starting material comprises two substances, for example two salts which have to be dissolved separately and whose solutions are ed separately into the RDC, an apparatus as shown in Figure 3 may be used. From the chamber 6 on top of the RDC vessel 1, liquid is discharged through line 7 to vessel 10, and concurrently through line 14 to a vessel 15· In the vessels 10 and 15, respectively, the two different starting materials are dissolved, and the solutions are recycled through line 11 with, pump 12, and through line 16 with pump 17, respectively.

The velocity of the ascending current in RDC 1 can be regulated by means of the 'recycle pumps (12 in Figure 2 and 12 and 17 in, Figure 3) * In this wa the crystal content of the liquid in the R20 vessel 1 can be maintained within a suitable range*, , The presence of a high crystal content lowers the super-saturation of the dissolved matter which is about to crystals predominates strongly over the formation of nascent crystals.

The size of the crystals which are removed is mostly between 0,05 and 0.5 mm. The crystal content of the liquid in vessel 1 is usually between 5 and 20 vol. . At a rate of settling of 1 to cm/sec. an RDC of 1 m diameter may have an output of the order of magnitude of 10,000 to 20,000 kg of crystals per hour.

The coarsely crystalline product which is removed from the bottom of the RDC vessel 1 may be separated from adhering mother lye in a seoond RDO. The second RDC may be connected with the first one by a side pipe but it is preferred to arrange the second RDC underneath the first one and to combine the two RDG's i one apparatus. In that case the rotor discs of both EDO's can be mounted on one single shaft. They may, however, be mounted on separate shafts so that the speed of rotation in either RDO may be adapted separately as conditions require. Both shafts can be mounted coaxially. The velocities of the ascendin current need not be equal in both RDG's. In the second RDC, for example, the velocity may be much lower than in the first one. I the RDC in which the ascending current is intended to classify the crystals a higher velocity is required than in the RDC in which the crystals are to be ashed by means of the ascending current. The diameters of both RDC's need not be equal either. For concentrating and effectively washing the crystalline product the seoond RDC has preferably a smaller diameter than the first RDC.

A second RDC which is used fo washing the crystals is of the RDC 18 a discharge orifice 20 is provided. Washing liquid can he introduced throug a pipe 21 connected to KDC 18 near its bottom.

As stated above, one of the most important applications of the invention is in the conversion of crude phosphates with sulphuric acid into phosphoric acid and gypsum.

As is known, this conversion is effected a temperatures of about 90 - 110°C in order to produce phosphoric acid of high concentration} the calcium sulphate is obtained in that case as hemihydrate. It is also known that this hemihydrate can be more readily filtered off if the crude phosphate is first dissolved in phosphoric acid produced in a preceding cycle and sulphuric acid is added subsequently. The hemihydrate obtained under such conditions still contains ?2°5» which is removed by recrystalllzation of the hemihydrate to dihydrate.

In accordance with the Invention the conversion of crude phosphates with sulphuric acid can be carried out in the apparatus represented in Figures 2 or 4. In the RDC 1 a solution containing calcium and phosphate Ions is introduced through line 11 by means of pump 12. 2hrough pipes 13 sulphuric acid is introduced. In RDC 1, some calcium sulphate hemihydrate is always present, and out of the feed solutions, more calcium and sulphate ions combine to form fresh hemihydrate which crystallizes on the existing crystal surfaces, so that the crystals grow. Fine crystals of hemihydrate are initially carried upwards by the ascending current , but in the course of this movement many of these crystals grow sufficiently and start to sink. The most favourable conditions are obtained at a solid content of the li uid in RDC w therein owing to the fact that the velocity of the ascendin liquid in that chamber is smaller. At 8 a clear solution of phosphoric acid is discharged, This phosphoric acid may serve for dissolving crude phosphate, but preferably the phosphoric acid which leaves the RDC through line 7 and still contains fine crystals of hemihydrate is used for this purpose. The dissolution of crude phosphate in the phosphoric acid Is carried out in vessel 10 which is provided with a stirrin device. Suitable temperatures for this dissolving process are between 80 and 115°C, particularly between 95 and 110°C. Th circulating amount of phosphoric acid must be sufficient for the dissolution of the entire feed of crude phosphate.

The relevant phase diagrams show that phosphoric acid of 30$ PgO^ dissolves 5 of CaO, phosphoric acid of 40$ PgO^ dissolves about 4$ of CaO, and phosphoric acid of 50 dissolves about 3 of CaO. The solution obtained, which contains calcium ions and phosphate ions and also fine crystals of hemihydrate, is transported to the KPC through line 11 by means of pump 12. The recirculated fine crystals of hemihydrate act as seed crystals in the BBC, The crystals leaving RDC 1 at 5 are coarse and uniform in size and also very pure. This product can easily be washed and concentrated.

The washing of the hemihydrate may be carried out in the second RDC 18 in an ascending current of water or dilute sulphuric acid. Filtration of the hemihydrate, which in known processes is a cumbersome intermediate step, is not required in the present process. Taking into account the composition of the liquid the temperature in BBC 18 is By reerystallization of hemihydrate to dihydrate in a subsequent step the product can- be freed from P2°5» recrystallization is carried out in a solution preferably containing a considerable amount of free sulphuric acid and usually also phosphoric acid (for example 2 to 25 by weight of HgS0^ and 0 to 20$ by weight of ^2°5^' 18 ^10^ tna 1x1 this recrystalllzation process the temperature should be below a limit which depends on the composition of the liquid* As a rule the limit is below 80^0.

The dihydrate which is prepared from the hemihydrate obtained by the process according to the present invention crystallizes in a medium which is rather pure. The absence of impurities makes for the formation of coarse crystals ' which can readily be filtered and washed. The mother liquor remaining after the reorystallization of hemihydrate to dihydrate is advantageously introduced into the bottom part of the second RDC and serves as a washing liquid. If desired, more sulphuric acid may be added in high concentration to this mother liquor in order to raise the temperature in the second HDD. This may be important in order to prevent premature crystallizatio of dihydrate in the pipe.

The process described yields phosphoric acids of any desired concentration up. to 55$ by weight of PgO^ without requiring the evaporation of water. The gypsum obtained can be used wet, without further purification, for the preparation of high-quality plaster of Paris. On the other hand it may serve for conversion into ammonium sulphate and calcium carbonate.

After drying and calcining it may be used as admixture to cement and for the preparation of cement clinker and sulphuric acid. - u A second very advantageous use of the invention is in the preparation of sodium silicofluoride according to the equation: HgSiPg + 2KaGl—-δ NagSiPg ♦ 2HC1 JJaCl is dissolved in vessel 10 in mother liquor previously obtained in this process (Figures 2 and 4) and reacts wit H2SiFg in RDC 1 in which the BgSlg is. introduced throug lines 13. I!iagSi crystallis.es in RDC 1 and is discharged from RDC 1 through orifice 5 (Figure 2) or through the taperliig tube 19 (Figure 4)· Mother liquor still containing fine crystals of NagSiFg flows through line 7 to vessel 10. Dilute hydrochloric acid, which contains any excess of HaO used, is discharged at 9· I may be desired to dilute the solution of HgSiFg with water before it is introduced into RDC 1· This may be done by adding this solution to circulating mother liquor in vessel 15 (Figure 3), or by mixing this solution with the washin liquid in the, washing vessel 18 (Figure 4) at about half the height of the latter. In the latter case the diameter of vesse 18 may be larger above the point; of introduction than below, it. This arrangement is shown in more detail in Figure 7 to t*hich Example II herein further relates.

K^Sl g may be prepared in an analogous way.

As a third application of the invention, its use in the so-called Somet-procesa ma be mentioned. (Sondershausen methanol process, Ή. Hoppe, A new method for th© production of chlorine-free potash fertilizers", Proc. of the XVIIth International Congress "Chemistry Days 1966" on Chemical Fertilize Milan, pp. 121-132). In this process potassium sulphate is 2 MgS04 + 2KC1 + 6H20 —3» %G12 + K2304.HgS04.6K2Q K2S04.I%S04#6H20 + 2Κ01-~->ί¾5θ12 ♦ 2E2S04 ♦ 6H20 In the last step methanol is added to the solution to decrease the solubility of K2S04. Instead of methanol, ethanol, dioxane, acetone and other organic liquids may be used.

According to the present invention the apparatus illustrated i Figure ~4 may be used in either or both steps of the Somet process. She performance of both steps in such re apparatus is schematicall /presented in Figure 5· The first step proceeds in RDC 51 from the top of which the mother liquor is discharged through lines 57 and 64· Line 57 leads to vessel 60 in which KCl is dissolved. The solution prepared in vessel 60 is introduced into EDO 51 through line 61 by means of pump 62, Line 64 leads to vessel 65 in which MgS04.6H20 is dissolved. This solution Is fed to RDC 51 through line 66 by means of pump 67. From RDG 51» crystalline schoenite (K2S04.M304.6H20> sinks into RDC 68 and is washed therein with a mother liquor containing an organic liquid, e.gt methanol.¾ This mother liquor is obtained in the second step of the process and is introduced into RDC 68 through line 71· A sohoenite suspension is fed from RBC 68 through line 70 to vessel 90 into which also KCl and (through line 87) a mother liquor discharged from the top of RBC 81 are introduced. The solution prepared in vessel 90 at an elevated temperature (for example, s dr"- 90 C) is fed through line 91 by means of pump 92 to the bottom part of RDC 81, into which also the organic result of the presence of methanol KgSO^ crystallizes and sinks down to RDC 98 In counter-current to ascending washing water introduced through line 101. Κ23Θ4 is removed as a product through line 100· On top of RDC 81 a chamber 86 is provided whic has no annular baffles nor rotor discs and in which fine crystals of KgSO^ can sink. From chamber 86» mother liquor overflows into line 71 which leads to RDC 68. Similarly there is a chambor 56 on top of RDC 51 without annular baffles and rotor discs, in which fine crystals of schoenite can sink* from chamber 56, mother liquor containing methanol overflows into line 59 which leads to a distillation column 75· From the latter, an aqueous mother liquor free from methanol and containing mainly Kl Clg is discharged through line 76.

Concentrated methanol (for example 85 - 90$ by weight) is discharged from column 75 through line 93 which leads to RDC 81.

It is eviden that by the applicatio of the invention in this conversion process a considerable simplification is achieved, since various filters and storing tanks are dispensed with.

A fourth application of the invention is in another known process , also for the preparation of ¾S0^ (L. Cavalli and M. ifeggiore, Industrial and technical aspects of the recovery of potash by means of syngenite, Proceedings of the XVIIth International Congress "Chemistry Days 1966" on Chemical Fertilizers , Mian, pp. 171 - 177). In this process kainite is first converted into schoenite by treating it with a mother liquor obtained elsewhere in the process* The schoenite is separated and extracted with a warm solution of K2S04 in wa¾er» whereby K2SO^ is made to crystallize from the mother liquor just mentioned. The latter, after cooling, is contacted with kainite. After being separated from the schoenite the liquid still contains about 3($ of the potassium present in the initial kainite. In order to recover this potassium,gypsum (dihydrate) is added which causes syngenite to crystallize.

Prom the separated syngenlte a solution of KgSO^ can be obtained by extraction with hot water, and this solution serves for the extraction of the schoenite. The various steps of this process, including the extraction and the washing of crystals, can advantageously be carried out in accordance with the present invention* As further examples of processes in which a separation 0 is obtained by crystallization and the invention can advantageously be applied, there may be mentioned: - The manufacture of pigments with aftertreatment of precipitates.

- Preparation of (MH^)2S0^ from gypsum, ΚΒ^' and C02* 1x1 which C&COj is wanted in coarsely crystalline condition for being easily washed out and separated off.

- The preparation of mono-ammonium phosphate from crude phosphoric acid and ammonia, in which process often hardly filterable and washable precipitates of complex iron and aluminium phosphates are produced. The EDO is very suitable for agglomerating such precipitates as a result of whioh they assume a sufficient velocity of settling, so that they can be washed in countercurrent. The washing phosphate and sodium polyphosphate) from the ammonium phosphate# - Preparation of iri which SiOg precipitates· - - The invention also consists in the preferred apparatus described above which comprises two upright RDC's of different diameter, the wider one being arranged vertically above the narrower one in open connection with the latterj a chamber without annular baffles and rotor discs disposed on top of the wider EDO in open connection therewith; and orifices for the introduction of feed liquids and the discharge of process products provided at or near the bottom of each RDC, at the top of the wider RDC and at the top of the chamber on top of the wide RDC. The internal diameter of the wider RDC may be, for example, between 2 and 5 times the internal diameter of the narrower RDC. The inventio also concerns the circulation lines connecting top and bottom of the wider RDC and including vessels with stirring devices for the preparation of solutions of starting materials. The rotor discs of both RDC* 8 usually can be mounted upon the same shaft. A third RDC, still narrower in diameter than the narrower RDC of the aforesaid combination, may be arranged vertically beneath the latter RDC and in open connection therewith. The shaft on which the rotor discs are mounted may extend through all three RDC,s. Orifices for supply and withdrawal of substances should be disposed at the bottom of the third RDC as. well. 33681/2 The process according to the invention is illustrated by the following examplesi Example I Conversion of Kola phosphate with sulphuric acid into phosphoric acid and calcium sulphate hemlhydrate kg per hour of Kola phosphate containing 50$ by weight of CaO and 35$ by weight of gO^ is reacted' with 17i kg HgSO^ in an apparatus as shown in Figure 6, which is similar to the apparatus of Figure 4, except that RDC 18 is connected at the bottom with a thickener 22, i.e. a chambe in which the suspended hemlhydrate concentrates. The concentrated suspension (30$ by weight of hemlhydrate, 70$ by weight of liquid) leaves the thickener 22 through line 23 towards an apparatus (not shown) in which the hemlhydrate is recrystallized to dihydrate. From the last-mentioned apparatus a stream of liquid is recycled to the thickener 22 through line 24· Water, sulphuric acid and phosphoric acid in quantities equal to those that are discharged with the suspension through line 23» are re-introduced through line 24 into thickener 22* Also an extra quantity of water is introduced into thickener 22 which is equal to the consumption of water. The water consumed is the quantity of water leaving the process as a diluting component of the phosphoric acid and as crystal water in the hemlhydrate, minus the quantity of water introduced into the process with the sulphuric acid (with 98$ sulphuric acidj 2> water). The extra quantity of water Introduced through line 24 may be used previously for washing the dihydrate after this has been separated by A part of the sulphuric acid required for the conversion may be fed through line 24, the remainder i© supplied through lines 13# The table below gives data of the preparation of phosphoric acid of two different concentrations, and either pure or containing some sulphuric acid.

In each case the liquid at the bottom of RDO 18 contained 10$ by weight o The temperature in vessel in RDC 18 on the average 85°Q TABLE I P2O5 In phosphoric acid produced ) jfert calculated on R- O^+HgO 4 ¾S0 in phosphoric acid produced ) Consumption of water, kg/h Quantity of HgSO^ supplied through lines 13, kg h 1 Quantity of ¾S04 supplied through line 24, kgh ; Quantity of CaO that dissolves in acid in vessel 10 together with quantity of CaO that is converted into CaS04 in vessel 10, calculated in jCwt on the acid.

Quantity of acid to vessel 10, kg/h 25 CaO, dissolved in vessel 10, kgh 1 CaO, precipitated as hemihydrate in vessel 10, kg/h "Dimensions of the apparatus • (length without space 6, cm 7 (diameter column, internally, cm 2 RDC 1 (number of compartments (diameter : "baffles. » internally, cm 2 (diameter rotor discs, cm 1 (length, cm 2 (diameter of column, internally, cm 1 RDC l8 (number of compartments (diameter ' ~¼affles^.:-, internally, cm (diameter rotor discs, cm Height of space 6, cm 3 Stirring speed, rotations per minute 20 Relative proportion of velocities of flow :RDC 1/RDC l8 Example II Preparation of sodium slllcofluorlde (150 kg/h) The apparatus used Is shown in Figure 7· It comprises an RDC 111 in which HgSiPg is reacted with NaCl* The precipitated Ha2SiF6 si*ks from RDC 111 successively through HBO 112 and RDC 113 in which the is washed* Through line 114· 460 kg of a 25$ by-weight aqueous solution of HgSiFg is supplied to the bottom part of RDC 112. 360 kg/hour of washing water are introduced at the bottom of RDC 113 through line 115· The KagSi g produced is discharged through line 116· A part of the mother liquor ascends from RDC 111 into chamber 117 which has no rotor discs and annular baffles and in which the fine orystals entrained with the mother liquor can sink down* Bother liquor free from crystals is discharged through line 118 (composition: 345 kg of water from the solution of HgSiFg, 360 kg of washing water, 58 kg HCl, 16 kg of NaCl, together 779 kg, specific weight 1*15).

Mother liquor (1530 litre per hour) which still contains fine crystalline NagSl g leaves RDC 111 at the top through line 119 to the stirred vessel 120 which is provided with a stirrer and in which also 110 kg/hour of NaCl is dissolved. It leaves vessel 120 at the bottom and is fed through line 121 by means of pump 122 to the bottom of RDC 111* The concentrated solution of HgSiFg in this example is diluted with washing water in order to avoid high concentrations of the SiFg-lonj to this end the solution is introduced into the apparatus at a point at which the concentration of sodium ions has decreased already so far that gABLE II Dimensions (cm) RDC 111 EDO 112 RDC 113 Vessel 117 Diameter of column (Internally) 90 40 25 90 Diameter of annular baffles (internally) 63 28 17.5 Diameter of rotor discs 40 20 12 Length of column 140 87.5 70 50 Number of compartments 7 7 7 -· 33681/2 \

Claims (1)

1. CLAIMS 1. A process for the preparation of chemical products, wherein two liqaids which are misclble with each other and of which at least one contains preoipitatable ions, are. separately fed to a rotating disc contactor (as herein defined, RDO for' and short), mixed in the latter to form a single liquid phase/at the same time to produce a crystalline precipitate; an upward flow of liquid is maintained in the RDC and made to contain the finer portions of the precipitate j_ mother liquor is discharged from the top end of the RDC; and the coarser fractions of the precipitate are allowed to settle in the RDC and discharged from the bottom part of the latter, 2· A process according to Claim 1, wherein the mother liquor discharged from the to end of the RDC ie made to flow upwards at' a lesser velocity of flow than in the RDC, through an upright chamber disposed on top of the RDC for allowing finer crystals, e¾t-¾ined by the mother liquor, to sink. 3· A process according to Claims 1 or 2, wherein mother liquor discharged from the RDC or the chamber on top of.it is used, wholl or in part* for dissolving further portions of starting materials. 4. A process according to Claim 3, wherein the mother liquor ie used for tfee dissolving startin material is one that has not been freed from finer fractions of the crystalline precipitate. 5. A process according to any one of Claims l to 4i wherein the crystalline precipitate discharged from the bottom part of the RDO is washed in a second RDC with an upward current of liquid. 33681/2 -r 22 - 6. A process according to Claim 5» wherein the washing operation takes place in a second HDO arranged below the first RBC in open connection with it. 7. A process according to Claims 5 or 6, wherein the velocity of the upward flow of washing liquid In the second j wherein one of the liquids fed to the RDC is sulphuric ac d, s · ' ■< \ and the other one a solution which contains calcium and phosphate ions and is produced by reaction of crude calcium phosphate with phosphoric acid. 10. A process according to Claim 9t wherei the mothe liquor discharged from the top end of the RDC and still containing ine crystals of gypsum is used for the dissolution of crude calcium phosphate and this solution is fed into the bottom part of the RDC.: 11. A process accordin to any one of Claims 1 to 8, wherein one of the liquids fed to the RDC is a solution of an . alkali metal salt, and the second one a solution of HgSiFg. 12. A process according to any one of Claims 1 to 8 for the production of potassium sulphate f om magnesium sulphate and potassium chloride, wherein: a. i a first stage, a solution of magnesium sulfate in water or in first-stage recycle mother liquor, and a solution of potassium chloride in water or in first-stage recycle mother and a second stage mother liquor liquo/are separatel fed to a first RDC, the mother liquor first-stage recycle mother liquor, and the first- stag© crystalline precipitate of the double salt K2S04.MgS04.6H20 is washed with a washing liquor comprising second-stage recycle mother liquor containing an organic liquid miscible with waterj and b, in a second-stage the said first-stage precipitate is admixed at an elevated temperature with KC1 and water and/or second-stage recycle mother liquor and the mixture as well as an organic liquid arc separately fed to a second-stage EDO, a second-^tage crystalline precipitate of KgSO^ is withdrawn from this RDC and, if desired, washed ; and the mother liquor produced in the second- stage RDC is used for washing the first-stage precipitate and admixture with the solutions in the first-, stage and admixture ¾rith KC1 in the second stage. 13. An apparatus for the preparation of crystalline precipitates, comprising a first RDC and a second RDC disposed verticall above the irst one in open connection with it and having a larger diameter than the first onei a chamber without annular baffles and rotor discs disposed on to the second RDC in open connection therewith» and openings for feeding and withdrawing substances provided in bottom parts of both RDC's, at the top of the wider RDC and at the top of the said chamber. 14. An apparatus according to Claim 13, comprising one or more circulation lines connecting the top of the upper RDC with the bottom of this RDC, and vessels provided with stirring devices disposed in those circulation lines. 33681/2 , - 24 - 15. An apparatus according to Claims 13 or 14» wherein the rotor discs of both RDC's are mounted on the same shaft. 16. Apparatus according to any one of Claims 13 to 15* wherein the inner diameter of the wider RDC is from 2 to 5 times the inner diameter of the narrower RDC. 17. An apparatus according to any one of Claims 13 to 16, wherein beneath the lower RDC a third RDC, of still smailer diameter than the first RDC, is disposed in open connection with the latter and has feeding and withdrawal openings in its bottom part. For the Applicants PARTNERS