WO1998047814A1

WO1998047814A1 - Solution crystallization process for the production of incongruently-soluble acid phosphates by incorporating a phosphate salt solution wash

Info

Publication number: WO1998047814A1
Application number: PCT/US1998/008140
Authority: WO
Inventors: David R. Gard
Original assignee: Solutia Inc.
Priority date: 1997-04-23
Filing date: 1998-04-23
Publication date: 1998-10-29
Also published as: AU7149398A

Abstract

A novel method for producing incongruently-soluble alkali metal acid phosphates in a solution crystallization process by incorporating a phosphate wash solution. The phosphate wash solution improves product purity by removing entrained free acid from the surface of the incrongruently-soluble phosphate crystals. Thus, the wash improves the physical propterties of the crystals, such as the hygroscopicity and handling characteristics. The phosphate wash solution significantly reduces the amount of crystals dissolved during the washing step and therefore results in significantly higher product recoveries when compared to conventional crystal washing techniques. Additionally, a solution crystallization method is disclosed for producing mixed cation, incongruently-soluble phosphate crystals. The process utilizes a reaction mixture having an excess of the theoretical molar amount of phosphorus required for the incongruently soluble phosphate salt. The entrained free acid from the excess amount of phosphorous is then washed from the crystal surface utilizing a phosphate solution.

Description

SOLUTION CRYSTALLIZATION PROCESS FOR THE PRODUCTION OF

INCONGRUENTLY-SOLUBLE ACID PHOSPHATES BY INCORPORATING

A PHOSPHATE SALT SOLUTION WASH

This invention relates to a method for producing incongruently-soluble acid phosphates. More particularly, this invention relates to a method for producing incongruently-soluble acid phosphates in a solution crystallization process by incorporating a phosphate solution wash to improve the recovery, the purity, and physical properties of the resulting finished product.

BACKGROUND OF THE INVENTION

With respect to their solubility in aqueous systems, salts may be classified as being either congruently-soluble or incongruently-soluble. A congruently-soluble salt dissolves in water to yield a solution which contains the ions present in the same proportion as in the salt. An example is a solution of sodium chloride, NaCI, wherein the mole ratio Na:Cl = 1 :1 at all proportions of water to NaCI, as in solid NaCI. A saturated solution of a congruently-soluble salt is in equilibrium with the salt itself. A congruently-soluble salt can be identified in an aqueous phase diagram by the intersection of the salt-water line with the two-phase envelope of the salt. Another example of a congruently-soluble salt is monopotassium phosphate, KH₂PO₄.

In contrast, an incongruently-soluble salt undergoes a disproportionation reaction as it dissolves. An example is the acidulant hemipotassium phosphate (HKP), expressed as KH₅(PO₄). Up to the proportion of water to HKP wherein all solids are dissolved, the system at equilibrium consists of a solution (more acidic than HKP) in equilibrium with solid monopotassium phosphate, KH₂PO₄. Some undissolved HKP may also be present. Dissolution is thereby accompanied by the following reaction shown schematically as: KH₅(PO₄)₂- — >H₃PO₄ + KH₂PO₄

The proportion of ions in a saturated solution of an incongruently- soluble salt is of different proportion than that of the salt itself. An incongruently- soluble salt can be identified in an aqueous phase diagram by the absence of an intersection of the salt-water line with the two-phase envelope of the salt. The saltwater line for the incongruently-soluble salt intersects with the two-phase envelope of another salt. Other examples of incongruently-soluble salts are the leavening agents sodium aluminum phosphates (SALP), expressed as Na3Al₂H₁₅(PO₄)₈ and monocalcium phosphate monohydrate, expressed as Ca(H₂PO₄)₂*H₂O. These compounds likewise dissociate in water to yield an acidic solution and more basic solid compounds.

One method of producing incongruently-soluble phosphates is crystallization from solution. An incongruently-soluble salt cannot be prepared by crystallization from a solution having the same ionic ratio as the salt itself. Thus, an incongruently-soluble salt is necessarily prepared by crystallization from a solution having a different ionic ratio as the salt. The ratio of ions and the concentration are adjusted such that the overall composition is supersaturated with respect to the desired salt. The compositions yielding incongruently-soluble salts by crystallization from solutions can be determined from a phase diagram of the appropriate system. A slurry is thereby produced wherein the solid phase is the crystallized phosphate salt. The ion ratio and the concentration required to crystallize an incongruently-soluble salt can be determined from an aqueous phase diagram representing the salt. The ion source may be (but is not limited to) phosphoric acid and the appropriate base or bases. Supersaturation may be achieved through cooling, evaporation of water, etc. The phosphate is recovered from the slurry through a solid-liquid separation technique such as centrifugation or filtration. The separation process generally leaves an amount of mother liquor entrained with the crystals. The entrained mother liquor, having a composition different than that of the product salt, may adversely impact the chemical and physical properties of the phosphate. The mother liquor for salts such as HKP and SALP contain phosphoric acid (free acid). The presence of phosphoric acid can adversely impact the purity and the physical properties of the product crystals, including the hygroscopicity, handling characteristics, and fiowability. It is therefore necessary to remove as much of the entrained mother liquor containing free phosphoric acid as possible from the crystals. The mother liquor is generally removed from the solids by washing. Drying the crystals alone is insufficient for the removal of free acid.

Conventional washing techniques to remove the mother liquor from the solids include the use of either water, a saturated solution, or a phosphoric-miscible organic solvent.

Washing the solids with water during or immediately after the separation step may result in a disproportionation reaction of incongruently-soluble solids with the water. The product crystals may therefore be contaminated with phosphoric acid or more basic phosphate compounds. Water washing may not sufficiently reduce the phosphoric acid content (free acid) because of the reaction of the incongruently-soluble salt with the water wash to generate additional phosphoric acid. In addition, if the salt is highly soluble, water washing of the separated salts results in low recoveries of the solids because of the dissolution of the salt.

U.S. Patent No. 4,329,326 discloses the use of a saturated monoammonium phosphate solution for washing monoammonium phosphate crystals. The monoammonium phosphate is a congruently-soluble compound which will not disproportionate upon dissolution. Therefore, washing with an aqueous monoammonium phosphate solution does not introduce impurities, does not reduce recovery of the solids, and does not undergo reaction with the solids. The monoammonium phosphate solution which is entrained in the washed cake is converted to solid monoammonium phosphate upon subsequent drying.

For incongruently-soluble compounds the crystallization mother liquor is the saturated solution of the salt. Washing with a saturated solution of the salt, as in U.S. Patent No. 4,329,326, therefore does not constitute a wash since the washed solids would have essentially the same quantity and composition of entrained mother liquor as the unwashed solids. The saturated solution wash would be of a different ionic ratio than the crystals and impurities would therefore be introduced upon subsequent drying. The saturated wash solution would contain a high level of free acid and would therefore be ineffective in removing free acid.

Use of an undersaturated solution is expected to produce complications of both a water wash and a saturated solution wash; i.e. there is less than adequate removal of free acid with accompanying difficulties in the physical properties of the solid, there may be impurities present in the washed solid, and there may be a lowered recovery of solids following the washing operation.

Washing with a phosphoric acid-miscible solvent such as acetone, methanol, etc. necessitates the additional handling and recovery of solvents. These solvents may be volatile and combustible. U.S. Patents Nos. 2,550,490 and 3,726,962 generally disclose the washing of incongruently-soluble phosphate crystals with either methanol or ethanol. The crystallization of the incongruently soluble phosphate from solution is generally induced by heating or by waiting over a period of several hours. The crystals are then recovered from the slurry and washed with the solvent.

The particular difficulties which manifest following washing may be dependent on the rate of reaction of the incongruently-soluble crystals with the wash. Highly soluble crystals such as HKP will tend to react almost immediately with the wash solution, resulting in high free acid levels and lowered wash recoveries. Lower solubility crystals such as SALP will tend to react primarily with the entrained wash liquor, resulting in impurity phosphates in the solids and physical properties which change with time.

Thus, the existing incongruently-soluble acid phosphate processes utilize crystal washing methods which adversely affect the recoveries and finished product purity and properties.

It would be advantageous to produce incongruently soluble acid phosphates in a solution crystallization process without obtaining a finished product having a high level of phosphoric acid. Phosphoric acid adversely impacts the purity, hygroscopicity, handling characteristics, and flowability of the finished product.

It would also be advantageous to improve crystal washing methods in incongruently soluble acid phosphates production processes to obtain higher finished product recoveries. Present cake washing techniques dissolve a large amount of incongruently soluble acid phosphate crystals. An improved washing method would significantly increase the overall product recovery.

It would also be advantageous to use an aqueous wash as opposed to a non-aqueous wash.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a novel method for producing incongruently-soluble acid phosphates in a solution crystallization process by incorporating a phosphate wash solution. The present inventive method improves the product purity by removing entrained free acid from the surface of the crystals. Furthermore, the method of the present invention results in significantly higher product recoveries compared to conventional crystal washing techniques. The method of the present invention generally involves washing incongruently-soluble acid phosphate crystals produced in a solution crystallization process. The crystals are formed in a reaction slurry and then separated from the solution. Some of the solution remains on the crystal surface in the form of entrained phosphoric acid or free acid. The entrained free acid must be removed in order to maintain product purity.

The method of the present invention utilizes a phosphate solution wash to displace the entrained free acid from the crystal surface. The phosphate solution generally has a common metal ion with the incongruently soluble acid phosphate to reduce impurities introduced into the process. The phosphate solution is passed over the wet crystals and displaces the entrained free acid without dissolving a significant amount of the crystals.

The phosphate solution used for washing in accordance with this invention may contain phosphoric acid as well as a salt such as e.g., monosodium phosphate or monopotassium phosphate. The presence of phosphoric acid in the wash solution may be noted by the wash solution having an M/P ratio <1.00, where M= equivalents of metal ion and P = moles of phosphate.

Since many incongruently-soluble salts such as HKP and SALP dissolve to yield a phosphoric acid, such as H3POΦ this invention also includes the use of phosphoric acid solution as a wash (common ion is hydrogen). A phosphoric acid wash would be used at a lower concentration than the free acid content of the entrained mother liquor in order to reduce the free acid content. For example, the entrained mother liquor in a SALP wet cake may typically be 62% P₂O₅; washing with a phosphoric acid having a concentration below 62% P₂O5 may substantially reduce the free acid content of the washed cake as the entrained wash has a lower density and viscosity than the mother liquor which it displaces. Additionally, the present invention includes a process for producing mixed cation, incongruently-soluble acid phosphates. The process involves the formation of an acidic solution having a molar excess of phosphorus over the theoretically required amount to form the specific mixed cation phosphate. The process eliminates the formation of intermediate phases of the mixed cation phosphate thus eliminating additional processing steps and resulting in improved production rates and better control of crystallization. The crystals are then washed with a phosphate solution in accordance with the present invention.

It is an object of the present invention to produce incongruently-soluble acid phosphates in a solution crystallization process by incorporating a phosphate wash solution. A phosphate wash solution is utilized upon separation of the incongruently-soluble acid phosphate crystals from the reaction slurry. The wash removes entrained free acid from the crystal surface. The removal of the entrained free acid from the crystal surface reduces the hygroscopicity and improves the product purity and physical properties of the incongruently-soluble acid phosphate.

It is also an object of the present invention to incorporate a phosphate wash solution in a solution crystallization process to improve the recovery of finished incongruently-soluble acid phosphates. A phosphate wash solution wash reduces the amount of incongruently-soluble acid phosphate crystals that dissolve upon washing.

It is also an object of this invention to incorporate a phosphoric acid wash in a solution crystallization process to improve the product purity or physical properties of the incongruently-soluble acid phosphate.

BRIEF DESCRIPTION OF THE DRAWINGS

The above, as well as other advantages of the present invention, will become readily apparent to those skilled in the art from the following detailed description of a preferred embodiment when considered in the light of the accompanying drawings in which:

Fig. 1 is a graph comparing the moisture pick up, in weight percent, over time for HKP crystals washed with water and HKP crystals washed with monopotassium phosphate;

Fig. 2 is a graph illustrating the effect of varying K/P mole ratios in the wash solution on the free acid content of the finished crystals;

Fig. 3 is a graph indicating the effect of varying K/P mole ratios in the wash solution on the K/P mole ratio of the finished crystal;

Fig. 4 is a graph illustrating the impact of the wash to wet cake ratio on the percent cake recovery after washing; and

Fig. 5 is a graph illustrating the effect of increasing wash to wet cake ratios on the free acid content of SALP crystals.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with the present invention, it has been discovered that a significant improvement in the purity and physical properties of incongruently-soluble acid phosphates, prepared by crystallization from solution, may be obtained by washing a solution crystallized incongruently-soluble product with a phosphate solution. The phosphate wash solution minimizes the reaction of the incongruently- soluble acid phosphate with the water from the wash solution. The present invention also contemplates a solution crystallization process for producing mixed cation, incongruently-soluble phosphates.

The method of the present invention involves production of solid incongruently-soluble acid phosphates which may, for example, be used in food products. The method of the present invention is suitable for use with incongruently- soluble compounds generally selected from the group consisting of hemipotassium phosphate, hemisodium phosphate, sodium aluminum phosphate, potassium aluminum phosphate, monocalcium phosphate, calcium potassium phosphate, or calcium ammonium phosphate. The sodium aluminum phosphate compounds are generally expressed as Na3Al₂H₁₅(PO₄)₈ NaAl3H_] (PO )₈, NaAl3H_{j 4}(PO₄)₈*4H₂O. The calcium potassium phosphates and calcium ammonium phosphates, wherein M=K or NH₄ are generally expressed as Ca₂MH₇(PO₄)₄*2H₂O. Other incongruent-soluble phosphates may also be produced in accordance with the present inventive process.

An example of an incongruently-soluble phosphate is hemipotassium phosphate, represented by the formulas KH₅(PO₄)₂ or H₃PO₄*KH₂PO₄. Hemipotassium phosphate is a discrete equilibrium solid present in a concentrated acid portion of an aqueous potassium phosphate system. The compound is present at least over the temperature range of 0-50 °C, with no hydrates present over this range. The compound is highly soluble in water and decomposes to monopotassium phosphate and phosphoric acid. Hemipotassium phosphate is in equilibrium with liquid at approximately 47 wt% P₂O₅ and 13 wt% K₂O to 57 wt% P₂O₅ and 7 wt% K₂O at 25°C. The solubility of the compound increases with increasing temperatures.

In general, the incongruently-soluble acid phosphates may be prepared by crystallization from aqueous solution. The process generally involves making a reaction mixture of a phosphate containing compound and a metal containing compound at the appropriate metal to phosphorus ratio. Conventional sources of the metal and P₂O₅ are suitable for use with the present invention. The reaction mixture is then crystallized to form a handleable slurry with a significant yield i.e., usually containing about 20-50 wt% solids. The solids are separated from solution and further processed before being collected. [(See refs.: B. Wendrow and K. A. Kobe, Chem.Rev. 54, 891 (1954); J. Myl and Z.Solc, Coll. Czech Chem. Comm.25,2414 (I960)].

With hemipotassium phosphate, the desired percent K₂O and percent P2O5 f°^{r me} reaction mixture are determined in accordance with a K₂O-P₂O5-H O phase diagram to yield 25-50% undissolved solids at about 25-30°C. The desired potassium to phosphorus ratio for hemipotassium phosphate is dependent upon the temperature, but is about 0.19 to 0.45 at 25 °C. The sources of potassium and phosphorus include, but are not limited to, potassium hydroxide, monopotassium phosphate, phosphoric acid. A typical reaction may include 45% KOH and 85%

H3PO Mother liquor recycled from a solid/liquid separation step may also be used as a source of potassium and phosphorus. Temperatures while mixing the reactants may be considerably hotter (>80°C) than the final crystallization temperature.

In accordance with the present invention, the reaction mixture is crystallized to form a slurry. The incongruently-soluble acid phosphates can be crystallized from solution provided the overall composition of the reaction mixture is within the phase envelope for the metal phosphate mixture. For hemipotassium phosphate, an approximate composition range, at 25 °C, is 51-55 wt% P O₅ to 11-15 wt% K₂O which yields approximately 20-50% solids.

In practicing the method of the present invention, it may be necessary to evaporate some water from the reaction mixture prior to cooling the solution to form the crystal slurry. The evaporation of water may be necessary in order to reach the appropriate concentration for the desired incongruently-soluble phosphate.

Crystallization of the incongruently-soluble acid phosphate will occur when the solution becomes supersaturated, for example, through cooling or evaporation. The appearance of crystals does not typically occur spontaneously when the solubility limit is exceeded. Crystallization may be initiated by seeding the mixture with previously produced crystals or with an unfiltered slurry from a previous reaction mixture. In general, seeding is accomplished by adding about 1 to 10 parts by weight of slurry to 100 parts total reaction mass. Alternatively, seeding may be accomplished by adding about 0.1 to 5 parts by weight of crystals to 100 parts total reaction mass. The seeding occurs at a temperature where the system is supersaturated. The addition of crystals at higher temperatures may result in the dissolution of the crystals.

In accordance with the present invention, a mixed cation, incongruently-soluble phosphates may be produced through a solution crystallization method. The process generally involves the formation of a reaction mixture having a different mixed cation to phosphorus ratio than theoretically required by the stoichiometry of the finished incongruently-soluble phosphate crystals. Conventional processes utilize drying a reaction mixture close to theoretical ratio requirement and may involve making an intermediate phase product to obtain a solid. The intermediate phase product requires additional processing in order to achieve the desired finished crystal. The present inventive process is capable of producing the desired finished product from a reaction mixture having a higher concentration of phosphorus.

The crystallization of mixed metal phosphates from solution is accomplished by preparing a composition which lies within a two phase envelope for the desired crystal form. Phase diagrams for four component systems, such as mixed cation phosphate salts, are generally not available. Therefore, practical ranges for the components and temperatures are established through equilibrium experimentation.

The overall composition of the system must lie within the phase envelope of the desired solid phase (crystal). The solution crystallization method of the present invention utilizes a reaction mixture having an excess molar amount of phosphorus that is still within the phase envelope of the desired incongruently-soluble crystals. Excess molar amounts of phosphorus in the reaction mixture are capable of producing the desired incongruently-soluble phosphate directly from solution without first forming intermediate phases of the salt.

The formation of the specific incongruently-soluble phosphate salts is dependent upon the particular system and the temperature employed. For example, a sodium aluminum phosphate, expressed as Na3Al₂H_{j 5}(PO₄)₈, may be crystallized at a temperature of about 80 °C when the reaction mixture has an approximate composition range of: 57-65% P O₅, 0.05 to 0.2 Al₂O₃/ P₂O₅mole ratio, and 1 to 4 Na O/Al₂O₃ mole ratio. The composition ranges for the crystallization of other mixed cation, incongruently-soluble phosphates at specific temperatures are generally determined through equilibrium experiments.

The process involves the formation of a reaction mixture containing source compounds for the desired mixed metal cations. Additionally, the phosphorus source, such as phosphoric acid, is added to the reaction mixture. Water may also be present in the reaction mixture.

An adjustment in the amount of water in the reaction mixture may be necessary in order to crystallize the incongruently-soluble phosphate. The adjustment may involve either the addition of water or the evaporation of water from the system.

Temperature limitations for evaporation may be necessary in order to prevent the formation or crystallization of an undesirable phase of mixed cation crystals or to minimize corrosion of the equipment. The temperature of the reaction mixture may be maintained by introducing sweep air across the surface of the reaction mixture.

Optionally, seed crystals may be introduced into the reaction mixture to facilitate crystallization. The seed crystals can be introduced during the evaporation step or anytime thereafter. The time at which the seeding occurs may have an impact upon the finished crystal size. Seeding during the evaporation step initiates nucleation under a relatively low degree of super saturation which favors growth of a fewer number of large crystals. If seeding is initiated later in the process, the reaction mixture is more supersaturated which favors nucleation over growth. Thus, smaller crystals are generated.

The mixed cation reaction mixture is cooled to further crystallize the incongruently-soluble phosphate. The resulting crystals are separated from the slurry utilizing conventional liquid/solid separation techniques and equipment. The use of an excess amount of phosphoric acid in the reaction mixture results in an acidic slurry. Thus, the separated crystals will have a high amount of entrained free acid on the crystal surface. Washing the crystals during the separation step or immediately thereafter is necessary to enhance the physical properties of the finished crystals. Washing is therefore accomplished in accordance with the present invention through the use of a phosphate solution having a common ion (metal or hydrogen ion) with the mixed cation, incongruently-soluble phosphate.

An example of a mixed cation, incongruently-soluble phosphate is a phase of sodium aluminum phosphate (SALP), expressed as Na3Al₂H_j5(PO₄)₈. A solution crystallization process for the compound may be initiated by forming a reaction mixture of phosphoric acid and alumina trihydrate in excess water. The mixture is heated to about 80 °C until the alumina is completely dissolved. Sodium hydroxide is then introduced over time into the reaction mixture. The temperature of the reaction mixture increases to about 115°C during the addition of the sodium hydroxide. A composition of the reaction mixture which is suitable for producing the phase of SALP expressed as Na₃Al₂H₁₅(PO₄)₈ is: 61-64% P₂O₅, 0.08 to 0.1 Al₂O3. P₂O5 mole ratio, and 1.5 to 1.6 Na₂O/Al₂O3 mole ratio. Other sodium, aluminum, and phosphorus sources may be suitable for practicing the method of the present invention.

The excess water in the reaction mixture is evaporated by heating the mixture to a temperature no greater than about 130 °C while utilizing sweep air to maintain the temperature. Seed crystals are added part way through the evaporation step to induce crystallization. The concentrated slurry is then cooled to a temperature between 70-90 °C for crystal growth. The crystals are then separated from the slurry and washed with the process of the present invention.

In general, the crystallization of all incongruently-soluble phosphates results in phosphate crystals having relatively large particles which are well suited for separation from the reaction slurry. The separation of the crystals is preferably accomplished through either filtration or centrifugation. However, other separation techniques are suitable for use in with the present inventive process.

The metal to phosphorus mole ratio of the reaction slurry can enhance the separation of the crystals from the slurry. With the hemipotassium phosphate compound, a mole ratio at the higher end of the 0.19 to 0.32 potassium to phosphorus range will result in a mother liquor with a lower P₂O₅ content and lower viscosity which improves the ability to remove the mother liquor from the crystals during the separation step of the process.

The separated crystals, in the form of a moist cake, generally have about 1.5 to about 15 percent entrained free acid depending upon the solution crystallization process utilized in forming the crystals. The entrained free acid is in the form of P₂O5-

The washing or displacement of the entrained free acid is important in recovering a dry, non-hygroscopic product. Washing with water to displace the entrained free acid is not desirable because of the solubility of the incongruently- soluble phosphate or reaction of water with the salt. A water wash will therefore result in a lower recovery of product or an impure product. Furthermore, the product will have a relatively high free acid content. In accordance with the method of the present invention, rapid washing of the cake with a phosphate solution displaces the entrained free acid or acidic mother liquor with reduced dissolution of, and reaction with, the highly soluble acid phosphate. The phosphate solution generally has a common ion with the incongruently-soluble phosphate crystal. The washing must occur rapidly to retard dissolution of the phosphate crystal. The washing is performed using conventional equipment during the separation step or immediately thereafter. The wash solution may be a saturated solution in order to minimize the amount of water.

Examples of the phosphate wash solutions for washing sodium aluminum phosphate include solutions of monosodium phosphate, mixtures of monosodium phosphate with either phosphoric acid or disodium phosphate, or phosphoric acid solutions. Examples of the phosphate wash solutions for washing hemipotassium phosphate include solutions of monopotassium phosphate, mixtures of monopotassium phosphate with either phosphoric acid or dipotassium phosphate, or phosphoric acid solutions. These examples are illustrative and the invention is not to be limited by these examples.

In practicing the method of the present invention, the metal to phosphorus mole ratio utilized in the wash solution is generally about 0.50 to about

1.5. Lower mole ratios in the wash solution will result in a higher free acid content in the finished product. Ratios lower than 0.5 may be preferred, however if properties other than the free acid content are also important (e.g. hygroscopicity, stability). Higher mole ratios in the wash solution will result in a product having a higher metal to phosphorus mole ratio.

The wash rate to wet cake ratio used in the method of the present invention is generally about 0.1 to about 1.0. Higher ratios may result in the dissolution of crystals while lower ratios result in the incomplete removal or displacement of entrained free acid. A preferred solution wash to wet cake ratio for hemipotassium phosphate is about 0.15 to about 0.5.

With hemipotassium phosphate, a wash solution of saturated potassium phosphate may be utilized. The mole ratio of the wash solution is about 0.5 to about

1.5, resulting in a product having a potassium to phosphorus mole ratio of about 0.50 to about 0.55 and less than about 1% entrained free acid.

The washed cake is dried to remove water from the saturated solution remaining on the surface of the crystal. Conventional drying techniques and equipment are suitable for practicing the method of the present invention. The drying temperature is selected to efficiently achieve the removal of water without degrading the crystals. For hemipotassium phosphate, the excess solution is removed by heating the cake to temperatures less than about 100°C, with a preferred drying range of about 80-95 °C. In general, the incongruently-soluble phosphates exhibit about a 2% to 10% weight loss during the drying step. The free acid content on the surface of the crystal product will also decrease during the drying step. The decrease in entrained free acid is due to the washed wet cake having some dissolved congruently-soluble phosphate and phosphoric acid on the crystal surface. Upon drying, the congruently soluble phosphate and the phosphoric acid react to form more incongruently-soluble acid phosphate.

The finished product is a dried crystalline incongruently-soluble acid phosphate. The product has a free acid content of less than about 1.0%. The hygroscopicity of the product is dependent upon the free acid content. A reduced free acid content on the crystal surface reduces the amount of moisture pick up by the crystals over time. The method of the present invention results in greater than 50% recovery of the incongruently-soluble acid phosphate crystals from the separated cake. With hemipotassium phosphate, the recovered product has a mean particle size of about 200-350 microns. Recovered sodium aluminum phosphates have a mean particle size from about 60 to about 95 microns.

The following examples, which constitute the best mode presently contemplated by the inventor for practicing the invention, are presented solely for the purpose of further illustrating and disclosing the present invention, and are not to be construed as a limitation on the invention:

Examples 1-4

Four hemipotassium phosphate samples were prepared in order to demonstrate the method of the present invention. Each sample was prepared by reacting approximately 34 grams of 45% KOH and approximately 86 grams of 85% H3PO The solution was then heated to evaporate approximately 20 grams of H₂0. The solution, of approximately 100 grams, contains 13 weight percent K₂O and 53 weight percent P₂O_$ Approximately 1.7 grams of a hemipotassium potassium phosphate reaction slurry from a previous reaction was added to the solution as seed crystal while stirring the solution maintained at 40 °C. The solution was then cooled to 25 °C to facilitate the precipitation of hemipotassium phosphate crystals. The reaction slurry was then vacuumed filtered to obtain approximately a 30 gram filter cake. The wet filter cake was then sprayed evenly with either 15 grams of saturated monopotassium solution at 25 °C or water at 25 °C while still under vacuum. The washed cake was then dried at 80°-95°C to remove the saturated solution remaining on the surface of the crystals.

The finished product was identified as hemipotassium phosphate by titration to determine the appropriate potassium to phosphorus mole ratio, which was approximately 0.5. The entrained free acid remaining on the surface of the crystal was determined by extracting a 5-10 gram sample of the hemipotassium phosphate in 100 milliliters of ethyl acetate while stirring for eight minutes. The ethyl acetate phase was separated by filtration and a portion titrated break-to-break with 0.1 N NaOH. The free acid was reported as wt% P O₅ The extracted solids were then utilized to determine the potassium to phosphorus mole ratio of the compound. The results are indicated in Table I.

The particle size measurements of the finished product were determined on a Coulter LS 130 particle size analyzer. The sample was suspended in air as a dry powder. The mean particle size for the samples were generally between 200 and 340 microns.

The hygroscopicity of a solution washed sample and a water washed sample, produced in accordance with the present example, were determined by subjecting the samples to a controlled atmosphere at 58% relative humidity and 25 °C. The surface area was determined for each sample in order to normalize the rate of moisture pick-up. The samples were maintained in the controlled atmosphere for over 24 hrs. The moisture pick-up, in weight percent, of the phosphate salt solution washed sample was less than the moisture pick-up of the water washed sample after

24 hours. Figure 1 is a graph of the moisture pick-up (weight percent) over time for both of the samples. The curve connecting the measured points was fitted utilizing a third order polynomial.

TABLE I

Examples 5-9

A hemipotassium phosphate solution was produced by reacting 561 grams of 45%) potassium hydroxide with 1391 grams of 85% phosphoric acid. Excess water was evaporated from the mixture by heating the solution until it reached a final weight of 1629 grams. The composition of the finished solution was 13 weight percent K₂O and 53 weight percent P O5 with a K/P ratio of about 0.37. The hot solution was divided into different portions. Each sample was then allowed to cool to 55 °C, upon which about 1 gram of HKP seed crystals were added to the solution to initiate crystallization. The solution was then cooled to 25 °C to form about 60 grams of crystals per 100 ml of solution.

The samples were vacuum filtered in coarse frit Buchner funnels for several minutes until most of the liquid was removed. A wash of saturated monopotassium phosphate solution at 25 °C was distributed over the cake while vacuum was applied. The wash K/P ratio was varied for each sample. The wash per wet cake ratio was 0.17 ml wash per gram wet cake. The cake was then dried in an oven at 90 °C. The dried cake was then weighed and analyzed for free acid content and K/P mole ratio.

The resulting properties for each sample are listed in Table II. The results indicate the effect of different phosphate salt solution wash ratios on the free acid content and the final composition of the dried, washed hemipotassium phosphate. Figure 2 is an illustration of the weight percent free acid plotted for wash solution with various K/P mole ratios. The graph generally indicates that free acid content appears to be minimized at a wash K/P ratio of about 1.0. Figure 3 is a plot of the K/P mole ratio of the finished product for solution washes having various K/P mole ratios. The graph indicates that higher wash ratios result in higher metal to phosphorus mole ratios in the finished product.

TABLE II

Examples 10-11

A Hemipotassium phosphate solution was produced by the reaction procedure described in Examples 5-9. The sample size during washing with the phosphate salt solution was larger than Examples 5-9 in order to demonstrate the impact of the wash on a larger crystal mass.

Hemipotassium phosphate crystals in solution, at an initial solution weight of 653 grams, were filtered on a 350 mm Buchner Filter while being washed with 54.6 grams of 1.0 K/P mole ratio solution of monopotassium phosphate. Approximately, 292 grams of wet cake, having a K/P mole ratio of 0.49, was recovered from the filtration and washing process. The wet cake had a free acid content of 2.3% (as wt% P₂O₅). The ratio of MKP wash per weight of wet cake was 0.19. Sixty-nine percent of the cake was recovered upon washing.

The sample was split into a 104 gram portion (Example 10) and a 69 gram portion (Example 11) for drying. Each washed cake example was dried by blowing hot air over the solids in a rotating can (to simulate a rotary drier). The bed temperature during drying was about 102-104°C for each example. The K/P mole ratio and free acid content for each example was determined. Example 10 had a K/P mole ratio of 0.54 and a free acid content of 0.31 wt% P₂O₅. Example 11 had a K/P mole ratio of 0.53 and a free acid content of 0.2 wt% P₂O₅.

Examples 12-17

Hemipotassium phosphate sample were produced, filtered, and washed in a similar manner as described in Examples 5-9. The samples were washed with a monopotassium phosphate solution at various wash to wet cake ratios. The results are listed in Table III. Fig. 4 is a graph of the percent cake recovery versus the wash to wet cake ratio. The results indicate that the percent recovery of finished crystals decreases with increasing wash to wet cake ratios. Additionally, the results indicate that a high wash to wet cake ratio may change the finished product K/P ratio (Example 17).

TABLE III

Examples 18-42

Sodium aluminum phosphate samples, expressed as Na Al₂H₁₅(PO₄)₈, were prepared in order to demonstrate the solution crystallization and phosphate salt solution wash methods of the present invention. A reaction flask containing 517 grams of 85% phosphoric acid and 30 grams of alumina trihydrate, Al₂O₃*3H₂O was agitated and heated to a temperature of about 60°C. The alumina trihydrate dissolved within 45 to 60 minutes as the temperature was increased up to 82 °C. Once the alumina trihydrate dissolved, 48 grams of 50% sodium hydroxide was added over 6 to 8 minutes with the use of a peristaltic pump. The reaction components resulted in a reaction mixture having a molar ratio of 2 moles Al: 3 moles Na: 22 moles of P.

The temperature of the solution was increased to about 115°C. Sweep air was then introduced at a rate of 20-25 SCFH over the reaction surface while the solution is heated to evaporate water. After the evaporation of 30 to 40 grams of water, crystallization was initiated by adding 10 grams of sodium aluminum phosphate from a previous sample as seed crystals. The evaporation of water continued until 65 to 80 grams was removed and the reaction temperature reached 115°C to l25°C.

The reaction slurry was cooled to 80 °C and held at that temperature for one hour. Slurry samples, of 30 ml each, were removed and vacuum filtered at 80°C on 60 ml coarse frit glass filters. The vacuum was applied until little additional liquid was removed from the underside of the filter. The filtered cake was at a bed depth of about 'Λ inch. Each sample was then weighed and recorded.

Each sample of filtered crystals was then washed with either water, a 40%) monosodium phosphate solution, or a 40% phosphoric acid solution. The wash solution for each sample was applied to the crystals through a wash bottle while the sample was vacuum filtered. The solution was applied in a sweeping motion in order to contact the entire surface area of the cake. The wash liquid was filtered through the cake after 30 to 45 seconds. The cake was then weighed a second time to determine the percent of cake lost during washing. The resulting crystals exhibited a mean particle size ranging from 60 to 95 microns.

The ratio of wet cake to wash solution was varied for different samples. Additionally, the monosodium phosphate concentration and the sodium to phosphorus ratio of the wash were varied to determine the impact on the free acid content and recovery of the finished sodium aluminum phosphate crystals.

The washing results with the 40%) monosodium phosphate solution, the 40%) phosphoric acid, and water are listed in Table IV. The results indicate that the free acid content of the samples washed with monosodium phosphate is significantly lower than the free acid content of the samples washed with either water or phosphoric acid. Examples 18-23 indicate a preferred free acid content on the finished sodium aluminum phosphate crystals, expressed as Na₃Al₂H_{j 5}(PO₄)₈, of less than about 1.0% (P₂O₅). Examples 18-23 indicate a more preferred free acid content on the finished sodium aluminum phosphate of less than about 0.5% (P₂O₅). Additionally, the samples washed with monosodium phosphate had lower cake losses than the samples washed with phosphoric acid or water.

The impact of various wash to wet cake ratios are illustrated in Table V. The results indicate that the cake loss increased with increasing amounts of wash. Additionally, the free acid content decreases with increasing wash to wet cake ratios. Fig. 5 illustrates the impact of increasing wash to wet cake ratios on the free acid content of the crystals.

Table VI represents the results of varying the sodium to phosphorus (Na/P) ratio of the 40%) monosodium phosphate wash. The free acid content was the lowest at a Na/P ratio of about 1. The table indicates that the percent cake loss decreased with increasing sodium to phosphorus ratios in the wash.

The results of utilizing various monosodium phosphate concentrations in the wash solution are illustrated in Table VII. Wash concentrations of 40% MSP result in free acid levels of below 0.5 wt%> P₂O₅. The percent cake loss decreased with increasing monosodium phosphate concentrations. The weight gain of cake washed with 60% MSP (Example 42) is associated with the precipitation of monosodium phosphate during the washing process. The 60%) MSP solution was warmed above ambient temperature to maintain a 60%) MSP concentration. TABLE IV

TABLE V

TABLE VI

TABLE VII

Examples 43-45

A reaction flask containing 775.5g of 85.7% phosphoric acid and 45.8g alumina trihydrate was agitated and heated to about 68°C to dissolve the alumina. 71.5 Ig of 50%) sodium hydroxide was added over 6 to 8 minutes, resulting in a solution with a Al₂O3. P₂O₅ mole ratio = 0.087 and a Na₂O/ Al₂O₃ mole ratio = 1.52. The solution was heated under an air sweep (20 SCFH) to evaporate water. 15.0g of sodium aluminum phosphate was added as seed crystal upon the evaporation of 36g water. The final temperature rose to 128°C and the final weight = 786.4g (total water evaporated about 121.4g), resulting in a final P₂O₅ concentration of about 62.4 wt%. The reaction slurry was cooled to 80°C and held at that temperature for one hour. The mean particle size was 93 micron. The reaction slurry was separated at 80°C into roughly three equal portions and each portion was vacuum filtered. The wet cake represented approximately 27%) of the reaction slurry by weight. Each portion was washed with either water, 40%) phosphoric acid, or 40%) MSP solution. The washed cakes were dried at 106°C.

Calcium carbonate was added to 24.01 g of each washed and dried sample at a level to neutralize 0.5% free acid in excess of the measured free acid content (neutralization to monocalcium phosphate). Calcium carbonate added = 0.26g for examples 43 and 45; = 0.926g for example 44. 0.22 g. Silica was added to each sample as a flow conditioner. The materials are mixed while milling in a small coffee grinder and redried at 106°C for one hour. The final material of example 44 had the lowest tendency towards caking upon storage. The pick up of moisture from the atmosphere for 4 g samples after 24 hr at 75%> relative humidity and ambient temperature are as follows.

The phosphoric acid-washed sample (Example 44) is the least hygroscopic while the water-washed sample (Example 45) is the most hygroscopic. Example 46

A reaction flask containing 625.4g of 86.4%) phosphoric acid and 37.4g alumina trihydrate was agitated and heated to about 65°C to dissolve the alumina. 57. Og of 50.6% sodium hydroxide was added over 6 to 8 minutes, resulting in a solution with a Al₂O₃/P₂O₅ mole ratio = 0.087 and a Na₂O/Al₂O₃ mole ratio= 1.50. The solution was heated under an air sweep (5 SCFH) to evaporate water. 0.5g of sodium aluminum phosphate was added as seed crystal upon the evaporation of ~23g water. The final temperature rose to 136°C and the total amount of water evaporated was about 90g (final P₂O₅ concentration of about 62.1 wt%>). The reaction slurry was cooled to 120°C and held at that temperature for one hour. The mean particle size was 102 micron. The 499g of the reaction slurry was vacuum filtered yielding lOOg of wet cake. The cake was washed with 75.5g of 80%> phosphoric acid at a rate of ~30ml/min. The washed cake was dried at 80°C overnight. The free acid content after washing and drying was 5.07 wt% (as P₂O₅).

In accordance with the provisions of the patent statutes, the present invention has been described in what is considered to represent its preferred embodiment. However, it should be noted that the invention may be practiced otherwise than as specifically illustrated and described without departing from the spirit and scope of the invention.

Claims

WHAT IS CLAIMED IS:

1. A solution crystallization process for producing an incongruently- soluble acid phosphates, comprising washing incongruently-soluble acid phosphate crystals upon separation from a reaction slurry with a phosphate solution, whereby said phosphate wash solution displaces entrained free acid on said incongruently- soluble acid phosphate crystals.

2. The process of claim 1 wherein said incongruently-soluble acid phosphate is selected from the group consisting of hemipotassium phosphate, hemisodium phosphate, sodium aluminum phosphate, sodium potassium phosphate, monocalcium phosphate, calcium potassium phosphate, and calcium ammonium phosphate.

3. The process of claim 2, wherein said sodium aluminum phosphate is selected from the group consisting of Na3Al₂H₁₅(PO₄)₈, NaAl₃H_{j 4}(PO₄)₈, and NaAl₃H₁₄(PO₄)₈*4H₂O.

4. The process of claim 2, wherein the calcium potassium phosphates and calcium ammonium phosphates are generally expressed as Ca₂MH₇(PO₄)₄*2H₂O, with M=K or NH₄.

5. The process of claim 1 , further comprising drying said incongruently- soluble acid phosphate crystals to remove free moisture from said crystals.

6. The process of claim 1 , wherein said phosphate wash solution has a common ion with the incongruently-soluble acid phosphate.

7. The process of claim 1 , wherein said phosphate wash solution is selected from the group consisting of monopotassium phosphate, monosodium phosphate, and phosphoric acid.

8. The process of claim 1 , wherein a wash solution to incongruently- soluble acid phosphate cake ratio is 0.1 to 1.0 on a weight basis.

9. The process of claim 1 , wherein the washed crystals have a free acid content of less than 1.0 percent by weight as P₂O₅.

10. The process of claim 1 , wherein the wash solution has a metal to phosphorus ratio of less than about 1.5.

11. The process of claim 1 , wherein said wash solution is a saturated solution.

12. The process of claim 1 , wherein said incongruently-soluble acid phosphate is recovered in yields greater than 50% upon washing.

13. A solution crystallization process for producing incongruently-soluble acid phosphates, comprising:

(a) forming a slurry by crystallizing a solution containing reaction components of a phosphorus compound and a metal compound to form incongruently- soluble acid phosphate crystals;

(b) separating said incongruently-soluble acid phosphate crystals from the slurry, an amount of solution from said slurry remaining on said crystals as entrained free acid; and

(c) washing said incongruently-soluble acid phosphate crystals with a phosphate solution, said wash solution having a common ion with said incongruently-soluble acid phosphate crystals, whereby said wash solution displaces the entrained free acid on the crystals.

14. The process of claim 13, wherein said incongruently-soluble acid phosphate is a mixed cation acid phosphate and the slurry is formed by crystallizing a reaction mixture having an excess of the theoretical molar amount of phosphorus required for the incongruently-soluble acid phosphate crystals.

15. The process of claim 13 , wherein said incongruently-soluble acid phosphate is selected from the group consisting of hemipotassium phosphate, hemisodium phosphate, sodium aluminum phosphate, sodium potassium phosphate, monocalcium phosphate, calcium potassium phosphate, or calcium ammonium phosphate.

16. The process of claim 13, further comprising drying said incongruently- soluble acid phosphate crystals to remove free moisture from said crystals.

17. The process of claim 13, wherein said phosphate wash solution is selected from the group consisting of monopotassium phosphate, monosodium phosphate, and phosphoric acid.

18. The process of claim 13 , wherein a said solution wash to incongruently-soluble acid phosphate cake ratio is 0.1 to 1.0 on a weight basis.

19. The process of claim 13, wherein the free acid content of washed crystals is less than 1.0 percent by weight as P₂O₅.

20. The process of claim 13, wherein the salt solution has a metal to phosphorus ratio of less than about 1.5.

21. The process of claim 13 , wherein said salt solution wash is a saturated solution.

22. The process of claim 13, wherein said incongruently-soluble acid phosphate is recovered in yields greater than 50% upon washing.

23. A solution crystallization process for producing hemipotassium phosphate, comprising:

(a) forming a slurry by crystallizing a solution containing reaction components of a phosphorus compound and a potassium compound to form hemipotassium phosphate crystals; (b) separating said hemipotassium phosphate crystals from the slurry, an amount of solution from said slurry remaining on said crystals as entrained free acid; and

(c) washing said hemipotassium phosphate crystals with a potassium phosphate solution, whereby said solution displaces the entrained free acid on the crystals.

24. The process of claim 23, further comprising drying said hemipotassium phosphate crystals at temperature of less than 100┬░C to remove free moisture from said crystals.

25. The process of claim 23, wherein a wash solution to hemipotassium phosphate cake ratio is 0.1 to 1.0 on a weight basis.

26. The process of claim 25, wherein said ratio is about 0.15 - 0.5.

27. The process of claim 23, wherein the washed crystals have a free acid content less than 1 percent by weight as P₂O₅.

28. The process of claim 23, wherein the wash solution has a potassium to phosphorus ratio of less than about 1.5.

29. The process of claim 23, wherein said wash solution is a saturated solution.

30. The process of claim 25, wherein said hemipotassium phosphate is recovered in yields greater than 50%) upon washing.

31. The process of claim 23, wherein said hemipotassium phosphate product has a potassium to phosphorus mole ratio of about 0.50 to 0.55.

32. A solution crystallization process for forming mixed cation, incongruently-soluble acid phosphates, comprising:

(a) forming a reaction mixture of a first metal compound, a second metal compound, and a phosphorus compound, said reaction mixture having an excess of the theoretical molar amount of phosphorus required for a mixed cation, incongruently-soluble acid phosphate crystal;

(b) adjusting an amount of water in said reaction mixture; (c) forming a slurry containing mixed cation, incongruently- soluble acid phosphate crystals by seeding said reaction mixture;

(d) separating said incongruently-soluble acid phosphate crystals from the slurry, an amount of solution from said slurry remaining on said crystals as entrained free acid; and (e) washing said incongruently-soluble acid phosphate crystals with a phosphate solution, said wash solution having a common ion with said incongruently- soluble acid phosphate crystals, whereby said wash solution displaces the entrained free acid on the crystals.

33. The process of claim 32, wherein the amount of water is adjusted by the addition of water to said reaction mixture.

34. The process of claim 32, herein the amount of water is adjusted by evaporation.

35. The process of claim 34, herein said seeding occurs during the evaporation step.

36. The process of claim 32, herein the first metal compound and the phosphorus compound are reacted in excess water before adding the second metal compound.

37. The process of claim 32, wherein said seeding is accomplished by adding 1 to 10 parts by weight of slurry to 100 parts total reaction mass.

38. The process of claim 32, wherein said seeding is accomplished by adding 0.1 to 5 parts by weight of crystals to 100 parts total reaction mass.

39. The process of claim 32, wherein said mixed cation, incongruently- soluble acid phosphate is selected from the group consisting of sodium aluminum phosphate, calcium potassium phosphate, and calcium ammonium phosphate.

40. The process of claim 39, wherein said sodium aluminum phosphate is expressed as Na3A_j2H₁₅(PO₄)₈ and the entrained free acid on the crystals after washing is less than about 0.5%> (P₂O₅).

41. The process of claim 32, wherein the wash solution has a metal to phosphorus ratio of less than about 1.5.

42. The process of claim 32, wherein a solution wash to incongruently- soluble acid phosphate cake ratio is 0.1 to 1.0 on a weight basis.

43. The process of claim 32, wherein the free acid content of washed crystals is less than 1.0 percent by weight as (P₂O₅).