PROCESS FOR THE PRODUCTION OF WATER SOLUBLE AMMONIUM PHOSPHATES

Abstract: A process is disclosed for the production of water solu­ ble ammonium phosphates, comprising the following steps: a) to react in a reaction step ammonia and untreated phosphoric acid with P₂O₅ concentration, comprised within the range of from 30 to 54% outcoming from wet processes; b) to filter under high temperature conditions the reac­ tion suspension, separating the impurity-containing sludges from an aqueous solution of ammonium phosphate; c) to cool and crystallize the aqueous solution of ammo­ nium phosphate; d) to separate the formed ammonium phosphate crys­ tals from the mother liquors.

Description: This invention relates to a process for preparing water soluble technical grade crystalline ammonium phosphates for the production of water soluble fertilizers: these latter must have a high purity, especially from the point of view of a low level of solubility-impairing metal cations. Generally speaking, the prior art processes require the purification of the phosphoric acid used as the starting material, or expensive ion-exchange treatments. US-A-2 891 856, while starting from phosphoric acid exiting a wet production process, requires a phosphoric acid which has a P₂O₅ concentration of from 20% to 28%, because, if the concentration of 28% is exceeded, the precipitate formed in the reaction with ammonia would be extremely difficult to separate: such a precipitate is generally a mud-like suspension of metal phosphates in a lattice of ammonium phosphate. GB-A-2 088 843 (Agrico Chemical Company) produces monoammonium phosphate from wet process phosphoric acid using two ammoniation steps, one at an extremely low pH (pH 1,5 to 2,5) and one at a more moderate pH (pH 4 to 5): the use of impure phosphoric acid results in a premature formation of monoammonium phosphate and retards the filtration of the precipitated impurities. Quite unexpectedly, this invention permits the use of impure phosphoric acid exiting a wet production process and having a concentration of P₂O₅ as high as up to 54%. To overcome this shortcoming and to be enabled to use phosphoric acid having a higher P₂O₅ concentrations, the present invention proposes a process for preparing water soluble ammonium phosphates from unpurified phosphoric acid, comprising the steps of: (a) reacting unpurified fluoride-containing phosphoric acid, exiting a wet production process and having a P₂O₅ concentration of from 30% to 54% by weight,at a pH of from 4 to 4,5, or from 7,5 to 8, directly with ammonia, at a temperature of from 80°C to 95°C, for a period of from 10 min to 30 min; (b) filtering at the same temperature the reaction suspension and separating the impurity-containing sludges from an aqueous solution of ammonium phosphate; (c) cooling and crystallizing the aqueous solution of ammonium phosphate, and (d) separating the as-formed crystals of ammonium phosphate from the mother liquor. By so doing, either monoammonium phosphate, or diammonium phosphate, can be obtained In order to obtain monoammonium phosphate, the reaction of phosphoric acid with ammonia is carried out at a pH of from 4 to 4,5, at a temperature of from 80°C to 95°C, and with a residence time of from 10 min to 30 min. The mother liquors, after having been optionally partly drained in order to reduce the content of organic matter, which would tend to accumulate, is recycled, if desired and after having been cooled, to the reaction step (c) as defined above, and/or to the reaction step (a) after having been heated. In order to produce diammonium phosphates, the reaction between ammonia and phosphoric acid is carried out at a pH value of from 7.5 to 8, at a temperature of from 80 to 95°C, and with a residence time of from 10 to 30 minutes. The mother liquors, after having been possibly partly drained, may be cooled and recycled to step (c). As an alternative, always to the purpose of producing diammonium phosphates, the reaction between ammonia and phosphoric acid may be carried out at a pH value comprised within the range of from 4 to 4,5, at a temperature of from 80 to 95°C, and with a residence time comprised within the range of from 10 to 30 minutes, i.e., by operating under the same conditions as for obtaining monoammonium phosphate, but after the high-temperature filtration, it is necessary to react again the aqueous solution of ammonium phosphate with ammonia, operating at a pH value comprised within the range of from 7.5 to 8, at a temperature comprised within the range of from 30 to 95°C and with a residence time of from 10 to 30 minutes. The mother liquors, after having been possibly drained, may be recycled to the step (c) after having been cooled, and/or to the step following the high-temperature filtration, wherein the aqueous solution of ammonium phosphate reacts with ammonia, after having been heated. Going back to the general process, the separation of formed crystals of monoammonium phosphate or of diammonium phosphate may occur by centrifugation. Said crystals, after the separation from the mother liquors, may be possibly dried. The invention shall be better disclosed with reference to the flow sheets of Figs 1 and 2 attached, which are not to be intended as being limitative of the invention. Fig. 1 shows a process flow sheet for the production of crystalline water soluble monoammonium phosphate. Untreated phosphoric acid (1) outcoming from a wet process is reacted with ammonia (2) in a reaction vessel (3) wherein fossil meal (4) may be possibly added to the purpose of favouring the subsequent filtration. The reaction suspension (5) leaving the reactor is filtered at high temperature in a filter (6), by means of which the impurity containing sludges (7) are separated from an acqueous solution of monoammonium phopshate (8) which is sent into a crystallizer (9) and then centrifuged (10). From the centrifugation, the ammonium phosphate crystals (11) are separated from the mother liquors (12). The crystals are then dried in a dryer (13), the crystalline monoammonium phosphate (14) being finally obtained. A portion (15) of mother liquors is drained in (16), the balance (17) thereof is sent to a mother liquor tank (18). From this tank the mother liquors are partly (19) recycled to the crystallizer (9) after having been cooled in (20), and partly (21) are recycled to the reaction vessel (3) after having been heated in (22). The drainage (16) of a portion of the mother liquors makes it possible to reduce the content of organic substances, which would otherways tend to accumulate within the reactor (3). Fig. 2 represents a process flow sheet for the production of crystalline water soluble diammonium phosphate. Untreated phosphoric acid (1) outcoming from a wet process is reacted with ammonia (2) within a reaction vessel (3) into which fossil flour (4) may be possibly added. The reaction suspension (5) leaving the reactor is filtered at high temperature in a filter (6) by means of which the sludges (7) containing impurities (Al, Fe, Mg and so on) are separated from an acqueous solution of ammonium phosphate (23). The aqueous solution (23) is in its turn reacted with ammonia (24) inside a reaction vessel (25). From the reactor (25) an aqueous solution of diammonium phosphate (26) comes out, which is delivered into a crystallizer (9) and then centrifuged (10). From the centrifugation, the diammonium phosphate crystals (27) are separated from the mother liquors (28). The crystals are subsequently dried in a dryer (13), crystalline diammonium phosphate (29) being finally obtained. A portion (30) of the mother liquors is drained in (16), the balance (31) thereof is sent into a mother liquor tank (18). From this tank, the mother liquors are partly (32) recycled to the crystallizer (9) after having been cooled in (20), and partly (33) recycled to the reactor (25) after having been heated in (34). Two Examples will be now given to the purpose of better illustrating the invention, such Examples however being not to be considered as being limitative of the invention. Example 1 An amount of 200.7 g of H₃PO₄ containing 45.4% of P₂O₅, whose composition is shown in Table 1, have been diluted with 102 g of a saturated solution of monoammonium phosphate containing 22% of P₂O₅ and reacted with ammonia up to a pH value of 4.4. The temperature has been kept at about 85°C. The reaction suspension has been filtered at the same temperature, 50.6 g of sludges containing about 30% of P₂O₅ and a monoammonium phosphate solution being obtained. From such a solution a crystalline monoammonium phosphate has been obtained by cooling to about 27 - 30°C, whose characteristics are reported in same Table 1. The P₂O₅ yield has been calculated to be of about 70%, but it can be increased by washing the sludges with a small amount of hot water (80 - 90°C). The mother liquors saturated at about 27 - 30°C contain about 20% of P₂O₅ and may be recycled into the reaction vessel. Example 2 In a test carried out in the continuously operating pilot plant, assembled according to the flow sheet of Fig. 1, to the reactor (3) 23 Kg/h of H₃PO₄ at 45% concentration (1), whose composition is reported in Table 2, 2.170 l/h of anhydrous NH₃ (2), 0.160 kg/h of fossil flour (4) and 2.510 kg/h of water have been fed. pH value has been maintained at 4.4 and the temperature at about 95° C. The suspension leaving the reactor (5) was delivered, at the same temperature, to the filter (6), from which 16.800 kg/h of saturated solution of monoammonium phosphate were separated, sent to the crystallizer (9) wherein they were cooled at 40°C by heat exchange with water. At the outlet from the crystallizer (9) 7.640 kg/h of moist monoammonium phosphate were obtained, which, when dried, yielded 7.160 kg/h of crystalline monoammonium phosphate, whose characteristics are reported in same Table 2.

Claims: Process for preparing water soluble ammonium phosphates from unpurified phosphoric acid, comprising the steps of: (a) reacting unpurified fluoride-containing phosphoric acid, exiting a wet production process and having a P₂O₅ concentration of from 30% to 54% by weight,at a pH of from 4 to 4,5, or from 7,5 to 8, directly with ammonia, at a temperature of from 80°C to 95°C, for a period of from 10 min to 30 min; (b) filtering at the same temperature the reaction suspension and separating the impurity-containing sludges from an aqueous solution of ammonium phosphate; (c) cooling and crystallizing the aqueous solution of ammonium phosphate, and (d) separating the as-formed crystals of ammonium phosphate from the mother liquor. Process according to Claim 1, wherein, in order to produce monoammonium phosphate, the mother liquor, after having been partly drained, is heated and recycled into the reaction step, as per item (a). Process according to Claim 1, wherein, in order to produce diammonium phosphate, following the filtration step, the aqueous solution of ammonium phosphate is reacted with ammonia once again, operating at a pH of from 7,5 to 8, at a temperature of from 80°C to 95°C and with a residence time of from 10 min to 30 min. Process according to Claim 1, wherein the mother liquor, after having been partly drained, is heated and recycled to the step, following the filtration step, in which the aqueous solution of ammonium phosphate reacts with ammonia. Process according to Claim 1, wherein the unpurified fluoride-containing phosphoric acid used in item (a) has a content of P₂O₅ of from 35% to 46%. Process according to Claim 1; wherein the mother liquor, after having been partly drained, is cooled and recycled to step (c). Process according to Claim 1, wherein the mother liquor, after having been partly drained, is partly cooled and partly recycled to step (c), and partly heated and recycled to the reaction step (a). Process according to Claim 1, wherein the mother liquor, after having been partly drained, is partly cooled and recycled to step (c), and partly heated and recycled to the step following the filtration step, in which the aqueous solution of ammonium phosphate reacts with ammonia. Process according to Claim 1, for preparing water soluble monoammonium phosphate from unpurified phosphoric acid, comprising the steps of: (a) reacting unpurified fluoride-containing phosphoric acid, exiting a wet production process and having a P₂O₅ concentration of from 30% to 54% by weight, at a pH of from 4 to 4,5, directly with ammonia, at a temperature of from 80°C to 95°C, for a period of from 10 min to 30 min; (b) filtering at the same temperature the reaction suspension and separating the impurity-containing sludges from an aqueous solution of monoammonium phosphate; (c) cooling and crystallizing the aqueous solution of monoammonium phosphate, and (d) separating the as-formed crystals of monoammonium phosphate from the mother liquor. Process according to Claim 1, for preparing water soluble diammonium phosphate from unpurified phosphoric acid, comprising the steps of: (a) reacting unpurified fluoride-containing phosphoric acid, exiting a wet production process and having a P₂O₅ concentration of from 30% to 54% by weight, at a pH of from 7,5 to 8, directly with ammonia, at a temperature of from 80°C to 95°C, for a period of from 10 min to 30 min; (b) filtering at the same temperature the reaction suspension and separating the impurity-containing sludges from an aqueous solution of diammonium phosphate; (c) cooling and crystallizing the aqueous solution of diammonium phosphate, and (d) separating the as-formed crystals of diammonium phosphate from the mother liquor.

Inventors: ANANIA, GUIDO, ANASTASI, FILIPPO, CARFI', GIUSEPPE, CULTRERA, VITO
Applicants: ENICHEM AGRICOLTURA S.P.A.
Application year: 1985
Main classification: C01B25
IPC codes: C01B25