Production of ammonium phosphates and product thereof

Abstract

Ammonium phosphate solutions free of precipitates are prepared from wet-process phosphoric acid containing incidental metallic impurities such as iron, aluminum and magnesium by concentration of the phosphoric acid to an anhydrous state and the formation of acyclic polyphosphoric acids therein which prevent the precipitation of the metallic impurities upon neutralization. The acid is heated to a temperature greater than 120° C. at atmospheric or subatmospheric pressures while removing the volatile impurities and thereafter is neutralized with ammonia. The neutralization step can be performed at elevated temperatures with anhydrous ammonia or the acid can be cooled and neutralized at ambient temperatures. The resulting ammonium phosphate is diluted with water to obtain an ammonium phosphate solution which is clear and free of precipitates.

I claim: .[.1. An ammonium phosphate solution produced by neutralization with ammonia of wet-process phosphoric acid containing incident metallic impurities comprising iron and aluminum; said ammonium phosphate solution being free of gelatinous precipitates and metaphosphates and having: 
     
        phosphorus calculated as P 2  O 5 ..]. .[.3.  The liquid ammonium phosphate of claim 1 having about 11 weight percent nitrogen and about 33 weight percent phosphorus calculated as P 2  O 5 ..]. .[.4. The ammonium phosphate solution of claim 1 wherein said impurities also comprise magnesium and wherein said acyclic polyphosphate also prevents the formation of insoluble salts of said magnesium impurity..]. .[.5. The ammonium phosphate solution of claim 1 wherein the amount of said acyclic polyphosphate comprises at least 2 atomic weights of said phosphorus present as acyclic polyphosphate for each atomic weight of said iron and aluminum impurities..]. .[.6. The ammonium phosphate solution of claim 1 which is obtained entirely by the neutralization with ammonium of wet-process phosphoric acid which has been concentrated to form said 
     
     
        acyclic polyphosphate in situ..]. 7. In the manufacture of ammonium phosphate from wet-process phosphoric acid containing between about 1 and about 10 weight percent of metallic and other incidental impurities, said impurities comprising iron and aluminum and causing the formation of precipitates and gelatinous bodies in said ammonium phosphate, the improved method of avoiding the formation of said precipitates and gelatinous bodies which comprises heating said acid .Iadd.to a temperature greater than 120°  C., .Iaddend..[.and.]..]. removing the volatilized impurities and sufficient water .[.from at least a portion thereof.]. .Iadd.therefrom .Iaddend.to obtain .Iadd.acyclic polyphosphoric acid in an amount sufficient to prevent the precipitation of all said metallic impurities upon neutralization of said acid, neutralizing said heat-treated acid with ammonia .Iaddend..[.at least two atomic weights of phosphorus present as acyclic polyphosphoric acid for each atomic weight of aluminum and iron in said acid neutralizing said heat-treated acid with ammonia.]. and adding sufficient water to obtain an aqueous ammonium 
     
     
        phosphate of the desired concentration. 8. The method of claim 7 where said acyclic polyphosphoric acid is obtained by passng .Iadd.the entire bulk of .Iaddend.said wet-process acid through a heating zone wherein the bulk of said wet-process acid is heated to a temperature less than about 200° C., and .[.said.]. .Iadd.a .Iaddend.portion of said acid is 
     
     
        heated to a temperature greater than about 200° C. 9. The process for preparing aqueous ammonium phosphate which comprises: (1) heating to a temperature between about 120° C. and about 400° C. crude wet-process phosphoric acid of between about 25 and about 55 percent P 2  O 5  concentration and containing between about 1 and about 10 weight percent of normally incident impurities including iron and aluminum which cause the formation of gelatinous products when said acid is reacted with ammonia to form aqueous ammonium phosphate; (2) removing the gaseous products which are evolved from the body of liquid acid during the course of said heating and forming an amount of polyphosphoric acid comprising at least two atomic weights of phosphorus per atomic weight of iron and aluminum present, and (3) reacting only the non-volatile products containing the non-volatile metal impurities so obtained with an aqueous ammonia solution of between about 10 and about 20 percent NH 3  concentration at a temperature between about 20° C. and about 100° C., the amount of ammonia employed being such that the product 
     
     
        of said reaction has a pH value between about 6.5 and about 8.5. 10. The process for preparing aqueous ammonium phosphate from crude wet-process phosphoric acid of between about 35 and about 55 weight percent P 2  O 5  content and containing between about 1 and about 10 weight percent of normally incident impurities including iron and aluminum which cause the formation of gelatinous precipitates when said acid is reacted directly with ammonia, which process comprises: (1) heating said acid at a temperature between about 120° C. and about 400° C. for a period of time sufficient to vaporize from said acid the components thereof which are volatile at said temperature and form an amount of polyphosphoric acid comprising at least two atomic weights of phosphorus per atomic weight of iron and aluminum present; (2) reacting the heat-treated acid containing the non-volatile metal impurities so obtained with ammonia at a temperature between about 150° C. and about 200° C.; and (3) adding sufficient water to the reaction product so obtained to form a solution containing between about 45 and about 50 percent by weight of ammonium phosphate; the amount of ammonia employed in step (2) being such that the solution obtained in step (3) has a pH value between about 6.5 and 8.5. .Iadd. 11. A process for the production of a high-analysis liquid mixed fertilizer which comprises the steps of continuously feeding commercially available wet-process phosphoric acid containing up to about 54 weight percent P 2  O 5  into evaporating means; continuously and simultaneously applying heat to said evaporating means; withdrawing from said evaporating means a highly concentrated, fluid, wet-process phosphoric acid substantially free of solids, containing about 65 to 70 weight percent P 2  O 5 , and containing in solution substantially all of the congeneric impurities ordinarily present in said commercially available wet-process acid fed to said evaporating means and sufficient acyclic polyphosphoric acid to prevent precipitation of said impurities upon neutralization of said acid with ammonia; cooling said concentrated acid to a temperature of about 20°  to 40°  C.; subsequently introducing ammoniating fluid to said highly concentrated acid in quantity sufficient to raise the pH of the resulting solution to about 7; maintaining the resulting solution of unhydrolyzed ammonium salts at a temperature in the range from about 30°  to 100°  C. during introduction of said ammoniating fluid; and withdrawing the resulting solution as product. .Iaddend. .Iadd. 12. A process for the production of high-analysis liquid mixed fertilizer which comprises the steps of continuously feeding commercially available wet-process phosphoric acid containing up to about 54 weight percent P 2  O 5  into evaporating means; maintaining the material in said evaporating means at a temperature in the range from about 120°  C. to about 400°  C.; withdrawing from said evaporating means a highly concentrated, fluid, wet-process phosphoric acid substantially free of solids, containing about 65 to 70 weight percent P 2  O 5 , and containing in solution substantially all of the congeneric impurities originally present in said commercially available wet-process acid fed to said evaporating means; and sufficient acyclic polyphosphoric acid to prevent precipitation of said impurities upon neutralization of said acid with ammonia cooling said concentrated acid to a temperature of about 20°  C. to 40°  C.; subsequently introducing an ammoniating fluid to said highly concentrated acid in quantity sufficient to raise the pH of the resulting solution to about 7; maintaining the resulting solution of unhydrolyzed ammonium salts at a temperature in the range of about 30°  C. to 100°  C. during introduction of said ammoniating fluid; and withdrawing the resulting solution as product. .Iaddend.