Potassium phosphate manufacture

Abstract

Fertilizer materials, especially potassium dihydrogen phosphate, are produced by the reaction of phosphate rock or a solubilized form thereof, such as mono calcium phosphate, dicalcium phosphate, etc., concentrated sulfuric acid, potassium hydrogen sulfate and, optionally fresh or recycle phosphoric acid, the resulting mixture is filtered to remove the calcium sulfate precipitate and the potassium dihydrogen phosphate is recovered from the filtrate, either by concentration and filtration or by extraction with an organic solvent.

What is claimed is: .[.1. A process for the preparation of potassium dihydrogen phosphate which comprises: 
     
        sulfate ion in the reacting mixture..]. .[.4.  A process according to claim 2 wherein the potassium hydrogen sulfate solution contains about 4% H 2  SO 4  and about 42% KHSO 4 ..]. .[.5. A process according to claim 1 wherein the filtrate, from step (4), after removal of the calcium sulfate, is treated with a calcium-bearing material selected from the group consisting of phosphate rock, tricalcium phosphate, dicalcium phosphate, calcium phosphate, calcium oxide and mixtures thereof, alternatively or successively, at a temperature of 40°-90° C., to precipitate additional sulfate ion and partially neutralize the mixture, and removing the solids to provide a lower-sulfate filtrate containing the potassium dihydrogen phosphate..]. .[.6. A process according to claim 1 wherein the filtrate from step (4), after removal of calcium sulfate, is concentrated and then cooled to precipitate the potassium dihydrogen phosphate..]. .[.7. A process according to claim 1 wherein the product is recovered by adding a water-miscible organic solvent to the filtrate from step (4), allowing the solid potassium dihydrogen phosphate to precipitate and separating the solid product..]. .[.8. A process according to claim 7 wherein the organic solvent is a 
     
     
        lower alkyl alcohol, a lower alkyl ketone or mixture thereof..]. 9. A process for the precipitation of potassium dihydrogen phoshate which comprises: (1) contacting phosphate rock or a solubilized form thereof with a sufficient excess of sulfuric acid to drive the reaction to completion at a temperature of about 40° to 90° C. in a main reactor;   (2) reacting potassium chloride and sulfuric acid in a separate reactor at a temperature of 250° to 300° C. with removal of the dry hydrogen chloride evolved and dissolving the resulting potassium hydrogen sulfate with water to form a 10-50% aqueous solution;   (3) adding said 10-50% aqueous solution of potassium hydrogen sulfate to said main reactor in sufficient amount to form a reaction mixture containing ions of potassium dihydrogen phosphate and at a rate so as to minimize the formation of insoluble salts of KHSO 4  and CaSO 4  and under such conditions that the concentration of dissolved solids in the reaction mixture does not exceed about 60%;   (4) maintaining the reaction mixture under these conditions for formation of an easily filterable solid, calcium sulfate precipitate;   (5) removing the solid calcium sulfate from the reaction mixture to provide a filtrate;   (6) contacting said filtrate with a calcium-bearing material selected from the group consisting of phosphate rock, tricalcium phosphate, dicalcium phosphate, calcium oxide and mixtures thereof, alternatively or successively, at a temperature of 40°-90° C. to precipitate additional sulfate ion and partially neutralize the mixture and removing the precipitated solids to provide a filtrate lower in sulfate ion and containing potassium dihydrogen phosphate:   (7) recovering the solid potassium dihydrogen phosphate from the filtrate to provide a phosphoric acid mother liquor; and   
     
     
       (8) recycling the mother liquor to the main reactor. 10. A process according to claim 9 wherein the filtrate from step (6), after removal of the solids, is concentrated and then cooled to precipitate the potassium 
     
     
        dihydrogen phosphate. 11. A process according to claim 9 wherein the filtrate from step (6), after removal of the solids is mixed with a water-miscible organic solvent, the solid potassium dihydrogen phosphate is allowed to precipitate and then the solid product is recovered by 
     
     
        filtration. 12. A process for the preparation of potassium dihydrogen phosphate which comprises: (1) contacting phosphate rock or a solubilized form thereof with a sufficient excess of sulfuric acid to drive the reaction to completion at a temperature of about 40° to 90° C. in a main reactor;   (2) adding thereto a 10-50% aqueous solution of potassium hydrogen sulfate in sufficient amount to form a reaction mixture containing ions of potassium dihydrogen phosphate and at a rate so as to minimize the formation of insoluble salts of KHSO 4  and CaSO 4  and under such conditions that the concentration of dissolved solids in the reaction mixture does not exceed about 60%;   (3) maintaining the reaction mixture under these conditions for formation of an easily filterable, solid, calcium sulfate precipitate;   (4) removing the solid calcium sulfate from the reaction mixture to provide a filtrate;   (5) contacting said filtrate with a calcium-bearing material selected from the group consisting of phosphate rock, tricalcium phosphate, dicalcium phosphate, calcium phosphate, calcium oxide and mixtures thereof, alternatively or successively, and removing the precipitated solids to provide a resultant solution lower in sulfate ion content;   (6) concentrating said resultant solution and cooling to crystallize the solid potassium dihydrogen phosphate;   (7) removing the solid potassium dihydrogen phosphate to provide a phosphoric acid mother liquor; and   
     
     
       (8) recycling the mother liquor to the main reactor. 13. A process for the preparation of potassium dihydrogen phosphate which comprises: (1) contacting phosphate rock or a solubilized form thereof with a sufficient excess of sulfuric acid to drive the reaction to completion at a temperature of about 40° to 90° C. in a main reactor;   (2) adding thereto a 10-50% aqueous solution of potassium hydrogen sulfate in sufficient amount to form a reaction mixture containing ions of potassium hydrogen phosphate and at a rate so as to minimize the formation of insoluble salts of KHSO 4  and CaSO 4  and under such conditions that the concentration of dissolved solids in the reaction mixture does not exceed about 60%;   (3) maintaining the reaction mixture under these conditions for formation of an easily filterable, solid, calcium sulfate precipitate;   (4) removing the solid calcium sulfate from the reaction mixture to provide a filtrate;   (5) contacting said filtrate with a calcium-bearing material selected from the group consisting of phosphate rock, tricalcium phosphate, dicalcium phosphate, calcium phosphate, calcium oxide and mixtures thereof, alternatively or successively, and removing the precipitate solids to provide a resultant solution lowered in sulfate ion;   (6) contacting said resultant solution with a substantial equal volume of a water-miscible organic solvent, allowing the potassium dihydrogen phosophate to precipitate and form solid potassium dihydrogen phosphate;   (7) removing the solid potassium dihydrogen phosphate to provide a phosphoric acid mother liquor;   (8) removing the organic solvent; and   (9) recycling the phosphoric acid mother liquor to the main reactor. .[.14. A continuous process for the preparation and recovery of potassium dihydrogen phosphate from the reaction of phosphate rock or more solubilized form thereof, sulfuric acid and potassium hydrogen sulfate, the improved steps which comprise:   (1) contacting phosphate rock or more solubilized form thereof with a sufficient excess of sulfuric acid to drive the reaction to completion to form a reacting slurry at a temperature of about 40° to 90° C. in a main reactor comprising a series of reactors with continuous recycle of a portion of the reacting slurry from the last reactors to the first or intermediate reactors;   (2) adding thereto a 10-50% aqueous solution of potassium hydrogen sulfate in sufficient amount to form a reaction mixture containing ion of potassium hydrogen phosphate and at a rate so as to minimize the formation of insoluble salts of KHSO 4  and CaSO 4  and under such conditions that the concentration of dissolved solids in the reaction mixture does not exceed about 60%;   (3) maintaining the reaction mixture under these conditions for formation of an easily filterable, solid, calcium sulfate precipitate;   (4) continuously removing a portion of the reacting slurry;   (5) separating the solid calcium sulfate from the reacting slurry to provide a solid calcium sulfate cake and a filtrate;   (6) washing the solid calcium sulfate cake with water and recycling the wash water to the main reactor;   (7) recovering the solid potassium dihydrogen phosphate from the filtrate to provide a phosphoric acid mother liquor; and   (8) recycling the mother liquor to the main reaction..]. .[.15. A continuous process according to claim 14 wherein the filtrate from step (5), after removal of the solid calcium sulfate cake, is contacted with a calcium-bearing material selected from the group consisting of phosphate rock, tricalcium phosphate, dicalcium phosphate, calcium oxide and mixtures thereof, alternatively or successively, at a temperature of 40° to 90° C. to precipitate additional sulfate ion and partially neutralize the mixture and removing the precipitated solids to provide a low-sulfate filtrate containing potassium dihydrogen phosphate, recovering the solid potassium dihydrogen phosphate from the low-sulfate filtrate to provide a phosphoric acid mother liquor, and recycling the mother liquor to the main reactor..]. .[.16. A process according to claim 14 wherein the low-sulfate filtrate containing the potassium dihydrogen phosphate is mixed with a substantially equal volume of a water-miscible organic solvent, the solid potassium dihydrogen phosphate is allowed to precipitate and is separated from a phosphoric acid mother liquor, the organic solvent is recovered from the mother liquor and cycled to storage for reuse in precipitating additional potassium dihydrogen phosphate, and the phosphoric acid mother liquor is recycled to the main reactor..]. .[.17. A process according to claim 15 wherein calcium oxide is added to the phosphoric acid aqueous solution from step (5) in sufficient amount to raise the pH of the solution and form potassium monohydrogen phosphate..]. .Iadd.18. In a process for the preparation of potassium dihydrogen phosphate by the reaction of phosphate rock or a solubilized form thereof with a reaction medium containing sulfuric acid and potassium hydrogen sulfate at a temperature of about 40° to 90° C., removing solid calcium sulfate from the resulting reaction mixture to provide a solution of potassium dihydrogen phosphate in phosphoric acid, and recovering solid potassium dihydrogen phosphate from the phosphoric acid solution, the improved steps comprising:   (1) forming said reacting medium by initially forming a mixture of phosphate rock or a solubilized form and an excess of sulfuric acid;   (2) adding thereto a 10-50% aqueous solution of potassium hydrogen sulfate in sufficient amount to form a reaction mixture containing ions of potassium dihydrogen phosphate and at a rate so as to minimize the formation of insoluble salts of KHSO 4  and CaSO 4  and under such conditions that the concentration of dissolved solids in the reaction mixture does not exceed about 60%; and   (3) maintaining the reaction mixture under these conditions for formation of an easily filterable calcium sulfate precipitate. .Iaddend..Iadd.19. A process according to claim 18 wherein the potassium hydrogen sulfate solution is prepared by the reaction of potassium chloride and sulfuric acid at a temperature of about 250°-300°  C., the hydrogen chloride is removed and the resulting potassium hydrogen sulfate is dissolved in water to form an aqueous solution of about 10-50%. .Iaddend. .Iadd.20. A process according to claim 19 wherein the sulfuric acid has a concentration of about 60 to 98% and is employed to provide an excess of about 5 to 10 weight percent excess sulfate ion in the reacting mixture. .Iaddend..Iadd.21. A process according to claim 19 wherein the potassium hydrogen sulfate solution contains about 4% H 2  SO 4  and about 42% KHSO 4 . .Iaddend..Iadd.22. A process according to claim 20 wherein the phosphoric acid filtrate containing potassium dihydrogen phosphate, after removal of the calcium sulfate, is treated with a calcium-bearing material selected from the group consisting of phosphate rock, tricalcium phosphate, dicalcium phosphate, calcium phosphate, calcium oxide and mixtures thereof, alternatively or successively, at a temperature of 40°-90° C., to precipitate additional sulfate ion and partially neutralize the mixture, and removing the solids to provide a lower sulfate filtrate containing the potassium dihydrogen phosphate. .Iaddend..Iadd.23. A process according to claim 18 wherein after removal of calcium sulfate from the filtrate, it is concentrated and then cooled to precipitate solid potassium dihydrogen phosphate. .Iaddend.