United States Patent (19) 11 Patent Number: 4,639,359 Michalski et al. 45) Date of Patent: Jan. 27, 1987

PROCESS OF REMOVING CATIONC (54) 4,493,820 l/1985 Clausen ...... 423/310 IMPURITIES FROM WET PROCESS 4,500,502 2/1985 McDonald et al...... 423/32 R PHOSPHORIC ACID 4,554,144 ll/1985 Ore ...... 423/321 R (75) Inventors: Dennis H. Michalski, Lakeland, Fla.; OTHER PUBLICATIONS Viswanathan Srinivasan, Terre Fertilizer Manual, International Fertilizer Development Haute, Ind. Center, United Nations Industrial Development Orga (73) Assignee: International Minerals & Chemical nization, Dec. 1979, pp. 191-193. Corp., Terre Haute, Ind. Primary Examiner-Gregory A. Heller 21) Appl. No.: 809,440 Attorney, Agent, or Firm-Wendell R. Guffey; Thomas L. Farquer 22 Filed: Dec. 16, 1985 57 ABSTRACT (51) Int. Cl." ...... C01B 25/16 52) U.S. C...... 423/321 R; 423/158; A method of removing aluminum, magnesium, iron and 423/317; 423/465 other impurities from wet process phosphoric acid is (58) Field of Search ...... 423/321 R, 320, 317, provided. The method comprises partially ammoniating 423/321 S, 158, 465,472 the acid and reacting the acid with a fluoride ion donat ing compound to precipitate aluminum- and magnesi 56) References Cited um-containing ralstonite and fluorosilicate U.S. PATENT DOCUMENTS which can be easily separated from the acid thereby 3,642,439 2/1972 Moore et al...... 423/321 R producing a partially ammoniated wet process phos 4,136,199 1/1979 Mills ...... 423/321 R phoric acid of reduced impurities content. 4,288,804 11/1981 Parks et al...... 423/321 R 4,435,372 3/1984 Frazier et al...... 423/321 R 20 Claims, No Drawings 4,639,359 1. 2 phate rock include minerals containing magnesium, PROCESS OF REMOVING CATIONC aluminum, iron and others. IMPURITIES FROM WET PROCESS U.S. Pat. No. 4,136,199 discloses a method for remov PHOSPHORIC ACID ing metal ion impurities from wet process phosphoric acid by treating waste pond water from a wet process BACKGROUND OF THE INVENTION phosphoric acid plant with lime or limestone to obtain a 1. Field of the Invention sludge high in calcium fluoride which, when added to wet process acid, causes the precipitation of solids con The present invention relates to a method of remov taining and metal ions such as magnesium and ing impurities from wet process phosphoric acid. More O aluminum. particularly, the present invention relates to a process of U.S. Pat. No. 4,500,502 discloses a method of prepar precipitating aluminum, magnesium, iron and other ing a purified ammoniated phosphoric acid composition cationic impurities from wet process phosphoric acid. by reacting wet process phosphoric acid containing 2. Description of the Prior Art magnesium, aluminum and fluorine impurities with am Wet process phosphoric acid is made by reacting 15 monium ions, including ammonia, in order to precipi phosphate rock with sulfuric acid. Phosphate rock is tate complex metal salts containing magnesium, alumi principally composed of fluoroapatite, but large num and fluorine. Unfortunately the complex metal amounts of contaminating substances are always pres salts also contain valuable phosphates which are lost. ent. These include silica and metal values such as iron, U.S. Pat. No. 4,299,804 discloses a method of remov aluminum and magnesium along with smaller amounts 20 ing magnesium and aluminum impurities from wet pro of trace impurities. During the reaction with sulfuric cess phosphoric acid. The disclosed process maintains acid, gypsum is precipitated and removed by filtration. the Al/Mg mole ratio within the range of 1.1-2.0 and After filtration of gypsum, many solubilized impurities the F/Al ratio within the range of 3.5-7 by adding remain in the acid. Because the quality of phosphate aluminum ion donating compounds (such as alum) and rock is declining as the better grades of the mineral are 25 fluoride ion donating compounds (such as hydrofluoric mined, the impurity levels in the wet process acid ob acid) which are reacted with wet process acid to precip tained from mined phosphate rock have been steadily itate crystalline compounds containing magnesium, increasing. aluminum and fluorine (MgAl2F8). The presence of impurities in wet process phosphoric U.S. Pat. No. 4,493,820 discloses a typical concentra acid results in a variety of problems for those who man 30 tion/clarification process. ufacture ammonium phosphate fertilizers. One problem U.S. Pat. No. 4,435,372 discloses the removal of alu is that the presence of impurities dilutes the nitrogen minum, magnesium and fluoride ion impurities from and P2O5 contents of the fertilizer ingredients thus re wet process phosphoric acid by hydrolizing and recy Sulting in a lower grade. Impurities also precipitate cling the off-gas scrubber solutions in the presence of a from the phosphoric acid and settle during storage 35 ferric iron catalyst. thereby resulting in slime accumulation in storage tanks U.S. Pat. No. 4,325,327 discloses a two-stage ammo and other apparatus resulting in reprocessing and equip niation process involving the precipitation of solids in ment cleaning costs, effective reduction in storage ca the pH regions of 1.5-2.5 and 4-5. The solids are filter pacity, production loss, the clogging of liquid fertilizer able and thus easily separated from the monoammonium application equipment, particularly spray nozzle ori phosphate solutions. fices through which the fertilizer is applied, and precipi SUMMARY OF THE INVENTION tates which tie up phosphate in a form that is unavail able to plants (i.e., citrate insoluble form). Accordingly, it is an important object of the present Because of the desirability of removing impurities invention to provide a process by which impurities, 45 especially metal cationic impurities such as aluminum, from wet process acid, a number of different impurity magnesium and iron, present in wet process phosphoric removal methods have been developed. One such acid can be substantially removed by precipitation method is solvent extraction, of which there are a num while at the same time maintaining a high P2O5 concen ber of variations. Solvent extraction involves the ex tration. traction of either phosphoric acid or impurities from 50 It is another important object of the present invention wet process phosphoric acid using an organic extract to provide an ammoniated liquid phosphate fertilizer ant, while leaving other components behind in the aque that can be stored for extended periods of time without ous solution. Significant disadvantages of solvent ex encountering solid precipitation problems. traction are the high capital and operating costs and the These and other important objects of the present fact that organic solvents must be handled. 55 invention are attained by a method of removing alumi Another type of impurity removal process is concen num, magnesium, iron and other impurities from wet tration/clarification of wet process phosphoric acid. In process phosphoric acid comprising partially ammoni the first step of this process, wet process acid is concen ating wet process phosphoric acid having a P2O5 con trated to about 54% P2O5 and some of the impurities are centration within the range of about 17-54%, reacting allowed to precipitate. However, there are several the acid with a fluoride ion donating compound to pre major disadvantages to the process, namely, high en cipitate compounds containing said impurities, thereby ergy consumption, difficulties in concentrating the acid producing a partially ammoniated phosphoric acid of from low quality phosphate rock, the cost of clarifica reduced impurities content. The impurity-containing tion equipment and the inability to reduce impurity compounds precipitated in connection with the present concentrations to acceptably low levels. 65 invention include: (i) aluminum- and magnesium-con Another general method of removing metal impuri taining ralstonite; and (ii) ammonium fluorosilicate or ties from wet process acid involves ammoniation of wet magnesium fluorosilicate; which precipitated com process acid. Metal impurities normally found in phos pounds can be easily separated from the acid. The puri 4,639,359 3 4. fied acid comprises an excellent feed for producing these, ammonium fluoride and hydrofluoric acid ar monoammonium or diammonium phosphate. preferred since these compounds add no undesirable While specific preferred embodiments of the inven metal ion impurities (such as sodium and potassium) to tion have been selected for description hereinbelow, it the wet process acid. Compounds such as fluosilicic will be appreciated by those skilled in the art that a wide 5 acid and fluorosilicate salts cannot be used in the pres variety of modifications and variations of the invention ent invention since these compounds do not freely dis can be made without departing from the scope of the sociate when in solution. invention, as defined in the appended claims. The wet process acid may be ammoniated in any number of ways, including sparging with ammonia gas DETAILED DESCRIPTION 10 or by the addition of hydrous ammonia to the wet pro The method of the present invention is effective for cess acid. The amount of ammonia added to the acid is removing aluminum, magnesium, iron and other impuri preferably calculated to give a N/P mole ratio in the ties from wet process phosphoric acid, having a P205 range of about 0.2-1.0. In the case when the fluoride ion concentration of from about 17 to 54% by causing pre donating compound contains nitrogen (such as ammo cipitates of ralstonite and fluorosilicates to form. Nor 15 nium fluoride) the N/P mole ratio should be within this mally, the phosphoric acid will have a P2O5 concentra range after both partial ammoniation and the addition of tion of from about 17% to 45%. Briefly, the method the fluoride ion donating compound. Preferably, the comprises the combined treatment of (i) partially am N/P mole ratio is within the range of about 0.2-0.3. moniating the wet process acid and (ii) adding a fluoride When using ammonium fluoride, the amount added ion donating compound. 20 to the wet process acid may be in concentrations up to The method of the present invention is particularly about 3.0 wt.%, preferably about 1.5 wt.%, based upon useful in connection with processes in which wet pro the total weight of the acid. If other fluoride donating cess phosphoric acid is ammoniated and granulated to compounds are used, the total added fluoride should be produce monoammonium or diammonium phosphate. in an amount of up to about 1.5% wt.% as fluoride. The Typically when making an ammoniated phosphate fer 25 amount of fluoride ion donating compound added to the tilizer, a mixture of wet process phosphoric acid, having wet process acid should be determined based upon the a P2O5 content of about 40-55%, and a scrubber liquor concentrations of magnesium and aluminum impurities (also a wet process phosphoric acid stream) is reacted in in the acid. Thus, in cases where the acid contains very a preneutralizer with ammonia to form a slurry having low magnesium and aluminum impurities, only small an N/P mole ratio between about 1.35 to 1.55. The amounts of the fluoride ion donating compound need be slurry then is further ammoniated in a drum granulator added. to produce granular diammonium phosphate. Scrubber The end product of the ammoniation of wet process liquor is produced when dilute wet phosphoric acid phosphoric acid is typically an ammonium phosphate (about 18%-30% P2O5) is used to scrub ammonia va fertilizer. Fertilizers are typically graded depending pors emanating from the granulator and the preneutral 35 upon their nitrogen, phosphorus and potassium con izer. The N/P mole ratio of the scrubber liquor often tents. The ammoniation of phosphoric acid produces a ranges from about 0.3 to about 1.6, typically about 0.4. N.P grade fertilizer. Thus, the use of ammonium fluo According to one embodiment of the present inven ride as the fluoride ion donating compound is preferable tion, a soluble fluoride ion donating compound capable from the standpoint that it contributes to the overall of releasing free fluoride ions, such as ammonium fluo nitrogen content of the fertilizer product. ride, is added to the scrubber liquor. The scrubber li In the processes of the present invention, three sepa quor constitutes a partially ammoniated wet process rate species of compounds may be precipitated. The phosphoric acid stream. Ammonium fluoride should be first is ralstonite which is a class of compounds in the added in an amount of up to about 3.0 wt.%, preferably cryolite family having the general formula: about 1.5 wt.%, based on the total weight of the scrub 45 ber liquor. If other fluoride ion donating compounds are used, the fluoride should be added in an amount up to about 1.5 wt.% as F. If necessary, additional ammonia wherein y typically equals 6, z typically equals 1, and X can be added to the scrubber liquid to increase its N/P typically equals 0-1. ratio to at least about 0.2. 50 The second class of compounds precipitated by the Seed crystals of ralstonite, which assist in the precipi processes of the present invention comprise fluorosili tation of ralstonite, also may be added at this time. cates of the general formula: The mixture is then sent to a settling tank where the mixture is allowed to sit preferably for up to about 8 (M)SiF6 hours. This allows the ralstonite crystals, as well as the 55 fluorosilicate crystals to form and settle to the bottom of wherein M is a cation such as ammonium, magnesium, the tank. The mixture can then be sent to any number of calcium, etc. and q is 1 or 2. known separation apparatus including filtering devices, A third class of precipitates comprises aluminum and centrifuges, decanters and the like in order to separate iron-containing compounds having the general formula the precipitated ralstonite and fluorosilicates from the 60 purified acid. The purified acid then can be used for manufacturing diammonium phosphate or monoammo nium phosphate. While this third class of precipitates is desirable from As examples of fluoride ion donating compounds the standpoint of removing iron impurities from the there can be mentioned any compound which freely 65 acid, it is undesirable from the standpoint that some donates fluoride ions when in solution. For example, phosphate values are lost. Unfortunately, the art has yet NH4F, HF, NaF, and NaHF2, NH4HF2, and KF may to devise any means of precipitating iron without also all be used as fluoride ion donating compounds. Of precipitating some phosphate values. However, the 4,639,359 5 6 processes of the present invention precipitate much less ing a P2O5 content of about 17-54%, comprising the phosphate values than the prior art processes which steps of: tended to lose phosphate values in the removal of mag- partially ammoniating the phosphoric acid; nesium and impurities, as well as in the re- adding a fluoride ion donating compound to the acid moval of iron impurities. 5 to form a precipitate; The advantages of the present invention will become separating the precipitate from the acid; and more apparent from the examples appearing hereinafter. recovering a partially ammoniated wet process phos phoric acid of reduced impurity content. EXAMPLES 1-4 2. The method of claim 1 wherein the phosphoric In Examples 1-4, 450 g of 27% acid was placed in a 10 acid has a P2O5 concentration of about 17 to 45% and a teflon coated stainless steel beaker with a teflon stir bar. precipitate containing a fluorosilicate, an aluminum-, A loose-fitting plexiglas cap having holes for a teflon magnesium- and fluorine-containing ralstonite and an coated thermometer was placed on the beaker. The iron-, potassium- and ammonium-phosphate is formed. beaker was also fitted with a gas sparger (the sparger 3. The method as defined in claim 1, wherein the was made from teflon tubing with 16-20 1.5' long razor 15 fluoride ion donating compound comprises HF. blade slashes near one end and was plugged with solid 4. The method as defined in claim 1, wherein the teflon). The contents of the beaker were heated to 75' fluoride ion donating compound is selected from the C. while stirring vigorously on a Corning PC-351 hot group consisting of ammonium fluoride and ammonium plate stirrer, after which time the quantity of NH4F bifluoride and both. indicated in Table 1 was quickly added to each sample. 20 5. The method as defined in claim 2, wherein the The beaker was heated to 85 C. and ammonia was ralstonite comprises NH4MgAlF6.H2O. sparged over a 22 minute period. Subsequently, the 6. The method as defined in claim 2, wherein the slurry was aged with slow stirring at 75 C., removed precipitated fluorosilicate comprises ammonium from the heat and its viscosity measured at 75 C. with fluorosilicate. a Thomas viscosimeter. The slurry was centrifuged at 25 7. The method as defined in claim 3, wherein the 600- g for 40 minutes. The supernatant was collected precipitated fluorosilicate comprises magnesium and weighed and the solids were washed with acetone, fluorosilicate. centrifuged twice, and then washed again with acetone, 8. The method as defined in claim 1, wherein the filtered and dried. The chemical analyses for the four fluoride ion donating compound is selected from the examples are presented in Table 1. 30 group consisting of , sodium bifluoride TABLE 1. Supernatant Liquid Chemical Analyses N/P Wt % Aging Example Mole NH4F Time % P2O5 No. Ratio Added (min.) 7% NH4 % Al % Fe % Mg 7% F % P2O5 (% AI + 7% Fe+% Mg) Untreated -- - - 0.069 0.495 0.89 0.275 2.5 26.7 16.08 Acid 1 0.3 3.0 60 3.22 0.17 0.80 0.0842 2. 26.8 25.42 2 0.3 1.5 60 3.28 0.17 0.82 0.0804 2. 26.8 25.04 3 0.3 1.5 240 3.23 0.080 0.12 0.0376 2.1 26.5 111.53 4. 0.5 1.5 240 4.63 0.099 0.0433 0.046 2.0 26.2 139.09

The SO4 content was 1.6% for the untreated acid, was and mixtures thereof. 1.7% for Examples 1 and 2 and was 1.8% in Examples 45 9. The method as defined in claim 1, wherein the 3 and 4. The acid of Example 2 was seeded with 0.6677 fluoride ion donating compound is selected from the g of the solids recovered from Example 1 prior to the group consisting of , potassium biflu addition of ammonium fluoride. Ammoniation and fluo- oride and mixture thereof. ride additive levels for Example 3 were similar to those 10. The method as defined in claim 1, wherein the of Example 2, but the aging time was increased to four 50 ammoniated phosphoric acid has an N/P mole ratio in hours and 1.02 g of solids recovered in Example 1 solids the range of 0.2-1.0. were used for seeding. Example 4 was similar to Exam- 11. The method as defined in claim 10, wherein the ple 3 but the N/P mole ratio achieved by ammoniation ammoniated phosphoric acid preferably has an N/P was increased from 0.3 to 0.5. mole ratio within the range of 0.2-0.3. X-ray diffraction analysis for the solids collected 55 12. The method as defined in claim 1, wherein the from Example 1 revealed significant amounts of ralston- fluoride ion added is in an amount of up to about 3.0 wt. ite, Fe3(K,NH4)H8(PO4)6.6H2O), ammonium fluorosili- % based upon the total weight of the acid. cate and . 13. The method as defined in claim 12, wherein the Increasing the aging time from one hour to four hours amount of fluoride ion added is up to about 1.5 wt.%. decreases the final magnesium and aluminum content of 60 14. The method as defined in claim 2, wherein the the liquid phase acid by over 50% and the final iron iron phosphate precipitate comprises a compound hav content by 85%. Increasing the N/P mole ratio to 0.5 at ing the formula of Fe3(K,NH4)H8(PO4)6.6H2O. a four hour aging time increased the magnesium and 15. The method as defined in claim 1, including the aluminum contents slightly, while reducing the iron additional step of allowing the ammoniated phosphoric content by an additional two-thirds. 65 acid containing the added fluoride ion donating com We claim: pound to settle. 1. A method for removing aluminum, magnesium and 16. The method as defined in claim 15, wherein the iron impurities from wet process phosphoric acid hav- settling is conducted for up to about 8 hours. 4,639,359 7 8 17. The method as defined in claim 1, wherein the recovering a partially ammoniated wet process phos ammoniating step comprises sparging ammonia gas phoric acid of reduced impurity content. through the wet process phosphoric acid. 20. A method for removing aluminum, magnesium, - - and iron impurities from wet process phosphoric acid 18. The method as defined in claim 1, wherein the 5 having a P2O5 content of about 17-54%, comprising the ammoniating step comprises adding- hydrous ammonia steps of: to the wet process phosphoric acid. partially ammoniating said phosphoric acid using a 19. A method for removing aluminum, magnesium, method selected from group consisting of sparging and iron impurities from wet process phosphoric acid ammonia gas through and adding anhydrous am having a P2O5 content of about 17-54%, comprising the 10 monia to said phosphoric acid thereby producing a steps of: phosphoric acid with a N/P ratio in the range from partially ammoniating said phosphoric acid thereby 0.2-1.0; producing a phosphoric acid with a N/P ratio in addingfrom thea fluoride group consistingion donating of HF, compound ammonium selected fluo the range from 0.2-1.0; 15 ride, ammonium bifluoride, a mixture of ammo adding a fluoride ion donating compound selected nium fluoride and ammonium bifluoride, sodium from the group consisting of HF, ammonium fluo fluoride, sodium bifluoride, a mixture of sodium fluoride and sodium bifluoride, potassium fluoride, ride, ammonium bifluoride, a mixture of ammo potassium bifluoride, and a mixture of potassium nium fluoride and ammonium bifluoride, sodium fluoride and potassium bifluoride to said ammoni fluoride, sodium bifluoride, a mixture of sodium 20 ated acid in an amount up to about 3.0 wt.% based fluoride and sodium bifluoride, potassium fluoride, upon total weight to form a precipitate; potassium bifluoride, and a mixture of potassium separating said precipitate from said ammoniated fluoride and potassium bifluoride to said ammoni acid; and ated acid to form a precipitate; 25 recovering a partially ammoniated wet process phos separating said precipitate from said ammoniated phoric acid of reduced impurity content. acid; and sk x 2k x 2k

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