Patented July 6, 1954 2,683,075

UNITED STATES PATENT OFFICE 2,683,075 DIGESTION OF PHOSPHATE ROCK Paul Caldwell, Evergreen Park, I., assignor, by mesne assignments, to Cannac Research and Development Company, Thornwood, N. Y., a joint venture No Drawing. Application June 5, 1951, Serial No. 230,07 Cains, (C. 23-09) 2 This invention relates to the digestion of phOS vention is further characterized by the elimina phate rock and, more particularly, to the recoV tion of a substantial portion of any fluorine con ery of the phosphatic value of such rock. The ponent of the phosphate rock so treated With the invention includes a method of recovering trical result that the end product is particularly Suit cium phosphate from phospate rock in a form 5 able for use as livestock feed Without requiring which is useful both as a fertilizer and as live pyrolytic treatment of the rock or of the final stock feed as well as a method of producing Such product for the elimination of fluorine pursu tricalcium phosphate in admixture with other ant to conventional practice. compounds of known fertilizer Value, and further The method of recovering a non-hygroscopic includes the novel products obtained thereby. O tricalcium phosphate product from phosphate It is conventional practice to recover the phos rock in accordance with my invention comprises phatic value of phosphate rock by digestion of dissolving the tricalcium phosphate component of the rock with Sulfuric acid. Regardless of the rock in an inorganic nitrogenous acid, Sep Whether this digestion is carried Out With Suf arating the resulting phosphorus- and calcium ficient acid to form a product consisting pre 5 containing solution from the acid-insoluble ima dominantly of monocalcium acid phosphate or terial, adding to said solution potassium chloride, with a lesser annount of acid to form a superphos Sodium chloride or sodium sulfate, neutralizing phate, the digestion requires the use of large the resulting phosphorus-, calcium- and potas quantities of the Sulfuric acid. This fact has sium- or SOdium-containing Solution with ann prompted numerous prior investigations to find 20 monia, and recovering the resulting precipitated an alternative reagent or procedure for recover tricalcium phosphate product. The added al ing the phosphatic value of phosphate rock. The kali metal Salt effects precipitation of a large recent critical shortage of Sulfuric acid has placed portion of any fluorine component of the rock a further prennium on the development of a phos in the form of potassium fluosilicate, the latter phate rock digestion operation which does not 25 being advantageously separated from the aque depend upon the use of this acid. ous phase prior to the ammonia-neutralizing step. All previous studies of this problenn have led The precipitated tricalcium phosphate product to the conclusion that such alternative phosphate may be recovered in a form largely free of other rock digestion reagents produce less satisfactory Compounds, or the ammonium and potassium end products than the product obtained by Sull 30 Or Sodium salts present in the solution from furic acid digestion. For example, attempts to which the tricalcium phosphate is precipitated produce a Suitable phosphatic material by diges may be crystallized therefrom so as to form an tion of phosphate rock with nitric acid have been addition to the precipitated tricalcium phosphate. thwarted by the substantial amount of calcium The novel phosphatic product of my invention. nitrate formed along the resulting acid phosphate 35 comprises a.. non-hygroscopic and substantially or Superphosphate. The hygroscopic nature of fluorine-free tricalcium phosphate obtained by calcium nitrate is largely responsible for the wet dissolving the tricalcium phosphate component of and gunny consistency of the phosphate product, phosphate rock in an inorganic nitrogenous acid, and has defeated prior attempts to produce a adding potassium chloride, sodium chloride or phosphatic material from phosphate rock by di 40 Sodium sulfate to the resulting acid solution to gestion with nitric acid. remove fluorine therefrom in the form of pre I have now devised a method of recovering cipitated potassium or sodium fluosilicate, and the phosphatic value of phosphate rock by a pro neutralizing the resulting solution with ammonia, cedure involving digestion of the rock with an to precipitate the tricalcium phosphate product. in Organic nitrogenous acid, such as nitric acid, This product may further contain, in accordance which is nevertheless characterized by the elim With the invention, an ammonium salt and po ination from the phosphatic end product of any taSSium or sodium nitrate obtained by crystal hygroScopic salts which might interfere With the lization from the ammonia-neutralized solution Stability and free flowing characteristics of this from which the tricalcium phosphate is precipi product. The method of my invention effects tated. The resulting product containing these Sequestering of the calcium component of the compounds comprises a highly effective, stable phosphate rock substantially only in the form of and non-hygroscopic fertilizer composition. the ultimate phosphatic compound which, as ex The product obtained by the method of my plained further herein, comprises essentially tri invention is Substantially non-hygroscopic in the calcium phosphate. The novel method of my in 55 accepted sense that it does not absorb a signifi 2,683,075 3. 4 cant amount of moisture from the atmosphere. fluorine component of the rock is largely in the The product does adsorb atmospheric noisture form of a calcium salt, and inasmuch as the rock to some extent but this adsorbed moisture does generally contains other calcium SaitS Such as not impart to the product any significant tend the carbonate, the solution which is neutralized ency to become wet, cake-like or gummy. Al 5 with annonia, will contain an excess of Calcium though the product obtained with the use of over and above that which will combine With the the potassium salt pursuant to the invention is phosphorus to precipitate tricalcium phosphate. less hygroscpic than that, obtained with the use Accordingly, the precipitated product Will Con of the sodium salt, both products can be accu tain other non-hygroscopic calcium compounds rately characterized as substantially non-hygro such as calcium hydrate or possibly tetracaiciun scopic under average United States Weather con phosphate, or both. If, as described an Ore fully ditions. hereinafter, the solution from which the trical The product of my invention is characterized cium phosphate product is precipitated is further by this non-hygroscopicity in the unwashed but suitably concentrated, an annonium Salt and dry state. If the precipitated product were 5 aikali metal nitrate are obtained either for Sepa washed, it Would be expected that it would he rate use or for addition to the precipitated tri non-hygroscopic, but drying of the washed pre calcium phosphate product in the preparation of cipitate causes a Substantial reversion of the a complete fertilizer material. In the latter phosphatic precipitate to an insoluble form event, the annoniura sat and the nitrate both whence it is not readily available for fertilizer 20 provide available nitrogen for the fertilizer, and value. However, when the precipitate is dried where potassium chloride is used as the added without Washing, there is no significant reversion alkali metal salt the resulting potassium initrate of its phosphatic content to an insoluble form forms a particularly Suitable Source of potash. and the resulting dried product is nevertheless The method of my invention is applicable to substantially non-hygroscopic. 25 the treatment of any phosphate rock. Prepa Digestion of the phosphate rock with the in ration of the rock for digestion. With nitric acid Organic nitrogenous acid effects dissolution of is substantially the Sarine as that which prevails the tricacium phosphate component of the rock in the conventional digestion of such rock With with the resulting production of a solution con Sulfuric acid. Thus, Satisfactory resultS can be prising phosphoric acid and calcium nitrate. 30 obtained with rock ground to such a degree Of The acid-insoluble cornponents of the rock settle fineness that, 80-90% will paSS through a 60 aesh out of this Solution and are preferably renoved screen (Tyler standard), although the rock inay before further treatnant of the phosphoric acid be more finely ground so that up to about 95% calciurn nitrate solution. The separated solu Will pass through a 100 resh Screen. As in the tion will further contain most if not all of any 35 case of other aethods of digesting phOSphate florine component of the rock in the form of rock, the degree of Subdivision of the rock is of fluosilicic acid. When the potassium chloride importance only in so far as it affects the late or sodium chloride or sulfate is added to this of its reaction. With the digesting agent. Where Solution in accordance with the invention, a the Speed of reaction can be Sacrificed Sonewhat large portion of this fluorine component of the 40 for ease in Separating the acid-inSouble poi Solution is removed in the form of a precipitate tion of the rock, a coarser starting material Such of the corresponding alkali metal fluosilicate. aS 20 resh rock can be used. The alkali metal salt (an expression which will The digesting agent used in the practice of the be used hereinafter to refer generically to the invention cornprises such inorganic nitrogenous potassium chloride, sodium chloride or sodium 45 acids aS nitric acid or the acid or acid iiixtutes Sulfate) is added in amount not only substan obtained by the passage of gaseous nitrogen tially equivalent stoichiometrically to the fluo Oxides through an aceous redill). Thus, the rine Component of the phosphoric acid solution nitrogenous acid may comprise nitric acid for but also in annount substantially equivalent a commercial Source or it may coprise it;og Stoichiometrically to the calcium nitrate content, 50 enous acids produced at the Site of the digestion of the acid Solution. Upon separation of the Operation by the passage of gaseous nitrogen precipitated alkali metal fluosilicate, the resulti Oxides through Water or other aceous liquor. ing Solution will contain phosphoric acid, calcium Alternatively, the nitrogenous digestion agent, nitrate and the added alkali metal chloride or may be produced in situ by passing gaseous Sulfate. Neutralization of this solution with 55 nitrogen oxides through an aqueous Suspension annonia will then precipitate the calcium and or slurry of the phosphate rock, the aqueous phosphorus components of the solution. The medium being either water, nitric acid of any Solution fron which these components have concentration ranging from dilute to concen been precipitated is therefore substantially free trated form, or either of these augmented by of calcium ions and contains ammonium ions, 60 recycled liquor from the annonia-neutralizing alkali netal ions, chloride ions and nitrate ions. Step. Under the neutral or slightly alkaline conditions The concentration of the nitrogenous digestion prevailing in the Solution, the equilibrium be-, agent, which Solely for convenience and not by tWeen these ions is Such that if the solution is Way of limitation will be referred to hereinafter concentrated to crystallize dissolved salts there 65 as "nitric acid,' may range from dilute to con centrated form while nevertheless producing ef fron, the crystallized salts will comprise essen fective results. Dilute solutions of the nitric tially ammonium chloride and the alkali metal acid require more prolonged digestion periods nitrate. than the concentrated acid, whereas the highly It will be seen from the foregoing description 0 concentrated acid requires more care than the of the chemical aspects of my novel method that dilute acid in controlling the elevation of the the tricalciuin phosphate component of the phos temperature of the reaction mass which, if un phate rock is ultimately obtained in the form of controlled, promotes excessive volatilization of tricalcium phosphate uncontaminated by any the acid. Thus, dilute nitric acid may be used hygroscopic calcium salts. Inasmuch as the 75 with particular effectiveness in digesting very 2,683,075. 5 6 finely ground phosphate rock-and-relatively con major portion of the fluosilicic acid present in centrated acid may be used satisfactorily, in di the Solution will be precipitated so that it can be gesting mere coarsely ground rock. - removed. The amount of nitric: acid required in the di The alkali metal salt, added to the phosphoric gestion stage will depend essentially upon eco acid-calcium nitrate solution as described may nomic considerations. Inasmuch as the nitric be either potassium chloride, Sodium chloride Or acid is more expensive than the phosphate rock, Sodium sulfate. Of these compounds, sodium it is generally preferred to use such relative chloride is the most abundant and least expen amounts of the rock and acid as will effect Sub Sive and is preferred where economical consid stantially complete utilization of the acid, but a 10 erations predominate. However, where it is de balance should obviously be... achieved between Sired to obtain an ultimate tricalcium phosphate the degree of utilization of the acid and the de product enhanced in fertilizer value by virtue of gree of recovery of the phosphatic component the presence of an additional potash content, and of the rock. Where such a balance, decided Where as mentioned previously the most non upon economic grounds, results in incomplete hygroscopic product is desired, I have found it dissolution of the available phosphatic: ; compo particularly advantageous to use potassium nent of the rock, the rock residue from the acid chloride in this stage of the method. The afore treatment may be added to a fresh charge-of rock negationed alkali metal salts are preferred in for further recovery of its phosphatic value. practicing the invention for the reason that they The temperature maintained during the di have generally satisfactory solubility character gestion operation is not critical although more istics and do not cause precipitation of the cal elevated temperatures are conducive to more cium component of the phosphoric acid-calcium rapid digestion. The action of the nitric acid nitrate Solution. Thus, calcium chloride will not on the phosphate rock, being exothermic, tends be precipitated if an alkali metal chloride is to promote the development of an elevated ten used, and calcium sulfate will not be precipitated perature throughout the reaction mass so that rapidly by the addition of sodium sulfate for the it is generally unnecessary to apply extraneous reason, at least in part, that calcium sulfate is heat to the digestion operation. The only pre Soluble in acidic Solutions and in solutions con caution which need be observed with respect to taining ammonium salts, both of these condi the temperature of the digestion operation is 30 tions prevailing at one stage or another in the that the rate of reaction should be controlled, practice of the method of my invention. On the either by the rate of addition of the reagents or other hand, the addition of potassium sulfate by their concentration, or by both, so as to avoid causes the innaediate precipitation of calcium the development of temperatures which cause Sulfate and thus precludes the possibility of sub excessive volatilization and loss of the acid from Stantially ammoniating the digestion liquor the reaction maSS. The development of Zones of While its calcium component is still in solution. local overheating is substantially eliminated by Accordingly, Solely for convenience and without mechanical agitation of the digestion mass, this any intention of limiting the invention thereto, agitation preferably continuing throughout the the practice of the invention will be described entire reaction period. 40 hereinafter with Specific reference to the use of During the course of the digestion operation, potassium chloride. the tricalcium phosphate component of the phos The amount of potassium chloride which is phate rock is decomposed to form phosphoric added to the phosphoric acid-calcium nitrate acid and calcium nitrate both of which go into Solution should be not only sufficient to precipi. solution in the aqueous medium. The reaction : tate a major portion of the fluorine content of mass resembles a thin slurry or suspension and the Solution in the form of potassium fiuosilicate does not set up as does the reaction mass re but should be substantially equivalent stoichi Sulting.from sulfuric acid digestion of phosphate Onetrically to the calcium nitrate conteit of the rock. Thus, sand and numerous other-extrane Solution. The presence of such a stoichiometric OUIS COIponents of the phosphate rock. Which are amount of potassium chloride in the solution in not soluble in the nitric acid remain in suspen Sures the Subsequent removal from the solution Sion while the digestion mass is being agitated of Substantially all of the calcium component and Settle readily when the reaction mass is al thereof in the form of tricalcium phosphate or lowed to become quiescent. The aqueous phase other non-hygroscopic calcium connagind when containing the phosphoric acid and calcium ni , the Solution is subsequently neutralized with trate may then be largely separated from the in annonia and the resulting precipitated i:iii, Soluble material by decantation followed, if de is separated and recovered. sired, by filtration to remove that portion of the Following completion of the dissolution of the aqueoLIS phase entrained in the sludge-like resi potassium chloride in the phosphoric agid-cal due, cium nitrate solution with the resulting precipi The Solution thus separated from the acid tation of potassium fluosilicate, advantageously insoluble residue contains not only phosphoric accelerated by the aforementioned agitation, the acid and calcium nitrate but further contains potassium fluosilicate precipitate is separated the acid-Soluble fluorine component of the phos from the resulting aqueous phase by decantation, phate rock in the form of fluosilicic acid. The filtering, or the like. The resulting solution is fluosilicic acid is effectively removed from the then delivered to a suitable vessel wherein it is Solution by the addition thereto of the alkali neutralized with ammonia. metal Salt. Although this result is readily Neutralization of the substantially fuorine achieved at Substantially ambient temperature free phosphoric acid-calcium nitrate-potassiin When the alkali metal salt is added in the form chloride Solution may be effected with either an of a Solution thereof, the solution may with ad hydrous or aqueous ammonia, although an vantage be heated in order to facilitate dissolu hydrous ammonia, is preferred ina.Sinch as it is tion. Of the alkali metal salt when the latter is more consistent with the maintenange of high added in Solid form. At such temperatures, and Salt concentrations as described hereinafter. The With the aid of vigorous mechanical agitation, a amount of ammonia added in this stage should be 2,683,075 7 8 Sufficient to completely. neutralize the Solution following the digestion operation itself. Thus, and preferably should be sufficient to provide a, the ammonium chloride-potassium nitrate Solu slightly alkaline pH. As this neutralization is tion can be admixed with the reaction in a SS Ob completed, accompanied by appropriate agitation tained as a result of the digestion, or the Solu of the Solution, the phosphatic component of the 5 tion may be introduced into the Separated phOS Solution is precipitated predominantly in the form phoric acid-calcium nitrate Solution either before of tricalcium phosphate and substantially all or after the addition of potassium chloride there of the calcium component is precipitated either to. On the other hand, the ammoniuin chloride Coimbined with the phosphorus or in the form of containing solution should not be recycled to any calcium hydrate. The resulting aqueous phase O stage of the operation prior to precipitation of is substantially free of dissolved calcium com the fluosilicate where this precipitation is effect pounds such as calcium nitrate so that the tri ed by a sodium salt ina.Smuch as Sodium fluo calcium phosphate Separated from the solution is silicate is unduly soluble in annonium Salt substantially free of any entrained calcium salt containing solutions. In practicing any of these which would tend to impart hygroscopic proper alternative recycling operations, the ammonium ties thereto, salt-containing solution may be recycled either It will be seen, accordingly, that the precipi in the form in which it is separated from the tated tricalcium phosphate product, separated precipitated tricalcium phosphate or the Solu from the aforementioned Solution and appropri tion may be concentrated by evaporation prior ately dried, is not only substantially inon-hygro 20 to being so recycled. However, where the am scopic but is also substantially free from fluorine. monia, neutralized solution containing precipi Thus, the product so obtained is particularly Suit tated tricalcium phosphate is Subjected to evap able for use as an adjunct to livestock feed With orative concentration to effect crystallization of out requiring any pyrolytic treatinent, as is usu ammonium chloride and potassium nitrate along ally the case with phosphatic feeds obtained with the precipitation of the tricalcium phoS from phosphate rock, for the elimination of phate, the liquor from which the Solids are Sepa fluorine to a tolerable inninimum value. The tri rated will generally be sufficiently concentrated calcium phosphate product obtained as described to be in Suitable condition for recycling directly hereinbefore is also particularly suitable for use to any earlier stage of the process. as a fertilizer. But when the product is to be 30 The building up of the concentration of anno used Soley as a fertilizer it will be clearly appar nium chloride and potassium nitrate in the ent that there is no need to remove, during its solution from which the tricalciun phosphate production, the various in Soluble residues and is precipitated is not adversely affected by the any alkali fluosilicate which is forined. Ac recycling of the ammonium chloride-pot2.SSirin cordingly, the tricalcium phosphate product ob nitrate solution to the acidic liquors prevailing in tained in this manner will contain the aforesaid earlier stages of the process. Ailillonia, is the insolubles as diluents, but the limited amount of only component of the recycled Salts which is these diluents does not adversely affect the value volatilized to any significant extent in these of the product as a fertilizer material. Although acidic liquors, and this component is readily and tricalcium phosphate is not as readily Soluble 40 adequately replaced in the final stage of the as the acid phosphate and Superphosphates, it is operation wherein ammonia is intiroduced to well established that my product is sufficiently effect precipitation of the tricalcium phosphate. soluble under conditions of use to meet all the The following specific example is merely illus requirements of an ideal fertilizer. trative of the practice of the method of my in The Solution from which the tricaciun phos 4 5 vention and gust not be considered as any lin phate product has been separated will contain itation thereof. In a stainless steel vessel, 155 Substantial concentrations of potassin, al parts by weight of a 35% solution of nitric acid raonium, nitrate and chloride ions. The equilib were added to 100 parts by weight of 77% B. P. T. rium constants of this solution, in its neutral rock ground to about 80% minus 100 mesh. The or slightly alkaline state, are such that when the O vessel was of such size that it was less than half solution is evaporated to increase the concentra filled by the acid-phosphate rock inixture. The tion of these ions there will be obtained crystals temperature of the rinixture rose to about 40° C. of annonium chloride and potassiut nitrate. It as the reaction proceeded and agitation if the will be apparent, accordingly, that the Solution mixture was continued until the evolution of gas may be concentrated by evaporation in such 5 5 ceased, thus indicating that the acid was Sub manner as to obtain both salts in admixture with stantially completely consumed. The remaining One another or in Such nanner as to effect sepa insoluble material was removed by filtering and rate recovery of the two salts. The ammonium was introduced along with another batgh of and potassium salts so obtained may be used for phosphate rock in a subsequent extraction Oper any purpose, although I have found it advan 60 ation. To the filtrate, comprising the Solubilized tageous to admix them with the tricalcium phos phosphate component or the rock, there were phate product, so as to form a more complete added 66 parts by weight of a potassium chloride fertilizer naterial. in the form of a Saturated solution at about 100 Concentration of the Solution from which the C. The mixture was agitated to facilitate re precipitated tricalcium phosphate is separated action and the resulting precipitate of potassium may also be effected by recycling the solution to fluosilicate was removed by filtering. The fil earlier steps in the process. For example, the trate, comprising the solubilized phosphatic ammonium chloride-potassium nitrate Solution component of the rock now largely freed of may be returned to the digestion Operation as fluorine, was then neutralized with anhydrous a source of at least a portion of the aqueous annonia, until a pH of was attained. The re medium in which the rock is digested. However, Sulting precipitate of the tricalciun phosphate if the introduction of such a solution into the di product was then removed by filtering and the gestion operation is incompatible with a desired unWashed filtel Cake was dried in an oven at a relatively high concentration of acid, the solu temperature of about 100° C. until firm and dry tion may be returned to any step immediately 75 in appearance. The dried product comprised

9. G about 175 parts by weight of a non-hygroscopic nitrate and phosphoric acid; adding to said cal tricalcium phosphate product containing by cium- and phosphorus-containing aqueous phase analysis 20% total P2O5, 19.5% available P3O5, an alkali metal salt of the group consisting of 5% N, 8% K2O and about 20% water. When the potassium chloride, sodium chloride and sodium moisture content of the product was further re 5 Sulfate in: an amount such that, in addition to duced by drying at a higher temperature, the the amount thereof consumed by the resulting phosphorus, nitrogen and potash contents of the precipitation of a substantial portion of any acid product were correspondingly increased in per Soluble fluorine component of the rock in the centage of the total weight of the product. The form of an insoluble alkali metal fiuosilicate, filtrate from which the tricalcium phosphate there will be further incorporated in said aqueous product had been operated was concentrated by phase an ariaount of the alkali metal salt at least evaporation, whereupon a portion of the potas Substantially equivalent stoichiometrically to the Sium nitrate and ammonium chloride compo calcium nitrate content of said aqueous phase; nents of the liquor were separated by crystalliza and after addition of said amount of the alkali tion and Were combined with the previously 15 metal Sait adding animonia, to the alkali metal separated tricalcium phosphate product in pro Salt-containing aqueous phase, thus precipitat ducing the complete fertilizer material. The ing therefrom the aforesaid dissolved calcium mother liquor from which the potassium nitrate and phosphorus components of the rock in the and ammonium chloride had been separated form of a Substantially non-hygroscopic calcium Contained excess potassium chloride. By re 20 phosphate product, turning this liquor to the next batch operation 2. The method according to claim. 1. Wherein in the form of the adueous medium by Which the alkali metal Salt is potassium chloride. potassium chloride was added to the acid-solu 3. The method according to claim 1 wherein bilized phosphate solution, the extraneous the nitrogenous acid is nitric acid. potassium chloride which had to be added to this 25 4. The method according to claim 1 wherein mother liquor was reduced to 60 parts by weight the phosphatic product precipitated by the am in lieu of the 66 parts used in the initial batch. moniation is Separated from the accompanying it will be seen, accordingly, that the method of aqueous phase, and the separated product is dried the present invention makes possible the recovery Without Washing. of the tricalcium phosphate component of phos 30 5. The method according to claim 1 whereir phate rock in a Substantially non-hygroscopic the annoniated aqueous phase is concentrated form either containing substantially all of the to promote precipitation of ammonium and al Original fluorine component of the rock or Sub kali metal salts therein, and all precipitated salts stantially free of fluorine. The method of the are recovered together. in Vention is also amenable to such building up 6. The method according to claim 1 wherein of the concentration of nitrogenous and potas the annoniated aqueous phase is separated from sium salts as to facilitate precipitation of these the precipitated phosphatic product and is re salts in addition to the ultimate precipitation of turned to an earlier step in the method in order the tricalcium phosphate with the resulting pro to effect concentration of the salts contained duction of a complete and high potency fertilizer 40 in Said annoniated aqueous phase and thereby material analyzing, for example, about 5% avail promote their precipitation along with the pre able nitrogen, 18-20% available phosphorus ex cipitated phosphatic product. preSSed as P2O5, and about 8% potash expressed 7. The method according to claim 1 wherein as K2O. It should be noted that this mixture the alkali metal fluosilicate precipitated by the Of nitrogenous and potassium salts with the tri incorporation of the alkali metal salt in the calcium phosphate product, like the tricalcium acqueous phase resulting from the treatment of phosphate product itself, is not only non-hygro the phosphate rock with the nitrogenous acid Scopic, non-caking and substantially free-flow is Separated from the aqueous phase prior to the ing, but is stable under storage conditions. In treatment of Said aqueous phase with ammonia. this respect, the complete fertilizer material of my invention offers a further advantage over 50 References Cited in the file of this patent other fertilizer materials in which the phosphatic UNITED STATES PATENTS material is present in an acidic form, the acidic nature of the phosphatic material in such pre Number Name Date viously available products tending to cause vola 1,487,205 Carothers ------Mar. 18, 1924 tilization of the acid components of the nitro 55 1788,828 Goldberg ------Jan. 13, 1931 genous SaltS Such as annonium nitrate and 1,806,029 Thor'ssell ------May 19, 1931 alkali metal nitrate admixed therewith and fur 1,849,703 Boller ------Mar. 15, 1932 ther tending to cause rotting of fibrous con 1849,989 Moore ------Mar. 15, 1932 tainers such as bags in which it is marketed. 1856,187 Johnson ------May 3, 1932 Thus, the precipitated and dried product, advan 60 1859,738 Johnson ------May 24, 1932 tageously broken up after drying, can be marketed 1,869,688 Heimann ------Aug. 2, 1932 directly Without any curing operation or the like 1948,520 Harvey ------Feb. 27, 1934 With the aSSurance that it will remain stable and 1983,024 Foss ------Dec. 4, 1934 in free-flowing condition up through the time of 2,057,025 Hogens et al. ------Oct. 13, 1936 its use. 65 2,102,831 Brill et al. ------Dec. 21, 1937 I claim: 2,114,600 Larsson ------Apr. 19, 1938 1. The method of recovering a substantially 2,164,627 Seyfried ------July 4, 1939 non-hygroscopic calcium phosphate product 2,176,464 Merchant ------Oct. 17, 1939 from phosphate rock containing calcium phos 2,384,814 Coleman ------Sept. 18, 1945 phate which comprises dissolving the phosphorus 70 2,614,040 Kaikinger ------Oct. 14, 1952 component of the rock in an inorganic nitrog FOREIGN PATENTS enous acid with the resulting formation of an acidic aqueous phase containing substantially all Number Country Date of the acid-Solubilized calcium and phosphorus 299,796 Great Britain ------Oct. 29, 1928 components of the rock in the form of calcium 75 (Other references on following page)

2,688,075 11. 12 FOREIGN PATENTS OTHER REFERENCES Number Country Date Frear et al., "Calcium metaphosphate: effect 376,934 Great Britain ------June 25, 1931 of impurities on fusibility, citrate solubility, and 357,473 Great Britain ------Sept. 24, 1931. 5 hygroScopicity' Industrial and Engineering Great Britain ------Jan. 6, 1936 Chemistry, vol. 36, No. 9, Sept. 1944, pages 467,843 835-840. 615,476 Great Britain ------Jan. 6, 1949 Hill, “Phosphate Fertilizers,’ Agricultural Chemicals, vol. W, No. 12, pages 55 and 86. 0 Sauchelli, “Manual on Fertilizer Manufac ture,' Davidson Chemical Corporation, Balti more, Md., page 94.