Some History

In 1840, Justus von Liebig, a physician and professor of chemistry at the University of Giessen, Germany, referred to the fact that the phosphate as stored in teeth and bones is ingested with plants which, in turn, take it from the soil, and that this way the soil becomes deprived of important minerals. He therefore recommended to replenish the soil with a fertilizer prepared by digesting powdered bones with dilute sulfuric acid and to spray this solution -after further dilution- to the fields. The sulfuric acid, he claimed, will be neutralized by reaction with the alkaline components of soil (1). 

Inventive spirits soon realized that rock phosphates which occur in abundance in certain deposits might also be used as a fertilizer. In 1851, Charles T. Jackson, M.D., Assayor to the State of Massachusetts, examined specimens of Eupyrchroite (2) sent to him by Mr. C. F. Hammond of Crown Point who wrote him that

While some people soon made their luck in patenting processes to convert rock phosphate into soluble phosphates which are easier available to plants, others became prosperous by exploiting phosphate mines or by land sales to those hoping for lucky discoveries. In his "Remarks on phosphate deposits in the United States", Otto Meyer reported in 1891, that in some U.S. states, especially Florida, there´s kind of a phosphate fever which reminds of the gold and silver booms in the western states (3): 

The phosphate has to be solubilized to facilitate absorption by plants. The solubilization process is essentially the same as proposed by Liebig for bone phosphate. But acid treatment of rock phosphate releases hydrogen fluoride and silicon fluoride. The presence of fluoride in natural phosphates was known since 1790 by the work of Pelletier and Donadei (see early European Fluoride research) and had been confirmed in several analyses since then. However, the fact that it would partly be released in the form of toxic gaseous compounds by treatment with sulfuric (or phosphoric) acid, as well as by the calcining processes developed later, was disregarded for some time. In 1894 S. Gabriel mentioned the trouble which the phosphate workers in the SILESIA experience from the hydrogen fluoride and silicon fluoride evolved in the solubilization process, as well as the damage visible on the glass windows in the neighborhood of the plant (4). Some patents issued after the turn of the century mentioned detrimental effects to "surrounding property" (Karl F. Stahl, 1917) or referred to observations that "noxious fumes accompanying the chemical reaction taking place in the den are very objectionable and injurious to workmen employed in removing the material from the den and in conveying the same to the curing pile" (William T. Doyle, 1921).

In 1934, the International Labour Office (5) published the following statements concerning health and environmental problems observed in the superphosphate industry:

Relative to agricultural workers who applied phosphate fertilizers to the fields, Bornschein (6) wrote in 1936 that they displayed not only problems with their eyes but also respiratory damage, as the eyes and respiratory organs were exposed to the dust carried with the wind.

Up to the 1930´s, and even afterwards, it was hard for victims to provide evidence that the damage they suffered was in fact caused by the toxic gases created by the phosphate plant operations. An early report by Enrico Ronzani, of the University of Padua, Italy, impressively attests to this (7). As the development of agriculture in Italy had led to a rapidly expanding phosphate fertilizer industry there, with a corresponding increase in damage suits filed against the plants by neighbours and workmen, Ronzani performed animal experiments to examine the toxic potential of the inhalation of low amounts of hydrogen fluoride. He found that even the lowest concentrations tested and reported to be harmless by earlier researchers, in association with the glass industry, were heavily toxic for the animals in his inhalation experiments (7). Likewise, toxic effects after ingestion of the finished product containing soluble fluorides had been described elsewhere: the addition, as proposed in some patents, of ammonia or sodium carbonate after treatment of the raw phosphate with acid led to the inclusion of easily soluble ammonium or sodium fluoride and silicofluoride (fluosilicate) in the product which was not only used as a fertilizer but also as a mineral supplement fed to cattle. As early as 1902, warnings were issued that phosphate supplements - due to soluble fluorides contained therein- caused severe illness in cattle fed the stuff, i.e. bleedings in the stomach and intestines, loss of appetite, stiffness of the joints, etc. (8,9).

References:

(1) Liebig J.: "Die organische Chemie in ihrer Anwendung auf Agricultur und Physiologie", Braunschweig 1840, pp. 165 and 180; (2) Edinb. New Phil. J. 51 (1851) 328); (3) Z. angew. Chem. (1891) pp. 111-114; (4) Hoppe Seylers Z. physiol. Chem. 18 (1894) 257; (5) Thiele A.: "The superphosphate industry", in "Occupation and Health, Vol. II, No. 24, International Labour Office, Geneva, 1934; (6) Bornschein L. W.: Inaug. Diss., Munich 1936; (7) Ronzani E.:   ; (8) Emmerling: Milch-Zeitung  31:No. 46 (1902) p. 728; (9) Dammann & Manegold: Dtsch. tierärztl. Wschr. 12 (1904) pp. 129-131 and 141-143;

---

The Patents

1859

J. J. MAPES, of Newark, New Jersey: "Improvement in fertilizers", US Patent 26,196; dated Nov. 22, 1859

"By the analysis of soils and plants it is well known what are their constituents, and it is also well known that the continued cropping of soils with less amounts of amendments than those removed as constituents of the crops must gradually impoverish the soil, and in consequence we find the uneven or unequal removal of these constituents to render the soil unfit for the raising of wheat and other crops requiring the presence of phosphoric acid, lime, potash, &c, ... The nature of my invention consists in saturating phosphate of lime - such as mineral apatite or calcined bones - with sulfuric acid to such extent as to form the superphosphate of lime by combining part of the lime with the sulfuric acid as sulphate of lime, leaving the whole of the phosphoric acid combined with the remaining portion of lime as superphosphate of lime, when such products are combined with guano and sulphate of ammonia, or the equivalents thereof, such as the ammoniacal liquor of gas works or other ammoniacal waste."

1889

SILESIA, Verein chemischer Fabriken in Ida- und Marienhuette bei Saarau (Schlesien): "Verfahren zur Nutzbarmachung des beim Aufschliessen fluorhaltiger Phosphate auftretenden Fluorsiliciums durch Darstellung kuenstlichen Kryoliths", German Patent DE 53,045; filed Sept. 19, 1889; pat. Sept. 8, 1890; update in German Patent DE 55,153; filed Sept. 19, 1889; pat. Jan. 7, 1891

patented in the U.S. as:

Engelbert RICHTERS, of Saarau, Germany: "Process of making artificial cryolite", US Patent 447,063; patented in Germany Sept. 19, 1889, No. 53,045; filed  in U.S. Nov. 7, 1890; pat. Feb. 24, 1891

"In the treatment of phosphates containing fluorine by means of sulfuric acid considerable quantity of silicic fluoride in gaseous form is generated, which is decomposed in part during this treatment and which combines with the water in the sulfuric acid into silicic acid and hydrofluosilicic acid according to the following formula:

3 SiF4 + 4 H2O == 2 SiF6H2 + H4SiO4

The troublesome fluorine compositions thus obtained could not be utilized in the arts. The object of this invention is to furnish an improved process by which these fluorine combinations may be utilized for the production of an double fluoride of an alkali and aluminium, which, according to numerous tests made with it, can be used in place of natural cryolite in the manufacture of enamels and opaque or enameled glass." 

Note: Because of the silica content this artificial cryolite cannot be used in aluminum production

1890

Allard MEMMINGER, of Charleston: "Verfahren zur Darstellung von schnell trocknendem Superphosphat"; German Patent DE 55,926; filed June 27, 1890; pat. Feb. 23, 1891

The process consists in adding to the ground phosphate, before it is treated with acid, either ground fluorspar or another fluoride.

1897

Georg SCHUELER, of Stettin: "Verfahren zur Darstellung von hochprocentigem Doppelsuperphosphat", German Patent DE 104,364; filed Nov. 12, 1897; pat. July 10, 1899

Use of phosphoric acid, instead of sulfuric acid, to increase the content in soluble phosphoric acid.

1899

Julius REICH, of Wien: "Verfahren zur Abscheidung des Fluorsiliciums aus solches enthaltenden Gasen", German Patent 123,885; filed Sept. 22, 1899; pat. Aug. 26, 1901

A procedure to bind silicon fluoride released in phosphate processing. "Hitherto, gases occurring in the fabrication of superphosphates have either been released into the air or rendered innocuous by conducting them in a solution of soda or lime. But the condensing of the silicon fluoride was still very difficult."

1902

Bruno TERNE, of Philadelphia, Pa.: "Process of making fertilizers", US Patent 709,185; filed May 7, 1902; pat. Sept. 16, 1902

"This invention relates to an improved process of making a high grade of ammoniated fertilizers from phosphate - rock and other substances containing phosphates - such as spent boneblack, highly steamed bones, &c., and crude ammoniacal liquors obtained from coke-ovens, gas-house retorts &c.; and for this purpose the invention consists of a process for making fertilizers by dissolving the phosphates with sulfuric acid, neutralizing the free phosphoric acid by a concentrated ammoniacal liquor run into the same, and then drying the mass preparatory to milling." -

The inventor apparently was not aware of the fact that this way his product would also contain an easily soluble ammonium fluoride.

1903

Emerson H. STRICKLER, of Baltimore, Md.: "Process of manufacturing trisodium phosphate", US Patent 744,128; filed Jan. 21, 1903; pat. Nov. 17, 1903

"At the present time trisodium phosphate is made from calcium phosphate in the following manner: The calcium phosphate is digested in an equivalent quantity of sulfuric acid, producing crude phosphoric acid. To this sodium carbonate is added to alkaline reaction, the disodium salt being formed. Finally, a slight excess of caustic soda over that required to form the trisodium salt is added and the solution is allowed to crystallize" - Here too a soluble fluoride is formed and included in the product: sodium fluoride.  "In my process I substitute for these three manufactured products a single material, niter cake, which has commonly been considered a waste product from the manufacture of sulfuric and nitric acids." (niter cake = NaHSO4)

1906

Rütgerswerke Akt.-Ges., of Berlin: "Verfahren zur Herstellung von Kieselfluornatrium unter gleichzeitiger Gewinnung von wertvollen Düngemitteln", German Patent DE 188,651; filed Nov. 2, 1906; pat. Sept. 10, 1907

Addition of a fluoride and/or silica to rock phosphate low in any of the two is suggested to increase the yield in fluosilicic acid for production of sodium fluorosilicate on a commercial scale, in combination with phosphate processing. Otherwise the variable amounts of fluorosilicate obtained - dependent on the source of the phosphate- make its retention and processing for further use uneconomic so that, according to prior technolgy, it was released with waste water into rivers.

1910

Emil TEISLER, of Dohna, Germany, assignor of one-half to Alex HUMANN, of Dohna, Germany: "Process of making sodium-aluminium fluorid", US Patent 1,015,220; filed Jan. 27, 1910; pat. Jan. 16, 1912

"... it has been found that the fluorin present in sodium silicofluorid which is a cheap by-product of the manufacture of super-phosphates, which can be conveniently and completely united with alumina and soda, in which case, it is true, no pure sodium aluminium fluorid results, but a preparation mixed with silicic acid in finely divided form is produced, which, however, although so mixed is found to be useful in the ceramic industry, for the preparation of milk-glass and enamel, as silica forms a large proportion of glasses and enamels."

1911

Henry V. DUNHAM, of Bainbridge, New York, assignor to Nacirema Chemical Company: "Art of converting phosphorites", US Patent 1,034,090; filed July 7, 1911; pat. July 30, 1912

"Some of these attempts have comprised calcining methods, the more usual method consisting in adding to the finely ground rock certain chemicals and suitable fluxes and then heating the mixture to varying degrees in a blast furnace or indirectly through heating ovens."

Karl F. STAHL, of Pittsburgh, Pa.: "Method of making sodium silicofluorid", US Patent 1,247,165; filed March 17, 1917; pat. Nov. 20, 1917

"In the process of making acid phosphate for use as a fertilizer the ground phosphate rock is mixed with sulfuric acid which has previously been diluted to the required strength. This converts the tertiary calcium phosphate into primary phosphate, rendering it soluble in water, but it also liberates the fluorin of the calcium fluorid, forming hydrofluoric acid. This acid in turn combines with the silica forming principally silicon fluorid (SiF4), which, as a gas, escapes into the air and is detrimental to surrounding property. In order to prevent the escape of this gas the silicon fluorid is brought into contact, in suitable apparatus, with sprays of water and this decomposes the silicon fluorid into hydrofluosilicic acid and silicic acid."

-

Karl F. STAHL, of Pittsburgh, Pa., assignor to General Chemical Company, of New York, NY: "Condenser or absorber for hydrofluosilicic-acid gas", US Patent 1,367,993; filed Aug. 18, 1917; pat. Feb. 8, 1921

"Hydrofluosilicic acid is a very energetic reagent and destroys the walls of the chamber or tower and also the wood filling contained therein. Furthermore, empty towers or those with baffles or cross pieces of wood are very inefficient, requiring a large space and much water to condense the gases, and the resulting hydrofluosilicic acid is too weak to be used for the manufacture of sodium silico fluorid." 

-

Abraham HENWOOD, of Cynwyd, Pa.: "Manufacture of phosphate fertilizers", US Patent 1,310,080; filed Dec. 11, 1917; pat. July 15, 1919

"A further purpose is to free the manufactured fertilizer from the injurious soluble hydrofluoric acid, a specific plant poison previously present in all fertilizers made from phosphate rock containing fluorids."

1918

John C. CAROTHERS, and William H. ROSS, (employees of the US Department of Agriculture) Washington, D.C.: "Process for the direct preparation of crystallized phosphoric acid", US Patent 1,283,398; filed Feb. 18, 1918; pat. Oct. 29, 1918

"The object of our invention is to obtain phosphoric acid directly in such a state as will permit its ready purification by crystallization and at the same time recover, as a by-product, any hydrofluoric acid associated with the phosphoric acid in its original state."

-

Ingenuin HECHENBLEIKNER, of Charlotte, N.C., assignor to Chemical Construction Company, of Charlotte, N.C.: "Recovery process for waste gases", US Patent 1,297,464; filed Oct. 26, 1918; pat. March 18, 1919

"Mineral phosphates usually contain 5 per cent to ten per cent calcium fluorid (CaF2). ... At an average in acudulating about 50 per cent to 75 per cent  of the calcium fluorid is decomposed and the fluorin escapes as a silicon fluorid (SiF4).. ... The silicon fluorid brought in contact with water is decomposed and forms mainly hydrofluosilicic acid (H2SiF6), gelatinous hydrosilicic acid (H2SiO3) and a small amount of hydrofluoric acid (HF). The reaction is very slow and is quicker with cold water than with steam. The reaction is considerably quickened and more efficient if a solution containing gelatinous hydrosilicic acid, water and hydrofluosilicic acid, are brought in intimate contact with the silicon fluorid gas. Upon this principle is based my process for absorbing waste gases from acidulating plants. ... also the recovered hydrofluosilicic acid up to 20 per cent strength can be produced economically."

His US patent 1,313,379; filed in June 9, 1919; pat. Aug. 19, 1919, "Process of making phosphoric acid" suggests the use of this hydrofluosilicic acid for the acidulation process instead of the sulfuric acid.

1919

William H. ROSS, of Washington, D.C., (employee of the US Department of Agriculture): "Process for the removal of hydrofluoric acid from phosphoric acid", US Patent 1,329,273; filed July 26, 1919; pat. Jan. 27, 1920

"The two sources of commercial phosphoric acid are charred bones and phosphate rock. Two methods known as the sulfuric acid and volatilization methods are also in use for preparing phosphoric acid from these raw materials. In the sulfuric acid method the phosphatic material is treated with a sufficient quantity of sulfuric acid to set free the phosphoric acid which is then filtered off, purified and concentrated to such a degree as the industry for which it is intended may demand. In the volatilization method phosphate rock is smelted in an electric furnace with a mixture of sand and coke and at such a temperature that the phosphorus is evolved either in the elementary state or as the oxid, depending upon whether or not air is admitted to the furnace. ... Of the impurities which do occur in the phosphoric acid by the volatilization method by fat the most objectionable is hydrofluoric acid."

1921

William T. DOYLE, of Boston, Ma., assignor to Sturtevant Mill Company, of Boston, Ma.: "Process for manufacturing acid phosphate or superphosphate", US Patent 1,383,912; filed Jan. 10, 1921; pat. July 5, 1921

"Heretofore, the ground phosphate rock and dilute sulfuric acid, for example 1,000 lbs. of each are introduced into a mixer where the masses are thoroughly mixed and then discharged into a large chamber generally built of concrete and known as a den. A chemical reation takes place in this den which generates heat, which in turn converts the water of the dilute sulfuric acid into steam. The mixture is allowed to stand about twelve hours in the den, and then it is broken down or disintegrated, removed and conveyed to a curing pile, where it is allowed to remain a sufficient length of time for the chemical reaction to continue, and further convert insoluble phosphoric acid into soluble phosphoric acid. There are certain objections to the method heretofore practiced. Noxious fumes accompanying the chemical reaction taking place in the den are very objectionable and injurious to workmen employed in removing the material from the den and in conveying the same to the curing pile. One of the purposes of the present invention, therefore, is to draw the steam and noxious fumes from the den, so that the workmen may remove the material from the den without injury to them from the poisonous gases."

1922

John N. CAROTHERS, and Arthur B. GERBER, of Anniston, Alabama, assignors to Federal Phosphorus Company, of Birmingham, Alabama: "Process for the removal of fluorine compounds from phosphoric acid", US Patent 1,487,205; filed Jan. 7, 1922; pat. Mar. 18, 1924

"When such rock is decomposed in a smelting furnace, the fluorine is liberated and carried off with the phosphoric acid fume as part of the furnace gases. As the phosphoric acid fume condenses and forms a liquid, the fluorine dissolves to a certain extent and remains in the phosphoric acid when it is collected,  notwithstanding the fact that the temperature of the phosphoric acid is between 100 and 200° C. when it is collected. ... Efforts have been made to remove the fluorine by bubbling air thorugh the hot phosphoric acid, and although this will reduce the fluorine somewhat, the reduction is incomplete. ... the method proposed in our present process removes the fluorine as a different compound, and more completely than the process as referred to above. ... In fact, it seems that the fluorine removal, when sodium carbonate is added, is due to small quantities of silicate in solution where the sodium carbonate supplies the necessary sodium to form a precipitate which probably is composed of sodium, silica, and fluorine."

-

Henry HOWARD, of Cleveland, Ohio, assignor to the Grasselli Chemical Company, of Cleveland, Ohio: "Process of making sodium-silico fluoride", US Patent 1,456,594; filed Apr. 2, 1922; pat. May 29, 1923

"In the usual process ... the phosphoric acid solution is treated with an alkali, usually sodium carbonate or soda ash, whereby sodium phosphate is formed and the impurities in the solution are precipitated.  This precipitate contains insoluble iron, aluminum and fluorine compounds, and sodium and phosphate compounds carried down with the precipitate. Heretofore the precipitate has been sold to fertilizer manufacturers at a price based upon its phosphate content. The fluorine content of the precipitate has been assumed to be in the form of artificial cryolite and has been regarded as not only valueless but as an impurity or diluent of the phosphate content of the precipiate. I have discovered that the fluorine content of the crude phosphoric acid solution is not precipitated in the form of artificial cryolite but in the form of sodium silico fluoride and that this material may be obtained in pure form available for use in the manufacture of white enamels, etc., by the process hereafter described." 

1923

Harry E. LA BOUR, of Homewood, Illinois: "Method of concentrating and purification of phosphoric acid", US Patent 1,597,984; filed Sept. 13, 1923; pat. Aug. 31, 1926

"The usual impurities which are encountered are some form of arsenic, lead, iron, alumina, fluorine, calcium, and organic matter. ... The removal of such impurities as iron, alumina, fluorine and organic matter is complicated and very expensive. ..."

1925

Henry HOWARD, of Cleveland, Ohio, assignor to the Grasselli Chemical Company, of Cleveland, Ohio: "Process for the preparation of sodium phosphate", US Patent 1,676,556; filed Dec. 4, 1925; pat. July 10, 1928

"Such crude phosphoric acid ordinarily contains silicofluoride compounds, sulfuric acid or sulfates and iron and aluminum and associated metal compounds such as compounds of manganese, chromium, nickel, copper, vanadium and silmilar metals, which must be removed in preparation of pure U.S.P. sodium phosphate."

1926

Bernard GEHAUF and Harold W. WALKER, of Edgewood, Md.: "Method of making silicofluorides and products thereof", US Patent 1,617,708; filed May 14, 1926; pat. Feb. 15,1927

"This invention ... also comprises a new composition of matter for insecticidal and other purposes ... made by neutralizing hydrofluosilicic acid with the appropriate base ... Hydrofluosilicic acid ordinarily is prepared by contacting various waste gases containing silicon fluorid with water.. Waste gases containing silicon fluorid arise in various industries, as in the manufacture of superphosphates"

1933

Eldon L. LARISON, of Anaconda, Montana: "Phosphatic material", US Patent 1,972,196; filed Jan. 14, 1933; pat. Sept. 4, 1934

"It has been demonstrated experimentally that the fluorine present in such material is very harmful to the animals eating it and consequently it cannot be used safely. Feeding tests have demonstrated that the ratio of fluorine to P2O5 in mineral supplements should be not more than 1 to 200."

1934

Harry A. CURTIS, Knoxville, Tenn., assignor to Tennessee Valley Authority, Wilson Dam, Ala.: "Manufacture of calcium phosphates", US Patent 2,053,266; filed May 21, 1934; pat. Sept. 8, 1936; (using phosphoric acid instead of sulfuric acid)

-

Harry A. CURTIS, Knoxville, Tenn., assignor to Tennessee Valley Authority, Wilson Dam, Ala.: "Treating phosphate rock to eliminate fluorine", US Patent 2,044,774; filed Nov. 15, 1934; pat. June 23, 1936

"I have found that by passing fine granular phosphate rock thru a suitable apparatus continuously and countercurrently to a stream of gas consisting essentially of steam produced by the direct combustion of hydrogen and oxygen, a product of high plant food availability is obtained."

1937

Milton H. MERCHANT, Dearborn, MI., assignor to the American Agricultural Chemical Company, Newark, N.J.: "Method of removal of fluorine compounds from bone liquors", US Patent 2,176,464; filed Aug. 5, 1937; pat. Oct. 17, 1939

"Good dicalcium phosphate of commercial grade preferably contains fluorine not more than about 0,03% or 300 parts per million, at the most, and it is desirable for some purposes that the fluorine content thereof be reduced even below that amount, preferably to as low a value as possible, say of the order of about 20 to 40 parts per million."

1938

Edward H. WIGHT, Baltimore, and David L. ANDERSON, Towson, Md., assignors to the Davison Chemical Corporation, Baltimore, Md.: "Animal feed supplement and process of manufacture", US Patent 2,234,511; filed March 5, 1938; pat. March 11, 1941

"It will be noted that the fluorine is substantially completely eliminated (about 95% removal) and the final product preferably contains not more than .1% fluorine" - which still is 1,000 p.p.m.!

1940

Mark SHOELD, of Baltimore, Md., assignor to The Davison Chemical Corporation, Baltimore, Md.: "Process of treating superphosphates", US Patent 2,288,112; filed Feb. 12, 1940; pat. June 30, 1942

"It will be further noted that the fluorine content of the original granulated superphosphate which was 1.4% was reduced to 0.09% at the end of the calcining operation, and that by the process of this invention the fluorine content was reduced to 0.015%, after complete processing." - 150 p.p.m.

-

Sihon C. OGBURN, Jr., Westfield, N.J., assignor to General Chemical Company, New York, N.Y.: "Fluorine removal from phosphate liquors", US Patent 2,312,047; filed March 18, 1940; pat. Feb. 23, 1943

"at the present time monocalcium phosphate containing more than 0.001% fluorine is considered unsatisfactory for food purposes. ... the production of such food grade monocalcium phosphate from crude phosphoric acid made by reacting phosphate rock with sulfuric acid has presented substantial difficulties. ... "

1945

Ernest J. BRETON, Arlington, and William H. WAGGAMAN, Alexandria, Va., assignors to the United States of America, as represented by the Secretary of the Interior: "Process for manufacturing calcium fluoride", US Patent 2,410,043; filed Jan. 4, 1945; pat. Oct. 29, 1946

"It is a well known fact that much fluorine is evolved in the treatment of phosphate rock to produce fertilizer and stock feed, but up to the present time only a small proportion has been collected and utilized. ... We have developed a process for the production of calcium fluoride from the gases evolved in the decomposition of materials containing both fluorine and silica."

1946

Charles A. BUTT, assignor to International Minerals & Chemical Corporation, Chicago, Ill.: "Manufacture of defluorinated tricalcium phosphate", US Patent 2,442,969; filed May 16, 1946; pat. June 8, 1948

"Because of the serious results in continued feeding of significant amounts of fluorine to animals, particularly to cattle, the value and saleability of a so-called defluorinated phosphate feed has been usually determined by the percentage of fluorine which it contains. A fluorine percentage of not more than .1% in a phosphate feed containing at least 30% of P2O5 is considered acceptable at the present time."

1947

Edward H. WIGHT, Baltimore, and Thomas O. TONGUE, Curtis Bay, Md., assignors to the Davison Chemical Corporation, Baltimore, Md.: "Superphosphate manufacture", US Patent 2,504,546; filed Jan. 28, 1947; pat. Apr. 18, 1950

"There is, however, sufficient active fluorine retained in the rock so that the resultant superphosphate will have a fluorine content high enough to cause marked deterioration of bags in which it is packed for handling and shipment."

1949

Walter H. McINTIRE, Knoxville, Tenn., assignor to American Zinc, Lead & Smelting Company, St. Louis, Mo.: "Treatment of fluoric effluents to obtain magnesium silicofluoride and hydrofluoric acid", US Patent 2,584,894; filed Oct. 28, 1949; pat. Feb. 5, 1952

"It is well known that fluoric materials are evolved in many major industrial operations, most important of which are probably the manufacture of fertilizers, steel, and aluminum. Governed by the nature of the operation and the materials used, fluorine is evolved in the form of silicon tetrafluoride (SiF4), as hydrofluosilicic acid (H2SiF6) and as hydrofluoric acid (H2F2, usually referred to as HF). The dispersion of such effluents into the atmosphere, even in relatively small amounts, tends to create a hazard to health and may result in serious injury to plant content, plant life and animal life and especially to horticultural and floral vegetation. The problem long has been recognized by the fertilizer industry which, over many years, has sought to diminish contamination of the atmosphere at points where superphosphate is manufactured by the passing of the effluent gases through scrubbing towers in contact with water and discharging the wash water through a marble or limestone-lined ditch. ... Meanwhile the growing use of fluoric materials in the ceramic industry, in the oil industry, in the manufacture of heavy metals and in the processing of uranium has resulted in a need for large quantities of fluorides."

1950

Edward A. HEISLER, assignor to Allied Chemical & Dye Corporation, New York, N.Y.: "Production of sodium fluoride", US Patent 2,585,387; filed July 8, 1950; pat. Feb. 12, 1952

"It is an object of this invention to provide a continuous process of producing sodium fluoride from sodium fluosilicate including sodium fluosilicate obtained as a by-product in the production of phosphoric acid or fertilizers, which process is efficient in operation and economical to carry out."

1967

Albert E. HENDERSON, Ponte Vedra Beach, Fla.: "Quick cure process for making superphosphates of low fluorine content", US Patent 3,519,387; filed Dec. 7, 1967; pat. July 7, 1970

"While single and intermediate superphosphates continue to evolve fluorides during curing and storage, the degree is less in triple superphosphate. In the latter case, serious air pollution problems result and producers are currently considering the difficult task of scrubbing the gases vented from very large storage and curing sheds common in the phosphate industry."

1982

Albert E. HENDERSON, Jacksonville, Fla.: "Method of defluorinating phosphates", US Patent 4,405,575; filed April 23, 1982; pat. Sept. 20, 1983

Presents a review of different processes