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Feb. 10, 1942. ' 2,272,402 G. DL_J B015 MANUFACTURE OF PHOSPHORIC ACID Filed May 22; 1959 - Flames NO. l Heuzé No. 5 Gas/on DUBa‘Q INVENTOR BY ATTORNEY atented Feb. 10, 1942 2,272,402 _ MANUFACTURE or rnosrnomc Acm Gaston Du Bois, Clayton, ’ Mo., assignor to Monsanto Chemical Company, a corporation of Delaware Application May 22, 1939, Serial No. 275,130 5 Claims. (jCl. 23—165) must be such as to provide ?owability and at the same time provide suf?cient viscosity to impart corrosion resistance to the metal. The property This invention relates to the manufacture of phosphoric acid and also to apparatus therefor. One of the objects of the present invention is the provision of a simpli?ed and economical process for the manufacture of strong phosphoric acids. Another object is the provision of a proc ess for the manufacture of various polyphos: phoric acids. A further object is the provision of apparatus suitable for carrying out such proc of ?owability is necessary to permit a sufficient ?ow of heat through the metal walls and viscous ?owing ?lm of acid in order that the cooler may function e?iciently. It is evident that if the vis cosity of the acid deposited upon the tubes is too great, an excessively thick ?lm is deposited which esses. 10 would greatly impede the flow of heat. On the other hand if the viscosity of the acid is too low In known processes for the manufacture of _then the rate of heat ?ow. would be so great that phosphoric acid involving the combustion of ele mental phosphorus and the subsequent hydra the metal surface would be heated to a tempera— ' tion of the combustion products, it has been cus ture at which corrosion takes place with rapid tomary to remove a large part of the heat gen erated by such combustion by the evaporation of , water. 15 destruction of the equipment. The nature of the acid ?lm, that is its viscos ity, is dependent upon the acid concentration and the temperature. It has been found that the In accordance with these earlier proc esses it has been the practice either to spray the requisite quantity of water into the hot combus acid should have a concentration of at least 95% tion gases to cool these gases, or else to pass the 20 HsPO-i and preferably higher. For most satis factory operation yielding an acid of high purity, combustion gases directly into a large‘ body of that is free of corrosion products, I prefer to op water or weak acid. Such prior processes pro erate with a minimum acid strength of 104.9% duced only the relatively weak grades of phos H3P04 (76% P205). Acid containing less than phoric acid, by which I mean-acids having a 15 p. p.’ m. of nickel may be condensed upon strength of less than 85% or 95% H3PO4. Be cooled Monel tubes provided that the acid con cause of the relatively large quantities of water tains at least 76% P205. ’ evaporated in cooling the combustion gases down The upper limit of the acid is 116% equiva to collecting temperatures, it was di?icult, if not lent H3PO4 content which limit is ?xed by the impossible, to produce acids having strengths ranging from 95% to as high as 116% H3PO4 30 viscosity of the acid. This acid is also known as tetraphcsphoric acid, H6P40l3. ’ The acid collected by my process, in addition to Other processes have been proposed in which ‘containing .metaphosphoric acid, has an appre- phosphorus combustion gases containing an ex ciable content of pyrophosphoric acid and also cess of water vapor are cooled to lower tempera tures to produce the meta, pyro and orthophos 35 orthophosphoric acid. The stoichiometric com position of the acid of the limits herein speci?ed phoric acids. Processes involving the condensa is as follows: tion of these higher acids upon cooled metallic surfaces cannot be carried out because of the high temperatures necessary to produce such . Calculated Concentration of—— acids in the presence of .excess quantities of 40 equivalent H3P 0 4 water vapor.. Should the temperatures of the gases be lowered then the acids stronger than equivalent. orthophosphoric acid cannot be obtained due to Percent Percent Percent Percent the ‘excess of water vapor. ' 104. 9 2. 0 50. 0 ‘43. 0 47. 0 55. 0 116. 0 _ 7 ~ In conducting experiments on the combustion 45 of elemental phosphorus and the hydration and cooling of the products of combustion down to ' collecting temperatures, I have found that phos phoric acids of certain strengths have the prop erty' of adhering to and covering with a continu ous, viscous, ?owing ?lm, cooled metallic sur faces without corroding such surfaces._.v . The nature of the acid ?lm deposited upon the 3. 0 The viscosity of acid having an equivalent H3PO4 content of 104.9% is 200 .Saybolt seconds, while that of an acid of 116.0% H3PO4 is 7000 60 Saybolt seconds, both determinations having, been made at 163° F. By controllof the moisture content of the air present during combustion of the phosphorus, or added'prior to condensation, I may condense di cooled metallic surfaces as obtained by my proc ess is important because the viscosity of the acid 55 rectly in ?owable form, upon the cooling sur 2 2,272,402 faces, various polyphosphoric acids. Such poly covery that when phosphorus is burned in air phosphoric acids as are directly condensable in containing a quantity of water vapor such that the acid contains at least three mols of water for each two mols phosphoric oxide (P205) an acid mist is produced which is easily condensed and liquid ?owable form are the triterohexaphos phoric acid H'IP3O11; the penterohexaphosphoric acid, H5P3O1o, and the tetraphosphoric acid HsP4O13. ' ?ltered out of the gas stream and consequently The triterohexaphosphoric acid has a viscosity in the neighborhood of 250 Saybolt seconds, and a greater efficiency of collection is obtained. While such processes are capable of operation the penterohexaphosphoric acid a viscosity of using the ordinary materials of construction, that 1500 Saybolt seconds, all measurements having 10 is carbon and acidproof brick, it has been found been made at a temperature of 163° F. > that the acid dissolves small amounts of such The workability of my herein described process construction, materials, that is, alumina, silica, depends upon the discovery that phosphoric acids within the concentration and temperature and iron oxides, which substances contaminate contact therewith and continually drops or flows oif therefrom. Because of the strength of the .phosphoric acid constituting the ?lm covering on nickel-chromium steel and high nickel, alloys is favored by the low viscosity of the acid, which the acid produced. In cases where carbon is used limits described above exhibit practically no cor 15 as a material of construction it must be kept at rosion upon certain metallic surfaces. At the a temperature below the oxidizing temperature same time the viscous ?lm of acid formed when or it must be maintained saturated with acid. acids within the above concentration limit are In construction of, plants for the process here deposited upon cooled metallic surfaces has a contemplated both of these requirements are dif~ sui?cient heat conductivity so as to permit of 20 ?cult to meet, with the result that the purity of the application of this discovery‘ to the construc the acid produced by prior known processes is tion of a practical cooler for cooling phosphoric somewhat lower than-that‘ produced by my pres acid containing gases down to the usual collect ent invention. ‘ ing temperatures. In the operation of my proc As a result of experiments which I have insti ess phosphoric acid is continually deposited upon 25 tuted on the process herein disclosed, I have dis the cooled metallic surfaces from the gases in covered that corrosion by ‘orthophosphoric acid prevents the formation of a ?lm of the acid ad the .metallic cooling surfaces, the layer of acid 30 hering to the metal, with the result that acid at immediately adjacent the metal is immobile high temperature is in contact with the metal while the layers of acid farther removed from surface. If the viscosity of the phosphoric acid the metal surfaces are ?owable due to their is increased by either increasing the strength of higher temperature; the, temperature and hence such acid, or by lowering the temperature there the flowability increasing with the distance the 35 of to below 180° C. and preferably below 140° C. layer is removed from the metal surface. This characteristic of an acid ?lm varying in ?ow ability as well as temperature outwardly from the metal surface is an important feature of my the corrosion may be substantially prevented be cause a ?lm of appreciable thickness will be formed on the metal surface. Consequently the acid at the interface of the metal and the acid process. I 40 ?lm would be of such low temperature due to By way of further explanation it may be stated the temperature drop across the ?lm that corro that the acid deposited upon the cooled metal sion is substantially eliminated. surfaces is pure and uncontaminated with solids In the application of this discovery to the here as in prior art processes. The presence of a pro tective viscous layer of phosphoric acid upon the metal cooling surfaces is dependent solely upon in described process I provide a surface of . chrome-nickel alloy or other suitable metal with the high strength of the acid thus deposited. Metals which may offer satisfactory resistance to corrosion at the temperatures which prevail at the metal-liquid interface when strong phos 50 phoric acid is maintained as a viscous ?lm on a ?lm of viscous acid' of such viscosity and strength as to be within the herein described limits. I maintain the viscosity and temperature of the ?lm of acid by the application of a cool ing ?uid to the chrome-nickel alloy in a manner “to be hereinafter more fully described. the surface of the metal, according to my inven Referring to Figure 1 of the drawing, I have tion, comprise nickel-chromium steels with or shown by numeral in a brick-lined combustion without silicon, titanium, molybdenum or man chamber provided with an inlet pipe II by. which ganese, alloys containing a high percentage of 55 molten yellow phosphorus is supplied to said nickel and certain casting alloys which maybe chamber. By means of pipe I2, I introduce a combinations of iron and silicon or complex mix quantity of previously humidi?ed air from a tures of iron with nickel, chromium, copper mo source not shown, the quantity of air thus sup lybdenum, manganese, tungsten and silicon. Al plied being so proportioned with respect to the though pure nickel itself is unsuited for the pres phosphorus supplied by pipe II, that a substan ent purpose, its alloys, especially those which 60 tial excess of oxygen over that required to burn contain iron, copper or ‘chromium, are suitable. such phosphorus is present in chamber 10. The Monel metal is particularly valuable for this pur amount of water ‘carried by said humidi?ed air is pose. such that an acid having a strength of from In accordance with the' present invention, I 104.9% to 116% H3PO4 equivalent is produced. . provide cooling or heat exchange units positioned 65 Combustion chamber It may contain ba?le within the combustion chamber or between the walls i 3, the function of which is to thoroughly combustion chamber and the absorbing system, mix the phosphorus, air and moisture undergoing such units being fabricated for the most part of reaction in said chamber. The products of the relatively thin alloy sheets of compositions as reaction taking place in said chamber In pass 70 hereinafter more particularly speci?ed. outwardly through duct H, which, in accordance My process is particularly applicable to those with my invention, is made of nickel-chromium phosphorus combustion processes wherein pure alloy steel sheet or drawn tube or of a high nickel elemental phosphorus is burned with prehumidi alloy in similar form and provided with cooling fled air. These processes are based upon the dis 16 water sprayed upon the exterior thereof, as by 2,272,402 pipe 15 supplying sprays It. The gases pass from duct or tube It into chamber ll which is pro by the prior art or as particularly pointed out in the appended claims. This application is a continuation in part of my earlier ?led application, Serial No. 160,398, vided with‘a ?lter medium I9 consisting of ?nely divided coke supported upon a grid [9. The gases pass upwardly through the ?lter bed l8 and out ?led August 23, 1937.- ' of the chamber I‘! by means of stack 20. . -What I claim is: ' 1. A process for producing pure phosphoric In accordance with my invention chamber I1 is constructed of nickel chromium steel sheet in a‘ acid of strength within the range of 104.9% to age tanks. tion between 104.9% and 116% equivalent H3PO4, said ?lm being substantially immobile adjacent 116% equivalent H3PO4 content, comprising similar manner to duct l4, and is provided with spray cooling means 2i, said means consisting in 10 burning pure yellow phosphorus in air, adjust ing the water vapor content of the hot combus this instance of a perforated pipe supplying cool tion gases so that the oxidation products are ing water to the exterior of the steel sheets or hydrated to the extent of containing phosphoric plates comprising ?lter chamber H. acid having a concentration of between 104.9 % The cooling water from sprays l6 and 2| is collected by means of pipe 22 and then pumped 15 and 116% equivalent H3PO4 content, contacting said hydrated oxidation products with a resistant back to pipes I5 and 2| by pumps not shown in metallic surface, said surface being maintained the drawing. constantly at a temperature below 180° 0., ‘con The strong acid collected in conduit I4 and ~densing upon said cooled metallic surface a ?ow ?lter chamber I1 is removed from the latter by means of outlet 23 and conveyed to suitable stor 20 able ?lm of phosphoric acid having a concentra v A further embodiment of my invention is illus said metallic surface and increasing in ?owability with distance outwardly from said metallic sur 25 face, and recovering acid from said ?owable ?lm. tion as ‘above described. 2. A process for producing‘ pure phosphoric In the operation of the apparatus shown in acid of strength within the range of 104.9% and Figure 2, the combustion gases pass out of com 116% equivalent H3PO4 content, comprising bustion chamber I0 by means of duct I4 and burning pure’ phosphorus in air, adjusting the enter bricklined chamber25. This chamber is provided with a multitudeof pipes or pipe coils 30 water~vap0rcontent of the hot combustion gases so that the oxidation products are hydrated to 26 connected to headers 21, which in 'turn are the extent of containing phosphoric acid having connected to a main header 28. The pipes 21 are a concentration within said limits, contacting maintained full of water‘, thermal circulation said hydrated oxidation products with the sur being maintained with the water in drum 28 in trated by Figure 2, wherein parts numbered sim . ilarly to thosein Figure 1 have the same func the usual manner. The temperature of the 35 face of a water cooled Monel tube, said cooling ‘water being maintained at a temperature below water in coils 26 is maintained below 180° C. and 180° C., condensing upon said cooled Monel tube preferably below 140° C. Said coils are con a ?owable ?lm of phosphoric acid within said structed of the metals or alloys hereinafter limits, said ?lm being substantially immobile ad enumerated and during operation are covered with a ?owable ?lm of viscous acid in accordance 40 jacent said Monel surface and increasing in ?ow vability with distance outwardly from said Monel with my herein described invention. Phosphoric surface, and maintaining said acid ?lm by con acid of high strength and of high viscosity is con densation from the hydrated combustion prod densed upon coils 26, said coils being below the nets. dew point of the vapors and the ?owable ?lm of acid serves to protect the material of said coils 45 3. A process for producing pilre phosphoric acid of strength within the range of 104.9% to against corrosion. ‘ 116% equivalent H3PO4 content, comprising The gases and vaporsrwhich are thus cooled by burning pure yellow phosphorus in air, adjust contact with said coils 26 pass into duct'29 and ing the water vapor content of the'resulting hot thence into chamber I'I provided with ?lter ma terial l8 supported upon grid l9. Acid condensed 50 combustion gases so as to contain an equivalent phosphoric acid of ' 104.9% to 116% H3PO4, con in chamber 25 passes into chamber l1 and to tacting said hot hydrated combustion gases with gether with acid collected in said chamber, ?ows the exterior surface of an internally Water-cooled out through port 23 into suitable storage tanks. Monel tube, said cooling water being maintained ‘As materials suitable for the construction of duct I4, chamber l1 and tubes 26, shown in the 65 below 180° C., condensing upon said cooled Monel tube a ?owing ?lm of acid of strength within said above apparatus in the described process, I recommend the following: ' limits, said- ?lm of acid being substantially im mobile adiacent said Monel surface-and increas ing in ?owability with distance outwardly from chromium, 8-35% nickel and small amounts of said Monel surface, and recovering" acid con molybdenum or titanium, the balance being sub 60 densed upon said Monel surface. stantially all ‘iron. Such ma?erial is known as 4. A process for producing pure phosphoric stabilized stainless steel. acid of strength within the range of 104.9% to Various alloys in which nickel and copper such 116% equivalent H3PO4 content, comprising as Monel metal are the predominating metals._ burning pure yellow‘ phosphorus in air, adjusting 65 These alloys may also contain appreciable'quan the water content of the hot combustion products tities of iron, manganese, silicon and aluminum. so that the contained phosphorus pentoxide is' Casting alloys which contain nickel, molybde hydrated to a degree corresponding to an acid of, num, chromium and iron as their main constit from 104.9% to 116% equivalent H3PO4, contact uents with smaller quantities of copper, manga ing said hydrated combustion products with one Nickel-chromium steel containing 10-30% nese, tungsten and silicon and casting alloys con sisting mainly of iron and silicon. Having now particularly described my inven 70 side of a resistant cooled metallic sheet, said metallic sheet being maintained below 180° C. by contact with cooling water maintained at a tem perature below 180° C. upon the other side there desire that it not be limited except as required 76 of, condensing upon said metallic sheet a ?lm of tion and the manner in which it can be used, I 4 - 2,272,402 phosphoric acid of strength within said limits, said ?lm being substantially immobile where it to said cooling water, and recovering acid or said strength from said ?lm. ‘contacts said metallic sheet and said ?lm in; 5. The process de?ned in claim 4 in which the creasing in'?owability and temperature with distemperature of the cooling water in contact with tance outwardly from said metallic sheet, and 5 said‘metallic sheet is maintained at a tempera conducting heat of condensation of said'acid through said ?lm and through said metallic sheet ture below 140° C. ‘ GASTON DU BOIS.