US2075212A - Manufacture of phosphoric acid - Google Patents

Description

March 30, 1937. c. 1.. LEVERMORE ET AL 21,075,212 MANUFACTURE OF PHOSPHORIC ACID Filed March 17, 1955 2 Sheets-Sheet l ywa 6 65 INVENTOR 61L Levermare R. E. Viv/an ATTORNEY March 30, 1937.

c. 1.. LEVERMORE ET AL 2,075,212

MANUFACTURE OF PHOSPHORIC ACID Filed March 17, 1933 2 Sheets-Sheet 2 INVENTCR CLLeL ermare RE. Viv/an BY ATTORNEY Patented Mar. 30, 1937 I 2,075,212 MANUFACTURE or ruosrnonro ACID Application March 17, 1933, Serial No. 661,266

6 Claims.

This invention relates to methods and apparatus for the recovery of phosphorus from'phosphate rock, and more particularly to the manufacture of phosphoric acid.

The process of the invention involves primarily reduction of phosphates contained in phosphate rock by means of carbonaceous reducing material to produce elemental phosphorus, and subsequent oxidation of the phosphorus to form phosphoric in acid. As one of the chief objects thereof, the invention contemplates a process by which the reduction of the phosphate, contained in the phosphate rock, to phosphorus and oxidation of the latter may be effected in a relatively short 15 period of time. To this end, the invention provides, in the preferred embodiments, a process by which phosphates of phosphate rock are reduced while the rock, in finely divided condition, is in suspension in a reducing atmosphere.

As another object, the invention aims to provide a process for the production of elemental phosphorus from phosphate rock by reduction of phosphates thereof in which process part of the fuel required to maintain proper temperature 25 conditions during the reaction is burned in the reducing zone. For this purpose, phosphates of the rock are reduced in the presence of an excess of reducing material over that required to eifect reduction of the phosphates of the rock to phos- 30 phorus and in the presence of sufiicient oxygen to support partial oxidation of the excess of reducing material, thus generating heat directly within the reduction zone.

In the preferred embodiments, the invention includes a procedure carried out in two relatively well defined zones, an initial reduction zone in hich phosphate of the rock is reduced, part of the fuel needed in the process is burned, producing carbon monoxide and phosphorus, and a subsequent oxidation zone in which the phosphorus carbon monoxide from the reduction zone, together with an added amount of extraneous uel if desired, are burned. Another object of invention lies in the provision of a process which the heat generated in the oxidation e by the oxidation of phosphorus and carbon monoxide may be advantageously utilized to aid in maintaining proper temperatures in the reduczone. It is still another important object 50 c. the invention to provide a process by which substantial economies in fuel requirements may be effected.

The invention accordingly comprises the several steps of the improved processes and the 55 relation of one or more of such steps with respect to each of the others, and the apparatus embodying features of construction, combination of elements and arrangement of parts adapted to carry out the processes of the invention.

A fuller understanding of the objects and ad- 5 vantages of the invention may be had from the following description taken in connection with. the accompanying drawings, which- Fig. 1 is a vertical section of a furnace in which the preferred embodiments of the process may be effectively carried out, I V

Fig. 2 is a longitudinal, vertical section of. a modified furnace construction, and

Fig. 3 represents a gas purification and absorp tion system. Refering to Fig. 1, the furnace indicated gen- I orally by referencenumeral Iii comprises a vertically disposed outer shell'i i, and an inner shell I2 preferably tapering toward the top end. Both shells are preferably circular in cross-section, shell l2 having therein a reduction chamber or zone It. The outer surface of shell 82 and the inner surface of shell I l form an annular o-xidizing zone or chamber I5. Shell l2 may be made of suitable refractory material such as carborundum, and shell H may be made of or lined with similar material to withstand high temperatures and the corrosive action of the reacting materials.

The lower end of shell II is provided with a frusto-conical shaped portion I6, and an adjoining cone-shaped section ll, together forming, for the furnace, a hopper bottom terminating in. an opening 3 through which slag may be tapped off from time to time. Under some operating conditions, the residue in the bottom of the furnace may be in the dry granular state, in which case provision may be made for discharging material of this nature from the apparatus. I

The top of the oxidizing chamber 15 is closed off by an annular crown i9 including one or more 40 outwardly opening doors or valves 26 which oper-. ate to open automatically in case of explosion in the furnace. Opening into theoxidizing chamber l5, near the base thereof, is a combustion chamber 2|, having a burner nozzle 22 by means of which additionalrequired amounts of fuel'may be introduced into chambers 2i and l5. Though combustion chamber 2| is shown for convenience opening radially into chamber I 5, it will be understood, in practice, combustion chamber 2i opens tangentially into chamber 15 to afford better mixing of gases therein. Air or other oxidizing gas, for purposes hereinafter noted, may be fedinto chambers 2| and I5 through pipes 23 from a bustle 24 connected through pipe 25 with heat exchangers shown iniFig. 3. Gaseousproducts formed 'duringthe reaction taking place in the furnace are withdrawn therefrom through an outlet pipe 26 opening into the upper end'of the 'oxidizingchamber l5, and communicating at the opposite end'with a purification and absorption rialsdrop from chamber l4 into the hopper bot-v system represented in Fig. 3. 1

As indicated on the drawings, the'inner shell l2fis cone-shaped, the bottom terminating at about the elevation of the upper end of bottom section [1.7 The bottom of shell. I2fiis of sub-' stantially the same diameteras thetop'of section l1, .so that fused, partially fusedor solid matetom of, the furnace and may be discharged through the opening l8. The bottom of the 're j duction chamber l4 communicates with the oxi dizing chamberi5 by means of openings 21in the lower end of shell 12. By this construction. it will be seenthat fumes and gases on' the one hand and fused or solidmaterialson'the other hand are separated .at the base of the reduction zone, the latter materials dropping. into section l1, while fumes and gases pass through openings 21into thelOWer'endof theo'xidizingtzo'ne I5,

Astream of finely divided phosphate rock,

carbonaceous material such as coalor .cokeyvand "a relatively small amount of air is fed into the reduction zone through an inlet conduit. 28,

opening at oneend intothe'topfof chamber l4 and having a valve controlled air jet 25in the opposite end. Rock and coal arefed into conduit 28 from a hopper 30 bya screw iconveyor 3|;the1shaft 32 of.which'car ries propellor like blades 33 acting tofeed rock and coal into con-' du'it 28 in a relatively steady streamw Further quantities of air, preferably preheated, 'are -in-l troduced into chamber 14 through annular 'in-, let 35, chamber '36 and pipe 3'l'communicating.

with an air preheater in the purification system of Fig. 3.,

In Fig. 2 there is illustrated another typeof vided at 5| and 52 to'substantially prevent gas leakage between the exterior of shell 4lland the adjacent edges of the walls of chambers 49 and 55. .Chamber'49 acts as a 'gas' outlet header,

and has at the upper end an outlet pipe 5310011- 7' responding with pipe 26 of Fig. l, through'which gases are conducted to the purification and'ab- V 1 sorption system of Fig. 3.

Carried rigidly by shell4ll is a 's econd rotary' shell 55, supported in: the position shown in the drawing Within shell 4ll,i by spiders 5 6. L'I'he in- V let'end of shellextends completely 'thr'ough,

- chamber 49 and projects through the outer wall 'shell40, a suitable gas-tight joint. connection is' 51 thereof. Asshell 55 rotateswith the outer made at 58 to prevent gas leakage.

The'interio-r of the inner drun155 constitutes a'reduction zone :or chamber59, theouter surface ofdrum55 and the inner surface of drum 40 forming an oxidizingzone orchamberfifl. The reduction zone'communicates with the oxidizing zone at the inner end B l of drum 55, the latter 75 terminating at a point within the outer shell 40.

Reference numeral 40. 111-.

The opposite end of oxidizing zone 60 opens into I chamber 49. A mixture of powdered phosphate.

rock and coal maybe injected, in a relatively small stream of carrying airginto the inlet end of drum 55 through a fixed pipe 62, correspond.

ing with pipe 28: of Fig. l. and connected to a rock-coal feed mechanism similar ,tothat shown in Fig. 1. "Also, as'in-Fig, 1, additional air is introduced into reduction zone 59 through annularinlet 63;.chamber 64, and pipe 65, corresponding with pipe 31 '(Fig; 1), and connected to the preheaters of Fig. 3. e I I -As indicated on the drawings, the end of shell 40 in chamber 49 is raised to facilitate movement of semi-fluid or solid materials through the'reduction chamber 59 toward the outletend 7 '6I' thereof. 7 In operation,'fiuid or solid material 'isfdischarged from the reduction chamber 59 intothelower end" of shell 40, and thence into {chamber 50. The latter is formed'with a hoppershaped bottom 65'tov facilitate discharge 'of'ma l terialfrom the apparatus through tap I8.

.Opening into chamber 50 is a combustion" chamber 2|, equipped with a burner 22, bustle 24 and pipe connections and 25 as in Fig. 1.

Fig. 3 illustrates thegas purification andphosphoric acid collection system; 'Suchsystem comrator 11 and hydration tower 18. The exchangers 75 and 15 may be constructed of refractory prises heat exchangers 15 and 16, a dust sepamateriaLand the internal parts exposed to the action of thefurnace gases may be lined with more resistantmaterial such carborundum.

The tube header arches 8| andthe tubes '82 are alsopreferably of resistant material such as carborundum. Each exchanger is formed with a relatively large chamber 83 in thelower end which, acts as dust chamber and facilitates re- J moval of dust fror'nthe. gas stream, Furnace M gases passfrompipe 26 into theupper end of cooler 15, in series through exchangers 15 and 16, and thence flow through a pipe connection;

85 into oneor moredust' separators 11, prefer ably of the cyclone type.

'Air, for use in oxidizing chambers I5 and 60" of the furnaces of Figs. 1 and 2, is pumped by blower 86 through the chambers 81 surrounding a the tubes 82 inexchangers 16 and 15. The air,

thus preheated by heat transfer with the hotv furnace gases flowing through the tubes of coolers l5 and 15, passes through pipe 81' into con-' duits 25 and 3'! controlled by valves '88 and ,89 respectively.

Gases discharged from dust collector 11, made forexample of nichrome steel, are conducted into I the bottom of hydrating tower 18, preferably lined with carbonaceous block and packed; 'if' desired; with preferably coarse coke- In tower 181 gases rise against-a downwardlyfiowing stream of phosphoric acidcirculatedflover the hydrating tower by means of a circulating system in: dicated generally by reference numeral 90.

Exit

gases and mist leaving the top. of tower 18 at a suitable ratethrough the apparatus is fiow through a cooler 91, and into Ia suitable 1 phosphoric acid'collectcr, not shown. Gas flow,

maintained by a blower; not. shown, interposed i between the cooler 19 and the phosphoric acid collector. orlocated at the end of the system.

' In carrying out the process of the invention,

phosphate rockand coal er coke may be crushed separately and subsequently admixed in proper proportions, or may be ground. together in proper proportions in a mill. 'The latter procedure is preferred since this operation effects a more F. in heat exchangers l5 and H5.

intimate admixture of rock and coal which in turn promotes rapid and completev reaction during the relatively short time of travel of the materials through the reduction zone in the furnace. The rock and coal are ground to pass, for example, 100 mesh, and proper proportions thereof are fed into hopper by conveyor mechanism not shown. On operation of conveyor 3|, a steady stream of finely divided rock and coal is fed into conduit 28. Just enough air is admitted through jet'29 to float the mixture into chamber I4, cool air being preferably used for this purpose to avoid premature ignition in pipe 28. At the beginning of operations, it will be understood the furnace It may be heated up to initial reaction temperature by any convenient means such as burner 22.

In accordance with the present invention, it is not essential, and in the preferred embodiments not desirable, that-all the extraneous fuel, as distinguished from the carbonaceous material involved as reducing material, required in the process be introduced into the reduction zone with the phosphate rock as part of the carbonaceous material. Preferably, however, an excess of carbonaceous material, over that theoretically required for reduction of phosphate, is charged into the furnace with the rock, and the heat generated by partial oxidation to carbon monoxide of this excess, which may be considered as extraneous fuel, is advantageously liberated directly within the reduction zone to aid in maintaining therein the desired high temperatures. As great an excess of carbona- 'teous material as practicably possible is desirably introduced into the furnace with the rock,

the excess of fuel being limited, preferably, so that it is unnecessary to unduly speed up the 7 amount of finely divided coal fed into the furnace with the powdered rock is that required to bring about reduction of phosphates plus as large an excess as may be convenient, taking into account the size of the furnace and the particular operating conditions.

Substantially all of the air needed for the reaction in chamber M is introduced through pipe 3i, chamber 36 and inlet 35, having been preheated to temperatures of, say, about 1500-1800 The volume of air thus entering the furnace may be regulated by valve 89 (Fig. 3) in pipe 31, and should be suificient to burn to carbon monoxide theccal or coke in excess of that needed for reduction of the phosphate in the reduction chamber M. Annular inlet has an injector action, and hence, a suspension of finely divided phosphate rock and coal in air is introduced into the reduction chamber I4. By introducing the major portion of the air directly into the top of the reduction zone, such air may be highly preheated, and as mixing of hot air and coal outside the furnace chamber is prevented, premature ignition of coal is avoided.

The temperature maintained in the reduction zone It is not desirably less than about 2400 F. and is preferably within the range of about 2600-3000 F. With such temperatures prevailing, the reduction of the phosphate of the rock,

by the coal, to elemental phosphorus takes place. Carbon monoxide is produced by the burning fuel which reacts with the phosphate rock in accordance with the following equation:

the carbon dioxide reacting with the excess carbon present to regenerate carbon monoxide as.

follows:

5CO2+5C+Heat=l0CO solid materials are separated from the stream.

of gas and phosphorus fume. Fume and gases leaving the reduction chamber through openings 27 comprise principally nitrogen, carbon monoxide and phosphorus, and materials dropping into the bottom I I include chiefly calcium oxide, with incidental amounts of coal ash and of calcium silicate, resulting from the silica content of most varieties of phosphate rock. and coal. V Y

This silica content of the phosphate rock or coal or both fed into the top of the'reduction chamber may be high enough so that the residue in the base of the furnace has the consistency of a semi-fluid slag, or the residue particles at least tend to stick together. .If the latter condition prevails, such residue may be made fluid by the addition of silica to the residue while in the base. of the furnace, thus facilitating discharge through tap hole 18. Where the residue is fused or partly fused, in this situation the mass also acts to catch solid particles impinged against the surface thereof by the gas stream from chamber i l. However, in order 'to avoid as much as possible the presence in the reduction chamber of molten material, which tends to shorten the life of shell !2, it is preferred that the silica content of the rock and coal should be low. In cases where the rock and coal contain substantially no silica, the residue in the furnace bottom may be in the solid condition. The base of the furnace is large enough to permit most of the normally fluid particles to drop out of the gas stream, or to impinge upon the surface of the pool of molten slag. and fume, substantially free of dust, after passing through openings 2'! in shell l2, rises through the oxidizing chamber I5 into the base of which preheated air, hot combustion gases, and unburned fuel are fed from combustion chamber 2|.

Air from bustle 24 is introduced into combustion chamber 2| in quantities sufiicient to support combustion of fuel introduced through burner 22, and combustion of phosphorus and carbon monoxide in chamber 55. As fume and gases from the reduction zone rise through chamber I5, the elemental phosphorus and carbon monoxide are oxidized respectively to phosphorus pentoxide and carbon dioxide. The oxidation of phosphorus to phosphorus pentoxide and of carbon monoxide to carbon dioxide liberates large quantities of heat. In accordance With oneof the important features of the invention, the heat generated during this oxidation operation is utilized economically to aid in maintaining the I The stream of gas.

proper temperatures in the reduction'zone'. To" accomplish this purpose, oxidation of phosphorus l ture in the furnaceis charged into combustion chamber 2| through burner 22. Under usual conditions, aboutone-third of .the quantity of extraneousfuel needed'in the process is fed, withtherock and coal. into chamber l4.. In this situation, about two-thirds'of extraneous fuel requirements are provided, for byregulation of burner 22. Airfor burning such fuel is intro- 'duced through bustle 24, and combustion of the fuel may take place principally in chamber 2! and burning completedin chamber 15., Ace cording to .particularfloperatingi conditions, the amounts of extraneousfuel introduced into rea I, ductionzone Hand combustionchamberll may vary considerably. Preferably, however, it is de sirable to introduce into the reductionzone less than a major portion of the total amount of V extraneous fuel utilized in the process. a

To further .r-aisethe temperaturesreached in the oxidizing chamber l5 and in the reduction chamber l4, and thereby increase theefiiciency of the operation, the air introduced through bustle 24, and employed in combustion chamber.

2| and in oxidizing zone l5 for burning phosphorus to phosphorus pentoxide and carbon monoxide to carbon dioxide is preferably preheated." Preheating of the air to .the desired degree, for example to about 1 500-1800" or highermay be advantageously effected bypassing such air through heat exchangers 16 and 15, the air outlet pipe 81? of the latter being connected through pipe 25 with bustle 24. "I'he quantity of air introduced into chamber I 5 from bustle 24 may be controlled by valve 88 inpipe 25; The gases leaving the top of oxidizing chamber i5, consisting principally in of nitrogen, carbon dioxide and phosphorus pentoxide, are conducted by pipe 25 to the'gas purification and absorption system shown in Fig. 3 of the drawings. The hot furnace gases flow through the heat exchangers l5 and 16 in series and arelcooled by'air passing countercurrent throughchambers: 8;! around the tubes 82. The furnace gases are thus cooled, and

the air employed in furnace H1 is preheated. This feature presents. another substantial advantage 6 10f the invention, inthat heat of the furnace gases, after discharge from the furnace, is recovered to a large degree and utilized to further the reduction reaction in the furnace. While passing through the heat exchangers, dust particles settle out of the gas stream and are recovered in dust chambers 83 provided in the base of both heat tr-ansferrers.

the latter, the gases are then introduced into a separator H, preferably of the cyclone type, and remaining dust is 'removed'from the gas stream. Any calcium oxide dust leaving the oxidizing chamber l5 with the gaseous phos-. phorus pentoxide may combine therewith to 775 some extent. Thus, it may well beof advantage After having passed through to return dust recovered from transferrers-15 and 16, and separator 11 to the furnace along with the fresh quantites of rock and'coal fed thereto.

The gasstream, substantially free of dust;lea'ving the top .of separator 11, is fed into the bot-' tom of hydrating tower 18, the latter'being pref-1 erablylined with carbonaceous material and The, ascending gases are contacted with adownwardly packed preferablywith very coarsecoke.

flowing stream of phosphoric acid maintained in circulation through the tower by" suitable pumps and pipe connections. In the hydratin tower,

the gas is cooled to or below hydrating tempera-I ture, e. g.; about400 F., and part of the water,

in the preferably weak circulated phosphoric acid, is flashed to water vaporwhich reacts; with P205 to form HaPO i-HzO mist,some of the acid.

thus formed being absorbed in the'facid of the circulating system. From time to time, water' may be added-tothe acid circulation. system in the usual manner.

. In cooler 9!, the mixtureof phosphoric acids I mist, Water'vap'or and gases is cooled to temperatures suitable for collection of acid mist, and the latteris collected by mechanical or electrical means, or in any desired manner. If desired, the exit gas of the phosphoric acid collector may be scrubbed for recovery of SiF4 vapors as HzSiFa The process whencarried out in the apparatus of Fig. 2 is, to a large degree, substantially the same as already described in connection with the apparatus of Figs. 1 and 3. In the furnaceof- Fig." 2,'powdered rock and coal, in a stream of carrying air, is'fedinto the inlet'end of the inner drum :55, through pipe 62, and, because of:

and residues are discharged into the chamber 50.

The atmosphere is reducing throughout the,

lengthof chamber 59, and phosphatesare reduced. to' form phosphorus, and the excess fuel above that necessary for reduction of the rock is burned with the limited amount of oxygen present to form carbon monoxide. The air required in reduction zone 59 is introduced, in preheated condition through pipe. 65, connected to As in the apparatus of Fig. 1,

pipe 37, Fig. 3. additional'air and fuel areintroduced, through bustle 24 and combustion chamber 2|; into chamber '50, and oxidation of phosphorus and the purification Fig. 3.

the rotation and pitch of drum 55, the materials are gradually worked toward the outlet end BI,

. carbon monoxide is effected in oxidizing zone lie. The fume and gaseous products'of the reaction' are discharged from the furnace into I chamber '49, and flow through conduit 53, to and absorptionsystem shown in Because of the rotation of drum s5, it' will be seen the finely divided'jrock and coal are dispersed ina reducing atmosphere. As the passage of material through reduction zone 59 is relatively slow,

under some circumstances, the process of the: invention may be carried out to advantage in apparatus of this type,-'wher'e .it is desired to maintaina relatively longer period 'of contact 7 between finely fuel.

We claim: 7 1 1. The method of producing phosphoric acid which comprises introducing finely divided phosphate rock and an excess carbonaceous reduc-' divided phosphate rocl; and the ing material in suspension in air into a reduction T zone, reducing the phosphate at elevated temperatures to phosphorous while passing the reacting materials co-current through the reduction zone, regulating the amount of air fed into the zone to afford oxygen sufficient to oxidize 'the excess of reducing material to carbon monoxide thereby supplying heat to the reducing reaction, separating gases containing phosphorus and carbon monoxide from non-gaseous residue of the reduction reaction, introducing into an oxidizing zone phosphorus, carbon monoxide and preheated air to support oxidation thereof together with sufficient extraneous heat to maintain reaction temperatures, oxidizing the phosphorus and carbon monoxide to phosphorous pentoxide and carbon dioxide while in heat transfer relation with the reduction zone, thereby utilizing the heat generated by the oxidation reaction to further the reduction reaction, and recovering phosphoric acid.

2. The method of producing phosphoric acid which comprises passing a suspension of phosphate rock, carbonaceous material, and a gas containing free oxygen through a reduction zone maintained at elevated temperatures, the carbonaceous material in the suspension being in excess of the amount necessary to convert the phosphate rock to phosphorous and the amount of gas containing free oxygen being regulated so as to convert the excess of carbonaceous material to carbon monoxide and supply heat for the reaction, oxidizing the resulting phosphorus and carbon monoxide to oxides of phosphorus and carbon dioxide in the presence of a heated gas containing free oxygen in an oxidation zone maintained in indirect heat exchange relationship with said reducing zone whereby the heat of oxidation is utilized in the reducing reaction, withdrawing the gaseous oxides of phosphorus and carbon from the oxidizing zone, and passing said gaseous mixture in heat exchange relationship with the gas containing free oxygen being supplied to said oxidation zone to preheat said gas.

3. A method of producing phosphorus pentoxide which comprises reacting a suspension of phosphate rock, carbonaceous material and a gas containing free oxygen at elevated temperatures in a reduction zone, the proportions of carbonaceous material and oxygen being adjusted so as to maintain reducing conditions in said reducing zone to release elemental phosphorus and to form carbon monoxide, separating gases containing phosphorus and carbon monoxide from the nongaseous residue of the reduction reaction, introducing said phosphorus and carbon monoxide together with oxygen into an oxidation zone maintained in indirect heat exchange relationship with said reduction zone, oxidizing the phosphorus to phosphorus pentoxide and carbon monoxide to carbon dioxide in said oxidation zone, whereby the heat generated by the oxidation reaction is utilized in effecting the reduction re action, and recovering phosphorus pentoxide.

4. A method of producing phosphorus pentoxide which comprises reacting a suspension of phosphate rock, carbonaceous material and a gas containing free oxygen at elevated temperatures in a reduction zone, the amount of carbonaceous material in said suspensionbeing in excessof that necessary to convert the phosphate rock to elemental phosphorus and the amount of oxygen being regulated so as to maintain reducing conditions in said zone, to release elemental phosphorous and to convert the excess carbonaceous material to carbon monoxide to supply heat for the reducing reaction, separating gases containing phosphorus and carbon monoxide from the non-gaseous residue of the reduction reaction, in-

troducing said phosphorous and carbon mon- V oxide along with extraneous fuel and free oxygen into an oxidation zone maintained in indirect heat exchange relationship with said reduction zone, oxidizing the phosphorus to phosphorus pentoxide and carbon monoxide to carbon dioxide and burning the fuel in said oxidation zone, whereby the heatgenerated in the oxidation zone is utilized in effecting the reduction reaction, and recovering phosphorus pentoxide.

5. The method of producing phosphorus pentoxide finely divided phosphate rock and relatively finely divided carbonaceous material into a reduction zone, reducing the phosphate rock at elevated temperatures while in suspension in a reducing gaseous atmosphere to release elemental phosphorus from the phosphate rock, separating the gaseous products containing phosphorus from the non-gaseous residue of the reduction reaction, introducing said gaseous products together with oxygen into an oxidation zone maintained in indirect heat exchange relation with said reduction zone, oxidizing the phosphorus to phosphorus pentoxide in said oxidation zone whereby the heat generated by the oxidation reaction is utilized in effecting the reduction reaction, and recovering phosphorus pentoxide.

6. The method of producing phosphorus pentoxide which comprises reacting a suspension of phosphate rock, carbonaceous material and oxidizing gas at elevated temperatures in a reduction zone, the amount of carbonaceous material in said suspension being in excess of that necessary to convert the phosphate rock to elemental phosphorus and the amount of oxygen being regulated so as to maintain reducing conditions in said zone, to release elemental phosphorus and to convert excess carbonaceous material to carbon monoxide to thereby supply heat for the reduction reaction, separating gases containing phosphorus and carbon monoxide from the nongaseous residue of the reduction reaction, introducing said phosphorus and carbon monoxide together with oxygen into an oxidation zone maintained in indirect heat exchange relation with said reduction zone, oxidizing the phosphorus to phosphorus pentoxide and the carbon monoxide to carbon dioxide in said oxidation zone whereby the heat generated in the oxidation zone is utilized in effecting the reduction reaction, and recovering phosphorus pentoxide.

CHARLES L. LEVER/MORE. ROBERT E. VIVIAN.

which comprises introducing relatively-

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