claim: 1. In a process for pr6ducing red iron oxide, the steps wliieh comprise heating ferrous sulfate out of direct contact with flame or combus25 tion gases in a substantially closed heating zone to temperatures between 1300 and 1550' F. to cause conversion of ferrous sulfate Into iron oxide of a predetermined quality, supplying air to . the decomposing material In an amount suffi. 30 cient to support the reaction' 2FeSO4.H20+1/202=Fe2O3+2SO3+2H20 restricting the supply of air so that the gases exhausted from the heating zone contain st Jeast 35 1OVo of sulfur oxides, and limiting the time of contact of such gases with decomposing qonditions in the heating tone such that the gases have at least 50% of the sulfur tri-oxide in said sulfur oxide content. 40 2. In a process for producing red'iron oxide, the steps which comprise heating ferrous sulfate indirectly in a substantially closed heating zone to a temperature between 1300 and 15501 P. to cause conversion of ferrous sulfate into ir6n 4;1 oxide, supplying air to the decomposing material in an amount sufficient to support the reaction 2FeSO4.H20+1/202=Fe2O3+2SO3+2H20 restricting the supply of air so that the gases exhausted from the heating@zone contain at least 10% of sulfur oxide8, limiting the time of contact of such, gases with decornposing e-onditions in the heating zone such that the gases have at least 50% of sulfur tri-oxide In said sulfur oxide ,content and convertiiig sulfur oxide from said gases into sulfuric acid. 3. In a process for producing red iron oxide, the steps which comprise passing ferrous sulfate continuously through a substantially closed heating zone heated externally to temperatures between 1300 and 1550' F. to cause conversion of ferrous sulfate into iron oxide of,predetermined quillity, suPPlying air. to the decomposing material in iin amount suffic'ient to support the reaction 2FeSO4.H20+ V2(32=Fe2O3+2SO3+2H20 restricting the supply of air so that the gases exhausted from the heating zone contain bt least 10% of sulfur oxides, liniiting the time of contact of such gases with decomposing conditions in the heating zone by restricting the voliame of gases held in said zoni@,so that the exhausted gases have at least 50% of sulfur tri-oicide in said sulfur oxide content and converting Sulfur oxide from said exhausted gases into sulfuric acid. 4. A cyclical process for Producing red irozi oxide which comprises dissolving iron in dilute sulfuric acid, crystall,izing and separating FeSO4.7H20 from the solution, drying and dehydrating the FeSO4.7H20 to obtain FeSO4,H20, decomposing the FeSO4.H20 by heating the same indirectly In a substantially closed heating zone to a temperature between 1300 and 15500 F. while supplying Air in an ariiount at least sufflcient to support decomposition according to the regetion 2FeSO4. H20+ Y202-Fe2O3+2SO3+2H20 restricting the supply of air to the decomposing material so as to have at least 10% of sulfur oxides in the decompositiori gases, flxing conditions such that the decomposition gases taken off from said zone have at least 50% of sulfur tri-oxide in said sulfur oxides, converting sulfur tri-oxide, from the decomposition gases, into sulfuric acid, and dissolving iron in the resulting sulfuric acid as aforesaid. 5. A cyclical process for producing iron oxide which com@rises dissolving iron in.dilute sulfuric acid-in a tank, crystallizing and sepaiating. FeSO4.7H20 from the solution, returning the FeSO4.7ft2O mother liquor into said tank, drying and dehydrating the FeSO4.7H20 to obtain FeSO4.H20, producing decomposition gases containing @,t least 10% of sulfur oxide and calcined material consisting essentially of ferric oxide and ferric sulfate by heating indirectly and decomposing the FeSO4.H20 out of contact with flame or combustion gases, in -an externally heated muffle, while supplying a restridted amount of oxygen-containing gas to the decgmposing material, fixing the eonditions of decomposition such that at least hdlf of the sulfur oxide content of said gases is sulfur trioxide, converting sulfur oxide from said decomposition gases into sulfuric acid, flowing the resulting sulfuric acid into said tank, washing said calcined niaterial to obtain ferric sulfate -solution and wet ferric oxide, flowing the ferric sulfate solution into said tank, and drying thi@ ferric oxide to obtain substantially plare iron oxide. JOSEPH W. AYFP.S.

Pat,ented Feb. 12t IN6 2,13949579 UNITED STATES PATENT OFF'ICE. 2,394,579 PROCESS FOR THE PRODUCTION OF IRON O)ME Joseph W. Ayers, Easton, Pa., assignor to C. R. Williams Co., a corporation of Pennsylvania Application June 8, 1939, Serial No. 278,028 5 Claims. (Cl. 23-200) This Invention relates to a new and improved end of the kiln is washed to remove soluble Improcess for producing red iron oxide and particularly to a new cyclical process for producing red iron oxide from ferrous sulfate. Pure red iron oxide is one of the principal col- 5 ored Pigments used by the paint, rubber, lin'oleum and similar industries. . . The process generally emplOY6d for the manufacture of iron oxide pigments involves the calcination or thermal decomposition of ferrous sul- 10 fate Ln a large rotary kiln. The usual kiln is a large, inclined, cylindrical tube lined with refractory material. The size of the kjjn varies: from 50 to 100 feet In length and from-5 to 10 feet in iameter. Ferrous sulfate is char-,ed into the 15 elevated end of the kiln and passes by gravity tbrough the entire tube during r6tation, to be discharged at the lower end. A flrebox is located adjacent the lower end of the kiln, and heat - and gases from the combustign of coal, coke, gas 20 or oil in this firebox sweep through the kiln in contact with the ferrous sulfate and exit through a flue at the upper end of the kiln. The ferrous sulfate crystals to be used in the purities, whereupon substantially pure red iron oxide suitable for use as a color pi-gment may be obtained. The decomposition gases exhausted from the upper end of the kiln corigist of combustion gases, water vapor and a very small concentration of sulfur oxides. These gases are genefally wasted, in. the air. This common process for producing red iron oxide has a number of defects and shortcomings that have not been overcome prior to my Invention. When treating the usual ferrous sulfate, which contains from .15 to .50% of manganese sulfate aii an Impurity, it Is impossible to obtain a high percent conversion of ferrous sulfate to ferric oxide while securing iron oxide sufficiently free of manganese oxide to be of satisfactory pfgment quality with respect to manganese oxide content and color properties. When producing valuable iron oxide pigments with low manganese oxide content the Yields do not exceed about 70%. Also, it is difflcult to vary the color properties of the product accurately; and the efficiency of the process with regard to heat consumption, appaproduction of iron oxide are sometimes obtained 25 ratus and material requirements, etc., is objecfrom scrap Iron by a series of treatments that tionably low. results In FeSO4.H20 as the end product. The A new and @inproved process overcoming these iron Is dissolved in a dilute sulfuric acid solution. and othersh@)rtcomings of the conventional procThe solution is then crystallized to form so ess Is disclosed and claimed in my copending appl@cation, Serial No. 278,027,,fUed of even date FeSO4.7H20 , which may be separated from the mother liquor in a centrifuge. The wet FeSO4.7H20 crystals are dried and then dehydrated to obtain FeSO4.H20 35 in a condition suitable for the decomposition process. The calcination or decomposition of the . ferrous sulfate in the rotary kiln involves treatmen-t of 4( the material under indeflnite conditions as to temperature and in the presence of an atrnosPhere consisting of combustion gases and an in-. determinate amount of air. 3:n order to secure sufficiently high temperature In the iipper or inlet end of the kiln, the lower or exit end must often be overheated. In practice decomposition Is allowed to proceed until most of the ferrous sulfate has been converted to ferric oxide, which may take place, at least partimlly, according to the reactiors: herewith. Another objectionab le feature of the convention al process for producing red iron oxide resides in the lack of efflcient utilization of process materials and byproducts. The decomposit i6n gases from the calcination kiln, for example, are of such nature that recovery of their valuable constituent s is economical ly impractica ble, so that thege gases are usually entirely wasted. Tbi8 not only involves an economic loss, but it also'freque ntly results in contaminat ion of the air in the neighborho od of the place where the process is being operated. Yet it would entail excessive costs to oper45 ate the process with special provision for preventing the release of the kiln gases into the air. It is ther(@fore Lin object of the present invention to provide a new and Improved cyclical process for producing red iron oxide which results In 5i), effleient production and utilization of materials and gases in the operation of the process. 6FeSO4.H20=2Fe2 O3+Pe2(SO4) 3+3SO2+ 6H20 Another object of lnven n Is to provi a process for producing red iron oxide by which Fe2(SO4)3=Fe2O3 +3SO3 the release of decomposition gases Into the air T'he calcined material issuing from the lower 55'and the consequent conta-in tion of the alr are

2 2,394,579 avoided, and in an economical and practical manner. . A further object of the invention is to provide such a process which at the same time overcomes the above mentioned defects and shortcomings of the conventional process for producing red iron oxide, giving a high conversion yield of iron oxide of high quality and low manganese oxide content, enabling ready control over and variation of color properties of th6 iron oxide, and resulting in materiany increased efficiency of process operations. One of the features of the present invention consists in carrying out the decomposition of ferrous sulfate, in the production of red iron oxide, under closely controlled temperature conditions Pnd in supplying to the decomposind material a regulated stream of air or other gas in an amount sufficient to obtain the desired quality and Yield of iron oxide while restricting 20 the supply of air or other gas so that the decomposition gases iriclude, comparatively, a very high concentration of sulfur oxides. The decomposition gases are then treated economically to convert sulfur oxide cont-ained in them into sulfuric 25 acid, which may be circulated advantageously to a starting point in the process and used In the production of ferrous sulfate from scrap iron or 1 the Uke. Another important feature of the present in- 30 vention resides in carrying out the decomposition of the ferrous sulfate in such manner as to obtain a relatively high percentage of sulfur trioxide in the sulfur oxides present in the decomposition gases. This is accomplished in part by 35 swpply',ng air or other oxygen containin-g L7,as In an amount sufftclent.to support decomposition of th6 ferrous sulfate aec'ording to the reaction: 2FeSO4.MO+ 1/202--Fe2O3+2.SO3+2MO 40 which results In the liberation from the decomposing material of gases the sulfur oxide content of which is substantially all sulfur tri-oxide. The sulfur tri-rxide generated according to this reaction is preserved to a large extent, notwith- 45 standing the strong tendency of sulfur tri-oxide to decompose into sulfur dioxide and oxygen at the temperatures encountered in the process, by maintaining appropriate control over the maximum heating temperatures in the zone where 50 conversion of ferrous sulfate is taking place, and by hastening the flow of the decomposition gases from this hot zore. According to the present process, I may produce decomposition gases containing more than 55 10% of sulfur oxide- in which more than 50% of the sulfur oxide co;;i@nt consists of sulfur trioxide. These gases are conveniently and economicauv flowed through a system for the recov- ' ery of their sulfur values, in which the sulfur 60 tri-oxide may be converted to sulfuric acid by absorption in wtiter and in which the remaining sulfur dioxide may be converted to sulfuric acid by the well known contact or catalytic process. The sulfuric acid recovered from the gases may 05 then be plassed to the starting point of the process and used for the digestion of iron to produce ferrous sulfate' The gases remaining after extraction of sulfur oxides mry be released into the air without danger of contaminating the sur- TO rounding atmosphere. The preferred embodiment of the lnventi6n is a cyclical process for the production of red iron oxide which results in maximum utilization of process materials and byproducts, eliminates the 75 5 I 0 15 econoinic loss and the atmospheric contamination caused by the release of decomposition gases according to the prior process, and at - the same ti 'iesult me s in economies, improvements in product quality and yield and savings that are not obtainable from the prior process. Other features and advantages of the invention vvill appear from the following detailed description of a preferred embodiment when considered In connection wi the accompanying drawings, in which: Mgure 1 is a diagrammatic flow chart indicating the paths of flow and the treatment of materials in the practice of the process; Mgure 2, is a vertical longitudinal section through one form of rotary muffle which may be employed in carrying out the calc'mation or decomposition of the ferrous sulfate; Flgure 3 Is a fragmentary end view of the muf-. fle, viewed from the right hand en of gure 2; and Mgure 4 is a vertical cross section through the muffle,- approximately along the hne 4-4 of Flgure 2. Referring pirticularly to Figure I of the drawings for illustrative details of a preferred embodiment, scrap iron or othe,r suita6le iron material is introduced into a digesting tank containing dilute sulfuric acid, and the iron is digested and dissolved in the acid to form a. solution Pf FeSO4.7ffio. This solution Is passed into a crystalhzer where the hydraied ferrous. sulfate is crystallized. The resulting mixture of crystals and mother liquor is separated, for example, In a centrifuge, the mother liquor being recirculated to the digesting tank, as indicated in Flgure 1, so that none of its ferrous 8ulfate content Is lost from the'process. The wet FeSO4.7H20 crystals are passed through a drier for the removal 0 water and then through a dehydrator where water of crystallization is extracted, In a well known manner, to produce FeSO4.I-120 In a condition suitable for conversion to ferric oxide. The ferroussulfate from the dehydrator is then passed into the calciner to be converted into Iron oxide. If desired, it may be coated or mixed with a small amount of an oxidation@retarding catalyst, such as an inorganic alkali nietal salt, the most common and inexpensive of which is sodium chloride, before being subjee'ted to the decomposition treatment. The calcination may be of any closed roaster type or form capoble of effecting the requisite treatment. For purposes of illustration, I have shown in Mgures 2, 3 and 4 a type of apparatus which is particularly weR -suited for this purpose. The apparatus as shown in these figures comprises a fumace or flrebox 10, of firebrick or other suitable refractory material, which provides an elongated heating chamber 12 through which exends a rotary muffle 30 for eirrying out,.the decomposition of the.ferrous sulfate. Opposite end portions of the muffle 30 prpject through and beyond oppositelydisposed openings 14 and 16 In the furnace waus. A. plurality of combustion chambers IS are located in the lower portion - of the furnace, and the mullle Is heated to controlled decomposition temperatures by the combustion of suitable fuel from burners 20, which extend Into thecombiistion chambers IS. The entire-muffle is mounted for roiation at a . suitable speed, for example by means of tiies 32 and 34 secured to the m@ffle waU and cooperating rollers 36 and 38. Its axis of rotation Is slightly incjined so that material introduced.into

2,394,570 the muffle at Its upper end gradually progresses to the lower end during rotation. In order that the compositioii of gases supplied to and exhausted from the rnuffle may be controlled, the muffle is made cubstantially gas-tight, r) while providing suitable means for feeding and discharging solid materials and for introducing and drawing off gases. As illustrated, the main body of the muffle comprises a charge header 40 at its upper end, a 10 multiplicity of elongated tubes 42 extendir.,g through furnace chamber 12 and aIl communicating with the charge header, and a common discharge header 44 communicating with the outlet ends of tubes 42 and having means therein for 15 the discharg6 of solid materials from the muffle during rotation. The ferrous sulfate may be fed into the charge header 40 by suitable feeding means, for example, an axially disposed screw conveyor 50 opening 20 into header 40 through a port in a stationary end wall section 52. Air or other gas may be introduced into header 40 by means of a supply Plz'L54 communicating with a second port in waR s@' : tion 52. 25 The ferrous sulfate accumulated in. charge header 40 is -picked up in the inlet ends of the several muffle tubes 42 during r6tation of the muffle, and the material contini-iously progresses lengthwise of th,- tubes, and through the reaction 30 zone maintained therein by heat from the furn,-,ce, until it is finally discharged from tubes 42 into header 44. 'ne calcined material entering the discharge header is lifted and dropped toward the axis of the muffle bY r'adial vanes 56 35 which rotate with the muffle. An axiaby disposed discharge cone 58 directs the material through a discharge opening 60 and thence into a stationary hood 62 that i@ arranged in slbstaritially gas-tight -relation to the muffle. From fhe hood 40 62 the -material may be withdrawn as desired b y means of sealed discharging means, such as a rotary star wheel 64. , Decomposition gases exhausted from the muffle may be drawn off through an exit pipe 66 connected wit'h hood 62. 45 The multiplicity of muffle tubes 42, a8 :[iIustrated, are arranged in parallel and circularly spaced relation,around the axis of rotaticn of the muffle, at substantially equal rpdii from the axis. For best results, each tube may be quadri50 lateral in cross section, as iiidicated, for example,' in Figure 4. With such a construction, the material passing through the tubes is continually . turned over and over during its progression from the charge header to the discharge header, anil 55 the transfer of heat to the material while in the decomposition zone takes Place at maximum efflcfency. For effectiveness of hep -t transfer the metal, and high strength and corrosion resistance are obtained by making the muffle tubes, as 'well as other parts of the muffle that are sub.Ject to contact with materials and gases gt high temperatures, from strong, corro ,Ive-reste,,ant, creep-resistant metals such as the high chromium 65 irons and steels alloyed in smaller percentages with metals such as nickel, tungsten, manganese and molybdenum. Tn the Practice of the present process, the heat- 7o ing temperatures for the material In the muffle tubes are kept at closely regulated points between about 1300 to 1550' P. The Particular ternperature wiU depend upon the desired coier and nianganese oxide content of the flnal Iron oxide prod- 75 uct and on the desired rate of conversion of the ferrous sulfate tc) Iron oxide. The ferrous suIfate from the dehydrator Ls fed intd the discharge, header 40 at a regulated rate, from which it passes through the muffle tubes 42 In substantially uniforrn, evenly distributed streams. At the same time, air or other oxygen-containing gas is introduced into header 40 and into the muffle tubes from supply pipe'54. The supply of gas is regulated to provide a'n ainount of oxygen sufdcient to support decomposition and conversion 6f the ferrous sulfate to ferric oxide according to the reaction: 2F@eSO4+1/202--Fe2O3+2SO3+2I-120 This gas supply, however, Is restricted so that the decomposition gases exhausted from the h eating zone of the muffle aiid drawn off through pipe 66 contain &- comparatively high concentration, usually about 12 to 15%, of sulfur oxides. By virtue of the higlily efficl-ent transfer of heat to the ferrous sulfate In the muffle tubes, the maximum temperatures to which materia-Is and go@ses in the tubes are subjected in the course of the treatment ate kept at 'the lowest possible polnt for the Production of each particular grade of iron oxide. Purtherinore, the rate of flow of gases through the.heating zone of the muffle Is enhanbed, and the lingering of sulfui tri-oxide at temperatures favorable to the production of sulfur dioxide is prevented to the maximum extent consistent with other requirements'of the process, by providlng little void space for the passage of gases through the conversibn zone of the muffle. In this manner I produce from the calcination stage of the process decomposition gases in which more than 50%, and usually about 60%, of the sulfur oxide content of the gases consists of @ sulfur trioxidi@. At the same time, conversion of the ferrous sulfate Into red iron oxide of high quality is car@. ried out to give extraordinarily high yields, of between 85 and 92%, or more, with a rnanganese oxide content belbw .05 %. Referring again to the ffow chart in ngure I of the drawings, the decomposition gases from the calcination apparatus are passed through a systein for the production of sulfuric acid, which may Include a unit of well known type, indicated at A, for the con"version of sulfur tri-oxide Into sulfuric acid by absorption and another unit of well known type, indicated at B, for the converslon of sulfur dioxide into sulftiric acid by the cohtact or catalytic process. When sulfur tri-oxide exceeds 50% of the sulfur oxide content of the gases, as in the Preferred Practice of ttte Invention, it may be desirable in some cases to convert only the sulfur tri-oxide i nto. sulfuric acid, provided that It is satisfactory to release age, where it Is tised for digesting the inconiing iron. The calcined materlal discharged from the calcination apparatus consists principauy of ferric oxide with a small percentage of ferric sulfate. This material Is passed to the thickener where it Is washed with water to dissolve soluble sulfate and where the residual insoluble ferric oxide Is partlauy separated from the resultiiig solution. The ferric sulfate solutior@, as indicated In Flgure 1, may then be flowed t6 the digesting'tank to be reduced to ferrous sulfate muffle tubes are preferebiy made entirely of sheet eo the remaining gases into the air. . The sulfuric acid obtained In this recovery, system Is passed -into the digesting tank at the head of the process, either directly or after stor-

4 21894,579 by the iron there present, and then to be reeirciilated throug - h the process. Ferric oxide separated at the thickener is filtered, dried and disintegrated, giviiig red iron oxide having valuable qualities for use iis a color pigment, in paints, rubber, linoleum and the like. For purposes of clarity and illustration I have described numerous details of a preferred embodiiiient of the present invention and have described and illustrated details of construction of 10 a form of apparatus which is particularly suitable for use in practicing the invention. . It wiR be uiiderstood, however, that v arious features . of the process may be modifled or avoided without sacrificin g contributions of the invention and jr, that the process clainied herein may be practiced with various types of apparatus. I therefore desire that the invention be accorded a scope fully commensurate with its novel teachings as limited only by the fair reqwrements of the ap- 20 pended claims. I