US3211538A

US3211538A - Concentration of sulfuric acid pickle liquor

Info

Publication number: US3211538A
Application number: US113828A
Authority: US
Inventors: James A Gross; Bernard G Mandelik
Original assignee: Chemical Construction Corp
Current assignee: Chemical Construction Corp
Priority date: 1961-05-31
Filing date: 1961-05-31
Publication date: 1965-10-12
Anticipated expiration: 1982-10-12

Description

Oct. 12, 1965 J. A. GROSS ETAL 3,211,538

CONCENTRATION OF SULFURIC ACID PICKLE LIQUOR Filed May 51, 1961 HOT DRYING GAS '2" 2: of, 5D |!I 21' 7 8 v 9 WEIR 2 II D U. 5 EXPANSION CHAMBER 5 DE-ENTRAINMENT SECTION 3 l6 LL! X (J E 0 DJ E m E O U we; SETTLING /5cm I VESSEL BAFFLE 26 1 FILTER 24 l FILTERED SULFURIC ACID V 25 souo FERROUS SULFATE CRYSTALS JAMES A. GROSS BERNARD G.MANDEL|K IN V EN TORS AGENT United States Patent 6 3,211,538 CONCENTRATION OF SULFURIC ACID PICKLE LIQUOR James A. Gross, East Brunswick, N.J., and Bernard G.

Mandelik, Pleasantville, N.Y., amignors to Chemical Construction Corporation, New York, N.Y., a corporation of Delaware Filed May 31, 1961, Ser. No. 113,828 8 Claims. (Cl. 23-300) This invention relates to the processing of spent pickling liquor containing ferrous sulfate and residual free sulfuric acid. A process and apparatus have been developed, which achieve the concentration of spent liquor in a novel manner, whereby usable strong sulfuric acid and an easily settled and readily filtered form of ferrous sulfate monohydrate are produced. In addition, heat utilization during concentration is very high and close to the theoretical, hence the process is quite economical compared to other processes of the prior art.

Pickle liquor is principally produced during the manufacture of iron and steel products. During the various steps of forming and shaping, the metal acquires a tenacious coating of iron oxide and scale, which must be completely removed from the final product. In order to remove such material from the surface of the metal, it is immersed in an aqueous bath containing sulfuric acid, which is generally designated as a pickling bath. The acid attacks and dissolves the surface material and thus cleanses the metal. The acid pickling bath reacts to some extent with the free metal which is present, forming ferrous sulfate in solution. Eventually a point is reached at which the pickling solution no longer functions with adequate speed, due to depletion of acid strength. This point Will vary, depending on economic considerations. In any case, the spent acid pickling solution is then removed from the bath and replaced with fresh acid solution.

The disposal of the spent pickling bath, known as pickle liquor, is a troublesome problem in the iron and steel industry. The pickle liquor is highly acid, generally containing principally ferrous sulfate in solution and also a substantial amount of residual free sulfuric acid. A detailed discussion of pickle liquor appears on p. 597 of The Making, Shaping and Treating of Steel, 7th (1957) edition, published by the United States Steel Corp. It is mentioned that typical pickle liquors may contain amounts of ferrous sulfate up to a concentration of about 25% as Well as 5% or less of free acid content. In some cases, of course, pickle liquors may also be produced having greater concentrations of ferrous sulfate or free sulfuric acid, depending on empirical optimum operating conditions at a particular industrial facility. An article by W. W. Hodge Waste Problems of the Iron and Steel Industry, which appeared in v. 31 #11 of Industrial and Engineering Chemistry 11/39) pp. 1364l380, also deals at some length with pickle liquor. At p. 1367 of Hodge, analyses of typical pickle liquors are given with free acid content ranging from 0.75 to 7% and ferrous sulfate content ranging from 4% to 30% Another major type of pickle liquor is produced during the manufacture of titanium pigments. In this case, a titanium-bearing ore containing iron as a principal impurity is digested with aqueous sulfuric acid. The iron is removed in the resulting leach solution as ferrous sulfate. According to p. 549 of Fairlie, Sulfuric Acid Manufacture (ACS Monograph #69, published 1936 by Reinhold Publ. Corp.), the waste pickle liquor produced by titanium pigments plants will usually range in solution concentration from about 18% to 25% free sulfuric acid and about 10% to 16% ferrous sulfate.

It is evident that in all cases, the Waste pickle liquor will contain a substantial proportion of ferrous sulfate, to-

3,211,538 Patented Oct. 12

gether with at least a minor amount of free sulfuric acid. The disposal of this waste product is a diflicult problem, since the pickle liquor is highly acid and cannot be discharged into streams or other bodies of water due to pollution restrictions. Consequently, much investigation has been carried out and numerous procedures have been suggested and developed in order to dispose of pickle liquor at reasonable cost. One basic approach involves concentration of the liquor to a crude strong acid having a free acid strength of about 30 to 70%. This causes precipitation of substantially all the ferrous sulfate as a hydrated solid crystalline material. The residual crude sulfuric acid is then re-used or further concentrated for other usages. None of the many processes in this field has gained universal or even widespread acceptance. Some idea of the adverse economic factors involved in re-processing the pickle liquor may be gained from the fact that, at current prices of crude oil or natural gas vs. sulfur, reconcentration of dilute sulfuric acid solutions to commercial strengths even in the absence of ferrous sulfate, does not become economically attractive compared to manufacture of fresh acid unless the dilute acid has an initial free acid strength of 30% or higher. However, since dumping of pickle liquor is prohibited by law in most areas due to pollution effects, processing of pickle liquor and reutilization of the sulfuric acid is usually adopted as the best overall solution to the problem. A comprehensive discussion by W. W. Hodge of the waste problems of the iron and steel industries, with particular reference to pickle liquor and its disposal, has been cited supra. An extensive bibliography, with 139 references, is also presented in Hodge.

The disposal of pickle liquor by concentration of the solution, so as to precipitate ferrous sulfate leaving a residual crude sulfuric acid solution, has been mentioned supra. The present invention provides an improved process and apparatus for accomplishing this concentrationprecipitation procedure. A reasonably complete precipitation of ferrous sulfate requires that the solution be concentrated to a free acid strength of at least about 30%. However, it has also been recognized that a preferable mode of operation involves addition of the spent pickle liquor to an existing concentrated acid bath, so that the ferrous sulfate is precipated from a strong acid solution as crystalline monohydrate rather than as the heptahydrate (copperas), since copperas is of much lower commercial value. Numerous types of concentration procedures and apparatus for this procedure have been developed in the past. Among these may be mentioned, for example, submerged combustion and spray drying. Others are described in the Hodge article cited supra, while U.S. Patent No. 2,616,790 to Swindin and U.S. Patent No. 2,662,812 to Shaw provide further advances in this art.

In the present invention, pickle liquor is added to a circulating body of strong acid solution. The mixed liquid stream is then suddenly concentrated by a type of flash evaporation of water. This evaporation is accomplished by projecting the liquid stream transverse to a highly accelerated stream of hot combustion gas or other heated gas. As a result, the liquid is dispersed into very fine droplets by the action of the hot gas, and sudden evaporation of Water takes place. The cooled and moisture-laden gas stream is then separated from the residual concentrated acid solution, which now contains the precipitated crystals of ferrous sulfate monohydrate. The solid crystals are readily settled to a slurry, and overhead strong acid solution containing fine seed crystals is recycled as the aforementioned circulating body of strong acid solution. The slurry is then filtered to recover product crystalline ferrous sulfate monohydrate, While the filtrate consisting of crude strong sulfuric acid solution is also recovered as the other major product of the process.

Under some conditions of pickle liquor composition, a portion of this filtrate may be recycled to the concentration step in order to maintain acid balance in the circulating solution.

Several noteworthy and novel results are achieved by this process. It has been found that the crystals which form are remarkably easy to filter and are quite freeflowing in the dry state, with a sandlike consistency. This result is apparently due to the sudden precipitation of the major portion of the crystal during the unique pickle liquor concentration step of the present invention. In addition, the fuel efficiency of the present invention is remarkably high. Heat utilization is about 85% of heat input, with most of the heat being utilized to evaporate water from the acid. This result is also due primarily to the essentially complete gas-liquid equilibrium which is achieved in the concentration step of the present invention. Surprisingly, it has been found that this concentration step does not result in significant loss of sulfuric acid into the gas stream as sulfur trioxide or as entrained mist. In fact, overall sulfuric acid losses in the present invention ran consistently below those reported for other processes.

Other relevant advantages may be mentioned. The cost of the apparatus employed in the present invention is quite low. As will appear infra, the novel apparatus of the present invention is quite simple and readily fabricated. Auxiliary units are also simple and standard sizes may be employed for pumps, motors, etc. Since the apparatus and general procedure are simpler than prior art procedures, with minimum holdup and tankage requirements, the process space required to install the units is comparatively small. Hence the apparatus may be readily installed within existing steel mills or other facilities. Finally, the process of the present invention is quite simple to control and modify, and it has been found that average crystal size and other characteristics of the precipitated ferrous sulfate monohydrate may be readily controlled within narrow limits. This is an important advantage, since it permits rapid filtration with low pressure buildup, and good separation of mother liquor from the solid crystals.

Itis an object of the present invention to concentrate pickle liquor in an improved manner.

Another object is to precipitate ferrous sulfate crystals having better physical characteristics from concentrated pickle liquor.

A further object is to separate pickle liquor into solid ferrous sulfate and crude sulfuric acid solution.

An additional object is to recover ferrous sulfate from pickle liquor as an improved form of ferrous sulfate monohydrate.

Still another object is to concentrate pickle liquor in an apparatus which provides improved contact between liquid solution and hot gas.

An object is to concentrate pickle liquor and precipitate ferrous sulfate monohydrate by means of a process and apparatus which achieves essentially complete utilization of heat.

These and other objects and advantages of the present invention will become evident from the description which follows. Referring to the figure, depleted pickle liquor stream 1 is added to recycle strong sulfuric acid stream 2 to form combined liquid solution stream 3. Stream 1 is a used pickle liquor containing ferrous sulfate and free sulfuric acid. As indicated supra, the proportions "of these two components may vary, depending on the process source from which the pickle liquor is generated. The usual composition of stream 1 will range from about to ferrous sulfate and from about 5% to 25% free sulfuric acid, however contents of ferrous sulfate or free acid outside of these ranges may be encountered in some cases. Recycle strong sulfuric acid stream 2 consists of a crude acid stream of etw e a t to 70% free acid strength, and preferably will also contain small seed crystals of solid ferrous sulfate.

The proportions of streams 1 and 2 are preferably regulated so as to provide combined liquid stream 3 with a minimum free acid strength of at least 30%. This initial free acid strength is usually required in order that the subsequent precipitation of ferrous sulfate will be fairly complete, since below 30% free acid strength a substantial proportion of ferrous sulfate will remain dissolved in the product acid solution. However, in some cases, such as when the product acid solution is to be reemployed in a pickling bath or other usage where dissolved iron content is not objectionable, stream 3 may be maintained at a free acid strength below 30%. An upper limit of free acid strength in stream 3 is usually considered to be at about since above this acid strength the solubility of ferrous sulfate increases. Therefore the maximum acid strength of 70% indicated supra constitutes a preferred upper limit based on solubility considerations.

Combined stream 3 is now circulated to the concentration section by pump 4, which discharges via 5. Filtered strong acid stream 6 may be added to stream 5 in some cases, to form a total combined stream 7. As will appear infra, addition of extra filtered strong acid via 6 may be necessary in some cases, in order to lower the final solids content of the slurry which is subsequently formed to a manageable level at which solids blockage does not become a problem.

Stream 7 is now passed via inlet 8 and over Weir 9 into chamber 11, and flows downwards on the inner surface of the converging walls 10 of chamber '11. Weir 9 serves to distribute the liquid stream evenly, so that the entire inner surface of the walls 10 is completely wette'd with downflowing liquid during the process. This is necessary in order to protect the walls from overheating and subsequent thermal or corrosive deterioration, since chamber 11 also contains hot gas stream 12 admitted via 13. Other liquid distributing means besides weir 9 may be employed to distribute liquid stream 7 over the inner surface of the walls 10, thus for example nozzles could be used for this purpose. In this case, the nozzles would preferably be oriented so as to discharge the liquid stream with a horizontal component of fiow direction tangential to the walls 10, so that the liquid stream would follow a spiral path to some extent as it flows downward, thus insuring complete wetting of the Walls 10. It should be understood that chamber 11 is preferably of circular crosssection in a plan view, in other words chamber 11 is preferably funnel-shaped with circular walls 10 sloping inwards. However, chamber 11 in some cases may be rec tangular in plan section. This configuration would be adopted when the available hot gas stream 12 is obtained in large volume with a relatively low pressure head. In this case a narrow slit-like constriction of stream flow at the bottom of chamber 11 would be preferable, in order to attain the desired gas-liquid contact of the present invention.

As mentioned supra, hot gas stream 12 is admitted into the top of chamber 11 via 13. Stream 12 may be derived from any convenient source, and is preferably obtained by combustion of a fluid hydrocarbon fuel such as natural gas, fuel oil, or residual heavy oil. When available at a steel mill installation, coke oven gas may be employed as a fuel to generate the hot gas. Solid fuel such as coke or coal may even be utilized for this purpose, as well as hot waste flue gases. Thus, hot gas stream 12 may be derived from optional sources, and will be of variable composition except of course that stream 12 must be fairly unsaturated with respect to water vapor. In gen-- eral, stream 12 will have a minimum temperature of' about 800 F. in order to provide suitable evaporation,

capacity and reasonable equipment size. Because of corrosion effects and other material of construction considerations, it is preferable to admit stream 12 into chamber 11 at a maximum temperature below about, 230.0. F.

Thus if the hot gas is generated at a temperature above 2300 F., it will usually be desirable to dilute the gas stream with air before it is passed into chamber 11, so as to lower the temperature of the gas stream and thereby avoid excessive corrosion and other adverse effects. A most preferable temperature range for stream 12, in view of the factors described supra, is between about 1000 F. to 1800 F.

Stream 12 now passes downwards inside chamber 11, and due to the convergence of walls the gas stream is accelerated to a high velocity. As discussed supra, the liquid stream flowing down the walls 10 prevents direct contact between the hot gas and the walls. The downflowing liquid stream is now projected transversely into the highly accelerated gas stream by substantially horizontal projection lips 14 at the lower end of walls 10. This direct sudden contact between the transversely flowing streams results in dispersion of the liquid into the hot gas stream in the form of very fine droplets, and sudden evaporation of water takes place. Due to the fact that the gas stream has been highly accelerated, the droplets are of small size and are evenly distributed in the gas stream. Consequently, equilibrium between the gas and liquid phases is practically instantly obtained, and with a sort of quench-cooling of the hot gas taking place.

The mixed gas-liquid stream now passes downwards into expansion chamber 15, which is preferably provided with gradually diverging walls so as to permit non-turbulent deceleration of the gas stream and thereby conserve energy by avoiding fluid pressure drop. The mixed stream now readily separates into liquid and gas phases. The gas phase, consisting of a relatively cool moistureladen gas stream, now passes upwards through de-entrainment section 16 which may be provided with baflles, not shown, or other suitable de-entrainment means. The gas stream finally passes via 17 to a stack or other disposal.

The liquid phase, now containing solid crystals of ferrous sulfate monohydrate, is collected as liquid body 18 in Settling vessel 19. It should be noted that the solid product of the present invention will be a crystal containing a variable proportion of combined water, usually between about 0.5 to 1.8 moles of water per mole of ferrous sulfate. However, in order to facilitate description of the process of the present invention, this solid product is referred to as ferrous sulfate monohydrate. It will be understood that this term refers to a crystal with a water content in the above range in the vicinity of 1:1 mol ratio, rather than a theoretically exact 1:1 proportion.

A portion of the liquid phase, relatively free of solid crystals except for relatively small seed crystals, now flows over solid bafile 20 and leaves vessel 19 via outlet 21. This portion of the liquid is now recycled as stream 2 described supra. The balance of the liquid stream together with solid crystals is collected as slurry 22, and passed via 23 to filter or Centrifuge 24. In 24, the slurry is separated into solid stream 25 consisting of crystals of ferrous sulfate monohydrate and filtered clear acid stream 26. Stream 25 may be further dried in a rotary solids dryer or other such apparatus, not shown, before shipment to product usage. Stream 26 also passes to product usage, via 27. As mentioned supra, a portion of stream 26 may be recycled to the process via 6, in order to adjust the final product slurry 22 to a pumpable consistency. This would be necessary in some cases if pickle liquor feed stream 1 has a high ratio of ferrous sulfate relative to free acid. Thus it has been found that a slurry having a solids content of about 33% represents an optimum concentration, since higher solids content readily led to plugging of the lower portion of vessel 19 due to solids blockage. less desirable, since it represents power loss due to excess circulation of liquor acid solution through the system,

Solids content lower than 33% is naturally above the minimum circulation rate required to maintain the slurry at a flowable consistency.

It will be evident that various process flow alternatives may be practiced within the scope of the present invention. Thus, stream 6 may alternatively be recycled and combined with stream 2 rather than stream 5. Another possible alternative would be to recycle stream 6 into vessel 19, so as to directly dilute and agitate the crystal slurry 22. This alternative is relatively less desirable, since it is preferable to recycle acid stream 6 into the process prior to the concentration step, in order to maintain the highest possible acid strength in the stream during concentration and crystal formation, which facilitates solid crystal deposition as monohydrate and also increases the rate of crystallization.

Another alternative aspect of the present invention should be noted. In some installations, particularly in steel mills, the pickle liquor is generated with a high proportion of ferrous sulfate, and therefore the net usage of sulfuric acid in such plants is high in proportion to the total acid inventory in the plant. Thus, a relatively large input flow of fresh acid is required. In addition, in such facilities a fresh acid bath with a small initial proportion of ferrous sulfate may be economically acceptable, in terms of operating cycles. In such a situation, it would be quite feasible to operate by combining fresh strong acid with pickle liquor to form a stream having a composition corresponding to stream 7 described supra. This ream would then be concentrated in accordance with the process and apparatus of the present invention, in order to deposit most of the ferrous sulfate as solid crystals. The residual liquid solution, corresponding to stream 2 and/or stream 26 of the process as described supra, would then be utilized as makeup acid for the pickling baths.

An example of the operation of the process of the present invention will now be. described.

Example A sample of pickle liquor was concentrated by means of the process and apparatus of the present invention. The input pickle liquor feed contained 8.5% free sulfuric acid and 13% ferrous sulfate. This stream was combined with recycle strong acid solution to provide a total feed stream to the concentrator of to 48% acid strength. This solution strength is preferable in most cases, since it is high enough to provide substantially complete precipitation of ferrous sulfate, and higher acid concentrations result in lower net heat utilization due to higher equilibrium gas saturation temperature. The feed steam was maintained at a temperature in the range of 200 F. to 220 F., since this temperature range avoided premature rapid precipitation of ferrous sulfate and plugging. Higher temperatures were not needed, and were avoided due to corrosion problems.

A hot gas stream was generated at 1250 F. by combustion of natural gas with excess air. This gas stream was passed to the concentrator, and quenched to 220 F. by the liquid feed solution. The liquid temperature rise across the concentrator amounted only to 1 to 2 F., while the gas pressure drop was 5 inches water gauge. The exit gas stream contained only 2 milligrams/sci. (standard cubic foot) of entrained acid mist. This could have been readily recovered if necessary by means of an entrainment separator or mist filter. Variation of relative stream flow rates produced a maximum of 4 milligrams/s.c.f. of entrained acid mist in the exit gas, well below prescribed limits for such processes. A calculated overall heat balance indicated that net heat utilization was over of heat input at all times. About of the net utilized heat was consumed as heat of vaporization in evaporating water from the liquid stream. The balance of the utilized heat appeared as heat of concentration and sensible heat in raising the temperature level of the acid feed steam.

The resulting solid ferrous sulfate was separated from the liquid solution using a crystallizer. The individual crystals averaged 20 microns in diameter, and these crystals had agglomerated into spherical particles of 40 to 80 mesh size. The particles were quite hard, with a physical nature similar to fine sand. The particles had excellent settling characteristics in the crystallizer, with the following settling rates in lab scale tests:

Ft./hr. Free settling 16 Hindered settling 8 Overall settling 10 The particles exhibited good flow characteristics in relatively thick test slurries having solids contents as high as 45%. The crystals were mainly ferrous sulfate monohydrate with only 1.5% ferric sulfate. The crude material analyzed as follows:

Percent Ferrous iron 28.16 Ferric iron 0.39 Total sulfate 57.75 Free sulfuric acid 8.85 Water of hydration (by difference) 8.01

A portion of the filtrate, consisting of 50% crude acid solution, was recycled with further fresh pickle liquor feed. The balance was recovered as crude acid product, essentially free of ferrous sulfate.

It is believed that the improved and relatively superior physical characteristics of the crystalline ferrous sulfate monohydrate produced by the process and apparatus of the present invention are due primarily to the sudden concentration of the solution which takes place in the gas-liquid contact section of the concentrator. Consequently, the resulting crystallization of solid crystals primarily consists of deposition onto the existing seed crystals. Thus the formation of smaller or irregular-shaped crystals is minimized. Compared to other concentration procedures, the crystals of the present invention are larger, better defined and possess better settling and filtering rates. Other methods such as submerged combustion, Vacuum evaporation or spray tower evaporation of the pickle liquor produce smaller, poorly shaped crystals due to the crystallization procedures inherent in these processes.

We claim:

1. Process for concentration of pickle liquor containing ferrous sulfate and free sulfuric acid, whereby said ferrous sulfate is substantially completely recovered as solid ferrous sulfate monohydrate, which comprises combining pickle liquor with strong sulfuric acid solution to form a combined liquid stream, generating a hot drying gas stream, accelerating said drying gas stream to a high velocity by passing said drying gas stream downwards through a passage defined by downwardly converging side walls, passing said combined liquid stream downwards on the side walls of said passage, projecting said combined liquid stream in transverse contact with said highly accelerated stream of hot drying gas at the lower terminus of said passage, thereby dispersing said combined liquid stream into said hot gas stream in the form of fine liquid droplets whereby sudden evaporation of liquid water into said gas stream takes place, separating the resulting moisture-laden gas stream from the remaining liquid phase, collecting said liquid phase as a slurry of solid crystals of ferrous sulfate monohydrate in strong sulfuric acid solution, and filtering said solid crystals from said slurry.

2. Process of claim 1, in which said combined liquid stream has an initial free acid strength of about 30% to about prior to said transverse contact with hot gas.

3. Process of claim 1, in which said hot gas is produced at a temperature in the range of 800 F. to 2300 F. by combustion of a fluid hydrocarbon with air.

4. Process for conversion of pickle liquor containing ferrous sulfate and free sulfuric acid into solid ferrous sulfate and crude strong sulfuric acid, which comprises combining pickle liquor with recycled strong sulfuric acid solution to form a combined liquid stream, generating a hot drying gas stream, accelerating said drying gas stream to a high velocity by passing said drying gas stream downwards through a passage defined by downwardly converging side walls, passing said combined liquid stream downwards on the side wall-s of said passage, projecting said combined liquid stream in transverse contact with said highly accelerated stream of hot drying gas at the lower terminus of said passage, thereby dispersing said combined liquid stream into said hot gas stream in the form of fine liquid droplets whereby sudden evaporation of liquid water into said gas stream takes place, separating the resulting moisture-laden gas stream from the remaining liquid phase, collecting said liquid phase comprising solid ferrous sulfate crystals in strong sulfuric acid solution, separating a portion of said strong sulfuric acid solution from said liquid phase leaving a residual :slurry of said crystals in liquid acid solution, recycling said portion as said recycled strong sulfuric acid solution, and filtering said slurry to produce solid ferrous sulfate and filtrate comprising product sulfuric acid.

5. Process for conversion of pickle liquor containing from about 4% to 30% ferrous sulfate and from about 0.75% to 25% free sulfuric acid into solid crystals of ferrous sulfate monohydrate and crude sulfuric acid of about 30% to 70% free acid strength which comprise-s combining said pickle liquor with recycled strong sulfuric acid solution to form a combined liquid stream having a free acid strength of at least about 30%, generating a hot drying gas stream by combustion of a fluid hydrocarbon with air, accelerating said drying gas stream to a high velocity by passing said drying gas stream downwards through a passage defined by downwardly converging side walls, passing said combined liquid stream downwards on the side Walls of said passage, projecting said combined liquid stream in transverse contact with said highly accelerated stream of drying gas at the lower terminus of said passage, said gas stream having an initial temperature in the range of 800 F. to 2300 F., thereby dispersing said combined liquid stream into said gas stream in the form of fine liquid droplets whereby sudden evaporation of liquid water into said gas stream takes place, separating the resulting moisture-laden gas stream from the remaining liquid phase, collecting said liquid phase comprising solid ferrous sulfate crystals in strong sulfuric acid solution, separating a portion of said strong sulfuric acid solution from said liquid phase leaving a residual slurry of said crystals in liquid acid solution, recycling said portion as said recycled strong sulfuric acid solution, and filtering said slurry to produce solid crystals of ferrous sulfate monohydrate and filtrate comprising product crude sulfuric acid of about 30% to 70% free acid stength.

6. Process of claim 5, in which said combined liquid stream has an initial free acid strength of about 45% to 48%, and an initial temperature in the range of 200 F. to 220 F., prior to said transverse contact with said gas stream.

7. Process of claim 6, in which the initial temperature of said gas stream is about 1000 F. to 1800" F., and said liquid phase comprising solid ferrous sulfate crystals in strong sulfuric acid solution is collected at a temperature in the range of 200 F. to 230 F.

8. Process of claim 5, in which a portion of said filtrate is recycled, and added to said combined liquid stream,

whereby said slurry is obtained with a solids content of about 33%.

References Cited by the Examiner UNITED STATES PATENTS Newlands 23-305 Sommer 23-305 Marquard et a1 159-4 Smith et a1 23-126 Mantius et a1 23-172 Smith 23-126 Fowler et a1. 23-126 Heath 159-4 Swindin 23-126 X Wiseman 23-275 Pluim et a1 23-273 Saeman 23-273 Sweet et a1 23-305 Gross 23-305 XR NORMAN YUDKOFF, Primary Examiner. 10 GEORGE D. MITCHELL, MAURICE A. BRINDISI,

Examiners.