PILOT-PLANT DEVELOPMENT OF T H E FOAM DISTRIBUTION PROCESS FOR PRODUCTION OF WET-PROCESS PHOSPHORIC ACID G. G. PATTERSON, J.
R. GAHAN, AND W . C. SCOTT
Tennessee Valley Authority, Muscle Shoals, Ala.
TVA's hemihy'drate foam process for producing wet-process phosphoric acid is being studied in a pilot plant. The product filter acid contains about 40% PzOj as compared with only about 30% PzOb for acid made b y the dihydrate! process. Sulfuric acid i s distributed onto a layer of foam that i s maintained in the reaction vessel on top of the acid slurry. The calcium sulfate is produced in the form of agglomerates of a stable hemihydrate with very good filtration characteristics. The reaction vessel i s 3 feet in diameter b y 8 feet deep. Agitation is provided by a slow turning, gate-type agitator. Retention time in the reactor i s in the range of 60 to 90 minutes. Premixed slurry of phosphate rock and recycled phosphoric acid i s introduced into the reactor several inches below the interface between the foam and slurry layers. Maximum temperatures usually are in the range of 225" to 235" F. Best results have been obtained at a rock feed rate of about 500 pounds per hour and a recycle acid to rock weight ratio o f 1.5 to 2.0. Extraction efficiencies as high as 97% and over-all recoveries o f 94% have been obtained. Successful development of this process should result in lower investment for equipment and product acid suitable for use in some maior fertilizer processes, such as production of diammonium phosphate, without further concentration.
studies of the wet process for manufacturing phosphoric acid were undertaken by T V A to increase filtering rates, increase concentration of the product acid. and decrease equipment rquirements. A promising method of achieving these objectives involved distribution of the sulfuric acid on the surface of a controlled layer of foam in a vessel where the foam and reaction slurry were gently agitated. The process yielded product acid containing about 407, P 2 0 5 and by-product calcium sulfate hemihydrate in the form of agglomerates of crvstals that filtered and washed well. Retention time of slurry in the reactor of about 1 hour was found to be adequate. whereas in conventional processes 4 or more hours (Hignett. .1962) are normally required for satisfactory crystal growth. The process is referred to as the hemihydrate foam distribution process. or more simply. the foam process. Results of the bench-scale studies have been reported (Davenport et a / , 1965). Subsequently, a pilot plant was constructed to continue development of the process. ENCH-SCALE
Equipment
T h e pilot-plant equipment was designed to process u p to about 800 pounds of phosphate rock per hour. However, most of the test work was done at rock feed rates of 500 to 700 pounds per hour. A schematic diagram of the pilot plant is shown in Figure 1. Finely ground phosphate rock is fed continuously to a Type 316 stainless steel premix tank by means of a belt feeder. Here, the rock is treated with recycle phosphoric acid containing about 30 to 339;) P205. The recycle acid is obtained by adding a portion of the product acid to the filtrate from the first wash cycle of the filtration step which normally contains about 28% P;Oj. Vigorous agitation is provided in the premixer by means of a turbine agitator. Retention time in the premix step of about 5 to 10 minutes is adequate. Slurry from the prernixer flows by gravity to the reactor (Figure 2), which is constructed of mild steel and has a lead lining. A layer of foam is naturally formed on top of the slurry in the reactor by the reaction between the phosphate rock and acid. Its thickness is controlled by use of a n antifoaming agent, most of which is added continuously to the slurry in the
premixer. The depth of the foam layer is regulated by intermittent direct addition of antifoam agent through a separate pump which is actuated by an electrical conductivity probe. The premixed slurry is fed into the reactor through a feed \vel1 that is concentric lvith the shaft of the agitator and extends through the foam layer, and flows onto a distributor plate mounted on the agitator shaft 2 inches below the end of the feed well. The interface between the slurry and foam layer is maintained at a level of from 5 to 9 inches above the bottom of the feed well. so that the incoming slurry: does not come in direct contact with the body of foam. The slurry and foam in the pilot-plant reactor are gently agitated by means of a sloivly turning anchor-type stirrer. Sulfuric acid is sprayed or dripped directly onto the top of the foam layer through either a fan-type nozzle or a perforated pipe distributor. Reacted slurry discharges from the bottom of the reactor throuqh an external vertical discharge leg that is adjustable in height to permit variation of the depth of slurry in the reactor and some variation of retention time without a change in the feed rate. Slurry from the reactor discharges by gravity onto a continuous belt filter Tvhich has a total area of 5 sq. feet available for filtering and Trashing the cake. The filter cloth is woven polypropylene monofilament. Originally the filter was equipped for txvo countercurrent ivashes. However, early runs shoived that two \\.ashes were not adequate, so the filter was modified to permit use of a third wash step. This markedly improved recovery of Ivater-soluble PzO; from the cake. A steam ejector is used as a source of vacuum for the filtration step, and filtrate-receiving tanks are placed betlveen the filter and the vacuum manifold. T h e filtrate receivers discharge into separate holding tanks. All of the tanks and piping are of AIS1 Type 316 stainless steel. Pilot-Plant Operation
T h e phosphate rock used in most of the pilot-plant tests was uncalcined Florida pebble phosphate which had been ground for use in the production of concentrated superphosphate. I t was about 7 5 7 , minus 200-mesh in size and contained 31.5 to 33.5% P205 (69 to 73% BPL). Ordinarily, sulfuric acid containing. about 93y0 H2S04 was distributed onto the foam layer in the reactor at rates adjusted to control the total SO3 content of the liquid phase of the slurry a t specified levels in the range of 0.5 to 3.57,. T h e sulfate concentration in the VOL. 6
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4 OCTOBER 1967
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ROCK FEEDER
V
SULFURIC1
t
15% P 2 0 5
J
Figure 1. TVA pilot plant for production of wet-process phosphoric acid by the foam distribution method
IT-
3 H.P MOTOR
U
REDUCER
shell of the reaction vessel was insulated to minimize such losses, but average temperatures in the insulated vessel were essentially the same as in the uninsulated one: slurry in the premixer, 115' to 135' F . ; foam-slurry interface in the reactor, 225' to 240' F. ; slurry in the reactor, 21 '5 to 240' F. ; and slurry discharging onto the filter, 200' to 210' F. Fluctuations within these ranges did not appear to have any important effects on operating results. For a series of tests. the pilot plant was operated on a n around-the-clock basis for 5 days. Twelve to 16 hours usually were allolved after startup for establishment of steady-state conditions before a test was begun. A test usually lasted at least 24 hours and frequently 48 hours or more. A period of 8 hours normally was allowed following a change in test conditions to permit the system to reach steady state. Study of Variables
Ld
Figure 2.
Sectional view of reactor
liquid phase of the slurry was checked hourly and adjustments were made as required. Recycle phosphoric acid was fed to the premixer at a rate of 1.5 to 2.2 pounds of acid per pound of rock fed. \\'ash water was supplied to the filter a t a rate of 0.8 to 1.3 pounds per pound of rock fed; the lower ratio was required for the production of acid containing about 4070 P205. Antifoam agent normally was added a t a rate of 2 to 3 pounds per ton of rock fed. T h e agent used in all pilot-plant work to date was a sulfonated oleic acid. Ordinarily, no heating or cooling was provided anywhere in the system. However, it was thought that heat losses through the shell of the small pilot-plant reactor might result in lower temperatures than would be encountered in a commercialscale vessel. Thus, after initial tests were completed, the 394
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PROCESS DESIGN A N D DEVELOPMENT
One of the main problems concerned the degree of extraction of P2Oj from the phosphate rock. In early tests, extraction efficiencies (conversion of rock P205 to lvater-soluble Pzo5) were generally lolver than those obtained in bench-scale work and in commercial units employing the conventional "dihydrate" processes. I n a n effort to improve the extraction efficiencies, a number of variables ivere studied to determine their effects on the degree of extraction and on filtration characteristics of the calcium sulfate formed. These studies together ivith improved operating and control techniquesdue mainly to longer operating experience-resulted in improvement in extraction efficiencies from a n average of about 92% in early work to 95 to 97% in more recent work. Two of the more important factors influencing good extraction and good over-all operation were the sulfate content in the liquid phase of the reactor slurry and the ratio of rock feed rate to cross-sectional area of the reactor (:'feed flux"). Tests made at various sulfate levels showed that best results were obtained when the SO3 content of the liquid phase of the slurry was in the range of 0.5 to 1.5%. Table I demonstrates the effect of SO3 content on degree of extraction of P205 from the rock. As the SO3 content increased, the extraction efficiency decreased significantly but the filterability of the slurry was not affected materially. The sulfate content was difficult to control a t levels above
Table 1.
Effect of SO3 Content of Slurry liquid Phase on Extraction and Filtration Efficiencies"
Extraction Eficiency , % Rock p?os zn WaterSoluble Form 95 0
Analysis of Liquid Phase, w t . yo $0 3 PlZ 0.3 41.1 0.9 40.4 1.3 39.8 2.1 39.2 3.3 38.1 4.4 37.0 a Rock feed rate 500 l b . / h r . i n all tests. sentially the same i n all tests.
Filtration Ejiciency, 70W a t e r Soluble P20 5 Filtered from Slurry 95.5
96 0
93.3 89.8 86.7 82.7 Other operating
Oh 7
91.3 97.3 97.8 97.4 conditions es-
about 2.57,. During a few periods (data not shown) when the sulfate content was unusually low (0 to 0.2%)> the agglomerates of hemihydrate were !smaller rhan usual, more individual crystals were present, and filtration rates and efficiencies were appreciably lower than normal. Extraction efficiencies liere not adversely affected by the low sulfate levels. In the pilot-plant reactor the feed flux \vas 70 pounds per hour per square foot of cross-sectional area at a rock feed rate of 500 pounds per hour. 85 a t 600 pounds per hour, and 100 a t 700 pounds per hour. When the feed flux was increased from 70 to 100, boiling in the reactor was much more violent and agitation of the b3am zone was more viqorous. T h e slurry was much more viscous, extraction efficiencies xvere lower, and filtration ra.tes were decreased. Results of tests with a feed flux of 85 were similar to those obtained with a feed flux of 100. Data are shown in Table I1 for tests at rock feed rates of 500, 600, and 700 pounds per hour; other operating conditions were essentially equal during the three tests. Efforts to improve the operation at feed fluxes higher than 70 pounds per hour per square foot were only partly successful. Some improvement in extraction efficiency was achieved by recycling a portion of the slurry discharged from the reactor back through the premix step (last test in Table 11). However, filtration was affected adversely and results still ivere less satisfactory than those obtained at the lower feed flux. Thus, it appeared that a feed flux of 70 pounds per hour per square foot of reactor cross section was about the maximum for good operation. I n studies of other variables, rather broad limits were defined for such factors as depth of the foam layer and point of introduction of the premixed slurry into the reactor. Satisfactory tests Tvere made in Lvhich the depth of foam was varied from about 5 to 30 inches. However, over-all results were best when the foam )vas 1 4 to 20 inches deep. Results were not so good when the foam layer was less than about 5 inches deep. I n all tests in which the premixed slurry was introduced into the foam layer, results ivere poor. Satisfactory results were obtained Ivhen the slurry was introduced into the reactor 2 to 12 inches below the foam-slurry interface, but operation was best w'len it \\'as introduced 5 to 9 inches below the interface. Several tests lvith a low grade of rock containing about 30.57, P205 (677, BPI,) showed no adverse effects. However, about twice as much antifoam agent was required to control the foam layer. Tests made with finely ground rock, about 90% minus 200 mesh as compared with about 7575 minus 200 mesh for the rock used most of the time, resulted in increased ex-
Table II.
Effect of Feed Flux" on Extraction and Filtration
Feed F l u x , L b. R o c k l H r . Sq. Ft.b 70 85 100
Rock Feed Rate, Lb./Hr. 500 600 700
Extraction Eficiency, 7c Rock P205 Converted to WaterSoluble Form 96.0 91.9 91.8
1o o c
700
93.2
b
Filtration Eficiency , 7cWaterSoluble PnOs
Filtered from Slurry 96.7 96.0 95.5 90.7
Rock feed rate per unit of cross-sectional area of reactor. Cross-sectional arPa of reactor. Approximately 50% of slurry f r o m reactor recycled to premixer.
traction efficiencies of about 1 percentage point. However, it was estimated that savings due to the increase in extraction Jvould be offset by the higher cost of fine grinding and no economic benefit Ivould be realized. In recent tests, the sulfuric acid was diluted Iiith a portion of the recycle acid (recycle acid to rock ratio of 2 pounds per pound) to loiver the sulfate concentration near the point of introduction and to maintain the water balance in the system Ivithout reducing the amount of water available for ivashing the filter cake. About 7570 of the recycle acid was used in the premix step and 2.57, to dilute the sulfuric acid. T h e resulting mixed acid contained about 587, HzSOI and 127, P205, I n tests \\ith rock feed rates of 500 and 700 pounds per hour, results were no better than when undiluted 93% sulfuric acid iias used. Table I11 shows operating conditions which have consistently given the most satisfactory results in pilot-plant work as well as the results of a typical pilot-plant run under similar conditions. Typically, microscopic examinations of the filter cake from the pilot plant have shown major to bulk phases of calcium sulfate hemihydrate and minor to very minor phases of calcium sulfate dihydrate or anhydrite. Most of the hemihydrate is present as agglomerates of crystals in the range of about 50 to 73 microns. There has been no evidence that the agglomerates contained occluded acid that could not be removed by normal washing. The hemihydrate is of a stable form that hydrates to gypsum only on long standing. Examination of material stored in the field at atmospheric conditions for a week showed that the major portion of the calcium sulfate was still in the hemihydrate form; after 2 months' storage, it was essentially all in the dihydrate form. At no time has there been any evidence of hydration of the calcium sulfate on the filter with resultant blinding of the filter cloth. Figure 3 shoivs photomicrographs of tivo samples of filter cake. The photograph at the left shows good agglomeration of the crystals with few individual crystals in evidence. This material filtered and washed well and \vas obtained when the pilot plant \vas operating normallv. The photograph a t the right shows small agglomerates and a high proportion of single crystals. Filterability of this material )\as poor. I t occurred during a period of operation without a satisfactory foam layer. Materials of Construction
A number of materials of construction were tested for their resistance to corrosion in the pilot-plant reactor. Since the conditions in the region of the foam layer differ greatly from VOL. 6
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Table 111.
Typical Operating Conditions and Results
Operating data Rock feed rate, Ib./hr. Feed flux, Ib. rock/hr./sq. ft. Sulfuric acid streneth. T , HSOd Recycle acid-rock \&hi ratio
500 71 93 2:l 14-20 5-9 100 0.5-1.5 0.8-0.85
SO1 content; liquid phase of reactor slurry, % SOa Wash water rate, Ib./lh. of rack fed Extraction and recovery Extraction efficiency, of P 2 0 sin rock Citrate-soluble PIOs losses, % of PsOs in rock Citrate-insoluble PeOi losses, yo of PeOj in rock
95.8 2.7 1.5 97.2 93.1
Washing efficiency,recovery of water-soluble P ~ O Syo , Over-all recovery of rock PeOs in acid, 7 ' Filtration rate (at 15 inches Hg, vacuum) Gal./hr./sq. ft. effective area Tons P 2 0 Ein product/day/sq. ft. Analytical. data
190 1.30
% by
PxOr
W.S.
Total
C.S.
Weitht
so.
cn 0
Rock Product acid Filter cake
F
1.2 0.8
1.4 1.6
c0z 3.4 -
Table IV. Corrosion of Alloys in Reaction Zone of Foam-Process Pilot-Plant Extraction Vessel
(Temperature: min. 221O F., max. 261' F., av. 231' F.)
In Ill H H H ci GI In Ill S. D
S. PI H H H
ulyIc
u"LL"lll
yL
*c"aLL,
IllaLLl.n>
."rlr
LLilLLy
a t both the foam-slurry interface and about 2 feet above the bottom of the reactor. Metals and Alloys. Results of tests in which a number of allays were exposed to the corrosive conditions in the reaction zone (near the foam-slurry interface) of the reactor are shown in Table IV. The purpose of these tests was to identify alloys that might be suitable for use in construction of the agitation equipment and slurry feed well. Although none of the alloys tested showed really good resistance (corrosion rates of less than 20 mils per year) a t this location, several were considered passable: Inconel 625 and Illium 98, 24 mils per year; Hastelloy G, 33 mils per year; and Hastelloy F, 44 mils per year. Hastelloy C and Carpenter 20 Cb-3 were attacked a t rates lower than 60 mils per year. AIS1 Type 316 stainless steel, exposed as a comparison standard, was attacked a t a rate of 260 mils per year. Several alloys of foreign manufacture were exposed in tests different hut 396
I & E C PROCESS D E S I G N A N D DEVELOPMENT
.
Tert separate jmm, bzt similar to, test A. AD. temp. 233" F .
similar to those mentioned above; Henricot HV9 and HV9A (Belgium) and Langalloy 7R (England) were attacked a t rates in the range of 30 to SO mils per year. T h e chemical lead lining of the pilot-plant reactor has shown good resistance during the entire pilot-plant program to date. I n tests made with specimens exposed near the bottom of the reaction vessel, Carpenter 20, Carpenter 20 Cb-3, and Nionel (Incoloy 825) were attacked a t rates lower than 20 mils per year; the rate of attack on Type 316 stainless steel was about 35 mils per year. Plastic and Rubber Materials. A number of plastic and ruhher-hase coating or lining materials were tested near the foam-slurry interface and near the bottom of the vessel. The materials rated as having good resistance a t both locations were:
Conclusions
Name Haveg 41
Heresite P-403 +L-66 Kel F Teflon Penton Buta-Bond Gates 50-H Viton
Type of Material Molded mixture of asbestos and a synthetic formaldehyde resin Thermosetting phenol-formaldehyde resinous coating Thermoplastic polymer of trifluoromonochloroethylene, rigid sheet Tetrafluoroethylene, sheet Chlorinated polyester, rigid sheet Butyl, flexible sheet Chemical resistant rubber, flexible sheet Copolymer of hexafluoropropylene and vinylidine fluoride, sheet
Several rubber materials were tested near the bottom of the reactor vessel where conditions are similar to those prevalent a t the filter feed zone, to identify materials suitable for use as filter mats. I n addition to the rubbers listed above, a butyl rubber, two chlorobutyl rubbers, and a chloroprene polymer were found 1.0 have good resistance. ,4 material composed of natural and butyl rubbers showed only fair resistance. Ceramic Materials. Four building materials-graphite brick, red shale brick: buff acid-resistant brick, and soapstone-were tested for their suitability as a lining for a largescale reaction vessel. T h e graphite brick was not visibly attacked, but it did sho7,v a small gain in weight, probably as a result of absorbing aci.d. T h e other three materials tested showed considerable loss in weight. Three high-temperature, acid-resistant mortars composed of a liquid resin and finely- ground carbon were also tested. One is known to be a furan resin-base mortar? but the type of resin base i n the other t\vo mortars is not kno\vn. There \\'as no visible attack on any, although a slight gain in weight occurred with one (furan-base resin) and slight losses in Iveight with the other two. I t is believed that any of the three mortars tested should be satisfactory for service i n the reaction vessel. General. AIS1 Types 304 and 316 stainless steels were tested in the laboratory thermal-block equipment a t 150' F. with acid solutions containing 1, 21, 31, and 357, P205 to evaluate their suitability for use in transportation and storage of the various acid solutions encountered in the foam process. T h e corrosion rates i n all tests \\'ere less than 1 mil per year.
T h e pilot-plant work to date has borne out the findings of the bench-scale studies (Davenport e6 al., 1965). T h e pilot plant was operated satisfactorily during production of acid more concentrated than that normally produced in conventional processes, and the by-product calcium sulfate hemihydrate had good filtering and washing characteristics. During the pilot-plant work, significant improvements were obtained in the degree of extraction of PzOS from the rock. Extraction efficiencies now compare favorably with those obtained by the conventional processes. T h e principal advantages of the foam process are the production of 40y0 PzO5 acid (as compared with the 28 to 3070 acid usually produced) and the requirement of smaller reaction equipment because of the shorter retention time. Without further concentration, the 40% P 2 0 5 product acid Jvould be suitable for use in some major fertilizer processes, such as production of diammonium phosphate. For applications requiring 52 to 5476 P2Oj acid, the size of evaporation equipment needed Tsould be substantially reduced. Preliminary rough estimates indicate that investment costs for a foam process plant jvould be 20 to 25% lower, and cost of evaporation equipment Ivould be 25 to 30% lower than for a conventional plant. Operating costs would be about the same as for a conventional process. TVork on the process is being continued and additional work is planned for the near future. This includes studies of the removal or recovery of fluorine from the process effluents, the scale-up of the reaction vessel and agitation equipment, and the performance of the process \vith different types of phosphate rock. Tests have been started Xvith rock from the North Carolina field with encouraging preliminary results. However, additional tests \vi11 be required with this rock before its suitability for the process can be fully evaluated. Literature Cited
Davenport, J. E., Getsinger, J. G., Carroll, Frank, IND. EXG. CHEM.PROCESS DESIGN DEVELOP. 4 , 84-8 (1965). Hignett, T. P., PhoJphorus and Potassium 4 , 24-9 (December 1962). RECEIVED for review February 25, 1966 ACCEPTED March 28, 1967 Division of Fertilizer and Soil Chemistry, 150th Meeting, ACS, Xtlantic City, N.J., September 1965.
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