Table IV.
Effect of an Antioxidant and a Free-Radical Initiator Phenol, Additive Tar, 7 0 0 Grams None (Standard) 974 1.33 E Antioxidant 2.00 896 F Initiator6 1.67 952 a Methanol insoluble material (see Experimental). This has a haif-life at 235’ C. of 4 hours (Hartzell and Huyser, 7964).
Two bands were present in all solution spectra a t 5.75 microns (1740 cm.-I) and 5.92 microns (1690 cm.-I). Inspection of the 7- to 20-micron region gave strong bands indicative of aromatic rings and carbonyl and ester groups. Broad diffuse bands from 3 to 4 microns and, in two of the solution spectra, weak bands at 3.23, 3.27, and 3.30 microns were found. I n some of the solution spectra, a very weak, broad band appeared a t 5.5 microns. The band a t 5.75 microns (1740 cm.-1) is the carbonyl-stretching frequency typical for ArCOOAr’ [phenyl benzoate gave 5.75 microns, Nakanishi (1962) lists 1735 cm.-’ for ArCOOAr, this is equivalent to 5.76 microns] while the band a t 5.92 microns (1690 cm.-I) is the carbonyl group of an aromatic acid [Szymanski (1963) lists 1690 cm.-l for benzoic acid]. The solution spectra are, with the exception of a few small bands, composites of the spectra of ArCOOAr’ and an aromatic acid. The broad diffuse bands from 3 to 4 microns are the same as in the benzoic acid spectrum. Phenyl benzoate has a peak a t about 3.25 microns. The weak, broad band a t 5.5 microns, if indicative of anything, can only be a n overtone band of either a n alkene (Rao 1963a) or a benzene derivative (Rao, 1963b). However, to be an overtone band, it must have a strong primary band around 1 1 microns. S o such band was seen in any of the spectra. Conclusions
I t is difficult to compare this and the previous work (Albright
et al., 1966), since they were run under different conditions. T h e results presented here are significant, however, since they were run under conditions similar to the process development work (Kaeding et al., 1961, 1965). One major difference, besides the scale of the experiments, is the amount
of water used. The previous work (Albright et al., 1966) passed about five times the amount of water through the reactor (based o n benzoic acid). This could have three effects. One would be to render the reactor solution less viscous; another could be more rapid hydrolysis of phenyl benzoate and subsequent faster removal of the resultant phenol thereby maintaining a lower phenol inventory in the reactor. Third, a n increase in the concentration of water would mean more effective competition of the water for the copper ion in solution relative to phenol. T o the extent that complexing of phenol with copper leads to tar, this should manifest itself in smaller amounts of tar being formed. The conclusions that can be drawn are that free-radical inhibitors have only a small effect on the oxidation of benzoic acid to phenol and that these effects are sensitive to the conditions and amounts of materials used. This is in agreement with kinetic work currently in progress which shows that this reaction is not free-radical in nature and therefore should not be grossly affected by either free-radical inhibitors or initiators. The infrared data seem to point to the fact that the tar is made up of material containing extensive aromatic ester and acid linkages. Acknowledgment The author thanks Paul Harris for obtaining the spectra and analytical results. 2,3-Dimethoxy-2,3-diphenylbutanewas supplied by Gordon Hartzell, E. C. Britton Laboratory, The Dow Chemical Co. literature Cited Albright, B. M., Perlaky, C., Masciantonio, P. X., IND.ENG. CHEM.PROCESS DESIGN DEVELOP. 5 , 71 (1966). Hartzell, G. E., Huyser, E., J . Org. Chem. 29, 3341 (1964). Kaeding, W. W., Lindblom, R. O., Temple, R. G., Znd. Eng.Chcm. 53, 805 (1961). Kaeding, W. W., Lindblom, R. O., Temple, R. G., Mahon, H. I., IND.END.CHEM.PROCESS DESIGN DEVELOP. 4, 97 (1965). Nakanishi, K., “Infrared Absorption Spectroscopy-Practical,” pp. 44, 5e, Holden-Day, Inc., San Francisco, 1962. Rao, C. N. R., “Chemical Applications of Infrared Spectroscopy,” p. 147, Academic Press, New York, 1963a. Rao, C. N. R., “Chemical Applications of Infrared Spectroscopy,” p. 159, Academic Press, New York, 1963b. Szymanski, H. A,, Ed., “Infrared Band Handbook,” p. 103, Plenum Press, New York, 1963. RECEIVED for review August 22, 1966 ACCEPTEDAugust 1, 1967
DEVELOPMENTS IN PRODUCTION OF GRAN U LAR U REA-AM MON I U M PHOSPHATE FERTILIZERS R . S . M E L I N E , G . C . H I C K S , T . M . K E L S O , A N D M. M . N O R T O N Tennessee Valley Authority, Muscle Shoals, Ala. ORLD
production of urea has increased a t a rapid pace in
W recent years, and there are indications that it will become the leading form of solid nitrogen fertilizer (Strelzoff and Cook, 1965). Improvements in technology of urea manufacture have led to less expensive and more efficient plants. Most new nitrogen fertilizer complexes being planned and built to cope with the world’s increased fertilizer needs include facilities for the production of urea. T h e favorable agronomic properties of urea for rice fertilization has been a stimulus to 124
l & E C PROCESS D E S I G N A N D DEVELOPMENT
production and use of this material in rice-growing countries which comprise a large part of the world’s future need for fertilizers. Popularity of ammonium phosphate fertilizers has increased spectacularly since 1960, and production is still rapidly increasing in the United States and other parts of the world. Established processes are used efficiently in a large number of plants (Newman and Hull, 1965). A combination of these proved fertilizers is a natural route
World production of urea has increased rapidly in recent years and production of diammonium phosphate has increased spectacularly since 1960. A combination of these developed processes is an attractive route for the formulation of high-analysis granular fertilizers. Pilot-plant development work was undertaken by TVA to determine the simplest and most economical means for combining urea and ammonium phosphate to produce grades such as 29-29-0, 25-35-0, 34-1 7-0, and 19-1 9-1 9. A conventional preneutralizer and rotary drum system was used to produce the ammonium phosphate component from wet-process phosphoric acid and ammonia. Urea was fed to the process as solutions that would be available from conventional urea synthesis plants or as solid products. Recycle ratios in the pilot-plant operation ranged from 3 : 1 to 5 : 1. The products were closely sized and very well rounded granules; drying to moisture content of about 1 .O% resulted in good storage properties when conditioned. Study of the drying step was emphasized, since this operation is the limiting factor in establishing the production rate for a plant system. Products are being evaluated in agronomic tests in the United States and several other countries.
to formulating high-analysis N-P and N-P-K grades with total plant food content of 50 to 60y0. As early as 1956, TVA produced granular urea-ammonium phosphates in pilot-plant tests, in which commercial ammoniating solutions that contained urea and free ammonia were used. I n 1959, laboratory work was started on the development of methods of production that would offer more flexibility in ratios and grades and allow more economical production of this type of fertilizer. Active pilot-plant development work was begun in 1961. Similar ratios and grades could be made by bulk-blending prilled urea and granular ammonium phosphates. T h e products would not be homogeneous and would be particularly susceptible to segregation because of the small particle size of prilled urea normally produced as compared with the larger particle size of granular ammonium phosphates. Complete facilities would be required to produce the two solid individual products, but this type of operation would allow higher rates of production from single-train systems because drying of the separate products would not be as difficult. T h e process developed by TV.4 for producing granular ureaammonium phosphate is a modification of the basic T V A granular diammonium phosphate process in which urea is cogranulated with ammonium phosphate to give higher N : P 2 0 s ratios than obtained in the straight diammonium phosphate. The basic TVA granular diammonium phosphate process uses a preneutralizer and rotary drum ammoniatorgranulator. I n some studies, simulated unstripped effluent from a urea reactor was fed to the process to provide the greatest process simplicity and to utilize both the urea and free ammonia as feed materials. I n other studies the simulated effluents from a “once-through,’’ “partial-recycle,” or “totalrecycle” urea process were used (Chenoweth, 1958). I n some tests, urea was fed in solid form as crystals or as prills. If the unstripped effluent from the reactor is used or a oncethrough urea process is utilized and all of the unreacted ammonia is combined with phosphoric acid, the ratio of ammonium phosphate to urea produces a 25-35-0 grade. Various partial-recycle processes are available that separate part of the ammonia from the reactor effluent and recycle it. The composition of the remaining solution is such that a 33-20-0 urea-ammonium phosphate can be produced. If either the once-through or partial-recycle process is utilized, it is necessary to have a n alternative and almost instantaneous use for the ammonia-carbon dioxide off-gas from the urea synthesis unit during any unscheduled shutdown of the granulation facilities. If a total recycle process is used, there is no export ammonia from the urea plant. This results in greater
flexibility in the operation of the granulation plant, and a variety of grades with N:P 2 0 ~ratios ranging from 1 : 2 to 3 : 1 can be produced. Pilot-Plant Facilities, Feed Materials Used, and Operating Procedures
All tests were made in a pilot plant using a preneutralizer, a 3- by 6-foot TVA-type ammoniator-granulator, and the accessory scrubbing, drying, cooling, and product-sizing equipment a t production rates of 0.5 to 0.75 ton per hour. I n most tests the acid used was wet-process phosphoric acid that had been made in the TVA pilot plant for development of the foam process which produces filter-grade acid containing about 40% PZOS. I n other tests wet-process phosphoric acid from several sources was used. Because of the concern over the impending shortage of sulfur and the trend toward the cheaper production of electric power, the use of thermal-process (electric-furnace) phosphoric acid is becoming more attractive in the production of fertilizers in some areas of the world. Therefore, several pilot-plant tests were made using thermal-process phosphoric acid as the principal source of PzOj. However, previous tests in the production of granular diamrnonium phosphate and ureaammonium phosphate had shown that the use of thermal acid as the only source of P 2 0 s resulted i n inferior granulation and severe caking of the bagged products. I n the current tests it was found that the addition of as little as 5% of the P205 as pulverized phosphate rock or 25y0 of the P205 as wet-process acid supplied the additives needed in the thermal acid for satisfactory granulation and improved storage properties of urea-ammonium phosphates. After the acid had passed through the scrubbing tower, it was mixed in a reaction tank with phosphate rock ground to about 70% minus 200 mesh, then fed to the preneutralizer. The retention time in the tank was approximately 12 minutes. Since urea production facilities were not available, the urea solutions were prepared by dissolving urea in water and concentrating the solution to the desired strength in steam-heated tanks. Simulated, unstripped solutions were prepared in a pressure tank by introducing free ammonia and carbon dioxide into the solution. And reactor off-gas was simulated by mixing ammonia and carbon dioxide from pressurized cylinders and heating the gaseous mixture to prevent the formation of carbamate. T h e technique of operation was about the same as in the production of granular diammonium phosphate by the T V A process (Young et al., 1962). T h e phosphoric acid was fed to the preneutralizer, where about two thirds of the ammonia was introduced to give a n VOL. 7
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”3: H Z 0 4 mole ratio of about 1.4, which is near the point of maximum solubility and therefore allows maximum concentration of the ammonium phosphate slurry. Use of the preneutralizer to remove heat and moisture decreased the recycle required to control granulation in the ammoniator-granulator. ‘The exhaust gases from the preneutralizer and from the ammoniator-granulator were scrubbed with the phosphoric acid feedstock to recover most of the evolved ammonia. The preneutralized slurry was fed into the ammoniator-granulator, where the remainder of the ammonia and the urea were added. Granulation was controlled easily by recycling adequate proportions of undersize material to the ammoniator-granulator. ‘Phe product was dried and then cooled in conventional rotary equipment. Drying was the most difficult and critical step in the process. Screening of the product was not difficult except when hot material at about 150’ F. was passed through the screens and caused coating on the wire. Oversize material and some product size (when needed for additional recycle) were crushed satisfactorily in a chain mill.
Mole Ratio Obtained
An effort was made to drive the final ammoniation in the drum as near as practical to a n “3: H3P04 mole ratio of 2.0 for diammonium phosphate by use of an excess of about 10% ammonia. When the ammonium phosphate slurry and urea solution were distributed over the same area in the ammoniH3P04 mole ratio of the product ator-granulator, the “3: generally was 1.7 to 1.9. T h e lo\z.er mole ratios obtained with this type of distribution may have been caused by the urea blinding the reaction between the ammonia and the phosphate slurry in the granulator. The mole ratio in the 25-35-0 grade was about 1.9 as compared with 1.8 in the 33-20-0 grade which contained a higher proportion of urea. However. higher mole ratios usually resulted in higher loss of ammonia in the dryer. During similar tests in production of straight diammonium phosphate (Young et al., 1962), a n S H a : H 3 P 0 4 mole ratio of 1.9 to 2.0 was obtained without difficulty. I n later tests when the ammonium phosphate slurry \vas introduced in the feed end of the ammoniatorgranulator and the urea solution was introduced near the discharge end, S H 3 : H3P04 mole ratios of about 2.0 were obtained in a grade containing a high proportion of urea. ’Phis change permitted the ammonia to react with the ammonium phosphate slurry before the urea formed a blockage, and such procedure is recommended. This type of blockage would not be expected to be as severe when urea is fed as a solid. Data from typical pilot-plant tests of granular urea-ammonium phosphate are given in Table I. The nominal grades shown are those that bvould result from using a typical unclarified wet-process phosphoric acid and 2% of a conditioner. The analyses of the pilot-plant products usually are higher than the nominal grade because the acid \vas settled and relatively clean, no conditioner was included, and no filler was used to adjust to the nearest even grade.
production of the ammonium phosphate component, a grade of about 25-35-0 would result. A 1 to 1 ratio of grade 27-27-0 could be produced by reaction of part of the ammonia with sulfuric acid, and a 29-29-0 grade could be produced by stripping part of the free ammonia from the solution and using it in another process. I n this type of operation about two thirds of the unstripped effluent would be fed to the preneutralizer and the remainder to the ammoniator-granulator drum. A flowsheet for this process is shown in Figure 1. Pilot-plant studies showed that these grades could be produced with a recycle ratio of 4 or 5 pounds per pound of product, but substantial hydrolysis of urea occurred in the preneutralizer. Hydrolysis of 6 to 10% of the urea resulted when wet-process acid was used. T h e hydrolysis of urea to carbamate and its subsequent decomposition are shown in the following reaction.
0
0
I1
;I
11
+ HzO % HzN-C-ONH4
HzN-C-NH2
(urea)
% COz
+ 2NH3
(carbamate)
Although most of the ammonia liberated by hydrolysis of the urea would be recovered in the scrubber, an increased amount of urea would be required to replace that decomposed and this would increase the unit cost of producing the urea-ammonium phosphate. For this reason the studies of the use of unstripped reactor effluent were terminated in favor of the use of stripped solutions which were fed directly to the granulator. Use of Once-Through or PartialRecycle Urea Synthesis Process
Operation with Once-Through Type of Urea Process. T h e once-through type of urea process requires the lowest investment for a standard process and is the simplest to operate. In such a process about 507, of the ammonia fed to the reactor is converted to urea. The remaining half of the ammonia and a proportional amount of carbon dioxide are evolved; usually the ammonia in this off-gas is utilized in some other process such as production of ammonium sulfate or ammonium nitrate. One attractive alternative for a urea-ammonium phosphate process system would be a once-through urea process in which the off-gas ammonia would be used in the granulation plant to react with phosphoric acid to produce the ammonium phosphate component. The stripped urea solu-
EXHAUST
WET-PROCESS H P 4
!r
Use of Unstripped Effluent from Urea Reactor, Once-Through Urea Process
Since a primary objective of this development work was to explore the use of plant systems having the simplest processes and lowest investment costs, early studies were madr with simulated, unstripped urea reactor effluent. This effluent contdins urea and \Later together with a large proportion of unreacted ammonia and carbon dioxide. The practical use of this solution directly from the synthesis reactor would allow the maximum in simplification of the urea system and result in substantial savings in investment. If the unstripped solution from such a simplified urea synthesis process were used and the free ammonia reacted with wet-process phosphoric acid for 126
iaEc P R O C E S S DESIGN A N D DEVELOPMENT
1I
PRENEUTRALIZER
EXHAUST GASES TO AMMONIA RECOVERY
REACTOR
GRANULATOR DRUM -
t
TO DRYING+ COOLING SIZING
PRO6UCT 25-35-0
Figure 1. Urea-ammonium phosphate process using unstripped reactor effluent from once-through urea plant
Table 1. Data from Typical Pilot-Plant Tests of Granular Urea-Ammonium Phosphate Nominal grade 25-35-0 33-20-0 34-17-0 38-1 3-0 29-2 9 -0 29-29-0 29-29-0 Solution Solution Prills Source of urea Solution Crystals Soh tion Solution 1 Test No. 2 3 6 7 4 5 Production rate, ton/hr. 0.75 0.5 0.67 0.64 0.5 0.5 0.5 Feed rate, Ib./ton of product To scrubber Phosphoric acid (% P205)a 1882 (40.1) 1051 (41 . O ) 961 (38.1) 689 (36. 9)b 1382 (42.9) 1312 (44.1) 1478 (37.4) To preneutralizer Ammonia (gaseous) 216 129 110 175e 173O 75 163 212 ... ... ... cos Phosphate rock (33y0 P206) 39d ... ... ... ... ... Scrubber effluent 1825 1037 957 652 1401 1319 1461 To granulator 115 Ammonia (gaseous) 115 71 60 37 129 113 89 ... Con 912(g3.0) Urea solution (yourea). 536 (97.1) 1169i97.6) 1459i97.4) 1519 i98.2) ... Urea solid ... ... ... ... 794 ... 827 1403 Preneutralizer slurry 984 898 I602 641 1376 1388 Recycle 4439 6676 5136 8236 8192 7628 10,598 Scrubber conditions Effluent temperature, O F . 152 128 130 128 155 157 135 Ammonia N in effluent, yo 1.3 1.4 1.6 1.9 2.3 1.6 3.1 0.01 0 . 2 N loss, Yo of total N 0.3 0.06 0.2 0.1 0.5 Scrubber efficiency, 7, 98.7 93.9 88.7 95.5 93.3 97.0 98.8 Preneutralizer conditions 240 238 230 229 228 Temperature, O F . 230 228 6.6 6.8 6.1 6.2 6.2 6.2 6.6 PH Composition of effluent, Ammonia N 12.8 12.0 11.7 12.0 11.7 12.5 12.2 Total PsO5 42.0 47.1 43.8 40.8 41.6 42.2 42.0 Calculated HzOe 22.2 21.6 (25.9) 14.3 (13.5) 19.5 (13.7) 20.8 (24.5) 24.6 (26.8) 21.8 N H 3 : H 3 P 0 4mole ratio 1.51 1.38 1.39 1.45 1.55 1.40 1.55 N evolved from preneutralizer, yc of total N 4.5 0.7 0.2 0.8 0.5 0.9 1.7 Granulator conditions Recycle Lb./lb. produc: 4.1 3.8 3.3 4.1 5.3 2.6 2.2 Temperature, F. 138 112 125 130 134 138 131 Calculated input moisture 2.7 content, yo 2.1 4.0 3.5 2.5 4.5 5.0 Granulator product 174 177 Temoerature. O F . 183 165 152 167 183 Cheniical analysis, yo Total N 24.9 33.3 35.3 38.9 29.0 29.8 31.9 38.5 22.7 19.6 13.5 29.8 Total P205 24.9 31.3 1.1 0 . 9 (1.7) 2.8 ( 3 . 4 ) 2.0 1.7 HsOe 1 .O (1.5) 3 . 3 (4.3) NH3:H3P04 mole ratio 1.91 1.79 1.78 1.62 1.89 1.87 1.91 Screen analysis, +6 mesh 10.9 2.9 11.3 7.8 7.0 2.9 8.8 36.8 12.4 28.8 -6 +10 mesh 43.3 24.0 17.9 18.8 -10 +I6 mesh 44.5 42.4 40.6 43.0 30.4 40.8 56.3 -16 mesh 15.8 33.9 22.8 32.1 22.0 15.4 26.4 N evolved from granulator, yo of total N 2.8 1.9 1 .o 1.4 3.5 5.9 5.0 Dryer product Temperature, O F . 190 195 207 196 200 205 194 0 . 3 (0.7) Moisture content, %e 1.1 0 . 4 (0.4) 1 . 1 (1.4) 0.5 1 .O (1.5) 0.8 1.1 4.0 2.8 2.0 0.6 N loss, yo of total N 0.7 3.3 Screen analysis, yo +6 mesh 1.8 11.6 14.5 8.1 8.8 3.2 10.7 -6 4-10 mesh 36.8 42.6 13.8 25.9 16.3 16.1 24.1 -10 +16 mesh 44.1 31 .O 44.7 42.8 40.0 57.5 37.0 17.3 27 . O 23.2 - 16 mesh 14.8 28.2 34.9 23.2 Screened product Chemical analysis, 70' Total N 25.3 33.9 37.0 38.6 28.7 29.6 31.2 7.8 6.6 5.9 11.1 13.3 Ammonia N 10.4 10.3 Urea N 25.1 29.4 32.2 17.3 19.2 11.6 20.8 22.6 18.2 14.4 32 .'7 31 . O 38.7 Total Pzos 28.6 22.3 37.8 Available P ~ 0 6 18.1 14.3 32.7 30.9 28.4 0 . 2 1 .o 0 . 5 (0.3) 0 . 3 (1 . O ) 0.9 1.1 H?Oe 1 .o NH3:HaPo4 mole ratio 1.80 1.84 1.75 1.96 1.77 1.70 1.85 Bulk density (untamped), 43 42 45 Ib./cu. ft. 40 46 44 44 a Value in parentheses is concentration of PZOS or urea solution. b Thermal-process acid. c Liquid instead of gaseous ammonia used in preneutralizer. d Phosphate rock added to phosphoric acid in a tank prior to being fed to preneutralizer. e Value in parentheses is Karl Fischer moisture. f This analysis is usually higher than the nominal grade because the pilot-plant products were made with relatively clean acid, no conditioner, and nofiller to adjust to the nearest ecen grade.
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tion would be fed to the ammoniator-granulator for cogranulation with the ammonium phosphate. Extensive pilot-plant tests were made of this type of system. T h e natural grade of urea-ammonium phosphate would be about 25-35-0 if all of the urea solution and off-gas ammonia were utilized with a proportional amount of wet-process acid in the process. A flow diagram of this type of system is shown i n Figure 2. Because of the problem with hydrolysis of urea when the solution was fed to the preneutralizer, the urea solution and ammonium phosphate slurry were fed as separate streams to the ammoniator-granulator. Tests showed that the streams could not be fed in the same pipeline or even mixed in the same spray nozzle without foaming that impaired flow. About two thirds of the simulated off-gas ammonia and carbon dioxide was fed to the preneutralizer to react with the wet-process acid and to provide slurry with an "3: H3PO4 mole ratio of about 1.4. The remainder was fed to the drum to complete the ammoniation to a mole ratio as near to 2.0 as was practical. The amount of ammonia evolved from the drum varied from 4 to 13% of the total fed. Most of this was recovered by scrubbing the exhaust gas together with that from the preneutralizer with the incoming phosphoric acid. The highest mole ratio achieved in the product was 1.92. .4 recycle ratio of about 3 was required when the concentration of urea solution was 96%. About 1% of the total nitrogen fed was evolved as NH3 in the dryer when the product temperature was 190' to 200' F. Increasing the dryer discharge temperature to 210' to 215' F. to obtain a moisture content of about 0.5% gave higher evolution of 2 to 37, of the nitrogen. lMost of this would be recovered in a plant-scale system by scrubbing the dryer and cooler exhaust gases. Data from a typical test are given under test 1 in Table I. A 16-22-22 grade was produced by adding to the 25-35-0 formulation potassium chloride, fed bvith the recycle to the ammoniator-granulator drum. Operation was about the same as for the 25-35-0 grade. I n practice, a somewhat lo\ver recycle requirement would be expected for grades that contain potash.
Operation with Partial-Recycle Type of Urea Process. A second standard type of urea process is the partial-recycle type. In the usual plant of this type, about 75% of the ammonia fed is converted to urea. T h e higher conversion is achieved by recycling part of the stripped ammonia and carbon dioxide to the reactor, usually as ammonium carbamate solution. If this type of urea synthesis unit were used in a ureaammonium phosphate system and all of the urea and off-gas ammonia were utilized in the process, a grade of about 33-20-0 would result. Several pilot-plant runs were made to test production of this grade. Operation generally was about the A flow diagram for same as in production of the 25-35-0. this type of system is shown in Figure 3. The recycle ratio was in the range of 3.5 or 4 to 1 for best operation when urea solution of about 98% concentration was used. Well-rounded, uniform granules were produced. The product dried a t a discharge temperature of 195' to 210' F. contained only 0.3 to 0.670 moisture. Data from a typical pilot-plant test are given under test 2 (Table I). A grade of 25-1 5-1 5 was produced by adding to the 33-20-0 formulation potassium chloride, fed to the amrnoniatorgranulator drum with the recycle. T h e recycle ratio used in the pilot-plant tests was about the same as for 33-20-0. Problems with Off-Gas Ammonia. T h e use of the carbon dioxide-ammonia mixture in the off-gas creates several problems which require special consideration in plant design. This mixture unless kept above the decomposition temperature (which varies with pressure) deposits as ammonium carbamate on exposed surfaces and causes stoppages in flow. Therefore, it is necessary to keep the pipelines hot. Decomposition temperature is plotted against pressure in Figure 4. Steamjacketed lines were satisfactory in the pilot plant. T h e mixture was kept a t 190' to 200' F., which is high enough to prevent the formation of carbamate a t pressures up to 30 p.s.i.g. The mixture of carbon dioxide and ammonia stripped out of the final decomposer vessel generally is a t a low pressure of 10 to 12 p.s.i.g. I t might be impractical to use the off-gas a t this low pressure because of insufficient driving force through a metering device, pipelines, and spargers, and because of the size of pipelines required. Compression of this gas would be EXHAUST
,___-_ I
REACTOR
-0
I
4
I I
SCRUBBER
_
UREA SOLUTION
4
Figure 2. Urea-ammonium phosphate process using stripped solution and offgases from once-through urea plant
- --- - - -- - 1 -
c
+ - - -- +
I
PRENEUTRALIZER
I I
I
I
EXHAUST GASES TO AMMONIA RECOVERY
I
I
t
-
DRUM GR A NUL ATOR
TO DRYING, COOLING SIZING
PRODUCT 25-35-0
128
l&EC PROCESS DESIGN A N D DEVELOPMENT
EXHAUST
4 1-
;,
A=,
"3
WET-PROCESS H3PO4 T
i
SCRUOBER
UREA
I
MREACTOR
CRYSTALS, PRILLS, OR SOLUTION
"3 - kI J n20
UREA SOLUTION
I
1
CONCENTRATOR EXHAUST GASES TO AMMONIA RECOVERY
I
I
EXHAUST GASES TO AMMONIA REC9VERY I
A
1 :
_ _ _ _ _ _ *_---_---
t' ._-I
RECYCLE
SIZING SIZING
440
400
360
320 280
VI
O'
J
240
;
200
W
a 160 120 80
Figure 4. Effect of temperature on dissociation Pressure-of ammonium carbamate ?* i
0
helpful. When the mixed gas was fed into the preneutralizer, the carbon dioxide was slow to escape and the resulting gasified slurry created a pumping problem. This was overcome by use of a n oversize pump with a large suction line from the bottom of the preneutralizer. I n a plant-scale preneutralizer more effective separation might occur i n the vessel because of the greater distance between the spargers and a bottom outlet. An appropriate antifoaming agent might also be helpful. Production of Grades Independent of Use of Off-Gas Ammonia
Urea-ammonium phosphate of various grades could be readily produced if no efforts were made to adapt the grade to fixed proportions of urea and off-gas ammonia that result from operation of once-through or partial-recycle urea processes. Pilot-plant studies were made of 29-29-0, 34-17-0, 38-1 3-0, 22-44-0, and 20-20-20 grades, which could readily be produced if a total-recycle type of urea unit were used. T h e ammonia required would be supplied from a source independent of the urea process. A flow diagram for this type of system is shown in Figure 5 . Tests Using 97y0Urea Solution. I n production of 34-17-0, the urea was supplied as a concentrated solution containing
34-17-0
38-13-0
Figure 5. Urea-ammonium phosphate process stripped solution from total-recycle urea plant
Figure 3. Urea-ammonium phosphate process using stripped solution and off-gases from partial-recycle urea plant
2
PRODUCT 22-44-0 29-29-0
PRODUCT 33-20-0
--
using
about 97y0 urea. Wet-process phosphoric acid was preneutralized to a mole ratio of about 1.4. Evolution of ammonia from the preneutralizer was about 0.5% of the total nitrogen fed. Ammoniation to a product mole ratio of about 1.8 was completed in the drum. Recycle requirement was about 4 for best operation. About 27, of the total nitrogen was evolved from the dryer when the product temperature was 200' F. and about 3% when the product temperature was 210' F. T h e dryer was operated with a cocurrent flow of air. T h e product was dried to about 0.5% moisture. The granules produced in this test were firm, well rounded, and uniform in size: Data from a typical pilot-plant test are given ucder test 3 (Table I). During other tests in which a 97% solution of urea was fed, thermal-process acid containing 5% of the P ~ 0 5as pulverized phosphate rock was used instead of wet-process acid. T h e phosphoric acid from the scrubber and the rock was assumed to react in the normal acidulation weight ratio of P205:CaO of 2.5 to form triple snperphosphate, which wlas assumed to fix 4 pounds of ammonia-per unit of P205. 'The phosphoric acid from the scrubber contained about 43% P205 and 0.3 to 1.4% nitrogen when added to the reaction tank. With a retention time of about 13 minutes and a temperature of about 130' F. in the reaction tank, essentially all of the Pz05 in the rock was solubilized. T h e over-all availability of the P205 in the extract was 99.9%. When 10% of the P205 was supplied as phosphate rock, the over-all availability of the P2Oj decreased to 98 to 99%, depending on the nitrogen content of the acid. Data indicated that increases in nitrogen and P2Ob contents of the acid decreased the solubilization of the phosphate rock. When thermal-process acid-rock mixtures were used in production of a 38-13-0 grade, the acid-rock mixture was fed to the preneutralizer where it was preneutralized to a n "3: Hap04 mole ratio of 1.36 to 1.65 before being fed into the granulator. T h e N H B evolved from the preneutralizer was about 1% of the total nitrogen fed. Granulation in these tests was good. T h e granules were firm and well rounded. T h e size distribution of the granules was such that very little crushing of oversize and product would be needed to produce recycle for a balanced condition. T h e recycle ratio was varied from about 3 to 4 pounds per pound of product. T h e temperature of the granulator product was 180' to 200' F. T h e ammonia evolved in the granulator was 0.5 to 1.7y0 of the total nitrogen fed. T h e NH3:H3P04 mole ratio in the granulator product was about VOL 7
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2.0, assuming that the triple superphosphate was ammoniated to 4.0 pounds of ammonia per unit of P ~ O S . Cocurrent drying was used during these tests and moisture contents of 0.2 to 1.0% were obtained in the screened products. Inlet gas temperatures to the dryer varied from 270' to 310' F. Air flow through the dryer was low, with a n average of about 1100 cubic feet per minute. This represents a n air velocity in the pilot-plant dryer of only 160 feet per minute. Data from a typical pilot-plant test are given under test 4 (Table I). Tests Using 83% Urea Solution. A 29-29-0 granular urea-ammonium phosphate was produced by using an 83Y0 solution of urea, prepared by dissolving urea prills in water. The solution was stored and fed to the granulator a t a temperature of 200' F. without difficulty. The pipelines were kept hot by jackets containing water a t about 200' F. Analyses of the solution indicated that about 1 to 3% of the urea was hydrolyzed to form ammonia during storage for 24 hours. There was no indicated increase in biuret during this storage. Wet-process phosphoric acid was preneutralized to a n "3: H3P04 mole ratio of 1.4 with anhydrous liquid ammonia. Evolution of ammonia from the preneutralizer was 0.5% of the total nitrogen fed. Temperature of the preneutralizer was 228' F. The p H was 6.2. Ammoniation to a product mole ratio of 1.77 was completed in the drum. A recycle ratio of 5.3 was required for best operation of the granulator. About 4% of the total nitrogen fed was evolved from the dryer when the product temperature was 205' F. T h e product from the dryer contained 1.1% moisture; the granules were well rounded and uniformly sized. Data from a pilot-plant test are given under test 5 (Table I). Use of Solid Forms of Urea as Feed
Most of the test work involved the use of solutions of urea ranging from 83 to 98% concentration. Use of solution would lower the investment and decrease production cost of the urea component. However, in some situations the feeding of urea in solid form to a urea-ammonium phosphate process might be desirable. If production of a substantial amount of "straight" urea in addition to urea-ammonium phosphate products is desired, it might be good planning to increase the production capacity for prilled urea and use an undersize fraction of the prills as feed for the urea-ammonium phosphate granulation system. The urea would already be dried and higher production rates of urea-ammonium phosphate grades might be expected. Similarly, crystal urea that is an intermediate in some urea processes could be fed to the granulator; this type of operation would produce products with very low biuret content. Pilot-plant tests were made in production of 29-29-0 grade with urea fed as either prills or crystals. Tests Using Prilled Urea. Wet-process acid was preneutralized with ammonia to a mole ratio of about 1.4 and the slurry was sprayed into the drum to coat prilled urea and recycled undersize to produce 29-29-0 grade. The mole ratio of the final product was about 1.7. Recycle requirement was about 2.6 for best operation. The granulator product contained 1.4 to 2.270 moisture. Drying a t a product temperature of 190' to 200' F. gave moisture content of about 1%; increasing the product temperature to 21 5' F. lowered the product moisture content to 0.6oI,. T h e screened product showed evidence of some uncoated prills during the limited period of pilot-plant operation. Extended operation and use of undersize prills should result in more complete and uniform coating of the urea. Crushing of the prills might be helpful. Data from a typical pilot-plant test are given under test 6 (Table I). 130
ILEC PROCESS DESIGN A N D DEVELOPMENT
Tests Using Crystal Urea. Crystal urea was fed in other tests of production of 29-29-0 grade. T h e granular product was not so well rounded as when urea solution was used, partly because of the comparatively low temperatures of 155' to 160' F. in the ammoniator-granulator drum. Best operation was obtained with a recycle ratio of about 3. T h e product dried a t a dryer discharge temperature of 207' F. contained about 1.2% moisture. Better shaped granules would be expected after additional experience has established the optimum operating technique and when the higher temperature encountered in a plant-scale drum is used. Data from a typical pilot-plant test are given under test 7 (Table I). Effect of Urea Source on Recycle Ratio
The source of urea affects the recycle ratio because of variations in the heat and moisture introduced by the various sources of urea. The required recycle ratios for 29-29-0 grade produced in the pilot plant when feeding urea as solids or as solutions are shown in Table 11. These data indicate that for the 29-29-0 grade sufficient recycle must be added to give about the following conditions for granulation, regardless of the source of urea. Input moisture, 4% Moisture exit granulator, 2% Temperature exit granulator, 160' to 170' F. !$h' en solid urea was fed, the moisture in materials fed to the granulator (exclusive of recycle) was about 12% and a recycle ratio of 2.6 was needed. T h e variation (11 us. 13y0) shown between the use of prills and crystals was caused by different moisture contents in the preneutralized slurry. When partially stripped urea reactor effluent was fed to the granulator, the recycle ratio increased to 4.3 because of the increased moisture and heat in the solution. When the urea was fed as 83% solution a t 200' F., still more moisture and heat were added; therefore, the recycle ratio increased to 5.3. Solution concentrated to 977& urea was not tested with this grade. Data for a test in production of a similar 25-35-0 grade with 97y0 solution show a n increase (3.3 us. 2.6) in recycle requirement between the use of 97% solution and solid urea, even though there was no significant increase in input moisture. Drying Tests
T h e main differences between the production of granular urea-ammonium phosphate grades and of ammonium phosphate such as 18-46-0 are the need for more thorough drying of the urea-ammonium phosphate products and the comparative difficulty of the drying step. I n production of 18-46-0 grade ammonium phosphate, drying is easy and a product with a
Table II. Recycle Ratios Required for 29-29-0 Grade Produced When Urea Fed as Solids or Solutions Input Moisture to Granulator Lb. Granulator, 7 0 Product Recycle/ 'Excluding Including 7, Tzmp., Lb. Source of Urea recycle recycle H20 F. Product Prills 13 4.5 1.7 167 2.6 Crystals 11 3.7 2.1 159 2.60 Partially stripped urea reactor effluentb 15 3.8 1.8 165 4.30 18 4.0 1.9 173 5.3 837, solution 977, solutionc 11 3.5 2.8 174 3.3 a Average of several tests. b Efluent added to preneutralizer and had following composition: 54% urea, 29.8% carbamate, and 16.27, water. e Data f o r test of 25-35-0 instead of 29-29-0 grade.
moisture content of about 1.5% stores well in bulk or in bags without a conditioner. When either urea or ammonium nitrate is combined with ammonium phosphate, the products must be dried to lower moisture contents. Also, the presence of the urea or ammonium nitrate as a substantial proportion of the product makes drying difficult because the temperature of drying gases must be kept lower to avoid melting the product, and a long retention time is required when drying materials of low moisture content to allow the moisture to diffuse to the surface. For these reasons the dryer becomes the key equipment in establishing the production rate for a plant system producing urea-ammonium phosphate products. With maximum size of a single dryer unit. the production capability is considerably lower than for an ammonium phosphate plant system of equal size. Because of the importance of the drying step, study of this operation was emphasized in pilot-plant tests of the various grades. Efforts were made to determine the maximum air temperature that could be utilized, the retention time required for drying the product to various moisture levels, and the storage properties of the products to establish the required amount of drying. Tests were made with countercurrent and cocurrent flow of air i n the pilot-plant dryer. Tests with Countercurrent Drying. Because experience in drying ammonium nitrate and ammonium phosphate nitrate products had indicated an advantage for countercurrent drying, this mode of operation was used in most of the test work. Previous experience had indicated that materials containing high proportions of highly soluble salts did not melt and stick in the dryer so severely with this type of drying. t'l'ith the countercurrent flow of air. the retention time in the dryer was about 25 minutes and the loading was about 3001,. I n dr) ing urea-ammonium phosphate products, the maximum inlet gas temperature that could be used without excessive melting was in the range of 300" to 350" F. Tests with Cocurrent Drying. In some of the later pilotplant tests the dryer was operated with a cocurrent flow of gases. Retention time was about 17 minutes and the loading was about 2070 of the volume. I n previous pilot-plant tests \vith urea-ammonium phosphate using countercurrent flow of gases in the dryer, enough of the material melted and stuck on the dryer shell and flights to cause a problem. T h e melting appeared to be caused by the material coming in contact with hot metal parts of the dryer rather than with the airstream. Therefore, it was thought that with cocurrent flow of air in the dryer the initial high rate of evaporation of moisture from the particles would cool them sufficiently to prevent this melting and sticking to the hot metal in the dryer and thus allow higher input gas temperatures. With countercurrent flow, the inlet gas temperature was kept in the range of 300" to 350" F. to prevent excessive melting. With the cocurrent flow, the inlet gas temperature used was as high as 400' F. with no increase in melting on the dryer shell. A few tests were made with the dryer operated under suction by using a higher flow of air through the fan a t the discharge end than through the blower a t the feed end. This unbalanced condition resulted in a n inleakage of cool air through the product feed chute and around the perimeter of the dry shell. This cool air lowered the temperature of the metal shell and chute and further decreased the amount of melted product collecting o n the hot surfaces. Thus it appears that urea-ammonium phosphate can be dried adequately with cocurrent flows of gases in the dryer, and more efficiently because higher inlet gas temperatures can be used without causing excessive melting of the products.
Physical Properties of Urea-Ammonium Phosphate
The critical humidity of urea-ammonium phosphate made in the pilot plant was about 55% as compared with 59 and 75% for ammonium nitrate and urea, respectively. Variations in grade and the use of thermal-process acid instead of wetprocess acid did not significantly affect the critical humidity. T h e addition of potassium chloride in the formulation lowered the critical humidity of the product to about 45%. However, laboratory tests made to simulate bulk exposure indicated that the rates of moisture pickup and penetration were significantly lower for urea-ammonium phosphate than for either ammonium nitrate or urea, even though the urea-ammonium phosphate had a lower critical humidity than either of the two other materials. When granular urea-ammonium phosphate (33-20-0) and ammonium nitrate were exposed in a humidity chamber a t 86" F. and 70Oj, relative humidity, the rates of moisture absorption and moisture penetration for the ureaammonium phosphate were only about one third as great during the first 10 hours of exposure as for the ammonium nitrate (see Figure 6). Furthermore, as the exposure was continued, these rates decreased for the urea-ammonium phosphate. T h e urea-ammonium phosphate was considerably more hygroscopic than diammonium phosphate. T h e top layers of all the materials except diammonium phosphate became soft and mushy and disintegrated upon handling. Because of the condition of the exposed granules and the probability that they would affect the bulk of the material when intermixed, a dehumidified building should be used for bulk storage of urea-ammonium phosphate products. The bulk densities of the granular products were relatively low. Untamped bulk densities ranged from 40 to 46 pounds per cubic foot. Products containing potassium chloride (1 5 to 22% K20) had bulk densities of 48 to 54 pounds per cubic foot. The angle of repose of the granulator products was about 36 degrees.
1
6 0 ALL
6
I
-
3 3 - 2 0 - 0 UREA-AMMONIUM PHOSPHATE PRODUCTS - 8 t12 MESH.
CONDITIONED
WITH 2 % KIESELGUHR
J 0
20 30 40 50 60 70 80 90 HOURS OF E X P O S U R E AT 86' E, 7 0 % R E L A T I V E H U M I D I T Y IO
Figure 6. Moisture absorption and penetration into bulk fertilizer during exposure at 86" F. (30" C.) and 70% relative humidity VOL 7
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Bag-Storage Tests. Bag-storage tests were carried out on urea-ammonium phosphate pilot-plant products a t various moisture contents. Products were stored with and without conditioning agents such as diatomaceous earth and calcined fuller’s earth. Portions of the products were bagged without being cured and other portions after they had been cured in bulk for 1 week. The bags had a capacity of 50 pounds and were valve-type, six-ply paper bags with two asphalt laminates. The bags were stacked 20 high and the bottom four bags were tested. T h e material was evaluated on the basis of the bag set before the bags were handled and the percentage of 1/2-inch or larger lumps after the bags had been dropped once from a height of 30 inches. Data from bag-storage tests of uncured pilot-plant products are given in Table 111. Cured products gave similar results. Unconditioned products dried to as low as 0.3 to 0.5% moisture generally stored satisfactorily for as long as 3 months, but caked during 6 months of storage. Products made with wetprocess acid dried to about 1.3 to 1.5% moisture and coated with 2% of diatomaceous earth stored satisfactorily for as long as 6 months. Kaolin clay was somewhat less effective as a conditioner. Biuret Formation
When urea is subjected to high temperature, part of it is converted to biuret by the following reaction. Urea
+ heat
+
biuret
+ ammonia
Since biuret may be toxic to plants in some methods of fertilization such as foliar application, efforts are made to minimize its
formation in processing urea. Pilot-plant experience in production of urea-ammonium phosphate products had indicated that the only appreciable formation of biuret occurs during concentration of the solution. Analyses of a large number of samples of material from the granulator and the dryer show no appreciable increases in biuret during these steps. For this reason it is assumed that by use of conventional equipment and a short retention time for evaporation of the solution, the biuret content of urea-ammonium phosphate products could be maintained a t a satisfactorily low level. If a biuret content of less than 0.5’% is desired, the urea could be fed to the granulator as crystals that contain very little biuret. Agronomic Tests
Although both urea and diammonium phosphate have been used extensively in fertilization, little is known of the agronomic effectiveness of a fertilizer composed of a combination of these two materials. Extensive agronomic tests are in progress to evaluate this new type of fertilizer. Agronomic tests of pilotplant products are being carried out in 27 states and seven foreign countries. Use in rice fertilization is being emphasized. Conclusions
Cogranulation of urea and diammonium phosphate is a n attractive way to produce high-analysis fertilizers containing 50 to 60% plant food. Products ranging in N:P*Os ratio from 3 : 1 to 1 : 2 can be made in a granular diammonium phosphate
u-20 U-27 u-20 U-32
Results of Bag-Storage Tests for Urea-Ammonium Phosphates Results of Inspections after Storage 3 Ma.-~ 6 Mo. Bagging Lumps* Lumpsb H20, Temp., OF. Bag sPta A B Bag seta A B Conditioner, % % 25-35-0 Grade ... M 10 L H 52 M 0.5 None 95 M M 1.5 29 M 36 M None M 0.5 L 0 0 11 L 1yo calcined fuller’s earth L 0 0 2y0 calcined fuller’s earth 104 L 1.3 0 0
U-45-46 U-56 U-45-46 U-64
None None 274 diatomaceous earth 2y0 diatomaceous earth
0.4 1.6 0.4 1.3
u-74
2% diatomaceous earth
0.3
Table 111.
Test
KO.
~
.
I
.
33-20-0 Grade 110 L 125 H 110 VL 80 L
M H VL L
76 0 9
VL H 0 VL
VL
0
0
29-29-0 (83y0 Solution) 120 LM 120 L
7
L
0
0
L
0 8 0 5
0 L 0 L
1 2
VL VL
9
38 69
L H
0
0
12
VL
5
VL
25 7
LM L
LM H L L
17 30
L M
0
0 L
L L
16 9
VL VL
...
.. .. ..
...
34-17-0 Grade
U-14 U-14
None 2.5 diatomaceous earth
0.9 1 .o
u-11 u-12 u-11 u-12
None None 2 , 5Yc diatomaceous earth 2. 57c diatomaceous earth
0.4 1.5
U-71 U-69
2Yc diatomaceous earth 2y0 diatomaceous earth
0.3 1.4
U-84 U-85 U-84 U-85
38-13-0 None None 2yc diatomaceous earth 2y0diatomaceous earth
0.4 1.2
...
29-29-0 (Prills) 120 L 120 M 120 L 118 LM 29-29-0
b
...
...
(Crystals) VL L
(Made with Thermal Acid and Phosphate Rock) H 77 H H 65 H 0.4 0.6 L 13 L 0.3 L 7 VL
0.7
0 = none; L = light set; M = medium set; H = hard set; V L = very light. A = 7 drop from height of 30 inches.
B = hardness of lumps; L
132
=
light, M
=
medium, H = hard.
l & E C PROCESS D E S I G N A N D D E V E L O P M E N T
L
M
... ...
...
3
..
..
...
...
plant having a rotary drum type of granulator. Potassium chloride can be added during granulation to give threecomponent grades, such as 38-1 3-0, 34-17-0, 26-1 3-1 3, 29-29-0, and 19-19-19. For a minimum of equipment the granulation plant should be adjacent to a urea plant. The urea can be supplied as a concentrated solution or as a solid. If the solutions are concentrated to 96 to 98y0 urea, the recycle requirements will be significantly less than for solutions containing about 85% urea. The use of solid urea further decreases the recycle requirement. However, additional equipment such as a crystallizer or prilling tower is required to produce the solid form and the product is not as homogeneous. T h e ammonia in the off-gas from the decomposer in the urea plant or gaseous or liquid anhydrous ammonia can be used to ammoniate the phosphoric acid. Products made with wet-process acid granulate and store much better than those made with straight thermal-process acid. However, satisfactory products can be made with ~ supplied as thermal-process acid, if about 5% of the P z O is phosphate rock reacted with the acid or 25% is supplied as wet-process acid. The presence of urea makes drying difficult because low
moisture contents are required for satisfactory storage and high gas temperatures cannot be used without melting the product. The design of the dryer is critical. Products require drying to about 1% moisture and coating with 2% of a n appropriate conditioner for satisfactory storage in bags. T h e products are less hygroscopic than ammonium nitrate or urea but considerably more hygroscopic than diammonium phosphate ; controlled humidity would be desirable for bulk storage. T h e biuret content of urea-ammonium phosphate products can be maintained a t a satisfactorily low level by the use of conventional equipment with short retention time for evaporation of the solution. literature Cited
Chenoweth, G. E., Chem. Eng. Progr. 54, No. 4, 55-8 (1958). Newman, E. L., Hull, L. H., Farm Chem. 128, 48-9 (June 1965). Strelzoff, S., Cook, L. H., Advan. Petrol. Chem. Rejrning 10, 315-406 11965). Young, R. D., Hicks, G. C., Davis, C. H., J . Agr. Food Chem. 10, 442-7 (1962). RECEIVED for review January 16, 1967 ACCEPTEDAugust 18, 1967 Division of Fertilizer and Soil Chemistry, 152nd Meeting, ACS, New York, N. Y . , September 1966.
CRYSTALLIZATION OF CALCIUM SULFATE FROM PHOSPHORIC ACID A. B. A M I N ' A N D M. A. LARSON Dejartment of Chemical Engineering and and Enginetring Research Institute, Iowa State University of Science and Technology, Ames, Iowa
The kinetics of the crystallization of calcium sulfate from phosphoric acid was studied using a laboratory continuous crystallizer. The apparatus was operated so that an unclassified suspension was achieved and an unclassified product was obtained. Nucleation and growth rates were determined from an analysis of crystal size distribution. Both reagent grade and wet process phosphoric acid were used. Nucleation rates were lower and growth rates were higher under conditions which produced hemihydrate crystals rather than gypsum cyrstals. When reagent grade raw materials were used, nucleation rates were generally higher and growth rates generally lower than when the raw materials were phosphate rock and wet process phosphoric acid. Phosphoric acid concentration had little effect on the kinetics of nucleation and growth, but increased suspension density increased the particle size.
N THE
production of phosphoric acid by the wet process, a n
I important but troublesome step is the separation of calcium sulfate from the product acid by crystallization. T h e conditions under which crystallization takes place and the type of crystal produced generally determine the amount of phosphate lost with the calcium sulfate, and largely determine the speed and efficiency of the subsequent filtration step. Depending upon the conditions during crystallization, appreciable amounts of phosphate may be incorporated in the crystal lattice and, because of undesirable crystal habit or small size, appreciable quantities of phosphate may not be washed free from the cake during filtration. Both phenomena result i n lost phosphate ; therefore, the crystallization step must be carried out to produce the best possible crystalline product. T h e crystallization step should be designed and operated Present address, American Cyanamid Co., Princeton, N. J.
to satisfy the following rather obvious criteria. T h e crystal growth rate should be a t a maximum consistent with good crystal formation, the nucleation rate should be a t a minimum, the crystal form should not be such that phosphate ions are incorporated i n the lattice, and the crystal form and habit should be such that filtration and cake washing rate are maximized. T h e relative kinetic rates of growth and nucleation determine the particle size distribution. T h e respective maximum growth rate and minimum nucleation rate sought would be those giving the largest particles in the minimum holding time. A part of a n investigation to find the optimum operating conditions consistent with these requirements requires the measurement of growth and nucleation rates and the determination of the effects of the operating conditions on these rates under conditions experienced in practice. This paper illustrates how such growth and nucleation kinetic data can VOL. 7
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133
