DIAMMONIUM PHOSPHATE H. L. THOMPSON', PHILIP MILLER2, R. M. JOHNSON, I. W. McCAMY, AND GEORGE HOFFNZEISTER, JR. Tennessee Valley Authority, Wilson Dam, Ala.
Crystalline diammonium phosphate, (NH&HPOa, of high purity was produced by a continuous process that consisted i n passing anhydrous ammonia gas and relatively pure phosphoric acid into saturated mother liquor kept at 60' t o 70" C. and pH 5.8 to 6.0. The heat of reaction vaporized water from the liquor, while crystals of diammonium phosphate developed and were withdrawn,
centrifuged, washed, and dried. The process was carried out successfully in (1) a saturator vessel of the ammonium sulfate type i n which air was used for cooling through vaporization of water from the mother liquor and (2) a vacuum crystallizer. The product contains about 759" plant nutrients and is deemed suitable for use either alone or i n mixtures with other fertilizer materials (13).
D
phosphoric acid mole ratios of about 2.0, which is the ratio of these constitutents in pure diammonium phosphate. Figure 1 shows that the composition of such a saturated solution, which is represented by point D in the figure, is close to the composition a t triple point T I . Thus, it is apparent that product contamination with the unstable salt is highly probable in a process based on diammonium phosphate crystallieation from solution of the composition represented by point D. The present authors believe that presence of the unstable salt in product produced in the past has caused many fertilizer technologists to conclude that diammonium phosphate is unstable and emits ammonia. Neither diammonium phosphate of reagent grade nor the present TVA product emits an odor of ammonia at normal temperatures. The process developed in the present work is based on continuous crystallization of diammonium phosphate from mother liquor that is maintained at an ammonia:phosphoric acid mole ratio only slightly greater than that represented by the diammoniummonoammonium phosphate triple point, which is shown as point T2in Figure 1; consequently, contamination of the product with the unstable salt was minimized. In considering some of the factors involved in the proposed process, it was recognized that continuous vaporization of water from the mother liquor would be required to prevent accumulation of water in the system and that this vaporization would also dissipate the heat of the following reaction:
IAMMONIUM phosphate, (NH4)2HPOd,'isa white, crystalline salt that contains 21.2'70 nitrogen and 53.8%phosphopus pentoxide (PgO5). The properties pertaining to fertilizer usage of this material were described in a previous publication ( 1 3 ) . This paper describes the pilot plant development of processes for producing diammonium phosphate by continuously proportioning ammonia and phosphoric acid into a saturated solution of controlled temperature and pH. INITIAL STUDIES
The mother-liquor temperatures and acidities that are required for the crystallization of several solid phases in the system ammonia-phosphoric acid-water ( NH3-H3P04-H20) have been established by the solubility measurements of previous investigators. Consistent and apparently reliable measurements at temperatures of 0", 25", and 50' C. have been reported by Muromtsev (8),Muromtsev and Nazarova ( 9 ) ,and Volfltovich et al. (14). More recently, Brosheer and Anderson @), in connection with the TVA study of ammonium phosphate for fertilizer use, ext,ended the knowledge of the system to 75' C. Figure 1 is a section of the solubility isotherm for the system at 60' C. as derived from the published data. The isotherm for 60' C. was chosen for presentation in Figure 1 because most of the pilot plant production of diammonium phosphate was carried out with the mother liquor at about this temperature. Figure 1shows that at 60" C. the acidity of liquor in equilibrium with only solid diammonium phosphate may vary from that represented by triple point TI to that represented by triple point T2; this corresponds to the animonia:phosphoric acid mole ratio range of from 1.51 to about 2.04. With mole ratios below 1.51, the stable solid phase is monoammonium phosphate, whereas with ratios above 2.04, the solid is a salt with the formula ("4)8PO4' ~ ( N H ~ ) ~ H P OThe I . last mentioned salt is relatively unstable ( 8 ) ; it decomposes with evolution of ammonia, and the presence of appreciable proportions of this salt in diammonium phosphate fertilizer wouId be undesirable. Previously described diammonium phosphate production techniques (7, 10, 11) have been based on crystallization from saturated solutions of ammonia: Present address, Missicsippi Chemical Corporation, Jackson 115, Miss Present address, H. K. Ferguson Company, Inc., 39 Broadway, New York 6,N. Y . 1 9
2NH3k)
+ HJ'Odaq)
-+ ( N H I ) J D " ~ ( S )
Calculations based on published thermal data (1, 4 ) indicate that the heat liberated by this reaction at 60' C. is about 2380 B.t.11. per pound of ammonia reacted, which is sufficient for vaporization of about 2.2 pounds of water. The acid fed into the reactor would introduce this proportion of water if it were about 54% HsPOa. With more concentrated acid, it would be necessary either to add make-up water to the reactor or to provide other means to effect heat dissipation. Electric-furnace acid of 75 to 85% Hap04 was used in the work described here, and make-up water (largely wash water from the centrifuging of crystals) was added to the mother liquor. Laboratory studies, the results of which are plotted in Figures 2 and 3, indicated that operation with relatively high liquor acidity and low liquor temperature would result in a relatively low ammo-
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The desired classification of crystals was not effected since the crystals withdrawn from the bottom of the leg and those recycled to the saturator were virtuall identical in particle size. However, d e slurry that was withdrawn periodically through a quick-acting valve a t the bottom of the leg contained about 50% crystals as compared to 30 to 35% crystals in slurry withdrawn from the top of the leg and recycled to the saturator. Crystals withdrawn from the bottom were centrifuged, washed in a basket centrifuge, and then dried in a rotary dryer. Mother liquor and wash water from the centrifuge were returned to the saturator. Fresh water also was added to the saturator in amounts required to maintain a constant liquor level. The phosphoric acid used in the pilot plant was a product of the electricfurnace process in the TVA plant (3, 18) and was essentially free of impurities. The acid was fed into the saturator below the liquor surface through two 0.5-inch stainless steel pipes. The ammonia was essentially pure anhydrous ammonia from TVA plant production Figure 1. Section of Solubility Isotherm for System Ammonia-Phosphoric (6)and was transported to the pilot plant Acid-Water at 60' C. in uressure tanks from which it was fed to the system as vapor, a t a constant rate, in mixture with air. The ammonia-air mixture entered the saturator through a submerged manifold that nia vapor pressure and a low ammonia: water ratio over the liquor. consisted of a 20-inch diameter circle of 1-inch steel pipe perfoIt was calculated that if the process were carried out a t 60' C. rated on the bottom with */,O-inch holes a t 1-inch intervals. In with a n ammonia:phosphoric acid mole ratio of about 1.5 in the initial studies, separate introduction of ammonia and air showed promise, but this method was not investigated thorou hly after liquor (minimum ratio commensurate with formation of diamsatisfactory absorption of ammonia was obtained with t i e ammomonium phosphate) evaporation of the amount of water required nia-air manifold. The air passed through the liquor in the satuto dissipate the heat of reaction (2.2 pounds of water per pound of rator and dissipated reaction heat chiefly through vaporization of ammonia) would result in volatilization of only about 1%of the ammonia fed. These results indicated the feasibility of a simple, single-stage process, whereas previously described processes, in which crystallization of diammonium phosphate was from mother liquors of ammonia :phosphoric acid mole ratio in the neighborhood of 2.0, necessarily included special provisions, such as multistage neutralization, to prevent excessiveammonia loss. The temperatures of 50' to 75" C. covered by Figures 2 and 3 are well below the normal boiling points of liquors saturated with respect to diammonium phosphate; thus, to carry out vaporization of water at these temperatures, it is necessary either to pass a stream of air through the liquor or to lower the boiling point of the liquor by use of reduced pressure. The first-mentioned method was studied extensively in this work. The production of diammonium phosphate through this method of evaporation will be referred to as the saturator process because it was carried out in a reaction veasel similar in general design to saturators employed in ammonium sulfate manufacture. The reduced-pressure method of evaporation waa tested briefly in a pilot plant-scale vacuum crystallizer, operation of which will be discussed also. SATURATOR P I u l T PLANT
.
A flow sheet of the saturator pilot plant for the production of diammonium phosphate is shown in Figure 4, and the plant is pictured in Figure 5. The saturator (Figure 6) wazl constructed of 0.25-iich carbon steel and waa 2 feet in diameter by 3 feet high. In operation, it contained 6 to 7 cubic feet of a suspension of diammonium phosphate crystals in saturated mother liquor. Phosphoric acid (75 to 85% HaPO4) and gaseous, anhydrous ammonia were fed continuously into the suspension in the ratio of 2 moles of ammonia permole of phosphoric acid, which resulted in formation and growth of diammonium phosphate crystals. Crystal suspension passed from the coned bottom of the saturator into a classifying le directly below the saturator. This leg was intended to provlde for settling of large, product crystals and allow return to the saturator of slurry that contained the h e r crystals.
NHa/HaPOd MOLE RATIO IN SOLUTION
Figure 2. Partial Pressure of A m m o n i a over Saturated Solutions in System AmmoniaPhosphoric Acid-Water
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Figure 8 show that operation at pH 6.6 gave a high ammonia partial pressure over the mother liquor, which resulted in about 10% of the ammonia fed leaving the system in the exhaust gas. When bperating at pH 5.8, only about 3% of the ammonia was lost in the exhaust gas. This reduction in loss was due to the effect of pH on the partial pressure of ammohia in the gas over the mother liquor (Figure 9). The rate of water evaporation required to dissipate the heat of reaction and the air rate required to obtai~ithis evaporation were essentially independent of the liquor pH. The average rate of evaporation was 2.3 pounds of water per pound of ammonia, and the air requirement averaged 673 pounds per hour (20.9 pounds per pound of ammonia). The humidity of the exhaust gas, as measured by wet- and dry-bulb thermometers, averaged 0.110 pound of water per pound of dry air; this represents about 98% saturation based on the laboratory vapor-pressure data that are presented in Figures 2 and 3. Mother-Liquor Temperature. As shown in Figure 9, for pH range of 5.8 to 6.6 and temperature of 61 'C., the partial pressures of ammonia in the saturator exhaust gas exceeded the equilibrium partial pressures that were determined in the laboratory study (Figure 2). However, in the pilot plant, a liquor temperature somewhat higher than 61' C. resulted in closer approach to equilibrium partial pressure, with resultant reduction in ammonia loss. In the pilot plant, mother-liquor temperatures of 51', 61', and 70' C. were obtained by varying the air rate to the saturator. Operation at these temperatures was investigated. In the tests of effect of temperaturqon production of diammonium phosphate, the mother-liquor pH and ammonia feed rate were held essentially constant as follows:
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peared that, t.hc ctLpari1.yof this type masuitable conatruction material for parts in contact, with erys
