Method for producing fertilizer grade DAP having an increased nitrogen concentration from recycle

A process for the preparation of granular fertilizer grade DAP (a product composed of ammonium phosphates, principally diammonium phosphate, resulting from the ammoniation of phosphoric acid, as defined in Official Publication No. 52 of the Association of American Plant Food Officials, dated 1999) comprising partially preneutralizing orthophosphoric acid with ammonia, completing the ammoniation of the orthophosphoric acid with ammonia in a rotary ammoniator-granulator to provide granular DAP, sizing the granular DAP to provide the granular DAP product, reducing the particle size of the oversized granular DAP, and recycling the undersized granular DAP and the sized-reduced oversized granular DAP to the ammoniator-granulator. The ammoniacal nitrogen concentration of the granular DAP recycle, hence the granular DAP product, is enhanced by subjecting the finely-divided recycled granular DAP to anhydrous gaseous ammonia which is at a super atmospheric pressure and which is at a temperature sufficient to maintain said anhydrous ammonia in the gaseous state. The increase in the ammoniacal nitrogen concentration is a function of the absolute ammonia pressure, the initial moisture content of the granular DAP recycle, and the contact time of the ammonium with the granular DAP recycle.

This application is related to the application entitled "Method for
 Producing Fertilizer Grade DAP Having an Increased Nitrogen Concentration
 In a Spray Column", filed concurrently herewith.
 BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The invention relates to a method of enhancing the nitrogen concentration
 of fertilizer grade DAP. Fertilizer grade DAP is defined in Official
 Publication No. 52 (1999) of the Association of American Plant Food
 Control Officials as a product composed of ammonium phosphates,
 principally diammonium phosphate, resulting from the ammoniation of
 phosphoric acid. It may contain up to 2 percent non-ammoniacal nitrogen.
 The guaranteed percentage of nitrogen and available phosphate shall be
 stated as part of the name.
 2. Background of the Invention
 Phosphorus as well as nitrogen are essential plant nutrients. Phosphorus
 and nitrogen ensure cell multiplication and thus growth since both are
 structural components of nucleic acids. Plants use the phosphorus and
 nitrogen to grow. Since fertilizers are generally formed by the reaction
 of phosphoric acid and ammonia, they are used to provide a rich source of
 nitrogen and phosphorus to the plants. Such fertilizers are generally
 applied to the soil and are readily assimilable by plants.
 The phosphoric acid used in fertilizers is usually manufactured from rock
 phosphate typically in one of two ways. Phosphate rock is composed chiefly
 of calcium phosphates and calcite.
 In the first prior art method, commonly known as the furnace acid process,
 the phosphoric acid is made by heating phosphate rock to a form of
 relatively pure elemental phosphorus which is then converted into the
 phosphoric acid. More particularly, this prior art process uses an
 electric furnace heated to about 1500.degree. K. The phosphate rock is
 reduced by coke in the presence of sand or silicon dioxide according to
 the following reaction:
EQU Ca.sub.3 (PO.sub.4).sub.2 +3SiO.sub.2 +5C.dbd.3CaSiO.sub.3 +5CO+P.sub.2
 Gaseous P.sub.2 condenses to form a solid P.sub.4. In the air, the P.sub.4
 converts to phosphorus pentoxide. Water is then added to the phosphorus
 pentoxide to form the phosphoric acid. The process usually yields 85
 percent phosphoric acid which is colorless and has a density of about 1.69
 kg/L. The phosphoric acid formed by this prior art process generally
 yields a purer phosphoric acid as compared to the phosphoric acid formed
 by the second prior art process, which is described below. However, this
 first process is also much more costly than the second prior art process.
 The second prior art method used to produce phosphoric acid is known as the
 wet process. Most of the phosphoric acid produced in the United States is
 produced by this process. In this wet process, finely ground phosphate
 rock is slurried with sulfuric acid. Sometimes, recycled dilute phosphoric
 acid is included in the slurry. The reaction that occurs in this wet
 process is set forth below:
EQU Ca.sub.3 (PO.sub.4).sub.2 +3H.sub.2 SO.sub.4 =3CaSO.sub.4 +2H.sub.3
 PO.sub.4
 In addition to the orthophosphoric acid, the reaction yields gypsum and
 numerous other suspended and dissolved impurities. The slurry is generally
 filtered to remove the solid impurities, mainly, gypsum. The resultant
 filtrate contains between 25 and 35 percent P.sub.2 O.sub.5 and between 1
 and 8 percent by weight of suspended solids or impurities that are not
 removed by the filtration process. Examples of wet process methods are
 disclosed in U.S. Pat. Nos. 4,487,750, 4,485,078, 4,657,559, 4,665,790 and
 4,655,789, which are incorporated herein in their entirety. U.S. Pat. No.
 4,710,366 discloses methods of removing further impurities from the
 filtrate; such patent is incorporated herein in its entirety.
 As used herein, the terms phosphoric acid and wet process phosphoric acid
 mean orthophosphoric acid.
 Phosphoric acid is reacted with anhydrous ammonia to form ammonium
 phosphates, which constitute a large class of phosphorus fertilizers. See
 Examples 7 and 9 of U.S. Pat. No. 4,485,078. The common ammonium
 phosphates include monoammonium phosphate and diammonium phosphate, which
 are the primary components of the fertilizers commonly known as MAP and
 DAP, respectively. DAP (fertilizer grade) is a product composed of
 ammonium phosphates, principally diammonium phosphate, resulting from the
 ammoniation of phosphoric acid. The phosphoric acid is preferably reacted
 with the proper proportion of anhydrous ammonia which primarily provides
 diammonium phosphate, which is a source of nitrogen and phosphorus readily
 assimilable by food crops.
 The TVA (Tennessee Valley Authority) in the early 1960s developed a process
 for the preparation of granular DAP from ammonia and phosphoric acid. The
 conventional TVA process for preparing granular DAP is described on pages
 248 to 251 of Manual of Fertilizer Processing, edited by Francis T.
 Nielsson, Marcel Dekker, Inc., (1987). The conventional TVA type of
 process has a preneutralizer for partial ammoniation of the phosphoric
 acid and completion of ammoniation is done in a rotary
 ammoniator-granulator. Granulation is controlled by recycling product
 fines to the drum. The basic TVA process involves partial
 preneutralization of the acid in a preneutralizer (reaction tank) followed
 by completion of ammoniation to DAP in the rotary ammoniator-granulator.
 Excess ammonia, which must be fed to the ammoniator-granulator to produce
 DAP, is recovered by scrubbing the off gases with the acid to be used in
 the process. The granular product is normally dried, cooled, and screened,
 having the undersized and crushed oversized granular DAP recycled to the
 granulator to control granulation.
 Pure diammonium phosphate [(NH.sub.4).sub.2 HPO.sub.4 ] is also termed
 dibasic ammonium phosphate. Page 561 of The Merck Index, .sub.10 th Ed.,
 (1983), states that diammonium phosphate gradually loses about 8 percent
 NH.sub.3 on exposure to air.
 Triammonium phosphate is (NH.sub.4).sub.3 PO.sub.4.
 BROAD DESCRIPTION OF THE INVENTION
 An object of the invention is to provide a process for enhancing the
 ammoniacal nitrogen concentration of DAP. Another object of the invention
 is to provide a process for producing DAP fertilizer with the nitrogen and
 phosphate concentrations required for international trade, using the lower
 quality orthophosphoric acid currently being produced, without using
 expensive nitrogen supplements. A further object of the invention is to
 provide a process for the production of fertilizer grade diammonium
 phosphate. Other objects and advantages of the invention are set forth
 herein or are obvious herefrom to one skilled in the art.
 The objects and advantages of the invention are achieved by the process of
 the invention.
 There currently exists a need for a process to produce ammonium phosphate
 from orthophosphoric acid having a high concentration of impurities.
 Furthermore, a need exists for increasing the nitrogen content of DAP used
 for fertilizers. The invention provides a solution to such needs.
 It has been found that reacting finely divided DAP with high pressure,
 anhydrous, gaseous ammonia will enhance or increase the ammoniacal
 nitrogen concentration of the DAP, but such treated DAP has the
 disadvantages that, as it ages, it tends to lose significant amounts of
 the added nitrogen and to gain excessive moisture.
 The invention involves a process for enhancing the ammoniacal nitrogen
 concentration of DAP. The DAP, which is in finely divided form, is
 subjected to anhydrous gaseous ammonia, which is at a super atmospheric
 pressure and which is at a temperature sufficient to maintain the ammonia
 in the gaseous state. The increase in the ammoniacal nitrogen
 concentration is a function of the absolute ammonia pressure, the initial
 moisture content of the DAP and the contact time of the ammonia with the
 DAP. The enhancement of the ammoniacal nitrogen content of the DAP is also
 a function of the particle size of the DAP.
 The DAP should be de-aerated before it is treated with the gaseous ammonia.
 The ammonia may have converted some of the DAP to triammonium phosphate
 and/or been adsorbed by the DAP.
 A preferred embodiment of the invention process involves modification or
 improvement of the conventional TVA process, or such type of process, for
 the preparation of granular DAP from ammonia and orthophosphoric acid.
 The conventional TVA type of process has a preneutralizer for partial
 ammoniation of the phosphoric acid and a rotary ammoniator-granulator for
 completion of the ammoniation. Granulation is controlled by recycling
 product fines to the drum. The TVA process involves partial
 preneutralization of the acid in a preneutralizer (reaction tank) followed
 by completion of ammoniation to DAP in the rotary ammoniator-granulator.
 Excess ammonia, which must be fed to the ammoniator-granulator to produce
 DAP, is recovered by scrubbing the off gases with the acid to be used in
 the process. The granular product is normally dried, cooled, and screened,
 with the undersized and crushed oversized granular DAP being recycled to
 the granulator to control granulation.
 Such preferred embodiment, as already stated, involves improving the
 conventional TVA type of process for the preparation of granular DAP
 product. The conventional TVA type of process involves partially
 preneutralizing orthophosphoric acid with ammonia, completing the
 ammoniation of the orthophosphoric acid in a rotary ammoniator-granulator
 to provide granular DAP, sizing the granular DAP to provide the granular
 DAP product, reducing the particle size of the oversized granular DAP, and
 recycling the undersized granular DAP and the sized-reduced oversized
 granular DAP to the ammoniator-granulator. The invention involves the
 improvement of enhancing the ammoniacal nitrogen concentration of the
 granulated DAP recycle, hence the granular DAP product, by subjecting the
 finely-divided granular DAP recycle to anhydrous gaseous ammonia which is
 at a super atmospheric pressure and which is at a temperature sufficient
 to maintain the anhydrous ammonia in the gaseous state. The increase in
 the ammoniacal nitrogen concentration is a function of the absolute
 ammonia pressure, the initial moisture content of the granular DAP
 recycle, and the contact time of the ammonia with the granular DAP
 recycle. The enhancement of the ammoniacal nitrogen content of the DAP is
 also a function of the particle size of the granular DAP recycle.
 Recycle in the invention process can include dust from the granulator.
 The independent functions (variables) which provide increase in ammoniacal
 nitrogen concentration in the invention process can be represented with
 more particularity by the following regression equation:
EQU % Nitrogen Increase=0.355.times.nitial % Moisture+0.027.times.Contact Time
 (min.)+0.009.times.NH.sub.3 Pressure (psia)-0.240
 wherein the coefficients associated with the three independent functions
 (variables), i.e., the absolute ammonia pressure, the initial percent
 moisture and the contact time, can each vary plus or minus up to 50
 percent (and still be within the scope of the regression equation). The
 dependent variable "% Nitrogen Increase" represents the increase of the
 ammoniacal nitrogen concentration. The coefficients in the above
 regression equation are based on the combined data from Tables 6 and 10
 below. As mentioned above, the coefficients of the above regression
 equation can each vary plus or minus 50 percent, but note that such
 coefficients may vary an even greater magnitude, and still be within the
 scope of the above regression equation, for wet process phosphoric acid
 with a significantly different combination of impurities.
 Conducting the invention process according to the above regression formula
 provides DAP which has enhanced ammoniacal concentration and which is
 stable (i.e., as to the enhanced ammoniacal concentration) upon aging.
 The invention process provides fertilizer grade DAP, a product composed
 principally of diammonium phosphate, as defined in Official Publication
 No. 52 of the Association of American Plant Food Control Officials, dated
 1999. The preferred process enhances the ammoniacal nitrogen concentration
 of DAP above what can normally be obtained, with a given orthophosphoric
 acid quality, in the standard TVA style DAP plant using a preneutralizer
 and granulator. The additional ammoniacal nitrogen is obtained by
 producing and incorporating a variable amount of triammonium phosphate
 into the DAP granular product and/or adsorbing a variable amount of
 ammonia on the surface and in the pores of the granular DAP, and
 incorporating this material into the DAP product.
 The DAP recycle treated according to the invention process is finely
 divided. The particle size of the finely-divided DAP recycle is generally
 -9 mesh or less. The broad range for the particle size is smaller than
 6.83 mm (3 mesh Tyler sieve size) with no theoretical lower limit. The
 preferred particle size range is smaller than 2.00 mm (9 mesh Tyler sieve
 size) and larger than about 0.01 mm (10 microns).
 The ammonia gas pressure used is super atmospheric, with the ammonia gas
 pressure being preferably 15 to 100 psia (and more preferably 30 to 85
 psia). There are two basic reasons that the upper limit for the preferred
 ammoniation pressure is 100 pounds per square inch gauge (psig) or 114.7
 pounds per square inch absolute (psia). First, the codes (ASTM) regulating
 the fabrication of pressure vessels make a vessel rated for pressures up
 to 100 psig. Second, once the pressure gets significantly about 100 psig,
 parts of the process may require the addition of heat (during periods of
 extremely cold ambient temperatures) to avoid condensation of the ammonia.
 The theoretical upper limit to the ammoniation pressure range would be the
 critical pressure for ammonia (11 1.5 atmospheres absolute or about 1624
 pounds per square inch gauge pressure). If the ammonia temperature is less
 than or equal to the critical temperature for ammonia (405.5.degree.
 Kelvin or 270.68.degree. F.), ammonia cannot exist as a gas at or above
 the critical pressure. In functional language, this can be stated as the
 region in terms of pressure and temperature where ammonia exists in the
 gaseous state.
 The contact time of the pressurized gaseous ammonia with the DAP recycle
 should be sufficient to allow the required amount of triammonium phosphate
 to be formed by chemical reaction and/or the required amount of ammonia to
 be adsorbed. The contact time is broadly between about 10 seconds and 2
 hours, preferably 5 minutes to one hour.
 The temperature of the pressurized gaseous ammonia is generally between
 about 450 and about 200.degree. F. The broad temperature range for the
 ammonia is at least 32.degree. F. (0.degree. C.) and not more than
 250.degree. F. (121.1.degree. C.) when it enters the pressure reactor. The
 preferred temperature range is at least 55.4.degree. F. (13.degree. C.)
 and not more than 175.degree. F. (79.4.degree. C.) when it enters the
 pressure reactor.
 The initial moisture (H.sub.2 O) content of the DAP recycle is generally
 between about 0.2 to about 4 percent, preferably about 0.5 to 3.5 percent.
 One especially preferred method involves de-aerating the granular DAP
 recycle in a first sealed vessel by the application of vacuum. The
 de-aerated granular DAP recycle is subjected in a second sealed vessel to
 an atmosphere consisting of anhydrous gaseous ammonia which is at a
 pressure of at least 15 psia, for a time period sufficient for the
 required amount of triammonium phosphate to be formed and/or the required
 amount of ammonia to be adsorbed. The ammonia-treated granular DAP recycle
 is transferred to a third sealed vessel and the excess ammonia is removed
 by vacuum applied to the third vessel. The vacuum-treated, ammonia-treated
 granular DAP recycle is recycled to the ammoniator-granulator.
 Another especially preferred method involves placing the granular DAP
 recycle in a sealed vessel, de-aerating the granular DAP recycle by the
 application of vacuum to the sealed vessel, feeding anhydrous gaseous
 ammonia into the sealed vessel until a pressure of at least 15 psia is
 obtained in the sealed vessel, and keeping the granular DAP recycle in
 contact with the ammonia in the sealed vessel for a time period sufficient
 for the required amount of triammonium phosphate to be formed and/or the
 required amount of ammonia to be adsorbed. At the end of the time period,
 the excess ammonia is removed by vacuum applied to the sealed vessel. The
 vacuum-treated, ammonia-treated DAP recycle is recycled to the ammoniator
 granulator.
 Another method of operating the process includes introducing a portion of
 the DAP recycle into an entry lock vessel at atmospheric pressure (with no
 effort to remove the air that enters the vessel with the recycle). This
 vessel is then pressurized with gaseous ammonia such that the total
 pressure is essentially equal to the total pressure in a second reactor
 vessel, the gas in this second reactor vessel being a mixture containing
 no more than 15 percent air and no less than 85 percent ammonia. A valve
 connecting the entry lock vessel and reactor vessel is then opened and the
 DAP recycle is allowed to flow into the reactor vessel. The valve
 connecting the entry lock vessel and reactor vessel is then closed and the
 gas in the entry lock vessel is vented to another part of the process,
 such that the ammonia contained in the gas can be recovered in a useful
 manner. The primary objective of the venting step is to reduce the
 pressure in the entry lock vessel to atmospheric pressure, such that
 additional DAP recycle can be introduced into the entry lock vessel. The
 DAP recycle that is now in the reactor vessel is kept in the reactor
 vessel, in contact with the ammonia atmosphere at elevated pressure, for a
 period of time (i.e., the reaction time) sufficient to allow the required
 amount of triammonium phosphate to be formed by chemical reaction, and/or
 the required amount of ammonia to be adsorbed. While the DAP recycle is in
 the reactor vessel, an exit lock vessel is pressurized with ammonia, such
 that the total pressure in the exit lock vessel is essentially equal to
 the total pressure in the reactor vessel. At the end of the reaction time
 a valve connecting the reaction vessel with the exit lock vessel is opened
 and the ammonia enriched recycle is allowed to flow into the exit lock
 vessel. The valve connecting the reaction vessel with the exit lock vessel
 is then closed. At this time, additional DAP recycle can be introduced
 into the reaction vessel in the manner indicated above. After the valve
 connecting the reaction vessel with the exit lock vessel is closed, the
 gas in the exit lock vessel is vented to another part of the process, such
 that the ammonia contained in the gas can be recovered in a useful manner.
 The primary objective of the venting step is to reduce the pressure in the
 exit lock vessel to atmospheric pressure. A second valve on the exit lock
 vessel is then opened and the ammonia enriched recycle is removed from the
 exit lock vessel and transferred to the ammoniator granulator where it is
 incorporated into the DAP product.

DETAILED DESCRIPTION OF THE INVENTION
 The invention provides a process for increasing the amount of nitrogen in a
 diammonium phosphate composition. The result is fertilizer grade DAP, as
 defined in Official Publication No. 52 of the Association of American
 Plant Food Control Officials, dated 1999.
 The invention provides a method for producing DAP fertilizer with the lower
 quality phosphoric acid currently being produced, without using expensive
 nitrogen supplements. The invention process can also produce a higher
 analysis fertilizer with, for example, a phosphate concentration
 (expressed as percent P.sub.2 O.sub.5) of about 48 percent and an ammonia
 concentration (expressed as percent N) of about 20 percent.
 The purpose of enhancing such nitrogen concentration of the DAP is so that
 it can be used as nitrogen rich fertilizer without having to use expensive
 nitrogen supplements. The reacting of finely divided DAP with high
 pressure, anhydrous, gaseous ammonia enhances or increases the ammoniacal
 nitrogen concentration of the DAP. Such ammonia-treated DAP is not
 suitable as a nitrogen rich fertilizer because it lacks the necessary
 stability since as it ages it tends to lose significant amounts of the
 added nitrogen and to gain excessive moisture. These disadvantages are
 present even after aging for only two weeks.
 The flow path of a conventional TVA (Tennessee Valley Authority) process
 for the production of granular DAP is shown in FIG. 1. Phosphoric acid (30
 to 54 percent P.sub.2 O.sub.5) is fed into the top of the preneutralizer
 (reaction tank) 100 via lines 102 and 104. Water is fed into the top of
 the preneutralizer 100 via line 106. Ammonia is fed into the
 preneutralizer 100 via lines 108 and 110, and is normally conveyed to a
 point below the liquid level in the tank. The phosphoric acid is partially
 preneutralized by the ammonia in preneutralizer 100. The heat of reaction
 of ammonia and phosphoric acid is used to evaporate water in
 preneutralizer 100. Preneutralizer 100 is vented to the atmosphere via top
 lines 122 and 124 with water vapor coming off via line 126. Further
 phosphoric acid is fed into the upper portion of scrubber 112 via lines
 102 and 114. Water is fed into line 114 via line 116. The solution of the
 partially preneutralized phosphoric acid is fed from preneutralizer 100
 via line 130 to rotary ammoniator-granulator 132. Further ammonia is fed
 via lines 108 and 134 into ammoniator-granulator 132, wherein the
 ammoniation is completed. Excess ammonia is required in
 ammoniator-granulator 132 to produce DAP. The granular product from
 ammoniator-granulator 132 is fed via line 136 into dryer 138. Exhaust
 containing ammonia and water vapor from ammoniator-granulator 132 is fed
 into the bottom of scrubber 112 via line 120. Water vapor comes off of
 scrubber 112 via line 118. The ammonia is scrubbed out of the off gases
 from ammoniator-granulator 132 and the phosphoric acid is fed from
 scrubber 112 via line 128 into the top of preneutralizer 100. (As an
 alternative, the granular material from dryer 138 can go directly to
 screen 146, with only the product size DAP going through cooler 142.) The
 dried granular DAP is fed via line 140 into cooler 142. The granular DAP
 is fed via line 144 into screen 146. The sized granular DAP product is
 removed via line 156 from screen 146. The undersized granular DAP exits
 screen 146 via line 152. The oversized granular DAP is removed from screen
 146 and fed via line 148 into crusher 150. The crushed DAP exits from
 crusher 150 via line 154 and is mixed with the undersized granular DAP
 from line 152, to form a mixture of fines. The fines mixture is recycled
 via line 158 into the front end of ammoniator-granulator 132. Product size
 DAP can also be included in the recycle stream to aid in
 ammoniator-granulator operation.
 Advantages can be taken of the maximum solubility of the ammonia/phosphoric
 acid mole ratio of about 1.45; therefore, the preneutralizer 100 is
 preferably operated at as near this point as is practical to obtain the
 most concentrated slurry having satisfactory fluidity. This slurry can
 either flow by gravity into a sawtooth weir pipe (not shown) for
 distribution in ammoniator-granulator 132, or be pumped into a sparged
 spray system (not shown) located over the moving bed (not shown) of dry
 recycle inside of ammoniator-granulator 132. The later procedure provides
 more consistent control and better slurry distribution. Ammoniation of the
 slurry in the ammoniator-granulator drum (not shown) to a mole ratio of
 about 2.0 lowers the solubility and causes crystallization of DAP. The
 heat of reaction between the monoammonium phosphate in the slurry and the
 ammonia causes the majority of the water present to vaporize and exit with
 the granulator off gases.
 Broadly speaking, a preferred embodiment of the invention process involves
 treating solid DAP recycle (in a TVA type operation) with gaseous ammonia
 at elevated pressure to increase the ammoniacal nitrogen concentration.
 This recycle is then returned to the DAP granulator and incorporated into
 the final DAP product.
 Preferably, one method of producing the triammonium phosphate or adsorbing
 the ammonia involves, first, introducing a portion of the DAP recycle
 material, normally present from the TVA process, into a vessel under
 vacuum, such that any air introduced with the recycle is removed. The DAP
 recycle material is then transferred to a second vessel in which there is
 an essentially 100 percent ammonia atmosphere at a pressure at or above 15
 psia and a temperature sufficiently high to insure that the ammonia exists
 in the gaseous state. The DAP recycle material is kept in this second
 vessel, in contact with the ammonia atmosphere, for a period of time
 sufficient to allow the required amount of triammonium phosphate to be
 formed by chemical reaction, and/or the required amount of ammonia to be
 adsorbed. The DAP recycle material is then transferred to a third vessel
 where the excess ammonia is removed by vacuum, or vented to another part
 of the process, such as, the preneutralizer or scrubber. The
 ammonia-enriched DAP recycle is then transferred to the granulator where
 it is incorporated into the DAP product.
 Preferably, another method for the production of triammonium phosphate
 and/or adsorption of ammonia is carried out in a single vessel as a batch
 process. In this case, the DAP recycle is placed in the vessel and the
 vessel is sealed. A vacuum is then applied to the vessel to remove the
 air. Anhydrous gaseous ammonia is then fed to the vessel until a pressure
 at or above 15 psia is obtained. The DAP recycle is then allowed to remain
 in the vessel, in contact with the ammonia, until the required amount of
 triammonium phosphate is formed and/or the required amount of ammonia is
 adsorbed. A vacuum is then applied to the vessel to remove the excess
 ammonia. The vessel is then opened and the ammonia-enriched DAP recycle is
 removed and transferred to the granulator where it is incorporated into
 the DAP product.
 The particle size of the finely-divided DAP recycle is generally -9 mesh or
 less.
 The ammonia gas pressure used is super atmospheric, with the ammonia gas
 pressure preferably being at least 15 psia and more preferably 30 to 95
 psia.
 The contact time of the pressurized gaseous ammonia with the DAP recycle
 should be sufficient to allow the required amount of triammonium phosphate
 to be formed by chemical reaction and/or the required amount of ammonia to
 be adsorbed. The contact time is generally between about 0.25 and about 90
 minutes, preferably 5 to 60 minutes.
 The initial moisture (H.sub.2 O) content of the DAP recycle is generally
 between about 0.2 to about 4 percent, preferably about 0.5 to 3.5 percent.
 The temperature of the pressurized gaseous ammonia can be as low as
 33.degree. F., but is generally between about 45.degree. and about
 200.degree. F.
 The independent functions (variables) which provide increase in ammoniacal
 nitrogen concentration in the invention process can be represented with
 more particularity by the following regression equation:
EQU % Nitrogen Increase=0.355.times.Initial % Moisture+0.0027.times.Contact
 Time (min.)+0.009.times.NH.sub.3 Pressure (psia)-0.240
 wherein the coefficients associated with the three independent functions
 (variables), i.e., the absolute ammonia pressure, the initial percent
 moisture and the contact time, can each vary plus or minus up to 50
 percent (and still be within the scope of the regression equation). The
 dependent variable "% Nitrogen Increase" represents the increase of the
 ammoniacal nitrogen concentration. The coefficients in the above
 regression equation are based upon the combined data from Tables 6 and 10
 below. As mentioned above, the coefficients of the above regression
 equation can each vary plus or minus 50 percent, but note that such
 coefficients may vary an even greater magnitude, and still be within the
 scope of the above regression equation, for wet process phosphoric acid
 with a significantly different combination of impurities.
 Conducting the invention process according to the above regression formula
 provides DAP which has enhanced ammoniacal concentration and which is
 stable (i.e., as to the enhanced ammoniacal concentration), when recycled
 to the granulator and incorporated into the DAP product, upon aging.
 The increase in the ammoniacal nitrogen concentration is also a function of
 the particle size of the granular DAP (recycle). Specifically, the smaller
 the particle size, the larger the increase in the percent nitrogen
 increase. The effect of the particle size is usually of small practical
 effect because the treated DAP (recycle) usually contains a relatively low
 amount of very small particles. The invention process includes treating
 recycle from a conventional TVA process which is composed of DAP dust, DAP
 undersized recycle material and crushed DAP oversize recycle material.
 As used herein, all percentages are on a weight basis unless otherwise
 stated herein or obvious herefrom to one skilled in the art.
 DAP is a product composed of ammonium phosphate, principally diammonium
 phosphate, resulting from the ammoniation of phosphoric acid. Diammonium
 phosphate is a chemical compound having the formula (NH.sub.4).sub.2
 HPO.sub.4.
 The following examples serve to further illustrate the invention in greater
 detail.
 In the following examples, the pressure ammoniation procedure of the
 (recycle) DAP was as follows:
 (a) The weighed quantity of (recycle) DAP was placed in a pressure vessel,
 which was then sealed.
 (b) A vacuum of 24 to 25" Hg was then applied to the sealed vessel to
 remove the air.
 (c) Gaseous ammonia was added to the sealed vessel until the desired
 pressure was obtained.
 (d) The (recycle) DAP was held in the sealed vessel for the required time,
 with additional ammonia being added to maintain the desired pressure (that
 is, to replace the ammonia consumed in the reaction).
 (e) A vacuum of 24 to 25" Hg was again applied to the sealed vessel to
 remove the unreacted ammonia.
 (f) Air was bled into the vessel to relieve the vacuum.
 (g) A vacuum of 24 to 25" Hg was applied to the vessel again to help remove
 the last traces of ammonia.
 (h) Air was bled into the vessel to relieve the vacuum.
 (i) The vessel was opened and the ammoniated (recycle) DAP was removed and
 weighed.
 EXAMPLE A
 Small particles of (granular) DAP (produced by a conventional TVA process),
 comprising 17.82 percent nitrogen and 46.70 percent phosphate, having a
 N/P mole rate of 1.93:1 and having a particle size between -4 mesh and +14
 mesh, were exposed to ammonia pressure of 15, 30 or45 psia for 30, 45 or
 60 minutes as set forth in Table 1 below. There were nine test samples.
 The above-described ammoniation procedure was used. Each of the nine DAP
 samples was separately placed in the pressure reactor and the pressure
 reactor was sealed. A vacuum was pulled on the sealed reactor to remove
 the air. The reactor was then filled with ammonia and pressurized to the
 desired test pressure. As necessary, additional ammonia was bled into the
 reactor to maintain the pressure. At the end of the specified contact time
 a vacuum was again applied to the reactor to remove the unreacted ammonia.
 Air was then bled into the reactor, the reactor was opened and the
 ammoniated DAP sample was removed. The results of these tests (high
 pressure ammonia treatment of DAP product) are set forth in Table 1 below.
 The results showed increases in the ammoniacal nitrogen concentration from
 0.48 to 1.0 percent, that were generally proportional to the ammonia
 pressure and contact time.
 TABLE 1
 Test Contact Ammonia
 Sample Time, Pressure, N, P.sub.2 O.sub.5, Molar % N
 No. min. psia % % Ratio Increase
 A-1 30 15 18.30 46.40 2.00 0.48
 A-2 45 15 18.30 46.51 1.99 0.48
 A-3 60 15 18.20 46.44 1.99 0.38
 A-4 30 30 18.36 46.41 2.00 0.54
 A-5 45 30 18.60 46.20 2.04 0.78
 A-6 60 30 18.47 46.09 2.03 0.65
 A-7 30 45 18.68 46.29 2.04 0.86
 A-8 45 45 18.82 46.09 2.07 1.00
 A-9 60 45 18.82 46.04 2.07 1.00
 Test Sample Nos. A-2, A-5 and A-8, as well as a control, were aged for two
 weeks to determine if the nitrogen gains were stable. [The control was
 small particles of (granular) DAP which had not been ammoniated.] A
 portion of each of the samples and the control was aged in a sealed
 container; and a portion of each of the samples and the control was aged
 in an open pan. The results from these tests are set forth in Table 2:
 TABLE 2
 Sealed Container Open Pan
 Test Ammonia Vac. Dry Vac.
 Dry Relative
 Sample Pressure, N, P.sub.2 O.sub.5, Oven Basis Mole N,
 P.sub.2 O.sub.5, Oven Basis Mole N Loss,
 No. psia % % Moist. % N Ratio % % Moist. %
 N Ratio %
 Control 17.95 47.03 0.83 18.10 1.93 17.23 47.44 1.96
 17.57 1.84 2.90
 (Duplicate 17.68 47.34
 18.03 1.89 0.37
 Analy.)
 A-2 15 18.32 46.6 0.45 18.40 1.99 16.98 44.83 6.80
 18.22 1.92 1.00
 A-5 30 18.53 46.34 0.68 18.66 2.03 16.19 42.66 9.57
 17.90 1.92 4.04
 (Duplicate 18.43 46.28 18.56 2.02
 3.52
 Analy.)
 A-8 45 19.02 46.02 1.43 19.30 2.09 16.27 42.09 10.78
 18.24 1.96 5.49
 (Duplicate 15.87 41.03
 17.79 1.96 7.82
 Analy.)
 Note: The relative nitrogen loss (percent) is based upon the dry basis
 percent nitrogen of the portion of the sample aged in the sealed
 container.
 The results in Table 2 of the aging test showed significant moisture gains
 and nitrogen losses for all of the treated samples. Thus, simply reacting
 the DAP product with high pressure ammonia would not be practical since
 such treated product tended to lose the added nitrogen and to gain
 excessive moisture.
 A particle size analysis for the starting DAP in this experiment was not
 run, however, since the starting DAP was obtained from a DAP production
 plant, a "typical" particle size distribution could be inferred. An
 example of a typical (30 day rolling average) particle size distribution
 is shown below:

Tyler 4 5 6 8 9 10 14
 Sieve Size
 Cumulative Percent 1.2 4.5 10.7 56.8 77.1 89.4 98.8
 Retained
 Standard Deviation 0.80 1.8 3.7 11.9 10.3 7.2 1.7
 Also, it should be noted that this material was product size DAP, not
 recycle. The recycle consists of undersize material and crushed oversize
 with typically about 97 percent being smaller than 9 mesh.
 EXAMPLE B
 DAP dust, from the dryer in a conventional TVA process, was exposed to a
 100 percent ammonia atmosphere at pressures from 15 psia to 90 psia. The
 DAP dust used in these tests was not mixed with any other DAP material.
 The size distribution of the DAP dust is shown below: