Production of urea phosphate

A two-stage continuous crystallization process for production of urea phosphate by reaction of impure wet-process orthophosphoric acid (about 54 percent P.sub.2 O.sub.5) and urea with simultaneous addition of a selected acidifying agent (sulfuric acid, hydrochloric acid, or phosphoric acid) to clarified mother liquor used as recycle in the process. Addition of the acidifying agent decreases pH in the crystallization process whereby the solubility of a contaminating water-insoluble iron phosphate-urea salt [FeH.sub.3 (PO.sub.4).sub.2.2CO(NH.sub.2).sub.2 ] is increased, purity of the crystalline urea phosphate product is improved significantly, and the useful storage life of the recycle mother liquor is prolonged.

INTRODUCTION 
Our invention relates to an improvement in method for production of urea 
phosphate. It relates, more particularly, to a two-stage continuous 
crystallization process for the production of urea phosphate by the 
reaction of impure merchant-grade wet-process orthophosphoric acid (about 
54 percent P.sub.2 O.sub.5) with urea; and more particularly to the 
addition of acidifying agents to decrease pH in the aforementioned 
crystallization process whereby the solubility of a contaminating 
precipitate in the recycle mother liquor is increased and the purity of 
the urea phosphate product is significantly improved. This contaminating 
phase has been identified and characterized by us and is a new compound 
consisting of an iron phosphate-urea salt to which we have assigned the 
formula FeH.sub.3 (PO.sub.4).sub.2.2CO(NH.sub.2).sub.2. Still more 
particularly our invention relates to the aforementioned crystallization 
process in which the addition of acidifying agents prolong the useful 
storage life of the recycle mother liquor. 
BACKGROUND OF THE INVENTION 
1. Field of the Invention 
Urea phosphate [CO(NH.sub.2).sub.2.H.sub.3 PO.sub.4 ] is a dry, white 
crystalline material that melts at 243.5.degree. F. It contains 17.7 
percent nitrogen and 19.6 percent phosphorus which is equivalent to 44.9 
percent P.sub.2 O.sub.5. Urea phosphate is acidic (pH of 1 percent 
solution, 1.8), very soluble in water, and has a specific gravity of 
1.759. The critical humidity of the material is 75 percent to 80 percent 
at 86.degree. F. and is similar to urea. It is well known that urea 
phosphate is a finished fertilizer. Agronomic tests of the material have 
shown that it is an efficient source of nitrogen and phosphate for plant 
growth. Numerous investigators of the prior art have proposed the use of 
urea phosphate as an intermediate in the production of solid and liquid 
fertilizers containing polyphosphate. 
Wet-process orthophosphoric acid is prepared by the acidulation of 
phosphate rock (maInly fluoroapatite) with sulfuric acid and separating 
the resulting calcium sulfate from the acidulate. The wet-process acid 
contains impurities from the phosphate rock such as iron, aluminum, 
magnesium, fluorine and calcium. In addition, unless the phosphate rock is 
calcined before it is acidulated, the acid contains dissolved and 
suspended carbonaceous matter which imparts a black, opaque color to the 
acid. It is difficult to make high quality liquids from wet-process acid 
containing all of its impurities. Methods for separating impurities from 
wet-process acid have been described by numerous investigators. Most of 
these methods are solvent extraction processes. Use of solvent extraction 
techniques require large quantities of expensive solvents which must be 
recovered. Also, the solvents only partially eliminate metallic impurities 
and sulfate from the acids. In the urea phosphate crystallization process, 
most of the impurities are separated from the phosphoric acid by reacting 
the acid with urea and crystallizing the adduct, urea phosphate, from the 
solution; most of the impurities from the acid remain in the mother 
liquor. The reaction of urea and phosphoric acid to form urea phosphate is 
shown by the following equation: 
EQU CO(NH.sub.2).sub.2 +H.sub.3 PO.sub.4 .fwdarw.CO(NH.sub.2).sub.2.H.sub.3 
PO.sub.4 
The urea phosphate is then crystallized from the solution. The 
urea-phosphoric acid reaction and the crystallization of urea phosphate 
are both exothermic. The enthalpy of producing crystalline urea phosphate 
from equimolar amounts of urea and phosphoric acid (75 percent H.sub.3 
PO.sub.4) at 77.degree. F. is -44 calories per gram. To produce solutions 
containing polyphosphate, the solid urea phosphate first is heated, either 
by external means or by chemical heat of ammoniation, to form 
urea-ammonium polyphosphate melt. This melt then is used to produce 
relatively pure solution fertilizers. Procedures similar to those involved 
in this process have been reported in the literature. 
Description of the Prior Art 
Heretofore, the reaction of phosphoric acid with urea has been studied by a 
number of investigators, beginning, we believe, with the work described in 
a German Patent (No. 286,491) granted to Badische Anilin and Soda-Fabrik 
in 1914. In this work, one mole of urea was reacted with one mole of 
phosphoric acid (50 percent). The solution was cooled to crystallize urea 
phosphate and the crystals were separated from the mother liquor. The 
patent claims the use of urea phosphate as a fertilizer. Clarkson and 
Braham (U.S. Pat. No. 1,440,056) added one mole of urea/mole H.sub.3 
PO.sub.4 to a solution containing 55-75 percent H.sub.3 PO.sub.4 and 
separated the resulting crystals of urea phosphate from the solution. 
According to the disclosure, evaporation of water from urea-phosphate 
solutions should be carried out at low temperatures since decomposition 
becomes rapid above 194.degree. F. 
Somewhat more recently, Keens (British Pat. No. 1,149,924) proposed 
production of urea phosphate continuously in a vacuum crystallization 
process. The process utilized unconcentrated phosphoric acid and urea 
solution as feed material and involved heating and evaporation of the 
solution to yield a supersaturated solution of urea phosphate which was 
contacted with a suspension of growing urea phosphate crystals in mother 
liquor. The mother liquor was recycled to the feed solution. The final 
product crystals consisted of larger crystals which were more suitable for 
the manufacture of fertilizers. Mansfield (German Offen. Pat. No. 
2,322,114) prepared urea phosphate by passing 90 percent H.sub.3 PO.sub.4 
at 95.degree. F. and 90 percent urea at 239.degree. F. into a tube of 6 m 
length and 5.degree. inclination with countercurrent passage of warmed 
air. By this method, urea phosphate of &lt;0.7 percent moisture cohtent at 
.about.95.degree. F. was obtained. Greidinger and Cytter (German Offen. 
No. 2,429,030) describe a process for the manufacture of urea phosphate, 
useful as a fertilizer, by reaction of urea with anhydrous H.sub.3 
PO.sub.4 optionally with the addition of Mg, CO, Fe, Zn, Cu or Mn trace 
elements. 
In still another patent, Koebner, Edwards and Williams (British No. 
1,191,635) prepared urea phosphate as an intermediate from wet-process 
H.sub.3 PO.sub.4 and treated the urea phosphate with an alkali metal or 
ammonium hydroxide or carbonate to produce orthophosphate and regenerate 
urea. The orthophosphate is separated as product and the urea is recycled. 
The reaction is preferably carried out in recycled mother liquor. 
Methods of utilization of Vrea phosphate as the starting material for 
production of ammonium polyphosphate-type fertilizers have been proposed. 
These processes involve thermal decomposition of urea phosphate and 
utilize the condensing action of urea in urea phosphate to form 
polyphosphates. Among those is one method described by Theobald (German 
Patent No. 2,308,408) who proposes a two-stage process where urea 
phosphate is melted in the first stage and pyrolyzed into polyphosphate in 
the second stage. 
In U.S. Pat. No. 3,713,802, Gittenait utilizes urea phosphate as the 
starting material for producing liquid and solid urea-ammonium 
polyphosphate. In the production of urea phosphate, unpurified wet-process 
phosphoric acid containing 30-60 percent P.sub.2 O.sub.5 is reacted 
directly with urea (solid or solution). Mother liquor is added to increase 
fluidity. The urea phosphate is crystallized out after one hour, for 
example, and removed from the mother liquor by centrifuging. Most of the 
mineral impurities accompanying the wet-process acid remain in the mother 
liquor. Stinson, Mann, and McCullough (U.S. Pat. No. 4,217,128) describe a 
process for production of urea-ammonium polyphosphates by pyrolysis of 
crystalline urea phosphate in one stage. Molten urea-ammonium 
polyphosphates that contain up to 95 percent of the phosphate as 
polyphosphate are obtained. These are then processed into high-analysis 
solid or liquid fertilizers. Addition of urea to the process to maintain a 
urea:biuret ratio of at least 16 prevents precipitation of biuret in the 
liquid fertilizers. 
Kozo Fukuba (Japanese Pat. No. 49-8498) describes a method of production of 
a highly purified, water-insoluble ammonium polyphosphate. The process 
involves calcination of dried urea phosphate, under an ammonia atmosphere, 
at 390.degree. F. to 570.degree. F. 
In the production of the intermediate urea phosphate by Fukuba, wet-process 
acid (29 percent P.sub.2 O.sub.5, as per example, produced by 
decomposition of Morrocan rock with sulfuric acid) was treated batchwise 
with caustic soda or sodium carbonate to remove some of the fluorine and 
silica; a large proportion of the organic materials in the acid are 
removed by using activated carbon and the acid is then concentrated to 45 
to 55 percent by weight P.sub.2 O.sub.5 content. Urea is reacted with the 
concentrated wet-process acid and recycle mother liquor is added for 
fluidity. The mole ratio of urea to H.sub.3 PO.sub.4 in the feed is 0.9 to 
1.5. The reaction to yield urea phosphate is carried out at about 
120.degree. l F. to 160.degree. F. The reaction mixture is cooled to about 
40.degree.0 F. to 86.degree. F. and crystalline urea phosphate is 
separated from the mother liquor. Fukuba's patent is characterized by the 
production of ammonium polyphosphate and the recycling of the mother 
liquor in the urea phosphate production system. Also, Fukuba discloses 
that addition of sulfuric acid increases yield of urea phosphate produced 
by the reaction of wet-process acid and urea. 
SUMMARY OF THE INVENTION 
Our invention is directed toward an improvement in processes for the 
production of urea phosphate. It, unlike the prior art referred to supra, 
involves the addition of acidifying agents such as phosphoric acid, 
sulfuric acid or hydrochloric acid in a two-stage continuous 
crystallization process for production of urea phosphate by the reaction 
of impure wet-process acid (about 54 percent P.sub.2 O.sub.5) with urea 
whereby: (1) pH in the crystallization process is decreased, (2) the 
solubility of the contaminating water-insoluble iron phosphate-urea salt, 
which normally precipitates in the mother liquor, is increased, (3) the 
purity of the crystalline urea phosphate product is improved 
significantly, and (4) the useful storage life of the recycle mother 
liquor is prolonged. In essence, these improvements, referred to supra, 
constitute the novelty of our invention. 
We have found that in our two-stage continuous crystallization process for 
the production of urea phosphate from impure merchant-grade wet-process 
acid and urea, a buildup of impurities occurs in the mother liquor used as 
recycle in the process. Recycle mother liquor is added to decrease 
suspension density and increase fluidity of the urea phosphate in each 
stage. Mother liquor also aids in separation in the centrifuge of product 
crystals from fine solid impurities containing iron, aluminum, magnesium, 
fluorine, and other impurities originally present in the feed wet-process 
acid. The product crystals in our continuous crystallization process 
contain only about 10 to 15 of these objectionable impurities and about 80 
percent of the acid P.sub.2 O.sub.5 and urea. The remainder of the 
phosphate and urea, and essentially all of the water and particulate 
carbonaceous material leave the process as byproduct mother liquor. The 
impurity level in the recycle mother liquor increases until steady state 
conditions are reached. In our production work with impure merchant-grade 
acids, Florida black acid derived from uncalcined phosphate rock and North 
Carolina green acid from calcined phosphate rock as examples, we found 
that at near steady state conditions the impurity level (impurity: P.sub.2 
O.sub.5 ratio) of the recycle mother liquor was about 3.5 times greater 
than in the feed wet-process acid; the Fe.sub.2 O.sub.3 content in the 
recycle mother liquor was in the range of 1.5 to 2.0 percent. We have 
discovered through our experiments that fine-grained crystals of the 
compound FeH.sub.3 (PO.sub.4).sub. 2.2CO(NH.sub.2).sub.2 precipitate when 
the iron content of the recycle mother liquor increases to approximately 
0.7 percent iron (1.00 percent Fe.sub.2 O.sub.3). Thus, it is readily 
apparent that in a continuous crystallization process precipitation of the 
iron phosphate-urea contaminant will occur in the recycle mother liquor 
before steady state conditions are reached. The presence of this 
contaminating compound is objectionable and limits the effectiveness of 
the process. Petrographic analysis indicates that it is present on the 
surface of the urea phosphate product crystals and is trapped between the 
crystals in the centrifuging step of the process to separate mother liquor 
from the urea phosphate crystals. The presence of the iron phosphate-urea 
salt leads to "blinding" in the centrifuge causing retention of an 
excessive amount of mother liquor and fine particulate carbonaceous 
material which results in decrease in product purity and loss of 
production time used to wash out these impurities from the screen or 
filter media in the centrifuge. 
OBJECTS OF THE INVENTION 
It is therefore an object of the present invention to produce urea 
phosphate in a two-stage continuous crystallization process by the 
reaction of impure merchant-grade wet-process acid with urea and 
simultaneous addition of an acidifying agent (H.sub.2 SO.sub.4, H.sub.3 
PO.sub.4, or HCl) to the recycle mother liquor wherein: (1) pH in the 
process [pH in the urea phosphate slurry in the second stage 
(crystallizer)] is decreased; (2) solubility of the contaminating iron 
phosphate-urea salt [FeH.sub.3 (PO.sub.4).sub.2.2CO(NH.sub.2).sub.2 ] is 
increased and its precipitation essentially eliminated; and (3) the 
average purification level of the urea phosphate product (reduction of 
impurity level calculated by comparing aluminum to P.sub.2 O.sub.5, iron 
to P.sub.2 O.sub.5, magnesium to P.sub.2 O.sub.5, and fluorine to P.sub.2 
O.sub.5 ratios of the product with those of the feed acid) is increased. 
A further object of the present invention is to prolong the storage life of 
urea-ammonium polyphosphate liquid fertilizers obtained by pyrolysis of 
urea phosphate followed by addition of water and ammonia gas. 
A still further object of the present invention is to prolong the useful 
storage life of the recycle mother liquor and thus increase the 
effectiveness of the process. 
Another object of the present invention and advantage of adding an 
acidifying agent (H.sub.2 SO.sub.4, for example) is that extra heat would 
be generated for evaporation of water during ammoniation of the byproduct 
mother liquor for conversion to a solid or granulated fertilizer product. 
Still another advantage of the present invention of adding sulfuric acid in 
the process is that the solid or suspension fertilizer made from the 
byproduct mother liquor will contain sufficient sulfur for supplying the 
needs of sulfur deficient soils which exist in about 60 percent of the 
farmland in the United States. 
Still further and more general objects and advantages of the present 
invention will appear from the more detailed description set forth below, 
it being understood, however, that this more detailed description is given 
by way of illustration and explanation only and not necessarily by way of 
limitation since various changes therein may be made by those skilled in 
the art without departing from the true spirit and scope of the present 
invention.

Referring now more specifically to FIG. 1, urea melt 
(275.degree.-285.degree. F.) from the concentrator of a urea production 
plant (not shown) or solid urea, source also not shown, is fed via line 1 
and means for control of feed rate 2 to first-stage reactor-crystallizer 
3. Merchant-grade wet-process orthophosphoric acid (54 percent P.sub.2 
O.sub.5) from a source not shown is fed to first stage 3 via line 4 and 
means of control 5. Simultaneously, clarified recycle mother liquor is 
mixed with an acidifying agent (H.sub.2 SO.sub.4, H.sub.3 PO.sub.4, or 
HCl) and fed to first stage 3. The clarified recycle mother liquor is fed 
to first stage 3 from centrifuge 6 via line 7 and means of control 8. The 
acidifying agent is fed from a source not shown via line 9 and means of 
control 10 to line 7 where it is introduced with the recycle mother liquor 
to first stage 3. The acidifying agent may be fed at a rate to maintain a 
predetermined percent by weight of the agent in the recycle mother liquor 
to give the desired pH in the second stage of the process. First-stage 
reactor-crystallizer 3 is cooled below saturation temperature of the 
solution; thus some crystallization occurs therein. The temperature of 
first-stage reactor-crystallizer 3 is controlled by any suitable means 
such as is shown in FIG. 2. Referring again to FIG. 1, cooling is 
illustrated by circulating the reaction mixture in first-stage 
reactor-crystallizer 3 via line 11 and means of control 12 through cooler 
13 with return to first-stage reactor-crystallizer 3 via line 14. Urea 
phosphate slurry from first-stage reactor-crystallizer 3 overflows to 
second-stage crystallizer 16 via line 15. Crystallization is completed in 
second-stage crystallizer 16 where the urea phosphate slurry is further 
cooled to the desired temperature (for example 68.degree. F., which gives 
a high recovery of urea phosphate and is close to ambient temperature) by 
circulation via line 17 and means of control 18 through cooler 19 with 
return via line 20 to second-stage crystallizer 16. The two stages that 
are used for the process give better control of crystallizing conditions 
and growth of larger crystals. Also, when urea melt is used, two stages 
are necessary to prevent excessive nucleation caused by the large 
temperature difference between the melt (275.degree.-285.degree. F.) and 
the desired crystallizing temperature in the second stage (for example, 
68.degree. F.). For a urea phosphate product of high purity relatively 
large crystals are necessary. The weight ratio of recycle mother liquor to 
feed acid and retention time requirements depend upon the degree of 
product purity desired and increase as the impurity content of the feed 
acid increases. Urea phosphate slurry is withdrawn from second-stage 
crystallizer 16 via line 21 and product crystals are separated from mother 
liquor in centrifuge 22. The product crystals, 17-44-0 in grade, are 
discharged from centrifuge 22 to a storage area not shown via line 23. 
Mother liquor from centrifuge 22 is discharged via line 24 and means of 
control 25 to surge and settling tank 26. Part of the mother liquor is 
used as recycle to first-stage reactor-crystallizer 3 after settling in 
surge tank 26 and discharge via line 27 and means of control 28 to 
centrifuge 6 where essentially all solid impurities are removed. The 
remainder of the mother liquor containing the major portion of the 
impurities that were originally present in the wet-process acid leaves the 
process as byproduct mother liquor via line 29 and means of control 30 and 
via line 31. The byproduct liquor is suitable for processing into 
suspension or granular fertilizers. 
Referring now more specifically to FIG. 2, there is shown a diagram of the 
equipment we used on tests of the scale smaller than that of a commercial 
plant and of a size generally referred to as bench scale. The equipment 
consisted of a mechanical feeder for metering urea, urea melter (FIG. 3), 
reactor-crystallizer for the first stage, crystallizer for the second 
stage, cooling and stirring mechanism for each stage, metering pumps and 
feed tanks. The reactor-crystallizer and crystallizer were of the 
mixed-bed type. The second stage was equipped with a meter for measuring 
pH. A batch-type centrifuge (basket 11 inches in diameter by 3.6 inches 
deep) was used for separating product crystals and mother liquor. A surge 
and settling tank plus a batch-type centrifuge were used for clarifying 
the recycle mother liquor. As may be seen in FIG. 2, urea prills 
(unconditioned, 46 percent N) are fed through a feeder to the first-stage 
reactor-crystallizer. Simultaneously, wet-process acid (54 percent P.sub.2 
O.sub.5) and recycle mother liquor, to which an acidifying agent was added 
batchwise prior to a test, are metered to the first stage. When urea melt 
instead of urea prills was used, the prills were fed to a melter, as 
illustrated in FIG. 3, and then to the first stage. The urea melt (about 
99 percent urea) simulated urea direct from the concentrator of a urea 
production plant. The melter was maintained at about 290.degree. F., which 
is about 20 degrees higher than the melting point of urea. The process 
operated satisfactorily with urea fed as hot melt, but some decomposition 
of urea occurred in the melter. This condition would not be present in a 
full-scale production plant. Again referring to FIG. 2, the first stage is 
cooled with a water jacket below saturation temperature of the solution, 
thus some crystallization occurs therein. The first stage was equipped 
with an "eggbeater" type foam breaker and a turbine-type agitator with 
slanted vanes. A stirring rate of about 2-5 feet per second, tip speed, 
was adequate for suspending the crystals. As may be seen, the crystals and 
mother liquor were discharged to the second stage crystallizer through an 
overflow tube; the tube was positioned about 3/4-inch off the bottom of 
the first stage. Crystallization is completed in the second stage by 
cooling to 68.degree. F. Coolant, refrigerated to maintain 68.degree. F. 
in the second stage, was circulated through an external jacket. The 
turbine-type agitator was operated at propeller tip speed of about 2-5 
feet per second which allowed drawoff of the largest crystals together 
with some of the medium size and smaller crystals. Excessive stirring 
rates were not used because they cause abrasion and cleavage of crystals 
and are detrimental to crystal growth. Product slurry, equivalent to about 
8.5 percent of the total volume of the second stage, is withdrawn at 
regular intervals. The intervals at 2.5, 5, or 10 minutes varied with 
retention times of 0.5, 1.0, or 2.0 hours, respectively. The urea 
phosphate crystals are separated from mother liquor in a batch-type 
centrifuge. Product crystals were saved for chemical analyses after near 
steady-state conditions were established by allowing sufficient production 
time for the second stage to be filled and emptied about five times. The 
size (length) of product crystals was determined by microscopic 
examination which was adequate for obtaining the average size of the bulk 
amount of the crystals. A polypropylene screen was used that would allow 
essentially no crystals to pass through the screening surface as 
undersize. Screens ranged from 150 to 42 mesh. Rate of addition of urea 
phosphate slurry to the centrifuge (about 120 cm.sup.3 /s) was such that 
the crystals were uniformly distributed on the screen. The method utilized 
more surface area and prevented blinding of screen openings with 
carbonaceous material. Again, as may be seen in FIG. 2, part of the mother 
liquor that is separated from the urea phosphate crystals is used as 
recycle to the first stage after clarification by settling in a surge tank 
and centrifuging to remove a high proportion of solid impurities. The 
remainder of the mother liquor is drawn off as byproduct (9-22-0 grade). 
The production rate of urea phosphate in our bench-scale equipment ranged 
from about one to seven pounds per hour and varied with retention time in 
the second stage and with weight ratio of mother liquor to feed acid. 
DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
1 Prior Disclosure of Earlier Related Work 
Prior to the instant invention and in our earlier work, we studied the 
effect of process variables on crystal size and product purity. High 
recycle rate and long retention time favor product quality; however, each 
increases size of equipment. A balance must be struck between size of 
equipment and product quality. Wet-process acids derived from uncalcined 
Florida phosphate ore and calcined North Carolina ore were tested in the 
process in our earlier work. The composition of the acids is listed below. 
______________________________________ 
Source 
Florida acid North Carolina acid 
Composition, % 
(Uncalcined rock) 
(Calcined rock) 
______________________________________ 
P.sub.2 O.sub.5 
52.5-54.0 52.8-53.6 
Al.sub.2 O.sub.3 
1.1-1.8 0.5-0.6 
Fe.sub.2 O.sub.3 
1.0-1.6 1.1-1.4 
MgO 0.5-0.7 0.9-1.0 
F 0.8-1.1 0.3-0.5 
SO.sub.4 2.0-4.1 2.3-3.6 
CaO Trace- 0.5 .sup. 
Trace 
C 0.23-0.34 &lt;0.01 
W.I. solids 
0.7-1.8 0.9-1.0 
______________________________________ 
The composition of the recycle mother liquors that we used in tests with 
these Florida and North Carolina acids corresponded to near steady-state 
conditions and is listed below. 
______________________________________ 
Recycle mother liquor made from 
Florida acid North Carolina acid 
Composition, % 
(Uncalcined rock) 
(Calcined rock) 
______________________________________ 
Total N.sup.a 
9.0-9.2 9.0-9.3 
Urea N 8.7-8.9 8.9-9.2 
P.sub.2 O.sub.5 
22.1-22.6 22.4-22.5 
Al.sub.2 O.sub.3 
2.2-3.0 0.9-1.0 
Fe.sub.2 O.sub.3 
1.5-2.0 1.8-2.0 
MgO 0.9-1.1 1.8-2.0 
F 0.9-1.6 0.4-0.6 
SO.sub.4 4.0-7.5 4.0-7.1 
______________________________________ 
.sup.a Total N includes urea N, ammonia N (0.1 to 0.2 percent). and biure 
N (0.05 to 0.1 percent) with urea fed to firststage reactorcrystallizer a 
prills. 
The weight ratio of the average impurity level of mother liquor:acid in the 
mother liquors made from the Florida and North Carolina acids, supra, was 
in the range of 3.3 to 3.8. Impurity level was calculated by comparing 
impurity: P.sub.2 O.sub.5 ratio in the mother liquor to that in the feed 
acid, including SO.sub.4. 
The conditions which we consider to be optimum for crystallization of urea 
phosphate in our bench-scale equipment are reported in TVA Bulletin Y-136 
("New Developments in Fertilizer Technology," 12th Demonstration, National 
Fertilizer Development Center, Muscle Shoals, Ala., 17-20, Oct. 18-19, 
______________________________________ 
First-stage reactor-crystallizer 
Mole ratio of urea to H.sub.3 PO.sub.4 in feed and mother 
1.0 
liquor 
Weight ratio of recycle mother liquor to 
2.0-3.0 
feed acid 
Retention time, h 1.0-2.0 
Operating temperature 
(temperatures are 16.degree. to 17.degree. F. below the 
saturation temperature of the feed solution) 
With recycle ratio of 2.0 
90.degree. F. 
With recycle ratio of 3.0 
85.degree. F. 
Agitator tip speed 3-4 ft/s 
Second-stage crystallizer 
Retention time 1.0-2.0 h 
Temperature 68.degree. F. 
Agitator tip speed 3-4 ft/s 
Centrifuge (batch-type; basket, 11 in diam by 
3.6 in deep for separating crystals and 
mother liquor) 
r/min 2,400-3,000 
G 900-1,400 
Rate of addition of urea phosphate slurry 
120 cm.sup.3 /s 
(crystals uniformly distributed on screen) 
Batch volume 600 cm.sup.3 
Batch time 10 s 
Filter screen 60-48 mesh 
Product cake thickness 0.2 in. 
Moisture content of crystals 
1%-2% by wt. 
______________________________________ 
The urea phosphate crystals, after air drying at about 100.degree. F. 
contain about 17.3 to 17.5 percent nitrogen and 44.0 to 44.5 percent 
P.sub.2 O.sub.5, compared with the theoretical composition of 17.7 and 
44.9 percent, respectively. About 80 percent of the acid P.sub.2 O.sub.5 
and urea is recovered as relatively pure urea phosphate. These crystals 
contain only about 10 percent to 15 percent of the iron, aluminum, 
magnesium, and fluorine from the feed acid. The average size (length) of 
the urea phosphate crystals ranges from 610 to 850 microns. The byproduct 
mother liquor contains about 20 percent of the acid P.sub.2 O.sub.5 and 85 
to 90 percent of the impurities. 
2. Work Relating Directly to the Instant Invention 
Referring now more specifically to FIG. 4, there is shown a curve showing 
purity of urea phosphate versus pH in the process for tests we made of 
addition of sulfuric acid to mother liquor. This curve shows the very 
significant increases in urea phosphate purity effected by decreasing pH 
in the process by addition of an acidifying agent. The urea phosphate was 
produced in batch-type tests, for convenience, and the mother liquor was 
made from merchant-grade Florida acid derived from uncalcined phosphate 
ore. A plot of product purity versus pH in the process shows a linear 
relationship, as represented by the equation 
EQU wt % product purity=-11.73 (pH)+89.23 
which has a correlation coefficient of 0.99. 
EXAMPLES 
In order that those skilled in the art may understand how the present 
invention can be practiced and more fully and definitely understood, the 
following examples that we have used for production of urea phosphate 
prepared according to our invention are given by way of illustration and 
not by way of limitation. 
EXAMPLE I 
Unconditioned urea prills (46 percent N) were fed to the first stage (FIG. 
2) through a feeder. Effective volume of the first stage was 2210 
cm.sup.3. Simultaneously, merchant-grade wet-process acid derived from 
calcined western ore and clarified recycle mother liquor to which sulfuric 
acid (95.8 percent H.sub.2 SO.sub.4) was added prior to the test were 
metered to the first stage. The clarified mother liquor before H.sub.2 
SO.sub.4 addition was of low-solids content and contained predominantly 
the fine grained crystals of the iron phosphate-urea salt. The first stage 
was maintained at 90.degree. F. (about 17.degree. F. below saturation 
temperature of the solution) by cooling with a water jacket. Stirring rate 
in the first stage was 4 feet per second (propeller tip speed). The feed 
rate of urea, wet-process acid, and mother liquor was 7.0, 16.0, and 32.1 
grams per minute, respectively. 
The feed acid contained all of the solid impurities present in the original 
acid. The acid contained 51.4 percent P.sub.2 O.sub.5, 1.5 percent 
Al.sub.2 O.sub.3, 1.0 percent Fe.sub.2 O.sub.3, 0.6 percent MgO, 0.9 
percent F, 3.1 percent SO.sub.4, 0.2 percent CaO, and 0.6 percent 
water-insoluble solids. The amount of sulfuric acid added (5.37 g of 95.8 
percent H.sub.2 SO.sub.4 /100 g mother liquor equivalent to about 103 lb 
H.sub.2 SO.sub.4 /ton mother liquor) raised the sulfate content of the 
recycle mother liquor from 5.5 to 10 percent and the specific gravity from 
1.44 to 1.47. The mother liquor, prior to H.sub.2 SO.sub.4 addition, was 
about 9-22-0 grade and was typical of mother liquor at near steady-state 
conditions (2.6 percent Al.sub.2 O.sub.3, 1.5 percent Fe.sub.2 O.sub.3, 
1.1 percent MgO, 1.1 percent F, and 5.5 percent SO.sub.4). The pH of the 
mother liquor was measured with a Leeds and Northrup instrument (Model No. 
7413) equipped with a reference electrode and calomel measuring electrode. 
The pH of the mother liquor in the feed tank was 0.8 at 80.degree. F. 
prior to addition of sulfuric acid and was 0.3 after adding sulfuric acid. 
Retention time in first stage and in the second stage was the same, 1.0 
hour. The mole ratio of urea to phosphoric acid in the feed was 1.0 and 
the weight ratio of recycle mother liquor to feed acid was 2.0. 
As may be seen in FIG. 2, supra, urea phosphate slurry (22 weight percent 
product crystals) overflowed from the first stage to the second stage 
(effective volume, 2210 cm.sup.3). Crystallization was completed in the 
second stage by refrigeration to 68.degree. F.; the slurry in the second 
stage contained 27 weight percent urea phosphate crystals. Stirring rate 
in the second stage was 4 feet per second (propeller tip speed). The pH in 
the urea phosphate slurry in the second stage was 0.3. Product slurry (184 
cm.sup.3 /5 min) was withdrawn from the second stage and the urea 
phosphate crystals were separated from mother liquor in the batch-type 
centrifuge operated at 900 G (2400 r/min) for 10 seconds; a polypropylene 
screen (48 mesh) was used in the centrifuge basket. Samples of product 
crystals were saved for chemical analyses after about five hours of 
production time. Product crystals as removed from the centrifuge were 
about 17-44-0 grade and contained about 2 percent moisture. Fresh mother 
liquor from the centrifuge was settled in a surge tank and clarified 
further by centrifuging. About 80 percent of the mother liquor was added 
to the feed tank as clarified mother liquor for use as recycle. The 
remaining 20 percent of the mother liquor, which contained most of the 
impurities originally present in the wet-process acid, was removed from 
the process as byproduct mother liquor of about 9-22-0 grade. The 
byproduct mother liquor was suitable for processing into a 16-38-0 grade 
solid fertilizer or a 15-22-0 grade suspension. 
The product crystals were white with a light green tinge and were 650 
micrometers in average crystal size. The crystals, air dried at 
110.degree. F., contained 17.5 percent nitrogen and 44.2 percent P.sub.2 
O.sub.5 as compared with the theoretical composition of 17.7-44.9-0 for 
urea phosphate. In impurities, the crystals contained 0.19 percent 
Al.sub.2 O.sub.3, 0.16 percent Fe.sub.2 O.sub.3, 0.06 percent MgO, 0.07 
percent F, 0.7 percent SO.sub.4, and 0.5 percent H.sub.2 O. The average 
purification level of the product (reduction of impurity level calculated 
by comparing aluminum to P.sub.2 O.sub.5, iron to P.sub.2 O.sub.5, 
magnesium to P.sub.2 O.sub.5, and fluorine to P.sub.2 O.sub.5 ratios of 
the product to those of the feed wet-process acid) was 86 percent. About 
80 percent of the acid P.sub.2 O.sub.5 and urea was recovered as product 
crystals. 
Petrographic examination showed that the product crystals were 99+ percent 
urea phosphate and contained &lt;1 percent of the iron-aluminum-potassium 
phosphate salt [(Fe,Al).sub.3 KH.sub.14 (PO.sub.4).sub.8.4H.sub.2 O] and 
chukhrovite (Ca.sub.4 SO.sub.4 SiAlF.sub.13.12H.sub.2 O) originally 
present in the feed wet-process acid. No iron phosphate-urea salt was 
present. Sulfuric acid solubilized the limited quantity of iron 
phosphate-urea salt initially present in the recycle mother liquor and 
prevented further precipitation during the test. 
Petrographic examination showed no evidence of the presence of urea 
sulfate. 
Other tests of the crystallization process involving addition of acidifying 
agents to mother liquor are shown in tables I, II, III, IV and V, infra. 
The crystallization tests were made in the bench-scale continuous 
equipment shown in FIG. 2, supra, and for convenience, in batch-type 
laboratory-scale equipment. 
These data (Table I, infra) were obtained in five continuous bench-scale 
tests (No. 1-5) that we made to study addition of sulfuric acid to 
clarified recycle mother liquor to improve purity of the crystalline urea 
phosphate produced from urea and merchant-grade wet-process acid derived 
from calcined western phosphate ore in the continuous two-stage 
crystallization process. Three of the five tests were made without 
sulfuric acid addition to the recycle mother liquor--two with retention 
time of 1.0 hour in each stage and one with increased retention time (2.0 
vs 1.0 h) in the second stage to further test the effect of retention time 
on size and purity of the product crystals. Retention time was 1.0 hour in 
each stage for the two tests that were made with sulfuric acid added to 
the recycle mother liquor. The amount of sulfuric acid added (about 103 lb 
H.sub.2 SO.sub.4 /ton mother liquor) increased the sulfate content of the 
mother liquor from 5.5 to 10 percent. We have found in our experimental 
work using Florida black acid (54 percent P.sub.2 O.sub. 5) that recovery 
at 68.degree. F. of acid P.sub.2 O.sub.5 and urea as solid urea phosphate 
varied only 1 to 2 percentage points when the sulfate content of the 
recycle mother liquor was varied over the range of 5.5 to 12.5 percent 
(about 4 to 8 percent SO.sub.4 in process). Although this variation in 
sulfate content appeared to have no appreciable effect on product 
recovery, it did result in very significant increases in product purity. 
The composition of the feed acid from calcined western ore and that of the 
recycle mother liquor are given in Example I, supra. Operating conditions 
and results of the tests are shown in Table I below. 
TABLE I 
______________________________________ 
Two-Stage Crystallization Process for Continuous Production of 
Urea Phosphate from Urea and Wet-Process Acid.sup.a 
Derived from Calcined Western Phosphate Ore 
Test No. 1 2 3 4 5 
______________________________________ 
Recycle mother liquor.sup.b 
(clarified) 
Conc H.sub.2 SO.sub.4 added.sup.c 
No No No Yes Yes 
SO.sub.4 level, % by wt 
5.5 5.5 5.5 10 10 
pH 0.8 0.8 0.8 0.3 0.3 
SO.sub.4 level in process, % by wt 
4.1 4.1 4.1 6.8 6.8 
First stage (reactor-crystallizer) 
Mole ratio urea:H.sub.3 PO.sub.4 in feed 
1.0 1.0 1.0 1.0 1.0 
and mother liquor 
Wt ratio recycle mother liquor 
2.0 2.0 2.0 2.0 2.0 
to feed acid 
Operating temperature, .degree.F. 
90 90 90 90 90 
Feed, g/min 
Urea (unconditioned prills, 
5.5 5.5 5.5 5.6 7.0 
46% N) 
Wet-process acid 12.7 12.7 12.7 12.9 16.0 
Mother liquor 25.4 25.4 25.5 25.7 32.1 
Volume, cm.sup.3 1773 1773 1773 1773 2210 
Second stage (crystallizer) 
Operating temperature, .degree.F. 
68 68 68 68 68 
pH in urea phosphate slurry 
0.7 0.7 0.7 0.3 0.3 
Volume, cm.sup.3 1773 1773 3546 1773 2210 
Retention time, h 
First stage 1.0 1.0 1.0 1.0 1.0 
Second stage 1.0 1.0 2.0 1.0 1.0 
Batch-type centrifuge for 
separating mother liquor 
and crystals 
R/min 2400 2400 2400 2400 2400 
G 900 900 900 900 900 
Batch time, s 10 10 10 10 10 
Filter screen, mesh 
48 48 48 48 48 
Product cake thickness, in. 
0.2 0.2 0.2 0.2 0.2 
Product urea phosphate.sup.d from 
second stage 
Percent by wt 
N 17.2 17.3 17.2 17.5 17.5 
P.sub.2 O.sub.5 44.0 44.0 43.9 44.1 44.2 
Al.sub.2 O.sub.3 0.30 0.33 0.29 0.21 0.19 
Fe.sub.2 O.sub.3 0.22 0.24 0.22 0.17 0.16 
MgO 0.11 0.13 0.10 0.07 0.06 
F 0.12 0.13 0.11 0.08 0.07 
SO.sub.4 0.6 0.7 0.6 0.8 0.7 
H.sub.2 O 0.7 0.7 0.6 0.5 0.5 
Avg crystal size, micrometers 
640 610 705 640 650 
Avg reduction of impurities 
78 76 79 85 86 
(Al, Fe, Mg, F),.sup.e % 
______________________________________ 
.sup.a Composition of wetprocess acid given in Example I. 
.sup.b Compositon of recycle mother liquor given in Example I. 
.sup.c Addition of acid (95.8% H.sub.2 SO.sub.4) equivalent ot about 103 
lb H.sub.2 SO.sub.4 /ton mother liquor. 
.sup.d Product crystals were air dried at 110.degree. F.; products were 
white in color with a light green tinge.? 
.sup.e Calculated by comparing impurity to P.sub.2 O.sub.5 ratio in the 
product with that in the feed acid. 
EXAMPLE II 
See Table I, supra 
In tests numbers 1 and 2 (no sulfuric acid added to the recycle mother 
liquor), pH of the mother liquor was 0.8; pH in the urea phosphate slurry 
in the second stage (68.degree. F.) was about the same, 0.7. Retention 
time in these tests was 1.0 hour in each stage. Product purity (average 
reduction of impurities calculated by comparing impurity to P.sub.2 
O.sub.5 ratio in the product with that in the feed acid) was 76-78 percent 
and average crystal size was 610-640 micrometers. The product crystals, 
air dried at 110.degree. F., contained 17.2 to 17.3 percent nitrogen and 
44.0 percent P.sub.2 O.sub.5 which is close to the theoretical composition 
of urea phosphate (17.7-44.9-0). The crystals were white in color with a 
light green tinge. 
EXAMPLE III 
See Table I, supra 
In test number 3 (no sulfuric acid added to recycle mother liquor), 
increasing the retention time from 1.0 to 2.0 in the second stage resulted 
in larger crystal size (705 vs 610 micrometers in test number 2); however, 
product purity (79 percent) was about the same as in tests 1 and 2. The 
product crystals, air dried at 110.degree. F., contained 17.2 percent 
nitrogen, 43.9 percent P.sub.2 O.sub.5, and were white in color with a 
light green tinge. 
EXAMPLE IV 
See Table I, supra 
Sulfuric acid was added to the recycle mother liquor in tests numbers 4 and 
5. Addition of the sulfuric acid increased the sulfate content of the 
mother liquor from 5.5 to 10 percent and decreased pH of the liquor from 
0.8 to 0.3. The pH in the urea phosphate slurry in the second stage was 
0.3. Decreasing pH by addition of sulfuric acid resulted in increased 
product purity compared with similar tests (Nos. 1 and 2) without sulfuric 
acid addition (85-86 percent vs 76-78 percent). Average crystal size was 
about 650 micrometers, about the same as in tests without sulfuric acid 
addition. The product crystals, air dried at 110.degree. F., contained 
17.5 percent nitrogen, 44.1-44.2 percent P.sub.2 O.sub.5, and were white 
in color with a light green tinge. 
EXAMPLE V 
Petrographic examinations were made of the product crystals from test 
numbers 1-5 (Table I). Results indicated that the.product crystals from 
tests 1, 2, and 3 (no sulfuric acid added to recycle mother liquor; 
product purity was 76-79 percent) contained 95+ percent urea phosphate, &lt;2 
percent of the iron phosphate urea salt [FeH.sub.3 
(PO.sub.4).sub.2.2CO(NH.sub.2).sub.2 ], and &lt;1 percent of the 
iron-aluminum-potassium phosphate solid and chukhrovite originally present 
in the feed wet-process acid. Similar petrographic examinations showed 
that product crystals from tests 4 and 5 (sulfuric acid added to recycle 
mother liquor; product purity was.85-86 percent) were 99+ percent urea 
phosphate and &lt;1 percent each of the iron-aluminum-potassium phosphate 
solid and chukhrovite originally present in the feed wet-process acid. No 
iron phosphate-urea salt was present. Sulfuric acid solubilized the 
limited quantity of iron phosphate-urea salt initially present in the 
recycle mother liquor and prevented further precipitation. 
EXAMPLE VI 
See Table I, supra 
The urea phosphate products from test numbers 1-5 were processed into 
urea-ammonium polyphosphate liquid fertilizers by batch-type pyrolysis of 
the crystals at 260.degree. to 275.degree. F. followed by addition of 
water and ammonia gas. Data in the following tabulation show that very 
significant increases in storage life of the liquids were obtained with 
increases in product purity by addition of sulfuric acid in the process. 
______________________________________ 
Urea phosphate Storage time at 80.degree. F. (months) 
products (Table I) 
of liquid fertilizer (15-28-0 grade, 
Test H.sub.2 SO.sub.4 added 
Product 50% of P.sub.2 O.sub.5 as polyphosphate) 
No. in process purity, % 
Very good condition.sup.a 
______________________________________ 
1 No 78 3/4 
2 No 76 3/4 
3 No 79 3/4-4 
4 Yes 85 4-6 
5 Yes 86 &gt;12 
______________________________________ 
.sup.a Liquids are considered to store in very good condition when they 
develop no more than 1% by volume of nonadheringtype crystals 
(carbonaceous material, ammonium phosphates, urea, and biuret) and a 
maximum of 0.1% by volume of adheringtype crystals [MgAl(NH.sub.4).sub.5 
--(P.sub.2 O.sub.7).sub.2 F.sub.2.6H.sub.2 O] - method developed at 
National Fertilizer Development Center, TVA, Muscle Shoals, Alabama. 
Data (Tables II through V, infra) were obtained in batch-type tests of 
crystallization of urea phosphate with addition of different acidifying 
agents to mother liquor. The tests were designed to show the effectiveness 
of varying the proportion of acidifying agent on pH in the process and 
product purity. Data and results of tests of addition of concentrated 
sulfuric acid (97.2 percent), concentrated hydrochloric acid (37.4 
percent), phosphoric acid (73.2 percent) from merchant-grade acid derived 
from uncalcined Florida phosphate ore, and concentrated nitric acid (71.0 
percent) to mother liquor are given below in Tables II, III, IV, and V, 
respectively. Merchant-grade acid derived from uncalcined Florida 
phosphate ore and urea (unconditioned prills, 46.4 percent urea N) were 
used as feed materials in the tests. 
TABLE II 
__________________________________________________________________________ 
Crystallization of Urea Phosphate from Urea and Wet-Process Acid.sup.a - 
Effect of Addition of Sulfuric Acid on pH of Mother Liquor and Purity of 
Product Crystals 
Urea phosphate 
Mother liquor slurry (68.degree. F.) 
Product urea phosphate.sup.e 
SO.sub.4 level, % by wt 
Conc H.sub.2 SO.sub.4 
pH pH of mother liquor 
Average reduction 
In mother (97.2%) 
after H.sub.2 SO.sub.4 
separated from cake 
% by wt 
of impurities 
Test No..sup.b 
liquor.sup.c 
In process.sup.d 
added, g 
addition 
by centrifugation 
N P.sub.2 O.sub.5 
(Al, Fe, Mg, F), 
__________________________________________________________________________ 
%.sup.f 
6 4 3.3 0 0.7 0.6 17.5 
44.5 
82 
7 7.5 5.4 10.7 0.5 0.3 17.5 
44.4 
86 
8 10 6.9 18.8 0.3 0.2 17.4 
44.4 
87 
9 12.5 8.5 27.5 0.2 0.sup.g 17.4 
44.4 
89 
__________________________________________________________________________ 
.sup.a Merchant-grade acid (53.0% P.sub.2 O.sub.5) derived from uncalcine 
Florida phosphate ore. 
.sup.b Charge: 60.4 g unconditioned urea prills (46.4% urea N); 133.95 g 
of merchantgrade Florida black acid; 267.9 g mother liquor; 0-27.5 g 
concentrated (97.2%) H.sub.2 SO.sub.4 added to mother liquor. Each test 
made with mole ratio urea:H.sub.3 PO.sub.4 = 1.0 and wt ratio mother 
liquor to feed acid, 2.0. 
.sup.c Calculated by comparing SO.sub.4 content in 267.9 g mother liquor 
(4% SO.sub.4) plus amount in added H.sub.2 SO.sub.4 with total weight of 
solution. 
.sup.d Calculated by comparing SO.sub.4 content in charge (footnote 
.sup.b) with total weight. 
.sup.e Product crystals air dried at 110.degree. F. Crystal size was in 
range of about 350 to 600 micrometers. 
.sup.f Calculated by comparing impurity to P.sub.2 O.sub.5 ratio in the 
product with that in the feed acid. 
.sup.g Estimated pH 0.01. 
TABLE III 
__________________________________________________________________________ 
Crystallization of Urea Phosphate from Urea and Wet-Process Acid.sup.a - 
Effect of Addition of Hydrochloric Acid on pH of Mother Liquor and Purity 
of Product Crystals 
Urea phosphate 
Mother liquor 
slurry (68.degree. F.) 
Product urea phosphate.sup.c 
HCl level in 
Conc HCl 
pH of mother liquor 
Average reduction 
mother liquor, 
(37.4%) separated from cake 
% by wt 
of impurities 
Test No..sup.b 
% by weight 
added, g 
by centrifugation 
N P.sub.2 O.sub.5 
(Al, Fe, Mg, F), %.sup.d 
__________________________________________________________________________ 
10 0 0 0.6 17.4 
44.4 
79 
11 1 7.4 0.5 17.5 
44.3 
79 
12 4 32.1 0.sup.e 17.6 
44.6 
87 
13 7.5 67.2 0.sup.e 17.5 
44.5 
88 
14 10 97.8 0.sup.e 17.5 
44.4 
88 
__________________________________________________________________________ 
.sup.a Merchant-grade acid (53.0% P.sub.2 O.sub.5) derived from uncalcine 
Florida phosphate ore. 
.sup.b Charge: 60.4 g unconditoned urea prills (46.4% urea N); 133.95 g o 
merchantgrade Florida black acid; 267.9 g mother liquor; 0-97.8 g 
concentrated (37.4%) HCl added to mother liquor. Each test made with mole 
ratio urea:H.sub.3 PO.sub.4 = 1.0 and weight ratio mother liquor to feed 
acid, 2.0. 
.sup.c Product crystals air dried at 110.degree. F. Crystal size was in 
range of 300 to 385 micrometers. 
.sup.d Calculated by comparing impurity to P.sub.2 O.sub.5 ratio in 
product with that in feed acid. 
.sup.e Estimated pH 0.01. 
TABLE IV 
__________________________________________________________________________ 
Crystallization of Urea Phosphate from Urea and Wet-Process Acid.sup.a - 
Effect of Addition of Phosphoric Acid on pH of Mother Liquor and Purity 
of Product Crystals 
Mother liquor Urea phosphate 
Merchant-grade slurry (68.degree. F.) 
Product urea phosphate.sup.c 
Florida black acid 
Increase in pH of mother liquor 
Average reduction 
(73.2% H.sub.3 PO.sub.4) 
H.sub.3 PO.sub.4 level, 
Mole ratio 
separated from cake 
% by wt 
of impurities 
Test No..sup.b 
added, g % by wt 
urea:H.sub.3 PO.sub.4 
by centrifugation 
N P.sub.2 O.sub.5 
(Al, Fe, Mg, F), 
__________________________________________________________________________ 
%.sup.d 
15 0 0 1.00 0.5 17.4 
44.4 
79 
16 15.5 4 0.90 0.4 17.4 
44.5 
80 
17 30.6 7.5 0.81 0.4 17.4 
44.5 
81 
18 42.4 10 0.76 0.3 17.4 
44.3 
83 
19 55.2 12.5 0.71 0.3 17.4 
44.6 
84 
__________________________________________________________________________ 
.sup.a Merchant-grade acid (53.0% P.sub.2 O.sub.5) derived from uncalcine 
Florida phosphate ore. 
.sup.b Charge: 60.4 g unconditioned urea prills (46.4% urea N); 133.95 g 
of merchantgrade Florida black acid; 267.9 g mother liquor; 0-55.2 g 
merchantgrade Florida black acid added to mother liquor. Each test made 
with mole ratio urea:H.sub.3 PO.sub.4 in feed = 1.0 and weight ratio 
mother liquor to feed acid, 2.0. 
.sup.c Product crystals air dried at 110.degree. F. Crystal size was in 
range of 475 to 530 micrometers. 
.sup.d Calculated by comparing impurity to P.sub.2 O.sub.5 ratio in the 
product with that in the feed acid. 
The batch-type equipment consisted of a 600-cubic-centimeter pyrex beaker 
(used as a reactor-crystallizer), a variable speed stirrer, a hot plate, a 
constant temperature bath operated at 68.degree. F., a centrifuge (IEC, 
Model 2K), and a Leeds and Northrup pH meter (Model No. 7413) equipped 
with a reference electrode and a calomel measuring electrode. The basket 
(8 in diameter by 3 in deep) in the centrifuge was equipped with a 
100-mesh polypropylene cloth for separating mother liquor from product 
crystals. 
A mole ratio of urea to phosphoric acid of 1.0 in the feed and a weight 
ratio of mother liquor to feed acid of 2.0 were used in the tests. The 
acidifying agent was added batchwise to the mother liquor prior to a test. 
Sulfuric acid was added in proportions to vary the sulfate level in the 
mother liquor from 4 to 12.5 percent; concentrated hydrochloric acid and 
nitric acid were added in proportions to give levels of either HCl or 
HNO.sub.3 of 0 to 10 percent by weight in the mother liquor. Phosphoric 
acid was added as the merchant-grade Florida feed acid in proportions to 
increase the H.sub.3 PO.sub.4 level of the mother liquor from 0 to 12.5 
percent by weight. 
The merchant-grade acid derived from uncalcined Florida phosphate ore 
contained 53.0 percent P.sub.2 O.sub.5, 1.8 percent Al.sub.2 O.sub.3, 1.3 
percent Fe.sub.2 O.sub.3, 0.6 percent MgO, 0.9 percent F, and 3.4 percent 
SO.sub.4. The average impurity level of the mother liquor used in these 
tests was lower than that of mother liquors at near steady-stage 
conditions (2.5 vs about 3.3-3.8). Prior to addition of a test acid 
(H.sub.2 SO.sub.4, HCl, H.sub.3 PO.sub.4 or HNO.sub.3) the mother liquor 
contained 8.6 percent urea N, 22.1 percent P.sub.2 O.sub.5 (mole ratio 
urea:H.sub.3 PO.sub.4 , 1.0), 1.6 percent Al.sub.2 O.sub.3, 1.4 percent 
Fe.sub.2 O.sub.3, 0.8 percent MgO, 0.7 percent F, and 4 percent SO.sub.4. 
The mother liquor was of relatively low-solids content and contained a 
higher proportion of the fine grained crystals of the iron phosphate-urea 
salt than normally present in clarified recycle mother liquor. After 
addition of a test acid to the mother liquor, the mixture was allowed to 
equilibrate overnight before a urea phosphate crystallization test was 
made. 
The following procedure was used in the batch-type tests. Urea (60.4 g), 
wet-process acid (133.95 g) and mother liquor (267.9 g), with the 
acidifying agent added or omitted, were mixed in a 600-cubic-centimeter 
pyrex beaker and heated to about 120.degree. F. to ensure dissolution of 
the prilled urea. The solution then was stirred (with addition of 0.5 g of 
seed crystals) and cooled slowly to 68.degree. F. The resulting urea 
phosphate slurry was maintained at 68.degree. F. for 2 hours to ensure 
maximum crystallization under test conditions. The slurry was centrifuged 
at 900 G for 10 seconds and the pH of the mother liquor separated from the 
crystals was measured. Product cake was air dried at 110.degree. F. and 
the dried crystals were submitted for chemical analysis and petrographic 
examination. 
EXAMPLE VII 
See Table II, supra 
In test 6 (made with no added sulfuric acid) the sulfate content of the 
mother liquor was 4 percent, pH in the urea phosphate slurry (process) was 
0.6 and product purity was 82 percent. Addition of sulfuric acid to 
increase sulfate level in the mother liquor over the range of from 4 to 
12.5 percent caused progressive decreases in pH from 0.6 to 0 and 
increases in product purity from 82 to 89 percent. The products air dried 
at 110.degree. F. contained 17.4 to 17.5 percent nitrogen and 44.4 to 44.5 
percent P.sub.2 O.sub.5 which is close to the theoretical composition of 
urea phosphate (17.7-44.9-0). 
Petrographic examinations were made of the product crystals from tests 6-9, 
Table II supra. Results indicated that product crystals from test 6 (no 
sulfuric acid added to mother liquor, 4 percent SO.sub.4 ; product purity 
was 82 percent) contained 95+ percent urea phosphate, &lt;2 percent of the 
iron phosphate-urea salt and &lt;1 percent chukhrovite originally present in 
the feed wet-process acid. Similar petrographic examinations showed that 
product crystals from tests 7 through 9 (sulfuric acid added to give 7.5, 
10, and 12.5 percent sulfate level in mother liquor; product purity was 
86-89 percent) contained 95+ percent urea phosphate, a lower proportion of 
the iron phosphate-urea salt (&lt;1 percent vs &lt;2 percent) and &lt;1 percent 
chukhrovite. 
EXAMPLE VIII 
See Table III, supra 
With no hydrochloric acid added to the mother liquor (test 10), pH in the 
process was 0.6 and product purity was 79 percent. Increasing the HCl 
level of the mother liquor to 1 percent decreased pH in the process only 
from 0.6 to 0.5 and product purity remained the same, 79 percent. 
Increasing the HCl level from 1 to 4 percent decreased pH in the process 
from 0.5 to 0 and increased product purity significantly (87 vs 79 
percent). Further addition of hydrochloric acid to increase its level in 
the mother liquor over the range of from 4 to 10 percent resulted in no 
appreciable increase in product purity. The products, air dried at 
110.degree. F., contained 17.4 to 17.6 percent nitrogen and 44.3 to 44.6 
percent P.sub.2 O.sub.5, which is close to the theoretical composition of 
urea phosphate (17.7-44.9-0). 
Petrographic examinations were made of the product crystals from tests 
10-14; see table III, supra. Results indicated that product crystals from 
tests 10 and 11 (0 and 1 percent HCl level, respectively, in mother 
liquor; product purity was 79 percent) contained 95+ percent urea 
phosphate, &lt;3 percent of the iron phosphate-urea salt and &lt;1 percent 
chukhrovite originally present in the feed wet-process acid. It is noted 
that the purification level of the blank (no HCl added) is lower than 
shown in Table II, supra (79 vs 82 percent product purity) because of 
continuous precipitation of the contaminating iron phosphate-urea salt in 
the mother liquor and the time interval of one week between the two series 
of tests. Product crystals from test 12 (HCI added to give a 4 percent 
level in mother liquor; product purity was 87 percent) contained 95+ 
percent urea phosphate, &lt;2 percent iron phosphate-urea salt, and &lt;1 
percent chukhrovite. Similar petrographic examination of product crystals 
from tests 13 and 14 (7.5 and 10 percent HCl level, respectively, in 
mother liquor; product purity was 88 percent) contained 95+ percent urea 
phosphate, &lt;1 percent of the iron phosphate-urea salt and &lt;1 percent 
chukhrovite. Thus, it can be seen that hydrochloric acid was an effective 
solvent for the iron phosphate-urea salt. 
EXAMPLE IX 
See Table IV, supra 
Increasing the phosphoric acid level of the mother liquor over the range of 
0 to 12.5 percent by additions of merchant-grade Florida black acid caused 
decreases in pH in the process from 0.5 to 0.3 and progressive increases 
in product purity from 79 to 84 percent. The products, air dried at 
110.degree. F., contained 17.4 percent nitrogen and 44.3 to 44.6 percent 
P.sub.2 O.sub.5, which is close to the theoretical composition of urea 
phosphate (17.7-44.9-0). 
Petrographic examinations indicated that product crystals from tests 15 
through 19 (table IV) contained 95+ percent urea phosphate, &lt;3 percent of 
the iron phosphate-urea salt and &lt;1 percent chukhrovite originally present 
in the feed wet-process acid. No appreciable differences in amounts of the 
iron phosphate-urea salt with different levels of phosphoric acid addition 
were detected. However, the purification level (79-84 percent) of the urea 
phosphate crystals indicated that phosphoric acid decreased pH and 
increased product purity; thus, it had some effectiveness in solubilizing 
the iron phosphate-urea salt. 
EXAMPLE X 
Addition of nitric acid to mother liquor was not an effective method of 
increasing purity of the urea phosphate crystals because of precipitation 
of fine grained urea nitrate. The data indicated that increasing the 
nitric acid level in the mother liquor over the range of 0 to 10 percent 
decreased pH in the process from 0.5 to 0, but increased NO.sub.3 -N 
content in the product crystals from nil to 2.9 percent and, unlike 
H.sub.2 SO.sub.4, HCl, and H.sub.3 PO.sub.4, decreased product purity from 
78 to 47 percent. 
Petrographic results were in good agreement with chemical analysis in 
showing a corresponding increase in urea nitrate crystals in the urea 
phosphate product with increasing nitric acid concentration. The formation 
of extremely fine grained urea nitrate crystals (&lt;10 micrometers) caused 
occlusion of urea phosphate mother liquor on the surface of the urea 
nitrate crystals as well as the urea phosphate product crystals, which 
resulted in poor separation in the centrifuge of mother liquor from 
product crystals and decreased product purity. 
The formation of urea-sulfuric acid and urea-hydrochloric acid did not 
occur when these acids were added to the urea phosphate system. Therefore, 
these acids (H.sub.2 SO.sub.4, HCl, and H.sub.3 PO.sub.4) did not produce 
any fine grained urea adducts besides the desired urea phosphate and 
consequently increased the solubility of the contaminating iron 
phosphate-urea salt [FeH.sub.3 (PO.sub.4).sub.2.2CO(NH.sub.2).sub.2 ] and 
purity of the urea phosphate crystals. The results are illustrated in 
Table V below. 
TABLE V 
__________________________________________________________________________ 
Crystallization of Urea Phosphate from Urea and Wet-Process 
Acid.sup.a - Effect of Addition of Nitric Acid on pH of Mother Liquor 
and Purity of Product Crystals 
Product urea phosphate.sup.c 
Mother liquor 
Urea phosphate Average 
Conc slurry (68.degree. F.) 
reduction 
HNO.sub.3 
HNO.sub.3 
pH of mother liquor 
% by wt of impurities 
Test 
level, 
(71.0%) 
separated from cake 
Total (Al, Fe, Mg, F), 
No..sup.b 
% by wt 
added, g 
by centrifugation 
N NO.sub.3 --N 
P.sub.2 O.sub.5 
%.sup.d 
__________________________________________________________________________ 
20 0 0 0.5 17.4 
Nil 44.1 
78 
21 1 3.8 0.5 17.4 
Nil 44.0 
78 
22 4 16.0 0.1 17.4 
0.1 43.8 
75 
23 7.5 31.6 0.1 18.5 
1.9 37.1 
61 
24 10 43.9 0.sup.e 19.5 
2.9 33.0 
47 
__________________________________________________________________________ 
.sup.a Merchant-grade acid (53.0% P.sub.2 O.sub.5) derived from Florida 
uncalcined phosphate ore. 
.sup.b Charge: 60.4 g unconditioned urea prills (46.4% urea N); 133.95 g 
merchantgrade Florida black acid; 267.9 g mother liquor; 0-43.9 g 
concentrated (71.0%) HNO.sub.3 added to mother liquor. Each test made wit 
mole ratio urea:H.sub.3 PO.sub.4 = 1.0 and weight ratio mother liquor to 
feed acid, 2.0. 
.sup.c Product crystals air dried at 110.degree. F. Crystal size was in 
range of 375-550 micrometers. 
.sup.d Calculated by comparing impurity to P.sub.2 O.sub.5 ratio in 
product with that in feed acid. 
.sup.e Estimated pH 0.01. 
EXAMPLE XI 
Storage of Mother Liquor 
Storage tests were made to show the effectiveness of addition of the 
different acidifying agents, supra, on prolonging the useful storage life 
of clarified-recycle mother liquor. The mother liquor, which was made from 
merchant-grade Florida black acid, contained about 7 percent SO.sub.4 and 
was typical of clarified recycle mother liquor at near steady-state 
conditions. 
Data from the tests indicated that sulfuric acid and hydrochloric acid were 
very satisfactory solvents for the iron phosphate-urea salt. Mother liquor 
to which sulfuric acid was added to increase sulfate level from 7 to 11 
percent by weight remained in very good condition (&lt;0.1 percent by volume 
of settled solids) during eight weeks quiescent storage at 80.degree. F. 
Similar results were obtained with a 4 percent level of hydrochloric acid 
in the mother liquor. With no acidifying agent added, the mother liquor 
contained about 13 percent by volume of settled solids after eight weeks 
of similar storage. Phosphoric acid was less effective than either 
hydrochloric acid or sulfuric acid. Nitric acid was unsatisfactory because 
of precipitation of urea nitrate. 
INVENTION AMETERS 
After sifting and winnowing through data presented above as well as other 
data available to us, we have determined that the operating limits as well 
as the preferred conditions for carrying out the instant invention related 
to use of acidifying agents in the production of urea phosphate are 
summarized below: 
______________________________________ 
Acidifying agent added to 
mother liquor Limits Preferred 
______________________________________ 
Sulfuric acid 
Increase in SO.sub.4 level of 
&gt;2 and &lt;10 4-6 
mother liquor, % by wt. 
pH in process &lt;1 and .gtoreq.0.01 
0.3-0.01 
Hydrochloric acid 
Level in mother liquor, % by wt 
&gt;1 and &lt;8 3-4 
pH in process &lt;1 and .gtoreq.0.01 
0.2-0.01 
Phosphoric acid 
Incrcase in H.sub.3 PO.sub.4 level of 
&gt;4 and &lt;13 10-12 
mother liquor, % by wt 
pH in process &lt;1 and .gtoreq.0.3 
0.4-0.3 
______________________________________ 
While we have shown and described particular embodiments of our invention 
modifications and variations thereof will occur to those skilled in the 
art. We wish it to be understood therefore that the appended claims are 
intended to cover such modifications and variations which are within the 
true scope and spirit of our invention.