Abstract:
Ammonium carbamate, in solution with urea, water and ammonia and under high temperature and pressure is cooled to below its crystallization temperature while being subjected to an elevated pressure. The crystallized substance formed is then dried, crushed, and pelletized with the aid of a binding agent to produce pellets having substantial strength and stability, and having particular utility as a deicer for roadways.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation-in-part application of pending U.S. patent application Ser. No. 07/892,924, filed Jun. 3, 1992 abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     Ammonium carbamate is not an available commercial product. It occurs as an intermediate product in the production of urea, and apparently it has never been separated and removed from urea plants due to the lack of any market. 
     A patent for using ammonium carbamate as a deicer was issued to Hansen as U.S. Pat. No. 4,698,173 on Oct. 6, 1987. This deicer does not corrode steel or aluminum, and instead of being toxic to plant life as are commonly used highway deicers, the contained nitrogen actually promotes plant growth. Tests at the Utah Department of Transportation also demonstrated that an ammonium carbamate deicer dramatically reduced the freezing damage to concrete. In 1976, the Environmental Protection Agency estimated that the use of nine million tons of salt per year as a deicer, costs the public about five billion dollars per year at a cost of about $555 per ton. With the potential money to be saved by using a deicer of moderate price and which does not damage the environment, and with a potential market of about nine million tons per year, it became evident to the present inventor that there would be numerous benefits if the ammonium carbamate could be commercially produced. Furthermore, urea production plants are a logical available source of ammonium carbamate. 
     The process of removing ammonium carbamate from a urea plant presented several problems, however. One of the principle problems was that it existed in the urea production plant under high temperature and pressure. A method had to be devised for removing the ammonium carbamate from the water and ammonia solvents it is in solution with and converting it to a useable state, which is crystalline in this case. 
     A second problem to be solved was to devise a method of making the ammonium carbamate crystals into granules or pellets which can be distributed on roads. In attempting to solve this problem, the present inventor discovered that the crystals produced contained too much moisture to be pelletized or granulated and so a method had to be devised to remove the water from the crystals. In addition, after learning how to remove the water from the crystals, a method of granulating or pelletizing the crystals had to be devised. Finally, a binder that would hold the pellets together, as well as a technique to keep the pellets stable for subsequent use, were also needed. 
     PRODUCTION OF AMMONIUM CARBAMATE 
     Two molal volumes of ammonia can be reacted at high pressure with one molal volume of carbon dioxide to produce ammonium carbamate in an autoclave reactor in a urea production plant. A separate facility for this purpose alone could be built for use according to the present invention, but such facility would require substantial financial investment. In an existing urea production plant, the ammonium carbamate is circulated through conversion towers, where about one-half of the ammonium carbamate is converted into urea and water. The remaining ammonium carbamate, along with the water released and excess ammonia, are pumped back against high pressure into the reactor. Such remaining ammonium carbamate may then be removed from the reactor for use in accordance with the present invention. 
     Thus, a urea production plant may be used to produce approximately twice as much useable product; that is, for every 100 pounds of urea previously produced, the plant would now produce approximately 100 pounds of urea and approximately 100 pounds of ammonium carbamate. The enhancement of production in a urea plant and the relatively inexpensive potential source of ammonium carbamate provided considerable incentives for the present inventor to learn how to recover ammonium carbamate from a urea production plant and then to produce a product from the ammonium carbamate which would be suitable for use as a deicer. 
     CRYSTALLIZING AMMONIUM CARBAMATE FROM RECYCLED SOLUTIONS 
     In the early part of the present inventor&#39;s research for a method of producing ammonium carbamate from a urea plant, it was necessary to determine the condition and composition of the chemicals which were being returned to the reactor. The returned ammonium carbamate and a small amount of urea were found to be dissolved in a solution of water and ammonia. 
     An analysis of a solution used in one test in which 1368 grams of solution were present in the crystallization chamber gave the following composition: 41.79% NH 3 , 27.19% CO 2 , 3.87% urea and 27.15% H 2  O. 
     Because ammonium carbamate is produced from two moles of ammonia and one mole of carbon dioxide, and there is an excess of ammonia in this composition, the contained ammonium carbamate is calculated from the content of carbon dioxide as follows: ##STR1## 
     
         ______________________________________Calculated Composition of the Absorber Bottoms Solution       Percent         Grams______________________________________(27.19/44) (78)         = 48.21   NH.sub.4 COONH.sub.2                               659.541.79 - (27.19/44) (34)         = 20.77   xs NH.sub.3 284.2         =  3.87   Urea         52.9         = 27.15   H.sub.2 O   371.4TOTAL         100.00                1368.0______________________________________ 
    
     With reference to table V-4 on pages 258-259 of the 1967 edition of &#34;Urea Its Properties and Manufacture&#34; by George Tseiyu Chao, it was determined that this solution would become super saturated during cooling and crystals would form. It was estimated, from this table, that in a crystallization chamber containing cooling water at 10° C., the temperature of the ammonium carbamate solution could be lowered to between 15° C.-20° C. A test was then set up from which the above composition of the ammonium carbamate solution was taken and the temperature of the solution being crystallized was lowered to 16.7° C. 
     In this test, the 1368 grams of absorber bottoms liquid was placed in the crystallization chamber to produce 960 grams of crystals having the following composition: ##STR2## 
     
         ______________________________________Calculated Composition of the Crystals Formed       Percent         Grams______________________________________(37.87/44) (78)         = 67.13   NH.sub.4 COONH.sub.2                               644.438.01 - (37.87/44) (34)         =  8.75   NH.sub.3     84.0         =  3.08   Urea         29.6         = 21.04   H.sub.2 O   202.0TOTAL         100.00                960.0______________________________________ 
    
     
         ______________________________________Recovery of Chemicals Into the Crystals______________________________________Recovery of ammonium carbamate                644.4/659.5                          97.7%Recovery of ammonia   84.0/284.2                          29.6%Recovery of urea     29.6/52.9 56.0%Recovery of water    202.0/371.4                          54.4%______________________________________ 
    
     EXPECTED RECOVERY OF AMMONIUM CARBAMATE INTO THE CRYSTALS 
     The following table gives the composition of the bottoms liquid with and without the contained urea. 
     
         ______________________________________      With Urea (%)                Without Urea (%)______________________________________NH.sub.4 COONH.sub.2        48.21       50.13NH.sub.3     20.77       21.62Urea          3.87       --H.sub.2 O    27.15       28.24TOTAL        100.00      100.00______________________________________ 
    
     From the table on pages 258-159 of the aforementioned book by Chao, it was expected that at a temperature of 16.7° C., the crystals which were formed from the above solution would have been in equilibrium with a solution having the following composition: 
     
         ______________________________________NH.sub.4 COONH.sub.2           30.0%NH.sub.3        30.0%H.sub.2 O       40.0%TOTAL           100.0%______________________________________ 
    
     Such a crystallization process would produce the following expected recovery of the ammonium carbamate, wherein: 
     Weight of original solution=100 g, 
     weight of solution in equilibrium with crystals=X; and 
     weight of crystals formed=(100-X). 
     Thus, 100×0.5013-0.30X (100-X)=weight of crystals formed, and X=71.24 g. Accordingly, the weight of the crystals equals 100-71.24=28.76 g, and the expected recovery was only 28.76/50.13=57.4%. 
     After measuring this unexpected increase in the actual recovery of the crystals as compared with the expected recovery, it was determined that additional tests should be run. After running these additional tests and studying their results, it became quite apparent that there are two main factors which are important in determining the percentage of ammonium carbamate which can be crystallized from the bottoms liquid solution: (1) a low final temperature and (2) a high pressure. The present invention comprises the discovery of the importance of maintaining a high pressure during the crystallization process. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the accompanying drawing: 
     FIG. 1 is a schematic drawing of a pilot plant for producing ammonium carbamate in accordance with the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Crystallizing Ammonium Carbamate 
     It was determined that an appropriate place to secure a sample of the solution being recycled in the urea plant was from the bottom of a first absorber. At this location the bottoms liquid is at a pressure of about 225 psig and a temperature of about 180° F. (about 82° C.). 
     The liquid sample was filled into an aluminum crystallization chamber which was formed from an eleven inch length piece of four inch, schedule 80 aluminum pipe, which was closed on the bottom with a four inch schedule 80 aluminum weld cap and on the top with two aluminum flanges. One of these flanges was welded to the pipe and then bolted to the top flange with eight one-half inch bolts. The four inch pipe was surrounded with a nine inch length piece of five inch, schedule 40 aluminum pipe, closed at both ends to form a cooling chamber. The line through which the crystallization chamber was filled and from which the gases were exhausted, entered through the side of the chamber at about two inches from the top. The openings for the pressure gauge and a thermal well for measuring temperature were defined by the top flange. Nine baffles were placed between the outside and inside aluminum pipes for the purpose of circulating the cooling water from the top to bottom as it cooled the contained bottoms liquid. The openings for the cooled water were through two one-fourth inch pipes in the side of the five inch aluminum pipe. 
     Before filling the crystallization chamber, the tare weight of the chamber was determined and recorded. As the crystallization chamber was filled, the pressure and temperature of the filled chamber were recorded. The gross weight of the filled chamber was then determined and recorded and the cooling process was started. The temperature and pressure of the contained solution were recorded at one minute intervals as the solution cooled. Except for the early tests, which were stopped at 16.7° C., the crystallization tests were stopped at one of three temperatures; namely, 30° C., 20° C. and 15.7° C. 
     At the conclusion of each test, the gases were exhausted and the crystallization chamber was then weighed. The chamber was then opened, the contained clear solution was decanted therefrom, and the chamber was reweighed. The contained crystals were then sampled and analyzed. 
     After it became apparent to the present inventor that it was important to maintain a high pressure in order to increase the recovery of crystals from the bottoms liquid, the recorded information was examined and the pressure, at the highest temperature at which the tests were concluded, 30° C., was noted. Following are the results of those tests. 
     
         ______________________________________EXAMPLE E14A1______________________________________Crystallization chamber filled at                      100 psiPressure at 30° C.  39 psiTemperature at completion of test                      30° C.Weight of absorber bottoms liquid                      318 gtaken into chamberWeight of crystals recovered                      114 gWeight of filtrate recovered                      86 gWeight of gases and liquid vented and lost                      118 g______________________________________Composition of Products (%)Name of Product        NH.sub.4 COONH.sub.2                    xs NH.sub.3                            Urea  Water______________________________________Absorber Bottoms        48.36       22.01   3.50  26.13LiquidCrystals     38.70       19.20   2.18  39.96Percent RecoveryPercent Into Crystals        28.7        31.2    22.3  54.8______________________________________EXAMPLE E15A1______________________________________Crystallization chamber filled at                      100 psiPressure at 30° C.  51 psiTemperature at completion of test                      15.7° C.Weight of absorber bottoms liquid                      485 gtaken into chamberWeight of crystals recovered                      249 gWeight of filtrate recovered                      126 gWeight of gases and liquid vented and lost                      110 g______________________________________Composition of Products (%) Name of Product        NH.sub.4 COONH.sub.2                    xs NH.sub.3                            Urea  Water______________________________________Absorber Bottoms        45.77       22.08   4.44  27.71LiquidCrystals     75.64        2.47   2.83  19.06Percent RecoveryPercent Into Crystals        84.7        5.7     32.7  35.3______________________________________EXAMPLE E15A2______________________________________Crystallization chamber filled at                      160 psiPressure at 30° C.  61 psiTemperature at finish of test                      20° C.Weight of absorber bottoms liquid                     1195 gtaken into chamberWeight of crystals recovered                      700 gWeight of filtrate recovered                      300 gWeight of gases and liquid vented and lost                      195 g______________________________________Composition of Products (%)Name of Product        NH.sub.4 COONH.sub.2                    xs NH.sub.3                            Urea  Water______________________________________Absorber Bottoms        46.67       20.22   4.64  28.46LiquidCrystals     74.92        3.93   2.78  18.37Percent RecoveryPercent Into Crystals        94.0        11.4    35.2  37.8______________________________________EXAMPLE E16A1______________________________________Crystallization chamber filled at                      160 psiPressure at 30° C.  74 psiTemperature at completion of test                      30° C.Weight of absorber bottoms liquid                     1433 gtaken into chamberWeight of crystals recovered                      803 gWeight of filtrate recovered                      365 gWeight of gases and liquid vented and lost                      265 g______________________________________Composition of Products (%)Name of Product        NH.sub.4 COONH.sub.2                    xs NH.sub.3                            Urea  Water______________________________________Absorber Bottoms        46.36       21.78   3.83  28.03LiquidCrystals     72.50        4.75   2.40  20.35Percent RecoveryPercent Into Crystals        87.6        12.2    35.2  40.7______________________________________EXAMPLE E16A2______________________________________Crystallization chamber filled at                      210 psiPressure at 30° C.  84 psiTemperature at completion of test                      20° C.Weight of absorber bottoms liquid                     1929 gtaken into chamberWeight of crystals recovered                     1158 gWeight of filtrate recovered                      437 gWeight of gases and liquid vented and lost                      334 g______________________________________Composition of Products (%)Name of Product        NH.sub.4 COONH.sub.2                    xs NH.sub.3                            Urea  Water______________________________________Absorber Bottoms        47.12       21.79   4.06  27.03LiquidCrystals     75.98        0.00   2.48  21.54Percent RecoveryPercent Into Crystals        96.8        00.0    36.7  47.8______________________________________EXAMPLE E17A1______________________________________Crystallization chamber filled at                      215 psiPressure at 30° C.  46 psiTemperature at completion of test                      15.7° C.Weight of absorber bottoms liquid                     2069 gtaken into chamberWeight of crystals recovered                     1199 gWeight of filtrate recovered                      442 gWeight of gases and liquid vented and lost                      428 g______________________________________Composition of Products (%)Name of Product        NH.sub.4 COONH.sub.2                    xs NH.sub.3                            Urea  Water______________________________________Absorber Bottoms        49.83       21.09   4.26  24.82LiquidCrystals     71.33        6.83   2.55  19.29Percent RecoveryPercent Into Crystals        82.9        18.8    34.7  45.0______________________________________EXAMPLE E17A2______________________________________Crystallization chamber filled at                      215 psiPressure at 30° C.  72 psiTemperature at completion of test                      30° C.Weight of absorber bottoms liquid                     1857 gtaken into chamberWeight of crystals recovered                      978 gWeight of filtrate recovered                      610 gWeight of gases and liquid vented and lost                      269 g______________________________________Composition of Products (%)Name of Product        NH.sub.4 COONH.sub.2                    xs NH.sub.3                            Urea  Water______________________________________Absorber Bottoms        44.46       22.37   5.03  28.14LiquidCrystals     76.58        4.01   2.47  16.94Percent RecoveryPercent Into Crystals        90.7        9.4     42.0  31.7______________________________________EXAMPLE D2A1______________________________________Crystallization chamber filled at                      200 psiPressure at 30° C.  62 psiTemperature at completion of test                      16.7° C.Weight of absorber bottoms liquid                     1368 gtaken into chamberWeight of crystals recovered                      960 gWeight of filtrate recovered                      408 g______________________________________Composition of Products (%)Name of Product        NH.sub.4 COONH.sub.2                    xs NH.sub.3                            Urea  Water______________________________________Absorber Bottoms        48.21       20.77   3.87  27.15LiquidCrystals     67.13        8.75   3.08  21.04Percent RecoveryPercent Into Crystals        97.7        29.6    56.0  54.4______________________________________ 
    
     DISCUSSION OF THE TEST RESULTS 
     As expected, the percentage of ammonium carbamate crystals recovered from the solution of ammonium carbamate in a water and ammonia solvent was a function of the temperature to which the solution was cooled during the crystallization process. Unexpectedly, it was also discovered that if the ammonium carbamate solution was maintained under pressure as the crystals were being formed, the percentage of ammonium carbamate recovered into the crystals was also a function of pressure. More particularly, the higher the pressure that was maintained in the crystallization chamber, the higher was the percentage of ammonium carbamate recovered into the crystals. 
     The following table compares the per cent recovery at the final temperatures of 30° C., 20° C. and 15.7° C. to the recovery made at the various pressures which were maintained until the temperature was reduced to the highest final temperature of crystallization, 30° C. 
     
         ______________________________________Percent Recovery at Temperature    PressureTest     MaintainedNo.      To 30° C.              30° C.                         20° C.                               15.7° C.______________________________________E16A2    84 psi               96.8E16A1    74 psi    87.6E17A2    72 psi    90.7D2A1     62 psi                     97.7E15A2    61 psi               94.0E15A1    51 psi                     84.7E17A1    46 psi                     82.9E14A1    39 psi    28.7______________________________________ 
    
     The tests which were completed at 30° C. demonstrated that as the pressure was increased from 39 psi to 72 psi and to 74 psi, the recovery increased from 28.7% to an average of about 89%. The difference in test results between the ending pressures of 61 psi and 84 psi for a temperature of 20° C. was not as great, but in both cases the per cent recovery was excellent. At 15.7° C., the difference between the 82.9% recovery at 46 psi and the 97.7% recovery at 62 psi was substantial. 
     These test results demonstrated that it is advantageous to maintain a high pressure during the crystallization process. Based on the results, this pressure should be above approximately 50 psig. Such pressure can be maintained by pressurizing the vessel with the liquid that is being crystallized or with any gas, such as ammonia or nitrogen, that is compatible with the ammonia and water solvents from which the ammonium carbamate crystallizes. 
     REMOVING WATER FROM AMMONIUM CARBAMATE CRYSTALS 
     Ammonium carbamate, like many other chemicals, has a vapor pressure which is a function of its temperature. According to Chao at page 299, the dissociation pressure of solid ammonium carbamate in the temperature range of 283° K to 355° K (10° C. to 82° C.) can be calculated from the equation, Log P=-2741.9/T+11.1448. To illustrate how the dissociation pressure changes with temperature, the pressures were calculated at the following temperatures. At temperatures below 10° C., the pressure are probably estimates: 
     
         ______________________________________         PressureTemperature     Log P      P       P°F.   °C.            °K.                   mm Hg    mm Hg psig______________________________________32.0     0.0     273    1.1012   12.62 0.24440.0     4.4     277    1.2621   18.29 0.35350.0    10.0     283    1.4561   28.58 0.55360.0    15.6     288.6  1.6426   43.92 0.84970.0    21.1     294.1  1.8221   66.39 1.28480.0    26.7     299.7  1.9950   98.85 1.91290.0    32.2     305.2  2.1615   145.05                                  2.806100.0   37.8     310.7  2.3221   209.90                                  4.061140.0   60.0     333    2.9109   814.5 15.75______________________________________ 
    
     An attempt was made to dry the crystals by heating them, but by comparing the vapor pressure of ammonium carbamate at 37.8° C. with the vapor pressure of water at this same temperature, that is 209.9 mm Hg versus 49.157 mm Hg, it became apparent that the ammonium carbamate would evaporate much faster than the water. 
     A chamber for filtering the ammonium carbamate crystals and then washing them with liquid ammonia was designed and manufactured. The chamber was constructed from an 18 inch length piece of 8 inch, schedule 40 aluminum pipe. To each end of the aluminum pipe was welded an 8 inch aluminum flange and the chamber was then closed with a blind 8 inch flange at each end. A filtering plate was welded into the interior of the chamber at a location 8 inches from the bottom of the chamber and 10 inches from the top. The filter plate contained 396 holes, each having a 1/16 inch diameter. The holes were drilled in concentric ring patterns about a center hole. The rings formed by the holes were about 1/3 of an inch apart. The outer ring contained 66 holes and each successively smaller ring contained six less holes than the previous ring, continuing to the center ring which contained 6 holes. A 20 cm filter paper was placed on this filter plate before the filtering and washing operation was conducted. 
     Into the top flange, about one inch from the inside wall of the chamber, a 1/4 inch hole was tapped for receiving a pressure gauge. The liquid ammonia for washing the ammonium carbamate crystals was circulated through a 1/8 inch pipe and a spray head which formed a solid cone-shaped spray. The spray head was placed at a height above the filter cake that allowed uniform washing of the filter cake. 
     A 3/8 inch hole was tapped in the center of the bottom flange and was used as the drain line for the chamber. About one inch from the inside edge of the chamber, a 1/4 inch hole was tapped in the bottom flange and a standpipe was placed inside the chamber. This standpipe and attached pipe to the outside were used to control the pressure in the base of the chamber. All pipes which led into or out of the chamber were closed with valves and a pressure gage was coupled to the standpipe line. 
     The following procedure was followed in washing the moisture from the ammonium carbamate crystals. After decanting the liquid from the crystals in the crystallization chamber, the crystals were transferred to the filtration and washing chamber. The crystals were spread out evenly and the wet crystals were packed onto the filter paper and filtered for ten minutes by applying a vacuum through the standpipe line of the chamber and closing the drain line. After filtering the crystals, the valve on the standpipe line was closed and the drain line valve was opened and the filtered liquid was allowed to drain. The cover of the filtration and washing chamber was closed, and prior to washing the crystals with ammonia, the pressure of the chamber was increased with nitrogen gas to about 40 psig. 
     As the crystals were being washed with ammonia, the opening of the drain valve was carefully regulated to maintain a pressure in the washing chamber at above 20 psig. Whenever the drain valve began to plug, the opening of the valve was washed with water. 
     After the last ammonia cylinder had been used to wash the crystals, the drain valve was opened to reduce the pressure in the chamber. When the pressure had been reduced, the chamber and contained filter cake were aerated by running compressed air into the ammonia line and out through the drain line. 
     The chamber was then opened and the crystals were removed and weighed. The crystals were spread over a shallow pan, and any lumps present were crushed and the contained gases were allowed to escape. The crystals were sampled and analyzed for water content. 
     For each test, the test number, weight of the wet crystals, and the number of washes were recorded. Following is a summary of the test results: 
     
         ______________________________________Summary of Filtering and Washing Results                    Washes  Weight            Per     % Water  of Wet   Number   1000 g  inExamp. Crystals of       of Wet  WashedNo.    (g)      Washes   Crystals                            Crystals                                   Remarks______________________________________K4B1   1180     9        7.6     1.8K4B2   1403     8        5.7      0.76K5B1   1480     8        5.4     1.4K6B1   1700     10       5.8     4.6    pressure                                   below 10                                   psiK6B2   1390     9        6.5      0.32K7B1   1345     7        5.0     1.3______________________________________ 
    
     Each of the pressure cylinders used in the washes had a capacity of 500 ml and contained about 400 ml of liquid ammonia. 
     An important aspect of the present invention is to wash the crystals with liquid ammonia so as to reduce the percentage of water contained therein. During the washing of crystals with liquid ammonia, it is important to maintain a sufficiently high pressure in the filtration and washing chamber to prevent the ammonia from flashing to a gas before it passes through the filter cake. 
     When the pressure in the washing chamber was allowed to drop below about 20 psig, the liquid ammonia entering the chamber flashed to a gas and instead of washing the crystals with a liquid, they were washed with a gas which was less effective in removing water from the crystals. Any ammonia which flashed to a gas in the washing chamber possibly lowered the temperature of the chamber so that at 20 psig, the ammonia sprayed therein remained sufficiently liquid to wash the water from the ammonium carbamate crystals. 
     Example No. K6B1 demonstrates how less effective washing with a gas is as compared to washing with liquid ammonia. Disregarding Test No. K6B1 , an average of 6.04 washes produced an average moisture content in the crystals of 1.12%. On this basis, it took about 6.8 washes to lower the moisture content of 1000 grams of wet crystals to a level of 1%. 
     CRUSHING AMMONIUM CARBAMATE CRYSTALS AND PRODUCING PELLETS THEREFROM 
     Before the present inventor learned that the crystals contained too much water to produce granules or pellets, many methods of granulation were attempted. The crystals were heated in the laboratory over a steam bath to their softening point, and then allowed to cool while being stirred. Using this method, granules of about 3/8 inch diameter were produced. These granules, however, did not retain their strength. 
     Two different devices for compacting the crystals were designed. Using these devices, the crystals were formed into rather strong cylindrical pellets, but it was discovered that after long standing the pellets lost their structural strength. 
     Crystals were next submitted to a commercial pellet mill to be tested for making pellets with its equipment, but in the pelletizing process, the crystals became so hot that they fumed. 
     Further experiments included placing crystals in the bottom of a beaker which was tilted and rotated. These experiments revealed that a pan granulator would be a good device for producing pellets from the ammonium carbamate crystals. Accordingly, arrangements were made with another company to conduct a pan granulation test. 
     PELLETIZATION TESTING 
     Using both ammonium and calcium lignosulfonate as binders, tests were conducted which produced unsuccessful results. It was learned from these tests that the lignosulfonates were poor binders for this system and that the crystals needed to be passed through a screen before being granulated. 
     The present inventor conducted tests to illustrate the discovery that humic acid is an effective binder for forming ammonium carbamate into pellets. Literature from SURFACTANT TECHNOLOGIES CORPORATION describes its product, Surtech L-101, as being a lignite and being composed of about 30% hydrophobic clay, 30% humic acid and the remainder being water plus the requisite organics (wax). Solutions comprising L-101 were made by adding 5%, 10% and 20%, respectively, of pulverized L-101 to aqua ammonia. Aqua ammonia was used as the solvent because it dissolves humic acid and the contained ammonia, and prevents the water from hydrolyzing the ammonium carbamate to ammonium carbonate. In making these solutions, only the humic acid was soluble. The undissolved clay and organics formed a deposit in the bottom of the container. 
     The 5% solution of L-101 contained the following: 
     (0.05×30)/(0.05×30+95)=1.55% humic acid dissolved in an aqua ammonia solution. 
     The 10% solution contained: 
     (0.1×30)/(0.1×30+90)=3.2% humic acid dissolved in an aqua ammonia solution. 
     The 20% solution contained: 
     (0.2×30)/(0.2×30+80)=7.0% humic acid dissolved in aqua ammonia. 
     It was determined that the 10% solution represented about the limit of solubility of the humic acid in aqua ammonia and accordingly the subsequent test work was performed using a 10% solution of Surtech L-101. 
     Using an eyedropper, and weighing 100 drops of the 10% solution, it was determined that after the addition of the first 20 drops, each 20 drop addition of solution increased the weight of the solution by 0.8782, 0.9384, 0.9308 and 0.9680 grams, or an average of 0.92885 grams per 20 drops, or an average of 0.0464 grams per drop. 
     One drop of this solution was then added to the following weights of pulverized ammonium carbamate crystals and each wetted mass was rolled into a pellet with the following results: 
     
         ______________________________________Ammonium Carbamate Bonded With a Solutionof Humic Acid in Aqua Ammonia    Grams of  PercentEXAMPLE  Ammonium  BondingNO.      Carbamate Solution Remarks______________________________________F16D1    0.3500    11.7     Pellet was too wet and                       took about 10 minutes                       to set upF16D2    0.3850    10.8     Pellet was still too                       wet and took about 5                       minutes to set upF16D3    0.4240    9.9      Pellet was somewhat wet                       and took about 5                       minutes to set upF16D4    0.4660    9.1      Pellet was less wet and                       took about 41/2                       minutes to set upF16D5    0.513     8.3      Pellet was less wet and                       took about 41/2                       minutes to set upF16D6    0.564     7.6      Pellet was somewhat dry                       and took about 41/2                       minutes to set upF16D7    0.620     7.0      Pellet was dry and took                       about 41/2 minutes to                       set upF16D8    0.682     6.4      There was not enough                       solution to bond the                       crystals into a pellet______________________________________ 
    
     Based on these results, a pellet containing approximately 8% of the bonding solution has the optimum characteristics. At about 3% humic acid in the bonding solution, the pellet contained approximately 0.03×8%=0.24% humic acid. From the above data, the humic acid is preferably present at from between 0.21% to 0.35%, and, more preferably, about 0.24%. 
     The present inventor also conducted tests to illustrate the discovery that a soy based protein bonds ammonium carbamate into pellets. This protein from SURFACTANT TECHNOLOGIES CORPORATION is identified as Surtech P-827 and is described as a 100% active modified polypeptide. It is, however, according to later information, a soy based protein with a molecular weight of 150,000 to 200,000. The manufacturer&#39;s literature indicates that a 5% or more solution of Surtech P-287 in water plus enough ammonia to raise the pH to about 9.5 to 10 can be made. In an aqua ammonia solution that would not hydrate the ammonium carbamate to ammonium carbonate, a 5% of the Surtech P-827 was determined to be about the maximum amount that could be put into solution. Accordingly, a 5% solution of Surtech P-827 was made up and 110 drops of the solution was weighed at 20 drop intervals. The increase in weight with each 20 drops was as follows: 0.6049+0.6046+0.6518+0.6186+0.6442+0.6698=3.7939 grams. The average weight of each 20 drops was 0.6323, or an average weight per drop of 0.0316 grams. 
     One drop of the 5% bonding solution was then added to various weights of pulverized ammonium carbamate and each wetted mass was then rolled into a ball to form pellets and the following results were obtained: 
     
         ______________________________________Ammonium Carbamate Bonded With a Solutionof a Soy Based Protein in Aqua Ammonia    Grams of  PercentEXAMPLE  Ammonium  BondingNO.      Carbamate Solution Remarks______________________________________F21D1    0.3630    8.0      Pellet was not wet and                       set up about 5 minutesF21D2    0.3990    7.3      Not sufficient bonding                       solution to form a                       pelletF21D3    0.3300    8.7      Pellet was not wet and                       took less than 5                       minutes to set upF21D4    0.3000    9.5      Pellet was not wet and                       took less than 5                       minutes to set upF21D5    0.2727    10.4     Pellet was not wet and                       took less than 5                       minutes to set upF21D6    0.2480    11.3     Pellet was not wet and                       took less than 5                       minutes to set upF21D7    0.2250    12.3     Pellet was not wet and                       took less than 5                       minutes to set upF21D8    0.2050    13.4     Pellet was not wet and                       took less than 5                       minutes to set up______________________________________ 
    
     The pellets formed over a broad range of contained bonding solution. It is generally advantageous to use the minimum amount of bonding solution that still produces a strong pellet. On this basis, such pellet would certain about 8% or more of the bonding solution and the percentage of soy based protein would be about 8%×0.05=0.4% or more. A preferred range of the soy based protein is from about 0.40% to 0.67%. 
     PELLETIZATION OF AMMONIUM CARBAMATE WITH SODIUM CHLORIDE 
     The present inventor discovered that a solution of humic acid, prepared by dissolving 10% of Surtech L-101 in aqua ammonia, as previously described, bonds a broad range of combinations of ammonium carbamate and sodium chloride into pellets. One drop of this solution was added to the following weights of pulverized and mixed, ammonium carbamate and sodium chloride and the wetted masses were then rolled into pellets with the following results: 
     
         ______________________________________50% Ammonium Carbamate with 50% Sodium ChlorideBonded with an Aqua Ammonia Solution of Humic Acid     Grams of PercentEXAMPLE   Crystal  BondingNO.       Mixture  Solution Remarks______________________________________G5D1      0.6800   6.4      Not enough solution to                       bond the crystals into                       a pelletG5D2      0.6180   7.0      Pellet was set up and dried                       in about 21/2 minutesG5D3      0.5110   8.3      Pellet set up and dried                       in about 3 minutesG5D4      0.4220   9.9      Pellet set up and dried                       in about 3 minutesG5D5      0.3490   11.7     Pellet set up and dried                       in about 3 minutesG5D6      0.2880   13.9     Pellet set up and dried                       in about 3 minutesG5D7      0.2380   16.3     Pellet set up and dried                       in about 41/2 minutes                       and became too wet______________________________________ 
    
     From these data, a preferred range for humic acid is from between about 0.21% to 0.49%. 
     In order to determine the upper limit on the amount of sodium chloride that can be bonded with a bonding solution containing humic acid dissolved in aqua ammonia, a mixture containing 10% ammonium carbamate and 90% sodium chloride was tested, and the following results were obtained: 
     
         ______________________________________10% Ammonium Carbamate with 90% Sodium ChlorideBonded with a Solution of Humic Acid in Aqua Ammonia     Grams of PercentEXAMPLE   Crystal  BondingNO.       Mixture  Solution Remarks______________________________________G5D16     0.4220   9.9      Not enough solution to                       bond the crystals into                       a pelletG5D17     0.3840   10.8     Pellet set up and dried                       in about 2 minutesG5D18     0.2380   16.3     Pellet set up and dried                       in about 21/2 minutes______________________________________ 
    
     As the percentage of sodium chloride in the crystal mix was increased, additional bonding solution was required in order to form a pellet. Humic acid additions between about 0.32% to 0.49% allowed pelletization. In an attempt to determine the upper limit of sodium chloride that would pelletize with this bonding solution, a mixture of 95% salt with 5% ammonium carbamate was tested. 
     
         ______________________________________5% Ammonium Carbamate with 95% Sodium ChlorideBonded with a Solution of Humic Acid in Aqua Ammonia      Grams of  PercentEXAMPLE    Crystal   BondingNO.        Mixture   Solution Remarks______________________________________G6D1       0.3840    10.8     Mixture would not                         pelletize even with                         additional bonding                         solution______________________________________ 
    
     A bonding solution composed of humic acid dissolved in aqua ammonia, will bond compositions of ammonium carbamate and sodium chloride in the range of from about 10% to 100% ammonium carbamate and from about 0% to 90% salt. As described above, as the percentage of sodium chloride in the mixture is increased, a higher percentage of bonding solution is required to produce a pellet. The maximum amount of salt in the mixture that can be bonded is between about 90% to 95%. Furthermore, 100% ammonium carbamate can be pelletized with about 7% of the bonding solution, while a mixture containing 90% salt and 10% ammonium carbamate requires over 10% of the bonding solution. 
     It was also discovered that a solution comprising a soy based protein which was prepared by dissolving 5% of Surtech P-827 in aqua ammonia, as previously described, bonds all proportions of ammonium carbamate and salt into pellets. One drop of this solution was added to the following weights of pulverized and mixed ammonium carbamate and sodium chloride, and the wetted mass was then rolled into pellets with the following results. 
     
         ______________________________________50% Ammonium Carbamate with 50% Sodium Chloride Bondedwith an Aqua Ammonia Solution of a Soy Based Protein      Grams of Percent      Crystal  BondingEXAMPLE NO.      Mixture  Solution Remarks______________________________________G5D8       0.4000   7.3      Pellet formed and dried                        in about 21/2 minutesG5D9       0.4400   6.7      Pellet formed and dried                        in about 21/2 minutesG5D10      0.4840   6.1      Not enough solution to                        bond the crystals into                        a pelletG5D11      0.3310   8.7      Pellet formed and dried                        in about 6 minutesG5D12      0.2740   10.3     Pellet formed and dried                        in about 2 minutesG5D13      0.2260   12.3     Pellet formed and dried                        in about 2 minutesG5D14      0.1870   14.5     Pellet formed and dried                        in about 2 minutesG5D15      0.1550   16.9     Pellet formed and dried                        in about 4 minutes______________________________________ 
    
     The soy based protein in aqua ammonia bonded the mixture of 50% ammonium carbamate and 50% sodium chloride so that it pelletized more readily than did either pure ammonium carbamate or the mixtures which contained 10% or less or ammonium carbamate. From these data, the preferred range for the soy based protein is from about 0.34% to 0.85%. 
     
         ______________________________________10% Ammonium Carbamate with 90% Sodium Chloride Bondedwith an Aqua Ammonia Solution of a Soy Based Protein      Grams of Percent      Crystal  BondingEXAMPLE NO.      Mixture  Solution Remarks______________________________________G5D19      0.2740   10.3     Not enough solution to                        bond the crystals                        togetherG5D20      0.2490   11.3     Not enough solution to                        bond the crystals                        togetherG5D21      0.2260   12.3     Not enough solution to                        bond the crystals                        togetherG5D22      0.2050   13.4     Pellet formed and dried                        in about 2 minutesG5D23      0.1860   14.5     Pellet formed and dried                        in about 2 minutesG5D24      0.1690   15.8     Pellet formed and dried                        in about 1 minuteG5D25      0.1540   17.0     Pellet formed and dried                        in about 1 minute______________________________________ 
    
     This mixture of 10% ammonium carbamate with 90% sodium chloride pelletized surprisingly well. Significantly, more of the bonding solution comprising the soy based protein was required to pelletize the crystal mix. From the data, the preferred range of the soy based protein for this mixture is form about 0.67% to 0.85%. 
     
         ______________________________________5% Ammonium Carbamate with 95% Sodium Chloride Bondedwith an Aqua Ammonia Solution of a Soy Based Protein      Grams of Percent      Crystal  BondingEXAMPLE NO.      Mixture  Solution Remarks______________________________________G6D2       0.2050   13.4     Pellet formed and dried                        in about 2 minutesG6D3       0.2260   12.2     Not enough solution to                        bond the crystals into                        a pelletG6D4       0.1860   14.5     Pellet formed and dried                        in about 2 minutesG6D5       0.1690   15.8     Pellet formed and dried                        in about 1 minuteG6D6       0.1540   17.0     Pellet formed and dried                        in about 1 minute______________________________________ 
    
     These test results were unexpected because the bonding solution made from humic acid would not bond a crystal mixture comprising only 5% ammonium carbamate. Accordingly, there was no expectation that the bonding solution made from the soy based protein would bond the same. After discovering that such bonding did in fact occur, the following tests related to bonding sodium chloride were conducted: 
     
         ______________________________________Sodium Chloride Bonded with an AquaAmmonia Solution and a Soy Based Protein      Grams of Percent      Crystal  BondingEXAMPLE NO.      Mixture  Solution Remarks______________________________________G6D8       0.1540   17.0     Not enough solution to                        bond the crystals into                        a pelletG6D9       0.1400   18.4     Pellet formed and dried                        in about 2 minutes______________________________________ 
    
     A higher percentage of bonding solution, namely 18.4%, was required to bond sodium chloride into a pellet. This value corresponds to 0.92% of the soy based protein in the pellet. 
     Based on the above results, all compositions containing sodium chloride and or ammonium carbamate can be bonded with an aqua ammonia solution containing a soy based protein. 
     CRUSHING TESTS 
     Believing that the crystals needed to be crushed to form a strong pellet, the present inventor conducted additional compaction tests in the laboratory. In these tests, the crystals were ground in a mortar and pestle before being compacted. It was then discovered that by placing the pellets in plastic vials and storing them in a pressurized container, the pellets would, on standing, actually increase in strength as opposed to losing strength. 
     The need to keep the crystals in a pressure tight container was thus determined to be important for the purpose of producing pellets that would remain stable. Based on this finding, arrangements were made with two associated companies to crush the ammonium carbamate crystals and then to pelletize the crushed crystals in a pan granulator. Before running the crushing tests, there was a concern that, because of the softness of the crystals, the crushed crystals might blind the screen in a hammer mill. A second concern was that the crushing operation might cause overheating of the crystals. 
     Example A7C1 was conducted in a hammer mill with the hammers turning at 9750 rpm, and it was found that the product produced remained cool and that the screens did not become blinded. The product produced, however, was too fine to adequately feed into a pan granulator. 
     In Example A7C2, the openings in the screens were enlarged, first to 3/32 inch and then to 3/16 inch, yet the product produced was still too fine. 
     In Example A7C3, the screen openings were 3/16 inch, and 20% of the material being ground passed through a 200 mesh screen, and the speed of the hammers was reduced to 5900 rpm. The product which was produced was coarse enough to feed well into the pan granulator, and also fine enough to form a strong spherical pellet having a diameter of about 1/8 inch. 
     An adjustable pan granulator, such as a DP-14 Agglo-Miser 14 inch pan granulator, may be used in accordance with the present invention. The tilt and speed of the pan can be adjusted to allow crushed crystals to run down the face of the pan and to break from the outside circumference of the pan at a selected position. 
     Based on the above results, the hammer mill is an excellent device for crushing the ammonium carbamate crystals. During the crushing operation the crystals remained cool and the screens did not become blinded. By adjusting the screens on the hammer mill and the speed of the hammers, the product so produced was free flowing. Such a product can be fed into a pan granulator. 
     SUMMARY OF METHOD OF PRODUCTION OF PELLETS 
     The method described above may be summarized as comprising the following steps: 
     Producing ammonium carbamate in a solution; 
     crystallizing the ammonium carbamate from the solution; 
     separating the ammonium carbamate crystals from the solution; 
     washing the separated crystals with liquid ammonia to dry the crystals; 
     crushing the dried crystals; 
     agglomerating the crushed crystals; and 
     storing the agglomerated crystals. 
     A method of forming pellets of ammonium carbonate which are especially useful as a roadway deicer and may be delivered by standard spreading vehicles has been described above. It will be understood by those skilled in the art, however, that modifications may be made in the crushing and pelletizing steps to produce products having different characteristics. 
     DESCRIPTION OF A PILOT PLANT 
     Referring to FIG. 1, first absorber 10 is conventionally employed in a urea production plant as described above. In accordance with the present invention, the solution in the first absorber herein referred to as the &#34;bottoms liquid&#34;, at a pressure of approximately 225 psi, is fed through pipe 11 under the control of valve 12 into a crystallization chamber 15 where the liquid pressure is reduced to and maintained at about 100 psig. 
     Conventional heat exchange elements (not shown) remove heat by way of liquid flowing through pipes 16 and 17. 
     The crystallized product is discharged through outlet 19 to diverter valve 20 and caused to flow through either of pipes 21 or 22 into one of pressure filters 23 or 24, which filters are maintained at a pressure below that of the crystallization chamber 15. 
     For drying the ammonium carbamate crystals, as described above, a liquid ammonia line 8, which is controlled by a diverter valve 9, alternately feeds ammonia liquid to the pressure filter at a pressure greater than the discharge pressure of the filter or the pressure at which the ammonia would become a gas. Filtrate from these filters is recirculated through lines 25 and 26 by pumps 27 and 28, and controlled by diverter valves 29 and 30 to the crystallization chamber 15. The diverter valves may be operated to cause the filtrate and any ammonia in the line to return to the first absorber 10 through lines 31, 32 such as, for example, during a shutdown of the crystallization chamber. 
     Crystals that are collected and dried in the pressure filters are discharged alternately through lines 34 and 35 to line 36 and to a hammer mill 38. 
     From the hammer mill 38, the crushed crystals are fed to a pan granulator 40 and then onto sizing screens 41. Undersize pellets pass through line 42 and to hammer mill 38. Pellets of selected size are transferred through line 44 to storage tank 45 for storage under a pressure sufficient to prevent volatilization of the ammonium carbamate. 
     The foregoing description of the preferred embodiment of the invention has been presented to illustrate the principles of the invention and not to limit the invention to the particular embodiment illustrated. It is intended that the scope of the invention be defined by all of the embodiments encompassed within the following claims, and their equivalents.