Properties of Granular and Monocrystalline Ammonium Nitrate WILLIAM H. ROSS, J. Y. YEE, AND STERLING B. HENDRICKS Division of Soil and Fertiliser InvestigatZoqts,
U. S. Department of Agriculture, Beltsuille, Md.
c
Ammonium nitrate conSuch inherent properties of ammonium nitrate as hygroAPACITY for producscopicity, melting point, solubility, and absolute density t a i n i n g synthetic nitrogen ing synthetic ammonia are independent of its method of manufacture. Other was first produced a t Notodand nitric acid in the United den, Norway, in 1910. In properties, such as its capacity to hold moisture, caking States and Canada has been the process used at that time tendency, crushing strength, and apparent density vary greatly expanded during the with the size, shape, and porosity of its particles; the latthe product was recovered in war. Postwar utilization of ter, in turn, vary with the process used in its preparation. the form of relatively small p a r t of t h e s e f a c i l i t i e s is This paper presents experimental data on the relation of crystals. The latest developplanned for production of porosity, size, and shape of ammonium nitrate to its bement in the crystallization ammonium nitrate for the havior in storage and its drillability in the field. The remethod is known as the Oslofertilizer trade. The marked sults show that ammonium nitrate in the form of porous ICrystal process (IO,14). The tendency of this material to solution from n e u t r a l i z e r s granules of uniform size and shape is more free flowing, cake and to absorb moisture c o n t a i n i n g 80-8570 of amunder humid conditions inexhibits less tendency to cake, and has a greater capacity to hold moisture before becoming undrillable than other monium nitrate at 75-80' C. terferes considerably with its types of the material which are less porous and more is mixed with mother liquor use for this purpose. The irregular in particle size and shape. of 65Y0 concentration and first synthetic ammonium nitrate on the market was made pumped to a vacuum e v a p in the form of monocrystals of orator. T h e discharge from this evaporator is passed a t 58-60' C. to a second evaporarelatively small particle size. Since then spraying and graining tor operating a t lower pressure. The bottom of this evaporator methods have been developed for producing this material in the communicates with a crystallizing tank. The supersaturated form of granules. The ammonium nitrate prepared by these different methods differs considerably in certain physical properties. solution entering this crystallizing tank from the second evaporaObservations are reported here on the properties of these different tor is uniformly conducted upward through a dense suspension of crystals. The classifying action in the crystallising tank keeps types of ammonium nitrate that affect particularly its behavior the fully grown crystals suspended in the bottom layer, and the in storage and ita drillability in the field. (The data presented smallest crystals in the top layer with the intermediate sizes in this paper relates only to the particular material submitted suspended between. The saturated solution leaving the crystalby the producers. Subsequent improvements in manufacture may lizing tank is mixed continuously with the solution from the neualter greatly the physical properties of the recovered products.) tralizers, the mixture is again supersaturated to the desired exI n the spraying method, a spray of molten ammonium nitrate tent, and the cycle is repeated. containing 4-5y0 of moisture is allowed to fall in a tower through a countercurrent flow of cool air. The liquid droplets congeal in PROPERTIES O F THREE TYPES their fall through the tower to form sphereshaped granules of Table I gives mechanical analyses of these three types of amapproximately 8-20 mesh size. As the congealed granules monium nitrate. The sprayed and grained materials represent reach the bottom of the tower, they are collected in a hopper with commercial production. The monocrystalline product was proan open bottom and are conveyed on an endless belt to the drying duced on a pilot plant scale. A microscopic examination of t h e equipment (5, 4, 6,1.9). three types showed that each particle of Krystal ammonium niIn the graining process a melt of ammonium nitrate containing trate consists of a single crystal, whereas the granules of the 1-2% of moisture is run into a circular shallow vessel equipped sprayed and grained materials are aggregates of crystals. Dewith mechanical plows and jacketed for cooling with water or termination of refractive indices with the petrographic microheating with steam. The rotation of the plows agitates the matescope showed that the particles of Krystal ammonium nitrate rial during the graining process. It thus prevents the formation were the orthorhombic form stable below 32' C., and had not of a solid mass as the rest of the moisture is being eliminated by passed through that transition (9). Differences in the physical the heat first developed by the crystallization of the matcrial from a state of fusion, and later supplied by steam or hot water in properties of the three types of ammonium nitrate are illustrated in Figures 1to 0. the jacket of the graining apparatus (7, 11). The product preThe apparent density of the particles of each type of ammonpared in this way is satisfactory with respect to the shape of parium nitrate in bulk (Table 11) represents the ratio of the weight of ticles, but the average particle size of the material now on the material required to fill a liter cylinder to the weight of the same market is smaller than is desired in a fertilizer intended for sepavolume of water at room temperature. The cylinder was filled rate application in the field. 1079
INDUSTRIAL A N D ENGINEERING CHEMISTRY
Vol. 37, No. 11
Figure 1(Left). Sprayed AmmoniumNitrate, Old Production, 14-16Mssh ( X 16) Figure 2 (Right). Sprayed Ammonium Nieate, Current Production, 14-16 Mesh ( X 16) MOISTURE M AMMONIUM NITRATE
TABLE I. SCREEN ANALYSISOF TYPICAL SAMPLES OF SPRAYED. GMINED, AND MONOCIZYSTALL~NE AMMON~UM N~TRITE
p .te"d
Of
+io
meab
Screen Frsetiona. % 10.14 14-20 20-40 mcah mcsh meah
-40
mssh
lloisture wa4 determined in the different types of ammonium nitrate by tho following methods: 1. Drying in an sir oven a t 70" C. for 20 hours. 2. Drawing air at 70" C. through the ssmple for 2 hours.
3. Dryingin avaeuum at room temperature over Anhydrona (anhydrous magnesium perchlorate) for 20 hours. a
Old produotion.
b Current produotion.
hy adding small increments of tho material to the cylinder while it was gently tapped. Esoh value represents the mean of five determinstions. The apparent density of the sverage particle was determined by noting the density of sn orgsnic liquid thst will hold the greater pari of the particles in suspension. The ahsolute density of ammonium nitnrte is 1.72 (6). The values obtained for the apparent density of the average particle of Sprayed, grained, and monocrystalline ammonium nitrate nre 1.56, 1.65, snd 1.69, respectively. These data indicate that the sprayed particles oontain an average of about 10% of voids, the grained psrticles about 4'%, and the ,monocrystalline particles leas than 2%. It WBS found that the particles of monocrystalline arqmonium nitrate of l&mesh sise contain more voids than do t h w of lFrmesh &e, an indication that the voids in this material increase with increase in orystal size. Inasmuch BS the grsnutes of sprayed ammonium nitrate eonsiat of aggregates of crystala while the particles of the monocrystalline material consist of only single crystals or q a t d fragmenta, it would be expected that the apparent density of the EWEtab would be higher than that of the granules. The results ob. tained in the density determmstiona show that this is the case.
Moisture hy these three methods wae determined in ( 0 ) sprayed ammonium nitrate that had not heen dried, (6) spraxed nitrste that had heen dried and then treated with B fine mist of waster while being rolled in a drum, (e) grained ammonium nitrate that had absorbed moisture siter being dried, and (8monocrystalline ammonium nitrate that had not heen dried. The results obtained (Table 111) show that the three methods agree quite cloaely in the determination of absorbed moisture in all four tm of ammonium nitrate, and that the vacnum method gives lower results than the other two methods for the determinstion of original moisture in ammonium nitrste. The dsts indicate that a portion of the original moisture in ammonium nitrate is present as oocluded moisture and that this moisture .is more difficultto remove than absorbed moisture.
TABLE 11. APPWNT DENSITY OF DIP~ERFNT TYPES OP
a% S*r.~sd~ Swryadb
*
0
Oreinad Monoaryatsi. Old pmduction. Currant production.
AXMONXUM NITRATE
-App*rent Density dPnrtiolas in hulk -47.wrticlc 0.883 0.859 iS8 0.978 1.85 0.973 1.89
Novamher, 1945
INDUSTRIAL A N D ENGINEERING CHEMISTRY
1081
Figure 3 ( f 4 t ) . Spray& .Ammonium Nitrate. Showing EReci of Drjinp Follo*ine: Sliph: Ab-rption of >loi*iure ( X 16) Figure I (Righr). G r a i n 4 Ammonium Nitrate, 11-16 Mesh ( X 16)
The moisture in monocrystalline ammonium nitrate, which the vacuum method over Anhydrone fails to remove, amounts to about 0.35% of the weight of the sample. The data in Table I1 ii.dicate that newly 2% of the volume of the crystals is void. I t must follow, therefore, that the greater part of the void space in the orystsls is filled with air rather than mother liquor. h s o n ~ r l OF o ~Mo~srUaE. Saturated solution8 of all three types of ammonium nitrate have the same vapor preasure at a given temperature (16). and all absorb moisture at essentially the same rate when exposed under the same conditions in a humid atmosphere (Table IV). The partides of granular ammonium nitrate, ae already explained, we somewhat porou~for the reaeon that they are made UP of nggregates of crystals. The partides of monmrystslline ammonium nitrste, on the other hand, are nonporous and are incapable of holding any moisture except OD thcir surfwe. A difference, therefore, exists in the quantity of moisture which the granular and monoeryatalline materials will ahsorh befcre they become dripping wet. Thus the solution phaee in
