154
INDUSTRIAL AND ENGINEERING CHEMISTRY
throughout this period. Part of the decrease in solution viscosity can no doubt be attributed to a real decrease in the average molecular weight of the soluble portion of tbe polymer as a result of removal of the higher molecular weight polymers selectively in forming the gel. However, it is believed that a further contributing factor is a change in the structure of the polymer particles as the polymerization proceeds. The rapid formation of gel at high conversions suggests that cross linking between the polymer chains is occurring, which no doubt affects the molecular structure of even that fraction of the polymer that remains in solution. It is believed that this effect may act to reduce the solution viscosity to a greater extent than it is increased by the accompanying rise in molecular weight. Solution viscosity values are little indication of the properties of these polymers made at conversions above that of the maximum point in the viscosity os. conversion curve, particularly if gel is present. A polymer of a given inherent viscosity made at a higher conversion may be very different from one of the same viscosity recovered at a conversion below that point. ACKNOWLEDGMENT
The authors wish to acknowledge the invaluable assistance of the various workers participating in the Rubber Reserve Research and Development Program whose numerous unpublished reports have contributed materially in this work.
Vol. 40, No. 1
LITERATURE CITED
(1) Baker, W.O.,Fuller, C. S., and Heiss, J. H., Jr., J . A m . Chem. Soc., 63,2142 (1941). (2) Cragg, L. H., J. Colloid Sci., 1, 261 (1946). (3) Craig, D.,U. S. Patent 2,362,052(Nov. 7, 1944). (4) Ewart, R. H., in Mark and Whitby’s “Advances in Colloid Science, Vol. 11, Scientific Progress in Field of Rubber and Synthetic Elastomers,” pp. 228-39, Interscience Publishers, Inc., 1946. (5) Fryling, C. F.,IND.ENG.CHEM.,ANAL.ED., 16, 1 (1944). (6) Hobson, R. W., Pierson, R. M., and Borders, A. M., Goodyear Tire & Rubber Co., private communication (iMar. 23,1943). (7) Xolthoff, I. M., and Harris, W. E., IND.ENG.CHEM.,ANAL. ED., 18, 161 (1946). (8) Xolthoff, I. M., and Harris, W. E., Univ. of Minn., private communication (June 15, 1943). (9) Kraemer, E. O.,IND.ENG.CHEM.,30, 1200 (1938). (10) Kraemer, E. O.,and Lansing, W. D., J . ‘Phys. Chem., 39, 153 (1935). CHEM.,ANAL.ED., 10,35 (1938). (11) Raaschou, P.E.,IND..ENQ. (12) Reynolds, W.B., Univ. of Cincinnati, private communication (Nov. 1944). (13) Smith, W.B., J. Am. Chem. SOC.,68,2064 (1946). (14) Starkweather, H.W., et al., IND.ENG.CHEM.,39,210 (1947). (15) Staudinger, H., “Die hochmolekularen organischen Verbindungen,” Berlin, Julius Springer, 1932. (16) Wollthan, H.,Drewer, M., and Becker, W., U. 6. Patent 2,281,613 (May 5, 1942). RECEIVED November 21, 1946.
Properties of Monocrystalline Ammonium Nitrate Fertilizer PHILIP MILLER’ AND W. C. SAEMAN Tennessee Valley Authority, Wilson Dam, Ala. Monocrystalline ammonium nitrate, produced on a pilot plant scale in a continuous vacuum crystallizer, was tested for resistance to shattering under impact and for behavior of the conditioned material during prolonged bag storage and when used in fertilizer distributors. These tests indicated that it was equal or superior to commercially available forms of ammonium nitrate for use as fertilizer. The previously reported adverse effect of low porosity and nonspherical shape on fertilizer properties for monocrystalline ammonium nitrate was not encountered with the present iinproved product. In addition to the favorable economics and product quality of cofitinuous vacuum crystallization, it is the least hazardous of the processes available for commercial use.
A
LARGE part of the United States and Canadian wartime capacity for producing synthetic ammonia arid nitric acid is now being utilized for the production of ammonium nitrate ,fertilizer. Such production has been limited by plant capacity for the production of solid ammonium nitrate in a form suitable for fertilizer use, and several producers have recently announced plans for construction of new plants for this purpose (9). Current production in the Cnited States is virtually all by the batch graining method ( 6 ) , which was designed originally to meet military requirements. A continuous spray granulation method (prilling} is used in Canadian plants. Although both processes give products that can be conditioned satisfactorily for 1
Present address, H . K. Ferguson Company, New York, N. Y .
fertilizer use by means of coating or dusting agents, the spraygranulated material is coarser and more free from fines, and requires less conditioning (IO). It is probable that new installations for ammonium nitrate fertilizer production will utilize some continuous process, such as spray granulation or crystallization, rather than batch graining. The Tennessee Valley Authority, which operates an ammonium nitrate graining plant, recently completed a pilot plant investigation in which the Oslo-Krystal crystallization process was successfully modified for the production of monocrystalline ammonium nitrate well suited for fertilizer use. This study of the crystallizing process was described in a recent paper ( 7 ) . Sufficient product was prepared in the pilot plant for a thorough evaluation of its storage and drillability properties, and the present paper gives the results of that study. Sprayed and grained ammonium nitrate and nitrate of soda, all obtained from commercial sources, were included in the tests to provide a basis of comparison. The properties of sprayed, grained, and monocrystalline ammonium nitrate intended for fertilizer use have been compared previously by Ross et al. (11, l a ) . The monocrystalline material tested in their work was produced in an earlier pilot plant investigation, made under the auspices of the Office of Production Research and Development ( I S ) , in which the crystallizing process differed significantly irom that used by TVA ( 7 ) . The OPRD material was quite different in appearance from the crystals tested in the present work and was, in general, markedly inferior to the latter in physical behavior, as indicated both by the
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1948
Prdurnd and photopraphsd *I 82" F. (form IVf
l'mdvclcd e t 82* F. and photowaphul a t 82" after healing to 97' and roulinp hack 10 82' (form IV after
double inversion)
Figure 1.
1'hotomit:mgraphs uf Ferm I l l and Form 1V Ammoninm Uilrate Crystals ( 8 tu 10 Mesh) Produced hy T V A
INDUSTRIAL AND ENGINEERING CHEMISTRY
IS6
T y y i d eryatsllizer w"diiet Let of blended oryatals for piant storage lest3
SprWedD
10
84
20
0
16
48 27
31 66 20
3 4
0
0
Grained
l'rystnls (form I V ) Grained produot S1"nycii p'od,*ct N II'XO,*
0
77 77 77 77
82
Vol. 40, No
are showi in Figure 2, wcic charact d by nllmPrOilS iiwu and voids, and, RS SMXI by compariso th Figure I, were mr more irrngular in shape than the TVA crystals. In the 01'1 wiwk attcni,ion was devoted chiefly to the p r a d u a t i o ~of~ cryst in form IVwIiich wcm dried below 90.1' F., in the bclief that I r:rystelu so produced would bo convidcrably strongor t h n u il that were permittcd to undergo inversion. IIowever, it v found in the prcsent work, sa will he sboum, tlmt by proper e< trol crystals produced above 90.1 " F. could be inverted to io 11' without significant a,eakening and that aftcr invr:riion tt were ELS strong 8 s or stronger than thosc produccd and ma tained in form IV. Etorage a n d drilling properties of ammoniuin nitrate ILZ heen iound to improvo nith jnerese in size and uniformity of 1 parf.iclcs (IO). The flcxibility oi vacuum cryst.alliaw operat. permit,s production of crystals of uniform size o w r i~ wide IBI 06 &os. A size range of 4 to 20 mesh was a d o p t d its the m practical and was conaistently obt.ainable in pilot p l i ~ noperatic l Screen a n a l p e ~of crystals and of other types oi aimnonil nitratc tested in this work are given in Tablo 11. Sincc 1 amount oi material required for tosting w s ratlsv l a w i n re tion lo the production eapaci1.y oi the crystiillizcr pilot pla some matrrinl produced in prciiminary operalion was ux:d, though part of it WBS somewhat outside the desired 1iinit.s particle size. The apparent density of particles of mnmonium nitrate various forms WBS determined by measuriiia the amount xylene displaced by a sample of know, w i g h t i n n calihial pyenometcr. The resulbs of these measurements %re shown Table 111. Bulk donsity tats were made by ir-cighing I amount of material conbained in a box holding 1 cubic io Cryst,& arid grains weighed 62 pounds per cubic iuol, and Nit, prills w-eighed 56 pouiids per cubic loot.
0
2
S 18
1.66-1.71 1.66 1.47 1.72 I 58-1 63 1.06
STORAGE PROPERTIES
Tlic storage proportios of monocrystslliiie ammonium nitn acre evaluated by storage to& made witlr 100-pwnd ha in st,acks 12 bags hi& over a 10-month period. DeliLiIs thc tost procedure have been given in n previous pnMieati deding with 8 study of grained ammonium nitrate ( 6 ) . I3aoat of the limited amount oi crystals availirhlc, only t,tie h i t o r n b (lo.12) oi each stack contained tho test nralciial. Tire degl oi erLking was evaluatcd by dropping tlx: bag oi fertilizer Eo
Figure 2. Sample of OPRD Form I V Ammoniunr Nitrate entsas high as 5.5y0 in individual tests, but no solution separated; after their moisture content exceeded about 4%, they became increasingly mushy and tended to clog the drill. This variation in moisture content' was not due to a difference in moisture absorption rate but to a difference in water-holding capacity, which \vas lower for t,he hard, relatively impervious cryst,alsthan for the much more porous prills. Ross et al. ( I @ , in their comparison of prills and OPRD erystals, studied this point in some detail and developed the hypothesis that the crystals, because of their lower water-holding capacity, are inherently less drillable under humid conditions than prills. They reported t,hat cryshls became undrillable a t a moisture content of 1.5%, whereas prills remained drillable until the moisture content exceeded 4%. Study of the data of Whittaker et al. ( l 4 ) ,from which the dat'a of Ross et al. were taken, indicates that the conclusion of the latt'er is supported by a single comparison between unconditioncd materials in one of three t'ypes of fertilizer distributor used in the tests. Data for unconditioncd ammonium nitrate in the other two types of distributor, and for variously conditioned ammonium nitrate in all t,hree types of distribut,ors, indicated that t,hc crystals remained if anything .more drillable than prills after severe exposure; condit>ionedcrystals showed almost no reduction in drillability alter absorbing 2% moisture. Whittaker et al. concluded from their data that "the untreated crystals stood up better in the gravity feed dispenser than did the Calgary product (prills). The results xith the two coated materials were about the same in all three dispensers." It seems clear that for practical purposes the drillability of different types of ammonium nitrate should be compared on the basis of comparable exposure rather than equal moisture content and in the conditioned rather than the unconditioned form. On
January 1948
INDUSTRIAL AND ENGINEERING CHEMISTRY
this basis it is concluded that OPRD crystals are equal, and TVA crystals superior, in drillability to spray-granulated ammonium nitraLe,which in turn is superior t o grained ammonium nitrate. STRENGTH OF CRYSTALS
v
The crushing strength of individual granules and monocrystals of ammonium ni\rate was studied by Ross et al. ( I $ ) , who used a device in which a particle is compressed between an anvil and a descending rack until it breaks ( 4 ) . They pointed out that the applicability of the method is limited by the plastic properties of ammonium nitrate, the anisotropic behavior of the monocrystals, and the need of relatively large particles for testing. After trial of this method it was found that for the present study an impact test gave more useful information. I n the impact test a sample of 8- to 10-mesh particles was fed into a stream of air discharging through a horizontal tube 0.5 inch in diameter and 6 inches long. The sample impinged a t a calculated velocity of 500 feet per second againstasteelplateperpendicular t o the path of flow. The plate was l S / a inches from the tube discharge; this distance was varied from 1 to 3 inches with little effect on the retjults. A screen analysis was made of the shattered sample. This method was used to compare the strength of monocrystals and other forms of ammonium nitrate, to determine the effect of the method of preparatibn on the strength of monocrystals, and to determine the effect of repeated inversion on the strength of monocrystals. The results obtained in impact tests made with prills and four differently prepared samples of monocrystals are shown in Figure 3. All of the crystal samples showed considerably less breakup than the prills. Similar tests with grained ammonium nitrate, not shown in Figure 3 because they were made with smaller particles, indicated that grains were intermediate between crystals and prills in resistance to shattering under impact.
2 4 6 8 NUMBER O F I N V E R S I O N S N 9 0 I T
Figure 4.
10
Effect of Repeated Inver-
sions on Strength of 8- to 10-Mesh
Crystals Produced in Form 111, Dried at 160' F., and Cooled below 90' F.
h
The four curves for crystals in Figure 3 show the relation of crystal strength t o several production factors. Crystals produced and dried above 90.1 F., and tested while still in form I11 (curve Z), were somewhat weaker than form IV crystals produced and dried below 90.1" F. (curve 3). However, when dried form I11 crystals were allowed to cool below 90.1 F. and invert to form IV (curve 4), their strength became equal to that of form IV crystals produced below 90.1 F. What is more important, their strength increased with age, so that 3 days after inversion (curve 5 ) the inverted crystals showed far Iess breakup under impact than the other samples. Other tests confirmed this behavior and indicated that maximum strength was appioached during the first week. O
O
159
Since it is more desirable to carry out the crystallizing and drying operations above rather than below 90.1 'F. (7), the fact that crystals produced in form 111 become stronger rather than weaker upon cooling below 90.1 ' F. is of considerable practical significance. Ross et al. (18) report the results of a tumbling test in which 8to 14-mesh samples were tumbled in 16-ounce bottles end over end for 48 hours, and the minus 14-mesh material was then determined. Values for percentage breakdown ranged from 0.95 to 9,1501, and were higher for OPRD monocrystals than for prills. I30
I I I ,CRYSTALS PROWCED As FIy(M Lp:
NUMBER OF INVERSION CYCLES COMRETED
Figure 5. Effect of Repeated Inversions on Bulk Volume of Ammonium Nitrate Produced in Various Forms
When this test was tried in the present work, using a Ro-tap machine for the screening, the weight of material passing the screen increased with the duration of screening and was considerably higher for prills than for crystals. It was learned (15) that hand screening had been used in the work of Ross et al. Although this may have minimized breakdown in preparing the sample and in evaluating the test, it is believed that the results given by their method of testing are misleading, because breakdown was measured by separation of minus 14-mesh material from a sample that was originally 8 to 14 mesh in size. Since the crystals were less regular in shape than the prills, there was more danger of incbmplete removal of minus 14-mesh material in the original screening. Furthermore, the removal of irregular points and edges from a crystal by tumbling would be likely to result in the passage of the entire crystal through the 14-mesh screen, so that the minus 14mesh fraction would not be a measure of the fines produced, whereas this would be less likely for the spherical prills. It is evident that the breakdown should be measured by a screen separation a t a smaller mesh size, such as 20 mesh, or preferably.by a complete screen analysis, as in Figure 3. EFFECTS O F REPEATED CRYSTAL INYERSION
In a recent bulletin of the United States Department of Agriculture (II), the possible effects of repeated fluctuations of the temperature above and below the 90.1 ' F. transition point on the behavior of ammonium nitrate in storage are discussed in some detail It is stated that, although summer temperatures frequently exceed 90.1 F., tests made under commercial conditions of storage indicate that the temperature of any considerable quantity of material in storage is not likely t o reach 90.1" F., and it is concluded that changes in crystal form actually encountered will be of little more than academic interest. Thase conclusions were corroborated by observations made by the present workers on crystals stored indoors in 100-pound bags a t Wilson Dam in summer. However, tests were made to determine the effect of repeated crystal inversions on the strength of crystals. A sample of crystals produced above 90.1" F. and cooled to form IV was alterO
I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY
160
nately heated to 95 F. and cooled to 85 F. for five complete cycles (24 hours per cycle), and the impact test was made on the crystals in form 111 and form IV during each cycle. The results are presented in Figure 4. There was a cumulative weakening of the crystals, the greatest change occurred after the first IV -,I11 inversion. The 111 IV inversion generally resulted in a slight increase in strength. Comparison with Figure 3 reveals that even after five cycles (10 inversions), the crystals were stronger than prills as purchased. Similar tests on the I11 f+I1 crystal inversion a t 183.6" F., which is of some interest in connection with the drying step in crystal manufacture, indicated no significaht eftect of a single cycle on strength or appearance. Repeated crystal inversion had a somewhat unexpected effect on the crystal volume. From the data of Table I, a reversible volume change of 3.8% is to be expected upon passage through the 90.1 F. transition point. It was found, however, that with repeated inversions a largely irreversible expansion was obtained. Ammonium nitrate samples in graduated 100-ml. cylinders were alternately heated to 113 O F. and cooled to 77 F. in 24-hour cycles. At the end of each cycle (at 77" F.) the cylinders were inverted to break up any cake formed, and after gentle tapping the volume was read. The results obtained with five different materials for 20 cycles are shown in Figure 5 as smooth curves that represent the actual data with sufficient accuracy. All the materials showed a cumulative increase in bulk volume for a number of cycles; the degree of expansion was greatest for the monocrystals, presumably because of their greater initial density. After about 10 cycles the crystals and prills began to break down into smaller particles with a consequent decrease in bulk volume. Fertilizer-grade TVA grained ammonium nitrate, which contains somewhat less than 0.1% Fez03 and AlzOa as impurities, continded to expand and showed no evidence of breakdown even after 20 cycles, whereas munitions-grade grained ammonium nitrate (substantially free of FezOs and AI2O3) broke down almost immediately. I n another series of tests, made a t constant volume and lasting 25 cycles, in which the tubes were left undisturbed between cycles, the interstices were gradually filled by expansion of the granules and the material caked, but no breakdown was observed. However, for the reasons stated earlier, these effects of crystal inversion are of academic interest only. The effect of crystal transformation on strength was considerably influenced by moisture content. This was most clearly evident in the pilot plant, production (7) of crystals above 90.1 F. where the damp crystals discharged from a ckntrifuge disintegrated rapidly if allowed to cool in their damp condition, whereas if they were dried first, they actually gained in strength upon cooling. General observation indicated that the detrimental cffeet gf moisture became evident when the moisture content exceeded about 0.2%. Study of the inversion behavior a t 90.1 ' F. in a petrographic microscope with polarized light showed that the number of transformation centers originating in a crystal was less for a dry crystal than a wet one, and this may account for the weakening effect of moisture. O
O
-+
O
O
CONCLUSIONS
On the basis of practical tests of the behavior of the conditioned material in storage and in fertilizer distributors, i t was concluded that monocrystaliine ammonium nitrate, 4 to 20 mesh as produced in a modified Oslo-Krystal pilot plant vacuum crystallizer, is equal or superior in physical behavior to commercially available forms of ammonium nitrate fertilizer. I n suitable bags, it should remain in satisfactory condition for direct application when stored in stacks 12 bags high for a t least a year in the most humid sections of the country. A satisfactory product was obtained from crystallizer operation either above or below the transition point of 90.1" F., but economy of crystallizer operation, ease and thor-
Vol. 40, No. 1
oughness of drying, and crystal strength favor the production of crystals above 90.1 'F. The crystals made by TVA in the modified Oslo-Krystal unit were significantly different and generally superior in appearance and physical behavior to monocrystals produced previously in an Oslo-Krystal unit by OPRD investigators and evaluated by USDA workers. The reported adverse effect of low porosity and nonspherical shape for the OPRD monocrystals ( I d ) was not encountered with the present product. Furthermore, review of the earlier studies indicated that conclusions reached on the relative merits of sprayed ammonium nitrate (Ritraprills) and OPRD crystals should have been more favorable to the crystals. Because of their large and uniform size, monocrystals, like sprayed and unlike grained ammonium nitrate, can be satisfactorily conditioned with a dust only; a water-repellent coating is not required and is not desirable. I n agreement with previous studies, kieselguhr is the most effective dust, and 3 to 4% is a satisfactory amount. Kaolin should also prove satisfactory but is less effective than kieselguhr. Crystals had higher resistance to shattering under impact than sprayed or grained ammonium nitrate. Particle weakening and cumulative volume increases were observe'd for all forms of ammonium nitrate upon repeated crystal inversions a t the 90.1 ' F. transition point, but these effects do not occur to a significant extent under commercial storage conditions in the United States. On the basis of the product quality, the crystallizer pilot plant investigation, and cost studies, continuous vacuum crystallization is an attractive method for the commercial production of ammonium nitrate fertilizer. Estimates indicate that the cost oi producing ammonium nitrate should be about the same for either crystallizing or spray granulation and significantly lower than for batch graining. Crystallizing has the important advantage of essentially eliminating the explosion hazard present in the evaporating steps of the graining and spray granulation processes ( 7 ) . ACKNOWLEDGMENT
Acknowledgment is made to W. A. Rice for performing microscopic studies of crystal transformations and to R. M. Johnson, T. R. Mitchell, Jr., and other members of the TVA staff who participated in the experimental work. LITERATURE CITED
(1) Adams, J. R., Love, K. S., and Ross, W. H., U. S. Dept. Agr., Div. Soil Fertilizer Investigations, Research Rept. 26, Supplcment 2 (1946). (2) Bowen, N. L., J . Phys. Chem., 30, 74-5 (1926). (3) Cordie, H. C., and Lawrence, R. W., U. S. Patent 2,399,987 (May 7, 1946). (4) ENG.CHEM.,30, 668-72 . . Hardestv. J. 0.. and Ross, W. H.. IND. (1938): ( 5 ) Hendricks, S. B., Posnjak, E., and Kracek, F. C., J . Am. Chem. SOC.,54, 2766-86 (1932). (6) Miller, P., Lepaeus, G. A., Saeman, W. C., and Dokken, M . N., IND. EHG.CHEM.,38,709-18 (1946). (7) Miller, P., and Saeman, W. C., Chem. Eng. Progress, 43, 667-90
(1947). (8) Morck, J. D., Brit. Patent 9687 (1899). (9) Private communications. (10) Ross, 14'. H., Adams, J. R., Yee, J. Y., and Whittaker, C. W., IND. ENG.CHEM.,36,1088-95 (1944). (11) Ross, W. H., Adams, J. R., Yee, J. Y., Whittaker, C. W., and Love, K. S.,U. S. Dept. Agr., Div. Soil Fertilizer Investigations, Tech. Bull. 912 (1946). (12) Ross, W. H.. Yee, J. Y., and Hendricks, S. B., IND. ENG.CHEM., 37, 1079-83 (1945). (13) Spencer, K. A., Rept. of War Production Board Project 122 (1944). (14) Whittaker, C. W., Lundstrom, F. O., Yee, J. Y . , Schoenleber, L. G., and Cummings, G. H., U. S. Dept. Agr., Div. Soil Fertilizer Investigations, Research Rept. 25 (1944). (15) Yee, J. Y., private communication. RECEIVEDSeptember 17, 1946. Presented before the Division of Fertilizer Chemistry at the 110th Meeting of the AMERICANCHEXICAL SOCIETY,
Chicago, 111.
