Reducing the Explosion Sensitivity of Ammonium Nitrate Fertilizer Carl Boyars Naval Surface Weapons Center, White Oak Laboratory, Silver Spring, Maryland 209 10
Introduction The likelihood of initiation of explosion in ammonium nitrate fertilizer by shock is increased by the increased porosity which is produced by cycling through the 32" phase change. Prevention of this phase change (and its consequent increase in sensitivity to detonation) is best accomplished by using a solid solution of potassium nitrate in ammonium nitrate. This is because the mechanism of phase stabilization in the solid solution is unaffected by moisture content, whereas inhomogeneously dispersed additives are effective phase stabilizers only until their desiccant capacity is exhausted. Ammonium nitrate (AN) finds extensive use as a fertilizer and as a major ingredient in commercial explosives, as well as occasional use in military explosives. The explosive capability of the fertilizer and the large quantities stored and shipped have led over the years to several disasters (Hansen and Berthold, 1972; Van Dolah, et al., 1966). The International Maritime Consultative Organization (IMCO) regulations require that AN be regarded as capable of explosion unless at least 18% of limestone is admixed (Hansen and Berthold, 1972). Many different factors can contribute to the likelihood of AN to undergo explosion. Operating procedures and manufacturing processes have been modified from time to time as a result of investigations following a disaster. For example, explosive charges are no longer used to break up a caked mass of AN, and the permissible content of combustible substances admixed in fertilizer AN is now very low. Each of these changes was preceded by accidents costing several hundred lives. It has been demonstrated that a high-velocity projectile (e.g., metal fragments resulting from another detonation nearby) can initiate detonation in AN in a manner similar to directly applied explosive shock (Van Dolah, et al., 1966). Recently, it has been shown (Hansen and Berthold, 1972) and confirmed (Goffart, 1973) that the sensitivity of AN to initiation by shock increases considerably after exposure to a few temperature cycles through 32 "C (90 O F ) , a condition which is often a normal daily temperature variation. AN "grows," decreasing in bulk density as a result of the 4% volume change that accompanies the 32 "C solid-solid IV-I11 phase change. Sensitivity of an explosive to initiation by shock is known to increase with heterogeneity of the explosive, porosity being one of the sensitizing heterogeneities (Price et al., 1974; Price, 1970). Mechanisms of hot spot formation such as adiabatic compression of gas or high-speed spalling of material at the surface of the pores may be operative. Increasing sensitivity of AN (Pasman et al., 1974) and of AN/fuel oil mixtures (Watson et al., 1974) accompanies decreasing density of AN. The IV-111 transformations of AN occur only in the presence of moisture; the mechanism apparently involves solution 308
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and recrystallization (Brown and McLaren, 1962). This explains why the bulk density of AN prills or grains continues to increase as the temperature cycle is repeated; i.e., the crystals do not go back to their original position in space as they are converted to the higher density form IV on cooling. As the water content of AN is reduced below 0.2%, the IV I11 transition temperature increases and the I11 IV transition temperature decreases (presumably a kinetic phenomenon); below 0.01% water, form IV does not go to I11 at all (Sjolin, 1971). AN can be stabilized against the IV-I11 transition by the use of additives (e.g., Mg(NO&, A12(S04)3)which function as phase stabilizers by taking up water from the AN (Sjolin, 1971). AN which is so stabilized does not become appreciably more sensitive to shock after five temperature cycles, in contrast to the behavior of unstabilized AN (Hansen and Berthold, 1972). However, when the stabilized AN is sprayed with only 0.5%water (an amount which it can readily pick up from the atmosphere) and then subjected to five temperature cycles, its sensitivity to shock is increased considerably, just as if it were not stabilized (Hansen and Berthold, 1972). Co-crystallization of AN with KN03 (KN) is known to produce material which shows relatively small density changes when exposed to temperature cycles (Roffey et al., 1945). Addition of KN to AN, followed by heating of the solids at 32-100 "C, forms a solid solution which is a phase I11 stable product (Whetstone, 1948; Taylor and Whetstone, 1952). The phase diagram of solid solutions of KN in AN shows that AN IV, in the absence of KN, is stable from -20 to +32 "C, that the temperature range of its stability decreases with increasing KN content, that AN IV does not exist when more than 15 wt % KN is in the solid solution, and that the high temperature limit of stability of AN I11 (for the 111-11 transformation) is increased by dissolved K N (Cady, 1974). Also, the rate of transformation of I11 to IV is decreased as the KN concentration increases (Cady, 1974).A reasonable mechanism of the stabilization of AN by KN, deduced from crystal structure data, is that the substitution of K+ ions in the crystal lattice reduces the hydrogen bond formation whose energy is necessary for stabilization of the AN-IV polymorph (Holden and Dickinson, 1975). It has been demonstrated that Mg(NO& (which is extremely hygroscopic) and A12(S04)3are inhomogeneously dispersed in AN and are effective as phase stabilizers only until the water content of the AN prill exceeds the capacity of those desiccants (Sjolin, 1971). It has also been demonstrated (Hansen and Berthold, 1972; Hansen, 1974) that AN prills, phase-stabilized with Mg(NO& or borate/phosphatehulfate (Brown et al., 1968a,b, 1967) (which presumably is similarly inhomogeneously dispersed) are susceptible to destabilization by a little water, as revealed by the detonability investigation mentioned above.
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Explosion sensitivity of ammonium nitrate is increased by the increased porosity produced by cycling through the 32 OC phase change. Using a solid solution of potassium nitrate in ammonium nitrate is the best means of preventing this.
It seems clear, therefore, that prevention of phase change (with consequent increase in sensitivity to detonation) on normal temperature cycling can be assured best by the solid solution of KN in AN, because water does not affect the mechanism of stabilization in this case. Solid solutions of KN in AN are easily prepared in the same equipment used for commercial manufacture of AN prills, with negligible modification of the production process. The KN is added to the molten AN before prilling, and the resulting solid solution prills are visually indistinguishable from ordinary AN prills. Pilot lots of solid solution prills, totaling about 25 000 lb, have been manufactured for this laboratory by Gulf Chemical and Ross Thermal Systems. When such. prills, containing 10% KN in solid solution in AN, are incorporated in military explosive compositions, a decrease in sensitivity to initiation of detonation is noted, as compared to similar compositions made with AN. As a fertilizer, 90/10 AN/KN contains 32.9% N compared to the 35.0% present in pure AN (and 34.5% or less in most commercial AN), but it also contains 4.7% K20, a valuable fertilizing ingredient, If the international maritime regulations were to permit the shipment of AN/KN solid solution without addition of 18%limestone, the AN/KN would have a competitive advantage over AN. Limestone may, however, be considered necessary to reduce thermal decomposition in the event of a fire, so there is no assurance that the regulations would be modified. While hot AN, as might exist near the surface in a fire, is capable of initiation to detonation by projectile impact (Van Dolah et al., 1966) (and, presumably, so is AN/KN), the propagation of such a detonation through the cooler mass of material would be less likely if the material were AN/KN rather than AN.
Brown, M. L., Green, A. W., Blanton, E. L. (to Mississippi Chemical Corp.) US. Patent 3 418 255 (Dec 24, 1968b). Brown, M. L., Green, A. W., Bianton, E. L. (to Mississippi Chemical Corp.) U.S. Patent 3 317 276 (May 2, 1967). Brown, R. N.. McLaren, A. C., Proc. Royal SOC.London, Ser. A, 266, 329 (1962). Cady, H. H., Los Alamos Scientific Lab., Los Alamos, New Mexico, unpublished data (which supplants a number of earlier phase diagrams by various authors), 1974. Goffart, P., Bull. Tech. Secur. Salubr., lnst. Mat. lnd. Exb. No. 6 (1973). Hansen, G., Berthold, W., Chem. Ztg., 96, 449 (1972). Hansen, G., Badische Aniiin- & Soda-Fabrik AG, Ludwigshafen. personal communication, 1974. Holden, J. R., Dickinson, C. W., J. Phys. Chem., 79, 249 (1975). Pasman, H. J., Groothuizen, T.M., Zwaneveld, J. H.C., Corbet, F. J., "Detonability and Sensitivity to Detonation of Four Samples of AN Prills Supplied by CERL." RVO/TNO, Rijswijk (Z.H.), Netherlands, Ass. # T 3000-11, Oct 1, 1974. Price, D., Clairmont, A. R., Jr., Erkman, J. O., "The NOL Large Scale Gap Test. 111. Compilation of Unclassified Data and Supplementary Information for Interpretation of Results." Naval Ordnance Laboratory (Silver Spring, Md.) NOLTR 74-40, 1974. Price, D., "Proceedings, Fifth Symposium (International) on Detonation," ONR ACR-184, pp 207-217. U.S. Government Printing Office, Washington, D.C., 1970. Roffey, F., Poole, H. J., Allmand, A. J., Style, D. W. G.. Drake, C. F., British Patent 573 147 (Nov 8, 1945). Sjolin, C., J. Agr. FoodChem., 19, 83 (1971). Taylor, J., Whetstone, J., (to Imperial Chemical Industries Ltd.), U.S. Patent 2 590 054 (Mar ~. > - 18. ~. 1952). Van Dolah, R . W., Mason, C . M., Perzak, F. J., Hay, J. E., Forshey, D. R.. "Explosion Hazards of Ammonium Nitrate Under Fire Exposure." U.S. Bureau of Mines, Pittsburgh Mining and Safety Research Center, R i 6773, 1966. Watson, R. W., Hay, J. E., Becker, K. R., "Sensitivity of Some Ammonium Nitrate-Based Explosive Compositions." U S . Bureau of Mines, Pittsburgh Mining and Safety Research Center, RI 7840, 1974. Whetstone, J.. Can. J. Res., 268, 499 (1948).
Literature Cited
T h i s paper was presented a t t h e D e p a r t m e n t of Commerce Seminar o n Agro-Chemical P r o d u c t s a n d Processes, Washington, D.C., M a y
Brown, M. L., Green, A. W., Blanton. E. L., J. Agr. Food Chem., 16, 373 (1968a).
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Receiued /or revieu,, June 23, 1976 Accepted August 17, 1976
20, 1976.
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