Ammonium Nitrate Kirk Othmer [PDF]

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Ammonium Nitrate. Broadly defined, fertilizer ammonium nitrates include straight ammonium nitrate (AN), containing 3334% N, ammonium sulfate nitrate (ASN) 26% N, and calcium ammonium nitrate [39368-85-9] (CAN) 2026% N. Worldwide, the estimated total production of these materials is about one-half straight AN. Most of the balance is CAN. The ASN and CAN grades are used primarily because of greater safety and better storage and handling qualities. Ammonium nitrate fertilizers were produced and used to some extent in Europe prior to 1942, but became of importance only after World War II when plants that had been producing ammonium nitrate for munitions were converted to fertilizer production. Ammonium nitrate rapidly became the leading solid nitrogen fertilizer both in the United States and worldwide, and held that position until about 1975, at which time its use was equalled and then surpassed by that of synthetic urea. Figure 6 gives world consumption of N fertilizers from 1955 through 1990. For the year ended June 30, 1990, world consumption of ammonium nitrate in solid form was 26:4 £ 106 t (8:86 £ 106 t N ) or about 11% of total world consumption of fertilizer nitrogen (9). United States consumption data for the same period (Fig. 3) show that only 7% of U.S. nitrogen consumption was furnished as solid ammonium nitrate, although an additional 20% was furnished as ureaammonium nitrate solutions.



Fig. 6. World trends in types of nitrogenous fertilizers consumed, where (| ) represents anhydrous ammonia, ammonium phosphates, cogranulated mixtures, fluid mixtures; ({ { { ), urea, including that in nitrogen solutions; (¢ ¢ ¢ ¢), ammonium nitrate, including that in nitrogen solutions; ({ ¢ { ), ammonium sulfate; and ({ ¢ ¢ { ) others. In the United States, only high grade (3334% N) ammonium nitrate is produced, and, because of the oxidizing nature and explosive potential of this material, strict handling precautions are required (10) (see EXPLOSIVES AND PROPELLANTS, EXPLOSIVES). Several other countries, France, Russia, Romania, and the UK also allow production of the high grade product, whereas others, for safety reasons, allow only the CAN grades (2026% N). Ammonium nitrate fertilizers are highly regarded because of the rapid agronomic response, especially true in the cooler latitudes. For assimilation by the roots of most plants (rice is an exception), all fertilizer nitrogen must first be converted in the soil to nitrate form (11,12). For non-nitrate fertilizers in the cooler areas, this process may be rather slow. Ammonium nitrate is produced from ammonia and nitric acid (qv). The acid also is produced from anydrous ammonia (see AMMONIUM COMPOUNDS). Prilling and Granulation. Ammonium nitrate is normally produced initially in the form of a 75 to 85% solution saturated at 40 to 77C. Conversion to solid prills or granules suitable for fertilizer use requires high temperature concentration of the solution to 96 99 + % ammonium nitrate content followed by solidification from that solution by cooling and drying in either a prilling tower or a granulation system. The usual prilling operation and the design of prilling towers have been described in detail (13). Basically, a highly concentrated solution of ammonium nitrate is sprayed as small droplets into the top of a tall tower countercurrent to an updraft of cool air. Solidification occurs in the form of small spheres (prills), usually of 1 to 3 mm particle diameter. A relatively short tower, typically about 21 to 30 m in height, and a feed of 99.599.8% solution is used to produce high density prills. The prill hardness and particle size are about optimum under those conditions. If there is proper control of solution concentration and air flow, no further drying is required. When lower (96 to 99%) solution concentrations are used, a taller tower is required, but the prills are of lower density, and passage through a heated dryer often is required. Treatment of prilling tower effluent air to avoid atmospheric pollution is a particularly difficult problem. Partly for this reason, granulation has, to some extent, been introduced as an alternative to prilling. Another advantage of granulation is greater particle-size



Kirk-Othmer Encyclopedia of Chemical Technology (4th Edition)



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flexibility, which assumes importance when producing closely sized product for use in bulk blending. The Spherodizer granulation process developed by Cominco Ltd. (Canada) and C & I/Girdler, Inc. (14,15) is one of the more advanced granulation processes. Very concentrated ammonium nitrate solution is sprayed onto a rolling bed of solid particles in a rotating drum, resulting in the formation of granules by layering and solidification. The granules contain only 0.10.5% moisture and require no further drying. From a screening operation, oversize granules are crushed and recycled to the granulator along with undersize particles to serve as nuclei for forming more product-size granules. The product size is controlled by choice of screen size. Air pollution (qv) is virtually eliminated by wet scrubbing in a manner that is simple and economical compared to that required for a standard prilling operation. The granules are harder than prills. Pan granulation of ammonium nitrate also has been developed commercially (16,17). Essentially water-free melt (