Dense prills

One of the more critical feed properties in the case of ammonium nitrate prilling is the moisture content of the melt. Evaporation of the melt to nearly 100% solids produces hard, non-porous prills. Larger water contents yield porous, less dense prills. 

Ammonium nitrate is hygroscopic and readily soluble in water (the saturated solution contains about 65% NH4N03). Transitions from one crystal form to another take place at + 125.2 °C = 257.4°F, +84.2 °C = 183.6°F, +32.3 °C = 90.1 °F and -16.9 °C = +1.6°F. The product shows a great tendency to cake. The difficulties therefore involved are avoided by transformation into Prills. Ammonium nitrate is marketed as dense prills and as porous prills. Both can be introduced in industrial explosives after milling except ANFO blasting agents, which need unmilled porous prills. 

Dense ammonium nitrate crystals are formed by spraying droplets of molten ammonium nitrate solution (>99.6%) down a short tower. The spray produces spherical particles known as prills . These crystals are non-absorbent and used in conjunction with nitroglycerine. An absorbent form of ammonium nitrate can be obtained by spraying a hot, 95% solution of ammonium nitrate down a high tower. The resultant spheres are carefully dried and cooled to prevent breakage during handling. These absorbent spheres are used with fuel oil. 

The density of the prills is reduced substantially when much evaporation occurs with 0.2-0.5% water in the feed, ammonium nitrate prills have a specific gravity of 0.95, but with 3-5% water it falls to 0.75. Prilled granules usually are less dense than those made by layering growth in drum or fluidized bed granulators. The latter processes also can make larger prills economically. To make large prills, a tall tower is needed to ensure solidification before the bottom is reached. The size distribution depends very much on the character of the atomization but can be made moderately uniform. Some commercial data of cumulative % less than size are ... 

This explosive is more quickly manufactured due to the less dense fuel sensitizer. This gives a more rapid and uniform absorbtion into the fertilizer prills. The manufacture is accomplished by placing the proper amount of AN prills in a container. Addition of the proper ratio of alcohol is the next step. These are stirred or tumbled together until a complete uniform mixture is obtained. The percentages are as follows ... 

J.J. Minnick G.W. Beil, USP 3394038 (1968) CA 69, 53326(1968) [High-density AN-FO expl compns contained a dense AN (d = 1,3—1.7), ground to 8 to 200 mesh and mixed immediately and thoroughly with fuel oil, A typical compn consisted of ground AN 60.2, whole prilled AN 30.0, No 2 fuel oil 2.5, powd coal 4.0, Ca chloride 2.0 and clay 1.3%. This compn had package d 1.11 and exhibited deton rate of 10500ft/sec when primed with Tetryl]... 

---