For its weight,it supplies more gas volume upon detonation than any other explosive. In pure form, ammonium nitrate (AN) is almost inert and is composed of 60% oxygen by weight, 33% nitrogen, and 7% hydrogen. With the addition of fuel oil, the ideal oxygen balanced reaction is:
Two characteristic make this compound both unpredictable and dangerous. Ammonium nitrate is water soluble and if uncoated can attract water from the atmosphere and slowly dissolve itself. For this reason, the spherical particles, prills, have a protective coating of silica flour, which offers some amount of water resistance. The second and most important characteristic is a phenomena called cycling. Cycling is the ability of a material to change its crystal form with temperature.
The two temperatures at which cycling will occur under normal conditions are 0 F and 90 F. This is to say that products which are stored over the winter or for a period of time during the summer most likely will undergo some amount of cycling. During the summer in a poorly ventilated powder magazine or storage bin located in the sun, the cycling temperature may be reached daily. The effect of cycling on AN when isolated from the humidity in the air is that the prills break down into finer and finer particles.
The prills are made up of pseudotetragonal crystals. When the temperature exceeds 90 F, each crystal breaks into smaller crystals of orthorhombic structure. When the temperature again falls below 90 F, the small crystals break into even finer crystals of the pseudotetragonal form. This process can continue until the density is no longer near 0.8 g/cm3, but can reach a density near 1.2 g/cm3. The density increase can make the product more sensitive and contain more energy per unit volume.
After the AN has undergone cycling, the water-resistant coating is broken and the water vapor in the air condenses the particles. As cycling continues water collects on the particles and the mass starts to dissolve, recrystalizing into large crystals can occur with a reduction of temperature.
Therefore, it is evident that a volume of AN after cycling may have very dense areas and areas of large crystals. This performance of this product may range from that of a very powerful explosive to one that deflagrates or one that will not shoot at all.
Figure 1: Cycled Prills