Ammonium nitrate (AN) has the chemical formula NH4NO3 and a molecular weight of 80.05. It is made in large quantities with principal applications as an agricultural fertilizer and as a constituent of explosives. As ammonium nitrate-fuel oil (ANFO) and related mixtures, it is the most common civilian explosive in the United States, but has also been involved in accidental disasters and as an ingredient in terrorist bombs.
It was first synthesized in 1654 by a German chemist, J. R. Glauber, a German chemist. In 1867-1870, it was used as an absorbent for dynamite. In 1899, it was learned that aluminium powder could greatly increase the power of explosive mixtures. 1958 saw the introduction of water-based slurry mixtures more powerful than ANFO.
There are several methods for producing ammonium nitrate, differing slightly depending if the desired end product is to be for explosive or fertilizer use. An important aspect of all is that the end product is a granule called a prill.
Fertilizer grade is 99% AN and has dense, nonporous prills, or prills with a hard coating. Explosive grade is 95% and is less dense and nonporous. Increased porosity allows the fuel oil or other explosive additives to mix more thoroughly, although agriculture grade still can be detonated. In 1998, 17,631 million pounds were manufactured.
The most common method is by passing ammonia gas into 40 percent to 60 percent nitric acid, both produced catalytically, in high purity, from atmospheric nitrogen. The solution is then concentrated in evaporating pans provided with air agitation and heating coils,  producing an 83 percent ammonium nitrate solution that is concentrated by evaporation to approximately 95 percent AN for explosive, and 99 percent for fertilizer.
Crystallization and prilling
The concentrated AN solution goes through sprayers at the top of a tall tower, against rising airflow. 95 percent produces porous prills while 99 percent produce nearly solid prills. In either case, the prills are then subjected to drying, coating, screening for size, and packaging.
Pure ammonium nitrate is quite difficult to detonate, requiring both high explosive boosters and particular environmental conditions. The Texas City blast, for example, was confined inside a ship, and in a fire that was fought improperly. Oppau involved a hardened mass of chemical that unwise people tried to break up with explosives.
Minimally, one needs a mixture such as ammonium nitrate-fuel oil (ANFO), but, under normal circumstances, a standard blasting cap using lead azide or a comparable primary explosive does not have the energy to detonate ANFO.
Military mixtures with other tertiary explosives, aluminium powder, or fuel components still tend to need a competent detonation system, but it is likely the other explosive, such as TNT or RDX, will explode first, and then set off the ammonium nitrate.
- Containing the Threat from Illegal Bombings: An Integrated National Strategy for Marking, Tagging, Rendering Inert, and Licensing Explosives and Their Precursors, National Academies of Science Press, 1998, pp. 96-
- Military Explosives, U.S. Department of the Army, September 1984, TM 9-1300-214, pp. 8-92 to 8-96