The Behavior of Ammonium Nitrate in a Fire, Recent Events, and Resulting Changes in Fire Code
Noel Hsu, PhD, Global Regulatory Affairs, Orica; Denver, Colorado, USA
Abstract
Ammonium nitrate is used as a fertilizer and as a primary feedstock for commercial explosives. Recent events have
shown that ammonium nitrate can mass explode when involved in a fire. This paper will highlight the chemical
behavior of AN in the context of recent events with a focus on the explosion at the West Fertilizer Company, and the
resulting actions being taken to update relevant Fire Codes and industry Codes of Practice.
Key Words: Ammonium nitrate, fire, explosion,
1. Introduction
Ammonium nitrate (AN) is a synthetic chemical that is used primarily in agriculture as a nitrogen source for plants,
as a precursor for commercial blasting explosives, and in other industrial applications such as the generation of
nitrous oxide gas.
The synthesis of ammonium nitrate is through the neutralization of nitric acid with ammonia. The resulting
ammonium nitrate solution is concentrated to 99.5% for agricultural use as a fertilizer (fertilizer grade AN, FGAN)
and 97.5% for use in the explosives industry, as technical grade AN, TGAN. The primary difference in the two
grades of AN is that the TGAN has a lower density than FGAN due to the particle having a porosity.
A schematic of the process for manufacturing AN is shown in Figure 1.
Figure 1. Schematic for the Manufacture of Ammonium Nitrate
Since the AN solution entering the prill tower for FGAN manufacture is almost a pure melt (99.5% AN), the solids
only undergo cooling. In contrast, the TGAN prill requires an AN solution of 97.5% and once the prill is formed in
the tower it then undergoes drying to remove the residual moisture and subsequent cooling. The AN prills can be
coated with a waxy substance to prevent caking. Most TGAN prills have this coating applied after the cooling step.
