Experimental and Theoretical Approach to the Assessment of Stability Criteria for Safe Transport of Ammonium Nitrate Based Emulsions

Ammonium nitrate (AN) is a crystalline solid with a melting point of 170 °C, which decomposes above 210 °C. It is extensively used in nitrogen fertilizer as a source of ammonia and NO3 – ion essential for the vegetal life, while in explosives and propellants nitrate ion constitutes an oxygen source. Since the 1870’s, ammonium nitrate has been widely used as a supplemental oxidant in dynamites1 and then in admixture with solid fuels and sensitizers, such as nytroglycerine to give more sensitive explosives. Ammonium nitrate and derived explosives show non-ideal detonation behaviour, due to the relatively low decomposition rate of AN, which cause a wide reaction zone, in combination with lateral heat losses and rarefaction which extinguish the decomposition reactions.2 Moreover, ammonium nitrate is a promising rocket propellant oxidizer as it produces, unlike the widely used ammonium perchlorate, completely eco-friendly products. AN-based oil in water and water in oil emulsion explosives have been developed since 1926, owing to their superior characteristics and cost-effectiveness. A typical formulation includes 20 % water, 12 % oil, 2 % microsphere, 1 % emulsifier and 65 % ammonium nitrate.3 Several major accidents involving ammonium nitrate were recorded in the first half of the XX century, (Oppau, Germany, September 21, 1921; Texas City, USA April 16, 1947; Brest, France, July 28, 1947) and the more recent explosion occurred on 21st September 2001 at Tolouse – AZF plant (30 persons killed, up to 2242 people injured and an estimated cost of 1.5 billion EUR)4 reveals that still much effort is to be devoted to investigate the hazards connected to energetic material storage and transport, despite the standards and procedures for safe storage and use. This evidence is confirmed by more recent high profile AN accidents during transportation, e.g.: on 24th May 2004, a major accident involved a cylindrical vessel used for AN transportation at Mihăileşti (Romania), when a truck loaded with 20 tons of ammonium nitrate rolled over, caught fire and exploded one hour later, causing 18 fatalities and 10 severely injured persons; on 12th September 2005, at Shengangzhai (China) a truck loaded with 18 t of ammonium nitrate exploded, destroying 17 village houses, killing Experimental and Theoretical Approach to the Assessment of Stability Criteria for Safe Transport of Ammonium Nitrate Based Emulsions