Hot spots in ammonium nitrate

Abstract

Ammonium nitrate (AN) is commonly used as an explosive and as a fertilizer. In both roles it is provided as prills or pellets, approximately spherical and a few millimetres in diameter. The microstructures of several commercially-available AN compositions were investigated using environmental scanning electron microscopy (ESEM) and X-ray microtomography. Those intended for explosive use were found to be more porous than those intended for fertilizer use. The pores in explosive prills were also found to form a connected network. The elemental composition of pellets of mixed AN and dolomite was investigated using energy-dispersive X-ray spectroscopy (EDX); the dolomite additive was found to take the form of grains roughly 50 μm in size. The compaction behaviour of confined cylindrical beds of these prills and pellets was studied at strain rates between 4 × 10−4 s−1 and 200 s−1. Quasi-static experiments were performed using a screw-driven instrumented press, while higher-rate experiments used a drop weight, instrumented with a line laser and load cell. The resistance of a bed to compaction was found to depend on the microstructure of its prills in most cases. Denser prills offered greater resistance to compaction. The exception to this rule was a pellet, rather than prill, formulation. Beds were also found to offer more resistance to compaction at higher strain rates. The Kawakita compaction model was found to agree well with the experimental data. A commercial fertilizer, not containing any AN, was assessed for use as an inert mock for AN prills and pellets. Prills of a suitable size for this purpose were found using EDX to consist of P2O5, with a coating of unknown composition. They were supplied mixed with smaller K2CO3 and urea prills. The mixture was found to have comparable compaction behaviour to AN compositions, indicating that it was useful as a mock for those compositions. In a plate impact experiment on a single layer of P2O5 prills, very little light was observed. This indicated that these prills were sufficiently inert for these purposes. The light produced by shocked granular ammonium nitrate beds and single prill layers was investigated using high-speed framing photography, photodiodes and gated visible-light spectroscopy. Framing photography of prill layers suggested that reaction in prill beds was dominated by effects internal to prills. This was further supported by the similarity between photodiode recordings of prill beds and beds of inert prills containing a single reactive prills. Framing photography of drop weight experiments searching for a mechanism for initiation of reaction by interaction between prills found nothing. Decay of the light output of the beds suggested that in both granular and prill beds this light output was due to small regions heated to thousands of kelvin, which then cooled. Spectroscopic study confirmed this. These regions were found to reach a peak temperature of 6660 ± 20 K, well in excess of the approximately 2000 K predicted by a simple chemical model. Investigation of spectral lines observed during this study indicated that the exothermic reaction that led to heating of these emitting regions involved NO.