Thermal decomposition of molten ammonium nitrate (AN)

Abstract

Raman spectra of ammonium nitrate (AN) above its melting point of 443 K and decomposition temperature of 533 K were acquired, and the thermal properties of AN were assessed using differential scanning calorimetry (DSC) so as to obtain a better understanding of the condensed phase decomposition. The Raman spectra of molten AN at both 443 and 533 K exhibited a strong peak at 1043 cm⁻¹, assigned to the ν₁ vibrational stretching mode of the NO ⁻₃ group, and a 710 cm⁻¹ peak attributed to the δ₄ bending mode of the NO ⁻₃ group, whereas no peak was observed at approximately 940 cm⁻¹ due to the O′–NO₂ stretching mode of HNO₃. Chemical equilibrium calculations based on the reaction NO ⁻₃  + NH ⁺₄ ⇌ HNO₃ + NH₃ demonstrated that the concentration of HNO₃ gradually increases with temperature, although the HNO₃ to NO ⁻₃ ratio was still only approximately 0.0034, even at 533 K. Thus, molten AN was found to contain primarily NO ⁻₃ and only a minor amount of liquid HNO₃ over the temperature range analyzed. The heat flow associated with condensed phase decomposition was simulated based on the assumption that the decomposition rate of AN is given by $- {\text{d}}C_{\text{AN}} /{\text{d}}t = 10^{9.4} { \exp }\left( { - 125520/{RT}} \right)C_{{{\text{NH}}_{4}^{ + } }} C_{{{\text{HNO}}_{3} }}$ and that $C_{{{\text{NH}}_{4}^{ + } }}$ and $C_{{{\text{HNO}}_{3} }}$ are constant due to chemical equilibrium. The heat flow curves thus calculated were in good agreement with the experimental DSC data acquired at 5.1 MPa.