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 NO3− group, and a 710 cm−1 peak attributed to the δ4 bending mode of the NO3− 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 NO3− + NH4+ ⇌ HNO3 + NH3 demonstrated that the concentration of HNO3 gradually increases with temperature, although the HNO3 to NO3− ratio was still only approximately 0.0034, even at 533 K. Thus, molten AN was found to contain primarily NO3− 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.