A kinetic and mechanistic study of the thermal decomposition of calcium nitrate

Anhydrous calcium nitrate melts at 836 +/- 2 K and subsequent decomposition occurs in the liquid phase. The temperature of NO3- anion breakdown under dynamic rising temperature conditions increases with the heating rate, being 949 K at 10 K min(-1). Kinetic studies of this pyrolysis are reported from observations obtained by two complementary isothermal techniques. From measurements of gaseous product evolution in an evacuated constant volume apparatus between 773 and 820 K (this reaction is approximately summarised as Ca(NO3)(2)-->CaO + 1(1/2)NO2 + O-2 + (1/4)N-2) it was shown that data were well expressed by the Avrami-Erofe'ev equation (n = 2) and the activation energy (E) was 229 +/- 10 kJ mol(-1) between 0.10 < fractional reaction, alpha, < 0.85. An alternative titration technique, based on the evolution of products (NO + NO2) between 818 and 848 K, identified a different pattern of kinetic behaviour. Two linear regions of the alpha-time curves were found, the first between 0.3 < alpha < 0.6 (E = 217 +/- 9 kJ mol(-1)) and the second, relatively more rapid, between 0.7 < alpha < 0.9 and E = 298 kJ mol(-1). Mechanistic interpretations lead us to conclude that anion breakdown occurs in two overlapping and interacting steps: nitrate-->nitrite-->oxide. Some environmental implications of these observations are discussed that may be of value in the development of technology for use in pollution abatement by removal of NOx from industrial flue gases.