The non-explosive and irrestorable fertilizer-grade ammonium nitrate (NEIFAN) is a kind of modified ammonium nitrate (AN), which is composed of AN and anti-explosive additives^[1-3]. Its preparation and properties have been reported^[2]. However, its thermal stability and adiabatic decomposition reaction kinetics have not yet been reported. The aim of this work is to study the thermal behavior and thermal safety of NEIFAN by thermogravimetry-differential thermal analysis (TG-DTA), derivative thermogravimetry (DTG), differential scanning calorimetry (DSC) and accelerating rate calorimetry (ARC). It is quite useful in the evaluation of its heat-resistance ability and thermal safety under non-isothermal and adiabatic conditions and in the better understands of its phenomenon, mechanism, and process from thermal decomposition and adiabatic decomposition to thermal explosion^[2].

Ammonium nitrate used in the experiment was of chemical purity. It was a commercial product of Liu-li dian chemical plant, Beijing, with purity higher than 98%. Its density and particle diameter (d₅₀) were 1.72 g·cm^-3 and 100-150 μm, respectively^[2];

Non-explosive and irrestorable fertilizer-grade ammonium nitrate (NEIFAN) with 8% anti-explosive additives as anti-explosive agents was from Beijing General Research Institute of Mining and Metallurgy ^{[2, 4-6]}. The measured values for density and particle diameter (d₅₀) were 1.83 g·cm^-3 and 140 μm, respectively^[2].

TG-DTA tests were carried out on a SDT2960 V3.OF TA instrument. The operation conditions of TG-DTA were: pan, closed cell of aluminum; atmosphere, a flowing rate of 40 mL·min^-1 of air; heating rate, 2.5 ℃·min^-1; temperature range, 25-500 ℃. DSC tests were performed on a MDSC instrument (Model TA2910 V4.4E TA Co. USA)The operation conditions of DSC were as follows: pan, closed cell of aluminum; atmosphere, a flowing rate of 40 mL·min^-1 of N₂; heating rate, 2.5 ℃·min^-1. Temperature calibrating of DTA and TG-TGA curves in the range of 25-500 ℃ were performed by running melting standards. In order to obtain calorimetric results, the same standards were used to calibrate the temperature of DSC curve. These calibrations were performed at a heating rate of 2.5 K·min^-1 by using a sample size of (3.71±0.05) mg. The gain of the thermobalance was chosen to give an approximate resolution of 0.3 μg. The samples were loaded into open alumina crucibles and a dry nitrogen purge flow of 40 mL·min^-1 at 0.1 MPa absolute pressure was used ARC measurements were made with an accelerating rate calorimeter manufactured by Columbia Scientific Industries and operated according to the conditions shown in Table 1^[7]. The principle and method of collecting, analyzing and treating the data from ARC curves and the corrected method of inertia factor Φ were same as those in Refs. [7-8].

表 1(Tab. 1)

Tab. 1 Mass of samples and measuring conditions in ARC characteristic parameters AN NEIFAN sample mass, m/g 0.367 0.504 bomb mass, mb/g 6.380 6.804 starting temperature, T0, s/℃ 150 150 slope sensitivity/℃·min-1 0.02 0.02 heat step temperature/℃ 3 3 wait time, t/min 5 5

Tab.1 Mass of samples and measuring conditions in ARC

The TG-DTA-DTG curves of AN and NEIFAN are shown in Fig. 1 and Fig. 2. The DTA curves of AN consists of three crystal transformation peaks at 53.45, 88.06, 126.03 ℃, one melting peak at 168.69 ℃ and one decomposition peak at 234.93 ℃. The DTA curve of NEIFAN consists of two crystal transformation peaks at 52.91, 127.16 ℃, one melting peak at 168.47 ℃ and one decomposition peak at 263.87 ℃.The phase transformation peak at about 88 ℃ disappears, which is attributed to the joint action of inorganic and organic additives in NEIFAN. The peak temperature of decomposition reaction for NEIFAN is much higher than that of decomposition reaction for AN, showing that the thermal stability of NEIFAN is better than that of AN. The DSC curves of two samples are shown in Fig. 3 and Fig. 4, which are similar to the DTA curves in Fig. 1 and Fig. 2. Mass loss processes of AN and NEIFAN with temperature under non-isothermal conditions show only one stage on TG and TGA curves.

图 1(Fig. 1)

Fig.1 TG-DTA-DTG curves for AN

图 2(Fig. 2)

Fig.2 TG-DTA-DTG curves for NEIFAN

图 3(Fig. 3)

Fig.3 DSC curve for AN

图 4(Fig. 4)

Fig.4 DSC curve for NEIFAN

Time-temperature-pressure curves for AN and NEIFAN are shown in Fig. 5.The test results and the derived adiabatic decomposition reaction characteristic data of AN and are summarized in Table 2. The curves of self-heating rate vs. temperature of AN and NEIFAN are shown in Fig. 6 and Fig. 7. The curves of the pseudo rate constant k as a function of temperature for the adiabatic decomposition reaction of AN and NEIFAN obtained by Eqs. (1), (2) and (3) are shown in Fig. 8 and Fig. 9. The apparent activation energy and pre-exponential constant of the pseudo zero order adiabatic decompo-sition reaction of AN and NEIFAN obtained by ARC data from Fig. 5-Fig. 7 and the calculated values of pseudo zero-order rate constant at 360 ℃^[9], k_{360 ℃} are listed in Table 3.

图 5(Fig. 5)

Fig.5 Time-temperature-pressure curves of AN and NEIFAN

图 6(Fig. 6)

Fig.6 The T VS.MT curve of AN

图 7(Fig. 7)

Fig.7 The T VS.MT curve of NEIFAN

图 8(Fig. 8)

Fig.8 The k* vs.T curves of AN

图 9(Fig. 9)

Fig.9 The k* vs. T curves of NEIFAN

表 2(Tab. 2)

Tab. 2 Measured thermal decomposition characteristic data of AN and NEIFAN characteristic parameter AN NEIFAN sample mass, m/g 0.367 0.504 inertia factor, Φ 5.875 4.550 onset temperature, T0, s/℃ 180.47 255.65 self-heat rate, m0, s/℃·min-1 0.022 0.031 maximum self-heat rate, mm, s/℃·min-1 0.04 0.14 maximum self-heat rate, mm, s/℃·min-1 (Φ corrected) 0.16 0.641 final temperature, Tf, s/℃ 287.51 388.12 final temperature, Tf, s(Φ corrected)/℃ 809.33 858.39 adiabatic temperature rise, ΔTad/℃ 107.04 132.47 adiabatic temperature rise, ΔTad/℃ (Φ corrected) 628.86 602.74 time to maxiumum rate, θm, s/min 1252.90 43.18 time to maxiumum rate, θm, s/min (Φ corrected) 213.26 9.49 temperature at maximum self-heat rate, Tm, s/℃ 273.59 267.65 maximum pressure rise, pm, s/MPa 2.69 3.86

Tab.2 Measured thermal decomposition characteristic data of AN and NEIFAN

表 3(Tab. 3)

Tab. 3 The kinetic parameters of the pseudo zero order adiabatic decomposition reaction of AN and NEIFAN samples A/s-1 E/kJ·mol-1 k360 ℃*/s-1 AN 3.5×1017 98.23 109.44 NEIFAN 2.75×1042 445.90 105.64 Note: 1) k* is a pseudo zero-order rate constant at 360 ℃.

Tab.3 The kinetic parameters of the pseudo zero order adiabatic decomposition reaction of AN and NEIFAN

Where k^* is a pseudo zero-order rate constant at temperature T, ℃; n is reaction order; C₀ is the initial concentration of the reactant; A is the pre-exponential constant, s^-1; E is the apparent activation energy, kJ·mol^-1; T_{f, s} is the final temperature, ℃; T_{0, s} is the initial temperature, ℃; T_{f, s}-T_{0, s} is the adiabatic temperature rise, ΔT_ad ^[10-11], ℃.

Fig. 5-Fig. 9 and Tables 2 and 3, the following observ-ations can be made.

(1) The adiabatic decomposition process can be derived into two exothermal stages

(2) The adiabatic decomposition reactions of AN and NEIFAN are exothermic, whereas the two reactions performed in non-sealed DSC conditions are endothermic.

(3) The facts of T_{0, s} (NEIFAN) > T_{0, s} (AN), T_{f, s} (NEIFAN) > T_{f, s} (AN), T_{f, s}(Φ corrected, NEIFAN) > T_{f, s}(Φ corrected, AN), E(NEIFAN) > E(AN) and k_{360 ℃}(NEIFAN) < k_{360 ℃}(AN) indicate that the thermal stability of NEIFAN is better than that of AN.

(4) Using ΔT_ad(Φ corrected) as criterion, the thermal safety of AN and NEIFAN decreases in the order of NEIFAN > AN.

(5) The kinetic equation describing the adiabatic decomposition reaction in studied temperature range is for AN.

lnk^*=40.35-11815/T

for NEIFAN

lnk^*=97.61-53632/T

(1) The decomposition reactions of AN and NEIFAN in adiabatic and sealed system are exothermic, whereas the two reactions performed in non-sealed DSC conditions are endothermic.

(2) Using the peak temperature of decomposition reaction and the apparent activation energy of adiabatic pseudo zero order decomposition reaction as criterions, the heat-resistant abilities of AN and NEIFAN decrease in the order of NEIFAN>AN, showing that NEIFAN has better thermal stability than AN.

(3) In comparison with AN, the disappearance of the phase transformation peak at about 88 ℃ of NEIFAN means that NEIFAN has better thermal physical stability.

(4) The relations of thermal decomposition temperature and pressure versus time, self-heating rate and pressure versus temperature describing the adiabatic decomposition reaction for AN and NEIFAN were presented.

(5) The increase of physicochemical stability of NEIFAN is due to the joint action of inorganic and organic additives in NEIFAN.

(6) The adiabatic decomposition processes of AN and NEIFAN accord with the pseudo zero order reaction theory model.