KINETICS AND MECHANISM OF THERMAL DECOMPOSITION
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that the pH values of the aqueous solution of AN remain
rather high; in an open ampule the AN melt is rapidly
acidified and the pH of its aqueous solution falls below
the admissible values. At the same time, it is known that,
when a granulation process is terminated, the AN melt
may be acidified above the equilibrium values for an
open ampule, which must not occur in an open system.
decomposition. This occurs because about 0.1 mol
of HNO3 accumulates from 1 mol of AN due to the
stoichiometry of oxidation of the ammonium ion by
nitric acid in AN decomposition [2].
Certain differences from other mixtures are observed
in the thermal decomposition kinetics for a mixture with
a higher content of water and a comparatively small
added amount of sulfuric acid (sample no. 6). In thermal
decomposition of this mixture at low temperatures
in the temperature range under study, heat absorption
is observed at the beginning of the process and gives
way to heat release after a certain time. Further, the
heat release rate grows, reaches a maximum, and
then decreases. At higher temperatures, heat release
is observed immediately after the ampule is heated to
the experimental temperature, and its rate also passes
through a maximum in the course of time. Heat variation
rate curves of this type can be regarded as an algebraic
sum of heat absorption rates in the endothermic
process of AS dissolution in an aqueous melt of AN
and a heat release rate in the exothermic process of AN
decomposition. The heat absorption in AS dissolution
in AN probably occurs because the energy expenditure
for disintegration of the crystal lattice of AS are not
compensated for by weak solvation effects in the
ionic melt of AN. According to published data, even
dissolution of 1 mol of AS in 400 mol of water, in which
case the hydration of ions being formed is important, is
accompanied by absorption of 10.0 kJ mol–1 of heat. If
solid AS particles are agitated in an aqueous melt of AN
only by gaseous products formed in AN decomposition,
the dissolution of AS takes a long time. As a result, two
parallel processes occur in the system in the course of
time: exothermic decomposition ofAN and endothermic
dissolution of AS in the AN melt. Only after the fusion
cake of AN is saturated with ammonium sulfate, the
true thermal decomposition rate will be recorded. With
the added amount of sulfuric acid raised at the same
content of water (sample no. 7), heat release is observed
in the whole temperature range under study from the
very beginning of the process. This can be attributed to
a higher heat release rate in thermal decomposition of
AN in sample no. 7 because of the higher equilibrium
concentration of nitric acid in this sample.
A general kinetic feature in decomposition of double
salts and AN and AS mixtures with various contents
of water and starting sulfuric acid is that the rate
decreases in the course of the decomposition process.
Figure 1 shows experimental dependences of the heat
release rates in thermal decomposition of a double salt
.
[(NH4)2SO4 2NH4NO3, sample no.1) and AN + AS
mixtures with the maximum and minimum concentration
ratios of added sulfuric acid and water (sample nos. 4
and 6, respectively) per gram of a sample on the running
process heat Q. The shape of the heat release rate curves
for sample nos. 2, 3, 5, and 7 is similar to that of the
curves for decomposition of sample nos. 1 and 4. The
value of Q was calculated by numerical integration of
the heat release rate curve,
It can be seen from the dependence of the heat
release rate on the running heat of the process that the
curve can be divided into two parts. In the initial stage of
the process, the heat release rate decreases in proportion
to the released amount of heat, i.e., proportionally to
the conversion in this stage of the reaction. The kinetic
equation for this stage has the form
dQ/dt = (dQ/dt)t=0 (1 – Q/Q1) ,
where Q1 is the total heat of the first stage of the reaction,
which is equal in this temperature range to only 8.5–
12.5 J kg–1 and very slowly increases with temperature;
and Q is the running heat in the first stage; as Q1 is
reached, the rate of the first stage is zero.
In the second stage, the heat release rate is
substantially, 9–10 times, lower and decreases further
only slightly, but there is no self-acceleration in this
stage, either. The decrease in rate in the course of the
process is a distinctive feature of the behavior of these
mixtures. As a rule, an increase in the heat release rate
is observed in the process of thermal decomposition
of AN manufactured by Kirovo-Chepetsk plant of
mineral fertilizers and AN from previously studied
batches, caused by accumulation of nitric acid in AN
Figure 2 shows temperature dependences of the ini-
tial heat release rates (dQ/dt)t=0 in thermal decomposi-
tion of sample nos. 1–5 and 7, plotted in the Arrhenius
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 84 No. 9 2011