Influence of lime-containing additives on the thermal behaviour of ammonium nitrate

Article abstract of DOI:10.1007/s10973-007-8769-1

Ammonium nitrate (AN) is one of the main nitrogen fertilizers used in fertilization programs. However, AN has some serious disadvantages - being well soluble in water hardly 50% of the N-species contained are assimilated by plants. The second disadvantage of AN is associated with its explosive properties. The aim of this paper was to clarify the influence of different lime-containing substances - mainly Estonian limestone and dolomite - as internal additives on thermal behaviour of AN. Commercial fertilizer grade AN was under investigation. The amount of additives used was 5, 10 or 20 mass%, or calculated on the mole ratio of AN/(CaO, MgO)=2:1 in the blends. Experiments were carried out under dynamic heating condition up to 900°C (10°C min^-1) in a stream of dry air or N₂ by using Setaram Labsys 2000 equipment coupled to Fourier transform infrared spectrometer (FTIR). The results of analyses of the gaseous compounds evolved at thermal treatment of neat AN indicated some differences in the decomposition of AN in air or in N₂. At the thermal treatment of AN's blends with CaCO₃, MgCO₃, limestone and dolomite samples the decomposition of AN proceeds through a completely different mechanism - depending on the origin and the content of additives, partially or completely, through the formation of Mg(NO₃)₂ and Ca(NO₃)₂.

Full text of DOI:10.1007/s10973-007-8769-1

Journal of Thermal Analysis and Calorimetry, Vol. 92 (2008) 1, 215–221
INFLUENCE OF LIME-CONTAINING ADDITIVES ON THE THERMAL
BEHAVIOUR OF AMMONIUM NITRATE
T. Kaljuvee*, E. Edro and R. Kuusik
Laboratory of Inorganic Materials, Tallinn University of Technology, Ehitajate tee 5, 19086 Tallinn, Estonia
Ammonium nitrate (AN) is one of the main nitrogen fertilizers used in fertilization programs. However, AN has some serious disad-
vantages – being well soluble in water hardly 50% of the N-species contained are assimilated by plants. The second disadvantage of
AN is associated with its explosive properties. The aim of this paper was to clarify the influence of different lime-containing sub-
stances – mainly Estonian limestone and dolomite – as internal additives on thermal behaviour of AN.
Commercial fertilizer grade AN was under investigation. The amount of additives used was 5, 10 or 20 mass%, or calculated on the
mole ratio of AN/(CaO, MgO)=2:1 in the blends. Experiments were carried out under dynamic heating condition up to 900°C (10°C min–1)
in a stream of dry air or N2 by using Setaram Labsys 2000 equipment coupled to Fourier transform infrared spectrometer (FTIR).
The results of analyses of the gaseous compounds evolved at thermal treatment of neat AN indicated some differences in the
decomposition of AN in air or in N2. At the thermal treatment of AN’s blends with CaCO3, MgCO3, limestone and dolomite samples
the decomposition of AN proceeds through a completely different mechanism – depending on the origin and the content of addi-
tives, partially or completely, through the formation of Mg(NO3)2 and Ca(NO3)2.
Keywords: ammonium nitrate, dolomite, limestone, TG-FTIR, thermal stability
Introduction
ANIV«ANIII associated with a large volume change of
the AN crystal causes cracking of the particles with the
following decrease in the thermal stability of AN. From
technological point of view at manufacturing or at han-
dling of AN the phase transition ANIV«ANII should be
followed, by-passing the form ANIII.
Ammonium nitrate (AN) is one of the main nitro-
gen (N) mineral fertilizers the consumption of which
in 2005 was on the level of 13 million tons [1]. AN is
characterized by high content of nitrogen (35%) and
relatively simple manufacturing technology. How-
ever, AN has two serious disadvantages – being well
soluble in water hardly 50% of N-species contained
are assimilated by plants [2–4]. The other part of N is
lost by leaching of N-species [3–5] or emission of
them into the atmosphere [3–7]. The second disad-
vantage of AN is associated with its explosive proper-
ties – AN is a light explosive with upset oxygen bal-
ance which, however, can be easily restored by
adding hydrocarbon components of any origin.
There are two widespread possibilities to reduce
above mentioned disadvantages of AN – internal ad-
ditives or coating agents can be used for modification
of AN to obtain the controlled-realized fertiliz-
ers (CRF) [2, 8–11].
The thermal behavior and the possibilities of en-
hancing the thermal stability of AN have been studied
by several authors [12–20]. AN has six temperature de-
pendent solid-state phases at normal pressure. The prin-
cipal transitions are ANVI«ANV«ANIV«ANIII«
ANII«ANI«ANmelt with equilibrium temperatures
Te –170, –17, 32...55, 84, 125 and 170°C, respectively.
In addition, a metastable transition ANIV«ANII is
known at Te 50°C [14]. The phase transition
In [12] the influence of reactive-grade sodium,
potassium, ammonium, calcium and magnesium sul-
phate, phosphate and carbonate additives on the ther-
mal stability of AN were studied. Most of these addi-
tives shifted the maximum of decomposition exotherm
of AN towards higher temperatures. The best stabiliz-
ing effect on AN was obtained with carbonate addi-
tives and at that calcium carbonate stabilized AN better
than sodium, potassium or magnesium carbonate.
Data on the influence of natural and technical
mineral additives on the thermal stability of AN is al-
most missing. Considering the positive influence of
reactive-grade Ca- and Mg-carbonates on the thermal
characteristics of AN, natural limestone and dolomite
which contain in addition to the main compounds
CaCO3 and CaCO3×MgCO3 also different impurities,
would be suitable minerals for improving thermal
stability of AN.
The aim of this paper was to study the effect of
different lime-containing substances as internal addi-
tives on AN thermal stability, mainly concentrating
on Estonian limestone and dolomite as potentially
suitable coating agents for AN granules.
*
Author for correspondence: tiidu@staff.ttu.ee
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2008 Akadémiai Kiadó, Budapest

KALJUVEE et al.
Experimental
conditions from 30 to 900°C, or in the case of neat
AN to 400°C, at a heating rate of 10°C min–1 in a
stream of dry air or N2 (flow rate 120 mL min–1). Stan-
dard 100 mL alumina crucibles were used. Sample
mass was calculated on the basis of constant mass of
AN (15±0.5 mg), the content of additives in the
blends being 5, 10 or 20 mass%, or calculated from
the mole ratio of AN/(CaO+MgO)=2:1.
Materials
Commercial granulated fertilizer-grade AN (34.4% N)
(Tserepovetski Azot Ltd., Russia) was under investiga-
tion. Three Estonian limestone (from VÞhmuta,
Vasalemma and Karinu deposits) and two dolomite
(Kurevere and Anelema) samples previously ground
(<45 mm) were used as additives to AN. Magnesium
and calcium carbonates used were of analytical-grade
(VWR International Ltd.). The chemical analyses pre-
sented in Table 1 show that all samples are relatively
pure limestone and dolomite. The content of total CaO
in limestone was between 52.9–54.2 mass% and of to-
tal MgO 1.1–2.8 mass%. In dolomite samples the con-
tent of total CaO was on the level of 29.0 mass% and of
total MgO 24.4–26.6 mass%.
To identify the gaseous compounds emitted dur-
ing the thermal treatment of samples, Interspec 2020
Fourier Transform Infrared Spectrometer (FTIR) was
used. The measurements were recorded in the
4000–600 cm–1 region with the resolution of 4 cm–1
and an average out of four scans was taken. To iden-
tify the gaseous compounds, the Bio-Rad (Sadtler)
KnowItAll search program and Gases & Vapours Da-
tabase (code GS) were used.
The SSA of the studied samples, given in Table 1
was relatively small (from 0.74 to 2.44 m2 g–1), being
somewhat bigger for dolomite samples. Hereby, the
selected samples were slightly porous and well crys-
tallized.
Results and discussion
Thermal analysis
The quantitative XRD study confirmed the pure
and well crystallized nature of the samples, espe-
cially, for the limestone ones. In limestone the content
of calcite [CaCO3] was between 93.4% (Karinu)
and 97.1% (Vasalemma) and the content of dolomite
[CaMg(CO3)2] between 2.2% (Vasalemma) and 6.3%
(Karinu). The content of [CaMg(CO3)2] in Kurevere
and Anelema samples was 96.4 and 92.6%, respec-
tively. The other minerals like quartz [SiO2],
From the DTA curves of ground granular AN samples
four endotherms between 30 and 180°C indicating to
phase changes could be observed (Fig. 1). The first,
ANIV«ANIII transition with minimums on DTA curve
in air and in nitrogen atmosphere was fixed at 53.0
and 53.7°C, respectively. The ANIII«ANII and
ANII«ANI transitions followed with minimums
at 88.1 and 90.8°C, and at 127.1 and 128.0°C. The
ANI«ANmelt transition endotherms had its minimums
at 164.8 in air and 168.4°C in nitrogen. Further in-
creasing of temperature reveals a massive exothermic
effect characterizing the decomposition of AN. The de-
composition exotherm had its maximums at 263.0°C
(with a shoulder at 246.4°C) in air and a double peak
with maximums at 276.0 and 286.3°C (and with a
shoulder at 269.1°C) in nitrogen. So, there was ob-
served not high but systematic increase (0.7–2.7°C) in
transition temperatures of AN in nitrogen atmosphere
as compared to these obtained in air, whereas the exo-
thermic maximum of AN decomposition in nitrogen
was moved 13°C towards higher temperature and, ac-
illite–smectite
[Na,Kx(Al,Mg)2Si4O10(OH)2xH2O],
K-feldspar [KAlSi3O8], albite and pyrite were present
on the level of 0.3–1.7% or as traces.
Methods
To study the influence of additives on the thermal sta-
bility of AN, thermogravimetric equipment (Setaram
Labsys 2000) capable of simultaneous recording of
mass loss (TG), differential mass loss (DTG) and dif-
ferential thermal analyses curve (DTA) was used. The
experiments were carried out under dynamic heating
Table 1 Chemical composition and specific surface area (SSA) of limestone and dolomite samples
Chemical composition/mass%*
Sample
BET SSA/m2 g–1
CaO
MgO
CO2
I.R.**
Al2O3
Fe2O3
Ssulfate
Karinu
52.92
53.17
54.22
29.04
28.85
2.76
1.50
38.98
39.87
40.45
41.87
40.81
0.80
1.33
0.75
3.17
5.83
1.72
1.81
1.50
0.64
0.84
0.08
0.04
0.09
0.21
0.14
0.04
0.06
0.09
0.09
0.10
0.74
1.56
0.89
1.70
2.44
VÞhmuta
Vasalemma
Kurevere
Anelema
1.14
24.40
26.63
*Per dry mass, **Insoluble residue in aqua regia
216
J. Therm. Anal. Cal., 92, 2008

INFLUENCE OF LIME-CONTAINING ADDITIVES ON THE THERMAL BEHAVIOUR OF AMMONIUM NITRATE
Fig. 1 Thermal curves of AN in a – air and b – nitrogen
sition temperature of AN were shifted (except with
Vasalemma limestone), respectively, 1.5–1.7 and
8–15°C towards higher temperatures (Table 2, Fig. 2).
Vasalemma limestone in the amount of 5 mass% moved
the temperature of ANII«ANI phase transition and the
melting point a few degrees towards lower, but the max-
imum of exotherm 45.8°C towards higher tempera-
ture – to 308.8°C. Vasalemma limestone was character-
ized as the purest and most well crystallized sample
studied (Table 1).
If the amount of additives was 20 mass% or more,
the ANIV«ANII phase transition instead ANIV«ANIII,
ANIII«ANII was observed on DTA curve, and
20 mass% of Kurevere dolomite shifted the exothermic
maximum 20°C towards higher temperature (Fig. 2).
The total mass loss of AN blends with 5, 10
and 20% Kurevere dolomite up to 315°C was 80, 71
Fig. 2 DTA curves of AN and its blends with Kurevere dolomite
and 56% from the initial sample mass, respectively.
On the DTA curves of AN blends with limestone
cording to the shape of the DTA curve, the decomposi-
tion had a multi-step character. The total mass loss up
to 315°C was, for both, 97–98%.
Limestone–dolomite additives in the amount
of 5–10 mass% did not affect the ANIV«ANIII,
ANIII«ANII and ANII«ANI phase changes in a sys-
tematic manner, but the melting point and the decompo-
or dolomite samples (as well as with reactive-grade
CaCO3 or MgCO3) at the mole ratio of AN/
(CaO+MgO)=2:1 the decomposition exotherm of AN
was disappeared and a multi-peaked endotherm was
fixed on the DTA curves in the temperature interval
of 200–300°C (Fig. 3), characterizing the interaction of
AN with CaCO3 and MgCO3×CaCO3, contained in nat-
Table 2 Thermal characteristics of AN with different limestone and dolomite samples (5 mass%) in air
Phase change/°C
Compound
Exotherm maximum/°C
ANIV«ANIII
ANIII«ANII
ANII«ANI
ANI«ANmelt
AN
53.6
53.0
52.5
53.0
52.5
50.7
88.1
88.4
90.9
88.1
87.6
89.2
127.1
127.0
127.8
125.8
127.8
126.2
164.8
166.5
166.3
161.4
166.4
166.3
263.0
274.3
274.3
308.8
273.8
270.7
AN+VÞhmuta
AN+Karinu
AN+Vasalemma
AN+Kurevere
AN+Anelema
J. Therm. Anal. Cal., 92, 2008
217

KALJUVEE et al.
Fig. 3 Thermal curves of AN blends with a – Karinu limestone and b – Kurevere dolomite at mole ratio of AN/(CaO+MgO)=2:1 in air
ural limestone and dolomite samples, with the forma-
tion of calcium and magnesium nitrates. The total mass
loss at heating up to 315°C was 54.1% for Kurevere
dolomite and 40.7% for Karinu limestone.
ping water peaks in the same region. The emission of
N2O and NO2 at 220–320°C was the most intensive
at 280°C. These temperatures correspond well to the
data received from thermal analysis (Fig. 1a). The
emission of the same compounds in the temperature
range of 230–320°C as in air, can be fixed also on the
FTIR spectra of neat AN in N2, but in addition, clear
and intensive NH3 peaks can be detected in the
1200–750 cm–1 region (Fig. 4b). This may be due to
the fact that the first step of AN’s decomposition is
dissociation into NH3 and HNO3 [12, 15]:
The followed endotherms with minimum on
DTA curves at 389°C (Fig. 3b) and double-peak at
528 and 545°C (Fig. 3b) and at 528 and 598°C
(Fig. 3a) characterized the decomposition of previ-
ously formed Mg(NO3)2 and Ca(NO3)2, respectively
[21, 22]. The last endotherms on DTA curves with
minimums at 751 (Fig. 3b) and 715°C (Fig. 3a) de-
scribe the decomposition of residual carbonates. The
total mass loss at heating of the samples up to 800°C
for Kurevere dolomite and Karinu limestone was 76.2
and 75.3%, respectively. For blends with lower con-
tent of limestone and dolomite more modest decom-
position endoeffects of nitrates and carbonates were
fixed at the same temperature intervals.
NH4NO3®NH3+HNO3
(1)
and N2 is suggested to be one by-product of AN’s de-
composition.
5NH3+3HNO3®4N2+9H2O
(2)
The presence of it in larger quantities inhibit the
further decomposition of AN into N2O and H2O by
the summary equation:
FTIR analysis
NH4NO3®N2O+2H2O
(3)
The results of the analysis of gaseous compounds
evolved at the thermal treatment of neat AN indicated
to some differences in the decomposition of AN car-
ried out in air or in N2. In air the main products of AN
decomposition were N2O (2235 and 2215 cm–1) and
H2O (bands in the broad ranges 3900–3500 and
1900–1300 cm–1) as the literature suggests [12, 15],
but also the peaks characteristic to NO2 (1630 and
1595 cm–1) and to NH3 as traces (967 and 392 cm–1)
could be fixed on the FTIR spectra (Fig. 4a). The
peaks at 1630 and 1595 cm–1 were partially overlap-
The results of the analysis of gaseous com-
pounds evolved at the thermal treatment of AN and
CaCO3 or Karinu limestone blends at the mole ratio of
AN/(CaO+MgO)=2:1 suggested that when mixed
with CaCO3 or Karinu limestone, the decomposition
of AN proceeds through a completely different mech-
anism. None of the peaks indicating to the emission of
N2O could be fixed on the FTIR spectra of the AN
blend with CaCO3 in N2, and the maximum intensities
of the characteristic peaks of N2O in air as well as for
218
J. Therm. Anal. Cal., 92, 2008

INFLUENCE OF LIME-CONTAINING ADDITIVES ON THE THERMAL BEHAVIOUR OF AMMONIUM NITRATE
Fig. 4 FTIR spectra of gaseous compounds evolved at thermal treatment of AN in a – air and b – nitrogen
Fig. 5 FTIR spectra of gaseous compounds evolved at thermal treatment of AN blends with a – Karinu limestone and
b – Kurevere dolomite at mole ratio of AN/(CaO+MgO)=2:1 in air
AN blend with Karinu limestone both in air (Fig. 5a)
and in nitrogen, were relatively low (Fig. 6).
when precedingly formed Ca(NO3)2 (Eq. (4)) begins
to decompose having at that with the maximum peak
intensities for the blends with CaCO3 and Karinu
limestone at 580 and 610°C, respectively.
In air in the temperature region from 180 to
300°C and from 200 to 320°C for AN blends with
CaCO3 and Karinu limestone, respectively, the peaks
characteristic to NH3 appeared in the 1200–750 cm–1
region. CO2 (2367 and 2354 cm–1) emitted most inten-
sively in the temperature range between 240 and
300°C, but traces of it can be detected on the FTIR
spectra up to 800°C due to the decomposition of re-
sidual carbonates.
2NH4NO3+CaCO3®Ca(NO3)2+2NH3+CO2+H2O (4)
In the temperature interval from 500 to 630°C
emission of a compound giving peaks in the range
of 1800–1950 cm–1 can be detected which is most
likely characteristic to NO [23] evolved during de-
composition of Ca(NO3)2.
In air modestly intensive characteristic peaks for
NO2 were fixed on the FTIR spectra in the tempera-
ture interval from 200 to 290°C, however, it starts to
emit more intensively at 500°C – at the temperature
The formed Ca(NO3)2 decomposes at tempera-
tures over 500°C. In [22] various decomposition reac-
tions for Ca(NO3)2 are suggested, of which proba-
bly Eqs (5) and (6) are dominating and Eq. (7) occurs
J. Therm. Anal. Cal., 92, 2008
219

KALJUVEE et al.
Fig. 6 Maximum intensities of characteristic peaks of gaseous compounds evolved at thermal treatment of AN and its blends with
different lime-containing additives at mole ratio of AN/(CaO+MgO)=2:1 in a – air and b – nitrogen; 1 – AN,
2 – AN+CaCO3, 3 – AN+MgCO3, 4 – AN+Karinu, 5 – AN+Kurevere
as a side reaction under the given experiment condi-
tions, because on the FTIR spectra the characteristic
peaks of NO2 are the most intensive and the ones of
NO somewhat less:
product of AN decomposition – was evolved and the
decomposition mechanism of AN was not heavily af-
fected by the atmosphere used in the experiment.
The results of the FTIR analysis of gaseous com-
pounds evolved at the thermal treatment of AN blends
with MgCO3 and Kurevere dolomite (Fig. 5b) at mole
ratio 2:1 differed from the ones of blends with CaCO3
and Karinu limestone (Fig. 6). At 250–270°C the
main product of AN decomposition N2O began to
evolve, having the most intensive peaks on the FTIR
spectra at 320–330°C both in air and in N2. The inten-
sities of the respective peaks for AN blends with
MgCO3 and Kurevere dolomite were much higher
than obtained with CaCO3 and Karinu limestone addi-
tives (Fig. 6). This indicates to the fact that the reac-
tion of AN with MgCO3 and dolomite was not as
complete as with CaCO3 and limestone, because
MgCO3 is less reactive towards AN than CaCO3, and
part of AN decomposed completely without reacting
with MgCO3. Also NH3 and CO2 emitting most inten-
Ca(NO3)2®CaO+NO2+NO+O2
Ca(NO3)2®CaO+2NO2+1/2O2
Ca(NO3)2®CaO+2NO+11/2O2
(5)
(6)
(7)
The results of the FTIR analysis of gaseous com-
pounds like CO2, NO2, NO evolved at the thermal treat-
ment of the same blends in N2 were similar to the ones in
air, with the exception that the beginning and the maxi-
mum intensities of characteristic peaks were shifted
10–40°C towards higher temperatures (Table 3).
So, FTIR analysis suggests that AN mixed with
CaCO3 or limestone at the equivalent mole ratio re-
acts almost completely with CaCO3 as in the tempera-
ture region of AN decomposition, in addition to NH3
and CO2, only modest amount of N2O – the main
Table 3 Temperatures (°C) of maximum intensities of characteristic absorption peaks of gaseous compounds evolved at ther-
mal treatment of AN and its blends with lime-containing additives at mole ratio of AN/(CaO+MgO)=2:1
N2O
(2230 cm–1)
NO2
(1630 cm–1)
NO
(1845 cm–1)
NH3
(967 cm–1)
CO2
(2367 cm–1)
H2O
(1735 cm–1)
Sample
In air
AN
280
330
330
280
330
300
580
480
610
550
–
280
280
260
280
280
–
230
280
330
230
330
AN+CaCO3
AN+MgCO3
AN+Karinu
AN+Kurevere
580
480
620
–
240
260
280
730
In nitrogen
AN
290
–
320
330
330
300
630
480
680
550
–
270
280
260
280
280
–
300
260
280
330
330
AN+CaCO3
AN+MgCO3
AN+Karinu
AN+Kurevere
630
480
680
–
280
330
280
730
220
J. Therm. Anal. Cal., 92, 2008

INFLUENCE OF LIME-CONTAINING ADDITIVES ON THE THERMAL BEHAVIOUR OF AMMONIUM NITRATE
sively from 180 to 300°C and NO2 from 350 to 480°C
for the blend with MgCO3, and from 350 to 550°C for
the blend with Kurevere dolomite, indicate that AN
reacts with MgCO3 to a limited extent according to
limestone samples, because MgCO3 is thermodynam-
ically less reactive towards AN than CaCO3.
Acknowledgements
2NH4NO3+MgCO3®
Mg(NO3)2+2NH3+CO2+H2O
This work was partly supported by Estonian Science of
Foundation (G7548).
(8)
with the following decomposition of Mg(NO3)2 at
higher temperatures.
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Emission of NO was fixed only when using
MgCO3, but not Kurevere dolomite (Fig. 6). The
peaks characteristic to CO2 could be observed on the
FTIR spectra of AN blend with Kurevere dolomite
being most intensive at 730°C (Table 3) indicating to
the decomposition of unreacted carbonates.
So, the proof of AN reaction with carbonates in dolo-
mite was even more solid and obvious than in the case
of limestone. The FTIR analysis of the gaseous com-
pounds evolved at the thermal treatment of AN
with 10–20 mass% of limestone–dolomite additives
showed analogues, but less expressed tendencies.
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