Thermal behaviour of ammonium nitrate prills coated with limestone and dolomite powder

Article abstract of DOI:10.1007/s10973-009-0391-y

The thermal behaviour of ammonium nitrate (AN) and its prills coated with limestone and dolomite powder was studied on the basis of commercial fertilizer-grade AN and six Estonian limestone and dolomite samples. Coating of AN prills was carried out on a plate granulator and a saturated solution of AN was used as a binding agent. The mass of AN prills and coating material was calculated based on the mole ratio of AN/(CaO + MgO) = 2:1. Thermal behaviour of AN and its coated prills was studied using combined TG-DTA-FTIR equipment. The experiments were carried out under dynamic heating conditions up to 900 °C at the heating rate of 10 °C min^-1 and for calculation of kinetic parameters, additionally, at 2, 5 and 20 °C min^-1 in a stream of dry air. A model-free kinetic analysis approach based on the differential isoconversional method of Friedman was used to calculate the kinetic parameters. The results of TG-DTA-FTIR analyses and the variation of the value of activation energy E along the reaction progress α indicate the complex character of the decomposition of neat AN as well as of the interactions occurring at thermal treatment of AN prills coated with limestone and dolomite powder.

Full text of DOI:10.1007/s10973-009-0391-y

J Therm Anal Calorim (2010) 99:749–754
DOI 10.1007/s10973-009-0391-y
Thermal behaviour of ammonium nitrate prills coated
with limestone and dolomite powder
I. Rudjak Æ T. Kaljuvee Æ A. Trikkel Æ
V. Mikli
MEDICTA2009 Conference
´
´
Ó Akademiai Kiado, Budapest, Hungary 2009
Abstract The thermal behaviour of ammonium nitrate
(AN) and its prills coated with limestone and dolomite
powder was studied on the basis of commercial fertilizer-
grade AN and six Estonian limestone and dolomite sam-
ples. Coating of AN prills was carried out on a plate
granulator and a saturated solution of AN was used as a
binding agent. The mass of AN prills and coating mate-
rial was calculated based on the mole ratio of
AN/(CaO ? MgO) = 2:1. Thermal behaviour of AN and
its coated prills was studied using combined TG-DTA-
FTIR equipment. The experiments were carried out under
dynamic heating conditions up to 900 °C at the heating rate
of 10 °C min-1 and for calculation of kinetic parameters,
additionally, at 2, 5 and 20 °C min-1 in a stream of dry air.
A model-free kinetic analysis approach based on the dif-
ferential isoconversional method of Friedman was used to
calculate the kinetic parameters. The results of TG-DTA-
FTIR analyses and the variation of the value of activation
energy E along the reaction progress a indicate the com-
plex character of the decomposition of neat AN as well as
of the interactions occurring at thermal treatment of AN
prills coated with limestone and dolomite powder.
Introduction
Ammonium nitrate (AN) is one of the most widespread
nitrogen mineral fertilizers used in the agriculture [1].
Unfortunately, thermal instability of AN, especially, if
contaminated with impurities even on the low level, makes
its handling and storage unpredictable which has led to
several catastrophic explosions [2–4].
The thermal behaviour and the possibilities of increasing
the thermal stability of AN have been studied by several
authors [4–11]. Normally, solid AN follows four poly-
morphic phase transitions at ambient pressure—the tran-
sition ANIV $ ANIII occurs at 32 °C and ANIII $ ANII at
84 °C. In addition, a metastable transition ANIV $ ANII
is possible at 50 °C [10–12]. The phase transition
ANIV $ ANIII causes drastic changes in the physical and
thermodynamic properties of AN (note that ANIV $ ANII
does not) [12] with the following decrease in the thermal
stability of AN. Therefore, avoiding the ANIV $ ANIII
transition at manufacturing, storing and handling of AN has
a great importance considering practical applications.
Another possibility to reduce the explosive potentiality
of AN is to use additives which retard the decomposition of
AN by increasing the reaction zone or stabilize thermal
properties of AN [4, 7, 11, 13]. In [4] the influence of
different reactive-grade sulphate, phosphate, and carbonate
additives on the thermal stability of AN were studied. The
maximum shift of the decomposition exotherm of AN
towards higher temperatures was obtained with calcium
carbonate additives.
Keywords Coated ammonium nitrate ꢀ Dolomite ꢀ
Kinetics ꢀ Limestone ꢀ TG-DTA ꢀ Thermal stability
Earlier, the influence of Estonian limestone and dolo-
mite from different deposits added to AN at the mole ratio
of AN/(CaO ? MgO) = 2:1 on the thermal behaviour of
AN was studied using combined TG-DTA-EGA (FTIR)
equipment [14].
I. Rudjak ꢀ T. Kaljuvee (&) ꢀ A. Trikkel ꢀ V. Mikli
Department of Chemical and Materials Technology, Tallinn
University of Technology, Ehitajate tee 5,
19086 Tallinn, Estonia
e-mail: tiidu@staff.ttu.ee
123

750
I. Rudjak et al.
The aim of the present research was to study the thermal
used. A single prill was analyzed, the mass of AN prills
was 15 0.2 mg, the mass of coated prills varied between
24 and 26 mg.
behaviour of AN prills coated with limestone and dolomite
powder and to calculate the kinetic parameters of decom-
position reactions of the coated AN prills.
FTIR measurements were recorded in the
4,000–600 cm-1 region with the resolution of 4 cm-1 and
an average out of four scans was taken. To identify the
gaseous compounds, the Bio-Rad (Sadtler) KnowItAll
search program and Gases and Vapours Database (code
GS) were used.
Experimental
Materials
The surface observations of the samples were carried out
with a scanning electron microscope Jeol JSM-8404.
A model-free kinetic analysis approach based on the
differential isoconversional method of Friedman [15] was
used to calculate the kinetic parameters. After baseline
correction and normalization, the TG signals were pro-
cessed with the AKTS Advanced Thermokinetics software
[16].
Commercial fertilizer-grade AN (34.4% N) (Tserepovetski
Azot Ltd, Russia) was under investigation. Six previously
ground Estonian limestone and dolomite samples
(\45 lm) were used as the coating material. Coating of
AN prills was carried out on a plate granulator with
diameter 0.30 m using saturated solution of AN as a
binding agent. The mass of AN prills and coating material
was calculated on the assumption of the mole ratio of
AN/(CaO ? MgO) = 2:1. SEM images of the cross-sec-
tions of an AN prill and the prill coated with Anelema
dolomite powder (thickness 200–300 lm) are presented in
Fig. 1.
Results and discussion
Thermal analysis
The chemical composition and specific surface area
(SSA) of the limestone and dolomite samples is presented
in Table 1. In limestone samples the content of total
CaO and MgO was in between 52.9–54.2 mass% and
1.1–2.8 mass%, and in dolomite samples in between
26.0–29.0 mass% and 24.3–26.6 mass%, respectively.
More precise characterization of the limestone and dolo-
mite samples used has been presented in [14].
On the DTA curves of thermally treated AN prills four
endotherms between 30 and 180 °C corresponding to the
respective phase transformations can be observed (Fig. 2a).
Depending on the heating rate, the first, ANIV $ ANIII
transition with a minimum on DTA curve, was fixed in a
temperature interval from 52 to 71 °C. The principal tran-
sitions ANIII $ ANII, ANII $ ANI and ANI $ ANmelt
with minimums on DTA curve occurred at temperatures
from 91 to 107 °C, 131 to 150 °C and 168 to 170 °C. At
higher temperatures these phase transitions were followed
with a modest exoeffect and massive endoeffect with
maximums and minimums on the DTA curve in between
226–258 °C and 280–324 °C, respectively, corresponding
to the decomposition of AN. Besides, the endoeffect has
two shoulders between 259–290 °C and 273–307 °C. The
curves indicate a complicated multi-step character of the
decomposition pathway of AN.
Methods
The thermal behaviour of AN and its coated prills was
studied using Setaram Labsys 2000 equipment coupled to
Interspec 2020 Fourier Transform Infrared Spectrometer
(FTIR) by a transfer line. The experiments were carried out
under dynamic heating conditions up to 900 °C at the
heating rate of 10 °C min-1 and for calculation of kinetic
parameters, additionally, at the heating rates of 2, 5 and
20 °C min-1 in
a stream of dry air (flow rate
120 mL min-1). Standard 100 lL alumina crucibles were
Differently, from the results with previously ground AN
prills [14], when using whole prills, the temperature min-
imums on DTA curve for ANIV $ ANIII $ ANII $ ANI
transitions were shifted at the heating rate of 10 °C min-1
10–14 °C and for ANI $ ANmelt transition 5.2 °C towards
higher temperatures, and the decomposition pathway of
ground AN prills was accompanied also by a very intensive
exoeffect on the DTA curve with maximum at 263 °C.
The thermograms of AN prills coated with limestone or
dolomite powder showed complicated but quite similar
pathway of interactions as compared to thermal behaviour
of AN blends with limestone or dolomite additives at the
same mole ratio of AN/(CaO ? MgO)—2:1 [14]. For
Fig. 1 SEM pictures of the cross-sections of neat AN prill (a) and
AN prill coated with Anelema dolomite powder (b) (magnification
910)
123

Thermal behaviour of ammonium nitrate prills
751
Table 1 Chemical composition and specific surface area (SSA) of limestone and dolomite samples
Sample
Chemical composition/mass%a
BET SSA/m2 g-1
CaO
MgO
CO2
I. R.b
Al2O3
Fe2O3
Ssulphate
Karinu
52.92
53.17
54.22
29.04
28.85
25.95
2.76
1.50
38.98
39.87
40.45
41.87
40.81
35.58
0.80
1.33
0.75
3.17
5.83
12.41
1.72
1.81
1.50
0.64
0.84
1.41
0.08
0.04
0.09
0.21
0.14
0.37
0.04
0.06
0.09
0.09
0.10
0.10
0.74
1.56
0.89
1.70
2.44
5.79
Vo˜hmuta
Vasalemma
Kurevere
1.14
24.40
26.63
24.29
Anelema
Anelema wastes
a
Per dry mass
b
Insoluble residue in aqua regia
The last endotherms on DTA curves with minimums at 746
and 728 °C (Fig. 2b), describe the decomposition of
residual carbonates.
The total mass loss at heating of the AN prills coated
with Vo˜hmuta limestone powder up to 610 and 770 °C was
65.1 and 71.7% and in the case of Anelema dolomite
powder coating when heated up to 440, 560 and 770 °C,
respectively, 59.1, 71.0 and 79.3%.
FTIR analysis
FTIR spectra of gaseous compounds evolved at thermal
treatment of neat AN prills showed peaks characteristic to
NH3 (967 and 932 cm-1), N2O (2,235 and 2,215 cm-1),
NO2 (1,630 and 1,595 cm-1) and H2O (bands in the broad
ranges 3,900–3,500 and 1,900–1,300 cm-1) having maxi-
mum intensity at 250, 280, 300 and 300 °C, respectively
(Fig. 3a). The peaks characteristic to NO2 and NH3 were
1.5–2 times more intensive as compared to these obtained
at thermal treatment of previously ground AN prills [14]. It
is in a good correlation with the results presented in [19,
20]—the decomposition of AN, after dissociation into NH3
and HNO3 in the first step, can follow different pathways
with the formation of N2O at lower temperatures
(\280 °C) and NO2 at higher temperatures. Compactable
AN prills follow preferably high-temperature route of AN
decomposition. So, the decomposition pathway of AN at
thermal treatment could be presented by the following
simplified equations:
Fig. 2 DTA curves of neat AN prills at different heating rates (a) and
AN prills coated with Anelema dolomite and Vo˜hmuta limestone
powder at the hating rate of 10 °C min-1 (b)
example, on the DTA curves of AN prills coated with
Vo˜hmuta limestone or Anelema dolomite powder, a multi-
peaked endotherm was fixed in the decomposition region of
AN (190–340 °C). With the prills coated with Anelema
dolomite, additionally, a modest exotherm with a maximum
at 317 °C was fixed (Fig. 2b). These peaks characterize the
interactions between AN and Mg, Ca-carbonates with the
formation of nitrates. The total mass loss at heating of the
AN prills coated with Vo˜hmuta limestone and Anelema
dolomite powder up to 340 °C was 35.4 and 56.2%,
respectively, indicating that dolomite (CaCO3ꢀMgCO3)
reacts with AN less actively than limestone (CaCO3).
The subsequent endotherms on the DTA curve of AN
prills coated with Vo˜hmuta limestone powder with mini-
mums at 527 and 602 °C (Fig. 2b) characterize, respec-
tively, the melting and decomposition of previously formed
Ca(NO3)2 [17]. The double-peak endotherms on the DTA
curve of AN prills coated with Anelema dolomite powder
with minimums at 392 and 422 °C, and at 522 and 552 °C
(Fig. 2b) characterize the decomposition of previously
formed Mg(NO3)2 and Ca(NO3)2, respectively [17, 18].
NH4NO3 ! NH3 þ HNO3
NH4NO3 ! N2O þ 2H2O
ð1Þ
ð2Þ
1
2HNO3 ! H2O þ 2NO2 þ = O2
ð3Þ
2
In the FTIR spectra of AN prills coated with limestone
powder, the peaks characteristic to NH3 and CO2 (2,358
and 2,354 cm-1) dominated in the temperature interval
from 200 to 300 °C. The characteristic peaks for H2O, NO2
and N2O were also present, but had lower intensity
123

752
I. Rudjak et al.
(Fig. 3b). In the FTIR spectra of AN prills coated with
dolomite powder, the peaks characteristic to NO2 and,
especially, to N2O in the same temperature interval were
presented more intensively (Fig. 3c). This indicates that at
these temperatures the following interaction between AN
and CaCO3 with the formation of Ca(NO3)2 dominated:
limestone coating. Corresponding tendencies in FTIR
spectra collected in the same temperature interval at ther-
mal treatment of AN blends with limestone and dolomite
additives were observed also in [14].
The second maximum on the absorbance profile of CO2
at 390–400 °C at thermal treatment of AN prills coated
with Anelema dolomite powder (Fig. 3c) points to the
partial interaction of formed Mg(NO3)2 with CaCO3:
2 NH4NO3 þ CaCO3 ! CaðNO3Þ2 þ 2 NH3 þ CO2
þ H2O
MgðNO3Þ2þ CaCO3 ! CaðNO3Þ2 þ MgO þ CO2 ð6Þ
ð4Þ
The peaks characteristic to NO2 were fixed in FTIR spectra
once again with maximum intensities at 400 and 540 °C
when using Anelema dolomite coating (Fig. 3c) and with
maximum intensity at 580 °C when using Vo˜hmuta
limestone coating (Fig. 3b). These are the temperatures
of decomposition of Mg, Ca-nitrates formed at lower
temperatures. The bands with maximum intensity at
540 °C when using dolomite and at 580 °C in the case of
limestone, fixed in FTIR spectra in the 1,800–1,950 cm-1
region, also indicate the formation and emission of NO
(Fig. 3b, c). The decomposition of Mg, Ca-nitrates can be
presented by the following simplified equations:
The reaction of AN with MgCO3
2 NH4NO3 þ MgCO3 ! MgðNO3Þ2 þ 2 NH3 þ CO2
þ H2O
ð5Þ
was not as complete as MgCO3 is thermodynamically less
reactive towards AN than CaCO3. In addition, using
dolomite as the coating material, more AN decomposed
with the formation of nitrous gases as compared to
MgðNO3Þ2! MgO þ NO2 þ NO þ O2
CaðNO3Þ2! CaO þ NO2 þ NO þ O2
ð7Þ
ð8Þ
The last maximum on the absorbance profile of CO2 at 680
and 730 °C obtained at thermal treatment of AN prills
coated, respectively, with Vo˜hmuta limestone (Fig. 3b) and
Anelema dolomite powder (Fig. 3c), is caused by the
decomposition of residual carbonates:
MgCO3 ꢀ CaCO3 ! MgO þ CaCO3 þ CO2
CaCO3 ! CaO þ CO2
ð9Þ
ð10Þ
Hereby, the results of EGA from the experiments with
coated AN prills confirmed that the interactions between
AN and carbonates contained in the coating materials used
proceeded mostly by similar pathway as compared to AN
blends with limestone and dolomite [14]—partially, or
almost completely through the formation of Mg(NO3)2 and
Ca(NO3)2 which decomposed thereupon in the temperature
range of 360–620 °C.
Determination of kinetic parameters
The reaction rates and the kinetic parameters A and E for
the decomposition of AN prills calculated using the dif-
ferential isoconversional method of Friedman are presented
in Fig. 4, and for AN prills coated with limestone or
dolomite powder in Table 2. Here, step I presents data for
low temperature region (depending on the heating rate
between 150 and 350 °C) where interactions between AN
and carbonates with the formation of Ca, Mg-nitrates take
Fig. 3 Absorbance profiles of gaseous compounds evolved at thermal
treatment of neat AN prill (a), AN prills coated with Vo˜hmuta
limestone (b) and Anelema dolomite (c) powder at the heating rate of
10 °C min-1
123

Thermal behaviour of ammonium nitrate prills
753
limestone powder having poor content of MgO (Table 1)
the step IIA is missing.
For neat AN prills the activation energy in the range of
conversion (decomposition) level 0.1 \ a\0.9 was in
between 66.7 and 142.7 kJ mole-1 (pre-exponential factor
A from 4.0 9 103 to 1.2 9 1011 s-1) (Fig. 4b) varying at
that more than this calculated in [14] in the case of pow-
dered neat AN (E between 92.7 and 106.8 kJ mole-1
)
indicating more complicated character of decomposition of
AN in prills than in powdered form.
For AN prills coated with limestone and dolomite
powder the value of activation energy in the same range of
a (decomposition and interactions of AN with carbonates—
step I) varied also in a wide range. For example, for AN
prills coated with Vo˜hmuta limestone powder from 82.8 to
119.6 kJ mole-1 (pre-exponential factor A in between
7.3 9 105 and 1.3 9 1010 s-1), with Kurevere dolomite
powder from 111.3 to 154.8 kJ mole-1 (A in between
2.0 9 108 and 2.9 9 1012 s-1). For step IIA (interactions
with Mg-nitrate) and IIB (decomposition of Ca-nitrate) the
value of activation energy also varied in a great extent. For
example, for AN prills coated with Kurevere dolomite
powder from 185.2 to 344.7 kJ mole-1 (A in between
1.3 9 1012 and 2.8 9 1024 s-1) during step IIA and from
154.6 to 194.1 kJ mole-1 (A in between 5.9 9 107 and
Fig. 4 Reaction rate da/dt (DTG, normalized signals) as a function of
the temperature for different heating rates (a), and activation energy
Ea and pre-exponential factor A determined by Friedman analysis as a
function of the reaction progress (b) for the decomposition of neat AN
prills. (The value of the heating rate in °C min-1 are marked on the
curves.)
place, but also unreacted AN, especially, in the case of
dolomite coating decomposes. Step IIA presents data
characterizing reactions occurring with Mg(NO3)2 in the
temperature region from 310 to 450 °C and step IIB data
characterizing decomposition of Ca(NO3)2 in the temper-
ature region of 420–620 °C. For AN prills coated with
Table 2 The activation energy E (kJ mole-1) and pre-exponent factor ln A versus reaction progress a
Sample
a
0.1
E
0.2
ln A E
0.3
ln A E
0.4
ln A E
0.5
ln A E
0.6
ln A E
0.7
ln A E
0.8
ln A E
0.9
ln A E
ln A
Step I:
Neat AN
66.7 8.3 81.0 11.9 114.4 19.3 136.7 24.2 142.7 25.5 141.0 25.2 138.7 24.8 138.7 24.5 129.6 23.2
119.6 23.3 114.4 21.7 110.9 20.5 106.5 19.3 104.6 18.6 101.1 17.7 95.1 16.3 87.2 14.4 82.8 13.5
106.7 20.0 90.7 20.6 96.0 14.5 83.9 13.8 81.7 13.2 79.0 12.5 80.2 12.8 83.6 13.5 87.6 14.4
109.7 19.3 91.9 15.2 85.5 13.6 85.8 13.6 92.2 14.9 106.8 17.9 116.3 19.9 119.2 20.5 118.7 20.4
111.3 19.1 118.8 20.9 131.5 23.7 154.8 28.7 153.3 28.2 152.6 28.0 144.9 26.2 133.9 23.8 131.4 23.2
107.2 18.1 101.5 16.9 97.9 16.1 97.9 16.2 97.9 16.2 99.6 16.6 101.7 17.1 103.9 17.5 107.2 18.2
AN ? Vo˜hmuta
AN ? Karinu
AN ? Vasalemma
AN ? Kurevere
AN ? Anelema
AN ? Anelema wastes 92.4 15.0 95.6 15.9 100.0 16.9 102.6 17.5 101.7 17.4 103.0 17.6 102.0 17.4 102.3 17.5 101.9 17.4
Step II:
AN ? Vo˜hmuta
124.8 11.6 120.4 10.7 206.9 23.5 247.0 29.5 255.3 30.8 230.2 27.5 223.7 26.8 219.9 26.4 206.6 24.7
170.3 17.9 205.1 23.1 200.8 22.6 170.0 18.4 156.6 16.7 173.1 19.3 178.3 20.2 182.8 21.0 191.3 22.4
192.8 22.3 194.0 22.3 192.1 21.7 197.0 22.3 219.5 25.9 229.6 27.7 218.0 26.3 223.5 27.2 228.1 28.1
253.8 40.0 335.5 54.8 344.7 56.3 308.2 49.7 265.7 42.0 234.9 36.5 219.1 33.6 205.3 31.3 185.2 27.9
160.3 18.7 154.6 18.0 158.9 18.9 165.4 19.8 169.0 20.3 168.1 20.0 155.9 17.9 172.1 20.3 194.1 23.8
246.1 39.3 292.5 47.7 321.1 52.8 316.3 51.9 297.4 48.4 285.7 46.2 283.4 17.9 172.1 20.3 194.1 23.8
177.4 20.9 153.8 17.3 145.8 16.3 164.8 19.3 175.0 20.9 174.2 20.6 163.0 18.8 123.6 12.9 135.5 14.9
253.8 41.0 299.2 49.4 110.1 51.3 304.0 50.1 287.7 47.1 273.0 44.5 263.9 42.8 249.2 40.1 228.4 36.4
AN ? Karinu
AN ? Vasalemma
AN ? Kurevere, IIA
AN ? Kurevere, IIB
AN ? Anelema, IIA
AN ? Anelema, IIB
AN ? Anelema
wastes, IIA
AN ? Anelema
wastes, IIB
160.3 18.7 154.6 18.0 158.9 18.8 165.4 19.3 169.0 20.3 168.1 20.0 155.9 17.9 172.1 20.3 194.1 23.8
123

754
I. Rudjak et al.
2.2 9 1010 s-1) during step IIB (Table 2). For AN prills
coated with Vo˜hmuta limestone powder the activation
energy for step II varied from 120.4 to 255.3 kJ mole-1 (A
in between 4.4 9 104 and 2.4 9 1013 s-1).
7. Remya Sudhakar AO, Mathew F. Thermal behaviour of CuO
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cations on the thermal decomposition of NH4NO3 in pure form
and supported on alumina. Thermochim Acta. 1993;216:137–46.
The results obtained indicate the complex character of
the decomposition of AN and the interactions occurring in
the AN prills coated with limestone or dolomite powder,
prove the formation (step I) and decomposition of nitrates
(step II), and point to the visible dependence of the values
of activation energy and pre-exponential factor on the
reaction extent.
¨
10. Kestila E, Harju MEE, Valkonen J. Differential scanning calo-
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decomposition of phase stabilized ammonium nitrate (PSAN) by
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ters. Thermochim Acta. 1998;319:55–65.
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stabilization: theoretical possibilities. Proceedings of the Inter-
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India. http://www.fertilizer.org/ifa/Library/Conference proceed-
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ammonium nitrate in the form of PVP-AN glass. Mater Lett.
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Conclusions
AN prills coated with limestone or dolomite powder follow
at thermal treatment predominantly the same complicated
but safe behaviour as AN blends with limestone or dolo-
mite additives—interactions between AN and Ca, Mg-
carbonates with the formation of Ca, Mg-nitrates excluding
exothermic explosive decomposition of AN and the fol-
lowing decomposition of the formed nitrates at higher
temperatures.
Acknowledgements This work was partly supported by the Esto-
nian Ministry of Education and Research (SF0140082s08) and the
Estonian Science Foundation (G7548).
16. AKTS Software and SETARAM Instruments: a global solution
for kinetic analysis and determination of the thermal stability of
materials. Switzerland: AKTS AG; 2006. p. 88.
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17. Madarasz J, Varga PP, Pokol G. Evolved gas analyses (TG/DTA-
MS and TG-FTIR) on dehydration and pyrolysis of magnesium
nitrate hexahydrate in air and nitrogen. J Anal Appl Pyrolysis.
2007;79:475–8.
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