This document summarizes research on factors that impact the thermal stability of ammonium nitrate (AN), including pH, impurities, and temperature. Testing with a differential scanning calorimeter (DSC) and adiabatic calorimeter (ARC) showed that lower pH decreases AN's onset temperature significantly. The acid used to adjust pH also influences stability. Impurities like chloride ions decrease onset temperature but have a greater impact on decomposition rates. Maintaining AN at a sufficiently high pH and low impurity levels is important for safe storage and transport.

1. 1
Espoon tutkimuskeskus
Impact of Impurities and
pH on AN Stability
Antti Vuori
Kemira Oyj
Espoo Research Centre
FIN-02271 Espoo
ANPSG Meeting
Oct. 10, 2001

2. 2
Espoon tutkimuskeskus
CONTENTS
Introduction
- decomposition reactions
- experimental methods
 Ammonium Nitrate Decomposition
- pH effect
- catalytic/impurity effect
- decomposition rate
CONCLUSIONS

3. 3
Espoon tutkimuskeskus
Thermal Decomposition
NH4NO3  N2 + 2H2O(g) + ½O2
H = - 1460 kJ/kg

4. 4
Espoon tutkimuskeskus
Decomposition Reactions of
Ammonium Nitrate 1
NH4NO3  NH3 + HNO3 +197 kJ/mol
NH4NO3  N2O + 2H2O -36.8 kJ/mol
2NH4NO3  2N2 + O2 + 4H2O -118 kJ/mol
2NH4NO3  N2 + 2NO + 4H2O -27.6 kJ/mol
3NH4NO3  2N2 + N2O3 + 6H2O -
4NH4NO3  3N2 + 2NO2 + 8H2O -92.9 kJ/mol
(5NH4NO3  4N2 + 2HNO3 + 9H2O) -123 kJ/mol
2NH4NO3 + C  N2 + CO2 + 4H2O -315 kJ/mol
Source: Nitrogen & Methanol No. 235, September-October 1998

5. 5
Espoon tutkimuskeskus
NH4NO3  N2 + 2H2O + 0.5O2 -1460 kJ/kg
1.5NH4NO3 + NH4H2PO4 
2N2 + 4.5H2O + H3PO4 -1540 kJ/kg
3NH4NO3 + (NH4)2HPO4 
4N2 + 9H2O + H3PO4 -2090 kJ/kg
4.5NH4NO3 + (NH4)3PO4 
6N2 + 13.5H2O + H3PO4 -2400 kJ/kg
2NH4NO3 + (NH4)2SO4 
3N2 + 8H2O + SO2 -2080 kJ/kg
source: Chem. & Eng. News July 24, 1995
Decomposition Reactions of
Ammonium Nitrate 2

6. 6
Espoon tutkimuskeskus
Test Methods for Thermal
Decomposition Studies
DIFFERENTIAL
SCANNING
CALORIMETER
REACTION
CALORIMETER
(Bomb Test)
ADIABATIC
CALORIMETER
(ARC)
SYSTEM PARAMETER
temperature
ramp
semibatch
batch
isothermal
adiabatic
Qr
concentration
volume / mass
temperature /
pressure

7. 7
Espoon tutkimuskeskus
Criteria for Thermal Risk
Evaluation
Risk Criteria Severity Probability
High Tad > 200 K or Tonset-Tp < 50 K or
Tmax-Tp > 200 K or TMRad < 8 h or
bp surpassed initiation/accumulation
Medium 50 K < Tad < 200 K or 100 K > Tonset-Tp > 50 K or
50 K < Tmax-Tp < 200 K 8 h < TMRad < 24 h
or gas evolution
Low Tad < 50 K or Tonset-Tp > 100 K or
Tmax-Tp < 50 K TMRad > 24 h

8. 8
Espoon tutkimuskeskus
Adiabatic Temperature
Increase
W.Hoyle, Pilot Plants and Scale-up of Chemical Processes 1997

9. 9
Espoon tutkimuskeskus
Ammonium Nitrate DSC
threshold at 0.01mW
Onset 255,68 °C
Integral 6729,53 mJ
normalized 1502,13 Jg^-1
Peak 320,41 °C
threshold at 1mW
Onset 290.29 °C
Pan: HP Gold Plated 50ul
Sample: perusnäyte/NH4NO3/6, 4.4800 mg
Module: DSC821 Au, 30.03.1998 17:10:37
Method: 27-5-420 Au
27.0-420.0°C 5.0°C/min
Experiment: perusnäyte/NH4NO3/6, 10.04.1998 01:35:33
Wg^-1
10
°C
140 160 180 200 220 240 260 280 300 320 340
^exo NH4NO3 02.10.2001 10:17:52
DSC: Kemira Agro Oy System
e
R
TA
METTLER TOLEDO S

10. 10
Espoon tutkimuskeskus
Repeatability of DSC
Measurements
NH4NO3 Tonset (C) Tmax (C) -deltaH (J/g)
290.9
287.4
293.3
290.5
289.4
287.5
322.6
323.3
325.7
323.1
320.4
321.0
1720.6
1743.2
1588.7
1583.7
1515.5
average 289.8 322.7 1630.3
stdev 2.25 1.87 97.4
stdev (%) 0.78 0.58 6.0

11. 11
Espoon tutkimuskeskus
Effect of pH
pH 4.7
Method: 30-5-420 Au
Onset 177,15 °C Integral 6767,71 mJ
normalized 1735,31 Jg^-1
Onset 259,69 °C
Peak 269,11 °C
Sample: NH4NO3 pH 1.0 5°C/min, 3,9000 mg
Sample: NH4NO3 pH 1.5 5°C/min, 4,9600 mg
Onset 189,38 °C
Integral 8770,33 mJ
normalized 1768,21 Jg^-1
Onset 249,18 °C
Peak 269,05 °C
Sample: NH4NO3 pH 2.3 5°/min, 3,6200 mg
Onset 191,74 °C Integral 6433,33 mJ
normalized 1777,16 Jg^-1
Onset 257,58 °C
Peak 269,80 °C
Onset 263,42 °C
Sample: perusnäyte/NH4NO3/6, 4.4800 mg
Integral 6848,98 mJ
normalized 1528,79 Jg^-1
Peak 320,41 °C
Wg^-1
20
°C
20 40 60 80 100 120 140 160 180 200 220 240 260 280 300 320 340 360 380 400
^exo NH4NO3 17.01.2000 15:32:25
DSC: Kemira Agro Oy System
e
R
TA
METTLER TOLEDO S

12. 12
Espoon tutkimuskeskus
pH Effect on AN
Decomposition
0
50
100
150
200
250
300
350
T (°C)
NH4NO3
pH
2.5
pH
2.0
pH
1.5
pH
1.0
Tonset/°C
Tpeak/°C

13. 13
Espoon tutkimuskeskus
pH Effect on AN
Decomposition
NH4NO3
Tonset
150
200
250
300
350
0 1 2 3 4 5 6
pH
T
/
ºC
Tonset/DSC
Tonset/ARC

14. 14
Espoon tutkimuskeskus
Acid Effect on AN pH
0
5
10
15
20
25
0 1 2 3 4 5
pH
ml
acid
/
100
g
10%
slurry
H3PO4
(52.1% as P2O5)
HNO3 (65%)
H2SO4 (96%)

15. 15
Espoon tutkimuskeskus
Acid Effect on AN Tonset
100
150
200
250
0 1 2 3 4 5
pH
Tonset
/
°C
HNO3
H3PO4
H2SO4

16. 16
Espoon tutkimuskeskus
AN Decomposition Rate at pH 1.5
Decomposition rate of NH4NO3
vs. used acid
0
0,05
0,1
0,15
0,2
0,25
0,3
130 150 170 190
Tr (°C)
rate
(%
AN/h)
H3PO4
HNO3

17. 17
Espoon tutkimuskeskus
Cl- Effect on AN Decomposition
0
50
100
150
200
250
300
350
T (°C)
NH4NO3
50
ppm
Cl-
100
ppm
Cl-
500
ppm
Cl-
Tonset/°C
Tpeak/°C

18. 18
Espoon tutkimuskeskus
Cl- Effect on AN Decomposition
NH4NO3+Cl
Tonset
120
160
200
240
280
0 100 200 300 400 500 600
Cl / ppm
T
/
°C
Tonset/DSC
Tonset/ARC
pH2,
Tonset/ARC
pH2,
Tonset/DSC

19. 19
Espoon tutkimuskeskus
pH and Cl- Effect on AN
Tonset
150
200
250
300
350
0 250 500
ppm Cl-
Tonset
(°C)
as such
pH 2
pH 1

20. 20
Espoon tutkimuskeskus
400 500 600 700 800 900 1000 1100 1200 1300 1400 1500 1600
140
160
180
200
220
240
260
280
300
50pp Cl
100ppm Cl
500ppm Cl
PH2+ 500ppm Cl
A R C C a l: 28.8.19101
EuroARC Data
Temperature as a Function of Time
Temperature
(°C)
Time (min)
NH4NO3 with Cl addition
AN Decomposition Rate by ARC

21. 21
Espoon tutkimuskeskus
0
0,01
0,02
0,03
0,04
0,05
0,06
0,07
0,08
140 145 150 155 160
T [°C]
rate
[mol/(kg*h)]
4.2 ppm Cl-
15.2 ppm Cl-
1998 IFA Technical Conference
Cl Effect on AN Decomposition Rate

22. 22
Espoon tutkimuskeskus
0
50
100
150
200
250
-5 -4 -3 -2 -1 0 1 2 3
log t (years)
Temperature
(C)
AN(238)
pH 2,5
pH 2,0
pH 1,5
pH 1,0
Cl 50
Cl 100
Cl 500
Cl 500/pH2
Time to Maximum Rate

23. 23
Espoon tutkimuskeskus
Effect of Cl-, Cu2+, and Fe3+ on AN Tonset
150
170
190
210
230
250
270
290
310
330
350
Tonset/°C
Tmax/°C

24. 24
Espoon tutkimuskeskus
DSC Example (Tonset)
1(3)
1. BASIC SAMPLE
200
210
220
230
240
250
260
270
as
such
1%
Cl
5%
Cl
10%
Cl
1%
MeU
5%
Cl &
1%
MeU
Tonset
(°C)
AN solution from process

25. 25
Espoon tutkimuskeskus
DSC Example (Tonset)
2(3)
2. SAMPLE AT pH 2.5
200
210
220
230
240
250
260
270
as such 1.2% Cl 5% Cl 12.6% Cl 1% MeU 5% Cl & 1%
MeU
Tonset
(°C)
AN solution from process

26. 26
Espoon tutkimuskeskus
DSC Example (Tonset)
3(3)
3. SAMPLE AT pH 1.0
200
210
220
230
240
250
260
270
as such 1% Cl 5% Cl 10% Cl 1% MeU 5% Cl & 1%
MeU
Tonset
(°C)
AN solution from process

27. 27
Espoon tutkimuskeskus
Conclusions 1
Tonset based both on DSC or ARC is a useful
measure of the relative stability of AN samples -
but the results may depend on the method used
pH is the main parameter in decreasing the
thermal stability of AN
Already at pH 2.5 the stability has decreased
significantly
The acid used in pH adjustment
(HNO3/H2SO4/H3PO4) has also an effect on AN
stability (pH vs. catalytic effect)

28. 28
Espoon tutkimuskeskus
Conclusions 2
The effect of impurities varies - e.g. for Cl- (ppm
level) it seems to be only secondary, while the
effect of Cu2+ and Fe3+ seems to be more
significant
The effect of Cl- on AN decomposition rate is
much higher than could be concluded on the
basis of the corresponding Tonset value
Dilution effects should also be considered

29. 29
Espoon tutkimuskeskus
Parameter 2
Safe area !
SAFETY LIMIT
Dangerous
area !
1998 IFA Technical Conference
LIMITS FOR THERMAL SAFETY
Parameter 1