AMMONIUM NITRATE PRILLS
Mechanism of Caking of Ammonium Nitrate (",NO,)
Prills
Christer Sjolin'
A study has been made with ammonium nitrate prills in order to determine the mechanism of caking. Methods of investigation used were X-ray diffractography, scanning-electron microscopy, and tests in a climate chamber. The results show that caking is mainly caused by the phase transition IV I11 at 32°C and depends on the dissolution and recrystallization of the solid at this phase transition. If no pressure is applied to the sample a porous powder is formed due to the volume increase of the phase
mmonium nitrate is finding increased use as a fertilizer and as an explosive. It is also well known that ammonium nitrate during storage will cake together. The problem of the caking of ammonium nitrate prills has been the subject of a large number of investigations because of its great importance with respect to the handling of ammonium nitrate prills. This problem has been partially solved by adding inorganic additives in an amount of 1 - 2 x and sometimes also by treating the prills with a surface active agent in order to prevent contact between the prills and surrounding humidity. Although good results have been reached by these methods, no unitary mechanism of the caking has been presented. An important factor is the particle size and the shape of the particle. It is known that crystallized ammonium nitrate will cake easier than prilled ammonium nitrate. It is known that ammonium nitrate is a very hygroscopic salt, which has sometimes b:en suggested as being the reason for caking. Some authors consider that the hygroscopicity is of no importance and that the reason for caking is the phase transition IV iS 111. This transition takes place at 32°C and involves a change in volume of 3 . 6 x (Nagatani et a/., 1967). Most producers refrigerate the ammonium nitrate to about 28°C after drying in order to prevent caking during storage (Kazakova et al., 1967). The purpose of this work is to describe the mechanism of caking of ammonium nitrate prills. Lowry and Hemmings (1920) have proposed that caking depends on recrystallizations on the surface of the ammonium nitrate crystals. Due to the phase transition IV Ft I11 at 32°C cracking takes place and the retained water in the crystals will be released and can thus take place in the recrystallization process. They have also shown that very carefully dried ammonium nitrate will not cake. This latter statement has been confirmed by Sjolin (1971). Shneerson et al. (1956) has stated that the phase transition IV + I11 does not cause caking, while the phase transition I11 -,IV is of very great importance for caking. They have also shown that the pressure is of importance. Like Lowry and Hemmings (1920) they stated that it is the water on the surface which, during recrystallization, causes salt bridges to be formed between the different crystals. This is intensified by
A
Department of Chemical Technology, The Royal Institute of Technology, Stockholm, Sweden. Present address: KemaNord AB, S-850 13 Sundsvall 13, Sweden.
transition and the formed salt bridges. If pressure is applied to the sample it will be pressed together, due to mechanical weakness of the prill during the phase transition, resulting in large crystal surfaces in contact with each other, and the dissolutionrecrystallization process gives a very hard product. A certain degree of caking may result from recrystallization phenomena on the surface of the prills due to changes in the relative humidity.
the increase of free ammonium nitrate surface created by the phase transition I11 -,IV. Erofeev and Mitskevich (1958) put forward that the phase transition IV iS I11 itself has nothing to do with caking. The phase transition only loosens the structure of ammonium nitrate, thus causing an increase in the specific volume. Wolf and Scharre (1967) have also reasoned in a similar manner. At the same time, contrary to Shneerson et al., they state that the phase transition IV -,I11 can cause caking. The caking mechanism is stated to merely be due to a difference in solubility of ammonium nitrate at different temperatures. The mass transport of water within and between the crystallites results in salt bridges, thus giving rise to caking. A similar argument has been put forward by Whetstone (1952). The importance of the water content and its effect on the phase transition temperature for the transition IV iS I11 (see Figure 1) has been demonstrated by Sjolin (1971) and Griffith (1963). Brown and McLaren (1962) have shown that the phase transition IV Ft I11 only takes place in the presence of water and that the phase transition is a dissolution and recrystallization of the solid ammonium nitrate. This latter theory is supported by the great difference in the structure of phase IV and phase 111. EXPERIMENTAL
The ammonium nitrate prills that have been used in this work are of commercial low-density type produced by KemaNord AB, Sweden. These experimental prills, however, have no additives in order not to disturb the effects of the different treatments. DETERMINATION OF THE MECHANISM OF THE PHASE TRANSITION IV F? 111
The X-Ray Investigations. If the mechanism of the phase transition IV Ft I11 is a dissolution and recrystallization of the solid ammonium nitrate, as stated by Brown and McLaren (1962), who noticed the large increase in electrical conductance at the phase transition, this could be studied by X-ray diffractography. During repeated transitions there is no reason to believe that the same nucleating center will be the seed of the recrystallization at each transition and, therefore, different crystal planes will be over- or under-represented in the sample each time the phase transition takes place, resulting in very different diffracted beam intensities each time. Ammonium nitrate is easy to grind and usually gives no orientating effects. The X-ray specimen holder was made of copper and the temperature could be changed by pumping water from a
J. AGR. FOOD CHEM., VOL. 20, NO. 4, 1972 895
SJOLIN
W o t e r - c o n t e n t ‘b
Figure 1. The phase transition temperature N-m as of the water content for ammonium nitrate prills
B
function
Table I. The Relative Intensities of the Diffracted Beam after 111 Cycling over the Phase Transition IV Intensity. Phase m Water CyPhase IV content, cling 100 peakC 75 peaki 100 peakb 75 peakk % no. (111) (OW (220) (120)
+
1.88
0.81
0 1
100 5
2
20 40
100
1 20
3 4 5
100
6
10
0 1 2
100 50
150 20 50 0 100 50 20
4
75
20
50
80
100 50 10 120 20 0
100 500
...
...
100 10 190
100 120
90
150
100 80 200
50
60
Intensity = % of line intensity for the initial phase IV and,Z of line intensity far the initial phase of that phase 111 which IS obtained after the first phase transition. b By “I00 p e a r and “75 peak,” r?spect/vely, is meant the peak with the highest intensity and 75 % of the Intensity of the highest peak, respectively, according to ASTM Index No. 8-499 for phase I11 and No. 8-452 for phase IV.
thermostated bath through small channels (loops) in the bolder. The temperature was measured with a thermocouple (Cu-constantan) placed in the middle of the ammonium nitrate. The experiments were made in the following ways. The sample was carefully ground and a diffractogram was registered (for Cu K a angle 20 = 35-25’) to study the initial phase. In all experiments the intensities of the different peaks in the initial diffractogram were in good agreement with those given in the ASTM Index. The sample was then heated until the phase transition IV 111took place and a diffractogram was registered (angle 20 = 41-20”) in order to determine the intensities of the different peaks of phase 111. Similarly, the sample was cooled again and the intensities of the peaks of phase IV were studied. By thus cycling over the phase transition temperature several times, the variations of the intensities could be studied. Samples with different water content were studied. The results from these experiments are shown in Table I. It is obvious that when the phase transition IV I11 has been passed, the different crystal planes are no longer statistically
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896 J. AGR. FOOD CHEM., VOL. 20, NO. 4, 1972
Figure 2. Ground prills (300x1
represented and the changes are much greater than can be expected by a common order-disorder phase transition. The same phenomenon takes place by the phase transition I11 + IV. No correlation or trend in the diffracted beam intensities of the different spectral lines was observed during repeated cycling through the phase transition. This indicates that different nucleating centers initiate the phase transition each time, as was predicted. As may be seen in Table I, samples of differentwater content were used, and as long as the phase transition IV Ft 111takes place, no difference dependent on the water content can be observed. The water content must he
