Process for preparing stabilized, ammonium nitrate containing granules

Stabilized NH.sub.4 NO.sub.3 containing granules are produced by dissolving in an aqueous NH.sub.4 NO.sub.3 solution Mg(NO.sub.3).sub.2 and, if desired, suspending mineral filler, spraying the resulting solution or suspension over nuclei maintained in spaced interrelationship and contacted with a hot stream of gas at a temperature of the nuclei of between 120.degree. and 135.degree. C., and cooling the resulting granules in such a manner that, between 70.degree. C. and 50.degree. C., they remain substantially homogeneous in temperature.

This invention relates to a process for preparing stabilized ammonium 
nitrate containing granules. 
Ammonium nitrate may occur in a number of different crystal modifications, 
depending on the temperature. The transitions from one crystal 
modification to another, and especially the transition at approximately 
32.degree. C. from modification III to modification IV and vice versa, is 
accompanied by effects of shrinkage and expansion of the crystals, whereby 
stresses are generated in the crystal structure. When ammonium nitrate 
granules pass the limit of approximately 32.degree. C. a number of times 
alternately in either direction, the results of the stresses which occur 
are that the granules become more porous, swell, loose their crushing 
strength, and in the end disintegrate into powder. These effects occur not 
only with granules consisting entirely of ammonium nitrate, but also with 
granules which contain ammonium nitrate and one or more mineral fillers, 
such as granules of calcium ammonium nitrate or of magnesium ammonium 
nitrate. Such effects of disintegration are very troublesome in regions 
where during the day the temperature generally rises above approximately 
32.degree. C. and in the night decreases below this value, but also in 
moderate regions, if the granules are stored in the open air, either in 
bulk or packed in plastic bags. 
High-porosity ammonium nitrate granules will generally explode in the 
E.E.C. explosibility test (see Official Journal of the European 
Communities, No. C 16/4 of Jan. 23, 1976). Conversely, low-porosity 
granules will generally not explode in this test; such granules are termed 
stable ammonium nitrate granules. Hitherto, ammonium nitrate granules 
having such a low porosity that they do not involve the risk of explosion 
can only be obtained by prilling or granulating a substantially anhydrous 
ammonium nitrate melt having a concentration of generally 99.8% by weight 
and higher, whereby high density prills or granules are formed. Such 
so-called stable granules, however, commonly loose their stability when 
they have repeatedly passed the limit of approximately 32.degree. C. by 
heating and cooling, for example, when they are subjected to five 
temperature cycles between 25.degree. C. and 50.degree. C. in the E.E.C. 
explosibility test. They then exhibit the effects described above, which 
lead to disintegration, and generally explode when subjected to the E.E.C. 
explosibility test. This applies not only to granules consisting entirely 
of ammonium nitrate, but also to granules containing ammonium nitrate and 
one or more mineral fillers to the extent they have a nitrogen content of 
at least 28%. According to E.E.C. directives, ammonium nitrate containing 
fertilizer granules having a nitrogen content of at least 28% may only be 
marketed if the granules are stable. 
It is known that ammonium nitrate granules of improved stability can be 
obtained by prilling or granulating a substantially anhydrous ammonium 
nitrate melt having a concentration of 99.8% by weight or higher, and to 
which a stabilizer has been added which specifically retards or prevents 
re-crystallization. Various stabilizers have been proposed, such as 
potassium nitrate, aluminium sulfate and magnesium nitrate (J. Agr. Food 
Chem. 19, No. 1 (1971), p. 83), a mixture of boric acid or an alkali metal 
or ammonium salt thereof with mono- or diammonium phosphate and diammonium 
sulfate (U.S. Pat. No. 3,317,276) and aluminium, magnesium, and/or calcium 
silicate containing clays in finely-divided condition (U.S. Pat. No. 
3,379,496). Some of these materials are used in practice for the 
production of stabilized ammonium nitrate prills and granules. 
A disadvantage of the prior art procedures for the production of stabilized 
ammonium nitrate granules is that they require a substantially anhydrous 
ammonium nitrate melt having a concentration of generally 99.8% by weight 
and higher. 
The reason is that for maximum stability the ammonium nitrate granules must 
have minimum porosity (i.e. maximum density). According as the ammonium 
nitrate melt used for the production of granules contains more water, more 
water must be evaporated from the granules formed by prilling or 
granulation, so that more pores and channels are left in the granules. 
It is an object of the present invention to provide a process using 
lower-concentration aqueous ammonium nitrate solutions having a 
concentration of at least 80% by weight to produce ammonium nitrate 
containing granules having a good roundness and a smooth closed surface, a 
high density, high crushing strength, a high resistance to the formation 
of fines upon impacts to the formation of fly dust resulting from rubbing 
together, and an adjustable grain size, for example, a diameter between 2 
and 12 mm, which granules remain free-flowing even upon prolonged storage, 
are resistant to repeated temperature fluctuations between -20.degree. C. 
and +60.degree. C., without thereby becoming weaker, swelling or 
disintegrating into powder and therefore permit storage in closed bags in 
the open air, without loss in quality under conditions varying from an 
arctic to a tropical climate, and when subjected to the E.E.C. 
explosibility test to not explode, even after five temperature cycles 
between 25.degree. C. and 50.degree. C. 
According to the present invention, this object is achieved by dissolving 
in an aqueous ammonium nitrate solution having an ammonium nitrate 
concentration of at least 80% by weight, Mg(NO.sub.3).sub.2 in a 
proportion of 0.5-3.0% by weight calculated on the ammonium nitrate in the 
solution, and suspending finely-divided mineral filler in a proportion of 
0-45% by weight, calculated on the total of ammonium nitrate and filler in 
the suspension, spraying the resulting solution or suspension over solid 
nuclei while these are maintained in essentially spaced interrelationship 
in an agitated particle bed or mass and contacted with a hot stream of gas 
while the temperature of the sprayed nuclei is maintained between 
120.degree. and 135.degree. C., to deposit droplets of the sprayed 
solution or suspension on the nuclei, which are dried in situ with removal 
of evaporated water by the stream of gas until a desired grain size has 
been reached, and thereafter cooling the resulting granules by means of a 
cooling medium to a temperature below 50.degree. C. in such a manner that 
the granules remain substantially homogeneous in temperature in the 
cooling range of from 70.degree. C. to 50.degree. C. 
In a first embodiment, the process according to the invention is used for 
the production of stabilized high-density ammonium nitrate granules, with 
the starting product being an aqueous ammonium nitrate solution in which 
Mg(NO.sub.3).sub.2 is dissolved. 
In a second embodiment, the process according to the invention is used for 
preparing stabilized ammonium nitrate containing fertilizer granules, such 
as calcium ammonium nitrate or magnesium ammonium nitrate granules, with 
the starting product being a suspension of mineral filler in an aqueous 
ammonium nitrate solution in which Mg(NO.sub.3).sub.2 is dissolved. 
Calcium ammonium nitrate is a chemically obtained product containing 
ammonium nitrate as its essential ingredient, and in addition mineral 
fillers, in particular calcium carbonate (limestone, marl, chalk), 
magnesium carbonate or calcium magnesium carbonate (dolomite). Calcium 
ammonium nitrate contains at least 20% by weight of nitrogen in the form 
of nitrate and ammoniacal nitrogen, each of which two forms should 
constitute approximately half of the nitrogen present, and at least 20% by 
weight of one or more of the carbonates listed, the purity level of which 
should be at least 90% by weight (see Official Journal of the Europeen 
Communities dated 30.1.76 No. L 24/25). 
Magnesium ammonium nitrate is a chemically obtained product containing 
nitrates and ammonium salts and magnesium compounds (dolomite, magnesium 
carbonate and/or magnesium sulfate) as essential ingredients. Magnesium 
ammonium nitrate contains at least 90% by weight of nitrogen in the form 
of ammoniacal and nitric nitrogen, with the content of nitric nitrogen 
being required to be at least 6% by weight, and at least 5% by weight of 
magnesium soluble in mineral acid, expressed as magnesium oxide (see 
Official Journal of the Eurpeen Communities dated 30.1.76, No. L. 24/26). 
In the second embodiment of the process according to the invention, the 
starting suspension is prepared by suspending in an aqueous ammonium 
nitrate solution having an ammonium nitrate concentration of at least 80% 
by weight, and in which 0.5-3.0% by weight of Mg(NO.sub.3).sub.2 has been 
dissolved, calculated on the ammonium nitrate in the solution, 
finely-divided mineral filler in a proportion of no more than 45% by 
weight, calculated on the total of ammonium nitrate and filler in the 
suspension. Preferred mineral fillers are limestone, marl, chalk, 
dolomite, magnesium carbonate and/or magnesium sulfate. It is also 
possible, however, in order to reduce the nitrogen content, to use other 
fillers, such as gypsum, clay and the like. The mineral filler preferably 
has a particle size of less than 0.2 mm with an average particle size of 
approximately 0.05 mm. 
In the process according to the invention, the granules are built up by 
alternately moistening solid particles with the ammonium nitrate 
containing solution or suspension and drying, during which process 
agglomeration of the moistened particles must be prevented. For this 
purpose the solution or suspension is sprayed into an agitated particle 
bed or mass in which during the spraying the particles are maintained in 
essentially spaced interrelationship and contacted with a hot stream of 
gas. The hot stream of gas supplies the heat required for the evaporation 
of water from the droplets deposited on the particles and removes 
evaporated water. The available quantity of heat should be adequate to 
sufficiently dry the sprayed particles in a short time to render 
agglomeration impossible when they come again into contact with each other 
by striking against each other or otherwise. All this can be realized 
using conventional techniques. Suitable examples of such techniques are 
the fluidization technique, the spouted-bed technique and the spherodizer 
technique. 
When the fluidization technique is used for the subject purpose, a bed of 
solid particles resting on a grid is fluidized and maintained in fluidized 
condition by means of a hot stream of gas supplied via the grid upwardly 
through the bed, while the solution or suspension is sprayed into the 
fluidized bed through one or more nozzles. By a suitable control of the 
quantities and temperature of the fluidization gas and the solution or 
suspension to be sprayed, it can be achieved that the particles in the bed 
are alternately moistened with the solution or suspension and dried. 
Further information about the fluidization technique is found for example, 
in the book by Daizo Kunii and Octave Levenspiel: "Fluidization 
Engineering", John Wiley & Sons, New York (1969). 
The spouted-bed technique is described in British Pat. No. 962,265. When 
that technique is used for the subject purpose there is provided a bed of 
solid particles contained in a vessel, and a hot stream of gas is supplied 
through a central aperture in the bottom of the vessel at such a velocity 
as to form a dilute phase of particles entrained by this stream of gas in 
the central portion of the bed, in which dilute phase the solution or 
suspension is sprayed, preferably in the bottom of the bed. In the dilute 
phase in the central portion of the bed the particles are entrained by the 
stream of gas to above the bed level, and then fall back on to the annular 
portion of the bed between the central portion of the bed and the vessel 
wall, in which annular portion they sink again until they are again 
entrained by the stream of gas and are sprayed. During their residence in 
the dilute phase, these moistened particles must be sufficiently dried to 
prevent agglomeration when they fall back onto the annular portion. This 
can easily be achieved by a suitable selection of the temperature of the 
stream of gas and of the quantity of solution or suspension to be sprayed 
per unit of time. A plurality of parallel spouted beds may be combined 
into one multiple spouted bed by having a bed in a large-diameter vessel, 
in which a plurality of gas streams are supplied through the vessel bottom 
at suitable distances from each other, and spraying the solution or 
suspension in each of the dilute phases formed. As the dimensions of the 
particles or granules to be treated cause no problems in a spouted bed, 
which they may in fuidized bed, it may be advantageous for the subject 
purpose to combine one or more fluidized beds with one or more spouted 
beds. Further information on the spouted-bed technique and on possible 
combination is found in the book by Kishan B. Mathur and Norman Epstein: 
"Spouted Beds", Academic Press, New York (1974). 
The spherodizer technique is described in British Pat. No. 894,773. In this 
technique, use is made of a rotary drum having longitudinal blades 
provided on its inner wall. Sprayers are arranged at suitable positions 
within the drum. When this technique is used for the subject purpose, the 
particles and a hot stream of gas are introduced at the supply end of the 
drum. The blades secured to the inner wall of the rotary drum carry along 
particles lying on the bottom and after a certain portion of the 
revolution allow these to slide off again, whereby the particles fall as 
rain in spaced interrelationship through the drum. During their fall, the 
particles are sprayed through the sprayers with the solution or 
suspension, and the thus moistened particles are dried by the hot stream 
of gas. By a suitable selection of the temperature of the gas stream and 
of the quantity of solution or suspension sprayed per unit of time, it can 
be achieved that before contacting each other again on the drum bottom the 
particles are sufficiently dried to prevent agglomeration. 
Naturally other suitable techniques may be employed. 
Various research workers assume that in the granulation of a substantially 
anhydrous Mg(NO).sub.2 containing ammonium nitrate melt, the 
Mg(NO.sub.3).sub.2 mainly functions as a moisture binder in that the water 
present in the granules formed is chemically bound to the 
Mg(NO.sub.3).sub.2 as water of crystallization. Such granules are then dry 
in a physical-chemical sense. As the transitions between crystal phases 
proceed through the mother liquor phase (see Proceedings of the Royal 
Society 266 (1962) 329), the phase transitions in such granules proceed so 
slowly that the granules practically do not suffer adverse effects from 
temperature fluctuation. 
According to the present invention it has been found that in the 
granulation of lower-concentration, Mg(NO.sub.3).sub.2 containing ammonium 
nitrate solutions, the Mg(NO.sub.3).sub.2 plays a further role, the result 
of which is that independently of the concentration of the sprayed 
ammonium nitrate solution, there are always obtained high-density 
granules. One explanation is that the (anhydrous) binary system NH.sub.4 
NO.sub.3 -Mg(NO.sub.3).sub.2 exhibits a eutectic point at approximately 
115.degree. C., above which temperature all Mg(NO.sub.3).sub.2 present is 
in solution in the ammonium nitrate. A granule consisting of ammonium 
nitrate containing 2% by weight of Mg(NO.sub.3).sub.2 contains at 
temperatures above the eutectic point a liquid phase, the proportion of 
which depends on the temperature as per the following table. 
______________________________________ 
composition liquid 
Temperature 
% by weight liquid 
phase 
.degree.C. 
phase in the granule 
Mg(NO.sub.3).sub.2 , % 
NH.sub.4 NO.sub.3, % 
______________________________________ 
120 9.4 21.1 78.9 
130 11.3 17.8 82.2 
140 14.0 14.2 85.8 
______________________________________ 
In practice the granules contain 0.1-0.5% water, so that the proportion of 
liquid phase in the granules will be still greater. 
In the process according to the present invention, the granules being built 
accordingly contain a considerable proportion of liquid phase, which is 
the cause of the granules being plastic and owing to the many instances of 
bumping into each other and rubbing against each other during their 
formation, getting a great roundness and a smooth, closed surface. When 
the granules are cooled in a next phase of the process, the ammonium 
nitrate and the magnesium nitrate crystallize in the pores of the 
granules, as a consequence of which there is obtained a product having a 
very high density and extremely low porosity, which is particularly hard 
and impact-resistant. 
It has further been experimentally found that the manner in which the 
granules produced according to the invention are cooled has a very 
important effect on the stability of the granules against disintegration. 
In particular, it has been found to be necessary for the granules to be 
cooled to a temperature below 50.degree. C. in such a manner that they 
remain substantially homogeneous in temperature in the range of from 
70.degree. C. to 50.degree. C. Preferably this is achieved by cooling the 
granules with a desired size between 70.degree. C. and 50.degree. C. at a 
uniform rate of at most 3.degree. C. per minute. 
This aspect of the invention is illustrated by the following tests. 
Ammonium nitrate granules having an average diameter of 4 mm were prepared 
in accordance with the present invention by spraying a 95% by weight 
ammonium nitrate solution containing 2% by weight Mg(NO.sub.3).sub.2 into 
a fluidized bed of solid ammonium nitrate particles, during which process 
the temperature of the granules being built was maintained between 
125.degree. and 130.degree. C. The granules were removed from the bed at a 
temperature of approximately 120.degree. C. and subsequently cooled with 
ambient air to approximately 90.degree. C. Thereafter the product was 
sieved and the granules having the desired diameter were further cooled. 
A portion of the granules (granules A) was cooled with ambient air to 
30.degree. C. over a period of 3 minutes. The granules A thus cooled 
exhibited the exceptionally high density of 1.68. 
Another portion of the granules (granules B) was cooled at a uniform rate 
to 50.degree. C. over a period of 15 minutes, using air of 50.degree. C. 
The granules B thus cooled had a density of 1.63-1.64, i.e. considerably 
lower than 1.68. 
On the ground of experience and of the literature, it could be expected 
that granules A which have the greater density, also exhibited the greater 
resistance against disintegration. When the cooled granules A and B were 
subjected to five temperature cycles between 25.degree. C. and 50.degree. 
C., however, it was found that the density of the rapidly cooled granules 
A decreased from 1.68 to 1.57, which is indicative of an increase in 
porosity and of a corresponding swelling, whereas the density of the 
granules B, cooled slowly and at a uniform rate, remained unchanged 
1.63-1.64. Against expectation, granules A turned out to be gravely caked 
together when stored in closed bags, whereas granules B even when stored 
for a long period of time in surroundings having a fluctuating temperature 
remained free flowing. 
One possible explanation for these effects is that when the granules are 
cooled too fast at least a portion of the magnesium nitrate cannot arrive 
at crystallization, but remains present within the granules as an amorphus 
solid. In that condition the Mg(NO.sub.3).sub.2 cannot bind water as water 
of crystallization, so that the granules continue to contain free water 
which upon storage causes caking together and owing to the formation of a 
mother liquor phase promotes the phase transition between the crystal 
modification and hence the disintegration of the granules. 
In the light of countless tests we have determined that the critical range 
within which the granules must be slowly and uniformly cooled in the 
manner described is between 70.degree. C. and 50.degree. C. The granules 
may be cooled from a high temperature to 70.degree. C. and from 50.degree. 
C. to ambient temperature at any desired rate without detracting from the 
quality or characteristics of the granules. 
The concentration of the ammonium nitrate solution to be used for the 
process according to the invention is basically not critical, but there 
are economic considerations which make the use of unduly low 
concentrations less desirable. According as the concentration of the 
solution is selected lower, the product yield per unit of time is lower 
and the quantity of water that must be evaporated per unit of time is 
larger. Practice has taught that an acceptable granule yield can be 
obtained using an ammonium nitrate solution having a concentration of at 
least 80% by weight without the drying of the sprayed granules presenting 
any problems. Preferably, however, solutions having a concentration of 
90-95% by weight are used, for one thing because such solutions are 
inexpensive compared with the anhydrous melt required for the prior art 
production of high-density ammonium nitrate granules, and for another 
because they give excellent granule yields. As an upper limit for the 
concentration, if any is needed, may be mentioned the concentration of the 
practically anhydrous ammonium nitrate melt required for the prior art 
processes, which is approximately 99.7% by weight and any rate at least 
99.5 % by weight. 
The ammonium nitrate containing solution or suspension to be used for the 
process according to the present invention contains 0.5-3.0% by weight of 
Mg(NO.sub.3).sub.2, which may be added to the solution or suspension as a 
hydrate or may formed in the solution in situ by adding MgO in a 
proportion corresponding to the desired magnesium nitrate content of the 
solution, followed by reaction to form magnesium nitrate. When, within the 
range of 0.5-3.0% by weight, the higher magnesium nitrate contents of the 
solution or suspension are used, the temperature of the granules during 
their formation is preferably selected lower within the range of 
120.degree. to 135.degree. C., as the quantity of liquid phase in the 
granules depends on both the magnesium nitrate content and the 
temperature. The result is that a combination of a high magnesium nitrate 
content of the solution of suspension with a high temperature of the 
granules during their formation may lead to the granules exhibiting undue 
plasticity owing to the presence of a large proportion of liquid phase. 
Preferably, an ammonium nitrate containing solution or suspension with an 
Mg(NO.sub.3).sub.2 content of 1.0-2.0% by weight is used, at which 
concentration no undue plasticity occurs within the range of 
120.degree.-135.degree. C. 
We have found that at a formation temperature in excess of 135.degree. C., 
the granules become so plastic as to agglomerate easily, especially the 
smaller granules, and to cake to the bottom plate, and that at a formation 
temperature below 120.degree. C. the formation of fines begins to occur. 
At temperatures of approximately 110.degree. C. and lower, granulation is 
impossible and fines are formed only. 
The drop size of the sprayed solution or suspension is not critical. In 
practice, excellent results are achieved with average drop diameters of 
between 0.01 and 0.1 mm, but larger average diameters have proved to be 
quite suitable, in particular in spouted beds. 
As nuclei, small ammonium nitrate prills or undersized sieved product 
granules may be used. It is also possible for oversized product granules 
to be ground and recycled to granulation. The nuclei may be also consist 
of other substances with have no adverse effect in the product. If 
desired, inert nuclei may be used. 
The granulation according to the present invention may be carried out 
continuously or batchwise. The granules having a desired granulometry are 
preferably cooled immediately after their production to reduce their 
plasticity. According to the invention, the product granules should be 
cooled so that, in the cooling range of 70.degree. C. to 50.degree. C., 
the granules remain substantially homogeneous in temperature. This is 
preferably achieved by cooling the granules in the said cooling range at a 
uniform cooling rate of no more than 3.degree. C. per minute. Cooling may 
be effected in conventional apparatus. Practice has shown, however, that 
when the granules are cooled in a fluidized bed a uniform cooling rate of 
no more than 3.degree. C. per minute is difficult to achieve, since the 
granules are cooled at a faster rate in the bottom portion of the bed than 
those in the top portion, unless the cooling gas, commonly consisting of 
air, has been pre-heated to approximately 50.degree. C., which may be an 
economic advantage. Partly in this connection it is preferable to perform 
the cooling of the product granules in the cooling range of 70.degree. C. 
to 50.degree. C. in a cooling drum with air of, for example, 
25.degree.-35.degree. C., which has been conditioned to reduce its water 
content, in such a manner that during the cooling process adsorption of 
moisture from the cooling air by the granules is minimized. 
To reduce their plasticity, the product granules are preferably cooled 
immediately after their production to a temperature located at a safe 
margin above 70.degree. C., for example, between 80.degree. and 90.degree. 
C. This cooling can be effected at any desired rate, for example, with air 
of ambient temperature. It is recommendable for the granules to be 
subsequently sieved, whereafter the undersized fraction can be directly 
recycled to granulation and the oversized fraction can first be broken and 
subsequently recycled to granulation, and then to subject the fraction 
having the desired dimensions to the above-described cooling through the 
range of 70.degree. C. to 50.degree. C. The cooling from 50.degree. C. to 
the ambient temperature can again take place at any desired rate. For 
example, the granules cooled to 50.degree. C. or a lower temperature may 
be packed in bags and allowed to cool in the surroundings. 
If desired, the sieving of the granules may be postponed until the granules 
have cooled to 50.degree. C. or a lower temperature. This, however, has 
the disadvantage that the undersized and oversized fraction must also be 
subjected to the particular cooling process through the temperature range 
of 70.degree. C. to 50.degree. C., and subsequently must be re-heated 
before being recycled to granulation. 
It is also possible for the product granules to be cooled from the 
temperature at which they are removed from the granulator to below 
50.degree. C. at a uniform cooling rate of no more than 3.degree. C. per 
minute, but this does not offer any particular advantages.

The invention is illustrated in and by the following examples. In all 
examples the density of the resulting granules was determined by "TVA 
Procedures for determining physical properties of fertilizers", Special 
Report No. S-444 (September, 1970), page 9 "Apparent density of fertilizer 
granules", Applied Research Branch, Division of Chemical Development, 
Tennessee Valley Authority, Muscle Shoals, Ala. 
EXAMPLE I 
To a 95% by weight ammonium nitrate solution, 0.6% by weight of MgO was 
added, whereafter the mixture was allowed to react at 170.degree. C. for 2 
hours. The solution then contained approximately 2% by weight of 
Mg(NO.sub.3).sub.2. 
In a fluid-bed granulator provided with two sprayers and a bottom plate 
having a passage area of 7%, 40 kg ammonium nitrate prills (33.5% N) 
having an average diameter of 2.4 mm was fluidized with approximately 1200 
Nm.sup.3 fluidization air per hour to a bed height of approximately 30 cm. 
The above described ammonium nitrate solution was sprayed into the 
fluidized bed at a temperature of 150.degree. C. and at a rate of 120 
kg/hour through the two sprayers, by means of air of 160.degree. C. and at 
a pressure of 245.2 kPa. The temperature of the bed was adjusted at 
130.degree. C. by controlling the temperature of the air of fluidization. 
After 15 minutes, the average diameter of the granules formed was 2.95 mm, 
after 30 minutes 3.75 mm, after 45 minutes 4.50 mm and after 1 hour 5.35 
mm. After an operating period of 1 hour the test was discontinued. The 
Granules were removed from the granulator, immediately cooled with outside 
air to approximately 90.degree. C., and subsequently sieved. Of the 
resulting product, 93 kg had a grain diameter of 4-6 mm, 31 kg a diameter 
of less than 4 mm and 36 kg a diameter of more than 6 mm. 
The product having a grain size of 4-6 mm was divided into three portions, 
which were cooled in a cooling drum at various cooling rates. The manners 
of cooling and the results thus obtained are summarized in Table A. 
TABLE A 
______________________________________ 
Portion 
A B C 
______________________________________ 
Temperature cooling air .degree.C. 
50 40 30 
Duration of the cooling process, min. 
14 14 10 
Final temperature 50 40 30 
Density after cooling 
1.634 1.658 1.662 
ditto after 5 cycles between 25.degree. C. and 
1.630 1.585 1.553 
50.degree. C. 
Swelling, % 
after 1 cycle 0 6 4 
after 2 cycles 0 12 8 
after 5 cycles 0 12 14 
after 10 cycles 0 20 22 
after 20 cycles 0 fines fines 
after 50 cycles 2 
______________________________________ 
The results show that the cooling procedure to which portion A was 
subjected was the only one that resulted in a stable product having a high 
density. 
The swelling of ammonium nitrate granules after being subjected to one or 
more temperature cycles between 25.degree. and 50.degree. C. can be 
measured in a simple manner by subjecting a given quantity by weight of 
the granules in a well-sealed bottle repeatedly to temperature cycles 
between 25.degree. and 50.degree. C., and after each cycle measuring the 
volume of the same quantity by weight of granules, for example, in a 
measuring cylinder. The increase in volume is a measure for the swelling. 
The granules of portion A produced in accordance with the present invention 
had an excellent roundness and a smooth closed surface. The product 
exhibited an oil retention (a measure for its porosity; see: Official 
Journal of the European Communities dated 23.1.76 No. C 16(4-7)) of 0.95% 
and contained 0.30% by weight of water and 1.65% by weight of 
Mg(NO.sub.3).sub.2. A 10% by weight aqueous solution of the product had a 
pH of 6.6. The granules having a diameter of 4 mm exhibited a crushing 
strength of 35.3 N. 
EXAMPLE II 
In a similar manner as described in Example I, a solution containing 
approximately 2% by weight Mg(NO.sub.3).sub.2 was prepared by adding MgO 
to a 97.5% by weight ammonium nitriate solution, and subsequent reaction. 
Portions of the resulting solution were diluted with water to form 
respective ammonium nitrate concentrations of 85%, 90% and 95% by weight. 
For purposes of comparison, there were prepared 95% by weight ammonium 
nitrate solutions respectively containing aluminum sulfate, bentonite, 
ammonium polyphosphate and a mixture of boric acid, diammonium phosphate 
and diammonium sulfate. 
In a series of tests, the resulting solutions were sprayed, in the manner 
described in Example I, into a fluidized bed of ammonium nitrate prills, 
with the understanding that the 85% solution was sprayed at a temperature 
of 110.degree. C. at a rate of 80 kg/hour, the 90% solution at a 
temperature of 120.degree. C. at a rate of 120 kg/hour, the 95% solution 
at a temperature of 150.degree. C. at a rate of 200 kg/hour and the 97.5% 
solution at a temperature of 170.degree. C. at a rate of 200 kg/hour. 
All tests were discontinued after an operating period of 1 hour, whereafter 
the product was removed from the bed, immediately thereafter cooled to 
80.degree. C. with outside air in a fluid-bed cooler and subsequently 
sieved. The fraction with a grain size of between 4 and 8 mm was then 
cooled to 50.degree. C. in a drum cooler with air of 30.degree. C. at a 
uniform cooling rate in the course of 15 minutes, whereafter the product 
was allowed to cool to ambient temperature in bags. 
The results of these tests are listed in Table B. These results clearly 
show that with stabilizers other than magnesium nitrate no stable ammonium 
nitrate granules are produced, if the starting product is a solution with 
an NH.sub.4 NO.sub.3 concentration of 95% by weight, but the process of 
the present invention produces stable ammonium nitrate granules of high 
density when solutions having NH.sub.4 NO.sub.3 concentrations of between 
85 and 97.5% by weight are used. 
TABLE B 
__________________________________________________________________________ 
Concentration 
NH.sub.4 NO.sub.3 solution, % by weight 
85 90 95 95 97.5 
97.5 
Stabilizer 1.75% by 
1.85% by 
none 
1.95% by 
none 
2% by 
weight 
weight weight weight 
Mg(NO.sub.3).sub.2 
Mg(NO.sub.3).sub.2 
Mg(NO.sub.3).sub.2 
Mg(NO.sub.3).sub.2 
Product 
moisture, % by weight 
0.34 0.30 0.13 
0.30 0.15 
0.25 
pH 10% by weight solution 
6.1 6.6 6.2 
6.0 6.1 
6.1 
crushing strength .phi. 4 mm, N 
37.3 35.3 32.4 
38.2 31.4 
39.2 
density 1.63 1.63 1.55 
1.63 1.58 
1.63 
oil retention, % 1.0 1.0 1.0 
1.0 0.9 
0.9 
ditto, after 5 cycles between 
25.degree. and 50.degree. C. 
1.1 1.0 6.1 
1.0 6.0 
1.1 
Swelling, % 
after 3 cycles 0 0 18 0 20 0 
after 5 cycles 2 2 40 0 43 0 
after 10 cycles 4 2 65 2 63 2 
Stability good good none 
good none 
good 
Concentration 
NH.sub.4 NO.sub.3 solution, % by weight 
95 95 95 95 
Stabilizer 1,5 wt. % 
2 wt. % 
1 wt. % 
0.15 wt. % boric acid + 
Al.sub.2 (SO.sub.4).sub.3 
bentonite 
NH.sub.4 -poly- 
0.2 wt. % di(NH.sub.4)phos- 
phosphate 
phate + 
0.1 wt. % di(NH.sub.4) sulfate 
Product 
Moisture, % by weight 
0.63 0.21 0.20 0.34 
pH 10% by weight solution 
4.9 6.2 5.2 5.2 
Crushing strength .phi. 4 mm, N 
50.0 29.4 55.9 45.1 
density 1.54 1.54 1.52 1.54 
Oil retention, % 0.5 1.0 0.9 0.8 
ditto after 5 cycles between 
25.degree. and 50.degree. C. 
5.8 9.8 6.7 5.2 
Swelling, % 
after 3 cycles 11 18 11 11 
after 5 cycles 26 36 38 26 
after 10 cycles 46 60 53 46 
Stability none none none none 
__________________________________________________________________________ 
EXAMPLE III 
In the manner described in Example I, a solution of approximately 2% by 
weight Mg(NO.sub.3).sub.2 in a 95% by weight aqueous ammonium nitrate 
solution was prepared. In this solution, 4% by weight of dolomite with a 
particle size of less than 0.2 mm was suspended. 
The resulting suspension was granulated in a spouted bed. The granulation 
was carried out in a cylindrical vessel having a conical bottom. The 
cylindrical portion was 25 cm in diameter and 50 cm high, and the conical 
portion was 20 cm high. The conical bottom was provided with a central 
opening having a diameter of 4 cm, to which was connected an air conduit 
having a diameter of 8 cm, restricted to a diameter of 4 cm at the end 
connected to the central opening. A liquid sprayer was disposed so that 
its nozzle was positioned in the restriction formed by the central 
opening. 
The vessel was filled to a bed height of 30 cm with calcium ammonium 
nitrate granules (33.5% N) with a grain size of 0.5-2.5 mm. To this bed, 
air of a temperature of 130.degree.-140.degree. C. was supplied at a gauge 
pressure of 9.8 kPa at a rate of 400 Nm.sup.3 /hour to form a spouted bed. 
Through the sprayer, the suspension, of a temperature of 160.degree. C., 
was sprayed at a pressure of 150 kPa at a rate of 120 kg/hour in coarse 
droplets into the stream of air accelerated by the restriction, which gave 
the granules in the dilute phase in the central portion of the bed a high 
velocity, owing to which the suspension was distributed over the granules 
substantially homogeneously. 
The temperature in the bed was 120.degree. C. 
After an operating period of 1 hour, the granulation was discontinued. 
Immediately thereafter the product was removed from the vessel and cooled 
in a drum cooler with air of 30.degree. C. to approximately 45.degree. C. 
in the course of 30 minutes. 
The product had the following properties: 
______________________________________ 
Moisture content 0.35% 
Crushing strength, .phi. 4 mm 
37.3 N 
Density, g/cm.sup.3 
1.63 
Oil retention 1.0% 
ditto after 5 cycles 
between 25.degree. C. and 50.degree. C. 
1.1% 
Swelling, % 
after 3 cycles 0 
after 5 cycles 2 
after 10 cycles 4 
Stability good 
Outward appearance 
round granules having 
a smooth surface 
______________________________________ 
EXAMPLE IV 
The suspension described in Example III was granulated in two tests in a 
combined fluidized and spouted bed. In both tests, a bed of calcium 
ammonium nitrate granules (33.5% N) with a grain size of 0.5-2.5 mm is 
fluidized and maintained in the fluidized state by means of air. 
In the first test, the suspension was sprayed into the bed by means of two 
pneumatic sprayers. The secondary air on the sprayers served not only to 
atomize the suspension, but also to give the granules in the spraying zone 
a higher velocity owing to the local formation of a spouted bed, which 
resulted in faster replacement of the granules and a substantially 
homogeneous distribution of the suspension over the granules. 
In the second test, a hydraulic sprayer was used, placed in a blanket of 
secondary air. The suspension was atomized under the influence of the 
liquid pressure. In this test, too, the secondary air around the sprayer 
served to give the granules in the spraying zone a higher velocity owing 
to the local formation of a spouted bed with the same effect as in the 
first test. 
Both tests were discontinued after an operating period of 1 hour. In both 
cases the entire product was immediately transferred to a drum cooler, in 
which it was cooled to approximately 45.degree. C. in the course of 30 
min., using air of 30.degree. C. 
The conditions and results of these tests are summarized in Table C. 
TABLE C 
______________________________________ 
Test 
1 2 
______________________________________ 
Conditions 
Air of fluidization 
Rate, Nm.sup.3 /hour 
1000-1200 1000 
Temperature, .degree.C. 
140-150 130-140 
Gauge pressure, kPa 
11.8 9.8 
Secondary air 
Rate, Nm.sup.3 /hour 
120 120 
Temperature, .degree.C. 
160 120 
Pressure, kPa 253 152 
Fluidized bed 
Bed height, cm 30-40 40-50 
Temperature, .degree.C. 
120-130 120 
Suspension 
Rate, kg/hour 120 180 
Temperature .degree.C. 
160 160 
Pressure, kPa 150 608 
Product 
Moisture content, % 
0.30 0.32 
Crushing strength, 
38.2 41.2 
.phi. 4 mm, N 
Density, g/cm.sup.3 
1.63 1.63 
Oil retention, % 
1.0 0.8 
ditto after 5 cycles 
between 25.degree. C. and 50.degree. C. 
1.1 0.8 
Swelling, % 
after 3 cycles 0 0 
after 5 cycles 0 0 
after 10 cycles 
2 2 
Stability good good 
Outward appearance 
round granules 
slightly angular 
having a smooth 
granules with a 
surface smooth surface 
______________________________________ 
EXAMPLE V 
In the manner described in Example I, there was prepared a solution of 
approximately 2% by weight Mg(NO.sub.3).sub.2 in a 95% by weight aqueous 
ammonium nitrate solution. In this solution, dolomite with a particle size 
of less than 0.2 mm was suspended in a proportion of 25% by weight, 
calculated on the total quantity of ammonium nitrate and dolomite in the 
suspension. 
The resulting suspension was granulated in a spouted bed under the 
conditions described in Example III, and using as nuclei calcium ammonium 
nitrate granules (26% N) with a grain size of 0.5-2.5 mm. 
After an operating period of 1 hour, the granulation was discontinued, and 
the product was cooled in the manner described in Example III. 
The resulting product had the following properties: 
______________________________________ 
Moisture content 0.35% 
Crushing strength .phi. 4 mm 
44.1 N 
Density, g/m.sup.3 
1.81 
Oil retention 0.9% 
ditto after 5 cycles 
between 25.degree. C. and 50.degree. C. 
1.1% 
Swelling, % 
after 3 cycles 0 
after 5 cycles 2 
after 10 cycles 3 
Stability good 
Outward appearance 
round granules having 
a smooth surface. 
______________________________________ 
EXAMPLE VI 
The suspension described in Example V was granulated in the manner 
described in Example IV, first test, using as nuclei calcium ammonium 
nitrate granules (26% N) with a grain size of 0.5-2.5 mm. After an 
operating period of 1 hour the granulation was discontinued and the 
product was cooled in the manner described in Example IV. 
The conditions and results are listed in Table D. 
TABLE D 
______________________________________ 
Conditions 
Air of fluidization 
Rate, Nm.sup.3 /hour 
1000-1200 
Temperature, .degree.C. 
130-140 
Gauge pressure, kPa 
9.8-11.8 
Secondary air 
Rate, Nm.sup.3 /hour 
120 
Temperature, .degree.C. 
160 
Pressure, kPa 253 
Fluidized bed 
Bed height, cm 40 
Temperature, .degree.C. 
120 
Suspension 
Rate, kg/hour 120 
Temperature, .degree.C. 
160 
Pressure, kPa 150 
Product 
Moisture content, % 
0.30 
Crushing strength, 
.phi. 4 mm N 53.9 
Density, g/cm.sup.3 
1.82 
Oil retention, % 1.0 
ditto after 5 cycles 
between 25.degree. C. and 50.degree. C. 
1.1 
Swelling, % 
after 3 cycles 0 
after 5 cycle 0 
after 10 cycles 2 
Stability good 
Outward appearance 
round granules having 
a smooth surface. 
______________________________________