IFA Technical Conference

Chennai, India
24-27 September 2002

International Fertilizer Industry Association - Secretariat: 28 rue Marbeuf - 75008 Paris - France
Tel. +33 1 53 93 05 00 - Fax +33 1 53 93 05 45/47 - ifa@fertilizer.org - www.fertilizer.org
 STRAIGHT AMMONIUM NITRATE FERTILIZER GRANULE-PRILL
STABILIZATION: THEORETICAL POSSIBILITIES (a)
Bernard Laurent, Kemira S.A., Belgium

1. Physicochemical Properties of Pure Ammonium Nitrate

The behaviour of ammonium nitrate (AN) against thermo-cycling is affected by its peculiar
physicochemical properties. The three main influencing properties are: water affinity, heat
transfer and polymorphism. They are detailed underneath so that the following chapters with
regard to fertilizers could be better understood.

1.1 Solubility-Critical relative humidity

Ammonium nitrate is strongly hygroscopic at the ambient temperature and has a lower critical
relative humidity (CRH) than other common salts. It also has exceptionally high water
solubility with the water held as a solution rather than combined as water of crystallization.
The temperature coefficient of solubility of the salt in water is also exceptionally high.
The solubility of ammonium nitrate decreases with the pressure. This particular behaviour
promotes the caking of AN fertilizers during piling in the storage: Indeed, the liquid phase in
the contact region between two granules starts to be in supersaturation ; new crystals appear
promoting some caking tendancy between granules.

Table1. Comparison of temperature, coefficient. of solubility, water solubility, and critical relative
humidity of some common salts.

Salt ∆S/∆T (20-30°C) Solubility (20°C) Critical relative humidity

(20°C)
Gr./ 100gr water Gr. / 100 ml solution %
NH4NO3 4.94 66.0 66.9
NaNO3 0.79 46.7 77.1
NH4Cl 0.42 27.1 79.3
(NH4)2SO4 0.26 42.85 81.0
KNO3 1.42 24.0 92.3
NaCl 0.025 26.37 77.6

Depending on the temperature and relative humidity of the air, ammonium nitrate can either
absorb or desorb moisture to the atmosphere. Table 2 gives the hygroscopic point, or the
relative humidity of air at which the vapour pressure of water in the air is equal to the vapour
pressure of water over saturated ammonium nitrate solutions, at different temperatures. For
instance, at 30°C if the relative humidity of the air is higher than 60%, AN absorbs moisture
in the form of a saturated solution.

Table 2.Influence of temperature on the CRH of ammonium nitrate

Temperature °C 10 15 20 25 30 40 50
Critical relative humidity of AN% 75.3 69.8 66.9 62.7 59.4 52.5 48.4
Gr H2O/kg dry air that air handle at the above 6 7 10 13 16 25 41
temperature and relative humidity

Email: Bernard.Laurent@kemira.com
(a) received on: 08/07/02
 2

Changes in temperature of the AN or the atmosphere cause moistening or drying leading to

recrystallisation of AN and, consequently, to the caking between the crystals.

M o llie r d ia g r a m in c lu d in g m o is te n in g a n d d r y in g z o n e o f A N

55

50
RH =40%
d r y in g z o n e

45

RH =60%
40

R H=80%
Temperature in °C

35

RH =100%

30

25

m o is te n in g z o n e
20

15

10
0 ,0 0 5 0 ,0 1 0 ,0 1 5 0 ,0 2 0 ,0 2 5 0 ,0 3 0 ,0 3 5 0 ,0 4

g r H 2 O /k g d r y a ir

In various ways explained in the following chapters, the hygroscopic nature of ammonium
nitrate together with thermo-cycling will affect the caking of fertilizers.

1.2 Thermal conductivity

The thermal conductivity of dry, granular AN fertilizer compared to other material is low
(Table 3). During the cooling of a fertilizer, the low thermic transfer affects the gradient of
temperature inside the granules.

This difference of temperature linked with change of volume during thermo-cycling can
promote cracks and affects the storage behaviour of the product.
 3

Table 3. Comparison of thermal conductivity of AN with other materials at 18°C

Material Rate of travel of heat through material

Cal/(sec).(cm2.).(°C/cm)
Iron 0.161
Soft glass 0.0025
Dry sand 0.00093
Paraffin 0.0006
Dry, granular ammonium nitrate (density 0.7g./cc.) 0.00057
AN33.5 Kemira Tertre (0.35%H2O-density 0.965) 0.000208
CAN 27% Kemira Tertre (0.10% H2O-density 1.020) 0.000344
Fire brick 0.00028

1.3 Polymorphism

1.3.1 Stable crystal forms-density-specific heat-heat of transition

Ammonium nitrate, NH4NO3 containing 35% nitrogen has the following properties: density
at 20°C, 1.725 gr/cm3; specific heat at 20°C, 0.422Kcal./kg°C ; heat of fusion 16.75-
18.3Kcal/kg and a melting point of 169.6°C.

Ammonium nitrate is polymorphic and can assume at least at ordinary pressure five
crystalline forms with different densities and enthalpies.
Every form, designed by a roman number, is stable over a certain temperature range. The
stable form at ambient temperature is the form IV.
During normal cooling at atmospheric pressure, ammonium nitrate passes successively
through the crystalline forms I to V. The change in crystal state occurs with a change of
volume and heat of transition.
The crystal changes, associated specific volume, energy changes and solubility in water are
shown in Table 4 and Graphics 1-2-3.

Table 4. Phase transitions during normal cooling

Crystalline form Temperature Specific Specific heat Solubility Transition Transition Heat of
range in volume temperature transition
which
crystalline
form is stable
% by
°C cm3/gr Kcal/kg°K °C Kcal/kg
weight
Liquid 169.6 0.697
I Cubic 169.6-125.2 0.642-0.627 -0.477 100-96.1 169.6 Melt I 16.75-18.3
II Tetragonal 125.2-84.2 0.612-0.603 0.454-0.441 96.1-87.4 125.2 I II 13.25
IIIOrthorhombic 84.2-32.2 0.613-0.605 0.386-0.356 87.4-70.9 84.2 II III 4.03
IV Orthorombic 32.3- -16.9 0.582-0.572 0.423-0.374 70.9-42.3 32.3 III IV 5.08
V Tetragonal <-16.9- 0.589- - 16.9 IV V 1.41
 4

1.3.2 Metastable transitions

Forms III and IV transform themselves quickly each one another and present rarely a delay to
transformation.
In another way, form II shows a considerable delay during cooling to form III. It can happen
that form II transforms immediately to form IV, by-passing the form III.
So, in addition to the four stable transitions V IV, IV III, III II and II I, ammonium
nitrate shows also a metastable transition IV II. This transition occurs at 51°C and can be
easily explained: the II and IV phases have close structures, differing only by the orientation
of the NO3 plans; form III being very different from the others. The direct transition II-IV
happens by simple rotation of the NO3 plans.
One can even realise the direct stable transition from II to IV without passing trough III form
if we prevent, under very high pressure, the volume dilatation requested to pass from II to III.

The impact of this transition on the specific volume, specific heat and variation of enthalpy is
shown in Graphs 1,2 and 3.
The direct transition II to IV is accompanied with 5.56 Kcal/kg of heat of transition compared
to 9.11 needed for the successive transitions II III IV.
Accordingly, to respect the thermodynamic low, the specific heat in the metastable zone is
higher than the one of phase III (Table 5 and Graph 2).

Table 5. Direct transition II IV

Crystalline form Temperature range in Specific volume Heat of Specific heat

which crystalline form is transition
stable
°C cm3/gr Kcal/kg Kcal/kg°K
II 84.2-51 0.603-0.598 0.441-0.431
II IV 51 5.56
IV 51- -16.9 0.585-0.572 0.442-0.374

Metastable form II which subsists during cooling of pure AN can transform to phase III just
below the transition point of II to IV at 51°C. The transition heat can be estimated to 1.41
Kcal/kg (table 6).

Table 6. Transition II III at 51°C and III IV at 32.2°C

Crystalline form Temperature range in which Specific volume Heat of Specific heat
crystalline form is stable transition

°C cm3/gr Kcal/kg Kcal/kg°K

II 84.2-51 0.603-0.598 0.441-0.431
II III 51 1.41
III 51-32.2 0.608-0.605 0.367-0.356
III- IV 5.08
IV 32.2- -16.9 0.582-0.572 0.423-0.374
 5

Graph1: Variation of specific volume for different crystalline forms of ammonium nitrate according to:

0,72

0,71

0,7

0,69 melt

0,68

0,67
Specific volume cm3/gr

0,66

0,65

0,64
I
0,63

0,62

0,61 III
II

0,6

V 0,59 II metastable
IV
0,58
IV metastable
0,57
-50 0 50 100 150 200
Temperature °C
 6

Graph 2: Specific heat of ammonium nitrate

0 ,5

I
0 ,4 8

0 ,4 6
II m e ta s ta b le II

IV m e ta s ta b le
specific heat kcal/(kg.°K)

0 ,4 4

1 2 5 .2 °c
IV
0 ,4 2

0 ,4

0 ,3 8
III
8 4 .2 °c
0 ,3 6

3 2 .2 °c
0 ,3 4
20 40 60 80 100 120 140
T e m p e ra tu re in °C
 7

Graph 3: Influence of type of transition:stable (IV-->III-->II) and metastable (IV-->II) on the enthalpy
variation of ammonium nitrate with temperature form IV)

70
Enthalpy variation in Kcal/kg (hyp.: enthalpy=0 at 20°C

60
I

50

40
II

30
m etastable II

20

III
10

0
20 IV 40 60 80 100 120 140
m etastable IV Tem perature in °C
 8

1.3.3 Transition delay – hysteresis

Most of the transformations show considerable delays. One can obtain metastable forms by
undercooling or superheating AN far away from its transition point. This is done by entering
more or less in the stability field of a neighboured crystalline form.

The delay is bigger when the change of temperature is fast and the temperature low.
The greatest gap is observed with the IV III inversion, which, as an example, can start at 36-
40°C with rising temperature and 28-25°C with falling temperature. When both phases are
initially present, the delay is very much less.

This delay and the fact that the transitions are slow close to the transition temperature have
complicated the determination of the transitions points and heat transitions from V to IV and
IV to III .

If the temperature rises above the II I transition at 125°C, then, on subsequent cooling, the
form II does not invert directly back to III as above , but it persists down to 48 or 45°C, when
it changes to form IV.

1.3.4 Kinetic of transition

The kinetic of transition between forms III and IV is influenced by:

the number of seeds of the form where the transition goes or in other words the
previous thermic treatment of the crystal

the humidity of the salt

the speed of change of temperature and the temperature

Depending probably upon these factors, it is practically observed that the transition point is
shifted in the way of a reduction of temperature after a certain number of transitions.

1.3.4.1. Number of seeds

The transition does not occur homogeneously in the mass.

It pre-exists points where this transition is generated which then diffuse further in the mass.

These points are made of crystals that for any reason have not change during the previous
transition in the opposite direction. The number of points depends upon the temperature and
duration at which the crystal was kept during the previous transitions.

As an example, analytical AN containing 0.5% H2O heated at 5°C/min. will make a IV II

transition while the same AN melted and cooled down at room temperature a few days before
will make at the same heating speed the IV III II transitions.
 9

The analytical AN heated at a slower rate of 1°C/min. will also show the IV III II
transitions.

1.3.4.2. Humidity

The III IV transition of moist ammonium nitrate is progressing faster than a dry one.

The kinetic at 31.1°C presented in Graph 4 is obtained by the dilatometric method: one
sample is poured in a glass vessel equipped with a capillary tube.

The dilatometer is filled with xylene and immerged in a bath kept at constant temperature.
One measures during the time the volume variation and expresses the transformation by
recording the graduation of the dilatometer.

The percentage of transformation at the time t1 is the ratio of volume variation at t1 divided by
total volume variation when transition is finished.
 10

Graph 4: Kinetic of transition III-->IV at 31.1°C

1 0 0 ,0

9 0 ,0
w e t A N

8 0 ,0

7 0 ,0
% of transformation

6 0 ,0

5 0 ,0

d ry A N
4 0 ,0

3 0 ,0

2 0 ,0

1 0 ,0

0 ,0
0 5 0 1 0 0 1 5 0 2 0 0 2 5 0 3 0 0 3 5 0 4 0 0 4 5 0 5 0 0

Time in minutes
 11

Moisture is contained in the form of a saturated solution: the higher the temperature, the
greater the amount of salt dissolved in the water contained in ammonium nitrate. Ammonium
nitrate which is very hygroscopic absorbs easily air humidity (Table 1).

The saturated solution enhances greatly the transition of one form to another because it can be
realised through the intermediate of the solution. It is suggested even that the IV-III transition
can only be made through the dissolution-recrystallisation route.

Practically, one observes that cooling a dry AN compared to a wet one lowers the transition
temperature especially when the cooling speed is bigger.

In that case, the salt has the opportunity to cool down before the kinetic of transition has got
the opportunity to be in a measurable magnitude.

This is shown in Graph 5 for 3 samples more or less dried in a dessicator where the cooling
curve temperature-time is plotted for AN more or less dried.

The III IV transition temperature is easely observed by the prolonged arrest in the
neighbourhood of 32-28-26°C.

1.3.4.3. Temperature

The kinetic of transition increases when passing the transition temperature if we impose a
broader difference of temperature between the surrounding media and the crystal (Graphs 6-7
obtained also by the dilatometric method).

The kinetic increases indefinitively when increasing the temperature while with a decrease of
temperature, it increases up to a certain temperature under which the speed of transition
reduces again. Undercooling and overheating may occur.
 12

Graph 5: Influence of humidity on undercooling of ammonium nitrate transition III

3 5

3 0 g r o f A N d r ie d a t 1 5 0 ° c d u r in g 1 h
3 4
s a m p le p o u r e d 1 - 7 a n d 1 9 d a y s in H 2 S O 4 d e s s ic a to r

3 3
Temperature in °C

1 d a y h e a t in g a t 4 5 ° c f o llo w e d b y s lo w c o o lin g

3 2

3 1

3 0
7 d a ys

2 9

2 8

2 7
1 9 d a ys

2 6

2 5
0 2 0 4 0 6 0 8 0 1 0 0 1 2 0 1 4 0

Time in minutes
 13

Graph 6: Influence of temperature on IV III transition kinetic on standard AN

450

3 5 °c
400

3 4 .5 ° c
Graduation of dilatometer

350
3 4 °c
300

250

200
3 3 .5 ° c
150

100

50
3 3 °c

0
0 50 100 150 200

Time in minutes

The minimum and maximum transformation speed obtained by the dilatometric method with
dry and wet AN-CAN fertilizers in function of the temperature around 32.2 and 84.2°C are
presented in Graphs 8 and 9.

The curves have been obtained by the dilatometric method on granules and therefore represent
a combination of the kinetic of transition but also of the thermic transfer from the granule to
the external media.
 14

Graph 7: Influence of temperature on IV-->III transition kinetic on dry AN

400
Graduation of dilatometer

3 5 °c
350

3 4 .5 ° c
300

3 4 °c
250

200

150
3 3 .5 ° c
100

50

3 3 °c

0
0 50 100 150 200
Time in minutes
They show that:

the transformations are fastened by humidity.

the transformation speed increases as much as one is far from 32 or 84°C. The kinetic
is bigger at 84°C compared to 32°C.

the transformation from III II and III IV are quicker than the inverse transitions.
This means that the transformations with contraction of volume are faster.
 15

Graph 8: Influence of temperature-humidity on the transformation speed at 32.1°C

250
Transformation speed in %/hour

F o r m IV F o r m III
m a x .w e t A N
200
m in . w e t A N

150

100
m in . d r y A N

50 m a x . d ry A N

0
26 27 28 29 30 31 32 33 34 35 36 37

Time in minutes
 16

Graph 9: Influence of temperature-humidity on the transformation speed at 84.2°C

250
F o rm III F o rm II
Transformation speed in %/hour

m in . w e t A N
200
m ax. w et A N

150

100

m in . d r y A N
m a x . d ry A N
50

0
78 79 80 81 82 83 84 85 86 87 88 89

Time in minutes

2. Definition of Thermocycling and Thermal Stability

Thermo-cycling is applied on discrete particles of ammonium nitrate fertilizers by raising and

lowering their temperature through the phase transition temperatures, of which the 32°C
transition point is the most significant in practice related to conservation properties.

The thermal stability is preventing or alleviating the well known phenomenon of thermal
shock and cracking of the particles due to deformation at phase transition temperature such as
32°C, in order to modify its properties so that the product may be safely stored for extended
periods of time without the development of dust, caking or low density which increases the
susceptibility to detonation.

The following equipment is needed to apply the thermo-cycles and to measure the thermal
stability.
 17

2.1 Equipment

A climate chamber equipped with the following devices: cooling and heating coils
thermostatically controlled, a fan, a heating-cooling timer and a multi-point temperature
recorder.

A sample sealed in a container can be thermo-cycled around 32°C or 84 °C several times in a

certain period of time. The example in Graph 10 shows a 15-50°C cycle made in 6 hours.
During heating and cooling, one observes stops in product temperature regularly distributed
but hardly observed without hysteresis.

Experience shows that rapid thermo-cycle enhance the transition temperature elevation by a
superheating effect, which breaks the corralation with field performance.

Thermo-cycles can also be done in a humidified atmosphere.

Graph 10: Example of one thermocycling in a climate chamber: 15-50°c during 6 hours

5 0

4 5
o v e n te m p e ra tu re p h a s e III
Temperature in °C

4 0
A N te m p e ra tu re
3 5

3 0

p h a s e IV
2 5

2 0

1 5
0 1 2 3 4 5 6

Time in minutes

2.2 Measures

The sample can then be further checked for caking, friability, hardness, porosity, swelling,
density,etc.
One can measure the percentage of unbroken granules/prills after each cycle.
 18

2.3 Related tests and index

One can tackle stability by the following parameters related to safety and quality:

Stability limit for safety: Maximum number of cycles supported by the product before
the density is reduced in such a way that the aptitude to detonation becomes apparent.

Stability limit for quality: maximum number of cycles supported by the granules
without destruction. In practice, one can consider as a stability limit the number of
cycles which reduces the initial hardness of granules at a hardness too low for handling.

Inertia: number of cycles supported by the granules without any modification of

hardness.

Hardness angle :mean modification of the initial hardness after one cycle, as per
following formule tg &=(P°-P)/(P(N-1)) where P°=initial hardness, P =hardness after N
cycles, N=number of cycles

3. Legal Requirements in Europe

Several accidents have ensured a poor reputation for ammonium nitrate.

AN has some aptitude to detonation which becomes apparent when it is mixed with some
form of organic material. One kg of AN is susceptible to decompose in 0.98 Nm3 releasing
360 Kcal.

Another problem arising from the chemical properties of ammonium nitrate is the risk of
decomposition. The onset-temperature is about 200°C, but this may be lowered by certain
sensitizing agents, such as chloride ions, copper salts or low pH.

The aptitude to detonation is favoured by the porosity of ammonium nitrate. The porosity can
be significantly increased by cracking of the crystals during the crystal transitions. This may
lead to the detonation of material which would not have detonated before cycling. The EEC
considers that it is necessary, especially when dealing hazardous aspects, to make a judgement
on the behaviour of ammonium nitrate under the conditions in which it is being handled.

Therefore, straight ammonium nitrate fertilizers with more than 80% of ammonium nitrate is
regarded as a special case on account of potential hazard on which detonation test is done not
on the product as such but after 5 thermo-cycles.

However, it is not only the risk of explosion or the possibility of decomposition that are
widely discussed nowadays but also the problems of degradation and caking. These are
commented in Chapter 4.

Regulations have been set fixing limits for porosity, pH, carbon, chloride and heavy metals.
The legislation of European Economic Community for Europe is resumed underneath.
 19

The EEC directive 80/876/EEC updated successively by the 87/94/EEC and 88126/EEC
directives require the straight ammonium nitrate based fertilizer containing more than 28% N
to fulfill limits and characteristics of Annex I-II in order to have the EEC label.
These characteristics including thermal stability guarantee the safety of the fertilizer.

Annex 1:
1.The porosity (oil retention) of a sample submitted to 2 TC from 25 to 50°C can not exceed
4% in weight
2.The percentage of combustible material, measured in a carbon form, cannot exceed 0.2% for
fertilizer >=31.5% N and 0.4% for fertilizers < 31.5% and >=28%.
3.The pH of 10gr of fertilizer in 100ml H2O must be equal or above 4.5
4.The fraction below 0.5 and 1mm cannot exceed respectively 3 and 5% by weight
5.The maximum chloride level is 0.02% by weight
6.The copper content cannot exceed 10ppm

Annex 2: The detonability of a sample submitted to 5 thermocycles is conclusive when the

deformation of one or more Pb supports is less than 5%

Method 1: Method to apply thermocycles

A 70gr sample poured into a 150 ml Erlenmeyer flask is brought through a thermostatic bath
to 50°C and maintained at that temperature for 2 hours. The sample is then cooled at 25°C and
maintained at that temperature for 2 hours.

These 2 successive phases at 50°C and 25°C defines a thermocycle.

After submitted to 2 or 5 thermocycles, the sample is maintained at 20°C before performing
the porosity or detonability tests.

Remarks: some additional requirements are made by some countries. France for instance
requests a detonation test every 6 months.
 20

4. The Impact of Thermocycling on AN Based Straight N Fertilisers Quality

Ammonium nitrate, as seen in the previous chapters, possesses a big affinity for moisture, a
low thermal conductivity and different crystal forms. All these properties affect the quality of
the fertilizers. Therefore, the influencing parameters promoting particles softening, dust and
setting are likely to be more complicated than with other inorganic salts. It is easier to
consider these parameters as being due to fundamental processes which can be treated
individually.

In this chapter, one emphasises particularly more on the impact of change of volume during
transitions on the quality and safety of AN-CAN fertilizers.
The impact of transitions during the cooling process at plant site and during the storage is
reviewed.

The nitrogen content of AN and CAN is respectively 33.0-34.5% and 20-27%.

NH4NO3 comprises 57-98.5% of the total weight. Water content is less than 0.5%. The rest
(limestone, dolomite…) is mostly inorganic additive applied to reduce hygroscopicity and
caking of the fertilizer and provide a pH buffer.

4.1 Impact of change of volume on behavior of AN cristals during successive

transitions

The forms of practical interest in the production and storage of AN are form IV, which is
stable between –16.9 and 32.3°C , and form III, which is stable between 32.3 and 84.2°C.

4.1.1 The cooling process

The cooling of AN-CAN granules which is part of the post-granulation treatment will affect
the storage properties of the final product by inducing mechanical stresses.

A granule submitted to cooling, by heat removal from its external surface, develops a thermal
gradient, the slope of which determined by the thermal conductivity of the product and the
cooling rate.

As a result of this gradient, the colder outside layers shrink more rapidly than the core of the
granule resulting in a tension of the crust; the external layer being pulled inwards. As further
cooling of the core proceeds, the tension is gradually relieved. When the heat removal is
stopped, temperature equalisation starts, which results in a slight increase of the temperature
of the outside layers.
 21

The polymorphous transitions during the cooling process around 84 and 32°C, occurring
respectively with a 1.3% volume increase and a 3.6% volume decrease, complicate the above
scenario.(Table 7).

Table 7. Impact of transition on specific volume

Transition temperature °C Volume variation %

I to II 2.1 decrease
II to III 1.3 increase
III to IV 3.6 decrease
IV to V 2.9 increase

The II III transition (84°C)

This transition produces a volume expansion of 1.3%.

After going through the transition at 84°C, the outer layer continues to cool down and starts to
contract more rapidly than the underlaying layers.On a further step, the shrinking outer layer
encounters an opposite force as the core of the granule starts to expands by going also through
the II III transition. The tension resulting from the contraction of the crust and the expansion
of the core will be relieved by the formation of micro-cracks and porosity generating fines.

The III IV transition (32°C)

This transition produces a volume contraction. The tension created in the crust by cooling is
relieved when the core goes through the transition and contracts. Consequently, less porosity
is normally created during the III IV transition.

The various cooling processes take account of this particular behaviour:

1. The cooling procedure can be adjusted to take into account the crystal transformations of
ammonium nitrate.
As an example, the back-mixing of granules up and down in a fluidised bed cooler will
expose their outlet layer to varying temperatures. This temperature cycling has no damaging
effect as long as the outer layer does not cycle through the 32°C transition point.

2. The drum cooler has the advantage that the product progress countercurrently to the cooling
air in such a way that it makes the transition only once.

3. In a drum granulator, severe degradation can occur if product leaving the granulator is
below 84°C and is reheated above in the drier in order to assist the drying. This will increase
the porosity and prolongs the cooling time as does the heat released during the crystal
transition at 32°C. The recycling of granules below 84°C in the granulator will also increase
the porosity.
 22

4.1.2 The storage of straight AN fertilizers

One can distinguish the caking coming immediately after production and the one appearing
after a prolonged storage. Both are influenced partly by the crystal transitions according the
following scenarios:

Fresh product leaving the plant

Caking is not only due to polymorphous transitions since dry ammonium nitrate does not
cake.
It is a speculation whether the freedom from setting of perfectly dry ammonium nitrate on
passage through 32°C is due to complete absence of moisture or to the non-occurrence of
transition : the latter being replaced by a II IV transition at about 50°C.

The caking due to the presence of water is explained as follow :

-During cooling, the saturated solution recrystallizes on the surface of the crystals.
This cements them together by a solid bonding of considerable strength.
For instance, 1 tonne of ammonium nitrate with 0.5% moisture at a temperature of
40°C holds in solution about 14 kg ammonium nitrate of which 3.5 kg recrystallizes
on cooling to 25°C.

-The effect of the pressure is to lower the solubility of ammonium nitrate. The sudden
application of pressure to moist ammonium nitrate during piling in the storage might
bring some crystallisation at the points of contact of granules.

The caking of product entering in the storage above 32°C can also be explained by
allotropic transformations during the storage:

-As it is suggested that the III IV transition can only be made through a dissolution-
recrystallisation process which as explained above cements the particles.

-The fact that the AN-CAN granules enter in the storage at a temperature below the
II III or III IV transitions does not guarantee that the transformation is complete.
As an example, based on the graph 7-8, one can say that AN requires at least 30 min.
to be completely converted when it is cooled down 5°C under 32.2°C.
The transition can therefore progress in the storage. Due to poor thermic transfer, the
release of heat is mainly used to increase the temperature of the mass.
For AN leaving the plant above the transition temperature, one calculates that in the
worst case, the release of 5.08 Kcal/kg during transition II III can increase the
temperature by 12°C. The subsequent dissolution-recrystallisation will promote
caking.

-A study of the solubility curves of ammonium nitrate in water gives the most
plausible explanation of the rapid setting following a transition III-IV. Normally there
is a considerable delay in this transition, in the upward direction it may not take place
until 45°C, and on cooling until 20°C, so that transition is likely to take place well
outside the normal temperature stability range of the salt present. There is a marked
difference in the solubilities of the metastable and stable forms of ammonium nitrate
 23

around 32°C. Therefore, in a slightly moist sample, a delayed transition will result in
the rapid crystallisation of part of the dissolved solid of the salt. Thus coalescence of
continuous granules may occur exactly in the same manner as with temperature
variation of moist ammonium nitrate, but in a very sudden and accentuated degree.

Prolonged storage
-Poor thermic exchanges with the external media will cool down the product slowly to the
transition temperature of 32.2°C. At that stage, the transition heat will maintain almost
indefinitively the temperature to 32°C before it goes down. This passage can cause some
cracked granules and dust formation.

On storage, the cycling of ammonium nitrate through the 32°C transition zone induced by
diurnal temperature variations is accompanied with a 3.6% expansion in volume. After
several cycles, AN prills or granules may soften, fracture, expand and finally convert to a fine
powder with an increased detonability properties (bagged products can even burst the bags).
Even if dust is absent from the product when packed, it will subsequently be formed on
handling and storage rendering the product more difficult to handle and the spreading pattern
uneven.
Another consequence of that physical weakness is that the product turn to a solid, hard cake
during storage.

The dissolution-recrystallization of the saturated solution on the surface of the crystals during
thermo-cycles even milder than those needed to cause transitions destroy the granules:
Indeed, the water migrates also across granule boundaries when the temperature cycles are
milder then those needed to cause transitions.
The disintegration of the external surface during thermocycles may serve to release included
saturated solution, and thereby give rise to caking, on further temperature variation of the salt.
These inclusions present as a mother liquor are located in elongated cavities included between
the long, needlike and often tabular ammonium nitrate crystals.

Ammonium nitrate is a very hygroscopic salt. The formation of a wet skin of saturated
solution on the surface of the crystal exposed to air during storage will therefore participate to
the destruction by increasing the kinetic of transition and recrystallisation phenomenon.
Coating oils that are used to suppress the immediate caking can increase the degradation
caused by temperature changes through the crystal transitions, especially when they contain
secondary amines and short length amines.

In conclusion, it is probable that the setting of ammonium nitrate on passage through the 32°C
transition must be explained by a cumulative effect of all the phenomenon described above.
A good resistance to thermocycle will improve the quality by reducing degradation during
transport and storage. The transition at 32.1°C is particularly harmful in climates where
ambient temperature is often close to that number or when bags are exposed to sunshine.

For this purpose, various cycling tests using laboratory oven were developed by the industry.
The stability against thermocycles can be characterised by several parameters based on
physical properties of granules and by establishing criteria for quality after each or several
thermocycles.
These measurements can be the density, granulometry, swelling, hardness, porosity, friability,
dust, etc.
 24

The minimum values of physical properties that are needed to overcome quality problems and
assure product safety are settled in order to be in accordance with a standard.
These tests are used to select additives in order to improve the thermal stability.
The additives that are proposed are presented in chapter 5.

5. Ways to Improve the Thermal Stability

As the change in volume is a primary cause of breakage, many experiments have been
centered on the elimination of transition III-IV, on the extension of the temperature range over
which the ammonium nitrate exists in the forms IV or on the shift of the transition point of the
crystal form to a temperature range never occurring during storage, handling and use of the
product.

It is done by deep drying or by inclusion of a small amount of conditioning agent to the

ammonium nitrate preferably in the melt prior to the particle forming.
It was also developed where insufficient cooling of the product has been carried out prior to
storage or bagging.

Many materials have been tested alone or in combinations like natural phosphates, potassium
metaphosphate, mono and diammonium phosphate, sulfates (ammonium, calcium,
potasssium), potassium chloride, magnesium salts, calcium salts, sodium silicate, clays,
sodium-calcium and potassium nitrates, iron cyanides and copper oxides.

Numerous salts equilibrium with AN have been studied in order to prepare double salts which
could be easier to use to prepare fertilizers.

The practical advantage is to increase discrete particles’s resistance to degradation by

temperature cycling and often also the quality of prills or granules.

There are several ways to overcome the cycling problems and many patents have been
granted for additives with varying effectiveness:

5.1 Increasing the transition temperature

5.1.1 Minimizing the free water content

5.1.1.1.By deep drying

Since the structure of phase III has no obvious similarities with the other ones, it is suggested
that the transition III –IV must take place by the dissolution and recrystallisation of the solid.

This theory is supported by the fact that the transition at 32°C disappears when AN is
exhaustively dried and is replaced by a transition at about 50-55°C. It is generally conceded
that this transition is a metastable transition between forms IV and II and therefore eliminates
form III from the phase diagram together with the transition at 84°C.
 25

As shown in Graph 11, the transition temperature from form IV to III is a continuous function
of the percentage of water contained in the nitrate between 32 and 55°C. So, depending of the
water in the sample, the transition from form IV to III may occur anywhere in the temperature
range between 32 and 55°C. The transition temperature form III IV plotted also as a
function of water content is the mirror-image of that for the phase transition in the opposite
direction. Drying the AN fertilizer below 0.03% H2O can therefore be a method of stabilizing
against thermo-cycles. Some salts like sodium nitrate (0.1%) overcome the influence of
dehydrating the ammonium nitrate.

Graph 11: Influence of water on the IV-->III transition temperature of pure ammonium nitrate

5 2
Temperature in °C

4 7

4 2

3 7

3 2
0 0 ,0 2 0 ,0 4 0 ,0 6 0 ,0 8 0 ,1 0 ,1 2 0 ,1 4 0 ,1 6

Water in % WT
 26

5.1.1.2. By addition of internal dessicants

Stability can be obtained by adding additives taking up moisture from the surrounding
ammonium nitrate bulk.
This results in a water content low enough to prevent the phase transition IV—III at 32°C.

5.1.1.2.1. Additives binding crystal water

Some inorganic additives bind reversivly crystal water. As an example, one can follow the
dehydratation of aluminium sulfate in function of the temperature in Graph 12. It shows
hydrates containing 18, 16, 10 and 6 moles of H2O.
During the granulation process, these salts can lose partly their crystal water in the AN melt
and pick it up again during the storage.

Graph 12: Al2(SO4)3.18H2O dehydratation

20

18

16
Molecules of cristal water, number

14

12

10

0
0 50 100 150 200 250
Temperature in °C
 27

Al2(SO4)3.17H2O at a rate equivalent to 2% Al2(SO4)3

The addition of aluminium is made by adding Al2(SO4)3.17H2O to a melt of NH4NO3.

A white colloidal precipitate of additive is obtained rapidly and the pH is lowered to about 1.

The decrease in pH is regulated to about 5 by adding NH3.

No growth of aluminium additive can be detected if pH is maintained below 6. Above 6, the

aluminium additive starts to agglomerate.

Aluminium is located in small aggregates of 1-10µ diameter in the prills; the following
formula being probable : Al2(SO4)3(NH4)2(OH)2(H2O)6-10 where NH3 is chemically
bound.

The amount of loosely bound zeolitic water of the additive in the AN melt depends of the
temperature. 2 to 3 moles of water could be bound with AN at 165°C while all the zeolitic
water disappears at 180°C after 8 hours.

In prills with 2% Al2(SO4)3, this means that a consumption of 0.40% H2O corresponds to an
increase of 4 moles of water in the Al-additive. This material has a lower buffer capacity in
the case of moisture uptake than Mg(NO3)2

MgO or Mg(NO3)2.xH2O (X=4-6) equivalent to 1-2% Mg(NO3)2

Magnesium nitrate is probably the most popular inorganic salt used to prevent caking.
Magnesium nitrate increases the IV to III transition continuously from 32 to 55°C and the III
to II transition from 84 to 90°C (Graph 13)
Graph 13: Phase diagram for the anhydrous system NH4NO3-Mg(NO3)2

1 6 0

1 4 0
Temperature in °C

1 2 0 N H 4 N O 3 II

N H 4 N O 3 II
1 0 0
(N H 4 )3 M g (N O 3 )5

8 0
N H 4 N O 3 III

6 0 (N H 4 )3 M g (N O 3 )5

4 0
N H 4 N O 3 IV
(N H 4 )3 M g (N O 3 )5
2 0
0 2 4 6 8 1 0
 28

Mg(NO3)2.2H2O is recognised to give better stabilisation than if Mg(NO3)2.6H2O is used.

It is therefore suggested that the additive can take up moisture from the ammonium nitrate.

Magnesium nitrate is added in the AN melt. At this stage, the high temperature and low
amount of water brings magnesium in a soluble and anhydrous form.

One observes that the additive does not distribute homogeneously throughout the solid prill
but in veins in the (NH4)3Mg(NO3)5 form.

This vein distribution is due to the different melting points of the salts : the AN that
crystallizes first excludes magnesium nitrate until it is all solidified.

After the granulation, magnesium nitrate pumps the remaining water in the NH4NO3 bulk
until it reaches an hydrostatic equilibrium. This is possible because magnesium nitrate with
less than 6 molecules of H2O is much more hygroscopic than AN. Thus the AN bulk will be
almost free from water, so that the transition IV III will be prevented.
 29

As an example, AN prills containing 2% Mg(NO3)2 and more than 2.5% H2O will show a
Mg(NO3)2 %WT
phase transition IV III taking place at a constant temperature of 37°C. With a water content
of less than 1.09%, the phase transition is received at a constant temperature of 52-53°C. In
the range 1.09 to 2.5%, the transition temperature decreases rather rapidly (Graph 14).

Graph 14: Phase transition temperature IV-->III;II as a function of the water content for magnesium-
ammonium nitrate (2%) prills

5 2

4 8

A N p r i lls + 2 % M g (N O 3 )2

4 4

A N p r i lls

4 0

3 6
0 ,0 1 0 ,1 1 1 0

Water Content %

This means that a consumption of the water-content of the AN bulk of 0.24% corresponds to
an increase of 1 mole of water of crystallization in the magnesium nitrate additive.

The position of the plateau in the curve in Graph 11 should correspond to 4.5 moles of water
in the magnesium nitrate.This may mean that a mixture of Mg(NO3)2.2H2O and
Mg(NO3)2.6H2O with an average water amount of 4.5 moles of water is the hydrostatic
equilibrium with almost water-free AN.

Normally, producers use an excess of Mg(NO3)2 so that their product contains a internal
drying capacity against moisture uptake, and any water picked up during storage and transport
is bound as water of crystallisation.
 30

Magnesium nitrate can be formed by adding MgO or MgCO3 to the nitric acid flow before
the neutralizer.

Adding as little as 0.1% MgO to the melted ammonium nitrate will move the transition
temperature between form IV to III to 40°C. Increasing the MgO to 1% raises the transition to
55°C. In that case, a sample cycled hundred of times in the temperature range between 20-
45°C will not show any transition, swelling or caking. This is due to a slow reaction that
occurs between AN and MgO to produce ammonia, water and magnesium nitrate.

A potential disadvantage to use soluble salt is their contribution to hygroscopicity.

As compared to standard prills, Mg(NO3)2 containing prills have a critical relative humidity
below 10%.

Since hygroscopicity is almost as important as particle stability, other methods of stabilisation

have been investigated for bulk storage as the initial fast water pick-up rate could be a factor
in serious caking if product is not quickly bagged.

Some companies have developed a hydrophobic coating which allows them to use the
powerful Mg(NO3)2 stabilisation without getting into troubles with hyroscopicity by bulk
handling.

Ca(NO3)2.4H2O
Calcium nitrate also extends the stability of form IV III up to a maximum of 50°C.
It has little influence on the temperature of the III II transition.
Under low pressure, the transition II to IV occur; after release of the pressure occurs an
immediate retransformation to form II.

A content of Ca(NO3)2 also leads to a very hygroscopic product.

Clays.
0.5 to 3% of porous inorganic substance with a particle size <100µ like alumina gel, silica gel,
bentonite, attapulgite, zeolites, montmorillonite are often used to increase the strength of the
particles and improve the resistance to transition.
Montmorillonite is also a nucleating agent (see chap.5.4.2.)
As an example, one can see in table 8 the effect of 15kg/T of various clays on CAN 27%
dolomite based.
It shows that the quarry from which the clay is originated has a dramatic effect on its
efficiency.
 31

Table8: Influence of clays on thermal stability of CAN 27% dolomite based.

Blanco Attapulgite Montmorillonite

Min-U-Gel African Tolsa
AN Kg/t 771 771 771 771 771
Dolomite Kg/t 229 214 214 214 214
Clay Kg/t 15 15 15 15
Hardness N 52 100 62 67 81
After 10TC N 12 72 36 48 68
After 20TC N 4 50 9 17 49
After 30TC N 2 44 4 10 49

5.1.2 Double salt formation

5.1.2.1. Ammonium sulfate (AS)

Small amount of sulfates improves the stability of AN only at moisture contents <0.3%.
As an example, K2SO4 is known to stabilize AN but not to improve the strength of the
particles.
In addition, ammonium sulfate (AS) increases also the strength even at 0.6% moisture by
favoring a more compact packing of the crystals.
Ammonium sulfate (AS) forms with AN 2 double salts having different molar ratios of nitrate
to sulfate, containing free AN and AS : 3NH4NO3.(NH4)2SO4 and 2NH4NO3.(NH4)2SO4
The phase diagram of AN-AS is presented in Graph 15.

3 Cases can be considered:

AS >5% molar: Above 5%, the melting point rises quickly from 181°C and AS
crystallises alone from the melt under cooling.
AN form I crystallises at 181°C and 3AN.1AS appears at 140 (5%AS) or 162°C
(10%AS). By heating above 162°C, 3AN.1AS decomposes into AN form I and AS.
The form III is suppressed: form IV transforms directly in form II at 51°C. The
transition I II is done at 107°C instead of 124°C.

3<AS<5% molar: idem except that all AS is dissolve in AN melt. The double salt appears at a
lower temperature.

AS<3% molar: melting point increases from 169 to 176°C. The transition I II+3AN.1AS is
lowered from 124 to 107°C. Between 0 and 3% AS, the transition point IV III gradually
increases from 32 to 51°C.
 32

Graph 15: Phase diagram pf NH4NO3-(NH4)2SO4 system

200
liquid + AS
liquid

180

160
I

140
Temperature in °C

I + 3AN.1AS
120

100

80
II + III + 3AN.1AS II + 3AN.1AS

60

III + IV + 3AN.1AS
40
IV + 3AN.1AS

20
0 1 2 3 4 5 6 7 8 9 10

NH4NO3-(NH4)2SO4 mole%
 33

5.1.3 H3BO3 0.2%-(NH4)2HPO4 0.2%-(NH4)2SO4 0.01%

Basic patents have been granted in the sixties in order to stabilize both low and high density
prills through the addition of boric acid (or ammonium or an alkaline salt) + mono or di-
ammonium phosphate + ammonium sulfate.

A mixture of 0.4% by weight of the 3 following ingredients is used to achieve the highest
degree of stabilisation: 0.135% H3BO3, 0.01% (NH4)2SO4 and 0.2% (NH4)2HPO4.
Large amounts of ammonium phosphate are usually required to improve the stability of
ammonium nitrate. However, when used in conjunction with the boric acid and diammonium
sulfate, very small amounts effectively improve the stability.

These prills withstand 10 cycles between 2-60°C and 1400 transitions between 21-43°C
without any breakage.

The transition temperature at 32.2 and 125.2°C are relocated to 43-45 and 124°C and the
transition at 84.2°C no longer occurs. It is claimed to reduce water absorption.

5.2 Lowering the transition temperature

5.2.1 Addition of cations that replace NH4+and blocks the transition

Ammonium nitrate can give solid solution by substitution of NH4+ ion. Attempts to substitute
NO3- have been unsuccessful.

The ionic radius of NH4 being 1.43A°, it can be substitute by ions having same radius: K, Rb,
Tl, Ca

The influence of these ions is linked to their radius value.

The IV III and III II transitions are facilitated, and their temperature lower when the ion is
smaller than ammonium (like K) and more difficult, and their temperature higher when the
ion is bigger than ammonium (Ca,Tl)

KNO3:

Potassium has a radius of 1.33A° which is lower than NH4+ radius: 1.43A°.
At ambient temperature, at least 25% of NH4+ in the form III can be substituted by K. A
double salt NH4NO3.2KNO3 appears when K is above 25%.

It can be added as potassium nitrate in the HNO3 before neutralisation or in the AN melt.
As shown in graph 16, potassium nitrate lowers the IV III transition about 6°C for each
percentage which is added. By adding sufficient KNO3, it is possible to render form III the
stable form throughout the range of normal temperature.

In order to obtain an appreciable effect, the amount of KNO3 should be higher than 5-10%.
 34

At a rate of 2% in a 34%N prilled fertilizer, it is said that it increases the hardness, density,
reduce the volume expansion at the transition temperature and hygroscopicity by increasing
the CRH.

It is interesting to note that a 10% KNO3-90% AN mixture, although not specially dried,
cannot be made to set hard on temperature variation through 32°C, any setting experienced
being slight and easily explained by the effect of simple solubility changes with temperature.
Another advantage is that it reduces the cooling needed as it is not needed to take out the
transition heat III IV.

In a dry sample, potassium nitrate is effective to cause the persistence of phase III below the
transition point of IV to III.

Graph 16: Phase diagram of NH4NO3-KNO3 system

180
liq u id

160
I
140
Temperature in °C

120
II
100

80
III
60

40
IV

20
0 1 2 3 4 5

N4NO3 KNO3 mole %

 35

5.3 Reduction of the kinetic

It is said that acid magenta reduces the kinetic of transformation from form IV to III.

5.4 Crystal habit modifiers

5.4.1 Tensioactives

Small amounts of external materials dissolved in mother liquid can modify crystal habit.
These materials can be incorporated by adsorption, solid solution, crystallisation in the
crystals when these are in formation. They modify the habit by reducing the growth speed of
some particular faces.

On AN, the habit modifiers induce crystallisation in forms such as prisms, plates or scales
rather than the usual, less desirable elongated prisms or needles. The surfactants reduce
surface tension of the saturated solution, thereby reducing the cross-sectional area of bridges
between adjacent particles.

Crystallisation of AN added with acid magenta results in the deposition of extremely soft
lamellar aggregates of the modified crystals, which when isolated are very fragile, thin plates
or scales. The setting tendency of the salt is very greatly reduced.

It modifies the habit of the dissolved salt during the subsequent re-crystallisation process
(which normally produces the hard setting), causing the formation of thin, fragile, platy
crystals instead of the usual more robust prisms and needles.

Thus the bonding between the crystals in the mass becomes of much reduced mechanical
strength, readily broken by light shearing forces.

By the addition of surface-active agents to the molten AN in very small amounts, between
0.0001 and 1%, rhombic, needle-shaped or plate-shapped crystals in form IV are obtained.
These are very brittle and friable, as they have a dendritic structure.

5.4.2 Nucleation agent

Recognising the influence of solid impurities on the ammonium nitrate prills, C&I/Girdler
Corp. have tested nucleating agents in the range of 0.1-10%. The aim is to produce a fine-
grained crystal, as compared to a large-stressed crystal, by eliminating super cooling in the
prilling tower.

They have found that it is now possible to discharge the product from the cooler below 51°C
without experiencing prill degradation during processing and storage.
 36

5.5 Various other experiments have been published

-2.5-5% ammonium polyphosphates or potassium polyphosphate shift the IV-III

conversion into a higher temperature range: 42-55°C. NH4 polyphosphates can be
obtained by addition of urea phosphate

-less than .5% each of Na2SiF6, (NH4)2HPO4 and (NH4)2SO4

-2-5% reactive dehydrated gypsum and 2-5% (NH4)2SO4

-0.5-2% potassium fluoride (KF)

-diammine dinitrate of Ni, Cu raises the transition above 50°C. The effect of the Zn
complex is less marked.

In nitrochalk, MgSO4 or (NH4)2SO4 stabilizes the NH4NO3

dispersive additives (1-3%) of natural minerals like glauconite, pyrophyllite or

clinoptilite increase the thermal stability and strenght of granules.

Zn(NO3)2 0.5% stabilises AN at 51°C

6 Pro’s and Con’s in Selecting the Optimal Stabilizer

Only a few additives have been successful commercial.

The selection of one additive from another is possible after establishing criteria for requested
product and a series of laboratory tests for measuring conformity to a standard.

The simple following tests are used: measurements of hardness, friability, bulk density,
caking, hygroscopicity, porosity applied before and after thermo-cycles.

The selected additive should also fit the best with the following requirements:

-prevent thermal shock and cracking at phase transition temperature during the process
and storage ; so reducing the fines formed during the granulation and avoid dust-
caking during storage

-be efficient at low amounts. This is particularly important in concentrated AN

fertilizers like 34.5%N where there is no place for a filler. The addition of several
additives working in combination can reduce the total amount: AS +AlS additives
containing nitrogen can be valorised: magnesium nitrate, ammonium sulfat

-incorporated in the molten ammonium nitrate prior to solidification. Advantage is that

it is better disseminated in the mass.
 37

inexpensive: clays can be inexpensive if quarrys are close to the plant; the same for
AS if there is caprolactam, cokery waste available (needs to be pure enough).

readily available at production sites. This is especially the case when it is part of the
used materials or by-products at fertilizer complexes like silicofluorides, phosphatic
shales…

safe: Aluminium sulfate is suspected to promote Alzheimer’s disease.

simple to apply: AS has to be dissolved in the AN melt in order to form double salts;
otherwise there will be no effect on thermal stability and some post-reaction during
storage generating caking.

improve the physical properties of the particles: like increasing the density, the
hardness; acting as a binding agent during the granulation….instead of bringing
disadvantages:

Indeed, some additives can bring disadvantages, despite their very good protection
against thermo-cycles, that render them unusable in some particular cases.
As shown in table 9, some additives increase the hygroscopicity of AN so much that
they are inappropriate in case of bulk storage in a humid atmosphere.

Table 9: Hygroscopic points of salts + ammonium nitrate mixtures

influence of soluble inorganic additives on CRH of ammonium nitrate

CRH at 30°C expressed in % of relative humidity

NH4NO3 59.4
K2SO4 96.3
NH4NO3+K2SO4 69.2
(NH4)2SO4 81.1
NH4NO3+(NH4)2SO4 62.3
KNO3 90.5
NH4NO3+KNO3 59.9
Ca(NO3)2.4H2O 46.7
NH4NO3+Ca(NO3)2.4H2O 23.5
Mg(NO3)2.6H2O 48
NH4NO3 +Mg(NO3)2 <10

One can see in table 10 a summary of the pro and con’s of some additives.
 38
Table10. Comparison of various additives: pro’s and con’s

Useable Thermal Physical quality Hygroscopicity Particular Safety Impact on N content Cost
amounts stability effect
Al sulfate 1-2% ++ -Improve hardness, -bound water -sensitive to high Reduce the N content +
granulating agent depending of pH :agglomerate above pH 6,
AN melt -lowers the pH of AN melt
temperature -Alhzeimer disease
Mg nitrate 2% +++ No caking, improve inappropriate NO3 valorized Less costly if from
hardness to bulk MgO,MgCO3
storage
without
special
coating
Boric acid 0.4% +++ More resistant to Sligtly less Increased porosity -Low amount of
phosphates+AS caking, lower hygroscopic additives needed.
friability, improve -doesn’t affect the N
hardness, reduce content
sensitivity to humidity
KNO3 5% +++ Reduce the NO3 valorized +++ loss of K
cooling
needed by
233

stabilising
form III
instead of IV
AS 5% ++ Improve hardness + NH3 + S valorized Low if waste from
caprolactam,
cokery…industries
available
K2SO4 5% +++ No effect on ++, loss of K
hardness
Ammonium Little NH3 valorized ++, loss of P
phosphates practical
experience
Clays 0.5-3% ++ Improve hardness, Contains often carbonates Variable
depending acts as granulating acting as pH buffer
clay agent
characterist
ic