1. Mechanics of caking
2. Factors affecting caking
3. Reduction of caking and water uptake

Mechanics of caking

Nearly every fertiliser which is stored and handled in bulk is prone to caking. The most important phenomena in caking are as the following.

Growth of small crystals

The solubility of the salts changes with the environmental conditions, and repeated dissolution and re-crystallisation cause crystal bridges to be formed between adjacent particles. After granulation or prilling, the crystals in the granules are relatively small, but strong crystal bridges, due to extensive crystal growth, can develop between the granules during storage.

Plastic deformation

High pressure can deform the granules, thereby increasing the contact surface. This phenomenon is promoted by high water content.

Decrease of the total surface area

A hygroscopic salt absorbs moisture above the critical relative humidity of the salt, which results in a film of saturated salt solution on the surface of the crystal, and this causes capillary adhesion. Capillary forces promote meniscus formation and the migration of ions to the contact point. This leads to a thermodynamically induced decrease in the surface area.

Double-salt formation within and between separate granules.

The formation of double salts will continue during storage if the chemical reactions between the different salts are uncompleted in the granulation process. Such exothermal reactions may lead to extensive re-crystallisation, increased stack temperature and severe caking.

Factors affecting caking

Several factors can contribute to caking:

Moisture content

The amount of water present in the fertiliser is often considered to be the most important factor in promoting caking. For most caking mechanisms, especially for dissolution and/or recrystallisation, water is essential.

There is a linear relationship between water content and caking (Thompson, 1972): caking increases linearly with water content from a threshold value of water to a maximum, after which caking decreased.

The variations in water content during production, as well as the performance of dryer and cooler are also important factors. Variation in moisture content of the fertiliser creates moisture gradients in the heap during storage, which can result in moisture migration which promotes caking.

Chemical composition

The composition of fertiliser affects the caking tendency. Whereas some fertilisers, such as potassium salts, are hardly sensitive to caking at all, others, such as ammonium phosphates, superphosphates, ammonium nitrate-based fertilisers and urea-based NPKs have a high or even very high caking index.

Some fertiliser salts strive to form more stable salt pairs after water absorption, for example:

NH₄NO₃ + KCl ↔ NH₄Cl + KNO₃ + heat

As this reaction is exothermic, it can lead to an increase of temperature in the bulk fertiliser. The presence of salt pairs results in several equilibria with their own specific characteristics towards caking and/or moisture uptake.

Impurities can affect caking. For example, it is known that the presence of traces of iron and aluminium in diammonium phosphate leads to a decrease in caking.

Different types of granulation processes will also influence the chemical composition at the surface and also to what extent after-reactions will take place during storage.

Moisture uptake

Most fertilisers are hygroscopic, varying from very hygroscopic, for example magnesium-stabilised ammonium nitrate, to moderately hygroscopic, such as urea to non hygroscopic, such as potassium sulphate K₂SO₄.  Depending on the production and storage conditions (temperature and humidity) a fertiliser can pick up water. It results in local dissolution and recrystallisation of the fertiliser. Usually the adjacent granule is affected as well, leading to crystal bridge formation (Figure 1). In addition to water uptake, migration of internal moisture can also give rise to recrystallisation of the granules. This moisture transfer can, for example, be caused by differences in temperature of the bulk material or by different temperatures of the surroundings, e.g. air, walls.

The rate of the moisture absorption depends on factors such as:

• The difference between the Relative Humidity (RH) of the air and the Critical Relative Humidity (CRH) of the fertiliser.
• The movement of air with a constant RH over the fertiliser.
• The granulometry of the fertiliser.
• The rate of penetration into the bulk of fertiliser.

Temperature

Owing to the large volumes produced, it is very difficult to maintain the same temperature throughout the whole bulk. Moreover, there can be strong fluctuations in temperature at the outer shell of the stored granules, often caused by differences between day and night temperatures. It can lead to moisture migration, especially for urea, and/or to changes in volume (expansion versus shrinkage) of e.g. ammonium nitrate-containing granules. The storage temperature is crucial, especially for ammonium nitrate. Unstabilised ammonium nitrate undergoes a transition in crystal structure at 32°C, the so-called phase III-IV transition (see for example Sjölin, 1971; Van Driel, 1994). When ammonium nitrate is subjected to several temperature cycles, the granules can lose their strength quickly, resulting in breakage and dust formation.

Furthermore, the CRH of most salts decreases with an increase in temperature. For instance, for ammonium nitrate an increase in temperature from 0 to 40°C decreases the CRH by 25% (from 78% to 53%) (Unido, 1998).

Particle size, size distribution, and presence of dust and fines

Smooth large fertiliser granules possess a small reactive surface, whereas the corresponding dust and fines are extremely sensitive to water uptake due to the large specific surface area. However, it is not economically feasible to produce only large and smooth granules. As a consequence, smaller and possibly irregular shaped granules will be present in the bulk as well. Small particles can easily fill the gaps between larger granules (Figure 2). Thus, the number of contact points increases, leading to promotion of (water-mediated) crystal bridge formation.

Hardness of the granules

It is important that the granules produced have a good hardness. When handling soft fragile particles, fines can be formed easily, especially at the bottom of the pile (lack of impact resistance). The negative effects of the resulting dust and fines on caking have been mentioned above.

Pressure

Especially at the bottom of a stack or pile the pressure can be high, resulting in deformation (flattening) of the granules. Hence, the contact area of the fertiliser increases.

Sometimes deformation of granules with very low water content is observed, especially for various NPK grades and NS fertiliser types. An explanation has not yet been found, but it should be stressed that the liquid phase in fertiliser granules is NOT equal to the amount of measured water. Several fertiliser salts are very water-soluble and their solubility increases with high pressure. Furthermore, many salts include water in their crystal lattice, which is therefore partly immobilised. Under pressure, this water might be liberated to its free form, allowing it to facilitate dissolution and recrystallisation as well as to influence various equilibria. Consequently, the liquid phase in a fertiliser may be much higher than might be expected! Deformation of the granule under pressure will increase its contact surface area with its neighbours and will possibly rupture an applied coating, causing severe caking.

Figure 3 shows a sample of a plastic NPK after storage in a cylinder under mechanical pressure (approx. 14 bar) for 24 hours at room temperature. It can be clearly seen that the top has been completely flattened and that the underlying granules have lost their original shape. When storing fertiliser in bulk, the opposite picture can be observed: flattening at the bottom and deformation of the more upper lying granules.

Time

The length of storage time influences caking. In most cases, caking is the most severe during the first weeks after production. However, if a fertiliser has a tendency to cake, prolonged storage usually leads to increased caking.

Excess or poor distribution of coating agent

When a coating has not been distributed sufficiently over the granules, the local concentration of the coating can be too low in some places and too high in other spots. In the case of an oil-based coating, this can give rise to ‘gluing’ of the granules, especially dust and fines.  On applying an excess of water-based coatings, dissolution and recrystallisation can occur, leading to an increased reactive surface. Figure 4 shows the typical effect of the coating dosage on the caking of fertiliser. It must be noted that the use of high amounts of coating agent on ammonium nitrates may be limited by safety requirements.

Reduction of caking and water uptake

Of all the factors mentioned above, only the moisture sensitivity of a defined fertiliser composition cannot be influenced: it is a characteristic of the product. All the other factors, such as moisture content, temperature, particle size and pressure, can be influenced.

Fertiliser manufacturers have succeeded in minimising these factors. Chemically, several improvements have been found: moisture migration and subsequent reactions can be prohibited by the addition of salts, which can include several equivalents of water in their crystal lattice.

Technically, several improvements have been made in the granulation processes, for example better grinding of the raw materials for NPK. As a consequence, more uniform and smoother granules can be produced. In addition, improved screening has led to a narrower particle distribution in the bulk. Hence, both the reactive surface and the number of contact points have been reduced substantially. For some processes it has been found that it is possible to optimise the size distribution of the granules by the addition of a chemical compound, usually a surface active agent. However, the balance between the additive and the fertiliser is very delicate. Hence, a small disruption can strongly affect the particle size distribution.

Moreover, production conditions have been optimised, especially the drying and cooling processes as well as storage conditions. Efficient drying will lead to low water content in the granules. Improved cooling of the fertiliser makes the products less sensitive to further reactions, such as moisture uptake, dissolution, recrystallisation and/or phase III-IV transitions (and vice versa). In addition, storage conditions have improved markedly, i.e. low relative humidity, diminishing fluctuations in temperature etc. It is of importance to maintain the good storage conditions over the total supply chain.

Last but not least, caking, water uptake and dust formation can be inhibited by applying a coating. It must be noted that this kind of treatment is not a substitute for good manufacturing practices. A good basic fertiliser quality is required. When treated with a suitable coating, the resulting fertiliser can be handled and stored well over a long period of time.

References

Sjölin, C. (1971). The influence of moisture on the structure and quality of NH₄NO³-prills. J. Agr. Food Chem. 19, 83-95.

Thompson, D.C. (1972). Fertiliser caking and its prevention, Proceedings International Fertiliser Society, 125.

UNIDO and International Fertilizer Development Center (1998), Fertilizer Manual, Kluwer Academic Publishers, ISBN 0-7923-5032-4.

Van Driel, C.A. (1994). Influence of additives on structure and thermal stability of ammonium nitrate, PhD Thesis, Technical University, Delft. ISBN 90-9006833-3.

Links to Related IFS Proceedings

5a, (1949), Condition in Fertilisers, E M Crowther

5b, (1949), Studies in the Caking of Fertilisers, G R Davies, J R Ditcham, W S Greaves

453, (2000), Fertiliser Coatings, A Ohlsson

584, (2006), New Developments in Fertiliser Coatings, E A Bijpost, J G Korver

783, (2016), Granulation of Complex Fertilisers, H Kiiski and A Kells