Physical properties of fertilisers

The most important fertiliser properties:

• Hygroscopicity 
• Caking 
• Particle shape and size distribution 
• Particle strength and mechanical resistance 
• Tendency to generate dust and fines 
• Bulk density 
• Compatibility (chemical and physical)

Hygroscopicity

Air contains moisture as water vapour and therefore exerts a water vapour pressure (p H2O) that is determined by humidity and temperature. Hot air can contain more water than cold air. The water content is expressed by the relative humidity (RH).

When the air is saturated with water vapour the relative humidity is 100 % and 50 % RH if half saturated. Water vapour will move from both high to low water vapour pressure. 

At 30˚C the air can contain 30.4 g of water pr m3 (100 % RH). The water vapour pressure of the air varies with humidity and temperature of the air.

All fertilisers are more or less hygroscopic which means that they start absorbing moisture at a specific humidity or at a certain water vapour pressure. Some very hygroscopic fertilisers attract moisture much more readily and at lower humidity than others. Water absorption takes place if the water vapour pressure of the air exceeds the water vapour pressure of the fertiliser.

Absorption of moisture during storage and handling will reduce the physical quality. By knowing the air temperature and humidity and the surface temperature of the fertiliser, it can be determined if water absorption will take place or not. 

Typically, a water absorption curve ascends slowly at low humidity (as illustrated), but at a certain humidity or humidity range it starts to increase steeply. This humidity is called the critical humidity of the fertiliser. The critical humidity goes down when the temperature increases. 

Significant water uptake has undesirable consequences for fertiliser products:

• Particles gradually become soft and sticky 
• Particles increase in volume 
• Particles start to crack 
• Bleaching, change of colour 
• Reduced particle strength 
• Caking tendency increases 
• Formation of dust and fines increases 
• Warehouse floors become damp and slippery 
• Stabilised straight ammonium nitrate loses thermostability 
• Quality of spreading can be affected 
• Clogging of equipment 
• Increased off-spec

A blend of two component can be more hygroscopic than the components on their own, as seen in the graph.

Caking

Most fertilisers tend to sinter or cake during storage. Such caking arises due to the formation of strong crystal bridges and adhesive forces between granules. Several mechanisms can be involved; those of most importance seem to be:

• Chemical reactions in the finished product
• Dissolution and re-crystallization of fertiliser salts on the particle surface
• Adhesive and capillary forces between surfaces

Caking is affected by several factors: 

• Air humidity 
• Temperature and ambient pressure 
• Moisture content of product 
• Particle strength and shape 
• Chemical composition 
• Storage time 

Caking tendency remains low if these parameters are controlled. In addition, application of an appropriate anti caking agent is often needed. There are small caking tendencies in calcium nitrate, but very important phenomenon in NPKs, AN and Urea. Coating of fertilisers reduces the products water absorption rate.

Particle shape and size distribution

fertiliser prills have a smooth and glassy surface, whereas the surface of the granules can vary a lot; normally granules are more rough and uneven than prills. The colour of the particle surface can vary according to raw materials applied in the process or due to mineral or organic pigments added to colour the particles. 

The particle size distribution is important for spreading properties and segregation tendencies. It is especially important if the component is in bulk blends.

Particle strength and mechanical resistance

The crushing strength of fertiliser particles differs greatly depending on the chemical composition. Crushing strength measured for various fertiliser types is illustrated in the table. Water absorption has negative effects on most fertilisers. Particles can become sticky and tend to disintegrate. 

Mechanical resistance is the ability of the fertiliser to resist the stresses imposed upon them in the handling chain. The mechanical resistance depends on surface structure and particle strength.

Dust formation

Large amounts of fertiliser dust causes discomfort in the work place. Therefore, in most countries dust emission from handling operations is restricted by law. Dust and fines normally arise during handling from:

• Water absorption 
• Poor surface structure and particle strength 
• Low mechanical resistance 
• Mechanical stresses in the handling chain 
• Wear and tear from equipment (scrapers, screw feeders, grain trimmers etc) See also how to prevent dust formation.

Bulk density

Bulk density or volume weight (kg/m3) differs between fertiliser types. Variations in particle distribution due to segregation will influence the bulk density. For mechanical spreading it is important that variations within a specific product are minimal.

Compatibility (chemical and physical)

Compatibility primarily relates to blending of different fertilisers, cross contamination and other problems in safety and/or quality; e.g. caking, weakening, dust formation, and loss of resistance to thermal cycling in the case of ammonium nitrate.

Ref: Guidance for the compatibility of fertiliser blending materials, EFMA, June 2006

1. Due to the hygroscopic behaviour of both products, the type of stabilisation of the ammonium nitrate grade could influence the storage properties. 
2. Consider the safety implications regarding the detonability of the blend (AN/AS mixtures) and legislative implications. 
3. Consider the safety implications regarding the detonability of the blend (AN/AS mixtures), the impact of free acid and organic impurities, if present, and legislative implications. 
4. Mixture will quickly become wet and absorb moisture resulting in the formation of liquid or slurry. There could also be safety implications. 
5. If free acid is present it could cause a very slow decomposition of AN, affecting, for example, the packaging. 
6. Consider the possibility of self-sustaining decomposition and the overall level of oil coating. 
7. Sulphur is combustible and can react with nitrates e.g. AN, KNO3 and NaNO3. 
8. Due to the hygroscopic behaviour of both products the type of stabilisation of the ammonium nitrate based fertiliser could influence the storage properties. 
9. Consider the moisture content of the SSP/TSP. 
10. Consider the relative humidity during blending. 
11. Risk of formation of gypsum. 
12. No experience but this can be expected to be compatible. Confirm by test and/or analysis. 
13. Consider impurities in AS and the drop in the critical relative humidity of the blend. 
14. Consider the likely impact of additional nitrate. 
15. Consider the possibility of ammonium phosphate/potassium nitrate reaction with urea and the relative humidity during blending, to avoid caking.
16. If free acid is present, there is a possibility of hydrolysis of urea giving ammonia and carbon dioxide. 
17. Formation of very sticky urea phosphate. 
18. Potential caking problem due to moisture. 
19. If free acid is present, consider the risk of a reaction e.g. neutralisation with ammonia and acid attack with carbonates.