Overview
The quality of the finished product depends almost entirely on the raw materials. Good blending starts with good raw materials. It is not realistic to expect to make good quality blended fertilisers from poor raw materials. The specification, purchase and checking of raw materials must be the first priority of the blender. The properties of materials commonly used for blending are shown in Table 1.
Every raw material should be bought to as tight a specification as possible. Deliveries must be checked regularly, preferably by independent inspectors, to ensure consistent quality. All sampling and testing should be carried out using methods agreed between supplier and purchaser, based either on National or European legislation or on accepted International Standards (CEN, ISO etc.). More information about sampling can be found here
Whilst many fertiliser raw materials may be considered to be commodities, rather than speciality chemicals, the opportunistic purchase of spot consignments of doubtful origin and quality is NOT RECOMMENDED. No raw material should be purchased without an agreed contractual specification covering, as a minimum, the registration to REACH, the chemical analysis and the particle size details.
Chemical Properties
The nutrient content of each raw material used must be known in order to prepare formulations for the different compounds required. Raw material suppliers should be asked to supply certificates of analysis for each large consignment. Where consignments differ markedly they should be stored separately and the formulations adjusted to take account of the true analysis figures, see here.
The water content of each raw material used must be known in order to ensure compatibility between raw materials.
In all cases it is advisable to make occasional random checks by arranging for representative samples to be taken by independent inspectors. These samples should be analysed as soon as possible and before the consignment is used.
Compatibility
Further information regarding compatibility may be found in Guidance for the Compatibility of Fertilizer Blending Materials. Published by Fertilizers Europe (2016)[1].
Notes for the numbers in the boxes in Figure 1:
Limited Compatibility
1. Due to the hygroscopic behaviour of both products, the type of stabilisation of the ammonium nitrate (AN) grade could influence storage properties.
2. Consider the safety implications regarding detonability of the blend (AN/AS mixtures) and legislative implications.
3. Consider the safety implications regarding detonability of the blend (AN/AS mixtures), impact of free acid and organic impurities, if present, and legislative implications.
4. If free acid is present it could cause very slow decomposition of AN, affecting, for example, packaging.
5. Consider the possibility of self-sustaining decomposition and the overall level of oil coating.
6. Due to the hygroscopic behaviour of both products, the type of stabilisation of the ammonium nitrate based fertilizer could influence the storage properties.
7. Consider the moisture content of the SSP/TSP.
8. Consider the relative humidity during blending.
9. Risk of formation of gypsum.
10. No experience but this can be expected to be compatible. Confirm by test and/or analysis.
11. Consider impurities in AS and the drop in the critical relative humidity of the blend.
12. Consider the likely impact of additional nitrate.
13. Consider the possibility of ammonium phosphate/potassium nitrate reaction with urea and relative humidity during blending to avoid caking.
14. If free acid present, there is a possibility of hydrolysis of urea giving ammonia and carbon dioxide.
15. Formation of very sticky urea phosphate.
16. Potential caking problem due to moisture.
17. If free acid is present, consider the risk of a reaction e.g. neutralisation with ammonia and acid attack with carbonates.
Not Compatible
NC1. Mixture will quickly become wet and absorb moisture resulting in formation of liquid or slurry. There could also be safety implications.
NC2. Sulphur is combustible and can react with nitrates e.g. AN, KNO3 and NaNO3.
From the chart, it is clear that urea and ammonium nitrate should never be used together as the mixture will quickly become wet and absorb moisture. Blends containing urea and single or triple superphosphate may also become sticky and cake. Such blends should never be bagged. Mixtures of di-ammonium phosphate and superphosphates should be avoided, as chemical reactions may take place which can lead to caking or changes in the solubility of the phosphate.
For reasons of safety, it is very important to avoid blending ammonium nitrate or raw materials containing ammonium nitrate with any organic materials.
DISCLAIMER : The information and guidance provided in this Figure is given in good faith. IFS, EFBA and Fertilizers Europe, their members, consultants and staff accept no liability for any loss or damage arising from the use of this guidance.
Physical Properties
The most important physical property – as far as blending is concerned – is the particle size distribution.
The particle size distribution must be known in some detail and the specification must include a full description of this property. At the very least, the mean particle size (as measured by the d50) must be specified. Ideally, the specification should include a measure of the granulometric spread index (GSI) and should also include maximum values for the amounts of oversize (for example > 5 mm) and fines (for example < 1 mm).
The shape and the density of particles could have an influence on the behaviour of the fertiliser during spreading.
Other physical characteristics to be specified must include “free flowing” and “dust free” and possibly hardness and impact resistance. However, these properties are much more difficult to assess using standard test methods. More realistically, it is better to specify that suitable anti- caking and/or anti-dust treatment is applied to the raw material and that it should not break down during handling.
In all cases it is advisable to make occasional random checks by arranging for representative samples to be taken and assessed by independent inspectors. It is also recommended that samples be taken during the delivery and tested for size distribution at the blending plant, see here for more details
Particle Size
The key factor in producing quality blends is the size compatibility of the raw materials. Unless all the ingredients are well matched, segregation will take place every time the blend is handled in bulk. This will lead to unevenness of chemical analysis and possibly, uneven spreading of nutrients on the crops.
The particle size distribution can be expressed in a number of ways but all rely on a sieve analysis of the material. It is essential therefore that blenders should have the facility to carry out a full sieve analysis of their raw materials, see here for more details.
A number of simple field test devices are available but these are limited in their ability to measure particle size distribution adequately. They are however, very useful to carry out spot checks on raw materials being fed to the blender.
The official method of test sieving is fully described in European Standard EN 1235 and Amendment A1 [2]. A number of numerical parameters may be calculated from the sieve analysis. These include the mean particle size (d50) and granulometric spread index (GSI). In view of the importance of size distribution, full descriptions of these parameters and the use of the various systems are described below.
However, for the best results, it is recommended that a full size distribution curve is plotted for all raw material samples tested. A description of the test sieving technique is given here. Size distribution curves can readily be superimposed to give a quick comparison and an indication of compatibility.
The mean particle size (d50) is determined using the following equation:
Where:
zn is the nominal sieve mesh in mm for which the cumulative undersize is nearest to but below 50% by mass
zn+1 is the nominal sieve mesh in mm for which the cumulative undersize is nearest to but above 50% by mass
cn is the cumulative percentage undersize for sieve n
cn+1 is the cumulative percentage undersize for sieve n+1
NOTE: d84 and d16 are calculated in the same way by substituting 84 and 16 respectively for 50 in the above equation.
An excellent measure of the spread of particles sizes can be obtained using the whole of the linear part (between d84 and d16) of the distribution curve obtained from the sieve analysis. The values of d84 and d16 may be found directly from the graph or by calculation. The spread is the difference between the two:
An important value, known as the Granulometric Spread Index (GSI), is derived from the following formula:
Bulk Density
The bulk density of the fertiliser may be measured in accordance with EN 1236 (Loose density) [3] or EN 1237 (tapped density) [4]. The general principle is to weigh the contents of a cylinder of a known volume. For the tapped density, the cylinder is subject to vibrations and compaction occurs. This value is always higher than the loose density. The density of the fertiliser can have an influence on the behaviour of the particles during spreading on the field. Severe segregation may occur if the densities are very different.
Generally, the loose bulk density of fertilisers is between 900 and 1,100 kg/m³ but extreme values can be between 750 and 1,350 kg/m³. In practice these extreme values rarely occur simultaneously.
Shape
The measurement of the shape of the fertiliser particles is not easy. Generally, it is necessary to use image analysis techniques. However, the measurement of the angle of repose of a heap formed by a fertiliser flowing from a funnel can be a useful guide to this parameter. The method is standardised in EN 12047 [5] and described here. Angles of repose vary from about 30° for the most spherical products to 40° for the most angular.
Particle Hardness
During handling and spreading, the fertiliser will be submitted to stresses which can break the particles, for example the impact with the vanes during spreading. This process leads to the production of small grains which cause some problems (segregation, caking). For this reason the particles should be of a sufficient hardness. Unfortunately, the test methods are not standardised because of the variability of the measurement and the evolution of these parameters with time. If the particle hardness is low, the fertiliser may contain too many small particles and thus may no longer meet the quality criteria, see here.
Dust Content
Some fertilisers have the tendency to produce large amounts of dust. This can cause problems in the neighbourhood of the plant and accentuates the risk of caking. There is no standard test method but with some experience the raw materials presenting this problem are easily detected.
Flow Rate
A standard method for the measure of the flow rate has been developed as EN 13299 [6]. About 2 kg of fertiliser is placed in a standard funnel which has a closed aperture of 25mm diameter. Then the aperture is opened and the time for 2 kg to flow out of the funnel is measured. The apparatus is calibrated with defined glass spheres.
Storage of raw materials
Raw material storage must be arranged to avoid:
- segregation within the materials.
- cross contamination.
- deterioration of the physical quality.
Storage must be arranged to ensure adequate identification of the raw materials.
The recommended type of storage is the horizontal or open bin layout. Ideally the bin should be fed from a conveyor belt system with the discharge fitted with an anti-segregation system such as a spinner or flow splitter (Figure 2)[7].
The Fertilizers Europe has published detailed guidance on the safe storage of fertilisers.[8]
Moisture Pick-up
Some fertiliser raw materials are hygroscopic which means they can pick up moisture from humid air. Stores holding these materials should be air-conditioned or the material should be covered when not being used. Figure 3 shows the critical relative humidity for a number of common blend components and mixtures[9]. The lower the critical relative humidity, the more moisture will be taken from the air. Generally the phosphates including the ammonium phosphates have a high critical relative humidity and thus almost never present hygroscopic problems. The opposite applies to nitrates such as calcium ammonium nitrate, ammonium nitrate, and especially calcium nitrate.
For blended and complex fertilisers the critical relative humidity in most cases is below the average derived from its components. This can be seen when looking at the data for PK and NPK fertilisers. An extreme example for this is demonstrated by the critical relative humidity of a blend consisting of urea and ammonium nitrate. Such a blend would pick up moisture so quickly that it would be impossible to handle it in a dry state, even if spreading occurs immediately after blending.
Contamination
Cross contamination of raw materials should be avoided as this will obviously affect their chemical analysis and hence the final analysis of the blends.
Ammonium nitrate and other materials containing ammonium nitrate must be kept well clear of organic materials.
Good housekeeping is vital to any blending operation. All spillages should be swept up as soon as possible and all equipment kept clean. Overhead conveyors must be kept in good condition to minimise spillage into other storage areas. The use of special chutes to avoid excessive dust is strongly recommended. Further information on the prevention of contamination is given in Reference 8.
Quality recommendations
To reduce segregation problems, it is recommended to blend raw materials having similar physical properties, the most important being the size of the particles. The European Fertilizer Blenders Association (EFBA) has defined quality recommendations for the particle sizes (Table 2). The recommendations concentrate on a d50 around 3.25 mm with a limited granulometric spread index (GSI). If the raw materials comply with these recommendations, flow segregation is not significant and spreading segregation only occurs if there are important density and/or shape differences. This happens only when certain specific fertilisers are used.
References
1 Guidance for the Compatibility of Fertilizer Blending Materials. Published by Fertilizers Europe (2014)
2 CEN, European Standard /A1. Test Sieving (2003)
3 CEN, European Standard EN 1236. Determination of bulk density (loose)
4 CEN, European Standard EN 1237. Determination of bulk density (tapped)
5 CEN, European Standard EN 12047. Measurement of static angle of repose
6 CEN, European Standard EN 13299. Determination of the flow rate
7 Lance G.E.N. (1996). Theory of Fertiliser Blending, Proceedings International Fertiliser Society,. 387.
8 Guidance for the Storage, Handling and Transportation of Solid Mineral Fertilisers. Published by Fertilizers Europe (2007)
9 Heege H. Quality of raw materials for fertilizer-blends: defining a standard. Rencontres Internationales de l’AFCOME, Strasbourg, Novembre 2003.
Links to related IFS Proceedings
448, (2000), Ammonium Nitrate: Safety Aspects of Blended and Granulated Compound Fertilisers based on Ammonium Nitrate, K Shah, D C Thompson
450, (2000), Self-Sustaining Decomposition of NPK Fertilisers Containing Ammonium Nitrate, H Kiiski
489, (2002), Measurement of Physical Characteristics of Fertilisers and their Influence on Handling and Application, S Dutzi
557, (2005), Procedures for Classifying the Physical Qualities of Fertilisers, P C H Miller, C S Parkin
Links to external resources
ECHA REACH Chemical Regulations
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