Overview
Segregation is the separation of particles due to differences in physical characteristics. For segregation to take place there must be movement between the particles. Various characteristics may cause segregation but the most common are particle size, particle density and particle shape. The size is certainly by far the most important factor to explain segregation during flow. During spreading, the three properties play a role, the shape being the least important and the size again having a dominating effect.
From the above it is seen that segregation may occur during manufacture, bulk transport (and handling) or during application to the soil or crops. The effects as seen by the crops may be the same in all cases unless remixing takes place during later handling stages. The following extract from Stairmand, 1962, deals with the problems of storage in hoppers and this is relevant to bulk handling, bagging operations and spreading.
“The two main factors are avoidance of segregation in filling and emptying and ensuring the hopper will be “self-clearing.” In examining the question of segregation in filling… if the material is charged into the hopper from a single point, the coarser particles will migrate to the outer edges and a central core of material containing an excess of fines will form. If now filling is discontinued and the hopper is allowed to discharge it will do so in zones… If the zones contain particles of different sizes due to segregation in filling, no device fitted at the cone discharge to remix can possibly be effective. If however, the hopper is filled without segregation, little segregation will occur when emptying.”
By applying the above to blending, it can be seen that the avoidance of segregation and the subsequent handling of the product, is the most important part of any blending process. This may be achieved in three ways:
- The careful matching of the blend components
- The design of hoppers (see Leonard)
- The loading methods of the hoppers and vehicles, etc.
The origin of the segregation is always a difference of physical properties leading to a physical separation of the particles which may or may not result in a chemical segregation. Size segregation may also occur within raw materials or complex fertilisers but this will have no (or very little) effect on the chemical content. However, in bulk blends, physical segregation often leads to chemical differences.
Flow segregation
Care must be taken when storing any material to avoid size segregation in the storage heaps. Whenever granular material falls freely to form a cone or part cone, the larger particles will tend to run down the outside with the smaller particles remaining near the centre of the cone. Should this happen, there may be quite large variations in size characteristics between various parts of a heap and this could have serious effects on all fertilisers (Figure 1) [1]. Batches taken from a segregated heap can thus differ in particle sizes, and this will affect the spreading width of a centrifugal spreader with straight, blended and complex fertilisers.
There may be some remixing at later handling stages. Such remixing may be caused simply by tipping a bag of fertiliser out into a hopper or even more simply by turning a loosely filled bag over. Remixing may also occur during loading and unloading of bulk fertiliser (Figure 2) [1]. However, systematic remixing can rarely be counted on.
It is essential that all handling equipment is constructed to minimise segregation. The use of specially designed chutes is recommended to avoid segregation and excessive amounts of free dust.
Examples of techniques which may be used to avoid segregation include (Figure 3) (Lance):
- “Egg crate” baffles in square or rectangular hoppers.
- Concentric cone distributors for cylindrical hoppers.
- Flexible spouts for direct loading equipment.
In the case of raw materials, the segregation is mainly a size segregation which has little effect on the chemical content. For blends, the risk is to have both size and chemical segregation.
Tests of filling and emptying containers have shown that granulometric segregation is well linked with the granulometric spread index (Figure 4) [2]. It is recommended that the granulometric spread index of the blend is limited to below 20. This means that the raw materials must have an even lower GSI (see Raw materials Table 2)
Figures 4 and 5 distinguish three ranges of segregation: Low segregation means that for the given difference of property the final segregation has little impact – Medium is the situation where the final segregation begins to be significant but if there is another unfavourable condition, for example low overlapping during spreading, the final result may be bad – High is the worst situation where there are negative economic and/or environmental consequences.
Chemical segregation is related to the absolute sum of the difference of d16 and d84 of each component (Figure 5). This means that if the fine particles come from one raw material and the coarse particles from another, the size segregation induces a chemical segregation.
Transport segregation
It is often said that blends are subjected to serious segregation during vehicle transport but this is not true. The level of vibration in transport vehicles is not high enough to induce a movement of the fertiliser. In fact, the segregation occurs when filling (and emptying) the container as described above. During transport, only percolation of very small grains between the larger ones can occur. With common fertilisers, this phenomenon appears if there are particles smaller than 0,5 mm. Above this size, the process is too slow to have a real impact on the final product.
Spreading segregation
Blenders should have some knowledge of the kinds of spreading equipment available and the advantages and disadvantages of these when using blended fertilisers. Blenders should be prepared and able to advise farmers on the type of equipment to use and the setting up, calibration and maintenance necessary. This means that they should be prepared to give as much information as possible about the physical quality of their products. Such information may include the composition, bulk density and flow characteristics as well as the particle size distribution. This is another reason for keeping adequate records of raw materials and product batches.
More detailed consideration of segregation during spreading is beyond the scope of this resource. More information is provided by the Handbook of Solid Fertilizer Blending, published by EFBA.
References
2 Miserque O. Analyse de l’influence des caractéristiques physiques des particules d’engrais sur le comportement des mélanges. PhD thesis – University of Liège, Faculty of Applied Sciences – Belgium – 2005.
Stairmand, C.J. (1962). The screening and Segregation of Fertiliser Materials, Proceedings International Fertiliser Society. 74.
Links to related IFS Proceedings
387, (1996), Theory of Fertiliser Blending, G E N Lance.
388, (1996), Technology of Fertiliser Blending, J E Leonard.
547, (2004), Non-Segregating Blended Fertiliser Development: A New Predictive Test for Optimising Granulometry, I A Bridle et al
553, (2005), Modelling to Aid Assessment of Fertiliser Handling and Spreading Characteristics, E Tijskens, P Van Liederkerke, H Ramon
554, (2005), Segregation of Blended Fertiliser During Spreading: The Effect of Differences in Ballistic Properties, M S A Bradley, R J Farnish
555, (2005), Photographic Imaging Systems to Measure Fertiliser Granule Velocity During Spreading, F Cointault, J Vangeyte
600, (2007), Segregation During Fertiliser Handling: Occurrence, Assessment and Control, R J Farnish, M S A Bradley
784 (2016), Potential for Post-Production Segregation of Blended Fertilisers, G Moitzi et al.
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