1. Agglomeration
2. Accretion

The formation of solid particles, particulation, in proper quality is of utmost importance in the production of solid fertiliser. The particles must have a size and strength suitable for mechanical handling, for example in spreaders. The particles should not contain or create dust, as dust may constitute a loss of nutrients and may be widely visible and/or annoying and may pose an environmental issue. The particulation is described below with a focus on multinutrient NPK compound and complex type fertilisers, the principles are equally valid for other, e.g. straight nitrogen, fertilisers.

The·method used for granule formation has a pronounced impact on the design and operation of the granulation process equipment. Therefore, a good knowledge of the primary mechanisms of granule formation, growth, and consolidation is essential in determining the design features of the granulation process. The following is a brief description of the two major granule formation mechanisms, agglomeration and accretion, encountered in most fertiliser granulation processes. Granulation by drop-formation (prilling) and pressure roll compaction is described separately.

Agglomeration

With most granular NPK products (excluding, for example, the slurry based nitrophosphate-type processes), agglomeration is the principal mechanism responsible for initial granule formation and subsequent growth, see Figure 1. In most agglomeration-type NPK formulations, 50-75% of the raw materials are fed to the granulator as “dry” solids. These solid particles are assembled and joined into agglomerates (granules) by a combination of mechanical interlocking and cementing – much as a stonemason fashions a stone wall by using stones of various sizes and shapes and mortar as the cementing agent. The cementing medium for fertiliser granules is derived from salt solutions, for example, a pre-neutralised ammonium phosphate slurry and/or the dissolution of salts on the moist surface of the soluble solid particles. Solutions of certain raw materials, water and steam can support the particle formation. If mainly steam is used, the process is called steam granulation. The size, shape, surface texture, strength, and solubility of the solid particles vary widely and have a profound influence on the granulation characteristics of the mixture.

Accretion

Accretion refers to the process in which layer upon layer of a fluid material (for example, an ammonium phosphate slurry) is applied to a solid particle causing it to grow in size. The slurry-type granulation processes used to produce DAP, MAP, TSP, and some nitro-phosphate compounds are examples of accretion-type granulation processes. The accretion process is quite different from the agglomeration process with respect to the mechanism of granule formation and growth. As a result, the required process parameters for optimum operation of these slurry-type accretion granulation processes are often quite different from those used in agglomeration processes. With a slurry-type granulation process, a relatively thin film of moist slurry, or a nearly anhydrous melt, is repeatedly applied, dried, and hardened to a relatively firm substrate consisting of granules that are often product size or nearly product size. The slurry or melt is usually sprayed onto the recycle stream, in which the large particles have been crushed prior to return to the granulator. In this process layer upon layer of new material is applied to a particle, giving the final granule an “onion-skin”-like structure, see Figure 1. In the process, of course, some agglomeration of particles also occurs, but this is not the-predominant granule formation mechanism.

The recycle-to-product ratio required for accretion type granulation is normally higher than that required for agglomeration-type processes. Accordingly, for a given production rate, the material handling capacity of the process equipment must be larger for accretion-type granulation plants than for most agglomeration-type plants. This likely applies to all equipment and transporting means within the recycle loop. However, because of particular temperature- and relative humidity-related processing requirements for some agglomerated NPKs, certain equipment, especially the dryer and process cooler, may actually be larger in some agglomeration-type plants to achieve the same production rate as in the accretion-type processes.

Granules formed by accretion are almost always harder, more spherical, and more durable than those formed by agglomeration. These properties lead to less attrition and less dust formation during handling. For example, a typical well formed NPK, DAP, or TSP granule produced by accretion-type granulation may have a crushing strength of about 4-8 kg, whereas the crushing strength of an agglomerated granule may not only be less (often less than 3 kg) but also more variable depending upon its raw material composition and a number of specific factors related to granule formation. Some examples of the variability in granule strength for a number of fertilizers are shown in Table 1.

A more detailed and mathematical treatment of this topic, with additional data, can be found in Kiiski, H. and Kells, A. (2016). Granulation of Complex Fertilisers, Proceedings International Fertiliser Society, 783.

Links to related IFS Proceedings and presentation recordings

186, (1979), The Pan Granulation Process,
Ø Skauli, O H Lie.

202, (1981), Granulation and Prilling for Fertilisers – Criteria for a Choice Between Various Technologies,
R Schoemaker, A C M Smit.

Recording of ‘Granulation of complex fertilisers‘, (2016), H Kiiski (Required password is 2016Tech05)

Links to external resources

IFDC. (1998). Fertilizer Manual.

Nitrophosphate-Based NPK Fertiliser: Air pollution control, R. E. Nitzschmann and J. G. A. Reuvers, in Pollution Control in Fertilizer Production, edited by C.A. Hodge and N.N. Popovici, Chapter 31, pages 419 – 431, 1994.

Pietsch, W. (1966). ‘Firmness and drying behaviour of granules with formation of salt bridges, ‘ Aufbereit. Tech. 38, no. 3, 371- 372.

Polo, J. (2001a). ‘Formulation of Compound Granular Fertilisers,’ IFDC, Advanced Fertiliser Production Technology Workshop, Corsendonk, Turnhout, Belgium, October 15-19.

Polo, J. (2001b). ‘Temperature Control During Agglomeration-Type Granulation,’ IFDC, Advanced Fertiliser Production Technology Workshop, Corsendonk, Turnhout, Belgium October 15-19.