The following brief overview includes terminologies used when discussing the formulation and production of complex granular fertilisers.

1. Agglomeration – particle growth
2. Accretion – particle growth
3. Ammoniation
4. Analysis
5. Compound fertiliser
6. Complex fertiliser
7. Critical relative humidity
8. Dry matter material calculations
9. Formulation
10. Liquid phase
11. Heat of reaction
12. Material compatibility
13. Material texture
14. Mole ratio
15. Particulation
16. Particulation processes
17. Pastillation
18. Pelletising
19. Raw Materials
20. Recipe
21. Recycle ratio
22. Reversion of phosphate water solubility
23. Sintering or thermal agglomeration
24. Size distribution
25. Size enlargement
26. Slurries and melts
27. Supplementary heat
28. Thixotropic mixtures
29. Wet basis or as-received material calculations

Agglomeration.

‘Agglomeration’ is a size enlargement process of fine particles to produce agglomerates or aggregates in which the original particles can still be identified. In the physical sense it can be defined as a method where fine particles are brought closely together by external forces (gravity, inertia, drag) to agglomerate together by short-range forces (van der Waals, electrostatic force). (Pietch, 1966; Rumpf, 1958; Kiiski, 2016b).

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. 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 fertilizer granules is derived from salt solutions, for example, a pre-neutralized 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. In the process, of course, some agglomeration of particles also occurs, but this is not the-predominant granule formation mechanism.

Ammoniation.

‘Ammoniation’ is the process of reacting (absorbing) ammonia with acidic materials such as sulphuric, phosphoric, or nitric acid, and/or with superphosphates. Ammoniation is a neutralisation process that converts gaseous or liquid ammonia into more stable solid ammonium-containing compounds. Ammoniation, often also called neutralization, is the major source of chemical heat in granulation processes.

Analysis.

‘Analysis’ means the analytical result of the determination of the quantities of the major, secondary, and minor plant nutrients, and the moisture contained in the raw materials and finished granular products. The quantities are expressed as percent by weight. The major nutrients (nitrogen, phosphate, and potash) are usually expressed as  N, P₂O₅, and K₂O, respectively; however, sometimes (for example in Scandinavian countries) the elemental basis (N, P, and K) is used.

Compound fertiliser.

Multinutrient or compound fertilisers containing two or more nutrients. In bulk-blend or blended fertilisers, two or more granular fertilisers, preferably of similar size, are mixed to form a compound fertiliser. Compound fertiliser means a fertiliser having a declarable content of at least two of the primary nutrients and obtained chemically or by blending or by a combination of both.

Complex fertiliser.

Multinutrient or compound fertilisers containing two or more nutrients. The term complex fertiliser refers to a compound fertiliser formed by mixing ingredients that react chemically. Complex fertiliser means a compound fertiliser, obtained by chemical reaction, by solution, or in its solid state by granulation, having a declarable content of at least two of the primary nutrients; in its solid state each granule contains all the nutrients in their declared composition.

Critical relative humidity (CRH)

CRH is the ambient relative humidity below which a material loses moisture to the atmosphere and above which the material absorbs moisture from the atmosphere. The CRH of a material varies with temperature.

Dry basis material calculations.

‘Dry basis’ calculations are made on the assumption that the materials used in the product formulation are completely dry and thus free of water.. Although completely dry materials are rarely encountered, the use of dry basis values simplifies the formulation calculation procedure. Once the dry basis values are known, they can be adjusted to reflect the actual moisture conditions.

Formulation.

‘Formulation’ is the process of performing mathematical calculations to arrive at the desired  mixture of raw materials – one that yields the desired nutrient content in the final product in a specific production process. A table showing the weight proportions of materials, which are required to produce a given granular product, is called a formula. Formulation includes the evaluation of alternative materials and their concentrations and considers the chemical reactions to produce a targeted granular product. It also reflects on the physical and quality requirements of raw materials and desired products.

Heat of reaction.

This describes the amount of heat resulting from the chemical reactions between two or more materials used to obtain the desired formulation. Some phase transitions are endothermic and may need to be compensated in the heat duty to e.g. drier. The total heat of reaction obtained by reacting ammonia with sulphuric, phosphoric, and nitric acids, for a specific formulation is often too high. Consequently,  certain amounts of already reacted ammonia (in the form of ammonium nitrate, ammonium sulphate, and/or ammonium phosphate) are used in the formulation. Some typical heats for ammonia reactions are shown in Table 1.

Liquid phase.

‘Liquid phase’ refers to the fluidity or plasticity that the materials are exposed to  during particulation, for example, granulation. The liquid phase of a given formulation depends primarily upon the solubility characteristics of the raw materials and the heat of reaction generated by the chemical reactions according to a given recipe

The liquid phase and finely divided (powdered) solids provide the plasticity and glue to promote good granulation and granule strength.

Material texture.

‘Texture’ generally refers to the physical properties of a material such as particle size, porosity, hardness, and other physical and mineralogical characteristics that affect the granulation properties of a material or mixture of materials.

Material compatibility.

The compatibility of materials is determined by the chemical and physical properties of each material. Compatibility of materials is an important aspect of fertilizers. Compatibility often focusses on the quality and safety of final products. It is an important concept and many organizations have published compatibility tables which one should duly consider.

Mole ratio

The mole ratio is the ratio between any two substances in a chemical reaction. It is the ratio between two coefficients in a balanced chemical equation and is used as a conversion factor between products and reactants. The coefficients are the numbers in front of the formula.

Particulation.

Particulation describes the industrial processes to produce solid particles out of various starting raw materials that can be solids, melts or solutions.  Particulation processes combine various raw materials into new particles that preferably have a uniform composition and shape.

Particulation processes.

Particulation processes comprise granulation, prilling, compaction and pastillation. These processes are described and explained in more detail elsewhere in the resource.

Pastillation

Relatively recently the technique of pastillation has been developed and applied in industrial-scale installations. Pastillation as a particulation technology has now been established for urea, ammonium nitrate and NPK formulations.

With the pastillation technique, a hot melt or slurry is pressed through a drop former onto a circulating cooled conveyor belt. Upon cooling on the belt, the particles, pastilles or tablets, are removed and can be packaged. The technique needs no (or very little) air to particulate the melt and needs no additional equipment for cooling. A number of variations and modifications, depending on the individual product involved, are known. The technique and its advantages are described in detail in the relevant literature (Brouwer, 2011; Swamy, Nanz and Kurt, 2012; Schromm and Kleinhans, 2016).

Pelletising.

Pelletising is the process of compressing or molding a material into the shape of a pellet. A wide range of different materials are pelletized including chemicals, iron ore, animal compound feed. Pelletizing is a method of agglomeration, or particle size enlargement, in which material fines are processed into pellets or granules. Pelletizing is used throughout a multitude of industries to process thousands of materials from difficult to handle powders and fines, into easy to handle pellets.

Raw Materials.

The term ‘raw materials’ refers to all of the materials that serve as input to the finished granular product. Steam and water added to promote granulation is not considered to be raw materials. However, such granulation aids need to be considered as energy input and cost.

Recipe.

Recipe is the formulation of raw materials for obtaining the desired product with its declared nutrient content. The recipe may include processing conditions and the needed energy requirements.

Recycle ratio.

In a continuous particulation process, the materials fed to the process normally pass through the equipment (granulator, dryer, cooler, screens, crushers) several times before obtaining the desired size and before being discharged as finished product.  A production plant producing 50 mtph of final product, at an internal recycle rate of 100 mtph, would be operating at a recycle-to-product ratio of 100:50 mtph or 2. Likewise, if the internal recycle rate increased to 125 mtph and the product rate remained steady at 50 mtph, the recycle-to-product ratio would be 125:50 mtph or 2.5. Most granulation plants are designed on the basis of a given recycle-to-product ratio. For example, a plant designed to produce 50 mtph of product at a recycle-to-product ratio of 2 (100 mtph recycle) would have the equipment designed to handle at least 130 mtph with perhaps a 10-30 mtph safety factor. A diagram showing the impact of recycle-to-product ratios on the net production rate is shown in Figure 1.

The recycle ratio is an important parameter for describing a particulation process. Low recycle ratios are preferred, simply because the production rate becomes higher. High recycle ratios can be necessary if melts or slurries contain much moisture – at the expense of the production rate.
Several definitions of recycle ratios do exist and care is needed if describing and discussing such ratios.

Reversion of phosphate water solubility.

‘Reversion’ refers to the conversion of water-soluble phosphate (monocalcium phosphate) into less soluble phosphate forms (di- and tricalcium phosphate) by the reaction of ammonia with superphosphate. Similarly, in nitrophosphate processes during the neutralisation (ammoniation) water soluble phosphate may turn into citrate soluble (but not water soluble) phosphate in the presence of calcium ions. An example of the conversion (reversion) of water-soluble superphosphate to the water-insoluble (but citrate soluble) form is shown in

Sintering or thermal agglomeration.

This is particulation by means of a heat treatment. Generally, strengthening and induration takes place through the bonding of particles by molecular or atomic attraction in the solid state, thus at a temperature below the melting point. Solidification can be the result of the drying of concentrated slurries and pastes and the cooling of melts.

Size distribution.

Size distribution refers to the distribution of particle sizes within a final product or raw material. Usually, the preferred size of particles is between 2 and 5 mm for most NPK formulations. The actual distribution of particles in this range may follow a Gaussian curve. An average particle size can be calculated, often expressed as d₅₀.

Size enlargement.

Basic mechanisms for size enlargement are agglomeration and accretion. Size enlargement is beneficial for the use of fine materials either downstream or as a final product agglomerate. The aim of particulation and size enlargement is to improve one or many fertilizer product characteristics such as flowability, particle strength, solubility, reactivity, segregation behaviour, size distribution, shape, spreadibility and appearance

Slurries and melts.

‘Slurry’ refers to the concentrated mixture of dissolved and suspended solids in water (usually ammonium phosphate, ammonium sulphate, urea or ammonium nitrate). Pumps are normally used to transfer slurries, and it is usually necessary to elevate and maintain the temperature of the slurry to prevent solidification of the highly concentrated mixture.

‘Melt’ refers to relatively anhydrous molten materials. A slurry at 150°C containing about 5% water would be normally referred to as a ‘melt’, although the term ‘slurry’ would also be possible. The handling characteristics and precautions used for fertiliser melts are basically the same as those used for slurries except that the temperature must be maintained at a higher level, usually through steam tracing.

Supplementary heat.

The heat that is deliberately added to a formula to obtain good granulation characteristics is called supplementary heat. The most common source of supplementary heat is steam, although sometimes hot air or water is added in order to improve granulation.

Thixotropic mixtures

Thixotropic mixtures show a sudden change in viscosity; sudden high viscosities may arise. This can happen during the ammoniation of mineral acids with ammonia in the presence of, for example, calcium salts. In the ammoniation of calcium phosphate / phosphoric / nitric acid mixtures thixotropic behaviour is much dependent on the pH value. It is a phenomenon that causes an occasional headache to fertilizer production operators.

Wet basis or as-received material calculations.

Calculations using ‘wet basis’ (also called ‘as received’) data are necessary to determine the actual amount of material that must be weighed or otherwise metered to obtain a predetermined product analysis. However, after processing (drying), the finished product weight and analysis will be equal to the dry basis material values except for the amount of moisture that is intentionally left in the finished product.

References

Brouwer, M. (2011). A single solution to four challenges. Nitrogen + Syngas, 313, September – October 2011, 53–57.

Kiiski, H. (2016a5a). (ed) Fertiliser Granulation, Ullmann’s Encyclopaedia of Industrial Chemistry, John Wiley and Sons.

Kiiski, H. (2016b5b). (ed) Fertiliser General, Ullmann’s Encyclopaedia of Industrial Chemistry, John Wiley and Sons.

Pietsch, W. (1991). Size enlargement by agglomeration, John Wiley & Sons, Chichester, p 532.

Rumpf, H. (1958). ‘Basic principles and methods of granulation,’ Chem. Ing. Technik, 30,144-158.

Schromm, H.-K. and Kleinhans, M. (2016). Method for producing tablets containing ammonium nitrate. US 9366485, 14.06.2016.

Swamy, K., Nanz, U. and Kurt, H. (2012). Fertilizer finishing process for AN and NPK’s with Rotoform HS 2000. Nitrogen & Syngas Conference, Athens, Greece, 2012.

Links to related IFS Proceedings

2, (1947), Granulation of Phosphatic Fertiliser – Theory and Practice,
S Nordengren.

47, (1957), Developments in Granulation Techniques, A T Brook .

59, (1960), New Developments in Granulation Techniques, P J van den Berg, G Hallie.

102, (1968), Production of High Nitrogen NPK Granular Fertilisers,
W C Weber, I S Mangat .

119, (1970), Control of Fertiliser Granulation Plants,
J A Bland, J Hawksley, W Perkins.

310, (1991). Applied Rule-Based Control of CAN/NPK Granulation,
K Sorth, P B Olsen, F Larsen, N K Larsen, N.K. .

450, (2000), Self Sustaining Decomposition of NPK Fertilisers Containing Ammonium Nitrate, H Kiiski.

535, (2004), Development and Use of Computer Simulation of Fertiliser Granulation, Vonk, P. and Schaafsma, S.H.

Links to external resources

Anon, (2006). Guidance for the compatibility of Fertiliser Blending Materials, EFMA.

De la Vega, J.R.L. (2014). ‘NPK Production alternatives’, Phosphate Fertiliser Production Technology Workshop, Berlin, Germany, October 5- 9.

Hignett, T.P., (ed.) (1985). Fertiliser Manual, Martinus Nijhoff/Dr W. Junk Publishers, Dordrecht, The Netherlands.

Kiiski, H. (2009). ‘Granulation of AN-Based Fertilisers,’ Advanced Fertiliser Production Technology Workshop, Rabat, Marocco, October 18-22.

ULLMANN’S Encyclopedia of Industrial Chemistry. Publishers J Wiley & Son

Schwehr, E.W. (1955). ‘Loss of Water Soluble Phosphates during Granulation of Superphosphates,’ ISMA Technical Conference, Aarhus, Denmark, 19-22 September.