1. Raw Material Particle Size and Surface Characteristics
2. Liquid Phase
3. Estimating Liquid Phase Values
4. Heat of Chemical Reaction
5. Insoluble Binders
6. Liquid phase control
7. Acid/ammonia neutralization methods in the granulator
8. Direct neutralization in granulator
9. Preparing the Production Formula

A discussion of the key physical and chemical parameters required for the production of agglomerated NPK products follows. Parameters specific to the production of most urea-based NPKs and other NPKs that exhibit similar high solubility, temperature sensitivity, and low critical relative humidity (CRH) characteristics are described separately.

Raw Material Particle Size and Surface Characteristics

Because much of the initial (in-process) and final mechanical strength of the agglomerated NPK granule is obtained by the mechanical interlocking, fitting, and cementing of the individual particles, the particle size, size distribution and surface characteristics of the solid raw materials are very important. If the final product is in the size range of 2-5 mm, then the raw materials should be quite widely distributed within a range of about 0.2-3 mm. An example of the particle size distribution of solid materials frequently used in agglomeration-type NPK granulation plants without further size reduction (crushing) is shown in Table 1. It is favourable for the raw materials to have approximately the same size and crushing of larger raw materials can be considered.

Liquid phase

Although mechanical interlocking of the particles within the granule structure must be optimised through selection and/or processing (crushing) of the raw materials (and sometimes recycled material), cementing is also needed to permanently bond the particles into strong agglomerates (granules). The liquid phase is derived from (1) soluble-salt solutions added to the granulator, for example, an ammonium phosphate slurry and/or a solution of urea or ammonium nitrate and (2) dissolution of a small portion of soluble material on the moist surface of the raw material and recycle particles. This dissolution is brought about by the combination of heat and water contained in the above solutions and/or by steam or water fed to the granulator. Solubility data for some of the more common fertiliser salts are given in Table 2. Liquid phase control is the key to achieving the desired level of granulation efficiency and product quality. Ideally, after drying, the liquid phase (salt solution) forms strong crystal bonds (salt bridges) between the particles of the agglomerate that are mechanically well-fitted and interlocked – again, the concept of a stone wall.

Estimating Liquid Phase Values

Experience has shown that there are considerable differences in the amount of liquid phase generated by the various materials normally used in the production of agglomerated NPKs. In the past, during the 1960s, the Tennessee Valley Authority (TVA) examined a wide variety of NPK production formulations that were known to granulate quite well and devised a numerical value to express empirically the “apparent” liquid-phase-contribution that could be expected from a number of materials commonly used to produce NPKs. These “liquid phase factors” are shown in Table 3. Experience has shown that, when these values are used as a guide, a liquid phase of about 300 kg/tonne of product is about optimal for most agglomeration-type NPK formulations. Of course, it should be appreciated that liquid phase, while important in granule formation, is only one of the criteria that must be carefully evaluated when the granulation characteristics of a particular formulation are estimated.

Heat of Chemical Reaction

The level of liquid phase is also closely linked with another criterion, i.e., the expected amount of heat created by various chemical reactions that occur during the granulation of a given NPK formulation. The amount of heat generated, particularly within the granulator, can have a marked  effect on the solubility of the fertilizer materials and the amount of liquid phase formed and, therefore, the resulting granulation characteristics of the mixture. In general, to achieve optimum granulation, the calculated total liquid phase for a formulation, using the data in Table 3, should be lowered if the formulation produces a large amount of chemical heat of reaction in the granulator. However, the optimum relationship between liquid phase and heat of reaction for a specific formulation must be learned from actual operating experience. The aspect of heat formation is of particular relevance if additional ammoniation is carried out in the granulator.

The most important heat-generating chemical reaction in most NPK granulation plants is the neutralization of acidic materials with ammonia inside the granulator. The approximate net amount of heat released when ammonia reacts with some common acids and other fertilizer materials is shown in Table 4. As with liquid phase, experience has shown that if the amount of heat released in the granulator is equivalent to about 45,000-50,000 kcal/tonne of product (not including recycle), conditions are generally favourable for obtaining optimum granulation. Of course, as with liquid phase, the proper level of heat is just another one of the many critical criteria that must be determined by experience and that must be met to obtain optimum granulation efficiency.

Insoluble binders

In some cases, the mechanical and crystal (salt bridge) bonding of particles can be greatly improved by adding a small amount of a finely divided insoluble binder powder, for example, kaolin or attapulgite clay or finely ground phosphate rock, to the granulating mixture. The binder powder helps to fill the many small voids between the fertilizer particles and acts as a saturated wick in helping to join the particles. This concept works particularly well with NPKs that contain large amounts of crystalline ammonium sulphate, potassium chloride, potassium sulphate, and/or kieserite (magnesium sulphate monohydrate) and relatively low levels of highly soluble salts or solid binders such as ammonium phosphate slurry or superphosphate, respectively. Depending upon the characteristics of the binder and the materials being granulated, the amount of binder used may need to be approximately 2-15% of the total formula weight to be effective. It is important to note, however, that some insoluble binders (some clays, for example) have the capacity to retain moisture and thus make subsequent drying more difficult. Finely ground phosphate rock that contains high levels of iron and aluminium impurities is a very effective binder that also adds a primary nutrient, albeit slowly available, to the product.

Liquid phase control

In all NPK granulation formulations, most of the liquid phase is obtained from materials that are introduced to the process at a fixed rate to achieve a final product with the desired chemical analysis. The resulting liquid phase can, of course, be adjusted within rather specific limits through the selection of raw materials or by controlling the free water content and/ or chemical composition of the slurry (for example, the NH₃:H₃PO₄ mole ratio as shown in Figure 1. However, once this is established (optimised), the flow rates of the liquids must remain constant to ensure the correct analysis of the final product. For this reason, all agglomeration-type formulations should allow for a moderate degree of liquid phase “tuning” performed by the operator using steam and/ or water fed directly to the granulator. The discretionary use of a small amount of steam and/or water by the operator helps to compensate for variations in granulation efficiency caused by changes in the temperature of the materials, quantity and particle size of the recycled material, and minor (but normal) upsets within the overall processing system. This fine tuning of the process by the operator is the basis for the observation: “NPK granulation by agglomeration is more of an art than a science.” The unique skill of an experienced granulator operator often greatly overrides the effectiveness of the most costly and sophisticated process design and engineering skills.

Acid/ammonia neutralisation methods in the granulator

As indicated earlier, the acid/ammonia neutralisation reactions create heat that contributes to the overall liquid phase conditions in the granulator and therefore greatly influences the efficiency of the granulation process. The method used for neutralisation (reacting ammonia) can significantly influence the overall performance of the granulation process. A brief. discussion of the most common methods used for neutralisation in agglomeration-type NPK granulation plants follows.

Direct neutralization in the granulator

This was one of the most common methods used for reacting with ammonia in the many NPK granulation plants that were operated in the United States and elsewhere during the 1960s and early 1970s. Direct neutralization in the granulator is particularly well suited for NPK grades containing large amounts of superphosphate (SSP or TSP) and a relatively low level of nitrogen. With direct neutralization, optimum operation is usually obtained, if the amount of ammonia reacted in the granulator does not exceed the equivalent of about 50 kg/tonne of product.

In the direct neutralization process, ammonia is distributed using a perforated pipe (sparger), positioned beneath the bed of maternal in the granulator. If sulfuric acid is used, it too is usually distributed through a sparger beneath the bed of material whereas the phosphoric acid, if used, is most often sprayed or dribbled on top of the bed of material. When sulfuric acid is used, precautions should be taken to ensure that the acid is added at a particular location with respect to the ammonia, this in order to ensure quick and uniform neutralization and thus minimise the unwanted reaction of sulfuric acid with muriate of potash (MOP) that is usually present in most NPK formulations. This acid/MOP reaction causes the formation of very corrosive hydrochloric acid, which reacts rapidly with ammonia to form a dense fume of ammonium chloride that is very difficult and costly to collect in the plant’s emission control (scrubbing) system.

Rotary drum-type granulators are best suited for direct neutralization because the submerged spargers can be more easily positioned beneath the bed of material and the reactions contained within the relatively deep bed of material. When using a pug mill or pan-type granulator, the positioning and effectiveness of submerged spargers are often less than optimum because of the mechanical configuration of the equipment and the relatively shallow bed of material.

The neutralisation / ammoniation reaction can also be performed prior to granulation in a tank-type neutraliser or in a pipe-type reactor . Often the ammoniation is done to a certain extent only and the tanks are referred to as pre-neutralizers.

Preparing the Production Formula

According to the foregoing discussion, a large number of raw material and process variables must be considered when developing NPK production formulations that result in good granulation characteristics. As with the operation of a NPK granulation plant, formulation too, requires a considerable amount of skill and an element of “art” to ensure that the particular formulation will yield the desired results in a specific situation. In addition, the composition as well as the operating conditions will reflect on the quality of the final fertilizer product.

A given NPK fertilizer can be formulated in many ways depending on the available raw materials and specific equipment system. Table 5 shows some examples of NPK production formulations that have been successfully produced in commercial practice, using agglomeration. Because the performance of these formulations will depend heavily upon a number of factors as described herein, these examples are offered only to illustrate the variability that should be considered in the planning and design of an NPK fertilizer granulation plant.

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 recordings

583, (2006), Phase Stabilisation of Ammonium Nitrate Fertilisers,
H Kiiski.

627, (2008), Ammonium Nitrate Fertilisers: Analysis and Appraisal of Classification Categories,
H Kiiski.

783, (2016), Granulation of complex fertilisers,
H Kiiski, A G Kells.

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

Links to external resources

Loste, R., Prats, F. and Pomares, F. (1972). ‘Intensive Ammoniation in the Preparation of Granulated Fertilisers’, ISMA Technical Conference, Seville, Spain, 20-24 November.

IFDC. (1998) Fertilizer Manual