The production technologies available for compound NP(K) fertilisers vary widely and many company- and site-specific variations have been developed. Over time most of these options have been described in literature and the reader is advised to consult this wealth of information. The options applicable are often set by the local conditions and limitations.

The most important relevant aspects and technology options are described on this page.

1. Agglomeration
2. Accretion
3. Heat of chemical reaction
4. Binders
5. Neutralisation methods
   1. Direct neutralisation in granulator
   2. Tank-type neutralisers
   3. Pipe reactors
6. Self-sustaining decomposition of certain NPK formulations
7. Particulation: drum granulation, prilling, compaction, pastillation

Agglomeration

With most granular NPK products (excluding 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 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 preneutralised ammonium phosphate slurry and/or the dissolution of salts on the moist surface of the soluble solid particles. 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 / sprayed onto a solid particle causing it to grow in size. The slurry-type granulation processes used to produce DAP, MAP, TSP, and some nitrophosphate 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. Layer upon layer of new material is applied to a particle, giving the final granule an “onion-skin”-like structure. 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.

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. 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.

Heat of chemical reaction

The level of liquid phase is 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 fertiliser 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 / estimated total liquid phase for a formulation, 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 most important heat-generating chemical reaction in most NPK granulation plants is the neutralisation of acidic materials with ammonia inside the granulator. The approximate net amount of heat released when ammonia reacts with some common acids and other fertiliser materials is known. 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. 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.

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 fertiliser 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 about 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.

Neutralisation methods

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 is given.

Direct neutralisation in the granulator

This was one of the most common methods used for reacting ammonia in the many NPK granulation plants that were operated worldwide during the 1960s and early 1970s.

Direct neutralisation 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 neutralisation, optimum operation is usually obtained if the amount of ammonia reacted in the granulator does not exceed the equivalent of about 50 kg/ton of product.

In the direct neutralisation process, ammonia is distributed using a perforated pipe / sparger positioned beneath the bed of material in the granulator (drum). Sulphuric acid, if used, is usually also distributed through a sparger beneath the bed of material. Phosphoric acid, if used, is most often sprayed or dribbled on top of the bed of material.

In using sulphuric acid, care should be taken as to the relative positions of the sulphuric acid and ammonia spargers to avoid the unwanted reaction between sulphuric acid and potassium chloride – leading to the formation of hydrochloric acid and, subsequently, ammonium chloride. The dense fume of ammonium chloride is difficult (and costly) to collect in a scrubbing system.

Rotary drum-type granulators are best suited or direct neutralisation 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 pugmill– 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.

Tank-type neutralisers

The use of an atmospheric or pressurised tank-type neutraliser offers maximum flexibility in managing the phosphoric acid / ammonia reactions and obtaining the critical heat/liquid phase criteria needed for good granulation when producing a wide variety of granular NPK grades.

Because the acid/ ammonia reactions are most often only partially completed in these tank-type neutralisers, they are often referred to as “preneutralisers”. Such preneutralisers are commonly used in many ammonium phosphate plants and in many NPK plants.

When a preneutraliser is used, large amounts of acid can be partially reacted with ammonia. The degree of reaction performed in the preneutraliser is determined by a number of factors; however, the most important criterion is the production of a fluid slurry that is easy to transport (pump) to the granulator for uniform distribution onto the rolling bed of material in the granulator.

The fluidity of the preneutralised slurry is maintained through careful control of the NH₃:H₃PO₄ mole ratio, temperature, and free water content of the reacted slurry. The neutralisation reactions that are only partially completed in the preneutraliser are completed in the granulator where additional ammonia is added beneath the rolling bed of material. In some cases, phosphoric acid may be added to the granulator to adjust the NH₃:H3PO₄ mole ratio to achieve optimum granulation.

When sulphuric acid is reacted in combination with phosphoric acid in the preneutraliser, special precautions must be taken to select construction materials that will resist the more corrosive environment caused by the presence of sulphuric acid. Type 316L stainless steel is not a suitable construction material if sulphuric acid is neutralised in combination with phosphoric acid; a more noble alloy is required. The selection of construction material for the preneutraliser system should be made only after careful testing of possible construction materials under actual operating conditions.

If sulphuric acid is used, and depending on mole ratio and pH, the formation of ammonium sulphate crystals may thicken the slurry and make pumping difficult.

Pipe reactors

In the early 1970s, the Tennessee Valley Authority (TVA) demonstrated the feasibility of replacing a conventional tank-type preneutraliser with a novel device referred to as a pipe or “tee” reactor. This type of reactor was a radical departure from the conventional tank-type preneutraliser normally used to react large amounts of ammonia with phosphoric acid. The pipe reactor consists basically of a length of corrosion-resistant pipe (about 5 15 m long), to which phosphoric acid, ammonia, and often water are simultaneously added to one end through a piping configuration resembling a tee, thus the name “tee reactor.” The acid and ammonia react quite violently, pressurising the unit and causing the superheated mixture of ammonium phosphate slurry (“melt”) and water vapour to forcefully discharge from the opposite end of the pipe that is positioned inside the granulator.

Uniform distribution of the “melt” on top of the rolling bed of material in the granulator is achieved  by varying the configuration and orientation of the discharge opening(s) of the pipe.

A primary advantage of the pipe reactor over conventional preneutralisers is that the preneutralised slurry pump and piping system is eliminated and a much more concentrated slurry can be delivered directly to the granulator. The chemical heat of reaction is more effectively used to evaporate unwanted water from the process compared with the operation of a conventional preneutraliser. The tee reactor was modified by TVA to also accept an additional flow of sulphuric acid through another pipe inlet located opposite the phosphoric acid inlet, giving the unit a “cross” configuration and thus the name “pipe cross reactor”.

Use of the pipe cross reactor makes it possible to react a wide variety of phosphoric/sulphuric acid mixtures with ammonia. This capability is particularly useful in NPK granulation plants and allows a greater choice in the selection of raw materials to improve granulation and optimise the overall cost of production. In addition, the problem of ammonium chloride fumes is eliminated because the sulphuric acid does not come in direct contact with the potassium chloride that may be in the formulation.

In general, the mixture discharged from the pipe cross reactor does not require further reaction with ammonia in the granulator. ln some cases, however, the level of reaction in the pipe cross reactor may be altered (decreased) to minimise the escape of ammonia or to obtain improved granulation characteristics of the “melt” when it is combined with the solids in the granulator.

Several variations of pipe-type reactors (and materials of construction) are currently used in NPK, DAP, and MAP plants; sometimes the pipe-type reactor is used in combination with a conventional tank-type preneutraliser.

Perhaps one of the greatest advantages offered by the use of pipe reactor technology in the NPK industry is that it provides an opportunity to effectively use a greater variety of raw materials including, for example, larger quantities of dilute acids and scrubber liquor. This added flexibility in raw material choices can often result in more favourable production costs and at the same time provide a method for disposing of, for example, excess scrubber liquor. However, the technology does not fit all situations equally well. Therefore, its potential should be carefully examined with regard to the individual circumstances.

Self-sustaining decomposition

Certain NPK formulations, in particular those that contain significant amounts of ammonium nitrate, may undergo a phenomenon known as self-sustaining decomposition. This phenomenon is also described under the section for ammonium nitrate.

Compound fertilisers containing ammonium nitrate may be subject to propagating decomposition called “cigar burning,” when ignited.

Once ignited the decomposition propagates through the mass of material at rates of 5 – 50 cm per hour and with a temperature in the decomposition zone of 300-500°C, often producing red-brown hazardous fumes. Compound fertilisers containing ammonium nitrate, having a content of 4% or more of KCl, are susceptible to this phenomenon.

The reaction may be inhibited or retarded by the presence of ammonium phosphates; therefore, NPK grades containing ammonium nitrate and ammonium phosphate may be less liable to this hazard.

In case of a self-sustaining decomposition its characteristics, for example the speed of propagation, the temperature in the decomposition zone and the amount of gas produced, depend on the composition of the fertiliser and on the extent of melting. The presence of compounds of trace elements such as copper salty and impurities such as chromium salts can increase this decomposition.

The speed of propagation (as well as the temperature in the decomposition zone) can be assessed by the so-called trough test.

With this type of fertiliser, the bulk form of handling presents a greater risk than does packaged handling due to the relatively greater ease of initiation from exposure to heat sources and the ability to propagate the decomposition throughout the heap. Minor heat sources such as a buried inspection lamp or self-heating resulting from contamination can be sufficient for the initiation of the decomposition.

Particulation: drum granulation, prilling, compaction, pastillation

Particulation can be achieved using various devices. The granulating devices used most often in the compound fertiliser industry are drums, pans, rollers, and pug mills.

In some selected cases pastilles are produced.

Recently the rather new 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

Fluid-bed granulation has become popular for the production of straight fertilisers such as urea and ammonium nitrate.

A modification of the drum granulation process is the spray-drum process, the spherodiser. In a rotating drum, preneutralised slurry is sprayed onto a dense curtain of granules cascading from baffles inside the drum. The water content of the slurry usually is in the order of 12–18% to allow good spray dispersion. During granulation, hot combustion gases flow through the drum in a co-current fashion, so drying of the granules takes place simultaneously.

The dried particles are then sprayed upon again. The grains grow in shell fashion with an onion structure and are very hard and round.

Compaction is used for the particulation of raw materials, for example potassium chloride, as well as of NPK formulations.

Compaction is a granulation technique that can be realised without the use of melts or slurries. In many operations, a mixture of solids is pressed between rollers and the resulting pressed cake is broken, screened, and treated, as and if necessary. Many raw materials can be included and ammonium sulphate may thereby serve as an easily available source of ammonium-N. A small amount of liquid is sometimes used to improve the adhesion between the raw materials are sometimes used. A rotating drum may be employed to round-off the irregular particles.

Prilling is used in particular for straight fertilisers but also for NP and NPK formulations.

In the 1960s, prilling was a favoured technology for a substantial range of fertilisers, including urea, AN, CAN, NP, NPK. Droplets of a liquid mass at high temperature are sprayed down a tower against a counter-current flow of mostly ambient air. Particles (prills) are formed during the cooling and drying of the droplets. The height of the prilling tower is a major factor in determining the size and size distribution of the prills. Only tall towers enable a mean prill diameter of above 2.5 mm to be obtained. Moreover, the water concentration of the sprayed melt or slurry needs to be low, usually < 2%, but in most cases preferably < 1.0%. With increasing amounts of insoluble components, for example in NPK or ASN slurries, prilling becomes more difficult and is no longer a possible or preferred technology. The low water concentrations at high melt / slurry temperatures have implications for safety and emissions.

Links to related IFS Proceedings

34, (1955), Compound Fertiliser Formulation, R Stewart.

49, (1958), Gaseous Effluents from Granulation Plants, F J Harris.

56, (1959), Concentration (of fertilisers)  – First Francis New Memorial Lecture, H U Cunningham.

61, (1960), Rotary Coolers and Driers – Some Related Aspects of Design, S J Porter, W G Masson.

91, (1966), NPK Fertiliser Production Using Superphosphoric Acid, G Bischofberger, R R Heck.

109, (1969), Prilling of Compound Fertilisers, F E Steenwinkel, J W Hoogendonk.

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

141, (1974), Solids Handling and Metering in an NPK Prilling Plant, W J Kelly.

146, (1975), Off-Line Data Logging for NPK Plants, I K Watson.

237, (1985), Production of Chloride-Free NPK Fertiliser and Feedgrade Dicalcium Phosphate, K C Knudsen.

243, (1986), Nitrophosphates with Variable Water Solubility: Preparation and Properties, L Diehl, K F Kummer, H Oertel.

244, (1986), Adapting a Pipe Reactor to a Blunger for NPK Production, R J Milborne, D W Philip.

245, (1986), New Diammonium Phosphate Technology – Powdered or Granular DAP, L M Marzo, J L Lopez-Nino.

245, (1986), Dual Pipe Reactor Process for DAP, NP and NPK Production, P Chinal, Y Cotonea, C Debateux, J F Priat.

271, (1988), On-Line Data Logging for NPK Plants, I K Watson, D W Philip.

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

451, (2000), Design of Rotary Driers and their Application in the Fertiliser Industry, I C Kemp, R J Milborne.

509, (2003), Energy Consumption and Greenhouse Gas Emissions in Fertiliser Production, t K Jenssen, G Kongshaug.

725, (2013), Urea-based NPK Granulation – Examination of Constraints and Potential Solutions, S R Doshi.

782, (2016), Granulation Technology with Flexibility to Produce a Range of Specialist Products, N Kargaeva.

783, (2016), Granulation of Complex Fertilisers, H Kiiski and A Kells.

803, 2017), Changes, challenges, and opportunities in fertiliser-manufacturing processes: A personal review and outlook, J G Reuvers.

805, (2019), The Carbon Footprint of Fertiliser Production: Regional Reference Values, A Hoxha, B Christensen.

856, (2021), Progress in Using Artificial Intelligence in Process Control to Reduce Energy Usage, S. Rademakers.

External resources

Fertilizer Manual, edited by the United Nations Industrial Development Organization (UNIDO) and the International Fertilizer Development Center (IFDC), Kluwer Academic Publishers, 1998.

IFA Technical Symposium on Innovation and Core Technologies for Sustainable Growth: Technical Developments in Fertiliser Production for Greater Efficiency and Environmental Stewardship, Vilnius, Lithuania, 25.-28. April, 2006.

The Uhde pugmill granulation: The process for safe and reliable production of CAN and other AN based fertilisers, P. Kamermann