1. Drum granulator
2. Pug mill
3. Spherodizer
4. Pan granulator

Granulation (particulation) equipment is closely connected to the various specific operations involved in a production plant, in particular within the granulation loop. The same specific operations may be provided for a prilling, compaction, or pastillation plant.

Particulation equipment:

• Granulator, which can be, for example, a drum granulator, a pug mill, or a pan granulator.
• Drying equipment, for example, a drying drum or a fluidised bed.
• Screens for filtering out the on-size particles.
• Crushers for crushing oversize and sometimes on-size product.
• Weighing equipment for determining the absolute and relative amounts of oversize, on-size and fines. The determination of the various fractions is relevant if advance process control is installed.
• Bucket (or other type) elevator.

The above specific operations are usually part of the granulation loop. The screened-out on-size product is further treated in additional units, from which a certain amount of product may be returned to the granulation loop:

• Cooling equipment, for example, a cooling drum, a fluidized bed cooler or a bulk flow cooler.
• Coating equipment.
• Final screening equipment and return of off-size to the granulator.
• Transport equipment, for example, conveyor belts, bucket elevators.

Granulation.
The condition for granulation is that a bed of solid particles moves, with simultaneous intensive mixing, in the presence of a liquid phase. This motion provides the particle collisions and bonding needed for granule growth. There are many types and models of granulating equipment, most of which use one of three basic intensive mixing mechanisms (Skauli, 1979):

1.  Rotation of one or more shafts carrying staggered paddles in a fixed trough (pug mill).

2.  Rotation of the whole device, such as drum or pan.

3.  Movement of particles by a third phase, as by blown air in a fluid-bed granulator. In slurry granulation the third phase is usually hot air or hot combustion off-gases, which can serve as a drying medium at the same time. In this way, two processing steps in the granulation loop can be carried out in a single apparatus, for example in a so-called Spherodizer (Polo, 2001).

In order to improve granulation conditions or granule qualities, binding agents can be added along with the granulation liquid. The binding agents may be solid or liquid, may form films, crusts, or crystals, and may harden at standard temperature or at higher temperature (Stokka, 1988).

The granulating devices used most often in the fertiliser industry are drums, pans, and pug mills. Fluid-bed granulation has become popular for the production of straight fertilisers.

Pug mill (Blunger)

The pug mill (also called a blunger) process is still one of the often used options for producing CAN (calcium ammonium nitrate, nitro-limestone, 26-28% N) and in earlier years for NPK fertilisers. One pug mill plant is capable of producing the whole range of N contents from 22-33.5% N. The switching from AN (ammonium nitrate, 33.5% N) to CAN is done quickly. The pug mill process is very tolerant with respect to the filler materials and additives. The addition of additives to achieve desired levels of sulphur or micronutrients is straightforward. These do not have to be added as solutions, but can be fed directly into the granulator. The high solid recycle makes the plant very stable and self-regulatory. All disturbances will balance over time and minor disturbances rarely require operator action. The kneading action of the pug mill produces a hard and uniform product. Scalping screens or a sophisticated seed preparation system is not required. With very few exceptions easily obtainable standard equipment is used in the plant design. The amount of proprietary equipment is very low, spare parts can normally be obtained or produced locally. The electrical power consumption is lower than for other granulation processes. The total plant costs are comparable or even lower than for other granulation processes (Kamermann, 2006).

However it should be borne in mind that dust emissions in the plant can be excessive and very high capacities (> 1.800 t/d) are better achieved for CAN in a drum granulator. In practice pug mills are mostly used for single nitrogen production, less so for the production of compound NPK products. Switching a pug mill over from AN / CAN to NPK products is cumbersome.

A pug mill (Figure 1) consists of a U-shaped trough and, inside the trough, one or two shafts bearing strong paddles staggered in a screw-thread fashion. Two-shaft pug mills are by far the most frequently used in industry (Anon, 1995). The shafts rotate at equal speeds in opposite directions in the horizontal or slightly inclined trough. The solid particles (fresh feed plus recycle) are fed in at one end of the trough and are thrown up in the middle of the trough, where they are wetted with the granulation liquid. In the trough, the paddles move, knead, and transport the moistened particles toward the discharge end. The particles can be given a better-rounded external shape either in a downstream tumbling drum or in the feed zone of the drying drum. Placement of a perforated NH₃ inlet pipe (or sparger) at the bottom of the trough makes it possible to ammoniate and agglomerate the fertiliser at the same time. The pug mill is sturdy and can adapt to variable service conditions; it produces hard granules of uniform composition. If the angle between the paddles and the shafts is optimised, the energy consumption can be reduced. The paddles are usually provided with a wear-resistant coating to prevent abrasion. Processes have been described for granulating in a pug mill an ammonium phosphate melt (12–57–0), with urea (28–28–0) (Lee, 1974), and also with KCl (19–19–19) (Anon, 1970a). The combination of a pipe reactor with a pug mill for the granulation of NPK has been reported (Milborne, 1986).

Drum granulator

The drum granulator (Figure 2), which is the type of granulator in widest use for fertilisers, in particular for NPK, is an inclined rotating cylinder.

Various features of such a drum are relevant for a good and efficient granulation effect:

• rotational speed,
• inclination,
• length to diameter ratio,
• type, direction of spray, and number of spray nozzles,
• distribution elements (vane, paddle, baffle, bucket),
• lifting flights,
• light flights for rounding the particles,
• the degree of filling of the drum, usually controlled by an internal ring separating the spraying and tumbling sections,
• possible further internal rings or dams control the degree of filling and therewith the residence time of a particle in the drum,
• hammers (or equivalent methods like balls moving up and down in a cylindrical shaft) on the outside of the rotating drum will remove (part of) the caking on the inside drum walls,
• proper flights at the exit end of the drum for removing the product into transport equipment,
• recycle ratio,
• air flow and air condition (amount, speed, temperature and humidity of the air).
• The rotation speed is usually adjustable. For a given drum and a given granular product, there is an optimal peripheral speed that gives the highest yield of granules.

An inclination of up to 10º from the horizontal ensures adequate movement of product toward the discharge end. Because, however, this inclination is not enough to effect classification, the discharged product has a fairly broad grain-size distribution, in contrast to a pan-granulator product.

Drums in which the lengthwise axis is inclined upward from the feed end to the discharge (Heinze, 1986) give a narrower particle-size spectrum. Such an upward inclination also increases the amount of material in the drum. Few such inclined granulation drums are operational for fertilizers.

In drums currently used in the fertiliser industry, the length-to-diameter ratio is ≥1 and may reach 6 : 1. The feed end may have empirically designed distributing elements on the inside wall to spread the feed material. In the adjacent part of the drum, where the granulation liquid is fed in, a good tumbling motion should be ensured. This can be achieved with light lifting flights, but the proper degree of lifting needs to be considered (often experimentally). In the remainder of the drum, the pre-granulated material should be tumbled to a round shape and further compacted. This is also achieved with ‘light flights’ on the drum wall and appropriate drum filling. The filling in the spray and tumbling areas can be controlled by means of internal ring dams. The cylinder may be either open ended or fitted with ring dams at the ends (Perry, 1973), to prevent overflowing at the feed end and to control the bed depth and thus residence time. Fixed or movable scrapers inside the drum, or hammers, or other rapping devices outside on the drum can be used to remove or reduce excessive product caking inside the drum. Some material build-up on the drum wall may promote granulation (Hardesty, 1955). Cylindrical drums are used for continuous granulation with and without internals.

As in the case of the pug mill, recycled product (undersize and crushed oversize, if needed even (crushed) on-size material) generates a moving bed of material in the drum; a slurry containing, say, 3–8% water can be sprayed onto the bed (Schmidt, 1972).

Powdered feed materials (mixed and wetted in an upstream mixer, if necessary, to provide granule nuclei) can be granulated in the drum through spraying with water, solutions, suspensions, and highly concentrated slurries, or through blowing with steam. The bed volume should be 20–30% of the cylinder capacity (Nielsson, 1987). The recycle ratios for drum granulation are generally between 1:3 and 1:6. Optimisation of these plant parameters for each product class is done by trial and error.

The drum granules are more rounded but less dense than the pug-mill granules. Drums 4.5 m in diameter and 16 m long are in use in the fertiliser industry.

TVA ammoniator-granulator
An important modification of drum granulation is the Tennessee Valley Authority (TVA) ammoniator – granulator (Figure 2).

This is a drum roughly equal in length and diameter, with ring dams at the ends but no lifting flights. Ammonia reacts with phosphoric and sulphuric acids below the surface of the tumbling bed of fresh feed and recycle. The reaction generates heat, which vaporises the water at the same time that granulation takes place. The heat is removed by injected air. The ammonia and the acids are supplied to the bed through perforated distribution pipes mounted parallel to the drum axis. The requisite bed depth is maintained by the ring dam at the drum discharge end.

In a modern process, a mixture of phosphoric and sulphuric acids and ammonia is neutralised in a pipe-cross reactor situated upstream of the granulating drum (Figure 3). The slurry is then fed to the drum along with recycle. While additional phosphoric acid is sprayed onto the tumbling bed, ammonia is fed into the bed for further neutralization. In this way, also NPK fertilisers can be produced (Anon, 1976).

In a specific variation, in the SA CROS process for monoammonium phosphate production, phosphoric acid and ammonia are mixed and reacted in a pipe reactor (Anon, 1977); the obtained slurry is distributed over the tumbling bed together with the steam generated; no subsequent ammoniation takes place in the bed (More, 1977). The use of the pipe reactor in combination with the granulating drum for the manufacture of granular ammonium phosphates was introduced by TVA in 1973 (Anon, 1976) and was later incorporated in many plants (Anon, 1986b). A possible improvement to the drum granulator is provided by the double-pipe granulator, which is especially well-suited to fertiliser mixtures with a high proportion of recycle. For example, by virtue of the high recycle ratio with corresponding residence times, an ammonium nitrate – ammonium phosphate mixture can be granulated which yields hard particles.

Spherodizer

Another modification of the drum granulation process described above is the spray-drum process (spherodiser, Figure 3). In a rotating drum, pre-neutralised 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 range of  12–18% to allow for good spray dispersion (Schmidt, 1972), depends on the quantity of additional solids added, and is often determined by trial and error. During granulation, hot combustion gases flow through the drum in co-current manner (Anon, 1987), so that drying takes place at the same time. The dried particles will pass through the granulation loop over and over until they obtain the desired size and are screened out. The grains grow in shell fashion with an onion structure and are very hard. Spherodiser units are built in capacities of up to 650 t/d. A spherodizer drum can have a diameter of 4.5 m and a length of 12 m (Schmidt, 1972). The spherodiser, developed by C & I Girdler (More, 1977), was first used on an industrial scale in 1959. The cold and hot versions of the spherodiser describe the condition of the air that flows through the drum. The cold version is used with melt feeds, especially ammonium nitrate and urea, while the hot version is for granulation and spraying with slurries (NPK fertilisers, nitrophosphates and ammonium phosphate – nitrate, urea – ammonium phosphate) (Anon, 1975; Anon, 1985). Granulation and (partial) drying thus take place in the same device. Under optimal service conditions, the recycle ratio is approximately 1 : 1. In many plants, however, an additional drier is installed.

In summary granulation is achieved through proper proportioning and controlled mixing of the liquid phase with dry material. Capillary action and surface tension hold the particles together in agglomerates when mechanical tumbling in the rotating cylinder brings them into contact. Continued mechanical contact strengthens the bond and forms the agglomerate into a more spherical configuration. The liquid phase, which is significantly affected by temperature, must be controlled by the operator to attain optimum granulation. The formulator must choose his raw materials very carefully, to provide the best combination of soluble salts to achieve strongly bonded granules (de la Vega, 2001).

Pan granulator

Dry recycled material is fed at a controlled rate to the inclined rotating pan granulator; in the granulator a hot melt, which is virtually moisture free, is sprayed onto the moving bed of solids and solidifies on the cool particles. Round granules are formed by agglomeration, and, as their size increases, they move upwards in the rotating pan, finally rolling over the rim. For a given pan size, if the inclination of the pan axis to the horizontal is increased, the granules roll upward less steeply but have a longer residence time in the pan. The granulation nuclei and the small granules initially move in the vicinity of the pan bottom. During granulation, the rotation of the pan and the force of gravity cause them to follow a spiral path. The tumbling motion of granules during agglomeration can also take place on a rotating inclined pan (Figure 4, lower).

Melts or slurries are often sprayed onto the bed, but water or solutions can also be used as granulation aids, and steam can be injected into the bed. Experience has shown that, the finer the solids, the more finely the spray liquid must be dispersed, for granulation (Klatt, 1958). Because the overflow product has a rather uniform grain size, downstream classification can often be dispensed with. By means of an advancing and retreating scraper blade, the pan bottom can be kept fairly clean and the formation of crusts can be avoided. Here, as in the drum, some material coating of the pan prevents wear and promotes the correct tumbling action (Perry, 1973). The pan can also be made in the shape of a truncated cone, or can have a tumbling ring at its periphery, onto which the granules fall from the pan rim. Coating agents can be applied. Pan granulators are manufactured with diameters of up to 7.5 m (Anon, 1985). Typically, the height of the rim is one-fifth of the diameter.

The granulation temperature is controlled by the rate at which solids are fed to the pan. An optimum temperature range for agglomeration, within which a high growth rate of the particles is obtained, is 5 – 25 °C below the temperature at which the fertiliser melt solidifies. The recycle ratio under these conditions is about 0.5 -0.7 : 1 in case of ammonium nitrate. Granules leaving the pan are plastic and have a somewhat irregular surface. They enter a polishing drum where they are exposed to mild mechanical forces and smoothed. A certain amount of cooling also occurs. Cooling to the desired product temperature is then performed in normal cooling equipment, such as a fluidised bed or a rotary drum. Depending on the climatic conditions at the plant site and the desired product temperature, the cooling air may be conditioned. Cooled granules are conveyed to a screen. Oversize material from the screen is fed to a crusher and the crushed material, undersize granules, and dust from the cyclones, are recycled to the pan.

Because of the high melt concentration and temperature, a ‘blue fume’ of submicron ammonium nitrate – similar to the fume during prilling of high-density ammonium nitrate – has to be recovered for treatment. However, the air flow from the pan and polishing drum is relatively small. A wet scrubber may be used for recovery; this is not usually practical in a prilling process with a high rate of air flow from the top of the prilling tower. The air from the product cooler is treated in wet or dry cyclones.

For the production of granular triple superphosphate, phosphoric acid is added to digest finely milled crude phosphate in a granulator – mixer; this step yields a moist, crumbly product, which is directly processed in a subsequent pan to the required pellet size with the injection of steam and the addition of hot phosphoric acid (Molerus, 1981; Ries, 1970).

Granulators, pug mills, drums and pans, usually require subsequent drying of the granules by means of, for example, rotary driers or fluidized beds. Often granulation continues in the co-current driers due to an increase in temperature while the water has not yet evaporated (Leger, 1953, Mutsers, 1983).

References

Anon, (1970a). New Developments in Fertiliser Technology, 8th Demonstration, Tennessee Valley Authority, Muscle Shoals, Ala., pp. 41–47.

Anon, (1975). Phosphorus Potassium, no. 76, 48–54.

Anon, (1976). New Developments in Fertiliser Technology, 11th Demonstration, Tennessee Valley Authority, Muscle Shoals, Ala., pp. 33–35.

Anon, (1977). ‘The Cross Fertiliser Granulation Process’, Phosphorus Potassium, no. 87, 33–36.

Anon, (1986). Phosphorus Potassium, no. 144, 27–33.

Anon, (1987). Nitrogen, no. 166, 39.

Anon, (1995a). Best Available Techniques for Pollution Prevention and Control in the European Fertiliser Industry, Booklet No 7 of 8; Production of NPK Fertilisers by the Nitrophosphate route, EFMA.

Anon, (1995b). Best Available Techniques for Pollution Prevention and Control in the European Fertiliser Industry, Booklet No 8 of 8; Production of NPK Fertilisers by the Mixed acid route, EFMA.

de la Vega, J.R.L. (2001). ‘Fundamentals of Granulation’. Prepared by IFDC, Advanced Fertiliser Production Technology Workshop, Corsendonk, Turnhout, Belgium, October 15-19.

Hawksley, J.L. (1971). ‘Some Advances in Compound Fertiliser Process Technology,’ Chem. Eng. (London) October, P.P. 364-369.

Heinze, G. (1986). ‘Pelletizing of carbon black,’ Chem. Tech. Heidelberg 15, no. 6, 16, 18, 21.

Klatt, H. (1958). Zem. Kalk Gips 11, no. 4, 144–154.

Lee, R.G., Norton, M.M. and Graham, H.G. (1974). Proc. Annu. Meet. Fert. Ind. Round Table, 24th, 79.

Leger, E.J. (1953). ‘Study of Granulation by Means of a Rotating Drier with Practical Application to the Designing of an Apparatus and Production Control,’ ISMA Technical Conference, Cambridge, United Kingdom, 15-17 September.

Milborne, R.J. and Philip, D.W. (1986). ‘Adapting a Pipe Reactor to a Blunger for NPK Production,’ Proceedings, International Fertiliser Society, 244.

Molerus, O. (1981). ‘3rd International Symposium on Agglomeration’, Part. Tech., Nürnberg, G2- G15.

Molerus, O. (1981). ‘3rd International Symposium on Agglomeration,’ Partikel Technologie Nürnberg, F2- F15.

More, A.J. (ed.) (1977). Granular Fertilisers and Their Production, The British Sulphur Corp., London.

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Perry, R.H. and Chilton, C.H. (1973). Chemical Engineers’ Handbook, 5th ed., McGraw-Hill, New York, pp. 57–65.

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.

Ries, H.B. (1970). ‘Apparatus and process for granulation,’ Aufbereit., Tech. 11, 262–280.

Schmidt, A. (1972). Chemie und Technologie der Düngemittelherstellung, Hüthig-Verlag, Heidelberg.

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Links to related IFS Proceedings and presentation recordings

215, (1983), Computer Simulation of Fertiliser Granulation Plants , S M P Mutsers, H J M Slangen, H.J. Rutten, I K Watson .

235, (1985), Fluid Bed Granulation of Ammonium Nitrate and Calcium Ammonium Nitrate, J P Bruynseels.

415, (1998), Fluid Drum Granulation for Ammonium Nitrate,
L Dall’Aglio.

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

767, (2015), Update on Fluidised Drum Granulation Technology and its Applicability for Different Fertilisers, S Valkov.

Recording of ‘Update on Fluidised Drum Granulation Technology and its Applicability for Different Fertilisers ‘, (2015), S Valkov (Required password is 2015Tech05)

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

Recording of ‘ Granulation Technology with Flexibility to produce a Range of Specialist Products‘, (2016), N. Kargaeva (Required password is 2016Tech04)

820, (2018), Fluidised bed granulation of ammonium sulphate – a new process,  C. Renk, P. Banik.

Links to external resources

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Davis, Ch., Meline, R.S. and Graham, H.G. (1968). ‘TVA mixed acid nitric phosphate process,’ Chem. Eng. Prog. 64, no. 5, 75–82.

Eimers, J. (1981). ‘Granuliertechniken bei der Düngemittelherstellung,’ paper presented at the congress AGRICHEM 81, Bratislava.

Formisani, C. (2006). ‘Modern Regional Granulation in Eastern Europe,’ 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.

Franzrahe, H. and Niehus, P. (2002). ‘Pugmill Granulation: A State of the Art Process for CAN and Other Ammonium Nitrate Based Fertilisers,’ IFA Technical Conference, Chennai, India, 24-27 September.

Hawksley, J.L. (1971). ‘Some Advances in Compound Fertiliser Process Technology,’ Chem. Eng. (London) October, P.P. 364-369.

Hicks, G.C., McCamy, I.W., Parker, B.R. and Norton, M.M. (1978). ‘Basis for Selection of Granulators at TVA,’ paper presented at the Proc. Annu. Meet. Fert. Ind. Round Table, 28th.

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Kamermann, P. (2006). ‘The Uhde Pugmill Granulation: The Process for Safe and Reliable Production of CAN and other AN Based Fertilisers,’ 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.

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Lagerholm, N. (1957). ‘Granulation of Fertilisers in a Conical Drum,’ ISMA Technical Conference, Madrid, Spain, 23-28 September.

Lauchard, D. and Kordek, M.A. (2000). ‘Granulation KT’s Progress Using Fluidised Drum Granulation (FDG) Technology,’ IFA Technical Conference, New Orleans, Louisiana, USA, 1-4 October.

Meline, R.S., McCamy, I.W., Graham, J.L. and Sloan, T.S. (1968). ‘Plant scale production of fertilisers in a pan granulation,’ J. Agr. Food Chem.16, no 2, 235–240.

Ries, H.B. (1977). ‘Industrial engineering possibilities of the granulation of lime and lime containing fertilisers, ‘ Aufbereit. Tech. 18 no 12, 633–640.

Ries, H.B. (1989). ‘Industrial applications of built-up agglomeration using pelletizing mixers and pelletizing disks. Part 2.,’ Aufbereit. Tech. 30, no 5, 295- 300.

Sauchelli, V. (1960). Chemistry and Technology of Fertilisers, ACS Monograph Series, Reinhold Publishing Corporation, New York; Chapman & Hall Ltd., London, p. 280.

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Valkov, S. and van Niekerk, T. (2014). ‘Fluidised Drum Granulation Technology and its New Industrial Application for Calcium Ammonium Nitrate,’ IFA Global Technical Symposium, Amsterdam, The Netherlands, 1-3 April.

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