Granulation is done to:

improve flow properties of the mix

improve compression properties of the mix

prevent segregation of components in powder mix

reduce production of toxic dust

reduce possibility of ‘cake’ formation

increase convenience of transport

Granulation is a particle design process whereby small particles are brought together to form physically strong agglomerates. Since powdered or crystalline materials give rise to a lot of dust, with all the attendant loss and nuisance (dust emission during handling at ports and in warehouses and workplaces, poor product flow-ability, caking tendency during storage and transportation, segregation of components, scattering by wind during the spreading on the fields), most of such materials are consolidated into larger particles with more suitable properties by means of granulation.

The compaction process is basically a dry granulation process, so there is no liquid medium and no binder. Instead, the fine particles of material are subjected to a sufficiently high pressure to squeeze them together and bring their surfaces close enough for short-range intermolecular and electrostatic forces to cause cohesion. The equipment used for the compaction is called a roller compactor.

The roller compactor comprises a feed unit and two parallel rolls rotating on horizontal axes and separated by a small but precisely determined gap (up to 25 mm). Usually the rolls speed is limited to 1 m/s. The rolls are mounted side by side with the feed section above. The powder is forced into the “nip” of the rolls by the combined forces of the feed system and the frictional force between the material and the rolls, which rotate so as to draw the material into the gap.

To ensure uniformity in the product it is most important that the feed system should maintain an absolutely steady flow of material to the rolls. Its design is therefore very critical. The feed system is fitted with a screw force-feeder which includes one or two conical screws. The rotational screw speed is variable and the feed screw is tapered to squash out air voids and pre-densify the powder before going through the rolls.

The powder passes out of the nip of the rolls in the form of a laminar sheet (thickness up to 25 mm), which breaks into flakes under its own weight.

Main steps of the compaction-granulation process

Raw feed preparation section

This section usually consists of the following components :

- Feed hopper with a rough screening system, for removing big lumps, and possibly a lump-breaking system.

- Raw material grinding system if required. It is advisable to have particles smaller than 1 mm to improve the

homogeneity of the final product as well as the production yield.

- Raw material storage hoppers .

- Weighing system (for batch operation) or dosing system (for continuous operation).

- Homogenizing system (mixing).

- Controlled feeding system to the roller compactor.

This is, in fact, much the same as the feed preparation system for a conventional wet granulation plant or a bulk blending plant.

Compaction section

This section comprises a number of sub-units :

- the compactor feed hopper

- the roller compactor with its force-feeder

- the flake-breaker, if required depending on the size of the flakes produced by the roller compactor

Granulation section

This section comprises the equipment to make the granules from the flakes: it is broken up into smaller fragments in a primary crusher-granulator installed downstream of the roller compactor. Particles in the desired size range (for example, between 2 and 4 mm) are screened and removed as product, while the oversize (above 4 mm) is returned to a secondary crusher granulator and the undersize (under 2 mm) to the roller compactor. This stage of the compaction-granulation process has a significant effect on plant capacity.

Finishing section

This section comprises a dry polishing unit and a coating unit. The dry polishing unit consists of a polishing drum and a finishing screen. The polishing drum improves the quality of the final product by rounding off the sharp edges of the granules and destroys any that are of substandard strength. The undersize finishing screen subsequently removes the dust generated by the abrasion of granules in the polishing drum. This limits the amount of dust that is formed when the product is handled and used.

Limitations of the compaction-granulation process

There are very strict limits for the free moisture content of mixtures before compaction. Super phosphates are difficult to compact, and pre-drying, or at least prolonged ageing, is required.

Granules produced by compaction-granulation are not as spherical as those obtained in a conventional granulation process. Furthermore, the granule size range is more irregular. The irregular particle surface and shape characteristics of compacted products may promote caking during storage and handling and because of the increased surface area of the irregular particle compared with the more rounded granules produced by other granulation processes.

Advantages of the compaction-granulation process

Compared with the others granulation processes, there are many advantages to be found with the compaction-granulation process, and among them :

- being a dry process, no water or binder consumption (or in some cases, 1 or 2 % of water are added to the product

before the roller compactor : due to the small rise in temperature (around 20°C) during run-through of product in the

roller compactor, the water is eliminated naturally).

- no energy is required for drying ; energy for the process is supplied in the form of electricity (around 30% less than the

wet granulation process).

- as there are few corrosion problems, maintenance costs are kept low.

- the level of pollution caused by the process is very low because there are no liquid or gas effluents to be processed. The

compaction process is environmentally friendly. In the compaction-granulation process, a dust collector with bag filter

is usually installed. With the wet granulation processes, a wet scrubber is required and this involves the problems

associated with the utilization/disposal of unwanted scrubber liquor.

- low investment cost is possible, no need to invest in a dryer and a cooler (expensive equipments).

- the process can use powdery raw materials which cannot be easily used in bulk blending.

- the process allows the manufacturing of fertilizers difficult, or even impossible, to produce by means of the wet

granulation process for technical reasons (for instance, high urea content fertilizer, potassium sulphate).

- the fertilizers manufactured using this process are less prone to caking (the compaction-granulation uses dry

components) and are more durable.

- the process can be switched over to different formulations quickly and efficiently.

- due to its high level of flexibility, the plant can adapt its production to the market demand

- as the plant uses simple circuits, plant layout is compact.

- operation and maintenance of the plant is easy ; therefore, staff do not require special training. And finally, only a very

few staff is required to operate the plant.

Steam/water granulation

In steam/water granulation process granules are formed by agglomeration. The solid raw materials are proportioned (weighed) and usually premixed before being fed to the granulator where agglomeration is initiated. In some plants, the raw materials are crushed either before or after weighing to obtain a more uniform particle size distribution. In the granulator (usually a rotary drum or a pug mill) steam and/or water or scrubber liquor is added to provide sufficient liquid phase and plasticity to cause the dry raw materials to agglomerate into product size granules. In some processes, the pug mill may be used to premix the solids and liquids prior to the granulation in rotary drum unit. The moist and plastic granules are dried, usually in a rotary drum type, fuel fired dryer and screened to remove the product size fraction.

The oversize material is crushed and recycled to the granulator along with the undersize fraction. To ensure a uniform particle size distribution of the material returned to the granulator, it is the best to return the crushed oversize material to the screening unit so that only material passing the product screen recycled to the granulator. In some cases it is necessary to cool the material before screening depending on the material grade and local conditions. If cooling is required, it is usually performed in a rotary drum type unit that is very similar to the rotary dryer. Fluid bed type coolers may be used to cool product size material, but they are not recommended for cooling material having a wide particle size range.

The irregular particle surface and shape characteristics of compacted products may promote caking during storage and handling and because of the increased surface area of the irregular particle compared with the more rounded granules produced by other granulation processes.

Wet granulation

In wet granulation, granules are formed by the addition of a granulation liquid onto a powder bed which is under the influence of an impeller (in a high-shear granulator, screws (in a twin screw granulator) or air (in a fluidized bed granulator). The agitation resulting in the system along with the wetting of the components within the formulation results in the aggregation of the primary powder particles to produce wet granules. The granulation liquid contains a solvent which must be volatile so that it can be removed by drying, and be non-toxic. Typical liquids include water, ethanol and isopropanol either alone or in combination. The liquid solution can be either aqueous based or solvent-based. Aqueous solutions have the advantage of being safer to deal with than solvents.

Water mixed into the powders can form bonds between powder particles that are strong enough to lock them together. However, once the water dries, the powders may fall apart. Therefore, water may not be strong enough to create and hold a bond. In such instances, a liquid solution that includes a binder is required. Once the solvent/water has been dried and the powders have formed a more densely held mass, then the granulation is milled. This process results in the formation of granules. In the traditional wet granulation method the wet mass is forced through a sieve to produce wet granules which are subsequently dried.

Drop formation (Prilling)

In this process of formation of granules by prilling, the mixture (solution) is concentrated to approximately 96%-98% solids before it is introduced to the top of the prilling tower through either a nozzle assembly or a perforated vessel referred to as a “prilling bucket”. The nozzle or prilling bucket assemblies are designed to produce droplets of the “molten” fertilizer mixture that, upon solidification, will yield the desired granule (prill) size. Granules (prills) produced by the prilling processes are very homogeneous. However, due to the practical

limitations of the drop formation and solidification process, prilled products usually exhibit a smaller particle (granule) size, typically 1.3 mm-3.0 in diameter, than those produced by other granulation processes.

Granulation Process Parameters

Impeller Speed

Higher Impeller speeds generally results in denser and smaller granules. Low impeller speeds generally result in more porous, large granules.

Chopper Speed

Generally Chopper speed has no significant effect on granule size and density but in cases where the chopper is large, it may function as a secondary impeller.

Water Addition Rate and Method

Water Addition Rate is critical to granule quality. Generally water addition rate is chosen such that local over-wetting of the powder mass is not a concern and at the same time the addition rate is fast enough to accommodate processing times (~2 –5 minutes for water addition).

Massing Time

Massing time is normally in the order of 1 to 10 minutes. Long massing times (> 20 mins) may lead to decreased dissolution rates due to the formation of denser granules.

Load of the Mixer

Generally, the load of the mixer is less than two-thirds the volume of the mixer.

Tumblers

The particles of a granular mass will cohere when they are tumbled and sprayed lightly with a liquid binder which often is water or a concentrated solution of the material being agglomerated. The growth may be due to agglomeration of small particles or to layering of material evaporated from the sprayed solution.

Rotary kilns of the kind used for drying or chemical reaction (cement or lime burning, for instance) are adapted to size enlarging service. Usually the tumbling action is less intense, only enough to expose the material to sprays. The sprays are fine and are applied to the surface of the bed of particles. The tumbling action then distributes the liquid uniformly through the mass.

Pan granulator is a shallow pan, inclined 45-700 to the horizontal and rotating at speeds of 10-30rpm. The ratio of pan diameter to collar height is 3-5. As the rotation proceeds, fresh solids and spray are charged continually. The finer particles settle to the bottom, the largest remain at the top and then overflow the collar and constitute the product. Because of the size stratification, the product of disk granulation is more uniform in size than of drum granulators which discharge a mixed product.

Rotating Drum Granulators

The equipments are largely free of internals to promote mixing, but provides just sufficient turnover to effect redistribution of the spray throughout the mass. With heavy sprays and little tumbling action, excess and non-uniform agglomeration will occur. Granules 4-6 mm diameter commonly are made by layering from the sprayed solutions. Fertilizer granules made this way are larger, more dense, and harder than those made by prilling. The trend in the industry has been for prilling towers to be replaced by drum granulators and for those in turn to be replaced by fluidized bed granulators in which dusting problems are most controllable. Length to diameter ratios of 2-3 is used and speeds of 10-20rpm. Recommended speeds are about 50% of the critical speed for the dry material; then adequate cascading occurs and the range of particle size distribution is narrowed.

WET GRANULATION

high shear granulator