1. Prills
2. Granules
3. Pastilles
4. Comparative properties of different particulate forms of urea
5. Urea Product Quality

Solid urea for fertiliser exists in different forms: prills, granules and pastilles. As a fertiliser, urea is a convenient form for fixed nitrogen and has the highest nitrogen content (46% by weight) available in a solid fertiliser. It is easy to produce in different solid forms and easily transported in bulk or bags with no explosive hazard by itself. It dissolves readily in water and leaves no salt residue after use on crops and can often be used for foliar feeding.

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Prills

About 75% of all 500+ urea plants worldwide produce prilled urea, Figure 1. Prill quality urea is suitable when it is to be applied locally and not transported in bulk. Prilled urea is a pure crystalline non-porous salt of low moisture content (0.15 to 0.25 wt%). Prills are essentially spherical, have a smooth surface and if produced by modern, efficient prilling devices, they are regularly sized, have a narrow size distribution with only minor fractions of undersize oversize, Figure 2. The typical average diameter is between 1.5 and 1.9 mm and 90 to 95% of the prills are between 1.2 and 2.2 mm in size. Prills are produced in a prill tower where in the top section urea melt is distributed via a rotating bucket or static sprayers (a kind of shower head). The urea melt droplets fall down by gravity and are cooled und solidified by the air flowing upwards. Prill towers can be natural draft or forced draft. The maximum average diameter is foremost determined by the free falling height of the prilling tower. Urea prills are typically smaller and have a lower particle strength compared to urea pastilles or granules. The lower strength of prills is caused by the fact that cooling of prills starts at the outside of the urea melt droplets and proceeds to the centre of the prill. As the density of solid urea is somewhat higher than the density of urea melt, a hole will form in the prill, obviously leading to a weak point with a lower strength in the product.

In prilling, after the droplet is formed from the prilling machine, the solidification of the urea melt starts. At first, the prill solidifies at the interface between the melt and the air, forming a shell of solidified melt which moves inward. However, there is a difference in density between the liquid urea melt and the solidified prill.

Usually, the solidified prill will take in a smaller volume as compared to the liquid droplet. As the solidification propagates the prill needs to shrink somewhere. As the shell is already formed, the prill will shrink in the middle, forming a cavity, the prill hole. This core makes for a porous structure of the urea prill. The bigger the prill becomes, the bigger the cavity in the core of the prill is. See Kreber 2020 for more information.

Granules

Most remaining urea plants use fluid bed granulation, with some drum granulators also still being used, Figure 3. Granules have a larger size and strength than prills and are therefore preferred when urea is to be transported in bulk, Figure 4. During the production of granules formaldehyde is added to improve the granule quality, to limit dust formation in the fluid bed granulator, and to avoid caking problems.

In granulation granules are formed via agglomeration of small particles or via a layered covering of particles; as a consequence. A granule does not have a hole and therefore is intrinsically stronger. As large amounts of air are typically required for production (both in prilling and in granulation technologies) this can lead to difficult and thus expensive measures to limit ammonia and dust emissions.

Pastilles

Pastillation of urea is a relatively new but promising technology, where urea melt is deposited as droplets onto a rotating cool steel belt, Figure 5. The product quality of pastilles is significant better than prills as the pastilles have a high strength, low moisture content, a very narrow size distribution and can be produced at any required size. This technology requires only a low power consumption and the pastilles can be produced basically free of any ammonia and dust emissions as no large air amounts are necessary. Another advantage of this technology is that it can easily produce different fertiliser grades, for example urea+S, urea+AS, urea+micronutrients, urea+nitrification inhibitors. The pastillation technology does not need formaldehyde for dust repression, so it can be used to produce higher specification technical grade urea for feed and industrial uses. However, to date, the usage of pastillation has not developed to the extent that was initially anticipated. This may be related to the limited daily capacities per production line, compared to those of fluid drum and other production technologies.

The solidification of the melt droplets on the steel belt starts from the bottom side and slowly progresses to the top. With pastilles no hole is formed in the final product. Although the shape of pastilles is half round (forming a ‘dome’ shape), farmers have been willing to accept the product as a fertiliser and seem prepared to pay a premium for pastilles.

Comparative properties of different particulate forms of urea

Table 1 provides an overview of the properties of the three main different types of solid urea. This indicates that granules and pastilles have some significant advantages over prills: granules and pastilles can be produced with a larger average diameter which can be chosen more or less freely in a relatively wide range, plus granules and pastilles are significantly stronger than prills. Depending on geographical location, markets served, capacity, and capital expenditures, all three particulation technologies may find industrial application.

Urea Product Quality

Strength

Strength is a key quality parameter as sufficient strength will avoid dust and fines formation during transportation and reduce caking problems.

There are three strength parameters, Figure 6:

1. Crushing strength: the static weight a urea particle can handle before it crushes. This parameter is of importance when storing urea in bulk.
2. Impact resistance: the dynamic force a urea particle can handle when bouncing with a certain velocity on a surface. This parameter is of importance when placing bulk urea in a ship or warehouse.
3. Abrasion resistance: the tendency that particles creates dust when these are rubbed against each other. This parameter is of importance when transporting urea for example on a conveyor belt.

Typically, only crushing strength is a requirement when urea is traded in the market. But the other strength parameters can cause complaints from a client. It is good practice to give good guidance to all parties in the logistic chain for the best practices how to handle urea. Many urea producers and fertiliser associations (for example Fertilizers Europe, IFDC) issue urea handling recommendations.

Urea export quality requirements

If urea is to be successfully exported in bulk, it should meet the following specification:

• Moisture : max 0.3 wt%
• Biuret : max 1 wt%
• Nitrogen : min 46.0 wt%
• Crushing strength : 3 kg for  a 3.15 mm granule
• Size distribution : min 90 wt% in the requested particle size range; (2-4 mm is preferred in case of bulk blending by a customer)
• Fines smaller than 1.25 mm : max 1 wt%
• Formaldehyde: as specified by the customer
• Free ammonia : max 150 wt-ppm
• No caking / free flowing

References

IFDC. (2005). Fertilizer Dealer Handbook – Products, Storage, and Handling, Reference Manual IFDC-R-15, D. W. Rutland and J. Polo, IFDC, Muscle Shoals, Alabama, USA.

Fertilizers Europe. (2015) GUIDANCE FOR THE STORAGE, HANDLING, AND TRANSPORTATION OF SOLID MINERAL FERTILIZERS.

Bibliography

IFA. (2002). Technical Conference, Chennai, India, 24-27 September 2002, Urea Caking Problems, How To Avoid Them, Pan Orphanides, Orphanco, Greece.

Kreber, The differences between prilling and granulation. 

IFDC. (1998). Fertilizer manual, pages 266 – 270.

Clayton, W. E. (1984). Humidity factors affecting storage and handling of fertilizers. International Fertilizer Development Center, Muscle Shoals, Alabama, USA.  

Related IFS Proceedings

131, (1972), Some Fundamental Aspects of Urea Technology, S M Lemkowitz, M G R T de Cooker, P J van den Berg

495, (2002), Urea Production Technology: Recent Developments, M J Brouwer

582, (2006), IPPC: The BAT Reference Document (BREF) for the Manufacture of Ammonia, Acids and Fertilisers, B Serr

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