For the particulation of fertiliser melts or solutions, a number of techniques can be considered, for example:
Granulation based on mere mixing of raw material supported by addition of steam and/or water
Granulation by exercising pressure on the mixture of raw materials, followed by breaking up (and possibly rounding) of the solid materials
Particulation by bringing out melt droplets onto a cool moving bed (pastillation)
This section is focused on the first two particulation techniques, involving granulation in a rotary drum. Other equipment can be used, and a separate section is provided for fluidised bed granulation.
Granulation based on the mixing of raw materials and supported by steam and/or water addition
In the steam/water granulation process (Figure 1), granules are predominantly formed by the mechanism of 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, a pugmill may be used to premix the solids and liquids prior to granulation in a rotary drum unit. ln other cases a small amount of ammonia may also be added during granulation to react with superphosphate or other acidic materials to promote granulation and improve product quality by decreasing the acidity and increasing the Critical Relative Humidity (CRH). 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 best to return the crushed oversize material to the screening unit so that only material passing the product fine screen is recycled to the granulator. In some cases it is necessary to cool the material before screening, depending on the fertiliser 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.
Granulation following prior ammoniation of acidic melts and benefiting from the heat of the chemical reaction (chemical granulation)
Chemical granulation is the most complex method for preparing granular NPKs (Figure 2). The chemical granulation process is very similar to the previously described steam/water granulation process except that most of the liquid phase required to achieve granulation is obtained by reacting ammonia with phosphoric, sulfuric, and/ or nitric acid prior to the particulation. In some cases, a concentrated solution of urea or ammonium nitrate may also be used . However, compatibility of urea and/or ammonium nitrate with the other materials needs to be carefully considered. Some processes also involve a significant reaction between ammonia and single or triple superphosphate (SSP or TSP). Much of the ammonia-acid reaction is often performed outside the granulator, either in a tank-type pre-neutralizer or in a pipe-type reactor. In many cases steam, water, scrubber liquor, and/or additional ammonia and acid are also fed to the granulator to optimise the granulation characteristics of each particular formulation. In most NPK chemical granulation plants, a significant amount of solid raw materials is also added to the (drum) granulator; therefore, the granules are formed primarily by agglomeration. The relationship between the consumption of solid and liquid raw materials is determined by a number of variables, including:
- the required nutrient ratio (fertiliser grade) and nutrient solubility requirements,
- liquid phase requirements
- heat of reaction and temperature limitations, and
- process plant equipment capacity and operating features.
The granulation process shown in Figure 2 also incorporates the design features recommended for the production of NPKs containing relatively large amounts of urea or other very soluble and hydroscopic-materials.
In some chemical granulation processes, the solid raw materials, for example muriate of potash, are partially dissolved in an ammonium phosphate and/or ammonium nitrate slurry and the mixture is then sprayed onto a bed or curtain of material in the granulator which is usually a heated rotary drum, for example, a spherodizer. In this process, granules are formed primarily by accretion.
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 J Rutten, I K Watson.
Recording of ‘Statistical production control of ammonium nitrate plant‘, (2015), M Langpape (Required password is 2015Tech09)
216, (1983), Granulation of Ammonium Phosphates – Recent Experiences,
K J Barnett, D M Ivell, S F Smith.
238, (1985), Granulation by Extrusion and Compaction Methods,
J P Bruynseels.
310, (1991), Applied Rule-Based Control of CAN/NPK Granulation,
A K Sorth, P B Olsen, F Larsen, N K Larse.
535, (2004), Development and Use of Computer Simulation of Fertiliser Granulation, P Vonk, S Schaafsma.
783, (2016), Granulation of Complex Fertilisers
H Kiiski, A Kells.
Recording of ‘Granulation of complex fertilisers‘, (2016), H Kiiski (Required password is 2016Tech05)
Recording of ‘Changes, challenges, and opportunities in fertiliser-manufacturing processes: A personal review and outlook ‘, (2017), H Reuvers (Required password is 2017Tech06)
807, (2017), Distributed control system implemented at a UK fertiliser complex: past, present and future Southerton, T
Recording of ‘Distributed control system implemented at a UK fertiliser complex: past, present and future‘, (2017), T Southerton (Required password is 2017Tech10)
Links to external resources
Autti, M., Loikkanen, M. and Suppanen, P. (1976). ‘Economic and Technical Aspects in some Differential NPK-Processes,’ ISMA Technical Conference, The Hague, The Netherlands, 13- 16 September.
Baynham, J.W. (1965). ‘The SAI-R Process for the Manufacture of Compound Fertilisers containing Ammonium Nitrate and Diammonium Phosphate,’ Proc. ISMA Tech. Conf., 14-16 September.
Benes, J.A., Hemm, A. (1985). ‘Modern Process Design in Compound Fertiliser Plants’, Seminar of Trends and Developments in the Fertiliser Industry.
Heiseler, G., Baier, D., Juling, D. and Kretschmer, W. (1973). ‘Process for manufacturing granulated phosphorous and potash fertilisers,’ Chem. Tech. (Leipzig) 25, no 7, 410–414.
Lee, R.G., Meline, R.S. and Young, R.D. (1970). ‘Pilot-Plant Studies of an Anhydrous Melt Granulation Process for Ammonium Phosphate Based Fertilisers,’ ISMA Technical Conference, Sandefjord, Norway, 8-11 September.
Lee, R.G., Meline, R.S. and Young, R.D. (1972). Pilot plant studies of anhydrous melt granulation process for ammonium phosphate-based fertilisers, Ind. Eng. Chem. Process. Des. Dev. 11, no1, 90–94.
Leyshon, D.W. and Mangat, I.S. (1976). ‘Innovations in Slurry Process Granulation Plants,’ ISMA Technical Conference, The Hague, The Netherlands, 13-16 September.
I. Makarenko, SOLEX, World Fertilizer Magazine, November-December 2019, pages 77-78, Saving energy while avoiding caking.
Passmore, R.I.O. (1965). ‘The Automatic Control of a Granulation Plant,’ ISMA Technical Conference, Edinburgh, United Kingdom, 14-16 September.
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