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Urea – Properties
Pure urea is a white crystalline solid. The melting point of urea is around 133 °C, decomposition of urea starts before reaching ist boiling point. Urea usually smells somewhat ammoniacal.
The relatively low specific density, compared with that of other fertilizer materials, must be considered in calculating storage space requirements. For instance, storage space requirements per unit of nitrogen are about the same for urea as for ammonium nitrate because the higher analysis of urea is offset by its lower specific density. As urea has a relatively low specific gravity, care needs to be taken to allow for this when formulating blended products. This characteristic also needs to be allowed for in relation to storage capacity planning – any given tonnage will require more volume.
Urea is less hygroscopic than ammonium nitrate but more hygroscopic than ammonium sulphate; it requires protection from humid atmosphere in some climates. The coating of prilled urea with clay, oil, or other coating agents gives relatively good protection. Many producers add small amounts of formaldehyde to the urea melt just before prilling and obtain very satisfactory results in storage. If the product is properly dedusted and the producer has made good prills with high impact and crushing strengths, it is possible to ship uncoated bulk urea long distances with no problems. Data on the critical relative humidity and solubility of urea can be found in literature. Urea has a low thermal conductivity and can provide a good insulating effect. Biuret is a chemical compound with the chemical formula (H2NC(O))2NH, Figure 1. It is a white solid that is soluble in hot water. Biuret results from the condensation of two equivalents of urea. As such, it is an undesirable impurity in urea-based fertilizers. As biuret is toxic to plants, its percentage in urea and urea-based fertilizers must be kept low.
References
Related IFS Proceedings
131, (1972), Some Fundamental Aspects of Urea Technology, S M Lemkowitz, M G R T de 166, (1977), Urea Stripping Process – Stripping Technology, Phase Equilibria and Thermodynamics, P J C Kaasenbrood, H A G Chermin, Cooker, P J van den Berg
167, (1977), An Integrated Process for Ammonia-Urea Manufacture, V Lagana, U Zardi
206, (1982), Materials of Construction for the Nitric Acid Process, K Nutall, A R Reid
257, (1987), Rationale for Mixed Ammonium Nitrate – Urea Fertilisers and Assessment of Granular Products, M K Garrett
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
770, (2015), World-Wide Trends in Urea Process Technologies, J M G Eijkenboom, M J Brouwer
805, (2019), The Carbon Footprint of Fertiliser Production: Regional Reference Values, A Hoxha, B Christensen
830, (2019), Principles and Applications of a Directory of Urea Safety Incidents, with Case Studies, M J Brouwer
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