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LAYERED AGGLOMERATION OF UREA GRANULES
Urea has been widely used as a crop fertilizer to increase crop yield. The low nutrient use efficiency (NUE) of urea, however, is a challenge. Coated fertilizers are considered a solution not only for enhancing the NUE but also for alleviating soil and water pollution. In this paper, the physical properties of coated fertilizers were analyzed, including their particle size distribution, fracture force, thermal behavior, envelope density, and apparent density (regular fertilizer: pure urea and the Anderson 12-6-6; slow release fertilizer: Osmocote 14-14-14, the Anderson 18-6-12; controlled release fertilizer: Environmentally smart nitrogen (ESN), Florikan 14-14-14, Everris 17-3-6). The granules’ closed and open pore number, pore volume, and total porosity were analyzed using X-ray micro-tomography (XRCT). The results demonstrated that pure urea and Florikan have a similar median particle size, around 4 mm, while ESN and Osmocote have a similar median particle size of around 3 mm. Finally, Everris, the Andersons 18-6-12, and the Andersons 12-6-6, have a similar median particle size of roughly 2.5 mm. The fracture pressure of ESN (4.58±0.98 MPa) and the NPK combination fertilizers (Florikan: 9.40±1.46 MPa and Osmocote: 8.94±2.09 MPa) were higher than pure urea. The envelope and apparent density of pure urea (envelope: 1.22±0.02 kg/m3 and apparent: 1.27±0.01 kg/m3) and ESN (envelope: 1.26±0.03 kg/m3 and apparent: 1.27±0.00 kg/m3) are similar, while all NPK fertilizers have a significantly higher density (envelope: 1.68–1.87 kg/m3 and apparent: 1.83–2.09 kg/m3). ESN had higher internal pore space and a higher total pore volume than pure urea, while NPK combination fertilizer showed lesser pores and significantly smaller pore volumes. The physical properties were also significantly different when comparing urea and NPK compound fertilizers, mainly because of the differences in their nutrient coatings and manufacturing methods. The coating of the urea increases the granule strength but does not alter the thermal properties; however, the overall porosity of the granules is influenced by the coating. In this thesis core, different binders were used to alter the internal structure of the urea granule to control the dissolution behavior and to make it a slow-release fertilizer. The layered agglomeration technique was used to manufacture the granules. The core of the granule was made by granulating technical urea powder in a drum granulator, with corn starch as the binder. A second layer of urea was added to the core by drum granulation in order to obtain a nutrient release pattern that matches with the crop demand. Corn starch, PEG 4000, and corn starch hydrogel were used as binders for the second layer. The density, thermal properties, strength, and internal porosity were measured to compare with market urea and coated slow-release fertilizer granules. All the dissolution rates of the double layer granules were slower than for market urea. Among these granule types, the dissolution rate curve of the granule with starch hydrogel in the second layer better matched the crop demand curve than those of the other two types of granules. Moreover, the strength of the double layer granules with hydrogel was the greatest of the three double layer granules. So, overall, the double layer granule manufactured with corn starch in the core and starch hydrogel in the second layer performed the best. Although the pattern of dissolution of the double layer granule was similar to the crop nitrate demand curve, a soil-based study is needed to verify the nitrate release characteristics.