Granulation by spray coating aqueous solution of ammonium sulfate to produce large spherical granules in a fluidized bed

Spherical 2–4 mm granules of ammonium sulfate (NH4)2SO4 are promising fertilizer for practical use, though only much smaller grains are being produced in industry. This work used coating granulation to produce large spherical granules of (NH4)2SO4 in a fluidized bed by spraying its aqueous solution onto 0.9–1.6 mm (NH4)2SO4 core particles. However, the overall coating efficiency was only 58% due to loss as dust by attrition of (NH4)2SO4 in the vent gas. By adding CaCO3 or SiO2 particles into the feed solution, the coating efficiency was increased to over 90%. This increase in coating efficiency was due to a change in the crystallization mechanism of (NH4)2SO4. The added CaCO3 or SiO2 particles provided a heterogeneous surface that induced (NH4)2SO4 to crystallize uniformly to form a more compact structure less susceptible to attrition.

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► High efficiency of (NH4)2SO4 aqueous solution coating granulation was achieved.
► Sand-like structure in the granules of pure (NH4)2SO4 was found.
► Compact structure of granules was achieved by adding CaCO3 or SiO2 additives.
► Granules with higher strength were produced from heterogeneous crystallization.