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A prill is a small aggregate of a material, most often a dry sphere, formed from a melted liquid. The material to be prilled must be a solid at room temperature and a low viscosity liquid when melted. Prills are formed by allowing drops of the melted prill substance to congeal or freeze in mid-air after being dripped from the top of a tall prilling tower. Fertilizers (ammonium nitrate, urea, NPK fertilizer) and some detergent powders are commonly manufactured as prills.
Melted material may also be atomized and then allowed to form smaller prills that are useful in cosmetics, food, and animal feed. Prills have also been used to protect active ingredients from exposure to environmental factors and to cover up the flavor of bitter nutraceuticals.