Prill

A prill is a solid particle of spherical shape obtained by allowing molten fertiliser droplets to solidify in free fall within a prilling tower. A spinning bucket or perforated drum at the top atomises the concentrated liquid; an upward flow of air cools the droplets by counter-current and enables them to harden^[1]. The product is subsequently screened to meet specifications; oversize and undersize material is recycled back into the process^[1].

This technique is employed in particular for urea, whose molten concentrate is sprayed through a rotating bucket or prilling head^[2]. The European regulation on fertilising products distinguishes prills from granules and pellets among the physical forms of solid fertilisers^[3]. Prills differ from granules produced by granulation, whereby molten concentrate is sprayed onto recycled seed particles circulating inside the granulator^[2].