A prill tower is the final key stage in the production of ammonium nitrate 'prills' - small spherical particles used as fertiliser or in the manufacture of explosives and rocket fuels. The first stage is to react ammonia with nitric acid to yield an aqueous solution of ammonium nitrate, which is then concentrated by evaporators to more than 99% ammonium nitrate content at a temperature of 180infinityC.
The concentrated solution is sprayed into the top of a prill tower and falls as droplets through an upwelling air stream. This dehydrates and concentrates the droplets, which arrive at the foot of the tower as dry, spherical particles of very uniform size. Low density, more porous prills go to explosives manufacture, high density prills are sold as fertilisers.