Urea and ammonium nitrate are prilled in a prill tower or granulated in a fluid bed granulator. Afterwards the granules are separated in the required grain size fractions by screening. Usually the grain size fraction between 2 mm and 4 mm is required because the machines for bringing the fertilizer on the fields are adopted to this particle size. In the production processes the screened fines are returned to the fluid bed granulator as “seed” resp. in case of using a prilling tower dissolved in water. The saturated solution is then pumped into the prilling tower. The coarse particles are crushed in a mill and then handled the same way as the fines (returning to granulator resp. dissolving in water).
The challenge in screening of Urea is the hygroscopic characteristic of the nitrogenous fertilizers. This leads to the adhesion of humidity mostly in dust. The humid dust adheres to the screen cloth and this in the course of time leads to clogging if the machine does not have a sufficient self cleaning system. Oftentimes used screening machines with long stroke resp. tumbling movement become clogged after short operation time and have to be cleaned.