Prilling and granulation

Prilling and granulation denote the two principal solidification techniques employed for solid fertilisers, notably prilled urea, ammonium nitrate (AN) and NPK fertilisers. Prilling involves spraying a concentrated solution from the top of a tower; the droplets, termed prills or pearls, fall counter-current to an air stream that solidifies them. Further cooling is subsequently required. Granulation projects a concentrated liquor onto recycle particles circulating within a drum, a pan granulator or a fluidised bed. Particle growth and drying proceed simultaneously. All industrial processes incorporate a recycle loop^[1].

Processes and equipment

For urea, prilling utilises a rotating head or a distributing bucket. Granulation calls upon drums or fluidised beds; subsequent drying and cooling are carried out in rotary drums or fluidised beds, with air purification^[1][2].

In the manufacture of AN and calcium ammonium nitrate (CAN), granulators comprise drums, spherodisers, pugmills and fluidised beds. For CAN, the mineral filler is incorporated before or during granulation, with AN introduced as a hot concentrated solution. Drying may prove unnecessary when residual heat suffices to drive evaporation: the autothermal process^[2]. Final cooling, common to both routes, employs rotary coolers or fluidised beds; dry-scrubbed air may be reused as secondary air for the dryer^[2].

Emissions and gas treatment

The finishing section constitutes the principal pollution source at urea installations. The pollutant load there represents 0.4 to 0.6% of total feed, i.e. at least ten times that of the wet section (0.005 to 0.05%). Extensive contact between air and hot urea loads the gases with ammonia and dust^[3].

Granulation demands less air; its dust is coarser and therefore easier to capture. The economic advantage of prilling — low capital expenditure — tends to be offset by effluent treatment costs^[3].

Dust emissions from urea prilling vary widely according to the treatment in place. A documented case with wet scrubbing and a gas flow of 300,000 to 350,000 Nm³/h records 270 g/t and a concentration of 15 to 23 mg/Nm³, against raw concentrations of 60 to 130 mg/Nm³. Conversely, installations lacking treatment report emissions of 500 to 2,200 g/t, with concentrations of 35 to 125 mg/m³. For urea granulation, the table indicates discharges of 100 to 550 g/t with scrubbing, and concentrations of 30 to 75 mg/m³^[4].

Ammonia abatement efficiency depends on the scrubbing medium (acid or water) and the number of stages. The scrubbing liquor may be recycled to the process when water scrubbing is employed^[3].

Energy integration

On an integrated site, the finishing sections rank among the net energy consumers, alongside AN neutralisation and evaporation. Conversely, AN neutralisation, ammonia and urea production may export heat depending on the design^[5].

The choice between the two routes hinges on economic and technical trade-offs. Certain downstream industrial applications still favour prilled urea. For AN, granulation dominates modern capacity owing to its environmental performance. Both routes produce freely flowing solids, an essential condition for bulk handling and any subsequent coating.

Regulatory framework

The European regulation on fertilising products prescribes sampling and test methods applicable indiscriminately to prills and granules. Annex IV details an abrasion resistance procedure: the sample is rolled between filter sheets through twenty-four circular movements, with corner lifting at defined intervals^[6].