Prilling is a method of producing reasonably uniform spherical particles from molten solids, strong solutions or slurries. It essentially consists of two operations, firstly producing liquid drops and secondly solidifying them individually by cooling as they fall through a rising ambient air stream. There is no agglomeration so the size distribution of the drops determines that of the product. The process is widely used in the fertilizer industry for making ammonium nitrate, calcium nitrate, urea and compound fertilizers containing N, P and K. A typical plant will produce 2000 te/day or more. Prilling is also used in the explosives industry to produce a porous prill of ammonium nitrate, which will absorb oil.
Prilling towers must be of sufficient height for the particles to be strong enough not to break on impact. Latent heat is transferred from the drop to the air as it falls, and if significant amounts of water are present evaporation also occurs, increasing the cooling effect on the drop. It is important for the temperature of the feed liquor to be as low as possible, just a degree or two above its solidification point. Higher temperatures require taller towers, as do larger particle sizes. Prilling towers in the fertilizer industry are typically over 50 m high for a mean particle size of about 2 mm. In the explosives industry the particle size is smaller, the feed wetter and towers of about 10 m are used.
The drop producer has to make uniformly sized drops and distribute them uniformly across the width of the tower. Various designs are used. For simple solutions an array of downward-pointing sprays is suitable. For more difficult materials such as slurries, spinning discs and spinning perforated baskets are used. Attention has to be paid to the ease of clearing blockages: in the fertilizer industry the feeds are liquids or slurries which solidify in the range 130°C to 200°C. Little further processing is required after prilling. Cooling is however necessary to prevent caking. Screening of small quantities of oversize and undersize prills is also done. In the ammonium nitrate explosives industry where the molten feed contains about 5% H2O further drying is essential to produce the porous structure for oil absorption.
NPK fertilizer products require an oil and dust anticaking treatment. Other products do not, e.g., in ammonium nitrate manufacture small quantities of salts of Mg or Al are added to the melt before prilling and these enhance the strength and anti-caking behavior without further treatment. In the mid-1960's a variation of the air prilling process was developed for manufacturing ammonium nitrate fertilizer. The drops fell through a rising cloud of finely ground mixed clay dusts and into a dense fluidized dust bed cooled by water radiators where the remaining heat was removed. The prilling operation was greatly intensified with much narrower towers, and tower heights of only 5-7 m. The process was used successfully for nearly 30 years, but is now closed down.
Until the 1980s hot air from prilling towers was discharged directly to the atmosphere. It contained small amounts of fine solid particles and also some very fine fume. Recent pollution regulations require this air to be cleaned, which is done using irrigated fiber candle filters.