A prill is a small, roughly spherical bead formed when a molten liquid is broken into droplets and cooled until it solidifies. Most prills range from about 1.5 to 3 mm in diameter, roughly the size of a coarse grain of sand or a small seed. You’ve likely encountered prills without knowing it: the tiny round pellets in a bag of fertilizer, the beads in certain cleaning products, or the small spheres of sulfur used across dozens of industries.
How Prills Are Made
The manufacturing process, called prilling, is elegantly simple in concept. A material is melted or dissolved into a liquid, then sprayed from the top of a tall structure called a prilling tower. These towers can be several tens of meters high. As the tiny liquid droplets fall, a stream of cool air rises from the bottom in the opposite direction. Surface tension pulls each droplet into a sphere, and the rushing air solidifies it before it reaches the ground. The finished prills are collected at the base of the tower.
The result is a large volume of uniformly shaped, free-flowing beads. Newer prilling towers use vibrating devices at the top to produce even more consistent droplets, yielding prills with a tighter size range (typically 2.5 to 3 mm) and a main fraction content of about 95%, meaning almost every bead that comes out is the target size.
Prilling can also be done at much smaller scales using cryogenic techniques, where liquid nitrogen replaces ambient air as the cooling agent. These methods can produce tiny particles as small as 60 to 300 micrometers, useful in pharmaceutical and food applications where precision matters more than bulk volume.
Prills vs. Granules
Prills and granules are often confused because they look similar at a glance, but they’re made differently and behave differently. Granules are built up layer by layer in a fluidized bed, where small seed particles are coated with successive layers of material. Prills are formed all at once from a single droplet. This difference in manufacturing creates real physical differences.
In urea fertilizer, for example, prills have a mean diameter of about 1.64 mm, while granules average 2.81 mm. Prills are also significantly softer. In crushing tests, urea prills fail at an average force of 3.8 newtons, while granules withstand 10 to 17 newtons before breaking. That makes granules better suited for rough handling, long-distance shipping, and mechanical spreading equipment that can shatter weaker particles. Prills, on the other hand, have a more uniform size and strength distribution, which can be an advantage when consistency matters more than durability.
The chemical properties of prills and granules made from the same material are identical. The differences are entirely physical: size, hardness, shape, and how they behave when spread, stored, or mixed with other materials.
Fertilizer: The Most Common Use
Urea in prill form is the single largest nitrogen fertilizer product in the world. Farmers and landscapers spread prilled urea because the uniform bead shape flows easily through broadcast spreaders and distributes evenly across soil. The round shape also reduces dust during handling, which is both a health benefit for workers and a practical one for storage facilities.
Caking is the main enemy of stored fertilizer. When prills absorb moisture and stick together, they become useless clumps. To prevent this, prills must be cooled to a specific temperature after production and before packaging. Modern prilling operations emphasize anti-caking properties as a key quality metric. Older prilling towers struggled with this, producing uneven sizes and generating significant ammonia and dust emissions. Updated designs have largely solved both problems.
Mining and Explosives
Ammonium nitrate prills are the foundation of ANFO, the most widely used commercial blasting agent in mining and construction. What makes prills work for explosives is their porosity. Porous prills have a lower density than fertilizer-grade ammonium nitrate, but their internal void spaces (about 20% of the total volume) absorb and hold fuel oil efficiently. That fuel oil absorption is what turns an otherwise stable fertilizer into a reliable explosive.
The size and structure of the pores matter. Larger pore diameters actually decrease detonation velocity, so manufacturers carefully control the prilling process to produce a specific pore structure optimized for blasting performance. Porous-grade prills are specifically recommended over fertilizer-grade prills for blasting companies because their higher absorption index makes them more energy-efficient and more predictable in detonation.
Sulfur, Detergents, and Other Industrial Prills
Fertilizer and explosives get the most attention, but prills show up across a wide range of industries. Sulfur prills are used in rubber vulcanization, asphalt production, paper bleaching, insecticide manufacturing, and dye production. Detergent ingredients are often prilled to create the small, uniform beads you see in powdered laundry and dishwasher products. Sodium hydroxide (caustic soda) is another chemical commonly sold in prill form for easier, safer handling compared to flakes or liquids.
In all these cases, the appeal is the same: prills flow freely, measure consistently, dissolve predictably, and generate less dust than powders or irregular chunks.
Prilling in Pharmaceuticals and Food
On the smaller end of the scale, prilling has become a valuable encapsulation technique for drug delivery and food technology. Instead of producing millimeter-sized beads, pharmaceutical prilling creates particles at the micrometer or even nanometer scale, trapping active ingredients inside a biodegradable shell.
In one application, researchers used prilling combined with solvent evaporation to create nanoparticles loaded with curcumin, an anti-inflammatory compound. The resulting particles achieved encapsulation efficiency between 66% and 76%, meaning most of the active ingredient was successfully trapped inside the bead rather than lost during manufacturing. By adjusting how much drug was loaded into each particle, researchers could control the release timeline: lower-loaded particles released 90% of their contents within 6 hours, while higher-loaded versions sustained a slow release over 7 days. That kind of tunability makes prilling attractive for wound healing applications and targeted drug delivery, where you want a medication to release gradually rather than all at once.
The FDA has approved the biodegradable polymers commonly used in these pharmaceutical prilling processes, and the technique is considered scalable enough for commercial production, not just laboratory experiments.