A prill is a bead of solid chemical compound created by spraying molten liquid and then cooling the droplets. Fertilizers, blasting agents for mining, and drug manufacturing are common uses for prills. They are not compressed forms of the chemical, such as what may be found in tablets, but are instead cooled liquids.
One method for producing prills is a combination of co-current and counter-current fluid bed technology. Co- or counter-current flow refers to the direction of flow of gas used to suspend the solid as it is produced. The molten liquid is first sprayed into a co-current bed, often using non-reactive nitrogen gas, creating small spheres of product moving in the same direction as the gas. Product then moves to a counter-current flow tank where the product is suspended in gas flowing in the opposite direction.
Ammonium nitrate is a common fertilizer used to add nitrogen to soils for plant growth. A mixture of ammonium nitrate and fuel oil, or ANFO, is used as a common blasting agent for mining. The mixture contains the fuel and oxidizer in a solid form, and can be ignited with a blasting cap or other ignition source. Ammonium nitrate fertilizer and ANFO are often produced as a prill that remains stable until needed.
Sulfur can be used as a fertilizer and chemical additive. A prill can be formed from melting and spraying the pure sulfur. Fertilizers can be produced from mixtures of molten sulfur and additives such as clay that are sprayed and cooled. Molten metals can be used to produce prill beads. Quantities of cooled metal beads can be mixed to form metal alloys which can then be re-melted and formed into desired shapes.
Another use for prill manufacturing is to produce beaded products that contain a central core surrounded by an inert or protective shell. The internal bead is produced first using a spray prilling process. Following this step, the bead can be coated with a second material in a mixing or spray-coating step. Drug capsules containing beads of active ingredients can be produced this way.
Production methods vary depending on the size of the beads that are needed. Static or non-moving spray nozzles create a range of beads larger than those produced from rotating nozzles. Droplet size can also be varied using different gas flow directions and temperatures. Smaller prill sizes may be more useful in drugs and cosmetics, and larger sizes for industrial uses and fertilizers.
