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Prills are solid chemical beads created by spraying molten liquid and cooling the droplets. They are used for fertilizers, mining blasting agents, and drug production. Co-current and counter-current fluidized bed technology is used to make them. Prills can be made from ammonium nitrate, sulfur, and molten metals. They can also be used to produce beaded products with a central core surrounded by an inert or protective shell. Production methods vary depending on the size of beads needed.
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 production are common uses for prills. They are not compressed forms of the chemical, such as can be found in tablets, but are instead cooled liquids.
One method of making prills is a combination of co-current and counter-current fluidized bed technology. Co- or counter-current flow refers to the direction of flow of the gas used to suspend the solid as it is being produced. The molten liquid is first sprayed into a co-current bed, often using non-reactive nitrogen gas, creating small balls of product that move in the same direction as the gas. The product then moves to a counter-flow tank where the product is suspended in the 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 solid form and can be ignited with a detonator 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 by melting and spraying pure sulfur. Fertilizers can be made from mixtures of molten sulfur and additives such as clay that are sprayed and cooled. Molten metals can be used to make prill beads. Quantities of cooled metal beads can be mixed to form metal alloys which can then be remelted and formed into desired shapes.
Another use for making prills is to produce beaded products that contain a central core surrounded by an inert or protective shell. The inner bead is first produced using a spray prilling process. Following this stage, the bead may be coated with a second material in a mixing or spray painting stage. Drug capsules containing active ingredient beads can be produced in this way.
Production methods vary depending on the size of beads needed. Static or non-moving spray nozzles create a larger array of beads than those produced by rotating nozzles. The droplet size can also be varied using different gas flow directions and temperatures. The smaller prill sizes may be more useful in pharmaceuticals and cosmetics, and the larger sizes for industrial and fertilizer uses.
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