Prilled urea is formed by spraying molten urea from the top of a prilling tower and letting the droplets solidify as they fall through a rising air stream. The result is a small, near-spherical prill of uniform composition. Prilling is a simpler and older process than granulation, and it produces a smaller and mechanically weaker particle — differences that determine where prilled urea is the right choice and where it is not.

Substance identity

Specification

— typical commercial ranges; not a guaranteed specification

Properties

Chemically identical to granular urea: solubility roughly 108 g per 100 g water at 20 °C, markedly endothermic dissolution, melting point approximately 133 °C, critical relative humidity approximately 72–75 % at 30 °C, hydrolysis by soil urease to ammonium with the associated risk of surface ammonia loss, and acidification on nitrification of roughly 1.8 kg CaCO₃ per kg N as a textbook approximation.

The physical differences are what matter. A prill has roughly an order of magnitude less mass than a 3 mm granule, so it dissolves faster, is thrown a much shorter distance by a spinning disc, and is more affected by wind. Prills are also weaker: the rapid solidification of a falling droplet gives a less consolidated structure than layered granulation, so prills abrade and shatter more readily in transfer and in the spreader, producing more dust and more fines than granular urea over the same handling chain.

Applications

Dissolution. Fast, complete dissolution makes prilled urea the practical choice for urea solution make-up, foliar sprays (specify low-biuret material) and dissolving-tank operations, where the small particle is a straightforward advantage.

Direct application at modest working widths. Where the spreader works at narrow widths, or where application is by drill, hand or drone, the short throw distance of prills is not a limitation.

Industrial and technical use. Urea-formaldehyde and melamine resin manufacture, ruminant feed as non-protein nitrogen, and other technical uses where the material is dissolved or melted and particle ballistics are irrelevant.

Prilled urea should not be used in bulk blends with 2–4 mm granular DAP, MAP, MOP or granular ammonium sulphate. The size mismatch causes segregation during every loading, transport and discharge step, and the field consequence is nutrient banding that cannot be corrected by spreader settings. It is also a poor choice for wide-swath spinning-disc application.

Handling and storage

Dry, covered storage; keep bags sealed and limit stack height, as prills deform and cake under sustained load more readily than granules. Minimise transfers and drop heights to limit fines. Do not store blended with ammonium nitrate — the mixture’s critical relative humidity is roughly 18 % at 30 °C. Blends with ammonium sulphate fall to roughly 56 % at 30 °C and should be spread promptly rather than stored. Keep away from strong alkalis and strong oxidisers. Not classified as dangerous goods for transport.

Notes

Prilled urea is not diesel exhaust fluid grade; automotive urea solution is specified under ISO 22241 and requires far tighter purity than fertiliser urea. Where the material is destined for foliar use, low-biuret grade must be specified explicitly — a standard ≤ 1.0 % biuret product can cause leaf injury on sensitive crops.

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Category page paragraph — N + S in ammonium sulphate vs straight N sources

Ammonium sulphate supplies nitrogen and sulfur in the same particle, both in the form roots take up: ammonium-N and sulfate-S. Straight nitrogen sources such as urea and ammonium nitrate carry no sulfur, and atmospheric sulfur deposition has fallen sharply in industrialised regions, so sulfur now limits yield and grain protein on many soils. Because plants build cysteine and methionine from sulfate, nitrogen applied without sulfur is used inefficiently. Ammonium sulphate corrects both nutrients in one application.

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