Freeze-coating process for producing sulfur-coated urea

Abstract

A freeze-coating process has been developed for the continuous production of sulfur-coated urea. This process consists essentially of immersing urea granules (initially at room temperature) in molten sulfur and removing the coated urea from the melt by centrifugal action. The entire coating operation is completed in about 1 s compared with 10 to 15 min for conventional spray-coating processes. The coating thickness and quality of coated urea produced in a laboratory-scale unit were found to be, inter alia, functions of residence time in the melt (0.6-0.8 s) and sulfur temperature (130-144 °C). Products containing 20 to 50 wt % sulfur were examined. Pinhole formation In the coats is not well understood and must be reduced to achieve high product quality.