A prilling technique was used to produce droplets of aluminum sulfate hydrates in the size range of 2-2.5 mm. Production of spherical particles of aluminum sulfate hydrates by using a prilling technique has never been studied, nor carried out. The effects of the orifice geometry and hydrostatic head on drop formation were measured. The flow rates of hydrated aluminum sulfate melt through bores of 0.08 and 0.11 cm were measured for liquid heads of 10-100 cm. It was found that the break-up of a liquid jet produced particles in the size range 2.0-2.5 mm with a 0.8 mm orifice. The modified Meister and Scheele correlation, which is the first application on a high viscous fluid in air system, was found to be satisfactory for estimating the particle size obtained by the jet break-up mechanism. (C) 2000 Society of Chemical Industry.