Ayub [28] prepared sulfur coated urea in a 2-D spouted bed and
evaluated the effects of spouting air temperature, atomizing air, and
liquefied sulfur flow rates on the quality of sulfur coated urea in terms
of the dissolution rate. He posited that the dissolution rate was a function
of spouting air temperature but the rate remained unaffected by
the atomizing air flow rate. For example, dissolution was 100% and
95.61% at spouting air temperatures of 69 °C and 82.5 °C at atomizing
air flow rates of 1.0 and 1.4 m3/h, respectively. The major dependence
of coating quality on spouting air temperature was determined in
terms of sulfur's behavior at different temperatures. At lower spouting
air temperatures, the exterior of the sulfur particles solidified prior to
coating the urea's surface. To the contrary, sulfur particles close to melting
point solidified after coating the urea granules resulting in a more
uniformcoating layer. The seven-day dissolution trial resulted in aminimal
level of 95.61% at the highest spouting air temperature (82.5 °C),
with a sulfur flow rate of 33.9 g/min and atomizing air flow rate of
1.4 m3/h. Orthorhombic (Beta) sulfur is amorphous in nature and
most suitable for encasing other polymer materials to enhance coating
longevity. Whereas monoclinic (Alpha) sulfur is crystalline and
subject to cracks and fissures which reduce coating life. Moreover,
beta sulfur readily converts to alpha sulfur at about 60 °C. To retard
the transformation from the amorphous to crystalline phase and
thereby strengthen sulfur coating against cracking and deformation,