Solidification of droplets of molten material free-falling against an upward stream of air in a tower. It is a process used extensively in nitrogen fertilizer manufacturing. Melt is dispersed in the top of the tower at a temperature just above the solidification point of the material being processed. The dispersion arrangement, air flow, tower dimensions, and feed material are selected so the droplets approach a spherical shape and solidify before reaching the bottom of the tower.