Agglomeration of poorly wetting powders is a common problem in pharmaceutical, minerals and fertilizer granulation. There are very few clear studies of poorly wetting systems, although many papers contain a statement to the effect that good wetting is a prerequisite for good nucleation in particle agglomeration. Although this is usually true, robust granulations with poorly wetting powder are possible.

A mixture containing approximately 67% by weight (90% by volume) of a micronized, hydrophobic powder was granulated in a high shear mixer using water. The microstructure of resultant agglomerates (granules) was studied using optical and electron microscopy as well as X-ray computed tomography (XRCT). The study revealed that granules are typically spherical or elliptical in shape and range in size from 200 to 500 microns. The granule structure is a consolidated powder shell surrounding an empty core. The hollow structure was strong enough to survive up to 15 minutes of granulation and downstream processing operations such as drying and milling. Based on the observed microstructure, a nucleation mechanism for hydrophobic systems is proposed. Implications for controlling granule growth and their properties are discussed