A systematic study has been performed to determine how the characteristics of granules prepared by spray drying aqueous alumina slurries are influenced by processing parameters: binder type (PEG Compound 20M or PEG-8000), solids loading (30 or 40 vol%), ammonium polyacrylate deflocculant level (0.35-1.00 wt%), and spray-dryer type. Correlations between slurry rheology and granule characteristics have been made, and a model for granule formation is presented. The packing density of the primary particles within the granules is lower for slurries with higher yield stress and is dependent on the slurry solids loading. Granules prepared using 0.35 wt% deflocculant (0.14 mg/m 2 ), which correspond to high slurry yield stress, are of solid morphology, whereas higher deflocculant levels result in hollow granules that contain a single large open pore or crater. The degree to which particles are able to rearrange during drying influences the final granule density and is determined by the strength of the floc structure, as indicated by the slurry yield stress. When the yield stress is low, a crater may form from the inward collapse of the surface of a forming granule when the particle packing density in a droplet continues to increase after the droplet size becomes fixed by the formation of a rigid shell, leaving an internal void with internal pressure lower than that of the surrounding atmosphere.

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Alumina specimens were prepared by compacting wellcharacterized spray-dried alumina granules of excellent deformability, followed by sintering. The strength and Weibull modulus were measured and microstructure was evaluated for specimens prepared from granules with different density and morphology. Compacts of spheroidal granules with lower density and lower granule strength sintered to 98% relative density and had fewer microstructural defects than compacts prepared from spheroidal or hollow granules of higher density. Flexural strength of sintered material was found to depend on initial granule size but not density or morphology of the granules. Weibull moduli of 18-20 were independent of particular granule characteristics.