Influence of slurry parameters on the characteristics of spray-dried granules

A systematic study has been performed to determine how the characteristics of granules prepared by spray drying aqueous alumina slurries are influence d by processing parameters: binder type (PEG Compound 20M or PEG-8000), sol ids 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 i s presented. The packing density of the primary particles within the granul es is lower for slurries with higher yield stress and is dependent on the s lurry solids loading. Granules prepared using 0.35 wt% deflocculant (0.14 m g/m(2)), which correspond to high slurry yield stress, are of solid morphol ogy, whereas higher deflocculant levels result in hollow granules that cont ain a single large open pore or crater. The degree to which particles are a ble to rearrange during drying influences the final granule density and is determined by the strength of the flee structure, as indicated by the slurr y yield stress. When the yield stress is low, a crater may form from the in ward 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 inte rnal pressure lower than that of the surrounding atmosphere.