Influence of Slurry Parameters on the Characteristics of Spray‐Dried Granules

Abstract

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.

Role of suspension preparation in the spray drying process to obtain nano/submicrostructured YSZ powders for atmospheric plasma spraying

“…Likewise, for the same solids content, granules formed by submicron-sized particles were denser than those made up of nanoparticles due to the higher packing efficiency of coarser particles. On the other hand, granule size increased when the viscosity of the starting suspension augmented owing to larger droplet size formed in the spray drying chamber [32,33]. Figure 13 displays the good linear correlation between granule size and suspension viscosity measured at shear rate of 1000 s -1 .…”

El proceso de secado por atomización: formación de gránulos y cinética de secado de gotas

“…La relación entre las condiciones de operación y las características del polvo atomizado se estudiaron para líquidos puros, soluciones acuosas (nitrato amónico, sulfato sódico, silicato sódico, cloruro sódico) y suspensiones de tipo alimentario (leche en polvo, café). A partir de los años 80 estos estudios se extendieron al uso de materiales utilizados en la industria cerámica como son SiO 2 , Al 2 O 3 , ZrO 2 y Si 3 N 4 (20)(21)(22)(23)(24)(25). En estos estudios se obtuvieron correlaciones que relacionaban las propiedades de las suspensiones (contenido en sólidos, temperatura y presencia de aditivos) con las propiedades finales del polvo atomizado obtenido (morfología, microestructura, densidad, resistencia mecánica y fluidez).…”

Freeze granulation: Powder processing for transparent alumina applications

“…powder load and additives) and drying temperature. 25,29 Binder migration during the drying can additionally reduce the homogeneity and thereby quality of the granules, leading to inhomogeneities in the green bodies. 30 Freeze granulation is an alternative to spray drying, solving many of its limitations.…”

“…When well controlled, self-flowing granulated powders allow automated production of highly dense and defect-free green bodies by relatively quick and simple uniaxial or isostatic pressing methods. The main granulation method used in ceramic processing is spray drying, [23][24][25] with freeze granulation attracting increasing interest. 26,27 During spray drying, a powder suspension is sprayed into a hot air cyclone.…”