Granulation of ultra-fine powders: Examination of granule microstructure, consolidation behavior, and powder feeding

Nathan B Davis, Purdue University

Abstract

Ultra-fine powders, sized between 0.1–10 µm, are commonly used materials in a variety of industries including detergents, catalysts, paint pigments, and agricultural products. Ultra-fine powders are known to have complex behaviors due to cohesive forces and are considered difficult to handle, feed, and form consistent final products. One specific issue is that wet granules formed from ultra-fine powders are difficult to densify and take significant amounts of time to reach a desired granule density. This thesis hypothesizes that formation of complex structures within ultra-fine powder granules is responsible for densification issues and that the granule microstructure can be controlled through careful powder handling and preparation prior to the granulation. Furthermore, this thesis hypothesizes that ultra-fine powders have unique behaviors separate from larger primary particles which requires specific studies in the applicable size range to understand ultra-fine powder granulation. In order to investigate this hypothesis, methods are developed to describe the granule microstructure and specifically the granule void phase distribution in 3D space. X-ray computed tomography (XRCT) and image analysis techniques are used to isolate, identify and describe the spatial distribution of the various granule phases. Additional methods for distinguishing between large macro-voids and the pore space between primary particles are created as well as tools for quantifying macro-void size, shape, volume fraction in the granule (ϵvoid) and distribution of the voids within the granule structure. Descriptions of macro-void size, shape, and volume fraction show that the macro-void properties depend upon primary particle size, powder history, liquid binder and method of granule formation. The granule microstructure measurement methods are developed to describe the internal structure of single-droplet nuclei granules formed in a static powder bed. Alumina powders with mean size varying from 0.5 to 100 µm are used as model powders with water and polymer solutions as the liquid binders. The size, shape, and macro-void volume fraction (ϵvoid) of the macro-voids is used to describe the effects of primary particle size and powder bed preparation on granule microstructure. Granules formed from ultra-fine powders show the presence of large spheroidal macro-voids distributed throughout a particle matrix. Granules formed from coarser powders show either no macro-voids or non-spheroidal macro-voids which are described as “cracks” within the granule microstructure. Smaller primary particles within the ultra-fine powder range are found to increase the size and ϵvoid of measured macro-voids and the complexity of the structure. The maximum void size and ϵvoid are dependent upon the powder bed preparation technique. Sifting the material through a 1.4 mm sieve produces a larger maximum macro-voids size and larger ϵvoid than producing powder beds by sifting through either 710 µm or 500 µm sieves. Sifting of 0.5 µm primary particles results in formation of stable, large, spheroidal agglomerates while other tested materials do not form stable structures from sifting. The developed methods are also applied to single-droplet granules formed in a tumbling drum to investigate granule microstructures from a moving bed. The effects of consolidation time and liquid binder viscosity are also evaluated and the results are compared to predictions from the surface-tension flow model of the nucleation immersion mechanism developed by Hounslow et al. As consolidation time increases, the thickness of the powder shell slowly increases, but the internal void structure is unchanged. Increasing the liquid binder viscosity increases the void size and ϵvoid. Granules formed from the 25 µm powder have a simpler structure. They have a uniform packing structure of the primary particles surrounding a central void. This structure forms within the first few seconds and is then unaffected by either time on liquid viscosity. Apart from the persistence of the central void, the kinetics of nucleation for the coarse powder agrees with Hounslow’s model. However, the ultrafine powder granules have a complex multiscale structure that is not predicted from a simple nucleation model. Finally, the feeding behavior of several ultra-fine and coarser powders used in the pharmaceutical industry is evaluated using the relative standard deviation (RSD) of the mass flow rate for a twin-screw feeder run in volumetric mode to predict quality of feeding. The RSD results are compared to bulk powder flow properties measured with the FT4 Powder Rheometer. The RSD of the mass flow rate does not correlate with any of the measured properties. The quality of material feeding and likelihood of failure remains a complicated endeavor with multiple types of failure that are not well described by a single material property. (Abstract shortened by ProQuest.)

Degree

Ph.D.

Advisors

Litster, Purdue University.

Subject Area

Chemical engineering

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