Dispersion of fine and ultrafine powders through surface modification and rapid expansion

Abstract

Improved dispersion of fine (<30μm) and ultrafine powders (<100nm) in gas medium is addressed. For fine powders, two surface modification approaches for producing dispersible powders are considered: first, suitable for powders >10μm where attrition is minimum, and second, suitable for finer particles including inhalable (2–5μm), where simultaneous micronization and surface modification is performed. In addition to improved dispersibility, surface modification is found to add significant corresponding benefits such as improved flowability, aeratibility or fluidizability and packing densities, leading to potential cost savings in handling and storage. Dispersibility of surface modified fine powders is assessed using Sympatec/Rodos through dispersion pressure titration. Flowability and bulk density improvements as corroborative measures are assessed using Hosokawa Powder Tester and FT4 Freeman Powder Rheometer. The indices such as flow function coefficient, angle of repose, bulk density and aeration are measured. For ultrafine powders (nano-particles <100nm), where surface modification is not applicable, deagglomeration via rapid expansion of high pressure or supercritical suspensions (REHPS) of nano-particle aggregates is considered and shown to be highly effective for their dispersion. The size distribution of fragmented nano-powders is characterized by online Scanning Mobility Particle Spectrometer (SMPS) and by offline Scanning Electron Microscopy (SEM). SMPS and SEM measurements indicate that the average agglomerate sizes are well below 1μm, consistent with the length scales observed in our complementary REHPS mixing experiments using alumina and silica nano-powders. In summary, industrially relevant powder dispersion approaches are presented, applicable to both fine and ultrafine powders.