The effect of naturally-occurring interparticle forces on the fluidisation characteristics of fine iron oxide powders

Abstract

Finely ground magnetite has some unusual fluidisation characteristics. This could be due to the fact that small particles can display natural magnetic dipoles, and such particles could align themselves. This investigation is aimed at examining the role of forces of this nature in a fluidised bed of milled magnetite. The experimental approach involved oxidising a batch of magnetic to haematite. A powder of similar size and density to the parent material is thus obtained, but this powder does not have any potential for magnetic interactions. Fluidisation tests were conducted on both powders. The results suggest that dense-phase voidage, measured by the bed collapse technique, is insensitive to the chemical structure of the solid, and is strongly influenced by the fraction of fine particles present. Entrainment rates, on the other hand, respond strongly to the change in chemical composition resulting in a sharp reduction in particle carryover in the case of magnetite. Flocculation in the freeboard, driven by magnetic effects, is likely responsible for this. It is suggested that capillary forces dominate the behaviour of the dense phase, and that the composition-based influence in the lean phase is related to the fact that the associated forces are active over a relatively longer range.