Separation of aluminum oxide powders with different degrees of aggregation by sedimentation in an aqueous medium

The dynamics of change in the average size of particles and their concentration in the sedimentation process of stable aqueous suspensions of two polydisperse nanopowders of aluminum oxide is studied. The specific surface area of nanopowder Al2O3-IAM obtained by the plasma-chemical method was 9 m2/g, which corresponds to a weighted average size of 169 nm; it was nonaggregated in a stabilized aqueous suspension. The specific surface area of nanopowder Al2O3-117ns obtained by the electric explosion method was 20 m2/g; the weighted average size was 80 nm, and it was aggregated in an aqueous suspension to a considerable degree. The method of dynamic light scattering was used to study the size distribution upon the sedimentation of suspensions with a concentration of 5 g/l for 500 h at various depths from the surface; it is shown that the dynamics of decrease in the average size of particles at any depth obeys the classical Stokes law on the assumption of the independence of the sedimentation of individual particles and aggregates. This counts in favor of the fact that no additional aggregation of particles occurs in the sedimentation process of diluted stable nanopowder suspensions. On the other hand, it was experimentally shown that the suspension concentration decreases during sedimentation much faster than it should according to the Stokes law, which indicates that the classical model has restrictions; however, their cause is not quite clear.