Compacting mechanism in high-speed centrifugal compaction process (part 1) - Observation of slips during compaction

The sedimentation behavior of particles during high-speed centrifugal compaction process (HCP) was investigated. Slips of alumina powders with various particle sizes (0.2-0.54 mum) were put into a glass tube and mounted in a high-speed centrifuge with a rotor radius of 120 mm. Then they were rotated at 3000-5000 rpm for 0.25-4 ks. The color change of slip was observed and the profiles of slip concentration in the glass tube was measured. During HCP, the slip divides into three zones with clear boundaries regardless of the compacting conditions. The upper part is merely water (clear zone), the middle is the slip (falling zone), and the lower is an unmovable sediment (compacted zone). Growing velocity of clear zone increases as the initial concentration of slip decreases. However, growing velocity of compacted zone is almost unaffected by the initial concentration. The growing velocity of the compacted zone, on the other hand, increases in proportion to the centrifugal force and to the square of particle size. This tendency is nearly the same as the Stokes' sedimentation law. Under various conditions, the concentration of slip at the boundary between the falling zone and the compacted zone is rather constant at around 50 vol%. This constancy is considered to be the main reason for the above mentioned characteristics of compacting velocity.