Float-Sink of Large Objects in a Gas-Solid Fluidized Bed Different from That in a Liquid

The gas-solid fluidized bed has liquid-like properties. Large objects float or sink in the fluidized bed based on density difference like in a liquid. Dry separation using the float-sink is in great demand as an alternative to the commonly used wet separations which have various environmental issues. The fundamental studies and applied research in the field of mineral processing and waste treatment has been actively carried out. The float-sink has been discussed based on Archimedes principle, considering the density difference between the fluidized bed and the objects. Although Archimedes principle properly works to explain most of the float-sink, we observed some float-sink phenomena deviated from Archimedes principle. Here we report the three anomalous float-sink phenomena. 1) A smaller density sphere sinks deeper than a larger density sphere when the air velocity is just below the minimum fluidization velocity (u₀/u_mf = 0.95) and the sphere density is close to the bulk density of the powder bed (ρ_sphere/ρ_bulk ≈ 1). The larger density sphere sinks instantly without air bubbles, whereas the smaller density sphere sinks slowly with air bubbles around it. 2) Spheres tend to sink with increasing u₀/u_mf from 2 to 4 due to the bed expansion and the decrease of the apparent fluidized bed density, whereas the spheres sinking becomes unstable at u₀/u_mf > 4. The float-sink depends on the fluidized powder bed height. 3) When the sphere density is just below the apparent fluidized bed density, the sphere floats if only one sphere is put onto the fluidized bed, whereas the spheres sink if many spheres are put onto the fluidized bed. The possible mechanisms will be discussed with considering the local fluidization around the sphere, the fluid force affected by the bubbles motion and the fluidized powder bed motion, many-body effect in the fluidized bed and so on.