Pressure filtration of alumina suspensions under alternating current field

The relation of applied pressure-height of piston during pressure filtration was measured for aqueous 7 vol.% suspensions of positively charged 150 nm alumina particles at 0.2 mm/min of crosshead speed of piston. The dispersed suspension changed to a flocculated suspension at a critical applied pressure (phase transition pressure, ΔPtc), and the flocculated suspension was consolidated at a low pressure (≈ΔPtc). Application of alternating current field in a frequency range from 0.01 to 1000 Hz at 1.5 or 10 V accelerated the colloidal phase transition and decreased the consolidation pressure of flocculated suspensions. This result was analyzed by calculating the migration distance of charged particles and ions included in a suspension. The periodic polarization of ions in an electrical double layer under AC field enhances the electrostatic attraction between polarized colloidal particles, accelerating the flocculation of alumina particles. New filtration theories were proposed to explain the applied pressure-suspension height relation for flocculated suspensions, and were in accordance with the experimental results.