Effects of pH and concentration of aqueous alumina suspensions on pressure filtration rate and green microstructure of consolidated powder cake

Pressure filtration rate, packing density and microstructural fluctuation in a consolidated powder cake were studied with 5-50 vol% aqueous suspensions of single crystal alumina particles of 0.5mum size under a pressure range of 200 to 400 kPa. The decreased height of the suspension during the pressure filtration followed t(m) of time, where m was in the range form 0.57 to 0.67 at 10 vol% solids (depending on pH of the suspensions) and approached 0.5 at around 20 vol% solids. The consolidation time was longest for the weakly charged particles near the isoelectric point because of the formation of a uniform distribution of small pore channels in the cake. The positively charged particles were densely agglomerated during pressure filtration but the formation of relatively large pore channels between dense particle clusters shortened the filtration time. The packing density of the formed cake increased in the following order: weakly charged particles near the isoelectric point < negatively charged particles < positively charged particles. Increase in the solid content over 30 vol% in the acidic suspensions enhanced the packing density to 65% of theoretical density and also improved the uniformity of green microstructure. The degree of microstructural uniformity (G) of a consolidated alumina compact was quantitatively defined by G= (dN/dS) / (N/S), where N is the number of particles included in a square of observed area, S, on a scanning electron microscope photograph and expressed by a polynomial approximation of S. N/S represents the average number of particles/unit area. The conditions G > 1 and G < 1 correspond to dense and porous packing of particles in an arbitrary square area, respectively. The microstructural fluctuation was evaluated with the G value.