Slip casting process for the manufacture of tubular alumina microfiltration membranes

A thorough investigation of the slip casting process for the manufacture of tubular alumina microfiltration membranes is presented. For this means an initial powder of an average particle size of 2 mu m and broad size distribution (up to 10 mu m) for imparting initial large pores during the slip casting process was used. The dispersing ability of sodium carboxymethylcellulose (Na-CMC) and Tiron (C(6)H(4)O(8)S(2)Na(2)center dot H(2)O) for slips containing 40, 50 and 60 wt. % of alumina was studied. It was shown that Na-CMC is not able to act as a proper dispersant. The kinetics of the slip casting process and time dependences of cast two-dimensional profile were investigated in function of slip concentration. The effect of sintering temperature on the pore microstructure of the final products was investigated. In all cases, the cross sections of the tubular membranes consisted of two regions: an inner thick section, consisting of relatively large pores, and a thin outer section, consisting of smaller pores. This phenomenon was attributed to Na-CMC migration during drying prior to sintering. The ratio of the thicknesses of the two sections depended on the slip concentration and sintering temperature. The average permeable pore size, based on dry nitrogen permeability experiments, was in the range of 0.13-0.24 mu m. The controlled use of partial binder migration during cast drying for the production of graded microfilter membranes was addressed.