Direct Coagulation Casting of Alumina Suspension by High Valence Counter Ions Using <scp> <scp>Ca</scp> </scp> ( <scp> <scp>IO</scp> </scp> ₃ ) ₂ as Coagulating Agent

Abstract

A new direct coagulation casting of aqueous alumina suspension was developed via controlled release of high valence counter ions from calcium iodate with increase in the temperature from 55°C to 70°C. The influence of calcium iodate on the rheology of alumina suspension was investigated. A small amount of calcium iodate increased the viscosity of the concentrated alumina suspension at high temperature and finally transformed it into a wet‐coagulated body. The mechanism of coagulation is proposed such as that the solubility of calcium iodate increases with increase in temperature. The high valence Ca 2+ ions diffuse into the double electrical layer of alumina particles surface through electrostatic attraction, reduces the zeta potential, hence decreases repulsive force between particles. Also reaction between Ca 2+ and citrate leads to insufficient dispersant coverage on the particle surface. Both factors contribute to the coagulation of the suspension. The coagulation time was from 1 to 4 h by maintaining the temperature in the range of 55°C–70°C. The wet‐coagulated bodies prepared from 50 vol% alumina suspension showed a high compressive strength of 2.6–3.2 MPa with uniform microstructure. The relative density of sintered sample is 99.4% at 1550 °C for 2 h with perfect microstructure.