MgO calcination for easy direct coagulation casting of aqueous alumina slurries

Abstract The reactivity of MgO with ammonium poly(acrylate) and diammonium hydrogen citrate dispersants was decreased by high-temperature calcination which enabled easy preparation of direct coagulation casting slurries without cooling. The decrease in reactivity of MgO with an increase of calcination temperature (30–1200 °C) was due to the decrease of surface area (52.7–0.7 m2/g) as a result of an increase of average particle size (285–2075 nm) as well as a change of particle morphology from flaky to near spherical. The MgO calcined at a temperature of 1000 °C and above provided sufficient time for mixing with aqueous alumina slurries by ball milling at room temperature (~30 °C) without producing an adverse increase in viscosity before casting. The setting time of 55 vol% alumina slurries was in the ranges of 260–1070 and 10–50 min at room temperature and at 70 °C, respectively, at MgO concentrations in the range of 0.1123–1.2 wt%. The faster setting at 70 °C was due to a combination of faster dispersant-MgO reaction, faster hydration of MgO and high valance counter ion effect.