Effect of kyanite on rheological properties of dense aqueous alumina suspensions

Abstract

The present work examines the effect of pH on aqueous suspensions of alumina and alumina–kyanite mixtures to assess the influence of kyanite additions on suspension properties. This system is of interest because on heating to temperatures above 1400°C the mineral kyanite (Al₂O₃ . SiO₂ ) reacts to form silica and mullite (3Al₂O₃ . 2SiO₂ ) and as the latter mineral has attractive elevated temperature properties, its incorporation into a common refractory material such as alumina is of importance in the field of technical ceramics. A popular method to produce such materials is to prepare a stable suspension of the components of interest and cast a green compact from the slurry; this compact is subsequently sintered at an appropriate temperature to densify the product. As the solids content of the original suspension influences the final product density, rheological properties are of critical importance. Both microelectrophoresis and rheological techniques suggested that a pH of 3–4 provided optimum stability. The flow behaviour of the binary mixture could be predicted by the Casson model and it is suggested that the surface characteristics of the kyanite were primarily responsible for the resulting stability regime.