Changes in the properties of aluminosilicate refractories during roasting and service

Abstract

The properties of aluminosilicate refractories and their service life depend on the composition and content of crystalline, amorphous (liquid), and gaseous phases. The principal crystalline phase of aluminosilicate refractories (chamotte and high-alumina refractories) is mullite. Above the composition of mullite, the refractories contain corundum; below the muilite composition, cristobalite and quartz are present. A second phase in the cold refractory is a vitreous substance; in the hot refractory, it is a melt, a third phase is air or a mixture of gases of complex composition; the gas phase composition may differ significantly in the open and closed pores. During roasting and service, each phase exerts an influence on the properties of the refractories and must be taken into account when developing heat-resistant linings.Much attention has been paid to the formation and change in the crystalline phases of refractories, but somewhat less to the vitreous substance, and even less to the role of the gas phase.This communication gives results of investigations, obtained during roasting of kaolin and kaolin--alumina briquets and aluminosilicate refractories in various gaseous media, and the calculated changes in the Gibbs free energies (AZ) over a wide range of partial pressures of oxygen for the reactions of formation of silica, silicon monoxide, and mullite.The quality of aluminosilicate refractories largely depends on the properties of chamotte. The principal requirement for the preparation of high-grade chamotte is purity of the clays, kaolins, and other initial materials. But the chemical composition of clays and kaolins cannot be the principal criterion for assessing the quality of the initial material. In a number of cases [1], chamotte and artifacts with different properties are obtained from kaolins of the same composition, roasted under the same conditions. This is due to the fact that certain kaolin varieties contain not only kaolinite but also admixtures of quartz sand and hydrargillite. In such cases the compositions of the initial material are equivalent, but the properties of the artifacts obtained differ. ~, 4 1 2O ~2 762, O 1Z