THE DE-VITRIFICATION OF ALUMINO-SILICATE CERAMIC FIBRE MATERIALS—THE KINETICS OF THE FORMATION OF DIFFERENT CRYSTALLINE PHASES

Abstract

Abstract The steel industry makes extensive use of alumino-silicate fibre products, particularly as linings in high temperature furnaces, where their outstanding heat insulation properties confer substantial energy conservation benefits. These products can have a range of compositions, and the fibre may be initially in a crystalline and/or amorphous material. De-vitrification may occur during the normal service life of the product and the crystalline phases formed may create health hazards during furnace relining operaticns. There is therefore a need both to characterise these fibres in terms of their rate of de-vitrification and the component phases that are formed. The de-vitrification process in five common grades of alumino-silicate based ceramic fibre insulation has been studied, through the use of X-ray diffraction methods, over a wide range of temperatures (1050–1500 °C) and times (up to 4004 h). The main phases that formed have been identified as mullite, α-cristobalite and a second form of cristobalite designated α′-cristobalite. Using previously determined calibration curves, the amounts of these three phases have been quantified in three of the ceramic fibre types (a standard alumino-silicate fibre, a chromium-containing variant, and a zirconium-containing variant) over the full range of temperatures/times examined. Unquantified details of the phases present in two other fibre grades (a crystalline mullite fibre and a mixed alumino-silicate based fibre) are also presented. Based on the results obtained, ideas are presented for the evolution of the phases observed. Scanning electron microscopy (SEM) has also been used to examine the effects of heat treatment on the physical structure of the fibres. Changes in the texture of the fibre were only observed after a significant quantity of the fibre had become crystalline. Large quantities of globular material (shot) were observed in the fibre types examined. X-ray diffraction studies of the behaviour of this shot has revealed that in some cases it behaves differently, in terms of its de-vitrification rate, to the bulk material when heat treated. This study of the crystalline phases present in ceramic fibre materials used in module form in working furnaces has demonstrated that it is possible to assess any potential health hazard of these materials prior to their removal.