Mineral and textural transformations in mixtures of Al-rich and Al–K-rich clays with firing: Refractory potential of the fired products

The present work analyses a set of cylinders manufactured by pressing and fired from 1000 °C to 1270 °C, which are composed of pure Al-rich rocks (bauxite) and mixtures of bauxite with Al–K-rich clays. The aim of the study is to determine the mineral and textural transformations that take place in the mixtures with firing and their influence on the physical properties of the final products in order to evaluate their refractory potential by comparing them with various fired commercial bauxites. To this end, raw and fired samples were analysed by X-ray diffraction, X-ray fluorescence, and optical and field emission scanning electron microscopy, and significant physical properties (e.g. density, linear shrinkage, porosity, colour, water absorption, thermal conductivity and point load resistance) were determined in the fired cylinders. The raw bauxite was mainly formed by boehmite, kaolinite and hematite whereas the illite- and kaolinite-rich clay was formed by quartz, illite and kaolinite. Kaolinite, illite and boehmite are not detected at 1000 °C and, from this temperature upwards, mullite, corundum, ¿-Al2O3, hercynite, ilmenite, cristobalite and vitreous phase are formed. Changes in physical properties with rising temperature are associated with the mineralogical changes. The increases in density and linear shrinkage are related to the formation of vitreous phase and the crystallization of mullite, and correlate with the decrease in porosity, water absorption and thermal conductivity. The changes in the colour of the samples are related to the hematite content at each temperature, whereas the point load resistance is greater with higher corundum content. Both the firing temperature and the clay content play an important role in the refractory potential of the mixtures, since samples mixed with illite- and kaolinite-rich clay present similar properties at lower temperatures to those of the fired commercial bauxites.