Phase and microstructural evolution of high TiO₂-containing Iranian bauxite at high temperatures in different atmospheres

R. Naghizadeh,H. R. Rezaie,F. Golestani-fard 한양대학교 세라믹연구소 2008 Journal of Ceramic Processing Research Vol.9 No.4

Calcined bauxite is one of the components of alumina-magnesia-carbon refractories for steelmaking applications when in these refractories two types atmosphere at the surface and inside of the refractories exist in service conditions. In this study, Iranian bauxite powders were pressed with and without graphite additions, fired at 1300 oC and 1600 oC in air or reducing atmospheres. The phase evolution of samples was investigated using X-ray diffraction. It was shown that the samples fired at 1600 oC in an air atmosphere, contain corundum, 3 : 2 mullite, tialite, rutile and in a reducing atmosphere corundum, alumina-rich mullite (Al1.7Si0.15O2.85), rutile and a reduced form of titanium oxide (Ti2O3) and titanium nitride (TiN) were present. For an explanation of alteration the 3 : 2 mullite to alumina-rich mullite and decomposition of tialite into the parent oxides, the microstructures of samples fired at 1600 oC were investigated. Also the effect of a reducing atmosphere at high temperatures on high purity TiO2 and a mixture of high purity TiO2 plus Al2O3 in the stoichiometric ratio of tialite were inspected. From these results, a new explanation for the mechanism of the alteration 3 : 2 mullite to alumina-rich type and decomposition of tialite were established. Calcined bauxite is one of the components of alumina-magnesia-carbon refractories for steelmaking applications when in these refractories two types atmosphere at the surface and inside of the refractories exist in service conditions. In this study, Iranian bauxite powders were pressed with and without graphite additions, fired at 1300 oC and 1600 oC in air or reducing atmospheres. The phase evolution of samples was investigated using X-ray diffraction. It was shown that the samples fired at 1600 oC in an air atmosphere, contain corundum, 3 : 2 mullite, tialite, rutile and in a reducing atmosphere corundum, alumina-rich mullite (Al1.7Si0.15O2.85), rutile and a reduced form of titanium oxide (Ti2O3) and titanium nitride (TiN) were present. For an explanation of alteration the 3 : 2 mullite to alumina-rich mullite and decomposition of tialite into the parent oxides, the microstructures of samples fired at 1600 oC were investigated. Also the effect of a reducing atmosphere at high temperatures on high purity TiO2 and a mixture of high purity TiO2 plus Al2O3 in the stoichiometric ratio of tialite were inspected. From these results, a new explanation for the mechanism of the alteration 3 : 2 mullite to alumina-rich type and decomposition of tialite were established.

An evaluation on sol-gel chemical processing of refractory barium hexa aluminate fibrous structures

H.R. Rezaie,R. Naghizadeh,F. Arianpour,R. Ghasemzadeh,S. Eslami 한양대학교 세라믹연구소 2009 Journal of Ceramic Processing Research Vol.10 No.2

In this research barium hexa aluminate fibrous refractory products were produced by a chemical processing route. Boehmite was used as aluminum precursor while barium nitrate was used as barium precursor and the final solution had made according to stoichiometric BaO.6Al2O3 composition. It was found that two different gels could be obtained during this route. Mainly the secondary gel was trapped into the initial gel structure. Finally gels were heated at various temperatures (1350, 1650℃) to following the formation of the barium hexa aluminate in fibrous structure due to sintering. Then the crystallization of transitional phases such as 3BaO.Al2O3 and BaO.Al2O3 to BaO.6Al2O3 were followed at different sintering temperatures by X-ray diffractometry (XRD) and Fourier transform infrared (FTIR) spectroscopy. Also scanning electron microscopy (SEM) was used to study the microstructural changes and micrographies. The produced gels were characterized by thermo gravimetrical and differential thermal analysis (TG/DTA). The results showed that BaO.6Al2O3 was the major phase at higher temperatures and fibrous structure appeared in final samples. In this research barium hexa aluminate fibrous refractory products were produced by a chemical processing route. Boehmite was used as aluminum precursor while barium nitrate was used as barium precursor and the final solution had made according to stoichiometric BaO.6Al2O3 composition. It was found that two different gels could be obtained during this route. Mainly the secondary gel was trapped into the initial gel structure. Finally gels were heated at various temperatures (1350, 1650℃) to following the formation of the barium hexa aluminate in fibrous structure due to sintering. Then the crystallization of transitional phases such as 3BaO.Al2O3 and BaO.Al2O3 to BaO.6Al2O3 were followed at different sintering temperatures by X-ray diffractometry (XRD) and Fourier transform infrared (FTIR) spectroscopy. Also scanning electron microscopy (SEM) was used to study the microstructural changes and micrographies. The produced gels were characterized by thermo gravimetrical and differential thermal analysis (TG/DTA). The results showed that BaO.6Al2O3 was the major phase at higher temperatures and fibrous structure appeared in final samples.

An investigation on the properties and microstructure of mullite-bonded cordierite ceramics

M.Nouri. Khezrabadi,R. Naghizadeh,P. Assadollahpour,S.H. Mirhosseini 한양대학교 세라믹연구소 2007 Journal of Ceramic Processing Research Vol.8 No.6

The fabrication of a mullite-bonded cordierite body suitable for use as kiln furniture has been investigated in this article. First of all cordierite powder was synthesized by mixing talc, kaolin and alumina as starting materials, pressing and firing up to 1325oC. The amount of cordierite phase was estimated by an X-ray diffraction technique. The synthesized cordierite was then mixed with certain amounts of kaolin and alumina as the starting materials for mullite formation and the mixture was shaped by a pressing method, following by firing at different temperatures. The changes of density, porosity and cold crushing strength of the samples were measured as a function of heat treatment temperature. The results showed that these properties improve by a rise in temperature. The mullite formation within the samples was confirmed by an XRD method. The thermal expansion coefficient of the samples fired at 1350oC was determined by a dilatometer having an average value of 3.18×10−6 (1/K), which indicates an excellent thermal shock resistant for the samples. The microstructure of the fired samples was studied using scanning electron microscopy (SEM), which showed interesting results. The fabrication of a mullite-bonded cordierite body suitable for use as kiln furniture has been investigated in this article. First of all cordierite powder was synthesized by mixing talc, kaolin and alumina as starting materials, pressing and firing up to 1325oC. The amount of cordierite phase was estimated by an X-ray diffraction technique. The synthesized cordierite was then mixed with certain amounts of kaolin and alumina as the starting materials for mullite formation and the mixture was shaped by a pressing method, following by firing at different temperatures. The changes of density, porosity and cold crushing strength of the samples were measured as a function of heat treatment temperature. The results showed that these properties improve by a rise in temperature. The mullite formation within the samples was confirmed by an XRD method. The thermal expansion coefficient of the samples fired at 1350oC was determined by a dilatometer having an average value of 3.18×10−6 (1/K), which indicates an excellent thermal shock resistant for the samples. The microstructure of the fired samples was studied using scanning electron microscopy (SEM), which showed interesting results.