Abstract The refractory materials used in the wall of the furnaces for glass melting can be prepared from mullite-zirconia composite material. The composite of mullite/zirconia was synthesized from Iraqi kaolin, γ-alumina, and zirconia using thermal decomposition with reaction sintering at 1600 °C. Several batches were prepared with various ratios of kaolin, γ-alumina, and zirconia, and the composite compositions were selected from the Al2O3-SiO2-ZrO2 phase diagram. The mullite-zirconia composite was prepared with different steps beginning with milling the starting materials, semi-dry uniaxial pressing, and then reactive sintering at various temperatures (1200, 1400, and 1600 °C). The predicted phases ZrO2 and Al6Si2O13 were identified by X-ray diffraction patterns according to the phase diagram for all the batches. The lower amount of the zirconia added to mullite reduced porosity and improved the bulk density of the mullite/zirconia composite. The thermal expansion coefficient slightly increased with the addition of zirconia. It also enhanced the thermal shock resistance of the composite. Finally, the mechanical properties were improved by increasing the amount of zirconia particles in a matrix of mullite due to the phase transformation of zirconia from tetragonal to monoclinic phase.

Resumo No presente estudo, um método de recristalização foi implementado para recuperar pó de alumina a partir de cristais de alúmen de amônio. O alúmen de amônio foi completamente dissolvido em água e tratado por ultrassonicação para evitar a aglomeração do cristal de alúmen. O precipitado branco foi seco a 150 °C por 6 h e as calcinações em diferentes temperaturas foram realizadas durante 2 h. Os resultados de DRX indicaram a estrutura cristalina da alumina em duas fases principais: γ-Al2O3 e α-Al2O3 a 800 e 1200 °C, respectivamente. Os resultados da isoterma de adsorção/dessorção de N2 indicaram que a área de superfície do pó na fase γ, que pode ser aplicada em catalisadores, foi de 142,5 m2/g, enquanto que na fase α foi de 15,3 m2/g. As morfologias elucidaram que as partículas de pó estavam amplamente distribuídas na faixa de ≤160 nm em diferentes temperaturas de calcinação e isso pode ser atribuído a incrementos na aglomeração de partículas à medida que a temperatura de calcinação aumentou.

Abstract In the present study, a recrystallization method was implemented to recover alumina powder from ammonium alum crystal. The ammonium alum was completely dissolved in water and treated by ultra-sonication to prevent the agglomeration of the alum crystal. The white precipitate was dried at 150 °C for 6 h, and calcinations at different temperatures were performed for 2 h. The XRD results indicated the crystalline structure of alumina with two main phases: γ-Al2O3 and α-Al2O3 at 800 and 1200 °C, respectively. The N2 adsorption/desorption isotherm results indicated that the surface area for the powder in the γ phase, which can be applied in catalysts, was 142.5 m2/g, while, in the α-phase, it was 15.3 m2/g. The morphologies elucidated that the powder particles were widely distributed in the range of ≤160 nm at different calcination temperatures and this may be attributed to increments in the particle agglomeration as the calcination temperature increased.