Microstructures and properties of microporous mullite‐corundum aggregates for lightweight refractories

Abstract

Five microporous mullite-corundum refractory aggregates were prepared from Al(OH) 3 and kaolinite gangue through in situ decomposition synthesis technique. The effects of the sintering temperature (1400-1600 • C) and the particle sizes of raw materials (20.6-94.5 μm) on the microstructures and strengths of the aggregates were investigated through X-ray diffractometer, scanning electron microscopy, and energy-dispersive spectrometer etc., to find out the technological conditions to be controlled in industrial production. The higher sintering temperature promoted the reaction between Al(OH) 3 and kaolinite gangue, leading to the development of primary-mullite as well as the generation of secondary-mullite, which promoted the formation of the neck and improved the strength. Meanwhile, the dense mullite layers were formed continuously on the surface of Al(OH) 3 pseudomorphs, making the micropores inside the pseudomorphs become closed pores, which increased the closed porosity of the aggregates. The reduction of the particle sizes of raw materials changed the particle packing behavior, accelerated the rearrangement of the Al(OH) 3 pseudomorph particles during the process of reactive sintering, and then reduced the closed porosity. To realize the industrial production of microporous mullitecorundum refractory aggregate with high strength (103 MPa) and high closed porosity (16.1%), the sintering temperature should be at about 1600 • C, and the median diameter of raw materials should be at 94.5 μm.

Effect of Tm2O3 on the properties of corundum‐mullite composite ceramics for solar heat transmission pipelines

“…At the same time, the bulk density increased as the temperature increased from 1400 to 1600°C and decreased to 0.18% for water absorption of the sintered sample AT5 at 1550°C, which significantly complied with the criterion for high‐density ceramics (water absorption < 0.5%). The rise in temperature will increase the amount of liquid phase, eliminating pores through the viscous flow of the liquid phase and accelerating grain rearrangement and growth, resulting in densification of the ceramic microstructure 29 . Furthermore, the addition of Tm 2 O 3 also significantly reduced the apparent porosity and water absorption, which are related to the internal evolution of the microstructure of ceramics, for which an explanation will be given in the SEM section.…”

“…The rise in temperature will increase the amount of liquid phase, eliminating pores through the viscous flow of the liquid phase and accelerating grain rearrangement and growth, resulting in densification of the ceramic microstructure. 29 Furthermore, the addition of Tm 2 O 3 also significantly reduced the apparent porosity and water absorption, which are related to the internal evolution of the microstructure of ceramics, for which an explanation will be given in the SEM section.…”

Properties and microstructures of porous corundum‐spinel aggregates fabricated via the Kirkendall effect

“…[6][7][8][9][10] To overcome these disadvantages, lightweight refractories can be used as working lining because they can increase ther-mal efficiency and decrease energy consumption while maintaining the slag resistance and mechanical properties of the refractories. [11][12][13][14][15][16] Currently, lightweight Al 2 O 3 -MgO refractory castables are primarily fabricated by substituting porous aggregates with tabular corundum aggregates. The pore-structural parameters and microstructures of porous corundum aggregates considerably affect the bulk density, thermal conductivity, and slag resistance of Al 2 O 3 -MgO refractory castables.…”

“…The pore-structural parameters and microstructures of porous corundum aggregates considerably affect the bulk density, thermal conductivity, and slag resistance of Al 2 O 3 -MgO refractory castables. [17][18][19] The preparation methods of porous corundum-based aggregates, including high-temperature foaming, 20 in situ decomposition, 12,19,21 and pore-forming agent addition, 7,18,22 have been extensively studied. Yuan et al 20 fabricated porous Al 2 O 3 -based aggregates with a closed porosity of ∼16% using submicron-sized Al 2 O 3 and MgO powders as raw materials and SiC as a hightemperature foaming agent.…”

“…The high bulk density and thermal conductivity of traditional Al 2 O 3 –MgO refractory castables are attributed to the use of dense tabular alumina aggregates during their preparation, which results in high energy consumption and considerable heat loss 6–10 . To overcome these disadvantages, lightweight refractories can be used as working lining because they can increase thermal efficiency and decrease energy consumption while maintaining the slag resistance and mechanical properties of the refractories 11–16 …”