Design and fabrication of alumina-mullite refractories with improved microstructure and insulation properties using direct ink writing

To address the challenge of preparing alumina-mullite refractory linings with complex geometries for sulfur incinerators, alumina-mullite thermal insulation refractories with controllable pore structures were fabricated using direct ink writing. This process used rice husks coated with different concentrations of alumina sol to effectively control their water absorption properties. The study explored the effects of alumina sol concentration on the rheological behavior and printability of the inks, as well as the influence of the alumina sol coating on the pore structure, phase composition, and key performance metrics (physical properties, thermal shock resistance, thermal conductivity, and acid corrosion resistance) of the final samples. Interestingly, increasing the alumina sol concentration initially decreased the viscosity and thixotropy of the ink, followed by an increased, while also affecting the shear-thinning behavior, yield stress and flow stress. Compared to refractories made with rice husk coated with a low concentration alumina sol, the presence of mullite whiskers formed in the pores left by the rice husk burnout not only mitigated the residual quartz phase issue but also enhanced the microporosity. This improvement led to increased strength, thermal insulation, and acid corrosion resistance. After heating to 1500 °C, the refractory made with rice husk coated with 15 wt% alumina sol achieved 25.8 % porosity, 73.6 MPa compressive strength, 3.21 W/m·K thermal conductivity, 85 % residual flexural strength ratio, and excellent resistance to acid corrosion. This method shows great potential for producing refractory linings with complex shapes and further enhancing material performance by optimizing the microstructure.