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High-strength and low-thermal mullite products

 The mullite corundum products have high strength, good thermal shock resistance, strong erosion resistance, and high temperature volume stability. The lightweight mullite corundum material has a porous or fibrous structure, and usually has a small thermal conductivity, thermal insulation and sound absorption effect. Well, it is widely used as the insulation layer of kiln furnaces in the iron and steel and chemical industries, as well as the fire-resistant insulation layer and sound insulation layer in the construction industry.
       High-alumina bauxite and metallurgical silica fume are used as raw materials, polystyrene balls are used as pore-forming agents, and high-strength and light-weight mullite-corundum products are prepared using the gel injection molding process, mainly to explore the optimum pH and dispersant of the slurry. The strength, porosity, thermal conductivity, phase composition, and microstructure of the sintered sample at 1550°C were measured. The mullite-corundum material prepared had a density of 0.37 g/cm3, porosity of 87.37%, and strength of 2.17 MPa. Thermal conductivity is only 0.22 W (m·k).
       The main materials used this time are high alumina bauxite (Al2O3 94.55%, D50=44um), silica fume (SiO2 92.32%) and polystyrene spheres (particle size <3mm). A polyacrylamide gel system solution was used as the binder, triammonium citrate was used as the dispersant, and the pH of the slurry was adjusted with hydrochloric acid and ammonia water.
       Mix 75% of high alumina bauxite and 25% of silica fume (mass fraction, the same below), add the dispersant and some of the binder to make the premixed liquid, stir the slurry for 1 hour, and adjust the material with hydrochloric acid and ammonia water. The pH of the pulp. The pore-forming agent and the remaining binder were added and stirred for 0.5 h to make a suspension for injection molding. After injecting the mold, it was naturally solidified for 12 hours. After demolding, it was dried at 80° C. for 12 hours, and then kept at 1550° C. for 5 hours to obtain the product.
Slurry performance measurement
       The effect of pH on the viscosity at different temperatures is shown in Figure 1. At 4 temperatures of 30, 40, 50, and 60°C, the viscosity decreases as the pH increases. With the increase of pH value, the decrease of viscosity has a tendency to slow down. When the PH value is about 9, the viscosity tends to be stable, with little change. According to the colloid chemistry, adjusting the pH value away from the isopotential point, the absolute value of the Zeta potential is large, and the repulsive ability is increased, which is favorable for the dispersion of the particles in the liquid phase, and thus the viscosity is small. However, as the pH value is further increased, due to the introduction of more OH- in the suspension, the concentration of the electrolyte in the suspension is increased, the electric double layer is compressed, the potential is lowered, and the particles are easily agglomerated and settled. In order to effectively control the preparation process, the best PH=9 was selected for this experiment.
       Effect of Dispersant Content on Viscosity of Slurry
       The effect of the amount of dispersant on the viscosity of the slurry is shown in Figure 2. When the amount of dispersant is small, the anionic clusters adsorbed on the surface of the particles are less, the surface charge density is lower, and the repulsion force between the particles is smaller, so the fluidity of the slurry is poor and the particle size is large. With the increase of the dispersant dosage, The anionic clusters adsorbed on the surface of the particles increase, the surface charge density increases, and the electrostatic repulsion between particles also increases. At the same time, more dispersants are added to increase the steric hindrance, the fluidity of the slurry is significantly improved, and the viscosity is significantly reduced.
  Effect of Ball Ratio on Product Bulk Density
       It can be seen from Table 1 that the addition amount of the pore-forming agent polystyrene sphere has a significant influence on the density of the sintered product. With the addition of the pore-forming agent, the bulk density of the material decreases and the porosity increases. When the ratio of balls is 1:1, the volume is 0.885 g/cm3, the porosity is 69.5%, and when the ratio of balls is 4:1, the density is 0.367 g/cm3 and the porosity is 87.37%.
       in conclusion
       (1) The optimum ph value of the slurry of high alumina bauxite and silica fume is 9 and the optimum amount of dispersant is 0.33%. At this time, the slurry has good fluidity.
       (2) An insulation material with a minimum bulk density of only 0.37 g/cm3 was prepared, the porosity was as high as 87.37%, the thermal conductivity was only 0.22 W/(m·K), and the strength was as high as 2.17 MPa.
       (3) The light mullite-corundum product prepared has a main crystalline phase of mullite and contains a small amount of corundum phase. The pores are evenly distributed. The walls of the mullite-corundum material are thin and complete, and small pores are enclosed to ensure that the material has a low density. , high strength, low thermal conductivity and other characteristics, excellent overall performance.\
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