Influence of Calcination Temperature on the Densification of Refractory Mullite Grains Synthesized from Recycled Materials
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Dense refractory mullite (3Al2O3·2SiO2) grains were synthesized by mechanical mixing of calculated amounts of pure ultra-fine Al-hydroxide and water-treated fumed silica powders. The powders were obtained by acid leaching of recycled Al-dross and boiling of fumed silica in water. Mixed powder samples were calcined at 900, 1000, 1100 and 1200°C and characterized with respect to phase composition, surface area, linear shrinkage and bulk density. They were then re-powdered, pressed and re-fired at 1650°C to determine which pre-calcined batch achieved maximum densification of its mullite grains. The solid phase composition, microstructure and microchemistry refractory properties of the dense, fired mullite samples were investigated, including volume stability and load-bearing capacity in terms of the temperature of maximum expansion (To), the temperature at beginning of subsidence under load (Ta), and the rate of creep at 1500°C. XRD, SEM and EDS were applied in the analysis.
Pre-calcination of the precipitated Al hydroxide/washed fumed silica at 1000–1100°C yielded a reactive amorphous Al2O3/SiO2 powder mixture with some crystalline major corundum and minor silica phases. This phase assemblage was best able to form dense stable mullite grains when re-fired at temperatures up to 1650°C. The mullite showed high refractory quality when tested for load-bearing capacity and volume stability. On firing under load up to 1500°C, the material linearly expanded 0.7%, with beginning of subsidence occurring above 1500°C. The sample also had minimal creep rate (0.2%/h) on firing under load for 1 h at 1500°C, and 0.0% permanent linear change after re-firing at 1600°C. As a result, the dense mullite samples prepared from mixed powders pre-calcined at 1000–1100°C can be recommended for applications at or above 1600°C service temperatures.
Keywordssynthetic mullite calcination densification refractory quality volume stability loadbearing capacity
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