Synthesis Of Mullite Materials From Waste Activated Alumina

In this research, waste activated alumina and longyan kaolin were used as rawmaterials to synthesize mullite materials. The essential component of waste activatedalumina is γ-Al2O3with high activity, which can accelerate the solid state reaction andthus increase the content of mullite. At the same time, reaction temperature can belowered, which leads to the decrease of cost and improvement of economic efficiency.So, this research belongs to compositive utilization of solid waste and possesses greatenvironmental protection and economic meaning.The influences of formula, calcination temperature, holding time and differentcontents of mineralizers on the phase composition, microstructures and properties ofsynthesized mullite materials were mainly investigated. X-ray diffraction and scanningelectron microscopy techniques were used to characterize the crystalline phases andmicrostructures of sintered specimen, and the content of crystalline phases weredetermined by the Rietveld Quantification method. Moreover, the properties such asbending strength, water adsorption and bulk density were also tested in this research.The results reveal that Al4.59Si1.41O9.7solid solution and α-Al2O3are formed indifferent samples. And Al4.59Si1.41O9.7is the main phase which presents in acicular andcolumnar shape, belonging to island structure, rhombic crystal system and pbam spacegroup. Meanwhile, the α-Al2O3presents in granular and deformed sheet shape,belonging to trigonal crystal system, R3c space group. The content of Al4.59Si1.41O9.7decreased gradually with the increase of waste activated alumina, while the content ofα-Al2O3increased. And the sample No.1has a better microstructure and excellentperformance. The content of mullite increased gradually with the increase of calcinationtemperature, while the content of α-Al2O3decreased. With the increase ofcalcination temperature, the content of liquid phase in samples increased also, and theviscosity of liquid phase decreased, which is beneficial to the growth of crystals. Andthe sample sintered at1390℃has good microstructure and excellent performance. The content of mullite with2h holding time is85.78%which is thehighest in sintered samples. Based on a comprehensive analysis, the optimalprescription was determined to be sample No.1, which consists of46wt%wasteactivated alumina and54wt%longyan kaolin. The sintering temperature of1390℃isdetermined to be the optimal and the preferred holding time is2hours, and the contentof mullite solid solution is85.78wt%. Meanwhile its bending strength is36.41MPa,bending strength maintenance after thermal shock is46.69%, water absorption is17.67%, apparent porosity is34.57%, bulk density is1.956g·cm-3. So, the properamount of CaF2and TiO2mineralizer can accelerate the diffusion of particles, whichcan help to form mullite in the solid state reaction and then increase its content. Thesetwo mineralizers improve the crystallization and grain growth，and they also canoptimize the performance and thermal stability of the samples. Addition of a littleamount of Na2SiF6and BaF2mineralizer can decrease the viscosity of liquid phasewhich is beneficial to the growth of crystals. And they can promote the growth of themullite grain, hlep form staggered network structure, thus can significantly improve thethermal stability of the samples. Based on a comprehensive analysis, the optimalamount of CaF2additive is determined to be1.0%. the optimal amount of Na2SiF6，BaF2and TiO2additive is all determined to be0.5%.