Properties And Reaction Sintering Mechanisms Of Mullite Ceramics Prepared From High Aluminum Fly Ash

Owing to the unique interlocking grain structure, excellent mechanical properties, favourable thermal and chemical stability, mullite and mullite based ceramics have been widely used in many fields such as refractory materials, anti-corrosion liners, sliding wear ceramics, etc. The huge amounts of fly ash from the coal combustion in China annually has put forwards the great challenge for the Chinese government to utilize it efficiently. In this work, author take the advantage of phase-mineral and chemical compositions of high-aluminum fly ash collected from North China Thermal Power Plant, and prepared mullite ceramics at lower sintering temperatures, which was aimed to apply in high strength, corrosion-resistance reinforced structure materials. This research focused on the correlations of the properties and structures of the mullite ceramics from point of mullitization thermodynamics, reaction kinetics and crystalloid chemistry. For comparison, this thesis also discussed the mechanism of the mullite formation from silicon-aluminium xerogel system.Processing parameter optimization and improved mechanical characteristics were obtained in two representative series of mullite ceramics named as AS and AD from series of experiments. At the molar ratio of Al₂O₃/SiO₂=0.9( fly ash mass content is about 70%), sintered temperature 1550℃, residence time 2～5h, the prepared AS90 sample exhibited the following properties of water absorption 0.62～0.91%, apparent porosity 1.72～2.45%, bulk density 2.76～2.82g/cm³, bending strength 80⁹0MPa, which is up to the National Standard of China "Acid-resisting bricks and tiles" GB/T 8488-2001. Moreover, M2 and CF2 samples in AD series ceramics displayed much higher bending strength at 169 MPa and 131 MPa respectively.The mineralogical phases analysis indicated that the sample sintered below 1300℃ consisted glass phase, mullite, corundum and cristobalite, and the products fired at above 1450℃ contained only mullite and glass phase in the batches of AS system. It is estimated by standard curve methods from XRD analysis results that mullite content is about 80% in AS90 ceramics. For AD ceramics, mullite content and grain size is greatly affected by additives and their contents, the aspect ratio of mullite grain obtained is 4～7. The correlation between bending strength and porosity obeys "Ryskewitsch" formula, and the relationship between bending strength and the grain size of crystallies agree well with "Orowan" semi-experienced functions. However, there is no obvious correlation among the oxygen hole rate, Al₂O₃ content, lattice parameters of synthesized mullite. This may be attributed to the high dopant content of Ti⁴⁺,Fe³⁺ that caused distortion or aberration of octahedron site in mullite crystals.At initial and intermediate sintering stage, the viscosity of the melt decreased along exponential gradient with sintering time, the constant of sintering rateincreased from 0.08mm^-1 at 1100℃ to 3.42 min^-1 at 1500℃; and the reaction exponent lies within 0.29～0.52, apparent active energy of sintering reaction is calculated at about 178 kJ/mol within the temperature of 1100℃～1450℃ and 295 kJ/mol within 1450℃～1500℃. At intermediate and final stage, the grain growth of mullite consists with general crystal theory, the constant of mullite growth rate climbed from 1100 to 1500℃. The grain growth exponent n is 2.13 at 1300℃, which is interpreted that grain growth is controlled by chemical potential gradients diffusion. At the temperature of 1500℃, the grain growth exponent n amounts to 3, which showed that the growth rate is dominated by pore transferring and elimination. The results indicated that the apparent active energy of grain growth decrease from 226 kJ/mol (1100～1300℃) to 69 kJ/mol (1300～1500℃).This research proposed a model of mullitization mechanism of Fly ash and Bauxite reactants couples reaction, which was termed as cenosphere mullitization to summarize the sintering process of the reactions from high aluminum fly ash. Firstly, crystallization of mullite takes place around the pristine mullite seeds in the shell of the cenosphere of fly ash with the formation of gas. And then the hollow microspheres were broken for the swelling of gas and leading to green ceramics volume shrinkage. Lastly, the mullites crystals grows as the reaction route among α-Al₂O₃, amorphous SiO₂. It was found that mullitization reaction takes place in the order of: Al₂O₃(liq)+SiO₂(liq)>Al₂O₃(s)+SiO₂(liq)>Al₂O₃(liq)+SiO₂(s)>Al₂O₃(s)+ SiO₂(s) vis thermadynamic and kinetics calculation. In AS systems, the apparent free engery is ～100kJ/mol lower than those of decomposition reactions of andalusite, sillimanite and kyanite, and also lower than that of utilization of α-Al₂O₃+quartz or γ-Al₂O₃+quartz. The apparent mullitization active energy is around 151 kJ/mol in the rage of 1100℃～1500℃. When 4w_B%Na₂O added into the initial batches of AS90, the apparent crystallization active energy was evaluated to decrease further to 92kJ/mol in the rage of 1100℃～1300℃.In addition, nanosized mullite was successfully prepared by supercritical CO₂ and subsequent supercritical fluid extraction from the xerogel of mullite precursors. The mineralogical phases tracing experiment showed that the mullitization process underwent from 1200℃ to 1400℃ as followed by the phase transition of γ-Al₂O₃→ δ-Al₂O₃ →α-Al₂O₃.The prepared mullite ceramics of series AS (utilization rate of fly ash is above around 70%) and AD might find its application in middle-low temperature, anti-corrosion and high strength areas, such as acid-resisting brick, refractory materials and friction-resistance liners in boilers. The results provide inventive data in preparation of low-cost, high performance mullite based composites from high aluminum fly ash, and enrich the sintering mechanism of fly ash system in view of mullitization thermaldynamics and kinetics.