| In oder to cope with the serious challeges of the scarcity of the high-quality bauxite mineral resources on alumina-silica system refractories and aim at the problems of Si3N4/Sialon matrix in traditional non-oxide composite refractories (i.e., high cost of synthetic raw materials) and the problems raised during the firing process of non-oxide composite refractories at high temperature (e.g., high energy consumption, unstable quality and high cost), in this doctoral dissertation, we advanced an idea, namely, the oxynitride transformation of natural minerals with high Al2O3/SiO2(including high-alumina-content solid waste resources) by carbothermal/aluminothermic reduction-nitridation technologies. The studies on firing-free technologies and performance optimization of the as-synthesized oxynitride composite SiC refractories were performed. Meanwhile, a new generation high-performance Si3N4/Sialon bonded SiC composite refractories by in-situ transition metal-catalytic nitride reaction was investigated. Many important researches were made.The process parameters effects, such as the raw materials components, synthesis temperature and reductant amount, on the phase compositions, morphologies/microstructures of the transformed products were analyzed systematically.The natural minerals, i.e., quartz, low-grade bauxite and magnesite, as well as industrial solid wastes, i.e., fly ash, coal gangue, aluminum ash were transformed to β-Si3N4, P-Sialon, Ca-a-Sialon and spinel-corundum-Sialon composite materials via CRN/ARN at1400℃-1600℃.The chemically pure raw materials for comparative study were transformed to Ca-Dy-a-Sialon and Li-a-Sialon at higher temperature of1700℃. The above results provided technical and theoretical basis for the application of the related non-oxides transformation of minerals in refractories.Transition metals Fe, Co and Ni could promote the nitridation of Si powders as well as Si-Al-Al2O3powders to form Si3N4/β-Sialon and improve their nano-fibers contents. One-dimensional α-Si3N4,β-Sialon nanowire/nanobelts were synthesized by catalytic thermal chemical vapor deposition method. Si3N4/Sialon bonded SiC composites were prepared by in-situ Co-catalytic nitridation reaction at1350-1450℃.When the added amount of Co is0.5%-1.0%, the bending strength of the composites fired at1400℃could be improved by50%-80%(reaching60MPa) compared to that without Co.The mechanisms of strength improvement in the Si3N4/Sialon bonded SiC composites through in-situ Co-catalytic nitridation reaction are proposed to be co-dominated by "catalyst promoting liquid phase sintering " and "strengthen via in-situ fibers (whiskers)" mechanisms.The oxynitride transformed from the natural minerals was applied in the firing-free refractories. The relationships among the material compositions, microstructure, mechanical properties and high temperature slag-resistance properties was discussed, and the slag erosion mechanism of prepared firing-free refractories also discussed by means of crystallography and mineralogy theory. Sialon-ZrN composites were synthesized from low-grade bauxite and zircon by CRN at1600℃, and the Sialon-ZrN-SiC firing-free composite refractories were prepared by using phenolic resin as the binder. The room-temperature strength of the as-prepared samples was provided by the resin, and the high-temperature strength is obtained by the sintering of the materials. The long-columnar Sialon grains with transgranular fracture are benefit to the strength improvement. There are two aspects erosion of blast furnace slag to firing-free Sialon-ZrN-SiC composite refractories:the refractories dissolved in the slag and slag penetrated into the refractories. In this work, the as-prepared Sialon-ZrN-SiC firing-free refractories have good slag erosion resistance, resulted from the ZrO2which was oxidated from ZrN and didn’t react with the oxides in the slag to the formation of low-melting substances, consequently play a key role in resistance to slag erosion. |
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