| China is one of the countries which are rich in boron resources in the world.64%boron resource is concentrated in Liaoning Province in the form of szaibelyite and ludwigite, except some in Qinghai Province and Tebit. Szaibelyite is mainly used as the raw material to produce borax and boric acid, which lead to the production of a number of waste residue of boron sludge. Ludwigite is a mine including Fe, B, Mg and other elements, but there will be lots of tailings after mineral processing. Boron sludge and the tailings are the typital solid wastes, and their main compositions are MgO and SiO2. Currently, boron sludge and tailings are not used effectively, and their numerous stacking have caused serious environmental impact, therefore, the comprehensive utilization of boron sludge and tailings need in-depth study urgently.In this paper, the assumption of preparing MgAlON/SiAlON composites ceramic with the tailings as raw material is put forward for the first time, and MgAlON/SiAlON composite ceramic is synthesized successfully. This is an effective way of overall value-added utilization of solid waste in industrial. The main researches of this subject are the preparation of MgAlON/SiAlON composite powder by carbothermal reduction-nitridation (CRN) form boron sludge and the milltrailings of ludwigite, the production of MgAlON/SiAlON composite ceramics using composite powders as starting materials. In this passage, the sintering process is different from that with pure substances as raw materials; this is a new subject, as well as the study of MgAlON/SiAlON composite ceramics preparation. This work is an innovation on both academic value and practical applications.Firstly, thermodynamics conditions and phase-equilibrium relationships of MgAlON and SiAlON were analyzed. The effects of carbon content, synthesis temperature, holding time, N2flow rate on the synthesis process were studied. The results show that the amount of MgAlON decrease with the increasing of the carbon content while the amount of SiAlON increase, but SiAlON is going to decompose when the carbon is excessive. The growth in synthesis temperature and holding time is favorable for the formation of MgAlON/SiAION phase. The volatilization of CO and SiO, the gas phase products, results in a mass loss of the samples, which is enhanced with the incease of the synthesis tempurature, holding time and N2flow rate. As a result, the optimum processing conditions for MgAlON/SiAlON composite powders synthesis with milltailings of ludwigite are as follows:sintering at1450～1500℃for4h with a N2flow rate of400ml/min and a carbon content of1.2time of the stoichiometric one. The optimum processing conditions for MgAlON/SiAlON composite powders synthesis with boron sludge as follows:sintering at1500℃for6h with a N2flow rate of400ml/min, a Si/Al of original proportion and a carbon content of1.5time of the stoichiometric one.Secondly, the MgAlON/SiAlON composite ceramics were prepared by high temperature sintering in a protective powder bed in high-purity nitrogen atmosphere with the MgAlON/SiAlON powder, using1wt%Y2O3as the additive. The effects of additive on material densifying, sintering temperature on phase constituent, mincrostructure, structural performance, mechanism property and oxidation resistance were studied. The results show that:the best sintering additive is Y2O3, and the value is lwt%; bulk density, apparent porosity, linear shrinkage rate, mass loss rate, vickers hardness, bending strength of the composite increase with the increasing of temperature; the optimum sintering temperature of the composites is1600℃. The MgAlON/SiAION composite ceramic prepared at1600℃has a bulk density3.44g/cm3, apparent porosity2.92%, mass loss rate12.00%, linear shrinkage rate10.38%, vickers hardness16.62GPa, bending strength171MPa, respectively. The grains of MgAlON/SiAION fully developed, and mix with lamellar and columnar crystals at1600℃. Oxidation ractions of the materials occur at1000℃in the air and acceletate at1200℃. A compacf"oxidation film"is formed on the surface of the materials after oxidation at1200～1300℃, which prevents further oxidation. Parabolic model, i.e.,(Am/S)2=kt+C1000～1300℃, can describe the oxidation process very well. The apparent activation energy for oxidation is1.598×105J/mol. |
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