Preparation Of ZrO₂ -nitride Composite Powder And It's Effects On The Properties Of Al₂O₃-C Refractories

Because of the carbon pickup of carbon-containing refractories used in continous casting, the production of clean steel calls for developing low carbon and carbon-free refractories increasingly. Owing to low wettability with molten slag and high thermal conductivity of carbon, Al₂O₃-C refractory materials have good thermal shock resistance and slag corrosion resistance. Therefore, how to ensure the properties of Al₂O₃-C refractories when reducing the carbon content simply is the current hot topics. In this work, low carbon Al₂O₃-C refractories used in clean steel production were developed by addition of ZrO₂ and nitride.In view of low resource utilization and unhomogeneous of ZrO₂ and nitride mixture, ZrO₂-nitride composite powder was synthesized by the method of carbothermal reduction and nitridation (CRN) of zircon. The process of CRN of zircon was discussed by thermodynamic analysis. Using zircon powder(-320 mush) as raw materails, N₂(99.999%)as the nitrogen source; the roles of technological parameters such as carbon source, carbon content, reaction time, reaction temperature, shaping pressure, nitorgen flow rate and mixing time on the CRN of zircon were studied. ZrO₂-nitride composite powder in large scale was synthesized by the method of CRN of zircon. The effects of the ZrO₂-nitride composite powder on the properties of the A1₂O₃-C refractories were investigated.The thermodynamic analysis shows that the stability domains of reaction products as well as the starting temperature of carbothermal reduction and nitridation of zircon have relations to the partial pressure of nitrogen and monoxide carbon in the furnace. The composites with different composions can be obtained by controlling the reaction temperature and the partial pressure of nitrogen and monoxide carbon. With increasing the partial pressure of N₂ and decreasing that of CO, the starting temperature of the reaction decreases.The results showed that the different carbon contents have different reaction products, the higher the carbon content, the lower the starting temperature of CRN of zircon. Reaction temperature has vital effect on reaction rate and product phase composition. In case of carbon content 22%, the zircon phase and m-ZrO₂ phase decrease with increasing temperature, while the Zr₇N₈O₄ phase increases firstly and then decreases and the ZrN phase increases continually. The effects of shaping pressure, nitorgen flow rate and mixing time have little effect on product phase composition, and increasing nitorgen flow rate and mixing time, decreasing shaping pressure can speed up the reaction rate. The XRD results of ZrO₂-nitride composite powder showed that the relative content of ZrO₂ and nitride reach about 30% and 70%, respectively. The mainly phases of the nitride are: Zr₇O₈N₄(30%), Si₂N₂O (20-30%), besides of them, some of ZrN(5%) andβ-Si3N4(3%) were also obtained.The oxidation resistance and thermal shock resistance were improved obviously by the addition of the synthesized ZrO₂-nitride composite powder, while the hot modulus of rupture decreased. Permanent linear change, bulk density, apparent porosity, cold crushing strength and cold modulus of rupture can be improved by the addition of the synthesized ZrO₂-nitride composite powder. The optimal addition content of composite powder is 5%.The results and achievements by this work deserve to be a refrence and guidance to further work in this connection.