High Temperature Stability And Densification Mechanism Of Zircon Refractories

The rapid development of alkali-free glass fiber industry has put forward higher requirements for refractories used in glass kilns.Refractories were required not to cause pollution to glass liquid and to form structural defects such as streaks and stones.At the same time,the implementation of"Green Manufacturing 2025","carbon peak,carbon neutral"and other policies required refractories to reduce energy consumption and saved energy in the service process.Therefore,it was of great practical significance to prepare a refractory material which could reduce energy consumption,meet service conditions,have long service life and do not pollute glass liquid.Zircon refractories could be widely used in alkali free fiberglass kilns due to the excellent resistance to glass liquid erosion,non-pollution of glass molten,non-streaking and stone formation,and excellent thermal shock resistance.However,zircon is not easy to densification and decomposition reaction at high temperature wouldl affect its service performance and service life.In view of the above problems,zircon was selected as the basic raw material in this topic,the main research work includes:investigating the decomposition reaction mechanism of zircon,establishing zircon decomposition reaction kinetics model by using thermodynamics and dynamics models,and exploring the relationship between the decomposition process and zircon sintering densification process;investigating the oxide decomposition process of zircon and the subsequent evolution of microstructure to set up the connection between the evolution of phases,microstructure and physical properties;preparing the high sphericity zircon aggregates with a core-shell double-layer structure by the granulation method to study the influence of sintering additives on aggregate properties;preparing zircon refractories made of zircon spherical aggregate and zircon fine powder by using different molding methods,so as to investigate the evolution of its properties and its interaction with glass fiber melt.The research conclusions are as follows:（1）The decomposition reaction of natural zircon could be divided into two stages.The temperature of the first stage decomposition reaction was between 1200-1400°C,at this time it was the interface reaction,and the mechanism equation is g（x）=1-（1-α）^1/3.Impurities and liquid phase were the main driving force of the decomposition of zircon.The temperature range of the second decomposition reaction was above 1400°C,and the reaction at this time was the controlled diffusion reaction.The mechanism equation is g（x）=（1-2/3α）-（1-α）^2/3.Temperature is the main driving force of the decomposition of zircon.Meanwhile,decomposition reaction of zircon at high temperature and the reverse reaction in the cooling process caused the volume expansion and contraction,which generated pores at the original grain location resulting in the sample density drop.The average size and number of pores were the main factors to affect the strength of zircon sample.（2）In the first stage of zircon decomposition reaction,Ca O and Mn O accelerated the decomposition rate of zircon mainly through the formation of liquid phase,while in the high temperature stage,Mn O promoted the growth of zircon grain,and the addition of Ca O further increased the formation of liquid phase and accelerated the decomposition of zircon.Mg O,Ti O₂and Al₂O₃had the little effect on the decomposition reaction of zircon in the first stage,and promoted the decomposition reaction of zircon in the second stage due to the solid phase reaction,but the solid phase reaction could fill part of the pores generated by the decomposition reaction.There was no reaction between Cr₂O₃and zircon,neither compounds nor solid solutions could be generated,but as lattice growth inhibitors hinder the process of zircon grain growth.（3）Zircon spherical aggregate with a double core-shell structure was prepared by granulation method and CMC as bonding system.The double layer structure was caused by the difference of capillary force between surface and inside of the aggregate by the granulation method and the sintering characteristics of zircon.In addition,the activation energy of reaction betweenγ-Al₂O₃and Si O₂was obviously higher,which resulted in more mullite phase in the system.And the higher the content of Na₂O caused the higher liquid phase content and the relatively low viscosity,which resulted in faster infiltration segregation behavior and accelerated the formation of mullite to improve the performance of zircon spherical aggregate.With the 3 wt%addition ofγ-Al₂O₃,zircon spherical aggregate has the best properties.（4）Zircon refractories were prepared by mechanical pressing,isostatic pressing and gel casting process,respectively.Although the sample prepared by isostatic pressing and gel injection molding process showed the apparent porosity were close.The closed pores existed in the zircon refractory prepared by the gel casting was the main reason for the lower bulk density.The zircon refractories prepared by isostatic pressing rendered the apparent porosity of 15.6%,the bulk density of 3.91 g/cm³and the high compressive strength of 188.9 MPa.Meanwhile,the introduction of zircon spherical aggregate could significantly enhance the glass fiber permeability resistance.