The Impact Of TiO₂ On The Phase Evolution And Properties Of Alumina-magnesia Refractory Castables

Due to the advantages of their components such as stable workability and good corrosion resistance,calcium aluminate bonded magnesium-aluminum castables become one of the most popular steel ladle lining materials.The in-situ spinel and calcium hexaluminate formation at high temperatures played a decisive role on the properties of castables.Recently,the regulation performance of alumina-magnesia refractory castables by adding different additives has been given more attention by researcher and engineers.TiO₂ is one of the most effective mineralizers.The effects of TiO₂ addition on the phase evolution and properties of alumina-magnesia refractory castables were studied,including the phase composition,microstructure,apparent porosity and cold modulus of rupture?CMOR?,as well as the kinetics of in situ spinel formation.In order to further understand the role of low melting phase in the phase evolution of castables,Na₂O-CaO-Al₂O₃-SiO₂?NCAS?and CaO-Al₂O₃-SiO₂?CAS?systems were selected as low melting phases.The effect of Na₂O and TiO₂ content on the physical properties of low melting phase and the reaction between low melting phase and?/?alumina were investigated.?1?The calcined magnesia was selected as the raw material of the castables by compared with the permanent linear changes.With TiO₂ addition,the initial formation temperature of CA₆ ws reduced,and the formation of spinel was accelerated.There were small amounts of C₂AS and nepheline in castables without TiO₂ after firing at 1250oC and 1350oC.C₂AS disappeared when the temperature rose to 1450oC.There were CaTiO₃ and unreacted Ti O₂ in castables with the addition of TiO₂ after firing at 1250oC.Accoding to the changes of the diffraction peak's position,TiO₂ dissolved into spinel at the initial stage.The more dissolution of TiO₂ into CA₆ was demonstrated with the increase of the amount of TiO₂ addition and the temperature.There were coarse CA₆platelets and particle spinel in castables without TiO₂.The addition of TiO₂ resulted in much finer CA₆ grains.Moreover,the spinel and CA₆ in the matrix of castables were bonded tightly with TiO₂ addition up to 3wt%.The structure of CA₆ platelets and the morphology of situ spinel significantly depended on TiO₂ addition.?2?The reaction rate constant of in-situ spinel formation in alumina-magnesia castables varied with the amount of TiO₂ addition and the increase of temperature.The maximum reaction rate constant and conversion rate of spinel were achieved in sample T2 after firing at 1450oC for 300 min.Due to the different distribution of TiO₂ in spinel and CA₆,the relation between the apparent activation energy of spinel formation and the amount of TiO₂ was not simple linearity.?3?A small amount of TiO₂?0.5-2wt%?added resulted in high permanent linear changes and apparent porosity firing at 1250oC and 1350oC.The castables with 2-3wt%TiO₂ had a certain shrinkage and low apparent porosity firing at 1450oC.The elastic modulus and CMOR of alumina-magnesia castables had been changed due to the combined effect of TiO₂ on in-situ reaction and sintering.?4?The softening and hemispherical point of low melting phases in Na₂O-CaO-Al₂O₃-SiO₂?NCAS?system increased with the addition of?-Al₂O₃.The phase transited from sodium melilite into Si-rich nepheline and CA₂.There was more obvious diffusion reaction between NCAS and?-Al₂O₃ at high temperatures.TiO₂ can reduce the melting and hemisphere point of the low melting point of CaO-Al₂O₃-SiO₂ system.It also can promote more liquid phase formation,resulting in the dissolution of alumina in the liquid phase.The results show that TiO₂ has a significant effect on the formation and evolution of the low melting phase.