Study On Sintering Behavior And Mechanical Properties Of Al₂O₃-MgO-CaO Refractories

Al2O3-MgO refractories are widely used as working lining and furnace functional components in metallurgical furnaces.Because their excellent properties,such as high-temperature resistance,good thermal shock resistance and slag erosion resistance.However,the larger bulk density and higher cost of production hinder the development of these refractories.To lighten the refractory and thus save the energy and reduce the emission,CaO is usually introduced into Al2O-MgO system,hence formihg Al2O3-MgO-CaO refractories.Still some fundamental problems about the sintering behavior,microstructure and properties have not been studied systematically in the preparation of Al2O3-MgO-CaO refractories.Therefore,Al2O3,MgO,CaCO3 reagents were used as the main materials in this work and made into Al2O3-MgO-CaO refractories by two-stage sintering process.The suitable addition quantity of CaO was determined.After that,the effect of ZrO2 addition on the sintering densification behavior and mechanical properties of the refractories was further investigated.The major conclusions are as follows:(1)After preheated at 1200?,magnesia-alumina(MA)spineI reaction is inhibited in the samples with CaO addition.After preheated at 1500?,reactions in the Al2O3-MgO-CaO refractories are fully completed,which is helpful to eliminate the adverse impact of volume effect on reactions.(2)The compactness of raw materials is greatly enhanced by preheating.Compared with the low-temperature preheating(1200?),?-Al2O3 and MA grains grow up significantly,the compactness of clinker increases and the relative density of green bodies is above 60%after preheated at 1500?,However,the grain size of new phase becomes bigger after preheated at 1500? and the sintering performance is worse and energy consumption becomes higher.Therefore,1200? is considered as the best preheating temperature.(3)After preheated at 1200? and calcinited at 1600?,the lowest relative density of 67.1%is attained for the refractory when the content of CaO is 4%.Nevertheless,the relative density of the refractory reaches 78.2%when the content of CaO is 8%,which is higher than the relative density of the refractory without CaO addition.This is because the growth of CA2 grain in the system can effectively promote the sintering and densification process.Therefore,8%is determined as the suitable content of CaO to make MA-CA2-CA6 refractory.(4)When added to MA-CA2-CA6 system refractory,ZrO2 can solubilize into CA6 grains by replacing Al3+ and thus result in lattice defects,i.e.,the concentration of ion vacancy increases.This could be helpful to promote ionic diffusion and therefore enhance the process of solid sintering.In addition,some solubilized ZrO2 precipitates at the grain boundary of CA6 in the process of cooling,which plays a role of binder phase.(5)Due to the anisotropic growth of CA6 grain caused by the development of large flaky morphology,elimination of the stomata between grains is inhibited.Consequently,a porous structure,which prevents the further densification of MA-CA2-CA6 refractory,is formed.It is found that the addition of ZrO2 can inhibit the abnormal growth of CA6 grains along the basal planes,which thus promotes the isotropic growth and eliminates the porous structure.Therefore,the densification of the refractory is greatly enhanced.When ZrO2 addition increases 4%and 6%,the apparent porosity decreases from the original 22.1%to 6.8%and 5.9%,which indicates a good densification process.(6)ZrO2 can greatly improve the compression strength in ambient temperature and the bending strength of MA-CA2-CA6 refractory.When ZrO2 addition is 4%,the compression strength in ambient temperature and the bending strength increase from 376 MPa and 83MPa to 576MPa and 154MPa,respectively.This is mainly caused by great increase in densification,decrease of porosity size and unobvious grain coarsening.The other reason is the precipitation of ZrO2 that can increase the strength of the grain bind.(7)Due to considerable increase in the microstructure densitification,square crystalloid zirconia(t-ZrO2)is totally preserved when ZrO2 is added to MA-CA2-CA6 refractory.The transmission and diffusion of cracks is therefore effectively inhibited,which is helpful to improve the fracture toughness of the refractory.When ZrO2 addition reaches 4%,the fracture toughness rises from 3.73MPa·m1/2 to 4.83MPa·m1/2.