Study On Reaction Behavior Of In-situ MgAl₂O₄ Spinel Formation

Spinel is generated during service at high temperature in in-situ alumina-magnesia spinel refractories,which are widely applied in refining processes of steel-making industry and cement calcining processes etc.due to their low cost,high melting point,great strength and excellent corrosion resistance against molten slag/metal.Expansion can be observed accompanying with the formation of spinel from alumina and magnesia.In the current study,powder compacts are employed for investigating the correlation between expansion behavior and spinel generation process.Such kind of experimental set-up allows quick reaction rate of spinel formation and monitoring both expansion behavior and reaction progress simultaneously to clarify their relationship.The base powder compact is homogeneous mixture of alumina and magnesia at a molar mixing ratio of 1:1.In some cases,additives are added to the base mixture to study the influence of additives on the process.Calcium aluminate cement?CAC?,a commonly-used binder in alumina-magnesia spinel castables,and zirconia,which is usually used to improve the thermal shock resistance of refractories,are chosen as the additive.Their effects on the expansion behavior and reaction mechanism of alumina-magnesia compacts are analyzed.The parameters controlled include pressing pressure,soaking temperature and time,and the amount of additives.Homogeneous powder mixtures are pressed under the pressures of 127?381?762 MPa into cylinders of?10 mm×4 mm,the soaking temperature is changed between 1300 and 1500?for a certain time period of no longer than 25.5 h.The amount of additives is varied in the range of 1.9 wt.%¹0.7 wt.%for CAC and 1wt.%⁹.1 wt.%for zirconia.Commercial grade fine powders of fused magnesia,calcined alumina,CAC Secar 71 and zirconia are used for the current study so that the current results are applicable for designing,formulating and manufacturing in-situ alumina-magnesia spinel refractories.Expansion ratios are utilized to evaluate the expansion behavior.The phase compositions are analyzed using XRD,the micro-structure and morphology of samples are determined by SEM-EDS.The following conclusions can be drawn based on the experimental results.?1?The expansion of powder compacts is largely determined by spinel formation from alumina and magnesia.The pores in magnesia particles originated from the much quicker diffusion of Mg²⁺ions than Al³⁺ions through spinel layer leads greater expansion ratio.The expansion ratio of powder compacts increases with increasing the pressing pressure,soaking temperature and time in all reaction systems of Al₂O₃-MgO,Al₂O₃-MgO-ZrO₂ and Al₂O₃-MgO-CAC.Higher pressing pressure results in larger initial density hence greater expansion ratio,while more spinel is generated when the soaking temperature and time are increased.?2?The permanent linear expansion ratio is proportional to spinel layer thickness in the case no sintering occurs.The apparent activation energy of spinel formation is calculated as 304 kJ/mol from the results of linear expansion ratio,which is very close to previously reported values.This indicates that spinel formation can be evaluated using permanent expansion ratio.?3?Al₂O₃-MgO-Zr O₂ compacts are observed larger expansion ratios than Al₂O₃-MgO ones.The apparent activation energy of spinel formation is significantly reduced by the addition of ZrO₂.This is probably because the inter-solution of MgO/Al₂O₃ with ZrO₂ at high temperatures promotes the diffusion of Mg²⁺/Al³⁺ions and hence accelerates the formation of spinel,resulting in larger expansion ratios of powder compacts.However,when more ZrO₂ is added,more MgO/Al₂O₃ dissolves in ZrO₂ phase,less MgO/Al₂O₃ is available for spinel formation.The expansion ratio reaches the largest when the content of ZrO₂ is 1.9 wt.%.?4?In Al₂O₃-MgO-CAC compacts,CAC can react with alumina as well.The influence of soaking temperature,time and CAC content on the reaction progress and expansion behavior is dependent on the product amount of CA₂?Ca O·2Al₂O₃?and CA₆?CaO·6Al₂O₃?.?a?in the temperature range of 1300¹350?,the main product is CA₂,the expansion ratio of Al₂O₃-MgO-CAC compacts is smaller than that of Al₂O₃-MgO and decreases with increasing CAC content;?b?in the temperature range of 1400¹500?,because in the initial stage CA₂ is the main product,the expansion ratio of Al₂O₃-MgO-CAC compacts is smaller than that of Al₂O₃-MgO;however,CA₂ reacts further with alumina leading to CA₆ formation if the samples are soaked longer,Al₂O₃-MgO-CAC compacts expands more than Al₂O₃-MgO system.The generation of CA₆ is enhanced when soaking temperature,time and CAC content are increased.As a summary,when the reaction between CAC and alumina produces mainly CA₂,the expansion ratio of Al₂O₃-MgO-CAC compacts is smaller than that of Al₂O₃-MgO,whereas when CA₆ is the dominant product,larger expansion ratio is observed for Al₂O₃-MgO-CAC compacts than Al₂O₃-MgO system.