Study On In-situ Pressureless Synthesis Of MgAlON-MgO Composites From Spent MgO-C Brick And Their Application Properties

MgAlON is a solid solution of spinel structure composed of AlN,Al2O3,and MgO.MgAlON and its composite materials show high mechanical properties,good slag resistance and low wettability to molten steel,which are expected to become new refractory materials for secondary steelmaking.However,their application is inhibited because of their high production cost.Moreover,the valuable graphite in spent MgO-C brick is often ignored in the process of recycling spent brick,and the obtained products present low additional value.To address the above issues,this thesis proposes a new approach to in-situ pressureless prepare MgAlON-MgO composites using spent MgO-C brick as raw material,and uses the composite as new refractory material for secondary steelmaking.The approach not only fully utilizes the valuable components in spent MgO-C brick,but also provides an effective way for the high value-added utilization of the spent brick.The mechanical properties,thermal shock resistance,and slag resistance of MgAlON-MgO composites were systematically studied,providing technical support for practical application of the composites.The fracture mechanism,thermal shock damage mechanism,and slag corrosion mechanism of MgAlON-MgO composites were revealed,laying a theoretical foundation for optimizing performance of the composites.Firstly,the Mg-Al-O-N system was thermodynamically analyzed to determine the process parameters for synthesizing MgAlON.Under the guidance of thermodynamics,MgAlON and MgAlON-MgO composites were in-situ synthesized by carbothermal reduction and nitridation method using spent MgO-C brick as raw material.MgAlON and MgAlON-MgO composites were characterized by XPS,SEM,TEM,and so on.The results show that MgAlONMgO composites are composed of the main crystal phase MgAlON,the secondary crystal phase MgO,and impurity phases CaMgSiO4 and MgFe2O4.MgAlON is indirectly bound with MgO,and the transient phase between them is glassy phase.CaMgSiO4 is directly bound with MgO,and MgFe2O4 is embedded in MgO.Secondly,the effects of raw material ratio and sintering temperature on the flexural strength of MgAlON-MgO composites were studied.The optimal process parameters for preparing MgAlON-MgO composites were determined,and the effect of impurity phases on its flexural strength was discussed.It is found that MgAlON-4.2 wt.%MgO with the highest flexural strength of 172 MPa is prepared when the sintering temperature is 1873 K,and the mass ratio of raw materials is 1:6:2(spent MgO-C brick:alumina micropowder:fused magnesite).The flexural strength of MgAlON-MgO composites using spent MgO-C brick as the primary magnesium source is related to the MgO content and the carbon content introduced by the spent MgO-C brick.When the utilization rate of spent MgO-C brick is 52.9 wt.%,the flexural strength of MgAlON-15.7 wt.%MgO could still reach 57 MPa.The impurity phase of CaMgSiO4 could increase the density of MgAlON-MgO composites,thus improving their room temperature flexural strength,while MgFe2O4 has no significant effect.Thirdly,the effects of thermal shock temperature difference,cooling rate,and number of thermal shock cycle on the thermal shock resistance of MgAlON-MgO composites were studied,and their thermal shock damage mechanism was explained based on the thermal shock resistance theory.The results show that the thermal shock resistance of MgAlON-MgO composites quenching in water is only affected by thermal shock at high thermal shock temperature difference(1100 K),while their thermal shock resistance is related to thermal shock and the hydration reaction of MgO at low thermal shock temperature difference(400 and 800 K).Under the thermal shock temperature difference of 1100 K,the retained strength of MgAlON-MgO composites quenching in air and water are about 55 and 6 MPa,respectively.As the thermal shock cycle increases,the retained strength of MgAlON-MgO composites with low MgO content quenching in air sharply decreases and then keeps unchanged,while that of the composites with high MgO content slowly decreases.The increase of MgO content could improve the ability of MgAlON-MgO composites to resist short crack propagation.Therefore,MgAlON-15.7 wt.%MgO shows the highest retained strength ratio(66%)after 5 thermal shock cycles.However,in terms of retained strength and retained strength ratio of the composites,MgAlON-4.2 wt.%MgO(retained strength ratio of 41%and retained strength of 55 MPa after 5 thermal shock cycles)is more promising for using as a lining material for secondary steelmaking.In addition,impurity phases CaMgSiO4 and MgFe2O4 could consume the thermal stress during thermal shock process,and play a positive role in the thermal shock resistance of MgAlONMgO composites.Finally,the slag resistance of MgAlON-MgO composites was studied,and the slag resistance mechanism of the composites was discussed from the aspects of wetting behavior,reaction mechanism,and penetration mechanism between the slag and the composites.The results show that MgAlON-MgO composites react with the molten slag to generate calcium aluminate phase,spinel phase,and TiN phase.The wetting angle of the composites decreases with the decrease of MgO content,and MgAlON shows the smallest contact angle(36°).MgAlON with the best slag wettability exhibits the best slag resistance(the thickness of slag erosion and infiltration was 780 μm).The reason for the abnormal result is that MgAlON presents low porosity and high slag viscosity,thus inhibiting slag infiltration.Moreover,impurity phases CaMgSiO4 and MgFe2O4 show no significant effect on slag resistance of MgAlON-MgO composites.In terms of the mechanical properties,thermal shock resistance,and slag resistance of MgAlON-MgO composites,MgAlON-4.2 wt.%MgO(utilization rate of spent MgO-C brick is 11.1 wt.%)is the most promising lining material for secondary steelmaking,and its flexural strength,retained strength after 5 thermal shock cycles,and thickness of slag erosion and infiltration are 172 MPa,55 MPa,and 1716 μm,respectively.