Study On Application Properties Of Novel Al-Al₂O₃-MgO Refractories For RH Refining Furnace

MgAlON bonded oxide refractories have become important research direction of environmentally-friendly refractories for RH refining furnaces due to its good thermal shock resistance,excellent slag resistance and slag penetration resistance.Based on the high temperature reaction activity and gradient reaction characteristics in oxide matrix,aluminum was introduced into Al2O3-MgO refractories in this work,and the phase,the microstructure and the properties of aluminum magnesium sol bonded Al-Al2O3-MgO refractories at high temperatures under nitrogen atmosphere were studied and novel MgAlON bonded MgO refractories were prepared.The service performance of the novel refractories was excellent:the comprehensive furnace life was 14%longer than that of the fused re-bonded magnesia chrome refractories.The research results are shown as follows.The oxygen partial pressure affects the reaction path and reaction mechanism of the sintered magnesium aluminate spinel used corundum and magnesia as raw materials.Compared with the magnesium aluminate spinel prepared by reaction sintering in air atmosphere,the high-temperature and nitrogen atmosphere promotes the decrease of the stability of MgO,gives the magnesium aluminate spinel a gas-solid mass transfer reaction path,improves the the reaction rate of magnesium aluminate spinel,and decreases the conversion temperature of magnesium aluminate spinel.The magnesium aluminate spinel generated by the gas-solid mass transfer method coats the corundum particles,as the binding phase,combines the aggregate and the matrix well,improves the properties of the refractory—the cold crushing strength and the cold modulus of rupture reached about 135 MPa and 17 MPa,respectively.Based on the high temperature reaction activity and structure gradient reaction characteristics of aluminum in oxide matrix,the Al-AlN core-film structure was constructed and the phase and reaction mechanism of Al-Al2O3-MgO refractories bonded by aluminum magnesium sol at high temperatures under nitrogen atmosphere were investigated.There are three different morphologies of MgAlON spinel in Al-Al2O3-MgO-granular shape,shell shape and flake shape.The formation mechanism of MgAlON can be expressed as follows:at low temperature,Al(s)reacts with nitrogen to form Al-AlN core-film structure.With the temperature increasing,Al(s)?Al(1)?Al(g)transformation reaction occurs,causing the Al-AlN core-film to rupture,Al(l/g)overflows or escapes,and reacts with nitrogen to form AlN.Then the AlN reacts with magnesium aluminate spinel to form granular shape MgAlON spinel.The Al-AlN core-film reacts with magnesium aluminate spinel in situ to form shell shape MgAlON spinel.In the gas-gas reaction stage,Mg(g)and Al2O(g),Al(g),N2(g)and O2(g)undergoes chemical vapor deposition to form MgAlON spinel nucleus.The MgAlON spinel nucleus attached to the granular shape and shell shape MgAlON spinel finally grow into flake shape MgAlON spinel solide solution.During the operation of the RH refining furnace,the refractory bears the load at high temperatures,so it is of great importance to investigate the phase composition and the evolution mechanism of Al-Al2O3-MgO refractories bonded by aluminum magnesium sol under load at high temperatures.Under the high temperature load,the Al-Al2O3-MgO refractory presents a stratification phenomenon—there is an Al-Al2O3 core-film structure on the peripheral part and an Al-AlN core-film structure on the central part.The outer Al-Al2O3 core-film structure is broken,and the metal aluminum escapes or overflows,which reduces the oxygen partial pressure and the reaction temperature of magnesium aluminate spinel.The internal phase evolution is different from the external.When the Al-AlN core-film structure is broken,Al(l/g)overflows or escapes,and forms three morphologies of MgAlON spinel with the system substances—granular shape MgAlON spinel formed by the reaction of Al(1),N2(g)and magnesium aluminate spinel;shell shape MgAlON spinel formed by in-situ transformation of Al-AIN core film;flake shape MgAlON spinel formed by gas-gas mass transfer reaction between Al2O(g),Al(g),Mg(g),N2(g)and O2(g).The volume effect of magnesium aluminate spinel and MgAlON spinel,the rigid dispersion enhancement effect of shell shape MgAlON spinel and the network bridging effect of flake shaple MgAlON spinel,all of which contribute to the excellent creep resistance of Al-Al2O3-MgO refractories.The creep results show that when aluminum is not added,the creep rate of Al2O3-MgO refractory is great,the curve change range is large,and the steady-state creep stage is not present in the experiment;after the introduction of metal aluminum,the creep rate of Al2O3-MgO refractory change is small,and the range of curve change is slow.Based on the effect of the Al addition on the phase,the structure and the properties of Al-Al2O3-MgO refractories bonded by aluminum magnesium sol,it is found that the Al-Al2O3-MgO refractories have excellent room temperature performance,thermal shock resistance and hot modulus of rupture when 6 wt%aluminum is added to the matrix.Novel Al(6 wt%)-Al2O3-MgO refractories were prepared and its on-site trial was conducted in the key parts of a 250 t RH refining furnace(submerged tube,immersion tube and bottom slot).The service life of the RH refining furnace on trial is 14%longer than that of the furnace using fused re-bonded magnesia chrome refractories.Thus the Al-Al2O3-MgO refractories are proved to be novel environment-friendly refractories for RH refining furnace.The novel Al-Al2O3-MgO refractories have excellent chemical resistance and penetration resistance to different types of RH refining slag(CaO/SiO2 ratios are 0.94 and 5.01,respectively).Different from fused re-bonded magnesia chrome refractories composed of periclase and(Mg,Fe)(Al,Cr)2O4 solide solution,Al-Al2O3-MgO refractories contain three different types of spinel,namely,magnesia alumina spinel,flake shape MgAlON spinel and non-layered shape MgAlON spinel When contacting with the molten steel and slag,MgO and spinel(magnesium aluminate spinel and non-layered shape MgAlON spinel)absorb Fe2+,Mn2+,Cr3+in molten steel or slag to form(Mg,Mn,Fe)O and(Mg,Mn,Fe)(Al,Cr)2O4 solid solution transition layers,which change the structure of materials and the composition of contact slag;the flake shape MgAlON and MgO,magnesium aluminate spinel dense layers improve the pore structure and reduce the pore size of Al-Al2O3-MgO refractories.Under the combined action of the above two factors,the Al-Al2O3-MgO refractories are given a new path for the penetration and erosion reaction of molten steel or slag,prolonging the reaction time with molten steel or slag,and improving the chemical resistance and penetration resistance of the material performance.During the RH refining intervals,Al-Al2O3-MgO refractories exfoliate due to the transition layers of(Mg,Mn,Fe)O and(Mg,Mn,Fe)(al,Cr)2O4 solid solution.The thickness of the spalling layer is thinner than that of the fused re-bonded magnesia chrome refractories,so the corrosion resistance of Al-Al2O3-MgO refractory is better.