Corrosion modeling of magnesia aggregates in contact with CaO–MgO–SiO2 slags

Ladle refining is an efficient process for improvement of quality of steel on secondary metallurgy under harsh conditions. Magnesia refractories with high purity are important raw materials for ladle lining in high-quality steel production. However, the penetration by CaO–MgO–SiO₂ slags damages magnesia refractories, which considerably limits their service life. Abundant grain boundaries in magnesia create channels for slag penetration and lead to the destruction of the structure. The effect of the microstructure on the slag corrosion behavior of magnesia aggregates requires further systematic investigation. In this study, a corrosion model was established to describe the slag penetration process of magnesia aggregates. The effects of the grain-boundary size and slag CaO/SiO₂ mass ratio (C/S ratio) on slag penetration were investigated, and the possibility of the microstructure optimization of magnesia aggregates was discussed. The results indicated that magnesia aggregates exhibited excellent slag resistance for slag with a C/S ratio above 1.5 or even 2.0. When the slag C/S ratio was lower than 1.5, the dissolution rate of magnesia decreased more rapidly with the increase in the slag C/S ratio. In addition, the much smaller grain-boundary size increased the slag penetration resistance by promoting the formation of a dense isolation layer and inhibiting further penetration processes. The calculation results agreed well with the experimental results, suggesting that the corrosion model is promising for predicting slag corrosion.     

Influence of FeO/SiO2 and CaO/SiO2 Ratios in Iron‐Saturated ZnO‐Rich Fayalite Slags on the Corrosion of MgO

The corrosion behavior of MgO in iron‐saturated ZnO‐rich fayalite (ZFS) slags having various FeO/SiO₂ ratio and CaO/SiO₂ ratio was investigated using MgO crucible tests for 12 h at 1200°C. The FeO/SiO₂ and CaO/SiO₂ ratios in the ZFS slags were varied from 1.0 to 2.2, and from 0.04 to 0.32, respectively. In all of the tests, it was observed that MgO dissolves into ZFS slags and that (Zn,Fe,Mg)₂SiO₄ olivine and (Zn,Fe,Mg)O solid solution are formed at the crucible/slag interface. The MgO dissolution decreased with the FeO/SiO₂ ratio up to a value of 1.7 and then slightly increased, whereas it continuously increased with the CaO/SiO₂ ratio. There is no obvious relationship between the amount of olivine and the FeO/SiO₂ ratio or CaO/SiO₂ ratio. In comparison, the formation of (Zn,Fe,Mg)O solid solution is enhanced by increasing the FeO/SiO₂ ratio or CaO/SiO₂ ratio in ZFS slags. The results suggest that MgO corrosion is the lowest for FeO/SiO₂ and CaO/SiO₂ ratios around 1.7 and 0, respectively.     

Investigation of slag-refractory interactions for the Ruhrstahl Heraeus (RH) vacuum degassing process in steelmaking

In order to determine the effect of slag composition during the RH process on refractory wear, magnesia–carbon and magnesia–chromite refractories were immersed for 10 min at 1600 °C in a ladle slag, two FeO-rich slags (20 and 40 wt% FeO) and two CaO–Al₂O₃ slags. Corrosion of magnesia–carbon refractory by the ladle and CaO–Al₂O₃ slags was limited as the refractory carbon phase efficiently prevented slag infiltration. Severe degradation was observed in contact with FeO-rich slags. FeO oxidized the carbon phase with formation of Fe droplets at the hot face. Regarding magnesia–chromite refractory, the corrosion mechanism consisted of severe slag infiltration, high temperature inactivation of the secondary chromite and primary chromite dissolution in the infiltrating slag. The FeO-rich slags seem to have generated more severe conditions as the infiltrating slag pushed apart the periclase grains, leading to severe refractory erosion. The degradation mechanisms are discussed by combining experimental results and thermodynamic calculations.     

Phase evolution and nature of oxide dissolution in metallurgical slags

The dissolution of solid lime particles into liquid slags at high temperatures was evaluated by means of confocal scanning laser microscopy. An additional solid layer around the lime particle was observed at the intermediate stage of the dissolution into CaO? Al₂O₃? SiO₂ slags. The dissolution rate was decelerated due to the existence of the additional layer and the dissolution profile could be clearly distinguished into three stages, that is, an early, intermediate, and late stage. By adding 10 wt % MgO, this layer could be effectively eliminated and the slope of the whole dissolution profile kept relatively constant. The dissolution path and mechanisms were subsequently evaluated by incorporating thermodynamic calculations. Both direct and indirect dissolutions could be distinguished. It was realized that the decrease in composition range for solid precipitating after adding MgO could significantly reduce the interfacial reaction (IR) layer formation. Post‐mortem analyses on quenched samples were further carried out to confirm the theoretical calculations. It was found that the solid layer in slags without MgO was (CaO)₂·SiO₂ and (CaO)₃·SiO₂ which is in line with the thermodynamic calculations. However, only (CaO)₂·SiO₂ was noticed in slags with MgO which both (CaO)₂·SiO₂ and MgO phases should be present according to the calculations. The nonequilibrium during dissolution may play an important role on phase transformation and MgO particles in much smaller quantity may have dissolved into (CaO)₂·SiO₂ phase. The diffusion of CaO in both slags with and without MgO was additionally investigated. The local CaO concentration distributions from the direct dissolution phase to the slag bulk could be well fitted with the theoretical model proposed via Fick's second law. As a result, the local diffusion coefficient in the dissolution region was obtained and the effect of MgO addition on diffusion could be assessed. © 2013 American Institute of Chemical Engineers AIChE J, 59: 2907–2916, 2013     

Slag resistance mechanism of MgO–Mg2SiO4–SiC–C refractories containing porous multiphase aggregates

The slag resistance of MgO–SiC–C (MSC) refractories should be improved because of the mismatch in the thermal expansion coefficient between the aggregates and matrix, as well as the defects caused by the affinity between periclase and slag. In this study, MgO–Mg₂SiO₄–SiC–C (MMSC) refractories were prepared using porous multiphase MgO–Mg₂SiO₄ (M-M₂S) aggregates to replace dense fused magnesia aggregates. Compared to MSC, the slag penetration index of MMSC decreased by 43.5%. The structure of the porous aggregates increased the surface roughness, and the multiphase composition of the aggregates decreased the mismatch of the thermal expansion coefficient with the matrix, thus reducing debonding between the aggregates and matrix. The aggregates and matrix in the MMSC formed an interlocking structure, which bound them more tightly to improve the slag resistance. The slag viscosity at different depths from the initial slag/refractory interface was calculated using the Ribond model. The M-M₂S aggregates increased Si_xO_y^z− in the slag, which increased the slag polymerization and slag viscosity. The aggregates and matrix in the MMSC reacted with the slag to form high melting point phases, which reduced the channel of the slag. In addition, the penetration depth and velocity derived from the Washburn Equation were modified for the CaO–SiO₂–Al₂O₃–MgO–FeO slag and magnesia based refractory to accurately evaluate slag penetration.     

Interfacial reactions between magnesia refractory and electric arc furnace (EAF) slag with use of direct reduced iron (DRI) as raw material

Interfacial reactions between the electric arc furnace (EAF) slag, i.e., CaO–SiO₂–FeO–MgO–Al₂O₃–MnO system, and the magnesia refractory as a function of direct reduced iron (DRI) addition (0, 10, 20, 30 wt%) were investigated at 1550 °C under an Ar atmosphere. MgO solubility increases with increasing DRI content by decreasing basicity (i.e., CaO/SiO₂ ratio), which is due to an increase in SiO₂ supplied from DRI. The measured MgO content was always lower than the theoretical MgO saturation level irrespective of DRI content because the magnesiowüstite (MW) intermediate layer, which formed at the slag/refractory interface, retarded the direct dissolution of the refractory by acting as a self-protective layer. The thickness of the MW intermediate layer and dissolution depth were proportional to DRI content. However, the penetrativity decreased with increasing DRI content by decreasing the fluidity of the slag. Several kinetic parameters were estimated, including the dissolution rate constant of the MW intermediate layer, the dissolution rate of the MgO refractory, and the rate constant of MW growth. Dissolution of MgO refractory is controlled by the dissolution of the MW intermediate layer. Increasing the growth rate is very important for protecting refractory after the formation of a MW intermediate layer. In addition, we provided a schematic diagram of the slag/refractory interfacial reaction phenomena that compares situations of low and high DRI content. The results of the present study show that it is necessary to control DRI content to minimize refractory degradation during the EAF process. If a large amount of DRI must be used in the EAF process, then MgO content in the slag should be at the saturation limit at first, which accelerates growth of the MW intermediate layer.     

Degradation of low‐carbon MgO‐C refractory by high‐alumina stainless steel slags in VOD ladle slagline

A new type of low‐carbon magnesia carbon refractory (LCMCR) substituting for MgO‐Cr₂O₃ refractory was successfully used in vacuum oxygen decarburization (VOD) ladle slagline, and the composition and microstructure of the used LCMCR were investigated. The results indicated that the decarburizing reaction (MgO‐C reaction) in the LCMCR under the VOD refining condition (high temperature, low pressure) was inhibited due to the low carbon content in the MgO‐C refractory and the dense layer formed between slag and original layer. The dense layer prevented the penetration of the external O₂ into the LCMCR inside because of the lower permeability of this layer, and thus, the direct burnout of the C in the LCMCR was substantially restrained. On the other hand, the large size crystal and the ultra‐low inclusions (SiO₂ and Fe₂O₃) content of the fused magnesia in the LCMCR made the magnesia more slag resistance, because the grain boundary in magnesia had higher slag penetration resistance and the contact area between the slag and the magnesia was reduced. The two aspects of the inhibited decarburizing reaction and the high quality magnesia synthetically contributed to the high slag resistance of the LCMCR.     

Dissolution behavior of silica in molten CaO–SiO2–Fe2O3–MgO–MnO slag

The modification of basic oxygen furnace (BOF) slag by adding silica can improve the properties of BOF slag for applications in the cement industry. The rapid dissolution of silica is essential to hot slag modification. In this work, the dissolution behavior of silica in the molten CaO–SiO₂–Fe₂O₃–MgO–MnO system as synthetic BOF slag was investigated by using the traditional rotating cylinder technique. Effects of rotation speed, temperature, immersion time, and slag basicity on the silica dissolution were studied. Scanning electron microscopy equipped with energy dispersive spectrometer (SEM-EDS) and FactSage simulations were employed to reveal the dissolution mechanism. It was found that the dissolution of the silica rod was affected by both the thermodynamic driving force and the slag viscosity. The silica dissolution rate in molten CaO–SiO₂–Fe₂O₃–MgO–MnO slag increased with increasing the rotation speed and temperature, but first increased and then decreased when decreasing the slag basicity from 2.5 to 1.5. A linear correlation between the logarithm of the dissolution rate and the logarithm of cylinder periphery velocity with a slope of 0.44 was observed, indicating the mass transfer within the boundary layer as the dissolution rate determining step. A direct dissolution way was found during the dissolution of silica in molten CaO–SiO₂–Fe₂O₃–MgO–MnO slag.     

Corrosion of spinel clinker by CaO–Al2O3–SiO2 ladle slag

Three different grades of sintered spinel clinker were used containing 47, 69 and 94 wt.% Al₂O₃, respectively, i.e. MgO-rich, stoichiometric and Al₂O₃-rich. Based on these clinkers, the corrosion mechanism of each spinel clinker by CaO–Al₂O₃–SiO₂ slag was investigated and the corrosion and penetration behavior of castables containing powdered spinel clinker examined. A layer of MgO·(Al, Fe)₂O₃ complex spinel formed at the slag-refractory interface was proportional to the MgO content of the spinel clinkers, and it depressed the slag corrosion. The free MgO and spinel minerals in each spinel clinker mainly trapped Fe₂O₃ from the slag. CaO–Al₂O₃ compounds were formed at the slag-clinker interface by the reaction between free Al₂O₃ in the Al₂O₃-spinel clinker and CaO from slag. Slag penetration into the spinel clinkers was retarded by these compounds. As a result of adding fine spinel powder to the matrix of Al₂O₃-based castables, it was observed that higher content of MgO in spinel clinker showed better resistance to slag corrosion but lower resistance to slag penetration.     

In situ observation of the direct and indirect dissolution of MgO particles in CaO–Al2O3–SiO2-based slags

The dissolution of magnesia particles in synthetic CaO–Al₂O₃–SiO₂ (CAS)-based slags with and without MgO addition was investigated in situ with a confocal scanning laser microscope (CSLM) at 1500 and 1600 °C. The dissolution process was recorded. The effects of slag composition and temperature on the dissolution process and the time dependency of the MgO particle size during dissolution were obtained. Increasing the temperature increases the dissolution rate. However, MgO addition to the slag retards the dissolution rate significantly. The rate limiting steps are discussed. It is shown that boundary layer diffusion is responsible for the dissolution. By combining in situ observations with post mortem analyses, thermodynamic calculations of local and global equilibrium, and kinetic considerations, the conditions under which MgAl₂O₄ spinel can be formed at the particle–slag interface are clarified.     

Radical reaction-induced Turing pattern corrosion of alumina refractory ceramics with CaO–Al2O3–SiO2–MgO slags

Quasi-volcanic corrosion occurs at the triple-phase interface of alumina refractory ceramics and MgO-containing CaO–Al₂O₃–SiO₂ slags in the air, causing severe damage to ceramics. To address this limitation, in this study, a slag corrosion experiment is performed on alumina refractory ceramics using CaO–Al₂O₃–SiO₂–MgO slags. Various spectroscopic techniques, including electron paramagnetic resonance spectroscopy, are used to investigate the influence of slag structures with varied MgO contents on the corrosion peaks and mechanism. The results show large quantities of reactive radicals, including superoxide radicals, in the slags. Free-radical reactions between refractory ceramics and slags lead to Turing pattern corrosion. An increase in the amount of non-bridged oxygen in the slag structure decreases the amount of original superoxide radicals. Consequently, the intensity of the free-radical reactions of alumina dissolution increases, thereby increasing the height of the corrosion peaks.     

Microstructure and phase evolution of Indian magnesite‐derived MgAl2O4 as a function of stoichiometry and ZrO2 doping

To aid development of cost‐effective sintered spinel as a refractory raw material, this paper presents an extensive analysis of microstructure and complex phase evolution of Al‐rich, Mg‐rich, and stoichiometric spinel aggregates derived from Indian magnesite and calcined alumina. Pore morphology in Al‐rich spinel was transformed upon sintering at 1650°C and corundum laths embedded in porous Al‐rich spinel matrix was formed. Stoichiometric spinel sintered at 1600°C consisted of mostly direct bonded angular equiaxed spinel grains which incorporated the impurities in solid solution. Mg‐rich spinel was composed of spinel grains with reduced angularity along with intergranular amorphous phase, small round monticellite grains, and periclase. EDS line scan revealed impurity‐free joins existed between direct bonded spinel grains. Mg‐rich spinel containing 0.65 wt% ZrO₂ formed cubic ZrO₂‐CaO‐MgO solid solution located along spinel boundaries, which reduced both intergranular amorphous phase and monticellite. This increased SiO₂ and MgO content in spinel solid solution triggering exsolution of metastable cubic forsterite manifested as split spinel peaks in XRD pattern. A 14.7% reduction in slag penetration was exhibited upon doping Mg‐rich spinel with 0.21% ZrO₂. Stoichiometric and Mg‐rich spinels attained 0.35% and 0.54% apparent porosity at 1600°C, which is better than most commercial sintered refractory spinels.     

Comparison study on effect of nano-sized Al2O3 addition on the corrosion resistance of microporous magnesia aggregates against tundish slag

The microporous magnesia aggregates show a promising application prospect as tundish lining, due to the excellent thermal insulation. In this study, the effect of nano-sized Al₂O₃ addition on the corrosion resistance of microporous magnesia aggregates against tundish slag is explored. The results show that the addition of nano-sized Al₂O₃ deteriorates the slag resistance of microporous magnesia aggregates, which is mainly because that the apparent porosity of aggregates increases with the addition of nano-sized Al₂O₃. Furthermore, MgO·Al₂O₃ spinel is formed in situ at the grain boundaries of Al₂O₃-bearing aggregates and the dissolution of MgO·Al₂O₃ spinel into molten slag damages the structure of aggregates. For the Al₂O₃-free microporous magnesia aggregates, as expected, the penetration of high basicity slag (CaO/SiO₂ = 9, mass ratio) into refractory is slighter than that of low basicity slag (CaO/SiO₂ = 4, mass ratio). But, for the Al₂O₃-bearing microporous magnesia aggregates, the corrosion of refractory by high basicity slag is severer. This is mainly because that MgO·Al₂O₃ spinel is more unstable in high basicity slag. Therefore, it is not suitable to add nano-sized Al₂O₃ for the preparation of microporous magnesia as tundish lining.     

Key features of alumina/magnesia/graphite refractories for steel ladle lining

The corrosion resistance of resin bonded alumina/magnesia/graphite refractories containing different kinds of aggregates were investigated when submitted to the action of slags of several CaO/SiO₂ ratios. The laboratory testing was performed by means of the rotary slag attack test. Specifically evaluated was the influence of alumina/carbon ratio and magnesia and silica contents on the refractories corrosion resistance. It was found that this property could be improved by increasing the refractory Al₂O₃/SiO₂ ratio as well as by choosing the appropriate Al₂O₃/C ratio.     

High‐temperature calorimetric study of oxide component dissolution in a CaO–MgO–Al2O3–SiO2 slag at 1450°C

Blast‐furnace slags are formed, as iron ore is reduced to metal, as a molten a mixture of refractory and not easily reducible oxides, largely silica, alumina, lime, and magnesia. Their relatively low silica content makes them basic and poor glass formers. Their thermodynamic properties, though important for modeling their formation and reactivity, as well as furnace heat balance, are poorly known. Solution calorimetry of small amounts of solid oxides in a molten oxide solvent at high temperature (up to about 1500°C) permits direct assessment of energetics of dissolution. The enthalpies of solution of slag forming oxides: CaO, SiO₂, Al₂O₃, MgO, and Fe₂O₃ in a simplified model slag of composition: CaO (45.9 mol%), SiO₂ (35.1 mol%), Al₂O₃ (8.3 mol%), MgO (10.7 mol%) were measured by high‐temperature drop solution calorimetry at 1450°C. For this slag composition, enthalpies of solution become more exothermic in the order: Fe₂O₃ (279.3 ± 20.8 kJ/mol), MgO (56.7 ± 9.1 kJ/mol), Al₂O, (41.6 ± 11.3 kJ/mol), CaO (?4.3 ± 2.3 kJ/mol), and SiO₂, (?20.4 ± 4.4 kJ/mol), reflecting the relatively basic character of this low‐silica melt. Within these fairly large experimental errors, characteristic of calorimetry at this high temperature, there is little or no discernible concentration dependence for these heats of solution. The trends seen for these five solutes parallel those seen for heats of solution of the same oxides in other melts at various temperatures, with changes in magnitude reflecting the differences in acid‐base character of the melts. The new data for quartz show systematic behavior which extends the range of basicity studied for the enthalpy of dissolution of silica. The results provide reliable data for future modeling of the thermal balance of steel‐making furnaces and geologic and ceramic systems.     

Interfacial reaction between magnesia refractory and “FeO”-rich slag: Formation of magnesiowüstite layer

This study investigated the reaction between CaO-SiO₂-Al₂O₃-xFeO-MgO-MnO (CaO/SiO₂?=?1.2, x?=?20–50?wt%) slag and magnesia refractory. Using SEM-EDS analysis, we confirmed the formation of a (Mg,Fe)O_{ss(solid_solution)}, called magesiowüstite (MW), intermediate layer at the slag-refractory interface. MgO dissolved from refractory and reacted with the bulk slag to form MW layer at the interface. Simultaneously, slag penetrated through micro-pores and reacted with the refractory to form MW layer. In other words, the MW layer built up in both directions from initial refractory-slag interface. The thickness of the MW layer increased as the FeO content in the slag increased, and using EDS line scanning, a Mg and Fe concentration gradient was confirmed within the MW layer. The slag, which penetrated into the refractory, had a chemical composition of the CaO-SiO₂-Al₂O₃-MgO system without FeO, indicating that FeO was consumed by forming a MW layer at the refractory hot face. The slag-refractory interfacial reaction was simulated using thermochemical software, FactSage?7.0. The results predicted a MW monoxide composed of MgO and FeO. A spinel phase was formed when FeO was greater than 40?wt%. These thermochemical computations were comparable to our experimental findings.     

A modified method for calculating the viscosity of multicomponent slags based on Kriging interpolation

Model prediction is an effective method to obtain the physicochemical properties data of molten slags, which is difficult to test experimentally due to their high melting points. Empirical models are prevalent at present; however, it needs large amount of experimental data to fit the empirical parameters with narrow application scope. In this paper, the Kriging interpolation method modified with oxide property weights, is firstly introduced into the viscosity prediction of multicomponent slags. The prediction results of CaO–Al₂O₃–SiO₂, CaO–Al₂O₃–CaF₂, CaO–Al₂O₃–SiO₂–MgO and CaO–Al₂O₃–SiO₂–MgO–CaF₂ systems showed that the predicted errors by the Modified Kriging Interpolation (MKI) method are smaller than those by various empirical models. It is anticipated that the MKI method can be extended to predicting the continuous physicochemical properties of multicomponent slags.     

Wetting Behavior between Fayalite-Type Slags and Solid Magnesia

The wetting behavior of single-crystalline magnesia (MgO) by fayalite-type slag melts has been studied by the sessile-drop technique, which incorporates high-temperature X-ray radiography. The experiments have been performed under oxygen partial pressures of 106 and 104 Pa at a temperature of 1473 K. The mass ratios of iron to silica, Fe/SiO₂, in the slag that has been used are 1.44 and 2.05. Infiltration behavior of a fayalite-type slag into MgO are discussed, based on the experimental results that involve wettability and surface tension. Two phases—magnesio-wustite and olivine—are observed at the interface. Interdiffusivity in the magnesiowustite solid solution has been determined as a function of the iron oxide content. To investigate the interfacial reaction, experiments that use high-density MgO crucibles also have been conducted. Phase relations between the magnesiowustite and the olivine, and the rate of growth of the phases, are discussed.     

Rationalizing the role of magnesia and titania on sintering of α-alumina

The rationalization of selection of sintering additives for α-alumina was investigated using two oxides (MgO and TiO₂) to discern their individual roles. Using both dynamic heating study in a thermomechanical analyzer and static heat treatment, the precise role of each oxide was established. Grain growth trajectory of different doped samples sintered at 1700 °C revealed that MgO neither significantly affected densification nor facilitated grain growth upto 1700 °C. MgO reacted with alumina to form spinel prior to the densification process. Thus it could not generate further extrinsic defects in corundum lattice during sintering, which usually facilitate densification. In contrast, TiO₂ significantly enhanced the densification and promoted grain growth in α-alumina. At 1700 °C, the average grain size of titania doped samples were 7.7x larger than undoped ones and 10x larger than magnesia dopes samples. The sintered grains developed higher aspect ratio when TiO₂ was used which may be ascribed to preferred growth of the 012 and 024 planes of corundum. The nearly perfect junction of grain boundaries meeting at ~120° indicates absence of liquid phase and that the entire sintering process most probably took place in solid state for both MgO and TiO₂ doped samples.     

Thermodynamic Stability of Spinel Phase at the Interface Between Alumina Refractory and CaO–CaF2–SiO2–Al2O3–MgO–MnO Slags

The interfacial reaction between alumina refractory and CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was observed at 1873 K to estimate the stability of the spinel phase using computational thermodynamics under refining conditions of Mn‐containing steels. The concentration of MnO formed by the slag–steel reaction in the CaO–CaF₂–SiO₂–Al₂O₃–MgO melts generally increased by decreasing the CaO/SiO₂ ratio of the initial melts. No intermediate compounds were formed at the refractory–slag interface when the initial CaO/SiO₂ ratio was 0.5, whereas CaAl₁₂O₁₉ (CA6) and Mg(Mn)Al₂O₄ (spinel), identified from TEM analysis using EDS mapping and SAED patterns, were observed at the refractory–slag interface when the CaO/SiO₂ ratio was 1.0 or greater. The (at.%Mg)/(at.%Mn) ratio in the spinel solution increased by increasing the CaO/SiO₂ ratio, which originated from the fact that MgO activity continuously increased as the CaO/SiO₂ ratio increased. From thermodynamic analysis considering the equilibrium constant (K_SP) and activity quotient (Q_SP) of the spinel formation reaction at the slag–refractory interface and the bulk slag phase, the precipitation–dissolution behavior of the spinel phase was predicted, which exhibited good consistency with the experimental results. Hence, the dissolutive corrosion mechanism of alumina refractory into the CaO–CaF₂–SiO₂–Al₂O₃–MgO–MnO slag was proposed.