Dissolution behavior of spent MgO–C refractory in the CaO–SiO2–FeO slag system as a steelmaking flux

A large amount of spent MgO–C refractory is generated in steel plant every year. Because of the similarities in chemical and mineralogical composition of slag formers and MgO–C refractory, it is possible to reuse the spent MgO–C refractory as a steelmaking flux. To achieve this goal, it should promote the dissolution of MgO–C refractory during slag forming. In this study, the effect of slag composition on the dissolution behavior of spent MgO–C refractory in the CaO–SiO₂–FeO slag system and the dissolution kinetics were investigated. It showed that the dissolution rate of MgO–C refractory was controlled by surface chemical reaction. The dissolution of MgO–C refractory led to an increase in the MgO content in slag while the FeO content decreased because the graphite in refractory was oxidized by FeO. Increasing temperature significantly promoted the dissolution of MgO–C refractory. The MgO–C refractory was readily dissolved in the low-basicity slag. A higher FeO content in slag was beneficial for the oxidation of graphite in refractory, resulting in better dissolution. The dissolution thickness of MgO–C refractory could exceed 4.0 mm under these conditions and its dissolution supplied some MgO to slag.     

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.     

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.     

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.     

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     

Effect of surface properties of MgO on its dissolution and spinel growth in CaO–SiO2–Al2O3 slag

The dissolution behavior of MgO in CaO–SiO₂–Al₂O₃ ternary slag at the interface of single-crystal, dense poly-crystal, and porous poly-crystal MgO was investigated to evaluate the effect of the surface properties of the MgO. The experimental results revealed that a detached spinel layer formed at the MgO interface due to the change in thermodynamic condition of the slag, which was independent of the surface properties. On the other hand, it was also confirmed that the growth rate and morphology of the detached spinel layer strongly depended on the surface properties, such as porosity and curvature of MgO. During the formation of the spinel layer at the interface during MgO dissolution, a kinetic approach adopting parabolic relation theory was employed to determine the correlation between the surface properties and the spinel growth mechanism.     

Penetration and corrosion of magnesia grain by silicate slags

Abstract

Penetration and corrosion resistance of high purity sintered and fused magnesia grain by model EAF (CaO/SiO₂ = 1·38) and BOF ‘late’ slags (CaO/SiO₂ = 3·29) at 1600 and 1700°C were investigated by SEM, EDS, and XRD analysis. Thermodynamic calculations were performed to assist interpretation of the reaction processes involved. At the test temperatures, Fe andMn ions from both model slags diffused into themagnesia grain to form a magnesiowustite, (Mg,Fe,Mn)O. The magnesiowustite directly adjacent to the slag had a much larger crystal size than that of the bulk MgO far from the MgO/slag interface. The large magnesiowustite grains limit the potential for grain boundary penetration into the sintered magnesia. The magnesiowustite layer formed with the EAF slag took up more Fe_xO from the slag than that formed with the BOF slag, which was partially responsible for a lower slag penetration into sintered magnesia grain since the remaining silica rich local liquid was rendered more viscous. The EAF slag was not saturated with respect to MgO, so the magnesiowustite which did form later reacted with Ca and Si ions remaining at the MgO/EAF slag interface to form low melting phases such as merwinite, C₃ MS₂, and then dissolved into the slag, rendering the dissolution process essentially indirect. The BOF late slag was already oversaturated with respect to MgO, so slag penetration only occurred in the sintered magnesia grains.     

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.     

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.     

Role of graphite on the corrosion resistance improvement of MgO–C bricks to MnO-rich slag

In order to clarify the effect of graphite content on the corrosion behavior of MgO–C refractories immersed in MnO-rich slag, the MgO–C refractory samples bearing 5 wt%, 10 wt% and 15 wt% graphite were prepared, and exposed in the slag composed of 40 wt% CaO, 40 wt% SiO₂ and 20 wt% MnO. The results show that metallic Mn particles and (Mg,Mn)O solid solution are formed at the slag/refractories interface. Whereas, no dense layer is formed by (Mg,Mn)O solid solution at the interface. The decrease in MnO content of slag is mainly attributed to the reaction with graphite to form liquid Mn. The graphite is found in the slag, and dissolved in the form of oxidation. The poor wetting limits the contact area of graphite and slag, reducing graphite oxidation and decarburized area. The graphite does not become the passage for slag to penetrate into the refractories due to the oxidation. On the contrary, the dissolution of MgO in slag is faster than graphite, thus is mainly responsible for the degradation of refractories. As a result, MnO and MgO contents change less in the slag contacted with the refractories bearing higher graphite content.     

The effect of phase formation during use on the chemical corrosion of magnesia–chromite refractories in contact with a non-ferrous PbO–SiO2 based slag

One of the main factors limiting the lining lifetime in pyrometallurgical smelters is continuous refractory oxides dissolution in the slag bath. The overall wear is accelerated when the slag infiltrates the porous brick and the dissolution thus occurs in a larger part of the lining. This work investigates the possibility of preventing deep infiltration by sealing off the pores with newly formed phases. Static finger tests at constant temperature (1200 °C) were performed in contact with a synthetic non-ferrous PbO–SiO₂–MgO slag, showing the formation of forsterite (Mg₂SiO₄) throughout the refractory sample by the reaction between SiO₂ (slag) and MgO (refractory). This phase grows with time, eventually sealing off the pores near the interface with the bath. The phase grows too slow to prevent full infiltration of the refractory but creates an equilibrium state in the sealed off part of the sample, ceasing the chemical corrosion in that part of the sample.     

Effect of varying Al2O3 contents of CaO–Al2O3–SiO2 slags on lumped MgO dissolution

This study utilized the single hot thermocouple technique to examine the dissolution behavior of lumped magnesium oxide (MgO) in CaO–Al₂O₃–SiO₂ ternary slags. The aluminum oxide (Al₂O₃) content in the slag (C/S = 1) varied from 10% to 30%; the MgO sphere with a diameter of 1 mm was placed in molten slags at 1,550 °C. Results showed that the dissolution rate decreased as the Al₂O₃ content increased up to 20%. Over 20% Al₂O₃, MgAl₂O₄ was formed at the interface of MgO and it did not fully melt at 30% Al₂O₃. The dissolution behavior and the formation of MgAl₂O₄ were analyzed by a phase diagram provided by Factsage 7.0 software. In the case of less than 20% Al₂O₃ content, apparent sphere radii were measured; the shrinking core model was then applied to understand the dissolution mechanism. The dissolution rate of both slags was controlled by boundary layer diffusion. The dissolution rate at 20% Al₂O₃ slag appeared to fit the behavior to the boundary layer diffusion, although it deviated during the middle stage of the dissolution because of MgAl₂O₄ formation. The 10% Al₂O₃ slag fitted well to the boundary layer diffusion curve; the obtained diffusion coefficient was 0.94 × 10⁻⁹ m²/s.     

Dissolution kinetics of alumina in molten CaO–Al2O3–FetO–MgO–SiO2 oxide representing the RH slag in steelmaking process

In order to effectively remove alumina inclusions suspending in ultra-low C steel during RH process, the dissolution kinetics of alumina in molten CaO–Al₂O₃–Fe_tO–MgO–SiO₂ oxide was investigated. A crucible dissolution technique was used where the alumina crucible was allowed to dissolve in the slag of various conditions ((% CaO)/(% Al₂O₃), (% Fe_tO), temperature). The obtained data were interpreted using a kinetic mass transport equation to obtain the mass transport coefficient (k_m) in each condition. Increasing (% CaO)/(% Al₂O₃), (% Fe_tO), and temperature increased the dissolution rate as well as the k_m provided that the slag composition is not close to its saturation composition by alumina. In order to simulate the dissolution of alumina inclusion in the RH slag, which cannot be measured by a confocal scanning laser microscopy (CSLM) at present due to the opaqueness of the slag, the modified invariant interface approximation was employed. Along with the obtained k_m, the viscosity of slag, and a reference experiment using the CSLM, the dissolution kinetics of alumina inclusion in the Fe_tO-containing RH slag was predicted. The time required for the dissolution of alumina inclusions from liquid steel to RH slag was discussed.     

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.     

Effect of applied voltage on wetting and corrosion of corundum refractory by CaO–SiO2–MgO molten slag

The wetting and corrosion behavior of the corundum substrate anode by CaO–SiO₂–MgO molten slag was investigated via the joint application of the sessile drop method with applied voltage and SEM-EDS technique. The slag drop exhibited a good wettability on the corundum substrate. The apparent contact angle at zero voltage slightly exceeded that at a 1 V applied voltage but was lower than those at 1.5 V and 2 V ones. Low applied voltage of 1 V had little effect on the corundum substrate's direct dissolution corrosion processes; high ones of not less than 1.5 V caused the electrode reaction to occur. The stirring effect of O₂ bubbles from the anode reaction aggravated the substrate's direct dissolution and physical stripping. It was found that the applied voltage could inhibit the slag penetration, and the apparent contact angle had no obvious relation with the direct dissolution thickness and penetration depth. A thin but almost continuous MgO?Al₂O₃ (MA) layer could form at the slag/substrate interface at the applied voltage of 1.5 V. These results indicate the positive effect of applied voltage on the distribution of interfacial products and the molten slag penetration in reducing the corrosion of corundum anode under certain conditions. However, when the applied voltage was too high, the vigorous electrode reaction could aggravate the direct dissolution and physical stripping of the corundum anode, and damage the continuation of the formed interface product layer with a high melting point.     

Dissolution mechanism of alumina inclusions into CaO–SiO2–Al2O3–FetO–MgO slag via a converter tapping process

Reducing the amount of inclusions during the steelmaking process as much as possible and much earlier plays a vital role in improving the quality of steel products. To reveal the dissolution mechanism of inclusions in slag during the converter tapping process, some comparison experiments were conducted by adding isolated spherical alumina balls as inclusions in CaO–SiO₂–Al₂O₃–Fe_tO–MgO slag, and Fe_tO content up to 10% was contained in slag. The results showed that the dissolution rate of alumina balls in the slag was mainly affected by the diffusion of Al₂O₃, and the diffusion coefficients of Al₂O₃ were 4.2 × 10^–11, 7.5 × 10^–11, and 1.5 × 10^–10 m²/s at 1500℃, 1550℃, and 1600℃, respectively. In addition, the upgraded diffusion-distance-controlled dissolution model (DDD-Model), in which Fe_tO content was introduced and applied in the study. The results illustrated that the Al₂O₃ inclusion apparent dissolution rate was improved by a high Fe_tO content, increasing CaO/SiO₂ and raising the temperature as soon as possible at the early stage of the converter tapping process. It is not necessary to increase the Fe_tO content in the slag to enhance the dissolution rate of the Al₂O₃ inclusion at the last tapping stage. The predicted complete dissolution time of spherical Al₂O₃ inclusions with 1000 µm in diameter based on the upgraded DDD-Model was approximately 1796 s during the actual converter tapping process.     

Influence of calcium aluminate cement in magnesia castable on total oxygen content of interstitial free steel

Abstract

Samples of interstitial free (IF) steel buried in MgO castable bonded by calcium aluminate cement (CA) in graphite crucibles were heated at 1600°C for 90 min. Total oxygen content (TOC) of the steel was examined after heating and the refractory was investigated by SEM and EDS. It was found that TOC was higher in IF steel samples in contact with MgO castables containing 3 or 5 wt-% CA than with castables containing 7 wt-% CA or without CA. A liquid layer formed between refractory oxide and molten steel separates the refractory oxides from molten steel and inhibits direct dissolution of oxides in the molten steel. Transfer of oxygen between the refractory oxide and molten steel occurs by the formation of CaO.Fe₂O₃ at the boundary between the refractory oxide and the liquid layer, diffusion of CaO.Fe₂O₃ in the liquid phase layer, decomposition of CaO.Fe₂O₃ and dissolution of FeO into the molten steel. W ith increasing CA content in MgO based castables the CaO content in molten steel increases, but iron oxide content decreases, leading to the result mentioned above.     

In situ observation of the dissolution phenomena of SiC particle in CaO–SiO2–MnO slag

The dissolution of SiC particle at 1600 °C in the CaO–SiO₂–MnO slag was observed in situ by means of confocal scanning laser microscopy in order to make the determination of dissolution mechanism. The SiC particle is initially wetted by molten slag from the outer surface and the wetting between SiC and slag phase is more dominant in the composition of higher CaO/SiO₂ ratio. When the SiC particle is wetted by molten slag, the gas bubbles that are mainly CO gas is generated by the reaction between SiC and MnO in slag phase and are continuously evolved at the wetted area, which is pronounced as the CaO/SiO₂ ratio increases. The dissolution of SiC particle in the slag through the reaction with MnO is enhanced in the composition of higher CaO/SiO₂ ratio not only due to greater thermodynamic driving force but also due to accelerated mass transport kinetics.     

Cathodoluminescence imaging for rapid identification of low-melting CaO–MgO–SiO2 phases in MgO-based refractories involving the steelmaking process

For MgO–C refractories used in the steelmaking process, identifying low-melting CaO–MgO–SiO₂ phases is crucial because they accelerate the corrosion of the refractories. However, electron probe microanalysis, a conventional method for identifying such phases, is time-consuming. Herein, cathodoluminescence (CL) imaging is proposed for the rapid identification of low-melting CaO–MgO–SiO₂ phases at the reaction interface between MgO-based refractory and steelmaking slag. Monticellite, merwinite, and melilite were identified as the low-melting phases, emitting green, red, and violet luminescence, respectively, in the CL images. Other mineral phases emitted luminescence whose colors differed from those of the low-melting phases (3CaO·2SiO₂ and 2CaO·SiO₂) or no luminescence (magnesiowüstite, MgO·Al₂O₃, Ca₂Fe₂O₅, 3CaO·SiO₂, MnS, and FeS). The CL images (area: 0.5 ×0.3 mm²) were obtained in 30 s. Therefore, CL imaging is effective for the rapid identification of mineral phases, which limit the service life of MgO–C refractories during steelmaking.     

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.