Corrosion mechanism of commercial doloma refractories in contact with CaO–Al2O3 –SiO2 – MgO slag

Abstract

The dissolution of three doloma based refractories in liquid CaO–Al₂O₃–SiO₂–MgO slag was studied. Cylindrical refractory specimens of doloma, carbon bonded doloma, and magnesia doloma were rotated in a stationary crucible of molten slag under forced convection conditions. Slag composition, temperature, rod rotation speed and rod immersion time were varied. The refractory dissolution rate was determined from the change in diameter of the cylindrical specimens. The corrosion rate was found to increase with temperature and rod rotation speed and decrease when the slag was nearly saturated with MgO. The findings of the study substantiate the assumption that the diffusion of magnesium oxide through the slag boundary layer controls the corrosion process. The results indicated the overall corrosion process to be the dissolution of refractory material into the slag, followed by slag penetration of the pores and grain boundaries and finally, dispersion of the grains into the slag.     

The Dissolution of Alumina in CaO‐SiO2‐Al2O3 Slags

The dissolution of alumina inclusions in CaO‐SiO₂‐Al₂O₃ based slags have been measured using laser scanning confocal microscopy (LSCM). Experiments were carried out over a temperature range of 1477 to 1577°C. It was found that diffusion coefficients calculated from the experimental results showed a dependence on the slag viscosity. This is considered strong evidence that the dissolution process is at least in part controlled by mass transfer in the slag phase. The diffusion coefficients for the alumina particles are estimated to be in the range 10^?11 to 10^?10 m²/s.     

Leaching of silica from vanadium-bearing steel slag in sodium hydroxide solution

The work aims to selectively extract silica from vanadium-bearing steel slag by a leaching process. The effects of the particle size, the ratio of solid to liquid, the concentration of sodium hydroxide solution and the leaching temperature on the leaching behavior of silica from vanadium-bearing steel slag were investigated. The leaching kinetics of silica from vanadium-bearing steel slag in 30-50% w/w NaOH solutions was studied at 240 °C and the shrinking-core model was established to express the leaching kinetics of silica. The data showed that the leaching rate was controlled by the chemical reaction on the system interface and the activation energy for the process was found to be 36.4 kJ mol^− 1. By the leaching process, the majority of silica could be removed effectively from the vanadium-bearing steel slag and a residue with a low SiO₂ content of 4.28% and a high V₂O₅ content of 11.15% was obtained. Under these conditions there was partial dissolution of Al and slight dissolution of Cr, Mn and Ti.     

Thermodynamic Behaviour of Chlorine in CaO‐SiO2‐MgO‐Al2O3(‐CaF2) Slags

The solubility of chlorine in CaO‐SiO₂‐Al₂O₃‐MgO(‐CaF₂) slag was measured at 1673 ‐1823 K. By estimating the chloride capacity of slags, thermodynamic behaviour of chlorine in the molten slag was investigated. Chloride capacity increased with increasing CaO / SiO₂ ratio (C/S). An increase in MgO content decreased chloride capacity at C/S≥1.0 because it lowered the activity of Ca²⁺ which seemed to have strong affinity with Cl^? in molten slag. Also, the chloride capacity decreased with increasing Al₂O₃ content. The affinity between the Ca²⁺ and Cl^? ions was confirmed by measuring the infrared spectra of slags. The dissolution reaction of chlorine into slag was exothermic and its molar enthalpy was evaluated from the experimental results at 1673 ‐ 1823 K. Based on the result obtained in the present study, the quantitative prediction of chlorine distribution during the blast furnace process was performed. It was estimated that almost all chlorine in the blast furnace would be absorbed into molten slag even if the PCI ratio was increased or low quality coal with chlorine content less than 1.0 mass% was injected.     

Modification of Inclusions in Molten Steel by Mg-Ca Transfer from Top Slag: Experimental Confirmation of the ‘Refractory-Slag-Metal-Inclusion (ReSMI)’ Multiphase Reaction Model

High-temperature experiments and Refractory-Slag-Metal-Inclusion (ReSMI) multiphase reaction simulations were carried out to determine the effect of the ladle slag composition on the formation behavior of non-metallic inclusions in molten steel. Immediately after the slag-metal reaction, magnesium migrated to the molten steel and a MgAl₂O₄ spinel inclusion was formed due to a reaction between magnesium and alumina inclusions. However, the spinel inclusion changed entirely into a liquid oxide inclusion via the transfer of calcium from slag to metal in the final stage of the reaction. Calcium transfer from slag to metal was more enhanced for lower SiO₂ content in the slag. Consequently, the spinel inclusion was modified to form a liquid CaO-Al₂O₃-MgO-SiO₂ inclusion, which is harmless under steelmaking conditions. The modification reaction was more efficient as the SiO₂ content in the slag decreases.     

Dissolution of Lime in Synthetic ‘FeO’‐SiO2 and CaO‐‘FeO’‐SiO2 Slags

Dissolution of different CaO samples into molten synthetic ‘FeO’‐SiO₂ and ‘FeO’‐SiO₂‐CaO slags was carried out in a closed tube furnace at 1873 K. The slag was kept stagnant. It was found that the dissolution rate was very fast when CaO rod was dipped into ‘FeO’‐SiO₂ slag. In the case of ‘FeO’‐SiO₂‐CaO slag, the dissolution of CaO rod in the stagnant slag was retarded after the initial period (2 minutes). Only less than 16 percent CaO reacted with the slag, irrespective of the type of lime. Three phase‐regions were identified in the reacted part of the lime rod by SEM‐EDS analysis. The formation of these regions was explained thermodynamically. A dense layer of 2CaO · SiO₂ was found to be responsible for the total stop of the dissolution. It could be concluded that constant removal of the 2CaO · SiO₂ layer would be of essence to obtain a high dissolution rate of lime. In this connection, it was found necessary to study the dissolution of lime in moving slag to reach a reliable conclusion regarding the relevance of the reactivity obtained by water ATSM test to the real reactivity of lime in high temperature slag.     

In situ studies of agglomeration between Al2O3–CaO inclusions at metal/gas,metal/slag interfaces and in slag

Abstract

Studies of inclusion behaviour at the metal/slag interface is of great importance for the steel industry in order to achieve better control of both the size and amount of the inclusions, as well as improving the steel quality and the casting process. In this work agglomeration of liquid Al₂O₃–CaO particles at both steel/argon gas and steel/slag interfaces was studied with a confocal scanning laser microscope. In addition, agglomeration of liquid Al₂O₃–CaO–SiO₂ inclusions present in the slag was investigated. The results showed that liquid inclusions more easily agglomerated to semiliquid inclusions than to liquid inclusions. Moreover, the agglomeration of liquid particles was found to be improved remarkably when the particles were present in the slag compared to when they were in the steel/slag interface.     

Dissolution behaviour of MgO based refractories in CaO–Al2O3–SiO2 slag

Abstract

Magnesium oxide (MgO) based refractories are widely used in secondary refining processes, and their dissolution into refining slag is the primary cause of their shortened lifespan. The dissolution rate was investigated for sintered MgO and commercial MgO–C and MgO–Cr₂O₃ refractories in a synthesised 50CaO–45Al₂O₃–5SiO₂ liquid (mass-%) slag. The change in slag composition was measured after a refractory sample was placed into the molten slag that was stirred by flowing argon gas at 1773?K. The dissolution rate of the sintered MgO was above those of the MgO–C and MgO–Cr₂O₃ refractories under the same gas flowrate, although the dissolution rate of all samples increased as the gas flowrate was increased from 25 to 75?mL·min^??1. The slag containing 5?mass-% FeO considerably promoted the dissolution of the MgO–C refractory because of the oxidation of carbon by FeO. The dissolution of all the refractories was greatly affected by penetration of the liquid slag, with the mass transfer of MgO in the penetrating slag at lower gas flowrates likely being the rate controlling step. At high gas flowrates, Ar bubbles covered the surface and blocked the contact between the liquid slag and the solid phase, reducing the dissolution rate.     

The Effects of CaF2, Basicity, and Atmospheric Conditions on the Solubility of Carbon and Nitrogen in the CaO-SiO2-Al2O3-Based Slag System

The solubility of carbon and nitrogen in the CaF₂-CaO-SiO₂-Al₂O₃ slag system was studied. The effects of the CaF₂, extended basicity (CaO/(SiO₂ + Al₂O₃)), and atmospheric conditions on the dissolution behavior of the carbon and nitrogen, as well as the correlations of the behaviors with the slag structure observed at 1773 K (1500 °C), are presented. Increases in the extended basicity and the CaF₂ increased the solubility of carbon in the slag. In the case of nitrogen dissolution, a characteristic parabolic curve with an identifiable minimum was observed for the slag. This curve shape correlated with a change in the dominant mechanism of dissolution from an incorporated to a free nitride. The solubility of carbon in the mixture of CO with N₂ was significantly higher than that of carbon in the mixture of CO with Ar and is likely due to the formation of cyanide. Thus, when carbon is present in significant quantities in the slag, the solubility of nitrogen in the slag increases. The degree of depolymerization of the slag with increased content of CaO/(SiO₂ + Al₂O₃) and CaF₂ was verified using Fourier transform infrared and Raman spectroscopy.     

Effects of transition metals on the kinetics of slag-refractory reactions

The dissolution kinetics of dense alumina discs in calcium aluminosilicate based melts was determined with a rotating disc technique at 1560 °C to 1590 °C, under a controlled atmosphere of Ar-CO-CO₂. The effects of rotation speed and the concentration of iron and manganese oxides on the dissolution rate of alumina into slags were measured by monitoring the concentration of species in the slag. Analysis of the results obtained indicated that at low concentrations of these transition metal oxides in 53 pct CaO-5 pct MgO-12 pct SiO₂-30 pct Al₂O₃ slags, the dissolution rate is most likely controlled by mass transfer in the slag phase. The rate data obtained also showed that the addition of iron oxide or manganese oxide results in considerable increase in the mass transfer by increasing the apparent diffusivities of species in the slag. Comparison of these results with published data on the diffusivities of species in similar slags are made and practical implications of the findings are briefly discussed.     

A study of the dissolution of Al2O3, MgO and MgAl2O4 particles in a CaO‐Al2O3‐SiO2 slag

The dissolution behaviour of alumina, MgO and MgAl₂O₄ particles in a 36%CaO‐21%Al₂O_3‐42%SiO₂ (mass contents in %) slag has been studied using a confocal scanning laser microscope in the temperature range of 1470 ‐ 1550 °C. The double hot thermocouple technique and metallographic examination was also used to characterize the reaction product between the particles and the slag. The total dissolution times for MgO and MgAl₂O₄ particles decreased with increased temperature and were almost identical (approximately 200 s for a 150 μm diameter particle at 1500 °C.). This phenomenon can be explained by the formation of MgAl₂O₄ layer on the surface of the MgO particle during dissolution. Alumina dissolution times were slightly longer (approximately 250 s for a 150 μm diameter particle at 1500 °C) than that of MgO and MgAl₂O₄ particles and no reaction layer was observed on the surface of the particles.     

Influence of SiO2 Addition on the Properties of Al2O3‐CaO‐CaF2 Slag

The Al₂O₃‐CaO‐CaF₂ slag system is used in making special quality steels by the electro‐slag re‐melting process (ESR). The purpose of our investigation was to analyse ESR slag that contained SiO₂. The slag samples with different SiO₂ fractions (0 ‐ 20 mass %) were examined by chemical analysis, differential thermal analysis, simultaneous thermal analysis, X‐ray diffraction, electron microscopy and wetting angle measurement. With addition of SiO₂ the polymerization of slags was increased due to the formation of new silicate complex compounds that influenced their melting points and wetting angles.     

Effect of changing slag composition on spinel inclusion dissolution

Abstract

The rate of MgAl₂O₄ spinel inclusion dissolution in CaO–SiO₂–Al₂O₃ slags at 1504°C has been measured using a laser scanning confocal microscope. It was found that the mechanism of spinel inclusion dissolution was at least in part controlled by mass transfer in the slag phase for the slag compositions used. Evidence in support of this finding was that the calculated diffusion coefficient was inversely proportional to the slag viscosity and that the diffusion coefficients were in reasonable agreement with those obtained in a separate study on alumina dissolution. The diffusion coefficients obtained were in the range of 0·76–2·2 × 10^?10 m² s^?1.     

Dissolution of alumina particles in CaO-Al2O3-SiO2-MgO slags

Abstract

The dissolution behaviour of alumina particles in CaO-Al₂O₃-SiO₂-MgO slags was studied by confocal scanning laser microscopy. Three different slag systems were investigated to evaluate the effect of changing the MgO content in a high silica tundish slag, and the effect of decreasing the silica content when dissolving the particles in a ladle slag. A particle of 80 mum diameter dissolved in 50 s in a simulated ladle slag but needed 100 s to dissolve in a high silica slag at the same temperature. When the silica content was decreased, a decrease in the alumina particle dissolution time was noted. During dissolution of alumina particles in a slag containing MgO, the alumina particle reacts with the slag to form an MgAlO₄ layer. No reaction layer was observed during the dissolution of alumina particles in slags that did not contain MgO. This work suggests that the dissolution time of large alumina particles is significant, increasing with particle size and with decreased temperature.     

Dissolution Mechanism of Indium in CaO-Al2O3-SiO2 Slag at Low Silica Region

The solubility of indium was measured in the low-silica region (<20?mass pct SiO₂) of the CaO-Al₂O₃-SiO₂ system by a thermochemical equilibration technique. The dissolution mechanism of indium into the CaO-Al₂O₃-SiO₂ slag at 1773?K (1500?°C) under a reducing atmosphere was elucidated. The indium solubility increases in the calcium silicate-based flux and decreases in the calcium aluminate-based flux with increasing oxygen partial pressure. Also, the solubility was found to decrease initially with increasing slag basicity until the basicity reached a critical level after which the solubility increases. This behavior is believed to indicate that the indium dissolution mechanism changes according to the basicity of the slag.     

Slag‐Droplet Model: a Dynamic Tool to Simulate and Optimise the Refining Conditions in BOF

Prior knowledge and control of the path of slag evolution during the blow are crucial for the optimal performance of the BOF process. This is, however, not an easy task owing to the complexity of the multicomponent‐multiphase reactions at high temperatures and the fact that the on‐line data available to simulate the process during the blow are limited. A slag‐droplet model was developed in the nineties on the basis of multi‐component mixed transport control theory, the production of iron‐oxide through jet‐metal interaction and droplet formation, waste gas analysis, dissolution rate of scrap, lime, ore, etc., and slag foaming. Recently, when new blowing practices had to be developed to cope with the changes needed in lining life and converter availability, the slag droplet model was used as a dynamic tool to guide the process adjustments. Operating practices were evaluated for retained slag practice and increased burned dolomite addition. Audiometer signals were recorded during the blow. The results obtained during plant trials, as described in this work, supported the predictions of the model and resulted in improvement of the practice. Precipitation rate of second phase particles (2CaO.SiO₂ and 3CaO.SiO₂) in slag substantially influences slag foaming behaviour.     

Removal of Silicon Impurity from Molten Brass Using an Innovative ZnO-containing Slag

The elimination kinetics of silicon impurity from molten brass by a novel ZnO-Na₂O-B₂O₃-Na₂CO₃ slag has been studied. The influence of temperature, initial concentrations of Si in molten brass, and initial concentrations of SiO₂ in liquid slag on the kinetics of the reaction was investigated. Experimental results showed that the reaction rate increases with increasing temperature and with decreasing initial concentration of silicon in the metal and silicon dioxide in the slag phase. Based on a kinetic analysis, it was concluded that the reaction is predominantly controlled by mass transfer in the slag phase. The activation energy was estimated to be approximately 197 kJ mol⁻¹.     

<Emphasis Type="Italic">In Situ</Emphasis> Observation of Calcium Aluminate Inclusions Dissolution into Steelmaking Slag

The dissolution rate of calcium aluminate inclusions in CaO-SiO₂-Al₂O₃ slags has been studied using confocal scanning laser microscopy (CSLM) at elevated temperatures: 1773 K, 1823 K, and 1873 K (1500 °C, 1550 °C, and 1600 °C). The inclusion particles used in this experimental work were produced in our laboratory and their production technique is explained in detail. Even though the particles had irregular shapes, there was no rotation observed. Further, the total dissolution time decreased with increasing temperature and decreasing SiO₂ content in the slag. The rate limiting steps are discussed in terms of shrinking core models and diffusion into a stagnant fluid model. It is shown that the rate limiting step for dissolution is mass transfer in the slag at 1823 K and 1873 K (1550 °C and 1600 °C). Further investigations are required to determine the dissolution mechanism at 1773 K (1500 °C). The calculated diffusion coefficients were inversely proportional to the slag viscosity and the obtained values for the systems studied ranged between 5.64 × 10^?12 and 5.8 × 10^?10 m²/s.     

Dissolution kinetics of chromium ore in slag system for stainless steelmaking

Abstract

To reveal the smelting reduction mechanism of chromium ore for producing stainless steel in converter, the dissolution behaviour of chromium ore in CaO–SiO₂–MgO–Al₂O₃ slag system was studied by laboratory experiments, and the effects of different temperatures and slag compositions on the dissolution rate of chromium ore in slag were investigated. A kinetic model for the dissolution process of chromium ore was developed for the first time. The dissolution process of chromium ore is controlled by the surface dissolution reaction under the conditions of the present experiments. The effects of temperature, slag composition and particle size of chromium ore on the dissolution rate of chromium ore are relatively great. The chromium ore sand is suggested to be used as raw material in the smelting reduction converter.

Afin de révéler le mécanisme de réduction de la fusion du minerai de chrome pour la production d'acier inoxydable dans un convertisseur, on a étudié le comportement de dissolution du minerai de chrome dans le système de laitier CaO-SiO₂-MgO-Al₂O₃, par des expériences en laboratoire. On a examiné l'effet des différentes températures et compositions de laitier sur la vitesse de dissolution du minerai de chrome dans le laitier. Pour la première fois, on a développé un modèle cinétique du procédé de dissolution du minerai de chrome. Le procédé de dissolution du minerai de chrome est contrôlé par la réaction de dissolution à la surface sous les conditions des présentes expériences. Les effets de la température, de la composition du laitier et de la taille de particule du minerai de chrome sur la vitesse de dissolution du minerai de chrome sont relativement importants. On suggère que le sable de minerai de chrome soit utilisé comme matériau brut dans le convertisseur de réduction de fusion.     

Study of Boronizing Mechanism of High‐Alumina Slag

As the Al₂O₃ content of the iron‐making raw materials increasing, the Al₂O₃ content in the slag increases as well, which is the certain result in the development of the iron‐making process. Based on the slag which was from the 2# blast furnace in Hebei Qianan of China, 7 different kinds of B compositions were added. The viscosity values of the CaO ‐MgO‐Al₂O₃‐SiO₂‐B₂O₃ system were measured by using the torque. The SEM and EDS apparatus were used to observe the variation characteristics of the boronizing slag of the blast furnace. Viscosity mechanism model of the boronazing high‐alumina slag was got from the analysis of the EDS compositions. These detailed analysis including the influence on the slag forming components result from different B contents which shows the conclusion that the suitable B content makes good to the viscosity, melting temperature and stability of high‐alumina slag.