Activity of Chromium Oxide in CaO‐SiO2 based Slags at 1873 K

Thermodynamic properties of chromium oxides in molten slags are very important for optimization of stainless steel refining processes as well as reduction processes of chromium ores. The solubility of chromite into molten slags has been found to vary drastically with oxygen partial pressure and slag composition in the former studies by the authors. In the present study, activity data and redox equilibria of chromium oxides measured under moderately reducing conditions, P_O2= 6.95×10^?11 atm, at 1873 K are summarized. For the CaO‐SiO₂‐CrO_x system, the activity coefficient of chromium oxide increased with increasing basicity and the optimized slag composition for stainless steel refining is assessed as that saturated with CaCr₂O₄ and Cr₂O₃ using the phase relations determined. On the other hand, the presence of MgO and Al₂O₃ brings about different behaviour of chromium oxide activity and redox equilibria and the 44 mass per cent CaO ‐ 39 mass per cent SiO₂ ‐11 mass per cent Al₂O₃ ‐ 6 mass per cent MgO slag is recommended to reduce the chromium oxidation loss in the practical stainless steel refining process at 1873 K.     

The effects of MgO and Al2O3 behaviours on softening–melting properties of high basicity sinter

Because the behaviours of MgO and Al₂O₃ during slag formation of high basicity sinter are not clearly understood, the effects of MgO and Al₂O₃ on softening-melting properties are always arguable. In this paper, four kinds of sinter containing different MgO and Al₂O₃ content are investigated. Some observations are obtained. The mechanism of the influence of MgO and Al₂O₃ on softening properties of sinter are different. Al₂O₃ has priority over MgO to enter into slag phase and forms low-melting point phase while MgO remains unslagged state and mainly exists in wustite as FeO–MgO solid solution. When sinter melts, the viscosity of the slag generated from sinter containing high MgO and Al₂O₃ content is low, which could result in low pressure drop. As MgO and Al₂O₃ content increase, the main minerals of residual slag change from 2CaO?SiO₂ to merwinte and melilite. The changes of the minerals in slag phase can well explain the trend of softening-melting characteristic temperatures.     

Experimental measurements and modelling of viscosity in CaO–Al2O3–MgO slag system

Abstract

The viscosity of the CaO–Al₂O₃–MgO slag system has been measured by the rotating cylinder method up to 1823 K. The MgO content was between 0·39 and 11·33 wt-%, and the mass ratio of CaO/Al₂O₃ was between 0·60 and 1·28. The results indicated that both the MgO content and the mass ratio of CaO/Al₂O₃ have an influence on the viscosity of the slag. The effect of the mass ratio of CaO/Al₂O₃ is larger than that of the MgO content. The viscosity decreased with increasing MgO content and then increased. The effect of the mass ratio of CaO/Al₂O₃ on the viscosity was similar. The main mineralogical compositions of the slag were determined by X-ray diffraction, and their effects on viscosity were investigated. The measured results were in good agreement with those given by the Iida model.     

Influence of MgO,Al2O3 and CaO/SiO2 on the viscosity of blast furnace type slag with high Al2O3 and 5 wt-% TiO2

The effect of MgO, Al₂O₃ and CaO/SiO₂ on the viscosity of CaO–SiO₂–Al₂O₃–MgO–5 wt-% TiO₂ slag was studied in the temperature range of 1673–1773?K. At a fixed CaO/SiO₂ ratio of 1·17 and 12 wt-% Al₂O₃, the viscosity of the slag decreased with increasing MgO content because of depolymerisation of the silicate structures. At a fixed CaO/SiO₂ ratio of 1·17 and 8 wt-% MgO, the viscosity of the slag increased with increasing Al₂O₃ content. At 8 wt-% MgO and 12 wt-% Al₂O₃ wt-%, increasing the CaO/SiO₂ ratio from 1·07 to 1·50 resulted in lower slag viscosity. The temperature dependencies of the viscosity on MgO addition, Al₂O₃ addition, and CaO/SiO₂ ratio were analyzed, and the apparent activation energies of each system were found to be between 178 and 232?kJ/mol, 273 and 360?kJ/mol, and 204 and 233?kJ/mol, respectively. Five different viscosity models were employed to predict slag viscosity, and the Riboud model was found to be the best for predicting this parameter.     

Immobilization of Chromium in Slags using MgO and Al2O3

Chromium containing slags from stainless steelmaking may be leached by acidic environments, therefore they should be treated before being stockpiled or land filled. In this work, synthetic slags were prepared and the effect of CaO/SiO₂, Cr₂O₃, MgO and Al₂O₃ contents on the stability of the mineralogical species formed was analysed. The morphology and composition of the slags were determined by XRD and SEM‐EDS, whilst their chemical stability was evaluated by leaching with an aqueous acetic acid solution. It was found that CaCr₂O₄ and CaCrO₄ are present in slags prepared with neither MgO nor Al₂O₃. The Al₂O₃‐based slags mainly produced Ca₂Al₂SiO₇ and the Cr(VI)‐containing oxide complex Ca₄Al₆CrO₁₆, whilst MgO‐based slags produced Mg Cr₂O₄ as main mineralogical species. Additionally, Eh‐pH diagrams for the Ca‐Cr‐H₂O and Mg‐Cr‐H₂O systems at 25°C were constructed. The results showed that the lowest chromium concentration levels in the leaching liquors corresponded to MgO‐based slags owing to the stable binding of chromium in spinel with MgO. It was also observed in the Al₂O₃‐based slags that when increasing the slag basicity from 1 to 2, the leachability of the slags was notably increased.     

Control of Non‐metallic Inclusions by Slag‐metal Reactions for High Strength Alloying Steels

A laboratory study was carried out to investigate non‐metallic inclusions in high strength alloying steel refined by high basicity slag. The results indicated that the inclusions were mainly of the CaO? MgO? Al₂O₃ system, Al₂O₃? MgO and MgO‐based inclusions. The steel/slag reaction time and Al₂O₃ content in slag had a great effect on inclusions characteristics. With the reaction time increasing from 30 to 180 minutes, inclusions experienced a transformation process: from mainly Al₂O₃? MgO system and MgO‐based inclusions to spherical CaO? MgO? Al₂O₃ system inclusions surrounded by a lower melting temperature surface layer of CaO? Al₂O₃. Formation and transformation mechanisms of the inclusions were given based on the results. It was also found that with Al₂O₃ content in slag reduced from 40% to 30%, [Mg] contents in steel melts were increased and MgO in slag reached saturation, which contributed to the formation of more MgO‐based inclusions and a more scattered inclusion composition distribution after 90 min reaction.     

Kinetics of Transformation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> to MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Spinel Inclusions in Mg-Containing Steel

During ladle furnace refining, initial Al₂O₃ inclusions generally transform into MgO·Al₂O₃ spinel inclusions; these generated spinel inclusions consequently deteriorate the product quality. In this study, the transformation from Al₂O₃ to MgO·Al₂O₃ was investigated by immersing an Al₂O₃ rod into molten steel, which was in equilibrium with both MgO and MgO·Al₂O₃ spinel-saturated slag. A spinel layer, with a thickness of 4 μm, was generated on the Al₂O₃ rod surface just 10 s after its immersion at 1873 K (1600 °C). The thickness of the formed spinel layer increased with the immersion period and temperature. Moreover, the MgO content of the generated spinel layer also increased with the immersion period. In this study, the chemical reaction rate at 1873 K (1600 °C) was assumed to be sufficiently high, and only diffusion was considered as a rate-controlling step for this transformation. By evaluating the activation energy, MgO diffusion in the generated spinel layer was found to be the rate-controlling step. In addition, this estimation was confirmed by observing the Mg and Al concentration gradients in the generated spinel layer. The results of this study suggest that the MgO diffusion in the spinel inclusions plays a substantial role with regard to their formation kinetics.     

Effect of cooling regime on phase transformation and chromium enrichment in stainless-steel slag

Cooling regime of stainless-steel slag has a great influence on the migration rule of chromium. Effect of cooling rate on chromium distribution in CaO–SiO₂–MgO–Al₂O₃–Fe_xO–Cr₂O₃ system has been studied by FactSage, XRD and SEM-EDS in this paper. Results showed that the main phases in stainless-steel slag are spinel, bredigite, wustite, lime and periclase. Chromium was enriched in the stable phases. With the decrease in cooling rate, the size of chromium-rich phase increased relatively while chromium content in spinel phase reduced. Rapid cooling was favourable to the chromium enrichment in spinel phase in high basicity (high than 2.0) stainless-steel slag.     

Kinetics of chromite ore reduction from MgO-CaO-SiO2-FeO-Cr2O3-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Kinetic experiments were performed in an induction furnace to investigate the reduction of chromite ore by carbon dissolved in a high carbon ferrochromium alloy melt under conditions of varying Cr₂O₃ concentration, slag basicity, and temperature. The results obtained show that chromite reduction by dissolved carbon in slag systems of the type MgO-CaO-SiO₂-FeO-Cr₂O₃- Al₂O₃ occurs principally by a stagewise process encompassing an intermediate reaction in which the divalent chromium oxide species is involved. During the fast period, Cr₂O₃ reduction is controlled by the diffusion of oxygen species in the slag for which a mass transfer coefficient of 0·003 cm s^-1 was calculated. An activation energy value of 117 kJ mol ^-1 obtained for the reduction of Cr₂O₃ implies the rate controlling step is mass transfer of Cr₂O₃ from the slag to the slag/metal interface, since activation energies for metal phase control are typically <70 kJ mol ^-1. The second period represents a pseudo-equilibrium condition with respect to Cr₂O₃ reduction that is probably under thermodynamic control by a step or mechanism involving the reduction of divalent chromium oxide to chromium.     

Optimisation of the MgO/Al2O3 ratio of high-alumina BF slag based on MOPSO algorithm

To reduce the content of MgO in the slag, which helps in lowering production costs, the chemical components of a SiO₂–CaO–MgO–Al₂O₃–TiO₂ blast furnace (BF) slag system with high Al₂O₃ content were optimised using the multi-objective particle swarm optimisation (MOPSO) algorithm. First, models for predicting the metallurgical properties of BF slag were established and verified, based on mixture experimental designs and the ion and molecule coexistence theory. Second, the problem was solved by MOPSO using the MATLAB software package. Optimisation results show that the appropriate content of MgO in the slag should be 4–7%, and the MgO/Al₂O₃ ratio should be 0.2–0.6. Finally, optimised solutions were used in the 3200?m³ BF of a steel company in China. MgO in the slag was reduced to less than 5%, the MgO/Al₂O₃ ratio was controlled at 0.3–0.4, and the fuel ratio remained at 515?kg?t^?1.     

Reduction of synthetic stainless steel slags by aluminium

One of the most efficient ways to eliminate the harm of chromium oxide in stainless steel slag is to reduce chromium oxide in stainless steel slag using aluminium. In the present work, the Al reduction of synthetic CaO–SiO₂–Al₂O₃–MgO–Fe₂O₃–Cr₂O₃ stainless steelmaking slags at different conditions, including temperature, slag basicity and Al amount was investigated to get optimal conditions for the reduction and the metal–slag separation. It was found that the agglomeration of metal droplets and metal–slag separations were improved by increasing temperature. The reduction degrees of SiO₂, Fe₂O₃ and Cr₂O₃ were enhanced with increasing basicity of slag. The addition of CaF₂ in slag leads to better agglomerations of metal droplets and metal–slag separations. The highest reduction degree of chromium could reach 99% in slag with basicity of 2 at 1873 K.     

Study on refining slags targeting high cleanliness and lower melting temperature inclusions in Al killed steel

Abstract

Three high basicity slags (A, B and C) were used in laboratory to refine Al killed steel to target high oxide cleanliness and low melting temperature inclusions. Inclusions were of CaO–MgO–Al₂O₃–SiO₂ system after 90 min reaction, parts of which were MgO based. Total oxygen were in the range of 0·0007–0·0010 and 0·0005–0·0010% respectively when slag A (CaO/SiO₂, 6–8; Al₂O₃, ～40%) and slag B (CaO/SiO₂, 6–8; Al₂O₃, ～30%) were applied, with inclusions all in spherical shape and mainly <5 μm. Inclusion composition concentrated in or around the lower melting point region (<1500°C) under slag A, while it became more scattered under slag B. Total oxygen varied between 0·0008 and 0·0011% under slag C (CaO/SiO₂, 3–4; Al₂O₃, about 20–25%). Many of the inclusions were in larger size, irregular morphology and located far away from the lower melting point region. Formation of MgO based inclusions closely related to solubility behaviour of MgO in the slag.     

Effects of FeO and CaO/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Ratio in Slag on the Cleanliness of Al-Killed Steel

The slag composition plays a critical role in the formation of inclusions and the cleanliness of steel. In this study, the effects of FeO content and the C/A (CaO/Al₂O₃) ratio in the slag on the formation of inclusions were investigated based on a 10-minute slag–steel reaction in a MgO crucible. The FeO content in the top slag was shown to have a significant effect on the formation of MgO·Al₂O₃ spinel inclusions, and critical content exists; when the initial FeO content in the slag was less than 2 pct, MgO·Al₂O₃ spinel inclusions formed, and the T.O (total oxygen) was 20 ppm; when the initial FeO content in the slag was more than 4 pct, only Al₂O₃ inclusions were observed and the T.O was 50 ppm. It was clarified that the main source of Mg for the MgO·Al₂O₃ spinel inclusion formation was the top slag rather than the MgO crucible. In addition, the cleanliness of the steel increased as the initial FeO content in the top slag decreased. As regards the effects of the C/A ratio, the MgO amount in the observed inclusions gradually increased, whereas the T.O content decreased gradually with the increasing C/A ratio. Slag with a composition close to the CaO-saturated region had the best effect on the inclusion absorption.     

Effect of Al2O3 and MgO on Interfacial Tension Between Calcium Silicate‐Based Melts and a Solid Steel Substrate

The effect of Al₂O₃ and MgO on the interfacial tension between the molten CaO–SiO₂‐based slag and solid steel at 1773 K was studied. The interfacial tension of molten slags slightly increased with increasing Al₂O₃, but no significant change of interfacial tension was observed with higher MgO. Fourier transform infrared (FTIR) of as‐quenched slag samples indicated the slag structure to polymerize with Al₂O₃ additions, but depolymerize with MgO additions. Further detailed studies of the slag surface using X‐ray photoelectron spectroscopy (XPS) showed the fraction of free oxygen ions to decrease with higher Al₂O₃ but remained constant at higher MgO. The results suggested that interfacial tension decreases not only with the depolymerization of the melt, but also with an increase in the free oxygen ions at the molten slag/solid steel interface.     

Influence of refining slag composition on cleanness and fatigue life of 60Si2MnA spring steel

The influence of basicity and Al₂O₃ content of LF refining slag on T.[O] (total oxygen) as well as type, number and size of non-metallic inclusions in Al killed 60Si2MnA spring steel was investigated. The results showed that with the increase of slag basicity R(CaO/SiO₂) or the decrease of Al₂O₃ content in slag, the T.[O], number and size of non-metallic inclusions decreased significantly. On the one hand, as the slag basicity increased, inclusions in steel were transformed from Al₂O₃–SiO₂–CaO–MgO quaternary system to Al₂O₃–SiO₂–CaO–MgO–CaS quinary system, which made the formation of voids between inclusions and steel matrix to decrease. Furthermore, thermodynamic calculations showed that CaS could only form in steel (R?≥?3.4). Al₂O₃–SiO₂–CaO–MgO came close to the compositions of the low melting point area, while Al₂O₃–SiO₂–CaO–MgO–CaS deviated from this. On the other hand, as the Al₂O₃ content in slag increased, Al₂O₃–SiO₂–CaO–MgO–CaS came close to the compositions of the low melting point area. In conclusion, the cleanness and fatigue life of 60Si2MnA spring steel had been improved by the increase of slag basicity or the decrease of Al₂O₃ content in slag.     

Experimental Study and Modelling of Viscosity of Chromium Containing Slags

Viscosities of slags containing Al₂O₃, CaO, CrO_x, MgO and SiO₂ were measured in contact with metallic chromium using the rotating cylinder method. The modified Urbain model, developed at the University of Queensland, was extended to include MgO, CrO and Cr₂O₃, in addition to existing Al₂O₃, CaO, FeO and SiO₂. Chromium oxides, in general, decrease the slag viscosity, although addition of trivalent oxide raises up the liquidus temperature and thus limits the measuring range. The model was able to describe the viscosity of complex slags reasonably well in most experimental cases.     

Viscosity of low silica CaO–5MgO–Al2O3–SiO2 slags

Abstract

The viscosity of CaO–5MgO–Al₂O₃–SiO₂ slag with low silica was measured by rotating cylinder method up to 1823 K. Slag compositions were chosen based on five different levels of SiO₂ content between 0 and 11·80%. The MgO content was 5·0%. The mass ratio of CaO/Al₂O₃ was varied from 0·66 to 1·95. It was shown that viscosity decreased with increasing temperature and decreased with increasing the mass ratio of CaO/Al₂O₃, following by an increase with further increasing the mass ratio of CaO/Al₂O₃. The viscosity decreased with the NBO/T ratio increasing, and the trend that flow activation energy changes with the NBO/T ratio of slag was the same as the trend that viscosity changes with the NBO/T ratio. Based on the experimental data as the boundary of the homogenous phase region, the mass triangle model was used to calculate the viscosity of low silica region.     

Effects of silica addition on chromium distribution in stainless-steel slag

The present study was aimed at highlighting the effect of silica on chromium distribution and enrichment in certain stainless-steel slag when basicity ranged from 0.96 to 1.96. Industrial slag samples of the CaO–Al₂O₃–MgO–SiO₂–CrO_X–Fe_XO system were investigated with different silica addition contents and soaked at the targeted regime. X-ray diffraction, electron probe micro-analyzer with wavelength-dispersive X-ray spectrometer, as well as Fourier-transform infrared spectroscopy and NIH ImageJ software were used for sample characterisation. FactSage version 6.4 was employed to calculate phase equilibria at 1200°C for the slag system to understand the crystallisation procedure. Results indicated that chromium was enriched in CaCr₂O₄ at high basicity, but in spinel at lower basicity. The spinel phase increased in size and chromium distribution with Fe(II) participated in growth with the decrease in depolymerisation of the slag structure.     

Measurement of FeO activity and solubility of MgO in smelting slags

In bath smelting, the FeO activity of the slag must be known to predict the equilibrium of slag-metal reactions and for effective control of the rate of reduction in the system. Also, knowledge of the solubility of MgO in these slags is useful for reducing refractory consumption. A series of measurements of the FeO activity in simulated bath smelting slags (CaO-SiO₂-Al₂O₃-MgO_sat-FeO) were conducted by the electromotive force (EMF) technique. The influence of the slag composition on the relationship between the FeO activity coefficient and FeO content was studied. It has been found that the measured FeO activity coefficient decreases with increasing FeO content in the slag and increases slightly with increasing slag basicity, which is defined as (CaO + MgO)/(SiO₂ + Al₂O₃) on a mole fraction basis. The measured values of the FeO activity coefficient are in reasonable agreement with previously published data. The solubility of MgO was also measured and found to rang from 16 to 30 pct and decrease with increasing basicity.     

Mobility of blast-furnace slag

Equipment for determining the mobility of blast-furnace slag in furnace operation has been developed and tested. On the basis of research at the casting doors of the blast-furnace shop, the MgO and Al₂O₃ content corresponding to high slag mobility is established. The dependence of the slag mobility on the hotmetal temperature, the basicity of the slag, and the MgO and Al₂O₃ content in the slag is determined.