Construction of viscosity diagrams for CaO–SiO<Subscript>2</Subscript>–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–8% MgO–4% B<Subscript>2</Subscript>O<Subscript>3</Subscript> slags by the simplex lattice method

The simplex lattice method of planning experiments is used to study the viscosities of CaO–SiO₂–Al₂O₃–8% MgO–4% B₂O₃ slags in a wide chemical composition range. For each viscosity, we developed an adequate mathematical model in the form of a reduced third-order polynomial. The results of mathematical simulation are presented in composition–viscosity diagrams. Composition regions with a high fluidity of slags, the viscosities of which are 0.8–1.2 Pa s in the temperature range 1500–1600°C, are indicated in the diagrams.     

Thermodynamic Simulation of the Influence of the Temperature and the Basicity of a Boron-Containing Slag on Steel Desulfurization

The results of thermodynamic simulation of the desulfurization of a medium-carbon steel by slags of the CaO–SiO₂–MgO–Al₂O₃–B₂O₃ system are presented. The HSC Chemistry 6.12 software package is used for the simulation. The thermodynamic simulation is performed for 20 various chemical compositions of slags with various B₂O₃ contents (1–4%)¹ and basicities ((CaO)/(SiO₂) = 2–5). The computations are performed using the Equilibrium Compositions module in the temperature range from 1500 to 1700°C with an increment of 50°C at a gas phase pressure of 0.1 MPa. The main results of the calculations are presented as the dependences of the change in the sulfur content in steel [S] on the temperature, the content of B₂O₃, and the slag basicity. An increase in the temperature of metal desulfurization from 1500 to 1700°C exerts a favorable effect on the sulfur content for the studied range of slag basicities. In particular, the sulfur content in steel decreases from 0.012 to 0.009% when steel is processed with the slag having 3% B₂O₃ and a basicity (CaO)/(SiO₂) = 2. A positive effect of an increase in the slag basicity from 2 to 5 on metal desulfurization is observed: the degree of desulfurization increases from 61.1 to 97.2% at 1600°C and 3% B₂O₃ content in the slag. As the B₂O₃ content in a slag increases from 1 to 4%, its refining properties decrease significantly in the range of basicity not higher than 2. In the range of high slag basicities (3–4), the negative effect of acidic oxide B₂O₃ on the refining properties of the slag decreases, providing low sulfur contents (which do not exceed [S] = 0.003–0.004% at 4% B₂O₃). At a slag basicity of 5, the sulfur content in steel decreases to 0.001%, all other things being equal. The simulation results can be used for the calculation of steel desulfurization processed by slags containing B₂O₃.     

Viscosities of the Quaternary Al2O3‐CaO‐MgO‐SiO2 Slags

Viscosities of some quaternary slags in the Al₂O₃‐CaO‐MgO‐SiO₂ system were measured using the rotating cylinder method. Eight different slag compositions were selected. These slag compositions ranging in the high basicity region were directly related to the secondary steel making operations. The measurements were carried out in the temperature range of 1720 to 1910 K. Viscosities in this system and its sub‐systems were expressed as a function of temperature and composition based on the viscosity model developed earlier at KTH. The iso‐viscosity contours in the Al₂O₃‐CaO‐MgO‐SiO₂ system relevant to ladle slags were calculated at 1823 K and 1873 K for 5 mass% MgO and 10 mass% MgO sections. The predicted results showed good agreement with experimental values and the literature data.     

Thermodynamic Modeling of Metal Desulfurization with Boron-Containing Slags of the CaO–SiO<Subscript>2</Subscript>–MgO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript> System

In thermodynamic modeling of the desulfurization of steel by CaO–SiO₂–MgO–Al₂O₃–B₂O₃ slag on the basis of HSC 6.12 Chemistry software (Outokumpu), the influence of the temperature (1500–1700°C), the slag basicity (2–5), and the B₂O₃ content (1–4%)1 on the desulfurization is analyzed. It is found that the sulfur content is reduced with increase in the temperature from 1500 to 1700°C, within the given range of slag basicity. At 1600°C, the sulfur content in the metal is 0.0052% for slag of basicity 2; at 1650°C, by contrast, its content is 0.0048%. Increase in slag basicity from 2 to 5 improves the desulfurization, which increases from 80.7 to 98.7% at 1600°C. If the B₂O₃ content in the slag rises, desulfurization is impaired. At 1600°C, the sulfur content in the metal may be reduced to 0.0052 and 0.0098% when using slag of basicity 2 with 1 and 4% B₂O₃, respectively; in the same conditions but with slag of basicity 5, the corresponding values are 0.00036 and 0.00088%, respectively. Note that desulfurization is better for slag without B₂O₃. According to thermodynamic modeling, metal with 0.0039 and 0.00019% S is obtained at 1600°C when using slag of basicity 2 and 5, respectively, that contains no B₂O₃. The results obtained by thermodynamic modeling for the desulfurization of metal by CaO–SiO₂–MgO–Al₂O₃–B₂O₃ slag of basicity 2–5 in the range 1500–1700°C are consistent with experimental data and may be used in improving the desulfurization of steel by slag that contains boron.     

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.     

Activity of MnO in MnO‐CaO‐MgO‐SiO2‐Al2O3 Slags at 1500 °C

A thermodynamic study was made on the MnO‐CaO‐MgO‐SiO₂‐Al₂O₃ slags that are typical of the production of ferromanganese in submerged arc furnaces. The Al₂O₃ content of the slags was kept constant at 5 per cent by mass. The activity‐composition relationship in Pt‐Mn binary alloys were re‐determined for calibration purposes at 1300, 1400 and 1500°C and po₂ values between 5.40×10^?6 and 4.54×10^?13 atm. A linear regression equation was derived to predict the activity coefficients of manganese, in Pt‐Mn alloys at 1500°C. The effect of concentration, basicity ratio and CaO‐to‐MgO ratio on MnO activities in above mentioned complex slags was investigated at 1500 °C and at two different po₂ values of 4.76×10^?7 and 5.80×10^?8 atm. It was found that a_Mno values increase with increasing MnO, and tend to increase with an increasing CaO‐to‐MgO ratio. The a_MnO values also increase with increasing basicity ratio. The activity coefficient of MnO increases with an increase in its mole fraction in the slag. Quadratic multivariable regression model equations which represent the activity data successfully and which can be used to predict the MnO activities in the compositional range of this study were developed. The MnO activity data was interpreted in terms of a slag model which describes the thermodynamic properties of the slag successfully.     

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.     

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.     

A Viscosity Estimation Model for Molten Slags in Al2O3‐CaO‐MgO‐SiO2 System

A model for viscosity estimation of molten slags in the Al₂O₂‐CaO‐MgO‐SiO₂ system is presented in this work. The model is an extension to the viscosity estimation model of molten slags in the CaO‐FeO‐MgO‐MnO‐SiO₂ system developed before by the present author. The present model has explicitly taken charge compensation into consideration. It is postulated that Al exists in a structural unit MAl₂O₄ when MO/ Al₂O₃ >1 for the Al₂O₃‐MO‐SiO₂ system (MO=CaO, MgO). MAl₂O₄ has a similar behaviour as SiO₂, i.e. it can form an Al‐O‐Al network and be depolymerised by network modifying oxides (CaO, MgO). The present model is applied in viscosity estimation of some slags within the Al₂O₃‐CaO‐MgO‐SiO₂ system. A mean deviation of less than 25% is achieved for the present model.     

Degradation of MgO refractory in CaO-SiO2-MgO-FeO x and CaO-SiO2-Al2O3-MgO-FeO x slags under forced convection

Experiments based on exposure of MgO to slags under forced convection flow conditions allowed the identification of different degradation mechanisms and the assessment of the role of Al₂O₃ in the degradation process. Slag with no alumina present resulted in direct dissolution. Samples immersed in alumina containing slag underwent indirect dissolution, with a spinel forming at the MgO-slag interface. At 1530 °C, the spinel was not effective in reducing the corrosion rate, as the scattered spinel grains were easily removed from the MgO surface. At 1500 °C, the loss of MgO was reduced due to the formation of a more cohesive spinel layer. Mechanical erosion then appears to play a greater role. Strength of the bond between the spinel and underlying MgO needs to be considered in strategies to reduce degradation of MgO refractories.     

Dissolution of water vapour in ESR-slags of the systems CaO–Al2O3 and CaF2–CaO–Al2O3 by application of Fourier-Transform-Infrared-Spectroscopy

In CaO–Al₂O₃- and CaF₂–CaO–Al₂O₃-slags the soluted water vapour was quantitatively determined with Fourier-Transform-lnfrared-(FT-IR-)spectroscopy. The slags were equilibrated at 1500 and 1600 °C with definite H₂O- and H₂O/HF-partial pressures, respectively. The liquid slags were quenched in liquid D₂O. They solidified glassily. Water vapour exists in the slags soluted as OH^?-ion. For all slags investigated a linear dependence of the integral extinction upon the square root of the H₂O-partial pressure, respectively upon HF-partial pressure, was determined. The values of the integral extinction (CaF₂–CaO–Al₂O₃-slags) are calibrated by a H₂O-doping-technique. The solution enthalpy of H₂O in CaO–Al₂O₃-slags was estimated: endothermal reactions. Interpretation of the vibrational range yielded in a mainly tetrahedral coordination of Al³⁺ with O^2?- and F^? -ions in the glassy state. By heating the glassily solidified CaO–Al₂O₃-slags were transformed into their crystalline products: shifting of IR-peaks. The CaO–Al₂O₃- and CaF₂–CaO-slags were compared with a CaO–SiO₂–Al₂O₃-slag. The FT-IR-method is quick. The water content can be estimated with an uncertainty of less than ± 5% and is, therefore, suitable for process control of metallurgical procedures.     

Equilibrium boron distribution between Fe–C–Si–Al melt and boron-bearing slag

HSC 6.1 Chemistry software (Outokumpu) and a simplex–lattice experiment design are employed in thermodynamic modeling of the equilibrium boron distribution between steel containing 0.2% C, 0.35% Si, and 0.028% Al (wt % are used throughout) and CaO–SiO₂–Al₂)₃–8% MgO–4% B₂O₃ slag over a broad range of chemical composition at 1550 and 1600°C. For each temperature, mathematical models (in the form of a reduced third-order polynomial) are obtained for the equilibrium boron distribution between the slag and the molten metal as a function of the slag composition. The results of simulation are presented as graphs of the composition and equilibrium distribution of boron. The slag basicity has considerable influence on the distribution coefficient of boron. For example, increase in slag basicity from 5 to 8 at 1550°C decreases the boron distribution coefficient from 160 to 120 and hence increases the boron content in the metal from 0.021% when L _B = 159 to 0.026% when L _B = 121. In other words, increase in slag basicity favorably affects the reduction of boron. Within the given range of chemical composition, the positive influence of the slag basicity on the reduction of boron may be explained in terms of the phase composition of the slag and the thermodynamics of boron reduction. Increase in metal temperature impairs the reduction of boron. With increase in temperature to 1600°C, the equilibrium distribution coefficient of boron increases by 10, on average. On the diagrams, we see regions of slag composition with 53–58% CaO, 8.5–10.5% SiO₂, and 20–27% Al₂O₃ corresponding to boron distribution coefficients of 140–170 at 1550 and 1600°C. Within those regions, when the initial slag contains 4% B₂O₃, we may expect boron concentrations in the metal of 0.020% when L _B = 168 and 0.023% when L _B = 139.     

Calculation and Analysis of Sulfide Capacities for CaO-Al2O3-SiO2-MgO-TiO2 Slags

The empirical models of sulfide capacity calculated by traditional optical basicity do not consider the charge compensation of alkaline metal ions to Al³⁺ in the molten slags, so that the deviations between the calculated values and measured ones of sulfide capacity are inevitable. The relation between sulfide capacity and the corrected optical basicity put forward by Mills considering the charge compensation was investigated. Combined with the relation between sulfide capacity and temperatures, a novel and accurate calculation model of sulfide capacity was proposed, which was applied to calculate the sulfide capacities of CaO-Al₂O₁-SiO₂-MgO and CaO-Al₂O₁-SiO₂-MgO-TiO₂ systems, where the sum of the CaO and MgO concentrations in the slags must be not lower than the Al₂O₃ concentration. It was also found that the calculated values were in a good agreement with the measured values, and the mean deviations were 2. 57% and 2. 65%, respectively.     

The sulphide capacity of CaO-SiO2-Al2O3-MgO(-FeO) smelting reduction slags

The effects of MgO and FeO contents on the sulphide capacity of Corex slags were investigated at 1773 K using gas/slag equilibrium technique as a fundamental study for stabilising Corex operational conditions. The gradual substitution of MgO for CaO at a fixed basicity [B = {(%CaO)+(%MgO)}/(%SiO₂)] decreased the sulphide capacities of CaO-SiO₂-Al₂C₃-MgO slags. The addition of FeO into the CaO-SiO₂-Al₂O₃-MgO slags at the fixed (%CaO)/(%SiO₂) ratio, MgO and Al₂O₃ contents significantly increased the sulphide capacities. The sulphide capacity decreased according to the increasing Al₂O₃ content at the fixed (%CaO)/(%SiO₂) ratio, MgO and FeO content. Based on the previously reported and present values of sulphide capacities, the sulphide capacity as a function of slag composition on the mole fraction base at 1773 K was expressed by the formula of log C_S = X_CaO – 3.89 X_SiO2 – 3.18 X_Al2O₃ – 0.55 X_MgO + 2.43 X_FeO – 2.61. In addition, the relationship between the sulphide capacity and optical basicity could be represented as the formula of log C_S = 12.51 A – 12.24 and the theoretical optical basicity of FeO was found be 0.94 from the correlation. The effect of FeO on the sulphur distribution ratio was estimated using the present sulphide capacity values and the oxygen activity in liquid iron, which could be determined with the help of Fe/FeO equilibrium. FeO activity in slag was well described by the quadratic formalism based on the regular solution model. The sulphur distribution ratio according to FeO content varies in an opposite way, compared with that of the sulphide capacity.     

Activities of SiO2 and Al2O3 and activity coefficients of FetO and MnO in CaO-SiO2-Al2O3-MgO slags

The activities of SiO₂ and Al₂O₃ in CaO-SiO₂-Al₂O₃-MgO slags were determined at 1873 K along the liquidus lines saturated with 2CaO · SiO₂, 2(Mg,Ca)O · SiO₂, MgO, and MgO · Al₂O₃ phases using a slag-metal equilibration technique. Based on these and previous results obtained in ternary and quaternary slags, the isoactivity lines of SiO₂ and Al₂O₃ over the liquid region on the 0, 10, 20, 30, and 40 mass pct Al₂O₃ planes and those on the 10 and 20 mass pct MgO planes were determined. The activity coefficients of Fe _t O and MnO, the phase boundary, and the solubility of MgO were also determined.     

Viscosity of SiO2-“FeO”-Al2O3 System in Equilibrium with Metallic Fe

The present study delivered the measurements of viscosities in SiO₂-“FeO”-Al₂O₃ system in equilibrium with metallic Fe. The rotational spindle technique was used in the measurements at the temperature range of 1473 K to 1773 K (1200 °C to 1500 °C). Molybdenum crucibles and spindles were employed in all measurements. The Fe saturation condition was maintained by an iron plate placed at the bottom of the crucible. The equilibrium compositions of the slags were measured by EPMA after the viscosity measurements. The effect of up to 20 mol. pct Al₂O₃ on the viscosity of the SiO₂-“FeO” slag was investigated. The “charge compensation effect” of the Al₂O₃ and FeO association has been discussed. The modified quasi-chemical viscosity model has been optimized in the SiO₂-“FeO”-Al₂O₃ system in equilibrium with metallic Fe to describe the viscosity measurements of the present study.     

The Effect of MgO on the Viscosity of the CaO‐SiO2‐20 wt%Al2O3‐MgO Slag System

The viscosities of CaO‐SiO₂‐20 wt%Al₂O₃‐MgO slags (CaO/SiO₂ = 1.0–1.2, wt%MgO = 5–13) were measured to estimate the effect of MgO on the viscous behaviour at elevated temperatures. The slag viscosity at 1773 K decreased with increasing MgO contents, which was typical of a basic oxide component at relatively low basicity (CaO/SiO₂) of 1.0. The FT‐IR spectroscopic analysis of the slag structure seems to verify this behaviour. However, an unexpected contradiction with the temperature dependence was observed above 10 wt%MgO and above CaO/SiO₂ of 1.2. Although the apparent activation energy was expected to decrease with additions of the basic oxide component MgO, the apparent activation energy increased. This unexpected behaviour seems to be related to the change in the primary phase field correlating to the phase diagram corresponding to the slag composition. Therefore, in order to understand the viscosity at both high Al₂O₃ and MgO, not only should the typical depolymerization of the slag structure with high MgO content be considered but also the primary phases of which the molten slag originates.     

Viscosity of Fluoride‐Free Mold Fluxes Containing B2O3 and TiO2

Viscosities of B₂O₃ and TiO₂‐bearing fluoride‐free mold fluxes have been measured by the rotating cylinder method in this work. Effects of different B₂O₃, TiO₂ content, and basicities on the viscosity characteristics have been examined. Viscosity of fluoride‐free mold fluxes containing B₂O₃ and TiO₂ was found to decrease with the increase of B₂O₃, TiO₂ content, and basicity. The values of apparent activation energy for viscous flow of slags decrease with additions of B₂O₃ and TiO₂ and the increase of basicity. Two parameters A and B in Riboud model were re‐evaluated based on the present experimental data, and the modified Riboud model was used to estimate the viscosity of fluoride‐free slag system investigated in present work. The viscosity values obtained by the experimental measurement were in good agreement with those calculated by the modified Riboud model.     

Activity of chromic oxide in the CaF2−CaO−Cr2O3 and the CaF2−Al2O3−Cr2O3 systems

The values of the activity of Cr₂O₃ in the slags based on the CaF₂−CaO−Cr₂O₃ and the CaF₂−Al₂O₃−Cr₂O₃ systems which may be used in the electroslag remelting (E.S.R.) process have been determined at 1450, 1500 and 1550°C by equilibrating the slags with Pt−Cr alloys of known chromium activity under known oxygen partial pressure and studying the equilibrium 2[Cr] alloy+3/2 O₂(g)=(Cr₂O₃)_slag. It was found that activity of Cr₂O₃ decreases with the addition of CaO and Al₂O₃ in the respective systems. In slags containing less than about 20 wt pct CaO and in the Al₂O₃ bearing slags, solutions of Cr₂O₃ showed a positive deviation from ideality and in slags containing more than 20 wt pct CaO, they showed a negative deviation. Both the authors were formerly with the Department of Metallurgy, University of Sheffield, England     

Dephosphorization equilibria between pure molten iron and CaO-saturated FeOn–CaO–SiO2 and FeOn–CaO–Al2O3 slags

Metal-slag refining reactions have been investigated to determine dephosphorization equilibria in steelmaking using CaO-saturated slags, low in P₂O₅–content, based on the systems FeO_n–CaO–SiO₂ and FeO_n–CaO–Al₂O₃. Slag compositions have been optimized with respect to basicity and oxygen potential to achieve maximum partition ratios wt.%(P₂O₅)/wt.%[P] and minimum phosphorus contents in pure molten iron at 1550, 1600 and 1700°C. Both slag systems prove to be effective dephosphorizers. Optimal slag compositions are around 10 wt.% SiO₂ near the CaO–3CaO · SiO₂ double saturation in the case of FeO_n–CaO–SiO₂ slags and at Al₂O₃ contents tending to zero in the case of FeO_n–CaO–Al₂O₃ slags. Attempts were also made to present phosphate capacities C_PO4^3?, fractions of free oxygen ions x_O^2? and theoretical optical basicities Λ as a function of the FeO_n content of slags.