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.     

Water Capacity Model of Al2O3‐CaO‐MgO‐SiO2 Quaternary Slag System

The focus of the present work was to develop a water capacity model for the quaternary slag system Al₂O₃‐CaO‐MgO‐SiO₂. In the model, a silicate melt was considered to consist of two ion groupings, viz. cation grouping and oxygen ion. The water capacity of a melt is supposed to depend on the interactions between the cations in the presence of oxygen ions. These interactions were determined on the basis of the experimentally measured water solubility data. Only binary interactions were employed in the model. For the system CaO‐SiO₂, disagreement in the literature data was found. Since the interaction between Ca²⁺ and Si⁴⁺ would play an important role, experiments were carried out to determine the water capacities of some CaO‐SiO₂ slags. For this purpose a thermogravimetric method was employed. Iso‐lines of water capacities at constant MgO contents were predicted by the model and compared with the experimental data from literature. The model calculations agreed well with the experimental results.     

Nitride capacities in CaO–SiO2 and CaO–SiO2–Al2O3 melts

Nitride capacities, , defined by (mass-% N) · in the CaO–SiO₂ and CaO–SiO₂–AI₂O₃ melts were measured in the temperature range of 1 723 to 1 923 K by a gas-slag equilibration technique using CaO, Al₂O₃, and Mo crucibles. Nitrogen content in slag, (mass-% N), was proportional to oxygen partial pressure, , to the power of ?3/4 at constant nitrogen partial pressure, . The values for increased linearly with increasing temperature and increased with the content of nitride formers, SiO₂ and Al₂O₃, but the effect of SiO₂ on value was found to be greater than that of Al₂O₃. The activity coefficients of Si_3/4N in the CaO–SiO₂ melts tended to increase with increasing the content of SiO₂.     

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.     

Sulphide Capacities of CaO‐SiO2‐Al2O3‐MgO Slags

In Japanese steelworks, hot metal is now being produced by a scrap melting process. With this process, removals of sulphur is very much handicapped because of very high sulphur levels (0.04‐ to 0.09‐ pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such handicaps, the authors explored on the respective phase diagrams. These explorations revealed that {CaO‐SiO₂‐Al₂O₃‐MgO} slags with Al₂O₃ contents of 30‐ to 35‐pct by weight would be good candidates as reagents for sulphur removal from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, sulphide capacities were determined through gas/slag equilibrium technique. The experimental results suggest that there would be, at least, a “window” to remove sulphur from high sulphur hot metal as relatively low temperatures.     

Das Hochofenschlackensystem Al2O3–CaO–MgO–SiO2 bei 1600, 1500 und 1400 °C

Beschreibung von Sättigungsflächen im System Al₂O₃–CaO–MgO–SiO₂ durch isotherme Schnitte im Konzentrationsbereich der Hochofenschlacken für 1600, 1500 und 1400 °C. Räumliche Tetraederdarstellungen und quantitative Grundrißprojektionen nach Überprüfung und Neufestlegung der Grenzflächen des homogenen Schlackenraumes durch Sättigungsschmelzen. Vollständige Darstellung des Grundsystems Al₂O₃–CaO–SiO₂ für 1600, 1500 und 1400 °C. Beispiel für die Anwendung der quantitativen Grundrißprojektion zur Einschätzung der Heterogenität von flüssigen Hochofenschlacken.     

Fundamental thermodynamic aspects of the CaO–Al2O3–SiO2 system

The most important metallurgical effects of ladle treatment of aluminium-killed steels with calcium, are associated with the modification of alumina inclusion. For the development of the deoxidation-control model for inclusions, the thermodynamic slag model, based on the Gibbs energy minimization and modelling approaches postulated from J. Hastie et al., was used to calculate component oxide activities in the system CaO–Al₂O₃ and part of systems 3CaO · Al₂O₃ – SiO₂, 12 CaO · 7Al₂O₃ – SiO₂ and CaO · Al₂O₃ – SiO₂ at 1600°C.     

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.     

Manganese Distribution between FeO‐MnO‐SiO2–CaO–MgO‐Al2O3 Slags and Molten Iron

Pretreatment of high manganese hot metal is suggested to produce hot metal suitable for further processing to steel in conventional LD converter and rich manganese slags satisfy the requirements for the production of silicomanganese alloys. Manganese distribution between slag and iron represents the efficiency of manganese oxidation from hot metal. The present study has been done to investigate the effect of temperature, slag basicity and composition of oxidizer mixture on the distribution coefficient of manganese between slag and iron. Ferrous oxide activity was determined in molten synthetic slag mixtures of FeO‐MnO‐SiO₂–CaO–MgO‐Al₂O₃. The investigated slags had chemical compositions similar to either oxidizer mixture or slags expected to result from the treatment of high manganese hot metal. The technique used to measure the ferrous oxide activity in the investigated slag systems was the well established one of gas‐slag‐metal equilibration in which molten slags contained in armco iron crucibles are exposed to a flowing gas mixture with a known oxygen potential until equilibrium has been attained. After equilibration, the final chemical analysis of the slags gave compositions having a particular ferrous oxide activity corresponding to the oxygen potential of the gas mixture. The determined values of ferrous oxide activity were used to calculate the equilibrium distribution of manganese between slag and iron. Higher manganese distribution between slag and iron was found to be obtained by using oxidizer containing high active iron oxide under acidic slag and relatively low temperature of about 1350°C.     

The Activities of FexO in {CaO‐SiO2‐Al2O3‐MgO‐FexO} Slags at 1723 K

In Japanese steelworks, hot metal is now produced by scrap melting process. With this process removal of sulphur is very much handicapped because of very high sulphur levels (0.04 to 0.09 pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such disadvantages, the authors explored on the phase diagrams of {CaO‐SiO₂‐Al₂O₃‐MgO} slags, and this research revealed that those slags at 35 wt%‐Al₂O₃ would be good candidates as reagents for the removal of sulphur from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, activities of Fe_xO were determined by using solid‐state electrochemical cells incorporating MgO‐stabilized zirconia and Mo + MoO₂ reference electrode.     

Thermodynamic Activity of Chromium Oxide in CaO‐SiO2‐MgO‐Al2O3‐CrOx Melts

Thermodynamic activities of chromium oxide contained in CaO‐SiO₂‐MgO‐Al₂O₃ melts were measured in the present work using gas‐slag equilibrium technique. The oxygen partial pressure was varied (10^‐3, 10^‐4, 10^‐5 Pa). Gas mixture of CO, CO₂ and Ar were used and investigated at 1803, 1873 and 1923 K. The activities of CrO showed a strong positive deviation from ideality and a decrease with increasing temperature and oxygen partial pressure. A mathematical expression relating the amount of chromium oxide in the slag phase with the activity of Cr in the metal phase based on the present experimental results is presented.     

Effect of FeO and Al2O3 on the MgO Solubility in CaO–SiO2–FeO–Al2O3–MgO Slag System at 1823 K

The MgO solubility in CaO–SiO₂–FeO–Al₂O₃–MgO quinary slag system at 1823 K was measured to evaluate the effect of FeO and Al₂O₃ on the MgO solubility. It was found that the MgO solubility was decreased with higher optical basicity, FeO concentration, and increased with higher Al₂O₃ concentration. The MgO solubility was affected by activity coefficient of Mg²⁺ ($\gamma _{{\rm Mg}_{{\rm 2 + }} } $ ). Increase of the activity coefficient of Mg²⁺ ($\gamma _{{\rm Mg}_{{\rm 2 + }} } $ ) with higher FeO or lower Al₂O₃ decreased the MgO solubility. The increment in MgO solubility is remarkably reduced beyond a critical $X_{{\rm Al}_{2} {\rm O}_{{\rm 3}} } /(X_{{\rm Al}_{2} {\rm O}_{{\rm 3}} } + X_{{\rm FeO}} )$ ratio. The significant decrease of the increment of MgO solubility is caused by the change of the molten slag structure. The excess stability function of Al₂O₃ and the Fourier transform infrared (FT‐IR) analysis were applied to indirectly verify the existence of the spinel structure in the CaO–SiO₂–FeO–Al₂O₃–MgO slag system.     

Verlauf der Kalksättigung im System FeO–Fe2O3–CaO–SiO2–P2O5–MgO–MnO beim Gleichgewicht mit einer Eisenschmelze

Durch Auswertung zahlreicher, in Laborversuchen bei 1580–1650°C erschmolzener kalkgesättigter Schlacken mit gleichzeitigen Gehalten an FeO, Fe₂O₃, SiO₂, P₂O₅, MgO und MnO ist die aus metallurgischen Gründen angestrebte Kalksättigung komplexer Schlacken der Stahlerzeugung für 1600°C neu festgelegt worden. Ausgehend von der Beschreibung der Schlackenzusammensetzungen in den Grundsystemen CaO′–FeO′_n–SiO′₂ und CaO′–FeO′_n–P₂O₅ wird dabei die Beeinflussung der Kalksättigung durch MgO und MnO sowie qualitativ durch dreiwertige Eisenanteile angegeben. Die untersuchten Schlacken lassen in beiden Grundsystemen einen stetigen, von hohen FeO′_n-Gehalten bis hinunter zu Massengehalten von 8% gültigen Verlauf der Kalksättigung ohne einsetzende Dicalciumsilikat- bzw. Tetracalciumphosphatausscheidung erkennen. Durch die bewertende Umrechnung der P₂O₅-, MgO- und MnO-Gehalte wird eine nahezu alle Komponenten erfassende Darstellung komplexer kalkgesättigter Schlacken in einem einfachen Quasidreistoffsystem CaO^*–FeO^*-SiO^*₂ erreicht und die resultierende, dicht belegte Kalksättigungslinie mathematisch beschrieben. Der Einfluß von Al₂O₃-Gehalten auf die Kalksättigung wird abgeschätzt sowie das maximale Lösungsvermögen der kalkgesättigten Schlacke für MgO durch die Berechnung der Schnittlinie zwischen der Kalk- und der Magnesiowüstitsättigungsfläche festgelegt. Ein Ansatz zur Erfassung des Temperatureinflusses auf die Kalksättigung erlaubt schließlich eine vollständige mathematische Beschreibung der Kalksättigungsgehalte im System CaO^*–FeO^*_n–SiO₂ sowohl in Abhängigkeit von der Schlackenzusammensetzung als auch von der Temperatur. Damit können die Kalkgehalte komplexer Betriebsschlacken unterschiedlicher Temperatur mit den erreichbaren Gleichgewichtssättigungswerten verglichen und die Sättigungszustände der Schlacken nach Berechnung eines Sättigungsgrades S_CaO beurteilt werden.