Equilibria between liquid Mn-Si alloys and MnO-SiO2-CaO-MgO slags

The activity of silicon in manganese-silicon melts was determined at 1500°C. The results are in general agreement with the thermodynamic data of the iron-silicon system. Equilibria between manganese-silicon melts and slags containing MnO, SiO₂, CaO, and MgO were studied at 1400 and 1500°C in silica and magnesia crucibles. An empirical relationship easy to use in practice was derived, expressing the manganese and silicon distribution ratio between slag and metal as a function of the slag basicity. This relationship describes equilibria pertinent to the silicothermic reduction of manganese oxide and the production of silicomanganese. The present knowledge of the activities in the slag and metal phase is adequate to explain the experimental results. The presence of up to about 10 pct CaF₂ in the slag makes it possible to maintain a higher slag basicity and therefore a lower activity of silica, resulting in lower silicon contents in the metal. Iron contents of up to about 20 pct in the metal cause a slight increase in the silicon content of the metal under otherwise similar conditions. The effect of 1 to 2 pct carbon in the metal on the equilibrium was roughly estimated and found to be almost negligible.     

Reduction of MnO from molten slags with liquid steel of high carbon content

This work estimated the reduction of MnO in slags of the CaO‐SiO₂‐FeO‐CaF₂‐MnO system and liquid steel with the initial composition (mass contents) 0.75 %Mn, 0.16 % Si and 0.5 to 2.0 % C, as an alternative to introducing Mn to the steel melt. The slag basicities (CaO/SiO₂) In the experiments were 2 and 3. MnO was obtained from manganese ore. The experiments were carried out in an open 10 kg induction furnace using Al₂O₃‐based refractory at 1873 K. The oxygen potential was measured throughout the experiments with a galvanic cell (ZrO₂‐solid electrolyte with a Cr/Cr₂O₃ reference electrode). The MnO reaction mechanism was analysed in terms of the slag basicity, the silicon and the initial carbon contents in the melt. The rate and the degree of MnO reduction were found to increase with the increasing of initial carbon content; however, the effect of slag basicity was less important. A kinetic analysis of the process was performed using a coupled reaction model.     

Parameters affecting manganese oxidation in top blown basic oxygen converter

The data obtained from 84 heats carried out in a 90-t top blown basic oxygen converter were used to study the effects of slag composition and temperature on the activity coefficient and activity of manganous oxide in the slag as well as on the manganate capacity and the manganese distribution between slag and metal. In addition, the dependence of manganese activity in the metal on the concentration of maganese and temperature was also investigated. The present study carried out in wide ranges of temperature, 1350–1690°C, and slag basicity expressed as (CaO)/(SiO₂), 1.4–10.6, clarifies the dependence of MnO activity coefficient mainly on temperature. The activity coefficient of MnO increases by decreasing the temperature. On the other hand, activity of MnO increases by increasing MnO concentration and temperature. Both activity coefficient and activity of MnO in the slag slightly increase by increasing the slag basicity. At constant temperature, the activity of Mn in the molten metal varies linearly with Mn concentration and tends also to increase with increasing temperature at constant Mn concentration. The increase in manganese activity by increasing Mn concentration is much steeper at high temperatures. The manganate capacity as well as manganese distribution ratio decrease with increasing temperature at constant basicity and tend also to slightly decrease with increasing slag basicity at constant temperature. Equations describing the parameters affecting activity coefficient and activity of manganous oxide in the slag, manganese activity in the metal, manganate capacity and manganese distribution ratio have been derived.     

Surface Selective Oxide Reduction During the Intercritical Annealing of Medium Mn Steel

Third generation advanced high-strength steels achieve an excellent strength–ductility balance using a cost-effective alloy composition. During the continuous annealing of medium Mn steel, the formation of an external selective oxide layer of MnO has a negative impact on the coating quality after galvanizing. A procedure to reduce the selective oxide was therefore developed. It involves annealing in the temperature range of 1073 K to 1323 K (800 °C to 1050 °C) in a HN_x gas atmosphere. Annealing at higher temperatures and the use of larger H₂ volume fractions are shown to make the gas atmosphere reducing with respect to MnO. The reduction of the surface MnO layer was observed by SEM, GDOES, and cross-sectional TEM analysis.     

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.     

Kinetics of Reduction of SiO2 in SiO2-Al2O3-CaO Slags by Al in Fe-Al(-Si) Melts

Kinetic models considering mass transport in, (i) metal phase only and (ii) both metal and slag phases (mixed control or two-phase mass transfer) were developed for the reduction of SiO₂ in a SiO₂-Al₂O₃-CaO slag by Al in an Al-Fe melt. The models were validated with experiments of the reaction with Fe-Al melt and SiO₂-Al₂O₃-CaO-MgO_sat slags at 1873 K (1600 °C). The models predict that the rate of reaction is slower in the mixed control model because of the added resistance of slag phase mass transport. The mixed control becomes applicable when the slag contains low amounts of SiO₂. In this case, when the initial Al content in the metal increases, the normalized rate of reaction decreases. The increased Al content in the metal retards the reaction due to the limited SiO₂ provided to the reaction interface in the mixed control model. Sensitivity analyses were done using the models for the ratios of mass transfer coefficients of Si to Al, and Al₂O₃ to Si, along with slag density, which did not impose a significant effect.     

Kinetics of simultaneous reactions between liquid iron-carbon alloys and slags containing MnO

The oxidation rates of carbon, phosphorus, and silicon; the desulfurization rate of liquid iron; and the simultaneous reduction rate of MnO from slag were examined at 1450 °C to 1550 °C by using high carbon iron alloys and CaO-SiO₂-CaF₂ slags containing MnO and FeO. The reaction rates were well reproduced by a kinetic model describing the reaction between the slag and multicomponent iron alloys. The controlling steps applied for the reactions considered in the present kinetic simulation were as follows. The rate of decarburization is controlled by the chemical reaction at the slag-metal interface, and those of the other reactions are controlled by the transport in slag and metal phases. Both observation and simulation results showed that MnO was not a strong oxidizer compared with FeO in the slag, but was an effective component for desulfurization. The simulation results also showed that the interfacial oxygen activity using MnO-based slag was much lower than that using FeO-based slag. The apparent equilibrium constants of phosphorus and sulfur, which were obtained by the kinetic modeling of experimental results, were found to increase with increasing the (MnO + CaO)/SiO₂ ratio of the slag. The controlling step(s) of each element transport across the slag-metal interface was discussed with the aid of the kinetic model.     

Optimization of Mold Flux for the Continuous Casting of Cr-Contained Steels

To compensate the negative effect caused by the absorption of chromium oxide inclusions during the casting process of Cr-contained steels, a new mold flux system has been designed and investigated. The melting temperature range of the newly designed mold flux system is from [1124 K to 1395 K (851 °C to 1122 °C)]. The viscosity at 1573 K (1300 °C) and the break temperature increase with the addition of MnO and Cr₂O₃ but decrease with the addition of B₂O₃. The crystalline fraction of mold flux decreases from 81 to 42.1 pct with the addition of MnO and Cr₂O₃, and then further decreases to 25.3 pct with the addition of B₂O₃; however, it improves from 54.4 to 81.5 pct when the basicity increases. Besides, the heat-transfer ability of mold flux is inverse to the crystallization ratio of the slag. The comprehensive study of the properties for the four designed mold fluxes suggests that the mold flux with 1.15 basicity-3.01 pct B₂O₃-1.10 pct MnO-2.10 pct Cr₂O₃ shows the best properties for the continuous casting of Cr-contained steels.     

Dissolution Behavior of Rhodium in the Na2O-SiO2 and CaO-SiO2 Slags

To understand the behavior of rhodium during its recovery process, the dissolution behaviors of rhodium in Na₂O-SiO₂ and in CaO-SiO₂ slags at temperatures ranging from 1423 K to 1623 K (from 1150 °C to 1350 °C) and from 1773 K to 1873 K (from 1500 °C to 1600 °C), respectively, in an oxidizing atmosphere were investigated. The solubility of rhodium in the slags was found to increase with increasing oxygen partial pressure, temperature, and the basic oxide content. The correlation between the solubility of rhodium and the oxygen partial pressure suggested that rhodium dissolved into the slags as RhO_1.5. The dissolution of rhodium was slightly endothermic: the enthalpy change of the dissolution of solid rhodium was determined to be 50 ± 10 kJ/mol for the 50(mass pct)Na₂O-50SiO₂; and 188 ± 94 kJ/mol for the 56(mass pct)CaO-44SiO₂ slag systems. The increase in the solubility of rhodium with the basic oxide content indicated that rhodium exhibits acidic behavior in slags. The correlation between the solubility of rhodium and the sulfide capacity of the slags suggested that the ionic species of rhodium in slags is the rhodate ion, RhO ₂ ^? . The rhodate capacity of the slags was defined, and its application to estimate the possible rhodium content in various slag systems was proposed.     

Selective Oxidation and Sub‐Surface Phase Transformation of TWIP Steel during Continuous Annealing

The selective oxidation of Twinning Induced Plasticity (TWIP) steel during annealing at 800 °C in a N₂ + 10%H₂ gas atmosphere with a dew point of ?17 °C and ?3 °C was investigated by means of high resolution transmission electron microscopy of cross‐sectional samples. The annealing resulted in the selective oxidation of Mn and Si and the austenite‐to‐ferrite phase transformation of the sub‐surface region. In the low dew point atmosphere, the annealing resulted in the formation of a MnO layer at the surface. Crystalline c –xMnO · SiO₂ (x ≥ 2) particles and amorphous a –xMnO · SiO₂ (x < 0.9) particles were found at the interface between the MnO layer and the steel matrix. In a narrow zone of the sub‐surface, the Mn depletion resulted in the transformation of the initial austenite. In the high dew point atmosphere, a thicker MnO layer was formed on the surface and no mixed manganese‐silicon oxides particles were observed at the MnO/steel matrix interface. In the sub‐surface, Mn was significantly depleted in the range of 2–3 µm below the surface and the initial austenite in this zone was transformed to ferrite. MnO particles were found at the grain boundaries and in the interior of grains.     

Experimental Verification of the Thermodynamics of Critical Aspects of the Carbothermic Reduction of Alumina. Part 1: Experimental Result of Reacting Al2O3-C Mixtures

The thermodynamics of several aspects of the carbothermic reduction of the alumina process were experimentally verified. The current thermodynamics are based on solution models for the Al₂O₃-Al₄C₃ system and the free energies of formation of a number of phases. In Part I of this paper, the results of reacting carbon and alumina mixtures at high temperatures 2173 K to 2323 K (1900 °C to 2050 °C) are presented. The amount of CO evolved was measured and slag samples analyzed. The initial slag or carbide formation at alumina saturation was measured to be 2220 K (1947 °C) and 1.76 pct C. The slag-making temperature and composition in the liquid region were determined and at carbide saturation were measured to be 2273 K (2000 °C) and 6.29 pct C, respectively. Considering the experimental challenges at the high temperature, with the exception of the initial slag composition, the agreement with the predicted thermodynamics is good.     

Mass Transfer in Slag Refining of Silicon with Mechanical Stirring: Transient Interfacial Phenomena

Experiments have been carried out to study the rates of mass transfer between liquid silicon and CaO-SiO₂ slag with impeller stirring at 1823 K (1550 °C). The occurrence of transient interfacial phenomena related to the mass transfer of calcium has been observed; the evidence suggests that the reduction of calcium oxide at the interface leads to a rapid, temporary drop in the apparent interfacial tension. At low apparent interfacial tension, mechanical agitation facilitates the dispersion of metal into the slag phase, which dramatically increases the interfacial area; here, it has been estimated to increase by at least one order of magnitude. As the reaction rate slows down, the apparent interfacial tension increases and the metal recoalesces. The incidental transfer of calcium very likely promotes the transfer of boron by increasing the interfacial area. Mechanical mixing appears to be an extremely effective means to increase the reaction rate of boron extraction and could feasibly be implemented in the industrial slag refining of silicon to improve reaction rates.     

Kinetics of mass transfer of manganese and silicon between liquid iron and slags

The kinetics of mass transfer of Mn and Si between liquid iron and slags were investigated in laboratory experiments at 1600°C in MgO crucibles with 1500 g iron and 250 g slag. Three different slags consisting of CaO-MgO-MnO-SiO₂, MgO-MnO-SiO₂ and MgO-MnO-Al₂O₃-SiO₂ were used. The concentration-vs.-time curves, experimentally measured under defined flow conditions generated by gas stirring, were evaluated by application of a multi-component transport model in order to obtain the mass transfer coefficients. The numerical values of the thus determined measured mass transfer coefficients were compared with values calculated by a theory of mass transfer at liquid-liquid interfaces. The measured and theoretical values were in good agreement with each other in the case of reduction of MnO from the slag by Si in the metal, provided that the measurements had been carried out below a critical stirring intensity, above which metal droplets were emulsified in the slag. Experiments, where sulphur was dissolved in the metal melt and where the sulphur contents were systematically varied, showed no changes of the mass transfer coefficient in comparison to sulphur-free melts. The experimental mass transfer coefficients for the reduction of silica from the slag by manganese in the metal were smaller than those calculated by the mentioned mass transfer theory. This could be explained by inhibition of surface renewal under the influence of solid reaction products precipitated at the interface.     

Electrically Enhanced Boron Removal from Silicon Using Slag

An approach to enhance silicon refining using slag has been developed. The enhancement of the process was carried out by applying electrical potential difference across the slag and the silicon phase. This resulted in a shift in the apparent equilibrium in favor of higher partition ratio for the impurities. The application of electrical potential difference also enhanced the mass transfer rate increasing the overall kinetics of the process. This has been demonstrated in laboratory experimentations for the removal of boron from silicon-boron melts using slag. A CaO-SiO₂-Al₂O₃ slag was reacted with Si-B melt at 1823 K (1550 °C). Electrical potential differences were applied through graphite rods immersed in each of the liquid phase. The results showed that the apparent B-partition ratio and the apparent slag mass transfer coefficient were increased by a factor of 1.2 and 1.4, respectively, when a potential difference of 3 V was applied to the phases. The technique has the potential to be used for improving the existing slag refining process by increasing the overall kinetics and the slag capacity to absorb the impurities.     

<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.     

Effect of CaO Addition on Iron Recovery from Copper Smelting Slags by Solid Carbon

We investigated the effect of flux (lime) addition on the reduction behavior of iron oxide in copper slag by solid carbon at 1773 K (1500 °C). In particular, we quantified the recovery of iron by performing typical kinetic analysis and considering slag foaming, which is strongly affected by the thermophysical properties of slags. The iron oxide in the copper slag was consistently reduced by solid carbon over time. In the kinetic analysis, we determined mass transfer coefficients with and without considering slag foaming using a gas holdup factor. The mass transfer of FeO was not significantly changed by CaO addition when slag foaming was ignored, whereas the mass transfer of FeO when slag foaming was considered was at a minimum in the 20 mass pct CaO system. Iron recovery, defined as the ratio of the amount of iron clearly transferred to the base metal ingot to the initial amount of iron in the slag phase before reduction, was maximal (about 90 pct) in the 20 mass pct CaO system. Various types of solid compounds, including Mg₂SiO₄ and Ca₂SiO₄, were precipitated in slags during the FeO reduction process, and these compounds strongly affected the reduction kinetics of FeO as well as iron recovery. Iron recovery was the greatest in the 20 mass pct CaO system because no solid compounds formed in this system, resulting in a highly fluid slag. This fluid slag allowed iron droplets to fall rapidly with high terminal velocity to the bottom of the crucible. A linear relationship between the mass transfer coefficient of FeO considering slag foaming and foam stability was obtained, from which we concluded that the mass transfer of FeO in slag was effectively promoted not only by gas evolution due to reduction reactions but also by foamy slag containing solid compounds. However, the reduced iron droplets were finely dispersed in foamy and viscous slags, making actual iron recovery a challenge.     

Effect of the Heat Treatment on the Chromium Partition in CaO-MgO-SiO2-Cr2O3 Synthetic Slags

Mg-spinel phase is known to be important for control of Cr leaching from Cr-containing slags. The objective of the present study is to get an understanding of the phase relationships in the CaO-MgO-SiO₂-Cr₂O₃ system with a view to control the precipitation of Cr-spinel in the slag phase. The equilibrium phases in CaO-MgO-SiO₂-Cr₂O₃ slag system in the range of 1673 K to 1873 K (1400 °C to 1600 °C) have been investigated experimentally and compared with the results from thermodynamic calculations. The slag compositions close to the industrial slag systems were chosen. The Cr₂O₃ and MgO contents in the slag were fixed to be 6 and 8 wt pct, respectively. The basicity (CaO/SiO₂) of the slag was varied in the range of 1.0 to 2.0. The slags were synthesized at a pre-determined oxygen partial pressure (10^?4) or air (2.13 × 10⁴ Pa) at a temperature above the liquidus temperature. The samples were then soaked at targeted temperatures for 24 hours in controlled atmosphere in order to achieve the equilibrium state before quenching in water. Four different heat-treatment regimes (defined as Ia, Ib, II.a and II.b) in Section II–D) were used in the present experiments. The lower oxygen partial pressure was maintained by a suitable mixture of CO and CO₂ gases. Phases present and their compositions in the quenched slags were studied using scanning electron microscopy coupled with energy-dispersive spectroscopy and X-ray diffraction techniques. The chromium content in the phases present was analyzed using wavelength-dispersive spectrometer. The experimental results obtained are compared with the calculation results from Factsage software. The size of spinel crystals increased drastically after slow-cooling from 1873 K (1600 °C) followed by annealing at 1673 K (1400 °C) for 24 hours (heating regimes II) compared to samples being quenched directly after soaking at 1873 K (1600 °C) (heating regime I.a). It was found that the amount of foreign elements in the spinel phase, and other phases decreased after soaking at oxygen partial pressure of 10^?4 Pa resulting in phases with less defects and foreign oxide contents compared to those treated in air. The size of spinel crystals was found to be larger in samples with lower basicity.     

Reduction kinetics of manganese oxide in basic oxygen furnace type slag

In order to improve manganese yield during the reduction of manganese ore, the reduction kinetics of manganese oxide in BOF type slag has been investigated on an experimental scale. The reduction rate of (MnO) was promoted for the slag of low basicity and high contents of (FeO). The maximum reduction rate of (MnO) has been found for an iron melt with carbon mass contents of 1.9 %. The silicon in metal may accelerate the reduction of manganese oxide in slag. The kinetic model for the reduction rate of (MnO) has been formulated based on the assumption that the reduction of (MnO) was controlled by the mass transfer through the metal and slag boundary layers at the metal/slag interface. The result calculated by the kinetic model showed a good agreement with the experimental one. The reduction behaviour of (MnO) can be described by the present model.     

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.