The Effect of Carbon in Slag on Steel Reoxidation by CaO‐SiO2‐Al2O3‐MgO‐MnO‐FetO Slags

An equilibrium study was carried out at 1873K to ascertain the effect of carbon in CaO‐SiO₂‐Al₂O₃‐MgO‐MnO‐Fe_tO slag systems on their Fe_tO and MnO activity coefficients, representing the slags’ thermodynamic potential for steel reoxidation. Both γf e_to and γm no showed not only a sharp increment but also a simultaneous slow decrement by increasing carbon content in slag, suggesting opposite roles of the carbon according to its stable forms. XPS (X‐ray photoelectron spectroscopy) was introduced to clarify the stable forms of carbon in slag. XPS results prove that carbon dissolves in slag as carbonate, and carbide ions under oxidizing and reducing atmospheres, respectively. It was concluded that carbonate ions increase γf e_to and γm no , but that carbide decreases them. This paper suggests an application method of the present results to actual ladle refining processes, in order to enhance steel cleanliness with maintaining (Fe_tO + MnO) in slag to some allowable amount.     

The effect of CaF<Subscript>2</Subscript> on thermodynamics of CaO-CaF<Subscript>2</Subscript>-SiO<Subscript>2</Subscript>(-MgO) slags

To address the role of CaF₂ in the CaO-CaF₂-SiO₂(-MgO) slag system employed for the production of low-pressure rotor steels, the thermodynamic aspects of the slag were investigated by equilibrating it with liquid iron at 1873 K in CaO or MgO crucibles. Presaturation of slag with an oxide block piece of CaO or MgO in a Pt crucible and application of a carbon paste to the outside of an oxide crucible were designed to prevent crucible failure during the slag-metal experiments. The liquidus isotherm and phase boundary of the preceding slag system were investigated using the slag-metal equilibria. Also, the effect of CaF₂ on the sulfide capacity and the activity coefficient of Fe _t O were of particular interest in controlling the sulfur level and cleanliness of low-pressure rotor steels.     

Thermodynamic behavior of nickel in CaO-SiO<Subscript>2</Subscript>-Fe<Subscript>t</Subscript>O slag

The distribution ratio of nickel between Ag-Ni alloy and CaO-SiO₂-Fe _t O slag at high temperatures was measured to clarify the dissolution mechanism of nickel in this melt. Also, the nickel oxide capacity was suggested and was compared to phosphate and sulfide capacities. The dissolution mechanism of nickel into the CaO-SiO₂-Fe _t O slags could be described by the following equation from the effect of oxygen potential and slag basicity on nickel dissolution behavior:

Thermodynamic behaviours of manganese and phosphorus between CaO‐MgOsat‐SiO2‐Al2O3‐FetO‐MnO‐P2O5 ladle slag and liquid iron

The thermodynamic equilibria of manganese and phosphorus between liquid iron and CaO‐MgO_Sat‐SiO₂‐Fe_tO‐MnO‐P₂O₅‐Al₂O₃ (0–33%) ladle slag have been investigated at 1873 K from the viewpoint of Mn and P yields for the production of high‐strength steels. The equilibrium distribution ratios of Mn and P were found to increase with increasing Fe_tO content; however, these ratios vary with basicity, but they do this the other way round. The addition of alumina into slag at a fixed basicity and Fe_tO content decreases both the equilibrium manganese and phosphorus distributions. The equilibrium distribution ratios were discussed in terms of the variation of activity coefficients of Fe_tO, MnO and PO_2.5, according to the slag basicity and Al₂O₃ content. The quantitative contributions of basicity and (%Fe_tO + %MnO) on L_Mn and L_P were empirically determined and their usefulness was discussed with the aid of plant data: To improve Mn and P yields in the practical RH operation, it is strongly recommended that Fe‐Mn and Fe‐P alloys be added after Al deoxidation treatment inducing relatively high Al₂O₃ in slag and maintaining low Fe_tO content. In addition, a ladle slag composition for the targeted Mn and P contents in liquid iron was substantially estimated using the empirical relationships.     

Activities of MnO in CaO-SiO2-Al2O3-MnO (<10 Pct)-FetO(<3 pct) slags saturated with liquid iron

Activity coefficients of MnO and Fe,0 in CaO-SiO₂-Al₂O₃-MnO(<10 mass pct)-Fe,O(<3 mass pct) slags were determined at 1873 K in an Al₂O₃ or CaO crucible by using the reported values for the activities of Al₂O₃ and SiO₂ or the analyzed contents of oxygen. The activity coefficients of MnO and Fe_tO were found to be constant in the studied concentration range of MnO and Fe_tO. The former increased with an increase in the CaO content, while the latter increased with an increase in the SiO₂ content.     

Thermodynamic assessment of CaO‐SiO2‐Al2O3‐MgO‐Cr2O3‐MnO‐FetO slags for refining chromium‐containing steels

The slag system of CaO‐SiO₂‐Al₂O₃‐MgO‐Cr₂O₃‐MnO‐Fe_tO relevant to refining chromium‐containing steels such as bearing steel is thermodynamically assessed at 1873 K. The activity coefficient of Fe_tO shows an initially rapid increment followed by a gradual reduction according to Cr₂O₃ content at a constant basicity, and decreases with increasing slag basicity. γ_MnO is decreased abruptly by increasing Cr₂O₃ content and thereafter, maintains a nearly constant level. From the standpoint of inclusion control, the Cr₂O₃ presence in ladle refining slags is thermodynamically harmful in that it minimizes the inclusion level by inducing the increment of γ_FetO even though Cr₂O₃ exists in extremely small amounts. However, it is beneficial in that it diminishes AI reoxidation by decreasing γ_MnO. The presence of carbon in slag decreases γ_FetO and γ_MnO, which turns out to be favourable for the reduction of Al reoxidation. The thermodynamic equilibria of chromium and manganese are quantified in terms of Fe_tO and Cr₂O₃ content as well as slag basicity by using multiple regression analysis. L_Cr and L_Mn are increased by the presence of Cr₂O₃, indicating a low recovery efficiency of Cr and Mn in the treatment of ferroalloy addition. In determining L_S values, Cr₂O₃ is not so important as the basicity of slags.     

Mechanism and rate of reaction of Al<Subscript>2</Subscript>O,Al, and CO vapors with carbon

During the production of aluminum by carbothermic reduction, large quantities of Al₂O and Al vapor are generated. For the process to be economical, the aluminum and energy associated with these species must be captured and used in the process. This is accomplished by reacting them with carbon to form Al₄C₃. The mechanism and rate of the reactions of gas containing Al and Al₂O with various forms of carbon was studied. The Al₂O-Al-CO gas was generated by reacting an Al₄C₃-Al₂O₃ melt with carbon at high temperatures (2000 °C to 2050 °C). The gas then reacted with carbon at lower temperatures (1900 °C to 1950 °C). The only form of carbon that reacted extensively, forming Al₄C₃, was wood charcoal; with other forms of carbon, such as metallurgical coke and petroleum coke, primarily only Al₂O₃ condensed on the surface formed. The rate of formation of Al₄C₃ on wood charcoal was found to be controlled by the diffusion of Al₂O and Al through the Al₄C₃ product layer, and their effective diffusivities were estimated to be 0.82 and 1.31 cm²/s, respectively. Over 90 pct of the carbide is formed by Al₂O and only 10 pct by Al vapor. When an Al₄C₃-Al₂O₃ dense slag was formed on the charcoal at lower temperatures (1920 °C to 1930 °C) and then reacted at a higher temperature, it appears that the slag and carbon reacted to form Al₄C₃ relatively fast. The volume of Al₄C₃ formed is much greater than that of the original carbon. It is believed that this is the reason the other forms of carbon with lower porosity (25 pct vs 60 pct) did not react significantly. Any amount of Al₄C₃ formed would quickly fill the pores of the more dense carbon, stopping any further reaction.     

Thermodynamics of MnO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MnS Liquid Oxysulfide: Experimental and Thermodynamic Modeling

The activities of MnO and MnS in a MnO-SiO₂-Al₂O₃(or AlO_1.5)-MnS liquid oxysulfide solution were investigated by employing the gas/liquid/Pt-Mn alloy chemical equilibration technique under a controlled atmosphere at 1773 K (1500 °C). Also, the sulfide capacity, defined as C _S = (wt pct S)(pO₂/pS₂)^1/2, in MnO-SiO₂-Al₂O₃ slag with a dilute MnS concentration was obtained from the measured experimental data. As X _SiO2/(X _MnO + X _SiO2) in liquid oxysulfide increases, the activity coefficient of MnO decreases, while that of MnS first increases and then decreases. As X(AlO_1.5) in liquid oxysulfide increases, the activity coefficient of MnS increases, while no remarkable change is observed for the activity coefficient of MnO. The behavior of the activity coefficient of MnS was qualitatively analyzed by considering MnO + A _x S _y (SiS₂ or Al₂S₃) = MnS + A _x O _y (SiO₂ or Al₂O₃) reciprocal exchange reactions in the oxysulfide solution. The behavior was shown to be consistent with phase diagram data, namely, the MnS saturation boundary. Quantitative analysis of the activity coefficient of the oxysulfide solution was also carried out by employing the modified quasichemical model in the quadruplet approximation.     

A Thermodynamic Model of Phosphorus Distribution Ratio between CaO-SiO2-MgO-FeO-Fe2O3-MnO-Al2O3-P2O5 Slags and Molten Steel during a Top–Bottom Combined Blown Converter Steelmaking Process Based on the Ion and Molecule Coexistence Theory

A thermodynamic model for calculating the phosphorus distribution ratio between top–bottom combined blown converter steelmaking slags and molten steel has been developed by coupling with a developed thermodynamic model for calculating mass action concentrations of structural units in the slags, i.e., CaO-SiO₂-MgO-FeO-Fe₂O₃-MnO-Al₂O₃-P₂O₅ slags, based on the ion and molecule coexistence theory (IMCT). Not only the total phosphorus distribution ratio but also the respective phosphorus distribution ratio among four basic oxides as components, i.e., CaO, MgO, FeO, and MnO, in the slags and molten steel can be predicted theoretically by the developed IMCT phosphorus distribution ratio prediction model after knowing the oxygen activity of molten steel at the slag–metal interface or the Fe _t O activity in the slags and the related mass action concentrations of structural units or ion couples in the slags. The calculated mass action concentrations of structural units or ion couples in the slags equilibrated or reacted with molten steel show that the calculated equilibrium mole numbers or mass action concentrations of structural units or ion couples, rather than the mass percentage of components, can present the reaction ability of the components in the slags. The predicted total phosphorus distribution ratio by the developed IMCT model shows a reliable agreement with the measured phosphorus distribution ratio by using the calculated mass action concentrations of iron oxides as presentation of slag oxidation ability. Meanwhile, the developed thermodynamic model for calculating the phosphorus distribution ratio can determine quantitatively the respective dephosphorization contribution ratio of Fe _t O, CaO + Fe _t O, MgO + Fe _t O, and MnO + Fe _t O in the slags. A significant difference of dephosphorization ability among Fe _t O, CaO + Fe _t O, MgO + Fe _t O, and MnO + Fe _t O has been found as approximately 0.0 pct, 99.996 pct, 0.0 pct, and 0.0 pct during a combined blown converter steelmaking process, respectively. There is a great gradient of oxygen activity of molten steel at the slag–metal interface and in a metal bath when carbon content in a metal bath is larger than 0.036 pct. The phosphorus in molten steel beneath the slag–metal interface can be extracted effectively by the comprehensive effect of CaO and Fe _t O in slags to form 3CaO·P₂O₅ and 4CaO·P₂O₅ until the carbon content is less than 0.036 pct during a top–bottom combined blown steelmaking process.     

The Thermodynamic Behavior of Copper in the CaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> and BaO-B<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag System

The Cu solubility was measured in the CaO-B₂O₃ and BaO-B₂O₃ slag systems to understand the dissolution mechanism of Cu in the slags. The Cu solubility had a linear relationship with oxygen partial pressure in the CaO-B₂O₃ slag system, which corresponds with previous studies. Also, the Cu solubilities in slag decreased with increasing the slag basicity, which value of slope was close to –0.5 in logarithmic form. From the results of experiment, the Cu dissolution mechanism established as follows:

Corrosion Behavior of Fe<Subscript>41</Subscript>Co<Subscript>7</Subscript>Cr<Subscript>15</Subscript>Mo<Subscript>14</Subscript>C<Subscript>15</Subscript>B<Subscript>6</Subscript>Y<Subscript>2</Subscript> Bulk Metallic Glass in Sulfuric Acid Solutions

An Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk metallic glass with a diameter of 5 mm was prepared with the copper-mold-casting method. The corrosion resistance of this amorphous steel in sulfuric-acid solutions was determined by electrochemical measurements. The passive film formed on the surface of the alloy after immersion in the 0.5-mol/l H₂SO₄ solution for 1 week was analyzed by X-ray photoelectron spectroscopy (XPS). Electrochemical measurements show that the corrosion resistance of the amorphous steel in the 1 mol/l-H₂SO₄ solution is superior to a stainless steel (SUS 321), and is almost the same as Ti6Al4V, which shows that the amorphous steel has an excellent corrosion resistance in sulfuric-acid solutions. As the concentration of the sulfuric-acid solutions increases from 0.5 mol/l to 4 mol/l, the corrosion resistance of the amorphous steel decreases. The XPS result reveals that a bilayer structure of protective film formed on the surface of the amorphous steel in a H₂SO₄ solution. The compositions of the inner part of the film are MoO₂, Cr₂O₃, CoO, and FeO, and those of the outer film are MoO₃, Cr(OH)₃, Co(OH)₂, and Fe(OH)₃.     

Distribution Ratios of Phosphorus Between CaO-FeO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>/Na<Subscript>2</Subscript>O/TiO<Subscript>2</Subscript> Slags and Carbon-Saturated Iron

In order to effectively enhance the efficiency of dephosphorization, the distribution ratios of phosphorus between CaO-FeO-SiO₂-Al₂O₃/Na₂O/TiO₂ slags and carbon-saturated iron (\( L_{\text{P}}^{\text{Fe-C}} \)) were examined through laboratory experiments in this study, along with the effects of different influencing factors such as the temperature and concentrations of the various slag components. Thermodynamic simulations showed that, with the addition of Na₂O and Al₂O₃, the liquid areas of the CaO-FeO-SiO₂ slag are enlarged significantly, with Al₂O₃ and Na₂O acting as fluxes when added to the slag in the appropriate concentrations. The experimental data suggested that \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the binary basicity of the slag, with the basicity having a greater effect than the temperature and FeO content; \( L_{\text{P}}^{\text{Fe-C}} \) increases with an increase in the Na₂O content and decrease in the Al₂O₃ content. In contrast to the case for the dephosphorization of molten steel, for the hot-metal dephosphorization process investigated in this study, the FeO content of the slag had a smaller effect on \( L_{\text{P}}^{\text{Fe-C}} \) than did the other factors such as the temperature and slag basicity. Based on the experimental data, by using regression analysis, \( \log L_{\text{P}}^{\text{Fe-C}} \) could be expressed as a function of the temperature and the slag component concentrations as follows:

$$ \begin{aligned} \log L_{\text{P}}^{\text{Fe-C}} & = 0.059({\text{pct}}\;{\text{CaO}}) + 1.583\log ({\text{TFe}}) - 0.052\left( {{\text{pct}}\;{\text{SiO}}_{2} } \right) - 0.014\left( {{\text{pct}}\;{\text{Al}}_{2} {\text{O}}_{3} } \right) \\ \, & \quad + 0.142\left( {{\text{pct}}\;{\text{Na}}_{2} {\text{O}}} \right) - 0.003\left( {{\text{pct}}\;{\text{TiO}}_{2} } \right) + 0.049\left( {{\text{pct}}\;{\text{P}}_{2} {\text{O}}_{5} } \right) + \frac{13{,}527}{T} - 9.87. \\ \end{aligned} $$

     

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.     

Kinetics of Reduction of CaO-FeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>-MgO-PbO-SiO<Subscript>2</Subscript> Slags by CO-CO<Subscript>2</Subscript> Gas Mixtures

Kinetics of the reaction of lead slags (PbO-CaO-SiO₂-FeO _x -MgO) with CO-CO₂ gas mixtures was studied by monitoring the changes in the slag composition when a stream of CO-CO₂ gas mixture was blown on the surface of thin layers of slags (3 to 10 mm) at temperatures in the range of 1453 K to 1593 K (1180 °C to 1320 °C). These measurements were carried out under conditions where mass transfer in the gas phase was not the rate-limiting step and the reduction rates were insensitive to factors affecting mass transfer in the slag phase. The results show simultaneous reduction of PbO and Fe₂O₃ in the slag. The measured specific rate of oxygen removal from the melts varied from about 1 × 10^?6 to 4 × 10^?5 mol O cm^?2 s^?1 and was strongly dependent on the slag chemistry and its oxidation state, partial pressure of CO in the reaction gas mixture, and temperature. The deduced apparent first-order rate constant increased with increasing iron oxide content, oxidation state of the slag, and temperature. The results indicate that under the employed experimental conditions, the rate of formation of CO₂ at the gas-slag interface is likely to be the rate-limiting step.     

Effect of Ce<Subscript>2</Subscript>O<Subscript>3</Subscript> on Structure,Viscosity, and Crystalline Phase of CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Li<Subscript>2</Subscript>O-Ce<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags

Aiming at devising new mold flux for Ce-bearing stainless steel, a fundamental investigation on the effect of Ce₂O₃ on properties of the CaO-Al₂O₃-Li₂O-Ce₂O₃ slag was provided by the present work. The results show that adding Ce₂O₃ could decrease the viscosity of the slag due to its effects on decreasing the polymerization of the slag. The crystalline process was restrained by increasing the content of Ce₂O₃, and the crystalline phases also can be influenced by the slag structure. The crystalline phases were transferred from LiAlO₂ and CaO to LiAlO₂ and CaCeAlO₄ with the addition of Ce₂O₃ to the slag, which could be well confirmed by the structure of the unit cell of the crystals.     

Evaluating the Diffusion Coefficient of Sulfur in Low-Silica CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag

The chemical diffusion coefficient of sulfur in the ternary slag of composition 51.5 pct CaO-9.6 pct SiO₂-38.9 pct Al₂O₃ slag was measured at 1680 K, 1700 K, and 1723 K (1403 °C, 1427 °C, and 1450 °C) using the experimental method proposed earlier by the authors. The P_\textS2 P_{{{\text{S}}_{2} }} and P_\textO2 P_{{{\text{O}}_{2} }} pressures were calculated from the Gibbs energy of the equilibrium reaction between CaO in the slag and solid CaS. The density of the slag was obtained from earlier experiments. Initially, the order of magnitude for the diffusion coefficient was taken from the works of Saito and Kawai but later was modified so that the concentration curve for sulfur obtained from the program was in good fit with the experimental results. The diffusion coefficient of sulfur in 51.5 pct CaO-9.6 pct SiO₂-38.9 pct Al₂O₃ slag was estimated to be in the range 3.98 to 4.14 × 10⁻⁶ cm²/s for the temperature range 1680 K to 1723 K (1403 °C to 1450 °C), which is in good agreement with the results available in literature     

Effect of carbon concentration on precipitation behavior of M<Subscript>23</Subscript>C<Subscript>6</Subscript> carbides and MX carbonitrides in martensitic 9Cr steel during heat treatment

The distributions and precipitated amounts of M₂₃C₆ carbides and MX-type carbonitrides with decreasing carbon content from 0.16 to 0.002 mass pct in 9Cr-3W steel, which is used as a heat-resistant steel, has been investigated. The microstructures of the steels are observed to be martensite. Distributions of precipitates differ greatly among the steels depending on carbon concentration. In the steels containing carbon at levels above 0.05 pct, M₂₃C₆ carbides precipitate along boundaries and fine MX carbonitrides precipitate mainly in the matrix after tempering. In 0.002 pct C steel, there are no M₂₃C₆ carbide precipitates, and instead, fine MX with sizes of 2 to 20 nm precipitate densely along boundaries. In 0.02 pct C steel, a small amount of M₂₃C₆ carbides precipitate, but the sizes are quite large and the main precipitates along boundaries are MX, as with 0.002 pct C steel. A combination of the removal of any carbide whose size is much larger than that of MX-type nitrides, and the fine distributions of MX-type nitrides along boundaries, is significantly effective for the stabilization of a variety of boundaries in the martensitic 9Cr steel.     

Lead production from recycled paste of lead acid batteries with SiC-Na<Subscript>2</Subscript>CO<Subscript>3</Subscript>

A novel pyrometallurgical route was used to reduce the lead paste to obtain metallic lead with different additions of sodium carbonate at 850°C in a silicon carbide crucible. The process allows to obtain a high recovery of metallic lead in a single step and a slag constituted mainly by lead silicate. Products obtained were characterized by atomic absorption, X-ray powder diffraction and SEM-EDS techniques. A thermodynamic study was carried out with the software FactSage to determinate the compounds formation to the experimental conditions. High amounts of Na₂CO₃ and SiC promoted the formation of a lead sulfide compound which decreased the lead recovery.     

The Viscous Behavior of FeO<Subscript>t</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> Copper Smelting Slags

Understanding the viscous behavior of copper smelting slags is essential in increasing the process efficiency and obtaining the discrete separation between the matte and the slag. The viscosity of the FeO_t-SiO₂-Al₂O₃ copper smelting slags was measured in the current study using the rotating spindle method. The viscosity at a fixed Al₂O₃ concentration decreased with increasing Fe/SiO₂ ratio because of the depolymerization of the molten slag by the network-modifying free oxygen ions (O²⁻) supplied by FeO. The Fourier transform infrared (FTIR) analyses of the slag samples with increasing Fe/SiO₂ ratio revealed that the amount of large silicate sheets decreased, whereas the amount of simpler silicate structures increased. Al₂O₃ additions to the ternary FeO_t-SiO₂-Al₂O₃ slag system at a fixed Fe/SiO₂ ratio showed a characteristic V-shaped pattern, where initial additions decreased the viscosity, reached a minimum, and increased subsequently with higher Al₂O₃ content. The effect of Al₂O₃ was considered to be related to the amphoteric behavior of Al₂O₃, where Al₂O₃ initially behaves as a basic oxide and changes to an acidic oxide with variation in slag composition. Furthermore, Al₂O₃ additions also resulted in the high temperature phase change between fayalite/hercynite and the modification of the liquidus temperature with Al₂O₃ additions affecting the viscosity of the copper smelting slag.     

Manganese Recovery by Silicothermic Reduction of MnO in BaO-MnO-MgO-CaF<Subscript>2</Subscript> (-SiO<Subscript>2</Subscript>) Slags

The effects of reducing agent, CaF₂ content, and reaction temperature upon the silicothermic reduction of MnO in the BaO-MnO-MgO-CaF₂ (-SiO₂) slags were investigated. Mn recovery was proportional to Si activity in the molten alloy. Moreover, 90 pct yield of Mn recovery was obtained under 5 mass pct CaF₂ content and 1873 K (1600 °C) reaction temperature. Increasing CaF₂ content above 5 pct yielded little or no further increase in Mn recovery, because it was accompanied by increased slag viscosity owing to the precipitation of high melting point compounds such as Ba₂SiO₄.