Thermodynamics of phosphorus and sulfur in the BaO-MnO flux system between 1573 and 1673 K

The oxidative dephosphorization of carbon-saturated Fe-Mn alloys was successfully demonstrated by measuring the phosphorus partition ratio between BaO-MnO, BaO-MnO-BaF₂, and BaO-CaO_satd.-MnO fluxes and liquid Fe-Mn-C_satd. alloy between 1573 and 1673 K. The phosphorus partition ratio increases with increasing BaO content of the flux. The phosphate capacity of the BaO-MnO flux is as high as that of the BaO-BaF₂ flux and is far larger than those of CaO-bearing fluxes. Addition of BaF₂ to the BaO-MnO flux increases BaO solubility, which increases the phosphate capacity. The manganese partition ratios between the BaO-MnO flux and Fe-Mn-C_satd. alloy were approximately constant at 0.64, 0.33, and 0.23 at 1573, 1623, and 1673 K, respectively. The carbon content of the BaO-MnO flux was measured as functions of slag composition, temperature, and partial pressure of CO. A stable species of carbon in the BaO-MnO flux was found to be BaC₂ experimentally and thermodynamically. The sulfide capacity of the BaO-MnO system at 1573 K has been shown to be far larger than any known flux systems and to be a few times larger than that for the BaO-BaF₂ system. Formerly Graduate Student, Department of Metallurgy, The University of Tokyo     

The Thermodynamic Properties of Fe‐Mn‐C Melt at Reduced Pressure

The solubility of carbon in the Fe‐Mn‐C system was measured under reduced pressure of 1 Pa at 1573 K and 1673 K, respectively. The carbon solubility in terms of mole fraction in the iron based solution was found varying with manganese mole fraction in the solution and with temperature, as x_C= 0.1819 + 0.2531 x_Mn at 1573 K and x_C = 0.1981 + 0.2515x_Mn at 1673K. Manganese has a stronger influence on the carbon solubility at lower pressure than it has at higher pressure. The pressure was found to have an insignificant effect on the carbon solubility when manganese in the melt was absent. The activity interaction parameters between manganese and carbon in the alloy were determined from carbon solubilities at 1573 and 1673 K. Analysis of the experimental results showed that under vacuum conditions, the activity interaction parameter of manganese with carbon is higher than that at atmospheric pressure.     

Thermodynamics of phosphorus in molten Si-Fe and Si-Mn alloys

The thermodynamics of phosphorus in molten Si-Fe and Si-Mn alloys has been investigated at 1723 K by equilibrating the alloys in a controlled phosphorus partial pressure. The activity coefficient of phosphorus in each alloy shows a maximum value at a certain composition due to a strong interaction between silicon and iron and between silicon and manganese. Interaction coefficients between phosphorus and iron in molten silicon were found to be ε _P ^Fe =7.43 and ρ _P ^Fe =−16.4 (0≦X _Fe≦0.65), and those between phosphorus and manganese were ε _P ^Mn =12.0 and ρ _P ^Mn =−22.2 (0≦X _Mn≦0.5). Further discussion has revealed that the Si-Fe-P and Si-Mn-P systems approximately conform to a regular solution within the composition ranges investigated in the present work.     

Phosphorus equilibrium between BaO-BaF2-MnO fluxes and ferro-manganese melts

The equilibrium distribution ratio of phosphorus between BaO-BaF₂-MnO slags and Mn(62–73%)-Fe-C_sat-P melts has been determined for different slag compositions, oxygen partial pressures and at 1573–1673 K. The results showed that with a certain content of BaO in the slags, the phosphorus distribution ratio increased with increasing oxygen partial pressure up to about 10^?17 atm. A high oxygen partial pressure resulted in a substantial oxidation of manganese from metal to slag. The phosphate capacity of BaO-BaF₂-MnO slags increased with the BaO content and decreased with the MnO content in the investigated slag composition range. A high temperature resulted in a low phosphorus distribution ratio. For a BaO(50%)-BaF₂(47%)-MnO(3%) slag, the effect of temperature on the phosphate capacity in the range of 1573–1673 K could be expressed as: (1) The heat of reaction: (2) was estimated to be ?1107 kJ/mole.     

Thermodynamics of Mn in Cu-Mn Melts

The present work aimed to measure the thermodynamic data of manganese in Cu-Mn melts over a broad manganese concentration range, using the equilibrium among the liquid copper/MnO_(s)/CO-CO₂ gas mixture in the temperature range from 1673 K to 1873 K (1400 °C to 1600 °C). Darken’s quadratic formalism was introduced to correlate the activity coefficient of manganese in copper to composition, and the excess molar Gibbs energy change of mixing of Cu-Mn melts was described satisfactorily by Redlich–Kister type polynomial.     

Thermodynamics of carbon in liquid manganese and ferromanganese alloys

The thermodynamics of carbon in manganese and ferromanganese melts were studied to predict the refining limit of carbon during the decarburization of molten ferromanganese. The equilibrium carbon content in a Mn-C melt was determined by the C-CO equilibrium in the presence of pure solid MnO at 1673 to 1773 K. The activities of manganese and carbon in the Mn-C melt were then calculated from the experimental results, the equilibrium constant for the reaction, and the Gibbs-Duhem equation integrated by the Belton-Fruehan treatment. The standard free-energy change of carbon dissolution in the manganese melt was determined to be 41,700 — 59.6 T J/g · atom, with the standard state taken as 1 wt pct carbon in solution. The effect of iron on the activity coefficient of carbon in ferromanganese was determined by measuring the carbon solubility in Mn-Fe melts. The first- and second-order interaction parameters between carbon and iron in ferromanganese melts were determined. The activity coefficient of carbon in the ferromanganese alloy melt can be expressed as

Dephosphorization of Mn-based alloys

The critical oxygen partial potential of dephosphorization in liquid Mn-base alloys under oxidizing or reducing conditions has been determined by thermodynamic analysis. Under oxidizing conditions, thermodynamic conditions to achieve dephosphorization and avoid manganese oxidizing were given. The results of thermodynamic analysis show that BaO-base slag can be an effective dephosphorization agent for Mn-base alloys. Under reducing conditions, the high degree of dephosphorization in Mn-base alloys can be obtained based on thermodynamic analysis. In experimental work, BaCO₃ was added to remove phosphorus from Mn–Fe–C melts. The time needed for equilibration of the dephosphorization reaction of Mn–Fe–C melts was determined at 1573, 1623 and 1673 K. The refining results were experienced as dephosphorization efficiency ≠_p = (%[P]₀ ? %[P])/%[P]₀. Moreover, the effect of the initial content of Si and C on ≠_p was investigated. ≠_p in the Mn–Fe–C melts was also given as a function of temperature. Dephosphorization reaction in Mn–Fe–C melts is of first order.     

The chemical diffusivity of oxygen in liquid iron oxide and a calcium ferrite

Measurements have been made of the chemical diffusion coefficient of oxygen in liquid iron oxide at temperatures from 1673 to 1888 K and in a calcium ferrite (Fe/Ca = 2.57) at temperatures from 1573 to 1873 K. A gravimetric method was used to measure the oxygen uptake during the oxidation of the melts by oxygen or CO₂-CO mixtures. The rate was shown to be controlled by mass transfer in the liquid melt. The chemical diffusivity of oxygen in liquid iron oxide at oxygen potential between air and oxygen was found to be 4.2±0.3 × 10⁻³ cm²/s at 1888 K. That in iron oxide at oxidation state close to iron saturation was established to be given by the empirical expression log D=−6220/T + 1.12 for temperatures between 1673 and 1773 K. For the calcium ferrite (Fe/Ca=2.57) at oxygen potential between air and oxygen, the diffusivity of oxygen was found to be given by log D=−1760/T−1.31 for temperatures between 1673 and 1873 K. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Activity-composition relations of MnO in MnO-CrO x -CaO-SiO2-containing melts

The MnO activities in (MnO-CrO _x -CaO-SiO₂)-containing melts, which were saturated with the (Mn, Cr)₃O₄ spinel phase, were determined at 1500 °C under an oxygen partial pressure of 10^−8.99 atm. This was done by equilibrating the samples with platinum. The activity of MnO in the melt was then calculated from the activity coefficient of manganese in the resultant Pt-Cr-Mn alloy. Darken’s quadratic formalism for ternary metallic solutions was used to calculate the activity coefficient of manganese in the Pt-Cr-Mn system, in which platinum was considered to be the solvent. It was found that an increase in the concentration of MnO in the melt increases both the MnO activity and the activity coefficient of MnO. For a constant MnO concentration in the (MnO-CrO _x -CaO-SiO₂)-containing melts, the activity of MnO can be increased by increasing the basicity of the melt. In order to obtain high-manganese recoveries from (MnO-CrO _x -CaO-SiO₂)-containing melts into an alloy phase, basic slags in which the activity coefficient of MnO is high should therefore be used.     

Thermodynamic properties of the BaO-MnO flux system

Although a great number of works on BaO-bearing fluxes for refining Fe-Cr and Fe-Mn alloys have been carried out, there still remain several unresolved problems on using them in the refining process. The principal aim of the present study is to understand the thermodynamic properties of the BaO-MnO system, which has been shown to be very effective for dephosphorization of Fe-Mn alloys. The activity of manganese oxide in the BaO-MnO flux was measured at 1573 and 1673 K by equilibrating the flux, a Ag-Mn alloy, and a gas mixture of CO and CO₂ as functions of the flux composition and temperature. The influence of BaF₂, which is an effective additive for lowering the melting temperature of the flux, on the thermodynamics of the BaO-MnO system, including the solubility of MnO in the BaO-BaF₂ system, was also investigated.     

Thermodynamic behavior of phosphorus in CaO-CaF2-SiO2 and CaO-Na2O-SiO2 systems

In order to understand the thermodynamic behavior of phosphorus, such as the polymerization of phosphate ions in slag melts, the influence of phosphorus content of slag melts on the partition of phosphorus between slags and carbon-saturated iron or silver was investigated for the CaO-CaF₂-SiO₂ and CaO-Na₂O-SiO₂ systems at 1573 and 1473 K, respectively. The predominant species changes from PO ₄ ³ to P₂O₇ ^4-approximately at 2 mass pct of phosphorus in slag melts, which is defined as the critical phosphorus content, for the CaO-CaF₂-SiO₂ system which is doubly saturated with CaO and 3CaO · SiO₂. The critical phosphorus content was found to be about 1 mass pct for the CaO-CaF₂-SiO₂ melts saturated with 2CaO · SiO₂. On the other hand, in the case of the 20CaO-35Na₂O-45SiO₂ system, PO₄/^3- is the predominant species until the slag becomes saturated with Ca₃(PO₄)₂. No effect of sulfur on the phosphorus partition ratio was observed for the CaO-CaF₂-SiO₂ system. Formerly Undergraduate Student, Department of Metallurgy, The University of Tokyo.     

Phase Equilibrium of the System CaO‐P2O5‐SiO2 between 1473 K and 1673 K

Phase equilibrium was investigated in the ternary system of CaO‐P₂O₅‐SiO₂ at 1473K, 1573K and 1673K.     

Chemical Potentials of Oxygen in Four‐Phase Assemblages of the System CaO‐P2O5‐CaF2‐FexO

In steelmaking processes, because of environmental requirements and health considerations, there is a strong incentive to reduce slag volume. The key to meet this requirement is the better understanding of phosphorus removal, which relies on the knowledge of the thermodynamic properties of slags and fluxes used for dephosphorization. In this study, the liquidus compositions of the four‐phase assemblages in the quaternary system CaO‐P₂O₅‐CaF₂‐Fe_xO were determined at 1573K by employing electron probe microanalysis. Measurements were also made on the Fe_xO activities at temperatures between 1523K and 1673K by employing an electrochemical technique involving stabilized zirconia electrolyte.     

Reaction Kinetics of FeO Containing Molten Slags

The kinetic study of FeO‐containing slag is valuable if we consider slag‐gas and slag‐metal reactions in steelmaking process. In the present work, the reduction rate of Fe_tO‐SiO₂–TiO₂–MO_x (MO_x = CaO, MgO, AlO_1.5, PO_2.5) melts in equilibrium with solid iron by CO gas was measured using the thermobalance system at 1673 K. A rate equation was developed based on the results obtained. The mechanisms of the reaction and the effect of P₂0₅ as a surfactant were discussed. Solid CaO was reacted with FeO‐containing slag at 1573 to 1673 K. The CaO–slag interface was analyzed by SEM and EDX, and the reacted layers were identified. The rate of solid CaO dissolution into a stagnant FeO‐containing slag at hot‐metal temperatures was explained by the FeO diffusion in slag phase.     

Oxygen and sulfur solubilities in Ni-Fe-S-O melts

Oxygen and sulfur solubilities were determined in Ni-Fe-S-O melts under the following conditions: 10^-11.50 ≤ P_O2 ≤ 10^-8.50 atm; 10^-3.00 ≤ P_{s 2} ≤ 10^-2.00 atm; 0.19 ≤ Ni/(Ni + Fe) ≤0.85; and 1473 K ≤T ≤ 1573 K. The oxygen solubility was found to increase with increasing partial pressure of oxygen up to a maximum value at oxide saturation and to decrease with increasing equilibrium partial pressure of sulfur. The ferrous metal content enhanced oxygen solubility. The trends in dissolution behavior of sulfur were opposite to those of oxygen with respect to changing P_{O 2} and P_{S 2} and to the Ni/(Ni + Fe) ratio; however, at high matte grades Ni/(Ni + Fe) > approximately 0.5, sulfur solubility appeared to decrease as a function of the Ni/(Ni + Fe) ratio, as did oxygen solubility. The standard Gibbs energy of oxygen dissolution in Ni-Fe-S-O melts Ni/(Ni + Fe) = 0.47 in the temperature range 1473 to 1573 K can be described by ΔG° = −202.5 + 0.0660 T(K) (±1.5 kJ/mol)     

Thermodynamics of oxygen solutions in Fe-Mn melts

Oxygen solutions in Fe-Mn melts are analyzed thermodynamically. The composition of the oxide phase is determined, and the equilibrium oxygen concentrations in Fe-Mn melts are calculated over a wide composition range. The oxide phase mainly contains MnO: even at a molar fraction of manganese of 0.02 in the melt, the molar fraction of manganese oxide in the slag is more than 0.9. This is due to a much higher oxygen affinity of manganese as compared to iron; that is, manganese additives to iron considerably decrease the oxygen solubility. When the manganse content in the melt is 19.32%, the oxygen solubility curve has a minimum corresponding to an oxygen concentration of 5.136 × 10^?3%. However, a further increase in the managanese content results in an increase in the oxygen concentration in the melt. In liquid manganese, the oxygen saturation concentration at 1873 K is 0.0472%. The interaction parameter e _o(Mn) ^o (?0.207) and the activity coefficient γ _o(Mn) ^o (1.131 × 10^?4) have been calculated for the first time.     

Experimental studies of the viscosities in the CaO-Fe n O-SiO2 slags

In the present work, the viscosities in the CaO-Fe _n O-SiO₂ ternary system have been measured by the rotating cylinder method involving a spindle and crucible made of iron. Nine slag compositions in the ternary system have been chosen with CaO varying between 5.5 and 45.5 pct mass, FeO between 10.0 and 70.0 pct mass, and one measurement each in the binary Fe _n O-SiO₂ and CaO-SiO₂ melts. The measurements have been carried out in the temperature range of 1423 to 1753 K. The viscosity in this system is described as a function of temperature and composition using the model approach developed earlier at the present laboratory. The isoviscosity lines have been predicated at 1573, 1673, and 1723 K. Good agreement between the calculated results and the experimental data has been obtained.     

Kinetics of Silicothermic Reduction of Manganese Oxide for Advanced High-Strength Steel Production

The kinetics of silicothermic reduction of manganese oxide from MnO–SiO₂–CaO–Al₂O₃ slags reacting with Fe-Si droplets were studied in the temperature range of 1823 K to 1923 K (1550 °C to 1650 °C). The effects of initial droplet mass, initial droplet silicon content, and initial slag manganese oxide content were studied. Data obtained for 15 pct silicon showed agreement with control by mass transport of MnO in the slag with a mass transfer coefficient (k _s) of 4.0 × 10^?5 m/s at 1873 K (1600 °C). However, when this rate-determining step was tested at different initial silicon contents, the agreement was lost, suggesting mixed control between silicon transport in the metal and manganese oxide transport in the slag. Increasing the temperature resulted in a decrease in the rate of reaction because of an increase in the favorability of SiO as a product. Significant gas generation was found during all experiments, as a result of silicon monoxide production. The ratio of silicon monoxide to silica formation was increased by factors favoring silicon transport over that of manganese, further supporting the conclusion that the reaction is under mixed control by transports of both silicon and manganese oxide.     

Nitrogen solubility in liquid manganese and ferromanganese alloys

The nitrogen solubilities in liquid manganese, manganese-iron, manganese-carbon, and manganese-iron-carbon alloys have been measured by the gas-liquid metal equilibration technique in the temperature range of 1623 to 1823 K. The equilibrium nitrogen content in pure liquid manganese at an atmospheric nitrogen pressure is high, and it does not follow Sievert’s law, i.e., f _N is not unity. The reduced nitrogen partial pressures by dilution with argon enabled us to obtain more reliable information on the thermodynamics of nitrogen in liquid manganese. The nitrogen dissolution follows Sievert’s law at nitrogen contents below 1 wt pct. The standard free-energy change for the dissolution of nitrogen in pure liquid manganese has been determined as −67,222+30.32T J/g atom, with the standard state of nitrogen taken as a 1 wt pct solution. Carbon and iron in manganese-rich melts decrease the nitrogen solubility significantly. The first- and second-order interaction parameters between nitrogen and other elements in manganese alloy melts have been determined. The activity coefficient of nitrogen in a ferromanganese alloy melt can be expressed as

Sulfide capacities of Na2O−SiO2 melts

Sulfide capacities of Na₂O−SiO₂ melts at 1473, 1523, 1573, 1623, and 1673 K were calculateda priori using the revised Reddy Blander model. An expression forC _S in the composition range of 0≤X _{SiO 2}<1.0 was derived. Our predictions ofC _S values are in very good agreement with the experimental data available in the range of 0<X _{SiO 2}<0.8. The sulfide capacities of slags are found to be directly related to two independent quantities: the equilibrium constant K and the activity of the base oxide.