Sulfide capacity of CaO-CaF2-SiO2 slags

The sulfide capacityC _S ^2- = (pct S^2-) · (P _{O 2}/P _{S 2})^1/2) of CaO-CaF₂-SiO₂ slags saturated with CaO, 3CaO · SiO₂ or 2CaOSiO₂ was determined at 1200 °C, 1250 °C, 1300 °C, and 1350 °C by equilibrating molten slag, molten silver, and CO-CO₂ gas mixtures. Higher sulfide capacities were obtained for CaO-saturated slags. A drastic decrease was observed in those values when the ratio pct CaO/pct SiO₂ is less than 2. The sulfur partition between carbon-saturated iron melts and presently investigated slags was calculated by using the sulfide capacities obtained and the activity coefficient of sulfur in carbon-saturated iron, which was also experimentally determined. For slags saturated with CaO, partitions of sulfur as high as 10,000 were obtained at 1300 °C and 1350 °C. Correlations between the sulfide capacity and other basicity indexes such as carbonate capacity and theoretical optical basicity were also discussed. Formerly with the Department of Metallurgy, The University of Tokyo.     

Kinetics studies on the dissolution of nitrogen in the CaO-Al2O3-SiO2 and CaO-Al2O3-TiO x melts

The rate of nitrogen dissolution in CaO-Al₂O₃-SiO₂ and CaO-Al₂O₃-TiO _x melts was measured by ¹⁴N–¹⁵N isotope exchange reaction. The rate constant for the CaO-Al₂O₃-SiO₂ melts at the ratio of mass pct CaO/mass pct Al₂O₃ = 1 increases as SiO₂ content increases, whereas the rate constant for the same melts at the ratio of mass pct CaO/mass pct SiO₂ = 1 increases as Al₂O₃ content increases. The rate constant for the CaO-Al₂O₃-TiO _x melts at the ratio of mass pct CaO/mass pct Al₂O₃ = 1 decreases as the TiO _x content increases. The activation energies of nitrogen dissolution in CaO-Al₂O₃-SiO₂ melts are about 1.5 to 3 times larger than that of molten pure iron. Moreover, the rate constant of nitrogen dissolution is independent of the ratio of Ti³⁺/Ti⁴⁺.     

Thermodynamics of Na2O in the molten CaO-CaF2-SiO2 system

The activity of a few percent of Na₂O in the CaO-CaF₂-SiO₂ system doubly saturated with CaO and 3CaO·SiO₂ has been determined between 1423 and 1623 K by a chemical equilibration technique. The temperature dependence of the activity coefficient of Na₂O may be expressed as follows. {fx553-01} It was found that Henry’s law holds up to 5 mass pct of Na₂O in the flux at 1473 K. The activity of a small amount of Na₂O in the CaO-CaF₂-SiO₂ flux is discussed in comparison with that for the Na₂O-SiO₂ system in terms of their refining characteristics.     

Thermodynamics of phosphate and phosphide in CaO-CaF2 melts

The thermodynamics of phosphate and phosphide in CaO-CaF₂ melts was studied by equilibrating the melts in a graphite boat with CO-Ar mixtures and Ag-P alloys between 1400 and 1550 ‡C. The dependence of phosphate and phosphide concentrations on the partial pressure of phosphorus, the partial pressure of oxygen, and slag composition confirmed the behavior of P³ and PO ₄ ³ in the melts as thermodynamically expected, giving 0.52 and 8.5 x 10⁻⁴ as the values for the activity coefficients of Ca_3/2PO₄ and Ca_3/2P, respectively.     

A thermodynamic study of dephosphorization using BaO-BaF2, CaO-CaF2, and BaO-CaO-CaF2 systems

Phosphorus partition ratios between BaO-BaF₂ fluxes and copper-phosphorus alloys have been measured as a function of slag composition at 1400 °C. A molybdenum sleeve has been used to avoid contact between the slag and the crucible in order to prevent the absorption of slag by the graphite crucible. The effects of additions of BaO to the CaO-CaF₂ system have been investigated by measuring the phosphorus partition ratios between these fluxes and Fe-C_sat-P alloy as a function of slag composition at 1400 °C. Also, the activity of BaO as a function of slag composition at 1400 °C has been determined by equilibrating a silver-barium alloy with the BaO-CaO-CaF₂ fluxes and CO in a graphite crucible. The results indicate that phosphorus partition ratios with carbon-saturated iron, L_P, for the BaO-BaF₂ system are relatively high, going up to 400, even for oxygen partial pressure as low as 7.8 × 10⁻¹⁶ atm. The phosphate capacity and the activity coefficient of PO_2.5 for this system were calculated from the experimental results using the available thermodynamic data. No effects of barium oxide addition in lime-based fluxes were observed for BaO contents less than 40 pct. One of the reasons for this phenomenon is that BaO activity in a lime-based flux is very small for BaO content less than 46 pct. However, for high BaO contents, increases significantly, as does the activity of BaO. Formerly Research Associate, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

The effect of Na2O on dephosphorization by CaO-based steelmaking slags

The effect of Na₂O on the equilibrium phosphorous distribution ratio between slag and iron or iron alloys, L_P, has been measured for CaO-SiO₂, CaO-FeOr-SiO₂ (CaO or 2CaO·SiO₂ saturated), and CaO-Al₂-SiO₂ slags. The addition of Na₂O to CaO-SiO₂ slags significantly increases L_P and the phosphate capacity. A 25 pct CaO-25 pct Na₂O-SiO₂ slag has a distribution ratio nearly two orders of magnitude greater than a comparable binary 50 pct CaO-SiO₂ slag. For the CaO-saturated slags containing 40 wt pct FeO_T, the addition of 6 wt pct Na₂O increases L_P by a factor of 5. For the 2CaO·SiO₂-saturated CaO-FeO_T-SiO₂ slag, there is an optimum FeOr content (20 wt pct) for dephosphorization, and 10 wt pct Na₂O increases L_P by a factor of 2. For reducing slags typically used in ladle metallurgy for Al-killed steels (50 pct CaO-40 pct Al₂O₃-10 pct SiO₂), as little as 3 wt pct Na₂O increases L_P by a factor of 100. The present results indicate that small additions of Na₂O to conventional steelmaking slags can greatly improve dephosphorization. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University.     

Thermodynamics of chromium oxides in CaO-SiO2-CaF2 slag

The distribution ratio of chromium between a CaO-SiO₂-CaF₂ slag and liquid silver under the oxygen partial pressure used in practical hot-metal dephosphorization treatment was measured at 1623 K. The distribution ratio was minimal when the basicity index of a slag, wt pct CaO/wt pct SiO₂, was about 2. The redox equilibrium between CrO (Cr²⁺) and CrO_1.5 (Cr³⁺) in the slag was also measured as a function of slag composition. The calculated activity coefficient of CrO had a maximum value at wt pct CaO/wt pct SiO₂=2, whereas that of CrO_1.5 decreased monotonously with the increase in slag basicity.     

Phase Relationship of CaO-SiO2-FeO-5 mass pct P2O5 System with Low Oxygen Partial Pressure at 1673?K (1400?°C)

Dephosphorization by using multiphase flux could considerably decrease the consumption of CaO and prevent the addition of fluorite. However, the equilibrium phase relationship within this system, which is of significant importance for understanding the formation mechanism of multiphase flux, remains unclear. Thus, it is required to provide reliable phase diagrams of the basic slag system of multiphase flux. In this research, the phase relationship of the CaO-SiO₂-FeO-5 mass pct P₂O₅ system at 1673?K (1400?°C) with $ {P}_{{{\text{O}}_{2} }} $ of 9.24?×?10^?11 atm has been studied by using the chemical equilibration method. It has been found that solid solution consists mainly of 2CaO·SiO₂-3CaO·P₂O₅, but occasionally it contains 3CaO·SiO₂. Liquidus saturated with solid solution shrinks toward the FeO corner compared with the isothermal at 1673?K (1400?°C) of the CaO-SiO₂-FeO system equilibrated with metallic iron. Thermodynamically stable CaO-FeO phase is confirmed, which could promote the condensation of 3CaO·P₂O₅ into the solid solution and increase the phosphorus partition ratio between the solid solution and molten slag. Based on the regular solution model, the effect of T.Fe and CaO content in the liquid phase on the phosphorus partition ratio between the solid solution and molten slag is discussed.     

Thermodynamics of the simultaneous desulfurization and dephosphorization processes of hot metal with CaO- and BaO-based slags bearing MnO

The technical feasibility of the simultaneous desulfurization and dephosphorization process of hot metal with high basicity slags of the BaO-BaF₂ system is discussed from the thermodynamic point of view. The partition ratios as well as the degrees of removal of sulphur and phosphorus from the hot metal obtained after a single refining step with BaO-saturated slags have been compared to those achieved when typical hot metal pretreatment slags of the system CaO-CaF₂-SiO₂ (double saturated with CaO and 3CaO?SiO₂) were used. The experiments were carried out in graphite crucibles at 1573 K, under CO atmosphere. Considering that both the desulfurization degree and the dephosphorization degree of hot metal achieved with BaO-based slags, around 95 %, were much higher than those obtained when CaO-based slags were used, maximum 45 %, it was concluded that the simultaneous desulfurization and dephosphorization process of hot metal with high basicity BaO-BaF₂ slags is technically feasible. It has been verified also that the high basicity BaO-based slags led to manganese recovery degrees as high as 86 % and prevented at the same time the undesirable oxidation of carbon from the metal phase.     

A thermodynamic study on cobalt containing calcium ferrite and calcium iron silicate slags at 1573 K

Redox equilibria, activities of cobalt, iron and their oxides in calcium ferrite and calcium ironsilicate slags, were measured through metal-slag-gas equilibrium experiments under controlled oxygen potentials (10⁻⁷ to 3 × 10⁻⁷ atm) at 1573 K. Results on the redox equilibria show that addition of CoO to calcium ferrite slag increases the equilibrium Fe³⁺/Fe²⁺ ratio in these melts. Measured activities of CoO and FeO showed positive deviations from ideal behavior, while that of Fe₂O₃ showed negative deviation. Partial substitution of CaO by SiO₂, by up to 4 wt pct SiO₂ in the calcium ferrite based melts, resulted in increases in the activity coefficients of CoO and Fe₂O₃. Phase equilibria studies on the cobalt containing CaO-FeO-Fe₂O₃-SiO₂ slags were also carried out using the drop-quench technique. Good agreement between the activity data and the liquidus temperature with respect to magnetite solid solution containing CoO was observed.     

Iron redox equilibria in CaO-Al2O3-SiO2 and MgO-CaO-Al2O3-SiO2 slags

Measurements have been made of the ratio of ferric to ferrous iron in CaO-Al₂O₃-SiO₂ and MgO-CaO-Al₂O₃-SiO₂ slags at oxygen activities ranging from equilibrium with pCO₂/pCO≈0.01 to as high as air at temperatures of 1573 to 1773 K. At 1773 K, values are given by $\begin{gathered} \log {\text{ }}\left( {\frac{{Fe^{3 + } }}{{Fe^{2 + } }}} \right) = 0.3( \pm {\text{ }}0.02){\text{ }}Y + {\text{ }}0.45( \pm {\text{ }}0.01){\text{ }}\log \hfill \\ \left( {\frac{{pCO_2 }}{{pCO}}} \right) - 1.24( \pm {\text{ }}0.01) \hfill \\ \end{gathered} $ where Y=(CaO+MgO)/SiO₂, for melts with the molar ratio of CaO/SiO₂=0.45 to 1.52, 10 to 15 mol pct Al₂O₃, up to 12 mol pct MgO (at CaO/SiO₂≈1.5), and with 3 to 10 wt pct total Fe. Available evidence suggests that, to a good approximation, these redox equilibria are independent of temperature when expressed with respect to pCO₂/pCO, probably from about 1573 to 1873 K. Limited studies have also been carried out on melts containing about 40 mol pct Al₂O₃, up to 12 mol pct MgO (at CaO/SiO₂≈1.5), and 3.6 to 4.7 wt pct Fe. These show a strongly nonideal behavior for the iron redox equilibrium, with $\frac{{Fe^{3 + } }}{{Fe^{2 + } }} \propto \left( {\frac{{pCO_2 }}{{pCO}}} \right)^{0.37} $ The nonideal behavior and the effects of basicity and Al₂O₃ concentration on the redox equilibria are discussed in terms of the charge balance model of alumino-silicates and the published structural information from Mössbauer and NMR (Nuclear Magnetic Resonance) spectroscopy of quenched melts.     

Effect of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Concentration on Density and Structure of (CaO-SiO<Subscript>2</Subscript>)-xAl<Subscript>2</Subscript>O<Subscript>3</Subscript> Slag

The effect of Al₂O₃ concentration on the density and structure of CaO-SiO₂-Al₂O₃ slag was investigated at multiple Al₂O₃ mole percentages and at a fixed CaO/SiO₂ ratio of 1. The experiments were conducted in the temperature range of 2154 K to 2423 K (1881 °C to 2150 °C) using the aerodynamic levitation technique. In order to understand the relationship between density and structure, structural analysis of the silicate melts was carried out using Raman spectroscopy. The density of each slag sample investigated in this study decreased linearly with increasing temperature. When the Al₂O₃ content was less than 15 mole pct, density decreased with increasing Al₂O₃ content due to the coupling of Si (Al), whereas above 20 mole pct density of the slag increased due to the role of Al³⁺ ion as a network modifier.     

Thermodynamics of Arsenic in FeOx-CaO-SiO<Subscript>2</Subscript> Slags

The distribution of arsenic between calcium ferrite slag and liquid silver (wt pct As in slag/ wt pct As in liquid silver) with 22 wt pct CaO and between iron silicate slag with 24 wt pct SiO₂ and calcium iron silicate slags was measured at 1573 K (1300 °C) under a controlled CO-CO₂-Ar atmosphere. For the calcium ferrite slags, a broad range of oxygen partial pressure (10^–11 to 0.21 atm) was covered, whereas for the silicate slags, the oxygen partial pressure was varied from 10^–9 to 3.1 × 10^–7 atm. The measured relations between the distribution ratio of As and the oxygen partial pressure indicates that the oxidation state of arsenic in these slags is predominantly As³⁺ or AsO_1.5. The measured distribution ratio of arsenic between the calcium ferrite slag and the liquid silver was about an order of magnitude higher than that of the iron silicate slag. In addition, an increasing concentration of SiO₂ in the calcium-ferrite-based melts resulted in decreases in the distribution of arsenic into the slag. Through the use of measured equilibrium data on the arsenic content of the metal and slag in conjunction with the composition dependent on the activity of arsenic in the metal, the activity of AsO_1.5 in the slags was deduced. These activity data on AsO_1.5 show a negative deviation from the ideal behavior in these slags.     

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.     

Redox Equilibrium of Niobium in Calcium Silicate Base Melts

The oxidation state of niobium has been determined at 1873 K (1600 °C) in CaO-SiO₂-NbO _x melts with CaO/SiO₂ ratios (mass pct) of 0.66, 0.93 and 1.10, and 5.72 to 11.44 pct Nb₂O₅ (initial). The slag samples were equilibrated with gas phases of controlled oxygen pressure, then quenched to room temperature and analyzed chemically. The niobium is mainly pentavalent with small amounts in the tetravalent state. It was found that the Nb⁵⁺/Nb⁴⁺ ratio increases with oxygen pressure at a constant CaO/SiO₂ ratio and constant content of total niobium, closely according to the ideal law of mass action, which is proportional to \textp_\textO2 ^1/4 . {\text{p}}_{{{\text{O}}_{2} }}^{1/4} . The ratio also increases with total niobium content, and it seems to have a maximum at a basicity of about 0.93. The color of the solidified slag samples is described and is explained with the help of transmission spectra.     

Determination of sulfide capacities of CaO-SiO2 slags containing CaF2 and B2O3 by an encapsulation method

The sulfide capacities of CaO-SiO₂ melts containing CaF₂ and B₂O₃ have been measured by simultaneous equilibration of samples within quartz capsules at 1503°C. In this method, the compositions of both the condensed phases and the static gas environment adjust to es-tablish a mutual equilibrium. Although the sulfurizing potential,P _{S 2}/P _{O 2}, is neither preset nor directly measured, the sulfide capacities,C _s, of all samples within the capsule are related by the following equation:C _S(j)=(pct S)j/(pct S)i · C_S(i)· Hence, the sulfide capacities of all samples may be calculated from the measurement of the final sulfur content of each sample and the inclusion of a “reference” slag sample for which C_S is known from previous measurements. In the present study, quartz capsules containing as many as fifty slag samples including several reference samples have been equilibrated. With considerable difficulty the encapsulation-equilibration method atl503°C has been developed into an acceptable procedure. The influences of CaF₂ and B₂O₃ on the sulfide capacities of several CaO-SiO₂ melts have been measured. The results may be summarized as follows: 1) for melts of fixed CaO/SiO₂ ratio, the addition of CaF₂ increases sulfide capacity and the addition of B₂O₃ decreases sulfide capacity, 2) the substitution of CaF₂ for CaO does not alter sulfide capacity significantly, and 3) the substitution of B₂O₃ for SiO₂ increases sulfide capacity slightly. These results have been obtained for melts with CaO/SiO₂ mass ratios ranging from 1.00 to 1.28 and with flux additions up to 10 wt pct.     

Kinetic studies on the dissolution of nitrogen in CaO-Al2O3, CaO-SiO2, and CaO-CaF2 melts

The rate of nitrogen dissolution in CaO-Al₂O₃, CaO-SiO₂, and CaO-CaF₂ melts was measured by ¹⁴N-¹⁵N isotope exchange reaction. The rate constant of nitrogen dissolution in CaO-based oxide melts, which is defined as first order with respect to nitrogen partial pressure, was found to be much smaller than that in molten iron-based alloys investigated in our previous work. The activation energies for nitrogen dissolution in 40 mass pct CaO-60 Al₂O₃ and 50 CaO-50 SiO₂ melts are 224 and 581 kJ/mol, respectively. For CaO-Al₂O₃ and CaO-SiO₂, dependence of the rate constant on composition is very similar to that of nitride capacities. Moreover, it was confirmed that the rate constant was not affected by oxygen or nitrogen partial pressure.     

Molecular dynamics simulations of microstructural properties of CaO–SiO2–TiO2 fluorine-free slag systems

The microstructural properties of F-free slag, the CaO–SiO₂–TiO₂ (CST) systems, are investigated by molecular dynamics (MD) simulations. The results show that in the CST system, the average bond length of Si–O remains in 1.61?Å. The addition of TiO₂ contributes to the increase in the concentrations of 4-coordinated Si and 4-coordinated Ti. Increasing the amount of CaO decreases the proportion of bridging oxygen (BO) atoms and the degree of network connectivity (Q³ and Q⁴), suggesting the simplification of melt polymerisation. Substituting CaO with SiO₂ and maintaining a constant TiO₂ level causes the microstructure of the slag to become more complex. Both SiO₂ and TiO₂ contribute to the more complex structures of the melts. Simultaneously, Si–O–Ti linkages are more favourable than Si–O–Si or Ti–O–Ti linkages. Thus, TiO₂ is regarded as network former in terms of its structure within CST system when the content of TiO₂ excessed 28 mass%.     

The equilibrium partitioning of titanium between Ti3+ and Ti4+ valency states in CaO-SiO2-TiO x slags

The redox behavior of titanium in CaO-SiO₂-TiO _x melts was investigated using a slag-gas equilibrium technique. Titanium partitioning between Ti³⁺ and Ti⁴⁺ valency states and the ratio of activity coefficients of TiO_1.5 and TiO₂ were determined as functions of oxygen partial pressure, temperature, and slag composition. The equilibrium experiments were carried out at temperatures between 1783 and 1903 K under CO-CO₂-Ar gas atmosphere with oxygen partial pressure ranging from 10⁻¹² to 10⁻⁷ atm (1.01×10⁻¹⁰ kPa to 1.01×10⁻⁵ kPa). The slags had CaO/SiO₂ ratios between 0.55 and 1.35 and total titanium oxide concentrations from 7 to 50 mass pct. Experimental results showed that the Ti³⁺/Ti⁴⁺ ratio in CaO-SiO₂-TiO _x slags, containing up to 50 mass pct TiO _x , increased with decreasing oxygen partial pressure and decreased with increasing CaO/SiO₂ ratio and decreasing temperature. Measured variation of the redox ratio Ti³⁺/Ti⁴⁺ with oxygen partial pressure closely followed the ideal behavior. Increasing the CaO/SiO₂ ratio increased the ratio of activity coefficients of TiO_1.5 and TiO₂. The effect of total titania content on this ratio was more complex and in accord with Raman spectroscopy data.     

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.