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.     

<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 FeO and CaO on the Sulfide Capacity of the Ferronickel Smelting Slag

The effect of FeO and CaO on the sulfide capacity in MgO-SiO₂-FeO based slags equilibrating with Fe-Ni alloys at 1773 K and 1873 K (1500 °C and 1600 °C) was investigated. The sulfide capacity in the MgO-SiO₂-FeO and MgO-SiO₂-CaO-FeO slags increased with higher FeO content and higher temperatures due to an increase in the activity of O^2? and a decrease in the activity coefficient of sulfide ion in slag. The sulfide capacity of the MgO-SiO₂-CaO-FeO slag also increased with an increase in the CaO content due largely to the increase in the activity of O^2?. Furthermore, CaO and FeO seem to be more effective than MgO in increasing the sulfide capacity in the MgO-SiO₂-CaO-FeO slag system. In addition, the comparison of the experimental results with the theoretical estimate using the modified empirical optical basicity showed relatively good linear agreement.     

Studies of Vaporization of Chromium from Thin Slag Films at Steelmaking Temperatures in Oxidizing Atmosphere

In the present work, the volatilization of chromium from thin chromium-containing slag film surfaces was studied in oxidizing atmosphere in the temperature range 1673 K to 1873 K (1400 °C to 1600 °C). The slag films on alumina rings were exposed to air or pure oxygen and the loss of Cr from the post-experiment sample films was examined by SEM/WDS analysis. The mass loss of the samples was also monitored during the heat-treatment. The results indicate that chromium loss increased with increase in temperature and oxygen partial pressure was found also to be relatively less as the sample thickness increased. The implications of chromium escape from slags during the tapping of stainless steel slags are discussed.     

Thermal Conductivity Measurements of Ladle Slag Using Transient Hot Wire Method

Thermal conductivities of four different ladle slags were measured at 1773 K, 1823 K, 1873 K, and 1923 K (1500 °C, 1550 °C, 1600 °C, and 1650 °C) using the transient hot wire method. Very good reproducibility was obtained. The thermal conductivity did not vary substantially with the variation of slag composition at 1873 K and 1923 K (1600 °C and 1650 °C), at which the slags were all entirely liquid. The thermal conductivities were low. It was found that the precipitation of solid phase resulted in considerable increase of thermal conductivity.     

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.     

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.     

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.     

The Rate of the Phosphorous Reaction Between Liquid Iron and Slag

In the current study, the rates of dephosphorization and rephosphorization of liquid iron with simulated steelmaking slags were investigated at 1873 K (1600° C). The experiments were conducted in an induction furnace with supplemental heating to maintain a consistent temperature within both the metal and slag phases. An integrated form of the rate equation was used to evaluate the results, assuming mass transfer in both the slag and metal was rate controlling. The results of the current and previous studies indicate that the mass transfer parameter, the slag-metal surface area, and the overall mass transfer coefficient (A*k ₀), decreased as the reaction proceeded. It is proposed that initially when the rate and oxygen flux are high, the interfacial energy decreases, and the interfacial fluid velocity increases causing disruption of the slag metal interface. The consequent increases in interfacial area and interfacial fluid flow cause A*k ₀ to be high initially and then decrease as the oxygen flux decreases.     

Phase relations and thermodynamics of the system Fe-Cr-0 in the temperature range of 1600 °C to 1825 °C (1873 to 2098 K) under strongly reducing conditions

Equilibrium relations involving alloy and oxide phases in the system Fe-Cr-O were determined in the temperature range from 1600 °C to 1825 °C (1873 to 2087 K). Compositions of coexisting alloy and spinel phases were established as a function of oxygen pressure by equilibrating liquid Fe-Cr alloys with iron chromite (Fe_3-xCr_xO₄) solid solutions at 1600 °C and 1700 °C. Combinations of these experimental data and thermodynamic calculations were used to construct composition-oxygen pressure diagrams for the system at 1600 °C and 1700 °C. Additional runs for selected mixtures were made at still higher temperatures (1700 °C to 1825 °C), and thermodynamic parameters were derived for spinel-containing phase assemblages at temperatures up to 1865 °C. The spinel phases occurring in the present system are typically in the high-chromium range of the solid-solution series Fe₃O₄-Cr₃O₄,i.e., in the range between stoichiometric iron chromite (FeCr₂O₄) and Cr₃O₄. The activities of the various oxide components of the spinel solid solution at 1600 °C were calculated from experimentally determined parameters for coexisting alloy and spinel phases, as well as by statistical-mechanical modeling of the same spinel solid solution based on crystal-chemical considerations. The agreement between the two sets of results was excellent. Temperature variation of parameters characterizing the univariant equilibria spinel + Cr₂O₃ + alloy and spinel + alloy + liquid oxide was established. The univariant curves were found to display temperature maxima of 1715 °C ± 5 °C and approximately 1865 °C, respectively. In analogy with relations in the Cr-O system, the increase in divalent chromium of the liquid oxide phase with decreasing oxygen potential was identified as the main cause of the sharp decrease in liquidus temperatures of chromites in contact with Fe-Cr alloys of high Cr contents. Formerly Graduate Research Assistant, Department of Metallurgy, The Pennsylvania State University L.S. DARKEN and ARNULF MUAN, formerly Professors of Geochemistry and Materials Science, The Pennsylvania State University, University Park, PA 16802, are deceased.     

Kinetics of Transformation of Al<Subscript>2</Subscript>O<Subscript>3</Subscript> to MgO·Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Spinel Inclusions in Mg-Containing Steel

During ladle furnace refining, initial Al₂O₃ inclusions generally transform into MgO·Al₂O₃ spinel inclusions; these generated spinel inclusions consequently deteriorate the product quality. In this study, the transformation from Al₂O₃ to MgO·Al₂O₃ was investigated by immersing an Al₂O₃ rod into molten steel, which was in equilibrium with both MgO and MgO·Al₂O₃ spinel-saturated slag. A spinel layer, with a thickness of 4 μm, was generated on the Al₂O₃ rod surface just 10 s after its immersion at 1873 K (1600 °C). The thickness of the formed spinel layer increased with the immersion period and temperature. Moreover, the MgO content of the generated spinel layer also increased with the immersion period. In this study, the chemical reaction rate at 1873 K (1600 °C) was assumed to be sufficiently high, and only diffusion was considered as a rate-controlling step for this transformation. By evaluating the activation energy, MgO diffusion in the generated spinel layer was found to be the rate-controlling step. In addition, this estimation was confirmed by observing the Mg and Al concentration gradients in the generated spinel layer. The results of this study suggest that the MgO diffusion in the spinel inclusions plays a substantial role with regard to their formation kinetics.     

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.     

Sulfide Capacities of CaO-Al2O3-SiO2 Slags in the Temperature Range 1673 K to 1773?K (1400?°C to 1500?°C)

With a goal to estimate the sulfide capacities of slags used in the pretreatment of hot metal, the sulfide capacities of CaO-Al₂O₃-SiO₂ slags were measured at 1673?K to 1773?K (1400?°C to 1500?°C). The gas?Cslag equilibrium technique has been used for this measurement. From the results obtained, it was found that the temperature dependence of the sulfide capacity of this slag is independent of the slag compositions. Therefore, a new empirical model based on optical basicity for sulfide capacity estimation of this slag was developed using the measured values of the current work and literature. With the use of the new model, the isosulfide capacity curves at 1673?K (1400?°C) were mapped.     

Effects of the Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Content on the Viscosity of CaO-SiO<Subscript>2</Subscript>-10 Pct Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> Quaternary Slag

The present study experimentally investigates the effect of Cr₂O₃ on the viscosity of molten slags. The viscosities of CaO-SiO₂-10 pct Al₂O₃-Cr₂O₃ quaternary slags with two different binary basicities (R, basic slag with R = 1.2 and acidic slag with R = 0.8) were measured by the rotating cylindrical method from 1813 K to 1953 K (1540 °C to 1680 °C). The results showed that the viscosity of both types of slag decreased as the Cr₂O₃ content increased, but the viscosity of acidic slags exhibited a greater decrease. The slags showed good Newtonian behavior at such high temperatures. Cr₂O₃ could act as a network modifier to simplify the Si-O-Si tetrahedral structure, as verified by the Raman spectral analysis, which was consistent with the decreasing trend of viscosity. The activation energy of viscous flow decreased slightly with increasing Cr₂O₃, but increasing the basicity seemed to be more effective in decreasing the viscosity than adding Cr₂O₃.     

Activity Coefficient of SiO2 in SiO2-Al2O3-CaO Slags with Low SiO2-Content at 1873 K (1600 °C)

The activity coefficient of SiO₂ in SiO₂-Al₂O₃-CaO slags with limited Al₂O₃ content was measured by equilibrating Fe-C-Si melt and slags at 1873 K (1600 °C). When the Al₂O₃ content was between 48 and 54 wt pct, the results show that $ \gamma_{{{\text{SiO}}_{ 2} }} $ rapidly decreases as the amount of SiO₂ in the slag decreases. The equilibrium amounts of Si and Al in a Fe melt in equilibrium with SiO₂-Al₂O₃-CaO slags were calculated based on the result of this study.     

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.     

Rate of interfacial reaction between molten CaO-SiO2-Al2O3-Fe x O and CO-CO2

Measurements of the rates of reduction of iron oxide from molten CaO-SiO₂-Al₂O₃-Fe _x O slags by Ar-CO mixtures have been made using a thermogravimetric method. The apparent first-order rate constant, with respect to the partial pressure of CO, of the gas/slag interfacial reaction was deduced from the measured rates, where the effects of the mass transfer in the gas and slag phases were minimized. It was found that the apparent first-order rate constant decreased with the concentration of ‘FeO’ from 100 to 20 wt pct, whereas it remained essentially constant in the range from 5 to 20 wt pct ‘FeO’. At a given iron oxide concentration, the reduction-rate constant increased significantly with an increase in the CaO/SiO₂ ratio. For fixed slag compositions, the reduction rate increased slightly with the oxidation state of the slags. When the rate constant is expressed in the form of k=k′(Fe³⁺/²⁺)^α, the values of α range from 0.15 to 0.25. The effect of temperature in the range from 1673 to 1873 K on the reduction rate of iron oxide in a 40.4CaO-40.4SiO₂-14.2Al₂O₃-5‘FeO’ (wt pct) slag was studied. The calculated activation energy, based on these results, is 165 kJ/mol.     

Interfacial Reaction Between CaO-SiO2-MgO-Al2O3 Flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) Steel at 1873 K (1600 °C)

This study investigated the interfacial reaction kinetics and related phenomena between CaO-SiO₂-MgO-Al₂O₃ flux and Fe-xMn-yAl (x = 10 and 20 mass pct, y = 1, 3, and 6 mass pct) steel, which simulates transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) steels at 1873 K (1600 °C). It also examines the effect of changes in the composition of the steel and slag phases on the interfacial reaction rate and the reaction mechanisms. The content of Al and Si in the 1 mass pct Al-containing steel was found to change rapidly within the first 15 minutes of the reaction, but then it remained relatively constant. The content of Al and Si in the 3 to 6 mass pct Al-containing steels, in contrast, changed continuously throughout the entire reaction time. In addition, the content of Mn in the 1 mass pct Al-containing steels initially decreased with increasing time, but the content did not change in the 3 to 6 mass pct Al-containing steels. Furthermore, the mass transfer coefficient of Al, k _Al, in the 1 mass pct Al-containing systems was significantly higher than that in other systems; i.e., the k _Al can be arranged such that 1 mass pct Al systems >> 3 mass pct Al systems ≥ 6 mass pct Al systems. The compositions of the final slags were close to the saturation lines of the [Mg,Mn]Al₂O₄ and MgAl₂O₄ spinels when the slags reacted with 1 mass pct Al and 3 to 6 mass pct Al-containing steels, respectively. These results, which show the effect of Al content on the reaction phenomena, can be explained by the significant increase in the apparent viscosity of the slags that reacted with the 3 to 6 mass pct Al-containing steels. This reaction was likely caused by the precipitation of solid compounds such as MgAl₂O₄ spinel and CaAl₄O₇ grossite at locally alumina-enriched areas in the slag phase. This analysis is in good accordance with the combination of Higbie’s surface renewal model and the Eyring equation.     

Confocal Microscopic Studies on Evolution of Crystals During Oxidation of the FeO-CaO-SiO<Subscript>2</Subscript>-MnO Slags

The current work investigates dynamic phenomena at the microstructural level during iron and manganese recovery from the liquid FeO-CaO-SiO₂-MnO slags using an oxidation method. A hot-stage-equipped confocal scanning laser microscope (CSLM) was used to analyze the kinetic behavior of crystallization in synthetic slags. Based on observed precipitations on cooling in the 1273 K (1000 °C) to 1873 K (1600 °C) temperature range, a time–temperature–transformation (TTT) diagram has been created. The crystallization studies were conducted in air.     

Mineralogical characterisation of vanadium slag under different treatment conditions

Abstract

Synthetic vanadium slags were melted at 1673 K and then cooled to a predetermined temperature at different cooling rates. The mineralogical phases and crystallisation behaviour were investigated based on optical microscopy, SEM, EDS and XRD analyses. The results show that spinels and silicates were the major phases in the complex vanadium slag samples, and V is enriched in the spinel phases, i.e. Fe_xV_3?xO₄ and Mg_x(V, Ti)_3?xO₄. The apparent diameter of spinel crystal grains decreases with increasing cooling rate, e.g. from 14 to 5 μm at 1523 K. Crystal grain size increases significantly with increasing holding time. It is also found that the grain sizes of spinels at higher temperatures, 1573 and 1623 K, are considerably smaller than at lower temperatures, 1473 and 1523 K. At lower temperature, large blocked spinel crystals between grey silicate phases are formed, whereas there only a few smaller blocked spinels when performed at higher temperature. Better control of the process cooling parameters will benefit vanadium slag quality.