Activity of Chromium Oxide in CaO‐SiO2 based Slags at 1873 K

Thermodynamic properties of chromium oxides in molten slags are very important for optimization of stainless steel refining processes as well as reduction processes of chromium ores. The solubility of chromite into molten slags has been found to vary drastically with oxygen partial pressure and slag composition in the former studies by the authors. In the present study, activity data and redox equilibria of chromium oxides measured under moderately reducing conditions, P_O2= 6.95×10^?11 atm, at 1873 K are summarized. For the CaO‐SiO₂‐CrO_x system, the activity coefficient of chromium oxide increased with increasing basicity and the optimized slag composition for stainless steel refining is assessed as that saturated with CaCr₂O₄ and Cr₂O₃ using the phase relations determined. On the other hand, the presence of MgO and Al₂O₃ brings about different behaviour of chromium oxide activity and redox equilibria and the 44 mass per cent CaO ‐ 39 mass per cent SiO₂ ‐11 mass per cent Al₂O₃ ‐ 6 mass per cent MgO slag is recommended to reduce the chromium oxidation loss in the practical stainless steel refining process at 1873 K.     

Oxidation state and activities of chromium oxides in CaO-SiO2-CrOx slag system

The activities of chromium oxides in a CaO-SiO₂-CrO _x slag system were determined with the electromotive force (EMF) method by equilibrating with metallic chromium at 1873 K. The effect of slag basicity on the activity coefficients of CrO and CrO_1.5 was analyzed. The results showed that increasing the slag basicity increased the activity coefficient of CrO; however, the effect on that of CrO_1.5 was not significant. The oxidation state of chromium in CaO-SiO₂-CrO _x slags was systematically investigated at both 1873 and 1863 K. It was found that divalent and trivalent chromium coexists in the slags. Divalent chromium oxide is favored, instead of trivalent chromium oxide, because of low slag basicity and low oxygen potential. It was concluded that the oxidation state of chromium in the slag system varied greatly from almost pure “CrO” to a composition corresponding to Cr₃O₄. In addition, the thermodynamic data in the slag system were assessed based on the regular solution model to mathematically describe the activities of chromium oxides in the slags. A group of model parameters were obtained. The calculated activities of chromium oxides were comparable to the measured data.     

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.     

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.     

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.     

Reduction of synthetic stainless steel slags by aluminium

One of the most efficient ways to eliminate the harm of chromium oxide in stainless steel slag is to reduce chromium oxide in stainless steel slag using aluminium. In the present work, the Al reduction of synthetic CaO–SiO₂–Al₂O₃–MgO–Fe₂O₃–Cr₂O₃ stainless steelmaking slags at different conditions, including temperature, slag basicity and Al amount was investigated to get optimal conditions for the reduction and the metal–slag separation. It was found that the agglomeration of metal droplets and metal–slag separations were improved by increasing temperature. The reduction degrees of SiO₂, Fe₂O₃ and Cr₂O₃ were enhanced with increasing basicity of slag. The addition of CaF₂ in slag leads to better agglomerations of metal droplets and metal–slag separations. The highest reduction degree of chromium could reach 99% in slag with basicity of 2 at 1873 K.     

Experimental Study and Modelling of Viscosity of Chromium Containing Slags

Viscosities of slags containing Al₂O₃, CaO, CrO_x, MgO and SiO₂ were measured in contact with metallic chromium using the rotating cylinder method. The modified Urbain model, developed at the University of Queensland, was extended to include MgO, CrO and Cr₂O₃, in addition to existing Al₂O₃, CaO, FeO and SiO₂. Chromium oxides, in general, decrease the slag viscosity, although addition of trivalent oxide raises up the liquidus temperature and thus limits the measuring range. The model was able to describe the viscosity of complex slags reasonably well in most experimental cases.     

Formation of hexavalent chromium by reaction between slag and magnesite-chrome refractory

The goal of this work was to understand how Cr⁶⁺ formation is affected by the interaction between chromite phases present in magnesite-chrome refractory and different slag compositions. In addition, the formation of Cr⁶⁺ as a function of chromite particle size and cooling rate due to the chromite phase/slag interaction was investigated. The following slag compositions were studied: calcium aluminate, calcium aluminum silicate, and calcium silicate. It was found that the presence of uncombined CaO in the calcium aluminate slags is responsible for a higher yield of Cr⁶⁺. However, the replacement of Al₂O₃ by SiO₂ in calcium aluminate slags decreases the formation of Cr⁶⁺. SiO₂ reacts with CaO to form stable 2CaO·Al₂O₃·SiO₂ and CaO·SiO₂ phases, consequently decreasing the amount of uncombined CaO available to react with the chromite phase to form Cr⁶⁺. Moreover, the content of Cr⁶⁺ is decreased by increasing chromite particle size and increasing the cooling rate of magnesite-chrome refractory.     

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.     

Influence of Cr2O3 on the Viscous Flow Behavior and Structure of Cr-Containing Molten Titanium Slag

Herein, the influence of Cr₂O₃ on the viscous flow behavior and structure of Cr-containing molten titanium slag is investigated in this work. The melting and viscosity properties are studied by hemisphere method and rotating cylinder method, respectively. In addition, the structure of Cr-containing molten titanium slag is investigated by Raman spectroscopy. It is shown in the results that the melting temperature and viscosity both increase with the increase of Cr₂O₃ content. The activation energy for viscous flow increases with increasing Cr₂O₃ content, which is in the same trend as viscosity. When the Cr₂O₃ content increases from 0% to 3%, the simpler 6-coordinated Ti⁴⁺ and TiO₄⁴⁻ monomer unit transformed into more complex O–Ti–O deformation and Ti₂O₆⁴⁻ chain unit. Furthermore, the fraction of relatively complex silicate structural units shows an increasing trend. Cr₂O₃ behaves as a network former and enhances the degree of polymerization of network structure, and the variation of Cr-containing molten titanium slag structure is consistent with the increasing trend of viscosity.     

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.     

Immobilization of Chromium in Slags using MgO and Al2O3

Chromium containing slags from stainless steelmaking may be leached by acidic environments, therefore they should be treated before being stockpiled or land filled. In this work, synthetic slags were prepared and the effect of CaO/SiO₂, Cr₂O₃, MgO and Al₂O₃ contents on the stability of the mineralogical species formed was analysed. The morphology and composition of the slags were determined by XRD and SEM‐EDS, whilst their chemical stability was evaluated by leaching with an aqueous acetic acid solution. It was found that CaCr₂O₄ and CaCrO₄ are present in slags prepared with neither MgO nor Al₂O₃. The Al₂O₃‐based slags mainly produced Ca₂Al₂SiO₇ and the Cr(VI)‐containing oxide complex Ca₄Al₆CrO₁₆, whilst MgO‐based slags produced Mg Cr₂O₄ as main mineralogical species. Additionally, Eh‐pH diagrams for the Ca‐Cr‐H₂O and Mg‐Cr‐H₂O systems at 25°C were constructed. The results showed that the lowest chromium concentration levels in the leaching liquors corresponded to MgO‐based slags owing to the stable binding of chromium in spinel with MgO. It was also observed in the Al₂O₃‐based slags that when increasing the slag basicity from 1 to 2, the leachability of the slags was notably increased.     

Rate of slag reduction in a laboratory electric furnace—alternating vs direct current

The objective of this laboratory investigation was to measure the reduction kinetics of nickel smelting and converting slags using alternating current (AC) and direct current (DC). The two slags tested contained 34 and 51 pct total iron in the form of FeO and Fe₃O₄. Laboratory experiments were carried out between 1200 °C and 1450 °C, and the rate of reduction was measured based on the CO and CO₂ contents in the off-gas from the furnace. Upon application of power to a pair of electrodes immersed in the molten slag, the reduction rate increased rapidly. This increase is explained by an increase in the electrode tip temperature enhancing the rate of the Boudouard reaction. The rate of reduction of the converter slag containing 29 pct Fe₃O₄ was 2 to 3 times faster than the smelting slag. With DC, the reduction rates at the anode and cathode were basically identical to each other, while for the smelting slag with only 8 pct Fe₃O₄, the anode and cathode reduction rates were quite different. With increasing current or power density, the temperatures of the electrodes increase above that of the bulk slag.     

Application of UV/VIS‐Reflection Spectroscopy for Determination of the Oxidation State of Liquid Slags with High Fe3+‐Contents

This work considers the reflectivity of liquid oxides (silicates and phosphates) in the ultraviolet and visible spectral range using a reflection angle of 0°. We have developed a spectroscopic reflection method (impulse‐flash‐technique) and have investigated CaO‐FeO_n‐SiO₂ and CaO‐FeO_n‐P₂O₅ with Fe₂O₃‐contents above 32 mol‐% (FeO_1.5) at a temperature of 1400°C applying oxygen partial pressures of p_o2 = 0.001 bar up to p_o2 = 0.21 bar. The increased reflectivity in the ultraviolet spectral range is based on the very intensive electron transfer (charge transfer bands, CT) from the oxide ion (bound to the respective matrix) to the Fe³⁺‐ion. The increased reflectivities in the visible spectral range are due to d‐d‐transitions in the Fe³⁺‐ion located in Fe³⁺‐O^2? ‐complexes. This can be proven in the following way. The reflection bands in the visible range are much less pronounced than the CT‐bands in the UV range. In the slag melts, complexes with the coordination number 4, Fe³⁺(O^2?)₄, have been found predominantly. The relation between the redox state of liquid slags and their UV‐reflectance has been evaluated quantitatively. The basic investigations for recording the redox state of liquid silicates and phosphates during a running metallurgic process are part of this work. Liquid silicates and phosphates are the most important slag systems in this context.     

A Sulfide Capacity Prediction Model of CaO-SiO2-MgO-FeO-MnO-Al2O3 Slags during the LF Refining Process Based on the Ion and Molecule Coexistence Theory

A sulfide capacity prediction model of CaO-SiO₂-MgO-FeO-MnO-Al₂O₃ ladle furnace (LF) refining slags has been developed based on the ion and molecule coexistence theory (IMCT). The predicted sulfide capacity of the LF refining slags has better accuracy than the measured sulfide capacity of the slags at the middle and final stages during the LF refining process. Increasing slag binary basicity, optical basicity, and the Mannesmann index can lead to an increase of the predicted sulfide capacity for the LF refining slags as well as to an increase of the sulfur distribution ratio between the slags and molten steel at the middle and final stages during the LF refining process. The calculated equilibrium mole numbers, mass action concentrations of structural units or ion couples, rather than mass percentages of components, are recommended to represent the slag composition for correlating with the sulfide capacity of the slags. The developed sulfide capacity IMCT model can calculate not only the total sulfide capacity of the slags but also the respective sulfide capacity of free CaO, MgO, FeO, and MnO in the slags. The comprehensive contribution of the combined ion couples (Ca²⁺ + O²⁻) and (Mn²⁺ + O²⁻) on the desulfurization reactions accounts for 96.23 pct; meanwhile, the average contribution of the ion couple (Fe²⁺ + O²⁻) and (Mg²⁺ + O²⁻) only has a negligible contribution as 3.13 pct and 0.25 pct during the LF refining process, respectively. The oxygen activity of bulk molten steel in LF is controlled by the [Al]–[O] equilibrium, and the oxygen activity of molten steel at the slag–metal interface is controlled by the (FeO)–[O] equilibrium. The ratio of the oxygen activity of molten steel at the slag–metal interface to the oxygen activity of bulk molten steel will decrease from 37 to 5 at the initial stage, and further decrease from 28 to 4 at the middle stage, but will maintain at a reliable constant as 5 to 14 at the final stage during the LF refining process. The proposed high-oxygen potential layer of molten steel beneath the slag–metal interface can be quantitatively verified.     

Experimental Studies on the Oxidation States of Chromium Oxides in Slag Systems

In view of the importance of the thermodynamic behavior of chromium in the slag phase as well as the serious discrepancies in the earlier reports on the valence state of chromium in slag melts, the oxidation state of chromium oxides in CaO-SiO₂-CrO_x and CaO-MgO-(FeO-) Al₂O₃-SiO₂-CrO_x were investigated experimentally in the present study using two different experimental techniques. The gas–slag equilibrium technique was adopted to study the CaO-SiO₂-CrO_x system between 1823 K (1550 °C) and 1923 K (1650 °C) and equilibrated with mixtures of CO-CO₂-Ar gases corresponding to three different oxygen partial pressures (between 10⁻⁴ and 10⁻⁵ Pa). After equilibrating, the samples were quenched and subjected to analysis using the X-ray absorption near edge structure method to determine the distribution ratio of Cr²⁺/Cr³⁺ in the slags. The second technique examined the applicability of the high-temperature mass spectrometric method combined with the Knuden effusion cell for quantifying the valence states of Cr in the multicomponent system CaO-MgO-(FeO-) Al₂O₃-SiO₂-CrO_x up to a maximum temperature of 2000 K (1727 °C). The results showed that the Knudsen cell-mass spectrometric method could be used successfully to estimate the valence ratio for Cr in silicate melts. According to the present study, the Cr²⁺/Cr³⁺ ratio increased with increasing temperature and a decreasing slag basicity as well as the oxygen partial pressure prevailing in the system. A mathematical correlation of X _CrO/X _CrO1.5 as a function of temperature, oxygen partial pressure, and basicity was developed in the present work based on the present results as well as on those assessed from earlier data.     

Effect of Al2O3 and MgO on Interfacial Tension Between Calcium Silicate‐Based Melts and a Solid Steel Substrate

The effect of Al₂O₃ and MgO on the interfacial tension between the molten CaO–SiO₂‐based slag and solid steel at 1773 K was studied. The interfacial tension of molten slags slightly increased with increasing Al₂O₃, but no significant change of interfacial tension was observed with higher MgO. Fourier transform infrared (FTIR) of as‐quenched slag samples indicated the slag structure to polymerize with Al₂O₃ additions, but depolymerize with MgO additions. Further detailed studies of the slag surface using X‐ray photoelectron spectroscopy (XPS) showed the fraction of free oxygen ions to decrease with higher Al₂O₃ but remained constant at higher MgO. The results suggested that interfacial tension decreases not only with the depolymerization of the melt, but also with an increase in the free oxygen ions at the molten slag/solid steel interface.     

Thermodynamic Behaviour of Chlorine in CaO‐SiO2‐MgO‐Al2O3(‐CaF2) Slags

The solubility of chlorine in CaO‐SiO₂‐Al₂O₃‐MgO(‐CaF₂) slag was measured at 1673 ‐1823 K. By estimating the chloride capacity of slags, thermodynamic behaviour of chlorine in the molten slag was investigated. Chloride capacity increased with increasing CaO / SiO₂ ratio (C/S). An increase in MgO content decreased chloride capacity at C/S≥1.0 because it lowered the activity of Ca²⁺ which seemed to have strong affinity with Cl^? in molten slag. Also, the chloride capacity decreased with increasing Al₂O₃ content. The affinity between the Ca²⁺ and Cl^? ions was confirmed by measuring the infrared spectra of slags. The dissolution reaction of chlorine into slag was exothermic and its molar enthalpy was evaluated from the experimental results at 1673 ‐ 1823 K. Based on the result obtained in the present study, the quantitative prediction of chlorine distribution during the blast furnace process was performed. It was estimated that almost all chlorine in the blast furnace would be absorbed into molten slag even if the PCI ratio was increased or low quality coal with chlorine content less than 1.0 mass% was injected.     

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.     

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.