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 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.     

Kinetics of Oxidation of Divalent Iron to Trivalent State in Liquid FeO-CaO-SiO<Subscript>2</Subscript> Slags

This work was devoted to the kinetics studies of the oxidation of divalent iron in liquid FeO-CaO-SiO₂ slags to the trivalent state. The experiments were carried out using a thermogravimetric technique (TGA) in the temperature range of 1623 K to 1773 K (1350 °C to 1500 °C) in an oxidizing atmosphere. The reaction products after oxidation were analyzed by X-ray diffraction and optical and scanning electron microscopy. The results obtained show that during the first 10 to 15 minutes of oxidation, 70 to 90 pct of the Fe²⁺ in the slag was oxidized. Kinetic analysis of the TGA results indicates that the oxidation process may consist of three distinct steps, viz an initial incubation period, followed by a chemical-reaction-controlled stage, and later, a diffusion-control stage. Appropriate mathematical relationships were set up for the first two consecutive steps. After combining these equations suitably as the mechanism of oxidation shifts from one form to another, the experimental results for the first two parts could be reproduced. A linear correlation was found between the thermodynamic activity of FeO in the slag and the degree of oxidation.     

Reduction rate of SiO2 in slag by carbon- saturated iron

The reduction rate of SiO₂ from CaO-SiO₂-Al₂O₃ and CaO-SiO₂-Al₂O₃-TiO₂ slags by carbon-saturated iron melts was investigated over the temperature range 1350 °C to 1600 °C under an argon atmosphere. It was found that the reduction rate of silicon increased with in-creasing temperature and decreased with increasing ratio of CaO/SiO₂ in these slags. A kinetic analysis of the experimental results developed on the basis of the two film theory showed that the silicon transport rate from slag to metal phase was controlled by the rate of chemical reaction at the slag-metal interface. The rate constants obtained for the reaction were 10 g m^-2 s^-1 at 1550 °C. The apparent activation energy was 238.0 kJ mol^-1.     

Solubilities of Chlorine in CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO Slags: Correlation Between Sulfide and Chloride Capacities

To derive a correlation between sulfide and chloride capacities through our own systematic experimental studies by using a gas equilibrium technique involving Ar-H₂-H₂O-HCl gas mixtures, the solubilities of chlorine were determined for CaO-SiO₂-MgO-Al₂O₃ slags at temperatures between 1673 K and 1823 K (1400 °C and 1550 °C). As a formula to correlate sulfide and chloride capacities, the following equation that is the function of temperature only was obtainable;

Kinetics of CO-CO<Subscript>2</Subscript> reaction with CaO-SiO<Subscript>2</Subscript>-FeOx melts

Measurements of the rate of interfacial reaction between CO₂-CO mixtures and CaO-SiO₂-FeOx slags have been made using the ¹³CO₂-CO isotope exchange technique. Ranges of slag compositions from 0 to 100 wt pct ‘FeO’ and CaO/SiO₂ between 0.3 and 2.0 were examined in the experiments. For each slag, the dependence of the apparent rate constant on temperature and equilibrium oxygen potential was studied. The relationship between the rate constant and oxygen potential was found to be in the form k _a=k _a ^o (a_o)^-α. The parameter a, with values between 0.5 and 0.9, was dependent on the slag composition. The activation energy of the reaction was independent of iron oxide content and dependent on slag basicity.     

Density Measurements of Low Silica CaO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags

Density measurements of a low-silica CaO-SiO₂-Al₂O₃ system were carried out using the Archimedes principle. A Pt 30 pct Rh bob and wire arrangement was used for this purpose. The results obtained were in good agreement with those obtained from the model developed in the current group as well as with other results reported earlier. The density for the CaO-SiO₂ and the CaO-Al₂O₃ binary slag systems also was estimated from the ternary values. The extrapolation of density values for high-silica systems also showed good agreement with previous works. An estimation for the density value of CaO was made from the current experimental data. The density decrease at high temperatures was interpreted based on the silicate structure. As the mole percent of SiO₂ was below the 33 pct required for the orthosilicate composition, discrete \textSiO₄^{4 -} {\text{SiO}}_{4}^{4 - } tetrahedral units in the silicate melt would exist along with O^2– ions. The change in melt expansivity may be attributed to the ionic expansions in the order of

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.     

Experimental Studies on the Sulfide Capacities of CaO-SiO<Subscript>2</Subscript>-CrO<Subscript>x</Subscript> Slags

To understand the desulfurization process during the refining of Cr-containing steel grades, this work was initiated to study the reactions between Cr-sulfur and chromium-containing slags. The sulfide capacities of CaO-SiO₂-CrO_x pseudo-ternary slags were measured using the traditional gas-slag equilibration technique between 1823 K and 1923 K. Sixteen different slag compositions were examined, and two different equilibrium oxygen partial pressures were used to understand the impact of the varying valence of Cr on the sulfide capacities. The results showed that log₁₀ Cs varied linearly with the reciprocal T, and the slope was higher than the corresponding value reported for the binary CaO-SiO₂ of corresponding composition. It was difficult to isolate the relative effects of the bi- and trivalent Cr in the slags because the Cr²⁺/Cr³⁺ ratio was influenced by the basicity of the slag. By using the equation developed by these authors earlier that related Cr²⁺/Cr³⁺ with basicity, oxygen partial pressure, and temperature, it was possible to obtain an approximate trend of the CrO effect on the sulfide capacities; viz. the sulfide shows a decreasing trend as Cr²⁺ replaces Ca²⁺ in the slag. With a continued increase of Cr²⁺ content, indications of the occurrence of a minimum point were observed; beyond which the sulfide capacities showed a slight increasing trend. The latter was attributed, based on slag-structure analysis by Gaskell et al., to the increasing extent of the polymerization reaction releasing oxygen ions for sulfide reactions.     

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.     

<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.     

The Thermal Stability of R<Subscript>3</Subscript>Pt<Subscript>4</Subscript> Compounds

Alloys of the rare earths R (including La, Ce, Pr, Nd, Sm, Gd, Tb, Dy, Y, Ho, Er) with platinum, having the composition R₃Pt₄, have been synthesized and investigated by X-ray diffraction (XRD) and differential thermal analysis (DTA). At temperatures above about 900 °C and below 250 °C, all the single phases R₃Pt₄ are formed, which crystallize with the same structure of the rhombohedral Pu₃Pd₄ type. Over the temperature range of about 250 °C to 900 °C, they occur at an eutectoid decomposition into RPt and RPt₂ compounds neighboring in the corresponding phase diagram, R₃Pt₄ → RPt + RPt₂. The stability of these phases R₃Pt₄ may be restricted to a radius ratio r _R/r _Pt range of 1.27 to 1.35.     

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: