A thermodynamic study of silica-saturated iron silicate slags in equilibrium with liquid copper

The thermodynamic properties of silica-saturated iron silicate slags in equilibrium with liquid copper have been studied from oxygen partial pressure measurements in the temperature range from 1490 to 1580 K by means of a solid electrolyte galvanic cell. The following cells were used: Pt, Ni-NiO/O⁼/slag-Cu(l), Cr₂O₃, Pt; Pt, Fe-FeO/O⁼/slag-Cu(Fe sat.), Fe. A strong correlation was found between oxygen pressure and the copper content of the slag; the copper content increased from less than 1 pct near iron saturation to about 4 pct at an oxygen partial pressure of 7.2 x 10^?3 Pa. A similar correlation was found between the ferric iron/total iron ratio and the oxygen pressure. The oxygen content in liquid copper decreased with increasing iron content in liquid copper and increased slightly near iron saturation. This behavior could be explained qualitatively by using the standard free energy of formation of FeO and the activities of components.     

Activities of titanium in molten copper at dilute concentrations measured by solid- state electrochemical cells at 1373 K

In order to obtain the activities of titanium in molten copper at dilute concentrations,i.e., between 5 x 10^-6 and 3.4 x 10^-3 titanium mole fractions, liquid copper was brought into equilibrium with molten {CaCl₂ + Ti₂O₃} slag saturated with Ti₂O₃ (s) at 1373 K and the equilibrium oxygen partial pressures were measured by means of a solid-oxide galvanic cell of the type Mo/Mo + MoO₂/ZrO₂(MgO)/(Cu + Ti)_alloy + Ti₂O₃ + CaCl₂ + Ti₂O_{3 slag}/Mo The free energy change for the dissolution of solid titanium in molten copper at infinite dilution referred to 1 wt pet was determined as Ti (s) = Ti(1 wt pet in Cu) ΔG°/J = -86,100 ± 8900 at 1373 K     

Lead solubility in FeO x -CaO-SiO2 slags at iron saturation

Experiments have been carried out to determine the equilibria between FeO _x -CaO-SiO₂ slag and lead metal in iron crucibles at temperatures ranging from 1473 to 1573 K. It has been found that the highest lead solubilities are observed in the silica-saturated iron silicate slags, while the lowest solubilities are observed in the CaO-saturated calcium ferrite slags. The activity coefficient of PbO varies from 0.15 to 3, depending on the slag composition. Changes in temperature do not have a significant impact on the activity coefficient. The activity of FeO and pct Fe³⁺/pct Fe²⁺ ratios have been determined as a function of slag composition. These new experimental data have been incorporated into an optimized thermodynamic slag model using the computer package FACT.     

Oxygen pressure dependence of tracer diffusivities of ca and fe in liquid CaO-SiO2-FexO system

The tracer diffusivities of calcium and iron in a steel-making slag of 33 pct CaO-27 pct SiO₂-40 pct Fe₂O₃ by charge composition have been measured at 1360 to 1460°C as a function of temperature and oxygen pressure in the gas phase. The results expressed in cm₂/s (in SI unit of m₂/s, the following equation should be divided by 10,000) are given by $$D^{tr} = D_0 [P_{O_2 } ]^{1/x} \exp \left[ { - \frac{E}{{RT}}} \right] (at 1360 to 1460^\circ C)$$ where for tracer diffusion of iron,D _o is 0.2,x is 8.5, andE is 26 kcal/mol (1.09 x 10⁴ J/ mol) and for tracer diffusion of calcium,D _o is 0.1,x is 12.5, andE is 28 kcal/mol (1.17 x 10⁴ J/mol). Prior to diffusion runs, the slag was equilibrated with the gas mixture of carbon monoxide and dioxide with an oxygen pressure of 10^-1 to 10^-8 atm. The diffusivity was measured by the instantaneous plane source method, using radioactive tracers of calcium and iron. The increase of the tracer diffusivities with the oxygen pressure was interpreted in relation to a probable increase of the divalent cation vacancies in the slag.     

Electronic and ionic transport in liquid PbO-SiO2 systems

The objective of the present work was to examine electronic transport in the PbO-SiO₂ melt. The experimental work consisted of studying the oxidation of liquid Pb covered by a layer of liquid PbO-SiO₂ using a thermogravimetric arrangement. Oxidation of liquid Pb was dependent on the melt height and was parabolic in nature, indicating the oxidation process to be diffusion controlled. We observed a moderate increase in the oxidation rate with additions of a transition metal oxide (Fe₂O₃) to the melt. However, when melt/gas and melt/metal interfaces were short circuited by Ir wires, a much higher increase in the oxidation rate was noticed. Additionally, the conductivity of the PbO-SiO₂ melt with Fe₂O₃ additions was measured as a function of P_o, to detect the nature of electronic contribution by the Fe₂O₃. Combining the results of the oxidation and conductivity experiments, we conclude that the oxidation of liquid lead covered by liquid slag occurs through ionic and rate controlling electronic (probably electron holes) transport in the melt. Uday Pal, formerly with the Department of Materials Science and Engineering, The Pennsylvania State University     

Interactions of gases in molten salts: Carbon dioxide and oxygen in cryolite alumina melts

A radiotracer method has been developed to measure the solubility of CO₂ in cryolite-based systems at temperatures above 1273 K and also a method to measure the solubility of oxygen. The high solubility of CO₂ in cryolite (0.34 pct A1₂O₃, 1.68 pct CaF₂), and the highly negativeAH of solution, suggest complex formation. In the presence of 5.33 pct excess A1₂O₃, the solubility of CO₂ is several times greater than in cryolite. In contrast, oxygen shows a solubility about a thousandth that of CO₂. The entropy of CO₂ in cryolite indicates a loss of translational degrees of freedom in a manner consistent with complexing. However, in the presence of increased A1₂O₃ (in spite of the increase of solubility) the entropy change upon solution appears to indicate little loss of translational freedom. It is suggested that CO₂ is complexed in both systems, but more strongly in the cryolite system. Complexing in cryolite produces the small carbonate (CO^- ₃)ion, and this compensates the loss of translational freedom arising from the CO₂ complexing with aluminofluorides. Measured solubilities for CO₂ were 1.0 X 10_-3 moles liter⁻¹ atm_-1 in cryolite at 1273 K, 4.6 x 10^-3 moles liter⁻¹ atm_-1 in (NaF)_2:3 A1F₃ + 5.33 wt pct A1₂O₃ at 1233 K. At 1230 K, oxygen showed a solubility of only 3.0 X 10₆ moles liter⁻¹ atm_-1 in the latter melt.     

Deoxidatlon rate of copper droplet levitated in Ar-H2 gas stream

Levitated copper droplets, 5 mm in diameter with initial oxygen contents of 0.036 to 1.9 wt pct, were deoxidized at about 1970 K in an Ar-H₂ gas stream. The Ar-H₂ gas mixture having hydrogen partial pressure less than 4 kPa was introduced into a silica reaction tube of 11-mm ID at gas flow rates up to 2 x 10^-4 Nm³s^-1. The effects of initial oxygen content of the droplets, hydrogen partial pressure, and gas flow rate on the deoxidation process were examined. A mixed control model for the deoxidation rate involving both gas and liquid film mass-transfer resistances was combined with a thermodynamic relationship for the dissolved species in molten copper. The value of 2 × 10^-4 m 73x00D7; s^-1 was assigned to the liquid film mass-transfer coefficient of dissolved oxygen throughout all experimental conditions. Under the experimental conditions of low initial oxygen content and high hydrogen partial pressure, the liquid film mass-transfer resistance was significant. When a droplet of high initial oxygen content was deoxidized, transition phenomena from gas to liquid film mass-transfer control were noticed in the later stage of reaction. It was deduced from the present model that the accumulation of dissolved hydrogen was indispensable to these phenomena. Formerly Student     

Oxygen pressure dependence of tracer diffusivities of Ca and Fe in liquid CaO-SiO2-FexO system

The tracer diffusivities of calcium and iron in a steel-making slag of 33 pct CaO-27 pct SiO₂-40 pct Fe₂O₃ by charge composition have been measured at 1360 to 1460°C as a function of temperature and oxygen pressure in the gas phase. The results expressed in cm²/s (in SI unit of m²/s, the following equation should be divided by 10,000) are given by $$D^{tr} = D_0 \left[ {P_{O_2 } } \right]^{1/\chi } \exp \left[ { - \frac{E}{{RT}}} \right](at 1360 to 1460^\circ C)$$ where for tracer diffusion of iron, D_o is 0.2,x is 8.5, andE is 26 kcal/mol (1.09 x 10⁴ J/ mol) and for tracer diffusion of calcium, D_o is 0.1,x is 12.5, andE is 28 kcal/mol (1.17 × 10⁴ J/mol). Prior to diffusion runs, the slag was equilibrated with the gas mixture of carbon monoxide and dioxide with an oxygen pressure of 10^?11 to 10^?8 atm. The diffusivity was measured by the instantaneous plane source method, using radioactive tracers of calcium and iron. The increase of the tracer diffusivities with the oxygen pressure was interpreted in relation to a probable increase of the divalent cation vacancies in the slag.     

High-temperature isopiestic studies on the ternary slag PbO-SiO2-B2O3 at 1273 K

An improved high-temperature isopiestic technique has been applied to the ternary slag PbO-SiO₂-B₂O₃ and its binary subsystems PbO-SiO₂ and PbO-B₂O₃ at 1273 K. The isopiestic measurements agree well with the literature data for the two binaries. Each entire isoactivity curve of PbO in the ternary slag was determined by only one experiment. The activities of SiO₂ and B₂O₃ were then calculated analytically. The measured and calculated results for the ternary slag approximately fit a partial ideal solution model proposed previously.     

Thermodynamic Activity of Chromium Oxide in CaO‐SiO2‐MgO‐Al2O3‐CrOx Melts

Thermodynamic activities of chromium oxide contained in CaO‐SiO₂‐MgO‐Al₂O₃ melts were measured in the present work using gas‐slag equilibrium technique. The oxygen partial pressure was varied (10^‐3, 10^‐4, 10^‐5 Pa). Gas mixture of CO, CO₂ and Ar were used and investigated at 1803, 1873 and 1923 K. The activities of CrO showed a strong positive deviation from ideality and a decrease with increasing temperature and oxygen partial pressure. A mathematical expression relating the amount of chromium oxide in the slag phase with the activity of Cr in the metal phase based on the present experimental results is presented.     

Determination of the chemical diffusion of oxygen in liquid iron oxide at 1615 °c

The chemical diffusion of oxygen in liquid iron oxide has been studied by the oxidation of a melt in a long capillary at 1615 °C. When pure oxygen was used as the oxidizing agent, the surface composition of the slag was found to be in close agreement with the expected gas-slag equilibrium, suggesting that diffusion is the controlling step. This was not the case when air, 5 pct oxygen in argon or pure CO₂ was used to oxidize the slag. The deviation of the surface composition from the expected equilibrium was in accordance with a mechanism of mixed control by both the gas-slag reaction and diffusion in the bulk. The average value of the chemical diffusivity of oxygen (or iron) in liquid iron oxide with Fe²⁺/Fe _T between 0.25 and 0.77 was established to be 3(±1) × 10^-7 m²/s. This value is one to two orders of magnitude higher than those from earlier studies. There seems to be a reasonable correlation between the chemical and the ionic self-diffusivities through the Darken equation. A quantitative analysis in this respect and on the role of electron hole migration depends on the availability of data on the ionic conductivity and the tracer diffusivities. Formerly Postgraduate Student     

Distribution equilibria of Sn,Se and Te between FeO-Fe2O3-SiO2-AI2O3-CuO0.5 slag and metallic copper

The distribution of tin, selenium and tellurium between alumina-containing fayalitic slags and metallic copper was measured at 1200 and 1300°C under controlled CO-CO₂ atmosphere with oxygen partial pressure (pO₂) in the rangePO ₂ = 10_-6 to 10_-11 atm (1 atm = 1.013 x 10² kPa). The solubility of Sn in slag was observed to increase linearly with increasing P^1/2O₂. It was deduced that Sn is present in the slag in the form of SnO or Sn² and the activity coefficient of SnO in the slag was calculated to be 1.9 at 1200°C and 0.8 at 1300°C. The solubility of Se in the slag decreases with increasing oxygen partial pressure up topO ₂l = 4 x 10₋₈ atm, but above this oxygen partial pressure it becomes practically constant and the ratio (pet Se in slag/pet Se in copper) = 0.018 (at 1200°C) and 0.036 (at 1300°C). The solubility of Te shows a similar variation with oxygen partial pressure and the ratio (pet Te in slag/pet Te in copper) = 0.026 (at 1200°C) and 0.032 (at 1300°C) abovepol ₂ = 10⁶ atm. A concept of molecular dissolution of chalcogen elements in slag was developed on the basis of thermodynamic properties of slag, and the observed solubility of Se and Te was explained in terms of the chemical stability of the molecular cluster FeSe and FeTe iη the slag. M. Nagamori is with Centre de Recherche Industrielle du Québec, Ste-Foy, Quebec and P. J. Mackey is Smelter Technical Superintendent, Noranda Mines Limited, Noranda, Quebec, Canada. Both authors were Formerly with Noranda Research Centre, Pointe Claire, Quebec.     

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.     

Water vapor solubility in ladle-refining slags

A thermo-gravimetric technique was used to determine the hydrogen solubilities of some Al₂O₃-CaO-MgO-SiO₂ quaternary slags. The focus of the work was to determine the water capacities in slags having lower SiO₂ concentrations, which were relevant to industrial practices. The majority of the experiments were carried out in the temperature interval 1747 to 1827 K with water pressure of 157 mbar. The reliability of the experiments was confirmed using a quenching technique. The temperature was found to have a negligible effect on the water solubilities. The composition of the slag did not seem to significantly affect the water capacity. However, as the slag composition approached CaO saturation, a considerable increase of the solubility was noticed.     

Reduction of MnO from molten slags with liquid steel of high carbon content

This work estimated the reduction of MnO in slags of the CaO‐SiO₂‐FeO‐CaF₂‐MnO system and liquid steel with the initial composition (mass contents) 0.75 %Mn, 0.16 % Si and 0.5 to 2.0 % C, as an alternative to introducing Mn to the steel melt. The slag basicities (CaO/SiO₂) In the experiments were 2 and 3. MnO was obtained from manganese ore. The experiments were carried out in an open 10 kg induction furnace using Al₂O₃‐based refractory at 1873 K. The oxygen potential was measured throughout the experiments with a galvanic cell (ZrO₂‐solid electrolyte with a Cr/Cr₂O₃ reference electrode). The MnO reaction mechanism was analysed in terms of the slag basicity, the silicon and the initial carbon contents in the melt. The rate and the degree of MnO reduction were found to increase with the increasing of initial carbon content; however, the effect of slag basicity was less important. A kinetic analysis of the process was performed using a coupled reaction model.     

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.     

Kinetics of reduction of lead oxide in liquid slag by carbon in iron

The reduction of lead oxide in dilute solution in CaO-Al₂O₂-SiO₃ slag by carbon dissolved in iron was investigated using a composite crucible as a container so as to exclude graphite from the system. The variables studied to elucidate the reaction mechanism were pressure inside the crucible, carbon content of the metal, lead oxide concentration in slag, and slag composition. The experimental results are best explained by postulating the existence of a gas film at the slag metal interface. It is suggested that the rate controlling step for the lead oxide reduction by carbon is a chemical reaction at the gas/slag interface. The rate constant for up to 3 wt pct PbO in the slag and 2.0 to 4.3 pct C in iron at 1400 °C as calculated from the present study is 4.6 x 10^-4 mol/cm²/min/atm.     

Dissolution behaviour of MgO based refractories in CaO–Al2O3–SiO2 slag

Abstract

Magnesium oxide (MgO) based refractories are widely used in secondary refining processes, and their dissolution into refining slag is the primary cause of their shortened lifespan. The dissolution rate was investigated for sintered MgO and commercial MgO–C and MgO–Cr₂O₃ refractories in a synthesised 50CaO–45Al₂O₃–5SiO₂ liquid (mass-%) slag. The change in slag composition was measured after a refractory sample was placed into the molten slag that was stirred by flowing argon gas at 1773?K. The dissolution rate of the sintered MgO was above those of the MgO–C and MgO–Cr₂O₃ refractories under the same gas flowrate, although the dissolution rate of all samples increased as the gas flowrate was increased from 25 to 75?mL·min^??1. The slag containing 5?mass-% FeO considerably promoted the dissolution of the MgO–C refractory because of the oxidation of carbon by FeO. The dissolution of all the refractories was greatly affected by penetration of the liquid slag, with the mass transfer of MgO in the penetrating slag at lower gas flowrates likely being the rate controlling step. At high gas flowrates, Ar bubbles covered the surface and blocked the contact between the liquid slag and the solid phase, reducing the dissolution rate.     

Activity of VO1.5 in CaO‐SiO2‐MgO‐Al2O3 Slags at Low Vanadium Contents and Low Oxygen Pressures

In the present work, the gas‐slag equilibration technique was employed for the measurement of the thermodynamic activity of vanadium oxide. The vanadium‐containing slag kept in a platinum crucible was equilibrated with a gas mixture of CO, CO₂ and Ar, with well‐defined oxygen partial pressure at a pre‐determined temperature. The slag sample was quenched and the composition of the final slag was determined by chemical analysis. From the value of the oxygen partial pressure, the thermodynamic activity of VO_1.5 could be calculated using the value for the activity of vanadium in V‐Pt alloy. The measurements were carried out in the temperature range 1823～1923K and the oxygen partial pressures employed were 10^‐3, 10^‐4, 10^‐5 Pa. The present results show that the activity of vanadium(III) oxide in slag exhibits a negative deviation from ideality in the present composition range. With increasing basicity of the slag, the final content of vanadium oxide in the slag was found to show an initial increase followed by a constant content. The activities of vanadium(III) oxide did not exhibit any significant change with increasing temperature. The activity coefficient of vanadium(III) oxide decreased sharply with slag basicity approximately up to a basicity of 1, beyond which it showed a near–constant value. Increase in basicity was found to cause a change in the distribution of vanadium between the slag and the alloy phases even though this effect was less pronounced. From the present results, a mathematical relationship for estimating the vanadium content in slag for a given activity of vanadium in the molten metal phase was developed.