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.     

Thermodynamics of iron-aluminum alloys at 1573 K

The activities of iron (Fe) and aluminum (Al) were measured in Fe-Al alloys at 1573 K using the ion-current-ratio technique in a high-temperature Knudsen cell mass spectrometer. The Fe-Al solutions exhibited negative deviations from ideality over the entire composition range. The activity coefficientsγ _Fe, andγ _A1 are given by the following equations as a function of mole fraction (x _Fe,x _Al): 1 $$\begin{gathered} 0< \chi _{A1}< 0.4 \hfill \\ ln \gamma _{Fe} = - 4.511 ( \pm 0.008)\chi _{A1}^2 \hfill \\ ln \gamma _{A1} = - 4.462 ( \pm 0.029)\chi _{Fe}^2 + 0.325( \pm 0.013) \hfill \\ 0.6< \chi _{A1}< 1.0 \hfill \\ ln \gamma _{Fe} = - 4.065 ( \pm 0.006)\chi _{A1}^2 + 0.099( \pm 0.003) \hfill \\ ln \gamma _{A1} = - 4.092 ( \pm 0.026)\chi _{Fe}^2 + 0.002( \pm 0.001) \hfill \\ \end{gathered} $$ The results showed good agreement with those obtained from previous investigations at other temperatures by extrapolation of the activity data to 1573 K.     

Phase equilibrium in the system ZrO2 + MgO + P2O5 at 1573 K

Phase equilibrium in the system ZrO₂ + MgO + P₂O₅ near ZrO₂—MgO edge was established at 1 573 K. Within the composition range investigated pseudoternary compounds were not detected, while two compatibility triangles for ZrO₂ + MgO + 3MgO · P₂O₅ and ZrO₂ + 3 MgO · P₂O₅ + 2ZrO₂ · P₂O₅ were determined.     

The solubility of Cr<Subscript>2</Subscript>O<Subscript>3</Subscript> in calcium ferrite slags at 1573 K

Continuous converting of copper matte based on calcium ferrite slag has many attractions for the copper smelting industry. However, a serious drawback is that this slag leads to shorter furnace campaigns because it is aggressive toward the magnesia-chromia refractories that form the furnace lining. As part of an investigation into the causes of this aggressiveness with a view to devising strategies to mitigate it, the solubility of Cr₂O₃ in calcium ferrite slag has been determined. The standard drop-quench experimental technique was employed at a temperature of 1573 K and a relatively high oxygen partial pressure of 3.7×10⁻⁴ atm, conditions similar to those used in continuous converting. It was found that approximately 2 wt pct of Cr₂O₃ can dissolve in calcium ferrite slag under these conditions. The Cr₂O₃ solubility was found to be unaffected by the Cu₂O content of the slag, but may decrease as CaO content decreases. The implications of these findings on the mechanism of attack of magnesia-chromia refractories by calcium ferrite slag are discussed.     

Phase relations in the system CaO-SiO2-ZrO2 at 1573 K

Zirconia-based solid electrolytes with zircon (ZrSiC₄) as the auxiliary electrode have been suggested of sensing silicon concentrations in iron and steel melts. A knowledge of phase relations in the ternary system MO-SiO₂-ZrO₂ (M = Ca, Mg) is useful for selecting an appropriate auxiliary electrode. In this investigation, an isothermal section for the phase diagram of the system CaO-SiO₂ZrO₂ at 1573 K has been established by equilibrating mixtures of component oxides in air, followed by quenching and phase identification by optical miroscopy, energy disperse analysis of X-rays (EDAX) and X-ray diffraction analysis (XRD). The equilibrium phase relations have also been confirmed by computation using the available thermodynamic data on condensed phases in the system. The results indicate that zircon is not in thermodynamic equilibrium with calcia-stabilized zirconia or calcium zirconate. The silica containing phase in equilibrium with stabilized zirconia is Ca₃ZrSi₂O₉. Calcium zirconate can coexist with Ca₃ZrSi₂Og and Ca₂SiO₄.     

Thermodynamics of the system iron-aluminum-oxygen between 1073 K and 1573 K

Equilibrium oxygen pressures of the univariant ternary coexistences hematite + spinel + alumina, alloy + wustite + spinel, and alloy + spinel + alumina were determined at temperatures in the range 1123 to 1423 K using solid state electrochemical cells. The compositions of alloys and oxide phases of this Fe-Al-0 system that equilibrated with one another at 1273, 1423, and 1573 K were determined. These results and those from the literature are used to determine stabilities of the ternary oxide phases as a function of oxygen pressure and composition by constructing equilibrium oxygen pressure diagrams at 1073, 1173, and 1273 K. Formerly with McMaster University     

Thermodynamics of iron oxide in Fe x O-dilute CaO + Al2O3 + MgO + Fe x O slags at 1873 K

In order to determine the ferrous and ferric ion capacities: 3

Experimental determination of sulphide capacities of blast furnace slags with higher MgO contents

Sulphide capacity measurements of slag with MgO content up to 18?mass% were carried out at 1713, 1743 and 1773?K to obtain reliable data for the blast furnace process. In the measurement, the slag is equilibrated with copper at a controlled oxygen partial pressure for 24?h. The sulphide capacities are calculated based on the sulphur analyses for both slag and copper.     

The effect of MgO on liquidus temperatures of fayalite slags

The effects of minor MgO additions on the liquidus temperatures of fayalite slag have been investigated experimentally in equilibrium with metallic iron. The synthetic slags were equilibrated, quenched, and subsequently examined by using optical microscopy and electron probe X-ray microanalysis. Liquidus isotherms in the fayalite primary field and boundary lines were determined in the multicomponent systems MgO-“FeO”-CaO-SiO₂ and Al₂O₃-MgO-“FeO”-CaO-SiO₂ in equilibrium with metallic iron. The experimental results show that the magnesia addition expands the fayalite primary phase field in size and increases the liquidus temperatures in the fayalite primary phase field. It has been found that the effects of alumina and magnesia additions on the liquidus temperatures in the fayalite primary phase field are independent of each other.     

Thermochemistry of calcium oxide and calcium hydroxide in fluoride slags

Calcium oxide activity in binary CaF₂-CaO and ternary CaF₂-CaO-Al₂O₃ and CaF₂-CaO-SiO₂ slags has been determined by CO₂-slag equilibrium experiments at 1400 °C. The carbonate ca-pacity of these slags has also been computed and compared with sulfide capacity data available in the literature. The similarity in trends suggests the possibility of characterizing carbonate capacity as an alternative basicity index for fluoride-base slags. Slag-D₂O equilibrium experi-ments are performed at 1400°C with different fluoride-base slags to determine water solubility at two different partial pressures of D₂O, employing a new slag sampling technique. A novel isotope tracer detection technique is employed to analyze water in the slags. The water solubility data found show higher values than the previous literature data by an order of magnitude but show a linear relationship with the square root of water vapor partial pressure. The activity of hydroxide computed from the data is shown to be helpful in estimating water solubility in in-dustrial electroslag remelting (ESR) slags. Formerly Graduate Student, University of British Columbia.     

Measurement of FeO activity and solubility of MgO in smelting slags

In bath smelting, the FeO activity of the slag must be known to predict the equilibrium of slag-metal reactions and for effective control of the rate of reduction in the system. Also, knowledge of the solubility of MgO in these slags is useful for reducing refractory consumption. A series of measurements of the FeO activity in simulated bath smelting slags (CaO-SiO₂-Al₂O₃-MgO_sat-FeO) were conducted by the electromotive force (EMF) technique. The influence of the slag composition on the relationship between the FeO activity coefficient and FeO content was studied. It has been found that the measured FeO activity coefficient decreases with increasing FeO content in the slag and increases slightly with increasing slag basicity, which is defined as (CaO + MgO)/(SiO₂ + Al₂O₃) on a mole fraction basis. The measured values of the FeO activity coefficient are in reasonable agreement with previously published data. The solubility of MgO was also measured and found to rang from 16 to 30 pct and decrease with increasing basicity.     

Observation of calcium aluminate inclusions at interfaces between Ca-treated,Al-killed steels and slags

The evolution of Al₂O₃-CaO inclusions on molten steel surfaces and at molten steel/slag interfaces was observed in-situ through a confocal scanning laser microscope (CSLM) equipped with a gold-image furnace. Depending on the slag chemistry, some of the initially liquid inclusions evolved into irregular Al₂O₃ or SiO₂-enriched inclusions during the separation across the interface. Inclusions were found to cluster at specific locations at the steel/slag interface. Unlike capillary-depression-driven clustering, which is observed on molten steel surfaces, a weak repulsive force opposes fluid-flow-driven clustering at the steel/slag interface. After clustering, the irregular solid inclusions were observed to agglomerate to form large aggregates. This article is based on a presentation given in the Mills Symposium entitled “Metals, Slags, Glasses: High Temperature Properties & Phenomena,” which took place at The Institute of Materials in London, England, on August 22–23, 2002.     

Liquidus temperatures in calcium ferrite slags in equilibrium with molten copper

Experimental laboratory methods have been developed that enable phase-equilibria studies to be carried out on slags in the system Ca-Cu-Fe-O in equilibrium with metallic copper. These techniques involve equilibration at temperature, rapid quenching, and chemical analysis of the phases using electron-probe X-ray microanalysis (EPMA). Equilibration experiments have been carried out in the temperature range of 1150 °C to 1250 °C (1423 to 1523 K) and in the composition range of 4 to 80 wt pct “Cu₂O,” 0 to 25 wt pct CaO, and 20 to 75 wt pct “Fe₂O₃” in equilibrium with metallic copper. Liquidus and solidus data are reported for the primary-phase fields of spinel (magnetite) and dicalcium ferrite. The resulting data have been used to construct liquidus isotherms of the CaO-“Cu₂O”-“Fe₂O₃” system at metallic copper saturation.     

Vaporization studies of calcium arsenate under neutral conditions and in contact with copper pyrometallurgical slags

. The feasibility of capturing As as calcium arsenate in reverberatory and Mitsubishi converter slags used in copper pyrometallurgy was studied by torsion-effusion, thermogravimetric, and mass spectrometric methods. In the temperature range from 1376 to 1489 K pure Ca₃ (AsO₄)₂ decomposed to yield very low equilibrium pressures of AsO and O₂. Mixtures of Ca₃ (AsO₄)₂ with slag exhibited rapid mass losses between 973 and 1173 K. The vapor phase species evolved under these conditions were As₂ and As₄, indicating that the arsenate had been reduced by the slag to volatile elemental arsenic.     

Thermodynamics of phosphorus and sulfur in the BaO-MnO flux system between 1573 and 1673 K

The oxidative dephosphorization of carbon-saturated Fe-Mn alloys was successfully demonstrated by measuring the phosphorus partition ratio between BaO-MnO, BaO-MnO-BaF₂, and BaO-CaO_satd.-MnO fluxes and liquid Fe-Mn-C_satd. alloy between 1573 and 1673 K. The phosphorus partition ratio increases with increasing BaO content of the flux. The phosphate capacity of the BaO-MnO flux is as high as that of the BaO-BaF₂ flux and is far larger than those of CaO-bearing fluxes. Addition of BaF₂ to the BaO-MnO flux increases BaO solubility, which increases the phosphate capacity. The manganese partition ratios between the BaO-MnO flux and Fe-Mn-C_satd. alloy were approximately constant at 0.64, 0.33, and 0.23 at 1573, 1623, and 1673 K, respectively. The carbon content of the BaO-MnO flux was measured as functions of slag composition, temperature, and partial pressure of CO. A stable species of carbon in the BaO-MnO flux was found to be BaC₂ experimentally and thermodynamically. The sulfide capacity of the BaO-MnO system at 1573 K has been shown to be far larger than any known flux systems and to be a few times larger than that for the BaO-BaF₂ system. Formerly Graduate Student, Department of Metallurgy, The University of Tokyo