Critical evaluation and optimization of the thermodynamic properties and phase diagrams of the CaO-Al2O3, Al2O3-SiO2, and CaO-Al2O3-SiO2 systems

All available thermodynamic and phase diagram data have been critically assessed for all phases in the CaO-Al₂O₃, Al₂O₃-SiO₂, and CaO-Al₂O₃-SiO₂ systems at 1 bar pressure from 298 K to above the liquidus temperatures. All reliable data for the binary systems have been simultaneously optimized to obtain, for each system, one set of model equations for the Gibbs energy of the liquid slag and all solid phases as functions of composition and temperature. The modified quasichemical model was used for the slag. With these binary parameters and those from the optimization of the CaO-SiO₂ system reported previously, the quasichemical model was used to predict the thermodynamic properties of the ternary slag. Two additional small ternary parameters were required to reproduce the ternary phase diagram and ternary activity data to within experimental error limits. The calculated optimized phase diagram and thermodynamic properties are self-consistent and are the most reliable currently available estimates of the true values.     

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     

Permeability,diffusivity, and solubility of oxygen gas in liquid slag

An experimental procedure for measurement of the permeability of dissolved oxygen gas in liquid slag has been developed using an oxygen concentration cell. The small amount of oxygen gas which penetrated through the liquid oxide from a pure oxygen compartment to a pure argon compartment was determined by the galvanic cell. The permeabilities of oxygen through liquid PbO-SiO₂ and FeO-PbO-SiO₂ were found to be in the range 3 x 10^-8 to 3 x 1O^-7 moles/cm s. The permeabilities were little influenced by temperature but more influenced by the composition. In separate experiments, the oxygen pressure change at the bottom of a column of slag was detected by another galvanic cell. By this method, it is not necessary to quench the specimen to determine the concentration profile of dissolved oxygen and to determine its diffusivity. Liquid oxides in the PbO-SiO₂, CaO-SiO₂-Al₂O₃and FeO-PbO-SiO₂ systems were studied. The oxygen diffusion coefficients (5 x 10^-5 to 3 x 10^-3 cm²/s) were found to increase with temperature for a fixed composition of slag, and with an increase of network-modifier oxide content at constant temperature. The solubility of oxygen gas in PbO-SiO₂ melts was estimated to be 2 x 10^-4 to 2 x 10^-5 moles/cm³ from the determined diffusivities and permeabilities. The solubilities decreased with increasing temperature in the composition range studied. Physical solubilities of gases and metals in slags determined by other investigators are compared with the present results.     

Interfacial effects in gaseous reduction of PbO-SiO2 melts

The reduction of PbO-SiO₂ melts by H₂-N₂ mixtures was investigated at 1173 K using a thermo-gravimetric arrangement. Some of the experiments were specially designed to assess the relative magnitudes of reaction rates at slag/gas, slag/refractory, and slag/metal interfaces. Analysis of the rate data revealed that the reaction rates at slag/gas and slag/refractory interfaces were comparable to each other. Reduction did not occur appreciably at the slag/metal interface. The concentration profiles of lead oxide in the partially reduced quenched slag samples were determined by electron microprobe. Significant concentration gradients of PbO existed at the slag/gas and slag/refractory interfaces. The diffusivity values of lead oxide in the slags were evaluated using the concentration profiles and the rate data. Additions of varied amounts of P₂O₅ to the slag were found to have a retarding effect on the reduction rate, which may be explained by a surface blockage mechanism.     

Thermodynamic properties of PbO-SiO2 slags by emf measurements

Abstract

The thermodynamic properties of PbO-SiO₂ liquid slags have been investigated by measuring the emfs of the following cell:

Pt, O₂(1 atm)/0.90 ZrO₂ + 0.10 CaO/(PbO-SiO₂)_liq.sol'n Pb_(l)

From the results, the activities and the other partial molar properties of the system have been calculated for compositions between 40 and 85 mole % PbO in the temperature range 720 to 110°C.

Résumé

Les propriétés thermodynamiques des scories liquids PbO-SiO₂ ont été recherchees en mesurant la f.e.m. de la pile suivante:

Pt,O₂(1 atm.)/0.90 ZrO₂ + 0.10 CaO/(PbO-SiO₂)_liq.sol'nPb_(l)

A partir des résultats, les activités et les autres propriétés molales partielles de ce système ont été calculées pour des compositions molales variant de 40 à 85% en PbO, sur un intelvalle de température s'etendant de 720 a 1100°C.     

Modeling the Viscosity of Silicate Melts Containing Lead Oxide

Our recently developed model for the viscosity of silicate melts is applied to describe and predict the viscosities of oxide melts containing lead oxide. The model requires three pairs of adjustable parameters that are fitted to the experimental viscosities in the following systems: pure PbO, PbO-SiO₂, and PbO-Al₂O₃-SiO₂. The viscosity of other ternary and multicomponent silicate melts containing PbO is then predicted by the model without any additional adjustable model parameters. Experimental viscosity data are reviewed for melts formed by PbO with SiO₂, Al₂O₃, CaO, MgO, Na₂O, and K₂O. The deviation of the available experimental data from the viscosities predicted by the model is within experimental error limits.     

The Viscous Behavior of FeO<Subscript>t</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-SiO<Subscript>2</Subscript> Copper Smelting Slags

Understanding the viscous behavior of copper smelting slags is essential in increasing the process efficiency and obtaining the discrete separation between the matte and the slag. The viscosity of the FeO_t-SiO₂-Al₂O₃ copper smelting slags was measured in the current study using the rotating spindle method. The viscosity at a fixed Al₂O₃ concentration decreased with increasing Fe/SiO₂ ratio because of the depolymerization of the molten slag by the network-modifying free oxygen ions (O²⁻) supplied by FeO. The Fourier transform infrared (FTIR) analyses of the slag samples with increasing Fe/SiO₂ ratio revealed that the amount of large silicate sheets decreased, whereas the amount of simpler silicate structures increased. Al₂O₃ additions to the ternary FeO_t-SiO₂-Al₂O₃ slag system at a fixed Fe/SiO₂ ratio showed a characteristic V-shaped pattern, where initial additions decreased the viscosity, reached a minimum, and increased subsequently with higher Al₂O₃ content. The effect of Al₂O₃ was considered to be related to the amphoteric behavior of Al₂O₃, where Al₂O₃ initially behaves as a basic oxide and changes to an acidic oxide with variation in slag composition. Furthermore, Al₂O₃ additions also resulted in the high temperature phase change between fayalite/hercynite and the modification of the liquidus temperature with Al₂O₃ additions affecting the viscosity of the copper smelting slag.     

Kinetics of chromite ore reduction from MgO-CaO-SiO2-FeO-Cr2O3-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Kinetic experiments were performed in an induction furnace to investigate the reduction of chromite ore by carbon dissolved in a high carbon ferrochromium alloy melt under conditions of varying Cr₂O₃ concentration, slag basicity, and temperature. The results obtained show that chromite reduction by dissolved carbon in slag systems of the type MgO-CaO-SiO₂-FeO-Cr₂O₃- Al₂O₃ occurs principally by a stagewise process encompassing an intermediate reaction in which the divalent chromium oxide species is involved. During the fast period, Cr₂O₃ reduction is controlled by the diffusion of oxygen species in the slag for which a mass transfer coefficient of 0·003 cm s^-1 was calculated. An activation energy value of 117 kJ mol ^-1 obtained for the reduction of Cr₂O₃ implies the rate controlling step is mass transfer of Cr₂O₃ from the slag to the slag/metal interface, since activation energies for metal phase control are typically <70 kJ mol ^-1. The second period represents a pseudo-equilibrium condition with respect to Cr₂O₃ reduction that is probably under thermodynamic control by a step or mechanism involving the reduction of divalent chromium oxide to chromium.     

Phase-diagram study for the Al2O3-CaF2-SiO2 system

Phase relations for the Al₂O₃-CaF₂-SiO₂ system were investigated from 1673 to 1723 K. The hotfilament technique was applied to observe a two-liquid region and liquidus for the ternary system. The liquidus saturated with SiO₂ was investigated at 1673 K by using the hot-filament technique and the chemical equilibrium technique. In this system, the addition of Al₂O₃ to the CaF₂-SiO₂ system reduces the congruent temperature. A small substitution of Al₂O₃ for CaF₂ increases the solubility of SiO₂, namely, if the region of liquid phase could be enlarged. These results suggest that Al₂O₃ would be an effective substitute of CaF₂ in slag for steelmaking.     

Desulphurisation ability of blast furnace slag containing high Al2O3 and 5 mass% TiO2 at 1773 K

The sulphide capacities (C_S) of CaO–SiO₂–Al₂O₃–MgO–TiO₂ blast furnace slags were experimentally measured at 1773?K, and the percentages of free oxygen ions (O^2?) and bridge oxygen (O⁰) of molten slag were calculated using molecular dynamics at 1773?K. The measured and calculated results were closely correlated with the change in CaO/SiO₂, MgO, and Al₂O₃ contents in the slag. The results show that C_S increases with increasing CaO/SiO₂ and MgO contents in the slag, but decreases with increasing Al₂O₃ content. The effects of O^2? and O⁰ on C_S were analysed using multiple linear regression, and results show that sulphide increases with the increase in the mass percentage of free oxygen and decreases with the increase in the mass percentage of bridge oxygen.     

The sulfur partition ratio with Fe-CSAT melts and the sulfide capacity of CaO-SiO2-Na2O- (Ai2O3) slags

The sulfur partition ratio between slag and carbon saturated iron and the sulfide capacity of CaO-Na₂O-SiO₂ slags and a 48 pet CaO-45 pet Al₂O₃-7 pet SiO₂-(Na₂O) slag have been mea-sured at 1400 °C. The addition of Na₂O to a CaO-SiO₂ slag increases the sulfur partition ratio and the sulfide capacity; however, Na₂O at low concentrations has no measurable effect on the sulfide capacity of a CaO-Al₂O₃-SiO₂ slag. To convert the sulfur partition ratio to the sulfide capacity, the oxygen potential was calculated assuming equilibrium between iron in the alloy and FeO in the slag with the activity of FeO calculated via a regular solution model. The optical basicity may be used to correlate the data, but at high Na₂O contents the data do not adhere to the correlation previously developed for CaO-based slags. Formerly Graduate Student at Carnegie Mellon University     

The electrical conductivity of some liquids in the system CaF2+CaO+Al2O3

The conductivity of slags in the binary systems CaF₂+Al₂O₃, CaF₂+CaO and the ternary system CaF₂+CaO+Al₂O₃ has been measured, using a four-lead electrode technique at a frequency of 1 kHz. The cell design used ensured that only molybdenum metal was in contact with the slag at high temperature and that the slag was wholly contained in molybdenum. No frequency dispersion could be detected at frequencies between 0.8 to 10 kHz. It is suggested that the formation of complex ions (e.g. AlO₂F₂ ³⁻ and AlOF ₂ ⁻ ) might account for the observed effects in CaF₂+Al₂O₃, CaF₂+CaO+Al₂O₃ liquids as has been previously suggested. an oxide ion clustering mechanism may explain the conductivities found in CaF₂+CaO.     

Activity Calculation Model for Ternary Slag System of Al2O3-BaO-B2O3

According to the ion and molecule coexistence theory, the activity model of Al₂O₃-BaO-B₂O₃ ternary slag system was established, and the influences of BaO/Al₂O₃ molar ratio, B₂O₃ mole fraction and temperature on the activity of the slag system were investigated. Finally, the equal activity curves were drawn with the model results. The results show that with the increase of BaO/Al₂O₃ ratio, the activity of Al₂O₃ is significantly reduced, the activity of BaO ? Al₂O₃ is increased obviously, and the activity of 2Al₂O₃ ? B₂O₃ is also decreased. With the increase of B₂O₃ mole fraction, the activity of BaO ? Al₂O₃ decreased significantly, while the activities of BaO ? 2B₂O₃ and 2Al₂O₃ ? B₂O₃ increased. In addition, the influence of temperature on the activities of different components is comparatively smaller than the influence of BaO/Al₂O₃ ratio and B₂O₃ mole fraction.     

Activity of components in the Al2O3-CaO system

When aluminum is used as a reducing agent in the production of low-carbon ferrochrome, Al₂O₃-CaO slag is formed. In thermodynamic simulation of the Al₂O₃-CaO system by Terra software, the composition of oxide melt is calculated within the approximation of associated solutions. Thermochemical data for the associates in the Al₂O₃-CaO system are systematized and corrected. The calculated and experimental results for the thermodynamic activities of the individual oxides Al₂O₃ and CaO are in agreement.     

Viscosity of PbO-SiO2 melts

Viscosities (η) of PbO-SiO₂ melts that contained 25.0 to 48.8 mole pct SiO₂ were measured at temperatures 928 to 1273 K by a rotating cylinder method. The data were analyzed in terms of the conventional polymer theory. The results followed Arrhenius behavior in the different temperature ranges despite the general belief of non-Arrhenius behavior of viscosity in slag melts. The calculated activation energies were a function of temperature and composition of the melts. The activation energies for viscous flow, in general, at above approximately 850 °C were lower than those below 850 °C and varied between 25 and 150 kJ per mole. Formerly with National Research Council of Canada, Atlantic Research Laboratory, Halifax, NS, Canada     

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     

Evaluation of the activity and molecular form of bi in cu smelting slags: Part I. ternary silicate slags

The thermodynamic behavior of bismuth in the chemical systems associated with copper processing is not well understood. This study was designed to further the understanding of the physical chemistry of bismuth in slags that have similar compositions to those found in copper extractive metallurgical processing. The silicate system investigated was the FeO-Fe₂O₃-SiO₂ ternary system in which bismuth was dissolved using an isopiestic experimental technique. Bismuth vapor pressures of 1 ? 10^?5 atm and 7.5 ? 10^?4 atm were used, and the silicates were equilibrated with this vapor at temperatures of 1458 K and 1523 K. In these experiments, the slag composition was varied such thatP _{O 2} ranged from 10^?12 to 10^?8 atm. Bismuth was found to enter the silicate slag in both neutral and oxidic molecular forms. The oxidic form identified was that of BiO. The data suggest that the activity coefficient of neutral bismuth, γ_Bi, is dependent on the solubility of that species in slag, even at the low concentrations observed in this study. It has been hypothesized, based on the large diameter of neutral Bi, that only a limited number of sites are available to accommodate neutral Bi, and that as the limit is approached γ_Bi increases significantly. That hypothesis is shown to be consistent with the experimental results obtained in the present work as well as the results obtained by other investigators.     

Effect of CaF2 and SiO2 modification on phosphorus utilisation in P-bearing slag

Abstract

In order to recycle phosphorus in P-bearing converter slag for slag phosphate fertiliser, the effect of CaF₂ and SiO₂ modification on P-bearing steelmaking slag on phosphorus-bearing phases, P₂O₅ solubility and magnetic separation behaviour has been researched. The results show that the phosphorus-rich phase is mainly n2CaO·SiO₂–3CaO·P₂O₅ (nC₂S–C₃P for short) and exists along with fluorapatite phase [Ca₅(PO₄)₃F] after the addition of CaF₂. The fluorapatite cannot be dissolved in 2% citric acid solution, which decreases the P₂O₅ solubility in slag. Although adding CaF₂ can increase the P₂O₅ content of phosphorus-rich phase, it can improve its metallisation and magnetisation of RO phase, increase the amount of recovered magnetic substances, and separate incompletely phosphorus and iron, therefore it is adverse to the phosphorus recovery from P-bearing slag by magnetic separation. P₂O₅ content of phosphorus-rich phase is more than 31% and P₂O₅ solubility of slag is increased to 96% in fluorine-free modified slag, which can meet the requirement of P₂O₅ content and P₂O₅ solubility of the phosphate fertiliser. Meanwhile, 87.1% of the phosphorus in the fluorine-free modified slag is contained in non-magnetic substance, and most of the phosphorus was recycled.     

Evaluation of the activity and molecular form of bi in cu smelting slags: Part I. ternary silicate slags

The thermodynamic behavior of bismuth in the chemical systems associated with copper processing is not well understood. This study was designed to further the understanding of the physical chemistry of bismuth in slags that have similar compositions to those found in copper extractive metallurgical processing. The silicate system investigated was the FeO-Fe₂O₃-SiO₂ ternary system in which bismuth was dissolved using an isopiestic experimental technique. Bismuth vapor pressures of 1 • 10⁻⁵ atm and 7.5 • 10⁻⁴ atm were used, and the silicates were equilibrated with this vapor at temperatures of 1458 K and 1523 K. In these experiments, the slag composition was varied such thatP _{O 2} ranged from 10⁻¹² to 10⁻⁸ atm. Bismuth was found to enter the silicate slag in both neutral and oxidic molecular forms. The oxidic form identified was that of BiO. The data suggest that the activity coefficient of neutral bismuth, γ_Bi, is dependent on the solubility of that species in slag, even at the low concentrations observed in this study. It has been hypothesized, based on the large diameter of neutral Bi, that only a limited number of sites are available to accommodate neutral Bi, and that as the limit is approached γ_Bi increases significantly. That hypothesis is shown to be consistent with the experimental results obtained in the present work as well as the results obtained by other investigators.     

Polythermal sections of the Al2O3-ZrO2-Y2O3 phase diagram

Polythermal sections of the Al₂O₃-ZrO₂-Y₂O₃ phase diagram confirm that the system is quasibinary and enable one to give a complete phase diagram for the system. The ternary eutectic Al₂O₃ + ZrO₂ (solid solution) + Y₃Al₅O₁₂ has the lowest melting point in the system, whose temperature is 1715°C. No single-phase regions for the ternary solid solutions have been identified, which indicates that Al₂O₃ is not soluble in solid solutions based on F-ZrO₂ and C-Y₂O₃. This gives theoretical evidence for the scope for making new ceramics in this ternary system, which may have a high level of mechanical characteristics.