Cryoscopic studies in fluoride-Oxide-Silica systems: Part II. systems containing Mg2+, Ca2+, Ba2+ and Pb2+

The depressions of the freezing temperatures of MgF₂, CaF₂, and BaF₂ by adding 2MO · SiO₂, 3MO · 2SiO₂, MO · SiO₂, 2MO · 3SiO₂, and MO · 2SiO₂ where M = Mg, Ca and Ba, have been measured, as have the depressions of the freezing temperature of PbF₂ resulting from additions of 2Pb · SiO₂, 3PbO · 2SiO₂ and PbO · SiO₂. The variations of the activities of the fluorides with liquidus composition have been calculated. These are shown to be in good agreement with a proposed theoretical model of the constitutions of these melts. In the alkaline earth systems with MO/SiO₂ > 1.5 linear chain silicate ions and free F⁻ ions are postulated and in melts of MO/SiO₂ < 1.5 reaction between F⁻ and silicate ions to form polyfluorosilicate anions is postulated. These conclusions are in agreement with those drawn from infrared absorption studies of CaF₂-CaO-SiO₂ glasses. The activity behavior in the reciprocal systems MF₂-M′O · SiO₂ and M′F₂-MO · SiO₂ is explained in terms of polymerization and preferred ionic association effects within the melts. In the lead fluoride-silicate systems fluorination of the silicate ions occurs at PbO/SiO₂ = 2 and, in contrast with the alkali and alkaline earth systems, it appears that polyfluorosilicate anions and free O²⁻ anions can coexist in lead fluorosilicate melts.     

Sulfide capacities of Na2O−SiO2 melts

Sulfide capacities of Na₂O−SiO₂ melts at 1473, 1523, 1573, 1623, and 1673 K were calculateda priori using the revised Reddy Blander model. An expression forC _S in the composition range of 0≤X _{SiO 2}<1.0 was derived. Our predictions ofC _S values are in very good agreement with the experimental data available in the range of 0<X _{SiO 2}<0.8. The sulfide capacities of slags are found to be directly related to two independent quantities: the equilibrium constant K and the activity of the base oxide.     

Thermodynamic study of Na2O-SiO2 melts at 1300° and 1400 °C

The vapor pressures of Na above stirred Na₂O-SiO₂ melts in equilibrium with graphite and CO were determined at 1300° and 1400 °C using the transpiration technique. Compositions studied ranged from about 60 mole pct SiO₂ to close to SiO₂ saturation. Activities of components Na₂O and SiO₂ were calculated from the data. Log a_Na2_O (pure liquid as standard state) varies from about −8.7 and −8.5 at silica saturation to −6.3 and −6.1 at 40 mole pct Na₂O at 1300° and 1400 °C, and the molar Gibbs energy of mixing, ΔG _m, at the disilicate composition (X_Na2_O = 0.33) at each of these temperatures is −83.0 and −85.4 kJ, respectively. The Toop and Samis, Yokokawa and Niwa, and Lin and Pelton solution models for binary silicates were applied to the ΔG _m data at 1350 °C and parameters for the models were estimated to give best fits. All three models show good correspondence with the measured ΔG _m curve. The capabilities of the models in predicting activity data in this system have been compared. D. N. Rego, Formerly Graduate Student at Carnegie-Mellon University, G.K. Sigworth, Formerly with Carnegie-Mellon University,     

Cryoscopic studies in fluoride-oxide-silica systems: Part I. systems containing Li+, Na+ and K+

The depressions of the freezing temperatures of NaF and KF by additions of 3M₂O · 2SiO₂, M₂O -SiO₂, M₂O · 2SiO₂ and SiO₂ (where M = Li, Na and K) have been measured and the variations of the activities of NaF and KF with liquidus composition have been calculated. These activities are shown to be in good agreement with a proposed theoretical model of the anionic constitutions of these melts. In melts of M₂O/SiO₂ > 1 linear chain silicate ions and free F⁻ ions are postulated and in melts of M₂O/SiO₂ < 1 reaction between F⁻ and silicate ions to produce polyfluorosilicate anions is postulated. The effect of alkali cation type on the deviation of the liquidus activities from the proposed ideal behavior in melts of M₂O/SiO₂ > 1 and the degrees of polymerization of the fluorosilicate ions in melts of M₂O/SiO₂ < 1 are explained in terms of coulombic interactions and the effects of cation size on the polarization behavior. In the reciprocal systems MF-M₂ ^′O · SiO₂ and MF-M₂ ^′O · 2SiO₂ deviations from proposed ideality are correlated with the signs and magnitudes of the standard free energy changes for the exchange reactions. The conclusions as to the dependency of solution mechanism of fluorides in their alkali silicates on M₂O/ SiO₂ ratio are in agreement with conclusions drawn from studies of infrared absorption in these systems.     

Modeling of sulfide capacities of silicate melts

An examination of the solution chemistry of sulfides in silicates leads to a method for the calculation of the sulfide capacities of silicate melts using the Flory model for polymeric silica chains. Calculations for the CaO−SiO₂, FeO−SiO₂, and MgO−SiO₂ binary systems at 1773 K and 1923 K are shown to be in good agreement with available experimental data and to be more reliable than calculations based on the empirical concept of basicity.     

The kinetics of oxidation of iron-silicon and iron-aluminum alloy melts by pure oxygen or oxygen-bearing gases

The kinetics of oxidation of Fe−Si and Fe−Al melts by pure oxygen, and that of pure Fe by He−O₂, N₂−O₂, or Ar−O₂ mixtures have been investigated by a modified Sieverts' method at 1600°C. Considerable decrease in the oxidation rate has been observed for the alloy melts containing a few percent of Si or Al since formation of a silica- or alumina-rich oxide layer on the melts prevents further progress of the exothermic chemical reaction. The oxidation rate for melts high in Al has been considered to be limited by the diffusion of ions through the oxide layer. Addition of diluents to O₂ markedly and continuously decreases the oxidation rate of a pure Fe melt. The latter rate has been show to be controlled by the diffusion of O₂ across the gaseous boundaries at gas/melt interfaces.     

Solubilities of carbon dioxide in sodium silicate melts

Carbonate solubilities in Na₂O-SiO₂ melts were measured over the composition range X_{Na 2}O/ (X_{Na 2}O + X_{SiO 2}) = 0.5 to 1.0 and the temperature range 1100 to 1300 ‡C. The solubility increased with increasing X_Na2_O and decreased with increasing temperature. Carbonate capacities calculated from the experimental results compared favorably with values for sulfide and phosphate capacities obtained from the literature. In addition, an excellent correlation was obtained between carbonate capacity and the activity of sodium oxide. The carbonate capacity, which is an easier parameter to obtain, is a good measure of the basicity of sodium silicate melts. It would appear that carbonate capacity could be an excellent basicity index for iron and steelmaking slags as well as for fluxes used in other high temperature technologies. Formerly with University of Toronto. Formerly with the Department of Metallurgy, University of Tokyo.     

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.     

Raman spectroscopy of borosilicate and germanate-silicate glasses and melts

33 mol % Na₂O · 67 mol % SiO₂ + xB₂O₃ (GeO₂) glasses and melts with x = 0 and 20 mol % are studied by high-temperature Raman spectroscopy. The laws of formation of the anion structure of glasses and melts of borosilicate and germanate-silicate systems are shown to be different. The dominant anion groups in the systems are identified, and the structural changes occurring with increasing temperature and during a glass-melt transition are detected.     

Thermodynamic behavior of Na2O-B2O3 melt

The activity of Na₂O in a Na₂O-B₂O₃ melt was measured at 1373 K by a chemical equilibration technique to understand the thermodynamic behavior of this slag system. Also, the activity coefficient of Na in Ag was preliminarily measured as fundamental thermodynamic data, to estimate the activity of Na₂O in the slag. Sodium in the silver melt, within the present concentration range, exhibits the Henrian behavior, and the Henrian activity coefficient of Na in Ag (γ ⁰ _Na) is estimated to be 37.5 at 1373 K, indicating a positive deviation from ideality. The activity of Na₂O in the slag varies from 1.86×10⁻⁶ to 4.99×10⁻⁴ when its content is increased from 11.0 to 64.9 mol pct; this indicates a significant negative deviation from ideality. The excess stability does not exhibit any pronounced peak, despite being drastically changed at specific slag compositions. Comparing the molar Gibbs free energy of mixing in various binary slags, Na₂O was considered to be more basic than BaO (CaO) was, followed next by MgO; also, P₂O₅ would be significantly more acidic than SiO₂ would be, followed next by B₂O₃. Comparing the heats of formation of solid compounds in binary slags, the larger the differences in the electronegativity values between slag components, the more the relative ionic characters of the melts seemed to be.     

Temperature dependence of the refractive index of Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Na<Subscript>2</Subscript>O-SiO<Subscript>2</Subscript> melts: Role of electronic polarizability of oxygon controlled by network structure

Refractive indexes for the Al₂O₃-Na₂O-SiO₂ system have been measured using an ellipsometer for a wavelength of 632.8 nm over a wide temperature range (1100 to 1800 K). Two kinds of sample were used: xAl₂O₃-(40-x)Na₂O-60SiO₂ and yAl₂O₃-yNa₂O-(100-2y)SiO₂, where x ranged between 6 and 20 mol pct and y between 12.5 and 25 mol pct. In the former samples, the temperature coefficient of refractive indexes changed from negative to positive on increasing the concentration of Al₂O₃. In the latter samples, the refractive indexes increased monotonically with decreasing concentration of SiO₂, and the temperature coefficient was always positive. It has been found that the temperature dependence of refractive indexes in these melts is determined by the coefficient of thermal expansion, which would be relevant to the degree of polymerization of the melts. In addition, the electronic polarizability of oxygen derived from the refractive indexes increased with increasing temperature in each melt. This suggests that the basicity of the alumino-silicate melts increases as temperature increases. The positive temperature coefficient of the electronic polarizability of oxygen can be attributed to an increase in the distance between cation and oxygen ion due to thermal expansion. The dependence of the electronic polarizability of oxygen on the concentration of Al₂O₃ has also been discussed in terms of the electronic polarizabilities of three types of oxygen contained in the melts. 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.     

Solubility of some transition metal oxides in cryolite-alumina melts: Part II. Solubility of TiO2

The solubility of TiO₂ in cryolite-alumina melts at 1020 °C was measured; it decreased with increasing alumina concentration up to ∼3.5 wt pct total oxide and then increased at higher alumina concentrations. The solubility was found to be 3.1 wt pct Ti in cryolite, and 2.7 wt pct Ti in an alumina-saturated melt. Modeling indicated that the most probable titanium species are TiOF₂ and Na₂TiO₃, which coexist in the solution; the former dominates at low alumina concentrations and the latter at high alumina concentrations. Additional unknown amounts of fluoride may also be associated with these species. Determination of the solubility of TiO₂ in alumina-saturated melts as a function of temperature showed that the solubility increased from 1.9 wt pct Ti at 975 °C to 2.8 wt pct Ti at 1035 °C, the apparent partial molar enthalpy of dissolution of TiO₂ being 88±4 kJ mol⁻¹.     

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.     

Computer study of structure, thermodynamic, and electrical transport properties of Na3AlF6-Al2O3 and CaF2-Al2O3 melts

Structure, thermodynamic, and electrical transport properties of Na₃AlF₆-Al₂O₃ and CaF₂-Al₂O₃ melts were examined by molecular dynamics. Ionic models were constructed for Na₃AlF₆-Al₂O₃ and CaF₂-Al₂O₃ melts at 1283 and 2000 K, respectively. It was found that in the Na₃AlF₆-Al₂O₃ melts, stable aluminum-fluorine-oxygen groups are formed. Although bonds between F⁻ and Al³⁺ ions in the first coordination shell are weaker than between O²⁻ and Al³⁺ ions, very stable negatively charged AlF ₆ ³⁻ groups are formed at low oxygen concentrations in the Na₃AlF₆-Al₂O₃. This results in migration of aluminum to the anode in an external electric field. In the CaF₂-Al₂O₃ melts, positively charged aluminum-oxygen groups dominate. This results in migration of aluminum to the cathode at almost all Al₂O₃ concentrations. Therefore, in Na₃AlF₆-Al₂O₃ melts, the Al³⁺ ion as a component of the complex anion has a negative partial conductivity and the O²⁻ ion has positive partial conductivity; in CaF₂-Al₂O₃ melts, Al³⁺ has a positive transport number while O²⁻ has a negative transport number.     

The thermodynamic properties of melts in the system MnO-SiO2

The activity of MnO in MnO-SiO₂ melts has been measured at 1400, 1500 and 1600 °C by means of the technique in which equilibrium is established between the melt, a gaseous atmosphere of fixed oxygen pressure and a Pt-Mn foil immersed in the melt. The variation of the activity of SiO₂ with composition and temperature and the integral free energies, and partial heats and entropies are derived. The activity of MnO and the free energy of formation of liquid Mn₂SiO₄ are well represented by Whiteway, Smith and Masson’s polymerization model of liquid silicates with Ink _1,1 = −5212/T + 1.011. Correlation of the results of the present study with previous measurements indicates that solubility of SiO₂ in MnO-SiO₂ melts is a significant function of oxygen pressure.     

The rate of reaction of solid iron with oxidized “FeO”-CaO-SiO2-Al2O3 slags at 1360 °C—the chemical diffusivity of iron oxide

Measurements have been made of the rate of reduction of oxidized iron oxide-containing 41CaO-38SiO₂-21Al₂O₃ (wt pct) slags at 1360 °C by a rotating disc of solid iron. For initial total iron concentrations of between 1.8 and 13.4 wt pct and rotation speeds up to 1000 rpm, the rate is shown to be determined by mass transfer in the liquid phase. The chemical diffusivity of iron oxide (in cm² s⁻¹) is found to be given by the empirical expression log D = −6.11 + 0.08 (wt pct Fe). It is concluded that the values of the diffusivity are for melts at close to iron saturation. It is shown that the available measurements of the diffusivity of iron oxide in liquid slags are consistent with increasing diffusivity with increasing state of oxidation, with about a tenfold increase between melts in equilibrium with iron and those in equilibrium with oxygen at 1 atm.     

The equilibrium partitioning of titanium between Ti3+ and Ti4+ valency states in CaO-SiO2-TiO x slags

The redox behavior of titanium in CaO-SiO₂-TiO _x melts was investigated using a slag-gas equilibrium technique. Titanium partitioning between Ti³⁺ and Ti⁴⁺ valency states and the ratio of activity coefficients of TiO_1.5 and TiO₂ were determined as functions of oxygen partial pressure, temperature, and slag composition. The equilibrium experiments were carried out at temperatures between 1783 and 1903 K under CO-CO₂-Ar gas atmosphere with oxygen partial pressure ranging from 10⁻¹² to 10⁻⁷ atm (1.01×10⁻¹⁰ kPa to 1.01×10⁻⁵ kPa). The slags had CaO/SiO₂ ratios between 0.55 and 1.35 and total titanium oxide concentrations from 7 to 50 mass pct. Experimental results showed that the Ti³⁺/Ti⁴⁺ ratio in CaO-SiO₂-TiO _x slags, containing up to 50 mass pct TiO _x , increased with decreasing oxygen partial pressure and decreased with increasing CaO/SiO₂ ratio and decreasing temperature. Measured variation of the redox ratio Ti³⁺/Ti⁴⁺ with oxygen partial pressure closely followed the ideal behavior. Increasing the CaO/SiO₂ ratio increased the ratio of activity coefficients of TiO_1.5 and TiO₂. The effect of total titania content on this ratio was more complex and in accord with Raman spectroscopy data.     

The densities and structures of melts in the system CaO-“FeO”-SiO2

The densities of melts in the systems CaO-SiO₂, CaO-“FeO,” “FeO”-SiO₂ and in ternary melts containing 33 and 44 mole pct SiO₂ have been measured from the liquidus temperature to 1600°C. Structural interpretation of melt density and expansivity has provided information on the nature of the ions present in these systems, and, in conjunction with known thermodynamic behavior, has indicated the natures of the interactions occurring among these ions. The influence of temperature, silica content andCaO/FeO ratio on the structures of ternary iron-calcium silicates is discussed.