PbO solubility in lead-blast furnace slags

A series of slags in the PbO-CaO-FeO-Fe₂O₃-SiO₂ system has been equilibrated in contact with molten Pb and Pb-Ag alloys in a closed system at 1473 K. The influences of lead activity and the CaO/SiO₂ and ferric/ferrous ion ratios on PbO solubility in slag have been examined. The PbO content is proportional to the lead activity and the ferric/ferrous ratio, but decreases with increasing values of the CaO/SiO₂ ratio. The solubility of lead in the slag can also be characterized by the following empirical equation, whereX _PbO is the lead oxide mole fraction,K ₂ the equilibrium constant for the formation of liquid PbO at 1473 K, andF andG are the calcia/silica and iron/silica weight ratios, respectively. This equation, developed from data in the literature, has been used with the experimental results of this investigation for comparison with values ofp _{O 2}predicted by Hollitt’s structural model. Further comparison with Hollitt’s model has been made with other experimental data in the literature. The values ofp _{O 2}predicted with the structural model are an order of magnitude lower than those predicted by the empirical equation and the values ofp _{O 2} used by other investigators. The reason for this discrepancy may lie in part in the use of questionable assumptions about the behavior of FeO in slag systems; reevaluation of the structural model to include more “basic” FeO improves agreement between the values ofp _{O 2}obtained with the structural model and those obtained with the empirical equation and the partial pressures of O₂ used by other investigators. In addition, the structural model has also been tested with regard to the influence of CaO and FeO additions on the solubility of PbO. The possible effect of interaction between the equilibrating slag melt and the calcia-stabilized zirconia crucible on the system equilibria has also been examined; such an effect may not be as negligible as previous researchers have claimed.     

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.     

The CaS solubility in the CaO bearing slags

The CaS solubility and sulfide capacity for the CaO–SiO₂–CaF₂ and CaO–Al₂O₃–SiO₂ systems have been measured at 1300°C and 1400°C, respectively. With the CaO–SiO₂–CaF₂ system, the slag doubly saturated with CaO and 3CaO · SiO₂ has the highest CaS solubility of 12.5 wt.%. On the liquidus the slag always has a higher CaS solubility than when it is not on the liquidus. The sulfide capacity was confirmed to decrease with increasing SiO₂ content. With the CaO–Al₂O₃–SiO₂ system, the CaS solubility was found to depend only on CaO content. A good correlation between the sulfide capacity and the CaS solubility was observed as expected from theory. The temperature dependence of CaS solubility in those systems was also discussed.     

Some aspects on MgO solubility in complex slags

A magnesia capacity is defined, C_Mg²⁺ = (%Mg²⁺)/a_MgO and its relation to optical basicity is examined and discussed. The solubility of MgO in complex slags, in equilibrium with magnesiowustite, is derived from the activity of MgO in the magnesiowustite and from the magnesia capacity as given by the slag's optical basicity. It is found that the activity of MgO in the magnesiowustite can be derived from [%O] assuming ideality in the FeO–MgO system at 1600°C. It is also found that the MgO saturation contents in complex slags are accurately derived from the equation: In addition, the relation between the magnesia capacity and the capacities of vanadium and phosphorus is examined and the relative basicity of the oxides is discussed.     

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.     

The solubility of nickel and cobalt in iron silicate slags

The solubility of nickel and cobalt in silica saturated iron silicate slags in equilibrium with nickel-gold and cobalt-gold alloys has been investigated under controlled oxygen pressures in the temperature range of 1250 to 1350°C. It was found that the metal solubility increased with 1) decreasing temperature, 2) increasing oxygen pressure, and 3) increasing metal content of the alloy. The solubility of cobalt in the slag was found to be much higher than that of nickel. The solubility of the metal in the slag from its alloy can be explained by a simple oxidation process.     

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.     

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.     

A new model on K2O solubility in blast-furnace slags

The solubility of K₂O in blast-furnace type slags in equilibrium with a reference slag is derived from the potassium oxide capacity as given by the slag's optical basicity. Optimizing of the blast-furnace slag and the relation between the elements is discussed. Die Löslichkeit von K₂O in Hochofenschlacken im Gleichgewicht mit einer Referenzschlacke wird aus der Kaliumoxidkapazität, die durch die optische Basizität der Schlacke gegeben ist, hergeleitet. Die Verbesserung der Hochofenschlacke und die Beziehung zwischen den Elementen wird diskutiert.     

Quantitative analysis of the relative basicity of CaO and BaO by silver solubility in slags

The solubility of silver in molten CaO-B₂O₃ and BaO-B₂O₃ slags at high temperatures was measured to seek a new measure of the basicity of slags. The B₂O₃-bearing fluxes, which have wide range of liquids at the temperature of interest, have been applied to investigate the effects of flux composition on the solubility of silver from the saturation of acidic or basic components. The solubility of silver in slags has a minimum value as a function of flux composition, and it is suggested that silver behaves as an amphoteric substance and dissolves by different mechanisms in acidic and basic fluxes. The solubility of silver decreased with the increasing temperature and content of basic oxides in the acidic region, and vice versa in the highly basic region. In the acidic region, BaO is more basic than CaO by, at most, about 33 pct, based on the difference of the solubility of silver in both slags, indicating that BaO is not a much-stronger basic oxide than CaO in the acidic slags. In the highly basic region, BaO is about 5 times more basic than CaO. The solubility of silver in slags was compared to the nitride capacity, showing that the dissolutions of silver and nitrogen into the melts are similar to each other. The relationship between Ag solubility and theoretical optical basicity was also discussed.     

The effect of temperature on nickel solubility in silica saturated fayalite slags from 1523 to 1623 K

The solubility of nickel in silica saturated fayalite slags has been measured at 1523 K and 1623 K through equilibration with liquid Ni-Au-Fe alloys and CO₂/CO gas. Solubility in weight percent is expressed as a function of oxygen pressure and activity of nickel and the data combined with previous measurements at 1573 K to quantify the effect of temperature on nickel solubility and to estimate the heat of solution of NiO in silica saturated slags.     

The effect of temperature on nickel solubility in silica saturated fayalite slags from 1523 to 1623 K

The solubility of nickel in silica saturated fayalite slags has been measured at 1523 K and 1623 K through equilibration with liquid Ni-Au-Fe alloys and CO₂/CO gas. Solubility in weight percent is expressed as a function of oxygen pressure and activity of nickel and the data combined with previous measurements at 1573 K to quantify the effect of temperature on nickel solubility and to estimate the heat of solution of NiO in silica saturated slags.     

Nitrogen solubility in metallurgical slags equilibrated with Fe–Al or Fe–Ca melts

Considerations are directed to the denitrogenation potential of metallurgical slags with respect to steel melts under reducing conditions. Experiments were made to determine partition ratios of nitrogen between molten slag and iron. The investigated systems were aluminate-based slags, containing CaO, MgO, SrO, BaO, CaF₂ or ZrO₂, in equilibrium with Fe–AI melts and Ca–CaO–CaF₂ slags equilibrated with Fe–Ca melts. Denitrogenation efficiency of aluminate-based slags is comparatively low and essentially determined by oxygen potential and basicity of the slag. Denitrogenation efficiency of Ca–CaO–CaF₂ slags is much higher and is dependent on calcium activity.     

The solubility of water vapour in CaO‐SiO2‐Al2O3‐MgO slag system

The solubility of water vapour in the CaO‐SiO₂‐Al₂O₃‐MgO quaternary slag system was measured using an inert gas fusion technique with thermal conductivity detection. The slags were equilibrated with argon‐water vapour mixture corresponding to 0.157 bar of water vapour pressure at 1873 K. The slag solubility of water vapour is proportional to the square root of vapour pressure. Since the hydroxyl capacity of slag, C_OH shows an independence on the relative amount of CaO or MgO in slag, the contributions of CaO and MgO on the hydroxyl capacity are equivalent on a molar basis. Whereas, Al₂O₃ shows a better effect on the hydroxyl capacity than SiO₂. A linear relationship between hydroxyl capacity and slag basicity in logarithmic scale was obtained with the slope of 1/2, confirming the water vapour dissolution reaction into a basic slag as (O^2‐) + H₂O(g) = 2(OH^?). The correlation between hydroxyl capacity and slag components was derived in terms of their contributing weight factors. The measured values of C^'_OH agree well with the calculated ones using the interaction energies of α_H‐Al = ?38300 and α_H‐Mg = ?22700 J determined with the aid of the regular solution model. In addition, the correlation between hydroxyl capacity and sulphide capacity was empirically derived as a formula of logC_OH = 1/2logC_S + (4.38 ± 0.25) through the thermodynamic expression of both capacities by virtue of the common oxygen ion activity.     

Effects of CaO, Al2O3, and MgO additions on the copper solubility, ferric/ferrous ratio, and minor-element behavior of iron-silicate slags

The effects of CaO, Al₂O₃, and MgO additions, singly or in combination, on the copper solubility, the Fe³⁺/Fe²⁺ ratio in slag, and on the minor-element behavior of silica-saturated iron silicate slags were examined at 1250 °C and a p _O2 of 10⁻¹² to 10⁻⁶ atm. The results indicated that copper solubility in slag was lowered with the addition of CaO, MgO, and Al₂O₃, in decreasing order. The Fe³⁺/Fe²⁺ ratio in the slag decreased with the additions, but this effect was smaller at lower oxygen potentials. The presence of small amounts (about 4 pct) of CaO, Al₂O₃, and MgO in the slag resulted in increased absorption of Bi and Sb into molten copper, but had a smaller effect at large additions (about 8 to 11 pct). The distribution behavior of Pb was a function of oxygen partial pressure, which indicates the oxidic dissolution of Pb in the slag as PbO, while the behavior of Bi, Sb, and As was found to be independent of oxygen potential, supporting the atomic (neutral) dissolution hypothesis of these elements in the slag. The distribution behavior of Pb and As was not significantly affected by the additions. The activity coefficients of Bi and Sb in the slags were determined to be as follows: (1) for no addition, γ _Bi=40 and γ _Sb=0.4; (2) for small additions (about 4.4 pct), γ _Bi=70 to 85 and γ _Sb=0.8; and (3) for large additions (about 8 to 11 pct), γ _Bi=60 to 75 and γ _Sb=0.5 to 0.7.     

Viscosities and activities in lead-smelting slags

Viscosity measurements at high temperature were made using a concentric cylinder viscometer. These measurements were conducted on a typical lead blast furnace slag, and this master slag doped with various amounts of SiO2, A12O3, CaO, MgO, and/or ZnO. It was found that additions of A1₂O₂ and/or SiO₂ increased the viscosity of the master slag over the entire temperature range of interest (1150 °C to 1350 °C). Additions of the basic oxides CaO, MgO, and/or ZnO decreased the viscosity at high temperatures but raised the slag liquidus temperature. The majority of the measured viscosities are accurate within ± 10 pct. The viscosity data of this study, along with that of several similar studies from the literature, were correlated with the weight parameter (WP), a composition-dependent function similar to a basicity ratio. Analytical expressions were developed relating viscosity and the WP at several temperatures between 1150 °C and 1350 °C. Viscosities calculated using these expressions have estimated accuracies of ±1 poise (within certain temperature and composition limits). PbO and ZnO activity coefficient data from the literature were collected and correlated with composition-dependent functions. The calculated activity coefficients are considered accurate within ±0.3 for γ_pbo and ±0.5 for γzn within certain composition and temperature limits. Formerly St. Joe Minerals Corporation Fellow in Extractive Metallurgy, Department of Metallurgical Engineering, Colorado School of Mines     

A compound-energy model of ordering in a phase with sites of different coordination numbers

When applied to a phase where all the sites do not have the same coordination number, the bond-energy model gives a term in u_AA − u_BB, which cannot be assigned a numerical value because the energy of different elements cannot be compared. As an alternative a compound-energy model is now developed from the well-known model for reciprocal systems. It gives a similar result but the u_AA − u_BB term does not appear. In addition, the new model takes into account two kinds of factors resulting in ordering, those which make different elements prefer different kinds of lattice sites and those which give a preference for unlike neighbors. The ordering in the sigma phase is considered in some detail.