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.     

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.     

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.     

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.     

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.     

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.     

MgO solubility in BOF slag equilibrated with ambient air

The MgO solubility in the CaO‐MgO‐Fe₂O₃‐FeO‐SiO₂‐(MnO)‐(Al₂O₃) slag was measured under the condition of equilibrium with the ambient air at 1873 K as a fundamental study for precise slag coating control in BOF operation. The CaO/SiO₂ mass ratios of the main slag were 1, 1.5, 2, 3 and 4, and total iron content was in the range of 10 to 35 %. Moreover, 1 to 13 % of MnO and 2 to 12 % of Al₂O₃ were added to the melt to evaluate their effects on the MgO solubility. The effect of slag composition on the MgO solubility was discussed and quantified by means of a newly developed formula. As the basicity in slag increases, the MgO solubility decreases. The effect of iron oxide content is observed to be dependent on the basicity of slag. An increase in iron oxide content makes the MgO solubility higher for basic slag but lower for acidic slag. It is revealed that the MgO solubility in steelmaking slag is controlled by the complex anion formation reaction of iron oxide. Both Al₂O₃ and P₂O₅ increase the MgO solubility by diluting the basic oxides as SiO₂ does, while manganese oxide affects the MgO solubility in a similar manner as iron oxide. The MgO solubility can be described as a function of slag composition, X = (%CaO) + 0.45(%Fe₂O₃+ %FeO) + 0.55(%MnO), in the equation of (%MgO) = 0.00816X^2‐1.404X + 62.31. Based on the results, the guidance for addition of MgO‐containing material could be suggested for best slag coating practice.     

Reactions of dense MgO with calcium ferrite-based slags at 1573 K

The drop-quench technique was used to investigate the solubility of dense MgO in calcium ferrite-based slags (CaO 20 wt pct) under oxygen potentials from 10⁻⁸ to 10⁻⁴ atm at 1573 K. The effect of copper oxide in the slag on the solubility of MgO was also examined in a CO₂ atmosphere. The results showed that MgO solubility in copper-free calcium ferrite slags was generally less than 2 wt pct and it increased with the addition of Cu₂O (up to 28.5 wt pct). It was found that magnesiowustite or magnesioferrite may form at the slag-refractory interface depending on the prevailing oxygen potential. The activity of MgO was estimated through equilibrium between the slag and the solid solution phases. The activity coefficient of MgO was found to be essentially independent of the oxygen potential within the range studied and to decrease from approximately 15 for the copper-free slag to 7 for slags with 28.5 wt pct Cu₂O.     

Structural statements,coordination numbers and water vapour solubility in aluminate slags

The IR emission of liquid CaO-Al₂O₃ slags is caused by IR active molecule vibrations of and -complexes. In the liquid CaO-Al₂O₃ slags the equilibrium dominates. At lower temperatures (1000°C) the formation of condensed -complexes was observed (glassy state). By the addition of B₂O₃ to CaO-Al₂O₃ slags B-O-Al associations were formed. Since the emission bands of the aluminate complexes overlap, a curve fitting with Gaussian functions was carried out to separate the bands. Furthermore, this quantitative evaluation leads to statements concerning the ratios between the different molecules. With the increase in temperature these associations are dissociated. In CaF₂-CaO-Al₂O₃ slags with higher contents of CaF₂, Al-O-F complexes with the coordination number 4 are present. Water vapour is dissoluted in liquid CaO-Al₂O₃ as hydroxide-ions (diffuse IR-emission-spectrum of ^?O-H^?…O^2?-molecule units).     

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.     

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.     

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.     

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.     

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.     

Some new aspects on inclusion separation in tundishes

The removal of non‐metallic inclusions due to buoyancy forces in tundishes in continuous casting systems is considered. The maximal theoretical removal rate is determined by the flow rate through the tundish, the magnitude of the tundish fluid surface area and the particle terminal rising velocity, depending on the particle size. Two reasons, why the particle separation is worse than the maximal possible one are an unsuitable fluid flow pattern and the turbulent particle diffusion. An analytical discussion of simple flow patterns (parallel flow with different velocity profiles, dead regions, swirling flows) and diffusion shows how they influence the particle removal. Using these simple considerations, it is demonstrated that the often used RTD (residence time distribution)‐curves are inappropriate to estimate the particle separation in tundishes; only the direct measurement and CFD‐simulation of particle removal should be used. The common CFD‐methods are affected with numerical errors such as numerical diffusion for particle concentration simulations with finite volume methods and interpolation errors for particle trajectory calculations. These errors influence significantly the calculated particle removal curves; they are non‐systematic and difficult to quantify.