Thermodynamic Behaviour of Chlorine in CaO‐SiO2‐MgO‐Al2O3(‐CaF2) Slags

The solubility of chlorine in CaO‐SiO₂‐Al₂O₃‐MgO(‐CaF₂) slag was measured at 1673 ‐1823 K. By estimating the chloride capacity of slags, thermodynamic behaviour of chlorine in the molten slag was investigated. Chloride capacity increased with increasing CaO / SiO₂ ratio (C/S). An increase in MgO content decreased chloride capacity at C/S≥1.0 because it lowered the activity of Ca²⁺ which seemed to have strong affinity with Cl^? in molten slag. Also, the chloride capacity decreased with increasing Al₂O₃ content. The affinity between the Ca²⁺ and Cl^? ions was confirmed by measuring the infrared spectra of slags. The dissolution reaction of chlorine into slag was exothermic and its molar enthalpy was evaluated from the experimental results at 1673 ‐ 1823 K. Based on the result obtained in the present study, the quantitative prediction of chlorine distribution during the blast furnace process was performed. It was estimated that almost all chlorine in the blast furnace would be absorbed into molten slag even if the PCI ratio was increased or low quality coal with chlorine content less than 1.0 mass% was injected.     

Manganese Distribution between FeO‐MnO‐SiO2–CaO–MgO‐Al2O3 Slags and Molten Iron

Pretreatment of high manganese hot metal is suggested to produce hot metal suitable for further processing to steel in conventional LD converter and rich manganese slags satisfy the requirements for the production of silicomanganese alloys. Manganese distribution between slag and iron represents the efficiency of manganese oxidation from hot metal. The present study has been done to investigate the effect of temperature, slag basicity and composition of oxidizer mixture on the distribution coefficient of manganese between slag and iron. Ferrous oxide activity was determined in molten synthetic slag mixtures of FeO‐MnO‐SiO₂–CaO–MgO‐Al₂O₃. The investigated slags had chemical compositions similar to either oxidizer mixture or slags expected to result from the treatment of high manganese hot metal. The technique used to measure the ferrous oxide activity in the investigated slag systems was the well established one of gas‐slag‐metal equilibration in which molten slags contained in armco iron crucibles are exposed to a flowing gas mixture with a known oxygen potential until equilibrium has been attained. After equilibration, the final chemical analysis of the slags gave compositions having a particular ferrous oxide activity corresponding to the oxygen potential of the gas mixture. The determined values of ferrous oxide activity were used to calculate the equilibrium distribution of manganese between slag and iron. Higher manganese distribution between slag and iron was found to be obtained by using oxidizer containing high active iron oxide under acidic slag and relatively low temperature of about 1350°C.     

Thermodynamic assessment of CaO‐SiO2‐Al2O3‐MgO‐Cr2O3‐MnO‐FetO slags for refining chromium‐containing steels

The slag system of CaO‐SiO₂‐Al₂O₃‐MgO‐Cr₂O₃‐MnO‐Fe_tO relevant to refining chromium‐containing steels such as bearing steel is thermodynamically assessed at 1873 K. The activity coefficient of Fe_tO shows an initially rapid increment followed by a gradual reduction according to Cr₂O₃ content at a constant basicity, and decreases with increasing slag basicity. γ_MnO is decreased abruptly by increasing Cr₂O₃ content and thereafter, maintains a nearly constant level. From the standpoint of inclusion control, the Cr₂O₃ presence in ladle refining slags is thermodynamically harmful in that it minimizes the inclusion level by inducing the increment of γ_FetO even though Cr₂O₃ exists in extremely small amounts. However, it is beneficial in that it diminishes AI reoxidation by decreasing γ_MnO. The presence of carbon in slag decreases γ_FetO and γ_MnO, which turns out to be favourable for the reduction of Al reoxidation. The thermodynamic equilibria of chromium and manganese are quantified in terms of Fe_tO and Cr₂O₃ content as well as slag basicity by using multiple regression analysis. L_Cr and L_Mn are increased by the presence of Cr₂O₃, indicating a low recovery efficiency of Cr and Mn in the treatment of ferroalloy addition. In determining L_S values, Cr₂O₃ is not so important as the basicity of slags.     

The Activities of FexO in {CaO‐SiO2‐Al2O3‐MgO‐FexO} Slags at 1723 K

In Japanese steelworks, hot metal is now produced by scrap melting process. With this process removal of sulphur is very much handicapped because of very high sulphur levels (0.04 to 0.09 pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such disadvantages, the authors explored on the phase diagrams of {CaO‐SiO₂‐Al₂O₃‐MgO} slags, and this research revealed that those slags at 35 wt%‐Al₂O₃ would be good candidates as reagents for the removal of sulphur from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, activities of Fe_xO were determined by using solid‐state electrochemical cells incorporating MgO‐stabilized zirconia and Mo + MoO₂ reference electrode.     

Oxidation State of Titanium in CaO‐SiO2‐TiOx Slags at 1873K

Oxidation state of titanium was determined in CaO‐SiO₂‐TiO_x slags in the composition range 25‐53 percent CaO, 27‐46 percent SiO₂, 10‐55 percent TiO_x at 1873K using gas equilibration method. In the experiments, slags with different titanium oxide contents were equilibrated with a known carbon monoxide and carbon dioxide ratio. The results were used to determine the Ti³⁺ and Ti⁴⁺ contents as well as the activity coefficient ratio of corresponding oxides in the slag. The dependence of the activity coefficient ratio as a function of oxygen partial pressure was determined.     

The Dissolution of Alumina in CaO‐SiO2‐Al2O3 Slags

The dissolution of alumina inclusions in CaO‐SiO₂‐Al₂O₃ based slags have been measured using laser scanning confocal microscopy (LSCM). Experiments were carried out over a temperature range of 1477 to 1577°C. It was found that diffusion coefficients calculated from the experimental results showed a dependence on the slag viscosity. This is considered strong evidence that the dissolution process is at least in part controlled by mass transfer in the slag phase. The diffusion coefficients for the alumina particles are estimated to be in the range 10^?11 to 10^?10 m²/s.     

Sulphide Capacities of CaO‐SiO2‐Al2O3‐MgO Slags

In Japanese steelworks, hot metal is now being produced by a scrap melting process. With this process, removals of sulphur is very much handicapped because of very high sulphur levels (0.04‐ to 0.09‐ pct by weight) and relatively low tapping temperatures (1623 to 1723 K). In order to overcome such handicaps, the authors explored on the respective phase diagrams. These explorations revealed that {CaO‐SiO₂‐Al₂O₃‐MgO} slags with Al₂O₃ contents of 30‐ to 35‐pct by weight would be good candidates as reagents for sulphur removal from high sulphur hot metal at relatively low temperatures. For better understanding of the thermodynamic properties of the candidate slags, in this study, sulphide capacities were determined through gas/slag equilibrium technique. The experimental results suggest that there would be, at least, a “window” to remove sulphur from high sulphur hot metal as relatively low temperatures.     

Electrochemical Measurements of Oxygen Potentials in the Slag System CaO‐P2O5‐SiO2‐FexO

The liquidus compositions of the four‐phase assemblages in the quaternary system of CaO‐P₂O₅‐SiO₂‐Fe_xO were determined at 1473 and 1573 K by employing electron probe microanalysis. Measurements were also made on the Fe_xO activities at temperatures between 1373K and 1699K by employing an electrochemical technique involving stabilized zirconia electrolyte.     

Reoxidation of Ni‐ and Ni‐Fe‐Alloys by Al2O3‐SiO2 Refractory Materials

Reactions at the refractory/melt interface during ingot casting of Ni‐ and Ni‐Fe‐alloys were studied. The casts were performed using different alumino‐silicate bricks as refractory materials. Samples taken from the casting channel before and after casting were investigated using light and scanning electron microscopy with XPS. Thermodynamic calculations were performed with FactSage and the results were compared with the results from industrial tests. After the melt has infiltrated the surface layer of the bricks, refractory corrosion starts with an attack of Mn and Mg of the melt on SiO₂ and Fe₂O₃ of the refractory bonding matrix. Despite the presence of elements with higher oxygen affinity in the melt, low‐melting alumino‐silicate phases are predominantly built by the reaction with Mn and Mg. In a second step this liquid phase either traps non‐metallic inclusions from the melt or, at higher contents of Zr, Ti, Mg, Y etc. in the melt, causes massive reoxidation and inclusion formation. The refractory materials investigated show an increasing trend for reoxidation with an increasing amount of SiO₂ in glassy phases of the refractory bonding matrix. By the use of a refractory material with higher mullite content in the bonding matrix or by use of alumina bricks a strong reoxidation of the melt and intense inclusion formation can be avoided. These observations are also valid for other alloys with higher contents of elements with high affinity to oxygen.     

Reaction between CaO‐SiO2‐Al2O3 Slags and H2 + HCl + H2O Gas Mixtures

To better understand the thermodynamic behaviour of chlorine in blast furnaces, CaO‐SiO₂‐Al₂O₃ slags were brought into equilibrium with Ar + H₂ + HCl + H₂O gas mixtures at temperatures between 1673 K and 1798 K. The dissolution of chlorine into molten slags could be well interpreted through a reaction: (1/2) {CaO}_slag + (HCl) = {Ca_1/2Cl}_slag + (1/2) (H₂O). Conforming to this formula, a linear relation was observed between log (%CI) and log with an anticipated slope of 1/2.     

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.     

Thermodynamic behaviours of manganese and phosphorus between CaO‐MgOsat‐SiO2‐Al2O3‐FetO‐MnO‐P2O5 ladle slag and liquid iron

The thermodynamic equilibria of manganese and phosphorus between liquid iron and CaO‐MgO_Sat‐SiO₂‐Fe_tO‐MnO‐P₂O₅‐Al₂O₃ (0–33%) ladle slag have been investigated at 1873 K from the viewpoint of Mn and P yields for the production of high‐strength steels. The equilibrium distribution ratios of Mn and P were found to increase with increasing Fe_tO content; however, these ratios vary with basicity, but they do this the other way round. The addition of alumina into slag at a fixed basicity and Fe_tO content decreases both the equilibrium manganese and phosphorus distributions. The equilibrium distribution ratios were discussed in terms of the variation of activity coefficients of Fe_tO, MnO and PO_2.5, according to the slag basicity and Al₂O₃ content. The quantitative contributions of basicity and (%Fe_tO + %MnO) on L_Mn and L_P were empirically determined and their usefulness was discussed with the aid of plant data: To improve Mn and P yields in the practical RH operation, it is strongly recommended that Fe‐Mn and Fe‐P alloys be added after Al deoxidation treatment inducing relatively high Al₂O₃ in slag and maintaining low Fe_tO content. In addition, a ladle slag composition for the targeted Mn and P contents in liquid iron was substantially estimated using the empirical relationships.     

Activity of Chromium Oxide in CaO‐SiO2 based Slags at 1873 K

Thermodynamic properties of chromium oxides in molten slags are very important for optimization of stainless steel refining processes as well as reduction processes of chromium ores. The solubility of chromite into molten slags has been found to vary drastically with oxygen partial pressure and slag composition in the former studies by the authors. In the present study, activity data and redox equilibria of chromium oxides measured under moderately reducing conditions, P_O2= 6.95×10^?11 atm, at 1873 K are summarized. For the CaO‐SiO₂‐CrO_x system, the activity coefficient of chromium oxide increased with increasing basicity and the optimized slag composition for stainless steel refining is assessed as that saturated with CaCr₂O₄ and Cr₂O₃ using the phase relations determined. On the other hand, the presence of MgO and Al₂O₃ brings about different behaviour of chromium oxide activity and redox equilibria and the 44 mass per cent CaO ‐ 39 mass per cent SiO₂ ‐11 mass per cent Al₂O₃ ‐ 6 mass per cent MgO slag is recommended to reduce the chromium oxidation loss in the practical stainless steel refining process at 1873 K.     

Deoxidization Equilibria with Zirconium and Sulfur Partition for Molten Iron/CaO‐Al2O3‐ZrO2 Slag at 1873K

The relationship of zirconium and oxygen concentrations with sulfide capacity was clarified using equilibrated molten iron with CaO‐Al₂O₃‐ZrO₂ slag at 1873 K. Al₂O₃ and CaO‐stabilized ZrO₂ crucible saturated compositions were equilibrated with CaO‐2Al₂O₃ and crucible solid solution (Css) in the CaO‐ZrO₂ system, and with CaOZrO₂ and Css, respectively. From the concentrations of zirconium and oxygen dissolved in molten iron, ZrO₂ activities of the slag of the Al₂O₃ crucible and ZrO₂ crucible compositions were determined to be 0.885 and 0.0343, respectively. On the other hand, the sulfur distribution ratio of the ZrO₂ crucible saturated composition was higher than that of the Al₂O₃ crucible saturated composition. Sulfide capacities were determined to be 6.30×10^?5 and 8.07×10^?5 in the Al₂O₃ and the ZrO₂ crucible saturated compositions, respectively.     

A study of the dissolution of Al2O3, MgO and MgAl2O4 particles in a CaO‐Al2O3‐SiO2 slag

The dissolution behaviour of alumina, MgO and MgAl₂O₄ particles in a 36%CaO‐21%Al₂O_3‐42%SiO₂ (mass contents in %) slag has been studied using a confocal scanning laser microscope in the temperature range of 1470 ‐ 1550 °C. The double hot thermocouple technique and metallographic examination was also used to characterize the reaction product between the particles and the slag. The total dissolution times for MgO and MgAl₂O₄ particles decreased with increased temperature and were almost identical (approximately 200 s for a 150 μm diameter particle at 1500 °C.). This phenomenon can be explained by the formation of MgAl₂O₄ layer on the surface of the MgO particle during dissolution. Alumina dissolution times were slightly longer (approximately 250 s for a 150 μm diameter particle at 1500 °C) than that of MgO and MgAl₂O₄ particles and no reaction layer was observed on the surface of the particles.     

Phase Equilibrium of the System CaO‐P2O5‐SiO2 between 1473 K and 1673 K

Phase equilibrium was investigated in the ternary system of CaO‐P₂O₅‐SiO₂ at 1473K, 1573K and 1673K.     

Evaluation of Desulphurization Flux in CaO‐Al2O3‐BaO‐CeO2‐MgO System

The flux of the CaO‐Al₂0₃‐BaO‐CeO₂‐MgO system as a desulphurization flux containing no fluorine for the secondary metallurgy process was evaluated in this study. The flux composition was designed using the eutectic compositions of the binary systems. The melting and desulphurization abilities of the fluxes were evaluated by measuring their liquidus temperatures and the distribution ratios of sulphur between the fluxes and the carbon‐saturated iron or stainless steel. The lowest liquidus temperature of 1325°C was obtained by adding 5.7 mass% MgO to the 80mass%A‐20mass%B flux. (A: 12CaO‐7Al₂O₃, B: BaCeO₃+12mass%Al₂O₃). The distribution ratios of sulphur and sulphide capacities of the fluxes in this study were higher than those of the commercial product of calcium aluminate flux. This means that the CaO‐Al₂O₃‐BaO‐CeO₂‐MgO fluxes developed in this study have higher desulphurization and melting abilities compared with the commercial product of calcium aluminate flux.     

高温条件下碳还原含SiO2熔渣的研究

研究了高温条件下用碳还原含ＳｉＯ２合成渣的过程。结果表明：温度是使氮化物（ＳｉＯ２,ＭｇＯ和Ａ１２Ｏ３）还原的重要因素,在较高的温度和碳过剩条件下,控制适当的熔渣碱度和增加ＭｇＯ含量,可增加渣中还原ＳｉＯ２所生成的ＳｉＣ并减少所形成的ＳｉＯ,实现在较高温度条件下抑制渣中ＳｉＯ２被还原的目的。     

高铝渣对济钢3200m~3高炉冶炼的影响

针对高铝渣特有的黏度高、流动性差、脱硫能力差的特点,济钢3200 m3高炉通过调整热制度和布料制度,在烧结时提高MgO含量,控制渣中镁铝比0.6,使渣中MgO含量在8%~11%,高炉的整体操作炉型适应了高铝渣的冶炼要求。在渣铁比升高43 kg/t的条件下,高炉生铁含硅降低,炉渣脱硫能力增强,基本杜绝了三类铁。