Theoretical concept on slag–oxygen sensors to measure oxide activities related to FeO,SiO2 , and CaO contents of steelmaking slags

Abstract

A theoretical concept is presented on the slag–oxygen sensors for in situ measurements of the FeO, SiO₂ , and CaO contents of steelmaking slags in the furnace and in the ladle. The purpose of this disclosure is to stimulate R & D interest in academia and technical institutes to further the development of new measuring devices applicable in steelmaking operations. The slag–oxygen sensor conceived* consists of two dissimilar electrodes such that when immersed in molten slag there will be a difference in oxygen potentials at the slag/ electrode interface between the two electrodes, registering an open circuit cell emf. Examples are given of different types of electrodes for different oxides in the slag; also, equations are derived for the theoretical relation between the oxide activity and sensor emf reading.     

Activity of VO1.5 in CaO‐SiO2‐MgO‐Al2O3 Slags at Low Vanadium Contents and Low Oxygen Pressures

In the present work, the gas‐slag equilibration technique was employed for the measurement of the thermodynamic activity of vanadium oxide. The vanadium‐containing slag kept in a platinum crucible was equilibrated with a gas mixture of CO, CO₂ and Ar, with well‐defined oxygen partial pressure at a pre‐determined temperature. The slag sample was quenched and the composition of the final slag was determined by chemical analysis. From the value of the oxygen partial pressure, the thermodynamic activity of VO_1.5 could be calculated using the value for the activity of vanadium in V‐Pt alloy. The measurements were carried out in the temperature range 1823～1923K and the oxygen partial pressures employed were 10^‐3, 10^‐4, 10^‐5 Pa. The present results show that the activity of vanadium(III) oxide in slag exhibits a negative deviation from ideality in the present composition range. With increasing basicity of the slag, the final content of vanadium oxide in the slag was found to show an initial increase followed by a constant content. The activities of vanadium(III) oxide did not exhibit any significant change with increasing temperature. The activity coefficient of vanadium(III) oxide decreased sharply with slag basicity approximately up to a basicity of 1, beyond which it showed a near–constant value. Increase in basicity was found to cause a change in the distribution of vanadium between the slag and the alloy phases even though this effect was less pronounced. From the present results, a mathematical relationship for estimating the vanadium content in slag for a given activity of vanadium in the molten metal phase was developed.     

Improvement of steel cleanliness by controlling slag composition

Abstract

The properties of slag are very important in the improvement of steel cleanliness. In particular, the value of %CaO/%Al₂O₃ ratio, slag basicity, fluidity, and oxygen activity in the slag are considered to affect the steel cleanliness directly. To control these values effectively, the method of deoxidation and slag composition control via additives has been changed. The effect of oxygen content before tapping was investigated. By optimising these factors, the total oxygen content of bearing steel was reduced from 12 to 8 ppm.     

Slag–graphite wettability and reaction kinetics Part 2 Wettability influenced by reduction kinetics

Abstract

The present paper reports results of experimental investigations into the wettability of graphite by molten slag containing FeO. The wettability was determined by measurement of the graphite–slag contact angle. A higher initial FeO content in the slag phase and higher temperature lead to better wettability of graphite owing to the chemical reaction between the molten FeO in the slag and the graphite surface. The presence of FeO also reduces the surface tension of the slag. The free energy of reaction released per unit area ΔG _r has been estimated and correlated with the wettability parameters. The overall contribution of this factor to lowering the interfacial tension has also been examined.     

Effect of Direct Reduced Iron on Oxygen Distribution between Slag and Bath in Electric Arc Furnaces

Test charges containing 40 to 95% direct reduced iron (DRI) were melted in two identical electric arc furnaces. Slag and the corresponding metal samples were collected from the furnace in the course of the steelmaking process and from the ladle after deoxidation and composition adjustment. The temperature was measured just before sampling. The activity coefficient and activity of total ferrous oxide in the slag were determined by using the theory of regular ionic solutions. The activity was used to assess the oxygen concentration in the metal. The effect of slag composition on Fe_tO activity coefficient and activity was investigated. The enthalpy of solution of total ferrous oxide in the slag was found to be 78 kJmol^?1. The ratio of activity to concentration of Fe_tO is equal to 0.0138. The effects of DRI proportion and degree of metallization on α^Fe,O were investigated. The activity coefficients of oxygen and carbon in the bath are 0.7243 and 1.0825, respectively. The activity coefficient and activity of oxygen in the metal decrease with increasing carbon concentration, carbon activity coefficient and activity. An “oxide capacity” has been developed which does not need the use of α^Fe,O. It correlates strongly with temperature over the range from 1500 to 1670°C. The basicity has only a small influence compared with the large temperature effect. The oxide capacity increases with growing DRI proportion and decreases with increasing metallization between 93.43 and 95.25%. Oxygen distribution between slag and metal was assessed by using the oxide capacity. Calculated values compare well with the corresponding data obtained from slag and metal analyses. The oxide capacity can be used in monitoring the steelmaking operation.     

Sulphur partition between hot metal and high alumina blast furnace slag

Abstract

The sulphur partition ratio between hot metal and high alumina blast furnace slag (>18% alumina) has been examined on cast by cast basis for G blast furnace of Tata Steel. Equilibrium sulphur partition ratio was calculated from sulphide capacity with the help of oxygen activity in the melt. Oxygen activity was calculated from SiO₂/Si, MnO/Mn and CO/C equilibria. The equilibrium sulphur distribution calculated by considering the reaction [C]+[O]=(CO)_g in equilibrium for estimation of oxygen activity was very close to measured sulphur distribution ratio on cast by cast basis. Use of MnO/Mn pairs gives very high oxygen activity compared with SiO₂/Si and CO/C pairs.     

The kinetics of reduction of iron oxide from molten slag

This investigation primarily consists of measurement of the rate of reduction of FeO from sulfur-free slag. Preliminary measurements of the effect of sulfur on the rate of reduction and the sulfide capacity of the iron-rich slags have also been made. The results show that with increasing SiO₂ contents the rate of reduction is decreased. The influence of sulfur could not definitely be clarified.     

The measurement of hydrogen activities in molten copper using an oxide protonic conductor

We have developed a hydrogen sensor for in situ measurements of hydrogen activities in molten copper. The sensor consists of a concentration cell utilizing a proton conductor, CaZr_0.9In_0.1O_3-δ, as the solid electrolyte. The electromotive force (emf) of the cell was generated by both hydrogen and oxygen activity gradients across the cell in a high-temperature region simulating the fire refining processes of copper. However, accurate hydrogen activity in molten copper could be evaluated from the emf if oxygen activity in molten copper was determined simultaneously by another concentration cell and if the hydrogen and the oxygen activities at the reference electrode were known. The performance of the sensor was studied under various conditions. The observed good response and reliability of the sensor show that it should be a powerful tool for improvement of the fire refining process of the molten copper. Theoretical treatment of the calculation of the emf of the concentration cells using a mixed ionic conductor, i.e., protonic and oxide ionic conductor, as solid electrolytes is also discussed briefly.     

Distribution behavior of cobalt, selenium, and tellurium between nickel-copper-iron matte and silica-saturated iron silicate slag

The distribution coefficients (D _X) of cobalt, selenium, and tellurium between nickel-copper-iron matte and silica-saturated iron silicate slag were determined as a function of matte and slag compositions, temperature, and the partial pressure of oxygen. The effect of slag additives, such as CaO, MgO, and Al₂O₃, on the distribution behavior of the minor elements was also investigated. Analysis of the data indicated that D _Co, D _Se, and D _Te were strongly dependent on the matte grade and slag additives. The effect of slag additives on the solubility of Co, Se, and Te in slag was discussed in terms of various experimental conditions. Cobalt distribution coefficients were found to decrease with increasing oxygen partial pressure, indicating the oxidic dissolution of cobalt in the slag. Based on the experimental results and available thermodynamic data, the activity coefficients of CoS in the nickel-copper-iron matte were estimated as a function of mole fraction of FeS in the matte at 1250 °C. Meanwhile, the distribution coefficients of both selenium and tellurium increased when raising the partial pressure of oxygen, implying that there was molecular dissolution of selenium and tellurium in the slag within the oxygen partial-pressure range investigated in this study.     

Effects of BOF top blowing and bottom stirring conditions on suppressing excessive oxidation

Abstract

The effects of top blowing and bottom stirring conditions during steelmaking in a 6 t basic oxygen furnace (BOF) were investigated in studies with the aim of suppressing excessive oxidation. With low oxygen feed rate and high stirring energy, the apparent partial pressure of CO P′_CO was calculated from the equilibrium of carbon and oxygen in molten steel as being <1 atm. The relationship between top blowing/bottom stirring conditions and mass transfer at the slag/metal interface was analysed. It is proposed that mass transfer at the hot spot is significantly affected by the reaching rate of oxygen to the steel bath and bottom stirring. Mass transfer at the slag/metal interface, outside the hot spot, is sufficient to allow equilibrium to be attained in combined blowing BOF processes. Thus, the oxygen that is not consumed for decarburisation is distributed between steel and slag, i.e. deoxidation from steel to slag takes place, which makes it possible to obtain P′_CO <1 atm under atmospheric conditions. The decarburisation model developed based on the analysis reproduces the suppression of excessive oxidation under a decreased, top blown oxygen feed rate and is in good agreement with results from both 6 t BOF experiments and 350 t commercial BOF operation.     

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.     

Cold model study of metal droplet descent through fluids including a treatment of the non-Newtonian behaviour as exhibited by molten slag system

Abstract

In the context of several high temperature metallurgical processes including blast furnaces, a cold model study simulating a metal droplet descent through the surrounding fluid system is presented. The study comprises an experimental programme employing wide range of fluids exhibiting both Newtonian and non-Newtonian behaviour. Such fluid systems are encountered in slag–metal droplet systems where viscosity of the slag system has a significant effect on the kinetics of refining reactions. Slag systems generally possess random network structures comprising internal regions of weak ordering and the presence of these regions may result in non-Newtonian behaviour of the slag. As the viscosity of the slag is very sensitive to structure, a treatment of non-Newtonian behaviour as exhibited by some molten slag systems is therefore required. Two parameters have been identified and estimated that help to determine the rheological characteristics of fluids in relation to their network structure. The underlying principle of the model development has been that the external pressure exerts a driving force that affects the motion of the fluid to a degree dependent on the rheological behaviour and the network structure of the fluid. The paper also describes some results of a cold model study of the momentum transfer to the fluid system by correlating the drag Reynolds number with the modified drag coefficient for non-Newtonian fluids

(N*_Rem)^½?n′ = (N _Rem)^½?n′ (1/2C _Dm )^½     

Oxidation state and activities of chromium oxides in CaO-SiO2-CrOx slag system

The activities of chromium oxides in a CaO-SiO₂-CrO _x slag system were determined with the electromotive force (EMF) method by equilibrating with metallic chromium at 1873 K. The effect of slag basicity on the activity coefficients of CrO and CrO_1.5 was analyzed. The results showed that increasing the slag basicity increased the activity coefficient of CrO; however, the effect on that of CrO_1.5 was not significant. The oxidation state of chromium in CaO-SiO₂-CrO _x slags was systematically investigated at both 1873 and 1863 K. It was found that divalent and trivalent chromium coexists in the slags. Divalent chromium oxide is favored, instead of trivalent chromium oxide, because of low slag basicity and low oxygen potential. It was concluded that the oxidation state of chromium in the slag system varied greatly from almost pure “CrO” to a composition corresponding to Cr₃O₄. In addition, the thermodynamic data in the slag system were assessed based on the regular solution model to mathematically describe the activities of chromium oxides in the slags. A group of model parameters were obtained. The calculated activities of chromium oxides were comparable to the measured data.     

A new model predicting phosphorus equilibrium and its applications

In this model, equations determining the ferric content of basic slags and a new definition of the activity a_FeO in slag are proposed. The application of this model gives a more precise prediction of phosphorus equilibrium, permits to evaluate the influence of both slag composition and temperature on dephosphorization, to show the optimal slag composition for dephosphorization. This model also permits to examine the possibility of dephosphorization at lower temperatures.     

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.     

Sulfur Transfer at Slag/Metal Interface—Impact of Oxygen Potential

In the present work, the interfacial movement resulting from sulfur mass transfer at the slag/metal interface was monitored by X-ray sessile drop method in dynamic mode at temperature 1873 K (1600 °C) under nonequilibrium conditions. The experiments were carried out with pure iron and CaO-SiO₂-Al₂O₃-FeO slag (alumina saturated at the experimental temperature) contained in alumina crucibles with well-controlled partial pressures of oxygen and sulfur. The impact of oxygen potential on the droplet oscillation as sulfur from the gas phase reaches the metal drop through the intermediate slag phase was monitored. The interfacial velocity was investigated. It was found that the increases of interfacial velocity and the maximum oscillation time were mainly attributed to the partial pressure of oxygen increases. The experiment results were explained by previous ab initio calculations. The thermo-physical and thermo-chemical properties of slag were also found to influence interfacial velocity.     

Variation of the partial thermodynamic properties of oxygen with composition in YBa2Cu3O7−δ

The variation of equilibrium oxygen potential with oxygen concentration inYBa ₂Cu₃O₇-δhas been measured in the temperature range of 773 to 1223 K. For temperatures up to 1073 K, the oxygen content of theYBa ₂Cu₃O₇-δsample, held in a stabilized-zirconia crucible, was altered by coulometric titration. The compound was in contact with the electrolyte, permitting direct exchange of oxygen ions. For measurements above 1073 K, the oxide was contained in a magnesia crucible placed inside a closed silica tube. The oxygen potential in the gas phase above the 123 compound was controlled and measured by a solid-state cell based on yttria-stabilized zirconia, which served both as a pump and sensor. Pure oxygen at a pressure of 1.01 × 10⁵ Pa was used as the reference electrode. The oxygen pressure over the sample was varied from 10^-1 to 10⁵ Pa. The oxygen concentrations of the sample equilibrated with pure oxygen at 1.01 × 10⁵ Pa at different temperatures were determined after quenching in liquid nitrogen by hydrogen reduction at 1223 K. The plot of chemical potential of oxygen as a function of oxygen non-stoichiometry shows an inflexion at δ ∼ 0.375 at 873 K. Data at 773 K indicate tendency for phase separation at lower temperatures. The partial enthalpy and entropy of oxygen derived from the temperature dependence of electromotive force (emf ) exhibit variation with composition. The partial enthalpy for °= 0.3, 0.4, and 0.5 also appears to be temperature dependent. The results are discussed in comparison with the data reported in the literature. An expression for the integral free energy of formation of YBa₂Cu₃O_6.5 is evaluated based on measurements reported in the literature. By integration of the partial Gibbs’ energy of oxygen obtained in this study, the variation of integral property with oxygen concentration is obtained at 873 K.     

Improvement of reaction efficiency in hot metal dephosphorisation

Abstract

In Japan, the hot metal pretreatment process has been developed to refine hot metal under conditions where each impurity can be removed most efficiently. At Nippon Steel, three types of hot metal pretreatment process, using torpedo car, hot metal ladle, or LD converter, are employed, that make a great contribution to the reduction of slag volume. Recently, Nippon Steel has developed a new hot metal pretreatment called the multirefining converter (MURC) process, in which dephosphorisation and decarburisation are carried out continuously in the same converter. Nevertheless, since the dephosphorisation efficiency of CaO is less than 30%, CaO is now being used in far greater quantities than that stoichiometrically required to make 3CaO.P₂O₅. As hot metal dephosphorisation is a non-equilibrium reaction, in which hot metal is in contact with slag whose oxygen activity differs greatly from that of hot metal, it is important to increase the interfacial oxygen activity. From the results of a fundamental experiment, the 2CaO.SiO solid phase, in which P₂O₅ can be dissolved, has a great effect on the dephosphorisation reaction.     

Distribution equilibria of Sn,Se and Te between FeO-Fe2O3-SiO2-AI2O3-CuO0.5 slag and metallic copper

The distribution of tin, selenium and tellurium between alumina-containing fayalitic slags and metallic copper was measured at 1200 and 1300°C under controlled CO-CO₂ atmosphere with oxygen partial pressure (pO₂) in the rangePO ₂ = 10_-6 to 10_-11 atm (1 atm = 1.013 x 10² kPa). The solubility of Sn in slag was observed to increase linearly with increasing P^1/2O₂. It was deduced that Sn is present in the slag in the form of SnO or Sn² and the activity coefficient of SnO in the slag was calculated to be 1.9 at 1200°C and 0.8 at 1300°C. The solubility of Se in the slag decreases with increasing oxygen partial pressure up topO ₂l = 4 x 10₋₈ atm, but above this oxygen partial pressure it becomes practically constant and the ratio (pet Se in slag/pet Se in copper) = 0.018 (at 1200°C) and 0.036 (at 1300°C). The solubility of Te shows a similar variation with oxygen partial pressure and the ratio (pet Te in slag/pet Te in copper) = 0.026 (at 1200°C) and 0.032 (at 1300°C) abovepol ₂ = 10⁶ atm. A concept of molecular dissolution of chalcogen elements in slag was developed on the basis of thermodynamic properties of slag, and the observed solubility of Se and Te was explained in terms of the chemical stability of the molecular cluster FeSe and FeTe iη the slag. M. Nagamori is with Centre de Recherche Industrielle du Québec, Ste-Foy, Quebec and P. J. Mackey is Smelter Technical Superintendent, Noranda Mines Limited, Noranda, Quebec, Canada. Both authors were Formerly with Noranda Research Centre, Pointe Claire, Quebec.     

Effect of slag composition on the cleanliness of drill rod steel

In order to obtain the 55SiMnMo drill rod steel with a high cleanliness, the slag refining has been simulated by laboratory experiments. More desired spherical-shaped complex inclusions with an average diameter of about 2.7?μm, total oxygen of 4?ppm and Mg of 10?ppm after refining were obtained with initial slag basicity of 2.1 and Al₂O₃ 15?wt-%. The relationship between the slag composition and the melting temperature and viscosities of slag was achieved based on a calculation by Factsage Software and Einstein–Roscoe Equation. The refractory–slag–metal–inclusion multiphase reactions were investigated from the viewpoint of thermodynamics and kinetics by the estimation of viscosities, MgO solubility, Al₂O₃ activity in slag and sulphur capacity of slags. It is experimentally confirmed that the corrosion of MgO crucible by slag was affected by the MgO solubility and viscosity of slag. The factors facilitating to obtain low oxygen and control sulphur content were also analysed. Finally, the composition transformation of inclusions during slag refining and cooling process was discussed based on thermodynamic calculation.