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.     

Measurement of oxygen potentials in Ag-Pb system employing oxygen sensor

Accurate and instantaneous analysis of dissolved nonmetallic or metallic species in molten metals at elevated temperatures using an online electrochemical sensor is important for continuous process control during metal refining and alloying operations in metallurgical industries. In this article, the application of long-life, solid-state electrochemical sensors for oxygen has been demonstrated to measure the oxygen potential as a function of the lead concentration in a molten Ag-Pb system at 1323 K. Yttria-stabilized zirconia (YSZ) in the form of one end-closed tube 20 mm long, 3 mm inner diameter, and 1 mm wall thickness has been used as a solid electrolyte in the oxygen sensor. Electromotive force (EMF) of the solid-state electrochemical cell (−)Pt, Ni-NiO//YSZ//O _Ag-Pb, Mo(+) has been measured at 1323 K. The reference electrode consisted of Ni + NiO biphasic mixture; the working electrode was composed of a molten Ag-Pb alloy of varying concentrations of lead. The concentrations of lead in silver ranged from 0.02 to 10.20 wt pct. Samples of the molten alloy were drawn after each addition of Pb to Ag and analyzed by induction coupled plasma (ICP).     

Analysis of the Deoxidation Process of Copper using an Electrochemical Cell

A solid electrolyte oxygen sensing cell employing yttria‐stabilized zirconia (YSZ) was used to determine the oxygen content and activity in molten copper on a laboratory scale at temperatures between 1200 and 1400 °C. The effects of temperature and time of treatment were investigated on the deoxidizing process with Cu₃P. The amount of Cu₃P added ranged from 1 to 3% of the mass of molten copper. At 1200 and 1300°C, fair agreement was obtained between the oxygen contents calculated from the measured probe EMF and those obtained by chemical analysis. It was found that the oxygen is more efficiently removed at lower temperatures. A metallographic analysis of rapidly cooled samples showed that oxygen and phosphorus are present in the copper mainly as oxide dendrites.     

Activities of titanium in molten copper at dilute concentrations measured by solid- state electrochemical cells at 1373 K

In order to obtain the activities of titanium in molten copper at dilute concentrations,i.e., between 5 x 10^-6 and 3.4 x 10^-3 titanium mole fractions, liquid copper was brought into equilibrium with molten {CaCl₂ + Ti₂O₃} slag saturated with Ti₂O₃ (s) at 1373 K and the equilibrium oxygen partial pressures were measured by means of a solid-oxide galvanic cell of the type Mo/Mo + MoO₂/ZrO₂(MgO)/(Cu + Ti)_alloy + Ti₂O₃ + CaCl₂ + Ti₂O_{3 slag}/Mo The free energy change for the dissolution of solid titanium in molten copper at infinite dilution referred to 1 wt pet was determined as Ti (s) = Ti(1 wt pet in Cu) ΔG°/J = -86,100 ± 8900 at 1373 K     

Sulfur determination in carbon-saturated iron by solid-state electrochemical sensor

In the frame of the continuous efforts to thein situ monitoring of the chemical composition of melts in steelmaking, the solid-state electrochemical probe (−)Pt/Nb-NbO/Caβ″-alumina-CaS/S, Fe (1)/C/Pt (+) was developed and tested to measure the sulfur content in carbon-saturated iron. The solid electrolyte used is calcium β″-Alumina, which behaves as a good ionic conductor by calcium vacancies at steelmaking temperatures. The calcium chemical-potential gradient is fixed at the electrolyte interfaces, on the left side, by keeping constant the oxygen chemical potential through the Nb-NbO coexistence mixture and, on the right side, by the activity of dissolved sulfurvia the solubility equilibrium of CaS. A theoretical equation of the probe electromotive force (emf), as a function of the dissolved concentration of sulfur in the bath, was derived. From this equation, which agrees well with the experimental results, it is clearly shown that the emf of the probe can be approximately linear against log ([S]/pct) in small intervals of sulfur concentration. The experimental emf s were taken up in the sulfur concentration range from 13 to 140 ppm. In this interval, the average slope of emfvs log ([S]/pct) curve is about −50 mV/decade. The effect of dissolved oxygen is discussed because it generates a mixed potential related to the CaS instability. The lower sulfur concentration limit, that can be detected, depends on the concentration of the oxygen dissolved in the melt because CaS tends to react with it. Some characteristic quantities of the theoretical equation were chosen as fitting parameters to fit the equation itself through the experimental emfvs log ([S]/pct) data. By this procedure, it was also found that the CO supersaturation in the bath was 10.2 bar.     

Possible failure of emf oxygen sensor in liquid iron containing dissolved calcium or magnesium

The solubility products of CaO and MgO in liquid iron, measured using the emf oxygen sensor, are several orders of magnitude greater than those calculated from the thermochemical data. This would imply that the values of Δ and Δ derived from the solubility products are, by a large amount, less negative than those given in the compiled thermochemical tables. As is shown in this paper, the values of Δ derived indirectly from numerous experimental data on various gas-solid (-liquid) reactions involving pure CaO, are in general accord with the compiled thermochemical data. It is surmised from these observations that the oxygen activities in liquid iron saturated with CaO or MgO, measured by the emf cell, giving high solubility products of CaO and MgO, may be attributed to the failure of the emf oxygen sensor in liquid iron containing dissolved calcium or magnesium. Two reaction mechanisms are discussed in the paper to account for the malfunction of the oxygen sensor under highly reducing conditions, which will prevail in liquid iron containing even a small amount of dissolved calcium or magnesium. These two mechanisms are (i) redox reaction at the (Y₂O₃)ThO₂ electrolyte/melt interface and (ii) oxygen flux through the electrolyte from the reference electrode Cr–Cr₂O₃ to the melt/electrolyte interface. Suggestions are made for some experimental work to confirm or refute the argument presented in this communication.     

A Study on the Electrochemical Deoxidation Using ZrO2 Based Solid Electrolyte and a Periodic DC Voltage

An electrochemical deoxidation using a ZrO₂ based solid electrolyte was investigated to control the interfacial oxygen concentration between the molten steel and ZrO₂. The electrochemical deoxidation cell consisted of an MgO stabilized ZrO₂ and an external power supply. In a previous study with constant external DC voltage, the oxygen concentration at the interface between the solid electrolyte and the molten steel was decreased to 2.2 ppm, which was the limit caused by the cathodic over‐potential when a constant external DC voltage was applied. In the present study, a novel process of using a periodic or cyclic voltage for the electrochemical deoxidation cell was developed, to surpass this limitation caused by the over‐optential of the electrochemical cell and thus decreasing the oxygen concentration to sub‐ppm levels at the interface between the molten Fe and the solid electrolyte.     

Electrochemical transport through a lime-silica-alumina slag

The transfer of oxygen in molten ionic slags is believed to be limited by the conduction of electrons in the slag. As a result, oxygen transfer might be enhanced by improving the electronic conduction capability of a predominantly ionic conducting slag. In order to examine this possibility, a series of experiments was undertaken whereby the oxygen transport across a CaO-SiO₂-Al₂O₂ slag was al-tered, both by placing an electronic conductor through the slag, and by adding water vapor to the atmosphere. The results indicate that oxygen transport across the slag was improved in both cases. In the presence of an electronic conductor, the rate controlling step was determined to be the discharge of electrons at the perimeter of the conductor; increasing this perimeter caused an increase in the oxygen transfer rate which was directly proportional to the conductor perimeter. With the addition of water vapor at a dew point of 273 K, this rate essentially doubled. Formerly Associate Professor, Technical University of Nova Scotia, Halifax, NS     

A thermodynamic study of MgO—P2O5 slags by means of solid-oxide galvanic cell at 1673 K

An electrochemical method was tested in order to find a suitable experimental technique for the determinations of the activities of P₂O₅ in molten fluxes used for external dephosphorization of hot metal. This technique consists of equilibrating phosphorus-containing copper with molten slag and measuring the equilibrium oxygen partial pressure by means of solid state galvanic cells. The activities of P²O₅ in MgO—P₂O₅ were determined with this technique.     

Activities of chromium in molten copper at dilute concentrations by solid-state electrochemical cell

In order to obtain the activities of chromium in molten copper at dilute concentrations (<0.008 chromium mole fractions), liquid copper was brought to equilibrium with molten CaCl₂ + Cr₂O₃ slag saturated with Cr₂O₃ (s), at temperatures between 1423 and 1573 K, and the equilibrium oxygen partial pressures were measured by means of solid-oxide galvanic cells of the type Mo/Mo + MoO₂/ZrO₂(MgO)/(Cu + Cr))alloy + Cr₂O₃ + (CaCl₂ + Cr₂O₃)_slag/Mo. The free energy changes for the dissolution of solid chromium in molten copper at infinite dilution referred to 1 wt pct were determined as Cr (s) = Cr(1 wt pct, in Cu) and ΔG° = + 97,000 + 73.3(T/K) ± 2,000 J mol⁻¹.     

A Thermodynamic Model of Sulfur Distribution Ratio between CaO–SiO<Subscript>2</Subscript>–MgO–FeO–MnO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Slags and Molten Steel during LF Refining Process Based on the Ion and Molecule Coexistence Theory

A thermodynamic model for calculating the sulfur distribution ratio between ladle furnace (LF) refining slags and molten steel has been developed by coupling with a developed thermodynamic model for calculating the mass action concentrations of structural units in LF refining slags, i.e., CaO–SiO₂–MgO–FeO–MnO–Al₂O₃ hexabasic slags, based on the ion and molecule coexistence theory (IMCT). The calculated mass action concentrations of structural units in CaO–SiO₂–MgO–FeO–Al₂O₃–MnO slags equilibrated or reacted with molten steel show that the calculated equilibrium mole numbers or mass action concentrations of structural units or ion couples, rather than mass percentage of components, in the slags can represent their reaction abilities. The calculated total sulfur distribution ratio shows a reliable agreement with the measured or the calculated sulfur distribution ratio between the slags and molten steel by other models under the condition of choosing oxygen activity based on (FeO)–[O] equilibrium. Meanwhile, the developed thermodynamic model for calculating sulfur distribution ratio can quantitatively determine the respective contribution of free CaO, MgO, FeO, and MnO in the LF refining slags. A significant difference of desulfurization ability among free component as CaO, MgO, FeO, and MnO has been found with approximately 87–93 pct, 11.43–5.85 pct, 0.81–0.60 pct and 0.30–0.27 pct at both middle and final stages during LF refining process, respectively. A large difference of oxygen activity is found in molten steel at the slag–metal interface and in bulk molten steel. The oxygen activity in molten steel at the slag–metal interface is controlled by (FeO)–[O] equilibrium, whereas the oxygen activity in bulk molten steel is controlled by [Al]–[O] equilibrium. Decreasing the high-oxygen-activity boundary layer beneath the slag–metal interface can promote the desulfurization reaction rate effectively or shorten the refining period during the LF refining process.     

Detailed assessment of partial thermodynamic quantities of tin in molten Bi-Sn alloys from electromotive force measurements

All the partial thermodynamic quantities, including the activity and activity coefficient of tin,a _Sn and γ_Sn, in the liquid Bi-Sn alloys between 300 °C and 500 °C are assessed in detail from electromotive force (emf) data. The emf values were obtained from a modified molten salt electrolyte concentration cell that had extraordinary stability within +0.005 mV. The composition dependence of γ_Sn did not follow Darken’s quadratic formalism, nor did the alloy behave as a regular solution. The excess partial quantities, ΔˉH _Sn ,ΔˉS _Sn ^xs , and ΔˉG _Sn ^xs , are oscillating across the composition, and the activity curve exhibits double, positive and negative, deviations from Raoult’s law. The a_Sn, γ_Sn, and ΔˉG _Sn ^xs change from positive to negative departure from ideality in the tin-rich region beyond about x_Sn = 0.82. All those partial quantities and γ _Sn ^o , including the β° (the α function of tin) at infinite dilution, are determined accurately by means of Chiotti’s method. ROBERTO CAMISOTTI, formerly Undergraduate Student, Central University of Venezuela. Formerly Assistant Professor, Central University of Venezuela.     

Tri-phasic zirconia electrolyte for the in-situ determinations of silicon activities in hot metal

Tri-phasic zirconia electrolyte, which consists of cubic ZrO₂–MgO solid solution, monoclinic ZrO₂ and 2MgO · SiO₂, was applied to solid state electrochemical sensor for rapid determinations of silicon activities in hot metal. The performance of the cell: Mo + MoO₂ / tri-phasic zirconia electrolyte / Fe + Si + C was tested with molten iron at 1723 K. The cell potentials showed good sensitivity to the variation of silicon content in the melt.     

Equilibria in reactions of CO and CO2 with dissolved oxygen and carbon in liquid iron

Carbon and oxygen activities have been established in molten iron using CO-CO₂ mixtures in the temperature range 1550 to 1750°C. Pressures of up to 70 atm were used to establish the required carbon activities up to carbon saturation at 1550°C in gas mixtures with CO₂ contents between 0.76 and 8.21 pct. The levitation technique was used to avoid any possible contamination and the effect of thermal diffusion in the gas mixture was measured and corrected for. The data obtained on carbon activities fall between those of previous workers at low carbon but measurements have been extended to the much higher carbon concentrations which had not been investigated previously. By using the same CO-CO₂ mixture and varying the total pressure it was possible to measure the effect of carbon on oxygen concentration at fixed oxygen activity and it was found that carbonincreases the activity coefficient of oxygen.     

Lead and nickel distribution in the Cup-PbO-SiO2 and the Cup-NiO-SiO2 Systems equilibrated with liquid copper

The distribution of lead and of nickel between molten copper and a ternary cuprous oxide-metal oxide-silica saturated slag was measured at 1498 K. The results are correlated using ion fractions, calculated either according to Temkin’s method, or as electrically equivalent ion fractions. The simpler method suggested by Temkin correlates slightly better than the other giving the following relations between the ionic concentration quotients and impurity concentration in the metal: logN _Pb ⁺⁺/N ² _Cu ⁺ = logX _Pb + 2.40 ± 0.05, valid up toX _Pb = 0.003 logN _Ni ⁺⁺/N ² _Cu ⁺ = 0.44 logX _Pb + 0.15 ± 0.07, valid up toX _Ni = 0.004. In the nickel systems, an Ni rich phase, identified as NiO, separates from the metal at X_Ni = 0.04 (0.4 wt pct). A tentative phase diagram of the Cu₂O-NiO-SiO₂ system at 1498 K is presented. The data found in this investigation explain why the impurities Pb or Ni cannot be practically reduced to low levels by oxidation. This results from the large amounts of copper that are oxidized in going to low impurity levels. Use of a two-slag refining process is shown to cut slag losses of copper to less than half those encountered with a single slag operation.     

The rate of antimony elimination from molten copper by the use of Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> slag

The rate of Sb elimination from molten copper by the use of Na₂CO₃ slag was measured at 1523 K. The results obtained under the present experimental conditions show that Sb in molten copper is eliminated in a tri-valent or a penta-valent form, depending on the oxygen concentration at the slag-metal interface, and its elimination rate increases with increasing initial oxygen concentration in molten copper. The overall elimination rate of Sb is affected by the stirring condition of the molten copper, which indicates a rate control by mass transfer in that phase. The mass-transfer coefficients of Sb and oxygen in molten copper at 1523 K without external stirring were determined, respectively, to be

Equilibria in reactions of Co and Co2 with dissolved oxygen and carbon in liquid iron

Carbon and oxygen activities have been established in molten iron using CO-CO₂ mixtures in the temperature range 1550 to 1750°C. Pressures of up to 70 atm were used to establish the required carbon activities up to carbon saturation at 1550°C in gas mixtures with CO₂ contents between 0.76 and 8.21 pct. The lévitation technique was used to avoid any possible contamination and the effect of thermal diffusion in the gas mixture was measured and corrected for. The data obtained on carbon activities fall between those of previous workers at low carbon but measurements have been extended to the much higher carbon concentrations which had not been investigated previously. By using the same CO-CO₂ mixture and varying the total pressure it was possible to measure the effect of carbon on oxygen concentration at fixed oxygen activity and it was found that carbonincreases the activity coefficient of oxygen.     

Hydrogen probe equipped with SrCeO3-based proton conductor and Ca/CaH2 reference electrode

The protonic transference number of SrCe_0.95Yb_0.05O_3-α (SCYB) ceramic pellets was determined to be above 0.9 in the temperature range of 873 to 1073 K using a hydrogen concentration cell. The density of the ceramic disk and tube is above 96 pct of the theoretical. Highly dense SCYB tube and glass seal improved the reliability and life of an electrochemical hydrogen probe. When the probe was immersed in molten aluminum, a hydrogen concentration cell was established as Mo, Ca/CaH₂|SrCe_0.95Yb_0.05O_3-α|[H]_{in molten Al}, Mo The probe under different conditions generated stable electromotive force within 5 minutes after immersion and responded rapidly to the change in the atmosphere. The thermodynamic stability of the solid electrolyte under the testing conditions is also discussed.     

Ionic conduction in BaxCe0.8Pr0.2O3-

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Further study of the electrochemical deoxidation of induction-stirred copper melts

Previous studies by Oberg, Friedman, Boorstein and Rapp on the electrochemical deoxidation of induction-stirred copper melts have been extended by the measurement and interpretation of the pumping current and the emf of an auxiliary galvanic cell to monitor oxygen contents at the pumping surface. Imperfect seals in the high-temperature apparatus allowed the pickup of gaseous oxygen by the melt to counteract partly the electrochemical deoxidation of the melt. The observed pumping current was divided into fractions accounting for this oxygen pickup as well as the oxygen removed from the bulk of the melt. Consistent with the deoxidation mechanism of Oberg,et al, the deoxidation rate is limited by ionic conduction in the zirconia electrolyte crucible at high oxygen contents, and by the mass transport of oxygen in the induction-stirred melt at low oxygen contents. A mass transport coefficient for oxygen was calculated by several methods.