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.     

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.     

Electronically driven transport of oxygen from liquid iron to CO + CO2 gas mixtures through stabilized zirconia

Transport of oxygen in the following electrochemical system was investigated;O (liquid iron) Oⁱⁿ²⁻ (in ZrO₂2−CaO) O₂ (CO + CO₂) An alumina crucible was charged with liquid iron containing 580 ± 10 ppm oxygen. A calcia-stabilized zirconia tube (closed at one end) was immersed in the liquid iron. The inside of the zirconia tube was flushed with a stream of CO + CO2 gas mixture. Oxygen was removed from liquid iron to the CO + COO₂ gas mixture without application of an external current. Kinetics of oxygen transport in this system are discussed in terms of mixed ionic and electronic conduction of the zirconia, and also diffusion of oxygen in liquid iron. The rate controlling step for this oxygen removal process was found to be transport of oxygen across a boundary layer in the melt at the melt/electrolyte interface. M. IWASE, on leave from the Department of Metallurgy, Kyoto University, Kyoto, Japan M. TANIDA, Formerly Graduate Student at Kyoto University     

A pilot-scale trial of an improved galvanic deoxidation process for refining molten copper

A laboratory-scale galvanic deoxidation technology developed by earlier workers has been improved, with the aim of developing a prototype pilot-scale deoxidation unit. Each deoxidation cell consists of a one end-closed yttria-stabilized zirconia (YSZ) tube coated with a Ni-YSZ cermet anode on the inner walls. The YSZ tube is immersed, with its closed end in the metallic melt, and an oxygen-chemical-potential gradient across the tube is established by passing a reducing gas through the tube. The melt is then deoxidized by short circuiting it with the anode. Through laboratory experimentation, the nature of the anode/electrolyte interface adhesion was identified to be an important factor in obtaining enhanced deoxidation kinetics. The kinetics of oxygen removal from the melt was increased by an order of magnitude with an improved anode/electrolyte interface. A pilot-scale refining unit consisting of 53 cells with the improved anode/electrolyte interface was manufactured, and a field evaluation of the galvanic deoxidation of copper was conducted. The deoxidation-process model was modified to include multiple deoxidation cells, which were required for the pilot-scale trials, and to analyze the effect of electrolyte/electrode adhesion on deoxidation kinetics. Preliminary studies on process component lifetimes were conducted by investigating the thermal cycling, corrosion behavior of the electrolyte, and stability of the cermet anode structure. Based on the results of the field trial and the analyses of the process component lifetime, future work needed toward commercializing the technology is discussed.     

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.     

Morphology of nonmetallic inclusions using silicon,aluminum, and calcium-silicon alloy in steel melt

The process of deoxidation of a 15 kg melt by silicon, aluminum, and calcium-silicon alloy was studied by continuously monitoring the dissolved oxygen potential using a solid electrolyte probe. Stable oxygen potential was established within two minutes, whether the deoxidizer was added onto the melt surface or plunged into the melt. Inclusion morphology was investigated with the help of optical and scanning electron microscopic studies. The inclusion index is dependent on the type of deoxidizer added, but was independent on the mode of addition. Thus, with ferrosilicon addition the index increased significantly which decreased very slowly on holding the melt while deoxidation with aluminum and calcium-silicon alloy resulted in a continuous decrease of the inclusion index. Evidence has been found to show that the reaction between the existing inclusion before deoxidizer addition and the deoxidizers added was also very fast.     

Dissolution of solid magnetite in Fe-S-O melt

The dissolution rate of solid magnetite in binary Fe-S and ternary Fe-S-O melts was measured at 1493 K using a rotating magnetite rod of 5 mm in diameter. At rotating speed higher than 52 rpm, the flow of sulfide melt was turbulent, and the effect of natural convection on the dissolution rate was negligible. Magnetite dissolution took place without the evolution of SO₂ gas at the rod surface immersed in the interior of the melt. On the other hand, SO₂ gas was evolved from a portion of magnetite rod which was in contact with the melt surface. The overall rate of dissolution was controlled by mass transfer through liquid boundary layer on the rod surface, and the dissolution rate of magnetite decreased with increasing oxygen concentration of the sulfide melt. Mass transfer coefficient was between 1.8 × 10⁻³ and 4.6 × 10⁻³ cm · s⁻¹ at the rotating speed of 156 rpm, and it decreased with increasing oxygen concentration of the melt. Formerly Graduate Student Formerly Graduate Student Formerly Graduate Student     

Improvement of the technology of the out-of-furnace treatment of wheel steel

Various versions of the deoxidation and out-of-furnace treatment of wheel steel under the OAO VMZ conditions are analyzed. The effective partial pressure of carbon monoxide over a melt in a 130-t ladle degasser is found to be 54 ± 9 kPa. Thermodynamic analysis of the deoxidation demonstrates that low oxygen concentrations in the melt of wheel steel can be achieved when it is deoxidized by aluminum, silicocalcium, aluminocalcium, or carbon in vacuum. Experiments and a thermodynamic calculation show that the vacuum-carbon deoxidation of a high-strength wheel steel provides oxygen concentrations in the metal that are comparable with the concentrations obtained by silicocalcium deoxidation (0.0023 ± 0.0005 wt %) and ensures the optimum morphology and concentration of oxide inclusions. The causes of the formation of the defects revealed by ultrasonic inspection in railway wheel templates are studied. The level of rejection controlled by these defects depends on the deoxidation method and is related to the number and morphology of the oxide inclusions that form during secondary oxidation.     

Kinetics of deoxidation of liquid copper by graphite particles during submerged injection

Liquid copper can be deoxidized by submerged injection of inert gas in the presence of graphite particles. This paper describes the results of experiments in which approximately 20 kg copper melts were deoxidized by injection of N₂, CO₂, and air in the presence of low sulfur graphite particles. The apparent kinetics of the deoxidation process are first order with respect to the concentration of dissolved oxygen, and the concentration of oxygen in the melt could be reduced to less than 10 ppm by weight after less than 30 minutes of injection. The kinetics are consistent with mixed rate control with both mass transfer and chemical reaction rate affecting the rate of deoxidation. In these experiments, the rate of deoxidation under a graphite covering was slower when particles were injected with the gas stream than when gas was injected alone, although this result may have been influenced by the small size of the melt. Y. W. Chang, formerly Graduate Student with the Department of Civil Engineering, Mechanics, and Metallurgy, University of Illinois at Chicago This paper is based on a presentation made in the T.B. King Memorial Symposium on “Physical Chemistry in Metals Processing” presented at the Annual Meeting of The Metallurgical Society, Denver, CO, February, 1987, under the auspices of the Physical Chemistry Committee and the PTD/ISS.     

“Chemical vacuum”—A concept to be refined

The concept “chemical vacuum” is discussed, as applied to the carbon deoxidation of metallic melts. Melting in an argon atmosphere, which is chemical vacuum for the reaction of carbon-oxygen interaction in a metallic melt, is shown not to be equivalent to melting in physical vacuum. For melting in an argon atmosphere, the total gas-phase pressure over a melt is important, and the fact that an unmixed gas layer is likely to exist over the molten metal surface should be taken into account. The CO content in this layer is determined by the carbon and oxygen contents in the melt.     

Analysis of the complex deoxidation of carbon steel melts

The joint complex deoxidation of carbon steel melts is analyzed. A procedure is proposed to calculate the equilibrium oxygen concentration in a melt. Rail steel is used as an example to study the joint complex deoxidation of a melt by aluminum and silicon. Mullite (2Al₂O₃ · 3SiO₂) and kyanite (Al₂O₃ · SiO₂) are considered as the reaction products. Thermodynamic calculations demonstrate that the deoxidizing capacity of aluminum is increased in the presence of silicon in a melt. In this case, a substantial increase in the deoxidizing capacity in the concentration range 0.001–0.1 wt % Al is achieved when kyanite (Al₂O₃ · SiO₂) forms in the reaction products. The results of laboratory and industrial experiments on complex deoxidation are shown to agree well with the calculated data. These results demonstrate that the proposed calculation procedure can be recommended to determine the equilibrium oxygen concentration in a melt in the presence of several deoxidizing elements.     

Kinetics of deoxidation of liquid Cu-O alloys by rotating graphite cylinders under argon

The rate of deoxidation of liquid Cu-O alloys by graphite cylinders rotating at about 250, 580, and 860 rpm was studied at 1400, 1482, and 1582 K under argon at 30, 58, and 99 kPa. Those at 1400 and 1482 K with about 580 and 860 rpm and at 1582 K do not depend on rotational speed. Under each argon pressure with these rotational speeds, at 1400 K, the apparent deoxidation-rate constantK _e defined by first order kinetics increases and then approaches a constant value with a decrease in the concentration of oxygen in the melt and at 1482 K,K _e is constant. At 1582 K,K _e is approximately constant. A plot of log₁₀ K _e at 1482 K and in very low oxygen concentration at 1400 and 1582 Kvs 1/T gives an apparent activation energy of 146 kJ · mol⁻¹. The dependence ofK _e on the concentration of oxygen can qualitatively be explained on the basis of a mechanism involving the chemisorption of oxygen from liquid copper onto the graphite surface, the release of carbon monoxide from the surface, and the interfacial reaction between oxygen in the melt and the carbon monoxide. This mechanism indicates that the activation energy is that for the chemisorption. The rate of deoxidation of liquid Cu-O alloys in graphite crucibles also was studied at 1473, 1573, and 1673 K under argon at 100 or 60 kPa by use of an induction furnace and is much smaller than that by the rotating graphite cylinders. The values ofK _e under both pressures of argon at 1473 K appear not to differ from each other and are close to theK _e value at 1573 K under argon at 100 kPa.     

Absorption of gaseous oxygen by liquid cobalt,copper, iron and nickel

An experimental investigation of the rates of oxygen solution in molten cobalt, copper, iron and nickel was carried out using pure oxygen and a constant-volume Sieverts’ method. It was found that the volume of gaseous oxygen which initially reacted with the inductively stirred metals was strongly dependent on the physical nature of the oxide film which formed during the first stage of reaction. The initial temperature of the molten iron, cobalt, and nickel was 1600°C, and for copper was 1250°C. For initial oxygen pressures above the melt of about one atmosphere both molten iron and copper, which formed liquid surface oxides, initially absorbed nearly 20 cm³ (STP) O₂/cm² of melt surface area, while molten cobalt and nickel, which formed solid oxides, absorbed about 6 cm³ (STP) O₂/cm² under the same experimental conditions. For approximately 30 s after the initial reaction between these liquid metals and gaseous oxygen, the oxygen absorption rate was proportional to the square root of the oxygen pressure above the melt, and proportional to the melt surface area, but independent of melt volume. The rate-limiting step for oxygen absorption by liquid iron, cobalt and copper can be described by dissociative adsorption of oxygen molecules at the gas/oxide interface. After 30 s of reaction, the rate of oxygen absorption became less dependent on the oxygen pressure above the melt. This indicated that the rate-controlling step was changing from a surface reaction to growth of the oxide layer by cationic diffusion in the bulk oxide. The oxidation rate of liquid nickel appears to be too complex to be described by models for dissociative adsorption of oxygen molecules at the gas/oxide interface and parabolic growth of the oxide layer. The formation of a thin layer of nickel oxide which allows oxygen to migrate through cracks or grain boundaries may be responsible for the relatively high oxygen absorption rate compared to that of liquid cobalt. R. H. RADZILOWSKI, formerly a Graduate Studient at The University of Michigan     

The kinetics of deoxidation of copper and copper alloys by carbon monoxide

The kinetics of deoxidation of molten copper, Cu-50 pct Ag and Cu-60 pct Sn alloys were studied. The solid electrolyte EMF technique was used to measure the oxygen content as a function of time. The gas was introduced as bubbles emitted from various-sized orifices placed in the melt. Liquid phase mass transfer control is implied by the results for deoxidation in the range 0.05 pct oxygen down to 0.005 pct, where the chemical reaction rate is inferred to become significant in controlling the overall rate. Hughmark’s mass transfer correlation is tested and found to be useful in predicting the liquid phase mass transfer coefficient. This paper is based on a portion of the Dissertation submitted by C. R. NANDA to the Graduate School of the University of Wisconsin in partial filfillment of the requirements for the Ph.D, degree. The research was conducted while the authors were, respectively, Research Assistant and Associate Professor in the Department of Minerals and Metals Engineering, University of Wisconsin, Madison, Wis.     

Preparation method of Ti powder with oxygen concentration of <1000 ppm using Ca

Abstract

To reduce the high oxygen concentration of commercial Ti powder and to obtain Ti powder with the oxygen concentration of <1000 ppm, we utilised two types of deoxidation pots to conduct deoxidation experiments: below the melting point of Ca using a contact deoxidation pot (experiment A), and above its melting point using a non-contact deoxidation pot (experiment B). To obtain Ti in powder form even after deoxidation above the melting point of Ca, we developed a new non-contact deoxidation pot in the experiments. In experiment A, the oxygen concentration in the Ti powder decreased down to 50% compared with the initial stage (2200 ppm). In experiment B, the oxygen concentration reduced to ～63% at the deoxidation temperature of 1000°C. As a result, Ti powder with 820 ppm of oxygen concentration could be prepared using the non-contact deoxidation pot with Ca.     

Thermodynamics of the oxygen solutions in aluminum-containing Fe-Co melts

Thermodynamic analysis of aluminum-containing Fe-Co melts is performed. The equilibrium constants of the deoxidation of iron-cobalt melts with aluminum, the activity coefficients during infinite dilution, and the interaction parameters in melts with various compositions are determined. The oxygen solubility in the melts under study is studied as a function of the cobalt and aluminum contents. Aluminum is characterized by a very high affinity to oxygen in iron-cobalt melts. The deoxidizing capacity of aluminum substantially increases with the cobalt content in the melt. The curves of the oxygen solubility in aluminum-containing iron-cobalt melts have a minimum, whose position shifts to lower aluminum contents as the cobalt content in the melt increases. Further aluminum additions increase the oxygen concentration in the melt: the higher the cobalt content in the melt, the sharper the increase in the oxygen concentration after the minimum when aluminum is added to the melt. The aluminum contents at the minimum points in the oxygen solubility curves are determined, and the corresponding minimum oxygen concentrations are found.     

A model for the silicon-manganese deoxidation of steel weld metals

From an analytical and theoretical study of flat and out-of-position gas metal arc (GMA) C-Mn steel welds containing varying additions of silicon and manganese, we conclude that the buoyancy effect (flotation obeying Stokes’ law) does not play a significant role in the separation of oxide inclusions during weld metal deoxidation. Consequently, the separation rate of the particles is controlled solely by the fluid flow pattern in the weld pool. A proposed two-step model for the weld metal deoxidation reactions suggests that inclusions formed in the hot, turbulent-flow region of the weld pool are rapidly brought to the upper surface behind the arc because of the high-velocity flow fields set up within the liquid metal. In contrast, those formed in the cooler, less-turbulent flow regions of the weld pool are to a large extent trapped in the weld metal as finely dispersed particles as a result of inadequate melt stirring. The boundary between “hot” and “cold” parts for possible inclusion removal is not well defined, but depends on the applied welding parameters, flux, and shielding gas composition. As a result of the intricate mechanism of inclusion separation, the final weld metal oxygen content depends on complex interactions among the following three main factors: (1) the operational conditions applied, (2) the total amount of silicon and manganese present, and (3) the resulting manganeseto-silicon ratio. The combined effect of the latter two contributions has been included in a new deoxidation parameter, ([pct Si][pct Mn])^−0.25. The small, negative exponent in the deoxidation parameter indicates that control of the weld metal oxygen concentrations through additions of silicon and manganese is limited and that choice of operational conditions in many instances is the primary factor in determining the final degree of deoxidation to be achieved.     

Inclusion control of ferritic stainless steel by aluminum deoxidation and calcium treatment

A thermodynamic equilibrium between aluminum and oxygen and inclusion morphology in the Fe-16Cr stainless steel were investigated to understand the fundamentals of the aluminum deoxidation technology for ferritic stainless steels. Further, the effect of calcium addition on the changes in chemistry and morphology of inclusions was discussed. The measured results for the aluminum-oxygen equilibria exhibit relatively good agreement with the calculated values, indicating that an introduction of the first-and second-order interaction parameters, recently reported, is reasonable to numerically express the aluminum deoxidation equilibrium in a ferritic stainless steel. In the composition of dissolved aluminum content greater than about 60 ppm, pure alumina particles were observed, while the alumino-manganese silicates containing Cr₂O₃ appeared at less than 20 mass ppm of dissolved aluminum. The formation of calcium aluminate inclusions after Ca treatment can be discussed based on the thermodynamic equilibria among calcium, aluminum, and oxygen in the steel melt. In the composition of steel melt with relatively high content of calcium and low aluminum, the log ( ) of inclusions linearly increases by increasing the log [a _Ca/a _Al ² ·a _O ² ] with the slope close to unity. However, the slope of the line is significantly lower than the expected value in the composition of steel melt with relatively low calcium and high aluminum contents.     

Absorption of gaseous oxygen by liquid cobalt, copper, iron and nickel

An experimental investigation of the rates of oxygen solution in molten cobalt, copper, iron and nickel was carried out using pure oxygen and a constant-volume Sieverts' method. It was found that the volume of gaseous oxygen which initially reacted with the inductively stirred metals was strongly dependent on the physical nature of the oxide film which formed during the first stage of reaction. The initial temperature of the molten iron, cobalt, and nickel was 1600‡C, and for copper was 1250‡C. For initial oxygen pres-sures above the melt of about one atmosphere both molten iron and copper, which formed liquid surface oxides, initially absorbed nearly 20 cm³ (STP) O₂/cm² of melt surface area, while molten cobalt and nickel, which formed solid oxides, absorbed about 6 cm³ (STP) 0₂/cm² under the same experimental conditions. For approximately 30 s after the initial reaction between these liquid metals and gaseous oxygen, the oxygen absorption rate was proportional to the square root of the oxygen pressure above the melt, and pro-portional to the melt surface area, but independent of melt volume. The rate-limiting step for oxygen absorption by liquid iron, cobalt and copper can be described by dissocia-tive adsorption of oxygen molecules at the gasJoxide interface. After 30 s of reaction, the rate of oxygen absorption became less dependent on the oxygen pressure above the melt. This indicated that the rate-controlling step was changing from a surface reaction to growth of the oxide layer by cationic diffusion in the bulk oxide. The oxidation rate of liquid nickel appears to be too complex to be described by models for dissociative ad-sorption of oxygen molecules at the gasJoxide interface and parabolic growth of the oxide layer. The formation of a thin layer of nickel oxide which allows oxygen to migrate through cracks or grain boundaries may be responsible for the relatively high oxygen ab-sorption rate compared to that of liquid cobalt. Formerly a Graduate Student at The University of Michigan     

Kinetics of manganous oxide sulfidization in carbonyl sulfide (COS) contaminated atmospheres

To contribute to the mitigation of man-made emission of sulfurous compounds, the susceptibility of manganous oxide for carbonyl sulfide under reducing atmospheres (C-O-S system) has been investigated over the temperature range 700 to 1010 °C (973 to 1283 K). The kinetic investigation employed thermogravimetry and anin situ solid electrolyte oxygen probe to follow the topochemical reaction of spherical MnO pellets under various experimental conditions. The apparent activation energy of sulfidization of manganous oxide, under measured oxygen potentials in the C-O-S system, was determined to be 12.06 (±1.5) kcal/mol (52.33 (±6.28) kJ/mol). Overall sulfidization appeared to proceed by mixed control involving convective mass transfer of COS across the boundary layer and diffusion through the product layer. Formerly with the Department of Theoretical Metallurgy, Royal Institute of Technology, Stockholm