The thermal characteristics of the electroslag process

The heat balance of a laboratory electroslag furnace working in quasisteady state is considered in detail. Results are presented for all possible melting mode combinations of direct and alternating (60Hz) current, with live and insulated molds, in air or argon atmospheres. The material studied is AISI 4340 steel, using a slag of CaF₂ + 25 wt pct A1₂O₃. The temperature and potential fields of the slag are determined, together with heat fluxes in the furnace. Heat balances are given for each subregion of the process, showing good agreement where results are available which permit cross checks of the balance. Suggested explanations are given for the differing behavior of the various melting modes based on variations in effective slag resistivity due to electrochemical reactions, and on variations in current path. The most important factors in determining the ingot heat balance are shown to be the electrode immersion, the slag volume’s dimensions, and the depth of the cylindrical liquid metal head on the ingot. Formerly Graduate Student, Department of Metallurgy, University of British Columbia,.     

The solution rate of alumina in CaF2-Al2 O3 slags

The rate of solution of A1₂O₃ in CaF₂ + 30 wt pct A1₂O₃ (at 1518° and 1509°C) and CaF₂ + 20 wt pct A1₂O₃ (at 1500°C) liquids has been determined. The operative process is diffusion-controlled, with an interdiffusion coefficient,D for the process varying between 8.5 and 8.1 x 10^-5 sq cms ^- 1 in the CaF₂ + 30 wt pct A1₂O₃ slags, and 4.0 × 10^-5 sq cms ^- 1 in the CaF₂ + 20 wt pct A1₂O₃ slag. Estimations of the rate at which alumina inclusions would react with these slag during the electroslag processing of steels, indicate that electrode inclusions approaching 100 μ in diam will be dissolved.     

Effect of Slag on Inclusions During Electroslag Remelting Process of Die Steel

Many factors influence the non-metallic inclusions in electroslag steel including furnace atmosphere and inclusions’ content in the consumable electrode, slag amount and its composition, power input, melting rate, filling ratio, and so on. Fluoride containing slag, which influences the non-metallic inclusions to a great extent, has been widely used for the electroslag remelting process. The current paper focuses on the effect of fluoride containing slag on the inclusions in electroslag ingots based on the interaction of the slag-metal interface and electroslag remelting process. In this work, die steel of CR-5A and several slags have been employed for investigating the effect of slag on inclusions in an electrical resistance furnace under argon atmosphere in order to eliminate the effect of ambient oxygen. Specimens were taken at different times for analyzing the content, dimensions, and type of non-metallic inclusions. Results of quantitative metallographic analysis indicate that a multi-component slag has better capacity for controlling the amount of inclusions; especially protective gas atmosphere has also been adopted. The findings of inclusions in electroslag steel by SEM–EDS analysis reveal that most non-metallic inclusions in electroslag steel are MgO-Al₂O₃ inclusions for multi-component slags, but it is Al₂O₃ inclusions when remelting using conventional 70 wt pct CaF₂-30 wt pct Al₂O₃ slag. The maximal inclusions’ size using multi-component slags is less than that using conventional binary slag. Small filling ratio as well as protective gas atmosphere is favorable for controlling the non-metallic inclusions in electroslag steel. All the results obtained will be compared to the original state inclusions in steel, which contribute to choice of slag for electroslag remelting.     

Electrode polarization in the DC electroslag melting of pure iron

It is evident from the known ionic properties of the slags used in electroslag melting, that the dc melting process must be accompanied by Faradaic reactions on the slag/ingot and slag/electrode interfaces. The present work has determined the magnitude of the overpotentials resulting from concentration polarization at these interfaces, in the case of pure iron/CaF₂+Al₂O₃, CaF₂+CaO slags using a galvanostatic pulsing technique in an electrolytic cell. The polarization overpotential existing on an electrode in an operating ESR unit has been measured by the same technique. It is found that the potentials observed on the ESR electrode agree well with the results from the electrolytic cell. The primary anodic process is postulated to be the corrosion of iron, leading to an Fe²⁺-saturated layer on the anode surface at sufficiently high current densities. The cathodic process is suggested to be the Faradaic reduction of Al³⁺ or Ca²⁺, to give a concentration of [Al]_Fe or (Ca)_slag in the cathode interface region. This observation is supported by the fact that the cathodic potentials with respect to a C/CO reference electrode are close to those predicted from the reactions: (Al₂O₃)+3C=3CO(g)+2Al(l) or (CaO)+C=CO(g)+Ca(g) At very high current densities both the anodic and cathodic processes may convert to arcs, leading to process instability. The chemical and thermal effects of the overpotentials are briefly discussed and compared with the present results on ESR ingots of pure iron.     

Kinetics of chromium oxide reduction from a basic steelmaking slag by silicon dissolved in liquid iron

The reduction of chromium oxide from a basic steelmaking slag (45 wt pct CaO, 35 wt pct SiO₂, 10 wt pct MgO, 10 wt pct A1₂O₃) by silicon dissolved in liquid iron at steelmaking temperatures was studied to determine the rate-limiting steps. The reduction is described by the reactions: (Cr₂O₃) + Si = (SiO₂) + (CrO) + Cr [1] and 2 (CrO) +Si = (SiO₂) + 2 Cr [2] The experiments were carried out under an argon atmosphere in a vertical resistance-heated tube furnace. The slag and metal phases were held in zirconia crucibles. The course of the reactions was followed by periodically sampling the slag phase and analyzing for total chromium, divalent chromium, and iron. Results obtained by varying stirring rate, temperature, and composition defined the rate-limiting mechanism for each reaction. The rate of reduction of trivalent chromium (reaction [1] above) increases with moderate increases in stirring of the slag, and increases markedly with increases in temperature. The effects of changes in composition identified the rate-limiting step for Cr⁺³ reduction as diffusion of Cr⁺³ from the bulk slag to the slag-metal interface. The rate of reduction of divalent chromium does not vary with changes in stirring of the slag, but increases in temperature markedly increase the reaction rate. Thus, this reaction is limited by the rate of an interfacial chemical reaction. The reduction of divalent chromium is linearly dependent on concentration of divalent chromium, but is independent of silicon content of the metal phase.     

Thermodynamics of chromium oxides in CaO-SiO2-CaF2 slag

The distribution ratio of chromium between a CaO-SiO₂-CaF₂ slag and liquid silver under the oxygen partial pressure used in practical hot-metal dephosphorization treatment was measured at 1623 K. The distribution ratio was minimal when the basicity index of a slag, wt pct CaO/wt pct SiO₂, was about 2. The redox equilibrium between CrO (Cr²⁺) and CrO_1.5 (Cr³⁺) in the slag was also measured as a function of slag composition. The calculated activity coefficient of CrO had a maximum value at wt pct CaO/wt pct SiO₂=2, whereas that of CrO_1.5 decreased monotonously with the increase in slag basicity.     

On the origin of oxide inclusions in ingots made by the electroslag process

Abstract

A study of the reactions leading to production of oxide inclusions in electroslag remelted ingots of pure iron. The material was prepared by 60Hz electroslag melting under argon using slags in the system CaF₂ + CaO + Al₂O₃. The possible chemical paths open to the system were calculated using available thermochemical data, but none of these paths could account for the experimental analyses. It is suggested that a small amount of rectification, arising in asymmetric electrochemical reactions, would lead to the observed ingot compositions. Examination of the ingot inclusion compositions suggests that the oxide particles are precipitated within the freezing metal interface. Since both the reaction path and the inclusion precipitation mechanism are inherent in the electroslag method, it is concluded that the small oxide particles are a necessary result of electro slag melting.

Résumé

Les auteurs ont étudié la formation d'inclusions d'oxides dans des lingots de fer pour être refondus sous laitier électroconducteurs. Ces lingots ont été préparés par fusion sous laitier électroconducteur à 60 Hz sous argon et en utilisant des laitiers du système CaF₂ + CaO + Al₂O₃. Les différentes voies chimiques possibles pour Ie système ont été calculées en utilisant les données thermodynamiques disponibles mais aucune ne ref1était les résultats. Il est alors suggéré qu'une faible rectification se produisant par des réactions électrochimiques assymétriques, conduise à la composition observée. La composition des inclusions permet de penser que les oxydes sont précipités à l'interface de solidification. Comme le chemin de la réaction et le mécanisme de précipitation des inclusions sont inhérents a cette méthode de fusion, il appert que les petites particules d'oxyde sont necesairement produites lors de la fusion sous laitier électroconducteur.     

Recovery Behavior of Rare Earth from Bayan Obo Complex Iron Ore

A novel process is presented for recovering rare earth from Bayan Obo complex iron ore. The iron ore was reduced and melting separated to produce iron nugget and rare-earth-rich slag. In order to investigate the influence of cooling rate on mineral components, especially the enrichment behavior of RE-containing mineral, the slag was remelted at 1673 K (1400 °C) and the liquid slags were cooled using three types of cooling conditions, water quenching, air cooling, and furnace cooling. Subsequently, the slags were leached by hydrochloric acid to evaluate the relations between leaching efficiency of rare earth and cooling conditions. The results indicated that the slags under different cooling conditions mainly contained fluorite, cefluosil, and cuspidine. The rare-earth mineral is more fully crystallized when the cooling rate of the liquid slag was decreased. The proportion of Ce (III) to Ce (IV) increases with the increase of heating time and decrease of cooling rate. It has been found that the influence of cooling rate on the leaching rate of the rare earth is slight. From water quenching to furnace cooling, the leaching rate of rare earth increases from 97.00 pct to 99.48 pct. After being filtered, filtrate can be used to produce rare-earth chloride. Leached residue, with CaF₂ of 64.45 pct and ThO₂ of 0.05 pct, can be used to recover CaF₂ and extract nuclear source material.     

Kinetics of reduction of lead oxide in liquid slag by carbon in iron

The reduction of lead oxide in dilute solution in CaO-Al₂O₂-SiO₃ slag by carbon dissolved in iron was investigated using a composite crucible as a container so as to exclude graphite from the system. The variables studied to elucidate the reaction mechanism were pressure inside the crucible, carbon content of the metal, lead oxide concentration in slag, and slag composition. The experimental results are best explained by postulating the existence of a gas film at the slag metal interface. It is suggested that the rate controlling step for the lead oxide reduction by carbon is a chemical reaction at the gas/slag interface. The rate constant for up to 3 wt pct PbO in the slag and 2.0 to 4.3 pct C in iron at 1400 °C as calculated from the present study is 4.6 x 10^-4 mol/cm²/min/atm.     

Interfacial phenomena in some slag-metal reactions

In the present work, the change of the interfacial tension at the slag-metal interface for sulfur transfer between molten iron, slag, and gas phases was monitored by X-ray sessile drop method in dynamic mode in the temperature range of 1830 to 1891 K. The experiments were carried out with pure iron samples immersed partly or fully in the slag phase. The slag consisted of 30 wt pct CaO, 50 wt pct Al₂O₃, and 20 wt pct SiO₂ (alumina saturated at the experimental temperatures) with additions of FeO. Metal and slag samples contained in alumina crucibles were exposed to a CO-CO₂-SO₂-Ar gas mixture with defined oxygen and sulfur partial pressures, and the change of the shape of the metal drop was determined as a function of time. The equipment and the technique were calibrated by measurements of the surface tensions of the pure Cu, Ni, and Fe containing two different amounts of dissolved oxygen. A theoretical model was developed to determine the sulfur content of the metal as a function of time on the basis of sulfur diffusion in the slag and metal phases as well as surface tension-induced flow on the metal drop surface. Attempts were made to compute the interfacial tensions on the basis of force balance. This article is based on a presentation made in the “Geoffrey Belton Memorial Symposium,” held in January 2000, in Sydney, Australia, under the joint sponsorship of ISS and TMS.     

Manganese Recovery by Silicothermic Reduction of MnO in BaO-MnO-MgO-CaF<Subscript>2</Subscript> (-SiO<Subscript>2</Subscript>) Slags

The effects of reducing agent, CaF₂ content, and reaction temperature upon the silicothermic reduction of MnO in the BaO-MnO-MgO-CaF₂ (-SiO₂) slags were investigated. Mn recovery was proportional to Si activity in the molten alloy. Moreover, 90 pct yield of Mn recovery was obtained under 5 mass pct CaF₂ content and 1873 K (1600 °C) reaction temperature. Increasing CaF₂ content above 5 pct yielded little or no further increase in Mn recovery, because it was accompanied by increased slag viscosity owing to the precipitation of high melting point compounds such as Ba₂SiO₄.     

The effect of Na2O on dephosphorization by CaO-based steelmaking slags

The effect of Na₂O on the equilibrium phosphorous distribution ratio between slag and iron or iron alloys, L_P, has been measured for CaO-SiO₂, CaO-FeOr-SiO₂ (CaO or 2CaO·SiO₂ saturated), and CaO-Al₂-SiO₂ slags. The addition of Na₂O to CaO-SiO₂ slags significantly increases L_P and the phosphate capacity. A 25 pct CaO-25 pct Na₂O-SiO₂ slag has a distribution ratio nearly two orders of magnitude greater than a comparable binary 50 pct CaO-SiO₂ slag. For the CaO-saturated slags containing 40 wt pct FeO_T, the addition of 6 wt pct Na₂O increases L_P by a factor of 5. For the 2CaO·SiO₂-saturated CaO-FeO_T-SiO₂ slag, there is an optimum FeOr content (20 wt pct) for dephosphorization, and 10 wt pct Na₂O increases L_P by a factor of 2. For reducing slags typically used in ladle metallurgy for Al-killed steels (50 pct CaO-40 pct Al₂O₃-10 pct SiO₂), as little as 3 wt pct Na₂O increases L_P by a factor of 100. The present results indicate that small additions of Na₂O to conventional steelmaking slags can greatly improve dephosphorization. Formerly Graduate Student, Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University.     

Experimental evidence for electrochemical nature of the reaction between iron oxide in calcia-silica-alumina slag and carbon in liquid iron

The electrochemical nature of the reaction between iron oxide in calcia-silica-alumina slag and carbon in liquid iron has been studied by measuring the kinetics of the slag-metal reaction. A base slag (48 pct CaO-40 pct SiO₂-12 pct Al₂O₃) containing iron oxide (≤8 wt pct FeO _t ) was reduced by an Fe-C metal bath (∼4 wt pct C) at 1400 °C. The reaction rate was calculated from measurements of the total inlet gas flow rate and the CO concentration in the outlet gas stream. The slag was “internally short circuited” by dipping an iron plate through the slag layer, and this resulted in an increase in the rate of CO evolution. An external circuit was produced by dipping a graphite rod (shielded from the slag) into the metal bath and a steel or molybdenum rod into the slag layer; the open-circuit voltage and short-circuit current were measured when iron oxide was added to the base slag layer. The reaction rate was enhanced by applying a voltage across the slag layer, and an electric arc cathode was employed in some of these “electrolysis” experiments.     

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.     

The reaction behavior of Fe-C-S droplets in CaO-SiO2-MgO-FeO slags

The rate of reduction of FeO in the slag by carbon in iron droplets (2.9 wt pct C, 0.01 wt pct S) was studied for CaO-SiO₂-MgO slags containing between 3 and 35 wt pct FeO and temperatures ranging from 1643 to 1763 K. The effects of Fe₂O₃ additions to the slag and sulfur variations in the metal on the reaction rate were also studied. It was found that the behavior of the metal droplets in the slag, as observed by X-ray fluoroscopy, changed significantly with FeO content in the slag. Below 10 wt pct FeO, the droplet remained intact while reacting with the slag; however, above this FeO concentration, the droplet became emulsified within the slag. The large increase in surface area of the metal droplet due to emulsification caused the rate of reaction to be one to two orders of magnitude faster than for droplets that did not become emulsified. It was suggested that when the droplet is emulsified, the surface area and reaction kinetics are greatly increased, and the rate becomes controlled by mass transfer of FeO as Fe²⁺ and O²⁻ ions in the slag to the emulsified droplet. At low FeO contents for which the droplet does not emulsify, the rate is controlled by dissociation of CO₂ on the metal. It was also found that a critical temperature exists for a given FeO content at which point the rate of CO evolution increases dramatically. Additions of Fe₂O₃ to the slag and sulfur to the metal caused significant changes to the rate of reaction possibly by affecting the emulsification behavior of the droplet.     

Some observations on current transfer at iron/calcium fluoride electrodes – Model experiments for current transfer in the ESR process

Measurement of current density-potential curves with direct and alternating current at iron electrodes immersed into CaF₂ or CaF₂/10wt.% CaO slag. Theoretical interpretation of overpotentials in low current range. Dependence of overpotential on FeO content of slag.     

Development of Low-Fluoride Slag for Electroslag Remelting: Role of Li<Subscript>2</Subscript>O on the Viscosity and Structure of the Slag

The effect of the substitution of CaF₂ with Li₂O on the viscosity and structure of low-fluoride CaF₂-CaO-Al₂O₃-MgO slag was studied with an aim to develop low-fluoride slag for electroslag remelting. Increasing Li₂O addition up to 4.5 mass pct was observed to significantly reduce the slag viscosity monotonically. Increasing temperature significantly lowered the viscosity of slag, whereas this influence is less effective with increasing Li₂O content especially above 3.5 mass pct. The activation energy for viscous flow decreases with increasing Li₂O content. The polymerization degree of aluminate networks decreased with increasing Li₂O content, as demonstrated by Raman analysis. The dominant structural unit in [AlO₄]^5?-tetrahedral network is \( {\text{Q}}_{\text{Al}}^{4} \). The amount of symmetric Al-O⁰ stretching vibrations significantly decreased with increasing Li₂O content. The relative fraction of \( {\text{Q}}_{\text{Al}}^{4} \) in the [AlO₄]^5?-tetrahedral units shows a decreasing trend, whereas \( {\text{Q}}_{\text{Al}}^{2} \) increases with the increase in Li₂O content accordingly. The change in slag viscosity with chemistry variation agrees well with the changes in slag structural units.     

Solid-State Metalized Reduction of Magnesium-Rich Low-Nickel Oxide Ores Using Coal as the Reductant Based on Thermodynamic Analysis

The solid-state metalized reduction of magnesium-rich low-nickel oxide ore using coal as a reductant was studied based on thermodynamic analysis. The major constituent minerals of the ore were silicates and goethite. The former was the main nickel-bearing mineral, and the latter was the main iron-bearing mineral. Single factor tests were conducted to investigate the effects of reduction temperature, duration, and coal dosage on the beneficiation of nickel and iron such that optimal conditions were achieved. Considering the low recoveries of nickel and iron (Ni, 13.9 pct; Fe, 30.3 pct) under the obtained optimal conditions, an improved process, adding CaF₂ before the reaction, was proposed to modify the solid-state metalized process. The results showed that the recoveries of nickel and iron reached to 96.5 and 73.4 pct, respectively, and that the grades of nickel and iron in the concentrate increased from 2.5 and 62.6 wt pct to 6.9 and 71.4 wt pct, respectively. Nickel and iron in the absence of CaF₂ were metalized; nevertheless, the size of ferronickel particles was only 1 μm. Furthermore, alloys in the presence of CaF₂ aggregated and exhibited bands with a length greater than 200 µm. These observations suggested that CaF₂ could effectively reduce the surface tension of the newly generated alloy interface and promote the migration and polymerization of the alloy particles, which improves the beneficiation of nickel and iron by magnetic separation.     

Rate of interfacial reaction between molten CaO-SiO2-Al2O3-Fe x O and CO-CO2

Measurements of the rates of reduction of iron oxide from molten CaO-SiO₂-Al₂O₃-Fe _x O slags by Ar-CO mixtures have been made using a thermogravimetric method. The apparent first-order rate constant, with respect to the partial pressure of CO, of the gas/slag interfacial reaction was deduced from the measured rates, where the effects of the mass transfer in the gas and slag phases were minimized. It was found that the apparent first-order rate constant decreased with the concentration of ‘FeO’ from 100 to 20 wt pct, whereas it remained essentially constant in the range from 5 to 20 wt pct ‘FeO’. At a given iron oxide concentration, the reduction-rate constant increased significantly with an increase in the CaO/SiO₂ ratio. For fixed slag compositions, the reduction rate increased slightly with the oxidation state of the slags. When the rate constant is expressed in the form of k=k′(Fe³⁺/²⁺)^α, the values of α range from 0.15 to 0.25. The effect of temperature in the range from 1673 to 1873 K on the reduction rate of iron oxide in a 40.4CaO-40.4SiO₂-14.2Al₂O₃-5‘FeO’ (wt pct) slag was studied. The calculated activation energy, based on these results, is 165 kJ/mol.     

The apparent solubility of aluminum in cryolite melts

The apparent solubility of aluminum in cryolite melts saturated with A1₂O₃ has been determined by titration with electrolytically generated O₂. The results may be expressed by wt pct Al = − 0.2877 + 0.0268 (NaF/AlF₃ wt ratio) + 2.992 × 10⁻⁴ (temp °C) − 0.00192 (% CaF₂) −0.00174 (% Li₃AlF₆) −0.00288 (% NaCl) with a standard deviation of ±0.017. Ranges covered were ratio 0.8 to 2.3, temperatures 969° to 1054°C, CaF₂ ≤ 14 pct, Li₃AlF₆ ≤ 20 pct, and NaCl ≤ 10 pct. There was no significant effect of adding 0 to 38. pct K₃A1F₆ or 0 to 10 pct MgF₂. It was found that solubility was approximately proportional to activity of aluminum when Al-Cu alloys were used. Possible mechanisms of solution are discussed. Monovalent aluminum is ruled out on the basis of the variation of solubility with NaF/AlF₃ ratio and a_Al. The favored, but not proven, mechanism involves formation of both sodium atoms and a colloidal dispersion of aluminum.