Study on electrical conductivity of FexO–CaO–SiO2–Al2O3 slags

The composition dependences of electrical conductivity of Fe_xO–CaO–SiO₂–Al₂O₃ slags at different oxygen potentials and temperatures have been studied experimentally in the present work. From the experimental results, the total electrical conductivity and electronic conductivity for all the slags monotonously decrease as increasing CO/CO₂ ratio from about 0 to 1. With the increase of Fe_xO content, the total electrical conductivity and electronic conductivity increase at a fixed CO/CO₂ ratio. It is also found that the ionic conductivity of all the studied slags increases as increasing the CO/CO₂ ratio, which is resulted from the increase of Fe²⁺ ion concentration. In addition, the temperature dependences of ionic, electronic and total conductivity for different compositions obey the Arrhenius law. The electronic transference number exhibits a strong relationship with oxygen potential, but is independent of temperature.     

Determination of the chemical diffusion of oxygen in liquid iron oxide at 1615 °c

The chemical diffusion of oxygen in liquid iron oxide has been studied by the oxidation of a melt in a long capillary at 1615 °C. When pure oxygen was used as the oxidizing agent, the surface composition of the slag was found to be in close agreement with the expected gas-slag equilibrium, suggesting that diffusion is the controlling step. This was not the case when air, 5 pct oxygen in argon or pure CO₂ was used to oxidize the slag. The deviation of the surface composition from the expected equilibrium was in accordance with a mechanism of mixed control by both the gas-slag reaction and diffusion in the bulk. The average value of the chemical diffusivity of oxygen (or iron) in liquid iron oxide with Fe²⁺/Fe _T between 0.25 and 0.77 was established to be 3(±1) × 10^-7 m²/s. This value is one to two orders of magnitude higher than those from earlier studies. There seems to be a reasonable correlation between the chemical and the ionic self-diffusivities through the Darken equation. A quantitative analysis in this respect and on the role of electron hole migration depends on the availability of data on the ionic conductivity and the tracer diffusivities. Formerly Postgraduate Student     

Charge transfer at Fe/FeO(CaF2) electrodes at 1450 ‡C: Exchange current density,electrode capacitance,diffusivity

The kinetics of the electrode reaction Fe = Fe _slag ²⁺ + 2e ^- has been investigated using the single and double pulse techniques. It was found that the exchange current densities are very large increasing from 2 to 10 A cm^-2 in the range of Fe²⁺-concentration from 2 · 10^-6 to 30 · 10^-6 mole cm⁻³. The electrode capacitance increases from 50 ΜF cm^-2 approximately linearly with the square root of Fe²⁺-concentration. An attempt was made to explain the concentration dependent part of the capacitance in terms of a model involving electrosorption of the speciesFe _ad ⁿ⁺ at the slag/iron interface. If the adsorbed iron ions are the intermediates in a consecutive charge transfer mechanism, the rate determining step is the transfer of iron between the adsorbed layer and the slag. On the other hand, it may be possible that the transfer is in one step with adsorption occurring in parallel. Formerly with Max-Planck Institut für Eisenforschung, D-4000 Düsseldorf, Germany     

The reduction of zinc from slags by an iron-carbon melt

An experimental investigation was undertaken to study the mechanism of reduction of zinc from slags in the presence of a carbon-saturated iron melt. Batch tests were performed at 1400 °C, and the variation of the zinc and iron concentration in the slag during reduction was determined by sampling the slag at intervals during the test. In graphite crucibles, zinc in slags containing iron was reduced faster than zinc in iron-free slags, both when an iron bath was present and when it was absent. Zinc was reduced faster from slags containing iron when an iron bath was present than when an iron bath was absent. The dominant mechanism of reduction of zinc from slags containing iron appears to be the reaction of Zn²⁺ ions with Fe²⁺ ions to form zinc vapor and Fe³⁺ ions. When an iron bath is present, the Fe³⁺ ions are reduced back to Fe²⁺ predominantly by reaction with iron from the bath. Mass transfer of Fe³⁺ ions in the slag appears to be the rate-controlling step. Reduction of iron from slag by carbon occurred in parallel with the reduction of zinc, and whether there was a net increase or decrease of iron in the slag depended on the relative rates of production and consumption of iron. Lead and copper in the slag were reduced to low levels. The lead volatilized and the copper dissolved in the alloy.     

Electrical properties of iron doped apatite-type lanthanum silicates

The effect of Fe doping on the electrical properties of lanthanum silicates was investigated. The apatite-type lanthanum silicates La10Si6-xFexO27-x/2 (x=0.2, 0.4, 0.6, 0.8, 1.0) were synthesized via sol-gel process. The unit cell volume increased with Fe doping because the ionic radius of Fe3+ ion is larger than that of Si4+ ion. The conductivities of La10Si6-xFexO27-x/2first increased and then decreased with the in-creasing of Fe content. The increase of the conductivity might be attributed to the distortion of the cell lattice, which assisted the migration of the interstitial oxygen ions. The decrease of the conductivity might be caused by the lower concentration of interstitial oxygen ions. The op-timum Fe doping content in lanthanum silicates was 0.6. La10Si5.4Fe0.6O26.7 exhibited the highest ionic conductivity of 2.712×10-2 S/cm at 800 ℃. The dependence of conductivity on oxygen partial pressure p(O2) suggested that the conductivity of La10Si6-xFexO27-x/2 was mainly con-tributed by ionic conductivity.     

New Methods for In‐Situ Determination of Iron Oxide in Steelmaking Slags

Although iron oxide is the most important component of steelmaking slags, no reliable technique for its in‐situ determination has been established yet. This paper however, presents data on the effect of iron oxide on the electrical conductivity of CaO‐SiO₂‐FeO‐Fe₂O₃ melts, and on limiting current and impedance in direct current or alternating current charge transfer at iron electrodes. The strong influence of iron oxide content can be utilized for in‐situ determination of total iron oxide content and Fe³⁺/Fe²⁺ ratio. The possible probe designs are presented and the principles and procedures of the measurement are explained.     

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     

Reaction mechanism for the acid ferric sulfate leaching of chalcopyrite

The acid ferric sulfate leaching of chalcopyrite, CuFeS₂ + 4Fe⁺³ = Cu⁺² + 5Fe⁺² + 2S⁰ was studied using monosize particles in a well stirred reactor at ambient pressure and dilute solid phase concentration in order to obtain fundamental details of the reaction kinetics. The principal rate limiting step for this electrochemical reaction appears to be a transport process through the elemental sulfur reaction product. This conclusion has been reached in other investigations and is supported by data from this investigation in which the reaction rate was found to have an inverse second order dependence on the initial particle diameter. Furthermore, the reaction kinetics were found to be independent of Fe⁺³, Fe⁺², Cu⁺² and H₂SO₄ in the range of additions studied. The unique aspect of this particular research effort is that data analysis, using the Wagner theory of oxidation, suggests that the rate limiting process may be the transport of electrons through the elemental sulfur layer. Predicted reaction rates calculated from first principles using the physicochemical properties of the system (conductivity of elemental sulfur and the free energy change for the reaction) agree satisfactorily with experimentally determined rates. Further evidence which supports this analysis includes an experimental activation energy of 20 kcal/mol (83.7 kJ/mol) which is approximately the same as the apparent activation energy for the transfer of electrons through elemental sulfur, 23 kcal/ mol (96.3 kJ/mol) calculated from both conductivity and electron mobility measurements reported in the literature. formerly Metallurgy Graduate Student, University of Utah.     

A mössbauer study of the behavior of iron cations in iron oxide-containing melts at 1400 °C

A Mössbauer spectroscopy technique has been developed to study the distribution of iron cations in slag melts as a function of distance from a reaction surface. A wide range of slag compositions of interest to the nonferrous smelting industry was reduced and oxidized using mixtures of CO, CO₂, and Ar. Mössbauer spectra were collected on quenched slag samples for varying conditions between iron saturation and magnetite saturation, l(10^-13) to 4(10^-6) atm P_O2 at 1400 °C. Slag sections were analyzed with a spatial resolution of approximately 500 /am, and details of the ferric and ferrous distribution at the surface and in subsurface regions were studied. In Fe_xO-SiO₂-Al₂O₃ melts exposed to a reducing gas mixture, there is a gradient in ferric iron concen-tration decreasing toward the surface. In the Fe_xO-Al₂O₃ melt, the reduction of ferric anions at the surface appears to release O_2- ions, which increases the coordination of Fe²⁺. However, in the presence of silica, which is surface active and acidic, reduction of ferric iron does not lead to an excess of O_2- at the surface. Lime has a dramatically different effect. It reverses the ferric gradient so that, even during reduction, the ferric concentration at the surface is enhanced. Ferric oxide is surface active in these melts, and under certain conditions, liquid-phase mass transfer is rapid enough to replenish the surface. The presence of a polymer-forming ion, Fe³⁺, at the surface results in a scarcity of O^2- ions and a reduced coordination of Fe²⁺. In the presence of both SiO₂ and CaO, the behavior of ferric iron during reduction depends, to an extent, on the CaO/SiO₂ ratio. The behavior of iron oxide melts during reduction is very complex, and the surface-active nature of SiO₂ and ferric oxide has a significant influence on the reaction. A quantitative analysis of the overall reaction would have to take these factors into account.     

Preparation and application of iron oxide/persimmon tannin/ graphene oxide nanocomposites for efficient adsorption of erbium from aqueous solution

Rare earth elements (REEs) are used for the development of new energy materials owing to their intrinsic physicochemical property. However, excess REEs in water threaten the safety of animals, plants and humans. An efficient way to separate REEs from the water is therefore needed. In this study, a biosorbent consisting of iron oxide (Fe₃O₄), persimmon tannin (PT), and graphene oxide (GO) as Fe₃O₄/PT/GO was prepared, and the adsorption of trivalent erbium (Er³⁺) ions from aqueous solution was investigated. The adsorption process for Er³⁺ ions conforms to pseudo-second order kinetic and the Langmuir isotherm model behavior. Thermodynamic studies indicate that the adsorption process is spontaneous and endothermic. Scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), thermogravimetric analysis (TGA), Fourier-transform infrared (FT-IR) spectroscopy, Brunauer-Emmett-Teller (BET) analysis, and vibrating sample magnetometer (VSM) were used to assess the adsorption mechanism of Er³⁺ ions onto the Fe₃O₄/PT/GO biosorbent. A combination of electrostatic interactions, redox reactivity and chelation are responsible for adsorption of Er³⁺ ions on the Fe₃O₄/PT/GO biosorbent. The magnetic Fe₃O₄/PT/GO biosorbent can be easily separated under the magnetic field for effective recycle of Er³⁺ ions from aqueous solution. Therefore, this new biomass composite holds great promise for wastewater treatment.     

The reduction mechanism of a natural chromite at 1416 °C

The behavior of a natural chromite from the Bushveld Complex, Transvaal, South Africa, during reduction at 1416 °C by graphite was studied by means of thermogravimetric analysis, X-ray diffraction (XRD) analysis, energy-dispersive X-ray analysis (EDAX), and metallographic analysis. Experimental runs were allowed to proceed up to 120 minutes, resulting in 99 pct reduction. The specific objective of this study was to delineate the reduction mechanism of chromite by graphite. Zoning was observed in partially reduced chromites with degrees of reduction of up to about 70 pct. The inner cores were rich in iron, while the outer cores were depleted of iron. Energy-dispersive X-ray analysis revealed that Fe²⁺ and Cr³⁺ ions had diffused outward, whereas Cr²⁺, Al³⁺, and Mg²⁺ ions had diffused inward. The following mechanism of reduction, which is based on the assumption that the composition of the spinel phase remains stoichiometric with increasing degree of reduction, is proposed, (a) Initially, Fe³⁺ and Fe²⁺ ions at the surface of the chromite particle are reduced to the metallic state. This is followed immediately by the reduction of Cr³⁺ ions to the divalent state, (b) Cr²⁺ ions diffusing toward the center of the particle reduce the Fe³⁺ ions in the spinel under the surface of the particle to Fe²⁺ at the interface between the inner and outer cores. Fe²⁺ ions diffuse toward the surface, where they are reduced to metallic iron, (c) After the iron has been completely reduced, Cr³⁺ and any Cr²⁺ that is present are reduced to the metallic state, leaving an iron- and chromium-free spinel, MgAl₂O₄. Formerly Postgraduate Student, Department of Metallurgy and Materials Engineering, University of the Witwatersrand. Formerly with the Department of Metallurgy and Materials Engineering, University of the Witwatersrand.     

Oxygen permeability of calcia-stabilized zirconia

Oxygen permeability of commercial calcia-stabilized zirconia has been measured at 1673 to 1823 K by the following cell; O₂ ZrO₂(CaO) N₂ - O₂ (P’tO₂ = 1 atm) solid electrolyte (P″O₂ = 0.39 - 10^10-3 atm). Oxygen permeability of calcia-stabilized zirconia is proportional to (1 - P″O₂ ^1/4). From the permeability measurement, the conduction properties of the electrolyte were log σ-_? ^b+ = 0.28- 5100/T and logP_b+ =-0.87 + 15,400/T where σ-_? ^b+ is the þ-@#@ type electronic conductivity at P_{O 2} = 1 atm, and P_b+ is the oxygen activity at which the þ-@#@ type electronic conductivity and the ionic conductivity are equal.     

Electrical conductivities of M3P2O8 (M = Ca,Ba)

Two-probe conductivity measurements made for M₃P₂O₈ (M = Ca, Ba) suggested that the electrical conduction of these phosphates would primarily be due to the migration of Ca²⁺ and Ba²⁺ ions. At relatively low temperatures and high oxygen partial pressures, in contrast to Ca₃P₂O₈, however, Ba₃P₂O₈ shows partial electronic conduction.     

Effect of dysprosium substitution on magnetic and structural properties of NiFe_2O_4 nanoparticles

This work investigated the effect of dysprosium(Dy)ions on the structural,microstructural and magnetic properties of nickel nanospinelferrite,NiFe_2 O_4.The nanoparticles(NPs)of NiDy_xFe_(2-x)O_4(0.0≤x≤0.1)were prepared via the hydrothermal method.The formation of cubic phase of Ni nanoferrite was confirmed through X-ray diffraction,field emission scanning and transmission electron microscopy.Moreover,the magnetic properties of NiDy_xFe_(2-x)P_4(0.01≤x≤0.10)NPs were discussed.The magnetization versus field,M(H)curves exhibit super paramagnetic nature at room temperature and ferrimagnetic nature at low temperature(10 K).A noticeable improvement in the different deduced magnetic parameters is obtained especially for the NiDy_(0.07)Fe_(1.93)O_4(x = 0.07)product.The obtained result is mostly derived from the substitution of Fe~(3+)ions of smaller ionic radii with Dy~(3+)ions of larger ionic radii that will strengthen the super exchange interactions among nanoparticles.The calculated squareness ratios are found to be much less than 0.5,due to the effect of spin disorder in the surface regions of NiDy_xFe_(2-x)O_4(0.01≤x≤0.10)NPs.The Dy~(3+) ions substitution increases the magnetic hardness(higher values of remanence M_r,coercivity H_c,and magnetic moment n_B)of nickel nanoferrite samples.     

Electronic and ionic transport in liquid PbO-SiO2 systems

The objective of the present work was to examine electronic transport in the PbO-SiO₂ melt. The experimental work consisted of studying the oxidation of liquid Pb covered by a layer of liquid PbO-SiO₂ using a thermogravimetric arrangement. Oxidation of liquid Pb was dependent on the melt height and was parabolic in nature, indicating the oxidation process to be diffusion controlled. We observed a moderate increase in the oxidation rate with additions of a transition metal oxide (Fe₂O₃) to the melt. However, when melt/gas and melt/metal interfaces were short circuited by Ir wires, a much higher increase in the oxidation rate was noticed. Additionally, the conductivity of the PbO-SiO₂ melt with Fe₂O₃ additions was measured as a function of P_o, to detect the nature of electronic contribution by the Fe₂O₃. Combining the results of the oxidation and conductivity experiments, we conclude that the oxidation of liquid lead covered by liquid slag occurs through ionic and rate controlling electronic (probably electron holes) transport in the melt. Uday Pal, formerly with the Department of Materials Science and Engineering, The Pennsylvania State University     

Oxygen permeability of calcia-stabilized zirconia

Oxygen permeability of commercial calcia-stabilized zirconia has been measured at 1673 to 1823 K by the following cell; O₂ ZrO₂(CaO) N₂ - O₂ (P’tO₂ = 1 atm) solid electrolyte (P″O₂ = 0.39 - 10^10-3 atm). Oxygen permeability of calcia-stabilized zirconia is proportional to (1 - P″O₂ ^1/4). From the permeability measurement, the conduction properties of the electrolyte were log σ-_‡ ^b+ = 0.28- 5100/T and logP_b+ =-0.87 + 15,400/T where σ-_‡ ^b+ is the t-@#@ type electronic conductivity at P_{O 2} = 1 atm, and P_b+ is the oxygen activity at which the t-@#@ type electronic conductivity and the ionic conductivity are equal.     

Chemical State, Site, Solid Solubility, and Magnetism of Fe in the Ferropericlase (Mg1–x Fe x )O Produced by Ball Milling of MgO and Fe

Ferropericlase (Mg_1–x Fe _x )O solid solution was prepared by ball milling of the mixture of MgO with a rock-salt structure and metal Fe powders in atmosphere and at room temperature. Differing from (Mg_1–x Fe _x )O prepared at high temperature by using MgO and FeO as starting materials, the solution of Fe in MgO is not continuous but limited in the ball milling process, and the solubility limit is less than 30 at. pct. About 92 pct of the Fe ions occupy the site of tetrahedral oxygen coordination in trivalent Fe (Fe³⁺) with high spin, whereas about 8 pct of the Fe ions occupy the site of octahedral oxygen coordination in bivalent Fe (Fe²⁺) with high spin. The Fe³⁺ and Fe²⁺ ions do not show a ferromagnetic but show a paramagnetic state. The as-milled (Mg_1–x Fe _x )O is metastable and decomposes to ferropericlase (Mg_1–y Fe _y )O (where y < x) and MgFe₂O₄ with spinel structure as annealed above 773 K (500  °C), and the content of Fe in the (Mg_1–y Fe _y )O increases with increasing annealing temperature. A bulk (Mg_1–x Fe _x )O was fabricated by annealing the as-milled (Mg_1–x Fe _x )O powders at 973 K (700  °C). It shows n-type conductivity, which is attributed to an electronic small polaron with an activation energy of 0.135 eV.     

Speciation of the Fe(II)–Fe(III)–H2SO4–H2O system at 25 and 50 °C

This work presents theoretical and experimental results on the speciation of the Fe(II)–Fe(III)–H₂SO₄–H₂O system in concentrated solutions (up to 2.2 m H₂SO₄ and 1.3 m Fe). The aim was to study the chemical equilibria of iron at 25 and 50 °C in synthetic aqueous sulphuric acid solutions that contain dissolved ferric and ferrous ion species. Raman spectroscopy, volumetric titration and conductivity measurements have been carried out in order to study the presence of specific ions and to characterize the ionic equilibrium. A thermochemical equilibrium model incorporating an extended Debye–Hückel relationship was used to calculate the activities of ionic species in solution. Model calculations were compared with experimental results. Model simulations indicate that anions, cations and neutral complexes coexisted in the studied system, where the dominant species were HSO₄⁻, H⁺, Fe²⁺ and FeH(SO₄)₂⁰. This indicated that these solutions showed a high buffer capacity due to the existence of bisulphate ions (HSO₄⁻), which presented the highest concentration. A decrease in the concentration of H⁺ and Fe³⁺ took place with increasing temperature due to the formation of complex species. Standard equilibrium constants for the formation of FeH(SO₄)₂⁰ were obtained in this work: log K_f⁰ = 8.1 ± 0.3 at 25 °C and 10.0 ± 0.3 at 50 °C.     

Iron redox equilibria in BOF slag equilibrated with ambient air

The Fe²⁺/Fe³⁺ ratio in the CaO‐MgO‐Fe₂O₃‐FeO‐SiO₂ based slag was measured under the condition of equilibrium with the ambient air at 1873 K as a fundamental study for precise slag coating control in BOF operation. The CaO/SiO₂ mass ratios of the main slag were 1, 1.5, 2, 3 and 4, and total iron mass content was in the range of 10 to 35 %. Moreover, mass contents of 1 to 13 % of MnO and 2 to 12 % of Al₂O₃ were added to the melt to evaluate their effects on the Fe²⁺/Fe³⁺ ratio. The effect of slag composition on the Fe²⁺/Fe³⁺ ratio was discussed and quantified into a form of formula. As the basicity in slag increases, the Fe²⁺/Fe³⁺ ratio decreases. The effect of iron oxide mass content is observed to be dependent on the basicity of slag. An increase in iron oxide mass content makes the Fe²⁺/Fe³⁺ ratio higher for basic slag but lower for acidic slag. It is revealed that the redox reaction of iron oxide in steelmaking slag under the ambient air is controlled by the complex anion formation reaction of iron oxide, and that the iron oxide in basic slag exists in the form of 2 or more kinds of complex anion controlling the oxygen anion content. Both Al₂O₃ and P₂O₅ increase the Fe²⁺/Fe³⁺ ratio by diluting the basic oxides as SiO₂ does, while manganese oxide lowers the Fe²⁺/Fe³⁺ ratio enormously down to nearly zero. The Fe²⁺/Fe³⁺ ratio can be described as a function of slag composition, X = (%CaO) + 0.38(%Fe₂O₃ + %FeO)+3.2(%MnO), in the equation of log(Fe²⁺/Fe³⁺) = ‐0.00107X² + 0.0721X ‐ 1.982.     

Kinetics of evolution of SO2 from hot metallurgical slags

The kinetics of the evolution of SO₂ gas from a liquid synthetic blast furnace -type slag (CaO-Al₂O₃-SiO₂) in an atmosphere of O₂ + Ar gas at a total pressure of 1 atm for 0.003 ≤ Po₂ ≤ 1.00 atm has been studied in the range 1360 to 1460°C. The process has been followed by collecting and analyzing the SO₂ as it forms, and also by observing the change in weight of the slag sample with time. The effect of slag composition has also been studied. For partial pressures of oxygen less than about 0.1 atm, the rate is very rapid and is controlled by transport in the gas phase. At greater values of Po₂, the rate is much slower and is controlled by a chemical process. In the high Po₂ region, the process is half-order with respect to the concentration of sulfur in the slag. This half-order dependence on sulfur concentration in the slag may be explained by an initial fast irreversible reaction to form two intermediate species which then decompose at equal rates to give the final products. Additions to the slag of iron or manganese oxides greatly accelerate the rate of evolution of SO₂ at Po₂ = 1.00 atm. This is interpreted to mean that a charge transfer process, possibly involving S^2- and O^2- ions, is rate-controlling at Po₂ = 1 atm. It is also apparent that Fe²⁺ and Fe³⁺ (or Mn²⁺ and Mn³⁺) ions can act as charge carriers. Some measurements with actual industrial blast furnace slags are also reported. Formerly Visiting Scientist, Massachusetts Institute of Technology