Kinetics of chromite ore reduction from MgO-CaO-SiO2-FeO-Cr2O3-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Kinetic experiments were performed in an induction furnace to investigate the reduction of chromite ore by carbon dissolved in a high carbon ferrochromium alloy melt under conditions of varying Cr₂O₃ concentration, slag basicity, and temperature. The results obtained show that chromite reduction by dissolved carbon in slag systems of the type MgO-CaO-SiO₂-FeO-Cr₂O₃- Al₂O₃ occurs principally by a stagewise process encompassing an intermediate reaction in which the divalent chromium oxide species is involved. During the fast period, Cr₂O₃ reduction is controlled by the diffusion of oxygen species in the slag for which a mass transfer coefficient of 0·003 cm s^-1 was calculated. An activation energy value of 117 kJ mol ^-1 obtained for the reduction of Cr₂O₃ implies the rate controlling step is mass transfer of Cr₂O₃ from the slag to the slag/metal interface, since activation energies for metal phase control are typically <70 kJ mol ^-1. The second period represents a pseudo-equilibrium condition with respect to Cr₂O₃ reduction that is probably under thermodynamic control by a step or mechanism involving the reduction of divalent chromium oxide to chromium.     

Performance of mesoporous CeO2-Cr2O3 mixed metal oxides applied to benzene catalytic combustion

In this paper,a hydrothermal method was used to prepare(Ce,Cr)-MOF with different Ce/Cr molar ratios and then a series of CeO₂-Cr₂O₃ mixed metal oxides(CeCr-MMO) with mesoporous structure were prepared by thermal decomposition of these MOFs at different temperatures.After a series of characterization techniques were applied to test the physicochemical properties of the materials,it is found that thermal decomposition temperature(TDT) and Ce/Cr molar ratios have i...     

Electrophoretic properties of the oxides NiO,Fe2O3, Cr2O3, MnO2 and TiO2 in aqueous suspension

Abstract

The electrophoretic properties of the oxides NiO, Fe₂O₃, Cr₂O₃, MnO₂ and TiO₂, present as an aqueous colloidal suspension in a polyacrylic acid-methylamine dispersant medium, have been investigated. Electrophoretic mobilities and zeta potentials were determined as a function of composition and pH of the solution. Optimum conditions for the electropainting of these oxides on steel have been determined. The colloidal particles depositing during the electropainting stage consist of hydrated acrylic acid -metal oxide agglomerates.

Résumé

On a étudié les propriétés d' électrophorèse des oxydes NiO, Fe₂O₃,Cr₂O₃, MnO₂ et TiO₂ en suspension colloïd ale dans un milieu aqueux en présence d'un mélange dispersant composé d'un acide polyacrylique et de la méthylanline.

Les mobilités électrophorétiques et les potentiels zéta ont été mesurés en fonction de la composition et du pH de la solution.

Les conditions optimales pour l' électrodéposition de ces oxydes sur l'acier ont été déternlinées.

Les particules colloidales se déposant sont des agglomérats formés d'acide acrylique hydraté et d'oxydes de métaux.     

Role of Cr2O3 on formation of reaction sintered dense ZrO2–Al2O3–SiO2 based refractory materials

Abstract

Extremely dense zirconia–mullite composites were developed by reaction sintering of microcalcined fine alumina powder and zircon flour taken in different ratios with Cr₂O₃ as an additive. The powder mixes were isostatically pressed and sintered at 1550°C. The sintered compacts were characterised by different physicomechanical properties, microstructure developed and phase compositions. Cr₂O₃ exhibited positive influence up to certain level of addition on the thermomechanical properties of reaction sintered zirconia–mullite composites including corrosion resistance and microstructure.

On a développé des composites extrêmement denses de zircone et mullite par frittage réactif de poudre fine d’alumine micro calcinée et de farine de zircone en différentes proportions, avec du Cr₂O₃ comme adjuvant. On a pressé les mélanges de poudres au moyen d’une presse isostatique et on les a frittés à 1550°C. On a caractérisé les différentes propriétés physico mécaniques, la microstructure développée et la composition de phases des compacts frittés. Le Cr₂O₃ exhibait une influence positive jusqu’à un certain niveau d’addition sur les propriétés thermomécaniques des composites de zircone et mullite à frittage réactif, incluant la résistance à la corrosion et la microstructure.     

On the solubility minimum in ternary oxide-metal equilibria

Abstract

Theoretical solubility relations have been used to interpret the experimental curves of oxygen solubility at 1600°C in Fe melts equilibrated with V₂O₃, Ti₂O₃, Al₂O₃, Cr₂O₃, B₂O₃, NbO₂, Ta₂O₅ and CO. The theory correctly predicts the detailed shape of the solubility curves and their minima. The fit of the curves to the data generates values for the Wagner cross-interaction parameter, ε₂₁, which are in good agreement with direct determinations, and values for the equilibrium constants which are in fair agreement with standard compilations.

Résumé

Les expressions théoriques de solubilité ont été employées pour l'interpretation des courbes expérimentales de la solubilite de l'oxygène dans des solutions en équilibre de V₂O₃, Ti₂O₃, Al₂O₃, Cr₂O₃, B₂O₃, NbO₂, Ta₂O₅ et CO dans du fer liquide. La théorie prédit précisément l'allure détaillée des courbes de solubilité et leurs minima. La bonne correspondance entre les courbes et les données fournit des valeurs pour les paramètres non-symétriques d'interaction de Wagner, ε₂₁, qui sont en bon accord avec les déterminations directes, ainsi que des valeurs pour les constantes d'équilibre, qui se comparent raisonnablement avec celles des tables de données.     

Aspects of smelting reduction of chromite ore fines in CaO-FeO-Cr2O3-SiO2-Al2O3 slag system by carbon dissolved in high carbon ferrochromium alloy bath

Abstract

Chromite reduction by carbon dissolved in a high carbon ferrochromium alloy melt has been investigated in the temperature range 1580-1640°C using a slag system based on CaO₂-FeO-Cr₂O₃-SiO₂-Al₂O₃. Although the reduction is essentially first order with respect to Cr₂O₃ concentration, it exhibits both zero order and first order reaction kinetics. The zero order period is occupied by the preferential reduction of iron oxide, during which time there is no significant change in the concentration of Cr₂O₃. The predominance of the divalent chromium oxide in the slag phase is seen to provide further evidence that the reduction of chromite occurs by a stagewise process, involving the thermodynamically stable CrO species. While high basicity slags may be recommended to minimise the generation of CrO, and hence improve reaction kinetics and the extent of Cr₂O₃ reduction, there is a limitation imposed by chemical erosion of the alumina crucible as the slag basicity is increased above unity, with the dissolving Al₂O₃ further retarding the reduction kinetics. There is also evidence to suggest the participation of a reductant other than carbon (possibly silicon) in the reduction of chromite.     

Effects of Transition Metal Oxides on Thermal Conductivity of Mould Fluxes

The thermal conductivity of the mould fluxes containing transition metal oxides was measured by hotline method at different temperatures. The relationship between the thermal conductivity of mold fluxes and the contents of transition metal oxides was discussed. The synthetic slags were composed of 30.0% — 35.4% CaO, 34.7% — 38.6% SiO₂, 6% Al₂O₃, 9% Na₂O, 14.4% CaF₂, 0–4% Cr₂O₃ and 0–8% MnO in mass percent. The results indicated that Cr₂O₃ and MnO had a negative effect on thermal conductivity of mold fluxes. The thermal conductivity of mold fluxes was about 0.25 — 0.55 W/(m K) when the temperature reached 1300 °C, and it increased sharply to about 1.32–1.99 W/(m K) when the temperature reduced from 1300 to 1000 °C. The thermal conductivity of mold fluxes containing Cr₂O₃ and MnO was 10%—25% lower than those of original fluxes. The decrease in thermal conductivity was attributed to the change of molecular structure of mold fluxes. In addition, the poor integrity and regulation of polycrystal structure, complexity of crystal structure, and effects of impurities in the boundary and lattice distortion leaded to the reduction in the thermal conductivity. Na₂CrO₄, Mn₂SiO₄ and other minor phases were also found in the samples containing Cr₂O₃ and MnO, respectively.     

Tunable luminescence in co-doped Zn3Al2Ge2O10:Cr3+ by controlling crystal field splitting and nephelauxetic effect

Ce³⁺/Dy³⁺/Tb³⁺/Eu³⁺/Mn²⁺ and Cr³⁺ ions co-doped Zn₃Al₂Ge₂O₁₀ phosphor were prepared by a high-temperature solid-state method. X-ray diffraction patterns prove the cubic phase structure of prepared Zn₃Al₂Ge₂O₁₀ phosphor. Emission, excitation spectra and decay curves confirm the tunable luminescence. Different degrees of the decrease of emission FWHM in Zn₃Al₂Ge₂O₁₀:0.02Cr³⁺,RE (RE = Ce³⁺, Dy³⁺, Tb³⁺, Eu³⁺) and Zn₃Al₂Ge₂O₁₀:0.02Cr³⁺,Mn²⁺ are observed. The reason of variable FWHM is the effect of crystal field splitting and nephelauxetic effect, and the nephelauxetic effect is dominant. Therefore, the emission FWHM decreases with the increasing concentration of Mn²⁺/Tb³⁺/Eu³⁺ in Zn₃Al₂Ge₂O₁₀:0.02Cr³⁺, and for Zn₃Al₂Ge₂O₁₀:0.02Cr³⁺,Ce³⁺ and Zn₃Al₂Ge₂O₁₀:0.02Cr³⁺,Dy³⁺, it is a constant. The variation of Zn₃Al₂Ge₂O₁₀:0.02Cr³⁺,Tb³⁺ is more obvious than that of Zn₃Al₂Ge₂O₁₀:0.02Cr³⁺,Eu³⁺, because Tb³⁺ ion has smaller electronegativity. Thus, the tunable luminescence of Cr³⁺ can be realized by co-doping different ions. And these phosphors have potential applications in light-emitting diodes for plant growth.     

Thermodynamic estimation on the reduction behavior of iron-chromium ore with carbon

The thermodynamics for reduction of iron-chromium ore by carbon is discussed. The thermodynamic properties of iron-chromium ore were evaluated from our previous work on the activities of constituents in the FeO·Cr₂O₃-MgO·Cr₂O₃-MgO·Al₂O₃ iron-chromite spinel-structure solid solution saturated with (Cr, Al)₂O₃, and those of the Fe-Cr-C alloy were estimated by a sublattice model. The stability diagrams were drawn for carbon reduction of pure FeO · Cr₂O₃, (Fe_0.5Mg_0.5)O·(Cr_0.8Al_0.2)₂O₃ iron-chromite solid solution, and South African iron-chromium ore. The evaluated stability diagrams agreed well with the literature data. It was concluded that the lowest temperature for reduction of FeO · Cr₂O₃ in the iron-chromium ore was 1390 K and a temperature higher than 1470 K would be necessary to reduce Cr₂O₃ in MgO·(Cr,Al)₂O₃ in the prereduction process of iron-chromium ore. The composition of liquid Fe-Cr-C alloy in equilibrium with iron-chromium ore was also estimated under 1 atm of CO at steelmaking temperature. The predicted metal composition showed reasonable agreement with the literature values.     

Production of Metallic Oxide and Metal Powders by Crystallization Stripping Applying Solvent Extraction

Abstract

The fundamental studies on production of metallic oxide and metal powders with crystallization-stripping using NH₄HF₄, (NH₄) ₂S04 and pressurized C0₂ gas as crystallization-stripping agent in solvent extraction were carried out in order to develop a new production method of fine powders.

The reaction significantly relates to the stripability of the extracted metal tons and the solubility of the metal compounds formed in crystallization process. Furthermore, the reaction depends on the various factors, such as extractant concentration, metal concentration in the organic phase, the sort of stripping solution and still more CO₂ pressure in the pressurized CO₂ process.

The resultant crystallized products can be converted to fine powders of high-purity metallic oxide and metal in micron order by thermal decomposition in air or hydrogen atmosphere, respectively.     

Non-Carbon Nanotubes (Review). Part 2. Types and Structure

Various types and structures of synthesized non-carbon nanotubes (N-NT) based on carbonitrides B _x C _y N _z , boron nitride BN, sulfides WS₂, MoS₂, selenides NbSe₂, halides NiCl₂, transition metal oxides SiO₂, TiO₂, MoO₃, V₂O₅ are considered, as well as theoretically predicted N-NT based on P, Si, Ge, and III-V semiconductors. General criteria for the stability of non-carbon nanotubes are analyzed.

     

Role of Silicon Carbide in Phase-Evolution and Oxidation Behaviors of Pulse Electrodeposited Nickel-Tungsten Coating

Silicon carbide (SiC) was reinforced in the pulse electrodeposited nickel-tungsten (Ni-W) coatings deposited on the steel substrate, and isothermal oxidation test was performed at 1273 K (1000 °C) for 24 hours. Addition of just 2 vol pct of SiC showed 26 pct increase in the relative oxidation resistance of Ni-W coating. The increased oxidation resistance was attributed to the phase evolution (SiO₂, Cr₂O₃, CrSi₂, Ni₂SiO₄, Cr₇C₃, Cr₃C₂, and Cr₃Si), which suppressed the spallation of the oxide scale in Ni-W-2 vol pct SiC. The presence of Fe₂O₃ phase in the oxidized Ni-W coating was mainly responsible for the major multiple spallations at the interface and in the bulk, which resulted in the degradation of oxidation resistance.

     

Processing and performance of Cr2O3–Fe2O3 reference electrode powders prepared by oxide coprecipitation

Abstract

Cr₂O₃–Fe₂O₃ based oxide mixtures for reference electrode powders of oxygen sensors were processed using oxide coprecipitation route. A special method for preparing reference electrode powders has been developed by mixing coarse Cr particles with the oxide mixture in the form of Cr–Fe hydroxide. Morphology and size of the mixed oxide powders were characterised by means of scanning electron microscopy and laser diffraction method. With the coprecipitation process, chemically homogeneous and very fine powders with a mean particle size of 1.53 μm were prepared. This powder mixture adhered and loosely coated to Cr particles. The processed reference electrode powders were tested in low level oxygen concentration measurements of steelmaking process under industrial scale. The reference electrode powders showed excellent results in terms of electromotive force reproducibility, response time and accuracy in soluble aluminium predictions at the oxygen concentration measurements. Most of the particles of the reference electrode powder remained separated after dipping to molten steel.     

Removal of nitrogen from steel using novel fluxes

The solubility of nitrogen and the nitride capacity of CaO-Al₂O₃-TiO₂ and CaO-BaO-Al₂O₃-TiO₂ slags were measured at 1873 K using a gas-slag-metal equilibration technique with carbon-saturated iron and gas mixtures of CO and N₂. The nitride capacity increased with increasing the TiO₂ and BaO content and is significantly higher than the nitride capacity for normal ladle slags. The activity coefficient of TiO₂ in CaO-Al₂O₃-TiO₂ and CaO-BaO-Al₂O₂-TiO₂ systems were measured. This is necessary to know in order to estimate the possible pickup of titanium in the metal when an aluminum-killed steel is treated with these slags. Also, the activity coefficient of Ti in carbon-saturated iron was measured. The kinetics of the nitrogen reaction between slag and metal is influenced by the oxygen potential in the metal and is primarily controlled by liquid-phase mass transfer of nitrogen in the metal. Formerly with the Department of Metallurgical Engineering and Materials Science, Carnegie Mellon University     

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.     

Reduction of synthetic stainless steel slags by aluminium

One of the most efficient ways to eliminate the harm of chromium oxide in stainless steel slag is to reduce chromium oxide in stainless steel slag using aluminium. In the present work, the Al reduction of synthetic CaO–SiO₂–Al₂O₃–MgO–Fe₂O₃–Cr₂O₃ stainless steelmaking slags at different conditions, including temperature, slag basicity and Al amount was investigated to get optimal conditions for the reduction and the metal–slag separation. It was found that the agglomeration of metal droplets and metal–slag separations were improved by increasing temperature. The reduction degrees of SiO₂, Fe₂O₃ and Cr₂O₃ were enhanced with increasing basicity of slag. The addition of CaF₂ in slag leads to better agglomerations of metal droplets and metal–slag separations. The highest reduction degree of chromium could reach 99% in slag with basicity of 2 at 1873 K.     

Corrosion and electrochemical properties of hot-pressed chromium nitride

Conclusions The conditions of synthesis of chromium nitride Cr₂N and of hot pressing of powder of it to obtain compact specimens of high density were determined more accurately. The kinetics of corrosion of the metallic and nonmetallic components of Cr₂N were studied in a broad range of potentials. The high reaction capacity of its nitrogen component, which is reduced intensely to ammonium ion in reducing and weakly oxidizing media, was established.It was concluded that there is a possible influence of Cr₂N on the corrosion behavior of stainless steels as a constituent of composite materials and as an excess phase.Translated from Poroshkovaya Metallurgiya, No. 5(293), pp. 46–50, May, 1987.     

The effect of different oxide additions on the electrical and magnetic properties of copper-manganese ferrites

Summary The electrical and magnetic parameters of copper-manganese ferrites with additions of other metal oxides are compared with those of the 0.15 Cu₂O·0.2 MnO₂·0.5 Fe₂O₃ ferrite without an addition. The magnetic parameters of the ferrites were determined by the method of oscillography of dynamic hysteresis loops. It was established that the introduction of the oxides Bi₂O₃, Sb₂O₃, TiO₂ Cr₂O₃, ZnO, and CaO leads to a substantial improvement of magnetic parameters, while the addition of the oxides BeO, CdO, BaO, Co₂O₂, SrO, and Al₂O₃ causes a deterioration of magnetic properties.     

Kinetic studies of the reaction between Cr23 C6 particles and Cr2O3 particles

The kinetics of reaction between Cr₂₃C₆ particles and Cr₂O₃ particles which yields metallic chromium was studied. This reaction is one of the basic reactions in the Simplex process and is also related to the carbon reduction of metal oxide. The rate of reaction between these particles in a pellet of the mixture was measured by thermogravimetry in vacuum at 1050, 1075, and 1100°C. The molar ratio of Cr₂₃C₆ to Cr₂O₃ in the pellet was maintained at 1:8. At the earlier stages of the reaction, a rate equation for interfacial reaction control was applied and the rate constant α was observed to be inversely proportional to the reciprocal effective radius of Cr₂₃C₆ particles. The indirect gaseous reactions are predominant: 1/6 Cr₂₃C₆ + CO₂ = 23/6 Cr + 2CO 1/3 Cr₂O₃+ CO = 2/3 Cr + CO₂ The reduction of Cr₂O₃ with CO gas is presumed to proceed so rapidly and the equilibrium is attained instantaneously. The reaction of CO₂ with Cr₂₃C₆ particles occurs in the sequence co₂ (g) +c< ⇋ c(o) + co(g) c(o) → CO(g) +c< The latter reaction was thought to be the rate determining step, and the activation energy of this reaction was estimated to be approximately 60 kcal per mole.     

Combustion synthesis of YAG:Ce phosphors via the thermite reaction of aluminum

Cerium-doped yttrium aluminum garnet(YAG:Ce) as a yellow phosphor for white light-emitting diodes(LEDs) was synthesized via a facile combustion method using Y_2 O_3, CeO_2, Al_2 O_3, Al,and NaClO_4 as raw materials. The combustion synthesis approach utilizes the strong exothermic oxidation of aluminum to realize a self-sustaining reaction. In this study, we investigated the effects of the ratios of Al_2 O_3 to AI,fluxes, and coprecipitated materials as raw materials on the luminescence properties of the synthesized YAG:Ce phosphors. When the amount of Al_2 O_3 x is varied, the combustion reaction proceeds at x ≤ 1.8,with x = 1.725 being the optimum condition for producing a high-performance product. When 5 wt%BaF2 is added, the luminescence intensity is significantly improved owing to a decrease of YAP(YAlO_3)formation with improved uniformity. However, the addition of CaF_2 and NaF does not improve the luminescence properties. To suppress the segregation of CeO_2, we used the coprecipitated material Y_2 O_3-CeO_2 as a raw material. Unlike with separate addition of Y_2 O_3 and CeO_2, Ce ions are uniformly distributed in the coprecipitated material, resulting in improved luminescence properties. The combination of BaF_2 and coprecipitated material significantly improves the internal quantum efficiency to83.0%, which is close to that of commercial phosphors.