Kinetics of the solid-state carbothermic reduction of wessel manganese ores

Reduction of manganese ores from the Wessel mine of South Africa has been investigated in the temperature range 1100 °C to 1350 °C with pure graphite as the reductant under argon atmosphere. The rate and degree of reduction were found to increase with increasing temperature and decreasing particle sizes of both the ore and the graphite. The reduction was found to occur in two stages: (1) The first stage includes the rapid reduction of higher oxides of manganese and iron to MnO and FeO. The rate control appears to be mixed, both inward diffusion of CO and outward diffusion of CO₂ across the porous product layer, and the reaction of carbon monoxide on the pore walls of the oxide phase play important roles. The values of effective CO-CO₂ diffusivities generated by the mathematical model are in the range from 2.15 x 10⁻⁵ to 6.17 X 10⁻⁵ cm².s⁻¹ for different ores at 1300 °C. Apparent activation energies range from 81. 3 to 94.6 kJ/kg/mol. (2) The second stage is slower during which MnO and FeO are reduced to mixed carbide of iron and manganese. The chemical reaction between the manganous oxide and carbon dissolved in the metal phase or metal carbide seems to be the rate-controlling process The rate constant of chemical reaction between MnO and carbide on the surface of the impervious core was found to lie in the range from 1.53 x 10⁻⁸ to 1.32 x 10⁻⁷ mol . s⁻¹ . cm⁻². Apparent activation energies calculated are in the range from 102.1 to 141.7 kJ/kg/mol. Formerly Doctoral Student, Department of Metallurgy and Materials Engineering, University of the Witwatersrand, Johannesburg,     

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.     

Reduction of FeO in smelting slags by solid carbon: Experimental results

The reduction of CaO-SiO₂-Al₂O₃-FeO slags containing less than 10 wt pct FeO by solid carbonaceous materials such as graphite, coke, and coal char was investigated at reaction temperatures of 1400 °C to 1450 °C. The carbon monoxide evolution rate from the system was measured using stationary and rotating carbon rods, stationary horizontal carbon surfaces, and pinned stationary spheres as the reductants. The measured reaction rate ranged from 3.25 × 10^?7 mol cm^?2 s^?1 at 2.1 pct FeO under static conditions to 3.6 × 10^?6 mol cm^?2 s^?1 at 9.5 pct FeO for a rotating rod experiment. Visualization of the experiment using X-ray fluoroscopy showed that gas evolution from the reduction reaction caused the slag to foam during the experiment and that a gas film formed between the carbon surface and the slag at all times during experimentation. The reaction rate increased with increased slag FeO contents under all experimental conditions; however, this variation was not linear with FeO content. The reaction rate also increased with the rotation speed of the carbon rod at a given FeO content. A small increase in the reaction rate, at a given FeO content, was found when horizontal coke surfaces and coke spheres were used as the reductant as compared to graphite and coal char. The results of these experiments do not fit the traditional mass transfer correlations due to the evolution of gas during the experiment. The experimental results are consistent, however, with the hypothesis that liquid phase mass transfer of iron oxide is a major factor in the rate of reduction of iron oxide from slags by carbonaceous materials. In a second article, the individual rates of the possible limiting steps will be compared and a mixed control model will be used to explain the measured reaction rates.     

Kinetics of smelting reduction of fluxed composite iron ore pellets

Kinetic studies on smelting reduction of unreduced fluxed composite pellets (FCP) and fluxed composite pre-reduced iron ore pellets (FCRIP) have been carried out in an induction furnace. The pellets are charged into the slag layer floating on the carbon saturated molten iron bath in a graphite crucible. The slag basicity was however varied such that it has the same value as that of the pellets charged. The temperature of the slag is varied within the range of 1623K to 1823K for the pellets of basicity 2.0. Kinetic studies show a mixed kinetic model of both diffusion and chemical reaction controlled. While the smelting of FCRIP follows the model expressed as G(or) = 1-(2/3)(α) -(1-α)^2/3, the unreduced FCP pellets initially follow the diffusion controlled model of G(α) = α² followed by a chemical reaction controlled first order model of -ln(1-α) at the latter stages of smelting reduction, where α denotes the degree of reduction. The basicity dependence on the kinetics is not very significant. Comparison of the activation energy values explains that the smelting reduction with pre-reduced pellets seems to be a rather less energy intensive process.     

Reduction of iron oxides from liquid slags using a graphite rotating disc

Research work has been carried out on the reduction of FeO from liquid slags of the CaO‐FeO‐SiO₂ ternary system using a graphite rotating disc technique. The investigations were conducted on slags with a basicity of CaO/SiO₂ = 1.27 and FeO contents of 20 and 60%, at temperatures of 1350 and 1420°C. The calculated viscosity range for these slags is within 2.53 – 0.43 dPa·s. It has been found that the factor controlling the reduction process is diffusion of FeO towards the disc surface, both in the case of the reduction from the slag with 20% FeO and in the case of the reduction from the slag with 60% FeO fraction. The diffusion coefficient of FeO at the reduction temperature of 1350°C is of the order of magnitudes of 10^?7 cm²/s, while at 1420°C it reaches the order of 10^?6 cm²/s. The calculated thickness values for the limiting diffusion layers range from 8.54·10^?3 to 0.70·10^?3 cm. It has been found that with increasing reduction rate also Boudouard's reaction starts to be important to the overall reduction rate. The limiting reduction rate at which Boudouard's reaction starts to be important to the entire process is dependent on temperature, being approximately 10.0·10^?6 mol FeO/cm² s at 1350°C, and approximately 15.0·10^?6 mol FeO/cm² s at 1420°C.     

Tribological properties of centrifugally cast copper alloy-graphite particle composite

Centrifugal casting technique was used to impart better tribological properties to the inner periphery of centrifugal castings of a C90300 copper alloy originally containing 13 vol pct graphite particles. Microstructural observation of centrifugally cast copper alloy containing graphite particles shows that a graphite-free zone and a graphite-rich zone (25 vol pct) with a unique microstructure are formed near the outer and the inner periphery of the centrifugally cast cylinders, respectively. Wear tests were conducted using a pin-on-disc apparatus running against a cast-iron counterface under dry conditions at applied loads between 44.5 and 267 N and at a sliding speed of 1 m/s. The worn surfaces of pin and counterface were analyzed using scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analysis. The wear rate (1.89×10⁻¹³ to 7.59 × 10⁻¹³ m³/m) and the temperature (50 °C to 170 °C) at the counterface for the pins from the graphite-rich zone of the centrifugal castings were found to be lower than the friction coefficient (0.52 to 0.75), the wear rate (6.32×10⁻¹² to 3.16 × 10⁻¹¹ m³/m), and the temperature (70 °C to 200 °C) at the counterface for the pins from the graphite-free zone (of the same centrifugal casting under similar conditions). A greater transfer of the copper phases from the pin to the cast-iron counterface was observed visually from the pin of the graphite-free zone than from the pin of the graphite-rich zone, which was also confirmed by EDX analysis. This leads to an increase in the weight of the counterface running against the pin from the graphite-free zone with an increase in the applied load. Despite the presence of graphite in cast iron, the presence of graphite in the matrix of mating copper alloys lead to improved tribological properties. The effect of graphite particles on tribological properties of the composites was discussed in terms of the transfer of iron and copper phases, the interparticle distance between graphite in cast-iron and copper-graphite alloys, and the deformability of the matrix containing graphite.     

The interfacial kinetics of the reaction of CO2 with nickel: Part II. the decarburization of liquid nickel

The kinetics of decarburization of liquid nickel in CO₂-CO mixtures have been studied at 1400 and 1500°C, using the experimental arrangement of the impinging jet. At carbon concentrations above about 1 wt pct, pressures of CO₂ ⪯ 0.1 atm, and for total gas flow-rates above about 40 l/min (STP) impinging on a metal surface of 2.08 cm², it is concluded that the interfacial reaction step controls the rate. Comparison with isotope exchange studies indicates that dissociative chemisorption of CO₂ is the rate determining step. Rate constants, based on the nominal surface area, are 1.2 ×10⁻³ and 1.4 × 10⁻³ mol/cm² · s · atm at 1400 and 1500°C, respectively. on leave of absence from the Homer Research Laboratories of the Bethlehem Steel Co.     

Study of solid mushroom formation in direct iron ore smelting reduction process using low temperature water model

Abstract

In the direct iron ore smelting reduction process, molten iron near the bottom blowing gas tuyere is cooled by low temperature/endothermic gas and forms a mushroom shaped solid on top of the tuyere. The formation of an appropriate solid mushroom, which covers the tuyere, can protect the tuyere and the surrounding refractory. In the present study, a water model with a low temperature gas system was established to investigate formation of the solid mushroom and the effects of operating conditions on its shape and dimensions. Transparent acrylic was used to construct the water model, which was 40% of the size of the actual furnace. Water was used to simulate the molten iron. Low temperature air, obtained by passing air through a heat exchanger cooled by liquid nitrogen, was blown into the water bath through a bottom tuyere. The air temperature was able to reach-188±1°C. In the water model experiments, water near the tuyere was cooled, and formed an ice mushroom surrounding the tuyere. The effects of operating conditions, mainly gas flowrate and mould material surrounding the tuyere, on the parameters of the solid mushroom were investigated. The parameters of the solid mushroom included whether it could be formed and duration of the solid mushroom, as well as the shape, dimensions, and weight of the solid mushroom. Attempts were also made to relate the temperature-time and pressure-time relationships of the blown gas to the parameters of the solid mushroom. With copper used as mould material surrounding the tuyere, the water model experiments were conducted with flowrate of the bottom blown gas set in the range 30-90 NL min^-1. The results show that as the gas flowrate was increased, the highest water temperature which allowed the solid mushroom to form in the water model was increased. Three different types of pressure-time curve were obtained under different gas flowrates in the present study. They also corresponded to different forms of solid mushroom. As peaks appeared in the pressure-time curve, they revealed ice capsulation and subsequent bursting to release the pressure. A gas flowrate of 80 NL min^-1 and water temperature of 19·2°C with copper plate as the bottom material are considered to be optimal conditions of the water model for growth of the appropriate ice mushroom. These data are rather consistent with the gas flowrate and superheat for the actual direct iron ore smelting reduction unit, which are 2700 NL min^-1 and 120°C (equivalent to 70 NL min^-1 and 22·7°C in the water model).     

Antimony activities in copper mattes

A mass spectrometric technique combined with a double Knudsen cell was used to determine the antimony and copper activities in the Cu-Sb binary system at 1373 K and in the two-melt composition range of the Cu−S−Sb ternary system at 1423 K. The antimony and copper activities were calculated based on the intensity ration of the gaseous Sb and Cu species, over the unknown and known activity samples, respectively. γ _Sb ^o were found to be 1.1×10⁻² in molten copper at 1373 K, and 1.8×10⁻² and 0.44 in a copper-rich phase and in a matter phase, of the Cu−S−Sb ternary system at 1423 K, respectively. These values indicate, that antimony can be removed during the matte smelting and slagging stage of the copper smelting process. Interaction parameters of antimony in molten copper slagging stage of the copper smelting process. Interaction parameters of antimony in molten copper at 1423 K were calculated and found to be 10.7, −5.4, and 6.3 for ε _Sb ^Sb · ρ_Sb ^Sb, and ε _Sb ^S , respectively. M. HINO, formerly Visiting Scientist at the University of Toronto     

Study on Efficient Use of High-Manganese Pig Iron

The smelting process of high-manganese pig iron is studied to determine ways to efficiently utilize the by-product of manganese-rich slag. This process consists of two stages: 1) demanganese and carbon retention and 2) decarbonization and dephosphorization. In the first stage, oxygen is supplied at 0.5–2.0 Nm³ t⁻¹ min⁻¹. The temperature is controlled between 1300 and 1400 °C by adding iron ore to the molten bath. Ultimately, high-manganese slag, which contains more than 50% manganese oxide and semisteel with 3.2% carbon, is produced. In the second stage, oxygen is supplied at 2.0−5.0 Nm³ t⁻¹ min⁻¹ and slag-forming materials are added to the molten bath for the dephosphorization and desulfurization of the hot metal. Consequently, the iron oxide, manganese oxide, and silicon oxide contents of the high-manganese slag and the carbon, manganese, phosphorus, and sulfur contents of the semisteel are 10.8–15.3%, 70–77%, and 3–7%, and 3.2–4.2%, 0.3–1.0%, 0.12–0.22%, and 0.01–0.024%, respectively. The main mineral phases of high-manganese slag, 2MnO·SiO₂ and FeO·MnO·SiO₂, are suitable for the preparation of high-carbon ferromanganese raw materials. After further smelting, clean molten steel containing 0.03−0.08%, 0.08−0.20%, 0.005−0.02%, and 0.01−0.024% carbon, manganese, phosphorus, and sulfur, respectively, is obtained.     

The reaction between solid iron and liquid Al-Zn baths

The reaction which occurred between iron panels and Al-Zn baths during hot dipping was investigated. Three baths were studied: 45Al-55Zn, 55Al-45Zn, and 75Al-25Zn (in wt pct) in the temperature range of 570 to 655 °C. The reaction between the iron panel and the Al-Zn bath was very severe and in all cases the iron panel was totally consumed by the bath in less than two minutes. The rapid attack of the iron panels by the Al-Zn baths was attributed to two separate causes depending on growth conditions. First, in some panels the intermediate layer which formed between the iron panel and the molten bath was nonadherent. This resulted in the direct contact of the molten bath with the iron panel at a nonequilibrium interface, which presented a large driving force and little inhibition for the reaction. Second, in panels containing an adherent alloy layer, the layer had channels of liquid Zn which extended from the molten bath to the iron panel. These channels allowed rapid transport of Zn and Al to the iron panel which resulted in a very high reaction rate. The controlling step in the reaction between the iron panel and molten Al-Zn bath was the diffusion rate of Al in the molten bath to the surface of the iron panel. The diffusion coefficient of Al in the molten bath was found to be in the range of 1 × 10^-5 to 5 × 10^-5 cm²/s. Microstructural, electron microprobe, and X-ray diffraction data are presented to support the above-mentioned mechanisms and conclusions.     

Diffusion of managenese and silicon in liquid iron over the whole range of composition

The measurement of the diffusivities of manganese and silicon in molten binary ferroalloys over the whole range of composition was undertaken to clarify existing but conflicting data at lower concentrations, to present new data at higher concentrations and to indirectly confirm the behavior of both systems observed in thermodynamic studies. The experiments were carried out under argon atmosphere in a Tammann furnace. The diffusion couples were held in 5 mm ID alumina tubes (98 pct Al₂O₃). Electron probe microanalysis of the samples led to a diffusion-penetration curve for the system under consideration. Results obtained over the whole range of composition showed a slight negative deviation for the Fe−Mn system and a very large positive deviation for the Fe−Si system. At lower concentrations (0 to 4 pct Mn), the temperature dependence of managanese diffusivity for the Fe−Mn binary alloy in the temperature range 1550° to 1700°C is as follows:D _Fe−Mn=1.8×10⁻³ exp (−13,000/RT) cm²/sec The concentration dependence of manganese diffusivity for the same system at 1600°C may be expressed asD _Fe−Mn={5.48−0.0137 (%Mn)+0.000276 (%Mn)²}×10⁻⁵ cm²/sec The temperature dependence of silicon diffusivity for the Fe−Si binary system in the temperature range 1550° to 1725°C at various concentrations is as follows:D _Fe−Si=2.8×10⁻³ exp (−11,900/RT) cm²/sec at 20 pct SiD _Fe−Si=2.1×10⁻³ exp (−13,200/RT) cm²/sec at 12.5 pct SiD _Fe−Si=5.1×10⁻⁴ exp (−9,150/RT) cm²/sec at 2.2 pct Si FELIPE P. CALDERON, formerly Graduate Student. University of Tokyo, Tokyo, Japan. This paper is based on a portion of a thesis submitted by FELIPE P. CALDERON in partial fulfillment of the requirements for the degree of Doctor of Engineering at University of Tokyo.     

The diffusivity of sulfur in wüstite and its solubility in wüstite and γ iron in equilibrium with each other and a liquid oxysulfide

The solubility of sulfur in wüstite in equilibrium with γ iron and liquid oxysulfide was found to be 0.011 wt pct at 1050°C. The sulfur solubility in γ iron in equilibrium with wüstite and liquid oxysulfide was also determined at 1050°, 1150°, and 1250°C and found to be 135, 165, and 160 ppm respectively. These values are considerably lower than the sulfur-solubility in y iron in the binary Fe-S system saturated with pyrrhotite. The diffusivity of sulfur in wüstite was determined by oxidizing Fe-S alloys in mixtures of CO and CO₂, and analyzing the entire sample for sulfur afterwards. From the amount of sulfur diffused through the growing wüstite layer into the gas phase, the diffusivity of sulfur in wüstite was evaluated, and found to be 4.1 × 10⁻⁸ and 9.6 × 10⁻⁷ cm²/s at 1050° and 1250°C respectively. These values are of the same order as the self-diffusivity of iron in wüstite in equilibrium with iron at the same temperatures.     

Evaluation of the reduction of iron oxide from liquid slags using a graphite rotating disk

The rotating disk methodology has been used for examination of the reduction of FeO from CaO-FeO-SiO₂ liquid slags (20 and 60 pct FeO) with a CaO/SiO₂ ratio equal to 0.66 and 1.27, in the temperature range 1350 °C to 1420 °C. It has been found that the reduction proceeds under diffusion control. The calculated diffusion coefficients fall in the range 0.76·10⁻⁷ to 1.6·10⁻⁶ cm²/s. Comparison of these values with those given in the literature suggests that the calculated coefficients are related to the diffusion of oxygen ions in the slag. The calculated thickness of the limiting diffusion layer, δ, ranges from 0.65·10⁻³ to 5.25·10⁻³ cm, depending on the reduction conditions. The largest decrease in the limiting diffusion layer thickness takes place at low rotational speeds, i.e., 100 and 400 rev/min. The maximum value of the mass transfer coefficient is 1.71·10⁻³ cm/s for reduction from slag with a CaO/SiO₂ ratio of 1.27, 60 pct FeO, at 1420 °C and 2000 rev/min, and the minimum value is 0.27·10⁻⁴ cm/s for reduction from slag with a CaO/SiO₂ ratio of 0.66, 20 pct FeO, at 1350 °C and 100 rev/min. Good agreement has been found between experimental and calculated reduction rates at low disk rotations (100 and 400 rev/min).     

Reduction of iron-silicon-oxysulfide by CO gas injection

The reduction of liquid oxysulfide in the Fe-Si-S-O system by CO gas injection has been studied by monitoring the exit gas composition. The reduction rate of oxygen was calculated from the volume of evolved CO₂. Sulfur-bearing species such as COS were close to the detection limit of the mass spectrometer, which indicated that the reduction of sulfur was very limited. The volume of evolved CO₂ reached steady values 1 minute after CO injection. The reduction reaction was controlled by a chemical reaction. The observed maximum reduction rate of oxygen at 1250 °C was 8.3×10⁻⁶ g-O/cm² s, which was within the range of the reduction rates in other melts such as iron oxide and iron silicates.     

Preparation of pure silicon by electrowinning in a bytownite-cryolite melt

Pure silicon was prepared by electrowinning in a bytownite-cryolite molten mixture at about 970 °C. In the electrolysis cell, the bottom of a graphite crucible was used as the anode. The cathode was also made of graphite. The reaction was carried out at the cell voltage of about 2.7 V, and the cathode current density was either 0.027 or 0.053 A/cm² for 10, 50, and 100 pct of the time for all Si deposited. The obtained silicon, which was analyzed by electron microprobe analysis (EMPA), contained 99.79 to 99.98 pct Si, which is a specially pure quality. CO₂ gas formed at the anode flushed silicon deposited at the cathode and purified it by oxidizing the impurities that ordinarily would be deposited at the cathode.     

ASSESSMENT OF REDUCTION BEHAVIOR OF HEMATITE IRON ORE PELLETS IN COAL FINES FOR APPLICATION IN SPONGE IRONMAKING

Studies on isothermal reduction kinetics (with F grade coal) in fired pellets of hematite iron ores, procured from four different mines of Orissa, were carried out in the temperature range of 850–1000°C to provide information for the Indian sponge iron plants. The rate of reduction in all the fired iron ore pellets increased markedly with a rise of temperature up to 950°C, and thereafter it decreased at 1000°C. The rate was more intense in the first 30 minutes. All iron ores exhibited almost complete reduction in their pellets at temperatures of 900 and 950°C in < 2 hours' heating time duration, and the final product morphologies consisted of prominent cracks. The kinetic model equation 1 ? (1 ? α)^1/3 = kt was found to fit best to the experimental data, and the values of apparent activation energy were evaluated. Reductions of D. R. Pattnaik and M. G. Mohanty iron ore pellets were characterized by higher activation energies (183 and 150 kJ mol^?1), indicating carbon gasification reaction to be the rate-controlling step. The results established lower values of activation energy (83 and 84 kJ mol^?1) for the reduction of G. M. OMC Ltd. and Sakaruddin iron ore pellets, proposing their overall rates to be controlled by indirect reduction reactions.     

Phase equilibria in the system Fe-Zn-O at intermediate conditions between metallic-iron saturation and air

The phase equilibria in the FeO-Fe₂O₃-ZnO system have been experimentally investigated at oxygen partial pressures between metallic iron saturation and air using a specially developed quenching technique, followed by electron probe X-ray microanalysis (EPMA) and then wet chemistry for determination of ferrous and ferric iron concentrations. Gas mixtures of H₂, N₂, and CO₂ or CO and CO₂ controlled the atmosphere in the furnace. The determined metal cation ratios in phases at equilibrium were used for the construction of the 1200 °C isothermal section of the Fe-Zn-O system. The univariant equilibria between the gas phase, spinel, wustite, and zincite was found to be close to pO₂=1 · 10⁻⁸ atm at 1200 °C. The ferric and ferrous iron concentrations in zincite and spinel at equilibrium were also determined at temperatures from 1200 °C to 1400 °C at pO₂ = 1·10⁻⁶ atm and at 1200 °C at pO₂ values ranging from 1 · 10⁻⁴ to 1 · 10⁻⁸ atm. Implications of the phase equilibria in the Fe-Zn-O system for the formation of the platelike zincite, especially important for the Imperial Smelting Process (ISP), are discussed.