Isothermal Reduction Experiments Research Articles

Smelting Reduction of HIsarna Slag with Methane

To study the smelting reduction behavior of methane in the HIsarna process, the isothermal reduction experiments of HIsarna slag with 6 wt% of FeO were conducted at the temperatures of 1450, 1475, and 1550 °C, using 5 vol% CH4‐Ar. The effect of the gas injection flow rate on the overall reduction rate was investigated in the range of 0.5–1.5 L min−1. The results confirm that the decomposed carbon black acts as the dominant reductant in the reduction process compared to hydrogen. Increased gas flow rate increases the reaction rate, while temperature shows no significant effect. The reduction process could be described by the JMAEK model: by fixing the gas flow rate at 1.0 L min−1 with different temperatures, the exponent n value is around 1.5, indicating FeO mass transfer is the limiting step since there are sufficient carbon blacks as nucleation sites. The reduction process follows the first‐order model controlled by the reactant concentration with the gas flow rate being at 0.5 L min−1 and in the second reaction stage of the gas flow rate at 1.5 L min−1. The determined activation energy for the methane reduction is 59.8 kJ mol−1.

Reduction of chengde vanadium titanium magnetite concentrate by microwave enhanced Ar–H2 atmosphere

In this study, a new method of microwave-enhanced hydrogen reduction was developed to carry out isothermal reduction experiments. The effects and mechanisms of different hydrogen volume fractions on the direct reduction behavior of a vanadium titanium magnetite concentrate (VTM) were investigated. The experimental results and theoretical analysis proved that combining the advantages of microwaves and hydrogen to reduce the VTM is feasible. The thermodynamics of the non-microwave field indicated that the hydrogen reduction of iron titanium oxides was more difficult than that of iron oxides and required a higher reaction temperature. Under microwave heating conditions, with an increase in hydrogen volume fraction, the metallization rate and reduction degree increased, the microscopic morphology developed into a porous sponge-like structure, the size of the internal pores increased, and the migration of metallic iron to the edges of the particles became obvious. When the hydrogen volume fractions exceeded 60%, the products did not contain TiO2, and the final products consisted of metallic iron containing a small amount of dissolved Ti. The reduction reaction was controlled by gas phase diffusion, and the average apparent activation energy was calculated to be 84.05 kJ/mol using a new kinetic model. Moreover, increased hydrogen volume fraction weakened the gas phase diffusion and was converted into internal diffusion.

Reduction Kinetics of Compact Hematite with Hydrogen from 600 to 1050 °C

Reduction of iron ores with hydrogen is a solution to replace fossil fuels. For this reason, it is important to discuss previous discrepancies. Some previous studies suggest a rate minimum with respect to temperature. Our research work indicates that a rate minimum can be avoided. Thermogravimetric isothermal reduction experiments were carried out from 600 to 1050 °C with pure reagent ferric oxide and hydrogen using a tubular furnace. The morphology and chemical composition of the initial sample, consisting of particulate hematite (Fe2O3), and the final product, consisting of metallic iron (Fe°), was defined using scanning electron microscopy (SEM) and X-ray diffraction (XRD). The reduction rate for the conversion from hematite to magnetite (Fe2O3 to Fe3O4) was the highest, around 5 %/min, decreasing to around 2–5%/min for the second stage of conversion from magnetite to wüstite (Fe3O4 to FeO). This reduction rate remains almost constant from about 20–80% reduction, decreasing to 0.3–1%/min for the completion of reduction from wüstite to metallic iron (FeO to Fe°). The reduction controlling mechanism was evaluated based on the calculated apparent activation energy and fitting the experimental data to one gas-solid reaction equation. Under the experimental conditions in this work, the reduction rate of pure hematite with hydrogen linearly increased with temperatures from 600 to 1000 °C, without a rate minimum in this temperature range. Above 1000 °C, the reduction rate decreased due to sintering phenomena. This result suggests a maximum reduction temperature of 1000 °C using pure hematite and hydrogen as the reducing gas. The reduction controlling mechanisms identified using hydrogen as a reducing gas were chemical reaction for the conversion from hematite to wüstite and diffusion control for the final reduction from wüstite to metallic iron. Since the reduction rate from wüstite to metallic iron is the one that affects the overall rate of reduction, overall changes in porosity were also evaluated. Finally, the reduction of wüstite is schematically described.

Kinetic Study on Reduction of FeO in a Molten HIsarna Slag by Various Solid Carbon Sources

To investigate the reduction behaviour of different reductants such as charcoal (CC), thermal coal (TC), and carbon black (CB) with HIsarna slag, a series of isothermal reduction experiments were performed in a vertical tube resistance furnace (VTF), coupled with a Quadrupole mass spectrometer (QMS) at 1450 °C, 1475 °C and 1500 °C. The results confirm that the highest overall reduction rate was achieved by CC, followed by TC and CB. The reduction mechanism between FeO containing molten slag and the selected carbonaceous materials is determined by studying the morphology of the water quenched samples at the intervals of 1.5, 3 and 5 minutes, using optical and scanning electron microscopes. The results reveal that the overall reaction is controlled by two main mechanisms: (1) nucleation and growth of CO bubbles, proceeded by the gaseous intermediates CO and CO2; and (2) diffusion of FeO in the molten slag. The initial reduction period in which chemical reaction control is dominant, can be described by the Avrami–Erofeev model, whereas the final period is described by the three-dimensional diffusion model.

Hydrogen reduction of spent lithium-ion battery cathode material for metal recovery: Mechanism and kinetics.

Hydrogen reduction is becoming a promising method for recycling lithium-ion battery cathode materials. However, the reaction mechanism and kinetics during hydrogen reduction are unclear, requiring further investigation. Therefore, non-isothermal and isothermal reduction experiments were conducted to evaluate the temperature dependence of the hydrogen reduction kinetics using simultaneous thermogravimetric and differential thermal analysis equipped with mass spectrometry. XRD and SEM were used to characterize the reduction products to understand the underlying reduction mechanisms. The hydrogen reduction profile could be divided into three main stages: decomposition of cathode materials, reduction of the resultant nickel and cobalt oxides, and reduction of LiMnO2 and residual nickel and cobalt oxides. The hydrogen reduction rate increased with increasing temperature, and 800°C was the optimum temperature for separating the magnetic Ni-Co alloy from the non-magnetic manganese oxide particles. The apparent activation energy for the isothermal tests in the range of 500–700°C was 84.86 kJ/mol, and the rate-controlling step was the inward diffusion of H2(g) within each particle. There was an downward progression of the reduction through the material bed for the isothermal tests in the range of 700–900°C, with an apparent activation energy of 51.82 kJ/mol.

Reduction kinetics of combusted iron powder using hydrogen

Despite extensive research on reduction of iron oxides in literature, there is no consensus on the most accurate reduction kinetics, especially for micron-sized iron oxide powders with high purity. Such data is particularly important for the application of metal fuels and chemical looping combustion, in which high purity iron powders function as dense energy carriers. Hence, in this work, hydrogen reduction of iron oxide fines, produced by iron combustion, were investigated using thermogravimetric analysis (TGA). The isothermal reduction experiments were conducted at the temperature range of 400–900 °C and at hydrogen partial pressures of 0.25–1.0 atm. Scanning electron microscopy (SEM) showed that the morphology of the reduction products depends on the reduction temperature but not on the hydrogen partial pressure. Reduction at higher temperatures leads to larger pore sizes. Based on an extended Hancock-Sharp “lnln”-method the appropriate gas-solid reaction models are determined, suggesting that the reduction can be described by a single-step phase boundary controlled reaction at temperatures below 600 °C, whereas a multistep mechanism is required for the description of reactions at higher temperatures.

Effect of CO–CO2 and H2–H2O on the reduction degree of fluxed pellets: reduction mechanism within hydrogen-rich blast furnace

ABSTRACT In order to explore the mechanism of hydrogen-rich blast furnace on improving the reduction behaviour of fluxed pellets, the isothermal reduction experiments of fluxed pellets in CO-CO2-N2 and CO-CO2-H2-H2O-N2 atmosphere were studied. The results show that the condition of hydrogen-rich blast furnace at 900°C has a significant effect on improving the reduction degree of fluxed pellets. It is helpful to increase the reduction rate of fluxed pellets at 1000~1100°C. The phenomenon of "reduction stagnation" at 800°C is due to the metallic iron layer formed on the periphery of the fluxed pellets, which hinders the entry of reducing gas. The morphological observation of the reduction products showed that the grid-like metallic iron structure is the key factor to improve the reduction rate and reduction degree of fluxed pellets in CO-CO2-H2-H2O-N2. Finally, the reduction mechanism of flux particles in hydrogen-rich blast furnace is summarized.

Reduction Behavior and Direct Reduction Kinetics of Red Mud-Biomass Composite Pellets

A large amount of red mud is discharged in the aluminum oxide production process, which contains a variety of valuable metals and is considered as a secondary resource. In order to reveal the mechanism of red mud carbon thermal reduction process, isothermal reduction experiments on carbon-bearing pellets of red mud were investigated with biomass carbon as a reducing agent. In this study, the reduction temperatures were conducted using a microwave stove in the temperature range from 850 to 1250 ℃, and the C/O molar ratio was 1.1. The results show that the reduction process of red mud is governed by a carbon gasification reaction, and the apparent activation energy is 88.44 kJ/mol. The optimum reduction process conditions were established to be 1150 ℃ for 13 min.

Isothermal reduction of V2O5 powder using H2 as oxygen carrier: Thermodynamic evaluation, reaction sequence, and kinetic analysis

V2O3 was prepared by reducing V2O5 powder using H2 as the oxygen carrier. To initially determine reasonable experimental for the H2-based reduction, oxygen potential and equilibrium calculations were performed. The reaction sequence of the V2O5 powder reduction was predicted by VO phase diagram assessments and was then verified by XRD and XPS measurements under various experimental condition. It was observed that the reduction followed the path of V2O5-VnO2n+1(n = 3 and 6)-VO2-VnO2n-1(n = 7, 6, 5, 4, and 3)-V2O3. Isothermal reduction experiments and kinetic analysis were performed using thermogravimetric analysis (TGA) at 813 K, 823 K, and 833 K, under a pure H2 gas flow. The entire reduction process, from V2O5 to V2O3, obeyed the unreacted shrinking core model (with random instant nucleation and three-dimensional growth of nuclei) and was controlled by chemical reactions. Mathematical fitting showed that the reduction process could be described as G(α) = [−ln(1-α)]1/3, and the apparent activation energy was determined to be 47.11 kJ·mol−1.

A clean process of preparing VO as LIBs anode materials via the reduction of V2O3 powder in a H2 atmosphere: Thermodynamic assessment, isothermal kinetic analysis, and electrochemistry performance evaluation

VO is one of the most promising anode materials for lithium ion batteries (LIBs). Herein, we present the clean preparation of VO powder via reduction of V2O3 powder under high-purity H2. Thermodynamic calculation and V–O phase diagram analysis were carried out to ensure reasonable oxygen potential conditions. Isothermal reduction experiments and kinetic analysis using thermogravimetric analysis (TGA) were then performed at 1623 K, 1648 K, and 1673 K under a pure H2 gas flow. The reduction of V2O3 powder appears to obey an unreacted shrinking core model and can be divided into two steps. The first step was controlled by a chemical reaction, the kinetics of which can be described as G(α) = [-ln(1-α)]1/3 with an apparent activation energy of 107.3 kJ·mol−1. The second stage was controlled by gas diffusion, the kinetics of which can be described as G(α) = [1-(1-α)1/3]1/2 with an apparent activation energy of 45.5 kJ·mol−1. Powder X-ray diffraction (XRD) and scanning electron microscopy (SEM) also were employed for characterizing the morphology of the prepared VO powder. As a result, when the VO was served as the LIBs anode material, the resulting electrodes exhibited a high specific capacity (843 mAh·g−1 after 30 cycles) and remarkable cyclic stability.

Powder reduction kinetics of dicalcium ferrite, calcium ferrite, and hematite: Measurement and modeling

Shrinking core model is widely applied to describe the reduction of iron ore pellets, but limited to the illustration on powder sample. The reduction of powder materials is commonly observed in blast furnace production but has been rarely investigated. In this study, thermal kinetics analysis was conducted to describe the powder reduction of dicalcium ferrite (2CaO⋅Fe2O3, C2F), calcium ferrite (CaO⋅Fe2O3, CF), and hematite (Fe2O3, H), with particle sizes below 70µm. Isothermal reduction experiments were performed through thermogravimetry analysis under CO atmosphere. The reduction degrees and reaction rate constants increased in the order of C2F, CF, and H at 1123, 1173, and 1223K. The reduction rate analysis illustrated that the reduction of C2F, CF, and H appeared as one-, two-, and three-stage reactions, respectively. Moreover, the reduction of C2F and CF proceeded as the 2D reaction mechanism described by Avrami–Erofeev (A-E) equation. The reduction of H was initially controlled by 2D, followed by the 3D A-E kinetics equation. Phase with superior reducibility could be reduced by CO in more dimensions of sample layers. The reduction degrees and rate change expressed by A-E equations were verified to be in accordance with the experimental data. A new kinetics model was proposed to elucidate the reduction of C2F, CF, and H in ultrafine powder compared with that in pellets. The reduction process in the powdered samples comprised independent reduction stages caused by uniform CO diffusion in powdered particles.

Effect of mechanical activation on the hydrogen reduction kinetics of magnetite concentrate

The effect of mechanical activation on the reduction kinetics of magnetite concentrate by hydrogen was studied. The magnetite concentrate was milled for 8 h in a planetary ball mill. After 8 h of milling, the average particle size was reduced from 14 to 4.4 µm, resulting in a lattice microstrain of 0.3. Isothermal reduction experiments were conducted by thermogravimetry to focus on the chemical reaction as the rate-controlling factor by eliminating external mass transfer effects and using a thin layer of particles to remove interstitial diffusion resistance. Thus, about 2 mg of magnetite powder was reduced at different temperatures under a sufficient flow of hydrogen. The magnetite concentrate and reduction products were analyzed by SEM and XRD. It was found that the onset reduction temperature decreased from 587 to 500 K (314–227 °C) due to the mechanical activation. The activation energy for hydrogen reduction of the activated concentrate decreased about 10% compared with the as-received concentrate. In view of the results, a reaction rate expression was established based on the nucleation and growth model with an Avrami parameter n = 2.5.

Oxygen Adsorption and Water Formation on Co(0001)

Oxygen adsorption and removal on flat and defective Co(0001) surfaces have been investigated experimentally using scanning tunneling microscopy, temperature-programmed and isothermal reduction, synchrotron X-ray photoemission spectroscopy, and work function measurements under ultrahigh vacuum conditions and H2/CO pressures in the 10–5 mbar regime. Exposure of the Co(0001) to O2(g) at 250 K leads to the formation of p(2 × 2) islands with a local coverage of 0.25 ML. Oxygen adsorption continues beyond 0.25 ML, reaching a saturation point of ∼0.39 ML Oad, without forming cobalt oxide. Chemisorbed oxygen adlayers can be reduced on both flat and defective Co(0001) surfaces by heating in the presence of ∼2.3 × 10–5 mbar H2(g). The onset of the oxygen removal as water during temperature-programmed reduction experiments (1 K s–1) is at around 450 K on flat Co(0001) and 550 K on defective Co(0001). By evaluation of isothermal reduction experiments using a kinetic model, the activation energy for water formation is...

Phase Development in Carbothermal Reduction and Nitridation of Ilmenite Concentrates

Abstract The phase development in the course of carbothermal reduction and nitridation of ilmentie concentrates and synthetic rutile was studied in temperature programmed reduction (623–1873 K) and isothermal reduction experiments. Ilmenites and synthetic rutile were reduced in a tube reactor with continuously flowing hydrogen-nitrogen mixture or pure nitrogen. The rate and extent of reduction were monitored by online off-gas analysis. Samples reduced to different extent were subjected to XRD and SEM/BSE analyses. Pseudorutile and ilmenite were the main phases in ilmenite concentrates; rutile was the main phase in synthetic rutile. Pseudorutile was first converted to ilmenite and titania which occurred at temperatures below 623 K; iron oxides in ilmenite were quickly reduced to metallic iron. Titania was reduced to titanium suboxides and further to titanium oxycarbonitride. Reduction of ilmenites and synthetic rutile in hydrogen-nitrogen mixture was much faster than in pure nitrogen. The rate of conversion of titanium oxides to oxycarbonitride was affected by iron content in the ilmenites. The rate of reduction increased with increasing iron content in ilmenite (decreasing grade) when ilmenites were reduced in the hydrogen-nitrogen gas mixture, but decreased with decreasing ilmenite grade in reduction experiments in nitrogen; reduction in nitrogen was the fastest for synthetic rutile. The difference in the reduction behaviour was attributed to different chemical compositions and morphologies of ilmenites of different grades.

Comprehensive Utilization of Paigeite Ore Using Iron Nugget Making Process

The feasibility of paigeite ore treatment with iron nugget making process is proved. The isothermal reduction experiments at laboratory scale were carried out, using carbon bearing pellets which were made of boron containing iron concentrate and pulverized coal mainly, from 1623 to 1723 K with different heating time. The results indicated that iron nugget making process depends mainly on heating time and temperature. And the iron nugget and slag can separate in a clean manner at 1673 K for 15 min. For the iron nugget, the C content is 3.57% (mass percent) and B is 0.065% (mass percent). The B2O3 content of slag is 20.01%, and the boron was concentrated into one phase which is identified as suanite (Mg2B3O5) during the solidification. With an extraction ratio of 80% under the atmospheric conditions, the activity of boron in slag is good. The boron-rich slag can be used to extract boric or boric acid and alleviate the shortage of boron resource. Through series of calculation and analysis related, it can be concluded that the recovery ratio of Fe and boron are about 98% and 97% respectively. The results show that this method is feasible and effective on the utilization of paigeite ore.

Innovative Methodology for Separating of Rare Earth and Iron from Bayan Obo Complex Iron Ore

A new process was proposed in this research, in order to address the problems of difficult treatment, low efficiency and heavy pollution of Bayan Obo complex iron ore. The isothermal reduction experiments, using carbon-bearing pellets which were mainly made of Bayan Obo complex iron ore and pulverized coal, were investigated in the temperature range of 1623–1723 K with different heating time. The results indicate that the pellets could not melt well at 1623 K and 1723 K, and the iron nugget and slag can separate in a clear manner at 1673 K for 12 min. The contents of C and S in iron nugget are 2.09% and 1.62% respectively. The iron nugget can be used partly to substitute the steel scrap for EAF steelmaking. The RE2O3 content is 14.19% in the rare-earth-rich slag. Nearly all rare earth is concentrated into one phase during solidification, which is identified as cefluosil ([7(Ca, Ce, La, Nd)2·SiO4] (F, O)10). The slag was leached by hydrochloric acid and the leaching efficiency of rare earth is 98.70%. After being filtered, the solution can be used to extract rare earth and the leached residue will be treated to recover CaF2 and ThO2.

Reduction Kinetics of Ag2MoO4 by Hydrogen

Ag2O and MoO3 powders were mechanically (ball) milled with the objective of obtaining a Ag2MoO4 phase for subsequent reduction with hydrogen gas. A thermogravimetric unit was used to follow the course of the reduction process aiming at the chemical reaction as the rate controlling step. Isothermal reduction experiments were performed on the individual oxides to establish a ground for the reduction parameters of silver molybdate. Consequently, a nonisothermal treatment was used on Ag2MoO4. It was found that Ag2MoO4 is reduced in three steps, e.g., Ag2MoO4 to MoO3 and Ag followed by MoO3 to MoO2 and finally MoO2 to Mo. By assuming a shrinking core model, the calculated activation energies for the first and second reduction steps were 69 and 29 kJ/mol, respectively. While the former value is in good agreement with the activation energy obtained from the isothermal reduction of Ag2O to Ag, 64 kJ/mol, the latter was found to be lower than the corresponding value for MoO3 to MoO2 under isothermal conditions, 124 kJ/mol. It was found that the presence of metallic silver obtained from the initial reduction step of Ag2MoO4 not only lowers the energy barrier but the reducing temperature of the subsequent reaction steps (i.e., MoO3 → MoO2 → Mo).

Mechanism of Carbothermic Reduction of Hematite in Hematite–Carbon Composite Pellets

Isothermal reduction experiments are carried out to study carbothermic reduction of hematite in hematite–graphite composite pellets. The carbothermic reduction is dominated by the direct reduction at the initial stage of reduction, and it is determined by the indirect reduction together with the carbon solution loss reaction when the CO partial pressure exceeds a certain value, which is dependent on the temperature. The reduction is greatly promoted due to the increment of the contact area between the hematite and graphite particles and enhancement of heat conduction in the hematite–graphite composite pellets.The indirect reduction together with the carbon solution loss reaction is markedly activated with increasing temperature. At a moderate carrier gas flow rate, the carbothermic reduction rate reaches the highest value. The reduction can also be effectively improved with decreasing the graphite particle size.The relationship of reduction products, heating temperature and holding time has been obtained from the XRD analysis results at the different stages for various temperatures under the present experimental conditions. The SEM observation indicates that the indirect reduction together with the carbon solution loss reaction should dominate the carbothermic reduction of hematite at the later stage of reduction.

Formation of stable Cu 2O from reduction of CuO nanoparticles

In situ time-resolved X-ray diffraction (TR-XRD) using synchrotron radiation has been used to capture the dynamics of the reduction of nanocrystalline CuO using a normal supply of CO gas. Copper(II) oxide nanoparticles 4–16 nm in width, as measured by XRD peak broadening, are synthesized using an aqueous organic-nitrate method and reduced in isothermal and temperature ramping reduction experiments. Temperature-programmed reduction of CuO nanoparticles using a ramping heating profile was observed to result in the sequential reduction process CuO → Cu 2O → Cu, with CuO reducing completely to the intermediate Cu 2O phase before further reduction to metallic copper. Isothermal reduction experiments at 250 °C show that CuO nanoparticles completely reduce to Cu 2O, and this phase remains stable without further reduction with continued exposure to CO. In contrast to what is typically observed in bulk CuO in both isothermal and ramping reduction conditions, nanocrystalline CuO reduces to a stable Cu 2O phase rather than forming metallic copper directly. The behavior of the CuO nanoparticles in temperature ramping reducing conditions is controlled by the particle size, with the smaller CuO nanoparticles exhibiting a greater stability and withstanding a higher temperature before their reduction to Cu 2O and then to metallic copper nanoparticles.

Reduction of Titania-Ferrous Ore by Carbon Monoxide

The reduction of titania-ferrous ore (ironsand) containing 57.2 wt% of iron and 7-8 wt% of TiO2 was investigated in non-isothermal and isothermal reduction experiments using CO-CO2-Ar gas mixtures in a laboratory fixed bed reactor. Samples in the course of reduction were characterised using XRD, EPMA and SEM.Two types of particles were identified in ironsand: 1) homogeneous particles of titano-magnetite with cubic spinel structure (a major type); and 2) non-homogeneous particles, characterised by lamellar structure of rhombohedral titano-hematite, exsoluted from the titano-magnetite.Titano-magnetite, which is a magnetite-ulvospinel solid solution (Fe3O3)1−x(Fe2TiO4)x, with x=0.27±0.02, was reduced to metallic iron and titanium sub-oxides. Titanium had a strong effect on the mechanism and rate of reduction of iron oxide in ironsand.