Reduction Kinectics of Hematite Powders in Non-Equilibrium Hydrogen Plasma

Abstract

The reduction kinetics of hematite (Fe₂O₃) powders was studied using nonequilibrium hydrogen plasma as reducing agent. Reduction experiments were performed in a DC pulsed plasma reactor, hydrogen flow-rate of 300 cm³/min, at pressure of 400 Pa, reduction times in range 30-120 minutes and in temperatures range 320-380 °C. Fe₂O₃ powders after reduction experiments were characterized by X-ray diffraction, weight loss of oxygen (gravimetric analyses), optic microscope and scanning electron microscope. The results showing that the reduction temperature of 380 °C after 120 min allows obtaining α-iron with a reduction fraction of about 0.93. The powder particles are transformed into two steps: Fe₂O₃→Fe₃O₄→Fe-α. The activation energy experimentally established for the reduction of Fe₂O₃ was about 98.4 kJ/mol.

Keywords:
Hematite reduction; Hydrogen plasma; Solid-state reaction; Activation energy

1. Introduction

The carbothermic reduction methods have been used since antiquity for extracting metals from their ores¹. New reduction routes have been proposed from 1970’s by several research groups, attempting to mitigate the generation of both solid wastes and greenhouse effect gases. Among the proposals, one can mention: metal oxide direct reduction using hydrogen gas as reducing agent, vacuum carbothermic reduction, direct electrochemical reduction of solid refractory oxide in molten magnesium chloride (Cambridge Process), metallothermic reduction, and high-energy milling²-⁷.

The development of process based on the use of hydrogen as reducing agent constitutes very interesting alternative, given the possibility of direct use of fine ore avoiding previous treatments such as sintering and pelletizing. It is possible the elimination of greenhouse gas emissions and generation of only water steam. However, the deployment and industrial rise of hydrogen reduction methods based on the high cost of production due to hydrocarbon reform steps or hydrolysis of water⁸.

The potential for application of cold plasma, also known as non-equilibrium plasma, in metal reduction has been demonstrated experimentally by different research groups over the last 10 years⁹-¹⁵. In 2003, Silva⁹ shows that between 600 °C and 800 °C the kinetics of reduction of tungsten oxide (WO₃) by cold plasma of hydrogen is much bigger than the hydrogen gas. The following year Zhang et al.¹⁰ published a work showing that at the temperature of 200 °C copper oxide (CuO) is totally reduced to metallic copper (Cu) after 2 h of experiment, that did not occur when hydrogen gas was used. Recently, Rajput et al.¹² showed that from the Hematite (Fe₂O₃) may be obtained with metallic iron reduced fractions (α) of 0.94 after 2 h at 300 °C temperature in microwave plasma reactor of 2.45 MHz. According to information contained in this work, the kinetics of reduction of Hematite by hydrogen gas (H₂) is comparable to the reduction by hydrogen plasma only from temperatures above 800 °C. Nakayama¹³ shows that the cold plasma of hydrogen as reducing agent provides a considerable increase in reduction kinetics of powders of cobalt oxide (Co₃O₄), for reducing temperatures between 250 °C and 300 °C. At 250 °C, the plasma allowed get fractional reduction around 0.90 after 50 min of reduction. In these same conditions, the use of gas resulted in fractions of the maximum reduction of just 0.30.

The Ellingham diagram-Richardson¹⁶ shows that the atomic hydrogen (H), one among the many species present in plasma, is capable of reducing the various existing oxides in low temperatures. The collision of these electrons, energy-dense, with gas molecules can result in excitation, ionization, dissociation, production of electrons and, among other reactive species, in the formation of atomic hydrogen¹¹,¹⁷. Dissociation reactions increase the kinetics of reduction since the same foster production of atomic hydrogen (H), species able to diffuse through the oxide structure¹⁸,¹⁹.

2. Materials and Methods

This work used samples of Hematite powder with a granulometry in the range from 1 µm to 5 µm which corresponded the mass 80% used with purity was above 99.0%. To promote the reduction was used hydrogen gas by the company White Martins®. The purity of the gas is 99.999% according to the supplier.

2.1. Plasma-assisted reduction experiments

A DC pulsed plasma reactor (Thor NP design, supplied by SDS Plasma Company®, Paranrá, Brazil) was used to carry out the reduction tests, Figure 1.

Figure 1
Schematic representation of the pulsed DC plasma reactor used of the plasma reduction experiments.

The reactor consists of a Board of 50 cm diameter by 75 cm high voltage pulsed voltage source 650 V maximum and 3.4 kHz frequency, mechanical pump with ability to achieve pressures below 1.0 Pa, with maximum flow meter H₂ of 500 cm³/min, thermocouples for temperature monitoring of samples and external heating system for heating of samples until the working temperature before the activation of the plasma. The heating was accomplished through an electrical resistance accoupled at the cathode.

The experimental conditions of the hematite reduction process by cold hydrogen plasma were employed the following parameters: voltage between electrodes of 540 V, 400 Pa pressure, and hydrogen flow of 300 cm³/min. The experiments were conducted in temperatures range 320-380 °C, and reductions times range in 30-120 minutes. Each experiment was used five samples contained in 316 L stainless steel crucibles, totaling eighty samples. The initial mass of each one of the samples was approximately 200 mg of hematite.

2.2. Gravimetry

After the hematite reduction process, the plasma was interrupted and the samples cooled still inside the reactor chamber under vacuum to room temperature. The mass of powders, before and after the reduction experiments, was determined by gravimetric method, using a scale from OHAUS Explorer precision model brand of 0.1 mg. The reduced fraction determination, α, was done as follows (Equation 1):

$\alpha =\frac{{\text{M}}_{\text{H}}}{\text{3}{\text{.M}}_{\text{O}}}.\left(\frac{{\text{m}}_{\text{i}}-{\text{m}}_{\text{f}}}{{\text{m}}_{\text{i}}}\right)\approx \mathrm{3,33.}\left(\frac{{\text{m}}_{\text{i}}-{\text{m}}_{\text{f}}}{{\text{m}}_{\text{i}}}\right)$ (1)

where: α is fraction of reduction, (mi) is the initial mass, (mf) is the final mass, M_H is the molecular weight of hematite worth approximately 160 g and M_O is the molecular weight of oxygen worth 16 g.

From the data of α, it was possible to trace the curves fraction of reduction (α) x time and reduction determine the activation energy (Ea) for the hematite reduction process particles by hydrogen plasma.

2.3. X-ray diffraction

The reduction products obtained in different experimental conditions were qualitatively identified through analysis by X-Ray diffraction (XRD). We used a model D2 diffractometer Phaser brand Bruker with K_α-Cu radiation with a wavelength of 1.54 Å. Powders samples were employed with mass between 700 and 1.000 mg. The parameters for measurements were as follows: time of 0.3 s, count with 2θ ranging from 10° to 80°, step of 0.02° in 2θ and 5 rotation speed rpm.

2.4. Microstructural analysis

After the experiments, the powders reduced samples were analyzed by o light microscope and scan electronic microscopy. The preparation consisted in polishing with alumina, 0.3 µm. The samples were not etching with reagent and the images were obtained with the aid of a light microscope NEDA Metallux model brand and scanning electron microscope (SEM) Zeiss model EVO MA10 Collection was performed using voltage of 25 kV, in secondary electron detection mode on the SE1 detector.

3. Results and Discussion

Figure 2 shows the diffractogram Hematite powders before and after the plasma reduction experiments at 320 °C. Figure 2b shows the Hematite reducing (Fe₂O₃) at temperature of 320 °C after 30 min. In this condition is possible to see that the magnetite phase (Fe₂O₃) appear. After 60 min, Figure 2c, shows the presence of metallic iron (Fe-α) and magnetite (Fe₂O₃), no detecting Hematite (Fe₂O₃). Increasing time for 120 minutes, Figure 2e, where the diffractogram is not differ from that submitted to reduction for 90 min (Figure 2d).

Figure 2
Phases generated in the process Hematite reduction by hydrogen at 320 °C in times of 30 min. (b), 60 min. (c), 90 min. (d) and (e) 120 min.

Figure 3 shows the diffractogram Hematite powders before and after the plasma reduction experiments at temperatures at 340 °C. Figure 3b shows that the temperature at 340 °C, during 30 min., the Fe (Fe-α) is associated with the Hematite (Fe₂O₃), evidencing that the increasing temperature at 320 °C to 340 °C favoring the appearing of Fe-α in a lower time. Until 60 min, the Fe₂O₃ is not present, evidencing only peaks related to magnetite (Fe₃O₄) and metallic iron (Fe-α).

Figure 3
Phases generated in the process Hematite reduction by hydrogen at 340 °C in times of 30 min. (b), 60 min. (c), 90 min. (d) and (e) 120 min.

Evolution of the phases present in Hematite powders before and after reduction in the temperature of plasma 340 °C for times of 30 min (b), 60 min (c), 90 min (d) and (e) 120 min.

Figure 4 shows the diffractogram hematite powders before and after the plasma reduction experiments at temperatures at 360 °C. Increasing the temperature to 360 °C (Figure 4), in the first 30 minutes of reduction, did not detect of Hematite traces (Fe₂O₃), appearing only peaks of magnetite (Fe₃O₄) and of metallic iron (Fe-α), Figure 4b. With the increase of reduction process time, Figures 4b, 4c, 4d and 4e, it’s possible to observe that there is an increase in the relative intensities of the peaks of Fe to the detriment of Fe₃O₄.

Figure 4
Phases generated in the process Hematite reduction by hydrogen at 360 °C in times of 30 min. (b), 60 min. (c), 90 min. (d) and (e) 120 min.

Figure 5 shows the diffractogram Hematite powders before and after the plasma reduction experiments at 380 °C. At this temperature, after 30 min of reduction (Figure 5b), the Fe-α is associated with magnetite (Fe₃O₄), not detecting the presence of Hematite (Fe₂O₃). Comparing Figure 5b and Figure 2e it is possible observe that for the reduced powder at 380 °C for 30 min., the relative peak to Fe-α (111) presents intensity higher than the same peak relative to powder reduced at 320 °C for 120 min, evidencing that the amount of phase Fe-α after reduction at 380 °C for 30 min is higher than that obtained after reduction at 320 °C for 120 minutes. Increasing reduction process time to 120 minutes, Figure 5e, it’s possible to observe only the presence of metallic iron (Fe-α) and some traces of magnetite (Fe₃O₄).

Figure 5
Phases generated in the process Hematite reduction by hydrogen at 380 °C in times of 30 min. (b), 60 min. (c), 90 min. (d) and (e) 120 min.

Therefore, the results of XRD indicate that the formation of the phase Fe-α is favored by increasing both the time and the temperature. The gaseous reduction of hematite occurred in two steps: Fe₂O_3(s) →Fe₃O₄ (s) →Fe_(α), as observed in Figure 5.

3.1. Kinetic model

Figure 6 shows the results of reacted fraction (α) versus time for each temperature of hematite reduction after the experiments of plasma reduction. According the results of x-ray diffraction, it is possible conclude that both the temperature and the time of reduction process are variables that have strong influence on reduction of Hematite. The different conditions of time and temperature possibility obtaining values of α that ranged from 0.08 (320 °C, 30 min) to 0.93 (380 °C, 120 minutes).

Figure 6
Reduction in Plasma, reacted fraction (α) versus time for each temperature.

3.1.1. Reaction rate constants and activation energy

Rajput et al.¹² presented that the hematite reduction by hydrogen at lower temperatures 573K is only possible with the presence of cold plasma. The use of the plasma reduces the activation energy for reduction of Hematite from 46.78 kJmol to 5.07 kJmol. Therefore, the presence of plasma possibility the reduction process at lower temperatures when compared with molecular hydrogen, H₂ (g). Although it is known that in the cold hydrogen plasma consists of various species such as H₂ molecules, excited hydrogen molecules (H₂*), atomic hydrogen (H), ionic hydrogen (H⁺), and free electrons., Rajput et al.¹² attributed the efficiency of the plasma reduction process relative to a gas reduction process to the vibrationally excited hydrogen molecules present in the plasma. Rajput et al.¹² does not explain clearly, as were obtained portions of reduction attributed to each of the possible species such as H, H⁺ and H_{2 (excited)}. In the case of reduction by atomic hydrogen (H), ionic hydrogen (H⁺), global reactions are as follows (Reactions 2 and 3):

${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3(s)}}+{\text{6H}}^{+}+{\text{6e}}^{\text{-}}\Rightarrow {\text{2Fe}}_{\text{(s)}}+{\text{3H}}_{\text{2}}{\text{O}}_{\text{(g)}}$ ΔG0= – 8847 – 78,8 × 10–3 T [kJ/mol](2)

${\text{Fe}}_{\text{2}}{\text{O}}_{\text{3(s)}}+\text{6H}\Rightarrow {\text{2Fe}}_{\text{(s)}}+{\text{3H}}_{\text{2}}{\text{O}}_{\text{(g)}}$ ΔG0= – 1072 – 88,4 × 10–3 T [kJ/mol](3)

In the step of reducing hematite by hydrogen at temperatures below 417°C, it was verified that the kinetic equation that best fits the experimental data is the Johnson-Mehl-Avrami-Erofe’ev-Kolmogorov²⁰-²² (JMAEK) equation, tridimensional model, phase boundary reaction (contracting sphere) (Equation 4):

$\text{1-(1-}\alpha {\text{)}}^{\raisebox{1ex}{$\text{1}$}\!\left/ \!\raisebox{-1ex}{$\text{3}$}\right.}=\text{k}\text{.t}$ (4)

where: α is the fraction of reduction and k is the temperature-dependent reaction rate constant and t is the time of reduction.

The hematite reduction particles by hydrogen plasma, it possible consider, that physically has a spherical solid is circled by a fluid. Using the unreacted nuclei model, widely used for molecular hydrogen reduction processes, Figure 7, adapted to the reduction process by cold plasma. In this case we have the central part surrounded by a layer of metallic iron product and finally reducing species as: H, H⁺ and others, according with the reports of Rajput et al.¹². In this work, the plasma reduction experiments were performed below 570° C, and it was not verified the formation of wustite (FeO). For the reduction of hematite particles with gaseous hydrogen, has been that the steps involved so that the process of reduction to occur are: I) diffusion of reagents; II) phase boundary reaction; III) nucleation and growth of a new phase; Each of these step’s work with a sort of "resistance" that added will determine the kinetics of the process.

Figure 7
Schematic of a partially reduced spherical Hematite particle in lower temperatures at 580 °C.

Figure 8 present evaluation of activation energy (E_a). Turkdogan and Vinters²³ and Pineau et al.²⁴ studied the influence of particle size and temperature on the reduction process of hematite by hydrogen. These authors demonstrated that the reduction process at low temperatures (less than 693 °C) and hematite fine ore with a particle size less than 0.1 mm is controlled by a chemical reaction in the solid-gas interface. The same study showed that the activation energy for reduction of Hematite was E_a = 87.5 kJmol for temperatures less than 690 K. According this study, Figure 8, it was obtained the value of E_a = 98.4 kJmol for global reaction, the same order of magnitude obtained by Pineau et al.²⁴. This difference may be associated with the plasma power used and type of equipment and particle size²³-³⁰.

Figure 8
Arrhenius plot - evaluation of activation energy of hematite reduction process by hydrogen.

3.2. Morphological aspects after hematite reduction

Figure 9 shows some particle agglomerates typically found after Hematite reduction by hydrogen plasma. At 380 °C, increasing the reduction time of 30 to 120 minutes, may observe that, practically all the darker portions of the agglomerates, turned into lighter portions. The lighter parts probably correspond to metallic iron while the dark match magnetite, as show the diffractograms presented in Figure 5b and Figure 5e. In this case, the value of the reacted fraction "α" increase for 0.40 (30 minutes) to 0.93 (120 minutes). These results obtained, through metallographic analysis, corroborate with the results obtained by mass balance and x-ray diffraction.

Figure 9
Cross-section of clusters of Hematite particles after plasma reduction in the temperature of 380 °C by: (a) 30 min (a) and (b) 120 min.

Figure 10a-c presents agglomerates of iron oxide after hematite reduction experiments at 380 °C for 120 minutes and the EDS results microanalysis performed in regions 1 (periphery) and 2 (center), respectively. The results of chemical microanalysis by energy dispersive spectroscopy (EDS) of region 1 (peripheral region) presents around 95% of iron and 5% oxygen, indicating that the region 1 is practically composed of Fe-α. The results of chemical microanalysis by energy dispersive spectroscopy (EDS) of region 2 (central region) indicating that the region 2 is composed of the Fe-α and Fe₃O₄ phases.

Figure 10
(a) Iron oxide cluster particles after plasma reduction at temperature of 380 °C for 120 min, (b) EDS microanalysis of region 1 (peripheral region), (c) microanalysis of region 2 (center region).

4. Conclusions

Through experimental data, this study concluded, conformed what was demonstrated in previous works, the reduction process at temperatures of 320 and 380 °C occurred in two steps: Fe₂O₃ → Fe₃O₄ → Fe. The high kinetics of oxide reduction at low temperature is associated with the action of reducing species derived from molecular hydrogen present in the plasma and the generation of vacancies during ion bombardment. At temperatures between 320 and 380 °C, the reduction mechanism of iron oxide particles is predominantly controlled by topochemical reactions. The range of 320 to 380 °C, the activation energy for the reduction of iron oxide, purity of 99.0% of Fe₂O₃, granulometry <5 μm, using cold hydrogen plasma as a reducing agent, was 97, 3 kJ/mol. X-ray diffraction analysis of partially agglomerated particles reduced under plasma revealed that on the periphery of the particle there are high concentrations of metallic iron, while in the interior region, there are high concentrations of oxygen, suggesting that the reduction process takes place from the surface to the nucleus.

5. Acknowledgments

We are grateful FAPES-Fundação de Amparo a Pesquisa-ES-Brasil, CNPq-Conselho Nacional de Desenvolvimento Científico e Tecnológico for the financial support. Capes-Coordenação de Aperfeiçoamento de Pessoal de Nível Superior, FINEP-Financiadora de Estudos e Projetos and IFES-Instituto Federal de Educação do Espirito Santo for their collaboration.The authors thank the direct financial support given by FINEP, CNPq through 2011-2012 Universal Edict and FAPES by granting master's scholarship.

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Publication Dates

• Publication in this collection
  24 June 2024
• Date of issue
  2024

History

• Received
  13 Nov 2023
• Reviewed
  18 Mar 2024
• Accepted
  02 May 2024