Effect of Fe₂O₃ on Electro-Deoxidation in Fe₂O₃-Al₂O₃-NaCl-KCl System

Abstract

The reduction of Fe₂O₃-Al₂O₃ is one of the important reactions in the resource utilization of iron-containing oxide waste. Fe₂O₃-Al₂O₃ was electro-deoxidized in the NaCl-KCl system by molten salt electrolysis to prepare FeO/Al₂O₃. The effect of the Fe₂O₃ content on the electro-deoxidation reaction process was studied. The results show that under the conditions of 850 °C, 2.3 V, and electro-deoxidation for 4 h, FeO/Al₂O₃ could be obtained by controlling the content of Fe₂O₃. The deoxidation process was divided into three stages: electric double layer charging, Fe₂O₃ electro-deoxidation to Fe₃O₄, and Fe₃O₄ electro-deoxidation to FeO. With the increase in the Fe₂O₃ content, the deoxidation reaction rate increased, and the low-valence iron oxide particles obtained by electro-deoxidation became larger. The mechanism of the influence of Fe₂O₃ on the electro-deoxygenation process was determined by analyzing the experimental results. The increase in the Fe₂O₃ content increased the concentration of activated molecules in the system, while it reduced the resistance of electro-deoxidation. The migration of active particles in the cathode was smoother, which increased the percentage of deoxygenation of activated molecules, thereby shortening the process of the deoxidation reaction.

1. Introduction

Fe₂O₃ and Al₂O₃ are the main components of solid waste such as red mud, blast furnace slag, and zinc kiln slag [1,2,3]. In order to treat these solid wastes as resources, researchers have reduced Fe₂O₃ to low-valent FeO or Fe₃O₄ through fire and wet methods and finally obtained metallic iron [4,5], ferroalloys [6], roadbed materials [7], and Fe-FeAl₂O₄ composite materials [3,8,9]. Therefore, the reduction reaction of the Fe₂O₃-Al₂O₃ system is an important reaction to realize the resource treatment of solid waste, and research on the system reaction is of great significance.

At present, the reduction reaction of the Fe₂O₃-Al₂O₃ system is mainly through C reduction [10,11], H₂ reduction [12,13], CO reduction [14], and CO-CO₂ [15] reduction. These methods have high energy consumption and CO₂ emission problems. In recent years, the widely used molten salt electrolysis method has been used as a typical electrochemical method with low energy consumption and no CO₂ emissions. It mainly includes the FFC method [16], the SOM method [17], the OS calcium thermal method [18], and the USTB method [19]. Through molten salt electrolysis, high-melting point metals, metal alloys, intermetallic compounds, and carbide ceramics were prepared in molten salt systems such as CaCl₂-NaCl and NaCl-KCl [20]. Electro-deoxidation research on the Fe₂O₃-Al₂O₃ system mainly focused on the preparation of FeAl and its intermetallic compounds [21,22,23,24], while the intermediate product FeO-Al₂O₃ has received fewer reports. The reduction reaction of Fe₂O₃-Al₂O₃ to FeO-Al₂O₃ is a necessary process for the reduction of Fe₂O₃-Al₂O₃ to metal Fe, FeAl, or its intermetallic compounds. It is one of the key links to obtaining the reduction mechanism of Fe₂O₃-Al₂O₃.

Herein, reduction of Fe₂O₃-Al₂O₃ to FeO-Al₂O₃ was studied by using molten salt electrolysis at 850 °C in a NaCl-KCl melt. The microstructure and phase composition of the deoxygenated products were examined by using SEM and XRD. By examining the changes in the composition and structure of the cathode during the electrolysis process, the influence law of the Fe₂O₃ content on the electrolysis process and the electrosynthesis pathway of FeO-Al₂O₃ in molten salt were also studied.

2. Experimental

The raw materials in this experiment, namely, pure Al₂O₃ (Macklin Group Co., Ltd., Shanghai, China, Analytical reagent) and Fe₂O₃ (Sinopharm Group Co., Ltd., Shanghai, China, Analytical reagent) powders, were used as the cathode precursors. First, the amounts of Al₂O₃ and Fe₂O₃ were ball milled in ethanol for 6 h according to the mass ratio in

Table 1. Specific experimental conditions in electrochemical deoxidation.

Samples	E (Decomposition Voltage) V	T (Temperature) °C	t (Reaction Time) h	mFe2O3/mAl2O3
S1	2.3	850	4	1:1
S2	2.3	850	4	3:2
S3	2.3	850	4	2:1
S4	2.3	850	4	4:1
S5	2.3	850	0.5	3:2
S6	2.3	850	1	3:2

. Then, the slurry was vacuum dried for 4 h, and the powder mixture was pressed into cylindrical pellets (10 × 3 × 1 mm³) under a uniaxial pressure of 8 MPa. The pellets were sintered at 800 °C for 4 h under argon flow to obtain cathode pellets. Then, the cathode pellets were connected to a 304 stainless-steel wire to assemble the cathode. A high-density graphite sheet (100 × 15 × 5 mm³) served as the anode. A eutectic mixture of NaCl-KCl (NaCl/KCl = 50.6:49.4 mol %, Sinopharm Group Co., Ltd., Shanghai, China, Analytical reagent) was used as the electrolyte, which was packed in a graphite crucible and dehydrated at 300 °C for 24 h. The graphite crucible, filled with the mixed salt, was introduced into the bottom of a tube furnace. Then, the furnace was sealed, and ultra-high purity argon gas was flushed into the reactor to provide a protective atmosphere. The furnace temperature was increased to 850 °C under continuous argon circulation. Then, a fresh graphite sheet and cathode were lowered into the melt. The electrolysis was carried out at 2.3 V for 4 h. After electrolysis, the samples were carefully rinsed with distilled water several times to remove the adhering salt and immersed in distilled water for 12 h, followed by drying at 150 °C for 0.5 h. The schematic illustration of the electrochemical deoxidation is shown in Figure 1.

The phase composition was identified by using X-ray diffraction (XRD, XPert PRO MPD, PANalytical, Almelo, The Netherlands). The microstructure and element analyses were carried out with SEM and EDS (GeminiSEM 300, Zeiss, Germany), respectively.

3. Results and Discussion

3.1. Thermodynamic Analysis and Electrochemical Reduction of Fe₂O₃-Al₂O₃

When energized, the Al₂O₃ and Fe₂O₃ in the molten salt would undergo the following electrolysis reactions:

Al₂O₃(s) = 2Al(s) + 2/3O₂(g)

(1)

Fe₂O₃(s) = 2/3Fe₃O₄ + 1/6O₂(g)

(2)

Fe₃O₄(s) = 3FeO + 1/2O₂(g)

(3)

FeO(s) = Fe + 1/2O₂(g)

(4)

The standard Gibbs free energy of the possible reaction of Fe₂O₃ and Al₂O₃ in the molten salt system at different temperatures could be calculated by using FactSage7.3. The standard theoretical decomposition voltage E^Θ was calculated by Formula (5).

ΔG^Θ = −nFE^Θ

(5)

where ΔG^Θ is the standard Gibbs free energy (kJ·mol⁻¹); E^Θ is the theoretical decomposition voltage in the standard state (V); F is the Faraday constant (96,485 C·mol⁻¹); and n is the number of electrons gained or lost in the reaction equation.

Figure 2 shows the change curve of E^Θ with T in different reactions. As the reaction temperature increases, E^Θ becomes more positive. At 600~1100 °C, the order of reaction among (1)~(4) is (2) > (3) > (4) > (1).

3.2. Effect on Deoxygenation Rate

Figure 3 shows the XRD pattern of the electrolysis under the experimental conditions in Table 1. The Fe₂O₃ content in the S₁~S₄ samples increased sequentially. S₁ and S₂ electrolyzed to Fe₃O₄, FeO, and Al₂O₃. S₃ and S₄ electrolyzed to FeO and Al₂O₃. The peaks of Fe₃O₄ gradually weakened from S₁ to S₄, the peaks of FeO became stronger, and the peaks of Al₂O₃ became weaker in turn. These experimental results show that with the increase in the Fe₂O₃ content, it was easier for FeO to be formed, and Al₂O₃ appeared to be reduced to a certain extent. This change of FeO was due to the increase in Fe₂O₃ in the samples. It increased the mass percentage of Fe₂O₃ activated molecules in the system that could participate in the electrical deoxidation reaction, thereby increasing the deoxidation reaction rate. As the external field conditions such as the temperature and voltage of the molten salt system had not changed, the diffusion flux of O²⁻ in the NaCl-KCl molten salt remained unchanged. As the deoxidation reaction rate increased, the O²⁻ produced by electrolysis could not be delivered to the anode in time, and more O²⁻ accumulated near the cathode. This part of O²⁻ reacted with Na⁺ and K⁺ to generate Na₂O and K₂O, and then it reacted with Al₂O₃ to generate NaAlO₂ and KAlO₂. As NaAlO₂ and KAlO₂ dissolved in the NaCl-KCl molten salt, this resulted in a decrease in Al₂O₃ in the cathode. The electrochemical reaction of Fe₂O₃ consisted of two steps. The first reaction electrolyzed to Fe₃O₄, and then the reaction of Fe₃O₄ electrolyzed to FeO. This conclusion is consistent with literature reports [21,25,26].

Figure 4 shows the change curves of the current with time in the process of S₁~S₄ electric deoxidation. The current is the amount of electricity per unit area, and it represents the rate of electrical deoxidation during the process of electrical deoxidation. The Fe₂O₃/Al₂O₃ electro-deoxygenation reaction process was divided into three stages: A, B, and C. Stage A was the charging process of the electric double layer. The current increased with time and reached the maximum current. At this time, the resistance was the smallest, the electric deoxidation rate was the highest, and the current was the largest, and the most active particles were produced. When the current reached the maximum current, this indicated that the electric double layer had been fully charged, and the electrical deoxidation reaction began to occur at 3PIs. Stage B was the Fe₂O₃ electro-deoxidation reaction process. Combined with the results of Figure 3, this stage should be the reaction of Fe₂O₃ electro-deoxidation to Fe₃O₄ (i.e., reaction (2)), and the current decreased with the reaction time. As the electric deoxidation of Fe₂O₃ to Fe₃O₄ progressed, the molten salt passed through the product layer and formed new 3PIs at the reaction product/unreacted material. The Fe³⁺ active particles obtained electrons at the 3PIs, and O²⁻ diffused into the NaCl-KCl molten salt and migrated to the anode. As with the above-mentioned Fe₂O₃ electro-deoxygenation to Fe₃O₄ reaction, 3PIs formation, O²⁻ transmission, and other reaction processes would increase the resistance of the system, and the current would decrease with time. Stage C was the process of Fe₂O₃ electro-deoxygenation to FeO (i.e., reactions (2) and (3)). The current decreased with the reaction time and was smaller than that in stage B. Compared with stage B, stage C increased the reaction of Fe₃O₄ electro-deoxidation to FeO (i.e., reaction (3)), and the resistance of the system further increased, meaning the current decreased with time and was less than the phenomenon of the stage B current.

For the I–t curves corresponding to S₁, S₂, S₃, and S₄, it was found that stage A, stage B, and stage C had certain changes. In stage A, the slopes of the corresponding curves for S₁~S₄ became larger, and the time to complete the electric double layer charging process was 0.78 h, 0.19 h, 0.16 h, and 0.09 h, which shows a gradual decrease, and the maximum current was 0.15A, 0.20 A, 0.24 A, and 0.3 A, which shows a gradual increase. Similarly, in stage B and stage C, the slopes of the corresponding curves of S₁~S₄ became larger, and the time required to complete the reaction of this stage became shorter. These results show that the rate of Fe₂O₃/Al₂O₃ electric deoxidation increased with the increase in the Fe₂O₃ content. It promoted the progress of the electric deoxidation reaction and shortened the time required to complete the corresponding reaction process. There are two reasons for the above phenomenon. One is that the increase in Fe₂O₃ increased the number of Fe₂O₃ activated molecules in the system that could participate in the electro-deoxygenation reaction. This increased the initial concentration of activated molecules in the Fe₂O₃ electro-deoxygenation reaction, thereby increasing the deoxygenation reaction rate. The second is that Al₂O₃ is an oxide with high resistivity and strong electrical insulation [27]. Al₂O₃ in the S₁~S₄ samples decreased sequentially. This meant that the resistance of the deoxidizing system decreased correspondingly and reduced the resistance of the free electrons to 3PIs. In turn, the deoxygenation reaction rate was increased. The effect of the Fe₂O₃ content on the deoxidation rate is consistent with the XRD results in Figure 3.

3.3. Effect on Organizational Structure

Figure 5, Figure 6 and Figure 7 showed the SEM morphology, spot scan, and surface scan of the Fe₂O₃/Al₂O₃ electro-deoxidation product. It can be seen that S₁ and S₂ are Fe₃O₄, FeO, and Al₂O₃. S₃ and S₄ are FeO and Al₂O₃. Al₂O₃ is the regular and smooth large particles, and the Fe₃O₄ and FeO particles are smaller. Fe₃O₄ and FeO adhered to Al₂O₃, nucleated, and grew. With the increase in the Fe₂O₃ content, the exposed Al₂O₃ decreased, and the low-valence iron oxides surrounding the surface of the Al₂O₃ particles increased. The low-valence iron oxide particles gradually became larger and angular polyhedrons. Under the same electrolysis conditions, with the increase in the Fe₂O₃ content, a low content of Fe₂O₃ could produce Fe₃O₄ and FeO, while a high content only produced FeO. This indicates that the increase in the Fe₂O₃ content increased the deoxidation reaction rate. The large, smooth, high-resistivity Al₂O₃ particles hindered the migration of active particles in the deoxidation process. Active particles needed to bypass Al₂O₃ and undergo the electrical deoxidation reaction on the Fe₂O₃ around Al₂O₃. Therefore, the Fe₃O₄ and FeO particles grew around Al₂O₃, and the decrease in the Al₂O₃ content was also beneficial to increase the deoxidation reaction rate. With the increase in the electro-deoxidation rate, the process of Fe₂O₃ electro-deoxidation to Fe₃O₄ (reaction (2)) was shortened, and the reaction of Fe₃O₄ electro-deoxidation to FeO (reaction (3)) was faster. At the same time, the FeO crystal nucleus had more time to grow, finally obtaining larger FeO particles.

3.4. Effect Mechanism

From the above XRD and SEM results, it can be seen that the increase in the Fe₂O₃ content increased the deoxidation reaction rate of FeO/Al₂O₃ prepared by Fe₂O₃/Al₂O₃ electro-deoxidation. In order to further clarify the deoxygenation reaction process, XRD analysis was performed on the deoxygenation products of S₂ at 0.5 h, 1 h, and 4 h, as shown in Figure 8. When the reaction reached 0.5 h, the sample color changed from red to black (Figure 9), and the deoxidation product was Fe₃O₄ and Al₂O₃. When the reaction reached 1 h, the deoxidation product was Fe₃O₄, FeO, and Al₂O₃. When the reaction reached 4 h, the sample color changed from black to dark green (Figure 9), and the deoxidized product was also Fe₃O₄, FeO, and Al₂O₃.

According to Figure 8 and Figure 9, it can be seen that the reaction from 0 to 0.19 h belonged to stage A, which was an electric double layer charging process. The active particles quickly gathered at 3PIs, and the Fe₂O₃ electro-deoxygenation reaction started at the maximum reaction rate. From 0.19 to 0.75 h, it belonged to stage B, and reaction (2) produced Fe₃O₄, accompanied by the formation of new 3PIs at the junction of Fe₃O₄/Fe₂O₃/NaCl-KCl molten salt. From 0.75 to 4 h, it belonged to stage C, and reaction (3) produced FeO, accompanied by the formation of new 3PIs at the junction of FeO/Fe₃O₄/NaCl-KCl molten salt. It can be seen from Figure 4 that the electro-deoxidation reaction with different Fe₂O₃ contents included three stages, A, B, and C. Combining the results of Figure 8 and Figure 9, the Fe₂O₃/Al₂O₃ electro-deoxidation reaction mechanism could be obtained, as shown in Figure 10. With the increase in the Fe₂O₃ content, the concentration of activated molecules in the system was increased, the resistance of Al₂O₃ in the system was reduced, and the reaction rate of reactions (2) and (3) was increased. This shortened the completion time of stages A, B, and C.

4. Conclusions

In this paper, molten salt electrolysis was used to study the reaction process of Fe₂O₃-Al₂O₃ electrolytic reduction of FeO-Al₂O₃ in the NaCl-KCl system. The influence of the Fe₂O₃ content on the electrolysis process was analyzed. When the electrolysis conditions were 850 °C, 2.3 V, and a reaction time of 4 h, Fe₂O₃-Al₂O₃ could be electrolyzed to Fe₃O₄, FeO, and Al₂O₃. FeO-Al₂O₃ could be obtained by adjusting the content of Fe₂O₃. The increasing content of Fe₂O₃ could increase the rate of the deoxidation reaction and shorten the time of Fe₂O₃ reduction to Fe₃O₄ and Fe₃O₄ reduction to FeO. The electrolysis product nucleated and grew around Al₂O₃, and it finally became angular polyhedrons. By studying the dynamic process of the electro-deoxygenation reaction, the electro-deoxygenation reaction mechanism was obtained. It included electric double layer charging, Fe₂O₃ electro-deoxygenation to produce Fe₃O₄, and Fe₃O₄ electro-deoxygenation to produce FeO, accompanied by the formation of new 3PIs, O²⁻ transport, and other reactions. The principle of obtaining FeO by adjusting the content of Fe₂O₃ was that the increase in the Fe₂O₃ content increased the concentration of activated molecules in the system. Additionally, it reduced the resistance of Al₂O₃ in the system. The active particles migrated more smoothly in the cathode and increased the reaction rate of reactions (2) and (3). The reaction rate shortened the reaction time of the electric double layer charging, Fe₃O₄ generation, FeO generation, and other stages. As we all know, in addition to the Fe₂O₃ content, the reaction temperature, reaction time, and voltage are all factors that affect the electrolytic reduction of Fe₂O₃-Al₂O₃ to FeO-Al₂O₃. Subsequent research will focus on the mechanism of the reaction temperature, reaction time, and voltage in the reduction process.