Study on the Influence of CaO on the Electrochemical Reduction of Fe₂O₃ in NaCl-CaCl₂ Molten Salt

Abstract

The presence of calcium-containing molten salts in the electrolysis of oxides for metal production can lead to the formation of CaO and, subsequently, the generation of intermediate products, affecting the reduction of metals. To investigate the impact of CaO on the reduction process, experiments were conducted using a Fe₂O₃-CaO cathode and a graphite anode in a NaCl-CaCl₂ molten salt electrolyte at 800 °C. The electrochemical reduction kinetics of the intermediate product Ca₂Fe₂O₅ were studied using cyclic voltammetry and I-t curve analysis. The phase composition and morphology of the electrolysis products were analyzed using XRD, SEM-EDS, and XPS. The experimental results demonstrate that upon addition of CaO to the Fe₂O₃ cathode, Ca₂Fe₂O₅ is formed instantly in the molten salt upon the application of an electrical current. Research conducted at different voltages, combined with electrochemical analysis, indicates that the reduction steps of Ca₂Fe₂O₅ in the NaCl-CaCl₂ molten salt are as follows: Ca₂Fe₂O₅ ⟶ Fe₃O₄ ⟶ FeO ⟶ Fe. The presence of CaO accelerates the electrochemical reduction rate, promoting the formation of Fe. At 0.6 V and after 600 min of electrolysis, all of the Ca₂Fe₂O₅ is converted into Fe, coexisting with CaCO₃. With an increase in the electrolysis voltage, the electrolysis product Fe particles visibly grow larger, exhibiting pronounced agglomeration effects. Under the conditions of a 1 V voltage, a study was conducted to investigate the influence of time on the reduction process of Ca₂Fe₂O₅. Gradually, it resulted in the formation of CaFe₃O₅, CaFe₅O₇, FeO, and metallic Fe. With an increased driving force, one gram of Fe₂O₃-CaO mixed oxide can completely turn into metal Fe by electrolysis for 300 min.

1. Introduction

The traditional method of iron ore reduction and smelting relies on carbon thermal reduction, a process that emits a significant amount of greenhouse gases. This poses a substantial challenge to achieving carbon neutrality and peak carbon goals [1,2]. Consequently, there is widespread interest in seeking a more environmentally friendly and efficient method for iron production. Hydrogen, as the most promising clean energy source, has garnered attention as a reducing agent in metal smelting processes [3]. Its utilization in metal production is considered an effective pathway toward achieving green and sustainable development [4,5,6,7,8,9]. Furthermore, electrochemical technologies driven by low-carbon electrical energy, generated from renewable sources such as wind energy [10], nuclear energy [11], and solar energy [12], play a vital role in energy conversion. Therefore, electrochemical metallurgy, which relies on these technologies, has become a key element in the steel industry’s efforts to reduce carbon emissions [13,14,15].

The Molten Oxide Electrolysis (MOE) method involves the reduction of iron oxides to liquid metallic iron, along with the release of O₂, in a molten oxide system at high temperatures, using an inert anode. However, due to the elevated process temperatures, some metals can be corroded during the anodic polarization process, making the selection of appropriate inert anodes a critical factor limiting their development [16,17]. The FFC Cambridge Process [18] proposes the use of solid TiO₂ as the cathode in a CaCl₂ molten salt system to electro-deoxidize and produce metallic titanium. In comparison to the MOE method, this process achieves the direct electrolytic production of metals and alloys at lower temperatures ranging from 800 °C to 850 °C [19,20]. The use of solid metal oxides as cathodes in chloride molten salts for a direct electrochemical reduction to produce elemental metals has been applied in the extraction of various metals, including Fe [21,22], Ti [23], Cr [24,25], Al [26], V [27,28], Se [29], titanium-based [30,31,32,33,34], aluminum-based alloys [35,36], high-entropy alloys [37], etc. Research has shown that the addition of a small amount of CaO to CaCl₂ can significantly increase the rate of reduction and deoxidation [38,39,40]. Therefore, it is believed that the primary factor influencing the reduction process in CaCl₂ molten salt is CaO. This is due to the fact that CaO dissolves in CaCl₂ molten salt and enables O²⁻ ion transport under electrolytic conditions [41]. During electrolysis, Ca²⁺ ions can combine with nearby O²⁻ ions to form CaO, which can then react with metal oxides to produce intermediate phases that are subsequently reduced during the electrolysis process [42,43,44,45,46,47,48,49]. In the electrolytic production of metallic titanium in a CaCl₂-NaCl mixture, CaO reacts with TiO₂ to form an intermediate phase, CaTiO₃. The use of sintered CaTiO₃ as a precursor material has been found to reduce the electrolysis time [50,51]. However, in experiments involving the electrolytic synthesis of SiC in molten CaCl₂, the interaction between SiO₂ and CaO can lead to the formation of intermediate phases that are difficult to remove, thereby hindering the progress of the reaction [52]. Therefore, the influence of CaO on the formation of intermediate products and the reduction process is a topic that warrants further research.

To investigate the influence of intermediate phases generated during the electrolytic production of metals in CaCl₂-containing molten salts, this study focuses on the reduction of metallic iron (Fe). Constant voltage electrolysis experiments were conducted using Fe₂O₃-CaO as the cathode in a NaCl-CaCl₂ mixed molten salt system. The research examined the effects of electrolysis voltage and electrolysis time on the reduction behavior of Fe₂O₃-CaO. Through the study of the formation of the intermediate product Ca₂Fe₂O₅ and its impact on the reduction process, in conjunction with electrochemical analysis methods, the electrode re-action mechanism was elucidated. This research aims to provide a theoretical basis for the preparation of other metals in calcium-containing molten salts.

2. Results and Discussion

2.1. Impact of Electrolysis Voltage

Voltage, acting as the driving force in the electrolysis process, exerts control over the formation of the final products and determines the thermodynamic conditions. The constant cell voltage electrolysis experiment was carried out at 800 °C by applying different voltages (0.5 V, 0.6 V, 0.8 V, 1.0 V, respectively), and the electrolysis time of each group was 600 min. The XRD (X-ray diffraction spectrum) patterns of the electrolysis products are depicted in Figure 1a. At a voltage of 0.5 V for the constant cell voltage electrolysis, the primary products of the electrolysis were CaFe₅O₇, accompanied by minor quantities of Ca₂Fe₂O₅ and FeO. As the voltage for the constant cell voltage electrolysis experiment was increased, iron oxides were entirely reduced to metallic iron. The heightened driving force led to an accelerated deoxidation rate, with CO₂ generated by anodic discharge dissolving in the molten salt to form CaCO₃. With the application of a voltage of 1.0 V for the constant cell voltage electrolysis, Ca₂Fe₂O₅ was completely electrolyzed, and the increased driving force allowed ample time for CO₃²⁻ ions to diffuse toward the anode, resulting in metallic iron as the ultimate product. According to the experimental results in reference [21], Fe₂O₃ is electrolyzed to metal Fe after 600 min at an electrolytic voltage greater than 1.2 V. In comparison, the presence of CaO reduces the electrolytic driving force and speeds up the electrolytic rate. Figure 1b,c shows the SEM and EDS images of the products obtained after 600 min of electrolysis at 1.0 V, which reveal well-defined particle clusters as the predominant components, primarily consisting of metallic iron.

To clarify the phases of Ca₂Fe₂O₅ and CaFe₅O₇ generated during constant-voltage electrolysis of Fe₂O₃-CaO at 0.5 V, Fe₂O₃ and prepared Fe₂O₃-CaO cathodes were immersed in NaCl-CaCl₂ molten salt and maintained for 10 min. The XRD results are shown in Figure 2a. The results indicate that Fe₂O₃ does not react with the molten salt without adding CaO. However, when adding CaO, Fe₂O₃ primarily forms the new phase Ca₂Fe₂O₅. During the initial stages of constant-voltage piezoelectric electrolysis of Fe₂O₃, Ca₂Fe₂O₅ is formed by the chemical reaction between CaO and Fe₂O₃ [53]. According to the thermodynamic calculations shown in Figure 2b, it can be inferred that at 800 °C, CaO undergoes a chemical reaction with Fe₂O₃ to produce Ca₂Fe₂O₅, while Fe₃O₄ does not react with FeO or CaO to form CaFe₅O₇. The presence of CaFe₅O₇ observed in the XRD results is likely due to the reaction of reduced Fe₃O₄ and FeO with CaO at room temperature.

2.2. Cyclic Voltammetry Measurements

Due to the formation of Ca₂Fe₂O₅ from Fe₂O₃-CaO at 800 °C, to investigate the electrochemical reduction mechanism of Fe₂O₃-CaO, Fe electrodes coated with Fe₂O₃ and Ca₂Fe₂O₅ were subjected to cyclic voltammetry tests in a NaCl-CaCl₂ molten salt at a scan rate of 0.1 V/s, as shown in Figure 3. The dashed line in the figure represents the cyclic voltammetry curve for the blank salt, with a scanning range from −2.5 V to 0.5 V. The solid blue line represents the cyclic voltammetry curve for the FCE-Fe₂O₃, and three reduction peaks, denoted as R1, R2, and R3, corresponding to the three-step reduction of Fe, were observed [21]. The solid red line in the figure represents the cyclic voltammetry curve for the FCE-Ca₂Fe₂O₅ with a scanning range from −2.0 V to −0.3 V. The reduction peaks R1’, R2’, and R3’ exhibit an overall positive shift, which is attributed to the faster reaction rate resulting from the addition of CaO. In comparison to FCE-Fe₂O₃, the reduction peaks in FCE-Ca₂Fe₂O₅ appear weaker during the scan. This is because of the fact that although the electrode areas are the same, there are fewer active particles participating in the reduction reaction in FCE-Ca₂Fe₂O₅, leading to smaller peak intensities. Additionally, narrowing the scan potential range results in a decrease in peak intensities. Combining XRD phase analysis of the products of Fe₂O₃-CaO at different voltages, it can be concluded that the reduction steps of Ca₂Fe₂O₅ are as follows: Ca₂Fe₂O₅ ⟶ Fe₃O₄ ⟶ FeO ⟶ Fe.

2.3. Constant Voltage Electrolysis and Phase Characterization

To further investigate the phase evolution process of CaO’s reduction of Fe₂O₃ during the electrolysis process, and to analyze the deoxidation kinetics, the experiment was conducted at 1.0 V for the constant cell voltage electrolysis for different durations (5~300 min). Figure 4a presents the XRD patterns of the electrolysis products. When the constant cell voltage electrolysis time ranged from 5 min to 10 min, the primary phases detected were Ca₂Fe₂O₅, along with small amounts of CaFe₃O₅ and CaFe₅O₇. This indicates that under the applied voltage, Fe₂O₃ rapidly formed Ca₂Fe₂O₅. At 30 min, FeO and Fe started to appear, while CaFe₃O₅ and CaFe₅O₇ disappeared. With the extension of the electrolysis time, the diffraction peaks of Ca₂Fe₂O₅ gradually weakened and disappeared at 240 min. Simultaneously, the diffraction peaks of Fe intensified, and at 300 min, Fe was completely reduced to metallic Fe.

During the electrolytic reduction process of Fe₂O₃-CaO for iron production, Ca₂Fe₂O₅ is initially formed. The phase evolution process leading to the reduction of metallic iron from Ca₂Fe₂O₅ involves the intermediate phases CaFe₃O₅, CaFe₅O₇, FeO, and ultimately Fe. Based on the E-pO₂ diagram of the Fe-O-Ca system shown in Figure 4b, it can be observed that at certain oxygen partial pressures, the reduction sequence of Ca₂Fe₂O₅ in molten salt is Ca₂Fe₂O₅ ⟶ Fe₃O₄ ⟶ FeO ⟶ Fe. When the oxygen partial pressure is low, Fe₃O₄ and FeO phases are not detected during the reduction process of Ca₂Fe₂O₅. In Figure 4a, the absence of Fe₃O₄ and FeO at 5 min to 10 min of constant cell voltage electrolysis is attributed to the fact that in the early stages of electrolysis, the electrochemical reaction rate is higher than the diffusion rate of O²⁻ to the anode. Consequently, O²⁻ generated during electrolysis accumulates near the cathode. When the sample is removed, O²⁻ forms CaO with Ca²⁺ in the molten salt. At room temperature, CaO reacts with Fe₃O₄ to produce CaFe₃O₅, and Fe₃O₄, FeO and CaO react to produce CaFe₅O₇, as shown in Figure 4b. The theoretical decomposition voltage for the reduction of Fe₃O₄ (−0.96 V) is lower than that for the reduction of FeO (−1.01 V). As the electrolysis time increases, Fe₃O₄ is reduced to FeO, and the phases CaFe₃O₅ and CaFe₅O₇ disappear. The thermodynamic calculation results of Equations (1), (3) and (5) show that the theoretical decomposition potential of Ca₂Fe₂O₅ is higher than the theoretical decomposition potential of Fe₃O₄ and FeO, and the reduction rate of Ca₂Fe₂O₅ is lower than that of Fe₃O₄ and FeO. Therefore, after 60~120 min of constant cell voltage electrolysis, the products are mainly Ca₂Fe₂O₅ and Fe. The ion migration during electrolysis is shown in Figure 5a. The reaction process is shown in Equations (1)–(5).

6Ca₂Fe₂O₅ = 4Fe₃O₄ + 12CaO + O₂(g) E^⊖ = −1.49 V, T = 800 °C

(1)

Fe₃O₄ + CaO = CaFe₃O₅ ΔG^Θ = −35.08 kJ·mol⁻¹, T = 20 °C

(2)

2Fe₃O₄ = 6FeO + O₂(g) E^⊖ = −0.96 V, T = 800 °C

(3)

Fe₃O₄ + 2FeO + CaO = CaFe₅O₇ ΔG^Θ = −24.63 kJ·mol⁻¹, T = 20 °C

(4)

2FeO = 2Fe + O₂(g) E^⊖ = −1.01 V, T = 800 °C

(5)

When combining the I-t curve during the electrolysis process, it can be observed that when a voltage is applied to the cathode of Fe₂O₃-CaO, there is an initial high current due to the double-layer charging process [36]. In the stage I, the current rapidly decreases, reflecting the fast deoxidation of Ca₂Fe₂O₅ on the cathode surface. As the three-phase reaction interface extends toward the cathode’s interior, the diffusion of O²⁻ to the cathode surface, hindered by the blocking effect, causes fluctuations in the current curve. In the stage Ⅱ, Fe³⁺ gradually reduces to Fe, and dissolved CO₂ in the molten salt near the cathode reacts with O²⁻ to form CO₃²⁻, further impeding the diffusion of O²⁻. Consequently, the current decreases. During the stage Ⅲ, an accumulation of O²⁻ near the cathode increases, and the concentration gradient between the cathode and anode accelerates the diffusion rate of O²⁻. The electrochemical rate becomes the limiting step in the reaction, leading to an increase in slope. In the stage Ⅳ, most of the Ca₂Fe₂O₅ has been reduced to Fe, and the reduction in active particles leads to a reduction in the removal of O²⁻ during electrolysis, causing the current curve to level off. Figure 5c illustrates the gradual reduction process of the cathode plate along the three-phase interface towards the interior.

Figure 6 depicts SEM-EDS images and XPS analyses of the products obtained after 10 min and 300 min of constant cell voltage electrolysis. After 10 min of constant cell voltage electrolysis, the products consist of large, flat plate-like particles of Ca₂Fe₂O₅, measuring approximately 10–20 μm in size. Surrounding these, there are smaller particles of CaFe₃O₅ and CaFe₅O₇, with sizes of around 4–5 μm. Small FeO particles are also observed on the surface of Ca₂Fe₂O₅ particles. The XPS results in Figure 6g indicate that the peaks at binding energies of 709.0 eV and 722.4 eV correspond to Fe(Ⅱ) 2p3/2 and Fe(Ⅱ) 2p1/2, respectively, while the peaks at binding energies of 711.0 eV and 724.4 eV correspond to Fe(Ⅲ) 2p3/2 and Fe(Ⅲ) 2p1/2, respectively. This suggests that some of the Fe(Ⅲ) has been reduced to Fe(Ⅱ) after 10 min of constant cell voltage electrolysis. Upon extending the electrolysis time to 300 min, the products consist of irregular particles with sizes of around 5 μm. Agglomeration between individual grains is observed, as shown in Figure 6e,f. The results in Figure 6h reveal the presence of metallic Fe and Fe(Ⅱ) in the products obtained after 300 min of electrolysis. The presence of metallic Fe aligns with the XRD results, while the appearance of Fe(Ⅱ) can be attributed to the ease of oxidation of metallic iron particles in the presence of air [50].

3. Experimental Section and Methods

3.1. Materials and Precursor Preparation

The experiments were carried out by using analytically pure Fe₂O₃, CaO, NaCl and CaCl₂ (Sinopharm Chemical Regent Co., Ltd., Shanghai, China, ≥99.95%). The high-purity graphite sheet (≥99.99%, 15 mm × 100 mm × 3 mm) was polished sequentially on 1000#, 1500#, and 2000# sandpaper to achieve a smooth surface, and its surface was repeatedly washed with deionized water and absolute ethanol. This process aims to minimize the detachment of carbon particles from the surface and prevent short-circuiting during the electrolysis process. A nickel wire (Φ1.5 mm) was used to connect the graphite sheet to a stainless steel conductor rod, serving as the anode. Fe₂O₃ and CaO (molar ratio 1:2) were mixed with 30 g of anhydrous ethanol in a ball mill. The mixture was blended at 160 r·min⁻¹ for 300 min to achieve uniform mixing. The uniformly mixed samples were examined by XRD, and the results are shown in Figure 7. The mixture was then dried at 160 °C for 480 min to completely evaporate the anhydrous ethanol. A total of 1 g of the mixed oxide was taken and compressed into a cylindrical cathode (Φ15 mm, 1.9~2.0 mm in thickness) under a pressure of 10 MPa. The cathode was wrapped with a stainless steel mesh (1000#), and a stainless steel wire (Φ0.5 mm) was used to connect it to the stainless steel conductor rod, serving as the cathode.

3.2. Constant Voltage Electrolysis and Cyclic Voltammetry Testing

A molten salt was formulated with a molar ratio of 48:52 (NaCl-CaCl₂), which had a eutectic point of 504 °C. An electrolysis temperature of 800 °C was chosen to ensure the full melting of NaCl-CaCl₂ and the low viscosity and fast reaction rate of the molten salt. The mixed molten salt (180 g) was dried at 250 °C for 600 min. The molten salt was heated at a rate of 4 °C/min to the experimental temperature and held for 60 min. Subsequently, a cathode nickel foil (40 mm × 40 mm × 1 mm) and an anode high-purity graphite sheet (120 mm × 40 mm × 5 mm) were used for pre-electrolysis at a voltage of 2.8 V. The reason for pre-electrolysis is to remove the residue moisture and metallic impurities from the molten salt and to decompose oxides and other compounds such as CaOHCl [54]. The experimental process was conducted under argon atmosphere, and the schematic diagram of the electrolysis setup is shown in Figure 8. The constant voltage electrolysis experiment was carried out at 800 °C. After the electrolysis, the obtained cathodic product was subjected to ultrasonic cleaning with deionized water and vacuum drying. Finally, the product was characterized.

Cyclic voltammetry testing was performed using NaCl-CaCl₂ as the electrolyte at 800 °C. A high-purity graphite sheet served as the auxiliary electrode. A Fe wire (Φ1 mm) coated electrode Fe₂O₃-CaO and Fe₂O₃ (FCE-Ca₂Fe₂O₅, FCE-Fe₂O₃) was used as the working electrode. An Ag/Ag⁺ reference electrode was prepared by filling a mullite tube (Φ5 mm) with a molar ratio of NaCl:CaCl₂:AgCl = 49:49:2 and silver wire (Φ0.5 mm, 99.99%). The experiments were conducted under argon atmosphere.

3.3. Characterization

The X-ray diffraction spectrum (XRD, X-ray 6000 with Cu Kα1 radiation λ = 1.5405 Å, scanning speed 10 °/min, Rigaku Corporation, Tokyo, Japan) was used for phase detection of the cathodic product. The microscopic morphology (SEM, JEM-2900F, Japan Electronics Co., Ltd., Tokyo, Japan) and elemental composition of the product were analyzed using a scanning electron microscope and an energy dispersive spectroscopy (EDS). X-ray photoelectron spectroscopy (XPS, Ulvac-Phi, Chigasaki, Japan) was employed to obtain information about the types of elements, the binding states of material atoms, and the distribution of charges. The equipment used for electrochemical testing was an electrochemical workstation model CHI660e (Shanghai Chenhua Co., Ltd., Shanghai, China).

4. Conclusions

By adding CaO to Fe₂O₃ for the preparation of a metal oxide cathode, two-electrode electrolysis experiments were conducted in NaCl-CaCl₂ molten salt at 800 °C. The influence of different voltages and durations on the phase composition, morphology, and valence state of the electrolysis products was studied. Combined with three-electrode cyclic voltammetry tests, the electrochemical deoxidation and reduction mechanisms of Fe₂O₃ in the presence of CaO were analyzed, leading to the following conclusions:

(1)

In NaCl-CaCl₂ molten salt, Fe₂O₃ and CaO can spontaneously undergo a chemical reaction to generate Ca₂Fe₂O₅ at 800 °C without applying electrolytic voltage. The reduction steps of Ca₂Fe₂O₅ are Ca₂Fe₂O₅ ⟶ Fe₃O₄ ⟶ FeO ⟶ Fe. Compared to the reduction of Fe₂O₃ alone, the addition of CaO reduces the electrolysis voltage required for the reduction to metallic Fe, facilitating the progress of the electrolysis process. Metallic Fe and CaCO₃ can be generated after electrolysis at 0.6 V for 600 min.

(2)

Under the condition of 1 V electrolysis, the cathodic deoxidation process of Fe₂O₃-CaO for the production of metallic iron results in the formation of CaFe₃O₅, CaFe₅O₇, FeO, and Fe phases over time. The appearance of CaFe₃O₅ and CaFe₅O₇ phases is attributed to the interaction of Fe₃O₄, Ca²⁺, and O²⁻ generated during the electrolysis process, as well as the subsequent reaction of Fe₃O₄, FeO, Ca²⁺, and O²⁻ at room temperature. Under the high driving force of 1 V, the reaction time for the electrolysis of Fe₂O₃-CaO to Fe is reduced, and after 300 min of electrolysis, it is entirely converted to metallic Fe.