The Electrochemical Reduction Mechanism of ZnFe₂O₄ in NaCl-CaCl₂ Melts

Abstract

The electrochemical reduction process of ZnFe₂O₄ in NaCl-CaCl₂ melts was studied. Thermodynamic analysis shows that the reduction process of ZnFe₂O₄ is carried out in multiple steps, and it is difficult to reduce Fe³⁺ to Fe in one step. Electrochemical tests revealed that the reduction process of ZnFe₂O₄ includes three steps: First, Fe³⁺ is reduced to Fe in two steps, then Zn²⁺ is reduced to Zn in one step. The reduction of Fe³⁺ on the Mo electrode is a reversible process controlled by diffusion, while the reduction of Zn²⁺ is an irreversible process controlled by diffusion. The influence of electrolysis voltage and temperature on the process of electric deoxidation has also been studied. It is indicated that properly increasing the temperature is conducive to the diffusion of oxygen ions, thereby increasing the deoxidation rate. With the gradual increase of voltage, the reduction process of ZnFe₂O₄ is ZnFe₂O₄ → FeO + ZnO → Fe + ZnO → Fe + Zn.

1. Introduction

Lots of solid slag containing ZnFe₂O₄ are inevitably produced in the process of zinc metallurgy [1,2]. ZnFe₂O₄ can cause a low zinc leaching rate in zinc hydrometallurgy. The massive accumulation of these solid residues also causes serious waste of resources and pollution to the air and soil [3]. Therefore, ZnFe₂O₄ in the waste can not only be reused as a resource, but is significant for environmental protection.

In order to obtain zinc and iron, ZnFe₂O₄ is subjected to zinc-iron separation by electro-deoxidation in melts [4,5]. This process can realize the reduction and extraction of two metals from the waste residue. Moreover, when preparing target products with high cost, high melting point, and difficult to shape, this process can effectively reduce the difficulty of preparation. Because of the electrolysis process carried out in the solid oxide, even if the melting point of the two metals in the product is quite different, the desired alloy can still be prepared [6]. In addition, the operation method is convenient and the equipment is easy, the production time is short, and the equipment is reduced. At present, the process can prepare a lot of products in the laboratory stage, but more research is needed to produce them on a large scale. Thermodynamic analysis and electrochemical testing methods are applied to explore the electrochemical mechanism of zinc and iron separation in melts. The reduction process of melts electrolysis ZnFe₂O₄ is studied.

2. Experimental

This experiment used NaCl-CaCl₂ melts, the reason is the theoretical decomposition voltage of this system is relatively high, the theoretical decomposition voltage of NaCl is −3.24 V, and the CaCl₂ is −3.29 V. The system has good conductivity, which is conducive to the progress of electrolytic deoxidation. The eutectic temperature is approximately 500 °C and below the experimental temperature [7].

2.1. Experimental Materials

The NaCl, CaCl₂ and ZnFe₂O₄ are analytically pure. The ratio of NaCl and CaCl₂ was 1:1. The reagents were dried at 200 °C for 8 h using DZF-6050 electric vacuum drying oven and cooled down to room temperature.

2.2. Experimental Method

2.2.1. Electrochemical Experiment

The CHI660E electrochemical workstation was used for experimental testing, and the reduction mechanism of ZnFe₂O₄ was analyzed by cyclic voltammetry (CV), square wave voltammetry (SWV), and open circuit potential (OCPT). The experiment used a three-electrode system, the working electrode was Mo wire (Φ0.5 mm) which was coated by ZnFe₂O₄ powder 3 g of ZnFe₂O₄ powders are added to 5 mL of alcohol to make a suspension, then immerse the Mo wire in the suspension for 15 min; A Ag/Ag⁺ reference electrode was prepared by filling an aluminum silicate tube (Φ5 mm) with a molar ratio of NaCl: CaCl₂: AgCl = 46: 50: 4 and silver wire (Φ0.5 mm, 99.99%); High-purity graphite flakes with good conductivity (99.99%, 80 mm × 20 mm × 5 mm) was polished with sandpaper, cleaned and dried with alcohol, a nickel wire is used to connect the graphite and flakes with Φ5 mm stainless steel rod as anode. Three-electrode system diagram is shown in Figure 1. All experiments were carried out under high purity argon (>99.999%).

The electrolyte was composed of CaCl₂ and NaCl. According to n (CaCl₂): n (NaCl) = 1:1 prepare 200 g. It was well mixed into the corundum crucible [8,9,10] and put the crucible into the tubular resistance heating furnace. Raised the temperature to 800 °C at 8 °C·min⁻¹, keep constant temperature for 1 h. Heating process under the protection of high-purity argon gas (>99.99%). The three electrodes are put into NaCl-CaCl₂ melts (the cathode is immersed in the melts depth of 2 mm, the anode is immersed in the meltsdepth of 20 mm), the electrochemical reduction behavior of ZnFe₂O₄ on the Mo working electrode is measured, and the reversibility of the reduction process and the number of transferred electrons are measured analyze.

2.2.2. ZnFe₂O₄ Electrolysis Experiment

The electrolysis experiment is used a two-electrode system, and the GWINSTEK PSM-3004 DC power supply is used to carry out the constant cell pressure electrolysis experiment. The cathode was prepared by grinding ZnFe₂O₄ powder and pressed it under a pressure of 200 MPa to form a ZnFe₂O₄ cylindrical sheet with diameter of 15 mm, thickness of 3 mm and weight of 1.2 g, which was placed in a tubular resistance furnace and sintered at 800 °C for 5 h to obtain sufficient mechanical strength. After cooling, it was wrapped with stainless steel mesh and connected with an iron rod to form a wire collector. The preparation of graphite anode is the same as the electrochemical test experiment.

The sample was slowly cooled with argon gas to 25 °C after the end of electrolysis, and the electrolysis product was ultrasonically washed in a beaker containing alcohol for 30 min. Characterization of the sample was analyzed by X-ray diffraction spectroscopy (XRD, X-ray 6000 with Cu Kα1 radiation λ = 1.5405 Å), scanning electron microscopy (SEM, JEM-2800F) and energy dispersive spectroscopy (EDX, EDAX Genesis 7000).

The schematic diagram of the electrode and the electrolysis device are shown in Figure 2.

3. Results and Discussion

3.1. The Calculation of Decomposition Voltage and Oxygen Partial Pressure of ZnFe₂O₄

The theoretical decomposition voltage E^Θ of ZnFe₂O₄ was calculated by Formula (1) [11].

ΔG^Θ = −nFE^Θ

(1)

where ΔG^Θ (kJ·mol⁻¹) is the standard Gibbs free energy; n is the number of exchanged electrons; F (96485C·mol⁻¹) is the Faraday’s constant.

Lg$P_{{\mathrm{O}}_2}$ was calculated by Formula (2) to analyze the difficulty of ZnFe₂O₄ reduction in stages.

$$\mathsf{\Delta }{G}^{\mathsf{\Theta }}=-2.303\mathrm{R}T\lg P_{{\mathrm{O}}_2}$$

(2)

where ΔG^Θ (kJ·mol⁻¹) is the standard Gibbs free energy; R (8.314 J·mol⁻¹·K⁻¹) is the molar gas constant; T (K) is the temperature; lg$P_{{\mathrm{O}}_2}$ is the oxygen partial pressure.

The more positive the theoretical reduction voltage of the metal, the easier it will be reduced. It can be seen from Figure 3a that the first reaction is 2ZnFe₂O₄ = 4FeO + 2ZnO + O₂ (g), the reduction products are mainly FeO and ZnO. Then, 2FeO = 2Fe + O₂ (g) and 2ZnO = 2Zn + O₂ (g) are occurred successively. Therefore, the reduction process of Fe in ZnFe₂O₄ is carried out in steps.

Figure 3b shows that the reaction ZnFe₂O₄ = 2Fe + ZnO + 1.5O₂ (g) has the lowest oxygen partial pressure, compared with stepwise reduction to Fe from ZnFe₂O₄, the conditions of one-step are more complicated. Therefore, under a certain oxygen partial pressure, ZnFe₂O₄ should first be reduced to FeO, then the reaction of FeO to Fe and ZnO to Zn are carried out successively.

The reduction process of ZnFe₂O₄ is carried out by multiple steps, which provides the basis for the following electrochemical test analysis and electro-deoxidation experiment.

3.2. Electrochemical Analysis of ZnFe₂O₄ in Melts

3.2.1. Cyclic Voltammetry

The reduction mechanism of ZnFe₂O₄ in NaCl-CaCl₂ melts was studied by CV, which was shown in Figure 4. Figure 4a is comparison chart of CV without and with ZnFe₂O₄, scan rate was 0.3 V·s⁻¹. The peak D and peak D shown by the dotted line were the oxidation and reduction processes of Na, and the peak E was the oxidation of chlorine inos [12].

In the electric potential range between −2.1 V and 0.3 V, Mo electrode coated with ZnFe₂O₄ powder, CV curves at different scan rates were shown in Figure 4b. Peak A (−0.7 V), Peak B (−0.9 V) and Peak C (−1.7 V) represent the three reduction processes, the oxidation peak A′ and peak B′ corresponding to the reduction peak A and peak B. The E_pc of peak A and peak B did not shift significantly with the increase of scanning speed, while the E_pc of peak C shifted. Therefore, it could be seen that peak A and peak B were reversible reaction processes [13,14], and peak C was an irreversible reaction process [15,16]. The relationship between i_pc ~ v^1/2 was calculated by the data of peak A, peak B and peak C as shown in Figure 5. The linear relationship between i_pc and v^1/2 indicated that the electrochemical reactions of peak A, peak B, and peak C were all diffusion-controlled reactions [12,17,18].

The number of exchanged electrons in the reduction reaction of peak A and peak B can be calculated by Formula (3) [13,14].

E_pc – E_pc/2 = −0.77(RT/nF)

(3)

where E_pc (V) is the cathodic peak potential; E_pc/2 (V) is the half-peak potential; n is the number of exchanged electrons; F(96485C·mol⁻¹) is the Faraday’s constant; R(8.314 J·mol⁻¹·K⁻¹) is the molar gas constant; T(1073 K) is the temperature.

The average value of E_pc – E_pc/2 under different scanning speeds of peak A was −0.05538 V, the n value was 1.28 (≈1) for peak A. The average value of E_pc – E_pc/2 under different scanning speeds of peak B was −0.04 V, the n value was 1.78 (≈2) for peak B. Therefore, peak C was the reduction of Zn, ZnFe₂O₄ undergoes three-step reduction on the Mo electrode.

3.2.2. Square Wave Voltammetry and Open Circuit Potential

SWV and OCPT were used to study the electrochemical reduction behavior of ZnFe₂O₄ deoxidation on the Mo electrode in NaCl-CaCl₂ melts, the experimental data is shown in Figure 6.

Three obvious reduction peak A (−0.62 V), peak B (−0.86 V) and peak C (−1.67 V) appeared on the curves in Figure 6a. Combining with CV, the peak A and peak B correspond to the two-step reduction reaction of Fe³⁺ on the Mo electrode, peak C was a one-step reduction of Zn²⁺. The stepwise reduction behavior of Fe³⁺ was studied by OCPT measurements, as shown in Figure 6b. An obvious platform appeared at −0.85 V, which was corresponding to the reaction of Fe³⁺ → Fe²⁺. Another platform appeared near −0.94 V, which was the reaction of Fe²⁺ → Fe.

For the reversible reaction of diffusion control, the relationship between i_pc and f^1/2 can be calculated using Formula (4).

$$i_{pc}=-031{\pi }^{-1/2}{\mathrm{R}}^{-1}{T}^{-1}{AC}_0{D}^{1/2}{n}^2{\mathrm{F}}^2{\Delta Ef}^{1/2}$$

(4)

where i_pc(A) is the cathodic peak current; A(cm²) is the area of electrode; C₀(mol·cm⁻³) is the molar concentration of electroactive species; D(cm³·s⁻¹) is the diffusion coefficient of electroactive species; $\Delta E$(V) is the potential amplitude; f^1/2 is the square root of the formula.

Theoretically, the linear relationship between i_pc and f^1/2 should pass through the origin [19,20,21]. However, some studies had found that the straight line fitted by the experimental data points will be shifted. There are two main reasons [22,23]. On the one hand, the electrode reaction will be accompanied by some other reaction processes. When fitting the experimental data points to a straight line, it will result in a small positive intercept. It cannot be denied that the process is affected by diffusion control due to the presence of a positive intercept. On the other hand, there is a certain experimental error. In order to reduce the error, the origin and experimental data points were put together to fit a straight line. Some researchers had also adopted this method [24,25,26].

It could be seen from Figure 7, the E_pc of peak A and peak B did not shift significantly, which further indicates that peak A and peak B were reversible reactions.

Because of other reaction processes and experimental errors, the experimental data of peak current density (including and excluding the origin) were fitted in this study [24], as shown in Figure 7. The ratio of the diffusion coefficients can be obtained from the slope of the linear fit. D₁/D₂ of peak A = 0.5715, D₁/D₂ of peak B = 1.393, D₁/D₂ of peak C = 0.8368. There was a little difference between the values of D₁ & D₂ which was in the same magnitude. Therefore, the number of exchanged electrons of ZnFe₂O₄ can be calculated according to the Formula (5) [27].

W_1/2 = 3.52RT/nF

(5)

where W_1/2(V) is the half-peak width.

The n value was 1.41 (≈1) for peak A, 2.04(≈2) for peak B, and 1.55 (≈2) for peak C. The calculation result was the same as the CV. Therefore, the step-reduction process of ZnFe₂O₄ is ZnFe₂O₄ → FeO + ZnO → Fe + ZnO → Fe + Zn.

3.3. The Electrolysis of ZnFe₂O₄ in Melts

3.3.1. Product Analysis under Different Electrolytic Voltages

According to the above electrochemical tests, the reduction process of Fe³⁺ in ZnFe₂O₄ was a two-step reduction, and the reduction process of Zn²⁺ was a one-step reduction. The melts temperature was 800 °C and different electrolysis voltages (0.6 V, 0.8 V, 1.0 V, 1.2 V, 1.6 V, 1.8 V) were used.

It can be seen from Figure 8 that in the voltage range of 0.6 V~1 V, the main products were FeO and ZnO, which corresponds to the first reduction reaction of ZnFe₂O₄. The crystal structure of ZnO had changed at 1 V, the structure of cubic sphalerite (a = 0.428 nm) gradually changed to hexagonal red zincite (a = 0.325 nm, b = 0.521 nm) [28].

According to XRD at 1.2 V, the main substance of metal shell was iron, the internal components were mainly FeO and ZnO, as shown in Figure 9a. Therefore, when the electrolysis voltage was 1.2 V, the reduction reactions of ZnFe₂O₄ from outside to inside were ZnFe₂O₄ → FeO → Fe. The product was a dense dark brown hard solid at 1.6 V, as shown in Figure 9b. It can be observed that there was a little black irregular solid embedded in the metal shell inside the product, the product obtained by the 1.6 V was mainly Fe and ZnO. In the voltage range of 1.2 V~1.6 V, the same reaction occurs at different electrolytic voltages, but the morphology of the products differs greatly. Electrolysis first occurs on the surface of massive ZnFe₂O₄, forming a three-phase interface of melts, stainless steel mesh and ZnFe₂O₄. While the reaction proceeds, the three-phase interface expands from the surface to the inside, and oxygen ions were precipitated, forming a new three-phase interface of melts, metal and solid oxygen-containing compound [29,30,31].

Figure 8 shows that the main product at 1.8 V was Fe. It indicated that the reduction reaction were ZnFe₂O₄ → FeO → Fe and ZnO → Zn. Figure 10 shows the SEM and EDS images of the cathode product at 1.8 V. It can be seen that the particle size of the product was uniform. Fe atoms formed by the reduction of Fe³⁺ aggregate to form crystal nuclei with crystal structure, the crystal particles continue to grow with the reduction process to form metallic Fe with a certain particle size, and the Fe grains form a loose and porous structure during the growth process. According to EDS analysis, the atomic percentages of Fe and Zn were 95.19% and 0.86% respectively, the atomic percentage of O was 3.95%. It indicated that a good deoxidation effect can be achieved at 1.8 V after 8 h of electrolysis. The driving force of electrical deoxidation increases with increasing voltage [32], and the reduction of Fe³⁺ and Zn²⁺ is easier.

3.3.2. Zinc Behavior during the Electrolysis of ZnFe₂O₄

There was no Zn detected in the product at 1.8 V, because the experimental temperature (800 °C) was closed to the boiling temperature of Zn (907 °C), and Zn has a significant vapor pressure at a temperature below the boiling point [33]. If zinc was reduced, a small part of the Zn had sunk to the bottom of the melts. A greater part of it would become zinc vapor volatilization, so the content of zinc in the product was less.

In order to study the behavior of Zn during electrolysis, a method will be adopted [34]. The ZnFe₂O₄ powder was placed in a small graphite crucible and electrically deoxidized for collecting the Zn, as shown in Figure 11a. After the electrical deoxidation was over, the product was taken out and cleaned, and the bottom of the product was detected by XRD. Figure 11b is the XRD analysis results, in addition to Zn, the presence of ZnO was also detected.

3.3.3. Product Analysis at Different Electrolytic Temperatures

When the temperature is the only variable, the diffusion rate of oxygen ions is mainly affected by the temperature, and the changed temperature would also affect the viscosity and conductivity of the melts. Therefore, the temperature of the melts in the electrolysis process is an important parameter [35], which would affect the deoxidation rate and deoxidation effect.

Figure 12 and Figure 13 are the XRD & SEM of the electrolytic products, which show that the main products were Fe and ZnO at 700 °C, the ZnO and Fe particles were bound to each other to form a relatively dense structure, indicating that the deoxidation of ZnFe₂O₄ was not complete. At 750 °C and 800 °C, the products were Fe. At 800 °C, the diffraction peak of XRD was sharper, indicating that the crystalline state of iron was better and Fe particle size increases. Figure 13 shows that the product had a larger particle size and pore spacing at 800 °C, which is more conducive to deoxidation. According to EDS spectrum analysis, the change of each element content with increasing temperature was shown in Figure 14, the atomic percentage of Fe was 95.19% at 800 °C, which had a lower oxygen content and zinc content than 86.55% at 750 °C. Therefore, the deoxidation effect is better at 800 °C. Within a suitable temperature range, as the temperature increases, the activation energy and diffusion coefficient are greater, and the viscosity of melts can be reduced, thereby increasing the reaction rate [36].

4. Conclusions

(1)

The electrochemical reduction process of ZnFe₂O₄ on the Mo electrode is a three-step reduction: ZnFe₂O₄ → FeO + ZnO → Fe + ZnO → Fe + Zn. Fe³⁺ → Fe²⁺ → Fe are reversible reduction processes controlled by diffusion, and Zn²⁺ → Zn is an irreversible reduction process controlled by diffusion.

(2)

The surface of the deoxidized product has high crystallinity at 1.8 V electrolysis for 8 h. The lower content of Zn and O indicates that the deoxidation effect is better.

(3)

Electrolysis voltage is the driving force for electro-deoxidation, ZnFe₂O₄ is reduced to FeO and ZnO during the voltage of 0.6 V~1 V, then FeO is reduced to Fe at 1.2 V~1.6 V, when the electrolysis voltage is 1.8 V, ZnO is also reduced to Zn. The increase of temperature is beneficial to the diffusion of oxygen ions, the deoxidation rate is accelerated and the occurrence of side reactions can be reduced.

(4)

Electrolysis experiments also verified the stepwise reduction of ZnFe₂O₄. The reduction of ZnFe₂O₄ is achieved by electrolysis at 800 °C and 1.8 V for 8 h.