Chlorination of ZnFe₂O₄ by Molten MgCl₂: Effect of Adding CaCl₂

Abstract

Natural Zn reserves might be exhausted in the following decades. Electric arc furnace (EAF) dust is an essential secondary Zn resource. The recovery of Zn from EAF dust is a critical factor in the sustainable development of the Zn industry and steelmaking industry. Efficient destruction of the ZnFe₂O₄ crystal structure and separation of Zn and Fe are the key issues to recycle EAF dust. Molten metal chlorides are believed to be effective in breaking the structure of ZnFe₂O₄. This work clarifies the distinct reaction behavior between ZnFe₂O₄ and MgCl₂ or CaCl₂. Experiments show that although MgCl₂ is a more powerful chlorinating agent, the capability of separating Zn and Fe is not as good as that of CaCl₂. Fast and selective chlorination of Zn from ZnFe₂O₄ happens by reacting ZnFe₂O₄ and MgCl₂–CaCl₂ mixture. CaCl₂ facilitates the formation of stable Fe-bearing phases, such as MgFe₂O₄ and Ca₂Fe₂O₅, thereby inhibiting the chlorination of Fe. Moreover, it becomes more effective in promoting the selectivity of chlorinating Zn by increasing temperature after adding CaCl₂ in MgCl₂. When ZnFe₂O₄ is subjected to chlorination using a MgCl₂–CaCl₂ mixture (MgCl₂–CaCl₂–ZnFe₂O₄ molar ratio = 1.5:1.5:1) at 950 °C for 120 min, the process results in a chlorination percentage of over 90% for Zn, with Fe chlorination percentage lower than 5%. These results demonstrate the prospect of efficient separation of Zn from ZnFe₂O₄ by reaction with molten MgCl₂–CaCl₂ mixture.

Graphical Abstract

Introduction

Zn is an essential element for modern society. The steel industry is the largest consumer of Zn. In 2019, the quantity of Zn used for galvanizing steel was 10,167 kt, which represented over 50% of the global Zn production [1]. Coating a thin Zn layer on the steel surface is a simple and efficient way to protect the base metal from corrosion and rusting. Galvanized steel is thus among the most popular steel types and is widely applied in various industrial sectors. When scrap steel is remelted in an electric arc furnace (EAF), all Zn evaporates and gets into the EAF dust. In these cases, the Zn content in EAF dust varies from 5 to 40% [2]. Figure 1 shows the schematic illustration of the mass flow of Zn in the steel industry. Complete recycling of Zn from EAF dust would increase the circularity of Zn and reduce the dependence on natural resources. However, the actual situation is that a large portion of EAF dust (around 37%) is sent to landfills because of the technical difficulties in recycling. Rostek et al. estimated that the loss of Zn in the steel industry was 500 kt in 2019 [1]. On the other hand, Zn is recognized as one of the elements with limited natural supplies. The US Geological Survey’s latest report suggested that the Zn mine might be exhausted within 20 years [3]. Due to the value of Zn and the gradual depletion of natural reserves, boosting the recycling ratio of Zn from EAF dust has become a vital issue in Zn sustainability.

Fig. 1

Schematic of the mass flow of Zn in steel industry

ZnFe₂O₄, ZnO, and Fe₂O₃ are the main components in EAF dust. The difficulty in recovering Zn from EAF dust is because of the high chemical stability of ZnFe₂O₄ [4, 5]. Both pyrometallurgical and hydrometallurgical techniques can treat EAF dust [6,7,8,9,10,11,12,13,14]. Pyrometallurgy is the primary choice due to the high metal recovery ratio and relatively short process [15]. The Waelz process and the rotary hearth furnace technology are two typical routes with commercial applications. Both are based on carbothermic reduction at high temperatures [16]. The Zn-bearing oxides in the dust are reduced by coke at around 1200 °C to form Zn vapor, which is later re-oxidized by air to ZnO. The final recycled product is crude ZnO powder instead of metallic Zn. However, high-energy consumption and massive emission of CO₂ are the main drawbacks, especially as the goal of achieving carbon neutrality has become an international consensus. Therefore, innovations in Zn recycling from EAF dust are urgently needed. Recently, Nagasaka et al. proposed a “CaO addition process” to replace the Waelz process after years of fundamental and systematic research input [17]. In this technology, EAF dust is mixed with CaO at a high temperature (e.g., 1000 °C). CaO would react with ZnFe₂O₄ in the dust to form ZnO and Ca₂Fe₂O₅. Because ZnO is far easier to dissolve than ZnFe₂O₄ in acid or alkali solutions, metallic Zn can be obtained after leaching and electrolysis [18, 19]. The most significant advantage of this technology is that carbothermic reduction is no longer necessary. However, the slow solid-state reaction between dust and CaO remains a challenge for industrial applications.

Chlorination-based pyrometallurgical processes have significant advantages in terms of efficiency and low carbon emissions [20,21,22,23,24,25]. Zn-bearing oxides can be converted into ZnCl₂ by Cl₂, polyvinyl chloride (PVC), or metal chlorides through a rapid gas–solid or liquid–solid reaction. Metallic Zn can be easily obtained by electrolyzing molten ZnCl₂. In principle, no carbon-based reducing agent is required for the chlorination process. Matsuura et al. [26,27,28] carried out systematic research on using Cl₂ as the chlorinating agent. As reported, Zn in EAF dust could be efficiently and selectively chlorinated utilizing a mixture of Cl₂ and O₂. However, the high corrosiveness and toxicity of Cl₂ pose challenges to equipment corrosion and safety issues. Finding a Cl₂-free substance as the chlorinating agent to make the process easier and safer is necessary. Apart from Cl₂, PVC has been utilized as a functional chlorinating additive to assess the feasibility of recycling EAF dust for metal extraction [29,30,31,32,33,34]. By reacting EAF dust with PVC, Zn in EAF dust is converted to ZnCl₂ by hydrogen chloride (HCl) gas evolved from PVC. However, generating dioxin and other pollutants from PVC at high temperatures is a disadvantage.

Molten metal chlorides can also be used as chlorinating agents in place of Cl₂ and PVC. Guo et al. clarified that molten CaCl₂ converts Zn in ZnFe₂O₄ into gaseous ZnCl₂ while making Fe remain in the solid residue as CaFe₂O₄ [35,36,37,38]. This result indicates a high selectivity of chlorinating Zn by CaCl₂. However, the sluggish reaction is not satisfied from a practical standpoint. Since MgCl₂ is another common chloride with higher reactivity to chlorinate solid oxides [39, 40], we propose an alternative technology of using MgCl₂ to treat EAF dust. The target product is ZnCl₂. Previously, we demonstrated the capability of MgCl₂ to chlorinate ZnFe₂O₄ [41]. MgCl₂ converts Zn in ZnFe₂O₄ to ZnCl₂ more efficiently than CaCl₂. Reaction temperature could be lower, and the rate is larger when using MgCl₂. However, a part of Fe in ZnFe₂O₄ is also chlorinated to form volatile iron chlorides, which means MgCl₂ is not as good as CaCl₂ in terms of selective chlorination of Zn.

Considering the distinct features of MgCl₂ and CaCl₂, using MgCl₂ with the addition of CaCl₂ may lead to fast and selective chlorination of Zn from ZnFe₂O₄. This work compares the chlorination behaviors of ZnFe₂O₄ between using MgCl₂ and using CaCl₂ and demonstrates the superior efficacy of using the MgCl₂–CaCl₂ mixture. The reaction mechanism and the effects of thermodynamic conditions (mixture composition, temperature, etc.) on the chlorination behaviors of Zn and Fe are clarified. This work may optimize the technology based on utilizing chlorides for Zn recovery from EAF dust.

Materials and Method

Materials

Reagent grade ZnO (99%, Aladdin), Fe₂O₃ (99%, Aladdin), MgCl₂ (99%, Aladdin), and CaCl₂ (97%, Aladdin) were used. ZnO and Fe₂O₃ (molar ratio of 1:1) were thoroughly mixed and sintered in air at 1100 °C for 24 h to prepare ZnFe₂O₄. The as-prepared ZnFe₂O₄ was ground, and particles in size range from 0.10 mm to 0.28 mm were used in the experiments. All chemicals were vacuum dried at 200 °C for at least 24 h before use.

Experimental Procedure

The experiment was performed in a horizontal tube furnace, as shown in Fig. 2. A MgO crucible (cuboid shape, length 60 mm, width 30 mm, depth 20 mm) containing a powdery sample was set in a quartz tube in the furnace. The samples were a mixture of MgCl₂ and ZnFe₂O₄ (0.2856 g and 0.2410 g, molar ratio = 3:1), a mixture of CaCl₂ and ZnFe₂O₄ (0.3329 g and 0.2410 g, molar ratio = 3:1), and mixtures of MgCl₂, CaCl₂, and ZnFe₂O₄ with different molar ratios. High-purity Ar was introduced as the protective gas at 200 mL min⁻¹. The reaction temperature was from 800 °C to 1000 °C.

Fig. 2

Schematic diagram of the experimental setup and post-treatment process for products analysis

The weight loss of the sample during the reaction was measured. As shown in Fig. 2, a corundum tube enclosing a thermocouple was placed inside the quartz tube, ensuring that the tube tip was at the center (hot zone). Once the furnace temperature reached the desired value, the reactant-filled MgO crucible was moved from the cold zone to the hot zone within the quartz tube. After a specific duration, the corundum tube was quickly pushed to return the crucible back to the quartz tube end (cold zone). The crucible was then retrieved using a stainless-steel clamp and weighed. To prevent the direct contact between the hot crucible and the balance, a quartz crucible was placed on the balance. After weighing, the crucible was placed back in the quartz tube center. These steps were repeated multiple times to plot the weight loss curve.

During reaction, volatiles would form, evaporate, and deposit on the inner quartz wall in the cold zone, while solid residues should remain in the crucible. In some cases, quartz wools were set in the quartz tube to collect the volatiles. After reaction, the quartz tube was rinsed with distilled water to dissolve the volatiles deposited on the inner wall. The crucible was also rinsed by immersing it in distilled water. Accordingly, substances after reaction were divided into four parts (Fig. 2): Rinse Solution-V (containing soluble phases in volatiles denoted by V) and Solids-V (insoluble phases in volatiles) from volatiles, Rinse Solution-R (containing soluble phases in residues in crucible denoted by R) and Solids-R (insoluble phases in residues in crucible) from the residues in crucible. These four parts of substances were analyzed by phase or chemical analysis, respectively.

Analysis and Characterization

Phase compositions of Solids-V and Solids-R were analyzed by an X-ray diffractometer (XRD, Cu K_α, 40 kV, 40 mA, 5° min⁻¹, Bruker, D8 Advance). When using quartz wools to collect volatiles, the quartz wools were ground into powder in a glove box after the reaction. The powdery sample was also analyzed by XRD. In addition, the microstructures of Solids-V and Solids-R were observed by a scanning electron microscope (SEM, Zeiss Gemini 450) equipped with an energy-dispersive spectrometer (EDS, Oxford instrument). Raman spectra were recorded on a spectrometer (WITec, Alpha300R) using a laser source with an excitation wavelength of 532 nm in the range of 100–900 cm⁻¹. Rinse Solution-V and Rinse Solution-R were subject to chemical analysis by an inductively coupled plasma mass spectrometer (ICP − MS, ThermoFisher, iCAP RQ). The fractions of Zn and Fe (F_M) in Rinse Solution-V and Rinse Solution-R can be calculated by

$$ F_{{\text{M}}} { }\left( {\text{\% }} \right) = \frac{{C_{{\text{M}}} \times V}}{{W_{{\text{M}}} }} \times 100{\text{\% }} $$

(1)

where C_M is the concentration of a metal ion in the solution (g L⁻¹), V is the total volume of the solution (L), and W_M is the total mass of Zn or Fe in initial ZnFe₂O₄. Although deposits on the quartz tube wall may contain water-insoluble FeOCl that is difficult to recover completely [41], this phase was formed in tiny quantities. Therefore, the sum of the fractions of Zn or Fe in Rinse Solution-V and Rinse Solution-R is defined as the chlorination percentage.

Results and Discussion

Distinct Reaction Behavior of ZnFe₂O₄ Between Using MgCl₂ and Using CaCl₂

Based on our previous experiments, when the molar ratio of MgCl₂ to ZnFe₂O₄ was 1:1, only less than 50% of Zn in ZnFe₂O₄ was chlorinated [41]. Therefore, a molar ratio of 3:1 was adopted to ensure sufficient chlorination of Zn in this work. Figure 3 shows the weight loss percentage curves of pure chlorides and chloride-oxide mixtures at 950 °C in Ar. The weight loss rate of chloride/oxide mixtures is more significant than their corresponding pure chlorides, suggesting the chlorination of ZnFe₂O₄ by MgCl₂ or CaCl₂ to form volatiles. The weight of MgCl₂–ZnFe₂O₄ mixture decreases rapidly and reaches a constant value within 10 min, while the weight loss of CaCl₂–ZnFe₂O₄ mixture is much slower. The result confirms that MgCl₂ is more potent in chlorination compared with CaCl₂.

Fig. 3

Weight loss percentage curves of different samples [MgCl₂ (0.2856 g), CaCl₂ (0.3330 g), MgCl₂–ZnFe₂O₄ mixture (0.2856 g–0.2410 g; molar ratio = 3:1) and CaCl₂–ZnFe₂O₄ mixture (0.3330 g–0.2410 g; molar ratio = 3:1)] at 950 °C in Ar

Figure 4a shows the variation of chlorination percentage of Zn and Fe for the MgCl₂–ZnFe₂O₄ and CaCl₂–ZnFe₂O₄ mixtures (molar ratio = 3:1 for both cases) at 950 °C in Ar. According to the increasing rate of chlorination percentage, chlorination of Zn is faster than that of Fe in both cases, suggesting the preference for chlorinating Zn over Fe. In the case of using MgCl₂, chlorination of Zn shows a tendency of “fast followed by slow,” while chlorination of Fe is “slow followed by fast.” As a result, the molar ratio of Zn to Fe in Rinse Solution-V (the solution after rinsing volatiles with water) is gradually declining (Fig. 4b). In the case of using CaCl₂, chlorination of Zn is slower, and it seems like that Fe is rarely chlorinated. Accordingly, a continuous increase in the molar ratio of Zn to Fe in Rinse Solution-V can be noticed. The ratio reaches more than 100 after reaction for 10 min, which is far larger than the values of using MgCl₂. These results confirm that although MgCl₂ is a potent chlorinating agent, the capability of separating Zn and Fe is not as good as that of CaCl₂.

Fig. 4

Variation of a chlorination percentages of Zn and Fe and b molar ratios of Zn to Fe in Rinse Solution-V (containing soluble phases in volatiles) during the reactions between ZnFe₂O₄ and MgCl₂ or CaCl₂ (molar ratio = 3:1 for both cases) at 950 °C in Ar

Figure 5 shows the XRD patterns of Solids-R (insoluble phases in residues in the crucible) after reacting ZnFe₂O₄ with MgCl₂ or CaCl₂ at 950 °C for 120 min. When reacting with MgCl₂, a spinel phase and MgO were detected. The spinel phase may be incompletely reacted ZnFe₂O₄ and newly formed MgFe₂O₄. When reacting with CaCl₂, CaFe₂O₄ and Ca₂Fe₂O₅ were detected. ZnFe₂O₄ should also remain in the CaCl₂ system. However, the presence of products such as CaFe₂O₄ and Ca₂Fe₂O₅ may envelop the remaining ZnFe₂O₄, making it challenging to detect through XRD. Figure 6 shows the SEM images and EDS mapping results of Solids-R in both cases. Fine particles can be observed in the products of the reaction between ZnFe₂O₄ and MgCl₂. EDS mapping indicates high concentrations of Mg and O while low concentrations of other elements in these particles (Fig. 6a). Although a small amount of MgFe₂O₄ forms, most of the solid phase products are MgO. In the case of reacting with CaCl₂, strips several hundred microns long can be detected. They were identified as Ca₂Fe₂O₅ (Fig. 6b). These observations are consistent with the previous findings of Guo et al. [37, 38].

Fig. 5

XRD patterns of Solids-R (insoluble phases in residues in the crucible) after reacting ZnFe₂O₄ with a MgCl₂ and b CaCl₂ (molar ratio = 3:1) for 120 min at 950 °C in Ar

Fig. 6

SEM images and EDS mapping results of Solids-R (insoluble phases in residues in the crucible) after reacting ZnFe₂O₄ with a MgCl₂ and b CaCl₂ (molar ratio = 3:1) for 120 min at 950 °C in Ar

We have clarified the reaction behavior between solid ZnFe₂O₄ and molten MgCl₂ in a previous work [41]. The chlorination of Zn is with higher priority

$$ {\text{MgCl}}_{{2}} ({\text{l}}) + {\text{ZnFe}}_{{2}} {\text{O}}_{{4}} \left( {\text{s}} \right) \to {\text{MgO}}({\text{s}}) + {\text{Fe}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) + {\text{ZnCl}}_{{2}} ({\text{g}}) $$

(2)

Although the priority is lower, the chlorination of Fe also occurs

$$ {\text{3MgCl}}_{{2}} ({\text{l}}) + {\text{Fe}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) \to {\text{3MgO}}({\text{s}}) + {\text{2FeCl}}_{{3}} \left( {\text{g}} \right) $$

(3)

In the case of reacting with CaCl₂, a similar reaction will occur

$$ {\text{CaCl}}_{{2}} {\text{(l}}) + {\text{ZnFe}}_{{2}} {\text{O}}_{{4}} \left( {\text{s}} \right) \to {\text{CaO}}({\text{s}}) + {\text{Fe}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) + {\text{ZnCl}}_{{2}} ({\text{g}}) $$

(4)

$$ \Delta_{r} G_{{950\;^\circ {\text{C}}}}^{0} = {87}.{24}\;{\text{kJ}}\;{\text{mol}}^{{ - {1}}} $$

Although reaction (4) is not spontaneous at standard state conditions, deviation from the standard state of products (CaO and ZnCl₂) may cause the reaction to proceed forward. Figure 7 shows the pH of the water-rinsed solutions of Solids-R. The increase in the pH after reacting with CaCl₂ is due to hydration of newly formed CaO, while the decrease in the pH after reacting with MgCl₂ is due to the hydration of MgO.

Fig. 7

The pH of the water-rinsed solutions of Solids-R after reacting ZnFe₂O₄ with MgCl₂ or CaCl₂ (molar ratio = 3:1) at 950 °C in Ar

The dissolution behavior of CaO in molten CaCl₂ and that of MgO in molten MgCl₂ are rather different, which might be the primary reason for the dissimilar performances in separating Zn and Fe between the two chlorides. Because CaO is highly soluble and dissociable in molten CaCl₂ (solubility > 20 mol% at 950 °C [42]), CaO formed in reaction (4) will immediately dissociate into Ca²⁺ and O²⁻ in molten CaCl₂ and react with Fe₂O₃ to form new compounds by

$$ {\text{CaO}}({\text{s}}) \to {\text{Ca}}^{{{2} + }} + {\text{O}}^{{{2} - }} $$

(5)

$$ {\text{Ca}}^{{{2} + }} + {\text{O}}^{{{2} - }} + {\text{Fe}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) \to {\text{CaFe}}_{{2}} {\text{O}}_{{4}} ({\text{s}}) $$

(6)

$$ {\text{2Ca}}^{{{2} + }} + {\text{2O}}^{{{2} - }} + {\text{Fe}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) \to {\text{Ca}}_{{2}} {\text{Fe}}_{{2}} {\text{O}}_{{5}} ({\text{s}}) $$

(7)

CaFe₂O₄ and Ca₂Fe₂O₅ are more stable than Fe₂O₃ [43], and thus, the chlorination of Fe is inhibited.

In contrast, O.²⁻ is almost insoluble in molten MgCl₂ [44]. Although the following reactions may turn Fe₂O₃ into more stable MgFe₂O₄

$$ {\text{Mg}}^{{{2} + }} + {\text{O}}^{{{2} - }} + {\text{Fe}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) \to {\text{MgFe}}_{{2}} {\text{O}}_{{4}} ({\text{s}}) $$

(8)

$$ {\text{MgO}}\left( {\text{s}} \right) + {\text{Fe}}_{{2}} {\text{O}}_{{3}} \left( {\text{s}} \right) \to {\text{MgFe}}_{{2}} {\text{O}}_{{4}} ({\text{s}}) $$

(9)

, the reaction rates must be low due to the low concentration of O²⁻ or limited surface area for reaction. As evidenced by the experimental results (Fig. 6), there was only a tiny amount of MgFe₂O₄ in Solids-R after reacting ZnFe₂O₄ with MgCl₂, and the particle size is far smaller than that of Ca₂Fe₂O₅ in the case of using CaCl₂. As a result, a considerable part of Fe was chlorinated to FeCl₃ when using MgCl₂.

Therefore, the mechanisms of chlorinating ZnFe₂O₄ by MgCl₂ and CaCl₂ can be summarized, as illustrated in Fig. 8. When MgCl₂ is used as the chlorinating agent, Zn in the lattice of ZnFe₂O₄ is first chlorinated to form ZnCl₂, MgO, and Fe₂O₃ [reaction (2)]. The newly formed MgO and Fe₂O₃ in contact with each other would react to produce a tiny amount of MgFe₂O₄. As Zn gets consumed, MgCl₂ gradually reacts with Fe₂O₃ to form FeCl₃ [reaction (3)]. Both ZnCl₂ and FeCl₃ leave the melt as volatiles soon after their formation. The final products remaining in the system as solid residues are MgO and MgFe₂O₄. In the case of using CaCl₂, after the reaction between CaCl₂ and ZnFe₂O₄ [reaction (4)], the newly formed CaO immediately dissolves in the melt and dissociates into mobile O²⁻. A concentration gradient layer of O²⁻ from the interface to the bulk will form. The melt film surrounding the newly formed Fe₂O₃ will instantly become rich in O²⁻, which turns the surface of Fe₂O₃ into more stable compounds such as Ca₂Fe₂O₅ and inhibits the further chlorination of Fe [reactions (5)–(7)]. Ca₂Fe₂O₅ is the major solid phase remaining in the system after sufficient reaction.

Fig. 8

Schematic representation of the reaction mechanisms between solid ZnFe₂O₄and a molten MgCl₂ or b molten CaCl₂

These findings suggest that the superior performance of CaCl₂ in separating Zn from Fe is because of the highly soluble and mobile nature of O²⁻ in molten CaCl₂. Therefore, we consider that adding CaCl₂ in MgCl₂ may promote the mobility of O²⁻ in the melt and thus achieve fast and selective separation of Zn from ZnFe₂O₄.

Chlorination of ZnFe₂O₄ by MgCl₂–CaCl₂

Figure 9 shows the chlorination percentages of Zn and Fe and molar ratios of Zn to Fe in Rinse Solution-V after the reactions between ZnFe₂O₄ and different MgCl₂–CaCl₂ mixtures at 950 °C for 120 min in Ar. The chlorination percentages of Fe decline clearly with the increase of CaCl₂ addition. The result demonstrates that adding CaCl₂ can enhance the selectivity of chlorinating Zn. Figure 10 shows the variation of chlorination percentages of Zn and Fe during the reactions between ZnFe₂O₄ and different MgCl₂–CaCl₂ mixtures at 950 °C in Ar. A decrease in chlorination rate with the increase in CaCl₂ addition can be noticed. It is considered that MgCl₂ plays the role of fast chlorination. CaCl₂ can inhibit the chlorination of Fe, yet it may slow down the overall reaction. There must be an optimum composition of the MgCl₂–CaCl₂ mixture to achieve the best chlorinating performance by balancing these two effects. It appears that the mixture of MgCl₂–CaCl₂ = 1:1 performs better than the others.

Fig. 9

a Chlorination percentages of Zn and Fe and b molar ratios of Zn to Fe in Rinse Solution-V (containing soluble phases in the volatiles) after the reactions between ZnFe₂O₄ and different MgCl₂–CaCl₂ mixtures at 950 °C for 120 min in Ar

Fig. 10

Variation of chlorination percentages of Zn and Fe during the reactions between ZnFe₂O₄ and different MgCl₂–CaCl₂ mixtures at 950 °C in Ar

Volatiles formed during the reaction between ZnFe₂O₄ and MgCl₂–CaCl₂ mixture or MgCl₂ were collected and characterized by XRD, as shown in Fig. 11. Due to the addition of CaCl₂, the diffraction peaks of FeCl₃ are obviously weakened, consistent with the chemical analysis results that the addition of CaCl₂ lowers the chlorination percentage of Fe (Fig. 10).

Fig. 11

XRD patterns of the volatiles formed after reacting ZnFe₂O₄ with MgCl₂ (MgCl₂–ZnFe₂O₄ molar ratio = 3:1) and MgCl₂–CaCl₂ mixture (MgCl₂–CaCl₂–ZnFe₂O₄ molar ratio = 2.5:0.5:1) at 950 °C for 45 min in Ar

As shown in Fig. 12, both the mass and mass fraction of MgCl₂ and CaCl₂ in the volatiles are analyzed. The results indicate that the volatilization loss of MgCl₂ is less than 2.5%, and that of CaCl₂ is below 1% in all cases. Due to the limited amount, CaCl₂ in the volatiles is difficult to detect by XRD. Additionally, the addition of CaCl₂ can further reduce the volatilization loss of MgCl₂. Therefore, the volatilization of either MgCl₂ or CaCl₂ would not cause significant problems for the chlorination of ZnFe₂O₄.

Fig. 12

a The mass of MgCl₂ and CaCl₂ in the volatiles, and b the mass fraction of MgCl₂ and CaCl₂ in the volatiles relative to the initial amount in the reactant after reaction at 950 °C for 120 min in Ar

Figure 13 shows XRD patterns and Raman spectra of Solids-R (insoluble phases in residues in the crucible) after reacting ZnFe₂O₄ with different MgCl₂–CaCl₂ mixtures for 120 min at 950 °C in Ar. Diffraction peaks belonging to a spinel phase and MgO are clearly observed in the XRD pattern. Ca₂Fe₂O₅ can also be detected, although the diffraction peaks are weak. The obtained Raman spectra have been deconvoluted into individual Lorentzian peaks for analysis. The peak of T_2g mode at ~ 210 cm⁻¹ and the peak of A_1g mode at ~ 700 cm⁻¹ indicate the occupation of Mg²⁺ in the octahedral and tetrahedral sites in the spinel structure [45, 46]. Thus, we infer that the spinel phase in Solids-R should be MgFe₂O₄ rather than ZnFe₂O₄ or Fe₂O₃. Figure 14 shows the SEM image and EDS mapping results of Solids-R (insoluble phases in residues in the crucible) after reacting ZnFe₂O₄ with MgCl₂–CaCl₂ mixtures for 120 min at 950 °C in Ar. Phases with compositions close to that of MgFe₂O₄ (Point A) and MgO (Point B) can be observed. We also notice that with the increase in CaCl₂ addition, more MgFe₂O₄ is generated. These results suggest that the addition of CaCl₂ leads to the formation of MgFe₂O₄ and Ca₂Fe₂O₅, which inhibit the chlorination of Fe.

Fig. 13

a XRD patterns and b Raman spectra of Solids-R (insoluble phases in residues in the crucible) after reacting ZnFe₂O₄ with different MgCl₂–CaCl₂ mixtures for 120 min at 950 °C in Ar

Fig. 14

SEM image and EDS mapping results of Solids-R (insoluble phases in residues in the crucible) after reacting MgCl₂–CaCl₂–ZnFe₂O₄ [molar ratios are a 2.5:0.5:1 and b 1.5:1.5:1] for 120 min at 950 °C in Ar

It is considered that CaCl₂ in MgCl₂ influences the reaction with ZnFe₂O₄ from two aspects. At first, since CaCl₂ and MgCl₂ are perfectly miscible, the addition of CaCl₂ is equivalent to lowering the concentration and thermodynamic activity of MgCl₂. Assuming reactions (2) and (3) are one-order reactions, the forward reaction rate (r) can be written as r = k[MgCl₂], where k is the rate constant and [MgCl₂] is the concentration of MgCl₂. As the equation shows, a decrease in the concentration of MgCl₂ will slow the reaction. On the other hand, the mobility of O²⁻ in the molten salt may rise with the increase in CaCl₂ content. Figure 15 illustrates the mechanism. Solid ZnFe₂O₄ first reacts with MgCl₂ in the MgCl₂–CaCl₂ mixture to produce ZnCl₂, MgO, and Fe₂O₃. A concentration gradient layer of MgCl₂ from the interface to the bulk will form because of the fast consumption of MgCl₂. Deficiency of MgCl₂ at the interface favors the reaction between ZnFe₂O₄ and CaCl₂ to produce mobile O²⁻. Since the diffusion of MgCl₂ from the bulk to the interface promptly refill the deficiency, O²⁻ and Mg²⁺ turn Fe₂O₃ into more stable MgFe₂O₄ via reaction (8). When most MgCl₂ is consumed, a tiny amount of CaFe₂O₅ is also formed via reaction (7). MgO, MgFe₂O₄, and Ca₂Fe₂O₅ are the solid phases remaining in the system after sufficient reaction. As a result, chlorination of Fe is inhibited, and the selectivity of Zn chlorination is improved.

Fig. 15

Schematic representation of the reaction mechanism between solid ZnFe₂O₄ and molten MgCl₂–CaCl₂

Synergistic Effect of Temperature and Adding CaCl₂

Figure 16a shows the chlorination percentages of Zn and Fe after the reactions between ZnFe₂O₄ and MgCl₂ or MgCl₂–CaCl₂ mixture for 45 min in Ar at different temperatures. When using MgCl₂ only (MgCl₂–ZnFe₂O₄ molar ratio = 3:1), the chlorination percentage of Zn is mainly independent of temperature, while that of Fe decreases slightly as the temperature increases. In the case of using MgCl₂–CaCl₂ mixture (MgCl₂–CaCl₂–ZnFe₂O₄ molar ratio = 2.5:0.5:1), the chlorination percentage of Zn increases from 81 to 94%, and that of Fe decreases from 38 to 15%, as the temperature increases from 800 to 1000 °C. These results indicate that it becomes more effective to promote the selectivity of chlorinating Zn by increasing temperature after adding CaCl₂ in MgCl₂. Molar ratios of Zn to Fe in Rinse Solution-V in the case of using MgCl₂–CaCl₂ mixture at 950 °C and 1000 °C are larger than the values using MgCl₂ only (Fig. 16b). Figure 16c shows the variation of chlorination percentages of Zn and Fe during the reactions between ZnFe₂O₄ and MgCl₂–CaCl₂ mixtures (MgCl₂–CaCl₂–ZnFe₂O₄ molar ratio = 2.5:0.5:1) at different temperatures in Ar. A higher temperature favors the chlorination of Zn in terms of amount and rate, while the dependency of the chlorination rate of Fe on temperature is not apparent. Furthermore, the chlorination percentages of Fe exhibit fluctuations that are particularly pronounced at higher temperatures. Our previous study [41] suggested that these fluctuations may be attributed to the formation of insoluble FeOCl from FeCl_x in the volatiles at elevated temperatures. This reaction may also facilitate the separation of Zn and Fe. Based on the selectivity and rate of chlorination, the ideal temperature for chlorination is 950 °C when using the MgCl₂–CaCl₂ mixture.

Fig. 16

Reaction between ZnFe₂O₄ and MgCl₂ (MgCl₂–ZnFe₂O₄ molar ratio = 3:1) or MgCl₂–CaCl₂ mixture (MgCl₂–CaCl₂–ZnFe₂O₄ molar ratio = 2.5:0.5:1) at different temperatures in Ar. a Chlorination percentages of Zn and Fe, b molar ratios of Zn to Fe in Rinse Solution-V (containing soluble phases in volatiles) after reaction for 120 min; c variation of chlorination percentages of Zn and Fe during the reactions between ZnFe₂O₄ and MgCl₂–CaCl₂ mixture (MgCl₂–CaCl₂–ZnFe₂O₄ molar ratio = 2.5:0.5:1) in 45 min

Figure 17 shows the XRD patterns and Raman spectra of Solids-R (insoluble phases in residues in crucible) after reacting ZnFe₂O₄ with MgCl₂–CaCl₂ mixture (MgCl₂–CaCl₂–ZnFe₂O₄ molar ratio = 2.5:0.5:1) at different temperatures for 45 min in Ar. In all cases, formation of MgFe₂O₄ can be confirmed. As can be noticed, the diffraction peaks at 35.4° belonging to MgFe₂O₄ and the Raman spectra peak of A_1g mode at ~ 700 cm⁻¹ are significantly intensified as temperature increases. This trend indicates that a higher temperature promotes the formation of MgFe₂O₄.

Fig. 17

a XRD patterns and b Raman spectra of Solids-R (insoluble phases in residues in the crucible) after reacting ZnFe₂O₄ with MgCl₂–CaCl₂ mixtures (MgCl₂–CaCl₂–ZnFe₂O₄ molar ratio = 2.5:0.5:1) at different temperatures for 45 min in Ar

The formation of MgFe₂O₄ from Fe₂O₃ can be expressed by reaction (8). The formation rate can be written as r = k[Mg²⁺][O²⁻]. The rate constant k would increase with temperature, and the concentration of O²⁻ ([O²⁻]) will also rise due to the increase in solubility. Therefore, the formation rate of MgFe₂O₄ increases with the temperature. Fast conversion of Fe₂O₃ into more stable phases, such as MgFe₂O₄, in molten MgCl₂–CaCl₂ at higher temperatures could explain why the chlorination of Fe is precluded.

Conclusions

This work clarifies the distinct reaction behavior of ZnFe₂O₄ between using MgCl₂ and using CaCl₂. Although MgCl₂ is a more powerful chlorinating agent, the capability of separating Zn and Fe is not as good as that of CaCl₂. Fast and selective chlorination of Zn from ZnFe₂O₄ happens by reacting ZnFe₂O₄ and MgCl₂–CaCl₂ mixture. By raising the mobility of O²⁻ in the molten salt, CaCl₂ facilitates the formation of stable Fe-bearing phases such as MgFe₂O₄ and Ca₂Fe₂O₅, thereby inhibiting the chlorination of Fe. Moreover, it becomes more effective in promoting the selectivity of chlorinating Zn by increasing temperature after adding CaCl₂ in MgCl₂. These results demonstrate the prospect of efficient separation of Zn from ZnFe₂O₄ by reaction with molten MgCl₂–CaCl₂ mixture.