Review on Electrolytic Processes for Magnesium Metal Production: Industrial and Innovative Processes

Abstract

Magnesium (Mg) metal is an attractive material for various industries due to its superior properties. Currently, the Pidgeon process is commercially used for primary Mg metal production. However, due to its high carbon dioxide (CO₂) gas emissions, electrolytic processes have attracted attention in recent years. Industrial electrolytic processes that use anhydrous magnesium chloride (MgCl₂) feedstock have a lower impact on global warming. Unfortunately, toxic chlorine (Cl₂) gas is generated during metal production, and anhydrous MgCl₂ production is energy intensive. In response, efforts have been made to improve industrial electrolytic processes and develop new electrolytic processes. Among these processes, the electrolysis of magnesium oxide (MgO) in molten fluoride salt is currently considered a promising alternative for the environmentally sustainable production of Mg metal with the generation of oxygen (O₂) gas. This paper comprehensively reviews industrial and innovative electrolytic processes, aiming to propose the next-generation Mg metal production method.

1. Introduction

As global greenhouse gas (GHG) regulations become increasingly stringent, the aerospace, automotive and other transportation industries are seeking lightweight materials to improve fuel efficiency. Currently, magnesium (Mg) metal is gaining attention owing to its light weight and high strength-to-weight ratio [1, 2]. Compared to common metals such as aluminum (Al) and iron (Fe), Mg metal offers weight reductions of 33% and 75%, respectively [3].

Nonetheless, the GHG emissions associated with Mg metal production are higher than those associated with Al or Fe production. Currently, approximately 85% of the global primary Mg metal is produced using the Pidgeon process in China [4], which emits 21.8–47 kg of CO_2(eq) per kg of Mg [5, 6]. Conversely, approximately 16.5 kg of CO_2(eq) and 2.3 kg of CO_2(eq) are emitted per equivalent amount of Al and Fe produced, respectively [7]. Therefore, although Mg-based materials significantly reduce the overall GHG emissions during use [8], their production poses substantial challenges to environmental sustainability.

The higher GHG emissions for Mg metal production compared to other metals production are due to the characteristics of the Pidgeon process. The Pidgeon process produces Mg metal through the reduction of calcined dolomite (MgO·CaO) using ferrosilicon (Fe·Si) at 1373–1553 K under vacuum [9–11]. The processes for the calcination of dolomite (MgCO₃·CaCO₃), the preparation of Fe·Si, and the reduction are all responsible for high GHG emissions [12].

In addition to the Pidgeon process, electrolytic processes are also used for the commercial production of Mg metal. In this case, the GHG emissions for the production of kg of Mg metal can decrease to 5.3–8.5 kg of CO_2(eq) [12]. Current electrolytic processes produce Mg metal through electroreduction of anhydrous magnesium chloride (MgCl₂) at 928–993 K [9]. Unfortunately, complex and energy-intensive processing is required for anhydrous MgCl₂ feed production from Mg resources [8, 9]. Additionally, toxic chlorine (Cl₂) gas is generated during the production of Mg metal.

Under these circumstances, many studies have been carried out on the development of electrolytic processes for environmentally sound Mg metal production [13–37]. Among these processes, an electrolytic process using magnesium oxide (MgO) feed is promising. By using MgO as a feed, the need for the production of anhydrous MgCl₂ is not required [8]. In addition, Cl₂ gas is not generated during Mg metal production. Notably, the solid oxide membrane (SOM) process demonstrated the feasibility of green Mg metal production with the generation of oxygen (O₂) gas through the molten salt electrolysis of MgO [13].

Since the development of the SOM process, many studies on the molten salt electrolysis of MgO have been extensively conducted. These studies have focused on improving the SOM process [14–18] or developing new processes [19–37]. Among the studies on developing new processes, the electrolysis of MgO in molten fluoride salt using a high-density metal cathode, followed by distillation is promising [21, 30–37]. In this process, Mg alloy is produced at the bottom of the electrolytic cell through electrolysis. Afterward, the Mg metal is separated from the Mg alloy through vacuum distillation. For example, the electrolysis of MgO in molten fluoride salt using tin (Sn), copper (Cu), or silver (Ag) cathodes produced Mg alloys with a current efficiency above 90% under certain conditions. Following electrolysis, Mg metal with a purity of 99.999% was recovered through vacuum distillation of Mg alloys [30–34].

Owing to the demand for carbon neutrality, primary Mg metal production processes will face significant challenges in the near future. Among the industrial Mg metal production processes, the electrolytic process is superior to thermal reduction processes considering GHG emissions. With this background, in this paper, we comprehensively review industrial electrolytic processes for Mg metal production. In addition, advancements achieved in the development of green electrolytic Mg metal production processes, such as the SOM process and the electrolysis of MgO in molten fluoride salt using a high-density metal cathode, are discussed.

2. Industrial Electrolytic Processes

2.1 Anhydrous MgCl₂ production

Table 1 summarizes the industrial electrolytic processes used to produce MgCl₂ for Mg metal production. As shown in Table 1, various Mg resources were used to prepare the MgCl₂ feedstock. Notably, Table 1 shows that most electrolytic processes use anhydrous MgCl₂ as a feedstock for producing Mg metal, except the Dow process. This preference indicates the importance of using anhydrous MgCl₂ as a feedstock in industrial electrolytic processes.

Table 1 Summary of MgCl₂ production processes conducted in industrial electrolytic processes for Mg metal production.

In industrial electrolytic processes, the production of MgCl₂ feedstock is generally conducted in two steps, regardless of whether the MgCl₂ feedstock is in anhydrous or hydrous form: The preparation of aqueous MgCl₂ solution and drying followed by dehydration. After the aqueous MgCl₂ solution is prepared from various Mg resources through the treatments shown in Table 1, drying and dehydration are conducted to produce the MgCl₂ feedstock. Drying is simply carried out to produce MgCl₂ hydrates from aqueous MgCl₂ solution. Therefore, dehydration is mainly discussed in this study.

Figure 1 shows the dehydration of the MgCl₂ hydrate obtained from aqueous MgCl₂ solution [38, 39]. At 298 K, MgCl₂ hexahydrate (MgCl₂·6 H₂O) is a stable form of MgCl₂ hydrate. When heated, it converts to MgCl₂ tetrahydrate (MgCl₂·4 H₂O) and then to MgCl₂ dihydrate (MgCl₂·2 H₂O) at approximately 423–453 K, as shown in eqs. (1) and (2).

  

\begin{equation} \text{MgCl$_{2}{\cdot}$6 H$_{2}$O ($s$)} = \text{MgCl$_{2}{\cdot}$4 H$_{2}$O ($s$)} + \text{2 H$_{2}$O ($g$)} \end{equation} (1)

  

\begin{equation} \text{MgCl$_{2}{\cdot}$4 H$_{2}$O ($s$)} = \text{MgCl$_{2}{\cdot}$2 H$_{2}$O ($s$)} + \text{2 H$_{2}$O ($g$)} \end{equation} (2)

Fig. 1

Hydrates of MgCl₂ depending on the vapor pressures of H₂O and HCl at elevated temperatures (adapted from Ref. [39]).

Although these remove the water (H₂O) to some extent, further dehydration of MgCl₂·2 H₂O without controlling atmosphere leads to hydrolysis, as shown in Fig. 1. This results in the formation of undesirable compounds such as magnesium hydroxychloride (MgOHCl), as shown in eqs. (3) and (4). Therefore, to produce anhydrous MgCl₂ feedstock from MgCl₂·2 H₂O without hydrolysis, dehydration is generally conducted under hydrochloric acid (HCl) gas.

  

\begin{equation} \text{MgCl$_{2}{\cdot}$2 H$_{2}$O ($s$)} = \text{MgOHCl ($s$)} + \text{H$_{2}$O ($g$)} + \text{HCl ($g$)} \end{equation} (3)

  

\begin{equation} \text{MgCl$_{2}{\cdot}$H$_{2}$O ($s$)} = \text{MgOHCl ($s$)} + \text{HCl ($g$)} \end{equation} (4)

To bypass the use of HCl gas for full dehydration, the Dow process developed an electrolytic process to produce Mg metal directly from MgCl₂·1–2 H₂O, as shown in Fig. 2 (see Fig. A-1 in Appendix for the Dow cell) [9, 22, 38–54]. However, the presence of H₂O in the MgCl₂ feedstock during electrolysis leads to several problems, such as the consumption of graphite (C) anode and the formation of a large amount of sludge at the bottom of the electrolytic cell [22, 38, 51]. It was reported that approximately 0.1 kg of C is consumed per 1 kg of Mg metal produced [38, 40, 45, 54] and 0.18–0.32 kg of sludge is generated per 1 kg of Mg metal produced [38]. Consequently, current industrial electrolytic processes use only anhydrous MgCl₂ as a feedstock.

Fig. 2

Flowchart of the electrolytic Mg metal production process that uses hydrous MgCl₂ feedstock.

2.1.1 Anhydrous MgCl₂ production using dehydration

To avoid the negative influence of the H₂O in MgCl₂ feedstock on electrolysis, many electrolytic processes use anhydrous MgCl₂ as the feedstock. In this case, to produce anhydrous MgCl₂ feedstock, dehydration of MgCl₂·2 H₂O is conducted under HCl gas. When dehydration is conducted under HCl gas, the hydrolysis of MgCl₂·2 H₂O and MgCl₂ monohydrate (MgCl₂·H₂O) is prevented by increasing the partial pressure of HCl, as can be understood by the Le Chatelier’s principle. Carbo-chlorination of MgCl₂ using Cl₂ gas is another method for removing H₂O in the MgCl₂ feedstock to produce anhydrous MgCl₂ feedstock through dehydration.

Table 1 shows that the Norsk Hydro process and Magnola process use magnesite (MgCO₃) and serpentine (3 MgO·2 SiO₂·2 H₂O), respectively, as Mg resources to produce anhydrous MgCl₂ feedstock. Consequently, acid leaching, purification, and drying were conducted prior to dehydration. Figure 3 shows the example of the flowchart conducted in the Norsk Hydro process to produce anhydrous MgCl₂ feedstock [9, 41, 44, 55]. In the Norsk Hydro process, acid leaching of MgCO₃ is conducted in concentrated HCl solution at 343–373 K [55] to obtain aqueous MgCl₂ solution by the reaction shown in eq. (5). This allows the separation of impurities insoluble in the HCl solution, such as silicon dioxide (SiO₂), as residues.

  

\begin{align} &\text{MgCO$_{3}$ ($s$)} + \text{2 HCl ($l$)} \\ &\quad = \text{MgCl$_{2}$ ($aq.$)} + \text{H$_{2}$O ($l$)} + \text{CO$_{2}$ ($g$)} \end{align} (5)

Fig. 3

Flowchart of the electrolytic Mg metal production process that uses anhydrous MgCl₂ feedstock obtained from magnesite.

Purification is conducted to remove the dissolved impurities from the aqueous MgCl₂ solution. This consists of precipitation and filtration. For precipitation, the pH of the aqueous MgCl₂ solution is increased by adding excess MgO because several impurities dissolved in the aqueous MgCl₂ solution precipitate when the pH of the solution increases, as can be understood by the Pourbaix diagram [56]. Subsequent filtration separates the precipitated impurities from the purified aqueous MgCl₂ solution. The purified solution is then preheated to concentrate. For example, steam-heated evaporators have been used to concentrate aqueous MgCl₂ solution up to approximately 50% [44].

The concentrated aqueous MgCl₂ solution is then dried to produce MgCl₂·6 H₂O. For dehydration, MgCl₂·6 H₂O is first dried in a fluidized bed with air at approximately 423–453 K to produce MgCl₂·2 H₂O [57]. Afterward, MgCl₂·2 H₂O is dried in a fluidized bed with HCl gas at approximately 603 K [9, 57] to produce solid anhydrous MgCl₂.

In the Magnola process, 20–25% or 33% HCl leaching of 3 MgO·2 SiO₂·2 H₂O is conducted at 368–383 K to produce aqueous MgCl₂ solution by the reaction shown in eq. (6) (see Fig. A-2 in Appendix) [58–62]. Meanwhile, 3 MgO·2 SiO₂·2 H₂O used in the Magnola process is originated from asbestos mine tailing, which contains impurities containing Fe. Consequently, the Magnola process also involves a magnetic separation step before acid leaching to separate Fe-containing particles to increase the efficiency of the latter purification process [61–63].

  

\begin{align} & \text{3 MgO${\cdot}$2 SiO$_{2}{\cdot}$2 H$_{2}$O ($s$)} + \text{6 HCl ($l$)}\\ &\quad = \text{3 MgCl$_{2}$ ($aq.$)} + \text{5 H$_{2}$O ($l$)} + \text{2 SiO$_{2}$ ($s$)} \end{align} (6)

After neutralization of the obtained aqueous MgCl₂ solution with MgO and filtration to separate un-leached residue, purification is conducted. First, Cl₂ gas is injected to oxidize impurities, such as ferrous chloride (FeCl₂) and manganese chloride (MnCl₂). Subsequent treatment with MgO and sodium hydroxide (NaOH) precipitates the impurities as hydroxides. The impurity precipitates in the purified aqueous MgCl₂ solution are separated by filtration, and the purified solution is pumped into a storage.

For dehydration, the concentrated aqueous MgCl₂ solution is spray-dried in a fluidized bed to produce MgCl₂·x H₂O. Afterward, chlorination of MgCl₂·x H₂O using HCl gas is conducted using a proprietary chlorinator [58, 59, 62, 64]. The chlorinator utilizes MgCl₂-depleted molten salt for chlorination of MgCl₂·x H₂O, which is obtained from an electrolytic cell after electrolysis [64]. This lowers the activity of MgCl₂ and suppresses the hydrolysis of MgCl₂ according to the Le Chatelier’s principle, as shown in eq. (7). Consequently, the produced anhydrous MgCl₂ contains less than 0.1 mass% MgO [9].

  

\begin{align} &\text{MgCl$_{2}$ ($l$, in molten salt)} + \text{H$_{2}$O ($g$)} \\ &\quad = \text{MgO ($s$)} + \text{2 HCl ($g$)} \end{align} (7)

The US Mag process and DSM process use MgCl₂ brine, such as the Great Salt Lake in Utah and the Dead Sea, respectively, as an Mg resource to prepare anhydrous MgCl₂ feedstock. In this case, although acid leaching is not needed, the Mg concentration in the brine is low. For example, the Great Salt Lake contains 0.4% Mg. As a result, dehydration is conducted after concentrating through solar evaporation.

In the US Mag process, the Great Salt Lake is evaporated using solar energy as shown in Fig. 4, and stored in large holding ponds [9, 41, 65, 66]. Subsequently, purification is conducted on the concentrated MgCl₂ brine. Sulfate ions (SO₄²⁻) in the concentrated MgCl₂ brine are removed as calcium sulfate (CaSO₄) by adding CaCl₂. When boron (B) removal is needed, solvent extraction is performed. After purification, the MgCl₂ brine is concentrated by passing it through a preheater vessel that utilizes waste gases as a heat source. For drying and dehydration, the concentrated MgCl₂ brine is first spray-dried to produce MgCl₂·2 H₂O. Subsequently, melting in an electric furnace and carbo-chlorination using Cl₂ gas in the presence of C are conducted at 1088 K [9, 65]. This produces molten anhydrous MgCl₂ containing 0.1–0.3 mass% MgO [9].

Fig. 4

Flowchart of the electrolytic Mg metal production process that uses anhydrous MgCl₂ feedstock obtained from natural MgCl₂ brine.

In the DSM process, the evaporation of the Dead Sea produces carnallite (MgCl₂·KCl·6 H₂O) (see Fig. A-3 in Appendix) [9, 59, 64]. The dehydration of MgCl₂·KCl·6 H₂O is conducted in a fluidized bed by increasing temperature from 400 K to 473 K [59]. Subsequently, carbo-chlorination is conducted using Cl₂ gas in the presence of C at approximately 973–1023 K [9, 59]. Carbo-chlorination is conducted after melting the product. This produces molten anhydrous carnallite (MgCl₂·KCl) containing 0.2–0.6 mass% MgO [9].

It is noteworthy that MgCl₂ brine generated as a byproduct of other industrial processes is also used to prepare anhydrous MgCl₂ feedstock. For example, Qinghai Salt Lake Magnesium Co., Ltd. (QSLM) uses MgCl₂-rich salt generated at an adjacent potash production facility for the production of anhydrous MgCl₂ feedstock through dissolution, purification, and dehydration [6, 67, 68].

2.1.2 Anhydrous MgCl₂ production using carbo-chlorination

Although the dehydration of MgCl₂ hydrates obtained from ores or MgCl₂ brine produces anhydrous MgCl₂ feedstock with a low concentration of MgO impurity, the dehydration process is complex and energy intensive. It was reported that up to 50% of the total cost of Mg metal production in the electrolytic process is attributed to the preparation of anhydrous MgCl₂ feedstock through dehydration [69]. Consequently, the I.G. Farben process, which produces anhydrous MgCl₂ feedstock through the carbo-chlorination of MgO using Cl₂ gas in the presence of C was also developed, as shown in Fig. 5 [38–44, 70, 71].

Fig. 5

Flowchart of the electrolytic Mg metal production process that uses anhydrous MgCl₂ feedstock obtained via the carbo-chlorination of MgO.

In the I.G. Farben process, MgO is prepared by calcination of magnesium hydroxide (Mg(OH)₂), which is obtained from seawater through precipitation using an alkaline source, such as MgO·CaO. MgO is also prepared by calcination of MgCO₃. Subsequently, the obtained MgO is pelletized with C using MgCl₂ brine as the binder. It was reported that the obtained MgO consequently had a composition of 50% MgO, 15–20% MgCl₂, 15–20% H₂O, 10% C, and alkali metal chlorides as the balance [43, 70].

Chlorination of MgO is carried out using Cl₂ gas in the presence of C at approximately 1300 K in a chlorinator to produce molten anhydrous MgCl₂, as shown in eqs. (8) and (9) (see Fig. A-4 in Appendix) [38, 39, 42, 43]. MgCl₂ is produced by decreasing the partial pressure of oxygen ($p_{\text{O$_{2}$}}$) in the system using C [38, 39], as can be understood by the chemical potential diagram of the Mg–O–Cl system at 1300 K [72]. To facilitate the carbo-chlorination reaction, molten anhydrous MgCl₂ is moved to the bottom of the chlorinator against the Cl₂ gas injection direction. The molten anhydrous MgCl₂ typically contains less than 0.1 mass% MgO [43].

  

\begin{equation} \text{MgO ($s$)} + \text{Cl$_{2}$ ($g$)} + \text{C ($s$)} = \text{MgCl$_{2}$ ($l$)} + \text{CO ($g$)} \end{equation} (8)

  

\begin{equation} \text{MgO ($s$)} + \text{Cl$_{2}$ ($g$)} + \text{CO ($g$)} = \text{MgCl$_{2}$ ($l$)} + \text{CO$_{2}$ ($g$)} \end{equation} (9)

The impurities present in the MgO pellet are also chlorinated during carbo-chlorination at 1300 K. However, most chlorinated impurities are volatile at approximately 1300 K, except calcium chloride (CaCl₂), as shown in Fig. 6 [72]. As a result, volatile chlorides are separated from the molten anhydrous MgCl₂ produced during carbo-chlorination. However, some impurities remain. For example, SiO₂ remained because chlorination of SiO₂ does not easily occur kinetically. The remaining impurities form slag, which must be removed using a slag removal door.

Fig. 6

Vapor pressures of some selected chlorides at elevated temperatures.

Given the feasibility of the anhydrous MgCl₂ production directly from MgCO₃ through chlorination [9, 73–75], carbo-chlorination is a simple and effective method for producing anhydrous MgCl₂ feedstock from Mg resources. However, carbo-chlorination method is also energy and capital-intensive. In addition, toxic dioxins and furans have been generated [76].

2.2 Electrolysis of anhydrous MgCl₂

2.2.1 Common principle of electrolysis

Table 2 presents a comparison of the electrolytic processes used to produce Mg metal [9, 38, 40–45, 53–55, 58, 59, 64–68, 73–75, 77–86]. As shown in Table 2, electrolytic processes are largely divided into two types depending on the type of feedstock used: anhydrous MgCl₂ or hydrous MgCl₂. When anhydrous MgCl₂ feedstock is used, an I.G. Farben cell, a Norsk Hydro cell, an Alcan multi-polar cell, an M-cell, or a VAMI cell is used for electrolysis. When hydrous MgCl₂ feedstock is used, a Dow cell is used for electrolysis, as discussed above. The existence of various electrolytic cells for electrolytic processes reflects the industrial efforts to improve electrolytic Mg metal production.

Table 2 Comparison of electrolytic processes for Mg metal production [9, 38, 40–45, 53–55, 58, 59, 64–68, 73–75, 77–86].

As shown in Table 2, each process has different characteristics. Despite these variations, basic electrochemical reactions are consistent in all electrolytic processes: The reduction of magnesium ion (Mg²⁺) to liquid Mg metal at the Fe cathode and the oxidation of chloride ion (Cl⁻) to Cl₂ gas at the C anode, as shown in eq. (10).

  

\begin{equation} \text{Cathode: Mg$^{2+}$ (in molten salt)} + \text{2 e$^{-}$} = \text{Mg ($l$)} \end{equation} (10)

  

\begin{equation*} \text{Anode: 2 Cl$^{-}$ (in molten salt)} = \text{Cl$_{2}$ ($g$)} + \text{2 e$^{-}$} \end{equation*}

In addition, all electrolytic processes involve optimization of molten salt properties. For example, the melting temperature, electrical conductivity, density, and surface tension of molten salt in each electrolytic process are customized by optimization of the composition of the molten salt. In the case of melting temperature, the melting temperature of MgCl₂ is lowered by adding sodium chloride (NaCl), CaCl₂, or barium chloride (BaCl₂), as shown in Table 3 [38]. Notably, Fig. 7 shows the theoretical decomposition voltages of the selected chlorides listed in Table 3, which are higher than the decomposition voltage of MgCl₂ [72]. Therefore, during electrolysis, MgCl₂ mainly decomposed to produce Mg metal.

Table 3 Typical supporting electrolyte components in MgCl₂ electrolysis processes and their properties [38].

Fig. 7

Theoretical decomposition voltages of MgO and some selected chlorides and fluorides.

Moreover, in all electrolytic processes, the liquid Mg metal produced is buoyant in the molten salt owing to the lower density of the Mg metal compared to that of the molten chloride salt used. Meanwhile, the Cl₂ gas generated at the anode rose towards the surface of the molten salt. The molten salt that wets the Mg metal could act as a kinetic barrier to the reaction with the Cl₂ gas generated [38]. However, there is still a risk of recombination of the produced Mg metal and Cl₂ gas generated. Consequently, a physical barrier and/or circulation in the molten salt is applied to separate the produced Mg metal from the Cl₂ gas generated [38].

Electrolytic processes that use anhydrous MgCl₂ feedstock are further classified depending on whether the electrolytic cell technology used is based on Western or Russian technologies. Electrolytic cells based on Western technology refer to I.G. Farben cell, Norsk Hydro cell, Alcan multi-polar cell, and M-cell. An electrolytic cell based on Russian technology refers to a flow line cell, VAMI cell.

2.2.2 Electrolysis using diaphragm cell

Figure 8 shows a schematic of the I.G. Farben cell used in the I.G. Farben process [40, 43, 53, 80, 81]. As shown in Fig. 8, the I.G. Farben cell is equipped with barriers between the cathode and the anode to separate the produced Mg metal from the Cl₂ gas generated during electrolysis. These barriers, known as semi-walls or diaphragms, are mainly made of chamotte or fused alumina [38, 80].

Fig. 8

Schematic of the I.G. Farben cell (adapted from Ref. [43]).

In the I.G. Farben process, electrolysis is carried out in molten salt using Fe cathode and C anode at 1013 K with a current density of 0.35–0.50 A·cm⁻² [54]. The temperature is maintained via resistance heating [42]. The typical composition of the molten salt is 11 mass% MgCl₂–6 mass% CaCl₂–65 mass% NaCl–18 mass% potassium chloride (KCl). Anhydrous MgCl₂ produced from seawater is used as a feed.

During electrolysis, the produced liquid Mg metal moves to the metal collection compartments, as shown in Fig. 8. This is due to the low density of Mg metal and the circulation of the molten salt. Circulation of the molten salt occurs owing to the gas-lift effect of the Cl₂ gas generated at the anode and a small local density difference in the molten salt due to the depletion of MgCl₂ [38]. The Mg metal in the collection compartments is recovered one to four times daily [38].

In this process, the semi-walls effectively separate the produced Mg metal from the Cl₂ gas, and a current efficiency of approximately 90% is obtained. However, semi-walls result in a relatively large anode-to-cathode distance of up to 140 mm [38, 53], leading to a large IR drop [42]. Consequently, the cell voltage during the electrolysis is approximately 7.0 V, and the production of 1 kg of Mg metal requires 15.0–18.0 kWh [54, 82]. In addition, semi-walls are susceptible to thermal shocks caused by sudden variations in the level of the molten salt [38].

Consequently, other electrolytic cells based on Western technology are not equipped with semi-walls between the cathode and anode. Instead, the cell is divided into two compartments: one for metal collection and the other for electrolysis with Cl₂ gas collection. The produced Mg metal is directed to a metal collection compartment by the circulation of the molten salt and is separated from the Cl₂ gas generated.

2.2.3 Electrolysis using diaphragmless cell

Figure 9 shows a schematic of the Norsk Hydro cell used in the Norsk Hydro process [41–43, 64, 81, 83]. As shown in Fig. 9, the cell is divided into metal collection and electrolysis compartments, without a diaphragm.

Fig. 9

Schematic of the Norsk Hydro cell (adapted from Ref. [43]).

In the Norsk Hydro process, electrolysis is carried out in molten salt using Fe cathode and C anode at 973–993 K with a cathodic current density of 0.8 A·cm⁻² [78]. The molten salt consists of MgCl₂, NaCl, and CaCl₂ [55]. The Fe cathode is hollow, and both sides are utilized for electrolysis [83, 84]. The solid anhydrous MgCl₂ feedstock is charged into the electrolysis compartment during electrolysis to decrease the influence of air inclusions [84].

During electrolysis, the produced liquid Mg metal moves to the metal collection compartment because of the circulation of the molten salt. Unlike the I.G. Farben cell, the Norsk Hydro cell has only one metal collection compartment, as shown in Fig. 9. This cell structure facilitates metal recovery. The Mg metal in the metal collection compartment is extracted using a vacuum ladle and transported to a cast house. The Cl₂ gas is recovered from the top of the electrolysis compartment through a pipe. The recovered Cl₂ gas then reacts with the hydrogen-containing gas to produce HCl, which is used for the dissolution of MgCO₃ [85].

In the Norsk Hydro process, the current efficiency of electrolysis is 89–91% [84]. This result demonstrates the effective separation of the produced Mg metal from the Cl₂ gas through the circulation of the molten salt. The cell voltage during electrolysis is approximately 5.3 V [66, 84]. The production of 1 kg of Mg metal using the Norsk Hydro cell requires 12.0–13.0 kWh [44, 66, 84].

US Magnesium LLC, formerly MagCorp, initially used an I.G. Farben-based cell for Mg metal production. However, the conversion to a diaphragmless cell provided various advantages, such as an increase in daily Mg metal production from 1.0 t to 1.4 t [66]. Building on these advancements, the company developed the M cell, a larger and more efficient electrolytic cell designed for producing Mg metal from anhydrous MgCl₂ produced from the Great Salt Lake. The M cell operates with a cell voltage of 4.5–5.0 V, reducing the energy consumption to 12–14 kWh per kg of Mg metal produced [66]. The amount of Mg metal produced increased to 2.8 t per day [66].

2.2.4 Electrolysis with bipolar electrodes

When bipolar electrodes are used, an electrolytic cell is considered to multiple electrolytic cells. It allows increased quantity of Mg metal production under a constant current [40]. This principle was employed for the development of the Magnola process. Figure 10 shows a schematic of the Alcan multi-polar cell used in the Magnola process, featuring C bipolar electrodes placed between the cathode and anode [58, 87].

Fig. 10

Schematic of the Alcan multi-polar cell (adapted from Ref. [87]).

In the Magnola process, electrolysis is carried out in molten chloride salt using Fe cathode and C anode at 928–938 K [9]. The molten salt consists of MgCl₂, NaCl, and CaCl₂ [58]. Fluoride, such as calcium fluoride (CaF₂), has been added to the molten salt to assist the growth of produced Mg metal droplet and wetting of the cathode [9]. The liquid Mg metal produced during electrolysis is directed to a metal collection compartment where it is tapped, or the MgCl₂ feedstock is charged, owing to the molten salt circulation. The Cl₂ gas is collected from the compartment for electrolysis. The recovered Cl₂ gas is then converted to HCl for the HCl leaching of 3 MgO·2 SiO₂·2 H₂O. The electrolysis occurs at a cell voltage of approximately 17 V [9]. The production of 1 kg of Mg metal using the Alcan multi-polar cell requires 10.5–11.5 kWh [9, 58, 66].

2.2.5 Electrolysis with flow line cell

Unlike cells based on Western technology, the cell based on Russian technology features many electrolytic cells connected by channels, providing a path for the molten salt and Mg metal to pass from cell to cell. Gravitational flow was achieved by slightly differentiating the cell heights [9, 64]. Figure 11 shows a flow line cell [64, 79]. It should be noted that some of the cells shown in Fig. 11 are omitted in the DSM process [64, 79].

Fig. 11

Schematic of the flow line cell, VAMI cell (adapted from Ref. [64]).

The electrolytic process using the flow line cell, the DSM process, was developed for producing Mg metal from MgCl₂·KCl [9, 64, 77, 79]. In the flow line cell shown in Fig. 11, the molten MgCl₂·KCl is mixed with the molten salt in a head cell. The molten salt then moves next to a series of top-mounted anode electrolytic cells, where Mg metal and Cl₂ gas are produced [64].

In the DSM process, electrolysis is carried out in molten salt using Fe cathode and C anode at 953–973 K [9, 44]. During electrolysis, the cell voltage is approximately 4.8 V, and the production of 1 kg of Mg metal requires 13–14 kWh [9]. After electrolysis, the liquid Mg metal produced and the molten salt move to a series of bottom-mounted anode cells and then to a separator cell located at the end of the flow line. The produced Mg metal is separated and siphoned out for casting, whereas the molten salt is recirculated to the head cell [64]. Meanwhile, because the feedstock contains KCl as one of its components, KCl accumulates during electrolysis. Therefore, a portion of the spent molten salt is directed to a granulation unit, where it solidifies and is repurposed as a raw material for fertilizer. The rest of the spent molten salt is recirculated back to the head cell and combined with the molten MgCl₂·KCl feedstock, ensuring efficient utilization of resources [64].

2.2.6 Current trend in electrolysis

Recent trends in electrolytic Mg metal production have focused on decreasing GHG emissions and improving resource efficiency through the integration of renewable energy sources and the effective utilization of by-products, such as Cl₂ gas. A representative example of this effort is the QSLM process [67, 68].

The QSLM process demonstrates a significant decrease in GHG emissions during Mg metal production while using the conventional electrolytic cell, the Norsk Hydro cell. By utilizing MgCl₂-rich salt, a by-product of potash production, as feedstock and integrating renewable energy sources, such as hydropower, solar power, and wind energy, the QSLM process achieves a substantial decrease in GHG emissions. Moreover, the Cl₂ gas generated during electrolysis, approximately 2.5 kg per 1 kg of Mg metal, is used to produce polyvinyl chloride at a nearby plant. Consequently, the overall GHG emissions for Mg metal production decreased to approximately 5.3 kg CO_2(eq) per kg of Mg metal.

3. Molten Salt Electrolysis of MgO

Mg metal production through electrolytic processes using anhydrous MgCl₂ feedstock generally generates lower GHG emissions than the Pidgeon process, which is the most widely used process for Mg metal production (see Table A-1 in Appendix) [9, 11, 38, 40–43, 64, 73–75, 77, 88–91]. Therefore, the electrolytic process is considered to be a more environmentally friendly method for producing Mg metal than the Pidgeon process.

However, as mentioned in section 2.1, the current electrolytic processes have disadvantages owing to the complex and energy-intensive anhydrous MgCl₂ feedstock production processes. The use of MgCl₂ brine decreased the complexity and energy consumption of feedstock production. However, toxic Cl₂ gas is still generated when Mg metal is produced via the electroreduction of MgCl₂.

To resolve the drawbacks of current electrolytic processes using anhydrous MgCl₂ feedstock, many studies have investigated the use of MgO as a feedstock. When MgO is used, complex and energy-intensive procedures for producing anhydrous MgCl₂ are not needed [8]. Notably, MgO can be produced by the calcination of Mg(OH)₂ obtained from seawater using NaOH from the chlor-alkali process, reducing GHG emissions through the use of electrical energy [92]. In addition, toxic Cl₂ gas is not generated during electrolysis, as shown in eq. (11). Furthermore, O₂ gas can be generated when an inert anode is used for electrolysis.

  

\begin{equation} \text{Cathode: Mg$^{2+}$ (in molten salt)} + \text{2 e$^{-}$} = \text{Mg ($l$ or $g$)} \end{equation} (11)

  

\begin{align*} &\text{Anode: $x$ O$^{2-}$ (in molten salt)} + \text{C ($s$)} \\ &\qquad \quad = \text{CO$_{x}$ ($g$)} + \text{2$x$ e$^{-}$}\ (x = 1, 2)\\ &\qquad \quad \text{or}\\ &\qquad \quad \text{O$^{2-}$ (in molten salt)} = \text{1/2 O$_{2}$ ($g$)} + \text{2 e$^{-}$} \end{align*}

Table 4 lists the previous studies on the production of Mg metal through electrolysis of MgO in molten salt [13–37, 104]. As shown in Table 4, the electrolysis of MgO was carried out in MgCl₂-based molten chloride salt or magnesium fluoride (MgF₂)-based molten fluoride salt, respectively.

Table 4 Previously investigated Mg metal production processes involving the electrolysis of MgO in MgCl₂-based and MgF₂-based molten salt [13–37, 104].

3.1 Mg metal production via electroreduction of MgO

In 1908, Kugelgen and Seward reported the electrolysis of MgO in MgF₂–CaF₂ molten salt using Fe cathode and C anode [23]. This research was further improved by Seward [24]. With these improvements, the American Magnesium Corporation commercially produced Mg metal through the electrolysis of MgO in molten fluoride salt (MgF₂–barium fluoride (BaF₂)–sodium fluoride (NaF)) at 1223 K [93–95]. However, this process showed a relatively low current efficiency, ranging from 50% to 60% [96].

In 2005, Pal et al. reported the SOM process in which the electrolysis of MgO was carried out in MgF₂–CaF₂ molten salt using Fe cathode and Ag anode at 1373–1423 K [13]. Notably, in the SOM process, an yttria-stabilized zirconia (YSZ) membrane was used to hold the liquid Ag anode, as shown in Fig. 12. As a result, only oxygen ions (O²⁻) that pass the YSZ membrane are oxidized at the liquid Ag anode to generate O₂ gas during the electrolysis. This also prevents recombination of the O₂ gas generated and Mg metal produced during electrolysis. The Mg metal produced during electrolysis evaporates because of the high electrolysis temperature and condenses in a separate chamber.

Fig. 12

Schematic of the SOM process (adapted from Ref. [13]).

Palumbo et al., in 2015, reported a solar thermal electrolytic process [28], based on a previous study [27]. The process produced Mg metal through the electrolysis of MgO in MgF₂–CaF₂ molten salt using Fe cathode and C anode at 1250 K. Utilizing solar energy significantly decreased the GHG emissions and energy required for Mg metal production. In addition, the process showed a current efficiency higher than 80% under certain conditions. Nevertheless, in many cases, the current efficiency was as low as approximately 60%.

The developed processes show that the production of Mg metal through the molten salt electrolysis of MgO is feasible, along with the generation of O₂ gas when an inert anode is used. However, these processes have a challenge in that the current efficiencies are low.

In general, one of the main reasons for low current efficiency is the generation of electronic current in the molten salt, which occurs because of the dissolution of the metal in the molten salt [14, 28, 37]. Powell et al. reported the presence of electronic current during the electrolysis of MgO in molten fluoride salt by measuring the electronic transference number [14]. Therefore, the electronic current caused by the dissolution of the produced Mg metal in molten fluoride salt is considered to cause a decrease in current efficiency.

To resolve the decrease in current efficiency caused by the dissolution of produced Mg metal in molten salt, several strategies have been conducted. In the SOM process, decreasing the partial pressure of Mg metal in the molten salt by blowing argon (Ar) gas at the cathode [16], lowering the total pressure of the electrolytic cell to 0.08 atm [17], oxidation of Mg metal dissolved in the molten salt by the addition of iron oxide (Fe₂O₃), or periodic shorting of electrolysis [18] had been implemented. Consequently, current efficiencies of greater than 75% were obtained in some cases. The commercialization of the SOM process is still in progress.

3.2 Mg alloy production via electroreduction of MgO

To mitigate the dissolution of the produced Mg metal in the molten salt during electrolysis, the production of Mg alloy through the molten salt electrolysis of MgO was also investigated. This is because the solubility of the metal in the molten salt can be suppressed when the activity of the metal is decreased by alloying [97], according to eqs. (12) and (13).

  

\begin{equation} \text{Mg (cathode)} = \text{Mg (in molten salt)} \end{equation} (12)

  

\begin{equation} K_{\text{d}} = a_{\text{Mg (in molten salt)}}/a_{\text{Mg (cathode)}} \end{equation} (13)

Equation (12) describes the equilibrium state where Mg metal produced at the cathode begins to equilibrate with the molten salt. Equation (13) defines K_d, the equilibrium constant of the dissolution reaction, where a_{Mg (in molten salt)} is the activity of the Mg metal dissolved in the molten salt and a_{Mg (cathode)} is the activity of the Mg metal produced at the cathode. According to eq. (13), the activity of the Mg metal dissolved in the molten salt decreases by lowering the activity of the Mg metal produced at the cathode. Consequently, the production of Mg metal as an alloy decreases the dissolution of Mg metal in the molten salt.

Furthermore, Mg metal can be separated from the alloy by utilizing its thermodynamic properties such as high vapor pressure at elevated temperatures. Therefore, the production of Mg metal from MgO is feasible, even though Mg alloy is produced during electrolysis. Table 4 shows that various metals used as a cathode alloy with produced Mg metal at the electrolysis temperature. In addition, investigation of the production of Mg alloys through co-electroreduction of MgO and oxide of alloying element was also conducted.

The production of Mg alloy through the electrolysis of MgO in molten chloride salt was first proposed by Yerkes. In 1947, Yerkes conducted the electrolysis of MgO in MgCl₂ molten salt with the addition of boron oxide (B₂O₃) by applying 50–135 A using liquid lead (Pb) cathode and C anode at 1087–1149 K [20]. Because of the higher density of the liquid Pb cathode compared to that of the molten salt, the Mg–Pb alloy was produced at the bottom of the electrolytic cell. Notably, the structure of the electrolytic cell used was similar to that of the Hall-Héroult cell used for commercial Al metal production.

Gao et al., in 2012, conducted the electrolysis of MgO in 23 mass% MgCl₂–67 mass% KCl–10 mass% rare earth chloride (ReCl₃) molten salt with a cathodic current density of up to 0.8 A·cm⁻² using liquid Al cathode and C anode at 1023–1033 K [25]. In this process, Mg–Al alloy was produced at the top of the molten salt owing to the lower density of the Mg alloy compared to that of the molten salt.

In the experiments conducted by Yerkes, the current efficiency of electrolysis was not reported. However, Gao et al. reported a current efficiency of 81.3% when 1–6 mass% Mg–Al alloy was produced. Therefore, the electrolysis of MgO in molten chloride salt shows a relatively high current efficiency when producing Mg alloy.

However, when the electrolysis of MgO is conducted in MgCl₂-based molten chloride salt, the molten salt may decompose. This is because the difference between the theoretical decomposition voltages of MgO and the chlorides is not large, as shown in Fig. 7 [72]. Indeed, Yerkes reported that Cl₂ gas was detected in the gases generated at the anode during electrolysis [20].

To prevent the generation of Cl₂ gas due to the decomposition of the molten chloride salt, the electrolysis of MgO in the MgF₂-based molten fluoride salt was suggested. This is because fluorides generally have higher theoretical decomposition voltages than chlorides, as shown in Fig. 7 [72]. In 1908, Kugelgen and Seward reported the electrolysis of MgO in MgF₂-based molten salt using liquid Al cathode and C anode [19]. They are expected to produce Mg–Al alloy during electrolysis. However, specific details of the electrolysis, such as the composition of the molten salt and the electrolysis temperature, were not provided.

Hard et al., in 1970, conducted the electrolysis of MgO in approximately 70 mass% MgF₂–30 mass% BaF₂ molten salt using high-density liquid Fe·Si cathode and C anode with a cathodic current density of 0.40–0.77 A·cm⁻² at 1573 K to produce Mg–Fe·Si alloy at the bottom of the electrolytic cell [21]. Later, in 1981, Hard proposed a method for recovering Mg metal from molten Mg alloy by passing an inert gas such as Ar [98]. However, Hard et al. reported current efficiencies as low as 60%. Because the Mg–Fe·Si alloy was produced at the bottom of the electrolytic cell, the loss of Mg metal due to the recombination reaction and dissolution in the molten salt was expected to be insignificant. Therefore, it is considered that the current efficiency is low because Mg metal evaporates from the alloy at the high electrolysis temperature of 1573 K.

Gao et al. conducted the electrolysis of MgO in 40 mass% MgF₂–30 mass% lithium fluoride (LiF)–30 mass% KCl molten salt or MgF₂–LiF–BaF₂ molten salt using liquid Al cathode and C anode at temperatures of 1053 K and 1123 K, respectively [25, 26]. In this process, the use of a liquid Al cathode, which is less dense than the molten salt, led to the production of Mg–Al alloy at the top of the molten salt. In addition, a current efficiency of 87.7% was obtained after electrolysis under certain conditions. However, in some cases, the current efficiency was as low as approximately 67%. The large difference in the current efficiency is probably due to the vulnerability of the Mg alloy to contact with the gases generated at the anode, considering the structure of the cell used, as shown in Fig. 13. Meanwhile, they also did not suggest a method for recovering Mg metal from the produced Mg–Al alloy.

Fig. 13

Schematic of the electrolytic cell used by Gao et al. (adapted from Ref. [26]).

Under these circumstances, Kang et al. developed a novel Mg metal production process. Figures 14 and 15 show the flowchart and schematic of the developed process, respectively [30]. In this process, the electrolysis of MgO was conducted in 54 mass% MgF₂–46 mass% LiF molten salt using a high-density metal cathode, such as Sn, Cu, or Ag, at 1053–1083 K, to produce Mg alloy at the bottom of the electrolytic cell. Subsequently, Mg metal was recovered from the produced Mg alloy through vacuum distillation at 1200–1300 K [30].

Fig. 14

Flowchart of the novel Mg metal production process (adapted from Ref. [30]).

Fig. 15

Schematic of the novel Mg metal production process (adapted from Ref. [30]).

Notably, Kang et al. conducted fundamental research on the development of a Mg metal production process with thermodynamic considerations. Considering that Mg metal starts to evaporate actively when its vapor pressure approaches 0.01 atm under atmospheric pressure [99, 100], the isobaric line of the vapor pressure of Mg metal was plotted on the binary phase diagrams of the Mg–Sn, Mg–Cu, and Mg–Ag systems, as shown in Fig. 16 [30, 101]. Using Fig. 16, the maximum concentration of Mg metal electrodeposited on the Sn, Cu, and Ag cathodes to decrease the loss of Mg metal due to evaporation at the electrolysis temperature was determined. Consequently, they produced Mg alloys of 18.2–20.1 mass% Mg with high current efficiencies of 82.2–88.0%, even under 75.0–95.0 h of electrolysis [31].

Fig. 16

Binary phase diagrams of (a) Mg–Sn, (b) Mg–Cu, and (c) Mg–Ag systems with the isobaric line of the vapor pressure of Mg as a function of temperature and Mg concentration (adapted from Ref. [30]).

By using Fig. 16, the temperature for vacuum distillation was also determined. When the pressure of the system becomes lower than 1 atm, the vapor pressure required for Mg to evaporate decreases to below 0.01 atm, i.e., 0.0001–0.001 atm. Therefore, they expected to recover a sufficient amount of Mg metal from the Mg alloy via vacuum distillation at 1200–1300 K. Consequently, they recovered Mg metal from the Mg alloy produced after electrolysis through vacuum distillation with a recovery efficiency of above 99.4%, under certain conditions [34]. Notably, the recovered Mg metal had a purity of 99.999%, under certain conditions [30, 32, 34].

Recently, Kang et al. investigated the feasibility of Mg metal production directly from primary and secondary MgO resources using the developed process [35]. For the primary MgO resources, calcined magnesite, seawater MgO clinker, and MgO·CaO were used. For the secondary MgO resources, oxidized MgO-C refractory brick, ferronickel slag, and aged ferronickel slag were used. When primary MgO resources were electrolyzed, Mg alloys were produced with current efficiencies of 88.6–92.4%. When secondary MgO resources were electrolyzed, Mg alloys were produced with current efficiencies of 78.1–92.3%, under certain conditions. After vacuum distillation of the Mg alloys produced, Mg metal with a purity of 99.999% was obtained with a recovery efficiency of 95.3%, under certain conditions.

It is worth noting that the effectiveness of the electrolysis of MgO using a liquid Sn cathode in molten fluoride salt was recently verified by another researcher. Powell et al. conducted the electrolysis of MgO in MgF₂–CaF₂ molten salt with a eutectic composition using liquid Sn cathode and C or Ag anode at 1323 K, as shown in Fig. 17 [37]. When the Ag anode was used, a YSZ membrane in the SOM process was utilized, and yttrium oxide (Y₂O₃) was added to the molten salt. Powell et al. reported that Mg–Sn alloy was produced at the bottom of the electrolytic cell with a current efficiency above 84%, regardless of the anode material [37].

Fig. 17

Schematic of the electrolytic cell used by Powell et al. (adapted from Ref. [37]).

In this process, a current efficiency exceeding 84% was achieved during the production of Mg–Sn alloy containing up to 2 mass% Mg at 1323 K. The high current efficiency at 1323 K is thought to be due to the low vapor pressure of Mg in the Mg–Sn alloy, which contains up to 2 mass% Mg. At this Mg concentration, the vapor pressure of Mg in the Mg–Sn alloy is below 0.003 atm [72, 101], which is lower than 0.01 atm. Consequently, the evaporation-drived loss of Mg metal from the alloy was prevented, even at 1323 K.

Meanwhile, Powell et al. used a gravity-driven multiple-effect thermal system (G-METS) to recover Mg metal from Mg alloy. The G-METS was developed to reduce the energy use and cost of the batch-type distillation process, and it can decrease the energy consumption by up to 90% compared to the conventional distillation process [102, 103]. The G-METS is still in the stage of the research.

3.3 Mg alloy production via co-electroreduction with MgO

Mg alloy production through co-electroreduction with MgO in MgF₂-based molten fluoride salt has rarely been investigated, as shown in Table 4. In 1908, Kugelgen and Seward reported the production of Mg–Al alloy through the electrolysis of a mixture of MgO and Al₂O₃ in MgF₂-based molten salt [19]. They reported that Al₂O₃ dissolves easily in the molten fluoride salt. However, they did not include specific details of electrolysis.

Powell et al. conducted the electrolysis of MgO and neodymium oxide (Nd₂O₃) in 6.6 mass% MgF₂–28.4 mass% LiF–65 mass% neodymium fluoride (NdF₃) molten salt using tungsten (W) cathode and proprietary anode at 1123 K [104]. The material used as an anode was not reported. Notably, NdF₃ was added to increase the solubility of Nd₂O₃. After the electrolysis of MgO and Nd₂O₃, Mg–Nd alloys of 50–55 mass% Mg were produced with current efficiencies of 85–95%. They produced approximately 250 kg of Mg–4 mass% Al–2 mass% Nd alloy (AE42) from the produced 30 kg of Mg–Nd master alloy [104].

4. Summary and Conclusions

At present, approximately 85% of global Mg metal production occurs in China using a silicothermic reduction process, the Pidgeon process. However, the Pidgeon process is energy-intensive and has a high global warming impact. Electrolytic processes using MgCl₂ feed are less energy-intensive and have a lower global warming impact. However, the production of the MgCl₂ feed remains energy-intensive, and Cl₂ gas is generated during the process. Accordingly, technological improvements, such as the integration of renewable energy sources and utilization of by-products, have been made in industrial primary Mg metal production processes. However, the current industrial processes still have disadvantages from an environmental perspective. Consequently, extensive research has focused on developing environmentally sustainable Mg metal production processes. Currently, a promising alternative is the electrolytic process using MgO feed in molten fluoride salt because it eliminates the need for anhydrous MgCl₂ production and does not generate toxic Cl₂ gas during electrolysis. The SOM process and electrolytic process using a high-density metal cathode followed by vacuum distillation have been extensively investigated and are recognized as environmentally sound approaches for Mg metal production.

Acknowledgments

This work was supported by the New Faculty Startup Fund from Seoul National University and the Korea Institute of Energy Technology Evaluation and Planning (KETEP) (Project No.: 20024463) funded by the Ministry of Trade, Industry & Energy (MOTIE) of the Republic of Korea.

REFERENCES

• 1)   Y.  Yang,  X.  Xiong,  J.  Chen,  X.  Peng,  D.  Chen and  F.  Pan: Research advances in magnesium and magnesium alloys worldwide in 2020, J. Magnes. Alloy. 9 (2021) 705–747. doi:10.1016/j.jma.2021.04.001
• 2)   S.V.S.  Prasad,  S.B.  Prasad,  K.  Verma,  R.K.  Mishra,  V.  Kumar and  S.  Singh: The role and significance of Magnesium in modern day research-A review, J. Magnes. Alloy. 10 (2022) 1–61. doi:10.1016/j.jma.2021.05.012
• 3)   G.S.  Cole and  A.M.  Sherman: Light weight materials for automotive applications, Mater. Charact. 35 (1995) 3–9. doi:10.1016/1044-5803(95)00063-1
• 4)  “Magnesium Metal”. U.S. Geological Survey. https://pubs.usgs.gov/periodicals/mcs2024/mcs2024-magnesium-metal.pdf, (accessed 2024-12-31).
• 5)  L.H. Prentice and N. Haque: Magnesium Technology 2012, (Springer, Cham, 2012) pp. 37–41.
• 6)  “Carbon footprint of magnesium production and its use in transport applications”. International Magnesium Association. https://cdn.ymaws.com/www.intlmag.org/resource/resmgr/sustainability/LCA-update-summary-2020-Oct-.pdf, (accessed 2024-12-31).
• 7)  “Steel in the circular economy: A life cycle perspective”. World Steel Association. https://canadiansteel.ca/files/resources/Steel_20in_20the_20circular_20economy_20-_20A_20life_20cycle_20perspective.pdf, (accessed 2024-12-31).
• 8)   S.  Das: Primary magnesium production costs for automotive applications, JOM 60 (2008) 63–69. doi:10.1007/s11837-008-0151-7
• 9)  H.E. Friedrich and B.L. Mordike: Magnesium Technology-Metallurgy, Design Data, Applications, (Springer, Berlin, 2006).
• 10)   L.M.  Pidgeon and  W.A.  Alexander: Trans. Am. Inst. Min. Met. Eng. 159 (1944) 315–352.
• 11)   D.  Hou,  L.  Liu,  Y.  Ke,  X.  Zhang,  Q.  Yang,  H.  Qui and  Q.  Yang: Thermodynamic study of magnesium (Mg) production by the Pidgeon process: The distribution of MgO and axial thermal field, Appl. Therm. Eng. 236 (2024) 121885. doi:10.1016/j.applthermaleng.2023.121885
• 12)  “IMA launches new magnesium LCA study-setting sight on lowering CO₂ emissions”. Light Metal Age. https://cdn.ymaws.com/www.intlmag.org/resource/resmgr/sustainability/LMA_FEB_2021_IMA-reprint.pdf, (accessed 2024-12-31).
• 13)   A.  Krishnan,  U.B.  Pal and  X.G.  Lu: Solid oxide membrane process for magnesium production directly from magnesium oxide, Metall. Mater. Trans. B 36 (2005) 463–473. doi:10.1007/s11663-005-0037-9
• 14)   X.  Guan,  U.B.  Pal and  A.C.  Powell: Energy-Efficient and Environmentally Friendly Solid Oxide Membrane Electrolysis Process for Magnesium Oxide Reduction: Experiment and Modeling, Metall. Mater. Trans. E 1 (2014) 132–144. doi:10.1007/s40553-014-0013-x
• 15)   X.  Guan and  U.B.  Pal: Design of optimum solid oxide membrane electrolysis cells for metals production, Prog. Nat. Sci.: Mater. Int. 25 (2015) 591–594. doi:10.1016/j.pnsc.2015.11.004
• 16)  E. Gratz, S. Pati, J. Milshtein, A. Powell and U. Pal: Electrometallurgy 2012, (John Wiley & Sons, New Jersey, 2012) pp. 111–118.
• 17)   E.  Gratz,  X.  Guan,  J.D.  Milshtein,  U.B.  Pal and  A.C.  Powell: Mitigating Electronic Current in Molten Flux for the Magnesium SOM Process, Metall. Mater. Trans. B 45 (2014) 1325–1336. doi:10.1007/s11663-014-0060-9
• 18)   X.  Guan,  S.  Su,  U.B.  Pal and  A.C.  Powell: Periodic Shorting of SOM Cell to Remove Soluble Magnesium in Molten Flux and Improve Faradaic Efficiency, Metall. Mater. Trans. B 45 (2014) 2138–2144. doi:10.1007/s11663-014-0142-8
• 19)  F.V. Kugelgen and G.O. Seward: U.S. Patent No. 881,934, (1908).
• 20)  L.A. Yerkes: U.S. Patent No. 2,431,723, (1947).
• 21)  R.A. Hard, J.E. Wells III and D.J. Hansen: U.S. Patent No. 3,503,857, (1970).
• 22)   R.A.  Sharma: A new electrolytic magnesium production process, JOM 48 (1996) 39–43. doi:10.1007/BF03223100
• 23)  F.V. Kugelgen and G.O. Seward: U.S. Patent No. 880,489, (1908).
• 24)  G.O. Seward: U.S. Patent No. 1,310,450, (1919).
• 25)   F.L.  Yang,  S.H.  Yang,  Z.W.  Wang,  Z.N.  Shi and  B.L.  Gao: Preparation of Aluminum-Magnesium Alloys from Magnesium Oxide by Molten Salt Electrolysis Method, Mater. Sci. Forum 610–613 (2009) 874–879. doi:10.4028/www.scientific.net/MSF.610-613.874
• 26)  F. Yang, S. Yang, X. Hu, Z. Wang, Z. Shi and B. Gao: Supplemental Proceedings: General Paper Selections, Volume 3, (The Minerals, Metals & Materials Society, 2011) pp. 183–186.
• 27)   W.  Sheline,  L.  Matthews,  N.  Lindeke,  S.  Duncan and  R.  Palumbo: An exploratory study of the solar thermal electrolytic production of Mg from MgO, Energy 51 (2013) 163–170. doi:10.1016/j.energy.2012.11.053
• 28)  R. Palumbo et al.: Magnesium Technology 2015, (Springer, Cham, 2015) pp. 43–48.
• 29)  S. Yang, L. Wu, F. Yang, M. Li, X. Hu, Z. Wang, Z. Shi and B. Gao: Magnesium Technology 2012, (Springer, Cham, 2012) pp. 63–66.
• 30)   T.-H.  Lee,  T.H.  Okabe,  J.-Y.  Lee,  Y.M.  Kim and  J.  Kang: Molten Salt Electrolysis of Magnesium Oxide Using a Liquid–Metal Cathode for the Production of Magnesium Metal, Metall. Mater. Trans. B 51 (2020) 2993–3006. doi:10.1007/s11663-020-01976-9
• 31)   D.-H.  Lee,  H.-J.  Jeoung,  T.-H.  Lee,  K.W.  Yi,  J.-Y.  Lee,  Y.M.  Kim,  T.H.  Okabe and  J.  Kang: Scale-Up Study of Molten Salt Electrolysis using Cu or Ag Cathode and Vacuum Distillation for the Production of High-Purity Mg Metal from MgO, J. Sustain. Metall. 7 (2021) 883–897. doi:10.1007/s40831-021-00367-x
• 32)   T.-H.  Lee,  T.H.  Okabe,  J.-Y.  Lee,  Y.M.  Kim and  J.  Kang: Development of a novel electrolytic process for producing high-purity magnesium metal from magnesium oxide using a liquid tin cathode, J. Magnes. Alloy. 9 (2021) 1644–1655. doi:10.1016/j.jma.2021.01.004
• 33)   H.-J.  Jeoung,  Y.  Kim,  T.-H.  Lee,  J.-Y.  Lee,  Y.M.  Kim,  K.W.  Yi and  J.  Kang: A novel electrolytic process using a Cu cathode for the production of Mg metal from MgO, J. Appl. Electrochem. 52 (2022) 1535–1549. doi:10.1007/s10800-022-01725-7
• 34)   H.-J.  Jeoung,  T.-H.  Lee,  J.-Y.  Lee,  K.W.  Yi and  J.  Kang: An Electrolytic Process Using an Ag Cathode and Vacuum Distillation for Mg Metal Production from MgO, J. Sustain. Metall. 9 (2023) 688–699. doi:10.1007/s40831-023-00669-2
• 35)   H.-J.  Jeoung,  T.-H.  Lee,  Y.  Kim,  J.-Y.  Lee,  Y.M.  Kim,  T.H.  Okabe,  K.W.  Yi and  J.  Kang: Use of various MgO resources for high-purity Mg metal production through molten salt electrolysis and vacuum distillation, J. Magnes. Alloy. 11 (2023) 562–579. doi:10.1016/j.jma.2022.07.009
• 36)   H.-J.  Jeoung,  T.-H.  Lee,  J.-Y.  Lee,  K.W.  Yi and  J.  Kang: Production of high-purity Mg metal from dolomite through novel molten salt electrolysis and vacuum distillation, J. Magnes. Alloy. 11 (2023) 1308–1320. doi:10.1016/j.jma.2022.10.007
• 37)   A.E.  Telgerafchi,  M.  Rutherford,  G.  Espinosa,  D.  McArthur,  N.  Masse,  B.  Perrin,  Z.  Tang and  A.C.  Powell: Magnesium production by molten salt electrolysis with liquid tin cathode and multiple effect distillation, Front. Chem. 11 (2023) 1192202. doi:10.3389/fchem.2023.1192202
• 38)  G.J. Kipouros and D.R. Sadoway: Advances in Molten Salt Chemistry, (Elsevier, Amsterdam, 1987) pp. 127–209.
• 39)   G.J.  Kipouros and  D.R.  Sadoway: A thermochemical analysis of the production of anhydrous MgCl₂, J. Light Met. 1 (2001) 111–117. doi:10.1016/S1471-5317(01)00004-9
• 40)  O. Takeda, T. Uda and T.H. Okabe: Treatise on Process Metallurgy, Volume 3: Industrial Processes, (Elsevier, 2014) pp. 995–1069.
• 41)  R.E. Kirk and D.F. Othmer: Kirk-Othmer Encyclopedia of Chemical Technology, fourth ed., (Wiley & Sons, 1995).
• 42)  A.M. Cameron, A.V. Hatten and V.G. Aurich: Magnesium Technology: Proceedings of the Conference, (Institute of Metals, 1987) pp. 7–17.
• 43)  F. Habashi: Handbook of Extractive Metallurgy, Vol. 2, (VCH Verlagsgesellschaft mbH, Weinheim, 1997) pp. 981–1038.
• 44)  G. Demirci: Dissertation, Middle East Technical University, (2006).
• 45)  E. Aghion and S.C. Bartos: 65th Annual World Magnesium Conference, (2008) pp. 97–104.
• 46)  “A survey of electrochemical metal winning processes”. Battelle Memorial Institute, Columbus Laboratories. https://www.osti.gov/servlets/purl/6063735, (accessed 2024-12-31).
• 47)  E.N. Brandt: We Called It Mag-nificent: Dow Chemical and Magnesium, 1916–1998, (Michigan State University Press, Michigan, 2013).
• 48)   W.J.  Rave: Magnesium production in the southwest, J. Electrochem. Soc. 100 (1953) 179–181.
• 49)  Institute of Metals: J. Am. Soc. Nav. Eng. 69 (1957) 81–94.
• 50)  I.J. Polmear: ASM Specialty Handbook: Magnesium and Magnesium Alloys, (ASM International, USA, 1999) pp. 3–6.
• 51)  R. Neelameggham: Fundamentals of Magnesium Alloy Metallurgy, (Elsevier, 2013) pp. 1–32.
• 52)  P.D.V. Manning: Metal of the future, https://calteches.library.caltech.edu/95/1/Manning.pdf, (accessed 2024-12-31).
• 53)  A. Hafeez: Dissertation, McGill University, Montreal (1979).
• 54)  M.C. Flemings, G.B. Kenney, D.R. Sadoway, J.P. Clark and J. Szekely: DOE/CS/40284-T1, (Massachusetts Inst. of Technology 1981) (online).
• 55)  P.M.D. Pinfold: Extraction, Refining and Fabrication of Light Metals, (Pergamon Press, 1991) pp. 43–53.
• 56)  M. Pourbaix: Atlas of Electrochemical Equilibria in Aqueous Solutions, (National Association of Corrosion Engineers, 1974).
• 57)  K. Adham, C. Lee and K. O’Keefe: https://www.researchgate.net/publication/346014587, (accessed 2024-12-31).
• 58)  C. Celik, J. Peacey, G. Bishop, C. White, E. Giasson and M. Avedesian: Proceedings of the Third International Magnesium Conference, ed. by G.W. Lorimer, (INST MATERIALS, London, 1996) pp. 19–31.
• 59)  “Magnesium, Its Alloys and Compounds”. U.S. Geological Survey. https://pubs.usgs.gov/of/2001/of01-341/of01-341.pdf, (accessed by 2024-12-31).
• 60)  J.E. Dutrizac, T.T. Chen and C.W. White: Magnesium Technology 2000, (Wiley-TMS, 2000) pp. 41–52.
• 61)  M. Lamy: Dissertation, McGill University, Montreal, (2002).
• 62)   P.  Ficara,  E.  Chin,  T.  Walker,  D.  Laroche,  E.  Palumbo,  C.  Celik and  M.  Avedesian: Magnola : A novel commercial process for the primary production of magnesium, CIM Bull. 91 (1998) 75–80.
• 63)  “Magnesium extraction from asbestos mine tailings: a report”. Vermont Geological Survey Department of Environmental Conservation. https://anrweb.vt.gov/PubDocs/DEC/GEO/TechReports/VGTR1998-1Magnesium.pdf, (accessed 2024-12-31).
• 64)   G.C.  Holywell: Magnesium: The first quarter millennium, JOM 57 (2005) 26–33. doi:10.1007/s11837-005-0248-1
• 65)   T.G.  Tripp: Production of magnesium from Great Salt Lake, Utah USA, Nat. Resour. Environ. Issues 15 (2009) 54–61.
• 66)   R.L.  Thayer and  R.  Neelameggham: Improving the electrolytic process magnesium production, JOM 53 (2001) 15–17. doi:10.1007/s11837-001-0128-2
• 67)  “Die Magnesiumelektrolyse in Golmud (China)”. MAGONTEC. https://www.bgr.bund.de/DERA/DE/Downloads/vortrag_magnesium_gruber.pdf?__blob=publicationFile&v=2, (accessed 2024-12-31).
• 68)  “The world’s greenest magnesium alloy producer”. MAGONTEC. https://magontec.com/wp-content/uploads/2018/02/Magontec-Brochure_FINAL_Web2_SinglePages.pdf, (accessed 2024-12-31).
• 69)   H.-C.  Eom,  H.  Park and  H.-S.  Yoon: Preparation of anhydrous magnesium chloride from ammonium magnesium chloride hexahydrate, Adv. Powder Technol. 21 (2010) 125–130. doi:10.1016/j.apt.2010.01.003
• 70)  A. Ketil, K.A. Terje, B. Per, R.E. Hans, O.H. Johanna, N. Carlos, R. Christian, V.B. Sia and W. Oddmund: Ullmann’s Encyclopedia of Industrial Chemistry, (Wiley-VCH, Weinheim, 2012).
• 71)  F. Cardarelli: Materials Handbook-A Concise Desktop Reference, second ed., (Springer, London, 2008).
• 72)  I. Barin: Thermochemical Data of Pure Substances, third ed., (VCH Verlagsgesellschaft mbH, Weinheim, 1995).
• 73)  L.M. Pidgeon: Magnesium-Extractive Metallurgy, (American Society for Metals, 1946).
• 74)  W. Wulandari, A. Rhamdhani, G. Brooks and B.J. Monaghan: Proceedings of European Metallurgical Conference, (2009) pp. 1401–1415.
• 75)  Magnesium Metal: Global Industry, Markets and Outlook to 2020, twelfth ed., (Roskill Information Services Ltd., London, 2016).
• 76)  “Guidance by source category, Source Categories Magnesium production, part III”, P. Finaly. https://chm.pops.int/Portals/0/docs/from_old_website/documents/meetings/bat_bep/2nd_session/egb2_followup/draftguide/6B3MagnesiumproductionDRAFT.pdf, (accessed 2024-12-31).
• 77)  S. Ehrenberger, S. Schmid, S. Song and H.E. Friedrich: Proceedings of the 65th Annual World Magnesium Conference, (2008).
• 78)  G.M. Haarberg: Encyclopedia of Applied Electrochemistry, (Springer, New York, 2014) pp. 1210–1213.
• 79)  G. Shekhovtsov, V. Shchegolev, V. Devyatkin, A. Tatakin and I. Zabelin: Essential Readings in Magnesium Technology, (Springer, Cham, 2016) pp. 97–100.
• 80)   N.  Höy-Petersen: Some aspects of the electrolytic production of magnesium in IG cells, JOM 21 (1969) 43–49. doi:10.1007/BF03378785
• 81)   J.W.  Evans: The evolution of technology for light metals over the last 50 years: Al, Mg, and Li, JOM 59 (2007) 30–38. doi:10.1007/s11837-007-0020-9
• 82)  S.-Y. Yoon: Dissertation, Massachusetts Institute of Technology, (1987).
• 83)  K.A. Andreassen and K.B. Stiansen: U.S. Patent No. 3,907,651, (1975).
• 84)  O. Wallevik, K. Amundsen, A. Faucher and T. Mellerud: Essential Readings in Magnesium Technology, (Springer, Cham, 2016) pp. 93–96.
• 85)  H. Eklund, P.B. Engseth, B. Langseth, T. Mellerud and O. Wallevik: Essential Readings in Magnesium Technology, (Springer, Cham, 2016) pp. 141–144.
• 86)   R.  Soper: Going forward with the Dow Magnesium technology, Miner. Process. Extr. Metall.: Trans. Inst. Min. Metall. 111 (2002) 56–61. doi:10.1179/mpm.2002.111.2.56
• 87)  O.G. Sivilotti: U.S. Patent No. 4,514,269, (1985).
• 88)  M. Buğdayci, A. Turan, M. Alkan and O. Yücel: Magnesium and Its Alloys, (CRC Press, Boca Raton, 2019) pp. 47–56.
• 89)  L. Wu, F. Han and G. Liu: Comprehensive Utilization of Magnesium Slag by Pidgeon Process, (Springer, Singapore, 2021).
• 90)   S.  Ramakrishnan and  P.  Koltun: Global warming impact of the magnesium produced in China using the Pidgeon process, Resour. Conserv. Recycling 42 (2004) 49–64. doi:10.1016/j.resconrec.2004.02.003
• 91)   D.  Fu,  Y.  Wang,  T.  Zhang and  N.  Feng: Review on the Silicothermic Process for Primary Magnesium Production, Metall. Mater. Trans. B 54 (2023) 1–21. doi:10.1007/s11663-022-02695-z
• 92)  A.C. Powell: Magnesium Technology 2022, (Springer, Cham, 2022) pp. 121–126.
• 93)  R.E. Brown: Essential Readings in Magnesium Technology, (Springer, Cham, 2016) pp. 3–12.
• 94)  McGRAW-HILL company, Inc.: Eng. Min. J. 112 (1921) 460.
• 95)   Y.  Fushiya,  K.  Mori and  H.  Imamura: Determination of Freezing Points of the System MgF₂-BaF₂-CaF₂, Denki Kagaku 1 (1933) 109–115 (in Japanese). doi:10.5796/denka.1.109
• 96)  R.L. Martin: Dissertation, Oklahoma State University, (1957).
• 97)   H.  Kim,  D.A.  Boysen,  J.M.  Newhouse,  B.L.  Spatocco,  B.  Chung,  P.J.  Burke,  D.J.  Bradwell,  K.  Jiang,  A.A.  Tomaszowska,  K.  Wang,  W.  Wei,  L.A.  Ortiz,  S.A.  Barriga,  S.M.  Poizeau and  D.R.  Sadoway: Liquid Metal Batteries: Past, Present, and Future, Chem. Rev. 113 (2013) 2075–2099. doi:10.1021/cr300205k
• 98)  R.A. Hard: U.S. Patent No. 4,298,437, (1981).
• 99)   J.  Kang and  T.H.  Okabe: Thermodynamic Consideration of the Removal of Iron from Titanium Ore by Selective Chlorination, Metall. Mater. Trans. B 45 (2014) 1260–1271. doi:10.1007/s11663-014-0061-8
• 100)   T.H.  Okabe,  S.  Yamamoto,  Y.  Kayanuma and  M.  Maeda: Recovery of platinum using magnesium vapor, J. Mater. Res. 18 (2003) 1960–1967. doi:10.1557/JMR.2003.0272
• 101)   C.W.  Bale et al.: FactSage thermochemical software and databases, 2010–2016, Calphad 54 (2016) 35–53. doi:10.1016/j.calphad.2016.05.002
• 102)  M. Rutherford, A.E. Telgerafchi, G. Espinosa, A.C. Powell and D. Dussault: Magnesium Technology 2021, (Springer, Cham, 2021) pp. 139–144.
• 103)  A.E. Telgerafchi, G. Espinosa, M. Rutherford, A.C. Powell and D. Dussault: Magnesium Technology 2021, (Springer, Cham, 2021) pp. 145–152.
• 104)  A.C. Powell IV: INFINIUM-3008-VTP-Mg-Final-T2V3, (INFINIUM, Inc. 2018) (online).

Appendix

Fig. A-1

Schematic of the Dow cell (adapted from Ref. [43]).

Fig. A-2

Flowchart of the electrolytic Mg metal production process that uses anhydrous MgCl₂ feedstock obtained from serpentine.

Fig. A-3

Flowchart of the electrolytic Mg metal production process that uses anhydrous MgCl₂ feedstock obtained from carnallite.

Fig. A-4

Schematic of the I.G. Farben chlorinator (adapted from Ref. [43]).

Table A-1 Summary of the Pidgeon process [9, 11, 38, 40–43, 64, 73–75, 77, 88–91].