International Journal of Minerals, Metallurgy and Materials

Accepted manuscript, https://doi.org/10.1007/s12613-020-1971-x © University of Science and Technology Beijing and Springer-Verlag GmbH Germany, part of Springer Nature 2020

A review on liquid metal as cathode for molten salt/oxide electrolysis

Shu-qiang Jiao1,2), Han-dong Jiao2), Wei-li Song2), Ming-yong Wang1), and Ji-guo Tu1)

1) State Key Laboratory of Advanced Metallurgy, University of Science and Technology Beijing,

Beijing 100083, China

2) Institute of Advanced Structure Technology, Beijing Institute of Technology, Beijing 100081,

China

Corresponding author’s E-mails: sjiao@ustb.edu.cn (Shu-qiang Jiao)

Authors’ E-mail: jiaohandong_new@163.com (Han-dong Jiao), weilis@bit.edu.cn (Wei-li Song),

mywang@ustb.edu.cn (Ming-yong Wang), guo15@126.com (Ji-guo Tu)

Abstract: Compared with solid metals, liquid metals are considered as promising cathodes for

molten slat/oxide electrolysis due to many fascinating advantages, such as significant

depolarization effect, strong alloying effect, excellent selective separation and low operation

temperature properties. After briefly introducing the properties of the liquid metal cathodes and

their selection rules, in this review, we summarize the development of the liquid metal cathodes in

the molten salt electrolysis, specifically in the titanium extraction, separation of actinides and rare

earth metals in the halide melts. We review the recent attractive progress in preparation of liquid

titanium alloys via molten oxide electrolysis, which is configured with liquid metal cathodes.

Nevertheless, there are still some problems with the high-quality alloy production and large-scale

applications, and therefore several research directions were discussed for further improving the

quality of alloys, aiming to realize the industrial applications of the liquid metal cathodes.

Keywords: Liquid metal cathodes; Molten salts; Molten oxides; Electrolysis

 

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1. Introduction

Electrochemical reduction technology is one of the most important metallurgy methods,

which is widely used for the extraction, recovery and refine of metals or alloys [1,2]. Compared

with the carbothermic reduction and metallothermic reduction, the reduction ability and rate of the

electrochemical reduction technology is controllable, and it is an effective method for the

production of active elements, such as alkali metals [3,4], alkali earth elements [4,5], rare earth

elements [6] and aluminum [7]. Electrochemical reduction technology can be divided into three

types according to operating temperature, i.e. room temperature (0~200C), middle temperature

(200~1000C) and high temperature (1000~1600C) melt electrolysis. Room temperature melt

electrolysis includes three categories, depending on different types of solution, i.e. aqueous

solutions [8], room temperature ionic liquids [9] and organic solutions [10]. Middle temperature

melt electrolysis generally refers to the molten salt electrolysis (MSE) [11] and the last one is

molten oxide electrolysis (MOE) [12]. MSE attracts more attention in the industry because the

potential windows of the molten salts are larger than those of aqueous solutions, and the

conductivity, ion mobility and reaction rate of the molten salts are greater than those of room

temperature melt [11]. Inspired by the success of aluminum production using MSE [13], MOE is

expected to achieve production of refractory metals for obtaining great development recently [14].

In general, the electrolytic cell structure of the conventional MSE or MOE is very simple,

which consists two electrodes (anode and cathode) that are separated by molten salts/oxide

electrolytes (Fig. 1(a)). According to the actual requirements, the anodes include graphite [15],

metals [16], alloys [17] or ceramics [18]. However, graphite [19] or solid metals [20] could be

selected as the cathodes. During the electrolysis process, metal ions diffusion to the cathode and

 

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then they are electro-reduced on the surface of the solid cathodes. Ultimately, the products are

collected from the cathodes, and thus the cathodes are recyclable. Although the operation of the

solid cathodes is very convenient, the efficiency is not satisfied in certain special cases. For

example, it is difficult to electro-reduce the strong electronegativity elements using the solid

cathodes [21]. Moreover, some metals with high solubility will further dissolve into the electrolyte

even they have been already deposited on the solid cathodes, which results in the low current

efficiency [22]. Besides, the re-oxidation of the products could reduce the product purity. To

alleviate above problems, liquid metal cathodes were utilized in the MSE or MOE. Upon the

liquid metal cathodes, the electrolytic cell structures of the MSE or MOE are diverse, and their

schemes could be briefly illustrated in Fig. 1(b).

Fig. 1. Schematic diagrams of (a) the conventional MSE or MOE based on solid cathode and

(b) the novel MSE or MOE based on liquid metal cathode.

1.1. Advantages

Compared with the solid cathode, the liquid metal cathodes have many fascinating properties,

as summarized below:

(1) The deposition potential of the target metal ions at the liquid metal cathodes is more

negative than that at the solid metal cathodes. The reason for this depolarization effect could be

 

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explained by changing the activity of the target metals in different cathodes. According to the

Nernst equation, the relationship between the standard electrode potential and the actual electrode

potential could be given as Eq. (1) [23].

Electrode reaction: Mn++ne- → M

Actual electrode potential: E=Eo+(RT

nF)ln( M +

αM) (1)

where E is the actual electrode potential, Eo the standard electrode potential, R the ideal gas

constant, T the absolute temperature, n the number of exchange electron, F the Faraday constant,

αMn+ the target metal ions activity in the electrolyte and αM

the target metal activity at cathodes. In

the Eq. (1), Eo, R, T, n and αMn+

are constant under the same conditions, while αM is different at

different cathodes. In general, αM is 1.0 at the solid cathodes, but it is lower than 1.0 at the liquid

cathodes because the target metal could dissolve in the cathode matrices, and then form alloys.

Therefore, the depolarization deposition of the target metals occurs on the liquid cathodes.

The depolarization effect could decrease the cell voltage on the liquid cathodes in comparison

with the solid cathodes, which is helpful for reducing the energy consumption. Moreover, the

previous studies demonstrate that the depolarization intensity of various target elements at

different liquid metal cathodes is varied. Upon this feature, liquid metal cathodes have been used

for separating some rare earth elements [24-26]. Besides, the depolarization effect could obviously

simplify the electro-reduction process of certain multi-valence metal ions. For example, the

reduction of Ti3+ ion at solid W cathode possesses two-step processes, including converting Ti3+

into Ti2+ and then further reducing into Ti. At a liquid Sn cathode in the molten NaCl-KCl

electrolyte, however, it becomes a one-step process via directly converting Ti3+ into Ti alloys [27].

In this case, the disproportionation reaction of Ti2+ ions at the conventional solid cathodes would

 

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be eliminated, which is significant for improving the cathodic current efficiency.

(2) MSE with liquid metal cathodes can be directly used to prepare alloys. In contrast to the

conventional blended elemental powder metallurgy (mixing) method, the preparation temperature

is lower using MSE with liquid metal cathodes. For instance, the preparation temperature of the

electrolytic rare earth elements in the molten chlorides is higher than 1000C [28,29]. When liquid

Al is used as the cathode, on the contrary, the operation temperature is usually around 800C, and

the final products are low melting point alloys [30]. Lower temperature is helpful for reducing the

energy consumption, investment of equipment, evaporation of molten salt as well as decreasing

the manipulation difficulty.

(3) During the electrolysis process, the deposited active metals, including Ca, Ba, rare earth

elements, etc., could dissolve into the liquid cathode, which effectively inhibits the re-oxidation

and chemical dissolution of the active metals. Consequently, it finally leads to high current

efficiency and reduced oxygen content in the alloy products [31].

(4) When the active metals are used in the liquid cathodes, the target metal ions could be

reduced in two routes, i.e electro-reduction and metallothermic reduction [32]. As a consequence,

the current efficiency of electrolysis could be very high.

1.2. Description

In the Earth’s curst, there are more than 90 metal elements, but only a few elements are

qualified for electrolysis using liquid cathodes. The compositions of the liquid metals are

highlighted in the periodic table presented in Fig. 2, and they are generally limited by the

following requirements:

(1) The cathode should be maintained in the liquid state at practical temperatures.

 

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According to the MSE’s practical temperature (~ 200-1000C), the melting point of the metals for

the MSE’s liquid cathodes should be less than 1000C. Similarly, in the MOE, the melting point of

the liquid metal cathodes should be less than 1500C. Therefore, refractory metals, such as Mo, Cr,

Ta, W, V, Hf, Nb, Ti, Zr, Sc, Y, etc., are not well applicable.

(2) The liquid metal cathodes may be insufficiently active and the target metal ions could

not be completely reduced via through metallothermic reduction, which indicates that the alkali

metals are not appropriate for liquid metal cathodes.

(3) For safety and operation, the liquid metal cathodes should be nontoxic and

non-radioactive, and Hg, As, Uhb, Uht, etc., are out of the scope.

(4) To scale up to industry, the cost of the liquid metal cathodes must be affordable in the

large-scale production. Thus, the noble metals, i.e., Au, Ag, Pt, Ir, Rh, etc., and partial rare earth

elements may not be ideal candidates.

(5) To maintain the cathode utilization in some cases, the metals with low solubility and

volatility would be the options in the molten salts or oxides.

(6) In the practical operation, the liquid metal cathodes should be positioned at the bottom

of the crucible, which requires the liquid metal density to be larger than that of the molten

electrolytes.

 

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Fig. 2. Material candidates for the liquid metal cathodes. The melting point of the elements

marked with yellow backgrounds is less than 1000C, and that marked with red background

is larger than 1000C but less than 1500C.

2. MSE based on liquid metal cathodes

This section aims to review the application of the liquid metal cathodes in the MSE for the

preparation of the rare earth metals, recovery of the nuclear wastes, preparation of liquid alloys

and deposition of titanium. In addition, the first two issues will be combined into one section

because the main compositions of the nuclear wastes belong to the rare earth elements and

actinides.

2.1. Extraction of the rare earth metals and actinides

The liquid cathodes have been widely used in the nuclear applications for reducing the

radiotoxicity and, in the end of fuel cycle, decreasing the volume of nuclear wastes [33-36]. Since

1990s, many processes have been carried out to selectively reuse minor actinide elements from

nuclear fuels using the liquid cathodes, especially using liquid Cd in the molten chlorides [33-40].

In Japan, several institutes, such as the Central Research Institute of Electric Power Industry

[33,38,41], Toshiba Corporation [33,42,43], Japan Atomic Energy Research Institute [38,41,42],

Tokyo Institute of Technology [43-45], investigated the cathodic process of alkaline earth

elements (Sr, Ca . . .), rare earth elements (Ce, Nd . . .), and some minor actinide elements (Np,

Am . . .) on the liquid metal cathodes. The activity coefficients, separation effect and the mass

transfer coefficients of target elements between the electrolytes and liquid metal cathodes have

been studied, which is helpful for recovering the nuclear wastes. Around the same time in the US

[46-48], the researchers from Argonne National Laboratory and University of Missouri-Columbia

 

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studied some fundamental nature of U, Np, Pu, Am, etc., using liquid Ca cathodes. Parallel

developments were also published in Korea [49], Spain [50] and other countries [51]. Later in

2006, Idaho National Laboratory [52] reported the engineering-scale liquid Cd cathode

experiments. However, the liquid metal cathodes mainly devoted to the extraction of the rare earth

metals in China (date back to the 1960s), owing to the effective depolarization effect of the liquid

metals on the rare earth elements [53,54]. Up to date, many metals, e.g., Mg, Al, Zn, Bi, have been

employed to extract the rare earth elements including La, Eu, Sm, Yb, Nd, etc [55-58].

Fig. 3. (a) Linear sweep voltammetries at 10 mV∙s-1 on liquid metal cathodes (Ga, Pb, Sn, Sb,

Bi) and Mo in LiF-CaF2 at 850C, (b) Linear sweep voltammetries at 10 mV∙s-1 on liquid

 

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metal cathodes (Ga, Pb, Sn, Sb, Bi) and Mo in LiF-NaF at 750C [59].

The investigation above was mainly performed in the molten chlorides. In 2014, M. Gibilaro

et al. systematically studied the liquid metal cathodes (Sb, Bi, Pb, Sn, Ga) in the molten fluorides

[59]. As shown in Fig. 3, due to the linear sweep voltammetry, activity domains along with

reactivity (Sb > Bi > Pb > Sn > Ga > Mo) and inertness scale (Ga < Sn < Pb < Sb < Bi < Mo) were

determined, which is valid in both solvents. According to these results, the extraction rate of an

element could be calculated. Then, in-situ preparation of liquid Bi-Li, Na or Ca electrodes was

operated in the LiF-NaF and LiF-CaF2 using electrolysis under a constant current. The results

indicated that Li+ and Ca2+ ions were electro-reduced on the liquid Bi at the same rate in the

LiF-CaF2, and in comparison with Li, Na was preferentially dissolved into Bi cathode in the

LiF-NaF. In addition, some studies also reported the preparation of rare earth element alloys based

on the liquid cathodes in the molten fluoride.

Fig. 4. Schematic representation of the electrolysis process of V-Al alloy by molten salt

electrolysis of soluble NaVO3 on a liquid Al cathode [62].

 

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2.2. Preparation of liquid alloys

Besides the preparation of the rare earth element alloys, liquid metal cathodes are also used

for synthesizing some low melting point alloys or extracting refractory metals. In 1989, A.

Honders et al. filed a patent for the production of metals or alloys from a molten salt with a liquid

cathode through electrolysis [60], in which the liquid metal cathodes comprised one or more

metals, and the target metals or alloys comprised the lanthanide series and actinide series. M.

Zhang et al. have investigated the electrochemical process of Zr4+ on the liquid Mg cathode in the

LiCl-KCl-K2ZrF6 melt [61]. Their results indicated that Zr4+ was electro-reduced to metallic Zr

through a two-step process, which corresponds to the Zr4+ to Zr2+ and Zr2+ to Zr transitions. At the

liquid Mg cathode, Mg-Zr alloy with about 0.8wt% Zr was obtained by potentiostatic electrolysis.

More recently, our group has investigated the preparation of V-Al alloy by MSE of soluble

NaVO3 at the liquid Al electrode [62]. The results showed that VO3- ions can be electro-reduced to

V-Al alloys at the liquid Al electrode through a combined effect of aluminothermic reduction and

electroreduction (Fig. 4). Finally, high-quality Al3V alloy with less Al2O3 could be prepared after

a long-time electrolysis.

Moreover, some active metals, such as Al, Ni, Fe, Au, show a clear depolarization effect or

underpotential deposition behavior during electrolysis process, and they are also used for the

preparation of alloys in the molten salt, despite they are in the solid state at operating temperature

[63-65]. We will not discuss this topic in the present review since such case is beyond

the scope of our discussion.

2.3. Deposition of titanium

In recent years, the extraction of titanium based on the liquid metal cathodes attracts great

 

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interest, which motivates us to discuss this section as a separate unit. In 2009, S.K. Maity et al.

carried out a study of electro-deposition of titanium using TiO2 and C composite anodes and liquid

Al electrodes [66]. They found that titanium ions could deposit on the liquid Al cathode, followed

by sinking into the liquid phase. SEM images showed the presence of titanium-rich phase (i.e.

TiAl2) in the Al electrode. Since 2013, researchers at Kyoto University have continuously reported

their research results on the titanium preparation based on liquid Bi electrode [67-69]. Their

investigations mainly consisted of the electrolysis of TiO2 or TiCl2 in the molten CaCl2 using a Bi

liquid cathode [67]. Also, titanium could be electro-refined from Bi-Ti alloys [69] that are

prepared from the MSE or an advanced Kroll process, i.e., reduction of TiCl4 into liquid Bi

through a magnesiothermic process (Fig. 5). Apparently, the prepared Ti-Bi alloys could be

refined by vacuum distillation because the liquid Bi has a low saturation vapor pressure.

Fig. 5. Schematic of the new smelting process proposed by the Kyoto University [68].

Aiming to improve the cathodic current efficiency and increase the technology of the MSE

for the extraction of titanium, our group started to study the liquid metal cathodes four years ago

[27]. However, the fundamental study of titanium in the molten salt started nearly forty years ago

in our group. In the early 1980s, we have devoted to the extraction, refinement and plating of

titanium in the molten chlorides or fluorides. Later in 2006, we proposed a new process for the

 

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preparation of titanium in the molten salt, namely USTB process [70,71]. Back to the liquid metal

cathodes, at present, we have systematically studied the cathodic behavior of the various titanium

ions, i.e., Ti3+, TiF63- and TiF6

2-, on many liquid metal cathodes, including Sn, Bi, Pb, and in

various melts, such as NaCl-KCl, NaCl-KCl-KF and LiF-NaF-KF [71-73]. Fig. 6 shows the

schematic representation of the depolarizing deposition of TiF63- ions on the liquid metal cathode

in the fluoride melt [73]. Moreover, we have studied the depolarization intensity of different liquid

metals on the titanium ions, motivating us to extract titanium from the melt. However, there is a

serious drawback that we did not perform a long-time electrolysis for high-quality cathodic alloys

with high titanium content. It is still unclear whether the liquid metal cathodes could remain active

in the liquid state during a long-time electrolysis. Consequently, more studies, including

investigation of the diffusion dynamics process of the deposited titanium in the liquid metal

cathodes as well as optimization of the alloying process, should be applied in the next step.

Fig. 6. Schematic representation of the depolarizing deposition of TiF63- ions on a liquid

metal cathode in a fluoride melt [73].

As we reviewed above, there are many studies on the liquid metal cathodes, ranging from the

 

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recovery of the nuclear wastes, extraction of the rare elements, and preparation of liquid alloys to

extraction of titanium, while unfortunately we found that there was no consistence on the nature of

the depolarization effect using different liquid metals to various target metals. Therefore, it is

suggested to systematically study and establish a general formula that could effectively guide

colleagues to select an appropriate liquid metal cathode that enables the separation, extraction or

recovery of elements from different melts. Moreover, the preparation of high-quality cathodic

products and alloys is also important in the section 2.1 and 2.2.

3. MOE using liquid metal cathodes

The fundamental of the MOE is similar to that of the Al preparation process, i.e.,

Hall-Héroult process, including the electrochemical decomposition of metal oxides and the metal

deposition at the cathodes.

Fig. 7. Schematic representation of the molten oxide electrolysis given by D.R. Sadoway [76].

3.1. Previous studies of the MOE

The history of the MOE could date back to 1906, R.H. Aiken proposed the concept of the

 

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MOE for the electrolytic preparation of Fe from iron oxide [74]. More than half a century later,

MOE was studied for electro-winning of Ti in the molten fluoride and oxide mixtures by K.

Hashimoto et al in 1971 [75]. Subsequently, D.R. Sadoway et al. reported many results on the

extraction of liquid metals through a MOE process in the 1990s [76]. Fig. 7 gives a scheme of

their steelmaking cell through an electrolysis process [77]. A multicomponent electrolyte that

consists of the ore and other oxides or fluorides could be used as the electrolyte. The cathode is

liquid metal iron at the bottom of the crucible. During electrolysis, the electrochemical reduction

of iron occurs at the interface of metal products and electrolytes. In 2003, F. Cardarelli issued a

patent for titanium preparation in the molten salt with a titanium oxide electrode [78]. With an

electro-slag re-melting unit in the 2010s, S. Takenaka et al. have studied the production of Ti-Al,

Ti-Fe and titanium from the molten electrolyte of CaF2-CaO-TiO2-Al2O3, CaF-CaO-TiO2-FeO and

CaF2-CaO-TiO2, respectively [79,80]. In recent years, D.R. Sadoway, D. Wang and A. Allanore et

al. have published some fundamental research results on MOE, as well as the preparation of

oxygen by using an inert anode [81-83]. Most recently, our group has successfully prepared iron,

Fe-Ni and Fi-Ni-Cr alloys by using this method as well [84-86]. Meanwhile, we have understood

the cathodic process of iron ions in terms of electrochemical analysis, e.g., square wave

voltammetry and cyclic voltammetry [84]. It is noted that the MOE has been developed for the

in-situ utilization of lunar, including oxygen evolution and metal extraction [87]. Based on above

review, the MOE shows many advantages and important practical value. More efforts, such as

developing the inert anode, investigating the melt structure and understanding the cathodic

deposition process, are necessary in the future. Meanwhile, some engineering problems for

realizing the applications of the MOE in the industry could be solved in further studies.

 

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3.2. The extraction of titanium by MOE using liquid metal cathodes

Up to date, there is almost no literature on the MOE equipped with liquid metal cathodes for

the preparation of metals, except our recent studies on the extraction of Ti from Ti-bearing blast

furnace slags (Ti-BF slags) [88]. Therefore, in this section, the liquid metal cathodes for the MOE

are mainly reviewed on the electrolytic production of titanium. Before the discussion of the liquid

metal cathodes, we would like to explain the reason for selecting the Ti-BF slag as the electrolyte.

As is well known, the content of titanium is very high, which ranks 9th out of all the elements in

the Earth’s crust [89]. However, only ~ 8% titanium resources are stored in terms of TiO2, i.e.,

anatase and rutile. Most of them are stored in the complex oxides, and a large portion is

vanadium-titanium magnetite ore (VTM). Nowadays, the VTM is mainly used for the extraction

of iron and vanadium, but about half of the titanium is discarded and abandoned in the Ti-BF slags

(TiO2>20wt%) [90]. Obviously, from a commercial view, it would be valuable to prepare the Ti

from the Ti-BF slags. Despite the fact that MOE shows a bright future for the extraction of

titanium from this slag, some critical issues limit the applications. Titanium has multiple oxidation

states, and it is mainly in high valence. In general, the reduction of the high-valent titanium is a

multi-step process, which results in the production of low-valent titanium. However, the

low-valent titanium has good electrical conductivity [91], and its reduction is more difficult than

that of high-valent titanium, SiO2 or even Al2O3, which could be concluded from Fig. 8, Table 1

and our recent studies [88].

Table 1. The main chemical composition of the Ti-BF slag [88].

CaO / wt% MgO / wt% Al2O3 / wt% TiO2 / wt% SiO2 / wt%

26.46 9.4 15.35 22.03 26.76

 

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Fig. 8. The relationship diagram of Gibbs free energy with temperature [88].

To address above challenges, the liquid metal cathodes were introduced firstly in the MOE.

About two years ago, we selected the Ti-BF slag (CaO-MgO-Al2O3-TiO2-SiO2) as the electrolyte

and molten iron as the liquid metal cathode to extract titanium [88]. It is noticeable that the

depolarization also appeared at the molten iron cathode in this case. This is important because it

may inhibit the production of the low-valent titanium and significantly prevent the occurrence of

the disproportionation reaction, which is helpful for improving the current efficiency. The results

of the constant current electrolysis verified this observation. The current efficiency of 69.82% at

the iron cathode is larger than that at the solid cathode.

 

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Fig. 9. (a) Current–time curve recorded by potentiostatic electrolysis on a solid molybdenum

electrode at a potential of 1.5 V vs. Mo for 75 min; (b) XRD pattern of the cathodic product

(inset is the photograph of the molybdenum electrode); (c) Ti 2p spectrum of the molten

oxide electrolyte after electrolysis [92].

To further understand the cathodic process of the high-valent titanium at different electrodes,

i.e., solid cathodes and liquid cathodes, one year later we carried out in-situ electrochemical

analysis and ex-situ chemical analysis in the molten CaO-MgO-Al2O3-TiO2 quaternary oxide

electrolyte [92]. Cyclic voltammetry and XPS results (Fig. 9) demonstrated that the high-valent

titanium (TiO2) in the melt was reduced to low-valent titanium (Ti2O3 and TiO), while the latter

was not further reduced to titanium metal. In contrast, the high-valent titanium can be

electro-reduced to metallic titanium (Ti-Fe alloys) on the liquid iron electrode.

Liquid iron cathode shows an effective depolarization effect, while it will not change the fact

that the electro-reduction of SiO2 is prior to that of TiO2, which is not cost-effective from a

 

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commercial view. Fortunately, our recent work showed that the molten Cu has the different

depolarization intensity on the titanium and silicon [93]. As shown in Fig. 10, the EDS results

obtained at a liquid copper cathode after electrolysis showed that the precipitated phase mainly

consisted of Cu and Ti, which clearly demonstrates that the molten Cu has larger depolarization

effect to TiO2 than that to SiO2. Furthermore, we tried to understand the nature of the strong

depolarization effect of copper to titanium from binding force. The optimized Ti-Cu alloys and

Si-Cu alloys are given in Fig. 11, and the lattice constants and binding forces of the alloys are

listed in Table 2. Apparently, the binding force between Ti and Cu is much larger than that

between Si and Cu. It indicates that the binding force between Ti and Cu is stronger, which is

easier to form alloys. The result is in agreement with the experimental results. It may be one of

reasons why the depolarization of molten copper electrode to titanium dioxide is stronger than that

to silicon dioxide.

Fig. 10. (a) SEM image and (b) EDS images of electrolytic products after galvanostatic

electrolysis of 3 h; (c) XPS image of Ti 2p spectrum of the molten

Al2O3-MgO-CaO-TiO2-SiO2 electrolyte after electrolysis [93].

 

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Fig. 11. Optimized crystallographic models for Ti-Cu and Si-Cu binary system intermetallic

compounds [93].

The above results indicate that it is feasible to extract titanium from molten oxide solution at

the liquid metal cathodes, in which liquid copper shows selective depolarization for titanium. At

present, however, the content of titanium in the cathodic products is very small. Meanwhile,

plenty of fundamental nature including the electrolyte and liquid cathode is unclear. Therefore, we

suggest that the further work will focus on studying the nature of the molten electrolytes,

understanding the diffusion kinetics process of the Ti in the liquid electrode and further optimizing

the electrolytic parameters.

Table 2. The main chemical composition of the Ti-BF slag [93].

Phase Unit cell lattice parameters and angle

Ti3Cu a=b=4.158 Å, c=3.594 Å; α=β=γ=90°

Ti2Cu a=b=2.938 Å, c=10.786 Å; α=β=γ=90°

 

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TiCu a=b=3.140 Å, c=2.856 Å; α=β=γ=90°

Ti3Cu4 a=b=3.126 Å, c=19.964 Å; α=β=γ=90°

Ti2Cu3 a=b=3.130 Å, c=13.950 Å; α=β=γ=90°

SiCu3(P63) a=b=5.160 Å, c=4.141 Å; α=β=90°, γ=120°

SiCu3(I4) a=b=3.606 Å, c=7.364 Å; α=β=γ=90°

Si2Cu7 a=b=4.085 Å, c=7.587 Å; α=β=90°, γ=120°

4. Conclusions

In this review, we briefly summarized the progresses of liquid metal cathodes for the MSE

and MOE, including rare earth and actinide element separation, alloys preparation, titanium

extraction. Firstly, liquid metal cathode is briefly introduced in the view of the requirements of

electrolysis. Subsequently, some advantages of the liquid metal cathodes are demonstrated in

consistent with the theoretical point of view.

Low-melting-point metals, such as Sn, Bi, Pb, Mg, etc., were developed as the first

generation of the liquid metal cathodes in MSE. Rare earth elements, actinides, alloys and

titanium could be successfully collected at those cathodes because of the significant selective

depolarization effect of the molten metal cathodes. However, the content of the deposited elements

in the liquid metal cathodes is too low, which leads to a low utilization of the liquid metal

cathodes as well as a large rate of high-quality alloys. Therefore, additional study should be

carried out to optimize the alloying process during electrolysis, especially the diffusion dynamics

process of the deposited elements in the liquid metal cathodes.

Besides the MSE, the MOE configured with liquid metal cathodes was also reviewed. Liquid

 

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irons and coppers have been applied to the extraction of titanium from the slag with titanium

oxides and in-situ preparation of liquid titanium alloys. Although liquid iron cathodes showed

obvious depolarization effect on the deposition of titanium, it will not change the fact that the

electro-reduction of silicon is prior to titanium. In contrast, liquid coppers possess a much stronger

depolarization effect on titanium. As a result, liquid copper could be used to realize the selective

depolarization deposition of titanium, and the synthesis of liquid Cu-Ti binary alloys.

Unfortunately, the detailed electrode process was still unclear, and the content of titanium in alloys

is too small. More efforts should be drawn to further develop this method.

Based on the views, future studies should be mainly devoted to two aspects, i.e. fundamental

study on the essential mechanism of the selective depolarization effect of the different liquid metal

cathodes and the diffusion kinetics process of the target elements in the liquid cathodes.

Consequently, a systematical summary or a formula is expected to establish for guiding the

researchers to select a proper liquid metal cathode, which enables the effective separation of

elements from different melts. In addition, the preparation of high-quality cathodic products or

alloys with high content target elements is the ultimate goal of MSE and MOE using liquid metal

cathodes.

Acknowledgements

We acknowledge financial support from the National Natural Science Foundation of China (No.

51725401) and the Fundamental Research Funds for the Central Universities (FRF-TP-18-003C2).

 

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