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: [email protected] (Shu-qiang Jiao)
Authors’ E-mail: [email protected] (Han-dong Jiao), [email protected] (Wei-li Song),
[email protected] (Ming-yong Wang), [email protected] (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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