Minimising oxygen contamination through a liquid copper-aided group IV metal production process
Minimising oxygen contamination through a liquid copper-aided group IV metal production process
Young Jun Lee
Seong Hun Lee
Jong Hyeon Lee
OPEN This paper demonstrates for the first time the fabrication of Zr-Cu alloy ingots from a Hf- free ZrO2 precursor in a molten CaCl2 medium to recover nuclear-grade Zr. The reduction of ZrO2 in the presence of CaO was accelerated by the formation of Ca metal in the intermediate stage of the process. Tests conducted with various amounts of ZrO2 indicate that the ZrO2 was reduced to the metallic form at low potentials applied at the cathode, and the main part of the zirconium was converted to a CuZr alloy with a different composition. The maximum oxygen content values in the CuZr alloy and Zr samples upon using liquid Cu were less than 300 and 891 ppm, respectively. However, Al contamination was observed in the CuZr during the electroreduction process. In order to solve the Al contamination problem, the fabrication process of CuZr was performed using the metallothermic reduction process, and the produced CuZr was used for electrorefining. The CuZr alloy was further purified by a molten salt electrorefining process to recover pure nuclear-grade Zr in a LiF-Ba2ZrF8-based molten salt, the latter of which was fabricated from a waste pickling acid of a Zr clad tube. After the electrorefining process, the recovered Zr metal was fabricated into nuclear-grade Zr buttons through arc melting following a salt distillation process. The results suggest that the removal of oxygen from the reduction product is a key reason for the use of a liquid CaCu reduction agent.
Published; xx xx xxxx
Direct reduction could significantly simplify the isolation of group IV transition metals from their
corresponding oxides (for example TiO2, ZrO2, and HfO2). These metals are typically produced by the Kroll process1,2 but
the multiple stages, especially the chlorination step, reduces the effectiveness of the extraction, and increases
the cost of the final product and the environmental impact. Metal oxides have been directly reduced to their
corresponding metals by electrochemical3?12 and metallothermic approaches13,14. However, the conventional
electrochemical reduction only achieves partial oxygen removal7?9. This is a serious disadvantage for producing
ductile transition metals on a commercial scale. The electrochemical method is inadequate for nuclear-grade Zr
production because the metal loses its ductility under loss-of-coolant nuclear reactor accident conditions in the
presence of impurities, especially for oxygen contamination exceeding 1400 ppm15,16. Here, we report a metal
production technology that lowers the oxygen content of Zr to meet the requirements for nuclear-grade metal.
During this process, the target metal is kept mixed with another metal, in its liquid state, which shields it from
CaO-containing electrolytes. We demonstrate the indirect electro-fabrication of CuZr alloy ingots from a low-Hf
ZrO2 feedstock in molten CaCl2 using a liquid Cu cathode (LCC). Once the CuZr alloy ingot is prepared, pure Zr
can be readily obtained from electrorefining without the chlorination process. Density functional theory (DFT)
molecular dynamics (MD) simulations provided details about the key processes leading to CuZr formation after
ZrO2 reduction. As the oxide-containing Zr as a solid solution did not form a CuZr phase due to repulsive forces,
only oxygen-free Zr is expected in the Cu. The combined theory?experiment study suggests that this ZrO2
reduction method can be generalised to the simple reduction of metal oxides.
Results and Discussion
The production of nuclear-grade Zr by chlorination-free method starts from a preparation of CuZr ingot that
was initially obtained by ZrO2 through electroreduction or metallothermic using liquid Cu as the cathode. The
resulting CuZr is used for the electrorefining process in the second step. Figure?1a and c show the
electrochemical cell using solid and liquid copper cathodes, respectively. Cyclic voltammograms (CVs) were obtained in the
CaCl2 + 5 wt% CaO electrochemical system at 1080 K vs. the W reference electrode (Fig.?1b). The process
temperature was lower than the melting point of Cu (Tmelt. = 1358 K), and therefore, the Cu cathode was in the solid state.
The Ca2+ reduction process on the Cu solid cathode can be written as:
Ca2+ (from CaO) + 2e? ? Ca? (E = ?1.25 V)
The reduction potential of the CaCl2 + 5 wt% CaO system increased to ?0.8 V at 1380 K (above the melting
point of Cu) (Fig.?1d). Two main reasons for the change in the reduction potential are related to the process
temperature and the liquid electrode. Therefore, an applied potential of at least ?1.5V is needed to effectively reduce
ZrO2, and this value can be reached by applying a current density of 500 mA/cm2 to the cathode (see Fig.?S2).
According to experimental observations, the part of the Cu cathode that was immersed in the molten
chloride melted, despite the system temperature being below the melting point of Cu due to intermetallic compound
between Cu and Ca decrease the melting point. We assumed that this melting of the Cu(Ca) cathode provoked
a sharp change in the reduction potential due to formation of an intermetallic compound between Cu and Ca17.
The CVs (Fig.?1c and d) displayed minimal oxidation peaks because the reduced Ca metal immediately reacted
with the Cu electrode to produce a CaCu alloy. This phenomenon is noticeable in liquid Cu cathodes because the
diffusion of reduced Ca on a liquid rather than a solid Cu cathode leads to rapid redistribution inside the Cu. The
reduction potential change resulted from the temperature difference and the underpotential deposition
phenomenon due to negative Gibbs formation energy of CaCu intermetallic compound, which allowed the reduced Ca
to react more easily with the liquid Cu cathode than its solid analogue (see Fig.?S2). Thus, the CV tests confirmed
the formation of Ca metal in both solid and liquid cathode systems. Through this experiment, it was confirmed
that a reducing agent suitable for the reduction of ZrO2 can be prepared electrochemically because it is heavier
than the electrolyte.
The electrochemically produced CaCu is used simultaneously for the reduction of ZrO2. The
reduction of ZrO2 was monitored by chronopotentiometry at an applied current density of 500 mA/cm2 (Fig.?1e).
Chronopotentiometry measurements for various ZrO2 concentrations (E1;6.2, E2;15.2, E3;40.1 and E4;84.7 in
ZrO2/Cu mass ratio) are detailed in the Methods Section. The cell potential was defined as the potential difference
between the cathode and anode. The cathode potential presented a negligible increase from ?1.41 to ?1.35 V
vs. W over 11 h at an applied current density of 500 mA/cm2. In contrast, the graphite anode potential increased
from 0.62 to 1.29 V. This change can be explained by a decrease in the anode surface area during the
electroreduction18. In the case of an electroreduction in which solid matter exists before and after the reaction, such as
Fray-Farthing-Chen (FFC) Cambridge process and Ono-Suzuki (OS) process, the oxygen diffusion becomes
difficult as the reaction progresses, and the cathode potential tends to become more negative. Therefore, the stable
cathode potential in this experiment indicates that the rate of reduction dramatically increases through rapid
diffusion of electrodeposits resulting from the use of a liquid cathode. This method of recovering reduced Zr as a
liquid phase is considered as one of the most important requirements in the commercialisation of the
The ZrO2 reduction process in a liquid phase can be expressed through the following equations.
Ca + xCu = CaCux (liquid)
2CaCux (liquid) + ZrO2 (solid) = Cu2xZr (liquid) + 2CaO
For the understanding of the mechanism of ZrO2 reduction and CuZr liquid phase formation, a schematic
diagram is presented in the Fig.?2a?c. Because ZrO2 powder exhibits a lower density than liquid or solid Cu, it is
always located on the Cu cathode surface. As a result, it accumulates on the Cu cathode when added to the
electrochemical cell (Fig.?2a). Below the Cu melting point, most of the reduced Zr accumulates on the Cu cathode
surface while a small portion diffuses into the cathode (Fig.?2b). The extremely large surface area of the solid
particles of the Zr product hinders the complete removal of oxygen from ZrO2 when exposed to the CaO-enriched
molten salt due to the chemical equilibrium of Ca/CaO19. However, above the melting point of Cu, the oxygen
concentration in the product can be greatly reduced because Zr reacts with Cu to form intermetallic compounds
during the reduction process. This reaction prevents the re-oxidation of Zr and any side reaction with CO2
generated from the anode (Fig.?2c). Moreover, because the liquid CuZr phase is sufficiently denser than that of
CaCl2, phase separation spontaneously occurs, and washing the product with water to remove electrolytes is not
required; hence, re-oxidation of Zr during post-treatment can be essentially prevented.
The formation of a CuZr alloy at high temperatures was assessed by DFT-MD simulations. Figure?2f shows
that ZrO2 reduced by Ca penetrated into the Cu(111) substrate at 1380 K, which was not observed at lower
temperatures (Fig.?2d, 400 K and Fig.?2e, 1000 K). The Zr?Zr cohesive energy was greater than its Cu?Cu equivalent
and the Cu?Zr interaction energy lay between these energies20, meaning that the formation of CuZr alloy was
thermodynamically endothermic. The Cu?Zr alloying limit occurs above the melting point of Cu. Consequently,
melting-induced structural disorder would provide room for additional Cu?Zr bonds and initiate Zr penetration
Supplementary DFT-MD simulations also showed that a Zr cluster containing oxygen as the feedstock in the
electroreduction process that was initially supported on a Cu(111) substrate could not penetrate the liquid Cu
(see Fig.?2g) even as the temperature rises to 1380 K. The Zr that contains oxygen as a solid solution did not form
a CuZr phase due to repulsive forces. Thus, a significantly low oxygen concentration of less than 300 ppm in Zr
was expected in the Cu as observed in the oxygen analysis data.
Metal reduction by the conventional metallothermic process is determined by19:
O (in Ti) + Ca (in salt) = CaO (in salt)
[%O] = (aCaO/aCa)1/f0 ) exp(?G0/RT )
where ?G0 is the standard free energy change of the reaction (Eq. (
)), aCaO and aCa are the activities of CaO and
calcium, respectively, and f0 is the activity coefficient of oxygen in solid titanium. Because the oxygen content is
determined by the thermodynamic properties as long as the solid metal product is in contact with the salt,
existing processes having a large reaction surface area are limited in reducing the oxygen content. Therefore, a lower
oxygen content in the metal rests on decreasing in CaO activity in the electrolyte or increasing the Ca activity. The
key deoxidation steps of the liquid Cu-assisted reduction are indirect reduction and intermetallic compound
formation, which produces the liquid CuZr alloy, simultaneously with the Ca-mediated metallothermic reduction of
ZrO2. In addition, by separating the CaO-containing electrolyte from the liquid CuZr phase based on the specific
gravity difference, the CaO is not subject to the chemical equilibrium reaction mentioned in Eqs (
) and (
the present process, the chemical equilibrium reaction can occur at the interface between the salt and CuZr phase,
but, as suggested by the above-mentioned DFT-MD simulations, oxygen-bound Zr cannot penetrate into the Cu.
Therefore, an extremely low oxygen content can be achieved.
The energy dispersive X-ray (EDX) analysis of the recovered CuZr detected Cu, Zr, and Al elements but no
oxygen in the samples (see Table?S3). The presence of Al originated from the Al2O3 crucible during the reduction
process. The microstructures of CuZr alloys obtained under various electroreduction conditions were analysed by
back-scattered electron imaging (?500) in which heavy elements (Zr in this analysis) back-scatter electrons more
strongly than light elements (Cu and Al in this analysis), and thus appear brighter in the image (Fig.?3a?d). With
increasing Zr concentrations, the shape of the grains became columnar, which is typical for alloy systems21,22.
Also, a refinement of the grains was noticeable above a certain Zr concentration (>30 wt%, Fig.?3d). This may
result from the simultaneous formation of several local crystallisation centres in the alloy sample. The oxygen
content values of samples E1?E4 (Fig.?3e) analysed by Eltra ONH-2000 ranged from 142 to 249 ppm, which are
acceptable levels for nuclear-grade Zr23.
Through quantitative analysis, it was confirmed that the reduced Zr concentration increases as the amount
of ZrO2 is increased. Then, XRD pattern analysis was performed to observe the phase change of CuZr formed
as the amount of reduced Zr increased. The main phases detected in alloy E3 (40.1, ZrO2/Cu weight ratio) were
Cu, Cu5Zr, and CuZr (Fig.?3f). When the Zr content increased to 28 wt%, an alloy phase corresponding to CuZr2
appeared in the reaction product (Fig.?3f). The amount of the CuZr phase also increased sufficiently. Based on the
XRD data, it is difficult to determine the sequence of formation of the different alloy phases during the
electroreduction. Aluminum contamination from the Al2O3 crucible was observed in CuZr production by
electroreduction. Therefore, ZrO2 reduction was carried out by a metallothermic method using CaCu to prevent Al pollution.
As a result, it was possible to recover the Al-free CuZr ingot in quantities suitable for industrial use (see SI for
details, Figs?S3?S8). Electroreduction and CaCu-mediated metallothermic reduction can effectively remove
oxygen from low-Hf ZrO2. The presence of CaCu alloy in these processes can produce a large amount of CuZr alloy.
It is possible to prevent co-reduction of the Al2O3 crucible by using CaCu as a reductant and insoluble metallic
crucibles such as those made of Mo or W. This CuZr ingot was used as anode feedstock to obtain Zr.
A low-Hf ZrF4-containing electrolyte, such as Ba2ZrF8, was needed to recover pure Zr from the CuZr ingot by
electrorefining. An economic way to secure such electrolytes from waste pickling acid has already been reported
in our previous research24. In order to produce nuclear grade Zr, it is necessary to secure the electrochemical
potential condition capable of recovering pure Zr by selectively dissolving Zr from the CuZr alloy ingot through
the electrorefining. The behaviour of the Zr4+ ion in the LiF?Ba2ZrF8 molten salt system was evaluated by CV
(Fig.?4a). The reduction of Zr4+ ion consists of a 3-step electrorefining process. The number of reactive electrons
was confirmed by Eq. (
)25,26 using each reduction potential peak in Fig.?4a.
|Ep ? Ep/2| = 0.774(RT/nF)
In Eq. (
), Ep is the reduction potential peak and Ep/2 is half of the Ep. Electrode potentials were stable during
the electrorefining process. During this process, the Cu remained on the anode while the Zr dissolved before
depositing with the salt on the cathode. The anode behaviour is detailed in the Supplementary Information. The
anode and cathode after electrorefining are shown in Fig.?4c and d, respectively. Zirconium was recovered as a
powder using vacuum distillation (see Fig.?4e) and nuclear-grade Zr metal buttons were subsequently generated
by arc melting. Analysis of the resulting Zr metal button showed that the concentration of major impurities was
very low, and contamination of molybdenum, aluminium, copper, etc., which could be contaminated from the
crucible and the anode, was satisfactorily blocked. In addition, the contents of oxygen and nitrogen were 891 and
10 ppm respectively, which proved that this process is very effective for preventing pollution of gas impurities.
Theoretically, oxygen contamination should be smaller than the above values, but oxygen contamination may
have resulted from atmospheric exposure during transportation between the unit processes: the salt distillation
and ingot manufacturing processes after electrorefining. The purity of the recovered metal satisfied the ASTM
B349 specifications for nuclear-grade Zr (Table?S2)23.
In summary, a CuZr alloy ingot was prepared from a low-Hf ZrO2 precursor in a CaCl2?CaO molten salt
using an LCC at a temperature of 1380 K (Fig.?5). The ingot exhibited an extremely low oxygen concentration.
Zirconium metal was isolated from a CuZr ingot produced by metallothermic reduction via an electrorefining
process in Ba2ZrF8-based electrolyte derived from waste pickling acid. Despite the fact that the metal was
produced by a direct reduction process from the oxide without the chlorination process, the prepared nuclear-grade
Zr displayed superior quality. In addition to producing nuclear-grade Zr, this technology can be applied to the
preparation of group IV transition metals, such as Ti and Hf. And this process could be an attractive alternative
to the Kroll process by avoiding environmental problems.
Materials and equipment. Anhydrous CaCl2 powder (purity 98%), CaO powder (purity 99%), and low-Hf
ZrO2 (impurities are listed in Table?S2) were supplied by Samchun Chemicals (Korea), Chemicals (Japan) and
Alkane Resources (Australia), respectively. The process flowsheet for the Dubbo Project, Australia, which is a new
source of zirconium, hafnium, rare earth metals, and niobium, consisted of a sulphuric acid leaching of a
polymetallic orebody, followed by solvent-extraction recovery and refining. Low-Hf ZrO2 was produced by a proprietary
process to remove hafnium from a high-purity zirconium stream.
Copper chips (purity: 99.9%) and wire (1 mm, purity: 99.9%) were obtained from Junsei Chemicals (Japan)
and Alfa Acer (USA), respectively. A CaCl2 electrolyte was used, and CaO was added for the electroreduction
process. Ba2ZrF8 was synthesised using BaF2 (Alfa Aesar, purity >90%) and an acidic waste solution (H2O:HF:H
NO3:Zr = 84:1.2:14.8:1.3 wt%) from KNF (Korea Nuclear Fuel). The electrorefining electrolyte was prepared using
65 mol% LiF (Alfa Aesar, purity >99.9%) and 35 mol% Ba2ZrF8. All raw materials were preheated at 623 K for 24 h
to remove residual moisture.
Tungsten metal wires and graphite rods (diameter:1 mm) used as electrodes were 99.9% pure, as supplied by
Sigma Aldrich (USA) and Shin Sung Carbon (Korea) companies, respectively. Al2O3 crucibles and tubes were
supplied by Mesto, Korea. Every experiment was performed in a glove box with a stainless-steel container
constructed to prevent oxidation of the electrolyte components and structural materials. The glove box was operated
in an argon atmosphere in which the concentration of oxygen and moisture were controlled to be less than 2
ppm. Electrochemical measurements and electrolysis were performed using an Autolab model PGSTAT302N
and NOVA computer software.
Electrochemical procedures. Electroreduction. The electrochemical behaviour of Ca2+ ions in the
electrolyte was evaluated by CV using solid and liquid Cu cathodes and a graphite anode. Components of the
experimental apparatus are shown in Fig.?S1. A Cu wire was used as a solid cathode while a chip provided the liquid
electrode. Cathode and anode potentials were monitored using a W wire as a pseudo-reference electrode. The
electrolytic system was kept at 1080 and 1380 K during the CV tests to obtain solid and liquid Cu cathodes,
Figure?1a and b show the electrochemical cell used for electrolytic reduction tests on ZrO2. During these
tests, CaCl2 (1.25 kg) and CaO (62.5 g) were melted in an Al2O3 crucible with a 100-mm inner diameter placed
in a stainless steel vessel and heated externally using an electric furnace. The temperature was maintained at
1380 ? 10 K. The graphite anode, W reference electrode, and liquid Cu cathode were immersed in the salt a
type-K thermocouple sheathed in an Al2O3 tube. To prepare the liquid cathode, copper chips (10 g) and ZrO2
powder (0.62?8.47 g) were loaded into a small Al2O3 crucible, which was held in the electrolyte for 30 min. An
additional W wire (1.0-mm diameter) acting as a conductor was inserted into the small crucible. Experimental
conditions are summarised in Table?S1. After the electroreduction, part of the cathode product was removed from
the Al2O3 crucible and the electrolyte was washed off with distilled water.
Electrorefining. The reduction potential of Zr at 1053K was determined by CV using the binary electrolyte LiF?
Ba2ZrF8 (35:65, mol%). A Mo wire, W rod, and W wire electrode were used as the cathode, anode, and reference
electrode, respectively. The electrorefining of Zr from a CuZr ingot resulting from metallothermic reduction was
performed by chronopotentiometry using CaCu as a reductant. In this experiment, CuZr ingot, a Cu plate, and a
W wire electrode were used as the anode, cathode, and reference electrode, respectively.
Post-electrorefining treatment. The electrodeposited Zr was ground in a globe box with an Ar gas
atmosphere and the electrolyte was effectively removed by salt distillation for 24 h at 1573 K under vacuum (pressure:
10?2 Torr). Metal powders were recovered in the glove box (Fig.?4e) and Zr buttons (Fig.?4f) were prepared by arc
melting under vacuum (10?5 Torr).
Material characterization. Isolated materials, such as CuZr and Zr, were characterised by X-ray (CuK?
radiation) diffraction (XRD, D/MAX-2200) and field-emission scanning electron microscopy (FE-SEM, JEOL
JSM-6700F) in combination with energy dispersive X-ray analysis (EDX). The residual oxygen and nitrogen in
the CuZr alloy and produced Zr ingot were quantified using an Eltra ONH-2000 analyser (Germany), and the
low-Hf ZrO2 and produced Zr ingot were analysed using a glow discharge mass spectrometer (GD-MS, Msi
DFT-MD simulations. DFT-MD simulations were performed using the Vienna ab initio simulation package
(VASP)27 and Perdew?Burke?Ernzerhof (PBE) exchange-correlation function28 in canonical ensemble
conditions. The equation of motion, which is governed by Newton?s second law, was integrated in the simulations using
a Verlet algorithm with a time step of 1 fs. All MD simulations were performed for a total simulation time of 10 ps.
The interaction between the ionic core and the valence electrons was described by the projector augmented wave
method29, and the valence electrons were described using a plane wave basis up to an energy cut-off of 400 eV. The
Brillouin zone was sampled at the ?-point. The convergence criteria for the electronic structure and the geometry
were 10?5 eV and 0.05 eV/A, respectively. A Fermi smearing function with a finite temperature width of 0.2 eV
was applied to improve the convergence of states near the Fermi level.
The simulations were conducted using an optimised 4? 4 ? 10 Cu(111) slab. The bottom three layers of
the slab were fixed during the simulations. To model the Cu?Zr interaction, four Zr atoms were placed on the
three-fold hollow sites of the Cu(111) slab model surface. DFT-MD simulations were performed at 400, 1000,
and 1380 K.
This work was supported by a National Research Foundation of Korea (NRF) (NRF-2014R1A2A1A11049997)
grant from the Korean government (MSIP) and the Korea Institute of Energy Technology Evaluation and
Planning(KETEP) and the Ministry of Trade, Industry & Energy(MOTIE) of the Republic of Korea (No.
20155000000190). The authors also thank the Korea nuclear fuel company (KNFC) for supplying waste pickling
acid to prepare the LiF?Ba2ZrF8 electrolyte and Alkane Resources for providing the ZrO2 test samples.
B.Y., Y.L. and J.L. designed all experiments; H.K. was responsible for DFT-MD computational simulations; B.Y.
performed electroreduction; Y.L. and S.L. performed electrorefining; H.N., B.Y. and J.L. designed the chemical
reactions; B.Y. and V.R. performed and interpreted XRD and SEM measurements; J.L. supervised the project;
B.Y. wrote the manuscript; and N.E. and A.M. of Alkane Resources the low-Hf ZrO2 and described its process
recovery details. All of the authors discussed the results and reviewed the manuscript.
Supplementary information accompanies this paper at https://doi.org/10.1038/s41598-018-35739-z.
Competing Interests: The authors declare no competing interests.
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1. Kroll , W. , Schlechten , A. & Yerkes , L. Ductile zirconium from zircon sand . Transactions of The Electrochemical Society 89 , 263 - 276 ( 1946 ).
2. Kroll , W. The production of ductile titanium . Transactions of the Electrochemical Society 78 , 35 - 47 ( 1940 ).
3. Mohandas , K. & Fray , D. Electrochemical deoxidation of solid zirconium dioxide in molten calcium chloride . Metallurgical and materials transactions B 40 , 685 - 699 ( 2009 ).
4. Xiao , W. & Wang , D. The electrochemical reduction processes of solid compounds in high temperature molten salts . Chem Soc Rev 43 , 3215 - 3228 , https://doi.org/10.1039/c3cs60327j ( 2014 ).
5. Fray , D. J. Emerging molten salt technologies for metals production . Jom-J Min Met Mat S 53 , 26 - 31 ( 2001 ).
6. Allanore , A. , Yin , L. & Sadoway , D. R. A new anode material for oxygen evolution in molten oxide electrolysis . Nature 497 , 353 - 356 ( 2013 ).
7. Ono , K. & Suzuki , R. O. A new concept for producing Ti sponge: Calciothermic reduction . Jom-J Min Met Mat S 54 , 59 - 61 , https:// doi.org/10.1007/Bf02701078 ( 2002 ).
8. Okabe , T. H. , Suzuki , R. O. , Oishi , T. & Ono , K. Thermodynamic properties of dilute titanium-oxygen solid solution in beta phase . Materials Transactions, JIM 32 , 485 - 488 ( 1991 ).
9. Abdelkader , A. M. , Daher , A. , Abdelkareem , R. A. & El-Kashif , E. Preparation of zirconium metal by the electrochemical reduction of zirconium oxide . Metall Mater Trans B 38 , 35 - 44 , https://doi.org/10.1007/s11663-006-9016-z ( 2007 ).
10. Okabe , T. H. , Oda , T. & Mitsuda , Y. Titanium powder production by preform reduction process (PRP) . Journal of Alloys and Compounds 364 , 156 - 163 ( 2004 ).
11. Merwin , A. et al. Presence of Li clusters in molten LiCl-Li . Scientific reports 6 ( 2016 ).
12. Chen , G. Z. , Fray , D. J. & Farthing , T. W. Direct electrochemical reduction of titanium dioxide to titanium in molten calcium chloride . Nature 407 , 361 - 364 , https://doi.org/10.1038/35030069 ( 2000 ).
13. Nersisyan , H. H. , Won , H. I. , Won , C. W. , Jo , A. & Kim , J. H. Direct magnesiothermic reduction of titanium dioxide to titanium powder through combustion synthesis . Chem Eng J 235 , 67 - 74 , https://doi.org/10.1016/j.cej. 2013 . 08 .104 ( 2014 ).
14. Li , H. et al. Nuclear-grade zirconium prepared by combining combustion synthesis with molten-salt electrorefining technique . Journal of Nuclear Materials 413 , 107 - 113 ( 2011 ).
15. Nikulin , S. et al. In Zirconium in the Nuclear Industry: 15th International Symposium . (ASTM International).
16. Nikulin , S. , Rozhnov , A. , Belov , V. , Li , E. & Glazkina , V. Influence of chemical composition of zirconium alloy E110 on embrittlement under LOCA conditions-Part 1: Oxidation kinetics and macrocharacteristics of structure and fracture . Journal of Nuclear Materials 418 , 1 - 7 ( 2011 ).
17. Castrillejo , Y. et al. Electrochemical formation of Sc-Al intermetallic compounds in the eutectic LiCl-KCl . Determination of thermodynamic properties . Electrochimica Acta 118 , 58 - 66 ( 2014 ).
18. Chen , H. , Jin , X. , Yu , L. & Chen , G. Z. Influences of graphite anode area on electrolysis of solid metal oxides in molten salts . Journal of Solid State Electrochemistry 18 , 3317 - 3325 ( 2014 ).
19. Okabe , T. H. , Hamanaka , Y. & Taninouchi , Y. -k Direct oxygen removal technique for recycling titanium using molten MgCl 2 salt . Faraday discussions 190 , 109 - 126 ( 2016 ).
20. Gunawardana , K. G. S. H. , Wilson, S. R. , Mendelev , M. I. & Song , X. Theoretical calculation of the melting curve of Cu-Zr binary alloys . Physical Review E 90 , 052403 ( 2014 ).
21. Kim , K.-H., Ahn , J.-P. , Lee , J.-H. & Lee , J.-C. High-strength Cu-Zr binary alloy with an ultrafine eutectic microstructure . Journal of Materials Research 23 , 1987 - 1994 ( 2008 ).
22. Peng , L. J. , Mi , X. J. , Xiong , B. Q. , Xie , H. F. & Huang , G. J. Microstructure of phases in a Cu-Zr alloy . Rare Metals 34 , 706 - 709 , https://doi.org/10.1007/s12598-014-0324- 1 ( 2015 ).
23. (ASTM International, West Conshohocken, 2003 ).
24. Nersisyan , H. et al. Two-step process of regeneration of acid (s) from ZrF 4 containing spent pickle liquor and recovery of zirconium metal . Journal of Nuclear Materials 486 , 44 - 52 ( 2017 ).
25. Nicholson , R. S. & Shain , I. Theory of stationary electrode polarography. Single scan and cyclic methods applied to reversible, irreversible, and kinetic systems . Analytical Chemistry 36 , 706 - 723 ( 1964 ).
26. Bard , A. J. , Faulkner , L. R. , Leddy , J. & Zoski , C. G. Electrochemical methods: fundamentals and applications . Vol. 2 (wiley New York, 1980 ).
27. Kresse , G. & Furthm?ller , J. Efficient iterative schemes for ab initio total-energy calculations using a plane-wave basis set . Physical Review B 54 , 11169 - 11186 ( 1996 ).
28. Perdew , J. P. , Burke , K. & Ernzerhof , M. Generalized gradient approximation made simple . Physical Review Letters 77 , 3865 - 3868 , https://doi.org/10.1103/PhysRevLett.77.3865 ( 1996 ).
29. Blochl , P. E. Projector Augmented-Wave Method . Physical Review B 50 , 17953 - 17979 , https://doi.org/10.1103/PhysRevB.50.17953 ( 1994 ).
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