Investigation of the reaction progress between stannous chloride and zirconium in molten LiCl-KCl

Article abstract of DOI:10.1039/c5ra02476e

LiCl-KCl-ZrCl₄ melt was prepared by an in situ displacement reaction between SnCl₂ and Zr in LiCl-KCl melt at 773 K, and the progress of the reaction between SnCl₂ and Zr was also investigated by dynamic electrochemical measurements, such as cyclic voltammetry, square wave voltammetry and open circuit chronopotentiometry. The results reveal that the concentration of Zr(iv) increases gradually and reaches a maximum value with the reaction time increasing from 0 to 210 min, while the concentration of Sn(ii) decreases gradually and drops below the detection limit. In addition, the chemical analyses of Zr(iv) and Sn(ii) in the melt at various times were also carried out and the results are in good agreement with those of the electrochemical measurements. Finally, LiCl-KCl-ZrCl₄ melts with a low concentration of Sn(ii) (<0.01 wt%) were obtained, when the reaction time was prolonged to 210 min.

Full text of DOI:10.1039/c5ra02476e

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PAPER
Investigation of the reaction progress between
stannous chloride and zirconium in molten LiCl–
KCl
Cite this: RSC Adv., 2015, 5, 31648
a
a
b
Yanqing Cai, Hongxia Liu, Qian Xu, Qiushi Songa and Huijun Liuc
*
LiCl–KCl–ZrCl4 melt was prepared by an in situ displacement reaction between SnCl2 and Zr in LiCl–KCl
melt at 773 K, and the progress of the reaction between SnCl2 and Zr was also investigated by dynamic
electrochemical measurements, such as cyclic voltammetry, square wave voltammetry and open circuit
chronopotentiometry. The results reveal that the concentration of Zr(IV) increases gradually and reaches
a maximum value with the reaction time increasing from 0 to 210 min, while the concentration of Sn(II)
decreases gradually and drops below the detection limit. In addition, the chemical analyses of Zr(IV) and
Sn(II) in the melt at various times were also carried out and the results are in good agreement with those
of the electrochemical measurements. Finally, LiCl–KCl–ZrCl4 melts with a low concentration of Sn(II)
(<0.01 wt%) were obtained, when the reaction time was prolonged to 210 min.
Received 9th February 2015
Accepted 27th March 2015
DOI: 10.1039/c5ra02476e
www.rsc.org/advances
into the salt since it is easy to be atomized due to its low boiling
Introduction
point of 331 C.3,6,7
ꢀ
Several investigators7–10 have attempted to prepare the
molten chlorides by the oxidation of metal with CdCl2 in the
LiCl–KCl melt, and this method also can eliminate any gasi-
cation of the metal chloride with low boiling point because the
chloride will preferably dissolve in the melt rather than gasify
into the air. Murakami et al.7 have attempted to prepare LiCl–
KCl–ZrCl4 melt by the reaction between CdCl2 and zirconium
metal according to the eqn (1).
Zirconium, as a major component, is widely used in nuclear
reactors for the cladding of the nuclear fuels and the structural
materials due to its low neutron-capture cross-section and high
corrosion resistance under normal service conditions.1
However, it must be nearly free of hafnium because of
hafnium's large neutron capture cross-section. Hence, the
rening of zirconium becomes a technology required substan-
tially for producing hafnium-free zirconium metal.2 The elec-
trochemical rening of molten chlorides with ZrCl4 has been
developed as a promising option for zirconium rening from
impure zirconium metal, alloy or spent metal fuels.3,4
In fact, during the electro-rening process in the molten salt,
a low-melting point Cu–Sn–Zr alloy is proposed to be a liquid
anode, while the highly pure zirconium can be deposited on the
cathode.5 The method would not only reduce the operating
temperature, but also provide a low cost and semi-continuous
process for the production of the nuclear grade zirconium.
The electrorening of Zr should be carried out in LiCl–KCl–
ZrCl4 molten salt at the temperature higher than 773 K
considering the melting point of Cu–Sn–Zr alloy as an anode.
However, there is a big challenge for preparation of the molten
LiCl–KCl–ZrCl4 as an electrolyte, if ZrCl4 is directly introduced
2CdCl2(l) + Zr(s) ¼ ZrCl4(l) + 2Cd(l)
(1)
In our previous work,10 we had prepare LiCl–KCl–ZrCl4 in situ
with the equilibration reaction between cuprous chloride and
zirconium.
Here, considering that the liquid Cu–Sn–Zr can be used as a
liquid anode, and stannous chloride is non-toxic and easily
obtained, SnCl2 was selected as the reactant for the displace-
ment reaction with Zr in order to prepare LiCl–KCl–ZrCl4. The
liquid Sn displaced can aggregate into the liquid metal, and be
removed from the molten salt. The present study focused on in
situ catching the change of electrochemical behavior of the melt
with the time during the reaction between stannous chloride
and zirconium in the LiCl–KCl melt by the electrochemical
methods, such as cyclic voltammetry and square wave voltam-
metry. This dynamically electrochemical measurement and
analysis could be expected for monitoring the reaction progress
and determining the nal of the reaction. The prepared LiCl–
KCl–ZrCl4 melt was also characterized, which could be used as
the electrolyte in the electrorening of Zr with the low-melting
Cu–Sn–Zr liquid alloy anode in the future research.
aSchool of Materials Science and Metallurgy, Northeastern University, Shenyang,
110004, PR China
bSchool of Materials Science and Engineering, Shanghai University, Shanghai, 200072,
PR China. E-mail: qianxu201@mail.neu.edu.cn
cLaboratory for Corrosion and Protection, Institute of Metal Research, Chinese
Academy of Sciences, Shenyang, 110016, PR China
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Experimental
Electrochemical apparatus and chemicals
All of the electrochemical experiments were conducted under
high-purity argon atmosphere.
A three-electrode electro-
chemical cell was assembled in an alumina crucible, which was
positioned in a stainless steel vessel and heated with an electric
furnace.10 A graphite rod (spectrum pure) having a diameter of
14.0 mm served as the counter electrode (CE), which was
provided by the Shanghai new graphite material Co., Ltd. of
Sinosteel Corporation (Shanghai, China). A molybdenum wire
(99.95% purity) with a diameter of 1.0 mm was used as the
working electrode (WE) for the investigation of electrochemical
behaviour. The active electrode surface area was determined by
measuring the immersion depth of the electrode in the salt aer
each experiment. The reference electrode (RE) was LiCl–KCl–
1.0 wt% AgCl molten salt placed in a close-ended mullite tube
with a 1.0 mm (dia.) silver wire (99.999% purity) immersed in it
for electrical connections. All potentials in this work will be
referred to this electrode unless otherwise stated.
Fig. 1 Schematic diagram of the displacement reaction.
electrochemical behaviour of Zr(IV) and Sn(II) were investigated
to monitor the reaction progress by cyclic voltammetry, square
wave voltammetry and open circuit chronopotentiometry at
various times. Cyclic voltammetry was carried out by changing
the applied potential as a scan rate of 100 mV sꢁ1, and the
objective was to determine the redox peak potentials of zirco-
nium and tin in LiCl–KCl melt. Meanwhile, the melt was sucked
out at various times by a quartz tube during the reaction
process, and the concentrations of tin and zirconium in the
melt were determined by the ICP-AES analysis.
A mixture of 400 g LiCl–KCl eutectic salt (59 : 41 mol%) was
used as the electrolyte, which was dried for more than 72 h at
573 K to remove residual water and then melted under argon
atmosphere in an alumina crucible. In order to eliminate the
residual water and other possible redox-active impurities, pre-
electrolysis of the LiCl–KCl eutectic was conducted at 2.8 V for
2.0 h between two graphite rods prior to the electrochemical
experiments. Stannous(II) ions were introduced into the bath in
the form of dehydrated SnCl2 powders. Anhydrous zirconium
tetrachloride was obtained from Aladdin and used without
further purication. All chemicals used in this work were of
analytical grade. Molybdenum wire, silver wire, and zirconium
wire were supplied by Rare Metallic Co., Ltd of Shenyang, which
were used aer successively polishing with sandpapers and
washing with alcohol.
Results and discussion
Cyclic voltammograms for LiCl–KCl melt, LiCl–KCl–SnCl2
melt and LiCl–KCl–ZrCl4 melt
An AUTOLAB/PGSTAT320 potentiostat from M sꢁ1. EcoChe-
mie, Netherlands controlled with NOVA 1.6 soware for square
wave voltammetry and GPES 4.9 soware for cyclic voltammetry
and open circuit chronopotentiometry was employed to
conduct the electrochemical tests. A quantitative compositional
measurement of the melt at various times was made by using an
inductively coupled plasma-atomic emission spectrometer
(ICP-AES). In addition, an X-ray diffractometer (XRD) was used
to characterize the recovered precipitates.
A group of typical cyclic voltammograms (CVs) recorded at a
molybdenum working electrode are exhibited in Fig. 2, which
are performed in LiCl–KCl melt (curve 1) and the melt with the
Experimental procedure
Aer pre-electrolysis, certain amount of SnCl2 was introduced
into LiCl–KCl eutectic, and then a zirconium spiral (99.4%
purity) more than the stoichiometric amount was introduced
when the salt was in a stable condition. Then displacement
reaction between zirconium and SnCl2 occurred. The schematic
diagram of the displacement reaction process is shown in Fig. 1.
Zr(IV) ions were generated and entered the molten salt during
the displacement reaction, and tin was produced in the form of
droplets because of the low-melting point and then deposited at
Fig. 2 CVs of blank LiCl–KCl melt (curve 1), LiCl–KCl–0.36 wt% SnCl2
melt (curve 2) and LiCl–KCl–0.25 wt% ZrCl4 (curve 3, the inset) on
molybdenum electrode at 773 K. Apparent electrode area: 0.628 cm2,
the bottom of the crucible. During the reaction process, the scan rate: 100 mV sꢁ1
.
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addition of 0.36 wt% SnCl2 (curve 2) and 0.25 wt% ZrCl4 (curve
Due to low concentration and diffusion rate of the cations,
3), respectively. The purpose is to determine the redox peak e.g., Zr4+and Sn2+, the reductions are controlled by the diffu-
potentials of zirconium and tin in LiCl–KCl melt, which will sion process and the rates are relatively slow, leading to small
provide the basal potentials for the determination of zirconium reduction peaks, such as T0, E0 and C0. However, the reduc-
and tin redox peaks in the displacement reaction process. The tions continue when the potential keeps on sweeping to the
cathodic peak A0 at ꢁ2.50 V vs. Ag/AgCl and the corresponding negative direction, with more reduzates depositing. Subse-
anodic peak A are observed as shown in curve 1, which corre- quently, the oxidation rates of the deposited tin and zirco-
spond to the deposition and dissolution of metal Li.11 In the CV nium are relatively high, which are controlled by the
of LiCl–KCl–0.36 wt% SnCl2 melt as shown in curve 2, the small electrochemical process, and the corresponding anodic peaks
cathodic peak T0 at around ꢁ0.50 V is attributed to the reduc- are prominent.3
tion of Sn2+ to metallic Sn. Then the corresponding anodic peak
observed at ꢁ0.30 V is related to the oxidation of Sn to Sn(II).12,13
What's more, another three couples G1/G10 , G2/G20 and G3/G30 are
Displacement reaction process
observed at potentials between ꢁ2.0 and ꢁ2.5 V, which should The reaction between SnCl2 and Zr started when an excess of Zr
be attributed to the formation and dissolution of Sn–Li inter- was added into LiCl–KCl eutectic salt with 0.57 wt% SnCl2. The
metallic compounds.14 The results are in good agreement with reaction process was monitored by in situ recording CVs, as
the Sn–Li phase diagram,15 which has four intermetallic shown in Fig. 3. It is obvious that the electrochemical behav-
compounds, i.e., Li5Sn2, Li13Sn5, Li7Sn2, and Li22Sn5. Among iours of tin and zirconium are changed with the progress of the
them, the rst two are probably too close in composition that reaction, which would be analysed in detail below.
they could not be distinguished by means of cyclic voltammetry.
Redox behavior of tin. As shown in Fig. 3 (a, 0 min), a large
In the CV of LiCl–KCl–0.25 wt% ZrCl4 melt, several new cathodic peak T0 at ꢁ0.50 V and a high anodic peak T at around
peaks appear compared to the CV of LiCl–KCl melt (curve 1) as ꢁ0.30 V are observed in LiCl–KCl–0.57 wt% SnCl2 melt before
shown in curve 3. According to the references reported by the introduction of zirconium, which are attributed to Sn2+/Sn
several investigators,3,6,16–18 the two cathodic peaks E0 and C0 at redox couple according to Fig. 2 (curve 2).
around ꢁ1.15 and ꢁ1.22 V are related to the multi-step reduc-
Aer the introduction of zirconium spiral, a declining trend
tion of zirconium, which are corresponding to the reactions of the redox peaks of Sn2+/Sn couple is observed, as shown in
given in eqn (2)–(4). What's more, Sakamura17 has also reported Fig. 3 (0–210 min). The result proves that the reaction between
another peak D0 at ꢁ1.65 V in LiCl–KCl melt containing SnCl2 and Zr goes smoothly according to eqn (8), which is
1.07 wt% ZrCl4, which corresponds to the reduction of ZrCl to thermodynamically favorable according to the very high value of
Zr. Correspondingly, another reaction related to the reduction equilibrium constant.19
of Zr(IV) to ZrCl is also observed at peak C0. It might be explained
by the reduction mechanism of Zr(IV). Competitive reactions
[Zr] + 2(SnCl2) / 2[Sn] + (ZrCl4), KQ773 K ¼ 1.25 ꢂ 1019 (8)
between the reduction of Zr(IV) to ZrCl and Zr(IV) to Zr exist
during the reduction of Zr(IV). At high concentrations of Zr(IV),
Peak C0 is attributed to the reduction of Zr(IV) to ZrCl and Zr
simultaneously. However, at low Zr(IV) concentrations, Zr(IV)
would only reduce to Zr.17 Therefore, in the present work, the
ZrCl4 concentration is so low (0.25 wt%) that the reduction peak
of Zr(IV) to ZrCl at peak C0 and the following reduction peak of
ZrCl to Zr are not observed.
For the oxidation process, the corresponding anodic peaks C
and E are found at around ꢁ0.95 and ꢁ0.88 V, which relate to
the oxidation of Zr to Zr(II) and Zr(IV), and Zr(II) to Zr(IV),
respectively, as shown in eqn (5)–(7). Among them, peak E is not
very clear and always masked by peak C3.
Peak E0: Zr(IV) + 2e / Zr(II)
Peak C0: Zr(IV) + 4e / Zr
Zr(II) + 2e / Zr
(2)
(3)
(4)
(5)
(6)
(7)
Peak C: Zr ꢁ 4e / Zr(IV)
Zr ꢁ 2e / Zr(II)
Fig. 3 CVs of LiCl–KCl–0.57 wt% SnCl2 melt with excessive Zr spiral at
various times during the displacement reaction on molybdenum
electrode at 773 K. Apparent electrode area: 0.628 cm2, scan rate:
Peak E: Zr(II) ꢁ 2e / Zr(IV)
100 mV sꢁ1
.
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It is very important to detect the Sn(II) concentration because
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Table 1 The calculated and measured data of Sn(II) concentration at
various times
it can reveal the reaction progress. In addition, as an impurity of
the nal LiCl–KCl–ZrCl4 melt, the nal Sn(II) concentration
should be strictly controlled. To obtain more details about the
Sn(II) concentration, the high-resolution CVs of cathodic peak T0
at different times in Fig. 3 are presented in Fig. 4(a). Apparently,
a declining trend about the cathodic peak currents is observed.
To further explore the relationship between Sn(II) concentration
and peak current, the variation of the cathodic peak T0 at
various Sn(II) concentrations (co) was also studied and shown in
Fig. 4(b). A linear relationship between the peak current Ipc and
co is obtained and shown in the inset of Fig. 4(b), according to
the Berzins–Delahay equation as shown in eqn (9).20 The
calculated Sn(II) concentrations at various times in Fig. 4(a) are
exhibited in Table 1 according to eqn (9).
Time/min
Ip/A cmꢁ2
ccal/wt%
cICP/wt%
0
30
55
70
115
150
160
190
210
ꢁ0.0553
ꢁ0.0479
ꢁ0.0320
ꢁ0.0211
ꢁ0.0148
ꢁ0.0089
ꢁ0.0062
ꢁ0.0040
ꢁ0.0010
0.352
0.305
0.204
0.134
0.094
0.067
0.039
0.025
0.006
0.356
0.285
0.199
0.128
0.092
0.068
0.037
0.023
<0.01
Redox behavior of Zr and Sn–Zr intermetallic compounds.
New redox couples K/K0, C/C0 and E/E0 are observed at potentials
between ꢁ0.75 and ꢁ1.50 V with the introduction of zirconium,
as shown in Fig. 3. Their appearance suggests that Zr(IV) is
produced during the reaction between zirconium and SnCl2.
From 30 to 160 min, the couple K/K0 is observed at potentials
between ꢁ0.75 and ꢁ1.15 V. Since the peak potentials of K/K0 in
Fig. 3 are more positive than that of zirconium, they are
deduced to be the redox peaks of Sn–Zr intermetallic
compounds. This could be explained by the so-called “under-
potential deposition”,21–23 the deposition of Zr on the previously
deposited Sn surface could electrochemically occur at a more
positive potential compared to that on the pure Zr metal. It is
similar to the underpotential deposition of Zr on the Sn
cathode, with the formation of Sn–Zr intermetallic compounds.
According to the Sn–Zr phase diagram,19 three intermetallic
compounds are available, i.e. SnZr, Sn3Zr5 and SnZr4. However,
in the present study, their potentials are probably too close and
they could not be distinguished by means of cyclic voltammetry.
Then the couple K/K0 is used to represent all of them in the
following discussion.
Ipc ¼ ꢁ0.157co
(9)
For verifying the accuracy of the calculated results, Sn(II)
concentration was also determined by ICP-AES analysis, as
shown in Table 1. By comparison, good agreement is found
between the measured and calculated data under all conditions
examined, implying that eqn (9) can be used for assuming the
concentration of Sn(II) and monitoring the reaction progress.
Additionally, it shows that the reaction has fully completed
within 210 min.
The peaks for the couple K/K0 appear aer the displacement
reaction last for 30 min and then increase to the maximum
value until 115 min, and nally decrease and disappear. The
reason is that the formation of these intermetallic compounds
rstly increases and then declines with the passage of reaction
time, as the concentration of Sn(II) decreases and Zr(IV)
increases. Meanwhile, the peaks for couples C/C0 and E/E0
appear and increase gradually when the reaction time reaches
to 150 min. which are corresponding to the couples Zr(IV)/Zr(II),
and Zr(II)/Zr and Zr(IV)/Zr, respectively, according to Fig. 2. It
indicates that apart from the formation of Sn–Zr intermetallic
compounds, the reduction and oxidation of zirconium occur
when the Zr(IV) ions are excessive in the melt. Finally, the peak
currents of C/C0 and E/E0 increase continuously and go to the
maximum value while T/T0 and K/K0 disappear, which suggests
the completion of the reaction.
CV was performed when the displacement reaction time
reached 210 min, as shown in Fig. 3 (b, 210 min), and the peaks
for couples C/C0 and E/E0 are both observed. All peaks are
consistent with the behaviour of zirconium in Fig. 2 (curve 3),
which is measured by the directly introducing of the ZrCl4,
implying that the generated Zr ions in the melt are in the form
Fig. 4 (a) High-resolution CVs of the cathodic peak T0 in Fig. 3, and (b)
CVs of LiCl–KCl–SnCl2 melt at various Sn(II) concentrations on
molybdenum electrode at 773 K, the inset shows the relationship
between the peak current Ipc and co(Sn(II)). Scan rate: 100 mV sꢁ1
.
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of Zr(IV). Additionally, no current peaks corresponding to Sn are suggests the increase of the Zr(IV) concentrations in the melt.
observed, suggesting that all the Sn(II) ions have been reduced Finally, only peaks C0 and E0 remain until 210 min, and peaks T0
to metallic form within 210 min and only Zr(IV) remains in the and K0 have completely disappeared. It implies that the reaction
melt.
is completed, and Sn(II) ions are exhausted while only Zr(IV) ions
remain. The results of square wave voltammetry are in excellent
agreement with those obtained from the CV measurements.
Square wave voltammetry
To further conrm the cathodic peaks K0, C0 and E0, square wave
voltammetry, a method more sensitive than the conventional
cyclic voltammetry, was carried out to further monitor the
displacement reaction progress in situ.23–25
Open circuit chronopotentiometry
In order to further investigate the progress of displacement
reaction, open circuit chronopotentiometry was performed for
observing the formation and dissolution of metals and inter-
metallic compounds. During the reaction process, a thin layer
of alloy was rstly prepared by cathodic deposition on the
molybdenum electrode for a period of 10 s at ꢁ2.60 V. Then the
open-circuit potential of the electrode was registered as a
function of time aer switching off the potentiostatic control.
Aer that, the potential plateaus equal to biphasic equilibrium
of coexisting states at the electrode surface would be
observed.21,23
Fig. 6 illustrates a set of open circuit chronopotentiograms
(OCPs) obtained at different periods during displacement
reaction. Curve (I) provides an OCP measured in LiCl–KCl–
0.57 wt% SnCl2 melt before the introduction of Zr. In the
beginning, a stable potential at around ꢁ2.50 V (plateau a) is
observed, which refers to the Li+/Li equilibrium potential.
Aerwards, three new plateaus b1, b2, b3 at ꢁ2.05, ꢁ2.20, and
ꢁ2.32 V are obtained respectively, which are related to the
dissolution of Sn–Li intermetallic compounds. Finally, plateau c
located at ꢁ0.38 V appears, which is associated with the equi-
librium potential of Sn2+/Sn redox couple. What's more, plateau
k at ꢁ0.2 V should be associated with the open circuit potential
of molybdenum electrode. Curve (II) shows an OCP obtained
aer the reaction reaches 60 min. Apart from the plateaus
observed in curve (I), a new plateau d1 is observed at ꢁ0.75 V,
which should correspond to one of the Sn–Zr intermetallic
compounds. With the reaction reaching 140 min, new plateaus
Fig. 5 shows a series of square wave voltammograms (SWVs)
detected at different times during the reaction between SnCl2
and Zr in LiCl–KCl–0.57 wt% SnCl2 melt at a step potential of
1 mV and frequency of 15 Hz. Before the introduction of
zirconium (0 min), a large cathodic signal (peak T0) is observed
at ꢁ0.45 V, which is associated with the reduction of Sn(II) to
metallic Sn. Aer the introduction of zirconium (40–210 min),
the peak currents of T0 decrease gradually with the peak
potential shiing negatively, indicating that the Sn(II) concen-
tration declines with the progress of reaction. What's more, a
new reduction peak K0, corresponding to the Sn–Zr intermetallic
compounds obtained at ꢁ0.95 V, is observed from 40 to
190 min. The peak current of K0 rstly increases and then
decreases, and nally disappears from the curves. Simulta-
neously, when the reaction time prolongs to 70 min, peaks of C0
and E0 appear, which are attributed to the multi-step reduction
of zirconium. It indicates that there is excess Zr(IV) remaining in
the melt aer the formation of Sn–Zr intermetallics. From 70 to
210 min, peak currents of C0 and E0 increase gradually, which
Fig. 6 OCPs of LiCl–KCl–0.57 wt% SnCl2 melt after electrodepositing
Fig. 5 SWVs of LiCl–KCl–0.57 wt% SnCl2 melt with excessive Zr spiral at ꢁ2.60 V vs. Ag/AgCl for 10 s on molybdenum electrode at 773 K. (I)
at various times during the displacement reaction on molybdenum Before the introduction of Zr spiral, 0 min, (II) after the introduction of
electrode at 773 K. Apparent electrode area: 0.628 cm2, frequency: 20 excessive Zr spiral for 60 min, (III) 140 min and (IV) 210 min. Apparent
Hz, pulse height: 15 mV, potential step: 1 mV.
electrode area: 0.628 cm2.
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d2, d3, e and f are observed in the potential range from ꢁ0.85 to may be because the concentrations of the reactants (SnCl2, Zr)
ꢁ1.10 V in curve (III), respectively. Similarly, plateaus d2 and d3 in the initial stage are high, then the reaction is fast. In the
should be correlated with the formation of the other two Sn–Zr second stage, the concentration of Sn(II) decreases continuously
intermetallic compounds. Additionally, plateau f should be and drops to below the detection limit of the ICP-AES analysis
associated with the deposited Zr corresponding to the equilib- with the reaction time prolonging to 210 min. The concentra-
rium potential of Zr(IV)/Zr and Zr(II)/Zr redox couples, and tion of Zr increases to the maximum value, indicating that
plateau e might correspond to Zr(IV)/Zr(II) couple, which is almost all of Sn(II) has reacted with Zr metal and the reaction is
coincident with the signal of Sn–Zr intermetallic compound completely nished within 210 min. The ICP-AES analysis
(plateau d3). When the displacement reaction is over, the OCP results are well consistent with the electrochemical measure-
obtained at 210 min is shown in curve (IV). It is obvious that ment results.
plateau f becomes more dened than that in curve (III), which
However, with the reaction continuing, there is a slight
can be deduced that more metallic Zr has been deposited on the decline with the concentration of Zr(IV). The slight decline of
electrode surface with the increasing reaction time. It is rather Zr(IV) may be explained from two aspects. On the one hand, it
remarkable that plateaus c, d1 and d2 disappear from curve (IV), may be explained by the eqn (10). Zr(IV) is attacked by excessive
which indicates the absence of Sn(II) ions. Moreover, the plateau Zr and forming Zr(II), which might have a low solubility in the
at ꢁ1.0 V should only correspond to Zr(IV)/Zr(II) couple (plateau melt. In addition, Basile16 and Sakamura18 have also investi-
e) at this time. Apparently, the results obtained by the open gated the corrosion of Zr with ZrCl4, and the former insisted
circuit chronopotentiometry test are well consistent with those that Zr(II) chloride is insoluble. Both of them agree that the
observed in the CVs and SWVs.
ZrCl2 generated in eqn (10) is unstable and would dispropor-
tionate to ZrCl4 and metallic Zr powders, according to eqn (11).
In the authors' review, the reactions (10) and (11) might be
approximately reversible, and Zr(II) might have a low solubility
in the melt, leading the decline of Zr(IV). On the other hand, the
soluble Zr(IV) and partly soluble Zr(II) chlorides easily reacted
with oxygen, which may be introduced by the moisture in the
owing argon, resulting in the formation of insoluble ZrO2 and
the nal decrease of the zirconium concentration in the melt.1
In a word, the nal zirconium content remains nearly constant
and the measured CV does not change in shape aer a long time
contact between Zr and Zr(IV), indicating that the existence of
Zr(II) in the melt may do not have any inuence.18 Finally,
almost all the zirconium ions remained in the melt are in the
form of Zr(IV).
Characterization of the melt and byproducts
To further investigate the progress of the reaction, the
concentrations of tin and zirconium in the melt were deter-
mined by the ICP-AES analysis at various times. Fig. 7 exhibits
the variation of Sn and Zr concentrations with reaction time in
LiCl–KCl melt containing 1.14 wt% SnCl2. The concentration of
Sn decreases gradually while the concentration of Zr increases
as the reaction prolongs. What's more, the reaction rate in the
beginning (0–70 min) is higher than later (70–210 min). This
ZrCl4 + Zr / 2ZrCl2
2ZrCl2 / ZrCl4 + Zr
(10)
(11)
To further conrm the nal concentration of Zr(IV), a series
of displacement experiments with different initial SnCl2
concentrations (0.57 wt%, 1.14 wt% and 2.09 wt%) and excess
Zr were conducted. And the masses of Zr, before and aer the
displacement experiment and dissolved in the melt were
measured. Table 2 shows the comparison of the calculated
weight losses of zirconium (m1) and experimental weight losses
of zirconium (m2), and the nal masses of zirconium dissolved
in the melt (m3) with different initial SnCl2 concentrations.
Fig. 7 The effect of reaction time on concentration of Sn and Zr
dissolved in LiCl–KCl–1.14 wt% SnCl2 melt.
Table 2 Calculated and experimental weight losses of zirconium and the weight of zirconium dissolved in melt
c
Test no.
Initial concentration of SnCl2 (wt%)
m1a/g
m2b/g
m3 /g
Final concentration of ZrCl4 (wt%)
1
2
3
0.57
1.14
2.09
0.55
1.10
2.01
0.94
2.68
2.91
0.43
0.89
1.83
0.28
0.57
1.17
a
c
m1-calculated weight loss of zirconium. b m2-experimental weight loss of zirconium. m3-mass of zirconium dissolved in melt.
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Among them, test no. 1 is corresponding to the electro- avoided in the continuous operation. In the actual rening
chemical tests and test no. 2 is corresponding to the ICP-AES experiment, the recovered copper and zirconium would be
analysis in Fig. 7. By comparison, the experimental weight los- returned to the liquid Cu–Sn–Zr alloy at the bottom of the
ses of zirconium are higher than the calculated data by eqn (8), crucible for reusing continuously in the electrorening process.
while the dissolved masses of zirconium are lower than them.
Combing with the above interpretations, the possible reason is
that there are two kinds of contributions to the weight losses of
Conclusions
zirconium. One is due to the participation in the displacement
reaction, and the other is the participation in the reaction
shown in eqn (10). This results in the increase of the experi-
mental weight losses and the decrease of the dissolved zirco-
The in situ preparation of LiCl–KCl–ZrCl4 melt by the displace-
ment reaction between Zr and SnCl2 was investigated in LiCl–
KCl molten salt at 773 K, and the reaction progress was also
monitored in situ by a series of electrochemical methods, such
nium masses. However, due to the equilibrium between the eqn
as cyclic voltammetry, square wave voltammetry and open
(10) and (11), the dissolved zirconium is slightly less than the
circuit chronopotentiometry. The electrochemical results reveal
that the concentration of Zr(IV) gradually increases with the
decline of Sn(II) concentration and reaches to the maximum
value, while Sn(II) drops to below the detection limit when the
calculated value. Finally, the LiCl–KCl–ZrCl4 melts are obtained
with relatively stable concentrations of ZrCl4 (0.28–1.17 wt%).
The tin and zirconium precipitates were obtained and
characterized as shown in Fig. 8. A clear layer structure with
reaction time prolongs to 210 min. The chemical analyses of
dark grey precipitates at the bottom and the molten salt in the
Zr(IV) and Sn(II) are in good agreement with the electrochemical
results. As a result, the reaction progress between SnCl2 and
zirconium is clearly presented, and LiCl–KCl–ZrCl4 melts with
upper layer are observed according to their different densities.
Aer washing with water, the precipitates are determined as a
mixture of Sn, ZrSn2, ZrSn, ZrO2 and Zr by the XRD analysis as
low concentration of Sn(II) (<0.01 wt%) are obtained, which is
shown in Fig. 8(b). Undoubtedly, metallic Sn is obtained by the
proposed to be used as electrolyte in the electrorening of Zr
with the low-melting Cu–Sn–Zr liquid alloy anode in the future.
displacement reaction, while zirconium powders are caused by
the disproportionation reaction of ZrCl2. Then ZrSn2 and ZrSn
are formed spontaneously with low formation energies. ZrO2 is
obtained due to the oxidation of metallic zirconium powders Acknowledgements
during the washing and drying process, as they are easily
The authors acknowledge the nancial support of National
oxidized even at low temperatures in accordance with the
Natural Science Foundation of China (Grant no. 51174055).
calculation by the HSC code,3 as well as by the oxidization and
hydroxylation of zirconium chlorides. Its formation can be
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