D824
Journal of The Electrochemical Society, 161 (14) D824-D830 (2014)
0013-4651/2014/161(14)/D824/7/$31.00 © The Electrochemical Society
The Electrodeposition of Lead in LiCl-KCl-PbCl2 and
LiCl-KCl-PbCl2 -PbO Melts
P. Pershin,a Yu. Khalimullina,a P. Arkhipov,a,z and Yu. Zaikova,b
a Institute of High Temperature Electrochemistry, 620990 Yekaterinburg, Russia
b Ural Federal University named after the first President of Russia B. N. Yeltsin,
620002 Yekaterinburg, Russia
The mechanism for the electrode process that occurs on a molybdenum substrate in LiCl-KCl-PbCl2 and LiCl-KCl-PbCl2 -PbO melts
depending on the concentration of PbO was investigated over the temperature range of 723–823 K using stationary polarization
curves and voltammetry techniques. An increase in the concentration of PbO was found to decrease the diffusion-limiting current
density for the recovery of lead, which was identified by the appearance of an additional peak in the voltammograms; it can
be explained by a decrease in the concentration of Pb2+ ions decrease due to the formation of Pb2 O2+ complex ions in the
electrolyte. The evaluation of the equilibrium constant KE and the rate constant of the reaction suggests that this reaction is
strongly shifted toward the formation of the Pb2 O2+ complex ions. A theoretical polarization curve for the diffusion kinetics was
calculated.
© 2014 The Electrochemical Society. [DOI: 10.1149/2.0051501jes] All rights reserved.
Manuscript submitted August 19, 2014; revised manuscript received October 3, 2014. Published October 31, 2014.
Investigating the kinetics of electrode processes allows one to determine the kinetic parameters of reactions, to calculate the activation
energies of these reactions and to establish the limiting stages of
electrode processes.1 Additionally, data related to the mechanism of
the electrode processes is valuable for developing the scientific basis
for electrochemical technologies that meet the current ecological and
energy saving requirements.
In the literature, considerable attention is focused on the electrorecovery of lead from molten salts. The authors2,3 reported that the
electro recovery of Pb2+ ions from a LiCl-KCl melt is a diffusioncontrolled, single-stage reaction.
Haaberg et al.4 investigated the lead electrodeposition process in
LiCl-KCl melts and in pure PbCl2 and determined that the migration
of Pb2+ ions controls the rate of the cathode process in the pure PbCl2
melt at temperatures from 823 to 858 K. A diffusion regime of the
process was observed in the LiCl-KCl melt; the diffusion coefficient
for the Pb2+ ions was equal to 2 · 10−5 sm2 /s.
The cathode behavior of lead ions on a molybdenum substrate in
LiCl-KCl-PbCl2 and LiCl-KCl-PbCl2 -MgCl2 melts was studied using
voltammetry and chronopotentiometry techniques in later works.5 The
extraction of lithium from the initially deposited lead was found to
result in the formation of a Li-Pb alloy in LiCl-KCl-PbCl2 and LiPb-Mg melts with the addition of MgCl2 . The diffusion coefficient of
Pb2+ ions was determined using various techniques, and its value was
found to be 2.26 · 10−5 sm2 /s.
In a number of studies, there are considerable differences regarding
the mechanism of the cathode process. Therefore, Strenberg et al.6
established a two-stage mechanism for the recovery of Pb2+ ions
through the stage of the formation of monovalent Pb+ ions.
These works were primarily performed during the second half of
the XXth century and were related to dilute solutions of lead in alkali
metal chlorides, free of oxide ions. There is no information available
regarding the mechanisms of electrode processes in oxide-chloride
melts. The structures and compositions of ionic melts affect the electrode processes that occur during the production and refining of metals
via the electrolysis of chloride melts. The oxide admixtures introduced
into the melt together with the raw material significantly change the
melt structure due to the formation of oxide-chloride complexes. The
experimental results allow the regularities of the cathode process to be
established and the structures of the ionic groups in the oxide-chloride
melts to be defined.
The objective of the present work is to study the mechanism and
kinetics of the electrode reactions during the recovery of lead ions
fromLiCl-KCl-PbCl2 and LiCl-KCl-PbCl2 -PbO melts.
z
E-mail: [email protected]
Experimental
Experimental cell.— The cathode processes in LiCl-KCl-PbCl2
and LiCl-KCl-PbCl2 -PbO melts were investigated using stationary
galvanostatic polarization curves and voltammetry techniques. An
IPC-Pro potentiostat (VoltaProm LLC, Saint-Petersburg, Russian Federation) and an AutoLab PGSTAT 302N galvanostat-potentiostat
(Metrohm Autolab B.W. Utrecht, the Netherlands) were used to record
galvanostatic impulses and responses. An APPA-109 N multimeter
(Appa Technology Corporation, Taiwan) was used as an additional
recording device.
A schematic of the experimental electrochemical cell is presented
in Figure 1. The experiments were carried out in a quartz tube 3, which
was hermetically sealed with a fluoroplastic cover 2 that contained
openings for electrodes and a thermocouple.
An alundum crucible 10 was placed at the bottom of the cell on
a special support composed of heat-resistant bricks 15. Metallic lead
14 and the prepared electrolyte 11 were loaded into the crucible,
and then the reference electrode 7, working electrode 6 and alundum
sheath 8 with a thermocouple 9 were installed. A molybdenum rod in
an alundum tube-case served as the working electrode (cathode). The
electrode area was chosen in such a way that the electrode length / the
electrode diameter ratio to be more than 10. Thus, the molybdenum
rod length was 17 mm, its diameter was 1 mm and the total surface
area was 0.54 cm2 . Molybdenum was selected as the material for
the working electrode because it does not interact with lead under
the experimental conditions. Other materials, such as graphite and
glassy carbon, could not be used as electrode materials because of the
possibility to recover PbO to metallic lead.
Metallic lead placed at the bottom of the alundum crucible
was used as an auxiliary electrode. The measurements were performed relative to the liquid lead 13 contacting the melt of the same
composition. A molten eutectic mixture of lithium and potassium
chlorides (45–55 mol.%) with added lead oxide and lead chloride
was used as the electrolyte for both the auxiliary and the working electrodes. The testing electrolyte and the reference electrode
electrolyte were separated by the diaphragm of the Gooch asbestos
crucibles 12. Molybdenum rods 5, which were protected from contact with the melt by alundum tubes 4, were used as the current
leads to the liquid metal electrodes. The free ends of the current
leads were covered with rubber plugs 1 to keep the cell hermetically
sealed.
The cell was hermetically sealed, evacuated and filled with purified, dried argon. Then, the cell was placed into a resistance furnace
and heated to the desired temperature under excess argon pressure.
The assembly was equipped with an automatic temperature stabilizing system, which controlled temperature fluctuations within ± 2◦ . A
Chromel-Alumel thermocouple 9 was used to measure the temperature of the cell.
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Journal of The Electrochemical Society, 161 (14) D824-D830 (2014)
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Lithium chloride (chemically pure grade), potassium chloride
(chemically pure grade), lead chloride (analysis grade) and PbO (ultrahigh purity) were used to prepare the electrolyte (Closed Joint-Stock
Company “Khimreactivsnab”, Ufa, Russian Federation). Metallic lead
(purity of 99.985, Ural Mining Metallurgical Company, V. Pyshma,
Russian Federation) was used for the reference and auxiliary electrodes.
Preparation of salts and electrolyte.— Potassium chloride was
dried under vacuum at 673 K and then melted under an argon atmosphere. Lithium chloride was dried in a glassy carbon crucible,
first at 473 K and then at 673 K, and then it was melted at 973 K
under an argon atmosphere. The prepared KCl and LiCl salts were
mixed in a 0.55–0.45 molar ratio and melted at 723 K under an argon atmosphere. Lead chloride was dried at 473 K and purified using
the zone-melting method. Lead (II) oxide was dried under vacuum at
473 K. The salts were stored in a glove box under a dry atmosphere.
For each experiment, 100 g of the prepared salt mixture was used.
The appropriate salts and oxide samples were weighed on a Shimadzu
BL-220H (Shimadzu Corporation, Kyoto, Japan) balance with an
accuracy of 0.001 g.
Results
Figure 1. Schematic of the experimental cell: 1 – Rubber plugs; 2 – Fluoroplastic cover; 3 – Quartz test tube; 4 – Alundum tubes; 5 – Current leads to
electrodes; 6 – Working electrode (Mo); 7 – Reference electrode quartz sheath;
8 – Thermocouple sheath; 9 – Thermocouple; 10 – Alundum crucible; 11 –
Electrolyte; 12 – Gooch asbestos crucible; 13 – Reference electrode (Pb); 14
– Counter electrode (Pb); 15 – Heat-resistant brick support.
Stationary galvanostatic polarization curve technique.— The lead
electrodeposition process in the LiCl-KCl-PbCl2 melt on a molybdenum substrate was investigated at temperatures from 723 to 823 K
and lead chloride concentrations of 1.06 and 2.10 mole%. During the
experiment, the overvoltage values were determined at the moment of
steady-state current shutdown in galvanostatic mode. The polarization
curves are presented in Figure 2.
An insignificant deviation in the potential from its equilibrium
value was observed in the initial area of the polarization curves as the
cathode current density increased to 0.4 A/cm2 (Fig. 2B). The dependency of η on lg(1-i/id ) is presented on Figure 3. The experimental
points in the [η – lg(1-i/id )] coordinates remained within the straight
lines. The tangent of the inclination angle is equal to tgα = 0.032,
0.034 and 0.036 V.
Figure 2. Polarization curves for the recovery of Pb2+ ions from the LiCl-KCl-PbCl2 melt: A: Temperature dependency at a PbCl2 concentration of 1.06 mole%;
B: Temperature dependency at a PbCl2 concentration of 2.10 mole%; C: Concentration dependency at 723 K.
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Journal of The Electrochemical Society, 161 (14) D824-D830 (2014)
Figure 3. Dependence of n on lg(1-i/1d ): A – 723 K; B – 773 K; C – 823 K.
Table I. Values of the limiting current density (id , A/cm2 ) for the
electrodeposition of lead ions in the LiCl-KCl-PbCl2 melt.
CPbCl2 , mole%
T, K
723
773
823
1.06
2.10
0.220
0.520
0.335
0.700
0.480
0.900
The number of electrons participating in the electrode reaction was
obtained according to equation 1:
n=
RT
tgα · F
[1]
Values of n = 2.00, 1.96 and 1.97 were calculated for temperatures
723, 773 and 823 K, respectively.
The cathode reaction can be described as follows:
Pb2+ + 2e → Pb0
[2]
When the current density reached values of 0.184, 0.323 and 0.406
A/cm2 at 723, 773 and 823 K, respectively (Fig. 2B), a sharp negative
shift in the potential to values of −1.003, −1.284 and −1.316 V was
observed.
The diffusion-limiting current density for the recovery of lead
increased with temperature and the concentration of PbCl2 (Fig. 2).
At a PbCl2 concentration of 1.06 mole% and temperature of 723 K,
the limiting current density was equal to 0.220 A/cm2 , and at the
same PbCl2 concentration and temperatures of 773 K and 823 K, the
limiting current densities were 0.335 and 0.480 A/cm2 , respectively.
When the PbCl2 concentration increased to 2.10 mole%, the limiting
current density increased to 0.520 A/cm2 at 723 K. The id values are
presented in Table I.
The analysis of the obtained dependences and literature data2–6
revealed the diffusion nature of the limiting current density for the
recovery of lead.
At the negative potential shift to −1.0 ÷ −1.3 V (relative to the
lead reference electrode), the recovery of lithium ions (Li+ ) occurs
according to the following reaction:
Li+ + e = Li◦
[3]
The low potential values for the recovery of alkali metal can be
explained by the formation of a lead and lithium alloy. Zhang et al.5
reported that PbLi3 and Li7 Pb2 compounds may be formed at the
cathode surface in the LiCl-KCl-PbCl2 melt.
The polarization curves for the electrode process in the LiCl-KClPbCl2 -PbO melt as a function of the lead (II) oxide concentration and
temperature are given in Figure 4.
Figure 4. Polarization curves for the recovery of Pb2+ ions from the LiCl-KCl-PbCl2 -PbO melt with a lead chloride concentration of 2.10 mole%: A: Dependence
on the PbO concentration at 773 K; B: Dependence on the PbO concentration at 823 K; C: Temperature dependency at a PbO concentration of 1.20 mole%.
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Journal of The Electrochemical Society, 161 (14) D824-D830 (2014)
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Table II. Values of the limiting current density (id , A/cm2 ) for the
electrodeposition of lead ions in the LiCl-KCl-PbCl2 -PbO melt at
a PbCl2 concentration of 2.10 mole%.
CPbO , mole%
T, K
723
773
823
0.00
0.54
1.20
1.78
0.520
0.330
0.200
0.125
0.700
0.520
0.280
0.160
0.900
0.580
0.360
0.220
This figure clearly indicates that the dependences [i – η] have a
similar form as the curves obtained in the melts without lead oxide
(Fig. 2). However, the limiting current density values in the oxidechloride melts are considerably lower than in chloride melts.
The initial area of the polarization curves shows an increasing
cathode current density and an insignificant deviation in potential
from the equilibrium value.
The values of tgα for the [η – lg(1-i/id )] dependency are
0.031,0.033 and 0.036 V at 723, 773 and 823 K, respectively. The
number of electrons calculated according to equation 1 is equal to n
= 2.00, 2.02 and 1.98.
The form of the polarization curves and their temperature and
PbO concentration dependences indicate that the cathode reaction
is a diffusion-controlled process. The values of the limiting current
densities for the electrodeposition of lead (II) ions in the LiCl-KClPbCl2 -PbO melt are presented in Table II.
From the diffusion kinetics perspective, the diffusion-limiting current density should increase as the concentration of potential-forming
ions increases. However, the plot (Fig. 4) indicates that the diffusionlimiting current density for the recovery of lead ions in the oxidechloride melt is considerably lower than that in the pure chloride melt
under otherwise equal conditions.
The addition of PbO to the chloride melt results in a decreasing
limiting current density for the electrochemical recovery of lead ions.
This fact has never been mentioned in the literature. This phenomenon
is apparently associated with the physical and chemical properties of
the melt and its changes.
Voltammetry.— The electroreduction of lead (II) ions on a molybdenum substrate in LiCl-KCl-PbCl2 melts with lead chloride concentrations of 1.06 and 2.10 mole% and in LiCl-KCl-PbCl2 -PbO melts
with a lead chloride concentration of 2.10 mole% and lead oxide concentrations of 0.54, 1.20 and 1.78 mole% were studied using cyclic
Figure 5. Cyclic voltammograms for a Mo electrode in LiCl-KCl at 773 K
with various scan rates: 1–0.2 V/s; 2–0.1 V/s; 3–0.05 V/s.
voltammetry over the temperature range of 723–823 K and sweep
potential range of 50–650 mV/s.
The range of potentials was selected bytaking into account the
investigation of only the cathode process (from zero to −1.4 V relative to the lead reference electrode) because the molybdenum rod
will dissolve in the anode region of potentials. The impossibility of
using graphite and glassy carbon in our experiments was mentioned
above.
The background voltammograms in the LiCl-KCl melt at 773 K
(Fig. 5) confirm the purity of the studied electrolyte. In the potential
range from 0–1.0 V, no peaks were observed, and further shifting of
the potential leads to the recovery of lithium ions (Li+ ) according to
reaction 3.
Figure 6 presents the LiCl-KCl-PbCl2 voltammograms depending
on the temperature and potential sweep rate within the range from
0.10 to 0.65 V/s.
When the potential sweep shifts to the cathode side, only one
electroreduction peak of the lead ions is observed at all potential
sweeps, as described by reaction 2 for the recovery of lead.
The number of electrons participating in the cathode process was
calculated according to equation valid only for reversible processes:7
E P/2 − E p = 2.20 ·
RT
,
nF
[4]
Figure 6. Cyclic voltammograms for a Mo electrode in a LiCl-KCl-PbCl2 (1.06 mole%): A: At 823 K with various scan rates: 1–0.1 V/s; 2–0.2 V/s; 3–0.5 V/s;
4–0.65 V/s. B: Under a potential sweep rate of 0.10 V/s at different temperatures: 1–723 K; 2–773 K; 3–823 K.
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D828
Journal of The Electrochemical Society, 161 (14) D824-D830 (2014)
Figure 7. Dependence of the peak current on the square root of the sweep rate
in a LiCl-KCl-PbCl2 melt on a Mo electrode at 823 K at a PbCl2 concentration
of 2.10 mole%.
where Ep - peak potential;
EP/2 - half-peak potential;
F - Faraday constant, 96484 C/mole;
R - absolute gas constant, 8.314 J/mole;
T - temperature, K;
n - number of electrons.
Thus, the average value of the number of electrons for the cathode
process was determined to be 1.97 ± 0.04.
The study of the peak potential dependency on the logarithm of the
sweep rate revealed that the peak potential insignificantly shifts to the
cathode side, from −124 to −129 mV, as the sweep rate increases. This
result indicates that the reduction of lead ions in the LiCl-KCl-PbCl2
melt is a reversible process.5
The Ip – V1/2 dependency (Fig. 7) is a continuous straight line that
is extrapolated through the origin of the coordinates.
The obtained results confirm that the electroreduction of Pb2+ in
the chloride melt is controlled by diffusion throughout the potential
sweep range under the experimental conditions. The diffusion coefficient for the lead ions in the LiCl-KCl-PbCl2 melt was calculated as
described by the Berzins-Delahay equation:5,8
n F 1/2 1/2 1/2
I p = 0.61 · n F SC
D V ,
[5]
RT
where Ip - cathode current peak, A;
V - potential sweep rate, V/s;
S - cathode area, cm2 ;
D - diffusion coefficient, cm2 /s;
C - concentration of electroactive particles, mole/vm3 .
The calculated values of the diffusion coefficients for the Pb2+ ions
in the LiCl-KCl-PbCl2 melt are 1.91, 2.29, and 2.59 · 10−5 cm2 /s at
723, 773 and 823 K, respectively. These values are in good agreement
with the literature data.3,9
The obtained results confirm that the reduction of Pb2+ ions on the
Mo substrate in the LiCl-KCl-PbCl2 melt over the temperature range
of 723–823 K is a one-stage diffusion-controlled reaction.
Figure 8 presents voltammograms for the LiCl-KCl-PbCl2 -PbO
(1.78 mol.%) melt at 823 K depending on the potential sweep rate
within the range from 0.25 to 0.65 V/s.
Two peaks are observed in the voltammograms of the cathode
process when PbO is added to the LiCl-KCl-PbCl2 -PbO melt.
Figure 8. Cyclic voltammograms for a Mo electrode in a LiCl-KCl-PbCl2 PbO (1.78 mole%) melt at 823 K with various scan rates: 1–0.25 V/s; 2–0.35
V/s; 3–0.45 V/s; 4–0.55 V/s; 5–0.65 V/s.
hypothesize that the addition of lead (II) oxide to the LiCl-KCl-PbCl2
melt results in the following equilibrium:
PbCl2 + PbO ↔ Pb2 OCl2
[6]
We previously studied the thermodynamic properties of dilute PbO
solutions in the KCl-PbCl2 melt.12 These systems were found to have
a negative deviation from Raul’s law. Such behavior was explained by
the formation of complex compounds, particularly Pb2 OCl2 .
From analysis of the voltammograms in the LiCl-KCl-PbCl2 -PbO
(Fig. 8) melt, we conclude that the cathode process can be viewed as
the reduction of two different electroactive ions at different potential
values.
Cathode peak A corresponds to the reduction of lead ions, as
described by equation 2. The peak value increases according to the
module as the sweep rate increases, and the potential peak value shifts
to the cathode area.
Cathode peak B is related to the reduction of the Pb2 O2+ oxide
complex formed as a result of the interaction of lead oxide with
the chloride melt.11 This process is the electrochemical reaction for
the reduction of lead with the preliminary chemical decomplexation
reaction. Therefore, the total electrode reaction for peak B may be
described as follows:
Pb2 O2+ + 2e → PbO + Pb0
1/2
[7]
2+
The Ip/V dependences for the electroreduction of Pb and
Pb2 O2+ ions in the oxychloride melts (Fig. 9) are linear, i.e., the
limiting stage is constant throughout the sweep rate range.
Discussion
Previous works10,11 have reported that if PbO is added to the leadcontaining melt, the formation of the Pb2 OCl2 compound is thermodynamically possible. The X-ray phase analysis also showed the
presence of the Pb2 OCl2 compound in the frozen electrolyte. Therefore, based on the analyses in the above-cited works, it is possible to
Figure 9. Dependence of the peak current B on the square root of the sweep
rate in a LiCl-KCl-PbCl2 -PbO melt at 823 K.
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Journal of The Electrochemical Society, 161 (14) D824-D830 (2014)
D829
complex compound:
N Pb2 OCl2 = N PbO
[9]
2+
particles that participate
The concentration of electroactive Pb
in the electrode process is equal to:
0
− N Pb2 OCl2
N Pb2+ = N PbCl
2
[10]
0
where N PbCl2 – the initial molar fraction of PbCl2 .
The molar fraction was then recalculated to the molar concentration
according to the equation:
C Pb2+ = N Pb2+ ·
ρ LiCl−K Cl−PbCl2
μ LiCl−K Cl−PbCl2
[11]
where ρLiCl-KCl-PbCl2 – density of LiCl-KCl-PbCl2 , g/cm3 ;13
μ LiCl-KCl-PbCl2 – molar mass of LiCl-KCl-PbCl2 , g/mole.
Figure 10. Voltammograms from the LiCl-KCl-PbCl2 -PbO melt at 823 K
with different concentrations of PbO at a PbCl2 concentration of 2.10 mole%.
The increasing concentration of PbO in the LiCl-KCl-PbCl2 -PbO
melt caused the absolute magnitude of the peak A current to decrease,
which corresponds to the lead electroreduction according reaction 2
under otherwise equal conditions. The increase in the peak B current
corresponds to an increase in the PbO concentration in the melt, and the
peak A current simultaneously reflects a decrease in the concentration
of Pb2+ electroactive ions (Fig. 10). This result confirms the hypothesis
that the lead oxide (II) forms Pb2 O2+ oxychloride complexes while
interacting with the chloride melt. The KE constant of equilibrium (6)
was calculated according to the equation:
−G
[8]
KE = ex p
RT
where KE – constant of equilibrium;
G – the Gibbs energy of the equation 6;
The values for the KE constant are 74.34, 40.61 and 23.50 for
773, 823 and 873 K, respectively. These high values of KE assume
that equilibrium (6) is almost completely shifted toward the formation
of the Pb2 OCl2 complex compound. This fact also can be confirmed
with the linear dependency of id as a function of difference of molar
fractions of PbCl2 and PbO (Fig. 11).
Based on the previous discussion, we can conclude that the equilibrium (6) will be significantly shifted toward the formation of the
Pb2 OCl2 compound in the studied LiCl-KCl-PbCl2 -PbO melt.
In this case, the molar fraction concentration of the dissolved lead
oxide can be considered as the concentration of the formed Pb2 OCl2
Figure 11. Dependence of id on the molar fraction of PbCl2 and PbO difference in a LiCl-KCl-PbCl2 -PbO melt at 773 K.
At high values of KE the value of the registered current will be
proportional to the equilibrium concentration of the lead ions. The
nature of this current will be defined by diffusion.
The curve equation for the equilibrium conditions of the polarization will be written as follows:
zF
·η
,
[12]
i = id · 1 − exp
RT
where id - limiting diffusion current density, A/cm2 ;
n - number of electrons;
F - Faraday’s constant, 96485 C/mole;
R - ideal gas constant, 8.314 J/(mole · K);
T - temperature, K;
n - overvoltage, V.
The diffusion-limiting current density is determined according to
the following equation:
id =
n F D Pb2+ C Pb2+
,
δ
[13]
where, DPb2+ - diffusion coefficient of the lead ions, cm2 /s;
SPb2+ - concentration of the electroactive lead ions, mole/cm3 ;
δ - thickness of the diffusion layer.
Experimental determination of the diffusion layer thickness in the
conditions of “natural” convection for laboratory electrolyzers where
halide fusions were exposed to electrolysis, represent a values of δ
within 0.5 mm.14
For equation 13, the value of DPb2+ , calculated from voltammetry
data using Berzins-Delahay equation, equal to 2.29 · 10−5 cm2 /s for
the temperature of 773 K was used.
Thus, the final equation for the polarization curve can be presented
as follows:
nF
n F D Pb2+ C Pb2+
· 1 − exp
·
.
[14]
i=
δ
RT
The comparison of the polarization curve for electrorecovery of
lead ions in the LiCl-KCl-PbCl2 -PbO melt at a PbCl2 concentration
of 2.10 mole%, PbO concentration of 1.20 mole% and temperature of
773 K with the polarization curve calculated according to expression
(14) is presented in Figure 12.
This figure demonstrates that the experimental data and calculated polarization dependency are in good agreement, and the calculated curve is slightly above the experimental one only in the current
density range from 0.3 to 0.5 A/cm2 . Deviations of an experimental curve from theoretical one probably connected with lower values
of diffusion coefficient of lead-containing oxide ions in comparison
with diffusion coefficient of lead (II) ions in the same electrolyte.
Besides the formation of lead-containing oxide ions12 can increase
thickness of a diffusion layer, that also leads to decrease of current
density.
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D830
Journal of The Electrochemical Society, 161 (14) D824-D830 (2014)
2.
3.
4.
ode process in LiCl-KCl-PbCl2 and LiCl-KCl-PbCl2 -PbO melts
within the temperature range of 723–823 K.
The increase in the concentration of PbO in the electrolyte is
determined to reduce the diffusion-limiting current density for the
electrodeposition of lead. The decrease in the diffusion-limiting
current density appears to be due to the reduced concentration of
free Pb2+ ions resulting from the formation of the Pb2 O2+ ions in
the electrolyte.
One current peak corresponding to the recovery of Pb2+ ions
in the LiCl-KCl-PbCl2 melt is observed. It is shown that the
recovery of lead (II) ions in the chloride melt is eversible and
diffusion-controlled process.
The introduction of PbO into the LiCl-KCl-PbCl2 melt leads to
the appearance of a second current peak in the voltammograms.
The additional cathode peak is related to the reduction of the
Pb2 O2+ oxide complex that formed as a result of the interaction
of lead oxide with the chloride melt.
Acknowledgment
Figure 12. Polarization curves for the recovery of Pb2+ ions from a LiClKCl-PbCl2 -PbO melt with a lead chloride concentration of 2.10 mole% and
lead oxide concentration of 1.20 mole% at 773 K: Solid line - experimental
dependency; Dotted line - calculated dependency.
Thus, the addition of lead oxide to the lead chloride-containing
melt complicates the electrode process due to the formation of the
complex ion:
PbO + Pb2+ → Pb2 O2+
[15]
The evaluation of the equilibrium constant KE and the rate constant
of reaction15 allows one to observe that this reaction is strongly shifted
to the right toward the formation of the Pb2 O2+ complex ions. In this
case, the registered current has diffusion character and is defined by
the equilibrium concentration of free Pb2+ ions.
Conclusions
1.
Stationary galvanostatic polarization curves and voltammetry
techniques were used to investigate the mechanism for the cath-
The authors gratefully acknowledge the Shared Access Centre
“Composition of Compounds” for analytical support.
This work was financially supported by the Russian Federation
President Grant MK-5678.2014.3.
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