CATHODE PROCESSES IN KCl-PbCl2 MELT
Yu.P. Zaikov1, P.A. Arkhipov1, Yu.R.Khalimullina2, V.V.Ashikhin2
1
The Institute of High Temperature Electrochemistry, Ural Branch of
Russian Academy of Sciences,
S. Kovalevskaya/Academicheskaya St, 22/20, 620990 Yekaterinburg, email:dir@ihte.uran.ru
2
Open Joint-Stock Company ELECTROMED, Scientific Research
Centre, Lenin St, 1, 624091, Verkhnyaya Pyshma
Technology of crude lead refining is developed in the Institute of
High-temperature Electrochemistry. The crude lead was obtained from
the car battery wastes. While organizing the refinement in the molten
salts it is important to know deposition mechanisms [1] of lead ions in
the chloride melts containing oxychloride complexes. It is necessary to
study kinetics of electrode processes to understand this mechanism
Many authors studied kinetics of electrode processes of lead
electroreduction from chloride melts [2 – 10]. Diffusion coefficients of
ions in molten salts were measured by using radioactive isotopes [10]
and with the help of electrochemical parameters [2 -9].
V.P.Yurkinskyi, D.V. Makarov [2, 3] studied the mechanism and
determined kinetic parameters of Pb(II) ion at electrochemical
reduction process in various individual melts (NaCl, KCl и CsCl), as
well as in mixtures with various component content using linear
voltamperometry, chronopotentiometry and chronoamperometry
methods. Studies of lead ions reduction in lithium, sodium, potassium
and cesium chlorides showed, that cation composition causes significant
influence on the process. Electrochemical reduction is limited by Pb 2+
diffusion in LiCl and NaCl melts, when in the potassium and cesium
chlorides by chemical reaction of complex ion [PbCln]2-n. dissociation.
Diffusion coefficient value was found to decrease and lead(II) ion
diffusion activation energy to increase in the LiCl–CsCl row.
Y.M. Rybuhin, E.A. Ukshe [4] measured lead ions diffusion
coefficients in molten chlorides by chronopotentiometry method.
Measurements were carried out under the argon atmosphere. The
rectangular polished platinum plate about 1 cm2 square was used as the
working electrode. Molten lead placed into the quartz tube, connected by
capillary with the bulk melt was the anode and reference electrode.
186
NaCl, KCl, PbCl2 salts of chemically pure grade were used in the work.
They were melted under vacuum before the experiment. According to
the results of these studies the validity of the Stocks-Einstein equation
to ion diffusion in molten salts is limited by the systems, where the
process of complex formation is absent, that is why the significant
deviations from the equation take place in KCl – NaCl, and especially in
the pure KCl,:
D=KT/(6r),
(1)
where  - viscosity, r – ion radius according to Goldschmidt.
Using oscillographic method I.I. Naryshkin and V.P. Yurkovskyi
determined lead, silver and cadmium ions diffusion coefficients
depending on temperature against the equimolar mixtures NaCl-KCl
and LiCl-KCl. Platinum microelectrode, the platinum wire butt with 0.6
mm diameter, soldered into a quartz capillary was used. 400 mm2
platinum foil was used as anode. Chloride-silver electrode was used as
the reference electrode. Short circuit during two minutes was used to
renovate the electrode surfaces after each observation. For obtaining the
more reliable results each curve was observed several times and the
results were averaged out. Authors showed the direct dependence of the
peak current from the investigated ions concentration. This fact confirms
the conclusion of Hills, Ocsley and Terner [11] about the possibility of
the oscilligraphic voltamperometry for the rapid quantitative analysis in
the molten salts. Dependence of the peak potential from the logarithm of
investigated ion concentration for cadmium, lead and silver was found.
Independence of the peak potential from the concentration logarithm for
cadmium and lead chlorides corresponds to the dissolved matter
deposition. Linear dependence observed for the silver chloride
demonstrates the absence of solubility in the process of silver deposition.
The following valence values were found: for silver 1.16, for lead 2.4.
In the works [7-9] diffusion coefficients of lead, zinc and
cadmium ions in the LiCl – KCl и NaCl – KCl melts were determined.
Raymond J. Heus and James J.Egan [7] used polyrophic method to study
processes of lead, zinc, cadmium ions electroreduction in the molten
chlorides. Dropping bismuth electrode was a cathode. Silver chloride,
containing 2 mass. % of AgCl in KCl – LiCl (eutectics) was a reference
electrode. Authors obtained linear dependencies of the concentration of
187
the investigated chlorides from the diffusion current densities. Diffusion
coefficients were calculated with the Ilkovich equation.
Richard B. Stein [8] investigated the ion reduction reaction of
divalent lead in the NaCl – KCl melt with oscillographic polyrography
method. Platinum microelectrode, with 0.5 mm diameter, soldered into
the quartz tube with 1,89х10-3 cm2 square, was a cathode. Reference
electrode was silver chloride and the auxiliary electrode was graphite.
Author founded out that the lead ion diffusion coefficients obtained by
the experimental data differ from calculated according to the equation of
Stocks-Einstein. He derived the conclusion that the cation structure is
more complex than just a single ion.
H.A. Laitinen, H.C.Gaur [9] investigated lead, cobalt and thallium
ion reduction in the molten potassium and lithium chlorides with
chronopotenciometry method. Authors fixed the value of the transition
time for melts, containing the control values of ions under investigation.
According to the experimental data empiric dependences of
concentrations and transitional time were determined. Coefficients of
cadmium, cobalt lead and thallium ion diffusion were calculated with
Sand’s equation (2,08; 2,42; 2,18; 3,88*10-5 cm2/s correspondingly).
Cathode processes in chloride melts containing lead ions were
studied by chronopotentiometric and stationary galvanostatic
polarization curves methods.
Experiments were carried out in the cell made of quartz
hermetically closed fluoroplastic cover (2) with the holes for electrodes
and thermocouple with accordance to the Fig.1
Glassy-carbon was a working electrode (cathode). Glassy-carbon
container played a role of a counter electrode. Melted equimolar mixture
of lead, lithium and potassium chlorides was used as the electrolyte for
the reference and working electrodes. Electrolytes of the working
electrode and reference electrode were separated by the diaphragm from
the Gooch asbestos (7). Measurements were conducted relatively to the
lead reference electrode that is a metal lead of C1 grade being in contact
with the melt containing 5 mass. % of lead chloride.
Potassium chloride, lithium chloride chemically pure grade and
lead chloride of pure for analysis grade were used for electrolyte
preparation. Glassy-carbon container (4) was placed on the cell bottom
on the special fireproof brick support (8).
188
Current lead to liquid-metal reference electrode was realized in a
form of molybdenum rod and to glassy-carbon crucible through graphite
bar. Current leads were protected from the contact with melt by alundum
tubes closed with the rubber plugs (1) to keep the cell hermetically
closed.
1
Ar
2
Vacuum
9
3
4
5
10
6
7
11
8
Fig. 1. Electrolytic cell. 1 – rubber plugs; 2 – fluoroplastic cover; 3 – thermocouple; 4 – glassy-carbon container; 5 – quartz-glass sell; 6 – working
electrode; 7 – diaphragm; 8 – fireproof brick support; 9 – current leads to
electrodes; 10 – electrolyte; 11 – reference electrode.
189
The cell was pumped out and fullfilled with purified argon. Later
it was put into the resistance furnace and heated until the given
temperature under the abundant pressure of the inert gas.
The setup was equipped with the automatic system of temperature
stabilization. Temperature measurement was performed with the help of
chromyl-aluminum thermocouple. Content of components in electrolyte
were being controlled before and after the experiment with the atomicabsorption method.
Stationary polarization measurements
Lead ion deposition processes in eutectic melt of lithium and
potassium chlorides were studied at 0.4 to 3.0 mol. % lead chloride in
temperature range from 673 to 823 К. Polarization curves are given on
the fig. 2 and 3. Two characteristic areas are observed on the
polarization curves. On the first area little potential deviations from the
equilibrium value takes place with cathode current density increasing to
0.08 A/cm2.
Experimental points on the area with 0.4 mol % lead chloride
concentration are on straight lines described by equations:
E = - 0,0703lgi - 0,1203 and E = - 0,0775lgi - 0,091 for 673 and 773 К
correspondingly.
At temperature 673 К tg is 0,070 мВ, and at 773 К - 0,078 мВ.
According to the equation:
n
2,3 RT
tgF
(2)
we have n=1,9 for 673 К and n=2,0 for 773 К.
At lead chloride concentration 3,0 mol % experimental points on
the first area of the polarization curve is described by the equation
E= - 0,0779lgi - 0,0877.
Amount of electrons in the reaction calculated on the equation (2)
is equal 2.
Reaching current densities 0,11; 0,12; 0,20 и 0,32 А/сm2 on the
fig.3 for 673, 723, 773, 823 К temperatures correspondingly. Potential
is greatly shifted to the negative area to the values -0,84; -0,84; -1,06
and -1,10 correspondingly.
At small values of cathode current density there is one wave
correspondingly to the fig. 4. In some time after current rise potential
190
reaches its stationary value at current density 0,045 А/сm2 for 3,5 s, for
current density 0,060 А/сm2 for
3,0 s. After current disconnection,
potential comes back to its equilibrium value.
Fig. 2. Polarization curves of lead ions (II) deposition in LiCl – KCl – PbCl2
(0.4 mol. %) melt.
191
Fig. 3. Polarization curves of lead ions (II) deposition in LiCl – KCl – PbCl2
melt at 823 К depending on the lead chloride concentration. Concentration of
lead chloride in mol per cents: 1 - 0.4; 2 - 0.5; 3 – 3.0.
192
Fig. 4. Engaging curves at 823 К temperature and the different current density.
On the engaging curves at current density values corresponding to
the second characteristic area on the polarization curves on the figures 2
and 3 two waves on figure 5 are seen. Time of reaching stationary
potential tst decreases with the current density increasing (for current
density 0,12 А/сm2 tst equals 8,5 s, for current density 0,17 А/сm2 tst 4,5 s).
Fig. 5. Engaging curves at 0.4 mol. % lead chloride concentration, current
density 0.12; 0.13; 0.17 А/сm2 and 823 К.
193
Processes taking places on the electrode can be described in the
following way. On the first characteristic area of the polarization curve
lead ion deposition happens:
Pb2+ + 2e = Pb0
(3)
The limiting current density of lead reduction increases with the
temperature and lead chloride concentration. At 3.0 mol. % of lead
chloride concentration and 823 K limiting current density i lim is 1.2
A/cm2.
On the second characteristic area of the polarization curve
deposition of the alkaline metal is possible on the reaction:
K+ + e = K0 (Pb)
(4)
Low values of the alkaline metal reduction potentials might be
connected with the process of alloy formation of alkali metal with lead
K + 4Pb = KPb4
(5)
Chronopotentiometric measurements at lead deposition from LiCl
– KCl (45-55 mol. %) – PbCl2 melt at 0.4 mol. % lead chloride
concentration were performed at 823 K and current density range from
0.10 to 0.17 A/cm2. There is only one wave on chronopotentiometric
curves under these conditions. Values of product i1/2 depending on
current density are given in the table 1, where  - transition time.
Table 1. Values of product i1/2 at diverse current density
, s
i, mA/cm2
i1/2, mA/cm2s1/2
0,95
170
165
1,61
130
165
1,81
120
162
2,62
102
165
It is seen that the product i1/2 does not depend on current
0
density at constant concentration of depolarizator C Ox
. In the table 2
potential values Е/4 at time equaling the forth of the corresponding
values of transition time are given.
194
Table 2.Values of Е/4 potential of different current density
i, A/cm2
0,10
0,12
0,13
0,17
, s
2,64
1,81
1,61
0,95
/4, s
0,660
0,453
0,403
0,238
Е/4, V
-0,061
-0,600
-0,061
-0,062
It is seen that the potential Е/4 does not depend on the experiment
conditions, the current density in this case.
Equation for the reversible process can be as follows:
Е  Е / 4
1/ 2

RT   

ln    1 ,
nF  t 

(6)
for irreversible process:
1/ 2
0
0
RT nFCOx k fh
RT   t  
Е
ln

ln 1     ,
nF
i
nF     
(7)
where E – electrode potential, E / 4 - measurement potential at ¼
of transition time, R – gas constant, F – Faraday number,
n – number
0
of electrons, T – temperature,  - transition time, C Ox - depolarizator
concentration, k 0fh - deposition speed constant.

On the figure 6 dependencies Е - ln   
 t 
 t
ln 1   
   
1/ 2

 at 0.4 mol. % of

1/ 2

 1 and Е 
lead chloride concentration, current
density 0.1 A/cm2 and 823 K are given.
195
- E, В
0,162
0,142
0,122
0,102
y = -0,0835x + 0,0654
0,082
0,062
0,042
0,022
-1,15
-0,65
-0,15
0,35
1

Fig. 6. Dependencies 1–Е=f  ln   
 t 
0,002
0,85
2
1/ 2

 1

 and 2-Е =f 
 t
ln 1   
   
1/ 2
 .
 

From the analysis of given graphic dependencies follows that the
1/ 2


experimental points in coordinates E - ln     1 are in a straight line
 t 

with the confidence interval 0.95. The can be described by equation:
 
E  0,065  0,083 ln  
 t 
1/ 2

 1

(8)
The amount of electrons in the electrode reaction was calculated
from the equation:
n
RT
,
0,083F
(9)
hence n=2.
196
It follows from the experimental conditions on lead ion (II)
deposition that the process is reversible, i.e. it is controlled by the speed
of divalent lead ions mass transfer from the volume of melt to the
electrode surface.
Diffusion coefficient of lead dichloride at 823 K was calculated on
Sand’s equation:
2(i  ) 2
D
 (nFCox0 ) 2
(10)
Lead ions (II) diffusion coefficient are equal to
2,33·10сm /s. It is in good accordance with the data obtained by other authors
[5, 6].
5
2
References
1. Yurkinsky V., Makarov D. Electrochemical reduction of lead ions in
halide melts. Russian J. Applied Chem., 1994, 67, p. 1283-1286.
2. Yurkinsky V., Makarov D. The influence of cation composition on
kinetics of lead electrochemical reduction in chloride melts. Russian
J. Applied Chem., 1994, 68, p. 1474-1477.
3. Ryabukhin Yu. And Ukshe E. The diffusion coefficients of lead in
molten chlorides. DAN SSSR, 1962, 145, p. 366-368.
4. Naryshkin I., Yurkinsky V. Oscillographic investigation of
temperature coefficients for some chlorides diffusion in LiCl-KCl.
Russian J. Electrochemistry, 1968, 4, p. 871-872.
5. Naryshkin I., Yurkinsky V. Voltammetry in molten salts. Russian J.
Electrochemistry, 1968, 2, p. 856-866.
6. Raymond J. Heus, James J. Egan. Fused Salt Polarography Using a
Dropping Bismuth Cathode. – J. of the Electrochemical Society,
October 1960, p. 824-828.
7. Richard B. Stein. The Diffusion Coefficient of Lead ion in Fused
Sodium Chloride Eutectic. – J. Electrochem. Soc., 1959, vol. 106, p.
528.
8. Laitinen H. A., Gaur H. C. Chronopotentiometry in Fused Lithium
Chloride-potassium Chloride. - Anal. Chem. Acta, 1958, vol. 18, p.
1-13.
9. Hills G.I., Oxley I. E., Turner D. W., Silicates Ind., 1961, vol. 26, p.
559.
197