Possibility of depositing electrolytic manganese dioxide onto titanium anodes from manganese chloride solutions

Article abstract of DOI:10.1007/s11167-005-0415-8

Fundamental aspects of the deposition of electrolytic manganese dioxide from chloride electrolytes were studied. The optimal parameters of the electrolysis on titanium anodes were determined.

Full text of DOI:10.1007/s11167-005-0415-8

Russian Journal of Applied Chemistry, Vol. 78, No. 6, 2005, pp. 891 896. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 6, 2005,
pp. 912 917.
Original Russian Text Copyright
2005 by Kolosnitsyn, Minnikhanova, Karaseva, Dmitriev, Muratov.
APPLIED ELECTROCHEMISTRY
AND CORROSION PROTECTION OF METALS
Possibility of Depositing Electrolytic
Manganese Dioxide onto Titanium Anodes
from Manganese Chloride Solutions
V. S. Kolosnitsyn, E. A. Minnikhanova, E. V. Karaseva, Yu. K. Dmitriev, and M. M. Muratov
Institute of Organic Chemistry, Ural Scientific Center, Russian Academy of Sciences, Ufa, Bashkortostan, Russia
Kaustik Private Company, Sterlitamak, Bashkortostan, Russia
Received September 1, 2004; in final form, February 2005
Abstract Fundamental aspects of the deposition of electrolytic manganese dioxide from chloride electrolytes
were studied. The optimal parameters of the electrolysis on titanium anodes were determined.
In production of organochlorine compounds, a large
amount of hydrochloric acid is formed as a waste gas
of the anode material. As the anode material for elec-
trolysis of MnCl solutions were used iridium [3],
2
(
HCl ). Therefore, it is necessary to develop pro-
platinum [4], lead coated with lead dioxide [4], graph-
ite [5], ORTA, and titanium electrodes coated with
titanium carbide or manganese dioxide [6 8]. Be-
cause platinum and iridium are very expensive metals,
it is unreasonable to use them in industrial electro-
wg
cesses that would make it possible to utilize HClwg
to give a new commercial product. One of the main
ways to utilize HCl is its use for leaching-out of man-
ganese ions from oxidized manganese ores (substan-
dard manganese ores, floatation tails, and other man-
ganese-containing wastes).
lysis. Lead anodes contaminate MnO with lead di-
2
oxide through their dissolution in the course of elec-
trolysis. Graphite anodes gradually burn away because
of the partial evolution of hydrogen and should be
replaced at regular intervals.
When oxidized manganese ores are decomposed
with hydrochloric acid, chlorine and manganese chlo-
ride solutions are formed by the reaction:
We suggested that less expensive metal anodes,
widely used in electrolysis with evolution of chlorine,
can be used to obtain EDM, because a manganese
dioxide film formed on titanium at the beginning of
electrolysis will increase its corrosion resistance.
MnO2 + 4HClconc
MnCl2 + Cl2 + 2H O. (1)
2
Chlorine can be used in manufacture of organo-
chlorine compounds and MnCl solutions, electro-
lytic manganese dioxide (EMD), and electrolytic
manganese metal (EMM).
2
In this study, we examined the possibility of ap-
plication of titanium anodes for deposition of EMD
Thus, two problems can be solved in the same
technological process: utilization of HClwg and pro-
cessing of substandard manganese ores and other
manganese-containing wastes. The use of HClwg and
manganese-containing wastes as raw materials will
make it possible to develop a competitive process for
manufacture of EMD and EMM [1].
from MnCl solutions.
2
EXPERIMENTAL
Manganese(IV) oxide was synthesized in a Plexiglas
electrolyzer with plane-parallel electrodes. VT-1-00
titanium plates (150 70 2 mm) were used as the
anode, and Kh18N10T stainless steel, as the cathode.
The separation between the opposite electrodes was
5 mm.
It should also be noted that MnO obtained from
2
chloride electrolytes is a more electrochemically ac-
tive and pure material [2].
In developing a process for obtaining electrolytic
Prior to the electrolysis, the surface of the electrodes
was trimmed with an M-40 emery paper, washed
MnO , a considerable attention is paid to the choice
2
1
070-4272/05/7806-0891 2005 Pleiades Publishing, Inc.

892
KOLOSNITSYN et al.
The experiments were performed as follows. First,
the electrolyzer with the electrode system prepared
was placed in the bath of the thermostat and kept there
at a prescribed temperature for 1 h. The electrolysis
time ensured deposition of 20 30% manganese from
the electrolyte. The current efficiency (CE) was cal-
culated from the loss of Mn(II) by the electrolyte
solution after the electrolysis. The corrosion rate of
the polarized and unpolarized (initial) titanium anodes
was determined gravimetrically [10]. After the elec-
trolysis, the electrodes were dried in air, and MnO2
was carefully removed from their surface with a brush.
Then, the electrodes were washed with 5% hydro-
xylamine hydrochloride and distilled water, and dried
in air to constant mass. The accuracy in determining
the corrosion rate was 7%.
The EDM deposited during the electrolysis was
washed with distilled water to negative reaction for
chloride ions. The MnO deposit was dried to constant
2
mass under an IR lamp at 105 C.
The temperature dependences of the current efficien-
2
cy by MnO at a current density of 5 and 10 mA cm
2
and different HCl concentrations are presented in
Figs. 1a, 1b. It can be seen that the CE-vs.-tempera-
ture dependences are bell-shaped and pass through
Fig. 1. Current efficiency CE by MnO vs. temperature
2
a maximum at 65 75 C. At a low temperature (30 C)
T. MnCl concentration 1.0 M; the same for Figs. 2, 3.
and a current density of 5 mA cm 2 the current effi-
2
2
Current density (mA cm ): (a) 5.0 and (b) 10.0; the same
ciency decreases with increasing HCl concentration
for Fig. 2. HCl concentration (M): (1) 0, (2) 0.05, and
2
(
Fig. 1a). Raising the current density to 10 mA cm
(
3) 0.25.
makes the current efficiency significantly lower and
changes the character of the dependence of CE on
the HCl concentration (Fig. 1b). The lowest current
efficiency is observed in the absence of the acid, and
the highest, with 0.05 M of HCl.
with distilled water and acetone, and dried in air for
2 min. The thus prepared electrodes were kept in
a desiccator before the experiment.
1
The electrolyzer temperature was maintained con-
stant with a U-8 water thermostat (Hungary). The an-
ode and cathode potentials were measured relative
to a silver chloride reference electrode.
As temperature increases, the current efficiency
sharply grows. At temperatures close to the tempera-
ture of the maximum, the highest current efficiency
is reached at an acid concentration of 0.05 M, irre-
2
The electrolyzer was polarized with a stabilized
B5-47 dc power source. The voltage across the elec-
trolyzer was measured with a V7-38 digital voltmeter.
The electrode potentials and pH of the electrolytes
were determined with an HI-9025 C portable micro-
processor-controlled pH/mV/ C-meter (Portugal).
spective of the current density (5 or 10 mA cm ). At
2
90 C, the current efficiency grows at j = 5 mA cm
a
as the HCl concentration becomes higher, and, reaches
2
a the minimum at j = 10 mA cm .
a
It should also be noted that titanium anodes rapidly
disintegrate at low temperatures (30 40 C). The rate
of this process increases as the density of polarizing
current and the acid concentration become higher. In
these conditions, the electrolyzer voltage reaches its
maximum value of 10 12 V.
As electrolytes were used 1 M MnCl solutions
2
containing a varied amount of hydrochloric acid.
The solutions were prepared from analytically pure
MnCl2 4H O and chemically pure HCl.
2
The manganese(II) content in the electrolytes was
determined by back titration [9].
The dependences of the electrolyzer voltage on tem-
perature and anode and cathode potentials at differ-
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 6 2005

POSSIBILITY OF DEPOSITING ELECTROLYTIC MANGANESE DIOXIDE
ent HCl concentrations are shown in Figs. 2a, 2b.
893
It can be seen that the anode potential and the elec-
trolyzer voltage decrease as the temperature becomes
higher. The cathode potential is virtually tempera-
ture-independent. The electrolyzer voltage decreases
with increasing HCl concentration in the electrolyte at
2
a current density of 5 mA cm and exhibits a com-
2
plex dependence at 10 mA cm . These results show
that the electrolyzer voltage is mainly determined
by the anode potential.
In the course of the electrolysis, a film consisting
of the reaction products was formed on the anode. In
the absence of the acid, the film was matte, uniform,
black, and strongly adherent to the electrode surface.
In the course of the electrolysis, the thickness of
the product layer increased, but no flaking from the
anode surface was observed. Addition of hydrochloric
acid to the electrolyte changed the properties of the
film. It acquired a metallic luster and became flaky,
with flakes exfoliating from the electrode surface and
going down to the bottom of the electrolyzer as their
size increased. In the process, the initial film of
the reaction products was retained on the anode sur-
face. The surface film possessed a reasonably high
corrosion resistance. According to a visual inspection
and a gravimetric analysis, the film on the anode re-
mained unchanged during several hundred hours after
the current was switched off.
Fig. 2. Electrolyzer voltage U and the potential E of
the (4) anode and (5) cathode vs. temperature T. HCl con-
centration (M): (1) 0, (2) 0.05, (3) 0.25, and (4, 5) 0.05.
The variation of the electrolyzer voltage in the
course of electrolysis is shown in Fig. 3. It can be
seen that at 30 C the electrolyzer voltage first in-
creases and then decreases. At temperatures of 60
to 90 C, the electrolyzer voltage somewhat decreases
and then reaches a constant value or slightly in-
creases.
The electrolytic deposition of EMD and EMM
from MnCl solutions was studied in [3, 5, 11]. In
2
the course of electrolysis from manganese(II) chloride
solutions, several electrochemical and chemical re-
actions proceed simultaneously at the anode and in
the solution bulk, including the electrochemical oxida-
tion of Mn2+ ions to the highest degrees of oxidation
and the oxidation of chloride ions and water, which
can be described by the following reaction equations:
Mn2+ + 2H O 2e
MnO2 + 4H , E = +1.228 V, (2)
+
2
pH
Mn , E = +1.510 V,
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 6 2005
4 3,
Fig. 3. Electrolyzer voltage U vs. time . Current density
2
5
mA cm . Temperature ( C): (1) 30, (2) 60, (3) 75,
Mn2+
e
3+
and (4) 90.
(3)

894
KOLOSNITSYN et al.
Mn2+
Mn2+ + 4H O 5e
2e
Mn , E = +1.840 V,
4+
(4)
Apparently, the strong temperature dependence
of CE is due to several factors: instability of higher
manganese chlorides formed by electrolysis at high
temperatures and by intense anodic dissolution of ti-
tanium at low temperatures, with both these factors
being interrelated.
+
MnO + 8H , E = +1.507 V, (5)
2
4
pH
2,
2
Cl
Cl2 + H O
Cl2 + 2e, E = +1.36 V,
(6)
(7)
(8)
(9)
At low temperatures the hydrolysis rate of the
Mn(IV) formed is low. Therefore, the reaction main-
ly proceeds in the bulk of the electrolyte rather
than on the anode surface or in the near-anode space.
HClO + HCl,
2
2HClO + ClO
ClO3 + 2H+ + 2Cl ,
Mn(2 + x)+ + xCl .
Therefore, no MnO film possessing high protective
2
Mn2+ + x/2Cl2
properties is formed on titanium. As a result, Ti
undergoes at low temperatures a strong electrochem-
ical dissolution to give a dense passivation film
of titanium oxide. This is manifested in a fast in-
crease in the anode potential, and, consequently, in
the electrolyzer voltage. Owing to the intense pas-
sivation of titanium, the current is concentrated at
those places where the passivation film is damaged,
which results in a fast electroerosion disintegration
of the anode.
The resulting chlorine partly dissolves in the elec-
trolyte to form hypochlorous and hydrochloric acids
reaction (7)], which results in a pronounced acid-
[
ification of the electrolyte near the anode surface.
In addition, the forming Cl can oxidize Mn2
+
2
to Mn4+ [reaction (9)]. The relative rates of the in-
dividual anodic reactions depend on the electroly-
sis conditions and are determined by the electrode
potential and the temperature and acidity of the so-
lution.
Raising the temperature leads to an increase in the
rate of MnCl hydrolysis. Therefore, MnO is formed
4
2
The rate of reaction (9) in the solution bulk is
many times less than that of reactions (2) and (4) [3],
which means that reactions (2) and (4) are the main
source of manganese(IV).
on the surface of the titanium anode, rather than in
the bulk of the electrolyte, to give a dense protective
layer. This layer does not hinder reactions (2), (4), and
(6) owing to its high electrical conductivity, but pre-
In the course of the electrochemical oxidation of
vents the anodic dissolution of titanium.
2
+
Mn , the anode potential is the key parameter. Rais-
ing the anode potential favors successive oxidation of
It is known that the MnCl dissociation at room
4
temperature to form chlorine [reaction (12)] is ex-
tremely slow. The rate of this reaction sharply in-
creases as temperature is raised [5]. As a result, the
current efficiency starts to decrease above 60 70 C
owing to the lowering of the Mn(IV) concentration.
2
+
3+
4+
Mn to Mn and then to Mn . At a potentials of
2
+
approximately up to 1.5 V, Mn is oxidized in solu-
tion to Mn3+ [equation (3)]. Another electrode reac-
tion [equation (4)] takes place when the potential in-
creases to 1.8 V.
As chloride ions compete with manganese(II) ions
for the oxidation on the electrode, it may be sug-
gested that an increase in the acidity would make
Manganese(IV) can be also obtained by dispro-
portionation of Mn by the reaction
3
+
lower the current efficiency by MnO . However, it
Mn3+
Mn4+ + Mn2+.
2
2
(10)
can be seen from Figs. 1a, 1b that the dependence
of CE on the concentration of hydrochloric acid is
complex.
Manganese dioxide deposited from chloride solu-
tions can be regarded as a product of hydrolysis [reac-
tion (11)] of manganese(IV), which is formed by reac-
tions (2), (4), (9), and (10):
The decrease in CE upon an increase in the HCl
concentration at low temperatures is probably due to
a rise in the rate of anodic dissolution of titanium with
increasing electrolyte acidity. The increase in CE from
MnCl4 + 2H O
MnO2 + 4HCl.
(11)
2
8
0 to 95% as the HCl concentration is raised from 0
Manganese tetrachloride may also decompose to
yield MnCl and Cl via the inner-sphere redox reac-
tion
to 0.05 M may be due to a rise in the MnCl concen-
tration as a result of reaction (9).
4
2
2
The data obtained in studying the corrosion rate of
unpolarized and polarized titanium anodes are listed
MnCl4
MnCl2 + Cl2
.
(12)
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 6 2005

POSSIBILITY OF DEPOSITING ELECTROLYTIC MANGANESE DIOXIDE
895
in Tables 1, 2 in relation to temperature and HCl con-
centration.
Table 1. Chemical corrosion rate vch of VT-01-00 titanium
at various temperatures and electrolyte compositions
The results of the study show that titanium does
c
cHCl
MnCl2
v ,
g m 2 h 1
ch
not corrode in neutral MnCl solutions. In the pres-
T,
C
, h
2
ence of HCl, titanium corrodes, but the corrosion
rate is low. As the temperature and HCl concentra-
tion increase, the rate of titanium corrosion slightly
grows. The results obtained are in a good agree-
ment with reference data. For example, the corro-
sion rate of a titanium plate was zero after 200 h of
electrolysis at 20 C in the presence of 5% HCl,
M
1
1
1
1
1
1
1
.0
.0
.0
.0
.0
.0
.0
60
6
15
15
6
10
10
6
10
10
6
10
10
6
0
0
0
0
75
90
60
75
90
60
75
90
60
75
90
60
75
90
0.05
0.05
0.05
0.25
0.25
0.25
4
2.4 10
3.7 10
1.1 10
2.4 10
6.7 10
1.1 10
2.6 10
4.0 10
4
2
1
0
6
.05 g m h after the same time of electrolysis at
4
2
1
4
0 C, and 4.23 g m h after as short a time as 25 h
1.0
1.0
4
of electrolysis at 90 C [12].
4
0
0
0
.05
.05
.05
Application of an anodic polarization makes the rate
of titanium corrosion somewhat higher. As the tem-
perature increases, the corrosion rate first grows,
reaches a maximum at 75 C, and then falls, irrespec-
tive of the HCl concentration in the electrolyte. Rais-
ing the HCl concentration somewhat decreases the
corrosion rate.
4
4
4
0.25
0.25
0.25
8.0 10
7.5 10
9.3 10
4
10
10
4
Table 2. Electrochemical corrosion rate vech of VT-01-00
titanium at various temperatures and HCl concentrations
(Current density 5 mA cm , MnCl concentration 1 M)
A visual inspection of the electrodes demonstrated
2
that the MnO deposit formed in electrolysis at 60 C
2
2
is strongly adherent to the anode surface. Magnesium
dioxide deposited at higher temperatures (75 90 C)
has a weak adherence and exfoliates in the form of
black flakes in the course of electrolysis, with the
Uini
Ufin
cHCl
M
,
vech ,
g m 2 h 1
T,
C
, h
V
dense protective MnO film retained on the surface
of the titanium anode.
2
60
6
15
15
6
10
10
3.9
3.5
2.6
2.7
2.3
2.9
10.0
5.1
5.1
5.2
3.5
3.2
0
3
7
5
9.4 10
7.2 10
3
9
0
Several concurrent reactions, oxidation of titanium,
evolution of Cl and O , and formation of MnO ,
0
0
0
.25
.25
.25
60
75
90
0
3
2
2
2
6.8 10
2.8 10
occur in the case of anodic polarization of the titanium
electrode in HCl solutions [13]. A thin passivating
3
TiO film formed on the anode via oxidation at the
2
beginning of the electrolysis hinders further ioniza-
tion of titanium. As a result, the anode potential in-
the anode potential increases to reach the potential
of Cl evolution. The distribution of the polarizing
2
creases and reaches the potential of MnO deposition.
2
current along the anode surface is nonuniform. It is
predominantly concentrated in the damaged areas of
the passivating film, thus causing electroeroison of
the anode.
The thickness of the oxide film formed on the titani-
um electrode and, accordingly, its potential, depend
on the current density, temperature, and electrolyte pH.
First, a primary passivating TiO film exhibiting
2
Calculations demonstrated that the rate at which
the thickness of the titanium anodes decreases be-
cause of the chemical and electrochemical corrosion
in the course of the electrolysis in manganese chloride
electrolytes does not exceed 0.03 mm/year.
high protective properties is formed on the titanium
surface at low current densities and relatively high
temperatures. Then, the secondary passivating film,
mostly composed of freshly deposited -MnO is
2
formed and reliably protects the titanium surface from
disintegration. The protective properties of the MnO2
film increase with temperature.
CONCLUSIONS
At low temperatures and high current densities,
a titanium oxide layer with a high electrical resistance
rapidly grows on the titanium surface. As a result,
(1) VT-01-00 titanium can be used as anode in
deposition of electrolytic manganese dioxide from
chloride electrolytes.
RUSSIAN JOURNAL OF APPLIED CHEMISTRY Vol. 78 No. 6 2005

896
KOLOSNITSYN et al.
(
2) The optimal conditions for deposition of MnO2
5. Lisov, V.N., Gidroelektrometallurgiya khloridov
(Hydroelectrometallurgy of Chlorides), Kiev: Nauko-
va Dumka, 1964, pp. 76 98.
with a high current efficiency are as follows: temper-
ature 70 75 C, current density 5 mA cm , and HCl
2
concentration 0.05 M. Under these conditions, the
6
. JPN Patent 2423.
MnO deposit has the form of coarsely crystalline
2
7. USSR Inventor’s Certificate, no. 220254.
aggregates that can be easily detached from the anode
surface.
8
9
. USSR Inventor’s Certificate, no. 1661247.
. Charlot, G., Quantitative Inorganic Analysis, London:
Wiley, 1957.
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