Current Efficiency Increase in Zinc Electrodeposition …
103
process. The second stage of purification occurs by the cementation of metal ions
[2].
The solution fed to the zinc electrolysis proceeds from the recirculation of the
cell electrolyte ~50 g/L Zn, which must be enriched with the purified concentrated
neutral solution at an order concentration of 150 g/L Zn, raising the concentration
of feed electrolyte at order values from 55 to 60 g/L Zn. The energy consumption in
this stage varies generally from 3000 to 3500 kWh/t cathodic zinc, whereas about a
third becomes heat losses [3]. The potential difference in the cells is from the order
of 2.5–3.0 V and the cellroom operates at a current density of 550 A/m
2 .
The main reactions involved in the process, as well as the standard potential
correspondents, are represented by the cathodic Eqs. (1) and (2) and the anodic
Eqs. (3) and (4) [3], with their respective standard potentials of electrode:
Zn
2+
(aq) + 2e
−
= Zn
0
(s) E
0
= −0.76 V
(1)
H
+
(aq) + e
−
= 1/2H 2 (g) E
0
= 0.00 V
(2)
2H 2 O(aq) = 4H
+
(aq) + O 2 (g) + 4e
− E
0
= −1.23V
(3)
Mn
2+
(aq) + 2H 2 O(aq) = MnO 2 (s) + 4H
+
(aq) + 2e
− E
0
= +1.33V
(4)
At the cathode, the reduction of Zn
2+ to Zn
0 under standard conditions (Eq. 1)
implies a more negative potential than the potential for hydrogen reduction (Eq. 2).
Thus, if only thermodynamics is considered of the reaction, the electrolysis of Zn in
aqueous medium would not be viable, however, what makes this important reaction
possible in the aqueous medium is the slow kinetics of the reaction of the evolution
of hydrogen on the surface of zinc. Equation (3) represents the oxidation of water on
the anode, with oxygen release, while Eq. (2) represents the oxidation of manganese
ion to MnO 2 . The formation of a protective layer of MnO 2 on the surface of lead
and silver anodes helps to reduce its corrosion rate. As a result, we have less lead
contamination in the zinc cathodic deposit as well as reduced costs due to increased
service life [1].
Current Efficiency
Current efficiency is one of the most important parameters of the electrolysis process,
along with the specific energy consumption. This indicator assesses the relationship
between deposited metal mass and theoretical mass, calculated from Faraday’s law.
For a given applied current, the current efficiency (CE) can be calculated according
to Eq. (5)
103
process. The second stage of purification occurs by the cementation of metal ions
[2].
The solution fed to the zinc electrolysis proceeds from the recirculation of the
cell electrolyte ~50 g/L Zn, which must be enriched with the purified concentrated
neutral solution at an order concentration of 150 g/L Zn, raising the concentration
of feed electrolyte at order values from 55 to 60 g/L Zn. The energy consumption in
this stage varies generally from 3000 to 3500 kWh/t cathodic zinc, whereas about a
third becomes heat losses [3]. The potential difference in the cells is from the order
of 2.5–3.0 V and the cellroom operates at a current density of 550 A/m
2 .
The main reactions involved in the process, as well as the standard potential
correspondents, are represented by the cathodic Eqs. (1) and (2) and the anodic
Eqs. (3) and (4) [3], with their respective standard potentials of electrode:
Zn
2+
(aq) + 2e
−
= Zn
0
(s) E
0
= −0.76 V
(1)
H
+
(aq) + e
−
= 1/2H 2 (g) E
0
= 0.00 V
(2)
2H 2 O(aq) = 4H
+
(aq) + O 2 (g) + 4e
− E
0
= −1.23V
(3)
Mn
2+
(aq) + 2H 2 O(aq) = MnO 2 (s) + 4H
+
(aq) + 2e
− E
0
= +1.33V
(4)
At the cathode, the reduction of Zn
2+ to Zn
0 under standard conditions (Eq. 1)
implies a more negative potential than the potential for hydrogen reduction (Eq. 2).
Thus, if only thermodynamics is considered of the reaction, the electrolysis of Zn in
aqueous medium would not be viable, however, what makes this important reaction
possible in the aqueous medium is the slow kinetics of the reaction of the evolution
of hydrogen on the surface of zinc. Equation (3) represents the oxidation of water on
the anode, with oxygen release, while Eq. (2) represents the oxidation of manganese
ion to MnO 2 . The formation of a protective layer of MnO 2 on the surface of lead
and silver anodes helps to reduce its corrosion rate. As a result, we have less lead
contamination in the zinc cathodic deposit as well as reduced costs due to increased
service life [1].
Current Efficiency
Current efficiency is one of the most important parameters of the electrolysis process,
along with the specific energy consumption. This indicator assesses the relationship
between deposited metal mass and theoretical mass, calculated from Faraday’s law.
For a given applied current, the current efficiency (CE) can be calculated according
to Eq. (5)