Current Efficiency Increase in Zinc Electrodeposition …
105
• Mn
2+ ions in a concentration greater than 10 g/L significantly reduce current
efficiency [7]. This can be explained why as the concentration of the MnO 4 ion
increases, the hydrogen evolution reaction becomes dominant [8].
• A high concentration of Mn
2+ ion generates a large amount of MnO 2 on the anodes,
which if not removed in a timely manner causes short circuits since it is an electrical
conductor. This also implies a greater generation of anodic sludge, which forces
to increase the cleaning frequencies of anodes and cells.
Modifications Made
Minimize Side Reactions (Secondary Reactions)
Zinc Content Increase in Electrolyte
As explained in Sect. 2, one of the factors that impact current efficiency is the rate of
generation of H 2 (g). Therefore, tests were conducted in a pilot cell to determine the
current efficiency by varying the concentrations of zinc in the electrolyte to define a
new specification limit.
For the test, a process electrolyte was used which had a concentration of 60 g/L
of Zn, which was diluted using deionized water to obtain concentrations between
30 and 75 g/L of Zn. To keep the concentration of Mn, Mg, and acidity constant,
synthetic solutions were used, the electrolyte composition used in the different tests
is described in Table 2.
To simulate the operating conditions, a temperature of 40° in electrolyte and
current density of 550 A/m
2 were adjusted and the test time was 6 h, obtaining the
results shown in Table 3.
In Fig. 1, we can see how the current efficiency increases when the zinc content
in the electrolyte increases.
The tests carried out served as a basis for modifying the specification of the zinc
content in the spent electrolyte of at least 38 g/L to a new specification between 50 and
55 g/L. Also based on the results obtained, it was decided to change the operational
control logic by replacing the [H
+ ] concentration control by [Zn
2+ ] concentration
Table 2 Concentrations
determined for the test
Element
Unit
Value
Zn
Min g/L
30
Max g/L
70
Acid
g/L
185
Mn
g/L
22
Mg
g/L
10
Data Source Internal Data
105
• Mn
2+ ions in a concentration greater than 10 g/L significantly reduce current
efficiency [7]. This can be explained why as the concentration of the MnO 4 ion
increases, the hydrogen evolution reaction becomes dominant [8].
• A high concentration of Mn
2+ ion generates a large amount of MnO 2 on the anodes,
which if not removed in a timely manner causes short circuits since it is an electrical
conductor. This also implies a greater generation of anodic sludge, which forces
to increase the cleaning frequencies of anodes and cells.
Modifications Made
Minimize Side Reactions (Secondary Reactions)
Zinc Content Increase in Electrolyte
As explained in Sect. 2, one of the factors that impact current efficiency is the rate of
generation of H 2 (g). Therefore, tests were conducted in a pilot cell to determine the
current efficiency by varying the concentrations of zinc in the electrolyte to define a
new specification limit.
For the test, a process electrolyte was used which had a concentration of 60 g/L
of Zn, which was diluted using deionized water to obtain concentrations between
30 and 75 g/L of Zn. To keep the concentration of Mn, Mg, and acidity constant,
synthetic solutions were used, the electrolyte composition used in the different tests
is described in Table 2.
To simulate the operating conditions, a temperature of 40° in electrolyte and
current density of 550 A/m
2 were adjusted and the test time was 6 h, obtaining the
results shown in Table 3.
In Fig. 1, we can see how the current efficiency increases when the zinc content
in the electrolyte increases.
The tests carried out served as a basis for modifying the specification of the zinc
content in the spent electrolyte of at least 38 g/L to a new specification between 50 and
55 g/L. Also based on the results obtained, it was decided to change the operational
control logic by replacing the [H
+ ] concentration control by [Zn
2+ ] concentration
Table 2 Concentrations
determined for the test
Element
Unit
Value
Zn
Min g/L
30
Max g/L
70
Acid
g/L
185
Mn
g/L
22
Mg
g/L
10
Data Source Internal Data