790
M. Maccagni and E. Guerrini
The competing reaction of zinc dissolution, as shown in reaction (8) below, oxidized by the presence of dissolved air in the solution, is also possible. But at the neutral
operating pH, it is kinetically very slow and is therefore not a serious competitive
reaction for the cementation.
Zn + 1/2 O 2 + 2NH 4 Cl → Zn(NH 3 ) 2 Cl 2 + H 2 O
( 8 )
The complete precipitation of all impurities is fundamental for the chemical purity
of the deposited zinc. The design of this unit is very important when a high purity zinc
cathode is desired. A multistage unit design is essential for high removal efficiency
while optimizing the zinc consumption. A multistage design is also appropriate when
notable quantities of valuable metals, such as Ag and Cu, are contained in the treated
feed and need to be recovered separately in a relatively pure form.
The purified solution resulting from this operation is ready to be fed to the EW
unit.
The Zinc Electrowinning
The EZINEX
® EW unit is the core of this process. As already mentioned, one of
the problems of conventional chloride EW is the anodic evolution of chlorine that
requires a complex cell design and a complex system for the chlorine handling.
With the EZINEX
® Process, the chloride EW is highly simplified. It occurs in a
conventional open cell with permanent titanium cathode blanks and graphite anodes.
As with the leaching unit, the electrolysis unit needs to be run at temperatures higher
than 60 °C due to zinc solubility. Because of the low concentration of zinc in solution,
the electrolysis is assisted by an air sparging system in order to optimize and increase
the diffusion rate to a point that the electrolysis can be run at relatively high current
densities (>300 A/m
2 ) even with relatively low zinc concentrations (5–6 g/l).
At the titanium cathode, the zinc deposition occurs according to the reaction (9)
below:
Zn(NH 3 ) 2 Cl 2 + 2e
−
→ Zn + 2NH 3 + 2Cl
−
(9)
At the graphite anode, the chemistry is slightly more complicated because the
electrochemical reaction (10):
2Cl
−
→ Cl 2 + 2e
−
(10)
is followed by a very quick chemical reaction:
Cl 2 + 2/3 NH 3 → 2HCl + 1/3 N 2
(11)
which leads to the following overall anodic reaction
M. Maccagni and E. Guerrini
The competing reaction of zinc dissolution, as shown in reaction (8) below, oxidized by the presence of dissolved air in the solution, is also possible. But at the neutral
operating pH, it is kinetically very slow and is therefore not a serious competitive
reaction for the cementation.
Zn + 1/2 O 2 + 2NH 4 Cl → Zn(NH 3 ) 2 Cl 2 + H 2 O
( 8 )
The complete precipitation of all impurities is fundamental for the chemical purity
of the deposited zinc. The design of this unit is very important when a high purity zinc
cathode is desired. A multistage unit design is essential for high removal efficiency
while optimizing the zinc consumption. A multistage design is also appropriate when
notable quantities of valuable metals, such as Ag and Cu, are contained in the treated
feed and need to be recovered separately in a relatively pure form.
The purified solution resulting from this operation is ready to be fed to the EW
unit.
The Zinc Electrowinning
The EZINEX
® EW unit is the core of this process. As already mentioned, one of
the problems of conventional chloride EW is the anodic evolution of chlorine that
requires a complex cell design and a complex system for the chlorine handling.
With the EZINEX
® Process, the chloride EW is highly simplified. It occurs in a
conventional open cell with permanent titanium cathode blanks and graphite anodes.
As with the leaching unit, the electrolysis unit needs to be run at temperatures higher
than 60 °C due to zinc solubility. Because of the low concentration of zinc in solution,
the electrolysis is assisted by an air sparging system in order to optimize and increase
the diffusion rate to a point that the electrolysis can be run at relatively high current
densities (>300 A/m
2 ) even with relatively low zinc concentrations (5–6 g/l).
At the titanium cathode, the zinc deposition occurs according to the reaction (9)
below:
Zn(NH 3 ) 2 Cl 2 + 2e
−
→ Zn + 2NH 3 + 2Cl
−
(9)
At the graphite anode, the chemistry is slightly more complicated because the
electrochemical reaction (10):
2Cl
−
→ Cl 2 + 2e
−
(10)
is followed by a very quick chemical reaction:
Cl 2 + 2/3 NH 3 → 2HCl + 1/3 N 2
(11)
which leads to the following overall anodic reaction