The EZINEX ® Process for Secondary Zinc Bearing Materials
789
MeO n/2 + nNH 4 Cl → Me(NH 3 ) n Cl n + n/2 H 2 O
Me = Zn
2+
, Cd
2+
, Cu
2+
, Cu
+
, . . .
(1)
PbCl 2 + nXCl → (X) n PbCl (2+n)
X = Na
+
, K
+
, NH
+
4
(2)
PbO + nXCl → (X) n PbCl (2+n) + X 2 O
X = Na
+
, K
+
(3)
These reactions are very quick and virtually all the zinc contained as zinc oxide is
leached while the zinc ferrite remains unleached. Due to the relatively low solubility
of zinc amino-chloride, the leaching is operated at a moderately high temperature
(>70 °C). The significant advantage of this neutral leaching is that iron is essentially
not leached. Calcium tends to build up in solution and is particularly problematic
for zinc deposition because it seems to interfere with zinc migration to the cathodic
surface. To avoid this problem as well as unwanted crystallization, Ca is precipitated
by Na 2 CO 3 /NaHCO 3 addition according to the reactions (4) and (5) below:
NaHCO 3 + CaCl 2 → CaCO 3 + NaCl + HCl
(4)
Na 2 CO 3 + CaCl 2 → CaCO 3 + 2NaCl
(5)
Of course, some other chemicals can be used. Both in EAFD and in CZO, we
commonly see a measurable amount of fluorides, which must be kept under control.
Due to the presence of calcium, we do not see a build up of fluorides in solution.
They are precipitated during leaching according to the following reaction (6):
Ca
2+
+ 2F
−
→ CaF 2
(6)
The leaching residue is recycled back to the EAF or thermal generating system
to fully recover the unleached zinc remaining as zinc ferrite.
The Leachate Purification
Among the leached elements, metals more noble than Zn that must be removed ahead
of the electrowinning cell because they could co-deposit with Zn decreasing its quality. Their removal is achieved through their precipitation by contacting the solution
with zinc powder or granules according to the following well-known cementation or
displacement reaction (7):
Me
n+
+ n/2 Zn → Me + n/2 Zn
2+
Me = Pb
2+
, Cd
2+
, Cu
2+
, Ag
+
, . . .
(7)
789
MeO n/2 + nNH 4 Cl → Me(NH 3 ) n Cl n + n/2 H 2 O
Me = Zn
2+
, Cd
2+
, Cu
2+
, Cu
+
, . . .
(1)
PbCl 2 + nXCl → (X) n PbCl (2+n)
X = Na
+
, K
+
, NH
+
4
(2)
PbO + nXCl → (X) n PbCl (2+n) + X 2 O
X = Na
+
, K
+
(3)
These reactions are very quick and virtually all the zinc contained as zinc oxide is
leached while the zinc ferrite remains unleached. Due to the relatively low solubility
of zinc amino-chloride, the leaching is operated at a moderately high temperature
(>70 °C). The significant advantage of this neutral leaching is that iron is essentially
not leached. Calcium tends to build up in solution and is particularly problematic
for zinc deposition because it seems to interfere with zinc migration to the cathodic
surface. To avoid this problem as well as unwanted crystallization, Ca is precipitated
by Na 2 CO 3 /NaHCO 3 addition according to the reactions (4) and (5) below:
NaHCO 3 + CaCl 2 → CaCO 3 + NaCl + HCl
(4)
Na 2 CO 3 + CaCl 2 → CaCO 3 + 2NaCl
(5)
Of course, some other chemicals can be used. Both in EAFD and in CZO, we
commonly see a measurable amount of fluorides, which must be kept under control.
Due to the presence of calcium, we do not see a build up of fluorides in solution.
They are precipitated during leaching according to the following reaction (6):
Ca
2+
+ 2F
−
→ CaF 2
(6)
The leaching residue is recycled back to the EAF or thermal generating system
to fully recover the unleached zinc remaining as zinc ferrite.
The Leachate Purification
Among the leached elements, metals more noble than Zn that must be removed ahead
of the electrowinning cell because they could co-deposit with Zn decreasing its quality. Their removal is achieved through their precipitation by contacting the solution
with zinc powder or granules according to the following well-known cementation or
displacement reaction (7):
Me
n+
+ n/2 Zn → Me + n/2 Zn
2+
Me = Pb
2+
, Cd
2+
, Cu
2+
, Ag
+
, . . .
(7)