Simulation of an Alternative Direct Leaching Process …
413
Table 3 Composition of the
simulated sulfatation
products from high and low
iron content zinc concentrate
Low iron content (%) High iron content (%)
Fe 2 O 3
2.44
6.73
SiO 2
1.39
0.16
CaSO 4
1.44
1.12
FeSO 4
1.50
2.99
Fe 2 (SO 4 ) 3
1.08
5.61
ZnFe 2 O 4
0.68
1.39
ZnSO 4
87.74
76.90
Mass change +54.51
+162.49
Furthermore, the obtained iron products wüstite (FeO) and magnetite (Fe 3 O 4 ) can
further oxide to hematite (Fe 2 O 3 ), dislocating the final composition (Eq. 5).
Equation 5: Conversion of wüstite (FeO) and magnetite (Fe 3 O 4 ) to hematite
(Fe 2 O 3 ) at 950 °C.
FeO(s) + 0.25O 2 (g) → 0.50Fe 2 O 3 (s)
H = −141.511 kJ/mol G = −68.033 kJ/mol
Fe 3 O 4 (s) + 0.25O 2 (g) → 1.50Fe 2 O 3 (s)
H = −125.650 kJ/mol G = −35.357 kJ/mol
(5)
The Sulfatation Process
By taking the same concentrates composition, amount, and airflow mentioned above,
we now simulated the sulfatation process, at 600 °C. The product composition is
shown in Table 3.
Differently from the calcination process, the zinc at the sulfatation process is less
than 1% in the ferrite form (0.33% and 0.79% low iron content and high iron content,
respectively). Furthermore, most of the zinc is in an easy to wash form, zinc sulfate.
Regarding the choice of reactor, it is not possible to do sulfatation in a conventional
fluidized bed roaster. The sulfates formed in the reaction would stick in the reactor
walls, causing constant maintenance problems, and making the fluidized system
impractical. The sulfatation requires a wedge furnace (or similar), in order to control
the reaction in stages and continuously brush the walls, avoiding impregnation.
Another concern regarding the sulfatation process is the SO 2 gas outlet concentration. In a normal roasting process, the concentration of this gas is between 6.5
and 9%, which is the minimum for a conventional sulfuric acid plant to work. In the
sulfatation process, the concentration hardly reaches 1%.
This process requires strict temperature control: If the sulfatation temperature
is too low, most of the iron assumes the sulfate form, including any iron that was
413
Table 3 Composition of the
simulated sulfatation
products from high and low
iron content zinc concentrate
Low iron content (%) High iron content (%)
Fe 2 O 3
2.44
6.73
SiO 2
1.39
0.16
CaSO 4
1.44
1.12
FeSO 4
1.50
2.99
Fe 2 (SO 4 ) 3
1.08
5.61
ZnFe 2 O 4
0.68
1.39
ZnSO 4
87.74
76.90
Mass change +54.51
+162.49
Furthermore, the obtained iron products wüstite (FeO) and magnetite (Fe 3 O 4 ) can
further oxide to hematite (Fe 2 O 3 ), dislocating the final composition (Eq. 5).
Equation 5: Conversion of wüstite (FeO) and magnetite (Fe 3 O 4 ) to hematite
(Fe 2 O 3 ) at 950 °C.
FeO(s) + 0.25O 2 (g) → 0.50Fe 2 O 3 (s)
H = −141.511 kJ/mol G = −68.033 kJ/mol
Fe 3 O 4 (s) + 0.25O 2 (g) → 1.50Fe 2 O 3 (s)
H = −125.650 kJ/mol G = −35.357 kJ/mol
(5)
The Sulfatation Process
By taking the same concentrates composition, amount, and airflow mentioned above,
we now simulated the sulfatation process, at 600 °C. The product composition is
shown in Table 3.
Differently from the calcination process, the zinc at the sulfatation process is less
than 1% in the ferrite form (0.33% and 0.79% low iron content and high iron content,
respectively). Furthermore, most of the zinc is in an easy to wash form, zinc sulfate.
Regarding the choice of reactor, it is not possible to do sulfatation in a conventional
fluidized bed roaster. The sulfates formed in the reaction would stick in the reactor
walls, causing constant maintenance problems, and making the fluidized system
impractical. The sulfatation requires a wedge furnace (or similar), in order to control
the reaction in stages and continuously brush the walls, avoiding impregnation.
Another concern regarding the sulfatation process is the SO 2 gas outlet concentration. In a normal roasting process, the concentration of this gas is between 6.5
and 9%, which is the minimum for a conventional sulfuric acid plant to work. In the
sulfatation process, the concentration hardly reaches 1%.
This process requires strict temperature control: If the sulfatation temperature
is too low, most of the iron assumes the sulfate form, including any iron that was