A New Route for Treating Neutral Leaching Residue
829
Unfortunately, the final acidity is not suitable for the following processes, and
besides that, it makes iron and other undesirable impurities soluble (like germanium,
arsenic, among others). Because of that, the resulting flow must go to a neutralization
(usually with the calcine) and iron removal step, commonly, jarosite (Eq. 3).
Jarosite reaction
3Fe 2 (SO 4 ) 3 (aq) + (NH 4 ) 2 SO 4 (aq) + 12H 2 O(l) → 2(NH 4 )Fe 3 (SO 4 ) 2 (OH) 6 (s) + 6H 2 SO 4 (aq)
(3)
In this operation, iron is precipitated in this mineral form called jarosite, and while
forming, drag the mentioned impurities with it. Unluckily, some of the zinc is lost as
well. Spite the formation of sulfuric acid in the reaction, the jarosite mineral includes
some sulfate, removing sulfuric acid from the process. This reaction requires strict
pH control; otherwise, the mineral will not form. Because of that, calcine is usually
used to control de pH, yielding a compatible flow to be incorporated in the neutral
leaching (after some solid/liquid separation process).
Alternative to Conventional Leaching Process
Looking for improvements in the RLE process, this work evaluated the sulfatation
process as an alternative for the neutral leaching residue treatment. As it can be seen in
Eq. 1, the roasting process releases SO 2 , and at the right conditions (around 650 °C),
the zinc ferrite from the neutral leaching residue reacts with this gas producing zinc
sulfate and hematite (Eq. 4) [1–8].
Sulfatation reaction
2ZnFe 2 O 4 (s) + 2SO 2 (g) + O 2 (g) → 2ZnSO 4 (s) + 2Fe 2 O 3 (s)
(4)
Regarding Eq. 4, an acid phase would not be needed, meaning that no extra acid
would be required in order to obtain the zinc from the ferrite form. Besides that, the
jarosite step becomes unnecessary as well, as the impurities are not solubilized (and
no zinc would be lost). Adding Eqs. 2 and 3 and comparing with 2 and 4, it was
possible to look for further assumptions (Eq. 5):
Sulfuric acid overall balance in the conventional process
6ZnS(s) + 30H 2 O(1) + 12Fe 2 (s) + 57O 2 (g) + 2(NH 4 )SO 4 (aq)
→ 18H 2 SO 4 (aq) + 4(NH 4 )Fe 3 (SO 4 ) 2 (OH 6 (s) + 6ZnSO 4 (aq)
H 2 SO 4re = 1.5
Sulfuric acid overall balance in the sulfatation process
2ZnS(s) + 4FeS 2 (s) + 19O 2 (g) + 8H 2 O(l)
→ 8H 2 SO 4 (aq) + 2ZnSO 4 (s) + 2Fe 2 O 3 (s)
H 2 SO 4 : Fe = 2.0
( 5 )
829
Unfortunately, the final acidity is not suitable for the following processes, and
besides that, it makes iron and other undesirable impurities soluble (like germanium,
arsenic, among others). Because of that, the resulting flow must go to a neutralization
(usually with the calcine) and iron removal step, commonly, jarosite (Eq. 3).
Jarosite reaction
3Fe 2 (SO 4 ) 3 (aq) + (NH 4 ) 2 SO 4 (aq) + 12H 2 O(l) → 2(NH 4 )Fe 3 (SO 4 ) 2 (OH) 6 (s) + 6H 2 SO 4 (aq)
(3)
In this operation, iron is precipitated in this mineral form called jarosite, and while
forming, drag the mentioned impurities with it. Unluckily, some of the zinc is lost as
well. Spite the formation of sulfuric acid in the reaction, the jarosite mineral includes
some sulfate, removing sulfuric acid from the process. This reaction requires strict
pH control; otherwise, the mineral will not form. Because of that, calcine is usually
used to control de pH, yielding a compatible flow to be incorporated in the neutral
leaching (after some solid/liquid separation process).
Alternative to Conventional Leaching Process
Looking for improvements in the RLE process, this work evaluated the sulfatation
process as an alternative for the neutral leaching residue treatment. As it can be seen in
Eq. 1, the roasting process releases SO 2 , and at the right conditions (around 650 °C),
the zinc ferrite from the neutral leaching residue reacts with this gas producing zinc
sulfate and hematite (Eq. 4) [1–8].
Sulfatation reaction
2ZnFe 2 O 4 (s) + 2SO 2 (g) + O 2 (g) → 2ZnSO 4 (s) + 2Fe 2 O 3 (s)
(4)
Regarding Eq. 4, an acid phase would not be needed, meaning that no extra acid
would be required in order to obtain the zinc from the ferrite form. Besides that, the
jarosite step becomes unnecessary as well, as the impurities are not solubilized (and
no zinc would be lost). Adding Eqs. 2 and 3 and comparing with 2 and 4, it was
possible to look for further assumptions (Eq. 5):
Sulfuric acid overall balance in the conventional process
6ZnS(s) + 30H 2 O(1) + 12Fe 2 (s) + 57O 2 (g) + 2(NH 4 )SO 4 (aq)
→ 18H 2 SO 4 (aq) + 4(NH 4 )Fe 3 (SO 4 ) 2 (OH 6 (s) + 6ZnSO 4 (aq)
H 2 SO 4re = 1.5
Sulfuric acid overall balance in the sulfatation process
2ZnS(s) + 4FeS 2 (s) + 19O 2 (g) + 8H 2 O(l)
→ 8H 2 SO 4 (aq) + 2ZnSO 4 (s) + 2Fe 2 O 3 (s)
H 2 SO 4 : Fe = 2.0
( 5 )