Zinc Plant Expansion and Modification for Increased Metals …
367
Expansion by Atmospheric Direct Leaching
The zinc plant expansion is depicted below with a description of the DL process and
its integration into existing plant, an outline of the implementation as well as some
first operational results.
Process Description
Outotec atmospheric direct leaching of zinc concentrates was originally developed
by Outokumpu for Kokkola zinc plant (today Boliden Kokkola) in Finland during
the 1990s and has later been applied at Odda in Norway (today Boliden Odda) and
at Zhuzhou in China [1, 2]. The key feature of the process is the oxidative leaching
of sphalerite and other metal sulfides in the concentrate by means of ferric iron in
sulphate solution according to
2Fe
3+
(aq) + MeS(s) → 2Fe
2+
(aq) + Me
2+
(aq) + S
0
(s) (Me = Zn, Fe, Cu, Cd)
(1)
In parallel, the ferrous iron formed in leaching is re-oxidized by oxygen gas:
2Fe
2+
(aq) + 2H
+
(aq) + 0.5O 2 (g) → 2Fe
3+
(aq) + H 2 O(aq)
(2)
As seen from the reactions, iron ions are needed in solution, and sulfuric acid is
consumed. The sulfur of the metal sulfides is converted into elemental form. Excellent
zinc leaching recovery of 98% or more is normally achieved after 20–25 h retention
time. For providing the needed volume and at the same time ensuring efficient oxygen
utilization, tall reactors with internal down-flow tubes are used for the leaching [1].
With proper adjustment of process conditions, iron can be precipitated as jarosite
towards the end of the direct leaching step, where the acidity is lower and iron
concentration is higher. The jarosite precipitation reaction (3) is given in a later
section of the paper.
Prior to the DL expansion, the Torreón zinc plant was based on the roasting–leaching–electrowinning route having an annual zinc production capacity of 240,000 ton.
The expansion for increasing the annual zinc production to 360,000 ton was integrated into the existing leaching plant according to the flowsheet outlined in Fig. 1.
In addition to capacity, the DL addition gives higher flexibility in terms of raw materials for the total zinc plant. For instance, concentrates with high Si and Cu can be
problematic in roasting but are easily treated in DL, while concentrates rich in Ag
are better recovered through roasting–leaching.
The calcine leaching consists of three stages: neutral leaching, acid leaching, and
hot acid leaching. The lead–silver residue from hot acid leaching goes further to the
lead smelter for recovery. Iron is removed as jarosite, and there is an acid wash stage
367
Expansion by Atmospheric Direct Leaching
The zinc plant expansion is depicted below with a description of the DL process and
its integration into existing plant, an outline of the implementation as well as some
first operational results.
Process Description
Outotec atmospheric direct leaching of zinc concentrates was originally developed
by Outokumpu for Kokkola zinc plant (today Boliden Kokkola) in Finland during
the 1990s and has later been applied at Odda in Norway (today Boliden Odda) and
at Zhuzhou in China [1, 2]. The key feature of the process is the oxidative leaching
of sphalerite and other metal sulfides in the concentrate by means of ferric iron in
sulphate solution according to
2Fe
3+
(aq) + MeS(s) → 2Fe
2+
(aq) + Me
2+
(aq) + S
0
(s) (Me = Zn, Fe, Cu, Cd)
(1)
In parallel, the ferrous iron formed in leaching is re-oxidized by oxygen gas:
2Fe
2+
(aq) + 2H
+
(aq) + 0.5O 2 (g) → 2Fe
3+
(aq) + H 2 O(aq)
(2)
As seen from the reactions, iron ions are needed in solution, and sulfuric acid is
consumed. The sulfur of the metal sulfides is converted into elemental form. Excellent
zinc leaching recovery of 98% or more is normally achieved after 20–25 h retention
time. For providing the needed volume and at the same time ensuring efficient oxygen
utilization, tall reactors with internal down-flow tubes are used for the leaching [1].
With proper adjustment of process conditions, iron can be precipitated as jarosite
towards the end of the direct leaching step, where the acidity is lower and iron
concentration is higher. The jarosite precipitation reaction (3) is given in a later
section of the paper.
Prior to the DL expansion, the Torreón zinc plant was based on the roasting–leaching–electrowinning route having an annual zinc production capacity of 240,000 ton.
The expansion for increasing the annual zinc production to 360,000 ton was integrated into the existing leaching plant according to the flowsheet outlined in Fig. 1.
In addition to capacity, the DL addition gives higher flexibility in terms of raw materials for the total zinc plant. For instance, concentrates with high Si and Cu can be
problematic in roasting but are easily treated in DL, while concentrates rich in Ag
are better recovered through roasting–leaching.
The calcine leaching consists of three stages: neutral leaching, acid leaching, and
hot acid leaching. The lead–silver residue from hot acid leaching goes further to the
lead smelter for recovery. Iron is removed as jarosite, and there is an acid wash stage