WO 2006/133777 PCT/EP2006/004412
SEPARATION OF METAL VALUES IN ZINC LEACHING RESIDUES
This invention relates to the separation of metals in Fe-bearing zinc
leaching residues, in particular neutral and weak acid leach residues.
Blende, which is an impure ZnS ore, is the main starting material for
the production of Zn. The typical industrial practice encompasses an
oxidative roasting step, producing ZnO together with sulphates or oxides
of the impurities. In subsequent steps, the ZnO in roasted blende is
brought into solution by leaching in neutral conditions or in weak
acidic conditions, thereby producing Zn-depleted residues, respectively
referred to as neutral leach residue and as weak acid leach residue in
this description. These residues typically contain from 2 to 10 wt.% S,
up to 30 wt% Zn, 35 wt% Fe, 7 wt% Pb and 7 wt% SiO2.
However, during roasting, part of the Zn reacts with Fe, a typical
impurity present in blende, and forms relatively insoluble zinc ferrite.
The leach residues therefore contain, besides lead sulphate, calcium
sulphate and other impurities, a sizeable fraction of Zn in the form of
ferrite. According to present practice, the recovery of the Zn from
ferrite requires a specific hydro-metallurgical residue treatment using
high acid concentrations of 50 to 200 g/1 H2SO4. A disadvantage of this
acidic treatment is that besides Zn, almost all the Fe and also other
impurities such as As, Cu, Cd, Ni, Co, Tl, Sb are dissolved. As even low
concentrations of these elements interfere with the subsequent
electrowinning of Zn, they must be removed from the zinc sulphate
solution. While Cu, Cd, Co, Ni and Tl are precipitated by addition of Zn
powder, Fe is typically discarded as hematite, jarosite or goethite
through hydrolysis. Due to the danger of washout of heavy metals, these
Fe-bearing residues have to be disposed off in a well-controlled
landfill. Land-filling of such residues has however come under heavy
environmental pressure, rendering the sustainability of the process
questionable. Another drawback of the above treatment is the loss of
metals such as In, Ge, Ag and Zn in the Fe-bearing residue.
An alternative treatment of the ferrite-bearing residues is applied in
some plants, using Waelz kilns, which produce a slag and a Zn and Pb
containing fume. Such a process is described in 'Steelworks residues and

WO 2006/133777 PCT/EP2006/004412
2
the Waelz kiln treatment of electric arc furnace dust', G. Strohmeier
and J. Bonestell, Iron and Steel Engineer vol. 73, N°4, pp. 87-90. In
the Waelz kiln, zinc enters in the form of ferrites and sulphate, and is
vaporized after being reduced by CO generated by burning cokes. In the
reaction zone of the kiln, where iron is reduced to metal, the problem
of overheating occurs frequently. In such cases, the charge in the kiln
melts and accretions are formed, mainly due to the formation of the
eutectic 2FeO.SiO2 - FeO, which has a melting point of approximately 1180
°C. The dissolution of FeO further lowers the melting point and through
combination with zinc sulphide, reduced from zinc sulphate in the
earlier stages, solid crusts are formed. The furnace rotation is further
hampered by the formation of large balls consisting of carbonized iron,
which are formed as a molten metallic phase at approx. 1150°C. This
again leads to a decreased reduction of ZnO and iron oxide, which are
formed in the earlier stages of the furnace from reduced zinc ferrites.
Overheating accelerates the wear of the brick lining of the kiln. In
order to limit the risks of overheating, the CaO/SiO2 ratio in the feed
has to be monitored closely by setting it to a value of 0.8 to 1.8.
Although numerous Zn fuming processes have been described, recent
literature concentrates on the treatment of Zn-containing Fe secondary
residues, such as EAF dusts. In this respect, the Waelz kiln is well
suited, but its productivity is nevertheless hindered by its
sensitiveness to overheating.
In WO2005-005674 a process for the separation and recovery of non-
ferrous metals from zinc-bearing residues was disclosed. The process
comprises the steps of subjecting the residue to a direct reduction
step, extracting Zn- and Pb-bearing fumes, and subjecting the resulting
metallic Fe-bearing phase to an oxidising smelting step. The direct
reduction is performed in a multiple hearth furnace operating at 1100 °C
in the reduction zone. One disadvantage of the use of such a reduction
furnace is that the reduction kinetics are limited by the temperature.
Temperatures above 1100 °C can however not be reached in a multiple
hearth furnace.

WO 2006/133777 PCT/EP2006/004412
3
JP2004-107748 describes a process for the treatment of zinc leaching
residues in a rotary hearth furnace, at a reduction temperature up to
1250°C. The burner air ratio is set within a limited range.
In US5,9O6,671 Zn plant leach residues are treated in a rotary kiln at
temperatures up to 1150°C, after being agglomerated together with alkali
earth and alkali metal complexes of alumina and silica oxides and a
reducing agent.
In US5,667,553 neutral leach residue by-products of zinc electrowinning
are heat treated in a reduction furnace, in the same way as EAF dust.
The aim of the present invention is to provide a process for the
separation of the metals contained in Fa-bearing zinc leaching residues,
which does not have the disadvantages described above. This process
comprises the steps of:
- preparing agglomerates containing, besides the Zn leaching residue, at
least 5 wt% of carbon and 2 to 10 wt.% of S;
- fuming said agglomerates in a static bed at a temperature above
1250°C, thereby producing a reduced Fe-bearing phase and Zn-bearing
fumes; and
- extracting said Zn-bearing fumes.
The Zn leaching residue should preferably be dried to a moisture content
of less than 12 wt.% H2O, or even to less than 5 wt.% H2O, before
preparing the agglomerates.
A carbon content in the agglomerates of at least 15 wt.% is preferred,
as is a CaO equivalent of at least 10 wt.%, or even at least 15 wt. %.
The strength of the pellets, expressed as their Mass Pellet Strength,
should preferably be at least 5 kg, or even 10 kg. This way dust carry
over is avoided and the fusion of the charge is better prevented at the
high process temperatures.
The fuming should advantageously be performed at a temperature of at
least 1300 °C, in a carbon monoxide containing atmosphere

WO 2006/133777 PCT/EP2006/004412
4
The process is ideally suited for processing neutral or weak acid Zn
leach residues.
The invented process can be performed in a in a rotary hearth furnace;
it can optionally be followed by a process whereby the reduced Fe-
bearing phase is melted and oxidised.
It may thus be necessary to add a S-bearing component to the residue, so
as to bring its total S content into the required range. Gypsum would be
a typical additive in this case. Using a S-rich carbon source could also
be envisaged in this case.
As evidenced by the Examples below, the high S content of the feed
allows for a relatively high operating temperature without producing
molten phases. There is thus no danger for the formation of accretions
at the discharge port of the furnace. High temperatures guarantee fast
reduction and fuming kinetics, which permit the use of a compact
technology such as a static bed furnace. This type of furnace
furthermore preserves the integrity of the agglomerates, avoiding to a
large extent the production of dust and limiting the ensuing pollution
of the fumes.
Example 1
The following example illustrates the separation of different non-
ferrous metals contained in a roasted and subsequently leached blende.
About 1000 g of Weak Acid Leaching (WAL) residue which mainly consists
of zinc ferrite (2nO.Fe2O3), lead sulphate (PbSO4), calcium sulphate
(CaSO4), zinc sulphate (ZnSO4) and impurities like CaO, SiO2, MgO, A12O3,
CU2O,- SnO, was dried to a moisture content below 5 wt% H2O, and mixed
with 15 wt% of CaO or the equivalent gypsum and 25 wt% of PET cokes,
having a purity of >85% C. This mixture was compacted in briquettes by
pressing it between 2 hydraulic rolls at a pressure of 20 kN/cm2
resulting in hard, shiny briquettes, having a Mass Pellet Strength of 20
kg.
The fuming step was carried out in an induction furnace to simulate the
process occurring in a rotating hearth furnace. An Indutherm MU-3000

WO 2006/133777 PCT/EP2006/004412
5
furnace with a maximum power of 15 kW and a frequency of 2000 Hz was
used. The internal furnace diameter was 180 mm, and the graphite
crucible carrying the briquettes had an internal diameter of 140 mm.
Approximately 400 g briquettes was placed on the bottom of the clean
graphite crucible, in such a way that the crucible surface is covered
with a single layer of material. The crucible was then placed in the
induction furnace, and a monitoring thermocouple was mounted between the
briquettes without touching the crucible bottom. The crucible was
covered by a refractory plate. The fumed metals were post combusted
above the crucible and captured in a filter under the form of flue dust.
The reactor and the material were heated at to 1300 °C, as measured with
a Pt/PLRhlO thermocouple mounted between the briquettes, up to 600 °C,
heating was performed under a protective N2 gas atmosphere at a gas flow
rate of 200 1/h. From 600 °C to 1300 °C, CO was injected into the
crucible at a flow rate of 200 1/h.
Samples were taken after 30 minutes after reaching 1300 °C. These
samples were quenched in liquid N2, stopping all reactions and freezing
the mineralogy. The composition of feed and products is given in Table
1. The elemental distribution across products is shown in Table 2.
Table 1: Composition of feed and products

Component Mass (g) Composition (wt.%)
Feed Pb Cu As Zn Fe In CaO SiO2 S C F
SAL Residue 1000 5.1 1.74 0.1 28 15 0.02 1.61 5.46 5.9 0.05 0.01
PET-Cokes 250 5.27 87.8
Sypsum 190 41 24 0.1
Briquettes 1440 3.5 1.50 0.2 19 10.9 0.02 6.7 3.75 6.8 14.6 0.02
Products
Reduced residue 365 1.0 4.05 0.47 2.3 30 <0.01 18.0 10.5 14.6 7 0.02
Flue dust 270 11 0.03 0.07 66 0.15 0.07 <0.01 0.19 1.0 0.043 <0.03

WO 2006/133777 PCT/EP2006/004412
6
Table 2: Elemental distribution across products

Distribution (wt.%)
Products Pb Cu As Zn Fe In CaO SiO2 S C F
Reduced residue 10.9 99.5 90.1 4.5 99.6 <13 >99.2 98.7 95.2 99.5 >71
Flue dust 89.1 0.05 9.9 95.5 0.4 >87 <0.8 1.3 4.8 0.5 <29
The experimental results clearly show that after 30 minutes of roasting,
Zn, Pb and In are effectively fumed out of the briquettes, while Fe, Cu,
As and F are concentrated in the reduced residue. The good selectivity
towards As and F is particularly interesting in view of the subsequent
processing of the fumes by hydrometallurgical means.
Example 2
This example illustrates the crucial role of S the briquettes, as it
avoids the softening and melting of the material during the roasting
process without loss in the selectivity.
Two mixtures were prepared using a synthetic, S-free zinc leach residue
comprising zink ferrite with 5 wt.% SiO2, and:
- 15 wt.% CaO and 25 wt.% finely ground cokes (Mixture 1);
- 36.7 wt.% of gypsum and 25 wt.% finely ground cokes (Mixture 2).
Both mixtures were compacted to briquettes and fumed according to the
procedure of Example 1.
The briquettes corresponding to Mixture 1, containing only about 0.3
wt.% S, appeared to smelt, indicating the formation of low smelting
phases like 2FeO.SiO2. However, the briquettes corresponding to Mixture
2, containing about 6.5 wt.% S, did not show any formation of such
phases, thanks to the presence of an adequate amount of S.

WO 2006/133777 PCT/EP2006/004412
7
CLAIMS
1. Process for the separation of metal values in a Fe-bearing Zn
leaching residue comprising the steps of:
- preparing agglomerates containing, besides the Zn leaching residue, at
least 5 wt% of carbon and 2 to 10 wt.% of S;
- fuming said agglomerates in a static bed at a temperature above
1250°C, thereby producing a reduced Fe-bearing phase and Zn-bearing
fumes; and
- extracting said Zn-bearing fumes.

2. Process according to claim 1, further comprising the step of drying
the Zn leaching residue to a moisture content of less than 12 wt.% H2O,
and preferably to less than 5 wt.% H2O, before the step of the
preparation of agglomerates.
3. Process according to claims 1 or 2, characterised in that the
agglomerates comprise at least 15 wt.% of carbon.
4. Process according to any one of claims 1 to 3, characterised in that
the agglomerates further comprise a Ca compound, whereby said compound
provides for at least 10 wt.%, and preferably at least 15 wt.% of CaO
equivalent in the agglomerates.
5. Process according to any one of claims 1 to 4, characterised in that
the agglomerates are pellets having a Mass Pellet Strength of at least 5
kg, and preferably briquettes having a Mass Pellet Strength of at least
10 kg.
6. Process according to any one of claims 1 to 5, characterised in that
the fuming temperature is at least 1300 °C.
7. Process according to any one of claims 1 to 6, characterised in that
the fuming is carried out in a carbon monoxide containing atmosphere.
8. Process according to any one of claims 1 to 7, characterised in that
the Zn leaching residue is a neutral or weak acid Zn leach residue.

WO 2006/133777 PCT/EP2006/004412
8
9. Process according to any one of claims 1 to 8, characterised in that
the fuming step is carried out in a rotary hearth furnace.
10. Process according to any one of claims 1 to 9, further comprising
the step of subjecting the reduced Fe-bearing phase to an oxidising
smelting step.

The invention relates to the separation of metals in Fe-bearing zinc leaching residues, in particular neutral and weak
acid leach residues. The process comprises the steps of : - preparing agglomerates containing, besides the Zn leaching residue, at least 5 wt% of carbon and 2 to 10 wt. % of S; - fuming said agglomerates in a static bed at a temperature above 12500C, thereby producing a reduced Fe-bearing phase and Zn-bearing fumes; and - extracting said Zn-bearing fumes. The high S content of the feed
allows for a relatively high operating temperature without production of molten phases. This guarantees fast reduction and fuming kinetics, and permits the use of a compact technology such as a static bed furnace.