AU2006257458A1

AU2006257458A1 - Separation of metal values in zinc leaching residues

Info

Publication number: AU2006257458A1
Application number: AU2006257458A
Authority: AU
Inventors: Jonathan Aerts; Charles Geenen; Maurits Van Camp
Original assignee: Umicore NV SA
Current assignee: Umicore NV SA
Priority date: 2005-06-13
Filing date: 2006-05-11
Publication date: 2006-12-21
Also published as: EP1893779A1; CA2611925A1; EA013690B1; EA200800030A1; JP2008545888A; MX2007015812A; NO20080042L; KR20080022545A; PE20070088A1; ZA200710377B; CN101341265A; BRPI0612150A2; US20080196551A1; WO2006133777A1

Description

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. 5 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 10 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% Si02 15 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 20 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/l H 2

SO

4 . 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 25 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 30 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. 35 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 5 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.SiO 2 - FeO, which has a melting point of approximately 1180 'C. The dissolution of FeO further lowers the melting point and through 10 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 0 C. This again leads to a decreased reduction of ZnO and iron oxide, which are 15 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/SiO 2 ratio in the feed has to be monitored closely by setting it to a value of 0.8 to 1.8. 20 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. 25 In W02005-005674 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 30 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 35 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. 5 In US5,906,671 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. 10 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 Fe-bearing zinc leaching residues, 15 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 20 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 25 of less than 12 wt.% H 2 0, or even to less than 5 wt.% H 2 0, 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.%. 30 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. 35 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; 5 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 10 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 15 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 20 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 25 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 (ZnO.Fe 2

O

3 ), lead sulphate (PbSO 4 ), calcium sulphate (CaSO 4 ), zinc sulphate (ZnSO 4 ) and impurities like CaO, Si0 2 , MgO, A1 2 0 3 , 30 Cu 2 0, SnO, was dried to a moisture content below 5 wt% H 2 0, 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/cm 2 resulting in hard, shiny briquettes, having a Mass Pellet Strength of 20 35 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. 5 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 10 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/PtRhlO thermocouple mounted between the briquettes. Up to 600 *C, 15 heating was performed under a protective N 2 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 20 samples were quenched in liquid N 2 , 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 SiO 2 S C F WAL 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 Gypsum 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 25 WO 2006/133777 PCT/EP2006/004412 6 Table 2: Elemental distribution across products Distribution (wt.%) Products Pb Cu As Zn Fe In Cao Sio 2 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, 5 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 10 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 15 comprising zink ferrite with 5 wt.% SiO 2 , 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 20 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 25 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.

Claims

1. Process for the separation of metal values in a Fe-bearing Zn leaching residue comprising the steps of: 5 - 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 0 C, thereby producing a reduced Fe-bearing phase and Zn-bearing fumes; and 10 - 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.% H 2 0, and preferably to less than 5 wt.% H 2 0, before the step of the 15 preparation of agglomerates.

3. Process according to claims 1 or 2, characterised in that the agglomerates comprise at least 15 wt.% of carbon. 20

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. 25

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. 30

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. 35

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 5 the step of subjecting the reduced Fe-bearing phase to an oxidising smelting step.