SE1950033A1 - Treatment of ferric iron based material comprising zinc and sulphur - Google Patents

Abstract

A method for treating ferric-iron based material comprising at least zinc and sulphur comprises smelting and heating (S10) of the ferric-iron based material and added flux forming a liquid slag. The flux comprises a refractory material and CaO. An amount of the refractory material of the flux is added to give the slag a liquidus temperature of the slag above 1200°C. An amount of the CaO of the flux is added to give a basicity of the slag in the range between 0.5 and 0.9. An oxygen potential in gas provided during the smelting and heating is controlled (S12) to be in the range of 10to 10atm., whereby the sulphur is converted to gaseous SO. Gases fumed-off from the liquid slag are removed (S14). The gases fumed-off from the liquid slag comprises zinc and the gaseous SO.

Description

lO TREATMENT(H?FERRK2HHN¶BASEDIWATERUH,COMPRHHNGJHNCANDEHHPHUR TECHNKÄLFEED The present invention relates in general to arrangements and methods forrecovery of evaporable substances, and in particular to arrangements and methods for treating ferric iron-based material comprising zinc and sulphur.

BACKGROUND The invention has its background in the large tonnage of residues that areproduced during zinc metal production. The amount of residue is frequentlymore than 50% of the zinc metal production. Most of the residues are presentlydumped Without any further processing but this procedure Will not be an alternative for the future.

The conventional zinc metal production process is based on sulphideconcentrates that contains sphalerite (zinc sulphide, ZnS) as the mainmineral. A typical concentrate contains also some pyrite (FeSg), galena (PbS),silica (SiOg), alumina (AlgOg) and lime (CaO). Minor amounts of mineralscontaining copper, silver, antimony, arsenic, cadmium, mercury and thallium are frequently also present in a zinc concentrate.

A typical sulphide concentrate is a chemically rather complex material. Itcontains typically 55% Fe, 35% S, 5% SiOg, <1 % Pb, <0. 1% Ag, lto 3 % AlgOgand CaO, < O.5% Cu along With many types of minor elements, e.g. As, Cd, Hg, Sb, Ge, In, Se, Tl.

The standard production method for electrolytic zinc is the Roast-Leach- Electrowinning process, RLE. It consists of roasting the zinc concentrate into calcine, calcine leachíng in sulphuric acid solution, leach purification, precipitation and electrowinning of zinc.

An alternative zinc production method includes a direct leach process insteadof roasting. The direct leaching process recovers sulphur as elemental sulphurinstead of S02 and possibly sulphuric acid. The method eliminates the risk ofS02 emissions but create a heavy-metal contaminated elemental sulphurwithout any commercial use or value. The sulphur therefore ends up as aresidue from the zinc plant. Further processing, i.e. leaching, solutionpurification and electrowinning of zinc metal is similar to conventional roast- based processing and the direct leaching process.

All hydrometallurgical zinc production processes will produce a ferric íron richprecipitate as a residue. It contains, beside ferric iron, also lead sulphate,minor elements and gangue compounds. Iron can be precipitated as jarosite(NaFe3(SO4)2(OH)6, hematite (FegOg) or goethite (FeOOH). The three types ofresidue are produced by different zinc plant process layouts and have different composition as shown in the table below.

Process Jarosite Goethite HematiteWt/wt zinc metal 0.5 0.32 0.20Fe wt % 25-30 40-45 50-60Zn wt % 4-6 5-8 0.5-1.0Swt% 10-12 2.5-5 3 J arosite is the most common residue due its relatively simple production flowsheet. A few zinc smelters use the goethite rout and just one the hematiteroute. The table indicates the quantity of precipitate. It is normally large, 50%or even more, for the jarosite-based production, i.e. the vast majority of world-wide zinc production. The amount of residue is increased even more When thedirect leach process is applied, since sulphur is recovered in elemental form and sulphur is mixed with jarosite into a combined residue. The mixed residue will contain at least 30% S with 20 to 25 % being elemental sulphur. The rest lO 3 is typically 20% Fe, 3% Zn, 3% Pb, <0. 1% Ag, 1 to 3 % AlzOs and CaO, < O.5%Cu.

All types of zinc plant residues become environmentally hazardous due to theircontent of minor elements, e.g. As, Sb, Cd, Tl, Hg. These elements are beincorporated in the residues and makes them chemically non-stable. Theresidues are today normally put on a dump site but this practice will not bepossible in the future, both due to legal and economic reasons. The need foran efficient recycling method that recovers minor potentially hazardouselements and produces a clean residue to be used as raw material for new products has therefor become large.

Some zinc plants run smelting and fuming processes to recover metals and tostabilize the leach residue as a slag. The slag has a quality that doesrft passcoming demand since the content of both lead and zinc are far too high.Presently, standards specify that slag produced from treatment of zinc plantresidues must have a quality that makes the slag a candidate for newapplications and the quality is closely linked to its lead and zinc content. Thepresent demand on slag quality within the EU is < 1% Zn and < 0.03% Pb. Atthe present there doesnt exist any process that can produce such a clean slagin one Operating stage. Even frequently used Q-stage processes doesnt reach the new strict level.

Among todays used process routes for treating zinc plant residues, the mostused alternatives are the Waelz kiln process, the Q-stage TSL process and the ArcFume process.

The traditional method is the Waelz kiln process. lt faces severe environmentalissues since it is an old process with very limited possibility to be improved.

The Waelz process cannot produce a slag with present quality demand.

The single stage TSL process in smelting mode is also used by e.g. Nyrstar in Port Pirie, Australia, and at several Chinese smelters. The TSL is used as an lO 4 efficient smelting unit as described above but it is run in a combination of ashaft furnace and slag fuming furnace that are used as reduction units. Thisset up is preferred when old lead smelters are modernized and adopted fortreatment of zinc plant residues and existing equipment reused. Thecomposition of slag from either 2-stage TSL or a combined TSL-shaft furnace-slag fuming furnace is, however, not good enough to pass the new strict EU demand at < 1% Zn and < 003% Pb.

The TSL process can be used in a two-stage set-up. The first stage is anoxidizing smelting step that converts sulphur into SOg(g) and partly recoverszinc, lead and other volatile elements as a fume product. However, it producesa slag with a zinc and lead content far above the quality targets given above.A second reducing “traditional” fuming step must be added to reduce the metalcontent. The 2-stage TSL process is highly flexible and used by Korea Zinc innumerous installations. lt has proven to be a very efficient alternative forrecycling various kinds of residues and waste containing a wide range ofmetals, including zinc plant residues but the route is not capable of producing the high-quality slag with < 1% Zn and < 0.03°/o Pb.

The ArcFume process is used by Nyrstar in Höyanger, Norway, to treat zincplant residues. The ArcFume reactor runs in a mode that simultaneouslyproduce a fume product with recovered zinc, lead and minor elements, matteproduct with copper and silver and a slag product. The slag is low in zinc andlead but doesn't pass the strict quality demand given above. The presence ofmatte hinders the best possible removal of lead and zinc since matte drops easily will become dispersed in the slag.

The limitations of presently used smelting processes can be summarized below.

Oxidizing smelting will remove sulphur as S02 but will also produce a slag with rather high content of zinc and lead. The smelting slag needs to be further lO treated by a slag fuming unit making the oxidizing process route eventually a 2-stage operation.

Slightly reducing smelting is a single stage operation and it will distributesulphur between S02 and matte that collects copper and silver. The slag willhave a rather low zinc and lead content but still much too high to qualify theEU demand. The presence of dispersed matte in the slag makes the effect of a second fuming stage limited and a high-quality slag will not be produced.

Fuming a zinc and lead containing slag has been a conventional process forabout 100 years. It is a reducing process that reduce metal oxides intoelemental form, e.g. Zn(g) that boils off from the slag bath. Metallic copper andsilver are collected as a mixture of speiss and matte due to reduction ofsulphur, arsenic and antimony in the slag. The potential to produce a slagwith < 1% Zn is limited since such a low zinc content requires rather strongreduction and this will cause also iron oxide to become metallic iron. Metalliciron forms steel, which has a high smelting point and will solidify at thefurnace bottom. The fuming process is therefor stopped before it enters the iron reduction state Which corresponds to a zinc content in the slag of ~ 1%.

In summary, none of the presently operated processes routes that treats zincplant residues are capable of producing a slag with quality as demanded by the new EU standard.

SUMMARY A general object of the present invention is to provide a simplified method forrecovering valuable metals and for production of a high-quality slag suitable for new products.

The above object is achieved by methods and devices according to the independent claims. Preferred embodiments are defined in dependent claims. lO 6 In general words, a method for treatíng ferric-iron based material comprisingat least zinc and sulphur comprises smelting and heating of the ferric-ironbased material and added flux forming a liquid slag. The flux comprises arefractory material and CaO. An amount of the refractory material of the fluxis added to give the slag a liquidus temperature of the slag above l200°C. Anamount of the CaO of the flux is added to give a basicity of the slag in therange between 0.5 and 0.9. An oxygen potential in gas provided during thesmelting and heating is controlled to be in the range of lO*9 to 10-1 atm.,whereby the sulphur is converted to gaseous S02. Gases fumed-off from theliquid slag are removed. The gases fumed-off from the liquid slag comprises zinc and the gaseous S02.

One advantage with the proposed technology is that valuable metals arerecovered and high-quality slag suitable for new products are produced in aone-step process. The proposed technology is particularly advantageous forprocessing leach residues from electrolytic Zn metal production. The proposedtechnology makes a great simplification in the leach residue processing with a simultaneously improved product quality.

BRIEF DESCRIPTION OF THE DRAWINGS The invention, together with further objects and advantages thereof, may bestbe understood by making reference to the following description taken togetherwith the accompanying drawings, in which: FIG. 1 is a schematic illustration of an arrangement for recovery ofevaporable substances; and FIG. 2 is a flow diagram of steps of an embodiment of a method for treatíng ferric-iron based material comprising at least zinc and sulphur.

DETAILED DESCRIPTION Throughout the drawings, the same reference numbers are used for similar or corresponding elements.

For a better understanding of the proposed technology, it may be useful to begin With a brief overview of an example of a fuming process.

Fig. 1 illustrates schematically an embodiment of an arrangement 1 forrecovery of evaporable substances, typically referred to as a fuming furnace.The arrangement 1 comprises a furnace 10. Ferric-iron based material 22comprising at least zinc and sulphur is introduced through an inlet 21 intothe furnace 10. Also flux 19 is added into the furnace 10, via the inlet 21 or aseparate inlet. A smelting and heating arrangement, in this example asubmerged heater 20, is arranged for smelting the ferric-iron based material22 and flux 19 introduced into the furnace 10 into a liquid slag 24. In thepresent example, the submerged heater 20 comprises a plasma gun 28 and atuyere 29. The plasma gun 28 is thus arranged for supplying the energy necessary for smelting the ferric-iron based material 22 and flux 19.

In the embodiment of Fig. 1, the plasma gun 28 is via the tuyere 29 submergedinto the liquid slag 24. The plasma gun 28 is thereby also arranged foragitating the liquid slag 24 by means of a submerged jet 26 of hot gas. The hotgas 27 creates bubbles in the liquid slag 24, causing a stirring of the liquidslag 24 on their Way up to the surface 25 of the slag bath. At temperatures,high enough, evaporable metals and /or evaporable compounds are fumed off from the liquid slag 24 into a gas volume 12 above the liquid slag surface 25.

The present example further comprises a fume handling system 30. The fumehandling system 30 is configured to collect the evaporable metals and/ orevaporable compounds in the gas volume 12 that has been fuming off fromthe liquid slag 24. The metals and/ or metal compounds are handled inaccordance with prior art methods for valuation of the final metals and/ orcompounds 31. The particular Way in Which the evaporable metals and/ orevaporable metal compounds are handled is not crucial for the operation of the slag fuming arrangement as such and is therefore not further discussed. lO 8 The present example also comprises a slag outlet 40 allowing liquid slagdepleted in evaporable metals and/ or evaporable compounds 41 to be tappedoff. The present embodiment of the arrangement 1 has a furnace that isarranged for performing a continuous process. In other Words, the presentembodiment is intended for a continuous operation, Where the ferric-ironbased material 22 and flux 19 continuously or intermittently are introducedinto the furnace 10. The liquid slag depleted in evaporable metals and/ orevaporable compounds may continuously or intermittently be removed from the furnace 10 by the slag outlet 40.

In an alternative embodiment, the furnace 10 can also be operated in a batchmanner, Where the material 22 first is entered into the furnace 10, thentreated into a liquid slag depleted in evaporable metals and/ or evaporable metal compounds and finally removed from the furnace 10.

In one preferred embodiment, the submerged heater 20 comprises a controller23 arranged for Operating the submerged heater 20 for keeping the liquid slag24 at a predetermined average temperature. The predetermined average temperature is preferably selected in dependence of the slag composition.

In a particular example, the furnace 10 is equipped With a cooled Wall 15, inorder to create a freeze lining 16 and to be able to reduce the Wear of thefurnace Wall. The predetermined average temperature of the slag is then alsopreferably selected in dependence of the performance of the cooled Wall 15.The controller 23 is then arranged for balancing the predetermined averagetemperature of the slag to the reactor Wall cooling to create a suitable protective frozen slag layer or freeze lining 16 on the reactor Wall 15.

It has now, unexpectedly, been found that it is possible to, in one stage andbased on a ferric-iron based material comprising at least zinc and sulphur,e. g. a ferric-iron-rich zinc-plant residue, simultaneously recover an extremelyclean slag, to recover sulphur as an SOg(g) rich gas and to recover zinc as Well as other valuable and minor elements to a fume product. The extremely clean slag can be produced to pass the limits of < 1% Zn and < 003% Pb. The SO2(g) rich gas is furthermore suitable for H2SO4 production.

These properties can be achieved by a close control of the slag properties, i.e.a controlled oxygen potential and a controlled slag composition. The slagproperties are controlled to enable smelting of the ferric-iron based materialcomprising at least zinc and sulphur at a high slag temperature, at least abovel200°C. This combination has a unique effect on the process and solves the problems mentioned above.

The ferric-iron based material comprising at least zinc and sulphur, e.g. a zincplant residue, is thus smelted under controlled oxygen potential to obtain aslag With low residual sulphur content, typically below 1%. A low sulphurcontent is crucial to avoid formation of matte that will hinder an efficient removal of lead, zinc and minor elements from the slag.

The ferric-iron based material comprising at least zinc and sulphur, e.g. thezinc plant residue, is furthermore smelted to a slag with high liquidustemperature by controlling the content of a refractory material and lime in theslag. The refractory material content in the slag is preferably close tosaturation which makes it easy to form a freeze lining of the refractory materialin combination with the liquid slag formed from ferric-iron based material in the fuming furnace.

The lime content is adjusted to create a slag with low viscosity. This is typicallyobtained at a basicity of 0.5 to 0.9 where the basicity is defined as the weight percent ratio of CaO/ SiOg.

Fig. 2 is a flow diagram of steps of an embodiment of a method for treatingferric-iron based material comprising at least zinc and sulphur. In step S10,the ferric-iron based material and added flux is smelted and heated, therebyforming a liquid slag. The flux comprises a refractory material and lime (CaO).

An amount of the refractory material of the flux is added to give the slag a liquidus temperature of above l200°C. An amount of the lime of the flux isadded to give a basicity of the slag in the range between 0.5 and 0.9. In stepS12, an oxygen potential in the gas provided during the step of smelting andheating is controlled to be in the range of 10-9 to 104 atm. This leads to theConversion of the sulphur into gaseous S02. In step S14, gases fumed-off fromthe liquid slag are removed. The gases fumed~off from the liquid slag comprises Zn and the gaseous S02.

Preferably, the amount of the refractory material of the flux exceeds 50% of a saturation content of the refractory material in the liquid slag. ln a preferred embodiment, the method comprises the further step S16, inWhich an operation temperature of the liquid slag is controlled to give a freezelining of the refractory material in combination With the liquid slag formedfrom ferric-iron based material on a Wall of a furnace in Which the smelting takes place.

The refractory material is preferably selected in dependence of the material ofthe furnace itself. For instance, if a furnace is built by a high-alumina brickWork, it is advantageous to add alumina (AlgOg) as the refractory material. Afreeze lining of the refractory material in combination With the liquid slagformed from ferric-iron based material then creates a rather inert slag in contact With a high alumina brick Work.

Thus, depending on the furnace constructions, different refractory materialsmay be preferred. Preferably, the refractory material is selected from AlgOg, l\/lgO, and CrzOg.

During fuming, the slag should be liquid. However, operating at a too hightemperature Will impose Wear of the freeze lining. It is therefore preferred tohave an operating temperature of the liquid slag that is less than 100°C abovethe liquidus temperature of the liquid slag. Even more preferably, the Operating temperature is less than 50°C above the liquidus temperature of the lO ll liquid slag. However, as mentioned above, the Operating temperature has to be higher than the liquidus temperature of the liquid slag.

The high slag temperature ensures a high vapour pressure of volatile elementsand compounds being present in the slag. Non-exclusive examples of suchelements are e.g. Ag, As, Cd, Hg, Sb, Tl. The high vapour pressure favoursthereby their fuming.

Moreover, the equilibrium condition for the reaction: ZnO -> Zn(g)+% O2(g) is shifted to the right-hand side due to a very high flame temperature in theslag bath. This reaction thus results in zinc fuming and occurs spontaneously Without the addition of e.g. reductant carbon.

In fact, any addition of reductant carbon or coal alternatively a reducing hydrocarbon gas must be avoided due to its contra productive effect on sulphur.Such an addition Would hinder or limit the sulphur removal as SO2(g) and Willinstead cause matte formation. The matte will in such a case capture zinc aszinc sulphide and thereby reduce the formation of volatile Zn(g).HoWever, inthe present solution, there is provided a simultaneous removal of sulphur as SO2(g) and fuming of Zn(g) and possibly also other volatile elements.

The simultaneous formation of SO2(g) and volatile elements and compoundsat a high process temperature makes the slag depleted in zinc, lead and minor metal and becomes thereby chemically stable by relevant slag stability test.

The recovery of heavy metals and the production of a clean slag is therebyobtained in a one stage operation and elimínates the need for a separate reduction stage. 12 A high temperature is thus favourable for the final result. Therefore, in apreferred embodiment, the amount of the refractory material of the flux isadded to give the slag a liquidus temperature of above 1250°C. Even morepreferably, the amount of the refractory material of the flux is added to give the slag a liquidus temperature of above 1300°C.

Depending on the choice of refractory material in the flux, the preferred addedamount may differ. If alumina (AlgOg) is used, the amount added as flux to the slag is preferably in the range of 7~15% by Weight of the liquid slag.

If MgO is used, the amount added as flux to the slag is preferably in the rangeof 2-5% by Weight of the liquid slag.

If CrgOg, is used, the amount added as flux to the slag is preferably in the range of 0.05-1% by weight of the liquid slag.

As mentíoned above, the amount of added lime determines the viscosityproperties. Since a Well-performed mixing is requested, the viscosity shouldpreferably not be too high. It has been found that CaO provided in an amountof 13-20% by Weight of the liquid slag gives a preferred viscosity in most situations.

The agitating of the slag bath is of importance to obtain an efficient fuming. Ina preferred embodiment, the smelting and heating is thereby performed byheating and agitating the slag by gas from a submerged heater. Such asubmerged heater can be designed in different Ways. Some preferred embodiments are a plasma gun, an oxyfuel burner or a submerged top lance.

For operation temperatures above 1200°C and With Well agitated slag baths,the fumíng of Zn becomes very efficient. Furthermore, by removing thesulphur by means of the controlled oxygen potential, capture of Zn in a mattephase is prohibited. The requested levels of < 1% remaining Zn in the final slag can thus be reached Within reasonable operation times. This means that in a 13 preferred embodiment, the step of smelting and heating is performed until a remaining content of Zn in the liquid slag is lower than 1% by weight.

The conditions for possible lead (Pb) impurities in the ferric-iron basedmaterial are relatively similar analogue to the Zn case. At high operationtemperatures, and at a controlled oxygen potential of 10-9 to 104 atm, mostcontent of Pb will undergo a fuming process. In other words, when the ferric-iron based material further comprises Pb, the gases fumed-off from the liquid slag further comprises Pb.

The requested levels of <0.03% remaining Pb in the final slag can thus bereached within reasonable operation times. This means that in a preferredembodiment, the step of smelting and heating is performed until a remaining content of Pb in the liquid slag is lower than 003% by weight.

Furthermore, as mentioned above, also other volatile elements, such as e.g.As, ln, Ge, Ag, Cd, Hg, Sb and/ or Tl may be present in the original ferric-ironbased material. Also, these elements are easily fumed at the proposedtemperature and oxygen potential conditions. Thus, if the ferric-iron basedmaterial further comprises at least one of As, In, Ge, Ag, Cd, Hg, Sb and Tl,the gases fumed-off from the liquid slag further comprises the As, In, Ge, Ag,Cd, Hg, Sb and/or Tl.

The gas fumed off from the slag bath is, as was described further above,transferred to a fume handling system 30 (Fig. l). The fume handling systemis preferably arranged for 30 is configured to condense metals or otherelements from the removed gases fumed-off from the liquid slag. Such acondensing is, as such, know from prior art, and will not be described infurther detail. The condensed material is handled in accordance with prior art methods for valuation of e.g. the final metals.

The sulphur dioxide is also preferably taken care of in an environmentally friendly manner. The S02 is separated from the removed gases fumed-off from lO 14 the liquid slag. Thereafter, the separated S02 is transformed into sulphuric acid. Such processes are, as such, known in prior art.

The embodiments described above are to be understood as a few illustrativeexamples of the present invention. It Will be understood by those skilled in theart that various modifications, combinations and changes may be made to theembodiments Without departing from the scope of the present invention. Inparticular, different part solutions in the different embodiments can becombined in other confígurations, Where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims (18)

1. A method for treating ferric-iron based material comprising at leastzinc and sulphur, Wherein said method comprises the steps of: - smelting and heating (S10) said ferric-iron based material (22) andadded flux (19) forming a liquid slag (24); said flux (19) comprising a refractory material and CaO; Wherein an amount of said refractory material of said flux (19) is addedto give said slag a liquidus temperature of said slag above 1200°C; Wherein an amount of said CaO of said flux ( 19) is added to give abasicity of said slag in the range between 0.5 and 0.9; - controlling (S12) an oxygen potential in gas (27) provided during saidstep of smelting and heating (S10) to be in the range of 10-9 to 10-1 atm.,Whereby said sulphur is converted to gaseous S02; and - removing (S14) gases fumed-off from said liquid slag (24), said gases fumed-off from said liquid slag (24) comprising zinc and said gaseous S02.

2. The method according to claim 1, characterized by the further stepof: - controlling (S16) an operation temperature of said liquid slag (24) togive a freeze lining (16) of said refractory material in combination With saidliquid slag formed from ferric-iron based material on a Wall (15) of a furnace (10) in Which said smelting takes place.

3. The method according to claim 1 or 2, characterized in that saidamount of said refractory material of said flux (19) exceeds 50% of a saturation content of said refractory material in said liquid slag (24).

4. The method according to any of the claims 1 to 3, characterized inthat said refractory material is selected from:AlgOg;MgO; andCrgOg. 16

5. The method according to any of the claims 1 to 4, characterized inthat said amount of said refractory material of said flux (19) is added to givesaid slag a liquidus temperature of above 1250°C and preferably above 1300°C.

6. The method according to claim 5, characterized in that said refractorymaterial comprises AlgOg in an amount of 7 -15% by Weight of said liquid slag (24).

7. The method according to claim 5, characterized in that said refractorymaterial comprises MgO in an amount of 2-5% by Weight of said liquid slag (24).

8. The method according to claim 5, characterized in that said refractorymaterial comprises Cr2O3 in an amount of 0.05-1°/o by Weight of said liquid slag (24).

9. The method according to any of the claims 1 to 8, characterized inthat said CaO is provided in an amount of 13-20% by weight of said liquidslag (24).

10. The method according to any of the claims 1 to 9, characterized inthat said step of smelting and heating (S10) being performed by heating andagitating said liquid slag (24) by gas from a submerged heater (20).

11. The method according to claim 10, characterized in that saidsubmerged heater (20) is at least one of:a plasma gun (28);an oxyfuel burner; and a submerged top lance. 17

12. The method according to any of the claims 1 to 11, characterized inthat an operating temperature of said liquid slag (24) is less than lOO°C abovesaid liquidus temperature of said liquid slag (24) and preferably less than 50°C above said liquidus temperature of said liquid slag (24).

13. The method according to any of the claims 1 to 12, characterized inthat said step of smelting and heating (S10) is performed until a remaining content of Zn in said liquid slag (24) is lower than 1% by Weight.

14. The method according to any of the claims 1 to 13, characterized inthat said ferric~iron based material further comprises Pb, Whereby said gases fumed-off from said liquid slag (24) further comprises Pb.

15. The method according to claim 14, characterized in that said step ofsmelting and heating (S10) is performed until a remaining content of Pb in said liquid slag (24) is lower than 0.03°/o by Weight.

16. The method according to any of the claims 1 to 15, characterized inthat said ferric-iron based material further comprises at least one of As, In,Ge, Ag, Cd, Hg, Sb and Tl, Whereby said gases fumed-off from said liquid slag(24) further comprises said at least one of As, In, Ge, Ag, Cd, Hg, Sb and Tl.

17. The method according to any of the claims 1 to 16, characterized by the further step of: - condensing metals from said removed gases fumed-off from said liquid slag (24).

18. The method according to any of the claims 1 to 17, characterized by the further steps of:- separating said S02 from said removed gases fumed-off from saidliquid slag (24);- transforming said separated S02 to sulphuric acid.