WO2020149777A1

WO2020149777A1 - Treatment of ferric iron based material comprising zinc and sulphur

Info

Publication number: WO2020149777A1
Application number: PCT/SE2020/050019
Authority: WO
Inventors: Matej IMRIS; Åke HOLMSTRÖM; Sven Santén
Original assignee: Val'eas Recycling Solutions Ab
Priority date: 2019-01-14
Filing date: 2020-01-13
Publication date: 2020-07-23
Also published as: SE1950033A1; SE542917C2

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 10-9 to 10-1 atm., whereby the sulphur is converted to gaseous SO2. Gases fumed-off from the liquid slag are removed (S14). The gases fumed-off from the liquid slag comprises zinc and the gaseous S02.

Description

TREATMENT OF FERRIC IRON BASED MATERIAL

COMPRISING ZINC AND SULPHUR

TECHNICAL FIELD

The present invention relates in general to arrangements and methods for recovery 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 are produced during zinc metal production. The amount of residue is frequently more than 50% of the zinc metal production. Most of the residues are presently dumped without any further processing but this procedure will not be an alternative for the future.

The conventional zinc metal production process is based on sulphide concentrates that contains sphalerite (zinc sulphide, ZnS) as the main mineral. A typical concentrate contains also some pyrite (FeS2), galena (PbS), silica (SiCb), alumina (AI2O3) and lime (CaO). Minor amounts of minerals containing 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. It contains typically 55% Fe, 35% S, 5% S1O2, < 1 % Pb, <0. 1% Ag, lto 3 % AI2O3 and CaO, < 0.5% 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 leaching in sulphuric acid solution, leach purification, precipitation and electrowinning of zinc.

An alternative zinc production method includes a direct leach process instead of roasting. The direct leaching process recovers sulphur as elemental sulphur instead of SO2 and possibly sulphuric acid. The method eliminates the risk of SO2 emissions but create a heavy-metal contaminated elemental sulphur without any commercial use or value. The sulphur therefore ends up as a residue from the zinc plant. Further processing, i.e. leaching, solution purification 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 iron rich precipitate as a residue. It contains, beside ferric iron, also lead sulphate, minor elements and gangue compounds. Iron can be precipitated as jarosite

(NaFe3(SC>4)₂(OH)6, hematite (Fe₂C>3) or goethite (FeOOH). The three types of residue are produced by different zinc plant process layouts and have different composition as shown in the table below.

Jarosite is the most common residue due its relatively simple production flow sheet. A few zinc smelters use the goethite rout and just one the hematite route. 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 the direct 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 is typically 20% Fe, 3% Zn, 3% Pb, <0.1% Ag, 1 to 3 % AI2O3 and CaO, < 0.5% Cu.

All types of zinc plant residues become environmentally hazardous due to their content of minor elements, e.g. As, Sb, Cd, Tl, Hg. These elements are be incorporated in the residues and makes them chemically non-stable. The residues are today normally put on a dump site but this practice will not be possible in the future, both due to legal and economic reasons. The need for an efficient recycling method that recovers minor potentially hazardous elements 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 to stabilize the leach residue as a slag. The slag has a quality that doesn t pass coming demand since the content of both lead and zinc are far too high. Presently, standards specify that slag produced from treatment of zinc plant residues must have a quality that makes the slag a candidate for new applications and the quality is closely linked to its lead and zinc content. The present demand on slag quality within the EU is < 1% Zn and < 0.03% Pb. At the present there doesn t exist any process that can produce such a clean slag in one operating stage. Even frequently used 2-stage processes doesn t reach the new strict level.

Among todays used process routes for treating zinc plant residues, the most used alternatives are the Waelz kiln process, the 2-stage TSL process and the Arc Fume process.

The traditional method is the Waelz kiln process. It faces severe environmental issues since it is an old process with veiy 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 efficient smelting unit as described above but it is run in a combination of a shaft furnace and slag fuming furnace that are used as reduction units. This set up is preferred when old lead smelters are modernized and adopted for treatment of zinc plant residues and existing equipment reused. The composition 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 < 0.03% Pb.

The TSL process can be used in a two-stage set-up. The first stage is an oxidizing smelting step that converts sulphur into S02(g) and partly recovers zinc, lead and other volatile elements as a fume product. However, it produces a 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 metal content. The 2-stage TSL process is highly flexible and used by Korea Zinc in numerous installations. It has proven to be a very efficient alternative for recycling various kinds of residues and waste containing a wide range of metals, including zinc plant residues but the route is not capable of producing the high-quality slag with < 1% Zn and < 0.03% Pb.

The ArcFume process is used by Nyrstar in H byanger, Norway, to treat zinc plant residues. The ArcFume reactor runs in a mode that simultaneously produce a fume product with recovered zinc, lead and minor elements, matte product with copper and silver and a slag product. The slag is low in zinc and lead but doesn t pass the strict quality demand given above. The presence of matte 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 SO2 but will also produce a slag with rather high content of zinc and lead. The smelting slag needs to be further 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 distribute sulphur between SO2 and matte that collects copper and silver. The slag will have a rather low zinc and lead content but still much too high to qualify the EU 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 for about 100 years. It is a reducing process that reduce metal oxides into elemental form, e.g. Zn(g) that boils off from the slag bath. Metallic copper and silver are collected as a mixture of speiss and matte due to reduction of sulphur, arsenic and antimony in the slag. The potential to produce a slag with < 1% Zn is limited since such a low zinc content requires rather strong reduction and this will cause also iron oxide to become metallic iron. Metallic iron forms steel, which has a high smelting point and will solidify at the furnace 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 zinc plant 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 for recovering 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. In general words, a method for treating ferric-iron based material comprising at least zinc and sulphur comprises smelting and heating 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 to be in the range of lO⁹ to lO^{- 1} atm., whereby the sulphur is converted to gaseous SO2. Gases fumed-off from the liquid slag are removed. The gases fumed-off from the liquid slag comprises zinc and the gaseous SO2.

One advantage with the proposed technology is that valuable metals are recovered and high-quality slag suitable for new products are produced in a one-step process. The proposed technology is particularly advantageous for processing leach residues from electrolytic Zn metal production. The proposed technology 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 best be understood by making reference to the following description taken together with the accompanying drawings, in which:

FIG. 1 is a schematic illustration of an arrangement for recovery of evaporable substances; and

FIG. 2 is a flow diagram of steps of an embodiment of a method for treating 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 for recovery of evaporable substances, typically referred to as a fuming furnace. The arrangement 1 comprises a furnace 10. Ferric-iron based material 22 comprising at least zinc and sulphur is introduced through an inlet 21 into the furnace 10. Also flux 19 is added into the furnace 10, via the inlet 21 or a separate inlet. A smelting and heating arrangement, in this example a submerged heater 20, is arranged for smelting the ferric-iron based material 22 and flux 19 introduced into the furnace 10 into a liquid slag 24. In the present example, the submerged heater 20 comprises a plasma gun 28 and a tuyere 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 submerged into the liquid slag 24. The plasma gun 28 is thereby also arranged for agitating the liquid slag 24 by means of a submerged jet 26 of hot gas. The hot gas 27 creates bubbles in the liquid slag 24, causing a stirring of the liquid slag 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 fume handling system 30 is configured to collect the evaporable metals and/or evaporable compounds in the gas volume 12 that has been fuming off from the liquid slag 24. The metals and/or metal compounds are handled in accordance with prior art methods for valuation of the final metals and/or compounds 31. The particular way in which the evaporable metals and/or evaporable metal compounds are handled is not crucial for the operation of the slag fuming arrangement as such and is therefore not further discussed. The present example also comprises a slag outlet 40 allowing liquid slag depleted in evaporable metals and / or evaporable compounds 41 to be tapped off. The present embodiment of the arrangement 1 has a furnace that is arranged for performing a continuous process. In other words, the present embodiment is intended for a continuous operation, where the ferric-iron based material 22 and flux 19 continuously or intermittently are introduced into the furnace 10. The liquid slag depleted in evaporable metals and/or evaporable 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 batch manner, where the material 22 first is entered into the furnace 10, then treated 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 controller

23 arranged for operating the submerged heater 20 for keeping the liquid slag

24 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, in order to create a freeze lining 16 and to be able to reduce the wear of the furnace wall. The predetermined average temperature of the slag is then also preferably selected in dependence of the performance of the cooled wall 15. The controller 23 is then arranged for balancing the predetermined average temperature 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 and based on a ferric-iron based material comprising at least zinc and sulphur, e.g. a ferric-iron-rich zinc-plant residue, simultaneously recover an extremely clean slag, to recover sulphur as an S02(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 < 0.03% Pb. The SC>2(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 slag properties are controlled to enable smelting of the ferric-iron based material comprising at least zinc and sulphur at a high slag temperature, at least above 1200°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 zinc plant residue, is thus smelted under controlled oxygen potential to obtain a slag with low residual sulphur content, typically below 1%. A low sulphur content 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. the zinc plant residue, is furthermore smelted to a slag with high liquidus temperature by controlling the content of a refractory material and lime in the slag. The refractory material content in the slag is preferably close to saturation which makes it easy to form a freeze lining of the refractory material in 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 typically obtained at a basicity of 0.5 to 0.9 where the basicity is defined as the weight percent ratio of CaO/Si0₂.

Fig. 2 is a flow diagram of steps of an embodiment of a method for treating ferric-iron based material comprising at least zinc and sulphur. In step S 10, the ferric-iron based material and added flux is smelted and heated, thereby forming 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 1200°C. An amount of the lime of the flux is added to give a basicity of the slag in the range between 0.5 and 0.9. In step S 12, an oxygen potential in the gas provided during the step of smelting and heating is controlled to be in the range of lO⁹ to lO¹ atm. This leads to the conversion of the sulphur into gaseous SO2. In step S 14, gases fumed-off from the liquid slag are removed. The gases fumed-off from the liquid slag comprises Zn and the gaseous SO2.

Preferably, the amount of the refractory material of the flux exceeds 50% of a saturation content of the refractory material in the liquid slag.

In a preferred embodiment, the method comprises the further step S 16, in which an operation temperature of the liquid slag is controlled to give a freeze lining of the refractory material in combination with the liquid slag formed from 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 of the furnace itself. For instance, if a furnace is built by a high-alumina brick work, it is advantageous to add alumina (AI2O3) as the refractory material. A freeze lining of the refractory material in combination with the liquid slag formed 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 materials may be preferred. Preferably, the refractory material is selected from AI2O3, MgO, and Cr2C>3.

During fuming, the slag should be liquid. However, operating at a too high temperature will impose wear of the freeze lining. It is therefore preferred to have an operating temperature of the liquid slag that is less than 100°C above the liquidus temperature of the liquid slag. Even more preferably, the operating temperature is less than 50°C above the liquidus temperature of the 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 elements and compounds being present in the slag. Non-exclusive examples of such elements are e.g. Ag, As, Cd, Hg, Sb, Tl. The high vapour pressure favours thereby their fuming.

Moreover, the equilibrium condition for the reaction:

is shifted to the right-hand side due to a very high flame temperature in the slag 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 hydro carbon gas must be avoided due to its contra productive effect on sulphur. Such an addition would hinder or limit the sulphur removal as S0₂(g) and will instead cause matte formation. The matte will in such a case capture zinc as zinc sulphide and thereby reduce the formation of volatile Zn(g). However, in the present solution, there is provided a simultaneous removal of sulphur as S0₂(g) and fuming of Zn(g) and possibly also other volatile elements. The simultaneous formation of S0₂(g) and volatile elements and compounds at 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 thereby obtained in a one stage operation and eliminates the need for a separate reduction stage. A high temperature is thus favourable for the final result. Therefore, in a preferred embodiment, the amount of the refractory material of the flux is added to give the slag a liquidus temperature of above 1250°C. Even more preferably, 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 added amount may differ. If alumina (AI2O3) 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 range of 2-5% by weight of the liquid slag.

If Cr2C>3 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 mentioned above, the amount of added lime determines the viscosity properties. Since a well-performed mixing is requested, the viscosity should preferably not be too high. It has been found that CaO provided in an amount of 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. In a preferred embodiment, the smelting and heating is thereby performed by heating and agitating the slag by gas from a submerged heater. Such a submerged 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 fuming of Zn becomes veiy efficient. Furthermore, by removing the sulphur by means of the controlled oxygen potential, capture of Zn in a matte phase 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 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 based material are relatively similar analogue to the Zn case. At high operation temperatures, and at a controlled oxygen potential of 10 ⁹ to 10 ¹ atm, most content 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 be reached within reasonable operation times. This means that in a preferred embodiment, the step of smelting and heating is performed until a remaining content of Pb in the liquid slag is lower than 0.03% by weight.

Furthermore, as mentioned above, also other volatile elements, such as e.g. As, In, Ge, Ag, Cd, Hg, Sb and/or T1 may be present in the original ferric-iron based material. Also, these elements are easily fumed at the proposed temperature and oxygen potential conditions. Thus, if the ferric-iron based material 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. 1). The fume handling system is preferably arranged for 30 is configured to condense metals or other elements from the removed gases fumed-off from the liquid slag. Such a condensing is, as such, know from prior art, and will not be described in further 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 SO2 is separated from the removed gases fumed-off from the liquid slag. Thereafter, the separated SO2 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 illustrative examples of the present invention. It will be understood by those skilled in the art that various modifications, combinations and changes may be made to the embodiments without departing from the scope of the present invention. In particular, different part solutions in the different embodiments can be combined in other configurations, where technically possible. The scope of the present invention is, however, defined by the appended claims.

Claims

1. A method for treating ferric-iron based material comprising at least zinc and sulphur, wherein said method comprises the steps of:

- smelting and heating (S10) said ferric-iron based material (22) and added flux (19) forming a liquid slag (24);

said flux (19) comprising a refractory material and CaO;

wherein an amount of said CaO of said flux (19) is added to give a basicity of said slag in the range between 0.5 and 0.9; and

- removing (S I 4) gases fumed-off from said liquid slag (24),

characterized in that

wherein an amount of said refractory material of said flux (19) is added to give said slag a liquidus temperature of said slag above 1200°C;

wherein the method comprises the further step of:

- controlling (S I 2) an oxygen potential in gas (27) provided during said step of smelting and heating (S 10) to be in the range of 10^-9 to 10 ¹ atm., whereby said sulphur is converted to gaseous SO2; and

wherein said gases fumed-off from said liquid slag (24) comprising zinc and said gaseous SO2.

2. The method according to claim 1 , characterized by the further step of:

- controlling (SI 6) an operation temperature of said liquid slag (24) to give a freeze lining (16) of said refractory material in combination with said liquid 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 said amount 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 in that said refractory material is selected from: AI2O3;

MgO; and

Cr₂0₃.

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

6. The method according to claim 5, characterized in that said refractory material comprises AI2O3 in an amount of 7- 15% by weight of said liquid slag (24).

7. The method according to claim 5, characterized in that said refractory material 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 refractory material comprises Cr₂0₃ in an amount of 0.05- 1% by weight of said liquid slag (24).

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

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

1 1. The method according to claim 10, characterized in that said submerged heater (20) is at least one of:

a plasma gun (28);

an oxyfuel burner; and a submerged top lance.

12. The method according to any of the claims 1 to 1 1 , characterized in that an operating temperature of said liquid slag (24) is less than 100°C above said 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 in that said step of smelting and heating (S 10) 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 in that 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 of smelting and heating (S 10) is performed until a remaining content of Pb in said liquid slag (24) is lower than 0.03% by weight.

16. The method according to any of the claims 1 to 15, characterized in that 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 SO2 from said removed gases fumed-off from said liquid slag (24); - transforming said separated SO₂ to sulphuric acid.