DE2205206C2 - Process for desulfurization of steel - Google Patents

Description

The invention relates to a method for desulfurizing a steel melt after corresponding reduction of the oxygen content in a vessel outside the melting unit and after deslagging.

It is usually desirable to reduce the sulfur content of steel because high sulfur content produces red brittleness and other adverse effects in the final product. Many known desulfurization techniques have been developed, but almost all techniques in commercial use require significant treatment of the melt in the furnace after melting is complete. For example, it is known that desulfurization occurs most efficiently at high temperatures and low oxygen potential. Consequently, steel is usually held in the arc furnace or other melting unit for a significant period of time following melting for the purpose of superheating and deoxidation because the steel in the molten state may contain such a large amount of oxygen that effective desulfurization cannot take place. The additional furnace residence time enables steps to be taken in the melting unit, such as the addition of active deoxidizers, of which aluminum is a good example, to both reduce the oxygen content and raise the steel temperature.

A major disadvantage of this extended post-melt furnace treatment time is that it reduces the efficiency of the steelmaking process because the furnace or other melting unit is not fully utilized. For example, an arc furnace is a very efficient melting unit, but operates as a refining unit with comparatively poor efficiency, and the whole time it is operating to superheat and/or deoxidize the metal it is operating as a refining device. It is a recognized fact that the superheating erodes the furnace lining much faster than the normal rate, thereby increasing the cost of the process. Ideally, the melting unit should be for melting only, and the refining (including desulphurization), temperature control, deoxidation and alloying should be carried out in a subsequent unit, as this combination of process steps makes the most effective use of the furnace, minimizes costs, and maximizes the efficiency of the overall steelmaking process.

A process of the type mentioned at the beginning is known from DE-OS, in which the sulphur and oxygen purification is to take place in a vacuum ladle that can be heated by means of arcs, whereby the melt is treated in a container with a basic slag line and also with a lining that does not allow oxygen from the lining to penetrate into the melt. The melt is poured into this container and then heated, whereby lime and/or another basic slag former is added to the melt during heating in an amount that forms a highly basic slag and aluminium is added to the slag for reduction purposes. The melt is then well deoxidised by means of vacuum degassing and/or deoxidation under atmospheric pressure using additives, whereby the melt is stirred during the entire treatment, starting with the addition of lime. In this known process, particular importance is attached to the fact that work is carried out in a container with a basic slag line and the lining described above. This known process involves working in a series of sequential process steps.

Furthermore, US-PS 35 01 290 discloses a process for vacuum degassing of steel melts, in which the melt is first filled into a treatment vessel and is degassed there during heating by an alternating current arc which is ignited during at least part of the treatment time.

The invention is based on the object of creating a method for desulfurization of a steel melt after a corresponding reduction of the oxygen content in a container outside the melting unit and after deslagging, which makes it possible to optimize the desulfurization conditions using alternating current arc heating with a low load on the lining

• a) Erzeugung eines absoluten Drucks nicht höher als 300 mm Hg,
• b) Inberührunghalten der Schmelze mit einer Entschwefelungsschlacke auf Kalkbasis, die ein Flußmittel enthält,
• c) Anwenden eines Spülgases, welches durch die Schmelze von einem gewählten Ausgangspunkt im Boden des Gefäßes nach oben sprudeln gelassen wird, derart, daß eine örtlich begrenzte, heftig kochende Bewegung an der Oberfläche der Schmelze erzeugt wird, wobei dieser örtlich begrenzte Schmelze erzeugt wird, wobei dieser örtlich begrenzte Bereich direkt senkrecht oberhalb des gewählten Ausgangspunktes des Spülgases liegt; und
• d) Aussetzen der Schmelze der Wirkung eines Wechelstromlichtbogens, welcher direkt zwischen nicht selbstverzehrenden Elektroden und dem örtlich begrenzten Bereich der heftig kochenden Oberfläche der Schmelze gezogen wird.

This object is achieved according to the invention in that the melt is simultaneously subjected to the following conditions in a final treatment:

• a) generating an absolute pressure not exceeding 300 mm Hg,
• b) keeping the melt in contact with a lime-based desulphurisation slag containing a flux,
• c) applying a purge gas which is bubbled upwards through the melt from a selected starting point in the bottom of the vessel so as to produce a localised, vigorous boiling motion at the surface of the melt, said localised region being located directly vertically above the selected starting point of the purge gas; and
• d) Exposing the melt to the action of an alternating current arc drawn directly between non-consumable electrodes and the localised area of the violently boiling surface of the melt.

The invention provides the particular advantage that the final desulfurization treatment can be carried out in vessels with conventional linings and no complex special linings are required. This is due to the fact that the optimal desulfurization conditions, in which purge gas and alternating current arc are applied simultaneously together with a vacuum, are present in comparatively small, localized areas of the melt surface, so that local overheating and optimal desulfurization conditions occur in these areas, which, however, do not place a strain on the lining. In these areas, the desulfurization slag can be optimally effective without placing a strain on the lining.

A preferred embodiment according to the invention consists in that the desulfurization slag is withdrawn from the melt and that a second desulfurization slag is fed to the melt.

In particular, it is preferred that the slag consists of 90% lime and 10% fluorspar.

In the following, the invention is explained in more detail using an embodiment illustrated by way of example in the drawings. It shows

Fig. 1 is a schematic flow diagram, and

Fig. 2 is a view on an enlarged scale compared to Fig. 1 of an apparatus for carrying out the method according to the invention.

In the description, like reference numerals refer to like parts in both figures of the drawings.

A typical arc furnace is designated 10 in Fig. 1. In process step 1, the furnace, which has a tapping shell 11 and a sill 12 for the charge and deslagging, is illustrated in a deslagging position. That is, the furnace has been tilted counterclockwise and the furnace slag 13 is removed from the melt 15 , for example by a deslagger 14. Preferably, as much slag as possible is removed, since the presence of slag from the melting unit during the later desulfurization steps tends to reduce the efficiency of the subsequent desulfurization.

It should be emphasized that although an electric furnace is illustrated, the present invention is equally applicable to other smelting units, including without limitation converters and Siemens-Martin furnaces.

At this point in the process cycle, the molten steel, which may be carbon steel or alloy steel, may be at a relatively low temperature of about 1577°C. The steel is in an oxidized state and consequently contains large amounts of oxygen. It may contain, for example, between 100 to 500 ppm O₂, the exact amount depending on various conditions, the most important of which is normally the carbon content, since the lower the carbon content, the greater the oxygen uptake capacity of the steel.

Furthermore, at this point in the process cycle the steel may already have undergone a minor, preliminary or initial desulphurisation treatment. For example, it may be appropriate to provide an initial addition of aluminium to deoxidise the steel and an initial treatment to remove a small amount of the sulphur, but such treatment, if used, will not significantly increase the residence time of the steel in the furnace.

The condition of the steel at this point in the process cycle should be contrasted with known processes where the steel may be at a temperature of or above 1687°C just prior to tapping and may have undergone significant initial desulfurization treatments, both of which require significant residence times in the furnace after melting.

In process step No. 2 of Fig. 1, the charge is illustrated being tapped into a vessel 17. In this example, the vessel consists of a bottom pouring ladle having the usual refractory lining and a sliding plug 16. One of the main advantages of the present invention is that the desulfurization can be carried out in the presence of conventional refractory linings in the reaction vessel, such as fire clay, high performance alumina or other commonly used types of refractory linings. It may be desirable in some cases to use a higher quality refractory lining in the region of the slag line of the vessel and high strength alumina is often used for this purpose. With this exception, however, the linings of the vessel may be of the conventional type and the above embodiment, which is well known to those skilled in the art, is intended to be understood by the term "conventional lining" as used in the following description. and used in the claims. The sliding plug can be used to advantage when the vessel is subjected to long-term treatments under vacuum and when it is desired to pour directly from the vessel into the final receiving vessel, such as a chill mold, through the bottom pouring spout. However, it is emphasized that it is equally possible within the scope of the invention to use a normal plug ladle or a rim-casting ladle from which the treated metal is subsequently poured into a casting vessel.

It is noted that desulfurizing materials such as lime and a flux and deoxidizing material such as aluminum and/or silicon may be added to the vessel 17 either prior to tapping or during tapping. In the illustrated embodiment, an initial addition of desulfurizing and deoxidizing materials was made to the ladle prior to tapping and a mixing action was created using a gas purge system 18 in addition to the mixing inherent in the pouring operation. Alternatively, purging may be omitted during tapping, with purging instead being performed at a later time in the process cycle.

In the specific embodiment, a number of porous purge means are illustrated at 19 and 20. In the specific embodiment, two porous purge plug means or plugs, as are known in the art, are illustrated, but it will be apparent that a greater or lesser number may be used as required by the particular system used. The plugs are connected to a source of purge agent 23 via a line 21 and a control valve 22. In the embodiment, a bottle of argon gas has been illustrated schematically. Argon is readily available, inexpensive and is an effective purge agent which is widely used in industry today. Other purge agents such as CO, helium and the other inert gases and under certain carefully controlled conditions, dry air may equally be used.

As soon as the metal level in the vessel 17 reaches a sufficient height, for example only a few centimeters deep, the control valve 22 is opened to admit the flushing agent from the source 23 into the vessel via the lines 21 and the plugs 19 and 20. The flushing agent creates a vigorous mixing effect at the interface between the slag and the steel, which assists in desulphurisation during tapping.

Preferably, the volumetric rate of purge gas flow is varied during tapping and then, if desired, the rate is gradually reduced as the vessel 17 fills, with the maximum purge gas rate occurring at about the start of tapping. It will be appreciated that as the depth in the vessel increases there will be a progressive growth in the size of the individual gas bubbles admitted through the porous plugs 19 and 20 until they reach the surface. Consequently, if the initial maximum purge gas flow rate were maintained, extremely violent boiling would result at the surface when the vessel was completely full. The purge gas flow rate variation is within the skill of one skilled in the art. However, purging at this point in the process cycle is not essential since purging will occur at a later stage.

Following tapping, the vessel 17 is transferred to a finishing unit, which is generally designated 25 under step 3.

The finishing unit 25 has a vacuum chamber which consists of a pit section 26 and a lid section 27 , the pit and lid sections being sealed at 28. A vacuum line is indicated at 29 and a gas output at 30. An additive chamber is designated at 31 , this additive chamber being connected to the vacuum chamber via a chute 32 and an opening in the lid 27 , and the additive chamber 31 is arranged such that its contents are released into the vessel 17 .

A second similar addition chamber is designated 33. It will be apparent that a single addition chamber having multiple compartments may also be used. For purposes of description only, it will be assumed that chamber 31 contains alloying addition materials and chamber 33 contains other charge materials such as aluminum and/or other deoxidizers.

The means for supplying heat to the charge in the vessel 17 are designated 34, 35 , which represent two electrodes of a three-phase alternating current arc heating system operating with three non-consumable electrodes.

At this stage of the process cycle, the AC arc heating system is operated to drive desulfurization and to adjust the temperature of the charge. Additions are made by operating the addition chamber 33. The purge gas creates vigorous mixing at the slag-metal interface which drives the desulfurization process. Furthermore, the circulation in the charge created by the purge gas stirs the alloy additions and equalizes the temperature of the charge. In the absence of the vigorous mixing and agitation action created by the purge gas, the surface of the charge could overheat significantly to the point where a breakthrough of the lining could occur and an adverse temperature gradient could develop from the surface to the bottom.

If vacuum is used at this stage of the process, this vacuum is preferably of the order of 2 mm Hg or lower, particularly if the hydrogen content is to be removed, although higher values up to 50 mm Hg absolute or even 200 to 300 mm Hg absolute may be used. It is obvious that at the comparatively low vacuum values mentioned above, small quantities of purge gas expand to several hundred times their volume, resulting in a violent mixing action. In view of the high specific gravity of molten steel, it is preferable to arrange the porous plugs 19 and 20 at the lowest point of the vessel 17 or in the vicinity of the lowest point, in order to ensure good internal circulation of the melt.

The final finishing is illustrated in step 4. At this time, the deoxidation materials from the addition chamber 31 to the melt to ensure the maintenance of a low oxygen potential.

The duration of the arc burn depends on many factors including the duration and amount of purge gas flow, the vacuum system, the type of steel, the amount and type of slag present, if any, and the shape of the vessel, among many other factors. For a 55 to 60 ton charge of low alloy steel, a treatment time of approximately 20 to 30 minutes may be quite sufficient.

A significant advantage of desulfurization under vacuum is that the localized heating spot or window effect of the electric arc heating system is utilized to the maximum, as will be explained in more detail below.

Referring separately to process step 4 and to Fig. 2, it can be seen that the porous plugs 19 and 20 are each located substantially directly below one of the electrodes 34 and 35 , respectively. The cooking action at the surface of the molten metal is most severe in areas near and below the electrodes where the gas bubbles break through the surface. By appropriately adjusting the flow rate of the purge gas, the AC arc of each electrode can be limited to the areas 36, 37 of most severe cooking action directly below the electrode. Consequently, the heat input to the charge from the electrodes is concentrated at these localized "hot spots" or "windows". The upwardly moving gas and bubbles create internal circulation in the charge whereby fresh metal is continuously conveyed by internal currents to the window area where its temperature is raised to a level higher than the temperature of other, remote areas of the charge. This maximizes the desulfurization conditions. The window areas are thus continuously replenished with non-desulfurized or only partially desulfurized steel, as opposed to steel already treated in a "window" area and moved away from it by circulation. In this way, all portions of the charge which have already been deoxidized are raised in temperature in the presence of the slag, thus maximizing the desulfurization conditions without overheating the entire charge.

If desired, additions of aluminium and/or other active deoxidisers may be made from time to time during the cycle to maintain the oxidation level of the charge within acceptable limits.

Following the end of the desulphurisation treatment, the charge in the vessel 17 can either be poured directly into the final receiving vessel, for example a mould, or can be poured into a casting vessel.

If excess aluminium remains in the charge following desulfurization, the melt can be brought to the specified levels by merely purging under atmospheric conditions for a period of time sufficient to reduce the aluminium content to an acceptable level. It is obvious that the burnout of aluminium is a continuous process and consequently the aluminium content can be controlled within tight limits by the duration of the purging treatment.

It should be noted that the circulation effect schematically illustrated in Fig. 2 is merely sketchy and illustrative and that the actual circulation will undoubtedly differ considerably from that illustrated. The important point here, however, is that the combination of using a flushing agent in the presence of an AC arc and under vacuum ultimately brings the entire charge to the surface at least once where it can come into contact with the desulfurizing slag under optimum conditions.

The desulfurizing slag 38 consists essentially of a major proportion of an active desulfurizing agent and preferably a minor proportion of a flux and/or conditioning agent. Preferably, the desulfurizing slag contains between 50 and 100% lime, which may be added in calcined and packaged form and has a low moisture content, and 0 to 50% fluorspar or other slag conditioning agents such as Al₂O₃. The amount of fluorspar or other conditioning agents will depend on the amount of lime in the desulfurized slag, the efficiency of the agitation system, the amount and composition of slag carried over from the furnace, if any, and the alumina and silica levels taken up from the refractory linings. A particularly effective slag may consist of approximately 85% CaO, 10% Na₂CO₃ and 5% CaF₂. added in an amount of about 1% by weight of the melt together with a small amount of aluminium for the purpose of slag deoxidation if conditions require. Another effective composition may consist of 90% CaO and 10% CaF₂. Fluorspar is the preferred additional material as it appears to have a catalytic effect in addition to its action as a slag flux.

Preferably, the total amount of alumina and silica, if used, should be less than 40% of the slag weight because of the effect on the basicity of the desulphurising slag.

The weight of the slag 38 can be varied within certain limits, but should preferably be from about 0.5% to about 2% of the weight of the melt, depending on the amount of slag, if any, carried over from the melting vessel, the efficiency of the system in utilizing the total slag, and the degree of sulfur removal desired. Typically, an amount of slag on the order of about 0.5% to about 1.5% of the weight of the melt is most effective.

Here are some practical examples that were carried out following the principles mentioned above:

example 1

A charge of 4140 steel resulting in an ingot weight of 58 061 kg was melted in an electric furnace in air and yielded 0.020% S in the furnace and was subsequently treated in the following manner:

• 1. Abschlacken bis auf die blanke Metalloberfläche, Zugeben von 45,5 kg Aluminium im Ofen
• 2. Zugabe in der Gießpfanne in fester Form
  • a) Kalk 363 kg
  • b) Kalzium-Silizium 150 kg
  • c) Ferro-Vanadium 27 kg (Vanadium)
  • d) Ferro-Molybdän 39 kg
• 3. Abstich der Charge in Gießpfanne
• 4. Abpumpen in Vakuumtank zum Entgasen (zwei Stopfen)
• 5. Zuschläge bei 10 Minuten unter Vakuum- 90% Silizium 100 kg (teilweise zugegeben).
• 6. Vakuum bei 12 Minuten unterbrochen und 726 kg Kalk und 45,5 kg Aluminiumschrot wurden zugegeben.
• 7. Erneutes Evakuieren und Umrühren unter Spülgas für 5 Minuten.
• 8. Unterbrechung des Vakuums zur Probennahme.
• 9. Lichtbogenwiedererhitzung in zwei Stufen unter Vakuum für 13 Minuten und 5 Minuten.
• 10. Unterbrechung des Vakuums und normaler Abstich.

Procedure: Batch No. 107 551

• 1. Deslagging down to the bare metal surface, adding 45.5 kg of aluminium in the furnace
• 2. Addition in the ladle in solid form
  • a) Lime 363 kg
  • b) Calcium-silicon 150 kg
  • c) Ferro-vanadium 27 kg (vanadium)
  • d) Ferro-molybdenum 39 kg
• 3. Tapping of the charge into the ladle
• 4. Pump into vacuum tank for degassing (two plugs)
• 5. Additions at 10 minutes under vacuum - 90% silicon 100 kg (partially added).
• 6. Vacuum was broken at 12 minutes and 726 kg of lime and 45.5 kg of aluminum shot were added.
• 7. Evacuate again and stir under purge gas for 5 minutes.
• 8. Breaking the vacuum for sampling.
• 9. Arc reheating in two stages under vacuum for 13 minutes and 5 minutes.
• 10. Breaking the vacuum and normal tapping.

During process step 9, conditions a to d required according to claim 1 are simultaneously met.

The following results were achieved: Results °=c:100&udf54;H&udf53;ta10,6:25,6:28,6&udf54;&udf53;tz5,5&udf54;&udf53;tw,4&udf54;&udf53;sg8&udf54;\Bomb tests\ Oxygen&udf53;tz5,10&udf54;&udf53;tw,4&udf54;&udf53;sg9&udf54;\Before slagging\ 49&udf53;tz&udf54; \Before vacuum cycles\ 56,¤65&udf53;tz&udf54; \After 1st vacuum cycle\ 21&udf53;tz&udf54; \After 2nd vacuum cycle\ 22&udf53;tz&udf54; \After reheating with arc\ 20&udf53;tz10&udf54;&udf53;te&udf54;&udf53;sb37,6&udf54;&udf53;el1,6&udf54; slags
°=c:70&udf54;H&udf53;ta5,6:21:24,6:28:31,6:33,6&udf54;&udf53;tz5,5&udf54;H@&udf53;tw,4&udf54;&udf53;sg8&udf54;\Experiment\ AlÊOË\ SiOÊ\ Mn\ FeO&udf53;tz5,10&udf54;&udf53;tw,4&udf54;&udf53;sg9&udf54;\Oven\ Æ5.2\ 22.5\ 1.1\ 1.4&udf53;tz&udf54; \After reheating with arc\ 21.4\ 12.6\ 0.32\ 0.14&udf53;tz10&udf54;&udf53;te&udf54;&udf53;sb37.6&udf54;&udf53;el1.6&udf54;

For this melt, the purge gas was not introduced before vacuum treatment began. Pumping the vacuum down to the degassing range of about 2 mm Hg absolute or lower required about 6 minutes. The lowest absolute pressure recorded was 1100 microns.

Example 2

A charge of 1045 steel, resulting in an ingot weight of 54 568 kg, was melted in air in an electric furnace and the melt was then treated in the following manner:

• 1. Abschlacken bis zu blanker Metalloberfläche im Ofen, Zugeben von 45,5 kg Aluminium im Ofen.
• 2. Zuschläge in die Gießpfanne in fester Form
  • a) Kalk 726 kg
  • b) Flußspat 72,6 kg
  • c) 50% Ferro-Silizium 227 kg
  • d) Kalziumsilizium 104,4 kg
  • e) Ferro-Vanadium 45,5 kg (Vanadium)
  • f) Aufkohlung 20,4 kg
• 3. Abstich der Charge in die Gießpfanne gemäß 2.
• 4. Vakuumentgasung mit Argonspülgas von 15 Minuten Dauer
• 5. Unterbrechen des Vakuums, Temperaturmessung und Zugabe von 27 kg Aluminium
• 6. Wiedererhitzung unter Vakuum mittels Lichtbogen und Entschwefelung in zwei Stufen 12 Minuten und 8 Minuten Dauer (200-300 mm), wobei die gemäß Anspruch 1 geforderten Bedingungen a bis d gleichzeitig vorliegen.

Treatment: Melt No. 207 575

• 1. Deslagging to a bare metal surface in the furnace, adding 45.5 kg of aluminium to the furnace.
• 2. Additions to the ladle in solid form
  • a) Lime 726 kg
  • b) Fluorspar 72.6 kg
  • c) 50% ferro-silicon 227 kg
  • d) Calcium silicon 104.4 kg
  • e) Ferro-vanadium 45.5 kg (vanadium)
  • f) Carburization 20.4 kg
• 3. Tapping of the charge into the ladle according to 2.
• 4. Vacuum degassing with argon purge gas lasting 15 minutes
• 5. Break the vacuum, measure the temperature and add 27 kg of aluminium
• 6. Reheating under vacuum by means of an electric arc and desulfurization in two stages of 12 minutes and 8 minutes duration (200-300 mm), wherein conditions a to d required according to claim 1 are present simultaneously.

The following results were obtained: Results Analysis:

For this melt, the purge gas was not introduced before vacuum treatment began. Pumping down to the degassing range of approximately 2 mm absolute mercury or lower required approximately 6 minutes. The lowest absolute pressure recorded was 450 microns.

In summary, the amount of slag, the duration of treatment, the amount of flushing and the slag composition primarily control the level of sulphur content achieved under conditions where oxygen is maintained at a suitably low level by using aluminium or other deoxidising agents or other agents having the same effect.

The efficiency of the process can be further improved by multiple slag treatment, in which at least one new slag is added to the bare metal which has been exposed to a previous slag.

As an alternative, a steel melt may be tapped onto solid or molten ladle additives of the type described above, such as 363 kg CaO and 36.3 kg CaF2. This first slag is removed from the ladle after tapping, for example by means of a deslagging machine, and a second similar slag is added. The melt with the slag is then subjected to an appropriate vacuum treatment, for example a treatment as described in Examples 1 and 2 above.

As a further alternative, a steel melt may be tapped onto molten or solid ladle charges of the type described above, consisting for example of 363 kg CaO and 36.3 kg CaF2. After tapping, the melt and the above first post-furnace slag may be subjected to vacuum and contact layer mixing by purge gas of the type described above for a suitable period of time, for example 8 minutes. The vacuum is then broken and the first post-furnace slag is removed from the melt, for example by means of a deslagging machine, and a second post-furnace slag is added, which may be similar to the first slag. This is followed by a second period of time vacuum treatment and mixing of the contact layer by purge gas, which may include reheating with the arc if required.

It should be stressed that other ladle fill materials may of course be used and that other materials may, under certain circumstances, partially or completely supplement or replace the fluorspar, including alumina, silica and soda. However, to obtain very low sulphur levels, fluorspar is preferred.

From the above description it can be seen that the furnace is operated as a melting unit only, since all the refining operations take place outside the furnace. Since, as already mentioned, the electric furnace is a very efficient melting device but a relatively low efficient refining vessel, the overall efficiency of the manufacturing process is thereby maximized and the throughput of the plant is increased, since the furnace, which is usually the core of a steel plant's equipment, is utilized in an optimal manner.

Claims (3)

1. A process for desulphurising a steel melt after a corresponding reduction of the oxygen content in a vessel outside the melting unit and after slag removal, characterised in that the melt is first poured into a vessel with conventional lining and that the melt is simultaneously subjected to the following conditions in a final treatment: a) generating an absolute pressure not exceeding 300 mm Hg; (b) keeping the melt in contact with a lime-based desulphurisation slag containing a flux; c) applying a purge gas which is bubbled upwards through the melt from a selected starting point in the bottom of the vessel so as to produce a localised, vigorous boiling motion at the surface of the melt, said localised region being directly vertically above the selected starting point of the purge gas; and d) Exposing the melt to the action of an alternating current arc drawn directly between non-consumable electrodes and the localised area of the violently boiling surface of the melt. 2. Process according to claim 1, characterized in that the desulfurization slag is removed from the melt and that a second desulfurization slag is fed to the melt. 3. Process according to claim 1 or 2, characterized in that the slag consists of 90% lime and 10% fluorspar.