WO2024170677A1

WO2024170677A1 - Slag blocking device for a metallurgical vessel

Info

Publication number: WO2024170677A1
Application number: PCT/EP2024/053848
Authority: WO
Inventors: Rick KOMANECKY; Jeremy STICKAN; Pavan Shivaram
Original assignee: Refractory Intellectual Property Gmbh & Co. Kg
Priority date: 2023-02-15
Filing date: 2024-02-15
Publication date: 2024-08-22

Abstract

To prevent slag (S) from draining through an outlet (C) at the bottom (A) of a metallurgical vessel (2) at the beginning of a casting process, a slag blocking device (1) having an inner space (I) bordered by a circumferential wall (W) comprising a lower edge (E) is provided. The slag blocking device (1) has a density of 2.6 to 6 g/cm³, preferably 2.75 to 3.2 g/cm³.

Description

SLAG BLOCKING DEVICE FOR A METALLURGICAL VESSEL

The current disclosure relates to a slag blocking device for a metallurgical vessel and manufacturing and use of a slag blocking device.

During operation a metallurgical vessel contains molten metal, e.g., molten steel, and a layer of slag (having a lower density) floating on top of said molten metal to protect said molten metal from oxidation and contamination from the atmosphere. The metallurgical vessel can be provided with an outlet at its bottom through which the molten metal can be drained. The flow rate of the molten metal through the outlet is typically controlled by a valve-like mechanism, e.g., a slide gate. The molten metal can be drained through the outlet of the metallurgical vessel and poured into another metallurgical vessel for further processing, or, in case the metallurgical vessel is a tundish, the molten metal is drained through the outlet and a nozzle provided at said outlet into a casting assembly to solidify the molten metal. In case the molten metal is poured from a ladle into the tundish this is usually done via a ladle shroud. This ladle shroud is only immersed into molten metal inside the tundish when the tundish is already filled with molten metal.

In general, it is not desired to drain slag through the outlet of the metallurgical vessel during pouring or casting processes. To prevent slag from draining through the outlet, dams protruding from the bottom of the metallurgical vessel might be used but these might not always be effective, as slag might still partially drain through the outlet, leading in the worst case to disruption or termination of the casting process.

When the metallurgical vessel having an outlet at the bottom is filled with molten metal (i.e. , when a batch of molten metal is poured into the metallurgical vessel), in particular during the first heat not only slag and/or molten metal can enter the outlet, but also slag and/or molten metal that has entered the outlet might then solidify and block the outlet. For preventing this, a starter tube can be provided at the outlet, said starter tube protruding into the metallurgical vessel vertically and therefore preventing slag but also molten metal from entering the outlet and thus solidifying in the outlet. Therefore, a larger amount of molten metal and slag remains inside the metallurgical vessel before it can enter the starter tube. Only when the level of molten metal and slag, i.e. , the total filling level, in the metallurgical vessel reaches the height of the opening of said starter tube, it can enter the starter tube and thus the outlet. At this point, the outlet is preheated (actively by a heating apparatus and/or passively by the larger amount of molten metal already present inside the metallurgical vessel), and thus solidification of the material entering the starter tube and therefore the outlet is effectively prevented. On the other side, slag (having a lower density, thus floating on top of the molten metal and therefore being the upper layer) can still enter the starter tube and thus the outlet as soon as the total filling level is high enough. So, a starter tube only solves the problem of molten metal (and slag) entering the outlet early on and prevents solidification through above mentioned heating, but not the problem of slag entering the outlet in general. In fact, a starter tube might even increase the total amount of slag entering the outlet.

It is therefore an object of the invention to prevent slag from entering and draining through an outlet at the bottom of a metallurgical vessel at the beginning of a casting process.

This object has been achieved by the slag blocking device having an inner space bordered by a circumferential wall, the circumferential wall having a lower edge, wherein the slag blocking device has a density of 2.6 to 6 g/cm³. A density of 2.75 to 3.2 g/cm³ is preferred, as in this range the density of the slag blocking device is similar to the density of the slag and thus the movement of the slag blocking device lifting up from the bottom of the metallurgical vessel while the total filling level increases, is improved and a better-balanced position is achieved. Molten steel has a density of about 7 g/cm³ while slag has a density of 2.5 to 3.0 g/cm³. The density of the slag blocking device is chosen according to the anticipated slag density. Slag can, e.g., be present due to impurities in the metallurgical vessel, due to reoxidation during pouring of the molten metal and due to usage of fluxes and/or filler sand. Filler sand can be used for initially preventing molten metal from entering a valve-like mechanism, e.g., a slide gate in combination with a nozzle, to prevent solidification of molten metal inside a valve-like mechanism and/or the nozzle before said valve-like mechanism is opened. As the density of the slag blocking device is higher than the density of slag and lower than the density of molten steel, the slag blocking device being positioned around an outlet or a starter tube at the bottom of the metallurgical vessel, stays at its position when the casting process starts, i.e. , when molten metal is first poured into the metallurgical vessel. Thereby, slag is prevented from entering the outlet at the bottom of the metallurgical vessel while the lower edge of the circumferential wall of the slag blocking device is in contact with the bottom of the metallurgical vessel.

1 g/cm³ equals 1000 kg/m³, thus all densities in this disclosure can be converted into kg/m³. For example, the density of the slag blocking device is 2600 kg/m³ to 6000 kg/m³, preferably 2750 to 3200 kg/m³.

If no starter tube is used, the density of the slag blocking device can be chosen so that the lower edge of its circumferential wall in contact with the bottom of the metallurgical vessel stays positioned around the outlet while the total filling level of the metallurgical vessel rises until the molten metal layer is thick enough to prevent slag from entering the outlet, even when the slag blocking device lifts up from the outlet. So, the slag blocking device loses contact to the bottom of the metallurgical vessel only when the molten metal layer is thick enough for preventing slag floating on top of the molten metal from entering the outlet which depends on the molten metal flow and nozzle diameter. It can also be preferential if the slag blocking device is kept over the outlet of the metallurgical vessel for a longer time after it loses contact with the bottom of the metallurgical vessel. This can be achieved e.g., by mechanical means.

The slag blocking device can be used in combination with a starter tube protruding vertically from the bottom of a metallurgical vessel and thus expanding the outlet vertically into the metallurgical vessel. The slag blocking device can be placed around said starter tube before the beginning of the casting process. Without using a slag blocking device the molten metal and slag enters the starter tube as soon as the total filling level in the metallurgical vessel is high enough (i.e., higher than the level of the opening of the starter tube). In contrast, if a slag blocking device is used at the beginning of the casting process, the slag blocking device is in contact with the bottom of the metallurgical vessel and its circumferential wall prevents slag (and also molten metal) from entering its inner space. While the total filling level of the metallurgical vessel rises, the slag blocking device floats at the interface between the molten metal and the slag and continues blocking the slag from entering its inner space because the slag blocking device is kept around the starter tube as its wall partially surrounds said starter tube during rising levels inside the metallurgical vessel. Thus, the inner space of the slag blocking device, i.e. , the space between the outside surface of the starter tube and the inner surface of the slag blocking device is free from slag and only filled with molten metal. As soon as the total filling level reaches the opening of the starter tube, molten metal can enter the starter tube while the slag is still blocked from entering the starter tube by the circumferential wall of the slag blocking device. At this point, the slag blocking device floats further up, but is still kept around the starter tube as its wall partially still surrounds said starter tube during rising levels inside the metallurgical vessel. When the total filling level is sufficiently high and therefore the slag layer is distanced enough from the outlet, the slag blocking device might not be surrounding said starter tube anymore and therefore floats away.

The circumferential wall of the slag blocking device has an inner surface, facing and defining the inner space and an outer surface facing the outside. The slag blocking device has a bottom area which in its intended use faces the bottom of the metallurgical vessel. As the circumferential wall has a lower edge the lower edge is located in the bottom area.

The slag blocking device has a bottom opening bordered by the lower edge connecting the inner surface and the outer surface of the circumferential wall. The slag blocking device thus has an inner space bordered by the circumferential wall having a lower edge. The circumferential wall by its lower edge forms an opening at the bottom of the slag blocking device and therefore connects the inner and outer surface of the circumferential wall. In its intended use the slag blocking device is placed such that its lower edge surrounds the outlet of a metallurgical vessel. The slag blocking device also has an upper area opposing the bottom area. The slag blocking device can have an upper opening in the upper area bordered by an upper edge connecting the inner surface and the outer surface of the circumferential wall. Also, the upper area can be closed such that no upper opening is provided. An open upper area is preferred to allow pre-heating of the outlet by a heating unit placed above the outlet and thus said heat not being blocked by a closed upper area. If the bottom and the upper areas are open, i.e. , a bottom and an upper opening are provided, the slag blocking device preferably only comprises the circumferential wall.

Preferably the circumferential wall has a decreasing thickness towards the lower edge. On the one hand, this geometry results in that the lower edge penetrates deeper the molten metal layer when said slag blocking device is floating and thus helps to prevent slag from moving under the lower edge of the circumferential wall into the inner space of the slag blocking device and therefore into the outlet when turbulences occur, even if the slag layer is very thick. This is primarily beneficial when a starter tube is used and the slag blocking device is designed to float above the starter tube while keeping slag out of it. On the other hand, the slag blocking device having a circumferential wall with decreasing thickness towards the lower edge stays around the outlet for a longer time duration while the lower edge of the circumferential wall is in contact with the bottom. This can also be beneficial if an outlet without a starter tube is used.

Preferably the circumferential wall has a constant thickness, preferably between 1 to 3", i.e., 25.4 to 76.2 mm, most preferably 1 .5”, i.e., 38.1 mm, in an upper section and a decreasing thickness in a lower section (towards the lower edge). The height of the lower section preferably is at least 20%, preferably 50% of the total height of the circumferential wall. In this case the density of the slag blocking device can be chosen such that when the slag blocking device is floating the upper section blocks the slag and the lower section penetrates the molten metal.

The lower edge can comprise a lower surface, connecting the inner surface of the circumferential wall and the outer surface of the circumferential wall. The edge in this case can be ring-shaped. The lower surface can also be seen as part of the inner and/or outer surface, in particular if the interface between the surfaces is continuous.

Preferably the lower edge is pointed. This means the lower edge is has the shape of a sharp edge, similar to a knife edge. Preferably the lower edge comprises a line-like, very thin lower surface, basically equal to the tip of the pointed edge. By having a pointed lower edge, the penetration of the molten metal layer is further increased (for a floating slag blocking device around a starter tube) and the slag blocking device is even longer in contact with the bottom of the metallurgical vessel (which is also beneficial if no starter tube is used). A slag blocking device having a pointed lower edge can have a lower density than a slag blocking device having no pointed lower edge and still perform the same way regarding slag blocking properties.

It is preferable when the lower edge has a radius not above 2 mm, most preferable not above 1 mm. This allows for even further increased penetration of the molten metal layer and thus even longer contact with the bottom of the metallurgical vessel.

The decreasing thickness of the circumferential wall in the lower section can, e.g., be incorporated by a linear slope from the outside and/or inside, preferably with an angle of 20 to 70 degrees with reference to the vertical, perpendicular direction.

The circumferential wall can also have a constant diameter along the longitudinal axis, i.e. , from top to bottom.

Preferably the slag blocking device is ring-shaped, which is easy to produce and works best for blocking slag from entering circular outlets or starter tubes. If the slag blocking device is ring-shaped, its cross-section (when viewed from the top) can be oval but preferably is circular, i.e., the slag blocking device has a constant radial diameter. Preferably the ring-shaped slag blocking device has a diameter in the range of 105 to 325 mm. The diameter preferably is 5 to 30% larger than the diameter of the outlet or the starter tube, wherein the outlet or starter tube preferably has a diameter in the range from 100 to 250 mm. For an outlet or starter tube having 100 mm a ring-shaped slag blocking device having a diameter from 105 to 130 mm might be used. For an outlet or starter tube having 250 mm a ring- shaped slag blocking device having a diameter from 262.5 to 325 mm might be used.

If the slag blocking device is ring-shaped preferably the upper area is open such that an upper opening is provided. The slag blocking device can also have a ring- shaped circumferential wall and a closed upper area such that no upper opening is provided.

The slag blocking device preferably comprises a main material having a density from 2.7 g/cm³ to 2.9 g/cm³ (i.e. , 2700 kg/m³ to 2900 kg/m³) and a secondary material having a higher density compared to the main material, preferably from 3.0 g/cm³ to 12 g/cm³ (i.e., 3000 kg/m³ to 12000 kg/m³), most preferably from 7.6 g/cm³ to 7.8 g/cm³ (i.e., 7600 kg/m³ to 7800 kg/m³). Adding a secondary material is an easy way to increase the density of the slag blocking device to fit the desired range. It is preferred for the slag blocking device to have a density from 2.75 g/cm³ to 3.2 g/cm³ (i.e., 2750 kg/m³ to 3200 kg/m³) if it comprises a main material having a density from 2.7 g/cm³ to 2.9 g/cm³ (i.e., 2700 kg/m³ to 2900 kg/m³) and a secondary material having a higher density compared to the main material.

Preferably the secondary material is mostly, preferably fully, immersed in the primary material. In this way a slag blocking device having a homogenous distribution of the density and thus an improved floating behaviour is obtained.

The main material may comprise one or more of: magnesia, alumina, silica, olivine, zirconia. For a slag density of 2.6 g/cm³, a main material having a density of 2.7 g/cm³ might be used for the slag blocking device. Preferably the main material comprises alumina castable, preferably comprising from 68 to 90 weight- % of alumina.

The secondary material may comprise a higher density material like one or more of: titanium, iron, or steel. Preferably the secondary material has the shape of bolts and/or shot and/or fibers. Fibers not only help adjusting the density of the slag blocking device but also reinforce the durability and strength of the slag blocking device.

Preferably the slag blocking device comprises 1 to 3 weight-% of the secondary material.

The slag blocking device can be placed around an outlet, preferably a starter tube, of a metallurgical vessel at the beginning of a casting process, during which molten metal is poured into said metallurgical vessel to block slag from entering the outlet.

The secondary material can comprise bolts, preferably being immersed vertically in the lower part of the circumferential wall of the slag blocking device.

The slag blocking device can be produced by providing a mold, pouring material, e.g., main material and secondary material into the mold, demolding the material and drying the material, preferably in an oven at the temperature of 350°C for 22 hours.

The slag blocking device can also be produced by providing a mold, pouring material, e.g., main material and secondary material into the mold, pressing the material and then demolding and drying the material, preferably in an oven at the temperature of 350°C for 22 hours.

Figs. 1 a to 6c show exemplary, schematic, and non-limiting advantageous embodiments of the invention wherein

Figs. 1 a shows a front view and top view of a first preferred embodiment of the slag blocking device,

Figs. 1 b shows a front view and top view of a second preferred embodiment of the slag blocking device, Fig. 2a shows a cross-sectional view of a segment of an empty metallurgical vessel having an outlet with a starter tube and having the slag blocking device from Fig. 1a surrounding the starter tube,

Fig. 2b shows a cross-sectional view of a segment of an empty metallurgical vessel having an outlet with a starter tube and having the slag blocking device from Fig. 1 b surrounding a starter tube,

Fig. 3a shows a cross-sectional view of the segment of the metallurgical vessel from Fig. 2a having a first total filling level,

Fig. 3b a cross-sectional view of shows the segment of the metallurgical vessel from Fig. 2b having a first total filling level,

Fig. 4a shows a cross-sectional view of the segment of the metallurgical vessel from Fig. 3a having a second total filling level, wherein the slag blocking device floats at the interface between the molten metal and the slag,

Figs. 4b shows a cross-sectional view of the segment of the metallurgical vessel from Fig. 3b having a second total filling level, wherein the slag blocking device floats at the interface between the molten metal and the slag,

Fig. 5a shows a cross-sectional view of the segment of the metallurgical vessel from Fig. 4a having a third liquid level, wherein the molten metal is drained through the starter tube and the outlet,

Figs. 5b shows a cross-sectional view of the segment of the metallurgical vessel from Fig. 4b having a third liquid total filling level, wherein the molten metal is drained through the starter tube and the outlet while the slag is still blocked by the circumferential wall of the slag blocking device,

Figs. 6a, b, c show cross-sectional views of a segment of an empty metallurgical vessel and a metallurgical vessel having various total filling levels, the metallurgical vessel having an outlet without a starter tube and a slag blocking device.

To prevent slag S from entering an outlet C located at the bottom A of a metallurgical vessel 2 a slag blocking device 1 is provided. The slag blocking device 1 has an inner space I bordered by a circumferential wall W, the circumferential wall W having a lower edge E. The slag blocking device 1 has a density of 2.6 to 6 g/cm³, preferably 2.75 to 3.2 g/cm³

The slag blocking device 1 may comprise a main material 11 of a density from 2.7 g/cm³ to 2.9 g/cm³, and a secondary material 12 of higher density compared to the main material 11 , preferably from 3.0 g/cm³ to 12 g/cm³, most preferably from 7.6 g/cm³ to 7.8 g/cm³. The secondary material 12 can be used to adjust the density of the slag blocking device 1 , wherein the slag blocking device 1 preferably comprises 1 to 3 weight-% of the secondary material 12. Here advantageously the secondary material 12 is fully immersed in the primary material 11 .

The main material 11 might comprises one or more of: magnesia, alumina, silica, olivine, zirconia, and the secondary material 12 may comprise one or more of: steel, titanium, iron. The secondary material 12 has the shape of bolts and/or shot and/or fibers.

The circumferential wall W of the slag blocking device 1 has an inner surface, facing and defining the inner space I and an outer surface facing the outside. The slag blocking device 1 has a bottom area which in its intended use faces the bottom of the metallurgical vessel. As the circumferential wall W has a lower edge E the lower edge is located in the bottom area.

Fig. 1 a shows a front view and top view of a first embodiment of the slag blocking device 1 having a circumferential wall W with a constant thickness d and a lower edge E. Here the lower edge E comprises a ring-shaped lower surface, connecting the inner surface of the circumferential wall W and the outer surface of the circumferential wall. The lower edge E as lower surface can also be seen as part of the inner and/or outer surface, in particular if the interface between the surfaces is continuous.

Fig. 1 b shows a front view and top view of a second embodiment of the slag blocking device 1 having a circumferential wall Wwith a decreasing thickness d1 towards the lower edge E. This decreasing thickness d1 only by way of example is embodied pointed in a knife-edge manner and by a linear slope from the outside but may also have a non-linear slope from the outside. In addition to, or instead of the (linear or non-linear) outside slope a (linear or non-linear) inside slope might be provided. These slopes can be considered to be part of the inner or outer surface. Compared to the embodiment shown in of Fig. 1a the lower edge E comprises a very thin lower surface, basically equal to the tip of the pointed knife- edge. Preferably the lower edge E has a radius not above 2 mm, preferably not above 1 mm.

Only by way of example the slag blocking devices 1 shown in the Figures are ring- shaped and have a circular circumferential wall Wwith constant inside and outside diameter in vertical direction (except from the declining thickness d1 in the lower edge E of course). A slag blocking device 1 might have any shape and in particular can also have a rectangular or oval base. Also, the inner and/or outer diameter can vary in vertical direction.

Fig. 2a shows a vertical cross-sectional view of a segment of an empty metallurgical vessel 2, e.g., a tundish, with an outlet C at the bottom A of the metallurgical vessel 2. A starter tube 3 is positioned over the outlet C located at the bottom A of the metallurgical vessel 2 and a slag blocking device 1 is placed in the metallurgical vessel 2, surrounding the starter tube 3 with its circumferential wall W. The starter tube 3 can be fixed at the bottom A by lining refractory or rammable refractory. The slag blocking device 1 is in contact with the bottom A of the metallurgical vessel 2 with its lower edge E.

Fig. 2b shows a vertical cross-sectional view of the same empty metallurgical vessel 2 as Fig. 2a, wherein the slag blocking device 1 from Fig. 1 b having a circumferential wall Wwith decreasing thickness d1 is used. Liquid metal M is poured into the metallurgical vessel 2. Slag S can, e.g., be present due to impurities in the metallurgical vessel 2, to reoxidation during pouring of the molten metal M and due to usage of fluxes or filler sand. Filler sand can be used for initially preventing molten steel from entering a valve-like mechanism, e.g., a slide gate in combination with a nozzle, to prevent solidification of molten metal inside the valve-like mechanism and/or the nozzle before said valve-like mechanism is opened. Fig. 3a shows a cross-sectional view of the metallurgical vessel 2 from Fig. 2a but already having a first total filling level. The slag blocking device 1 is still placed in the metallurgical vessel 2 surrounding the starter tube 3. As the slag blocking device 1 has a density of 2.6 to 6 g/cm³, preferably 2.75 to 3.2 g/cm³, and molten steel M has a density of about 7 g/cm³ while slag S has a density of 2.5 to 3.0 g/cm³ and the total filling in the metallurgical vessel 2 has not reached the level for the slag blocking device 1 to float, the slag blocking device 1 is still in contact with the bottom A of the metallurgical vessel 2. The layer of molten metal M and slag S is blocked from entering the outlet C by the presence of the slag blocking device 1 .

Fig. 3b shows the same cross-sectional view as Fig. 3a but using the slag blocking device 1 from Fig. 2b, having a circumferential wall W with decreasing thickness d1.

Fig. 4a shows a cross-sectional view of the metallurgical vessel 2 from Fig. 2a, b and 3a, b but having a second total filling level that is higher than the first total filling level. The total filling level in the metallurgical vessel 2 is such that the slag blocking device 1 is not anymore in contact with the bottom A of the metallurgical vessel 2 but it is still surrounding the starter tube 3. As the slag blocking device 1 has a density between the molten metal M and the slag S, the slag blocking device 1 floats in the interface between the molten metal M and the slag S. Thus, molten metal M still fills the space between the outside surface of the starter tube 3 and the inner surface of the slag blocking device 1 while the slag S is still blocked by the circumferential wall W of the slag blocking device 1 . Fig. 4b shows the same cross-sectional view as Fig. 4a, but the slag blocking 1 device has a circumferential wall Wwith decreasing thickness d1 , according to Fig. 2b.

A slag blocking device 1 having a circumferential wall Wwith a decreasing thickness d1 towards the lower edge E penetrates deeper the molten metal layer M when said slag blocking device 1 is floating and thus helps to prevent slag S from moving under the lower edge E of the circumferential wall W into the inner space I and therefore into the outlet C when turbulences occur.

Fig. 5a, b show the same cross-sectional views as Fig. 4a, b but in this case the total filling level is such that the slag blocking device 1 floats above and away from the starter tube 3 and the molten metal M is allowed to enter the starter tube 3 and thus the outlet C at the bottom A of the metallurgical vessel 2.

If no starter tube 3 is used, a slag blocking device 1 can still be placed over an outlet C at a bottom of a metallurgical vessel 2. The lower edge E of the circumferential wall W is in contact with the bottom A of the metallurgical vessel 2 and due to its density, the slag blocking device 1 stays around the outlet C while the total filling level rises and only lifts up from the outlet C when the molten metal layer M is thick enough to prevent slag S from entering the outlet C. This works with a slag blocking device 1 having the abovementioned density, wherein a lower edge E with declining thickness d1 is preferred as it stays around the outlet C for a longer duration while the lower edge E of the circumferential wall W is in contact with the bottom A of the metallurgical vessel 2.

Fig. 6a shows a cross-sectional view of an empty metallurgical vessel 2 with an outlet C at the bottom A of the metallurgical vessel 2. A slag blocking device 1 is positioned over the outlet C at the bottom A of the metallurgical vessel 2, the circumferential wall W of the slag blocking device 1 having a decreasing thickness d1 . The slag blocking device 1 is in contact with the bottom A of the metallurgical vessel 2 with its lower edge E. Fig. 6b shows the cross-sectional view of the metallurgical vessel 2 from Fig. 6a with the vessel already having a first total filling level of molten metal. The slag blocking device 1 is still in contact with the bottom A of the metallurgical vessel 2 with its lower edge E. Fig. 6c shows the cross- sectional view of the metallurgical vessel 2 from Fig. 6b with the vessel having a second total filling level of molten metal that is higher than the first filling level. Here the slag blocking device 1 is not in contact anymore with the bottom A of the metallurgical vessel 2 with its lower edge E but it is lifted up and molten metal M is entering the outlet C. This happens only when the total filling level is high enough to prevent the slag S entering the outlet C.

Claims

1 . Slag blocking device (1 ) for a metallurgical vessel (2), wherein the slag blocking device (1 ) has an inner space (I) bordered by a circumferential wall (W), the circumferential wall (W) having a lower edge (E), characterized in that said slag blocking device (1 ) can be placed around an outlet (C), preferably a starter tube (3), of a metallurgical vessel (2) at the beginning of a casting process, during which molten metal (M) is poured into said metallurgical vessel (2), to block slag from entering the outlet (C), wherein the slag blocking device (1 ) has a density of 2.6 to 6 g/cm³, preferably 2.75 to 3.2 g/cm³, wherein the circumferential wall (W) has a decreasing thickness (d1 ) towards the lower edge (E).

2. Slag blocking device (1 ) according to claim 1 , characterized in that the lower edge (E) is pointed.

3. Slag blocking device (1 ) according to claim 2, characterized in that the lower edge (E) has a radius not above 2 mm, preferably not above 1 mm.

4. Slag blocking device (1 ) according to any one of claims 1 to 3, characterized in that the slag blocking device (1 ) is ring-shaped.

5. Slag blocking device (1 ) according to any one of claims 1 to 4, characterized in that the slag blocking device (1 ) comprises a main material (11 ) of a density from 2.7 g/cm³ to 2.9 g/cm³, and a secondary material (12) of higher density compared to the main material, preferably from 3.0 g/cm³ to 12 g/cm³, most preferably from 7.6 g/cm³ to 7.8 g/cm³.

6. Slag blocking device (1 ) according to claim 5, characterized in that the secondary material (12) is mostly, preferably fully, immersed in the main material (11 ).

7. Slag blocking device (1 ) according to claim 5 or 6, characterized in that the main material (11 ) comprises one or more of: magnesia, alumina, silica, olivine, zirconia.

8. Slag blocking device (1 ) according to any one of claims 5 to 7, characterized in that the secondary material (12) comprises one or more of steel, titanium, iron.

9. Slag blocking device (1 ) according to any one of claims 5 to 8, characterized in that at least part of secondary material (12) has the shape of bolts and/or shot.

10. Slag blocking device (1 ) according to any one of claims 5 to 9, characterized in that at least part of secondary material (12) has the shape of fibers.

11 . Slag blocking device (1 ) according to any one of claim 5 to 10, characterized in that the slag blocking device (1 ) comprises 1 to 3 weight- % of the secondary material (12).

12. Use of a slag blocking device (1 ) according to any one of claims 1 to 11 , characterized in that said slag blocking device (1 ) is placed around an outlet (C), preferably a starter tube (3), of a metallurgical vessel (2) at the beginning of a casting process, during which molten metal (M) is poured into said metallurgical vessel (2), to prevent slag (S) located inside the metallurgical vessel (2) from entering the outlet (C).

13. Manufacturing process for a slag blocking device (1 ) according to any one of claims 1 to 11 , characterized in that a mold is provided and material is poured into said mold, in that the material is demolded and dried, preferably in an oven.

14. Manufacturing process for a slag blocking device (1 ) according to claim 13, characterized in that the material is pressed after it is poured into the mold and before it is demolded and dried.

15. Use of a slag blocking device (1 ) in a metallurgical vessel (2) containing molten steel and slag, wherein the slag blocking device (1 ) has an inner space (I) bordered by a circumferential wall (W), the circumferential wall (W) having a lower edge (E), characterized in that said slag blocking device (1 ) is placed around an outlet (C), preferably a starter tube (3), of the metallurgical vessel (2) at the beginning of a casting process, during which molten metal (M) is poured into said metallurgical vessel (2), to block slag from entering the outlet (C), wherein the density of the slag blocking device is higher than the density of slag and lower than the density of molten steel, wherein the circumferential wall (W) has a decreasing thickness (d1 ) towards the lower edge (E).