GB2351297A

GB2351297A - Metallurgical treatment apparatus

Info

Publication number: GB2351297A
Application number: GB9914485A
Authority: GB
Inventors: John Rushe
Original assignee: Vacmetal Ltd; Vacmetal Gesellschaft fuer Vakuum Metallurgie mbH
Current assignee: Vacmetal Ltd; Vacmetal Gesellschaft fuer Vakuum Metallurgie mbH
Priority date: 1999-06-21
Filing date: 1999-06-21
Publication date: 2000-12-27
Anticipated expiration: 2019-06-21
Also published as: GB2351297B; GB9914485D0; WO2000078482A1; AU5549800A

Abstract

A vessel 20 and removable cover for processing molten ferrous metal 22, the cover comprising an outer cover 40 having an inner ring 42 depending therefrom along with a port (62, Fig. 5). In use, the cover is movable between a first position (Fig. 4), separated from the molten metal 22 and a second position (Fig. 5) where the inner ring 42 extends into the molten metal 22 below the level of the slag 24, thereby blocking the flow path between the edge of the vessel 20 and the port (62, Fig. 5). The inner ring 42 may be detachable from the outer cover 40. A fume collection hood (72, Fig. 5) may be provided above the cover and in which a port may be provided through which a lance (82, Fig. 6) may be inserted, a similar port being provided in the outer cover 40.

Description

2351297 40301 METALLURGICAL TREATMENT APPARATUS The present invention

relates to apparatus for processing ferrous material.

During treatment of molten ferrous material, the melt is held in a vessel. The vessel is normally protected by a cover, which performs several important functions. The cover reduces heat losses due to radiation during treatment, thereby providing an efficient working system as less energy input is 10 required to maintain temperature. Also, the cover prevents the escape of molten ferrous material and fume during treatment, creating a safer working environment, and by reducing the amount of ferrous material lost, a more efficient working system. 15 The cover reduces contact between the surface of the molten ferrous material and the surrounding atmosphere. By performing this function, the cover reduces the absorption of oxygen and nitrogen into the material. Absorption of oxygen and nitrogen into the molten material during its processing 20 can lead to the formation of impurities which can adversely affect its mechanical properties. During treatment to reduce the sulphur content of the material, it is very important to prevent absorption of oxygen and nitrogen, as it is metallurgically impossible to remove sulphur unless a very low 25 oxygen content is maintained. Also, the lower the residual sulphur content of the material, the greater the ability of the material to absorb nitrogen. US 4,634,104 discloses a cover for a ferrous material treatment vessel, in which a suction hood is lowered over the 30 vessel. Air is drawn in through a gap between the hood and the vessel. An inner hood or bell is carried by the suction 2 hood to prevent contact of air with the steel melt in the vessel. However, the inner and outer hoods are separate, thereby creating a convective air path through the gap between the outer hood, inner hood and upper gap, leading to increased absorption of oxygen and nitrogen into the metal. Another problem with US 4, 634, 104 is that fume is extracted directly from the outer hood, by the suction device, again increasing the flow of air through the gap between the outer cover and the lip of the vessel.

In US 4,634,104, although the outer hood is designed to sit on the upper rim of the vessel, the vessel lip may be uneven due to buildup of steel and slag deposits on the pouring lip of the vessel, so that gaps may exist between the outer cover and the lid of the vessel.

In US 4,634,104, the inner hood is lowered until the lower edge of the inner hood rests in the slag layer on top of the molten ferrous material in the vessel. However, problems arise with this design in that the slag layer is not always continuous and therefore air may be able to pass underneath the lower rim of the inner cover and therefore pass over the contents of the vessel, creating a convective path.

US 4,405,363 discloses a cover for a metallurgical vessel in which the outer cover rests below the surface of the rim of the vessel. However, gaps exist between the outer cover, inner cover and rim of the metallurgical vessel, allowing a through draught of air over the surface of the contents of the vessel.

In addition to air drawn into the system through various gaps by the extraction system, there is also the problem of natural convection. Since gas produced during treatment of molten ferrous material in the vessel is extremely hot, it rises vertically and escapes through any opening or gap in the upper 3 part of the vessel cover. Air is drawn by convection through any lower gaps between the vessel and the vessel cover, thus creating a chimney effect. Therefore, even without any fume extraction, there will be direct contact between air and the melt in the vessel, unless the seal between the vessel and the cover is completely airtight. This would require a great deal of expense in design and manufacture and very strict maintenance requirements, thus air tight sealing between the vessel and the cover is not a practical proposition.

During treatment of molten ferrous material in the vessel, the surface of the melt is disturbed, particularly in the case of powder injection. Where the surface of the melt is disturbed, the contact between the air and melt surface is increased, causing increased absorption of oxygen and nitrogen into the ferrous material.

In order to improve conventional vessel covers, shrouding rings have been added to some covers either to the lower periphery of the cover, or to the upper ports in the cover, or both. The purpose of the rings, which contain small holes or slots, is to blow an inert gas into the gap between the cover and vessel or port in an attempt to prevent the ingress of air. Problems occur with this approach in that air can still enter the inner chamber and contact the surface of the melt.

Also, as very large amounts of gas are required and the cost of inert gas is high, this type of sealing is extremely expensive to operate. Further, this approach can actually result in accelerated entrainment of air, should inadequate amounts of shrouding gas be used.

The invention in its various aspects is defined in independent claims 1 and 5. Preferred features of the invention are defined in the dependent claims 2, 3, 4, 6, 7 and 8.

4 According to the present invention, there is provided an apparatus for processing ferrous material including a vessel for receiving the molten ferrous material and a removable cover for the vessel, the cover comprising:

an outer cover which has an upper port, and an inner ring depending from the outer cover; wherein, when in position over the vessel, the inner ring is moveable between a first position when it is separated from the molten ferrous material, and a second position when it extends into the material below the layer of slag; the arrangement being such that when the inner ring extends into the material, the flow path between the edge of the vessel and the upper port is blocked.

In a preferred embodiment of the apparatus, when the apparatus is in the second position, the lower edge of the outer cover extends below the upper edge of the vessel. In another embodiment of the apparatus the inner ring is detachable from the outer cover. In a further embodiment of the apparatus there exists fume collection apparatus comprising a fume collection hood above the cover, for collection of fumes generated during the treatment of ferrous material in the vessel.

In another embodiment of the apparatus, there may exist a port in the fume collection apparatus and the outer cover through which a lance may be lowered.

In another embodiment of the apparatus the inner ring has a large inner diameter within the reaction area, but in use sufficient volume of ferrous material is between the outer surface of the inner ring and the inner surface of the metallurgical vessel to prevent freezing of this volume in treatment.

According to another aspect of the present invention, there is provided a method of processing ferrous materials in a vessel, the method including:

moving a cover having an upper fume port, onto a vessel containing molten ferrous material; providing a barrier between the edge of the vessel and the fume port such that air cannot flow therebetween over the surface of molten ferrous metal in the vessel; and treating the molten ferrous material held in the vessel.

A preferred embodiment of the method of the invention exists wherein the treating of the molten ferrous material comprises inserting a lance through a port in the cover into the melt such that it extends downwards into the melt for injection of treatment agents into the melt. The treatment can be extended by allowing the treatment agents, which rise to the surface of the molten ferrous material and are retained within the inner ring, to continue to react with the molten ferrous material by continued stirring with inert gas, either by top lance or bottom plug stirring.

A further preferred embodiment of the method of the invention exists wherein fume generated during the treatment and escaping through the fume port is collected by the fume collection apparatus.

A further preferred embodiment of the method of the invention exists wherein a layer of slag covering the surface of the molten ferrous material between the barrier and the inner wall of the vessel is provided.

A preferred embodiment of the invention will now be described in more detail, by way of example, with reference to the drawings, in which:

Figure 1 is a cross-section through an existing metallurgical treatment apparatus with the vessel cover 6 lowered and resting on the top of the vessel, showing the convective air path created by direct fume extraction from the ladle cover when there is no blockade between the gap between the ladle and the cover and the fume extraction system and; Figure 2 is a cross-section through an existing metallurgical treatment apparatus with the vessel cover lowered and overhanging the top of the vessel, showing the convective air path created by direct fume extraction from the ladle cover when there is no blockade between the gap between the ladle and the cover and the fume extraction system and; Figure 3 is a cross-section through an existing cover which has a separate fume collection system.

The cover when lowered fits over the rim of the vessel, and has an upper port through wh ich fume can escape into the fume collection hood and; Figure 4 is a cross-section through metallurgical treatment apparatus with the vessel cover raised and; Fiqure 5 is a view similar to Figure 1, but with the vessel cover lowered and; Figure 6 is a view similar to Figure 2, but with an injection lance lowered into the melt.

The apparatus shown in the drawings comprises a vessel 20 containing molten ferrous material 22, upon which is generally a layer of slag 24. Previously known examples of such apparatus are shown in Figures 1, 2 and 3. Figure 1 illustrates a cover for a vessel in which the cover 26 sits on the rim of the vessel. As described previously, gaps 28 generally exist between the vessel and the cover, and air is 7 drawn in through these gaps, over the surface of the metal, and out through the fume extraction system 30,32. Therefore, a convective path is formed over the surface of the metal (shown by arrows). Figure 2 illustrates a similar example, in which the cover has a skirt 34 which overhangs the edge of the vessel, creating a similar convective air path. Figure 3 illustrates a cover which has a separate fume collection system 36. The cover fits over the rim of the vessel, and has an upper port 38 through which fume can escape into the fume collection hood. However, a similar convective air path forms to those described above, as there is no blockade between the gaps in the outer cover and the upper port of the cover, air is drawn over the contents of the vessel and out through the fume extraction system.

Figures 4, 5 and 6, which illustrate the invention, are described below. Apparatus 10 comprises a removable cover, illustrated in the raised position above the vessel in Figure 4 comprises an outer cover 40 and an inner ring 42. In the example illustrated, the outer cover is of a generally conical shape, and is connected to the inner ring by a dowel, wedge and locking pin arrangement 44. The inner ring is detachable, when required, in order to replace the inner ring and prevent build-up of slag and/or ferrous material on the surface of the inner ring, thereby maintaining efficiency of the system.

The cover is attached by a cable 46 and pulley 48 to a winch (not shown) so it may be raised and lowered.

When the cover is positioned according to Figure 5, the lower edge of the inner ring 50 has passed through the layer of slag 24 into the melt 22, ensuring that air or other gases cannot pass through from the gap 54 to the annular space 56 above the slag within the ring. The layer of slag covers the surface of molten ferrous material not included within the inner ring 53 to prevent contact between the molten material and air. A 8 layer of slag covers the surface of the melt outside of the inner ring 53 to ensure no contact with atmospheric gases.

The top of the outer cover may be flat, with a central raised section 60 incorporating a central port 62. Gussets 64 between the central raised section and the flat top provide stiffness and mechanical stability.

Fume collection apparatus 70 is located above the vessel cover and is separate from the moving cover. There is no tendency of the fume collection apparatus to cause a flow of air across the surface of the melt in the metallurgical vessel. The barrier 61 which exists between the gap 54 and the port 62 ensures no through draught of air across the surface of the melt.

In operation, the vessel containing the material to be treated is positioned below the vessel cover. The freeboard 80, that is, the distance from the lip of the metallurgical vessel to the surface of the metal, is measured. The cover is then lowered until the lower edge of the inner ring is a pre-set distance below the level of the surface of the melt in the metallurgical vessel.

An injection lance 82, as shown in Figure 6 may be lowered through the port in the fixed fume collection hood 72, through the port in the outer cover 62, and into the melt 22 in the metallurgical vessel for injection of agents into the melt.

Desulphurisation agents may be injected into the ferrous material through the lance using a carrier gas. The reaction products rise to the surface of the melt, and most are contained within the inner ring. A further desulphurisation reaction then takes place between the.steel and the slag material on the surface of the melt. For this reason, the inner ring has a large inner diameter, so a large proportion 9 of the slag is within the inner ring.

Since the lower edge of the inner ring is in the ferrous material, the presence of a gap 54 between the outer cover and the lip of the vessel is no longer relevant. The volume between the outer cover and the inner ring is a trapped volume, with no escape path. Therefore, there is no chimney effect and no tendency for air from the surrounding atmosphere to be drawn into the gap between the metallurgical vessel and the outer cover. The heating of air in a trapped volume leads to its expansion and small outward flow, consequently further reducing inward air flow.

Furthermore, the surface of the melt between the outer surface of the inner ring and inner surface of the metallurgical vessel is quiescent and covered by a static layer of slag. As a result, there is no contact between the surface of the melt and gases outside the inner ring.

The immersion depth of the inner ring can be varied to suit different applications, and the same principle applies whether desulphurisation is achieved by injection through an immersed lance, by top slag mixing using top lance stirring or bottom plug stirring, or a combination of these processes. The large diameter inner ring gives a large surface area for top slag reaction.

However, the outer diameter of the inner ring is limited by the necessity to ensure sufficient steel volume between the outer surface of the inner ring and the inner surface.of the metallurgical vessel to prevent freezing of the melt during treatment. If the inner ring is too large, the volume of molten metal between the outer wall of the inner ring and the inner wall of the metallurgical vessel is relatively small.

The inner ring and lining of the metallurgical vessel are refractory and absorb heat from the molten metal. A 1 relatively small volume of metal between these two refractory materials would have a small thermal capacity. Therefore, the temperature in this volume would fall rapidly as heat was absorbed, leading to possible freezing of this volume of metal. A build up of material on the inner ring and wall of the metallurgical vessel can result, causing the ring to become trapped in the melt.

The size of the inner ring is dependent on several parameters, which include: the inner diameter of the metallurgical vessel over the range of working metal levels; the type of treatment to be carried out; the superheat of the metal to be treated; the slag composition and quality; the thermal properties of the metallurgical vessel refractories; the thermal properties of the inner ring and its associated refractories; the likely condition of the metallurgical vessel with respect to side wall build up of metal and slag; the temperature of the metallurgical vessel refractories; the immersion time of the inner ring and the properties and position of elements positioned in the melt, for example the injection lance, stirring lance and porous plugs. Only by detailed study and calculation on a case by case basis can the optimum ring size by defined.

Fume generated during treatment escapes through the port in the top of the outer cover. During treatment the volume enclosed by the inner ring and the top of the outer cover is under a positive pressure, generated by the hot gases emerging from the melt, preventing any ingress of air at the port.

Claims

1. Apparatus for processing ferrous material including a vessel for receiving the molten ferrous material and a removable cover for the vessel, the cover comprising:

an outer cover which has an upper port, and an inner ring depending from the outer cover; wherein, when in position over the vessel, the outer cover and the inner ring are moveable between a first position when it is separated from the molten ferrous material, and a second position when the inner ring extends into the material below the layer of slag; the arrangement being such that when the inner ring extends into the material, the flow path between the edge of the vessel and the upper port is blocked.

2. Apparatus for processing ferrous material according to claim 1 wherein, when in the second position, the lower edge of the outer cover extends below the upper edge of the vessel.

3. Apparatus for processing ferrous material according to claim 1 or claim 2, in which the inner ring is detachable from the outer cover.

4. Apparatus for processing ferrous material according to any preceding claim, including apparatus for fume collection comprising a fume collection hood above the cover, for collection of fumes generated during the treatment of ferrous material in the vessel.

5. Apparatus for processing ferrous material according to any preceding claim in which a port exists in the fume collection apparatus and the outer cover through which a lance may be lowered.

6. Apparatus for processing ferrous material according to any 0 1 12 preceding claim in which the inner ring has a large inner diameter within the reaction area, but in use sufficient volume of ferrous material is between the outer surface of the inner ring and the inner surface of the metallurgical vessel to prevent freezing of this volume in treatment.

7. A method of processing ferrous materials in a vessel, the method including:

moving a cover having an upper fume port, onto a vessel containing molten ferrous material; providing a barrier between the edge of the vessel and the fume port such that air cannot flow therebetween over the surface of molten ferrous metal over the vessel; and treating the molten ferrous material held in the vessel.

8. The method of claim 7 wherein the treating of the molten ferrous material comprises inserting a lance through a port in the cover into the melt such that it extends downwards into the melt for injection of treatment agents into the melt.

9. The method of claim 7 or claim 8 wherein fume generated during the treatment and escaping through the fume port is collected by the fume collection apparatus.

10. A method of processing ferrous materials according to any of claims 7 to 9 including providing a layer of slag covering the surface of the molten ferrous material between the barrier and the inner wall of the vessel.

11. Apparatus as hereinbefore described with reference to Figures 4, 5 and 6.

12. Method as hereinbefore described with reference to Figures 4, 5 and 6.