WO1999055920A1

WO1999055920A1 - A cover for a metallurgical vessel

Info

Publication number: WO1999055920A1
Application number: PCT/GB1999/001289
Authority: WO
Inventors: Heinz Mair
Original assignee: Foseco International Ltd
Current assignee: Foseco International Ltd
Priority date: 1998-04-29
Filing date: 1999-04-23
Publication date: 1999-11-04
Anticipated expiration: 2000-10-29
Also published as: AU3717799A; GB9809094D0

Abstract

A cover for a metallurgical treatment vessel characterised in that the cover has an inner surface which is substantially dome-shaped.

Description

A COVER FOR A METALLURGICAL VESSEL

This invention relates to a cover for a metallurgical treatment vessel, particularly a cover for a treatment vessel, such as a ladle, in which molten ferrous metal is treated with a treatment agent, such as magnesium or calcium, which volatilises at a temperature below the temperature of the molten ferrous metal.

Although the cover can be used on vessels such as ladles, which are used to treat molten ferrous metals with other elements, such as calcium, and with other forms of the elements, the use of the cover will be described with reference to the treatment of molten iron with magnesium in the form of wire.

As its boiling point is well below the temperature of molten iron magnesium is a difficult metal to introduce into molten iron because it volatilises almost immediately, and reacts with the iron extremely rapidly and violently. In addition magnesium has a relatively low solubility in iron, it has a much lower density than iron, and there is a high risk of magnesium being lost as magnesium oxide or magnesium vapour during the treatment process.

Due to the high and violent reactivity of the magnesium with the molten iron, it is normal practice to provide the treatment vessel, usually a ladle, with a cover or lid which, during the treatment process, protects the surroundings from the splashing which occurs, and which also avoids material losses from the vessel.

DE 3818000 C2 describes equipment in which substances, such as magnesium, aluminium, molybdenum or calcium, in the form of wire are introduced into molten steel in a vessel which is provided with a lid. In order for treatment in such equipment to achieve reproducible results, and at the same time be economic, it is necessary that the wire melts at a point close to the bottom of the treatment vessel, and it is highly desirable that the treatment substance is used in a pure form containing no diluent substances. These considerations require that the wire is introduced at a certain minimum speed, and as a result the intensity of the reaction of the treatment substance is also a necessary feature of the technological function of the process.

In order to diminish the intensity of the reaction of the treatment substance with the molten metal it is frequently necessary to dilute the treatment substance by incorporating other substances in the wire. The manufacturing cost of the wire and the cost of the metal treatment process are thus increased. In addition the diluent substances can increase heat loss from the molten metal, and also have an adverse effect on the quality of the molten metal.

If the advantages of such a treatment process using a treatment substance such as magnesium are to be fully utilised, then it is necessary to eliminate, or at least partially eliminate the disadvantages resulting from the intense reactivity when the magnesium is added to the molten metal.

Figure 1 of the accompanying drawings is a vertical section of the top part of a ladle for treating molten iron with a magnesium wire treatment agent and a conventional cover. During treatment splashing of the molten iron occurs and molten metal can be ejected between the cover (1) and the ladle (2). The cover (1) often has an aperture (3) for relieving pressure in the ladle (2) so molten metal can also be ejected through the aperture (3). The cover (1) also has an aperture (7) through which the magnesium wire is fed into the ladle. Splashing of molten metal can also occur through aperture (7), and becomes more of a problem the nearer the aperture (7) is located to the surface of the molten metal (4).

In order to minimise the above disadvantages the distance (6) between the surface of the melt (4) and the rim (5) of the ladle (2) can be increased. While this measure reduces the problems caused by splashing of the molten iron, it entails making the size of the ladle considerably larger, and as a result there is a significantly greater loss of heat from the molten iron. Increasing the size of the ladle also increases the amount of refractory material which is needed to line the ladle, and transportation costs are increased due to the increased weight of the ladle. Due to restraints on space on foundry plant it is frequently not possible to increase the height of ladles, and in these circumstances the problems caused by splashing of the molten metal are alleviated by reducing the quantity of metal which is treated. However this measure also has disadvantages of its own, namely higher heat loss and increased expenditure on transportation.

It has now been found that the splashing problems can be eliminated or considerably reduced, without the need to enlarge the ladle or reduce the weight of the melt to be treated, if the interior of the cover has a particular shape and configuration.

According to the invention there is provided a cover for a metallurgical treatment vessel characterised in that the cover has an inner surface which is substantially dome shaped.

The height of the dome shape will usually be between 0.1 to 3 times the inner diameter of the cover at the base of the cover.

The shape of the inner surface of the cover enlarges the volume of the space above the melt in the treatment vessel up to the underside of the cover compared with the space when using a conventional cover and treatment vessel. When the cover of the invention is fixed to a treatment vessel, which is preferably a ladle, and magnesium, preferably in the form of wire, is introduced into molten metal, such as molten iron, contained in the vessel, any molten metal which splashes up the walls of the vessel and the cover, is deflected by the substantially dome shaped inner surface of the cover towards the central axis of the bath when it falls down again.

The dome shape of the inside of the cover has the effect that the temperature immediately under the cover closure is appreciably lower than the temperature of the surface of the molten metal contained in the vessel. If any vapour from the treatment agent, for example calcium and/or magnesium vapour, escapes from the melt the vapour cools under the cover and a volume reduction occurs. This results in a reduction in the pressure in the space under the inside of the cover, and alleviates any tendency for the vapour pressure of the treatment agent to lift the cover from the treatment vessel, as would happen with a conventional cover.

The cover preferably includes an aperture through which a treatment agent, for example magnesium wire, may be fed into the vessel from its exterior. The aperture is preferably located substantially in the apex of the dome. The aperture may, for example, extend through an inwardly projecting portion of the cover.

When using a conventional cover the parting line between the cover and the vessel is in line with the height to which the molten metal splashes so metal can leak between the cover and the vessel. When using the cover of the invention the height to which the metal splashes is much higher than the parting line between the cover and the ladle so that metal does not leak at the joint between the cover and the ladle.

However, in instances in which the bottom of the cover does not fit flush with the top of the vessel there may be a tendency for molten metal to leak at the joint between the two. In such instances it is desirable to provide some form of sealing means between the cover and the vessel. The sealing means may be a gas cushion seal, for example an air cushion seal, preferably consisting of a system of jets in the cover into which a gas, e.g. air, is injected.

In practice it often happens that the treatment vessel is positioned in such a way that its base, and hence its top, are not horizontal. For this reason the cover is hung from chains or suspended in gimbals as shown in DE 3818000 C2. Hanging the cover from chains suffers from the disadvantage that there is little control over the guidance of the cover into position. As a result, when metal in the ladle is to treated with a wire contained in a feeding tube, the tube can become bent and damaged in the aperture through which it passes through the cover. This above problem can be overcome by incorporating into the cover means for controlling the movement of the cover. Such means may take the form of supporting spigots which are located in grooves which are provided with slots, the size of the slot determining the movement of the cover about the horizontal axis, and bosses which contact carrier arms, the distance between the bosses and the carrier arms determining the degree of rotation about the vertical axis. By these means swinging and rotation of the cover can be restricted.

The invention is illustrated with reference to Figures 2 to 6 of the accompanying drawings in which

Figure 2 is a plan view from above of a cover according to the invention,

Figure 3 is a diagrammatic part sectional side elevation of the cover of

Figure 2,

Figure 4 is another diagrammatic part sectional side elevation of the cover of Figure 2 at 90° to that of Figure 3,

Figure 5 is diagrammatic part sectional side elevation of a ladle fitted with a cover according to the invention and

Figure 6 is a diagrammatic part sectional side elevation of the ladle and cover of Figure 5 at 90° to that of Figure 5.

Referring to Figures 2 to 6 of the drawings a cover (21) has an inner surface (28) which is substantially dome shaped such that when the cover (21) is fixed to a ladle (22) containing molten metal such that height (26) of the space from the surface of the molten metal bath to the top of the inner surface (28) is increased appreciably compared with the height of the space when using a ladle with a conventional cover. The cover (21) also has an aperture (23) for relieving pressure in the ladle (22) and an aperture (27) through which a treatment agent such as magnesium wire may be fed into the ladle.

The parting line (30) between the cover (21) and the ladle (22) is sealed by means of an air cushion seal consisting of a system of jets (31 , 32, 33) in the cover (21) into which air is injected.

The cover (21) has supporting spigots (34) which rest in grooves (35) which are provided with a slot. The size of the slot determines the movement along the X axis. Additionally, the cover (21) has two bosses (36) which contact carrier arm (37) when the cover rotates about the Z axis. The distance between the bosses (36) and the carrier arm (37) determines the magnitude of the rotational movement. When the cover (21) is set on the ladle (22) the cover (21) is lowered so that the supporting spigots (34) are located substantially centrally in the slot. As a result the cover (21) can largely adjust to any ladle position, for example when the ladle (22) is tilted slightly as shown in Figure 6.

In tests to compare a cover according to the invention as shown in

Figures 2 to 6 of the drawings with a conventional cover as shown in Figure 1, no splashing was observed with the cover according to the invention, whereas with the conventional cover, under the same process conditions, splashing amounting to about 5% of the weight of the metal being treated was observed.

Claims

1. A cover for a metallurgical treatment vessel, characterised in that the cover has an inner surface which is substantially dome shaped.

2. A cover according to Claim 1 , in which the dome shaped inner surface has a maximum height in the range 0.1 to 3 times the inner diameter of the cover at the base of the cover.

3. A cover according to Claim 1 or Claim 2, which includes an aperture through which a treatment agent may be fed into the vessel from the exterior thereof.

4. A cover according to Claim 3, in which the aperture is located substantially in the apex of the dome.

5. A cover according to Claim 3 or Claim 4, in which the aperture extends through an inwardly projecting portion of the cover.

6. A treatment vessel for molten metal, the vessel having a cover according to any preceding claim.

7. A treatment vessel according to Claim 6, which comprises a ladle.

8. A treatment vessel according to Claim 6 or Claim 7, further comprising sealing means between the cover and the vessel.

9. A treatment vessel according to Claim 8, in which the sealing means is a gas cushion seal, preferably an air cushion seal.

10. A treatment vessel according to Claim 9, in which the gas cushion seal is formed by a system of jets in the cover into which a gas, preferably air, is injected.

11. The use of a treatment vessel according to any one of claims 6 to 10, in the treatment of molten iron with magnesium, preferably magnesium wire.