GB2239828A - Tundish - Google Patents

Abstract

A tundish (1) for molten metal, particularly a tundish which is in combination with an atomiser (40), has a plasma torch assembly (30) whereby, in use, heat can be supplied by the torch to molten metal stored in the tundish. Current flowing between the plasma torch (30) and an anode (33) set in the tundish wall creates the plasma between the torch and the molten metal. Argon gas is bubbled into and through the metal from a bubbler (45) and two stopper rods regulate outflow from the tundish. <IMAGE>

Description

TUNDISH This invention relates to a tundish for receiving molten metal. A particular, but not sole, application of the invention is to a tundish for use in a metal atomising system.

When a tundish is used to hold a quantity of molten metal for a period of time, there is the problem that the temperature of the metal may fall below its required level.

According to the present invention, a tundish is provided with a plasma torch assembly whereby, in use, heat can be supplied to molten metal in the tundish.

By employing a plasma torch, heat can be introduced into the molten metal contained in the tundish in order to maintain its temperature or to increase it.

The problem of loss of heat from molten metal is severe in the operation of large metal atomising systems, particularly systems handling molten steel.

In the case of gas atomisation, it is difficult and expensive to arrange for the rate at which molten metal can be poured into the atomiser to be increased above 20 - 100 kgs/min (1 - 6 t/hr). Consequently, if the contents of a large furnace of, say, several tons capacity is to be poured directly into the atomiser, the contents are poured slowly from the furnace into the atomiser over a period of up to about one hour and this means that, during this time, the metal in the furnace has to be kept at its required temperature and this increases the attack of the molten metal on the furnace refractories. Furthermore, during this time, the furnace cannot be used for heating additional metal.If, to avoid the furnace being occupied for this length of time, the contents of the furnace are poured into a conventional tundish which is sufficiently large to hold the entire batch from the furnace, then the metal in the tundish will start to freeze well before the atomisation process is completed.

When it is desirable to increase the output of an atomiser by operating it continuously, it is not feasible to keep supplying the atomiser with molten metal brought to the atomiser in ladles because, again, the molten metal in the ladles would start to freeze during the 1 - 3 hours needed to atomise the metal.

In the case of an atomiser which is designed to produce metal powders in small volume, even if there already exists a large, low operating cost, melting furnace, the atomiser cannot be put directly underneath the furnace since only one melt in, say, twenty from the furnace is to be atomised. If the atomiser is sited remote from the melting furnace, then the metal has to be transferred from the furnace to the atomiser in ladles and will freeze unless the atomiser is built with a very large capacity, in which case it is only used for short periods of time.

Furthermore, there is an increasing demand for fine gas atomised powders which are produced by lowering the rate at which metal is poured into the gas atomiser. The low rate of pouring increases the problem of the contents of the tundish freezing and it is also inefficient in that there is very low furnace and atomiser utilisation as it is not possible for the conventional atomising system to melt and atomise metal simultaneously.

The provision of the plasma torch with the tundish allows valuable improvements in production costs, as well as improvements in the inclusion content of the atomised powders, due to the long holding time of the molten metal in the tundish which is feasible with the heated tundish and the fact that the plasma torch operates on argon or other inert gas protects the metal in the tundish from reaction with air.

The torch is conveniently supported on a support arm which allows the torch to enter into the tundish through an opening in the lid. The support arm conveniently allows the torch, after it has been withdrawn from the tundish, to be swung to one side clear of the tundish.

One or more openings are provided in the base of the tundish and the tundish can be mounted directly on the top of an atomiser whereby the or each opening in the base of the tundish is aligned with an inlet to the atomiser. The or each opening is held closed with a removable stopper and, when atomising is to commence, the or each stopper is withdrawn to allow the molten metal to flow through the small opening into the atomiser where it is atomised by blasting it with a liquid or gas.

In order that the invention may be more readily understood, it will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a diagrammatic sectional side elevation of a tundish; Figure 2 is a section on the line A-A of Figure 1; and Figure 3 is a plan of the tundish.

A tundish for holding molten metal comprises an open-topped generally rectangular base member 1 fitted with a removable lid 3. Both the base member and the lid comprise a steel shell lined with refractory material 5. The base member 1 carries pairs of trunnions 7 on its outside to enable the tundish to be lifted and tilted. A spout 9 is formed at one end of the tundish and it is filled with a removable plug.

This spout is for use in emergencies only when it would be possible to knock out the plug to enable the contents to be poured out of the tundish.

In the lid 3 there is a hole 11 for receiving a plasma torch and two openings 13 each for receiving a removable stopper rod assembly. Each assembly is associated with an outlet port 15 formed in the base member 1. The stopper rod assembly comprises a refractory plug 17 mounted on an elongate rod 19 which projects through an opening 13 in the lid of the tundish. A support bracket 21 serves to hold the stopper rod in position and a lever mechanism 23 is connected, by means not shown in detail, to the stopper rod to urge the rod into the outlet port. The lever also enables a user to raise the stopper rod sufficiently to allow molten metal to flow through the outlet 15.Once the flow through the outlet has commenced, the stopper rod assembly can be completely withdrawn from the tundish through the opening 13 by means of an overhead crane which is connected to a loop 25 on the upper end of the stopper rod.

A plasma torch assembly 30, which may be obtained from Tectronics R & D Ltd., of Faringdon, Oxfordshire, comprises a torch 31 of elongate form which can be lowered into and removed from the tundish through the opening 11 in the lid. An anode structure 33 is built into the base or side wall of the base member of the tundish so that the anode forms part of the inside wall of the base member. The torch 31 is supported on the end of a pivoted arm 35 and, when the torch has been withdrawn from the tundish, the arm 35 can be pivoted to displace the torch away from the tundish.

The tundish shown in the figures is for retaining molten metal prior to it entering into an atomiser. The tundish is mounted on the top of the atomiser indicated by reference numeral 40 with the outlets 15 from the tundish being aligned with corresponding inlets to the atomiser. At least one furnace, indicated generally by reference numeral 43, is positioned at the side of the tundish.

Alternatively, the furnace(s) can be remote from the tundish and molten metal transferred from the furnace to the tundish by means of ladles.

In use, the two stopper rods are lowered through the openings 13 to close off the outlets 15.

The plasma torch is removed from the tundish. A quantity of molten metal is poured directly from the furnace or ladle into the atomiser through an opening in the lid. The plasma torch is pivoted on the arm 35 and lowered into the tundish through the opening 11.

The plasma torch which is a D.C. unit is energised and supplied with argon gas. The anode structure is used to keep the molten metal at or near to earth potential. Argon is bubbled into the tundish through a port 45 in the base to stir the metal and to reduce temperature stratification within the tundish. The metal is held for a time to allow for inclusion materials to float to the surface. The current flows between the torch and the anode to form a plasma between the torch and the metal which heats the metal. The argon bubbling continues during the heating operation.

When the metal is of a desired temperature and the atomiser is ready, the stoppers are lifted by the lift mechanism 23 to allow the molten metal to flow through the outlets 15 into the atomiser. Heating will continue at controlled power levels to maintain the metal at the desired temperature throughout the atomisation process. When the atomisation is complete, the plasma torch is switched off and the nozzles are stopped off with a metal bar to prevent slag from entering into the atomiser.

The plasma torch 30 passes through an electrically insulating collar (not shown) located in the opening 11 and this partially seals the tundish and also insulates the torch nozzle from the tundish roof.

The plasma torch cathode and nozzle are cooled by deionised water. In turn, the de-ionised water is cooled by process water in a heat exchanger. The anode in the wall of the tundish is backed-up with a water jacket for cooling the anode.

In use, the plasma torch is normally operated with the anode at earth potential. The power supply to the plasma torch should be current limiting and thyristor control is provided for. cutting back the supply very rapidly in the event of mal-functioning taking place. For plasma initiation and subsequent operator control, the current demand level is controlled by a conventional plasma control system.

Claims (8)

Claims:

1. A tundish for molten metal having a plasma torch assembly whereby, in use, heat can be supplied by the torch to molten metal in the tundish.

2. A tundish as claimed in claim 1, in which there is at least one opening in the base of the tundish which can be opened and closed by the action of a vertically movable refractory stopper rod, said stopper rod being electrically insulated from earth.

3. A tundish as claimed in claim 2, in combination with an atomiser, the or each opening in the base of the tundish being aligned with a corresponding inlet of the atomiser.

4. A tundish as claimed in any preceding claim, wherein the torch assembly comprises an elongate plasma torch removable into and out of the tundish and an electrode permanently mounted in the base or wall of the tundish.

5. A tundish as claimed in claim 4, in which the tundish has a removable lid with an aperture in the lid, the aperture being defined by electrically insulating material which permits the torch to pass through the aperture.

6. A tundish as claimed in claim 4 or 5, wherein provision is made for liquid cooling the electrode in the wall of the tundish.

7. A tundish as claimed in claim 4, 5 or 6, wherein the elongate plasma torch is supported on a support arm which allows the elongate torch to be swung clear of the tundish when it has been withdrawn from the tundish.

8. A tundish substantially as hereinbefore described with reference to the accompanying drawings.