GB2114721A - Injection lances for molten metal - Google Patents

Abstract

An injection lance for injecting gas and/or powders into molten metals comprises a central steel tube 6 and a protective refractory sheath 11 fixed thereto at one location along the length of the tube by projections 12. By provision of a winding plastics tape 13, the refractory sheath and the metal tube 6 are free to expand and contract independently of one another in response to temperature change, thus reducing stresses in the refractory material. <IMAGE>

Description

SPECIFICATION Injection lances for molten metal This invention relates to injection lances for injecting materials for instance gases and/or powders into molten metal.

It has long been the practice to stir a ladle of molten metal by means of injecting a gas into it, often to try to ensure that the whole ladle is of uniform temperature and to improve the homogeneity of the molten metal in the ladle. A practice which is becoming increasingly common is the injection of various powders in a gaseous carrier into the molten metal in order to affect some metallurgical change in the molten metal.

An example of this is the desulphurising of molten iron in a ladle by means of an injection of lime mixtures or mixtures containing calcium carbide.

This practice has grown up with the increasing demand for low sulphur steels.

Gas is primarily injected by one of two means.

The first is a porous plug which is most commonly placed in the bottom of the ladle as illustrated in Figure 1 of the accompanying drawings. The second method is by means of a lance as is illustrated schematically in Figure 2 of the accompanying drawings.

The porous plug allows gas under sufficient pressure to overcome the head of molten metal to enter the ladle but at the same time is so constructed that no molten metal is allowed to flow out of the ladle through the plug.

In the case of the lance injection system, gas is again injected at a sufficiently high pressure to overcome the pressure of the head of molten metal at the end of the lance. The gas is injected continuously whilst the lance is submerged. Unlike the porous plug which essentially acts as a oneway valve, many lances are open ended so that a cessation of gas injection or a drop in gas pressure below that exerted by the head of molten metal results in metal entering the lance head and blocking it. The gas on entering the body of molten metal forms bubbles and these expand as they naturally move towards the surface of the bath or ladle. In so doing they both stir up the colder metal from the region of the ladle bottom and help mix it with the remainder of the molten metal in the ladle.This helps to obtain a more uniform temperature throughout the molten metal and helps make it a more metallurgically homogeneous metal.

When the injection lance is used to inject a powder in a gaseous carrier the gas acts in a similar way to that just described. The powder is intimately mixed with the molten metal, the mixing being aided both by the action of the gas described above and any reactivity which may occur between the powder and the metal. The powder is invariably in practice of a lower melting point than the molten metal and dissolves into it in some manner, most usualiy to change the metallurgical characteristics of the molten metal.

The injection lances are frequently fabricated in the plants in which they are used to inject materials into molten metal. Such lances often comprise a steel tube protected from the effects of the molten metal by the use of a series of relatively short refractory sleeves arranged along the length of the steel tube and joined by refractory cement. Such a lance is illustrated in Figure 3. The refractory sleeves of the same kind as are used in the casting ladle as part of the stopper rod assembly controlling metal outflow from the ladle. These have been used mainly because they are readily available on the plant.

Often, these made up sleeve lances, whilst relatively inexpensive, only last for one application of the injectant (gas or gas/powder) and a new lance must be made ready for the next ladle. With this type of lance, failure often occurs caused by the molten metal or slag penetrating one of the joints between the sleeves. Sometimes, failure occurs during the actual process of injecting the gas which causes expensive delays in the steelmaking (where that is the metal concerned) and casting processes.

Both to increase the number of times a lance can be used and to increase its reliability during use, lances have been made and used consisting of a central steel tube protected by a layer of suitable refractory concrete. Such a lance is illustrated in Figure 4 of the accompanying drawings. Whilst the lance is immersed in the molten metal, it is heated up by the molten metal reaching the temperature of the molten metal at its outer surface with a decreasing temperature towards the centre. The temperatures present cause both refractory concrete and the steel to expand and due to the fact that they usually have differing coefficients of linear expansion, and due to the temperature differential mentioned above, some relative movement between the steel and the concrete may result, breaking down any bond between the two at the outside circumference of the steel tube.The process of gas or gas powder injection into molten metal alinost invariably causes vibration along the lance and movement of it in the molten metal. The movement is often most pronounced at the extreme tip of the lance immersed in the metal. This tends to shake the refractory concrete away from the tube.

To inhibit the breaking down of the bond between the steel tube and the refractory concrete due to the differences in thermal expansion between the steel tube and the concrete and the physical forces tending to break up the concrete as the lance flexes in use, steel reinforcing of various types has been used in the concrete in an effort to hold the tube and the concrete together.

Sometimes the reinforcing is anchored to the steel tube. The type of reinforcing which has been extensively used in of a "barbed-wire" type wound in helical fashion around the steel tube and anchored to cleats welded to the tube in the manner shown in Figure 4 of the accompanying drawings.

Such reinforcement does help to hold the concrete together under tensile strain but of itself cannot prevent the stresses and strains occurring due to the differences in thermal expansion or contraction the steel tube and the refractory concrete. Lance failure is generally brought about by refractory concrete spalling off the lance at some point brought about by metal or slag penetration which is in turn allowed by cracks forming in the refractory concrete where it is put under tensile stress and strain.

The present invention now provides a lance for injecting material into molten metal, comprising a tubular member in a protective sheath of refractory material when the tubular member and the protective sheath are able to expand and contract longitudinally substantially independently of one another in response to temperature change.

This capability for independent expansion and contraction alleviates the stress and strain engendered normally in both the inner tube and the protective sheath when, as outlined above, it is attempted to hold them together.

Preferably, the tubular members of steel and preferably the protective sheath is a refractory concrete and such refractory concrete is preferably provided with reinforcement, e.g. steel reinforcement.

Preferably the tubular member and protective sheath are fixedly connected at one location along their length. This has the effect of holding the two components firmly together whilst still allowing longitudinal expansion and contraction to take place in each member independently. Fixedly connecting the two components at two points longitudinally spaced along their length would, of course, prevent independent expansion and contraction between the two points of fixing.

Preferably, the connection between the protective sheath and the tubular member is provided by one or more radially extending members attached to the tubular member. Such radially extending members may for instance be welded to the tubular member and may be of the same material as the tubular member, for instance steel. There may be a number of radially extending members arranged around the circumference of the tubular member or there may be a continuous flange provided on the tubular member.

Where the protective sheath is of reinforced refractory concrete, the reinforcement may be fixed to the radially extending member or members, e.g. by welding.

The invention includes a method for producing a lance for injecting material into molten metal which method comprises casting refractory concrete over the surface of a tubular member to provide a protective sheath thereon, wherein means are provided on the surface of the tubular member to prevent the refractory concrete forming a bond with the tubular member, whereby the tubular member and the protective sheath are able to expand and contract longitudinally substantially independently of one another in response to temperature change.

The means for preventing the formation of the bond between the surface of the tubular member and the refractory concrete may for instance be a refractory concrete-impermeable sheathing. This may be a sheathing of plastics material, e.g. a helical winding of plastics tape. Additionally, means may be provided to further prevent the refractory concrete forming a bond to the sheathing. For instance, the sheathing may be greased.

The invention also includes a method for producing a lance for injecting material into molten metal which method comprises casting a refractory concrete tubular sleeve and introducing into the pre-cast sleeve a tubular member so that the sleeve forms a protective sheath for the tubular member and so joining the sleeve and tubular member that they are able to expand and contract longitudinally substantially independently of one another in response to temperature change.

The pre-cast sleeve may be fired prior to introducing the tubular member therein.

The invention will be further illustrated by the following specific description of preferred embodiments and comparison with prior art devices with reference to the accompanying drawings in which: Figure 1 illustrates a ladle for molten metal equipped with a porous plug for gas injection.

Figure 2 illustrates a ladle for molten metal provided with an injection lance for injecting gas or gas/powder mixtures.

Figure 3 shows a prior art injection lance.

Figure 4 shows a second type of known injection lance, and Figure 5 is a cross-section through a lance according to the present invention.

Figure 1 illustrates the practice of injecting gas through a porous plug 1 into a ladle 2 containing molten metal 3.

Figure 2 illustrates schematically the practice of injecting gas into a ladle 2 of molten metal 3 through an injection lance 4 supported by means schematically shown at 5. The introduction of gas into the lance is illustrated by the arrow.

Figure 3 shows a lance of the kind conventionally made at the site of injection. The lance 4 comprises a central steel tube 6 along which are arranged a series of close-fitting refractory concrete sleeves 7 joined together by refractory cement 8.

Figure 4 illustrates a further type of known injection lance in which a central steel tube 6 is provided with radially extending welded on cleats 9 over which is wound steel reinforcing 10 to form a cage of reinforcement over which refractory concrete 11 is cast.

Figure 5 illustrates a lance according to the present invention. The lance 4 comprises a central steel tube 6 having at one location along its length radially extending members 1 2 in the form of strong steel angle welded to the tube. Four pieces of angle are provided spaced at 900 around the tube circumference. A winding of plastics water repellant tape 1 3 is provided over the tube surface where the tube 6 is to be protected by a refractory sheath, and the plastics tape is lightly covered with grease to ensure that no bond is developed between the steel tube 6 and the refractory sheath which is to be provided thereon. A cylindrical cage of wire reinforcement 10 is welded or otherwise fixed to the members 12 and extends along the outside of the tube 6.A protective sheath of refractory concrete 11 is cast over the reinforcement.

A suitable mould for casting the protective sheath may be formed of any suitably shaped tubular material, and it has been found that cardboard is particularly effective for this purpose, and has the added advantage of being relatively inexpensive, such that each mould need be used only once and may then be discarded. Cardboard moulds are useful in providing some protection to the refractory concrete after it has developed its initial set and before it is fully dried out.

Suitable cardboard tubes, for example of circular cross section, are, furthermore freely commercially available.

The refractory concrete sheath is fixedly attached to the steel tube 6 only at the region where the projecting members 1 2 are provided.

So for the remainder of the length of the steel tube 6, a refractory concrete is prevented from bonding to the surface of the steel tube 6 by the plastics tape 13 and its coating of grease. This results in the formation of reinforced hollow refractory concrete tube surrounding the steel tube and 'anchored to the latter at one point but otherwise free to expand or contract relative to it. The reinforcing of the concrete sleeve is designed to give a high resistance to break up under the stresses and strains inherent in the operation of the lance.

Whilst the refractory sleeve cah be cast with the steel tube in situ as described above, it is also possible to pre-cast a concrete sleeve and to attach a steel tube therein subsequently in such a manner as to provide independent ability for expansion and contraction.

Because lance failure tends to occur where the lance is in contact with slag because slag is generally more penetrating than metal, it may be desirable to provide extra protection at that part of the lance which is expected to be most in contact with the slag. Accordingly, as a further refinement on the present invention, to help alleviate slag attack on the refractory at the vulnerable slag line, a preformed pre-cast high quality dense refractory sleeve can be set in that part of the lance close to the slag line. Whilst a preformed sleeve is likely to give the best results in casting a lance, a simiiar effect may be achieved by casting special high quality refractory in the slag line area of the lance.

Although there is no particular limitation on the type of refractory material which may be used, refractory castables of the tabular alumina type have been found to be particularly suitable.

The tabular member and protective sheath may be anchored to each other at any one point along their length, and Ms advantageous if the point of anchorage is towards the upper end of the lance since in certain circumstances the lower end of the lance may be destroyed in use.

It has also been found that lance life may be enhanced by applying a refractory material in liquid form to the exterior of the refractory sheath.

Suitable examples of such materials are silica containing materials such as mixtures of fused or colloidal silica and mixtures containing steelwork slag. Such coatings should be thoroughly dried before the lance is used.

Claims (18)

1. A lance for injecting material into molten metal comprising a tubular member in a protective sheath of refractory material, wherein the tubular member and the protective sheath are able to expand and contract longitudinally substantially independently of one another in response to temperature change.

2. A lance as claimed in claim 1 wherein the tubular member is of steel.

3. A lance as claimed in claim 1 or claim 2 wherein the protective sheath is of refractory concrete.

4. A lance as claimed in claim 3 wherein the protective sheath is of reinforced refractoryconcrete.

5. A lance as claimed in any preceding claim wherein the tubular member and the protective sheath are fixedly connected at one location along their length.

6. A lance as claimed in claim 5 wherein the said connection between the protective sheath and tubular member is provided by one or more radially extending members attached to the tubular member.

7. A lance as claimed in claim 6 wherein the protective sheath is of reinforced refractory concrete and the said radially extending member or members are fixed to both the tubular member and the reinforcement of the sheath.

8. A lance substantially as hereinbefore described with reference to and-as illustrated in Figure 5 of the accompanying drawings.

9. A method for producing a lance for injecting material into molten metal which method comprises casting refractory concrete over the surface of a tubular member to provide a protective sheath thereon, wherein means are provided on the surface of the tubular member to prevent the refractory concrete forming a bond with the tubular member, whereby the tubular member and the protective sheath are able to expand and contract longitudinally substantially independently of one another in respect to temperature change.

10. A method as claimed in claim 9 wherein a radially extending member or members are provided at one location along the length of the tubular member so that the protective sheath when cast is fixed to the tubular member at said one location along the length of the tube.

1 A method as claimed in claim 9 or claim 10 wherein the means on the surface of the tubular member whereby a bond with the refractory concrete is prevented is a refractory concrete impermeable sheathing.

12. A method as claimed in claim 11 wherein the sheathing is a plastics material.

13. A method for producing a lance for injecting material into molten metal which method comprises casting a refractory concrete tubular sleeve and introducing into the pre-cast sleeve a tubular member so that the sleeve forms a protective sheath for the tubular member and so joining the sleeve and tubular member that they are able to expand and contract longitudinally substantially independently of one another in response to temperature change.

14. A method as claimed in claim 13 wherein the precast sleeve is fired prior to the introducing of the tubular member.

1 5. A method as claimed in any one of claims 9 to 14 wherein the tubular member is of steel.

16. A method as claimed in any one of claims 9 to 15, wherein the casting is carried out utilizing a tubular cardboard mould.

17. A method for producing a lance substantially as hereinbefore described with reference to Figure 5 of the accompanying drawings.

18. A lance produced by a method as claimed in any one of claims 9 to 17.