GB2130695A - Insulation of ladles - Google Patents

Abstract

A method of providing a refractory, insulating lining 16 on the working in a surface of a ladle 19 which comprises applying a layer of refractory, insulating composition to the ladle surface as a slurry from a sprayhead 23 or else by trowelling and setting the slurry to form the lining. The lining composition may be applied to a permanent refractory lining or to a refractory porting layer of ceramic fibrous material. <IMAGE>

Description

SPECIFICATION Insulation of ladles This invention relates to methods of providing an insulating refractory lining in a ladle for molten metal.

Molten metal is traditionally transported from its place of melting where it is to be discharged in a refractory lined ladle.

Molten iron and steel is typically carried in ladles lines with either acid or basic refractory.

The thickness of the refractory lining varies according to a number of factors including the type of molten metal being carried, the size of the ladle, the time the molten metal is held in the ladle and an acceptable working temperature of the containing ladle steel shell. Another important if not overriding consideration is the provision of adequate refractory thickness to ensure the safe operation of the ladle i.e. adequate thickness in that refractory layer which has direct contact with the molten metal and the usually steel shell of the ladle itself.

The working lining of the ladle is worn away with use and it is replaced at such intervals as will ensure safe operation of the ladle, often together with the back-up or safety lining.

The refractory lining of the ladle is perforce of a dense material and of a relatively high heat capacity so that if molten metal is poured into a ladle, which when empty has been at ambient temperature, a temperature drop will occur in the metal as heat is transferred from it to the refractory lining and the steel shell. In order to compensate for this temperature drop the temperature at which the molten metal is poured into the ladle is raised which can result in undesirable metallurgical effects as well as adding to both the costs of melting and the time taken to achieve the desired temperature.

To help overcome the heat loss to the refractory lining it is common practice to heat the empty ladle prior to its use to carry molten metal.

This is often done with oil or gas fired burners sized to give the appropriate heat output into the ladle.

A pre-heating system for a typical steelworks steel carrying ladle of about 1 OOT steel capacity is a burner arranged to send a flame into the ladle through its top when the ladle is laid on its side.

For ladles of over 1 SOT, capacity the burner is usually employed with the ladle upright. Whilst ladle pre-heating does enable a better control to be maintained of the temperature of the molten metal poured into the ladle it is generally expensive of energy and time. Generally it takes some hours for a ladle on a pre-heater to be brought up to the required temperature for use.

Whilst pre-heating of ladles has been shown to be advantageous in decreasing the initial temperature drop in the molten metal in the ladle it has no effect of the heat losses from the molten metal which subsequently occur through the refractory lining to the steel shell and to the atmosphere.

The refractory lining is generally a poor insulator so that the heat losses through it from the molten metal can be significant during the residence time of the molten metal in the ladle. To help alleviate this heat loss an insulating refractory lining can be provided over the safety or back-up lining which in turn lies over the steel shell. For a typical steel ladle, this insulating lining might be of refractory insulating bricks.

From the metallurgical point of view a disadvantage of a refractory lining which wears away in service is the possibility that pieces of the lining will stay in the molten metal thus forming undesirable non-metallic inclusions.

This invention is concerned with a method for the lining of ladles to form a working surface of a refractory insulating material. Refractory, insulating material can have a low thermal capacity so that pre-heating of the ladle may be avoided. The refractory insulating material can if desired be so formed as to enable it to be replaced easily after one or a few uses of the ladle thus keeping the inside of the ladle clean and obviating any refractory pick-up by the molten metal thus giving the possibility of having cleaner metal.

For some time tundishes used in the continuous-casting of steel have been lined with refractory insulating boards generally 25 mm to 40 mm thick to give an effectively insulated bath of steel feeding the continuous casting machines.

These boards enable the bath of molten metal to be controlled to close temperature limits during casting with advantage to the steel cast. With reasonable practice at the end of each cast the tundish boards fall out on inversion of the tundish leaving a permanent refractory lining over which to construct the next refractory/insulating board lining for the next cast. In certain steel foundry ladle applications the practice has grown using a refractory/insulating board lining in a similar way.

These are usually fixed in position by a wedging action of the pieces forming the inner lining. The joints between the boards are generally sealed with a suitable refractory cement to inhibit metal penetration. An infill of free flowing refractory material is used to fill any gaps between the boards and the permanent lining. Sand is often used. This can be firmed into position if desired by using a mixture of sand and sodium silicate and applying CO2 to the layer. These linings are quick and easy to fit, require no pre-heating and are typically used once or twice before the whole ladle is inverted the lining tipped out and replaced. Board thickness is typically 25 to 32 mm.Experience has shown that the use of these formed linings has given significant advantages in that ladle pre-heating has been eliminated the heat loss from the molten metal whilst it is resident in the ladle has been considerably reduced. In some cases this has resulting in the tapping temperature from furnace to ladle being reduced by 200C with resultant energy and time savings and metallurgical advantages. There have also been recorded significant improvements in metal cleanliness, i.e. the number of non-metallic inclusions in the cast metal has been reduced as compared with the previously used refractory lined ladle. With no ladle pre-heating required and the speed of putting in a new formed lining ladle availability has been greatly improved as have ladle preparation area environmental conditions.

The formed board refractory/insulating linings have been used generally on foundry ladles between typically 0.5 to 5T capacity. As ladle size increases towards typical steelworks sized ladles from say 50T to 300T the use of formed board linings becomes somewhat less attractive due to the more difficult and time consuming installation in the larger ladles. One reason for this is that unlike for the smaller foundry sized ladles where a single board can be easily made to stretch from the top to the bottom of the ladle in the case of the larger ladles this is generally not feasible so that several layers of boards have to be used.

With layered construction steps may have to be taken to ensure that the boards do not float up with the molten metal as the ladle is filled. One approach, which I believe to be novel would be to assist the wedging action of the boards by placing restraining hoops e.g. of steel or other suitable material tightly against the rings of refractory insulating boards.

The joints between the boards are sealed with a suitable refractory cement to inhibit metal penetration. Any gaps between the back of the boards and the ladle permanent refractory lining can be filled with a suitable refractory infill. If sand is used it can be held in place with sodium silicate and CO2 as described above. Alternatively a more granular refractory infill can be employed.

The formed board lining in the large ladles can be used in the same way as for the smaller foundary ladles with the same inherent advantage. However, it may be deleterious from a metallurgical point of view to have a loose infill material as described between the boards and the permanent lining. In these cases an alternative parting material such as a covering of ceramic fiber sheet can be used.

For the larger steelworks type ladle it may be desirable to have a faster and less cumbersome method of applying a working lining of refractory/ insulating properties.

The present invention now provides a method of providing a refractory, insulating lining on the working inner surface of a ladle which method comprises applying a layer of refractory, insulating composition to at least one inner ladle surface as a slurry and drying or setting the slurry to form the lining.

The slurry may be applied by such methods as spraying or "slinging" or by trowelling.

In a preferred method of spray application the solid components of the composition are blown as a powder to a spray head and are mixed with water to make the slurry immediately before issuing from a spray nozzle.

Not all refractory materials capable of being applied by these methods will provide a lining that is "insulating" i.e. has a sufficiently low chemical capacity and a sufficiently low conductivity. A low density material is required, i.e. one having a density in the finished state of 2 g/cm3 or less, eg.

1.7 g/cm3 or less, preferably 1.4 g/cm3 or less.

The slurry may be applied directly over a permanent refractory lining.

Alternatively an intermediate refractory lining may be provided between the slurry layer and the permanent refractory lining to prevent the slurry layer becoming bonded to the permanent lining.

The working refractory/insulating lining may then be removed after each ladle used or after a number of uses. Because of the presence of a surface between the sprayed trowelled lining and the permanent ladle lining the inner lining may easily be tipped out as required. This situation may be achieved by placing a lining of thin refractory material of high melting point between the two. A continuous lining of ceramic fibre would serve this purpose. This must be contiguous to but not fixed to the permanent lining. The refractory insulating mix is then applied, e.g. sprayed or slung on the ceramic sheet or other suitable material covering the ladle permanent lining.

The thickness of the lining applied as a slurry can be varied to suit the ladle operating conditions but will generally lie between 25 and 40 mm.

The base of the ladle can either be coated with slurry mix or covered with preformed refractory insulating boards or left uncoated.

The mix applied on can be formulated to meet each specific plant ladle operating condition and can be made to give smaller refractory/insulating properties as the formed boards.

The binder in the mix will preferably be such that the mix will air dry in place. As an example of this a magnesite based mix can be mixed with sodium silicate through a continuous type foundry mixer and slung on to the ceramic fiber sheath to form the inner working lining. This mix will air dry reasonably quickly to allow the ladle to be used. For certain mixes it may be advantageous to employ a small amount of heat into the ladle to harden off the refractory/ insulating layer before it is used.

After the ladle has been used the inner lining plus ceramic fibre sheath and any residue metal or slag can be tipped out leaving a clean refractory permanent lining for the deposition of the next ceramic fibre/refractory/insulating inner layer. Alternatively the ladle may if appropriate be used again. Should this be the case it may be desirable to apply a further amount of new refractory/insulating mix to those areas of the ladle requiring it. This can be done over a number of ladle lives until it is deemed prudent to tip out the inner lining and start again. The use of the sprayed or trowelled on lining gives the advantages in ladle operation as previously enumerated.

For a trowelled application the ceramic fibre or other suitable sheath may be used as before but the appropriate refractory/insulating mix is trowelled on and dried as before.

The use of a refractory/insulating lining to the ladles give a much cooler ladle after use than with a typical refractory lining. This means that the ladle can be worked on more quickly and that working conditions are improved.

One advantage of replacing the refractoryl insulating frequently is the likely improvement in metal quality with fewer inclusions resulting from a cleaner ladle. Where this is perhaps of not overriding importance the inner refractory/insulating lining can be applied, e.g. sprayed or trowelled direct on to the permanent refractory ladle lining without an intermediate parting layer. With this method the ladle is sprayed up with fresh mix as required after each use to maintain a satisfactorily performing refractory/insulating layer.

The present invention includes ladles lined by the method of the invention and metal castings produced using such ladles.

The invention will now be illustrated by the following description of a preferred embodiment with reference being made to the accompanying drawing in which Figure 1 shows a perspective cut away of a ladle lined with boards in the manner taught by the prior art; Figure 1 A shows a plan view of the ladle of Figure 1.

Figure 2 shows a large ladle in cut-away perspective view, lined with several layers of insulating refractory boards in accordance with the prior art.

Figure 3 shows portion of the lining of the ladle of Figure 2 but modified in a novel manner to mitigate its disadvantages.

Figure 4 shows a schematic view of apparatus for the application of a lining to a ladle by the method of the present invention.

Figure 5 shows an alternative apparatus for use in the present invention.

Figure 1 shows a typical arrangement for providing an insulating refractory lining in a ladle as presently used. The ladle shell 1 2 is first lined with a permanent protective refractory (but not insulating) lining 11. Boards 9 of insulating refractory material are arranged to line the interior of the sides and base of the ladle to provide a working lining and the spaces left behind the boards because of the curvature of the ladle walls is filled by infill 10, e.g. sand or sand and sodium silicate.

Figure 2 shows the same method extended to deal with a large ladle. There are now three layers of boards 9 required to line the ladle walls and there is a danger of some boards becoming detached in use.

Figure 3 shows one method I have devised for overcoming this. Hoops 13 of steel or some other suitable material are arranged tightly over the centre of each row of boards 9 to hold the boards against the infill and permanent refractory lining.

Figure 4 shows the method of providing an insulating refractory lining of the present invention. A suitable slurry is supplied from a mixing station 20 by a pump 21 through a pipe 15 to an applicator 14. Applicator 14 comprises a hollow shaft 22 terminating in a rotatable spray head 23. Spray head 23 is inside a ladle having a steel shell 19 and a permanent refractory lining 1 8. A parting layer 1 7 of ceramic fibre or shown lying over the permanent lining 1 8.

As spray head 22 sprays the insulating refractory mix it rotates and is gradually raised and lowered in the ladle so that a layer 16 of insulating refractory mix is built up over the ladles walls.

As shown in Figure 5, in an alternative spray technique, powder in a hopper 25 is mixed with air or other gas in a chamber 26 and the powder is thereby blown through pipe 15 to the spray head 23. A separate feed pipe 24 conveys water to the spray head. Mixing of water and powder taken place immediately before spraying in the spray head.

The refractory mix may also be sprayed onto the base of the ladle or else a refractory insulating board may be provided on the base.

Alternatively the refractory mix may be applied by trowelling instead of spraying.

Examples of suitable refractory insulating mixes for spray application are as follows Percentage Formulation 1 of weight Alumino silicate grog 78.0 Expanded perlite 4.5 Paper pulp 2.5 Sodium silicate 5.0 Clay 3.0 Water 7.0 Percentage Formulation 2 of weight Crushed magnesite 78.0 Expanded perlite 5.0 Paper pulp 2.0 Sodium silicate 7.0 Water 8.0 The % of water can be adjusted to give the desired consistency of application. Thus the trowelling mixes may be as shown but with say 2% less water in each case.

Claims (10)

Claims

1. A method of providing a refractory, insulating lining on the working inner surface of a ladle which method comprises applying a layer of a refractory, insulating composition to at least one inner ladle surface as a slurry and drying or setting the slurry to form the lining.

2. A method as claimed in claim 1 wherein the slurry is applied by spraying or by trowelling.

3. A method as claimed in claim 1 or claim 2 wherein the slurry is applied over a refractory parting layer which lies on but is not fixed to a permanent refractory lining of the ladle.

4. A method as claimed in claim 3 wherein the parting layer is of ceramic fibre.

5. A method as claimed in any one of the preceding claims wherein the slurry comprises, as a binder, an alkali metal silicate.

6. A method as claimed in any one of the preceding claims wherein the slurry comprises magnesite as a refractory.

7. A method as claimed in any one of claims 1 to 4 wherein the slurry has substantially the composition given as Formulation 1 or Formulation 2 above.

8. A method as claimed in any one of the preceding claims wherein the setting of the slurry is aided by the application of heat.

9. A method of providing a refractory, insulating lining substantially as hereinbefore described with reference to and as illustrated in Figure 4 of the accompanying drawings.

10. A ladle lined by a method claimed in any one of the preceding claims.