EP0093039B1 - Process for increasing the gas-permeability of fire-proof elements in the bottoms of metallurgical vessels, and materials for carrying it out - Google Patents

Abstract

For improving permeability of a metallurgical vessel bottom provided with permeable refractory elements for controlled injection of a stirring fluid into a molten metal bath, the metallurgical vessel is emptied of its contents, a castable composed of a refractory material compatible with the refractory material of the bottom is deposited in the bottom and spread over the latter, and the castable is left to dry and set, while maintaining in the permeable refractory elements a sufficient pressure to provide a permanent flow of a stirring fluid. A hydraulic magnesian refractory castable used in this method has a content of water of substantially between 8 and 10% by weight.

Description

The present invention relates to the field of the production of metals, in particular steel. It relates more precisely to metallurgical vessels, particularly refining converters, the bottom of which is provided with permeable refractory elements.

Metallurgical treatments are known which consist in subjecting a bath of molten metal to pneumatic stirring by controlled injection of a stirring fluid, usually an inert gas such as nitrogen or argon, through permeable refractory elements. incorporated into the usual refractory lining of the container containing the bath and opening under the surface of the latter. More generally, these blowing elements are housed in the bottom of the container (FR-A-2.322.202, US no. 3.259.484).

The application of such a brewing technique to a converter of steelworks to refining oxygen blown from above is currently developing worldwide under the trade name "LBE process" (Lance - Brewing - Balance). This process tends to achieve, as its name suggests, the balance between metal and slag and thus allows to combine, to a large extent, the respective advantages of conventional refining processes with blowing of oxygen from above and oxygen blowing from below.

Numerous solutions have already been proposed aiming to give the refractory elements a sufficient selective permeability to ensure a satisfactory flow rate of stirring fluid, while avoiding penetration in the opposite direction of the molten metal. Among the various solutions proposed in this regard, or may in particular note that described in the published European patent application No. 0021861 and which consists in forming very small dimension passage zones in a usual compact refractory material. This is obtained either by incorporating longitudinal foreign bodies (blowing direction) within a monolithic refractory mass, or by juxtaposition of refractory plates with interposition of calibrated spacers between them.

Furthermore, these elements, like any refractory material, inevitably wear out in contact with the molten metal. This wear is further accelerated by the very gas blowing which causes very sensitive convection movements at the level of the blowing elements and whose induced effects are also felt over the service life of the surrounding conventional refractory. However, today we succeed, in particular thanks to the type elements mentioned above, in limiting their wear speed to little more than that of the conventional refractory lining constituting the bottom, which thus finds a life comparable to that that it presents in conventional converters with top blown oxygen (type LD).

Another problem arises in practice consisting in the fact that the permeability of the blowing elements tends to decrease during use. This phenomenon also appears somewhat paradoxical, because it accompanies the normal progressive wear of the bottom, and that we are therefore entitled to think that, the elements wearing out at almost the same speed as the bottom, their permeability should on the contrary increase over time, following a reduction in pressure losses in the blowing spaces.

If not to frequently replace the permeable elements when their permeability no longer makes it possible to pass the desired gas flow rates, (which would not only be highly penalizing, but would also take away all the interest provided by a lifetime of the elements equal to that of the background), the problem comes back to knowing if there is a simple, effective, and inexpensive method allowing to raise the level of permeability of these elements, without having to intervene directly on them, and in particular without having to replace them by new elements .

In order to provide a solution to this problem, the subject of the invention is a treatment method for improving the permeability of the bottoms of metallurgical vessels, in particular steel converters with top blowing oxygen blowing, said bottoms being provided with permeable refractory elements for the controlled introduction of a stirring fluid into the bath of molten metal contained in the container, process characterized in that, after emptying the container of its content at the end of the refining of any load, a concrete of refractory material compatible with the refractory material constituting the bottom is deposited in the bottom, said concrete having sufficient fluidity to ensure its spread over the bottom; and in that the concrete is left to dry and ensure its setting, while maintaining in the permeable refractory elements a sufficient pressure to provide a permanent flow of mixing fluid. not exceeding approximately 30 m ³ / h per element counted in gas _m ³ .

In accordance with a particular preferred operating method, a well pourable refractory concrete is prepared, capable of reaching the bottom of the container from the spout by flowing along the side wall; this concrete is poured into the container through the spout, the container being in an inclined position, for example in an intermediate position between the upright position and the completely tilted position which it presents at the end of casting of the molten metal, then it is straightened to vertical to ensure the distribution of the concrete on the bottom and the concrete is allowed to dry and ensure its setting, while maintaining in the permeable refractory elements a sufficient pressure to ensure a flow of mixing fluid.

If necessary, the container can be tilted on either side of its vertical position to perfect the spreading of the concrete on the bottom.

In what follows, it will be considered that the metallurgical container is a refining converter for oxygen blown from above by means of an emerging vertical lance, it being understood that the in vention also applies to any other metallurgical vessel, for example, ladles or arc furnaces.

In addition, it is appropriate to qualify as "concrete" both traditional concretes with cold hydraulic setting (temperature of use less than 100 ° C) as tarred refractory products, such as dolomite or tarred magnesia for example , therefore with a "carbon" bond and which is generally used between 130 and 180 ° C. approximately.

The expression "a concrete of refractory material compatible with the refractory material constituting the bottom" is intended to denote any refractory material capable, taking into account the nature of the bottom, of sticking to the latter during its solidification. It is for example a magnesian concrete if the bottom has a predominance in magnesia, or a dolomitic concrete if the bottom is based on dolomite, etc ...

In addition, by the expression "well pourable refractory concrete" is meant to qualify a preparation of the latter which makes it more fluid than the fluidity which would result from a preparation in accordance with the prescriptions of the concrete manufacturer. In other words, it is generally a question of making this concrete very moist, that is to say containing an excess of water compared to the usual prescriptions so as to achieve a content of the order of 10% by weight.

In addition, it is clear that the more the water is in excess the longer the drying time. On the other hand, the lower limit of the humidity rate to be adopted must take into account the capacity, that is to say the size of the container, in particular its height and the diameter of the bottom, as well as its thermal mass, so that the concrete can, by being introduced by the upper open end (the spout) reach the bottom, then once the bottom is reached, spread there before solidifying.

To fix the ideas, series of tests carried out on a 240 t converter have shown that the water content is preferably between 8 and 10% by weight, that is to say 1 to 2 points more than that recommended by the manufacturer as much as possible (up to 7%, but more generally between 3 and 6%).

• I. Béton magnésien hydraulique
  
  H₂0: 8 à 10% du poids du béton où R₁0₃ représente l'ensemble des oxydes présents de métaux tels que AI, Ti, Cr ...
• Il. Béton magnésien goudronné
  
  Goudron: 10% en poids du béton
• III. Béton dolomitique qoudronné
  
  Goudron: 10% du poids du béton.

We will now give three examples of concrete weight compositions that can be used according to the invention. The first two are intended to cover a bottom of converter in magnesia bricks, the last can be provided for a dolomitic bottom.

• I. Hydraulic magnesian concrete
  
  H ₂ 0: 8 to 10% of the weight of the concrete where R ₁ 0 ₃ represents all the oxides present in metals such as AI, Ti, Cr ...
• He. Asphalt magnesium concrete
  
  Tar: 10% by weight of the concrete
• III. Powdered dolomitic concrete
  
  Tar: 10% of the weight of the concrete.

As can be seen, the method according to the invention is simple, inexpensive, and poses no uncontrolled difficulty. The presence of permeable refractory elements housed in the bottom does not imply any other requirement during the drying of the concrete than that which consists in maintaining through them a minimum flow of the mixing fluid, a flow which can be qualified " of security".

In addition, this flow which can be considered lost (that is to say not used for the treatment of the bath itself) increases only very slightly the overall cost of the operation, given its value relatively minimal compared to that used during the mixing of the bath (of the order of 150 m ³ / h per element). One can even say that the consequences on the cost are practically negligible, if one takes care to choose a gas widely available on the market, like nitrogen for example, or if necessary, a recovery gas produced in the factory it - even, like CO ₂ .

Once dried, the concrete is in mechanical engagement on the bottom and forms a refractory layer which can reach in the central area, an average thickness of between 5 and 20 cm approximately (240 t converter). The converter is then ready for processing new loads. We notes from the first treated load that not only is the permeability of the bottom preserved, but that it has also very significantly increased compared to the level it had before the concrete was added.

It is recalled for all practical purposes, that a possible indicator of the "level" of permeability can be constituted by the pressure / flow ratio of stirring fluid in the supply line of the latter to the permeable refractory element. This ratio can be determined from a reference value, the permeable element being taken in new condition with vacuum blowing or during the refining of the first charge to the converter.

• - l'observation semble montrer, que la préservation de la perméabilité est asurée par la présence d'un réséau de canaux reliant la face de soufflage de l'élément à la surface libre au fond au travers de la couche de coulis rapportée, ce réseau se formant lors du séchage de cette couche grâce au soufflage permanent du fluide de brassage.
• - quant à l'amélioration de cette perméabilité, il pourrait s'agir d'un phénomène interne à l'élément réfractaire perméable lui-même. On est en effet en droit de penser que l'origine se trouve vraisemblablement dans les effets de choc thermique provoqués au sein des éléments de soufflage par la coulée de la masse de béton froid (température inférieure à 100°C ou à 200°C selon la nature du béton) et amplifiés encore par le flux permanent de fluide de brassage. On peut supposer que les cntraintes thermiques qui en résultent au sein des éléments de soufflage par contraction de la matière provoque, en se libérant, une formation de micro-fissures s'amorçant préférentiellement en paroi des passages originels prévus pour le fluide de brassage.

The explanation of the results obtained is not yet fully understood:

• - the observation seems to show, that the preservation of the permeability is ensured by the presence of a network of channels connecting the blowing face of the element to the free surface at the bottom through the added grout layer, this network forming during the drying of this layer thanks to the permanent blowing of the stirring fluid.
• - As for the improvement of this permeability, it could be a phenomenon internal to the permeable refractory element itself. We are indeed entitled to think that the origin is probably in the thermal shock effects caused within the blowing elements by the pouring of the mass of cold concrete (temperature below 100 ° C or 200 ° C depending the nature of the concrete) and further amplified by the permanent flow of mixing fluid. It can be assumed that the thermal stresses which result therefrom within the blowing elements by contraction of the material causes, when released, the formation of micro-cracks preferentially initiating on the wall of the original passages provided for the stirring fluid.

These assumptions are based, among other things, on the fact that there is a statistically greater improvement in the permeability of these elements when the entire mass of liquid concrete intended to cover the bottom is poured quickly, all at once. the container (this procedure, moreover, constituting a preferred implementation of the invention).

On the other hand, it was noted, taking into account the significant thermal mass of the bottom, that the temperature of the concrete brought did not influence. sensitive way on permeability.

But we can also think of a purely aeromechanical explanation, the mixing fluid being able to circulate on the one hand laterally in zones of least pressure drop which possibly form at the interface of the deposited concrete layer and the pre-existing refractory bottom.

The technique according to the invention can be implemented at any time, either between two refining campaigns, or between two charges of the same campaign, or even before the first charge, on a converter in the state new.

It is understood that incidentally the invention also provides repair or renovation of the worn funds.

Furthermore, the invention applies whatever the type of permeable refractory elements mounted in the bottom. It should be emphasized, however, that excellent results have been obtained with elements such as those mentioned at the beginning, and of which we can have more knowledge by referring to the detailed description which is made of it in European patent application no. 0021861 already cited.

Claims (4)

1. A treating method for improving the permeability of the bottoms of metallurgical vessels, in particular top-blowing steel converters, said bottoms being provided with permeable refractory elements for controlled injection of a stirring fluid into the molten metal bath, characterized by the steps of depositing in the bottom of the vessel, after it has been emptied of its contents, a concrete made of refractory material compatible with that of the bottom, said concrete having a liquidity sufficient to ensure its spread over the surface of the bottom, and of letting the concrete dry and set while maintaining in the permeable refractory elements a pressure sufficient for obtaining a permanent delivery of stirring fluid not exceeding approximately 30 M3 /h per element, calculated in _M ³ of gas.

2. A method according to claim 1, characterized in that a refractory concrete is prepared which is readily pourable i.e. capable of reaching the bottom of the vessel, from the spout, flowing along the lateral wall; in that, after the vessel is emptied of its contents, this concrete is poured into the vessel via the spout, the vessel being in a tipped position, it is then rectified to the vertical to assure the spread of the concrete over the bottom, and it is allowed to dry and set while a pressure sufficient for obtaining a permanent delivery of stirring fluid is maintained in the permeable refractory elements.

3. A method according to claim 2, characterized in that, after the concrete is poured into the vessel, the latter is tipped from one side to the other from its vertical position in order to complete the spread of the concrete over the bottom.

4. A method according to claim 2, characterized in that the concrete used is a hydraulic magnesian refractory concrete having a content by weight of water between 8 and 10%.