EP0180491B1 - Refractory-lining repair process for a metallurgical vessel by flame gunning - Google Patents

Abstract

1. Process for repairing the refractory lining of a metallurgical vessel by spraying a refractory Gunite material through a flame, characterized in that two superposed layers of the same material are placed in position, the first layer having, on the one hand, a total porosity of 15 to 25% so as to promote its adhesion to the lining by impregnation with the stag present at the surface of the said lining and, on the other hand thickness such that all the stag impregnates the said first layer, the second layer having an apparent density of at least 3.1 kg/dm**3 in order to promote the resistance of the sprayed material against chemical and mechanical attacks by the molten bath which will subsequently be formed in the said vessel.

Description

The invention relates to the repair of the refractory lining of a metallurgical container (in particular a steelworks converter or a transfer and / or treatment pocket) by spraying through a flame a gunning material.

It is known that the material projecting through a flame must contain at least two types of constituents, a first - the main aggregate, of the same nature as that of the material making up the coating to be repaired - which remains solid through the flame and which constitutes the reinforcement of the sprayed product, and a second - the binder: voluntary addition or impurities present in the product - which serves as cement for the main aggregate. The material is delivered in powder form to a burner, which is advantageously a "multidard" burner as described in the French patent document FR-A-2 533 020 in the name of the Applicant.

It will be recalled that the main aggregate is conventionally a magnesian (based on MgO and CaO) or aluminous (based on alumina AI ₂ 0 ₃ ) material. The binder, in the first case, is generally based on phosphates or sodium or calcium silicates, whereas in the case of an aluminous aggregate, a binder based on calcium aluminates is usually used. It is also known that a binder consisting of oxygen converter slag (LD, OLP, etc.) can be suitable for both a magnesium aggregate and an aluminous aggregate.

It has been observed that the behavior of the conventionally sprayed layers is often not satisfactory when the container is put back into service following the spraying: in some cases, the sprayed product detaches from the wall, in other cases, it undergoes strong chemical corrosion and mechanical erosion from the molten metal bath contained in the container.

The object of the invention is to find a remedy for the poor behavior of the layers projected in the conventional manner.

This object is achieved according to the invention by the fact that one puts in place, by projection of a refractory material of gunning through a flame, two superimposed layers of the same material, the first layer having, on the one hand , a total porosity of 15 to 25% in order to promote its adhesion to the coating by impregnation with the slag present on the surface of this coating, and on the other hand, a thickness sufficient to be able to be impregnated with all of this slag, the second layer having an apparent density of at least 3.1 kg / dm ^{3 in} order to promote the resistance of the sprayed material to chemical and mechanical attack by the molten bath which the metallurgical container will contain.

The inventors have in fact discovered that the quality of the sprayed layer depends closely on its density (and therefore vice versa, on its porosity). They were able to show that when the sprayed layer is very dense, there is no reaction between the sprayed product and the slag layer remaining on the coating to be repaired. It follows that during the metallurgical operation (refining or bag treatment) following the projection, the intermediate layer of slag melts and causes the detachment of the projected layer.

When, on the contrary, the sprayed layer is porous, there is reaction and interdiffusion between the slag and the sprayed layer, which ensures the latter a good adhesion by impregnation by the slag. Unfortunately, the porosity of the layer makes it vulnerable to chemical corrosion and mechanical erosion.

According to the invention, an excellent behavior of the sprayed product is obtained by first spraying a porous layer, intended to ensure adhesion by impregnation with the slag, then over, a dense layer, intended to ensure the high resistance to corrosion. and subsequent erosion by the molten metal bath.

For a good understanding of the invention, it is recalled that the term “total porosity” designates the ratio of the volume of the pores (open and closed) to the total apparent volume (i.e. matter + pores). This ratio is usually multiplied by 100 in order to be able to express the total porosity in percentages. For flame gunning products, the porosity is almost exclusively open. "Apparent density" is the ratio of the mass of the body to its apparent volume. For the same material, apparent density and porosity are linked by the following relationship: the apparent density is equal to the product of the absolute density by the difference 1-porosity.

Porosity is a geometric characteristic independent of the nature of the material, unlike density. Strictly speaking, it is therefore necessary to compare the porosities and not the densities of various products. However, in the remainder of this thesis, taking into account the fact that the products studied have close absolute densities, we will use either concept.

• - en figure 1, l'influence de la teneur en liant présent dans le produit projeté sur sa porosité,
• - en figure 2, le rôle de la puissance spécifique sur la densité apparente et la porosité totale des couches projetées ;
• - en figure 3, l'influence de la teneur en liant sur la densité apparente des produits projetés ;

en figure 4, la puissance nécessaire pour obtenir une densité apparente donnée en fonction d'une teneur en liant.Other advantages and characteristics will appear on reading the following description, where reference will be made to the attached graphics which represent:

• in FIG. 1, the influence of the content of binder present in the sprayed product on its porosity,
• - in Figure 2, the role of the specific power on the apparent density and the total porosity of the projected layers;
• - In Figure 3, the influence of the binder content on the apparent density of the sprayed products;

in FIG. 4, the power necessary to obtain a given apparent density as a function of a binder content.

The inventors have established in FIG. 1 a graph showing on the ordinate the porosity of the projected product (magnesium aggregate MgO-CaO) expressed in%, and on the abscissa the content of binder (calcium phosphate) expressed in% by in relation to the weight of the aggregate. The experiment was carried out with a specific gunning power constant at 2.5 kW / kg. The graph shows that there is a direct relationship of inverse proportionality between the binder content and the porosity.

Experiments carried out in an experimental furnace, as well as in a steel ladle and in a converter, made it possible to determine the relationship between the specific power (quantity of heat supplied by the burner per kilogram of powder of gunning material) and the density or, conversely, the porosity of the sprayed layer.

The graph in FIG. 2 was obtained for a product based on MgO, CaO containing 7.5% of binder consisting of calcium phosphate.

When the specific power is close to 0, the apparent density is that of the product sprayed cold; its value is close to that of the bulk product (density = 1.50 or about 60% porosity). When the specific power increases, the apparent density increases linearly to a maximum of 3.30 (or about 7% of porosity) for a power of 6 kW / kg, then stabilizes at this value when the specific power still increases.

This value of 3.30 in fact represents the maximum possible value that the density which is measured when cold can reach; the difference between this value and the theoretical density (3.55) of the product seems to correspond to the shrinkage due to the volume contraction of the product during its cooling.

Thus, for this type of material, it is useless to operate with a specific power greater than 6 kW / kg.

The linear relationship between apparent density and specific power reflects the fact that when the latter increases (and therefore the particle concentration in the flame decreases), the number of binder particles reaching their melting temperature increases due to a better heat exchange in the flame (convection) and between particles (radiation).

To increase the density, while reducing energy consumption, tests have been carried out on products having various contents of bonding elements. FIG. 3 gives the results obtained on products containing 2.5 - 7.5 - 12 - 15 and 20% of binder.

When the binder content increases, there is a shift in the maximum allowable specific power towards low values for a density of 3.30, the maximum is close to 7.3 kW / kg for a binder content of 2.5 %, and is only 3 kW / kg for 20% of binder. By combining these data, it is possible to define, for a type of product and according to its composition, the specific power necessary to reach the desired density value d (fig. 4).

It is clear from FIG. 4 that for low values of specific power, a small variation in power is sufficient (by variation of the powder flow rate for example, to cause a significant variation in density) whereas for more powerful identical variation has less marked effects on density.

A first conclusion is that increasing the specific power is a good way to improve the homogeneity of the projected layers, but it also results in an increase in the energy cost.

A second conclusion is that it is possible to carry out the deposition, in accordance with the invention, by varying the specific power of the burner and in particular by varying the powder flow rate.

• - avant projection, réchauffage de la paroi à réparer à l'aide du brûleur pour en éliminer le laitier superflu (si l'épaisseur de laitier est supérieure à quelques millimètres) ;
• - en début de projection, mise en place d'une couche de quelques millimètres (avantageusement 5 mm, épaisseur optimale pour permettre, par diffusion, une imprégnation de toute la couche par du laitier sans compromettre la résistance de l'ensemble) de produit à porosité comprise entre 15 et 25 % (un écart avec cette gamme conduite soit à des risques de décollement en-deçà de 150/0, soit à une usure plus rapide et à des écaillages au-delà de 25%);
• - ensuite, projection d'un produit très dense (3,1 à 3,3 kg/dm³), la différence de densité étant obtenue en faisant varier le débit de poudre dans le brûleur. L'épaisseur de cette deuxième couche est de 1 à 2 cm en projection classique mais peut atteindre, si on le souhaite, des valeurs supérieures sans inconvénient pour l'invention.

Thus, in its most advantageous embodiment, the method of the invention comprises the following steps.

• - before projection, reheating of the wall to be repaired using the burner to remove excess slag (if the slag thickness is greater than a few millimeters);
• - at the start of the projection, placement of a layer of a few millimeters (advantageously 5 mm, optimal thickness to allow, by diffusion, an impregnation of the entire layer by slag without compromising the resistance of the whole) of product to porosity between 15 and 25% (a deviation from this range leads either to risks of delamination below 150/0, or to faster wear and flaking beyond 25%);
• - then, projection of a very dense product (3.1 to 3.3 kg / dm ³ ), the difference in density being obtained by varying the powder flow rate in the burner. The thickness of this second layer is 1 to 2 cm in conventional projection but can reach, if desired, higher values without disadvantage for the invention.

It is pointed out that the recommended thickness of approximately 5 mm for the first layer is in the middle of the range of determined values going from 3 to 8 mm. In fact, below 3 mm, the layer may be insufficiently thick to incorporate, by diffusion, all of the slag, so that detachments are to be feared following a melting of the residual slag at the interface between the pre refractory lining -existent and the projected layer. Beyond 8 mm, on the contrary, the slag may be in insufficient quantity to impregnate the layer throughout its thickness, which can lead to detachments due to the poor mechanical resistance of the surface portion not impregnated with the sprayed layer. .

• - dans le cas des poches de transfert ou de traitement, le facteur prépondérant étant les chocs thermiques répétitifs dus à la fréquence relativement élevée des opérations de remplissage et de vidange, on choisira de préférence des valeurs de densité moins élevées, i.e. de 3,1 à 3,2 kg/dm3 ;
• - à l'inverse, pour les convertisseurs d'affinage, le facteur d'usure prépondérant étant l'érosion à la fois chimique et mécanique de la part du bain en fusion, on optera pour les densités les plus élevées, i.e. supérieures à 3,2 kg/dm³.

Regarding the density range recommended for the second layer (between 3.1 and 3.3 kg / dm ³ ), it is indicated that:

• - in the case of transfer or treatment bags, the predominant factor being repetitive thermal shocks due to the relatively high frequency of filling and emptying operations, lower density values will preferably be chosen, ie from 3.1 to 3.2 kg / dm3;
• - conversely, for refining converters, the predominant wear factor being both chemical and mechanical erosion on the part of the molten bath, we will opt for the highest densities, ie greater than 3 , 2 kg / dm ³ .

As for the thicknesses recommended for this second layer, they are dictated in a nonlimiting manner only by production requirements. Below 1 cm in thickness, repetitive repairs may become too frequent compared to the usual practice which is about one repair every 8 h. Beyond 2 cm, taking into account the powder flow limits, the repair time may exceed the 10 to 15 minutes generally available in the production cycle at the steelworks.

Finally, to achieve the high specific powers necessary for the explanation of this second high density layer, it is strongly advised to use the gun of gunning described in the French patent application n ° 82/15261 already cited and schematically constituted by a bundle of numerous small diameter joined tubes acting like so many small burners and each receiving at their outlet end an equal fraction of the total powder flow rate.

Claims (9)

1. Process for repairing the refractory lining of a metallurgical vessel by spraying a refractory Gunite material through a flame, characterized in that two superposed layers of the same material are placed in position, the first layer having, on the one hand, a total porosity of 15 to 25 % so as to promote its adhesion to the lining by impregnation with the stag present at the surface of the said lining and, on the other hand thickness such that all the stag impregnates the said first layer, the second layer having an apparent density of at least 3.1 kg/ dm³ in order to promote the resistance of the sprayed material against chemical and mechanical attacks by the molten bath which will subsequently be formed in the said vessel.

2. Process according to Claim 1, characterized in that the thickness of the said first layer is between 3 and 8 mm.

3. Process according to Claim 2, characterized in that the said thickness is 5 mm.

4. Process according to Claim 1, for the repair of the refractory lining of a metallurgical transfer ladle or treatment ladle, characterized in that the apparent density of the second layer does not exceed 3.2 kg/dm3.

5. Process according to Claim 1 for the repair of the refractory lining of a steelworks converter, characterized in that the apparent density of the second layer is at least 3.2 kg/dm³.

6. Process according to Claim 1, 2 or 3, characterized in that the thickness of the second layer is between 1 ans 2 cm.

7. Pricess according to Claim 1, characterized in that the said two superposed layers are placed in position by means of a gun for applying Gunite, which gun consists of a bundle of adjoining tubes, each receiving at its outlet end an equal proportion of the total flow of Gunite material in powder form.

8. Process according to Claim 1, characterized in that the density of the superposed layers is varied by varying the specific power of the Gunite application.

9. Process according to Claim 8, characterized in that the specific power of the Gunite application is varied by varying the flow rate of the Gunite material.

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