EP0422192B1 - Process for coating a metallurgical container with a purifying layer and composition relating thereto, and protective coating thus obtained - Google Patents

Abstract

A composition containing an alkaline-earth metal carbonate together with a certain concentration of refractory grain material is applied to the inner wall of a metallurgical container. Said coating is then heated so as to eliminate the CO2 and obtain an oxide layer of said alkaline-earth metal with a porous structure. The concentration of refractory material is sufficient to ensure that the coating is able to withstand the erosion of the liquid metal cast in the container. Use: for obtaining a purifying coating with a high resistance to erosion.

Description

The present invention relates to a process for coating a metallurgical container, such as a tundish or ladle, with a purifying coating.

The present invention also relates to a use, for the implementation of the above process, of a composition known per se.

By purifying coating is meant a coating capable of reducing pollution of the liquid metal by impurities such as oxides.

To limit the oxidation of the liquid metal, means are currently used such as pouring tubes interposed between the ladle and the tundish and covering powders disposed on the surface of the molten metal.

However, these means do not make it possible to completely reduce the pollution of the metal bath.

DE-C-966 136 discloses a method for producing a basic insulating-refractory brick having a porosity greater than 45%, by compression and baking of a raw mass containing a natural carbonate. According to this method, a base mass comprising a large proportion of fine-grained constituents, such as chromite, magnesia, zirconia, etc., is mixed with a coarser particle size carbonate containing approximately 50% by weight of CO₂, then , if necessary after forming, the base mass is cooked, until a sintering or a ceramic bond is obtained.

Such a brick is adapted to be placed between the metal frame of a furnace and the basic refractory lining of this furnace. It is not suitable for being brought into contact with molten metal.

JP-A- 86-33743 also discloses a refractory material in the form of plates comprising two layers, an interior purifying layer of limestone material containing at least 10% by weight of CaO or CO₃ Ca or the mixture of two, and an outer layer in contact with the permanent coating and containing at least 80% MgO.

It is known that the installation of these plates inside a hot distributor is both delicate and painful work given the high temperatures encountered.

Also known are coatings for metallurgical vessels, in particular pouring distributors, obtained by spraying a composition in the form of aqueous mud containing refractory particles, such as SiO₂, Al₂O₃, MgO or a mixture thereof, and an organic or inorganic binder. These particles sinter under the effect of the heat of the liquid metal poured into the container, which ensures the cohesion of the coating. However, such coatings do not have a purifying function with respect to the liquid metal, some being even polluting.

There is also known a coating obtained by applying baked dolomite, that is to say essentially CaO and MgO in the form of grains mixed with a binder such as for example a thermosetting resin.

Coatings obtained by applying raw dolomite or calcium carbonate are also known.

After application of these coatings, they undergo heating which decomposes the carbonate and / or the binder. There thus remains a porous layer essentially consisting of CaO and / or MgO having a density of less than 1.

Such coatings have purifying properties with respect to liquid metal. These properties are mainly based on the greed of CaO for oxides which pollute the metal such as Al₂O₃, FeO, Fe₂O₃, giving compounds which are more fusible than the constituents and which migrate towards the slag.

However, these coatings have the drawback of having very low mechanical strength, so that they do not resist the abrasion caused by the liquid metal.

To remedy these drawbacks, the applicant has carried out the following two tests:

In a first test, the applicant applied in an pouring distributor an aqueous mixture comprising approximately 98% of raw dolomite and 2% of sodium silicate as a binder.

After spraying, the coating was fully baked to around 900 ° C, in order to release the CO₂ from the dolomite.

The coating thus cooked had a porous structure whose mechanical strength was insufficient to resist the abrasion caused by the liquid metal.

In a second test, the applicant poured the above mixture into the space between the inner wall of the tundish and a wall forming a mold.

After drying at 180 ° C, it was found that the coating had no internal mechanical resistance and was destroyed when the wall was removed forming a mold so that this procedure is of no industrial interest.

The Applicant has continued its research in order to discover an industrially acceptable solution making it possible to produce a purifying coating from raw dolomite or calcite.

The object of the present invention is precisely to propose a process for producing such a purifying coating, and a use of a known composition for the implementation of this process.

The invention thus relates to a method for coating the interior wall of a metallurgical container for transferring liquid metal with a purifying coating based on at least one oxide of an alkaline earth metal.

• on applique sur la paroi intérieure du récipient une composition renfermant au moins un carbonate simple ou multiple de métal alcalino-terreux additionné d'une proportion prédéterminée de matière réfractaire en grains, cette composition étant susceptible de faire prise, et
• on chauffe ce revêtement pour éliminer le CO₂ et éventuellement l'eau de constitution et/ou d'hydratation et obtenir une couche de structure poreuse du ou des oxydes de métal alcalino-terreux et de matière réfractaire en grains, la proportion de matière réfractaire étant prédéterminée pour conférer au revêtement une résistance prédéterminée à l'érosion du métal liquide coulé dans le récipient par suite du frittage de ce revêtement au contact du métal liquide.

According to the invention, this process is characterized by the following stages:

• a composition is applied to the inner wall of the container containing at least one simple or multiple carbonate of alkaline earth metal added with a predetermined proportion of refractory grains, this composition being capable of setting, and
• this coating is heated to remove the CO₂ and possibly the water of constitution and / or hydration and to obtain a layer of porous structure of the oxide (s) of alkaline earth metal and of refractory material in grains, the proportion of refractory material being predetermined to give the coating a predetermined resistance to erosion of the liquid metal poured into the container as a result of the sintering of this coating on contact with the liquid metal.

According to another aspect of the invention, it also relates to the use, for producing a purifying coating by the implementation of the aforementioned process, of a composition comprising at least one simple or multiple carbonate of alkaline earth metal added of a predetermined proportion of refractory grain, this composition being capable of setting and sintering in contact with the liquid metal.

The composition may also contain a certain proportion of either alkaline earth hydroxides or alkaline earth oxides capable of hydrating during wetting of the composition and of activating lime or magnesia sufficiently "in situ" during preheating said composition, again with the aim of imparting sufficient resistance to the coating.

The process according to the invention thus makes it possible to obtain an economical purifying coating since it uses as starting material one or more alkaline earth carbonates such as dolomite or calcite.

In addition, the coating obtained has good resistance to erosion caused by the metal poured into the metallurgical vessel.

Thus, the coating according to the invention has a double function, namely the purifying function and the function of wear coating, so that it can advantageously replace all the wear and protective coatings applied so far which do not have no purifying function.

The merit of the present invention lies in the fact that it has surprisingly discovered that the addition of a certain proportion of refractory material to one or more simple or multiple alkaline earth carbonates made it possible to obtain, after baking, that is to say after the elimination of CO₂, a structure having an erosion resistance markedly greater than that obtained in the absence of this refractory material.

In addition, the coating obtained, in addition to its purifying properties due to the alkaline earth oxide which is obtained after baking, has a structure which is sufficiently porous to give it thermal insulation properties making it possible to limit the cooling of the liquid metal contained in the pouring container.

Furthermore, the coating obtained according to the process according to the invention does not adhere strongly to the permanent refractory coating to which it is applied so that at the end of casting, it can be easily removed in order to apply a new coating .

The aforementioned composition can advantageously be sprayed in the form of aqueous mud, with approximately 15 to 20% by weight of water.

The composition can also be poured between the interior surface of the metallurgical container and a wall forming a mold.

After application, the coating is heated to the core around 900 ° C, to release the CO₂ of the alkaline earth carbonate and optionally the water of constitution and / or hydration. This heating can be carried out by any means such as a burner, electric heating elements by Joule effect or with emission of infrared radiation.

The single or multiple alkaline earth carbonate used in the composition is preferably a carbonate chosen from the following compounds: raw dolomite, raw calcite and their mixtures, dolomite being however preferred because of its low cost in the raw state (not calcined).

To facilitate its application, the composition used preferably contains a binder chosen from mineral binders such as, for example: alkali metal silicate or phosphate, organic such as, for example, phenolic resin, hydraulic, such as, for example, cements, and mixtures thereof.

The refractory material added to the alkaline earth carbonate is preferably chosen from the following compounds:
- calcined dolomite, lime, magnesia, chromium magnesia, chromium oxide, zirconium silicate, silica, silico-aluminous and / or mixtures thereof.

• . carbonate alcalino-terreux   : 30 à 95%
• . matière réfractaire   : 70 à 5%
• . liant   : 0 à 15%.

The weight composition of the main constituents is advantageously as follows:

• . alkaline earth carbonate: 30 to 95%
• . refractory material: 70 to 5%
• . binder: 0 to 15%.

The optimal proportions will depend on the desired effect. The higher the proportion of alkaline earth carbonate, the greater the purifying effect.

Conversely, the higher the proportion of refractory material, the greater the resistance to erosion of the coating.

There is however an advantage in that the proportion of refractory material is barely sufficient to give the coating the desired resistance to erosion so as to preserve the purifying properties due to the alkaline earth oxide remaining after release of CO₂. In order to increase the purifying properties of the coating, the elements containing calcium will be favored in the composition.

In some cases, the presence of the binder is not compulsory, because a setting effect with water already exists in its absence.

• . 60 à 80% de dolomie crue
• . 10 à 40% de matière réfractaire ; et
• . 1 à 5% de liant.

The best results are obtained with the following composition:

• . 60 to 80% raw dolomite
• . 10 to 40% of refractory material; and
• . 1 to 5% of binder.

The alkaline earth carbonate can be pure carbonate of calcium or associated with another carbonate, for example magnesium carbonate, in the form of dolomite.

Furthermore, the alkaline earth oxides cited as examples of refractory material may be replaced in whole or in part by hydroxides and / or sulfates, the latter being transformed into oxides during cooking.

Preferably, the grains of the refractory material have at least in part a dimension greater than that of the grains of carbonates and / or hydroxides and / or alkaline earth sulfates.

In fact, the grains of carbonates, hydroxides and sulfates decompose the more easily and quickly the smaller they are.

In general, the particle size can be adjusted as a function of the desired result, namely the porosity which gives insulating and / or sintering properties which ensures cohesion and resistance to erosion.

The composition according to the invention also preferably contains up to 20% by weight of organic and / or mineral fibers.

The composition according to the invention may also contain a carbonaceous matter in grains.

Preferably, the composition additionally contains up to 5% by weight of a surfactant.

The aforementioned composition can be applied to the bottom and the lateral faces of a tundish, by spraying or molding in one or more layers a few centimeters thick.

In the case where it is desired to accelerate the appearance of sintering on the surface of the purifying layer during its first contact with the liquid steel, it is useful to add to the mixture one or more fluxes such as, for example: olivine, colemanite, borocalcite (calcium borate), iron oxide, fluoride, boric acid and its salts (borax) and their mixtures. We can thus choose a flux which migrates towards the surface with water during drying, such as boric acid, for example, to obtain a rapid sintering of the surface which comes into contact with liquid steel, and seek a slower or less active sintering in the mass of the purifying coating thanks to a flux which does not migrate such as, for example, colemanite.

Liant

: 0 à 15%

Carbone

: 0 à 5%

Fondants

: 0 à 10%

The fineness of the grains constituting the coating also plays a role in the sintering process, and the following formulas can advantageously be used, the percentages being given by weight:
Dolomite and / or calcite and / or hydroxides and / or
sulfates: 30 to 95%, grain diameter <3 mm, preferably <1 mm
Refractory material: 70 to 5%, grain diameter <5 mm, preferably <2 mm

Binder

: 0 to 15%

Carbon

: 0 to 5%

Fondants

: 0 to 10%

• un fondant facilement soluble, susceptible de migrer vers la surface,
• un fondant peu ou pas soluble,
• un mélange des deux types de fondants dans une proportion prédéterminée permettant un certain frittage sélectif en surface et dans les différentes épaisseurs de la couche.

We can choose as fondant, as appropriate:

• an easily soluble flux, capable of migrating to the surface,
• a poorly or not soluble fondant,
• a mixture of the two types of fluxes in a predetermined proportion allowing a certain selective sintering on the surface and in the different thicknesses of the layer.

It is also possible to obtain a maximum compactness of the layer by optimizing the respective particle sizes of the various constituents in a known manner, for example by applying the Bolomey law or the Furnas compactness or else by means of the triangle rule: can thus increase the resistance of the purifying layer to erosion by liquid metal without having to use any mechanical compaction process which is impracticable in practice at low cost.

In the case of a coating in several layers, the composition and the grain size of the grains can vary from one layer to another depending on the desired properties.

Thus, if we want the inner layer (the one intended to come into contact with the liquid metal) to have a high resistance to erosion, it will contain more refractory material, and will contain fine particles to promote sintering.

Conversely, it may be preferable for the outer layer adjacent to the permanent refractory lining to sinter less than the inner layer, so that it can be easily detached from the permanent lining at the end of casting.

Likewise, the basicity of the inner layer may be favored by a high calcium content in order to obtain optimal purifying properties.

• une couche intérieure épurante d'usure venant au contact de l'acier liquide, à base d'oxyde d'un métal alcalino-terreux (dolomie, hydroxydes, sulfates, calcite, carbonates),
• une couche extérieure de sécurité, mise en place entre la couche épurante d'usure et le revêtement permanent ; cette couche peut être, soit une couche frittable aux températures en jeu, à base de grains d'oxydes réfractaires stables (par exemple à base de magnésie et/ou de silice et/ou d'alumine et/ou de zircon et/ou de zircone), soit en variante une couche extérieure peu ou très peu frittable de sécurité à base d'oxydes réfractaires stables additionnés de carbone (moins de 35% de carbone). Cette dernière couche peut également jouer le rôle de troisième couche peu ou très peu frittable mise en place entre la couche précitée de sécurité et le revêtement permanent pour éviter tout accrochage du revêtement permanent, comme toute autre couche partiellement, peu ou très peu frittable.

A coating in several layers to protect the interior wall of a metallurgical vessel can thus include:

• a purifying inner layer of wear coming into contact with liquid steel, based on an oxide of an alkaline earth metal (dolomite, hydroxides, sulfates, calcite, carbonates),
• an outer safety layer, placed between the purifying wear layer and the permanent coating; this layer may be either a sinterable layer at the temperatures in play, based on grains of stable refractory oxides (for example based on magnesia and / or silica and / or alumina and / or zircon and / or zirconia), or alternatively an outer layer with little or very little sinterability of security based on stable refractory oxides added with carbon (less than 35% carbon). This last layer can also play the role of third layer with little or very little sinterability placed between the aforementioned safety layer and the permanent coating to avoid any attachment of the permanent coating, like any other partially, little or very little sinterable layer.

It is thus possible, for example, to take for the outer layer (s) of security, the compositions indicated respectively in the patents French No. 2,393,637 and 2,585,273 in the name of the plaintiff.

It is also possible, in the case of three layers, to have a first purifying inner layer, a second layer which is little or very little sinterable or not sinterable, and a third outer sinterable layer or at least partially sinterable.

Claims (22)

1. A method of lining the inside wall of a metallurgical vessel for transferring liquid metal, with a purifying lining based on the oxide of an alkaline-earth metal, characterised by the following steps:

   - there is applied to the inside wall of the vessel a composition containing at least one single or multiple carbonate of an alkaline-earth metal plus a predetermined proportion of granular refractory material, this composition being adapted to set, and

   - this lining is heated to eliminate the CO₂ and any constitutional and/or hydration water, and to obtain a porous-structure layer of the or each oxide of alkaline-earth metal and granular refractory material, the proportion of refractory material being predetermined to give the lining a predetermined resistance to the erosion of the metal liquid poured into the vessel as a result of sintering of said lining in contact with the liquid metal.
2. A method according to claim 1, characterised in that the composition is sprayed in the form of an aqueous slurry.
3. A method according to claim 1, characterised in that the composition is poured between the inside surface of the vessel and a wall which forms a mould.
4. A method according to any one of claims 1 to 3, characterised in that the lining is heated throughout to about 900°C.
5. A method according to any one of claims 1 to 4, characterised in that there is applied, before the purifying lining, at least one first layer of a refractory material based on stable refractory oxides, said material being adapted to sinter at least partially at the temperatures involved.
6. A method according to any one of claims 1 to 4, characterised in that there are applied, before the purifying lining, two layers of refractory materials based on stable refractory oxides, the material of one of the layers being little or very little adapted to sintering, the material of the other layer being at least partially adapted to sintering at the temperatures involved.
7. The use, to produce a purifying lining by the practice of the method according to any one of claims 1 to 6, of a composition comprising at least one single or multiple carbonate of alkaline-earth metal plus a predetermined proportion of granular refractory material, said composition being adapted to set and sinter in contact with the liquid metal.
8. The use according to claim 7, characterised in that the said alkaline-earth carbonate is selected from the following compounds: non-calcined dolomite, calcite and their mixtures.
9. The use according to claim 7 or 8, characterised in that the composition used also contains a binder selected from the mineral, organic, and hydraulic binders, and their mixtures.
10. The use according to any one of claims 7 to 9, characterised in that the refractory material is selected from the following compounds:
    - calcined dolomite, lime, magnesia, chromium-magnesia, chromium oxide, zirconium silicate, silica, silico-aluminous and their mixtures.
11. The use according to any one of claims 7 to 10, characterised in that the composition contains by weight:

    . alkaline-earth carbonate:   30 to 95%

    . refractory material   70 to 5%

    . binder   0 to 15%.
12. The use according to claim 11, characterised in that the composition contains by weight:

    . 60 to 80% of non-calcined dolomite

    . 10 to 40% of refractory material and

    . 1 to 5% of binder.
13. The use according to any one of claims 7 to 12, characterised in that the grains of refractory material have at least partly a larger dimension than that of the grains of alkaline-earth carbonate.
14. The use according to any one of claims 7 to 13, characterised in that the composition also contains up to 20% by weight of organic and/or mineral fibres.
15. The use according to any one of claims 7 to 14, characterised in that the composition also contains up to 5% by weight of a surfactant.
16. The use according to any one of claims 7 to 13, characterised in that the composition also contains a certain proportion of alkaline-earth hydroxide(s) and/or alkaline-earth oxide(s) adapted to hydrate when the composition is wetted, and/or alkaline-earth sulphates.
17. The use according to any one of claims 7 to 16, characterised in that the composition also contains from 0 to 10% by weight of a flux.
18. The use according to claim 17, characterised in that the flux is selected from olivine, colemanite, borocalcite (calcium borate), iron oxide, fluorides, boric acid and its salts (borax) and their mixtures.
19. The use according to claim 17 or 18, characterised in that the composition contains a readily soluble flux.
20. The use according to any one of claims 7 to 19, characterised in that the composition contains by weight:

    . dolomite and/or calcite and/or hydroxides and/or sulphates of alkaline-earth metals   30 to 95%

    . refractory grains:   70 to 5%

    . binder:   0 to 15%

    . carbon:   0 to 5%

    . flux:   0 to 10%.
21. The use according to any one of claims 7 to 20, characterised in that the main constituents have the following particle sizes:

    - carbonates, hydroxides, oxides and sulphates of alkaline-earth metals: diameter less than 3 mm and preferably less than 1 mm;

    - refractory grains: diameter less than 5 mm and preferably less than 2 mm.
22. The use according to any one of claims 7 to 21, characterised in that the respective grain sizes of the different constituents are selected to give the purifying lining maximum compactness.