LU88560A1 - Process and mixture of powders for the repair of refractory bodies based on oxides. - Google Patents

Description

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Process and mixture of powders for the repair of refractory bodies based on oxides

The present invention relates to a method of repairing an oxide-based refractory body by a ceramic welding process.

The oxides of silicon, zirconium, aluminum and magnesium are refractory oxides customary in the industry. In particular, aluminum and magnesium oxides are commonly used in the metallurgical industry, where they are chosen for their resistance to high temperatures, erosion and corrosion by materials such as metal, slag and molten slag.

Magnesium oxide refractories, otherwise known as basic refractories, can form the lining of molten steel transport bags. Such linings wear out on contact with molten steel and slag. The erosion of the lining occurs mainly at the surface of the liquid. It is therefore necessary to repair from time to time such refractory bodies based on oxides.

It has been proposed to repair refractory bodies by the use of a "ceramic welding" technique. In this technique, the refractory body to be repaired is maintained at an elevated temperature, and a mixture of powders is sprayed in the presence of oxygen, the said mixture of powders comprising particles of refractory material and combustible particles which react exothermically with oxygen to form a refractory oxide. By this process, a refractory mass is formed and adheres to the refractory body at the location to be repaired. The ceramic welding technique is illustrated in British patents No. GB 1 330 894 (Glaverbel) and GB 2 170 191 (Glaverbel). Combustible particles are particles whose composition and particle size are such that they react exothermically with oxygen, forming a refractory oxide and releasing the heat necessary to melt, at least superficially, the projected refractory particles.

However, when a mixture of powders consisting of oxide particles and combustible particles is used to repair an refractory body based on oxides, and in particular a refractory body based on oxides with a high melting point such as magnesium oxide and aluminum oxide, it happens that the resulting refractory mass is porous. If there is significant apparent porosity, the repair mass will not be useful for some applications, particularly if the repair mass is to be subjected to erosion or corrosion by molten materials.

One of the objects of the present invention therefore consists in providing a method of repairing oxide-based refractory bodies which allows the formation of a repair refractory mass having an acceptable porosity.

It has surprisingly been discovered that, when the combustible particles are chosen from magnesium, aluminum, silicon and their mixtures, this objective can be obtained by incorporating, in the mixture of powders, a specific amount of silicon carbide. This is contrary to the generally accepted principle of harmonizing the composition of the refractory repair material with the composition of the surface of the refractory material that is being repaired. In addition, silicon carbide is considered to be an inert material in the ceramic welding process and is not wetted by the liquid phase which forms during the reaction. The effect of silicon carbide on the porosity of the mass is therefore surprising.

Without wishing to be bound by any theory, we believe that additional silicon carbide particles conduct heat into the repair refractory mass and that prolonged exposure to high temperatures causes decomposition of the carbon-generating silicon carbide particles. elementary, which is known to give the refractory repair mass good resistance to corrosion by slag. Therefore, the present invention, in its first aspect, relates to a method of repairing an refractory body based on oxides by spraying at high temperature and in the presence of oxygen of a mixture of powders on a surface of the said body, the said mixture of powders comprising refractory oxide particles and combustible particles which react exothermically with oxygen to form a refractory oxide, characterized in that the combustible particles are chosen from magnesium, aluminum , silicon and their mixtures and in that the powder mixture additionally contains up to 10% by weight of particles of silicon carbide.

The silicon carbide content of said mixture of powders is preferably at least 1% by weight. If too high a content of silicon carbide is included, this may result in an absence of repair mass formation because the repair material flows out of the site to be repaired. Without wishing to be bound by a theory, it can be assumed that this is due to too much heat retention after the repair process, leading to a low viscosity liquid phase. If too little silicon carbide is used, the advantages of the invention are no longer obtained at a significant level.

Preferably, the size of the silicon carbide particles is small, for example less than 200 μιη. The term "particle size" as used herein means that the material in question has a particle size distribution such that at least 90% by weight of the particles conforms to the given limits. The dull "average dimension", as used herein, denotes a dimension such that 50% by weight of the particles has a smaller dimension than this average.

The refractory oxide particles can comprise at least one oxide of which the refractory body is made up. Therefore, when the refractory body is a body containing aluminum oxide, the refractory oxide particles may include alumina particles. When the refractory body is a body containing magnesium oxide, the refractory oxide particles may include particles of magnesia.

Preferably, the major part of said mixture of powders is formed of refractory oxide particles chosen from magnesia, alumina and their mixtures. These are the oxides in the presence of which the exothermic reaction is most intense and therefore presents a greater risk of producing a very porous repair mass. Preferably, the refractory oxide particles are less than 2.5 mm in size, with substantially no particles larger than 4 mm in size.

The combustible particles are chosen from magnesium, aluminum, silicon and their mixtures. A mixture of aluminum and silicon is particularly advantageous. The combustible particles used in the mixture preferably have an average dimension of less than 50 μτη. The repair operation is generally carried out when the refractory body is hot. This makes it possible to repair eroded refractory bodies while the equipment remains substantially at its working temperature.

The elevated temperature can be greater than 600 ° C, measured at the surface of the refractory body to be repaired. At this temperature, the combustible particles burn in the presence of oxygen to release a refractory oxide and generate sufficient heat for the oxide particles, along with the fuel combustion product, to form the refractory mass which constitutes the repair. The invention also relates, in its second aspect, to a mixture of powders for the repair of refractory bodies based on oxides, the said mixture comprising: - from 80% to 95% by weight of refractory particles comprising a refractory oxide ; and - from 5% to 20% by weight of combustible particles which react exothermically with oxygen to form a refractory oxide, characterized in that the combustible particles are chosen from magnesium, aluminum, silicon and their mixtures and in that the refractory particles contain up to 10% by weight (calculated relative to the total of the mixture) of particles of silicon carbide.

To obtain a homogeneous repair mass, an amount of at least 80% by weight of refractory particles, including the oxide particles, must be present in the powder mixture.

In a preferred embodiment, the mixture comprises: - from 80% to 94% by weight of refractory oxide particles chosen from particles of alumina, of magnesia and their mixtures; - from 1% to 5% by weight of particles of silicon carbide; and - from 5% to 15% by weight of combustible particles.

Preferably, the refractory particles in the powder mixture, including the particles of silicon carbide, have a dimension of at least 10 μτη. If too small particles are used, there is a risk that they will be lost during the reaction.

An easy technique for bringing the powder mixture onto a surface of the refractory body to be repaired is to spray the powder mixture with an oxygen-containing gas. In general, it is recommended to spray the particles in the presence of a high oxygen concentration, for example using commercial grade oxygen as the carrier gas. In this way, the repair mass is easily formed and adheres to the surface on which the particles are projected. Because of the very high temperatures that the ceramic welding reaction achieves, it can penetrate the slag which may be on the surface of the refractory body to be treated, and it can soften or melt the surface in such a way that a good connection is established between the treated surface and the newly formed refractory repair mass.

This process is advantageously implemented by means of a lance. A lance suitable for carrying out the method of the invention comprises one or more orifice (s) for the powder stream, optionally with one or more orifice (s) for additional gas. For repairs carried out in a hot environment, the gas streams can be discharged by a lance cooled by circulation of fluid. Such cooling can be easily obtained by providing the lance with a water jacket. Such lances are suitable for spraying powder at flow rates ranging from 30 to 500 kg / hour.

In order to facilitate the formation of a regular jet of powder, the refractory particles preferably do not include substantially particles whose size is greater than 4 mm, preferably no particles greater than 2.5 mm. The invention is particularly suitable for the repair or maintenance of pockets for molten steel because it can be implemented quickly, at high temperature, between the charges of the pockets. The refractory bodies forming part of such pockets are particularly stressed by contact with molten metal or slag. The area requiring the most significant repairs is located at the surface of the liquid. The invention will now be further described in the following nonlimiting examples: EXAMPLE 1

A refractory repair mass is formed on a wall of the lining, based on magnesium oxide, of a pocket for molten steel. These bricks are sprayed with a mixture of refractory particles and combustible particles. The wall temperature is around 850 ° C. The mixture is sprayed at a rate of 150 kg / hour in a stream of pure oxygen. The mixture has the following composition:

The particles of Mgu have a maximum dimension of approximately 2 mm. The particles of silicon carbide have a dimension of 125 μηη, with an average dimension of 57 μτη. The silicon and aluminum particles have a maximum dimension of less than 45 μιη. EXAMPLE IA (Comparative) For comparison, the same repair is carried out in the same way as in Example 1, but using a mixture of powders of the following composition:

The apparent specific mass and the apparent porosity (that is to say the open porosity) of the refractory repair masses formed in Examples 1 and 1 are measured and the following results are obtained:

In a variant of Example 1, the refractory containing aluminum oxide is repaired in a similar manner, but by replacing in the powder mixture the particles of magnesia with the same quantity of alumina particles of the same particle size. EXAMPLES 2 to 4

Refractory repair masses are formed on a wall of the lining, based on magnesium oxide, of a pocket for molten steel. These bricks are sprayed with a mixture of refractory particles and combustible particles. The wall temperature is around 850 ° C. The mixture is sprayed at a rate of 60 kg / hour in a stream of pure oxygen. The mixtures have the following compositions (by weight):

The MgO particles have

a maximum dimension of approximately 2 mm. The particles of silicon carbide have a dimension of 125 μπι, with an average dimension of 57 μm. The particles of silicon and aluminum have a maximum dimension less than 45 μηη.

The apparent specific mass and the apparent porosity (that is to say the open porosity) of the refractory repair masses formed in Examples 1 and 1 are measured and the following results are obtained:

EXAMPLE 5

A ceramic welding powder has the following composition (by weight):

Alumina 87%

5% silicon carbide

Aluminum 6%

Magnesium 2% The alumina used is fused alumina. The alumina has a maximum nominal dimension of 700 μπι, the silicon carbide has the same particle size as in Example 1, the aluminum particles have a maximum dimension less than 45 μπι and the magnesium particles have a smaller maximum dimension at 75 μπι.

The above powder mixture can be used in a manner analogous to that described in Example 1, to repair a Corhart (™) Zac refractory block (composition: alumina / zirconia / zircon) located in a basin of melting furnace of glass below the level of the surface of the molten material, after the oven has been partially emptied to give access to the place of repair.

Claims (9)

1. Method for repairing an refractory body based on oxides by spraying at high temperature and in the presence of oxygen of a mixture of powders on a surface of said body, said mixture of powders comprising oxide particles refractory and combustible particles which react exothermically with oxygen to form a refractory oxide, characterized in that the combustible particles are chosen from magnesium, aluminum, silicon and their mixtures and that the mixture of powders additionally contains up to 10% by weight of particles of silicon carbide. 2. Method according to claim 1, characterized in that the silicon carbide content of said mixture of powders is at least 1% by weight. 3. Method according to one of claims 1 or 2, characterized in that the size of the particles of silicon carbide is less than 200 μπι. 4. Method according to one of claims 1 to 3, characterized in that the refractory oxide particles comprise at least one oxide of which the refractory body is made. 5. Method according to one of claims 1 to 4, characterized in that said refractory body is chosen from bodies containing aluminum oxide and bodies containing magnesium oxide. 6. Method according to one of claims 1 to 5, characterized in that the major part of said mixture of powders is formed of refractory oxide particles chosen from magnesia, alumina and their mixtures. 7. Method according to one of claims 1 to 6, characterized in that said refractory body to be repaired is part of a pocket for molten steel. 8. Mixture of powders for the repair of refractory bodies based on oxides, the said mixture comprising: - from 80% to 95% by weight of refractory particles comprising a refractory oxide; and - from 5% to 20% by weight of combustible particles which react exothermically with oxygen to form a refractory oxide, characterized in that the combustible particles are chosen from magnesium, aluminum, silicon and their mixtures and in that the refractory particles contain up to 10% by weight (calculated relative to the total of the mixture) of particles of silicon carbide. 9. Mixture of powders according to claim 8, characterized in that it comprises: - from 80% to 94% by weight of refractory oxide particles chosen from particles of alumina, of magnesia and their mixtures; - from 1% to 5% by weight of particles of silicon carbide; and - from 5% to 15% by weight of combustible particles.