ITTO940907A1 - METHOD AND POWDER MIXTURE TO REPAIR REFRACTORY BODIES BASED ON OD SIDI. - Google Patents

Abstract

A method for repairing an oxide-based refractory body is disclosed. The method provides for a mixture of powders to be projected against the surface of said body at high temperature in the presence of oxygen. The powder mixture comprises oxide particles and combustible particles which react exothermically with oxygen to form a refractory oxide. The invention is characterized in that the combustible particles are selected from magnesium, aluminum, silicon and their mixtures and in the fact that the powder mixture also comprises up to 10% by weight of silicon carbide particles. The invention provides a method for repairing oxide-based refractory bodies, which allows to form a refractory repair mass with acceptable porosity.

Description

DESCRIPTION

of the industrial invention entitled:

"Method and powder mixture to repair oxide-based refractory bodies"

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

Oxides of silicon, zirconium, aluminum and magnesium are used as industrial refractory oxides. 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 molten metal and slag.

Magnesium oxide based refractory materials, also known as basic refractory materials, can form the liner of a ladle for transporting molten steel. Such coatings are subjected to abrasion from molten steel and slag during use.

The erosion of the coating occurs in particular at the liquid level. There is thus a need to repair such oxide-based refractory bodies from time to time.

It has been proposed to repair the refractory bodies using a "ceramic welding" technique. In this technique the refractory body to be repaired is kept at a high temperature, and a mixture of powders is projected in the presence of oxygen, which includes particles of a refractory material and combustible particles that react exothermically with oxygen to form an oxide refractory. With this method, a refractory mass is formed which adheres to the refractory body at the point of repair. The ceramic welding technique is illustrated in British Patent Nos. 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, it has been found that when a powder mixture consisting of oxide particles and combustible particles is used to repair an oxide-based refractory body, and in particular a high-melting oxide-based refractory body, such as oxide of magnesium and aluminum oxide, the resulting refractory mass can be porous. If there is significant visible porosity, the repair mass is not useful for certain applications, particularly if it is subjected to erosion or corrosion by molten materials.

Therefore, an object of the present invention is to provide a method for repairing oxide-based refractory bodies which allows to form a refractory repair mass with acceptable porosity.

We have surprisingly found that, when the combustible particles are chosen from particles of magnesium, aluminum, silicon and their mixtures, this objective can be achieved by incorporating a specific amount of silicon carbide into the powder mixture. This contrasts with the generally accepted principle of matching the composition of the refractory mass of repair with that of the surface of the refractory material being repaired.

Furthermore, silicon carbide is seen as an inert material in this ceramic welding process and is not wetted by the liquid phase that forms during the reaction. The effect of silicon carbide on the porosity of the mass is therefore somewhat surprising.

While not wishing to be bound by a theory, we believe that the additional silicon carbide particles conduct heat in the refractory repair mass and that, over time, prolonged exposure to high temperatures causes the silicon carbide particles to decompose to generate elemental carbon which, as known, provides the refractory repair mass with good resistance to corrosion by slag.

Thus, according to a first aspect of the invention, a method is provided for repairing an oxide-based refractory body by projecting a powder mixture against a surface of such a body at a high temperature and in the presence of oxygen, such a powder mixture comprising particles of refractory oxide and combustible particles which react exothermically with oxygen to form a refractory oxide, characterized in that the combustible particles are selected from magnesium, aluminum, silicon and their mixtures and by the fact that the powder mixture also comprises up to 10% by weight of silicon carbide particles.

The level of the silicon carbide in the powder mixture is preferably at least 1% by weight. If too much silicon carbide is included, we have found that the result may be that no repair mass is formed at all because the repair material slips off the repair site. Without being bound by a theory, it could be assumed that this is due to the fact that too much heat is retained after the repair process, which gives rise to a low viscosity liquid phase. If too little silicon carbide is used, the benefits of the invention are no longer achieved significantly.

Preferably the silicon carbide has a small particle size, for example less than 200 µm. By the term "particle size" as used herein, we mean that the material in question has a particle size distribution such that at least 90% by weight of these falls within the given limits. "Average size", as used herein, means a size such that 50% by weight of the particles are smaller than this average.

The refractory oxide particles can comprise at least one oxide of which the refractory body is composed. Thus, when the refractory oxide body is a body containing aluminum oxide, the refractory oxide particles may comprise alumina particles. When the refractory oxide body is a body that contains magnesium oxide, the refractory oxide particles may comprise magnesia particles.

Preferably, a greater part of said powder mixture is formed by refractory oxide particles selected from magnesia, alumina and their mixtures. These are the oxides in the presence of which the exothermic reaction is much more lively, and therefore has a greater risk of producing a highly porous repair mass. Preferably, the refractory oxide particles are less than 2.5mm in size, and substantially no particles are more than 4mm in size.

The combustible particles are selected from particles of 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 size of less than 50 μm.

The repair operation is generally performed 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 above 600 ° C, measured at the surface of the refractory body to be repaired. At this temperature, the combustible particles will burn in the presence of oxygen to release a refractory oxide and produce sufficient heat to cause the oxide particles, along with the combustion product of the fuel, to form the refractory repair mass that makes up the repair.

The invention also provides, according to another aspect thereof, a mixture of powders for repairing refractory bodies based on oxides, said mixture comprising:

80% to 95% by weight of refractory particles comprising a refractory oxide; And

- 5% to 20% by weight of combustible particles which react exothermically with oxygen to form a refractory oxide,

characterized in that said combustible particles are selected from magnesium, aluminum, silicon and their mixtures and in that said refractory particles comprise up to 10% by weight of the total mixture of carbide and silicon particles.

In order to obtain a homogeneous repair mass, a percentage of at least 1180% by weight of refractory particles, including oxide particles, should be present in the powder mixture. .

In a preferred embodiment, the mixture comprises:

- from 80% to 94% by weight of refractory oxide particles selected from alumina, magnesia particles and their mixtures;

- from 1% to 5% by weight of silicon carbide particles, e

- from 5% to 15% by weight of said combustible particles.

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

A useful technique for bringing the powder mixture against a surface of the refractory body to be repaired is to project the powder mixture together with a gas containing oxygen. In general it is recommended to perform particle projection in the presence of a high concentration of oxygen, for example, using commercial grade oxygen as the carrier gas. In this way, a repair mass is easily formed which adheres to the surface onto which the particles are projected. Due to the very high temperatures that the ceramic welding reaction can reach, the reaction can penetrate through slag that may be present on the surface of the refractory body to be treated, and it can also soften or melt the surface in such a way that a good bond between the treated surface and the newly formed refractory refractory mass.

This procedure is conveniently carried out with the use of a lance. A lance suitable for use in the process of the invention comprises one or more outlets for discharging the powder jet, optionally together with one or more outlets for additional gases. For repairs, carried out in a hot environment, the gas streams can be discharged from a lance which is cooled by fluid circulating through it. Such cooling can easily be achieved by providing the lance with a water jacket. These lances are suitable for spraying powders at flow rates from 30 to 500 kg / hour.

To facilitate the formation of a regular powder jet, the refractory particles preferably do not substantially comprise particles larger than 4 mm, and more preferably do not comprise particles larger than 2.5 mm.

The invention is particularly useful for the repair or maintenance of ladles for cast steel, since said repair can be carried out rapidly at a high temperature, between ladle charges, while the refractory bodies which form part of such ladles are particularly damaged by the ladle. contact with molten metal and slag. The area that requires the most repair tends to be the liquid surface line.

The invention will now be further described in the following non-limiting examples.

EXAMPLE 1

A refractory refractory mass is formed on a wall of the magnesium oxide liner of a molten steel ladle. A mixture of refractory particles and fuel particles is projected onto these bricks. The wall temperature is around 850 ° C. The mixture is projected - at a flow rate of 150 kg / hour in a stream of pure oxygen. The mixture has the following composition:

MgO 87% by weight

SiC 5%

Yes 4%

At 4%

The MgO particles have a maximum size of about 2 mm. The silicon carbide particles have a particle size of 125 μm, with an average size of .57 μm. Silicon particles and aluminum particles have a maximum size below 45 μm.

EXAMPLE 1A (Comparison)

For comparison, the same repair was carried out in the same way as described in Example 1, but using a mixture of powders of the following composition:

MgO 92% by weight

Yes 4%

At 4%

The bulk density and apparent porosity (i.e., open porosity) of the refractory repair masses formed in Examples 1 and 1A were measured and the results were as follows:

In a modification of Example 1, a refractory containing aluminum oxide can be repaired in a similar manner, however the magnesia particles in the powder mixture are replaced with the same amount of alumina particles of the same particle size.

EXAMPLES 2 to 4

Refractory refractory masses are formed on a wall of the magnesium oxide coating of a molten steel ladle. Mixtures of refractory particles e. combustible particles are projected onto these bricks.

The wall temperature is around 850 ° C. The mixtures are projected at a flow rate of 60 kg / h in a stream of pure oxygen. The mixtures had the following compositions (by weight):

The MgO particles have a maximum size of about 2 mm. The silicon carbide particles have a particle size of 125 μm, with an average size of 57 μm. Silicon particles and aluminum particles have a maximum size below 45 μm.

The bulk density and apparent porosity (i.e. open porosity) of the refractory repair masses formed in Examples 2 to 4 were measured and the results were as follows:

EXAMPLE 5

A ceramic solder powder comprises the following composition (wt%):

Alumina 87%

5% silicon carbide

Aluminum 6%

Magnesium 2%

The alumina used was electro-fused alumina. The alumina had a nominal maximum grain size of 700 µm, the silicon carbide had the same particle size as in Example 1 above, the aluminum particles had a maximum size of less than 45 µm and the magnesium particles had a size maximum of 75 jim.

The above powder mixture can be used as described in Example 1, to repair a Corhart (Registered Trademark) Zac refractory block (composition: alumina / zircon / zirconium oxide) in a tank furnace for melting the glass below the level of the work surface of the melt, after the tank had been partially emptied to give access to the repair site.

Claims (9)

R IV E N D I C A Z I O N I 1. Method of repairing an oxide-based refractory body by projecting a powder mixture against a surface of that body at a high temperature and in the presence of oxygen, such a powder mixture comprising refractory oxide particles and combustible particles which react exothermically with oxygen to form a refractory oxide, characterized in that the combustible particles are selected from magnesium, aluminum, silicon and their mixtures and in that the powder mixture also comprises up to 10% by weight of silicon carbide particles. 2. Method according to rev. 1, in which the level of silicon carbide in the powder mixture is at least 1% by weight. 3. Method according to rev. 1 or 2, where the silicon carbide has a particle size of less than 200 μn. Method according to any one of the preceding claims, wherein the refractory oxide particles comprise at least one oxide of which the refractory body is formed. Method according to any one of the preceding claims, wherein the refractory oxide-based body is selected from bodies containing aluminum oxide and bodies containing magnesium oxide. Method according to any one of the preceding claims, in which a greater part of the powder mixture is formed of refractory oxide particles selected from magnesia, alumina and their mixtures. Method according to any one of the preceding claims, wherein the refractory body to be repaired is part of a ladle for molten steel. 8. Mixture of powders for repairing oxide-based refractory bodies, said mixture comprising: 80% to 95% by weight of refractory particles comprising a refractory oxide; And - 5% to 20% by weight of combustible particles which react exothermically with oxygen to form a refractory oxide, characterized in that said combustible particles are selected from magnesium, aluminum, silicon and their mixtures and in that said refractory particles comprise up to 10% by weight of the total mixture of silicon carbide particles. 9. Mixture of powders according to claim 8 comprising: - from 80% to 94% by weight of refractory oxide particles selected from alumina, magnesia particles and their mixtures; - 1% to 5% by weight of silicon carbide particles, e - from 5% to 15% by weight of said combustible particles.