GB2284415A - Repairing oxide based refractory bodies - Google Patents

Abstract

A method of repairing an oxide-based refractory body comprises projecting a powder mixture against the surface of said body at an elevated temperature in the presence of oxygen. The powder mixture comprises oxide particles and fuel particles which react in an exothermic manner with the oxygen to form a refractory oxide. The fuel particles are selected from magnesium, aluminium, silicon and mixtures thereof and the powder mixture additionally contains up to 10% by weight of silicon carbide particles. This method enables a refractory repair mass to be formed with acceptable porosity.

Description

A METHOD AND POWDER MIXTURE FOR REPAIRING OXIDE BASED REFRACTORY BODIES The present invention relates to a method of repairing an oxide-based refractory body by a ceramic welding process.

Oxides of silicon, zirconium, aluminium and magnesium are used as industrial refractory oxides. In particular, oxides of aluminium and magnesium are currently used in the metallurgy industry, where they are chosen for their resistance to high temperature, to erosion and to corrosion by materials such as molten metal, slag and dross.

Magnesium oxide-based refractory materials, otherwise known as basic refractory materials, may form the lining of a ladle for the transport of molten steel. Such linings become abraded by the molten steel and the slag in use. Erosion of the lining occurs particularly at the level of the liquid.

There is thus a need to repair such oxide based refractory bodies from time to time.

It has been proposed to repair refractory bodies by use of a "ceramic welding" technique. In this technique, the refractory body to be repaired is maintained at an elevated temperature, and a powder mixture is projected in the presence of oxygen, said powder mixture comprising particles of a refractory material and fuel particles which react in an exothermic manner with the oxygen to form a refractory oxide.

By this method a refractory mass builds up and adheres to the refractory body at the repair site. The technique of ceramic welding is illustrated in British patent nos. GB 1,330,894 (Glaverbel) and GB 2,170,191 (Glaverbel). The fuel particles are particles whose composition and granulometry are such that they react in an exothermic manner with the oxygen while forming a refractory oxide and while releasing the necessary heat for melting, at least superficially, the projected refractory particles.

However, it has been found that when a powder mixture consisting of oxide particles and fuel particles is used to repair an oxide-based refractory body, and in particular a refractory body based on high melting point oxides such as magnesium oxide and aluminium oxide, the resulting refractory mass may be porous. If there is a significant apparent porosity, the repair mass is not useful for certain applications, particularly if the repair mass is subject to erosion or to corrosion by molten materials.

It is therefore an object of the present invention to provide a method of repairing oxide-based refractory bodies which enables a refractory repair mass to be formed with acceptable porosity.

We have surprisingly discovered that, where the fuel particles are selected from particles of magnesium, aluminium, silicon and mixtures thereof, this objective may be achieved by the incorporation, in the powder mixture, of a specific quantity of silicon carbide. This is contrary to the generally accepted principle of matching the composition of the refractory repair mass with the composition of the refractory material surface being repaired. Further, silicon carbide is seen as an inert material in this ceramic welding process and is not wetted by the liquid phase which is formed during the reaction. The effect of the silicon carbide on the porosity of the mass is therefore somewhat surprising.

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

Thus, according to a first aspect of the invention there is provided a method of repairing an oxide-based refractory body by projecting a powder mixture against a surface of said body at an elevated temperature and in the presence of oxygen, said powder mixture comprising refractory oxide particles and fuel particles which react in an exothermic manner with the oxygen to form a refractory oxide, characterised in that the fuel particles are selected from magnesium, aluminium, silicon and mixtures thereof and in that the powder mixture additionally contains up to 10% by weight of silicon carbide particles.

The level of silicon carbide in said 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 at all is formed because the repair material flows away from the repair site. Without wishing to be bound by theory, one might expect that this may be due to the retention of too much heat following the repair process, leading to a low viscosity liquid phase. If too little silicon carbide is used, the benefits of the invention are no longer obtained to a significant degree.

The silicon carbide preferably has a small particle size, such as less than 200 pm. By "particle size" as used herein, we mean that the material concerned has a particle size distribution such that at least 90% by weight of particles conform to the given limits. "Average dimension", as used herein, designates a dimension such that 50t by weight of the particles have a smaller dimension than this average.

The refractory oxide particles may comprise at least one oxide of which the refractory body is formed. Thus, where the refractory oxide body is an aluminium oxide-containing body, the refractory oxide particles may comprise particles of alumina. Where the refractory oxide body is a magnesium oxide-containing body, the refractory oxide particles may comprise particles of magnesia.

Preferably, a major portion of said powder mixture is formed of refractory oxide particles selected from magnesia, alumina and mixtures thereof. These are the oxides in the presence of which the exothermic reaction is most lively, and therefore has a higher risk of resulting in a highly porous repair mass. Preferably, the oxide refractory particles have a size below 2.5 mm, with substantially no particles having a size above 4 mm.

The fuel particles are selected from particles of magnesium, aluminium, silicon and mixtures thereof. A mixture of aluminium and silicon is particularly advantageous. The fuel particles used in the mixture preferably have an average dimension of less than 50 pm.

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 may be above 6000C as measured at the surface of the refractory body to be repaired. At this temperature, the fuel particles will combust in the presence of oxygen to liberate a refractory oxide and to generate sufficient heat to cause the oxide particles, together with the combustion product of the fuel, to form into the refractory repair mass which constitutes the repair.

The invention also provides, according to a second aspect thereof, a powder mixture for the repair of oxide-based refractory bodies, said mixture comprising: - from 80% to 95% by weight of refractory particles comprising a refractory oxide; and - from 5% to 20% by weight of fuel particles which react in an exothermic manner with the oxygen to form a refractory oxide, characterised in that said fuel particles are selected from magnesium, aluminium, silicon and mixtures thereof and in that said refractory particles include up to 10% by weight of silicon carbide particles, based on the total mixture.

In order to obtain a homogeneous repair mass, an amount of at least 80% by weight of refractory particles, including the 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 particles of alumina, magnesia and mixtures thereof; from 1% to 5% by weight silicon carbide particles; and from 5% to 15% by weight of said fuel particles.

Preferably, the refractory particles in the powder mixture, including the silicon carbide particles, have a size of at least 10pm. If particles which are too small are employed, 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 an oxygen-containing gas.

In general it is recommended to perform the projection of particles in the presence of a high concentration of oxygen, for example, by using oxygen of commercial quality as a gas carrier. In this manner a repair mass is easily formed that adheres to the surface onto which the particles are projected. Because of the very high temperatures that the ceramic welding reaction can reach, it can penetrate slag which might be present on the surface of the refractory body being treated, and it can soften or melt the surface in such a way that a good bond is produced between the treated surface and the newly formed refractory repair mass.

This process is conveniently carried out with the use of a lance. A suitable lance for use in the process of the invention comprises one or more outlets for the discharge of the powder stream, optionally together with one or more outlets for supplementary gas. For repairs carried out in a hot environment, the gas streams may be discharged from a lance which is cooled by fluid circulating through it. Such cooling may easily be achieved by providing the lance with a water jacket. Such lances are suitable for projecting powder at rates of 30 to 500 kg/h.

In order to facilitate the formation of a regular jet of powder, the refractory particles preferably comprise substantially no particles with a size greater than 4 mm, most preferably not greater than 2.5 mm.

The invention is particularly useful for the repair or maintenance of molten-steel ladles because it can be carried out rapidly, at a high temperature, between ladle charges, while the refractory bodies which form part of such ladles are particularly affected by contact with molten metal and slag. The region which requires the greatest repair tends to be the line of the liquid surface.

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

EXAMPLE 1 A refractory repair mass is formed on a wall of the magnesium oxide-based lining of a molten steel ladle. A mixture of refractory particles and particles of a fuel is projected onto these bricks. The temperature of the wall is about 8500C. The mixture is projected at the rate of 150 kg/h into a stream of pure oxygen. The mixture has the following composition: MgO 87% by weight SiC 5% Si 4% Al 4% The MgO particles have a maximum dimension of approximately 2 mm. The silicon carbide particles have a particle size of 125 pm, with an average dimension of 57 pm. The silicon particles and the aluminium particles have a maximum dimension below 45 pm.

EXAMPLE 1A (Comparative) By way of comparison, the same repair was carried out in the same manner as described in Example 1, but using a powder mixture of the following composition: MgO 92% by weight Si 4% Al 4% The apparent 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: Density Example No kg/dm3 Porosity (%) 1 2.9 about 8% 1A 2 - 2.4 about 20% In a modification of Example 1, an aluminium oxide-containing refractory may be repaired in a similar way, but where the magnesia particles in the powder mixture are replaced by the same amount of alumina particles of the same granulometry.

EXAMPLES 2 to 4 Refractory repair masses are formed on a wall of the magnesium oxide-based lining of a molten steel ladle.

Mixtures of refractory particles and particles of a fuel are projected onto these bricks. The temperature of the wall is about 8500C. The mixtures are projected at the rate of 60 kg/h into a stream of pure oxygen. The mixtures had the following compositions (by weight): Example No: 2 3 4 Si 4% 4% 4% Al 4% 4% 4% SiC 2% 5% 10% MgO 90% 87% 82% The MgO particles have a maximum dimension of approximately 2 mm. The silicon carbide particles have a particle size of 125 pm, with an average dimension of 57 pm. The silicon particles and the aluminium particles have a maximum dimension below 45 pm.

The apparent 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: Density Example No kg/dm3 Porosity (%) 2 2.6 14% 3 2.7 10% 4 2.9 8% EXAMPLE 5 A ceramic welding powder comprises the following composition (% by weight): Alumina 87% Silicon carbide 5% Aluminium 6% Magnesium 2% The alumina used was an electrocast alumina. The alumina had a nominal maximum grain size of 700 pm, the silicon carbide had the same granulometry as given in Example 1 above, the aluminium particles had a maximum dimension below 45 pm and the magnesium particles had a maximum dimension of 75 pm.

The above powder mixture may be used as described in Example 1, to repair a Corhart (Trade Mark) Zac refractory block (composition: alumina/zircon/zirconia) in a glass melting tank furnace beneath the working surface level of the melt after the tank has been partially drained to give access to the repair site.

Claims (9)

1. A method of repairing an oxide-based refractory body by projecting a powder mixture against a surface of said body at an elevated temperature and in the presence of oxygen, said powder mixture comprising refractory oxide particles and fuel particles which react in an exothermic manner with the oxygen to form a refractory oxide, characterised in that the fuel particles are selected from magnesium, aluminium, silicon and mixtures thereof and in that the powder mixture additionally contains up to 10% by weight of silicon carbide particles.

2. A method according to claim 1, wherein the level of silicon carbide in said powder mixture is at least 1% by weight.

3. A method according to claim 1 or 2, wherein the silicon carbide has a particle size of less than 200 pm.

4. A method according to any preceding claim, wherein said refractory oxide particles comprise at least one oxide of which the refractory body is formed.

5. A method according to any preceding claim, wherein said refractory oxide body is selected from aluminium oxidecontaining bodies and magnesium oxide-containing bodies.

6. A method according to any preceding claim, wherein a major portion of said powder mixture is formed of refractory oxide particles selected from magnesia, alumina and mixtures thereof.

7. A method according to any preceding claim, wherein said refractory body to be repaired is part of a molten-steel ladle.

8. A powder mixture for the repair of oxide-based refractory bodies, said mixture comprising: - from 80% to 95% by weight of refractory particles comprising a refractory oxide; and - from 5% to 20% by weight of fuel particles which react in an exothermic manner with the oxygen to form a refractory oxide, characterised in that said fuel particles are selected from magnesium, aluminium, silicon and mixtures thereof and in that said refractory particles include up to 10% by weight of silicon carbide particles, based on the total mixture.

9. A powder mixture according to claim 8, comprising: from 80% to 94% by weight of refractory oxide particles selected from particles of alumina, magnesia and mixtures thereof; from 1% to 5% by weight silicon carbide particles; and from 5% to 15% by weight of said fuel particles.