LU87749A1 - PROCESS FOR CERAMIC WELDING AND MIXTURE OF POWDERS FOR SUCH A PROCESS - Google Patents

Description

ANNEX

INVENTORS Léon Philippe MOTTET 814 B Wadsworth Road US. Medina, OHIO 44256 U.S.A.

Charles Michaël ZVOSEC 923 Middle Ridge Road US. Lorain, OHIO 44053 U.S.A.

Stephen D. CHERICO 12465 Deer Creek Drive, 205 U.S. North Royalton, OHIO 44133 U.S.A.

Alexandre ZIVKOVIC Rue de la bascule, 21 B-1180 UCCLE

Guy VAN MARCKE DE LUMMEN Chaussée de la Croix, 72 B B-1340 OTTIGNIES

Jean MOREAU Rue de France, 19 B-6100 CHARLEROI

Pierre ROBYN

Rue de Charleroi, 11

B-1400 NIVELLES

DESCRIPTIVE MEMORY attached to a request for

PATENT OF INVENTION filed by so-called Companies

GLAVERBEL 166, Chaussée de la Hulpe B. 1170 - Brussels (Watermael-Boitsfort)

Belgium and FOSBEL INTERNATIONAL Ltd. 285, Long Acre Nechells GB. Birmingham B7 5JR - Great Britain for:

Ceramic welding process and mixture of powders intended for such a process.

Priority: Great Britain N ° 89 16 951.0 of July 25, 1989.

Inventors: Léon Philippe ΜΟΤΓΕΤ

- Charles Michaël ZVOSEC

Stephen D. CHERICO

Alexandre ZIVKOVIC

Guy van MARCKE from LUMMEN

Jean MOREAU

Pierre ROBYN

Ceramic welding process and mixture of powders intended for such a process

The present invention relates to a ceramic welding process in which oxidizing gas and a mixture of refractory and combustible powders are sprayed against a surface, the fuel burns giving off enough heat to melt or at least partially soften the refractory powder and a coherent refractory mass is gradually formed against this surface. The invention also relates to a powder for ceramic welding, consisting of a mixture of refractory and combustible powders, intended for use in a ceramic welding process.

Ceramic welding processes can be useful for the manufacture of new refractory parts, for example parts with fairly complicated shapes, but in current commercial practice, they are most often used to coat or repair hot refractory structures, such as ovens of different types. They allow repair of eroded refractory surfaces (provided these surfaces are accessible) while the oven remains substantially at its working temperature and, in some cases, even while it is in operation. It is in any case desirable for this work not to have to deliberately cool the refractory structure below its normal working temperature. The fact of being able to avoid such cooling promotes the efficiency of the ceramic welding reactions, avoids deterioration of the structure due to thermal stresses created by such cooling and / or by the subsequent heating up to the working temperature, and also helps reduce oven downtime. .

In ceramic welding repair methods, refractory powder, combustible powder and oxidizing gas are projected against the place to be repaired, the fuel burns so that the refractory powder is at least partially melted or softened and a mass Refractory repair is gradually formed at the location to be repaired. The fuel used is typically silicon and / or aluminum, although other materials such as magnesium and zirconium can be used. The refractory powder can be chosen so that the chemical composition of the repair mass is as close as possible to the composition of the refractory to be repaired, it can therefore differ from it, for example so as to deposit a coating of greater refractoriness on the basic structure. In usual practice, the combustible and refractory powders are sprayed by means of a lance in the form of a mixture in a stream of oxidizing carrier gas.

Due to the intense heat released by the combustion of combustible powders at or near the surface to be repaired, this surface also softens or melts, and as a result the repair mass, which is itself largely melted, adheres firmly to the surface to be repaired, and effective and lasting repairs are obtained. Descriptions of these ceramic welding repair techniques can be found in British Patents Nos. 1,330,894 and 2,110,200.

To date, one of the most widespread uses of repair methods using the ceramic welding technique has been the refurbishment of coke ovens, which are made of silica refractories. The most widely used standard ceramic welding powder is that for repairing silica refractories and includes silica, with silicon and, optionally, aluminum as combustible powders. Silica refractories are actually the easiest to repair by the ceramic welding technique. This is at least partially due to the fact that the silica refractories have a relatively low refractant level, so that the temperatures (e.g. 1800 ° C or higher) reached in the area of the ceramic weld reaction easily allow the formation of 'an adherent and consistent repair mass, and the refractoriness requirements of this repair mass are usually no higher than those of the original silica refractory structure.

We have found, however, that certain problems arise when repairing very refractory materials or in other cases when the requirements of refractoriness of the ceramic weld mass are particularly restrictive. As examples of very refractory materials, mention may be made of refractories of chromium-magnesia, magnesia-alumina, alumina-chromium, magnesia-chromium, chromium and magnesia, refractories with a high alumina content , and refractories containing a considerable proportion of zirconium such as Corhart (trademark) Zac (a fused alumina-zirconia-zircon refractory). To obtain the formation of a ceramic weld mass which has a refractoriness and / or a composition approaching or equivalent to those of such very refractory materials, it is not enough to always use a powder for ceramic weld standard as described above.

A particular problem, which arises in the case of a ceramic welding repair mass which must be subjected to very high temperatures during use, is to avoid the presence of a phase inside the mass. repair, which has an insufficiently high softening or melting point. The cohesion of a repair compound containing such a phase is deteriorated at elevated temperatures and its resistance to corrosion at elevated temperatures is not as good as one would expect. In general, a refractory phase which is relatively less physically resistant to heat will also be more easily attacked chemically at high temperatures.

One of the objects of the present invention is to provide a ceramic welding process, as well as a ceramic welding powder for application in such a process, which results in the formation of a welding in which the appearance of a phase of low refractoriness is reduced, and can even, in certain embodiments of the invention, be avoided.

The present invention relates to a ceramic welding process in which oxidizing gas and a mixture of refractory and combustible powders are sprayed against a surface, the fuel burns giving off enough heat to melt or at least partially soften the refractory powder and a coherent refractory mass is gradually formed against this surface, characterized in that the combustible powder is present in a proportion which is not more than 15% by weight of the total mixture and comprises at least two metals chosen from aluminum, magnesium , chromium and zirconium, in that at least the major part by weight of the refractory powder consists of one or more oxide (s) among magnesia, alumina and chromium oxide, and in that the proportions molars of silica and calcium oxide present in the refractory powder, if any, satisfy the following expression: [Si02]% <0.2 + [CaO]%. The invention also relates to a powder for ceramic welding, consisting of a mixture of refractory and combustible powders, intended for use in a ceramic welding process in which oxidizing gas and the mixture of refractory and combustible powders are sprayed against a surface, the fuel burns giving off enough heat to melt or at least partially soften the refractory powder and a coherent refractory mass is gradually formed against this surface, characterized in that the combustible powder is present in a proportion which is not greater than 15% by weight of the total mixture and comprises at least two metals chosen from aluminum, magnesium, chromium and zirconium, in that at least the major part by weight of the refractory powder consists of one or more oxide (s) among magnesia, alumina and chromium oxide, and in that the molar proportions of silica e t of calcium oxide present in the refractory powder, if it occurs, satisfy the following expression: [Si02]% <0.2 + [CaO]%. The use of such a powder in such a process gives rise to a ceramic solder mass which is highly resistant to molten materials such as molten metals and metallurgical slag, and molten glass. Such welds have good resistance to liquids and corrosive gases at high temperatures such as those encountered for example in the work or manufacture of steel, copper, ahiminium, nickel and glass, and in crucibles or other chemical reactors exposed to the action of a flame. Such solder masses are also able to adhere well to basic structures with a high degree of refractoriness.

The occasional loss of refractoriness in a ceramic solder mass is often observed when using a solder powder containing appreciable amounts of silica or silica formers, and may be due to the formation of a glassy phase, in the solder mass, at very high temperatures which can be reached during ceramic solder reactions. Such a glassy phase often has a relatively low melting point and it can also be attacked relatively easily by molten material such as molten metals, slag and molten glass, and its presence would therefore adversely affect the quality of the welding as a whole. Silica is often present in refractories, either as a deliberately added constituent or as an impurity. By adopting the present invention, the proportion of silica allowed is limited to an amount which tends to form a mass of refractory weld in which such a vitreous phase is greatly reduced or avoided and whose refractoriness is improved.

The refractoriness of the weld mass formed is improved if, as is preferred, the molar proportions of silica and calcium oxide present in the refractory powder, if any, satisfy the following expression: [Si02 ]% <[CaO]%. This prevents the presence of an acid phase in the weld and improves its resistance to corrosion by molten glass or metallurgical slag.

Preferably, the refractory powder is substantially free of silica. The adoption of this characteristic also militates against the formation of any silica-based vitreous phase in the weld mass formed.

Advantageously, the projected refractory powder consists substantially of one or more oxide (s) among zirconia, magnesia, alumina and chromium oxide. Such materials are capable of forming masses of very high refractoriness.

According to the present invention, the combustible powder comprises at least two metals chosen from aluminum, magnesium, chromium and zirconium. Such fuels burn giving oxides which are of good refractory quality and which are either amphoteric (alumina and zirconia) or basic (magnesia or chromium oxide) and therefore such fuels contribute to the formation of a refractory mass which is very resistant to corrosion by molten glass or metallurgical slag. This characteristic of the invention also allows very great flexibility in the choice of combustible elements, and therefore of the refractory oxide produced by the combustion of these elements, so that the composition of the possible refractory weld mass can be modified. as desired.

Advantageously, the combustible powder comprises aluminum with one or more element (s) from magnesium, chromium and zirconium. Aluminum has excellent combustion characteristics for the intended purpose, and it is also relatively readily available in powder form.

Preferably, no element constitutes more than 80% by weight of said combustible powder. It has been noted that this has the advantage of allowing the conditions under which combustion occurs to be controlled. For example, by adopting this preferred characteristic, a highly reactive main combustible ingredient is limited to 80% of the total fuel, and the remainder of the fuel, at least 20% by weight, can consist of a combustible element which reacts more slowly to control the rate of combustion. Conversely, a less active main combustible ingredient may have its reaction speed increased by the addition of at least 20% by weight of one or more faster reacting combustible element (s).

Advantageously, the combustible powder comprises an alloy containing at least 30% by weight of a metal chosen from aluminum, magnesium, chromium and zirconium, the rest of the alloy consisting of at least one element other than this metal chosen, this element also being oxidizable to form a refractory oxide. The use of alloy particles as fuel is particularly advantageous since it allows the conditions under which combustion occurs to be controlled.

The sprayed powder mixture does not necessarily have to be completely free of silicon to avoid or reduce the formation of acidic or vitreous siliceous phases with relatively low refractoriness. Under certain circumstances, silicon may be present in the combustible powder. In fact, it has been found that the use of silicon as a fuel can be advantageous because it stabilizes the way in which the ceramic welding reactions take place. For this reason, in certain preferred embodiments of the invention, silicon is present in the fuel in the form of a silicon alloy and at least one metal among aluminum, magnesium, chromium and zirconium . Silicon as an alloying component is capable of having a favorable effect on the way in which the combustion reactions take place during the implementation of the process of the invention. For example, silicon in an alloy with magnesium can have the effect of moderating the way in which highly active magnesium burns. In addition, because an alloy is an intimate mixture of its constituents, the intimacy of the reaction products is favored, and this militates against the tendency of silicon to give rise to a distinct acid or glassy phase inside the ceramic solder mass formed.

In other preferred embodiments of the invention, again to avoid the introduction of an acidic or vitreous siliceous phase into the solder mass formed, it is preferred that the molar quantity of silicon present, if it occurs, in the projected mixture is not greater than the molar amount, if any, of zirconium calculated as elemental zirconium. For example, the refractory powder could contain a proportion of zirconium orthosilicate (zircon) which is an entirely acceptable ingredient of high refractoriness. As a variant, or in addition, the combustible powder could contain a proportion of elemental silicon which could combine with zirconium in the mixture (in the form of elementary zirconium or zirconia) to form zircon, without introducing an acid phase into the mass of weld formed. Consequently, in some of these preferred embodiments of the invention, the fuel comprises elementary silicon in the form of particles having an average dimension of less than 10 μm, preferably less than 5 μm, and in that the mixture comprises particles of zirconia having dimensions of less than 150 μm, these particles of zirconia being present in a molar quantity which is at least equal to the molar quantity of elementary silicon in the mixture. It has been found that the use of this optional feature of the invention facilitates the formation of zircon (zirconium orthosilicate) in the solder mass formed, this resulting from ceramic solder reactions, so that this mass is substantially free silica as such, and the risk of a weakly refractory vitreous phase being formed is low. In this way, one can benefit from the advantages of using silicon as a fuel without the risk of encountering the disadvantages of the possible incorporation of a glassy or acidic siliceous phase into the solder mass.

In other preferred embodiments of the invention, the sprayed combustible powder is substantially free of silicon. Adopting this characteristic will prevent the formation of any silica-based glassy phase in the weld mass formed.

In certain preferred embodiments of the invention, the projected combustible powder comprises magnesium and aluminum. The oxidation of aluminum and magnesium in appropriate proportions can generate sufficient heat for carrying out the process of the invention, and gives rise to the formation of refractory oxides which can be incorporated into a solder mass. strongly refractory.

Preferably, the projected combustible powder comprises, by weight, more aluminum than magnesium; for example, aluminum may be present in the fuel in a molar amount approximately twice that of magnesium. This promotes the formation of spinel (magnesium aluminate) in the weld. Spinel is a very useful highly refractory material.

Advantageously, the magnesium is incorporated into the combustible powder sprayed in the form of a magnesium / aluminum alloy. The use of an alloy powder of these metals rather than a mixture of powders also promotes the formation of spinel rather than separate oxides as a result of the ceramic welding reactions. The composition of the alloy may vary, or additional aluminum or magnesium may be added thereto in order to adjust the relative proportions of aluminum and magnesium in the combustible powder if desired.

In other preferred embodiments of the invention, the sprayed combustible powder comprises chromium and aluminum. Such combustible powders are useful for forming refractory solder masses with a high chromium content, and advantageously, the projected combustible powder comprises, by weight, more chromium than aluminum.

Preferably, at least 60%, and in certain embodiments, at least 90% by weight of the projected combustible powder is composed of grains having a dimension less than 50 μm. This characteristic promotes rapid and efficient combustion of the combustible powder to form a coherent refractory weld mass.

The method of the invention is particularly advantageous for treating refractories which themselves are basic rather than acidic, and therefore, the application of the method to repairing a structure made of basic refractory material is preferred.

Different powders for specific ceramic welding, in accordance with the invention will now be described by way of example only. EXAMPLE 1

A powder for ceramic welding includes, by weight:

Magnesia 82%

Zirconia 10%

Alloy Mg / Al 5%

Aluminum grains 3%

The magnesia used consists of grains having a dimension of up to 2mm. Zirconia is made up of grains smaller than 150pm. The Mg / Al alloy nominally contains 30% by weight of magnesium and 70% of aluminum; the grains have a dimension less than 100 μm and their average dimension is approximately 42 μm, and the aluminum is in the form of grains having a maximum nominal dimension of 45 μm.

The magnesia used has a purity of 99% by weight. It contains 0.8% by weight of calcium oxide, and approximately 0.05% by weight of silica. The SiC ^ / CaO molar ratio in magnesia is therefore 1: 17.4.

Another suitable magnesia composition has a purity of 98% by weight. It contains 0.6% by weight of calcium oxide, and 0.5% by weight of silica. The SiC ^ / CaO molar ratio in magnesia is therefore 1: 1.28.

This powder can be sprayed at a rate of 1 to 2 tonnes per hour by means of a lance, well known in the ceramic welding technique using oxygen as a carrier gas, to repair a steelworks converter formed from basic magnesian refractories, the place of repair being at a temperature of 1400 ° C immediately before the projection. EXAMPLE 2

A powder for ceramic welding includes, by weight:

Magnesia 82%

Zirconia 10%

Aluminum grains 3%

Aluminum flakes 3.5%

1.5% magnesium grains

Magnesia, zirconia and aluminum grains have the dimensions indicated in Example 1. The composition of magnesia is one of those given in Example 1. Magnesium has a maximum nominal grain size of approximately 75 μm and an average grain size less than 45pm. The aluminum flakes have a specific surface (measured by permeametry 9

Griffin) greater than 7000cm / g.

This powder can be sprayed, as described in Example 1, to repair a steel converter formed from magnesium-chromium refractories, the place of repair being at a temperature of 1400 ° C. immediately before such a projection. EXAMPLE 3

A powder for ceramic welding includes, by weight:

82% chromic oxide

Zirconia 10%

Alloy Mg / Al 5%

Aluminum grains 3% The chromic oxide used includes grains up to 2mm in size. The other materials are as described in Example 1. The chromic oxide used is substantially free of silica, only traces being simply detected on analysis.

This powder can be sprayed at a rate of 150 to 200 kg / h by means of a lance, well known in the ceramic welding technique, using oxygen as a carrier gas to repair a converter for copper formed of refractories magnesia-chromium, the location of the repair being at a temperature of 1100 ° C. immediately before such a projection. EXAMPLE 4

A powder for ceramic welding includes, by weight:

82% chromic oxide

Zirconia 10%

Aluminum grains 3%

Aluminum flakes 3.5%

1.5% magnesium grains The chromic oxide is as described in Example 3. The other materials are as described in Example 2.

This powder can be sprayed at a rate of 150 to 200 kg / h using a lance, well known in the ceramic welding technique, using oxygen as a carrier gas to repair a steel degassing nozzle. formed of magnesium-chrome refractories, the place of repair being at a temperature of 1100 ° C. immediately before such a projection. As a variant of this example, magnesium is replaced by zirconium having an average grain size of approximately 10 to 15 μm, taking all the precautions necessary having regard to the well known high reactivity of zirconium. EXAMPLE 5

A powder for ceramic welding includes, by weight:

90% chromic oxide

Chrome 8%

Aluminum flake 2%

The chromium is in the form of grains having a maximum nominal dimension of approximately 100 μm and an average dimension between 25 and 30 μm. The chromic oxide is as described in example 3. The aluminum flakes have a specific surface (measured by Griffin permeametry) greater than 7000 cm 2 / g.

This powder can be sprayed at a rate of 40 kg / h using a lance, well known in the ceramic welding technique, using oxygen as a carrier gas to repair refractory blocks from Corhart (trademark). Zac (alumina-zircon-molten zirconia) placed at the surface of the melt in a glass melting furnace, the place of repair being at a temperature of 1500 ° C to 1600 ° C immediately before such a projection .

The powder is also suitable for repairing chromium refractory (i.e. a refractory containing more than 25% chromic oxide and less than 25% magnesia) also located at the surface of the melt in a glass melting furnace. EXAMPLE 6

A powder for ceramic welding includes, by weight:

Magnesia 72%

Zirconia 10%

Carbon 3%

Aluminum grains 3%

1.5% Mg / Al alloy

Carbon is coke having an average diameter of about 1.25mm. The other materials are as described in Example 1. This powder can be sprayed as described in Example 1 to repair a steelworks converter formed of magnesia-carbon refractories. EXAMPLE 7

A powder for ceramic welding includes, by weight:

Magnesia 82%

Zirconia 10%

2% silicon

Magnesium 4%

Aluminum flake 2%

The silicon is in the form of grains having an average dimension of 4 μηχ The zirone has a maximum nominal dimension of 150 μm. The other materials are as described in the preceding examples. This powder can be sprayed at a rate of 150 kg / h to repair a pocket of basic magnesian refractory steel. EXAMPLE 8

A powder for ceramic welding includes, by weight:

Alumina 92%

Magnesium 2%

Aluminum grains 6% The alumina used is an electrofused alumina containing, by weight, 99.6% of A ^ O ^. It contains 0.05% CaO and 0.02% Si02. The SiO2 / Ca0 molar ratio in this alumina is therefore 1: 2.68. Alumina has a maximum nominal dimension of 700 μm and aluminum and magnesium have dimensions as described in Example 2. Such a powder can be used as described in Example 5 to repair refractory blocks of Corhart (trademark) Zac arranged below the surface level of the melt in a glass melting furnace after the furnace has been partially emptied to provide access to the repair site.

As a variant of this example, the fused alumina is replaced by tabular alumina. The tabular alumina used has a maximum nominal size of 2 mm, and it contains, by weight, 99.5% of A ^ O ^. It contains 0.073% of CaO and 0.085% of SiO ^ The molar ratio Si02 / CaO in this alumina is therefore 1: 0.86, but it is clear that it satisfies the expression: [Si02]% <0.2 + [CaO]%. EXAMPLE 9

A powder for ceramic welding includes, by weight:

Magnesia 80%

Zirconia 10%

Alloy Mg / Si 5%

5% Mg / Al alloy The magnesium / silicon alloy contains equal proportions by weight of the two elements and has an average dimension of approximately 40 μm. The other materials are as described in Example 1. This powder can be sprayed as described in Example 1 to repair a refractory wall formed of a basic magnesian refractory. EXAMPLES 10 TO 16 As a variant of Examples 1 to 4, 6, 7 and 9, the zirconia is replaced by tabular alumina as described in Example 8.

As a variant of Examples 1, 3, 6, 9, 10, 12, 14 and 16, the alloy particles containing 30% magnesium and 70% aluminum have a maximum dimension which is not greater than 75 μm and a average size less than 45pm. In other variations, the alloy contains equal weights of magnesium and aluminum.

Claims (33)

1. Ceramic welding method in which oxidizing gas and a mixture of refractory and combustible powders are sprayed against a surface, the fuel burns giving off enough heat to melt or at least partially soften the refractory powder and a coherent refractory mass is formed progressively against this surface, characterized in that the combustible powder is present in a proportion which is not more than 15% by weight of the total mixture and comprises at least two metals chosen from Γ aluminum, magnesium, chromium and zirconium , in that at least the major part by weight of the refractory powder consists of one or more oxide (s) among magnesia, alumina and chromium oxide, and in that the molar proportions of silica and calcium oxide present in the refractory powder, if it occurs, satisfy the following expression: [Si02]% <0.2 + [CaO]%. 2. Ceramic welding method according to claim 1, characterized in that the molar proportions of silica and calcium oxide present in the refractory powder, if any, satisfy the following expression: [Si02]% <[ CaO]%. 3. Ceramic welding method according to one of claims 1 or 2, characterized in that the refractory powder is substantially free of silica. 4. Ceramic welding method according to one of claims 1 to 3, characterized in that the sprayed refractory powder consists substantially of one or more oxide (s) among zirconia, magnesia, alumina and chromium oxide . 5. Ceramic welding method according to one of claims 1 to 4, characterized in that the combustible powder comprises aluminum with one or more element (s) among magnesium, chromium and zirconium. 6. Ceramic welding method according to one of claims 1 to 5, characterized in that no element constitutes more than 80% by weight of said combustible powder. 7. Ceramic welding method according to one of claims 1 to 6, characterized in that the combustible powder comprises an alloy containing at least 30% by weight of a metal chosen from aluminum, magnesium, chromium and zirconium, the rest of the alloy consisting of at least one element other than this chosen metal, this element also being oxidizable to form a refractory oxide. 8. Ceramic welding method according to one of claims 1 to 7, characterized in that any silicon present in the said fuel is in the form of a silicon alloy and at least one metal among aluminum, magnesium , chromium and zirconium. 9. Ceramic welding method according to one of claims 1 to 8, characterized in that the molar quantity of silicon present, if it occurs, in the sprayed mixture is not greater than the molar quantity, if it does , of zirconium calculated as elementary zirconium. 10. Ceramic welding method according to claim 9, characterized in that the fuel comprises elementary silicon in the form of particles having an average size less than 10 pm, preferably less than 5 pm, and in that the mixture comprises particles of zirconia having dimensions less than 150 μm, these zirconia particles being present in a molar quantity which is at least equal to the molar quantity of elementary silicon in the mixture. 11. Ceramic welding method according to one of claims 1 to 7, characterized in that the projected combustible powder is substantially free of silicon. 12. Ceramic welding method according to one of claims 1 to 11, characterized in that the projected combustible powder comprises magnesium and aluminum 13. A ceramic welding method according to claim 12, characterized in that the projected combustible powder comprises, by weight, more aluminum than magnesium 14. Ceramic welding method according to one of claims 12 or 13, characterized in that the magnesium is incorporated in the projected combustible powder, in the form of a magnesium / aluminum alloy 15. Ceramic welding method according to one of claims 1 to 11, characterized in that the projected combustible powder comprises chromium and aluminum 16. Ceramic welding method according to claim 15, characterized in that the projected combustible powder comprises, by weight, more chromium than aluminum 17. Ceramic welding method according to one of claims 1 to 16, used for repairing a structure made of basic refractory material. 18. Powder for ceramic welding, consisting of a mixture of refractory and combustible powders, intended for use in a ceramic welding process in which oxidizing gas and the mixture of refractory and combustible powders are sprayed against a surface, the fuel burns by giving off sufficient heat to melt or at least partially soften the refractory powder and a coherent refractory mass is gradually formed against this surface, characterized in that the combustible powder is present in a proportion which is not more than 15% by weight of the total mixture and comprises at least two metals chosen from aluminum, magnesium, chromium and zirconium, in that at least the major part by weight of the refractory powder consists of one or more oxide (s) from magnesia , alumina and chromium oxide, and in that the molar proportions of silica and calcium oxide present in the refractory powder, if any, satisfy the following expression: [Si02]% <0.2 + [CaO]%. 19. Powder for ceramic welding according to claim 18, characterized in that the molar proportions of silica and calcium oxide present in the refractory powder, if any, satisfy the following expression: [Si02]% <[ CaO]%. 20. Powder for ceramic welding according to one of claims 18 or 19, characterized in that the refractory powder is substantially free of silica. 21. Powder for ceramic welding according to one of claims 18 to 20, characterized in that the refractory powder consists substantially of one or more oxide (s) among zirconia, magnesia, alumina and chromium oxide. 22. Powder for ceramic welding according to one of claims 18 to 21, characterized in that the combustible powder comprises aluminum with one or more elements) among magnesium, chromium and zirconium. 23. Powder for ceramic welding according to one of claims 18 to 22, characterized in that no element constitutes more than 80% by weight of said combustible powder. 24. Powder for ceramic welding according to one of claims 18 to 23, characterized in that the combustible powder comprises an alloy containing at least 30% by weight of a metal chosen from aluminum, magnesium, chromium and zirconium, the rest of the alloy consisting of at least one element other than this chosen metal, this element also being oxidizable to form a refractory oxide. 25. Ceramic welding powder according to one of claims 18 to 24, characterized in that any silicon present in the said fuel is in the form of an alloy of silicon and at least one metal among aluminum, magnesium , chromium and zirconium. 26. Ceramic welding powder according to one of claims 18 to 25, characterized in that the molar quantity of silicon, if it occurs, present in the sprayed mixture is not greater than the molar quantity, if it does , of zirconium calculated as elementary zirconium. 27. Ceramic welding powder according to claim 26, characterized in that the said fuel comprises elementary silicon in the form of particles having an average dimension of less than 10 pm, preferably less than 5 pm, and in that the mixture comprises particles of zirconia having dimensions of less than 150 μm, these particles of zirconia being present in a molar quantity which is at least equal to the molar quantity of elementary silicon in the mixture. 28. Ceramic welding powder according to one of claims 18 to 24, characterized in that the projected combustible powder is substantially free of silicon. 29. Ceramic welding powder according to one of claims 18 to 28, characterized in that the projected combustible powder comprises magnesium and aluminum. 30. Ceramic welding powder according to claim 29, characterized in that the projected combustible powder comprises, by weight, more aluminum than magnesium. 31. Ceramic welding powder according to one of claims 29 or 30, characterized in that the magnesium is incorporated in the combustible powder sprayed in the form of a magnesium / aluminum alloy. 32. Ceramic welding powder according to one of claims 18 to 28, characterized in that the projected combustible powder comprises chromium and aluminum. 33. Ceramic welding powder according to claim 32, characterized in that the projected combustible powder comprises, by weight, more chromium than aluminum.