NL9001330A - CERAMIC MELTING PROCESS AND POWDER MIXTURE FOR USE BEFORE IT. - Google Patents

Description

Ceramic melting process and powder mixture for the user for.

This invention relates to a ceramic melting process in which oxidizing gas and a mixture of refractory particles and fuel powder are projected onto a surface and the fuel is burned to generate sufficient heat so that the refractory powder is at least partially melted or softened and a coherent fire solid mass is gradually built up against that surface.

This invention also relates to a ceramic melting powder mixture comprising a refractory powder and a fuel powder for use in such a ceramic melting process.

Ceramic melting processes are useful for the production of new refractory bodies, for example bodies of rather complicated shapes, but in current commercial practice they are often used for coating or repairing hot refractory structures, such as melting furnaces or furnaces of various kinds, and they allow eroded areas of the refractory structure (provided these areas are accessible) to be repaired while the structure is essentially at its operating temperature and in some cases even while the structure is still operating. In any case, it is desirable that there is no intentional cooling of the refractory structure from its normal operating temperature. Prevention of such deliberate cooling tends to promote the efficiency of ceramic melting reactions, avoids further structural damage due to the thermal stresses caused by such cooling and / or subsequent reheating to operating temperature and also helps to reduce the melting furnace "down time".

In ceramic melt repair processes, refractory powder, fuel powder and oxidizing gas are projected against the repair site and the fuel is burned such that the refractory powder is at least partially melted or softened and a refractory repair mass is gradually built up at the repair site. The fuel used typically consists of silicon and / or aluminum, although other materials such as magnesium and zirconium can also be used. The refractory powder can be selected so that the chemical composition of the repair mass is as close as possible to the composition of the refractory furnace to be repaired, although it can be varied, for example, such that a higher quality refractory mass coating on the base structure is being dropped off. In conventional practice, the fuel and refractory powder is projected from a lance as a mixture in a stream of oxidant carrier gas.

As a result of the intense heat generated during the combustion of fuel powders at or near the surface to be repaired, that surface also becomes softer or melted, and as a result, the repair materials, which themselves are highly melted together, become strongly bonded to the repaired wall, and results in a highly effective and durable repair. Previous descriptions of ceramic melt repair techniques can be found in British Patent Nos. 1,330,894 and 2,110,200.

So far, one of the most widespread uses of ceramic melt repair methods has been to refurbish coke ovens formed from silica refractories. The standard ceramic melting powder most commonly used for the repair of silicon oxide refractory furnaces includes silicon oxide along with silicon and optionally aluminum as the fuel powder. In fact, silicon oxide refractory furnaces are easiest to repair by at least partial ceramic melting, because silicon oxide refractory furnaces are of a relatively low refractory quality material, so that the temperatures (eg 1800 ° C or more) which are reached in the ceramic melt reaction zone, easily allow the formation of an adherent coherent repair mass, and the refractory quality requirements of that repair mass usually do not exceed those of the original silicon oxide refractory structure.

However, we have found that certain problems arise in repairing better quality refractory furnaces or in other cases when the refractory quality requirements of the ceramic melt are particularly stringent. Examples of high quality refractories include chromium magnesite, magnesite alumina, alumina chromium, magnesite chromium, chromium, and magnesite refractory masses, high alumina refractory masses, and refractory masses containing a significant amount of zirconium, such as Corhart (trademark ) Zac (a fused aluminum oxide-zircon-zirconia refractory mass). To achieve the formation of a ceramic melting mass having a refractory quality and / or composition approximating or corresponding to such high quality refractory masses, it is not always sufficient to use a standard ceramic melting powder as described above.

A particular problem that arises in the case of a ceramic melt repair mass which must be subjected to very high temperatures during its working life is the prevention of a phase in the repair mass which has an insufficiently high softening or melting point. The coherence of a repair mass containing such a phase is reduced at high temperatures and its resistance to corrosion at high temperatures is also not as hoped for. In general, a refractory phase, which is relatively less physically resistant to heat, is also more easily chemically attacked at high temperatures.

It is an object of this invention to provide a ceramic melting process, and a ceramic melting powder for use in such a process, which results in the formation of a melting mass in which the occurrence of such low quality refractory phase tends to be reduced, and in some embodiments of the invention can even be prevented.

According to the present invention, there is provided a ceramic melting process in which oxidizing gas and a mixture of refractory and fuel powders are projected against a surface and the fuel is burned to generate sufficient heat so that the refractory powder at least partially melts or softens and becomes a coherent refractory mass built up gradually against the surface, characterized in that the refractory powder is present in a ratio of not more than 15% by weight of the total mixtures comprising at least two metals selected from aluminum, magnesium, chromium and zirconium, and that at least the largest weight part of the refractory powder consists of one or more of magnesium oxide, alumina and chromium oxide, and that the molar ratios of silicon oxide and calcium oxide present in the refractory powder (if any) meet the following expression: [SiO2]% <0, 2 + [CaO]%.

The invention also provides a ceramic melting powder, which is a mixture of refractory and fuel powders, for use in a ceramic melting process, in which oxidizing gas and the mixture of refractory and fuel powders are projected against a surface and the fuel is burned in order to generate sufficient heat so that the refractory powder at least partially melts or softens, and a coherent refractory mass is gradually built up against the surface, characterized in that the fuel powder is present in a ratio of not more than 15 wt. % of the total mixture and comprising at least two metals selected from aluminum, magnesium, chromium and zirconium, and that at least the major part by weight of the refractory powder comprises one or more of magnesium oxide, aluminum oxide and chromium oxide, and that the molar ratios of silicon oxide and calcium oxide are present in the refractory powder (submit) comply with the following expressions ng: [SiO2]% <0.2 + [CaO]%.

The use of such a powder in such a process gives rise to a ceramic melt mass which is highly resistant to molten materials, such as molten metals and metal slag, and molten glass. Such melt masses have good resistance to corrosive liquids and gases at elevated temperatures, such as those encountered, for example, in the machining or fabrication of steel, copper, aluminum, nickel and glass, and in crucibles or other chemical reactors that are exposed to flame. Such melts are also capable of adhering well to high quality refractory base structures.

The incidental loss of refractory quality in the molded ceramic melt is often observed when using a melting powder containing significant amounts of silica or silica-forming materials and may contribute to the formation of a glassy phase in the melt at very high temperatures , which can be achieved during the ceramic melting reactions. Such a glassy phase often has a relatively low melting point, and it can also be relatively easily attacked by molten material such as molten metals, slag and molten glass, and thus its presence would degrade the quality of the melt mass as a whole. Silicon oxide is often present in refractory masses, either as an intentionally added ingredient or as an impurity. By applying the present invention, we limit the allowable ratio of silicon oxide to an amount that tends to form a refractory melt mass, thereby greatly reducing or preventing such a glassy phase and the refractory quality of the melt mass formed is improved.

The refractory quality of the formed melt mass is improved if, as preferred, the molar ratios of silica and calcium oxide contained in the refractory powder (if any) meet the following expression: [SiO 2]% <[CaO]%. This promotes the prevention of an acid phase in the melt and improves its resistance to corrosion by molten glass or metallurgical slag.

It is preferred that the refractory powder be essentially free of silicon oxide. The adoption of this feature also contradicts the formation of any silica-based glassy phase in the melt melt formed.

Advantageously, the projected refractory powder essentially contains one or more of zirconia, magnesium oxide, aluminum oxide and chromium oxide. Such materials are capable of forming very high quality refractory materials.

In accordance with the invention, the fuel powder comprises at least two metals selected from aluminum, magnesium, chromium and zirconium. Such fuels burn to give oxides, which have good refractory quality and which are either amphoteric (aluminum oxide and zirconia)

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or be basic (magnesium oxide or chromium oxide), and accordingly such fuels will contribute to the formation of a refractory mass, which is highly resistant to corrosion by molten glass or metallurgical slag. This characteristic also provides significant flexibility in the selection of fuel elements, and thus in the refractory oxide product obtained upon combustion of those elements so that the composition of the final refractory melt mass formed can be varied if desired.

The fuel powder advantageously comprises aluminum together with one or more of magnesium, chromium and zirconium. Aluminum has excellent combustion properties for its intended purpose, and it is also relatively easily available as a powder.

Preferably, no element constitutes more than 80% by weight of the said fuel powder. This has been found to be beneficial in controlling the conditions under which combustion takes place. Thus, for example, in adopting this preferred feature, a highly reactive main fuel component is limited to 80% of the total fuel and the remaining part of the fuel, at least 20% by weight, may be formed by a fuel element which reacts more slowly in order to control the combustion rate. Conversely, the reaction rate of a less active main fuel component can be increased by adding at least 20% by weight of one or more fuel elements, which react faster.

Advantageously, the fuel powder comprises an alloy comprising at least 30% by weight of a metal selected from aluminum, magnesium, chromium and zirconium, the balance of the alloy being formed by at least one element other than such a selected metal, which element is also oxidizable to form a refractory oxide. The use of alloy particles as fuel is particularly valuable in controlling the conditions under which combustion takes place.

The projected mixture of powders need not necessarily be completely free of silicon, in order to prevent or reduce the formation of relatively low-quality acidic or glassy silicon-containing phases.

Under some circumstances, silicon may be present in the fuel powder. Indeed, we have found that the use of silicon as a fuel component can have advantages in stabilizing the manner in which the ceramic melting reactions proceed. Therefore, in some preferred embodiments of the invention, silicon is present in said fuel in the form of an alloy of silicon with at least one of aluminum, magnesium, chromium and zirconium. The use of silicon as an alloying component can have a beneficial effect on the rate at which the combustion reactions take place while carrying out the process of the invention. For example, silicon in a magnesium alloy can have the effect of tempering the rate at which the highly active magnesium burns. Furthermore, intimate mixing of the reaction products is promoted because an alloy is an intimate mixture of its components, and this argues against the silicon, which may give rise to a distinct acid or glassy phase within the refractory melting mass formed.

In other preferred embodiments of the invention, again to promote the prevention of inducing a silicon-containing acid or glassy phase in the melt mass formed, it is preferred that the molar amount of silicon (if any) be present in the mixture is not more than the molar ratio (if any) of zirconium. For example, the refractory powder may contain a portion of zirconium orthosilicate (zircon) which is a very acceptable high quality refractory component. Alternatively, or in addition, the fuel powder may contain a portion of elemental silicon which may bond with zirconium (or as elemental zirconium or as zirconium oxide) in the mixture to form zircon without inducing an acid phase in the molten melt formed .

Thus, in some such preferred embodiments of the invention, said fuel comprises elemental silicon in the form of particles having an average grain size of less than 10 microns, preferably less than 5 microns, and the mixture comprises zirconia particles with grain sizes below 150 micrometer, such zirconia particles being present in a molar ratio at least equal to the molar ratio of elemental silicon in the mixture. We have found that using this optical feature of the invention promotes the formation of zircon (zirconium orthosilicate) in the formed melt mass as a result of the ceramic melt reactions, so that the mass in itself is essentially free of silicon oxide, and the risk of forming a low quality glassy refractory phase is small. In this way, the advantages of using silicon as a fuel can be achieved without also taking into account the disadvantages of incorporating a possible glassy acid phase of silicon oxide into the melt mass.

In other preferred embodiments of the invention, the projected fuel powder is essentially free of silicon. The adoption of this feature will prevent the formation of any silica-based glassy phase in the melt mass formed.

In some preferred embodiments of the invention, the projected fuel powder includes magnesium and aluminum. The oxidation of aluminum and magnesium in suitable proportions can generate sufficient heat for the operation of the process of the invention and gives rise to the formation of refractory oxides which can be incorporated into a highly refractory melt mass.

Preferably, the projected fuel powder, by weight, comprises more aluminum than magnesium, for example, aluminum may be present in the fuel in a molar ratio of about twice that of magnesium. This promotes the formation of spinel (magnesium aluminate) in the melt mass. Spinel is a very useful high quality refractory material.

Magnesium is advantageously incorporated into the projected fuel powder in the form of a magnesium / aluminum alloy. The use of a powdered alloy of these metals in place of a mixture of powders further promotes spinel formation rather than the individual oxides as a result of the ceramic melting reactions. The alloy composition may be varied, or additions of additional aluminum or magnesium may be effected to control the relative proportions of aluminum and magnesium in the fuel powder, as desired.

In other preferred embodiments of the invention, the projected fuel powder comprises chromium and aluminum. Such fuel powders are useful for forming high chromium refractory melting masses, and advantageously such a projected fuel powder, by weight, contains more chromium than aluminum.

Preferably at least 60% by weight and in some embodiments at least 90% by weight of the projected fuel powder has a grain size below 50 microns. This promotes rapid and effective combustion of the fuel powder to form a coherent refractory melt mass.

The method of the invention is particularly advantageous when used for the treatment of refractory furnaces, which themselves have a basic character rather than an acidic character, and accordingly it is preferred that the method be used for the repair of a structure that has been built of a refractory basic material.

Various specific ceramic melting powders according to the invention will now be described by way of example only.

Example 1

A ceramic melting powder, by weight, includes the following:

Magnesium Oxide 82% Mg / Al Alloy 5%

Zirconia 10% Al granules 3%

The magnesium oxide used had grain sizes up to 2 mm. The zirconia had grain sizes below 150 microns. The Mg / Al alloy contained a nominal 30 wt% magnesium and 70 wt% aluminum, with grain sizes below 100 microns and an average grain size of about 42 microns, and the aluminum in the form of grains was a nominal maximum size of 45 microns.

The magnesium oxide used had a purity of 99% by weight. It contained 0.8 wt% calcium oxide, and 0.05 wt% silicon oxide. The molar ratio of SiO 2 to CaO in the magnesium oxide was therefore 1: 17.4.

Another magnesium oxide composition suitable for use had a purity of 98% by weight. It contains 0.6 wt% calcium oxide, and 0.5 wt% silicon oxide. The molar ratio of SiO 2 to CaO in this magnesium oxide composition is therefore 1: 1.28.

Such a powder can be projected from a lance at a rate of 1 to 2 tons per hour, as is well known in the art of the ceramic melting technique, using oxygen as the carrier gas for the repair of a steel converter formed of basic magnesium oxide refractory material, where the repair site has a temperature of 1400 ° C immediately prior to such projection.

Example 2

A ceramic melting powder, by weight, includes the following:

Magnesium oxide 82% Al granules 3%

Zirconia 10% Al Flak 3.5%

Mg granules 1.5%

The magnesium oxide, zirconia and aluminum granules had grain sizes as shown in Example 1. The magnesium oxide composition was one of those shown in Example 1. The magnesium had a nominal maximum size of about 75 microns and an average grain size of less than 45 microns. The aluminum chip had a specific surface area (measured by Grif fin permeametry) of more than 7,000 cm / g.

Such a powder can be projected as described in Example 1 for the repair of a steel converter formed from magnesium oxide-chromium refractory material, the repair site having a temperature of 1400 ° C immediately prior to such projection.

Example 3

A ceramic melting powder, by weight, includes the following:

Chromium oxide 82% Mg / Al alloy 5%

Zirconia 10% Al granules 3%

The chromium oxide had grain sizes of up to 2mm. The other materials were specified as in Example 1.

The chromium oxide was essentially free of silicon oxide and the smallest traces were found upon analysis.

Such a powder can be projected from a lance at a rate of 150 to 200 kg / h, in a manner known per se in the ceramic melting technique, using oxygen as the carrier gas, for the repair of a copper converter formed of magnesium oxide-chromium- refractory material, the repair site having a temperature of 1100 ° C immediately prior to such projection.

Example 4

A ceramic melting powder, by weight, includes the following:

Chromium oxide 82% Al granules 3%

Zirconia 10% Al Flak 3.5%

Mg granules 1.5%

The chromium oxide was specified as in Example 3. The other materials were specified as in Example 2.

Such a powder can be projected from a lance at a rate of 150 to 200 kg / h, in a manner known per se in the ceramic melting technique, using oxygen as the carrier gas, for the repair of a steel degassing nozzle, formed from magnesium oxide -chromic refractory material, the repair site has a temperature of 1100 ° C immediately prior to such projection.

In a variant of this example, the magnesium was replaced with zirconium with an average particle size of about 10 to 15 micrometers, taking all desirable precautions with regard to the well-known higher activity of zirconium.

Example 5

A ceramic melting powder, by weight, includes the following:

Chromium oxide 90% Cr 8%

Al chip 2%

The chromium was in the form of granules with a nominal maximum grain size of about 100 microns and an average grain size of between 25 and 30 microns. The chromium oxide was as specified in Example 3. The aluminum chip had a specific surface area (measured by Griffin permeametry) of more than 7000 cm 2 / g.

Such a powder can be projected from a lance at a rate of 40kg / h, in a manner known per se in the ceramic melting technique, using oxygen as the carrier gas, for the repair of Corhart (trademark) Zac (fused alumina-zircon-zircon -Nium oxide) refractory blocks placed at the level of the surface of the melt in a glass melting furnace, the repair site having a temperature of 1500 ° C to 1600 ° C immediately prior to such projection.

The powder is also suitable for the repair of a chromium refractory material (ie a refractory material containing more than 25% chromium oxide and less than 25% magnesium oxide), relocated to the level of the surface of the melt in a glass melting furnace.

Example 6

A ceramic melting powder, by weight, includes the following:

Magnesium Moxide 72% Al Granules 3%

Zirconia 10% Mg / Al Alloy 5%

Carbon 10%

The carbon was coke with an average diameter of about 1.25 mm. The other materials were as specified in Example 1. Such a powder can be projected as described in Example 1 for the repair of a steel converter formed from a magnesium oxide-carbon refractory material.

Example 7

A ceramic melting powder, by weight, includes the following:

Magnesium oxide 82% Si 2%

Zirconia 10% Mg 4%

Al chip 2%

The silicon was in the form of granules with an average particle size of 4 microns. The zirconia had a nominal maximum grain size of 150 μm. The other materials were specified as in the previous examples. Such a powder can be projected at a rate of 150 kg / hour for the repair of a magnesium oxide basic refractory steel ladle.

Example 8

A ceramic melting powder, by weight, includes the following:

Aluminum oxide 92% Mg 2%

Al granules 6%

The alumina used was an electro-cast alumina containing 99.6% Al2 O3 by weight. It contained 0.05% CaO, and 0.02% SiO2. The molar ratio of SiO2 to CaO in this alumina is therefore 1: 2.68.

The alumina had a nominal maximum grain size of 700 microns and the aluminum and magnesium had grain sizes as specified in Example 2. Such powder may be used as described in Example 5 for the repair of Corhart (trademark) Zac refractory blocks in a glass melting furnace, below the working surface level of the melt after the tank has been partially drained to allow access to the repair place.

In a variant of this example, the electro-cast aluminum oxide was replaced with platelets made of aluminum oxide.

The aluminum oxide plates used had a nominal maximum grain size of 2 mm, and contained 99.5 wt% Al2 O3. It contained 0.073 mol% CaO, and 0.085 mol% SiO2. The SiO 2 to CaO ratio in this alumina was accordingly 1: 0.86, although it clearly meets the expression: [SiO 2]% <0.2 + [CaO]%.

Example 9

A ceramic melting powder, by weight, includes the following:

Magnesium oxide 80% Mg / Si alloy 2%

Zirconia 10% Mg / Al Alloy 5%

The magnesium / silicon alloy contained equal weight ratios of the two elements and had an average particle size of about 40 microns. The other materials were as specified in Example 1. Such a powder can be projected as described in Example 1 for the repair of a refractory wall formed from a basic magnesium oxide refractory.

Examples 10 to 16

In variants of Examples 1 to 4, 6, 7 and 9, the zirconia was replaced with platelets made of aluminum oxide as described in Example 8.

In variants of Examples 1, 3, 6, 9, 10, 12, 14, and 16, the alloy containing 30% magnesium and 70% aluminum had a maximum grain size of no more than 75 microns and an average grain size less than 45 μm. In still further variants, the alloy contained equal weight amounts of magnesium and aluminum.

Claims (33)

1. Ceramic melting method in which oxidizing gas and a mixture of refractory and fuel powders are projected against a surface and the fuel is burned to generate sufficient heat so that the refractory powder is at least partially melted or softened and a coated refractory mass is gradually built up against that surface, characterized in that the fuel powder is present in a ratio of no more than 15% by weight of the total mixture and comprises at least two metals selected from aluminum, magnesium, chromium and zirconium, and at least the largest weight portion of the refractory powder consists of one or more of magnesium oxide, alumina and chromium oxide, and that the molar ratios of silicon oxide and calcium oxide present in the refractory powder (if any) meet the following expression: [SiO 2]% <0.2 + [CaO]%. The ceramic melting process according to claim 1, wherein the molar ratios of silica and calcium oxide contained in the refractory powder (submissive) satisfy the following expression: [SiO2]% <[CaO]%. A ceramic melting process according to claim 1 or 2, wherein the refractory powder is essentially free of silicon oxide. A ceramic melting process according to any one of the preceding claims, wherein the projected refractory powder consists essentially of one or more of zirconia, magnesium oxide, aluminum oxide and chromium oxide. Ceramic melting process according to any one of the preceding claims, wherein the fuel powder comprises aluminum together with one or more of magnesium, chromium and zirconium. Ceramic melting process according to any one of the preceding claims, wherein no element constitutes more than 80% by weight of said fuel powder. A ceramic melting process according to any one of the preceding claims, wherein the fuel powder comprises an alloy containing at least 30% by weight of a metal selected from aluminum, magnesium, chromium and zirconium, the balance of the alloy being supplemented with at least one element other than such a selected metal, which element is also oxidizable to form a refractory oxide. A ceramic melting process according to any one of the preceding claims, wherein any silicon contained in said fuel is in the form of an alloy of silicon with at least one of aluminum, magnesium, chromium and zirconium. A ceramic melting process according to any one of the preceding claims, wherein the molar ratio of the silicon present in the projected mixture (if any) is no more than the molar ratio (if any) of zirconium, calculated as elementary zirconium. The ceramic melting process according to claim 9, wherein said fuel comprises elemental silicon in the form of particles having an average grain size of less than 10 microns, preferably less than 5 microns, and the mixture comprises zirconia particles having grain sizes below 150 microns, such zirconia particles in a molar amount is present which is at least equal to the molar amount of elemental silicon in the mixture. 11. The ceramic melting process according to any one of claims 1 to 7, wherein the projected fuel powder is essentially free of silicon. A ceramic melting process according to any one of the preceding claims, wherein the projected fuel powder comprises magnesium and aluminum. The ceramic melting process according to claim 12, wherein the projected fuel powder on a weight basis comprises aluminum rather than magnesium. A ceramic melting process according to claim 12 or 13, wherein magnesium is incorporated into the projected magnesium / aluminum alloy fuel powder. A ceramic melting process according to any one of claims 1 to 11, wherein it comprises fuel powder chromium and aluminum projected. A ceramic melting process according to claim 15, wherein the projected fuel powder on a weight basis comprises chromium than aluminum. Ceramic melting method according to any one of the preceding claims, used for the repair of a structure built of basic refractory material. 18. Ceramic melting powder, which is a mixture of refractory and fuel powders, for use in a ceramic melting process, in which oxidizing gas and the mixture of refractory and fuel powders are projected against a surface and the fuel is burned to generate sufficient heat so that the refractory powder becomes at least partially melted or softened and a coherent refractory mass builds up gradually against that surface, characterized in that the fuel powder is present in a ratio of not more than 15% by weight of the total mixture and comprises at least two metals selected from aluminum, magnesium, chromium and zirconium, and that at least the largest part by weight of the refractory powder consists of one or more of magnesium oxide, aluminum oxide and chromium oxide, and that the molar ratios of silicon oxide and calcium oxide present in the refractory powder (if any) meet the following expression: [SiO2]% <0.2 + [CaO]%. A ceramic melting powder according to claim 18, wherein the molar ratios of silica and calcium oxide present in the refractory powder (in part) satisfy the following expression: [SiO 2]% <[CaO]%. Ceramic melting powder according to claim 18 or 19, wherein the refractory powder is essentially free of silicon oxide. Ceramic melting powder according to any one of claims 18 to 20, wherein the projected refractory powder consists essentially of one or more of zirconia, magnesium oxide, aluminum oxide and chromium oxide. 22. A ceramic melting powder according to any one of claims 18 to 21, wherein the fuel powder comprises aluminum together with one or more of magnesium, chromium and zirconium. 23. Ceramic melting powder according to any one of claims 18 to 22, wherein no element comprises more than 80% by weight of the said fuel powder. A ceramic melting powder according to any one of claims 18 to 23, wherein the fuel powder comprises an alloy comprising at least 30% by weight of one metal selected from aluminum, magnesium, chromium and zirconium, the balance of the alloy is formed by at least one element other than such a selected metal, which element is also oxidizable to form a refractory oxide. 25. A ceramic melting powder according to any one of claims 18 to 24, wherein any silicon present in said fuel is in the form of an alloy of silicon with at least one of aluminum, magnesium, chromium and zirconium. 26. A ceramic melting powder according to any one of claims 18 to 25, wherein the molar amount of silicon (deposited) present in the projected mixture is no longer than the molar amount (if any) of zirconium, calculated as elementary zirconium. 27. A ceramic melting powder according to claim 26, wherein said fuel comprises elemental silicon in the form of particles having an average grain size of less than 10 µm, preferably less than 5 µm, and the mixture comprises zirconia particles having grain sizes below 150 µm, wherein such zirconia particles are present in a molar amount, which is at least equal to the molar amount of elemental silicon in the mixture. 28. Ceramic melting powder according to any one of claims 18 to 24, wherein the projected fuel powder is essentially free of silicon. 29. Ceramic melting powder according to any one of claims 18 to 28, wherein the projected fuel powder comprises magnesium and aluminum. 30. A ceramic melting powder according to claim 29, wherein the projected fuel powder on a weight basis comprises more aluminum than magnesium. 31. A ceramic melting powder according to claim 29 or 30, wherein magnesium is incorporated into the projected fuel powder in the form of a magnesium / aluminum alloy. 32. Ceramic melting powder according to any one of claims 18 to 28, wherein the projected fuel powder comprises chromium and aluminum. 33. A ceramic melting powder according to claim 32, wherein the projected fuel powder on a weight basis comprises more chromium than aluminum.