GB2213812A

GB2213812A - Ceramic welding composition and process

Info

Publication number: GB2213812A
Application number: GB8729419A
Authority: GB
Inventors: Francois Toussaint
Original assignee: Glaverbel Belgium SA
Current assignee: AGC Glass Europe SA
Priority date: 1987-12-17
Filing date: 1987-12-17
Publication date: 1989-08-23
Also published as: GB8729419D0

Abstract

A particulate mixture for use in forming a refractory mass by a ceramic welding technique comprises refractory particles and particles of one or more fuel elements which is or are oxidisable to form a refractory oxide; the fuel being selected so that the average grain size of the total fuel particles is greater than 50 mu m and at least 10% by weight of the fuel is constituted by fuel particles having grain sizes below 50 mu m. The fuel particles, e.g. of Al or Si, are present in such proportion as to release sufficient heat on combustion that at least the surfaces of the refractory particles become softened or melted to allow formation of the weld mass. The mixture additionally incorporates particles of at least one oxidising agent.

Description

CERAMIC WELDING COMPOSITION AND PROCESS This invention relates to a particulate mixture suitable for use in the in situ formation of a coherent refractory mass by a ceramic welding technique in which is projected against a surface a mixture comprising refractory particles and particles of one or more fuel elements which is or are oxidisable to form a refractory oxide, such fuel particles being present in such proportion that on combustion they will release sufficient heat to soften or melt at least the surfaces of the refractory particles to allow the formation of said refractory mass.The invention also relates to a ceramic welding process in which a coherent refractory mass is formed in situ on a surface by projecting against that surface a mixture comprising refractory particles and particles of one or more fuel elements which is or are oxidisable to form a refractory oxide, such fuel particles being present in such proportion that on combustion they will release sufficient heat to soften or melt at least the surfaces of the refractory particles and bring about the formation of said refractory mass.

Known ceramic welding processes are illustrated by British Patent Specifications Nos 1,330,894 and 2,170,191 A (Glaverbel). Such known ceramic welding processes may be used for the formation of a refractory element, for example a block of special shape, but they are most commonly used for forming adherent coatings or repairs on refractory blocks or walls, and they are particularly useful for repairing or reinforcing existing refractory structures, for example for repairing walls or wall-coatings which have become eroded during use of a glass-melting furnace, coke oven or other refractory equipment such as that used in the metallurgical industry.

Because of the high temperatures generated in the ceramic welding reaction zone, the reaction tends to-be able to cut through any slag which may be present on the surface of a refractory being treated and soften or melt that surface, so that a good joint can be made between the substrate being treated and the newly formed refractory weld mass.

It is desirable to effect repairs on a refractory surface while it is hot because this promotes the formation of a high quality repair, and in particular of a good bond between the weld mass and the base refractory.

Hot repair also has the advantage of reducing the "down-time" of the refractdry apparatus being repaired and avoids problems of differential contraction or expansion as the refractory apparatus is cooled and then reheated to working temperature. In some cases it is even possible to effect the repair or reinforcement without interrupting normal operation of the equipment.

In order to initiate and sustain the ceramic welding reaction, such known processes make use of very fine fuel particles having an average grain size of less than 50 micrometres. Indeed, commercially practised ceramic welding techniques make use of fuel particles having a maximum nominal grain size of less than 50 micrometres. Aluminium and silicon are examples of such fuels. Such particles are, because of their high specific surface area, highly reactive. It is necessary to use very fine and therefore highly reactive fuel particles in order to ensure a continuous reaction at the weld site so that the weld mass formed can be of good and uniform quality.However, when the recommended very fine fuel particles are used, there is a risk that the rate of propagation of the ceramic welding reactions may exceed what is desirable with the result that the reactions become uncontrolled, and this may also have an adverse effect on the quality of the deposited refractory mass. If, on the other hand, the fuel particles used are much coarser, the ceramic welding reactions proceed unevenly and are difficult to sustain. Also there is a high risk that unreacted fuel particles will be incorporated in the deposit where they may have an adverse effect on the quality of the deposited refractory mass. For example if some fuel elements in view become incorporated in a weld mass of certain refractory materials, there may be a risk of the fuel element acting as a reducing agent on the refractory material.

It is an object of this invention to provide a particulate mixture for use in ceramic welding which allows a better control of the conditions under which the ceramic welding reactions take place so as more easily to allow the formation of a refractory weld mass of high quality.

According to the present invention, there is provided a particulate mixture suitable for use in the in situ formation of a coherent refractory mass by a ceramic welding technique in which is projected against a surface a mixture comprising refractory particles and particles of one or more fuel elements which is or are oxidisable to form a refractory oxide, such fuel particles being present in such proportion that on combustion they will release sufficient heat to soften or melt at least the surfaces of the refractory particles to allow the formation of said refractory mass, characterised in that said mixture comprises: refractory particles, particles of at least one oxidising agent, and fuel particles, the fuel being selected so that the average grain size of the total fuel particles is greater than 50pom, and at least 10% by weight of the fuel is constituted by fuel particles having grain sizes below 50cm.

The use of such a mixture in a ceramic welding process has the advantage of enabling better control of the conditions under which the ceramic welding reactions take place while still and more easily permitting the formation of a high quality refractory weld mass.

We have found that by making use of the present invention, we can reduce the proportion in the mixture of the very fine fuel particles which is necessary to sustain the ceramic welding reaction. As a result, because the powder mixture contains less very fine fuel particles there is a reduced tendency for the reactions to run out of control. Indeed storage and handling of the mixture also becomes safer. We have also found that any fuel from the coarser particles which initially is incorporated in the weld mass can still react with the oxidising agent with the result that the adverse consequences usually associated with using such coarse fuel particles are avoided or reduced.Indeed in some circumstances the yield of the ceramic welding process, measured as the proportion of powder mixture projected which becomes incorporated into the coherent refractory weld mass, can be improved by virtue of the entrapment of fuel particles in the mass while they oxidise. This entrapment of fuel particles which continue to oxidise can also generate additional heat within the just formed weld mass and can give rise to an improved cohesiveness of that mass, or the ability to make use of refractory particles of a higher softening or melting temperature, or the ability to use a lower total proportion of fuel in the projected mixture. (It may be noted that the fuel used is often the most expensive part of that mixture.) A further very important advantage of this invention is that it makes performance of the ceramic welding operation easier. When using the conventional very fine particles as sole fuel, we have found that the speed at which the reaction zone traverses the surface on which material is being deposited is very important for sustaining the ceramic welding reactions. For the best adherence of the deposit to the substrate, it was desirable that the material should be projected against a zone on the surface which was strongly heated by the reactions taking place. Thus the reaction zone could only be moved across that surface slowly, to allow time for the new impingement zone to become heated by the reaction and allow deposition of weld mass from the reaction zone before it was moved across the work surface to a new impingement zone.When using a mixture according to this invention, we have found that a wider and more rapid scan of the reaction zone across the surface being treated can be tolerated without adverse effects on the quality of the weld mass and its adherence to that surface.

Advantageously, not more than 5% of the mixture is made up by fuel particles having grain sizes below 50cm. This is particularly beneficial for reducing any risk of uncontrolled reaction.

In preferred embodiments of the invention, at least 5% of the mixture is made up by fuel particles having grain sizes above 50cm. The adoption of this feature has the advantage of allowing the generation of an ample supply of heat for melting or softening the refractory particles of the mixture notwithstanding the reduced proportion of very fine fuel particles, and without adding substantially to any risk of uncontrolled reaction.

The advantages afforded by the invention are most easily achieved by incorporating into the particulate mixture two distinct fractions of fuel.

Such two distinct fractions or components of fuel may easily he added in relative and total proportions to achieve a particular desired result in any given mixture. Advantageously, said fuel comprises finer and coarser fractions such that a grain size distribution curve of the fuel particles has two maxima which are at least lOOpm apart. If the quasi-normal distribution curve exhibits two maxima, it may be concluded that two fractions of fuel particles have been mixed.

In the most preferred embodiments of the invention, said fuel comprises a fraction of fuel particles which has an average grain size finer than 50irm and a fraction of fuel particles which has an average grain size coarser than 50cm. Mixing two such fractions of fuel particles together is a very simple way of achieving the fuel granulometry required by the present invention.

Such two fractions of the fuel particles in a mixture according to this invention may be distinguishable from one another in various ways. If all the fuel particles are of the same element and of similar shape, then a notional separation of the two fractions may be made after they have been mixed. This may be done by plotting a distribution curve of the grain sizes of the fuel particles as aforesaid.

In many embodiments of the invention however it is possible to make a real distinction between two fractions of fuel particles. This may be done on the basis of the chemical composition of the fuel particles in the two fractions. As an example, some preferred embodiments of the invention provide that said fuel comprises finer and coarser fractions made up of particles of different elements. By way of example, the finer fraction may consist of aluminium while the coarser fraction consists of silicon grains.

The use of different fuel elements in a mixture according to the invention affords a further parameter which may be varied in order to control the ceramic welding reactions which take place in -s mixture, Alternatively, or in addition, the of the form of the individual grains of fuel wh ermine by the method by which the fuel was comminuted. In some preferred embodiments of the invention, said fuel comprises finer and coarser fractions made up of particles of a same element which fractions are distinguishable by granular shape. The shape of the particles is usually a characteristic of the production method used for comminuting the fuel material. By way of example aluminium powders having generally globular grains may be made by atomising the molten metal.Very fine aluminium may be made by milling globular grains so that they are in the form of flakes. Thus for example a finer fraction of the fuel may consist of aluminium flake while a coarser fraction consists of globular grains of aluminium which are easily distinguished from the flake by inspection. In some other cases, there may be fractions which are distinguishable by a combination of these methods, for example a finer fraction may consist of a mixture of aluminium flakes and silicon powder.

while a coarser fraction consists of aluminium globular grains.

The size distribution of the fuel particles of both fractions is of importance for the way in which the ceramic welding reactions proceed. In general, the finer is the first fine fraction of fuel particles the better is that fraction able to sustain the reactions which take place as the fuel is in course of its trajectory towards the surface being treated. It is preferred that at least 80% by weight of such finer fuel fraction has a grain size of less than 50m and preferably less than 30cm.

of course the actual optimum size of the fuel particles will depend on the nature of the fuel used. The three fuel materials particularly in view are aluminium and silicon as have already been referred to, and also magnesium. Because of the way in which the combustion reactions of grains of these three materials proceed, we have found that the optimum grain size for magnesium fuel particles is somewhat coarser that it is for silicon or aluminium particles.

It-is preferred that the average grain size of said coarser fraction of fuel particles is at least lOOpm. Advantageously, at least 80% by weight of the particles of such coarser fuel fraction has a grain size of at least 75pm and preferably at least lOOpm. Such particles are sufficiently coarse to afford a very low risk of uncontrolled reaction. They are also sufficiently coarse to allow unburnt fuel to become embedded in the weld mass being deposited so that such fuel can continue to react there with the projected oxidising agent in order to maintain a hot zone on the working surface.

In preferred embodiments of the invention, at least 80% by weight of the particles of such coarser fuel fraction has a grain size of less than 400pm and preferably less than 300cm. For example it is preferred that the average grain size of such coarser fuel fraction is less than 300cm.

and preferably less than 200cm. This promotes a good reaction of those particles when used in a ceramic welding process, and limits the amount of unreacted fuel which remains after repair.

The amounts and relative amounts of the fractions of fuel particles in the mixture also has an important effect on the degree of safety with which the mixture can be handled and on the usefulness of the mixture in a ceramic welding process.

It is advantageous that such coarser fuel fraction is present in an amount of at least 1% and preferably at least 5% by weight of the mixture.

This provides additional fuel with small increase in uncontrolled reaction hazard, and gives quite sufficient fuel for the softening or melting of many refractory particles.

In order to achieve the best compromise between safety and the ease with which a ceramic welding reaction can be maintained, it is preferred that such finer fuel fraction is present in an amount of at least 1% and preferably at most 10% by weight of the mixture.

In order to facilitate the smooth and even course of a ceramic welding reaction, it is preferable that such finer fraction of fuel particles is present in an amount of at least 10% by weight of the total fuel particles present in the mixture.

In order to limit uncontrolled reaction hazard and indeed for the sake of economy, it is preferred that the fuel particles are present in a total amount of at most 20% by weight of the mixture.

In preferred embodiments of the invention, said particles of oxidising agent are present in an amount of at least 5% by weight of the mixture, and it is preferred that said particles of oxidising agent are present in an amount of at most 20% by weight of the mixture. The presence of such quantities of oxidising agent is quite sufficient for the purposes in view.

There are many oxidising agents which could be used in a mixture according to this invention. Among suitable oxidising agents are various metal compounds. It is desirable to avoid deliquescent materials and those which decompose at too low a temperature to allow them to become incorporated in the weld mass which results from the performance of a ceramic welding process using the mixture in question. Suitable oxidising agents include higher oxides, for example ferro-ferric oxide and ferric oxide, nitrates, for example potassium and sodium nitrates, perhalates, such as sodium perchlorate and sodium periodate, and peroxides such as barium, sodium and magnesium peroxides. The actual oxidising agent to be used in any particular mixture may be selected for its effect on the refractory mass which will be formed on use in a ceramic welding process.For example sodium, calcium and iron containing compounds act as fluxes which assist in lowering the melting or softening points of many refractory material. Their use may or may not be desirable depending on circumstances. If a very high grade refractory was required, it might be desirable to use magnesium peroxide as oxidising agent.

The granulometry of the oxidising agent is preferably such that said particles of oxidising agent have an average grain size which is between half and double the average grain size of the refractory particles of the mixture. This militates against desegregation of the oxidising agent within the particle mixture during handling.

Preferably, at least 80% by weight of said particles of oxidising agent have a grain size below 500cm, and advantageously, said particles of oxidising agent have an average grain size below 20Opm. This allows the particles to be well distributed in a refractory weld mass formed using the powder mixture so that their oxygen content is readily available to oxidise any unburnt fuel in that mass.

The present invention also provides a ceramic welding process in which a coherent refractory mass is formed in situ on a surface by projecting against that surface a mixture comprising refractory particles and particles of one or more fuel elements which is or are oxidisable to form a refractory oxide, such fuel particles being present in such proportion that on combustion they will release sufficient heat to soften or melt at least the surfaces of the refractory particles and bring about the formation of said refractory mass.The process according to the invention is characterised in that the mixture which is projected comprises: refractory particles, particles of at least one oxidising agent, and fuel particles, the fuel being selected so that the average grain size of the total fuel particles is greater than 50cm. and at least 10% by weight of the fuel is constituted by fuel particles having grain sizes below 50cm.

The principal advantages of such a process are that it is easier to perform rapidly on a large surface for forming a thick weld deposit and that there is a reduced risk of uncontrolled reaction as compared with working with the very fine fuel particles alone. In some circumstances, a greater yield of refractory weld mass product can be obtained, and/or a better quality product can be obtained.

In the most preferred embodiments of the process of the invention, the mixture of particles is projected in a stream of carrier gas which contains proportionately at least as much oxygen as does air. The use of such a carrier gas stream is beneficial for supplying oxygen to the reaction zone for the proper maintenance of the ceramic welding reactions.

In preferred embodiments of the process, use is made of a powder mixture having one or more of the preferred features hereinbefore defined.

Certain preferred embodiments of the mixture and process of the invention will now be described by way of Example. In each process use was made of a ceramic welding machine substantially in accordance with British Patent Specification No 1,330,895, as also referred to in British Patent Specification No 1,330,894.

EXAMPLE 1 A powder mixture was made up of the following constituents: 4% by weight aluminium flakes having an average grain size of 8pm, 13% by weight aluminium powder which was composed of grains of globular form having a granulometry such that at least 80% by weight had a grain size above 160pm and no more than 15% had a grain size above 400cm, 15% by weight barium peroxide 68% by weight alumina having an average grain size of about lOOpm This mixture was used for forming a ceramic weld repair patch on an alumina refractory structure. The temperature of the structure when repair commenced was above 10000C. The mixture was projected at a rate of 30kg/hr using oxygen as carrier gas fed at a rate of about 160 litres/minute.

EXAMPLE 2 A powder mixture was made up of the following constituents: 3% by weight aluminium flakes having an average grain size of 8pm, 15% by weight silicon powder which had a granulometry such that at least 95% by weight had a grain size above 75pm and no more than 5% had a grain size above 150cm, 10% by weight sodium nitrate with an average grain size of 400-500pm, 72% by weight silica made up of three parts cristoballite to two parts tridymite and having an average grain size of 450m This mixture was used for forming a ceramic weld repair patch on a silica refractory structure. The temperature of the structure when repair commenced was 11500C.The mixture was projected at a rate of 60kg/hr using oxygen as carrier gas fed at a rate of about 200 litres/minute.

EXAMPLE 3 A powder mixture was made up of the following constituents: 4% by weight of a first very fine fuel fraction made up magnesium powder having a granulometry such that less than 40% by weight had a grain size above 63cm, and the average grain size was below 50pm 10% by weight aluminium powder which was composed of grains of globular form having a granulometry such that at least 80% by weight had a grain size above 160pm and no more than 15% had a grain size above 400cm, 10% by weight sodium peroxide with an average grain size of about 50m 768 by weight refractory oxide made up of about two parts zirconia having an average grain size of 150vm to three parts a-alumina having an average grain size of 100um This mixture was used for forming a ceramic weld repair patch on a refractory structure made of a refractory sold under the Trade Name Corhart Zac. This refractory has an approximate composition of 8-12% silica, 65-75% alumina and 15-20% zirconia. The temperature of the structure when repair commenced was 1200 OC. The mixture was projected at a rate of 30kg/hr usiny oxygen as carrier gas fed at a rate of about 160 litres/minute.

EXAMPLE 4 A powder mixture was made up of the following constituents: 6% by weight of a first very fine fuel fraction made up of three parts silicon having an average grain size of 6.3pm and one part of the aluminium flakes as used in Example 1 8% by weight aluminium powder which was composed of grains of globular form having a granulometry such that at least 80% by weight had a grain size above l60pm and no more than 15% had a grain size above 400pm.

8% by weight ferric oxide 78% by weight of a silico-aluminous refractory oxide chamotte having an average grain size of 50pm This mixture was used for forming a ceramic weld repair patch on a refractory structure made of a refractory sold under the Trade Name Corhart Standard. This refractory has an approximate composition of 28% silica and 70% alumina. The temperature of the structure when repair commenced was above 10000C. The mixture was projected at a rate of 60kg/hr using oxygen as carrier gas fed at a rate of about 180 litres/minute.

EXAMPLE 5 A powder mixture was made up of the following constituents: 4% by weight of a first very fine fuel fraction made up magnesium powder having a granulometry such that less than 40% by weight had a grain size above 63pm, and the average grain size was below 50m 8% by weight magnesium powder which was composed of grains having a granulometry such that at least 90% by weight had a grain size above 63pm and no more than 20% had a grain size above 250pm, 6% by weight magnesium peroxide 82% by weight of electrocast magnesia with an average grain size of 250pm This mixture was used for forming a ceramic weld repair patch on a basic refractory structure made of magnesia. The temperature of the structure when repair commenced was above 10000C. The mixture was projected at a rate of 30kg/hr using air as carrier gas fed at a rate of about 160 litres/minute.

Claims

1. A particulate mixture suitable for use in the in situ formation of a coherent refractory mass by a ceramic welding technique in which is projected against a surface a mixture comprising refractory particles and particles of one or more fuel elements which is or are oxidisable to form a refractory oxide, such fuel particles being present in such proportion that on combustion they will release sufficient heat to soften or melt at least the surfaces of the refractory particles to allow the formation of said refractory mass, characterised in that said mixture comprises: refractory particles, particles of at least one oxidising agent, and fuel particles, the fuel being selected so that the average grain size of the total fuel particles is greater than 50pom, and at least 10% by weight of the fuel is constituted by fuel particles having grain sir

2.A mixture according to claim 1, wherein adore than 5% by weight of the mixture is made up by fuel particles with grain sizes below 50pm.

3. A mixture according to claim 1 or 2, wherein at least 5% by weight of the mixture is made up by fuel particles having grain sizes above 50cm.

4. A mixture according to any preceding claim. wherein said fuel comprises finer and coarser fractions such that a grain size distribution curve of the fuel particles has two maxima which are at least lOOpm apart.

5. A mixture according to any preceding claim. wherein said fuel comprises a fraction of fuel particles which has an average grain size finer than 50pm and a fraction of fuel particles which has an average grain size coarser than 50pm.

6. A mixture according to any preceding claim, wherein said fuel comprises finer and coarser fractions made up of particles of different elements.

7. A mixture according to any preceding claim, wherein said fuel comprises finer and coarser fractions made up of particles of a same element which fractions are distinguishable by granular shape.

8. A mixture according to any of claims 4 to 7, wherein at least 80% by weight of such finer fuel fraction has a grain size of less than 50pm.

9. A mixture according to any of claims 4 to 8, wherein at least 80% by weight of such coarser fuel fraction has a grain size of more than 50pm.

10; A mixture according to claim 9, wherein at least 80% by weight of such coarser fuel fraction has a grain size of at least 75pm and preferably at least 100pm.

11. A mixture according to any of claims 4 to 10, wherein at least 80% by weight of such coarser fuel fraction has a grain size of less than 400pm and preferably less than 300pm.

12. A mixture according to any of claims 4 to 11, wherein such coarser fuel fraction is present in an amount of at least 1% and preferably at least 5% by weight of the mixture.

13. A mixture according to any of claims 4 to 12, wherein such finer fuel fraction is present in an amount of at least 1% and preferably at most 10% by weight of the mixture.

14. A mixture according to any of claims 4 to 13, wherein such finer fraction of fuel particles is present in an amount of at least 10% by weight of the total fuel particles present in the mixture.

15. A mixture according to any preceding claim, wherein the fuel particles are present in a total amount of at most 20% by weight of the mixture.

16. A mixture according to any preceding claim, wherein said particles of oxidising agent are present in an amount of at least 5% by weight of the mixture.

17. A mixture according to any preceding claim, wherein said particles of oxidising agent are present in an amount of at most 20% by weight of the mixture.

18. A mixture according to any preceding claim, wherein said particles of oxidising agent have an average grain size which is between half and double the average grain size of the refractory particles of the mixture.

19. A mixture according to any preceding claim, wherein at least 80% by weight of said particles of oxidising agent have a grain size below 500pm.

20. A mixture according to any preceding claim, wherein said particles of oxidising agent have an average grain size below 200pm.

21. A ceramic welding process in which a coherent refractory mass is formed in situ on a surface by projecting against that surface a mixture comprising refractory particles and particles of one or more fuel elements which is or are oxidisable to form a refractory oxide, such fuel particles being present in such proportion that on combustion they will release sufficient heat to soften or melt at least the surfaces of the refractory particles and bring about the formation of said refractory mass, characterised in that the mixture which is projected comprises: refractory particles, particles of at least one oxidising agent, and fuel particles, the fuel being selected so that the average grain size of the total fuel particles is greater than 50cm, and at least 10% by weight of the fuel is constituted by fuel particles having grain sizes below 50cm.

22. A process according to claim 21, wherein the mixture of particles is projected in a stream of carrier gas which contains proportionately at least as much oxygen as does air.

23. A process according to claim 21 or 22, wherein the projected mixture of particles is a mixture according to any of claims 2 to 20.