MXPA97003791A - Production of a siliceous refractory mass - Google Patents

Abstract

A process is provided producing a crystalline siliceous refractory máss by projecting solid refractory particles, solid silicon particles and gaseous oxygen against a surface in such a way that reaction between the silicon particles and gaseous oxygen occurs against the surface, thereby releasing the heat of reaction against the surface so that a coherent refractory máss comprising cristobalite is formed, wherein the solid refractory particles comprise silica in the form of vitreous silica and in that the surface against which they are projected is at a temperature of a least 1000øC. The process can be used for in situ repair of high temperature furnaces such as glassmaking furnaces or for the manufacture of high quality refractory bricks.

Description

PRODUCTION OF REFRACTORY MASS SILICEOUS The present invention relates to improvements in a process for manufacturing a crystalline silicon refractory mass containing cristobalite. This method can have as its purpose the production of the mass as such, for example for the manufacture of bricks or refractory building blocks to be used in the construction or repair of furnaces, or it can be a procedure for the in situ repair of surfaces Eroded refractory material containing silicon, for example in the case of industrial furnaces such as furnaces for glass manufacture. This method makes use of a technique of the type generally known as "ceramic welding", in which a mixture of solid particles of refractory material and solid particles of combustible material of a metal or semimetal such as aluminum or silicon is projected against a surface to repair, wherein they are reacted therein with a gas rich in oxygen, usually substantially pure oxygen, so that the heat of reaction of the fuel is transferred to the surface in order to form a coherent refractory mass of repair. Such ceramic welding is described in British patent 1,330,894 (Glaverbel) and GB 2,170,191 (Glaverbel), in which a coherent refractory mass is formed on a surface by projecting thereon a mixture of refractory particles and combustible particles in the presence of oxygen .

The combustible particles are particles whose composition and granulometry are such that they react exothermically with oxygen to form a refractory oxide generating sufficient heat to melt at least superficially the projected refractory particles. The projection of the particles is conveniently and surely achieved by the use of oxygen as a carrier gas for the mixture of particles. In this way a coherent refractory mass can be formed on the surface against which the particles project. These known methods of ceramic welding can be used for the formation of a refractory article, for example a block having a particular configuration, however they are used more frequently for the formation of coatings or the repair of bricks or walls and are particularly useful for the repair or reinforcement of existing refractory structures, for example walls in glass-making or coking ovens. The process is particularly suitable for the repair of hot surface sub-strata. The foregoing makes it possible to repair eroded surfaces while the equipment remains substantially at its operating temperature and in numerous cases while the whole furnace remains in operation. Such repairs while the furnace remains in operation are particularly useful in glass and coke-making furnaces since the life of the furnaces is measured in years, often as long as twenty years, with the furnace continuously maintained in operation throughout period. The composition of the ceramic welding mixture is generally selected to produce a repair mass having a chemical composition similar or close to that of the basic refractory. The above helps to ensure compatibility with and adhesion to the new material and the base material on which it is formed. Even with such chemical compatibility some problems may arise to ensure adhesion of the repair mass to the substratum, especially if the adhesion is to be maintained for prolonged periods. The problem tends to increase if the repaired surface is subjected to very high temperatures. In this case, a high grade refractory is required, for example for the vaulted ceiling of a molten glass tank. The breaking-off of a portion of the repair mass should be avoided if possible. In the manufacture of glass, the masses detached by breakage will possibly fall on the molten glass and will introduce into it unacceptable impurities., it being necessary sometimes to discard a full load of large volume of molten glass. It has been found inventively that it is possible to easily form highly refractory masses containing cristobalite, provided the temperature is maintained at a very high level, from solid particles of refractory material which had been traditionally discarded because it was incompatible with the base material. Specifically, the material that can now be used according to the present invention for successful repair is vitreous silica. According to the present invention there has been provided a process for producing a crystalline silica refractory mass, projecting oxygen gas, solid refractory particles and solid combustible particles comprising silicon particles against a surface so that the reaction between the combustible particles and the gas oxygen takes place on the surface, releasing the heat of reaction towards the surface so as to form a coherent refractory mass comprising cristobalite, which is characterized in that the solid refractory particles contain silica in the form of vitreous silica and the surface being against the which particles are projected at a temperature of at least 1000 ° C. The high temperature at the surface ensures that the silica formed by the combustion of the silicon particles is incorporated into a crystalline network in the refractory mass.

The presence of the crystalline network creates different advantages with respect to the internal cohesion of the refractory mass and with respect to its ability to adhere, if required, to a surface under repair. The explanation given in the present as to how these advantages are achieved is by its nature a hypothesis. Whatever the accuracy of this hypothesis, the advantages have been clearly evidenced in the practical application of the present invention. It is assumed that the crystal lattice acts as a bonding phase that extends through the refractory mass. The network creates a continuous structure that extends through the entire mass, ensuring a dense structure with high mechanical resistance. If the procedure is used for the repair of an eroded refractory surface, the crystal lattice spreads and adheres to the surface. The projected refractory particles may have a structure different from that of the ligation phase. The adhesion to the surface is essentially provided by the ligation phase. The exposure of the refractory mass to very high temperatures in situ in a hot oven transforms said mass into cristobalite. In the case of the formation of a discrete refractory block or brick, for example by projecting the mass into a mold, the refractory mass thus formed is preferably flamed to a temperature of at least 1000 ° C. The high temperature of the oven and flaming transform the residual vitreous phase into cristobalite. The above has the particular advantage that cristobalite is stable at elevated temperatures. The process according to the present invention is especially suitable to be used in the in situ repair of furnaces for the manufacture of glass due to the very high temperatures prevailing therein. For example, the surface of the vaulted ceiling above the molten glass tank may be greater than 1500 ° C. The mixture of particles used according to the present invention, namely a mixture of solid combustible particles and solid refractory particles comprising vitreous silica *, can be used for the repair of surfaces that are at temperatures above 1000 ° C, provided that said mixture comprises an additive as defined in British patent application 2 257 136 (Glaverbel). The silica to be used according to the present invention must be of high purity, for example at least 95% by weight of purity, preferably at least 99% by weight of purity. The mass thus obtained is highly refractory and also reduces the risk of pollution in a glass making tank in case some portion of the mass falls into the glass. Vitreous silica is advantageously used in the inventive process in the form of solid refractory particles both for its ready availability and also because it can be obtained with a high degree of purity. The total amount of silicon is preferably not greater than 15% by weight. The foregoing is desirable to limit the amount of combustible material that does not react and would be retained in the refractory mass thus formed since the presence of a significant proportion of combustible material that does not react in the refractory mass thus formed can affect its quality. The refractory particles preferably do not comprise particles with a size greater than 4 mm, even more preferably not greater than 2.5 mm, in order to facilitate the smooth projection of the powder. The dispersion factor of the size range f (G) of the refractory particles is preferably not less than 1.2. Said factor fG) is used in the present in relation to a certain species of particles to denote the factor: 2 (G80 - G20 f (G) = (G80 + G20) in which G80 means the grain size of 80% of the particles of said species and G20 the grain size of 20% of the particles of said species. it preferably has an average particle diameter of not more than 50 μm. The term "average particle diameter" as used herein denotes a dimension such that 50% by weight of the particles have a smaller dimension than said average value. The melting point of the refractory mass obtained according to the present invention is close to that of pure silica. The bricks produced according to the present invention have a deformation coefficient T0r5 according to ISO R1893 of more than 1650 ° C. The above can be compared with the T0 / 5 of about 1550 ° C for ordinary silica bricks prepared by conventional methods. The refractory bricks consist predominantly of a solid solution of highly stabilized cristobalite (for example silica in which part of it is replaced by Al with Ca or Ca in addition to other cations) obtained according to U.S. patent 4 073 655 (Owens- Illinois, Inc) by devitrification of glass, are suitable for use at a temperature of up to 1250 ° C. The present invention will be illustrated below with reference to the following examples. It should be noted, however, that the present invention is not limited to the specific amounts and particular methods described therein. Example 1 A mixture of (by weight) 88% vitreous silica particles having a purity of 99.7% silica, and 12% silicon particles were projected in a commercially pure oxygen stream against the vaulted ceiling of a glass making tank to form a refractory mass on it. The vaulted ceiling was at a temperature of approximately 1600 ° C. The maximum size of vitreous silica particles was 2 mm. Its G? 0 was 950 μm and its G20 was 225 μm, which gave a dispersion factor of the size range f (G) of 1.23. The silicon particles had an average grain diameter of less than 45 μm and a specific surface area of about 2,500 to 8,000 cm 2 / g. After seven days a sample was extracted from the mass thus formed for analysis purposes, having determined the following characteristics: Melting point - 1723 ° C. Cristobalite structure T0 (5 (ISO R1893) almost 1700 ° C * * The best commercially available bricks normally used in this vaulted ceiling location (high purity silica bricks Hepworth Refractories "HEPSIL SV") have a T0 / 5 of 1640 ° C. Example 2 A pulverulent mixture with the same composition as described in Example 1 was projected in a commercially pure oxygen stream into a mold to form a brick.The mold was preheated to 1600 ° C to receive said pulverulent mixture, once the brick was formed, it was maintained at a temperature of 1450 ° C for 6 days.The brick was then analyzed and it was determined that it had the same melting point, structure and T0? 5 as the sample of Example 1. The bricks, such as those produced according to Example 2, can be employed directly and without any special precaution in the repair of roofs above eroded dies from glassmaking ovens, and if necessary with additional welding. Any other type of crystalline silica brick placed under the same conditions without precautions such as careful preheating, will immediately suffer breakage due to the formation of multiple fissures.

Claims (9)

1. CLAIMS 1.- Improvements in a process for producing a crystalline siliceous refractory mass, projecting oxygen gas, solid refractory particles and solid combustible particles comprising silicon particles against a surface so that the reaction between the combustible particles and the oxygen gas takes place on the surface, releasing the heat of reaction towards the surface so as to form a coherent refractory mass comprising cristobalite, characterized in that the solid refractory particles contain silica in the form of vitreous silica and the surface against which said particles are projected onto a temperature of at least 1000 ° C.
2. 2.- Improvements in a procedure according to the claim 1, characterized in that after the projection of the particles, the resulting refractory mass is flamed at a temperature of at least 1000 ° C.
3. 3. Improvements in a process according to claim 1 or 2, characterized in that the solid refractory particles are composed of silica having a purity of at least 95% by weight.
4. 4. Improvements in a process according to at least one of the preceding claims, characterized in that the solid refractory particles are constituted by silica having a purity of at least 99% by weight.
5. 5. Improvements in a process according to at least one of the preceding claims, characterized in that the total amount of silicon is not greater than 15% by weight of the total weight of the projected particles.
6. 6. Improvements in a process according to at least one of the preceding claims, characterized in that the silicon has an average diameter of grains not greater than 50 μm.
7. 7. Improvements in a process according to at least one of the preceding claims, characterized in that the solid refractory particles do not substantially comprise particles with a size greater than 4 mm.
8. 8. Improvements in a method according to at least one of the preceding claims, characterized in that the dispersion factor of the size range f (G) (as defined above) of the refractory particles is not less than 1.2.
9. 9. - Improvements in a refractory brick which has a cristobalite structure and a deformation coefficient of more than 1650 ° C, prepared by a process as defined in at least one of claims 2 to 8. RESUME Improvements in a procedure to produce a refractory siliceous crystalline mass projecting: solid refractory particles, solid particles of silicon and oxygen gas against a surface so that the reaction between the silicon particles and the oxygen gas takes place on the surface, releasing the heat of reaction on the surface in a of forming a coherent refractory mass containing cristobalite, in which the solid refractory particles comprise silica in the form of vitreous silica and in which the temperature of the surface against which said particles are projected is at a temperature of at least 1000 ° C. The process can be used for in situ repairs of furnaces that are at elevated temperatures, such as glassmaking ovens or for the manufacture of high quality refractory bricks.