MXPA06011413A - Refractory articles for quiding or conveying a solidified material and process for the manufacture thereof - Google Patents

Abstract

The present invention relates to refractory articles for guiding or conveying a solidified material comprising a vitreous silica basis, in particular rollers or guiding elements and to a process for the manufacture of said articles which do not show the pickup problems normally observed with the articles of the art. The refractory articles of the invention comprise a vitreous bases and, homogeneously distributed therein, a carbonaceous material.

Description

REFRACTORY ARTICLES TO GUIDE OR TRANSPORT A SOLIDIFIED MATERIAL AND PROCEDURE FOR ITS MANUFACTURE DESCRIPTIVE MEMORY The present invention relates to refractory articles for guiding or transporting a solidified material, comprising a vitreous silica base, and to a process for the manufacture of said articles. Vitreous silica is the generic term for the glassy (amorphous or non-crystalline) form of silicon dioxide. High purity sand or quartz deposits provide the raw material that is fused by electric arc at a very low temperature, to provide fused silica or fused quartz, respectively. Vitrea silica can routinely withstand temperatures of more than 1250 ° C, and due to its very low coefficient of thermal expansion, it can be heated and cooled rapidly, virtually without risk of breakage by thermal shock. It is leathery and hard, in such a way that articles made of it exhibit a good resistance to surface damage and superior resistance to deterioration. Typically, vitreous silica exhibits a (apparent) density of 1.8 to 2.2 g / cm3, a coefficient of thermal expansion (at room temperature) of 0.50 to 0.95 - 10"6 / ° C, a thermal conductivity of 0.62 to 1.38 W / m. ° K, and an apparent porosity of 7 to 16% Several industrial applications of vitreous silica are known to take advantage of these properties.For example, it can be used as a conveyor roller to transfer solidified material (such as metal or glass) into sheet form, strip or thin sheet, in an oven, or as a guide to a solidified metal wire in a galvanizing bath It has been found that the surface of the vitreous silica rollers forming the conveyors used to transfer sheets, strips or thin sheets through an oven, or the articles that form the guide of a wire in a galvanization bath, tend to collect deposits of material from the leaves, strips, thin sheets or wires, in such a way that the leaves,Strips, thin sheets or wires that pass over the rollers are marked, scraped or dented. The deposition formation phenomenon is complex and is affected by the composition of the sheets, strips, thin sheets or wire transported or guided by the article, and by the composition and temperature of the installation, as well as by the character of the surface of the Article. Such a deposit is referred to as a figure or adherence, and is referred to as adherence. Of course, such sheets, strips, thin sheets or marked wire are not perfect, and should be scraped or given a lower grade. It is not always possible to simply replace these items keeping the installation hot, so that when the articles reach this stage of adhesion, it is often necessary to turn off the installation until the article can be polished with grinders or even replaced. This shutdown is a long process and represents a serious reduction in production. A period of several days may be necessary to cool the installation, and additional time is required for actual polishing or replacement of the items before the installation can be put back into service. Even in cases where it is possible to replace the articles without having to turn off the entire installation, this requires not very easy handling of hot and heavy items and other problems arise. In a known application "scraper" rollers are used to convey a flat glass ribbon between the end of a molten tin bath and the beginning of an annealing line. Carbon blocks configured under the scraper rollers are provided to scrape the surface of the rollers and remove any tin particles carried by the glass strip, and release it on the surface of the roll. In fact, it has been observed that the scraping blocks force some tin particles towards the porosity of the roll. After years of service, a substantial proportion of tin is rusty. The resulting tin oxide damages the surface of the roller and marks the glass ribbon. In another known application (described for example in USP 4,412,503) refractory segments of vitreous silica are used to guide a steel wire in a galvanization bath. After some time it can be observed an important adhesion of mixed tin, and oxides of iron on the surface of the segment in contact with the wire, resulting in a severe marking of the wire. Various attempts have been made to improve the properties of vitreous silica with respect to adhesion. Until now, the most common proposal has been to use a material other than vitreous silica for particularly demanding applications (such as high silicon steel, for example). Thus, it has already been suggested to provide the rolls with special alloy coatings (USP 2,695,248), or to use an arrow made from a particular steel grade (USP 4,470,802). It has also been proposed to use a roller made of a different material, such as graphite, or having a layer made of a material with a laminar structure, such as talc, graphite or boron nitride (FR-A1-2672586). Some good results have been obtained with relatively "soft" graphite rollers and with graphite coated rollers that do not tend to accumulate adhesion on their surface. With these articles, it has been observed that the outer layer of the articles on which the adhesion is formed tends to be eroded faster by the sheets, strips or thin sheets transported by the articles, than the formation of the deposit, in such a way that no adhesion is observed. An obvious disadvantage of these articles is that, due to their weak resistance to erosion, they must also be replaced frequently, with all the aforementioned problems. Therefore, it is an object of the present invention to provide refractory articles for guiding or transporting a solidified material possessing the excellent mechanical properties of the vitreous silica articles, without showing the adhesion problems normally observed with the articles of the prior art. The items searched must also have a long useful life. These problems and others have been solved with articles for guiding or transporting a solidified material comprising a base of vitreous silica and a carbonaceous material distributed homogeneously in said base. According to a first embodiment, the vitreous silica base is comprised of a vitreous silica aggregate chemically bonded (bound with cement or bound with resin). Normally, the aggregate of chemically bonded vitreous silica is prepared from a mixture comprising (i) at least 75% by weight, preferably more than 85% by weight, of amorphous silica, (ii) from 2 to 23% by weight chemical binder; and (iii) water. Suitable chemical binders are calcium aluminate, calcium silicate, polyalkoxysiloxanes such as polydiethoxysiloxane (ethylsilicate), colloidal silica, aluminum or zirconium acetate, magnesium oxide, and the like, or mixtures thereof. Calcium aluminate is the preferred binder. The mixture is formed and then dried. It is generally not necessary to calcify said added vitreous silica chemically bound. The aggregate of chemically bonded vitreous silica, dry, generally comprises from 75 to 96% by weight of vitreous silica, from 2 to 23% by weight of the chemical binder and from 2 to 4% by weight of water. According to a second preferred embodiment, the vitreous silica base generally comprises at least 60% by weight of amorphous silica, preferably more than 90% by weight, preferably more than 95% by weight, and usually more than 99% in weigh. The vitreous silica forms a matrix and can be obtained by any known method of preparing a vitreous silica matrix, such as slip casting or injection molding. The vitreous silica, once conformed, it is burned. The shape is generally densified by sintering at a temperature above 1000 ° CS. According to the invention, said articles can be prepared by a process according to what is described in claim 6. This procedure can be carried out on an article newly formed or on a recycled item (after having machined its surface). Advantageously, refractory articles comprising a vitreous silica base are impregnated with a liquid carbonaceous material, such as tar or resin. Impregnation with carbon reduces the apparent porosity to about 2% or less, which in addition to reducing adhesion, also serves to further protect the refractory silicon oxide from the corrosive attack that may occur otherwise. The articles to be impregnated are placed in a container and the air is evacuated. The vacuum is maintained between 15 minutes and 1 hour. This ensures that the air trapped within the internal pores of the item is removed. At this point the liquid resin or tar is introduced into the container. The required viscosity of the impregnating material depends on the pore size of the article. A piece with finely distributed porosity requires a low viscosity impregnation material to ensure adequate impregnation. The viscosity scale is usually 10-100 centipoise. Higher viscosity resins can be used if they are thinned with the appropriate solvents. Once the impregnation material has been introduced into the container, a pressure of between 5 and 25 bar is usually applied to force the resin or tar into the porosity. This completes the impregnation process. Carbonaceous materials suitable for the impregnation of the vitreous silica base are tar or pitch and also resins (for example phenolic resins). Optionally, the article can be heated to 300 ° C before or during the impregnation process to ensure adequate impregnation. Then the impregnated article is optionally dried (for example at 90 ° C) and then heated from 200 ° C to 750 ° C for 10 hours to remove volatile compounds at low temperature. The resin or cured tar can be carbonized to fix the coal, heating up 950 ° C in a reducing or inert atmosphere for up to 24 hours. Advantageously, the container can be subjected to high pressure (up to 25 bar) to promote cracking of the cured resin or tar. The impregnated article comprises from 1 to 6% by weight of carbonaceous material. If necessary, the article can be subjected to several impregnation steps to achieve the desired amount of carbonaceous material. It is noteworthy that the article can be impregnated in several millimeters of its surface or in all its thickness. In particular, said article exhibits a surprisingly low tendency of adhesion presenting all the excellent properties previously discussed of a vitreous silica article, in particular the resistance to erosion. As a result, these items have a particularly long lifespan before requiring grinding or replacement. Conveyor units comprising a plurality of said rollers are advantageously used for the transport of material in the form of a sheet, strip or thin sheet in a very demanding application, such as the transport of sheet, strip or thin sheet of high-content steel. silicon (oriented grains), stainless steel in an annealing furnace, or in a galvanizing line, or for the transport of sheet, strip or thin sheet of glass. As an example, two vitreous silica rollers were manufactured according to the invention, and compared with the same roller but without carbonaceous material. Table I shows several measured properties of the rolls according to the invention (rolls 3 and 4) compared to the same roll before being impregnated with the carbonaceous material (roll 1). Another roller (roller 2, comparative example) is identical to roller 1, but with a coating of 0.2 mm of graphite. The roller 3 is impregnated with pitch; the roller 4 is impregnated with a phenolic resin (in 3 mm).

TABLE 1 Roller 1 Roller 2 Roller 3 Roller 4 Absolute (apparent) density 2.208 2.208 2.161 2.039 (kg / dm3) Relative density (kg / dm3) 1.984 1.984 2.030 1.977 Open porosity 10.14% 7% 6.1% 3.05% Average rupture modulus 32,674 32,674 33,984 35,123 (MPa) Average pore diameter 0.11 0.050 0.057 0.050 (μm) Carbon content (% in 0 Surface: 2.16% Surface weight) 100% 4.30% Core: 0 Core: 0 Rollers 1 and 2 were installed in a conveyor unit for the transport of high silicon steel strips. The surface of rolls 1 and 2, and of the strips transported, was checked regularly during its useful life, and the results are shown in table II.

TABLE II Roller Time 1 Roller 2 Roller 3 Roller 4 control 15 days A, E A, E A, E A, E 1 month B, E B, E A, E A, E 2 months B, F B, E A, E A, E 3 months C, G C, F A, E A, E 6 months C, G C, G A, E A, E 12 months D, G D, G B, E A, E 18 months - - B. F B, E Caption: A: adherence is not observed.

B: some adhesion can be observed with an amplifier. C: some adherence can be observed visually. D: important adhesion; the roller has to be replaced. E: the transported strips are not marked. F: the transported strips are marked slightly. G: the transported strips are so marked that they have to be given a lower grade.

Claims (7)

NOVELTY OF THE INVENTION CLAIMS

1. - Refractory article for guiding or transporting a solidified material, comprising a vitreous silica base impregnated with a carbonaceous material.

2. The refractory article according to claim 1, further characterized in that the article comprises from 1 to 6% by weight of carbonaceous material.

3. The article according to claim 1 or 2, further characterized in that it consists of a chemically bonded vitreous silica comprising from 75 to 96% by weight of vitreous silica, from 2 to 23% by weight of a chemical binder, and from 2 to 4% by weight of water.

4. The article according to claim 3, further characterized in that the chemical binder is selected from the group consisting of calcium aluminate, calcium silicate, polyalkoxysiloxanes, colloidal silica, zirconium acetate, magnesium acetate, magnesium oxide, and their mixtures, and preferably is calcium aluminate.

5. The article according to claim 1 or 2, further characterized in that the refractory article is sintered and comprises at least 60% by weight, preferably more than 90% by weight, preferably more than 95% by weight, and most preferably more than 99% by weight of amorphous silica.

6. Process for the preparation of a refractory article according to any of claims 1 to 5, characterized in that it comprises the step of: (b) impregnating a base of vitreous silica with a carbonaceous material, preferably under heat or pressure.

7. The process according to claim 6, further characterized in that after the impregnation step follows an additional step of: (c) cracking the impregnated carbonaceous material, under heat and preferably under pressure.