KR20020042710A - Carbon-containing refractory article having a protective coating - Google Patents

Abstract

The present invention discloses a carbon-containing fire resistant article having a protective coating. Protective coatings reduce oxidation of the article during preheating and casting operations. Hot wetting aids comprising oxides of metals selected from the group consisting of arsenic, antimony, bismuth, lead, vanadium and tin improve the wettability of the protective coating on the article, thereby reducing defects in the dissolved protective coating. The coating protects the article from oxidation and decarbonization that may occur with the article at high temperatures in an oxidizing atmosphere. Wet aids are metal oxides such as bismuth oxide that reduce the surface tension of the protective coating at high temperatures and promote the spreading of the protective coating on the surface of the article.

Description

CARBON-CONTAINING REFRACTORY ARTICLE HAVING A PROTECTIVE COATING}

Refractory articles used in continuous casting of steel often include refractory oxides and significant amounts of elemental carbon. Carbon, in contact with the molten steel, can improve the article's resistance to thermochemical and thermomechanical stresses. For example, sub-entry shrouds (SES) transfer molten steel from a tundish to a mold and often contain 5-40 weight percent graphite.

Carbon is susceptible to oxidation and can seriously impair the performance of refractory articles. Temperature rises accelerate oxidation and can occur at least during ignition, preheating and casting operations. Carbon-containing articles are often ignited in a reducing, non-oxidizing atmosphere so that oxidation cannot occur thereafter. In contrast, reducing atmospheres are not feasible in preheating and casting operations. In order to suppress oxidation, it is common to coat the article with a protective coating that does not allow relatively oxygen to pass. The protective coating can be formed from a solid or a liquid that dissolves to form a protective oxygen barrier layer. Importantly, the protective coating must be “wet” to completely cover the article without defects or “pinholes”. The pinholes in the protective coating allow oxygen to be contacted with the carbon containing refractory article. Unfortunately, carbon itself can act to inhibit wetting by the protective coating.

Attempts have been made to reduce pinholes and improve wettability with coatings not intended for carbon containing refractory materials. For example, U.S. Pat.No. 5,013,697 (Akhtar) discloses a semiconductor industrial seal using up to 30% by weight of bismuth oxide to lower the melting temperature to below 400 ° C and to improve the wettability of the varnish in ceramic substrates, especially metal-coated ceramic materials. Teaching Glass. The sealing glass joins the two ceramic surfaces together to form a sealed container that is resistant to breakage and fracture. Lower melting temperatures reduce damage to the semiconductor. Akhtar makes no mention of protective coatings for carbon-containing substrates, reduced pinhole formation or use in high temperature applications.

EP-A-0252621 discloses a protective coating for a carbon containing fire resistant article. The coating includes a solid particulate polish for the graphite electrode in an electric arc furnace. Polishing agents include 20-40% by weight of soluble materials that wet the graphite, 35-70% by weight of refractory fillers and 10-25% by weight of metals or metalloids and can be oxidized to refractory oxides while the electrodes are at high temperatures. Soluble materials that wet the graphite are boron compounds or boron containing glasses such as boron silicate glass. The metal or metalloid is preferably aluminum and reacts with any oxygen before oxygen reaches the refractory article. The solid particulate polish is sprayed onto the hot electrode to dissolve. Similar solid compositions comprising alkali and boron oxide are known from document DE-A1-4216934. These compositions are troweled, chopped and sprayed. Such spraying, trowelling or ramming cannot easily and quickly cover complex geometries or cavities, including, for example, holes in the injection tube.

Other protective compositions comprising boron oxide are also disclosed in US Pat. No. 3,460,987. However, these compositions cannot be used directly on substrates containing less than 5% boron oxide and thus are not useful for most carbon containing refractory articles used in the glass or metal industry.

Applicant incorporated by reference in U.S. Patent 5,856,015 (Buchanan), which teaches protective coatings for carbon containing articles. Protective coatings are aqueous suspensions comprising clay, glass frits, aluminum, silicon or alloy powders thereof and inhibitors. This frit is boron silicate, boron phosphate or phosphate glass with a softening point of 600 ° C. or less. Powders are disclosed as antioxidants that inhibit the oxidation of carbon. Inhibitors are disclosed to reduce the reactivity of the powder with water. The reaction between the inhibitor and water releases hydrogen gas that causes bubbles and pinholes in the coating. Inhibitors include some acids and salts, preferably sodium triphosphate. Advantages of the present invention include formation of an improved protective film and improvement of shelf life.

Despite the addition of wetting aids and inhibitors, pinhole formation and the resulting oxidation can still occur in carbon containing refractory articles, in particular articles further comprising alumina, magnesia and zirconia. There is a continuing need for protective coatings that reduce pinhole formation on carbon containing refractory articles at high temperatures and protect the article from oxidation during preheating and casting operations.

The present invention relates to a carbon-containing fire resistant article having a protective coating. More specifically, the present invention discloses a protective coating that reduces carbon oxidation.

Summary of the Invention

The present invention relates to a carbon-containing fire resistant article having a coating that protects the article from oxidation. In a broad aspect, the article comprises carbon, preferably in the form of graphite and one or more other refractory components such as oxides, carbides or nitrides. Protective coatings include silica, boron oxide, alumina and wetting aids. Alternatively, it may be replaced with calcia or magnesia instead of all or part of the alumina.

Most commonly, carbon containing refractory articles are described as articles that come into contact with molten metals during the casting process, such as, for example, shrouds, nozzles, monoblocks or battery rods. Alternatively, the article may be a crucible or other refractory article comprising or in contact with molten metal.

Protective coatings are described as being particularly effective at the point where it is difficult to wet the fire resistant composition, such as articles comprising magnesia and zirconia. Wetting aids have been disclosed to reduce the surface energy of the protective coating at high temperatures, thereby improving wetting of the carbon containing refractory and reducing pinhole formation and oxidation. In one aspect, the wetting aid is disclosed as a metal oxide of a metal selected from the group consisting of arsenic, antimony, bismuth, tin, vanadium and lead. The metal oxide is preferably bismuth oxide, and preferably present at 10 wt% or less.

In a further embodiment of the invention, the protective coating comprises a flux that improves its fluidity. The solvent may be an oxide of an alkali metal, preferably an oxide of lithium, sodium or potassium.

Other details, objects, and advantages of the invention will be apparent from the detailed description of the preferred method of practicing the invention.

Detailed description of the invention

The present invention relates to a carbon-containing fire resistant article having a protective coating. Refractory articles include any article that includes an amount of carbon, where oxidation of carbon has a significant impact on the performance of the article. Typically, the refractory article comprises 2 to 40 weight percent carbon, the carbon being in the form of graphite. Such articles are often carbon bonded and in direct contact with the molten metal. The article will comprise one or more refractory components, such as refractory oxides, carbides or nitrides, in addition to carbon. Such refractory components may be selected by those skilled in the art. Examples of refractory articles include submersible inlet nozzles, submerged inlet shrouds, slide gate plates, battery rods, graphite electrodes, crucibles, and other components used in the steel and metallurgical industries. Refractory articles may include "unitary" articles, which are integral parts comprising a slide gate plate secured to a tube, such as a nozzle or shroud.

The protective coating contains a small amount of high temperature wetting aid, the main part of the coating comprising silica, boron oxide and suitable oxides such as oxides of aluminum, calcium or magnesium. A major part of the present invention is substantially disclosed in US Pat. No. 5,856,015, which is incorporated by reference. The main part is formed from glass particles free of lead and barium including refractory oxides, in particular boron silicate, boron phosphate and phosphate having an expansion softening point of 600 ° C. or lower. The main part comprises 40 to 80% by weight silica, 5 to 20% by weight boron oxide and 5 to 20% by weight of one or more refractory oxides selected from the group consisting of alumina, calcia and magnesia. In addition, the main part may comprise up to 4% by weight of cobalt oxide. The major part is generally present in up to about 99% by weight of the protective coating.

The main part may comprise an antioxidant that is oxidized before elemental carbon in the refractory article. If it contains an antioxidant, it is recommended to use an inhibitor that delays the oxidation of the antioxidant. Antioxidants and inhibitors may comprise up to about 30% by weight of the main portion. Suitable antioxidants include powders of metals, metalloids, carbides, nitrides or combinations thereof. The term "powder" includes powder and flake material. The powder preferably has a particle size of 1 to 250 μm, most preferably 10 to 50 μm. If a thick protective film is desired, the larger the particle size, the better. Common antioxidants include aluminum, magnesium, boron, calcium, silicon and carbides of silicon, calcium, zirconium, boron, tantalum, and titanium.

High temperature wetting aids improve the wettability of protective coatings in carbon containing refractory articles at temperatures of about 300 ° C. or higher. Improved wettability reduces pinhole formation and improves the ability of the protective coating to protect the article from oxides. Without being bound to this description, wetting aids can reduce the surface energy of the protective coating, thereby improving wettability and reducing pinhole formation and diffusion of oxygen into the carbon containing refractory article. Wetting aids include oxides of metals selected from the group consisting of arsenic, antimony, bismuth, tin, vanadium and lead. Bismuth oxide is particularly useful as a wet aid. These metals can often be present in the reduced state as well as as oxides. Ignition of the carbon containing refractory article under a reducing atmosphere may result in the presence of a reducing metal. As is customary in the industry, the presence of the reducing metal is reported in weight percent as if it were present as an oxide.

Sufficient amount of wetting aid is present to improve wettability and reduce pinhole formation. Effective concentrations vary depending on the specific conditions such as, for example, the composition of the refractory article, the thickness of the protective coating and the dissolution temperature. The amount of wetting aid impairs the barrier layer properties of the main part and limits the amount of oxygen diffusion through the coating into the carbon containing refractory article. The amount of the wet aid is preferably 0.5 to 10% by weight of the protective coating.

The protective coating may include a solvent that reduces the viscosity of the protective coating at high temperature. Low viscosities increase the fluidity and can easily seal small defects in the coating with a protective coating. Solvents should not cause harmful reactions with other components of the protective coating, especially antioxidants. Solvent addition levels depend on the particular properties desired and can be readily determined by one skilled in the art. Oxides and fluorides of alkali metals such as lithium, sodium and potassium are particularly useful solvents. Solvents can be used at up to about 20% by weight, but up to about 5% by weight is most effective.

The protective coating can be applied to the refractory article by any suitable application method, such as slip casting, dipping, dry or wet spraying or infiltration. Depending on the method, additives such as liquid carriers and flow modifiers may be required. The type and amount of additives required depends on the method. Preferably the wetting aid and the main part are suspended in a liquid carrier. Flow modifiers can be used to thicken and stabilize the suspension in the liquid carrier, thus facilitating coating the suspension on refractory articles. The liquid carrier is most commonly water and may be other suitable liquids such as alcohols and other organic solvents. Sufficient liquid carrier will be needed to suspend the wetting aid, main portion, and flow modifier to coat the protective coating on the refractory article. The amount of liquid carrier and flow modifier can be determined experimentally by mixing with other components of the protective coating until the desired consistency is achieved.

A carbon bonded battery rod having a protective coating was prepared. The body of the battery rod was carbon-bonded and contained alumina and at least 20 wt% graphite. The anode of the battery rod contained magnesia and more than 20% by weight graphite. The protective coating mainly consists of the following composition.

material weight%

Silica62

Boron Oxide13

Cobalt Oxide 1

Potassium oxide 2

Sodium oxide6

Lithium Oxide 2

Calcium Oxide 3

Alumina 9

Bismuth Oxide 2

The protective coating was applied to the battery rods before heating the article in an oxidizing atmosphere of 800 ° C. or higher as an aqueous suspension. The protective coating appeared to completely wet the rods and no pinholes were observed. The battery rod with the protective coating was heated in air at 1100 ° C. for 90 minutes. Oxidation and decarbonization were not observed. In contrast, rods without protective coatings exhibited extensive oxidation and softened due to deteriorated carbon bonds.

It is apparent that many variations and modifications of the invention are possible. Accordingly, it will be appreciated that the invention may be practiced in addition to those specifically disclosed within the following claims.

Claims (9)

(a) 40-80 weight percent silica; (b) 5-20% by weight of boron oxide; (c) 5-20% by weight of an oxide selected from the group consisting of alumina, calcia and magnesia; And (d) a carbon-containing fire resistant article having a protective coating comprising from 0.5 to 10% by weight of a high temperature wetting aid, wherein the high temperature wetting aid comprises a metal oxide, the metal being arsenic, antimony, bismuth, lead, vanadium and tin. A carbon-containing fire resistant article, characterized in that it is selected from the group consisting of. The article of claim 1 wherein the metal oxide is bismuth oxide. The article of claim 1, wherein the protective coating further comprises up to about 30% by weight of an antioxidant selected from the group consisting of metals, metalloids, carbides, nitrides, and combinations thereof. The article of claim 3, wherein the antioxidant is selected from the group consisting of aluminum, magnesium, silicon, boron, silicon carbide, boron nitride, aluminum nitride, and combinations thereof. 5. The article of claim 1, wherein the protective coating further comprises about 20% by weight or less of flux. 6. The article according to claim 5, wherein the solvent is an oxide of an alkali metal. The article of claim 6, wherein the alkali metal is selected from the group consisting of sodium, potassium and lithium. The article of claim 1, wherein the article is carbon bonded. The article of claim 1, wherein the article is selected from the group consisting of battery rods, shrouds, nozzles, slide gate plates, monoblocks and crucibles. Phosphorus goods.