CHROMIUM OXIDE COATED FIBRES, ARTICLES MADE THEREFROM AND METHOD OF COATING THE FIBRES

Abstract: A ceramic fibre and a shrink resistant article manufactured therefrom. The fibre comprises at least 80 weight percent of a refractory compound selected from the group consisting of silica, alumina, aluminum silicate, titania, zirconia, zirconium silicate and mixtures thereof and which comprises less than 5 combined weight percent of alkali or alkaline metal oxide or alkali or alkaline metal silicate. The fibre is uniformly coated with from about 0.01 to about 5 weight percent of Cr₂O₃. The coating is effected by precipitating Cr₂O₃.xH₂O on the fibre followed by conversion of the precipitated Cr₂O₃.xH₂O to Cr₂O₃.

Description: CHROMIUM OXIDE COATED REFRACTORY FIBRES. This invention concerns high temperature fibres and shrink resistant articles manufactured therefrom. In the prior art, amorphous or polycrystalline fibres were manufactured either from molten ceramic materials or from materials which would convert to ceramic materials upon the application of sufficient heat. Examples of sud fibres are mineral wool manufactured by blouiing fibres from molten slag obtanned as a by-produ.6t·from'metal refining, glass ucol manufactured by -blowing fibres from molten clay, silica and alumina; refined -mineral fibres blown from molten iron silicates; ceramic fibres made from molten aluminum silicates; drain glass fibres manufactured from alkali metal silicates, alkaline earth metal silicates and borosilicates; and spun fibrous from inorganic or organic compositions which convert to ceramic fibres upon the application of heat such as solutions of aluminum chlorhydrate. Of the foregoing fibres, aluminum sil cats, silica and alumina-silica fibres are generally considered to have the highest temperature resistance. Such fibres, however, have disadvantages uhich have never been completely overcome. In particular, the heat resistance is still not as high as is desired and shrink resistance of fibre articles manufactured from the fibres was not as good as desired. Numerous approeches have been taken to improve heat resistance and shrink resistance in inorganic fibres at high temperatures. The most common of such approaches has been to incorporate additives into the composition from uhich the fibres are made. Among such additives, chromium oxide has been used to increase temperature resistance. For example, U.S. Pat'ents 3,007,806 to Hartwig and 3,449,137 to Ekdahl; U.S.,Patent 4,125,406 to Sowman and British Patent Specification 495,654 all disclose that chromium oxide can incorporated into a fusion containing silica and alumina followed by formation of fibres from the fused material. While the incorporation of the chromium oxide into the fibres results in fibres having higher temperature resistance and improvement in shrink resistance of the fibres uhen formed into mats or other fibre articles, the process has serious disadvantages. In particular, oxygen is released by the chromium oxide in the melt which creates corrosion problems and results in the presence of chromium metal which creates an unstable fibre product. U.S. Patent 3,019,117 to Labino discloses that a mass of glass fibres may be dipped into a saturated solution of a metal nitrate such as chromium, iron, nickel or cobalt nitrates followed by drying the -fibre mass and subjecting the fibre mass to sufficient pressure and temperature to fuse the fibres. While the resulting fibre block has reasonably good temperature resistance, the flexibility and shrink resistance at elevated temperatures is not as good as desired. This is believed, in accordance with the present invention, to be due to migration of the solution prior to complete drying which results in non-uniform coating of the fibres. U.S. Patent 2,639,424 to Labino discloses that fibres of an alkali silicate could be treated with an acidic salt such as an aqueous solution of calcium, zinc or barium chloride so that the alkaline metal of the chloride replaces at least a portion of the alkali metal in the fibre fowed by heating to drive off water and treating the resulting fibres with a solution such as chromic anhydride to fill the resulting pores in the fibre with chromic oxide upon heating to a sufficient temperature. The resulting fibre is characterised by pockets of chromic oxide and according to the patent, is resistant to temperatures in excess of 2200 0F and, under certain circumstances, when the fibres are completely covered, i.e., encased. with chromic oxide, will resist temperatures of about 3000 C without deformation. It is to be noted that the process for treating the fibres is compiex utilising at least two liquid treatment steps and numerous drying and heating steps. It is to be further noted that the only specific teachings with respect to a method for obtaining complete coverage of the fibre with chromium oxide is by incorporating the fibre into a refractory brick structure containing between about 6 to about 16 percent of temperatureresistant oxides such as chromium oxide by weight of product. Such a brick structure is not flexible. Additionally, the fibres discussed in U,S. Patent 2,839,424 are fibres of an alkali silicate which is a substance suitable for utilisation in accordance with the process of the Labino patent but which is highly undesirable if resistance to humidity and chemical action is to be obtained. In accordance with the invention, there is provided a flexible shrink resistant ceramic fibre article, for example a fibre mat, comprising ceramic fibres which comprise at least 80 and preferably at least 90 and most preferably at least 95 weight percent of a refractory compound selected from the group consisting of silica, alumina, aluminum silicate, titania, titanium silicate, zirconia, zirconium silicate and mixtures thereof and which comprise less than S combined weight percent of an alkali or alkaline metal oxide or alkali or alkaline metal silicate, said fibres being uniformly coated with from about 0.01 to about 5 weight percent of Cr203. Alkali metals include lithium, sodium and potassium. Alkaline metals include barium, calcium and magnesium. The invention also comprises the ceramic fibre uniformly coated with from about 0.01 to about 5 weight percent of Cr203 from which the shrink resistant ceramic fibre article such as a mat is manufactured and the method for uniformly coating a ceramic fibre comprising at least 80, preferably at least 90 and most preferably at least 95 weight percent of a refractory compound selected from the group consisting of silica, alumina, aluminum silicate, titania, zirconia, zirconium silicate and mixtures thereof and which comprises less than S combined weight percent of an alkali or alkaline metal oxide or alkali or alkaline metal silicate. The invention also provides a method for uniformly coating a ceramic fibre with Cr203 comprising precipitating Cr203.xH2O upon the fibre followed by converting the precipitated Cr203.xH20 to Cr203. One method contemplated by the invention comprises contacting the fibre with a solution containing from about 0.1 to about 20 weight percent of chromium ion; raising the pH of the solution to above about 9.5 while the fibre is in contact with the solution to precipitate a chromium oxide hydrate upon the fibre; removing the 'fibre from the solution; vaporizing remaining free water from the fibre and heating the fibre to from about 200 to about 12000C to convert the chromium oxide hydrate to chromium oxide. Another method contemplated hy the invention for uniformly coating the ceramic fibres with Cr203 comprises the above method wherein the pH is not upwardly adjusted but the solution contains urea or another compound which thermally decomposes to produce ammonia or other basic compound. Plternatively, the pH of the chromium ion solution may be adjusted with ammonium hydroxide or other basic compound so that the adjusted pH is slightly less than that required to precipitate the hydrous chromium compound. Upon exposure to heat, water evaporation produces a higher pH solution in which the hydrous chromium compound precipitates. Another method contemplated in accordance with the invention for uniformly coating a ceramic fibre with from about 0.01 to about 5 weight percent of Cr203 is to spray a solution of chromium ion on the ceramic fibre during the fiberisation step and prior to initial cooling to ambient temperature. The temperature at which the solution is applied is sufficient to immediately vaporise the water and form a uniform chromium containing compound on the fibre surface which calcines to form a Cr203 coated ceramic fibre. Ceramic" as used herein means an inorganic compound having a decomposition or softening point above 1100 C. "Ceramic fibres", as used herein, means synthetic inorganic fibres which, both before and after treatment in -accordance with the present inventionS have a decomposition temperature or softening point greater than 11000C and preferably greater than 15000 C and a continuous maximum operating temperature greater than 1000 0C and preferably greater than 13000 C. The ceramic fibres with which this invention is concerned, are glass or microcrystalline fibres having diameters of less than 20 microns. Glass fibres are defined herein as fibres having no true crystalline structure; i.e., amorphous fibres, and microcrystalline fibres are defined herein as fibres comprising true crystals which, on the average, have a size of less than one-fourth of the diameter of the fibre. Examples of inorganic compounds, from which ceramic fibres are made, in accordance with this invention, are silicon dioxide (silica, Six2), aluminum silicate, aluminum oxide (alumina, Al203), titanium oxide (titania, TiO2), zirconium oxide (zirconia, ZrO2), zirconium silicate, other high temperature oxides and silicates and mixtures and complexes thereof, and lower melting oxides and silicates such as iron oxides, feldspar and bentonite in low percentages; i.e., less than -10 percent, provided that less than 5 combined weight percent alkali and alkaline metal oxides and silicates are used. The most preferred fibre for use in accordance with the present invention is a fibre which contains from about 40 to 100, preferably from 45 to 65 and most preferably 45 to 55 weight percent aluminum oxide with between 0 and 60 and preferably from 35 to 55 weight percent silica. Other compounds such as magnesia, iron oxide and ceramic oxides and silicates may be used provided that greater than 5 weight percent of combined alkali and alkaline metal oxides and alkali and alkaline metal silicates are not present since these compound reduce maximum operating temperatures. Also, generally purer materials (e.g. purified- silica and alumina) are frequently desirable. The most preferred fibre contains at least 90 < of combined Al20 3 and SiO which are usually at least partially combined with each other in the form of mullite. In accordance with the invention, the fibre is uniformly coated with from about 0.01 to about 5 weight percent and usually 0.1 to 0.8 weight percent of chromium oxide (chromia, Cr203) which both raises heat resistance of the fibre and unexpectedly permits the formation of shrink resistant mats, blankets and other particles to be formed from the fibres. "Uniformly Coated, as used herein, means that chromium oxide on the coated fibres is uniformly distributed along the length of the fibre in the form of substantially equally spaced particles or crystals without chemically removing portions of the fibre surface. In'accordance with the invention, there is two major methods for accomplishing a uniform coating of Cr203 upon the fibre. The first method comprises precipitating chromium oxide hydrate (Cr203.xH20) upon the fibres by means of a change in pH of a chromium ion solution followed by conversion to Cr203 and the second method comprises spraying the fibre at an elevated temperature with an aqueous chromium ion solution to very rapidly vaporise the water, leaving a uniform deposit of Cur203. In the first method, an aqueous solution containing from about 0.1 to about 7 weight percent of chromium ion is- treated with a base to raise its pH to above about 9.5 in the presence of the fibre to be treated thus precipitating chromic oxide hydrate upon the fibre The chromium ion is provided by any suitable water soluble salt such as chromium nitrate or chromium acetate. The base used to raise the pH may be any suitable base such as ammonium hydroxide or sodium hydroxide or may be a compound which will thermally decompose to a compound which will prov-ide hydroxy ions upon heating of the solution. An example of such a compound is urea which thermally decomposes to form N143 which in water forms NH40H. Alternatively, the pH of the solution may be raised to a level below 9.5, but above about 8 and the solution concentrated by evappration thus raising the pH to above about 9.5 to p-recipitate Cr203exH20 upon the fibres. After precipitation of Cr2O3.xH2O the Cr2O3.xH2O is converted to Cr203 by heating frdm about 200 C to about 12000C for a sufficient time to remove both free water and water of hydration leaving a uniform chromic oxide coating on the surface of the fibre. The time for conversion is dependent- upon the conversion temperature which may be from 0.5 to 2 hours at 2000C and from 1 minute to one second at 12000C depending upon the quantity of free water on the fibre, the initial temperature of the free water, the initial temperature of the fibre and Cur20 3' the quantity of Cr203, the circulation and transfer of heat and the number of fibres per unit heating area. The time usually is between 2.5 and 35 minutes at about 700 C depending upon mat thicknesses varying between 0.5 and 5 centimeters and densities varying between 0.06 and 0.13 grams per cc. In the second method, a chromium 'ion solution similar to the solution previously described, prior to precipitation of chromium oxide hydrate, is sprayed in be form of very fine droplets or mist upon the fibre; e.g., in droplet or particle diameter of from between molecular size to ten times- the diameter of the fibre. The chromium ion solution preferably contains from about 0.1 to about 7 weight percent chromium ion. The droplets may be initially somewhat larger but become smaller as they approach the fibre due to vaporisation. The temperature of the fibre struck by the droplets or mist is generally from the melting temperature of the fibre down to 350 C and in the case of alumina-silica fibres when the solution is applied at the time of blowing is usually from about 15000C to about 17500C. The fibres, uniformly coated with Cr203 in accordance with the invention, are characterised in having better heat resistance than uncoated fibres or non-unformly coated fibres of the same composition and are characterised in that mats and other articles formed from the fibres have better shrink resistance at elevated temperatures than prior art mats or articles formed from uncoated or non-uniformly coated¯fibres otherwise hauing'ths- same composition. As used herein, elevated temperature means above about 0.8T where T is the maximum continuous operating temperature in Or It has been further discovered that shrink resistance of articles made from the fibres can be additionally improved by pretreating the fibres at an elevated temperature in addition to coating them with chromium oxide. The heating at an elevated temperature must be for an insufficient time to cause the fibres to become brittle thus making them difficult to handle. Generally the fibre is pretreated at a temperature between about 1150 and about 1350 C for from 1 to about 10 minutes. The higher pretreating temperatures require the shorter times. Fibre articles or mats made from fibres of the invention are usually manufactured by compressing and needling the fibres to form a three dimensional structure which may be used as/is or cut to a desired final shape. The invention will be illustrated by reference to the following Examples. EXAMPLE 1. A mixture of 48 percent Bayer process alumina and 52 percent high purity silica sand is melted in a furnace. A stream of the molten mixture at about 1850 C is then passed through an orifice at a rate of about 340kg. per hour. The molten stream is then first impinged by an air stream at sufficient pressure to turn the stream about 900. The molten-stream then passes through a secondary stream of air which impinges -the molten stream at sufficient force and volume to form fibres having a diameter of about 3 microns. About 65 litres per hour of a liquid containing a lubricant is applied to the stream at the primary nozzle to assist needling. The fibres are then collected on a moving conveyor mesh and compressed and needled te form a 2.5 cm. thick mat having a density of eight pounds per cubic foot (about 0.13 grams per cc). The mat is then heated to oxidise any remaining lubricant. The resulting mat is then heated to about 1425so from ambient temperature over a period of about 12 hours and held at that temperature for 7 and 14 days. The mat is found to have a linear shrinkage of 9.35% after 7 days and 10.44% after 14 days. EXAMPLE 2. Example 1 is repeated except an aqueous solution, contassLno about 1.74 weight percent chromium, provided by dissolving chromium acetate and 4 weight percent lubricant, is substituted for the lubricant solution of Example 1. The resulting mat after heating is uniformly coated with Cr2O3 and when tested is found to have a linear shrinkage of 7.48% after 7 days and 9.04% after 14 days. The mat is found to have a chromium oxide (cur203) content of 0.47 weight percent. EXAMPLE 3. Example 2 is repeated except prior to testing, the mat is prefired at about 12000 C for 5 minutes. The resulting mat retains flexibility and is found to have a linear shrinkage of 4.65 after 7 days and 5.72 after 14 days. EXAMPLE 4. Example 2 is repeated except about 86 litres per hour of solution is applied to the fibre. The resulting mat is found to have a Cr203 content of about 0.61 weight percent and a linear shrinkage of about 6,61 after 7 days and 8.10% after -14 days at about 1425 C. EXAMPLE 5. Example 4 is repeated except prior to testing for shrinkage, the mat is prefired at about 12000C for 5 minutes. The resulting mat retains flexibility and is found to have a linear shrinkage of about 5.16% after 7 days and about 6.51 after 14 days. EXAMPLE 6. The process of Examp'le i is followed except that the fibres are needled and compressed to form a 2.5 cm thick mat having 2 density of 6 pounds per cubic foot (about 0.1 grams per cc). The resulting mat is found to have a linear shrinkage of about 6.65% after 9.5 hours at 1425 C. EXAMPLE 7. The mat of Example 6 is saturated with a chromium acetate solution containing 87 grams of chromium acetate per 1000 ml of water. Prior to treatment, the solution was adjusted to a pH of 9.5. The blanket is allowed to drain and is dried at about 1700C which results in a uniform precipitate. After heating at 1425 C for 9.5 hours, the mat is found to have a linear shrinkage of 5.82. EXAMPLE 8. Example 7 is repeated except the mat is prefired at. 1200 C for 5 minutes prior to testing. The resulting mat retains flexihility and after heating at 1425oC for 9.5 hours, the mat is found to have a linear shrinkage of 2.39%. EXAMPLE 9. Example 6 is repeated except kaolin clay fibres are used instead of the alumina and silica. The resulting fibre mat has a linear shrinkage of 11.61% after heating at 14250C for 9.5 hours. EXAMPLE 10. Example 7 is repeated except kaolin clay fibres are used. -4The resulting fibre mat has a linear shrinkage of 5.09% after heating at 14250C for 9.5 hours. EXAMPLE 11. Example 8 is repeated except kaolin clay fibres are used. The resulting fibre mat retains flexibility and has a linear shrinkage of 3,79.

Claims: CLAIMS 1. A shrink resistant ceramic fibre article comprising ceramic fibres which comprise at least 80 weight percent of a refractory compound selected from the group consisting of silicon oxide, aluminum silicate, aluminum oxide, titanium oxide, zirconium oxide, zirconium silicate and mixtures thereof and which comprise less than 5 combined weight percent of an alkali or alkaline metal oxide or alkali or alkaline metal silicate, said fibres being uniformly coated with from about 0.01 to about 5 weight percent of Cr2 3+ 2. An article as claimed in claim 1 wherein the fibres are micro-crystalline fibres. 3. An article as claimed in claim 2 wherein prior to coating uith Cr203, the fibres comprise from about 40 to 100 weight percent alumina and from 0 to about 60 weight percent silica. 4. An article as claimed in claim 3 wherein the fibre contains up to about 65 weight percent Al203. 5. An article as claimed in claim 2 wherein the fibres are treated by heating them to between about 1150 and about 1350or for from about 10 to about 1 minutes. 6. An article as claimed in any one of claims 1 to 5 which is a fibre mat. 7. A ceramic fibre which comprises at least 80 weight percent of a refractory compound selected from the group consisting of silica, alumina, aluminum silicate, titania, zirconia, zirconium silicate and mixtures thereof and which comprises less than 5 combined weight percent of alkali or alkaline metal oxide or alkali or alkaline metal silicate, said fibre being uniformly coated with from about 0.01 to about 5 weight percent of Cur203. 8. A fibre as claimed in claim 7 wherein said fibre is a micro-crystalline fibre. 9. A fibre as claimed in claim 8 wherein the fibre comprises from about 40 to 100 weight percent alumina and 0 to about 60 weight percent silica. 10. A method for uniformly coating a ceramic fibre with Cry03 compr-ising precipitating Cr203.xH20 upon the fibre followed by converting the precipitated Cr203.xH2O te Cur203. 11. A method for uniformly coating a ceramic fibre with Cr203 comprising: 1) contacting the fibre with à solution containing from about 0.1 to about 20 weight percent of chromium ion; 2) while said fibre is in contact with said solution, raising the pH to above about 9.5 to precipitate Cr203.xH2O upon the fibre; 3) vaporising the remaining water from the fibre; and 4) heating of said fibre to from about 200 to about 12000C to convert said Cr203.xH20 to Cur203 . 12. A method as ciaimed in claim 10 wherein said solution contains about 0.1 to about 7 weight percent of chromium ion. 13. A method as claimed in claim 11 or 12 wherein the fibre is a microcrystalline fibre. 14. A method as claimed in claim 13 wherein the fibre, prior to coating, comprises from about 40 to 100 weight percent Al203 and from 0 to about 60 weight percent Six2. 15. A method as claimed in any one of claims 11 to 14 wherein said pH is raised by adjusting it upward with NH4OH. 16. A method as claimed in any one of claims 11 to 14 wherein said solution contains a compound which will thermally decompose to No3 or free hydroxyl ion. -17. A method. as claimed in claim 16 wherein said compound is urea and the pH is adjusted to above about 9.5 by heating said solution to decompose the urea to NH3. 18. A method for uniformly coating a ceramic fibre comprising spraying a solution containing from about 0.1 to about 20 weight percent chromium ion upon the fibre at a fibre temperature of about 350 0C to the melting temperature of the fibre. 19. A method as claimed in claim 18 wherein the solution contains from about 0.1 to about 7 weight percent chromium ion. 20. A method as claimed in claim 18 or 19 wherein the fibre is formed from a composition comprising at least 90 weight percent of a mixture of alumina and silica and the fibre temperature at the time of spraying is from about 1500 to about 1750 0Ct 21. A method as claimed in claim 20 wherein the fibres comprise from about 40 to 100 weight percent alumina and from 0 to 60 weight percent silica. 22. A method as claimed in claim 21 wherein the fibres comprise from about 45 to about 65 weight percent alumina and from about 35 to about 55 weight percent silica. 23. A method as claimed in claim 22 wherein the fibres comprise from about 45 to about 55 weight percent alumina and from about 45 to about 55 weight percent silica.

Inventors: SMITH, RUSSELL DAVID, TRESSLER, RICHARD ERNEST
Applicants: KENNECOTT CORPORATION
Application year: 1980
Main classification: C04B41
IPC codes: C03C13, D06M11, C04B20, C04B41, C04B38, D06M23, D04H1, C04B35, D06M13, D06M101, C03C25