MAGNESIUM ALUMINATE FIBER COMPOSITION AND PROCESS THEREFOR

Abstract: Strong, fine-grained microstructure refractory magnesium aluminate fibers, preferably having an aluminum to magnesium molar ratio of 2.03 to 2.20, are produced by fiberizing a viscous precursor solution prepared by the ordered steps of: (1) dissolving an aluminum salt in a polar solvent; (2) dissolving a magnesium compound in this solution; and (3) dissolving an aluminum compound in this solution. A dopant, stabilizing agent, and spinning aid are optionally comprised in the precursor solution.

Description: This invention concerns magnesium aluminate fibers and more particularly a composition and process for preparing magnesium aluminate (MgAl₂O₄) fibers useful as refractory fibers. Energy conservation efforts by industry in recent years have led to an increase in the use of ceramic fibers for high temperature insulation applications. Furnace lining products fabricated from ceramic fibers have a lower heat capacity and are better insulators than traditionally used dense firebrick refractories. The replacement of firebrick with a fibrous refractory lining can reduce thermal conductivity by more than 50 percent, and the lower heat capacity of the fibrous refractory lining reduces heat-up cost and allows faster heating/­ cooling cycle rates. Currently available ceramic fiber refractory products, which are fabricated mainly from aluminosilicate or alumina fibers, are generally limited to use tempera­ tures below 1600°C and are inappropriate in a corrosive environment. Products made from fibrous zirconia are useful at temperatures up to 2,200°C and are stable in most corrosive environments, but are relatively expen­ sive for normal industrial furnace lining applications. Consequently, a fiber that exhibits good corrosion resistance and/or is usable above 1,600°C for long periods is much desired. MgAl₂O₄ fibers possess both of these properties and have a relatively lower raw material cost than some other currently produced fibers, such as ZrO₂ fibers. Previous art, notably U. S. Patents 4,020,233; 3,992,498; and 3,322,865, describe various methods of making magnesium aluminate fibers from solution. In each case the solution is unstable and a precipitate frequently results within a short time. Once this happens, the solution becomes incapable of being spun into fiber. When the fiberizing procedure can be performed, the resultant fibers are often extremely fragile and therefore difficult to handle. Surprisingly, the present invention provides a fiberizable composition that is suitable for producing magnesium aluminate fibers that can be readily handled, and a process for their preparation that is simple and convenient. Accordingly, the present invention comprehends a composition which comprises a fiberizable precursor solution suitable for producing magnesium aluminate fibers having a fine grained microstructure with a molar ratio of aluminum to magnesium within the range of from 2.03 to 2.20. The present invention further comprehends a process for preparing a fiberizable precursor solution suitable for the preparation of magnesium aluminate fibers which comprises the ordered steps of: (a) preparing an aluminum salt solution; (b) dissolving a magnesium compound in the solution prepared in step (a); and (c) dissolving an aluminum compound in the solution prepared in step (b). The present invention still further compre­ hends a process for preparing magnesium aluminate fibers which comprises: (a) forming a precursor solution by the ordered steps of (1) dissolving an aluminum salt in a polar solvent; (2) dissolving a magnesium compound in the solution prepared in step (1); and (3) dissolving an aluminum compound in the solution prepared in step (2); (b) dissolving a dopant and a stabilizing agent in the solution prepared in step (a); such that this solution is capable of decomposing to magnesium aluminate fibers; (c) concentrating this solution to a viscous material of from 25 to 500 poise (2.5-50 Pa.s); (d) fiberizing the viscous material to form fibers; and (e) heating these fibers sufficiently to form crystalline magnesium aluminate fibers. In general, the present invention comprehends the production of magnesium aluminate fibers of novel composition by a process that involves forming a fiber­ izable solution of specific composition; fiberizing the solution; and then heating the resultant fibers at a temperature sufficient to crystallize the magnesium aluminate fibers. By "fiberizable solution" it is meant a solution which is capable of being formed into fibers by known methods. Providing a precursor, fiberizable solution capable of decomposing upon heating to form crystalline fibers of magnesium aluminate having a fine-grained microstructure is an important feature of the present invention. By "fine-grained" is meant a microstructure in which grains have average diameters in the range of from 0.01 to 5 micrometers (µm), preferably from 0.01 to 0.1 µm. An important aspect of forming the fiberizable solution is control of the step order of addition of materials. In general, the procedure involves, first, dissolution of an aluminum salt in a polar solvent, followed by addition of a magnesium compound, prefer­ ably magnesium metal. The magnesium is allowed to dissolve, and then an aluminum compound, preferably aluminum metal, is added to the solution which is now a precursor fiberizable magnesium aluminate solution. Other additives may include a dopant, which serves to substantially densify the final fiber and which may be added at any point; a stabilizing agent; and a spinning aid. Of these, the stabilizing agent and spinning aid must be added after the basic precursor solution is formed. The solution may then be concentrated as necessary for fiberizing by known methods. The reason for the order of addition (i.e., formation of the aluminum salt solution, then dissolu­ tion of a magnesium compound, followed by dissolution of an aluminum compound) is that, by following this order, the pH of the solution is maintained below about 7, i.e., the solution is to some degree acidic through­ out its preparation. This acidity encourages complete dissolution of the magnesium and therefore helps to reduce or eliminate entirely the possibility of a precipitate of magnesium forming. It is preferred that the magnesium be given time to completely dissolve prior to addition of the aluminum compound, since dissolution of the magnesium will stop when the alu­ minum compound is added. Since the solution's essential nature is thereby preserved, there is increased solu­ tion stability with time and therefore also increased suitability for fiberizing, since formation of preci­ pitates generally makes fiberizing difficult or impos­ sible and disrupts the stoichiometry of the final product desired. The stability of the present inven­ tion also offers commerical advantage and economy, since storage and shipment of the solution are facilitated. Further economical advantage may be obtained through judicious selection of starting materials. Any polar solvent is efficacious in the formation of the precursor solution, with the provision that the aluminum salt, magnesium compound and aluminum compound are soluble in it. It may be organic or inorganic. Among possible choices are, for example: an alcohol, in particular, methanol or ethanol; glacial acetic acid; dimethylsulphoxide; dimethylformamide; water; or mixtures of these. It is preferred to use water as the solvent, from the standpoint of conveni­ ence and expense. The compounds of magnesium and aluminum as used herein include the metals of magnesium and alu­ minum, their suitable inorganic metal salts such as sulfates, nitrates, chlorides and phosphates, and mixtures thereof. A sufficient amount of magnesium and aluminum metal or compounds are dissolved in the pre­ viously specified order in the solvent to provide a resulting solution with the desired aluminum to mag­ nesium molar ratio. This ratio should be carefully calculated. The theoretical aluminum to magnesium molar ratio for stoichiometric magnesium aluminate is 2.00; however, in this invention the aluminum to mag­ nesium molar ratio is desired to be between 2.03 and 2.20, more preferably between 2.03 and 2.10, and most preferably about 2.05. It is further preferred that the cation (i.e., aluminum and magnesium) to anion (i.e., chloride, nitrate, sulfate) molar ratio be as high as possible. The cation to anion molar ratio is desired to be from 0.33 to 1.0, and preferably about 1.0. In general, the high cation to anion ratios result in increased metals loading, which results in better quality fiber that is less susceptible to shrinkage upon calcination. This is because of reduced grain growth and porosity in the microstructure. In carrying out one process of the present invention, it is preferred to use, e.g., magnesium and aluminum and an aqueous aluminum salt solution to form the fiberizable solution. The magnesium and aluminum metals used are in the form of powders, small particles or turnings in order to facilitate dissolution of the metals in the solution. Generally, the metals are added to the solution in a form such as powders having a high surface area. The aluminum salt used is a halide of aluminum such as aluminum chloride, bromide or iodide; sulphate; nitrate; or mixtures thereof. As an example of the present process, the magnesium metal may be dissolved directly in an aluminum chloride salt solution, then the aluminum metal may be added to the mixture, preferably not until the magnesium has dis­ solved completely, to provide the ratio of Al:Mg required for the solution. A precipitate will occur, however, when the aluminum metal is dissolved in the aluminum chloride salt solution prior to adding the magnesium metal. As noted above, the precipitate adversely affects the spinnability of the solution, the stability of the solution, and the stoichiometry of the final fibers when the solution is fiberized. A dopant is optionally added to the precursor solution. This is a soluble metal compound and serves to decrease both the grain size and shrinkage potential of the final fibers. The dopant component is added as a soluble compound capable of forming an oxide upon heating. The dopant aids in the densification of the final fiber and substantially eliminates the formation of pores in the final fiber. In addition to minimizing shrinkage, it also adds strength to the fibers. These advantages are believed to result because the presence of dopant in the final product speeds the formation of a MgAl₂O₄ phase by increasing the defect concentration. At higher (i.e. > 1200°C) temperatures the dopant decreases the sintering rate of the MgAl₂O₄ which prevents the formation of trapped porosity and rela­ tively larger grains, for example, grains larger than about one µm in diameter. Suitable dopants include any compound or compounds which decompose to the fol­ lowing: ZrO₂; MnO₂; TiO₂; Cr₂O₃; Y₂O₃; or mixtures thereof. Preferably, the solution is doped with a compound which will decompose upon heating to ZrO₂, such as ZrCl₄, because ZrO₂ has desirable refractory properties such as a high melting point (∼2700°C). The solution should have a sufficient amount of dopant such that the final product, i.e., the fibers, has a concen­ tration of dopant lying in the range from 500 parts per million (ppm) up to 50,000 ppm. Preferably, the level of dopant should be sufficient to provide a concen­ tration in the final product lying in the range from 2,000 to 5,000 ppm. The dopant may be added at any point in preparation of the solution. The solution according to the present inven­ tion also contains a "stabilizing agent" or "stabilizer," in a concentration preferably lying in the range of from 10 to 100 grams/mole of MgAl₂O₄ in solution. This stabilizer is always added prior to concentrating the solution. More preferably, the stabilizer is present in the solution in a concentration of between 30 to 70 grams/mole of MgAl₂O₄ in solution, most preferably between 45 to 55 grams/mole of MgAl₂O₄ in solution. Suitable materials used as the stabilizing agent may include, for example, carboxyl type organic acids such as lactic, acetic, formic, oxalic, citric, tartaric, maleic, adipic, itaconic, their halogenated derivatives and the like. The preferred stabilizer is lactic acid. Preferably, the stabilizer used in the solution does not react with the metal compounds, such that the stabilizer is not thereby depleted. The stabilizer provides a stable solution with a higher solids con­ tent, i.e., about 30 percent by weight based on MgAl₂O₄, such that fibers can easily be formed by known methods. By "stable solution" is meant a solution that does not crystallize or gel and remains fluid. Typically, a stable solution will remain fluid for approximately one month under ambient conditions. Addition of more solvent will allow the solution to remain fluid indefinately. Once the precusor solution is formed, it may be concentrated as needed to produce a solution sufficiently viscous for fiberizing. The viscosity of the solution preferably lies in the range from 25 to 500 poise (2.5 to 50 Pa.s) and, more preferably from 60 to 150 poise (6 to 15 Pa.s). Concentration may be by evaporation of the solvent or other effective means, such as a rotary evaporator or other apparatus, and is preferably carried out at a temperature of 90 to 110°C and at atmospheric pressure (0.1 MPa). Lower temperatures can be used at reduced pressures. This concentration step serves to maximize the solids content of the solution, which generally is up to about 30 percent by weight based on MgAl₂O₄. Optionally, other additives may be present in the solution, for example, an additive for providing corrosion resistance to the fiber, such as a chromium compound, or an additive for facilitating the spinning of the solution such as an organic polymeric spinning aid. These are generally added after the concentration step, for reasons of convenience, but may be added prior to concentration, if desired. Various spinning aids which may be used in the present invention are known in the art such as those described in U. S. Patent 3,992,498, including partially hydrolyzed poly­ vinyl acetate, polyvinyl alcohol, polyacrylamide and partially hydrolyzed polyacrylamide, polyacrylic acids, polyethylene oxides, carboxyalkyl celluloses (for example, carboxymethyl cellulose), hydroxyalkyl cel­ luloses (for example, hydroxymethyl cellulose), alkyl celluloses (for example, methyl cellulose), hydrolyzed starches, dextrans, guar gum, polyvinyl pyrrolidones, polyethylene glycols, alginic acids, polyisobutylene derivatives, polyurethanes and esters, copolymers or mixtures thereof. Preferred spinning aids are straight-­ chain polyhydroxylate organic polymers, for example, polyvinyl alcohol, partially hydrolyzed polyvinyl acetate, polyethylene oxide, or polyethylene glycol; most preferred, because of convenience and economy is polyethylene oxide. The spinning aid or aids should preferably be added after the basic precursor solution is formed. The reason for the specific order of addition of the three main starting materials given above is the tendency of a precipitate of metals to form when a different order of steps is followed. The precipitation results in a suspension which is generally incapable of being fiberized. It is preferred to maintain the solution at 70 to 110°C, more preferably 90 to 110°C, until the starting components and the dopant are incor­ porated. This temperature range is sufficient to hasten the reaction while maintaining the solution's essential nature without encouraging premature decom­ position of the components. This helps to ensure a fiberizable resultant solution. The temperature of this solution is then preferably lowered to between 25 to 70°C, more preferably 40 to 70°C, and the stabilizer and spinning aid are then incorporated into the solu­ tion. This minimizes the likelyhood of reaction between the stabilizer or spinning aid and the precursor solu­ tion. The concentrated fiberizable precursor solu­ tion, which may contain the additional materials such as dopant, stabilizer, and spinning aid, is now ready for fiberizing. The solution may be formed into green or precursor fibers by any of the known dry spinning techniques, for example, by blow-spinning or centri­ fugal spinning. Preferably, the precursor fibers from the solution are formed by a blow-spinning operation in which the solution is extruded through orifices into converging (e.g. 30°), high velocity streams of dry nitrogen gas which elongates the fiber. The fibers are dried by exposure to hot air of low relative humidity present in the spinning chamber. An example of a suitable process for fiberizing the solution is described in U. S. Patent 3,992,498. The precursor fibers formed by the fiberizing step may then be collected and heated sufficiently to remove any volatiles from the fibers and convert the fibers into their crystalline form, i.e., their oxide form. The precursor fibers, once heated or sintered, form MgAl₂O₄ fibers. Generally, the fibers are heated at from 100 to 150°C for one to four hours to dry the fibers. The fibers are then fired at from 800 to 1,600°C, preferably from 1,300 to 1,500°C, for one minute to four hours to form the magnesium aluminate fibers which may be used as refractory material. More preferably the fibers are held at temperature for 1 minute to 1 hour. Furthermore, it is possible, in order to obtain superior fibers useful as refractory material, to subject the fibers to a heating cycle, i.e., the fibers are heated in successively higher steps in a plateau-like cycle, for example, as described in Table I of Example 2. The cyclic heating serves to maintain slow outgassing which prevents disruption of the fiber structure. Having formed the fibers above, the fibers may be useful in high temperature (i.e. ≧ 1,500°C) thermal insulation applications such as for furnace lining. The fibers may also be useful as a reinforcing additive in other ceramics or plastics. In addition the fibers may be used as a catalyst support. The fibers formed in accordance with the present invention, in addition to having high temperature and corrosion resistance, are characterized as being quite "handle­ able," i.e., they are not fragile and do not tend to break up easily or crumble upon handling. The refractory MgAl₂O₄ fibers prepared by the present invention have a relatively higher friability index than MgAl₂O₄ fibers prepared by other known methods. The "friability index" of the fibers is a measure of the fiber's fracture resistance or non-­ friable nature. A test procedure which may be used to measure the friability index of a fiber product formed in accordance with the present invention is disclosed in U. S. Patent 4,047,965. While there are other commonly known friability tests, this one has been selected for use with the examples, infra , because of its ease and relative reliability. General Friability Test Procedure The friability index of the fibers produced by the process of the present invention is determined by placing a sample of fibers weighing 0.5 to 2.0 grams in a 1 pint (0.57 l) glass jar containing 25 size No. 00 stan­ dard laboratory rubber stoppers. The jar is sealed with a screw top, placed on a ball mill roller and rotated for 4 minutes. The fibers are then rinsed onto a 50 mesh test screen with water containing a detergent such as Calgon® (a trademark of the Calgon Corporation, Pittsburgh Activated Carbon Division). The screen and the fibers retained thereon are heated at 125°C until dry. The fibers are then removed from the screen and weighed. The "friability index" is that number obtained by dividing the weight of dry fibers retained on the screen by the total weight of the initial fibers placed in the jar. A friability index of "zero" means the fiber is so fragile or weak that it breaks into short fragments or disintegrates to dust when rubbed even gently between the index finger and thumb. A test sample of fiber product having a friability index of "zero" washes completely through the test screen, i.e., essentially no fibers are retained on the screen. A friability index of "one" represents a sample which does not break into short fragments upon milling and is therefore retained in its entirety on the screen. The following examples are merely illustra­ tive of the present invention and are not intended to limit the scope of the invention. Example 1 A 48.28 gram (g) (0.2 mole) sample of AlCl₃·6H₂O was dissolved in 150 g H₂O and heated to 90°C. As the hot (90°C) solution was stirred, 4.86 g (0.2 mole) of Mg metal turnings were added in 0.5 g portions over a 2 hour period with evolution of H₂ gas. After all the Mg metal was dissolved in the AlCl₃ solution, 5.39 g (0.2 mole) of Al metal turnings were added to the stirred, hot solution in 0.5 g portions over a 2 hour period. Again H₂ gas was evolved. A total of 4 percent by weight of lactic acid was then added. The hot solution was filtered to remove insol­ uble residue and evaporated at 90°C until a scum or film formed on the surface. The resulting solution had an Al:Mg molar ratio equal to 2.00. This solution was cooled to 40°C and treated with 1 percent by weight of a high molecular weight polyethylene oxide to give a clear, viscous solution. The viscosified solution was extruded at a pressure of about 30 psig (0.2 MPa) through a 10 mil (0.25 mm) orifice into a high velocity stream of dry N₂ where the solution was blown into fibers having a diameter of 5 to 10 µm which were collected as a fibrous mat on a 10 mesh screen. The mat of fibers was dried for 1 hour at 180°C and then fired at 1,000°C for 1 hour. A sample of the fibers were ground to a powder. The X-ray diffraction pattern of the powdered sample of the resultant ceramic fibers matched the pattern of MgAl₂O₄. Example 2 A chloride solution was prepared by dissolv­ ing a 48.28 g sample of AlCl₃·6H₂O in 150 milliliters (ml) of deionized water while stirring. A 4.86 g sample of magnesium metal was added to the chloride solution. Then the solution was heated to 100°C under reflux conditions. After the magnesium was completely dissolved, 5.39 g of aluminum metal was added to the solution to form a clear solution. The solution had an Al:Mg molar ratio of 2.00. Then sufficient ZrCl₄ was added to the clear solution to form 500 ppm ZrO₂ in the final fiber product. The solution temperature was lowered to 60-70°C and then 10 g of lactic acid was added to the solution to prevent gelling or crystal­ lization of the solution. The solution was then concentrated by evaporation using a rotary evaporator until the viscosity of the solution was 100 to 200 poise (10 to 20 Pa.s). The viscous solution was used to form fibers using the following procedure: The clear solution was fiberized by extruding the solution at 40 psig (0.27 MPa) through a 16 mil (0.4 mm) orifice into three high velocity jets of dry N₂ issuing from 3 holes situated concentrically, 120° from each other around an orifice. The jets of N₂ angled at 20° toward the center line of the orifice so that the solution coming from the orifice was picked up, elongated, and blown into precursor fibers having a diameter of 5 µm to 10 µm. The precursor fibers were collected on a screen mounted 6 feet (1.8 m) from the spray nozzle. Two heat guns were mounted near the spray nozzle to help dry the fibers as they were formed. The mat of precursor fibers collected on the screen was dried at 25°C for 1 hour. The dried mat was removed from the screen and fired to 1,500°C for 1 hour using a rapid temperature furnace with an automatic controller. The firing was carried out to drive off the chloride and form oxide fibers. The precursor fiber firing schedule is shown in Table II below. The resultant product was found to be MgAl₂O₄ as determined by X-ray diffraction analysis. The MgAl₂O₄ fibers produced after firing was subjected to the general friability test procedure described above. The MgAl₂O₄ fibers had a friability index of 0.1. Example 3 A magnesium aluminate fiber with an aluminum to magnesium ratio of about 2.00 having a diameter of from 1 to 10 µm and length up to about 5 inches (12.7 cm), was made by the following procedure: 241.43 g of aluminum chloride hexahydrate were dissolved in 500 ml of deionized water and heated to reflux, approximately 100°C. After complete solution had taken place 24.305 g of magnesium metal was slowly added to the chloride solution over a 2 hour period with the evolution of H₂ gas. After the reaction was complete and the solution clear, 26.98 g of aluminum metal was slowly added to the chloride solution over a 2 hour period with the evolution of H₂ gas. After the reac­ tion was complete and a clear solution resulted, 0.535 g of ZrCl₄, zirconium chloride, sufficient to produce a 2,000 ppm ZrO₂ level in the final oxide fiber, was added to the chloride solution. This solu­ tion was then cooled to 40°C and 49.6 g of 88 percent lactic acid was added. The aluminum magnesium oxy­ chloride solution was carefully filtered through a fine acid-resistant filter paper. A 2.75 percent by weight solution of poly­ ethylene oxide was made by stirring in distilled water overnight. Once the polymer was in solution, 0.1 per­ cent MnCl₂·4H₂O was added as a stabilizer. 5.00 g of the polyethylene oxide solution was added to the above aluminum magnesium oxychloride solution. The resultant mixture was evaporated on a standard laboratory rotary evaporator at 60°C under reduced pressure. This was continued until a room temperature viscosity of 80 poise (8 Pa.s), as measured by a Brookfield viscometer with a number 5 RVT spindle at 10 rpm, was produced. The resulting oxychloride solution was transparent with a slight yellowish tint. The viscosified solution was then extruded at a pressure of about 30 psig (0.2 MPa) through a 10 ml (0.25 mm) orifice into a high velocity stream of dry N₂ where the solution was blown into fibers having diameters of 1 to 10 µm which were collected as a fibrous mat on a 10 mesh screen. The mat of fibers was dried overnight at 100°C and then heated to 1,400°C over a 4 hour cycle. The X-ray diffraction pattern of the resultant fiber indi­ cated pure magnesium aluminate. The fiber was very fragile and could not be handled without crumbling. Fiber microstructure was very coarse having single grains the size of the diameter of the fiber. The friability index was 0.10. Example 4 Fibers were made as described in Example 3 except that the following formulation was used: 241.43 g aluminum chloride hexahydrate 23.91 g magnesium metal 26.98 g aluminum metal 49.6 g lactic acid 88 percent .535 g zirconium chloride 5.00 g 2.75 percent solution of poly­ ethylene oxide The resultant fiber had an aluminum to magnesium molar ratio of 2.03. The fiber microstructure consisted of small, about 02. µm diameter, grains with numerous slip planes visible. The fiber had a friability index of 0.30. Example 5 Fibers were made as described in Example 3 except that the following formulation was used: 241.43 g aluminum chloride hexahydrate 23.71 g magnesium metal 26.98 g aluminum metal 49.6 g lactic acid 88 percent .535 g zirconium chloride 5.00 g 2.75 percent solution of poly­ ethylene oxide The resultant fiber had an aluminum to magnesium molar ratio of 2.05. The resultant fiber microstructure was smooth with individual grains difficult to distinguish. The fiber had a friability index of 0.60. Example 6 Fibers were made as described in Example 4 except that the following formulation was used: 241.43 g aluminum chloride hexahydrate 23.48 g magnesium metal 26.98 g aluminum metal 49.6 g lactic acid 88 percent .535 g zirconium chloride 5.00 g 2.75 percent solution of poly­ ethylene oxide The resultant fiber had an aluminum to magnesium molar ratio of 2.07. The resultant fiber microstructure was smooth with individual grains difficult to distinguish. The fiber had a friability index of 0.50. Example 7 Fibers were made as described in Example 3 except that the following formulation was used: 241.43 g aluminum chloride hexahydrate 23.14 g magnesium metal 26.98 g aluminum metal 49.6 g lactic acid 88 percent .535 g zirconium chloride 5.00 g 2.75 percent solution of poly­ ethylene oxide The resultant fiber had an aluminum to magnesium molar ratio of 2.10. The resultant fiber microstructure was smooth with individual grains difficult to distinguish. The fiber had a friability index of 0.35. Example 8 Fibers were made as described in Example 3 except that the following formulation was used: 241.43 g aluminum chloride hexahydrate 23.09 g magnesium metal 26.98 g aluminum metal 49.6 g lactic acid 88 percent .535 g zirconium chloride 5.00 g 2.75 percent solution of poly­ ethylene oxide The resultant fiber had an aluminum to magnesium molar ratio of 2.20. The resultant fiber microstructure was smooth with individual grains difficult to distinguish. The fiber had a friability index of 0.40. Example 9 Fibers were made as described in Example 3 except that no ZrCl₄ was added. The resultant fiber had a microstructure exhibiting obvious grain boundaries and slip planes in grains. The fiber had a friability index of 0.05 Example A (Comparative Example) An attempt was made to produce a solution suitable for spinning using the following procedure: 241.43 g of aluminum chloride hexahydrate were dissolved in 500 ml of deionized water and heated to reflux, approximately 100°C. After complete solution had taken place 26.98 g of aluminum metal was slowly added to the chloride solution over a 2 hour period with the evolution of H₂ gas. After the reaction was complete and the solution clear, 24.305 g of magnesium metal was slowly added to the solution over 2 hours. The resultant solution consisted of a white gel precipitate with much unreacted magnesium metal. No fibers could be formed.

Claims: 1. A composition which comprises a fiberizable precursor solution suitable for producing magnesium aluminate fibers having a fine-grained microstructure with a molar ratio of aluminum to magnesium with the range of from 2.03 to 2.20. 2. A composition as claimed in Claim 1, wherein a dopant is also present in said solution. 3. A composition as claimed in Claim 2, wherein said dopant comprises at least one compound that decomposes upon heating to form ZrO₂, MnO₂, TiO₂, Cr₂O₃, Y₂O₃ or mixtures thereof. 4. A composition as claimed in Claim 3, wherein said dopant is zirconium chloride. 5. A composition as claimed in any one of the preceding claims, wherein a stabilizing agent is present in said solution. 6. A composition as claimed in Claim 5, wherein said stabilizing agent is lactic acid. 7. A composition as claimed in any one of the preceding claims, wherein a spinning aid is present in said solution. 8. A composition as claimed in Claim 7, wherein said spinning aid is polyethylene oxide. 9. A composition as claimed in any one of the preceding claims, wherein said molar ratio is within the range from 2.03 to 2.10. 10. A composition as claimed in any one of the preceding claims, wherein sufficient amounts of magnesium and aluminum metals are dissolved in the solution such that the cation to anion molar ratio is in the range of 0.33:1 to 1:1. 11. A process for preparing a fiberizable precursor solution as defined in any of Claims 1 to 10, which comprises the ordered steps of: (a) preparing an aluminum salt solution; (b) dissolving a magnesium compound in the solution prepared in step (a); and (c) dissolving an aluminum compound in the solution prepared in step (b). 12. A process as claimed in Claim 11, including one or more of the following further steps (i) an unordered step of adding a dopant to the solution of step (a), (b), or (c); (ii) an ordered step of adding a stabilizing agent to the solution after step (c) or a subsequent step; (iii) an ordered step of concentrating the solution prepared in step (c) or a subsequent step; (iv) an ordered step of adding an organic polymeric spinning aid to the solution prepared in step (c) or a subsequent step. 13. A process as claimed in Claim 11 or Claim 12, wherein the temperature in steps (a), (b) and (c) is from 70° to 110°C. 14. A process for preparing magnesium aluminate fibers which comprises: (a) forming a precursor solution by the ordered steps of (1) dissolving an aluminum salt in a polar solvent; (2) dissolving a magnesium compound in the solution prepared in step (1); and (3) dissolving an aluminum compound in the solution prepared in step (2); (b) dissolving a dopant and a stabilizing agent in the solution prepared in step (a); such that said solution is capable of decomposing to magnesium aluminate fibers; (c) concentrating said solution to a viscous material of from 2.5 to 50 Pa.s (25 to 500 poise); (d) fiberizing said viscous material to form fibers; and (e) heating said fibers sufficiently to form crystalline magnesium aluminate fibers. 15. A process as claimed in Claim 14, wherein said heating is at a temperature within the range of 800°C to 1,600°C. 16. A process for preparing magnesium aluminate fibers which comprises: (a) dissolving an inorganic aluminum salt and a magnesium compound in water at a temperature to form a salt solution; (b) dissolving an aluminum compound in said salt solution; (c) dissolving a dopant in said salt solution; (d) dissolving a stabilizing agent in said salt solution; (e) concentrating said solution to form a viscous material of from 2.5 to 50 Pa.s (25 to 500 poise); (f) fiberizing said viscous material to form fibers; and (g) heating said fibers at a temperature in the range of 800 to 1,600°C to sufficiently form crystalline magnesium aluminate fibers. 17. A process as claimed in any one of Claims 11 to 16, wherein the molar ratio of aluminum to magnesium is within the range 2.03 to 2.20. 18. A process as claimed in Claim 17, wherein said ratio is within the range 2.03 to 2.10. 19. A process as claimed in any one of Claims 11 to 18, wherein the magnesium compound is magnesium metal and the aluminum compound is aluminum metal. 20. A process as claimed in any one of Claims 11 to 19, wherein the aluminum salt is aluminum chloride. 21. A process as claimed in any one of Claims 11 to 20, wherein the solvent of the precursor solution is water. 22. A process as claimed in any one of Claims 12 to 21, wherein the dopant is zirconium chloride. 23. A process as claimed in any one of Claims 12 to 22, wherein the stabilizing agent is lactic acid.

Inventors: DAHAR, STEPHEN L., STEVENS, REX R.
Applicants: THE DOW CHEMICAL COMPANY
Application year: 1986
Main classification: D01F9
IPC codes: D01F9, C04B35, C01F7