Title of Invention

METHOD FOR PRODUCING ALUMINA WITH LOW SODA CONTENT

Abstract

A method for producing an alumina, the method comprising the steps of mixing a soda-containing aluminum compound with a soda-reducing agent containing a silica-based material and another silica-based material coated with a non-silica- based ceramic to obtain a mixture thereof, calcining the mixture at a temperature of 1100°C or higher and separating the resulting alumina from the soda-reducing agent.

Full Text

I The present invention relates to a method for producing an alumina with a low Na2o) content. More particularly, the invention relates to a method for producing an alumina with a low Na20 content using an aluminum compound with a high Na20 content. DESCRIPTION OF THE RELATED ART Alumina is one of ceramics, and is employed widely in a structural component requiring a heat resistance. An alumina containing a reduced amount of Na20 (hereinaller, reierred to as "soda") as impurities is employed in an insulator of spark plug for an engine and a ceramic for electronic parts since it has an excellent electricity insulating performance. An alumina can be industrially produced, for example, by a method in which a bauxite is dissolved in a caustic soda to obtain an aqueous solution of sodium aluminatc, the sodium aluminate in the solution is hydroly/ed to obtain aluminum hydroxide, and the aluminum hydroxide is then calcined to obtain an alumina. In this method, the aluminum hydroxide to be calcined contains soda, which are needed to be removed. Such a soda removal is conducted, for example, by mixing the aluminum hydroxide with a soda-reducing agent consisting of a silica-based material, calcining the mixture and separating the resulting alumina from the soda-reducing agent. In the above-described method for producing an alumina, a silica contamination tends to be recognized in the alumina and may lead to an increase in silica content in the final product, an alumina, with a low soda content. In order to suppress the silica contamination, an attempt was made in which a silica-based material employed as a soda-reducing agent is washed off with water or in which microparticulate components in the alumina are removed. However, the attempt is not satisfactory against the silica contamination. In view of an improvement in an insulating ability at a high temperature and/or strength at a high temperature, an alumina as a final product is demanded to have a low soda content. At the same time, in view of an improvement in physical properties of a ceramic obtained by molding the alumina, the alumina is demanded to have a low silica content. AS described above, a method of mixing a soda-reducing agent consisting of a silica-based material and calcining the mixture poses a difficulty in reducing both of the soda and the silica contents in the alumina, simultaneously. That is, an increase in the soda-reducing agent results in an increase in a silica content although it serves to reduce a soda content in the resultant alumina, while a reduction in the soda-reducing agent results in a difficulty in reducing the soda content although it serves to reduce the silica content. SUMMARY OF THE INVENTION Under such a circumstance, the inventors of the present invention have found a method by which an alumina having a low soda content with a reduced silica content can be obtained, and have established the present invention. The present invention provides a method for producing an alumina, the method comprising the steps of mixing a soda-containing aluminum compound with a soda-reducing agent containing a silica-based material and another silica-based material coated with a non-silica-based ceramic to obtain a mixture thereof, calcining the mixture and separating the resulting alumina from the soda-reducing agent. DESCRIPTION OF THE INVENTION In the present invention, a soda-containing aluminum compound is mixed with a soda-reducing agent containing a silica-based material and another silica- based material which is coated with a non-silica-based ceramic. The resulting mixture is calcined and is separated into the soda-reducing agent and an alumina with a low soda content. An aluminum compound to be used for mixing a soda-reducing agent in the present invention may be an aluminum compound capable of being converted upon heating into an alumina having an a-type crystalline structure and containing soda. Examples of the aluminum compound include an aluminum hydroxide having a gibbsite-type, bayerite-type, boemite-type or diaspore-type crystalline structure or a transition alumina having y-type, x-type, 0-type, 6-type or K-type type crystalline structure. Typically, the aluminum hydroxide having a gibbsite-type crystalline structure or the transition alumina having y type crystalline structure may be utilized. The aluminum hydroxide having a gibbsite-type crystalline structure may be produced by dissolving an aluminum-containing mineral stone such as bauxite and laterite in a solution of an alkali such as sodium hydroxide and sodium carbonate to obtain an aqueous solution of sodium aluminate, hydrolyzing the sodium aluminate in the solution to obtain an aluminum hydroxide and washing the aluminum hydroxide. The transition alumina having y-type crystalline structure may be produced by preliminary calcining the above-described aluminum hydroxide with a gibbsite-type crystalline structure. The aluminum compound to be used in the present invention may have an Na20 content of about 0.2 % by weight or higher, and may have a mean particle size of from about 10 μm to about 200 μm A soda-reducing agent to be used for mixing with 10 the aluminum compound in the present invention may contain a silica-based material and a silica-based material which is coated with a non-silica-based ceramic. The usage of a soda-reducing agent consisting of a non- coated silica-based material tends to result in an increase in the silica content in a resultant alumina, although it can reduce the soda content. On the other hand, the usage of a soda-reducing agent consisting of a ceramic-coated silica-based material tends to pose a difficulty in reducing the soda content, sufficiently. It is preferred in the present invention that a ceramic-coated silica-based material and a non-coated silica-based material are employed in amounts of about 20 % by weight to about 80 % by weight for the former and about 80 % by weight to about 20 % by weight for the latter, respectively, based on the total amount of them. It seems that there are at least two ways of removing soda from an aluminum compound, one way in which soda volatizes whereby reacting with and being immobilized into silica inside a silica-based material and the other way in which soda is brought into a direct contact with the surface of a silica-based material whereby migrating into and being immobilized on the silica-based material. Accordingly, it is not clear, but is assumed that soda can be successfully reduced in the present invention while preventing a silica contamination since, in the present invention, the amount of the silica-based material to be brought into a direct contact with the aluminum compound is reduced by replacement of the silica-based material for immobilizing the volatizing soda with the ceramic-coated silica-based material. One of the constituents of the soda-reducing agent to be used in the present invention may be a silica-based material. Such a silica-based material may be a particulate material containing a silica. Examples of the silica-based material includes materials comprising a quartzite, a quartz, a silica sand, a shamotte, a nulite, a silimanite, a magnesium silicate, an alumina silicate or the like. Among these, a quartzite, a quartz and a silica sand are preferred since these tend to have high soda-reducing effects. The mean particle size of the silica-based material is preferably in the range of from about 0.3 mm to about 2 mm. Another one of the constituents of the soda-reducing agent to be used in the present invention may be a silica-based material which is coated with a non-silica-based ceramic. Such a ceramic-coated silica-based material can be obtained by coating the surface of a silica-based material as a substrate with a ceramic other than a silica-based material. A silica-based material employed as a substrate here may be a quartzite, a quartz, a silica sand, a shamotte, a nulite, a silimanite, a magnesium silicate or an alumina silicate, as mentioned above, and may be the same kind of or a different kind from that of the non-coated silica-based material to be used together therewith. The ceramic employed as a coating may be an oxide other than a silica-based material as a substrate. Examples of the ceramic include a transition alumina having y-, %-, 6-, 8- or K-type crystalline structure, an a-alumina and the like. Among these, it is preferred to use a transition alumina or an a-alumina, each of which has a purity of about 95 % by weight or higher in terms of Al2O3. By using the silica-based material coated with such an alumina, impurities derived from the ceramic can be reduced. A non-silica-based ceramic, as a starting material of the coating. preferably has particulate shape, of which mean particle size may be about 2 μm or more, preferably about 20 \xia or more, and may be about 200 μm or less, preferably about 100 μm or less. By using a ceramic-coated silica-based material obtained from the coating with a non-silica-based ceramic having a mean particle size within the above-described range, the silica contamination can be reduced, whereby reducing a silica content in the resulting alumina with a low soda content. The amount of a non-silica-based ceramic which forms the coating may be about 5 % by weight or more based on the ceramic-coated silica-based material. In view of low silica content in the final product, a higher ceramic content in the ceramic-coated silica-based material is more preferred since it leads to a higher ability of reducing the silica contamination, which results in reducing a silica content in the resulting alumina with a low soda content. In this point, the amount of the non-silica-based ceramic is preferably about 20 % by weight or more based on the ceramic-coated silica-based material. On the other hand, an excessive amount of the ceramic which forms the coating tends to reduce soda-reducing effect and, therefore, the ceramic-coated silica-based material preferably has an SiO2 content of 50 % or more based on the ceramic-coated silica-based material. The ceramic- coated silica-based material may have a mean particle size of from about 0.3 mm to about 2 mm, which is equivalent to or slightly larger than the mean particle size of the noh-ceramic-coated silica-based material to be used together therewith. The ceramic-coated silica-based material may be prepared by a method in which the above-described silica-based material is mixed with the above-described ceramic, and the resulting mixture is heated using a heating furnace such as an electric furnace, rotary kiln, roller heath kiln, tunnel kiln and the like. The heating may be effected at a temperature capable of fusing or calcining the silica in the silica-based material and a component (for example, alumina) in the ceramic. The heating temperature may be about 1,000°C or higher, and is preferably in the range of from abcTut 1,100°C to abotit 1,300°C. The heating may be effected in the presence of a fluorine-based substance such as hydrogen fluoride and ammonium fluoride, or a chlorine-based substance such as hydrogen chloride, ammonium chloride and chlorine. In the present invention, a soda-containing aluminum compound are mixed with a soda-reducing agent containing a silica-based material and another silica-based material coated with a non-silica-based ceramic. The mixing can be accomplished by using a rotary container mixer such as a horizontal cylindrical mixer, V-shaped mixer and dual cone mixer, or a mechanical stirring mixer such as a ribbon mixer, screw mixer and pin mixer. When the following calcination is effected in a continuous rotary kiln, the mixing may also be effected in the rotary kiln. The amounts of the alumina compound and the soda-reducing agent are abejut 70 % by weight to about 95 % by weight for the former and about 5 % by weight to about 30 % by weight for the latter, respectively, based on the total of them. The thus obtained mixture of the soda-containing aluminum compound and the soda-reducing agent is calcined. The calcination may be carried out while mixing the soda-containing aluminum compound with the soda-reducing agent. The calcination can be accomplished in a calcining furnace such as a rotary kiln, roller heath kiln, tunnel kiln, electric furnace and the like. The calcining furnace may be of a batch type or continuous type. The calcination can be accomplished at a temperature sufficient to effect a phase transfer of the aluminum compound into an a-alumina. The calcination temperature may be about 1,000°C or higher, and is preferably in the range of from about 1,100°C to about 1,400°C. The period of time for the calcination may vary depending on the type of the calcining furnace to be employed and the calcination temperature. The period of time may be two minutes or longer, and is preferably in the range of from about 10 minutes to about 10 hours. The calcinaiton may be carried out in the presence of a mineralizer. By using a mineralizer, the calcination can be accomplished at a lower temperature. Examples of the mineralizer to be employed include a fluorine-based substance such as hydrogen fluoride, ammonium fluoride, magnesium fluoride and aluminum fluoride, a chlorine-based substance such as hydrogen chloride, ammonium chloride and chlorine and a boron-based substance such as boron oxide and boric acid. The amount of the mineralizer to be used may vary depending on the type of the mineralizer, and may be about 0.001 % by weight or more based on the aluminum compound as a starting material. The larger the amounts of the mineralizer becomes, the lower the calcination temperature can be, which is desirable. The amount of the mineralizer may be about 0.005 % by weight or higher based on the aluminum compound. On the other hand, an excessively larger amounts of the mineralizer do not give any further reduction in the calcination temperature and, therefore, the amount of the mineralizer is preferably about 0.5 % by weight or less based on the aluminum compound. After calcinations, the calcined mixture may then be separated into the alumina and the soda-reducing agent. The separation may be effected, for example, by using a dry classifier such as a sieving machine. When a sieve is employed, the sieve may have a mesh size which is between the particle size of the soda-reducing agent and the particle size of the alumina. The mesh size may appropriately be selected depending on the particle size of the soda-reducing agent and the particle size of the alumina, and may be in the range of from about 100 \ixa to about 300 \jim. Since the particle size of the soda-reducing agent may be larger than that of the alumina, the soda-reducing agent may be recovered from the outlet for coarse particles of the dry classifier and the alumina may be recovered from the outlet for fine particles. The soda-reducing agent thus separated may be recycled as a ceramic-coated silica-based material. After the separation, an alumina which is represented by Formula AI2O3 is obtained. The obtained alumina may have an a-type crystalline structure and may have a soda (Na20) content of about 0.1 % by weight or less and a silica (Si02) content of about 0.1 % by weight or less. Preferably, the alumina has a silica content of about 0.05 % by weight or less. The alumina may be pulverized, if necessary, and may be used as a starting material for a ceramic. In accordance with the present invention, an alumina having a low soda content with a low silica content can be obtained from a soda-containing aluminum compound. The invention being thus described, it will be apparent that the same may be varied in many ways. Such variations are to be regarded as within the spirit and scope of the invention, and all such modifications as would be apparent to one skilled in the art are intended to be within the scope of the present invention. The entire disclosure of the Japanese Patent Application No. 2001-192463 filed on June 26, 2001, is incorporated herein by reference in its entirety. EXAMPLES The present invention is described in more detail by reference to the following Examples, which should not be construed as a limitation upon the scope of the present invention. In Examples and Comparative Examples, a Na20 content, a Si02 content, a AI2O purity, a mean particle size and crystalline structure were determined as described below. Na20 content and Si02 content: A Na20 content (% by weight) and a Si02 content (% by weight) were measured using a fluorescent X-ray spectrometry. Al2O3 purity: Using a fluorescent X-ray spectrometry, a Na2O content (% by weight), a SiO2 content (% by weight) and a FeaOa content (% by weight) were measured. Then, a calculation was made in accordance with the following equation, to obtain an Al2O3 purity. AI2O3 purity (%) = lOO-CNaaO content + Si02 content + Fe203 content) Mean particle size (jxm) : A particle-size distribution of a sample to be measured was obtained in a sieving method. Based on the particle size distribution, a cumulative particle-size distribution curve was obtained with the particle size on the abscissa and the cumulative particle weight on the ordinate. The particle size at which the cumulative particle weight in the cumulative particle size distribution curve becomes 50 % by weight is regarded as a mean particle size of the sample. Crystalline structure: An X-ray diffraction spectrum of a sample to be measured was obtained in X-ray diffraction method. Based on the X-ray diffraction spectrum, a crystalline structure of the major component in the sample was determined. Example 1 Preparation of a soda-reducing agent: 100 Parts by weight of a transition alumina {manufactured by Sumitomo Chemical Co., Ltd.) which has a mean particle size of 50 ^im, a AI2O3 purity of 99 % and a y-type crystalline structure was mixed with 14 parts by weight of a silica sand which has a mean particle size of 1 mm to obtain a mixture thereof. The mixture was supplied to a continuous rotary kiln and was heated. The maximum temperature within the heating area of the rotary kiln employed here was 1300°C and the mean residence time of the mixture in the kiln was 4 hours. After cooling, any excessive alumina was removed through a sieve which has a mesh size of 149 lun to obtain an alumina-coated silica sand. The alumina-coated silica sand has 25 % by weight of the alumina, which forms the coating layer, and 75 % by weight of SiOa- 100 Parts by weight of the alumina-coated silica sand was mixed with 100 parts by weight of the same kind of silica sand as used above, whereby preparing a soda-reducing agent. Calcination of alumina: A sodium aluminate in an aqueous solution thereof was hydrolyzed to obtain an aluminum hydroxide which has a mean particle size of 50 μm and has Na20 and SiOa contents of 0.2 % by weight and 0.01 % by weight, respectively. 100 Parts by weight of the obtained aluminum hydroxide and 9 parts by weight of the soda-reducing agent prepared above were supplied to a continuous rotary kiln and were mixed with each other in the kiln, while calcining the resulting mixture. The maximum temperature within the heating area of the rotary kiln employed here was 1300°C and the mean residence time of the mixture in the kiln was 4 hours. After calcining, the mixture was cooled, and sieved through a sieve which has a mesh size of 149 ^im, to separate into the soda-reducing agent and the resulting alumina with a low soda content. The obtained alumina has 0.032 % by weight of NaaO and 0.032 % by weight of SiOa, and has an a-type crystalline structure. Comparative Example 1 The same procedures as in Example 1 were employed except for using, as a soda-reducing agent, only a silica sand which has a mean particle size of 1 mm, to obtain an alumina. The alumina has 0.074 % by weight of Na20 and 0.044 % by weight of Si02, and has an a-type crystalline structure. WE CLAIM: 1. A method for producing an alumina, the method comprising the steps of mixing a soda-containing aluminum compound with a soda-reducing agent containing a silica-based material and another silica-based material coated with a non-silica- based ceramic to obtain a mixture thereof, calcining the mixture at a temperature of 1100°C or higher and separating the resulting alumina from the soda-reducing agent. 2. The method as claimed in claim 1, wherein the soda-containing aluminum compound has a mean particle size of from lO μm to 200 μm. 3. The method as claimed in claim 1 or 2, wherein at least one of the silica- based materials comprises at least one compound selected from quartizite, a quartz and a silica sand. 4. The method as claimed in claim 1 or 2, wherein the obtained alumina has a NaiO content of 0.1% by weight or less and a Si02 content of 0.1% by weight or less. 5. A method for producing an alumina substantially as herein described and exemplified.

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482-mas-2002 correspondence others.pdf

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482-mas-2002 description (complete) duplicate.pdf

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482-mas-2002 form-1.pdf

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