ALUMINA POROUS BODY AND PRODUCTION OF THE SAME

Abstract: An alumina porous body which comprises coarse alumina particles (1) as a filler and SiO₂, said alumina and SiO₂ forming an Al₂O₃-SiO₂ compound (mullite (2) on the surface of the coarse alumina particles through the reaction between them that takes place at the time of firing, said compound binding said coarse alumina particles, forming pores between them. A process for producing an alumina porous body which comprises mixing coarse alumina particles with SiO₂, molding the mixture, and firing the molding at a tempera­ ture below 1700°C, thereby sintering the coarse alumina particles.

Description: The present invention relates to an alumina porous body that can be used as a filter or a carrier for immobilized microorganisms and enzymes, and also to a process for producing the same. A ceramics porous body to be used as a filter is required to have as high a porosity as possible and as uniform a pore diameter as possible so that it permits the medium to pass through with a minimum resistance. According to the conventional process, this ceramics porous body is produced by mixing a filler (such as alumina Al₂O₃, titania TiO₂, silicon carbide SiC, and chamotte) of proper particle diameter and particle size distribution with a small amount of binder (such as clay) and vitreous flux, and firing the resulting mixture. The binding of a filler needs none of or only a very small amount of vitreous flux in the case where the filler is composed of fine particles or the filler has a low refractoriness. In such a case, the particles of the filler readily sinter at the time of firing. This is not the case with a filler such as alumina having a high refractoriness or a filler composed of coarse particles. Particles of such a filler do not sinter easily. They have to be bound by the aid of a vitreous flux or by firing at a high temperature (above 2000°C) at which the particles sinter. The porous body produced in this manner has a low porosity (due to the flux) or a high production cost (due to the high firing temperature). The present invention aims to solve the above-mentioned problem involved in the prior art technology. The present invention provides an alumina porous body which comprises alumina particles of average diameter 3 to 85µm as a filler and SiO₂, said alumina and SiO₂ forming a mullite (3Al₂O₃.2SiO₂) layer on the surface of the alumina particles through the reaction between them that takes place at the time of firing, said mullite layer binding together said alumina particles, forming pores between them. The present invention also provides a process for producing an alumina porous body which comprises mixing alumina particles of average diameter 3 to 85µm with SiO₂, molding the mixture, and firing the molding at a temperature below 1700°C, thereby sintering the alumina particles, the firing being carried out in such a manner that the SiO₂ reacts with the alumina particles, forming a mullite (3Al₂O₃.2SiO₂) layer on the surface of the alumina particles, and the said mullite layer binds the alumina particles to one another. Embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings, in which:- Fig. 1 shows powder X-ray diffraction patterns indicating the temperature at which mullite is formed in the mixture of 90 parts by weight of alumina filler (240 mesh, with an average particle diameter of 80.5 µm) and 10 parts by weight of silica stone powder or ultrafine silica particles. Fig. 2 is a schematic diagram showing the internal structure of the fired body. Fig. 3 is a graph showing the pore size distribution of the fired bodies obtained in Examples 1 to 3. The embodiments of the present invention will be described with reference to the accompanying drawings. According to an embodiment of the present invention, the alumina porous body comprises coarse alumina particles as a filler and ultrafine silica particles (colloidal silica), said alumina and silica forming mullite (3Al₂O₃·2SiO₂) on the surface of the coarse alumina particles through the reaction between them that takes place at the time of firing, said mullite binding said coarse alumina particles, forming pores between them. The alumina porous body is produced by firing a mixture of coarse alumina particles (as a filler) and ultrafine silica particles (colloidal silica) at a firing temperature below 1700°C. The amount of the colloidal silica is so adjusted as to account for not more than 2U wt% of Al₂O₃. According to another embodiment of the present invention, the alumina porous body comprises coarse alumina particles as a filler and silica stone powder, said alumina and silica stone powder forming mullite (3Al₂O₃·2SiO₂) on the surface of the coarse alumina particles through the reaction between them that takes place at the time of firing, said mullite binding said coarse alumina particles, forming pores between them. The alumina porous body is produced by firing a mixture of coarse alumina particles (as a filler) and silica stone powder having an average particle diameter not greater than 5 µm at a firing temperature below 1700°C. After experiments with alumina having different particle diameters, the present inventors found that alumina particles are completely bound only when they are fired at 1700°C or above if they have a particle diameter larger than about 6 µm. In the case of alumina having a particle diameter larger than about 10 µm, alumina particles sinter incompletely when fired at 1600°C, resulting in a weak fired body having a flexural strength lower than about 200 kgf/cm². The present inventors continued their investigation in search of a method for binding coarse alumina particles firmly at a firing temperature below 1700°C and yet forming uniform pores without decreasing the porosity. As the result, it was found that if coarse alumina particles (as a filler) are incorporated with silica stone powder (containing more than 99% of SiO₂) or ultrafine silica particles (colloidal silica), the coarse alumina particles are firmly bound by mullite (3Al₂O₃·2SiO₂) which is formed on the surface of the coarse alumina particles at the time of firing. Fig. 1 shows powder X-ray diffraction patterns of a mixture of 90 parts by weight of alumina filler (with a particle diameter of about 80 µm) and 10 parts by weight of silica stone powder (containing more than 99% of SiO₂) or ultrafine silica particles (colloidal silica). It is noted from Fig. 1 that the silica stone powder or colloidal silica added to the alumina filler changes into cristobalite when the firing is carried out at about 1300°C, whereas it reacts with the alumina filler, forming mullite on the surface thereof, if the firing temperature is raised to 1600°C. The fired body obtained by firing at 1600°C has a flexural strength of 400 kgf/cm², a porosity of 45%, and an average particle diameter of 36 µm. Fig. 2 schematically shows the internal structure of the fired body in which mullite is formed. It is noted that the fired body is composed of the alumina particles (filler) 1 and the mullite 2 formed on the surface thereof, with the former being firmly bound by the latter, and the pores formed between the alumina particles 1 . Tests for the amount of the silica stone powder or ultrafine silica particles (colloidal silica) to be added to the alumina indicate that the firm binding of alumina particles is achieved if it is not more than 20 wt%, preferably 3 wt% to 20 wt%, (in terms of SiO₂) of Al₂O₃. Incidentally, the silica stone powder should have an average particle diameter not greater than 5 µm, preferably 1 to 5 µm. The firm binding of alumina particles is achieved at a firing temperature below 1700°C, preferably from 1600 to 1650°C. The coarse alumina particles as a filler should have an average particle diameter of 3 to 85 µm, preferably 5 to 85 µm, so that they form a fired body having an average pore diameter larger than 2 µm and a porosity not less than 40%, preferably 40 to 45%. The alumina porous body of the present invention is formed by incorporating coarse alumina particles as a filler with ultrafine silica particles or silica stone powder as an SiO₂ source, and firing the resulting mixture at a comparatively low temperature. The firing causes the coarse alumina particles to react with the ultrafine silica particles or silica stone powder, forming mullite on the surface of the former. The mullite firmly binds the coarse alumina particles to one another. For this reason, it is possible to bind the coarse alumina particles a comparatively low temperature and to form uniform pores between the coarse alumina particles. Thus, the process of the present invention does not need a high firing temperature for the binding of coarse alumina particles unlike the conventional process. In addition, it provides a porous body having a high bond strength without using a large amount of flux even in the case where the coarse alumina particles have a large particle diameter. It also provides an alumina porous body having a high porosity and uniform pores. According to the process of the present invention, the alumina porous body can be produced at a low production cost because firing can be achieved at a low temperature below 1700°C on account of the addition of SiO₂ which contributes to the high mechanical strength of the alumina porous body. According to the process of the present invention, it is possible to produce the alumina porous body having a high strength and high porosity at a low production cost on account of the ultrafine silica particles (colloidal silica) or silica stone powder as an SiO₂ source added to the coarse alumina particles as a filler. The alumina porous body may be provided with pores of desired diameter if the particle diameter of the coarse alumina particles is adequately selected. The alumina porous body having a proper pore diameter will find use as not only filters but also carriers for immobilized microorganisms and enzymes. The one having an extremely small pore diameter will find use as a support of an ultrafilter membrane. The invention will be described in more detail with reference to the following examples. Example 1 The following four components were mixed using a mixing machine and the resulting mixture was extruded into a pipe 20 mm in diameter and 3 mm thick. 90 parts by weight of white electrofused alumina containing more than 99% of Al₂O₃ and having a particle size of 240 mesh and an average particle diameter of 80.5 µm, made by Nikkei Kako Co., Ltd. 10 parts by weight of aerosil containing more than 99.8% of SiO₂ and having an average particle diameter of 7 µm, made by Nippon Aerosil Co., Ltd. 30 parts by weight of water. 12 parts by weight "Cerander" (a methyl-cellulose, marketed under that trade mark by Yukon Kogyo Co., Ltd). The pipe was heated to 1600°C at a rate of about 5°C/minute and fired at 1600°C for 1 hour. The fired pipe was allowed to cool to room temperature. The fired pipe was found to have an average pore diameter of 36 µm, a porosity of 45%, and a flexural strength of 400 kgf/cm². Example 2 The following four components were mixed using a mixing machine and the resulting mixture was extruded into a pipe 20 mm in diameter and 3 mm thick in the same manner as in Example 1. 95 parts by weight of white electrofused alumina having a particle size of 1200 mesh and an average particle diameter of 13 µm. 5 parts by weight of aerosil. 30 parts by weight of water. 12 parts by weight of "Cerander". The pipe was heated to 1600°C at a rate of about 5°C/minute and fired at 1600°C for 1 hour. The fired pipe was allowed to cool to room temperature. The fired pipe was found to have an average pore diameter of 4.9 µm, a porosity of 46%, and a flexural strength of 390 kgf/cm². Example 3 Fired pipes were prepared in the same manner as in Examples 1 and 2 except that the alumina was replaced by those which have an average particle diameter of 5.3 µm, 20 µm, and 40.5 µm. The fired pipes prepared in Examples 1 to 3 were tested for pore size distribution with a mercury porosimeter. The results are shown in Fig. 3. Example 4 The following four components were mixed using a mixing machine and the resulting mixture was extruded into a pipe 20 mm in diameter and 3 mm thick. 90 parts by weight of white electrofused alumina containing more than 99% of Al₂O₃ and having a particle size of 240 mesh and an average particle diameter of 80.5 µm made by Nikkei Kako Co., Ltd. 10 parts by weight of silica stone powder containing more than 99% of SiO₂ and having an average particle diameter of 1 µm. 20 parts by weight of water. 12 parts by weight of "Cerander" (binder) made by Yuken Kogyo Co., Ltd. The pipe was heated to 1600°C at a rate of about 5°C/minute and fired at 1600°C for 1 hour. The fired pipe was allowed to cool to room temperature. The fired pipe was found to have an average pore diameter of 36 µm, a porosity of 45%, and a flexural strength of 450 kgf/cm². Example 5 The following four components were mixed using a mixing machine and the resulting mixture was extruded into a pipe 20 mm in diameter and 3 mm thick in the same manner as in Example 4. 95 parts by weight of white electrofused alumina having a particle size of 1200 mesh and an average particle diameter of 13 µm. 5 parts by weight of silica stone powder containing more than 99% of SiO₂ and having an average particle diameter of 1 µm. 22 parts by weight of water. 12 parts by weight of "Cerander". The pipe was heated to 1600°C at a rate of about 5°C/minute and fired at 1600°C for 1 hour. The fired pipe was allowed to cool to room temperature. The fired pipe was found to have an average pore diameter of 4.9 µm, a porosity of 46%, and a flexural strength of 450 kgf/cm². Example 6 Fired pipes were prepared in the same manner as in Examples 4 and 5 except that the alumina was replaced by those which have an average particle diameter of 5.3 µm, 20 µm, and 40 µm. The fired pipes prepared in Examples 4 to 6 were tested for pore size distribution with a mercury porosimeter. The results were similar to those shown in Fig. 3. The preferred embodiments of the present invention can provide an alumina porous body which has a high porosity and a high strength and can be produced at a low firing temperature and hence at a low production cost.

Claims: An alumina porous body which comprises alumina particles (1) of average diameter 3 to 85µm as a filler and SiO₂, said alumina and SiO₂ forming a mullite (3Al₂O₃.2SiO₂) layer (2) on the surface of the alumina particles (1) through the reaction between them that takes place at the time of firing, said mullite layer (2) binding together said alumina particles (1), forming pores (3) between them. An alumina porous body as claimed in claim 1, wherein the amount of SiO₂ is not more than 20 wt% of the alumina particles (1). An alumina porous body as claimed in claim 2, wherein the amount of SiO₂ is from 3 to 20 wt% of the alumina particles (1). An alumina porous body as claimed in any foregoing claim, wherein the alumina particles (1) have an average particle diameter of from 5 to 85µm. An alumina porous body as claimed in any foregoing claim, which has a porosity not less than 40%. An alumina porous body as claimed in claim 5, which has a porosity of from 40 to 45%. A process for producing an alumina porous body which comprises mixing alumina particles (1) of average diameter 3 to 85µm with SiO₂, molding the mixture, and firing the molding at a temperature below 1700°C, thereby sintering the alumina particles (1), the firing being carried out in such a manner that the SiO₂ reacts with the alumina particles (1), forming a mullite (3Al₂O₃.2SiO₂) layer (2) on the surface of the alumina particles (1), and the said mullite layer (2) binds the alumina particles to one another. A process as claimed in claim 7, wherein the SiO₂ is in the form of ultrafine silica particles (colloidal silica). A process as claimed in claim 7, wherein the SiO₂ is in the form of silica stone powder. A process as claimed in claim 9, wherein the silica stone powder has an average particle diameter not greater than 5µm. A process as claimed in claim 10, wherein the silica stone powder has an average particle diameter of from 1 to 5µm. A process as claimed in any one of claims 7 to 11, wherein the amount of SiO₂ is not more than 20 wt% of the alumina particles (1). A process as claimed in claim 12, wherein the amount of SiO₂ is from 3 to 20 wt% of the alumina particles (1). A process as claimed in any one of claims 7 to 13, wherein the alumina particles (1) have an average particle diameter of from 5 to 85µm. A process as claimed in any one of claims 7 to 14, wherein the alumina porous body has a porosity not less than 40%. A process as claimed in claim 15, wherein the alumina porous body has a porosity of from 40 to 45%. A process as claimed in any one of claims 7 to 16, wherein the firing temperature is from 1600 to 1650°C.

Inventors: ITO, TAKAAKI, MURAGUCHI, YUKITO, NAKAJIMA, HIROHITO
Applicants: INAX CORPORATION
Application year: 1989
Main classification: C04B38
IPC codes: C04B35, C04B38