Patent Application
20070243994
The invention will be described in detail below.
The fused siliceous refractory of the invention is a molded product containing fused silica and a fluoride. In order to produce such a fused siliceous refractory, a slurry containing raw material solid matter comprising a fused silica powder and a fluoride powder is first prepared.
The fused silica powder is usually a mixture having an average particle size of 1 to 500 μm, and may be used as raw material solid matter as it is. However, it is preferred to use a mixture of a relatively fine-grained first fused silica powder and a relatively coarse-grained second fused silica powder. Thereby, the first fused silica powder enters clearances formed among particles of the second fused silica powder, and when the fused silica powder is formed into a molded article, filling properties thereof become high, which causes a texture of the fused siliceous refractory after burning to be densified, thereby increasing heat resistance. This is therefore preferred. Further, when the fused siliceous refractory is used as a lining material or the like, such improvement in heat resistance makes it possible to decrease its thickness, thereby reducing the mass to improve workability and further to decrease the specific heat (thermal capacity). This is therefore preferred.
When the mixture of the first fused silica powder and the second fused silica powder is used, the average particle size of the first fused silica powder is preferably from 1 to 10 μm, and more preferably from 2 to 6 μm. On the other hand, the average particle size of the second fused silica powder is preferably from 50 to 500 μm, and more preferably from 100 to 300 μm. Further, as for the blending ratio of the first fused silica powder and the second fused silica powder in the fused silica powder, their proportions are preferably such that 10 to 50% by weight of the second fused silica powder and 50 to 90% by weight of the first fused silica powder, and more preferably such that 20 to 40% by weight of the second fused silica powder and 60 to 80% by weight of the first fused silica powder. When the respective average particle sizes of the first fused silica powder and the second fused silica powder and the blending ratio thereof are selected within the above-mentioned ranges, filling properties of the fused silica powder in the molded article are preferably improved.
The fluorides are not particularly limited, but include inorganic fluorides such as calcium fluoride (CaF2), magnesium fluoride (MgF2), cryolite (Na3AlF6), lithium fluoride (LiF), barium fluoride (BaF2), aluminum fluoride (AlF3), strontium fluoride (SrF2), cerium fluoride (CeF3), yttrium fluoride (YF3), sodium fluoride (NaF), potassium fluoride (KF), sodium silicofluoride (Na2SiF6) and ammonium silicofluoride ((NH4)2SiF6). In the invention, at least one member selected from calcium fluoride (CaF2), magnesium fluoride (MgF2) and cryolite (Na3AlF6) is preferably used because of low cost.
The average particle size of the fluoride is preferably from 1 to 10 μm, and more preferably from 2 to 6 μm. When the particle size is selected within the above-mentioned range, the fluoride powder can be easily homogeneously dispersed in the fused silica powder. This is therefore preferred. In particular, when the fused silica powder is the mixture of the first fused silica powder and the second fused silica powder, the fluoride powder becomes easy to enter clearances formed among particles of the second fused silica powder by using the fluoride powder having a particle size similar to that of the first fused silica powder, thereby becoming easy to disperse more homogeneously. This is therefore more preferred.
The blending ratio of the fused silica powder and the fluoride powder in the raw material solid matter is not particularly limited so long as the fluorine content in the resulting fused siliceous refractory falls within the range of preferably 0.01 to 10% by weight, more preferably 0.1 to 3.0% by weight. For example, 0.01 to 10% by weight of the fluoride powder and 90 to 99.99% by weight of the fused silica powder are blended. More preferably, 1 to 5% by weight of the fluoride powder and 95 to 99% by weight of the fused silica powder are blended. When the blending ratio is within the above-mentioned range, the strength, thermal shock resistance (the coefficient of thermal expansion) and corrosion resistance of the fused siliceous refractory is satisfactory, so that this is preferred.
The slurry is prepared by mixing the above-mentioned raw material solid matter with water. As a mixing method, there can be employed a known method. The blending ratio of the raw material solid matter and water in the slurry is adjusted preferably to 10 to 40 parts by weight of water, and more preferably to 20 to 30 parts by weight, based on 100 parts by weight of the raw material solid matter.
Further, a molding aid, a binder or the like may be added to the slurry as needed. The molding aids used in the invention include, for example, PVA (polyvinyl alcohol) and CMC (carboxymethylcellulose). In addition, the binders used in the invention include, for example, silicate glass and caustic soda. The use of the molding aid is preferred because moldability is improved, and the use of the binder is preferred because shape retention of the molded article is improved.
Then, a molding step is performed to obtain the molded article having a desired shape from the above-mentioned slurry. A method for obtaining the molded article is not particularly limited, and there can be used, for example, cast molding, press molding or extrusion molding. Of these, cast molding is preferred, because the slurry can be densely filled in a mold and the resulting molded article easily becomes high in density.
The resulting molded article may be burned subsequently moving to a burning step as it is. However, when a large amount of water remains in the molded article, or when the molded article is rapidly elevated in temperature in the burning step, a drying step may be employed before the burning step is performed, as needed, for fear of rapid evaporation of water in the molded article to generate cracks or the like in a burned article. The drying step may be performed under conditions under which water in the molded article is gradually evaporated, and a known method can be employed.
Then, the burning step is performed to obtain a fused siliceous refractory from the above-mentioned molded article. There is no limitation on the burning temperature, as long as it is a temperature at which fused silica powder particles are sintered together. However, it is suitably from 1050 to 1250° C., and preferably from 1100 to 1200° C. When the burning temperature is less than 1050° C., the fused silica powder particles are unfavorably difficult to be sintered together. On the other hand, when the burning temperature exceeds 1250° C., cristobalite is unfavorably formed to increase the coefficient of thermal expansion of the fused siliceous refractory. There is no limitation on the burning time, as long as burning of the above-mentioned molded article is completed. However, it is suitably from 0.5 to 20 hours, and preferably from 1 to 5 hours. When the burning time is 0.5 hour, sufficient sintered strength is not obtained. This is therefore unfavorable. Even when the burning time exceeds 20 hours, the sintering effect scarcely changes.
The thus-obtained fused siliceous refractory according to the invention comprises an amorphous fused silica phase obtained by fusing and sintering the fused silica powder, and the fluoride nearly homogeneously dispersed in the fused silica phase. When the mixture of the first fused silica powder and the second fused silica powder is used as the fused silica powder, the first fused silica powder and the fluoride enter clearances formed among particles of the second fused silica powder. As the content ratio of the fused silica and the fluoride, the blending ratio in the raw material solid matter is maintained as it is.
The bulk density of the fused siliceous refractory according to the invention is preferably from 1.3 to 2.2 g/cm3, and more preferably from 1.4 to 1.8 g/cm3. When the bulk density is less than 1.3 g/cm3, the strength unfavorably decreases. On the other hand, exceeding 2.2 g/cm3 unfavorably results in an increase in mass.
Further, taking processability and strength into consideration, the bending strength of the fused siliceous refractory according to the invention is preferably 5 MPa or more, and more preferably 6 MPa or more.
Furthermore, the fused siliceous refractory according to the invention is joined to a base material composed of another material at an application site. Accordingly, in order to inhibit separation from the base material, the coefficient of thermal expansion thereof is preferably 2.50×10−6° C.−1 or less, more preferably 2.0×10−6° C.−1 or less, still more preferably 1.5×10−6° C.−1 or less, and particularly preferably 1.0×10−6° C.−1 or less.
Such a bulk density, bending strength and coefficient of thermal expansion is can be attained by adjusting the composition of the raw material solid matter, molding conditions and burning conditions.
The fused siliceous refractory according to the invention, to which excellent corrosion resistance is imparted by the fluoride, is most suitably used particularly at a site in contact with a molten metal of magnesium or a magnesium-containing alloy. Accordingly, the fused siliceous refractory of the invention is suitable as a lining materials for a teeming box, a trough and a retaining furnace of a machine for casting magnesium or a magnesium-containing alloy, or as attached members such as a float, a spout, a hot-top ring and a transition plate.
The present invention will be illustrated in greater detail with reference to the following Examples and comparative Examples, but the invention should not be construed as being limited thereto.
As shown in Tables 1 and 2, fused silica powder A, fused silica powder B and a calcium fluoride powder or a magnesium fluoride were mixed to prepare a raw material solid matter, and further, 20 parts by weight of water was added to 100 parts by weight of this raw material solid matter, followed by kneading to obtain each slurry. Details of the ingredients are as shown below. This slurry was poured into a gypsum mold, and cast molding was performed. The resulting molded articles were burned in a nitrogen gas atmosphere at 1150° C. for 3 hours to obtain tabular test specimens 150 mm long, 30 mm wide and 15 mm thick.
The bulk density, three-point bending strength and coefficient of thermal expansion of the respective test specimens were measured, and a corrosion test were made. The results thereof are shown together in Tables 1 and 2.
The coefficient of thermal expansion is a value measured according to JIS-R1618 at 1,000° C.
Further, the tree-point bending strength was measured at a distance between fulcrums of 100 mm and a loading rate of 2 mm/min using an autograph “AG-50 kNG” manufactured by Shimadzu Corporation, for a test piece 150 mm long, 20 mm wide and 7 mm thick cut out from the test specimen.
For the corrosion test, a test piece having a rectangular shape with a side of about 70 mm and a thickness of 25 mm was cut out from the test specimen. As schematically shown in
The respective test specimens are substantially improved in corrosion resistance, compared to Comparative Example 1 in which no fluoride is contained. Further, a decrease in bending strength is also small, and the coefficient of thermal expansion is also suppressed low. The test specimens of Examples 4 and 8 are somewhat poor in corrosion resistance, but they are suitably used in a member requiring high spalling resistance because of their low coefficient of thermal expansion.
While the present invention has been described in detail and with reference to specific embodiments thereof, it will be apparent to one skilled in the art that various changes and modifications can be made therein without departing from the spirit and scope thereof.
This application is based on Japanese patent application No. 2006-100492 filed Mar. 31, 2006, and the contents thereof are herein incorporated by reference.
| Number | Date | Country | Kind |
|---|---|---|---|
| P.2006-100492 | Mar 2006 | JP | national |
