Compositions: ceramic – Ceramic compositions – Pore-forming
Patent
1984-10-02
1987-07-07
Bell, Mark L.
Compositions: ceramic
Ceramic compositions
Pore-forming
2105101, C04B 3808, B01D 3920
Patent
active
046787585
DESCRIPTION:
BRIEF SUMMARY
The invention describes a open ceramic, porous filter body for filtration of molten metals, especially iron and iron alloys, and a method to fabricate such a filter body.
Well known are porous ceramic bodys consisting of regular or unregular spherical refractory grains which are formed together with a temporary binder to a porous body and subsequently sintered together with an inorganic binder to a refractory porous material. During the heat treatment the temporary binder is removed. Examples of the application of those filter media are filtration of molten metals especially liquid aluminum.
However, these filter media have only limited temperature stability, if the binding face is a glass consisting of the usual glass components like Al.sub.2 O.sub.3, SiO.sub.2, B.sub.2 O.sub.3, CaO, MgO and alkali oxides. The temperature stability of the filter media does not exceed the softening point of the bonding glass. In addition the mechanical stability degrades below the softening point of the glass during longer use at high temperatures. These disadvantageous properties are the reasons for the limited usability of such filter media at high temperatures, especially with the use of these materials being impossible in the temperature region of 1000.degree.-1200.degree. C.
The South African Patent No. 82/7115, filed Sept. 29, 1982 and published Aug. 31, 1983 corresponding to German Publication No. 3140098, published Apr. 21, 1983, describes the use of hollow spherical fused alumina as starting material and a reactive alumina powder as a binder to produce such a porous body wherein the powdered alumina bonds together the fused alumina spheres. The disadvantage of this method lies in the high sintering temperatures which are necessary to achieve a bond and therefore lies above 1700.degree. C. Another disadvantage of this method lies in the number of perfect sintering necks which can be achieved by this method which is too low to achieve a strong bond at high temperature. In addition another disadvantage is the high thermal expansion of those porous bodies made out of aluminum oxide which results in a low thermal shock resistance which does not allow the use of this filtration media for filtration of high melting metals especially iron without isothermal preheating which is impossible and impractical in most cases.
This explains why high temperature stable thermal shock resistant porous filter media are not available. Especially there are no porous, mechanically stable, thermal stable and thermal shock resistant filter media available especially for filtration of molten metals in the temperature region up to above 1600.degree. C. The object of the following invention is to overcome this drawback.
The solution to this object is a porous filter media with characteristic of a ceramic, open, porous body for the filtration of metals, preferentially of ferrous alloys and iron, with a temperature resistance of 1600.degree. C., in which the macroscopically homogeneous structure remains unchanged after the temperature treatment of the non-shrinked filter body with a linear coefficient of thermal expansion lying between 3.times.10.sup.-6 and 7.times.10.sup.-6 /.degree.C., and which is formed from spherical particles of minimum one refractory material in homogeneous distribution and another refractory material as binding phase, which combines at high temperature by chemical reaction with the spheres.
The new material made by the chemical reaction between the binder and the fused alumina spheres is more refractory than the binder alone. The microscopic homogeneous material after sintering has a melting point which lies in between the melting point of the binding material and that of the refractory fused alumina grains before sintering. The grains are bonded together during sintering by chemical reaction without a macroscopic shrinkage of the body. Instead of a volume shrinkage of 20-40 Vol %, which is a normal shrinkage during sintering, the macroscopic structure and the dimensions of the foregoing described body do not change dur
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patent: 3269850 (1966-08-01), Miller
patent: 3939079 (1976-02-01), Uchiyama et al.
patent: 4251377 (1981-02-01), Schleinitz
patent: 4278544 (1981-07-01), Takashima
patent: 4528099 (1985-07-01), Rieger et al.
Rankin, G. A. et al., "The Ternary System MgO-Al.sub.2 O.sub.3 -SiO.sub.2 ", Am. Journal of Science, vol. XLV, pp. 301-325, Apr. 1918.
Dellapina Marco
Gauckler Ludwig
Kampfer Konrad
Rieger Wolfhart
Bell Mark L.
Swiss Aluminum Ltd.
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