Glass manufacturing – Means providing special atmosphere
Reexamination Certificate
1999-12-29
2002-06-18
Vincent, Sean (Department: 1731)
Glass manufacturing
Means providing special atmosphere
C065S134200, C065S374130
Reexamination Certificate
active
06405564
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a vacuum degassing apparatus for molten glass which removes bubbles from molten glass continuously supplied.
2. Discussion of Background
In order to improve the quality of formed glass products, there has been used a vacuum degassing apparatus which removes bubbles generated in molten glass before the molten glass which has been molten in a melting furnace is formed by a forming apparatus. Such a conventional vacuum degassing apparatus is shown in FIG.
12
.
The vacuum degassing apparatus
200
shown in
FIG. 12
is used in a process wherein molten glass G in a melting tank
212
is vacuum-degassed and is continuously supplied to a successive treating vessel (not shown), e.g. a treating vessel for plate glass such as a floating bath and an operating vessel for bottles. A vacuum housing
202
where a vacuum is created has a vacuum degassing vessel
204
substantially horizontally housed therein, and an uprising pipe
206
and a downfalling pipe
208
housed in both ends thereof so as to extend vertically and downwardly. The uprising pipe
206
has a lower end immersed in the molten glass G in an upstream pit
214
which communicates with the melting tank
212
. The downfalling pipe
208
also has a lower end immersed in the molten glass G in a downstream pit
216
which communicates with the successive treating vessel (not shown).
The uprising pipe
206
communicates with the vacuum degassing vessel
204
. The molten glass G before degassing is drawn up from the melting tank
212
into the vacuum degassing vessel
204
. The downfallng pipe
208
communicates with the vacuum degassing vessel
204
. The molten glass G after degassing is drawn down from the vacuum degassing vessel
204
and is led to the successive treating vessel (not shown). In the vacuum housing
202
, thermal insulation material
210
such as bricks for thermal insulation is provided around the vacuum degassing vessel
204
, the uprising pipe
206
and the downfalling pipe
208
to cover these parts for thermal insulation. The vacuum housing
202
may be made of metal such as stainless steel. The vacuum housing is evacuated by a vacuum pump (not shown) to maintain the inside of the vacuum degassing vessel
204
therein in a depressurized state such as a pressure of {fraction (1/20)}-⅓ atmosphere. As a result, the molten glass G before degassing in the upstream pit
214
is sucked up by the uprising pipe
206
to be introduced into the vacuum degassing vessel
204
. After the molten glass is vacuum-degassed in the vacuum degassing vessel
204
, the molten glass is withdrawn down by the downfalling pipe
208
to be led into the downstream pit
216
.
In the conventional vacuum degassing apparatus
200
, the molten glass G that has a high temperature such as a temperature between 1200-1400° C. is treated. In order to deal with such a high temperature treatment, portions in direct contact with the molten glass G such as the vacuum degassing vessel
204
, the uprising pipe
206
and the downfalling pipe
208
are constituted by circular shells which are normally made of noble metal such as platinum, and platinum-rhodium and platinum-palladium as platinum alloy, as disclosed in JP-A-2221129 in the name of the applicants. The applicants have used circular shells of platinum alloy for these members to put the vacuum degassing apparatus into practice.
The reason why these members are constituted by the circular shells made of noble metal such as platinum alloy is that not only the molten glass G is at a high temperature but also a low reactivity of the noble metal with the molten glass at a high temperature prevents the molten glass from being made to be heterogeneous by reaction with the molten glass, that there is no possibility of mixing impurities into the molten glass G, and that required strength can be ensured to some degree at a high temperature. In particular, the reason why the vacuum degassing vessel
204
is constituted by a circular shell made of noble metal is that the circular shell is self-heated by flowing an electric current in the circular shell per se, and the molten glass G in the shell is uniformly heated to maintain the temperature of the molten glass G at a certain temperature, in addition to the reasons as just stated.
When the vacuum degassing vessel
204
is made of noble metal, a circular shell is appropriate in terms of mechanical strength such as high temperature strength. Since noble metal such as platinum is too expensive to increase the wall thickness, the circular shell has a limited diameter and can not be formed in a large size because of both of cost and strength. This has created a problem in that the vacuum degassing apparatus can not be formed so as to have a large quantity of flow because of the presence of a limited quantity of flow of the molten glass G which can be degassed by the vacuum degassing vessel
204
. If the vacuum degassing vessel
204
in a circular shell has the entire length thereof extended and the current of the molten glass is increased to make the volume larger so as to increase a degassing throughput, there has been created a problem in that the apparatus is extended and cost is raised. That is to say, there has been created in a problem in that the degassing throughput (the quantity of flow) of the molten glass G in the vacuum degassing apparatus can not be made large.
Since the molten glass G is obtained by dissolution reaction of powdered raw materials, it is preferable that the temperature in the melting vessel
212
is high in terms of dissolution and that the viscosity of the molten glass is low or the temperature of the molten glass is high in terms of vacuum-degassing. Although the conventional vacuum degassing apparatus
200
requires to use alloy made of noble metal in the vacuum degassing vessel
204
and the like in terms of high temperature strength, it is difficult to increase the wall thickness of the circular shells in terms of cost because such alloy is expensive. Even if noble metal such as platinum is used, the temperature of the molten glass at an inlet of the vacuum degassing apparatus
200
has been limited to a certain temperature (1200-1400° C.) as stated earlier.
The appropriate temperature at which a forming machine (forming treatment vessel) forms the molten glass after degassing has been limited to a certain temperature though the temperature varies depending on articles to be formed, such as plate glass and bottles to be formed. When noble metal is used to form the vacuum degassing vessel
204
, the temperature of the molten glass G at the inlet of the vacuum degassing apparatus
200
has been restricted to a temperature lower than 1400° C. This has created a problem in that a drop in temperature of the molten glass G in the vacuum degassing apparatus
200
decreases the temperature of the molten glass G at an outlet of the vacuum degassing apparatus
200
to a lower temperature than the temperature required for forming since the quantity of flow (throughput) can not be made greater and the quantity of heat carried in by the molten glass G is not so large. This requires that the molten glass G in the vacuum degassing vessel
204
be uniformly heated as stated earlier. For this uniform heating, the vacuum degassing vessel
204
per se is required to be constituted by a circular shell made of noble metal, causing the problem in that it is difficult to increase the throughput as stated earlier.
In order to cope with these problems, a proposal has been made to use inexpensive refractory material such as firebricks in paths of the vacuum degassing vessel
204
, the uprising pipe
206
and the downfalling pipe
208
instead of using expensive noble metal material such as platinum alloy.
There has been known a bubble forming phenomenon that use of refractory material in the melting furnace generates fine bubbles from the surface of the refractory at an initial stage when the refractory used as the refractory material directly contacts with the
Imamaki Katsuharu
Ishino Toshihiro
Kawaguchi Toshiyasu
Kijima Takashi
Matsuwaki Masataka
Asahi Glass Company Ltd.
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Vincent Sean
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