Vacuum degassing apparatus for molten glass

Glass manufacturing – Processes – Fining or homogenizing molten glass

Reexamination Certificate

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C065S134900, C065S346000, C065S347000, C065S374130

Reexamination Certificate

active

06560996

ABSTRACT:

The present invention relates to a vacuum degassing apparatus for molten glass, which removes bubbles from molten glass continuously supplied.
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 vessel is formed by a forming apparatus, as shown in FIG.
4
.
The vacuum degassing apparatus
410
shown in
FIG. 4
is used in a process wherein molten glass G in the melting vessel
420
is vacuum-degassed and is continuously supplied to a subsequent treatment vessel. The vacuum degassing apparatus includes a vacuum housing
412
which is evacuated to be depressurized therein for vacuum-degassing, a vacuum degassing vessel
414
which is depressurized along with the vacuum housing
412
, and an uprising pipe
416
and a downfalling pipe
418
which are connected to both end portions of the vacuum degassing vessel so as to extend downwardly and vertically. The uprising pipe
416
has a lower end immersed in the molten glass G in an upstream pit
422
in communication with the melting vessel
420
. Likewise, the downfalling pipe
418
has a lower end immersed in the molten glass G in a downstream pit
424
in communication with the subsequent treatment vessel (not shown).
The vacuum degassing vessel
414
is provided in the vacuum housing
412
in a substantially horizontal direction, which is evacuated by a vacuum pump, not shown, to be depressurized therein. The vacuum degassing vessel
414
as well as the vacuum housing
412
is depressurized to a pressure of {fraction (1/20)}-⅓ atmosphere therein. As a result, the molten glass G before vacuum-degassing in the upstream pit
422
is sucked and drawn up by the uprising pipe
416
to be introduced into the vacuum degassing vessel
414
, the molten glass is vacuum-degassed in the vacuum degassing vessel
414
, and the molten glass is drawn down by the downfalling pipe
418
to be discharged into the downstream pit
424
.
In order to evacuate the vacuum housing
412
through a suction port
412
c
by the vacuum pump or the like, not shown, to depressurize the inside of the vacuum degassing vessel
414
to a certain pressure and to maintain the certain pressure therein, the vacuum degassing vessel
414
has suction ports
414
a
,
414
b
formed in an upper portion thereof so as to be open toward the inside of the vacuum housing
412
.
The vacuum housing
412
is a metallic casing, such as an ordinary steel casing, a stainless steel casing or a heat-resistant steel casing. The vacuum housing is evacuated by the vacuum pump (not shown) or the like from outside to be depressurized therein, depressurizing the inside of the vacuum degassing vessel
414
provided therein to the certain reduced pressure, such as a pressure of {fraction (1/20)}-⅓ atmosphere and maintaining the reduced pressure in the vacuum degassing vessel.
Around the vacuum degassing vessel
414
, the uprising pipe
416
and the downfalling pipe
418
in the vacuum housing
412
is provided thermal insulation material
430
, such as refractory bricks, to cover these members for thermal insulation.
Since the conventional vacuum degassing apparatus
410
is used to deal with the molten glass G at a high temperature, such as a temperature in a range of 1,200-1,400° C., paths for molten glass in direct contact with the molten glass G, such as the vacuum degassing vessel
414
, the uprising pipe
416
and the downfalling pipe
418
, are constituted by circular shells made of noble metal, such as platinum and platinum alloy like platinum rhodium, as disclosed in JP-A-2221129 in the name of the applicants.
The reason why the paths for molten glass, such as the vacuum degassing vessel
414
, the uprising pipe
416
and the downfalling pipe
418
, are constituted by a circular shell made of noble metal, such as platinum and platinum alloy, is that there is no inclusion of impurities into the molten glass G and a certain strength is ensured at high temperatures since it is hardly possible due to low reactivity of the noble metal with the molten glass at a high temperature that, when the noble metal contacts the molten glass G at such a high temperature, the noble metal does not elute by reaction with the molten glass G.
When the paths for molten glass, such as the vacuum degassing vessel
414
, the uprising pipe
416
and the downfalling pipe
418
, are made of noble metal, such as platinum and platinum alloy, the following problems are created especially in building the vacuum degassing apparatus
410
in a large size:
(1) It is required that the temperature of the molten glass G at an inlet of the vacuum degassing apparatus
410
be not higher than 1,400° C.
When the temperature is higher than 1,400° C., the strength of the noble metal lowers. In order to set the temperature at a temperature not higher than 1,400° C., the temperature in the melting vessel
420
can not be raised in a sufficient way. This could lead to insufficient
25
dissolution of glass material in the melting vessel
410
.
(2) The cost greatly increases.
In order to deal with a large quantity of molten glass, the paths are required to have a large sectional area. This requires that the paths have a thick wall to maintain a required strength for the paths, remarkably increasing the required quantity of the expensive noble metal and significantly raising the cost.
In terms of mainly cost reduction, a proposal has been made to constitute the paths for molten glass in the vacuum degassing vessel
414
, the uprising pipe
416
and the downfalling pipe
418
in the conventional vacuum degassing apparatus
410
shown in
FIG. 4
by refractory bricks more inexpensive than noble metal, such as platinum and platinum alloy, in order to build the apparatus in a large size and increase the degassing throughput of the molten glass.
However, there are limits to which refractory bricks are formed in a large size. It is absolutely impossible to build each of the vacuum degassing vessel
414
, the uprising pipe
416
and the downfalling pipe
418
from a single refractory brick. In order to constitute the vacuum degassing vessel
414
, the uprising pipe
416
and the downfalling pipe
418
of the vacuum degassing apparatus
410
by refractory bricks, many refractory bricks are required to be combined. This means that joints are inevitably formed between the refractory bricks in the paths in direct contact with the molten glass.
If a joint is in touch with atmosphere, there is a possibility that air enters into the paths through the joint to make it difficult to maintain the depressurized state in the paths since the inside of the paths in direct contact with the molten glass is depressurized. There is no possibility that this problem is created at the vacuum degassing vessel
414
because the vacuum degassing vessel is housed in the depressurized vacuum housing
412
. However, there is left a possibility that air enters through a joint because it is difficult to house the uprising pipe
416
and the downfalling pipe
418
in their entirety in the vacuum housing.
It is an object of the present invention to solve the problems of the conventional apparatus and to provide a large size of practical vacuum degassing apparatus capable of dealing with a large quantity of molten glass.
In order to attain the object, the present invention provides a vacuum degassing apparatus for molten glass, comprising a vacuum housing which is evacuated to be depressurized therein; a vacuum degassing vessel made of refractory material, which is provided in the vacuum housing to vacuum-degas molten glass; an uprising pipe made of refractory material, which connects to the vacuum degassing vessel in the vacuum housing, and sucks and draws up undegassed molten glass to introduce the undegassed molten glass into the vacuum degassing vessel; a downfalling pipe made of refractory material, which connects to the vacuum degassing vessel in the vacuum housing and draws down the degassed mol

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