Glass manufacturing – Processes – Fining or homogenizing molten glass
Reexamination Certificate
2000-01-18
2002-10-08
Derrington, James (Department: 1731)
Glass manufacturing
Processes
Fining or homogenizing molten glass
C065S134500, C065S134700, C065S157000, C065S337000
Reexamination Certificate
active
06460376
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for melting and refining vitrifiable materials for the purpose of continuously feeding glass-forming plants with molten glass.
2. Discussion of the Background
More particularly intended are plants for forming flat glass such as float or rolling plants, but also plants for forming glassware of the bottle or flask type, plants for forming glass fibres of the mineral wool type for thermal or acoustic insulation or else textile glass fibres called reinforcing fibres.
A great deal of research has been carried out on these processes, which schematically comprise a first melting step followed by a refining step intended to condition the molten glass thermally and chemically and in eliminating therefrom any batch stone, bubbles or any cause of defects appearing after forming.
In the melting range, it has thus been sought, for example, to speed up the melting process or to improve its energy efficiency. Mention may thus be made of the process consisting in rapidly heating the vitrifiable materials in a homogeneous and controlled manner while carrying out intense mechanical stirring allowing the still-solid vitrifiable materials to be brought into intimate contact with the already-liquid phase. This process is especially detailed in Patents FR-2,423,452, FR-2,281,902, FR-2,340,911 and FR-2,551,746 and generally uses electrical heating means of the submerged-electrode type.
Another type of melting process has been developed, for example of the type of those described in Patents U.S. Pat. No. 3,627,504, U.S. Pat. No. 3,260,587 or U.S. Pat. No. 4,539,034 which consist in using, as heating means, submerged burners, that is to say burners fed with gas and air, these generally being placed so as to be flush with the bottom wall so that the flame develops within the mass of vitrifiable materials during liquefaction.
In either case, although it is possible actually to very significantly reduce the residence time of the vitrifiable materials in the melting chamber and to considerably increase the production efficiency compared with “conventional” melting operations, the molten glass being molten is, on the other hand, in the form of a foam which is difficult to refine—it is especially difficult to guarantee the quality of the final glass, especially optical glass.
Research has also been conducted in the refining field. Thus, it is, for example, known from Patent EP-775,671 and Patent U.S. Pat. No. 4,919,697 to carry out at least part of the refining operation under reduced pressure, thereby making it possible, for example, to obtain glass very low in sulphates and with a high redox. However, such refining causes intense foaming, which may be difficult to control and remove.
SUMMARY OF THE INVENTION
The object of the invention is therefore to improve melting and refining processes, aiming especially to use plants which are more compact and/or have greater operating flexibility and/or greater production efficiency, etc., without these industrial advantages being obtained to the detriment of the quality of the glass produced.
The subject of the invention is firstly a process for melting and refining vitrifiable materials, which is characterized by the combination of two characteristics:
on the one hand, all or part of the thermal energy necessary for melting the vitrifiable materials is supplied by the combustion of fossil fuel(s) with at least one oxidizer gas, the said fuels/gas or the gaseous products resulting from the combustion being injected below the level of the mass of vitrifiable materials,
on the other hand, the refining of the vitrifiable materials after melting comprises at least one step of subjecting them to subatmospheric pressure.
There has in fact proved to be an extremely advantageous synergy from an industrial standpoint between the use of melting called hereinafter “melting by submerged burners” for the sake of simplicity and that of refining at reduced pressure.
However, since this combination is far from being imposed as evidence, it might be expected that all these advantages mentioned above would be obtained only at the price of mediocre glass quality, which has not been the case. This is because, although the principle of reduced-pressure refining was known in its generality, it remained difficult to use and the user was not sure of obtaining the same acceptable residual level of bubbles/batch stones as with conventional refining. In the invention, this very particular refining is used by changing a size parameter, namely instead of feeding the refining zone with “conventional” molten glass to be refined, it is fed here in fact with a glass obtained by melting by submerged burners, that is to say with glass having very special characteristics in the sense that it is foamy throughout, with a relatively low density compared with that of a standard glass. Nothing would suggest that it would be possible to refine an initially relatively foamy glass at reduced pressure.
Surprisingly, this has proved to be possible as it has been discovered that this foamy glass resulting from melting by submerged burners also had the characteristic of containing only an extremely small amount of sulphates, which may or may not have been present initially. The sulphate content is generally less than 600 and even less than 200 or less than 100 ppm, or indeed less than 50 ppm, expressed by weight of SO
3
in the glass leaving the melting chamber, this being so without having to control or reduce the amount of sulphate normally contained in the batch materials used, unintentionally, or even by intentionally adding sulphates to the vitrifiable materials. It is this low amount of sulphate which allows effective refining under reduced pressure without any problem. In contrast, a high or simply “standard” sulphate content in the glass to be refined would have caused, during reduced-pressure refining, a very high expansion of the foam by desulphation, which expansion would have been very difficult to control. The fact that there is almost no sulphate in the glass leaving the melting chamber may especially be explained by the partial pressure of water generated by the combustion by burners submerged in the vitrifiable materials.
It should be noted that a desulphated glass gives fewer problems of volatile compounds in the float bath, fewer risks of the formation of tin sulphide and therefore, finally, fewer risks of a tin defect in the sheet of glass.
Another highly advantageous characteristic of the glass leaving the melting chamber according to the invention should also be noted: although it is actually in the form of a kind of foam which remains to be refined, it is possible to control the size of the bubbles which it contains and, especially in certain cases, to remove almost all the smallest bubbles, that is to say those having a diameter of less than 200 &mgr;m, by carrying out, on this glass while it is being melted, a kind of “microrefining” prior to the actual refining after the melting, this microrefining facilitating the coalescence of the bubbles and the disappearance of the smaller bubbles in favour of the larger ones and being promoted by the addition into the vitrifiable materials of refining promoters of the coke or sulphate type. Furthermore, this glass leaving the melting chamber generally has a particularly low residual amount of batch stone, thereby also, just like the size of the bubbles, facilitating the refining operation after the melting operation.
The invention therefore makes it possible to have glasses which are very low in sulphate even before the refining operation, therefore glasses which are at least as low, or even deplete, in sulphate after refining, this being so without having to purify/select vitrifiable materials so that they are low in sulphate. On the contrary, it is even possible to add sulphate at the start, something which is completely surprising and advantageous.
One advantageous effect obtained by the combination according to the invention relates to the energy cost of t
Berthereau Anne
Jeanvoine Pierre
Massart Tanguy
Derrington James
Oblon & Spivak, McClelland, Maier & Neustadt P.C.
Saint-Gobain Glass France
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