Glass manufacturing – Processes – Fining or homogenizing molten glass
Reexamination Certificate
2002-02-22
2003-07-08
Derrington, James (Department: 1731)
Glass manufacturing
Processes
Fining or homogenizing molten glass
C065S134100, C065S134900, C065S135100
Reexamination Certificate
active
06588233
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a process for physical refinement of a liquid, especially a glass melt, which has dissolved gases and bubble-shaped gas inclusions, by varying the pressure on the liquid in a refining chamber.
The present invention also relates to an apparatus for physical refinement of a liquid, which has dissolved gases and bubble-shaped gas inclusions, with at least one processing container for treatment of the liquid and with at least one refining chamber in which there is a change of the pressure exerted on the liquid relative to the pressure in the processing container.
2. Prior Art
Liquids in which gases are dissolved, which, in part, form bubbles in the liquids, are participants in many engineering processes. Since these gases or gas bubbles can interfere with further processing or impair the properties and thus the quality of the manufactured product disadvantageously, it is necessary to free the liquid from these gas bubbles. This priority is designated as degassing or refinement.
In the following the refinement of a glass melt is used as an example of the refinement of a liquid in general, but the invention should not be limited here to that example. The analogous problem arises in many engineering applications.
As a result of the decomposition of the starting materials in the initial batch considerable amounts of gases are generated during the melting of glass. As a crude estimate it is said that about 1 kg of glass results from melting 1.2 kg of batch, i.e. during the melting about ⅕ of the batch weight is released in the form of gas. Other gases physically accompany the batch or starting mixture or are introduced by the combustion heat source used in the glass melting process.
Most gases escape of course during the initial melting of the glass, however a considerably portion of the gases are entrapped in the melt. A portion of the trapped gases is dissolved in the glass melt, while the other portion remains as local gas inclusions, as so-called bubbles, in the melt. The bubbles shrink or grow when the bubble internal pressure is higher or lower than the equilibrium pressure of the dissolved gas. The gas bubbles have different sizes.
Since these gas bubbles disadvantageously impair the quality of a glass or glass-ceramic body made from the glass melt, the glass melt must be refined of the gas, i.e. which means that the gas must be removed from the glass melt.
The term “refining of the glass” means a melt process step subsequently connected with the melting in the so-called refining chamber, which
largely results in a removal of a definite size class of gas bubbles and
guarantees a desired adjustment of the gas content of the glass melt and at the same time
is integrated in a complex sequence of melting process steps.
The refining of the glass is then of highest significance for the quality of the product resulting from the melting process.
A variety of methods have been used in the prior art for the refining.
The gas bubbles have the tendency to rise in the melt and to escape from containment into the surroundings because of their static buoyancy. This process however requires considerable time without external influence, which would be costly for the production process because of the long idle time. It is thus known to provide higher temperatures in a refining zone in order to increase the viscosity of the melt and thus the rising speed and bubble diameter of the gas bubbles. This additional temperature rise requires a considerable amount of energy, which adds comparatively large additional costs to the production process.
The methods of chemical refining of glass with oxides by means of temperature-dependent oxidation steps are well tested and largely optimized. These methods especially involve refining agents such as NaCl, Sb(V)-oxide, As(V)-oxide and Sn(IV)-oxide.
During chemical refining especially the rising speed of smaller bubbles is increased because the refining gas O
2
that is generated from the refining agent is pumped into them.
Chemical refining methods comprise a sequence of elementary steps interwoven with each other in time and space. The finely dispersed bubbles in the crude melt are expanded to such a great extent by the refining gas O
2
that a drastic shortening of the rising time occurs. Simultaneously the refined bubbles extract gas dissolved in the glass. In subsequent cooling steps as complete as possible a resorption of the unavoidable remaining bubbles occurs. Among others, satisfactory color, water-content and concentration limits of O
2
and SO
2
are major goals for a successful control of the gas content of the glass. A once-achieved satisfactory bubble quality may not be impaired again during cooling or shaping processes.
Chemical refining has several disadvantages in principle limiting it:
the method does not function for every glass system, especially in NaCl refining, or only at higher temperatures;
the refining process requires much time, since gas diffusion occurs slowly in the melt. Because of that the refining chambers must have a comparatively large extent which increases the production costs;
the refining agents change the chemistry of the glass and thus its properties; furthermore they are toxic (arsenic, antimony).
Because of these disadvantages the so-called physical refining processes were introduced which do not disadvantageously change the glass chemistry. The physical refining of the glass melt is based on “forcing” the bubbles with physical methods to climb to the surface of the melt. The bubbles then burst at the surface and release their gas content or dissolve in the melt.
The so-called low pressure refining is a physical refining method in wide spread use, which is described in numerous literature references, for example in U.S. Pat. No. 4,738,938 and European Patent Document EP 0 231 518 B1.
In low pressure refining the bubbles present in the melt grow when the equilibrium pressure on the gas dissolved in the melt drops below their internal pressure (hydrostatic pressure in the melt plus the surface tension pressure of the bubble). Because of that the bubbles become larger, growing more rapidly at the surface of the melt, bursting there or are “skimmed off”. The growth speed is determined primarily by melt-bubble gas transport.
The prior art methods of low pressure refining has the following disadvantages in principle limiting them:
a) The growth speed of the bubbles is primarily determined by gas diffusion and by the equilibrium pressure of the gas dissolved in the melt; low pressure refining operates only economically then, i.e. sufficiently rapidly, when additional chemical refining agents are used. Thus low pressure refining usually does not permit the complete abandonment of chemical refining agents, but allows only a gradual reduction of the chemical refining agents. Thus U.S. Pat. Nos. 4,886,539, 4,919,697 and 4,919,700 describe a method for vigorous bubbling of glass melts at low pressure for removal of gas ingredients.
b) Furthermore low pressure refining is based on the rise of bubbles to the surface of the melt. Thus the viscosity of the melt must be sufficiently low or its temperature must be sufficiently high. Glass melts being refined at refining conditions usually have a viscosity of definitely less than 10 Pas, which means that the associated temperatures of the glass melts must certainly be over 1400° C., in special glasses definitely over 1500° C.; low pressure refining requires a comparatively large-surface and long refining chamber and a comparatively long refining time, which disadvantageously effects production costs.
c) Small bubbles grow only very slowly during low pressure refining and mainly do not climb to the surface of the melt. All the bubbles are hardly removed from the melt by low pressure refining, furthermore only the bubble spectrum in the melt (gross dispersion of bubbles) changes. The engineering goal is then to beneficially transform the bubble spectrum. A complete debubbling or bubble removal fr
Derrington James
Schott Glas
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