Glass manufacturing – Processes – Fining or homogenizing molten glass
Reexamination Certificate
2001-02-26
2004-08-03
Griffin, Steven P. (Department: 1731)
Glass manufacturing
Processes
Fining or homogenizing molten glass
C065S029120, C065S135300, C065S160000, C065S178000, C065S374120
Reexamination Certificate
active
06769272
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to methods and processes for oxygen refinement of water and/or liquids containing hydroxyl groups, especially glass melts, to an apparatus for performing these methods and to glass obtained using these methods as part of the glass production process.
Liquids, in which gases are dissolved, which in part form bubbles in the liquid, participate in many technical production processes. Since these gases or gas bubbles interfere with further processing and/or disadvantageously impair the properties and thus the quality of the product, it is necessary to free the liquid of these bubbles. This process is called bubble removal or refinement.
In the following the problems of refinement of liquids are described using the example of refinement of a glass melt, but the invention should not be considered as limited to this example. The same problem is present in other applications.
2. Prior Art
During manufacture of glass from inorganic material raw materials and of course silica or glass sand, soda, limestone, marble or calcereous clay, are mixed and melted in a continuously running process. The melting process takes place in different stages, in which chemical reactions and physical processes occur side-by-side. At higher temperatures solid state reactions occur at the points of contact between neighboring grains. Moreover CO
2
and H
2
O are released from the crystalline phase. The CO
2
is generated by decomposition of the glass-forming salts. In so far as sulfates are present also SO
2
is released.
As a result of the decomposition of the starting materials present in the reaction mixture a considerable quantity of gas is generated during melting of glass. As a rough estimate, it is said that about 1 kg of glass results from melting 1.2 kg of the mixture, i.e. about ⅕ of the mixture weight is released in the form of gas during the melting. Furthermore also other gases are conducted through the mixture during the melting or introduced into the melting glass by the combustion oven.
The release of the gases, especially of CO
2
, causes good pre-mixing of the glass melt. The gas generation and thus the pre-mixing are concluded at temperatures of about 800° C. to 1100° C.
The largest amount of gas escapes of course during the initial melting of the glass, but a considerable portion of the gas is captured by the melt. A portion of the captured gases is dissolved in the glass melt, but another portion remains as local gas inclusions, as so-called bubbles, in the glass melt. The bubbles shrink or grow when the bubble internal pressure is lower or higher than the equilibrium pressure of the dissolved gas. The gas bubbles have different sizes.
The resulting melt is thus designated a raw melt. However it has very distinct streaking or regions of varying index of refraction and many bubbles, which impair the quality of a glass and/or glass-ceramic body that is made from the glass melt. Because of this reason the still strongly streaked and bubble containing glass melt is heated further and homogenized by means of mechanical stirring elements, by chopping with fine needles or by blowing oxygen into it through fine nozzles. These methods refine the glass melt of the gas bubbles.
The term “refinement” or “refining” of the glass means a melting process comprising sequential melting process steps performed in so-called refining chambers, which
removes gas bubbles of a predetermined size class; and
guarantees a certain adjustment of the gas content of the glass melt and at the same time
is to be integrated in a complex sequence of melt processing steps.
The refinement of the glass is thus of the greatest significance for the quality of the end product of the melting process.
Different methods have been formulated for the refinement.
The gas bubbles have the tendency to rise in the melt due to their buoyancy and to escape into the atmosphere from the vessel containing the melt. Since this process however takes a considerable time without other influences, it would make a production process using it expensive because of its long dwell time for the refinement. It is thus known however to produce higher temperatures in the refining zone so that the viscosity of the melt and thus the bubble rising speed is increased and so that the bubble diameter also increases. These additional temperature increases however require considerable energy, which similarly greatly increases the process costs.
The chemical refinement of glass has been well tested and further optimized. Chemical refining agents and of course oxides are added to the melt in temperature-dependent oxidation stages. Common refining agents include Sb(V) oxide, As(V) oxide and Sn(IV)-oxide. An increased mixing of the melt and thus an improved homogenization is obtained by oxygen released in situ by the refining agents or by additional mechanical gas introduction. Moreover the additional release of oxygen causes growth of small gas bubbles present already in the glass melt.
Small bubbles are pumped up with the refining gas O
2
arising from the refining agent during chemical refinement. The resulting larger bubbles formed more rapidly rise in the glass melt. The refinement thus leads to removal of the glass inclusions, which leads to a higher quality product.
The chemical refinement also comprises a sequence of elementary steps interlaced with each other spatially and temporally. First the finely dispersed bubbles in the raw melt are greatly inflated by the refining oxygen gas so that a drastic shortening of the bubble rise time occurs. At the same time the refining bubbles extract gas dissolved in the glass. As much as possible, resorption of the unavoidably present residual bubbles occurs in the subsequent cooling steps. Color, moisture content and the so-called reboil conditions for O
2
and SO
2
are the targeted parameters for a successful adjustment of the gas content of the glass. Once a satisfactory bubble quality is obtained it is not impaired in subsequent cool-down or casting processes.
Chemical refinement has several principal disadvantages. First the methods. First these methods do not function well for every glass system, especially during NaCI refinement, or only at higher temperature, which also requires much time, since gas diffusion in the melt takes too long. Thus the refining chambers must be comparatively large, which further increases product costs. Finally the chemical refining agents change the chemistry of the glass and thus its properties. Moreover arsenic oxide is extremely poisonous and the required purity is not reached without more work. Major environmental problems are also connected both with its manufacture and its use. They also occur for antimony oxide. Cerium oxide itself is of course not poisonous, but it is extremely expensive so that its use is limited to specialty glass.
So-called physical refining methods, which do not impair or damage the chemistry of the glass, are also known because of these disadvantages for chemical methods. The physical refinement of a glass melt is based on the “forcing” of the bubbles to the surface of the melt, where they are destroyed and their gas content released or on their dissolution in the melt.
DE-A 3 022 091 described an apparatus for melting glass in a melt oven with a refining device for refinement of the glass and at least one receiver in which a direct current is applied to a heated electrode formerly or usually operated with alternating current.
A method and apparatus for melting of those glasses which have a high reducing action in a melted state are described in DE-A 3 906 270. Especially in phosphate glass, whose reducing action is even more pronounced in the melted state, erosion by immersion of platinum parts should be avoided. The reducing action of phosphate glass is so great that the glass reacts with platinum or a platinum alloy of the melt-containing vessel to form a platinum-phosphorous alloy. This platinum-phosphorous alloy however has a melting point of 588° C., which is
Duch Klaus-Dieter
Pfeiffer Thomas
Roeth Gernot
Griffin Steven P.
Lopez Carlos
Schott Glas
Striker Michael J.
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