Glass manufacturing – Processes – Fining or homogenizing molten glass
Reexamination Certificate
2000-08-18
2004-03-02
Vincent, Sean (Department: 1731)
Glass manufacturing
Processes
Fining or homogenizing molten glass
C065S134900, C065S135200
Reexamination Certificate
active
06698244
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to a method refining molten glass, in which refining gas is generated by refining agents in the molten glass.
In the context of molten glass, the term refining is understood to mean the removal of gas bubbles from the molten material. To achieve the maximum possible freedom from foreign gases and bubbles, it is necessary for the molten mix to be thoroughly mixed and degassed.
The behaviour of gases or bubbles in molten glass and the way in which they are removed are described, for example, in “Glautechnische Fabrikationsfebler” [Glass Engineering Manufacturing Errors], edited by B. Jebsen-Marwedel and R. Bruckner, 3rd edition, 1990, springer Vertag, on pages 195 ff.
In general terms, two refining principles which differ significantly through the way in which the refining gas is generated are known.
In the physical refining methods, by way of example the viscosity of the molten glass is reduced by increasing the temperature. Therefore, to reduce the viscosity higher temperatures of the molten glass are set during the refining than in the melting and cooling-down period. The higher the refining temperature selected can be, the more effective the removal of bubbles from the molten material. If possible, the viscosity of the molten material should be below 10
2
dPas. However, the maximum permissible refining temperature is limited by the ability of the wall material of the melting unit used to withstand such temperatures and, where Pt alloys are used, is at most 1600° C. while where refractory bricks are used, it is at most 1650° C. to
1700° C.
Further physical refining methods are distinguished by the fact that the flow inside the molten glass is influenced by causing mechanical movements of the molten material by poling or by blowing in gas, by the fact that the molten material is mechanically vibrated by the action of sound or ultrasound or bubbles are removed by means of centrifuging. Furthermore, vacuum (vacuum refining) or pressure (high-pressure refining) is employed, or intensified bubble nucleation is initiated by roughening of the surface.
Most commonly, chemical refining methods are employed. The principle of such methods consists in adding to the molten material compounds which decompose and split off gases, or compounds which are volatile at elevated temperatures, or compounds which release gases in an equilibrium reaction at elevated temperatures.
The first group of compounds includes, for example, sodium sulphate, which is used for refining soda-lime glass materials. In this case, SO
2
and O
2
are released in a temperature range of from 1300° C. to 1450° C., with a maximum at 1380° C. This temperature range approximately corresponds to the refining range for glass materials of this type.
Compounds which are volatile at high refining temperatures owing to their vapour pressure and act in this way include halides. By way of example, a series of borosilicate glass materials are refined using NaCl.
Finally, the last group of substances comprises the so-called redox refining agents, such as for example arsenic oxide and antimony oxide. In this case, the redox refining agents used are polyvalent ions which can occur in at least two oxidation states which are in a temperature-dependent equilibrium with respect to one another, a gas, generally oxygen, being released at high temperatures.
The redox equilibrium of the substance dissolved in the molten material can be demonstrated with reference to the example of arsenic oxide using the equation (I)
AS
2
O
5
⇄As
2
O
3
+O
2
↑ (I).
The equilibrium constant K for (I) can be formulated as shown in equation (II):
K
(
T
)
=
As
2
a
O
3
·
PO
2
As
2
a
O
5
(
II
)
.
In this equation, aAs
2
O
3
and aAs
2
O
5
denote the activities of arsenic trioxide and arsenic pentoxide, respectively, and pO
2
denotes the fugacity of oxygen.
The equilibrium constant K is highly temperature-dependent, and a defined oxygen fugacity pO
2
can be set using the temperature and the activity of the oxidic arsenic compounds.
For the chemical refining, it is possible to distinguish between substantially three refining effects:
1) a primary refining effect, under which the gases which are formed during decomposition of the refining agents added, for example oxygen from redox refining agents, diffuse into the bubbles which are formed during the decomposition of the mix, for example CO
2
, N
2
, H
2
O, NO, NO
2
bubbles;
2) a secondary refining effect, under which gases are removed from the molten glass, involving the spontaneous formation of gas bubbles by the refining agents added, e.g. O
2
bubbles from redox refining agents. Foreign gases, such as CO
2
, H
2
O, N
2
, NO, NO
2
, can diffuse into these refining bubbles even if their partial pressure is below 10
5
Pa, and
3) a so-called resorption effect, under which bubbles which have formed as described in 1) or 2), and in the event of a temperature reduction expanded bubbles of, for example, oxygen which are still in the molten material, are dissolved, for example in the case of the redox equilibrium (I) through a shift of the equilibrium towards the starting material.
For high-melting glass material which only have a viscosity of <10
2
dPas above 1700° C., the known refining agents, such as Na
2
SO
4
. NaCl, As
2
O
5
or Sb
2
O
5
, are ineffective. The refining gases are released as early as during melting, and therefore the refining gases are no longer available for the secondary refining effect. Only the primary refining effect takes place. Standard redox refining agents, such as As
2
O
5
or Sb
2
O
5
, are effective at releasing refining oxygen between 1150° C. and 1500° C., with a maximum at 1220° C. to 1250° C., the release of oxygen outside the refining temperature being substantially dependent on the glass composition and on the refining-agent composition (one or more refining agents). Particularly for high-melting glass materials, it is necessary to add larger amounts of refining agent than are actually required in order to achieve a refining effect at all. The large amounts of refining agent represent a particular drawback with arsenic oxide and antimony oxide, since these are highly toxic and expensive compounds. Moreover, the addition of refining agents may have an adverse effect on the properties of the glass material and may increase the production costs, since they are generally expensive compounds owing to the interaction with the tin float bath, arsenic oxide and antimony oxide cannot be used in the float glass process. The fact that high-melting glass materials only reach the viscosity of <10
2
dPas, which is advantageous for the refining, at temperatures above those which are conventionally achievable means that such materials are difficult to refine or else effective refining is altogether impossible.
A series of patents have attempted to use SnO
2
, which releases its maximum level of refining gas at elevated temperatures as refining agent for high-melting glass materials.
By way of example, DE 196 03 698 C1 which corresponds to U.S. Pat. No. 5,770,535 issued on Jun. 23, 1998 to Brix, et al. and entitled “Alkali-free aluminoborosilicate glass and its use,” has disclosed the use of from 0.5 to 2.0% by weight SnO
2
as refining agent for refining alkali-metal-free aluminoborosilicate glass, the refining of the molten glass being carried out at 1600° C.
The use of from 0.02 to 1.0 mol % SnO
2
and of from 0.02 to 0.5 mol % CeO
2
as refining agents for refining aluminosilicate glass materials which can be chemically tempered is known from DE 196 16 633 C1, which corresponds to U.S. Pat. No. 5,895,768 issued on Apr. 20, 1999 to Speit and entitled “Chemically prestressable aluminosilicate glass and products made therefrom”. The refining is carried out at 1580° C. in a Pt crucible.
The use of from 0.5 to 2.0% by weight SnO
2
, preferably together with nitrates, as refining agents for refining alkali-metal-free aluminoborosilicate glass is also
Kiefer Werner
Kolberg Uwe
Köpsel Detlef
Nass Peter
Pfeiffer Thomas
Nils H. Ljungman & Associates
Schott Glas
LandOfFree
Method for refining molten glass does not yet have a rating. At this time, there are no reviews or comments for this patent.
If you have personal experience with Method for refining molten glass, we encourage you to share that experience with our LandOfFree.com community. Your opinion is very important and Method for refining molten glass will most certainly appreciate the feedback.
Profile ID: LFUS-PAI-O-3251337