Method of melting glass on molten metal alloys

When using a copper-containing molten metal support upon which glass is melted, diffusion of copper into the melting glass is inhibited by including in the molten metal an element less noble than copper at the melter temperature. The element may be silicon, aluminum, boron, magnesium, manganese, molybdenum, iron, or cobalt.

BACKGROUND OF THE INVENTION 
This invention relates to melting of glassmaking batch materials on a 
molten metal support, in particular, on molten copper alloys. 
In a conventional glass melting process, glass batch materials are 
deposited onto a pool of molten glass where they float on the surface. 
Heat for melting is usually provided by overhead flames and, therefore, 
the great majority of heat for melting is imparted to the batch materials 
from the upper side only, which is relatively inefficient. Limits on the 
thermal durability of the roof of such a melting furnace restricts the 
amount of thermal input to the furnace and, thus, can limit the throughput 
of the furnace. 
For these reasons, it has been proposed that glass be melted on a molten 
metal support wherein the high thermal conductivity of the molten metal 
would provide greater amounts of thermal energy to the underside of the 
batch layer and thereby provide a more efficient process. A molten metal 
support for melting is also advantageous for the sake of reducing the area 
of contact between the molten glass and ceramic containment elements which 
can contaminate the glass. Another advantage of melting on a molten metal 
support is that the glass melt may be maintained as a relatively thin 
layer rather than the deep pool maintained in a conventional tank type 
glass furnace. As a result, the furance may be reduced in size, less 
energy is consumed in maintaining the smaller volume of glass in a molten 
state, and color changes are made easier. Examples of molten metal 
supports for melting glass may be found in U.S. Pat. Nos. 3,450,516 
(Emhiser et al.) and 3,764,287 (Brocious). The former discloses tin, gold 
or silver as the molten support, and the latter discloses the use of tin. 
Gold and silver are obviously economically impractical to employ as molten 
metal supports in large scale commercial glass melting operations, but an 
element that is less expensive than tin and which possesses advantages 
over tin is copper. Potentially, copper could be less reactive to the 
glass compared to tin because of the lower reducing potential of copper. 
Also, the higher boiling point of copper entails a lower vapor pressure at 
a given temperature compared to tin at the same temperature. However, a 
major drawback of copper is its strong tendency to color glass. Cupric 
ions yield blue colored glass, and cuprous ions result in the development 
or ruby red coloration upon subsequent heat treatment. It would be 
desirable to control the coloration effects of copper so that molten 
copper could be used as a support for melting glass. 
The use of copper alloyed with tin or other metals has been suggested for 
use as the molten metal support in the float process wherein molten glass 
is delivered onto a pool of molten metal and spreads to form a flat sheet, 
e.g., U.S. Pat. No. 710,357 (Heal). The temperatures involved in the float 
process, however, are considerably lower than those required for melting 
glass, and thus, infusion of unwanted coloring elements into the glass is 
less of a concern in the float process than in a melting process. In 
addition to higher temperatures a melting operation involves localized 
concentrations of alkalinity that promote chemical activity of the support 
metal. For these reasons, molten copper is considerably more difficult to 
employ as a support in a melting environment than in a float environment. 
Furthermore, for float forming, major proportions of copper are not 
employed because of its high melting temperature. 
U.S. Pat. No. 3,127,261 (Long) discloses the use of a molten 
copper-containing alloy as a heat transfer medium for cooling molten glass 
in a conditioning section of a glass melter downstream from the melting 
zone. 
In U.S. Pat. No. 3,670,179 (Loukes et al.), glass forming elements are 
reacted within a molten metal pool that may include copper alloys so as to 
synthesize a glass. The glass synthesis method is carried out at 
temperatures below those required for fusion melting of glass. 
Preventing migration of tin into a glass ribbon being formed in a float 
process by incorporating trace amounts of various elements into the molten 
is disclosed in U.S. Pat. Nos. 3,305,337 (Loukes et al.); 3,337,323 
(Loukes et al.); and 3,954,432 (Hummel et al.). 
SUMMARY OF THE INVENTION 
In the present invention, incorportion of cuprous or cupric ions into glass 
during melting thereof, while supported on a molten metal support 
including copper, is inhibited by incorporating into the molten 
copper-containing support a minor amount of an element less noble than 
copper at the melting conditions. The less noble element is sacrificially 
oxidized rather than the copper, thereby helping to maintain the copper in 
the elemental state, in which state it is not prone to migrate into the 
melting glass. The sacrificial elements include silicon, aluminum, 
magnesium, boron, manganese, molybdenum, iron, and cobalt. 
This invention pertains to operations in which glass is being melted, as 
opposed to being formed. Melting may be characterized by exchange of heat 
from the molten metal substrate to the melting glass materials. With 
conventional flat glass compositions, a temperature above about 
2000.degree. F. (1100.degree. C.) would be characteristic of a melting 
operation. The presence of unmelted glass batch materials in substantial 
amounts is also indicative of a melting process. In a melting process as 
characterized above, the invention is applicable to any copper-containing 
molten metal alloy, including those in which copper is not the major 
constituent. However, an alloy which is predominantly copper and, 
therefore, by virtue of its melting temperature restricted to use in 
melting processes, is a particularly suitable application for the present 
invention.

DETAILED DESCRIPTION OF THE INVENTION 
The present invention relates to the manufacture of clear glass and to any 
colored glass which does not intentionally include substantial amounts of 
copper. The invention is not limited to particular glass compositions but 
it is contemplated that a major application will be for melting 
soda-silica glasses, and references herein to specific glass properties 
relate to conventional flat glass compositions. 
Since Cu.sup.+ has the same charge and almost the same ionic radius as 
Na.sup.+, the relatively great mobility of Na.sup.+ in silicate glasses 
and some refractories is nearly duplicated by Cu.sup.+. Thus, to minimize 
copper incorporation into the glass and refractories, it is necessary to 
prevent Cu.sup.+ from accumulating in a copper-containing molten 
substrate. Although the mobility of Cu.sup.++ is not as great, a portion 
of Cu.sup.++ is converted to Cu.sup.+ in the presence of metallic copper 
and, therefore, any mode of oxidation of the copper should be kept at a 
minimum. 
The object of the present invention is to sacrifice a less noble metal to 
intercept and possibly reverse the formation of the fast diffusing 
Cu.sup.+. It should be understood that the term "less noble" applies at 
the temperature of operation, where the standard order of nobility may not 
apply. In Table I, there are listed the proposed sacrificial elements in 
ascending order of nobility at two selected temperatures. These relative 
nobilities were determined by calculating reaction probabilities for the 
formation of the designated oxides which were then compared with the 
probability of Cu.sub.2 O formation. The "preferred alloy ratio" in Table 
I is the ratio of the sacrificial element to copper which would 
theoretically reduce copper migration into the glass to 1 percent of that 
from pure copper under the same assumed ideal conditions. Of course, 
satisfactory performance need not require reduction to the 1 percent 
level, and satisfactory performance may be obtained for a given 
application with less of the sacrificial element. Additionally, it should 
be noted that estimated interpolation between the two temperatures 
reported in Table I may be required for specific operating temperatures. 
For many glass compositions, 1300.degree. K. is lower than would be 
desired for melting, and 2000.degree. K. is above the temperature to which 
most glass melts are heated. But the data of Table I are useful for 
estimating alloy ratios at melting temperatures lying between the reported 
extremes. 
TABLE I 
______________________________________ 
Temperature Most Probable 
Preferred Alloy 
(.degree.K.) 
Element Oxide Ratio Element:Cu) 
______________________________________ 
1300 Si SiO.sub.2 7.43 .multidot. 10.sup.-21 
Al Al.sub.2 O.sub.3 
1.80 .multidot. 10.sup.-20 
Mg MgO 4.73 .multidot. 10.sup.-15 
B B.sub.2 O.sub.3 
3.18 .multidot. 10.sup.-14 
Mn MnO 2.70 .multidot. 10.sup.-8 
Mo MoO.sub.2 5.24 .multidot. 10.sup.-8 
Fe FeO 3.26 .multidot. 10.sup.-4 
Sn SnO 1.09 .multidot. 10.sup.-2 
Co CoO 1.73 .multidot. 10.sup.-1 
2000 Si SiO.sub.2 4.61 .multidot. 10.sup.-12 
Al Al.sub.2 O.sub.3 
1.05 .multidot. 10.sup.-11 
B B.sub.2 O.sub.3 
1.13 .multidot. 10.sup.-8 
Mg MgO 4.56 .multidot. 10.sup.-7 
Mn MnO 1.02 .multidot. 10.sup.-4 
Mo MoO.sub.2 3.82 .multidot. 10.sup.-4 
Fe FeO 1.42 .multidot. 10.sup.-2 
Co CoO 8.47 .multidot. 10.sup.-1 
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Other considerations enter the selection of a sacrificial alloy in addition 
to its relative activity. Silicon and aluminum, although highest in 
activity, may be undesirable for some applications due to their strong 
reducing effect on molten glass. On the other hand, tin and cobalt are 
only marginally effective and at higher temperatures, tin is not effective 
to prevent oxidation of copper. Economics are also a factor to be 
considered, in which case the use of iron is a preferred embodiment in 
spite of its relatively low activity. Some of the sacrificial element in 
its oxide form will become incorporated into the molten glass and its 
effect on the glass should be considered in selecting the sacrificial 
element. Incorporation of small amounts of iron oxide into the glass is 
not a drawback in most cases and, therefore, iron is a preferred 
embodiment for this reason as well. 
Because the object of the invention is to avoid oxidation of copper, it is 
contemplated that heat for melting would preferably be provided by 
electric heating means rather than by combustion so as to avoid oxidizing 
conditions within the melter. A suitable arrangement for carrying out the 
melting process of the present invention is disclosed in the 
aforementioned U.S. Pat. No. 3,450,516 (Emhiser et al.). 
It should be understood that other variations and modifications as are 
known to those of skill in the art are included within the scope of the 
invention as defined by the claims which follow.