Selenium encapsulation for producing colored glass

The present invention provides a glass forming composition for encapsulating selenium which includes, by weight percent of raw materials, 20 to 50% silica and 50 to 80% alkali and alkaline earth materials. The composition has a liquidus temperature between 600.degree. to 1200.degree. C., preferably up to 1000.degree. C., and a viscosity up to 10,000 Poise at said liquidus temperature, preferably up to 5,000 Poise. The alkali and alkaline materials preferably include at least one group of materials combined in an approximate eutectic molar ratio. In one particular embodiment of the invention, the alkali and alkaline earth materials include groups of nitrates, such as KNO.sub.3, NaNO.sub.3 and/or Ca(NO.sub.3).sub.2, and/or carbonates, such as K.sub.2 CO.sub.3, Na.sub.2 C0.sub.3 and/or Li.sub.2 CO.sub.3.

BACKGROUND OF THE INVENTION 
This invention relates to the preparation of selenium for incorporation 
into a glass batch. 
Selenium is an important additive for making various heat absorbing and 
colored glass. It has been used in the glass industry for more than 100 
years to produce colors from pink to red to blue and even as a 
de-colorizing agent by compensating for the blue green color resulting 
from the incorporation of ferrous (Fe.sup.2+) material. Its desirable 
coloring properties and long history of wide spread use, however, do not 
speak of the difficulties involved in its utilization. 
Selenium is considered one of the most expensive components of a soda lime 
silica glass batch. Fortunately, only parts per million (PPM) levels are 
needed for most coloring needs. However, concerns about selenium's extreme 
volatility outweigh the small amounts required for coloration. 
The volatilization of selenium and its compounds are of important 
technological significance to glass makers. According to conventional 
practice, in most cases the selenium, in the form of metal or selenium 
compounds, is mixed and melted with other total amounts of the raw 
materials for the glass. Selenium losses of nearly 85% are common in glass 
production facilities. Selenium is so thermally unstable that 
volatilization for the metallic state begins at about 200.degree. C. 
(which is below its melting point of 217.degree. C.). The economic and 
corresponding environmental costs of selenium volatility are therefore of 
great concern. 
There are only two forms of selenium which produce color in soda lime 
silica glass, namely, selenium in its elemental state (Se.sup.o) and 
polyselenide state (Se.sub.x.sup.2-). The neutral form creates a pink 
color whereas the polyselenide's contribution depends on the associated 
species, for example, FeSe produces red brown color. The other possible 
forms of selenium in the vitreous state Se.sup.2-, Se.sup.4+ and Se.sup.6+ 
are colorless. Glass redox, therefore, will have great influence on the 
final color of the glass. Clearly the problems associated with selenium go 
beyond just its retention in the melt and must address the efficient 
development of the correct color of the final glass product. As a starting 
point, however, selenium must be retained before dealing with the 
resulting color. 
One method of retaining selenium is to combine the selenium with either 
glass cullet or a portion of the glass batch material and sinter the 
resulting mixture as disclosed in U.S. Pat. Nos. 3,291,585 and 3,628,932. 
The sintered material is then recombined with the glass batch and melted. 
It would be advantageous to provide a system of adding selenium to a glass 
batch without the necessity of pre-reacting the selenium prior to adding 
it to the glass batch. 
SUMMARY OF THE INVENTION 
The present invention provides a glass forming composition for 
encapsulating selenium which includes, by weight percent of raw materials, 
20 to 50% silica and 50 to 80% alkali and alkaline earth materials. The 
composition has a liquidus temperature between 600.degree. to 1200.degree. 
C., preferably up to 1000.degree. C., and a viscosity up to 10,000 Poise 
at its liquidus temperature, preferably up to 5,000 Poise. The alkali and 
alkaline materials preferably include at least one group of materials 
combined in an approximate eutectic molar ratio. In one particular 
embodiment of the invention, the alkali and alkaline earth materials 
include groups of nitrates, such as KNO.sub.3, NaNO.sub.3 and/or 
Ca(NO.sub.3).sub.2, and/or carbonates, such as K.sub.2 CO.sub.3, Na.sub.2 
C0.sub.3 and/or Li.sub.2 CO.sub.3. 
The present invention also includes a selenium containing glass forming 
composition which includes, by weight percent of raw materials, 18 to 50% 
silica, 45 to 80% alkali and alkaline earth materials and up to 10% 
selenium. The combination of the silica and the alkali and alkaline earth 
materials in these glass forming compositions has a liquidus temperature 
between 600 to 1200.degree. C. and a viscosity up to 10,000 Poise at said 
liquidus temperature. The alkali and alkaline materials preferably include 
at least one group of materials combined in an approximate eutectic molar 
ratio. 
The present invention also includes an improved method of producing 
selenium containing glasses, and in particular, soda lime silica glasses. 
Prior to combining selenium and batch materials and melting the combined 
materials, the selenium is encapsulated in a low silica, high alkali glass 
forming composition that promotes oxidation of selenium and has a liquidus 
temperature between 600 to 1200.degree. C. and a viscosity up to 10,000 
Poise at the liquidus temperature.

DETAILED DESCRIPTION OF THE INVENTION 
Selenium will be retained in a glass batch only in an oxide form. When 
selenium is added to a typical soda lime silica batch of a type well known 
in the art, it undergoes a solid state oxidation reaction with alkali and 
alkali earth materials. The following illustrates one such series of 
reactions: 
EQU at.about.290.degree. C.: Na.sub.2 CO.sub.3 (soda)+Se+O.sub.2 
(air).fwdarw.Na.sub.2 SeO.sub.3 (selenite Se.sup.4+)+CO.sub.2(Eq. 1) 
EQU at.about.600.degree. C.: Na.sub.2 SeO.sub.3 +1/2 O.sub.2 (air)Na.sub.2 
SeO.sub.4 (selenate Se.sup.6+) (Eq. 2) 
These forms of selenium are more thermally stable than metallic selenium 
and will remain in a liquefied glass batch material. 
In the present invention, selenium is encapsulated in a material that will 
drive the selenium oxidation reactions to form compositions that better 
retain selenium at high temperature and better distribute the selenium 
throughout the glass melt. By providing such a system, the selenium is 
protected from the high temperatures that would result in its early 
volatilization soon after being heated so as to increase its potential to 
be assimilated in the glass melt. 
In the present invention, the encapsulating material is a low silica, high 
alkali material having a low liquidus temperature and high fluidity at its 
liquidus temperature. The low silica, high alkali nature of the material 
favors the selenium oxidation reactions as discussed earlier. The low 
liquidus temperature allows the selenium compounds to be dissolved in the 
liquefied material earlier in the batch melting process for better 
retention. The highly fluid nature of the liquefied encapsulant provides 
high diffusion and dispersion of the selenium ions. All three of these 
factors combine to yield an environment more suitable for assimilation of 
the selenium into the glass batch than that found in typical 
soda-lime-silica glass compositions. 
As used herein, low silica, high alkali material means a glass forming 
composition that is about 20-50% silica and 50-80% alkali and alkaline 
earth materials measured by weight percent of the raw material used to 
form the encapsulant. Low liquidus temperature means a temperature in the 
range of about 600.degree. to 1200.degree. C. (1112.degree. to 
2192.degree. F.), preferably less than 1000.degree. C. (1832.degree. F.). 
High fluidity means a viscosity of about 10,000 Poise (P) or less at its 
liquidus temperature, preferably less than 5000 Poise. 
The following low silica, high alkali compositions were used to encapsulate 
selenium and were tested to evaluate their effectiveness in retaining 
selenium in a glass batch during melting. The compositions are disclosed 
as discrete groupings of chemical compounds. The mole ratios (expressed as 
a %) describe a nearly eutectic molar ratio for each specific grouping and 
not the final composition. Similarly, the liquidus temperatures given are 
for each specific group, e.g. a nitrate group or a carbonate group. The 
group liquidus temperatures are shown to illustrate that the combined 
liquidus temperature for the encapsulant is greatly influenced by the 
nearly eutectic mixtures of alkali and alkaline earth material groups. The 
viscosity for each composition was measured at its combined liquidus 
temperature. The measured temperatures and viscosities are approximate due 
to the influence of water and impurities on the melting temperature. 
______________________________________ 
Mole Ratio 
Liquidus Wt. % of 
Material (%) Temp. (.degree.C.) 
Raw Mat'l 
______________________________________ 
COMPOSITION A 
KNO.sub.3 36) 20% 
NaNO.sub.3 30) .about.200.degree. C. 
20% 
Ca(NO.sub.3).sub.2 
34) 20% 
SiO.sub.2 (-200 mesh sand) 
.about.1720.degree. C. 
40% 
Combined Liquidus 
Temperature: .about.1204.degree. C. 
Viscosity: .about.-5000P 
______________________________________ 
COMPOSITION B 
KNO.sub.3 36) 25% 
NaNO.sub.3 30) .about.200.degree. C. 
25% 
Ca(NO.sub.3).sub.2 
34) 25% 
SiO.sub.2 (-200 mesh sand) 
.about.1720.degree. C. 
25% 
Combined Liquidus 
Temperature: .about.1204.degree. C. 
Viscosity: .about.-1000P 
______________________________________ 
COMPOSITION C 
KNO.sub.3 36) 10% 
NaNO.sub.3 30) .about.200.degree. C. 
10% 
Ca(NO.sub.3).sub.2 
34) 10% 
Na.sub.2 CO.sub.3 
57) .about.740.degree. C. 
15% 
K.sub.2 CO.sub.3 
43) 15% 
SiO.sub.2 (-200 mesh sand) 
.about.1720.degree. C. 
40% 
Combined Liquidus 
Temperature: .about.785.degree. C. 
Viscosity; .about.2000P 
______________________________________ 
COMPOSITION D 
KNO.sub.3 36) 16.67% 
NaNO.sub.3 30) .about.200.degree. C. 
16.67% 
Ca(NO.sub.3).sub.2 
34) 16.67% 
Na.sub.2 CO.sub.3 
57) .about.740.degree. C. 
13.33 
K.sub.2 CO.sub.3 
43) 13.33% 
SiO.sub.2 (-200 mesh sand) 
.about.1720.degree. C. 
23.33% 
Combined Liquidus 
Temperature: .about.710.degree. C. 
Viscosity; .about.1590P 
______________________________________ 
COMPOSITION E 
KNO.sub.3 36) 12.5% 
NaNO.sub.3 30) .about.200.degree. C. 
12.5% 
Ca(NO.sub.3).sub.2 
34) 12.5% 
Na.sub.2 CO.sub.3 
39) 15.0% 
K.sub.2 CO.sub.3 
30) .about.400.degree. C. 
15.0% 
Li.sub.2 CO.sub.3 
31) 10.0% 
SiO.sub.2 (-200 mesh sand) 
.about.1720.degree. C. 
22.5% 
Combined Liquidus 
Temperature: .about.750.degree. C. 
Viscosity: .about.1000P 
______________________________________ 
It should be appreciated that although the silica in the above compositions 
was provided in the form of fine sand, other SiO.sub.2 sources may be 
used, e.g. clay, feldspar and glass cullet, as well as powdered or liquid 
(aqueous) alkali silicates. 
To evaluate the effectiveness of the encapsulation on selenium retention, 
metallic selenium was added to each of the compositions at an amount equal 
to 1% of the total weight of the encapsulant so that the initial selenium 
concentration was 10,000 PPM. 
The encapsulated selenium was prepared as follows. All of the raw materials 
are initially combined as dry granular material or powders. To achieve the 
greatest state of mixing, all materials should be as fine as possible, 
preferably about -120 mesh or less. This is important for materials such 
as nitrates since these are typically prilled materials. The source of 
selenium metal is typically -200 mesh. All of the materials are blended in 
a non-stick container for approximately 1 minute to homogenize the 
powders. Next, an amount of warm (about 85.degree. C.) deionized water 
equal to about 1/3 the total dry weight of the powders is slowly added to 
the mixture to obtain a slurry type mixture. Stirring is continued for 
about 2 minutes. Mixing with the warm water partially dissolves these 
alkali materials and aids in mixing on an atomic scale. The nitrates and 
carbonates are at least partially soluble in warm water. The slurry is 
then heated in a 100.degree.-130.degree. C. oven until completely dry 
(experiments have shown about 5% of the water remains). The material is 
then crushed into a coarse, granular material between 20 to 40 mesh. Based 
on the selenium content of the mixture, the required amount of selenium 
may be added to the glass batch. The encapsulated selenium was added to a 
soda-lime-silica glass batch of the type shown in Table 1 in the amount 
required to provide an initial selenium concentration of 100 PPM in the 
glass batch. 
TABLE 1 
______________________________________ 
Material Weight 
______________________________________ 
Sand - SiO.sub.2 1000 
Soda Ash - Na.sub.2 CO.sub.3 
310 
Limestone - CaCO.sub.3 
86.7 
Dolomite - MgCO.sub.3 CaCO.sub.3 
246.6 
Salt Cake - Na.sub.2 SO.sub.4 
5.0 
Niter - NaNO.sub.3 15.0 
Rouge - Fe.sub.2 O.sub.3 
2.28 
blast furnace slag 10.6 
CO.sub.3 O.sub.4 0.053 
______________________________________ 
The selenium retention in PPM (and the corresponding retained percentage) 
for each of the compositions is as follows: Composition A--26, 
Composition B--31, Composition C--29, Composition D--36 and Composition 
E--38. As can be seen, those encapsulants having a lower liquidus 
temperature, due in part to the greater proportion of nitrates, and having 
greater fluidity, due in part to the lower proportion of silica, provide 
the greatest selenium retention in the glass batch. 
In using the low silica, high alkali glass forming compositions of the 
present invention to encapsulate selenium, with other typical glass batch 
materials to form soda lime silica glasses it is recommended that the 
selenium be no more than 10% by weight of the raw materials used to make 
the encapsulated selenium, and preferably no more than 5%. It is believed 
that this limitation is dictated by the ability of the encapsulant to 
assimilate the selenium. 
It should be appreciated that the selenium will also be oxidized by other 
alkali materials in the encapsulating material. For example, 
EQU Ca(NO.sub.3).sub.2 +Se.fwdarw.CaSeO.sub.3 +NO.sub.x .uparw.(Eq. 3) 
EQU CaSeO.sub.3 +1/2O.sub.2 .fwdarw.CaSeO.sub.4 (Eq. 4) 
and 
EQU K.sub.2 CO.sub.3 +Se+O.sub.2 .fwdarw.K.sub.2 SeO.sub.3 +CO.sub.2 
.uparw.(Eq. 5) 
EQU K.sub.2 SeO.sub.3 +1/2O.sub.2 .fwdarw.K.sub.2 SeO.sub.4 (Eq. 6) 
However, the selenium will preferentially react with the Na (if present) as 
shown in Equations 1 and 2. 
It is also contemplated in the present invention that selenium oxides, such 
as Na.sub.2 SeO.sub.3 and CaSeO.sub.3, may be encapsulated by low silica, 
high alkali materials of the type described earlier. One advantage of this 
type of approach is that when the encapsulated material is added to the 
batch, there is no delay associated with the oxidation of the selenium. In 
addition, it reduces the potential of selenium loss through volatilization 
during oxidation, since the selenium will have already been oxidized. The 
oxidized selenium compounds are readily assimilated into the encapsulating 
material so that the selenium compounds properly diffuse throughout the 
glass batch.