Apparatus and method for removing gaseous inclusions from molten material

A process and apparatus for improving the removal of gaseous inclusions from molten materials is disclosed. The inclusions, e.g., seeds or bubbles, are removed by applying vibrations to a restricted zone for discharging molten material from the melting tank. The seeds are slowed during a "pile up" in the restricted channel and are more easily brought to the surface of the molten material.

TECHNICAL FIELD 
This invention relates to a process and apparatus for making 
heat-softenable materials using high frequency vibrations. 
BACKGROUND ART 
Previous efforts in the field of improving the removal of gaseous 
inclusions in molten materials such as molten glass having included the 
application of ultrasonics. For example, U.S. Pat. No. 2,635,388 issued on 
Apr. 23, 1953, discloses a process for glassmaking that includes the step 
of applying simultaneous heating and vibrating to a glass bath. Another 
patent assigned to Battelle Memorial Institute, U.S. Pat. No. 4,316,734 
issued on Feb. 23, 1982, discloses a method for removing bubbles from 
molten glass by applying low intensity sonic energy to remove bubbles from 
the molten glass without creating additional cavitation. The disclosure in 
the Battelle patent describes driving the growing bubbles away from the 
source of the sonic energy. Their research was carried out in a model at 
room temperature with acoustic horns in the floor of the model tank 
driving the growing bubbles toward the surface of the liquid. 
DISCLOSURE OF THE INVENTION 
We have developed a method and apparatus for making heat-softenable 
material using high frequency vibrations. After forming a pool of molten 
material in the tank of a melting furnace, we have found that applying the 
vibrations to the molten material in a restricted zone for discharging 
molten material from the tank improves the bubble removal. The "seeds" or 
inclusions are slowed down or "pile up" in the channel and are more easily 
brought to the surface of the molten material. While applying the 
vibrations to the molten material in the tank may be effective, we have 
found the convection currents in a large tank interfer with bringing the 
bubbles to the surface. By applying the high frequency energy to the 
molten glass in the channel, bubble removal is more effective possibly due 
to the fact that the molten material flows in only one direction in the 
channel. 
This invention pins the seeds against the unidirectional flow of molten 
material in the channel to coalesce them into larger bubbles that will 
bouyantly rise to the surface.

BEST MODE OF CARRYING OUT THE INVENTION 
The sonic probe is cylindrical rod of molybdenum that uses piezoelectric 
ceramics to generate acoustic waves. The probes are placed in glass 
furnace channel between the melting tank and the forehearths. 
The probes enter the channel through the refractory roof or sidewalls 
openings, into the combustion space and then into the glass through its 
surface. Since the probes pass through the combustion space, an oxidation 
protection sheath must surround the probe to protect it from oxidation. 
The current protective sheath is a platinum alloy tube with a high grade 
alumina sleeve insert. The probes will extend approximately 2" under the 
glass surface with the protective sheath approximately extending 1/2" 
under the glass surface. When the probes are powered, a spherical area, 
approximately 3" radius is effected around each probe. The effect is to 
push seeds away from the probe. Since the probe is positioned in the 
channel, seeds will push against the glass flow and collide with seeds 
moving into the sphere of influence. As the seeds collide they grow in 
size and become large enough to float to the glass surface and break. 
The top view in FIG. 1 shows a typical outline of a glass melting furnace 
including tank 10, forehearths 12 and channel 14. Sonic probes 16 are 
located in channel 14. A row of 3 probes 16 appears to be the best 
arrangement. 
FIG. 2 shows a close up of probe 16 in more detail. As shown, the equipment 
utilized for the imparting of sonic vibration into the glass included 
sonic probes, probe oxidation protection, and electronic power unit. Sonic 
probes 16 were 1 inch diameter molybdenum rods with piezoelectric ceramic 
disks 18 built into one end. Piezoelectric ceramic disks 18 were 
sandwiched between plate electrodes 24 that had a sine wave voltage being 
applied to them by the power unit 20. The piezoelectric ceramics were 
caused to vibrate by this electric signal, which sent vibrations down the 
molybdenum rod into the glass. Electrical power unit 20, simply provided 
the correct frequency and voltage input to probes 16. The oxidation 
protection system was a set of 11/2 inch diameter alloy tubes 22 held by 
an alloy manifold (not shown). Each tube extended down through the 
combustion space into the glass to form a purged protection for each 
molybdenum probe 16. 
In an embodiment not shown, probes 16 may enter the channel through the 
side walls above the surface of the molten material. In this embodiment 
probes 16 probably would be at an angle of 45.degree. and would still 
require protective sheath 22 since they would be passing through 
combustion space before entering the molten material. 
In another embodiment, probes 16 may be located elsewhere in channel 14 or 
in one of the forehearths 12. However, our experience has shown that 
probes 16 should be located before any branching of channel 14 takes 
place. 
The following work has been accomplished towards developing the use of 
sonic vibration to enhance the fining in glass. Trials were run to 
quantify the effect sonic vibration has on the seed level for a production 
furnace. 
For the trial, three probes 16 were placed through three predrilled 2 inch 
diameter evenly spaced holes in the inboard channel roof (not shown). The 
set of holes were downstream from the furnace skimmer. The electronic 
power units 20 were installed and lead wire (not shown) runs to the probe 
position. Compressed air was used by Vortex tranvectors (air horns) to 
blow air over the piezoelectric transducers for cooling. The alloy 
tube-manifold system was purged with nitrogen supplied from a bottle of 
compressed nitrogen. The alloy tubes 22 were in the glass approximately 
1/2 inch, and the molybdenun probe 16 extended 1/2 inch below the alloy 
tube. 
The probes were inserted in the inboard channel (as shown in FIG. 1). The 
first three days of operation were successful with lowered seed counts and 
no effect on forming performance. The attached tables give some specific 
results for the trial. The table summarizes the seed level by giving 
average values. The information on the first three days shows good 
operation with a close to 60 percent decrease in seeds. 
Since the data available on seed count with the sonic probe working 
properly before the protection system failure was only over 3 samplings, 
the sampling was continued. This additional sampling was necessary to 
determine the post trial seed count after the furnace returned to steady 
conditions to see if the seed level returned to pre-trial levels. The 
average values show the seed levels more than doubled after the trial and 
returned to levels very close to pre-trial seed levels. Therefore, the 
return to similar seed levels shows the reduction that occurred during 
sonic fining was not a fluctuation in seed level, but a real effect caused 
by the sonic probes. 
TABLE 
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FURNACE AVERAGE SEED COUNT 
Bushing 
Trial Phase A B C D 
______________________________________ 
Pre-Trial 67 -- 2 22 
During Trial 31 18 4.7 2.3 
Post Trial 54 61 2.1 12 
______________________________________ 
Industrial Applicability 
Typically, the viscosity of the molten material ranges from 50 to 1000 
poise with the above trials being carried out at 160 poise. The energy 
intensity of the sonic energy generally ranges from 10 to 100 watts per 
square centimeter with trials being carried out at 20 watts/cm.sup.2. The 
frequency of the sonic energy ranged from 30 to 36 kHz and the molten 
glass had a temperature of 1400.degree. C. More specifically, the horn was 
operated at a frequency of 33 kHz with 100 watts of input power to each 
horn. 
The above conditions were employed in a typical large horizontal glass 
melting furnace for the continuous manufacture of glass fibers. The glass 
undergoes the stages of being prepared by melting batch materials, which 
float on the molten glass at one end of the furnace, fining the molten 
glass in a succeeding zone, which in certain cases may be isolated from 
the first zone by a wall, and conditioning, quieting and cooling the 
molten glass to a temperature suitable for manufacture into glass products 
in a conditioning or working zone, which may also be substantially 
isolated. These furnaces may be gas fired or electric melt furnaces. 
A common system for producing glass filaments or fibers includes a furnace 
having forehearths extending therefrom through which molten glass in the 
furnace is carried to a plurality of spaced apart bushings or other 
devices, such as spinners, located along the bottom wall of the 
forehearths. The forehearths can extend directly from the furnace or can 
extend as branches from one or more main channels carrying the molten 
glass from the furnace. The glass from each forehearth flows through 
openings by gravity into the bushings therebelow with molten glass streams 
from the bushings or spinners being formed into glass filaments or fibers. 
Glass fibers used in the practice of this invention can be "E" glass 
fibers, well known to those skilled in the art; such fibers are described 
in U.S. Pat. No. 2,334,961. 
Strands of glass fibers are produced by pulling several hundred or more 
tiny molten streams of glass which issue from holes in the bottom of a 
molten glass tank over a size applying apron to a gathering shoe which 
groups the fibers together into a strand. This strand then proceeds to a 
traverse mechanism and winding drum which provides the pulling action 
which attenuates the molten glass and coils the strand into a package. The 
fibers are individually separated at the time that they pass over the size 
applicator, so that the surfaces of the fibers are substantially 
completely coated before they are drawn together into a strand. This size 
acts as a lubricant which separates the individual filaments, and if the 
filaments are not separate by the size, they will scratch each other and 
break as they are flexed and drawn over guide eyes in the subsequent 
twisting, meaning and finishing operations.