Note: Descriptions are shown in the official language in which they were submitted.
3313
BACKGROUND OF T~E INVENTION
In Canadian Patent Application No. 233,203 filed
August 11, 1975 and entitled "Method and Apparatus for the ~anu-
facture of Glass", a rapid process of melting and refining glass
is described in which a vitrifiable material is melted and brought
to an elevated temperature while maintaining the viscosity of the
molten mass at less than 1000 poises. As soon as the melting has
been achieved, an intense foaming of the molten mass is effected
throughout its entire thickness while keeping the viscosity at a
value less than 1000 poises. The rate of expansion of the mass
is at least 1.5 (preferably between 2 and 3). After the foaming
subsides, a perfectly refined glass is collected.
According to the process disclosed in said copending
application, the foaming operation is performed in a channel in
which the molten material progresses, without back currents, from
a first location where the raw vitreous material is received from
a premelting apparatus to a second location where the refined
glass is recovered.
To ensure the intense and complete foaming required,
a number of steps may be taken. For example, foaming agents can
be incorporated into the raw materials. The foaming agents give
rise, in the temperature range, corresponding to the desired
viscosities, to the formation of gas bubbles inside the glass.
The gases produced by the foaming agents are soluble in glass,
and preferably their solubility inthe molten glass increases as
its temperature decreases. It is also recommended that a refining
agent be present, at least in the final phàse. After the elimi-
nation of most of the gases, the refining agents aid in the
readsorption of the bubbles which remain on cooling. The foaming -
agents are selected such that they do not induce foaming of the
~'" ~
1088~11J
- I .
1 ¦vitreous material until that material has reached a desired
2 ¦temperature, which temperature is maintained in the refining
3 ¦channel. The following foaming agents are useful in the process
¦disclosed in said copending application: arsenic compounds, such
5 ¦as arsenic trioxide; antimony compounds such as antimony trioxide;
6 ¦sulfur compounds, such as sodium sulfate; and halogen salts such
7 ¦as potassium chloride. Other agents useful in the process will
¦ be apparent to those skilled in the art.
91 Another method disclosed in said copending application
10¦ for ensuring the thorough foaming of the molten mass involves
11¦ subjecting the batch to rapid uniform heating during the foaming
12¦ operation of about 20C per minute or more.
13¦ In a discontinuous melting installation, the heating
14 means are employed at a time when the vitreous batch contains a
large number of solid or gaseous nuclei and a sufficient amount
16 of foaming agents to ensure an expansion of at least 1.5, and
17 preferably above 2 times the normal volume of the mass in the
18 unfoamed molten state.
19 In a continuous melting installation similar heating
means can be employed. The predefined time sequence corresponds
21 to the rate of treatment of the vitreous mass.
22 To aid the foaming process, it is also recommended
23 that the raw materials contain a large number of nuclei, such as
24 unmelted particules or small gas bubbles, capable of inducing
the foaming. These nuclei essentially act as nucleation sites.
26 The nuclei should be distributed throughout the molten mass at a
27 concentration of at least 10 nuclei per cc. Generally , it is
desirable that the raw materials be agglomerated. The agglomer~
29 makes it possible to preheat the materials before actual melting.
The preheating is accomplished by a brief and intense heat
'~ ~088313 ~
transfer (less than 10 minutes) while simultaneously keeping the
temperature of the materials below the foaming temperature. This
permits the maintenance of a high number of nuclei consisting of
unmelted particles and gas bubbles in the vitreous mass introduced
into the total foaming stage.
To assure the presence of sufficient nuclei, outside
nuclei, for example, cullet or colored cullet can be added to the
raw materials. In relation to the usual glass refining processes,
the process disclosed in said copending application, requiring the
presence of gas producing agents and foaming nuclei, can employ
unrefined vitreous materials. It has been discovered that 1 to 2
mm. grains originating from the limestone and dolomite in the
material introduced in the refining tank, are totally digested
at the end of the total foaming phase. The process is therefore -
not dependent on the use of a vitreous batch of high quality.
The channel in which the molten mass flows can be of very
simple geometry. Preferably, it has a slight width in relation --
to its length, in a ratio of 1:5 at least. This construction
limits-undesirable back currents. Also, for this same purpose,
it is possible to use baffles, barriers, bottlenecks or even
cascades along the path traveled by the vitreous molten mass
during treatment in the channel of the refining apparatus.
SU.~ARY OF THE PRESENT IWVENTION
According to the present invention, the channel of the
refining apparatus is constructed to increase the homogeneity of
the foaming of the molten mass throughout its entire thickness.
The construction of the channel also enhances the uniform flow
of the molten mass. More particularly, the apparatus includes -
an elongated continuous flow refining channel in which a molten
vitreous mass is introduced at one end thereof and flowed
--4--
~3313
horizontally therethrough to an exit at its other end. A
localized foaming zone extends along a predetermined length of
said channel and is spaced from the one end thereof. The foaming
zone has a width appreciably wider than the channel upstream
thereof. Heating means are submerged along the length of the
channel for subjecting the flowing molten mass in the foaming
zone to an expansion of at least 50% of its initial molten volume
by foaming the flowing molten mass in the foaming zone throughout
its entire volume as the mass moves along the channel from its
one end to the other end.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 is a plan view of the channel constructed in
accordance with the present invention; and
Fig. 2 is a cross-sectional view of the channel, in the
widened or foaming zone as taken along lines II-II of Fig. 1
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
In Fig. 1, the direction of the arrow indicates the
direction of flow of the molten mass between electrodes El. The
channel walls diverge at 1 to define the entrance of the foaming
or widened zone 2. The electrodes E2 in the foaming zone are
placed at a distance apart greater than the width of the channel
upstream. Thus, the totality of the molten glass mass that comes
from there enters between electrodes E2. The length of the widened
foaming zone corresponds to the period of the intense foaming phase
disclosed in the above-mentioned copending application. The
dissipation of energy by Joule effect is produced within the
vitreous mass itself to control the temperature of the mass all
along the channel.
The wall of the foaming zone is moderately heat
insulated to maintain its temperature at a rather low level (on
, ,
10~3313
the order of 1350C), whereas the glass in foam state between
electrodes E2 is around 1550C. From this important heat
gradient, there results, around each electrode E2, a notable
convection current, helicoidal in shape in the direction indi-
cated by the arrows represented in Fig. 2. This causes an
intense mixing of the molten glass mass particularly favorable
to its refining.
The glass has free passage around the electrodes along
the hearth and side walls. Passage of the current from one
electrode to the other produces active thermal convection which
favors the crosswise homogenization of the molten mass and
eliminates any major longitudinal currents. The result is a
uniform flow of glass called a "piston" flow. In Fig. 2, the
level of molten mass before foaming is shown by the broken lines
while that of the foam is shown by the solid lines.
By way of example, an embodiment of the invention for
refining glass at a production capacity on the order of 120 to 250
kg/hour, for the usual silica-soda-lime glass, is given below.
The walls and hearth of the channel are made up of
blocks of electro-melted refractory 3 with a base of alumina and
zirconia about 10 cm. thick, heat insulated by a lining 4 of
refractory bricks. To obtain a moderate heat insulation in the
widened foaming zone 2, a thinner lining thickness 4 is used.
The hearth 5 of the channel is level on its entire surface.
The depth of the channel is 25 cm., uniformly over
its entire length, which totals 2.5 m. The narrow upstream zone,
in which receipt of the premelted mass occurs, is 30 cm. wide
and 40 cm. long. The two electrodes El in this zone are made of
molybdenum rods 40 mm. in diameter and 40 cm. long. They are
placed symmetrically and 150 mm. apart.
~ .
.
108~i313
1 ¦ After this upstream zone, the widened foaming zone
2 lextends for a total length of 80 cm. It inciudes the entrance 1
3 ¦where the walls diverge over a length of 15 cm. This increases
4 ¦the width of the channel from 30 to 60 cm., this latter value
5 ¦being maintained over a length of 50 cm. The channel then com-
6 ¦prises a narrowing portion 6 where, over a length of 15 cm., the
¦walls converge to reduce the width of the channel from 60 to 30
8 ¦cm. This reduced width is then maintained over the downstream
9¦ zone, for a length of 1.3 m., to the drawing off orifice 7, whose
10¦ output is controlled by a needle system, not shown.
11¦ Electrodes E2 in the widened foaming zone are made of
12¦ cylindrical molybdenum rods 40 mm. in diameter and are 70 cm.
13¦ long. The pairs of electordes E3, E4 and E5 in the downstream
14¦ zone of the channel are of the same diameter (40 mm.) and are 30
15 cm. long. For the areas where the molybdenum is in contact with
16 the molten mass, even in the upstream zone and in the widened
17 foaming zone where the mass is charged with bubbles of various
18 gases, it has been found tha~ with the usual compositions of
19 silica-soda-lime glass, no particular precautions need be taken
20 for the protecting of this metal from oxidation.
21 The current lead-ins 8 to the electrodes are also moly-
22 bdenum rods, but their diameters are only 25 mm. Assembly of the
23 lead-in and electrode is accomplished by screwing of one into the
24 other. In the areas where there is a danger of oxidation of the
25 molybdenum of the lead-ins, they are protected,-as is known, by
26 a reducing gas such as town gas. The connecting clamps to the
27 electric supply are cooled by circulation of liquid. The lead-ins
28 can slide in passages 8a which are made through the walls of the
29 channel. The height of/these passages is 5 cm. above the level of
l~ 30 the hearth except for electrodes E2 whose height is 2 cm. greater.
. .
. ~ 108Ei31;~ ~:
¦ Between the pair of electrodes E4 and E5, a barrier of .-
2¦ refractory material or platinum is placed to provide a passage of
31 adjustable height between its lower part and the hearth 5. The
41 barrier blocks possible surface currents; and is slidably mounted
i 51 in guides 10 in the lateral walls of the channel to control the
61 flow of molten mass.
71 Heating of the molten mass contained in the channel is .
81 assured by means of the immersed electrodes, previously described,
91 with independent electric power supply for each pair of electrodes
10¦ An example of the rated electrical characteristics of the power .
11¦ supply is as follows:
12¦ Power Current
13¦ Powers Used C ~ ~ Amperage
14¦ electrodes El 40 80 500
15¦ electrodes E2 40 80 500
16¦ -electrodes E3 7.5 120 62.5
17¦ .electrodes E4 7.5 120 62.5
18¦ electrodes E5 15 120 125
19¦ The above-described construction and power supply permit~ ; :~
20¦ refining of about 150 kg/hour of silica-soda-lime glass under ¦
. 21¦ operating conditions shown in the following table, the temperatures
22¦ being those indicated by pyrométers going into the molten mass at
231 points indicated Tl to T5 in Fig. 1:
24 .
251Powers Used Values Temperatures Measured Values
26¦electrodes El 15 Point of Measurement Tl 1300
271electrodes E2 22 Point of Measurement T2 1480
28¦electrodes E3 1 Point of Measurement T3 1540
29¦electrodes E4 1 Point of Measurement T4 1400
30 ¦electrodes E5 0 Point of Measurement Ts 1250
88J13
1 The operating conditions correspond to a supply of
2 premelted paste delivered at about 1350C by a melting apparatus
3 of the type disclosed in the above-mentioned copending application
in which the following vitrifiable mixture (in kg. per 100 kg. of
5 glass) is introduced in the form of agglomerates.
6 sand 67.0
7 limestone 9.47
8 dolomite 16.2
9 feldspar 6.13
sodium carbonate 7.58
11 50% caustic soda 22.5
12 sodium sulfate 1.0
13 The minimum level of the unexpanded molten glass mass
14 should be on the order of 10 cm. to cover, and therefore protect
15 from oxidation, the totality of the various pairs of electrodes
16 disposed along the channel, even if the rate of expansion is
17 small. In practice, in the installation described, the rate of
expansion of about2 permits optimal functioning, while leaving a
19 safety space of 5 cm. above the molten mass.
Devices with greater production capacity can be made in
21 a way similar to the above-described embodiment. Appropriate
22 electrical heating means must be provided in relation to the
23 contemplated output, i.e., heating means with a capacity to assure
24 an elevation of temperature of the vitreous mass of at least 20C/
25 minute-at the level of the widened foaming zone. Also, if the ,
26 thickness of the molten glass mass is increased, the electrodes
27 are still kept close to the hearth, as indicated above, so that
28 the heat will directly affect the deepest layers of the mass to
29 be treated. Thus, unwanted currents of longitudinal convection
30 are reduced, to the benefit of the quality of the refining. For
1088313
1 similar reasons, particular care is given to heat insulation of
2 the hearth while the arch and walls are, optionally, as stated
3 above, slightly less heat insulated to favor transverse convection -.
5~movements
19 . .
21 .
22 .
231
26
28 . .
29
.~. : - . . . ~ .
, . . . .: . :, . , : .
.
