U.S. patent application number 10/537702 was filed with the patent office on 2006-07-06 for method and apparatus for heating melts.
Invention is credited to Frank-Jurgen Druschke, Rainer Eichholz, Holger Hunnius, Frank-Thomas Lentes, Volker Ohmstede, Guido Rake, Hildegard Romer, Ernst-Walter Schafer, Jorg Schollmayer, Thomas Stelle, Gunter Weidmann.
Application Number | 20060144089 10/537702 |
Document ID | / |
Family ID | 32471497 |
Filed Date | 2006-07-06 |
United States Patent
Application |
20060144089 |
Kind Code |
A1 |
Eichholz; Rainer ; et
al. |
July 6, 2006 |
Method and apparatus for heating melts
Abstract
A method and an apparatus for heating a melt in a melting vessel
with cooled walls is provided. The melt is heated conductively by
current flowing between at least two cooled electrodes, which each
replace part of the wall of the melting vessel.
Inventors: |
Eichholz; Rainer; (Duisburg,
DE) ; Rake; Guido; (Pfaffen-Schwabenheim, DE)
; Ohmstede; Volker; (Bingen, DE) ; Weidmann;
Gunter; (Flonheim, DE) ; Lentes; Frank-Thomas;
(Bingen, DE) ; Stelle; Thomas; (Mainz, DE)
; Schafer; Ernst-Walter; (Welgesheim, DE) ; Romer;
Hildegard; (Foarsheinn, DE) ; Schollmayer; Jorg;
(Mainz, DE) ; Hunnius; Holger; (Mainz, DE)
; Druschke; Frank-Jurgen; (Mainz, DE) |
Correspondence
Address: |
OHLANDT, GREELEY, RUGGIERO & PERLE, LLP
ONE LANDMARK SQUARE, 10TH FLOOR
STAMFORD
CT
06901
US
|
Family ID: |
32471497 |
Appl. No.: |
10/537702 |
Filed: |
November 27, 2003 |
PCT Filed: |
November 27, 2003 |
PCT NO: |
PCT/EP03/13353 |
371 Date: |
February 17, 2006 |
Current U.S.
Class: |
65/29.21 ;
65/135.7; 65/162; 65/DIG.4 |
Current CPC
Class: |
C03B 5/235 20130101;
C03B 5/027 20130101; C03B 5/42 20130101; C03B 2211/70 20130101;
C03B 5/44 20130101; C03B 5/03 20130101; H05B 3/03 20130101; Y02P
40/57 20151101 |
Class at
Publication: |
065/029.21 ;
065/135.7; 065/162; 065/DIG.004 |
International
Class: |
C03B 18/18 20060101
C03B018/18; C03B 5/027 20060101 C03B005/027; C03B 5/24 20060101
C03B005/24 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 2002 |
DE |
10256657.7 |
Dec 4, 2002 |
DE |
10256594.5 |
Claims
1. A method for heating a melt in a melting vessel with cooled
walls, comprising: conductively heating the melt by flowing current
between at least two cooled electrodes, wherein the at least two
cooled electrodes each replace part of the cooled walls of the
melting vessel so that a melt contact surface of the at least two
cooled electrodes forms a wall region of the melting vessel.
2. The method as claimed in claim 1, further comprising
conductively heating a region of the melt to a temperature that is
above a melting or decomposition temperature of at least one of the
at least two cooled electrodes.
3. The method as claimed in claim 1, further comprising cooling the
at least two cooled electrodes so that the at least two cooled
electrodes can be set and/or controlled separately.
4. The method as claimed in claim 1, wherein the at least two
cooled electrodes are inserted into cutouts in cooled walls of the
melting vessel.
5. The method as claimed in claim 1, further comprising passing at
least one cooling fluid through the at least two cooled electrodes
to effectuate cooling.
6. The method as claimed in claim 5, wherein the at least one
cooling fluid comprises a gaseous cooling fluid.
7. The method as claimed in claim 6, further comprising passing the
gaseous cooling fluid through the at least two cooled electrodes
with a pressure difference of less than 1,000 mbar.
8. The method as claimed in claim 1, further comprising
additionally heating the melt by the introduction of radiant
energy.
9. The method as claimed in claim 8, wherein the melt is heated by
alternating current with an alternating current frequency in a
range from 50 Hz to 50 kHz.
10. The method as claimed in claim 1, further comprising keeping a
temperature of the cooled walls of the melting vessel and of the at
least two cooled electrodes below a temperature at which high
levels of corrosion occur.
11. The method as claimed in claim 1, further comprising keeping a
temperature of the melt above 1,600.degree. C.
12. The method as claimed in claim 1, further comprising keeping a
temperature of the melt-contact surface of the at least two cooled
electrodes below 1,650.degree. C.
13. The method as claimed in claim 1, further comprising
maintaining a temperature difference between the melt in an edge
region of the melting vessel and the melt in a central region of
the melting vessel to more than 150.degree. K.
14. The method as claimed in claim 1, wherein the melt at a melting
temperature has an electrical conductivity in a range from
10.sup.-3 to 10.sup.2.OMEGA..sup.-1*cm.sup.-1.
15. The method as claimed in claim 1, wherein the current that
emerges from the at least two cooled electrodes into the melt does
not exceed a current density of 5 A/cm.sup.2.
16. The method as claimed in claim 1, further comprising
continuously supplying a melting material to the melting vessel and
continuously discharging the melt from the melting vessel.
17. The method as claimed in claim 16, wherein the melting material
is supplied in molten form via an inlet and the melt is discharged
in molten form via an outlet.
18. The method as claimed in claim 16, wherein the current flows
between the at least two cooled electrodes substantially along a
main direction of flow of the melt or perpendicular with respect
thereto.
19. The method as claimed in claim 18, further comprising setting a
temperature difference of more than 150.degree. K. between the
melt-contact surface of the at least two cooled electrodes and a
region of the melt located substantially centrally between the at
least two cooled electrodes.
20. The method as claimed in claim 17, wherein the inlet supplies
the melting material and the outlet discharges the melt at a
surface of the melt.
21. The method as claimed in claim 1, further comprising heating at
least one electrode of the at least two cooled electrodes at least
from time to time.
22. The method as claimed in claim 21, wherein the heating of the
at least one electrode is effected by transverse application of
current to the melt-contact surface of the at least one
electrode.
23. The method as claimed in claim 1, further comprising providing
a melt path of sufficient electrical conductivity between the at
least two cooled electrodes in the melting vessel during a starting
operation.
24. The method as claimed in claim 23, further comprising heating
the at least two cooled electrodes and/or parts of the cooled walls
during the starting operation to a sufficient temperature for their
temperature to be above a dew point of the melting vessel.
25. The method as claimed in claim 23, further comprising
introducing starting electrodes into the melting vessel and passing
a current through the melting material via the starting electrodes
so that the melting material is melted down.
26. The method as claimed in claim 25, further comprising moving
the starting electrodes away from one another during the starting
operation.
27. The method as claimed in claim 23, further comprising
converting from a melt with a higher electrical conductivity to a
melt with a lower electrical conductivity.
28. The method as claimed in wherein claim 25, further comprising
pushing the staring electrodes together before the starting
operation and pulling the starting electrodes apart during the
starting operation.
29. The method as claimed in claim 23, further comprising feeding
radiant energy to the melting material in order for the latter to
be melted down during the starting operation.
30. An apparatus for the heating of melts, comprising: a melting
vessel with cooled walls for receiving a melting material, and at
least two electrodes for conductively heating a melt, wherein the
at least two electrodes each have a melt contact surface that
replaces part of the cooled walls of the melting vessel.
31-71. (canceled)
Description
[0001] The invention relates to a method and an apparatus for
heating melts, in particular to a method and apparatus for
conductively heating melts.
[0002] The melting of glasses or glass-ceramics in conventional
tanks that are known from the prior art is generally restricted to
melting temperatures of 1 600.degree. C. to at most 1 650.degree.
C. The service life of the tanks is very greatly restricted even at
melting temperatures of 1 700.degree. C.
[0003] The heating of the glass melt in conventional tanks is
usually effected by oil or gas burners located in the top part of
the furnace. In this case, the heat is introduced into the glass
via the surface of the glass. In the case of strongly colored
glasses, the absorption is so high just in the region of the
surface of the glass that only relatively thin glass layers are
heated through. In such cases, additional electrical heating is
often provided by electrodes which are introduced through the base
of the tank.
[0004] In the case of the electrically heated tanks, the glass melt
is heated conductively using alternating current by electrodes,
i.e. the glass melt is heated directly. The electrodes are
introduced into the glass melt either through the tank base or
through the side walls of the tank and are surrounded by the glass
melt on all sides.
[0005] The electrode material used is often molybdenum or platinum.
The Mo electrodes have a very strong tendency to be oxidized, and
therefore should not generally come into contact with air. Glass
melts containing redox elements, such as for example
Sb.sub.2O.sub.5 and AS.sub.2O.sub.5, can also attack the Mo or Pt
electrodes.
[0006] Pt electrodes are significantly more resistant to oxidation
but can only be used stably for a long time up to temperatures of 1
500.degree. C., or for a short time up to 1 650.degree. C.
[0007] Patents GB 644,463 and DE 100 05 821 have disclosed rod
electrodes cooled by water alone. However, on account of the
maximum current which can be applied per unit area of the
electrodes, only a limited amount of power can be supplied using
cooled rod electrodes. Heating of a melting unit with strongly
cooled walls is only possible to a very restricted extent using an
electrode of this type, since it is impossible to introduce higher
power densities.
[0008] Electrodes with larger surfaces--known as plate
electrodes--are described, inter alia, in patents SU 1016259 or DE
2705618. Electrodes of this type have the advantage that they can
be exposed to higher current loads on account of the large
electrode surface area. However, these plate electrodes are not
cooled, and consequently in this case too the maximum melting
temperature which can be reached is restricted to the application
limit temperature of the electrode material. Although in SU 1016259
the walls of the melting unit are made cooler than the temperature
in the center of the melting unit by virtue of the positioning of
the electrodes in the interior of the melting unit, the electrodes
are at the same temperature as the melt and are therefore the
limiting factor for the maximum temperature which can be reached in
the melt.
[0009] A similar approach is described in patents U.S. Pat. No.
5,961,686 and U.S. Pat. No. 6,044,667, in which only the inner
region of the melting unit is heated using cooled top electrodes.
The walls are additionally water-cooled. This arrangement allows
the wall temperature to be kept at temperatures below 1 482.degree.
C., in order to avoid extensive corrosion. However, the maximum
melting temperature is limited to 1 788.degree. C. by the
electrodes being positioned in the hottest zone. The structure of
the unit has the crucial drawback that only a small part of the
total melting volume can be used for melting at the high
temperatures. Furthermore, the unit is not suitable for refining,
on account of the surface being covered by batch.
[0010] To allow the melting of glasses, glass-ceramics, ceramics or
crystals at over 1 700.degree. C., on the one hand the walls of the
melting units and the electrodes located in the melt have to be
cooled, and on the other hand more energy has to be supplied to the
melt than is withdrawn from the melt through the cooled walls and
electrodes.
[0011] The patent literature describes melting using radiofrequency
in skull crucibles for the melting of high-melting glasses or
crystals. For example, DE 199 39 779 describes the continuous
melting of high-melting glasses using a radiofrequency-heated skull
crucible.
[0012] In this context, the term skull crucible is to be understood
as meaning a vessel whereof the walls comprise water-cooled metal
tubes arranged close together. The sealing of the crucible is
ensured by the melt freezing in the immediate vicinity of the
tubes. This makes it possible to dispense with the need for
refractory material.
[0013] The skull crucible is surrounded by a radiofrequency coil.
Between the metallic tubes there has to be space, so that the
radiofrequency can be introduced into the melt. The glass melt is
heated directly with the aid of the radiofrequency.
[0014] The advantages of melting using radiofrequency in a skull
crucible consists in the fact that glass melts can be heated even
to temperatures of over 1 700.degree. C.
[0015] The direct introduction of the radiofrequency into the melt
allows the melt to be cooler in the edge region of the melting unit
than in the center. The cooled metal tubes cause a skull layer of
material of the same type as the melt to form, and this skull layer
can be constantly renewed. Consequently, even high-melting or very
aggressive glasses can be melted down and refined successfully.
[0016] A further advantage of melting using radiofrequency consists
in the fact that other refining agents, known as high-temperature
refining agents, can also be used for refining at high
temperatures. This makes it possible for example, as described in
DE 19939771, to dispense with the use of toxic refining agents,
such as arsenic oxide or antimony oxide.
[0017] However, melting using radiofrequency has the drawback that
the glasses, glass-ceramics, ceramics or crystals to be melted have
to have a sufficiently high electrical conductivity at the melting
temperature. The electrical conductivity of the melt must be high
enough for the energy introduced by the radiofrequency to be
greater than the thermal energy dissipated via the walls and in
particular via the skull walls. Although the limit on the
electrical conductivity required also depends on a range of
apparatus parameters, in practice it has been found that the
electrical conductivity of the melt should be above
10.sup.-1.OMEGA..sup.-1 cm.sup.-1.
[0018] The electrical conductivity of glasses and glass-ceramics is
generally determined to a very significant extent by the alkali
metal content and only to a lesser extent by the alkaline-earth
metal contents of these glasses.
[0019] In practice, however, it has emerged that in particular the
high-melting glasses, for which radiofrequency melting in the skull
crucible would be particular suitable on account of the high
temperatures, in most cases in fact have an electrical conductivity
of below 10.sup.-1.OMEGA..sup.-1 cm.sup.-1, which is too low.
Therefore, a range of important technical glasses cannot be
processed using the radiofrequency melting technique.
[0020] These glasses include, for example, the glasses with a high
thermal stability and a high resistance to temperature changes,
which are required, for example, for pharmaceutical packaging
applications, lamps that can be exposed to high temperatures and
fireproof glasses. Glasses of this type have to have both a high
transformation temperature and a low linear thermal expansion,
these glasses generally having a high melting temperature and a low
electrical conductivity.
[0021] A further group of glasses, such as for example display
glasses, have to be coated during further processing. In this case,
it is undesirable for the glasses to contain alkali metals, such
alkali metals can easily diffuse out of the glasses and in this
way, for example, enter the functional layers of the display. On
account of the low or absence alkali metal content, these glasses
likewise have an electrical conductivity which is too low to be,
for example, sufficiently coupled to radiofrequency.
[0022] Accordingly, there is a demand for a method and an apparatus
which make it possible to melt glasses, glass-ceramics, ceramics
and crystals even at high temperatures, for example above 1
600.degree. C., preferably above 1 700.degree. C., while the
glasses, glass-ceramics, ceramics and crystals may also have an
electrical conductivity lower than 10.sup.-1.OMEGA..sup.-1
cm.sup.-1.
[0023] With the method and apparatus, on the one hand it should be
possible to sufficiently cool the walls of the melting unit to
prevent the melt from chemically attacking the walls, and on the
other hand to supply the melt with more energy than is removed from
it through the cooled walls.
[0024] According to the invention, this object is achieved by a
method having the features of claim 1 and an apparatus having the
features of claim 30.
[0025] Accordingly, the invention provides a method for heating a
melt in a melting vessel with cooled walls, the melt being
conductively heated, and the current flowing between at least two
cooled electrodes, wherein the electrodes each replace part of the
wall of the melting vessel. In the context of the invention, this
is also to be understood as meaning that during introduction or
insertion of electrodes into a melting vessel of predetermined
geometry, the sum of the surface areas of melting vessel and
electrodes in the region of the melt remains constant. For example,
if a defined geometry of the melting vessel is selected, the
electrodes take up part of the walls of the melting vessel, with
the selected geometry being retained. By contrast, hitherto the
wall surface area has been increased, for example by the additional
introduction of finger electrodes, so that the cooling power also
increases accordingly.
[0026] For this purpose, the electrodes may advantageously be
inserted into cutouts in the cooled walls of the melting vessel.
The arrangement according to the invention creates a favorable
ratio of the surface proportion of the melting vessel through which
energy is introduced into the melt to the surface fraction formed
by cooled walls. This also makes it possible, by suitably setting
or controlling the cooling power, to heat at least a region of the
melt by the current to a temperature which may be significantly
above the temperature of the surface of the melt-contact material.
In particular, in the melt it is possible to reach a temperature
which is above the application limit temperature, such as in
particular above the melting or decomposition temperature of the
melt-contact material of at least one of the electrodes or of the
wall material. The decomposition is to be understood in this
context in particular as meaning chemical decomposition, such as
corrosive attack, sublimation or evaporation.
[0027] Application limit temperatures for different melt-contact
materials are known, inter alia, from the publications [0028] [1]
Johnson Matthey Noble Metals: "Platinum Sheet Material for the
Glass Industry", [0029] [2] Glass Science and Technology 13:
"Metals in Glassmaking", Roland Kirsch (Ed.), Elsevier, Amsterdam,
London, New York, Tokyo, 1993, [0030] [3] E. Drost, H. Golitzer, M.
Poniatowski, S. Zeuner: "Platinwerkstoffe fur
Hochtemperatur-Einsatz" [Platinum materials for high-temperature
use], Metall--Internationale Zeitschrift fur Technik und Wirtschaft
No. 7/8 1996, pages 492-498, Metallverlag Berlin/Heidelberg 7/8
1996, and [0031] [4] "Precious Metals Science and Technology": L.
S. Benner, T. Suzuki, K. Meguro, S. Tanaka (Eds.), The
International Precious Metals Institute, USA, 1991, the content of
disclosure of which in this respect is hereby incorporated in the
subject matter of the present invention.
[0032] In this context, the application limit temperature of
melt-contact materials is determined, inter alia, by the chemical
composition, the grain growth, the resistance to oxidation, the
resistance to corrosion in the melt, the hot strength, the creep
rupture strength, the creep rate, the duration of use and the type
of heating. For pure platinum, despite the good chemical stability,
a maximum long-term application temperature of 1 400.degree. C. is
stipulated on account of the low hot strength (cf. in this respect,
by way of example, publication [1]). In many cases, as is known,
inter alia, from publications [2] and [4], platinum can also
reliably be used up to 1 500.degree. C.
[0033] In order, for example, to achieve particularly rapid and
effective refining of the melting material, it is expedient if the
temperature of the melt is kept at least in a range above 1
600.degree. C., preferably above 1 700.degree. C.
[0034] By virtue of the electrodes being arranged in the melting
vessel in accordance with the invention, it is also possible to use
a material such as platinum as melt-contact material, which
otherwise could not generally be used for a prolonged period for
temperatures above 1 600.degree. C. on account of the ability of
this material to withstand thermal loads.
[0035] The temperature of the melt-contact surface of the
electrodes, in particular of electrodes comprising platinum as
melt-contact material, is preferably restricted to at most 1
500.degree. C. This prevents electrode material which influences
the properties of the melt material, such as for example the color,
from migrating into the melt in relatively large quantities.
Moreover, this considerably lengthens the service life of the
electrode.
[0036] Adding other platinum group metals to Pt, such as for
example Rh, Ir, or Ru, to the alloy in individual cases also allows
even higher hot strengths to be achieved, as disclosed, for
example, in publications [1], [2] or [3]. However, crystal growth
generally commences at temperatures higher than 60% of the melting
temperature T.sub.s, which leads to a reduction in the hot strength
and therefore limits the application limit temperature for PtRh10
to 1 550.degree. C., for PtRh20 to 1 650.degree. C. and for PtRh40
to at most 1 700.degree. C. Moreover, PtRh alloys are generally
ruled out if it is unacceptable for the glass to be colored by
rhodium.
[0037] The use of fine-grain-stabilized platinum or platinum alloys
of this type, such as for example PtRh10, allows application limit
temperatures of 1 500.degree. C. or 1 650.degree. C., since the
coarse grain formation in this case only begins at temperatures of
use higher than 85% of the melting temperature T.sub.s (cf. in this
respect also publications [1], [2] and [3]). Fine-grain-stabilized
materials, such as the fine-grain-stabilized platinum mentioned
above, are also known as dispersion-strengthened materials or
oxide-dispersion-hardened materials.
[0038] Moreover, the application limit temperature may be limited
by the chemical stability of the melt-contact materials, which
depends, inter alia, on the presence of polyvalent elements in the
melt. These often form low-melting alloys with the melt-contact
materials, which can considerably reduce the application limit
temperature. If there are no intended additions of polyvalent
elements, for example in the form of refining agents, impurities
are generally still present in the glasses.
[0039] In the case of ceramic materials, application limit
temperatures may result inter alia from chemical decomposition in
contact with a glass melt. For example, fused-cast ceramic
materials, such as high zirconium-containing ceramic material
(HZFC) or AZS (aluminum-zirconia silica) can generally only be used
as melt-contact material up to at most 1 650.degree. C.
[0040] The application limit temperature may also be reduced
through oxidation. In particular when using iridium, the sulfur
content in the glass also has to be taken into account, since the
formation of IrS.sub.2 may considerably restrict the use of
[0041] Mo can usually be used in glass melts at up to 1 700.degree.
C. With Mo, however, spontaneous grain growth commences between 1
600.degree. C. and 1 800.degree. C., which is associated with a
considerable drop in the strength, so that this may produce an
application limit temperature within this temperature range. A
similar statement applies to W. In this case, total
recrystallization has concluded at just 1 500.degree. C. The
application limit temperature for Mo and W is, however, determined
to a lesser extent by the mechanical properties at high
temperatures than by the chemical stability. Mo and W oxidize in
air or oxygen at 400.degree. C. It is also known from publication
[2] that polyvalent compounds dissolved in the glass likewise lead
to the formation of MoO.sub.3 and/or WO.sub.3. Mo reacts with
SiO.sub.2 at 1 650.degree. C. and with Al.sub.2O.sub.3 at 1
700.degree. C., to form MoO.sub.3. As a result, above these
temperatures undesirable dissolution and migration of molybdenum
into the melt may occur, and consequently application limits are
likewise reached at these temperatures.
[0042] If Sb.sub.2O.sub.3 is present in the glass as a refining
agent, Mo.sub.3Sb.sub.7 may be formed beyond 600.degree. C. to
900.degree. C., and the application limit temperature of Mo may be
reduced considerably on account of the formation of a liquid phase
and associated destruction of the component. As.sub.2O.sub.3
likewise reacts with Mo to form low-melting eutectics. An important
factor in the application limit temperature of Mo and W is
presented by molybdates and tungstates, which may be formed in
melts containing alkali metals. These compounds have lower melting
points than the pure oxides. Therefore, the application limit
temperatures may also depend on the alkali metal content of the
respective glasses.
[0043] The invention provides an apparatus and a method in which
the current density on the surface of the melt-contact material can
be kept at a particularly low level, so that there is little
heating of the melt-contact material. In the electrode according to
the invention, the total surface area, made up of the electrode
surface area and vessel surface area, is not increased compared to
conventional electrodes, since the electrode takes up part of the
vessel surface area. However, the proportion of the surface area of
the melting vessel which introduces energy into the melt does
increase. By contrast, with conventional electrodes, in order not
to exceed the maximum possible current density, the electrode
surface area had to be increased, which resulted in a larger total
surface area, made up of the electrode and vessel surface areas,
which in turn leads to an increased removal of heat.
[0044] An apparatus according to the invention for the heating of
melts, in particular for the high-temperature refining of melts, by
contrast, accordingly comprises
[0045] a melting vessel with cooled walls for receiving melting
material, and
[0046] at least two electrodes for the conductive heating of the
melt, which in each case replace part of the walls of the melting
vessel. By way of example, these electrodes can be inserted into
cutouts in the wall of the melting vessel.
[0047] The arrangement according to the invention of the electrodes
and the direct conductive heating of the melting material makes it
possible to set a temperature gradient in which at least a region
of the melt can be kept at a temperature which is significantly
higher than the temperature of the surface of the melt-contact
material, even, for example, higher than the application limit
temperature of the melt-contact material of the electrodes. It is
in this way possible in particular to accelerate refining
operations. For this purpose, the electrode surface area
advantageously takes up more than 5%, preferably more than 10%, and
particularly preferably more than 15%, of the wall surface of the
melting vessel. Such large-area electrodes keep the current density
and therefore the heating of the electrodes to a low level.
Furthermore, a homogeneous introduction of power over the entire
melt volume, without any dead volumes, is achieved.
[0048] The inventors have discovered that it is possible to set a
temperature gradient even in melts with an electrical conductivity
of lower than 10.sup.-1.OMEGA..sup.-1 cm.sup.-1. One advantage of
the method compared, for example, to inductive radiofrequency
heating therefore resides, inter alia, in the fact that
high-temperature refining, in particular with at least regions of
the melt at temperatures over 1 600.degree. C., preferably over 1
700.degree. C., can be carried out even in the case of melts with
an electrical conductivity of lower than 10.sup.-1.OMEGA..sup.-1
cm.sup.-1. An advantageous high-temperature refining process is
described, inter alia, in DE 199 39 771, the content of disclosure
of which in this respect is hereby also incorporated in the subject
matter of the present invention.
[0049] The required conductivity of the melt is subject to scarcely
any restrictions with regard to the feasibility of the method
according to the invention, since the operating voltage of the
electrodes can be adapted accordingly. At the melting temperature,
the electrical conductivity of the melt is preferably in a range
from 10.sup.-3 to 10.sup.2.OMEGA..sup.-1 cm.sup.-1, particularly
preferably in a range from 10.sup.-2 to 10.sup.1.OMEGA..sup.-1
cm.sup.-1.
[0050] The electrodes used for the apparatus according to the
invention for the heating of melts, in particular for the
high-temperature refining of melts, may particularly advantageously
also be heating apparatuses as described in the PCT application,
filed on the same date as the present invention, in the name of the
Applicant entitled "Heating apparatus with electrode for the
conductive heating of melts", the content of disclosure of which
with regard to the electrodes of the apparatus described here is
hereby also incorporated in its entirety in the subject matter of
the present application.
[0051] The structure of the electrodes can be divided into three
groups: each of the groups has to perform different roles.
[0052] The first group comprises the melt-contact material and the
electrical supply conductors.
[0053] The role of the melt-contact material is to feed the
electrical energy into the melt. One surface of the material is in
direct contact with the melt, and therefore the material has to be
as inert as possible with respect to the melt. Current can be
supplied from the outside via suitable terminal lugs.
[0054] The second group comprises the cooling. The role of the
cooling is in particular to set defined temperature profiles in the
materials of the electrode structure. The temperature profiles are
selected in such a manner that overheating and excessive corrosion
of the materials is ruled out. To enable the temperature to be set,
the electrode advantageously comprises at least one controllable
cooling circuit.
[0055] A preferred structure of the electrode consists in the
electrode being equipped with a double cooling system for two
different cooling media. The device for cooling the electrodes may
therefore advantageously comprise at least two cooling circuits,
which in particular can be set or controlled independently of one
another. The cooling circuits may advantageously be designed for
two different cooling media; in particular air, aerosols and water
are suitable cooling media. A further role of the cooling,
moreover, is to prevent medium from running out between cooled
walls and cooled electrodes.
[0056] The third group comprises the supporting structure. The
supporting structure serves to realize the electrical supply
conductors, the melt-contact material and the cooling in a
structure suitable for the electrode function. This primarily
involves ensuring the exchange of thermal energy between the
cooling and the melt-contact material. Suitable materials for the
thermal and mechanical stability have to be used. Ceramic
materials, inter alia, are suitable for this purpose.
[0057] The supporting structure may comprise one or a plurality of
different layers or parts, in order to ensure the exchange of
thermal energy. A suitable melt-contact material in particular for
glass melts is, for example, a material which comprises
electrically conductive ceramic, such as for example SnO.sub.2
ceramic and/or refractory metals, such as in particular platinum
metals, for example iridium, rhodium, platinum and their alloys or
high-melting refractory metals, such as tungsten, molybdenum,
tantalum, osmium, hafnium and their alloys. Moreover, the
melt-contact material may comprise a fine-grain-stabilized
material. These are generally distinguished by a high strength and
a good long-term stability. Fine-grain-stabilized materials of this
type may, for example, comprise high-strength platinum or iridium
materials.
[0058] At least one of the electrodes may advantageously also
include at least two electrode segments. The electrode segments or
electrode units are in this case preferably well electrically
insulated from one another, and the electrode units or segments are
preferably arranged at such a short distance from one another that
the melt cannot flow through the spaces between the electrodes.
[0059] Cooling the preferably large-area electrodes combined, at
the same time, with conductive heating can create in the melt a
temperature distribution whereby large regions of the melt are
significantly hotter than the melt-contact surface of the
electrodes. Even with active cooling of the electrodes, the
electrode arrangement and configuration according to the invention
allows more energy to be introduced into the melt than is
dissipated by the cooled electrodes and the further walls.
[0060] Accordingly, the apparatus according to the invention may
advantageously also comprise a device for cooling the electrodes,
in particular for cooling the melt-contact material of the
electrodes, in order, for example, to prevent the melt-contact
material from overheating.
[0061] In this context, it is also particularly advantageous if the
cooling is controlled, so that the electrodes are prevented from
overheating and the cooling power can be optimized for minimum
dissipation of heat.
[0062] The cooling is preferably effected by passing a cooling
fluid, such as in particular air and/or water, through the
electrodes. For this purpose, the apparatus according to the
invention may advantageously accordingly comprise a fluid-conveying
device. In this case, the cooling is particularly effected by
passing through a gaseous cooling fluid, such as for example air,
by means of a low-pressure blower. The coolant is advantageously
passed through the electrodes at a pressure difference of less than
1 000 mbar preferably than 500 mbar, particularly preferably less
than 150 mbar. The passages for routing the cooling fluid in the
electrodes are for this purpose dimensioned such that a sufficient
flow of coolant is achieved even at such a low pressure difference
built up by the low-pressure blower.
[0063] To allow a defined temperature gradient to be produced from
the hottest region of the melt to the melt-contact material of the
electrodes, the apparatus according to the invention preferably
also comprises a device for controlling the cooling power of the
electrodes. The electrodes integrated in the wall of the melting
vessel and the device for controlling the cooling power are
preferably designed in such a way that the temperature of the
melt-contact surface of the electrodes can be controlled very
accurately within a wide temperature range by means of the
installed cooling system.
[0064] The temperature of the electrodes is controlled in such a
way that during the melting operation the temperature of the
electrodes is always below the temperature at which the electrode
material is chemically attacked to a disruptive extent by the
melt.
[0065] In the case of platinum being used as electrode material,
for example, the temperature of 1 550.degree. C. should not be
exceeded.
[0066] If there is a high level of convection in the melt, the
temperature of the electrodes may under certain circumstances have
to be reduced further, so that in this case too the electrodes are
not attacked to a disruptive extent by the melt.
[0067] A significant factor in the heating of the electrode surface
and the dissipation of heating power is also the absorption of
infrared radiation from the melt. Whereas an increase in the
temperature of the melt compared to the walls of the melting vessel
can be achieved by cooling, on the other hand it can also be
achieved by virtue of the melting vessel having a surface which
reflects infrared. The infrared-reflecting surface reduces the
heating of the walls. Accordingly, an infrared-reflecting surface
is equivalent in terms of its effect to cooling of the walls within
certain limits. Accordingly, the invention also provides an
apparatus for heating melts which comprises a melting, conditioning
and/or refining vessel for receiving melting material. In this
case, the melting vessel has a surface which reflects infrared at
least in regions. An apparatus of this type may likewise be
provided with electrodes for conductive heating. However, it is
also possible for other heating methods, for example radiofrequency
heating, to be used. To achieve good reflection properties, the
infrared-reflecting surface may be polished. It may also be
provided with an infrared-reflecting coating, in which context in
particular a gold, platinum, nickel, chromium or rhodium coating is
suitable. By way of example, gold-coated walls have already reduced
the heating power required by up to 20%.
[0068] If an apparatus of this type is designed for conductive
heating, the infrared-reflecting surface may in particular comprise
the surface of the melt-contact material of at least two electrodes
for the conductive heating of the melt which replace part of the
walls. In this case, a significant effect is achieved even if only
the surface of the melt-contact material is designed to reflect
infrared.
[0069] The method according to the invention also has the advantage
over melting units which are operated with radiofrequency and in
which the melting material is completely surrounded by water-cooled
skull walls in skull crucibles, that the preferably cooled
electrodes installed in the melt extract significantly less heat
from the melt.
[0070] Furthermore, the electrical converters for frequencies in a
range from 50 Hz to 50 kHz have a considerably better efficiency
than the converters for radiofrequency which are customarily used
in skull crucibles, allowing operating costs to be significantly
reduced.
[0071] Moreover, it is advantageous if the wall of the melting
vessel is also cooled, at least in a region, in order on the one
hand to protect the walls from overheating and on the other hand to
cause a temperature gradient within the melt. The apparatus
according to the invention, as is the case, for example, with skull
crucibles, therefore preferably comprises a device for cooling at
least a region of the wall of the melting vessel.
[0072] It is advantageous in particular if the preferably
large-area electrodes are arranged in electrically insulated
fashion, so that the current for the conductive heating can flow
only through the glass melt and not across the walls.
[0073] Depending on the arrangement of the electrodes and the
geometry of the melting vessel, the electrodes may advantageously
comprise plate and/or button and/or rod electrodes. Since the
electrodes are exposed to a certain amount of wear even when
operating below the temperature range at which decomposition of the
electrode commences, it is furthermore advantageous if the
electrodes are secured exchangeably to the apparatus.
[0074] The electrodes are preferably operated with an alternating
current of 50 Hz to 50 kHz, particularly preferably from 2 kHz to
10 kHz. The alternating current substantially prevents corrosion to
the electrodes, since corrosion decreases with increasing frequency
of the current. Accordingly, an apparatus according to the
invention may also advantageously comprise a device for generating
alternating current, such as for example a medium-frequency
transformer or an alternating current generator, the alternating
current preferably having a frequency within one of the ranges
mentioned above.
[0075] Tests have shown that in the case of melts with a poor
electrical conductivity, the introduction of energy via large-area
cooled electrodes which are not integrated in the walls is
insufficient to heat these melts if the cooled electrodes are
located in a melting unit whereof the walls are likewise cooled.
The removal of energy from the melt via the cooled walls and the
additional removal of energy via the cooled electrodes is then
higher than the introduction of energy into the melt in the case of
melts with a low electrical conductivity. In this case, it is also
not sufficient to make the melt-contact surfaces of the electrodes
larger, since this not only increases the introduction of energy,
but also causes additional heat to be dissipated via the larger
electrode. On the contrary, there is a risk that in apparatuses
known from the prior art a melt which is already hot will be cooled
to such an extent by the dissipation of heat that it will be
disconnected on account of the falling temperature and the
associated falling electrical conductivity.
[0076] On account of the fact that, by contrast, the cooled,
preferably large-area electrodes according to the invention are
integrated in the cooled walls in such a way that they themselves
form part of the walls, the cooling total surface area of the unit
is not increased, and consequently even in the case of melts of a
poor electrical conductivity the introduction of energy can be kept
at a higher level than the removal of energy.
[0077] For the method for heating the melt, it is advantageous if
the temperature of the walls of the melting vessel and the
electrodes are kept below the temperature at which the materials of
the walls and of the electrodes are chemically attacked to a
significant degree by melt. In order, therefore, to ensure a long
service life of the melting unit at high melting temperatures, the
cooled walls, in particular made from refractory material, have to
be cooled by cooling systems to such an extent that the chemical
attack of the melt on the refractory material is negligible.
[0078] At high melting temperatures, the convection of the melt
also increases, and therefore so does the chemical attack on the
refractory material. In this case, the temperature of the walls
should be reduced further, in order to avoid increased corrosion of
the walls as a result of the strong convection. However, it is
generally advantageous if the electrodes and/or walls of the
melting vessel, for example when using refractory ceramic as
electrode or wall material, are substantially chemically resistant
to the melt, so that the walls do not have to be cooled too far and
the removal of energy through the walls does not become too great.
Moreover, it is advantageous if the material of the electrodes and
walls has a good thermal conductivity, to allow the walls to be
effectively cooled.
[0079] The cooled walls used may preferably also be skull walls.
The skull walls have the advantage that a skull layer of the same
type of material as the melt is formed on the cooled metallic tubes
and is constantly reformed at high temperatures. The skull walls
have also proven suitable for melts which are chemically highly
aggressive with respect to ceramic refractory materials.
[0080] Despite the thin thermally insulating skull layer, skull
walls still extract very large amounts of heat from the melt.
Therefore, inter alia refractory ceramics may advantageously also
be used as material for the walls of the melting vessel, in which
case it is preferable also for the walls made from refractory
ceramic to be cooled.
[0081] It has proven advantageous, at least during the start-up
process, for the skull walls, which preferably comprise cooled
metallic tubes, such as for example copper tubes or steel tubes, to
be lined with a material with a poor electrical and thermal
conductivity, preferably in the form of ceramic plates or slip, in
particular SiO.sub.2 slip, on the side facing the melt. This allows
the dissipation of heat to be reduced until the supply of
electrical energy has reached a sufficiently high level at
relatively high melting temperatures.
[0082] It has likewise proven advantageous for the cooled tubes of
the skull walls to be such that they do not release any coloring
ions into the melt through the skull layer. This can be achieved on
the one hand by the metal tubes either consisting of platinum or
being coated with platinum. Metal tubes made from aluminum can
easily be oxidized at the surface but do not release any coloring
ions into the surface.
[0083] Furthermore, it is also possible for the metal tubes to be
coated with plastic, as is, described, for example, in DE 100 02
019. Plastic coatings can be very chemically resistant and do not
generally release any metal ions to the melt which can cause
undesirable discoloration in the fully melted and processed
product.
[0084] Moreover, particularly when using skull walls, it is
important for there to be no conductive connection between the
electrodes used and the electrically conductive skull tubes, since
otherwise heating of the melt is not possible on account of the
resistance distribution in the system. Therefore, suitable
resistance bridges have to be inserted in the skull and toward the
electrodes, so that the current for conductive heating does not
flow across the skull, but rather substantially through the
melt.
[0085] As well as the frequency of the current, the current density
at the interface with the melt also has a strong influence on the
electrolysis and therefore on the corrosion of the electrodes.
Tests have shown that it is advantageous to use electrodes with as
large an area as possible and/or to create the largest possible
melt-contact surface, in order to keep the current density as low
as possible. To prevent damage to the electrodes or the
introduction of electrode material into the melt, it is
advantageous if the electrodes are dimensioned in such a way that a
current density of 5 A/cm.sup.2 is not exceeded for a given heating
power.
[0086] Moreover, tests have shown that at high melting
temperatures, in particular at melting temperatures over 1
700.degree. C., a significant part of the energy in the melt is
transported through convection. Since the density of the glass
decreases with the temperature, the hottest zone is usually located
in the upper part of the melting unit.
[0087] As the temperature rises, the electrical conductivity of a
melt increases exponentially. Therefore, the electrical
conductivity is highest in the upper part of the melting unit,
where the current density on the electrodes is particularly high.
In this context, it has been found that it is possible to prevent
overloading of the electrodes if the electrodes are arranged in the
lower part of the melting unit. Furthermore, arranging the
electrodes in the lower part of the melting unit has the advantage
that sufficient convection is present even in the lower part of the
melting unit, and therefore flow dead zones are avoided or reduced.
Therefore, even the top edge of the electrodes is always below the
melt bath surface, which avoids an air-melt-electrode three-phase
boundary, which is particularly critical in terms of corrosion. In
this context, it has proven advantageous in particular for the
electrodes in the lower part of the melting vessel to be arranged
below the melt bath surface in the region of the lower two thirds
of the height of the melting vessel.
[0088] Overloading of the electrodes can also be avoided if the
melt-contact surfaces of the electrodes are arranged obliquely with
respect to one another, in which case the melt-contact surfaces
tend to diverge toward the melt bath surface, so that the
resistance path between the electrodes is longer in the upper part
near the melt bath surface than in the lower part. This makes it
possible, for example, for the higher conductivity to achieve a
homogeneous current density in the vicinity of the melt bath
surface, caused by the higher temperature in the upper region of
the melt, to be at least partially compensated for. By way of
example, for this purpose the melting vessel may be of a form which
widens upward in frustopyramidal or frustoconical form. As a
further measure for reducing the maximum current loading on the
electrodes, it is also possible, for example, for the upper edge
thereof to be rounded.
[0089] In the case of relatively large installations, the heating
may also be effected by more than one electrode pair. Therefore,
one embodiment of the apparatus provides a plurality of electrode
pairs and/or a plurality of pairs of electrode segments. These may
in particular be operated with a plurality of heating circuits that
can be controlled independently. For example, two circuits can be
operated in the form of a Scott circuit.
[0090] In the present context, the term an electrode pair is to be
understood in the electrical sense as a pole pair. In this context,
it is also quite possible to combine a plurality of electrodes
and/or a plurality of electrode segments on one electrical pole,
and/or one electrical terminal. It is also possible for a plurality
of electrode pairs to be arranged above one another at the side
walls of the melting vessel. The vertical temperature
stratification can be deliberately set in this way. Suitable
electronic actuation also allows heating circuits that can
advantageously be controlled independently and each of which are
assigned an electrode pair to be operated successively at offset
times.
[0091] To avoid excessively high voltages in combination with a low
electrical conductivity of the melt, it is also possible for the
electrode surface to be increased by working with one or more
bottom electrodes and one or more electrodes in the side walls. The
electrode pairs may also be supplied by a plurality of current
sources.
[0092] A plurality of electrode pairs arranged vertically or
horizontally next to one another can also be arranged so as to
allow targeted influencing of the spatial temperature distribution.
By way of example, the vertical temperature stratification can be
set in such a way by two electrode pairs and heating circuits
operated independently that flow dead zones are avoided in the
lower part of the melting unit.
[0093] Furthermore, the electrodes may advantageously be arranged
and connected in such a way that the majority of the electrical
power drops predominantly in the lower region of the melting
unit.
[0094] The melting vessel may advantageously have a square or
rectangular basic contour, so that the electrodes can have planar
melt-contact surfaces. However, to reduce the specific surface area
of the melt for the same volume, the melt vessel may also be of
cylindrical design with a circular or oval basic contour. In this
case, the electrodes may also be configured, inter alia, in ring
form and, for example, take up a segment of the height of the wall
of the melting vessel. The apparatus may also have one or more
electrodes arranged at the base of the melting vessel. An electrode
for a melting vessel shaped in this way may also, for example, form
a ring segment of the wall of the melting vessel. Bottom electrodes
may advantageously also be arranged in such a way that there is the
possibility of the inlet or outlet for melting material to be at
the base.
[0095] The melting vessel may also have a polygonal basic contour
which, for example, closely approximates to a round or oval basic
contour and can be produced in a simple way from planar wall
segments.
[0096] The melting unit according to the invention with the cooled
walls and the large-area electrodes integrated therein can be used
both to melt down and to refine glasses, glass-ceramics, ceramics
or crystals.
[0097] It is also possible for two of these units, for example one
for melting-down and another for refining, to be connected in
series. Furthermore, the units can be used for both discontinuous
and continuous melting.
[0098] If the unit is used for melting down, the melting-down
operation can be accelerated by bubbling with a gas, for example
with oxygen or noble gas. For this purpose, the apparatus may
advantageously have at least one blowing nozzle or bubbling nozzle,
preferably arranged at the base of the melting vessel.
[0099] If the walls of the melting unit comprise skull walls and
cooled electrodes, the upper part of the melting unit, to protect
the metal tubes of the skull, may be configured in the form of a
mushroom at the melt bath surface, as described, for example, in DE
199 39 772.
[0100] For discontinuous melting and for complete emptying of the
melting vessel, for example in order to carry out maintenance work,
it is advantageously possible for at least one outlet nozzle to be
fitted in the base of the melting unit.
[0101] Moreover, the apparatus may advantageously have a cooled
bridge, which is preferably arranged in such a way that it is
immersed in the melt from above through the melt bath surface. This
makes it possible both to melt down the batch continuously and
refine the melt continuously in just one melting unit. In this
case, the melting unit can be divided into a melt-down part and a
refining part by a bridge formed from water-cooled metal tubes.
[0102] As is also the case when melting using radiofrequency, in
the method according to the invention the melt first of all has to
be heated to a temperature at which the electrical conduction of
the melt is high enough for the melt to be sufficiently well
connected, for conductive heating, for the energy which is
introduced to be higher than the energy which is dissipated via the
cooled walls and electrodes.
[0103] When melting using radiofrequency, it is most cases
sufficient for a small part in the interior of the melt to have a
sufficient electrical conductivity and to be well connected. The
radiofrequency energy is concentrated in this region and heats it
further, with the result that this region propagates continuously
until it fills the whole of the melting unit.
[0104] The melting method according to the invention is based on
conductive heating of the melt, which means that electrical
conduction across the melt has to be produced between the
electrodes. It is therefore not sufficient for a melting region in
which the electrical conduction is high enough to be present in the
interior of the melt. Rather, the resistance of the melt between
the electrodes has to be sufficiently lowered at least in a region,
or the electrical conductivity of the melt has to be increased
sufficiently far at least within a region, for the applied voltage
or the current density to be sufficient to heat the melt to a
greater extent than the heat loss through the walls. For this
purpose, the apparatus according to the invention may moreover
advantageously have a device for additional heating. The device for
additional heating may in this case expediently comprise, for
example, at least one fossil burner and/or at least one plasma
torch and/or at least one resistance heating element and/or at
least one infrared radiator.
[0105] By way of example, if the additional heating is fitted above
the crucible, to start up the conductive heating process first of
all a melt path with sufficient electrical conductivity has to be
heated for a sufficient extent for the conductivity to be high
enough for a sufficiently high current to flow. In this case, for
example when using large crucibles, correspondingly high-power
additional heaters can be installed.
[0106] Even when using additional heating, the dissipation of heat
via the electrodes, even without active cooling of the electrodes,
can cause the melt-contact surface of the electrodes to remain at a
lower temperature than inner regions of the melt or of the melting
material, and consequently this effect impedes the flow of current,
in particular in the vicinity of the electrode surface. It is
therefore advantageous if at least one of the electrodes also
comprises a heating apparatus, by which the melting material at the
electrode surface and/or the melt-contact surface can be heated
directly. This allows the electrode to be heated automatically at
least from time to time until, for example, the heating power
provided by conductive heating of the melt is sufficiently
high.
[0107] Moreover, when restarting the apparatus, the problem often
arises whereby an insulating air gap is present between
melt-contact material and cooled melting material. This air gap is
formed through shrinkage of the melting material during cooling. If
a voltage is applied to the electrode when the air gap is present,
the air gap can be locally bridged by ionization and the current
can shoot through at this location, which can lead to damage to the
melt-contact material. With direct heating of the electrodes, the
melting material can be initially melted in the region of the
melt-contact material, and in this way comes back into electrical
contact with the electrode by forming a large-area conductive
bridge.
[0108] A particularly suitable heating apparatus is an ohmic
heating device. This device may preferably comprise a current
source which is connected to the melt-contact material or a
conductive material beneath it, thereby forcing a current through
the melt-contact material or the conductive material beneath
it.
[0109] However, as an alternative or in addition, the heating
apparatus may also comprise an apparatus for heating a cooling
fluid. In this way, the electrode can be both heated and cooled
using the same fluid. Accordingly, the heating device also does not
have to be arranged directly beneath the melt-contact surface, but
rather can be arranged at virtually any desired suitable point in
the cooling fluid circuit. The heating apparatus may, for example,
comprise an electrical and/or fossil heater and/or a waste heat
heater.
[0110] Preheating by heating the coolant in particular by means of
electrical energy, waste heat or preferably using fossil energy
carriers is also advantageous, for example, in order to prevent the
precipitation of moisture on the electrodes by virtue of it being
heated above the dew point of the furnace upper atmosphere of the
melting apparatus. Moisture may be formed in the melting apparatus,
for example, in relatively large quantities during initial
operation of the unit when the melting material is being preheated
using fossil burners.
[0111] There are a range of methods for heating the melt to a
sufficient extent for it to reach a temperature at which the
electrical conduction of the melt is sufficient to allow further
heating with the aid of the electrodes or connection of the
melt.
[0112] For example, for initial operation of the melting unit, the
method according to the invention may advantageously comprise a
starting operation in which solid melting material located in the
melting vessel is melted down or introduced into the melting vessel
in molten form. The following text is to describe, by way of
example, this type of melt start-up. However, the invention is not
restricted to this specific start-up method.
[0113] To start up the melting unit according to the invention, by
way of example, charge cullet or batch is placed into the melting
unit and partially melted by means of one or more fossil burners,
plasma torches or infrared heating in the upper furnace until the
electrical conduction is sufficient to-start the direct electrical
heating. The technical limit in this context is the maximum voltage
which can be provided by a device for generating alternating
current, such as for example a medium-frequency transformer. To
allow current to flow at a lower contact resistance, the
electrodes, during the starting operation, can be resistance-heated
or operated without or with only little cooling.
[0114] To facilitate the starting operation, the electrodes may
also be arranged displaceably. In this case, the electrodes can be
pushed together prior to the starting operation, so that the volume
between the melt-contact surfaces is reduced. This smaller melt
volume can then easily be heated to a temperature at which the melt
has a conductivity sufficient for the conductive heating by the
electrodes. During the starting operation, the electrodes can be
pulled apart again to their operating positions, increasing the
melting volume.
[0115] The electrodes and/or cooled walls, during the start-up
process or the starting operation, can be heated by a heating
apparatus to a sufficient extent for their temperature to be above
the dew point of the furnace upper atmosphere. This prevents
moisture from precipitating on the electrodes; such moisture, as a
film of water, leads to short circuits during initial operation of
the electrodes. It is particularly preferable for preheating of
this type to comprise cooling-water preheating.
[0116] According to one embodiment of the invention, to melt down
the melting material, starting electrodes are introduced into the
melting vessel and a current is passed through the melting material
via the starting electrodes. In this way, a melt can initially be
produced in a small region of the melting unit. Moreover, the
starting electrodes can be moved apart during the starting
operation, so that the region comprising molten material increases
in size. This region of molten material which increases in size
during the starting operation can then ultimately come into contact
with the actual electrodes or molten regions in the vicinity
thereof, which have been melted, for example, by means of a heating
apparatus for heating the electrodes. It is in this way possible to
produce a melting path with a sufficient conductivity of the
melting material between the electrodes, so that the conductive
heating of the melt can start operating. The electrical
conductivity of the melt increases exponentially as the temperature
of the melt rises, and a transformer can be used to switch to a
lower voltage, since a strong current is required to introduce a
high electrical power.
[0117] In parallel with the rise in the introduction of electrical
power, it is possible to lower the power of the upper furnace
heating accordingly as the electrical conductivity of the melt
rises. Since the electrical conductivity of the melts, for example
in the case of glass melts, rises very rapidly with the
temperature, the electrodes can easily burn through. Therefore, the
cooling of the electrodes and the heating power can be controlled
accurately as a function of the temperature and the composition of
the melt.
[0118] According to the invention, this can be achieved by the
temperatures of the electrodes being accurately monitored and the
temperature of the electrodes being accurately set with the aid of
a control system and the cooling system fitted in the electrodes.
On the one hand, the electrodes must not extract too much heat from
the melt, while on the other hand they must not become sufficiently
hot for them to corrode or even burn through. If the walls of the
melting unit comprise skull walls, the skull walls can be covered
with ceramic materials or with slip. The slip may, for example,
consist of milled quartz material. Covering the skull walls in the
start-up phase prevents an excessive loss of heat through the
walls.
[0119] Furthermore, for initial operation of the melting unit or
for the starting operation, it is possible to convert the melt from
a melt with a high electrical conductivity to a melt with a low
electrical conductivity.
[0120] It has been found that strong convection commences in the
melting unit or the melting vessel if the temperature difference
between the melt in the edge region of the melting vessel and the
melt in the center region of the melting unit amounts to more than
150.degree. K., preferably more than 250.degree. K. Strong
convection is advantageous to recirculate the melting material, so
that the entire melt, or at least the majority thereof, passes
through the region which is at an elevated temperature compared to
the electrode surface. When the melt-contact surface of the
electrodes is at a temperature of 1 500.degree. C., the melting
temperature in a region preferably arranged in the center of the
melting vessel can accordingly be over 1 650.degree. C., preferably
over 1 750.degree. C.
[0121] The possibility, according to the invention, of heating a
melt to high temperatures even above loading limits for electrode
and wall material brings with it a range of advantages. Firstly,
even high-melting glasses, glass-ceramics, ceramics or crystals can
be melted. A further advantage is that the chemical reactions and
physical processes take place significantly more quickly at higher
temperatures than at lower temperatures. For example, both the
melt-down processes and the refining operations take place
significantly more quickly at higher temperatures. Increasing the
temperature by 200.degree. K. accelerates the chemical reactions
and the physical processes by a factor of 3 or even higher. As a
result, by way of example, included bubbles in the melting material
can be removed more quickly. For example, the rate at which bubbles
rise in a melt is given by: v = 2 9 .times. .rho. g r 2 .eta.
.times. .times. ( T ) . ##EQU1##
[0122] In the above, .rho. denotes the density of the melting
material, g denotes the gravitational acceleration, r denotes the
bubble radius and .eta.(T) denotes the temperature-dependent
dynamic viscosity. If the temperature of the melt is increased, the
thermal expansion causes the bubble radius to increase, the
diffusion of gases out of the melt into the bubble to be
accelerated and the viscosity of the melt to be reduced. To
additionally accelerate the chemical and physical reactions, it is
advantageous if, as described above, strong convection of the melt
takes place in the melting unit.
[0123] According to one embodiment of the invention, for continuous
processing of the melting material, melting material is fed to and
discharged from the melting vessel continuously. In particular, the
melting material may also be supplied in molten form via an inlet
and discharged likewise in molten form via an outlet. This
realization of the method, and of a corresponding apparatus, is
particularly expedient for the refining of a melt in a continuous
production process, since an apparatus which is correspondingly
configured as a refining unit can easily be connected downstream
of, for example, a melting-down furnace, with the melt being
removed from the unit via the outlet after refining.
[0124] Moreover, it may be advantageous for the electrodes to be
arranged in such a way in the melting unit that the electrodes are
positioned opposite one another in the direction of flow or
transversely with respect to the direction of flow. If continuous
melting is taking place, for example when using the apparatus as a
refining unit with a continuous inflow and outflow, the electrodes
can be installed in such a way that the electrical heating current
between the electrodes flows substantially in the main direction of
flow of the melt or transversely with respect thereto. These two
arrangements or directions of current flow are advantageous, inter
alia, in order to promote the formation of suitable convection
circulation zones in the melt, by means of which the melting
material is transported through the melting vessel. It is
particularly expedient to form convection circulation which rotates
with the axis of rotation perpendicular to the main direction of
flow of the melt. Which of the two electrode arrangements, i.e.
opposite one another in or perpendicular to the main direction of
flow of the melt, is more expedient for the formation of a
convection circulation of this nature depends in the individual
circumstances on the geometry of the melting vessel. To promote the
formation of convection circulation, it is also expedient for the
electrodes to be arranged in the lower region of the melting
vessel.
[0125] In this case, a temperature difference of more than
150.degree. K., preferably more than 250.degree. K., can
advantageously be set between the melt-contact surface of the
electrodes and a region of the melt located substantially centrally
between the electrodes. In this way, convection circulation is set
in motion, promoting the transfer of melting material from the
inlet to the outlet and passing the melting material through the
melting vessel without any short-circuit flow at the melt bath
surface.
[0126] With this opposite arrangement of the electrodes, the melt,
to be refined, no longer has to be introduced into the refining
unit from below, but rather can advantageously be introduced into
the refining unit and withdrawn again from it from above, in the
region of the melt bath surface, via inlet and outlet. This
arrangement is in technical terms significantly simpler to realize
than the introduction of the melt from below.
[0127] The invention is explained in more detail below on the basis
of preferred embodiments and with reference to the appended
drawings, in which identical reference symbols denote identical or
similar parts. In the drawings:
[0128] FIGS. 1A to 1C: show views of a first embodiment of the
invention,
[0129] FIG. 2 shows a diagrammatic cross-sectional view of an
electrode,
[0130] FIGS. 3A to 3F: diagrammatically depict electrode
configurations, melting vessel forms and electrode connections for
various embodiments of the invention,
[0131] FIG. 4 shows a further embodiment of the apparatus according
to the invention,
[0132] FIG. 5 show measured diagrams of temperatures in the melting
unit as a function of the heating current, and
[0133] FIGS. 6A to 6C use diagrammatic cross sections through a
melting unit to illustrate method steps of a starting
operation.
[0134] FIGS. 1A to 1C illustrate various views of a first
embodiment of an apparatus according to the invention for heating
melts. The apparatus is denoted overall by 1. FIG. 1B shows a view
of the apparatus 1 as seen in the direction of arrow B in FIG. 1A.
FIG. 1C represents a top view as seen in the direction of arrow C
in FIG. 1B. The apparatus 1 comprises a melting vessel 3 designed
as a skull crucible. The crucible is produced from tubes 7 through
which coolant is passed in the region of the apparatus for cooling
the vessel. Copper, among other materials, is a suitable material
for the tubes, on account of its good thermal conductivity.
However, on the other hand the strength of copper is not
particularly high, and consequently tubes made from metal with a
high mechanical strength or thermal stability, in particular tubes
made from high-strength or heat-resistant steel, may also be
suitable.
[0135] To minimize the heat loss via the skull walls, the walls may
moreover be provided with a surface which reflects infrared. By way
of example, the tubes 7 may for this purpose be provided with a
platinum or gold coating, which may in particular also be polished
in order to increase the reflectance. Rhodium, chromium, nickel or
palladium, as well as alloys thereof, can also be used for this
purpose.
[0136] A refractory collar 13, which is preferably made from
chemically resistant material, is arranged at the crucible in the
region of the melt bath surface, in order to suppress reactions at
the three-phase boundary which is formed at the boundary of the
melt bath surface with the furnace upper atmosphere. This collar
may, for example, comprise a fused-cast ceramic material. An inlet
9 and an outlet 10 with melting-material channels 11, via which the
melting material is supplied and discharged continuously in the
region of the melt bath surface of the melting vessel, are machined
into the refractory collar. Moreover, a bottom outlet 15, through
which the crucible can be emptied, is arranged at the base 14 of
the melting vessel. Unlike in the embodiment shown in FIG. 1, the
skull tubes may also project out of the melt bath surface and be
coated with plastic, for example, at least in the region of the
melt-bath surface, in order to increase the chemical resistance. A
suitable plastic for this purpose is in particular Teflon.
[0137] At the side wall of this embodiment of the apparatus 1, two
electrodes 5, which have coolant connections 6 as part of a device
for cooling the electrodes and via which coolant is passed through
passages in the interior of the electrodes, are arranged in
corresponding cutouts in the side wall 16 of the melting vessel. On
account of the arrangement according to the invention, the
electrodes 5 replace part of the side wall 16 of the melting vessel
3, the electrodes 5 moreover being arranged opposite one another at
the melting vessel. To keep the current density in the melt-contact
material of the electrodes low, the electrodes 5 maintain a large
surface area. It is preferable for the electrodes 5 in this case to
replace at least 15% of the wall surface area of the melting vessel
in the region of the melt.
[0138] To heat the melt, a heating current is passed through the
melt via the electrodes 5, with the electrodes 5 being insulated
with respect to the melting vessel 3, so that no current can flow
via the walls of the melting vessel and reduce the heating power.
In order nevertheless to maintain resistance to short circuits in
the event of a defect in the insulation, it is also possible,
moreover, for the wall elements of the melting vessel to be divided
into segments that are insulated from one another. Furthermore, the
electrodes are arranged on the same sides of the apparatus 1 as
inlet 9 and outlet 10 for the melt, so that the heating current
between the electrodes flows substantially in the direction of the
main direction of flow of the melt or in the opposite direction
thereto.
[0139] The electrodes are of large-area dimensions, so that the
current which emerges from the electrodes into the melt does not
exceed a current density of 5 A/cm.sup.2 at any point on the
melt-contact surface. A heating current, preferably with an
alternating current frequency in a range from 50 Hz to 50 kHz,
particularly preferably with an alternating current frequency in a
range from 2 kHz to 10 kHz, is passed through the melt located in
the melting vessel 3 via the electrodes 5 by means of a device for
generating alternating current.
[0140] FIG. 2 shows a diagrammatic cross-sectional view of an
electrode 5. The electrode 5 has electrical supply conductors 52,
which are connected to the melt-contact material 53 and can be
connected to a power supply, preferably a medium-frequency
transformer, for conductive heating of the melt. The melt-contact
material 53 of the electrode 5 has a melt-contact surface 51 which
is in contact with the melt. To stabilize the melt-contact material
53 in the melt with respect to the hydrostatic pressure of the
melt, the melt-contact material 53 is secured to a supporting
apparatus 54. The supporting apparatus 54 may be produced, for
example, from refractory ceramic. Moreover, in this embodiment the
supporting apparatus 54 is also provided with metal holding plates
56 which are used to fit and secure the electrode 5. The metal
holding plates secure the electrode to the melting unit or the
melting vessel, making it easy to exchange the electrode 5.
[0141] The electrode 5 is integrated in a melting or refining unit
in such a way that the melt-contact surface 51 forms a wall region
of the melting vessel. The melt-contact material 53 is preferably
made from refractory metal, such as platinum or a platinum alloy,
which has only a low susceptibility to corrosion and migration of
electrode material into the melt at temperatures below 1
600.degree. C. A conductive, refractory ceramic, such as for
example SnO.sub.2 ceramic, is also suitable for some melts.
[0142] The supporting apparatus 54 has a multiplicity of
connections 6 which are connected to fluid-carrying passages in the
interior of the supporting apparatus 54. A first group of
connections 61 is connected to a first cooling circuit, and a
second group of connections 62 is connected to a second cooling
circuit. The first cooling circuit is preferably an air-cooling
circuit, and the second cooling circuit is preferably a
water-cooling circuit. The fluid-carrying passages of the first
cooling circuit in the interior of the supporting structure 54 are,
moreover, arranged in such a way that the coolant comes into direct
contact with the melt-contact material on the opposite side from
the melt-contact surface 51. These cooling circuits allow the
cooling capacities of the electrodes to be controlled or set
separately.
[0143] Moreover, the electrode has an ohmic heating device. This
device comprises a current source 33, which is connected to the
melt-contact material 53 via supply conductors 34. For ohmic
heating of the electrode 5, therefore, a current can be passed
through the melt-contact material 53, which is then heated. This
type of heating of the melt-contact material 53 in transverse
current mode is particularly advantageous since in this way heating
is effected particularly close to the melt, and therefore the
heating has only a very low inertia.
[0144] FIGS. 3A to 3F show diagrammatic illustrations of possible
electrode configurations, melting-vessel forms and electrode
connections of various embodiments of the invention.
[0145] FIG. 3A shows a first embodiment of an apparatus which,
similarly to the embodiment explained with reference to FIGS. 1A to
1C, has a melting vessel 3 which is substantially square in terms
of its basic contour. The side walls 16 of the melting vessel are
in this case each arranged at right angles to the base, so that the
melting vessel 3 is cuboidal in form. The electrodes 501 and 502
are fitted at opposite sides of the melting vessel, so that the
heating current has to pass through the melt between the electrodes
in the melting vessel 3 over the entire width of the melting
vessel, resulting in a uniform distribution of the heating power in
the melt. To heat the melt, the electrodes 501 and 502 are
connected to the poles of an alternating current source 18 as a
device for generating alternating current and form an electrode
pair.
[0146] Furthermore, the electrodes 501 and 502 are arranged in the
lower part of the melting vessel, beneath the melt bath surface, in
the region of the lower two thirds of the overall height of the
melting vessel. As a result, the electrodes 501, 502 are always
completely immersed in the melt, and a three-phase boundary that is
critical in terms of corrosion is avoided. Arranging the electrodes
in the lower region of the melting vessel moreover increases the
size of the region which is at an elevated temperature compared to
the melt-contact surfaces of the electrodes, since the dissipation
of heat from the melt-bath surface in the upper part is
considerably lower than the dissipation of heat via the walls and
electrodes.
[0147] FIG. 3B illustrates an embodiment of the apparatus with two
opposite, obliquely arranged side walls 161 and 162 of the melting
vessel. The electrodes 501 and 502 each take up a planar region of
these side walls. The oblique arrangement of the electrodes with
respect to one another means that the current covers a longer
distance in the upper region of the melting vessel, where the melt
is at a higher temperature and consequently has a better
conductivity. This causes the ohmic resistances along the different
paths to be at least partially matched to one another, which leads
to a more homogeneous distribution of the heating power in the melt
and on the electrode surface.
[0148] FIG. 3C shows an embodiment of the apparatus 1 with a
plurality of electrode pairs. In each case two of the electrodes
501, 502, 503 and 504, which are arranged on opposite sides of the
wall 16, are combined to form an electrode pair which is in each
case supplied by an alternating current source 18 or 20. The
electrodes 501 and 502 are in this case connected to the
alternating current source 18, and the electrodes 503 and 504 are
in this case connected to the alternating current source 20.
[0149] The electrode configuration of this embodiment is
particularly suitable for heating melts with a high electrical
conductivity, since independent operation of a plurality of
electrode pairs effectively increases the electrode surface area
and therefore allows high current densities to be achieved within
the melt, in order to achieve sufficient heating powers. However,
the electrode pairs have to be arranged in such a way that a shunt
connection via electrically conductive wall elements, specifically
melting-vessel parts or electrodes, is avoided.
[0150] FIG. 3D shows a further embodiment of the apparatus having a
plurality of electrode pairs. Unlike the embodiment of the
apparatus explained with reference to FIG. 3C, however, in this
case the electrodes are not arranged next to one another, but
rather above one another. In this case, the two electrodes 501 and
502 form an electrode pair which is supplied by the alternating
current source 18 and is arranged closer to the melt bath surface,
above the electrodes 503 and 504 which are supplied by the
alternating current source 20 and are arranged on the same sides of
the melting vessel 3. An arrangement of this type is particularly
suitable for influencing the spatial temperature distribution in
the melt by separate operation of the electrode pairs arranged
above one another, thereby allowing, for example, the shape and
extent of one or more convection circulation zones in the melt to
be controlled.
[0151] FIG. 3E illustrates an embodiment with an annular electrode
501 and a second electrode 502 in the form of a bottom electrode.
The electrode 501 in this case takes up an annular region of the
side wall 16 of the cylindrical melting vessel 3 with a circular or
oval basic contour. As an alternative to the circular or oval basic
contour illustrated, it may also be in the form of a polygon.
[0152] Of course, the other embodiments illustrated by way of
example may also have one or more bottom electrodes. Bottom
electrodes of this type are advantageous, inter alia, for the
targeted delivery of the heating power in the lower region of the
melting vessel. This ensures an optimum utilization of the volume
and/or avoids dead volumes.
[0153] The embodiment of the apparatus illustrated in FIG. 3F, like
the embodiment shown in FIG. 3E, comprises a cylindrical melting
vessel 3 with a circular or oval basic contour. The electrodes 501
and 502 are arranged in the cylindrical side wall and each form a
region of the side wall 16, in the form of a ring segment. A
cylindrical melting vessel as shown in the embodiments presented in
FIGS. 3E and 3F, has a smaller surface area of the inner wall
compared to cuboidal melting vessels, which reduces the dissipation
of thermal energy. However, electrodes in the form of ring segments
produce very different resistance paths through the melt. This can
be compensated for, for example, by the electrodes being divided
again, for example into separately operated segments.
[0154] FIG. 4 shows as further embodiment of the apparatus 1
according to the invention, which is designed in particular as a
continuous melting-down unit. In this embodiment too, the melting
vessel 3 is preferably designed as a skull crucible. The electrodes
5 are arranged on opposite sides of the melting vessel 3 and form
planar regions of the side wall 16.
[0155] A cover 27, in which there is a feeder 30 for adding melting
material, is located on the melting vessel 3. The melt is
discharged via an outlet 10. Moreover, a gas burner is arranged in
the outlet, preventing the melt from cooling as it is discharged
through the outlet. In addition, a cooled bridge 26 is arranged
between outlet and feeder, in such a way that it is immersed in the
melt 22 from above, through the melt bath surface 24. This prevents
melting-down material which has not yet been melted from passing
directly into the outlet 10, but rather this material remains in
the melting vessel 3 for a sufficient length of time. Moreover, a
gas burner, by which, for example when starting up, the melt 22 can
be preheated to a sufficient temperature for it to have a
conductivity sufficient for the conductive heating, is arranged in
the upper furnace above the melt bath surface 24. The conductive
heating by the electrodes 5 and the simultaneous cooling of the
skull walls and the electrodes 5 leads to a temperature drop within
the melt from the center region toward the cooled walls. As a
result, a hot zone 23, the temperature of which can be set in such
a way by the interaction of cooling and heating power that it is
more than 150.degree. K., preferably more than 250.degree. K.,
higher than the temperature of the electrode surfaces, is formed in
the center region of the melt 22. As a result, moreover, a strong
convection flow is formed in the melt, producing one or more
convection circulation zones 25. In this way, the melting material
is passed through the melting vessel, and dead zones, in which the
melting material remains in the melting vessel for an excessive
length of time, are avoided in the melt 22. Furthermore, the
convection can be boosted by a blowing nozzle 32, through which,
for example, oxygen or noble gases can be blown into the melt
22.
[0156] FIG. 5 shows measured diagrams of temperatures as a function
of the heating current. The melting material used was display glass
without any alkali metals. The curve illustrated in dotted lines
shows the temperature dependency in the hot zone 23 of the melt.
The measured curve illustrated by a solid line indicates the
temperature measured values at the melt-bath surface 24, and the
measurement curve illustrated by dashed lines represents the
measured values at the electrode surface.
[0157] In this case, cooling with two cooling circuits was used for
the electrode. For this purpose, one of the cooling circuits was
operated with air as coolant, which comes into direct contact, via
fluid-carrying passages, with a plate with a good thermal
conductivity, on which the melt-contact material is arranged. The
measurement curves demonstrate that the method according to the
invention produced a temperature difference of 242.degree. C.
between hot zone 25 and electrode surface. In this context, the
temperature difference can also be increased further, inter alia,
by water being added to the cooling air so as to form an
aerosol.
[0158] FIGS. 6A to 6C show diagrammatic cross sections through a
melting unit 1 to illustrate method steps of a starting operation
or of initial operation. FIG. 6A for this purpose shows the
starting state, in which the melting vessel 3 of the melting unit 1
has been filled with solid melting material 35. The melting
material 35 may be added, for example, in the form of charge cullet
or batch.
[0159] Next, as shown in FIG. 6B, starting electrodes 37 and 39 are
introduced into the melting material. The starting electrodes 37
and 39 are connected to a current source or power supply 41. Then,
a small region of the melting material, for example at the surface,
is heated between the electrodes 37 and 39 by means of a fossil
burner until the conductivity of this small region is sufficient
for conductive heating via the starting electrodes. Next, the
conductive heating between the electrodes 37 and 39 forms a molten
region 220. The electrodes 37, 39 can slowly be moved apart,
correspondingly increasing the size of the region 220 between
them.
[0160] At the same time, the electrodes 51 and 52 which are
integrated in the wall of the melting vessel 3 can be preheated.
This is effected by means of in each case one power supply 33
connected to the melt-contact material of the electrodes, effecting
a transverse current mode and therefore ohmic heating of the
melt-contact material. The walls of the melting vessel 3, on the
side facing the melt, can additionally be lined with a material of
poor electrical and thermal conductivity, for example with ceramic
plates or SiO.sub.2 slip.
[0161] The transverse current mode also causes melting material to
be melted in the region of the electrodes, forming molten regions
221 and 222. When the starting electrodes have been moved so far
apart that they reach the vicinity of the electrodes 51 and 52, the
molten regions 221 and 222 finally in each case come into contact
with the molten region 220. This produces a molten region which
extends from one of the electrodes 51 and 52 to the other, thereby
forming a conductive bridge. After that, the power supply 18 for
the two electrodes 51 and 52 can start to operate, and in this way
the melt can be heated with a high power by means of these
electrodes.
LIST OF REFERENCE SYMBOLS
[0162] 1 Apparatus for heating a melt [0163] 3 Melting vessel
[0164] 51, 501, 502 Electrode [0165] 51 Melt-contact surface of the
electrode 5 [0166] 52 Electrical supply conductors [0167] 53
Melt-contact material of the electrode 5 [0168] 54 Supporting
structure for melt-contact material 53 [0169] 46 Metal holding
plate [0170] 6 Coolant connections [0171] 61 Connections for the
first cooling circuit [0172] 62 Connections for the second cooling
circuit [0173] 7 Tubes of the skull crucible [0174] 9 Inlet [0175]
10 Outlet [0176] 11 Melting material channel [0177] 13 Refractory
collar [0178] 14 Base of the melting vessel [0179] 15 Bottom outlet
[0180] 16 Side wall of the melting vessel 3 [0181] 161, 162
Obliquely positioned side walls of the melting vessel 3 [0182] 18,
20, 33 Alternating current source [0183] 22 Melt [0184] 220, 221,
222 Molten regions [0185] 23 Hot region [0186] 24 Melt bath surface
[0187] 25 Convection circulation [0188] 26 Bridge [0189] 27 Cover
[0190] 28, 29 Gas burner [0191] 30 Feeder [0192] 32 Blowing nozzle
[0193] 34 Supply conductors [0194] 35 Solid melting material [0195]
37, 39 Starting electrodes [0196] 41 Power supply for starting
electrodes 37, 39.
* * * * *