METHOD AND APPARATUS FOR MAKING A GLASS PRODUCT AND CORRESPONDING GLASS PRODUCT

Abstract

A method of making a glass product includes: melting a batch of raw materials to form a glass melt in a melting tank; heating the batch and/or the glass melt using two or more electrodes, the electrodes including an electrode material and heating the batch and/or the glass melt includes operating the electrodes at a current frequency of at least 1,000 Hz and at most 5,000 Hz; withdrawing the glass melt from the melting tank; and forming the glass melt into the glass product.

Claims

1. A method of making a glass product, comprising: melting a batch of raw materials to form a glass melt in a melting tank; heating the batch and/or the glass melt using two or more electrodes, the electrodes comprising an electrode material, wherein heating the batch and/or the glass melt comprises operating the electrodes at a current frequency of at least 1,000 Hz and at most 5,000 Hz; withdrawing the glass melt from the melting tank; and forming the glass melt into the glass product.

2. The method of claim 1, wherein the current frequency is less than 3,000 Hz.

3. The method of claim 1, wherein at least one of the following is satisfied: the method is a continuous process or a batch process; the method further comprises providing additional heating by a fuel burner or no fuel burner is used for additional heating; or the method is a continuous process having a throughput of at least 1 t/d.Math.m.sup.2 of a melting tank cross-section.

4. The method of claim 1, wherein a temperature of the glass melt, a withdrawal rate of glass melt from the melting tank, and an electrical operation frequency of the electrodes is such that a corrosion rate of the electrodes is less than 2.5 mm/a with a current density of 0.5 A/cm.sup.2 and a glass melt temperature of 1,600? C.

5. The method of claim 1, wherein at least one of the following is satisfied: the electrode material is selected from the group consisting of Pt, Rh, Ir, Pd, alloys of these noble metals, Ta, Mo, MoSi.sub.2, MoZrO.sub.2, W, SnO.sub.2, and combinations thereof; a current density used at the electrodes is 0.2 A/cm.sup.2 to 2.0 A/cm.sup.2; or a ratio of a current frequency to an electric conductivity of the glass melt at a temperature T2 is 3 kHz.Math.?.Math.m to 140 kHz.Math.?.Math.m, wherein the temperature T2 is a temperature at which the glass melt has a viscosity of 10.sup.2 dPas.

6. The method of claim 1, wherein an electric conductivity of the glass melt at a temperature T2 is at least 3 S/m, wherein the temperature T2 is a temperature at which the glass melt has a viscosity of 10.sup.2 dPas.

7. An apparatus for glass melting comprising: a melting tank having walls and a bottom for holding a glass melt; a supporting structure for the walls and/or the bottom of the melting tank, the supporting structure comprising non-ferromagnetic materials; two or more electrodes immersible into the glass melt held in the melting tank; a transformer and a frequency changer; and conductors connecting the transformer, frequency changer, and electrodes, where the conductors connecting the electrodes with the frequency changer comprise coaxial shielding.

8. The apparatus of claim 7, wherein the melting tank further comprises a cover.

9. The apparatus of claim 7, further comprising an outlet for the glass melt.

10. The apparatus of claim 7, wherein the non-ferommagnetic material has a relative permeability ?.sub.T of 1.0 to 80.

11. The apparatus of claim 7, wherein the supporting structure comprises bracings and/or guy-wires arranged and/or constructed in a manner interrupting inductive loops.

12. The apparatus of claim 7, wherein at least one of the following is satisfied: one or more electrodes are located partially or completely in or on a wall of the melting tank and/or in or on the bottom of the melting tank and/or constitute a wall section and/or a bottom section of the melting tank; or the electrodes comprise an electrode material selected from the group consisting of Pt, Rh, Ir, Pd, alloys of these noble metals, Ta, Mo, MoSi.sub.2, MoZrO.sub.2, W, SnO.sub.2, and combinations thereof.

13. The apparatus of claim 7, wherein the frequency changer is set to provide a current frequency of at least 1,000 Hz and at most 5,000 Hz.

14. The apparatus of claim 7, wherein a total power dissipation is less than 0.5 kWh/kg glass.

15. The apparatus of claim 7, wherein a power dissipation determined between the frequency changer and the glass melt is less than 35%.

16. A glass product, comprising: a glass composition having a fining agent and an electrode material, wherein the fining agent is present in an amount of at least 300 ppm and the electrode material is present as an oxide in an amount of less than 5 ppm, the glass product comprising less than 2 bubbles per 10 g of glass.

17. The glass product of claim 16, wherein the glass composition has a total carbon content of less than 310 ppm, based on to weight of carbon atoms with respect to a weight of the glass product.

18. The glass product of claim 16, wherein the glass product is produced by a method comprising: melting a batch of raw materials to form a glass melt in a melting tank, heating the batch and/or the glass melt using two or more electrodes, the electrodes comprising an electrode material, wherein heating the batch and/or the glass melt comprises operating the electrodes at a current frequency of at least 1,000 Hz and at most 5,000 Hz; withdrawing the glass melt from the melting tank; and forming the glass melt into the glass product.

19. The glass product of claim 16, wherein: the amount of the electrode material in the form of an oxide is from 0.1 ppm to 5 ppm; and/or the glass composition contains alkali metal oxides in amounts of less than 20% by weight and/or alkaline earth metal oxides in amounts of less than 20% by weight.

20. The glass product of claim 16, wherein the glass composition has at least one of the following: a conductivity for thermal radiation at 1,580? C. of at least 300 W/m.Math.K; an electric conductivity of at least 3 S/m in the molten state at a temperature T2; or a temperature T2 at 1,580? C. or higher and/or a temperature T4 at 1,000? C. or higher; wherein T2 is a temperature where the glass has a viscosity of 10.sup.2 dPas and T4 is a temperature where the glass has a viscosity of 10.sup.4 dPas.

21. The glass product of claim 16, wherein the fining agent is selected from the group consisting of As.sub.2O.sub.3, Sb.sub.2O.sub.3, CeO.sub.2, SnO.sub.2, Fe.sub.2O.sub.3, chloride, fluoride, sulfate, and combinations thereof.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0017] The above-mentioned and other features and advantages of this invention, and the manner of attaining them, will become more apparent and the invention will be better understood by reference to the following description of an embodiment of the invention taken in conjunction with the accompanying drawing, wherein:

[0018] the sole FIGURE illustrates a schematic view of an exemplary embodiment of a glass melting vessel provided according to the invention.

[0019] The exemplification set out herein illustrates an exemplary embodiment of the invention and such exemplification is not to be construed as limiting the scope of the invention in any manner.

DETAILED DESCRIPTION OF THE INVENTION

Definitions

[0020] A glass melt is a volume of a batch of glass raw materials that has a viscosity of less than 10.sup.7.6 dPas.

[0021] Such a viscosity can be measured using the fiber elongation method, e.g. as described in DIN ISO 7884-6:1998-02, where the elongation speed of a fiber with a defined diameter is determined with different weights at different temperatures.

[0022] The temperature at which the glass melt has a viscosity of 10.sup.2 dPas is herein called temperature T2. Similarly, the temperature at which the glass melt has a viscosity of 10.sup.4 dPas is herein called temperature T4. Temperature T2 is less than 1,500? C. for glass compositions with high contents of alkali metal oxides or alkaline earth metal oxides, such as soda lime glass and other glass compositions.

[0023] These viscosities can be measured using a rotational viscosimeter, e.g. as described in DIN ISO 7884-2:1998-2. The dependence of viscosity on temperature is determined according to the VFT equation (Vogel-Fulcher-Tammann). The VFT equation is shown below.

[00001]$\lg \left(?/\mathrm{dPas}\right)=A+\frac{B}{\left(t-t_0\right)}$

[0024] In the VFT equation, t is the temperature under consideration. A, B and to are the so-called VFT constants that are specific for each glass composition.

[0025] A melting tank is a vessel used for melting glass. The vessel defines a volume that can contain a glass melt. The melting tank may have a substantially rectangular base, or bottom plate. It may have walls to keep the melt within the tank. Typically, a melting tank will not be filled to the rim. A melting tank may have a cover above the glass melt surface (covered melting tank). The cover may be vaulted. The melting tank may be a part of a larger melting facility, which may comprise further parts such as a refining tank or refining area. Some melting facilities have a combined tank with different sections, one section for melting and one for refining, in which case the melting tank according to this disclosure relates to the whole combined tank including the refining section.

[0026] The bottom plate is a part of the melting tank that forms the bottom of the tank. The bottom plate may be a single piece of material. Alternatively, the bottom plate may be composed of a plurality of parts or sections. The bottom plate may be closed, i.e. essentially impermeable to the glass melt. Alternatively, the bottom plate may have a closable opening so that glass melt may be withdrawn from the melting tank through the bottom opening.

[0027] A bubble is a gaseous inclusion within the glass or the glass melt, optionally having a diameter of at least 10 ?m. The diameter means the largest diameter of the gaseous inclusion.

[0028] Dwelling time is the time that a given portion of the glass melt spends in the melting tank before being withdrawn from the melting tank. Dwelling time can be measured using so-called tracers, i.e. components that are added to the glass melt so that they can be detected in the product, allowing conclusions as to the time spent in the melting tank. Examples of tracer compounds are Ca, Sr and Y. The average dwelling time is defined as:

[00002]$\frac{\mathrm{melting}\mathrm{tank}\mathrm{volume}\left[m^3\right]}{\mathrm{melting}\mathrm{tank}\mathrm{throughput}\left[\frac{m^3}{h}\right]}$

[0029] The relative permeability ?.sub.T is defined as the ratio of the permeability ? of a specific medium to the permeability ?.sub.0 of free space, wherein ?.sub.0 of vacuum, also called the magnetic constant, is defined as 1.25663706212.Math.10.sup.?6 H/m:

[00003]${?}_T=\frac{?\left[\frac{H}{m}\right]}{{?}_0\left[\frac{H}{m}\right]}$

[0030] The total power dissipation, as referred to within this application, is defined as the sum of the power dissipation of the electric melting process, which may be determined between the frequency converter and the glass melt, and the amount of energy spent for the production of new electrodes and re-heating the melting tank to operation temperature after a shutdown for exchanging the spent electrodes. The term is intended to express the amount of energy spent during the overall process including its auxiliary processes which does not result in heating the melt. The production of new electrodes is very energy consuming. The number of electrodes used for a considered melting tank is of course relevant for the required energy to be spent for the production of the electrodes. Further, the exact amount of energy required differs with the chosen electrode material and size. The same applies for the energy required for re-heating the melting tank. It depends on the type of glass and its respective melt temperature and the size of the melting tank.

DETAILED DESCRIPTION

[0031] This disclosure relates to glass making processes that are as energy efficient as possible, even though the molten glass compositions melt at very high temperatures. They are characterized by a comparatively low carbon footprint. The processes do not sacrifice glass product quality for energy efficiency.

[0032] In some embodiments, this disclosure relates to a method of making a glass product, comprising the steps: [0033] melting a batch of raw materials to form a glass melt in a melting tank, [0034] heating the batch and/or the glass melt using two or more electrodes, the electrodes comprising an electrode material, [0035] withdrawing the glass melt from the melting tank, [0036] forming the glass melt into a glass product, wherein the electrode is operated at a current frequency of at least 1,000 Hz and at most 5,000 Hz.

[0037] The glass product can be a sheet, wafer, plate, tube, rod, ingot, strip or block.

[0038] The inventors found that a particularly low corrosion rate of electrodes is achieved, if the operation frequency of the electrodes is optimized. Prior art processes typically employ frequencies of 50 Hz (line frequency) or slightly above. The inventors analyzed electrode corrosion behavior at these frequencies and other electrode operation frequencies. They found that electrode corrosion can be reduced at operation frequencies significantly higher than 50 Hz and lower than 10 kHz.

[0039] However, it was also found that while increasing the frequencies up to 10 kHz has reduced the corrosion of the electrodes, the increasing reduction in corrosion is linked with an increasing loss of power which can effectively be used for heating because of electric and magnetic stray fields. At the currents required for melting glass types having good electric conductivity, the losses by emission and coupling into surrounding equipment parts are so high that the process is not feasible anymore. The coupling into equipment parts even leads to structural damage of the facility because the amount of heat generated by this induction process in the parts is large enough to let them melt or at least deform.

[0040] The inventors have discovered that the claimed current frequency range will achieve both a good glass quality as well as a low carbon footprint. Particularly when reducing the number of electrodes in a melting tank or producing glass melts having a high electric conductivity, these objects are difficult to achieve. However, a reduction of the number of the electrodes is very desirable because every electrode holder is provided with a water cooling in order to prevent the electrodes from melting which causes a considerable energy loss. The reduced number of electrodes results in a higher current density since the same or even a higher current has to flow through the reduced total surface of the electrodes in order to sufficiently heat the glass melt. Similarly, the current density has to be higher for glass melts having a high electric conductivity in order to generate enough heat. The application of higher current frequencies up to 10 kHz will allow for a higher current density while keeping the corrosion of the electrode at low levels.

[0041] However, at high frequencies there is more loss of power in the cables, the frequency changer, and at the electrodes. The efficiency of frequency changers is decreasing with increasing frequency. So, there will already be a considerable loss of power at the supply of the electrodes. Moreover, the high frequency current creates magnetic alternating fields which will induce eddy currents in the surrounding facility parts which are made of electrically conductive materials. The supporting structures, like beams and traverses, as well as the bracings or guy-wires stabilizing the refractory material of the melting tank walls are usually made of steel. All of them cause a power dissipation by inductive coupling and generation of heat. At 10 kHz, already about a third of the mains power is lost for heating the melt. While the losses would be smaller at lower currents, these would then be insufficient for adequately heating the melt.

[0042] Another aspect of a reduced number of electrodes is the larger distance between them. In a typical production sized glass melting tank, they may then easily be several meters apart from each other. This increased distance between the electrodes leads to an increase in the loss of power due to radio frequency emission. At higher current frequencies, the spaced apart electrodes act like an antenna.

[0043] The electrode may be operated at a current frequency of less than 5,000 Hz, less than 4,500 Hz, less than 4,000 Hz, less than 3,500 Hz, or less than 3,000 Hz. For example, the electrode may be operated at a current frequency of less than 3,000 Hz. The electrode may be operated at a current frequency of at least 1,000 Hz, at least 1,500 Hz, at least 2,000 Hz, or at least 2,500 Hz. A lower limit of 1,000 Hz ascertains that the electrode corrosion is sufficiently low at the required current density and the formation of bubbles is adequately suppressed. An upper limit of 5,000 Hz ascertains that the power loss due to emission and induction remains at an acceptable level.

[0044] In some embodiments, this disclosure relates to a method of making a glass product, comprising the steps: [0045] melting a batch of raw materials to form a glass melt, [0046] heating the batch and/or the glass melt using two or more electrodes, the electrodes comprising an electrode material, [0047] withdrawing the glass melt from the melting tank, [0048] forming the glass melt into a glass product, wherein a temperature of the melt, a withdrawal rate of glass melt from the melting tank, and an operation frequency of the electrodes is such that the corrosion rate of the electrodes is less than 2.5 mm/a with a current density of 0.5 A/cm.sup.2 and a glass melt temperature of 1,600? C.

[0049] It has been found by the inventors to be advantageous to select the listed parameters in combination such that the corrosion rate of the electrodes is less than 2.5 mm/a for optimal results. The corrosion rate may be less than 2.5 mm/a, less than 2.25 mm/a, less than 2.0 mm/a, less than 1.75 mm/a, or less than 1.5 mm/a. The corrosion rate may be at least 0.05 mm/a, at least 0.1 mm/a, at least 0.15 mm/a, at least 0.2 mm/a, at least 0.25 mm/a, or at least 0.3 mm/a.

[0050] In some embodiments, this disclosure relates to a method of making a glass product, comprising the steps: [0051] melting a batch of raw materials to form a glass melt, [0052] heating the batch and/or the glass melt using one or more electrodes, the electrodes comprising an electrode material, [0053] withdrawing the glass melt from the melting tank, [0054] forming the glass melt into a glass product having a glass composition, wherein a fining agent is present in an amount of at least 300 ppm, the electrode material is present as an oxide in an amount of less than 5 ppm, the glass product comprising less than 2 bubbles per 10 g of glass, and wherein the fining agent has an alloy forming property towards the electrode material characterized by the formation of a substitutional alloy at temperature T2.5 of the product's glass composition, wherein optionally the glass composition has a total carbon content of less than 310 ppm, based on the weight of the carbon atoms with respect to the weight of the glass product.

[0055] The fining agent may be present in an amount of at least 300 ppm or at least 400 ppm or at least 500 ppm or at least 600 ppm or at least 700 ppm or at least 800 ppm or at least 900 ppm or at least 1000 ppm. The fining agent may be present in an amount of at most 1000 ppm or at most 900 ppm or at most 800 ppm or at most 700 ppm or at most 600 ppm or at most 500 ppm or at most 400 ppm or at most 300 ppm. The fining agent may be present in an amount of 300 ppm to 1000 ppm or 400 ppm to 1000 ppm or 500 ppm to 1000 ppm or 600 ppm to 1000 ppm or 700 ppm to 1000 ppm.

[0056] The electrode material may be present as an oxide in an amount of less than 5 ppm or less than 4.5 ppm or less than 4 ppm or less than 3.5 ppm or less than 3 ppm or less than 2.5 ppm or less than 2 ppm. The electrode material may be present as an oxide in an amount of more than 0.1 ppm or more than 0.2 ppm or more than 0.3 ppm or more than 0.4 ppm or more than 0.5 ppm or more than 0.75 ppm or more than 1 ppm.

[0057] The total carbon content may be of less than 310 ppm or of less than 300 ppm or of less than 290 ppm or of less than 280 ppm or of less than 270 ppm or of less than 260 ppm or of less than 250 ppm.

[0058] The method provided according to the invention may be a continuous process or a batch process. It is equally suitable for both operation modes and reduces the carbon footprint in both of them. However, since for a batch process a new initial heating step is required for each new batch, the advantage of reduction of the carbon footprint is greater in a continuous process.

[0059] The method may be a continuous process having a throughput of at least 1 t/d.Math.m.sup.2 of melting tank cross-section. The throughput may be at least 1 t/d.Math.m.sup.2 or at least 2 t/d.Math.m.sup.2 or at least 3 t/d.Math.m.sup.2 or at least 4 t/d.Math.m.sup.2. The throughput may be at most 10 t/d.Math.m.sup.2 or at most 9 t/d.Math.m.sup.2 or at most 8 t/d.Math.m.sup.2. The throughput may be 1 t/d.Math.m.sup.2 to 10 t/d.Math.m.sup.2 or 2 t/d.Math.m.sup.2 to 9 t/d.Math.m.sup.2 or 3 t/d.Math.m.sup.2 to 8 t/d.Math.m.sup.2 or 4 t/d.Math.m.sup.2 to 8 t/d.Math.m.sup.2. Such a throughput may aid in reducing the content of the electrode material in the resulting glass product. The increased flow of the glass melt shortens the dwelling time of the melt in the melting tank and consequently the time during which the melt may be contaminated with the dissolved electrode material. Meanwhile, the corrosion rate of the electrodes remains essentially constant since the main factors of influence are the composition of the glass melt, the electrode material, and the current density.

[0060] In some embodiments, additional heating can be provided by a fuel burner, or no fuel burner is used for additional heating. Optionally, no fuel burner is used for additional heating. If the amount of heat required by the glass melt is very high, it may be more favorable in view of the carbon footprint not to increase the current density, as it causes more corrosion and power loss, but to provide additional heat by a burner. This burner may then be fueled by a green gas, i.e. hydrogen produced from renewable energies. However, in general no fuel burner should be used. As an alternative to a fuel burner, also an electrical additional heater can be used.

[0061] An exemplary current density used at the electrodes is 0.2 A/cm.sup.2-2.0 A/cm.sup.2. This range provides the best balance between power input into the melt and corrosion as well as reduction of the number of electrodes. The current density may be 0.2 A/cm.sup.2-2.0 A/cm.sup.2 or 0.3 A/cm.sup.2-1.8 A/cm.sup.2 or 0.4 A/cm.sup.2-1.65 A/cm.sup.2 or 0.5 A/cm.sup.2-1.5 A/cm.sup.2. The current density may be at least 0.2 A/cm.sup.2 or at least 0.3 A/cm.sup.2 or at least 0.4 A/cm.sup.2 or at least 0.5 A/cm.sup.2. The current density may be at most 2.0 A/cm.sup.2 or at most 1.8 A/cm.sup.2 or at most 1.65 A/cm.sup.2 or at most 1.5 A/cm.sup.2.

[0062] In some embodiments, a ratio of the current frequency to the electric conductivity of the melt at temperature T2 is 3 kHz.Math.?.Math.m to 140 kHz.Math.?.Math.m. It has been found by the inventors that by using this ratio range, the current frequency may be set to provide a low carbon footprint for a given glass composition to be melted having a certain electric conductivity. The ratio may be at least 3 kHz.Math.?.Math.m, at least 5 kHz.Math.?.Math.m, at least 7 kHz.Math.?.Math.m, at least 10 kHz.Math.?.Math.m, at least 15 kHz.Math.?.Math.m, at least 20 kHz.Math.?.Math.m, or at least 25 kHz.Math.?.Math.m. The ratio may be at most 140 kHz.Math.?.Math.m, at most 130 kHz.Math.?.Math.m, at most 120 kHz.Math.?.Math.m, at most 110 kHz.Math.?.Math.m, at most 100 kHz.Math.?.Math.m, at most 75 kHz.Math.?.Math.m, at most 50 kHz.Math.?.Math.m. The ratio may be 3 kHz.Math.?.Math.m-140 kHz.Math.?.Math.m, 5 kHz.Math.?.Math.m-130 kHz.Math.?.Math.m, 7 kHz.Math.?.Math.m-120 kHz.Math.?.Math.m, 10 kHz.Math.?.Math.m-110 kHz.Math.?.Math.m, 15 kHz.Math.?.Math.m-100 kHz.Math.?.Math.m, 20 kHz.Math.?.Math.m-75 kHz.Math.?.Math.m, or 25 kHz.Math.?.Math.m-50 kHz.Math.?.Math.m.

[0063] In some embodiments, the electric conductivity of the melt at temperature T2 is at least 3 S/m. The current method is particularly useful for glass compositions which form a melt having a high electric conductivity because the problem of an increased carbon footprint is very pronounced in these types of glass and not yet satisfactorily solved in the state of the art. The electric conductivity may be at least 3 S/m, at least 4 S/m, at least 5 S/m, at least 10 S/m, at least 15 S/m, at least 20 S/m, at least 25 S/m, or at least 30 S/m. The electric conductivity may be at most 45 S/m, at most 44 S/m, at most 43 S/m, at most 42 S/m, at most 41 S/m, at most 40 S/m, at most 38 S/m, or at most 36 S/m. The electric conductivity may be 3 S/m-45 S/m, 4 S/m-44 S/m, 5 S/m-43 S/m, 10 S/m-42 S/m, 15 S/m-41 S/m, 20 S/m-40 S/m, 25 S/m-38 S/m, or 30 S/m-36 S/m.

[0064] In some embodiments, this disclosure relates to an apparatus for glass melting comprising: [0065] a melting tank having walls and a bottom, and optionally a cover, for holding a glass melt, [0066] a supporting structure for the walls and/or the bottom and/or the cover of the melting tank, [0067] two or more electrodes immersible into the glass melt, [0068] a transformer and a frequency changer, [0069] conductors connecting the transformer, frequency changer and electrodes, and [0070] optionally an outlet for the glass melt, wherein [0071] the conductors connecting the electrodes with the frequency changer comprise coaxial shielding means, and/or [0072] the supporting structure comprises, optionally consists of, non-ferromagnetic materials, optionally a material having a relative permeability ?.sub.T of 1.0 to 80, such as stainless steel.

[0073] The power dissipation and, hence, the carbon footprint of the method may further be reduced by providing the conductors with a shielding and/or using non-ferromagnetic materials for the supporting structure. Both features avoid or at least reduce the electromagnetic coupling of the high frequency current into the surrounding parts of the apparatus.

[0074] Optionally, the frequency changer is set to provide a current frequency of at least 1,000 Hz and at most 5,000 Hz.

[0075] Optionally, the supporting structure comprises bracings and/or guy-wires being arranged and/or constructed in a manner interrupting inductive loops. This means that those parts of the apparatus are designed and/or attached such that they avoid the induction of eddy currents resulting in a heating of them. This additional measure may further reduce the power dissipation or vice versa allow for higher currents.

[0076] The electrode material may be selected from Pt, Rh, Ir, Pd, alloys of these noble metals, Ta, Mo, MoSi.sub.2, MoZrO.sub.2, W, SnO.sub.2, and combinations thereof. By using one of these materials, the overall carbon footprint can be optimized. Those electrodes provide an optimal combination of low corrosion, comparatively low carbon footprint during manufacture, and suitability for the operation at the claimed current frequencies.

[0077] Some fining agents have a tendency to form alloys with electrode material. These combinations of fining agent and electrode material require most attention by the glassmaker because the fining agent may accelerate dissolution of electrode material into the glass melt. Resulting glass products may be unusable for the intended purpose.

[0078] In some embodiments, one or more electrodes are located partially or completely in or on a wall of the melting tank and/or in or on the bottom of the melting tank and/or constitute a wall section and/or a bottom section of the melting tank. One or more electrodes may be located partially or completely in or on a wall of the melting tank. One or more electrodes may be located partially or completely in or on a bottom plate of the melting tank. In some embodiments, one or more electrodes constitute a wall section and/or a bottom plate section of the melting tank. In some embodiments, one or more of the electrodes may be extending upwardly from the bottom of the melting tank up to at least 50% glass melt depth, optionally at least 60%, at least 70% or at least 80% of the glass melt depth. Optionally, the one or more electrodes may extend up to 100%, up to 95% or up to 90% of the glass melt depth from the bottom of the melting tank. Optionally, the one or more electrodes may extend from 50% to 100%, from 60% to 95%, or from 70% to 90% of the glass melt depth from the bottom of the melting tank.

[0079] In some embodiments, the total power dissipation is less than 0.5 kWh/kg glass. The power dissipation of the melting process may be determined between the frequency converter and the glass melt. The total power dissipation is the sum of this power dissipation and the amount of energy spent for the production of new electrodes and re-heating the melting tank to operation temperature after a shutdown for exchanging the spent electrodes. Optionally, the power dissipation may be less than 0.5 kWh/kg glass, less than 0.4 kWh/kg glass, less than 0.3 kWh/kg glass, less than 0.2 kWh/kg glass or less than 0.1 kWh/kg glass. Optionally, the power dissipation may be more than 0.01 kWh/kg glass, more than 0.02 kWh/kg glass, more than 0.03 kWh/kg glass, more than 0.04 kWh/kg glass or more than 0.05 kWh/kg glass.

[0080] In some embodiments, the power dissipation is less than 35%. The power dissipation of the melting process may be determined between the frequency converter and the glass melt. The total power dissipation is the sum of this power dissipation and the amount of energy spent for the production of new electrodes and re-heating the melting tank to operation temperature after a shutdown for exchanging the spent electrodes. Optionally, the power dissipation may be less than 35%, less than 30%, less than 25%, less than 20% or less than 10%. Optionally, the power dissipation may be more than 1%, more than 2%, more than 4%, more than 6% or more than 8%.

[0081] In some embodiments, this disclosure relates to a glass product, optionally obtainable according to the method disclosed herein, comprising a glass composition having a fining agent and electrode material, wherein the fining agent is present in an amount of at least 300 ppm and the electrode material is present as an oxide in an amount of less than 5 ppm, the glass product comprising less than 2 bubbles per 10 g of glass, wherein optionally the glass composition has a total carbon content of less than 310 ppm, based on the weight of the carbon atoms with respect to the weight of the glass product.

[0082] The fining agent may be present in an amount of at least 300 ppm or at least 400 ppm or at least 500 ppm or at least 600 ppm or at least 700 ppm or at least 800 ppm or at least 900 ppm or at least 1000 ppm. The fining agent may be present in an amount of at most 1000 ppm or at most 900 ppm or at most 800 ppm or at most 700 ppm or at most 600 ppm or at most 500 ppm or at most 400 ppm or at most 300 ppm. The fining agent may be present in an amount of 300 ppm to 1000 ppm or 400 ppm to 1000 ppm or 500 ppm to 1000 ppm or 600 ppm to 1000 ppm or 700 ppm to 1000 ppm.

[0083] The electrode material may be present as an oxide in an amount of less than 5 ppm or less than 4.5 ppm or less than 4 ppm or less than 3.5 ppm or less than 3 ppm or less than 2.5 ppm or less than 2 ppm. The electrode material may be present as an oxide in an amount of more than 0.1 ppm or more than 0.2 ppm or more than 0.3 ppm or more than 0.4 ppm or more than 0.5 ppm or more than 0.75 ppm or more than 1 ppm.

[0084] The total carbon content may be of less than 310 ppm or of less than 300 ppm or of less than 290 ppm or of less than 280 ppm or of less than 270 ppm or of less than 260 ppm or of less than 250 ppm.

[0085] In some embodiments, this disclosure relates to a glass product comprising a fining agent and electrode material, wherein the fining agent is present in an amount of at least 300 ppm, the electrode material is present as an oxide in an amount of less than 5 ppm, the glass product comprising less than 2 bubbles per 10 g of glass, and wherein fining agent has an alloy forming property towards the electrode material characterized by the formation of a substitutional alloy at temperature T2.5 of the product's glass composition, wherein optionally the glass composition has a total carbon content of less than 310 ppm, based on the weight of the carbon atoms with respect to the weight of the glass product.

[0086] The fining agent may be present in an amount of at least 300 ppm or at least 400 ppm or at least 500 ppm or at least 600 ppm or at least 700 ppm or at least 800 ppm or at least 900 ppm or at least 1000 ppm. The fining agent may be present in an amount of at most 1000 ppm or at most 900 ppm or at most 800 ppm or at most 700 ppm or at most 600 ppm or at most 500 ppm or at most 400 ppm or at most 300 ppm. The fining agent may be present in an amount of 300 ppm to 1000 ppm or 400 ppm to 1000 ppm or 500 ppm to 1000 ppm or 600 ppm to 1000 ppm or 700 ppm to 1000 ppm.

[0087] The electrode material may be present as an oxide in an amount of less than 5 ppm or less than 4.5 ppm or less than 4 ppm or less than 3.5 ppm or less than 3 ppm or less than 2.5 ppm or less than 2 ppm. The electrode material may be present as an oxide in an amount of more than 0.1 ppm or more than 0.2 ppm or more than 0.3 ppm or more than 0.4 ppm or more than 0.5 ppm or more than 0.75 ppm or more than 1 ppm.

[0088] The total carbon content may be of less than 310 ppm or of less than 300 ppm or of less than 290 ppm or of less than 280 ppm or of less than 270 ppm or of less than 260 ppm or of less than 250 ppm.

[0089] Using the method disclosed herein, glass products are available that contain less electrode material, despite being fined with alloy-forming fining agents. This allows for efficient bubble removal even in these sensitive glass melts. Moreover, the glass product may have a low total carbon content because the improved heating technology presented herein does not introduce carbon dioxide into the melt.

[0090] In some embodiments, the glass product may comprise a glass composition having an amount of an electrode material in the form of an oxide of from 0.1 ppm to 5 ppm, wherein optionally the electrode material is selected from Pt, Rh, Jr, Pd, alloys of these noble metals, Ta, Mo, MoSi.sub.2, MoZrO.sub.2, W, SnO.sub.2 and combinations thereof.

[0091] The electrode material may be present as an oxide in an amount of from 0.1 ppm to 5 ppm, from 0.2 ppm to 4.5 ppm, from 0.3 ppm to 4 ppm, from 0.4 ppm to 3.5 ppm, from 0.5 ppm to 3 ppm, from 0.75 ppm to 2.5 ppm, or from 1 ppm to 2.5 ppm, The electrode material may be present as an oxide in an amount of less than 5 ppm or less than 4.5 ppm or less than 4 ppm or less than 3.5 ppm or less than 3 ppm or less than 2.5 ppm or less than 2 ppm. The electrode material may be present as an oxide in an amount of more than 0.1 ppm or more than 0.2 ppm or more than 0.3 ppm or more than 0.4 ppm or more than 0.5 ppm or more than 0.75 ppm or more than 1 ppm.

[0092] In some embodiments, the glass product may comprise a glass composition having a conductivity for thermal radiation at 1,580? C. of at least 300 W/m.Math.K. The conductivity for thermal radiation may be at least 300 W/m.Math.K, at least 310 W/m.Math.K, at least 320 W/m.Math.K, or at least 330 W/m.Math.K. The conductivity for thermal radiation may be at most 800 W/m.Math.K, at most 700 W/m.Math.K, at most 600 W/m.Math.K, or at most 500 W/m.Math.K.

[0093] In some embodiments, the glass product may comprise a fining agent, which is selected from the group consisting of As.sub.2O.sub.3, Sb.sub.2O.sub.3, CeO.sub.2, SnO.sub.2, Fe.sub.2O.sub.3, chloride, fluoride, sulfate, and combinations thereof.

[0094] In some embodiments, the glass product may comprise a glass composition having a T2 at 1,580? C. or higher, and/or a temperature T4 at 1,000? C. or higher.

[0095] The method provided according to the invention is particularly useful for glass compositions with very high melting temperatures, such as glass compositions comprising only limited amounts of alkali and alkaline earth metal oxides. The composition of the glass may, for example, be such that the melt has a viscosity of 100 dPas at temperatures above 1,580? C. When heating the melt to a temperature sufficiently high that the viscosity is 10.sup.2.5 dPas or less, a lot of energy is required. However, low viscosities and the corresponding high temperatures are desirable for bubbles to leave the melt.

[0096] Optionally, the glass compositions used according to the invention have T2 temperatures much higher than 1,500? C. The T2 temperature for the glass melt in the melting tank during the method provided according to this disclosure may be above 1,580? C. and optionally even above 1,600? C. or above 1,620? C. In some embodiments, T2 temperature of the glass compositions may be less than 1,800? C., less than 1,750? C. or less than 1,700? C. Glass compositions with very high T2 temperatures are very difficult to process and require a lot of energy for melting.

[0097] The T4 temperature of the glass composition in the melting tank during the method provided according to this disclosure may be above 1,000? C. and optionally even above 1,050? C. or above 1,120? C. In some embodiments, T4 temperature of the glass compositions may be less than 1,400? C., less than 1,350? C. or less than 1,300? C. Glass compositions with very high T4 temperatures are very difficult to process and require a lot of energy for melting.

[0098] In some embodiments, the glass product may comprise a glass composition having an electric conductivity of at least 3 S/m in the molten state at temperature T2. The electric conductivity may be at least 3 S/m, at least 4 S/m, at least 5 S/m, at least 10 S/m, at least 15 S/m, at least 20 S/m, at least 25 S/m, or at least 30 S/m. The electric conductivity may be at most 45 S/m, at most 44 S/m, at most 43 S/m, at most 42 S/m, at most 41 S/m, at most 40 S/m, at most 38 S/m, or at most 36 S/m. The electric conductivity may be 3 S/m-45 S/m, 4 S/m-44 S/m, 5 S/m-43 S/m, 10 S/m-42 S/m, 15 S/m-41 S/m, 20 S/m-40 S/m, 25 S/m-38 S/m, or 30 S/m-36 S/m.

[0099] In some embodiments, the glass product may comprise a glass composition which contains alkali metal oxides in amounts of less than 20% by weight, less than 15% by weight, less than 12% by weight, less than 10% by weight or less than 5% by weight. Optionally, the glass composition may be free of alkali metal oxides. In some embodiments, the amount of alkali metal oxides in the glass composition may be at least 1% by weight or at least 2% by weight. Optionally, the amount of alkali metal oxides in the glass composition may be ?1% by weight and <20% by weight or ?2% by weight<10% by weight.

[0100] The glass composition may be a borosilicate, alumino-borosilicate, or aluminosilicate glass.

[0101] In some embodiments, the glass composition may contain alkaline earth metal oxides in amounts of less than 20% by weight, less than 15% by weight, less than 12% by weight, less than 10% by weight, or less than 5% by weight. Optionally, the glass composition may be free of alkaline earth metal oxides. In some embodiments, the amount of alkaline earth metal oxides in the glass composition may be at least 1% by weight or at least 2% by weight. Optionally, the amount of alkaline earth metal oxides in the glass composition may be ?1% by weight and <20% by weight or ?2% by weight<10% by weight.

[0102] Optional glass compositions include Al.sub.2O.sub.3 in an amount of at least 1.5% by weight or at least 5.0% by weight or even at least 10.0% by weight. The amount of Al.sub.2O.sub.3 may be up to 23.0% by weight, up to 20.0% by weight or up to 18.0% by weight. In some embodiments, the amount of Al.sub.2O.sub.3 may range from 1.5% to 23.0% by weight, from 5.0% to 20.0% by weight or from 10.0% to 18.0% by weight.

[0103] Additionally or alternatively, the glass composition may include B.sub.2O.sub.3 in an amount of at least 0.0% by weight or at least 8.0% by weight or even at least 10.0% by weight. The amount of B.sub.2O.sub.3 may be up to 20.0% by weight, up to 16.0% by weight or up to 14.0% by weight. In some embodiments, the amount of B.sub.2O.sub.3 may range from 0.0% to 20.0% by weight, from 8.0% to 16.0% by weight or from 10.0% to 14.0% by weight.

[0104] Many highly viscous glass compositions contain significant amounts of SiO.sub.2, Al.sub.2O.sub.3 and B.sub.2O.sub.3. Optionally, the glass compositions used according to the invention have a total content of SiO.sub.2, Al.sub.2O.sub.3 and B.sub.2O.sub.3 of at least 75.0% by weight, at least 78.0% by weight or even at least 85.0% by weight. The total amount of SiO.sub.2, Al.sub.2O.sub.3 and B.sub.2O.sub.3 may be limited to not more than 97.0% by weight, up to 93.5% by weight or up to 90.0% by weight. Optionally, the amount of SiO.sub.2, Al.sub.2O.sub.3 and B.sub.2O.sub.3 may range from 75.0% to 95.0% by weight, from 78.0% to 92.5% by weight or from 85.0% to 90.0% by weight.

[0105] Optionally, glass compositions used according to the invention may comprise (in % by weight, the composition summing up to 100%):

TABLE-US-00001 SiO.sub.2 71-77 B.sub.2O.sub.3 9-12 Al.sub.2O.sub.3 5.5-8.sup. Na.sub.2O 6-8 K.sub.2O 0.1-0.9 Li.sub.2O .sup.0-0.3 CaO .sup.0-1.5 BaO 0-1 F .sup.0-0.3 Cl .sup.0-0.3 MgO + CaO + BaO + SrO 0-2 or SiO.sub.2 71-77 B.sub.2O.sub.3 9-12 Al.sub.2O.sub.3 3.5-6.sup. Na.sub.2O 5.5-8.sup. K.sub.2O .sup.0-0.5 Li.sub.2O .sup.0-0.3 CaO 0-3 BaO .sup.0-1.5 F .sup.0-0.3 Cl .sup.0-0.3 MgO + CaO + BaO + SrO 0-2 or SiO.sub.2 60-85 Al.sub.2O.sub.3 0-10 B.sub.2O.sub.3 5-20 Li.sub.2O + Na.sub.2O + K.sub.2O 2-16 MgO + CaO + SrO + BaO + ZnO 0-15 TiO.sub.2 + ZrO.sub.2 0-6 P.sub.2O.sub.5 0-2 [0106] optionally further comprising [0107] coloring oxides, such as Nd.sub.2O.sub.3, Fe.sub.2O.sub.3, CoO, NiO, V.sub.2O.sub.5, MnO.sub.2, TiO.sub.2, CuO, CeO.sub.2, Cr.sub.2O.sub.3, [0108] 0-2% by weight fining agents, such as As.sub.2O.sub.3, Sb.sub.2O.sub.3, SnO.sub.2, SO.sub.3, Cl, F and/or CeO.sub.2, and [0109] 0-5% by weight rare earth metal oxides.

[0110] In some embodiments, the glass composition canalternatively or additionally to the compositions described abovebe described by the following composition ranges.

[0111] In some embodiments, the glass composition can be a borosilicate glass which contains the following components in wt.-%:

TABLE-US-00002 SiO.sub.2 70.0 to 87.0 B.sub.2O.sub.3 7.0 to 25.0 Na.sub.2O + K.sub.2O 0.5 to 9.0 Al.sub.2O.sub.3 0.0 to 7.0 CaO 0.0 to 3.0.

[0112] In some embodiments, the glass composition can be a borosilicate glass which contains the following components in wt.-%:

TABLE-US-00003 SiO.sub.2 70.0 to 86.0 Al.sub.2O.sub.3 0.0 to 8.0, or 0.0 to 5.0 B.sub.2O.sub.3 9.0 to 25.0 Na.sub.2O 0.5 to 8.0, or 0.5 to 5.0 K.sub.2O 0.0 to 1.0 Li.sub.2O 0.0 to 2.0, or 0.0 to 1.0.

[0113] In some embodiments, the glass composition can be a borosilicate glass which contains the following components in wt.-%:

TABLE-US-00004 SiO.sub.2 70.0 to 80.0, or 71.0 to 77.0 Al.sub.2O.sub.3 3.0 to 8.0, or 3.5 to 8.0 B.sub.2O.sub.3 9.0 to 15.0, or 9.0 to 12.0 Na.sub.2O 5.5 to 8.0 K.sub.2O 0.0 to 1.0, or 0.1 to 0.5 Li.sub.2O 0.0 to 0.5, or 0.0 to 0.3 CaO 0.0 to 3.0, or 0.0 to 1.5 BaO 0.0 to 1.5 F.sup.? 0.0 to 0.3 Cl.sup.? 0.0 to 0.3 MgO + CaO + BaO + SrO 0.0 to 2.0.

[0114] In some embodiments, the glass composition can be an alkali borosilicate glass which contains the following components in wt.-%:

TABLE-US-00005 SiO.sub.2 78.3 to 81.0 Al.sub.2O.sub.3 3.5 to 5.3 B.sub.2O.sub.3 9.0 to 13.0 Na.sub.2O 3.5 to 6.5 K.sub.2O 0.3 to 2.0 CaO 0.0 to 2.0.

[0115] In some embodiments, the glass composition can be an alkali borosilicate glass which contains the following components in wt.-%:

TABLE-US-00006 SiO.sub.2 55.0 to 85.0 Al.sub.2O.sub.3 0.0 to 15.0 B.sub.2O.sub.3 3.0 to 20.0 Na.sub.2O 3.0 to 15.0 K.sub.2O 3.0 to 15.0 ZnO 0.0 to 12.0 TiO.sub.2 0.5 to 10.0 CaO 0.0 to 0.1.

[0116] In some embodiments, the glass composition can contain the following components in wt.-%:

TABLE-US-00007 SiO.sub.2 58.0 to 75.0 Al.sub.2O.sub.3 18.0 to 25.0 Li.sub.2O 3.0 to 6.0 Na.sub.2O + K.sub.2O 0.1 to 2.0 MgO + CaO + BaO + ZnO 1.5 to 6.0 TiO.sub.2 + ZrO.sub.2 2.0 to 6.0
and optionally one or more of the oxides from Co, Ni, Fe, Nd, Mo, and optionally one or more refining agents selected from the group of SnO.sub.2, chlorides, As.sub.2O.sub.5, Sb.sub.2O.sub.5, optionally 0.1 to 1.5 wt.-% SnO.sub.2, or optionally 0.1 to 1.5 wt.-% As.sub.2O.sub.5, or optionally 0.1 to 1.5 wt.-% Sb.sub.2O.sub.5.

[0117] In some embodiments, the glass composition can contain the following components in wt.-%:

TABLE-US-00008 SiO.sub.2 58.0 to 65.0 Al.sub.2O.sub.3 14.0 to 25.0 B.sub.2O.sub.3 6.0 to 10.5 MgO 0.0 to 3.0 CaO 0.0 to 9.0 BaO 3.0 to 8.0 ZnO 0.0 to 2.0, [0118] wherein the sum of MgO, CaO and Bao is from 8.0 to 18.0 wt.-%.

[0119] In some embodiments, the glass composition can contain the following components in wt.-%:

TABLE-US-00009 SiO.sub.2 50.0 to 68.0, or 55.0 to 68.0 Al.sub.2O.sub.3 15.0 to 20.0 B.sub.2O.sub.3 0.0 to 6.0 Li.sub.2O 0.0 to 6.0 Na.sub.2O 1.5 to 16.0, or 8.0 to 16.0 K.sub.2O 0.0 to 5.0 MgO 0.0 to 5.0 CaO 0.0 to 7.0, or 0.0 to 1.0 ZnO 0.0 to 4.0, or 0.0 to 1.0 ZrO.sub.2 0.0 to 4.0 TiO.sub.2 0.0 to 1.0, or substantially free from TiO.sub.2.

Examples

[0120] Dissolution of Electrode Material in a Glass Melt

[0121] A glass melt for a borosilicate glass for pharmaceutical packaging, comprising 74-77% by weight SiO.sub.2, 9-12% by weight B.sub.2O.sub.3, 3.5-6% by weight Al.sub.2O.sub.3, 5.5-8% by weight Na.sub.2O, 0-0.5% by weight K.sub.2O, 0-0.3% by weight Li.sub.2O, and 0-3% by weight CaO, was heated to 1,620? C. using molybdenum electrodes immersed into the melt. Two electrodes were used. Different current frequencies were applied, namely 50 Hz, 1 kHz, and 10 kHz. Current density was 0.5 A/cm.sup.2 for all experiments.

[0122] Electrode dissolution was the highest at 50 Hz. Dissolved electrode material, mainly in the form of MoO.sub.3, gave the glass a green color. Dissolution of electrode material was far less at 10 kHz but still noticeable. The measured content of electrode material in the glass has been 45 ppm for the 50 Hz melt and 3 ppm for the 1 kHz and the 10 kHz melt.

[0123] Referring now to the sole FIGURE, the sole FIGURE shows a scheme of an exemplary embodiment of a glass melting vessel 1, which may also be referred to as a glass melting apparatus and/or an apparatus for glass melting, provided according to the invention with a single heating circuit using one pair of electrodes 6. The purpose of the sole FIGURE is to exemplify the constituents of a heating circuit as used within this application.

[0124] The upper left corner of the sole FIGURE indicates the three conductors L1, L2, L3 of a three-phase power line provided from the power grid. These are connected to a frequency changer 2 which converts the power grid frequency of 50/60 Hz to a medium range frequency of from 50 Hz to 25,000 Hz. The frequency changer 2 is connected via a power factor correction 3 for prevention of harmonic currents to a transformer 4. In a melting vessel 5, which may also be referred to as a melting tank, a pair of electrodes 6 is arranged below a surface 7 of a glass melt 8 such that the electrodes 6 coming from below are fully immersed in the glass melt 8. Conductors 12 connect the transformer 4, frequency changer 2, and electrodes 6 whereas the conductors connecting the electrodes 6 comprise coaxial shielding 13. The melting tank 5 has walls 9 and a bottom 10, optionally a cover, and a supporting structure 11 for the walls 9 and/or the bottom 10 and/or the cover of the melting tank 5, and optionally an outlet for the glass melt 9. The supporting structure 11 may comprise bracings and/or guy-wires 14 being arranged and/or constructed in a manner interrupting inductive loops.

[0125] While this invention has been described with respect to at least one embodiment, the present invention can be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles. Further, this application is intended to cover such departures from the present disclosure as come within known or customary practice in the art to which this invention pertains and which fall within the limits of the appended claims.