Abstract:
A float glass apparatus typically includes a float tank for a metal bath, the metal bath, a glass melt producing unit, which includes a melting tank for making a glass melt and a refining, tank for degassing the glass melt, and conducting devices for conducting the glass melt from the glass melt producing unit to the metal bath of the float tank and auxiliary devices as needed. In order to minimize bubble defects and extend service life of Pt units by a suitable electrical device the float tank with the metal bath is connected electrically with ground and at least one auxiliary electrode is connected with ground and put in electrical contact with the glass melt in the melting tank.

Description:
BACKGROUND OF THE INVENTION  
       [0001]     1. Field of the Invention  
         [0002]     The present invention relates to a device for electrically grounding a float glass production apparatus comprising a float tank with a metal bath; a glass melt producing unit, which comprises a melting tank for making the glass melt and a refining tank for degassing the glass melt; conducting devices for conducting the glass melt from the glass melt producing unit to the metal bath in the float tank and, as required, auxiliary units.  
         [0003]     2. Related Art  
         [0004]     The production of float glass, especially of special glass, occurs in a so-called flat glass production apparatus of flat glass plant. The term “float glass production apparatus or plant” in the sense of the present invention means the entire apparatus of a typical structure, i.e. a production unit, which includes the entire hot region from the melting tank and refining tank up to the float bath upstream of the annealing system. Since the hot glass is a good ion conductor, i.e. behaves like an electrolyte, the production steps of the “hot region” are unavoidable, also electrolytically connected with each other. Because of the typical structure of the production unit also part of the glass melt has at least locally direct conducting contact to the metal parts of this apparatus. These metal parts thus can be characterized as or have the character of electrodes, at least in an electrochemical sense. A short-circuited electrochemical battery is realized with each exterior low-resistance connection between each pair or several of these electrodes, e.g. by a common ground. The associated short-circuit current flows as a direct current through the electrolyte glass and can be the cause of bubble-forming or alloy-forming side reactions at the so-called interface, the region between the melt producing vessel and the float tank.  
         [0005]     Examples of parts, which have direct contact with the glass and which have the potential to act as electrodes, include grounding electrodes, heated electrodes from Pt, Mo, Ir, etc., glass level meters (operating according to the principle of resistance measurement), direct thermocouple elements, Mo enclosure (for corrosion of the fire-resistant surrounding wall), bottom outlet, outlet pipes, stirrers and shut-off slide (also called “tweel”) made of platinum or its alloys with other noble metals and/or coated with these other metals, and the tin bath.  
         [0006]     Besides these so-called “notorious” direct electrode types other potential electrodes exists, which indeed have no direct contact with the glass but in spite of that are electrochemically connected to the glass surface. That means all metallic structures with free surfaces in the superstructure of the melting tank and refining tank or of the troughs. Included among these parts are, among others, direct thermo-elements in arched structures, viewing flaps or lids, parts of the overflow, gas and oil burners and parts of the loading apparatus. The electrolytic contact of these indirect electrodes occurs in this case by means of the hot gas atmosphere. Electrochemical equilibrium between the free glass surface and the concerned metal parts is guaranteed when the gas atmosphere contains sufficient amounts of redox active chemical components. Mixtures of redox pairs, water/hydrogen, water/methane or CO 2 /CO, are examples of redox active chemical components. Appropriate boundary surface reactions and rapid transport of gaseous species in the atmosphere above the melt guarantee exchange and transmission of charge equivalents over large distances—the gas atmosphere thus has quasi-electrolytic properties.  
         [0007]     A similar situation exists in a typically grounded float bath, the float tank. The grounding of the glass typically occurs via the liquid tin and the metallic housing. The forming gas atmosphere has direct, surface contact with both and it fulfills the requirements of a buffering redox system with its component mixture, water/hydrogen. Normally because of that it would be guaranteed that the platinum coated tweel at the entrance to the float tank is electrically connected with the float bath ground. The extent of the short circuit can be controlled by the local composition of the forming gas: high hydrogen content in the vicinity of the tweel promotes e.g. current flow from the melting tank to the float tank ground.  
         [0008]     The huge difference in the concentration of oxygen in the melting tank and the float bath is the main situation that a direct voltage arises in the apparatus. Oxygen pressure of one to up to two bar must be produced in the refining tank with oxygen-refined special glass, in order to guarantee formation of refined bubbles. In contrast the oxygen pressure in the float bath must be 10 −15  to 10 −18  bar, to prevent coating of the tin bath with a tin oxide layer. A reduction of the concentration difference is thus not possible.  
         [0009]     Thus a short circuit electrochemical chain arises so that direct currents flow from anode to cathode (positive current flow) in the glass. The interior resistance of this short-circuited battery is the sum of all partial resistances for charge transport and boundary surface conversion. The current strength is then the quotient of the battery voltage and the total resistance.  
         [0010]     The current flow has consequences for 
        local bubble formation at all phase transitions between glass and metal, e.g. at joining points between outlet pipe segments made of Pt, at stirrer surfaces, at the tweel and at glass/Sn contacts;     local corrosion of Pt components at the interface because of increased incorporation of alloyable glass components, especially Sn, Si, . . . ;     the monitoring of the electrochemical potential of metallic components, special stirrers, outlet pipe segments or tweel. The direct currents in the measurement path, reference/component are always considered in evaluating electromotive forces, which are measured between the concerned component and a suitable reference electrode.        
 
         [0014]     The essential prerequisite for occurrence of direct current flow with disadvantageous effects is the existence of a sufficiently low-resistance connection between at least two potential electrodes. This sort of connection is realized intentionally or unintentionally by a so-called ground connection in the daily production routine.  
         [0015]     Intentional ground conditions are frequently closely connected with regulations for guaranteeing personal safety. They replace laborious or elaborate safety measures for routine handling. Likewise frequently ground-free construction of an apparatus means an expensive selection of materials during design or it simply is not a useful solution.  
         [0016]     If the principle of ground-free construction is not part of the specifications of the apparatus, there are consequences. Once built up, many ground connections may not be undone with reasonable effort or expense.  
         [0017]     The most important example here is the tin bath in the float tank, whose subsequent ground disconnection is not possible. Generally the liquid tin in the completely hot region remains the single direct connection to ground, but all would still be in order from an electrochemical viewpoint, when the current circuit is not closed on account of the absent second and further ground connections.  
         [0018]     However other grounds may exist based on that. The supply apparatus, the overflow and the burners in the melting tank come into question. Subsequent ground disconnection is indeed possible in principle but only with a great effort or expense. The electrical contact here occurs by means of the furnace atmosphere, i.e. an actual ground circuit formed is high resistance. Typically also the ground is not definitely localized and it is characterized as a virtual ground.  
         [0019]     Usually all measuring units (thermocouple elements, glass level meters), viewing closures, stirrers and all components at the interface, which are made of platinum or its alloys with other noble metals, are built-in without grounding without further effort.  
         [0020]     Every noble metal uncoated tweel is designed “weakly” grounded by the forming gas atmosphere (i.e. a high resistance ground).  
         [0021]     Additional grounds may exist at the so-called cold end, i.e. at the cooling sheet; however they are all not damaging from an electrochemical viewpoint and thus must be not considered further.  
       SUMMARY OF THE INVENTION  
       [0022]     It is an object of the present invention to provide a device as characterized herein for electrical grounding of a float glass production apparatus, so that the direct current flow dependent on electrochemical effects between the melting tank and the downstream float tank is at least minimized by suppression of the boundary surface reactions (minimizing the bubble formation and corrosion) in the production process.  
         [0023]     These objects and others which will be made more apparent hereinafter are attained in a device for electrical grounding of a float glass apparatus comprising a float tank for a metal bath, the metal bath; a glass melt producing unit that includes a melting tank for making a glass melt and a refining tank for degassing the glass melt; conducting devices for feeding the glass melt from the glass melt producing unit to the metal bath of the float tank and auxiliary devices as needed.  
         [0024]     According to the invention the float tank with the metal bath is connected electrically with ground and at least one auxiliary electrode connected with ground is put in electrical contact with the glass melt in the melting tank.  
         [0025]     The troublesome direct current flows can be minimized in a surprising way by the features according to the invention.  
         [0026]     Furthermore embodiments and additional aspects of the invention are set forth in the appended dependent claims, and in the following description of the preferred embodiments. 
     
    
     BRIEF DESCRIPTION OF THE DRAWING  
       [0027]     The objects, features and advantages of the invention will now be illustrated in more detail with the aid of the following description of the preferred embodiments, with reference to the accompanying figures in which:  
         [0028]      FIG. 1  is a diagrammatic cross-sectional view of the entire hot region of a float glass production apparatus comprising a glass melt producing unit, interface and float tank, along with the chemical reactions occurring therein;  
         [0029]      FIG. 2  is a diagrammatic cross-sectional view of another embodiment of a float glass production apparatus similar to that of  FIG. 1 , but with a modified interface, with a direct measurement of the current, which flows through the interface;  
         [0030]      FIG. 3  is a diagrammatic cross-sectional view of another embodiment of a float glass production apparatus similar to that of  FIG. 2 , but with an indirect measurement of the current, which flows through the interface;  
         [0031]      FIG. 4  is a glass melting tank part of the float glass producing apparatus according to  FIG. 1  with a tank equipped with two auxiliary electrodes, in which rectifiers or diodes are built into each auxiliary electrode circuit to prevent current from flowing through both auxiliary electrodes in different directions (short circuiting current circuit); and  
         [0032]      FIG. 5  is a glass float producing apparatus according to  FIGS. 2 and 3  with automatic current regulation. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0033]     The first production unit of a float glass production apparatus according to the invention comprises a glass melt producing unit with a melting tank  1 , in which a predetermined batch of starting materials is melted in a known way to form the glass to be floated. The required melt energy is supplied in a known manner. The melting tank  1  is structured in a known manner in the longitudinal direction (not shown). There is first a feed region for the batch. A melt region up to an outlet nozzle or up to a flow-through wall (in case it is present) forms the actual melting vessel, which is associated with a so-called superstructure, in which the gas burner for melting the batch is arranged. The connected region between the outlet nozzle and the overflow wall is the refining tank  1 . The region between the overflow wall and the flow-through unit is the so-called refining tank II.  
         [0034]     An electro-additional heating device can be provided in the melting tank or in the refining tank  1 .  
         [0035]     The refining tank and the melting tank form the so-called glass melt producing unit.  
         [0036]     The melting tank  1  has a known structure made from fire-resistant material, which has largely unknown grounds with a similarly unknown high internal resistance.  
         [0037]     As shown from  FIG. 1  the fire-resistant material of the melting tank  1  has symbolically indicated steel supports  17 , which for their part are embedded in the bottom of the melting tank  1  and have a conducting contact to equipotential busbars (safety grounds) of the apparatus. The unknown total resistance of this arrangement is indicated symbolically by the resistance  4  with a “?” in the figures. This high internal resistance is largely determined by the oven atmosphere  1   a  above the melt bath. Furthermore at least one electrode  2  immersed in the melt bath is mounted on the melting tank, whose function is described later.  
         [0038]     The principle electrochemical processes occurring at the symbolic ground point  3  shown in  FIG. 1 , at the melting tank and at the electrodes  2 , which preferably comprise molybdenum, are shown in the circular detail cutouts in  FIG. 1 .  
         [0039]     At least one stirred crucible  5  each with an unshown metallic stirrer is connected to the melting tank  1 . The embodiments shown in FIGS.  1  to  3  have two stirred crucibles.  
         [0040]     The glass melt  6  flows into a longitudinally extended pipe, which comprises at least one electrically heated noble metal pipe segment  7 . In the embodiment shown in  FIG. 1  there are three pipe segments  7  and the embodiments shown in  FIGS. 2 and 3  there are two pipe segments. The glass melt  6  flows over an overflow at the end of the longitudinally extend pipe under flow control by means of a controlling slider  7   a , the so-called tweel, and over a downwardly inclined lip block  8 , the so-called “spout lip”, to a tin bath  9  in the float tank  10 . The glass melt then spreads out over the tin bath  9  and is brought to the desired thickness.  
         [0041]     In order to minimize the undesirable direct current through the so-called interface comprising the at least one stirred crucible  5 , at least one pipe segment  7  and the tweel  7   a , disks  15  made from electrically insulating ceramic material are mounted or placed between all platinum components or parts.  
         [0042]     After the glass ribbon or sheet has obtained the desired fire polishing, it is continuously drawn from the tin surface, relaxed in an annealing lehr and packaged in the cutting section. However these steps are not illustrated or explained further herein because they do not have any significance for the grounding problem according to the invention.  
         [0043]     The float tank  10  comprises a steel basin  11 , which is connected with the ground  3 , and which is provided with a fire-resistant material cladding for receiving a tin bath  9  and a superstructure for receiving a suspended cover and a float bath heater, which have not been illustrated. Furthermore a reducing atmosphere (6-12% H 2 , 88-94% N 2 ) is required in the float tank. The essential electrochemical processes arising between the glass melt  6  loaded with negatively charged oxygen ions and the tin bath  9  are illustrated in detail in circular detail cutouts in  FIG. 1 .  
         [0044]     As explained earlier, the electrochemical process damage the glass melt  6  in harmful ways because of the “unintended” ground circuit, especially in the melting tank  1 . Furthermore it is clearly explained that a subsequent complete disconnection of the melting tank  1  from ground in order to avoid these detrimental effects is no longer possible both in construction of new production apparatus and also in optimizing existing plants.  
         [0045]     The quasi “electrochemically correct” structure for an ideal float plant simply comprises in this respect only a single ground connection in the entire apparatus, e.g. via the metal housing of the steel basin  11  of the float tank  10 .  
         [0046]     At least one further ground of the melting tank  1  besides the float bath ground exists in a real apparatus under participation of the superstructure atmosphere  1   a . However the comparatively high self-resistance of the gas and the boundary surface glass/gas offers the possibility for successful use of “equivalent or artificial circuits”. The term “equivalent or artificial circuits” means that the charge flows are diverted so that they produce no damaging consequences at the interface, at tweel  7   a  or in the float tank  10 .  
         [0047]     The self-resistance of the superstructure ground largely determines the interior resistance  4  of the corresponding battery circuit, so that each additional current load must in principle lead to a drop of the clamping voltage. Naturally this is the same for each battery, however after a first attempt and measurement at the float tank it has been shown that the potentially available charge amounts are actually greatly reduced.  
         [0048]     The invention consists, among other things, in recognizing that required limitation of these battery currents in the conventional melting tank can be easily achieved by construction of additional auxiliary grounds. The auxiliary grounds guide or conduct damaging direct currents from the superstructure directly to the float bath via the grounded auxiliary electrodes, while at the same time weakening the undesired currents in the interface.  
         [0049]     The additional grounds are most advantageously constructed as grounded Mo or W electrodes  2 . The Pt electrodes used as temporary expedients however are greatly endangered by alloy formation and subsequent dissolving. The best electrode position for the melting tank is in the refining tank  11 . Every additional melting tank auxiliary ground made by means of the grounded auxiliary electrode  2 , as shown in  FIG. 4 , especially in the vicinity of the feed apparatus, burner or overflow, increases the desired effect. The best position in the distributor (in case a heated superstructure exists there) is at the level of the burner.  
         [0050]     Mo and W have a side effect when used as auxiliary electrodes. They function like an active electrode with the boundary surface Mo/glass or W/glass and they generate an additional battery circuit between the auxiliary electrode  2  and tweel/float tank in the grounded state. This current is the opposite from the usual current between the superstructure and the tweel/float tank. The currents may be balanced out by building in a controlling ohmic resistance between the auxiliary electrode  2  and the ground  3 , so that the residual current in part between the melting tank and the float tank, the interface, is zero.  
         [0051]     In the melting tank with additional electro-heating it is necessary to block the alternating-current-side ground of the auxiliary electrode(s)  2 . For this purpose suitable impedance  13  is connected in series with the ohmic resistance  12 .  
         [0052]     In principle it is also possible that the current flows through one auxiliary electrode  2  from the tank to ground and through the other from the ground to the tank when several auxiliary electrodes  2  are used ( FIG. 4 ). That is especially possible when the auxiliary electrodes are made from different materials or different temperatures clearly exist at auxiliary electrodes or the composition of the glass melt differs significantly at the respective locations of the auxiliary electrodes. In the worst case all three effects mutually reinforce and augment each other. In this case the embodiment shown in  FIG. 4  is employed. This problem is easily eliminated by building in a rectifier or a diode  16 , which permits current flow in only one direction.  
         [0053]     The invention additionally includes an embodiment with a current meter or measuring device in the “interface”. Two insulated, electrically conducting, non-corrosive and chemically identical components are put in direct contact with the glass for this purpose. Pt or a suitable Pt alloy is the preferred material. The components are arranged parallel to the glass and current flow, they should have the same temperatures and they should “see” the same chemical environment.  FIG. 2  shows the structure of a suitable current measuring device  20 ,  21 . A current-voltage converter  20  is connected between an entrance flange  22  of the first stirred crucible  5  and an outlet flange  23  of the final pipe segment  7 . The current flowing between the melting tank  1  and the float bath  10  flows to the ground via the lower input resistance of the current-voltage converter almost exclusively and no longer through the glass melt. The output voltage of the converter  20  is measured with the voltmeter  21  and converted to a current value.  
         [0054]     A current meter  14  indicates the direct current to be controlled.  
         [0055]     An additional embodiment of the invention considers changing electrolytic properties of the superstructure atmosphere under changing firing conditions (fuel and oxidizer amounts) and generally under changing temperature distributions in the furnace superstructure and in the melting tank (additional electro-heating, throughput) or the chemical composition of the melt.  
         [0056]     All these changes have a principal effect both on the source voltage and also on the internal resistance of the “battery”. It would then be significant to control the protective currents flowing through the auxiliary electrodes automatically so that the damaging currents in the interface remain minimal.  
         [0057]      FIG. 5  shows the structure of this sort of automated current controller using only a single auxiliary electrode  2 . The current measurement is as described above in relation to the embodiment shown in  FIG. 2 . The measured value of the digital volt meter  20  is stored via a conductor  19  as an actual value in a current controller  18 , which controls the current flowing to ground  3  from the melt  6  via the electrode  2 , the impedance  13  and the meter  14 , so that the current flow between the entrance flange  22  of the first stirred crucible and the outlet flange  23  of the second pipe segment  7  is minimized.  
         [0058]     Alternatively to the direct current measurement according to  FIGS. 2 and 5  the current measurement can also occur indirectly by means of a voltage measurement between two suitable reference electrodes (e.g. ZrO 2   − , Mo or W electrodes) with respective exactly known temperatures. They should be likewise positioned parallel for testing the current flow, analogous to the positions in  FIGS. 2 and 5 .  
         [0059]      FIG. 3  shows the structure of this embodiment using indirect current measurement. ZrO 2  reference electrodes  30 ,  31  are used as the electrodes. The voltage between the reference electrodes  30 ,  31  is measured with the help of a digital volt meter  32 , which is connected to the respective reference electrodes  30  and  31  by means of conductors  34  and  35 . Their resistances are calculated from length, cross-section, specific resistance and temperature of the melt. The current can be calculated without problem from resistance and voltage.  
         [0060]     Auxiliary electrodes  2 , controlling resistor  12 , impedance  13  and current meter  14  form a unit according to the invention. The regulation of the effective resistance between tweel and float bath ground operates by local adjustment of the float bath atmosphere independently in the sense of an additional current limitation. Additional features or measures of the invention include elimination of grounds of all remaining metal parts in direct contract to the melt and putting as manner metal parts in direct contact with the hot superstructure gasses is helpful.  
         [0061]     The invention provides an advantageous grounding of the melting tank for suppressing or minimizing current flow between the melting tank and the float tank in order to control the boundary surface reactions (minimize bubble formation and corrosion) in the production process. Particularly minimizing corrosion especially extends the service live of Pt parts.  
         [0062]     The grounding of the melting tank causes, as observed electrically, the float glass production apparatus to act as an electrochemical concentration cell with current distribution or division.  
       PARTS LIST  
       [0000]    
       
           1  melting tank  
           1   a  superstructure atmosphere  
           2  electrode  
           3  ground  
           4  internal resistance  
           5  stirred crucible  
           6  glass melt  
           7  noble metal pipe segment  
           7   a  tweel  
           8  lip block  
           9  tin bath  
           10  float tank  
           11  steel basin  
          12 ohmic resistance  
           13  impedance  
           14  current meter  
           15  ceramic material  
           16  rectifier or diode  
           17  steel support  
           18  current controller  
           19  electrical conductor  
           20  current-voltage converter  
           21  digital voltmeter  
           22  entrance flange  
           23  outlet flange  
           30 ,  31  reference electrode  
           32  digital voltmeter  
           34 ,  35  conductors  
       
     
         [0091]     The disclosure in German Patent Application 10 2004 033 714.4-45 of Jul. 13, 2004 is incorporated here by reference. This German Patent Application describes the invention described hereinabove and claimed in the claims appended hereinbelow and provides the basis for a claim of priority for the instant invention under 35 U.S.C. 119.  
         [0092]     While the invention has been illustrated and described as embodied in an apparatus for electrically grounding a float glass production apparatus, it is not intended to be limited to the details shown, since various modifications and changes may be made without departing in any way from the spirit of the present invention.  
         [0093]     Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features that, from the standpoint of prior art, fairly constitute essential characteristics of the generic or specific aspects of this invention.  
         [0094]     What is claimed is new and is set forth in the following appended claims.