Abstract:
The improvement in an apparatus for refining glass being fed along a channel wherein the molten glass is foamed throughout its thickness, the improvement involving an increase in the width of the channel at the location of foaming. Also, the channel is provided with submerged electrodes disposed on opposite sides of the channel adjacent the bottom for heating the molten mass to produce the foaming.

Description:
RELATED APPLICATION 
     This application is a continuation-in-part of applicant&#39;s copending application Ser. No. 602,670, filed Aug. 7, 1975, entitled &#34;Method and Apparatus for the Manufacture of Glass&#34;, now abandoned. 
    
    
     BACKGROUND OF THE INVENTION 
     In copending U.S. application Ser. No. 602,670 filed Aug. 7, 1975 now abandoned and entitled &#34;Method and Apparatus for the Manufacture of Glass&#34;, a rapid process of melting and refining glass is described in which a vitrifiable material is melted and brought to an elevated temperature while maintaining the viscosity of the molten mass at less than 1000 poises. As soon as the melting has been achieved, an intense foaming of the molten mass is effected throughout its entire thickness while keeping the viscosity at a value less than 1000 poises. The rate of expansion of the mass is at least 1.5 (preferably between 2 and 3). After the foaming subsides, a perfectly refined glass is collected. 
     According to the process disclosed in said copending application, the foaming operation is performed in a channel in which the molten material progresses, without back currents, from a first location where the raw vitreous material is received from a premelting apparatus and a second location where the refined glass is recovered. 
     To ensure the intense and complete foaming required, a number of steps may be taken. For example, foaming agents can be incorporated into the raw materials. The foaming agents give rise, in the temperature range, corresponding to the desired viscosities, to the formation of gas bubbles inside the glass. The gases produced by the foaming agents are soluble in glass, and preferably their solubility in the molten glass increases as its temperature decreases. It is also recommended that a refining agent be present, at least in the final phase. After the elimination of most of the gases, the refining agents aid in the readsorption of the bubbles which remain on cooling. The foaming agents are selected such that they do not induce foaming of the vitreous material until that material has reached a desired temperature, which temperature is maintained in the refining channel. The following foaming agents are useful in the process disclosed in said copending application: arsenic compounds, such as arsenic trioxide; antimony compounds such as antimony trioxide; sulfur compounds, such as sodium sulfate; and halogen salts such as potassium chloride. Other agents useful in the process will be apparent to those skilled in the art. 
     Another method disclosed in said copending application for ensuring the thorough foaming of the molten mass involves subjecting the batch to rapid uniform heating during the foaming operation of about 20° C. per minute or more. 
     In a discontinuous melting installation, the heating means are employed at a time when the vitreous batch contains a large number of solid or gaseous nuclei and a sufficient amount of foaming agents to ensure an expansion of at least 1.5, and preferably above 2 times the normal volume of the mass in the unfoamed molten state. 
     In a continuous melting installation similar heating means can be employed. The predefined time sequence corresponds to the rate of treatment of the vitreous mass. 
     To aid the foaming process, it is also recommended that the raw materials contain a large number of nuclei, such as unmelted particules or small gas bubbles, capable of inducing the foaming. These nuclei essentially act as nucleation sites. The nuclei should be distributed throughout the molten mass at a concentration of at least 10 nuclei per cc. Generally, it is desirable that the raw materials be agglomerated. The agglomeration makes it possible to preheat the materials before actual melting. The preheating is accomplished by a brief and intense heat transfer (less than 10 minutes) while simultaneously keeping the temperature of the materials below the foaming temperature. This permits the maintenance of a high number of nuclei consisting of unmelted particles and gas bubbles in the vitreous mass introduced into the total foaming stage. 
     To assure the presence of sufficient nuclei, outside nuclei, for example, cullet or colored cullet can be added to the raw materials. In relation to the usual glass refining processes, the process disclosed in said copending application, requiring the presence of gas producing agents and foaming nuclei, can employ unrefined vitreous materials. It has been discovered that 1 to 2 mm. grains originating from the limestone and dolomite in the material introduced in the refining tank, are totally digested at the end of the total foaming phase. The process is therefore not dependent on the use of a vitreous batch of high quality. 
     The channel in which the molten mass flows can be of very simple geometry. Preferably, it has a slight width in relation to its length, in a ratio of 1:5 at least. This construction limits undesirable back currents. Also, for this same purpose, it is possible to use baffles, barriers, bottlenecks or even cascades along the path traveled by the vitreous molten mass during treatment in the channel of the refining apparatus. 
     SUMMARY OF THE PRESENT INVENTION 
     According to the present invention, the channel of the refining apparatus is constructed to increase the homogeneity of the foaming of the molten mass throughout its entire thickness. The construction of the channel also enhances the uniform flow of the molten mass. More particularly, the channel is constructed with a widening in the middle zone where, due to the rapid heating of the molten mass, the great expansion and foaming of the mass is produced. The width of the channel at the zone of foaming is about double that of the upstream zone. Downstream from the foaming zone, the channel returns to a narrow width, for example, on the same order as that of the upstream zone. 
     The channel is equipped with pairs of opposed electrodes placed on opposite sides of the channel along the longitudinal refractory walls of the apparatus. The dissipation of energy by Joule effect is produced within the vitreous mass itself to control the temperature of the mass all along the channel. In the foaming zone, the spacing of opposed electrodes is greater than the spacing between the upstream electrodes. Advantageously, the spacing between the electrodes in the foaming zone is at least equal to the width of the channel of the upstream zone. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of the channel constructed in accordance with the present invention; and 
     FIG. 2 is a cross-sectional view of the channel, in the widened or foaming zone as taken along lines II--II of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, the direction of the arrow indicates the direction of flow of the molten mass between electrodes E 1 . The channel walls diverge at 1 to define the entrance of the foaming or widened zone 2. The electrodes E 2  in the foaming zone are placed at a distance apart greater than the width of the channel upstream. Thus, the totality of the molten glass mass that comes from there enters between electrodes E 2 . The length of the widened foaming zone corresponds to the period of the intense foaming phase disclosed in the above-mentioned copending application. 
     The wall of the foaming zone is moderately heat insulated to maintain its temperature at a rather low level (on the order of 1350° C.), whereas the glass in foam state between electrodes E 2  is around 1550° C. From this important heat gradient, there results, around each electrode E 2 , a notable convection current, helicoidal in shape in the direction indicated by the arrows represented in FIG. 2. This causes an intense mixing of the molten glass mass particularly favorable to its refining. 
     The glass has free passage around the electrodes along the hearth and side walls. Passage of the current from one electrode to the other produces active thermal convection which favors the crosswise homogenization of the molten mass and eliminates any major longitudinal currents. The result is a uniform flow of glass called a &#34;piston&#34; flow. In FIG. 2, the level of molten mass before foaming is shown by the broken lines while that of the foam is shown by the solid lines. 
     By way of example, an embodiment of the invention for refining glass at a production capacity on the order of 120 to 250 kg/hour, for the usual silica-soda-lime glass, is given below. 
     The walls and hearth of the channel are made up of blocks of electro-melted refractory 3 with a base of alumina and zirconia about 10 cm. thick, heat insulated by a lining 4 of refractory bricks. To obtain a moderate heat insulation in the widened foaming zone 2, a thiner lining thickness 4 is used. The hearth 5 of the channel is level on its entire surface. 
     The depth of the channel is 25 cm., uniformly over its entire length, which totals 2.5 m. The narrow upstream zone, in which receipt of the premelted mass occurs, is 30 cm. wide and 40 cm. long. The two electrodes E 1  in this zone are made of molybdenum rods 40 mm. in diameter and 40 cm. long. They are placed symmetrically and 150 mm. apart. 
     After this upstream zone, the widened foaming zone extends for a total length of 80 cm. It includes the entrance 1 where the walls diverge over a length of 15 cm. This increases the width of the channel from 30 to 60 cm., this latter value being maintained over a length of 50 cm. The channel then comprises a narrowing portion 6 where, over a length of 15 cm., the walls converge to reduce the width of the channel from 60 to 30 cm. This reduced width is then maintained over the downstream zone, for a length of 1.3 m., to the drawing off orifice 7, whose output is controlled by a needle system, not shown. 
     Electrodes E 2  in the widened foaming zone are made of cylindrical molybdenum rods 40 mm. in diameter and are 70 cm. long. The pairs of electordes E 3 , E 4  and E 5  in the downstream zone of the channel are of the same diameter (40 mm.) and are 30 cm. long. For the areas where the molybdenum is in contact with the molten mass, even in the upstream zone and in the widened foaming zone where the mass is charged with bubbles of various gases, it has been found that with the usual compositions of silica-soda-lime glass, no particular precautions need be taken for the protecting of this metal from oxidation. 
     The current lead-ins 8 to the electrodes are also molybdenum rods, but their diameters are only 25 mm. Assembly of the lead-in and electrode is accomplished by screwing of one into the other. In the areas where there is a danger of oxidation of the molybdenum of the lead-ins, they are protected, as is known, by a reducing gas such as town gas. The connecting clamps to the electric supply are cooled by circulation of liquid. The lead-ins can slide in passages 8a which are made through the walls of the channel. The height of the axes of these passages is 5 cm. above the level of the hearth except for electrodes E 2  whose height is 2 cm. greater. 
     Between the pair of electrodes E 4  and E 5 , a barrier of refractory material or platinum is placed to provide a passage of adjustable height between its lower part and the hearth 5. The barrier blocks possible surface currents; and is slidably mounted in guides 10 in the lateral walls of the channel to control the flow of molten mass. 
     Heating of the molten mass contained in the channel is assured by means of the immersed electrodes, previously described, with independent electric power supply for each pair of electrodes. An example of the rated electrical characteristics of the power supply is as follows: 
     
         ______________________________________       Power               Current       Capacity  Voltage   AmperagePowers Used (kVA)     (V)       (A)______________________________________electrodes E.sub.1       40        80        500electrodes E.sub.2       40        80        500electrodes E.sub.3       7.5       120       62.5electrodes E.sub.4       7.5       120       62.5electrodes E.sub.5       15        120       125______________________________________ 
    
     The above-described construction and power supply permits refining of about 150 kg/hour of silica-soda-lime glass under operating conditions shown in the following table, the temperatures being those indicated by pyrometers going into the molten mass at points indicated T 1  to T 5  in FIG. 1: 
     
         ______________________________________     Values                     ValuesPowers Used     (kVA)    Temperatures Measured                                (° C)______________________________________electrodes E.sub.1     15       Point of Measurement T.sub.1                                1300electrodes E.sub.2     22       Point of Measurement T.sub.2                                1480electrodes E.sub.3      1       Point of Measurement T.sub.3                                1540electrodes E.sub.4      1       Point of Measurement T.sub.4                                1400electrodes E.sub.5      0       Point of Measurement T.sub.5                                1250______________________________________ 
    
     The operating conditions correspond to a supply of premelted paste delivered at about 1350° C. by a melting apparatus of the type disclosed in the above-mentioned copending application in which the following vitrifiable mixture (in kg. per 100 kg. of glass) is introduced in the form of agglomerates. 
     
         ______________________________________sand                    67.0limestone               9.47dolomite                16.2feldspar                6.13sodium carbonate        7.5850% caustic soda        22.5sodium sulfate          1.0______________________________________ 
    
     The minimum level of the unexpanded molten glass mass should be on the order of 10 cm. to cover, and therefore protect from oxidation, the totality of the various pairs of electrodes disposed along the channel, even if the rate of expansion is small. In practice, in the installation described, the rate of expansion of about 2 permits optimal functioning, while leaving a safety space of 5 cm. above the molten mass. 
     Devices with greater production capacity can be made in a way similar to the above-described embodiment. Appropriate electrical heating means must be provided in relation to the contemplated output, i.e., heating means with a capacity to assure an elevation of temperature of the vitreous mass of at least 20° C./minute at the level of the widened foaming zone. Also, if the thickness of the molten glass mass is increased, the electrodes are still kept close to the hearth, as indicated above, so that the heat will directly affect the deepest layers of the mass to be treated. Thus, unwanted currents of longitudinal convection are reduced, to the benefit of the quality of the refining. For similar reasons, particular care is given to heat insulation of the hearth while the arch and walls are, optionally, as stated above, slightly less heat insulated to favor transverse convection movements.