EP0944555A1

EP0944555A1 - Glass furnace and installation comprising same

Info

Publication number: EP0944555A1
Application number: EP98939727A
Authority: EP
Inventors: Stéphane Maugendre; Tanguy Massart; François Szalata
Original assignee: Saint Gobain Isover SA France
Current assignee: Saint Gobain Isover SA France
Priority date: 1997-07-22
Filing date: 1998-07-21
Publication date: 1999-09-29
Also published as: AU746124B2; JP2001501167A; IS4994A; CA2266648A1; PL332381A1; AR016528A1; BR9806062A; CZ101099A3; WO1999005068A1; NO991187L; US6125658A; AU746124C; CN1234784A; NO991187D0; KR20000068601A; TR199900631T1; AU8813998A

Abstract

The invention concerns the technique of electric melting wherein the melting energy is dissipated by Joule effect in the molten glass bath by means of electrodes dipping through the bath surface. The invention is characterised in that the electrodes are immersed into a molten glass bath from a height h less than 800 mm and from a surface S such that the ratio h/S is less than 0.5 m/m2. In another embodiment, the exchange surface between the electrodes and the bath is more than 0.075 m2/m3 of glass. The invention is useful for making glass-based products such as fibre glass insulating materials.

Description

GLASS OVEN AND INSTALLATION COMPRISING THE OVEN

The present invention relates to electric glass melting techniques in which the conductivity of molten glass is used to develop by Joule effect the energy necessary for the melting of raw materials. In a first type of electric glass melting furnace, the energy is supplied by means of electrodes totally immersed in the mass of molten glass (hereinafter called the bath), arranged vertically on the bottom of the oven and / or horizontally on the side walls of the oven, the composition to be melted being introduced from above so as to maintain a surface layer constituting both a permanent reserve of raw materials and a protection of the bath against surface heat losses.

The molten glass bath is the seat of convection movements, due to changes in the density of the glass according to its degree of heating, which participate in the transport of heat to the surface layer where the fusion must take place as well as in the rest of the volume of the bath. These movements are particularly intense along the electrode due to the temperature gradient existing with the neighboring mass of glass.

In this configuration where the electrodes are completely submerged, the energy exchange surface between the electrode and the bath is distributed over almost the entire height of the bath. Consequently, the temperature difference of the glass according to the height of the bath is not very pronounced and the convection movements are of a very large amplitude, the current of hot glass rising along the electrode then along the layer of material to be melted to bring it the fusion energy. This circulation results in a continuous stirring of the bath which makes it possible to homogenize the mass of molten glass both in composition and in temperature. In general, ovens of this type comprise a tank made of very deep refractory material, usually of the order of at least 1.5 m, in which a thickness of molten glass of the order of at least 1.2 at 1, 4 m is provided so that the materials which melt under the surface layer have a sufficient residence time in the bath to reach a state of uniform composition and temperature, and thus develop a satisfactory glass.

The total immersion of the electrodes considered to be advantageous because it allows a fairly uniform energy supply throughout the volume of the bath, however proves to be constraining because the violent stirring movements which have been discussed above cause the erosion of the sole and measures must be taken to protect the sole against this wear which can also affect the electrodes themselves.

More recently, the technique of electric fusion has undergone an important modification, consisting in immersing the electrodes in the bath through the free surface of the latter, instead of causing them to spurt out inside the bath from the sole. . This made it possible to resolve the delicate problems of replacing worn electrodes and of sealing linked to the passage of the electrodes through the refractory of the sole. The wear of the refractories could also be reduced because the use of plunging electrodes eliminates direct heating at the level of the bottom, the hot zones being located in an upper part of the molten bath, and therefore makes it possible to limit the development of currents. convection in contact with the bottom. This configuration also made it possible to increase the possibilities of adjusting the production parameters. For more details on this technique and its advantages, reference may be made in particular to document FR-A-2 599 734.

The design of the ovens has not been significantly modified with the use of plunging electrodes, and a minimum depth has hitherto been recommended to properly develop the temperature gradient necessary for establishing the temperature at the bottom. relatively low desired: the temperature profile in the bath is indeed such that the temperature is higher near the electrode and decreases relatively slowly towards the floor. This minimum depth was also considered necessary for the production of good quality glass. The object of the invention is to improve the definition of electric melting furnaces with plunging electrodes, so as to optimize the production conditions in particular by reducing, for equal production, the investment and / or operating cost, for improved profitability. . Quite surprisingly, it appeared to the present inventors that, for an equal production capacity, the volume of the molten bath and, in an equivalent manner, the residence time of the materials in the bath could be considerably reduced compared to the prior art. without the qualities of the glass being appreciably affected. Thus, contrary to the generally accepted idea that a minimum bath height is necessary to produce a homogeneous glass in composition and in temperature, the inventors have succeeded in producing completely satisfactory glasses in an oven whose depth has been reduced by - very significant way.

In this regard, the subject of the invention is an oven for the preparation of glass by electric melting in which the melting energy is dissipated by the Joule effect in the molten mass, comprising means for supplying vitrifiable materials depositing said materials in layer on the surface of a molten glass bath and submerged melting electrodes immersed from the surface of the bath through the layer of melting composition covering the molten bath, characterized in that the height h of the molten bath is less than 800 mm and the ratio of the height h to the surface S of the bath is less than 0.5 m / m ² .

For the purposes of this description, the height h of the molten bath actually designates the useful height of the molten bath, namely the height between the upper level of liquid in the tank and the bottom of the oven, or possibly in certain cases where the racking glass is made at a higher level than that of the sole, between the upper level of the liquid in the tank and the lower level of the glass withdrawal orifice. Indeed, for certain reasons, in particular when pollutants are likely to be deposited at the level of the floor and soil the mass of glass at the bottom of the bath, it may be preferable to withdraw the glass in an area located a little above the level sole. It is established that the mass of glass thus isolated in the lower part of the furnace does not participate in the production of the glass and does not enter the useful mass of the bath. It is generally difficult to accurately measure the exact level at which the liquid phase begins in the tank, due to the fact that the surface layer of materials covering the bath (called "crust") is the seat of equilibria between several phases ( solids, liquids, gases) resulting from the fusion of the composition. In general, an indirect measurement is carried out using the principle of communicating vessels, by detecting the upper level of liquid in a compartment located downstream of the tank.

Against all expectations, it has been found by the present inventors that by carrying out the electrical fusion in an oven with plunging electrodes much less deep than usual, all other things being equal, a completely elaborated glass is produced, not containing no solid particles, and having a satisfactory homogeneity of composition, despite a residence time of the materials in the molten bath much lower than in the usual melting techniques. As a main advantage, the reduction in size of the oven offered by the invention allows a substantial saving on the quantity of refractory material necessary for the constitution of the side walls of the oven, and therefore on the investment cost of the installation. The reduction in the amount of glass present in the molten bath also allows for a more rational use of energy by limiting the energy losses in the mass of glass, thereby saving operating costs, in particular when it comes to making glasses with low heat conductivity. Other advantages of the invention will appear in the following of this description.

The oven according to the invention has a characteristic aspect ratio with a bath height limited to a value less than 800 mm and low compared to the surface of the bath. In a preferred variant, the height h of molten glass can advantageously be reduced to a value less than 500 mm, in particular less than or equal to 450 mm, with a very significant reduction in the cost of the furnace. Heights less than or equal to 400 mm, in particular of the order of 300 mm or less, are particularly preferred.

In a particular embodiment, the h / S ratio defined above is moreover less than or equal to 0.05, for example of the order of 0.03 or less. According to preferred variants where the oven has a large surface area for a high production capacity, the h / S ratio can even be less than or equal to

0.02 or even 0.01 or 0.005.

The height of the tank will advantageously be limited accordingly, preferably to a value greater than the height of the bath from 100 to 200 mm, in particular of the order of 150 mm.

No lower limit should theoretically be set for the height h of molten glass and those skilled in the art should be free to choose heights as low as of the order of a few centimeters, subject to preserving a contact surface between the electrodes and the molten glass sufficient to provide the necessary melting energy. It has in fact been observed by the inventors that, as soon as one is placed more than 2 to 3 cm below the upper level of liquid in the tank (as defined above), there does not remain any solid particle in the bath (that is to say that the fusion is carried out) and the glass is completely elaborated. In this regard, it can be indicated that any bath height of 20 to 300 mm is included in the invention, the corresponding ovens being essentially melting devices, producing elaborate glasses but relatively less refined than traditional ovens.

The invention therefore has the remarkable feature that it makes it possible to process the same quantity of materials as a traditional oven with the same specific draft, but in an oven of reduced height: in fact, if the electrodes provide the same quantity of energy, they transmit it to a lower bath volume. By thus modifying the conditions of energy supply in the bath, the temperature profile in the bath is such that it is established in the mass of molten glass of the glass circulation currents conducive to the development of a homogeneous glass.

Thus, according to another aspect of the invention, the furnace is characterized by a bath height of less than 800 mm, in particular less than 500 mm, in particular less than 450 mm, and by an exchange surface between the electrodes and the molten glass ( this surface being constituted by the lateral electrode surface immersed under the upper level of liquid present in the tank, per unit of bath volume) greater than 0.075 m ² of electrode per m ³ of glass.

According to the invention, the mass of glass has a higher energy exchange surface than usual. There is therefore a quantity greater relative glass exposure to electrodes than in traditional ovens.

The electrode surface per unit volume of bath is advantageously greater than 0.1 m ² per m ³ , preferably greater than or equal to 0.15 m ² per m ³ , in particular of the order of 0.2 m ² per m ³ or more.

The depth of immersion of the electrodes in the molten bath is necessarily limited to a value less than the height of the bath to avoid contact between the electrode and the bottom. This immersion depth must however be sufficient to provide the exchange surface necessary to dissipate the desired power.

In particular to limit the wear of the sole, in particular by erosion caused by the convective currents of glass in the bath, it may be preferable that the immersed length of the electrodes is less than or equal to two thirds of the height of the bath, preferably at half the height of the bath, the depth of immersion also depends on the height of the bath. This makes it possible to locate the hottest areas in the vicinity of the surface of the bath, the fusion energy being dissipated where it is most needed. This precaution also proves to be favorable for a circulation of the molten materials along a path allowing the rapid production and homogenization of the glass within the shallow bath. Advantageously, the shape of the electrodes can be adapted so that they have a very high lateral surface for a minimum of length. Electrodes, in particular of substantially cylindrical outline, whose dimensions are such that their lateral surface S _e , and their submerged length I are in a ratio S _é , / I greater than or equal to 0.45, advantageously 0, will therefore be advantageously used. , 6.

In a particularly advantageous embodiment, at least one electrode can comprise at least one substantially planar conducting element. In particular, such an electrode may have the form of a plate or comprise several plates associated with one another. A substantially planar conducting element may nevertheless also have the form of a ribbon or be composed of a plurality of juxtaposed wires.

Preferably, such plates are square or rectangular, in particular for reasons of ease of manufacture, although any other shape of plate also makes it possible to supply the glass bath with electric current for the creation of a Joule effect.

The cylindrical type electrodes are in fact less preferred according to the invention as soon as the depth of the glass bath becomes increasingly shallow since, to obtain a sufficient lateral exchange surface with a short submerged length, it is necessary to use a large diameter cylinder, therefore relatively heavy. One solution may consist in using a hollow cylinder, since only the lateral surface of the electrode participates in the electrical exchange, the internal part being completely inert in this respect. This solution is however of no economic interest in the current state of the technology of materials capable of constituting the electrodes (such as molybdenum) because a hollow cylinder can only be manufactured by milling, the material eliminated, lost, entering all the same cost of manufacturing such a hollow electrode.

The use of flat or plate electrodes makes it possible, compared to conventional electrodes in the form of a cylindrical bar, to considerably reduce the weight of the electrode, for an equal lateral surface. This weight gain is a major advantage in the oven of the invention where the electrodes are plunging, that is to say suspended from a support element.

The use of plates overcomes the economic handicap mentioned above for hollow cylinders.

The dimensions of the plates are chosen as a function of the desired exchange surface, the thickness being chosen to ensure that the electrode has sufficient longevity as a function of the wear kinetics by consumption of the conductive material constituting the electrode under the conditions the oven.

Advantageously, when the depth of the furnace is very small, the electrode comprises at least one plate, in particular rectangular, arranged in such a way that its larger side is oriented in a substantially horizontal direction. Thus, the support element is connected to the larger dimension side of the plate or plates. The thickness of the plate can be chosen to ensure a resistant fixing when the support element enters the plate in particular by screwing. It is possible to combine conductive plates in a wide variety of configurations:

- two square or rectangular plates can be arranged in L;

- three square or rectangular plates can be arranged in a U; - four square or rectangular plates can be arranged to form a hollow parallelepipedal electrode.

Preferably, the associated plates for example as above are assembled together, in particular by screwing or any other means.

Electrodes of different configurations can be used in combination in the same oven to provide a particular distribution of current lines. It is thus possible to install, in an oven for example, both cylindrical electrodes and plate-shaped electrodes, or else both L-shaped and U-shaped electrodes.

The electrodes made up of plate (s) can also be provided with means for adjusting the orientation of the electrical exchange surfaces, in particular by pivoting around at least one axis, in particular a horizontal axis and / or a vertical axis, so as to adjust the distribution of the current lines in the molten glass bath.

As mentioned above, an essential feature of the melting technique according to the invention is the short average residence time of the molten materials in the oven in the bath, relative to the rate of production which is generally expressed by the specific _draw T _spec which is the quantity of glass (in tonnes) drawn from the oven per day compared to an oven surface of 1 m ² . Indeed, for a constant specific draw, the residence time of the materials in the bath is proportional to the volume of the bath and the reduction in the height of the bath results in a corresponding reduction in the residence time.

In this regard, the subject of the invention is also a process for the electrical melting of glass in which the energy is dissipated by the Joule effect in the molten mass from plunging electrodes, comprising the steps consisting in distributing the materials constituting the composition to melt in a layer on the surface of the bath, immerse the electrodes from the surface of the bath through said layer of composition to be melted, supply the electrodes with an electric current, the materials melting and combining in the bath to form the glass and withdraw the molten glass with a flow rate expressed by the specific draw T spec

, characterized in that the average residence time (in days) of the materials in the

bath between the surface layer and the racking area is less than 2

Tspec

,. 1.2 „0.8 advantageously a, or even a

Tspec Tspec As an indication, under preferred operating conditions, the average residence time of the materials in the bath between the surface layer and the withdrawal zone is less than or equal to 0.7 days, advantageously 0.5 days, for example of the order of 0.25 to 0.4 days, for a specific pulling of the order of 3 t / m ² / d. The oven according to the invention proves to be particularly advantageous for the production of “opaque” glasses with infrared radiation, such as for example glasses containing a relatively high proportion of iron oxide (for example of the order of at least 0 , 60% Fe ₂ O ₃ , up to 10 - 12% or more) in which the radiation develops in a limited way. Usually in conventional size ovens, the low conductivity of the radiation creates marked temperature differences between the zones of the bath, with in particular relatively cold zones at the bottom of the bath where the glass tends to devitrify. Because of its shallow depth, the oven according to the invention allows the establishment of a thermal gradient avoiding the creation of these cold zones and limiting the risks of devitrification of these particular glasses.

The melting technique according to the invention makes it possible to produce, with less expensive apparatus and under more economical operating conditions, good quality glass usable for very numerous applications with results as satisfactory as glass produced in traditional ovens. . For example, the molten glass according to the invention can be transformed into glass wool, in particular for the production of insulating products, of quality equivalent to existing wools.

Consequently, the invention also relates to an installation for manufacturing glass wool comprising a glass melting furnace, a fiberizing device and means for supplying the fiberizing device with glass. melt produced in said furnace, characterized in that the furnace is a furnace with plunging electrodes of shallow depth as described above.

Other possible applications are in particular the manufacture of reinforcing fibers or glass substrates of various shapes. For applications where very high quality glass is required, it is possible to couple the furnace according to the invention with an additional device which will carry out rehomogenization and / or refining at the last moment before the processing of the molten glass. Such an installation can be advantageous compared to an installation comprising a very deep oven calculated to produce glass directly from the oven with the very high degree of homogenization and refining desired: thus placing such a complementary device just upstream of the final processing device, it is possible to make up for any defects (inclusion of gas bubbles) which may appear within the molten glass during its transportation from the oven to the final processing stage. The use of two separate devices instead of a single high-performance oven generally remains economically advantageous because of the substantial gain made on the cost of building the oven.

Other advantages and features of the invention will emerge from the detailed description which follows, given with reference to the appended drawings, in which:

- Figure 1: shows schematically an installation for manufacturing glass wool using an electric oven according to the invention;

- Figure 2: shows a schematic view in longitudinal section along the axis ll-ll of the oven of Figure 1; - Figure 3: shows a schematic view in longitudinal section along the axis III-III of the oven of Figure 1;

- Figure 4: shows a side view in partial section of a plate-shaped electrode usable in an oven according to the invention;

- Figure 5: shows another side view of the electrode of Figure 4

- Figure 6: shows a side view in partial section of an electrode consisting of an assembly of plates usable in an oven according to the invention; - Figure 7: shows an elevational view of the plate assembly shown in Figure 6.

Note that the figures are schematic representations on which do not appear all the details of realization within the reach of the skilled person and on which the scale is not necessarily respected, unless he does be specified otherwise later.

The installation shown in Figure 1 is intended to produce glass wool for the production of thermal insulation materials. It essentially comprises a glass melting furnace 1 supplied with a mixture of vitrifiable materials by a supply system 2, a channel 3 for transporting the molten glass produced in the furnace 1 and a fiberizing machine 4 supplied with molten glass by the channel 3. In the fiberizing machine, the molten glass falls into a fiberizing plate 5, the side wall of which is pierced with a multitude of orifices, rotated around a vertical axis 6, so as to eject centrifugally the glass melted through said orifices in the form of glass filaments 7 solidifying by cooling.

The constitution of the furnace 1 appears in detail in the two sectional views of FIGS. 2 and 3. The furnace comprises a tank 8 made of refractory material constituted by a hearth 9 and vertical walls 10, and surmounted by a vault 11. The tank shown has a horizontal sole 9. The tank 8 of the oven according to the invention can take any traditional general shape, but is distinguished from traditional ovens by the low height of the walls 10. By way of example, the tank shown has an area of approximately 10 m ² for a 0.4 m high.

As shown in FIG. 2, the tank 8 contains a mass of molten glass 12 constituting the melt, covered with a layer 13 of solid raw materials distributed continuously by the supply system 2. This layer as uniform as possible may be more or less thick depending on the operating regime. In operation, a thickness of at least 100 mm is preferably maintained to thermally isolate the molten bath from the atmosphere. Preferably, this thickness should not exceed approximately 300 mm since this does not provide any advantage for the melting and would therefore unnecessarily overload the surface of the bath. The height h of the bath is calculated by measuring the difference between the level of free glass in the transport channel 3 and the level of the floor 9. In the mode shown, it is approximately 300 mm; the h / S ratio is therefore 0.03 (in m / m ² ).

In the mode shown, the molten material is discharged through a groove 14 located on one side of the tank 8 and at the same level as the floor 9, this groove communicating with the channel 3.

Fusion electrodes 15, six in number in this example, are arranged in the upper part of the oven, carried by supports 16 of the traditional type. Their arrangement, of the type described in EP-A-0 140 745, is more particularly suitable for a three-phase current supply, the distribution of the phases (R, S, T) being as indicated in FIG. 3. This arrangement allows good phase balancing. Any other usual feeding method can however be envisaged in the context of the invention.

The electrodes 15 pass through the surface layer of raw materials and enter the molten bath. The lowest possible immersion depth is preferred, provided that the necessary exchange surface is provided. In practice, an immersion depth less than 2/3 the height of the bath and even preferably half this height will generally be advantageous.

In the embodiment shown, the electrodes are cylindrical in shape, short but of relatively large diameter to provide a large exchange surface. With a diameter of approximately 200 mm and a useful length I of approximately 150 mm, the lateral exchange surface S _é , is approximately 0.095 m ² per electrode and the ratio S _é , / I is 0.63. In operation, the electrodes are immersed over their entire useful length 150 mm, that is to say half the height of the bath, the exchange surface per unit volume of bath therefore being 0.190 m ² per m ³ of bath.

In general, this oven can be supplied with a current density on the electrode of the order of 1 to 3 A / cm ² .

In a particular example of operation, the furnace supplied with a current density of 2 to 2.5 A / cm ² , makes it possible to produce glass with a specific draw on the order of 3 t / d / m ² , i.e. total drawdown of 30 t / d: with a glass whose density is 2.4 t / m ³ , the volume of glass produced is 12.5 m ³ per day. Knowing that the volume of the bath is 3 m ³ , the residence time of the materials in the bath is about 0.25 day with a floor temperature in the usual range.

For comparison, compared to a standard plunging electrode oven comprising a deep tank containing a glass bath 1 m high, the oven according to the invention has a reduced construction cost of approximately 40% corresponding to the reduction in the height of the side walls. Furthermore, in operation under the same production conditions (same specific draw), the advantageous distribution of the energy in the bath according to the invention makes it possible to reduce the energy consumption by approximately 5%. FIGS. 4 and 5 represent an electrode 19 in the form of a plate usable in the furnace 1 in place of at least one of the cylindrical electrodes 15.

The actual electrode consists of a rectangular molybdenum plate 20 connected to a steel extension 21 by screwing. The plate 20 is provided with a thread 22 formed in the thickness of the plate in the middle of the side of greatest dimension (length). The extension 22 is provided with a corresponding threaded end 23.

The extension 21 is the connection means between the electrode and the support arm of the assembly: its function is to support the electrode and to bring the electric current to the electrode. In operation, it passes through the layer 13 of raw materials surmounting the bath, the lower portion with the thread 23 inserted in the plate 20 being approximately at a level corresponding to half the thickness of the surface layer 13.

To avoid melting of the fixing between the electrode and the steel extension, and to avoid wear of the molybdenum in the fixing zone in the upper part of the plate 20, a cooling system 24 of the "water-jacket" type integrated in the extension 21.

This system comprises a circuit 25 for circulating cooling water inside the extension between an inlet orifice 26 and an outlet orifice 27.

The extension 21 provided with the cooling system 24 is equipped with a plate 28 for connection to a support element (arm) not shown penetrating through the side walls of the oven. As an example, a plate electrode 19 functionally equivalent to the cylindrical electrode 15 described above has a length of 300 mm, a height of 150 mm and a thickness of 45 mm.

The diameter of the tapping 22 should preferably be as large as possible in order to have the strongest possible attachment of the electrode. This furthermore results in better cooling of the entire supported electrode area since the threaded end 23 of the extension 21 enclosing the end of the cooling system 24 provides a higher water flow rate.

Under these circumstances, it is preferable that the shoulder 29 on the extension 21 is such that it projects beyond the thickness of the plate 20. In fact, it is on this front contact surface that the electrical contact supplying the electrode. It is therefore advantageous for this contact surface to be as large as possible in order to avoid excessive current densities on this connection surface of the electrode. With the dimensions indicated above, the lateral surface of the plate

20 is 0.103 m ² per electrode, which is very close to the lateral surface S _e , of the electrode 15.

On the other hand, the weight of the plate 20 is only about 21 kg instead of

50 kg approximately for the cylindrical bar 15. For the same current supply capacity, the plate electrode 19 is more than twice as heavy as the cylindrical electrode 15.

The reduction in the weight of the electrode leads to a reduction in the lever arm on the electrode support and therefore allows a simplification of the construction of the oven. This electrode 19 was tested under the same operating conditions as those described above with the electrode 15 with a current density at the electrode of approximately 2 A / cm ² .

Under these conditions, the fusion has the same qualities with the two types of electrodes. While the cylindrical electrode 15 wears with a loss of 3.1 grams of molybdenum per tonne of glass produced, the electrode 19 wears with a loss of 2.9 grams of molybdenum per tonne of glass produced. Thanks to the cooling of the threaded end of the extension 21, the wear of the plate takes place from the external rectangular faces without prejudice to the electrical contact. The thickness of 45 mm of the plate 20 is sufficient to ensure satisfactory longevity of the electrode. This could be explained in particular by the fact that, with equal loss of mass, the surface of a plate would decrease less quickly than the surface of a cylinder. Figures 6 and 7 show a hollow electrode 30 with square section, consisting of the assembly of four plates 31, 32, 33, 34 of molybdenum fixed to a support plate 35 of molybdenum by screws 36 also of molybdenum.

The support plate 35 is provided with a thread 37 making it possible to fix the electrode on an extension not shown, which may be of structure similar to the extension 21 provided with a cooling system.

Compared to the plate electrode 19 of Figures 4 and 5, the square electrode 30 allows the diffusion of electric current in four perpendicular directions instead of two opposite directions.

In a particular example, the dimensions of the plates 31, 32, 33 and 34 are such that each face of the electrode measures 160 mm wide by 150 mm high, which gives a total lateral exchange surface of 0.096 m ² , that is to say of the order of that of electrode 15.

With a total weight of 32 kg, the electrode 30 is also twice as efficient as the cylindrical electrode 15. The electrodes 19 and 30 can be provided with means for adjusting their orientation, in particular by pivoting about a vertical axis or a horizontal axis, to adjust the distribution of the current lines in the molten bath.

These means can in particular be carried by the extension 21 or else by the carrier element from which the extension is suspended. In FIGS. 4 and 5, the electrode 19 includes such means of orientation around a vertical axis, in the form of the union fitting 40 between the extension 21 and the support 28.

The glass produced in the oven 1, once led to the fiberizing machine 4, is transformed into glass wool with a proportion of fiber-like materials as low as with the glasses obtained from traditional ovens.

The step of transporting the molten glass to the processing device can advantageously be used to homogenize and refine the glass. However, even if the routing conditions were not optimal or if the glass produced in oven 1 was not sufficiently homogeneous or refined due to uncontrolled variations in production parameters, it was observed that the fiber product nevertheless exhibits satisfactory qualities.

The association of the furnace according to the invention with a processing device such as a fiber-drawing machine with internal centrifugation appears to be particularly advantageous probably because the centrifugal force applied to the molten glass within the fiber-drawing machine, gives the glass a higher degree of homogeneity, this ultimate homogenization improving the ability of glass to be transformed into wool. Very low height furnaces according to the invention, in particular in which the height h of molten glass can range from 20 to 300 or 400 mm, preferably from 200 to 300 or 400 mm, therefore find a particularly advantageous application in this type of installation, which they considerably reduce costs (investment and operation). The invention, which has just been described in the context of an installation for manufacturing insulation materials based on glass wool, is in no way limited to this particular embodiment, and other glass products can be manufactured using the fusion device according to the invention, coupled with the appropriate processing means.

Claims

1. Furnace for the preparation of glass by electric melting in which the fusion energy is dissipated by the Joule effect in the melt, comprising means for supplying vitrifiable materials depositing said materials in layer on the surface of a molten glass and submerged melting electrodes immersed from the surface of the bath through the layer of melting composition covering the molten bath, characterized in that the height h of the molten bath is less than 800 mm and the ratio of the height h at the surface S of the bath is less than 0.5 m / m ² .

2. Oven according to claim 1, characterized in that the h / S ratio is less than or equal to 0.05, in particular 0.03.

3. Furnace for the preparation of glass by electric melting in which the fusion energy is dissipated by the Joule effect in the melt, comprising means for supplying vitrifiable materials depositing said materials in layer on the surface of a molten glass and submerged melting electrodes immersed from the surface of the bath through the layer of melting composition covering the molten bath, characterized in that the height h of the molten bath is less than 800 mm and the surface of immersed electrode per unit of bath volume is greater than 0.075, advantageously 0.1, in particular 0.15.

4. Oven according to any one of the preceding claims, characterized in that the height h of the bath is less than 500 mm, in particular less than or equal to 450 mm, in particular of the order of 200 to 400 mm.

5. Oven according to any one of the preceding claims, characterized in that the electrodes are immersed in the bath over a height less than or equal to two thirds, advantageously half the height h of the bath.

6. Oven according to any one of the preceding claims, characterized in that the electrodes have a shape such that their lateral surface S _é , and their useful length I are in a ratio S _é , / I greater than or equal to 0.45 , advantageously at 0.6.

7. Oven according to any one of the preceding claims, characterized in that at least one electrode (19, 30) comprises at least one substantially planar conducting element (20; 31, 32, 33, 34).

8. Oven according to claim 7, characterized in that the electrode (19) has the form of a plate (20) or comprises several plates (31, 32, 33, 34) associated with each other.

9. Oven according to claim 7 or 8, characterized in that the electrode comprises at least one plate, in particular rectangular (20), having its larger side disposed substantially horizontally.

10. Oven according to any one of claims 7 to 9, characterized in that at least one electrode (19, 30) is provided with means for adjusting the orientation of the electrical exchange surfaces, in particular by pivoting around at minus one axis.

11. An electrical glass melting process in which the energy is dissipated by the Joule effect in the melt from plunging electrodes, comprising the steps consisting in distributing the materials constituting the composition to be melted into a layer on the surface of the bath , immerse the electrodes from the surface of the bath through said layer of composition to be melted, supply the electrodes with an electric current, the materials melting and combining in the bath to form the glass and withdrawing the molten glass with an expressed flow rate by the specific _draw T _spec , characterized in that the average residence time of the materials in the bath between the surface layer and the zone of

racking is less than, advantageously - - -, or even less

Tspec Tspec Tspec

12. Method according to claim 11, characterized in that the average residence time of the materials in the bath between the surface layer and the withdrawal zone is less than or equal to 0.7 days, advantageously 0.5 days, for a specific draw of around 3 t / m ² / d.

13. Installation for manufacturing glass wool comprising a glass melting furnace, a fiberizing device and means for supplying the fiberizing device with molten glass produced in said oven, characterized in that the oven is an oven according to l any of claims 1 to 10.

14. Installation according to claim 13, characterized in that the fiberizing device is of the internal centrifugation type.