GB2057828A - Furnace electrode assembly - Google Patents

Abstract

Where an electrode 4, for example of molybdenum, is located in an aperture 3 of a furnace wall 1 for feeding current to molten material 2, e.g. glass, within the furnace, hot parts of the electrode 4 which are exposed to air are apt to corrode. Such corrosion may be substantially prevented by surrounding the electrode 4 with a sleeve 5 formed from one or more refractory oxygen- containing compounds (e.g. of the system Al2O3-ZrO2-SiO2). A bush 19 is sealingly fitted to the electrode 4, and a seal is provided between the bush 19 and the sleeve 5 which is independent of the furnace wall 1 so that cracks or porosity in the furnace wall do not affect sealing of the space 16 between the sleeve 5 and the electrode 4. <IMAGE>

Description

SPECIFICATION Furnace electrode assembly The present invention relates to an electrode assembly for insertion into an aperture in a furnace wall which assembly comprises an elongate electrode surrounded by a substantially gas-impermeable protective sleeve.

The invention is particularly concerned with glass-melting furnaces in which a plurality of electrodes pass through one or more side walls or the base of the furnace to conduct electric current to the molten glass contained by the furnace. Molten glass is of course highly corrosive, and even though the electrode material may be chosen to withstand such corrosion to some extent, the electrodes generally become eroded and provision is accordingly made for them to be replaceable and/or longitudinally movable so that a desired length projecting into the molten glass can be maintained. In fact, the present generally preferred electrode material is molybdenum. In use, a molybdenum electrode may be introduced through an aperture in a furnace wall, and sealing of the aperture can be achieved by glass from the belt in the furnace.

Molten glass penetrates the aperture, and because of the temperature gradient across the furnace wall it will freeze in the aperture thus sealing it. Such a solid glass plug will be formed at a region in the thickness of the wall where the temperature is, in the case of sodalime glass, about 600"C, so that cooler regions of the electrode are exposed to the atmosphere within the aperture and outside the furnace. This however leaves the molybdenum electrode open to attack along its exposed regions, and it is well known that molybdenum is reactive with atmospheric oxygen especially at temperatures above 300 C, so that portions of the electrode become corroded prior to their contact with the molten glass. This corrosion reduces the strength of the electrode and can eventually lead to mechanical failure.One proposal has been to heat temporarily the region of the electrode within the furnace wall so that molten glass from the furnace can penetrate the wall aperture further to provide a more extensive shield for the electrode. Unfortunately when the temporary heat source is removed or shut down so that such additional glass can solidify, the thermal gradient along the resulting solid glass sleeve is such that the glass almost always cracks so that it becomes ineffective to protect the electrode from the atmosphere.

Accordingly certain other proposals have been made. These include providing a metal sleeve for covering that part of the electrode which is apt to be corroded. The sleeve is of a corrosion resistant metal such as nickel or a nickel alloy: it is known to make such a sleeve as a water jacket or to provide an additional water jacket or finned radiator sleeve to cool the electrode to improve its resistance to corrosion, and it is also known to flush any space between the sleeve and the electrode with an inert or reducing atmosphere. In order to be effective, these proposals rely on an air-tight joint between the wall of the furnace and the sleeve, and this is extremely difficult to maintain under the usual conditions of furnace operation.A further difficulty which is of particular importance is that when a furnace is fired up, its walls become highly thermally stressed, and this results in the development of fissures in the walls along which the atmosphere can permeate to attack an electrode. In addition, it should be noted that the usual refractory materials from which glass melting furnaces are made are themselves slightly porous.

It is an object of the present invention to provide an alternative construction of electrode assembly which protects the electrode from atmospheric corrosion while it is in use.

According to the present invention, there is provided an electrode assembly for insertion into an aperture in a furnace wall which assembly comprises an elongate electrode surrounded by a substantially gas-impermeable protective sleeve, characterised in that said assembly comprises in combination a said sleeve formed from one or more refractory oxygen-containing compounds and surrounding said electrode in spaced relation therewith, a bush sealingly fitted onto the electrode, and means providing a seal between said bush and the sleeve whereby access to the space between the sleeve and the electrode may be had only via one end of the sleeve prior to insertion of that end of the sleeve into a said furnace wall aperture.

The length of sleeve required will depend on the operating conditions of the furnace in which it is to be used. In general it is convenient to make the length of the sleeve equal to the thickness of the furnace wall in which it is to be fixed. In any event, the sleeve should penetrate the furnace wall to a region where, when the furnace is in use, molten glass from the melt in the furnace can flow between the electrode and its surrounding sleeve. The bush should be located at a position on the electrode which, under operating conditions, is at a temperature sufficiently low that there is no substantial risk of atmospheric corrosion of the electrode.

Thus when operating in accordance with the invention an electrode being used will be protected from attack by the atmosphere outside the furnace and will also be protected against any gases which may diffuse into the furnace wall. Furthermore, the need for a troublesome seal between the sleeve and the furnace wall is eliminated.

Preferably, said sleeve comprises a flange at one end thereof. Such a flange can for exam ple be used to limit insertion of the sleeve into the furnace wall aperture, and in highly advantageous embodiments of the invention, said sealing means is clamped against said flange.

Preferably, said sleeve is constituted as a moulding of fused refractory material, since this is a very simple way of conferring the necessary gas-impermeability.

Various oxygen-containing compounds can be used for forming the sleeve, but preferably the sleeve forming material comprises a refractory metal silicate. A refractory material of the system Al203-ZrO2-SiO2, available under the trade designation "zac 1681" is an especially preferred material for forming a said sleeve.

Preferably, said electrode is formed of molybdenum and advantageously such molybdenum electrode bears a coating of MoSi2.

Such a coating serves as additional protection against oxidation of the electrode.

In some preferred embodiments of the invention, said sealing means includes a water jacket interposed between the end of the sleeve and the bush. Such water jacket may be used to promote cooling of that part of the electrode which is outside the furnace, and a principal advantage of this preferred feature of the invention is that heat-resistance requirements for the material of which the bush is made become less stringent. Of course such cooling also renders exposed parts of the electrode less susceptible to oxidation.

In some embodiments of the invention means is provided for the introduction of gas into the space between the electrode and its sleeve. Such gas may for example be a substantially inert gas such as nitrogen or it may be a reducing gas mixture such as a mixture of nitrogen and hydrogen. A mixture containing 5 to 10% hydrogen is very suitable. The presence of a reducing gas will ensure that any oxygen entering the space will be sequestered. This provides an additional assurance against oxidation of the most vulnerable regions of the electrode. If the space between the electrode and its sleeve is fed with such a gas or gas mixture, the vulnerable regions of the electrode will be protected even if the sleeve should crack.Moreover, if such space is charged to above atmospheric pressure, any minor leakage at the bush will be in an outward direction so that atmospheric oxygen will not enter the space between the electrode and the sleeve. Advantageously said electrode is constituted as a bar of substantially uniform cross section along the major part of its length, and said sealing means includes a reieasable clamping means clamping said bush in sealing engagement with the electrode whereby on release of such clamping means, said bush may be moved along the electrode with the sleeve and sealing means and clamped to the electrode at a different position. This provides a very simple and convenient way of allowing the electrode to be pushed further into the furnace, for example in order to compensate for erosion of the electrode by molten glass within the furnace.

The invention includes a glass-melting furnace incorporating at least one electrode asembly as herein defined.

A preferred embodiment of the invention will now be described by way of example witht reference to the accompanying diagrammatic drawing which shows a cross-section through a furnace wall in which an electrode assembly is located.

In the drawing, a glass melting furnace has a bottom wall 1 and holds a body of molten glass 2. An aperture 3 is provided in the furnace wall 1 for an electrode 4 so that electric current can be supplied to the melt 2.

In order to protect the electrode 4 against destructive oxidation of its hotter regions, a sleeve 5 surrounds the electrode 4 where it leads through the hole 3 in the furnace wall 1. The sleeve 5 is a loose fit within the hole 3 in the wall 1, leaving a space 6. If desired the space 6 may be filled e.g. using an acid mortar, for example made from zircon, zirconia and phosphoric acid. Normally however the sleeve 5 is inserted into the aperture 3 without using any mortar. The space 6 between the furnace wall 1 and the sleeve 5 becomes partially filled with glass which solidifies owing to the temperature gradient along the sleeve. The end of the sleeve 5 outside the furnace has a flange 7, and an annular clamping member 8 of metal is located behind the flange 7. A packing washer 9 may be provided between the flange 7 and the clamping member 8 if desired.The clamping member 8 carries a plurality of bolts 10 which are tightened to secure a second annular member 11 against a sealing washer 1 2 which in turn bears on the front face of the flange 7 at the end of the sleeve 5. This second annular member 11 is provided with an annular cavity 13, an inlet 14 and an outlet (not shown) for the circulation of cooling water. In other words, the second annular member 11 is constituted as a water jacket.

The water jacket 11 also includes an optional gas inlet 1 5 so that the space 16 between the sleeve 5 and the electrode 4 can be filled with any desired gas or gas mixture.

Secured to, or integral with the water jacket 11 is an annular distance piece 1 7 also surrounding the electrode 4, and the end of the distance piece 1 7 bears an external screw thread for the receipt of an annular cap clamping member 18. A sealing bush 1 9 is provided internally of the cap 1 8 so that when the latter is tightened, the bush 1 9 is clamped to form a seal against the electrode 4.

In use, glass from the melt 2 within the furnace penetrates into the space 1 6 between the electrode 4 and the sleeve 5 until it reaches a region 20 where the temperature is low enough that no further outward flow of glass is possible.

In this way, that part of the electrode 4 which is vulnerable to destructive oxidation, that is, the region in the figure which is above the sealing bush 19, is completely isolated from the ambient atmosphere, and it will remain isolated even if the furnace wall 1 develops fissures which intersect the aperture 3.

In a particular practical example, the furnace wall 1 is 300 mm thick, and the electrode 4 is a bar of molybdenum 1 050 mm long and 50 mm in diameter. Prior to use, the electrode is subjected to a silicon diffusion treatment at 11 00'C to provide an oxidation resisting coating of MoSi2. The electrode may be set to penetrate 240 mm into the molten glass 2 in the furnace. The sleeve 5 is a moulding of fused zirconium silicate (zac 1681) and comprises a cylindrical portion 300 mm long and of external and internal diameter 96 mm and 56 mm respectively, and an integral flange portion 7 which has a diameter of 11 6 mm and a thickness of 30 mm.The sealing washer 1 2 and the bush 1 9 are each formed of graphited asbestos, though the bush 1 9 may instead be formed of pure graphite as known under the trade name "Grafoil".

The space 1 6 may be filled with a reducing gas mixture of nitrogen and hydrogen as a further protection against oxidation of the electrode 4.

Under these conditions it has been found that substantially no corrosion of the electrode 4 will take place outside the body of molten glass 2 in the furnace.

This means that the electrode maintains its physical strength. Also, it is very easy to shift the electrode 4 further into the furnace as it becomes eroded in use, simply by unscrewing the clamping cap 1 8 so that the electrode can slide through the bush 19, whereafter the clamping cap 1 8 is screwed up again to reseal the space 16.

Claims (13)

1. An electrode assembly for insertion into an aperture in a furnace wall which assembly comprises an elongate electrode surrounded by a substantially gas-impermeable protective sleeve, characterised in that said assembly comprises in combination a said sleeve formed from one or more refractory oxygencontaining compounds and surrounding said electrode in spaced relation therewith, a bush sealingly fitted onto the electrode, and means providing a seal between said bush and the sleeve whereby access to the space between the sleeve and the electrode may be had only via one end of the sleeve prior to insertion of that end of the sleeve into a said furnace wall aperture.

2. An electrode assembly according to Claim 1, characterised in that said sleeve comprises a flange at one end thereof.

3. An electrode assembly according to Claim 2, characterised in that said sealing means is clamped against said flange.

4. An electrode assembly according to any preceding claim, characterised in that said sleeve is constituted as a moulding of fused refractory material.

5. An electrode assembly according to any preceding claim, characterised in that the sleeve forming material comprises a refractory metal silicate.

6. An electrode assembly according to Claim 5, characterised in that the sleeve forming material comprises a refractory material of the system A12O3-ZrO2-SiO2.

7. An electrode assembly according to any preceding claim, characterised in that said electrode is of molybdenum.

8. An electrode assembly according to Claim 7, characterised in that said electrode bears a coating of MoSi2.

9. An electrode assembly according to any preceding claim, characterised in that said sealing means includes a water jacket interposed between the end of the sleeve and the bush.

1 0. An electrode assembly according to any preceding claim, characterised in that means is provided for the introduction of gas into the space between the electrode and its sleeve.

11. An electrode assembly according to any preceding claim, characterised in that said electrode is constituted as a bar of substantially uniform cross section along the major part of its length, and said sealing means includes releasable clamping means clamping said bush in sealing engagement with the electrode whereby on release of such clamping means, said bush may be moved along the electrode with the sleeve and sealing means and clamped to the electrode at a different position.

1 2. An electrode assembly substantially as herein described with reference to the accompanying drawings.

13. A glass melting furnace incorporating at least one electrode assembly according to any of the preceding claims.