AU705587B2 - Method and furnace for making a molten product - Google Patents

Abstract

A method and a furnace for melting a solid material to form an electrocast product, including at least two electrodes (4, 5). According to the method, the melting process is started by contacting the tops (13) of the electrodes (4, 5) with said solid material (23) to be melted while holding the electrodes close enough together to enable an electric current to flow therebetween, an electric arc is then generated between said electrodes to melt the solid material adjacent the tops (13) of the electrodes (4, 5), and said tops are subsequently gradually moved apart without breaking the contact with the material or cutting off the current flow between the electrodes.

Description

METHOD AND FURNACE FOR MAKING A MOLTEN PRODUCT The present invention concerns a process for melting a solid material, in particular a metallic or ceramic charge, in an electric furnace, with the aim of forming an electro-molten product, comprising at least two electrodes between whose free ends an electric current can be created, for example in the form of an arc.

One of the essential aims of the present invention is to offer a process permitting the fabrication, in a very simple and economically justified manner, of an electro-molten product on the basis of very diverse materials, said materials being electrically conductive or otherwise.

In particular, it concerns a process making it possible to prepare electro-molten products at relatively high temperature.

In the known processes of the aforementioned type, the charge of solid material to be molten must preferably be electrically conducting. If the charge is non-conducting, special measures must be taken to initiate melting, such as adding carbon or graphite, in order to permit the creation of an electric current through the charge.

Moreover, these known processes are in general only applicable at relatively low temperatures, for example of the order of 1,500 to 1,600 0 C, making them unsuitable for processing of refractory materials.

Accordingly, the present invention has as its aim to propose a process which avoids the disadvantages of 2 the known processes, and which is suitable both for electrically conducting and non-conducting charges, without any special measures having to be taken in the case of the latter.

To this end, according to the invention, in order to start the melting, said ends of the electrodes are put into contact with said solid material to be molten; by bringing them sufficiently close together to be able to create an electric current between said electrodes, an electric arc is struck, so as to melt the solid material in proximity to the ends of the electrodes; the ends are then moved gradually away from each other as melting of the solid material progresses, while maintaining contact with said material so as to ensure the flow of electric current between the electrodes and across the molten fraction of the charge formed between them.

The invention also concerns an electric furnace for preparing an electro-molten product, in particular for implementing the above-mentioned process.

The furnace is characterised by the fact that the electrodes are inclined with respect to each other, and can move with respect to each other between a close-up position, where their free ends may even be in contact with each other, and a separated position, where said free ends are at a certain distance from each other, with means being provided to enable said free ends to be moved in a substantially continuous manner between said two positions.

The electrodes can advantageously each be mounted on a support which is insulated electrically.

In a particular embodiment of the invention, the electrodes are mounted laterally in the furnace crucible in such a way that they can undergo a translation between the two above-mentioned positions.

Other details and characteristics of the invention will be clear from the non-limitative description below, given by way of example, of a particular embodiment of the invention, with reference to the attached drawings.

Fig. 1 is a schematic view of a vertical crosssection of a furnace according to this particular embodiment.

Fig. 2 is a view along line II-II in fig. 1.

Fig. 3 is a schematic view of the electrode supply circuit.

In the different drawings, the same figures refer to identical components.

The invention concerns, in a general manner, a process for melting a solid material which can be very varied in nature, but which more in particular is formed by a charge of refractory products destined to be submitted to oxidation, for the purpose of forming an electro-molten refractory product.

Melting takes place in an electric furnace comprising at least two electrodes, between whose free ends an electric current can be created which provides the necessary energy for said melting.

In particular, when the charge to be molten is non-conducting or poorly conducting, as is the case for example with solid ceramic materials, when starting the melting, said free electrode ends are put in contact with each other and with said solid material to be molten. At this moment, an electric arc is struck between said electrodes, so as to obtain heating of the solid material located in proximity to the ends of the electrodes, and so to melt said material.

Next, said electrodes are gradually moved away from each other as melting of the solid material progresses, while keeping the electrodes in contact with the solid material and making sure that the electric current is maintained between the electrodes and through the molten part of the charge forming between said electrodes. In this stage of the process, the heating is essentially due to the Joule effect of the resistance to the passage of the electric current presented by the charge in which the electrodes are partially submerged.

It is generally the case that a solid material which is electrically non-conducting becomes conducting when it is brought into a liquid state.

If the material to be molten is sufficiently electrically conducting, being e.g. formed of a metallic charge, it is not indispensable for the ends of the electrode to be in contact with each other in order to start the process; it is sufficient to bring them close enough together for an electric current to be created between them. The heating then obtained is partly due to the heat of the arcs projecting into the solid material, and partly to the Joule effect of the resistance to passage of the current posed by the charge in which the electrodes are partially submerged.

It results from the foregoing that the close-up and far-off positions can be varied to suit the nature of ,x~m LI q the charge to be molten. For example, in the case of charges which are not electrically conducting, the electrodes are practically touching or are very near to one another in the close-up position, so as to form an arc between the ends opposite each other, while this is not necessarily the case with a conducting charge.

The same applies to the far-off position, which can also depend on the conductivity of the molten charge and the power of the electric installation. Said far-off position in fact corresponds to the position for optimum yield.

Further, according to the invention, in order to homogenise the material thus molten, convection stirring is created in said material before evacuating it. Said stirring is advantageously created by bringing the electrodes closer together again during a certain period after all the material has been molten.

The attached drawings concern an electric furnace for manufacturing an electro-molten product by the implementation of the process described above.

The furnace is of the type that uses the submerged electrodes system.

Said furnace comprises a crucible 1 whose upper part is closed by a vault 2 in which there is an opening 3 for introducing the charge to be molten into the crucible 1.

Two electrodes 4 and 5, inclined with respect to each other, are each mounted on an electrically insulated mounting 6 and are located laterally with respect to the crucible 1, on either side of said crucible, such that they can undergo a translation between a close-up position in which their free ends possibly not be in contact with each other, and a far-off position in which their ends are at a certain distance from each other. For this purpose, the electrodes penetrate freely through openings in the side walls 1' of the crucible 1, the cross-section of said openings being such as to form an annular passage 19 permitting air to enter into the furnace and the electrodes to tilt. In general, the angle o formed between the axes of the electrodes may vary between 150 and 1650.

The tilting can advantageously take place around a pivot point 28 external to the furnace and sufficiently far off from the wall 1' of the latter, such as to obtain an as long as possible tilting arm of the electrodes 4 and in the furnace, and thus perfect control of the development of melting, independently of the quantity of material employed. Said pivot point 28 is in practice located on the support 6.

More specifically, each support 6 comprises a base 7 on which is fixed a column 8, at the top end of which, forming the pivot point 28, is articulated a cradle 9 in which an electrode may be fixed in movable manner by means of circlips 10. In addition, an adjustment wheel 11, which may or may not be powered, is fitted on the cradle 9, enabling the electrodes to undergo a translation in the direction of the arrows 12 and thus to vary the spacing between the free ends 13 of the electrodes 4 and inside the crucible 1. This spacing can also be adjusted by a tilting of the electrodes about the point 28, as already mentioned above.

V

0 The bottom of the crucible 1 has a cylindrical external wall 15 with which it rests, by the intermediary of wheels 16, on a pedestal 14, said wheels being able to move in rails 17 located on said cylindrical wall Finally, a pouring hole 18 is arranged in the side wall of the crucible 1, more or less half way up said side wall.

To empty the crucible, it is tilted on its pedestal 14 in the direction of the arrow 26, as shown in fig. 2.

The fact that the electrodes 4 and 5 extend completely freely through the walls 1' of the furnace, without forming any contact with said walls, constitutes a very important characteristic which distinguishes this furnace from the known electro-melt furnaces.

In these known furnaces, the electrodes are generally mounted in the walls of the furnace and are tilt-suspended by relatively complex devices which themselves are submitted to high temperatures, for which accordingly very great precautions have to be taken, in particular to protect these electrodes against the high temperatures. The presence of these devices is often the reason for which these known furnaces can only work at temperatures of the order of 1,600 0

C.

By contrast, due to the fact that the furnace according to the invention has a sufficiently large annular opening 19 around the electrodes, permitting cold air to circulate around them through said passage, and that moreover said electrodes are each mounted on a lateral support 6 far enough away from the walls no particular precautions have to be taken to protect the electrodes and their supports against the high temperatures prevailing in the crucible.

This has the consequence that it is possible to work at temperatures higher than 2,5000C, and thus to submit refractory materials to an electro-melting process.

Fig. 3 is a schematic diagram of the electric supply circuit of the electrodes 4 and 5, which is connected at point 29 to the mains in the traditional manner by a circuit breaker (not shown), and which comprises a self-induction coil 20 which can be connected in series with the electrodes 4 and 5 when the latter are in their above-mentioned close-up position.

A switch 21 is provided to short-circuit said coil 20 when the electrodes 4 and 5 are in their abovementioned far-off position.

This circuit also comprises a transformer 27 enabling the voltage to be applied to the terminals of the electrodes and to ensure the necessary current density to create melting. More specifically, said transformer can be a conventional transformer with a fixed voltage ratio, e.g. 220V/11,000 V.

Finally, a main switch 22 enables the electric circuit to be closed, thus applying voltage to the electrodes 4 and To start up the process, the switch 22 is first closed, taking care that the switch 21 is in the open position and that the electrodes 4 and 5 have their ends submerged in the charge to be molten or in contact with same, being in their close-up position.

Once a certain quantity 24 of the solid material 23 has been molten, the electrodes 4 and 5 are gradually ciT moved apart, and the switch 21 is closed so as to shortcircuit the self-induction coil As the melting progresses, the separation of the electrodes is increased, taking care that the density of the current between the electrodes passing through the molten part 24 remains sufficiently large to create the heating necessary for progressive melting of the neighbouring solid material 23.

At the moment when all the solid material is molten, the electrodes 4 and 5 can advantageously be moved closer together once more, preferably to a distance sufficiently large below the level of said molten material, and if necessary the switch 21 is opened to prevent too large electric currents in the circuit. This leads to a relatively high concentration of energy in a relatively small volume within the molten material, thus creating a local increase in the temperature of the latter. This causes convection currents, resulting in vigorous stirring of said molten material, thus yielding a substantially homogeneous mass of high quality.

Below are given some specific examples illustrating the application of the process according to the invention within the furnace, as described above and shown in the attached drawings.

Examples A charge of 1,500 kg was introduced into the furnace, with the following composition: 33% zirconium oxide, 50% aluminium oxide, 14% silicon oxide and 3% alkaline salt consisting of sodium bicarbonate, with an average particle size ranging from 0.5 mm to 15 cm diameter.

To begin with, the free ends 13 of the two electrodes 4 and 5, which in this case were made of graphite, were brought close together at the level of the solid mass previously introduced into the crucible 1, and the free ends 13 of said electrodes were covered with part of the charge, so that they were completely submerged.

Next, the switch 22 was closed, taking care that the switch 21 was in its open position, and an electric arc was formed between the electrodes 4 and The power at the electrodes was of the order of 300 kW. After about 5 minutes, a sufficiently large amount of molten charge 24 was formed around the ends 13 of the electrodes 4 and 5 for the self-induction coil to be shorted, and so the switch 21 was closed while at the same time gradually moving the electrodes away from each other.

This mass of liquid 24 was electrically conducting, and so the electric current propagated between the electrodes 4 and 5 through the molten part 24 of the charge. The total duration of the melting was of the order of 45 minutes, while the temperature was around 2,250°C.

The heat thus produced was sufficient to gradually melt the charge while continually increasing the distance between the electrodes until the charge had been completely molten.

Other types of charge were processed in this furnace in a similar way.

These included a charge of 50% iron and cobalt, a charge of 95% aluminium oxide and 5% alkaline salt, a charge of 50% cobalt and 50% nickel, a charge of bronze, a charge of brass, etc.

Another important example concerns the melting of cullet, such as recycled glass.

In this case, electrodes made of e.g. molybdenum and graphite were used.

The cullet was molten in the furnace as described above and shown in the attached drawings, together with incineration waste containing in some cases heavy metals, more specifically solid waste left over from incineration of detritus.

As a result of this melting, a vitreous product was obtained in which said heavy metals were incorporated, and which could be used as a raw material for manufacturing glass marbles according to known techniques, for the purpose of inerting heavy metals.

This melting was advantageously carried out in a continuous matter by maintaining the crucible 1 in an inclined position such that the molten vitreous mass could overflow through the pouring hole 18 as the melting progressed, while charge to be molten was added to the crucible at the same time through the opening 3 in the vault of the furnace.

This continuous process was also applied to any other type of charge to be molten.

The process according to the invention, as described above, and the electric furnace for implementing the process, have the advantage that no particular precaution has to be taken during start-up or shut-down of the furnace.

For example, it is possible to let part of the non-conducting charge solidify in the furnace, provided care is taken that the electrodes 4 and 5 are brought to their far-off position above the bath before this solidification takes place, so as to permit start-up as described above. This precaution does not have to be taken if the charge is electrically conductive.

Further, the operating conditions of the furnace can be adjusted so as to maintain a certain layer of electro-molten product 25 along the walls of the crucible, so providing permanent protection of the inside walls of the crucible 1.

The current can work either with direct current or with single-phase current or with three-phase current.

Finally, the mounting of the electrodes 4 and with respect to the crucible 1 can advantageously be such that it is possible to adjust their angle of inclination a with respect to the level of the charge to be molten. In this way, it is possible to ensure that the electrodes can also undergo a certain tilting on the column 8 of the support 6, with a relatively large amplitude, thanks in particular to the pivot point being far away from the wall of the furnace.

The invention is of course not limited to the particular embodiment described above; many variants can be envisaged while remaining within the scope of the invention.

For example, any type of electrode used in known electric furnaces comprising a system of submerged 421' 0_o/ 13 electrodes can be employed, and the construction of the support 6 for the electrodes can be of very varying design.

As regards the charge to be molten, not only can the composition be very variable, but so also can its particle size, which can vary from powder to blocks with diameters of several tens of centimetres.

Any use of the words "comprising", "comprises" or "comprise: in the specification is meant to mean "including" or "amongst other things" and is not limited to the claimed integers.

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Claims (10)

1. Process for melting a solid material, in particular a metallic or ceramic charge, in an electric furnace, for the purpose of forming an electro-melted product, which furnace comprise at least two electrodes between free ends of which an electric current can be created, in particular an electric arc, said ends of the electrodes are put in contact with said solid material to be melted, by bringing them sufficiently close together to start the melting, in order to form an electric current between these electrodes, possibly in the form of an arc, so as to melt the solid material in proximity to the ends of said electrodes, said current subsequently passing as well through the melted part of the charge being formed e between these ends, the process being characterised in that said ends are then gradually moved apart as melting of the solid material progresses, while keeping them in contact Sg: 20 with said solid material and taking care that the passage of current between the electrodes and through the melted part of the charge being formed between these is maintained.

2. Process according to claim 1, characterised S 25 in that during melting, the charge to be melted is gradually brought close to the electrodes, in particular between said electrodes, while the melted material is simultaneously evacuated, so as to obtain a substantially continuous process.

3. Process according to claim 1 or 2, characterised in that melting is carried out in an oxidising environment.

4. Process according to any of claims 1 to 3, characterised in that convection stirring of the melted material is set up before evacuating it.

Electric furnace for preparing an electro- melted product by melting of charges of solid materials, in H:\ARymer\Keep\speci\27087-95.doc 23/09/98 15 particular metallic and ceramic materials, comprising a crucible, at least two electrodes extending through the wall of the furnace, and means for creating an electric current between the free ends of said electrodes, the electrodes being inclined with respect to each other and mobile with respect to each other, between a close-up position in which their free ends are possibly in contact with each other, and a far-off position in which said ends are at a certain distance from each other, while maintaining contact with the charge to be melted, means being provided to allow displacement of these ends in a substantially continuos manner between said two positions, the furnace being characterised in that the electrodes are each mounted on a support, thereby being able to tilt around a point external to the furnace at a distance from the wall of the latter and to translate in the direction of "their respective axis, and in that the electrodes are extending freely through this wall each through a respective opening provided in the latter of which the 20 cross-section being such as to form an annular passage around the electrodes, the angle formed between the axis of these electrodes being able to vary between 15' and 1650, i: the crucible being closed in its upper part by a vault in which there is an opening for introducing the charge into the crucible.

6. Furnace according to claim 5, characterised in that each support is electrically insulated and comprises a base on which is fixed a column, at the top end of which, forming the pivot point, is articulated a cradle in which an electrode is fixed in a movable manner, enabling the electrodes to undergo a translation and a tilting, and thus to vary the spacing between the free ends of the electrodes.

7. Furnace according to any one of claims 5 or 6, characterised in that the electric supply circuit for the electrodes comprises a self-induction coil which can be connected in series with the electrodes when said H: \ARymer\Keep\Speci\2708 7 9 5.doc 23/09/98 16 electrodes are in their above-mentioned close-up position, where said coil can be short-circuited when the electrodes are in their far-off position.

8. Furnace according any one of claims 5 to 7, characterised in that means are provided to maintain the crucible in an inclined position during melting of the charge, such that the melted material can be evacuated in a substantially continuous manner while material to be melted is gradually introduced into the crucible.

9. Process for melting a solid material substantially as herein described with reference to the accompanying drawings.

10. Electric furnace for preparing an electro- melted product substantially as herein described with 15 reference to the accompanying drawings. Dated this 23rd day of September 1998 JEAN-MARIE DERKENNE By their Patent Attorneys 20 GRIFFITH HACK Fellows Institute of Patent Attorneys of Australia H:\ARymer\Keep\Speci\27087-95.doc 23/09/98 I