EP0768017A1

EP0768017A1 - Method and furnace for making a molten product

Info

Publication number: EP0768017A1
Application number: EP95922361A
Authority: EP
Inventors: Jean-Marie Derkenne
Original assignee: Individual
Current assignee: Individual
Priority date: 1994-06-24
Filing date: 1995-06-19
Publication date: 1997-04-16
Anticipated expiration: 2015-06-19
Also published as: PL317937A1; CZ376096A3; PL176908B1; HU220470B1; DE69504350D1; CZ289969B6; ATE170356T1; AU2708795A; HU9603424D0; ES2124557T3; BG62150B1; CA2192068A1; SK283103B6; BG101072A; BE1008485A3; SK165496A3; RU2144285C1; EP0768017B1; WO1996000489A1; DE69504350T2

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

PROCESS AND OVEN FOR THE MANUFACTURE OF A MOLTEN PRODUCT.

The present invention relates to a method for melting a solid material, in particular a metallic or ceramic charge, in an electric furnace, with the aim of forming an electrofused product, comprising at least two electrodes between the free ends of which 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 propose a process allowing the manufacture, in a very simple and economically justified manner, of an electrofused product from very diverse solid materials electrically conductive or not.

It is more particularly a process which allows the preparation of electrofused products at relatively high temperature.

In known processes of the aforementioned type, the charge of the solid material to be melted must preferably be electrically conductive. If the load is non-conductive, special precautions must be taken to start the fusion, such as the addition of carbon or graphite, to allow the creation of an electric current through the load.

In addition, in general these known methods are only applicable at relatively low temperatures, e.g. of the order of 1500 to 1600 ° C., so that they are not suitable for the treatment of refractory materials.

Thus, the invention aims to propose a method which makes it possible to remedy the drawbacks of known methods and which may also be suitable for an electrically conductive or non-conductive load without the latter, special precautions must be taken.

To this end, according to the invention, for starting the melting, said ends of the electrodes are brought into contact with said solid material to be melted, bringing them close enough to one another to be able to create between these electrodes a current electric, an electric arc is created between them so as to be able to melt the solid material located near the ends of the electrodes and the ends are then gradually moved apart from each other as the progress of the melting of the solid material, while maintaining them in contact with this material and ensuring the passage of electric current between the electrodes and through the molten part of the charge forming between them.

The invention also relates to an electric furnace for the preparation of an electrofused product, in particular for the implementation of the above-mentioned process. This furnace is characterized by the fact that the electrodes are inclined with respect to each other and movable with respect to each other between a close position, where their free ends are possibly in contact with one another. other, and a spaced position, where these ends are at a certain distance from each other, means being provided to allow these ends to be moved in a substantially continuous manner between these two positions.

Advantageously, the electrodes are each mounted on an electrically insulated support.

According to a particular embodiment of the invention, the electrodes are mounted laterally in the crucible of the furnace so as to be able to undergo a translation between the two aforementioned positions. Other details and particularities of the invention will emerge from the description given below, by way of nonlimiting example, of a form of particular embodiment of the method and the oven according to the invention with reference to the appended figures.

Figure 1 is a schematic view of a vertical section of an oven according to this particular embodiment.

Figure 2 is a view along line II-II of Figure 1.

Figure 3 is a schematic representation of the electrode supply circuit. In the various figures, the same reference numbers relate to identical elements.

The invention relates, in general, to a process for the melting of a solid material which can be of very varied nature, but which is more particularly formed by a charge of refractory products intended to be subjected to oxidation, in the purpose of forming an electrofused refractory product.

The fusion takes place in an electric oven comprising at least two electrodes between the free ends of which can be created an electric current supplying the energy necessary for said fusion.

Above all, when the load to be melted is not electrically conductive or not very conductive, as is for example the case for solid ceramic materials, when the fusion starts, the said ends of the electrodes are brought into contact with one another. other and with said solid matter to be melted. At this time, an electric arc is created between these electrodes so as to obtain the heating of the solid material located near the ends of the electrodes and thus be able to melt the latter.

Then, these ends are gradually moved apart from one another as the solid material progresses, while keeping these electrodes in contact with the solid material and ensuring that the electric current may remain between the electrodes and through the molten part of the charge forming between them. In this stage of the process, the heating is essentially due to the Joule effect of the resistance opposed by the load, in which the electrodes are partially immersed, to the passage of the electric current. It is, in fact, thus that a solid material which is not electrically conductive generally becomes electrically conductive when the latter is brought to the liquid state.

If the material to be melted is sufficiently electrically conductive and is, for example, formed of a metallic charge, it is not essential that, for starting, the ends of the electrodes are in contact with each other , but it suffices to bring them close enough together to be able to create an electric current between them. The heating obtained is thus due, in part to the heat of the arcs sprung up within the solid matter and, in part, to the Joule effect of the resistance opposed by the charge, in which the electrodes are partially immersed, in passing electric current.

It follows from the above that the close and apart positions may vary depending on the nature of the load to be melted. Thus, for example, for non-electrically conductive charges, in the close position, the electrodes practically touch or are very close to each other so as to be able to create an arc between their opposite ends, while this is not necessarily the case for a conductive load.

It is the same for the spread position which can also depend on the conductivity of the molten charge and the power of the electrical installation. This discarded position actually corresponds to that in which the yield is optimal.

Furthermore, according to the invention, in order to homogenize the material thus melted, a convection stirring is created in the latter before discharging it. This mixing is advantageously created by bringing together again the electrodes from each other for a while after all of the material is melted.

The appended figures relate to an electric furnace for the manufacture of an electrofused product and in particular for the implementation of the method described above.

It is a furnace of the type using the submerged electrode system.

This furnace comprises a crucible 1 closed at its upper part by a vault 2 in which an opening 3 is provided for introducing the charge to be melted into the crucible 1.

Two electrodes 4 and 5, inclined with respect to each other, are each mounted on an electrically insulated support 6 and are located laterally with respect to the crucible 1, on either side of the latter, and this is in such a way as to be able to undergo a translation between a close position, where their free ends are possibly in contact with one another, and a separated position, where these ends are at a certain distance from each other . In this regard, the electrodes freely penetrate into openings provided in the side walls 1 ′ of the crucible 1 and the cross section of which is such as to form an annular passage 19 allowing the entry of air into the furnace and the tilting of the electrodes. Generally, the angle α formed between the axes of the electrodes can vary between 15 ° and 165 °.

The tilting advantageously takes place around a point 28 outside the oven and sufficiently spaced from the wall l 'of the latter, so as to obtain a rocking arm as large as possible of the electrodes 4 and 5 in the oven and thus a control perfect 1 evolution of the fusion regardless of the amount of material used. This tilting point 28 is located in practice on the support 6.

More particularly, each support 6 comprises a base 7 on which is fixed a column 8 to the upper end of which, forming the tilting point 28, is articulated a cradle 9 in which an electrode can be fixed in a removable manner by hoops 10. In addition, an adjustment flywheel 11 motorized or not is provided at cradle 9 making it possible to subject the electrodes to translation in the direction of the arrows 12 and thus to vary the spacing between the free ends 13 of the electrodes 4 and 5 inside the crucible 1. This spacing can also be adjusted by a tilting of the electrodes around point 28, as already indicated above.

The bottom of the crucible 1 has a cylindrical outer wall 15 with which it rests, by means of rollers 16, on a base 14, these rollers being able to move in rails 17 provided on this cylindrical wall 15.

Finally, a tap hole 18 is formed in the side wall of the crucible 1, substantially halfway up the latter. Thus, to empty the crucible, it is tilted on its base 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, forming no contact with the latter is a very important characteristic which distinguishes this furnace from known electrofusion furnaces.

Indeed, in these known ovens, the electrodes are generally mounted in the walls of the latter and are suspended to tilt in relatively complex devices subjected to high temperatures for which, therefore, very important precautions must be taken, in particular for protection of the electrodes against these high temperatures. The presence of such devices is also often the reason why these known ovens can only work at temperatures of the order of 1600 ° C. On the other hand, thanks to the fact that in the furnace, according to the invention, a sufficiently large annular passage 19 is provided around the electrodes thus allowing a circulation of cold air around the latter through this passage and that, moreover, they are each mounted on a lateral support 6 quite distant from the walls 1 ′, no particular precaution must be taken for the protection of these electrodes and of their support with respect to the high temperatures prevailing in the crucible. This has the consequence that it is possible to work at temperatures above 2500 ° C. and therefore to subject refractory products to an electrofusion treatment.

Figure 3, which schematically shows the electrical supply circuit of the electrodes 4 and 5, is connected, at 29, to the network in a traditional way by means of a circuit breaker, not shown, and comprises a coil of self-induction 20 which can be connected in series with the electrodes 4 and 5 when the latter are in their above-mentioned close position.

A switch 21 is provided for short-circuiting this coil 20 when the electrodes 4 and 5 are in their above-mentioned separated position.

This circuit further comprises a transformer 27 for applying the voltage across the electrodes and ensuring the density of the current necessary to create the fusion. It can, more particularly, be a conventional transformer with fixed voltage ratio, e.g. 220V / 11000V. Finally, a main switch 22 makes it possible to close the electrical circuit and thus to energize the electrodes 4 and 5.

Thus, during startup, the switch 22 is first closed, making sure that the switch 21 is in its open position and that the electrodes 4 and 5 are immersed by their free ends in the charge to melt or at least in contact with it, while in their close position.

Once a certain quantity 24 of the solid material 23 is melted, the electrodes 4 and 5 are gradually moved apart from each other and the switch 21 is closed so as to short-circuit the self-winding coil. induction 20.

As to measurement of the melt progression is increased one electrode gap and this taking care that the current density between the electrodes through the molten portion 24 is enough ^'important to create the heating necessary for melting progressive of the neighboring solid 23.

When all the solid material is melted, the electrodes 4 and 5 are advantageously brought closer to one another, preferably at a sufficiently large distance below the level of this molten material, and, if necessary, we open the switch

21 to avoid excessively high electric currents in the circuit. This results in a relatively large concentration of energy in a reduced volume within the molten material creating a local increase in temperature in the latter. This therefore results, as a result of convection currents produced, a significant mixing of this molten material making it possible to obtain a substantially homogeneous mass of high quality.

Below are given concrete examples to illustrate the application of the method according to the invention in the oven, as described above and shown in the accompanying drawings.

Examples.

A 1500 kg load was introduced into the oven which had the following composition: 33% zirconium oxide, 50% aluminum oxide, 14% silicon oxide and 3% alkali salt formed of bicarbonate sodium, while its average particle size varied from 0.5 mm to 15 cm (diameter). First, the free ends 13 of the two electrodes 4 and 5 were brought, which, for this particular case, were made of graphite, one near the other at the level of the solid mass previously introduced into the crucible 1 and the free ends 13 of the latter have been partially covered with a portion of the load, so that they are submerged.

Then, 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 5.

The energy at the electrodes was around 300k. After about 5 minutes, a quantity of the molten charge 24 was obtained around the ends 13 of the electrodes 4 and 5 sufficient to allow the self-induction coil 20 to be short-circuited and the switch 21 was therefore closed in at the same time gradually moving the electrodes away from each other.

This liquid mass 24 being electrically conductive, the electric current therefore propagated between these electrodes 4 and 5 through the molten part 24 of the charge. The total duration of the fusion was around 45 minutes, while the temperature was around 2250 ° C. The heat thus produced was sufficient to allow the charge to gradually melt while the distance between the electrodes was continued to increase, until the complete charge was melted. Other types of fillers have been treated in this oven in a similar manner.

These included a charge of 50% iron and 50% cobalt, a charge of 95% aluminum oxide and 5% alkali salt, a charge of 50% cobalt and 50% nickel, bronze filler, brass filler, etc. Another important example concerns the melting of groisils, such as recovery glass.

In this case, use was made of molybdenum or graphite electrodes. These groïsils were melted in the furnace as described above and shown in the appended figures, together with incineration waste possibly containing heavy metals. It was more particularly waste remaining in the solid state in trash incinerators.

As a result of this fusion, a glassy product was obtained in which said heavy metals were incorporated and which could serve as a base material for the manufacture of beads according to techniques known per se, for the purpose of inerting heavy metals.

Advantageously, this melting was carried out continuously by maintaining the crucible 1 in an inclined position in such a way that the molten vitreous mass could flow by overflowing through the tap hole 18 as the progression of melting while at the same time adding the charge to melt in the crucible through opening 3 in the roof of the furnace.

This continuous process has also been applied to any other type of load to be melted. The method according to the invention, as described above, and the electric oven for the implementation of this method have the advantage that no particular precaution must be taken when starting or stopping the oven. Thus, it is for example possible to let a part of the non-conductive filler solidify in the furnace provided that care is taken that the electrodes 4 and 5 are brought into their separated position above the bath before this solidification for allow start-up, as described above. If the load is electrically conductive, this precaution should not be taken. Furthermore, the working conditions of the furnace can be adjusted so as to maintain a certain layer of electrofused product 25 along the walls of the crucible, thus constituting permanent protection of the interior walls of the crucible 1.

The oven can work both in direct current as in single-phase current as in three-phase current.

Finally, advantageously, the mounting of the electrodes 4 and 5 relative to the crucible 1 is such that it is possible to adjust their angle of incidence α relative to the level of the charge to be melted. In this way, it is ensured that the electrodes can also undergo a certain tilting on the column 8 of the support 6 with a relatively large amplitude, especially thanks to the fact that the tilting point is distant from the wall of the furnace.

It is understood that the invention is not limited to the particular embodiment described above and that many variants can be envisaged without departing from the scope of the present invention.

Thus it is possible to make use of any type of electrode used in known electric ovens comprising a submerged electrode system and that the construction of the support 6 for the electrodes can be of very different designs.

With regard to the load to be melted, not only its composition can be very variable, but also its particle size. It can in particular be both powder and blocks having diameters of several tens of centimeters.

Claims

1. Method for melting a solid material, in particular a metallic or ceramic charge, in an electric furnace, with the aim of forming an electrofused product, comprising at least two electrodes between the free ends of which a current can be created electric, in particular an electric arc, characterized in that, for the start of the fusion, said ends of the electrodes are brought into contact with said solid material to be melted by bringing them sufficiently close to one another to be able to create between these electrodes an electric current, optionally in the form of an arc, and so that an electric arc is then created between these electrodes so as to melt the solid material located near the ends of the electrodes and so that it is separated then gradually these ends from each other as the solid material progresses, while keeping them in contact with this the material and ensuring that the current flow between the electrodes persists.

2. Method according to claim 1, characterized in that one brings, during the melting, gradually the charge to melt near the electrodes and in particular between the latter while simultaneously evacuating molten material, so as to achieve thus a substantially continuous process.

3. Method according to one or other of claims 1 and 2, characterized in that the fusion is carried out in an oxidizing medium.

4. Method according to any one of claims 1 to 3, characterized in that one creates a stirring, by convection, of the molten material before discharging it.

5. Electric oven for the preparation of an electrofused product by the fusion of charges of materials solids (23), in particular metallic and ceramic fillers, more particularly for implementing the method according to either of the preceding claims, comprising a crucible (1), at least two electrodes (4) and (5 ) extending through the wall (1 ') of the furnace and means (20,21,22) to create between the free ends of these electrodes an electric current, characterized in that the electrodes (4) and (5) are inclined with respect to each other and movable with respect to each other between a close position, where their free ends (13) are possibly in contact with each other, and a separated position , where these ends (13) are at a certain distance from each other, means (9,10,11) being provided to allow these ends (13) to be moved in a substantially continuous manner between these two positions.

6. Oven according to claim 5, characterized in that the electrodes (4) and (5) are each mounted on a support (6) electrically insulated fixed on the wall (1 ') of the oven.

7. Oven according to either of Claims 5 or 6, characterized in that the electrodes (4) and (5) are mounted laterally with respect to the crucible (1) in such a way as to be able to undergo translation between the two aforementioned positions.

8. Oven according to claim 7, characterized in that each of the electrodes (4) and (5) is mounted on a support (6) in such a way as to allow a certain tilting of the latter in the crucible (1).

9. Oven according to any one of claims 5 to 8, characterized in that each of the electrodes (4) and (5) extends freely through the wall (1 ') of the oven in an opening provided in the latter, the section is such as to form around the electrode an annular passage (19) allowing the entry of air into the furnace.

10. Oven according to any one of claims 5 to 9, characterized in that each of the electrodes is tiltably mounted around a point (26) outside the oven.

11. Oven according to any one of claims 5 to 10, characterized in that the crucible (1) is closed at its upper part by a vault (2) in which is provided an opening (3) for introducing the load (23 ) in the crucible (1).

12. Oven according to any one of claims 5 to 11, characterized in that the electrical supply circuit of the electrodes (4) and (5) 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 position, this coil (20) being able to be short-circuited when the electrodes (4) and (5) are in their separated position.

13. Oven according to any one of claims 5 to 12, characterized in that means are provided to maintain, during the melting of the load, the crucible (1) in an inclined position, in such a way as to allow a substantially continuous evacuation of the molten material obtained while gradually introducing the material to be melted in the crucible (1).