WO1998043030A1

WO1998043030A1 - Device to protect graphite electrodes in an electric arc furnace

Info

Publication number: WO1998043030A1
Application number: PCT/EP1998/001582
Authority: WO
Inventors: Carlo Vecchiato
Original assignee: Acciai Speciali Terni S.P.A.
Priority date: 1997-03-25
Filing date: 1998-03-16
Publication date: 1998-10-01
Also published as: IT1291117B1; AU6730498A; ITRM970164A1

Abstract

When operating an electric furnace, each electrode is surrounded by an open-ring duct, placed on the furnace roof and fixed with respect thereto, from which pressurized water is delivered, with different angles with respect to the electrode surface itself. A cooling effect is thus obtained that is additional with respect to the one obtained, in a known way, from water delivered immediately below the clamping vice. In this way the oxidation consumption of the electrodes further highly decreases.

Description

DEVICE TO PROTECT GRAPHITE ELECTRODES IN AN ELECTRIC ARC FURNACE

FIELD OF THE INVENTION

The present invention refers to a device to protect graphite electrodes in metallurgic electric furnaces and, more particularly, refers to a procedure to decrease the consumption of electrode lateral face when operating the furnace. BACKGROUND ART

Use of electric furnaces is very widespread when producing metallic alloys, particularly steel. In this type of process, the furnace is loaded with solid metal scraps, with a liquid alloy bed and with other materials like slags, additional alloys (to obtain the desired final composition), etc.: at that time, through suitable holes on the furnace roof, one or more electrodes, generally made of graphite, are lowered till they contact the charged metal material, and finally current is allowed to flow in order to generate an electric arc between electrodes and charge, thus melting the latter. The electric arc generates a pseudo-plasma, so that nearby temperatures are extremely high, up to 3000-3500 °C. Moreover, the atmosphere inside the furnace is oxidizing.

The number of electrodes essentially depends on the current being used; usually, a three-phase alternate current is preferred, so that there will be three electrodes, but it is also possible to use direct current furnaces, which require only one electrode. In the present description, anyway, for ease of understanding, only one electrode will be referred to, being understood that what is referred thereto is valid for each one of the electrodes being used. The above-mentioned operating conditions cause the electrode to be subjected to a consumption due, as regards its later face , essentially to oxidation.

So far, this electrode consumption and its effects on the cost of the whole process, though known, have not been taken greatly into account, above all because they were deemed to be unimportant as compared with problems and costs related to the remaining production process. Nowadays, however, the electrical furnace metal production process, particularly of steels, has been improved and its costs have been rationalized. In order to further lower them, therefore, it is necessary to take into account those aspects that so far have been neglected.

Among them, the electrode consumption is quite important, because it can sum up to a few percentage points over the total process cost.

So far, to the extent of our knowledge, there is nothing specifically addressed to decrease the electrode consumption; in practice, some actions, for example to optimize the arc shape and the current density being used, actually affect the electrode consumption, but are effectively carried out for other reasons such as, for example, to decrease energy consumption. On the contrary, some actions like use of foamy slags screening the furnace walls from the intense electric arc radiation, in practice insulate the electrode and promote its consumption.

It has been proposed to cool the electrode lateral face by means of devices, integral with the clamping vice for each electrode, that spray water onto the electrode itself. With such a device, cooling is possible only for a limited extension of the electrode, so that when extracting the electrode from the furnace, the farthest part from the clamping vice, that is also the hottest one, is not cooled at all and is thereby further subjected to strong oxidizing. DETAILED DESCRIPTION OF THE INVENTION

According to the present invention, in an electric furnace containing a solid metal charge to be melted, equipped with at least an electrode passing through an opening in the furnace roof and extending inside the furnace towards the metal charge, and wherein an electric arc is established between said at least one electrode and said metal charge, the electric arc being maintained till completion of melting of the solid charge and of further possible operations, such as melted material composition correction, refinement and the like, a device is used for protecting said at least one electrode from oxidation consumption, characterized in that it is composed of a ring-shaped water delivering device, placed next to the furnace roof, external thereto and around the opening housing each electrode, and equipped with a plurality of spraying nozzles, oriented towards the side electrode surface to deliver pressurized water against said surface. Preferably, said delivering device is composed of a pipe, bent in a semicircle, and at least part of the spraying holes is upwardly oriented, with an angle of about 45° with respect to the electrode surface. The nozzles are such as to deliver semi-nebulized water jets with spraying angle comprised between 0 and 30 degrees. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail with reference to an embodiment thereof, that is purely an example and that by no way is intended to limit the spirit and scopes of the invention itself, referring to a three-phase arc electric furnace and shown in the enclosed table of drawings, in which: ■ Fig. 1 shows a schematic sectional view of part of a furnace roof, including an electrode; ■ Fig. 2 shows a schematic plan view of the central part, called arch, of the arc electric furnace roof, with the three electrodes. DESCRIPTION OF THE PREFERRED EMBODIMENT

With reference to the enclosed Figures, roof 1 of the furnace includes a central part 2, called arch, in which bodies 3, generally frustoconically shaped, are housed, these bodies being arranged with their own vertical axis passing through the arch and having an axial hole to allow the passage of electrodes 4; the axial hole of the body 3 has a diameter that is slightly greater than the electrode one, in order to allow the electrode itself to be placed into the hole and to slide therein without problems, but at the same time to prevent an excessive exchange of atmosphere with the outside environment.

Around each electrode and resting on crown 2 and on body 3, a tubular member 5 is located, closed as a semicircle and connected to a water-feeding duct 6; each duct 6 is in turn connected to a manifold 7, supplied by a single tubing 8. Tubular members 5 are not closed as a ring, but have two opposed ends, separated by a gap 9, to prevent them from acting as spires and emanating electric fields generated by the strong electromagnetic field existing around the electrodes.

As schematically shown in Fig. 1 , tubular members 5 are equipped with a plurality of holes, oriented towards the electrodes, at least part of them being oriented upwardly with an angle of about 45 degrees, with respect to the electrode surfaces. With such an arrangement, a layer of cooling water is created that flows downwards on the electrode surface and penetrates inside the furnace through the gap existing between the electrode and the frustoconical body 5, thereby cooling the electrode for a length, inside the furnace, of about two meters. In this way, according to experiments carried out on industrial furnaces, it is possible to further decrease by at last 10% the oxidation consumption of the electrodes.

Claims

CLAIMS 1. Device for protecting from oxidation electrodes into an arc electric furnace containing a solid metal charge to be melted, equipped with at least an electrode passing through an opening on the furnace roof and extending inside the furnace itself towards the metal charge, and wherein an electric arc is established between said at least one electrode and said metal charge, the electric arc being maintained till completion of melting of the solid charge and of further possible operations, such as melted material composition correction, refinement and the like, characterized in that it is composed of a ring-shaped water delivering device, placed next to the furnace roof, outside of it and around the opening housing each electrode, and equipped with a plurality of spraying nozzles, oriented towards the side electrode surface to deliver pressurized water against said surface. 2. Device according to claim 1 , characterized in that said is a semicircle-bent pipe, and in that at least part of the spraying nozzles is upwardly oriented, with an angle of about 45° with respect to the electrode surface. 3. Device according to any one of the previous claims, characterized in that the nozzles are such as to deliver semi-nebulized water jets with a spraying angle comprised between 0 and 30 degrees.