EP0473809B1 - Direct-current arc furnace - Google Patents

Abstract

In a direct-current arc furnace, the arc is deflected under the influence of the high-current lines (18), which run below the furnace vessel, in the direction away from the power supply device (19). This leads to thermal overloading of a part of the furnace vessel wall. If the lining layers are constructed such that, in a first sector (21) which opens towards the power supply device (19), they are formed from a material having a lower specific conductivity than the lining layer in the second sector (22), the arc burns symmetrically again. A lining layer which is inhomogeneous in this sense can furthermore be used for deliberate deflection of the arc. <IMAGE>

Description

TECHNICAL AREA

The invention relates to a direct current arc furnace with a furnace vessel which is surrounded by a metallic jacket, with at least one electrode connected as a cathode and at least one bottom contact, the bottom of the furnace consisting of a single or multi-layer lining layer which is electrically conductive Brick or other inserts with the same effect, which lining layer rests on a contact plate covering most of the bottom, which contact plate forms the ground contact switched as an anode and rests on a bottom plate, said contact plate is provided with a plurality of connection fittings, which are provided by openings reach through in the base plate and are connected via electrical lines to a power supply device provided next to the furnace vessel.

The invention relates to a state of the art, as it results, for example, from US Pat. No. 4,550,413.

TECHNOLOGICAL BACKGROUND AND PRIOR ART

In high-performance DC arc furnaces, the high currents flowing in the power supply and discharge lines lead to deflections of the arc. The arc is not burned vertically. Rather, the arc is directed against the furnace wall and there leads to overheating.

The arc can be "centered" by special guiding of the power supply and discharge lines under and next to the furnace vessel. It is proposed in US-A-4,550,413 and US-A-4,577,326 to lay these lines in such a way that the magnetic fields caused by the flowing direct current act symmetrically on the arc. However, these measures are complex and also increase the costs of the furnace. Another solution is to make the electrode, including the electrode holder, horizontally displaceable relative to the furnace vessel, in order to compensate for asymmetries in the current supply and dissipation. This measure is also very complex because there must be sufficient space in the furnace cover for the travel of the electrode.

While the power supply leads to unwanted deflection of the arc, in practice it can happen that the arc should be deflected in one direction or the other in order to e.g. to bring in more heat in the area of an eccentric floor cut or in furnaces with continuous charging in said areas. This would only be possible by moving the electrode horizontally relative to the furnace vessel, but this would be very expensive.

SUMMARY OF THE INVENTION

The invention has for its object to provide a DC arc furnace in which a targeted deflection and / or symmetry of the arc is achieved with simple means.

According to the invention, this object is achieved in that one or more sections of the lining layer consist of a material for the targeted deflection of the arc has a lower specific electrical conductivity than the lining layer in the rest of the section.

The lining layer in its section facing the power supply device preferably consists at least partially of a material which has a lower specific electrical conductivity than the lining layer in the rest of the section.

In the case of arc furnaces with an eccentric floor cut, it is expedient if the lining layer in the area of the floor cut has a lower electrical conductivity than in the rest of the area, in order in this way to avoid a deflection of the arc. In this way, the arc is deflected in the direction of the floor tapping and thus more heat is introduced into the melt.

In arc furnaces for the continuous charging of sponge iron or scrap, deflection of the arc can be accomplished if the lining layer in the area in which charging is carried out has a lower specific electrical conductivity than in the rest of the area. Analogous to the aforementioned, this leads to deflection of the arc towards charging and thus to an increased supply of heat.

The advantage of the invention is to be seen in particular in the fact that without complex wiring under or next to the furnace vessel or method of the electrode for the targeted deflection of the arc, this deflection, if necessary, leading to symmetry, or deliberately deflection of the arc in a predetermined direction . Since the lining layer has to be renewed at regular intervals anyway, existing arc furnaces can also be equipped with the lining layer according to the invention.

Embodiments of the invention and the advantages which can be achieved thereby are explained in more detail below with reference to the drawing.

BRIEF DESCRIPTION OF THE DRAWING

Fig.1

ein Ausführungsbeispiel eines Gleichstrom-Lichtbogenofen mit exzentrischen Bodenabstich im Längsschnitt;

Fig.1a

ein Detail aus Fig.1, das den elektrischen Anschluss am Ofenboden verdeutlicht;

Fig.2

eine Draufsicht auf den Ofengefässboden des Lichtbogenofens gemäss Fig.1;

Fig.3

eine Draufsicht auf die Futterschicht des Gleichstrom-Lichtbogenofens gemäss Fig.1 mit zusätzlichen Vorkehrungen zur vermehrten Wärmezufuhr im Bereich des Bodenabstichs;

Fig.4

eine Draufsicht auf die Futterschicht des Gleichtrom-Lichtbogenofens gemäss Fig.1 mit zusätzlichen Vorkehrungen zur vermehrten Wärmezufuhr im Bereich der Chargierung.

In the drawing, an embodiment of the invention is shown schematically, namely:

Fig. 1

an embodiment of a direct current arc furnace with eccentric bottom cut in longitudinal section;

Fig.1a

a detail from Figure 1, which illustrates the electrical connection to the furnace bottom;

Fig. 2

a plan view of the furnace bottom of the arc furnace according to Figure 1;

Fig. 3

a plan view of the lining layer of the DC electric arc furnace according to Figure 1 with additional precautions for increased heat supply in the area of the floor tapping;

Fig. 4

a plan view of the lining of the DC electric arc furnace according to Figure 1 with additional precautions for increased heat supply in the area of the batch.

WAYS OF CARRYING OUT THE INVENTION

A direct current arc furnace according to FIG. 1 has a furnace vessel 1 which is provided with a jacket 2 made of metal. The furnace lid and the electrode holder are omitted been. In the exemplary embodiment, the furnace has only one solid electrode 3 connected as a cathode, but this number can also be two, three or more. An electrode spot, ie a slag-free surface of the melt 4, is obtained under the electrode 3 in the usual way. The furnace has a tapping device in the form of an eccentric bottom cut 5 in an oriel-like projection 6 of the furnace vessel. There is contact with the floor in the bottom of the furnace. The ground contact consists in the example of three lining layers 7a, 7b and 7c graphite or graphite-containing bricks 8a, 8b, 8c, which rest on a dome-shaped contact plate 9. Connection fittings 10 (FIG. 1a) on the contact plate 9 project downwards through openings 11 in the vessel bottom 12 into the open.
The conventional furnace lining 13 adjoins the floor lining layer on the outside. The vessel bottom 12 can be provided with a cooling device (not shown) in order to keep it at the lowest possible temperature. The bricks 8a, 8b and 8c of the lining layers 7a, 7b and 7c serve as current conductors between the melt 14 and the contact plate 9.
To this extent, the direct current arc furnace corresponds to the prior art and is described in detail, for example, in US Pat. No. 4,228,314, DE Pat. No. 30 22 566, GB-A 21 33 125 or DE-A-32 41 987, wherein the first-mentioned documents relate to classic arc furnaces, the latter-mentioned arc furnaces with eccentric floor tapping.

The jacket 2 of the furnace vessel is drawn radially inwards and forms an inwardly projecting collar 15, the end 16 of which is bent upwards. The base plate 12 projects beyond the collar 15 in the radial direction. A ring 17 made of insulating material is arranged in the overlap area. In this way, the entire bottom part of the furnace is supported on the collar 15 in an electrically insulated manner. The bottom part of the furnace virtually floats in the furnace vessel 1. At the same time, the electrical insulation between the furnace shell 2 and the base plate 12 and thus the ground contact is brought about via the insulating material.

The distribution of the connection fittings 9 can be seen from the top view of the underside of the furnace vessel 1 according to FIG. One recognizes four fittings 10 regularly distributed over the floor and the high-current lines 18 to the power supply device 19 of the arc furnace.
The top view of the upper lining layer 7a according to FIG. 3 shows the distribution of the bricks 8a: in a first sector 21 with an opening angle α of typically 45 ° to 90 °, which opens symmetrically towards the power supply device 12, the bricks 8a exist, 8b and / or 8c of the lining layers 7a, 7b and 7c made of a material with a lower carbon content than the bricks of the second sector 22, which typically have a carbon content of 10-20% by weight carbon. The electrical conductivity in the first sector 21 is accordingly lower than outside this area.

Without this measure and a line routing as shown in FIG. 2 (the line routing and the position of the power supply device 19 are only indicated schematically in FIG. 1), the electric arc would be influenced by those flowing in the electrode 3 and the high-current lines 18 Current deflected in the direction away from the power supply device 19. On the other hand, with the composition of the lining layer according to the invention, the electrical / magical center of the ground contact - viewed in isolation - is shifted out of the geometric center. In this way, the current distribution in the melt is influenced in such a way that more current penetrates into it in the region of the second sector 22 and thus compensates for the deflecting direct field that comes from the high-current lines 18 overlaid. The consequence of this is a deflection-free arc operation.

Both the "normal-conductive" and the "weakly conductive" bricks consistently correspond to the state of the art and are offered by relevant companies with a wide variety of specifications. However, bricks can also be used which have electrical conductors other than graphite, e.g. those in which the electrical conductivity is determined by the boride content. Bricks can also be used which consist of a substantially non-conductive core which is completely or only partially covered with a metallic shell.

Instead of sectors 21, 22 of different conductivity, the said lining layers can also be designed differently in terms of their electrical conductivity in another way, for example by adding bricks to the lining layer (s) in the section of the lining layer which faces the power supply device (12) lower conductivity or non-conductive bricks are interspersed.

It could be considered a disadvantage that the proposed measures do not lead to a complete elimination of the distraction in the case of a new installation, for example because the opening angle α was chosen too small or too large, or the conductivity of the lining layer (s) in the first sector 21 is incorrect was dimensioned. However, since lining layers have to be renewed regularly anyway, the test phase is comparatively short compared to the life of the furnace and accordingly only has a minor impact on furnace operation and its economy.

For arc furnaces with eccentric floor tapping or for arc furnaces in which scrap or sponge iron is continuously charged, the area of the floor tapping or the charge, the temperature in the melt is lower than in the rest of the melt. By selecting different electrically conductive sections of the lining layer, a specific deflection of the arc can also be achieved for such special purposes, in order in this way to define certain zones of the melt:

In FIG. 4, in addition to the sector 21, a second sector 23 with bricks that are less electrically conductive is provided, which sector opens symmetrically with an opening angle β toward the floor cut 5. The same considerations apply to the dimensioning of the opening angle β and the conductivity of the bricks as were mentioned above in connection with the symmetrization. Of course, the targeted deflection due to the construction of the sector 23 can also be used on its own if, for example, a line guide according to the prior art according to US Pat. No. 4,577,326 or US Pat. No. 4,550,413 is used.

FIG. 3 also indicates a third possibility of influencing the arc. It applies to arc furnaces with continuous charging with sponge iron pellets or scrap. When charging in relation to the power supply device 19 - indicated by the arrow 24 - a deflection in the direction of the batch is achieved in that in a sector 25 with the opening angle φ the material of the lining layer has a lower conductivity than in the section (s) 22. Here too, this measure can be taken on its own if necessary.

Claims (8)

1. Direct-current arc furnace having a furnace vessel (1) which is surrounded by a metal shell (2), having at least one electrode (3) connected as the cathode, and at least one bottom contact, the bottom of the furnace consisting of a single or multiple lining layer (7a, 7b, 7c) which possesses electrically conducting bricks (8a, 8b, 8c) or other equally acting inserts, which lining layer lies on a contact plate (9) covering most of the bottom, which contact plate forms the bottom contact connected as the anode and lies on a bottom plate (12), said contact plate (9) is equipped with a plurality of connection fittings (10) which pass through openings (11) in the bottom plate (12) and are connected via electric lines (18) to a current-supplying device (19) provided next to the furnace vessel, characterised in that, for the intentional deflection of the arc, one or more sections (21, 23; 21, 25) of the lining layer (7a, 7b, 7c) are composed of a material which possesses a lower electrical conductivity than the lining layer in the remaining section (22).
2. Arc furnace according to Claim 1, characterised in that the lining layer (7a, 7b, 7c) is composed, in their section (21) facing the current-supplying device (19), at least partly of a material which possesses a lower electrically conductivity than the lining layer(s) in the remaining section (22).
3. Arc furnace according to Claim 2, characterised in that the lining layer (7a, 7b, 7c) consist, in a third sector (21) which opens towards the current-supplying device (19), of a material which possesses a lower electrical conductivity than the lining layer in the second sector (22).
4. Arc furnace according to Claim 3, characterised in that the opening angle (α) of the first sector (21) is between 20° and 180°, preferably between 45° and 90°.
5. Arc furnace according to one of Claims 2 to 4, characterised in that the electrical conductivity of the lining layer (7a, 7b, 7c) in the first section or sector (21) is at least 25% lower than the conductivity of the lining layer (7a, 7b, 7c) in the other section or sector (22).
6. Arc furnace according to Claim 1 or 2 having an eccentric bottom taphole (5), characterised in that the lining layer in the region of the bottom taphole (5) possesses a lower electrical conductivity than in the remaining region.
7. Arc furnace according to Claim 1 or 2 for the continuous charging of spongy iron or scrap, characterised in that the lining layer in the region (25) being charged possesses a lower electrical conductivity than in the remaining region.
8. Arc furnace according to one of Claims 1 to 7, characterised in that the lining layer is composed of one or more courses of bricks (14) [sic] which contain graphite, borides or metal as the electrical conductor, or of at least partly bricks enveloped in metal.

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