WO1998048060A1

WO1998048060A1 - Electric arc furnace system

Info

Publication number: WO1998048060A1
Application number: PCT/IB1998/000474
Authority: WO
Inventors: Alfredo Poloni; Milorad Pavlicevic; Alessandro Martinis
Original assignee: Danieli & C. Officine Meccaniche S.P.A.
Priority date: 1997-04-21
Filing date: 1998-04-01
Publication date: 1998-10-29
Also published as: ITUD970076A0; AU6632898A; ITUD970076A1; IT1295102B1

Abstract

Electric arc furnace system to melt scrap (11), the system including at least an electrode functioning as a cathode and one or more electrodes functioning as anodes in order to generate the electric arc, the system including at the upper part movable covering means which can be moved from a first closed position to at least a second position of non-contact and at least an aperture for the discharge of the fumes (22) produced during the melting cycles, the system including at least a lower portion functioning as a melting chamber, the lower portion being defined by a floor (14) and by cooled sidewalls (30), the system having the cathode (12) arranged in the floor (14) of the lower portion (10b) of the furnace (10) in a substantially central position and the anodes (13) being positioned in the floor (14) around the cathode (12).

Description

"ELECTRIC ARC FURNACE SYSTEM" * * * * *

FIELD OF APPLICATION This invention concerns an electric arc furnace system as set forth in the main claim.

The invention is applied in the field of steel production and particularly, but not only, to electric arc furnaces which are loaded with scrap which has been pre-heated by the fumes from the furnace itself during the melting cycles. The electric furnace system according to the invention makes it possible to accelerate the loading operations, drastically reduce the times of the cycle and to limit to a minimum any heat losses and leakages of gases and powders from the furnace towards the outside environment. Moreover, the invention makes it possible to leave the area above the furnace free, so that it can thus be used to pre-heat the scrap which is to be loaded into the furnace.

Furthermore, with this invention, the loading operations of the furnace can be completely automated, the spaces occupied can be kept to a minimum both above and at the side of the furnace, and also it is possible to drastically reduce the times, the equipment and the movements involved in the loading process.

The invention can work either in direct current only, or in a mixed system with both direct and alternating current.

STATE OF THE ART The state of the art covers electric arc furnaces, fed either by direct or alternating current, used to produce steels starting from scrap and salvaged materials of various kinds.

The scrap is loaded into the furnace in a single lot, or semi-continuously , by means of baskets or containers, or continuously by means of carrier devices, for example belts. When the furnace is loaded by means of baskets or containers, they are first filled with scrap, then transported by means of lifting and/or moving means to the mouth of the furnace which is kept momentarily open, and then their content is unloaded into the furnace.

Such a loading system has very long operating times because of the combination of movements of opening the roof and positioning the baskets; this not only reduces the productivity of the furnace, it also causes a huge loss of heat to the outside environment, and also the leakage of fumes and gases containing noxious and polluting substances.

Moreover, in this system, opening the roof necessarily involves removing the electrodes; this causes a further increase in the loading times, oxidation of the electrode and thus a greater consumption thereof, risks of damage for the electrodes, space occupied at the sides of the furnace^", difficult and complex movements and yet other problems.

At present, in order to increase the productivity of the furnaces and reduce energy consumption by decreasing the melting times, there is a tendency to pre-heat the scrap which constitutes the load by using the heat of the fumes discharged from the furnace during the melting cycles.

Among the various loading systems which include preheating the scrap, the state of the art provides to make the fumes leaving the furnace circulate inside the baskets containing the scrap which is still to be loaded, by means of conduits connected on one side to the fourth hole of the roof and on the other side to the structure of the basket . This pre-heating system has various disadvantages; for example, it requires complex and costly constructions, considerable installation spaces, complex moving systems and also problems in the automation of the loading steps of the furnace . The present applicant has designed, tested and embodied this invention to overcome the shortcomings of the state of the art and to achieve further advantages.

DISCLOSURE OF THE INVENTION The invention is set forth and characterised in the main claim, while the dependent claims describe variants of the idea of the main embodiment.

The purpose of the invention is to provide an electric arc furnace system able to drastically reduce the times and movements required by the scrap-loading cycles.

Another purpose is to limit the space occupied at the side of the furnace and free the part above the furnace in such a way as to make it usable for the direct pre-heating of the scrap which is to be unloaded into the furnace. A further purpose is to minimise the actions and movements needed, and also the spaces occupied, to momentarily free the mouth of the furnace during the loading step.

Yet another purpose is to limit to a minimum the leakages of fumes, gases and powders from the furnace during loading and to minimise heat losses. Another purpose of the invention is to reduce the flickers which are generated when the electric arc is generated, to reduce heat stresses and therefore wear on the cooling panels associated with the sidewalls of the furnace. The electric furnace according to the invention comprises at least one electrode functioning as a cathode and one or more electrodes functioning as anodes.

According to the invention, the cathode is positioned, together with the anodes, in correspondence with the bottom or floor of the furnace.

According to one embodiment, the cathode is positioned at the centre and the anodes are arranged around the cathode. According to a variant, the anodes are distributed symmetrically with respect to the central cathode.

According to another variant, the cathode extends upwards to a height greater than that of the meniscus of the steel; this encourages the electric arc to spread and propagate, and thus to act on a considerable part of the mass of scrap inside the furnace.

Both the cathode and the anodes, in a preferential embodiment, are of a type whose upper part melts during the melting cycle of the furnace, and solidifies during the inactive cycles of the furnace, which guarantees a substantially unlimited duration both of the cathode and of the anodes .

According to a variant, the cathode and/or the anodes are associated at their lower part with a cooling system which extends upwards as far as at least inside the floor of the furnace.

According to a further variant, the lower part of the cathode and/or the anodes, associated with the cooling system, is made of material with high heat conductivity, for example, copper, while the upper part, that is to say, the part which melts and then re-solidifies, is made of steel or similar material.

According to one embodiment of the invention, the electric furnace has a lower portion where the scrap is melted and an upper portion where the scrap is temporally contained and pre-heated. When the upper electrodes and the relative support, moving and feeding systems are not included, the upper, pre-heating portion can be obtained as a continuous structural part of the lower portion. According to a first embodiment, the upper and lower portions are made in a single body; according to a variant, the portions are independent and can be associated/ disassociated at least temporally. According to the invention, the upper portion, functioning as a temporary container and pre-heating chamber, is equipped at the lower part with movable means to retain the scrap. Moreover, the upper portion has at the top at least an aperture to discharge the fumes leaving the furnace after they have lapped the scrap, and an aperture to load the scrap to be pre-heated.

According to a variant, the loading aperture cooperates with air-tight movable closing means which cover the furnace during the melting cycles.

According to the invention, the movable means to retain the scrap are structured in such a way as to allow the passage of the fumes produced during the melting cycles in the lower portion of the furnace.

Before being discharged through the discharge aperture, the fumes lap the scrap which is temporally retained by the movable means in the upper portion of the furnace, and give up at least part of their heat to the scrap. Once the melting cycle is finished, the melted metal produced is unloaded from the furnace; subsequently, when the movable retaining means are opened, the furnace is immediately reloaded with the pre-heated scrap temporally retained in the upper portion of the furnace itself. Then, the movable retaining means are closed, so that new scrap loaded from above can be retained and pre-heated.

According to the invention, the scrap which is to be preheated is unloaded into the upper portion of the furnace by means of feeder means cooperating with the afore-said air- tight movable closing means. According to one embodiment, the feeder means are of the conveyor belt type .

In other possible embodiments, the upper portion of the furnace can be loaded by means of baskets, containers, with arm-type carrier means movable on a bridge crane or in other ways .

According to a variant, the walls of the upper containing and pre-heating portion and the walls of the lower melting portion at least partially consist of cooled panels.

The furnace system according to the invention makes it possible to load the pre-heated scrap, where the lower part of the scrap has the highest temperature, in extremely limited times and with drastically limited movements. This guarantees a reduction in the flicker which is generated when the electric arc is struck between the cathode and the anodes, which are all arranged on the floor of the furnace.

Moreover, the generation of an electric arc in the lower portion of the furnace drastically reduces those problems on the cooling panels which are caused by the passage of the current between the cathode and the anodes .

Furthermore, the absence of the moving and feeding systems and of the upper electrodes, and therefore of the relative electrode-bearing arms, considerably reduces the overall construction, installation and management costs of the electric furnace.

ILLUSTRATION OF THE DRAWINGS The attached figures are given as a non-restrictive example and show some preferred embodiments of the invention as follows: Fig. 1 shows a first embodiment of the electric arc furnace system according to the invention; Fig. 2 shows a variant of Fig. 1; Figs. 3 and 4 show in diagram form some further applications of the invention.

DESCRIPTION OF THE DRAWINGS The electric arc furnace 10 shown in the embodiment of Fig. 1 comprises a lower portion 10b, which functions as a chamber to melt the scrap 11, and an upper portion 10a, which functions as a chamber to contain and pre-heat the scrap which is to be unloaded into the lower portion 10b. The variant shown in Fig. 3 shows a conventional furnace 10 with a fourth hole 33 connected to a conduit to discharge the fumes 23, while the variant shown in Fig. 4 shows a furnace 10 whose fourth hole 33 is connected by means of the conduit 23 to a basket 34 to pre-heat the scrap 11 arranged in a position at the side of and in proximity to the furnace 10.

In the embodiment shown in Fig. 2, the mouth of the furnace 10 is associated directly with loading means 18 including a conveyor belt 19 for a possible continuous loading of the scrap 11.

The furnace 10 comprises a floor 14 made of refractory material and sidewalls 30 equipped with cooling panels 29.

In this case, the cathode 12 and the anodes 13 are arranged on the floor 14; the cathode 12 is arranged in a substantially central position with respect to the floor 14 and the anodes 13 are arranged symmetrically around the central cathode 12.

According to a variant which is not shown here, the anodes 13 are arranged asymmetrically with respect to the central cathode 12.

The respective upper segments 12a and 13a of both the cathode 12 and the anodes 13 are made of a first material, for example steel, which melts during the melting cycles and solidifies when the furnace 10 is inactive. The respective lower segments 12b and 13b are made of a second material with a high heat conductivity, for example copper, and are associated with a cooling system 31, for example of the type with circulating water, which extends as far as a defined height inside the refractory floor 14.

In this case, the top of the central cathode 12 is placed at a greater height than the anodes 13 and above the level of the meniscus of the molten metal; this encourages the generation of electric arcs 17 which develop from the top downwards and therefore act on a considerable part of the scrap 11 which has accumulated in the lower portion 10b of the furnace 10.

Moreover, in this case, at least the top of the central cathode 12 is protected by a covering 28 made of high resistance material which protects it from falling scrap 11 during the loading cycle and facilitates extraction for operations of replacement and/or maintenance.

In Fig. 1, the lower portion 10b and the upper portion 10a of the furnace 10 are separated by movable means 15 suitable to momentarily retain the scrap 11 which is contained in the upper portion 10a and which is to be pre-heated; this allows the fumes 22 produced during the melting cycles to pass upwards . In this case, the movable means 15 consist of sliding, counter-opposed grids 16; as the grids 16 slide towards the outside of the furnace 10, and take the position of non- contact as shown by the line of dashes in Fig. 1, it is possible to unload the pre-heated scrap 11 into the lower portion 10b of the furnace 10.

In this case, the grids 16 cooperate with retaining means 116 which prevent the heat and fumes inside the furnace 10 from escaping towards the outside environment .

In the case of Fig. 1, the scrap 11 is loaded into the upper portion 10a of the furnace 10 by means of loading means 18 consisting of a conveyor belt 19 which cooperates with air-tight closing means 20 associated with the top of the upper portion 10a. The function of the air-tight closing means 20 is to cover the furnace 10 at the top during the melting cycles.

In this case, in cooperation with the top of the upper portion 10a there is a ring-type chamber to take in the fumes 32 which causes a cyclone-type circulation of the fumes 22 before they are discharged through the duct 23.

The ring-type fume-intake chamber encourages the fumes to rise uniformly over the whole volume of the upper portion 10a and therefore the scrap 11 placed in the pre-heating position in the upper portion 10a is heated homogeneously.

In this case, the air-tight closing means 20 are able to slide and are taken to the open position, indicated by a line of dashes, when the loading means 18 unload the scrap 11 into the upper portion 10a of the furnace 10. In this case, the loading means 18 also comprise covering and containing means 21 which facilitate the delivery of the scrap 11 into the upper portion 10a of the furnace 10 and limit heat losses into the outer environment.

In this case, the furnace 10 has a circular section and a height/diameter ratio which is suitable to exploit as much as possible the heat generated by the fumes 22 so as to preheat the scrap 11 retained by the retaining means 15.

According to the invention, the fumes 22 produced during the melting cycles in the lower portion 10b pass through the retaining means 15 and ascend into the upper portion 10a, lap the scrap 11 and give up at least part of their heat; they are subsequently discharged through the fume-intake chamber 32 and the duct 23 towards the purifying plants and discharged into the atmosphere. In this case, moreover, in order to obtain a homogeneous distribution of the temperature, to improve and accelerate the descent of the scrap 11 into the lower portion 10b, and to accelerate the melting cycles, on the sidewall of the lower portion 10b there are burners 24 arranged on one or more levels.

In the embodiment shown in Fig. 1, there are also burners 24 on the upper part of the upper portion 10a, in order to achieve a process of post-combustion of the fumes 22 so as to abate the noxious and polluting substances present therein before the fumes 22 are discharged through the duct 23.

The lower portion 10b is also equipped with supersonic lances 25 to inject oxygen and with tuyeres 26 to blow oxygen and/or carbon into the liquid metal from below.

In this case, moreover, in order to mix the molten metal efficiently so as to achieve a homogeneous bath, a uniform temperature, and also to accelerate the chemical reactions, there are also stirring means 27 of the electromagnetic type in the lower portion 10b.

In Fig. 2, the loading means 18 not only feed but also momentarily retain and possibly pre-heat the scrap 11, at least part of the fumes 22 deriving from the melting process lapping the scrap 11 on the conveyor belts 19.

Claims

1 - Electric arc furnace system to melt scrap (11), the system including at least an electrode functioning as a cathode and one or more electrodes functioning as anodes in order to generate the electric arc, the system including at the upper part covering means which can be moved from a first closed position to at least a second position of non- contact and at least an aperture to discharge the fumes (22) produced during the melting cycles, the system including at least a lower portion functioning as a melting chamber, the lower portion being defined by a floor (14) and by cooled sidewalls (30), the system being characterised in that the cathode (12) is arranged in the floor (14) of the lower portion (10b) of the furnace (10) in a substantially central position and the anodes (13) are positioned in the floor (14) around the cathode (12).

2 - Electric furnace system as in Claim 1, in which the top of the cathode (12) is located at a greater height than the height of the meniscus of the molten metal . 3 - Electric furnace system as in Claim 1 or 2 , in which the upper segment (12a) of the cathode (12) melts during the melting cycles and solidifies when the furnace is inactive.

4 - Electric furnace system as in any claim hereinbefore, in which the upper segment (12a) of the cathode (12) is made of a first material and the lower segment (12b) is made of a second material which has a greater heat conductivity than the first material, the lower segment (12b) cooperating with a cooling system (31) which extends for at least a defined height inside the floor (14) of the lower portion (10b) of the furnace (10) .

5 - Electric furnace system as in any claim hereinbefore, which comprises an upper portion (10a) arranged above the lower portion (10b) constituting a substantially continuous structure therewith, the upper portion (10a) serving to momentarily contain and pre-heat the scrap (11), the upper

(10a) and lower (10b) portions being separated by movable means (15) to momentarily retain the scrap (11) to be pre- heated, the movable retaining means (15) allowing the fumes

(22) to pass from the lower melting portion (10b) to the upper pre-heating portion (10a) .

6 - Electric furnace system as in Claim 5, in which the upper portion (10a) includes air-tight movable covering means (20) .

7 - Electric furnace system as in any claim hereinbefore, in which the upper portion (10a) includes, in its upper part, a ring-type fume-intake chamber (32) connected to the discharge conduit (23). 8 - Electric furnace system as in any claim hereinbefore, in which the lower portion (10b) and the upper portion (10a) are made in a single body.

9 - Electric furnace system as in any claim from 1 to 7 inclusive, in which the lower portion (10b) and the upper portion (10a) are separate and can be associated/disassociated at least momentarily.

10 - Electric furnace system as in any claim hereinbefore, in which the means (15) to retain the scrap (11) which is to be pre-heated can be moved from a first retaining and pre- heating position to at least a second position of non- contact wherein the scrap (11) is unloaded into the lower portion (10b) .

11 - Electric furnace system as in any claim from 1 to 4 inclusive, in which the top of the lower portion (10b) is directly associated with means (18) to load the scrap (11).

12 - Electric furnace system as in Claim 11, in which between the loading means (18) and the top of the lower portion (10b) there are closing means (20) which can be moved from a position wherein the scrap (11) is pre-heated to an unloading position.

13 - Electric furnace system as in any claim from 1 to 4 inclusive, in which the furnace (10) has a fourth hole (33) to discharge the fumes.

14 - Electric furnace system as in Claim 13 , in which the fourth hole (33) for the discharge of the fumes is associated with a basket (34) to contain and pre-heat the scrap (11) . 15 - Electric furnace system as in any claim hereinbefore, in which in cooperation with the lower portion (10b) of the furnace (10) there are electromagnetic stirring means (27), tuyeres (26) to blow oxygen and/or carbon in from the bottom, heating burners (24) and/or supersonic lances (25) to blow in oxygen.

16 - Electric furnace system as in Claim 5, in which in the upper part of the upper portion (10a) there are post- combustion burners (24) .