Abstract:
The invention relates to a metallurgical furnace with a vessel ( 10 ), a cover ( 20 ) for the vessel, a charging device ( 30 ) for charging material that will be melted in said vessel, which charging device has a rotatable retaining means ( 31 ) and a volume (C) for receiving charging material, and a projection ( 40 ) provided in the charging opening ( 42 ) and which is arranged on the cover ( 20 ) or on the vessel ( 10 ). The furnace has a maximum filling level (H 2 ), the rotatable retaining means is pivoted for charging into the projection, the vessel, the cover, the charging device, the retaining element and the projection are dimensioned in such a way that the pivoting range of the retaining means is higher than the maximum filling level (H 2 ). The invention is characterised in that the charging device comprises at least two self-supporting material baskets ( 32 ) that can be positioned in a removable manner above the projection ( 40 ), which comprise respectively an inner chamber that can be closed on the lower side of the material basket ( 32 ) by the retaining means ( 31 ) and also a volume (C) for receiving the charging materials and a change device ( 33 ) provided for interchanging and positioning the material basket ( 32 ).

Description:
CROSS-REFERENCE  
         [0001]    This application is the national stage filing of International Application No. PCT/EP02/09255, which was filed Aug. 19, 2002 and claimed priority to German Patent Application No. 101 40 805.6 filed Aug. 20, 2001.  
         TECHNICAL FIELD  
         [0002]    The invention concerns a metallurgical furnace, in particular an electric arc furnace, and a material basket for the metallurgical art.  
         DESCRIPTION OF THE RELATED ART  
         [0003]    A metallurgical furnace, such as the electric arc furnace known from WO 98/08041 A1, has a vessel that comprises a lower vessel portion and an upper vessel portion, a cover for the vessel and a charging apparatus, which is in the form of a shaft and in which a pivotable retaining means is arranged. In the smelting operation for such apparatus, the entire charging material (for example, scrap) is charged in one operation. Because the volume of the smelted material, particularly in the case of scrap, is substantially less than in the non-molten condition, the volume defined by the vessel and the cover is not sufficient to receive the entire amount of charging material for a smelting operation. Therefore, the portion of the charging material that is not in the molten condition remains standing as a column within the shaft.  
           [0004]    Consequently, the retaining means, which is formed by a plurality of pivotably mounted fingers, cannot be pivoted back into the closed condition until the column of the charging material has melted together to such an extent that the top side of the column is below the range of pivotal movement of the retaining means (see in particular FIG. 4 of WO 98/08041 A1).  
           [0005]    In order to charge the entire amount of the charging material into the furnace all at once, the shaft must have a corresponding volume, which means that the shaft must be very high due to the spatial requirements, in particular the arrangement of the electrodes, in the arc furnace. In the alternative, the shaft may be designed only to receive the amount of the charging material that will be charged in one operation. In that case, additional charging material must be charged via the shaft by means of a crane and a material basket (for example, a scrap basket). In any event, the shaft is loaded with the charging material from above, thereby meaning that sufficient space above the shaft must be provided for the loading apparatus, such as a crane or the like. All in all, such an arrangement requires a very large height to be available above the shaft when the steel works are built.  
           [0006]    When loading the shaft with the charging material, for example with scrap, the charging material drops from a height of 4 to 6 m, because the material must be dropped into the shaft from above. As a result, there is a possibility of damaging the retaining member and/or the shaft walls.  
           [0007]    Because of the necessary structural height, an arc furnace of the kind described in WO 98/08041 A1 is normally constructed with a shaft that is designed for half the necessary amount of charging material. The balance of the charging material for the smelting operation is charged into the furnace through the shaft from above by means of a material basket (scrap basket). Because only a portion of the charging material (scrap) that was stored in the shaft during the refining period for the preceding charge is preheated, a considerable portion of the charging material is at a relatively low temperature at the beginning of the smelting operation; thus the waste gas that passes through the shaft is at a very low temperature. For known reasons, that low temperature is disadvantageous with respect to pollutants, in particular so-called VOCs (Volatile Organic Compounds). For example, VOCs must be incinerated at a temperature of about 700 to 800° C. and the resulting waste gas then must be quenched (rapidly cooled down) so that the renewed formation of pollutants does not occur. The waste gas that passes through the shaft and the cold charging material is far from being at the appropriate temperature, such that a considerable introduction of energy is required into a post-combustion chamber in order to destroy the VOCs. The additional energy requirements results in increased operating costs.  
           [0008]    Because the shaft and the retaining means formed by the fingers are supported on the furnace during all operating cycles, cooling of the retaining means and at least the lower part of the shaft is necessary for thermal reasons. The corresponding design configuration of the shaft means that a carrier structure is required for the shaft, which carrier structure, on the one hand, holds the cooling system and the necessary supply arrangements and, on the other hand, forms a sufficient support device for the scrap that is charged via the shaft. In addition, during the charging operation of the charging material, in particular scrap, through the shaft, considerable dynamic forces occur due to the great dropping height, and those forces entail the risk of damage to the shaft and the fingers, in particular the cooling arrangement. Due to the high dynamic forces involved, it is also necessary for the fingers of the retaining member to be individually mounted and to be replaceable in an expensive and complicated configuration, for example with spring packs. That results in the structure having an increased complexity, and an increased volume necessary due to the increased complexity, with respect to the retaining means.  
           [0009]    Due to the foregoing aspects, the volume of the shaft, in particular the presence of the carrier structure, the cooling arrangement, etc. is relatively large, such that, in order to maintain the necessary spacing relative to the electrodes at the centre of the cover of the furnace vessel, the shaft must be disposed relatively far away from the center in the direction of the edge of the cover or the vessel.  
           [0010]    EP 0 672 881 A1 discloses an arc furnace having two charging shafts, which shafts have charging openings that are arranged laterally at the side walls of the unit formed by the vessel and the cover.  
           [0011]    DE 44 24 324 A1 and DE 43 26 369 A1 each disclose retaining means formed by divided fingers for a shaft. FIG. 1 of DE 44 24 324 A1 illustrates an example of a conventional arc furnace in which a material basket  27  is disposed above the shaft for further charging of charging material, thereby requiring the above-described necessary internal height above the shaft.  
           [0012]    JP 7-332836 (A) discloses an arc furnace having a shaft in which two charges are held one above the other, each by appropriate retaining members. The Steel Times International, November 1995, discloses the so-called ‘Daido MSP-DCArc Furnace’ that is also published in a paper of the SEAISI 1996 THAILAND CONFERENCE under the title “Development of MSP-DC EAF Process.” That furnace has a charging apparatus in the form of a shaft. Two chambers are arranged in a perpendicularly mutually superposed relationship in the shaft. Each of the chambers can be closed at its underside by a pivotable retaining means. The entire shaft can be displaced laterally with respect to the furnace vessel for maintenance purposes or the like. The chambers of the shaft are loaded from above by means of a scrap basket.  
         SUMMARY OF THE INVENTION  
         [0013]    The object of the present invention is to provide an improved metallurgical furnace and a material basket suitable for such a furnace.  
           [0014]    This object is attained by the furnaces and material baskets disclosed herein.  
           [0015]    In a metallurgical furnace according to the present teachings, the retaining means can be pivoted back immediately after the charging operation in order to close the charging apparatus. This design results in a configuration of the charging apparatus with suitable interchangeable material baskets, in which the corresponding material basket can be exchanged immediately after the unloading operation. In addition, this design provides an empty space in the upper region of the projection, in which space it is possible to influence the waste gas.  
           [0016]    When the charging apparatus is designed with the interchangeable material baskets, there is no need to provide cooling means for the retaining means and the shaft, because they do not heat up so greatly so as to require cooling, due to relatively short residence time above the projection.  
           [0017]    In addition, when the charging apparatus is designed with the interchangeable material baskets, the need for a large internal height above the furnace is eliminated, because the material baskets can be replaced by moving them laterally over the projection.  
           [0018]    Because the empty space within the projection above the charged material is normally always above the firing temperature for combusting coal dust and/or CO, the waste gas temperature/waste gas composition can be adjusted in a very simple and inexpensive manner by simply injecting coal dust and/or oxygen into the waste gas.  
           [0019]    In such a configuration of the charging apparatus having interchangeable material baskets, there is no longer any need for a shaft carrier structure and a water cooling arrangement. Therefore, when the charging volume is the same, the corresponding material baskets can be arranged substantially closer to the electrodes than known shafts. Therefore, it is possible to use a vessel shape that is round in plan view, instead of a horseshoe shape.  
           [0020]    Further features and advantages will be apparent from the description of embodiments by way of example with reference to the Figures.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0021]    [0021]FIG. 1 shows a metallurgical furnace of a first embodiment of the invention in a front view a) in cross-section, a side view b) in cross-section, which is taken from the left in FIG. 1 a , and a plan view c) in cross-section.  
         [0022]    [0022]FIG. 2 shows the front view of FIG. 1 a ) in an enlarged scale.  
         [0023]    [0023]FIG. 3 shows the side view of FIG. 1 b ) in an enlarged scale.  
         [0024]    [0024]FIG. 4 shows the plan view of FIG. 1 c ) in an enlarged scale.  
         [0025]    [0025]FIG. 5 shows the metallurgical furnace of the first embodiment of FIG. 1 which, for operation as an arc furnace without a shaft, is closed by means of a cover, as a front view a) in cross-section, a side view b) in cross-section, which is taken from the left in FIG. 5 a ), and a plan view c) in cross-section.  
         [0026]    [0026]FIG. 6 shows the front view of FIG. 5 a ) in an enlarged scale.  
         [0027]    [0027]FIG. 7 shows the side view of FIG. 5 b ) in an enlarged scale.  
         [0028]    [0028]FIG. 8 shows the plan view of FIG. 5 c ) in an enlarged scale.  
         [0029]    [0029]FIG. 9 shows a second embodiment of the metallurgical furnace as a front view a) in cross-section in a steel works, a side view b) in cross-section, which is taken from the left in FIG. 9 a ), and a plan view c) in cross-section.  
         [0030]    [0030]FIG. 10 is a cross-sectional view of a material basket in accordance with an embodiment of the invention, the material basket being disposed on a transport carriage. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0031]    Described hereinafter with reference to FIGS.  1  to  8  is a first embodiment of a metallurgical furnace according to the invention, which represents the preferred embodiment.  
         [0032]    FIGS.  1  to  4  show the first embodiment in a first configuration in which material baskets  32 , which can be positioned by means of a changing device  30  above a projection  40 , are used as the shaft.  
         [0033]    FIGS.  5  to  8  show a second configuration of the first embodiment, in which the charging opening  42  of the projection is closed by means of a cover  43  in the melting operation in the manner of a conventional arc furnace that does not have a shaft. That configuration can be used to continue operation when maintenance procedures are to be performed on the changing device and/or the material baskets or the like.  
         [0034]    The first configuration will now be described with reference to FIGS.  1  to  4 .  
         [0035]    The first embodiment is formed as an arc furnace  1  with a furnace vessel  10  supported on a furnace cradle  2 . The furnace vessel  10  comprises a lower vessel portion  11  formed by a brick-built furnace hearth and an upper vessel portion  12  that is formed in the usual manner with water-cooled elements. The furnace vessel has a bay  13  with a tap hole  15  and a closure device  16  for the tap hole  15 .  
         [0036]    The furnace also has a cover  20  that is formed in the usual manner with water-cooled elements and, which in the usual manner, has openings for the electrodes  71  to be introduced into and removed from the furnace vessel. In the first embodiment shown in FIGS.  1  to  4 , the cover has a projection  40  that, in the plan view of the vessel  10  and the cover  20 , is disposed on one side of the electrodes. The projection  40  is made of water-cooled elements and is joined to the cover  20 .  
         [0037]    In accordance with an alternative embodiment, the projection  40  can also be formed separately from the cover  20 . The projection  40  can then be in the form of a separate part or part of the vessel  10 .  
         [0038]    Preferably, the projection  40  is part of the cover  20 . In that case, the projection  40  can be lifted off together with the cover  20  by the lifting device for the cover, for example during maintenance operations, and can be pivoted or moved away.  
         [0039]    The projection  40  is substantially cylindrical with a cross-section that is substantially rectangular in the horizontal direction. In the first embodiment shown in FIGS.  1  to  4 , the projection  40  has substantially perpendicular walls in the vertical direction of the furnace vessel. As can be seen from FIGS. 2 and 4, adaptation of the substantially rectangular shape of the projection to the substantially round cross-section (broken line  14  in FIG. 4), as viewed in cross-section in the horizontal direction, of the furnace vessel, is substantially effected by the side walls of the furnace vessel  12  having an appropriate configuration.  
         [0040]    In the alternative, the corresponding adaptation can be effected by modifying the shape of the projection  40  so that the walls of the vessel  12  are perpendicular and adapted to the shape of the lower vessel portion. This alternative embodiment, which is not shown in FIGS.  1  to  4 , is preferred because it reduces the height from which the charging material drops on to the inclined walls, and the resulting dynamic loading thereon.  
         [0041]    The projection  40  has an opening  41  laterally above the height H2 and below the height H1, which opening  41  serves as an extraction opening for suctioning away waste gas. The extraction opening  41  is connected via a waste gas conduit  61  to a waste gas main conduit  62  that communicates with a post-combustion chamber  60 . A controlled valve flap  64  is provided in the waste gas conduit  61 . The projection  40  has an upwardly open opening  42 , which is a charging opening for the furnace.  
         [0042]    A charging apparatus  30 , which has a rotary tower or turret  33  as an embodiment of a changing device, is provided for the operation of charging the charging material. In the present case, scrap is exemplified as a batch material that is charged into the furnace using the charging apparatus  30 .  
         [0043]    In the first embodiment, an electrode arrangement  70  having an electrode holding and displacement device  72  for holding and displacing the electrodes  71  is arranged on one side (the left-hand side in FIG. 2) of the furnace vessel  10 . The charging apparatus  30  is arranged on the corresponding other side of the furnace vessel  10  (the right-hand side in FIG. 2).  
         [0044]    The changing device  33 , which is in the form of a rotary tower, has two cantilever arms  34  that are displaced 180° about the vertical rotational axis  35  of the rotary tower  33 , each being designed to hold a respective material basket  32 . The cantilever arms  34  are dimensioned so that a material basket  32  held thereby can be positioned over the charging opening  42  of the projection  40 . The changing device is designed so that the arms  34 , and also the material baskets  32 , can be lifted upwardly in the direction of arrow A (see FIG. 2) or can be lowered downwardly in the opposite direction.  
         [0045]    In that respect, the charging apparatus  30  is designed so that the empty material basket  32  (at the left in FIG. 2) can be lifted by a lifting movement in the direction of the arrow A. In addition, the empty material basked  32  can be exchanged for the full material basket  32  (at the right in FIG. 2) by rotating the changing device 180° about the axis  35  and the full material basket  32  can then be lowered onto the projection  40  over the charging opening  42  by a lowering movement in the direction opposite of arrow A. Therefore, the material baskets  32  are always held by the arms  34  of the changing device  30  and are not carried by the projection  40 . The transition between the material baskets  32  and the projection  40  can be sealed off in a suitable manner, for example by using skirts.  
         [0046]    In the embodiment shown in FIGS.  1  to  4 , the arms  34  are lifted upwardly as a whole in the direction of arrow A and are lowered in the opposite direction. In the alternative, as can be seen from FIG. 9, the changing device can be provided with cantilever arms  34  that are individually pivotable about a horizontal axis  36  or with cantilever arms  34 , which are pivotable in the manner of a rocker member about a horizontal axis  36 . This design configuration, in which the arms  34  are pivotable about the horizontal axis, affords a further saving in terms of reducing the height requirements of the building of the steel works. This embodiment is therefore to be preferred when there are particular requirements for a reduction in height.  
         [0047]    An embodiment of the material basket  32 , which is in the form of a scrap basket, is shown in greater detail in FIG. 10 in the condition in which the scrap basket  32  is supported on a carriage  80 . As can be clearly seen from the plan view in FIG. 4, the scrap basket  32  has a rectangular cross-section in the horizontal direction. Projections  32   a  are provided at the side walls corresponding to the longer sides of the rectangle, which projections  32   a  enable engagement with the holding elements  91  of a crane  90  (see FIGS. 2 and 9). A shaft  31   a  is provided at the underside of those longer side walls and fingers  31  of the retaining means (retaining member) are mounted/pivoted thereon. The fingers  31  have a length that substantially corresponds to half the length (preferably somewhat more) of the short side of the rectangular shape, as can be clearly seen from FIG. 10. The fingers are laterally spaced from each other in such a way, for example at a spacing corresponding to their width, that an intervening space is provided for gas to pass therethrough. The fingers  31  are pivotable about the shafts  31   a  by means of an actuating member  35  in order to close and open the underside of the scrap basket  32 . The actuating member  35  has a hydraulic cylinder  35   a  and a lever arrangement  35   b , which pivots the fingers  31  about the shaft  31   a  in the described manner, as can be seen from FIG. 10. The actuating member  35  can be provided either only on one short side or on both short sides of the scrap basket  32 .  
         [0048]    The scrap basket  32  is loaded at a storage area for the charging material, for example at a storage area where the collected scrap is stored for the steel works. When the scrap basket  32  is loaded, the fingers  31  are supported from below at their free ends by a support  81  that is provided on the carriage  80 . Therefore, during the loading operation of the scrap basket  32 , both ends of the fingers  31  are firmly supported, thereby forming a stable lower boundary for the scrap basket  32 .  
         [0049]    The internal space of the scrap basket  32  has a predetermined volume C and is lined with heat-retaining panels (thermopanels)  37 , which are cast steel panels in the preferred embodiment. The fingers  31  are cut from steel ingots in the preferred embodiment and have a cross-section of about 300 mm (horizontal)×200 mm. Depending on the respective requirements involved, the lengthwise dimension can be selected to be different, but the thickness should be not less than 100 mm.  
         [0050]    The scrap basket  32  does not have water cooling either at the fingers  31  or at the side walls  37 .  
         [0051]    In addition, at the scrap storage location, the scrap basket  32  is loaded in portions of the total filling quantity, e.g., by a procedure in which 3 to 5 tonnes of scrap is loaded in one operation by an excavator or the like into the scrap basket, which has a capacity for example of 50 t or 80 t, thereby minimizing the dynamic forces involved during the loading operation. Therefore, the self-supporting structure of the scrap basket  32  is not required to bear the high dynamic forces that occur when charging 50 or 80 tonnes of scrap all at once into the shaft of a conventional furnace. The structure of the scrap basket  32  is therefore only designed to hold the weight, and not to receive the dynamic forces that are produced when 80 tonnes of scrap drops from a height of 6 meters. This structure saves on costs and considerably reduces the outer dimensions in comparison with a shaft having the same internal volume.  
         [0052]    The scrap basket  32  shown in FIG. 10 has an approximately rectangular cross-section. In another embodiment of the scrap basket, at least one side wall, which in operation in the position on the projection  40 , faces towards the electrode arrangement  70  (the side wall, at the left in FIG. 4, of the scrap basket disposed above the projection  40 ) has a convex configuration. The outside wall has a radius of curvature in the horizontal direction that corresponds to the radius R (see FIG. 4) of the pivotal movement of the outside wall about the axis  35  during the rotational movement of the rotary tower  33 . By virtue of this arrangement, the basket can have a greater volume and can be mounted closer to the electrodes without colliding with the electrodes  71  or the electrode arrangement  70  during pivotal movement.  
         [0053]    It is clear that one or more of the other outside walls can also have a convex configuration in order to increase the volume of the internal space. In that case, it is possible to select the same radius of curvature so that the scrap basket can be used in both possible orientations. In the alternative, the side wall, which arranged in opposite relationship to the electrode arrangement, can be, for example, conformed to the outside shape of the vessel wall. It will be appreciated that, in this embodiment, the shape of the projection  40  is also conformed in horizontal cross-section to the horizontal cross-section of the scrap basket  32  to the one or more convex outside walls.  
         [0054]    Alternatively, it is also possible for the scrap basket  32  and the projection  40  to be designed such that the scrap basket  32  has a rectangular cross-section, as viewed in section in the horizontal direction, such that the corners of the scrap basket in the position above the projection  40  project beyond the circle of radius R (see FIG. 4). In this case, during the pivotal movement of the basket about the axis  35 , the corners of the scrap basket  32  would collide with the electrodes. To avoid that situation, the arm  34  is designed so that, prior to the rotational movement, the scrap basket is moved radially in the direction of the rotational axis  35  until the corners are on or within the radius R, which then precisely no longer collides with the electrodes. Thus, collision avoidance can be effected, on the one hand, by movement in the horizontal direction or, as is possible for example with the embodiment shown in FIG. 9, by pivotal movement about a horizontal axis  36  that also has a radial component.  
         [0055]    The furnace of the first embodiment also has an extraction hood  50  that is movable upwardly and downwardly in the vertical direction and that is suspended substantially perpendicularly above the projection  40 . The extraction hood  50  is connected via a waste gas conduit  63  to the main waste gas conduit  62 . A controllable valve flap  65  is provided in the waste gas conduit  63 . The extraction hood  50  can be lowered together with the waste gas conduit  63  in the direction of arrow B (see FIG. 3) or can be lifted in the opposite direction. That lifting and lowering movement is necessary to permit adequate free space for lifting and lowering the scrap basket  32  above the projection  40  when changing the scrap basket  32  or when tilting the furnace vessel for the tapping operation (see FIG. 3).  
         [0056]    The extraction hood  50  has a lower intake opening. The cross-section of the lower intake opening is matched to the shape of the upper portion of the material basket  32 , so that the extraction hood  50  seals off the upper portion of the basket when it is lowered onto the material basket  32 .  
         [0057]    As can be clearly seen from FIG. 3, the furnace vessel  10  can be tilted by means of the cradle  2  for the tapping operation (see the broken-line illustration of the projection  40  in FIG. 3). In this case, the waste gas conduit  61  is designed so that it is variable in length by a suitable displaceable portion that slides within a sleeve.  
         [0058]    Nozzles are provided on the projection  40  for injecting coal dust (or another fuel) and oxygen. A waste gas analyzer (not shown) is provided at the post-combustion chamber  60 , which waste gas analyzer measures in real time the composition of the waste gas that is exhausted from the post-combustion chamber  60 . The nozzles (feed device) for fuel and oxygen, as well as the valves  64  and  65  and the waste gas analyzer, are connected to a control system that can be programmed to control and regulate the waste gas composition.  
         [0059]    The configuration of the first embodiment, as shown in FIG. 1, enables the arc furnace to operate as a shaft furnace. In this case, the material basket  32 , which is positioned above the projection  40 , performs the function of the shaft, which is covered at its upper position by the extraction hood  50 . The changing device can change the material basket  32  in about 30 seconds. As a result, the material in a first material basket can be preheated during the refining period of the preceding smelting procedure. When the furnace is refilled after the tapping operation of the preceding smelting procedure, that preheated charge of material can be charged into the furnace (see FIG. 2). Then, a second material basket  32  (at the right in FIG. 2), which is filled with a second charge, can be positioned over the projection  40  within 30 seconds so that that charge is preheated during the smelting operation of the first charge. Once again, the material basket  32  serves as a shaft.  
         [0060]    If maintenance or repair operations will be carried out for the charging apparatus  30  or the material baskets  32  or the like, the arc furnace of the first embodiment, in the second configuration shown in FIGS.  5  to  8 , can be operated as a conventional furnace with a cover (not as a shaft furnace) in order to avoid a complete stoppage of operation.  
         [0061]    For that purpose, the charging opening  42  of the projection  40  is closed with a cover  43 . For the charging operation, the cover  20  with the projection  40  (and the cover  43 ), as well as the electrodes  71 , can be lifted in a conventional manner and pivoted away. In the alternative, charging can be effected through the projection  40  after the cover  43  has been lifted off.  
         [0062]    The empty space in the projection  40 , which is closed off by the cover  43 , serves in that situation as a conventional furnace waste gas discharge conduit.  
         [0063]    As the second configuration does not differ in other respects from the first configuration, a further description is not provided herein.  
         [0064]    A second embodiment of the arc furnace of the invention is shown in FIG. 9. As has already been described above, in this embodiment the changing device  33  is provided with cantilever arms  34  that are individually pivotable about a horizontal axis  36 . The arms  34  also can be jointly pivoted as an arm of a rocker member about the axis  36 .  
         [0065]    As can be clearly seen from FIG. 9, and this applies with respect to all embodiments of the invention, the structural height above the shaft can be markedly reduced because there is no need to provide space for an additional scrap basket or charging material container that is held by the crane  90  above the shaft or material basket  32  during the charging operation.  
         [0066]    [0066]FIG. 9 shows a cover suction arrangement  100  that is connected via a waste gas conduit  101  to a waste gas disposal system. The waste gas disposal system can be, for example, the post-combustion chamber  60  or another apparatus.  
         [0067]    In all embodiments, such an arc furnace is designed for a specific amount of material to be smelted. Thus, for example, there are 100 ton furnaces or 160 ton furnaces. Such an amount of material to be smelted corresponds to a specific volume of the batch material, for example scrap.  
         [0068]    In the embodiments of the invention, the furnace vessel and the projection preferably are dimensioned such that the amount of scrap that corresponds to half the charge to be smelted (that is to say 50 tonnes in the case of a 100 ton furnace) extends in a loose material column, which is introduced through the projection  40  in the non-molten condition, at a maximum as far as the height H2 (maximum filling height) of the projection  40 . Therefore, the approximate volume of the furnace vessel that is occupied by a loose material column which extends from the lower edge  44  of the projection  40 , which is closest to the electrode arrangement  70 , at an angle of 45° to the bottom of the lower vessel portion  11  and fills the part of the vessel that is below the projection  40 , substantially corresponds to the volume C of a material basket  32  (FIG. 2).  
         [0069]    The lower edge of the suction intake opening  41 , which is at the height H2, is preferably slightly higher than the lower edge  44 , which is disposed towards the electrodes, of the projection  40 , as can be clearly seen from FIG. 2. In FIG. 2, the loose material column forming the charge C is slightly higher than the lower edge  44 , which is disposed towards the electrodes, of the projection  40 , and this represents an amount which is still tolerable, as will be clear hereinafter with reference to the description of furnace operation.  
         [0070]    Operation of the arc furnace of the embodiments of the invention in the first configuration will now be described.  
         [0071]    During the refining period of a preceding smelting procedure, a filled scrap basket  32  is held by the changing device  33  above the charging opening  42  of the projection  40  with the fingers  31  in the closed position. As a result, the material in the scrap basket  32  is heated by the hot gases that rise between the fingers  31  and through the scrap basket  32 . The residence time of the scrap basket  32  is about 20 minutes, depending on what is referred to in the art as the “tap-to-tap” time (the time period between two tapping operations). In addition, the cast panels  37  lining the internal space also absorb heat and heat is communicated to the scrap. The top side of the scrap basket  32  is closed by the cover hood  50 , whereby the gases rising through the scrap basket  32  are passed to the post-combustion chamber  60 .  
         [0072]    After the tapping operation of the preceding smelting procedure has been performed, the charge in the material basket  32  is charged by opening the fingers  31  when the electrodes  71  have been withdrawn from the vessel, which results in the condition illustrated in FIG. 2. Because the electrodes  71  have been withdrawn from the vessel, the electrodes cannot suffer from breakage due to scrap falling on them.  
         [0073]    After the unloading operation, the column of scrap is not so high that the fingers  31  are prevented from pivoting back into the closed position.  
         [0074]    Therefore, the emptied material basket  32  is immediately closed and replaced by the next filled scrap basket  32  by appropriate operation of the changing device  33 . For that replacement operation, the hood  50  and the scrap baskets  32  are lifted, the cantilever arms  34  of the scrap changing device  33  are then rotated by 180° about the perpendicular rotational axis  35 , and then the filled scrap basket  32  and the extraction hood  50  are lowered in the direction of the projection  40  (see the arrows B in FIG. 3). In the meantime, the electrodes  71  have been lowered again into the furnace vessel and, after the second scrap basket  32  is in position, power is supplied to the electrodes for melting the charge from the first scrap basket.  
         [0075]    During the rotational movement of the material baskets  32  about the axis  35  (pivotal movement), the electrode arrangement  70  is not moved in the horizontal direction away from the projection  40  or the material baskets  32 . As was already described above, if the material basket  32  has a rectangular cross-section in the horizontal direction, the material basket  32  is designed such that the corners just avoid touching the electrodes  71  during the pivotal movement. In the alternative, the material basket is first moved away from the electrode arrangement  70  in the horizontal direction (by horizontal movement or by pivotal movement about the horizontal axis  36 ) until the corners no longer collide with the electrode arrangement  70  during the pivotal movement. In another alternative, at least the side wall of the material basket  32  that is disposed towards the electrodes  71  has a convex shape with the radius of curvature R of the circular path, which convex shape avoids collision with the electrode arrangement  70  with a small clearance. In all three cases, this design prevents the electrode arrangement  70  from having to be moved away from the projection  40  in the horizontal direction.  
         [0076]    When the charge from the first basket is being smelted, the material in the second scrap basket  32 , which is positioned above the charging apparatus  42 , is preheated in the same manner.  
         [0077]    Further, when the material from the first basket has been smelted, the charge from the second basket is charged into the furnace vessel by opening the fingers  31  after the electrodes  71  have again been withdrawn from the vessel (in order to avoid electrode breakage due to scrap falling thereon).  
         [0078]    As there is a relatively large molten bath in the lower vessel portion  11 , the scrap of the second charge floats more strongly in the direction of the side of the vessel that is remote from the projection  40  in a horizontal direction. As a result, the column of solid scrap does not reach the height that is shown in FIG. 2 but is markedly lower.  
         [0079]    At this time, the empty scrap basket  32  and the hood  50  are once again lifted and the empty scrap basket  32  is replaced by a full scrap basket  32 , which in the meantime has been placed on the other cantilever arm  34  by the crane  90 . In that way, while the second charge is being smelted, the next charge, which is already provided for the next smelting operation, is preheated.  
         [0080]    After the electrodes  71  have been lowered again, the second charge is now smelted so that thereafter a molten bath is formed from the first and the second charges.  
         [0081]    In the described embodiment, the furnace vessel and the scrap baskets are designed so that two charges are sufficient to completely fill the furnace. It will be appreciated that it is also possible for the corresponding dimensions of the furnace vessel to be modified in such a way that a third or another fraction of the total furnace filling amount is charged in one operation.  
         [0082]    When changing the material baskets  32 , the charging opening  42  is opened upwardly. In that condition, about 40 to 50% of the waste gases are suctioned away through the extraction opening  41 . The remainder of the waste gases rises upwardly due to the temperature thereof and the majority is collected by the upper extraction hood  50 . The remaining waste gases pass to the cover extraction arrangement  100  (see FIG. 9).  
         [0083]    In comparison with known shaft furnaces, the extraction opening  41  and the upper extraction hood  50  substantially reduce the loading on the cover extraction arrangement  100 , as is described hereinafter.  
         [0084]    After the refining period for the molten bath consisting of the smelted first and second charges, a tapping operation is performed in a conventional manner.  
         [0085]    Thereafter, the above-described procedure is repeated.  
         [0086]    If it is not possible to change the scrap baskets, for example because repair operations are being performed, then the arc furnace can be operated, in accordance with the embodiments of the invention, in the second configuration that is shown in FIGS.  5  to  8 . Operation in this configuration does not differ from the operation of a conventional arc furnace. The description thereof will therefore not be provided herein.  
         [0087]    The design of the arc furnace in accordance with the embodiments of the invention, and the resulting possibility of the correspondingly described operation thereof, permit the following advantages.  
         [0088]    By virtue of the relatively short residence time of the material baskets  32  above the projection  40  (about 20 minutes in a smelting operation) and the replacement thereof by another material basket that is filled with charging material at another location, the thermal loading on the fingers  31  and the side walls  37  is so low that it is possible to omit water cooling.  
         [0089]    In the present embodiments, the interchangeable material baskets  32  are used instead of the conventional shaft and therefore, during the charging operation, there is no need, as is the conventional practice, for the entire amount of the scrap to be charged into the shaft from above in one operation. Instead, because charging is effected in relatively small portions into the material basket at the scrap storage location, the drop height of the scrap that will be charged in the direction of the fingers  31 , as well as the amount that will be charged into the material basket at the same time, are drastically reduced. It is therefore possible to forego a complicated and expensive mounting arrangement for the fingers  31 , and that results in a smaller structure and a considerable cost saving. As it is possible to forego water cooling for the fingers  31  and the side walls, there are also no problems due to leakages that can be caused by scrap falling thereon.  
         [0090]    The material baskets  32  replace the scrap baskets that are required in any event in scrap-processing steel works. Thus, no additional expenditure is involved in this respect.  
         [0091]    In conventional shaft furnaces, it was necessary to provide a way to move the shaft for repair purposes. That function is now performed by the changing device so that no additional expenditure worth mentioning is involved here.  
         [0092]    In conventional shaft furnaces, a column of scrap extends into the shaft at the beginning of the smelting operation. Therefore, on the one hand, the waste gas passing over the column of scrap was cold and, on the other hand, there was no possible way to regulate the waste gas temperature and/or the waste gas composition utilizing the heat that is always present in the furnace.  
         [0093]    The empty space that is always present in the projection  40 , and in which there are the extraction opening  41  and the nozzles for fuel and/or oxygen, easily makes it possible to control and regulate the waste gas composition at any time during the smelting procedure. If for example the waste gas temperature and in particular also that portion which is removed through the hood  50  and/or the cover extraction arrangement  100  is too low, then above the column of scrap, above which there exists always an ignition temperature (flash point) of about 500° C. (this applies simply because of the scrap which is preheated to about 500° C.), by injecting carbon and/or oxygen, it is possible to produce hot waste gas that is fed to the post-combustion chamber  60  via the extraction opening  41  and the waste gas conduit  61 . The post-combustion chamber  60  includes the waste gas analyzer so that the optimum composition of the waste gases that are to be subjected to post-combustion in the post-combustion chamber  60  can be adjusted by controlling the flow of the fuel and the oxygen and the position of the valves  64  and  65 . In the same way for example an excess of CO can be eliminated by injecting oxygen into the projection  40 .  
         [0094]    The saving in terms of structural height above the furnace, which is afforded by virtue of the horizontal interchangeability of the material baskets  32 , has already been discussed above.  
         [0095]    In comparison with a conventional arc furnace, the overall power of the waste gas cleaning system can be reduced by about 60%, because the majority of the waste gases that escape are removed by the extraction opening  41  and the extraction hood  50 , even when no material basket  32  is present.  
         [0096]    The absence of the support structure for the shaft means that the material basket  32  can be placed significantly closer to the electrode arrangement  70 . As a result, the horseshoe shape that is required in the case of conventional shaft furnaces can be greatly reduced to the round shapes that are more advantageous in terms of energy and structure.