Patent Publication Number: US-6219371-B1

Title: System to plug the delta area of the roof of an electric arc furnace

Description:
FIELD OF THE INVENTION 
     The invention concerns a system to plug the delta of the roof of an electric arc furnace (EAF) used in steel works to melt iron materials or other metals. 
     To be more exact, the invention refers to a plugging system which prevents the fumes produced during the melting process from leaking from the hole or holes made in the roof through which the electrodes are inserted, and also to prevent the outside air from entering the central chamber of the furnace and interfering with the inner heat conditions. 
     BACKGROUND OF THE INVENTION 
     In the state of the art, the area of the roof of an electric arc furnace where the electrodes are positioned is known as the delta. It is achieved with a plugging element made of refractory material, with one or more holes to allow the electrodes to be inserted into the furnace. 
     The function of the plugging element is to prevent enormous quantities of fumes leaking from the furnace and to electrically insulate the electrodes from the cooled metal body which constitutes the structure of the roof. 
     There are two main problems in the state of the art, which limit the duration of the elements which make up the roof itself. 
     The first problem is that the high surface temperature which occurs on the refractory itself and which takes the refractory to partial melting and hence the plugging element is consumed. 
     The second problem is that metallic particles adhere on the surface of the central part of the roof. These semi-melted particles are transported by the fumes and adhere to the surface of the refractory element as they move upwards. This reduces the insulating capacity of the element. Moreover, when the furnace is working, the metal particles transported by the gases encourage ionisation thereof, creating working conditions suitable for the formation of micro-discharges between the electrodes and the refractory element itself. The higher the load of particles transported by the fumes, the more frequent and intense these micro-discharges are. 
     This phenomenon of wear takes the name of electro-erosion and is widely known to workers in this field. 
     Moreover, the holes produced in the plugging element for the insertion of the electrodes have a diameter suitably larger than that of the electrodes themselves, to prevent them from coming into direct contact. In order to prevent the fumes produced in the course of the melting process from leaking out through the interstices between the plugging element and the electrodes, in the surrounding area a slight depression is created in correspondence with the two. Therefore, there is a considerable inlet of air through these apertures from outside and to the melting volume, which interferes with the inner heating conditions and encourages the oxidation of the electrodes. 
     In many electric arc furnaces, where the power applied is very high (from 20MVA to 100MVA), it is necessary to increase the size of the gap between the wall of the hole and the corresponding electrode, and therefore there are frequent leakages of large quantities of fumes from the furnace, which are dangerous for the workers managing the plant and harmful for the work environment. 
     In applications of furnaces fed with Direct Current with two or more electrodes, there is also a deflection of the arcs towards the center of the furnace, due to the attraction between the two lines of current. This entails a direct radiance towards the refractory element and an intensification of the heat load thereon. 
     The present Applicant has devised, tested and embodied the plugging system according to the invention to solve all these disadvantages. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a plugging system for electric arc furnaces. 
     One purpose of the invention is to achieve a plugging system for electric arc furnaces wherein the fumes produced by the melting metal do not leak from the aperture or apertures made on the top of the roof to allow the electrodes to be introduced, and wherein, at the same time, the outer air does not enter the main chamber of the furnace through the same apertures. 
     Another purpose of the invention is to achieve a plugging system which will prevent the formation of electric discharges onto the structure of the roof, due to the massive presence of ionised powders in the atmosphere of the furnace, and will also prevent the “sticking” of the metallic particles oh the refractory materials, which would entail an obvious reduction in the level of electric insulation. 
     In accordance with these purposes, the plugging system according to the invention comprises an upper element or auxiliary roof element consisting of a plurality of cooling pipes, arranged on the top of the main roof, shaped so as to define an inner chamber and provided with apertures through which the electrode or electrodes can be moved. The inner chamber of the auxiliary roof element is separated from the main chamber of the furnace by a cooling grid and is connected laterally to a vertical turret with a main aspiration function to create a vortex or cyclone effect at least around the upper part of each electrode. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other characteristics of the invention will be clear from the following description of a preferred form of embodiment, given as a non-restrictive example, with the help of the attached drawings wherein: 
     FIG. 1 is a longitudinal section, in diagram form, of an electric arc furnace adopting a plugging system known to the state of the art; 
     FIG. 2 is a longitudinal part section, in diagram form, of the roof of an electric arc furnace adopting the plugging system according to the invention; 
     FIG. 3 is a view from above of the roof shown in FIG. 2; 
     FIG. 4 is a longitudinal part section, in diagram form, of a variant of the plugging system shown in FIG. 2; 
     FIG. 5 is a view from above of the roof of a DC furnace with two electrodes adopting a plugging system according to the invention; 
     FIG. 6 is a view from above, in diagram form, of the upper part of the plugging system shown in FIG. 4 in a first form of embodiment; 
     FIG. 7 is a view from above, in diagram form, of the upper part of the plugging system shown in FIG. 4 in a second form of embodiment. 
    
    
     DETAILED DESCRIPTION OF PREFERRED FORMS OF EMBODIMENT 
     FIG. 1 shows a plugging system for a roof  10  of an electric arc furnace  11  made according to the state of the art. This system provides to use a plugging element  12  made of refractory material arranged on the top of the roof  10  and provided with one or more apertures or holes  13  which allow one or more electrodes  15  to be inserted into the furnace  11 . 
     In accordance with this invention, as shown in FIG. 2, an upper element or auxiliary roof element  20 , shaped so as to define an inner chamber  21 , is arranged on the top of the roof  10 . 
     The auxiliary roof element  20  consists of a plurality of pipes  23  in which a cooling fluid, such as water for example, circulates under pressure, and is provided with apertures  113  through which the electrodes  15  can be moved. 
     The apertures  113 , with the same diameter as the electrodes  15 , are much bigger than the apertures  13  of the refractory elements  12 , and such as to prevent the formation of discharges from the electrodes  15  onto the cooled pipes  23 . 
     The chamber  21  is separated from the underlying main chamber  25  of the furnace  11  by a grid  26  of cooling pipes and is connected laterally, by means of an aperture or window  29  of appropriate size, to a vertical turret  27  with a main aspiration function, through which the fumes produced by the melting of the metal emerge. 
     A cyclone effect is created in the chamber  21  due to its shape and the fact that it is directly connected to the turret  27 ; this cyclone effect conveys the air arriving from the outside directly towards the turret  27 , preventing the air from entering the main chamber  25 . In a similar way, the fumes produced by the melting of the metal which enter the chamber  21  are discharged to the turret  27  and are not dispersed to the outside, through the apertures  113 . 
     The turret  27  is provided with a side aperture  28 , also known as the fourth hole, which is connected to an aspiration system of a known type and is not shown in the drawings. The aspiration system can be for example of the type described in the application for a patent of industrial invention n°. UD96A000066 filed by the present Applicant on Apr. 30, 1996. 
     The function of the aspiration system is to create a region of uniform aspiration in the roof of the furnace in order to reduce the speed at which the fumes are aspired with a vertical ascending motion up through the melting volume and at the same time to induce a rotatory movement of the fumes along their path towards the fourth hole  28 , in order to perform a further filtering thereof. 
     The plugging system also comprises a plurality of pipes  30  (FIG. 4) arranged in the upper part of the main chamber  25 , in correspondence with the roof  10 . Cooling fluid is also made to flow under pressure inside the pipes  30 . 
     The pipes  30  are arranged in substantially radial spirals (FIG. 5) which define a circular crown arranged to almost totally cover the upper part of the central chamber  25 , except for a central aperture  31  through which the electrodes  15  pass. 
     The spirals of the pipes  30  have a substantially trapezoid cross section (FIG.  4 ), except for those arranged in correspondence with the turret  27 , which open upwards to allow the fumes to flow towards the latter. 
     The density of the spirals of the pipes  30  varies according to the zones of the roof  10 , and is greater in correspondence with the turret  27 , where the volume of the fumes is greatest and the prevalence of the aspiration is highest. 
     The grid  26  is shaped so as to conform to the outer profile of the electrodes  15 . 
     To improve the yield of the electrodes  15  and increase their duration, a vortex or cyclone effect is created around them, so that the air and the fumes are discharged from the chamber  21  towards the turret  27  peripherally, far from the surface of the electrodes  15  themselves. 
     According to a first form of embodiment, shown in FIG. 6, a single vortex of air is created around the electrodes  15 , arranging the turret  27  in a substantially median position with respect thereto, with the relative outlet collector  32  arranged tangentially with respect to the turret  27 , at its highest part. The turret  27  and the chamber  21  communicate by means of an aperture or duct  34  of an appropriately sized section. In FIG. 6 the arrows indicate the movement of the fumes before they exit from the furnace  11 . 
     According to a different embodiment, shown in FIG. 7, the inner chamber  21  is shaped so as to define two substantially cylindrical zones  21   a  and  21   b  coaxial to the two electrodes  15  and connected to each other by an intermediate zone  21   c,  while the turret  27  is shaped so as to define a first substantially cylindrical zone  27   a  and a second substantially cylindrical zone  27   b,  connected by a central zone  27   c.  In this case, the outlet collector  32  is arranged in correspondence with the central zone  27   c  while the cylindrical zones  27   a  and  27   b  are each in correspondence with an electrode  15 . In this case the two volumes  21  and  27  are put into communication by means of two distinct apertures  33   a  and  33   b  of a size suitable to set off the two complementary vortexes around the two electrodes  15  by means of vortexes induced directly in the turret  27  with the appropriate median connection to the discharge collector  32 . In this way two distinct vortexes are formed before the fumes exit from the furnace  11 , as shown by the arrows. 
     It is obvious that modifications and additions may be made to the plugging system for electric arc furnaces as described heretofore, but these shall remain within the field and scope of the invention.