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.