Industrial electric heating furnaces – Environmental control – Arc furnace
Reexamination Certificate
2000-01-14
2001-01-16
Hoang, Tu Ba (Department: 3742)
Industrial electric heating furnaces
Environmental control
Arc furnace
C373S002000, C373S008000, C266S158000
Reexamination Certificate
active
06175584
ABSTRACT:
FIELD OF THE INVENTION
This invention concerns an aspiration system to reduce losses of fine materials and powders in an electric arc furnace (EAF) used in steel works to melt ferrous materials or other metals.
To be more exact, the invention refers to a system to aspirate the fumes produced during the melting process and convey them towards the outside, the system being suitable to be used both in furnaces with electrodes fed on direct current (DC) and on alternating current (AC).
BACKGROUND OF THE INVENTION
The state of the art includes various aspiration and plugging systems for melting volumes, performed by means of adjacent cooling pipes through which water flows, or by means of walls formed by sheet metal cooled by sprayed water, or again by means of refractory materials able to resist high temperatures.
The gases produced during the melting process are aspirated by means of a pipe and an aperture made in the roof of the furnace itself, commonly known as the fourth hole.
In existing systems, it has been noticed that with the direct aspiration of the fumes huge quantities of solid particles are transported into the plant; these particles increase the consumption of electric energy of the auxiliary equipment, such as the fans, used for the aspiration of the particles, and limit the duration of the filters through which the fumes pass. Above all, since a large part of the particles transported by the fumes consists of metallic material, this also reduces the productivity of the furnace. In addition, when the material loaded is fine material, such as for example DRI (directly reduced iron) or IC (iron carbide, or material obtained from the reduction of iron material containing a high percentage of Fe
3
C), the yields are further diminished.
SUMMARY OF THE INVENTION
The present invention relates to an aspiration system to reduce the losses of fine materials and powders from an electric arc furnace
One purpose of the invention is to achieve an innovative aspiration system which will efficiently and drastically reduce the losses of fine materials loaded into an electric furnace.
In accordance with this purpose, the aspiration system according to the invention substantially consists of three sub-systems cooperating with each other: a sub-horizontal aspiration sub-system, one to collect the fumes and a cyclone sub-system to discharge the fumes.
The sub-horizontal aspiration sub-system is connected with the discharge sub-system by means of the collection sub-system.
The movement of the fumes generated in the bath of liquid material is prevalently horizontal. Moreover the fumes possess a strong component of vertical ascensional movement, caused by their high temperature.
In this type of movement, which is substantially curved, the fumes are partly separated from the solid suspended particles due to the effect of the different density and the action of the centrifugal force.
Moreover, since the fumes pass through a grid of cooling pipes arranged in a coil inside the central chamber of the hearth, a natural filtering action occurs.
The solid particles impact on the pipes and fall back into the melting volume or adhere to the pipes on their outer surface.
The interaxis between the pipes is sized in a suitable manner in order to prevent there being any blockage of the empty spaces between the pipes. Experience has shown that a film of transported material, mostly consisting of oxides, is deposited on the pipes, protecting them from the peaks of heat flow and increasing their duration.
When the heat flow imposed by the working conditions of the furnace becomes high, part of the deposited film liquefies, and thus diminishes the apparent heat flow extracted from the water.
The interaxis between the pipes is also sized so that the fumes have an adequate local speed in the interspace between the side wall of the furnace and the pipes themselves, to prevent any solid material from being blocked between the said pipes.
On the contrary, the speed of the fumes in the empty spaces between the pipes depends on the total aspiration section, which in the system according to the invention is much greater than the conventional section found in a usual furnace. Therefore, the transportation of the solid particles is per se reduced, since the speed is lower and since the quantity of particles transported by the fumes is directly proportionate to the speed of the fumes.
The distance between the cooling pipes and the side wall of the furnace is sized in such a way as to allow a suitable, balancing ascensional speed of the fumes. Therefore, the ascensional speed is variable from position to position, and changes both on the sections of height, because the volume of gases increases, and also on the azimuth sections, in order to balance the aspiration.
The fumes collected in the interspace between the inner wall of the cooling pipes, which are not dense, and the outer wall of the furnace, possibly consisting of other adjacent cooled pipes or of sheet metal cooled by sprayed water or another plugging element, are then aspirated upwards and circumferentially towards the area of discharge.
The resulting movement is therefore of the helical type with a prevalently vertical component and a tangential component. This movement can be managed by means of an appropriate sizing of the sections of the interspace.
The helical movement of the fumes entails a further filtering of the fumes from the suspended particles due to the cyclone effect.
The particles fall downwards where they are re-melted and re-enter the bath.
The collection sub-system is a cyclone proper. It has the double function of transforming the helical movement into a tangential one, with a consequent further filtering, and of aspirating the residual fumes from the region of the roof of the furnace.
Finally, the discharge sub-system is achieved by means of a cooled cylinder which is able to induce a helical movement in the inner volume: in fact, it aspirates from the bottom and is connected tangentially with the discharge aperture of the fumes.
The fumes pass from below through a cooled grid and are further filtered. The solid particles fall back at this point into the melting volume.
Another purpose of the invention is to achieve an aspiration system for an electric arc furnace wherein the fumes are rapidly cooled already inside the furnace and are conveyed towards the roof in such a way that the particulate cools sufficiently to make it re-acquire consistency, through coalescence, so as to make it precipitate into the underlying bath of melted metal, preventing it from exiting afterwards through the chimney and dispersing into the atmosphere.
Another purpose of the invention is to achieve an aspiration system which will allow to use pre-reduced metal material in the furnace, in pellets of around 15-20 mm in diameter, with very small particles which therefore do not participate in the formation of the molten metal but which remain suspended in the fumes.
Another purpose of the invention is to achieve an aspiration system which will prevent the formation of a substantially static cloud of particulate around the electrodes of the furnace; this cloud would encourage the dissipation of energy towards the walls of the central chamber and the roof, with a consequent rapid wear of the said walls and of the insulating component arranged around each electrode.
Another purpose of the invention is to achieve an aspiration system which will reduce the passage of gas and air on the surface of the electrodes, limiting their consumption through oxidation.
Another purpose of the invention is to achieve an aspiration system which will be valid for furnaces fed on direct current and also those fed on alternating current.
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patent: 4526678 (1985-07-01), Myhren et al.
patent: 4634104 (1987-01-01), Wamser
patent: 4720837 (1988-01-01), Kanada
patent: 5787108 (1998-07-01), Pavlicevic et al.
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patent: 0807793 (1997-11
Della Negra Angelico
Pavlicevic Milorad
Poloni Alfredo
Tishchenko Peter
Danieli & C. Officine Meccaniche SpA
Hoang Tu Ba
Stevens Davis Miller & Mosher LLP
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