ITUD20120142A1 - CONTINUOUS POWER SUPPLY AND PRE-HEATING SYSTEM FOR ELECTRIC ARC FURNACE - Google Patents

Description

Description of the Industrial Invention Patent entitled:

"CONTINUOUS FEEDING SYSTEM AND PREHEATING CHARGE FOR ELECTRIC ARC OVEN"

Application sector

This invention refers to a continuous feeding and preheating system of the charge for electric arc furnaces used for the production of liquid steel with steel scrap, cast iron and pre-reduced ferrous material. The heat used for preheating the charge is recovered from the hot fumes sucked in by the kiln dedusting system and can, if necessary, be partially supplied by special burners installed on the continuous power supply system of the kiln.

The production of steel with an electric arc furnace is a method of primary importance for the global steel industry, because it allows the recycling of ferrous scrap available in large quantities and, compared to the blast furnace process, does not require large quantities of carbon coking , whose production still generates severe air pollution today.

Despite their intrinsic environmental qualities, electric arc furnaces are heavy energy consumers, but progress has been made in the last few years to contain consumption. The reduction of energy consumption, in addition to leading to savings on the operating costs of electric arc furnaces, is now a universally felt need, because with a lower energy consumption the overheating of the earth's atmosphere due to the emission of gas is reduced. with greenhouse effect and, for this reason, the environmental authorization procedures in force in many countries require the use of BAT - Best Available Techniques for the construction of new steel plants.

The present invention solves the negative aspects of the known systems of continuous feeding and preheating of the charge for electric arc furnaces, fed both with alternating current and with direct current, and is also suitable for use with other types of furnaces, such as for example the submerged arc, plasma or induction furnaces, both in the steel industry and in other sectors.

The state of the art

In the current state of the art, one of the most widespread continuous feed and charge preheating systems for electric arc furnaces is represented by the invention of John A. Vallomy, described in US patent 4,543,124, which has been developed in the last thirty years with the improvements described in US 6,155,333, US 6,450,804 and US 7,767,136. The basis of the systems cited in the patents indicated above is a conveyor of variable length connected to an opening made on the side of the electric arc furnace. A first section of the conveyor, open in the upper part, is used for loading the scrap from above, by means of a crane with lifting electromagnet or other system, while the second part of the conveyor is enclosed in a preheating tunnel, which is run through counter-current from the hot fumes sucked in by the oven's dedusting system. The position of the fumes intake port divides the conveyor into its two essential parts: the open part for loading from above and the closed part for conveying the fumes drawn from the oven. To limit the entry of external air into the preheating section, US patent 6,155,333 uses (Fig. 5) a combination of mechanical seals and a "dynamic seal", which is substantially another intake inlet located downstream of the load of the transporter. The fumes sucked in by this outlet, after treatment in a cyclone, are discharged as "clean air". The operation of this sealing system was found to be problematic and in the invention US 7,767,136 the "dynamic seal" was in fact replaced by a series of mechanical seals, used in groups comprising up to five elements, whose maintenance is expensive.

It is evident that the infiltration of cold external air into the system involves a lower efficiency of heat recovery from the fumes generated by the furnace and an increase in the gaseous volume treated by the dedusting system serving the furnace itself, with consequent greater investment costs. , maintenance and energy consumption of the fume extraction and filtration system. In addition, the cooling caused by the outside air makes it difficult to reach the temperatures necessary for an efficient control of carbon monoxide, volatile organic compounds and dioxins that are formed inside the furnace and in the scrap preheating tunnel.

US patent 6,155,333 claims the use of a conveyor consisting of a long vibrating channel for the continuous transport of the scrap into the furnace, a solution that poses the problem of sealing against the infiltration of external air along the two sides of the conveyor. This problem has been faced with longitudinal water seals, described in US patent 6,450,804 (Pos. 100 of Fig. 8, 9 and 10). Water seals are complex, because the water level must be continuously restored due to evaporation, and their maintenance is demanding.

In addition to the side seals point of view, the vibrating channel conveyor is problematic for the fatigue breaking of the suspension elements and the production of vibrations and noise, not to mention the demanding structural sizing of the entire system and its foundations.

During some phases of the process, for example when the slag is discharged from the furnace, the furnace must be able to rotate by a certain angle on itself, and therefore the known systems mentioned above include a system for disengaging the scrap conveyor from the furnace, which is made in various ways. This system is also used to facilitate maintenance. For example, in US patent 6,450,804 the entire feeding and preheating system is placed on trolleys with wheels, which make it move backwards by sliding on rails, while in US patents 6,155,333 and US 7,767,136 the inclination of the furnace is allowed by the retraction of the only the terminal part of the preheating conveyor, which moves backwards thanks to a trolley that runs on rails. It is evident that all these solutions for disconnecting the system from the oven, as well as being expensive, pose reliability problems due to their complication.

The most serious problem of the above-mentioned findings is their poor heat exchange efficiency, deriving from the fact that the hot fumes extracted from the furnace pass over the scrap bed transported by the vibrating channel and do not pass through it. The reduced surface of the scrap that comes into contact with the hot fumes and the minimum contact time do not allow to obtain a high energy recovery from the hot fumes sucked from the furnace. This consideration, which is a direct consequence of the heat exchange mechanism, is evident in the device represented with Pos. 36 of Fig. 7 of US patent 6,155,333, which consists of a deflector intended to direct the hot fumes towards the internal mass of the scrap . Admitting that this device can be used in practice, it is evident that the hot fumes can only affect a small part of the scrap present along the entire length of the vibrating preheating channel. Furthermore, the transport with vibrating channel does not allow to have a high height of the scrap layer, with the consequence that the contact time with the hot smoke is very limited, because the speed of advancement of the scrap along the channel must be sustained for guarantee the high volumetric flow required to ensure the production capacity of the furnace. An attempt was made to remedy this latter drawback by lengthening the length of the conveyor preheating section, with a consequent increase in the overall dimensions of the system. The problem is particularly felt in steel mill revamping interventions, when the available spaces are often insufficient to accommodate a system of excessive length.

The recent publication “The Evolution of the Consteel EAF” by C. Giavani et al. (AIM Workshop, Milan 29-30 March 2012), which describes the improvements made to John Vallomy's invention, does not present a solution for any of the aforementioned drawbacks. This publication, illustrating the research carried out on the penetration of the flame inside the scrap present on the vibrating channel, indirectly confirms the need to use auxiliary burners with strong penetration to heat even the scrap that is not exposed to the flow of hot fumes drawn from the furnace. .

This patent allows to overcome all the disadvantages and drawbacks listed above, realizing a new system characterized by high efficiency of heat recovery of the fumes, excellent seal against the infiltration of external air and the escape of polluting fumes, better control of atmospheric emissions of carbon monoxide and toxic organic compounds, limited size, absence of vibrations, construction simplicity, reliability and low cost of construction and management.

Essence of the invention

The objects of the invention are achieved according to the characteristics of the main claim and / or of any other claim reported in this patent text.

The main purpose of the present invention is to provide a continuous loading system for the electric arc furnace with preheating of the loaded scrap, which allows the heat of the fumes extracted from the dedusting system serving the furnace to be recovered with high efficiency, increasing in at the same time the production capacity of the oven.

The system uses a mobile floor conveyor, consisting of longitudinal elements (slats) 3 with programmed reciprocating movement, divided into two sections of variable length. The first section of the conveyor 4, provided with side containment walls 9, is open in the upper part, where there is a hopper to facilitate the loading of the scrap from above by means of overhead cranes, or articulated arm cranes, equipped with a lifting electromagnet 10 or bucket. In the second section of the same conveyor 5, which is completely closed by two side walls and a cover, the heat exchange takes place between the fumes extracted from the furnace and the scrap which advances in the opposite direction. The end of the conveyor opposite the loading end from above interfaces with an opening made in the side of the vat of the kiln 1, and partly also in the opening vault of the same. The slats of the mobile transport floor protrude slightly from the preheating tunnel and unload the scrap directly into the liquid bath contained by the furnace.

In the section that comes into contact with the hot fumes drawn from the oven, the slats of the movable floor, the side walls and the conveyor cover have a heat-resistant construction, for example they are cooled with water or air, or made of resistant steel at high temperatures. In particular, the conveyor cover, which does not come into contact with the scrap and therefore is not subject to wear, can conveniently be made of refractory material so as to limit heat losses through it.

The operation of the mobile floor conveyor is known and takes place in four phases. The slats of the floor are moved longitudinally by electric, pneumatic or hydraulic actuators that control three, four or more groups of slats integral with each other. In the case of three groups, the stick n. 1, n. 4, n. 7 and so on. In the case of four groups, the stick n. 1, n. 5, no. 9 and so on. Alternatively, an actuator can be provided for each slat, without prejudice to the synchronized movement of an entire group of slats made virtually integral as indicated above. The displacement of a third, or a quarter, of the floor surface fails to longitudinally move the material deposited above it, which remains stationary due to friction with the remaining two thirds, or three quarters, of the slats. The groups of slats are moved in sequence in the same direction for about 30-50 cm, according to times programmed by the floor control system. During the movement phases of the individual groups of slats, a part of the scrap falls into the furnace through the openings that are created under the scrap when the slats move in the opposite direction to that of loading into the furnace. When the slats are all aligned again, they are moved simultaneously towards the oven and a new advancement cycle, carried out in four or five phases, can begin. The slats of the movable floor are supported by a series of transversal trestle supports 14, spaced longitudinally, over which the slats slide, sliding on lubricated or low-friction contact surfaces. The slats are positioned in contact with each other, so as to create a horizontal gas-tight surface. Downstream of the top 4 scrap loading area, positioned between two successive supports below the movable floor, there are sealed chambers, made with a heat-resistant construction (water or air-cooled walls, or made with materials resistant to high temperatures). The lower chambers are put in communication with each other through openings 15, so as to form a single suction plenum, which is connected by one or more pipes to the de-dusting system of the furnace. The chambers underneath the movable floor are thus kept at negative pressure, so as to allow the hot fumes sucked by the furnace to flow into them, which, after passing through the scrap on the movable floor, pass through two longitudinal slits 8 placed at the sides of the movable floor itself. The pressure difference due to the passage of fumes through the two communication slots between the conveyor and the underlying plenum, allows the flow of fumes to be evenly distributed along the entire length of the preheating section of the mobile floor.

The two slats located on the external sides of the movable floor can be shaped with two reliefs (Fig. 3), one of which slides inside a groove made on the support crosspiece of the slats, in order to prevent their movement. lateral. The upper relief, on the other hand, serves to prevent the entry of material into the underlying chamber.

As shown in Fig. 10, the longitudinal slots are absent in the loading section of the conveyor from above 4, in which the end slats slide practically in contact with the walls of the loading hopper 9.

One of the advantages of the system described above is represented by the fact that the smoke crosses the entire height of the scrap and therefore the scrap surface affected by the heat exchange is much larger than in the case of the vibrating channel conveyor used in known systems. The heat exchange efficiency is thus higher, both due to the fact that the hot fumes come into contact with a much higher scrap surface, and because the time available for the fumes-scrap heat exchange is greater, thanks to the lower average speed. advancement of the scrap, which can be loaded on the mobile floor conveyor with a layer of greater height than that allowed by vibrating channel conveyors.

The 7 lower chambers are equipped with sealed access doors to allow their scheduled cleaning.

The lower chambers can be used as post-combustion chambers for the abatement of polluting compounds carried by the furnace and due to the heating of the scrap. In this case, the controlled entry of a certain amount of combustion air into the chambers can be envisaged. Or, post-combustion can take place in a special chamber located downstream of the preheating system.

The problem of precarious lateral seals, typical of the vibrating channel conveyor system, is solved by the floor with movable slats, which does not require any particular sealing in the longitudinal direction. If the system is used only as a continuous feeding system of the furnace, without particular purpose of preheating the scrap, the lower chambers and the relative longitudinal connection slots are absent and the longitudinal seal is ensured by two simple sliding seals placed between the walls side of the conveyor and the two external floor slats.

When the conveyor has to disengage from the oven to allow its inclination, as required by the production needs, all the slats of the floor are set back for a certain distance (Figures 6 and 7). This occurs by providing three different end-of-stroke points for the slats: the first is the one more advanced towards the oven, the second corresponds to the working position of the mobile floor and is placed about 30-50 cm from the previous point (the slats move alternatively between these two points), while the third, more distant end-of-stroke point is used only when the conveyor must be decoupled from the oven. In the latter circumstance, the heating tunnel must also disconnect from the furnace and this is done by means of a mobile sleeve (Figures 6 and 7), or thanks to the simultaneous lifting and retraction of the final part of the tunnel (Figures 4 and 5). The disconnection of the tunnel from the furnace takes place by means of electric or hydraulic actuators other than those that activate the mobile floor. For the retraction of the mobile floor, hydraulic or electric actuators can be provided, with three end-of-stroke positions, two for work and one for disconnection from the oven, or independent actuators can be used which act in series with respect to the actuators used for the advancement of the scrap.

A sealing system is provided to prevent the infiltration of external air at the entrance opening of the scrap into the heating tunnel. The seal is made by a group of elements 19 side by side, individually counterweighted, which can rotate upwards by a certain angle when pushed by the scrap during the advancement phase envisaged by the programmed transport cycle (Figures 8 and 9). The sealing system described minimizes the free area between the scrap and the upper part of the heating tunnel and is also able to compensate for an uneven distribution of the scrap in the transverse direction of the conveyor (Fig. 10). For an even more effective seal against external air infiltrations, the system described above can be equipped with air curtains 22 integrated into the mobile devices that ensure the seal. In this way, the operating distance of the air curtains is reduced, compared to a system with a fixed air curtain, and therefore the air flow required to achieve a good seal is significantly lower, with consequent lower energy consumption of the fan. at the service of the air curtain system.

The mobile air curtains, one for each sealing element, are fed by a common blower, while the distribution of the air through the various elements is obtained by coupling the hollow axis 23 of rotation of the same with a pressurized pipe by means of a fan . The seal between the hollow shaft and the fixed pipe is obtained with an axial sealing system. Along the hollow axis of rotation there is a series of openings, one for each sealing element, which connect the inside of the axis with the various sealing elements. In this way, the air curtains are active in all positions of the angular work sector, without the need to resort to flexible pipes for the supply of pressurized air.

Drawings illustrations

These and other characteristics of the present invention will appear evident from the following description by way of preferential example of construction, however not limiting, and in the seven attached drawing tables, where:

- Fig. 1 shows, in plan view, the electric arc furnace with the continuous feeding and preheating system of the charge;

- Fig. 2 shows, in plan view, the operation of the mobile floor with alternating movement slats;

- Fig. 3 shows, in cross-sectional view, the mobile floor conveyor sectioned in correspondence with a lower chamber and the fumes suction intake;

- Fig. 4 shows, in longitudinal section, the electric arc furnace with the continuous feeding and preheating system of the charge;

- Fig. 5 shows the same section as Fig. 4 with the system decoupled from the electric oven following the lifting of the terminal element of the preheating chamber; - Fig. 6 shows, in partial longitudinal section view, the decoupling system of the preheating tunnel from the furnace by means of a mobile sleeve;

- Fig. 7 shows, in partial longitudinal section view, the decoupling system of the preheating tunnel from the furnace by retracting the terminal section of the tunnel;

Fig. 8 shows, in longitudinal section, an enlargement of the sealing system placed at the entrance to the preheating tunnel (sealing system raised);

- Fig. 9 shows, in longitudinal section view, an enlargement of the sealing system placed at the entrance to the preheating tunnel (sealing system in the working position);

- Fig. 10 shows, in cross-sectional view, the sealing device placed at the entrance to the preheating tunnel.

As can be seen from the attached figures, this is a new system for the continuous feeding and preheating of scrap for electric arc furnace 1 substantially characterized by being formed by a mobile floor conveyor 2, with reciprocating movement slats 3. The conveyor is divided into two parts, an open one for loading the scrap from the top 4 and a closed one for preheating (tunnel) 5. The two parts of the conveyor are separated by a sealing system 6 against infiltration of external air and by a series of chambers lower (plenum) 7 communicating with the scrap preheating zone through two lateral longitudinal slots 8. The suction plenums are connected to the fume extraction and purification system of the electric furnace, which is not shown in the drawings.

The system uses a mobile floor conveyor 2, consisting of longitudinal elements (slats) with programmed reciprocating movement 3, divided into two sections of variable length. The first section of the conveyor, equipped with side containment walls, is open at the top, where there is a hopper 9 to facilitate the loading of scrap from above by means of an overhead crane or articulated boom crane equipped with a lifting electromagnet 10 or bucket. In the second section of the same conveyor, which is completely closed by two side walls and a cover, the heat exchange takes place between the fumes extracted from the furnace and the scrap which advances in the opposite direction 11. The end of the conveyor opposite to the loading one from above it interfaces with an opening 12 obtained in the side of the oven, and partly also in the opening vault of the same. The slats 3 of the mobile transport floor protrude slightly from the preheating tunnel and unload the scrap directly into the furnace.

In the section that comes into contact with the hot fumes drawn from the oven, the slats of the movable floor, the side walls and the conveyor cover have a heat-resistant construction, for example they are cooled with water or air, or are made of steel resistant to high temperatures. In particular, the conveyor cover 13, which does not come into contact with the scrap and therefore is not subject to wear, can conveniently be made of refractory material so as to limit heat losses through it.

The operation of the mobile floor conveyor is known and takes place in four phases, as illustrated in Fig. 2. The floor slats are moved longitudinally by electric, pneumatic or hydraulic actuators which control three, four or more groups of slats integral with each other. In the case of three groups, the stick n. 1, n. 4, n. 7 and so on; this solution is illustrated in Fig. 2. In the case of four groups, the slat n. 1, n. 5, no. 9 and so on. Alternatively, an actuator can be provided for each slat, without prejudice to the synchronized movement of an entire group of slats made virtually integral as indicated above. The displacement of a third, or a quarter, of the floor surface fails to advance the material deposited on it, which remains stationary due to friction with the remaining two thirds, or three quarters, of the slats. The different groups of slats are moved in sequence in the same direction for about 30-50 cm, according to times programmed by the automatic floor drive system. During the displacement phases of the individual groups of slats (phases 1, 2 and 3 of Fig. 2), the scrap located on the terminal part of the floor falls by gravity into the furnace due to the free spaces that form under the scrap when the slats move in the opposite direction to that of loading into the oven. When the slats are all aligned again (position D), they are moved simultaneously towards the oven (phase 4: from position D to position A) and a new advancement cycle, carried out in four or five phases depending on the number of groups of slats in solidarity with each other, can begin. The mobile floor described above is already widely used to move goods transported by trucks (both during loading and unloading); the high reliability of the system has made it possible to spread it also for the transport of materials that are particularly difficult to handle with other types of conveyors, such as waste and scrap metal. The volumetric handling capacity, as well as the size of the system, depends on the height of the layer of material and the average speed of advancement, which is determined by the stroke of the floor slats and the times of the activation cycle of the same.

As shown in Fig. 4, the slats of the movable floor 3 are supported by a series of transversal trestle supports 14, spaced longitudinally, over which the slats slide and slide on lubricated or low-friction contact surfaces. The slats are positioned in contact with each other, so as to create a horizontal surface which is substantially gas-tight. When the material loaded into the oven does not present the risk of clogging, as in the case of loading pre-reduced pellets, the slats can be spaced apart to allow the fumes coming from the oven to pass through the longitudinal slits created between the slats. Below the mobile floor, positioned between two successive supports, there are one or more sealed chambers 7, made with a heat-resistant construction (water or air-cooled walls, or made with materials resistant to high temperatures). The lower chambers are put in communication with each other through openings 15, so as to form a single suction plenum, which is connected by one or more suction inlets 16 to a pipe that carries the suctioned fumes to the dedusting system of the oven. The chambers underneath the movable floor are thus kept at negative pressure, so as to allow the hot fumes sucked by the furnace to flow into them, which, after passing through the scrap on the movable floor, pass through two longitudinal slits 8 placed at the sides of the movable floor itself. The pressure drop due to the passage of fumes through the two communication slots between the conveyor and the underlying plenum, allows the flow of fumes to be evenly distributed along the entire length of the preheating section of the mobile floor.

One of the advantages of the system described above is represented by the fact that the smoke crosses the entire height of the scrap and therefore the scrap surface affected by the heat exchange is much larger than in the case of the vibrating channel conveyor used in known systems. The heat exchange efficiency is thus higher, both due to the fact that the hot fumes come into contact with a much higher scrap surface, and because the time available for the fumes-scrap heat exchange is greater, thanks to the lower average speed. advancement of the scrap, which can be loaded on the mobile floor conveyor with a layer of greater height than that allowed by vibrating channel conveyors.

The lower chambers can be used as post-combustion chambers for the abatement of polluting compounds carried by the furnace and due to the heating of the scrap. In this case, the controlled entry of a certain amount of combustion air into the chambers can be envisaged. Or, post-combustion can take place in a special chamber located downstream of the preheating system.

The problem of lateral seals, typical of the vibrating channel conveyor system, is solved by the floor with movable slats, which does not require any particular seal in the longitudinal direction. If the system is used only as a continuous feeding system of the furnace, without particular purpose of preheating the scrap, the lower chambers and the relative longitudinal connection slits are absent and the longitudinal seal is ensured by two simple contact seals placed between the walls side of the conveyor and the two external floor slats.

When the moving floor conveyor has to disengage from the oven to allow it to be tilted, all floor slats are set back for a certain distance. This occurs by providing three different end-of-stroke points for the slats: the first is the one more advanced towards the oven, the second corresponds to the working position of the mobile floor and is placed at about 30-50 cm from the previous point (in the normal condition of loading the furnace, the slats move alternately between these two points), while the third end-of-stroke point, more distant, is used only when the conveyor must be uncoupled from the furnace. In the latter circumstance, the side walls and the cover of the preheating tunnel must also be disconnected from the furnace and this is done by means of a telescopic sleeve 17 (Figures 6 and 7), or thanks to the simultaneous lifting and retraction of the terminal element 18 of the tunnel. (Figures 4 and 5).

The disconnection of the terminal element of the preheating tunnel from the furnace takes place by means of electric or hydraulic actuators other than those that operate the mobile floor. For the retraction of the mobile floor, hydraulic, pneumatic or electric actuators can be provided, with three end-of-stroke positions, two for work and one for disconnection from the oven, or independent actuators acting in series with respect to the actuators used can be used. for the advancement of scrap.

Due to the action of the fans of the purification plant of the fumes produced by the furnace, the preheating tunnel is in a slight depression with respect to the atmospheric pressure and therefore a sealing system 6 is provided to prevent the infiltration of external air in correspondence the entrance opening of the scrap into the preheating tunnel. The sealing system is detailed in Figures 8, 9 and 10.

The seal is made by a group of side-by-side elements, individually counterweighted, which can rotate upwards by a certain angle when pushed by the scrap during the advance phase foreseen by the programmed transport cycle (phase 4 of Fig. 2), this sealing system minimizes the surface of the free cross-section between the scrap and the upper part of the heating tunnel and, as shown in Fig. 10, is also able to compensate for an uneven distribution of the scrap in the transverse direction of the conveyor. Thanks to the movable floor, the resistance that the sealing elements exert on the scrap advancing in the tunnel is negligible and therefore cannot hinder the advancement of the scrap, as would happen in the case of a vibration conveyor.

The counterweight 20 connected to each oscillating sealing element by means of a steel cable and a system of pulleys 21. In place of the gravity counterweights, it is also possible to use electronic weight compensation systems, which cancel the weight of the seal 19 creating a slightly lower force on the suspension cable than that deriving from the weight of the element.

For an even more effective seal against the infiltration of external air at the entrance of the scrap into the preheating tunnel, the system described above can be equipped with air curtains 22 integrated in the mobile sealing devices 19. With the solution provided by the present invention, the operating distance of the air curtains is reduced compared to a system with a fixed air curtain, and therefore the air flow necessary to achieve a good seal is significantly lower, with consequent lower energy consumption of the fan. service of the air curtain system.

The movable air curtains, one for each sealing element, are fed by a common fan, while the distribution of the air through the various sealing elements 19 is obtained by coupling the hollow axis 23 of their rotation with a pressurized pipe. connected to the fan. Along the hollow axis of rotation there is a series of openings, one for each sealing element, which put the inside of the axis in communication with the various sealing elements. In this way, the air curtains are active in all positions of the angular working sector.

As an alternative to the air distribution system described above, it is possible to use flexible pipes that connect the various (movable) sealing elements to a (fixed) pipe connected to the outlet of the fan which generates the required air flow. for the operation of the curtain.

The sealing system described above effectively reduces the infiltration of external air into the system and therefore allows to reduce the flow of fumes treated by the fume purification system, thus limiting the consumption of the smoke extraction fans.

The passage of the scrap by the fumes involves a certain loss of pressure for the smoke circuit, which must be taken into account when calculating the pressure of the fans of the fumes purification system. If the nature of the material loaded into the kiln involves a loss of pressure higher than the desired one, it is possible to send part of the fumes sucked from the kiln directly to the fume purification system by means of a by-pass pipe 24, whose opening can be adjusted so automatic acting on the damper 25 located on the by-pass pipe. The opening of the by-pass shutter can be adjusted automatically according to the value set for the depression inside the oven. This prevents fumes from escaping from the furnace when the scrap is not sufficiently permeable to the fumes. The by-pass system described above can also be used to avoid localized melting of the scrap in the preheating tunnel due to an excessive heating temperature. When the by-pass damper is completely open, the flow of fumes passing through the scrap is practically zero and from this point of view the system behaves like known systems.

It is also possible to provide another damper in correspondence with the section of pipe 26, upstream of the confluence between the suction pipe from the plenum and the by-pass pipe (Fig. 3). This second damper allows you to exclude the entry of fumes into the plenum 7 through the longitudinal slits.

The invention, of course, is not limited to the exemplary embodiment described above, starting from which other forms and other embodiments can be provided, and the details of execution can in any case vary without thereby departing from the essence of the invention as well as stated and claimed below.

Claims (5)

CLAIMS 1 ^ Continuous feeding and charge preheating system for electric arc furnace 1; the charge consists of ferrous material 11, of controlled size, which before being loaded into the kiln vat is preheated using the heat recovered from the hot fumes sucked by the kiln dedusting system and / or the heat produced by special burners, or by introducing in the external air preheating area to oxidize the unburnt gases present in the said fumes sucked by the oven. The preheating takes place in the final part of the conveyor 2, in a particular preheating tunnel 5, cooperating downstream in coordination with the operating cycles of said furnace 1 and, upstream, with the loading systems 10 of the ferrous material on the conveyor, characterized by the fact that said conveyor 2 is constituted by a plurality of metal rods 3 suitable for transporting the ferrous material from an uncovered area of loading from above by gravity 10 to a downstream unloading area inside the vat of the kiln 1, passing the scrap through a tunnel 5 located in the second part of the conveyor and made in such a way as to withstand the high temperatures, where the preheating of the feed materials takes place. Said conveyor 2, of the rectilinear type, moves the material forward in a longitudinal direction towards the furnace and is constituted by a plurality of metal rods 3 side by side with each other in a parallel way in a transversal direction, a few millimeters apart, until forming the entire basic plan of advancement of the scrap. Said slats 3, internally cooled by water or air, or made of a material resistant to temperature, have a length equal to the length of the conveyor and are moved horizontally back and forth so as to advance the material of the charge. The rectilinear movement of the slats, automatically controlled with suitable systems, is single and / or by groups and occurs alternately forward and backward with various possible combinations. The slats 3 are supported by a series of transversal trestle supports 14, spaced lengthwise, over which the slats slide, sliding on lubricated or low coefficient of friction contact surfaces. The displacement of the material on the upper surface of the slats 3 occurs in that all or some of them are moved longitudinally forward in synchrony, thus moving the supported material by about 30-50 cm, while the return of the slats to the initial position occurs individually. or in groups, without carrying the same material back with them, as only a part of the slats moves backwards and the friction with the stationary slats prevents the material from moving backwards. 2 <Λ> System of continuous feeding and preheating charge of the electric arc furnace as in the first claim, characterized by the fact that the forward and backward movement of the single ribs 3, and / or of the groups of ribs forming the basic transport plane of the material, takes place in a variously combined manner, simultaneously and / or alternately, at constant or variable rates according to the optimum speed of advancement of the material in order to finish the desired hourly mass of material 11 to be loaded into the vat of the kiln 1; this occurs automatically with the aid of suitable known means and drive systems. 3 <Λ> System of continuous feeding and preheating charge of the electric arc furnace as in one or more of the preceding claims, characterized by the fact that the conveyor 2, in addition to having the particular movable floor, has two fixed side walls 9, vertical or inclined, for the lateral containment of materials. One wall is on the right and the other on the left along the entire transport route. 4 <Λ> System of continuous feeding and preheating charge of the electric arc furnace as in one or more of the preceding claims, characterized in that the slats 3 of the conveyor 2 are formed by longitudinal elements of rectangular or square cross-section or other suitable profiling for this purpose, solid and / or tubular, side by side parallel to each other, with a span of a few millimeters in a transverse direction (span less than the size of the materials transported) to allow the relative motion between them forward and backward, so as to determine an upper surface mobile, discontinuous but flat, horizontal or slightly inclined towards the oven. 5 <Λ> System of continuous feeding and preheating charge of the electric arc furnace as in one or more of the preceding claims, characterized by the fact that the movable slats 3 which constitute the material transfer floor 11 are side by side and slightly spaced transversely from each other , with a fixed distance in relation to the size of the transported materials. Inside the preheating tunnel 5, the slots between the slats allow the passage of hot fumes from the upper floor to the lower floor formed by one or more sealed chambers 7 equipped with a smoke intake 16. Before entering the chambers 7, the hot fumes pass through the entire layer of material 11 and heat it. 6 <Λ> System of continuous feeding and preheating charge of the electric arc furnace as in one or more of the preceding claims, characterized by the fact that the two fixed side walls 9 each have, in proximity to the mobile transport surface 2, a lateral groove along the entire length affected by the preheating tunnel 5, so as to allow the passage of fumes from the upper area towards the lower chamber 7 through the two slits 8 which are created between the side walls 9 and the outer edges of the mobile surface 2. 7 <Λ> System of continuous feeding and preheating charge of the electric arc furnace as in the previous claim characterized by the fact that the two perimeter slats of the mobile surface 2, those in proximity to the two fixed lateral vertical walls 9, have a different section shape with respect to the others, having side edges fi projecting upwards to prevent the accidental fall downwards, through the two slits between the transport surface 2 and the side walls 9, of small pieces of the transported material. 8 <Λ> Continuous feeding and preheating charging system of the electric arc furnace as in claims 6 <Λ> or 7 <Λ>, characterized by the fact that the two perimeter slats of the mobile surface 2, those in proximity to the two vertical side walls fixed elements 9, have a different section shape than the others, presenting at the bottom a longitudinal groove, or a longitudinal relief, suitable for coupling with complementary fixed elements (relief or groove) of the trestle supports 14, so as to contain in the transverse direction the two external slats, without however hindering the longitudinal sliding of all the slats. 9 <Λ> Continuous feeding and charging preheating system of the electric arc furnace as in one or more of the preceding claims, where the conveyor 2 consists of side-by-side slats 3 constituting the feed base of the materials 11, characterized in that said straight conveyor, with movable floor and fixed containment side walls 9, has an open area at one end for loading the material 11, while the remaining part is covered by a thermally insulated tunnel 5 13, which does not come into contact with the transported material, in which the latter is heated and directed inside the vat of the kiln 1; between the two areas of the conveyor, the open one 4 at atmospheric pressure, and the one for heating the material, which is in depression, there is a hinged sealing device 6, counterweighted 20 with a rope and pulley system 21 to prevent the infiltration of outside air inside the preheating tunnel 5. 10 <Λ> Continuous feeding and charge preheating system of the electric arc furnace as in claim 9 <Λ>, characterized by the fact that the sealing device 6 is composed of a plurality of plates 19, metal or other material, side by side , supported independently of each other by ropes, pulleys 21 and counterweights 20 and hinged 23 at the end on the arrival side of the material 11, while the end on the side of the kiln 1 is free, so that the seal 19 constantly follow the transverse profile of the advancing layer of material 11. The hinged plates are free to descend by gravity or to move upwards as the material passes, raised by the advancement of the material itself, by rotating slightly in the same direction of advance as the loading material. The plates move upwards pushed by a higher layer of material II and descend towards the mobile transport surface when the level of the material is lowered; in this way the free passage of air is kept to a minimum and the infiltration of cold external air in the preheating tunnel is greatly reduced 5. 11 <Λ> Continuous feeding and charging preheating system of the electric arc furnace as in claims 9 <Λ> or 10 <Λ>, characterized by the fact that the weight of the movable sealing elements 19 is not compensated by a mechanical counterweight system , but it is almost completely zeroed with an equivalent electronic system, which uses for each sealing element a suspension cable with load sensor and an electric, pneumatic or hydraulic actuator. The actuators are controlled by a microprocessor. 12 <Λ> Continuous feeding system and pre-heating charge of the electric arc furnace as in claims 9 <Λ>, 10 <Λ> or 11 <Λ>, characterized by the fact that the system provides for an improvement to the apparatus 6, which consists of an air curtain 22 for an even more effective seal against the infiltration of external air at the entrance of the charge 11 in the preheating tunnel 5. Said air curtain transversely affects the entire width of the entrance to the way material transport; in particular, if there are more sealing elements side by side, each sealing element 19 is equipped with an individual air jet fed by a suitable known system. 13 <Λ> System of continuous feeding and preheating charge of the electric arc furnace as in one or more of the preceding claims characterized by the fact that the suction and treatment system for the fumes produced by the furnace is not connected only to the chambers 7 located below to the mobile floor 2 for transporting the charge 11, but also to the overlying preheating tunnel 5. This connection is made by means of a by-pass pipe 24, equipped with a flow rate adjustment damper 25 and is used to regulate the flow of hot fumes that pass through the charge material 11, in order to limit the pressure drops of the fume extraction system and / or avoid overheating of the transported material. The adjustment of the damper 25 takes place automatically according to the pressure detected in the preheating tunnel, or in the smoke ducts upstream thereof, and / or on the basis of the temperature of the fumes. 14 <Λ> Continuous feeding and charging preheating system of the electric arc furnace as in one or more of the preceding claims characterized by the fact that it is equipped with an automatic disconnection system of the terminal element 18 which interfaces with the opening 12 of the oven 1, providing for its retraction by means of oblique lifting upwards.