TECHNICAL FIELD

The invention relates to a melting furnace and a process for the production of steel or cast iron via melting metal loads distinguished by high energy efficiency. In particular, the invention relates to electric arc melting furnaces.

BACKGROUND ART

In the steel industry, the use of electric arc furnaces (operating with AC alternating current or DC direct current) is known for melting metal loads, consisting for example of scrap metal, cast iron, direct reduced iron and combinations thereof. Said furnaces are possibly equipped with auxiliary melting elements such as: burners, to enhance the supply of chemical energy to be associated with the electrical energy, coal injectors, oxygen injectors, but also structures such as the scorification door which allows the elimination of process components such as slag, but also useful for the introduction of samplers, the furnaces have a translatable vault for introducing part of the material to be melted and which is normally connected to the fumes system. The known furnaces, also thanks to these elements, have slits or gaps from which there is a continuous introduction of air, since the furnaces work in vacuum, thanks to the action of the aforementioned fumes system, which by sucking in the melting fumes, therefore allows the air to enter the furnace.

The optimal melting in an electric arc furnace is aimed at striking a balance, to be sought between the amounts of oxygen and coal which are fed during the process, as known from the state of the art and illustrated in figure 4. This search for balance is the main cause of the consumption of coal and O2 during the process, since they are subject to a continuous “chase”, which is perpetrated until the steel is tapped at the end of the melting, considering that the final molten product must have a desired carbon concentration. The chase between oxygen and coal takes place in particular to be able to deoxidize the metal load to be melted (operation allowed by the injection of coal), which however is re-oxidized by the oxygen present in the air entering the furnace (in jargon, called false air) and that of the burners, which help the melting. Furthermore, excessive charcoal may be introduced, thus the introduction of oxygen attempts to oxidize the excess amount in order to obtain the desired composition of the steel.

The combination of all the operating conditions applied in electric furnaces for melting scrap is the cause of a number of disadvantages. In fact, the consumption of O2 and coal is high, greater than the thermodynamically necessary values. The carbon monoxide CO developed is subject to oxidation, forming CO2 inside the furnace (partially) or fume treatment apparatus, and only part of the energy developed is recovered in the melting bath. There is an over-oxidation of the slag, resulting in a decrease in the furnace yield. There is also a significant consumption of the protective refractory materials of the melting vessel, this being favoured by the high FeO content in the slag. Furthermore, there is a high transport of fines in the fumes treatment system due to the high gas speeds. Finally, it should be noted that there is a loss of sensible and latent energy in the exhaust gases (off-gases) which also reaches from 200 to 400 kWh/tls as a function of the carbon present in the load. In the event of preheating the scrap by the exhaust gases, only a small part of this energy is recovered.

The furnaces of known type have a tapping hole for the liquid material located in the bottom of the melting vessel in an eccentric position (EBT, Eccentric Bottom Tapping). The scorification instead occurs through a side door. Normally when they must be scored/tapped, they are tilted laterally on guides, to convey slag or liquid steel respectively towards the scorification door or the tapping hole.

DISCLOSURE OF THE INVENTION

The object of the invention is to overcome the aforementioned drawbacks and to propose a melting furnace and a related melting process which allows to better exploit the latent energy generated by the process and potentially available in the exhaust gases. A further object of the invention is the reduction in the wear of the refractory materials used in the melting furnaces. Said objects are achieved by means of a melting furnace, in particular of the electric arc type, for the production of cast iron or steel by melting metal loads (in particular scrap and/or direct reduced iron (DRI)), which comprises:

(a) a bottom and side walls which define a melting vessel; (b) a vault adapted to close the vessel, forming a melting chamber which is protected from gas exchanges with the external environment, being adapted to be managed in overpressure or at atmospheric pressure;

(c) at least one inlet for feeding the metal loads to be melted and optionally for that of additives;

(d) a suction system for removing gases, in particular CO, formed during the melting process; and

(e) at least one outlet for discharging slag and/or molten metal material.

In the case of a metal load consisting of direct reduced iron, it can be generated by any existing technology in the art and is fed hot or cold, through a buffer or intermediate storage silo.

The alternative to the traditional furnaces proposed by the invention consists in the use of a new type of melting furnace, in particular electric arc furnaces for melting metal loads, such as scrap, preferably crushed, direct reduced iron, briquettes and combinations thereof, so that said furnace is able to limit the supply of oxygen to the furnace, by creating a melting chamber insulated from the external environment and managed in overpressure.

Advantageously, said furnace will also have the characteristic of being fixed and of not having to be tilted in the different steps of the melting process and/or for tapping. Since it is not a tilting furnace, the discharge of slag and steel to the outside occurs by gravity. In fact, molten metal and slag can exit through one or more outlets located at suitable heights in the melting chamber. Preferably in the walls of the melting furnace, or melter, two or more through outlets are provided, obtained at different heights: one or more than one for the discharge of the slag at a higher position and one or more than one for the liquid metal at a lower position, since usually the slag is discharged first and then the molten metal, an order which would be difficult to obtain with a single outlet.

An alternative of the invention can foresee a tiltable furnace for the discharge of slag and molten metal.

The protection of the furnace with respect to the external environment avoids the entry of air and therefore of O ₂ , which would transform the CO into CO _2, thus subtracting the CO from future uses as a fuel or reducing medium in other applications, such as for example heating furnaces, room heating, direct reduction reactors, etc. The protection against the exchange of gas with the outside, such as the entry of air or a leakage of process gas, occurs mainly because the melting chamber is held at atmospheric pressure or in slight overpressure, thus helping to prevent air from entering from the outside, effectively creating an insulation between the furnace and the external environment. In this regard, the furnace pressure is managed so as to avoid the suction of air, for example by setting an internal pressure of 0-2 mm H2O (0-19.61 Pa) above atmospheric pressure. An overpressure is preferable to atmospheric or ambient pressure because it further reduces the risk of the entrance of air, for example from diffusing processes. A dynamic control of the seal of the gap of the electrodes with respect to the vault can be useful in this regard.

Furthermore, the person skilled in the art readily applies with his general knowledge further measures to ensure a closure as complete as possible, including by way of example sealing the furnace parts where feasible, using pressurized feeding inlets, preferably obtained in the vault, or interposing chambers containing inert gases around the feeding inlets, as illustrated below. The electric furnace can be of the alternating current (AC) or direct current (DC) type. Advantageously, it is an open arc furnace, preferably with short arc to limit the energy loss by radiation. The number of electrodes can vary, the skilled person readily identifies with his general knowledge the appropriate configuration of the electrodes as a function of furnace productivity; in a preferred embodiment, there are two electrodes in service.

As a function of the process type, the arc can also be shielded, in particular in the case where the furnace is internally completely coated with refractory material.

In the ideal implementation thereof, the melting furnace does not involve, and does not require according to the inventive concept, burners and the use thereof inside the melting chamber. The presence of burners, which would require further oxygen introduction, would mean a waste of excess CO formed and would not be useful for the reduction of iron oxides; in other words, the absence of burners avoids the transformation of CO into CO2 and the waste of latent energy resulting from the presence of CO. However, particular embodiments which could induce the user to introduce burners at certain steps of the melting process are not excluded, as for example hydrogen burners.

In case of need, a controlled supply of O2 can still be provided with injectors dedicated to metal decarburization before tapping. In the case of cast iron production, the supply of O2 is not necessary.

As anticipated, preferably, the melting furnace according to the invention cannot be tilted, i.e., it consists of a fixed structure. This choice of a non-tilting vessel allows to create a sealed furnace of much larger dimensions than the known furnaces (e.g., 10 m diameter), this favouring the solidification of the slag on the furnace walls and further reducing the wear of the refractory material, as will be illustrated below. The structural simplification of the furnace obviously also has economic advantages.

Advantageously, the furnace walls and vault according to the invention are essentially made of refractory material to limit thermal dispersion, which is instead typical of the water panel cooling of the known electric furnaces. Advantageously, the refractory material can still be cooled with FhO panel cooling on the outer side thereof.

In an advantageous embodiment of the invention, cooled inserts, preferably of copper, are provided on the internal walls of the furnace in the formation zone of the slag layer. Their purpose is to quickly cool the slag in order to make it become a further insulating layer to protect the refractory walls of the furnace with respect to the radiation of the electrodes.

The outlet(s), i.e., the hole(s) included for tapping cast iron/steel and scorification can, by way of example, be made with dedicated sealing doors (e.g., so-called tap-holes), which are blocked with special refractory cones applied for example by robotic arms.

In a preferred embodiment of the invention, the at least one outlet for discharging the slag and/or the molten metallic material, is realized with a sealing closure, for example in form of a sliding gate or shutter or of the guillotine type, comparable to a flood-gate or dam door, suitable to hermetically close the outlet. For this purpose, also conceivable are doors that open and close the furnace in perimetral direction (like sliding doors) or the use of valves that allow the regulation of the flow of the slag or of the molten metallic material.

Also conceivable is a tapping from the bottom of the furnace by known EBT systems (eccentric bottom tap-holes).

In a very advantageous embodiment of the invention, the melting gas suction system is connected to a recirculation system, in particular of CO, so as to use the latent energy in a better manner, for example in heating furnaces or heating systems of buildings and/or reactors to produce direct reduced iron, etc.

The gases produced by the furnace can be taken by suction devices after cooling and cleaning treatment, and can thus be used in other utilities of the plant and/or stored for possible transport. A furnace equipped with a gas treatment and CO separation apparatus, so that it can be intended for different users present in the plant in at least partial replacement of the natural gas which would normally be used thereby, reduces CO2 emissions per ton of liquid steel or cast iron produced.

In a particular embodiment of the invention, a specific power value referring to the surface of the bath (with units of measurement kW/m ² ) lower than that of the standard furnaces for melting scrap is applied to the melter, equal to about 1/3 of the standard furnaces. The innovative furnace can also operate at standard power densities, enhancing wall cooling. Said furnace according to the invention can advantageously have a larger diameter than traditional furnaces so as to greatly decrease the wear of the refractory material and operate continuously without maintenance stops, for very long times. In this sense, the melter can continue to be loaded and melt even while it is tapped or during scorification from the dedicated holes.

In a preferred embodiment of the invention, the furnace may be provided with electromagnetic stirrers on the bottom.

A further aspect of the invention relates to a melting process of metal loads, preferably scrap and/or direct reduced iron, which comprises the following steps:

(i) continuous feeding of metal loads in a melting furnace with a melting chamber which is protected from gas exchanges with the external environment, being managed in overpressure or at atmospheric pressure;

(ii) heating and melting, preferably by continuous electrical energy, of the metal loads,

(iii) suction of the CO produced by the melting furnace during the melting process;

(iv) discontinuous discharge of the molten metal material and slag; and

(v) use of the CO removed in step (iii) for other uses exploiting the latent energy thereof. Such uses may be for example obtaining energy by combustion or reducing metal oxides, in particular iron oxides, to metals.

The metal loads can be fed at room temperature, but also hot. The mainly continuous management of the system is preferable, however, interruptions of the process which require discontinuous management may occur.

In a particularly preferred embodiment of the invention, the furnace is then managed to be continuously fed with the metallic load and possibly with additives, such as coal (if necessary) or scorification agents or other additives useful for melting, or for slag formation, while the tapping/scorification advantageously takes place discontinuously. For example, the tapping may be managed automatically with the measurement of molten metal and slag levels and/or with mass scales. The loading takes place advantageously from the vault.

When the molten bath reaches the desired height, for example, a predetermined amount of the molten metal is tapped. Preferably, the furnace according to the invention is constructed for continuous processing, with longer life of the refractory material than that of the standard furnaces.

In summary, it can be seen that the advantages of the type of furnace according to the invention are: the low total energy consumption compared to standard furnaces with air inlet and also use of chemical energy; and the removal of the CO formed during the reduction of metal oxides, so as to exploit the latent energy of CO in other applications.

There are mainly two reactions responsible for CO generation within the furnace:

The first concerns the reduction of the residual FeO contained in the load by the carbon present in the load itself or with the carbon fed into the load:

FeO + C - CO (g) + Fe (1)

The second concerns the oxidation of the carbon residue present in the bath with injectors (under the slag) dedicated to decarburization, in the case of steel production:

C + ½ 0 ₂ (g) - CO(g) (2).

Fig. 5 shows the trend of the latent energy possessed by the CO developed by the reactions occurring in the melting process. This energy with a furnace according to the invention, which avoids oxygen excesses by sealing the furnace, working in vacuum and removing CO before further oxidation of the same, is totally recoverable outside the furnace for other applications. The features described for one aspect of the invention may be transferred mutatis mutandis to the other aspects of the invention.

The embodiments of the invention described reach the objects of the invention. In particular, they allow the direct reduced iron and/or scrap to be melted continuously, with the highest possible yield, with the lowest possible energy consumption, recovering the latent energy of the CO produced, producing a liquid metal with controllable carbon concentrations and reducing the wear of the refractory material inside the furnace. Said objects and advantages will be further highlighted in the description of preferred embodiment examples of the invention given by way of example and not of limitation.

Variant embodiments of the invention are the object of the dependent claims. The description of the preferred exemplary embodiments of the melting furnace, the melting process and the use of the furnace is given by way of example and not of limitation, with reference to the attached drawings.

DESCRIPTION OF PREFERRED EMBODIMENT EXAMPLES

Fig. 1 shows a front view of a first embodiment of a melting furnace according to the invention.

Fig. 2 shows a front view of a second embodiment of a melting furnace according to the invention.

Fig. 3 shows a side view of the two embodiments of the melting furnaces of figures 1 and 2.

Fig. 4 shows the oxygen-carbon balance which is established in an electric arc furnace as known in the state of the art.

Fig. 5 shows the trend of the latent energy possessed by the CO developed by the reactions occurring within the melting process.

Fig. 6A shows in a perspective view a section through a wall of a melting furnace with an outlet with a sealing sliding closure in the closed state.

Fig. 6B shows in a perspective view a section through a wall of a melting furnace with an outlet with a sealing sliding closure in the open state.

Fig. 1 shows a front view of a first embodiment of a melting furnace 2 according to the invention. The main parts of the furnace are the bottom 4, the side walls 6 and the vault 8 which are sealed together so as to create an essentially closed melting chamber. A supporting structure 10 supports the furnace 2 and anchors it in the floor 12. In the case of a DC furnace, to close the electrical circuit inside the furnace 2, for example, an anode 14 and cathode(s) 16 are provided supported by electrode carrier arms 18 (see fig. 3) which allows the axial displacement of the electrodes 16. Alternatively, if the furnace were AC, the electrical circuit would close between the electrodes and the material and there would be no need for an anode. However, this aspect is common to current furnaces. In the side walls 6, cooled inserts are provided at a height corresponding to the slag forming zone, preferably in copper 20 to allow the solidification of the slag 22 floating above the molten metal 24. The furnace may be fed through a feeding system 26 with material to be melted 29 from a first storage container 28 and possibly with additives 31 from a container 30. The amounts delivered are adjustable by respective valve systems or similar systems for feeding regulation 32 and 34. The slag can be discharged through a first hole 38a, the molten metal can be discharged through a second hole 38b. A temperature/sampling probe 40 may be provided in order to adjust the process.

Fig. 2 shows a front view of a second embodiment of a melting furnace according to the invention. The melting furnace 2 corresponds to that shown for the first embodiment in figure 1 with the only difference regarding the loading with materials/additives to be melted. A conveyor belt 42 is included here, which transports the material to be melted with which it has been loaded from a container 44. The material to be melted, before entering through the opening 49 into the furnace 2, passes through a chamber 46 fed with an inert gas 48 so as to avoid the introduction of O2 into the melting chamber of the furnace 2.

Fig. 3 shows a side view of the two embodiments of the melting furnaces of figures 1 and 2. The gases are removed through a process gas suction duct 36. Along the path thereof, they pass through a gas cooling system 37 which separates the solid parts (arrow 39), while the cooled gases leave (arrow 41) the cooling system 37 for use in other parts of the plant, such as for example burners or other furnaces or heating systems, or for the reduction of metallic minerals or metal oxides.

Fig. 4 shows, as already explained above, the oxygen-carbon balance which is established in an electric arc furnace as known in the state of the art.

Fig. 5 shows the trend of the latent energy possessed by the CO developed by the reactions occurring within the melting process. The CO contained in the exhaust gases can be used as already described above in other parts of the plant.

Figures 6A and 6B show in a perspective view a section through a wall of a melting furnace with a sealing sliding outlet in the closed and open state respectively. At the height between the bottom 111 and the wall 110 of the furnace there is an outlet delimited by a corresponding frame 138 that can be closed tightly with a sloping sliding element 139 that can be lowered (closed) and raised (open) with a dedicated mechanism 141. These doors can be provided in the position and number best suited to the requirements of the production process, and can for example be provided on opposite sides of the furnace or one on top of the other such as outlets 38a, 38b in figures 1 and 2.

During implementation, further embodiment modifications or variants of the melting furnace, melting process and use object of the invention may be implemented. If such modifications or such variants should fall within the scope of the following claims, they should all be considered protected by the present patent. In practice, the materials employed, as well as the dimensions, numbers and shapes, provided that they are compatible with the specific use and not otherwise specified, may be different, according to requirements. In addition, all the details can be replaced by other technically equivalent elements.