Method and installation for steel production

Abstract: The invention relates to a method to produce steel from a ferrous material, by using one furnace divided into, at least, two vessels which are connected to each other at least by ducts for the off-gases and ducts for the melted metal. The vessels have a growing capacity starting from the first one (11); the material to be cast is divided into a first charge for the first vessel and a second charge for, at least, a second vessel (12) of the furnace, the charge of the material in the first vessel of the furnace is melted using electric energy and/or combustion energy; the off-gases from the first vessel of the furnace are sent to the second vessel of the furnace in order to heat the charge of material in said second vessel, and the off-gases in the second vessel are sent to the first vessel to heat the material in this first vessel; the metal melted in the first vessel is poured in the second vessel of the furnace.

Description: The present invention concerns the steel industry and particularly the steel production and equipment for steel production. Nowadays, the Electric Arc Furnaces ( EAF ) are the most frequently used method for the steel production. With such method there are however problems related to energy consumption, thermal losses, electrode consumption, maintenance costs, quality of the steel obtained in this way and, last but not least, the environmental situation for the workers. Till now the steel plants management has tried to increase the EAF capacity in order to increase the quantity of the produced steel and consequently to reduce the unit cost, dividing the total cost on a larger quantity of produced material. The electric steel industry demand is a technology to improve the product quality and to reach higher productivity, also considering: lower costs and consumption for the electric energy; lower electrode consumption; lower maintenance requirements, i.e. higher availability production time; max. flexibility in the utilisation of alternative power sources as gas, carbon, post-combustion energy, etc; max. utilisation of the off-gases to pre-heat the scrap to be melted; environmental situation improvement for the steel-making facility in respect to noise, off-gases volume, amount of dust, etc; reduction of the flicker in connection with higher productivity; possibility to retrofit in the existing steel plants. One part of these requirements are already fulfilled by the several developments in the last years, but always with certain compromises without complete answers to all demands. For example, document DE 4302285 A discloses a steel making plant with a double furnace arrangment and a process for operating such a plant in an environmentally safe manner. The furnace includes two arc furnace vessels which are connected via a line and which can be closed by covers. The process uses the gases produced in a first vessel for heating the charging material located in the second vessel in order to increase the efficiency of the plant and reduce the consumption of the electric energy. On other hand the present invention was developed to find an optimum compromise to fulfil all the above stated requirements by a new original steel production methodology. The present invention proposes a steel production method based on a plant which includes one furnace divided into at least two parts, or vessels, interconnected by a duct system for the off-gases and with possibility to tap molten steel from a vessel to another one, and wherein: the capacity of the different vessels increases starting from the first one, and the raw material ( scrap ) to be charged is divided into a charge for the first vessel and a second charge for the at least second vessel of the furnace; the charge of the first vessel of the furnace is melted using electric energy and alternative combustion energy; the off-gases from the first vessel can be conveyed to the second vessel of the furnace in order to pre-heat the scrap present in the second vessel, and the off-gases of the second vessel, in different time, can reach the first vessel to pre-heat the present scrap; the molten steel of the first vessel is poured in the second (at least) vessel to contribute, with its thermal energy, to cast the scrap present in the second vessel; the melted metal of the second vessel of the furnace is discharged for the use. The vessels of the furnace can be more than two, for example three and, in this case, to facilitate the passage of the off-gases and to pour the liquid metal from one vessel to another (the off-gases tend to move upward and the liquid metal tends to move downward) at least the second vessel can be moved from a lower level to an upper level in comparison with the first one or to the third vessel of the furnace. The advantages of this invention are essentially the following: A furnace which is divided into some parts, or vessels, can be installed also in the existing steel plants, with the possibility to have larger steel production without using larger capacity furnaces. The total furnace capacity can be easily fitted to the production requirements according to the subdivision of the furnace in more vessels. The total productivity is higher and the tap to tap time is led back to the first vessel tapping time (are not considered the starting and the final cycles). The total electric energy consumption is lower, related to the first vessel casting operations. The remaining energy required can be supplied by combustible material as gas, carbon and oxygen, CO post-combustion and also coming from a possible aluminothermic process or similar. The electrodes consumption is lower according to the less quantity of electrical energy required in the total balance of energy utilised. The vessels of the furnace placed after the first one, because of electric arc lack, do not require water cooled panels, with consequent decrease in energy dispersion. The off-gases are utilised in the pre-heating of the material in the various vessels of the furnace. The total investment for the equipment is reduced compared to conventional EAF as the operations require a less expensive electrical equipment, the electrodes are not present in the vessels after the first one, simple loading devices, etc. The economical engagement for the electrical energy is reduced because of the lower electric power required and the utilisation of smaller transformers. The off-gases volume is reduced as they are conveyed and used from one vessel to the others before the final exhausting, and that decreases also the off-gases dusts. The flicker is reduced due to the lower electric power, engaged only for the first vessel. More details of the invention are shown in the annexed drawings: Fig. 1 shows a plan view of an equipment to realise the method of the invention; Fig. 2 shows a section of a furnace with two vessels. The drawings and the description show an example of steel plant with a furnace composed by two vessels. The vessels could be three or more. In case of two vessels, the furnace of the invention include a first vessel (11) and a second vessel (12); the second vessel (12) is in lower position compared to the first vessel (11). The first vessel has a lower capacity than the second one. The sum of the row material charged in the single vessels (11) and (12) allows the final amount of molten steel at every melting cycle, starting from one total charge of solid material to be divided in the two vessels. Therefore, the second vessel can contain the quantity of steel produced in the first vessel added to the steel produced by itself. For instance, for a production cycle of 80 tons, the first vessel can have a production capacity of 66 tons and the second vessel a production capacity of 22 tons. Therefore, the first vessel capacity will be 60 tons of liquid steel and 80 tons for the second one. Any other combination among the production capacities of different vessels is allowed provided the compatibility with the final result to be obtained. The charge of the solid material will be properly divided between the different vessels. In particular the size of the material to be charged in the first vessel should be smaller than the material to be charged in the second vessel, because of the different main energy utilised: electrical for the first vessel, fuel burners of liquid, gaseous or solid for the second. The off-gases produced in the first vessel (11) can be conveyed through a connecting duct (15) in the second vessel (12). The off-gases in the second vessel can be conveyed through a connecting duct (16) back to the first vessel (11). Both, the first and the second vessel have direct connecting ducts to the final exhausting system. The first vessel (11) has a tapping hole (19) in order to pour, by an hole (20) the liquid steel in the second vessel (12). The second vessel (12) has the same tapping system (21) for the melted metal towards a ladle (23) and an outlet for the slag towards a pot. To facilitate the melted metal discharge each vessel (11 and 12) can oscillate on a base and can be reclining using an hydraulic actuator or similar. In this furnace type the solid material charged in the first vessel is cast at the desired temperature, using the electric energy transformed in thermal energy by the voltaic arc of the electrodes. At the same time the hot off-gases produced in the first vessel are conveyed for pre-heating the solid material charged in the second vessel. When the total charge melting in the first vessel is completed, the liquid metal and the slag are poured in the second vessel of the furnace, obtaining with own thermic energy, together with the combustion thermic energy, the melting of the pre-heated charge in the second vessel. For instance, with a furnace and process according to the invention, the first vessel (11) of the furnace includes three electrodes and the second vessel (12) oxygen/carbon gas burners. The first vessel (11) is a normal EAF with tapping weight of 60 ton of liquid steel, equipped with a 60 MVA transformer and three side burners with a capacity of 2,8 MW each and a door burner of 3,5 MW. In the first vessel (11) high carbon steel, approx. 2,5 % C, is produced. The charge is metallic scrap with an average density of 0,7 t/m3. The vessel is equipped by tapping hole with sliding gate. The second vessel (12) is completely lined with refractory, instead water cooled panels, to avoid the liquid steel cooling. In a production process tapping 80 ton of liquid steel, the first vessel (11) is charged with 66 ton scrap, the second vessel (12) with 22 ton of scrap. At the process beginning, in the first vessel (11) carbon steel (2,5 %) is melted. After approx. 34 min., the liquid metal can be tapped at approx. 1500° from the tapping sliding gate (giving minimum temperature loss), discharging it into the second vessel (12), previously charged with approx. 22 ton. scrap. The molten steel, with high carbon content, is decarburised in the second vessel (12) creating energy which contributes to the melting of the scrap charged in the second vessel in order to produce steel with carbon content of 0,1 % or less, similar to a converter production. The liquid steel produced (80 ton approx.) is tapped, for the use, in a ladle placed on the suitable ladle car. After the first phase, the scrap newly charged in the first vessel (11) will be pre-heated with the off-gases of the second vessel (12), and in different time the scrap in the second vessel will be pre-heated by the off-gases of the first vessel. And so on for any following cycle. The cycle is programmed so that when the melting begins in a vessel (for instance n.11) in the other vessel (for instance n.12) the scrap in pre-heating takes place using the hot off-gases coming from the melting material. The operation takes place alternatively. It has to be remarked that the furnace can have three, or more, vessels placed so that the starting scrap can be charged in each vessel in decreasing quantities from the first to the last and the melted steel is poured from the first to the second and the third vessel, and so on, and the off-gases of each vessel can be used for the pre-heating of the scrap in the other vessels according to the pre-set cycle.

Claims: A method to produce steel from a ferrous material, using one furnace divided into, at least, two vessels (11, 12) which are connected to each other at least by ducts (15, 16) for the off-gases and ducts 19 for the melted metal, the vessels having a growing capacity starting from the first one, and wherein: the material to be charged is divided into a first charge for the first vessel and a second charge for, at least, a second vessel of the furnace; the charge of the material in the first vessel of the furnace is melted using electric energy or electric and combustion energy; the off-gases coming from the first vessel of the furnace are sent to the, at least, second vessel of the furnace in order to heat the charge of material in said second vessel, and the off-gases in the, at least, second vessel are sent to the first vessel to heat the material in this first vessel; the metal melted in the first vessel is poured in the, at least, second vessel of the furnace in order to contribute with its own thermal energy and with combustion energy to the melting of the material charged in the, at least, second vessel; the metal melted in the, at least, second vessel is poured for the use. The method to produce steel in accordance with the claim 1, wherein the furnace includes, at least, three vessels with growing capacity starting from the first one and fitted to receive material charges decreasing proportionally; the second vessel being designed to contain also the liquid metal cast from the first vessel and the third vessel being designed to receive the material contained in the second vessel, including the liquid metal cast from the first vessel. The method in accordance with the claim 2, wherein the second vessel of the furnace is movable between a lowered position and a raised position in comparison to the other vessels of the furnace. A plant to produce steel with the method of claims 1 to 3, comprising a furnace divided into, at least, two vessels (11 and 12) connected by ducts (15, 16, 19) for fumes and for melted metal, wherein the first vessel of the furnace has a capacity less than the capacity of the following furnace vessels, the last vessel of the furnace having a capacity equal to the sum of raw material charged in all furnace vessels a first vessel being arranged in order to receive a charge of starting material to be melted which is greater than the one of the, at least, second vessel, said first vessel being heated with electric power and combustion power and the, at least, second vessel of the furnace being heated by combustion energy and thermal energy contained in the melted material of the previous vessel, being the melted material in the first vessel discharged in the second vessel of the furnace.

Inventors: BONOMETTI NATALE, BONOMETTI, NATALE
Applicants: BONOMETTI NATALE, BONOMETTI, NATALE
Application year: 1998
Main classification: C21C5
IPC codes: C21C5