WO2009095942A1

WO2009095942A1 - Iron metallurgical plant

Info

Publication number: WO2009095942A1
Application number: PCT/IT2008/000064
Authority: WO
Inventors: Stefano Miani; Bruno Rubeo
Original assignee: Concast Ag
Priority date: 2008-02-01
Filing date: 2008-02-01
Publication date: 2009-08-06

Abstract

The present invention relates to an iron metallurgical plant 1 comprising: a shredder (3) adapted to mill the undifferentiated scrap; downstream the shredder, an electric arc furnace (7) adapted to melt the milled scrap; and transport means (6) adapted to transport the milled scrap and adapted to essentially continuously feed the electric arc furnace with the scrap milled by the shredder.

Description

DESCRIPTION

IRON METALLURGICAL PLANT

[0001] The present invention relates to an iron metallurgical plant, in particular a plant for the production of steel by an electrical cycle, also called secondary iron metallurgy, comprising an electric arc furnace for the production of steel. The invention also relates to a secondary iron metallurgy method.

[0002] Nowadays, the production of steel is carried out by means of three basic processes.

[0003] A first process is called the integrated cycle, or primary iron metallurgy. According to this, ferrous mineral (usually an iron oxide, in particular Fe₂O₃) turns to cast iron by means of the blast furnace, where it contacts the coke and other additives (mostly CaCO₃) . Downstream the blast furnace, the cast iron is processed in order to reduce the carbon percentage thereof, thereby turning it to steel. [0004] A second steel production cycle, called the secondary iron metallurgy, starts from the ferrous scrap in order to regenerate it and obtaining new steel . The core of such secondary iron metallurgy is the electric arc furnace (EAF) , where the scrap is molten. Downstream the electric furnace, successive processing operations result in the production of semifinished products. [0005] Finally, an intermediate process takes place which consists in the direct reduction of the iron oxides to metal iron, without the melting of the latter. In this way, the so-called direct reduced iron (DRI) is obtained. Subsequently, such direct reduced iron can be processed in order to eliminate the impurities (for example, inclusions of inerts) and to form iron briquettes, or hot briquetted iron (HBI) which feed the secondary iron metallurgy cycle. [0006] Currently, the global production of steel is similarly split between primary and secondary iron metallurgy, while the amount of steel which is produced using the raw material coming from the direct reduction is greatly lower. [0007] Each of the processes described above has advantages and drawbacks. For example, the primary iron metallurgy requires heavy initial investments, due to the implementation cost for the blast furnace and the infrastructures necessary to the procurement of raw materials. On the other hand, a blast furnace has relatively low management costs and high production capacities, on a scale of several thousands tons of cast iron per day. [0008] On the other hand, the steel obtained by primary iron metallurgy is usually of an excellent quality, and it is preferred for a number of uses. For example, in a field of strategic importance as the automotive field, the blast furnace steel is markedly preferred. In fact, in the motor vehicles production, the processing of relatively thin sheets and the conformation with even quite reduced radiuses of curvature must give aesthetically pleasant results. Such characteristics are ensured by the use of the blast furnace steel. [0009] The secondary iron metallurgy is actually based on plants the dimensions of which are lower than those of the blast furnace. Therefore, the individual plant requires lesser initial investments and has lower production capacities, usually on a scale ranging between hundreds and a few thousands tons of steel per day. [0010] However, the electric arc furnace has considerable advantages compared to the blast furnace. First of all, it is fed with scrap, actually playing a fundamental role in the recycle of the raw materials, with evident environmental advantages. Furthermore, the arc furnace underwent a continuous progress in the last forty years, until becoming an extremely efficient plant. In particular, there have been progressive improvements in the energy efficiency and constant decreases of the management costs and environmental impact. [0011] While having such indubitable advantages, the secondary iron metallurgy suffers however from some drawbacks, most of all due to the fact that the quality of the available metallic scrap greatly affects logistics, cycle efficiency, environmental impact, and quality of the produced steel.

[0012] From the logistics point of view, the scrap is usually stored in a yard, from where it is loaded in buckets intended to feed the furnace. Currently, an average cycle of an electric furnace approximately ranges between 40÷60 minutes in tap-to-tap cycle. During a single cycle, the furnace melts in average the contents of two or three buckets. Since the loading of the single bucket in the scrap yard can take tens of minutes, the presence of more loading buckets is necessary for a same furnace. Of course, each bucket requires a dedicated crane to load the scrap, and a dedicated crane to handle the same bucket. From this it can be deduced the complexity of the logistic management of the scrap yard. [0013] Furthermore, the loading of each bucket involves the switching off and the removal of the electrodes, and the lifting of the furnace vault. Such operations result in an overall power off time equal to about 10-15% of the cycle duration. [0014] Furthermore, it is necessary to take into account that scrap is in general extremely inhomogeneous . For example, the scrap can comprise batches of material having a very low bulk density (shavings, off-cuts of sheets, etc.), and batches having a much higher bulk density (rails, beams, etc.). [0015] Therefore, while loading the bucket it is necessary to distinguish the scrap bulk density so as to obtain, while keeping the volume constant, a predetermined effective mass. Such need further increases the loading times for a single bucket, further complicating the logistics.

[0016] Furthermore, the scrap, most of all deriving from the scrapping of motor vehicles and/or electrical appliances, comprises also a quantity of other materials beside the steel: plastics, glasses, inerts, paints, oils, and several metals such as aluminium, lead, copper, and tin.

[0017] The presence of organic material (polymers, oils, paints) in the metallic scrap results in the generation of pollutants such as dioxins, furans, and odours. The generation of these pollutants involves the need to check an optional scrap preheating step in a particularly stringent manner. Such step could bring in a significant advantage as regards the overall efficiency of the cycle. In fact, it is possible to heat the scraps by exploiting the hot fumes exiting the furnace, thus reducing the energy needed to their melting. Today, such preheating is prevented, in -practice, by the difficulty in managing the pollutants generated from batches of dirty scrap. [0018] Moreover, the presence of non-ferrous metallic fractions (copper, lead, aluminium, tin) determines a poor quality, thus a reduced value for the steel exiting the electric furnace.

[0019] For such reasons, shredders have been provided in the demolition plants, which are adapted to mill any type of scrap so as to crush it into fragments having homogeneous dimensions in the order of cubic decimetre.

[0020] Inside the shredder, the energies of the breakings, strains, frictions, and rubbings determine a local temperature increase up to 800° C. Such temperature increase determines a substantial evaporation of most of the oils and paints contaminating the scrap. The resultant formation of pollutants, localized inside the shredder can be easily managed by means of a suitable suction and filtering plant. [0021] Downstream the actual shredder, automatic selection plants first of all adapted to divide the metallic from the non-metallic mass have been subsequently developed. Furthermore, a further automatic separation of the ferrous fraction from the non-ferrous fraction inside the metallic mass is known. [0022] In view of a large availability on the market of dirty and undifferentiated scrap, the crushed, cleaned, and selected ferrous scrap is commercially available in amounts that are absolutely insufficient for the feeding of the existing electric furnaces. Furthermore, the high quality of the cleaned scrap and the poor availability thereof result in its extremely high price on the market . In addition, it is very difficult for the manager of the electric furnace to verify that the purchased scrap has been really properly treated and selected.

[0023] Therefore, the object of the present invention is to provide an iron metallurgical plant and a method which at least partially overcome the drawbacks complained with reference to the prior art. [0024] Therefore, the task of the present invention is to provide a secondary iron metallurgy plant which is characterized by simplified logistics, high efficiency, low environmental impact, and high quality of the produced steel. Further task of the present invention is to provide a plant which does not suffers from the shortage of clean scrap on the raw material market. [0025] Such object and such task are achieved by an iron metallurgical plant in accordance with the claim 1. [0026] Further characteristics and the advantages of the present invention will be more clearly understood from the description of some exemplary embodiments, given by way of indicative and non-limiting example herein below, with reference to the following Figures:

[0027] Pig. 1 represents a block diagram of the iron metallurgical plant according to the invention;

[0028] Fig. 2 represents an overall schematic view of the iron metallurgical plant according to the invention; [0029] Fig. 3 represents an overall schematic view of a variation of the iron metallurgical plant according to the invention;

[0030] Fig. 4 represents an overall schematic view of a variation of the iron metallurgical plant according to the invention;

[0031] Fig. 5 represents an overall schematic view of a variation of the iron metallurgical plant according to the invention.

[0032] With reference to the Figures, the iron metallurgical plant according to the invention is generally indicated with 1. [0033] The iron metallurgical plant 1 according to the invention comprises: at least one shredder 3 adapted to mill the undifferentiated scrap;

- downstream the shredder 3, an electric arc furnace 7 adapted to melt the scrap milled by the shredder 3; and - transport means 6 adapted to transport the milled scrap and adapted to essentially continuously feed the electric arc furnace 7 with the scrap milled by the shredder 3. [0034] By "continuous" or "essentially continuous" feeding is meant herein below that the transport means transport the scrap with a flow having a relatively reduced and nearly constant flow rate. Such term is to distinguish the operation of the transport means employed in the plant according to the invention from the buckets employed in the plants of known type. In fact, the buckets result in a discrete or batch feeding.

[0G35] Nonetheless the term "continuous feeding", as it will be appreciated by those skilled in the art, in some steps of the melting cycle, such as for example the refining necessary to bring the melt to the tap temperature, the flow can be temporarily interrupted, and that the scrap is not fed to the furnace in such steps . [0036] The embodiment of the plant 1 described above is schematized in Fig. 1 in solid line and it is represented schematically in Fig. 2. The dashed elements in Fig. 1 are optional elements of the iron metallurgical plant 1 which can be present in some embodiments in order to meet specific needs. [0037] In accordance with an embodiment, the plant 1 also comprises, upstream the shredder 3, a scrap yard 2 adapted to store undifferentiated scrap.

[0038] In accordance with such embodiment, the plant 1 also comprises handling means 4 adapted to transport the undifferentiated scrap from the scrap yard 2 to the shredder 3.

[0039] In a per se known manner, the scrap yard 2 is a foreground where the scraps to be processed are situated. Differently from what happens in the known plants, in the scrap yard 2 according to the invention a division of the scrap on the basis of its bulk density is not necessary. In fact, the next milling step has the effect of uniforming the scrap bulk density for the most part, so as to be able to disregard the differences therein in the loading step of the furnace 7. In this way, the logistics of the scrap management is considerably simplified.

[0040] The handling means 4 can comprise in a per se known manner cranes, overhead travelling cranes, rail trucks, tyred trucks, or the like. [0041] In accordance with some embodiments, for example those in the Figures 4 and 5, the plant 1 also comprises, downstream the shredder 3 , a separator 5 adapted to separate the ferrous milled scrap by the non-ferrous milled scrap. [0042] In accordance with such embodiment, the transport means 6 are adapted to transport the undifferentiated milled scrap from the shredder 3 to the separator 5, and from the separator 5 to the electric furnace 7 in a continuous manner.

[0043] In accordance with some embodiments, for example the one in the Figures 3 and 4, the plant 1 also comprises, downstream the shredder 3 and, when present, downstream the separator 5, a buffer 8' which acts as a storage system. [0044] In accordance with such embodiment, the transport means 6 are adapted to transport the undifferentiated milled scrap from the shredder 3 to the buffer 8' (optionally through the separator 5) , and from the buffer 8' to the electric furnace 7 in a continuous manner . [0045] Here and below, we refer to a material flow that, in the most complete version thereof, begins in the scrap yard 2 and ends in the arc furnace 7 after covering the plant 1. Consequently, terms such as "preceding", "upstream" , or the like indicate a position which is relatively near to the scrap yard 2. On the other hand, terms such as "successive", "downstream", or the like indicate a position which is relatively near to the furnace 7. [0046] The shredder 3 or crusher, per se known, is able to reduce the scrap into fragments which are quite homogeneous in size, for example allowing the exiting thereof only through a calibrated screen. For example, it can be a hammer shredder or a counter-rotating shaft waste ripper. [0047] The separator 5 consists, in a manner known per se, in a sequence of stations inside which the scrap fragments are divided according to simple physical principles. For example, a magnetic station draws from the undifferentiated scrap only the ferrous fraction, which is that of interest for the plant 1 feeding. Other stations can separate the different fractions through countercurrent air blows, by floating in a water tank, by centrifugation and the like. [0048] Therefore, a plurality of different materials will come out from the separator 5. The non-ferrous metals such as aluminium, copper, lead, etc., can be, in turn, separated and retrieved in suitable re-melting cycles. The so-called fluff, an heterogeneous material composed of polymers, elastomers, wood, foamed materials, and the like, has a fair calorific power and can be suitably combusted in cogeneration plants. The inert materials such as earth, sand, cement, and the like, must be otherwise disposed of. Finally, the ferrous scrap, or proler, represents the raw material for the secondary iron metallurgy. At this point, such scrap turns out to be homogeneous, clean, selected and with a bulk density ranging between 0.8-1.2 Kg/dm³.

[0049] Upon exiting the separator 5, the ferrous milled scrap is directed to the buffer 8', when present, and then, from there to the furnace 7 through the continuous transport means 6, per se known.

[0050] As stated above, the final length of the transport of the processed (milled and optionally- selected) scrap takes place in a continuous manner. In other terms, in the plant 1 according to the invention the loading is not carried out in the conventional discrete manner by means of two or three buckets for each tap-to-tap cycle. On the contrary, in the plant 1 according to the invention, the furnace loading takes place through a continuous flow having a relatively low flow rate of scrap. To the purpose, the transport means 6, at least in the final length thereof just upstream the electric furnace 7, must be adapted to provide a continuous feeding. For example, they can comprise conveyor belts, chains of scoops, lines of oscillating plates or the like.

[0051] The loading of the furnace 7 in a continuous manner during the cycle rather than in a discrete manner allows avoiding the power off time associated to the switching off and removal of the electrodes for the lifting of the furnace vault. Therefore, the continuous loading allows, while keeping other furnace characteristics constant, a marked increase of the productivity of the same furnace. [0052] As described above, the plant 1 according to the invention comprises a buffer 8' in some embodiments thereof. In such embodiments, the transport means 6 are adapted to hold the milled (and optionally selected) scrap in the buffer 8' which is arranged downstream the separator 5, when present, and upstream the electric furnace 7.

[0053] The buffer 8' serves to make the milled scrap flow on the transport means 6 regular in order to continuously feed the furnace 7. In fact, it is likely that the feeding of the scrap to the shredder 3 is not regular and provides for flow rate peaks alternated with moments in which the flow rate is reduced or null, for example for the shredder maintenance. In view of this, it is preferable to interpose a buffer 8' which makes the flow rate of milled scrap entering the furnace 7 regular, so as to optimize the advantages achieved by the continuous feeding of the same furnace.

[0054] Furthermore, the shredder 3 must periodically undergo a maintenance in order to ensure proper operation. For example, the replacement of the hummers and/or of the cutting edges which mill the scrap is needed at regular operational intervals . If during the operative phases of the shredder 3 the milled scrap is stored in the buffer 8', during the shredder 3 downtime periods, the same buffer 8' can ensure feeding the scrap to the transport means 6, hence to the electric furnace 7.

[0055] A possible alternative to ensure the continuous feeding to the electric furnace 7 in the absence of the buffer 8' is to provide two or more shredders 3 arranged in parallel and intended to alternate in the operative and downtime steps so as to constantly ensure the provision of milled scrap to the furnace 7.

[0056] In accordance with some embodiments, for example the one in Pig. 5, the plant 1 according to the invention further comprises a station 8" for the milled scrap preheating.

[0057] Thanks to the use of the shredder 3 and, when present, of the separator 5, the selected scrap is essentially free of plastics and oils. For this reason the scrap preheating process, otherwise challenging from the environmental point of view, can be carried out without incurring in this kind of problems.

[0058] In such embodiment, the transport means 6 are adapted to hold the milled scrap in the preheating station 8" arranged downstream the separator 5, when present , and upstream the electric furnace 7. [0059] In accordance with an embodiment of the plant according to the invention, the preheating station 8" exploits the fumes exiting the electric arc furnace 7. In fact, such fumes exit the furnace 7 at a high temperature, around 1000° C in average.

[0060] Furthermore, the plant according to the invention preferably comprises a fume processing system 9. Such fume processing system 9 comprises a special post- combustion chamber for safety reasons, where the carbon monoxide (CO) present in the fumes is combusted.

[0061] Therefore, the heat contained in the fumes can be employed to preheat the milled scrap before its feeding to the furnace 7, so as to promote the melting and reduce the electrical power needed by the same furnace 7 in order to perform the cycle.

[0062] In accordance with a different embodiment of the plant according to the invention, the milled scrap preheating station 8" is fed by heat sources independent from the furnace 7, for example from methane burners. [0063] In accordance with an embodiment of the plant 1 according to the invention, the scrap preheating station 8" is adapted to manage two different preheating steps. [0064] In the first preheating step, the temperature is kept below 300° C for the period of time (for example, 20-30 minutes) needed to evaporate the possible oil and/or paint residues in a controlled manner. [0065] In the second preheating step, in which the scrap is by now clean, the temperature can be raised to the reachable maximum value by means of the fumes arriving from the electric furnace 7, or by means of the other forms of heating being used. [0066] In accordance with an embodiment, the preheating station 8" comprises a depuration apparatus for the fumes originating from the oil and paint evaporation which can take place during the first step. In such depuration apparatus, the fumes are sucked and suitably processed, for example by means of activated carbons. [0067] Furthermore, in accordance with other embodiments, the preheating station 8" is adapted to keep therein, during the first step, a modified atmosphere, for example an oxygen-depleted one. [0068] In accordance with some embodiments of the plant 1, the transport means 6 comprise a suitable thermal insulation. In this manner the dissipation into the environment of the heat stored in the scrap is avoided during the transport to the furnace 7. [0069] Of course, such solution is advantageous when a scrap preheating station 8" is present. The thermal insulation of the transport means 6 can also be advantageous When the dissipation of the heat originated in the shredder 3 during the scrap crushing step is not desired. In fact, it shall be noticed that such heat originating from the energy released by the breakings and frictions generated by the shredder 3 in the scrap, can locally result in temperatures near to 800° C. [0070] In accordance with an embodiment of the plant 1 according to the invention, the buffer 8' and the preheating station 8" coincide. For example, the buffer

8' can be a scrap deposit adapted to allow the passage of the hot fumes through the mass of the stored scrap.

[0071] The advantages of the iron metallurgical plant 1 which is the object of the present invention are understood and have been partially discussed above.

[0072] As those skilled in the art will be undoubtedly able to understand from the description discussed above, the scrap processing in the same melting plant allows ensuring a more efficient and cleaner successive production cycle. In fact, the crushing into homogeneous dimensions and densities drastically simplifies the logistics upstream the furnace.

[0073] Furthermore, the removal of the non-metallic mass allows preheating the scrap, thus improving the cycle efficiency and obtaining a molten bath with less slag. [0074] The removal of the non-ferrous metallic fraction allows retrieving other metals as a by-product, and obtaining steel of an excellent quality, comparable to that obtained by the primary iron metallurgy. [0075] The continuous loading of the furnace allows having a fume suction plant with reduced dimensions, the loading of the buckets in the furnace with the consequent opening of the vault being omitted. [0076] Finally, and most importantly, the continuous loading of the furnace, which is made possible thanks to the particular configuration of the plant according to the invention, allows employing a more stable and regular electric arc in the furnace, while originating a more stable melting process and reducing electric network instability phenomena (flicker) .

[0077] It will be appreciated that only some particular embodiments of the iron metallurgical plant being the object of the present invention have been described, to which those skilled in the art will be able to make all the modifications which are necessary to the adaptation thereof to particular applications, without however departing from the protection scope of the present invention, as defined by the following claims.

Claims

1. An iron metallurgical plant (1) comprising:

- a shredder (3) adapted to mill the undifferentiated scrap; - downstream the shredder (3) , an electric arc furnace (7) adapted to melt the scrap milled by the shredder (3) ; and transport means (6) adapted to transport the milled scrap and adapted to essentially continuously feed the electric arc furnace (7) with the scrap milled by the shredder (3) .

2. The plant according to claim 1, further comprising a scrap yard (2) upstream the shredder (3) , which is adapted to storage undifferentiated scrap.

3. The plant according to the preceding claim, further comprising handling means (4) adapted to transport the undifferentiated scrap from the scrap yard (2) to the shredder (3) .

4. The plant according to any preceding claim, further comprising a separator (5) downstream the shredder (3) , which is adapted to separate the ferrous milled scrap from the non-ferrous milled scrap.

5. The plant according to the preceding claim, wherein the transport means (6) are adapted to transport the undifferentiated milled scrap from the shredder (3) to the separator (5) and from the separator (5) to the electric furnace (7) in a continuous manner.

6. The plant according to any preceding claim, further comprising a buffer (8') downstream the shredder (3), which serves as a storage system adapted to make the milled scrap flow on the transport means (6) regular in order to continuously feed the furnace (7) .

7. The plant according to the preceding claim, wherein the transport means (6) are adapted to transport the milled scrap from the shredder (3) to the buffer (8') and from the buffer (8') to the electric furnace (7) in a continuous manner.

8. The plant according to claim 6, wherein the transport means (6) are adapted to hold the milled scrap in the buffer (8')_# then to transport it to the electric furnace (7) in a continuous manner.

9. The plant according to any preceding claim, wherein the shredder (3) is of the hammer or counter-rotating shaft type, and is able to reduce the scrap to fragments which are homogeneous in size.

10. The plant according to any preceding claim, wherein the transport means (6) , at least in the length just upstream the electric furnace (7) comprise conveyor belts, chains of scoops or lines of oscillating plates adapted to generate a continuous flow of scrap having a relatively low flow rate.

11. The plant according to any preceding claim, comprising two or more shredders (3) which are arranged in parallel and intended to alternate in the operative and downtime steps, so as to constantly ensure the provision of milled scrap to the electric furnace (7) .

12. The plant according to any preceding claim, further comprising a station (8") for the preheating of the milled scrap.

13. The plant according to the preceding claim, wherein the transport means (6) are adapted to hold the milled scrap in the preheating station (8") , then to transport it to the electric furnace (7) in a continuous manner.

14. The plant according to claim 12, wherein the scrap preheating station (8") exploits the fumes around 1000° C in average which exit the electric arc furnace (7) .

15. The plant according to claim 12, wherein the scrap preheating station (8") exploits heat sources independent from the furnace (7) , such as methane burners.

16. The plant according to any preceding claim, further comprising a fume processing system (9) comprising a special post-combustion chamber adapted to burn the carbon monoxide (CO) present in the fumes exiting the furnace (7) .

17. The plant according to any of claims 12 to 16, wherein the scrap preheating station (8") is adapted to manage two different preheating steps:

- a first step in which the temperature is kept below 300° C for 20-30 minutes, adapted to evaporate oil and/or paint residues in a controlled manner;

- a second preheating step, in which the temperature is raised to the reachable maximum value.

18. The plant according to the preceding claim, wherein the preheating station (8") comprises a fume depuration apparatus adapted to suck, and process the fumes by means of activated carbons during the first step.

19. The plant according to claim 17, wherein the preheating station (8") is adapted to keep an oxygen- depleted atmosphere therein during the first step.

20. The plant according to any preceding claim, wherein the transport means (6) comprise a suitable thermal insulation in order to avoid the dissipation of the heat stored by the scrap during transport.

21. The plant according to claims 6 and 12, wherein the buffer (8') and the preheating station (8") coincide.

22. A secondary iron metallurgy method comprising the steps of:

- milling the undifferentiated scrap in a shredder (3) ; essentially continuously transporting the milled scrap from the shredder (3) to an electric arc furnace (7) ; - melting the scrap milled by the shredder (3) in the electric arc furnace (7) .

23. The method according to the preceding claim, further comprising, after the scrap milling step, the step of separating the ferrous milled scrap from the non-ferrous milled scrap by means of a separator (5) .

24. The method according to claim 22 further comprising, after the scrap milling step, the step of storing the scrap in a buffer (8') in order to make the continuous flow to the furnace (7) regular.

25. The method according to claim 22 further comprising, after the scrap milling step, the scrap preheating step.

26. The method according to the preceding claim, wherein the preheating step comprises two steps: - a first step in which the temperature is kept below 300° C for 20-30 minutes;

- a second step in which the temperature is raised to the reachable maximum value.