Preheating device, apparatus for treating waste powders comprising said preheating device, and method for heating a mixture of powders by using said preheating device

Abstract

A preheating device (60), an apparatus (100) for treating waste powders (19) comprising the preheating device (60), and a method for heating a mixture of powders (13) by using the preheating device (60). The preheating device (60) is for heating a mixture of powders (13) during the production of steel (82) and / or during the treatment of scrap (49, 83) resulting from steel production (82). The preheating device (60) comprises a heat exchange chamber (69) configured to: receive exhaust gases (15) resulting from steel production (82) and / or from the treatment of the scrap (49, 83), the preheating device (60) being configured to receive the exhaust gases (15) from a melting furnace (28); and transmit heat from the exhaust gases (15) to the mixture of powders (13), so as to heat the mixture of powders (13) by recovering heat from the exhaust gases (15).

Description

[0001] PREHEATING DEVICE, APPARATUS FOR TREATING WASTE POWDERS COMPRISING SAID PREHEATING DEVICE, AND METHOD FOR HEATING A MIXTURE OF POWDERS BY USING SAID PREHEATING DEVICE

[0002] The present invention relates to a preheating device and to an apparatus for treating waste powders which comprises said preheating device. The present invention also relates to a method for heating a mixture of powders by using the preheating device.

[0003] The preheating device, apparatus, and method according to the present invention are particularly, but not exclusively, useful and practical in the field of treating waste powders produced by electric arc furnaces (a known type of melting furnace for steel production). Typically, these waste powders are known as “electric furnace dust” or, more simply, as “EAFD” (acronym of Electric Arc Furnace Dusts).

[0004] In practice, waste powders can therefore be steel mill powders, possibly mixed with other inorganic powders (reagents).

[0005] It is known that the steel production process generates a mix of slag and waste, including waste powders resulting from the suppression of exhaust gases, such as for example EAFD.

[0006] These waste powders can have highly variable chemical compositions depending on the type of steel from which they derive, the quality of the scrap and ferroalloys used as raw materials, and the conditions of the steel production process. Typically, waste powders emitted from melting furnaces that produce ferritic steels have high amounts of oxides of iron, zinc, and lead, while waste powders emitted from melting furnaces that produce stainless steels have high amounts of oxides of iron, nickel, chromium, manganese, and, in some cases, molybdenum.

[0007] Due to the presence of these oxides, waste powders such as EAFD are considered as particularly hazardous special waste.

[0008] Currently, waste powders derived from ferritic steels (in short: ferritic powders) are either disposed of directly in special landfills, or treated using systems that implement the so-called Waelz process or similar systems (for example, rotary hearth furnaces), which recover a zinc oxide concentrate, generating an equal amount of waste containing iron oxides and other metals, which is disposed of in special landfills. Due to these iron oxides and other metals, this waste can also be usefully treated to generate waste powders that can be used by the preheating device, apparatus, and method according to the present invention.

[0009] Waste powders from stainless steels are instead disposed of directly in special landfills, treated in plasma furnaces, or melted directly (or briquetted and remelted) in electric arc furnaces. The latter treatment systems, which use plasma furnaces or electric arc furnaces, recover almost all of the iron and ferrous metals, generating slag and exhaust gases that have a high concentration of zinc oxides and other non-ferrous metals.

[0010] In the field of steelmaking, processes are also known which involve treating waste powders from an exhaust gas treatment system in order to recover their metal content and inertize the residue.

[0011] However, known solutions are not without drawbacks, including the fact that they are particularly costly both in terms of energy and from the economic standpoint. For example, plasma furnaces entail a high energy consumption. As a further example, the briquetting process, which is typically carried out before treatment with EAF electric furnaces, is very expensive in terms of energy and requires the suppression of dioxins produced at high temperatures by agglomerating additives. Moreover, all these steps entail very high economic costs, to such an extent that in many cases it is preferable to entrust the disposal of the powder to special landfills (at a cost on the order of 250 euros / ton). As a further example, the Waelz process is currently the only process capable of recovering zinc oxide from ferritic powders, but at the cost of even higher waste volumes than those just described. Another drawback resides in that in the known and above-mentioned processes a considerable amount of hot exhaust gases is generated at the outlet of a melting furnace, which must first be diluted with air at room temperature, then cooled in jacketed ducts, and then sent to exhaust gas treatment to be cleaned before being released into the atmosphere.

[0012] Moreover, in known types of apparatus, when the melting furnace is an electric arc, induction, or plasma furnace, the melting processes that take place inside the furnace generate turbulence that tends to entrain a significant portion of the waste dust into the exhaust gas treatment system (e.g., through a suction hole) without melting this same portion of the dust. However, this entrainment causes a loss of efficiency of the treatment process and therefore a low percentage of recovery of the metals contained in the waste powders. To mitigate these losses, waste powders and their reagents are traditionally mixed with agglomerating additives and converted into briquettes or pellets by using specific machines (e.g., briquetting or pelletizing machines) before being introduced into the furnace. Although effective, this traditional approach entails additional costs and operational complexity.

[0013] Therefore, known solutions offer poor cost-effectiveness, both in relation to the waste powder treatment process and in relation to the consequent recovery of materials for subsequent reuse. Moreover, known solutions have a negative impact on both the environmental sustainability and overall profitability of a steel mill.

[0014] The aim of the present invention is to overcome the limitations of the background art described above, devising a preheating device, a waste powder treatment apparatus, and a method that allow to achieve higher levels of productivity and material recovery than achievable with known solutions and / or similar levels of productivity and material recovery at a lower cost.

[0015] Within the scope of this aim, an object of the present invention is to devise a preheating device, a waste powder treatment apparatus, and a method that allow to improve the energy efficiency and cost-effectiveness of a steel mill.

[0016] Another object of the present invention is to devise a preheating device, a waste powder treatment apparatus, and a method that allow to give added economic value to waste powders (for example, EAFD) and exhaust gases that are generated during steel production and / or during the treatment of waste derived from the production of said steel.

[0017] A further object of the present invention is to devise a preheating device, a waste powder treatment apparatus, and a method that allow to produce atmospheric emissions within the limits of current environmental regulations.

[0018] Furthermore, an object of the present invention is to devise a preheating device, a waste powder treatment apparatus, and a method that allow to optimize energy consumption (in particular, reducing electric power consumption) related to the treatment of said waste powders, such as for example EAFD.

[0019] A further object of the present invention is to eliminate the briquetting or pelletizing step.

[0020] Not least object of the present invention is to provide a preheating device, a waste powder treatment apparatus, and a method that are highly reliable, relatively simple to provide, and economically competitive when compared to the background art.

[0021] This aim, as well as these and other objects that will become better apparent hereinafter, are achieved by a preheating device according to independent claim 1 , preferably provided with one or more of the additional characteristics defined by the dependent claims.

[0022] The intended aim and objects are also achieved by an apparatus for treating waste powders which comprises said preheating device, as well as by a method for heating a mixture of powders defined as in the respective independent claim.

[0023] Further characteristics and advantages of the invention will become better apparent from the description of a preferred but not exclusive embodiment of the preheating device, apparatus, and method according to the invention, illustrated by way of non-limiting example in the accompanying drawings, wherein:

[0024] Figure 1 is a sectional perspective view of a preheating device according to the invention;

[0025] Figure 2 is a longitudinal sectional view of the preheating device of Figure 1 in a condition of use in which said preheating device is crossed by exhaust gases and a mixture of powders, with the addition of arrows indicating the respective directions of entry and exit of said exhaust gases and mixture of powders;

[0026] Figure 3 is a block diagram schematically showing an embodiment of an apparatus for treating waste powders generated during steel production and / or during the treatment of waste derived from steel production, the apparatus comprising the preheating device according to the invention; and

[0027] Figure 4 is a flow diagram schematically showing the operation of a preferred embodiment of the apparatus of Figure 3 and, therefore, a method for heating a mixture of powders using the preheating device according to the invention.

[0028] In the cited figures, the preheating device (hereinafter, in short: preheater) according to the invention is designated by the reference numeral 60.

[0029] In some embodiments, the preheater 60 is a separate device.

[0030] In other embodiments, the preheater 60 is comprised in an apparatus 100 for treating waste powders 19.

[0031] The preheater 60 is (suitable) for heating a mixture of powders 13, in particular by recovering heat from exhaust gases 15.

[0032] The exhaust gases 15 may be generated during the production of steel 82. In addition or alternatively, the exhaust gases 15 may be generated during the treatment of waste resulting from the production of steel 82 (i.e., during the treatment of by-products derived from the production of steel 82).

[0033] In the preferred and illustrated embodiments, the waste derived from steel production 82 may be EAFD dust, inorganic slag, and / or foundry waste, which will be discussed in greater detail hereinafter.

[0034] In practice, the exhaust gases 15 generated during the production of steel 82 and / or during the treatment of production waste are very “dirty” (i.e., they have a high concentration of oxides of iron, zinc, and other ferrous and nonferrous metals). Moreover, said exhaust gases 15 are very hot. For example, at the outlet of a postcombustion chamber (hereinafter, in short: postcombustor) 36 of the apparatus 100, the exhaust gases 15 typically have a temperature of about 600 °C.

[0035] The powder mixture 13 comprises waste powders 19 such as, for example, EAFD. The waste powders 19 can be generated during the production of steel 82. In addition or alternatively, the waste powders 19 can be generated during the treatment of the production waste of steel 82. In practice, the powder mixture 13 comprises highly volatile material.

[0036] The waste powders 19 may be stored within a first primary silo 21 A belonging to the apparatus 100. Typically, the waste powders 19 stored in the primary silo 21 A have a temperature substantially equal to ambient temperature (for example, 15°C or 25°C).

[0037] Advantageously, the preheater 60 comprises a heat exchange chamber 69. The heat exchange chamber 69 is configured to:

[0038] - receive exhaust gases 15 resulting from the production of steel 82 and / or from the treatment of waste from the production of steel 82; and

[0039] - transfer heat from the exhaust gases 15 to the powder mixture 13, so as to heat the powder mixture 13 by recovering heat from the exhaust gases 15.

[0040] In practice, the exhaust gases 15 that are supplied to the preheater 60 are significantly hotter than the powder mixture 13. Consequently, when the preheater 60 is fed with the powder mixture 13 and the exhaust gases 15, the heat exchange chamber 69 allows heat to be transferred from the exhaust gases 15 to the powder mixture 13.

[0041] In some preferred embodiments, the preheating device 60 (and in particular the heat exchange chamber 69) is configured to provide a temperature difference comprised between 550 °C and 650 °C between the exhaust gases 15 entering the preheater 60 and the powder mixture 13 entering said preheater 60. This temperature difference may be comprised between 550 °C and 620 °C, preferably comprised between 560 °C and 610 °C, and even more preferably comprised between 580 °C and 590 °C (for example, approximately 575 °C or 585 °C). Consequently, it is preferable that the preheater 60 be made of material designed to withstand - without plastic deformation or breakage - temperatures above 600 °C.

[0042] In practice, the heat exchange chamber 69 is configured to heat the powder mixture 13 with the heat recovered from the exhaust gases 15.

[0043] Moreover, when the preheater 60 is integrated into the apparatus 100, the preheater 60 allows to increase the energy efficiency of the apparatus 100.

[0044] In practice, injecting the exhaust gases 15 into the heat exchange chamber 69 to heat the powder mixture 13 entails energy savings of approximately 10% of the energy consumption of the apparatus 100.

[0045] In some preferred embodiments, the preheater 60 comprises a jacket (or main body) 62 and a drum 64.

[0046] The jacket 62 comprises a first intake port 65 and a first discharge port 67. The first intake port 65 is configured to introduce the exhaust gases 15 into the preheater 60. The first discharge port 67 is configured to discharge said exhaust gases 15 downstream of the preheater 60.

[0047] In the preferred and illustrated embodiments, the first intake port 65 and the first discharge port 67 are angularly arranged (in particular, at right angles) with respect to an axial direction X of the preheater 60.

[0048] More preferably, the first intake port 65 and the first discharge port 67 are arranged transversely with respect to the axial direction X.

[0049] The drum 64 comprises a second intake port 61 and a second discharge port 63. The second intake port 61 is configured to feed the powder mixture 13 into the preheater 60. The second discharge port 63 is configured to discharge the powder mixture 13 downstream of the preheater 60.

[0050] Preferably, the second intake port 61 and the second discharge port 63 are aligned along the axial direction X.

[0051] In the preferred and illustrated embodiments, the second intake port 61 is arranged in the vicinity of the first discharge port 67, while the second discharge port 63 is arranged in the vicinity of the first intake port 65. However, alternative embodiments (not shown) are also conceivable, in which the second intake port 61 is arranged in the vicinity of the first intake port 65, while the second discharge port 63 is arranged in the vicinity of the first discharge port 67.

[0052] The main body 62 and the drum 64 collectively form the heat exchange chamber 69. In particular, the drum 64 extends through the main body 62.

[0053] In practice, the heat exchange chamber 69 is configured to receive in input the exhaust gases 15 that enter the preheater 60 from the first intake port 65 (as shown, for example, by the arrow F3 in Figure 2) and to discharge said exhaust gases 15 from the first discharge port 67 (as shown, for example, by the arrow F4 in Figure 2). In other words, the first intake port 65 and the first discharge port 67 form respective ends of the heat exchange chamber 69, and the heat exchange chamber 69 places the first intake port 65 in fluid communication with the first discharge port 67.

[0054] In some preferred embodiments, the second intake port 61 is arranged externally to the jacket 62. In particular, the drum 64 may further comprise: a first portion 64A, arranged outside the jacket 62, the first portion 64A comprising the second intake port 61; and a second portion 64B, arranged inside the jacket 62.

[0055] In practice, the jacket 62 covers the second portion 64B but leaves the first portion 64A exposed.

[0056] In some preferred embodiments, the first portion 64A is (approximately) 2.5 m long. For example, the first portion 64A may have a length L64A of 2.48 m.

[0057] Preferably, the second portion 64B has a geometry similar to that of the first portion 64A.

[0058] In the preferred and illustrated embodiments, the heat exchange chamber 69 is formed between an internal surface 62A of the jacket 62 and an external surface 64C (of the second portion 64B) of the drum 64. In this way, the heat exchange chamber 69 allows to heat the powder mixture 13 with the exhaust gases 15, avoiding direct contact between said exhaust gases 15 (which, as said, are very “dirty”) and the powder mixture 13 (which has a fine part that is highly volatile).

[0059] In practice, when the powder mixture 13 passes through the drum 64 (as shown, for example, by the arrows Fl and F2 in Figure 2), and the exhaust gases 15 pass through the heat exchange chamber 69, heat is transferred from the exhaust gases 15 to the powder mixture 13 in an “indirect” manner, via thermal conduction on the part of a wall 64D that extends from the external surface 64C to an internal surface 64E of the drum 64.

[0060] Advantageously, when the preheater 60 is integrated into the apparatus 100, the first portion 64A allows dehumidification or drying of the powder mixture 13. In other words, the first portion 64A acts as a drying portion (or “dryer”). In this way, the preheater 60 acts as an integrated device for drying and preheating the powder mixture 13.

[0061] In practice, the powder mixture 13 can: enter the first portion 64 A with a certain level of humidity (e.g., a relative humidity of 3-5%); and exit from the first portion 64 A (substantially) free of humidity. During its stay inside the first portion 64A, the powder mixture 13 may undergo preliminary heating (e.g., up to 30 °C). The powder mixture 13 that exits from the first portion 64 A enters the second portion 64B, where the powder mixture 13 is further heated by the heat of the exhaust gases 15 rising along the heat exchange chamber 69. Upon exiting the second portion 64B, the powder mixture 13 may have a temperature comprised between 300 °C and 400 °C, thus being free of humidity and ready to be fed into a melting furnace 28, discussed in greater detail hereinafter.

[0062] Advantageously, the drum 64 can (be configured to) rotate relative to the jacket 62. In this way, the powder mixture 13 can remain inside the drum 64 for a period of time that is long enough to allow heat exchange between the exhaust gases 15 and said powder mixture 13.

[0063] In the preferred and illustrated embodiments, the rotation of the drum 64 can be achieved by operating a motor 79 which is functionally connected to said drum 64 (see Figure 4).

[0064] The rotation of the drum 64 can be facilitated by one or more bearings 66, interposed between the drum 64 and the jacket 62. For example, in Figures 2 and 3, the preheater 60 further comprises a first bearing 66A, arranged proximate to the first discharge port 67, and a second bearing 66B, arranged proximate to the first intake port 65.

[0065] In some preferred embodiments, the preheater 60 further comprises a screw-like member 68.

[0066] The screw-like member 68 is configured to move the powder mixture 13 along the drum 64 toward the second discharge port 63.

[0067] In practice, the screw-like member 68 serves to guide the powder mixture 13 as it advances along the drum 62.

[0068] In some preferred embodiments, the screw-like member 68 is a body made of metal (in particular, steel). In this way, the screw-like member 68 allows to increase the heat exchange surface of the drum 64, thereby improving the ability of the drum 64 to transfer heat by conduction and radiation.

[0069] In the preferred and illustrated embodiments, the screw-like member 68 is a spiral ridge fixed to the internal surface 64E. This particular shape of the screw-like member 68 allows to facilitate the powder mixture 13 in passing through the drum 64.

[0070] In practice, during the rotation of the drum 64 the spiral ridge moves the mixture of powders 13 towards the second discharge port 63.

[0071] In some preferred embodiments, the spiral forming the spiral ridge of the screw-like member 68 has the following dimensions:

[0072] - height H: 20 cm;

[0073] - thickness W: 10 cm;

[0074] - pitch P: 50 cm.

[0075] Preferably, in a suboptimal embodiment, the drum 64 has an inside diameter D of 1.2 m and an exchange length L69 of 6 m. This suboptimal embodiment is the simplest way to prevent possible “packing” of the powder mixture 13 inside the preheating device 60, as well as to minimize the complexity of the elements that form said preheating device 60.

[0076] In an optimal embodiment, the inside diameter D of the drum 64 is 1.4 m, while the length L64 of the drum 64 is 12 m. In the optimal embodiment, these specific dimensions ensure (with residence times of 30 minutes or more) temperatures of at least 320-350 °C for the powder mixture 13 exiting the drum 64.

[0077] In some preferred embodiments, the apparatus 100 comprises a melting system 12, an exhaust gas management system 14, and a recovery system 16 for nonvolatile slag.

[0078] The melting system 12 is configured to produce the powder mixture 13, in particular by mixing the waste powders 19 with one or more reagents R1, R2. In some preferred embodiments, the powder mixture 13 is provided by mixing steel mill powders with coke, silica sand, and / or lime. A suitable amount of water (for agglomeration purposes) is preferably added to the powder mixture 13.

[0079] Moreover, the melting system 12 is configured to melt the powder mixture 13.

[0080] In some preferred embodiments, the melting system 12 comprises a storage group 20.

[0081] The storage group 20 comprises at least one primary silo 21 for storing recycled material, and at least one secondary silo 22 for storing one or more of the reagents R1 and R2.

[0082] In the preferred and illustrated embodiments, the storage group 20 comprises:

[0083] - a first primary silo 21 A for storing the waste powders 19, as well as one or more secondary silos 22 for storing the reagents Rl, R2;

[0084] - a second primary silo 2 IB, for storing byproducts 71 such as, for example, mill scale;

[0085] - a first secondary silo 22 A, for storing a first reagent Rl from among said reagents; and

[0086] - a second secondary silo 22B, for storing a second reagent R2 from among said reagents.

[0087] The first reagent Rl and / or the second reagent R2 may be selected from sand, silicon, carbon coke (preferably in the form of coke fines), and lime.

[0088] In some preferred embodiments, the apparatus 100 (and, in particular, the melting system 12) further comprises a mixer 25 and a melting furnace 28.

[0089] The mixer 25 is configured to generate the powder mixture 13 for the preheater 60.

[0090] In particular, the mixer 25 is equipped with (high-speed) agitators configured to distribute the various components of the powder mixture 13 homogeneously.

[0091] In practice, when the water and the powder mixture 13 are subjected to the action of the mixer 25, in particular of the corresponding agitators, the powder mixture 13 spontaneously agglomerates, thus forming small spheres (with a granule size of less than 1 cm).

[0092] In some preferred embodiments, the mixer 25 is functionally connected to appropriate automation means 23 of the melting system 12.

[0093] The automation means 23 are preferably configured to supply the mixer 25 with the contents of the at least one primary silo 21 and the at least one secondary silo 22, in predetermined quantities.

[0094] In practice, the mixer 25 generates the powder mixture 13, in particular by uniformly mixing the waste powders 19 with one or more of the reagents Rl, R2 (with the possible addition of the byproducts 71).

[0095] In some particularly advanced embodiments, the automation means 23 are further configured to regulate a quantity of the powder mixture 13 entering the preheater 60 (and, in particular, the second intake port 61).

[0096] The melting furnace 28 is configured to generate the exhaust gases 15 for the preheater 60. Therefore, the melting furnace 28 is functionally connected to the preheater 60 (and vice versa). More precisely, the melting furnace 28 is configured to generate the exhaust gases 15 by melting the powder mixture 13.

[0097] The melting furnace 28 - which is per se known - may correspond to one of the furnaces mentioned above (e.g., an electric arc, induction, or plasma furnace).

[0098] In particular, the melting furnace 28 is configured to melt the powder mixture 13, obtaining:

[0099] - steel 82, in the form of ferroalloy in which ferrous metals are concentrated;

[0100] - inorganic slag, in the form of lava 49; and - the exhaust gases 15, comprising secondary waste powders 32 in which oxides of zinc and other non-ferrous metals are concentrated.

[0101] The secondary waste powders 32 are commonly known as crude zinc oxide or, more simply, as “CZO” (acronym of Crude Zinc Oxide).

[0102] Preferably, the melting furnace 28 is of the electric type.

[0103] Advantageously, the melting furnace 28 may comprise electrodes that are electronically controlled. Electronic control of the electrodes of the melting furnace 28 allows to improve the quality of the melting of the powder mixture 13.

[0104] In the preferred embodiments in which at least the mixer 15 and the melting furnace 28 are present, the preheater 60 is configured to:

[0105] - receive the powder mixture 13 generated by the mixer 15; and

[0106] - heat the powder mixture 13 with the exhaust gases 15 generated by the melting furnace 28.

[0107] In preferred but not illustrated embodiments, the powder mixture 13 may be transferred (in the form of an agglomerate) from the mixer 15 to the preheater 60 via a conveyor belt.

[0108] In some preferred embodiments, the apparatus 100 (and, in particular, the melting system 12) further comprises a collection hopper 27.

[0109] The collection hopper 27 is configured to feed the melting furnace 28 with the powder mixture 13.

[0110] In the preferred embodiments in which the collection hopper 27 is present, the preheater 60 is configured to discharge the powder mixture 13 into the collection hopper 27.

[0111] In some preferred embodiments, the apparatus 100 (and, in particular, the melting system 12) further comprises a powder transfer line 26.

[0112] Advantageously, the powder transfer line 26 is configured to transfer the powder mixture 13 from the mixer 25 toward the melting furnace 28, bypassing the preheater 60. In this way, the powder transfer line 26 ensures that the apparatus 100 is operable even in the event of failure or maintenance of the preheater 60.

[0113] The melting system 12 may further comprise an ingot mold 80 functionally connected to the melting furnace 28.

[0114] The ingot mold 80 receives the ferroalloy 82 produced by the melting furnace 28, preferably via a ladle.

[0115] The ingot mold 80 is preferably configured to form ingots and / or metal pigs 81 by cooling the ferroalloy 82 cast into said ingot mold 30.

[0116] The forming of the ingots and / or metal pigs 81 may moreover entail the generation of foundry scrap 83, such as for example, granulated metal.

[0117] The exhaust gas management system 14 is configured to manage the exhaust gases 15 generated by the melting furnace 28.

[0118] In particular, the exhaust gas management system 14 is functionally connected to the melting system 12 via the melting furnace 28 and the preheater 60.

[0119] In practice, the preheater 60 acts as an interface member between the melting system 12 and the exhaust gas management system 14. For example, in Figure 4, the preheater 60 can be considered as part of the melting system 12 and / or the exhaust gas management system 14.

[0120] Advantageously, the exhaust gas management system 14 is a closed and automated system to prevent dangerous dispersals of materials comprising pollutants into the outside environment.

[0121] In some preferred embodiments, the apparatus 100 (and, in particular, the exhaust gas management system 14) comprises a radiating duct 37 and / or a bag filter 38.

[0122] The radiating duct 37 is configured to receive the exhaust gases 15 from the preheater 60 and convey the exhaust gases 15 toward the bag filter 38. In other words, the radiating duct 37 is configured to convey the “dirty” exhaust gases exiting the heat exchange chamber 69 toward the bag filter 38.

[0123] In some preferred embodiments, the exhaust gas management system 14 is further configured to capture the secondary waste powders 32. In particular, the bag filter 38 may be configured to physically separate the secondary waste powders 32 (which are lighter or finer) and a portion of gas 33 (mainly carbon dioxide) that are comprised in the exhaust gases 15 exiting the radiating duct 37.

[0124] Advantageously, the bag filter 38 may comprise one or more hoppers 38A-D, and each hopper 38A-D is configured to discharge the secondary waste dust 32 filtered by the bag filter 38 into an appropriate silo (not shown).

[0125] In some preferred embodiments, the apparatus 100 (and, in particular, the exhaust gas management system 14) comprises an exhaust chimney 34.

[0126] The exhaust chimney 34 is configured to discharge the portion of gas 33 exiting the bag filter 38, and to disperse the same portion of gas 33 into the external environment, in particular into the atmosphere.

[0127] Advantageously, the chimney 34 may comprise a motorized suction fan 31 configured to move and cool the portion of gas 33 exiting the bag filter 38.

[0128] The motorized suction fan 31 may be part of an appropriate suction system (not shown) of the apparatus 100. Although not visible in the figures mentioned, the suction system of the apparatus 100 is configured to suck the exhaust gases 15 from the melting furnace 28 and / or from one or more components for the treatment of waste resulting from the production of steel 82.

[0129] In practice, the suction system of apparatus 100 is configured to move the exhaust gases 15 through the various components of the exhaust gas management system 14.

[0130] In some preferred embodiments, apparatus 100 (and, in particular, the exhaust gas management system 14) further comprises the postcombustor 36 mentioned above.

[0131] In particular, the postcombustor 36 is functionally connected to the melting furnace 28, and the preheater 60 is configured to receive the exhaust gases 15 from the melting furnace 28 via the postcombustor 36.

[0132] In practice, the postcombustor 36 is provided with a hot fume outlet (or exhaust pipe) that allows the preheater 60 to be fed with the exhaust gases 15.

[0133] Advantageously, the postcombustor 36 can (be configured to) facilitate complete combustion of the carbon monoxide comprising the exhaust gases 15 exiting the melting furnace 28. Moreover, the postcombustor 36 can facilitate deposition of a heavier / thicker portion of the secondary waste powders 32 included in the exhaust gases 15 exiting the melting furnace 28.

[0134] In some preferred embodiments, the apparatus 100 (and, in particular, the exhaust gas management system 14) further comprises an exhaust gas transfer line 39.

[0135] Advantageously, the exhaust gas transfer line 39 is configured to transfer the exhaust gas 15 from the postcombustor 36 to the bag filter 38, bypassing the preheater 60. In this way, the exhaust gas transfer line 39 acts as a safety bypass to ensure the operability of the apparatus 100 even in the event of failure or maintenance of the preheater 60.

[0136] In practice, the exhaust gas transfer line 39 can be activated automatically in the event of malfunction of the postcombustor 36 and / or the preheater 60.

[0137] In simpler variants, the exhaust gas transfer line 39 is derived from (or corresponds to) a conveyor line through which the heaviest / thickest part of the secondary waste powders 32 can pass from the postcombustor 36 to the bag filter 38.

[0138] In particular, the exhaust gas transfer line 39 can connect the postcombustor 36 directly to the radiating duct 37, as shown, for example, in Figure 4. For example, in Figure 4, the radiating duct 37 can convey the “clean” exhaust gases exiting the postcombustor 36 to the bag filter 38. However, other embodiments are also conceivable in which the exhaust gas transfer line 39 connects the postcombustor 36 directly to the bag filter 38.

[0139] In practice, the exhaust gases exiting the postcombustor 36 can therefore alternatively, in a controlled manner, flow to the bag filter 38 (either directly or via the radiating duct 37), or feed the preheater 60.

[0140] In more technologically advanced variants, the exhaust gas transfer line 39 can be implemented independently of said conveyance line.

[0141] Advantageously, the radiating duct 37 can be functionally connected to an exhaust gas cooling line (not shown) configured to cool with water (all) the exhaust gases 15 passing through the radiating duct 37.

[0142] In the preferred and shown embodiments, said water enters the radiating duct 37 as an inlet flow CWm, and exits the same radiating duct 37 as an outlet flow CWout.

[0143] Obviously, the radiating duct 37 can be functionally connected to (or comprise) a motorized suction fan adapted to cool and move the exhaust gases 15. This latter motorized suction fan may also be part of the suction system of the apparatus 100.

[0144] The recovery system 16 for non-volatile slag is functionally connected to the melting system 12 (in particular, to the melting furnace 28), and is configured to produce granulated slag 55 from (at least part of) the inorganic slag obtained from the melting of the powder mixture 13 (i.e., from at least part of the lava 49).

[0145] In particular, the recovery system 16 for non-volatile slag comprises a granulation apparatus 17.

[0146] In the preferred and shown embodiments, the granulation apparatus 17 comprises, in sequence: a loading device 56, a treatment tank 57, a discharge device 58, and a storage device 59.

[0147] The loading device 56 is configured to receive at least part (and, preferably, all) of the lava 49 coming from the melting furnace 28, for example via a ladle.

[0148] Moreover, the loading device 56 is configured to load the lava 49 (still in a molten state) into the treatment tank 57.

[0149] The treatment tank 57 is configured to treat the lava 49 with a suitable liquid in order to obtain the granulated slag 55.

[0150] The discharge device 58 is configured to extract the granulated slag 55 from the treatment tank 57 and feed the storage device 59 with the same granulated slag 55.

[0151] Preferably, in particularly advantageous embodiments, the recovery system 16 is part of a system for storing thermal energy obtained from high- temperature solid materials (where these high-temperature solid materials may be, for example, foundry waste 83 and / or the part of lava 49 that is supplied to the recovery system 16). In this way, it is possible to obtain a so- called “recu-regenerator” (i.e., a recovery and regeneration system) that allows for increased circularity and, therefore, optimization of both resource consumption and energy efficiency of the apparatus 100.

[0152] In some preferred embodiments, the apparatus 100 may further comprise a fiberizing system 18, configured to convert at least part of the inorganic slag 49 into mineral wool 53.

[0153] The fiberizing system 18 is functionally connected to the melting system 12 (in particular to the melting furnace 28), preferably via a ladle.

[0154] Advantageously, the fiberizing system 18 is arranged downstream of the melting system 12, so that the inorganic slag 49 flowing out of the latter, poured into a suitable ladle, can be treated while still molten by the fiberizing system 18.

[0155] In embodiments in which both the recovery system 16 and the fiberizing system 18 are to have, the ladle of the recovery system 16 is preferably arranged downstream with respect to that used by the fiberizing system 18.

[0156] In a preferred embodiment, the fiberizing system 18 comprises, in sequence: a feed tank 50, a rotary disc fiberizer 54, a cutting apparatus 51, and a winding unit 52. The operation of the preheater 60 - i.e., the method for heating the powder mixture 13 using the preheater 60 - is clear and evident from the description.

[0157] Essentially, the method comprises recovering heat from the exhaust gases 15 that are generated during the production of steel (in particular: ferroalloy) 82 and / or during the treatment of waste (in particular: lava 49 and / or foundry waste 83) resulting from the production of steel 82.

[0158] In particular, the powder mixture 13 can be supplied to the preheater 60 via the mixer 25. Moreover, the exhaust gases 15 can be supplied to the preheater 60 via said suction system of the apparatus 100.

[0159] In practice, the melting system 12 of the apparatus 100 produces the powder mixture 13, which is supplied to the melting furnace 28 via the preheater 60. Subsequently, the melting system 12 melts the powder mixture 13 (in particular, in the melting furnace 28) in order to produce steel 82. The production of steel 82 entails the generation of exhaust gases 15, either “directly” (i.e., in the melting furnace 28) and / or “indirectly” (i.e., during the treatment of waste resulting from the production of steel 82). The exhaust gases 15 are then sent to the preheater 60, where the heat exchange chamber 69 heats the mixture of powders 15 with the heat of the exhaust gases 15.

[0160] In some preferred embodiments, the method according to the invention further comprises one or more of the following steps a-c: a) naturally agglomerating the powder mixture 13 with (a suitable amount of) water to obtain the agglomerate; b) drying and preheating the agglomerate using the preheater 60; c) melting the dried and preheated agglomerate using the melting furnace 28.

[0161] In this way, it is possible to feed the preheater 60 with the exhaust gases 15 generated by the melting of the dried and preheated agglomerate, so as to obtain heat exchange between the exhaust gases 15 and the agglomerate.

[0162] In practice, it has been found that the invention fully fulfills the preset aim and purposes. In particular, it has been seen that the heating device, the system, and the method thus conceived make it possible to overcome the qualitative limitations of the background art, since they make it possible to obtain better effects with respect to those obtainable with known solutions and / or similar effects at lower cost and with higher performance.

[0163] For example, using the preheating device according to the suboptimal embodiment, the inventors have demonstrated that, when:

[0164] - the preheater 60 contains the powder mixture 13 in an amount of 1.8 m3;

[0165] - the average thermal conductivity of the material to be heated (i.e., the average thermal conductivity of the powder mixture 13) is 1.13 W / m K;

[0166] - the above average thermal conductivity value was obtained assuming that:

[0167] - (A) the thermal conductivity of the material to be heated is equal to that of silica sand, (B) the thermal conductivity of air has the typical value expected at room temperature, and (C) there is a 20% percentage of voids in powder mixture 13;

[0168] - an exchange surface between powder mixture 13 and preheater 60 is equal to 6.9 m2;

[0169] - the powder mixture 13 is kept rotating inside the preheater 60 at a rotation speed of 2 revolutions per minute for a residence time of 35 minutes (thus obtaining a total exchange surface area of 1,580 m2);

[0170] - the heat exchange chamber 69 is fed with an exhaust gas flow rate of 6450 Nm3 / h; and

[0171] - the exhaust gases 15 entering the heat exchange chamber 69 have a temperature of approximately 600 °C; the energy absorbed per ton of material is 508 MJ per ton. Consequently, when the negative contribution of an estimated 30% fraction of non-heatable material is also considered (material larger than 0.6 mm that cannot reach the required temperature due to its low conductivity), this absorbed energy allows the powder mixture 13 to be heated so that, at the outlet of the preheater 60, the temperature of the same powder mixture 13 is approximately 450 °C. In particular, the hot exhaust gases 15 generated by the melting furnace 28 can flow in countercurrent between the drum 64 and the jacket 62, so as to heat the drum 64 to a temperature comprised between 450 °C and 550 °C.

[0172] An advantage of the preheating device, the apparatus, and the method according to the present invention is that they allow the resource consumption, cost-effectiveness, and energy efficiency of a steel mill to be optimized.

[0173] Another advantage of the preheating device, apparatus, and method according to the invention is that they allow for the economic valorization of waste powders (for example, EAFD) and exhaust gases generated during steel production and / or during the treatment of waste from steel production, using these same exhaust gases to preheat said waste powders.

[0174] Moreover, the preheating device, apparatus, and method according to the invention promote the optimization of a circular economy within a steel mill.

[0175] A further advantage of the preheating device, apparatus, and method according to the present invention resides in that they allow to produce limited emissions into the atmosphere, which are undoubtedly within the limits of current environmental regulations.

[0176] Furthermore, an advantage of the preheating device, apparatus, and method according to the invention resides in that they allow to reduce the energy consumption (in particular, the electricity consumption) of a steel mill.

[0177] Moreover, the apparatus according to the invention eliminates the need for briquetting or pelletizing machines, thus reducing operating costs and the complexity of processes such as, for example, a steelmaking process and / or metal recovery.

[0178] In practice, the preheating device, apparatus, and method according to the invention can be applied industrially in a systematic manner within a steel mill and / or metal recovery center, thereby reducing current operating costs and generating high value-added byproducts.

[0179] Moreover, the high temperature and (substantial) absence of humidity of the powder mixture exiting the preheater produces the following benefits:

[0180] - increased productivity of the apparatus (in particular, of the melting furnace);

[0181] - significant reduction of the energy consumption of the apparatus (in particular, thanks to the use of the heating power of the melting furnace to preheat the powder mixture);

[0182] - lower particulate emissions, thus obtaining cleaner gases coming out of the exhaust flue;

[0183] - in the case of steel mill dust, obtaining a byproduct with a high concentration of zinc oxide (e.g., Waelz oxide);

[0184] - production of inert slag that can be used as a byproduct for cement factories, road beds, or raw material for the production of mineral wool.

[0185] Although the preheating device, apparatus, and method according to the invention have been conceived in particular to heat waste powders generated during steel production and / or during the treatment of waste from steel production using electric arc furnaces (i.e., powders known as EAFD), they can in any case be used more generally for the treatment of waste powders produced by any type of melting furnace for steel production.

[0186] Moreover, the preheating device, apparatus, and method according to the invention could also treat other steel mill production waste (for example, rolling scale and sweepings) that is problematic since it is difficult to process.

[0187] The invention thus conceived is susceptible of numerous modifications and variations, all of which are within the scope of the accompanying claims. For example, the spiral member of the preheater may be replaced by a plurality of guides arranged inside the drum.

[0188] Unless otherwise indicated, the various embodiments described above may be combined to provide additional and / or alternative embodiments.

[0189] Moreover, all the details may be replaced with other technically equivalent elements.

[0190] In practice, the materials used, so long as they are compatible with the specific use, as well as the dimensions and contingent shapes, may be any according to the requirements and the state of the art.

[0191] To conclude, the scope of the protection of the claims must not be limited by the illustrations or preferred embodiments shown in the description by way of example, but rather the claims must comprise all the characteristics of patentable novelty that reside in the present invention, including all the characteristics that would be treated as equivalents by the person skilled in the art.

[0192] The disclosures in Italian Patent Application No. 102024000022515, from which this application claims priority, are incorporated herein by reference.

[0193] Where technical features mentioned in any claim are followed by reference signs, those reference signs have been included for the sole purpose of increasing the intelligibility of the claims and accordingly such reference signs do not have any limiting effect on the interpretation of each element identified by way of example by such reference signs.

Claims

CLAIMS1. A preheating device (60) for heating a mixture of powders (13) during the production of steel (82) and / or during the treatment of scrap (49, 83) resulting from steel production (82), said preheating device (60) comprising a heat exchange chamber (69) configured to: receive exhaust gases (15) resulting from said steel production (82) and / or said treatment of said scrap (49, 83), said preheating device (60) being configured to receive said exhaust gases (15) from a melting furnace (28); and transmit said heat from said exhaust gases (15) to said mixture of powders (13), so as to heat said mixture of powders (13) by recovering said heat from said exhaust gases (15).

2. The preheating device (60) according to claim 1, comprising: a main body (62), which comprises a first intake port (65) and a first discharge port (67), said first intake port (65) being configured to feed said exhaust gases (15) into said preheating device (60), and said first discharge port (67) being configured to discharge said exhaust gases (15) downstream of said preheating device (60); and a drum (64), which comprises a second intake port (61) and a second discharge port (63), said second intake port (61) being configured to feed said mixture of powders (13) into said preheating device (60), and said second discharge port (63) being configured to discharge said mixture of powders (13) downstream of said preheating device (60); wherein said main body (62) and said drum (64) collectively form said heat exchange chamber (69).

3. The preheating device (60) according to claim 2, wherein said drum (64) is configured to rotate with respect to the main body (62).

4. The preheating device (60) according to any one of the preceding claims, further comprising a screw-like member (68) configured to move said mixture of powders (13) along said drum (64) toward said seconddischarge port (63).

5. The preheating device (60) according to claim 4, wherein said screw-like member (68) is a spiral ridge fixed to an internal surface (64E) of said drum (64).

6. The preheating device (60) according to any one of claims 2 to 5, wherein said drum (64) has an inside diameter (D) of 1.2 m and an exchange length (L69) of 6 m.

7. An apparatus (100) for treating waste powders (19) during the production of steel (82) and / or during the treatment of scrap (49, 83) resulting from steel production (82), said apparatus (100) comprising the preheating device (60) according to any one of the preceding claims.

8. The apparatus (100) according to claim 7, further comprising: a mixer (25), configured to generate the mixture of powders (13) for said preheating device (60); and said melting furnace (28), configured to generate said exhaust gases (15) for said preheating device (60) by melting said mixture of powders (13); wherein said preheating device (60) is configured to:- receive said mixture of powders (13), and- heat said mixture of powders (13) with said exhaust gases (15).

9. The apparatus (100) according to claim 8, further comprising a collection hopper (27) configured to feed said melting furnace (28) with said mixture of powders (13), said preheating device (60) being configured to discharge said mixture of powders (13) into said collection hopper (27).

10. The apparatus (100) according to claim 9, further comprising a powder transfer line (26) configured to transfer said mixture of powders (13) from said mixer (25) toward said melting furnace (28), bypassing said preheating device (60).

11. The apparatus (100) according to any one of claims 8 to 10, further comprising a radiating duct (37) and a bag filter (38), said radiatingduct (37) being configured to:- receive the exhaust gases (15) from said preheating device (60); and- convey said exhaust gases (15) toward said bag filter (38).

12. The apparatus (100) according to any one of claims 9 to 11, further comprising a postcombustion chamber (36) functionally connected to said melting furnace (28), said preheating device (60) being configured to receive said exhaust gases (15) from said melting furnace (28) via said postcombustion chamber (36).

13. The apparatus (100) according to claims 11 and 12, further comprising an exhaust gas transfer line (39) configured to transfer said exhaust gases (15) from said postcombustion chamber (36) toward said bag filter (38) bypassing said preheating device (60).

14. The apparatus (100) according to any one of claims 9 to 13, wherein: said preheating device (60) is configured to provide a temperature difference between said exhaust gases (15) in input to said preheating device (60) and said mixture of powders (13) in input to said preheating device (60); and said temperature difference is comprised between 550 °C and 650 °C.

15. A method for heating a mixture of powders (13) by using said preheating device (60) according to any of the preceding claims, said mixture of powders (13) comprising waste powders (19) generated during the production of steel (82) and / or during the treatment of scrap (49, 83) resulting from said production of said steel (82), said method comprising a step of recovering heat from exhaust gases (15) generated during said production of said steel (82) and / or during said treatment of said scrap (49, 83).

16. The method according to claim 15, further comprising the steps of: a) naturally agglomerating said powder mixture (13) with water, so asto obtain an agglomerate; b) drying and preheating said agglomerate, by means of said preheating device (60); c) melting said dried and preheated agglomerate, by means of said melting furnace (28).

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