Plant and process for the production of secondary zinc

Abstract

The plant (1) for the production of secondary zinc comprises: at least one melting assembly (2) adapted to contain a molten zinc bath (B) and to heat and melt metal residues in the form of shredded scrap and / or grain, the metal residues being composed of metallic zinc; at least one feeding assembly (3) adapted to supply the metal residues to the melting assembly (2); at least one mixing and separation assembly (4) connected to the feeding assembly (3) and associated with the melting assembly (2) and adapted to receive the metal residues, mixing them in the molten zinc bath and to move the process scraps away from the melting assembly (2); at least one pick-up assembly (5) adapted to transfer molten zinc from the melting assembly (2) to a machine for forming secondary solid zinc.

Description

[0001] PLANT AND PROCESS FOR THE PRODUCTION OF SECONDARY ZINC

[0002] Technical Field

[0003] The present invention relates to a plant and a process for the production of secondary zinc.

[0004] Background Art

[0005] In the metallurgical sector, the recovery of metals from previous machining residues and metal waste is well known.

[0006] In particular, zinc, thanks to its chemical and physical characteristics, is widely used for coating metal structures and components in order to prevent corrosion. This machining process produces a waste product called “zinc skimmings”, which is a powdery material composed of a finer part and a coarser part called “zinc grains”.

[0007] Zinc is also used in many European countries to make metal sheets for roofing buildings and to produce gutters and other accessories. When these roofs are dismantled after several years of wear and tear due to weathering, they are sent to special disposal plants where they are recovered to produce second-melted zinc called “secondary zinc” with a minimum zinc content of 98.5%.

[0008] The plants of known type comprise a melting furnace which is maintained at a temperature sufficient to allow the zinc to melt, a feeding system adapted to supply the metal residues (zinc grains and scraps) to the melting furnace, and a pick-up system through which the molten zinc is picked from the tank and fed to a forming machine called an “ingot maker” for the purpose of creating solid zinc ingots.

[0009] The metal residues are then loaded within the melting furnace and dipped in a bath of molten zinc. In detail, the sheets are generally compacted so as to reduce the volume thereof and to form compacted packs. The grains, on the other hand, are fed through hoppers.

[0010] Inside the melting furnace, the metal residues are brought to the melting point and the liquid zinc obtained is conveyed to the forming machine.

[0011] The plants of known type for the production of secondary zinc are however susceptible to improvements to obtain better melting yields and higher purities. In fact, although zinc is a low-melting metal, residues of other metals contained in the scrap can also be partly melted or eroded and / or corroded in the melting tank, contaminating the molten zinc and affecting its chemical analysis.

[0012] Furthermore, the plants of known type require very long machining times due to the melting times of the sheet metal packs, which consequently lead to suboptimal production yields.

[0013] Description of the Invention

[0014] The main aim of the present invention is to devise a plant and a process for the production of secondary zinc which allow a finished product with a high degree of purity to be obtained.

[0015] Another object of the present invention is to devise a plant and a process for the production of secondary zinc which allow for a reduction in machining times, improved operating conditions for operators and optimized production yields, significantly reducing slag production and consequently increasing the quantity of zinc produced.

[0016] Another object of the present invention is to devise a plant and a process for the production of secondary zinc which allow the aforementioned drawbacks of the prior art to be overcome in a simple, rational, easy and effective to use as well as inexpensive solution.

[0017] The aforementioned objects are achieved by this plant for the production of secondary zinc having the characteristics of claim 1.

[0018] The aforementioned objects are further achieved by this process for the production of secondary zinc having the characteristics of claim 13.

[0019] Brief Description of the Drawings

[0020] Other characteristics and advantages of the present invention will be more evident from the description of a preferred, but not exclusive, embodiment of a plant and a process for the production of secondary zinc, illustrated by way of an indicative, yet non-limiting example, in the accompanying drawings, in which:

[0021] Figure 1 is a side view of the plant according to the invention;

[0022] Figure 2 is a plan view of the plant according to the invention; Figure 3 is a lateral cross-sectional view of the plant according to the invention. Embodiments of the invention

[0023] With particular reference to these figures, reference numeral 1 globally denotes a plant for the production of secondary zinc.

[0024] The plant 1 according to the invention comprises at least one melting assembly 2 adapted to contain a molten zinc bath B and to heat and melt metal residues in the form of shredded scrap and / or grains, which are composed of metallic zinc.

[0025] In this context, the term “metal residues in the form of shredded scrap and / or grains” refers to metal waste mainly consisting of metallic zinc and undergoing a recovery process in order to obtain purified metallic zinc, also known as secondary zinc. In particular, residues in the form of grains come from zinc coating processes on metal structures and components. Residues in the form of shredded scrap, on the other hand, come from metal sheets used for roofing buildings and / or for the production of gutters, which is then shredded into thin strips approximately 15 cm wide.

[0026] The plant 1 also comprises: at least one feeding assembly 3 adapted to supply the metal residues to the melting assembly 2; at least one mixing and separation assembly 4 connected to the feeding assembly 3 and associated with the melting assembly 2 and adapted to receive the metal residues, mixing them in the molten zinc bath and to move the process scraps away from the melting assembly 2; at least one pick-up assembly 5 adapted to transfer molten zinc from the melting assembly 2 to a machine for forming secondary solid zinc.

[0027] In actual facts, by means of this plant 1, after being introduced into the melting assembly 2, the metal residues are moved within the molten zinc bath B and the metallic zinc contained therein melts in turn. In detail, the movement of the metal residues in the molten zinc promotes and speeds up the melting of the zinc contained therein.

[0028] In addition, the mixing and separation assembly 4 determines the forward movement of the process scraps, i.e., what remains of the metal residues after the zinc has melted and moves it away from the melting assembly 2.

[0029] In this way, the molten zinc bath B is free of components which could cause contamination and is substantially composed of pure zinc.

[0030] Specifically, the secondary zinc obtained by means of the plant 1 according to the invention has a purity of more than 99%, preferably 99.5%.

[0031] In addition, this plant 1 allows the machining and relevant extraction of zinc from the metal residues to be optimized and, consequently, process scraps to be reduced. In this case, the process scraps (ash) resulting from the machining of metal residues using this plant 1 contains less than 50% zinc (metal).

[0032] The plant 1 is described in more detail below.

[0033] The melting assembly 2 comprises a partly underground melting tank 6 having depth of, e.g., 2 meters, which is adapted to contain the molten zinc bath B.

[0034] The melting assembly also comprises heating means, not shown in detail in the figures, associated with the melting tank 6 and adapted to heat the latter and to maintain the contents thereof at a substantially constant temperature.

[0035] In detail, when in use, the melting tank 6 constantly contains a predefined amount of molten zinc, maintained at the respective melting temperature, which allows the melting of metal residues. In actual facts, the heating means are adapted to indirectly heat the metal residues, which are instead heated by the molten metal itself, in a sort of water bath.

[0036] When a predefined amount of liquid is achieved in the melting tank 6, the pickup assembly 5 causes the molten zinc to flow out of the tank itself.

[0037] The pick-up assembly 5 comprises an outlet duct 7 connected to the melting tank 6 in a fluid-operated maimer and connectable to the forming machine.

[0038] In particular, the forming machine, not shown in detail in the figures, comprises a plurality of molds into which the molten zinc is poured as it cools and solidifies, thus forming solid zinc ingots.

[0039] The pick-up assembly 5 also comprises a valve device associated with the outlet duct 7 and adapted to adjust the flow of molten zinc leaving the melting tank 6. In particular, the valve device is adjusted so that the level of molten zinc within the melting tank 6 is always above a certain limit, so as to ensure the heating of the new metal residues to be recovered.

[0040] The feeding assembly 3 comprises at least one hopper 8a, 8b adapted to contain the metal residues and at least one transfer device 9, of the type e.g. of a paddle conveyor belt, adapted to transfer the metal residues from the hopper 8a, 8b to the melting tank 6.

[0041] In accordance with a preferred embodiment shown in the figures, the feeding assembly 3 comprises a first hopper 8a adapted to contain the metal residues in the form of grains and a second hopper 8b adapted to contain the metal residues in the form of strips.

[0042] The hoppers 8a, 8b are activated to release their respective contents onto the transfer device 9 depending on a predefined ratio between them.

[0043] It cannot, however, be ruled out that the feeding assembly 3 may be of a different type.

[0044] Thanks to the presence of the mixing and separation assembly 4, which allows speeding up the melting of the zinc and removing process scraps, the feeding assembly 3 is configured to feed the melting tank 6 in a substantially continuous maimer.

[0045] Usefully, the mixing and separation assembly 4 comprises at least one movement unit 10 configured to move the metal residues in the melting assembly 2 along a direction of forward movement A.

[0046] The movement along the direction of forward movement A allows the metal residues to be mixed with the molten zinc in the zinc bath B and the gradual forward movement of the process scraps.

[0047] The movement unit 10 comprises at least one inlet area 11 for the metal residues and one outlet area 12 for the process scraps which is arranged downstream of the inlet area 11 with respect to the direction of forward movement A.

[0048] The feeding assembly 3 is arranged where the inlet area 11 is located.

[0049] The mixing and separation assembly 4 also comprises at least one removal unit 18 adapted to receive the process scraps from the movement unit 10 and to convey them to the outside of the melting assembly 2.

[0050] The removal unit 18 is arranged where the outlet area 12 is located. The removal unit 18 comprises a movable plane extending towards the outside of the melting tank 6.

[0051] In accordance with the embodiment shown in the figures, the movable plane is of the type of a vibrating plane. It cannot, however, be ruled out that the movable plane may be of a different type.

[0052] Usefully, the movement unit 10 is arranged in the melting tank 6 at a predefined elevation with respect to an upper edge of the melting tank itself and is configured, in use, to be partly dipped in the molten zinc.

[0053] In fact, given the specific weight of the molten zinc, the metal residues, after being introduced into the melting tank 6, tend to float and remain on the free surface of the liquid.

[0054] The movement unit 10 is configured to bring the metal residues into direct contact with the molten zinc and to cause the movement thereof along the direction of forward movement A. During the path, the zinc in the metal residues melts in turn and mixes with the zinc already melted in the melting tank 6, leaving the process waste. The latter, after reaching the outlet area 12, are removed from the melting tank 6 by means of the removal unit 18.

[0055] In accordance with the embodiment shown in the figures, the movement unit 10 comprises an auger body 13 extending along the direction of forward movement A.

[0056] The movement unit 10 also comprises an actuator 14 connected to the auger body 13 and adapted to set the latter in rotation. The rotation of the auger body 13 causes the molten zinc and metal residues to be moved along the direction of forward movement A.

[0057] It cannot, however, be ruled out that the movement unit 10 may be of a different type, e.g. of the paddle conveyor type.

[0058] Specifically, the auger body 13 comprises a shaft 15 rotatable around a respective axis of rotation R substantially parallel to the direction of forward movement A and at least one spiral 16 associated with the shaft 15.

[0059] In accordance with the embodiment shown in the figures, the spiral 16 is of the type of a sector spiral. It cannot, however, be ruled out that the spiral 16 may be of the type of a continuous spiral.

[0060] In detail, the removal unit 18 comprises a pair of paddles associated with and opposite the shaft 15 of the auger body 13 and rotatable locked therewith. Specifically, the paddles are arranged on a plane passing through the axis of rotation R and are configured to cause the movement of the process scraps along a direction transverse to the direction of forward movement A, towards the outside of the melting tank 6.

[0061] Advantageously, the auger body 13 comprises at least one mixing fin 17 associated with at least one loop of the spiral 16.

[0062] The mixing fin 17 promotes the mixing of the metal residues in the molten zinc and, specifically, causes them to be temporarily dipped, thus ensuring optimal and substantially complete melting of the zinc contained therein.

[0063] In particular, the mixing fin 17 is associated with the outer edge of the spiral 16. In accordance with a possible embodiment, the mixing fin 17 is arranged transversely with respect to the direction of forward movement A. In detail, the mixing fin 17 is folded towards the shaft 15.

[0064] Preferably, the auger body 13 comprises at least one pair of mixing fins 17 associated with at least one loop of the spiral 16.

[0065] Specifically, the auger body 13 comprises a plurality of mixing fins 17 associated with respective loops of the spiral 16.

[0066] It is easy to appreciate that this plant 1 can also be used for machining metals other than zinc.

[0067] According to a further aspect, the present invention also relates to a process for the production of secondary zinc.

[0068] The process according to the invention comprises at least the following phases: supplying a molten zinc bath B; supplying metal residues in the form of shredded scraps and / or grain, the metal residues being composed of metallic zinc; feeding the metal residues in the molten zinc bath B; mixing the metal residues in the molten zinc bath B and removal of the process scraps from the molten zinc bath B, resulting from the melting of the zinc contained in the metal residues; picking molten zinc from the molten zinc bath B to obtain secondary solid zinc. The process can be implemented using the plant 1 according to the invention.

[0069] The mixing phase allows speeding up and optimizing the melting of the metallic zinc contained in the metal residues. Thanks to this, the feeding phase can take place substantially on a continuous basis.

[0070] The mixing and removal phase allows high-purity secondary zinc to be obtained, even up to 99.5%, and process scraps to be reduced.

[0071] It has in practice been ascertained that the described invention achieves the intended objects and, in particular, it should be emphasized that the plant and process according to the invention for the production of secondary zinc allow a finished product with high purity to be obtained. Furthermore, the plant and process according to the invention make it possible to reduce machining times and to achieve high production yields.

Claims

CLAIMS1) Plant (1) for the production of secondary zinc, characterized by the fact that it comprises: at least one melting assembly (2) adapted to contain a molten zinc bath (B) and to heat and melt metal residues in the form of shredded scrap and / or grain, said metal residues being composed of metallic zinc; at least one feeding assembly (3) adapted to supply said metal residues to said melting assembly (2); at least one mixing and separation assembly (4) connected to said feeding assembly (3) and associated with said melting assembly (2) and adapted to receive said metal residues, mixing them in said molten zinc bath and to move the process scraps away from said melting assembly (2); at least one pick-up assembly (5) adapted to transfer molten zinc from said melting assembly (2) to a machine for forming secondary solid zinc.2) Plant (1) according to claim 1, characterized by the fact that said mixing and separation assembly (4) comprises at least one movement unit (10) configured to move said metal residues in said melting assembly (2) along a direction of forward movement (A).3) Plant (1) according to one or more of the preceding claims, characterized by the fact that said movement unit (10) is provided with at least one inlet area (11) for said metal residues and at least one outlet area (12) for said process scraps, arranged downstream of said inlet area (11) with respect to said direction of forward movement (A), said feeding assembly (3) being arranged where said inlet area (11) is located.4) Plant (1) according to one or more of the preceding claims, characterized by the fact that said mixing and separation assembly (4) comprises at least one removal unit (18) adapted to receive said process scraps from said movement unit (10) and to convey them to the outside of said melting assembly (2), said removal unit (18) being arranged where said outlet area (12) is located.5) Plant (1) according to one or more of the preceding claims, characterized by the fact that said melting assembly (2) comprises at least one melting tank (6)adapted to contain said molten zinc bath (B) and by the fact that said movement unit (10) is arranged in said melting tank (6) at a predefined elevation with respect to an upper edge of said melting tank (6) and is configured, in use, to be partly dipped in said molten zinc bath (B).6) Plant (1) according to one or more of the preceding claims, characterized by the fact that said movement unit (10) comprises at least one auger body (13) extending along said direction of forward movement (A).7) Plant (1) according to one or more of the preceding claims, characterized by the fact that said auger body (13) comprises a shaft (15) rotatable around a respective axis of rotation (R) substantially parallel to said direction of forward movement (A) and at least one spiral (16) associated with said shaft (15).8) Plant (1) according to one or more of the preceding claims, characterized by the fact that said spiral (16) is of the type of a sector spiral.9) Plant (1) according to one or more of the preceding claims, characterized by the fact that said auger body (13) comprises at least one mixing fin (17) associated with at least one loop of said spiral (16).10) Plant (1) according to one or more of the preceding claims, characterized by the fact that said mixing fin (17) is associated with the outer edge of said spiral(16).11) Plant (1) according to one or more of the preceding claims, characterized by the fact that said mixing fin (17) is arranged transversely to said direction of forward movement (A).12) Plant (1) according to one or more of the preceding claims, characterized by the fact that said auger body (13) comprises at least one pair of mixing fins(17) associated with at least one loop of said spiral (16).13) Process for the production of secondary zinc, characterized by the fact that it comprises at least the following phases: supplying a molten zinc bath (B); supplying metal residues in the form of shredded scrap and / or grain, said metal residues being composed of metallic zinc; feeding said metal residues in said molten zinc bath (B);mixing said metal residues in said molten zinc bath (B) and removal of the process scraps from said molten zinc bath (B), resulting from the melting of zinc contained in said metal residues; picking molten zinc from said molten zinc bath (B) to obtain secondary solid zinc.

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