WO2025233369A1

WO2025233369A1 - Melting furnace for aluminium and its alloys

Info

Publication number: WO2025233369A1
Application number: PCT/EP2025/062408
Authority: WO
Inventors: Marco CHECCACCI
Original assignee: Individual
Current assignee: Individual
Priority date: 2024-05-10
Filing date: 2025-05-06
Publication date: 2025-11-13
Anticipated expiration: 2026-11-10

Abstract

This melting furnace comprises a first stage (10) which comprises a vertically-developing melting shaft (11) and a second stage (20) which comprises a horizontally-developing melting basin (21), placed below the melting shaft (11) in such a position to receive the aluminium by falling from the melting shaft (11). The first stage (10) takes on the structure of a tower furnace but has no heating members, while the second stage (20) takes on the structure of a basin furnace and is provided with heating members capable also to send hot gas towards the first stage of the furnace. The result is a very versatile furnace.

Description

Melting furnace for aluminium and its alloys

DESCRIPTION

Technical Field

The present invention concerns a furnace for melting aluminium and aluminium alloys. In the following, any reference to aluminium must be understood as a reference not only to the pure Al metal, but also - and indeed above all - to a metal alloy based on aluminium; in the following, also the generic terms "material" and "metal" must be understood as referring to aluminium or an alloy thereof, possibly also comprising impurities or foreign substances, both in the solid state and in the liquid state.

Background Art

Aluminium melting has been a metallurgical process known and practiced for a long time, to make available aluminium in the molten state, to then be used in casting plants of various kind to obtain the desired pieces. As a result, various processes and plants have been developed over the years that allow aluminium to be melted. It should be noted that in aluminium melting, two temperatures are normally taken into consideration: a relatively low melting temperature at which the metal passes from the solid state to the liquid state (generally comprised between 550 and 700 °C, depending on the type of alloy) and a relatively high casting temperature (generally 150-200 °C higher) at which the liquid metal reaches the fluidity suitable for use in casting plants.

The perhaps most traditional melting system still in use is the one including the so-called crucible furnaces. In such a system, a crucible of refractory material (e.g. silicon carbide) is placed in a heating chamber. The solid material is loaded into the crucible, which is melted (by bringing it up to the casting temperature) thanks to the high temperatures that are obtained by heating the crucible with suitable means, either electric or combustion. This system is not optimal from a metallurgical point of view, because of the non-constant temperatures in the crucible; moreover, its productivity is low, the management is complex and potentially dangerous, and energy costs are high. However, this system is still widely used, as it requires a low initial investment.

An example of a crucible furnace is disclosed in US 2009/0130619 Al.

A particular case of crucible furnaces is represented by the induction furnaces, in which the heating of the metal is operated electromagnetically, using the same aluminium charge as a secondary coil. However, their success is greatly penalised by the high energy and investment costs, the management complexity and the modest quantities of metal they allow to melt.

There are then the basin furnaces, where the solid metal is loaded into a large tank where there is already present a bath of liquid aluminium at the casting temperature. The tank is inserted in a chamber that is heated by a burner system. This system allows to have available large quantities of molten metal, but it is not without issues. In particular, the metallurgical quality is low, because the molten metal is often oxidized and with a high gas content, because of the non-constant temperature. The only way to have an acceptable quality (without having to intervene downstream with complex and expensive treatments) is to have a really very large basin compared to the required melting productivity, so as to reduce the temperature fluctuations induced at each entry of solid material into the molten metal bath. In addition, cleaning is problematic and maintenance costs are high. Finally, the energy cost is high and so is the melting loss, i.e. the loss of material compared to the theoretical quantity, due to the burnt material. It is therefore a system that is now very little used.

A particular case of basin furnaces is represented by the dry-sole furnaces, in which the solid material is introduced into the basin by first passing through a slide, also heated. The dirt that inevitably accompanies the solid material therefore does not end up directly in the molten metal bath, but deposits on the slide, improving the quality of the product a little. The other defects of the basin furnaces remain, though.

The most modern and popular furnaces are currently the so-called tower or shaft furnaces. These furnaces have a funnel-shaped, vertically-developing melting shaft in which the solid material is introduced from the top; a controlled flow of hot gas, generated by a combustion system provided in the lowest portion of the vertical melting shaft, hits the charge of solid material from the bottom upwards, heating it up to the melting temperature and then causing it to melt. Melting, therefore, takes place, so to speak, dry: in fact, the metal - as soon as it melts - slides downwards into the funnel-shaped melting shaft and arrives on an inclined bottom, from which it is conveyed towards a holding chamber, flanked to (or located below) the melting chamber, located at a lower height. In the holding chamber, one or more burners are provided that heat the holding chamber itself, so as to bring the molten metal to the casting temperature, thus making it suitable for use in the casting systems provided downstream. The molten metal is withdrawn from the holding chamber in various ways, depending on the intended use.

Examples of tower furnaces are disclosed in US 4664702 A or EP 0400214 Al.

The tower furnaces reduce both energy consumptions and melting losses, but increase plant costs. In addition, they cannot easily be adapted to treat solid material coming from the recycling of aluminium objects, due to the high quantity of fine or very fine particle sized material, which in the vertical melting shaft easily tends to oxidize and generate a considerable quantity of slag. Disclosure of Invention

Aim of the present invention is to make available a new melting furnace for melting aluminium that allows to overcome at least in part the drawbacks of the aforementioned known furnaces.

Such a furnace is defined in claim 1; preferred characteristics thereof are reported in the dependent claims.

More specifically, this melting furnace for aluminium and its alloys comprises: a vertically-developing melting shaft adapted to be fed with aluminium in the solid state, heating members capable of generating a flow of hot gas that rises in the melting shaft, a hood above the melting shaft, provided with an upper opening for the discharge of hot gas from the melting shaft, characterized in that it further comprises a horizontally-developing melting basin, placed below the melting shaft in such a position to receive the molten aluminium by falling from the melting shaft and retain it, wherein the heating members are provided out of the melting shaft, in the melting basin.

This furnace somehow combines the structure of a tower furnace with the structure of a basin furnace. The result is a very versatile furnace. The loaded solid material fills the melting shaft completely. Since the heating members are provided in the melting basin and not in the melting shaft, the latter results to be less warm than in a traditional tower furnace; therefore, a part of the solid aluminium will melt like in a traditional tower furnace, but a part thereof (which is mainly the finest fraction, that tends to fall more easily and thus to cross more quickly the melting shaft) will arrive at the melting basin while still in the solid state and will melt only there, like in a traditional basin furnace, by immersion in the already molten metal, thus undergoing less oxidation. The heating members are sized and adjusted so that they can both generate the flow of hot gas suitable for partially melting the aluminium in the melting shaft, and keeping the aluminium in the molten state in the melting basin.

Preferably, the heating members are adapted to keep the molten aluminium present in the melting basin at a melting temperature. The melting temperature depends on the specific material that is processed; for most aluminium alloys, the melting temperature is between 550 and 700 °C.

Preferably, the heating members comprise gas burners placed in an upper vault of the melting basin. This type of heating member is particularly suitable for generating the flow of hot gas necessary for melting the solid material in the melting shaft.

Preferably, the melting furnace also comprises a holding basin, flanked to the melting basin and in communication with the melting basin through a window, and further heating members adapted to heat and keep the molten aluminium contained in the holding basin at a casting temperature, wherein the casting temperature is higher than the melting temperature. The casting temperature depends on the specific material that is processed; for most aluminium alloys, the casting temperature is higher than the melting temperature by 150-200 °C.

The holding basin provided with the further heating members improves the efficiency of the furnace. In fact, the heat generated by the further heating members remains confined in the holding basin alone and allows the molten material present therein to be heated up to the casting temperature; the heat produced is not instead dispersed in the melting shaft, where it is not necessary to raise the temperature beyond the melting temperature, which as mentioned is lower than the casting temperature.

Preferably, the further heating members comprise electric heaters placed at an upper vault of the holding basin. Since it is not necessary or wanted to generate a flow of hot gases (as is the case in the melting basin), it is advantageous to use electric heaters, which have a significantly lower environmental impact.

Preferably, the window is formed in a septum between the melting basin and the holding basin and extends vertically both downwards and upwards: downwards, up to a bottom of the holding basin and up to a bottom of the melting basin, upwards, up to a height lower than the upper vault of the holding basin and the upper vault of the melting basin, so that during operation of the melting furnace the window can be submerged, completely below the level of the molten aluminium contained in the melting basin and in the holding basin. In this way, the window behaves like a siphon between the two basins; these basins, which form two communicating vessels containing the same liquid aluminium, are thus separated from each other by the siphon and therefore the heat dispersed from the holding basin (at casting temperature) towards the melting basin (at melting temperature, lower than the casting temperature) is reduced.

Preferably, in the presence of the submerged window forming a siphon, the furnace comprises a tubular duct for the exit of the molten aluminium from the holding basin, extended from a first end inside the holding basin located near the bottom thereof to a second end outside the holding basin located higher than the first end, and a pneumatic system for generating an increase in pressure in the holding basin, such as to push the molten aluminium into the tubular duct until it exits from the second end thereof. Thanks to this, the withdrawal of molten aluminium can be managed with the pneumatic system, easily and safely for operators.

It should also be noted that an increase in the pressure in the holding basin at the time of withdrawing a quantity of molten aluminium also causes a rise in the level of the molten metal in the upstream melting basin; therefore, during operation of the melting furnace, the periodic rise in the level of the liquid aluminium in the melting basin favours the wetting of the finer fractions of the solid aluminium introduced into the melting shaft, thus hindering its oxidation and improving the quality of the molten aluminium produced.

Preferably, the second end of the tubular duct is provided with a drain spout. The molten aluminium can then be easily withdrawn by a suitable conveyor to take it to the subsequent plant, or even by a simple ladle, brought under the drain spout.

Preferably, the bottom of the holding basin is provided with an emptying valve. Such a valve allows, if necessary, the complete emptying of the holding basin, for example for maintenance and cleaning operations. In addition, such a valve can be used for the normal delivery of molten aluminium, if the tubular duct for the exit of the molten aluminium is not present in the third stage.

Preferably, the melting basin is provided with a first cleaning door, placed on a side wall of the melting basin. This first door facilitates the operations of maintenance and periodic cleaning of the furnace.

Preferably, the holding basin is provided with a second cleaning door, placed on a side wall of the holding basin. This second door also facilitates the operations of maintenance and periodic cleaning of the furnace.

Brief Description of Drawings

Further characteristics and advantages of a melting furnace according to the invention will be more evident from the following description of a preferred embodiment thereof, made with reference to the enclosed drawings. In such drawings: fig. 1 is a schematic sectional plan view of a melting furnace according to the invention, according to plane G-H in fig. 2; fig. 2 is a schematic vertical sectional view of the melting furnace of fig. 1, according to plane A-B in fig. 1; fig. 3 is a schematic vertical sectional view of the melting furnace of fig. 1, according to plane C-D in fig. 1; fig. 4 is a schematic vertical sectional view of the melting furnace of fig. 1, according to plane E-F in fig. 1.

Best Mode for Carrying Out the Invention

The figures show a melting furnace 1 for melting aluminium and aluminium alloys, schematically represented according to different section planes. The furnace 1 is intended to produce molten aluminium starting from input material formed by a charge of solid aluminium, typically supplied in ingots (tendentially of high purity) or recovered scrap, in which in addition to aluminium there are more or less significant quantities of other materials, either metallic or not; the ingots have well-defined sizes and shape, whereas the scrap has extremely variable shapes and sizes, from a few centimetres up to sizes even greater than those of the ingots.

The melting furnace 1 comprises a first stage 10, a second stage 20 and a third stage 30, in which the material is treated in sequence.

The first stage 10 comprises a melting shaft 11 with a substantially vertical development, surmounted by a hood 12 extended upwards up to an opening 13, from which the fumes generated in the melting of the aluminium in the melting shaft 11 exit. Downwards, the melting shaft 11 comprises a bottom zone 14 towards which the material descends while it is being treated. The bottom zone 14 of the melting shaft 11 is tilted, so as to convey the material that reaches it by force of gravity.

The hood 12 is provided with a loading door 18, through which the solid material to be melted is introduced into the melting shaft 11.

The second stage 20 comprises a melting basin 21, which is flanked to the bottom zone 14 of the melting shaft 11 and is open directly on this bottom zone 14, so as to receive by falling (thanks to the force of gravity) both the molten aluminium that is pouring on the bottom zone 14 itself, and the aluminium that is still in the solid state. The melting basin 21 has a lower bottom 22 and an upper vault 23, as well as side walls 24. In the melting shaft 11, no heating members are provided. In the melting basin 21, in particular near its vault 23, heating members 25, preferably gas burners, are provided, which are sized and adjusted so as to generate a flow of hot gas that rises also in the melting shaft 11, hitting and partially melting the aluminium present therein; furthermore, the heating members 25 allow to keep the aluminium poured in the melting basin 21 in the molten state. The temperature of the molten aluminium contained in the melting basin 21 is kept slightly higher than the melting temperature, i.e. it is not raised up to the casting temperature; in figures 2, 3 and 4, the level of the molten aluminium contained in the melting basin 21 is indicated with LI.

The third stage 30 comprises a holding basin 31, flanked to the melting basin 21. The holding basin 31 has a lower bottom 32 and an upper vault 33, as well as side walls 34. The bottom 32 of the holding basin is substantially at the same height as the bottom 22 of the melting basin 21; preferably both bottoms are inclined towards withdrawal points. The holding basin 31 is separated from the melting basin 21 by a septum 35, extended from the bottom 32 up to the vault 33.

A window 36, which puts the melting basin 21 in communication with the holding basin 31, is formed in the septum 35. The window 36 extends downwards up to the bottom 32 and to the bottom 22, while upwards it extends only as far as a height lower than the vault 23 of the melting basin 31, such as to remain submerged below a level L2 reached by the molten metal present in the holding basin 31 during operation of the furnace 1. The window 36 then operates as a siphon and allows the melting basin 21 and the holding basin 31 to form a system of communicating but separate vessels. Further heating members 37 are provided in the holding basin 31, for example and preferably electric heaters placed at the vault 33. These third heating members 37 are sized and adjusted so as to bring and keep the aluminium contained in the holding basin 31 at the casting temperature, higher than the melting temperature at which the molten aluminium is kept in the melting basin 21; the molten aluminium at the casting temperature has the fluidity suitable to be used for the subsequent casting.

The third stage 30 then comprises a tubular duct 41 for the exit of the molten aluminium from the holding basin 31. The tubular duct 41 extends from a first end 42 inside the holding basin 31 and located near the bottom thereof to a second end 43 outside the holding basin 31, located higher than the first end 42. The second end 43 is optionally provided with a drain spout 44.

The third stage 30 further comprises a pneumatic system 45, for example a compressed air intake duct, capable of generating an increase in pressure in the holding basin 31, such as to push the molten aluminium into the tubular duct 41 until it exits from the second end 43. The molten aluminium thus dispensed can be collected for using it, for example directly by means of a casting plant placed in the immediate vicinity of the melting furnace 1, or indirectly by means of a ladle (not shown in the figures) for its transfer where necessary. Depending on how the molten aluminium is withdrawn, the drain spout 44 may or may not be present.

The holding basin 31 is then provided with an emptying valve 46, on the bottom 32. Through this emptying valve 46 it is possible both to completely empty the holding basin 31 (for example, for maintenance, cleaning or melting change operations), and to dispense the molten aluminium if the tubular duct 41 is not present (or cannot be used).

The melting furnace 1 also comprises doors that allow access to its interior with special tools for cleaning operations or (with the furnace stopped) maintenance operations; in the example illustrated, a first cleaning door 28, located on one of the side walls 24 of the melting basin 21, and a second cleaning door 38, located on one of the side walls 34 of the holding basin are provided.

In operation, a charge of material to be melted is introduced into the first stage 10, loading it into the melting shaft 11 through the loading door 18. Thanks to the action of the heating members 25, the aluminium in the melting shaft 11 is hit by the hot gases and starts melting; both the molten fraction and the fraction still in the solid state fall, on the inclined bottom zone 14. The molten aluminium is then collected in the melting basin 21, where it is kept in the molten state - at the melting temperature - by the heating members 25 themselves. Fractions of the material still in the solid state (typically the very fine fractions) can directly reach the melting basin 21 before melting; these material fractions therefore do not melt by direct action of the heating members 25, but by immersion in the molten material present in the melting basin 21. In the melting basin 21, the heating members 25 keep the material in the molten state and contribute to melting (indirectly, through molten aluminium) fractions of material that have reached the melting basin 21 still in the solid state.

The melting furnace 1 can be operated semi-continuously, by inserting a charge of solid material into the melting shaft 11 as soon as the previous charge has been partially melted and a sufficient portion of the melting shaft 11 has then been freed.

Not all the material coming from the melting shaft 11 to the melting basin 21 is already in the molten state; in particular, it is possible that small-sized solid material quickly falls down into the melting shaft 11, before it can be heated up to the melting point. This solid material thus falls into the liquid material contained in the melting basin 21 where it melts by immersion.

Therefore, most of the solid material is melted like in a traditional tower furnace, in the melting shaft 11 thanks to the hot gases that arrive from the heating members 25 that are provided in the melting basin 21; these hot gases in fact reach the melting shaft 11 where they hit the material, partially melting it. The smaller-sized fractions, instead, are melted like in a traditional basin furnace, by immersion in the already melted material present in the melting basin 21. Since the quantity of material that is melted by immersion is relatively small, the temperature of the melted material in the melting basin 21 does not undergo significant variations.

From the melting basin 21, the molten aluminium also passes into the holding basin 31, through the window 36, which thus ensures that - when the pneumatic system 45 is not activated - the level L2 of the molten aluminium in the holding basin 31 is the same as the level LI of the molten aluminium in the melting basin 21. The further heating members 37 keep the material in the holding basin 31 in the molten state and at the desired casting temperature.

To withdraw a quantity of molten aluminium from the holding basin 31, the pressure in the holding basin itself is increased, by means of the pneumatic system 45. The thrust of the pressure increase causes the molten aluminium to be pushed upwards where this thrust does not act, i.e. in the tubular duct 41 but also in the melting basin 21; in fact, the fact that the window 36 is below the level L2 of the molten aluminium prevents the pressure increase caused by the pneumatic system 45 on the molten aluminium in the holding basin 31 from also reaching the space above the molten aluminium present in the melting basin 21. Therefore, while molten aluminium is being "pumped" into the tubular duct 41 and is then being dispensed to the second end 43 thereof, the level LI of the molten aluminium present in the melting basin 21 also rises, for example up to a level LI'. This rise - which may arrive as far as the bottom zone 14 of the melting shaft 11 - favours the contact between the molten metal and the one still in the solid state, in particular the smaller-sized fractions of solid material which, as already mentioned, may tend to fall rapidly downwards. Since typically the molten aluminium is withdrawn in a discontinuous manner, in the melting basin 21 and in the melting shaft 11 there is a periodic rise in the level LI of the molten material, which reaches and gathers all the fallen material that is not yet molten or is not yet completely molten, operating as a sort of "rinsing" of the bottom zone 14 of the melting shaft 11 with the already molten material.

Optionally it is possible to maintain a certain pressurization inside the holding basin 31 to stably (or almost stably) raise the level LI of the liquid metal in the melting basin 21 to accommodate portions of solid metal at a higher level.

Therefore, thanks to the presence of the melting basin 21, aluminium melting is better, even if the solid material used comprises very small-sized fractions: these fractions can in fact quickly fall into the molten metal present in the melting basin 21 and melt by immersion, reducing the risk that they may oxidize or burn.

This beneficial effect on the quality of the molten metal is made even more marked if the holding basin 31 is also present, with the window 36 and the pneumatic system 45 for the withdrawal of the molten aluminium, which - as seen - in addition to the delivery of the molten metal from the tubular duct 41 causes a rise in the level LI of the molten metal in the melting basin 21 and in the bottom zone 14 of the melting shaft 11.

Claims

1. Melting furnace for aluminium and its alloys, comprising: a vertically-developing melting shaft (11) adapted to be fed with aluminium in the solid state, heating members (25) capable of generating a flow of hot gas that rises in the melting shaft (11), a hood (12) above the melting shaft (11), provided with an upper opening (13) for the discharge of hot gas from the melting shaft (11), characterized in that it further comprises a horizontally-developing melting basin (21), placed below the melting shaft (11) in such a position to receive the aluminium by falling from the melting shaft (11) and retain it, wherein the heating members (25) are provided out of the melting shaft (11), in the melting basin (21).

2. Melting furnace according to claim 1, wherein the heating members (25) are adapted to keep the molten aluminium present in the melting basin (21) at a melting temperature.

3. Melting furnace according to claim 1 or 2, wherein the heating members (25) comprise gas burners placed in an upper vault (23) of the melting basin (21).

4. Melting furnace according to claim 1, further comprising a holding basin (31), flanked to the melting basin (21) and in communication with the melting basin (21) through a window (36), and further heating members (37), adapted to heati and keep the molten aluminium contained in the holding basin (31) at a casting temperature, wherein the casting temperature is higher than the melting temperature.

5. Melting furnace according to claim 4, wherein the further heating members (37) comprise electric heaters placed at an upper vault (33) of the holding basin (31).

6. Melting furnace according to claim 4, wherein the window (36) is formed in a septum (35) between the melting basin (21) and the holding basin (31) and extends vertically both downwards and upwards: downwards, up to a bottom (32) of the holding basin (31) and up to a bottom (22) of the melting basin (21), upwards, up to a height lower than the upper vault (33) of the holding basin (31) and the upper vault (23) of the melting basin (21), so that - during operation of the melting furnace - the window (36) can be completely submerged below the level (L2) of the molten aluminium contained in the holding basin (31).

7. Melting furnace according to claim 6, comprising a tubular duct (41) for the exit of the molten aluminium from the holding basin (31), extended from a first end (42) inside the holding basin (31) located near the bottom (32) thereof to a second end (43) outside the holding basin (31) located higher than the first end (42), and a pneumatic system (45) for generating an increase in pressure in the holding basin (31), such as to push the molten aluminium into the tubular duct (41) until it exits from the second end (43) thereof.

8. Melting furnace according to claim 7, wherein the second end (43) of the tubular duct (41) is provided with a drain spout (44).

9. Melting furnace according to claim 2, wherein the bottom (32) of the holding basin (31) is provided with an emptying valve (46).

10. Melting furnace according to claim 4, wherein the holding basin (31) is provided with a second cleaning door (38), placed on a side wall (34) of the holding basin (31).