TECHNICAL FIELD

The present invention relates to a melting device for aluminium and aluminium alloys, and a melting process for production of aluminium and aluminium alloys.

BACKGROUND ART

It is known to melt aluminium and aluminium alloys in a furnace, in which an aluminium containing material such as aluminium ingots and/or aluminium scrap are supplied into the furnace. The material is melted by heating using at least one burner supplied with oxidizer and with fuel, in order to obtain molten aluminium. In the field of aluminium melting, this melting operation is normally carried out in a rotary or reverberatory furnace. Although this melting process may be continuous, it is usually carried out in batches: the materials are charged into the furnace, in one or more successive cycles, before the molten metal is poured towards its place of use.

During melting/remelting of aluminium, it is also known to use electromagnetic stirrers placed below or at the side of the furnace vessel to obtain a stirring of the molten metal in the furnace vessel and to reduce the temperature and the concentration gradients in the molten metal and thereby increase the productivity of the furnace plant. From US patent US 4294435 and WO 1996/34244 it is known to arrange electromagnetic stirrers for achieving stirring of the molten metal in furnace plants for melting/remelting of aluminium. Typically, this gives an increase of the effective coefficient of heat conduction by a factor of 25-35.

The remelting of scrap is a very energy intensive process, and additional energy is required to convert and keep the metal scrap into the molten state. The form of energy used is heat, generated by an electric or combustion source. The introduction of metal scrap to the heat generating areas is problematic in that great amounts of heat energy are lost both when relatively cold scrap metal is introduced into those heat generating areas and as those heat generating areas are exposed to the colder ambient air during the introduction of the scrap.

Many new technologies have been applied to the aluminum remelting process in order to increase its efficiency and productivity.

Two examples can be outlined as below:

Combustion technology. The oxy-fuel and regenerative burners have drastically increased the power input rate and power efficiency. It is known that oxy-fuel burners can improve the energy efficiency by up to 66%.

Electromagnetic stirring has showed its prominent effect in increasing the productivity by 10-15% and also reducing the quantity of dross.

However, remelting/reverberatory furnaces, used for aluminium melting, typically have a low efficiency because the liquid aluminium reflects a large fraction of the available heat. Although a number of exhaust gas recovery techniques have been developed, enveloped and deployed, furnace efficiency remains low, often a value around 30%. For the operation of aluminum reverberatory furnaces, the aim is to melt down the solid aluminum while at the same time avoiding the aluminum loss caused by dross formation. For this reason, the burner flame is controlled to minimize the direct contact with the molten aluminum. The local high temperature zone is also under tight control to avoid local oxidizing of the molten aluminum.

Economic and environmental analyses have found that the greatest cost savings from an improved combustion system are realized from the reduction in dross. Aluminium dross, which is a mass of solid impurities, is formed in the presence of oxygen and this reaction (Al + O ₂ = > AI ₂ O ₃ ) is accelerated in the molten aluminium body in a remelting/reverberatory furnace because of the high temperature. However, in a remelting/reverberatory furnace dross is also formed at the surface of the molten metal when the melt reacts with the atmosphere in the furnace.

From US Patent No. 5421856 it is known to introduce an appropriate mixture of inert and hydrocarbon gases continuously into a furnace bottom to reduce the formation of dross throughout the melt as well as on the surface of the melt.

THE OBJECT OF THE INVENTION

The object of the invention is to find a way to improve the energy efficiency and productivity in an aluminum remelting process.

SUMMARY OF THE INVENTION

The object of the invention is achieved by an aluminium melting device as defined in claim 1. The device is characterised in that it comprises a gas-supplying device arranged to supply gas comprising oxygen into the volume of molten aluminium kept in the melting chamber and thereby establish an exothermic reaction between the molten aluminium and the oxygen. Further, an electromagnetic stirrer is arranged to stir the molten aluminium by applying an electromagnetic field thereon, so that the heat established by the exothermic reduction is distributed in the melt.

A direct consequence of blowing a gas-containing oxygen into the aluminium melt is that there will be a local hot area around the gas plume since an exothermic reaction between molten aluminium and oxygen (Al + O ₂ = > AI ₂ O ₃ ) will take place inside the melt. Thereby dross is formed and the exothermic heat, which is very large, of the aluminium oxidation can be used for speeding up the melting of remaining solid aluminium in the melting furnace. At the same time, dross will also be formed at the surface of the molten metal when the melt reacts with the atmosphere in the furnace. However, by speeding up the melting of remaining solid aluminium in the furnace, the total amount of dross formed will be significantly reduced since the time used for the melting of remaining solid aluminium in the furnace will be reduced. Hence, due to the time reduction, less dross will be formed at the surface of the molten metal and hence more aluminium can be produced during each melting cycle.

Also, by speeding up the melting process, less energy will be used and consequently both the energy efficiency and the productivity will be improved by the above mentioned idea.

According to an embodiment of the invention, a gas-supplying device is arranged to supply a gas comprising oxygen at the bottom of the melting chamber. Thereby the heat, caused by the exothermic reaction, is easily distributed in the melt.

According to another embodiment of the invention, the gas- supplying device comprises a plurality of nozzles arranged distributed in the bottom of the melting chamber. Thereby the total size of the gas plume in the melt will increase and subsequently the heat, caused by the reaction between the injected gas comprising oxygen and the molten aluminium, is distributed in the melt.

According to another embodiment of the invention, the electromagnetic stirrer is arranged below and adjacent to the bottom of the melting chamber.

According to another embodiment of the invention, the electromagnetic stirrer is arranged to generate a magnetic field in the melt which is essentially perpendicular to the direction of the gas flowing through said melt. This arrangement will enable the heat from the exothermic reaction to be easily distributed in the melt in order to melt the remains of solid aluminium in the melting chamber. The object of the invention is also achieved by a melting process, characterised by the steps of injecting a gas comprising oxygen into the melting furnace with at least one gas-supplying device for establishing of an exothermic reaction between the molten aluminium and the oxygen. Further, an electromagnetic stirrer is used for the purpose of stirring the molten aluminium so that the heat established by the exothermic reduction is easily distributed in the melt.

According to yet another embodiment of the invention, the electromagnetic stirrer is arranged to supply a magnetic field in the molten aluminium which is essentially perpendicular to the flow of gas supplied to the molten aluminium.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be described in more detail in connection with the enclosed drawings.

Figure 1 shows a side view of an aluminium melting device according to the invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Fig. 1 shows a melting device 1 according to an embodiment of the invention. The melting device 1 is arranged to be used in association with aluminium melting and especially for the purpose of melting solid aluminium or aluminum alloys, such as aluminum ingots and/or aluminium scrap.

The melting device 1 comprises a melting furnace 2 having a melting chamber 3, arranged to keep a volume of aluminium. The melting chamber 3, which is adapted to be filled with molten 13 and/or solid 14 metal, comprises an outer shell, preferably of a metallic material, and a lining of refractory material covering the inner side of the outer shell. The lining protects the outer shell from the heat, wear and chemical reactivity present inside the melting chamber 3. The melting chamber 3 comprises an opening for emptying the furnace (not shown), a bottom part 4, side walls 5 and a roof 6 with a charge chute 7, through which aluminum ingots and/or aluminium scrap are loaded into the melting chamber 3, and an opening for flue gases 8.

Normally, the maximum bath dept 12 in a furnace for melting and/or holding of aluminium is below one metre in known furnaces; most often, the maximum bath dept 12 for this type of furnaces varies within the interval 0.3 to 0.9 metres.

Further the melting chamber is provided with burners 9 arranged in the side walls 5 for the purpose of heating and melting solid aluminium, charged into the melting chamber 3, by radiation and convection. The choice of heat source is of no significance for the present invention and, of course, other types of heat sources, such as electric resistor elements, may be used in those cases a sufficient heating capacity can be achieved by such means.

A gas-supplying device is arranged to supply a gas comprising oxygen into the volume of molten aluminium kept in the melting chamber and thereby establish an exothermic reaction between the molten aluminium and the oxygen. The gas-supplying device comprises nozzles 10, a gas cistern or gas tubes, pipes and means, such as valves, for the purpose of regulating the flow of gas. The nozzles 10 are distributed in the bottom of the melting chamber 3. The exothermic reaction (Al + O ₂ = > AI ₂ O ₃ ), caused by the reaction between the molten aluminium and the gas comprising oxygen, causes so much energy that, together with the heat content of the melt, it is sufficient to melt the remaining solid pieces of metal in the melting chamber 3. It is also to be understood that the preferred gas is pure oxygen; however, also air or a mixture between oxygen and nitrogen can be used. The powerful reactions also create a stirring effect on the melt; however, the movement or stirring of the molten metal in the vicinity of solid pieces of metal is limited, at least at the beginning of a melting procedure.

The melting device is further arranged with an electromagnetic stirrer 11, arranged adjacent to or inside the bottom 4 of the melting chamber 3. The stirrer 11 comprises coils of electric conductors wound around an iron core. The stirrer 11 is arranged adjacent to the outer shell for the purpose of inducing an electromagnetic travelling field, through the outer shell and the lining, to interact with the molten aluminium 13 and thereby cause a stirring of the molten aluminium 13 inside the melting chamber 3.

The stirrer 11 is arranged to support and strengthen the stirring of the molten aluminium 13 so that the heat established by the exothermic reaction is spread in the melt. Thereby the melting speed of the remaining solid 14 aluminium in the melting furnace will increase, and hence the total process cycle time will decrease.

At the same time, dross will also be formed at the surface of the molten metal when the melt reacts with the atmosphere in the furnace. However, by speeding up the melting of remaining solid aluminium 14 in the furnace, the total amount of dross formed at the surface will be significantly reduced since the time used for the melting of remaining solid aluminium 14 in the furnace will be reduced.

The total formation of dross (Al + O ₂ => AI ₂ O ₃ ) will be reduced (e.g. from 3 - 5 % of the total volume to 2.5 - 4 % of the total volume). Hence, due to the time reduction, less dross will be formed at the surface of the molten metal and hence more aluminium can be produced during each melting cycle.

As can be seen in figure 1, the electromagnetic stirrer 11 is arranged adjacent to and on the outside of the bottom part of the melting chamber 3. Another possible stirrer arrangement is to arrange a stirrer adjacent to and outside the side walls of the melting chamber. An alternative solution is to arrange one electromagnetic stirrer in or near the sidewall of the melting chamber, and also to arrange another electromagnetic stirrer below and adjacent to the bottom of the melting chamber 3.

As can also be seen in figure 1, one or more gas-supplying devices, such as nozzles 10, are arranged in the bottom part of the melting chamber. Thereby a vertical gas flow, comprising oxygen, is being supplied to the melt kept in the melting chamber causing an exothermic reaction in the melt.

One additional possibility, regarding the location of the gas- supplying devices, is to arrange the nozzles 11 in the side walls of the melting chamber, thereby supplying a horizontal gas flow into the melt.

Another possibility regarding the location of the gas-supplying device is to arrange the nozzles 11 both in the bottom and in the side walls of the melting chamber. This makes it possible to supply both a vertical and a horizontal gas flow into the melt.

Still another possibility, regarding the gas supplying-device, is to use a metallurgical lance, which is brought into the melt from above, for the purpose of supplying a gas flow into the melt.

Since all parts and situations of the melting process are believed to occur at about the same time in each production cycle, the application of the electromagnetic travelling field together with the flow of gas-containing oxygen, can advantageously be carried out according to a predetermined schedule. In order to build a certain determined flow pattern, but rather achieve a more random stirring of the melt, the electromagnetic travelling field can be applied with a strength and/or frequency which varies with time, e.g. in a periodical manner. An additional stirring of varying power on the melt may in certain applications be more efficient to mix the melt than a continuous one. When the end of the gas blow phase of the melting process is reached, the gas jet is turned off. In such a situation, the intense gas blow has oversaturated the melt with oxygen. By applying an electromagnetic travelling field to the melt a certain time after the end of the gas injection, an additional stirring is achieved. This additional stirring will cause some of the excess oxygen to react with the molten aluminium, bringing down the excess oxygen concentration. The reduction of the oxygen content is thus promoted by post-blow stirring.

The invention is not limited to the embodiments shown but a person skilled in the art may, of course, modify it in a plurality of ways within the scope of the invention as defined by the claims.