METHOD FOR THE RECOVERY OF ALUMINIUM FROM ALUMINIUM SCRAP, AND MULTICHAMBER MELTING FURNACE

Abstract

Aluminum scrap having organic adhesions is processed to recover aluminum. A hearth of scrap chamber of a multi-chamber melting furnace is batchwise loaded with aluminum scrap where it is heated in low oxygen to convert the organic adhesions on the aluminum scrap into a pyrolysis gas. In a second pretreatment phase, the scrap chamber is heated to the auto-ignition temperature of the pyrolysis gas, wherein at least one air flow is provided in the scrap chamber to produce an ignitable substoichiometric pyrolysis gas/combustion air mixture which is reacted in the scrap chamber in a combustion process. The atmosphere from the scrap chamber is transferred to a post-combustion. A corresponding multi-chamber melting furnace is also provided.

Claims

1. A method for recovering aluminum from aluminum scrap, which has organic adhesions, in a multi-chamber melting furnace, having a scrap chamber which is set up to receive melt, wherein the scrap chamber has a hearth which can be loaded in batches with the aluminum scrap, which is located above the level of the melt and wherein a loading door is located in the wall of the scrap chamber and with at least one heating chamber, which is set up to receive melt and which has at least one combustion device, at least with the following method steps: batchwise loading of the hearth of the scrap chamber with aluminum scrap, thermal pretreatment of the aluminum scrap in the scrap chamber during a first pretreatment phase at a predetermined first temperature and in an atmosphere free of oxygen to convert the organic adhesions on the aluminum scrap into a pyrolysis gas, thermal pretreatment of the aluminum scrap in the scrap chamber during a second pretreatment phase at a predetermined second temperature, wherein the scrap chamber is heated to the auto-ignition temperature of the pyrolysis gas, wherein at least one air flow is provided in the scrap chamber to produce an ignitable substoichiometric pyrolysis gas/combustion air mixture, which is reacted in the scrap chamber in a combustion process, and transferring the atmosphere from the scrap chamber to a post-combustion.

2. The method according to claim 1, wherein during the second pretreatment phase the air flow is provided in such a way that a pyrolysis gas/combustion air mixture with an air number in the range of 0.3 to 0.6 is achieved.

3. The method according to claim 1, wherein the melt recirculates between the heating chamber and the scrap chamber to heat the melt in the scrap chamber.

4. The method according to claim 1, wherein during the second pretreatment phase the following method step is additionally carried out: when the temperature in the scrap chamber is lower than the auto-ignition temperature of the pyrolysis gas, generating at least one flame in the scrap chamber by a burner, to which fuel and combustion air are supplied.

5. The method according to claim 1, wherein the atmosphere from the scrap chamber is transferred to the heating chamber for post-combustion.

6. The method according to claim 5, wherein a characteristic value for the mixing ratio of the gas/air mixture in the heating chamber is measured by a sensor in an exhaust gas outlet of the heating chamber, and a signal for supplying more or less fuel and/or combustion air to the combustion device is generated as a function of the deviation of the measured characteristic value from a set value.

7. The method according to claim 6, wherein an actuating variable for providing and/or terminating the provision of the air flow in the scrap chamber and/or for generating the flame in the scrap chamber is derived from the measured characteristic value, or from the signal as a function of the deviation of the measured characteristic value from the set value.

8. The method according to claim 1, wherein in the second pretreatment phase the air flow is provided by directing the air flow between loading door and aluminum scrap into the scrap chamber or by directing one air flow each between loading door and aluminum scrap into the scrap chamber from opposite walls of the scrap chamber.

9. The method according to claim 4, wherein the flame is generated adjacent to the air stream provided in the scrap chamber.

10. The method according to claim 4, wherein the air flow in the scrap chamber is provided by means of the burner, which is operated with excess air when the temperature in the scrap chamber is lower than the auto-ignition temperature of the pyrolysis gas and/or its fuel supply is interrupted and its combustion air supply is thus reduced, that a substoichiometric pyrolysis gas/combustion air mixture is generated in the scrap chamber when the temperature in the scrap chamber has reached or exceeds the auto-ignition temperature of the pyrolysis gas.

11. A multi-chamber melting furnace for recovering aluminum from aluminum scrap which has organic adhesions, comprising: a scrap chamber which is set up to receive melt, wherein the scrap chamber has a hearth which can be loaded in batches with the aluminum scrap and is located above the level of the melt, and a loading door being arranged in the wall of the scrap chamber, the scrap chamber being set up for thermal pretreatment of the aluminum scrap, and, during a first pretreatment phase, at a predetermined first temperature in an atmosphere which is free of oxygen, the organic adhesions on the aluminum scrap can be converted into a pyrolysis gas, at least one heating chamber which is arranged to receive melt and which has at least one combustion device at least one air inlet in the wall of the scrap chamber for providing at least one air flow in the scrap chamber during a second pretreatment phase at a predetermined second temperature, the scrap chamber being heatable to the auto-ignition temperature of the pyrolysis gas, a control/regulating unit which is arranged to providing the air flow in the scrap chamber in such a way that an ignitable substoichiometric pyrolysis gas/combustion air mixture is formed in the scrap chamber, which mixture can be reacted in the scrap chamber in a combustion process, and an atmosphere outlet in the wall of the scrap chamber for discharging the atmosphere from the scrap chamber for post-combustion.

12. The multi-chamber melting furnace according to claim 11, wherein a partition wall is located between the scrap chamber and heating chamber, and the partition wall has at least one opening for recirculation of the melt between the heating chamber and the scrap chamber and/or that the atmosphere outlet is designed as a connecting line between the scrap chamber and the heating chamber, in order to transfer the atmosphere from the scrap chamber to the heating chamber for post-combustion.

13. The multi-chamber melting furnace according to claim 11, wherein the scrap chamber comprises at least one burner to which fuel is supplied by a fuel supply and combustion air is supplied a combustion air supply.

14. The multi-chamber melting furnace according to claim 13, wherein the air inlet is designed as an air lance and/or that the burner is arranged next to the air inlet.

15. The multi-chamber melting furnace according to claim 13, wherein the burner forms the air inlet and that the control/regulating unit is arranged to operate the burner with excess air, when the temperature in the scrap chamber is lower than the auto-ignition temperature of the pyrolysis gas and/or to interrupt the fuel supply to the burner and to reduce its combustion air supply when the temperature in the scrap chamber has reached or exceeds the auto-ignition temperature of the pyrolysis gas.

16. The multi-chamber melting furnace according to claim 11, wherein at least one circulation channel with an inlet opening and an outlet opening is connected to the scrap chamber in order to circulate the atmosphere inside the scrap chamber.

17. The method according to claim 1, wherein during the second pretreatment phase the air flow is provided in such a way that a pyrolysis gas/combustion air mixture with an air number () of about 0.5 is achieved.

18. The method according to claim 5 wherein the combustion device in the heating chamber is operated with excess air.

19. The method according to claim 9, wherein the distance between the flame and the air stream is chosen such that the flame heats the air stream, and the flame and the air stream are directed into the scrap chamber in the same manner.

20. The multi-chamber melting furnace according to claim 16, wherein the outlet opening is located in the wall of the scrap chamber between the loading door and the hearth, and the air inlet and/or the burner is/are arranged within the outlet opening and/or that the burner is set up in such a way that its outlet velocity is between 60 m/s and 130 m/s and/or in that the air inlet is set up in such a way that the outlet velocity of the air stream is between m/s and 60 m/s.

Description

[0073] The invention, further advantages and the technical environment are explained in more detail below by way of examples of preferred embodiments with reference to the accompanying drawings. The invention is not intended to be limited by the embodiments shown. It is also possible to combine partial aspects of the features explained in the figures with other features from other figures and/or the description.

[0074] It shows in schematic representation:

[0075] FIG. 1 shows a multi-chamber melting furnace for recovering aluminum from aluminum scrap in a sectional view from above;

[0076] FIG. 2 shows the multi-chamber melting furnace from FIG. 1 in a lateral sectional view;

[0077] FIG. 3 shows a cross-sectional view from the front side of the multi-chamber melting furnace shown in FIG. 1;

[0078] FIG. 4 shows a further embodiment of the multi-chamber melting furnace according to the invention in a sectional view from above;

[0079] FIG. 5 shows a further embodiment of the multi-chamber melting furnace according to the invention in a lateral sectional view.

[0080] In all figures, identical components have identical reference numerals. FIG. 1 shows a sectional view, viewed from above, of a multi-chamber melting furnace 1 in the form of a two-chamber melting furnace for recovering aluminum from aluminum scrap which has organic adhesions. FIG. 2 shows a lateral sectional view and FIG. 3 a sectional view across the width of the multi-chamber melting furnace as seen from the front. Only those components of the multi-chamber melting furnace 1 which are essential to the invention are shown.

[0081] FIGS. 1 to 4 schematically show that the multi-chamber melting furnace 1 has a scrap chamber 2 and a heating chamber 3 with a wall 4 closed to the outside atmosphere with walls 4a and 4b.

[0082] The scrap chamber is set up for pretreatment of the aluminum scrap prior to melting. The scrap chamber 2 has a lockable loading door 5 at the front end, through which the scrap chamber 2 can be loaded with the aluminum scrap 6 in batches. The loading door 5 is horizontally movable and extends substantially over the entire width of the scrap chamber 2 or the multi-chamber melting furnace 1. A hearth 7, which is loaded with the aluminum scrap 6, is located in the scrap chamber 2. The aluminum scrap 6 is assembled as a batch. The hearth 7 is at least partially inclined and is adjacent to the liquid melt 8 under production conditions, wherein the hearth 7 is located above the level N of the melt 8. After pretreatment, the aluminum scrap is manually inserted into the liquid melt 8 and melted.

[0083] The heating chamber 3 extends, viewed from the loading door 5, behind the scrap chamber 2 over the entire width of the multi-chamber melting furnace 1. Liquid melt 8 is located in the heating chamber 3 and the heating chamber 3 has a combustion device 9 and an exhaust gas outlet 10. The combustion device 9 is designed as a gas burner directed into a heating zone 3a above the melt 8 in the melting chamber. Several combustion devices 9 in the form of gas burners are used, of which only one combustion device 9 is shown in FIG. 2.

[0084] The scrap chamber 2 and the heating chamber 3 are arranged one behind the other in the longitudinal direction and are separated from each other by means of a partition wall 11. The chambers can alternatively be arranged side by side or in an L-shape. It may be a suspended partition wall, for example. The partition wall 11 projects into the melt 8 under operating conditions. The partition wall 11 has below the level N or the surface of the melt 8 at least one opening 12 or channel for recirculating the melt 8 between the heating chamber 3 in the scrap chamber 2, in order to heat the melt in the scrap chamber 2 and thus to heat the scrap chamber 2.

[0085] The hearth 7 of the scrap chamber 2 is loaded with aluminum scrap in batches by means of an automatic charging machine not shown or a bucket wheel loader. A charging machine is used here which is sealed off from the melting furnace so that the entry of oxygen into the scrap chamber is largely prevented during loading. If the entry of oxygen into the scrap chamber during loading cannot be completely avoided, the oxygen is eliminated before the first pretreatment phase, preferably by means of a short combustion step. Experience has shown that a combustion of between 30 seconds and two minutes is sufficient to largely eliminate oxygen introduced into the scrap chamber during loading. The scrap chamber 2 is first heated to a predetermined first temperature preferably 550 C., by means of recirculation of the melt 8 from the heating chamber 3 into the scrap chamber. The heating could be accelerated by means of a suitable external heating.

[0086] No oxygen is supplied to the scrap chamber during the first pretreatment phase. The aluminum scrap is pretreated during the first pretreatment phase at the predetermined first temperature in a reducing atmosphere, i.e., an atmosphere substantially free of oxygen, to convert the organic adhesions on the aluminum scrap to a pyrolysis gas.

[0087] By free of oxygen is meant the absence of oxygen such that the air number A is essentially=0. A large part of the adhesions is transferred to the gas phase during this first pretreatment phase.

[0088] In each of the walls 4a, 4b of the scrap chamber 2 there is an air inlet 13a, 13b for providing an air flow L in the scrap chamber 2 in order to generate an ignitable pyrolysis gas/air mixture in the scrap chamber in a second pretreatment phase.

[0089] During the second pretreatment phase, the provision of the air flow L is controlled such that the air number of the pyrolysis gas/air mixture in the scrap chamber 2 is between 0.3 and 0.6, preferably 0.5.

[0090] In the two embodiments, starting from opposite walls 4a and 4b of the wall 4 of the scrap chamber 2, one air inlet 13a, 13b each in the form of an air lance is arranged such that the air flow L from each air inlet 13a, 13b is directed into the scrap chamber 2 between the loading door 5 and the aluminum scrap 6. This is intended to provide the airflow in the coldest area of the scrap chamber 3 near the aluminum scrap 6, while not directing the airflow directly at the aluminum scrap 6 to prevent melting of the metal and thus metal burnup. In addition, a uniformity of the temperature distribution in the melting chamber 3 is achieved.

[0091] During the second pretreatment phase, the scrap chamber 2 is heated to the auto-ignition temperature of the pyrolysis gas. In FIGS. 2 and 3, a control/regulation unit 14 is shown, which is arranged to control or regulate the provision of the air flow from the air outlets 13a, 13b in the scrap chamber 2 in such a way that an ignitable substoichiometric air/pyrolysis gas mixture is formed in the scrap chamber 2, which reacts in the scrap chamber 2 in a combustion process to form a combustion gas. The substoichiometric combustion ensures that no undesired oxidation of the aluminum scrap occurs.

[0092] The scrap chamber has a burner 15a, 15b on each of opposite walls 4a, 4b to initiate the combustion process during the second pretreatment phase when the temperature in the scrap chamber 2 is lower than the auto-ignition temperature of the pyrolysis gas. These burners are used to remove oxygen, which may be introduced into the scrap chamber during loading, from the scrap chamber during or immediately after loading by means of a combustion reaction before the first pretreatment phase begins. Only enough fuel is supplied to the scrap chamber by means of the burners until an ignitable mixture is formed with the oxygen present in the scrap chamber, which is brought to ignition.

[0093] Each burner 15a, 15b is assigned adjacent to an air inlet 13a, 13b. Preferably, the distance between each air inlet 13a, 13b and a burner 15a, 15b is so small that in each case the flame of the burner heats the adjacent air stream L from the air inlet. Adjacent burners and air outlets are directed in the same direction, preferably substantially parallel, to each other, into the scrap chamber 2. The exit velocity of the air streams L from the air outlets 13a, 13b is between 30 m/s and 60 m/s. This ensures that an optimum mixture of air and the combustible components of the pyrolysis gas is achieved. The exit velocity of the burners 15a, 15b is between 60 m/s and 130 m/s.

[0094] In the embodiment according to FIG. 4, it is shown that the scrap chamber 2 has a circulation channel 16a, 16b on both sides, each with a fan 17a, 17b, for circulating the atmosphere inside the scrap chamber 2. The atmosphere is circulated in the longitudinal direction of the scrap chamber between the partition wall 11 and the front loading door. Each circulation channel 16a, 16b has an inlet opening 18a, 18b and an outlet opening 19a, 19b in each of the walls 4a, 4b. The inlet openings 18a, 18b are arranged adjacent to the partition wall 11. The outlet opening 19a, 19b are each located in the walls 4a, 4b between the front loading door and the hearth 7. The air inlet 13a and the burner 15a are arranged within the outlet opening 19a and the air inlet 13b and the burner 14b are arranged within the outlet opening 19b.

[0095] FIG. 5 shows an embodiment in which the burners 15a, 15b also serve as air outlets. Each burner has a fuel supply 22a, 22b and an air supply 23a, 23b. Each burner 15a, is operated with excess air as long as the temperature in the scrap chamber 2 is lower than the self-ignition temperature of the pyrolysis gas. By means of the control/regulation unit 14, the fuel supply 22a, 22b to the burner is interrupted and its combustion air supply 23a, 23b is reduced in such a way that a substoichiometric pyrolysis gas/combustion air mixture is generated in the scrap chamber 2 when the temperature in the scrap chamber 2 has reached or exceeds the auto-ignition temperature of the pyrolysis gas. In this operating condition, shown in FIG. 5, only air flows from burners 15a, 15b. Because air lances can be dispensed with, this embodiment is particularly simple in terms of design.

[0096] In FIG. 2 it is shown that a connecting line 20, which has a blower 21, is located between the scrap chamber 2 and the heating chamber 3 in order to supply the atmosphere from the scrap chamber 2 to the post-combustion in the heating chamber. The atmosphere has the exhaust gas from the combustion reaction during the second pretreatment phase and unburned pyrolysis gas. The thermal energy generated during post-combustion is used to heat the heating chamber.

[0097] The combustion device 9 is connected to a fuel line 24 and a combustion line 25. The combustion device 9 in the heating chamber 3 is set up for superstoichiometric operation, so that the combustion air for post combustion in the heating chamber is supplied to the atmosphere from the scrap chamber 2 by means of the combustion device 9. This is a structurally particularly simple solution to the provision of combustion air for post-combustion. However, the atmosphere in the heating chamber 3 could also be supplied with the combustion air required for post-combustion by means of an air line.

[0098] In other words, the combustion air line 26 of the combustion device 9 in the heating zone 3a of the melting chamber 3 is arranged to provide combustion air for post-combustion of the atmosphere from the scrap chamber 2 in the heating zone 3a in addition to the combustion air for the fuel, which is supplied to the combustion device by means of the fuel line 25.

[0099] A sensor 26 (FIG. 2) is arranged in the exhaust gas outlet 10 of the heating chamber 3 for measuring the oxygen content in the exhaust gas outlet 10 of the heating chamber 3. The oxygen content represents a characteristic value for the mixing ratio of the gas/air mixture in the heating chamber 3. Depending on the deviation of the measured oxygen content from a setpoint value, the control/regulation unit 14 generates a signal for supplying more or less fuel and/or combustion air to the combustion device 9.

[0100] From the measured characteristic value or from the signal in dependence on the deviation of the measured characteristic value from the setpoint value, a control variable for providing and/or stopping the provision of the air streams L in the melting chamber is further derived. Within the scope of the invention, a signal for generating the flame in the scrap chamber (2) could also be derived.

[0101] During post-combustion, the atmosphere from scrap chamber 2 is post-combusted in heating chamber 3 at a long residence time and high temperatures in a safe and environmentally friendly manner. The exhaust gases from heating chamber 3 finally pass from exhaust outlet 10 to a special exhaust gas cleaning process, in which the exhaust gas is cleaned of dust and harmful gas components.

[0102] After completion of the second pretreatment phase, the pretreated aluminum scrap batch 6 is pushed into the melt in the scrap chamber 2.

[0103] According to the invention, optimum pretreatment conditions are created in order to transfer organic adhesions as completely as possible into the gas phase. This leads to reduced dross formation and thus to an increased metal yield with simultaneous energy savings.

[0104] The embodiments described above are to be understood as examples. The features which are described together with other features, whether disclosed in the description, the claims, the figures or otherwise, also individually define essential elements of the invention.

[0105] Within the scope of the invention, variations are readily possible. For example, the multi-chamber melting furnace may have more than two chambers. Within the scope of the invention, only a single air inlet/burner combination or multiple air inlet/burner combinations may be provided.

[0106] Finally, it is noted that the term comprising does not exclude any component or element, that reference signs in the claims are not limiting, and that a or one includes a plurality.

LIST OF REFERENCE SIGNS

[0107] 1 Multi-chamber melting furnace [0108] 2 Scrap chamber [0109] 3 Heating chamber [0110] 3a Heating zone of the heating chamber [0111] 4 Wall [0112] 4a, 4b Wall [0113] 5 Loading door [0114] 6 Aluminum scrap [0115] 7 Hearth [0116] 8 Melt [0117] 9 Combustion device [0118] 10 Exhaust outlet [0119] 11 Partition wall [0120] 12 Opening [0121] 13a, 13b Air inlet [0122] 14 Control/regulation unit [0123] 15b Burner [0124] 15b Burner [0125] 16a, 16b Circulation channel [0126] 17a, 17b Fan [0127] 18a, 18b Inlet opening [0128] 19a, 19b Outlet opening [0129] 20 Atmosphere outlet/connecting line [0130] 21 Blower [0131] 22a, 22b Fuel supply [0132] 23a, 22b Combustion air supply [0133] 24 Fuel line [0134] 25 Combustion air line [0135] 26 Sensor [0136] Air number [0137] L Air flow [0138] N Melt level