EP3228403B1 - Method and device for keeping liquid metals warm - Google Patents

Description

The invention relates to a method for keeping liquid metals warm according to the preamble of claim 1. The invention further relates to a corresponding device.

In the foundry, it is often necessary to keep a molten metal, for example a melt of aluminum or iron, in a liquid state at a high temperature for a certain period of time. In particular, there is a need to transport the liquid metal from the place of liquefaction to the place of further processing. Initially, the transport was only carried out within one plant, but today molten metals are transported over a distance of 200 km and more by road or rail vehicles. Special transport containers, also called "transport pans", are used for the transport, which, depending on the type of vehicle and the metal to be transported, can hold between 500 kg and 200 t of molten metal.

In order to keep the liquid metal at a predetermined temperature of, for example, 700 to 1000 ° C. over the entire duration of the transport, liquid metal transport containers are generally preheated. For example, from the EP 1 078 704 B1 a device is known in which a transport pan between its uses is heated by an air-natural gas burner. In the DE 10 2007 022 684 A1 describes a liquid metal transport container with a preheating device in which the container is preheated by means of a field-type pore burner before it is used. The pore burner is either arranged in the lid of the transport vessel or extends along a column which is inserted into the center of the empty container during the heating process. However, this arrangement is not suitable for heating a liquid melt, since the pores of the burner are quickly clogged by penetrating liquid metal, which would make it impossible for the smoke gases to penetrate.

The temperature of liquid metal decreases by around 5-15 K per hour in conventional transport containers. Since at the same time the initial temperature has a certain value for metallurgical reasons, in the case of liquid aluminum around 950 ° C, should not exceed, the temperature of the melt proves to be not unproblematic, particularly in road transport, since unforeseeable events, such as a traffic jam, can lead to considerable delays in transport, for example.

In order to ensure the maintenance of a predetermined temperature of the melt even during longer transport times or the length of time the melt stays in the transport container, it is already known to equip the containers with heating devices.

For example, it is known to provide ladles with an arc heater; especially in the case of small pans, however, the required pitch circle diameter of the electrodes leads to only small distances between the electrodes and the refractory wear lining of the pan wall, as a result of which the lining wear is extremely high.

Another known measure consists in induction heating of the pan. However, this requires a minimum height of the liquid metal bath in order to ensure an acceptable level of efficiency of this heating. Special pans are also required for this type of heating.

From the DE 3 637 065 A1 electric heating by means of a graphite rod extending over the melt in the pan is known. The pan is heated with or without content only by heat radiation, with a radiation shield arranged above the graphite rod serving as a reflector for better utilization of the radiation energy. However, this device is associated with considerable design effort and high energy consumption during operation.

With the subject of EP 0 217 094 A1 a fuel is burned in a - filled or empty - furnace with air or another oxygen-containing gas and the heat released is transferred directly to the melt or to the walls of the pan. The burner is attached to a holder in a lid of the pan and heats the surface of the with its flame Melt or the inside of the empty pan. Compared to electrical heating by means of an arc, the investment costs of a burner and the running costs for providing the energy are relatively low. A disadvantage of this object, however, is the low efficiency when heating a pan filled with melt, since the heat radiation from the flame has only a small depth of penetration into the melt.

In order to achieve a certain depth of penetration of the energy into the melt, attempts have already been made to arrange a fuel-air burner in a downward-closed immersion tube made of a ceramic or non-ceramic material, which is inserted in the middle of a crucible holding the melt pool. The hot fuel gases come into indirect thermal contact with the melt via the outer walls of the immersion tube, which act as heat exchanger surfaces. The flue gas escapes from the dip tube above the surface of the bath and is discharged into the atmosphere. However, the homogenization of the temperature in the melt is inadequate for this object, since a sharply falling temperature profile results from the hotter dip tube to the cooler crucible wall.

In the WO 2006/133679 A2 describes a method and a device for setting predetermined melting properties in a liquid metal, in particular aluminum. The container holding the liquid metal is equipped with a heating device which can be lowered into the melt and by means of which the temperature of the liquid metal can be maintained even during longer transport times. An electrical heating element or a burner, which is accommodated in a protective tube, serves as the heating device. In addition, a gas purging plug is provided on the bottom of the container, by means of which a gas can be introduced into the liquid metal. The rising gas bubbles create a flow through which the temperature distribution in the melt is homogenized. With this arrangement, the temperature of the melt can be maintained over a longer period of time with satisfactory homogeneity. However, the device is very complex in construction and still needs improvement in terms of energy efficiency.

The invention is therefore based on the object of increasing the energy utilization when keeping a melt warm and at the same time improving the homogeneity of the temperature profile of the melt.

This object is achieved with a method with the features of claim 1 and with a device with the features of claim 4.

A method for keeping liquid metals warm, in which a metallic melt is accommodated in a container and is heated by means of a burner, which is accommodated in an immersion tube inserted vertically or obliquely in the melt, by burning a fuel with oxygen, is characterized in that during the combustion of the fuel in the burner, the flue gases generated are introduced into the melt through at least one flow opening of the dip tube.

In the method according to the invention, the burner is ignited in the immersion tube which is initially still arranged above the melt, forming a flame. The dip tube is then lowered so deep into the melt that at least the at least one flow opening, but preferably a substantial section of the dip tube that surrounds the flow opening, is located below the surface of the melt. The lower part of the immersion tube is preferably inserted into the melt up to at least the level of the flame generated by the burner. The flue gases produced during the combustion of the fuel penetrate into the melt from the at least one flow opening and rise in the form of gas bubbles in the melt.

In this case, the flow opening is preferably arranged on the lower end face of the dip tube, that is to say immersed in the melt; the parts of the immersion tube that protrude from the melt in the intended use, on the other hand, are preferably sealed gas-tight, so that the flue gases can only escape through the melt. The liquid metal is therefore heated not only via the walls of the immersion tube heated by the burner, but also via the surfaces of the gas bubbles of the flue gases rising in the melt.

Compared to the prior art, the total surface area available for heat transfer is thus significantly increased. At the same time, the rising gas bubbles, after the mammoth pump effect, circulate the melt and thus homogenize the temperature distribution. Since essentially only water and / or carbon dioxide and / or oxygen are present in the flue gases when using natural gas and / or hydrogen as fuel, there is no contamination of the liquid metal.

The flue gases are preferably introduced into the melt in a lower region of the container in order to ensure that the ascending gas bubbles remain in the melt for as long as possible. For this purpose, the dip tube has one or more flow openings on its lower end face and / or in an end section bordering this end face. For example, the immersion tube is inserted so deep into the melt that at least two thirds, preferably at least 80%, of the volume of the melt is above the flow opening or the flow openings. As a result, a large part of the melt is caught by the flow induced by the rising gas bubbles and a particularly good homogenization of the temperature distribution in the melt is achieved.

The supply of fuel and / or oxygen to the burner is expediently regulated as a function of physical or chemical parameters of the melt. The temperature of the melt, for example, serves as a controlled variable, which is recorded continuously or at predetermined time intervals using a suitable measuring probe. The measured values are transmitted to a control unit, by means of which they are used to regulate the supply of fuel and / or oxygen to the burner. The burner is preferably controlled in two stages or in proportion to the deviation of the measured variable from a predetermined target value. When regulating the supply, however, care must be taken to ensure that no liquid metal comes into contact with the burner orifice if the burner output is reduced or the burner is switched off. In this case, the immersion depth of the immersion tube should also be regulated accordingly and the immersion tube should be completely lifted out of the melt for the duration of a burner standstill.

The object of the invention is also achieved with a device having the features of claim 4. The device has a container intended for holding a metallic melt and a heating device which can be moved vertically or obliquely from above into the interior of the container by means of a feed device and which comprises a burner accommodated in an immersion tube and connected to a feed line for a fuel and a feed line for oxygen , wherein the immersion tube is equipped in its lower section with at least one flow opening which is permeable to fuel gases of the burner. According to the invention, the device is characterized in that the immersion tube is made gas-tight in a section arranged outside the melt when the device is in use, with the exception of at least one passage opening for the burner and / or for the feed lines for the fuel and / or the oxidizing agent.

The burner of the heating device is accommodated in an immersion tube, the upper section of which, with the exception of the feed lines for the fuel and oxidant of the burner, is sealed gas-tight and allows the flue gases flowing out of the burner to escape only through one or more flow openings which are in a lower section of the Immersion tube is / are arranged, for example in an end face opposite the feed lines of the burner or in a part of the side wall of the dip tube adjacent to this end face. The heating device is guided through an opening in a cover or a wall of the container and is designed to be movable in the axial direction between two adjustment positions during operation of the device by means of the feed device. In the first setting position, at least the mouth of the burner accommodated in the immersion tube, preferably the entire immersion tube, is arranged vertically spaced from a predetermined level, at which the surface of a molten bath filled into the container is present during operation of the device. This means that at least the burner has no contact with the weld pool. The burner is switched on or off in this position. When the burner is switched off, the heating device is also in this position. In the second setting position, the immersion tube is immersed at least with a lower section into the melt pool, at least to such an extent that the flow opening or the flow openings are located below the surface of the melt pool. The heating device is fed into the melt either vertically or obliquely from above. Before the heater is moved to the second position, the burner must be lit so that it flows through the flow opening (s) escaping smoke gases prevent the liquid metal from entering the dip tube and direct contact of the liquid metal with the burner. After the heating process has ended, the heating device is moved back to the first position and then the burner is switched off.

The burner is fixed in the immersion tube and is arranged, for example, along the axis of the immersion tube. The preferably cylindrical immersion tube is equipped with one or more flow openings, which is / are arranged in a section immersed in the melt when the device is used, preferably in the lower end face of the immersion tube. In the simplest case, the lower end face of the dip tube is completely open. However, it is also conceivable within the scope of the invention to provide a plurality of flow openings in the lower end face and / or in the side face of the immersing section of the dip tube.

In order to ensure that the flue gases generated during combustion are introduced as completely as possible into the melt, the dip tube is designed to be gas-tight on its upper end face and in the jacket section which is not below the surface of the melt after the dip tube has been immersed. The immersion tube has only one through opening or more through openings for the burner and / or the feed lines for the fuel and / or the oxidizing agent, the connection between the burner or the feed lines and the walls of the dip tube likewise being at least largely gas-tight and no or only negligible amount of flue gas can escape there.

A particularly advantageous embodiment of the invention provides that the container is designed as a transport container for transporting a metallic melt. For example, it is a container permanently mounted or mountable on a road or rail vehicle for transporting liquid metals, such as liquid iron or liquid aluminum. The heating device mounted on the container enables continuous temperature control of the melt even during transport. Due to the vertical adjustability of the heating device, the burner can also be switched off completely.

The only drawing ( Fig. 1 ) schematically illustrates a device according to the invention.

The device 1 for keeping a liquid metal, for example liquid aluminum, warm comprises a container 2, for example a crucible or a pan, which is suitable, for example, for transport on a road or rail vehicle, and a removable lid 3 which closes the container 2 when used as intended serves the container 2 for receiving a melt 4 up to the level of a level 5. The container 2 and lid 3 each consist of a refractory, heat-resistant material or are each lined with a refractory material (not shown here). A recess 6 is provided in the cover 3 for a heating device 7 described in more detail below.

The heating device 7 comprises a burner 10 accommodated in an immersion tube 9, for example in the form of a circular cylinder, and is arranged in the vertical direction by means of a feed device 11 which is arranged on the outside of the cover 3 and is not explained in detail here. into the interior of the container 3 and out of the interior of the container 3, movable. By means of the heating device 7, for example, the melt 4 is to be kept at a temperature of, for example, 780.degree.

The burner 10 is a fuel-oxygen burner with a central feed 13, connected to a fuel line 12, for a gaseous or liquid fuel, for example natural gas, and a connection, connected to an oxygen line 14, on the radially outside to the feed for the fuel subsequent oxidant supply 15 and an ignition device 16. The burner 10 is fixed in a manner not shown here in the dip tube 9 and aligned along its axis. The fuel line 12 and the oxygen line 14 are connected to sources of fuel and oxygen, not shown here.

The dip tube 9 is made of a ceramic or non-ceramic material and is open at its lower end face 17. Instead of or in addition to a completely open end face 17, one or more openings can moreover be arranged in a lower section of the immersion tube 9, that is, when used as intended, below the level 5 of a melt 4, in particular those with which smoke gases escaping from the immersion tube 9 in the lateral direction be directed into the melt. The length of the immersion tube 9 is dimensioned such that when moving upwards with the aid of the feed device 11, a position is reached in which the lower end face 17, or at least the burner 10, is above the level 5 of the melt during the movement downward, however, the immersion tube is immersed deeply in the melt 4, and the burner 10 is positioned with its mouth below the level 5.

On its upper end face 18, the immersion tube 9 is designed to be largely gas-tight and only has bushings 21, 22 for the lines 12, 14, in which these in turn are at least largely gas-tight, so that flue gas does not emit in this direction, or only to an insignificant amount can escape the dip tube 9.

During operation of the device 1, the crucible 2 is filled with a melt 4 up to the level 5 and then the cover 3 with the heating device 6 is placed on the crucible 2. The heating device 7 is initially still in an upper position in which the lower end face 17 of the immersion tube 9, or at least the burner 10, is above the level 5. Subsequently, fuel, preferably natural gas, and oxygen, preferably oxygen with a purity of at least 95% by volume, are introduced via lines 12, 14 and ignited by means of ignition 16, whereupon the fuel burns to form a flame 19. The resulting flue gases escape completely, or almost completely, via the open end face 17 of the dip tube. Subsequently, the heating device 7 is lowered by means of the feed device 11 until a lower section of the immersion tube 9 and in particular the end face 17 is inside the melt 4, as in FIG Fig. 1 shown. The pressure of the flue gases produced during combustion prevents the melt from penetrating into the interior of the immersion tube 9; the flame 19 is therefore not in direct contact with the melt 4 or only at its outermost tip. Instead, the smoke gases are pushing - As indicated by arrows 23 - from the open, lower end face 17 of the dip tube 9, penetrate into the melt 4 and bead up in the form of gas bubbles 24. Due to the large number of ascending gas bubbles 24, a large area is available for heat transfer from the flue gas into the melt 4. At the same time, the rising gas bubbles 24 lead to a continuous circulation of the melt 4 due to the mammoth pump effect, through which the temperature distribution in the melt 4 is homogenized. In addition to the heat transfer on the surface of the gas bubbles 24, heat transfer also takes place on the wall of the immersion tube 9. The smoke gases emerging from the melt 4 are then discharged via a trigger 25 in the cover 3.

The supply of thermal energy in the melt 4 can, moreover, in the embodiment according to Fig. 1 can be regulated depending on the temperature of the melt 4. For this purpose, a temperature measuring probe 27, which is guided through the wall of the cover 3, is in data connection with a control unit 28 - as indicated by a dashed line. The probe 27 enables the temperature of the melt 4 to be measured continuously. The control unit is in data communication with control valves 29, 30 in the lines 12, 14. In this way, by influencing the supply of fuel and / or oxygen, the output of the burner 10 can be controlled proportionally or stepwise as a function of the temperature of the melt 4 detected at the temperature measuring probe 27 and used, for example, to keep the temperature of the melt 4 at a constant level hold predetermined value. As a result, automated heating is also possible during transport.

The device according to the invention is suitable for keeping various metals, in particular iron or aluminum, warm up to a temperature of 1000.degree. The large heat exchange surface on the gas bubbles rising in the melt ensures a high heating efficiency as well as good temperature homogeneity in the melt due to the constant movement induced by the rising gas bubbles. When using a fuel-oxygen burner, it is ensured that the flue gases consist largely of CO ₂ , H ₂ O and O ₂ , which do not negatively affect the melt.

Reference symbol list

1.

contraption

2nd

container

3rd

cover

4th

melt

5.

level

6.

Recess

7.

Heater

8th.

-

9.

Dip tube

10th

burner

11.

Feeding device

12.

Fuel line

13.

Feeder for fuel

14.

Oxygen line

15.

Supply for oxygen

16.

Ignition device

17th

Lower front

18th

Upper face

19th

flame

20th

-

21.

execution

22.

execution

23.

arrow

24th

Gas bubble

25th

Deduction

26.

-

27th

Temperature measuring probe

28

Control unit

29.

Control valve

30th

Control valve

Claims (6)

1. Method for keeping liquid metals warm, in which a metallic melt (4) is held in a container (2) and is heated by burning a fuel with oxygen by means of a burner (10) held in an immersion pipe (9) introduced into the melt (4) vertically or at an angle, characterized
   in that flue gases produced during the burning of the fuel in the burner (10) are introduced into the melt through a flow opening of the immersion pipe (9).
2. Method according to Claim 1, characterized in that the flue gases are introduced into the melt (4) in a lower region of the container (2).
3. Method according to Claim 1 or 2, characterized in that the feeding of fuel and/or oxygen to the burner (10) is controlled in dependence on parameters in the melt (4) that are detected at a measuring probe (27) inside the container (2).
4. Apparatus for keeping liquid metals warm, with a container (2), intended for receiving a metallic melt (4), and also with a heating device (7), which can be moved into the interior of the container vertically or at an angle by means of a feeding device (11) and comprises a burner (10), which is held in an immersion pipe (9) and connected to a supply line (12) for a fuel and a supply line (14) for oxygen, wherein the immersion pipe (9) is equipped in its lower portion with at least one flow opening that allows combustion gases of the burner (10) to pass through,
   characterized
   in that, in a portion arranged outside the melt (4) when the apparatus is in use, the immersion pipe (9) is formed in a gastight manner with the exception of at least one passage (21, 22) for the burner (10) and/or for the supply lines (12, 14) for the fuel and/or the oxidizing agent.
5. Apparatus according to Claim 4, characterized in that the flow opening is arranged in the lower end face (17) of the immersion pipe.
6. Apparatus according to Claim 5 or 6, characterized in that the container (2) is formed as a transporting container for transporting a metallic melt (4).

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