DEVICE FOR MELTING METALS

Description

[0001] The invention relates to a device for melting metals, in particular non-ferrous metals. The term "metals" shall also include corresponding alloys whose melting point should preferably be below 1000 °C. The invention is particularly suitable for melting aluminum and its alloys.

[0002] Currently, it is common practice to use oil or gas burners to melt these metals in differently configured melting furnaces. The hot flame of these burners converts the respective metal into a liquid phase. During the combustion of the respective hydrocarbon compounds, relatively large quantities of CO₂ are released into the Earth's atmosphere through chemical oxidation, which is extremely detrimental in terms of climate change.

[0003] Furthermore, it is also known to carry out inductive heating of metal as a melt. However, due to the alternating electric fields that occur, a strong stirring effect develops in the resulting melt. This leads to a high degree of oxide inclusions in the metal, so that the quality of the components produced with the melt obtained in this way is severely impaired. Inductively heated melting furnaces are also generally poorly suited for melting coarse recycled material or casting fractures due to unfavorable coupling conditions.

[0004] Electrically resistance-heated furnaces are also known. These usually have low power output and are therefore generally only suitable for keeping already molten metal warm.

[0005] There have also been previous attempts to use plasma for melting metal. These methods utilize an electric arc to generate the plasma. An electrode was contacted with the unmelted material. However, this leads to contamination of the metal being melted, which cannot be avoided or controlled. A loss of contact causes the arc to immediately terminate during the melting process and therefore must be prevented through very complex process control.

[0006] It was known from US 6 362 449 B1 to generate a plasma using microwaves and to use its energy, in particular to carry out waste treatment.

[0007] US 2005 / 0163696 A1 concerns possibilities for the synthesis of carbon nanotubes using microwave-generated plasma.

[0008] It is therefore an object of the invention to provide possibilities for melting metal in which the plant engineering effort is kept within limits, the release of CO2 on site and contamination of the resulting melt can be avoided to the greatest extent possible.

[0009] According to the invention, this problem is solved with a device having the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features specified in dependent claims.

[0010] In the device according to the invention for melting metals, in particular non-ferrous metals whose melting point is preferably less than 1000 °C, a device for generating a plasma in the form of a free gas flare and transferring heat to the material being melted by radiation is arranged on a melting furnace. This can be a melting furnace that is otherwise known and configured unchanged. Shaft, hearth, and crucible furnaces are preferably suitable.

[0011] The device is connected to an electrical power supply and also has at least one first supply for a plasma gas with which the plasma can be formed.

[0012] The device is designed, dimensioned, arranged, and / or aligned such that the generated plasma is positioned at a distance from the metallic melt, and a hot gas stream can be generated with the plasma, directed towards the melt. The generated plasma thus never comes into direct contact with the unmelted material or the molten metal. This also eliminates the need for an electrode that would contact the material or molten metal, as no such electrode should be present.

[0013] The generated plasma should function solely as a heat source. This allows the heating of the molten metal to be achieved exclusively through the thermal energy of the hot gas stream and thermal radiation emitted by the plasma.

[0014] The generated plasma should be designed and arranged in the device in such a way that virtually no free electrically charged particles (especially ions and electrons) can come into direct contact with the metal to be melted. The volumetric flow rate and flow velocity of at least one other supplied gas, which essentially forms the hot gas stream, can also be adjusted or controlled accordingly.

[0015] The plasma can also be mixed with another gas (secondary gas) to form a stable plasma or gas flare that also emits radiant energy towards the material being melted. The material being melted can then be heated and melted in the furnace chamber using the hot gas from the plasma or gas flare and the additional radiant energy.

[0016] A melting pan or crucible is arranged in the melting furnace to receive the molten metal.

[0017] In an alternative version of the invention, the device can be configured with a microwave generator and a resonator connected thereto, having at least one reflection plate for generated microwaves, which is designed as a waveguide. Furthermore, an electrical ignition device with an ignition electrode electrically insulated from a housing should be part of the device. The ignition device serves exclusively to ignite a plasma and can be switched on when a sufficiently large quantity of free charge carriers has been generated in the plasma gas used, following the generation of free charge carriers by the microwaves.

[0018] The plasma should be generated within the resonator in the region of stationary microwaves, in front of at least one reflection plate with flowing plasma gas. Translational movement of the generated plasma can be largely avoided, allowing it to form a stationary heat source for generating a hot gas stream.

[0019] In this alternative, the plasma should be generated within the housing of the device, with the hot gas flow directed towards the melting material in the interior of the melting furnace via at least one flow guide element. Tubular or channel-shaped elements can be used as flow guide element(s), through which the hot gas can flow towards the melting material. Flow guide elements can be made of glass, glass-ceramic, or pure ceramic material. At least two flow guide elements can also be present. In this case, one flow guide element can be located in at least one area within a flow guide element with a larger inner diameter or a larger free cross-sectional area, thus forming a heat shield. Flow guide elements should not be in direct contact with each other.

[0020] In another alternative configuration, the device can consist of two electrodes positioned at a distance from each other, between which a plasma gas flows towards the material being melted, resulting in an electric arc discharge. This device can be designed analogously to a conventional plasma torch used for cutting and welding materials. In this configuration, one electrode is typically made of tungsten, hafnium, or an alloy thereof. The counter electrode can be a housing through which the plasma gas flows. Only the dimensions and operating parameters need to be adapted to the specific application of melting metal. However, even in this case, the generated plasma should not come into direct contact with the material being melted and should merely serve as a heat source for heating a gas that can be used for melting as a hot gas stream or a free gas flare.

[0021] The housing can be connected to at least one additional inlet for plasma gas or another gas (secondary gas). This additional inlet can be arranged at a distance from the first inlet. Preferably, the additional inlet can be arranged within the area of ​​the formed plasma or downstream of it in the flow direction.

[0022] Advantageously, the electrical power of the microwave generator of the device or the electric arc discharge, the volumetric flow rate of the plasma gas, and / or the volumetric flow rate of other gases can be controlled. For example, a measured temperature can serve as the controlled variable. This could be the temperature of the hot gas stream, the plasma, the material being melted, or the melt itself. The temperature should preferably be determined non-contact, for example, using thermography or a pyrometer. However, the temperature can also be controlled. Depending on the process, this can be done, for example, to melt or maintain a melt at a specific temperature.

[0023] It is also possible to control the microwave generator in such a way that the length of the gas flare formed with the plasma or the length of the plasma flare formed in the direction of the molten material is adjusted, so that in particular the proportion of usable radiation energy can be influenced.

[0024] Advantageously, plasma gas can be allowed to flow tangentially into the housing with a swirl before it comes under the influence of the microwaves. This allows the contact time to be extended and the free charge carriers (ions, electrons) to be brought to a higher energy level more effectively, thus increasing the efficiency.

[0025] However, another gas can also flow tangentially into the housing, either alone or additionally. Plasma gas can be introduced into the housing parallel to the longitudinal axis of the device's housing or the flow direction of the hot gas stream. It can enter the housing through an inlet opening, which may be located directly next to the ignition device.

[0026] Inlets for plasma gas can be arranged around the circumference of the housing, through which plasma gas can flow from the supply into the housing.

[0027] Argon can be advantageously used as a plasma gas and / or other gas because it is completely inert with respect to the material being melted and the melt itself. Nitrogen, as a plasma or other gas, should be avoided, especially when melting aluminum or its alloys. Oxygen or air promote oxidation and are therefore detrimental in this respect.

[0028] However, a gas mixture can also be used as the plasma gas or as an additional gas. The respective gas proportions of the mixture can be adjusted to suit the metal being melted. For example, argon can be mixed with air, with the air proportion being smaller than the argon proportion.

[0029] A system can also be provided for recirculating hot gas extracted from the melting furnace, allowing this gas to be reused as plasma gas and / or as additional gas in the cycle, or for other uses of the residual heat. A closed-loop system reduces the required quantity of plasma gas or other gas to be supplied, which is particularly cost-effective for argon.

[0030] However, the residual heat of the extracted hot gas can also be used, for example to keep the resulting melt warm or to preheat, in particular, further gas.

[0031] The invention allows the use of microwaves with a frequency in the range of 500 MHz to 5000 MHz and an electrical power in the range of 5 kW to 3000 kW.

[0032] The total volume flow of plasma gas and / or other gas should be chosen to be at least large enough that the hot gas flowing into the melting furnace reaches the unmelted material, or at least comes close to it, so that the material is melted as a result of thermal radiation.

[0033] It is also advantageous to arrange the ignition electrode of a plasma ignition device, which generates plasma using microwaves, within a radiation trap. This ignition electrode can be positioned in a tubular or channel-shaped element whose inner diameter or free cross-sectional area is smaller than that of the device housing in which the plasma is generated. It is particularly beneficial if the tip of this electrode is recessed within the tubular or channel-shaped element, i.e., located inside the radiation trap. Such a design increases the service life of the ignition electrode and completely prevents contamination of the melt with the electrode material.

[0034] The plasma generation device can advantageously be mounted in a pivotable fixture on the furnace body, thus enabling targeted and variable guidance of the gas flare, plasma flare, or hot gas stream within the furnace chamber. This allows the direction of a gas flare or plasma flare to be changed, enabling locally targeted heating of the respective material being molten within the furnace chamber. For example, the outer edges or the center of the material being molten within the furnace chamber can be heated more or less as needed.

[0035] The invention allows for at least partial cooling of the housing. Temperature control, particularly at the flow guide element(s), can also be advantageous in order to reduce the effects of strong temperature fluctuations in this area; it is especially beneficial to avoid large, short-term temperature differences.

[0036] As already explained, the invention significantly reduces the amount of CO2 released. Existing melting furnaces can be converted or retrofitted with very little effort. The quality of the molten metal is at least equivalent to that achievable with conventional gas or oil burners. Contamination and oxidation of the molten metal can be largely, if not completely, avoided.

[0037] The invention will now be explained in more detail by way of example. Features can be combined independently of the specific example or the corresponding representation in a figure. The individual features are not limited to the respective example or representation.

[0038] This shows: Figure 1 is a schematic representation of an example of a device according to the invention and Figure 2 is a sectional view through a part of an example of a device for generating a plasma using microwaves.

[0039] In Figure 1 A schematic example of a device according to the invention with a melting furnace 1 is shown. A door (not shown) is provided on one side of the melting furnace 1, through which the melting furnace 1 can be charged with unmelted material 9. The unmelted material 9 can be placed on a melting platform 4, which is inclined at an angle, in the example shown an angle of 10°, so that molten metal can drip from the melting platform 4 into the crucible 5 or a melting tank (not shown).

[0040] A device 2 for generating a plasma is flanged to the housing 6 of the melting furnace 1, and at least one flow guide element (not shown) for a hot gas stream is led through the housing wall of the melting furnace 1 into the interior of the melting furnace 1, so that at least one hot gas stream can be directed onto the unmelted material 9. The housing is pivotally mounted, thus enabling the gas or plasma flare or a hot gas stream generated by the plasma 8 to be repositioned during the melting process.

[0041] By means of a viewing window embedded in the housing wall 6 of the melting furnace 1, the melting process can be observed from the outside, or the temperature inside the melting furnace 1 can also be determined from there.

[0042] In Figure 1Furthermore, a hot exhaust vent 7 is provided at melting furnace 1, through which hot exhaust gas can be extracted from melting furnace 1. The hot exhaust gas can be recirculated and, for example, returned as plasma or another gas.

[0043] The extracted hot exhaust gas can also be used to keep molten material warm or for other uses where the thermal energy can be utilized.

[0044] Hot exhaust gas can also be passed through a heat exchanger.

[0045] In Figure 2 The essential elements of a device 2 for generating a plasma are shown. The illustration of a microwave generator, which could be a commercially available product, has been omitted. It is flanged to the resonator 10.

[0046] The microwaves 11 generated by the microwave generator can be obtained as standing waves in the resonator 10. For this purpose, a reflection plate 10.1 is arranged in a flange of the housing 13 of the device 2 opposite a second flange 21. The microwave generator is connected to the second flange 21.

[0047] The reflective plate 10.1 can be made of glass. Near the microwave reflective plate 10.1, a supply 17 for a cooling gas is provided in the housing 13 of the device 2. In addition to its cooling effect, the cooling gas can also flow along the surface of the reflective plate 10.1 inside the housing 13, cleaning it and keeping it free of particles.

[0048] In Figure 2On the left side of the housing 13 of the device 2, an ignition device with a rod-shaped ignition electrode 12 is visible. This electrode is connected to one pole of an electrical voltage source (not shown). Briefly applying an electrical voltage to this ignition electrode 12 can increase the energy of the supplied plasma gas, leading to the ignition of a plasma 8 in the region of the resonator 10 and the formation of standing microwaves 11 there. After the plasma 8 has been ignited, the ignition device can be switched off.

[0049] The housing 13 can be designed in the area of ​​the ignition device with the ignition electrode 12 as a beam trap, as explained in the general part of the description.

[0050] Plasma gas can pass through the beam trap alone. or solely through inlets 18 distributed around the circumference of the housing 13. However, a combination of these is also possible.

[0051] A swirl effect can be achieved and utilized by a tangential inflow through preferably several inlets 18.

[0052] In the example shown, a further gas supply has been omitted. However, at least one additional gas can be introduced into the housing 13 of the device 2, preferably in the region of the formed plasma 8. This additional gas can then be used, at least predominantly, for the hot gas stream.

[0053] The hot gas stream leaves the device 2 in the direction of the arrow shown. In this example, three tubular flow guide elements 14, 15, and 16 are provided for this purpose. The quartz glass tube 14, with the smallest diameter, encloses the formed plasma 8. In its section facing the melting furnace 1, which is located opposite the ignition device, it is enclosed by another tubular flow guide element 15, which can simultaneously form a shield against thermal radiation.

[0054] In the area of ​​the flange of the housing 13, which is arranged facing the melting furnace 1, a third tubular flow guide element 16, with the largest diameter, is arranged. The third flow guide element 16 can extend at least to the wall of the housing 6 of the melting furnace 1, so that the hot gas flow can be directed through an opening in the wall of the housing 6 onto the material to be melted 9 in the melting furnace 1. Its length can also be chosen so that it projects into the interior of the melting furnace 1.

[0055] The third tubular flow guide element 16 can be guided and held in a flange 19 of the housing 13 of the device 2. The flow guide elements 14, 15, and 16 are nested inside one another. However, they should not touch each other.

[0056] In addition to the supply 17 for a cooling gas, further areas of the housing 13 of the device 2 can be designed and used for cooling. A cooling medium (gas or liquid) can flow through these areas. These areas should be located at least in the vicinity of the plasma 8 being generated.

[0057] In the example shown, a flange-shaped cooling element 20 is provided in an area of ​​the housing 13 of the device 2.

Claims

1. Apparatus for melting metals, in particular non-ferrous metals, in which a device (2) for forming a plasma (8) is arranged on a melting furnace (1), wherein the device (2) is connected to an electrical voltage supply and at least one first feed for a plasma gas, with which the plasma (8) can be formed, is connected to the device (2), characterized in that the device (2) is designed, dimensioned, arranged and / or aligned in such a way that the plasma (8) formed is arranged at a distance from the metal as the material to be melted (9), and in this case a hot gas stream can be formed with the plasma (8), which hot gas stream is aligned in the direction of the material to be melted (9), and a melting tank or crucible (5) is arranged in the melting furnace (1) to receive the molten metal.

2. The apparatus according to claim 1, characterized in that the device (2) is designed in such a way that plasma gas of the plasma (8) and further gas form a free gas torch or plasma torch in the furnace chamber of the melting furnace (1) which can use its hot gases and radiation energy for heat transfer and melting of a respective metal.

3. The apparatus according to claim 1, characterized in that the device (2) is provided with a microwave generator and resonator (10) connected thereto, which is designed as a waveguide and has at least one reflection plate (10.1) for generated microwaves (11).

4. The apparatus according to the preceding claim, characterized in that the device (2) is formed with an electrical ignition device, with an ignition electrode (12) electrically insulated from a housing (13), the plasma (8) being formed in the region of standing microwaves inside the resonator (10) in front of the at least one reflection plate (10.1) with plasma gas flowing there, and the plasma (8) being formed in the housing (13).

5. The apparatus according to one of the two preceding claims, characterized in that the hot gas flow is directed in the direction of the melt material (9) via at least one flow guide element (14, 15, 16).

6. The apparatus according to any one of the three preceding claims, characterized in that the ignition electrode (12) of the electric ignition device for plasma is arranged in a radiation trap.

7. The apparatus according to one of the preceding claims, characterized in that the power, the length, the temperature and / or the length of the free gas flare or plasma flare can be varied with a controllable microwave generator.

8. The apparatus according to one of the preceding claims, characterized in that the device (2) is formed with two electrodes which are arranged at a distance from one another and between which a plasma gas flows in the direction of the material to be melted (9) and an electric arc discharge takes place.

9. The apparatus according to one of the preceding claims, characterized in that at least one further supply for plasma gas or a further gas is connected to the housing (13) of the device (2).

10. The apparatus according to any one of claims 3 to 6, characterized in that the electrical power of the microwave generator or the electrical arc discharge of the device (2), the volumetric flow of the plasma gas and / or the volumetric flow of the further gas can be controlled.

11. The apparatus according to one of the preceding claims, characterized in that at least the plasma gas and / or a further gas flows tangentially into the housing (13) with a swirl.

12. The apparatus according to any one of the preceding claims, characterized in that argon is used as plasma gas and / or further gas.

13. The apparatus according to any one of claims 3 to 10, characterized in that microwaves having a frequency in the range 500 MHz to 5000 MHz, at an electrical power in the range 5 kW to 3000 kW are used.

14. The apparatus according to any one of the preceding claims, characterized in that a gas mixture is used as plasma gas and / or further gas.

15. The apparatus according to one of the preceding claims, characterized in that a recirculation system for hot gas withdrawn from the melting furnace (1) is provided, by means of which a renewed use of this gas as plasma gas and / or further gas in the cycle or another use of the residual heat is achievable.

16. The apparatus according to one of the preceding claims, characterized in that the device (2) is fixed to the furnace body in a pivotable device, so that a targeted and variable guidance of the gas flare, plasma flare or hot gas flow in the furnace chamber is made possible.