Method for melting and heat-treating solids

Abstract

In the method, a device for forming a plasma is positioned on a furnace. Said device is connected to an electrical power supply and to a plasma gas supply. The device is designed, dimensioned, positioned and / or orientated in such a way that the plasma formed is positioned at a distance from the solid or a melting tank. A hot gas stream is formed using the plasma, and the melted product or a melting tank is heated exclusively by means of thermal energy from the hot gas stream and by emitted electromagnetic radiation. At least one chemical element and / or one chemical compound is / are introduced into the plasma and / or the hot gas stream, ions from said at least one chemical element and / or chemical compound being converted to an excited state by the heat of the plasma or of the hot gas stream, or at least one chemical reaction being initiated such that the proportion of electromagnetic radiation which is directed onto the melted product, having a wavelength of ≥ 575 nm, is increased by at least 10 % of the total spectrum of the electromagnetic radiation.

Description

[0001] METHOD FOR MELTING AND HEAT TREATMENT OF SOLIDS

[0002] The invention relates to a method for melting and heat-treating solids in a melting furnace or melting tank. Heat treatment or melting can be carried out individually, or both can be carried out simultaneously.

[0003] Until now, it has been common practice to melt these solids in variously configured melting furnaces using oil or gas burners, whose hot flames can convert the material into the liquid phase. During the combustion of the respective hydrocarbon compounds, which originate from fossil sources, relatively large amounts of CO2 formed by chemical oxidation are released into the Earth's atmosphere with the flue gas, which is particularly detrimental from the perspective of climate change and the consumption of natural fossil resources.

[0004] Furthermore, it is also known to perform inductive heating of conductive solids, especially metal, as melt material. However, the resulting alternating electric fields cause a strong stirring effect of the resulting melt. This leads to a significant amount of oxide inclusions in the metal, which has a severe adverse effect on the quality of the components produced with the resulting melt. Furthermore, due to unfavorable coupling conditions, inductively heated melting furnaces are generally poorly suited for melting coarse recycle material or cast scrap.

[0005] Electrically resistance-heated furnaces are also common. These usually have low power and are therefore generally only suitable for keeping already molten metal hot by suppressing heat loss.

[0006] In recent years, however, technical possibilities have been developed that utilize electrical energy to heat the melt via a plasma generated by the plasma, which directs a hot gas stream onto the melt to melt it. This is described in WO 2021 / 170652 A1.

[0007] A disadvantage, however, is that the melt can only be heated using the thermal energy of the hot gas stream and electromagnetic radiation. The wavelength spectrum of the electromagnetic radiation also plays a role, as different wavelengths are known to be absorbed differently by a correspondingly irradiated material and thus contribute to its heating to varying degrees. For example, a plasma and a hot gas stream emit short-wave electromagnetic radiation, which impinges on the respective melt or a melting tank, for example, to melt it. This radiation, which predominantly contains discrete (blue or green) radiation components, is absorbed with relatively low efficiency by the solids (metals, glasses) to be melted or heat-treated or by the melting tank, so that the overall efficiency is correspondingly reduced.

[0008] It is therefore an object of the invention to provide possibilities by which the efficiency of melting or heat treatment of metallic melt or glass can be easily increased by using a plasma from a plasma source for heating.

[0009] For the sake of completeness, it should be noted that the term "heat treatment" is used in accordance with technical usage. This term therefore does not include, for example, the application of a coating to a substrate in plasma per se, i.e., processes that exclusively produce coatings. Examples of heat treatment include recrystallization annealing, aging at elevated temperatures, a phase transformation in or of a material, the hardening of a material, etc.

[0010] According to the invention, the object is achieved by a method having the features of claim 1. Advantageous embodiments and further developments of the invention can be realized with features defined in the dependent claims.

[0011] In the invention, a device for generating a plasma, which is arranged on a melting furnace, is used to heat the solid (e.g., melting material). The device is connected to an electrical power supply and to at least one first supply for a plasma gas with which the plasma is generated.The device is designed, dimensioned, arranged and / or aligned such that the plasma formed is arranged at a distance from the solid (metal or glass) or a melting tank, and a hot gas flow is formed with the plasma, which is aligned in the direction of the solid, in particular a melting material, or at a distance along the surface of the solid or in the direction of or along the surface of a melting tank, such that the heating of the solid or a melting tank is achieved exclusively by means of thermal energy from the hot gas flow and by emitted electromagnetic radiation. The formed plasma should function exclusively as a heat source for heating the hot gas flow and, if necessary, as an energy source for the chemical decomposition of hydrocarbon compounds or salts through chemical reaction(s). The hot gas flow should preferably be at an angle in the range 0° to 20°. 0be aligned with respect to the surface of the solid or melting tank.

[0012] According to the invention, at least one chemical element and / or at least one chemical compound is / are introduced into the plasma and / or the hot gas stream and / or into the treatment chamber (in particular the furnace chamber) in which the solid is arranged for treatment, and ions from the chemical element or chemical compound are converted into an excited state using the heat of the plasma and / or the hot gas stream. In combination with the conversion into an excited state, photons are released. Alone or in addition to the excitation of the ions, at least one exothermic chemical reaction can be initiated, the energy of which can additionally heat the hot gas stream.

[0013] In any case, the proportion of electromagnetic radiation with wavelengths > 575 nm directed at the melt is increased by at least 10%, preferably by at least 30% of the entire spectrum of the electromagnetic radiation, with a transition from a discrete to a continuous radiation spectrum being preferably aimed for. The transition to a continuous spectrum enables the long-wave portion of the electromagnetic radiation in particular to penetrate into the melt, thereby enabling highly efficient heat input. With short-wave, discrete electromagnetic radiation, on the other hand, reflection at the surface of the respective solid dominates, so that a large part of the radiation energy heats the furnace walls and is dissipated by convection with the flowing gases. By introducing the at least one chemical element orof the at least one chemical compound, the proportion of radiation with wavelengths > 575 nm is increased by at least 10% (relative to the proportion of radiation with wavelengths > 575 nm before the introduction of the at least one chemical element or the at least one chemical compound), whereby the remaining conditions or parameters preferably remain at least approximately unchanged. By increasing this proportion of radiation energy, in particular the temperature of the plasma itself is at least approximately unchanged. The change in the temperature of the plasma can in particular be less than 10%, preferably less than 5%, relative to the temperature of the plasma without the addition of the at least one chemical element or the at least one chemical compound. These details relate to the average core temperature of the plasma.

[0014] A solid melted in this way by plasma and / or the hot gas stream can then be transferred to a melting tank or crucible in the melting furnace. The molten solid (metal or glass) in the melting tank can be removed or discharged from it.

[0015] The chemical element used can be Na, Ca, Sr, Li, Rb or Mg, or the chemical compound can be at least one chemical compound containing at least one of these chemical elements. Of the alkali metals, their salts can be used, for example, in aqueous salt solutions. The use of sodium or a sodium compound leads to a yellow discoloration of electromagnetic radiation; with Ca or a calcium compound, to orange-red, Sr or a strontium compound to red, Li or a lithium compound to red, and Rb or a rubidium compound also to red due to the well-known effect of flame coloration. The use of carbon-containing compounds leads to a continuous spectrum with a maximum in the yellow radiation range.

[0016] A carbon compound can also be used. However, a metal selected from iron, copper, and aluminum, and pure carbon should not be used as a chemical element.

[0017] The at least one chemical element and / or the at least one chemical compound should be supplied in a proportion of greater than 0 to 15 vol.%, preferably greater than 0.2 vol.%, or greater than 0.5 vol.%, up to a maximum of 10 vol.% in relation to the supplied plasma gas and hot gas stream.

[0018] The addition can be in a solid, gaseous, or liquid state. A particularly preferred option is to add a chemical element or compound in a solvent, i.e., in dissolved form, particularly as a salt solution.

[0019] The at least one chemical element or at least one chemical compound, if not already introduced into the plasma, should be introduced into a region of the hot gas stream and / or the treatment chamber where a minimum temperature of 750 °C is maintained. This is particularly important if the proportion of long-wave electromagnetic radiation with wavelengths of 575 nm and greater is to be achieved by introducing a corresponding chemical element and / or one of the chemical compounds that must react chemically for this purpose. This is particularly advantageous when introducing hydrocarbon compounds.

[0020] In addition to oils, hydrocarbon-containing process gases such as pyrolysis gases (e.g., from painted scrap), gasification gases or flare gases, or other gaseous hydrocarbons such as propane or butane, carbon dioxide, carbon monoxide, ammonia, or other homologous nitrogen-hydrogen compounds, or water vapor, or nitrogen, or mixtures of at least two of the aforementioned gases, such as a mixture of nitrogen and a gaseous hydrocarbon, etc., can also be used to advantageously influence the spectrum of the electromagnetic radiation used for melting. Mixtures of at least one gas and at least one solid can also be used.

[0021] Particulate Al2O3, lime, soot, coal dust, iron can also be added as solids, although soot, coal dust, and iron are not preferred.

[0022] If contaminated or toxic hydrocarbon compounds are introduced, they should be introduced into the plasma, the hot gas stream, in such a way that these chemical bonds are broken down due to the prevailing temperatures and gas composition. Contaminated or toxic hydrocarbon compounds can include, for example, oils used for cooling in transformers. These can also be used thermal oils for heating or cooling high-temperature processes, oil or tar condensates from pyrolysis processes (biogenic residues), solvent waste, biogenic or mineral oils, or waste oils.

[0023] Thus, the process according to the invention can also be used for safe disposal, since the hazardous components can be chemically converted into non-hazardous or significantly less hazardous components and at the same time can be used to increase the efficiency of the melting process.

[0024] In particular, hydrocarbon compounds can be introduced directly into a developed plasma, so that sufficiently high temperatures are available for decomposition by chemical reaction(s).

[0025] In the plasma gas, the hydrocarbons are completely decomposed and oxidized, for example, aromatics such as toluene, benzene, phenol, xylene, or furan.

[0026] (e.g. toluene: C7H8+ 9O2-+ 4H2O + 7CO2) as well as polycyclic aromatic hydrocarbons such as naphthalene, fluorene or pyrene

[0027] (Example: Naphthalene: C 10 H8 + 12024H20 + 10CO2) can be rendered harmless.

[0028] Chemical elements and chemical compounds which are supplied according to the invention can, however, also be supplied directly or at a distance of preferably a maximum of 50 mm after the hot gas stream emerges from a device with which a plasma can be formed.

[0029] A device for generating a plasma can be configured with a microwave generator and a resonator connected to it, which is configured as a waveguide and has at least one reflection plate for generated microwaves. Furthermore, an electrical ignition device with an ignition electrode electrically insulated from a housing can be part of the device. The ignition device serves exclusively to ignite a plasma and can be switched on when a plasma has been formed in sufficient quantities after the generation of free charge carriers in the plasma gas used with the generated microwaves.

[0030] The plasma should be formed in the area of ​​standing microwaves within the resonator in front of at least one reflection plate with plasma gas flowing there. Translational movement of the formed plasma can be largely avoided, allowing it to form a stationary heat source for generating a hot gas flow.

[0031] The plasma should be formed in a housing of the device, with the hot gas flow being directed via at least one flow guide element towards the material to be treated, the melting material, or the melting tank into the interior of the melting furnace. Tubular or channel-shaped elements can be used as flow guide(s), through which the hot gas can flow towards the melting material or the melting tank. Flow guide elements can be made of glass, a glass-ceramic, or a pure ceramic material. At least two flow guide elements can also be present. In this case, one flow guide element can be arranged in at least one region within a flow guide element with a larger inner diameter or larger free cross-sectional area, where it forms a shield against heat. Flow guide elements should not be in direct contact with one another.

[0032] In a further alternative, the device can be formed with two spaced-apart electrodes, between which a plasma gas flows towards the melting material or melting tank, creating an electric arc discharge. The device can be designed analogously to a conventional plasma torch used for cutting and welding materials. One electrode is typically made of graphite, copper, tungsten, hafnium, or an alloy thereof. The counter electrode can form a housing through which plasma gas flows. Only the dimensions and operating parameters should be adapted to the application for melting metal or glass. However, even in this case, the generated plasma should not come into direct contact with the melting material and should serve solely as a heat source for heating a gas that can be used as a hot gas stream or a free gas flare for melting.

[0033] The plasma gas used can be air, nitrogen, carbon dioxide, water vapor, flue gas, inert gas, hydrogen, methane, carbon monoxide or a mixture of these gases.

[0034] Such a device is described in WO 2021 / 170652 A1, to the disclosure of which reference is made in its entirety.

[0035] With such a device, in tests in which a plasma was generated using a microwave generator at an electrical power of 6 kW and a supply of 50 l / min of air as the plasma gas without the supply of a chemical element or a chemical compound, compared to a test procedure in which butane was introduced into the hot gas stream at 5 l / min immediately after the outlet from the device, at a distance of 100 mm from the exit from the device, a temperature of the hot gas stream increased by approximately 200 °C, at a distance of 150 mm from the exit, a temperature increased by approximately 165 °C and at a distance of 200 mm, a temperature increased by approximately 125 °C, whereby the effect desired according to the invention was reliably achieved.

Claims

P a t e n t a n s p r ü c h e 1. A method for heat treating or melting solids, in particular metal or glass, in which a device for forming a plasma is arranged on a furnace, the device being connected to an electrical voltage supply and to the device at least one first supply for a plasma gas with which the plasma is formed, and the device being designed, dimensioned, arranged and / or aligned such that the formed plasma is arranged at a distance from the solid, in particular metal or glass as melting material or a melting tank, and in the process a hot gas flow is formed with the plasma, which is aligned in the direction of the solid or melting material or at a distance along the surface of the melting material or in the direction or along the surface of a melting tank, so that the heating of the solid,Melting material or a melting tank is achieved exclusively by means of thermal energy from the hot gas stream and by emitted electromagnetic radiation, wherein at least one chemical element and / or at least one chemical compound is introduced into the plasma and / or the hot gas stream and / or into a treatment chamber in which the solid is arranged for treatment, the ions of which are converted into an excited state with the heat of the plasma and / or the hot gas stream or at least one chemical reaction is initiated, so that the proportion of electromagnetic radiation directed onto the melting material with wavelengths of > 575 nm is increased by at least 10% of the total spectrum of electromagnetic radiation.

2. Method according to claim 1, characterized in that the hot gas stream is directed at an angle between 0° and 20 0directed relative to the surface of the solid to be heat-treated or melted or the surface of a melting tank.

3. Method according to one of the preceding claims, characterized in that Na, Ca, Sr, Li, Rb or Mg is used as the chemical element or as the chemical compound containing at least one of these chemical elements or at least one carbon compound.

4. Method according to one of the preceding claims, characterized in that the at least one chemical element and / or the at least one chemical compound is supplied in a proportion of greater than 0 vol. to 15 vol.% in relation to the supplied plasma gas. Method according to one of the preceding claims, characterized in that the at least one chemical element or the at least one chemical compound is fed into a region of the hot gas stream in which a minimum temperature of 750°C is maintained. Method according to one of the preceding claims, characterized in that the at least one chemical compound is fed in a solvent as a solution, in particular a salt solution. Method according to one of the preceding claims, characterized in that the proportion of electromagnetic radiation with wavelengths > 575 nm is increased by at least 30%.Method according to one of the preceding claims, characterized in that contaminated or toxic hydrocarbon compounds are introduced into the sphere of influence of the plasma or the hot gas stream in such a way that, due to the high temperatures and the prevailing gas composition, chemical bonds are broken down, thus significantly reducing the proportions of contaminated or toxic compounds contained in the hot gas stream. Method according to one of the preceding claims, characterized in that the plasma is generated using microwaves or at least one electric arc. Method according to one of the preceding claims, characterized in that the plasma is generated in such a way that it never comes into direct contact with unmelted melt material or a melt.Process according to one of the preceding claims, characterized in that gaseous or liquid products from the thermal cracking or gasification of biomass, residues, waste or fossil energy sources are used as chemical compounds.