Method for producing thermal insulation for a high-temperature tube furnace and use

Abstract

A method for producing thermal insulation for a high-temperature tube furnace comprising at least the following steps: a) providing at least one heating conductor, wherein the at least one heating conductor is a wire-shaped heating conductor made of tungsten or molybdenum, b) fixing the at least one heating conductor with a slip containing aluminum oxide, c) drying the slip applied in b), d) applying a first layer of aluminum wool to the fixed at least one heating conductor, e) inserting the at least one heating conductor coated in this way into a sealable housing in such a way that a circumferential gap is formed between the sealable housing and the at least one coated heating conductor, f) introducing a second layer of a ceramic concrete mass containing at least 66% by mass of hollow spherical corundum, 2 to 12% by mass of hydraulically setting alumina cement and 1 to 2% by mass of at least one surfactant into the space formed to form a furnace body, g) Thermal treatment of the formed furnace body.

Description

[0001] The invention relates to a method for producing thermal insulation for a high-temperature tube furnace.

[0002] High-temperature tube furnaces are used for the thermal treatment and sintering of materials, for conducting chemical reactions, or for heating measuring systems. These high-temperature tube furnaces consist of a furnace housing, a muffle with a heating element located within it, and thermal insulation between the heating element and the housing. The heating elements are primarily heating coils made of molybdenum or tungsten and are suitable for temperatures up to 1900 °C. They are mounted on a ceramic substrate or embedded in a ceramic mass. The heating elements are surrounded by electrically insulating thermal insulation.

[0003] Ceramic wools are preferred for thermal insulation. High-temperature wools (HTW) include amorphous aluminum silicate and AES (high-temperature glass wools), as well as polycrystalline wools (PCW). Aluminum silicate wools, formerly known as ceramic fibers, are amorphous fibers produced by melting a combination of Al2O3 and SiO2, usually in a 50:50 weight ratio. ZrO2 may also be present. AES (alkaline earth silicate wools) consist of amorphous fibers produced by melting a combination of CaO, MgO, and SiO2 and are intended for high-temperature applications. Products made from AES wool are typically used at temperatures up to a maximum of 1200°C. Polycrystalline wools (PCW) consist of fibers with an Al2O3 content > 63 wt.% and a SiO2 content < 37 wt.%; they are produced from aqueous spinning solutions using the sol-gel process.The initially produced water-soluble green fibers (precursor product) become polycrystalline wools through subsequent heat treatment and are generally used at operating temperatures > 1300 °C and under critical chemical and physical application conditions.

[0004] Activities involving aluminum silicate and polycrystalline wools can release fiber dusts with carcinogenic potential. Based on current scientific knowledge, a cancer risk from inhaling these fiber dusts cannot be ruled out. The released fiber dusts are classified as Category 2 or Category 3 carcinogens according to TRGS 905, the "List of Carcinogenic, Mutagenic, or Reproductively Toxic Substances." According to this, fiber dusts from aluminum silicate wools (ASW) are classified as Category 2 carcinogens. Fiber dusts from polycrystalline wools (PCW) are classified as Category 3 carcinogens according to TRGS 905. Fiber dusts from AES wools are not classified as carcinogenic.

[0005] The fiber-free, refractory products currently used for thermal insulation are non-metallic ceramic materials. Commonly used materials are lightweight chamotte, perlite, vermiculite, expanded clay, hollow spherical corundum, or calcium hexaaluminate. A distinction is made between shaped and unshaped products: Shaped products (e.g., bricks, panels, molded parts) have a defined geometry, are ready for installation, and largely already heat-treated. The rigid geometry can be disadvantageous if the thermal insulation must be adapted to specific kiln shapes. Unshaped products (e.g., concretes, masses) are brought into their final shape by pouring, stamping, or spraying with the addition of a binder and are heat-treated after hardening. Unshaped products also include mortars and putties. The thermal insulation of the materials currently used in the high-temperature range is significantly lower than the insulating properties of ceramic wool.To improve this, WO 93 / 04010 A1 proposed increasing thermal insulation for certain applications by pouring an inorganic oxide-based foam with water glass as a hardener and hydrogen peroxide as a blowing agent. Ceramic coating systems, particularly for the protection of metallic components, are frequently proposed, for example, in EP 0 075 228 A2 and DE 10 2010 060 944 B3.

[0006] DE 196 06 007 A1 discloses a lining for kilns, mills, and containers and a method for producing such a lining. The lining is produced by laying composite bricks consisting of at least one facing layer and at least one supporting layer. The at least one supporting layer is firmly bonded to the facing layer by prior firing of a reactive mortar through a ceramic bond.

[0007] These suggestions are not suitable for furnace construction, especially for smaller tube furnaces.

[0008] The objective is therefore to provide a thermal insulation and a method for producing thermal insulation for high-temperature tube furnaces that overcome the disadvantages of the state of the art. In particular, the thermal insulation should not require ceramic wool or fibers, but should exhibit approximately the same thermal insulation properties and good compatibility with metallic heating conductors, and be suitable for temperatures up to 1800 °C.

[0009] According to the invention, the object is achieved by a method for producing thermal insulation according to independent claim 1. Advantageous embodiments of the invention are specified in the dependent claims.

[0010] According to the invention, the method for producing thermal insulation for a high-temperature tube furnace comprises at least the following steps: a) providing at least one heating conductor, wherein the at least one heating conductor is a wire-shaped heating conductor based on tungsten or molybdenum, b) fixing the at least one heating conductor with a slip containing aluminum oxide, c) drying the slip applied in b), d) applying a first layer of aluminum wool to the fixed at least one heating conductor, e) inserting the at least one heating conductor coated in this way into a sealable housing in such a way that a circumferential gap is formed between the sealable housing and the at least one coated heating conductor, f) introducing a second layer of a ceramic concrete mass into the space formed to form a furnace body, g) Thermal treatment of the formed furnace body.

[0011] This advantageously produces thermal insulation without ceramic wool or fibers, which has almost identical thermal insulation properties and good compatibility with metallic heating conductors, and is suitable for temperatures up to 1800 °C. Furthermore, it is advantageous to produce thermal insulation that is individually adapted to different furnace types and dimensions.

[0012] In embodiments, in step a), the at least one heating conductor is provided on a heating conductor support tube. Advantageously, the heating conductor support tube is a ceramic heating conductor support tube onto which the at least one heating conductor is wound. In further embodiments, the at least one wire-shaped heating conductor has a diameter in the range of 0.5 mm to 3 mm.

[0013] Based on tungsten or molybdenum, heating conductors made of tungsten, tungsten alloys, molybdenum and molybdenum alloys.

[0014] A slip, as defined in the invention, refers to a suspension of water and powdered material, such as aluminum oxide powder. A slip may additionally contain dispersing aids, binders, stabilizers, wetting agents, and / or defoamers known to those skilled in the art.

[0015] Advantageously, in step b) the at least one heating conductor is fixed to the heating conductor support tube by applying the slip containing aluminum oxide to the at least one heating conductor, for example by coating it.

[0016] In further embodiments, the at least one heating conductor is at least partially coated in step b). If the at least one heating conductor is provided on a heating conductor support tube, at least those regions of the at least one heating conductor that are not in contact with the heating conductor support tube are coated in step b). If the at least one heating conductor is provided on a heating conductor support tube in the form of a winding, it is advantageous if the regions of the heating conductor that are not in contact with the heating conductor support tube and the regions between the windings of the heating conductor are coated in step b).

[0017] In embodiments, the drying in step c) is carried out at a temperature in the range of 20 °C to 150 °C for a duration in the range of 60 to 600 min in air.

[0018] Aluminium wool in the sense of the invention means wool made of aluminium or aluminium alloys, turnings made of aluminium or aluminium alloys and / or mats formed from wires made of aluminium or aluminium alloys.

[0019] In embodiments, the aluminum wool applied in step d) can be impregnated with an aluminum nitride-containing slurry. Advantageously, the impregnated aluminum wool contains up to 10 wt.% aluminum nitride. In embodiments, an aluminum nitride-containing slurry contains at least 90 wt.% aluminum nitride in the dried state. In further embodiments, the aluminum nitride contained in the slurry has a grain size of ≤ 20 µm.

[0020] A sealable housing within the meaning of the invention means a metallic housing, preferably a cylindrical metallic housing, sealable, for example, with lids. In further embodiments, the sealable housing has devices for supplying and discharging gases, a power connection, and a vacuum connection. This advantageously allows for the establishment of a defined atmosphere within the sealable housing and the direct heating of the heating conductor. This is particularly advantageous for the thermal treatment in step g).

[0021] The second layer introduced in step f) into the space between the sealable housing and the at least one coated heating conductor, together with the first layer of aluminum wool, forms the thermal insulation of the high-temperature tube furnace. The radial dimension of the space must be selected depending on the dimension of the resulting furnace body in order to achieve sufficient thermal insulation.

[0022] In embodiments, in step b) a slurry containing at least 80 mass% aluminum oxide and free of silicon is applied.

[0023] In further embodiments, the aluminum oxide contained in the slip has a grain size ≤ 100 µm.

[0024] In embodiments, in step d) an aluminum wool having a thread thickness in the range of ≤ 0.5 mm is applied.

[0025] This advantageously prevents the formation of a melting body of wool.

[0026] According to the invention, in step f) a ceramic concrete mass containing at least 66 mass% of hollow spherical corundum is introduced.

[0027] Advantageously, such a ceramic concrete mass has good dimensional stability at high temperatures.

[0028] In embodiments, the hollow spherical corundum has a grain size in the range of 0.1 to 4 mm.

[0029] In embodiments, the ceramic concrete mass further contains 5 to 20 mass% of at least one hydrophobic metal soap.

[0030] This advantageously generates microporosity in the ceramic concrete mass, which significantly determines the insulating properties of the ceramic concrete mass. In some embodiments, microporosity in the range of 30 to 70 vol.% is generated.

[0031] In embodiments, the at least one hydrophobic metal soap comprises aluminum stearate and calcium stearate.

[0032] According to the invention, the ceramic concrete mass further contains 2 to 12 mass% of hydraulically setting alumina cement and 1 to 2 mass% of at least one surfactant.

[0033] A surfactant is a metal- and sulfur-free compound, such as fatty alcohol ethoxylates or alkyl polyglycosides. The ceramic concrete mass also contains 15 to 25% water by mass, with all components totaling 100% by mass. The surfactant advantageously facilitates the solubility of the hydrophobic metal soap. Such a ceramic concrete mass is vibratable or pourable, allowing it to be easily incorporated in step f).

[0034] In embodiments, the thermal treatment in step g) is carried out in at least two stages, with the thermal treatment taking place in a first stage in air up to a maximum temperature of 450°C and in a second stage in a nitrogen-containing, oxygen-free atmosphere at a temperature above 450°C up to at least 900°C. Advantageously, in the first stage, the water contained in the ceramic concrete mass is removed and the at least one hydrophobic metal soap - aluminum or calcium stearate - is decomposed. Further advantageously, the second stage involves the formation of aluminum nitride. During heating, the aluminum of the introduced aluminum wool becomes liquid at approximately 660°C. Apparently due to the naturally present oxide skin on the aluminum, the aluminum wool does not melt together.

[0035] In some embodiments, the atmosphere is exchanged after the first stage of thermal treatment. For example, the sealable housing can be evacuated and then flooded with the atmosphere required for the second stage of thermal treatment.

[0036] In further embodiments, the thermal treatment in the second stage takes place in a nitrogen-containing, oxygen-free atmosphere at a temperature above 450°C up to a temperature of 1800°C.

[0037] A nitrogen-containing, oxygen-free atmosphere within the meaning of the invention means an atmosphere containing at least 20 vol.% nitrogen.

[0038] In embodiments, the thermal treatment in step g) is carried out by direct heating of the at least one heating conductor or by indirect heating of the formed furnace body in a furnace.

[0039] In further embodiments, the thermal treatment is carried out at a heating rate ≤ 300 K / h.

[0040] The duration of the thermal treatment is chosen depending on the dimensions of the formed furnace body.

[0041] Furthermore, a thermal insulation for a high-temperature tube furnace is shown, comprising at least a first layer and a second layer, characterized in that the first layer contains aluminum nitride, and the second layer is a highly porous aluminum oxide concrete layer, and has a total porosity in the range of 30 to 70 vol.%.

[0042] Advantageously, such thermal insulation is free of ceramic wool and fibers and exhibits almost the same thermal insulation properties as thermal insulation with ceramic wool and fibers. Furthermore, such thermal insulation exhibits good compatibility with metallic heating conductors at temperatures up to 1800 °C.

[0043] In embodiments, the first aluminum nitride-containing layer is loosely packed and thus advantageously capable of absorbing expansion changes, e.g., in diameter, and thus preventing cracking in the aluminum oxide concrete layer when the temperature increases. Furthermore, the first aluminum nitride-containing layer advantageously serves as an additional thermal insulation barrier, significantly reducing the heat flow from the at least one heating conductor to the outside. Furthermore, the first aluminum nitride-containing layer protects the at least one heating conductor from oxidation and acts as a getter layer. In the process, the aluminum nitride is converted into a highly temperature-resistant oxide.

[0044] In embodiments, the first layer initially contains at least 90 mass% aluminum nitride or consists entirely of aluminum nitride.

[0045] In embodiments, the highly porous aluminum oxide layer contains at least 66 mass% Al2O3 in the form of hollow spherical corundum bound in an alumina cement binder phase.

[0046] In some embodiments, the first layer has a thickness in the range of 5 mm to 20 mm. In other embodiments, the thickness of the second layer depends on the dimensions of the respective furnace and the desired insulation effect.

[0047] The invention further includes the use of a method according to the invention for producing thermal insulation of a high-temperature tube furnace and / or thermal insulation produced in this way in a high-temperature tube furnace.

[0048] The invention is not limited to the embodiments illustrated and described, but also encompasses all embodiments having the same effect within the meaning of the invention. Furthermore, the invention is not limited to the specifically described combinations of features, but can also be defined by any other combination of specific features of all the individual features disclosed as a whole, provided that the individual features are not mutually exclusive or a specific combination of individual features is not explicitly excluded. Examples of implementation

[0049] The invention will be explained in more detail below using an exemplary embodiment. This exemplary embodiment is intended to describe the invention without limiting it.

[0050] In step a), at least one heating element is provided on a heating element support tube. The heating element support tube is a ceramic tube made of KER 710 (Al2O3) with a length of 240 mm and an outer diameter of 50 mm. A wire-shaped heating element made of molybdenum with a diameter of 1 mm is wound onto the heating element support tube over a length of 200 mm with a pitch of 2.5 mm. In step b), the heating element is coated with an aluminum oxide-containing slurry and thus fixed to the heating element support tube. The heating element is at least partially coated so that the spaces between the heating element windings are partially filled with the aluminum oxide-containing slurry. Subsequently, in step c), the applied coating is dried in air at 120°C for 60 minutes. Subsequently, in step d), a first layer of aluminum wool approximately 10 mm thick is applied.The aluminum wool is 0.5 mm thick aluminum wool soaked in a slip containing aluminum nitride. Subsequently, in step e), the wrapped heating conductor is inserted into a sealable housing in such a way that a circumferential gap is formed between the sealable housing and the inserted heating conductor. The sealable housing is a sheet metal cylinder, sealable with a lid and with a diameter of 160 mm. The sheet metal cylinder has a vacuum connection, a gas inlet and outlet, and a power connection. A ceramic tube, closed on one side and containing an internal type B thermocouple, protrudes through the lid of the sheet metal cylinder into the heating conductor support tube wound with molybdenum wire. Subsequently, in step f), a second layer of a ceramic concrete mass is introduced into the resulting gap to form a furnace body.The ceramic concrete mass consists of 67 mass% hollow spherical corundum, 4 mass% alumina cement, 8 mass% aluminum stearate, 1 mass% surfactant, and 20 mass% water. The resulting furnace body is then thermally treated in step g). For this purpose, in a first step, the temperature is increased to 400°C at a heating rate of 150 K / h by external, indirect heating with air purging. This removes the water from the ceramic concrete mass and decomposes the aluminum stearate. This is followed by evacuation via the vacuum connection of the sheet metal cylinder and flooding with nitrogen. The molybdenum heating element is directly subjected to voltage, and the temperature is increased to 1750°C at a heating rate of 300 K / h. Aluminum nitride formation begins at temperatures above 800°C and is completed at temperatures around 1300°C.

Claims

[1] A method for producing thermal insulation for a high-temperature tube furnace comprising at least the following steps: a) providing at least one heating conductor, wherein the at least one heating conductor is a wire-shaped heating conductor made of tungsten or molybdenum, b) fixing the at least one heating conductor with a slip containing aluminum oxide, c) drying the slip applied in b), d) applying a first layer of aluminum wool to the fixed at least one heating conductor, e) inserting the at least one heating conductor coated in this way into a sealable housing in such a way that a circumferential gap is formed between the sealable housing and the at least one coated heating conductor, f) introducing a second layer of a ceramic concrete mass containing at least 66% by mass of hollow spherical corundum, 2 to 12% by mass of hydraulically setting alumina cement and 1 to 2% by mass of at least one surfactant into the space formed to form a furnace body, g) Thermal treatment of the formed furnace body. [2] Method according to claim 1, characterized by that in step b) a silicon-free slip containing at least 80 mass% aluminum oxide is applied. [3] Method according to claim 1 or 2, characterized by that in step d) an aluminum wool having a thread thickness in the range of ≤ 0.5 mm is applied. [4] Method according to one of claims 1 to 3, characterized by that the ceramic concrete mass further contains 5 to 20 mass% of at least one hydrophobic metal soap. [5] Method according to claim 4, characterized bythat the at least one hydrophobic metal soap comprises aluminum stearate and calcium stearate. [6] Method according to one of claims 1 to 5, characterized by that the thermal treatment in step g) is carried out in at least two stages, wherein in a first stage the thermal treatment is carried out in air up to a maximum temperature of 450°C and in a second stage in a nitrogen-containing, oxygen-free atmosphere at a temperature above 450°C up to at least 900°C. [7] Use of a method according to one of claims 1 to 6 for producing thermal insulation of a high-temperature tube furnace.