Silicon carbide grit and method for producing same

Abstract

The invention relates to the field of industrial ceramic and relates to silicon carbide grit that can be used, for example, for refractory products, and to a method that can be used for the production of different qualities and grain sizes of silicon carbide grit. The object of the present invention is to provide silicon carbide grit in which the SiC particles have a high density. This object is achieved by silicon carbide grit of which the particles consist to an extent of at least 85 mass% of SiC waste products and have a density of 2.89 to 3.20 g / cm3, a compressive strength of > 2500 MPa, a fraction of < 5% of pores with an equivalent diameter of > 100 µm and a fraction of open porosity of < 10%, wherein the silicon carbide grit consists at least predominantly of particles with particle sizes of greater than or equal to 2 mm and the particles have an irregular shape produced by a mechanical loading.

Description

[0001] Silicon carbide grit and process for its production

[0002] The invention relates to the field of technical ceramics and concerns silicon carbide grit, which can be used, for example, for refractory products, and a process for producing silicon carbide grit, which can be used for producing different qualities and grain sizes of silicon carbide grit.

[0003] Silicon carbide (SiC) is a synthetic industrial mineral used in many industries due to its outstanding properties (hardness, high-temperature properties, chemical resistance). Its use in the form of fractionated ultrafine powder grains (0.5 to approximately 250 pm) is particularly important in microelectronics / photovoltaics (wafer sawing), for the production of technical ceramics (mechanical seals, ballistic protective ceramics for military technology), automotive and environmental technology (diesel particulate filters), as an abrasive material for high-quality surface finishing in mechanical engineering, and as a component in refractory materials for the lining of waste incineration plants.Raw SiC is produced using the Acheson process (DE 76629 A, DE 85197 A), which has been used for approximately 100 years. In this process, SiC is synthesized by carbothermal reduction of SiO2 (quartz sand) with carbon (usually petroleum coke) in an electric furnace. This process is significantly tied to the price of electricity and oil (petrol coke being the raw material) and also incurs relatively high environmental costs (due to high dust, CO / CO2, and SO2 emissions).

[0004] Despite many attempts, alternative manufacturing processes have not been successful, mostly for economic reasons, and will not be available in the foreseeable future.

[0005] The desired SiC powder grains are produced from the raw SiC after SiC production by grinding, purification and fractionation.

[0006] An increase in grain demand therefore always requires an increase in raw SiC production, which leads to a further increase in raw capacity and thus to structural scarcity and price inelasticity.

[0007] Although SiC is considered a globally available commodity raw material, shortages and price increases for strategically important high-quality (HQ) grains have been observed for years. An even greater problem with specialty grains, however, is that large quantities of individual grain size ranges are required for high-tech applications. Both of these factors lead to price premiums and supply bottlenecks for these specialty grains due to the aforementioned price inelasticity.

[0008] The fractionation of SiC powders, especially the F4 to F220 grain sizes, is typically verified using sieving methods according to FEPA Standard 42-1:2006 or ISO 8486 and using test sieves. The coarsest SiC powder size currently available according to these standards is F4 grain, also called macrograin, which has an average particle size of approximately 4.8 mm.

[0009] Coarser fractions of SiC powder are not available on the market. Numerous semi-finished and finished products are manufactured from raw SiC.

[0010] According to DD 300 288 A7, abrasion-resistant ceramic grinding balls and a process for their production based on SiSn4 and SiC are known. The grinding balls are formed from very fine SiC powder mixtures with grain sizes < 1 pm, produced by pressing or granulation, and then sintered into SiC ceramic. They have densities of 2.93–3.11 g / cm³. 3

[0011] These grinding balls have a high roundness of >0.95 as intended. Roundness is defined and measured according to DIN ISO 13322 Parts 1 and 2.

[0012] During the production of SiC products, various SiC waste is generated, such as so-called SiC sinter scrap or other production-related SiC waste.

[0013] But even after the products have been used and their service life has expired or after the products have failed, SiC waste remains, such as grinding sludge or used kiln furniture, or used SiC diesel particulate filters.

[0014] Various processes are already known for recycling SiC products.

[0015] According to DE 10 2013 218 450 A1, a process for recycling powdered silicon carbide waste products is known, in which powdered SiC waste products containing at least 50 mass% SiC and an average grain size d50 between 0.5 and 500 pm are subjected to a temperature treatment under vacuum or oxygen-free atmosphere at temperatures of at least 2000 °C.

[0016] As a result, the SiC particles become coarser again to a few mm and can therefore be used again for a number of applications in which they would otherwise not have been usable due to their small particle size.

[0017] Furthermore, according to DE 10 2020 102 512.2 A1, a process for separating impurities from silicon carbide is known, in which powdery SiC waste products which have at least 50 mass% SiC and an average grain size d50 between 0.5 and 1000 pm, are subjected to a temperature treatment under vacuum or in a non-oxidizing atmosphere at temperatures of 1400 - 2600 °C and cooled, are subsequently mechanically treated and physically separated into two fractions, of which the mass of impurities in one fraction is at least a factor of 2 higher than in the other fraction.

[0018] A disadvantage of the known processes for recycling products containing SiC is that only a few specific SiC waste products have been recycled so far.

[0019] The object of the present invention is to provide silicon carbide grit in which the SiC particles have a high density and further to provide a simple and cost-effective process for producing silicon carbide grit.

[0020] The object is achieved by the invention defined in the claims. Advantageous embodiments are the subject of the dependent claims, whereby the invention also includes combinations of the individual claims in the sense of an "and" connection, as long as they are not mutually exclusive.

[0021] The silicon carbide gravel according to the invention, the particles of which consist of at least 85 mass % silicon carbide from SiC waste products and have a density of 2.89 to 3.20 g / cm 3, a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, consists at least predominantly of particles with particle sizes greater than or equal to 2 mm, and the particles have an irregular shape with a roundness of 0.5 to 0.8 produced by mechanical stress with an energy input of between 0.1 and 5 MJ / kg.

[0022] Advantageously, SiC waste products from the Acheson process or SiC sinter scrap or manufacturing-related SiC waste from product manufacturing are present as SiC waste products, the particles of which consist of at least 85 mass% silicon carbide and have a density of 2.89 to 3.20 g / cm 3, a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, wherein the particles at least predominantly have particle sizes greater than or equal to 2 mm and the particles have an irregular shape with a roundness of 0.5 to 0.8 produced by mechanical stress with an energy input of between 0.1 and 5 MJ / kg, wherein such products made of SSiC, LPS-SiC, SiSiC, RSiC, NSiC and / or OBSiC ceramics and / or fiber composites made of C-SiC and / or SiC-SiC are also advantageously present as SiC waste products.

[0023] Furthermore, the silicon carbide gravel according to the invention advantageously consists of at least 90 mass%, advantageously 95 mass%, even more advantageously 98 mass% silicon carbide.

[0024] Also advantageously, the particles of the silicon carbide gravel according to the invention have a density of 3.05 to 3.20 g / cm 3

[0025] And also advantageously, the silicon carbide grit according to the invention consists at least predominantly, advantageously at least 85%, even more advantageously at least 95%, of particles with a particle size of greater than or equal to 2 mm.

[0026] It is also advantageous if the particle sizes of the silicon carbide gravel according to the invention are between 2 mm and 20 mm, or 5 mm and 63 mm.

[0027] It is furthermore advantageous if the particle shapes of the silicon carbide gravel according to the invention, which are realized after mechanical stressing of the SiC waste products by applying a mechanical impulse, have been produced by mixing, grinding, more advantageously by autogenous grinding, or by the use of eddy currents and / or ultrasound, or by grinding, hammering, breaking or by means of electrical discharges or shock waves.

[0028] It is also advantageous if the silicon carbide gravel according to the invention has irregular, completely and / or partially sharp-edged and / or irregular, completely or partially rounded particle shapes.

[0029] In the process according to the invention for producing silicon carbide gravel, SiC waste products are subjected to mechanical stress with an energy input of between 0.1 and 5 MJ / kg to SiC particles which consist of at least 85 wt. % silicon carbide and have a density of 2.89 to 3.20 g / cm 3 , a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, and with particle sizes greater than or equal to 2 mm and an irregular shape with a roundness of 0.5 to 0.8, or SiC waste products are processed before and / or after a temperature treatment under vacuum or non-oxidizing atmosphere at temperatures of 1400 to 2600 °C by means of mechanical stress with an energy input of between 0.1 and 5 MJ / kg to SiC particles which consist of at least 85 mass% silicon carbide and have a density of 2.89 to 3.20 g / cm 3, a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, and with particle sizes greater than or equal to 2 mm and an irregular shape with a roundness of 0.5 to 0.8, or SiC waste products are processed before and / or after a temperature treatment under vacuum or non-oxidizing atmosphere at temperatures of 1400 to 2600 °C by means of mechanical stress with an energy input of between 0.1 and 5 MJ / kg to SiC particles which consist of at least 85 mass% silicon carbide and have a density of 2.89 to 3.20 g / cm 3, a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, and with particle sizes of greater than or equal to 2 mm and an irregular shape with a roundness of 0.5 to 0.8, whereby these particles can be mechanically treated again and are physically separated during or after this into two fractions, of which the mass of impurities in one fraction is at least a factor of 2 higher than in the other fraction.

[0030] Advantageously, at least after one or the last treatment under mechanical stress of the SiC particles, a physical separation of the SiC particles into fractions is carried out.

[0031] Furthermore, SiC waste products are advantageously subjected to a temperature treatment under vacuum or non-oxidizing atmosphere at temperatures of 1400 to 2600 °C, followed by a treatment under mechanical stress and then a physical separation of the SiC particles into fractions.

[0032] Also advantageously, SiC waste products with a density of 3.05 to 3.20 g / cm 3 used.

[0033] And also advantageously, the mechanical stressing of the SiC waste products is realized by applying a mechanical impulse, advantageously by mixing, grinding, even more advantageously by autogenous grinding, or by the use of eddy currents and / or ultrasound or by means of grinding, hammering, breaking or by means of electrical discharges or shock waves.

[0034] It is also advantageous if particles with particle sizes between greater than or equal to 2 mm and 20 mm, or 5 mm and 63 mm are realized.

[0035] It is also advantageous if a temperature treatment is carried out at temperatures between 2000 °C and 2600 °C.

[0036] It is also advantageous if a temperature treatment is carried out under an argon or nitrogen atmosphere.

[0037] It is also advantageous if a temperature treatment is carried out at temperatures of at least 2000 °C for between 10 and 300 minutes.

[0038] It is also advantageous if the physical separation of the particles after the temperature treatment is carried out according to the particle size, the particle shape, the density and / or the physical and / or chemical surface properties of the particles.

[0039] It is also advantageous if the separation according to particle size and / or particle shape is carried out by sieving, sifting and / or cyclone processes, or the separation is carried out by the action of mass forces with respect to the particle density by means of flotation, sedimentation, sifting, centrifugation and / or cyclone processes, or the separation according to the density of the particles is carried out by flotation and / or cyclone processes.

[0040] According to the invention, the silicon carbide grit according to the invention, which is produced according to the invention, is used for the production of SiC-containing ceramics, in particular refractory ceramics.

[0041] With the solution according to the invention, it is possible for the first time to provide silicon carbide grit whose particles have a high mechanical strength and, furthermore, to provide a simple and cost-effective process for producing silicon carbide grit.

[0042] This is achieved by silicon carbide gravel, consisting of particles that consist of at least 85 mass% silicon carbide from SiC waste products and have a density of 2.89 to 3.20 g / cm 3 , a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%.

[0043] Gravel is a term for a grain size and for a loose sediment.

[0044] According to DIN 4022 and DIN EN ISO 14668, the grain size for gravel is specified as a grain diameter between 2 mm and 63 mm. A distinction is also made between coarse gravel with grain sizes of 20–63 mm, medium gravel with grain sizes of 6.3–20 mm, and fine gravel with grain sizes of 2–6.3 mm (Wikipedia, keyword "Gravel").

[0045] Gravel as loose sediment, or loose rock, is a consolidated heap with little grain binding (Wikipedia, keyword loose sediment).

[0046] In the context of the present invention, the term silicon carbide grit refers to SiC particles with grain diameters of greater than or equal to 2 mm to 63 mm. The SiC particles according to the invention are irregularly shaped due to mechanical stress with an energy input of between 0.1 and 5 MJ / kg, but have predominantly convex surfaces. Average roundnesses in the range of 0.5 to 0.8 are measured using known grain shape analyzers.

[0047] The grain size, grain shape and roundness are defined and measured according to DIN-ISO 13322 Parts 1 and 2.

[0048] Advantageously, the silicon carbide gravel according to the invention consists of particles with at least 85 mass%, advantageously 90 mass%, even more advantageously 95 mass%, and even more advantageously 98 mass% of silicon carbide from SiC waste products.

[0049] Of particular importance according to the invention is that the particles of essentially silicon carbide of the silicon carbide gravel according to the invention are produced from and consist of SiC waste products.

[0050] For the purposes of the present invention, SiC waste products are understood to mean SiC products that arise during the production of raw SiC or from semi-finished and finished products made from SiC that were originally produced from raw SiC, sintered, and processed into shaped bodies. Likewise, SiC waste products used according to the invention are products that arise as waste during the production of SiC semi-finished and finished products made from SiC when the material exhibits cracks, deformations, poor dimensional accuracy, or other defects and is sorted out as rejects, and is then known as SiC sintered scrap. However, SiC waste, such as used kiln furniture or used SiC diesel particulate filters, also occurs after the products have been used, at the end of their service life, or after they have failed or been destroyed.

[0051] For the purposes of the present invention, SiC waste products are always understood to mean lumpy SiC waste products that are processed to the desired particle size for SiC gravel, advantageously crushed by mechanical stress. In any case, the SiC waste products according to the invention do not relate to SiC waste products that require grain growth to coarsen the particle size of the SiC waste products in order to achieve the desired particle size for SiC gravel. This applies, for example, to SiC waste products with particle sizes of less than 2 mm, such as SiC powder, SiC grinding sludge, or SiC dust.

[0052] With the solution according to the invention, no grain growth is realized but rather a chemical conversion of the SiC waste products in order to realize a higher SiC content, as well as a reduction of the open porosity and / or the realization of a certain grain shape with regard to roundness.

[0053] All such SiC products can be used according to the invention as SiC waste products.

[0054] The particles of silicon carbide gravel have a density of 2.89 to 3.20 g / cm 3 Advantageously, the particles of the silicon carbide gravel according to the invention have a density of 3.05 to 3.20 g / cm 3 , measured using pycnometric methods.

[0055] Furthermore, the SiC particles of the silicon carbide gravel according to the invention have a very high compressive strength of > 2500 MPa due to the very low content of < 5% of coarse pores with an equivalent diameter of > 100 pm, which crushed raw SiC from the Acheson process does not achieve.

[0056] The determination of grain size and grain shape can be carried out according to DIN-ISO 13322 Parts 1 and 2 (static and dynamic analysis).

[0057] The grain strength can be measured using the method according to ASTM D5731-16 (Standard Test Method for Determination of the Point Load Strength Index of Rock and Application to Rock Strength Classifications) or the analogous method according to “ISRM - International Society for Rock Mechanics 2007. Rock characterization testing & monitoring - ISRM suggested methods. Ed. ET Brown, Pergamon Press, London, 211 p.” according to the test specimen shape / dimension d) “irregular lump”.

[0058] The reference to the breaking force is made by determining an equivalent diameter.

[0059] These methods can be applied equivalently for SiC particles according to the present invention. The strength is standardized according to the aforementioned methods and expressed as a point load strength index ls(50) in MPa. Using a correlation method, a compressive strength of c can be calculated in MPa.

[0060] The compressive strength of the particles of the SiC gravel according to the invention determined in this way is >2500 MPa, while coarse SiC particles crushed from Acheson raw material have compressive strengths of significantly less than 1000 MPa due to their high fracture fracture and porosity.

[0061] Therefore, the use of such raw SiC as a starting material for various applications, such as refractory products, is also out of the question. Due to the high fracture and porosity of the SiC particles, high quality refractory products in terms of strength and durability cannot be achieved. SiC ceramic bodies manufactured specifically from special powders using a ceramic manufacturing process, such as grinding balls, do not achieve the required packing density of the SiC particles and are also not used due to the high price resulting from the complex manufacturing process.

[0062] Of particular importance is that the SiC particles according to the invention have only very few large pores with an equivalent diameter of > 100 pm. These pores are irregularly shaped cavities in the structure and are determined by image analysis using polished sections. The size is determined here as the equivalent diameter with the same area. The quantity of such pores is given as a percentage according to the area fraction determined by image analysis, which can be expressed as a volume fraction in percent using stereological principles. It is also possible, albeit more complex, to produce so-called micro-X-ray computed tomography images of the gravel particles and thus determine the size and volume fraction of the large pores with an equivalent diameter of > 100 pm.

[0063] Furthermore, the particles of the gravels according to the invention advantageously have a very low open porosity of <10%. In the context of the present invention, these pores include all pores, cavities, and cracks in the SiC particles that are accessible from the surface. Open porosity is commonly measured using liquid or gas pycnometry or mercury porosimetry.

[0064] The SiC waste products according to the invention can advantageously be products made of SSiC, LPS-SiC, SiSiC, RSiC, NSiC, and / or OBSiC ceramics and / or fiber composites made of C-SiC and / or SiC-SiC. The abbreviations and designations cited for the various SiC ceramics are familiar to those skilled in the art and can be found, for example, in the Brevier Technische Keramik (Brevier Technische Keramik: Ed. von Verband der Keramischen Industrie, 4th edition 2004, ISBN: 9783924158361). Waste from high-SiC mineral concretes, also called mineral castings, can also be advantageous SiC waste products. SiC mineral concretes consist of SiC particles in a polymer matrix, usually made of polyester resins.

[0065] When selecting the SiC waste products used according to the invention, it is also possible to carry out a pre-sorting according to their composition prior to use in the processes according to the invention. This allows the production of silicon carbide grit according to the invention of high purity and quality.

[0066] If SSiC waste products are used, usually no additives need to be added in the production of silicon carbide grit according to the invention and silicon carbide grit with particle sizes greater than or equal to 2 mm of high purity > 99% and with a pore-freeness of almost 100% is achieved.

[0067] For LPS-SiC waste products, no additives need to be added in the production of silicon carbide grit according to the invention, and silicon carbide grit with particle sizes greater than or equal to 2 mm of high purity > 90% and with a pore-freeness of almost 100% is achieved.

[0068] For SiSiC waste products, the free Si content is quantitatively determined using known methods, and an appropriate amount of carbon-containing additives is added to the waste products to achieve a stoichiometric conversion of Si+C to SiC. The optimal amount can be determined by a specialist through simple test series. This results in silicon carbide grit with particle sizes greater than or equal to 2 mm, high purity > 98%, and a content of < 5% of pores with an equivalent diameter of > 100 pm.

[0069] For RSiC waste products, 2-20% of fine SiC powder with a grain size of 0.2-10 pm is added, resulting in silicon carbide grit with particle sizes greater than or equal to 2 mm and a high purity of > 99%, containing < 5% pores with an equivalent diameter of > 100 pm. The addition of the fine SiC powder seals the open porosity of the RSiC particles during thermal treatment. The optimal amount and particle size of the added fine SiC particles can be determined by a specialist through simple series of tests until an open porosity of < 10% is achieved.

[0070] For NSiC waste products, which also include SiON, SiAlON, SiSn, and similar nitride-bonded SiC waste products, the oxide content is primarily determined, and an appropriate amount of carbon-containing additives is added to achieve oxide reduction. The optimal amount can be determined by a specialist through simple test series. The resulting silicon carbide grit has particle sizes greater than or equal to 2 mm, a SiC content of > 85%, and a pore content of < 5% with an equivalent diameter of > 100 pm.

[0071] For OBSiC waste products, which include SiO2 and aluminosilicate-bonded SiC waste products, the oxide content is primarily determined, and an appropriate amount of carbon-containing additives is added to achieve oxide reduction. The optimal amount can be determined by a specialist through simple test series. The resulting silicon carbide grit has a particle size greater than or equal to 2 mm and a high purity of > 85%, with a pore content of < 5% with an equivalent diameter of > 100 pm.

[0072] If C-SiC waste products, i.e., waste from carbon fiber-reinforced SiC composites, such as short- or long-fiber-reinforced materials, or composites produced via siliconization or precursor infiltration, are used, the free Si and C contents must be analyzed during the inventive production of the silicon carbide gravel, and an appropriate amount of Si or C additives must be used to create a stoichiometric composition. Since C-SiC waste products usually have a significantly higher C content than a free Si content, Si additives must usually be added to convert the excess carbon to SiC. The addition takes place in the amount required to achieve a stoichiometric composition to SiC.The optimum quantity can be determined by a specialist through simple test series and silicon carbide gravel with particle sizes greater than or equal to 2 mm of high purity > 95% with a content of < 5% of pores with an equivalent diameter of > 100 pm is achieved.

[0073] If SiC-SiC waste products, i.e. waste from silicon carbide fiber-reinforced SiC composites, are used, 2-20% of fine SiC powder with a grain size of 0.5-5 pm is added, resulting in silicon carbide grit with particle sizes greater than or equal to 2 mm and a high purity of > 99%, with a content of < 5% of pores with an equivalent diameter of > 100 pm. The addition of the fine SiC powder closes the open porosity of the SiC-SiC particles during thermal treatment. The optimal amount and particle size of the added fine SiC particles can be determined by a person skilled in the art through simple test series until an amount of open porosity of < 10% is achieved.

[0074] For SiC mineral concrete waste products, a temperature treatment at 600–1000 °C under exclusion of air (pyrolysis) is first carried out. The carbon content is then quantitatively determined using known methods, and an appropriate amount of silicon-containing additives is added to the waste products to achieve a stoichiometric conversion of Si+C to SiC. The optimal amount can be determined by a specialist through simple test series. The resulting silicon carbide grit has a particle size greater than or equal to 2 mm, a high purity of > 98%, and a pore content of < 5% with an equivalent diameter of > 100 pm.

[0075] For coarse waste products from raw SiC production, 5-20% of fine SiC powder with a grain size of 0.2-10 pm is added, resulting in silicon carbide grit with particle sizes greater than or equal to 2 mm and a high purity of > 99%, containing < 5% pores with an equivalent diameter of > 100 pm. The addition of the fine SiC powder seals the open porosity of the SiC particles in the raw SiC during thermal treatment. The optimal amount and particle size of the added fine SiC particles can be determined by a specialist through simple series of tests until an amount of open porosity of < 10% is achieved.

[0076] Mixtures of the waste products mentioned can also be used very advantageously, especially if this reduces the type and amount of additives required. For example, it is advantageous to produce mixtures of SiSiC waste II products with C-SiC and / or pyrolyzed SiC mineral concrete waste products and subsequently treat these thermally. The components are then mixed according to the analyzed free Si and C contents in order to add as few or no further C- or Si-containing additives as possible. Mixtures of the various SiC waste products are used in this way to produce silicon carbide gravel with particle sizes greater than or equal to 2 mm, a purity of > 85% SiC and a content of < 5% of pores with an equivalent diameter of > 100 pm.

[0077] Furthermore, it is of particular importance according to the invention that the SiC waste products have been subjected to mechanical stress with an energy input between 0.1 and 5 MJ / kg and subsequently exhibit at least predominantly particle sizes greater than or equal to 2 mm and an irregular shape with a roundness of 0.5 to 0.8 produced by mechanical stress. The mechanical stress can occur either before the thermal treatment or after the thermal treatment, or even before and after the thermal treatment.

[0078] Treating SiC waste products with mechanical stress results in products with a wide range of particle sizes. Therefore, to produce the silicon carbide grit according to the invention, physical separation into different fractions is usually required. The yield of particles with particle sizes greater than or equal to 2 mm is at least 50%, usually 80 to 95%, due to the mechanical stress processes.

[0079] This is a clear difference from raw SiC obtained using the Acheson process, where the SiC crystals can reach sizes up to the millimeter range. However, these SiC crystals are highly fractured and intergrown, and exhibit significantly more than 5% coarse pores ranging in size from a few hundred micrometers up to the millimeter range of the equivalent diameter. This macrograin structure of raw SiC results in very low mechanical strength, making their application very limited or even impossible. Such macrograins cannot be used as abrasives, as they would break very easily, and likewise, they cannot be used in applications that are exposed to chemical, particularly oxidative, attack, or attack by aggressive media or atmospheres, as their high porosity offers a large surface area for attack and destruction of the material.

[0080] The particles of the silicon carbide gravel according to the invention have no or < 5% of pores with an equivalent diameter of greater than 100 pm.

[0081] The silicon carbide grit according to the invention contains SiC which, due to its starting materials as SiC waste products and due to the mechanical stress, is present in a macrograin of particles with particle sizes greater than or equal to 2 mm, which do not have the disadvantages of the macrograin of raw SiC.

[0082] Advantageously, the silicon carbide grit according to the invention consists at least predominantly, advantageously at least 85%, more advantageously at least 95%, of particles with a particle size of greater than or equal to 2 mm.

[0083] Furthermore, the silicon carbide grit according to the invention advantageously has particle sizes between 2 mm and 20 mm, or between 5 mm and 63 mm. Likewise, the silicon carbide grit according to the invention differs from silicon carbide products with dimensions greater than or equal to 2 mm in that the silicon carbide grit according to the invention consists of particles that have an irregular shape created by mechanical stress.

[0084] The produced irregular shape with a roundness of 0.5 to 0.8 of the silicon carbide gravel according to the invention is present after mechanical stressing of the SiC waste products with an energy input between 0.1 and 5 MJ / kg by applying a mechanical pulse, advantageously this has been realized by mixing, grinding, even more advantageously by autogenous grinding, or by the use of eddy currents and / or by ultrasound, or by grinding, hammering, breaking or by means of electrical discharges or shock waves, for example by means of electric pulse fragmentation or pulsed power processing.

[0085] The particles may advantageously be in the form of irregular, completely and / or partially sharp-edged and / or irregular, completely or partially rounded shapes.

[0086] The irregularity according to the invention relates to SiC particles that are irregularly shaped and have a roundness of 0.5 to 0.8. The irregularity refers to the shape, which according to the invention should not correspond to any geometric shape, such as spheres, cylinders, cubes, cuboids, pyramids, cones, pyramids, or truncated cones.

[0087] Furthermore, the silicon carbide grit according to the invention has only a low impurity content, since the starting materials, the SiC waste products, generally have a high SiC purity. Impurities still present, especially metallic impurities or impurities introduced by mechanical stress, can be reduced or eliminated by applying known impurity removal methods.

[0088] In the process according to the invention for producing silicon carbide gravel, SiC waste products are converted by mechanical stress into SiC particles which consist of at least 85 mass% silicon carbide and have a density of 2.89 to 3.20 g / cm 3, a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, with particle sizes of greater than or equal to 2 mm and an irregular shape, wherein a physical separation into different particle fractions can take place before and / or after mechanical stressing. Alternatively, silicon carbide gravel is produced according to the invention by converting SiC waste II products before and / or after a temperature treatment under vacuum or a non-oxidizing atmosphere at temperatures of 1400 to 2600 °C by means of mechanical stress to SiC particles which consist of at least 85 mass% silicon carbide and have a density of 2.89 to 3.20 g / cm 3, a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, and can be processed with particle sizes of greater than or equal to 2 mm and an irregular shape.

[0089] Even with this alternative process, physical separation into different particle fractions can occur both before and after mechanical stress.

[0090] Another alternative method according to the invention for producing silicon carbide gravel consists in that SiC waste products, before and / or after a temperature treatment under vacuum or non-oxidizing atmosphere at temperatures of 1400 to 2600 °C, are converted by means of mechanical stress into SiC particles which consist of at least 85 mass% silicon carbide and have a density of 2.89 to 3.20 g / cm 3, a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, and with particle sizes of greater than or equal to 2 mm and an irregular shape, it being possible for these particles to be subsequently mechanically treated again and, during or after this, to be physically separated into two fractions, of which the mass of impurities in one fraction is at least a factor of 2 higher than in the other fraction. In accordance with the other alternative production processes according to the invention, a physical separation into different particle fractions can take place both before and after the mechanical stress(es).

[0091] For all process alternatives according to the invention for the production of silicon carbide gravel, SiC waste products with a density of 3.05 to 3.20 g / cm 3Likewise, for all process alternatives according to the invention for the production of silicon carbide grit, the mechanical stressing of the SiC waste products with an energy input of between 0.1 and 5 MJ / kg is advantageously realized by applying a mechanical impulse, even more advantageously by mixing, grinding, such as by autogenous grinding, or even more advantageously by using eddy currents and / or ultrasound, or by grinding, hammering, crushing, or by means of electrical discharges and shock waves.

[0092] With the process variants according to the invention, SiC particles with particle sizes between greater than or equal to 2 mm and 20 mm, or 5 mm and 63 mm are advantageously realized in all cases.

[0093] In the case of a temperature treatment of the SiC particles, this is advantageously carried out at temperatures between 2000 °C and 2600 °C, even more advantageously at temperatures of at least 2000 °C between 10 and 300 min.

[0094] The temperature treatment is also advantageously carried out under an argon or nitrogen atmosphere.

[0095] During the temperature treatment, silicon or silicon dioxide powder and carbon powder can be added to the particles from the SiC waste II products, whereby an increase in the SiC content in the silicon carbide gravel according to the invention can be achieved.

[0096] Likewise, SiC powders with smaller particle sizes can be added during the heat treatment, which can then lead to a significant reduction in the porosity of the SiC powders from the SiC waste products. This is particularly advantageous for waste from raw SiC from the Acheson process. It is possible to achieve a significant reduction in the coarse pores in very porous SiC powders with particle sizes of > 2 mm from raw SiC from the Acheson process with a low SiC content and / or poorly crystallized SiC (so-called beta-SiC) by admixing finer SiC powders and other additives during the heat treatment. As a result, these silicon carbide grit particles have a high SiC content and good crystallization from so-called alpha-SiC.If, after a temperature treatment according to the invention, a physical separation of the particles according to the invention is carried out, this can advantageously be carried out according to the particle size, the particle shape, the density and / or the physical and / or chemical surface properties of the particles.

[0097] However, it is also possible if, after a temperature treatment according to the invention and in particular after the treatment(s) of the SiC waste II products with mechanical stress, a physical separation of the particles into fractions according to the invention is carried out, wherein this separation is advantageously carried out according to the particle size and / or particle shape by sieving, sifting and / or cyclone processes, or the separation is carried out by the action of mass forces with regard to the particle density by means of flotation, sedimentation, sifting, centrifugation and / or cyclone processes, or the separation is carried out according to the density of the particles by flotation and / or cyclone processes.

[0098] After a physical separation realized according to the invention, the SiC powder is separated into at least two fractions according to the invention, wherein, in the case that a treatment of the SiC waste products under mechanical stress has been carried out before a physical separation, a separation into at least two fractions takes place, of which the mass of impurities in one fraction is at least a factor of 2 higher than in the other fraction.

[0099] The removal of impurities in the form of Si and / or C is realized by the temperature treatment, since carbon, advantageously carbon black, graphite and / or coke powder, and / or silicon and / or silicon dioxide (SiC>2), which are added during the SiC production, can then be further converted to SiC and even added to achieve a composition as close to stoichiometric as possible.

[0100] Actual impurities for SiC are essentially metallic impurities, which accumulate predominantly in one fraction due to physical separation after heat treatment and can thus be easily removed.

[0101] AI and B are not considered as impurities in the production of raw SiC and also in the production of silicon carbide grit according to the invention, since they are incorporated into the SiC lattice as doping elements and are not disruptive for most applications of SiC.

[0102] The impurities, such as C-free, Si-free, SiO2, and iron, as well as the SiC content, are determined using known analytical methods, for example, according to DIN EN ISO 9286: 2021-10 and DIN EN ISO 21068 Parts 1-3. Spectroscopic methods are also used for the analysis of impurities, including DIN EN 15991.

[0103] The invention is explained in more detail below using several exemplary embodiments.

[0104] Example 1

[0105] 100 kg of lumpy SSiC sintered scrap with a SiC content of 99.5 wt.% is crushed using roller mills with metallic rollers at an energy input of 0.5 MJ / kg within 20 minutes. The resulting sharp-edged SSiC particles are sieved, and a fraction with a particle size of > 10 mm is separated with a yield of > 80%. This fraction is the silicon carbide grit according to the invention.

[0106] The individual particles in this fraction have SiC contents of 99.5 wt.% and a density of 3.10 g / cm 3 , a compressive strength of 3200 MPa, a proportion of < 0.1 % of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of 0.1 %.

[0107] The irregularly shaped particles have an average roundness of 0.6. Example 2

[0108] 200 kg of NSiC from refractory applications in lump form with a composition of 90 wt.% SiC, 9.6 wt.% SiO2, and free carbon contents of 0.15 wt.%, 0.14 wt.% Si, and 0.128 wt.% Fe is crushed using a jaw crusher with an energy input of 2 MJ / kg within 10 minutes. 79 g of C per kg of NSiC are added during the mechanical loading in the jaw crusher to ensure a stoichiometric composition of the SiC waste product.

[0109] The mixed material is then treated under vacuum at 2200 °C for 300 min.

[0110] Due to partial sintering, the SiC particles are separated by a pneumatic energy input of 1 MJ / kg in an autogenous mill and then fractionated by screening. Irregularly shaped particles with sharp and partially rounded edges, ranging in size from 2 to 5 mm, are obtained with a yield of 90%.

[0111] The SiC particles resulting from the process have a SiC content of > 98 wt.% and a density of 3.02 g / cm 3 , a compressive strength of 2850 MPa, a proportion of 2.3% pores with an equivalent diameter of > 100 pm and a proportion of open porosity of 3%.

[0112] The irregularly shaped particles have an average roundness of 0.7.

[0113] Example 3

[0114] 100 kg of coarse Si-SiC sintered scrap with a composition of 80 wt% SiC, 14 wt% Si, a free carbon content of 0.13 wt%, a SiO2 content of 3.97 wt%, and an iron content of 0.968 wt%, and 33 kg of pyrolyzed mineral concrete waste with 65 wt% SiC, 30 wt% C, and 5 wt% ash were crushed using a hammer mill with an energy input of 5 MJ / kg. The material subsequently had an additional iron concentration of 2 wt%. The ground material was then treated at 2250°C for 40 min under an argon atmosphere.

[0115] The particles are separated by pneumatic energy input of 2 MJ / kg in autogenous mills. During subsequent air separation, impurities accumulate in a fine fraction <500 pm. This fine fraction then contains Fe and Si impurities of >5 wt.%.

[0116] The coarse fraction with particle sizes of 4 mm with a yield >95% has a SiC content of 98 wt.%.

[0117] The resulting irregularly shaped particles have sharp and partially rounded edges and a roundness of 0.75.

[0118] The partially sintered particles resulting from the process have a SiC content of 98 mass% and a density of 2.95 g / cm 3 , a compressive strength of 2600 MPa, a proportion of 4% pores with an equivalent diameter of > 100 pm and a proportion of open porosity of 3%.

Claims

Patent claims 1. Silicon carbide gravel, the particles of which consist of at least 85 wt.% silicon carbide from SiC waste products and have a density of 2.89 to 3.20 g / cm³ 3 , a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, wherein the silicon carbide gravel consists at least predominantly of particles with particle sizes greater than or equal to 2 mm and the particles have an irregular shape with a roundness of 0.5 to 0.8 produced by a mechanical stress with an energy input between 0.1 and 5 MJ / kg.

2. Silicon carbide gravel according to claim 1, wherein the SiC waste products are products from the Acheson process or SiC sintered scrap or manufacturing-related SiC waste from product production, the particles of which consist of at least 85 wt. % silicon carbide and have a density of 2.89 to 3.20 g / cm³ 3 , a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, wherein the particles have at least predominantly particle sizes greater than or equal to 2 mm and the particles have an irregular shape with a roundness of 0.5 to 0.8 produced by a mechanical stress with an energy input between 0.1 and 5 MJ / kg.

3. Silicon carbide pyrite according to claim 2, wherein the SiC waste products are such products made of SsiC, LPS-SiC, SiSiC, RsiC, NsiC and / or OBSiC ceramics and / or fiber composites made of C-SiC and / or SiC-SiC.

4. Silicon carbide pyrite according to claim 1, which consists of at least 90 wt.%, advantageously 95 wt.%, and more advantageously 98 wt.% silicon carbide.

5. Silicon carbide pyrite according to claim 1, the particles of which have a density of 3.05 to 3.20 g / cm³ 3 own.

6. Silicon carbide gravel according to claim 1, which consists at least predominantly, advantageously at least 85%, or more advantageously at least 95%, of particles with a particle size greater than or equal to 2 mm.

7. Silicon carbide gravel according to claim 1, wherein particle sizes are greater than or equal to 2 to 20 mm, or 5 mm to 63 mm.

8. Silicon carbide pyrite according to claim 1, which has particle shapes that are realized after mechanical stress on the SiC waste products by applying a mechanical impulse, advantageously by mixing, grinding, more advantageously by oxyfuel grinding, or by the use of eddy currents and / or ultrasound, or by grinding, hammering, breaking or by means of electrical discharges or shock waves.

9. Silicon carbide gravel according to claim 1, which has irregular particle shapes that are entirely and / or partially sharp-edged and / or irregular particles that are entirely or partially rounded.

10. Process for the production of silicon carbide pyrite, in which SiC waste products, excluding SiC dusts, are processed by mechanical stress with an energy input between 0.1 and 5 MJ / kg to produce SiC particles which consist of at least 85 wt. % silicon carbide and have a density of 2.89 to 3.20 g / cm³ 3, exhibiting a compressive strength of > 2500 MPa, a pore size of < 5% with an equivalent diameter of > 100 pm and an open porosity of < 10%, and processed with particle sizes greater than or equal to 2 mm and an irregular shape with a roundness of 0.5 to 0.8, or in which, for the production of silicon carbide pyrite, SiC waste products are subjected to mechanical stress with an energy input between 0.1 and 5 MJ / kg before and / or after heat treatment under vacuum or non-oxidizing atmosphere at temperatures of 1400 to 2600 °C to produce SiC particles that consist of at least 85 wt% silicon carbide and have a density of 2.89 to 3.20 g / cm³ 3 , a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10% exhibiting, and are processed with particle sizes greater than or equal to 2 mm and an irregular shape with a roundness of 0.5 to 0.8, or in the production of silicon carbide pyrite, in which SiC waste products are subjected to mechanical stress with an energy input between 0.1 and 5 MJ / kg before and / or after heat treatment under vacuum or non-oxidizing atmosphere at temperatures of 1400 to 2600 °C to produce SiC particles that consist of at least 85 wt% silicon carbide and have a density of 2.89 to 3.20 g / cm³ 3, have a compressive strength of > 2500 MPa, a proportion of < 5% of pores with an equivalent diameter of > 100 pm and a proportion of open porosity of < 10%, and are processed with particle sizes greater than or equal to 2 mm and an irregular shape with a roundness of 0.5 to 0.8, whereby these particles may be mechanically treated again, and thereby or afterwards are physically separated into two fractions, one of which has a mass of impurities at least twice as high as the other fraction.

11. Method according to claim 10, wherein at least after one or the last treatment under mechanical stress of the SiC particles a physical separation of the SiC particles into fractions is carried out.

12. Method according to claim 10, wherein SiC waste products are subjected to a temperature treatment under vacuum or non-oxidizing atmosphere at temperatures of 1400 to 2600 °C, followed by treatment under mechanical stress and then by physical separation of the SiC particles into fractions.

13. The method of claim 10, wherein SiC waste products with a density of 3.05 to 3.20 g / cm³ 3 be used.

14. The method of claim 10, wherein the mechanical stress on the SiC waste products is realized by applying a mechanical impulse, advantageously by mixing, grinding, even more advantageously by oxyfuel grinding, or by the use of eddy currents and / or ultrasound or by grinding, hammering, breaking, or by means of electrical discharges or shock waves.

15. Method according to claim 10, wherein particles with particle sizes between greater than or equal to 2 mm and 20 mm, or 5 mm and 63 mm are realized.

16. The method of claim 10, wherein a temperature treatment is carried out at temperatures between 2000 °C and 2600 °C.

17. The method of claim 10, wherein a temperature treatment is carried out under an argon or nitrogen atmosphere.

18. Method according to claim 10, wherein a temperature treatment is carried out at temperatures of at least 2000 °C for between 10 and 300 min.

19. The method of claim 10, wherein the physical separation of the particles after the temperature treatment is carried out according to the particle size, particle shape, density and / or the physical and / or chemical surface properties of the particles.

20. The method of claim 10, wherein the separation according to particle size and / or particle shape is carried out by sieving, screening, and / or cyclone processes, or the separation is carried out by the action of inertial forces with respect to particle density by means of flotation, sedimentation, screening, centrifugation, and / or cyclone processes, or the separation according to the density of the particles is carried out by flotation and / or cyclone processes.

21. Use of silicon carbide pyrite according to claims 1 to 9 and produced according to a method according to claims 10 to 20 for the production of SiC-containing ceramics, in particular refractory ceramics.