Silicon carbide grit and method for producing the same

Abstract

Silicon carbide grit and method for producing the same. The present invention relates to the field of industrial ceramics, and relates to, for example, silicon carbide grit that can be used in refractory products, and a method that can be used for producing silicon carbide grit of various qualities and grains. The problem of the present invention is to present silicon carbide grit in which SiC particles have a high density. This problem is solved by silicon carbide grit, wherein at least 85% by mass of the particles of the silicon carbide grit consists of silicon carbide derived from SiC waste products, and the density is 2.89 to 3.20 g / cm 3 3, the compressive strength is >2500 MPa, the proportion of pores with an equivalent diameter >100 μm is <5%, and the open porosity is <10%, and at least the majority of the silicon carbide grit consists of particles with a particle size of 2 mm or more, and the particles have an irregular shape generated by mechanical stress.

Description

Technical Field

[0001] The present invention relates to the field of industrial ceramics, for example, silicon carbide grit that can be used in refractory products, and a method for manufacturing silicon carbide grit that can be used in the manufacture of silicon carbide grit of various qualities and grains.

[0002] Silicon carbide (SiC) is a synthetic industrial mineral used in many industrial fields due to its excellent properties (hardness, high-temperature properties, chemical resistance). Of particular importance is its use in the form of classified micropowder grains (0.5 to about 250 μm) in the manufacture of industrial ceramics (mechanical seals, ballistic defense ceramics for military technology), in the automotive / environmental technology (diesel particulate filter), in high-quality surface treatment abrasives in the entire field of mechanical engineering, and as a component of refractory materials for the lining of waste incinerators in microelectronics / solar cells (wafer sawing).

[0003] The production of raw SiC has been carried out by an electrosynthesis process, the so-called Acheson process (German Patent Application Publication No. 76629, German Patent Application Publication No. 85197), which has been used for about 100 years. In this process, SiC is synthesized by carbothermal reduction of SiO 2 (quartz sand) with carbon (usually petroleum coke). This process is clearly affected by the prices of electricity and petroleum (petroleum coke as a raw material), and is also a factor that makes the environmental cost relatively high (due to a large amount of dust, CO / CO 2 and SO 2 emissions).

[0004] Alternative manufacturing processes, despite many attempts, have generally not been successful for economic reasons and cannot be utilized in the short term.

[0005] After manufacturing SiC from raw SiC, desirable SiC powder grains are produced by grinding, purification, and classification. Therefore, due to the increasing demand for grains, it is now always necessary to increase the production volume of raw SiC, which leads to a further increase in the capacity of raw materials, and ultimately results in structural shortages and price inelasticity.

[0006] SiC is considered a globally available bulk raw material, but for strategically important high-quality grains (HQ), supply bottlenecks and price increases have been observed over the years. However, an even greater problem with special grains is that in high-tech applications, large quantities of individual particle size ranges are required. Due to the aforementioned price inelasticity, both lead to price increases and supply bottlenecks for these special grains.

[0007] The classification of SiC powder, especially for grains from F4 to F220, is usually tested by a sieving process and test sieves compliant with FEPA standard 42-1:2006 or ISO 8486. The coarsest Si powder grains currently available compliant with these standards are F4 grains, also called macro grains, with an average particle size of about 4.8 mm.

[0008] Coarser SiC powder fractions are not commercially available.

[0009] Numerous semi-finished and finished products are manufactured from raw SiC.

[0010] According to the specification of the former East German economic patent No. 300288, wear-resistant ceramic grinding spheres made of the base material Si 3 N 4 and SiC and a method for manufacturing the same are known. The grinding spheres are manufactured, for example, by press-molding or build-up granulation of a very fine SiC powder mixture with a particle size of <1 μm, and then sintering to obtain SiC ceramics. These grinding spheres have a density of 2.93 - 3.11 g / cm 3 of.

[0011] These spherical grinding bodies are characterized by a high sphericity of >0.95. The sphericity is defined and measured in accordance with DIN-ISO 13322 Part 1 and Part 2.

[0012] During the production of SiC products, various types of SiC waste materials are generated, such as so-called SiC sintering scraps and other SiC waste materials resulting from production. However, even after the product has been used and reached its service life, or after the product has lost its function, there are SiC waste materials, such as grinding sludge, used combustion aids, and used SiC diesel particulate filters.

[0013] Regarding the recycling of SiC products, various methods are already known.

[0014] According to German Patent Application Publication No. 102013218450, as a method for recycling powdery silicon carbide waste products, a powdery SiC waste product having at least 50% by mass of SiC and an average particle size d50 of 0.5 to 500 μm is heat-treated at a temperature of at least 2000°C in a vacuum or an oxygen-free atmosphere. As a result, the SiC particles coarsen to several millimeters, so that the particles can be used in many applications where they could not be used because their particle size would otherwise be too small.

[0015] Furthermore, according to DE102020102512.2A1, as a method for separating impurities from silicon carbide, a powdery SiC waste product having at least 50% by mass of SiC and an average particle size d50 of 0.5 to 1000 μm is heat-treated at a temperature of 1400 to 2600°C in a vacuum or a non-oxidizing atmosphere and then cooled, and then mechanically processed to physically separate it into two fractions, with one fraction having at least twice as low a mass of impurities as the other fraction.

[0016] The drawback of the known recycling methods for products containing SiC is that only a very small number of special SiC waste products have been recycled so far.

[0017] The object of the present invention is to provide silicon carbide grit with a high density of SiC particles and a simple and cost-effective manufacturing method for the silicon carbide grit.

[0018] This object is solved by the present invention as described in the claims. Advantageous configurations are the subject matter of the dependent claims, and the present invention includes combinations of individual claims in the sense of an AND operation as long as they are not mutually exclusive.

[0019] The particles of the silicon carbide grit according to the present invention are composed of silicon carbide derived from SiC waste products with at least 85% by mass, and the density is 2.89 - 3.20 g / cm 3 and the compressive strength is >2500 MPa, the proportion of pores with an equivalent diameter >100 μm is <5%, and the open porosity is <10%. At least the majority of the silicon carbide grit according to the present invention consists of particles with a particle size of 2 mm or more, and the particles have an irregular shape with a sphericity of 0.5 - 0.8 generated by mechanical stress at an energy input of 0.1 - 5 MJ / kg.

[0020] Advantageously, as the SiC waste product, there is a product derived from the Acheson process, or SiC sintered scrap, or SiC waste material derived from product manufacturing due to manufacturing, and the particles are composed of at least 85% by mass of silicon carbide, and the density is 2.89 - 3.20 g / cm 3and having a compressive strength > 2500 MPa, a proportion of pores with an equivalent diameter > 100 μm < 5%, and an open porosity < 10%, with at least the majority of the particles having a particle size of 2 mm or more, the particles having an irregular shape with a sphericity of 0.5 - 0.8 produced by mechanical stress at an energy input of 0.1 - 5 MJ / kg, and more preferably, as SiC waste products, there are products of this kind made of SSiC ceramics, LPS - SiC ceramics, SiSiC ceramics, RSiC ceramics, NSiC ceramics and / or OBSiC ceramics and / or fiber composites made of C - SiC and / or SiC - SiC.

[0021] More preferably, at least 90% by mass, preferably 95% by mass, and still more preferably 98% by mass of the silicon carbide grit according to the present invention consists of silicon carbide.

[0022] Similarly preferably, the particles of the silicon carbide grit according to the present invention have a density of 3.05 - 3.20 g / cm 3 is.

[0023] Also preferably, at least the majority of the silicon carbide grit according to the present invention preferably consists of at least 85%, and still more preferably at least 95% of particles with a particle size of 2 mm or more.

[0024] Also, it is advantageous when the particle size of the silicon carbide grit according to the present invention is 2 mm - 20 mm or 5 mm - 63 mm.

[0025] Furthermore, it is also advantageous when the particle shape of the silicon carbide grit according to the present invention realized after mechanical stress of SiC waste products by application of mechanical shock is obtained by mixing, grinding, still more preferably by autogenous grinding, or by using eddy current and / or ultrasonic waves, or by grinding, hammering, crushing, or by electric discharge or shock wave.

[0026] Moreover, it is also advantageous if the silicon carbide grit according to the present invention has an irregular and completely and / or partially pointed angular shape and / or an irregular and completely and / or partially rounded shape.

[0027] In the method for producing silicon carbide grit according to the present invention, an SiC waste product is processed by mechanical stress with an energy input of 0.1 to 5 MJ / kg to obtain SiC particles, where at least 85% by mass of the SiC particles consists of silicon carbide, and the density is 2.89 to 3.20 g / cm 3 and the compressive strength is >2500 MPa, the proportion of pores with an equivalent diameter >100 μm is <5%, the open porosity is <10%, the particle size is 2 mm or more, and it has an irregular shape with a sphericity of 0.5 to 0.8, or alternatively, an SiC waste product is processed by mechanical stress with an energy input of 0.1 to 5 MJ / kg before and / or after heat treatment at a temperature of 1400 to 2600 °C in a vacuum or non-oxidizing atmosphere to obtain SiC particles, where at least 85% by mass of the SiC particles consists of silicon carbide, and the density is 2.89 to 3.20 g / cm 3 and the compressive strength is >2500 MPa, the proportion of pores with an equivalent diameter >100 μm is <5%, the open porosity is <10%, the particle size is 2 mm or more, and it has an irregular shape with a sphericity of 0.5 to 0.8, or alternatively, an SiC waste product is processed by mechanical stress with an energy input of 0.1 to 5 MJ / kg before and / or after heat treatment at a temperature of 1400 to 2600 °C in a vacuum or non-oxidizing atmosphere to obtain SiC particles, where at least 85% by mass of the SiC particles consists of silicon carbide, and the density is 2.89 to 3.20 g / cm 3 and the compressive strength is >2500 MPa, the proportion of pores with an equivalent diameter >100 μm is <5%, the open porosity is <10%, the particle size is 2 mm or more, and it has an irregular shape with a sphericity of 0.5 to 0.8, and the particles can be further mechanically processed and physically separated into two fractions during or after that, with one fraction having at least twice as low an impurity mass as the other fraction.

[0028] Advantageously, physical separation of the SiC particles into the respective fractions is carried out at least once or finally after the SiC particles have been processed under mechanical stress.

[0029] Even more advantageously, the SiC waste product is heat-treated at a temperature of 1400 to 2600 °C under vacuum or in a non-oxidizing atmosphere, then processed under mechanical stress, and thereafter the SiC particles are physically separated into the respective fractions.

[0030] Similarly advantageously, a SiC waste product having a density of 3.05 to 3.20 g / cm 3 is used.

[0031] Also advantageously, the mechanical stress on the SiC waste product is achieved by applying mechanical shock, which is advantageously by mixing, grinding, even more advantageously by autogenous grinding, or by using eddy currents and / or ultrasound, or by grinding, hammering, crushing, or by electric discharge or shock waves.

[0032] It is similarly advantageous when particles having a particle size of 2 mm to 20 mm or 5 mm to 63 mm are achieved.

[0033] Furthermore, it is also advantageous when the heat treatment is carried out at a temperature of 2000 °C to 2600 °C.

[0034] It is also advantageous when the heat treatment is carried out in an argon or nitrogen atmosphere.

[0035] It is also advantageous when the heat treatment is carried out at a temperature of at least 2000 °C for 10 to 300 minutes.

[0036] It is also advantageous when the physical separation of the particles after heat treatment is carried out according to the particle size, particle shape, particle density and / or physical and / or chemical surface properties.

[0037] Separation is equally advantageous if it is carried out by sieving, sorting and / or cyclone process according to particle size and / or particle shape, or if separation is carried out by the action of inertial forces with respect to particle density by floating, sedimentation, sorting, centrifugation and / or cyclone process, or if separation is carried out by floating and / or cyclone process according to the density of the particles.

[0038] According to the present invention, the silicon carbide grit according to the present invention produced by the present invention is used for the production of SiC-containing ceramics, particularly refractory ceramics.

[0039] The solution according to the present invention makes it possible for the first time to present silicon carbide grit in which the particles have high mechanical strength and, furthermore, to present a simple and cost-effective production method for silicon carbide grit.

[0040] This is achieved by silicon carbide grit consisting of at least 85% by mass of silicon carbide derived from SiC waste products, having a density of 2.89 to 3.20 g / cm 3 and having a compressive strength of > 2500 MPa, a proportion of pores with an equivalent diameter > 100 μm of < 5%, and an open porosity of < 10%.

[0041] Grit is a term for particle size and unconsolidated sediment.

[0042] The particle size of the grit is defined as a particle diameter of 2 mm to 63 mm in accordance with DIN 4022 and DIN EN ISO 14668. Here, it is also distinguished into coarse grit with a particle size of 20 to 63 mm, medium grit with a particle size of 6.3 to 20 mm, and fine grit with a particle size of 2 to 6.3 mm (Wikipedia, keyword "grit").

[0043] Grit as unconsolidated sediment (also called unconsolidated rock) is a solidified aggregate with little bonding of the particles (Wikipedia, keyword "unconsolidated sediment").

[0044] Within the scope of the present invention, the term silicon carbide grit means SiC particles with a particle size of 2 mm to 63 mm in terms of particle diameter.

[0045] The SiC particles according to the present invention become irregular in shape due to mechanical stress at an energy input of 0.1 to 5 MJ / kg, but mainly have convex surfaces. When using a known particle shape analyzer, the measured average sphericity ranges from 0.5 to 0.8.

[0046] The particle size, particle shape, and sphericity are defined and measured in accordance with DIN-ISO 13322 Part 1 and Part 2.

[0047] Advantageously, the silicon carbide grit according to the present invention composed of particles consists of at least 85% by mass, preferably 90% by mass, more preferably 95% by mass, and even more preferably 98% by mass of silicon carbide derived from SiC waste products.

[0048] Particularly important for the present invention is that the particles substantially composed of silicon carbide of the silicon carbide grit according to the present invention are manufactured from SiC waste products and consist of SiC waste products.

[0049] SiC waste products mean, within the scope of the present invention, SiC products generated during the production of raw SiC, or SiC products derived from semi-finished and finished products made of SiC that were originally manufactured and sintered from raw SiC and processed into molded bodies. Similarly, the SiC waste products used according to the present invention are products that occur as waste materials when, during the production of SiC semi-finished products and finished SiC products, defects such as cracks, distortions, or poor dimensional accuracy are found in the materials, and after being sorted as scrap, they become so-called SiC sintered scrap. However, even after the product has been used, after reaching the end of its service life, or after the product has lost its function or been damaged, there are SiC waste materials such as used combustion aids and used SiC diesel particulate filters.

[0050] By SiC waste products is further meant within the scope of the present invention always agglomerated SiC waste products which have been processed, advantageously crushed by mechanical stress to the desired particle size for the SiC grit.

[0051] In any case, the SiC waste product according to the invention does not correspond to SiC waste products that require an increase in the particle size of the SiC waste product by grain growth to obtain the desired particle size of the SiC grit. This corresponds, for example, to SiC waste products with a particle size of less than 2 mm, such as SiC powder, SiC grinding sludge or SiC dust. The solution according to the invention realizes a chemical conversion of the SiC waste product rather than grain growth to achieve a higher SiC content, and furthermore a reduction in open porosity and / or a specific grain shape with respect to sphericity. All such SiC products can be used as SiC waste products according to the invention.

[0052] Silicon carbide grit particles have a density of 2.89-3.20g / cm 3 Advantageously, the particles of silicon carbide grit according to the invention have a density, as measured by pycnometer, of 3.05 to 3.20 g / cm. 3 It is.

[0053] Furthermore, the SiC particles of the silicon carbide grit according to the invention have a very low content of <5% macropores with an equivalent diameter >100 μm, resulting in a very high compressive strength of >2500 MPa, which is not achieved by crushing raw SiC from the Acheson process.

[0054] It is known that particle size and shape measurements can be carried out in accordance with DIN-ISO 13322 Part 1 and Part 2 (static and dynamic analysis).

[0055] The particle strength can be measured by the same methodology as in ASTM D 5731-16 (Standard Test Method for Measurement of the Point Load Strength Index of Rock and Its Application to Rock Strength Classification) or "ISRM - International Society for Rock Mechanics 2007. Rock characterization testing & monitoring - ISRM suggested methods. Ed. E. T. Brown, Pergamon Press, London, 211 p.", with the test piece shape / dimension d) "irregular lump". By determining the equivalent diameter, reference is made to the breaking force. These methods can be used in the same way for the SiC particles according to the present invention.

[0056] The strength is normalized according to size by the method described above and is expressed in MPa as the "point load strength index" l s(50) From this, the compressive strength σ c can be calculated in MPa by the correlation method.

[0057] The compressive strength of the SiC grit particles according to the present invention thus obtained is > 2500 MPa, while the coarse SiC particles crushed from the Acheson raw material have a high cracking and porosity rate, so the compressive strength is far below 1000 MPa. Therefore, it goes without saying that such unprocessed SiC cannot be used as a starting material for various applications, for example as refractory products. This is because, due to the high cracking and porosity rate of the SiC particles, high quality in terms of strength and durability cannot be achieved in such refractory products. In contrast, SiC ceramic bodies specially manufactured in the ceramics manufacturing process from specific powders, such as grinding spheres, are not used because the required packing density of the SiC particles cannot be obtained and the manufacturing is complicated and thus expensive.

[0058] Particularly important is that the SiC particles according to the invention have very few large pores with an equivalent diameter > 100 μm. These pores are irregularly shaped voids within the structure and are measured by an image analysis method using a polished cross-section. As size data, the equivalent diameter of the same area is determined in this specification. The expression of the amount of such pores is carried out in percentage units according to the area ratio obtained by image analysis, which can be expressed in percentage units as a volume ratio using the principles of stereology. Also, although more laborious, it is also possible to create so-called micro X-ray computed tomography images of the grit particles and similarly determine the size and volume ratio of large pores with an equivalent diameter > 100 μm.

[0059] Even more advantageously, the grit particles according to the invention have a very low open porosity of < 10%. These pores include, within the scope of the invention, all pores, voids and cracks within the SiC particles that are accessible from the surface.

[0060] It is known that the measurement of open porosity is carried out by liquid or gas pycnometry or mercury porosimetry.

[0061] The SiC waste material products according to the present invention may advantageously be products made of SSiC ceramics, LPS-SiC ceramics, SiSiC ceramics, RSiC ceramics, NSiC ceramics and / or OBSiC ceramics and / or fiber composites made of C-SiC and / or SiC-SiC. The various abbreviations and names of SiC ceramics listed are well known to those skilled in the art and are described, for example, in Brevier Technische Keramik (Brevier Technische Keramik: Hrsg. v. Verband der Keramischen Industrie, 4. Auflage 2004, ISBN: 9783924158361). Waste materials derived from mineral-based concrete with a high SiC content, also referred to as mineral cast, can also be advantageous SiC waste material products. SiC mineral-based concrete usually consists of SiC particles contained in a polymer matrix made of polyester resin.

[0062] When selecting the SiC waste material products used according to the present invention, it is also possible to perform preliminary sorting according to their composition before use by the method according to the present invention. Thereby, it becomes possible to realize high-purity and high-quality silicon carbide grit according to the present invention.

[0063] When using waste material products, usually, it is not necessary to add additives during the production of silicon carbide grit according to the present invention, and silicon carbide grit with a particle size of 2 mm or more, a high purity of >99% and a porosity of almost 100% can be obtained.

[0064] Similarly, with respect to LPS-SiC waste material products, it is not necessary to add additives during the production of silicon carbide grit according to the present invention, and silicon carbide grit with a particle size of 2 mm or more, a high purity of >90% and a porosity of almost 100% can be obtained.

[0065] Regarding SiSiC waste products, the free Si content is quantitatively determined by known methods, and to achieve stoichiometric conversion from Si + C to SiC, the corresponding amount of carbon-containing additive is added to the waste product. The optimal amount can be confirmed by a series of simple experiments by those skilled in the art. Silicon carbide grit with a particle size of 2 mm or more, a high purity of >98%, and a pore content with an equivalent diameter of >100 μm of <5% can be obtained.

[0066] Regarding RSiC waste products, fine SiC powder with a particle size of 0.2 - 10 μm is added in an amount of 2 - 20%, and silicon carbide grit with a particle size of 2 mm or more, a high purity of >99%, and a pore content with an equivalent diameter of >100 μm of <5% can be obtained. When fine SiC powder is added, the closure of open pores in RSiC particles occurs during heat treatment. The optimal amount and particle size of the fine SiC particles added can be confirmed by those skilled in the art through a series of simple experiments until the open porosity reaches an amount of <10%.

[0067] Regarding NSiC waste products (which also include those containing SiON, SiAlON, Si 3 N 4 containing and similar nitride-bonded SiC waste products), the oxide content is preferentially determined, and the reduction of oxides is achieved by adding the corresponding amount of carbon-containing additive. The optimal amount can be confirmed by a series of simple experiments by those skilled in the art. Silicon carbide grit with a particle size of 2 mm or more, a SiC content of >85%, and a pore content with an equivalent diameter of >100 μm of <5% can be obtained.

[0068] Regarding OBSiC waste products (which also include those containing SiO 2 and aluminosilicate-bonded SiC waste products), the oxide content is preferentially determined, and the reduction of oxides is achieved by adding the corresponding amount of carbon-containing additive. The optimal amount can be confirmed by a series of simple experiments by those skilled in the art. Silicon carbide grit with a particle size of 2 mm or more, a high purity of >85%, and a pore content with an equivalent diameter of >100 μm of <5% can be obtained.

[0069] When C-SiC waste products, i.e., waste materials derived from carbon fiber reinforced SiC composites, such as short fiber reinforced materials or long fiber reinforced materials, or composites produced by silicon treatment or precursor infiltration, are used, when producing silicon carbide grit according to the present invention, in order to generate a stoichiometric composition, the ratios of free Si and C must be analyzed and the corresponding amounts of Si additive and C additive must be used. Usually, C-SiC waste products have a much higher C ratio than the free Si ratio, so in order to convert the excess carbon to SiC, it is usually necessary to add Si additives. The required amount necessary to adjust the stoichiometric composition of SiC is added. The optimal amount can be confirmed by a series of simple experiments by those skilled in the art, and silicon carbide grit with a particle size of 2 mm or more, a high purity of >95%, and a pore content with an equivalent diameter >100 μm of <5% can be obtained.

[0070] When SiC-SiC waste products, i.e., waste materials derived from silicon carbide fiber reinforced SiC composites, are used, fine SiC powder with a particle size of 0.5 - 5 μm is added in an amount of 2 - 20%, and silicon carbide grit with a particle size of 2 mm or more, a high purity of >99%, and a pore content with an equivalent diameter >100 μm of <5% can be obtained. When the fine SiC powder is added, the open pores of the SiC-SiC particles are blocked during heat treatment. The optimal amount and particle size of the added fine SiC particles can be confirmed by a series of simple experiments by those skilled in the art until the open porosity reaches an amount of <10%.

[0071] Regarding SiC mineral-based concrete waste products, first, heat treatment (pyrolysis) at 600 - 1000 °C is carried out under airtight conditions, and then the free carbon content is quantitatively determined by a known method. In order to achieve the stoichiometric conversion from Si + C to SiC, the corresponding amount of silicon-containing additive is added to the waste product. The optimal amount can be confirmed by a series of simple experiments by those skilled in the art. Silicon carbide grit with a particle size of 2 mm or more, a high purity of >98%, and a pore content with an equivalent diameter >100 μm of <5% can be obtained.

[0072] Regarding the coarse waste products derived from the production of unprocessed SiC, fine SiC powder with a particle size of 0.2 to 10 μm is added in an amount of 5 to 20%, and silicon carbide grit with a particle size of 2 mm or more, a high purity of >99%, and a content of pores with an equivalent diameter of >100 μm of <5% can be obtained. When the fine SiC powder is added, the open pores of the SiC particles in the unprocessed SiC are blocked during the heat treatment. The optimal amount and particle size of the added fine SiC particles can be confirmed by those skilled in the art through a series of simple experiments until the open porosity reaches an amount of <10%.

[0073] Very advantageously, mixtures of the above-mentioned waste products can also be used, especially when this can reduce the types and amounts of the necessary additive substances as a result. For example, it is advantageous to produce a mixture of SiSiC waste products and C-SiC and / or pyrolytic SiC mineral-based concrete waste products, and then heat-treat this. Thereafter, depending on the analyzed free Si and C content rates, by mixing the respective components, the addition of further C-containing or Si-containing additives can be made with as little amount as possible or not added at all. In this way, silicon carbide grit with a particle size of 2 mm or more, a purity of >85% SiC, and a content of pores with an equivalent diameter of >100 μm of <5% can be obtained from a mixture of different SiC waste products.

[0074] Furthermore, what is particularly important according to the present invention is that the SiC waste product has been subjected to mechanical stress at an energy input of 0.1 to 5 MJ / kg, and then at least most of it has a particle size of 2 mm or more and has an irregular shape with a sphericity of 0.5 to 0.8 generated by the mechanical stress. At this time, the mechanical stress can be applied before the heat treatment, after the heat treatment, or either before or after the heat treatment.

[0075] By the process of applying mechanical stress to the SiC waste product, products with a wide range of particle sizes can be obtained. Therefore, in most cases, physical separation into different fractions is required to produce silicon carbide grit according to the present invention, and by the mechanical stress process, the yield of particles with a particle size of 2 mm or more is at least 50%, usually 80 to 95%.

[0076] This is a major difference from as - received SiC obtained by the Acheson process, where SiC crystals can be up to the size of units in the mm range. However, these SiC crystals are highly prone to cracking and inter - growth, and the proportion of macropores with a diameter of several hundred micrometers to several millimeters significantly exceeds 5%. Due to the structure of such macroparticles of as - received SiC, their mechanical strength is very low, so their use is very limited or impossible. Such macroparticles are very easily damaged and thus cannot be used as abrasives. Also, due to their porosity, they provide a large surface area for attacking and destroying materials, so they cannot be used in applications that are subject to chemical attack, especially oxidation attack, or attack by aggressive media or the atmosphere.

[0077] The silicon carbide grit particles according to the present invention have no pores with an equivalent diameter exceeding 100 μm or have no more than 5%.

[0078] By using the silicon carbide grit according to the present invention, based on its starting material as an SiC waste product and based on mechanical stress, SiC present in macroparticles of particles with a particle size of 2 mm or more, which do not have the drawbacks of macroparticles derived from as - received SiC, can be obtained.

[0079] Advantageously, at least the majority of the silicon carbide grit according to the present invention, advantageously at least 85%, and still more advantageously at least 95% consists of particles with a particle size of 2 mm or more.

[0080] Even more advantageously, the silicon carbide grit according to the present invention has a particle size of 2 mm to 20 mm or 5 mm to 63 mm. Also, the silicon carbide grit according to the present invention differs from silicon carbide products with dimensions of 2 mm or more in that it consists of particles with an irregular shape generated by mechanical stress.

[0081] The irregular shape of the silicon carbide grit according to the present invention, with a sphericity of 0.5 to 0.8, is obtained after mechanical stressing of the SiC waste material product at an energy input of 0.1 to 5 MJ / kg by the application of mechanical shock, and advantageously this is achieved by mixing, grinding, more advantageously by autogenous grinding, or by the use of eddy currents and / or by ultrasonic waves, and / or by grinding, hammering, crushing, or by fragmentation by electric discharge or shock waves, for example by fragmentation by electric pulses or pulsed power treatment.

[0082] The particles can advantageously be present in an irregular and completely and / or partially angular and / or irregular and completely and / or partially rounded shape.

[0083] The irregularity according to the present invention relates to SiC particles having an irregular shape and a sphericity of 0.5 to 0.8. Here, the irregularity relates to the shape, which according to the present invention does not apply to geometric shapes such as spheres, cylinders, cubes, cuboids, pyramids, cones, truncated pyramids or truncated cones.

[0084] Furthermore, since the starting SiC waste material product is basically of very high purity SiC, the silicon carbide grit according to the present invention has a very low impurity content. Although impurities are still present, especially metal impurities and impurities introduced by mechanical stress, they can be reduced or removed by using known impurity removal methods.

[0085] In the method for producing silicon carbide grit according to the present invention, an SiC waste material product is processed by mechanical stress to form SiC particles, and at least 85% by mass of the SiC particles consists of silicon carbide, and the density is 2.89 to 3.20 g / cm 3 and the compressive strength is >2500 MPa, the proportion of pores with an equivalent diameter >100 μm is <5%, the open porosity is <10%, the particle size is 2 mm or more, it has an irregular shape, and physical separation into different particle fractions can be carried out before and / or after mechanical stress.

[0086] Also according to the present invention, silicon carbide grit is produced by processing SiC waste products with mechanical stress before and / or after heat treatment at a temperature of 1400 to 2600 °C in a vacuum or non-oxidizing atmosphere into SiC particles, where the SiC particles consist of at least 85% by mass of silicon carbide and have a density of 2.89 to 3.20 g / cm 3 and have a compressive strength > 2500 MPa, a proportion of pores with an equivalent diameter > 100 μm of < 5%, an open porosity of < 10%, a particle size of 2 mm or more, and an irregular shape.

[0087] In this alternative method too, it is possible to perform physical separation into different particle fractions before and after mechanical stress.

[0088] In a further alternative production method according to the present invention for silicon carbide grit, SiC waste products are processed with mechanical stress before and / or after heat treatment at a temperature of 1400 to 2600 °C in a vacuum or non-oxidizing atmosphere into SiC particles, where the SiC particles consist of at least 85% by mass of silicon carbide and have a density of 2.89 to 3.20 g / cm 3 and have a compressive strength > 2500 MPa, a proportion of pores with an equivalent diameter > 100 μm of < 5%, an open porosity of < 10%, a particle size of 2 mm or more, and an irregular shape, and the particles can then be further mechanically processed and physically separated into two fractions during or after that, where in one fraction the mass of impurities is at least twice as high as in the other fraction. As with other alternative production methods according to the present invention, it is possible to perform physical separation into different particle fractions before and after mechanical stress.

[0089] Regarding all alternative production methods according to the present invention for silicon carbide grit, advantageously SiC waste products with a density of 3.05 to 3.20 g / cm 3 are used.

[0090] Regarding all alternative manufacturing methods of silicon carbide grit according to the present invention, advantageously, the mechanical stress of the SiC waste material product is achieved by applying mechanical shock at an energy input of 0.1 to 5 MJ / kg, which is also advantageously carried out by mixing, grinding, for example, by self-grinding, or also advantageously by using eddy currents and / or ultrasonic waves, or by grinding, hammering, crushing, or by electric discharge and shock waves.

[0091] According to a method variant of the present invention, in any case advantageously, SiC particles with a particle size of 2 mm to 20 mm or 5 mm to 63 mm are realized.

[0092] When the SiC particles are heat-treated, this is advantageously carried out at a temperature of 2000 °C to 2600 °C, and still more advantageously at a temperature of at least 2000 °C for 10 to 300 minutes.

[0093] Similarly advantageously, the heat treatment is carried out under an argon or nitrogen atmosphere.

[0094] During the heat treatment, on the one hand, powders of silicon or silicon dioxide and carbon powder can be added to the particles derived from the SiC waste material product, thereby achieving an increase in the SiC content in the silicon carbide grit according to the present invention. Similarly, SiC powder with a smaller particle size can also be added during the heat treatment, which can significantly reduce the pores in the SiC powder derived from the SiC waste material product. This is particularly advantageous in the waste material of raw SiC by the Acheson process. From SiC powder with a particle size > 2 mm containing a very large number of pores derived from raw SiC by the Acheson method with a low SiC content and / or SiC with low crystallinity (so-called β-SiC), it is possible to significantly reduce large pores by adding fine SiC powder and other additives during the heat treatment. As a result, these silicon carbide grit particles have a high SiC content and good crystallization of so-called α-SiC.

[0095] If, after the heat treatment according to the invention, a physical separation of the particles according to the invention is further carried out, this can advantageously be carried out according to the particle size, particle shape, particle density and / or physical and / or chemical surface properties.

[0096] However, after the heat treatment according to the invention, in particular after the treatment of SiC waste products by mechanical stress, it is also possible to carry out a physical separation of the particles according to the invention into the respective fractions, and advantageously this separation is carried out by sieving, sorting and / or cyclone process according to the particle size and / or particle shape, or this separation is carried out by the action of inertial forces with respect to the particle density by floating, sedimentation, sorting, centrifugation and / or cyclone process, or this separation is carried out by floating and / or cyclone process according to the particle density.

[0097] After the physical separation achieved according to the invention, the SiC powder is separated into at least two fractions according to the invention, where, if the treatment of the SiC waste product under mechanical stress has been carried out before the physical separation, the separation into at least two fractions takes place, and in one fraction the mass of impurities is at least twice as high as in the other fraction.

[0098] The removal of impurities in the form of Si and / or C is achieved by heat treatment. This is because the carbon added during the production of SiC, advantageously carbon black, graphite and / or coke powder and / or silicon and / or silicon dioxide (SiO 2 ) can then be further converted to SiC and can also be added, thereby achieving a stoichiometric composition as far as possible.

[0099] The actual impurities in SiC are substantially metal impurities, which can be easily removed because they mainly accumulate in one fraction by physical separation after heat treatment.

[0100] Al and B are not considered impurities either in the production of raw SiC or in the production of silicon carbide grit according to the present invention. This is because they are incorporated into the SiC lattice as doping elements and are not harmful in most applications of SiC. Impurities, such as C 不含 , S 不含 , SiO 2 and iron, as well as the SiC content, are determined by known analytical methods in accordance with, for example, DIN EN ISO 9286:2021-10, DIN EN ISO 21068 Parts 1 to 3. Spectroscopy, in particular DIN EN 15991, is also used for the analysis of impurities.

[0101] Hereinafter, the present invention will be described in more detail with reference to several examples.

[0102] Example 1 100 kg of massive SSiC sintered scrap with a SiC content of 99.5% by mass is crushed in a roller mill equipped with a metal roller within 20 minutes with an energy input of 0.5 MJ / kg. The sharp crushed SiC particles thus obtained are sieved, and the fraction with a particle size > 10 mm is separated with a yield > 80%. This is the silicon carbide grit according to the present invention. The individual particles of this fraction have a SiC content of 99.5% by mass, a density of 3.10 g / cm 3 , a compressive strength of 3200 MPa, a proportion of pores with an equivalent diameter > 100 μm of < 0.1%, and an open porosity of 0.1%. These irregularly shaped particles have an average sphericity of 0.6.

[0103] Example 2 90% by mass of SiC, SiO 2A lump of NSiC for refractory use with a composition of 9.6% by mass of [substance], 0.15% by mass of free C, 0.14% by mass of free Si, and 0.128% by mass of free Fe, weighing 200 kg, is crushed by a jaw crusher within 10 minutes with an energy input of 2 MJ / kg. 79 g of C per kg of NSiC is added to the jaw crusher during mechanical stress to achieve the stoichiometric composition of the SiC waste product. This mixed material is then treated at 2200 °C for 300 minutes under vacuum.

[0104] Based on partial sintering, SiC particles are dispersed in a self - grinding mill with an aerodynamic energy input of 1 MJ / kg, and then classified by screening. In this case, particles with a particle size of 2 - 5 mm, sharp corners, and a partially rounded irregular shape are obtained with a yield of 90%.

[0105] The SiC particles obtained by this process have a SiC content of >98% by mass, a density of 3.02 g / cm 3 and a compressive strength of 2850 MPa, a proportion of pores with an equivalent diameter >100 μm of 2.3%, and an open porosity of 3%. These irregular - shaped particles have an average sphericity of 0.7.

[0106] Example 3 100 kg of coarse lump - shaped Si - SiC sintered scrap with a composition of 80% by mass of SiC, 14% by mass of Si, 0.13% by mass of free C, 3.97% by mass of SiO 2 and 0.968% by mass of Fe, and 33 kg of thermally decomposed mineral - based concrete waste with a composition of 65% by mass of SiC, 30% by mass of C, and 5% by mass of ash are crushed by a hammer mill with an energy input of 5 MJ / kg. Then, this material further has an Fe concentration of 2 mass%.

[0107] This crushed material is then treated at 2250 °C for 40 minutes in an argon atmosphere.

[0108] The particles are shredded in a self-grinding mill with an aerodynamic energy input of 2 MJ / kg. During subsequent air classification, the impurities collect in the <500 μm fine fraction. This fine fraction has Fe and Si impurities far exceeding 5 mass%.

[0109] The coarse fraction with a particle size of 4 mm and a yield > 95% has a SiC content of 98 mass%.

[0110] The irregularly shaped particles produced in this case have sharp corners and are partially rounded, with a sphericity of 0.75.

[0111] The partially sintered particles obtained by this process have a SiC content of 98 mass%, a density of 2.95 g / cm 3 and a compressive strength of 2600 MPa, a proportion of pores with an equivalent diameter > 100 μm of 4%, and an open porosity of 3%.

Claims

1. Silicon carbide grit, wherein at least 85% by mass of the particles of the silicon carbide grit consist of silicon carbide derived from SiC waste products, and the density is 2.89 to 3.20 g / cm ３ and the compressive strength is >2500 MPa, the proportion of pores with an equivalent diameter >100 μm is <5%, and the open porosity is <10%. At least the majority of the silicon carbide grit consists of particles with a particle size of 2 mm or more. The particles have an irregular shape with a sphericity of 0.5 to 0.8 generated by mechanical stress at an energy input of 0.1 to 5 MJ / kg. Silicon carbide grit.

2. As a SiC waste product, there exists a product derived from the Acheson process, or SiC sintered scrap, or SiC waste material derived from product manufacturing due to production, and at least 85% by mass of its particles consists of silicon carbide and the density is 2.89 to 3.20 g / cm ３ and the compression strength is > 2500 MPa, the proportion of pores with an equivalent diameter > 100 μm is < 5%, and the open porosity is < 10%. At least the majority of the particles have a particle size of 2 mm or more, and the particles have an irregular shape with a sphericity of 0.5 to 0.8 generated by mechanical stress at an energy input of 0.1 to 5 MJ / kg. The silicon carbide grit according to claim 1.

3. The silicon carbide grit according to claim 2, wherein as SiC waste products, there are products made of SS-SiC ceramics, LPS-SiC ceramics, SiSiC ceramics, RSiC ceramics, NS-SiC ceramics and / or OBSiC ceramics and / or fiber composites made of C-SiC and / or SiC-SiC.

4. The silicon carbide grit according to claim 1, wherein at least 90% by mass, preferably 95% by mass, and still more preferably 98% by mass of the silicon carbide grit is composed of silicon carbide.

5. The particles of the silicon carbide grit have a density of 3.05 to 3.20 g / cm ３ The silicon carbide grit according to claim 1, which is as described above.

6. The silicon carbide grit according to claim 1, wherein at least most of the silicon carbide grit, preferably at least 85%, and still more preferably at least 95% is composed of particles with a particle size of 2 mm or more.

7. The silicon carbide grit according to claim 1, wherein the particle size is 2 mm to 20 mm or 5 mm to 63 mm.

8. The particle shape of the silicon carbide grit is realized after mechanical stress on the SiC waste product by applying mechanical impact, preferably by mixing, grinding, still more preferably by self-grinding, or by using eddy current and / or ultrasonic waves, or by grinding, hammering, crushing, or by electric discharge or shock wave, and is obtained as described in claim 1.

9. The silicon carbide grit according to claim 1, having an irregular and completely and / or partially pointed angular shape, and / or an irregular and completely and / or partially rounded shape.

10. A method for manufacturing silicon carbide grits, wherein an SiC waste product excluding SiC dust is processed by mechanical stress at an energy input of 0.1 to 5 MJ / kg to obtain SiC particles, where at least 85% by mass of the SiC particles consists of silicon carbide and the density is 2.89 to 3.20 g / cm ３ and the compressive strength is >2500 MPa, the proportion of pores with an equivalent diameter >100 μm is <5%, the open porosity is <10%, the particle size is 2 mm or more, and the particles have an irregular shape with a sphericity of 0.5 to 0.8, or Alternatively, to produce silicon carbide grit, SiC waste products are processed into SiC particles by mechanical stress with an energy input of 0.1 to 5 MJ / kg before and / or after heat treatment at a temperature of 1400 to 2600 °C in a vacuum or non-oxidizing atmosphere, where at least 85% by mass of the SiC particles consists of silicon carbide and has a density of 2.89 to 3.20 g / cm ３ and has a compressive strength of > 2500 MPa, a proportion of pores with an equivalent diameter > 100 μm of < 5%, an open porosity of < 10%, a particle size of 2 mm or more, and an irregular shape with a sphericity of 0.5 to 0.8, or Alternatively, to produce silicon carbide grit, SiC waste products are processed into SiC particles by mechanical stress with an energy input of 0.1 to 5 MJ / kg before and / or after heat treatment at a temperature of 1400 to 2600 °C in a vacuum or non-oxidizing atmosphere, where the SiC particles consist of at least 85% by mass of silicon carbide and have a density of 2.89 to 3.20 g / cm ３ and have a compressive strength of > 2500 MPa, a proportion of pores with an equivalent diameter > 100 μm of < 5%, an open porosity of < 10%, a particle size of 2 mm or more, and an irregular shape with a sphericity of 0.5 to 0.8, and the particles can be further mechanically processed and physically separated into two fractions during or after that, with one fraction having at least twice the mass of impurities higher than the other fraction, a method.

11. The method according to claim 10, wherein after subjecting at least the SiC particles to mechanical stress once or finally, physical separation of the SiC particles into each fraction is carried out.

12. The method according to claim 10, wherein the SiC waste product is heat-treated at a temperature of 1400 to 2600 °C in a vacuum or non-oxidizing atmosphere, then treated under mechanical stress, and thereafter the SiC particles are physically separated into each fraction.

13. The method according to claim 10, using an SiC waste material product having a density of 3.05 to 3.20 g / cm ３ .

14. The mechanical stress of the SiC waste product is realized by applying mechanical shock, which is preferably carried out by mixing, grinding, more preferably by autogenous grinding, or by using eddy currents and / or ultrasonic waves, or by grinding, hammering, crushing, or by electric discharge or shock waves, according to the method of claim 10.

15. The method according to claim 10, which realizes particles with a particle size of 2 mm to 20 mm or 5 mm to 63 mm.

16. The method according to claim 10, wherein the heat treatment is carried out at a temperature of 2000 °C to 2600 °C.

17. The method according to claim 10, wherein the heat treatment is carried out in an argon or nitrogen atmosphere.

18. The method according to claim 10, wherein the heat treatment is carried out at a temperature of at least 2000 °C for 10 to 300 minutes.

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

20. The separation is carried out by sieving, sorting and / or cyclone process according to the particle size and / or particle shape, or the separation is carried out by the action of inertial force with respect to the particle density by floating, sedimentation, sorting, centrifugation and / or cyclone process, or the separation is carried out by floating and / or cyclone process according to the density of the particles, according to the method of claim 10.

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

Images