WO2025219273A1 - Method for producing silicon carbide, and silicon carbide - Google Patents

Abstract

The invention is concerned with the area of material management and relates to a method which can be used, for example, to produce refractory ceramic. The aim of the invention is to provide a method and silicon carbide, wherein raw silicon carbide is produced in a production process with high conversion rates and a high yield. This is achieved by a method in which silicon- and carbon-containing starting powders are mixed, introduced into a first refractory container with a bulk porosity ranging from 0.3 to 0.6, and heat-treated at 1400 to 1650 °C in a vacuum or in an oxygen-free atmosphere. The SiC reaction mixture is then analyzed with respect to the content of free carbon in the mixture, silicon- and/or carbon-containing powders are added to achieve a stoichiometric composition, and the mixture is introduced into a second refractory container with a bulk porosity ranging from 0.5 to 0.7 and heat-treated at 1850 to 2500 °C in a vacuum or in an oxygen-free atmosphere.

Description

Process for the production of silicon carbide and silicon carbide

The invention relates to the fields of materials management, material synthesis and technical ceramics and concerns a process for producing silicon carbide (SiC), in particular raw SiC, which can be used, for example, for producing refractory ceramics for linings in waste incineration plants or in industrial furnaces.

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 special, highly pure, and narrowly fractionated fine powder grains (0.5 to approximately 250 pm) is particularly important in microelectronics/photovoltaics (wafer sawing), for the production of ballistic protective ceramics for military technology, and in automotive/environmental technology (diesel particulate filters). Its coarser form (2 pm to 2000 pm) is also used as an abrasive material for high-quality surface finishing throughout mechanical engineering.

SiC powder granules are produced from special raw SiC through grinding, purification, and fractionation. This process results in high and consistent amounts of low-quality, poorly recyclable SiC. For approximately 120 years, crude SiC has been produced using an electrosynthesis process, the so-called Acheson process (DE 76629 A, DE 85197 A, EG Acheson, Carborundum: Its history, manufacture and uses, J. of the Franklin Institute, Vol. 136, Issue 3, 1893, pp. 194-203, ISSN 0016-0032). In this process, crude SiC is produced on a technically relevant scale via the carbothermal reduction of SiC>2 with carbon. Quartz sand is typically used as the starting material for SiC>2, and petroleum coke is typically used as the starting material for carbon.

The carbothermal reduction occurs according to the molecular formula:

However, in reality, the carbothermal reduction does not proceed completely according to the molecular formula, but depending on the temperature and composition of the starting materials, partial reactions also take place, such as a gas phase reaction of emerging SiO, the formation of SiO2 melts, the reduction of _SiO2 to molten Si, the reverse reaction of SiC with _SiO2 or SiO.

To produce high-quality raw SiC that is well crystallized, has a high purity of > 98% SiC and consists predominantly of the high-temperature modification of SiC, the so-called Alpha SiC, temperatures of > 2000 °C are required in this Acheson process.

Accordingly, this manufacturing process, based on the Acheson process, requires high energy input and is therefore tied to the price of electricity and oil (petrol coke as the raw material). It also generates high levels of dust, CO2, and SO2 emissions, which incur additional costs.

In addition, due to the high production temperatures required, the process has so far only been carried out discontinuously, and this also leads to further technical problems with the equipment used for production, such as gas evolution or reactions with components of the production equipment. Discontinuous production also limits the yield and quality of the raw SiC.

Alternative, especially continuous, manufacturing processes have not been successful so far, despite many attempts, mostly for economic reasons. J.C. McMullen, J. Electrochem. Soc. 104, 462-465, 1957, provides an overview of the state of the art in raw SiC production.

Furthermore, US Pat. No. 2,178,773 A discloses the production of SiC by moving a SiC>2-C mixture through an inductively heated furnace. In particular, a briquetted mixture is used, or the mixture is processed in a furnace in refractory containers. Only partially reacted mixtures at lower temperatures can be further reacted with additional heat treatment.

According to US 2,729,542 A, a raw material mixture for SiC in interconnected containers, which together form a tube, is passed through a tube furnace at a temperature of at least 1800 °C.

Using divided containers does not lead to the desired result.

From DE 1 186447 C a process for the production of silicon carbide is known in which carbon and sand are mixed with an adhesive liquid and continuously passed through a vertical furnace at temperatures up to 2100 °C until 80% conversion is achieved, and then the reaction mixture is cooled and stacked in a second furnace and treated under protective gas at temperatures above 2200 °C.

This procedure was proposed because the quartz-carbon mixture sinters heavily during silicon carbide production, severely impeding the further movement of the reaction mixture in a furnace. By achieving up to 80% conversion in a vertical furnace, further transport of the reaction mixture is possible. However, a second process step is required for complete conversion.

The disadvantage of this solution is that a high level of effort is required for grinding and mixing the starting products and for briquetting to stack the reaction material in the second furnace, and safe transport is not guaranteed, even in a vertical furnace.

WO 95/33683 A also describes a two-step process for producing alpha-SiC, using approximately stoichiometric ratios of SiO2 and carbon. The mixture, in pellet form, is converted to almost 100% ß-SiC at temperatures in the range of 1500-1800°C in a continuously operating rotary kiln. This is then converted to alpha-SiC in a second, conventional discontinuous heat treatment between 1800 and 2300°C. Furthermore, according to DE 30 50 136 C2, a process for producing ß-silicon carbide is known in which very fine carbon-containing powder and very fine silicon dioxide powder are reacted at a temperature of up to 1650 °C in an inert atmosphere, wherein the reaction mixture is conveyed by a screw conveyor through a low-temperature zone, then through a high-temperature zone up to a maximum of 1650 °C and again through a low-temperature zone and this cycle is repeated until the conversion into ß-silicon carbide is complete.

The disadvantage of this process is that very fine powders are required as starting powder and at least several repetitions of the process are necessary for each reaction product.

Despite ongoing investigations, no process is known according to the state of the art with which a substantially complete conversion of silicon carbide can be achieved in one process.

The object of the present invention is to provide a process for producing silicon carbide and silicon carbide in which crude silicon carbide is produced in one production process with high conversion rates and a high yield.

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.

In the process according to the invention for producing silicon carbide with a SiC content of more than 98 to 99.95 wt.%, silicon and carbon-containing starting powders with particle sizes having a dgs value of up to 1.6 mm are mixed in a Si:C ratio of 37:63 to 50:50 wt.% and filled into at least one first refractory container, wherein a bulk porosity of the mixture is set between 0.3 and 0.6 in the first container, and subsequently the container with the mixture is heat-treated at temperatures of 1400 to 1650 °C under vacuum or oxygen-free atmosphere, then the SiC reaction mixture is cooled, homogenized and analyzed with regard to its free carbon content, depending on the analysis result to achieve a substantially stoichiometric composition of the homogenized SiC reaction mixture, silicon and/or carbon-containing powders are added and subsequently this mixture is at least one second refractory container and a bulk porosity of this mixture between 0.5 and 0.7 in the second container is set, and then the second container filled in this way is heat-treated at temperatures of 1850 to 2500 °C under vacuum or oxygen-free atmosphere and the resulting raw SiC is cooled.

Advantageously, powders made of and/or with predominantly SiO2 and/or Si particles, advantageously made of quartz sands or pure SiO2 or pure metallic or metallurgical Si, are used as silicon-containing starting powders.

Also advantageously used as carbon-containing starting powders are powders made from soot, graphite, coal, particularly in the form of anthracite, coke, such as petroleum coke, pitch coke, coke from lignite or hard coal, recycled carbon, such as so-called thermolysis coke or recovered soot (rCB - recovered carbon black), biochar from wood gasification, or carbon from methane pyrolysis.

Furthermore, the starting powders are advantageously used in a Si : C ratio of 42 : 58 to 44 : 56 mass %.

And also advantageously, the starting powders are used with particle sizes with a dgs value of 100 pm to 1.6 mm, advantageously from 500 pm to 1000 pm.

It is also advantageous if, when mixing the starting powders in the at least one first refractory container, a bulk porosity of 0.32 to 0.45, advantageously of 0.35 to 0.4, is set.

It is furthermore advantageous if, in the case of the SiC reaction mixture, after the first heat treatment and after the addition of additives of silicon- and/or carbon-containing powders to achieve a stoichiometric composition in the at least one second refractory container, a bulk porosity of this mixture is set between 0.55 and 0.68 in the second container.

It is also advantageous if first and second fireproof containers made of graphite and/or carbon fiber reinforced carbon (CFC) are used.

It is also advantageous if the mixture filled into the first refractory container is covered with a layer of carbon-containing powder, whereby the carbon-containing powder of the covering layer is part of the total carbon content in the mixture and amounts to a maximum of 5 mass% thereof. It is also advantageous if first fireproof containers with a filling volume of up to 100 liters and second fireproof containers with a filling volume of up to 20 liters are used.

It is also advantageous if the heat treatments are carried out in a continuous furnace.

It is also advantageous if a large number of first and second refractory containers are filled with the respective mixtures.

It is also advantageous if the first and/or at least the second refractory container(s) are continuously passed through a continuous furnace one after the other and are heat-treated in the process.

Advantageously, the heat treatment of the at least one first refractory container with the mixture is carried out at temperatures between 1560 and 1650 °C, and/or the heat treatment of the at least one second refractory container with the reaction mixture from the at least one first refractory container with additives for realizing a stoichiometric mixture is carried out at temperatures between 2050 and 2350 °C.

The silicon carbide according to the invention with a SiC content of more than 98 to 99.95 wt. % is produced by the process according to the invention and has a predominant content of isometric and/or congruent SiC particles in predominantly cubic particle shape with an average roundness of 0.5 to 0.8 and essentially convex surfaces and no plate-like portions, wherein the SiC particles are essentially isolated and have no or essentially no closed porosity and a bulk density of the silicon carbide of at least 3.05 g/cm ³ is present.

Advantageously, particles are present as isometric and/or congruent SiC particles which have a ratio of dimensions in all spatial directions of > 0.5 to 1.

Also advantageous are SiC particles with a predominantly cubic particle shape with convex surfaces and no plate-like portions, which have an average roundness of 0.65 to 0.78. And also advantageously, the silicon carbide has a bulk density of 3.10 g/cm ³ , advantageously of 3.18 g/cm ³ .

With the solution according to the invention, it is possible for the first time to provide a method for producing silicon carbide in which crude silicon carbide is produced in one production process with high conversion rates and a high yield.

This is achieved for the first time with a process for producing silicon carbide with a SiC content of more than 98 to 99.95 mass%, in which silicon and carbon-containing starting powders with particle sizes with a dgs value of up to 1.6 mm are mixed in a Si:C ratio of 37:63 to 50:50 mass% and filled into at least one first refractory container.

Advantageously, silicon-containing starting powders are powders made of and/or containing predominantly SiO2 and/or Si particles. Quartz sands are advantageously used as SiO2 powders.

In case of using SiO2 powders, their silicon content is used to calculate the Si:C ratio of the powder mixture.

In pure SiO2 the Si content is 47 mass%.

In pure metallic silicon, the Si content is 100 mass%.

In metallurgical silicon, the Si content is at least 90 wt.%. Typical impurities in metallurgical silicon are iron, aluminum, and calcium. Other powders containing silicon particles are produced during the recycling of solar cells and during the grinding of single-crystal scrap from the semiconductor industry.

Carbon-containing starting powders which are advantageously used are powders made from soot, graphite, coal, particularly in the form of anthracite, coke, such as petroleum coke, pitch coke, coke from lignite or hard coal, recycled carbon, such as so-called thermolysis coke or recovered soot (rCB - recovered carbon black), biochar from wood gasification, or carbon from methane pyrolysis.

The carbon-containing starting powders advantageously have a carbon content of at least 85 mass% carbon, more advantageously at least 90 mass% carbon, even more advantageously at least 95 mass%.

It is also possible to use materials that already contain silicon and/or silicon oxides and carbon as starting powder, such as pyrolyzed Rice husks, or which only contain silicon from grinding sludge from the solar and microelectronics industries, or which already contain silicon carbide, or which contain carbon from organic contaminants.

The starting powders are also advantageously used in a mixing ratio of Si : C of 42 : 58 to 44 : 56 mass %.

It is particularly preferred that the silicon-containing starting powder used predominantly consists of SiCl particles, with only a minor proportion of Si-containing starting powder. Mixtures with a proportion of > 50 wt.% SiCl>2 are particularly preferred, and even more preferably > 80 wt.%.

The mixing of the silicon and carbon-containing starting powders is carried out, for example, using stirrers or in a mill.

Of particular importance in the process according to the invention is that, in contrast to prior art processes, relatively coarse starting powders can be used. Advantageously, the starting powders have particle sizes with a dgs value of 100 pm up to 1.6 mm, advantageously from 500 pm to 1000 pm. These particle size values indicate the upper limit of the particle size distribution (D95), at which 95% of the particles in the powder have a particle size up to the upper limit. Such particle size distributions are technically realized, for example, by sieving or classifying the starting powder. As an average particle size d50, the starting powders have particle sizes of 150 pm to 1.2 mm, advantageously from 200 pm to 650 pm. In the lower range of the particle size distribution, very fine particles with particle sizes < 45 pm may also be present in the starting powders according to the invention, but it is also advantageous that this content of very fine particles in the starting powders is low, since this proportion of very fine particles is unfavorable for the safe handling of the powders during transport, mixing and filling.

The particle sizes of silicon-containing starting powders and carbon-containing starting powders can differ, i.e. coarser carbon-containing starting powders can be mixed with finer silicon-containing starting powders or vice versa.

Preferably, finer silicon-containing starting powders are used and mixed over the carbon-containing starting powders. Even coarser particles of the silicon-containing and/or carbon-containing starting powders with a size of >1.6 mm may also be present in isolated cases, up to a maximum of 5% of the particles in the starting powders, but this slows down the reaction of the particles to SiC and the content of unreacted SiC>2 or carbon in the SiC increases unfavorably.

The mixture of starting powders is then poured into at least one first refractory container.

Advantageously, the at least one first and/or at least one second refractory container consists of graphite and/or carbon fiber reinforced carbon (CFC). However, other materials, such as silicon carbide ceramics or CFC containers with a SiC coating, can also be used as container materials.

Advantageously, the at least one first refractory container consists of carbon-, nitride- or reaction-bonded SiC, while recrystallized SiC can be used for the at least one second refractory container.

It is particularly important that the mixture of starting powders in the at least one first refractory container has a bulk porosity of 0.3 to 0.6, advantageously of 0.32 to 0.45, even more advantageously of 0.35 to 0.4.

The aggregate porosity is defined as

Mass of the mixture per filled volume of the mixture

1 . -

True density of the mixture

The pure density of the mixture can be determined mathematically or experimentally using gas pycnometry if the density of the starting materials and their proportions is known.

The mass of the mixture can be easily determined by weighing.

The volume of the containers can be easily determined by calculation for simple container geometries or by filling with liquid and measuring the volume of the liquid for unclear container geometries.

The containers can be provided with fill level markings to ensure that the same volume of mixture is always added to the containers. After the mixture of starting powders is filled into at least one first refractory container, the container is heat-treated with the mixture at temperatures ranging from 1400 to 1650 °C under vacuum or in an oxygen-free atmosphere. Technical protective gas atmospheres such as argon or nitrogen are used as the oxygen-free atmosphere.

Advantageously, the heat treatment of the at least one first refractory container with the mixture of starting powders takes place at temperatures between 1560 and 1650 °C.

The heat treatment can advantageously be carried out in a continuous furnace. It is also advantageous if a plurality of first refractory containers are filled with the mixtures of starting powders and are continuously passed through a continuous furnace, where they are heat-treated.

After heat treatment of the mixture in the at least one first refractory container, the container is cooled, resulting in a reaction mixture that has already been predominantly converted to SiC. The SiC content of the SiC reaction mixture is essentially still < 98 mass%, and there are still portions of Si and C in the reaction mixture that have not yet been converted or have reacted with each other.

The SiC reaction mixture is then homogenized into a powder, for example by grinding, pressing, or breaking, which on the one hand restores a particle size distribution of the reaction mixture of dgs = 1.6 mm and on the other hand evens out and homogenizes the composition and particle size of the powder.

For the homogenized SiC reaction mixture from the first process step of the process according to the invention, the chemical composition is then analyzed according to the invention.

The analysis of the SiC content is determined using the method according to FEPA Standard 45-1:2011.

The impurities can be differentiated using known analytical methods, for example Cfree, Sifree, SiO ₂ and iron according to DIN EN ISO 9286:2023-10, DIN EN ISO 21068:2024 Part 1-3 and FEPA Standard 45-1:2011. Spectroscopic methods can also be used for the Analysis of impurities can be applied, for example according to the procedure of DIN EN 15991:2024.

In particular, the chemical analysis of the homogenized SiC reaction mixture is intended to determine the free carbon content (Cf _re i) using the specified analytical methods.

Based on the analysis of the SiC reaction mixture, the reaction to form SiC and/or

Si + C -> SiC

Silicon and/or carbon-containing powders, advantageously those powders which are also used as starting powders, are added in order to realize a stoichiometric composition of the SiC reaction mixture for filling the at least one second container.

This measure ensures that the SiC reaction mixture is essentially completely converted to more than 98 mass% SiC.

For this purpose, according to the invention, the SiC reaction mixture with the additions of silicon- and/or carbon-containing powders for a stoichiometric composition is filled into the at least one second refractory container and a bulk porosity of this mixture is set between 0.5 and 0.7, advantageously between 0.55 and 0.68, in the second container.

The determination and adjustment of the aggregate porosity is carried out according to the same formulas and with the same procedures as for the aggregate porosity in the at least one first refractory container.

After adjusting the aggregate porosity according to the invention, the at least one second refractory container thus filled is heat-treated at temperatures of 1850 to 2500 °C under vacuum or in an oxygen-free atmosphere. Technical protective gas atmospheres, such as argon or nitrogen, are used as the oxygen-free atmosphere. Advantageously, the heat treatment of the at least one second refractory container with the mixture of starting powders takes place at temperatures between 2050 and 2350 °C. The heat treatment can advantageously be carried out in a continuous furnace. It is also advantageous if a plurality of second refractory containers are filled with the SiC reaction mixtures with the additives required to achieve the stoichiometric composition and are continuously passed through a continuous furnace, where they are heat-treated.

The heat treatment of the at least one first and at least one second refractory container with the respective mixtures can be carried out in one furnace one after the other or in two different furnaces.

After heat treatment, the resulting raw SiC is cooled and can be further processed.

The adjustment of the aggregate porosity mentioned according to the invention in the refractory containers both of the starting powder mixture in the at least one first refractory container and of the SiC reaction mixture with the additives for realizing a stoichiometric mixture in the at least one second refractory container is realized by filling the mixtures and by a mechanical treatment of the aggregate, for example by vibrating, indenting, pressing, tamping or by a combination of such mechanical treatments, such as vibratory pressing.

The invention expressly does not envisage forming the mixtures into any molded parts before filling the containers, thereby filling the containers. The resulting compaction prevents the desired aggregate porosity from being achieved, thus preventing the desired high conversion rates and yields of crude SiC from being achieved.

Likewise, the present invention expressly does not provide for the necessary aggregate porosity to be achieved by the addition of auxiliary materials such as binders or other burnout materials.

It has been found according to the invention that if a larger aggregate porosity is set than specified for the respective containers according to the invention, partial losses of the SiC content occur during SiC production and thus the yields of crude SiC are significantly lower, both for the SiC reaction mixture from the first container and for the crude SiC from the second container, wherein in particular the crude SiC from The second container contains significantly more raw SiC with SiC contents < 98%. In addition, the yield of raw SiC is lower because less material can be introduced into the containers, especially the first container.

Furthermore, it has been found according to the invention that if a lower aggregate porosity is set than that specified for the respective containers according to the invention, there is a lower and in particular more inhomogeneous conversion of the starting powder to SiC in the first container and a strong caking or sintering of SiC particles in the second container, wherein the caking or sintered SiC particles can then no longer be easily separated.

It has been particularly surprisingly found that, for a high SiC content of >98 wt.% and uniformity of composition and particle size in the containers, different aggregate porosities in the first and second containers are advantageous. Surprisingly, low aggregate porosities of 0.35-0.45 are preferred in the first containers, while higher aggregate porosities of 0.55-0.68 are preferred in the second containers.

In addition to the mechanical treatment of the aggregate, the aggregate porosity also depends on the particle size distribution of the starting powder. For the first process step in at least one first container, the particle size distribution can also be varied within the limits specified in the invention to achieve the aggregate porosity according to the invention. This can be determined using simple test series.

For the second process step in at least one second container, the aggregate porosity depends not only on the mechanical treatment of the aggregate but also on the particle size distribution of the SiC particles achieved in the first process step, their degree of agglomeration, and the quantity and particle size of the admixtures. Here, too, the aggregate porosity according to the invention can be determined and adjusted through simple test series.

It is also possible to use the first refractory containers as second refractory containers, i.e. after emptying the first refractory containers from the first heat treatment, the SiC reaction mixture with the determined additives can be refilled into these refractory containers for the second heat treatment and then heat treated. However, it is preferred that different containers with different geometries are used for the first refractory container and the second refractory container.

It is very advantageous according to the invention if the first fireproof container has a filling volume of up to a maximum of 100 liters and the second fireproof container has a filling volume of up to a maximum of 20 liters.

It is also advantageous if the mixture filled into the first refractory container is covered with a layer of carbon-containing powder.

The enrichment of carbon in a covering layer on the heap in the first refractory container promotes the conversion to SiC, since, for example, escaping vaporous Si and/or SiO can react with the carbon of the covering layer to form SiC and does not enter the atmosphere outside the container.

According to the invention, it is important that the carbon in the cover layer is part of the total carbon content in the mixture in the first container and amounts to a maximum of 5 mass% thereof.

Due to the small amount of carbon in the top layer, a maximum of 5 mass%, only a thin layer is formed, the thickness of which depends on the bulk density of the carbon material used. The impact of this thin layer on the aggregate porosity of the mixture is therefore negligible; i.e., the aggregate porosity data in the first container refer to the aggregate without the thin carbon top layer.

However, to calculate the Si:C ratio of the mixture, the amount of carbon in the top layer must be taken into account by adding the amount of carbon in the top layer to the total amount of carbon in the mixture.

The process according to the invention produces silicon carbide with an SiC content of more than 98 to 99.95 mass %. This raw SiC is further processed into SiC powder, which can then be used, for example, to produce SiC ceramics.

It has been shown that under the conditions stated in the invention, a particularly uniform and homogeneous conversion of the starting powder to the SiC reaction product and during the conversion of the SiC reaction product to crude SiC takes place and a significantly higher yield of crude SiC with SiC contents of > 98 mass % is realized. The silicon carbide according to the invention with a SiC content of more than 98 to 99.95 wt. % has a predominant content of isometric and/or congruent SiC particles in predominantly cubic particle shape with an average roundness of 0.5 to 0.8 and essentially convex surfaces and no plate-like portions, wherein the SiC particles are essentially isolated and have no or essentially no closed porosity and a bulk density of the silicon carbide of at least 3.05 g/cm ³ is present.

The silicon carbide according to the invention is a raw SiC, which is further processed into SiC powder.

For all information within the scope of the present invention which is stated to be essentially the same, it should be understood that the values deviate only very slightly from the information and that the deviations have been caused by a technical realization or technical processing.

Advantageously, SiC particles are present as isometric and/or congruent SiC particles which have a ratio of dimensions in all spatial directions of > 0.5 to 1.

The determination of isometric particle size, congruent particle shape and roundness can be carried out according to DIN-ISO 13322-1:2014-5 Parts 1 and 2 (static and dynamic analysis) or determined in accordance with DIN-ISO 14688-1:2020-11.

Also advantageously, the SiC particles of the silicon carbide according to the invention have a predominantly cubic particle shape with convex surfaces and no plate-like portions and show an average roundness of 0.65 to 0.78.

The SiC particles according to the invention are rounded, semi-rounded and edge-rounded and have essentially convex surfaces.

It is also advantageous if the silicon carbide has a bulk density of 3.10 g/cm ³ , even more advantageously of 3.18 g/cm ³ .

The bulk density can be determined using the well-known method of gas pycnometry.

Furthermore, the porosity of the silicon carbide according to the invention can be determined from the bulk density in relation to the theoretical density of the silicon carbide Bulk density

Porosity [in Vol. -%] = (1 - - ) *100 theoretical density

The theoretical density of silicon carbide is usually assumed to be 3.21 g/cm ³ .

The bulk density of the silicon carbide according to the invention, measured by gas pycnometric means, is therefore at least 3.05 g/cm ³ , which corresponds to a porosity of <5 vol.%, advantageously at least 3.10 g/cm ³ , which corresponds to a porosity of <3.5 vol.%, even more advantageously at least 3.18 g/cm ³ , which corresponds to a porosity of <1 vol.%.

The silicon carbide according to the invention exhibits highly isometric and/or congruent, i.e., very uniform and consistent, SiC particles in terms of shape and size. A further feature of the silicon carbide according to the invention is that the SiC particles are only slightly intergrown, with no or essentially no conglomerates present.

Likewise, the SiC particles of the silicon carbide according to the invention show no or only very slight closed porosity.

The silicon carbide according to the invention can be produced by the process according to the invention for producing silicon carbide.

For example, X-ray analysis of the silicon carbide according to the invention shows that the SiC consists of 100% alpha polytypes, predominantly of the 6H, 4H, and 15R variants. Furthermore, the crystallite size of the particles is very uniform, meaning the average crystallite size deviates from the mean by only approximately +/- 20%. The mean value itself depends on the manufacturing conditions, for example, the temperature, the holding time, and the protective gas atmosphere of the second heat treatment. High temperatures and long holding times lead to a coarser powder, and an argon atmosphere also produces coarser powders than a nitrogen atmosphere. The silicon carbide according to the invention and produced according to the invention differs from the raw SiC produced by the known Acheson process on the one hand by very high SiC contents of more than 98 mass% and on the other hand the silicon carbide according to the invention and produced according to the invention has very uniform properties, such as the fact that the silicon carbide consists almost exclusively or exclusively of alpha polytypes and has a very uniform crystallite size of the SiC particles.

Likewise, the silicon carbide according to the invention and produced according to the invention also differs from the high-purity silicon carbides produced by known processes, which have contents of more than 99.95 mass% SiC, essentially by an even higher purity.

Likewise, the raw SiC from the Acheson manufacturing process is known to show very non-uniform properties with regard to SiC content, polytype content, particle size and particle shape within a furnace batch, when viewed over the entire batch process.

SiC crushed from the inner zone of a furnace charge using the Acheson process can also exhibit SiC contents of more than 98 wt. %, but this SiC material is highly irregular in terms of particle size and shape. Very large, platelet-shaped and sharp-edged crystals up to 20 mm in size can occur, alongside very small (<200 pm), highly intergrown, and irregular platelet-shaped or even columnar crystals. These intergrowths are so strong that the crystallites also enclose pores and can only be crushed by the application of strong mechanical forces, resulting in a highly irregular and splintery particle shape in the resulting powder.

Due to the porosity of the intergrowths, the bulk density of the coarsely broken raw SiC with a particle size between 1 and 3 mm from the Acheson process, measured by gas pycnometry, is far from the true density of SiC of 3.21 g/cm ³ and usually shows values for the bulk density of less than or equal to 2.97 g/cm ³ , which corresponds to a porosity of 7.5 vol.-%, or even only 2.59 g/cm ³ , which corresponds to a porosity of 19 vol.% (EHP Wecht, Feuerfest-Siliconcarbid, Applied Mineralogy Vol 11 , Springer-Verlag 1977, p. 41 and p. 246 ISBN 978-3-7091-7069-4).

It can be assumed that the properties of the silicon carbide according to the invention arise from the fact that, during the production according to the invention, in particular the adjustment of the aggregate porosity according to the invention prevents excessive sintering and intergrowth of the silicon carbide crystals formed and thus the inclusion of porosity. At the same time, the inventive use of the Starting powder and the heat treatment according to the invention achieve very uniform temperatures and chemical reaction conditions, especially in the second refractory containers according to the invention, so that very uniform SiC crystals are formed.

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

Comparison example 1

With a portion of a mixture as in Example 1 consisting of 1180 kg of quartz sand (SiO2 content 99 mass%, gas pycnometrically determined true density 2.65 g/cm ³ , particle size dgs = 0.3 mm, with a maximum of 20% of the particles having a particle size of <45 pm) and 820 kg of petroleum coke (carbon content 89 mass%, gas pycnometrically determined true density 1.40 g/cm ³ , particle size dgs = 0.6 mm, with a maximum of 15% of the particles having a particle size of <45 pm), dry mixed according to a Si : C ratio of 42.8 : 57.2 with a true density of the mixture of 1.94 g/cm ³ , 5 of the first cuboidal refractory containers according to Example 1 (graphite with a length of 350 mm, width 350 mm and height 200 mm with a wall thickness of 15 mm) loosely filled to a filling height of 180 mm corresponding to a filling volume of 18432 cm ³ , analogous to Example 1. A differential weighing of empty and filled container results in a filling quantity of the mixture of 13.5 kg. This results in a corresponding aggregate porosity of 0.62. The filled first refractory container is introduced into the same resistance-heated continuous walking beam furnace as in Example 1 and there subjected to heat treatment at a maximum temperature of 1630 °C with a residence time of 2 h under a nitrogen atmosphere, and discharged after passing through the cooling section. A whitish, fluffy layer has formed on the surface of the reaction mixture, which indicates the escape of SiO during the process. The loose powder is removed from the first refractory container as the reaction mixture. The amount of powder removed, determined by weighing, is 5620 g.

Subsequently, four additional identical graphite refractory containers containing the same amount of powder mixture are filled to a fill level of 180 mm and subsequently subjected to the same heat treatment at a maximum temperature of 1630°C for a residence time of 2 hours under a nitrogen atmosphere in the continuous furnace. The powder of the reaction mixture taken from the five refractory containers is poured into the drum of a roller mixer, where it is homogenized over a period of 2 hours with the addition of 5 mm diameter SiC grinding balls. A sample of the homogenized reaction mixture is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, DIN EN ISO 21068:2024 Part 1-3 and consists of 66.5 wt.% SiC, 5.4 wt.% SiO2, and 27.7 wt.% C-free. The remainder consists of other impurities, such as iron. 41 wt.% of the same quartz sand as used as the starting material must be added to the powder to achieve the stoichiometric reaction to SiC. The entire reaction mixture is dry-mixed in a roller mixer for a further 2 h. The reaction mixture is then removed, and the grinding balls are separated. The true density of the mixture is determined by gas pycnometric analysis to be 2.167 g/ ^cm3 .

2.9 kg of the mixture are loosely poured into a second cuboid-shaped refractory graphite container with a length of 300 mm, a width of 200 mm, and a height of 100 mm, with a wall thickness of 10 mm (identical to Example 1), resulting in a reaction mixture filling height of 95 mm, corresponding to a filling volume of 4788 ^cm³ . This results in a corresponding aggregate porosity of 0.72. The filled second refractory container is introduced into a second resistance-heated pusher furnace as in Example 1, where it is heat-treated at a maximum temperature of 2350 °C with a residence time of 30 minutes under a nitrogen atmosphere. After passing through the cooling section, the container is discharged. The slightly sintered but easily separated powder is removed from the second refractory container. The amount of powder removed, determined by weighing, is 1970 g.

Subsequently, four additional identical second refractory graphite containers are filled with the same amount of reaction mixture to a fill level of 95 mm and subjected to the same heat treatment at a maximum temperature of 2350°C with a residence time of 30 minutes under a nitrogen atmosphere in the continuous furnace. The powder removed from the five second refractory containers is transferred to a tumbler mixer, where it is homogenized with the addition of 5 mm diameter SiC grinding balls over a period of 1 hour.

The powder is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, DIN EN ISO 21068:2024 Part 1-3 and consists of 94.2 wt.% SiC, 1.3 wt.% SiO2, and 4.1 wt.% C-free. The remainder consists of minor impurities such as iron, free silicon, aluminum, and other trace elements. X-ray analysis shows a SiC content of 100% polytypes of the alpha modification, predominantly of the 6H polytype. Scanning electron micrographs show an average crystallite size of approximately 80 pm. The SiC particles are present as isolated and partially intergrown particles with a partially platelet-like particle shape with an average roundness of 0.4.

Sampling from different areas of the second refractory vessel (top/middle, bottom/edge) revealed very large compositional deviations of >4 wt.%. SEM analyses revealed significant differences in mean crystallite size.

Comparison example 2

From the mixture of example 1 consisting of 1180 kg of quartz sand (SiO2 content 99 mass%, gas pycnometrically determined pure density 2.65 g/cm ³ , particle size dgs = 0.3 mm, with a maximum of 20% of the particles having a particle size of <45 pm) and 820 kg of petroleum coke (carbon content 89 mass%, gas pycnometrically determined pure density 1.40 g/cm ³ , particle size dgs = 0.6 mm, with a maximum of 15% of the particles having a particle size of <45 pm); After the dry mix, 200 kg of the mixture (corresponding to a Si:C ratio of 42.8:57.2) with a true density of 1.94 g/cm ³ ) are taken and mixed with 4 kg of polyvinyl alcohol (PVA) dissolved as a 30% solution in water for a further 2 hours. The wet mixture is then pressed into cuboids 40 mm high using a hydraulic press in a steel tool with die dimensions 80x80 mm at a pressure of 120 MPa. The 80x80x40 mm cuboids weigh 352 g each after drying in air at 120 °C for 2 hours. 80 of these cuboids are filled into a first cuboid-shaped refractory container from Example 1 (graphite with a length of 350 mm, width of 350 mm, and a height of 200 mm, with a wall thickness of 15 mm) to a filling height of 200 mm, corresponding to a filling volume of 20,480 ^cm³ . The total mass of 80 cuboids results in a filling quantity of 28.2 kg. This results in a corresponding aggregate porosity of 0.29. The filled first refractory container is introduced into the same resistance-heated continuous walking beam furnace as in Example 1, where it is heat-treated at a maximum temperature of 1630 °C with a residence time of 2 hours under a nitrogen atmosphere. After passing through the cooling section, the cuboids are discharged. The cuboids have shrunk slightly in size by approximately 1 mm, are heavily cracked, and disintegrate into irregular chunks upon removal. It can be seen that the blocks located in the center of the container have a dark to black color, while the blocks at the surface have a white surface layer. The total amount of the removed blocks, determined by weighing, is 17,180 g. Subsequently, four additional identical graphite refractory containers are filled with the same quantity of blocks and subjected to the same heat treatment at a maximum temperature of 1630 °C for a residence time of 2 hours under a nitrogen atmosphere in the continuous furnace. The lumps of the reaction mixture removed from the five refractory containers are filled into the drum of a roller mixer, where they are homogenized over a period of 8 hours with the addition of 5 mm diameter SiC grinding balls.

A sample of the homogenized reaction mixture is analyzed using FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, DIN EN ISO 21068:2024 Part 1-3 and consists of 36.2 wt.% SiC, 30.9 wt.% SiO2 and 32.4 wt.% C-free. The remainder is other impurities such as iron. 23.2 wt.% of the same quartz sand as used as the starting material must be added to the powder to achieve the stoichiometric reaction to SiC. The entire reaction mixture is mixed dry in a roller mixer for a further 2 h. The true density of the reaction mixture, determined by gas pycnometric analysis, is 1.88 g/ ^cm3 . 4 kg of polyvinyl alcohol (PVA) dissolved as a 30% solution in water is then added to the mixture, and the mixture is mixed wet for a further 2 h. The reaction mixture is then removed and the grinding beads separated. The moist mixture is pressed into 25 mm high cubes using a hydraulic press in a steel die with die dimensions of 60 x 70 mm at a pressure of 100 MPa. The 60 x 70 x 25 mm cubes weigh 105 g each after drying in air at 120 °C for 2 hours.

48 of these cuboids are stacked into a second cuboid-shaped refractory container from Example 1 (graphite with a length of 300 mm, width of 200 mm, and a height of 100 mm, with a wall thickness of 10 mm) to a filling height of 200 mm, corresponding to a filling volume of 5040 ^cm³ . The total mass of 48 cuboids results in a filling quantity of 5 kg. This results in a corresponding aggregate porosity of 0.47. The filled second refractory container is introduced into a second resistance-heated pusher furnace as in Example 1, where it is subjected to heat treatment at a maximum temperature of 2350 °C with a residence time of 30 minutes under a nitrogen atmosphere. After passing through the cooling section, it is discharged. The heavily sintered cuboids are removed from the second refractory container. The quantity of cuboids removed, determined by weighing, is 2.97 kg.

In the following, another 4 identical second refractory containers made of graphite, each containing 48 pieces of the reaction mixture, are brought to a filling height of 100 mm and are subjected to the same heat treatment at a maximum temperature of 2350 °C at a residence time of 30 minutes under a nitrogen atmosphere in the continuous furnace. The blocks removed from the five second refractory containers are filled into a drum mill, where they are crushed over a period of 10 hours with the addition of 5 mm diameter SiC grinding balls.

The crushed powder is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, DIN EN ISO 21068:2024 Part 1-3 and consists of 92.7 wt.% SiC, 1.6 wt.% SiO2 and 5.2 wt.% C-free. The remainder consists of minor impurities such as iron, aluminum and other elements in traces. X-ray analysis shows a SiC content of 100% polytypes of the alpha modification, predominantly of the 6H polytype. Scanning electron microscopy images show an average crystallite size of approximately 100 pm. The SiC particles are predominantly intergrown particles in a predominantly platelet-like, partly sharp-edged or irregular particle shape. The bulk density of the powder determined by gas pycnometric analysis is 3.03 g/ ^cm3 . Closed pores were found between the intergrown particles in the polished section.

Sampling from blocks in various areas of the second refractory vessel (top/middle, bottom/edge) reveals very large compositional deviations of >5 wt.%. SEM analyses reveal significant differences in mean crystallite size.

Example 1

1180 kg of quartz sand (SiO2 content 99 mass%, gas pycnometrically determined true density 2.65 g/cm ³ , particle size dgs = 0.3 mm, with a maximum of 20% of the particles having a particle size of <45 pm) and 820 kg of petroleum coke (carbon content 89 mass%, gas pycnometrically determined true density 1.40 g/cm ³ , particle size dgs = 0.6 mm, with a maximum of 15% of the particles having a particle size of <45 pm), corresponding to a Si:C ratio of 42.8:57.2, are dry mixed in several batches in a drum mixer for 8 hours each. The true density of the mixture is 1.94 g/cm ³ . 21.8 kg of the mixture are filled into a first cuboid-shaped refractory container made of graphite with a length of 350 mm, a width of 350 mm and a height of 200 mm with a wall thickness of 15 mm, and compacted by vibratory tamping to a filling height of 180 mm for the powder mixture, corresponding to a filling volume of 18432 ^cm³ . This results in a corresponding aggregate porosity of 0.39. The filled first refractory container is introduced into a resistance-heated continuous walking beam furnace and subjected to heat treatment at a maximum temperature The powder is subjected to a 2-hour residence time under a nitrogen atmosphere at 1630 °C and discharged after passing through the cooling section. The slightly sintered but easily separated powder is removed from the first refractory container as a reaction mixture. The amount of powder removed, determined by weighing, is 7652 g.

Subsequently, another 89 identical graphite refractory containers containing the same amount of powder mixture are filled to a fill level of 180 mm and subsequently subjected to the same heat treatment at a maximum temperature of 1630 °C for a residence time of 2 hours under a nitrogen atmosphere in the continuous furnace. The powder of the reaction mixture taken from the 90 refractory containers is filled into the drum of a roller mixer, where it is homogenized over a period of 2 hours with the addition of 5 mm diameter SiC grinding balls.

A sample of the homogenized reaction mixture is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, and DIN EN ISO 21068:2024 Part 1-3 and consists of 95.0 wt.% SiC, 1.4 wt.% SiO2, and 3.1 wt.% C-free. The remainder consists of other impurities such as iron. 3.77 wt.% of the same quartz sand as used as the starting material is added to the powder; corresponding to the stoichiometric reaction to SiC, and dry-mixed in a roller mixer for a further 2 h. The reaction mixture is then removed, and the grinding balls are separated. The true density of the mixture is determined by gas pycnometric analysis to be 3.03 g/ ^cm3 .

4.9 kg of the mixture are poured into a second cuboid graphite refractory container measuring 300 mm long, 200 mm wide, and 100 mm high with a wall thickness of 10 mm on a vibrating table, resulting in a reaction mixture filling height of 76 mm, corresponding to a filling volume of 3830 ^cm³ . This results in a bulk porosity of 0.58. The filled second refractory container is transferred to a second resistance-heated pusher furnace, where it is heat-treated at a maximum temperature of 2350 °C with a residence time of 30 minutes under a nitrogen atmosphere. After passing through the cooling section, the container is discharged. The slightly sintered, but easily separated powder is removed from the second refractory container. The amount of powder removed, determined by weighing, is 4660 g.

In the following, another 119 identical second refractory containers made of graphite are filled with the same amount of reaction mixture to a filling height of 76 mm and are subjected to the same heat treatment at a maximum temperature of 2350 °C at a residence time of 30 minutes under a nitrogen atmosphere in the continuous furnace. The powder taken from the 120 second refractory containers is poured into a tumbler mixer, where it is homogenized with the addition of 5 mm diameter SiC grinding balls over a period of 1 hour.

The powder is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, DIN EN ISO 21068:2024 Part 1-3 and consists of 99.3 wt.% SiC, 0.1 wt.% SiO2 and 0.2 wt.% C-free. The remainder consists of minor impurities such as iron, free silicon, aluminum and other elements in trace amounts. X-ray analysis shows a SiC content of 100% polytypes of the alpha modification, predominantly of the 6H polytype. Scanning electron microscopy images show an average crystallite size of approximately 120 pm. The SiC particles are predominantly isolated, isometric and congruent particles in a predominantly cubic shape with an average roundness of 0.7. The bulk density of the powder determined by gas pycnometric analysis is 3.12 g/ ^cm3 . In the polished section, almost no closed pores were found in the particles.

Sampling from different areas of the second refractory vessel (top/middle, bottom/edge) revealed very small compositional deviations of <0.1 wt.%. Both X-ray analysis and SEM investigations show no differences in polytype content or mean crystallite size.

Example 2

1450 g of quartz sand (SiO2 content 98 wt. %, gas pycnometrically determined true density 2.65 g/cm ³ , particle size dgs = 0.8 mm, with a maximum of 10% of the particles having a particle size of <45 pm) and 970 g of calcined petroleum coke (carbon content 98 wt. %, gas pycnometrically determined true density 2.1 g/cm ³ , particle size dgs = 1 mm, with a maximum of 5% of the particles having a particle size of <45 pm) are mixed dry in a tumble mixer for 4 h. The true density of the mixture is 2.4 g/cm ³ . 1320 g of the mixture are filled in three layers of 440 g each into a first cylindrical refractory graphite container with a wall thickness of 5 mm, an inner diameter of 185 mm and an inner height of 50 mm, each by light manual pressing, so that the powder filling height is 35 mm, corresponding to a filling volume of 940 cm ³ . This results in a corresponding aggregate porosity of 0.42. The surface of the powder filling is evenly covered with 30 g of acetylene black (carbon content 99.9 wt%), in This is sprinkled on with a sieve and then lightly pressed down. The total amount of carbon (the sum of petroleum coke and soot) in the mixture is thus 1000 g. The Si:C ratio is thus 40.4 to 59.6.

The filled first refractory container is heat-treated in a resistance-heated furnace at 1600 °C for 5 hours under an argon atmosphere, cooled in the furnace, and the slightly sintered but easily separated powder is removed from the first refractory container as a reaction mixture. The amount of powder removed, determined by weighing, is 554 g.

The powder of the reaction mixture is treated dry in a tumbler mixer for 2 h, during which 20 SiC grinding balls with a diameter of 5 mm are added.

A sample of the ground powder is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, and DIN EN ISO 21068:2024 Part 1-3 and consists of 94.1 wt.% SiC, 1.1 wt.% SiO2, and 4.2 wt.% C-free. The remainder consists of other impurities such as iron. 8.3 wt.% silicon powder (Si content 98 wt.%, dgs = 150 pm) is added to the powder; corresponding to the stoichiometric reaction to SiC, and dry-mixed in a tumbler mixer for a further 2 h. The powder is then removed, and the grinding balls are separated. The true density of the mixture is determined by gas pycnometric analysis to be 2.93 g/ ^cm3 .

560 g of this mixture is refilled into the first refractory container, which serves as the second refractory container, on a vibrating table to a height of 18 mm, corresponding to a volume of 484 ^cm³ . This results in a bulk porosity of 0.60. The filled second refractory container is heat-treated in a resistance-heated furnace at 2450 °C for 3 hours under an argon atmosphere, cooled in the furnace, and the slightly sintered but easily separated powder is removed from the second refractory container.

The quantity of powder taken, determined by weighing, is 526 g. The powder is analyzed according to FEPA Standard 45-1:2011, DIN EN ISO 9286:2023-10, DIN EN ISO 21068:2024 Part 1-3 and consists of 99.4 wt.% SiC, 0.1 wt.% Si-free, and 0.1 wt.% C-free. The remainder consists of minor impurities such as iron, aluminum, and other elements in trace amounts. X-ray analysis shows a SiC content of 100% polytypes of the alpha modification, predominantly of the 6H polytype. Scanning electron micrographs show an average crystallite size of approximately 400 pm. The SiC particles are predominantly isolated, isometric, and congruent particles in a predominantly cubic shape with predominantly rounded edges and an average circularity of 0.6. The bulk density of the powder, determined by gas pycnometric analysis, is 3.18 g/cm ³ . The polished section shows almost no closed pores in the particles.

Sampling from different areas of the crucible (top/center of the crucible, bottom/rim of the crucible) reveals very small deviations of <0.1 wt.% in composition. Both X-ray analysis and SEM investigations show no differences in polytype content or average crystallite size.

Claims

Patent claims 1. A process for producing silicon carbide with a SiC content of more than 98 to 99.95 wt.%, in which silicon and carbon-containing starting powders with particle sizes with a dgs value of up to 1.6 mm in a ratio of Si:C of 37:63 to 50:50 wt.% are mixed and filled into at least one first refractory container, wherein a bulk porosity of the mixture between 0.3 and 0.6 is set in the first container, and subsequently the container with the mixture is heat-treated at temperatures of 1400 to 1650 °C under vacuum or oxygen-free atmosphere, then the SiC reaction mixture is cooled, homogenized and analyzed with regard to its content of free carbon, depending on the analysis result to achieve an essentially stoichiometric composition of the homogenized SiC reaction mixture, silicon and/or carbon-containing powders are added and subsequently this mixture into at least a second refractory container and a bulk porosity of this mixture is adjusted between 0.5 and 0.7 in the second container, and then the thus filled second container is heat-treated at temperatures of 1850 to 2500 °C under vacuum or oxygen-free atmosphere and the resulting raw SiC is cooled. 2. Process according to claim 1, in which powders made of and/or with predominantly SiO2 and/or Si particles, advantageously made of quartz sands or pure SiO2 or pure metallic or metallurgical Si, are used as silicon-containing starting powders. 3. Process according to claim 1, in which powder made of soot, graphite, coal, in particular in the form of anthracite, coke, such as petroleum coke, pitch coke, coke from brown or hard coal, recycled carbon, such as so-called thermolysis coke or recovered soot (rCB - recovered carbon black), biochar from wood gasification, or carbon from methane pyrolysis are used as carbon-containing starting powder. 4. A process according to claim 1, wherein the starting powders are used in a Si:C ratio of 42:58 to 44:56 mass%. 5. A process according to claim 1, wherein the starting powders are used with particle sizes having a dgs value of 100 pm to 1.6 mm, advantageously from 500 pm to 1000 pm. 6. Process according to claim 1, wherein, during the mixing of the starting powders in the at least one first refractory container, a bulk porosity of 0.32 to 0.45, advantageously of 0.35 to 0.4, is set. 7. The method according to claim 1, wherein in the SiC reaction mixture, after the first heat treatment and after the addition of additives of silicon and/or carbon-containing powders to achieve a stoichiometric composition in the at least one second refractory container, a bulk porosity of this mixture is set between 0.55 and 0.68 in the second container. 8. A method according to claim 1, wherein first and second refractory containers are used which consist of graphite and/or carbon fiber reinforced carbon (CFG). 9. The method according to claim 1, wherein the filled mixture in the first refractory container is covered with a layer of carbon-containing powder, wherein the carbon-containing powder of the covering layer is a component of the total carbon content in the mixture and amounts to a maximum of 5 mass% thereof. 10. The method according to claim 1, wherein first fireproof containers with a filling volume of up to 100 liters and second fireproof containers with a filling volume of up to 20 liters are used. 11. A method according to claim 1, wherein the heat treatments are each carried out in a continuous furnace. 12. The method according to claim 1, wherein a plurality of first and second refractory containers are filled with the respective mixtures. 13. A method according to claim 1, wherein the first and/or at least second refractory container(s) are continuously passed through a continuous furnace one after the other and are thereby heat-treated. 14. The method according to claim 1, wherein the heat treatment of the at least one first refractory container with the mixture is carried out at temperatures between 1560 and 1650 °C, and/or the heat treatment of the at least one second refractory container with the reaction mixture from the at least one first refractory container with additives to achieve a stoichiometric mixture at temperatures between 2050 and 2350 °C. 15. Silicon carbide with an SiC content of more than 98 to 99.95 mass%, which has a predominant content of isometric and/or congruent SiC particles in predominantly cubic particle shape with an average roundness of 0.5 to 0.8 and essentially convex surfaces and no plate-like portions, wherein the SiC particles are essentially isolated and have no or essentially no closed porosity and a bulk density of the silicon carbide of at least 3.05 g/cm ³ is present. 16. Silicon carbide according to claim 15, wherein the isometric and/or congruent SiC particles are particles having a ratio of dimensions in all spatial directions of > 0.5 to 1. 17. Silicon carbide according to claim 15, wherein the SiC particles are predominantly cubic in shape with convex surfaces and no plate-like portions, and have an average roundness of 0.65 to 0.78. 18. Silicon carbide according to claim 15, wherein the silicon carbide has a bulk density of 3.10 g/cm ³ , advantageously of 3.18 g/cm ³ .