METHOD FOR SEPARATING IMPURITIES FROM SILICON CARBIDE, AND TEMPERATURE-TREATED AND PURIFIED SILICON CARBIDE POWDER

Abstract

The invention concerns the area of ceramics an relates to a method for separating impurities from silicon carbide, said method being applicable to SiC powders from grinding sludges, and to temperature-treated and purified silicon carbide powder. The aim of the invention is to provide a method with which different impurities are substantially completely removed using a simple and economical process. This is achieved by a method in which pulverulent SiC waste products that have a mass percent of SiC of at least 50% and an average grain size d.sub.50 ranging from 0.5 to 1000 μm and have been subjected to a temperature treatment and cooled are mechanically treated and physically separated. The physically separated SiC powder is then divided into two fractions, one of which has a mass of impurities that is greater than the mass of impurities in the other fraction at least by a factor of 2.

Claims

1. A method for separating impurities from silicon carbide, in which powdery SiC waste products having at least 50% by mass SiC and an average grain size d.sub.50 between 0.5 and 1000 μm, measured by laser diffraction, and a minimum content of 0.1% by mass iron and 0.1% by mass metallic silicon are subjected to a temperature treatment under a vacuum or in a non-oxidizing atmosphere at temperatures of 1400-2600° C. and cooled, and are then mechanically treated and physically separated, and subsequently a division of the physically separated SiC powder into two fractions is performed, of which the mass of impurities in one fraction is at least twice as high as in the other fraction.

2. The method according to claim 1, in which the mass of impurities in one fraction is at least 10 times higher, advantageously at least 20 times higher, than in the other fraction.

3. The method according to claim 1, in which the powdery SiC waste products have at least 75% by mass SiC, advantageously 80% by mass SiC, more advantageously 85% by mass SiC, more advantageously 90% by mass SiC.

4. The method according to claim 1, in which powdery SiC waste products having at least 50% by mass SiC and an average particle size d.sub.50 between 0.5 and 500 μm, measured by laser diffraction, are used.

5. The method according to claim 1, in which powdery SiC waste products having at least 50% by mass SiC and an average particle size d.sub.50 between >500 and 1000 μm, measured by laser diffraction, are advantageously used.

6. The method according to claim 1, in which powdery SiC waste products having at least 50% by mass SiC and an average particle size d.sub.50 between >500 and 1000 μm, measured by laser diffraction, are subjected to a temperature treatment at temperatures of 1400 to <2000° C.

7. The method according to claim 1, in which powdery SiC waste products having at least 50% by mass SiC and an average particle size d s between 0.5 and 1000 μm, measured by laser diffraction, and a content of 0.5 to 5.0% by mass iron and 0.5 to 5.0% mass metallic silicon are used.

8. The method according to claim 1, in which the temperature treatment of the SiC waste products is carried out at temperatures of 1400-2000° C.

9. The method according to claim 1, in which the temperature treatment of the SiC waste products is carried out at temperatures of 2000-2600° C.

10. The method according to claim 1, in which the temperature treatment is performed under a vacuum or a non-oxidizing atmosphere during the heating phase in the temperature range between 1200° C. and <1400° C. and from 1400° C. to 1800° C. with heating rates of less than or equal to 8 K/min.

11. The method according to claim 1, in which the temperature treatment is performed under a vacuum or a non-oxidizing atmosphere during the heating phase over 1800° C. with heating rates of less than or equal to 5 K/min.

12. The method according to claim 1, in which the temperature treatment is performed under a vacuum or a non-oxidizing atmosphere with holding times at the maximum temperature of 10 min to 300 min.

13. The method according to claim 1, in which the temperature treatment is performed under a non-oxidizing atmosphere with an amount of non-oxidizing gases of 0.5 to 30 l/h.

14. The method according to claim 1, in which the temperature treatment is performed under a vacuum or a non-oxidizing atmosphere while dissipating gaseous reaction products.

15. The method according to claim 1, in which the cooling of the powdery SiC is performed at a cooling rate of 0.1 to 100 K/min.

16. The method according to claim 1, in which the cooling of the powdery SiC is performed in a temperature range between 1200° C. and 800° C. at a cooling rate of 0.5 to 10 K/min.

17. The method according to claim 1, in which the mechanical treatment of the recycled powdery SiC is implemented by applying a mechanical impulse, advantageously by mixing, grinding, even more advantageously by autogenous grinding, or by using eddy currents and/or ultrasound.

18. The method according to claim 17, in which the mechanical treatment is carried out with an energy input between 0.1 and 5 MJ/kg.

19. The method according to claim 1, in which the physical separation of the recycled powdery SiC is carried out according to the particle size, the particle shape, the density, and/or the physical and/or chemical surface properties of the particles.

20. The method according to claim 19, in which the separation according to the particle size and/or particle shape is carried out by sieving, sifting, and/or cyclone methods.

21. The method according to claim 19, in which the separation is carried out by the effect of mass forces with regard to the particle density by flotation, sedimentation, sifting, centrifugation, and/or cyclone methods, or when the separation is carried out according to the density of the particles through flotation and/or cyclone methods.

22. The method according to claim 19, in which the separation is realized in a fraction containing at least 95% by mass, advantageously at least 98% by mass, even more advantageously at least 99% by mass silicon carbide.

23. The method according to claim 1, in which substantially metallic impurities are separated as the impurities.

24. The method according to claim 1, in which, in order to remove impurities in the form of Si and/or C, carbon, advantageously soot, graphite, and/or coke powder, and/or silicon and/or silicon dioxide (SiO.sub.2), is added during the temperature treatment in order to achieve a composition that is as stoichiometric as possible.

25. A temperature-treated silicon carbide powder containing SiC powder particles and substantially metallic impurities in the form of metallic mixed phases and having metallic impurities on the planar and/or convex surfaces of the silicon carbide powder particles, which are disposed in an island-like configuration or in the interstices between silicon carbide powder particles and are permanently bonded to one or more silicon carbide powder particles, wherein the impurities have a wetting angle between 10° C. and 90° C.

26. A temperature-treated silicon carbide powder having at least 98% by mass SiC and a maximum of 2% by mass substantially metallic impurities, wherein the impurities are disposed substantially on the surface of the silicon carbide powder particles.

27. The temperature-treated silicon carbide powder according to claim 26, in which the purified silicon carbide powder has at least 99% by mass SiC.

28. The temperature-treated silicon carbide powder according to claim 26, in which the impurities are present in the form of island-like melts of metallic mixed phases on the surface of the silicon carbide powder particles, which are permanently bonded to the surface of the particles after a mechanical treatment.

29. The temperature-treated silicon carbide powder according to claim 26, in which metallic impurities are disposed on the surface in primary particles of silicon carbide and, in secondary particles of silicon carbide, on the surface and/or in the interstices of the particles.

30. The temperature-treated silicon carbide powder according to claim 26, in which the impurities are disposed on the surface of the silicon carbide powder particles substantially on the convexly shaped parts of the surface of silicon carbide powder particles.

Description

EXAMPLE 1

[0144] 10 kg of an SiC powder as a by-product from SiC primary crushing, containing 91.8% by mass SiC, 1.3% by mass C.sub.free, 1.8% by mass Si, 3.7% by mass SiO.sub.2, 0.4% by mass Fe, and impurities with Al, V, Ti, and Ca in the range of >300 ppm each, is mixed with 180 g of coke powder, 100 g Si, and 40 g Fe. The powder mixture has an average particle size of 9.5 μm, determined by means of laser diffraction. The bed density of the powder mixture is 0.6 g/cm3. The powder mixture is loosely filled in graphite crucibles and compressed at 50 MPa in a plunger. The crucibles are heated in an inert gas furnace under an argon atmosphere at 8 K/min up to 2500° C. and held there at 2500° C. for 60 minutes, wherein a reduced heating rate of 3 K/min is used between 1200° C. and 2000° C. During the entire temperature treatment, an Ar gas flow is guided around the graphite crucible at 20 l/h.

[0145] Furthermore, carbon monoxide (CO) and silicon monoxide produced from the inert gas furnace is dissipated.

[0146] The cooling is performed at an overall speed of 2.5 K/min.

[0147] After cooling, the powdered crucible contents are treated in an air mill at 0.1 MJ/kg and, after a sifting step, split into three powders with particle sizes of <10, 10-60 μm, >60 μm.

[0148] Before the mechanical treatment, the SiC powder has an SiC content of 97.8% by mass. In particular, the metallic impurities are present in the same concentration order of magnitude as the starting powder.

[0149] After the mechanical treatment in the air mill, the powder with a particle size >60 μm has an SiC content of 99.1% by mass SiC. The content of Si and SiO.sub.2 is 0.22% by mass respectively, that of C.sub.free is 0.12% by mass, the Fe content is 0.16% by mass, and the other contents of metallic impurities each amount to significantly <100 ppm.

[0150] In the other two powders that were combined, 8.6 times the amount of impurities has been found.

[0151] The average grain size after the temperature treatment and the mechanical purification in the “clean” fraction is 92.7 μm. Thus, the grains are on average 9.75 times larger than in the starting material placed in the inert gas furnaces.

[0152] As a result of the method according to the invention, the “clean” fraction has a mass percentage of 81% by mass.

[0153] Before the mechanical treatment, the temperature-treated powder had a large number of secondary particles in whose interstices the impurities had accumulated. After the mechanical treatment, the “clean” fraction contains almost exclusively primary particles. Island-shaped or fragmented island-shaped metallic melts of primarily Fe.sub.5Si.sub.3 can be found on these particles on the convexly shaped parts of the particle surfaces and in the places where, due to the mechanical forces introduced, the secondary particles have been converted back into primary particles. The remaining secondary particles have impurities in the interstices of the intergrown particle agglomerates.

[0154] The existence of these impurities, which are permanently bonded to the SiC particles, has been proven by means of REM.

[0155] In the “impurified” fraction, the impurities are also present in powder form, in addition to the forms described here.

EXAMPLE 2

[0156] 10 kg of an SiC powder as a by-product from SiC processing, containing 95.8% by mass SiC, 0.2% by mass C.sub.free. 1.2% by mass Si, 1.2% by mass SiO.sub.2, 1.4% by mass Fe, and impurities with Al, V, Ti, and Ca in the range of >100 ppm each, is mixed with 80 g of coke powder. The powder mixture has an average particle size of 41.5 μm, determined by means of laser diffraction. After the compression step, the powder mixture introduced into the crucibles has a density of 1.3 g/cm.sup.3. The crucibles are heated in an inert gas furnace under an argon atmosphere at 70000 Pa under-pressure at 5 K/min up to 2000° C. and at 6 K/min up to 2300° C. and held at this temperature for 180 minutes. During the entire temperature treatment, an Ar gas flow is guided around the graphite crucible at 5 l/h.

[0157] Furthermore, carbon monoxide (CO) and silicon monoxide produced from the inert gas furnace is dissipated in the temperature range between 1200° C. and 2000° C.

[0158] The cooling is performed at an overall speed of 2.5 K/min. In the range between 1200° C. and 800° C., the cooling occurs at a rate of 8 K/min.

[0159] After cooling and before the mechanical treatment, the SiC powder has an SiC content of 96.1% by mass. The metallic impurities are present unchanged.

[0160] The powdered crucible contents are treated in a mill at 0.3 MJ/kg and, after sifting, split into five powders with particle sizes of <40 μm, 40-63 μm, 63-125 μm, >250 μm.

[0161] Subsequently the powders with particle sizes of <40 and 40-63 μm are mixed into one fraction, and the powders with particle sizes of 63-125 μm, 125-250 μm, and >250 μm are mixed into the second fraction. The “clean” fraction is the fraction with the particle sizes of 63-125 μm, 125-250 μm, and >250 μm, which has a SiC content of 98.8% SiC. The content of Si and SiO.sub.2 is 0.2% by mass respectively, that of C.sub.free is 0.14% by mass, the Fe content is 0.25% by mass, and the other contents of metallic impurities were each significantly reduced to <50 ppm.

[0162] In the “impurified” fractions with the particle sizes of <40 and 40-63 μm, there are 13.8 times more impurities compared to the “clean” fraction.

[0163] The average grain size after the temperature treatment and the mechanical purification in the “clean” fraction is 100.4 μm. Thus, the particles are on average 2.4 times larger than in the starting material placed in the inert gas furnaces.

[0164] As a result of the method according to the invention, the “clean” fraction has a mass percentage of 82.5% by mass.

[0165] Before the mechanical treatment, the temperature-treated powder had a large number of secondary particles in whose interstices the impurities had accumulated. After the mechanical treatment, the “clean” fraction contains almost exclusively primary particles. Fragments of metallic melts of Fe.sub.3Si and Fe.sub.5Si.sub.3 can be found on these particles on the planarly and convexly shaped parts of the particle surfaces and in the places where, due to the mechanical forces introduced, the secondary particles have been converted back into primary particles. The remaining secondary particles have impurities with the same silicides in the interstices of the intergrown particle agglomerates.

[0166] The existence of these impurities, which are permanently bonded to the SiC particles, has been proven by means of REM-EDX.

[0167] In the “impurified” fraction, the impurities are also present in powder form, in addition to the forms described here.

EXAMPLE 3

[0168] 10 kg of a dusty SiC powder as a by-product from SiC processing, containing 98.5% by mass SiC, 0.3% by mass C.sub.free, 0.6% by mass Si, 0.4% by mass SiO.sub.2, 0.1% by mass Fe, and impurities with Al, V, Ti, and Ca in the range of >100 ppm each. This powder is mixed with 20 g of Fe.

[0169] The powder mixture has an average particle size of 16.4 μm, determined by means of laser diffraction. The powder bed is placed into a crucible and compressed to a density of >1.2 g/cm.sup.3.

[0170] The crucible is heated in a furnace under a nitrogen atmosphere at 10 l/min nitrogen throughput and an under-pressure of 0.9 bar, at 5 K/min up to 1800° C. and from there to 2400° C. at 3 K/min. At 2400° C., it is held for 100 minutes.

[0171] The cooling is performed at a speed of 10 K/min.

[0172] After cooling, the powdered crucible contents are treated in an air mill at 1 MJ/kg and subsequently split into three powders by means of sedimentation with densities of 2.5-3.9 g/cm.sup.3.

[0173] Before the mechanical treatment, the SiC powder has an SiC content of 99.5% by mass. In particular, the metallic impurities are present in the same concentration order of magnitude as the starting powder.

[0174] After the mechanical treatment in a mill, the powder with a density of 3.2 g/cm.sup.3 has 99.8% by mass SiC and thus forms the “clean” fraction. The content of Si is 0.03% and the content of SiO.sub.2 is 0.02% mass, that of C.sub.free is 0.11% by mass, the Fe content is 0.02% by mass, and the other contents of metallic impurities were all significantly reduced to <20 ppm.

[0175] The powders with the densities of 2.5 g/cm.sup.3 and 3.9 g/cm.sup.3 were combined and form the “impurified” fraction. In this fraction, there are 10.4 times more impurities compared to the “clean” fraction.

[0176] The average particle size after the temperature treatment and the mechanical purification in the “clean” fraction is 59.7 μm. Thus, the grains are on average 3.6 times larger than in the starting material placed in the inert gas furnaces.

[0177] As a result of the method according to the invention, the “clean” fraction has a mass percentage of 84% by mass.

[0178] Before the mechanical treatment, the temperature-treated powder had a large number of secondary particles in whose interstices the impurities had accumulated. After the mechanical treatment, the “clean” fraction contains almost exclusively primary particles. Island-shaped metallic melts of Fe.sub.5Si.sub.3 can be found on these particles on the convexly shaped parts of the particle surfaces and in the places where, due to the mechanical forces introduced, the secondary particles have been converted back into primary particles. The remaining secondary particles have impurities in the interstices of the intergrown particle agglomerates.

[0179] The existence of these impurities, which arc permanently bonded to the SiC particles, has been proven by means of REM. The metallic impurities have wetting angles to the SiC particle surfaces between 30 and 75°, proven by analytical image evaluations of bevels of the powder particles.

[0180] In the “impurified” fraction, the impurities are also present in powder form, in addition to the forms described here.

EXAMPLE 4

[0181] 10 kg of a dusty SiC powder as a by-product from SiC processing, containing 97.5% by mass SiC, 0.4% by mass C.sub.free, 0.6% by mass Si, 0.5% by mass SiO.sub.2, 0.2% by mass Fe, and impurities with Al, V, Ti, and Ca in the range of >100 ppm each, is mixed with 180 g of iron powder and 60 g of Si powder. The powder mixture has an average particle size of 16.4 μm, determined by means of laser diffraction. The bed density of the powder mixture is 1 g/cm3. The powder mixture is loosely filled in graphite crucibles. The crucibles are heated in a furnace under a vacuum at 7.5 K/min up to 2050° C. and held there at 2050° C. for 270 minutes.

[0182] The cooling is performed at a speed of 10 K/min.

[0183] After cooling, the powdered crucible contents are treated in a mill at 0.2 MJ/kg and, on the basis of different surface potentials in the electric field, split into two fractions.

[0184] Before the mechanical treatment, the SiC powder has an SiC content of 98.3% by mass. In particular, the metallic impurities are present in the same concentration order of magnitude as the starting powder.

[0185] After the mechanical treatment in the mill, the “clean” fraction has 99.2% by mass SiC. The content of Si and SiO.sub.2 is 0.3% and 0.2% by mass respectively, that of C.sub.free is 0.2% by mass, the Fe content in the clean fraction is 0.1% by mass, and the other contents of metallic impurities were all significant reduced to <100 ppm.

[0186] In the “impurified” fractions, there is 9.6 times the amount of impurities compared to the “clean” fraction.

[0187] The average grain size after the temperature treatment and the mechanical purification in the “clean” fraction is 33 μm. Thus, the grains are on average twice as large as the starting material placed in the inert gas furnaces.

[0188] As a result of the method according to the invention, the “clean” fraction has a mass percentage of 87% by mass.

[0189] Before the mechanical treatment, the temperature-treated powder had a large number of secondary particles in whose interstices the impurities had accumulated. After the mechanical treatment, the “clean” fraction contains almost exclusively primary particles. Island-shaped metallic melts of Fe.sub.5Si.sub.3, FeSi, and FeSi.sub.2 can be found on these particles on the convexly shaped parts of the particle surfaces and in the places where, due to the mechanical forces introduced, the secondary particles have been converted back into primary particles. The remaining secondary particles have impurities in the interstices of the intergrown particle agglomerates.

[0190] The existence of these impurities, which are permanently bonded to the SiC particles, has been proven by means of TEM.

[0191] In the “impurified” fraction, the impurities are also present in powder form, in addition to the forms described here.

EXAMPLE 5

[0192] 10 kg of a powdery SiC powder as a by-product from the SiC raw material production, containing 92.6% by mass SiC, 2.75% by mass C.sub.free, 0.1% by mass Si, 3.5% by mass SiO.sub.2, 0.2% by mass Fe, and impurities with Al, V, Ti, and Ca in the range of >400 ppm is mixed with 150 g of sand and 25 g of graphite powder. The powder mixture has an average particle size of 100 m, determined by means of laser diffraction. The powder mixture is loosely filled in graphite crucibles and subsequently compressed to >1 g/cm.sup.3. The crucibles are heated in an inert gas furnace under an atmosphere at 5 K/min up to 1900° C., and the pressure is set to 70000 Pa under-pressure. At 1900° C., the temperature is held for 180 minutes.

[0193] The cooling is performed at a speed of 25 K/min.

[0194] After cooling, the powdered crucible contents are treated in a mill at 0.1 MJ/kg and, by means of a cyclone series connection, split into three powders with particle sizes of <20 μm, 20-70 μm, >70 μm.

[0195] Before the mechanical treatment, the SiC powder has an SiC content of 98.3% by mass. In particular, the metallic impurities are present in the same concentration order of magnitude as the starting powder.

[0196] After the mechanical treatment in the mill, the powder with the particle size of >70 μm, as the “clean fraction,” has a SiC content of 98.5% by mass. The content of Si and SiO.sub.2 is 0.1% and 0.1% by mass respectively, that of Cr, is 1% by mass, the Fe content is 0.1% by mass, and the other contents of metallic impurities were each significantly reduced to <200 ppm.

[0197] The powders with the particle sizes <20 μm and 20-70 μm were combined into the “unpurified” fraction.

[0198] In the “impurified” fractions, there are 2.3 times more impurities compared to the “clean” fraction.

[0199] The average grain size after the temperature treatment and the mechanical purification in the “clean” fraction is 125 μm. Thus, the particles are on average 1.25 times larger than in the starting material placed in the inert gas furnaces.

[0200] As a result of the method according to the invention, the “clean” fraction has a mass percentage of 90% by mass.

[0201] Before the mechanical treatment, the temperature-treated powder had a large number of secondary particles in whose interstices the impurities had accumulated. After the mechanical treatment, the “clean” fraction contains almost exclusively primary particles. Island-like metallic melts of carbides and silicides of the silicon and vanadium can be found on these particles on the convexly shaped parts of the particle surfaces and in the places where, due to the mechanical forces introduced, the secondary particles have been converted back into primary particles. The remaining secondary particles have impurities in the interstices of the intergrown particle agglomerates.

[0202] The existence of these impurities, which are permanently bonded to the SiC particles, has been proven by means of REM-EDX.

[0203] In the “impurified” fraction, the impurities are also present in powder form, in addition to the forms described here.