1
FORM 2
THE PATENTS ACT 1970
(39 of 1970)
&
The Patents Rules, 2003
COMPLETE SPECIFICATION
(See section 10 and rule 13)
1. METHOD FOR SEPARATING IMPURITIES FROM SILICON CARBIDE, AND
TEMPERATURE-TREATED AND PURIFIED SILICON CARBIDE POWDER
2.
1. (A) FRAUNHOFER-GESELLSCHAFT ZUR FÖRDERUNG DER
ANGEWANDTEN FORSCHUNG E.V.
(B) Germany
(C) Hansastr. 27c 80686 München GERMANY
2. (A) ESK-SIC-GMBH
(B) Germany
(C) Günter-Wiebke-Straße 1 50226 Frechen GERMANY
The following specification particularly describes the invention and the manner in
which it is to be performed.

2
The invention pertains to the field of technical ceramics and relates to a method for
separating impurities from silicon carbide, which can be used, for example, on SiC
powders of grinding sludges or on production-related silicon carbide waste or on socalled SiC sinter scrap, as well as a temperature-treated and purified silicon carbide
powder.
Silicon carbide (SiC) is a synthetic industrial mineral that is used in many industrial
sectors due to its outstanding properties (hardness, high temperature properties,
chemical resistance). Of particular importance is its use in the form of special, highly
pure, and finely fractionated fine powder grains (0.5 to approx. 250 μm) in
microelectronics/photovoltaics (wafer saws), in ceramics production, for the
production of ballistic protective ceramics for military technology, in automotive and
environmental technology (diesel particle filters), and as an abrasive material for
high-quality surface treatment throughout the field of mechanical engineering.
SiC powder grains are produced from especially raw SiC by grinding, purification,
and fractionating. This results in high and constant amounts of low-grade, poorly
usable SiC.

An increase in the demand for grains therefore always requires an increase in raw SiC
production. In this respect, an increase in raw capacity is not lucrative for the
producers, which leads to structural scarcity and price inelasticity.
The raw SiC production via the electrosynthesis process which has been used for
approximately 120 years, the so-called Acheson process (DE 76629 A, DE 85197 A),
is linked to the electricity and oil price (raw material: petroleum coke) and the
environmental costs (due to high dust, CO/CO2 and SO2 emissions). Despite many
attempts, alternative production methods have not been successful, mostly for
economic reasons, and will not be available in the foreseeable future.
Although SiC is a mass raw material available worldwide, bottlenecks and price
increases for the strategically important high-quality grains (HQ) have been seen for
some years. In 2008, the deficit of HQ SiC raw material in Europe was estimated to
be 40-60,000 t (Silicon Carbide & More #24, 2008, pg. 3). However, an even greater
problem with the special grains is that high-tech applications require large quantities
of individual grain size bands. Both lead to price surcharges and supply bottlenecks
for these HQ special grains due to the price inelasticity conditions mentioned above.
3
SiC powders in abrasive applications are subject to wear in terms of cutting
performance and grain size. A majority of the SiC is lost through dissipative
processes. In many cases where SiC-containing waste products can be detected, the
material separation is technically extremely difficult and preparation is not
worthwhile economically.
Overall, there is a large amount of very differently impurified SiC powders.
Depending on their origin, the impurities are of very different types and consist of
organic and inorganic non-metallic and inorganic metallic impurities. Organic
impurities include organic polymer residues (mostly hydrocarbon compounds) such
as liquid oils, polyethylene glycol (PEG), solvents, lubricants, as well as solid
polymers such as plastic debris.
The inorganic, non-metallic impurities include primarily free carbon and silicon
oxides (mainly SiO2) and Ca-Al-Si-O compounds.
Metallic impurities include iron and iron alloys, boron, vanadium, aluminum,
titanium, copper, manganese, tungsten, chromium, nickel, and their compounds.
Furthermore, so-called free silicon (Sifree) can be contained, which is understood to
mean metallic or alloyed silicon that is not bound in SiC or SiO2.
SiC waste occurs during the production of SiC, for example in the Acheson process,
or as fragments of SiC molded parts, which cannot then be processed industrially
without further preparation steps.
The impurities are differentiated using known analysis methods, for example Cfree,
Sifree, SiO2 and iron according to DIN EN ISO 9286, DIN EN ISO 21068 Part 1-3,
and FEPA Standard 45-1:2011. Spectroscopic methods are also used in order to
analyze the impurities, including DIN EN 15991.
A large number of methods for separating the impurities and thus for purification, i.e.
increasing the concentration of the SiC portion of impurified SiC, are already known.
These can be divided into physical and chemical methods.
4
Physical methods take advantage of different densities, particle sizes, and other
physical properties, such as magnetic properties or wetting behavior, of some
impurities.
For example, magnetic iron impurities are removed by magnetic separation.
Impurities having different particle sizes and densities are separated by cyclone and
hydrocyclone methods, [and] impurities having different densities and wetting
behavior through flotation methods.
Chemical methods substantially use the solubility of various impurities in chemicals
such as solvents, acids, or alkalis. Because SiC is very stable in such chemicals, even
very aggressive chemicals such as hydrofluoric acid can be used in order to dissolve
the impurities.
Thermal methods can also be used in order to remove impurities, for example the
oxidation of free carbon at temperatures of 400-800 °C in air.
It is also known that NaCl is sometimes added during the SiC synthesis, whereby
undesirable impurities react with the chlorine at high temperatures in order to form
volatile chlorides and are thus removed from the SiC. However, post-treatment of
impurified powders using this method is not known, because it is very complex and
the chlorine gas has an aggressive effect on the system technology and is not
environmentally friendly.
A disadvantage of the known methods is the high cost of removing the impurities
from the SiC, because the specific impurities must be removed using a method that is
specific for this purpose. In order to achieve a high concentration of SiC that is as
pure as possible, a plurality of methods must always be combined.
The problem of the present invention is to provide a method for separating impurities
from silicon carbide, with which various impurities can be removed with a simple and
economical method substantially completely or to such an extent that the purified
silicon carbide powder can be reused industrially, and a temperature-treated silicon
carbide powder that can be easily purified in a simple and cost-effective method, and
purified silicon carbide powder that can be industrially resupplied, in particular for
high-tech applications.
5
The problem is achieved by the invention specified in the claims. Advantageous
configurations are the subject-matter of the subclaims, wherein the invention also
includes combinations of the individual claims in the sense of an “AND” conjunction,
as long as they are not mutually exclusive.
In the method according to the invention for separating impurities from silicon
carbide, powdery SiC waste products that contain at least 50% by mass SiC and an
average grain size d50 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.
Advantageously, the mass of impurities in one fraction is at least 10 times higher,
advantageously at least 20 times higher, than in the other fraction.
Furthermore advantageously, 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.
Likewise advantageously, powdery SiC waste products having at least 50% by mass
SiC and an average particle size d50 between 0.5 and 500 μm, measured by laser
diffraction, are advantageously used.
Also advantageously, powdery SiC waste products having at least 50% by mass SiC
and an average particle size d50 between > 500 and 1000 μm, measured by laser
diffraction, are advantageously used.
It is advantageous when powdery SiC waste products having at least 50% by mass
SiC and an average particle size d50 between > 500 and 1000 μm, measured by laser
diffraction, are subjected to a temperature treatment at temperatures of 1400 to <
2000 °C.
It is likewise advantageous when powdery SiC waste products having at least 50% by
mass SiC and an average particle size d50 between 0.5 and 1000 μm, measured by
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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.
It is likewise advantageous when the temperature treatment of the SiC waste products
is carried out at temperatures of 1400 - 2000 °C.
It is also advantageous when the temperature treatment of the SiC waste products is
carried out at temperatures of 2000 - 2600 °C.
It is also advantageous when 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.
It is furthermore advantageous when 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.
It is likewise advantageous when 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.
It is also advantageous when the temperature treatment is performed under a nonoxidizing atmosphere with an amount of non-oxidizing gases of 0.5 to 30 l/h.
Advantageously, the temperature treatment is performed under a vacuum or a nonoxidizing atmosphere while dissipating gaseous reaction products.
Also advantageously, the cooling of the powdery SiC is performed at a cooling rate
of 0.1 to 100 K/min.
Furthermore advantageously, 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.
It is furthermore advantageous when the mechanical treatment of the recycled
powdery SiC is implemented by applying a mechanical impulse, advantageously by
7
mixing, grinding, even more advantageously by autogenous grinding, or by using
eddy currents and/or ultrasound.
It is also advantageous when the mechanical treatment is carried out with an energy
input between 0.1 and 5 MJ/kg.
It is furthermore advantageous when 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.
It is likewise advantageous when the separation according to the particle size and/or
particle shape is carried out by sieving, sifting, and/or cyclone methods
It is also advantageous when the separation is carried out by the effect of mass forces
with regard to the particle density by means of 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.
It is also advantageous when 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.
It is also advantageous when substantially metallic impurities are separated as the
impurities.
It is also advantageous when, 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 (SiO2), is added during the temperature treatment in order to achieve a
composition that is as stoichiometric as possible.
The silicon carbide powder that is temperature-treated according to the invention
contains SiC powder particles and substantially metallic impurities in the form of
metallic mixed phases and has 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.
8
Furthermore, the purified silicon carbide powder according to the invention has 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.
Advantageously, the purified silicon carbide powder has at least 99% by mass SiC.
Likewise advantageously, in the case of the purified SiC powder, 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.
Furthermore advantageously, 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.
Also advantageously, 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.
With the solution according to the invention, it is possible for the first time to remove
various impurities from silicon carbide with a simple and economical method
substantially completely or to such an extent that the purified silicon carbide powder
can be reused industrially. It is also possible for the first time to provide temperaturetreated SiC powder, which can be easily purified in a simple and cost-effective
method, as well as purified silicon carbide powder that can be reused industrially, in
particular for high-tech applications.
This is achieved by a method for separating impurities from silicon carbide, in which
powdery SiC waste products that contain at least 50% by mass SiC and an average
grain size d50 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
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impurities in one fraction (the “impurified fraction”) is at least twice as high as in the
other fraction (the “clean fraction”).
Thus, according to the invention, there is a “clean fraction” and an “impurified
fraction.”
In the context of the present invention, the term “fraction” is to be understood as a
division of SiC powders, produced by physical separation, according to their
impurities.
The physical separation can be carried out according to various physical properties,
such as particle size, density, mass, or concentration.
For example, after the physical separation of the powdery SiC waste products
according to particle sizes, powder groups having, for example, three, four, or five
different particle sizes can be obtained. Depending on the subdivision and
composition of the impurities in the powder groups, these are then assigned to the
two fractions, i.e. the clean and the impurified fraction. It is readily possible for the
applicable person skilled in the art to carry out the separation of the respective
powder groups with regard to the impurities into the two fractions according to the
invention with only a few fractionation attempts and content analyses, so that a
separation into a “clean” and an “impurified” fraction is achieved with the parameters
according to the invention.
Advantageously, the mass of impurities in one fraction, the “impure” fraction, is ten
times higher, more advantageously 20 times higher, than in the other “clean” fraction.
Advantageously, based on the total mass of the two fractions, the “clean” fraction has
a mass percentage of ≥ 60% by mass, advantageously ≥ 70% by mass, and more
advantageously ≥ 80% by mass.
Furthermore, within the scope of the present invention, the mass of impurities in a
fraction should be understood to mean the mass concentration of all of the
components that are not SiC.
10
The primary quality feature of an SiC powder is the concentration of SiC, which is
determined using FEPA Standard 45-1:2011. The remainder up to 100% is assumed
to be impurity content.
The doping elements built into the SiC lattice, such as Al and Bor, are not regarded as
impurities in the context of the present invention. These impurities are not
detrimental or disruptive to most SiC applications.
According to the invention, the SiC concentration in the “clean” fraction is increased
by comparison to the starting SiC powder due to the separation of the impurities. An
increase in the SiC concentration from 85% in the starting powder to 95% in the
“clean” fraction or from a concentration of 96% in the starting powder to a
concentration of at least 99% SiC in the “clean” fraction is advantageously achieved.
The mass of impurities in the starting material is at least 3 to 100 times higher than
the mass of impurities in the “clean” fraction.
With the method according to the invention for separating impurities from silicon
carbide, powdery SiC waste products having at least 50% by mass SiC and an
average particle size d50 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 used
as the starting materials.
Here, it is advantageous when the powdery SiC waste products have at least 75% by
mass SiC, better 80% by mass SiC, even better 85% by mass SiC, or 90% by mass
SiC. In the case of powdery SiC waste products, the proportion of SiC is usually not
as high, however, it is possible to perform known preparation and homogenization
methods before the use of the powdery SiC waste products in order to already remove
the impurities that are separable with these methods. Thus, the mass of starting
materials for the method according to the invention can be reduced overall and
limited to the powdery SiC waste products, whose impurities are thus not separable.
Furthermore, it can be advantageous according to the invention that powdery SiC
waste products having at least 50% by mass SiC and an average particle size d50
between 0.5 and 500 μm or between > 500 and 1000 µm, measured by laser
diffraction, are used.
11
According the solution according to the invention, powdery SiC waste products are
used that have at least 50% by mass SiC and an average particle size d50 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, which advantageously have a content of 0.5
to 5.0% by mass iron and 0.5 to 5.0% by mass metallic silicon. According to the
invention, the SiC waste products must have a content of 0.1% by mass iron, which
can be present in the form of iron, iron alloys, or iron compounds. The at least 0.1%
by mass metallic silicon which is present according to the invention can be present in
the form of free silicon. If the SiC waste products which are to be used according to
the invention do not originally have these contents of at least 0.1% by mass iron and
0.1% by mass metallic silicon, these contents must be realized by the addition of iron,
iron alloys, iron compounds, and/or metallic silicon.
Accordingly, it is advantageous when the SiC waste products are analyzed for their
material composition before their use in the method according to the invention, so
that during the subsequent temperature treatment, silicon, SiO2, iron, iron-containing
compounds, and/or carbon can be added in order to powdery SiC that is as
completely recycled as possible as the end product of the temperature treatment. Soot
and/or coke powder can advantageously be added as the carbon. The SiO2 can also be
present through an oxidation of the SiC.
It is particularly advantageous when the amount of free carbon, SiO2, free silicon, and
iron is adjusted before the temperature treatment such that the free carbon can
stoichiometrically react with the SiO2 and the free silicon during the temperature
treatment in order to form SiC, wherein the amount of free silicon is adjusted
excessively with respect to this reaction such that the free silicon and iron are
contained in a mass ratio of 60:40 to 20:80. Thus, during the temperature treatment,
chemical compounds, in particular iron silicide, can form, which have a high linear
thermal expansion coefficient of >6 x10-6 1/K in the temperature range between 20
and 1400 °C, which is decisive for the subsequent purification.
It is also particularly advantageous when the free silicon and iron is contained in mass
ratio of 35:65 to 20:80. Here, during the temperature treatment, it is primarily the
metastable iron silicide Fe5Si3 that forms, which leads to a very good purification
capability during the subsequent preparation due to its very high expansion
coefficient.
12
Powdery SiC waste products as a starting material for the production of recycled
powdered silicon carbide can be used as powder in a loose bed. Powders having a low
compaction, as is the case due to production technology and/or the storage of the
powders, can also be used as the starting materials.
It is advantageous when the SiC waste products are not used in a loose bed, but rather
subject to slight compression forces before use in the method according to the
invention. It is particularly advantageous when compression forces between 0.002
and 100 MPa are applied in order to compress the powder.
The density of the bed is determined by weighing and finding the volume of the bed.
The true density of the powder is determined, for example, by gas pycnometry. If the
composition is known, it can also be calculated from the known true density of the
components. The true density of silicon carbide is 3.21 g/cm3
.
These powdery SiC waste products for the production of recycled powdered silicon
carbide are then subjected to a temperature treatment under a vacuum or nonoxidizing atmosphere at temperatures of 1400-2600°C.
The temperatures are advantageously between 2000 °C and 2600 °C.
During the temperature treatment of the powdery SiC waste products, low heating
rates of less than or equal to 8 K/min are advantageously used when heating in the
temperature range between 1200 °C to < 1400 °C and from 1400 °C to 1800 °C,
advantageously heating rates of less than or equal to 5 K/min for temperatures above
1800 °C.
In the context of the present invention, the heating rate is understood to be the
temperature difference at the start and end of the temperature range, divided by the
total time of the heating in this temperature range.
However, in the respective temperature range, various heating rates and also various
dwell times can be adjusted.
In the context of the present invention, dwell time is understood to be a heating rate
of 0 K/min during this time.
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The dwell times at the temperatures of the method according to the invention are
advantageously between 10 minutes and 300 minutes, wherein this also depends on
the powder volume to be treated and on the temperature.
It is also true that the lower the maximum temperature is realized, the longer the
dwell time at the respective maximum temperature.
Temperatures in the range between 1400 and 2000 °C are also advantageously
suitable for powdery SiC waste products having at least 50% by mass SiC and an
average grain size d50 between 500 and 1000 μm, measured by laser diffraction.
During the temperature treatment under a non-oxidizing atmosphere, a throughput of
flowing, non-oxidizing gas with an average flow rate of 0.5 to 30 l/min is set for the
heating. It is particularly advantageous when a throughput of the flowing, nonoxidizing gas with an average flow rate of 0.5 to 30 l/min is the set in the temperature
range between 1200 and 2000 °C.
In the context of the present invention, the average flow rate is understood to be the
entire throughput gas amount in the aforementioned temperature range from the start
to the end of the temperature range, divided by the total time of the heating in this
temperature range.
In the respective temperature range, different flow rates of the flowing, non-oxidizing
gas can be set. The flowing gas atmosphere can be set in an over-pressure or underpressure, wherein an under-pressure for example between 70,000 and 90,000 Pa is
preferred.
Depending upon the apparatus for the temperature treatment according to the
invention of the powdery SiC waste products, the method can be performed
differently.
Typically, the temperature treatment is performed in a closed furnace body, into
which the powdery SiC waste products are introduced and present in fireproof vessels
(crucibles).
Either a vacuum is produced in such a closed furnace body, or a non-oxidizing gas
atmosphere is produced. In the case of a non-oxidizing gas atmosphere, the gas is
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advantageously guided through the furnace body in such a way that it flows around
the vessels and the powder as completely as possible. For example, this can be
achieved in that the gas flows in on one side of the furnace body and flows out on the
opposite side, wherein the vessels are disposed between the inlet and the outlet. This
gas guidance or a vacuum is possible in both a so-called batch furnace for a
discontinuous temperature treatment as well as in running furnaces for a continuous
temperature treatment.
Technical inert gas atmospheres, such as argon or nitrogen atmospheres, which have
a residual oxygen content of <100 ppm, are used as the non-oxidizing atmosphere.
However, mixtures with CO can also be used, for example, because these do not lead
to the oxidation of the SiC. The temperature treatment is possible under a slight
overpressure as well as under negative pressure, up to a vacuum. The temperature
treatment is advantageously carried out under an argon atmosphere. The temperature
treatment is possible both in batch furnaces as well as in continuous operation.
The temperature treatment of the SiC waste products under a vacuum or nonoxidizing atmosphere can be performed while dissipating gaseous reaction products.
Advantageously, carbon monoxide (CO) and silicon monoxide (SiO) are dissipated.
Advantageously, the dissipation of gaseous reaction products occurs during the
heating phase and preferably in the temperature range between 1200 °C in 2000 °C,
particularly preferably in the temperature range between 1350 °C and 1800 °C
With the method according to the invention, a separation of impurities from recycled,
powdered silicon carbide is achieved.
During the temperature treatment of the SiC waste products for the production of
recycled, powdered silicon carbide, the substantially powdered structure of the
products is retained. The powder particles are either in the form of isolated primary
crystals and crystallites, i.e. not connected to one another, or slightly intergrown as
secondary crystallites, i.e. with a connection of a few primary crystals or crystallites
that have intergrown.
Crystallites are understood to mean individual grains which are homogeneous in
terms of their crystal structure and which, in their external form, do not have the
crystal structure or have it only partially.
15
Following the temperature treatment, the recycled, powdered silicon carbide is
cooled.
Cooling is advantageously carried out at a cooling rate of 0.1 to 100 K/min,
particularly advantageously until a temperature of 200 °C is reached. In industrial
processes, the furnace can typically be opened from this temperature of 200 °C, and
the powder can be further cool in the air.
In the case of a temperature treatment between 2000 and an end temperature of
2600°C, a fast cooling from the respective end temperature to 1800 °C at a cooling
rate of 10 to 50 K/min is particularly advantageous. Likewise, the cooling
advantageously occurs in a temperature range between 1200 °C and 800 °C at a
cooling rate of 5 to 25 K/min. Such cooling rates can technically be achieved by the
use of fast cooling devices in that, for example, cold non-oxidizing gas is introduced
into the furnace, circulated, and guided over a heat exchanger.
The subsequent mechanical treatment according to the invention is advantageously
carried out by applying a mechanical impulse, advantageously by mixing or grinding
or by using eddy currents and/or ultrasound.
The grinding can in turn advantageously be carried out by means of autogenous
grinding under a reduced pressure (inter-particular movements).
Furthermore advantageously, the mechanical treatment is carried out with an energy
input of between 0.1 and 5 MJ/kg.
The mechanically treated SiC powder is then physically separated.
The recycled powdery SiC can be separated according to the particle size, the particle
shape, the density, and/or the physical and/or chemical surface properties of the
particles.
A separation according to particle size and/or particle shape can advantageously be
carried out by sieving, sifting and/or cyclone methods.
A separation can be carried out through the effect of mass forces with regard to the
particle density by means of flotation, sedimentation, sifting, centrifugation, and/or
16
cyclone methods or separation according to the density of the particles by flotation
and/or cyclone methods.
Surface properties can be used in separation methods that separate by means of
electric field strengths or the like. Solid body properties can be used in separating
methods that use electrical material properties, e.g. magnetizability.
After the physical separation, the SiC powder obtained is divided into two fractions,
wherein the division is based on the mass of impurities and, according to the
invention, the mass of impurities in one fraction is at least twice as high as in the
other fraction.
The division into two fractions according to the invention has surprisingly shown that
the one fraction contains SiC powder with impurities of less than 5% by mass,
advantageously less than 2% by mass, more advantageously less than 1% by mass,
and the other fraction contains substantially all of the remaining impurities.
The division into at least one “clean” fraction and at least one fraction with a higher
impurity content is carried out using one or more classic mechanical separation
methods, advantageously a dry classification method, such as sieving or sifting.
Depending on the number of separating cuts in the method, a plurality of purified,
clean, and impurified fractions can also be produced, which, however, are
recombined at the end of the division/fractionation to form the two fractions
according to the invention.
The purification factor can be determined in each fraction by dividing/fractionating
into a plurality of fractions before combining, and the difference in the purification
factors of the two fractions according to the invention can be adjusted by combining
the fractions at the end of the method according to the invention.
With the method according to the invention, at least 95% by mass, advantageously at
least 98% by mass, more advantageously 99% by mass silicon carbide is achieved in
at least one fraction.
Surprisingly, it has been found that the metallic impurities melt during the
temperature treatment and at least partially wet the SiC grains, so that after cooling,
the metallic impurities form a permanent bond with the SiC grains between the
17
neighboring SiC grains and in the interstices of a plurality of SiC grains. The metallic
impurities are thus present as metal silicides or ternary metal-Si-C compounds or
alloys or as mixtures thereof, so-called metallic mixed phases.
Surprisingly, the mechanical treatment causes the metallic impurities to flake off or
break off, whereby these impurities are separated from the SiC and are made even
smaller during the further mechanical treatment.
As a result, an enrichment of these metallic impurities in a fraction of the
mechanically treated, recycled, powdered silicon carbide is achieved.
If the SiC waste products as a starting material for the production of purified silicon
carbide powder do not contain any carbon, it is then advantageous to add carbon
particles, for example in the form of soot, graphite, or coke powder.
Such an addition can advantageously be 0.3 to 0.5 times the proportion-by-weight of
carbon, based on the free Si content and/or the Si content of the SiO2.

With the method according to the invention, purified SiC powders are to be produced
which can be reused industrially, in particular for high-tech applications, and which
can be produced with a high yield in a simple and economical method.
The SiC powder purified using the method according to the invention can then be
subjected to a chemical purification.
The recycled, powdered silicon carbide is separated according to the invention into
two fractions, wherein the recycled silicon carbide powder in one fraction has at most
5% by mass impurities, advantageously at most 2% by mass, more advantageously
less than 1% by mass, and the remaining impurities are contained in the other
fractions.
The advantage of the method according to the invention is that the mechanical
treatment and the fractionation of the powder can be implemented using conventional
methods which are known and already in use for a preparation of SiC powders, for
example for use as abrasives.
18
The purified silicon carbide powder according to the invention with at least 98% by
mass SiC, advantageously 99% by mass SiC, and maximum 2% by mass impurities
has residues of the melted impurities still adhered to the surface of the purified silicon
carbide powder.
Surprisingly, only a maximum of 2% by mass impurities remain on the surface of the
SiC powder particles after the method according to the invention.
After the temperature treatment according to the invention, SiC particles are formed,
which surprisingly have unique structures for a material separation in the mechanical
treatment according to the invention. These structures are characterized in that the
metallic impurities can be present in the form of metallic mixed phases, which are
concentrated and deposited on the surfaces of the silicon carbide powder particles and
are usually so permanently bonded to the surface of the SiC powder particles that a
mechanical separation of the SiC powder particles from the largest part of the
impurities and in particular the metallic impurities in two or more fractions is easily
realizable. Such an accumulation of the impurities on the surface of the SiC powder
articles has not been observed thus far. It is likewise surprising that these structures of
the impurities accumulated on the surface of the SiC powder particles can be
removed to a very large extent by a comparatively simple mechanical separation
and/or physical treatment, so that highly to extremely pure SiC powder having at least
98% by mass SiC can be produced in one fraction.
The impurities are substantially present in the form of mostly drop-like melts of the
metallic mixed phases, which are deposited in an island-like manner on the surface of
the silicon carbide powder particles. After the mechanical treatment and physical
separation according to the invention, the remaining impurities are permanently
bonded to the surface of the SiC powder particles.
According to the invention, these impurities are advantageously disposed on the
surface of the individual silicon carbide powder particles after the temperature
treatment substantially on the planarly and/or convexly shaped parts of the surface of
silicon carbide powder particles. The wetting angle of these impurities on the surface
of the silicon carbide powder particles varies significantly, but is typically between
10° and 90°.
19
Furthermore, the impurities can also be disposed as metallic mixed phases between
SiC powder particles, which on the one hand are permanently bonded to the planar
and/or convex shape of the surface of the individual SiC powder particles, and on the
other hand are also bonded to one another as metallic mixed phases and thus fill
interstices between the SiC powder particles. At these points and/or grain boundaries,
the metallic mixed phases form shapes that are visually reminiscent of weld seams in
fillet weld shapes (flat weld, hollow weld, arched weld).
The metallic mixed phases formed as impurities during the temperature treatment of
the method according to the invention are characterized in that they are primarily
silicides or carbides of the present metals. Above all, the method is aimed at
producing the iron silicides Fe3Si (α2 phase), FeSi (ε phase), FeSi2 (ζ2 phase or
Fe3Si7) and in particular the metastable Fe5Si3 (η phase). These silicides typically
have a linear thermal expansion coefficient of > 6 x 10-6 1/K in the range of 20-1400
°C. This forms the basis for the separation method according to the invention, as they
can be separated relatively easily from the surfaces of the silicon carbide powder
particles.
In addition to the aforementioned silicides, ternary Fe-Si-C compounds can also
occur to a lesser extent. Other metallic impurities such as Ti, V, Al can also be
dissolved in the silicides and carbides or form mixed crystals.
Surprisingly, as a result of the mechanical treatment of the temperature-treated SiC
waste products, the impurities both on the surfaces of the SiC powder particles and
between the SiC powder particles break off relatively cleanly from the SiC powder
particles, so that substantially only the droplet-like melts of the metallic mixed phases
deposited in an island-like manner remain as impurities on the planarly and/or
convexly shaped surface of the silicon carbide powder particles. If the island-like
melts also break off completely or partially, permanently adhering fragments of these
melts remain on the planar and/or convex surfaces of the silicon carbide powder
particles. The break points of the permanently adhering fragments have a typical
shell-like break (conchoidal break).
The SiC powders temperature-treated according to the invention contain SiC powder
particles and substantially metallic impurities, wherein these SiC powders have a
significantly higher proportion of intergrown secondary crystallites connected by the
metallic impurities before the mechanical treatment according to the invention than
20
after the mechanical treatment. During the mechanical treatment, these intergrown
secondary crystallites are isolated in order to form primary crystallites, and a majority
of the adhering metallic impurity is blown off.
After the method according to the invention, the impurities deposited in an island-like
on the surface of the silicon carbide powder particles, which are permanently bonded
to the SiC powder particles, have a wetting angle of these metallic mixed phases on
the planar and/or convex SiC powder particle surfaces between 10 and 90°.
In the context of the invention, the wetting angle of the metallic mixed phases on the
SiC particle surfaces means the angle that occurs at the phase boundary between the
solid surface of a SiC particle, the molten metallic mixed phase, and the surrounding
gas atmosphere during the temperature treatment as a result of the interfacial tensions
(Young's Equation) and is retained even after the melt has solidified. Even when the
metallic mixed phases are arranged as seams between SiC particles and in the form of
interstices, the wetting angle is present at the phase boundary between the SiC
particle surface, the metallic mixed phase, and the surrounding atmosphere.
The wetting angle is preferably measured using microscopic images, particularly
preferably by means of image evaluation of images taken by way of raster electron
microscope of ceramographically/metallographically prepared bevels of the
temperature-treated powder particles, which produce a cross-section through the
phase boundaries.
The wetting angle of remaining, permanently adhering impurities in the form of
metallic mixed phases and their fragments on the SiC particle surfaces can also be
measured on the mechanically treated, cleaned SiC powders according to the
invention.
The break pattern can be used to easily distinguish between the surfaces of the mixed
metal phases and mixed phase fragments on the cleaned SiC powders that were
broken during the mechanical treatment and the surfaces of the mixed metal phases
on the temperature-treated SiC powders that were caused by the melting during the
temperature treatment.
In the case of the mechanically treated, cleaned SiC powder, only the wetting angles
at the phase boundary between the molten mixed phase, the surface of the SiC
21
particles, and the surrounding atmosphere are used in order to determine the wetting
angle, but not the angles between the break surfaces of the metallic mixed phase
fragments created by the mechanical treatment and the SiC particle and the
surrounding surface.
These cleaned silicon carbide powders are thus distinguished from known recycled
SiC powders, in which the impurities are predominantly present separately from the
SiC particles.
The purified silicon carbide powder according to the invention can therefore be
distinguished from known recycled powders by light and electron microscopic
methods such as REM, TEM, or REM-EDX.
Likewise, after the mechanical processing according to the invention, in particular the
reduction, of the SiC particles after the temperature treatment, characteristic break
points and break surfaces have formed on the SiC powder particles treated according
to the invention, which can be easily determined using the examination methods
mentioned. These typical break points and surfaces of the impurities that have arisen
according to the invention are a characteristic feature of the purified silicon carbide
powder according to the invention.
The silicon carbide powder particles that are present according to the method
according to the invention are mostly broken at the grain boundaries of the individual
SiC particles and thus also ensure an isolation of the particles.
A particular advantage of the solution according to the invention is that the removal
of the melted impurities and a clear fractionation can be better achieved with a
successively larger amount of impurities in the SiC waste products.
The invention will now be explained in further detail by way of several exemplary
embodiments.

Example 1
10 kg of an SiC powder as a by-product from SiC primary crushing, containing
91.8% by mass SiC, 1.3% by mass Cfree, 1.8% by mass Si, 3.7% by mass SiO2, 0.4%
22
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.
Furthermore, carbon monoxide (CO) and silicon monoxide produced from the inert
gas furnace is dissipated.
The cooling is performed at an overall speed of 2.5 K/min.
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.
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.
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 SiO2 is 0.22%
by mass respectively, that of Cfree 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.
In the other two powders that were combined, 8.6 times the amount of impurities has
been found.
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.
23
As a result of the method according to the invention, the “clean” fraction has a mass
percentage of 81% by mass.
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
Fe5Si3 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.
The existence of these impurities, which are permanently bonded to the SiC particles,
has been proven by means of REM.
In the “impurified” fraction, the impurities are also present in powder form, in
addition to the forms described here.
Example 2
10 kg of an SiC powder as a by-product from SiC processing, containing 95.8% by
mass SiC, 0.2% by mass Cfree, 1.2% by mass Si, 1.2% by mass SiO2, 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/cm3
. 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.
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.
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.
24
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.
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.
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 SiO2 is 0.2% by mass respectively, that of Cfree
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.
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.
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.
As a result of the method according to the invention, the “clean” fraction has a mass
percentage of 82.5% by mass.

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 Fe3Si and Fe5Si3 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.

25
The existence of these impurities, which are permanently bonded to the SiC particles,
has been proven by means of REM-EDX.
In the “impurified” fraction, the impurities are also present in powder form, in
addition to the forms described here.
Example 3
10 kg of a dusty SiC powder as a by-product from SiC processing, containing 98.5%
by mass SiC, 0.3% by mass Cfree, 0.6% by mass Si, 0.4% by mass SiO2, 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. 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/cm3
. The crucible is heated in a
furnace under a nitrogen atmosphere at 10 l/min nitrogen throughput and an underpressure 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.
The cooling is performed at a speed of 10 K/min.
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/cm3
.
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.
After the mechanical treatment in a mill, the powder with a density of 3.2 g/cm3
has
99.8% by mass SiC and thus forms the “clean” fraction. The content of Si is 0.03%
and the content of SiO2 is 0.02% mass, that of Cfree 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.
The powders with the densities of 2.5 g/cm3
and 3.9 g/cm3
were combined and form
the “impurified” fraction. In this fraction, there are 10.4 times more impurities
compared to the “clean” fraction.
26
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.
As a result of the method according to the invention, the “clean” fraction has a mass
percentage of 84% by mass.
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 Fe5Si3 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.
The existence of these impurities, which are 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.
In the “impurified” fraction, the impurities are also present in powder form, in
addition to the forms described here.
Example 4
10 kg of a dusty SiC powder as a by-product from SiC processing, containing 97.5%
by mass SiC, 0.4% by mass Cfree, 0.6% by mass Si, 0.5% by mass SiO2, 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.
27
The cooling is performed at a speed of 10 K/min.
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.
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.
After the mechanical treatment in the mill, the “clean” fraction has 99.2% by mass
SiC. The content of Si and SiO2 is 0.3% and 0.2% by mass respectively, that of Cfree
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.
In the “impurified” fractions, there is 9.6 times the amount of impurities compared to
the “clean” fraction.
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.
As a result of the method according to the invention, the “clean” fraction has a mass
percentage of 87% by mass.
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 Fe5Si3, FeSi, and FeSi2 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.
The existence of these impurities, which are permanently bonded to the SiC particles,
has been proven by means of TEM.
28
In the “impurified” fraction, the impurities are also present in powder form, in
addition to the forms described here.
Example 5
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 Cfree, 0.1% by mass Si,
3.5% by mass SiO2, 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 μm, determined by means of laser
diffraction. The powder mixture is loosely filled in graphite crucibles and
subsequently compressed to > 1 g/cm3
. 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.
The cooling is performed at a speed of 25 K/min.
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.
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.
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 SiO2 is 0.1% and 0.1% by mass respectively, that of Cfree 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.
The powders with the particle sizes < 20 µm and 20-70 μm were combined into the
“unpurified” fraction.
In the “impurified” fractions, there are 2.3 times more impurities compared to the
“clean” fraction.
29
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.
As a result of the method according to the invention, the “clean” fraction has a mass
percentage of 90% by mass.

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.
The existence of these impurities, which are permanently bonded to the SiC particles,
has been proven by means of REM-EDX.
In the “impurified” fraction, the impurities are also present in powder form, in
addition to the forms described here.

30
We Claim:-
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 d50
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 d50 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 d50 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 d50 between > 500 and
31
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 d50 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 or 10, 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.
32
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 means of flotation,
sedimentation, sifting, centrifugation, and/or cyclone methods, or when the separation
33
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 (SiO2), 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.