|Title of Invention||
"PROCESS FOR PREPARING SILICON CARBIDE"
|Abstract||Process for preparing silicon carbide wherein silicate and/or quartz containing rocks are subjected to electrolysis in a salt molt consisting of a fluoride-containing electrolysis bath, whereby silicon is formed in the same bath and in a deposit on the cathode; carbon powder from the cathode material and/or from external sources is added directly to the molten bath or frozen bath in addition to the cathode deposit, the frozen bath and the cathode deposit being crushed before or after the addition of carbon particles; concentrated sulfuric acid and then hydrochloric acid and water are added to the product from step II; the obtained mixture of liberated Si-grains and carbon particles floating to the surface together with some slag, is melted at a temperature above 1420°C, and SiC is crystallized by cooling.|
|Full Text||The present invention relates to a process for preparing silicon carbide
and optionally aluminum and silumin (aluminum silicon alloy) in a salt melt. Silica
and silicate rocks and/or aluminum containing silicate rocks are used as raw material,
with/without soda (Na2C03) and/or limestone (CaC03) dissolved in fluorides,
in particular cryolite.
The products prepared are of high purity.
WO 95/33870 (EP patent 763151), in the following designated as "WO
95", discloses a process for continuous preparation and batch preparation in one
or more steps in one or more furnaces, of silicon (Si), optionally silumin (AlSialloys)
and/or aluminum metal (Al) in a melting bath using feldspar or feldspar
containing rocks dissolved in fluoride. In said process Si of high purity is prepared
by electrolysis (step I) in a first furnace with a replaceable carbon anode arranged
underneath the cathode, and a carbon cathode arranged at the top of the
furnace. For the preparation of silumin the silicon-reduced residual electrolyte
from step I is transferred to another furnace, and Al is added (step II). Then Al is
prepared in a third furnace (step III) by electrolysis after Si has been removed in
step I and possibly in step II. It also describes combinations of furnaces with a
partition wall in the preparation of the same substances. Further, process equipment
for the procedure is described.
The present invention represents a further development and improvement
of the above-mentioned process.
The most important further development is that SiC of high purity is prepared
in molten Si, as explained below.
A great improvement is that it is possible to prepare pure Si (which is converted
to SiC), pure low-iron low-alloyed Al-alloys (AlSi-alloys) and pure lowphosphorus
high-alloyed Al-alloys (SiAI-alloys) in the same furnace (step I) by
varying such parameters as the choice of raw material, current density (voltage)
and time. The proportions of the Si and Al-products are adjusted by the choice of
raw material and cathodic current density (voltage) in the electrolysis bath and
mechanical manipulation of the cathodes. Further the; r.nmnn.sition of the Al2
(AlSi-alloy) as referred to herein, is an Al-alloy with an amount of Si which is
lower than that of an eutectic mixture (12% Si, 88% Al). Correspondingly, a highalloyed
alloy (SiAI-alloy) as.referred to herein is an alloy having a Si-content
above that of an eutectic mixture.
According to the present invention there is provided a process for preparing
silicon carbide and optionally aluminum and silumin (aluminum silicon alloy)
in the same cell, step I. Carbon is added to the Si fraction in step II, and SiC is
crystallized from molten Si and C in step III. The process takes place by
I. subjecting silicate and/or quartz containing rocks to electrolysis in a salt
melt consisting of a fluoride-containing electrolysis bath, whereby silicon
and aluminum are formed in the same bath, and aluminum
formed, which may be low alloyed, flows downwards to the bottom and
is optionally drawn off,
II. carbon powder from the cathode material and/or from external sources
is added directly to the molten bath or frozen bath in addition to cathode
deposit, the frozen, bath and the cathode deposit being crushed
before or after the addition of carbon particles;
III. the obtained mixture is melted at a temperature above 1420°C, and
SiC is crystallized by cooling.
Soda may be added to the electrolysis bath so that said bath will be basic
if quartz is used, in order to avoid loss of Si in the form of volatile Sip4. With high
concentrations of soda the melting point of the mixture is reduced, and the use of
added fluorides goes down. Limestone is added if necessary to reduce the absorption
of phosphorus in the Si deposited on the cathode.
In connection with the crystallization the fluorides should preferably be
A new feature according to the invention is that carbon which has either
been taken from the cathode (e.g. as turnings) or from external sources, is mixed
with silicon (in electrolyte). The carbon powder may then either be added to the
molten bath and/or added to the solid frozen bath and/or the cathode deposit. If
molten bath or frozen bath is used the cathode deposit must in advance be
scraped into the bath. The carbon is mixed with the desired fractions and
crushed to desired grain size. The obtained mixture consisting of Si, electrolyte
and C is either melted directly or preferably subjected to acid treatment as described
below. If acid treatment is employed, the melting of the acid-treated powder
will correspond to step III. Carbon must in any case be added in stoichiometric
excess to obtain a complete conversion to SiC in the Si melting process, steps
II and III.
An advantageous alternative is that concentrated H2SO4 is added to the
untreated, pulverized cathode deposit (step II) containing 20% Si, and/or the pulverized
bath (electrolyte) containing 20% Si after the cathode deposit has been
scraped into the bath, and carbon. The powder fractions initially result in a concentration
of Si to about 50% as the sulfuric acid has a good dissolving effect on
cryolite. This mixture of 50% Si and other residual products, i.a. acidic sulfates,
represents a sticky substance which must be treated further. By diluting the mixture
with water and adding HCI in dilute amounts for some time a very good liberation
of Si-grains floating to the surface is achieved. The HCI addition has the
effect in addition to the refining of Si, that the powder mixture does not remain
sticky. In this manner it is possible to obtain an increase in concentration of Si
together with C in a Si/C/electrolyte grain mixture with a sand-water consistency.
This sand-water consistency has the effect that the mixture is easy to filter and is
washed with water and dried at room temperature. As a consequence of the increase
of the concentration of Si and C in the powder mixture, the use of jig as a
separator (WO 97) becomes superfluous. What happens is that the acidic mixture
gradually reacts with the electrolyte and dissolves it. The Si-grains which are
partly embedded in electrolyte, are gradually liberated and get in contact with the
acid/water mixture. The acidic water attacks the contaminations in Si and C,
which primarily consist of metals. Hydrogen gas is formed on the surface and in
the pores of the Si- and C-grains, which results in an uplift even in very dilute
acid. In addition to the fact that Si (d = 2,3 g/cm3) and C (d = 2,1 g/cm3) float up
to the surface of the water, the Si- and C-grains will be hanging there until they
are scraped away from the surface. The refining of the Si-grains has also been
improved in addition to the concentration, since the acids over a longer time get
in better contact with the liberated Si- and C-grains. (The Si- and C-grains are so
pure that one gets below the detection limit for all the elements analyzed with
microprobe equipment. This means that there is not any analysis method which
can determine Si purer than about 99.99% as long it is impossible to concentrate
Si to -100% from a Si/C/electrolyte grain mixture).
Example 1 (from WO 95)
I. A feldspar of the type CaAI2Si208 containing 50% SiO2) 31% AI2O3 and
0,8% Fe2Os, was dissolved in cryolite and electrolyzed with a cathodic current
density of 0,05 A7cm2 (U = 2,5-3,0 V) for 18,5 hours. In the deposit around the
cathode highly purified Si was formed separate from small FeSi-grains. In the
electrolyte dissolved AI203 was formed. Al is not formed when the current density
is so low.
Since Al was not formed in the bath (Al^-containing electrolyte) this was
the reason why bath was drawn off from this furnace (step I) and to another furnace
(step II) in which residues of Si and Si(IV) were removed by addition of Al
before the electrolysis and the preparation of Al in a third furnace (step III). (See
II. The cathode deposit which contained about 20% Si, was knocked off the
cathode. In addition carbon (graphite, C) from the cathode followed. The obtained
mixture of powder was acid washed as described above to obtain a mixture
in which the concentration of Si and C had been increased.
III. The mixture from II together with residues of acid fluorides was melted
above 1420°. The crystallized Si contained large areas with "highly purified" SIC
as demonstrated by microprobe analysis. The purity in the sample was 99,997 %
SiC in a matrix consisting of 99,997-99,99999 % Si.
Conclusion: The reason why not only SiC was obtained, was a stoichiometric
deficit of carbon in the Si-melt, steps II-III. The reason why Si only and not Al was
formed in step I in this case, was the low current density (voltage).
Example 2 (Alternative process for step I with formation of Al)
A diorite (rock) containing feldspar and quartz, analyzed to contain 72%
SiO2, 16% AI2O3 and 1,4% Fe2O3l was dissolved in cryolite and electrolyzed at a
cathodic current density of 0,5-1,6 A/cm2 (U = 2,5-8,0 V)for 16.5 hours. In the
deposit around the cathode highly purified Si and many small separate FeSigrains
were formed. Underneath the electrolyte Al (low-alloyed AlSi-alloy) was
formed, and this had a low iron content.
Conclusion: The reason why both Si and Al were formed in step I was the high
current density (voltage). The reason why the Al (AlSi-alloy) has a low iron content,
was that the FeSi-grains remain in the deposit on the cathode.
Carbon was not added in the experiment. SiC could therefore not be
It must be expected that if carbon is added to a steadily purer Si, the purity
of the resulting SiC will also be increased.
Example 3 (Alternative process for step I with formation of Al)
Quartz containing close to 99,9% Si02 was dissolved in cryolite (NasAIFe),
mixed with 5% soda (Na2CO3) and electrolyzed with a cathodic current density of
0.5 A/cm2 (U = 6-7 V) for 44 hours. In the deposit around the cathode highly purified
Si was formed. Most of (12 kg) of the cathode deposit was pushed into the
bath (the electrolyte). The remaining cathode deposit (8 kg) was lifted out with
the cathodes together with the residues of the anode. The cathode deposit was
easily knocked off the cathodes and was mixed with the electrolyte in the bath.
Both contained 20% Si. Small amounts of Al (low alloyed AlSi-alloy) were formed,
which were low in iron and phosphorus. Iron and phosphorus poor AlSi-alloys are
defined as 110 ppm Fe and 0,08 ppm P. The crystallized silicon contained a total of 3 ppm
contamination corresponding to 99,9997 % Si.
Conclusion: The reason why both Si and Al were formed in step I was the high
current density (voltage). Al originates from electrolyzed cryolite. The reason why
Al (the AlSi-alloy) was now high alloyed with Si, was that Si from the cathode deposit
starts to dissolve in Al. The reason why the Al-alloy is iron and phosphorus
poor was that the raw materials initially are low in iron and phosphorus.
The silicon together with residues of small grains of FeSi prepared by acid
refining as described above without carbon addition, contains a total of 75 ppm
Fe and about 15 ppm P. The concentrated Si powder mixture contained 80% Si
or more than 80% Si. After crystallization of Si from the Si-melt, Si contained only
3,0 ppm contaminations. A mixing of slag with the acidic fluorides in Si will promote
the formation of an even purer SiC. In a possible crystallization of SiC from
a fluoride-refined Si-melt which also contains carbon, one must expect an even
lower content of contaminations in SiC than demonstrated in example 1.
If it is desired to prepare Al together with Si, the cathodic current density
should be relatively high, at least above 0,05 A/cm2, preferably above 0,1, in particular
above 0,2 A/cm2. An upper limit is about 2, preferably about 1,6 A/cm2. In
addition to the formation of aluminum with a high current density, the electrolysis
rate also increases with increasing cathodic current density.
With electrolysis it was found that the purity of Si was in the range 99,92 -
99,99%. Previously (W095), in order to concentrate Si further above 20% from
the cathode deposit, the cathode deposit was crushed so that as much as possible
of free and partly not free Si-grains would float up and could be taken up on
the surface in a heavy liquid consisting of different C2H2Br4/acetone mixtures with
a density of up to 2,96 g/cm3. Si in solid form has a density of 2.3 g/cm3 and will
float up, while solids of cryolite have a density of 3 g/cm3 and will remain at a bottom.
After filtration and drying of the powder for removal of heavy liquid, the different
concentration fractions were mixed with water/H2SO4/HCI for refining Si.
In WO 97/27143, in the following designated as "WO 97", water, HCI and
H2SO4 in this order were added to crushed cathode deposit, containing 20% Si,
to refine Si with a dilute NaOH which was formed by adding water. Then it was
tried to concentrate the powder containing Si refined with HCI, with concentrated
Neither in WO 95 nor in WO 97 Si was concentrated more than to about
40%. The reason for this is that the fluorooxosilicate complexes in the cathode
deposit were hydrolyzed in water and NaOH to form a difficultly soluble hydrated
silica. As a consequence of this an addition of H2SO4 after the treatment with water
did not accomplish the concentration effect which it has when added directly
to untreated dry powder. Concentrated HCI does not have any essential concentrating
effect as it contains much water in contrast to concentrated H2S04. In WO
97 a jig was used to concentrate Si further. This resulted only in an insignificant
When it is primarily desired to prepare SiC, a quartz containing rock is
suitably used as starting material. If Al is also of interest, a rock containing an Alrich
feldspar, for instance anorthite (CaAI2Si2O8) is suitably used.
Si may be melted together with Al prepared in the electrolysis (step I), to
form Fe-poor, P-poor, low alloyed A|Si-alloys and/or high alloyed SiAI-alloys,
which are desired alloys in many connections.
Both the high alloyed SiAI-alloys and the low-alioyed AtSi-alloys may be
dissolved in HCI or H2SO4. Al goes into solution and "pure"-Si-powder (-100%
Si), free of electrolyte, is formed. From dissolved Al pure products of AlCb and
Al2(SO4)3 are formed.
With respect to equipment it is suitable that the waits consisting of graphite
in the electrolysis furnace advantageously can be replaced by SiC or silicon nitride-
The walls of the electrolysis furnace do not have to consist of Si (WO 95,
figure 2 number 4). Further, Si does not have to cover the anode stem, since a
current jump does not take place between the cathode and anode, even when
they grow together.
1. Process for preparing silicon carbide wherein
I. silicate and/or quartz containing rocks are subjected to electrolysis in
a salt molt consisting of a fluoride-containing electrolysis bath,
whereby silicon is formed in the same bath and in a deposit on the
II. carbon powder from the cathode material and/or from external sources is added directly to the molten bath or frozen bath in addition to the cathode deposit, the frozen bath and the cathode deposit being crushed before or after the addition of carbon particles;
III. concentrated sulfuric acid and then hydrochloric acid and water are added to the product from step II;
IV. the obtained mixture of liberated Si-grains and carbon particles floating to the surface together with some slag, is melted at a temperature above 1420°C, and SiC is crystallized by cooling.
2. Process as claimed in claim 1, wherein both the anode and the cathode are prepared from graphite, and the anode is placed under the cathode.
3. Process as claimed in claim 2, wherein an amount of graphite is removed from the cathode and/or is added externally so that the amount of carbon is greater than the stoichiometric amount of carbon in SiC.
4. Process as claimed in any of claims 1-3, wherein the fluoride-containing electrolysis bath comprises cryolite.
5. Process as claimed in any of claims 1-4, wherein soda (Na2CO3) and limestone (CaCO3) are used in the electrolysis bath.
6. Process as claimed in any of claims 1-5, wherein quartz-containing rocks are used as a starting material.
7. Process as claimed in any of claims 1-7, wherein a basic, neutral or preferably acidic fluoride-containing electrolyte in step III is stirred into a mixture of molten silicon and carbon powder, which gradually crystallizes to SiC.
|Indian Patent Application Number||01415/DELNP/2003|
|PG Journal Number||37/2008|
|Date of Filing||03-Sep-2003|
|Name of Patentee||NORWEGIAN SILICON REFINERY AS|
|Applicant Address||FESTEVEIEN 10, 1525 MOSS, NORWAY.|
|PCT International Classification Number||C25B 1/00|
|PCT International Application Number||PCT/NO02/00074|
|PCT International Filing date||2002-02-21|