Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to a process for the
production of beta-silicon carbide from silicic acid and
carbon.
Silicon carbide has been recovered for many
decades in basically the same manner, according to the
Achason process. The Acheson process involves the reaction
of silicon dioxide, in the form of quartz sand, and carbon,
in the form of oil coke, in an electrical resistance
furnace at temperatures above 2000C. A drawback of this
method is that the product thus obtained is lumpy and,
depending on the intended use, has to be finely crushed to
varying degrees. Where an extremely fine product is
necessary, the crushing operation requires much energy and
considerable equipment. Such is the case, ~or instance,
where the resultant product is intended as a sinter powder
for the production of silicon carbide ceramics, an ever
increasingly important use. Other drawbacks of the Acheson
process are that the furnace used has to be very large to
make economical operation possible and that the yield per
furnace run is only in the order of magnitude of 20
~ercent, so that a large part of the mat~rial used has to
be recycled. Moreover, the Acheson process yields the
hexagonal high-temperature modification of silicon carbide,
which generally is designated as the alpha-modification,
while the cubic low-temperature modification, pre~erred for
certain uses, and usually designated as the beta-
modification, is not accessible according to this prior
process.
Processes for the production of beta-silicon
carbide are also known which start from silicon dioxide and
carbon. ~or this purpose, the reactants have to be in fine
particulate ~orm and intimately mixed. The reaction
temperatures are generally below 1800C. The reaction rate
can be increased and the particle size of the product
reduced by the addition of fine-particle beta-SiC, as a
nucleating agent, and/or metals or metal oxides.
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Previously, both finely ground quartz and precipitated
silicic acids or pyrogenic silicic acid was used as the
source of silicon dioxide, while, for example, oil coke or
carbon black was used as carbon. However, the fine
grinding of quartz is very energy-intensive, and particle
sizes below 1 micron, which are desirable for the
production of SiC, can be achieved only at high costs.
Moreover, where pr~cipitated silicic acids, such as those
described in, for example, U.S. Patent No. 4,377,563, or
pyrogenic silicic acid are used, the specific properties of
these products make technical performance difficult since,
by virtue of their pronounced thickening action, they can
only be poorly processed in aqueous suspension. Yet the
formation of suspensions with high solid content and low
viscosity is desirable or necessary for mixing,
deagglomerating or spray drying.
Another drawback of the pyroganic silicic acids is
their relatively high price.
The main object of the invention is to provide a
process for the production of beta-silicon carbide, which
starts from an inexpensive silicon dioxide having
favoura~le particle size and good processing properties.
Accordingly, the present invention provides a
process for the production of beta-silicon carbide powder
from silicon dioxide and carbon black by heating both
starting materials to a temperature of from 1200 to 2000~C
in the presence of beta-silicon carbide nuclei and then
removing excess carbon. Amorphous silicon dioxide,
resulting from the reaction of hexafluorosilicic acid with
aluminium hydroxide, is used as the silicon dioxide.
It was found in the production of aluminum
fluoride from hexafluorosilicic acid and aluminum
hydroxide, according to the reaction equation:
H2SiF6 + 2Al(OH)3 > 2AlF3 + SiOz + 4H2O
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that the resultant amorphous silicon dioxide not only has
a favourable particle size, which makes possible a rapid
reaction with carbon, but also forms aqueous suspensions
with solid contents of up to over 40 percent by weight at
low viscosity.
Silicon dioxide is a by-product in the industrial
production of aluminum fluoride and, therefore, is very
inexpensive and available in large quantities. The
recovery on a laboratory scale is described, for example,
in U.S. Patent No. ~,693,878 (Example 1).
For the production of beta-silicon carbon, the
silicon dioxide is suitably washed with acid to reduce the
aluminum content to harmless values. A harmless value is
a level of aluminum content which, when achieved, obviates
the need for a similar acid treatment of the silicon
carbide product. Hexafluorosilicic acid, necessary in
aluminum fluoride production, can be used as the washing
liquid, but other acids, such as, for example, hydrochloric
acid can also be used. Special measures for the reduction
of the fluorine content are not necessary, since the
fluorine escapes at the operating temperatures for silicon
carbide production. However, it can be advantageous to
remove the fluorine before production of silicon carbide
begins, in the manner described in U.S. Patent No.
4,693,878.
In the process of the present invention gas black
or furnace black is the preferred carbon source, however,
any carbon blac~ may be suitably used. Where furnace black
is used as the carbon source, preferably, it is washed to
remove metal traces and thereby avoid the introduction of
impurities into the end product.
In a preferred embodiment of the present
invention, the washed silicon dioxide is first ground or
deagglomerated in a stirred ball mill or attrition mill.
Water is preferably used as the grinding liquid. The
preferred grinding elements are those made from silicon
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dioxide, for example, balls rnade from quartz glass or
rounded quartz sand. A quartz sand, known by the name
"Ottawa sand", with a particle size of about 1 mm has
proved to be especially advantageous.
In order to avoid the introduction of impurities
by metal abrasion, a stirred ball mill or an attrition mill
with an elastomer lining is preferably used. The elastomer
lining may, for example, be made from polyurethane. In
this grinding or deagglomeration process, it is preferable
that the beta-silicon carbide powder, necessary for
nucleating, be present. To make the grinding or
deagglomerating process easier, the usual auxiliary agents,
especially liquefiers and defoaming agents, can be added.
Moreover, it is also possible to introduce additives, such
as sintering auxiliary agents, desirable for the later use
of the silicon carbide produced according to the present
invention.
The carbon, preferably in the form of gas black or
furnace black, is also introduced lnto the stirred ball
mill or attrition mill to achieve an optimal mixing. Then
the suspension is dehydrated according to ona of the known
methods and put into a form suitable for the production of
silicon carbide. Preferably, the dehydration is performed
by spray drying, which can optionally be followed by a
granulation st~p, if even coarser agglomerates are desired.
The reaction can be performad in any furnace which
allows for the necessary temperature and retention times.
On a laboratory scale, the furnace can be a crucible
` furnace or a muffle furnace, while, on an industrial scale,
; 30 suitable examples are shaft furnaces, rotary kilns or
fluidized-bed furnaces. Rotary kilns are especially
advantageous since they make possible an adecluate heat
transfer without the need for an auxiliary medium. The
reaction temperatures and retention times are known
generally those customary in the prior ark process. The
reaction can be performed, for example, at a temperature of
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from 1500 to 1800C with a retention time in the order of
1 hour.
After completion of the reaction it i5 often
necessary to remove the excess carbon. There are several
methods known in the art for th:is purpose. For example, it
is possible to either burn the carbon in the presence of
oxygen or to react it with hydrogen, under pressure at 600
to 1400C, to form methane. Preferably, the carbon is
removed by treatment with a~lmonia at 800 to 1400C,
especially at 1000 to 1200C (F.K. van Dilen, J.
Pluijmakers, J. Eur. Ceram. Soc., 5, 385 (1989), and
literature cited therein). ~hen, in the process of the
~ present invention treatment with ammonia is performed, the
1 use of a fluidized-bed furnace is preferred so as to ensure
an optimal interaction between the gas and solid and an
efficient transfer of heat.
After removal of excess carbon, the product can be
used as such or subjected to another grinding and
deagglomerating process. For this purpose, again a stirred
ball mill or an attrition mill is preferably used. Silicon
carbide balls are preferably used as the grinding elements
ù so as not to introduce any foreign substances through
abrasion. For the same reason, the attrition mill is
preferably lined with plastic or (SiC) ceramic. Either
water or an organic liquid, such as isopropanol or heptane,
can be used as the grinding liquid. In this further
grinding or deagglomerating step, additiYes such as sinter
additives can be introduced where appropriate to the later
use of the silicon carbide powder.
The following Examples illustrate the performance
of the process according to the invention.
EXAMPLE 1
Purification of the starting material
(a) Silicon dioxide: Crude silicon dioxide powder
from the procluction of aluminum fluoride was first stirred
at 95C for 4 hours with a diluted (0.6 percent by weight~
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hexafluorosilicic acid (6 1 to 1 kg o~ silicon dioxide) and
then filtered off. The filter cake was washed with
desalted water (10 1 to 1 kg of silicon dioxide) and dried.
The aluminum content was reduced by this treatment from 1.5
percent by weight to 130 ppm and the fluorine content from
4.5 percent by weight to 3.3 percent by weight. The
specific surface of the powcler was 3 m2/g, and the
agglomerate size was smaller than O.1 mm.
(b) Furnace black: Commercial furnace black
(Elftex~ 470) was stirred at 90C for 60 minutes with a
diluted (0.6 percent by weight) aqueous hexafluorosilicic
acid (6 1 to 1 kg of furnace black) and then filtered off.
The filter cake was washed with desalted water (5 1 to 1 kg
of furnace black) and dried. The metal content was reduced
by this treatment from 2000 ppm to 100 ppm.
EX~NPLB 2
Production of be~a-5iC
10.5 kg of washed silicon dioxide from Example 1
was ground with 0.5 kg of beta-SiC powder (as nucleating
agent), 0.5 kg of Triton~ X-100 (liquefier), 0.1 kg of
silicone defoaming agent and 0.4 kg of poly~inyl alcohol
(binding agent) in 25 1 of desalted water in a stirred ball
mill lined with polyurethane and with Ottawa sand (0.8 -
1.1 mm) for 20 minutes (effective grinding period). Then
6.9 kg of gas black (Printex~ U) was added and ground for
another 15 minutes. After separation of the sand by a
filter cartridge, the suspension was spray dried so that a
granular material with 0.4 mm average diameter and 2
percent residual moisture was obtained. The granular
material was poured into graphite crucibles with a diameter
of 50 mm and heated to 1700C for 60 minutes in an argon
atmosphere. After cooling, the silicon carbide powder was
heated to 1100C in a fluidized-bed furnace with ammonia
for 5 hours to remove the residual carbon. The silicon
carbide powder thus obtained exhibited the following
properties:
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phase composition (x-ray diffraction) 100% beta
specific surface (BET) 5 m2/g
carbon content (total) 29.8 % by weight
nitrogen content 0.2% by weight
oxygen content 0.3% by weight
metals 220 ppm
EXANPLE 3
A suspension of 10.5 ]cg of washed silicon dioxide
and the additives described in Example 2 were ground in 15
l of desalted water for 20 minutes in a stirred ball mill
as described in Example 2. Then 6.6 kg of furnace black
(Elftex 470), washed according to Example 1, and lO l of
desalted water was added. The other steps took place as
described in Example 2. The silicon carbide powder thus
obtained exhibited the following properties:
phase composition (x-ray diffraction) 100% beta
specific surface (BET) 5 m2/g
carbon content (total) 29~8% by weight
nitrogen content 0.2% by weight
oxygen content 0.3% by weight
.l metals 300 ppm
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