PROCESS FOR PREPARING TRICHLOROSILANE

METHODE POUR PREPARER LE TRICHLOROSILANE

English Abstract

A process for preparing trichlorosilane by reducing
tetrachlorosilane in a fluidized bed reactor has the following
steps: a) establishing in the reactor, a fluidized bed of
silicon particles; b) heating the silicon particles to a
temperature of 300°C to 1100°C by directing microwave radiation
into the reactor; c) passing a reaction gas containing
tetrachlorosilane and hydrogen through the fluidized bed and
reacting the reaction gas with the silicon particles, to form a
product gas which contains trichlorosilane and d) removing the
product gas from the reactor.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for preparing of trichlorosilane by reducing
tetrachlorosilane in a fluidized bed reactor, which comprises
(a) establishing in the reactor a fluidized bed of
silicon particles;
(b) heating the silicon particles to a reaction
temperature of 300°C to 1100°C by directing
microwave radiation into the reactor;
(c) passing a reaction gas containing
tetrachlorosilane and hydrogen through the
fluidized bed and reacting the reaction gas with
the silicon particles, to form a product gas which
contains trichlorosilane; and
(d) removing the product gas from the reactor.
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2. The process as claimed in claim 1, comprising
compensating for a reaction-related consumption of
silicon by continuously feeding silicon particles to the reactor.
3. The process as claimed in claim 1, comprising
separating the product gas into trichlorosilane and a
residual gas; and
recycling the residual gas to the reactor as reaction
gas.
4. The process as claimed in claim 1, comprising
separating the product gas into trichlorosilane and a
residual gas; and
thermally cleaving the trichlorosilane in a deposition
reactor (CVD reactor), with elemental silicon being formed.
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5. The process as claimed in claim 1,
wherein the reaction temperature is 500°C to 700°C.
6. The process as claimed in claim 1,
wherein the reaction gas has an initial temperature of
20° to 50°C.
7. The process as claimed in claim 1,
wherein the microwave radiation has a frequency of 500
to 5000 MHz.
8. The process as claimed in claim 1,
wherein the microwave radiation has a frequency of 1000
to 1500 MHz.
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9. The process as claimed in claim 1,
wherein the silicon particles have a mean diameter of
50 to 5000 µm.
10. The process as claimed in claim 1,
wherein the silicon particles have a mean diameter of
500 to 3000 µm.
11. The process as claimed in claim 1,
wherein the fluidized bed reactor is free from a
catalyst.
12. The process as claimed in claim 1,
wherein the fluidized bed reactor is free from a copper
chloride catalyst.
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13. The process as claimed in claim 1,
wherein the molar ratio of tetrachlorosilane to
hydrogen in the reaction gas is 3:1 to 1:10.
14. The process as claimed in claim 1,
wherein the molar ratio of tetrachlorosilane to
hydrogen in the reaction gas is 3:2 to 5:3.
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Description

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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the preparation of
trichlorosilane by reducing tetrachlorosilane in a fluidized bed
reactor.
2. The Prior Art
Tetrachlorosilane is a chemical compound which is formed, in
particular, in the deposition of high-purity silicon by thermal
decomposition of trichlorosilane. It also arises as a by-product
in the preparation of trichlorosilane, which is produced by
reacting hydrogen chloride and silicon in a fluidized bed
reactor. Large amounts of the industrially produced
tetrachlorosilane are reduced to trichlorosilane during the
production of high-purity silicon.
According to US Patent No. 4,526,769, the process is carried
out in a fluidized bed reactor, in which case the following,
reversible reaction equation applies:
3SiCl4 + 2H2 + Si ~ 4HSiCl3
This patent publication discloses that the conversion rate
of tetrachlorosilane is a function in particular of the reaction

2 1 8598 1
temperature, the ratio which is set of tetrachlorosilane to
hydrogen and the presence of a copper chloride catalyst.
SUMMARY OF THE INVENTION
It is an object of the present invention to improve the
process and to achieve an increase in conversion rate.
The above object is achieved by a process for the
preparation of trichlorosilane by reducing tetrachlorosilane in a
fluidized bed reactor, which comprises
a) establishing in the reactor a fluidized bed of silicon
particles;
b) heating the silicon particles to a reaction temperature of
300C to 1100C by directing microwave radiation into the
reactor;
c) passing a reaction gas containing tetrachlorosilane and
hydrogen through the fluidized bed and reacting the reaction
gas with the silicon particles, to form a product gas which
contains trichlorosilane; and
d) removing the product gas from the reactor.
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Trichlorosilane is produced by the process of the invention
with enhanced conversion rates and lower energy consumption. The
use of a catalyst such as a copper chloride catalyst, which
contaminates the product, is not necessary. Thus, the reactor is
devoid of, and free from, a catalyst.
The energy savings primarily result from the microwave
radiation interacting directly with the silicon particles and
heating them to the reaction temperature without the reactor wall
and reaction gas having to be heated conjointly. This also spares
the reactor material, so that longer service life is possible, or
less stringent requirements can be made of the reaction material
with respect to its corrosion resistance.
The fluidized bed is established by forcing reaction gas or,
if appropriate, inert gas or hydrogen, at a defined pressure,
from the bottom upwardly through a bed of the silicon particles
introduced into the reactor. The silicon particles are brought to
the reaction temperature by microwave irradiation. This process
can, if appropriate, be accelerated by preheating the gas which
is passed through the bed of the silicon particles. The
conversion reaction starts when the silicon particles have the
necessary reaction temperature in the range of 300C to 1100C,

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preferably 500OC to 700OC, and reaction gas at an initial
temperature of 20C to 50C is passed through the bed. The
reaction gas is then heated up to this necessary reaction
temperature range of 300C to 1100C.
The frequency of the microwave radiation used for the
heating is 500 to 5000 MHz, preferably 1000 to 1500 MHz. The
mean diameter of the silicon particles introduced into the
reactor is 50 to 5000 ~m, particularly preferably 500 to 3000 ~m.
The reaction gas contains tetrachlorosilane and hydrogen
and, if appropriate, a carrier gas which is not a participant in
the reaction, for example nitrogen or argon. The molar ratio of
tetrachlorosilane to hydrogen in the reaction gas is 3:1 to 1:10,
preferably 3:2 to 5:3. Tetrachlorosilane is incompletely
converted in the reaction, so that the product gas which leaves
the reactor further contains compounds of the reaction gas, in
addition to the desired trichlorosilane. It is therefore provided
to separate the trichlorosilane from the product gas and to
recycle the residual gas to the reactor as reaction gas.
Since silicon is consumed in the reduction of
tetrachlorosilane in the above-mentioned reaction equation, it is

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further provided to compensate for the silicon consumption
continuously or batchwise. Preferably, silicon particles are
continuously fed to the reactor as a function of the reaction-
related silicon consumption. The trichlorosilane separated off
from the product gas is preferably used for the preparation of
high-purity silicon, this preparation being particularly
preferably accomplished by the Siemens process or by a process
which is described in US Patent No. 4,900,411. Thus, for
example, the product gas can be separated into trichlorosilane
and a residual gas. Then the trichlorosilane is thermally
cleaved in a deposition reaction (CVD reactor) with elemental
silicon being formed.
The apparatus shown in this patent and in US Patent No.
5,382,412 for preparing polycrystalline silicon can also be used
in principle for carrying out the process of the invention.
Therefore, the disclosure of each of U.S. Patent Nos. 4,900,411
and 5,382,412 is herewith incorporated by reference. Obviously,
the apparatus is to be adapted to the requirements of the process
of the invention and these modifications are within the
capability of those skilled in the art. This includes, for
example, dispensing with the need for a device for taking off
silicon particles from the reactor. It has also proved to be

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advantageous if the lower part of the reactor, which receives the
silicon particle bed, is designed so as to taper conically.
Conversely, the invention can also be used to clean off
undesired silicon deposits from an apparatus used for the
preparation of polycrystalline silicon. No fluidized bed is
required in this case, since the deposits replace the silicon
particles and supply the silicon necessary for the course of the
reaction.
While several embodiments of the present invention have been
shown and described, it is to be understood that many changes and
modifications may be made thereunto without departing from the
spirit and scope of the invention as defined in the appended
claims.