Process for the manufacture of trichlorosilane and silicon tetrachloride

Trichlorosilane or silicon tetrachloride is prepared by reacting metallic uminum-containing silicon in a reaction zone with hydrogen chloride or chlorine at a temperature between about 260.degree. and about 1200.degree. C, the reaction gas is then cooled to 40.degree. to 130.degree. in a cooling zone through which the velocity of flow is maintained at between 3 and 30 meters per second, and finally filtered.

BACKGROUND OF THE INVENTION 
The subject matter of the present invention is a process for the 
manufacture of trichlorosilane and/or silicon tetracholoride by reacting 
metallic, aluminum-containing silicon at 260.degree. to 1200.degree. C. 
with hydrogen chloride or chlorine and subsequently separating the 
resulting reaction mixture. 
Crude silicon contains up to 2% by weight of aluminum and iron in the form 
of impurities. In the manufacture of trichlorosilane and silicon 
tetrachloride from crude silicon and hydrogen chloride, these impurities 
are converted to aluminum chloride (AlCl.sub.3) and ferrous chloride 
(FeCl.sub.2). Under normal reaction conditions, unlike the ferrous 
chloride, all the aluminum chloride emerges from the reaction zone at 
temperatures above 260.degree. C. in gaseous form owing to its relatively 
high vapor pressure, and is deposited in the form of a white to yellowish 
crystal mass on the walls of the separation and condensation system 
connected to the reaction zone. In the reaction of crude silicon with 
chlorine, in addition to aluminum chloride, ferric chloride (FeCl.sub.3) 
is formed which behaves in a corresponding manner. As a result, the 
insides of the pipes become encrusted and clogged and the cooling devices 
used become obstructed. Consequently, it is often necessary after a short 
time for the pipe systems to be opened and freed of the aluminum chloride 
that has crystallized out onto them. As a result of the fact that 
trichlorosilane, silicon tetrachloride and aluminum chloride are extremely 
readily hydrolysable and owing to the combustibility of trichlorosilane 
and the high-boiling chlorosilanes produced in side reactions, apart for 
there being general work-hygiene problems, considerable costs are also 
incurred. In addition it is necessary to use coolers that have a large 
surface area, or scraper coolers in which the wall deposit which forms is 
continuously scraped off by means of built-in rotating components. Rinsing 
out the solid material that has been scraped off and covering the shaft 
guide means in particular, present problems here, however. 
GENERAL DESCRIPTION OF THE INVENTION 
The object underlying the invention is therefore to find a process for the 
manufacture of trichlorosilane and/or silicon tetrachloride, in which it 
is possible for aluminum chloride and, where applicable, ferric chloride, 
to be separated from the reaction gas without problem. 
This object is achieved by inserting directly after the reaction zone, a 
cooling zone which cools the reaction gas to a temperature of 40.degree. 
to 130.degree. C. for its entry into the filters arranged upstream of the 
following condensation system.

Referring to the drawing: 
Gaseous hydrogen chloride or chlorine is blown into the lower portion 1 of 
the reactor 20 through an inlet pipe 2. The gas flows upward through the 
sieve plate 3 through a fluidized bed 4 of ground crude silicon, which is 
introduced continually through an inlet pipe 5 into the upper portion 6 of 
the reactor 20 in accordance with its consumption. There is connected to 
the lower portion 1 of the reactor a removable dust container 7, which 
collects the solid materials that have been separated by the sieve plate 
3. 
The chlorine gas or the gaseous hydrogen chloride is converted by the crude 
silicon in the fluidized bed 4 to silicon tetrachloride or trichlorosilane 
respectively. In addition, a series of other gaseous reaction products, 
such as higher chlorosilanes, are produced and, in the case of hydrogen 
chloride, of course, hydrogen is produced as a reaction gas. The iron and 
aluminum contained in the crude silicon are converted to iron chloride and 
aluminum chloride and are removed with the reaction gas through the pipe 
8. Since the reaction of hydrogen chloride or chlorine is carried out at a 
temperature of 260.degree. to 1200.degree. C., when the reaction product 
gas leaves the reactor, it has a temperature of at least 250.degree. C. 
In accordance with the invention, this reaction gas is then cooled, in the 
path between the reactor and the filter housing 9, arranged upstream of 
the condensation system 10, to a temperature of 40.degree. to 130.degree. 
C., preferably 60.degree. to 80.degree. C. For this purpose, a tube 10 
provided with a cooling jacket through which a suitable cooling liquid, 
for example, water, is pumped by way of the pipes 11 and 12, is preferably 
used. 
The selection of the correct tube cross-section and the size of the total 
cooling surface area, are particularly important here, since, if the tube 
diameters are too large and therefore the rate of flow too slow, and if 
the cooling is too intense, dust and aluminum chloride will be deposited 
on the walls of the tube. 
The internal diameter of the tube 10 must therefore be so selected that in 
a given plant, a stream velocity of the reaction gas of 3 to 30 m/sec, 
preferably 6 to 15 m/sec is insured. The cooling is so adjusted that the 
reaction gas has a temperature of 40.degree. to 130.degree. C., preferably 
60.degree. to 80.degree. C., when it enters the inlet pipe 13 of the 
filter housing 9. Within this temperature range, in the case of a high 
aluminum content in the crude silicon used, a higher temperature is 
selected and in the case of a low aluminum content, a correspondingly 
lower temperature is selected. The same applies to the reaction with 
chlorine, in which, in addition to the aluminum content, it is also 
necessary to observe the iron content. 
Most of the aluminum chloride is precipitated out in this cooling path. The 
individual aluminum chloride crystals are prevented from joining together, 
however, by the silicon and iron chloride dust emerging directly from the 
reactor, irrespective of whether this is a solid bed or fluidized bed 
reactor, so that the aluminum chloride cannot become attached to the 
cooled walls of the tube 10, but is deposited in the filters 14 of the 
filter housing 9. The dust, consisting of silicon, iron chloride and 
aluminum chloride, continuously falls out of the filters 14, on account of 
its weight, into the dust container 15, attached to the lower portion of 
the filter housing 9, but some of the dust also falls directly into the 
dust container immediately after entering the filter housing. From time to 
time, the filter is blown free by means of a stream of gas from the 
opposite direction. The dust container 15 can be emptied simply by 
tipping. 
The reaction gas, which is free of solid substances and in the case of 
reaction of silicon with hydrogen chloride contains approximately 45 to 
50% by volume of hydrogen, approximately 45% by volume of trichlorosilane, 
approximately 5% by volume of silicon tetrachloride and a few % by volume 
of hydrogen chloride, leaves the filter housing 9 through the gas outlet 
pipe 16 and is separated into its constituents in the condensation system 
connected thereto. 
The process also renders possible, by simple means, an essentially 
quantitative separation of aluminum chloride (AlCl.sub.3) and, in the case 
of the reaction of silicon with chlorine, also of ferric chloride 
(FeCl.sub.3), which behaves in a similar manner, before the reaction gas 
enters the condensation system. 
As a result of the considerable increase in the life of reactor systems 
brought about thereby, a marked increase in productivity and capacity can 
be achieved.