Process for the production of silicon tetrachloride

A silica-containing material such as silica sand, a carbonized biomass or biomass ash is converted into carbon tetrachloride by reaction with chlorine gas at an elevated temperature in the presence of a carbonaceous material such as coke, and sulfur or a sulfur compound. The reaction may be suitably performed in the further presence of a potassium compound.

This invention relates to a process for the production of silicon 
tetrachloride by chlorination of a silica-containing material. 
Silicon tetrachloride which is an important raw material for the production 
of organosilicon compounds, silicone resins, fine particulate silica, high 
purity artificial quartz, silicon nitride, silicon carbide, etc. is 
generally prepared by the following three methods: (1) a method in which 
elemental silicon powder is chlorinated by reaction with HCl to produce 
trichlorosilane as a main product and silicon tetrachloride as a 
by-product; (2) a method in which ferrosilicon or silicon carbide is 
reacted with chlorine; and (3) a method in which a mixture of a 
silica-containing material such as silica stone and carbon is reacted with 
chlorine. 
These conventional methods involve the following problems. The Method (1) 
in which silicon tetrachloride is produced as a by-product has a problem 
because the economy thereof depends on the demand for trichlorosilane 
produced as a major product. Further, elemental silicon is not available 
at low costs due to the consumption of much electrical energy for the 
production thereof which is generally performed by reduction of silica at 
a temperature of 2000.degree. C. or more in an electric furnace. The 
Method (2) is also disadvantageous from the standpoint of economy since 
the raw material, i.e. ferrosilicon or silicon carbide is expensive 
because the production thereof requires much electrical energy. The Method 
(3) has a problem because the reactivity of a silica-carbon mixture with 
chlorine is low and requires a high reaction temperature, though the raw 
materials are easily available at a low price. 
The present invention has been made with the foregoing problems of the 
conventional methods in view and has as its object the provision of an 
improved method which can produce silicon tetrachloride with a high yield 
using a silica-containing material such as silica stone as a raw material. 
In accordance with the present invention, there is provided a process for 
the production of silicon tetrachloride, comprising reacting a 
silica-containing material with chlorine in the presence of a carbonaceous 
material and sulfur or a sulfur compound. 
The silica-containing material to be used as a raw material in the process 
of the present invention may be, for example, silica sand, silica stone or 
combustion residues (ash) or carbonized materials or a biomass. The 
biomass is a silica-containing plant and may be, for example, leaves, 
stalks, straw, chaff of various plants. Illustrative of suitable biomass 
are wheat chaff, rice hull, straw of wheat and rice plant, and leaves and 
stalks of corn, sugar cane, bamboo and rush. 
The biomass combustion residues may be obtained by combusting the biomass 
at a temperature of 500.degree.-1100 .degree. C., preferably 
600.degree.-900 .degree. C. Since the silica in the ash obtained at a high 
temperature combustion tends to form crystals and since the reactivity of 
silica is lowered when it forms crystals, the combustion is desirably 
performed at a temperature below 900.degree. C. The carbonized biomass to 
be used as the silica-containing material in the process of the present 
invention may be obtained by carbonizing the biomass at a temperature of 
300.degree.-1200 .degree. C., preferably 600.degree.-1000 .degree. C. in 
an inert atmosphere such as in the atmosphere of nitrogen, argon or 
helium. 
The silica contained in the silica-containing material is preferably 
amorphous though cystalline silica such as cristobalite, tridymite or 
quartz may be used. 
In the present invention, the chlorination of the silica-containing 
material is performed in the presence of a carbonaceous material and 
sulfur or a sulfur-containing compound. 
The carbonaceous material to be used in the present invention may be, for 
example, pitch, coke, carbon black, activated carbon or a carbonized 
biomass and may be in the form of a liquid or solid. When a 
silica-containing, carbonized biomass is used as the silica-containing 
material, the carbonaceous material may be omitted. The carbonaceous 
material is preferably used in such an amount as to provide a weight ratio 
of SiO.sub.2 to carbon of about 10:1 to 1:1, more preferably 2.5:1 to 
1.5:1. 
The sulfur compound may be, for example, carbon disulfide, hydrogen sulfide 
or sulfur dioxide and may be in the form of a gas, liquid or solid. Of 
these, the use of carbon disulfide is preferred. Since, carbon disulfide 
can provide carbon necessary in the chlorination of silica, the 
carbonaceous material may be omitted or the amount thereof may be 
decreased when the chlorination of the silica-containing material is 
performed in the presence of carbon disulfide. The amount of sulfur or the 
sulfur compound is 1:1000 to 1:1, preferably 1:1000 to 1:2 in terms of the 
weight ratio thereof to the chlorine fed for the chlorination to the 
reaction zone. 
It is preferred that the chlorination be performed in the additional 
presence of potassium compound so as to further improve the yield of 
silicon tetrachloride. Examples of the potassium compounds include 
potassium carbonate, potassium chloride, potassium hydrogensulfate, 
potassium hydroxide, potassium nitrate and potassium sulfate. The 
potassium compound may be in the form of a solid or liquid. The potassium 
compound is preferably used in an amount of 0.05-50 weight %, more 
preferably 0.1-10 weight % based on the weight of silica in the 
silica-containing material. 
The chlorination of the silica-containing material is performed at a 
temperature of 400.degree.-1100.degree. C., preferably 600.degree.-1000 
.degree. C. and may be performed in various manners. For example, the 
carbonaceous material, the sulfur or sulfur compound and the potassium 
compound (which is an optional ingredient) may be mixed with the 
slica-containing material prior to the commencement of the chlorination. 
It is preferred that the mixture be well commingled by means of, for 
example, a ball mill prior to the chlorination for reasons of improved 
conversion of the silica-containing material. In a preferred embodiment 
according to the present invention, the above mixture is pulverized so 
that the particle sizes of the silica-containing material and the 
carbonaceous material are, for example, about 10 .mu.m or less and about 
50 .mu.m or less, respectively, more preferably about 4 .mu.m or less and 
15 .mu.m or less, respectively. The mixture which may be in the form of 
powder or may be shaped into pellets is supported in the form of a bed in 
a reaction zone to which a chlorine-containing gas which may be a pure 
chlorine gas or a chlorine gas diluted with nitrogen or the like inert gas 
is fed for contact with the bed. The bed may be a fixed bed or a fluidized 
bed. This method may be adopted even when one or more of the carbonaceous 
material, the sulfur or sulfur compound, and the potassium compound are 
liquid. Chlorine is generally used at least in a stoichiometric amount. 
When the sulfur or sulfur compound is in a gaseous form, it may be supplied 
to the reaction zone simultaneously with the chlorine-containing gas for 
contacting with the bed of a mixture of the silica-containing material, 
carbonaceous material and, optionally, potassium compound. The sulfur or 
sulfur compound even when it is in the form of a liquid or a solid may 
also be fed to the reaction zone simultaneously with the 
chlorine-containing gas using a liquid pump or powder feeder. 
The gas discharged from the reaction zone and containing unreacted chlorine 
and silicon tetrachloride is fed to a separator where the silicon 
tetrachloride is separated, for example, by condensation. The gas from 
which silicon tetrachloride is separated may be recycled to the reaction 
zone, if desired. 
The following examples will further illustrate the present invention. In 
the examples, combustion residues of rice hull, a carbonized product from 
rice hull and natural silica sand were used as silica-containing raw 
materials. 
The combustion residues of rice hull were those obtained by combusting rice 
hull in a furnace at a temperature in the range of 800.degree.-1000 
.degree. C. at an excess air ratio with a rice hull feed rate of 450 kg/hr 
and with a residence time of 4 hours. The rice hull residues (ash) were 
found to contain 96.5% by weight of SiO.sub.2 and 2.1 % by weight of 
residual carbon. 
The carbonized rice hull was that obtained by heating rice hull (20 g), 
packed in a quartz glass tube with an inside diameter of 55 mm, at 700 
.degree. C. for 1 hour under a nitrogen stream supplied at a rate of 2 
liter/minute. The carbonized product was found to contain 47.4% by wight 
of SiO.sub.2 and the balance of essentially carbon. 
The natural silica sand contained 97.7% by weight of SiO.sub.2. 
The SiO.sub.2 content herein was analyzed after treating the sample with 
nitric acid and perchloric acid. The carbon content was determined by 
ignition loss by thorough combustion in air. 
Oil coke (petroleum oil coke) having the following analytical values or the 
above-mentioned carbonized rice hull was used as carbonaceous material. 
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Proximate analysis (equilibrated sample) 
Moisture content 0.5% by weight 
Ash content 0.1 
Volatile matter content 
6.3 
Fixed carbon content 
93.1 
Elementary analysis (water- and ash-free sample) 
C 95.1% by weight 
H 3.7 
N 0.6 
O 0.2 
S 0.5 
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The silica-containing materials and the carbonaceous materials were each 
ground for 5 minutes with a planetary ball mill having an inside volume of 
12 ml.

EXAMPLES 1-8 
A quantity of the silica-containing material as shown in Table 1 was mixed 
with a quantity of the carbonaceous material as shown in Table 1 to obtain 
a mixture having a weight ratio of SiO.sub.2 to C of 2:1. The mixing was 
performed with a planetary ball mill for 1 minute (Examples 1-3) or 10 
minutes Examples 4-8). Each mixture (or carbonized rice hull in Example 6) 
was then subjected to chlorination in the presence of sulfur (Example 5) 
or CS.sub.2 (Examples 1-4 and 6-8). Thus, the above mixture (1g) was put 
in a boat made of alumina and this boat was set in the center of a quartz 
glass reaction tube with an inside diameter of 30 mm. The quartz glass 
tube supporting the mixture-containing boat therein was heated to 
900.degree. C. at a heating rate of 26.degree. C./minute in an argon gas 
stream flowing at a rate of 100 ml/minute through the reaction tube. After 
maintaining the reaction tube at 900.degree. C. for 30 minutes, the feed 
of argon gas was stopped and chlorine gas was fed at a rate of 100 
ml/minute for 1 hour while maintaining the reaction tube at 900.degree. C. 
Simultaneously with the feed of the chlorine gas, CS.sub.2 was also 
introduced into the reaction tube by means of a liquid pump at a feed rate 
as shown in Table 1, in which the amount of the CS.sub.2 is given as 
percentage based on the weight of the chlorine feed and calculated on 
elemental sulfur basis. In Example 5, in which elemental sulfur was used 
in place of CS.sub.2, the reaction tube was set in a vertical position and 
the sulfur was supplied by means of a powder feeder. 
The downstream of the reaction tube was connected to a condenser 
(methanol-dry ice trap) so that the gas discharged from the reaction tube 
was cooled for the recovery of silicon tetrachloride. After the 
termination of the chlorination, the reaction mixture remaining in the 
alumina boat was allowed to be cooled to room temperature while feeding 
argon and then analyzed for determining the conversion of SiO.sub.2 into 
SiCl.sub.4. The conversion was determined from the contents of SiO.sub.2 
before and after the chlorination. The results are summarized in Table 1. 
COMATIVE EXAMPLES 1-4 
Examples 1 and 6-8 were each repeated in the same manner as described 
except that CS.sub.2 was not supplied to the reaction tube. The results 
are also shown in Table 1. From the results shown in Table 1, it will be 
appreciated that the addition of sulfur or CS.sub.2 improves the 
conversion. 
TABLE 1 
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Example 
Silica-containing 
Carbonaceous 
Sulfur or Sulfur-containing material 
Conversion 
No. raw material 
material 
(amount of S based on weight of Cl.sub.2) 
(%) 
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1 hull ash coke CS.sub.2 (50%) 58.9 
2 hull ash coke CS.sub.2 (30%) 56.4 
3 hull ash coke CS.sub.2 (10%) 42.0 
4 hull ash coke CS.sub.2 (10%) 59.1 
5 hull ash coke S.sup. (10%) 57.8 
Comp. 1 
hull ash coke -- (0) 31.5 
6 carbonized hull 
-- CS.sub.2 (10%) 90.5 
Comp. 2 
carbonized hull 
-- -- (0) 81.0 
7 silica sand 
coke CS.sub.2 (10%) 15.7 
Comp. 3 
silica sand 
coke -- (0) 6.8 
8 hull ash carbonized 
CS.sub.2 (10%) 93.1 
hull 
Comp. 4 
hull ash carbonized 
-- (0) 66.8 
hull 
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EXAMPLES 9-14 
Example 4 was repeated in the same manner as described except that each of 
the potassium compounds shown in Table 2 was further incorporated into the 
mixture of the rice hull ash and the coke in an amount of 10% based on the 
weight of the SiO.sub.2 contained in the rice hull ash. The results were 
as shown in Table 2 together with the result of Example 4. 
EXAMPLE 15 
Example 6 was repeated in the same manner as described except that 
potassium hydrogensulfate was mixed with the carbonized rice hull in an 
amount of 10% based on the weight of the SiO.sub.2 contained in the 
carbonized rice hull. The result is shown in Table 2 together with that of 
Example 6. 
EXAMPLE 16 
Example 7 was repeated in the same manner as described except that 
potassium hydrogensulfate was further incorporated into the mixture of the 
natural silica sand and the oil coke in an amount of 10% based on the 
weight of the SiO.sub.2 contained in the silica sand. The results were as 
shown in Table 2 together with the result of Example 7. 
TABLE 2 
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Conver- 
Exam- Silica-containing 
Carbonaceous 
Potassium 
sion 
ple material material compound 
(%) 
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4 hull ash coke -- 59.1 
9 hull ash coke KHSO.sub.4 
93.3 
10 hull ash coke KCl 95.1 
11 hull ash coke K.sub.2 CO.sub.3 
88.0 
12 hull ash coke KOH 91.9 
13 hull ash coke KNO.sub.3 
89.0 
14 hull ash coke K.sub.2 SO.sub.4 
91.7 
6 carbonized hull 
-- -- 90.5 
15 carbonized hull 
-- KHSO.sub.4 
97.7 
7 silica sand coke -- 15.7 
16 silica sand coke KHSO.sub.4 
25.9 
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From the results shown in Table 2, it will be apparent that the addition of 
a potassium compound improves the conversion. 
EXAMPLES 17-21 
Example 9 was repeated in the same manner as described except that the 
amount of the coke in the mixture was varied to provide a SiO.sub.2 /C 
weight ratio as shown in Table 3 and that the mixing time was changed from 
10 minutes to 1 minute. In Example 21, no coke was used. The results are 
shown in Table 3. 
EXAMPLES 22 AND 23 
Example 21 was repeated in the same manner as described except that the 
feed rate of the CS.sub.2 was increased as shown in Table 3. The results 
are shown in Table 3. 
COMATIVE EXAMPLE 5 
Example 21 was repeated in the same manner as described except that the 
CS.sub.2 was not supplied. The result was as shown in Table 3. 
TABLE 3 
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SiO.sub.2 :C 
Amount of S based on 
Conversion 
Example ratio weight of Cl.sub.2 (%) 
(%) 
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17 2:1 10 78.8 
18 4:1 10 70.7 
19 8:1 10 59.1 
20 20:1 10 48.6 
21 1:0 10 35.6 
22 1:0 30 58.7 
23 1:0 50 67.0 
Comp. 5 1:0 0 4.6 
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From the results shown above, it will be seen that the conversion becomes 
higher as the amount of the coke is increased. Even if no coke is used, 
the conversion can be improved by increasing the amount of CS.sub.2.