Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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The present invention relates to the production of
silica and in particular to the production of silica by flame
hydrolysis.
The production of silica by pyrogenic means, for ex-
ample, by subjecting silicon tetrachloride to a flame hydrolysis
is known. These kinds of silica are, for example, the different
silica types marketed under the name of Aerosil ~ They have
varying particle sizes ranging from 7 to 40 nm and, therefore can
be used in greatly varying fields of application, as for example
for the thickening of liquid systems.
The specific surface area, measured in sq m per gram
according to BET (not the particle si~e) is usually used as the
typical characteristic quantity. Insofar as silicas free from
pores are concerned these two quantities are closely correlated.
A characteristic which is just as important is the
thickening effect of the different silica types in a liquid system,
since most of these silica types are used as thickening and thixo-
tropic agents. This characteristic is a function of the BET sur-
face area so that in conventional silica types which are produced
by means of a flame hydrolysis a specific thickening effect can
also be attributed to a specific surface area.
This kind of correlation is shown graphically in
Figure 1 of the accompanying drawings, in which:-
Figure 1 shows the thickening effect as a function ofthe BET surface area for conventional silicas;
Figure 2 shows the thickening effect as a function of
the quantity of added water vapour in correlation to the BET
surface area for silicas produced by means of the process accord-
ing to the invention when the additional water vapour is passed in-
to the reaction mixture before reaching the combustion chamber;and
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Figure 3 shows the thickening effect as a function of
the quantity of added water vapour in correlation to the B~T sur-
face area for silicas produced by means of the process according
to the invention when the additional water vapour is fed into the
flame.
In -the known processes of flame hydrolysis of silicon
halogen compounds in a hydrogen flame using the hydrolysis of
silicon tetrachloride-air or oxygen as an example, hydrogen and
silicon tetrachloride are mixed with one another and burned off
in such a ratio that the hydrogen can burn completely while form-
ing water vapour and that the silicon tetrachloride can react
quantitatively with the water vapour formed while forming SiO2.
The reactions which proceed consecutively and concurrently can be
represented by the equations 1, 2 and 3:
) 2 H2 2 ~~ 2 H20
2) 2 H20 + SiC14 -~ SiO2 + 4 HCl
-
H2 + 2 + SiC14 ~ Si2 + 4 HCl
These equations are also applicable when other silicon
halogen compounds are used as starting substances. For this pur-
pose vaporizable inorganic halogen compounds and/or organic halo-
gen compounds of silicon are used. For example SiHC13, SiC12H2
or SiC14 are useful as inorganie halogen eompounds and CH3SiC13
(CH3)2 SiC12, (CH3)3SiCl, CH3-CH2-SiC13 or (CH3-CH2)3SiC12 as
organie halogen compounds.
For earrying out the combustion hydrolysis the compon-
ents hydrogen, oxygen or air and silicon tetrachloride are fed,
either separately or premixed, to a type of burner sueh as that
shown diagrammatieally in the US Patent 2,990,249. The quantity
of hydrogen is so ealeulated that, during the formation of water
vapour, it is suffieient for a quantitative reaetion of the
ehlorine atoms on the silieon atom while forming hydrogen ehloride.
A slight exeess assures that the reaetion proeeeds not only
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quantita-tlvely but also exceedingly fast. It is not possible to
use an arbitrarily high excess of hydrogen, reiative to the
amount of silicon tetrachloride. Quite apart from the fact that
this measure would unnecessarily increase the costs of the pro-
cess~the hydrogen excess is limited in that not only does this
reaction component constitute the component required for the
hydrolysis of the chloride but it also supplies the energy. If
the increase of the hydrogen excess is too large, the effect
would be that the temperature of the flame rises with unfavour-
able consequences for the quality of the SiO2 reaction products.
It is also possible to reduce the reaction temperature by adding
quantities of air or oxygen which exceed the stoichiometric
amount. With this measure the reaction temperature is usually
influenced and the fine division or the specific surface area of
the reaction products thus is fixed. However, this possibility
is limited since the discharge velocity from the burner orifice
must move within relatively narrow limits and the increase of
the quantity of inert gas is at the expense of the performance
of the apparatus.
The known process for producing silicas by means of
flame hydrolysis according to the US Patent 2,990,249 has the
disadvantage that it is not possihle therewith to change the
correlation between specific surface area and thickening pattern
and to adjust the thickening effect of the silica independently
of the value of the specific surface area.
The present invention provides a process for the con-
trolled production of silica by means of flame hydrolysis, char-
acterized in that additional amounts of water vapour, which do
not result from the combustion of hydrogen or of hydrogen-
containing gases required for the flame hydrolysis, are intro-
duced into the reaction mixture.
The introduction of the additional water vapour can be
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carried out in various ways. Thus, the additional water vapour
can be passed into the mixing chamber of the burner via a sep-
arate conduit. In another preferred embodiment of the process
according to the invention the additional water vapour may be
fed into the hydrogen or air supply to the burner whereby a
mixture of hydrogen and water vapour or a mixture of air and
water vapour is fed to the burner. For the mixing with water
vapour the hydrogen or the oxygen-containing gas may be passed
through a water evaporator at a temperature ranging from 20C
to the boiling temperature of the water.
In a further preferred embodiment the additional water
vapour can be admixed with the chloro silicon compound before
the latter compound enters the burner. However, the temperature
of the mixture from chloro silicon compound must be kept above
the dew point in order to avoid separation of silica.
In yet another embodiment of the process according to
the invention the additional water vapour can also be fed into
the flame, the region of the actual silica formation. This can
be done by means of a lance, which is passed axially through the
burner and is allowed to project from the burner orifice. How-
ever, it is important that the mixing procedure is as fast and
homogeneous as possible so that the in~luence of the additional ~-
water vapour on the occurrence of the reaction and on the form-
tion of the silica can be fully effective since the influence of
the partial pressure of the water vapour decreases with the dis-
tance from the burner orifice. At the outlet of the so-called
flame tube, i.e. a heat exchanger zone, into which the flame
gases are usually injected no influence on the formation of pro-
perties of the silica upon admixing additional quantities of
water vapour is detectable.
The admixed quantity of water vapour may be varied
within wide limits.
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The wa-ter vapour is preferably added at a temperature
from 150 to 250C and at an excess pressure of 10 to 20 atmos-
pheres, particularly at a temperature from 185 to 210C and at
an excess pressure of 12 to 13~atmospheres.
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Any known inorganic/and/or organic silicon h~e~e~
compound can be used as the starting substance.
The ratio of water vapour to starting substance may be
from 0.1 to 1 kg of water vapour per kg of starting substance.
In a further embodiment of the invention, e.g. a hydro-
carbon, instead of pure hydrogen, may be used as the burner gas.
These hydrocarbons may be for exam le, propane and/or butane.
An apparatus like that described in the US Patent
2,990,249 can be used as the burner. However, a closed burner
system, in which no secondary air can penetrate the fîame, may
also be usea.
The controllability of the thickening effect is evident
from the accompanying drawings.
According to Figure 1 the thickening effect as a func~
tion of the BET surface area is shown graphically for silicas
~0 producèd b~ means of conventional processes. The following
values are obtained for the individual silicas:
surface area thickening
130 sq m/g 2000 mPas
150 sq m/g 2700 mPas
200 sq m/g 3100 mPas
300 sq m/g 3500 mPas
380 sq m/g 3000 mPas
These thickening values were determined from a polyester
reference system.
This poiyester reference system is produced by mixing
80 parts by weight of Ludopal P6 (a trademark) with 11.4 parts
by weight of monostyrene and 7 parts by weight of styrene con-
,,
taining 1 part by weight of paraffin. This system is also used
in all the further determinations of the thickening.
According to Figure 2 the entire pattern of properties,
particularly the specific surface area and the thickening pattern
of the silicas obtained shifts as the amount of water vapour
- added increases.
The curve a) thus shows the correlation between the
specific BET surface area and the thickening pattern of silicas
obtained by means of conventional processes by varying the
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excess of air. Thus, this measure makes it possible only to
- attain the combination of properties of the silicas shown in
Figure 1.
;~ However it is evident from the curve b) that as the
' addition of water vapour to the reaction mixture prior to the
combustion increases both the thickening pattern and the specific
BET surface area first extend far beyond the values known from
the conventional silicas according to the curve a) whereupon
they substantially decrease with larger quantities o~ water vapour
, and new combinations of properties are attained.
;~ 20 For example, it is thus possible to produce silicas
~ having identical BET surface areas but greatly varying patterns
,` of thickening. -
` According to Figure 3 the entire pattern of properties
` of the silicas obtained, particularly the specific BET surface
area and the pattern of thickening, shifts as the ~uantity of
water vapour added increases.
Thus, the curve a) shows the correlation between
specific BET surface area and the pattern of thickening in
silicas obtained by means of conventional processes by varying
i 30 the excess of air. Thus, this measure makes it possible (as
shown in Fig. 1 and 2) to attain only a combination of properties
in which the thickening effect is a function of the specific BET
surface area.
,
However, it is evident from the curve b) that as the
addition of water vapour to the flame increases both the pattern
- of thickening and the specific BET surface area of the silicas
obtained by means of the process according to the invention
i follow a path which differs completely from that of the curve a)
corresponding to the known silicas, as compared with a basic
standard. New combirations of properties can thus be attained.
For example, it is thus possible to produce silicas
having identical BET surface areas but greatly varying patterns
of thickening.
The process according to the invention is further des-
cribed by way of the following Examples.
Example 1
6.2 kg of silicon tetrachloride are evaporated and
, mixed with 2.2 cu m of hydrogen and 5.8 cu m of air in the mixing
chamber of a burner. The gas mixture burns from the outlet and
is sucked into the cooliny system by means of vacuum. After
separation from the hydrogen chloride-containing gas mixture 2.2
kg of a highly dispersed silica having a specific surface area
of 200 sq m per gram and a thickening of a polyester reference
system of 3100 mpascals is obtained.
Example 2
The same procedure as in Example 1 is followed, but in
addition to the substances mentioned therein, 0.5 kg of water
vapour per hour is injected into the mixing chamber of the
burner. The silica obtained has a specific surface of 466 sq m
and a thickening value of 3910 mpascals.
Example_3
.
The same procedure as in Example 1, is followed by 1.~
kg of water vapour per hour are additionally added in the manner
described in Example 2. The silica has a surface area of 277
sq m per gram and a thickening value of 10~0 mpascals.
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Example 4
. . _
The same procedure as in Example 1 is followed, but 0.5
kg of water vapour per hour is injected with a probe into the
flame axis at a distance of 1 cm from the burner orifice. The
silica has a specific surface of 309 sq m per gram and a thicken-
ing value of 4040 mpascals.
_xample 5
The same procedure is followed as in Example 1 but with
the difference that 3 kg of water vapour are injected into the
.
flame axis at a distance of 10 cm from the burner orifice. The
BET surface area of the silica is 212 sq per gram and the thick-
- ening value 1105 mpascals.
; The values obtained in the above examples of the pro-
:' eess according to the invention have been listed in Table I.
The values for the specific surface area and for the
thiekening eorrespond to the eurves b) in Figure 2 and 3.
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