SPRAYABLE PRECIPITATING SILICA SUSPENSION

SUSPENSION DE SILICE PRECIPITEE, APPLICABLE EN AEROSOL

English Abstract

ABSTRACT OF THE DISCLOSURE
The present invention provides a process for producing
a sprayable highly concentrated precipitating silica suspension
which has a low grit content and contains 12%, preferably more
than 18% of solids, which comprises continuously feeding a
silica filter cake and a dispersing agent by means of feed members
into a dissolver tank while maintaining a pH value of not more
than 4, and stirring the mixture in said tank by means of a
stirring dissolver, the ratio of the tank diameter to the dia-
meter of the stirring dissolver being 3:1, the maximum ratio of
dissolver tank diameter to height of filling of the tank being
1:2, and at least 1.5 to 2 kg of silica suspension being present
in the tank per 1 kg of silica filter cake fed therein, said
stirring being carried out by the dissolver at a peripheral speed
of 20 to 30 metres per second, the silica filter cake and
dispersing agent being withdrawn from a zone of maximum turbu-
lence by a separating member or a mixer.

Claims

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A process for producing a sprayable highly concen-
trated precipitating silica suspension which has a low grit
content and contains at least 12% of solids,
which comprises continuously feeding a silica filter cake and a
dispersing agent by means of feed members into a dissolver
tank while maintaining a pH value of not more than 4, and stirring
the mixture in said tank by means of a stirring dissolver, the
ratio of the tank diameter to the diameter of the stirring
dissolver being 3:1, the maximum ratio of dissolver tank diameter
to height of filling of the tank being 1:2, and at least 1.5 to
2 kg of silica suspension being present in the tank per 1 kg of
silica filter cake fed therein, said stirring being carried out
by the dissolver at a peripheral speed of 20 to 30 metres per
second, the silica filter cake and dispersing agent being with-
drawn from a zone of maximum turbulence by a separating member or
a mixer.
2. A process according to claim 1, in which the feed
member is a single screw.
3. A process according to claim 1, in which the feed
member is a double screw.
4. A process according to the claim 1, 2 or 3, in
which the dispersing agent is an acid.
5. A process according to the claim 1, 2 or 3, in
which the dispersing agent is dilute aqueous sulphuric acid.
6. A process as claimed in claim 1, 2 or 3, in which
the dispersing agent is 20 to 30% sulphuric acid solution.
7. A process according to the claim 1, 2 or 3, in
which the dispersing agent and the silica filter cake are
simultaneously fed into the dissolver tank by a feed member.

8. A process according to the claim 1, 2 or 3, in
which the ratio of the diameter of the dissolver tank to that
of the stirring dissolver is 2.5:1.
9. A process according to the claim 1, 2 or 3, in
which the ratio of filling height of the tank to the tank diameter
is 1:1.
10. A process according to the claim 1, 2 or 3, in
which the dissolver is a double-disc dissolver.
11. A process according to the claim 1, 2 or 3 in which
the separating member is a wire strainer.
12. A process according to the claim 1, 2 or 3, in
which the suspension is drawn off by a forced circulation mixer
connected at an outlet end or by means of a mixer pump.
13. A process according to the claim 1, 2 or 3, in
which a portion of the withdrawn silica suspensions passing
through a wire strainer or through the mixer is returned to the
dissolver tank.

Description

1061056
The present invention relates to a process for producing
- a sprayable, highly concentrated precipitating silica suspension
which has a low grit content and contains at least 12%, preferably
more than 18% of solids.
Precipitating silica, which is produced in a conventional
manner by wet-chemical processes, must be dried in order to satisfy
engineering requirements. For this purpose a spray dryer may be
used. However, in this kind of dryer the nozzle and the fan
blower become clogged due to both the high solids content and
the structure of the silica filter cake. Therefore, the structure
of the silica filter cake must be such that the nozzle or the
fan blower of the spray dryer will not be obstructed.
It is known that in order to adapt t~e structure of
the silica filter cake to the re`quirements of the spray dryer,
the filter cake is diluted with water. However, for a favourable
dilution with water for the process a great amount of energy
must be used in order to remove the water from the silica filter
cake during the drying. In this method of treating the silica
filter cake the structure of the filter mass obtained is inhomo-
geneous and contains grit, i.e., it contains particles havingsizes of more than 45 ~m.
The present invention provides a process which so struc-
tures a silica filter cake that the consumption of energy of
vaporization required for the spray drying is only small and
that the filter mass contains a homogenous grit-free silica.
According to the present invention therefore there is
provided a process for producing a sprayable highly concentrated
precipitating silica suspension which has a low grit content and
contains at least 12%, preferably more than 18% of solids which
process comprises continuously feeding by means of feed elements
silica filter cake and dispersing agent into a dissolver tank while
maintaining a pH value of not more than 4, and stirring the
- 1 - ~

lQ61056
mixture in said tank by means of a stirring dissolver, the
ratio of the tank diameter to the diameter of the stirring
dissolver being 3:1, the maximum ratio of dissolver tank diameter
to the height of filling of the tank being 1:2, and at least
1.5 to 2 kg of silica suspension being present in the tank per
1 kg of silica filter cake fed in, said stirring being carried
out by means of the dissolver at a peripheral velocity of 20
to 30 metres per second, the silica filter cake and dispersing
agent being withdrawn from a zone of maximum turbulence by way
of a separating member or a mixer.
A single screw or a double screw may be used as the feed
member.
The dispersing agent, which is preferably an aqueous
dilute acid, and more preferably a 20 to 30% H2SO4, can be fed
to the dissolver by a feed member simultaneously with the silica
filter cake. The ratio of the diameter of the dissolver tank
to that of the stirring dissolver may be approximately 2.5:1 while
the ratio of filling height to tank diameter can be 1:1. A
double-disc dissolver can be used as the stirring dissolver. A
wire strainer can be used as the separator or the suspension
can be drawn off by a forced circulation mixer connected at the
outlet end or by means of a mixer pump. A portion of the silica
suspension passing through the wire strainer or through the
mixer can be returned to the dissolver tank. Because of this
recycling shearing of the silica present in the filter cake the
suspension has the desired freedom from lumps and grit.
The advantages of the process according to the invention
lie inter alia in a restructuring of the silica filter cake such
that the resulting silica suspension is homogeneous and free from
grit and can be subjected to spray drying without additional
consumption of energy and additional work.
The present invention will be further illustrated by

1061056
! way of the following Example in conjunction with the accompanying
drawing in which Eig. 1 is a schematic representation of an
apparatus for effecting a process according to one embodiment of
the present in~ention.
Example 1
260 kg of silica filter cake having a water content of
82% by weight are fed into a 1.5 cu m dissolver tank 4 per
minute by means of the double screw,l operating at 295 r.p.m.
through the narrowed discharge 2 simultaneously with 0.6 litre
of a dilute aqueous 30% sulphuric acid passing from pipe 3 in
order to increase the mixing-kneading effect. The concentration
of acid is maintained such that the silica suspension has a pH
value of 3. The dissolver tank 4 is cylindrical and has a diame-
ter of 1.2 m, a height of 1.5 m. The dissolver 5, which is
a double-disc dissolver, has a diameter of the discs 6 and 7
of 0.4 m and is driven by the electric motor 8, which has an out-
put of 36.7 kw (50 hp). The rotational speed of the two discs
6 and 7 is 21 metres per second (1000 r.p.m.). By means of the
immersion probes 9 and 10 the height of the suspension in the
tank 4 is kept between 0.8 and 1.20 metres, so that for 1 kg
of silica filter cake fed there are 1.5 kg of silica suspension
in the dissolver tank 4. The silica suspension is drawn off from
the zone of maximum turbulence by means of the wire strainer 12.
A portion of this silica suspension is recycled through pipe
13 and the pump 14 in order to further decrease the proportion of
grit and lumps. The fact that the liquid phase remains intact
and is sprayable in this process if a critical concentration is
not exceeded is surprising. The silica suspension obtained by
means of the process according to the invention has a viscosity
of approximately 200 cp, as measured in a Haake viscotester.