Note: Descriptions are shown in the official language in which they were submitted.
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A method for the production of porous silica granules which
have mechanical strength and withstand moisture
The present invention relates to a method for the production
of porous silica granules which have mechanical strength
and withstand moisture, by granulating a mixture of a
finely-ground silica-containing raw material, such as
synthetic silica or a silica skeleton separated from a
silicate such as phlogopite, of a binder, and of water, and
by drying the thus obtained granules. Granules prepared by
the method according to the invention are well suited for
the production of chemical adsorbent, for example.
Finely-divided materials can be agglomerated, or granulated,
in very many different ways, for example by mixing,
compression, thermal treatment, by spray and dispersion
methods, and by agglomeration from a liquid medium. The
joining of small particles to form agglomerates can occur
with the help of solid-material bridges, stationary or moving
liquids, internal and external forces of molecules, and
mechanical bonds. Many kinds of additives can be used for
increasing the particle size, such as binders, lubricants,
and wetting agents. Only adsorbent having a large
internal surface have technical importance. As a result of
activation or a special preparation method, a large number
of small pores and a large specific surface area can be
obtained in the adsorbent. In addition to the surface area,
the size of the pores is decisively important. Most of the
active surface is within micro pores. The presence of
macro pores is important for diffusion velocity, In addition
to the surface properties, the mix must fulfill certain
technical requirements, especially as regards mechanical
strength. The more porous the material is, the more binder
it in general requires in order to cohere. A high amount of
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binder for its part tends to clog the pores in the granule
and to decrease its specific surface area.
Synthetic silicas are good aggregates for adsorbent with
respect to their chemical and physical properties. They are
well resistant to moisture, to changes in temperature and
pressure, and to most substances used in adsorption
applications. They have a large specific surface area, a
high pore volume, and a high micro-meso-macroporosity.
However, for adsorption applications they must first be
granulated. The granulating method should be one which
affects the porosity of the initial material as little as
possible but which at the same time gives sufficient strength
to the granules so that they withstand mechanical handling.
The methods presented in the literature for the granulating
of silicas are of the kinds which produce either porous but
too soft ~ranules,or strong granules which are too dense
for adsorption applications. The strength of porous granules
can be increased by using more binder; in this case, however,
the amount of micro porous adsorbent decreases and its
effectiveness is reduced.
In addition to synthetic silica, a silica skeleton separated
from a silicate has a pore distribution suitable for
adsorption applications. However, it is necessary to use
a binder in granulating it in order for the granules to
obtain a mechanical strength sufficient for their further
use. The literature presents different methods for the
preparation of silica granules, but they have been found
unsuitable for adsorption applications, mainly for the
following reasons:
binders (e.g. lye, water-glass and starch) by means of
which sufficiently strong granules were obtained either
- significantly decreased the specific surface area of the
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silica (lye, water-glass) and thereby decreased the usability
of the granules for adsorption applications, or
- the binder (starch) itself acted on the chemicals used
for the impregnation of adsorption mixes (starch is a
reducer), thereby limiting the possibilities for use of the
product. The reducing action of starch can, it is true, be
eliminated by thermal treatment, but then the preparation
costs of the product also increase.
By using binders which did not decisively alter the porosity
of the initial product (for example silica sol), sufficiently
strong granules were obtained only when binder had been
used at over 10 % of the weight of silica. Even then the
resistance of the granules to abrasion was not sufficiently
high. The costs of the binder also rose too high.
The object of the present invention is therefore to provide
an economical method for the preparation of silica granules
which are at the same time both very porous and have
mechanical strength and withstand moisture. The object of
the invention is in particular to provide a method for
the preparation of silica structures suitable for adsorption
mixes.
The main characteristics of the invention are given in the
accompanying claims.
It has now been observed surprisingly that by using burnt
lime and/or cement as the binder it is possible to obtain
from a finely-ground silica-containing raw material, such
as synthetic silica and a silica skeleton separated from a
silicate such as phlogo~ite, very porous silica granules
which are at the same time mechanically very strong and
withstand moistening.
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Burnt lime and/or cement is added at 2 % at minimum,
preferably at approximately Lowe I, calculated from the
weight of the silica-containing raw material.
It has been observed in particular that by using burnt lime
as the binder it is possible to granulate from silica, for
example by means of a tray granulator, porous spherical granules
which withstand mechanical handling well, in such a manner
that the specific surface area of the silica being
granulated does not change to a noteworthy degree. When the
granulation is carried out from a paste, cement can
advantageously be used as a binder besides, or instead of,
burnt lime. This procedure it an especially advantageous
alternative when the silica is not dried but the granulation
is carried out from a slurry or a filter-dry cake. Tray
granulation is suitable above all for dried silica.
With respect to the strength of the granules it is important
that they are allowed to prudery slowly in the presence of
air, for example for 1-2 days at approximately room
temperature. The result is further improved if the redrying
is carried out in air which contains carbon dioxide. The
final drying can be carried out at elevated temperature by
keeping the granules, for example, for 1-2 hours at above
lo C, preferably at Lowe C. Thereby the impregnability
of the granules is maximized.
By the method according to the invention it is possible to
prepare granules which are well suited for adsorption
applications for example. The desired pore and strength
properties have been achieved without chancing the original
advantageous properties of the silica. The mechanical strength
of the prepared granules is, in addition, so high that they
can be used for air purification applications. In addition,
the granules give off very little dust.
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The invention is described below in greater detail with
the aid of examples.
Reference Example 1
A silica sol having a dry matter content of 15 by weight
was added to 25 g dried and ground silica skeleton
separated from phlogopite, in such an amount that the silica
sol content of the mix, calculated as dry matter, was a) 5,
b) 10, c) 15, d) 20 and e) 30 of the weight of the silica.
When necessary, water was added to the mixture in such an
amount that a suitably moist paste was obtained (water
approximately 60 % by weight).
my using a perforated plate, cylindrical granules (diameter
3 mm, height 4 mm) were formed from the mixes, and the
granules were dried first for a few days at room temperature,
then overnight at 110 C. The specific surface area of the
prepared granules was determined by N2-adsorption (BET).
The resistance of the granules to compression and abrasion
was checked by the hand.
Granules which contained dry matter of silica sol at most
10 % of the weight of silica fractured easily when pressed.
Their surface gave off a large amount of dust when they were
chosen. When the binder amounted to more than 10 % of the
weight of silica, the resistance of the granules to
compression was relatively good but their resistance to
abrasion continued to be poor. The BET surface area of the
granules decreased when the amount of binder increased from
205 mug to 110 mug corresponding to 5 % and 30 % silica
sol as dry matter of the weight of silica.
Reference Example 2
1.25 g starch was mixed with 100 g silica slurry having a
solids content of 25 percent by weight. While mixing by
Jo
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means of a magnetic mixer the temperature was raised close
to 100 C, at which time the mixture converted to a Mel. By
means ox a perforated plate cylindrical pieces (diameter 3
mm, height 4 mm) were formed from the cooled mix. The
granules were dried first at room temperature, then at 110
C. The resistance of the granules to compression and
abrasion was checked by the hand. The specific surface area was
determined by N2-adsorption (BET).
When pressed, the granules felt strong and, when they were
shaken, they gave off very little dust. Their BET surface
area was 310 mug
When the granules prepared in the above manner were
impregnated with a KMnO4 solution, the starch used as the
binder reduced the permanganate to manganese dioxide. At
this time a significant proportion of the effectiveness of
the mix intended for adsorption applications was lost. The
reducing action of the starch was almost completely
eliminated when the granules were heat treated at 500 C.
Reference Example 3
12.5, 35.5, 72.6 and 126 ml 2-N Noah solution were added
to 150 g moist silica (HO 65 % by weight). The mix was
stirred well and dried to produce a moldable paste.
Cylindrical pieces (diameter 3 mm, height 4 mm) were
prepared from the paste by means of a perforated plate. The
pieces were dried at 110 C, and the resistance of the thus
obtained granules to abrasion and compression was evaluated
by the hand. Their specific surface area was determined by
N2-adsorption. The results are shown in Table 1 below.
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Table l
ON Wash solution BET Strength
ml N go
a) 12.5 205 Brittle, crumbled easily
b) 35.5 110 " " "
c) 72.6 25 Relatively strong, did not give off dust
d) 125 I Strong, did not give off dust
A similar decrease in the surface area and a similar
improvement in strength were observable when corresponding
sodium additions were made in the form of water-glass. In
this case, a water-glass solution was used which had a dry
matter content of 25 % by weight and Sue : NATO = 2.5
(molar ratio).
Example 4
Finely-ground burnt lime at 5 % by weight was added to
finely-ground silica. This mixture was fed onto a rotating
granulating tray while water was sprayed onto it. The
granulation conditions were adjusted in such a way that a
maximum number of granules having a size of 2-5 mm was
obtained. The granules were dried at room temperature in
the presence of air for 1-2 days and finally at 105 C for
about 2 hours.
The pore distribution of the granules thus prepared and
their specific surface area were analyzed both by using a
Hug porosimeter and by nitrogen adsorption. By using a Hug
porosimeter the surface area obtained was 158 mug and the
pore volume 1.38 ml/g, and respectively by N2-adsorption
the surface area obtained was 149 m go When felt by the
hand the granules were strong, and when they were screened,
very little dust detached from them. The granules were
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impregnated with a KMnO4 solution. No changes were observed
in the strength of the granules. It should be pointed out
that, when granules had been prepared from both natural
silicates and from synthetic silicates, by using burnt lime
as the binder, the granules broke during impregnation.
Reference Example 5
Finely-ground cement was mixed at 5 %, calculated from the
weight of the silica, with an aqueous slurry of a wet-ground
silica skeleton of phlogopite. The tough paste thus obtained
was formed into cylindrical pieces (diameter 3 mm, height
3-5 mm) by means of an extruder-type granulator. The
granules were first dried in the presence of air for 2 days
at room temperature and thereafter for 2 hours at 110 C.
The pore distribution and specific surface area of the
granules prepared in this manner were analyzed both by means
of a Hug porosimeter and by nitrogen adsorption. A surface
area of 70 mug and a pore volume of 1.06 ml/g were obtained
by means of a Hug porosimeter and respectively a surface
area of 220 m go by nitrogen adsorption. When felt by the
hand the granules were found to be even stronger than those
prepared in Example 4. Their resistance to impregnation was
also good. Cement-bonded silicate granules did not withstand
moistening.
The pore size distributions of the granules prepared in
Examples 4 and 5 are shown graphically in Figure 1.
The difference between the surface areas measured by nitrogen
adsorption and a Hug porosimeter describes the micro porosity
of the product. The granules prepared by the method according
to the invention differ from commercial adsorbent above
all with respect to Marco porosity (pore diameter over 200 no).
The MicroPro volume of commercial adsorbent (for example
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XMnO4-impregnated Allah mixes presented in US. Patent
3,226,332) is only 10-30 % of the macro porosity of granules
prepared by the method according to the present invention.
The pore volume of granules intended as adsorption mixes is
important both for their action and for their impregnability
(the higher the pore volume, the more impregnation liquid
can be caused to be absorbed into the granules). Those
properties of granules prepared by the methods described
in the examples above which are crucial with respect to
adsorption applications have been compiled into Table 2.
Table 2
Resistance to Resistance to Specific Pore Suitability
compression abrasion surface volume for adsorbing
M go ml/g mixes
Reference
example l
a) poor poor 205 no
b) good poor 110 no
" 2 good good 310 xx)no
" 3
a) poor poor 110 xxx)no
b) good good I xxxx)no
Example
4 good good 173 1.32 yes
" 5 good good 220 1.03 "
x) binder costs too high, resistance to abrasion poor
xx) reducing properties of starch are . hindrance
xxx) mechanical resistance poor
xxxx) surface area too small
