Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Pelletized silica
The subject of the invention is pelletized silica
particles, the method to produces them and their use.
It is known how to produce quartz (silica) glass powder by
hydrolyzing alkoxysilane at a specific pH to prepare a gel,
powdering the gel, and after drying, calcining the powder
(Japanese Patent Application Laid-open (KOKA1)
No. 62-17 69 28 (1987)).
Furtheron it is known how to produce synthetic quartz glass
powder by the following steps:
a) hydrolyzing an alkoxysilane to form a gel thereof
b) finely dividing the gel and then drying or drying the
gel and then finely dividing to form a powder, and
c) calcining the powder of step b) (US 5,516,350).
The particle size of the gel before the calcinaton step was
adjusted to a diameter of 60 to 900 pm. The reference is
silent about the particle size of the sintered quartz glass
powder and the feature on its flowability.
Furtheron it is known how to produce a monolith silica
glass article by the following steps:
- hydrolyzing a silicon alkoxide in solution to form a
hydrolyzed solution
- adding an effective amount of fumed silica to the
hydrolyzed solution to form a sol solution
- gelling the sol solution to form a gel
- drying the gel to form a dry gel and
- sintering the dry gel to form a glass thereby to form a
large monolithic silica glass article (US 4,801,318)
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This process does not produce a free flowing powder of
synthetic quartz.
Furtheron it is known ho to prepare inorganic oxide-based
materials of spherical form with substantial monomodal
distribution, by the following steps:
- forming a sol of at least one of the inorganic oxides by
hydrolyzing a tetraalkoxysilane
- adding to said sol a solvent immiscible with the sol
- finely dispersing the obtained two-phase mixture into a
dispersion of particles of equal diameter
- growing said particles by limited coalescence to the
desired size and gelling said dispersion (of coalesced
droplets) by adding a second solvent containing a
gelling agent, and
- removing the solvent.
The spherical material shows in the case of monomodal
distribution a diameter between lnm and 1.000 nm. It can be
used as a support for catalysts for the polymerization and
copolymerization of olefinically unsaturated compounds (EP
0 537 850 Al).
Furtheron it is known how to prepare spherical silica
particles by the following step:
(A) preparing a colloidal silica solution by hydrolyzing a
silicon alkoxide on an aqueous media in the presence
of mineral or organic acids
(B) possible dispersing fumed silica in the resulting
colloidal silica solution
(C) mixing either the pure (step A) or the hybrid (step B)
silica sol in an organic media constituted by
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monofunctional aliphatic alcohols R-OH or mixtures
thereof
(D) emulsifying the so obtained mixture
(E) gelifying either the pure or the hybrid silica sol by
bringing the previously obtained emulsion into contact
with a basic solution
(F) heat treatment of the resulting gel
The spherical silica particles show a particle diameter
within the range of from 10 to 100 micrometers
(EP 0 653 378 Al).
The transportation, handling and storage of powder material
is largely affected by the flowability and hardness of the
particles, which consequently have a big commercial impact
on the price and the quality of the final product. In
particularly the efficiency of the mixing of powders could
be strongly driven by the inhomogeneous particle size
distribution, particles agglomeration and caking issues.
With the expression "free flowing powder composition" used
throughout this specification is meant a powder (made by
milling, micropelletizing, or similar technique) of which
the particles consist of a composition as defined above and
of which the particles do not adhere to one other. The size
of the powder particles is expressed in terms of the
particles diameter. In general, this size is determined by
sieving and is independent of the shape of the particle.
On the contrary cohesive powder are those powders in which
the cohesive forces among the particles are very important
(strengthwise). As per definition a free flowing powder
does not contains fines.
A conventional spray drying technique in which a spray
dryer mixes a heated gas with an atomized (sprayed) liquid
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stream within a vessel (drying chamber) to accomplish
evaporation and produce a free flowing dry powder with a
controlled average particle size, is that available for
instance by SDS Spray Drying Limited. With such spray drier
it is possible to produce particle with averaged dimension
of 300 micron with rather narrow particle size
distribution.
Another even more elegant way to get the pelletization of
metal oxides particles is to coat with chemicals like
polymers or oils or waxes. In this case the properties of
the granules are significately changed for this reason very
little is reported in the scientific literature.
Even though in the literature is reported a plethora of
methods for the pelletization of metal oxides, at
industrial level operation like: the mixing and the
transportation, along i.e. moving belt or in pneumatic
conveyors are still affected by the well known problems
such as:
a) segregation: mainly due to differences on size of the
particles, and to a minor extend differences in
particle density. More in detail the forces that drive
the segregation are: Van der Walls forces,
electrostatic forces, liquid bridges, solid matter
bridges and entanglement.
b) Percolation, during transit test granules with small
sizes can gradually move under the bigger ones and
thus leads to a separation of the differently sized
particles.
When it comes to purity of the particle a major importance
plays the silica. In fact nowadays there is a growing
demand of silica with high purity grade for new high end
applications, such as: the inner part of crucibles, optical
fibers and components for microelectronics among the
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others. Unfortunately on the market there isn't a large
availability of such a product, because of the very high
costs and the very complex production procedures.
The subject of the invention are pelletized silica
5 particles, which are characterized by a round shape and a
monomodal particle size distribution.
Particles of silica according to the invention have the
said silica as core with all around Si02 obtained via
hydrolysis of liquid alkoxisilane.
Especially preferred the silica can be Si02 as fumed silica
or natural quartz. The natural quartz can be i. e. JOTA 4
Type from JOTA Corporation and natural quartz from
Norwegian Crystallites.
In a preferred feature of the invention the pelletized
inorganic oxide particles the core can consist of natural
quartz which is surrounded by silicondioxid, obtained via
hydrolysis of liquid alkoxisilane. The alkoxisilane can be
preferably tetraalkolisilane like tetraethoxy silane.
The said pelletized inorganic oxide particles according to
the invention can be characterized by a surface area
exceeding 50 m2/g.
The said pelletized inorganic oxide particles can have a
size by which at least 90% are bigger than 100 micron. At
least 90% of the pores can have a diameter lying between 50
and 1000 Angstrom.
Furthermore the alkoxisilane can be mixed with a soluble
salt in order to obtained pelletized inorganic oxide
particles doped with metals.
The free-flowing value is in the range from to more
preferably from 5 to 2 according to the method developed by
Degussa GmbH and published in the Technical Bulletin fine
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Particles Number 11 "Basic Characterisation of Aerosil Fumed
silica. Powders with bad flow behavior have a mark 5 while
powder with very good flow behavior are rated 1.
It is the object of this invention a method for the production of
the pelletized silica particles having a round shape and a
monomodal particle size distribution. According to the
invention, which is characterized in that inorganic silica, to be
pelletized, is added under stirring in a vessel containing acidic
water, when the dispersion is clearly homogeneous and without
lumps liquid alkoxide silane such as i. e. tetramethoxisilane
and/or tetraethoxysilane is then added very slowly to the
mixture. As consequence of the exothermic reaction the
temperature raises. The so obtained dispersion is then
transferred slowly by means, for example, of a cannula in a
vessel containing an organic solvent or a silicon oil previously
mixed with an ammonia derivatives kept under strong stirring.
The drops of silica dispersion in contact with the alkaline
organic solution form gelly particles that are collected on the
bottom of the reactor and then transferred to another vessel to
be washed abundantly with water or with a water/acetone solution.
The organic solvent or the silicon oil is then used for further
runs. The particles are then filtered and then the solvent is
extracted under supercritical or slightly subcritical conditions.
Alternatively the solvent can be removed by control drying under
controlled conditions (% humidity and T). The dried particles
are then calcinated at high temperature with oxygen for at least
1 hour in order to eliminate traces of the solvents from the
silica particles.
More in detail, the invention relates to a method for the
production of silica particles out of powder with broad size
distribution. The pelletization can be obtained by using
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sol-gel techniques which has been partially described in
the EP 0 537 850 Al.
According to one aspect of the invention there is provided a
method for the production of pelletized silica particles
having a round shape and a monomodal particle size
distribution, the method comprising:
dispersing silica particles in water at acidic pH
under stirring; adding a liquid alkoxisilane; transferring
the so obtained dispersion under strong stirring in to a
vessel containing an organic solvent; raising the pH by
addition of an inorganic base ammonia derivative; washing
the obtained gelly particles abundantly with water or with a
water/acetone solution; filtering and then calcinating the
particles at high temperature for at least 1 hour in order
to eliminate traces of the solvent from the inorganic oxide
particles.
According to another aspect of the invention there is
provided a method for the production of pelletized natural
quartz particles having a round shape and a monomodal
particle size distribution, the method comprising:
adding a liquid alkoxide silane to water at acidic pH
under stirring; and adding under strong stirring natural
quartz powder to the solution and transferring the so
obtained dispersion under strong stirring into a vessel
containing an organic solvent; raising the pH by the
addition of an inorganic base ammonia derivative; washing
the obtained gelly particles abundantly with water or with a
water/acetone solution; filtering and then calcinating the
particles at high temperature for at least 1 hour in order
to eliminate traces of the solvent from the inorganic oxide
particles.
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In a preferred subject of the invention the method can
comprise the following features:
At room temperature an acid is added to water in a vessel
until an acidic pH (2) is reached. Under strong stirring is
then added the silica powder very slowly and afterwards the
liquid siliconalkoxide like TEOS(Dynasil A from DEGUSSA
AG)= As consequence of the exothermic hydrolysis reaction
the temperature rises of few degrees. The mixture is then
kept under stirring for at least 20 minutes.
After vigorous stirring the solution is then poured in a
vessel containing an organic solvent by means of a cannula.
The pH is then risen by addition of an amine such as the
Primene type supplied by Rohm and Haas, till very alkaline
conditions are created, for example pH for 10% in water
solution. The temperature further rises. The particles are
then removed in continuous from the reaction batch and the
obtained particles are then washed abundantly with water,
in order to eliminate the residual solvent. The material so
obtained is then calcinated in a vertical furnace at
600 1500 C for 6 2 hours in order to eliminate residual
solvents.
The treated particles have a size in at least 90% of the
particle have a size higher than 100 micron. The
pelletization is obtained by using tetraethoxysilane as
pelletizasing agent.
In case of pelletization of natural quartz powder is
slightly different. At room temperature an acid is added to
water in a vessel until a very acidic pH is reached (2).
Under stirring the liquid siliconalkoxide like TEOS(Dynasil
A from DEGUSSA AG) is added to the mixture, as consequence
of the exothermic hydrolysis reaction the temperature rises
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of few degrees. Afterwards the natural quartz powder is
then added to the mixture and then kept under stirring for
at least 20 minutes.
After vigorous stirring the solution is then poured in a
vessel containing an organic solvent kept under strong
stirring. The pH is then risen by addition of an amine such
as the Primene type supplied by Rohm and Haas, till very
alkaline conditions are created, for example pH for 10% in
water solution. The temperature further rises. The
particles are then removed in continouos from obtained
particles are then washed abundantly with water, in order
to eliminate the residual solvent. The material so obtained
is then calcinated in a vertical furnace at 600 150 C
for 6 2 hours in order to eliminate residual solvents.
Elsewhere the patents relates to a free flowing powder
composition comprising hydrolyzed silica alkoxide like TEOS
and metal oxides and/or a mixture of thereof.
Particles are characterized by the fact that there is an
almost continuous phase between the pellettizasing agent
(the hydrolised silica alkoxide like TEOS) and the core
material, since the pelletisating agent is able to
impregnate the inorganic oxide particles.
The glassy particles obtained with this method according to
the invention are characterized in terms of: flowability,
microporosity and size distribution. Elsewhere, when the
said inorganic oxide is silica or silica quartz the method
allows to obtain a higher purity of the final product when
compared to that of the starting particles. The purity
increases approximately as direct function of the quantity
of pelletizasing agent silica alkoxide like TEOS used.
The starting pH of the aqueous solution with inorganic
oxide particles can be in the rage 1 to 4.
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The liquid alkoxi silane can be tetramethoxisilane
(CH3O)4Si and/or tetraethoxysilane (CH3-CH2-0)4Si.
The organic solvent can be an apolar organic solvent, which
has a dielectric constant lower than 60 at 20 C.
The salt to be added in order to obtain the doped silica
particles can be inter alias, aluminum acetate, aluminum
sulfate, aluminum ammonium sulfate, lead acetate, boric
acid, ammonium fluoride sulfate, ammonium fluoride.
The apolar organic solvent can be a liquid alkane such as
hexane, eptane, octane, nonane and alcohol such as:
propanol, butanol, pentanol, hexanol, eptanol, octanol,
nonanol, decanol and a aromatic compound like toluene,
benzene, nitrobenzene, chlorobenzene, dichlorobenzene,
quinoline, decaline and/or a mixture thereof.
Furtheron as organic solvent silicon oil can be used. The
silicon oil can be polydimethylsiloxane fluids like
Dimethicone from Wacker Chemie AG listed under the brand
name Wacker AK 50.
The organic base ammonia derivative can be
cyclohexyilamine, t-alkyl amine.
The pH after the addition of the organic base can be in the
range from 8 to 13, more preferably from 10 to 11 expressed
as 10% water solution.
The calcinations temperature can be between 300 and 700 C
more preferably between 300 and 600 .
The pelletized silica particles are characterized by round
shape and they are virtually without fines.
Analysis carried out on the pelletized silica particles
showed that the method according to the invention allows
obtain a monomodal distribution.
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The microporosity and surface area of the material have
been determined according to the method DIN 66131 by means
of ASAP 2010 instrument from Micromeritics; measurements
are carried out in liquid nitrogen.
5 Before the analysis, the material has been degassed at
300 C for 4h (P= 1*10ex-6).
Flowability has been determined via an extremely simple,
but nevertheless meaningful measurement method with
viscosity vessels resembling hourglasses. When this method
10 is employed, powder with good flow behaviour still flow out
of the glass vessels via a small discharge opening (see
Degussa Aerosil Silanes Technical Bulletin Fine particle,
pag 56-57, 2005). When the powder/pellets flows only
through the very big vessels it is consider a mark 5 while
when the powder/pellets flows very easily even through a
very thin vessel it has a mark of 1.
Solvents used to disperse the gelly particles during
titration with ammonia derivatives are apolar, with
dielectric constant not higher than 60. The list of tested
solvents includes: alkane such as hexane, eptane, octane,
nonane and alcohol such as: propanol, butanol, pentanol,
hexanol, eptanol, octanol, nonanol, decanol and a aromatic
compound like toluene, benzene, nitrobenzene,
chlorobenzene, dichlorobenzene, quinoline and decaline.
The pelletized silica particles purity has been checked via
ICP-MAS.
The pelletized silica particles, according to the invention
can significantly raise the yield of transportation of the
material by reducing the quantity of fines and narrowing
the particle size distribution.
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Example 1:
At room temperature (19 C) hydrochloric acid 37 Gew.-%
conc., is added to 900 ml of water in a 4 1 vessel till pH
2 is reached. Under stirring are then added very slowly
650g of fumed silica, Aerosil EG50 supplied by Degussa, to
be pelletized. When the dispersion is clear and homogeneous
and without lumps, 650g TEOS (Dynasil 40 fom DEGUSSA AG)
are then added very slowly to the mixture. As consequence
of the exothermic hydrolysis reaction the temperature rises
up to 24 C. After 1 hour of vigorous stirring the solution
is then poured slowly dropwise by means of a cannula in a
22 1 vessel containing 15 1 of silicon oil (Wacker AK 50
from Wacker Chemie) mixed with a tertiary amine that had
given a pH of 11 (expressed for a 10% water solution) :
Primene JM-T (supplied by Rohm and Haas) . The temperature
further rises up to 31 C. The emulsion containing the gelly
particles is then filtered and the so obtained particle are
then washed abundantly with water/acetone solution, in
order to eliminate the residual silicon oil. The material
so obtained is then calcinated in a vertical furnace at
600 C for 8 hour in order to eliminate residual solvents.
Characterization:
Particles size: The material is characterized by
monomodal size distribution
The averaged diameter is 430 micron
Dimension of the starting material 3.7
micron
Porosity: Pores diameter 60 Angstrom, surface
area 99 m2/g which is almost twice of
the surface area of the starting
material.
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Elemental Analysis:
Impurities in starting material (ppm):
Na 1.6
K 0.3
Li 3.8
Al 23
Ca 0.5
Fe 0.6
Ti 2.4
Co 0.01
Cu 0.01
Cr 0.02
Impurities in final material (ppm):
Na 0.6
K 0.05
Li 3.0
Al 12
Ca 0.03
Fe 0.01
Ti 1.0
Co <0.01
Cu <0.01
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Cr <0.01
The pelletisation improves greatly the dimension of the
particles and its dispersion as well (monomodal
distribution), not only, because the purity of the final
material is much better than that of the starting
particles.
Flowability: starting material mark is 5 while the
palletized silica has a mark of 2, which means the
pelletization process has improved the free flowing
behaviour.
Example 2:
At room temperature (19 C) hydrochloric acid 37 Gew.-%
conc., is added to 900 ml of water in a 4 1 vessel till pH
2 is reached. Under strong stirring 650 g Tetraethoxysilane
(TEOS) (Dynasil 40 from DEGUSSA AG) are added very slowly
to the mixture. After 20 minutes stirring are then added
very slowly 650 g of natural quartz to be pelletized As
consequence of the exothermic hydrolysis reaction the
temperature rises up to 22 C. After 1 hour of vigorous
stirring the solution is then poured dropwise in a 22 1
vessel containing 15 1 of silicon oil (Wacker AK 50 from
Wacker Chemie) The pH is then risen by addition of a
tertiary amine: Primene JM-T (supplied by Rohm and Haas),
till pH 11 (10% water solution) is reached, that pH
corresponds to a 10% w/w of Primene in silicon oil. The
temperature further rises up to 31 C. The emulsion
containing the gelly particles is then filtered and the so
obtained particle are then washed abundantly with water, in
order to eliminate the residual silicon oil. The material
so obtained is then calcinated in a vertical furnace at
600 C for 8 hour in order to eliminate residual solvents.
Characterization:
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Particles size: The material is characterized by
monomodal size distribution
The averaged diameter is 500 micron
Dimension of the starting material 5.7
micron
Porosity: Surface area 74 m2/g which has to be
compared with the almost indetectable
low surface area of the natural quartz.
Elemental Analysis:
Impurities in starting material (ppm):
Na 1.9
K 0.6
Li 3.8
Al 36.0
Ca 1.0
Fe 0.4
Ti 3.2
Co <0.001
Cu 0.009
Cr 0.03
Impurities in final material (ppm) before Calcination
Na 2.0
K 0.78
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Li 3.1
Al 20
Ca 1.7
Fe 0.51
5 Ti 3.2
Co <0.01
Cu <0.015
Cr 0.43
Impurities in final material (ppm) after calcination:
10 Na 0.9
K 0.59
Li 2.01
Al 21.0
Ca 3.40
15 Fe 0.01
Ti 2.9
Co <0.01
Cu <0.01
Cr <0.02
The pelletisation improves greatly the dimension of the
particles and its dispersion as well (monomodal
distribution), not only, because the purity of the final
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material is much better than that of the starting
particles.
Flowability: starting material mark is 5 while the
palletized natural quartz before the calcinations process
has a mark of 4 whereas after the calcinations process the
mark is between 2 and 3 which means that the pelletization
process has improved the free flowing behaviour.
Example 3:
At room temperature (19 C) hydrochloric acid 37 Gew.-%
conc., is added to 900 ml of water in a 4 1 vessel till pH
4 is reached. Under strong stirring 85 g of NH4F are
dissolved and always under stirring are then added very
slowly 585g of fumed silica, Aerosil EG50 supplied by
Degussa, to be pelletized. When the dispersion is clearly
homogeneous without lumps 650g TEOS (Dynasil 40 fom DEGUSSA
AG) are then added very slowly to the mixture. As
consequence of the exothermic hydrolysis reaction the
temperature rises up to 24 C. After 1 hour of vigorous
stirring the solution is then poured very slowly and drop-
wise in a 22 1 vessel containing 15 1 of silicon oil
(Wacker AK 50 from Wacker Chemie). The pH of the silicon
oil bath has been risen by addition of a tertiary amine:
Primene JM-T (supplied by Rohm and Haas), till pH 11 (10%
water solution) is reached, that pH corresponds to a 10%
w/w of Primene in silicon oil. The temperature further
rises up to 31 C. The emulsion containing the gelly
particles is then filtered and the so obtained particle are
then washed abundantly with acetone/water solution, in
order to eliminate the residual silicon oil. The material
so obtained is then calcinated in a vertical furnace at
600 C for 8 hour in order to eliminate residual solvents.
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Characterization:
Particles size: The material is characterized by
monomodal size distribution
The averaged diameter is 300 micron
Dimension of the starting material 5.7
micron
