Note: Descriptions are shown in the official language in which they were submitted.
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Proc~s~ for Preparing ~lumi~m oxi~e Particle~,
an Aluminum O~ide Powder Prepaxed ~caor~i~g
to thi~ Proce~, as well as i~s ~se ~ :
8pe~ifi~atio~
The present invention pertains to a process for
preparing sinter-active, very extensively spherical
aluminum oxide particles with a mean particle diameter of
< 1.0 ~m, to an aluminum oxide powder prepared according
to this process, as well as to its use.
Aluminum oxide powders are used as pigments,
abrasives and polishing agents, in refractory or fire-
resistant products, in ceramics, as catalyst materials,
or as fillers~ as well as for coatings.
The chemical resistance, good mechanical strengths,
especially its favorable wear property, its high
electrical resistance, and its good temperature
resistance are decisive for the industrial application of
aluminum oxide.
. Especially the following properties are required for
the preparation of high quality ceramic, especially
re~ractory products:
- high sintering activity (especially due to small
particle sizes),
- minimization of the impurities that inhibit the
sintering process or promote an undesirable particle . .
growth,
- lowest possible content of melt phase~forming ~ :
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accompanying substances,
- good processability (pressability), and ~ ;
- high green strength.
Low porosity of the individual particles (powder
particles) is also necessary for reaching high green `-~
strengths (green densities). ;
Various thermal, wet-chemical, mechanical, and
physical processes for preparing sinter-active,
microcrystalline Al2O3 particles and powders have been
known.
They include the thermal decomposition and
subsequent calcination of purified alum (ammonium
aluminum sulfate), or the thermal decomposition of
aluminum chloride according to the so-called spray
roasting processO The disadvantages of such thermal
decompositions of aluminum salts are the high price of
the corresponding plants and the salt residues remaining
in the oxide, which may contribute to an increased
particle growth during the sintering process.
It has also been known that an alumina prepared
according to the so-called Bayer process can be ground to
prepare finely dispersed aluminum oxide. However, this
fine grinding is very expensive, and the more finely the
material is to be ground, the more time-consuming is the
process, so that particles finer than 1 ~m can be
prepared, if at all, with an extrem~ly high technical
effort only. In addition, the morphology of the powder
particles thus prepared is splintery and granular. This
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may cause disadvantages in terms of the rheological
behavior.
The preparation of spherical Al2O3 particles
according to a process, in which water-containing
aluminum oxide is subjected to a special, multistep, heat
treatment, has been known ~rom U.S.-A-4,818,515. The
process reguires a starting material that can be prepared
at a considerable cost only because of the required
purity.
Finally, finely dispersed calcinates can be prepared
by the hydrolysis of aluminum alkoxides; however,
depending on the degree of calcination, these calcinates
sometimes have a considerable microporosity, which leads
to a corresponding (undesired) shrinkage during a
subsequent sintering process.
The above-described processes have therefore not
become successful for many large-scale applications for
technical and economic reasons.
Therefore, the basic task of the present invention
is to provide a process which makes it possible to
prepare ver~ fine aluminum oxide particles in the
submicron range (< 1.0 ~m) in a relatively inexpensive
manner, wherein very extensively spherical particle
shape, low porosity, and consequently good compaction and
sintering properties are desirable in order to make
possible or optimize application for the purposes
mentioned in the introduction.
This goal is achieved by a process of the class
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described in the introduction, which is characterized by
the following steps: :
- introduction of an aluminum carrier, such as Al or :~
Al203, into a furnace unit,
- heating the aluminum carrier, ~-~
- reduction of the aluminum carrier, unless it is
introduced as metallic aluminum, into metallic aluminum
and/or aluminum carbides (including aluminum
oxicarbides),
- increasing the furnace temperature to a value at which
the metallic aluminum or the aluminum carbides evaporate,
- subsequent oxidation of the metallic aluminum or its
carbides into aluminum oxide in a gas ~low, and
~ introduction of the gas flow into a filter, wherein
- the temperature, the atmosphere, and the hold time of
the aluminum oxide particles in the gas flow are adjusted ~:
according to the desired particle size.
Consequently, using, e.g., lumpy aluminum oxide as
the starting product, this aluminum oxide is first
reduced into metallic aluminum and/or aluminum carbides,
these are subsequently or simultaneously evaporated, and
finally reoxidizPd in a suitable manner, before the
aluminum oxide particles thus formed secondarily are
separated in a filter. .
What is very important for reaching the fine
particle size of the aluminum oxide particles, which was
formiulated according to the task, is to introduce the
Alz03 particles, carried in the gas flow, in the gas flow -
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into the subsequent filter. The shorter the hold time of
the Al2O3 particles in the gas flow, the smaller is the
particle size, which can also be controlled secondarily
~ia the tempera~ure and the (oxidizing) atmosphere of the
gas flow.
To obtain the fineist Al203 particles possible, the
filter is consequently connected directly to the above-
described oxidation step.
An electric arc furnace proved to be particularly
advantageous as a furnace unit. According to an
embodiment of the present invention, it is operated at
current densities between 10 and 50 A/cm2l and in the
preferred range between 15 and 30 A/cm2.
It proved to be advantageous to add a reducing agent
(such as carbon) during the reduction reaction of
aluminum oxide, aluminum carbide, or aluminum oxicarbide
in order to reach the effective evaporation capacity. It
is also possible to use carbon-releasing compounds in
this sense.
The subsequent oxidation of the vapor-form aluminum
and/or condensed aluminum particles can be accelerated by
feeding in ex~ernal oxygen. This makes it possible to
subsequently separate the particles, which have a short
hold time in the oxidation step, in a suitable filter,
e.g., a bag filter.
As an alternative, the oxidation step may be
designed such that aluminum particles are introduced into
a furnace section, in which oxidizing atmosphere is
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present.
Sinter-active, spherical aluminum oxide particles
with a density of up to 3.97 g~cm3 and a specific surface
between 0.5 and 60 m2/g can be prepared by the process
according to the present invention.
The process makes it possible to form aluminum oxide
particles with a mean particle size of marXedly less than
1 ~m, and even as small as 0.10 ~m, by correspondingly
adjusting the process parameters, such as the
temperature, the atmosphere, and the hold time of the
Al2O3 particles in the gas flow.
One particular advantage is the fact that the Al2O3
particles prepared according to the process described
have a nearly ideal spherical (ball-shaped~
configuration, so that the material can be used
particularly advantageously for, e.g., abrasives and
polishing agants, or in refractory ceramic materials (in
which latter it is used as a binder or binder component).
The sphsrical shape is the major factor for the
contribution of the particles to the exc llent
rheological properties of corresponding systems.
If an electric arc furnace is used, the charge may
readily be in the lumpy form. The evaporation capacity
of the arc generated depends on its energy content and
the local arc temperature. The evaporation capacity is
in the range of 40 to 100 g of Al2O3 per kWh at current
densities in the range of 10 to 50 A/cm2.
The composition of the Al2~3 particles obtainad
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according to this process depends on the aluminum-
containing raw material (aluminum carrier) charged in and
the reducing agent used. If the raw material and/or the
reducing agent contain alkali and/or alkaline earth
oxides, sio2, iron oxide, or the like, these impurities
can be found in the final product nearly quantitatively.
If carbon is used as the reducing agent, or sometimes
also due to carbon from the furnace electrodes, small
amounts of carbon are released, or carbides or
oxicarbides are formed. If the reoxidation does not take
place completely, low carbon contents of up to ca. 0.5
wt.%, which can be further reduced, if needed, by heat
aftertreatment (eOg., annealing treatment), may occur in
the final product in this case~
The present invention will be described in greater
detail below on the basis of an exemplary embodiment:
A mixture of 85 parts by weight of lumpy aluminum
oxide and 15 parts by weiyht of graphite chips were
charged into an electric arc furnace equipped with ;
graphite electrodes. After the arc had been ignited, a
melt sump o~ aluminum oxide, Al404C and Al4C3, which is
advantayeous as a protective layer for the bottom lining
of the ~urnace (which consisted of carbon bricks in this
case), was initially formed. The capacity of the arc was
2~ in the range of 150-180 kVA. The current density was
between 16 and 23 A/cm2.
The lumpy starting material subsequently ~vaporated,
and metallic aluminum and aluminum carbides were formed;
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the metallic aluminum and the aluminum carbides were
subsequently reoxidized into Al203 particles in the
atmosphere or by supplying oxygen, before these were
introduced into a fabric filter, where they were
separated at a rate exceeding 99 wt.%. The mean size
of the predominantly spherical aluminum oxide powder
particles obtained was 0.2 ~m. The density of the
material was 3.8 g/cm3. The specific surface was 9.8
m2/g -
In the case of an Al2O3 charge of the composition of
0.03 wt.% Na2O ~ K2O,
0.014 wt.% Fe2O3,
0~03 wt.% MgO,
0.03 wt.% sio2,
remainder Al2O3, `
an Al2O3 powder of the following particle composition was
obtained:
0.037 wt.% Na2O ~ K2O,
0.03 wt.% Fe2O3,
0.05 wt~% MgO,
0.08 wt.% SiO2, `
0.37 wt.~ C,
remainder Al2O3.
The increase in the impurity level was due to the
percentage of ash in the graphite used for the reduction,
as well as to the furnace electrodes.
