SK138493A3 - Method of manufacturing of sintering, mainly round shaped particles of oxide aluminate - Google Patents

Abstract

The invention relates to a process for the preparation of aluminium oxide particles and also an aluminium oxide powder prepared by this process and also use thereof.

Description

BACKGROUND OF THE INVENTION

The alumina powder is used in the form of pigments, abrasives and polishes, in fire-resistant or fire-resistant products, in ceramics as a catalyst or fuel material and for coating.

The decisive factor for the technical application of alumina is its stability, good mechanical properties, especially its favorable abrasion behavior, high electrical resistance and good temperature resistance. In addition, alumina is not toxic.

In particular, the following properties are required for the production of high-quality, fire-resistant ceramic products:

high sinterability (especially due to small grain sizes), minimization of impurities impeding the sintering process or promoting undesirable grain growth, the smallest contents of melt-phase co-builders, good processability (compressibility), high wet density.

Low porosity of the individual particles (powder particles) is also necessary to achieve high wet conditions.

A variety of thermal processes, wet chemical processes, mechanical and physical processes for the production of sinterable, microcrystalline particles and Al 2 O 4 powders are known.

This includes the thermal decomposition and subsequent calcination of the purified alum (ammonium aluminum sulphate) or the thermal decomposition of the aluminum chloride by the so-called sputtering method. The drawbacks of such thermal decomposition of aluminum salts reside in the high cost of suitable equipment and in the fact that salt remains in the oxide, which may contribute to an increase in grain growth during the sintering process.

For the production of fine particle alumina, it is also known that the alumina produced by the so-called Sayer process is ground. This fine grinding is, however, very expensive and even slower, which makes the material finely comminuted, so that particles below 1 µm can either not be produced at all or only at the expense of extreme technical expense. The morphology of the powder particles thus produced is also fragmented-grained. This may lead to lack of in theological behavior.

From US-A-4,818. 515, it is known to produce spherical particles Αΐ 3 in a process in which alumina containing water is subjected to a special, multi-stage, heat treatment. The process requires a starting material which, in view of the necessary purity, can only be produced at high expense.

Finally, calcinates can be produced by hydrolysis of aluminum aluminum alkoxides, but after the calcination stage they exhibit in part considerable microporosity, leading to a corresponding / undesirable / shrinkage in the subsequent sintering process.

For technical and economic reasons, these methods have therefore not been established in many mass uses.

It is therefore an object of the present invention to provide a method which makes it possible to produce very fine alumina particles in the submicro-region (<1.0 µm) at relatively favorable costs, while aiming to spherically grain, low porosity and thus good densification and sintering. properties to allow use for the above purposes or to optimize. SUMMARY OF THE INVENTION

This objective is achieved in a manner of the kind described above, characterized by the following steps:

4 by introducing an aluminum support such as A1 or AlgOO into the furnace aggregate, heating the aluminum support, reducing the aluminum support, unless it is introduced as metallic aluminum, to metallic aluminum and / or aluminum carbides (including aluminum oxycarbides), wherein the metallic aluminum or aluminum carbides are evaporated by the subsequent oxidation of the metallic aluminum or its carbides to the aluminum oxide in the gas stream and introducing a gas stream into the filter, wherein. the temperature, atmosphere and inflection of the alumina particles in the gas stream are set in accordance with the desired particle size.

Starting from, for example, lumpy alumina, this is first reduced to metallic aluminum and / or aluminum carbides, which are then evaporated in parallel in the event of a fall and then reoxidized in a suitable manner before the secondary alumina particles so formed are separated by a filter.

In order to achieve the fineness of the alumina particles formulated according to the invention, it is of great importance that the Al2O3 particles guided in the gas stream lead to a non-following filter in the gas stream. The shorter the inflection of the particles A'1 ^0 O-j in the gas stream, the smaller the particles, the size being

- 5 particles can also be controlled secondary by the temperature and / oxidizing / atmosphere of the gas stream.

In order to achieve the finest possible particles A ^O ^, the filter is therefore connected directly to the oxidation stage described above.

An electric arc furnace has proven to be particularly advantageous as a furnace aggregate. It is operated according to one embodiment of the invention with current densities between 10 and 50 A / cm, in a preferred 2 region between 15 and 30 A / cm.

The addition of a reducing agent (such as carbon) in the reduction reaction of alumina to aluminum, aluminum carbide or aluminum oxycarbide has proven advantageous to increase the effective evaporation performance. Carbon-releasing compounds can also be used in this sense.

The subsequent oxidation of aluminum in the form of steam and / or condensed aluminum particles can be accelerated by an external oxygen supply. This makes it possible for the particles to be immediately separated in a suitable filter, for example a bag filter, with only a very short inflexion in the oxidation stage.

Alternatively, the oxidation stage may be formed by conducting aluminum particles in a section of the furnace in which an oxidizing atmosphere exists.

With the method described, sinterable, spherical alumina particles with a density of up to 3.97 g / cm @ 2 and a specific surface area of between 0.5 to 60 m @ 2 / g can be produced.

The process allows the formation of alumina particles having a mean particle size of clearly below 1 µm, by appropriately adjusting process parameters such as temperature, atmosphere and inflection of Al 2 O 2 particles ⁱⁿ the gas stream, and even below this size up to 0.10 µm.

It is particularly advantageous that the Al 2 O 4 particles produced in the manner described have an almost ideal spherical / spherical configuration, since the material can be used particularly advantageously for example for glazing and polishing compositions or in fire-resistant ceramic materials (there and also as binder or binder components. The spherical shape is responsible, to a decisive degree, for the particles to contribute to the excellent theological properties of the corresponding systems.

With the use of an electric arc furnace, the feed material can be supplied in unitary form without any restrictions. The evaporation performance of an existing arc depends on its energy content and the local arc temperature. At current densities in the range of 10 to 50 A / cm ^d , the evaporation power is in the range of 40 to 100 g AlgO 4 / kwh.

The composition of the A? Op particles obtained by this process is dependent on the feedstock containing the clinic (aluminum support) and the reducing agent used. If the raw material and / or reducing agent contains alkali metal oxides and / or alkaline earth oxides, Sif4, iron oxide 8 or the like, then these impurities are found almost quantitatively

- 7 again in the final product. When carbon is used as a reducing agent, partly but also due to carbon from the furnace electrodes, small amounts of carbon are released or carbides or oxicarbides are formed.

In the final product, if the reoxidation does not proceed perfectly, small carbon contents of up to about 0.5% by weight are possible, which can be further reduced if necessary by further heat treatment (e.g., baking).

DETAILED DESCRIPTION OF THE INVENTION

The invention is described in more detail below by way of example:

A mixture composed of a graphite electrode is introduced into an electric arc furnace with graphite electrodes. 85 parts by weight of lumpy alumina and 15 parts by weight. graphite grits. After the arc is ignited, it is initially formed mainly of alumina mud, Al2O4C and Al2Cp, which is suitable as a protective layer for the lining of the furnace bottom (which here consists of carbon bricks). The power of the arc is between 150 and 160 kVA. The current density is between 16 and 23 A / cm ² .

As a result, the lumpy starting material evaporates to form metallic aluminum and aluminum carbides which are then re-oxidized by the atmosphere or oxygen supply to the Al ₂ O 4 particles before being introduced into the tissue filter where they are separated by more than 99 wt%.

The average size of the predominantly spherical alumina powder particles obtained is about 0.2 µm. The density of the material is 3.8 g / cnP. The specific surface area is about 9.8 m / g.

When composing the material used ΑΙ ,, θβ

0.03 wt.%. 0.014 wt% 0.03 wt%. 0.03 wt.%. Rest

Na ₂ 0 + ^Fe 2 ° 3

MgO

SiO ₂

A1 ₂ O ₃

K ₂ 0 gives an Al ₂ O ₃ powder having the following particle composition:

0.037 wt%. Na ₂ 0 + K ₂ 0

0.03 wt.%. Pe ₂ O ₃

0.05 wt%. MgO

0.08% by weight. SiO ₂

0.37% by weight. C residue Α1 ₂ 0β.

The increase in impurities results from the proportion of graphite ash used for the reduction as well as the furnace electrode.

Claims (16)

PATENT CLAIMS A process for producing sinterable, as spherical as possible alumina particles with a mean particle diameter of <1.0 µm, preferably <0.5 µm, with the following steps: 1.1. introducing an aluminum support such as metal aluminum or aluminum oxide into the furnace aggregate, 1.2. heating the aluminum carrier, 1.3 reduction of the aluminum support, unless metal aluminum is introduced, to metal aluminum and / or aluminum carbides, 1.4. increasing the kiln temperature to a value at which the aluminum metal or the aluminum carbides evaporate, 1.5. following oxidation of the metal aluminum or its carbides to alumina in the gas stream, whereby 1.6. adjusts the temperature, atmosphere and inflection of the alumina particles in the gas stream in accordance with the desired particle size. Method according to claim 1, characterized in that the aluminum support is used in spherical form. Method according to claim 1 or 2, characterized in that the evaporation is carried out in an electric arc furnace. A method according to claim 3, characterized in that the current density is 10 to 50 A / cm. A method according to claim 4, characterized in that the current density is 15 to 30 A / cm. 6. The method according to ^- claim 1 to 5, in yznačujúci in that the reagent is employed aluminum or aluminum-releasing compound. Method according to one of claims 1 to 6, characterized in that oxygen is blown into the gas stream in the oxidation stage. Method according to one of Claims 1 to 6, characterized in that the oxidation of aluminum in the form of steam or of aluminumkerbido to aluminum oxide is carried out by introducing an aerosol into the furnace section of the oxidizing atmosphere. Method according to one of Claims 1 to 8, characterized in that the separation of the alumina particles takes place in a bag filter. Sinterable, as spherical as possible alumina powder produced by the process according to any one of claims 1 to 9, characterized in that it has a density of 2.5 to 3.97 g / cm and a specific surface of 0.5 to 60 m. / g. An alumina powder according to claim 10, characterized in that it has a density of between 3.2 and 3.97 g / cm 2 and a specific surface area of 4 to 20 m 2 / g. Alumina powder according to claim 10 or 11, characterized in that it has a mean particle size of between 0.05 and 0.3 µm. Use of an alumina powder according to any one of claims 10 to 12 as an abrasive or polishing agent. Use of an alumina powder according to one of claims 10 to 12 as a binder in fire-resistant ceramic materials. Use of an alumina powder according to one of claims 10 to 12 as a filler. Use of an alumina powder according to any one of claims 10 to 12 as a material for catalysts.