WO2013026688A1 - Process for preparing alpha-aluminium oxide - Google Patents

Abstract

Process for preparing aluminium oxide which has a proportion of alpha-aluminium oxide of at least 98% by weight and is largely present in the form of isolated particles having an average diameter of from 1 to 50 pm, in which granules which - comprise essentially aggregated primary particles of one or more transition aluminium oxides, in each case having an average primary particle diameter of from 5 to 50 nm, - have an average diameter of from 500 to 5000 pm and - have a tamped density of at least 250 g/l, are treated at from 800°C to 1200°C in an atmosphere containing at least 70% by volume of hydrogen chloride gas or at least 70% by volume of chlorine gas for a period of from 0.5 to 5 hours.

Description

 Process for the production of alpha-alumina

The invention relates to a process for the production of alpha-alumina.

It is widely used, for example, as an abrasive, in the

technical ceramics and as a catalyst carrier. Another one

Field of application are fluorescent lamps. Between phosphor and glass tube there is a protective layer containing aluminum oxide. This reflects on the one hand not absorbed by the phosphor UV radiation. On the other hand, it prevents the diffusion of mercury and thus one

Graying of the glass. It also ensures better adhesion of the phosphor to the glass. For this purpose, US Pat. No. 5,552,665 uses gamma-aluminum oxide; US Pat. No. 5,604,444 uses a mixture of gamma and alpha-aluminum oxides.

The production of alpha alumina is mostly carried out by means of the Bayer process, in which bauxite is converted into aluminum hydroxide or transition aluminum oxides ("transitional alumina") .Additionally, hydrothermal processes still exist.

Furthermore, it was tried to produce alpha-alumina by converting pyrogenically produced aluminum oxides or by a pyrogenic process itself. Due to the good availability and the high purity of pyrogenically prepared aluminum oxides, these should be ideal starting materials for the production of alpha-alumina.

EP-A-395925 discloses a process in which

Alumina powder having an alpha content of 20 to 80 wt .-% directly from aluminum trichloride in an oxygen-A / Hydrogen flame can be obtained. Again, higher levels of alpha alumina can not be achieved.

WO 92/16595 discloses a process in which an aluminum hydroxide obtainable from the Bayer process is made into compacts having a

Diameter of 1 to 3 mm compacted and then heat treated. Furthermore, the compacted aluminum hydroxide may initially be gamma Alumina are transferred, and this are converted in a separate step in alpha-alumina. The gamma-alumina is, as stated above, a material having a BET surface area of typically 200 to 250 m ² / g. The alpha-alumina obtainable by this process has a narrow particle size distribution.

Furthermore, it was tried to produce alpha-alumina by converting pyrogenically produced aluminum oxides or by a pyrogenic process itself. Due to the good availability and the comparatively high purity of pyrogenically prepared aluminum oxides, these should be ideal starting materials for the production of alpha-alumina. EP-A-355481 discloses a process in which a pyrogen-based gamma-alumina is thermally treated in an oxygen / hydrogen flame. Although it can be obtained with alpha-alumina-enriched powder, contents of more than 70 wt .-% alpha alumina can not be achieved. EP-A-395925 discloses a process in which alumina powder having an alpha content of 20 to 80% by weight can be obtained directly from aluminum trichloride in an oxygen-hydrogen flame. Again, higher levels of alpha alumina can not be achieved. In both processes a material is obtained whose structure and composition is not suitable for further processing.

EP-A-644277 discloses a process for producing high purity alpha alumina having a particle diameter of 0.1 to 30 μιτι in which a transitional alumina in an HCl or C ^ -containing atmosphere at relatively low temperatures of at least 600 ° C, preferably 800 to 1200 ° C, is calcined.

WO 2010/069690 discloses a process for the preparation of a

Alumina powder which is in the form of aggregated primary particles and consists of at least 85% by weight of alpha alumina. This is based on a transitional alumina, which largely already has the structure of the later product, namely by in shape Aggregated primary particles present. However, this structure is for many

Applications not ideal.

The technical problem of the present invention was a

To provide a process for the preparation of predominantly or exclusively alpha-alumina, which comprises the use of aggregated primary particles of transitional aluminum oxides, as found, for example, in pyrogenic

The process should additionally lead to a change in the structure of the alpha alumina to largely isolated particles present .. It has now surprisingly been found that the technical problem is solved by a method in which is a coarse-grained granules of finely divided, aggregated primary particles of transitional aluminas with high

Tapped density in the presence of hydrogen chloride or chlorine thermally treated.

The invention relates to a process for the production of aluminum oxide with a proportion of alpha-alumina of at least 98 wt .-%, which largely in the form of isolated particles having an average diameter of 1 to 50 μιτη, preferably 3 to 20 μιτη, more preferably 5 to 15 μιτη, is present, characterized in that one granules, which

essentially aggregated primary particles of one or more

 Transitional aluminas comprises, each with a middle

 Primary particle diameter of 5 to 50 nm, - a mean diameter of 500 to 5000 μιτη, preferably 1000 to 4000 μιτη, more preferably 2200 to 3200 μιτι and

a tamped density of at least 250 g / l, preferably 300 to 800 g / l, at 800 ° C to 1200 ° C, preferably 900 ° C to 1000 ° C, in an at least 70 vol .-%, preferably 90 to 100 vol .-%, hydrogen chloride gas or at least 70 vol .-%, preferably 90 to 100 vol .-%, chlorine gas-containing atmosphere over a period of 0.5 to 5 hours, preferably 1 to 3 hours treated. By transitional aluminas are meant those which can be converted by heating in alpha-alumina. In particular, these are gamma-alumina, theta-alumina, delta-alumina, X-ray amorphous alumina and mixtures of the aforementioned

Links. Primary particles are to be understood as meaning the smallest particles which obviously can not be broken down further, for example by means of transmission electron spectroscopy (TEM). The mean primary particle diameter can be determined, for example, by counting the particles in TEM images. Several primary particles can firmly grow together at their contact points to aggregates.

The granules used consists essentially of

Transition aluminas. This is understood to mean that their content is at least 98% by weight, preferably at least 99% by weight, in each case based on the granules. In particular, the granules are free of detectable amounts of alpha alumina. The determination of the alpha-alumina content is carried out by X-ray analysis in comparison with powder mixtures defined composition of alpha-alumina and said crystal modifications. In addition to alpha-alumina, the process product may contain minor amounts of the gamma, theta, delta crystal modifications and amorphous constituents, with the gamma crystal modification usually being the main component of the minor constituents.

Furthermore, the content of iron, nickel and chromium in total is preferably not more than 500 ppm, usually from 50 to 100 ppm. The

inventive method can be used to reduce this

Contaminate at least 2/3. Due to the low impurities may preferably pyrogenic

transitional aluminas produced are used. These are usually offered as alumina konnnnerziell. In general, the gamma modification is the main component of pyrogenic aluminas. However, it is also possible to produce pyrogenic alumina powders in which delta and theta phases predominate. Finally, X-ray amorphous aluminas can also be used. Pyrogenic means the processes of flame hydrolysis and flame oxidation. This aluminum compounds, usually aluminum chloride, at high

Temperatures to form one or more transitional aluminas hydrolyzed and / or oxidized. Commercially available alumina powders include SpectrAI ™ 51, SpectrAI ™ 81, SpectrAI ™ 100, Fa. Cabot, AEROXIDE Alu ^® 65, AEROXIDE ^® Alu C, Fa. Evonik Degussa. Pyrogenic alumina powders are characterized by a very high purity. Due to the manufacturing process, the primary particles have a compact, almost spherical shape, which is largely free of internal pores.

The granules used are preferably a through

Compression obtained pressed granules with a tamped density of at least 250 g / l. The tamped densities mentioned in the application are determined in accordance with DIN EN ISO 787-1 1. The pressed granules can be compacted by

Breaking and classification of alumina powder can be obtained. The aluminum oxide powder used for this purpose generally has a tamped density of about 30 to 150 g / l. By compacting is meant the mechanical compression without the addition of binder. In the compaction, a uniform pressure of the alumina powder should be ensured in order to obtain granules with a largely homogeneous density. The

Compaction to scabs is usually done by means of two rollers, one or both of which can simultaneously have a venting function. The rollers can be smooth or profiled. In another

Embodiment of the invention, at least one of the rollers a

Be a vacuum roller. For compacting, in particular, a method in which the alumina powder to be compacted by means of two Compacting rollers, at least one of which is rotatably drivable and the specific contact pressures of 0.25 kN / cm to 50 kN / cm cause compacted, wherein the surface of the compaction rollers consists of a material which is predominantly or completely free of metals and / or metal compounds the surface is made of a very hard material. Suitable materials are engineering ceramics such as silicon carbide, silicon nitride, coated metals or alumina. After compaction, the pressed granules can be broken. For this purpose, a sieve granulator can be used to obtain the required granule dimensions.

The granules are at 800 ° C to 1200 ° C, preferably, in one

Hydrogen chloride gas or chlorine gas containing atmosphere treated. The treatment, which may be continuous or discontinuous, is preferably carried out at 900 ° C to 1000 ° C. It is preferably carried out in a rotary kiln, which is fed continuously. The throughput can be varied with the dimension of the rotary kiln. It is usually 1 to 100 kg / h.

The process according to the invention is characterized in that it succeeds in converting finely divided powders of transitional aluminas via well-handled granules of defined size and ramming weight into polyhedral crystals of alpha-alumina having an average diameter of 1 to 50 μm. A direct conversion of finely divided powders of

Transitional aluminas in polyhedral crystals of alpha-alumina do not succeed. The inventive method is further characterized by the fact that it usually with a significant reduction of metallic

Contaminates. The condition is that the finely divided

Alumina powder is previously converted into a granulate defined size and tamped density. Examples

Example 1: Preparation of granules of a transitional aluminum oxide

A pyrogenic alumina powder having a BET surface area of 100 m ² / g, a tamped density of 50 g / l and an alumina content of> 99.6 wt .-%. The crystalline portion consists of about 2/3 of gamma alumina and about 1/3 of delta-alumina. The powder is granulated with a roller compactor. Granule diameters of 1000 to 2000 μm are isolated. The tamped density of the granules is 662 g / l.

EXAMPLE 2 Production of granules of a transitional aluminum oxide A pyrogenically prepared aluminum oxide powder having a BET surface area of 65 m ² / g, a tamped density of 50 g / l and an aluminum oxide content of> 99.6% by weight. The main crystalline components are theta and delta alumina, along with low levels of gamma alumina. The powder is granulated with a roller compactor. There are granules diameter of 2200 to 3200 μιτι isolated. The tamped density of the granules is 51 1 g / l.

Example 3 (according to the invention):

 Production of alpha-alumina - discontinuous procedure

A quartz tube with a diameter of 100 mm containing the granulate from Example 2 is heated to 900 ° C. by means of a furnace. Over a period of 3 h, an HCl volume flow of 40 Nl / h is passed through the quartz tube. The differential pressure before and after the bed is about 5 mbar.

X-ray diffractograms show the complete conversion of the granules into alpha-alumina. SEM images show that the material is present in the form of polyhedral crystals with an average diameter of 10 μιτι. Example 4 (according to the invention):

 Production of alpha-alumina - continuous driving

A quartz tube with a diameter of 50 mm, which can be continuously loaded and unloaded with granules according to Example 1, by means of a Furnace heated to 1000 ° C. The mean residence time of the material in the quartz tube is 2 hours. The quartz tube is flowed through with an HCI volume flow of 20 Nl / h. The differential pressure before and after the bed is about 5 mbar. X-ray diffractograms show complete conversion of the bed to alpha alumina. SEM images show that the material is present in the form of polyhedral crystals with an average diameter of 10 μιτι.

Example 5 (Comparative Example)

Procedure analogous to Example 3, but using the pyrogenically prepared aluminum oxide powder used in Example 1. The experiment had to be stopped because of blockage of the quartz tube. The conversion to alpha alumina was only minor. They were not

to detect polyhedral crystals.

Claims

claims: 1 . Process for the preparation of aluminum oxide with a proportion of alpha-alumina of at least 98 wt .-%, which is largely in the form of isolated particles having an average diameter of 1 to 50 μιτι present, characterized in that one granules, which essentially aggregated primary particles of one or more  Transitional aluminas comprises, each with a middle  Primary particle diameter of 5 to 50 nm, - a mean diameter of 500 to 5000 μιτι and - Has a tamped density of at least 250 g / l, treated at 800 ° C to 1200 ° C in a at least 70 vol .-% hydrogen chloride gas or at least 70 vol .-% chlorine gas-containing atmosphere over a period of 0.5 to 5 hours. 2. The method according to claim 1, characterized in that  you use a fumed alumina. 3. Process according to claims 1 or 2, characterized in that the aluminum oxide used is a pressed granulate.