WO2009007200A2

WO2009007200A2 - Process for preparing metal oxide granules

Info

Publication number: WO2009007200A2
Application number: PCT/EP2008/057566
Authority: WO
Inventors: Christian Schulze-Isfort; Björn BRAUN; Monika Oswald; Kai Schumacher; Ralph Hofmann
Original assignee: Evonik Degussa Gmbh
Priority date: 2007-07-06
Filing date: 2008-06-16
Publication date: 2009-01-15
Also published as: DE102007031635A1; TW200920698A; WO2009007200A3

Abstract

Process for preparing metal oxide granules having a particle diameter of 200 to 1500 μm, characterized in that a pyrogenic metal oxide powder selected from the group consisting of the oxides of Al, B, Ce, Cs, Er, Fe, In, Ga, Ge, Ni, Pb, Sn, Ta, Zr and/or Zn with a tamped density of 10 to 1200 g/l is compacted to slugs which are subsequently crushed and optionally classified, the slug fragments having a tamped density of 210 to 2000 g/1.

Description

Process for preparing metal oxide granules

The invention relates to a process for preparing metal oxide granules, and to the granules themselves.

Numerous methods for preparing metal oxide granules are known. Suitable starting materials may be metal oxides prepared by sol-gel processes, metal oxides obtained by precipitation or pyrogenic metal oxides.

The preparation usually comprises a wet granulation. In this method, a sol and, from this, with gradual removal of the moisture, a crumbly material are obtained from a colloidal metal oxide dispersion by constant mixing or stirring. The preparation by means of wet granulation is complicated and costly, especially when high demands are made on a low contamination of the granule.

It is additionally possible to obtain metal oxide granules by spray-drying a dispersion. For instance, WO-A-03/014021 and EP-A-1266864 disclose granules of aluminium oxide or of silicon dioxide doped with aluminium oxide, which have a mean particle diameter of 10 to 150 μm and a tamped density of 400 to 1200 g/1. A disadvantage of the process disclosed is the fact that the (surface) properties of the powders used can be altered by the treatment in a liquid medium. Moreover, these processes do not allow the preparation of granules with a particle diameter of more than 150 μm. Flowability and dust nuisance are improved compared to an ungranulated powder, but there is nevertheless an interest in a further optimization of these parameters.

It is additionally possible to obtain granules by compacting metal oxides. The compaction without binder of pyrogenic metal oxides is difficult because pyrogenic metal oxides are very dry and no capillary forces can bring about the particle binding. Pyrogenic metal oxides are notable for extreme fineness, a low bulk density, a high specific surface area, a high purity, a very substantially spherical primary particle shape and the absence of pores. Pyrogenic metal oxides frequently have a high surface charge which complicates the agglomeration in electrostatic terms.

The compaction of pyrogenic metal oxides has to date not constituted a usable route to the preparation of high-value granules .

It was an object of the present invention to provide a process for preparing metal oxide granules, in which no binders are required. The process should allow the preparation of large amounts and afford products with high purity, which additionally have a high flowability with simultaneously low dust nuisance.

The invention provides a process for preparing metal oxide granules having a particle diameter of 200 to 1500 μm, characterized in that a pyrogenic metal oxide powder selected from the group consisting of the oxides of Al, B, Ce, Cs, Er, Fe, In, Ga, Ge, Ni, Pb, Sn, Ta, Zr and/or Zn with a tamped density of 10 to 1200 g/1 is compacted to slugs which are subsequently crushed and optionally classified, the slug fragments having a tamped density of 210 to 2000 g/1.

The tamped densities depend on the type of the metal oxide powder. In principle, for the process according to the invention, the tamped density of any metal oxide powder is lower than that of the slug fragments prepared.

The BET surface area of the pyrogenic metal oxide powder used may pr :eeferably be 10 to 500 m²/g and more preferably 30 to 150 m²/g. The pyrogenic metal oxide powder used may preferably be a pyrogenic aluminium oxide powder. This may preferably have a tamped density of 10 to 180 g/1. In addition, it may preferably have a BET surface area of 30 to 150 m²/g. The pyrogenic metal oxide powder used may likewise be a mixed oxide powder or a doped metal oxide powder. The mixed oxide components or dopant components used may be one or more oxides selected from the group consisting of Ca, K, Li, Mn, Na, P, Si, Ti or Y.

According to the invention, mixed oxide powders refer to powders whose content of the mixed oxide component is >3 to 50% by weight, based on the mixed oxide powder. Powders whose content of the dopant component is 10 ppm to 3% by weight, based on the doped metal oxide powder, are referred to as doped metal oxide powders. Silicon is considered as a metal within the invention. The mixed oxide powders preferably have a mixed oxide component, and the doped metal oxide powders a dopant component.

The mixed oxide powders and doped metal oxide powders are those which have bonds between metal oxide and mixed oxide component or dopant component, for example Si-O-Al or Si-O-Ti.

Preferably, pyrogenic silicon-aluminium mixed oxide powders may be used. The aluminium/silicon weight ratio is preferably 70:30 to 90:10.

Preferably, pyrogenic yttrium-zirconium mixed oxide powders may also be used. The zirconium/yttrium weight ratio is preferably 85:15 to 95:5.

The pyrogenic metal oxide powder with a tamped density of 10 to 1200 g/1 is compacted to slugs. Slugs refer to the more or less strip-like intermediates which arise in the course of roll compaction through the pressing of the starting material. They are crushed in a second step. The properties of the slugs and slug fragments can be influenced by the process parameters, such as the selected process control mode, the compacting force, the width of the gap between the two rolls and the pressure hold time which is established by the corresponding change in the rotation speeds of the pressing rolls.

The pyrogenic metal oxide powder used has a tamped density of 10 to 1200 g/1. Preferably, a pyrogenic metal oxide powder having a tamped density of 15 to 600 g/1, more preferably one of 20 to 400 g/1 and most preferably one of 40 to 200 g/1 is used. The tamped densities specified in the invention are determined to DIN EN ISO 787-11. The tamped density of the pyrogenic metal oxide powder can be compressed to these values by means of known processes and apparatus. For example, the apparatus according to US4325686, US4877595, US3838785, US3742566, US3762851, US3860682 can be used. In a preferred embodiment of the invention, it is possible to use a pyrogenic metal oxide powder which has been compressed by means of a pressing belt filter according to EP-A-0280851 or US 4,877,595.

The pyrogenic metal oxide powder having the tamped density of 15 to 1200 g/1 is subsequently compacted to slugs. Compaction is understood to mean mechanical compression without addition of binders. The compaction should ensure simultaneous pressing of the pyrogenic metal oxide powder in order to obtain slugs with a very substantially homogeneous density.

The compaction to slugs can be effected by means of two rolls, of which one or else both may simultaneously have a venting function.

Preferably, two compacting rolls may be used, which may be smooth or profiled. The profile may be present either only on one compacting roll or on both compacting rolls. The profile may consist of axially parallel corrugations or any arrangement of recesses (depressions) in any configuration. In a further embodiment of the invention, at least one of the rolls may be a vacuum roll. For the compaction, a suitable process is especially one in which the pyrogenic metal oxide powder to be compacted is compacted by means of two compacting rolls of which at least one is arranged so as to be drivable with rotation and which bring about specific pressures of 0.5 kN/cm to

50 kN/cm, the surface of the compacting rolls consisting of a material which is predominantly or completely free of metals and/or metal compounds, or the surface consisting of a very hard material. Suitable materials are industrial ceramics, for example silicon carbide, silicon nitride, coated metals or aluminium oxide. The process is suitable for minimizing the contamination of the slug fragments and of the metal oxide granule.

After the compaction, the slugs are crushed. To this end, it is possible to use a screen granulator which, with its mesh width of the screen, defines the particle size. The mesh width may be 250 μm to 20 mm.

The slug fragments have a tamped density of 210 to 2000 g/1. Preference is given to an embodiment in which the slug fragments have a tamped density of 280 to 1500 g/1. Particular preference is given to an embodiment in which the slugs have a tamped density of 400 to 1000 g/1.

The slug fragments generally have a tamped density higher by 10 to 40% than the uncrushed slugs.

The slug fragments can subsequently be classified by means of a sifter, of a screen or of a classifier. The slug fragments have a particle diameter, determined by means of screen analysis, of 200 to 1500 μm. Preferably, the particle diameter may be 300 to 700 μm. The fines fraction (particles smaller than 200 μm) can be removed. The classifiers used may be crossflow classifiers, countercurrent deflection classifiers, etc. The classifier used may be a cyclone. The fines fraction removed in the classification (particles smaller than 200 μm) can be recycled into the process according to the invention.

The classified slug fragments can subsequently be exposed at temperature of 400 to 1100⁰C to an atmosphere which comprises one or more reactive compounds which are suitable for removing hydroxyl groups and impurities from the slug fragments. These may preferably be chlorine, hydrochloric acid, sulphur halides and/or sulphur oxide halides. More preferably, chlorine, hydrochloric acid, disulphur dichloride or thionyl chloride may be used. Usually, the reactive compounds are used in combination with air, oxygen, helium, nitrogen, argon and/or carbon dioxide. The proportion of the reactive compounds may be 0.5 to 20% by volume .

Subsequently, depending on the composition of the slugs, it is also possible to sinter at 1200⁰C to 1700°C.

The invention further provides a metal oxide granule which is obtainable by the process according to the invention.

The sum of the impurities in the inventive metal oxide granule may preferably be <50 ppm. The sum of the impurities may preferably be less than 10 ppm and more preferably less than 5 ppm. The proportion of metallic impurities may preferably be <5 ppm and more preferably <1 ppm.

Particular preference may be given to a granule which has the following contents of impurities, all in ppb : Al < 600, Ca < 300, Cr < 250, Cu < 10, Fe < 800, K < 80, Li < 10, Mg < 20, Mn < 20, Na < 80, Ni < 800, Ti < 200, V < 5 and Zr < 80. A distinction should be drawn between an impurity and a dopant component. A dopant component is a substance which has been introduced deliberately into a raw material. An impurity is a substance which was present in the feedstocks from the start and/or is introduced unintentionally during the process. Very particular preference may be given to a granule which has the following contents of impurities, all in ppb : Al < 350, Ca < 90, Cr < 40, Cu < 3, Fe < 100, K < 50, Li < 1, Mg < 10, Mn < 5, Na < 50, Ni < 80, Ti < 100, V < 1, Zr < 3.

The metal content is determined by means of inductively coupled plasma mass spectrometry (ICP-MS). The precision is approx. 10%.

In particular, an advantageous metal oxide granule may be an aluminium oxide granule having a mean particle diameter of 210 to 600 μm and a tamped density of 280 to 550 g/1.

The invention further provides for the use of the metal oxide granule for producing ceramic materials, composite materials, catalysts and catalyst supports.

Examples

The examples are carried out according to the following procedure. Feedstocks, reaction conditions and apparatus settings are reproduced in Table 1. The metal oxide powders used are prepared pyrogenically .

The powder from Example 1 corresponds to that disclosed in EP-A-1083151, Example 1. The powder from Example 2 corresponds to that disclosed in EP-A-585544, Example 1. The powder from Example 3 corresponds to that disclosed in DE-A-102004061702, Example 1. The powder from Example 4 corresponds to that disclosed in DE-A-102004039139, Example 1.

The resulting rod-shaped slugs are crushed by means of a comminution machine (Frewitt MG-633) equipped with a screen fabric (size 800 μm) . After the fines removal, stable slug fragments are obtained. Subsequently, the slug fragments are purified in an HCl gas stream in a reactor. In each case, a high-purity metal oxide granule is obtained with the dimensions and impurities listed in Table 1. The inventive metal oxide granule is highly pure. It does not comprise a binder. The dust content is reduced significantly compared to the powder used.

The metal oxide granule has the necessary cohesion in order not to decompose again prematurely in subsequent steps of an application. Nevertheless, the metal oxide granules exhibit good incorporability .

Table 1: Feedstocks, reaction conditions and apparatus settings

1) Compactor: L 200/50 P, from Hosokawa BEPEX GmbH; Working width: 50 mm; with preliminary venting; equipped with a 12 mm hardened steel roll with a wave profile, closed at the side; 2) before classification; 3) after classification; 4) screen analysis

Claims

1. Process for preparing metal oxide granules having a particle diameter of 200 to 1500 μm, characterized in that a pyrogenic metal oxide powder selected from the group consisting of the oxides of Al, B, Ce, Cs, Er, Fe, In, Ga, Ge, Ni, Pb, Sn, Ta, Zr and/or Zn with a tamped density of 10 to 1200 g/1 is compacted to slugs which are subsequently crushed and optionally classified, the slug fragments having a tamped density of 210 to 2000 g/1.

2. Process according to Claim 1, characterized in that the BET surface area of the pyrogenic metal oxide powder used is 10 to 500 m²/g.

3. Process according to Claims 1 and 2, characterized in that a pyrogenic aluminium oxide powder is used.

4. Process according to Claim 3, characterized in that the pyrogenic aluminium oxide powder has a tamped density of 10 to 180 g/1.

5. Process according to Claims 3 and 4, characterized in that the pyrogenic aluminium oxide powder has a BET surface area of 30 to 150 m²/g.

6. Process according to Claims 1 to 5, characterized in that a pyrogenic mixed oxide powder or a pyrogenic doped metal oxide powder which comprises, as a mixed oxide component or dopant component, one or more oxides selected from the group consisting of Ca, K, Li, Mn, Na, P, Si, Ti or Y is used.

7. Process according to Claims 1 to 6, characterized in that the slug fragments are treated with one or more reactive compounds at 400⁰C to 1100⁰C.

8. Process according to Claims 1 to 7, characterized in that the slug fragments are sintered after the treatment with the reactive compounds.

9. Metal oxide granule obtainable by the process according to Claims 1 to 8.

10. Metal oxide granule according to Claim 9, characterized in that the proportion of metallic impurities is less than 50 ppm.

11. Metal oxide granule according to Claims 9 and 10, characterized in that it is an aluminium oxide granule having a mean particle diameter of 210 to 600 μm and a tamped density of 280 to 550 g/1.

12. Use of the metal oxide granule according to Claims 9 to 11 for producing ceramic materials, composite materials, catalysts and catalyst supports.