Agglomeration of alumina material

Details Agglomeration of alumina material Agglomeration of alumina material

1994-08-22

1999-01-12

Bos, Steven

Chemistry of inorganic compounds

Oxygen or compound thereof

Metal containing

23313R, C01F 702, C22B 114

Patent

active

058583254

DESCRIPTION:

BRIEF SUMMARY
BACKGROUND OF THE INVENTION

1. Field of the Invention
This invention relates to a process for the agglomeration of alumina containing material substantially comprising Al.sub.2 O.sub.3.nH.sub.2 O where n is in the range of from zero to 3. The invention also relates to agglomerated granules produced by that process.
2.Description of the Prior Art
Fine powder or dust of such alumina containing material (herein called "alumina powder") is difficult to handle and has poor flow characteristics. As a consequence, by-product alumina powder from the Bayer process presents difficulties. In the Bayer process, precipitated alumina trihydrate is filtered, dried and calcined to yield high purity alumina product of a relatively narrow size spectrum, for use in an electrolytic smelting operation. By-product alumina powder, referred to as fines, superfines and sometimes as ESP dust, is recovered by multicyclones and/or electrostatic precipitator collectors from the calcining stage and typically has an average particle size of less than 30 .mu.m. In addition to being difficult to handle and having poor flow characteristics, by-product alumina powder can not be readily digested if recycled to the hot caustic digestion stage of the Bayer process. Also, if added to the alumina product for use in the smelting operation, it increases the size range and dustiness of the product.
There is a need to be able to agglomerate alumina powder into a coarser product of a size range which approximates the preferred size range for the smelting operation. However, there also can be benefit in ceramics manufacturing in being able to agglomerate fine alumina powder, whether this is ESP dust or is from another source.
In ceramics manufacturing, fine micron sized ceramic powders are agglomerated by spray drying, using an organic polymer, such as PVA, as a binder. The purpose, however, is to make weak granules that act as a flowable precursor to facilitate pressing of low porosity green bodies of ceramics, prior to firing. However, these granules are essentially friable and weakly bonded, and may be degraded when handled or transported. Thus, agglomeration with such organic polymers is of limited benefit.
We have found that alumina powder can be agglomerated by use of a suitable inorganic binder.


SUMMARY OF THE INVENTION

According to the invention, there is provided a process for agglomerating powder (herein referred to as "alumina powder") comprising alumina containing material, using a binder comprising a polymer form of a hydroxy salt of aluminium. In the process, an aqueous slurry of the alumina powder, containing a sufficient quantity of the binder, is subjected to a spray drying operation to form agglomerated granules, and the granules then are calcined at an elevated temperature for their consolidation; the alumina material comprising Al.sub.2 O.sub.3.nH.sub.2 O where n is in the range of from zero to 3.


DETAILED DESCRIPTION OF THE INVENTION

The alumina containing material may be fully dehydrated alumina, fully hydrated alumina, partially hydrated alumina or a mixture of these forms. Where resulting from calcination of alumina trihydrate, the material may be of high purity. However, the material may be other than of high purity, comprising for example relatively high grade bauxite fines or powder. Where the alumina containing material is other than of high purity, it preferably has an alumina content (calculated as Al.sub.2 O.sub.3, i.e. with n being zero) of at least about 80 wt %.
The alumina powder may comprise ESP dust resulting from calcining trihydrate in the procedure of the Bayer process. However, the alumina powder may be from other sources. The alumina powder may have an average particle size of less than 30 .mu.m, although alumina powder of larger particle size can be used, subject to its suitability for spray drying.
The polymer used as the binder preferably is one based on units such as of the form Al.sub.x (OH).sub.y.sup.(3x-y)+. The polymer can be formed by the action of a base such as NaOH on a suitable aluminium sa

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