Chemistry of inorganic compounds – Oxygen or compound thereof – Metal containing
Reexamination Certificate
2000-07-05
2002-05-21
Bos, Steven (Department: 1754)
Chemistry of inorganic compounds
Oxygen or compound thereof
Metal containing
C423S625000, C423S121000, C423S124000
Reexamination Certificate
active
06391277
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for manufacturing of alumina trihydrate by precipitation in the presence of a seed consisting of a pregnant sodium aluminate liquor derived from the BAYER process, in order to separately control the sodium content and the particle size distribution of the precipitated alumina trihydrate, while maintaining high productivity.
The physicochemical characteristics of alumina trihydrate, in particular the purity, particle size distribution, the size of elementary crystallites and the degree to which they agglomerate in more or less friable grains, are parameters that control the properties of alumina hydrate and of the resulting alumina. These characteristics must be adapted as a function of product applications; metallurgical alumina for the production of aluminum by igneous electrolysis or technical aluminas for use in a wide variety of fields such as flame retarding fillers for plastics, refractory materials, abrasives, ceramics and catalysts.
These characteristics are interdependent in a conventional BAYER process of the American or European type, which makes it difficult to adapt the alumina perfectly to its final application. Thus, an alumina producer who has a Bayer process with which he can make an alumina trihydrate while adjusting the particle size distribution independently of the residual sodium content will find that this manufacturing process gives him a considerable advantage in terms of operating flexibility. This advantage will be particularly appreciable if the performances of the process in terms of productivity and reliability are maintained, or even improved, compared with values obtained with known processes.
DESCRIPTION OF RELATED ART
The Bayer process is widely described in the specialized literature, and is the essential technique used for manufacturing of alumina for transformation into aluminum by igneous electrolysis or for use in the state of a hydrate, a transition alumina, calcinated alumina, sintered or molten alumina, within a wide range of applications in the engineering aluminas field.
According to this process, the bauxite ore is digested when hot by means of an aqueous solution of sodium hydroxide with an appropriate concentration, thus causing extraction of the alumina and resulting in a slurry composed of particles of undigested residue in a sodium aluminate solution called “aluminate liquor”.
In general, this slurry is then diluted to separate undigested residues in the aluminate liquor, by settling. Once “cleaned”, this liquor is cooled to a temperature at which it is in a strongly unbalanced supersaturated state. At this stage, it is called “pregnant liquor”. The liquor saturation or stability state, is characterized by the following ratio by weight:
R
p
=
concentration
of
dissolved
Al
2
O
3
(
in
g
/
l
)
caustic
concentration
Na
2
O
(
in
g
/
l
)
This strong unbalance results in the crystallization of alumina trihydrate in a phenomenon called “precipitation”, which is unstable but reacts slowly, and it is essential that it must be perfectly controlled. After precipitation, the sodium aluminate liquor, depleted in alumina due to the precipitation and called the “spent liquor”, is recycled after concentration to ore digestion.
Precipitation is a complex phenomenon including crystallization, crystal growth and agglomeration of crystallites leading to the production of particles of alumina trihydrate. This phenomenon takes place slowly, so that this pregnant liquor has to be circulated in a sequence of tanks with partial recycling of the trihydrate produced, this recycling being designed to facilitate precipitation of crystallites with respect to the seeds or “nuclei”. The “aluminate slurry” is the medium that circulates in the precipitation line; it is a mix of aluminate liquor and solid alumina trihydrate particles which have just precipitated or which are resulting from agglomeration or crystal growth of trihydrate particles drawn off near the downstream end of the precipitation line and reinjected at the upstream end of the precipitation line. Particles recycled due to their small size are called “fines”. In practice, precipitation is carried out differently in European BAYER lines and in American BAYER lines.
American BAYER lines use an aluminate liquor with a lower caustic concentration; although the soda is less productive (less alumina trihydrate is produced for the same volume of aluminate liquor) however it is rich in seeds. The added recycled trihydrate is a seed and a regulating makeup carried out at the beginning of precipitation, both in the first step called “agglomeration” and in subsequent “feed” or “growth” steps.
European BAYER lines use an aluminate liquor with a much higher caustic concentration. This means that more trihydrate is obtained for a given volume (see below for the definition of productivity) but makes precipitation more difficult since there are far fewer crystallization seeds. In general, the pregnant liquor is mixed with seeds in a first tank, called the seed tank, to form a “pregnant slurry”. Precipitation continues in subsequent tanks called “precipitators” either by feeding existing grains or crystallites, or by the appearance of new seeds, or by the formation of particles by agglomeration of crystallites.
In the American and European processes, direct recycling from alumina trihydrate produced proves to be inadequate when it is required to make a product with precise characteristics. U.S. Pat. No. 3,486,850 and U.S. Pat. No. 4,311,486 recommend that the trihydrate produced should be sorted before recycling fines at several steps of the precipitation. In European processes, simply recycling alumina trihydrate is not sufficient to give good control over the size and structure of precipitates. “Particle size crises”, in which the trihydrate produced is either too fine or too coarse, occur suddenly, and the remedy must be applied very cautiously without any immediate effect—other characteristics of the trihydrate produced must not be disturbed, and the productivity of the pregnant aluminate liquor during precipitation must not be reduced.
It is known that the addition of seeds produced outside the BAYER line can make process control more flexible. Thus, FR-A-2 709 302 proposes recovering sorted fines from another precipitation line. FR-A-1 525 302 proposes seeding of an auxiliary pre-calibrated seed in the form of an ultrafines gel at the beginning of precipitation, and EP-A-0 344 469 proposes ground trihydrate as an auxiliary pre-calibrated seed.
But these solutions are not economic. The solution in FR-A-1 525 302 is expensive, and the solution recommended in FR-A-2 709 302 is only attractive if there is a second precipitation line on the same industrial site. Finally, the solution provided by EP-A-0 344 469 is very expensive since it is necessary to obtain a very closely controlled particle size distribution of the ground seeds, in order to obtain a satisfactory quality of alumina trihydrate.
Thus, even making use of prior art, the expert in the field has no reliable and inexpensive industrial process by which he can make a trihydrate, with independent control over the BAYER line operating parameters, particularly with an influence on the particle size distribution and the residual sodium content, while maintaining good productivity.
Reliability includes precise and stable long term reproduction of the target characteristics starting from adjustment set values, therefore requiring low dispersion of the characteristics obtained compared with the target characteristics.
Productivity means productivity of the pregnant sodium aluminate liquor derived from alkaline digestion of bauxite using the Bayer process which precipitates during precipitation in the presence of a seed. The weight ratio Rp characteristic of the saturation state of alumina dissolved in the liquor in the B
Clerin Philippe
El Kadi Bassam
Aluminium Pechiney
Bos Steven
Dennison, Schultz & Dougherty
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