DD139833A1 - PROCESS FOR PREPARING DUST ARMED GROB CRYSTALLINE ALUMINUM FLUORIDE TRIYDRATE - Google Patents

Abstract

Coarsely crystalline low-dust aluminum fluoride with a dust index below 10 can be obtained by bringing a supersaturated Aluminiumfluoridlösung under the conditions of DD-PS 88 080 for crystallization and the crystal slurry is then separated with a peeler centrifuge, wherein during the Ausschälprozeß the centrifuge coefficient to a value of 3 to 30, preferably 5 to 15, is set.

Description

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Process for the preparation of low-dust coarsely crystalline aluminum fluoride trihydrate

Field of application of the invention

The invention relates to a process for the production of coarsely crystalline low-dust aluminum fluoride trihydrate,

Characteristic of the known technical solutions

It is already known that the production of coarsely crystalline monohydric aluminum fluoride trihydrate by crystallization from a hot supersaturated solution of aluminum fluoride is characterized in that during the crystallization period under turbulent conditions

-2 conditions a Froude number range of 1.5 · 10 to

8,0 .10 is set (WP 88,080). In this case, a crystal slurry is obtained, which consists of Aluminiumfluoridtrihydratkristallen whose grain shape and grain size is largely retained in the subsequent calcination, so that only small dusting losses during thermal dehydration to aluminum fluoride on.

The separation of the obtained according to DL-PS 88,080 coarse Aluminiumfluoridkristalle from the mother liquor is carried out in practice by filtration or centrifugation, which is given in the industrial implementation of the process of centrifugation preference. It is observed in the application of continuous peeler centrifuges, the phenomenon that the peeled

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Crystal pulp after calcination gives a product containing a coarse fraction of ^ 6O * The balance of about 5 % consists of very fine aluminum fluoride particles, which cause the product despite its coarsely crystalline nature, a dust index of more than 30 has. Under the dust index is the amount of dust to understand * which is deposited in the free fall of a product sample of 10 g from 1 m height by aspirating 50 1 of air at an air. Speed of 1 nr / h in a downpipe.

Taking into account the fact that in a chronic intake of 20 mg daily in the form of fluorine-containing dusts in a human there is already a risk of fluorosis disease, the aluminum manufacturers demanded that the aluminum fluoride either granulated or in coarse crystalline form with a dust index by Max. 10 is used.

Object of the invention

The aim of the invention is to eliminate the above-mentioned deficiency.

Explanation of the essence of the invention

The object of the invention is the production of an aluminum fluoride with a dust index of <T10, which meets the occupational hygiene requirements of the aluminum industry.

It has now been found that the recovery of a low-dust AlFo product with a dust index of <^ 10 is possible, if under the crystallization conditions of. DL-PS 88.080 is separated off before the calcination at ca, 60U ⁰ C by means of a peeler centrifuge, wherein during the Ausschälprozesses the centrifuged crystals the Ver¬

from centrifugal acceleration to gravitational acceleration is adjusted to give a centrifuge index of 3 to 30, preferably 5 to 1b. This centrifuge code range is calculated according to the formula

n ² ♦ R

Z = 900

calculated, where Z = centrifuge index, η = centrifuge speed in min and R is the radius of the centrifuge drum. When a centrifuge coefficient of less than 3 is used, the stripping process proceeds so slowly that it is no longer possible to carry out the process on an industrial scale, whereas with a centrifuge indentation of more than 30, significant mechanical destruction of the coarsely crystalline aluminum fluoride trihydrate takes place Product after thermal dewatering a dust index of 20 - 50 and higher results, the use of which in the melt electrolysis of alumina for the production of aluminum does not meet the occupational hygiene requirements. The relationship between the centrifuge index during stripping of the aluminum fluoride trihydrate and the dust index in the calcined aluminum fluoride can be seen from the following comparison:

Centrifuge index when peeling Dust code na © Calcining 5 6 10 7 20 8th 30 10 40 20 50 40

Ausführungsbei game .

The invention will now be explained with reference to an embodiment,

6 m of a 13% supersaturated AIF ^ solution are dissolved in

Crystallized from a crystallizer at 95 C with the addition of 200 kg of aluminum fluoride trihydrate seed crystal and uniform stirring, whereby turbulent stirring

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conditions a Froude number of 5.0. 10 set

becomes. (Pr = η d / g)

After a crystallization period of 6 hours 920 kg of aluminum fluoride trihydrate are deposited, which is centrifuged in a peeler centrifuge from the adhering mother liquor using a centrifuge coefficient of 270 (n = 570 U / min, d = 1.5 m). Subsequently, a strong deceleration of the centrifuge to a speed of 100 rev / min, which corresponds to a centrifuge coefficient of 8. After calcination at 600 ⁰ C results in a coarsely crystalline aluminum fluoride having a dust index of 7, U. ·

Claims (1)

-s · - 2 03 8 invention claim Process for the preparation of a coarsely crystalline low-dust aluminum fluoride by crystallization of
Alf ^. 3 HpO from a hot, supersaturated aluminum fluoride solution, characterized in that the crystallized product is separated from the solution by means of a peeler centrifuge
Mother liquor separated and at a centrifuge index of 3 - 30, preferably 5-15, is peeled off.