DE2345615C3 - Process for reducing the dusting of aluminum oxide - Google Patents

Description

The invention relates to a method for reducing the dusting of aluminum oxide with a BET surface area from 20 to 50 mVg.

Up to now, there has not been any particular preparation for aluminum electrolysis and its dry exhaust gas cleaning Oxide used. The oxide used there is said to be readily soluble in the electrolyte as well easy uptake of foreign constituents from the electrolysis furnace exhaust gases, where appropriate reintroduction of the oxide then loaded with fluorine compounds (adsorption from the Exhaust gases) is possible in the aluminum electrolysis. The corresponding reactivity of the oxide is usually determined with the help of the BET surface area. Surface areas of 20-5OmVg are considered required specified.

The generation of suitable alumina with this surface is by the Bayer process, not a problem, if one carries out a classification of the complex hydroxide, and calcined this rough hydroxide at low temperatures (1000-1050 ^r C). This is the path taken in America. It leads to an oxide with a surface area of 50 mVg and an angle of repose <40 °.

In Europe, an oxide has become established that is obtained without the expensive classification of the hydroxide, is finer in the entire grain structure and above all contains the fine fraction below 40 μm, which is not contained in the American hydroxide. This hydroxide is Ix ¹ I Tfiiipfiiiiiiren of 1200 IJOO C kiil / inieri and gives the "mealy" eiimpiiische oxide. KS has a surface area of up to about 2 ni / g and an angle of attack of> 40. This oxide is unsuitable for dry exhaust gas cleaning.

To get from this hydroxide ntis / u a suitable Oxide / u must come, the calcining tempcra'.ur be reduced. But then you get an oxide with more or less dusty properties. This can lead to operator amusement, increased oxide losses during electrolysis and Difficulties in transshipment because of its dust formation.

The invention solves the problem of reducing the dusting of aluminum oxide by simple means.

The inventive method is characterized in that, added to a calcined alumina at HOO ⁰ C a calcined in the range 1200 to 1300 ⁰ C alumina whose particle size is 90% less than 30 μηι in amounts of I to 10%.

Preferably, the amount of the fine grain oxide added is 5%. As a fine grain oxide appropriately used calciner dust.

To an oxide of the appropriate reactivity, the required surface and above all with the appropriate To obtain a low degree of dustiness, it would have made sense to add the fine fractions from the hydroxide remove and calcine at low temperature. As summarized in the table below Tests 2 and 3 show, however, that the dusting does not decrease.

Surprisingly, however, it turned out that an addition of fine-grained oxide to weakly calcined Oxide, the z. B. is obtained in a fluidized bed calciner, the degree of dust considerably lets sink. Experiments 4, 5, 6, 7 show that when small amounts of particularly fine-grained oxide are added, the Dusting is greatly reduced and an oxide is obtained whose tendency to dust is even lower than that is of highly calcined oxide. An aluminum oxide can be used as the fine-grained oxide has a fineness of 90% <30 μm. This fine fraction preferably has a high degree of calcination on. Ground or sifted oxide is particularly suitable.

The following table gives an overview of practical dust collection experiments to explain the Invention.

Dust collection attempts 2 Alumina at HOO ⁰ C
calcined 3 4th 5 6th 7th I. at 1250 ° C
calcined none none 8.6
37.0
70.8
82.2
17.8 at HOO ⁰ C
calcined at HOO ⁰ C
calcined at 1100 ° C
calcined at HOO ⁰ C
calcined at 1100 ° C
calcined 5.3
27.7
60.8
73.4
26.6 3.4 sifted none 5% fine
oxide to
mixed 7.5% fine
oxide to
mixed 10% fine
oxide to
mixed treatment 1.6 11.3
48.5
100.0 6.3
30.9
57.2
70.6
29.4 6.0
30.2
57.0
68.0
32.0 5.3
28.8
53.5
64.5
35.5 5.0
28.0
54.3
61.8
38.2 Sieve analysis
> 90μηι
> 60μηι
> 40μιΐι
> 33 μm
<33 μm 3.3 3.6 0.7 ·. 0.4 0.5 Dust loss,%

Claims (2)

Patent claims: 1. A process for reducing the dusting of alumina having a BET Oberfliiche from 20 to ^ 50 mVfe, characterized in that a calcined in IIDO ⁰ C alumina is a calcined in the range 1200 to 1300 ⁰ C alumina whose particle size to 90% is less than 30 μηι , added ια in amounts of I to 10% 2. The method according to claim I, characterized in that the amount of fine-grained added Oxids 5% cheats. J. The method according to claim I or 2, characterized in that the fine-grained oxide calcining ι ·> oven dust is used.