DE2346537A1 - PROCESS FOR PROCESSING ALUMINUM OXIDE, SODIUM ALUMINATE AND / OR ALKALIZED ALUMINUM OXIDE USED TO SEPARATE FLUORINE COMPOUNDS FROM EXHAUST GASES - Google Patents

Description

Process for the processing of aluminum oxide used to separate fluorine compounds from exhaust gases, Sodium aluminate and / or alkali-safe tem

The invention relates to a method for working up for Separation of fluorine compounds from waste gases used aluminum oxide, sodium aluminate and / or alkalized aluminum oxide .

Such substances become particularly dry aasorptive Cleaning of fluorine-containing exhaust gases from aluminum electrolysis cells used, e.g. E. in such a way that the dry adsorption takes place in a sta ^ k expanded aluminum oxide fluidized bed.

The Schrne.l ζΐ lußelektrolyse for the production of cottage "> aluminum is technically described at temperatures close to 1000 ⁰ C, a melt is of cryolite, frequently as the electrolyte of an excess of j.uorid Alurniniumfluorid as well as other additives such as Caiciumfluorid and Lithiumf added will.

With the relatively high electrolysis temperature, the electrolyte has a noticeable vapor pressure. As a result, the exhaust gases from the electrolysis cells contain volatile fluorine compounds or *. whose ^{TJmwandlungsprodu:} rte that in the cooled Dep;, chiolite-h, öind contain as fine dust beside dusty alumina as solid in the form of compounds VIIc cryolite, Alumixiiumfluo ^ id,

509817/0897

BAD ORiGiNAL

In addition, the exhaust gas contains gaseous hydrogen fluoride, the secondary to hydrolysis of the fluorine compounds mentioned. Depending on the type of suction and dilution, the exhaust gases contain up to 1000 mg HF / Nm, but usually less than 100 mg HF / Nm. The dust content in the exhaust? is of the same order of magnitude .

While the removal of hydrogen fluoride in practice has hitherto mainly been carried out by wet washing, various dry adsorption processes have recently become known. The method described in DT-OS 2 O56 O96, in which the dry adsorption takes place in a strongly expanded aluminum oxide fluidized bed, is particularly favorable and economical. When the latter method is used in practice, it has been found that only an amount of 10 % or less of the total oxide required for the electrolysis needs to be used for gas cleaning.

Together with the fluorine compounds, other compounds, e.g. JrS. of phosphorus, vanadium, K.Lsens, Titans, sulfur, silicon, etc. It is not in detail clarified whether these connections are primarily z. T, as gaseous compounds attack; Secondly, they are found as solid compounds in that used for fluorine adsorption Alumina or the like again. When this oxide is returned to the electrolysis cells, these impurities also get into the Electrolytes and cause deterioration in the operation of the electrolysis or the quality of the metal produced. A method is already known through an after-treatment of the fluorine-laden Oxides a disruptive carbon content largely thereby to remove the solids used for exhaust gas purification and then intended for use in the Elektrölyseöfan in addition to fluorine also contain other compounds and elements, in particular also carbon in the form of coal dust, one thermal treatment in which the carbon dunch combustion is removed urici the vjei of exhaust gas purification

509817/0897

predominantly absorbed by adsorption as hydrogen fluoride and not firmly bound-fluorine into a stable chemical aluminum-fluorine compound is convicted.

The invention is based on the object of creating a method as explained at the beginning with which the above-mentioned impurities, ie compounds %. B. of phosphorus, vanadium, iron, titanium, sulfur, silicon, etc., can be removed from the system electrolysis furnace / adsorber or effectively reduced therein. According to the invention this is achieved in that the adsorbents are treated by pyrohydrolysis or chlorination. In this way it is possible to largely or completely eliminate the contamination.

In one embodiment of the invention it is provided that the adsorbents are divided into grain fractions before working up. It is expedient to remove fractions below 20 μm, preferably below 15 μm, which contain the main amount of impurities. This is because it has been found that the aforementioned harmful impurities are accumulated in the fine fraction of the aluminum oxide / dust mixture produced by fluorine adsorption. A sieve analysis showed z. B. that> 70 % of the iron and vanadium contained in the fluorine-laden oxide are contained in the very fine fraction <20 μm, preferably <15 μm, which makes up about 20 % of the oxide dust Gerniöches. This procedure is also advantageous when using coarse-grained adsorbents.

A variant of the method as described above is that the division into grain fractions by '..' indichtung is made. The following example may serve for this purpose:

Xorn fractionation of a fluorine-laden oxide by air classification. Le.1stnng of the wind sifter ^: 1 t / h.
Tronriüchnitt! <■ 20 / bm.

509817/0897

pollution Analysis of the
unfractionated
th mixture Coarse fraction
(78% by weight ) Fine action
(£ 22 wt.) Pe ₀ O, wt. #
2 3 0.27 0.19 0.59 ν _ο 0 _κ wt. % 0.027 0.016 0.063 0.058 0.034 0.151

But it is also possible to split the grain into fractions by sieving. An example of this is the following:

Grain fractionation of a fluorine-laden oxide-dust mixture by sieving.

Grain fraction impurity (wt / ^)

/ around wt.% ^Fe 2 ° 3 ^P 2 O 5 ^V 2 O 5

90 4th 0.18 0.06 0.002 90-75 7th 0.06 0.02 0.001 75-60 8th 0.04 0.03 0.001 60-45 12th 0.06 0.02 0, Cü2 45-30 25th 0.06 0.02 0.004 30-15 24 0.13 0.03 0.007 15th 19th 0.95 0.18 - 0.008 100 0.25 0.05 0.02

Wt.% Impurity
in faction

ν 15 / around 72 57 95

One embodiment of the method according to the invention is given therein sees that the division into grain fractions is already carried out when the adsorbents are discharged from the adsorber. It is also possible that the division with the help of a with several fields and 3t-.? vorgHi'iOinmen.

509817/0897

Suitably happens pyrohydrolysis at temperatures above 500 ⁰ C, preferably above 600 ⁰ CZB this is done in such a manner that the oxide used for the adsorption after loading a pyrohydrolysis, for example in a rotary kiln, at about 650-700 ⁰ C is subjected. In the process, hydrogen fluoride escapes in concentrated form, which can be absorbed in a small dry or wet cleaning apparatus. For this it is necessary to apply not essential in the pyrohydrolysis temperatures above 750 ⁰ C, as in the other case, the specific surface area and the adsorption capacity would be reduced. The largely defluorinated oxide can be reused to adsorb hydrogen fluoride from the furnace exhaust gases. This cycle can be repeated 6-10 times without a disruptive deterioration in the adsorption capacity. After the last cycle, the oxide is subjected to pyrohydrolysis at 1000 ⁰ C one may, for example, and a use for example in the ceramic or refractory industry be supplied.

Experiments have shown that the chlorination with chlorine gas and / or Hydrogen chloride gas is expediently carried out at temperatures above 600 C. As tests have shown, the Chlorination largely removes the impurities such as iron, titanium, etc., so that the chlorinated adsorbents are immediately removed Electrolysis can be supplied.

Finally, it should be mentioned that the measure of pyrohydrolysis makes it possible to reduce the effective amount of the oxide used for adsorption to a small fraction, for example 1-2 %, of the oxide required for electrolysis.

Execution; examples »

1. In a fixed bed, the oxide used as an adsorbent (►30 g-? Roben) in the air stream (0.3 NirrVl-uft / hour) in the presence heated by steam (pyrohydrolysis).

- 6 -5098t7 / 0897

conditions Temp. Water content -, 1600 F content BET-Ober- _p Duration/ ⁰ C of the air g / Nnr 76O Weight $ area g / m Hours. of pyrohydrolysis 280 15th Oxide before 650 1.5 15th 0.5 700 1.2 12th 0.5 750 1.2 7th 0.5

Determination of the surface according to Brunauer-Emmet-Teller

2. In a plague bed, an oxide used as an adsorbent was treated with chlorine gas / air or HCl / air. It changed the content of impurities as follows:

Chlorination conditions

Temperature ⁰ C

Impurities in the oxide (wt

^Fe 2 ° 3

TiO,

Oxide before chlorination 1.2 0.28

a) Chlorine-air mixture (1: 1)

400 ° C 0.7 0.25

500 ⁰ C 0.7 0.21

600 ⁰ C 1.0 0.05

700 ⁰ C 0.6 o, o 4

800 ° C - 0.02

b) HCl-air mixture (1: 1)

65O ⁰ C - 0.04

0.02}

0.023

< 0.003 <0.003 < 0.003

, 003

0.024

0.026

0.011 0.0Co 0.006

0.009

Similar results are achieved when working up loaded sodium aluminate.

509817/0897

Claims (9)

Claims 1. Process for reprocessing for the deposition of fluorine compounds aluminum oxide, sodium aluminate and / or alkalized aluminum oxide used from exhaust gases, characterized in that the adsorbents by pyrohydrolysis treated or chlorinated. 2. The method according to claim 1, characterized in that the Adsorbents are divided into grain fractions before processing. 5 · The method according to claim 2, characterized in that the Division into grain fractions is made by air sifting. 4. The method according to claim 2, characterized in that the division into grain fractions is carried out by sieving. 5. λ'erfahren according to claim 2, characterized in that the The adsorbents are divided into grain fractions when the adsorbents are discharged from the adsorber. 6. The method according to claim 5 *, characterized in that the Division is recorded with the help of an electric fliter equipped with several fields and dust bunkers. ?. Method according to one OAER plurality of dc-r claims 1-6, characterized in that the pyrohydrolysis at temperatures above 500 ⁰ C, preferably above 600 ° C, is carried out. 8. Ancestors according to a cd ^ r several of claims 1 - 6 " characterized by the fact that the chlorination r> i with Chlorjras and / or hydrogen chloride gas is carried out at temperatures above 600.degree. S09B17 / 0897 _{BAD 0RlelNAL} 9. The method according to one or more of claims 1-7 * characterized in that the adsorbents treated by pyrohydrolysis are returned to the adsorber for separation of fluorine compounds are entered. 509817/0897