Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Halogenous component
Patent
1997-06-09
1999-03-23
Straub, Gary P.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Halogenous component
423240R, 42324401, B01D 5368
Patent
active
058855394
DESCRIPTION:
BRIEF SUMMARY
This application is a national stage application filed under 35 U.S.C. .sctn.371 of PCT/SE95/01392 filed Nov. 22, 1995.
FIELD OF THE INVENTION
This invention relates to a method for separating, by adsorption and for recovery purposes, impurities, such as fluorine-containing gases and fluorine-containing dust, from a gas that is emitted from a process for aluminium production. The gas emitted from the process is brought into contact with an adsorbent in the form of particulate aluminium oxide, which can be recycled as raw material to the process. To be more specific, the invention concerns a multi-stage countercurrent process combining effective cleaning of the gas with a high degree of concentration of fluorine-containing substances on the adsorbent. In an environmentally advantageous embodiment of the invention, sulphur dioxide is simultaneously removed from the gas.
DESCRIPTION OF THE PRIOR ART
In a process for electrolytic production of aluminium, such as the Hall-Heroult process where aluminium is produced by reducing aluminium oxide in a melted electrolyte in the form of a fluoride-containing mineral to which aluminium oxide is supplied, the process gases are loaded with fluorine-containing substances, such as hydrogen fluoride and fluorine-containing dust. Being extremely damaging to the environment, these substances have to be separated before the process gases can be discharged into the surrounding atmosphere, while at the same time the fluorine-containing melt is essential to the electrolytic process.
The recovery of fluorine-containing compounds from gases generated in aluminium production suffers from the inconvenience that the process gas usually is loaded also with other substances, such as sulphur dioxide, which originate chiefly from the oxidation of electrodes but to some extent also from impurities found in the raw material. If recycled to the process along with the adsorbent, these substances will be emitted to the process gases and thus be concentrated in a cycle arising in the electrolytic process and the gas treatment. If concentrated in the process, these substances often have an adverse effect on the yield of the process or interfere with the process in some other way, thereby adversely affecting the process economy. Consequently, these substances should be removed from the adsorbent before this is recycled to the process. For environmental reasons, the amount of sulphur dioxide discharged from the process should be reduced.
It is previously known to use dry adsorption processes for cleaning gases generated in aluminium production, in which case aluminium oxide may be used as adsorbent. Aluminium oxide (Al.sub.2 O.sub.3), which as raw material is supplied to the process for aluminium production, has a great capacity to adsorb (more specifically, to chemically adsorb) hydrogen fluoride. Aluminium-oxide powder of commercial qualities and of a particle size in the range of 0.03-0.16 mm has a porous structure and an active surface of 40-80 m.sup.2 /g, such that large amounts of hydrogen fluoride can be adsorbed before the aluminium oxide is saturated. It is, however, true that the adsorption capacity diminishes when the active surface is all but covered by adsorbed hydrogen-fluoride molecules, i.e. when the aluminium oxide is saturated with hydrogen fluoride. Usually, particulate aluminium oxide is efficiently and turbulently mixed with the gases from the aluminium-production process in a fluidised bed or some other contact reactor, hydrogen fluoride being then adsorbed on the aluminium oxide. The aluminium oxide, which now contains adsorbed fluorides, is separated downstream from the contact reactor with the aid of one or more filters. The aluminium oxide is then supplied to the aluminium-production process, and the fluorides are recovered. However, also sulphur dioxide is to a certain extent (as a rule 10-30%) adsorbed in these processes, and the sulphur dioxide thus accompanies the aluminium oxide back to the aluminium-production process, where it is released to the process gase
REFERENCES:
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Baverez et al. "Adsorption of Hydrogen Fluoride . . . " Journal of Metals, vol. 32 No. 1, pp. 10-14, Jan. 1980.
Bjarn.o slashed. Odd E.
Wedde Geir
ABB Flakt AB
Straub Gary P.
Vanoy Timothy C
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