Metallurgical apparatus – Means for melting or vaporizing metal or treating liquefied... – By means introducing treating material
Patent
1999-03-08
2000-05-09
Kastler, Scott
Metallurgical apparatus
Means for melting or vaporizing metal or treating liquefied...
By means introducing treating material
266233, 266235, C21C 700
Patent
active
060600136
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The invention relates to a rotary gas dispersion device for the treatment of a bath of liquid aluminum or aluminum alloys. In the rest of this text, the word "aluminum" will be used in the generic sense to mean "aluminum and its alloys."
STATE OF THE ART
Liquid aluminum output from electrolysis cells or remelting furnaces contains dissolved impurities and impurities in suspension. The most important of these impurities are hydrogen, alkaline elements such as sodium or calcium and oxides (and particularly aluminum oxide itself).
Liquid aluminum is subject to various treatments to eliminate these impurities which have negative consequences on subsequent properties of the partly finished product. The most widespread of these treatments that uses a combination of chemical reactions and flotation phenomena, consists of adding an inert or reactive gas into the bath in the form of small bubbles. For example, an argon bubble will entrain a solid inclusion in suspension with it to the bath surface. Similarly, a chlorine bubble will react with the sodium contained in the bath and produce a sodium salt that will also be transported to the surface of the bath. These types of treatment by the action of gases can be carried out discontinuously in a furnace or in a crucible. But it is usually done continuously between the furnace and the casting machine in a treatment ladle like that shown diagrammatically in FIG. 1.
The liquid metal to be treated enters the first compartment (2) of the ladle through an inlet spout (1). As it passes through it is treated by gas bubbles (4) dispersed by the rotary device (3). The metal thus treated overflows into an outlet compartment (5) equipped with a baffle (6) and exits from the ladle through the outlet spout (7).
The gas to be dispersed in the liquid bath is sometimes still injected using simple tubes, but the most widespread technique consists of using a rotary dispersion device composed of a hollow rotation shaft through which the gas is inlet and a rotor with the most appropriate shape to disperse gas bubbles in the bath. Obviously, the treatment is most efficient when the exchange surface area between the bath and the gas is a maximum. This is obtained by designing the rotor to produce very small bubbles, to project these bubbles throughout the volume (with the smallest possible dead volume) and create recirculation within the bath itself so that the liquid comes into contact with the bubbles (always the smallest possible dead volume).
This search for maximum treatment efficiency by intense stirring in the volume of the bath results in permanent surface agitation, often called "surface waves," by splashes from the bath caused by large bubbles rising and by a vortex phenomenon around the rotation axis. These three phenomena create a risk of adding new inclusions into the bath and generating annoying oxidation of the liquid aluminum.
An attempt has been made to eliminate or reduce these disadvantages.
For example, U.S. Pat. No. 4,618,427 suggests a radical change in the technology of gas dispersion devices. This device does not have the disadvantages mentioned above, but this type of rotor only creates very slow recirculation of the liquid metal, which is equivalent to reducing the metal/gas interface and consequently the efficiency of the process.
Patent Application EP 0347108 proposes combining gas treatment and filtration in the same device. A filter layer is inserted between the gas injection rotor and the surface of the liquid metal. Gas bubbles pass through the filter and rise to the surface, and it is understandable that surface turbulence should be very small, since the filter distributes bubbles and interrupts any vigorous bubbling. However, this device has serious disadvantages: firstly, the filter layer is an expensive device, is difficult to use, gets clogged and must be periodically replaced; secondly, the size of the rotor is obviously small to facilitate its passage through the filter layer and to assure a seal in this position. The conica
REFERENCES:
patent: 3227547 (1966-01-01), Szekely
patent: 3982913 (1976-09-01), Feichtinger
patent: 4401295 (1983-08-01), Yoshida
patent: 5160693 (1992-11-01), Eckert et al.
Bertrand Joel
Le Brun Pierre
Xuereb Catherine
Aluminium Pechiney
Kastler Scott
Pechiney Rhenalu
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