Specialized metallurgical processes – compositions for use therei – Processes – Electrothermic processes
Patent
1984-11-21
1986-03-11
Andrews, Melvyn J.
Specialized metallurgical processes, compositions for use therei
Processes
Electrothermic processes
75 68R, 100 35, 210773, 210808, C22B 700, C22B 2100
Patent
active
045753920
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a process and an apparatus for the recovery of liquid aluminium by the compression of hot dross. It should be noted that the term aluminium used in the specification of this application also embraces all the alloys of that element.
The cycle of production of aluminium from its ores or recycled waste material generally comprises a phase involving melting of the metal followed by a casting phase. The man skilled in the art is aware that, in the melting phase, formed at the surface of the metal bath in furnaces is a layer of a substance which is referred to as `dross`, analysis of which shows that the dross comprises non-metallic particles such as oxides, carbides, nitrides, etc, between which droplets of metal are trapped. If the contents of the furnace are poured as they are, the dross will be entrained therewith and will form inclusions in the solidified product; in subsequent transformation operations, for example rolling, such inclusions will give rise to the occurrence, if not of internal or surface defects, at least local areas in which the properties of the material suffer from heterogeneity, which is detrimental to high-quality products.
It is therefore found to be necessary for the dross to be removed from the metal before the casting operation, and that is generally done by skimming the bath by means of a scraper in the course of an operation which is referred to as the dross removal operation. However, the dross may contain a substantial proportion of metal, up to 95% of the mass of dross, and discharging such dross as it stands would result in a serious reduction in the level of efficiency of making use of the metal produced.
It is for that reason that the man skilled in the art turned to seeking ways of recovering the maximum possible amount of metal contained in such dross. One solution comprises treating the dross in the melting furnace itself by means of a flux, the effect of which is to cause the metal droplets to coalesce and to facilitate transfer thereof from the dross to the metal bath. However, apart from the harmful influence of the flux materials on the properties of certain alloys, the efficiency thereof in regard to the level of recovery remains limited so that, irrespective of the possible forms of treatment in the furnace, it is inevitably necessary to treat or re-treat the dross in special pieces of equipment after the dross has issued from the furnace.
Thus, it is known to use a working apparatus in which the dross is agitated in the presence of air and possibly flux so as to cause oxidation of a part of the metal that it contains. The highly exothermic oxidation reaction makes it possible for the dross to be maintained at a sufficient temperature for the metal droplets to be able to coalesce and to flow downwardly in the vessel where they are easily separated from the dross.
The hot dross which is drained off then goes into a hammering apparatus and is then separated in accordance with its granulometry so that a fresh portion of the metal which had escaped separation in the working apparatus is then extracted.
That type of treatment suffers from the disadvantage that, in order to ensure oxidation, it requires a loss of aluminium which is generally higher than 10% of the amount of aluminium contained in the dross. In addition, recovery of the material after the hammering phase is incomplete and more than 5% of the aluminium remains in the dross which is then discharged.
Besides the questions relating to the metal yield, that type of treatment is carried out under conditions which are often unsatisfactory from the point of view of health, by virtue of the fumes which may be emitted in the oxidation phase and the operations involving handling hot dross between the various pieces of apparatus for treating same, without mentioning the problems of noise and water pollution raised by discharging substances containing relatively soluble flux materials which emit harmful ions.
It is for that reason that this art has seen the appearance of other processes
REFERENCES:
patent: 2278135 (1942-03-01), Osborn
patent: 4057232 (1977-11-01), Ross et al.
patent: 4137073 (1979-01-01), Singleton
patent: 4386956 (1983-06-01), Roth et al.
patent: 4527779 (1985-07-01), Roth et al.
Julliard Jacques
Tirilly Louis
Vigier Pierre
Andrews Melvyn J.
Cegedur Societe de l'Aluminium Pechiney
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