Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Patent
1993-01-04
1995-04-11
Wyszomierski, George
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
75715, 75754, 148710, C23C 2234
Patent
active
054054281
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to the decoating of contaminated scrap metals, particularly aluminum scrap, and/or to the surface treatment of metals, particularly aluminum and alloys thereof, in order to reduce the susceptibility of such metals to oxidation during heat treatments.
BACKGROUND ART
The recycling of contaminated metal scrap material, particularly aluminum scrap, is a large and important industry nowadays but it depends to a very large extent for its commercial viability on the percentage of metal that can be successfully recovered from the scrap. Metal scrap material may be of a variety of types, such as turnings from machining operations, used food and beverage containers, large castings, and the like, but in virtually all cases the major contaminants are organic materials such as oil, paints, adhesives and paper. In order to maximize the recovery and minimize the emission of pollutants, it is necessary to remove such organic contaminants from the scrap material before subjecting the scrap to remelting for metal recovery. By removing or substantially reducing organic contaminants, it is possible to increase the percentage of recovered metal by as much as 10-15% by weight, or sometimes more.
One of the best known and most used processes for the decontamination of scrap utilizes the pyrolysis of the organic contaminants in a hot air current, e.g. as carried out in a rotary kiln or packed bed conveyor decoater. This involves contacting the scrap with a moving current of air at a temperature of 400.degree.-600.degree. C. in order to bring about heating of the scrap and combustion of the contaminants. Because of a limited rate of heat transfer when large pieces of scrap are processed, the scrap must first be shredded or ground to increase the surface to volume ratio, which requires a troublesome and costly additional step. Even when shredding is carried out, however, it is difficult to control the temperature of the scrap during the decontamination process and it is found that the temperature can vary considerably according to the position of the scrap in the load, i.e. due to channelling of the heated air, some of the scrap may be overheated and other parts may remain unsuitably cool. Temperature variation of this type results in some of the scrap being unduly oxidized while other parts of the scrap may be only partially decoated. Aluminum alloys having high magnesium contents (e.g. 4-5% by weight) are especially susceptible to oxidation or even melting when overheated and metal recovery can be considerably reduced when such metals are subjected to the conventional decoating process.
The conventional hot air process encounters further difficulties when attempts are made to decoat thin gauge aluminum scrap (e.g. up to 100 microns, and usually 6-7 microns, in thickness) containing from 15 to 70% by weight of aluminum and a high proportion of flammable material, such as paper, e.g. as is the case for thin laminated materials used for the manufacture of hermetically sealed food and beverage containers, and the like. Processing of such material in a stream of hot air is generally not possible because the rapid combustion of the flammable material causes overheating and oxidation of the thin aluminum and may result in partial fusion. Given the thinness of the sheets, the least amount of surface oxidation dramatically decreases the metal recovery and complete oxidation or melting of the metal may result.
Alternative apparatus for decoating metal scrap has been suggested, but not put into practice on a wide scale. For example, British patent publication no. 2,046,888 B of Nov. 19, 1980 in the name of Tolltreck Ltd. discloses the use of a heated fluidized bed for the decoating of contaminated scrap. Similar procedures are also mentioned in U.S. Pat. No. 4,508,564 issued on Apr. 2, 1985 to Kennedy and in U.S. Pat. No. 3,250,643 issued on May 10, 1966 to Sergent. However, while the use of a fluidized bed improves the heat distribution throughout the charge of scrap undergoing the decoating treat
REFERENCES:
patent: 3250521 (1966-05-01), Sergent
patent: 3250643 (1966-05-01), Sergent
patent: 4508564 (1985-04-01), Kennedy
patent: 4654088 (1987-03-01), Fitzpatrick et al.
patent: 4929283 (1990-05-01), King et al.
Dube Ghyslain
Tremblay Francois
Alcan International Limited
Wyszomierski George
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