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Selective anodic dissolution and cathodic
redeposition of Al
- Impurities more noble than Al are
"trapped" in the impure anodic layer (i.e Si, Fe)
- Impurities more oxydable than Al are
"trapped" in the electrolytic refining bath (i.e Mg, Ba)
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- The first refining process referred to
as the "three layers" process was developped by the
American Thomas Betts in 1905. It was not industrialized until 1922
when perfected by Hoopes.
- This method consisted of electrolysis
in a fluoride salt solution, with a soluble anode of standard liquid
commercial purity aluminum.The metals less electropositive than
aluminum were trapped at the anode and only aluminum metal is
transferred to the cathode which can thus reach a purity of some
99.90%. The purity level was still considerably off the mark.
- The method met serious technological
problems at the industrialization level, because of still very high
operating temperatures (> 900°C) which favored attack of the
refractories, pollution of the metal, and environnmental troubles.
- A major step was overcome in 1932,
when the french Pechiney engineer Robert Gadeau invented a much more
efficient bath composition, able to sustain electrolytical refining
at much lower temperatures (700°C-800°C).
- PECHINEY then erected a small series
of refining vessels at the Saint Jean de Maurienne plant in the Alps
(600 tons/year of 99.99% pure metal referred to as 4N).
- PECHINEY perfected this technology at
its MERCUS plant. As this production was
very successfull, a first refining plant was then built and put into
service in 1954.
- 1980, the number of vessels
doubled.The purity was in the
4N-5N level (from 99.99% to 99.999%).
- In 1960,PECHINEY began an extensive
research into radically new methods to improve the purity level,
while continuing its production using traditional techniques. This
was called segregation .
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