Specialized metallurgical processes – compositions for use therei – Processes – Producing or treating free metal
Patent
1995-06-13
1996-11-12
Simmons, David A.
Specialized metallurgical processes, compositions for use therei
Processes
Producing or treating free metal
75671, 75678, 75679, 75686, 205364, 205372, C22B 900, C22B 2100
Patent
active
055735746
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
The invention relates to an aluminium of above 99.998% purity which is a product of electrolytic refining, and the invention also relates to a process for obtaining this aluminium.
DESCRIPTION OF RELATED ART
High purity aluminium which is used for metallization of electronic circuits, for example, needs to have aft extremely low content of impurities, particularly those which tend to emit alpha particles which can modify the content of the circuit memories.
Generally speaking, at least 3 categories of high purity aluminium are used by circuit manufacturers, after mostly adding high purity alloy elements: content of less than 20 ppb (parts per milliard 10.sup.-3 ppm) and a minimum aluminium content of above 99.998%. less than 5 ppb and a minimum Al content of above 99.999%. content of above 99.9995%, (and even sometimes above 99.9997%) and a U+Th content of less than 1 ppb, and even sometimes less than 0.3 or 0.1 ppb.
Rare earths, some of which, like samarium, have a significant alpha radioactivity, are also undesirable. By way of example, 10 ppb of natural samarium emits as many alpha particles as 0.1 ppb of uranium 238. The high purity aluminium used for this application is usually obtained by way of two successive stages:
a) electrolytic refining by way of the process known as 3 layer electrolysis, as described in French Patent Nos. 759588 and 832528, for example.
These 3 layers which are kept at a temperature of at least 700.degree. C. are successively: potential. fluorides and possible chlorides floating on the base alloy.
The refining is done by transferring the aluminium from the bottom alloy to the surface layer, leaving in the impure bottom alloy impurities which are nobler than aluminium, and by oxidising the metal impurities which are less noble than aluminium and which are trapped in the bath.
However, it is seen that although electrolytic refining is very effective in eliminating major impurities from the primary aluminium, such as Fe, Si, Ga, Zn, it is not very effective for eliminating certain impurities in particular such as uranium, thorium and rare earths which are always present in the aluminium to be refined but also in the halogenated salts which constitute or correct the electrorefining bath. In certain cases, these latter can even lead to an increase in the content of these elements. The reason for the inefficiency with respect to these particular impurities is still unaccounted for.
Thus, the metal which has been electrolytically refined from the primary aluminium, even high purity aluminium, or even from aluminium which has already been refined electrolytically, still contains more than 50 ppb U+Th and more than 200 ppb of rare earths (mainly La, Ce, Nd, Pt, Sm). For this reason, this stage of electrolytic refining, for the application indicated, is always followed by a second purification stage to eliminate these harmful impurities.
b) physical purification of the electroredefined metal by way of various processes, such as the CZOCHRALSKY method of pulling crystals, zone fusion or segregation. This latter process is described in French Patent 1594154 (=U.S. Pat. No. 3,671,229). It consists in cooling the aluminium to be purified in a crucible under properly laid down thermic conditions in order to recover and separate the crystals formed which are heavier and of greater purity than the surrounding liquid. By way of successive refusion and resolidification operations, accompanied by compression which enables the impure liquid remaining trapped between the crystals to be expelled, a very pure metal is obtained at the bottom of the crucible, whilst the upper part which has remained liquid contains the greatest part of eutectic impurities initially contained in the metal to be purified.
An example of the successive application of these two techniques to obtain an aluminium of 99.999% purity is illustrated in French Patent No. 2445380 (=U.S. Pat. No. 4,222,830).
As mentioned in that patent, the fractionation crystallization stage usually has a mediocre me
REFERENCES:
patent: 4273627 (1981-06-01), Dawless et al.
Kondo, et al., "The Production of High-Purity Aluminum in Japan", JOM, Nov. 1990, pp. 36-37.
Hannibal, et al., Metallwissenschat und Technik, vol. 27, No. 3 (Mar. 1973), pp. 203-211.
Aluminium Pechiney
Koehler Robert R.
Simmons David A.
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