Thixoforming : Semi-solid Metal Processing

4.8
Recycling of Aluminium–Lithium Alloys from Thixoforming Processes
4.8.1
State of the Art
Since the commercial introduction of Al–Li moulding alloys in the late 1980s, the
question of recycling has presented a challenge in many respects [42]:
. The lithium contained in Al–Li residues represents a value far in excess of the
aluminium content.
. The residues can cause problems if they enter the usual recycling route, especially
in the foil sector.
. To date, the specifications for secondary casting alloys do not indicate any lithium
content and an agglomeration of Al–Li residues is not without consequences,
because in moist conditions it can react with the hydrogen being released.
In order to secure a wider use of this group of alloys, a complete recycling
concept must be developed for the residues that would facilitate 100% reuse of the
recycled Al–Li residues. For recycling, first as far as possible a closed system
( closed-loop recycling ) is desirable, in which the residues can be returned directly
to the existing process and need not be diverted through the entire secondary cycle
for aluminium. In the case of scrap recycling this would probably not be feasible:
because of the problems involved in sorting of separated types of scrap, the lithium
would be lost.
In principle, there are two possibilities for recycling of the thixo-materials. The first
and by far the most sustainable solution would be to retain the valuable alloying
elements within the alloy. This is primarily of interest in the case of clean scrap
materials that could be melted down in molten salt. The second alternative would be
to transfer the alloying elements (Mg, Li) into molten salt in order to reclaim them
subsequently from there if required. Such a procedure could be applied for both clean
and low-quality scrap materials. From the point of view of economy, the same
molten salt should be used that is used in the secondary aluminium industry (70%
NaCl þ 30% KCl with 2–3% CaF 2 additive), in contrast to the very hygroscopic
mixture of LiCl and KCl that has been proposed [43, 44]. Then recycling of the salt slag
produced could be performed in a largely conventional manner.
4.8.2
Thermochemical Metal–Salt Equilibria
4.8 Recycling of Aluminium–Lithium Alloys from Thixoforming Processesj139
In order to decide which salt mixture compositions come into consideration for
the two above-mentioned recycling alternatives, thermochemical calculations were
performed in respect of the equilibria between the relevant alloys and various molten
salts, using the FACTSAGE program. Four alloy types were combined with salt
mixtures, on the one hand based on NaCl and KCl with added CaF2,MgCl2, LiF and
on the other based on KCl and LiCl.