Zinc Removal from Aircraft Aluminium-alloy scrap
C.R. Boeree
Y. Yang – Mentor
Y. Xiao – Mentor
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Abstract
This report introduces a method for the removal of zinc from aircraft aluminium-alloy scrap. The driving force for this research is the demand from the secondary aluminium industry for a low zinc content in the Al-alloy scrap, and the growing demand for aluminium in the world. Meanwhile large amounts of obsolete aircraft are stored because of the problems during recycling of the aluminium. The influence of coating on the aircraft scrap recycling is studied by treating the scrap in a de-coating process, regarding its influence on the melting process of the scrap. The coating accounts for approximately 1,6 wt% of the scrap. On a small scale, the melting of de-coated scrap shows much better results in coalescence and the separation of alloy from slag. On a larger scale, the melting results are also better for de-coated scrap, while the melting process of not de-coated scrap forms a reasonably coalescent alloy piece. The recyclability of aircraft scrap is studied in the presence of different salt fluxes. The addition of 10 wt% cryolite promotes the coalescence of the alloy and the separation of alloy and slag. However, this salt flux removes magnesium from the alloy. Addition of magnesium fluoride maintains or even increases the magnesium content in the alloy, but gives poor results in the melting process. The use of a higher salts-to-alloy ratio does not improve the melting results and possibly even counteracts the evaporation of zinc for both cryolite and magnesium fluoride as an addition. To improve the zinc removal from the aluminium alloy, a lance is used to blow argon gas into the alloy melt. The argon gas reduces the partial pressure of zinc, thereby promoting the evaporation of zinc from the melt. The tests are performed on an alloy with an initial zinc content of 2,42 %. A test with argon blowing lowers the zinc content to 1,88 %. However, a similar test without argon blowing results into an alloy with a zinc content of 2,11 %. The argon blowing only reduces the zinc content with 0,23 % compared to the similar test in which the same alloy is molten but no argon is blown into the melt.