Residues of aluminum dross, an unavoidable by-product of aluminum production, have been discharged over than 200,000 tons per year in Japan. Aluminum dross is composed of aluminum, aluminum nitride, Al₂O₃, chlorine and other oxides. The main purpose of this study is to investigate the feasibility of nitrogen removal from residues of aluminum dross by hydrothermal reactions. Aluminum nitride and aluminum dross were treated at temperature ranging from 100 to 400°C and corresponding saturated vapor pressure. The reaction time was varied from 0 to 120 min. For aluminum nitride, 91% of nitrogen was converted into ammonia under the reaction conditions of 200°C and 10 min. In the case of aluminum dross, it was obtained a reduction ratio of 53% in the nitrogen contents at 250°C for 10 min. A bench plant of aluminum dross treatment system was developed in terms of above results. The treatment capacity was 60 kg of aluminum dross per unit in the bench plant. These results demonstrated that hydrothermal reactions could be successfully used as a treatment strategy of aluminum dross.

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To remove magnesium, one of the main impurity elements in aluminum scrap, compound-separation method by addition of Shirasu (natural mineral from Kagoshima) consisting of SiO₂ and Al₂O₃ into the molten metal was conducted. As additional method, effects of the stepped additional method and continuous additional method of Shirasu on the removal efficiency was studied. Aluminum scrap containing 4.63 mass% magnesium was melted using an electric furnace in air atmosphere, When Shirasu is added into molten scrap, MgAl₂O₄ and MgO are formed as the dross by the reaction between Shirasu and magnesium in the molten scrap. By removing the dross, it is feasible to remove magnesium in molten scrap. In the stepped additional method, Mg₂Si is formed by the reaction between magnesium and silicon produced by decomposition of SiO₂ during solidification after each stepped addition. The amount of Mg₂Si in molten scrap increases with the increase in the number of Shirasu addition, resulting in the saturation of magnesium removal. In the continuous additional method, the amount of removed magnesium increases with the increase in the number of Shirasu addition because Mg₂Si is not formed in molten scrap.

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Machined chips of the Mg–Al–Zn alloy were converted to a raw material, and the recycling from Mg–Al–Zn system alloys to high-purity Mg–Zn alloys by the vacuum distillation method was examined. For 100 g of raw material AZ31 magnesium base alloy machined chips, a raw material temperature of 600°C, a condenser temperature of 355°C, and a purification time 7 h, the evaporation ratio is nearly 95%. The condenser condensate becomes about 93%, and Mg and Zn in the raw material can be almost completely recovered in the condenser as high purity Mg–Zn alloy that is free of impurities such as Fe, Ni and Cu. The corrosion rate of recycling extrusions became a little worse in comparison with about 0.5 mm/y for 4 N magnesium extrusion samples, and the corrosion resistance became better than that of AZ91D magnesium base alloy die castings. The recycling extrusions was elongated a little less in comparison with the 4 N magnesium extrusions, and the tensile strength and the 0.2% proof stress increased to about 50 MPa.

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Porous aluminum has the advantages of good lightweight property and high specific strength. By applying porous aluminum as materials for automobile and structural component, both improvement of fuel efficiency and lightening can be realized. Authors have proposed the new fabrication method of porous aluminum using friction stir processing, i.e. Friction stir processing route (FSP route), which has low manufacturing cost and high productivity. Pore structure in porous aluminum depends on a holding temperature and a holding time when the blowing agent is foamed, and affects the mechanical properties. In this study, changing a holding temperature and a holding time, porous aluminum is fabricated by FSP route. Porosity of porous aluminum rises at short holding time when the holding temperature is high. Pore structures change even if the porosity is almost same and also changes quickly with the passage of holding time.

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Closed-cell porous aluminum is expected to be used in automobile components, because porous aluminum has good lightweight properties and can realize the improvement of fuel efficiency. Recently, a new processing route for fabricating porous aluminum, in which friction stir processing (FSP) is used, has been developed. In the FSP route, a precursor is fabricated by mixing blowing agent powder and stabilization agent powder into aluminum alloy plates by the significant stirring action of FSP. In this contribution, ADC12 porous aluminum is fabricated by the FSP route using high recyclability ADC12 aluminum alloy die castings and the effect of the tool rotating rate and the number of passes on the porosity and morphology of the pores is investigated. It is shown that groove-like defect was formed because of abnormal stirring action at any tool rotating rate. However, by applying multi-pass FSP, the defect disappeared at 800 and 1000 rpm. On the other hand, the defect remained at 1200 and 1400 rpm instead of carrying out 4 pass FSP. The porosity and the morphology of pores of obtained porous aluminum were affected by these defects. It is shown that closed-cell porous aluminum with uniform pore size distribution and with porosity of about 50–60% is successfully fabricated using ADC12 aluminum alloy die castings at the tool rotating rate of 800–1000 rpm and a holding time of 10 minutes and a holding temperature of 933 K.

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